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Microwave assisted synthesis of ETS-4, ETS-10, ZIF-8, NTHU-4and fluorinated tin oxide materials

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MICROWAVE ASSISTED SYNTHESIS OF ETS-4, ETS-10, ZIF-8, NTHU-4 AND
FLUORINATED TIN OXIDE MATERIALS
APPROVED BY SUPERVISORY COMMITTEE:
___________________________________________
Dr. Kenneth J. Balkus Jr, Chair
___________________________________________
Dr. John P. Ferraris
___________________________________________
Dr. Bruce E. Gnade
Copyright 2006
Jose Antonio Losilla Yamasaki
All Rights Reserved
This work is dedicated to my extremely supporting and loving parents Jorge, Patricia, to
my brothers, Jorge and Juan K, to my family and my friends. To everyone kind enough to
put a smile on my face.
MICROWAVE ASSISTED SYNTHESIS OF ETS-4, ETS-10, ZIF-8, NTHU-4 AND
FLUORINATED TIN OXIDE MATERIALS
by
JOSE ANTONIO LOSILLA YAMASAKI, B.S.
THESIS
Presented to the Faculty of
The University of Texas at Dallas
in Partial Fulfillment
of the Requirements
for the Degree of
MASTER OF SCIENCE IN
CHEMISTRY
THE UNIVERSITY OF TEXAS AT DALLAS
DECEMBER, 2006
iv
UMI Number: 1442437
UMI Microform 1442437
Copyright 2007 by ProQuest Information and Learning Company.
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unauthorized copying under Title 17, United States Code.
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PREFACE
This thesis was produced in accordance with guidelines which permit the inclusion as part of
the thesis the text of an original paper or papers submitted for publication. The thesis must
still conform to all other requirements explained in the “Guide for the Preparation of
Master’s Theses and Doctoral Dissertations at The University of Texas at Dallas.” It must
include a comprehensive abstract, a full introduction and literature review and a final overall
conclusion. Additional material (procedural and design data as well as descriptions of
equipment) must be provided in sufficient detail to allow a clear and precise judgment to be
made of the importance and originality of the research reported.
It is acceptable for this thesis to include as chapters authentic copies of papers already
published, provided these meet type size, margin and legibility requirements. In such cases,
connecting texts which provide logical bridges between different manuscripts are mandatory.
Where the student is not the sole author of a manuscript, the student is required to make an
explicit statement in the introductory material to that manuscript describing the student’s
contribution to the work and acknowledging the contribution of the other author(s).The
signatures of the Supervising Committee which precede all other material in the thesis attest
to the accuracy of this statement.
v
ACKNOWLEDGEMENTS
I thank my advisor, Dr Kenneth J. Balkus, Jr. his exceptional guidance and support
during the research and writing of this thesis. I also thank my committee members, Dr. John
Ferraris, and Dr. Bruce Gnade for their contributions and assistance. Next, I will like to
acknowledge the Dr. Balkus group members, Dr. Ferraris group members, colleagues, and
the chemistry department for their kind assistance.
November, 2006
vi
MICROWAVE ASSISTED SYNTHESIS OF ETS-4, ETS-10, ZIF-8, AND
FLUORINATED TIN OXIDE MATERIALS
Publication No. ___________________
Jose Antonio Losilla Yamasaki, M.S.
The University of Texas at Dallas, 2006
Supervising Professor:
Dr. Kenneth J. Balkus, Jr.
ABSTRACT
Microwave heating is a relatively new and unexplored field in molecular sieve synthesis.
Microwave radiation allows a fast and efficient way to obtain diverse products. The rapid
microwave synthesis of ETS-4, ETS-10, ZIF-8, NTHU-4 and FTO particles compared with
conventional heating is presented. ETS-4, ETS-10 and ZIF-8 are promising nanoporous
materials for applications such as gas adsorption, while the white light emitting NTHU-4 and
conductive FTO are related to optical applications.
vii
TABLE OF CONTENTS
Acknowledgements............................................................................................................ vi
Abstract ............................................................................................................................. vii
List of Tables .......................................................................................................................x
Chapter 2 ..................................................................................................................x
Chapter 4 ..................................................................................................................x
List of Figures .................................................................................................................... xi
Chapter 1 ................................................................................................................ xi
Chapter 2 ............................................................................................................... xii
Chapter 3 .............................................................................................................. xiii
Chapter 4 .............................................................................................................. xiv
Chapter 5 ................................................................................................................xv
CHAPTER 1 MICROWAVE SYNTHESIS OF ETS-4 AND ETS-4 THIN FILMS†*....1
1.1
Introduction..................................................................................................2
1.2
Experimental ................................................................................................4
1.3
Results and discussion .................................................................................7
1.4
Conclusions................................................................................................13
1.5
Acknowledgement .....................................................................................14
APPENDIX ............................................................................................................15
REFERENCES.......................................................................................................23
CHAPTER 2 MICROWAVE ASSISTED SYNTHESIS OF ETS-10.............................27
2.1
Introduction................................................................................................28
2.2
Experimental Section .................................................................................31
2.3
Results and discussion ...............................................................................32
2.4
Conclusions................................................................................................35
2.5
Acknowledgement .....................................................................................35
viii
APPENDIX ............................................................................................................36
REFERENCES.......................................................................................................48
CHAPTER 3 MICROWAVE SYNTHESIS OF ZIF-8 ..................................................51
3.1
Introduction................................................................................................52
3.2
Experimental Section .................................................................................54
3.3
Results and Discussion ..............................................................................55
3.4
Conclusions................................................................................................59
3.5
Acknowledgement .....................................................................................59
APPENDIX ............................................................................................................60
REFERENCES.......................................................................................................73
CHAPTER 4 MICROWAVE SYNTHESIS OF GALLIUM ZINC PHOSPHATE NTHU-4
75
4.1
Introduction................................................................................................76
4.2
Experimental section..................................................................................78
4.3
Results and Discussion ..............................................................................80
4.4
Conclusions................................................................................................84
4.5
Acknowledgement .....................................................................................85
APPENDIX ............................................................................................................86
REFERENCES.....................................................................................................104
CHAPTER 5 HYDROTHERMAL SYNTHESIS OF FLUORINATED TIN OXIDE
NANOPARTICLES.........................................................................................................107
5.1
Introduction..............................................................................................108
5.2
Experimental Section ...............................................................................113
5.3
Results and discussion .............................................................................114
5.4
Conclusions..............................................................................................122
5.5
Acknowledgement ...................................................................................122
APPENDIX ..........................................................................................................123
REFERENCES.....................................................................................................133
VITA
ix
LIST OF TABLES
TABLE
PAGE
CHAPTER 2
Table 1. Synthesis conditions for ETS-10 products using conventional heating. .......36
Table 2. Synthesis conditions for ETS-10 products using microwave heating. ..........37
CHAPTER 4
Table 1. The synthesis conditions for NTHU-4 using conventional heating at
160°C and 400 watts. (OA= Oxalic Acid, EG= Ethylene glycol, Y=yellow,
O=orange, W=white) .......................................................................................... 86
Table 2. The synthesis conditions for NTHU-4 using microwave oven at
160°C and 400 watts. (OA= Oxalic Acid, EG= Ethylene glycol, Y=yellow,
O=orange, W=white) .......................................................................................... 87
x
LIST OF FIGURES
FIGURE
PAGE
CHAPTER 1
Figure 1 ETS-4 structure modeled using Materials Studio and crystallographic
data from reference 14. (A) In box Ti-O-Ti chains shown, (a-b plane),
and (B) 8 membered rings (a-c plane). (black: Si; grey: Ti; white: O).................15
Figure 2. Powder XRD patterns of ETS-4. A) pattern calculated using Materials
Studio and the data from reference 14, and ETS-4 synthesized by
microwave heating for B) 40 minutes, C) 50 minutes, D) 60 minutes,
and E) 120 minutes. ...............................................................................................16
Figure 3. SEM images of ETS-4 prepared by microwave heating for A and B)
40 minutes; C and D) 50 minutes; E and F) 120 minutes......................................17
Figure 4. A) Powder XRD pattern calculated using Materials Studio
and the data from reference 14; and powder XRD patterns of ETS-4 s
ynthesized by conventional heating at 200°C for (B) 14 hours, (C) 24 hours
and (D) 48 hours. ...................................................................................................18
Figure 5. SEM images of ETS-4 prepared by conventional heating at 200°C.
A and B) 14 hours; C and D) 24 hours; E and F) 48 hours....................................19
Figure 6. SEM image of PLD ETS-4 films after microwave synthesis for
60 minutes at 235°C. Images a) and b) show a cross section view
while images c) and d) show a top surface view. ..................................................20
Figure 7. SEM image of an ETS-4 films after post hydrothermal treatment in a
conventional oven for 24 hours. A, B, and C) film grown at 180ºC; D, E,
and F) film grown at 150ºC....................................................................................21
Figure 8 .XRD patterns of A) ETS-4 film on alumina disk after hydrothermal
treatment in microwave oven for 60 minutes at 235˚C; B) ETS-4 films
on alumina disc grown at 180ºC for 24 hours; C) film grown at 150ºC
for 24 hours. ...........................................................................................................22
xi
LIST OF FIGURES
FIGURE
PAGE
CHAPTER 2
Figure 1. ETS-10 structure calculated using JADE software and data
from reference [2]. (A) View through <110> axis. (B) View through
<001> axis........................................................................................................38
Figure 2. SEM image of ETS-10 sample CH2 after 96 h, at 230°C in a
conventional oven. ...........................................................................................39
Figure 3. XRD pattern for ETS-10 using conventional heating; (A) sample CH6
as reference, (B) sample CH2. .........................................................................40
Figure 4. SEM image of ETS-10 sample CH4 after 72 h, at 230°C, using
Degussa P25 as titanium precursor, with a pH of 10.45 in presence
of hydrogen peroxide. ......................................................................................41
Figure 5. SEM image of ETS-10 sample CH5 after 72 h, at 230°C, using
Degussa P25 as titanium precursor, with a pH of 10.6 without
hydrogen peroxide. ..........................................................................................42
Figure 6. XRD patterns of ETS-10 samples using P25 as titanium source at
230°C, for 3 days with; (A) sample CH6 as reference, (B) sample CH4
with a pH of 10.5, (C) sample CH5 with a pH of 10.6 without hydrogen
peroxide. Arrows show quartz. ........................................................................43
Figure 7. SEM image of ETS-10 sample MW2 after 15 h at 210°C by
microwave heating. ..........................................................................................44
Figure 8. XRD patterns of microwave products after (A) sample MW3
after 22 h and (B) sample MW2 after 15 h at 210°C showing formation
of ETS-10 and ETS-4.......................................................................................45
Figure 9. SEM images for ETS-10 sample MW4 after 18 h of microwave
heating at 230°C...............................................................................................46
Figure 10. XRD pattern of ETS-10 (A) Sample CH6 as reference, (B) Sample
MW5 using microwave oven, at 230°C for 24 h. ............................................47
xii
LIST OF FIGURES
FIGURE
PAGE
CHAPTER 3
Figure 1. Structure of ZIF-8 calculated using Materials Studio and the crystal
data in ref [9]. A) View along the 001 axis, B) View along the 111 axis.
and C) View along the 100 axis.......................................................................60
Figure 2. XRD patterns of ZIF-8 material obtained by conventional heating at
140°C and 20 hours; A) Calculated pattern using data from ref [9],
B) ZIF-8 orange crust, and C) the ZIF-8 white powder after s
edimentation.....................................................................................................61
Figure 3. SEM image of the ZIF-8 crystals present in the
orange crust......................................................................................................62
Figure 4. SEM image of the ZIF-8 crystals in the white powder obtained from
the synthesis in regular oven at 140°C for 20 hr..............................................63
Figure 5. Simulated ZIF-8 pattern and temperature effect on ZIF-8 samples on
syntheses, at 140°C, 145°C, 155°C, 160°C, 170°C, and 180°C for 1 hour.
(Arrows mark ZnO) .........................................................................................64
Figure 6. SEM image showing a ZIF-8 crystal after heating at 180°C for 1 hour. .....65
Figure 7. Simulated ZIF-8 pattern and temperature effect on ZIF-8 samples on
syntheses, at 140°C,150°C, 160°C, 170°C, and 180°C for 2 h.
(Arrows mark ZnO). ........................................................................................66
Figure 8. Simulated ZIF-8 pattern and time effect on ZIF-8 samples when
heated at 140°C for b) 1.0 h, c) 1.5 h, d) 2.0 h, and e) 2.5 h..........................67
Figure 9. SEM image showing the unknown product obtained after heating
the reaction mixture at 140°C for 1.5 h. ..........................................................68
Figure 10. XRD pattern for: simulated ZIF-8, products obtained after heating
at 180°C for 0.5 h, 1.0 h, 1.5 h, 2.0 h and crystals collected from
filtrate after three days. ....................................................................................69
Figure 11. SEM image showing a ZIF-8 crystal after heating at 180°C
for 0.5 h............................................................................................................70
Figure 12. XRD patterns showing the effect of sonication with MeOH on the
ZIF-8 sample for 180°C and 1 h. .....................................................................71
Figure 13. SEM image for ZIF-8 crystals produced at 180°C for 1 h after
sonication with MeOH.....................................................................................72
xiii
LIST OF FIGURES
FIGURE
PAGE
CHAPTER 4
Figure 1. NTHU-4 structure calculated using with Materials Studio, and the
crystallographic data in ref [21].......................................................................... 88
Figure 2. Powder XRD patterns of NTHU-4. (A) Pattern calculated using
Materials Studio and the data from Ref. [21], and NTHU-4 synthesized by
conventional heating for (B) 1day, (C) 2days, (D) 3days, (E) 4days,
(F) 5days, (G) 6days, (H) 7days.......................................................................... 89
Figure 3. Emission spectra under a 390nm excitation for NTHU-4 samples
produced by regular heating after (A) 1 day, (B) 2 day, (C) 5 day,
(D) 7 day. ............................................................................................................ 90
Figure 4. SEM image for NTHU-4 product by conventional heating after 1 day
at 160°C. ............................................................................................................. 91
Figure 5. SEM image for NTHU-4 product by conventional heating after 2 days
at 160°C. ............................................................................................................. 92
Figure 6. SEM image for NTHU-4 product by conventional heating after 3 days
at 160°C. ............................................................................................................. 93
Figure 7. SEM image for NTHU-4 product by conventional heating after 4 days
at 160°C .............................................................................................................. 94
Figure 8. SEM image of NTHU-4 laminates (left) and gallium phosphate (right)
after 7 day at 160°C using conventional oven .................................................... 95
Figure 9. Powder XRD patterns of simulated NTHU-4, and the microwave
products for 1 h, 2 h, 3 h, 4 h, 5 h, 7 h, and 10 h. ............................................... 96
Figure 10. SEM image for a NTHU-4 sample, with crossing stick morphology
after 2h at 160°C using microwave oven............................................................ 97
Figure 11. Emission spectra for microwave NTHU-4 material for 2 hour
(A) 365 nm excitation, (B) 390 nm excitation and 10 hour (C) 365 nm
excitation and (D) 390nm excitation................................................................... 98
Figure 12. SEM image for NTHU-4 material after 4 hour synthesis by
microwave oven .................................................................................................. 99
Figure 13. SEM image for a NTHU-4 sample after 10h at 160C, using the
microwave oven. ............................................................................................... 100
Figure 14. XRD pattern of simulated NTHU-4 pattern and microwave synthesis
powder after 4 hours at 155°C,160°C, 165°C and 170°C................................. 101
Figure 15. Powder XRD patterns of NTHU-4 by microwave heating at 160°C
with after mixing the precursor for 24 h. .......................................................... 102
xiv
Figure 16. Emission spectra for NTHU-4 samples synthesized by microwave
heating with a mixing time of (A) 48hr, (B) 56hr, (C) 64hr, (D) 72 hr,
excitation at 390nm........................................................................................... 103
CHAPTER 5
Figure 1. Unit cell of cassiterite (SnO2) crystalline structure. Sn atoms
(gray) are sixfold coordinated to three-fold coordinated oxygen atoms
(red). Data and image by Materials Studio. ...................................................123
Figure 2. TEM image showing the crystal lattices of a FTO particle and
Selected Area Diffraction (SAD)after heating at 160°C for 24 hours. ..........124
Figure 3. FTO particles stirred overnight and then heated at 165°C for 24 h. .........125
Figure 4. FTO particles made using SnCl4·5H2O as tin precursor, after
heating at 185°C for 3 days............................................................................126
Figure 5. SEM image of FTO particles after calcination at 450°C. for a
sample produced using EtOH and DME and heating at 165°C
for 24 hours. ...................................................................................................127
Figure 6. SEM image of FTO particles using DME, heating at 165°C
for 24 h...........................................................................................................128
Figure 7. TGA weight loss percent graph for;(A) with DME (black)
and (B) without DME (gray)..........................................................................129
Figure 8. TGA derivative graph for FTO particles; a) with DME (black)
and b) without DME (gray)............................................................................130
Figure 9. XRD pattern for calcined FTO particles produced with ethanol
and DME........................................................................................................131
Figure 10. XRD pattern for calcined FTO particles produced without DME . .........132
xv
CHAPTER 1
MICROWAVE SYNTHESIS OF ETS-4 AND ETS-4 THIN FILMS†*§
Decio Coutinho, Jose A. Losilla, and Kenneth J. Balkus Jr.*
Department of Chemistry and the UTD NanoTech Institute, University of Texas at Dallas,
Richardson TX 75083-0688 United States
Abstract. Engelhard titanosilicate (ETS-4) was successfully synthesized by microwave
heating at 235ºC within 50 minutes. ETS-4 was synthesized using titanium(IV) butoxide as
the titanium source. Microwave irradiation shortened the synthesis time considerably as
compared to conventional heating which resulted in the rapid synthesis of ETS-4 in less than
1 hour compared to 36-48 hours for the traditional synthesis. Pulsed laser deposition and
microwave treatment was also studied for the preparation of ETS-4 films on alumina..
Keywords: Microwave synthesis; ETS-4; Zeolites; Titanosilicates; PLD
†Dedicated to the late George Kokotailo
*Corresponding authors: Tel: 972-883-2659
Fax: 972-883-2925
E-mail address: balkus@utdallas.edu
1
2
1.1
Introduction
The titanosilicate ETS-4 is a small pore member of the Engelhard Titanosilicate (ETS)
family. Since Kuznicki [1] first reported the synthesis of ETS-4 in 1990, there has been a
growing interest in ETS-4 as evidenced by several studies reported in the literature [2-14].
The ETS-4 structure consists of an interconnected octahedral–tetrahedral framework with
narrow eight-membered rings pore openings as shown in Figure 1 [2]. The ETS-4 structure
involves corner sharing SiO4 tetahedra and TiO6 octahedra units as well as TiO5 units [2].
The ETS crystals have been studied for adsorptive separations [2,15-19]. This makes ETS-4
an attractive material for important commercial applications such as separations of gas
mixtures with similar molecular size [2,15,16,18], as well as safe storage and removal of
radioactive elements from nuclear waste [19].
Access to the crystal interior of ETS-4 occurs through the eight-membered rings. This
produces a two dimensional transport within the b-c plane [2]. By exchanging cations and
thermal treatment, a “molecular gate” effect can be achieved. This effect allows for pore size
manipulation and separation of molecules of very similar size such as nitrogen and methane.
The presence of TiO6 chains (Figure 1) formed within the channel direction, could also find
application as quantum wires [4-7]. Some potential applications include chemical sensors,
photochemistry and nonlinear optics [7]. Therefore there is interest in controlling the crystal
growth purity and morphology of ETS-4.
Microwave heating techniques are now widely used in many applications of chemical
research including organic/inorganic synthesis [20-26]. Microwaves have been used for
§
Reprinted from Microporous and Mesoporous Materials, Vol. 90, Decio Coutinho, Jose A.
Losilla, Kenneth J. Balkus Jr. , Microwave Synthesis of ETS-4 and ETS-4 thin films, 229236, Copyright (2006), with permission from Elsevier.
3
accelerating chemical reactions because microwave syntheses are generally faster, simpler,
and energy efficient [21,26-34]. An important application of microwave radiation is in the
synthesis of nanoporous materials, such as zeolites, from aqueous media [27-44]. This
technique can be combined with hydrothermal crystallization to shorten significantly the
synthesis time for zeolites. For the synthesis of zeolites in a microwave, volumetric heating
produces a rapid nucleation and growth as well as fast supersaturation of the reaction
mixture, which leads to shorter reaction times. Additionally, microwave radiation enables
the suppression of undesired phases compared with conventional heating [33]. Several
zeolites have been prepared using microwave heating. For example, microwave synthesis
has being successfully used to synthesize mesoporous silica [36], zeolite A [30], MCM-41
[29], beta [28], ZMS-5 [27], TS-2 [41] and metal substituted aluminophosphates [37]. In all
instances there was a significant reduction in the synthesis time. For example, zeolite beta
was produced in 4 hours using a microwave assisted synthesis, while the synthesis using
conventional heating takes ~60 hours [28].
Microwave heating may also assist in the preparation of ETS-4 films. Applications for ETS4 films such as gas separation or catalysis may require a thin, continuous layer of the crystals
on a substrate. By using pulsed laser deposition (PLD) and hydrothermal treatment such
characteristics were achieved for materials such as UTD-1 [45,46], MCM-22 [47], MCM-41
[48], NaX [49,50], MAPO-39 [51], FeAPO-5 [52], AlPO4-5 [53], mordenite [54], silicalite
[55]. In the PLD method, a tightly packed layer of molecular sieve fragments is deposited
onto a substrate via laser ablation. The laser-deposited film is subsequently subjected to a
hydrothermal treatment to reorganize the PLD film producing crystalline molecular sieve
thin films. In this paper, we report a faster and more effective synthesis of ETS-4 by using
4
microwave heating.
The ETS-4 products show a high degree of crystallinity free of
undesired amorphous SiO2, TiO2 or other titanosilicate phases. Microwave heated ETS-4
laser deposited films on alumina are compared with oven heated as well as other reported
seed methods for preparing ETS-4 films.
1.2
Experimental
1.2.1
Microwave synthesis
The synthesis recipe for ETS-4 was adapted from a published literature procedure [16,17]. In
a typical synthesis, two separate solutions were prepared. Solution 1 was prepared by
dissolving 3.6 g of silica gel (Alfa Aesar, 60Å) and 3 g of NaOH (EMD) in 10 mL of
deionized water. Solution 2 was prepared by adding 1.8 g of titanium(IV) butoxide (Aldrich,
98%) to a solution containing 0.37 g of NaOH and 24 mL of deionized water. To this
solution, 5 mL of H2O2 (EM Science, 30%) was added and solution 2 was stirred at room
temperature until a light yellow solution was obtained. The light yellow solution 2 and an
extra 35 mL of deionized water were added to solution 1 and stirred for an additional ~30
minutes. 30mL of the resulting clear yellow reaction mixture having molar composition of
16 NaOH : 1 TiO2 : 11.33 SiO2 : 761.3 H2O : 8.5 H2O2 (pH ~12.1) were placed in a 95 mL
CEM XP-1500 Plus Teflon reaction vessel and heated to 180-235ºC in a CEM
MARSXpress™ microwave (CEM Corporation) at 2.45 GHz. The heating time was varied
from 20 minutes to 5 hours.
A ten minute ramp time was used to reach the desired
temperature and the power did not exceed 300W. The reaction temperature was controlled
via a reference vessel. The temperature probe was placed inside a glass thermowell in the
reference vessel. Because of the high basicity of the reaction mixture (pH ~12.5), the pH of
5
the mixture in the reference vessel was adjusted to ~7 with 4 M HCl to avoid the dissolution
of the probe’s glass thermowell.
During the synthesis the temperature was controlled
automatically and the pressure was monitored. The zeolite product was then filtered, washed
with deionized water and dried at 70ºC for 24 hours.
1.2.2
Conventional heating synthesis
ETS-4 was also prepared by regular heating. The clear yellow reaction mixture of the same
composition as described above was placed in a 23 mL Teflon-lined Parr autoclave and
heated to 200°C for a period of 14-48 hours. The zeolite product was then filtered, washed
with deionized water and dried at 70ºC for 24 hours.
1.2.3
Pulsed laser deposition of films
ETS-4 powder was impregnated with ferrocene by sublimation. 1 g of ferrocene (Aldrich,
98%) was placed at the bottom of the test tube (200 ml) and covered with a layer of glass
wool. A layer of ETS-4 (1.5 g) was then placed on top of the glass wool and the test tube
was placed in an oven at 135ºC for ~48 h. After cooling the material to room temperature,
the resulting brown solids were washed with dichloromethane (EM Science, 99.9%) until a
colorless filtrate was obtained and the brown product was dried at room temperature.
A pressed pellet (diameter ~2.5 cm, thickness ~3 mm) of ferrocene-ETS-4 was mounted in a
controlled atmosphere chamber as previously described [45,46]. Laser depositions were
performed using a pulsed Lambda Physik Compex 102 excimer laser operating at 248 nm
(KrF) and at a frequency of 10 Hz. The target pellet was rotated at ~48 rpm to provide a
continuously fresh target surface. The laser was focused into a spot of size 0.001 cm2 upon
entering the ablation chamber. The substrate was alumina discs (Keir Manufacturing, Inc.).
6
The substrate was heated in vacuo at 400–500ºC for 30 min prior to laser ablation to remove
any absorbed organics. Typical experimental conditions were as follows: laser power, 50–70
mJ/pulse; repetition rate 10 Hz; substrate temperature 200–250ºC; oxygen background
pressure 180–220 mTorr; and deposition time of 40 min. The rate of growth of the ETS-4
film was ~27 nm/min.
1.2.4
Hydrothermal Treatment
In the microwave heating hydrothermal treatment; the PLD coated substrates were suspended
horizontally with the ablated side facing down (~1 cm from the bottom) in a 95 mL CEM
XP-1500 Plus Teflon reaction vessel and heated at 235ºC in a CEM MARSXpress™
microwave (CEM Corporation) at 2.45 GHz. The heating time was varied from 20 minutes
to 60 minutes. For the hydrothermal treatment using conventional heating, the PLD films
were suspended as described above in a 23 mL Teflon-lined Parr autoclave and heated to 150
and 180ºC for 24 hours.
1.2.5
Analysis
Powder X-ray diffraction patterns were obtained using a Rigaku Ultima III X-ray
diffractometer using CuKα radiation.
Samples for scanning electron microscopy were
coated with Pd/Au and micrographs obtained on a Leo 1530 VP electron microscope. The
degree of crystallinity was estimated by comparing the area below the peaks with d-spacing
of 11.7, 6.99, 5.29, 4.47, 3.43, 3.39, 3.05, 2.99, and 2.59 Å with that of a reference sample.
The reference sample used was a sample prepared by conventional heating at 200ºC for 48
days.
7
1.3
Results and discussion
1.3.1
Bulk ETS-4 Crystals
ETS-4 was synthesized by conventional and microwave heating from clear synthesis
mixtures as described in the experimental section. The effects of reaction temperature and
reaction time were studied and all syntheses were carried out using titanium(IV) butoxide
and silica gel as the titanium and silica sources. This particular synthesis was used because it
has been shown to produce highly crystalline ETS-4 with an intergrown plate-like
morphology [16,17]. The syntheses by conventional heating were carried out at 200ºC for
12–48 hours, while the microwave syntheses were performed at 180, 200, and 235ºC. For
the microwave synthesis, the temperature was ramped from room temperature to the desired
temperature in 10 minutes and then held at the desired temperature for 20 minutes to 5 hours.
Therefore, for a 1-hour synthesis, there was a 10-minute ramp time to the desired temperature
and then the temperature was held constant at the desired temperature for 1 hour. The
microwave power was adjusted automatically to maintain this heating rate but did not exceed
300W.
The first microwave synthesis was attempted at 180ºC for ~5 hours, which yielded an
amorphous product.
ETS-4 microwave synthesis at 200ºC for 5 hours yielded poorly
crystalline powder. Then the synthesis temperature was increased to 235ºC for 2 hours.
During the synthesis, the pressure in the vessel was monitored and measured at 26.7 bars.
Figure 2 shows the powder X-ray diffraction patterns for ETS-4 crystals obtained by
microwave synthesis.
As shown in Figure 2e, the 2-hour synthesis produced highly
crystalline ETS-4. The XRD pattern compares well with published XRD patterns for ETS-4
and with a calculated XRD pattern for ETS-4 using the data in reference 14. The XRD
8
pattern in Figure 2e does not indicate the presence of any obvious impurities. Therefore, the
synthesis time was decreased to 1 hour. This synthesis also produced crystalline ETS-4
without any detectable impurities as confirmed by the XRD pattern in Figure 2d. However, a
synthesis time of only 30 minutes yielded amorphous materials. When the synthesis time
was increased to 40 minutes, the product was ETS-4 as shown in Figure 2b. The ETS-4
produced by the 40-minute synthesis does not appear to be as crystalline as that for the 1- and
2-hour syntheses. The crystallinity of this sample was estimated to be ~48%. The XRD
pattern does not indicate that there are other crystalline phases such as ETS-10 or the dense
phase (Grace Titanosilicate one, GTS-1) present [13,14,57]. These phases are normally the
impurities formed during the synthesis of ETS-4. Increasing the reaction time to 50 minutes,
yielded highly crystalline ETS-4 as evidenced by the XRD pattern in Figure 2c.
The
crystallinity of this sample was estimated at ~90%. Therefore, pure ETS-4 can be prepared
in less than one hour by microwave heating.
Figure 3 shows the SEM images of the ETS-4 produced by the microwave synthesis. As
shown in Figures 3a and 3b, the 40-minute synthesis yielded polycrystalline ETS-4.
However, there were large amounts of unreacted amorphous material present in the product.
The ETS-4 crystals are composed of aggregated thin plate-like crystallites (~100-200 nm
thick, and ~10 µm long). The 50-minute synthesis yielded polycrystalline ETS-4, and shows
minor amounts of unreacted materials (Figures 3c and 3d). The 50-minute synthesis ETS-4
crystals are composed of stacked thin plates and in some cases these plates are starting to
form the intergrown spherulite morphology, which is common for ETS-4 crystals synthesized
by conventional heating [4]. The 1-hour synthesis also produced the stacked plate-like
crystallites (not shown), and the 2-hour synthesis product was composed mainly of
9
spherulites of intergrown crystals. (Figures 3e 3f). There were no noticeable unreacted
materials after 1 hour.
For comparison, ETS-4 was synthesized by conventional oven heating, Figure 4 shows the
XRD patterns for the products. Three samples were synthesized at 200ºC for 14, 24, and 48
hours. As seen in Figure 4, all three syntheses produced ETS-4. However, the 14 and 24hour syntheses also produced GTS-1 phase as evidenced by the appearance of a reflection at
~11.4º 2θ. The 24-hour synthesis shows an even more pronounced presence of this impurity
phase as evidenced by the sharp increase in the intensity of the peak at ~11.4 º2θ. The 48hour synthesis does not show any evidence of this impurity phase. The XRD pattern for the
48-hour synthesis shows a pure and fully crystalline ETS-4. Using a similar synthesis
composition at 200ºC, Nair and coworkers [14] showed that the dense GTS-1 forms first and
then a phase transformation to ETS-4 occurs. Initially, most of the titanium present is
consumed by the formation of GTS-1 (Si:Ti = 0.75). It is believed that this inhibits the
formation of ETS-4 (Si:Ti ~2.4). The growth of ETS-4 is then enhanced by the dissolution
of GTS-1, which provides the nutrients.
Figure 5 shows the SEM images of the syntheses performed using conventional heating at
200°C. As shown in Figures 5a and 5b, there are some amorphous silicon and titanium oxide
present in the product for the 14-hour synthesis. The crystal morphology is similar to the one
seen in the 40-minute microwave synthesis with intergrown plate-like crystals. From the
SEM images, it is difficult to distinguish the GTS-1 crystals. For the 24-hour synthesis, there
was less unreacted titanium and silica present in the product (Figures 5c and d).
As
confirmed by the XRD analysis, there was an increase in the amount of the dense phase
GTS-1 for the 24-hour synthesis. The 48-hour conventional heating synthesis yielded pure
10
ETS-4 as confirmed by XRD and SEM. The SEM images for the 48-hour synthesis product
are shown in Figures 5e and f. The ETS-4 crystals are composed of intergrown plate like
crystallites. These cauliflower crystal shapes are similar to the 2-hour microwave synthesis.
3.2 ETS-4 Films
ETS-4 films were prepared by pulsed laser deposition by irradiating a 2.5 cm ETS-4 pressed
pellet that had been impregnated with ferrocene. The organometallic serves to assist the
ablation process [56]. The resulting PLD films were amorphous to x-rays and, therefore,
were subjected to hydrothermal treatment to reorganize the ETS-4 fragments to the
crystalline zeolite. The PLD layer is essential to the growth of a crystalline zeolite film and
this has been previously demonstrated during the preparation of various zeolite films [45-56].
The microwave hydrothermal treatments were studied for 20, 30, 40, 50 and 60 minutes at
235°C. The PLD ETS-4 films on alumina discs were introduced in the synthesis mixture
using a Teflon holder. Both the film and the product from the reaction mixture were
analyzed after each synthesis. Under 40 minutes no ETS-4 was evident by means of XRD.
For the syntheses over 40 minutes, ETS-4 was noticeable after XRD analysis of the film as
well as the bulk powder in the bottom of the autoclave.
The SEM images in Figure 6 show cross section and surface views for ETS-4 on alumina
discs after hydrothermal treatment. In Figure 6a there are several rod shaped structures that
are ~3 µm long and ~0.6 µm thick as well as the expected plate like crystallites. Figure 6b
shows a ~7.5 µm thick film of ETS-4 crystals. Figures 6c and 6d show a top view of the
ETS-4 film. It can be seen that coverage is not completely homogeneous and there are spots
that were not fully covered. Increasing the laser deposition time should produce a more
11
continuous film. While the PLD derived ETS-4 film is reasonably crystalline with no
obvious impurities, there is no evidence of any preferred orientation.
The films prepared by conventional heating Figure 7 contain the aluminosilicate phase
analcime in addition to ETS-4. The hydrothermal treatment of ETS-4 PLD films on alumina
were performed at two temperatures (150 and 180ºC) for 24 hours. The films prepared by
hydrothermal treatment at 180ºC for 24 hours (Figures 7a, 7b, and 7c) appear to be
continuous and the cross section view of the film (Figure 7b) does not indicate a preferred
orientation. The SEM images also show the aluminosilicate phase analcime as evidenced by
the presence of the large crystals on top of the ETS-4 films. In an attempt to curb the
formation of analcime, the hydrothermal treatment was performed at 150ºC for 24 hours.
The SEM images of a film prepared at 150ºC are shown in Figures 7d, 7e, and 7f. As in the
previous case, the ETS-4 film appears to be continuous (at this magnification) and it is
composed of plate-like crystals with a thickness in the 10 µm range. This film also shows
the presence of analcime. The analcime crystals in this case are not as prevalent.
Figure 8 shows the XRD pattern of an ETS-4 film after microwave hydrothermal treatment at
235ºC for 60 minutes. The XRD pattern for the film (Figure 8a) compares well with the
XRD pattern for the bulk powder (Figure 2d).
The extra peaks are from the alumina
substrate (corundum phase). The XRD pattern for the film does not indicate the presence of
a preferred orientation.
The XRD patterns for ETS-4 films grown on alumina by
conventional heating are shown in Figures 8b and 8c. The XRD pattern for the film grown at
180ºC for 24 hours (Figure 8b) indicates a substantial presence of analcime. In fact, the
peaks corresponding to ETS-4 are of very low intensity when compared to the ones for
analcime. The bulk powder for the synthesis at 180ºC shows the presence of the impurity
12
phase GTS-1 as evidenced by the appearance of the peak at 11.4º 2θ (not shown). The XRD
pattern for the film grown at 150ºC for 24 hours also shows the presence of analcime, and the
XRD pattern also shows the presence of an unknown impurity as indicated by the peak at
~6.8º 2θ. However, there is no evidence of the presence of GTS-1 in the films. The XRD
pattern for the film grown at 150ºC suggests that this film is partially oriented as evidenced
by emergence of the peak at ~12.85º 2θ, which correspond to the (110) reflection. This
reflection appears as a shoulder to the peak corresponding to the (001) reflection in the bulk
powder. This was demonstrated in a recent publication where oriented ETS-4 films on
alumina were reported [58]. Tsapatsis et al [17] also reported a b-out-of-plane orientation
with significant (110), (020) and (221) reflections for ETS-4, using a double hydrothermal
procedure on titania substrates. In the present study, because of the presence of the analcime
phase, it is difficult to tell with certainty that oriented ETS-4 films were formed.
The main difference between the two methods of PLD film reorganization (microwave
versus conventional oven) is the appearance of impurity phases in the regular heating
synthesis. For the ETS-4 films prepared by regular heating, the dense aluminosilicate,
analcime was formed on the surface as evidenced by XRD and SEM.
The formation
occurred even when the post hydrothermal treatment was performed at 150ºC for 24 hours.
The analcime was likely formed by partial dissolution of alumina from the alumina disc
substrates. The as deposited PLD film had a thickness of ~1 µm; only one side of the
alumina disk was coated with the PLD film. This phase did not appear in the microwave
heated ETS-4 films.
13
1.4
Conclusions
It is apparent from this study that microwave heating accelerates the crystallization of ETS-4
while avoiding the formation of undesired impurities. The accelerated ETS-4 crystallization
can be assigned to the increased heating rate of the synthesis mixture. In the microwave
synthesis of zeolites, this has been attributed to the fact that water absorbs microwave
radiation strongly due to its high dielectric constant [28]. In fact, in a recent study on the
synthesis of silicalite, Conner et al. [31] concluded that the rapid heating and the creation of
hot spots contributed to an increase in the reaction rates of silicalite. In addition, microwave
irradiation enhances the dissolution of the gel [27]. It has been suggested that overheating
accounts for the observed increased reaction rates [32]. In this study, microwave syntheses
were performed at 235ºC, while conventional heating syntheses were performed at 200ºC.
This overheating coupled with the rapid heating of the reaction mixture led to the led to
increased reaction rates.
Pure ETS-4 can be produced in 1 hour, which is a vast improvement over the standard
reaction methods. The fastest ETS-4 crystallization procedure previously reported was in ~8
hours at 210ºC and ~20 hours at 190ºC [13]. In addition to accelerating the crystallization,
the microwave heating did not produce the GTS-1 phase that resulted from the conventional
heating. Finally, pulsed laser deposition and post hydrothermal microwave treatment allows
for the formation of a thin film of ETS-4 over an alumina substrate without the formation of
GTS-1 or analcime.
14
1.5
Acknowledgement
We thank the Robert A. Welch Foundation and SPRING, for the supporting of this project.
APPENDIX
Figure 1. ETS-4 structure modeled using Materials Studio and crystallographic data from
reference 14. (A) In box Ti-O-Ti chains shown, (a-b plane), and (B) 8 membered rings (a-c
plane). (black: Si; grey: Ti; white: O).
15
16
Figure 2. Powder XRD patterns of ETS-4. A) pattern calculated using Materials Studio and
the data from reference 14, and ETS-4 synthesized by microwave heating for B) 40 minutes,
C) 50 minutes, D) 60 minutes, and E) 120 minutes.
17
Figure 3. SEM images of ETS-4 prepared by microwave heating for A and B) 40 minutes; C
and D) 50 minutes; E and F) 120 minutes
18
Figure 4. A) Powder XRD pattern calculated using Materials Studio and the data from
reference 14; and powder XRD patterns of ETS-4 synthesized by conventional heating at
200°C for (B) 14 hours, (C) 24 hours and (D) 48 hours.
19
Figure 5. SEM images of ETS-4 prepared by conventional heating at 200°C. A and B) 14
hours; C and D) 24 hours; E and F) 48 hours.
20
Figure 6. SEM image of PLD ETS-4 films after microwave synthesis for 60 minutes at
235°C. Images a) and b) show a cross section view while images c) and d) show a top
surface view.
21
Figure 7. SEM image of an ETS-4 films after post hydrothermal treatment in a conventional
oven for 24 hours. A, B, and C) film grown at 180ºC; D, E, and F) film grown at 150ºC.
22
Figure 8. XRD patterns of A) ETS-4 film on alumina disk after hydrothermal treatment in
microwave oven for 60 minutes at 235˚C; B) ETS-4 films on alumina disc grown at 180ºC
for 24 hours; C) film grown at 150ºC for 24 hours.
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(2005) article in press.
CHAPTER 2
MICROWAVE ASSISTED SYNTHESIS OF ETS-10
Abstract. Engelhard Titanium Silicate-10 (ETS-10) has attracted interest for gas adsorption,
catalysis, separations and ion-exchange. Practical application of ETS-10 would benefit from
improvements in the synthesis. In this paper we discuss a 1 day microwave assisted synthesis
at 230°C which can be compared with the 4 days required by conventional heating. The
effects of different titanium precursors, pH, heating time, and organic templates on ETS-10
were also investigated.
Keywords: Microwave synthesis, ETS-10, microporous.
27
28
2.1
Introduction
Engelhard Titanium Silicate-10 (ETS-10) is part of a unique family of molecular sieve
materials, first reported Kuznicki [1]. In 1994 the structure of ETS-10 having the general
formula M2TiSi5O13·nH2O (M = K, Na) [2, 3] was reported. The structure of ETS-10
consists on tetrahedral SiO4 and octahedral TiO6 units that form 12, 7, 5, and 3-membered
rings as shown in Figure 1. The one dimensional pores are defined by 12-membered rings
having dimension of 7.6Å x 4.9Å. The titanium atoms form a continuous set of orthogonal
Ti-O-Ti chains. ETS-10 has been reported to be thermally stable up to 480°C. [4]
ETS-10 has attracted interest in areas such as catalysis, adsorption, separations and ionexchange. For example, there have been reports of ETS-10 catalyzed conversion of 2propanol to acetone [5], the photocatalytic degradation of alcohols [6], the dehydration of nbutanol [7], the isomerization of 1,3,5-trimethylbenzene [7], and the isomerization of mxylene [7]. ETS-10 has also been used for the adsorption of nitrogen, hydrogen, carbon
dioxide and nitric oxide [8], as well as the separation of ethanol/water mixtures [9]. The ionexchange properties of ETS-10 have been exploited in the adsorption of metals such as Pb2+,
Cd2+,Cu2+, and Zn2+ with a faster and higher capacity compared with Zeolite NaY [10].
There have been several attempts to optimize the synthesis of ETS-10, which includes
studying the effect of molar ratios, reagents, pH, synthesis time and temperature, as well as
controlling the crystal size, purity, and costs. There is interest in finding new titanium
sources to replace the relatively expensive TiCl3 reported in the original procedure [1].
Recently, TiO2 Anatase was used as a starting material and ETS-10 formed after 24 h [11].
However, an anatase impurity was present in the product. When TiCl4 was used [4], to make
ETS-10, it was found that at a pH of 11.7 ETS-10 could be made at 190°C in 72 h [4]. TiF4
29
was also compared with TiO2 as starting material by Yang et al. [12] for the synthesis of
ETS-10. In the case of TiO2, ETS-10 seeds were used, and the crystallization was monitored
from 14-168h at 200°C with a pH of 10.5. It was shown that in less than 24 h quartz and
anatase were present, while at 42 h only ETS-10 was formed. After 48 hours ETS-4 started to
form, and at 168h ETS-4 was the only product present. In addition, Degussa P25 produced
~1µm crystals after 42 h while TiF4 produces ~1µm crystals with tetramethylammonium
chloride (TMACl) and ~4µm without TMACl after 168 h. When using anatase, rutile and
Degussa P25 as the titanium source, Liu and Thomas [13] reported that F- was essential,
however, Rocha et al showed that ETS-10 could be made from anatase without F- [11].
Other Ti sources for ETS-10 reported include (NH4)2F6Ti [14], TiOSO4 [15] , and Na2TiF6
[16]. When Ti(SO4)2 was used as the titanium source, largely to determine the importance of
K+, Na+, and F- on the nucleation of ETS-10 at pH of ~10.6 and 473 K [17, 18], it was
shown that KF or KOH can produce ETS-10 in absence of ETS-10 seeds. The (Na + K)/Na
ratio was studied by Noh et al. [15], and it was found that a ratio of at least 1.26 was
necessary to avoid the formation of AM-3 and AM-1 impurities. There have been several
studies on the role of Na+ and K+ on the formation of ETS-10. While the presence of Na+ has
been shown to be necessary for the synthesis of ETS-10, an excess of Na+ normally produces
ETS-4 [19]. This observation however could be attributed to a higher pH. Generally KF or
KOH are used as the K+ source. In the absence of K+, ETS-10 can be formed using Na+ and
tetrapropylammonium bromide [20].
Several templates have been reported for the synthesis of ETS-10. These include: pyrrolidine,
tetramethylammonium
chloride,
tetraethylammonium
chloride,
tetrapropylammonium
bromide, tetrabutylammonium chloride, 1,2-diaminoethane and 1,6-diaminohexane, choline
30
chloride [OHCH2CH2(CH3 )3N+C1-] and the bromide salt of hexaethyl diquat-5 [BrC2H5)3N+(CH2)5N+(C2H5)3Br-] [7, 21].
The effect of different templates to reduce impurities and synthesis time has also been
studied. In the article by Valtchev, [21], the following organic bases were considered:
pyrrolidine, tetramethylammonium chloride, tetraethylammonium chloride (TMACl),
tetrapropylammonium bromide, tetrabutylammonium chloride, 1,2-diaminoethane and 1,6diaminohexane. TMACl lowered the required synthesis time from 7 days to 2 days at 200°C.
It was suggested that, ETS-10 was stabilized by a pore filling effect due to the size of
TMACl (7.4 Å) versus pore size of ETS-10 (~8 Å) [21].
Of all synthesis parameters previously studied to optimize the formation of ETS-10, the
method of heating has not been explored. Microwave heating has been used for several
organic and inorganic products [22-35]. Through the absorption of microwave radiation,
many solvents and materials produce a faster and more efficient heating of the reaction
mixtures. In this form, reaction products can be obtained in shorter time without impurities.
[35] Recently we reported the microwave synthesis of ETS-4 in as few as 40 min [34]. There
has been a report on the leaching effect of hydrogen peroxide and microwave radiation to
form supermicropores and mesopores on ETS-10 [36] but not on the synthesis. We have
prepared ETS-10 for the first time, by microwave heating in 1 day compared with 4 days by
conventional heating under the same conditions. The effect of pH, temperature, template, Ti
source, and molar ratios were also studied.
31
2.2
Experimental Section
2.2.1
Precursor solution
The synthesis of ETS-10 involves the combination of two precursor solutions. Solution 1
was prepared by dissolving 2.4g of KCl (Fisher, 99%) in 10mL of deionized water. Then
1.4g of NaCl (Sigma, 99.5%), 1.6g of NaOH (EMD, 97%) and 3.6g of SiO2 (Alfa Aesar,
60Å) were slowly added with stirring at room temperature upon complete dissolution of KCl.
Solution #2 was prepared by dissolving 0.48g of NaOH in 15mL of deionized water followed
by the addition of 3.7 g of Ti(OBu)4 (Sigma, 97%) to form a white cloudy suspension. The
mixture turned light yellow with effervescence after adding 10mL of H2O2 (Sigma, 30%).
Both solutions were stirred at least for 30 minutes, then combined and stirred at room
temperature for 45 minutes. The pH was adjusted to 10.64 using 4M HCl. The resulting
precursor solution was pale yellow color having a molar ratio of 0.62Na2O : 0.56KCl :
0.18TiO2 : 1.00SiO2 : 29.63H2O.
2.2.2
Microwave Heating
For the microwave syntheses, 30mL of the precursor solution were placed in a 95mL CEM
XP-1500 Plus Teflon reaction vessel and heated to 180-235°C in a CEM MARSXpress™
microwave (CEM Corporation) at 2.45 GHz. The heating time was varied from 1 hour to 24
hours. A 20 min ramp time was used to reach the desired temperature and the power did not
exceed 400 watts. The reaction temperature was controlled via a reference vessel. A
temperature probe was placed inside a glass thermowell in the reference vessel. Because of
the high pH of the reaction mixture (~12), the solution was adjusted to pH~7 with 4M HCl to
avoid dissolution of the probe’s glass thermowell. During the synthesis the temperature was
32
controlled automatically and the pressure was monitored. The products were filtered, washed
with deionized water and dried at 60°C for 24 h.
2.2.3
Conventional Heating
ETS-10 was also prepared by heating in an oven. The clear yellow precursor solution was
placed in a 23mL Teflon-lined Parr autoclave and heated to 200°C-230°C for a period of 48
h to 96 h. After cooling to room temperature, the products were the filtered, washed with
deionized water and dried at 60°C for 24 h.
2.2.4
Analysis
Powder X-ray diffraction patterns were obtained using a Rigaku Ultima III X-ray
diffractometer using CuKα radiation.
Samples for scanning electron microscopy were
coated with Pd/Au and micrographs obtained on a Leo 1530 VP Field emission scanning
electron microscope.
2.3
Results and discussion
We recently reported the microwave assisted synthesis of ETS-4 which dramatically reduced
the synthesis time and improved the purity [34]. The synthesis of ETS-10 by microwave
heating proved to be a much greater challenge. This maybe because its formation is
thermodynamically less favorable and the range of reagents and the reaction conditions
including temperature and pH are narrower. Therefore, to find the optimum conditions for
microwave synthesis of ETS-10, several synthesis parameters were evaluated, including
molar ratios, time and temperature.
33
2.3.1
Molar Composition
The different synthesis conditions for the products obtained using conventional heating
(Table 1) and microwave heating (Table 2) are provided. The first recipe tested employed
Ti(OBu)4 in a precursor solution MW1 having a molar ratio of 0.77Na2O : 0.16KF : 0.09TiO2
: 1.00SiO2 : 63.8H2O, with the pH fixed at 11.5 as suggested by Pavel et al [4]. This mixture
was heated at 230°C for two hours using the microwave oven. The resulting product showed
no characteristics of ETS-10. At the same temperature other samples were attempted,
changing the Ti source to TiCl3; the F- source from KF to KF·2H2O and NaF, and K+ source
from KF to KCl, the pH 11.5, 12.87 and 10.63, and none of them produced ETS-10.
Using conventional heating, sample CH1 was maintained at 220°C for three days and
produced ETS-4 and impurities. The second sample CH2, whose water content was doubled,
was heated for 4 days at 230°C and produced ETS-10 crystals up to 22µm in size as shown in
Figure 2. The XRD pattern for sample CH2 is presented in Figure 3, and shows a XRD
pattern matching ETS-10.
Replacing the titanium source of Ti(OBu)4 with Degussa P25, resulted in ETS-4 at a pH of
11.19 (CH3) and ETS-10 at a pH of 10.45 (CH4).The ETS-10 crystals (CH4) shown in
Figure 4 range from ~100 nm to ~300 nm with quartz as an impurity. A sample made with
P25 and pH 10.6 with no hydrogen peroxide (CH5), produced ETS-10 crystals of about ~750
nm, but with a decreased amount of quartz as shown in Figure 5. The XRD patterns for
sample CH4 and CH5 in Figure 6 shows the characteristic pattern of ETS-10 with a quartz
impurity shown with a peak at ~ 26.6°.
These results show that when P25 is used, SiO2 is not easily incorporated into the ETS-10
structure, or the gel composition was titanium deficient. In addition, with P25 the presence of
34
hydrogen peroxide was not necessary to produce ETS-10 but quartz was also produced as an
impurity. The synthesis of ETS-10 with P25 also provides a route to produce ETS-10 crystals
under 1 µm in size.
From Table 1, it can be seen that the observed Ti/Si ratio was 0.09/1 for CH2, and 0.18/1 for
CH3, in the precursor solution. With this in mind, the amount of Ti(OBu)4, and H2O2 was
doubled CH6, giving the molar ratio of 0.62Na2O : 0.56KCl : 0.18TiO2 : 1.00SiO2 :
29.63H2O in the precursor gel. After four days at 230°C, with a pH of 10.64, ETS-10 crystals
were formed with no evidence of quartz.
2.3.2
Microwave Oven Heating
The formation of ETS-10 was verified after microwave heating at 210°C for 15 h (MW2) as
shown in Figure 7. The XRD pattern in Figure 8 shows that this sample is a mixture of ETS10 and ETS-4. Further heating of a similar precursor solution for 22 h (MW3) also resulted
in a mixture of ETS-4 and ETS-10. In this case there is ~60% ETS-10 after 22 h relative to
ETS-4 as observed in Figure 8. If the temperature is increased with a pH of 10.6, the
resulting product after 18 hrs (MW4) shows ETS-10 crystals with fiber like impurities on the
surface, as seen in the SEM image in Figure 9. If ETS-10 synthesis was attempted at 230°C
was extended to 24 h pure ETS-10 was obtained. The XRD pattern for sample MW5, shown
in Figure 10 matches ETS-10.
35
2.4
Conclusions
Compared to the microwave synthesis of ETS-4 the synthesis of ETS-10 proved to be more
difficult. From a thermodynamic point of view, the magnitude of the enthalpy of formation
for ETS-10 is lower than for ETS-4, which reflected in the higher temperatures and heating
times required to produce ETS-10. In the same way, when time and temperature are reduced,
there is a strong tendency to produce impurities. From the obtained results, it is showed that
ETS-10 can be obtained in as few as 24 h heating in a microwave oven at 230°C.
2.5
Acknowledgement
We thank the Robert A. Welch Foundation and SPRING, for the supporting of this project.
APPENDIX
Table 1. Synthesis conditions for ETS-10 products using conventional heating.
Sample
CH1
CH2
CH3
CH4
CH5
CH6
CH7
KCl
0.55
0.55
0.56
0.56
0.55
0.55
0.55
H2O
23.08
46.17
23.15
23.15
23.08
23.15
46.30
Na2O
0.62
0.62
0.63
0.63
0.63
0.63
0.63
SiO2
1.00
1.00
1.00
1.00
1.00
1.00
1.00
TiO2
0.09
0.09
0.19
0.19
0.19
0.18
0.18
H2O2
0.73
0.73
0.80
0.80
0.00
1.47
1.47
10.41
10.41
11.19
10.45
10.60
10.64
10.64
220
230
230
230
230
230
230
3
4
3
3
3
4
4
pH
Temp (°C)
Time (d)
36
37
Table 2. Synthesis conditions for ETS-10 products using microwave heating.
Sample
MW1
MW2
MW3
MW4
KCl
0.16 (KF)
0.54
0.55
0.55
0.55
H2O
63.83
41.44
32.23
27.62
23.02
Na2O
0.77
0.67
0.64
0.65
0.65
SiO2
1.00
1.00
1.00
1.00
1.00
TiO2
0.09
0.18
0.18
0.18
0.18
H2O2
0.78
1.46
1.46
1.46
1.46
11.54
10.64
10.64
10.64
10.65
220
210
210
230
230
2
15
22
18
24
pH
Temp (°C)
Time (h)
MW5
38
A
B
Figure 1. ETS-10 structure calculated using JADE software and data from reference [2]. (A)
View through <110> axis. (B) View through <001> axis.
39
Figure 2. SEM image of ETS-10 sample CH2 after 96 h, at 230°C in a conventional oven.
Relative Intensity
40
B
A
5
10
15
20
25
30
Two-Theta (deg)
35
40
45
50
Figure 3. XRD pattern for ETS-10 using conventional heating; (A) sample CH6 as reference,
(B) sample CH2.
41
Figure 4. SEM image of ETS-10 sample CH4 after 72 h, at 230°C, using Degussa P25 as
titanium precursor, with a pH of 10.45 in presence of hydrogen peroxide.
42
Figure 5. SEM image of ETS-10 sample CH5 after 72 h, at 230°C, using Degussa P25 as
titanium precursor, with a pH of 10.6 without hydrogen peroxide.
Relative Intensity
43
x3 C
B
A
5
10
15
20
25
30
35
40
45
Two-Theta (deg)
Figure 6. XRD patterns of ETS-10 samples using P25 as titanium source at 230°C, for 3 days
with; (A) sample CH6 as reference, (B) sample CH4 with a pH of 10.5, (C) sample CH5 with
a pH of 10.6 without hydrogen peroxide. Arrows show quartz.
44
Figure 7. SEM image of ETS-10 sample MW2 after 15 h at 210°C by microwave heating.
45
Relative Intensity
ETS-4
ETS-4
ETS-4
A
B
5
10
15
20
25
30
35
40
45
Two-Theta (deg)
Figure 8. XRD patterns of microwave products after (A) sample MW3 after 22 h and (B)
sample MW2 after 15 h at 210°C showing formation of ETS-10 and ETS-4.
46
Figure 9. SEM images for ETS-10 sample MW4 after 18 h of microwave heating at 230°C.
Relative Intensity
47
B
A
5
10
15
20
25
30
35
40
45
Two-Theta (deg)
Figure 10. XRD pattern of ETS-10 (A) Sample CH6 as reference, (B) Sample MW5 using
microwave oven, at 230°C for 24 h.
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CHAPTER 3
MICROWAVE SYNTHESIS OF ZIF-8
Abstract. Metal organic frameworks (MOF), such as the family of Zeolitic Imidazole
Frameworks (ZIF), show a promising future for important applications such as gas
adsorption. In this paper, a synthesis method based on microwave heating, was developed for
the rapid synthesis of the pure Zn2+, 2-methylimidazole based, ZIF-8. ZIF-8 was produced
after 1 h, at 180°C compared with 24 h by conventional heating. The ZIF-8 was characterized
by XRD, TGA, gas adsorption and SEM
Keywords: ZIF, MOF, zeolite, microwave synthesis.
51
52
3.1
Introduction
The Metal Organic Frameworks (MOF), are built by using organic ligands that bind metal
ions or clusters together [1-15]. An advantage of MOFs compared to zeolites is the very high
surface areas, in some cases > 3000 m2/g [1]. The organic linkers in an MOF can also be
functionalized achieving more specialized applications [2-6]. However, most of the MOFs
lack the physical and chemical stability of zeolites [2, 4-8] .
The Zeolitic Imidazole Frameworks (ZIFs) comprise a new family of porous materials that
share many of the desirable characteristics of metal organic frameworks. ZIF materials have
been reported to be quite stable, in some cases > 450°C [9]. Many of the ZIF materials
exhibit framework structures analogous to zeolites and related molecular sieves [9].
The imidazolate linkers in ZIFs are aromatic heterocycles, based on a 1,3-diazole ring [10].
Due to the aromatic character of the lone electron pair at N-1, the only coordinating,
unshared pair of electrons is located at N-3 [10]. The imidazolium cation can be formed, if
N-3 is protonated (pKa = 7.1), and the anionic imidazole forms in basic conditions when N-1
is deprotonated (pKa from 14.2 to 14.6) [10]. In the anionic form, two equivalent sites are
produced, while the imidazole remains aromatic [10].
Of particular interest among the Zeolitic Imidazole Frameworks, is the product of 2methylimidazolate and zinc (II) nitrate, ZIF-8. Huang et al [7] first reported the synthesis of
this metal-organic framework using zinc (II) and 2-methylimidazole. They noted that
utilization of substituted imidazoles, allowed the formation of three dimensional structures
instead of dense materials [11, 12]. The ZIF-8 synthesis was accomplished after 30 days
using methanol and aqueous ammonia as solvents [7].
53
More recently, Park et al [9] reported a more extended series of imidazolate based MOF
materials, using zinc (II), cobalt (II) and indium (III) as metals, with various imidazolate
linkers [9].
The synthesis of 2-methylimidazole-zinc (II) framework (ZIF-8), was
accomplished in 24 hours after collecting crystals from chloroform [9]. The main advantage
of ZIF-8 compared to other MOFs is the high stability in refluxing organic solvents as well as
boiling water and alkaline solutions for up to 7 days without considerable decomposition [9].
ZIF-8 was reported to have a surface area of 1,630 m2g-1 by the Brunauer-Emmett-Teller
(BET) method, which is higher than most zeolites and ordered mesoporous silica materials
[9].
The ZIF-8 structure is described as a sodalite cage, in which the metal-imidazole-metal
angles are close to 145°. This angle is comparable to the Si-O-Si angle formed in many
zeolites [9]. The ZIF-8 structure shown in Figure 1 reveals the SOD topology formed by 4
membered rings and 6 membered rings. The sodalite like cage has a pore diameter of 11.6 Å
with pore openings of 3.4 Å [9].
Sodalite as a mineral, is present in nature as Na8[AlSiO4]6Cl2 [13]. The sodalite cages are
also present in zeolites A, X and Y [5, 14-17]. The sodalite framework is adjustable to
different volumes, based on the size of the species inside the cage. In this way, the Si-O-Al
angle can vary from 123° to 160.5° [13]. However, the utility of these sodalite molecular
sieves is limited because access to the cages is through a six membered ring. In contrast, the
ZIF-8 cage windows are 3.4 Å such that small molecules such as H2 might be adsorbed.
While the original 30 day synthesis reported has been reduced to 24 hours [9], there would be
advantages to further optimization of the synthesis including time in the reactor.
54
Microwave heating has been studied to improve and speed up the synthesis of organic and
inorganic materials [18-30]. Despite the important advantages of microwave synthesis, only
there has been one report describing the formation of three MOFs, IRMOF-1, IRMOF-2 and
IRMOF-3 by Ni and Masel [30]. In the present paper, the rapid microwave synthesis of pure
ZIF-8 after 1 hour of heating at 180°C is reported. In addition an intermediate phase formed
after 30min at 180°C was identified which may have the AST type morphology.[31]
3.2
Experimental Section
3.2.1
Conventional heating synthesis
In a typical synthesis, based on the reported procedures [9], the ZIF-8 precursor solution was
prepared by dissolving 1.07g of Zn(NO3)2·4H2O (EMD, 98,5%) and 0.30g of 2methylimidazole (Aldrich, 99%) in 100mL of N,N-Dimethylformamide (DMF) (EMD,
99.97%) and stirring for 5 minutes. The reaction mixture was placed in a 200mL Teflon
bottle and heated to 140°C for a period of 20 hours. The product was allowed to cool to room
temperature. The resulting product was filtered, washed two times with 10 mL of DMF and
then twice with 10mL chloroform (VWR, 99.8%) and dried overnight at 50°C.
3.2.2
Microwave Synthesis
For the microwave synthesis, the solution was prepared by dissolving 0.77g of
Zn(NO3)2·4H2O and 0.23g of 2-methylimidazole in 30mL of DMF and stirring for 5 minutes
at room temperature.
The resulting transparent gel was placed in a 95 mL CEM XP-1500 Plus Teflon reaction
vessel and heated in a CEM MARSXpressTM microwave (CEM Corporation) at 2.45 GHz.
The heating time was varied from 30 to 240 min. A 20 min ramp time was used to reach the
55
desired temperature and the power did not exceed 400 W. The reaction temperature was
varied form 140°C to 180°C, and was controlled via a reference vessel. The resulting brown
solid product was filtered, washed two times with 10 mL of DMF and then twice with 10mL
chloroform and dried overnight at 50°C. When required, sedimentation was used to separate
amorphous debris from the main product. The separation was achieved by placing the
product in DI water and sonicating for 60 seconds. The cloudy solution was decanted and the
solid was washed with methanol (EMD, 99.8%) and dried overnight at 90°C.
3.2.3
Analysis
Powder X-ray diffraction patterns were obtained using a Rigaku Ultima III X-ray
diffractometer using CuKα radiation.
Samples for scanning electron microscopy were
coated with Pd/Au and micrographs obtained on a Leo 1530 VP Field emission scanning
electron microscope. Nitrogen adsorption was measured at 77 K on a Quantachrome
Autosorb 1. Thermogravimetric analyses were performed using a Perkin Elmer PYRIS 1
Thermogravimetric Analyzer.
3.3
Results and Discussion
For comparison purposes, the synthesis of ZIF-8 was first performed using a conventional
oven. After heating for 20 hours at 140°C, two different solids were obtained. A thin orange
crust was formed, along with a fine, white powder. The XRD pattern for these products and
the calculated pattern are shown in Figure 2. Besides difference in intensity the XRD patterns
in Figure 2b and c generally match quite well the calculated pattern. The intensity difference
could reflect the presence of adsorbed solvent. For both the orange crystals and the white
56
powder, there are small peaks at 31.7°, 34.4° and 36.2°, that are characteristic of ZnO,
present as an impurity.
The orange crust and the white powder of ZIF-8, are quite different in terms of crystal size
and distribution. Figure 3 shows the scanning electron microscope (SEM) image of the
orange crystals which are composed of large cubic-rhombohedral crystals ~100 µm in
diameter. From the SEM images, the crystals present to square faces on the top and the
bottom of the crystal. Each side of these squares connects to parallelograms, and the corners
of the squares connect to rhombic faces. In this way a hexagonal shape is formed in the
middle of the crystal. In contrast, the white powder is composed of much smaller crystals as
shown in Figure 4. The ZIF-8 crystals are ~ 3 µm with morphology similar to that formed in
the crystals in the orange crust shown in Figure 3.
3.3.1
Microwave Heating
Once a reference material was obtained with the conventional oven, the material was
produced in the microwave oven. The effect of temperature was studied by performing
syntheses, at 140°C, 150°C, 160°C, 170°C, and 180°C for 1 hour and 2 hours. The XRD
patterns for the products for 1 h are shown in Figure 5. Starting with the temperature used in
the conventional oven, 140°C it can be observed that the only product is zinc oxide with
peaks at 31.8°, 34.5° and 36.3° two-theta. After increasing the temperature to 145°C,
additional peaks appear at 11.1°, 17.1° and 19.3° two-theta. It is not until the temperature
reaches 155°C that a ZIF-8 pattern starts to appear. However, after 1 h, the ZnO impurity
persists, even after heating at 160°C and 170°C. It is not, until the synthesis gel is heated to
180°C, that ZIF-8 is formed with trace amounts of ZnO. The SEM image in Figure 6 for the
57
ZIF-8 product at 180°C shows the morphology of the ZIF-8 crystals which are similar to the
orange crust obtained in the conventional oven.
Figure 7 shows the XRD patterns for ZIF-8 products obtained after 2 hrs of microwave
heating at 140°C, 150°C, 160°C, 170°C, and 180°C. After two hours, there are two peaks, at
10.6° and 11.3° two-theta. The additional peak at 11.3° is assigned as unknown impurity. If
we compare 1 and 2 hr syntheses products at 180°C it would appear that the 1 hour may have
more ZnO, but the 3 hour product has a new phase growing in. These results suggest that the
synthesis of ZIF-8 through microwave heating could be achieved in a short period of time, by
increasing the reaction temperature.
The ZIF-8 synthesis was also studied at 140°C with increasing time. The XRD shown in
Figure 8 shows the presence of ZnO until heated for 2.5 h, when the amount of ZnO is
considerably reduced. ZIF-8 is formed after 1.5 hr but an impurity phase with reflections
10.6° and 11.3° are also present. After 2.5 hours this new phase is the main product as shown
in Figure 8e. SEM images of the 2.5 h product are shown in Figure 9. Interestingly the
morphology is similar to an AlPO-16 (AST) cage described as 46612.
When the ZIF synthesis was carried out at 155°C in the microwave oven for period of 1, 2,
3, and 4 hours, similar results were obtained where a mixture of ZIF-8, ZnO and impurity
phase were obtained.
The effect of time on ZIF-8 synthesis was also studied at 180°C. The XRD patterns for the
products after heating for 0.5 h, 1.0 h, 1.5 h, and 2.0 h are presented along with the simulated
pattern and the pattern for crystals collected from the filtrate after three days in Figure 10.
The first product, obtained after 0.5 h exhibits an XRD pattern quite similar to ZIF-8.
However, after examining the SEM images, the morphology resembles the AST morphology.
58
(Figure 11) After 1.0 h the product showed the ZIF-8 XRD pattern and the rhombic
hexahedral morphology. After 1.5 h a XRD peak at 11.0° begins to show. In the article by
Park et al [9], the original procedure detailed how the ZIF-8 crystals were obtained from
chloroform. After filtering and washing the ZIF-8 crystals, a solid was clearly formed in the
filtrate. After three days this solid was analyzed, showing an interesting XRD pattern, which
doesn’t correspond to ZnO. The three peaks at 7.4°, 10.5° and 12.9° partially match the ZIF8 pattern. However, the rest of the XRD pattern for ZIF-8, and ZIF-8 crystals are not
distinguished by SEM. This solid could be made of very small ZIF-8 crystals or some other
by-product or impurity.
3.3.2
Stability and Purification
When TGA was performed on the sample prepared at 180°C for 1 h, the resulting plot (not
shown) is very similar to the one presented by Park et al. The first step, up to 450°C shows a
weight loss of 12.2% compared to 28.3% as reported by Park. This further confirms the
thermal stability of ZIF-8.
The as synthesized ZIF-8 samples, consisting of a fine brown powder, were sonicated for 1
minute in methanol. After sonication, the sample was allowed to settle for 30 minutes before
carefully decanting. The cloudy liquid was discarded and the solid was dried at 90°C
overnight. This simple purification method, proved to be very effective for getting bigger
ZIF-8 crystals with less debris. The XRD pattern show how compared to the as synthesized
material, the sonicated ZIF-8, shows peaks of bigger intensity, implying bigger particle size.
(Figure 12) The as synthesized ZIF-8 showed a surface area of 600 m2/g using the BET
nitrogen adsorption at 77K and selecting points from P/Po 0.1 – 0.10. SEM images for the
sonicated product are presented (Figure 13). Gas adsorption analysis was performed on the
59
sonicated and dried product. The material was outgassed at 300°C for 36 hours and showed a
surface area (BET) of 951 m2/g and a pore volume (DFT) of 10.6Å.
3.4
Conclusions
The rapid microwave synthesis of ZIF-8 was accomplished after heating at 180°C for 1 h.,
compared to the previous 24 h synthesis at 140°C by Park. et al. The XRD pattern of the
obtained material matches the simulated pattern for ZIF-8, and the SEM images give the first
known report of the aspect of the ZIF-8 crystals. Microwave heating provided a fast route to
obtain high purity ZIF-8, which can be even improved after sonication with MeOH.
Microwave synthesis shows its efficiency into reducing synthesis time without detriment on
the material’s quality.
3.5
Acknowledgement
We thank the Robert A. Welch Foundation and SPRING, for the supporting of this project.
APPENDIX
A
B
C
Figure 1. Structure of ZIF-8 calculated using Materials Studio and the crystal data in ref [9].
A) View along the 001 axis, B) View along the 111 axis. and C) View along the 100 axis.
60
61
Relative Intensity
C
B
A
5
10
15
20
25
30
35
40
45
Two-Theta (deg)
Figure 2. XRD patterns of ZIF-8 material obtained by conventional heating at 140°C and 20
hours; A) Calculated pattern using data from ref [9], B) ZIF-8 orange crust, and C) the ZIF-8
white powder after sedimentation.
62
Figure 3. SEM image of the ZIF-8 crystals present in the orange crust.
63
Figure 4. SEM image of the ZIF-8 crystals in the white powder obtained from the synthesis
in regular oven at 140°C for 20 hr.
64
180°C
170°C
160°C
155°C
145°C
140°C
simulated
Two-Theta (deg)
Figure 5. Simulated ZIF-8 pattern and temperature effect on ZIF-8 samples on syntheses, at
140°C, 145°C, 155°C, 160°C, 170°C, and 180°C for 1 hour. (Arrows mark ZnO)
65
Figure 6. SEM image showing a ZIF-8 crystal after heating at 180°C for 1 hour.
66
180°C
170°C
160°C
150°C
140°C
Simulated
Two-Theta (deg)
Figure 7. Simulated ZIF-8 pattern and temperature effect on ZIF-8 samples on syntheses, at
140°C,150°C, 160°C, 170°C, and 180°C for 2 h. (Arrows mark ZnO).
67
2.5 h
2.0 h
1.5 h
1.0 h
Simulated
Two-Theta (deg)
Figure 8. Simulated ZIF-8 pattern and time effect on ZIF-8 samples when heated at 140°C
for b) 1.0 h, c) 1.5 h, d) 2.0 h, and e) 2.5 h.
68
Figure 9. SEM image showing the unknown product obtained after heating the reaction
mixture at 140°C for 1.5 h.
69
Filtrate
2.0 h
1.5 h
1.0 h
0.5 h
Simulated
Two-Theta (deg)
Figure 10. XRD pattern for: simulated ZIF-8, products obtained after heating at 180°C for
0.5 h, 1.0 h, 1.5 h, 2.0 h and crystals collected from filtrate after three days.
70
Figure 11. SEM image showing a ZIF-8 crystal after heating at 180°C for 0.5 h.
71
Simulated
Sonicated with MeOH
As synthesized
Two-Theta (deg)
Figure 12. XRD patterns showing the effect of sonication with MeOH on the ZIF-8 sample
for 180°C and 1 h.
72
Figure 13. SEM image for ZIF-8 crystals produced at 180°C for 1 h after sonication with
MeOH.
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CHAPTER 4
MICROWAVE SYNTHESIS OF GALLIUM ZINC PHOSPHATE NTHU-4
Abstract. There is growing interest in white light emitting diodes for solid-state lighting.
Novel and low cost approaches for the synthesis of white light emitting materials is essential.
The large pore gallium zinc phosphate NTHU-4 has been shown to emit white light when
photoexcited. NTHU-4 was successfully synthesized after only 4 hours at 160°C using a
microwave oven, compared to 48 hours at 160°C by conventional heating. The effects of
synthesis parameters on phase purity, crystal morphology and emission properties were
explored.
Keywords: Microwave synthesis, NTHU-4, LED, Gallium Zinc Phosphate, white light.
75
76
4.1
Introduction
.The average person is exposed to an increasing amount of artificial color and light. The
scientific interest in light can be traced as far back as 1671 with Isaac Newton’s Theory of
Light [1] showing white light decomposition into colors, or the slit interference experiments
in 1804 by Thomas Young [2]. In 1931 the International Commission on Illumination (CIE)
[3] established a system of colorimetry based on the experiments of W. David Wright [4, 5]
and J Guild [6]. Despite the evolution, from oil lamps to high resolution displays, the
perception and development of artificial light sources is still a challenging subject.
At the present time, many technological devices such as cell phones, digital cameras,
televisions, computer monitors, traffic signals, automotive applications, indoor and outdoor
building lighting, etc. are based on displaying images or emitting light. The growing
applications demand greater economic and energy conservation. At the same time, energy
and economical considerations get more demanding. According to the US Department of
Energy lighting represented 16 percent of the total primary energy consumption in buildings
in 2004 [7]. Electrical energy consumption is expected to increase 1.6 percent annually,
going from 3,729 billion kilowatt hours in 2004 to 5,208 billion kilowatt hours in 2025 [7].
Finally, unlike space heating, water heating and refrigeration, the trend of energy
consumption for lighting and other devices will increase per household despite increasing
efficiency [7]. The need for efficient lighting requires great progress in the design of new
materials.
Compared to incandescent bulbs and fluorescent lamps, new devices and associated materials
are expected to provide higher brightness, reliability, lower power consumption, and longer
life. For instance, light emitting diodes (LEDs) report a luminous efficacy around 100 lumens
77
per watt (lm/W) [8], compared to 16 lm/W for incandescent and 85 lm/W for fluorescent
bulbs [9]. Comparable to Moore’s law for semiconductors [10], Haitz’s law states that the
LED luminous output doubles every 18-24 months [11]. At this rate, the theoretical
maximum for LEDs of ~200 lm/W will be achieved around year 2020 [9, 12, 13]. For 2001,
the value of a white LED was $0.20/lm; for 2012 the projected price is $0.01/lm [13]. With
the expected drop in production costs, increased luminous output and increased life-time of
up to 100,000 hours, [9] LED technology appears as the favorite alternative for lighting.
Human perception of color is based on three photoreceptor cones that perceive red, green and
blue. The cones are divided into three classes based on their wavelength sensitivity: the short
(S) expressed by genes on chromosome 7, medium (M) and long (L) both expressed by
chromosome X. The ratio and distribution of L and M cones varies between individuals but S
cones are present in a smaller amount (under 5%) as described by Roorda.[14] Short cones
perceive radiation up to 520nm, allowing a good blue-yellow but a limited green-red
discrimination on color blind individuals, which lack L or M cones. [3, 14-17]
The design of LED devices is related to the study of photoreceptor cones. The emission of
white light can be achieved by three different methods. The first approach involves the
combination of three monochromatic sources; one red, one green and one blue and is called
the RGB approach. This approach presents the highest theoretical efficiency for the emission
of white light, and presents the best color rendering properties [11, 18]. As different light
emitting sources contribute in different amounts, a big challenge is to balance and control
each source intensity. In the second approach, a GaN LED pump provides blue light that also
excites a yellow phosphor layer. This relatively simple and common device provides white
light of irregular spectral composition due to the absence of red light, with poor color
78
rendering [19]. The third method uses a UV-LED to excite a RGB phosphor layer. The color
is controlled by the phosphor mixture, giving good color rendering. However finding a nearUV high absorption phosphor and the degradation of materials by UV radiation are important
challenges to overcome. [11, 18-20]
It is clear that a single source emission of white light that provides low cost, high efficiency
and long life would represent a great advance in LED technology. One possible alternative
may be found with NTHU-4, which was first reported by Liao et al [21]. This
gallozincphosphate uses 4,4’-trimethylenedipyridine amine as a template, forming 14membered ring channels along the c-axis. The building blocks are GaO4 and ZnO4 tetrahedra,
with PO4 and HPO4 units on the corners. (Figure 1) This promising material, which was
synthesized after heating for 7 days at 160°C, emits white light when excited by UV
radiation [21]. Reducing the synthesis time will considerably lower the production cost of
NTHU-4. Microwave heating has been successfully used for several organic and inorganic
products, dramatically reducing heating time [22-39]. In this paper we describe the successful
synthesis of NTHU-4 by microwave heating in less than 4 hours.
4.2
Experimental section
4.2.1
Gel Preparation
In a typical synthesis 4.4g of H3PO4 (EM Science, 85.0%) and 0.88g of ZnCl2 (Fisher
Scientific, 98.0%) were combined with 33mL of deionized water. 0.6g of Ga2O3 (Aldrich,
99.99%) were added and stirred at room temperature for 20 minutes. In a separate vessel,
8.16g of 4,4’-trimethylenedipyridine (TMDP) (Aldrich, 98%) were mixed with 28.4mL of
ethylene glycol (Fisher Scientific, 99+%) and 10mL of deionized water. Then, 0.96g of
79
oxalic acid dihydrate (Aldrich, 99%) were added and the solution stirred for 20 minutes at
room temperature. The TMDP solution was combined with the zinc gallophosphate solution
and stirred from 30 minutes to 72 hours at room temperature.
4.2.2
Microwave synthesis of NTHU-4
The thick white, NTHU-4 precursor gel was placed in a 95 mL CEM XP-1500 Plus Teflon
reaction vessel and heated to 160-200°C in a CEM MARSXpressTM microwave (CEM
Corporation) at 2.45 GHz. The heating time was varied from 60 to 600 min. A 20 min ramp
time was used to reach the desired temperature and the power used was 400 watts. The
reaction temperature was controlled via a reference vessel. The resulting product was filtered,
washed with deionized water and dried at 45°C for 24 h.
4.2.3
Conventional heating synthesis
The NTHU-4 precursor solution was placed in a 23 mL Teflon-lined Parr autoclave and
heated at 160°C for a period of 1-7days. The product was then filtered, washed with
deionized water and dried at 45°C for 24 h.
4.2.4
Analysis
Powder X-ray diffraction patterns were obtained using a Rigaku Ultima III X-ray
diffractometer using CuKα radiation.
Samples for scanning electron microscopy were
coated with Pd/Au and micrographs obtained on a Leo 1530 VP Field emission scanning
electron microscope. Fluorescence spectra were recorded using a SPEX FLUROLOG 1680
0.22 Double Spectrometer.
80
4.3
Results and Discussion
Liao et al [21], reported a mixture of TMDP, Ga2O3, ZnCl2, oxalic acid, and H3PO4 within
the molar ratio of 12.8:1:2:2.4:12 in combination with a certain amount of water and ethylene
glycol produced NTHU-4. Table 1 show a series of samples where the synthesis variables
including molar ratios, temperature and time were explored for a conventional oven synthesis
of NTHU-4. Non-luminescent and luminescent materials of different colors; orange, yellow,
blue and white resulted from different conditions and molar ratios while trying to obtain the
NTHU-4 material.
Using conventional heating the synthesis of NTHU-4 was studied from 1 to 7 days at 160°C.
Impure NTHU-4 material formed after 24h. However, pure NTHU-4 was obtained after 48h.
In contrast with the results reported by Liao [21], our product started disappearing with time,
until no NTHU-4 material was left after 7 days of heating. The XRD patterns for the samples
from 1 to 7 days are shown in Figure 2. Sample CH2, the 2 day product, best matches the
simulated XRD pattern. With increasing time, the NTHU-4 material starts converting into
unknown materials.
According to Liao et al [21], substitution of water in the synthesis precursor by ethylene
glycol, produced a white light (NTHU-4w) instead of the original yellow light (NTHU-4y).
White light emission could also be obtained from NTHU-4y, after reducing crystal defects,
by heating at 280°C for 4 hours.
Sample CH2 also showed a low intensity emission of yellow color light under UV excitation
at 390 nm as shown in Figure 3. The products in Table 1 were also analyzed by SEM to
observe the change in morphology. After one day at 160°C intercrossing aggregates of plate
like crystals are observed for NTHU-4 as shown for Sample CH1 in Figure 4. For the two
81
day synthesis (Sample CH2), the NTHU-4 crystals were bigger with dimensions of ~ 150 x
160 µm as shown in Figure 5. After three days, the crystal morphology starts to change, from
stacked plates to intercrossing crystals as shown in Figure 6. After 4 or 5 days, samples CH4
and CH6 exhibit a new phase or rod shaped crystals as shown in Figure 7. From the XRD
patterns in Figure 2 it is clear that there is a mixture of NTHU-4 and some impurity phase.
The SEM in Figure 8 shows sample CH8 after 7 days of conventional heating. In this case
both, square laminates (right) as small aggregates of plate like crystals (right) are observed.
4.3.1
Microwave Oven Synthesis
Once the reaction profile by conventional heating was obtained, the microwave synthesis of
NTHU-4 was studied as shown in Table 2. First, the product obtained after stirring the
precursor for 30 minutes at room temperature was studied. The synthesis was performed for
2, 3, 4, 5, 7, 8 and 10 hours. After 2 to 10 hrs, samples MW1 to MW7, consisted of whiteyellow intense light emitting NTHU-4 particles and a mixture of NTHU-4 fine yellow
powder and other less emissive material. However, based on the particle size, the larger
NTHU-4 crystals could be easily separated from the fine powder. After separation, the XRD
patterns for the big particles of NTHU-4 material were obtained as shown in Figure 9. The
formation of NTHU-4 by microwave heating starts to occur after two hours at 160°C,
however, there is an impurity peak at ~7.6° similar to conventional heating.. The SEM image
for MW1, in Figure 10 show the rod like morphology. The emission spectra for the sample
MW1 with a white-yellow emission color, showed an emission maxima at 420nm and 540nm
for 365nm and 390nm excitation as shown in Figure 11. For sample MW2, the 3 hour
synthesis, the impurity peak at ~7.6° diminishes in Figure 9, but a new impurity appears at
~10.8° which is attributed to zinc phosphate. This impurity is present in the as-synthesized
82
material, before separation. After 4 hours, the NTHU-4 crystals continue to grow into bigger
(~ 60 µm) laminates as shown in Figure 12. This tendency continues for the 5 hour, 7 hour,
and 8 hour syntheses. Finally for the sample MW7, the 10 hour synthesis, the SEM images
in Figure 13 show both small intercrossed laminates (< 20 µm), along with big laminates
(~120 µm) of NTHU-4. The resulting solid presents a white-yellow emission. In general the
plate like crystals increase in size without greatly changing the basic morphology, suggesting
an Ostwald ripening process.
It should be noted that the NTHU-4 synthesis without zinc produced sword-like crystals that
exhibited no photoluminescence and an XRD pattern very different. If gallium is removed
from the NTHU-4 recipe, white crystals and an XRD pattern consistent with the impurity in
sample MW2 at ~10.8° are obtained.
4.3.2
Heating Temperature
As a very important variable in order to reduce synthesis time, the effect of heating
temperature was also studied. The XRD patterns obtained for the NTHU-4 as-synthesized
powders after microwave heating for 4 hours at 155, 160, 165 and 175 °C are shown in
Figure 14 . It can be observed that heating at a temperature of 155°C, results in a powder that
is the closest to the NTHU-4 pattern, with some impurities. This suggests that a longer
heating time will be required at this temperature. The sample at 160°C, presents impurity
peaks as well as a deficient peak at ~7.2°. This shows that little NTHU-4 is present in the
product, because most of it is forming the previously described bigger NTHU-4
agglomerates. The samples at 165°C and 170°C, show the appearance of an extra peak at
~7.6°. This condition is comparable to the degradation of NTHU-4 material after 3 days
under conventional heating conditions. In this sense is established that under the current
83
molar ratios, it is not viable to reduce the synthesis time for NTHU-4 by heating at higher
temperature using microwave heating.
4.3.3
Mixing Time
The most homogeneous samples were obtained with a longer mixing time. The synthesis
was performed for 1, 2, 3, and 5 hours with mixing times of 24, 48, 56, 64 and 72 hours.
XRD patterns shown in Figure 15, match with the simulated pattern without the presence of
additional peaks or impurities. From the XRD patterns in Figure 14 there are no apparent
changes with heating time of mixing time after 24 hours. Once the mixing time is increased
over 24 hours, the samples will present a white powder of uniform appearance, with a pure
NTHU-4 XRD pattern. After excitation with a 365 nm UV light, the samples with a mixing
time of 48 h and 56 h showed a white-orange color. In the other hand, the samples with a
mixing time of 64 h and 72 h showed a white light emission.
The emission spectra for samples heated for 1-5 h after mixing for 24, 48, 56, 64 and 72 h are
shown in Figure 16. When the emission spectra are compared, it is evident that the samples
show two main peaks: one at ~420nm and the other at ~530 nm. Under 365 nm excitation,
the intensity of the 420 nm peak is higher than the 530 nm in all cases. However, under 390
nm excitation, both peaks present very similar intensities after 48h and 56h, producing a
balanced white color. For longer periods of time, 64h and 72h, the intensity of the 420 nm
peak is higher than the 530 nm peak. However for these two samples, the perceivable color is
white.
The white light emitting products, were compared to the one reported by Liao et al [21]. It
was confirmed that to obtain a white light emission, it is necessary to decrease the water
content while adding ethylene glycol. In the case, of microwave heating, mixing time proved
84
to be the critical factor in obtaining pure NTHU-4. Liao et al reported that NTHU-4 was
brown in color while the microwave samples were generally white in color, especially after
mixing for more than 24 h. This color difference could be caused by template decomposition.
However, compared to the reported brown product, our solids presented a white color under
visible light, especially after mixing for more than 24 h. This color difference could be
caused by template residues or poor incorporation due to an incomplete mixing.
4.4
Conclusions
The adaptation of a hydrothermal synthesis of NTHU-4 from conventional to microwave
heating is not a trivial transition. Some articles have proposed several explanations on a
microwave effect, but most are particular to each system and are based on vibrations and
rotations produced by microwaves.
Even when the details on how the microwaves affect
the reagents are not described here, the proof of this particular interaction evidences through
of a shorter reaction time. The main goal on this project was accomplished, providing the
microwave synthesis of NTHU-4, showing comparable characteristics compared to the
reported material.
The photoluminescent characteristics of several gallophosphates are attributable to the
template agent and the presence of defects.[21] In this sense, by modifying synthesis
conditions, both intensity and emission wavelength should be tunable. With appropriate
parameters, NTHU-4 could provide a bright future for LED applications.
experimental conditions are currently under study to determine these effects.
Several
85
4.5
Acknowledgement
We thank the Robert A. Welch Foundation and SPRING, for the supporting of this project.
APPENDIX
Table 1. The synthesis conditions for NTHU-4 using conventional heating at 160°C and 400
watts. (OA= Oxalic Acid, EG= Ethylene glycol, Y=yellow, O=orange, W=white)
Sample
TMDP
Ga2O3
ZnCl2
OA
H3PO4
H2O
EG
CH1
12.7
1.0
2.0
2.4
12.2
1157.8
109.7
24
O+Y
CH2
12.5
1.0
2.0
2.4
11.9
802.8
107.7
48
Y
CH3
12.5
1.0
2.0
2.4
11.9
802.8
107.7
72
O
CH4
12.5
1.0
2.0
2.4
11.9
802.8
107.7
96
O
CH5
5.3
0.0
1.4
1.0
5.1
478.9
45.4
96
O+W
CH6
12.5
1.0
2.0
2.4
11.9
802.8
107.7
120
Y
CH7
12.5
1.0
2.0
2.4
11.9
802.8
107.7
144
O+Y
CH8
12.5
1.0
2.0
2.4
11.9
802.8
107.7
168
O+W
CH9
12.5
1.0
2.0
2.4
11.9
802.8
107.7
192
Y+O
86
Time (h)
emission
87
Table 2. The synthesis conditions for NTHU-4 using microwave oven at 160°C and 400
watts. (OA= Oxalic Acid, EG= Ethylene glycol, Y=yellow, O=orange, W=white)
Sample
TMDP
Ga2O3
ZnCl2
OA
H3PO4
H2O
EG
Time
(h)
emission
Mix
Time
MW1
12.8
1.0
2.0
2.3
11.9
812.2
107.2
2.0
Y
0.5
MW2
12.6
1.0
2.0
2.4
12.0
808.5
106.3
3.0
W-O
0.5
MW3
12.6
1.0
2.0
2.3
11.8
806.7
106.1
4.0
W-O
0.5
MW4
12.6
1.0
2.0
2.3
12.0
807.5
106.2
5.0
W-O
0.5
MW5
10.3
1.0
1.4
1.6
8.1
547.8
72.0
7.0
Y
0.5
MW6
10.8
1.0
1.7
2.0
10.2
691.5
90.9
8.0
W-O
0.5
MW7
10.8
1.0
1.7
2.0
10.2
691.5
90.9
10.0
Y-W
0.5
MW8
12.9
1.0
2.0
2.4
12.0
845.0
160.0
2.0
Y
24.0
MW9
12.9
1.0
2.1
2.4
12.1
845.0
159.0
5.0
W-O
48.0
MW10
12.9
1.0
2.1
2.4
12.1
845.0
159.0
2.0
W-O
56.0
MW11
12.9
1.0
2.1
2.4
12.1
845.0
159.0
1.0
W
64.0
MW12
12.9
1.0
2.1
2.4
12.1
845.0
159.0
3.0
W
72.0
88
Figure 1. NTHU-4 structure calculated using with Materials Studio, and the crystallographic
data in ref [21]
89
CH7
CH6
CH5
CH4
CH3
CH2
CH1
simulated
Two-Theta (deg)
Figure 2. Powder XRD patterns of NTHU-4. 1) Pattern calculated using Materials Studio
and the data from Ref. [21], and NTHU-4 synthesized by conventional heating for 2) 1 day,
3) 2 days, 4) 3 days, 5) 4 days, 6) 5 days, 7) 6 days, 8) 7 days.
90
Intensity
B
C
A
D
400
450
500
550
600
650
700
Wavelength (nm)
Figure 3. Emission spectra under a 390nm excitation for NTHU-4 samples produced by
regular heating after A) 1 day, B) 2 day, C) 5 day, D) 7 day.
91
Figure 4. SEM image for NTHU-4 sample CH1 by conventional heating after 1 day at
160°C.
92
Figure 5. SEM image for NTHU-4 sample CH2 by conventional heating after 2 days at
160°C.
93
Figure 6. SEM image for NTHU-4 sample CH3 by conventional heating after 3 days at
160°C.
94
Figure 7. SEM image for NTHU-4 sample CH4 by conventional heating after 4 days at
160°C.
95
Figure 8. SEM image of NTHU-4 laminates (left) and gallium phosphate (right) after 7 day at
160°C using conventional oven ( sampleCH7).
96
MW7
MW6
MW5
MW4
MW3
MW2
MW1
Simulated
Two-Theta (deg)
Figure 9. Powder XRD patterns of 1) simulated NTHU-4, and the microwave products for 2)
1 h, 3) 2 h, 4) 3 h, 5) 4 h, 6) 5 h, 7) 7 h, and 8) 10 h.
97
Figure 10. SEM image for a NTHU-4 sample MW1, with crossing stick morphology after 2h
at 160°C using microwave oven.
98
Intensity
B
D
A
C
400
450
500
550
600
650
700
Wavelength (nm)
Figure 11. Emission spectra for sample MW1 for 2 hour A) 365 nm excitation, B) 390 nm
excitation and 10 hour C) 365 nm excitation and D) 390nm excitation.
99
Figure 12. SEM image for NTHU-4 sample MW3 after 4 hour synthesis by microwave oven
100
Figure 13. SEM image for a NTHU-4 sample after 10h at 160C, using the microwave oven.
101
170°C
165°C
160°C
155°C
Simulated
Two-Theta (deg)
Figure 14. XRD pattern of simulated NTHU-4 pattern and microwave synthesis powder after
4 hours at 155°C,160°C, 165°C and 170°C.
102
MW12
MW11
MW10
MW9
MW8
Simulated
Two-Theta (deg)
Figure 15. Powder XRD patterns of NTHU-4 by microwave heating at 160°C with after
mixing the precursor for 24 h.
103
Intensity
D
C
B
A
400
450
500
550
600
650
700
Wavelength (nm)
Figure 16. Emission spectra for NTHU-4 samples synthesized by microwave heating with a
mixing time of (A) 48hr, (B) 56hr, (C) 64hr, (D) 72 hr, excitation at 390nm.
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CHAPTER 5
HYDROTHERMAL SYNTHESIS OF FLUORINATED TIN OXIDE
NANOPARTICLES
Abstract. There is a growing demand for materials that permit a better utilization of energy.
Transparent Conducting Oxides (TCO) like Fluorinated Tin Oxide (FTO) can be used in
several applications like smart windows, light emitting diodes, and flat panel displays. The
synthesis of FTO particles under 200 nm was achieved, by hydrothermal treatment at 165°C
for 24 hours. The fluorine doping of this particles was accomplished by using LiF, with a
fluorine content of 3.9 %. According to TGA and XRD analysis, the crystalline structure is
stable at least up to 1300°C.
.
Keywords: Fluorinated Tin Oxide, P123, FTO, Transparent Conducting Oxide, TCO.
107
108
5.1
Introduction
Tin Oxide IV is considered a promising material for optic and electric applications due to its
high electrical conductivity and transparency. Some applications include transparent
conductive electrodes [1-5], photovoltaic devices [6-10], oxidation catalyst [11-15], gas
sensors [11, 16-20], and conductive layers [21, 22]. Such diversity of applications suggests
that as material, tin oxide presents very particular and important properties.
In nature, tin forms two different oxides, stannous oxide SnO and stannic oxide SnO2.
According to the heat of formation, stannic oxide (∆H= -138 cal/mol) is thermodynamically
more stable than stannous oxide (∆H= -68 cal/mol) [11]. As a mineral, SnO2 is known as
Cassiterite and as some other metal oxides like TiO2, RuO2, VO2 and CrO2 presents a rutile
structure, with a tetragonal unit cell. In this lattice, all Sn atoms are sixfold coordinated to
three-fold coordinated oxygen atoms. (Figure 1)
Besides the naturally occurring structures, tin oxide has also been synthesized in novel
shapes by different techniques. Novel nanostructures such nanoboxes [23], nanotubes [18,
23-25] , nanorods [25-28], nanowires [29, 30], nanodiskettes [22, 31], nanobelts [32, 33],
and nanoparticles [2, 16, 19, 20, 22, 25, 34-43] have been reported. While these novel
nanostructures have been made using mainly vapor deposition, other techniques like
electrospray, laser ablation, glow discharge, plasma discharge, combustion chemical vapor
deposition, r.f. magnetron sputtering, spray pyrolisis and hydrothermal treatments have been
used for particles and thin films of tin oxide.[11, 44] However using most these techniques
requires a special furnace setup, high temperature and pressure control, and with special
cooling and gas transport systems. [11]
109
To get a good idea of the potential of tin oxide’s applications is important to observe the
unique properties and characteristics of this material. There are circumstances when an
electrode is required to provide a good electric contact without blocking light. In that
situation the bulk properties of the material will define its performance. This condition is
present in devices such as solar cells, light emitting diodes and flat panel displays. A
Transparent Conducting Oxide (TCO) such as Fluorinated Tin Oxide (FTO) is a good
alternative. Several articles report an optical transparency in the visible region from 80% to
90% for undoped and doped tin oxide thin films [2, 11, 45-49].
Andersson et al, compared FTO and ITO to determine the feasibility of replacing ITO with
FTO as the hole-injecting electrode in polymer Light Emitting Diodes (LEDs)[1]. ITO was
presented as more expensive than FTO and indium presents the undesirable trend to diffuse
into the emissive polymer in LEDs. For a potential of 8V and after been washed with acetone
and isopropanol FTO films showed a luminance of 210 Cd/m2 while ITO presented only 15
Cd/m2. After been washed with a DI water, hydrogen peroxide and ammonia mixture, the
values for FTO films were 190Cd/m2 and ITO films 51 Cd/m2. It was also determined that
FTO presented a higher resistance to oxidation than ITO, this because FTO showed a lower
but stable work function of 4.4 eV compared to the ITO values of 4.4-4.8 eV depending on
the cleaning method. Despite providing more light at the same voltage, FTO films presented
a leakage current of 1 mA/cm2 which is a high value for display applications because it
produces cross-talking among pixels [1]. Compared to FTO, ITO films produced by spin
coating of ITO nanoparticles and with a Sn/In ratio of 5 at.% presented a transmittance of
~90% [49].
110
The optical transparency in the visible region combined with the reflectance of infrared
radiation also provides novel applications such as windows coatings. These coatings allow
taking advantage of sunlight illumination without the undesirable heating effects saving
energy on cooling applications. In addition there are also smart windows that change their
color by applying a voltage (electrochromic), after been exposed to sunlight (photochromic)
or both (electrophotochromic).[8, 10, 50-58] The importance of the heat reflecting and
dissipating materials becomes more and more important as cooling applications become
more energy demanding in an energy-limited world. The European Union reported a 17%
rise a year between 1995 and 2003 in air-conditioning demand [9]. In China, 51% of
electricity household use is for air-conditioning, refrigeration and water cooling. [9] While
some scientists work to find new energy sources, others work for more efficient and smart
ways to use energy such as solar cells and smart windows [9].
The uncommon property of optical transparency in FTO occurs between wavelengths of 0.4
µm and 1.5 µm. As conductivity and the number of charge carrier increase, the transparency
at long wavelengths decrease. In this way there is a trade-off in TCOs between conductivity
and transparency. Even when tin oxide is less conductive than indium tin oxide, it presents
the advantage of been less transparent on the infrared region. [11]
The remarkable combination of optical transparency and conductivity in SnO2 is the result of
four basic conditions. First the presence of a wide optical band gap that avoids interband
transitions in the visible range. Second, oxygen vacancies or the presence of dopants donate
electrons to the conduction band. Third, the presence of a single s-type conduction band
allows a high mobility of conducting electrons with low scattering and uniform distribution.
111
Fourth, inter-conduction-band adsorption of photons is prevented by a large internal gap in
the conduction band. [11]
Optical transparency and electrical conduction is the main feature of TCOs such as SnO2,
ZnO, In2O3, Ga2O3 and CdO. As the optical transparency is hard to improve, most of the time
efforts are made to increase electrical conductivity. Tin oxide is a wide band-gap (Eg = 3.62
eV at 300 K) [11, 59] semiconductor and electrical insulator (ρ~107 Ω cm) [60]. However, as
oxygen deficiencies are produced in SnO2 its electrical conductivity rises. An even higher
conductivity is achievable through increasing the charge carrier concentration via dopant
addition. The most common dopants for SnO2 are Sb and F as cation and anion dopant
respectively. In the case of FTO, when O2- is replaced by F- anions the amount of free
electrons (free carrier concentration) increases and the material shows a higher conductivity.
Fluorine presents three characteristics to be a good substitute for O2- in the SnO2 crystal
lattice [61]. First the ionic size of F- (0.133 nm) is very close to O2- (0.132 nm). Second the
Sn-F bond energy is (~26.75D°/kJ mol-1) close to the Sn-O (31.05D°/kJ mol-1) [61]. Third,
the Coulomb forces that bind the lattice together are reduced because the fluorine ion
presents a charge that is half of the oxygen ion [11, 60-63].
The resistivity of thin films has been reported by Kawashima et al as 1.2 x 10-4 Ω cm for
ITO, 6.5 x 10-4 Ω cm for FTO and 1.4 x 10-4 Ω cm for a FTO film on ITO. [46] According to
Gamard et al, FTO powder prepared from alkoxyfluoro(β-diketonate)tin(IV) compounds
presents a resistivity of 6 Ω cm compared to 30 Ω cm of ITO powder. [44] As presented by
Cachet et al. depending of the technique and the amount of F content, the resistivity values
can oscillate between 0.11 Ω cm with F/Sn 5 at.% and 8 x 10-4 Ω cm with F/Sn 13 at.% for
spray deposition and 20 Ω cm for spin coated films with F/Sn 3 at.%. [47] However, in the
112
article, they also prevent that an excess of F atoms tend to isolate on grain boundaries
increasing resistivity. [47] In the article by Shanti et al, the resistivity minimum was reported
for sprayed films with a F/Sn ratio of 57 at.%. [62] Elangovan and Ramamurthi reported a
resistivity minimum at 15 wt.% for SnO2:F and 2 wt.% for SnO2:Sb.[61]
According to the previously presented properties, the production of nanoparticles of
fluorinated tin oxide can generate new opportunities in the optical and electrical areas. If in
addition these particles are mesoporous, there is even a bigger possibility of coupling the tin
oxide substrate with other material with good photoelectrical properties such as TiO2.
In the article by Yang [64] it is stated that a good option to produce metallic oxide
mesoporous structures is by using nonionic amphiphilic block copolymers. These block
copolymers present the characteristic of allowing an interaction just adequate to promote a
self-assembly of a stable 3D mesophase, but at the same time the interaction is weak enough
to allow the removal of the template without collapsing the formation. The use of low
molecular weigh surfactants normally produce structures that collapse during calcination. It
is also recommended not to use metal alkoxide or aqueous solutions, because these
conditions tend to increase the rate of hydrolysis, condensation, and crystallization, harming
the mesoporous structure’s assembly.
The synthesis of mesoporous tin oxide films and particles, has been reported by using several
templates including polybutanodiene-block-poly(ethylene oxide) (PB-PEO) in benzyl
alcohol[36], different glycols and cetyltrimethylammonium bromide (CTAB)[65], amixture
of
CTAB
and
neutral
dodecylamine
(DDA)
in
water[59],
N-cetyl-N,N,N-
trimethylammonium bromide in water [66], block co-polymer Pluronic P-123 in ethanol [64],
113
CTAB in DI water [17], n-cetylpyridinium chloride in DI water [67], tetradecylamine and
tetraorthosilicate [68], stearic acid and 1-propanol [69], within others.
To accomplish the goal of fluorinated nanoparticles hydrothermal synthesis was selected due
to its relatively simple conditions. The main approach was first to get submicron-particles,
then find a way to fluorinate them and finally select the best conditions for the formation of
mesoporous particles. This research has successfully developed a method for the
hydrothermal synthesis of fluorinated tin oxide particles.
5.2
Experimental Section
5.2.1
Gel Preparation
In a typical synthesis 1.11g (BASF) of Pluronic P123 were dissolved while stirring in 60 mL
of absolute ethanol (AAPER) and 24mL of 1,2-dimethoxyethane (EMD, 99.5%). Then 0.41g
of LiF (Fluka, 99%) were added to the solution, followed by 12.2g of SnCl4·4H2O (SigmaAldrich, 98%). The resulting solution was stirred for 1 h or 24 hr at room temperaure.
The incorporation of fluoride into the tin oxide nanoparticles required the study of the
following reagents NH4F (Aldrich, 97%), LiF (Fluka, 99%), NaF (Aldrich, 99%), KF·2H2O
(Sigma-Aldrich, 98%), SbF3 (Aldrich, 99.8%), and 2,2,2-trifluoroethanol ( Aldrich, 99%).
Using the hydrothermal approach several solvents were studied. The solvents used, include
hexanes, ethanol, p-dioxane, DI water, t-butanol, 1,2-dimethoxyethane, isopropanol, nbutanol, t-butanol and sec-butanol. Surfactants such as P123, CTAB and vitamin E were used
as templating agents. Both SnCl4 and SnCl4·5H2O, were used as tin precursors. A mixture of
ethanol and 1,2-dimethoxyethane with P123 gave the best result for nanoparticle formation.
114
5.2.2
Microwave synthesis of FTO particles
The clear precursor solution, was placed in a 95 mL CEM XP-1500 Plus Teflon reaction
vessel and heated at 160°C-200°C in a CEM MARSXpressTM microwave (CEM
Corporation) at 2.45 GHz. The heating time was varied from 60 to 240 min. A 20 min ramp
time was used to reach the desired temperature and the power used was 400 watts. The
reaction temperature was controlled via a reference vessel. The resulting product was
centrifuged, and washed with deionized water and ethanol and dried at 90°C for 24 h.
5.2.3
Conventional heating synthesis
The clear precursor solution was placed in a 23 mL Teflon-lined Parr autoclave and heated at
160°C under static condition for a period of 1-2 days. The product was centrifuged, and
washed with deionized water and ethanol and dried at 90°C for 24 h.
5.2.4
Analysis
Powder X-ray diffraction patterns were obtained using a Rigaku Ultima III X-ray
diffractometer using CuKα radiation.
Samples for scanning electron microscopy were
coated with Pd/Au and images obtained on a Leo 1530 VP Field emission scanning electron
microscope. TEM images were obtained using a High Resolution Structural Analysis JEOL
2100 F which is a 200kV field emission TEM. Thermogravimetric analyses were performed
using a Perkin Elmer PYRIS 1 Thermogravimetric Analyzer.
5.3
Results and discussion
Based on the article by Yang et al.[64], the first approach was to use the mesoporous thin
film approach to form mesoporous nanoparticles. With this in mind, the first surfactant
115
studied was the Block Copolymer P123, along with a SnCl4 as Sn source, and 95% ethanol
and hexanes as solvents. Organic solvents like hexanes and p-dioxane were studied. Ethanol
was also taken into consideration because it has shown to promote a slow hydrolysis and
condensation for mesoporous structures [64].
The reaction mixture was heated at 225°C for 2 h using a microwave oven. In presence of
hexanes, the results were not favourable, giving irregular shaped SnO2 agglomerates. The
reaction conditions were modified, replacing hexanes for p-dioxane and then by using
ethanol alone. The heating conditions were also modified, heating first at 175°C for 1h an
then at 200°C for 4 h to try to achieve a more progressive build up of the mesoporous
structure. With the elimination of hexanes and the new heating scheme spherical aggregates
of ~2µm of tin oxide started to form in presence of p-dioxane. Other organic solvents such
as t-butanol, and dimethoxyethane were tried. The presence of these oxygen-containing
organic solvents resulted in the formation of irregular shaped aggregates made up of
spherical collections of particles between 20 nm and 150 nm. According to XRD analysis, all
the samples showed a cassiterite pattern, but any possibility of a stable mesoporous structure
was discarded by the absence of low angle peaks.
After the initial stage of the effect of different organic solvent in the microwave synthesis of
SnO2 the next step was to study the incorporation of fluoride as a dopant. The first attempt
was with NH4F in the microwave oven. With a very violent reaction in the microwave
vessels, it was decided to continue using Teflon liners in steel autoclaves in a regular oven.
Using ethanol and 1,4-dimethoxyethane as solvents, P123 as template, SnCl4 as tin source,
the effect of different fluoride sources; NH4F, NaF, LiF and KF·2H2O was studied. Every
precursor was added in the same amount 0.003 mol per mixture. The reaction mixtures were
116
heated at 120°C for 24 h. For these products the XRD showed a non-identified pattern. The
SEM images and EDAX analysis for the sample with NH4F showed rhombohedra and platelike structures with an approximate composition of 44.0 at.% Cl, 38.5at.% C, 9.1 at.% Sn and
8.4 at.% O. When NaF was used, rhombohedra shaped structures appeared with a
composition of 54.4 at.% Cl, 37.8 at.% Na, 6.1 at.% O, and 1.7 at.% Sn. When LiF was used,
no distinguishable pattern was obtained with the XRD. The SEM and EDAX analysis
showed irregular shaped particles with up to 83.1 at.% of C and just 1.6 at.% on Sn. For the
last sample, KF·2H2O was examined. From the XRD patterns, SEM images and EDAX
analysis, it was evident that the main product was potassium fluoride with cubic crystals with
49.3 at.% Cl and 46.1 at.% of K and no evidence of tin.
From these experiments important information was obtained. First, that the samples required
a more rigorous elimination of surfactant and ionic salts by washing with DI water or any
necessary techniques. This was evidenced by the high carbon and alkaline halide content.
Second, that the temperature and time selected were not appropriate to form tin oxide,
because no XRD pattern evidenced a presence of tin oxide. And third, that under that amount
of impurities, and lack of tin oxide, it wasn’t possible to determine the best fluorinating
agent.
Considering the preceding results, a new experiment was designed, maintaining the same
molar ratios, but increasing the reaction time from 24 h to 48 h, and the temperature from
120°C to 160°C. In addition, trifluoroethanol was also tried as fluorinating agent. This time
all the samples showed some signal on the low angle pattern of the XRD analysis, as an
indication of a possible mesoporous structure. At high angle, two samples, one with KF and
the other with NaF showed an irregular pattern before been calcined. After calcination, only
117
the KF sample showed, additional to the cassiterite structure, an impurity that matches the
pattern of barium tin oxide. The SEM samples showed agglomerates of spherical particles of
tin oxide. The sample with NH4F showed irregular shaped agglomerates of ~7 µm with no
evidence of fluorine according to EDAX. After calcination, the particles showed a smoother
surface, and a fluorine content between 6.4 and 9.4 at.%. The trial with NaF showed
agglomerates of ~ 3 µm in size and a fluorine content of ~15.7 at.% in the surface and 0 at.%
in the interior of the structure. After calcination there was no measurable fluorine content.
The sample with LiF showed homogeneous aggregates from ~50-100 nm of spherical
nanoparticles. The fluorine content was between 8.0 at.% and 11.6 at.%. After calcination
the aggregates collapsed, making images hard to focus. However, the fluorine content was
still at 10.8 at.%. As presented before, the sample with KF showed a crystalline impurity
according to XRD. In the SEM images, aggregates of ~3 µm presented 9.4 at.% of fluorine
before calcination. Before and after calcination, unidentified chlorine containing species were
present as impurities. Calcined samples showed no evidence of fluorine. When
trifluoroethanol was used; spherical structures with a porous layer around them were formed.
This layer was composed mainly by oxygen 43.7 at.%, tin 26.9 at.% and carbon 19.8 at.%.
No fluorine content was detectable before or after calcination. This was probably due to a
mayor loss of trifluoroethanol during the washing and centrifuging processes.
Only LiF and NaF showed a doping effect over tin oxide. However LiF shows higher
fluorine content on the final material according to EDX, so it was selected as the fluorinating
agent. A probable cause of the higher fluoride content can be attributed to its lower
solubility. In the case of trifluoroethanol, the C-F bond is thermally stable and presents high
118
bond energy (116 kcal mol-1) [70, 71] , and doping of tin oxide was unlikely at a low
temperature of 160°C.
Other solvents and compounds were also formed by heating at 165°C two days. One trial
replaced ethanol with 1,4-dimethoxyethane (DME).This sample produced a fine, black
powder that under SEM showed smooth ~6µm spheres with low crystallinity according to
XRD pattern. It was considered that DME produced smaller aggregates of tin oxide,
compared to just ethanol.
The effect of an oxidating agent was also studied. Two samples were compared, form which
one contained hydrogen peroxide 30%. The samples were heated at 160°C for 24 h, SnCl4
was used as tin source, and LiF as the fluorine source. The sample without peroxide formed
spheres from 150 nm to 550 nm, with a chlorine content of 1.3 at.%. The sample with
hydrogen peroxide presented bigger agglomerates containing spheres mixed with other
irregular structures. The presence of hydrogen peroxide did produce a low angle peak of
higher intensity than the sample without peroxide. However, this can be attributed to the
presence of particles of irregular shape and distribution, compared to the desirable spherical
particles. At high angle, both samples showed a cassiterite pattern.
In the continued effort of optimizing the synthesis of fluorinated nanoparticles the reaction
was studied at 90°C, 120°C, 140°C and 160°C for a period of 24 hours using SnCl4·5H2O.
According to the XRD patterns, all the samples, except the one at 90°C, presented a clear
cassiterite pattern at high angle. At low angle, all the samples showed a peak, however this
peaks were of very low intensity. Under the SEM, the sample produced at 90°C showed an
amorphous appearance of small particles with a high content of chlorine (11.1 at.%), carbon
(22.1 at.%) and fluorine (7.2 at.%). The sample heated at 120°C showed blocks of particles
119
with no evidence of fluorine. This blocks presented high amounts of carbon (between 30.0
and 50.0 at.%). Agglomerates of nanoparticles with diameter size around ~100nm were
present in the sample at 140°C. This sample showed a carbon content of (24.9 at.%), fluorine
(5.6 at.%)
and chlorine (1.7 at.%). Finally the sample heated at 160°C also formed
agglomerates of spherical nanoparticles. However, the samples still presented a high carbon
content of (34.0 at.%) along with chlorine (1.0 at.%) and fluorine (6.0 atm.%).The sample
synthesized at 160°C showed a single crystal structure according to the Selected Area
Diffraction (SAD) image on the Transmission Electron Microscope (TEM) (Figure 2). On
the imaging mode, it is possible to observe the crystal lattices of tin oxide, but not evident
mesoporous material was observed.
Even when crystalline SnO2 fluorinated nanoparticles could be formed at 160°C after heating
for 24 h it was clear that impurity content (carbon and chlorine) was too high, and the
fluorine content too low to give a satisfactory product. However it was determined that
heating at 90°C for 24 h wasn’t enough to produce crystalline SnO2 under the reaction
conditions. It was also determined that there wasn’t a clear trend relating fluorine
incorporation with heating temperature.
Keeping in mine the goal of a mesoporous material, other surfactants were tried. CTAB was
studied, at room temperature, in presence of ethanol water and in a 50% v/v mixture of
ethanol and water. Comparing the reaction mixtures, all of them formed a white colloid when
the SnCl4·5H2O was added. However the samples with a 50% v/v concentration of ethanol in
water formed a completely white solution, instead of a white cloudiness. This white
appearance was attributed to a condensation process of tin chloride. The three samples
produced at room temperature showed no crystalline XRD pattern and an amorphous
120
conformation according to SEM images. These reactions were also studied after heating at
160°C for 72 h. These samples synthesized, showed a cassiterite pattern at high angle, and at
low angle some low intensity peaks. However the sample with the higher intensity was the
one obtained using just ethanol as a solvent. This sample was also the only to show a fluorine
content of (5.7 at.%) but with a high carbon content (27.3 at.%). According to SEM the
agglomerates of spheres were interconnected forming a bigger, disordered structure. SEM
images of CTAB samples with water and 50% v/v EtOH showed blocks or disordered
structures.
The effect of stirring time was also studied. A sample with pH ~ 0.80 was stirred overnight
before heating at 165°C, for 24 h. The effect of these extended stirring time, produced two
structures. The first one is a spherical aggregate with a diameter between 2 µm and 12 µm
(Figure 3), made up of small particles of ~50nm in size. The fluorine content was determined
at 10.22 at. %. The second is a irregular shaped ~7 µm aggregate, made up of spherical
particles, which didn’t evidenced fluorine content. According to XRD analysis, the sample
presented very sharp, intense peaks at high angle and a small peak at low angle.
Two different tin precursors were compared SnCl4 and SnCl4·5H2O, along with the effect of
a 3day heating at 185°C. Lithium fluoride was used as fluorinating agent. Ethanol and 1,2dimethoxyethane were the organic solvents. For both samples XRD analysis showed a
crystalline, well-defined cassiterite pattern at high angle, and no peaks at low angle. SEM
images of the sample made with SnCl4 showed a tendency to produce big spherical
agglomerates (~7 µm) of small nanoparticles (~20 nm). In contrast, irregular shaped
agglomerates of spherical particles (~100 nm) (Figure 4) were formed when SnCl4·5H2O was
used. This result illustrates the expected trend of forming agglomerates of very small
121
particles, when more reactive tin precursors are employed due to the fast formation of
nucleation centers.
The effect of dimetoxyethane on the synthesis of tin oxide fluorinated particles was also
studied. The first synthesis was performed at 165°C for 24 hours, containing both ethanol
and dimethoxyethane. The SEM images show the sample after calcination at 450°C, in which
groups of smooth spheres (~200nm in diameter) present a fluorine content of 3.9 at. %
(Figure 5). When the synthesis is performed without DME, fluorine content is increased, but
the particle size is reduce, and the morphology and size will become more irregular and
rough. Particles can be present from ~50 nm to ~200 nm, with a fluorine content of 5.6 at. %
(Figure 6).
The thermal stability and template content is this samples has been studied by Thermal
Gravimetric Analysis (TGA). In the TGA graph of percent weight loss against temperature,
both samples, with and without DME, show a high stability up to 1300°C. The sample
without DME shows a higher stability with weight loss of 5.2% and the sample with DME
presents a weight loss of 6.2%. (Figure 7) In both cases, the weight loss indicates that the
template content in the samples is very small, under 6.2%. This condition, indicates first that
washing with ethanol and DI water eliminates most of the organic and inorganic impurities.
Then it also explains why the spheres are not mesoporous. If the template is easily removed
by washing, then it is unlikely that the metal oxide spheres are formed from the inside to the
outside. By the contrary, the template, could be surrounding the metallic particles, producing
a spherical shape. When the derivative weight graph is considered, the FTO particles present
three main peaks in similar positions.(Figure 8) The first peak ~ 100°C represents water and
ethanol loss. The second one ~575°C is considered to be produced by P123 residues. The
122
third peak around 900 and 1000°C has not been assigned yet. According to XRD patterns,
after calcination, there is no effect on the peaks position. Instead the intensity of the peaks
becomes higher, and the peaks become thinner, as the temperature increases. According to
Scherrer’s formula, this will imply that with increasing temperature the particle size
increases. (Figure 9) (Figure 10)
5.4
Conclusions
The hydrothermal synthesis of FTO particles was accomplished after heating at 165°C for 24
hours, with fluorine content from 3.9 to 10.0 at. % and diameter under 200 nm. The effect of
different solvents, templates and fluorinating agents was studied, but the best results were
obtained when using ethanol, DME, P123 and LiF. Stirring overnight proved effective to
produce a better morphology and increase the fluorine content of the particles. According to
XRD and TGA analysis, FTO particles obtained presented a high stability, and remain
crystalline up to 1300°C. The obtained FTO particles can greatly contribute to the important
field of energy efficient materials
5.5
Acknowledgement
We thank the Robert A. Welch Foundation and SPRING, for the supporting of this project.
APPENDIX
Figure 1. Unit cell of cassiterite (SnO2) crystalline structure. Sn atoms (gray) are sixfold
coordinated to three-fold coordinated oxygen atoms (red). Data and image by Materials
Studio.
123
124
Figure 2. TEM image showing the crystal lattices of a FTO particle and Selected Area
Diffraction (SAD)after heating at 160°C for 24 hours.
125
Figure 3. FTO particles stirred overnight and then heated at 165°C for 24 h.
126
Figure 4. FTO particles made using SnCl4·5H2O as tin precursor, after heating at 185°C for 3
days
127
Figure 5. SEM image of FTO particles after calcination at 450°C. for a sample produced
using EtOH and DME and heating at 165°C for 24 hours.
128
Figure 6. SEM image of FTO particles using DME, heating at 165°C for 24 h.
129
100
99
Weight % (%)
98
97
96
B
95
94
A
93
92
91
90
0
100
200
300
400
500
600
700
800
900 1000 1100 1200 1300
Temperature (°C)
Figure 7. TGA weight loss percent graph for;(A) with DME (black) and (B) without DME
(gray)
130
0
Derivative Weight % (%/m)
-0.02
0
150
300
450
600
750
900
1050
1200
1350
-0.04
-0.06
-0.08
-0.1
B
-0.12
-0.14
A
-0.16
Tem perature (°C)
Figure 8. TGA derivative graph for FTO particles; a) with DME (black) and b) without DME
(gray).
131
1300°C
800°C
450°C
300°C
Uncalcined
Figure 9. XRD pattern for calcined FTO particles produced with ethanol and DME.
132
1300°C
800°C
Uncalcined
Figure 10. XRD pattern for calcined FTO particles produced without DME .
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VITA
José Antonio Losilla Yamasaki was born in San Jose, Costa Rica, on September 26, 1981. He
is the son of Jorge Alberto Losilla Penón and Patricia Yamasaki Camacho. He graduated
with honors from Saint Paul College in 1998. He entered Universidad de Costa Rica, in
March 1999, where he received the degree of Bachelor of Science on Chemistry in July,
2004. In August 2004, he entered the PhD program on Chemistry in the University of Texas
at Dallas.
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