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Surfactant-Free Nonaqueous Synthesis of Metal Oxide Nanostructures.

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Minireviews
N. Pinna and M. Niederberger
DOI: 10.1002/anie.200704541
Sol–Gel Processes
Surfactant-Free Nonaqueous Synthesis of Metal Oxide
Nanostructures
Nicola Pinna* and Markus Niederberger*
metal oxides · nanostructures ·
organic–inorganic hybrid composites ·
sol–gel processes · surfactant-free synthesis
S
urfactant-free nonaqueous (and/or nonhydrolytic) sol–gel routes
constitute one of the most versatile and powerful synthesis methodologies for nanocrystalline metal oxides with high compositional
homogeneity and purity. Although the synthesis protocols are particularly simple, involving only metal oxide precursors and common
organic solvents, the obtained uniform nanocrystals exhibit an
immense variety of sizes and shapes. The small number of reactants in
these routes enables the study of the chemical mechanisms involved in
metal oxide formation. Nonhydrolytic routes to inorganic nanomaterials that used surfactants as size- and shape-controlling agents
have been discussed recently. This Minireview supplements this topic
by discussing surfactant-free processes, which have become a valuable
alternative to surfactant-assisted as well as to traditional aqueous
sol–gel chemistry routes.
1. Introduction
This Minireview discusses the nonaqueous liquid-phase
synthesis of metal oxide nanostructures in the absence of
surfactants, and thus supplements two recent reviews in
Angewandte Chemie on nonhydrolytic and surfactant-controlled routes to inorganic nanomaterials.[1, 2] Research on
nanoparticles, which encompasses synthesis, characterization
of the structural, chemical, and physical properties, assembly
into larger structures extending over several lengths scales,
[*] Dr. N. Pinna
Department of Chemistry, CICECO
University of Aveiro
3810-193 Aveiro (Portugal)
Fax: (+ 351) 234-370-004
E-mail: pinna@ua.pt
Homepage: http://www.pinna.info
http://www.nanodesigners.net
Prof. Dr. M. Niederberger
Laboratory for Multifunctional Materials
Department of Materials, ETH Z<rich
Wolfgang-Pauli-Strasse 10, 8093 Z<rich (Switzerland)
Fax: (+ 41) 44-632-1101
E-mail: markus.niederberger@mat.ethz.ch
Homepage: http://www.multimat.mat.ethz.ch
http://www.nanodesigners.net
5292
and application in various fields of
technology, represents a fundamental
cornerstone of nanoscience and nanotechnology. The large number of different synthesis techniques developed
in the last few years gave access to
nanomaterials with a wide range of
compositions, monodisperse crystallite
sizes, sophisticated crystallite shapes, and with complex
assembly properties. Although vapor-phase processes have
also successfully been employed for the preparation of
nanomaterials, in particular for one-dimensional nanostructures such as nanowires or nanobelts,[3, 4] it seems that liquidphase syntheses are more versatile with regard to the
controlled variation of structural, compositional, and morphological features of the final products.[5] Liquid-phase
routes employed to date include coprecipitation, hydrolytic
as well as nonhydrolytic sol–gel processes, hydrothermal or
solvothermal methods, template synthesis, and biomimetic
approaches.[5] However, often the synthesis protocol for a
particular nanomaterial involves not just one, but a combination of several of these methods.
Although the topic of this Minireview is set on the
surfactant-free nonaqueous synthesis of metal oxide nanoparticles and nanostructures, some other selected methods
shall briefly be discussed. One of the most powerful
preparation routes to semiconductor and metal oxide nanocrystals is represented by nonhydrolytic colloidal routes, in
which surfactants in the reaction mixture provide excellent
control over crystallite size, shape, and dispersibility.[1, 2] The
use of surfactants and surfactant assemblies has a long
tradition in colloid chemistry[6] and is not at all restricted to
nonhydrolytic reaction conditions. An early example is the
2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chemie
Metal Oxide Nanostructures
aqueous synthesis of size-controlled CdS quantum dots in the
presence of thiols.[7] More recent work includes uniform
hematite,[8] ceria,[9] indium oxide[10] nanocubes, and many
other nanoparticles that were obtained either in aqueous
systems, or in two-phase systems consisting of an aqueous and
an organic phase.[11–13] Other variations of surfactant-directed
approaches include the aqueous synthesis of metal oxide sols
and their subsequent injection into a surfactant solution,[14] or
the room-temperature decomposition of organometallic precursors in surfactant solutions using the moisture in air.[15, 16]
Another recently reported approach utilizes metallomesogens, that is, metal-containing amphiphiles, as metal oxide
precursors instead of a metal species and surfactants.[17] The
use of ionic liquids as organic solvents constitutes another
fast-developing research field, which enables the synthesis of
various inorganic nanoparticles and nanostructures either in
the presence or under exclusion of water.[18–21]
Although surfactant-free aqueous systems have been used
in chemistry for decades for the preparation of highly
monodisperse colloids,[22, 23] they showed some limitations
when it came to the preparation of smaller particles in the
nanometer range. One of the main problems was the lack of
crystallinity. However, in the last few years several interesting
examples were reported that were able to overcome this
detrimental issue (under strict control of the physicochemical
conditions of the precipitation process).[24–26] Also biomimetic
synthesis approaches enabled the synthesis of various metal
oxide nanoparticles and nanostructures at low temperatures.[27–29] In addition to the problem of low crystallinity,
aqueous processes are rather complex, mainly due to the high
reactivity of the metal oxide precursors and the dual role of
water as ligand and solvent. Slight changes in experimental
conditions result in altered particle morphologies, hampering
the reproducibility of a synthesis protocol as well as its scaleup to industrial quantities.
The rapidly growing number of inorganic nanoparticles
that were prepared by nonaqueous and/or nonhydrolytic
processes clearly indicates that synthesis routes in organic
solvents under exclusion of water represent a versatile
alternative to aqueous methods.[1, 2, 30–36] Although the success
of these synthesis approaches is strongly connected to the
manifold role of the organic species (either initially present in
the reaction solution or formed in situ during the reaction
course), their detailed function on a molecular level is not yet
completely understood. On the one hand, this is due to the
difficulty in gaining experimental information about the
organic–inorganic interphase,[37] and on the other hand it
seems that the role of the organic species is much more
complex than originally thought. Whereas the influence of
organic surfactants on the size and shape of the inorganic
nanocrystals is relatively well understood on the basis of
dynamic, sometimes face-selective, surface adsorption and
desorption processes,[1, 38–41] the investigation of the organic
reaction pathways, that is, the chemical transformation of the
organic components in the reaction mixture with longer
reaction time, have only been started recently.[32, 42, 43] Nevertheless, there is no doubt that the organic compounds act as
oxygen source during the formation of oxidic nanomaterials,[32] influence the size, shape, surface, and assembly properties of the crystallites, and, in some cases, also affect the
composition and the crystal structure.[44]
A comparison of the literature on surfactant-free and
surfactant-assisted approaches clearly shows that surfactant
routes permit outstanding control over the growth of metal
oxide nanoparticles, leading to almost perfectly monodisperse
samples.[2] Additionally, the ability of surfactants to cap the
surface of the nanoparticles provides several advantages such
as low agglomeration tendency, good dispersibility in organic
solvents, and the potential to tailor the surface properties.
However, drawbacks resulting from surface-adsorbed surfactants are the unpredictable influence on the toxicity of the
nanoparticles (the cytotoxicity of nanoparticles not only
strongly depends on particle size, but also on the surfactants
bound to the nanoparticle surface),[45–47] and the diminished
accessibility of the particle surface, which is a serious issue
when considering applications in gas sensing or catalysis. If
the amount of surfactants on the surface of the nanoparticles
has to be minimized, but good dispersibility properties need
to be retained, it is possible to use surfactant-free synthesis
routes with a subsequent post-functionalization step. It was
found that in these cases a tiny amount of surfactants is
enough to lead to completely transparent nanoparticle
dispersions. Examples along these lines are magnetite nanoparticles that are redispersible in water or in hexane, depending on the surfactants added,[48] or zirconium oxide nanoparticles that can be incorporated in a polymer matrix after
post-functionalization.[49, 50] Another feature, which makes
surfactant-free routes economically particularly interesting, is
Nicola Pinna studied chemistry at the
Universit Pierre et Marie Curie (Paris). He
received his Ph.D. in 2001, and in 2002, he
moved to the Fritz Haber Institute of the
Max Planck Society (Berlin). In 2003, he
joined the Max Planck Institute of Colloids
and Interfaces (Potsdam). In 2005, he
moved to the Martin Luther University,
Halle-Wittenberg, as an Assistant Professor
of Inorganic Chemistry. Since 2006 he is
senior researcher in the Department of
Chemistry and CICECO of the University of
Aveiro (Portugal). His research activity is
focused on the synthesis of nanomaterials by nonaqueous sol–gel routes,
their characterization, and the study of their physical properties.
Markus Niederberger studied chemistry at
the Swiss Federal Institute of Technology
(ETH) Z:rich, where he also received his
Ph.D. in 2000. After a postdoctoral stay at
the University of California at Santa Barbara, he became group leader at the Max
Planck Institute of Colloids and Interfaces at
Potsdam in 2002. Since 2007 he is Assistant Professor in the Department of Materials at the ETH Z:rich. His primary research
interests are the development of general
synthesis concepts for inorganic nanoparticles, investigation of formation and crystallization mechanisms, and nanoparticle
assembly.
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Minireviews
N. Pinna and M. Niederberger
Table 1: Pros and cons of surfactant-directed and solvent-controlled nonaqueous liquid-phase routes to metal oxide nanoparticles.
pros
cons
Surfactant-directed approaches
Solvent-controlled approaches
excellent control over crystal size
narrow size distribution
good control over crystal shape
low agglomeration tendency
good redispersibility
large amount of organic impurities
toxicity of surfactants
restricted accessibility of the nanoparticle surface
complex reaction mixtures
low amount of organic impurities
nontoxic solvents
simple, robust, and widely applicable synthesis protocols
good accessibility of the nanoparticle surface
the yields and concentration of the synthetic approaches.
Yields higher than 80 % in inorganic content are readily
achievable. From a scientific point of view, the absence of
surfactants in the reaction batch simplifies the investigation of
possible chemical reaction pathways.[32] Nevertheless, surfactant-assisted and surfactant-free synthesis approaches both
have advantages and limitations. Table 1 summarizes the pros
and cons of the two strategies, clearly underlining that they
are rather complementary. Of course, one always has to be
careful with such general trends, because selected reaction
systems can behave in a completely opposite way.
This Minireview on surfactant-free synthesis routes to
metal oxide nanoparticles is primarily meant to supplement
the two reviews[1, 2] recently published in Angewandte Chemie
on surfactant-assisted approaches in the very active research
area of inorganic nanoparticle synthesis in nonaqueous and/or
nonhydrolytic reaction media. In Section 2, we provide a
quite exhaustive overview of surfactant-free nonaqueous
liquid phase routes to metal oxide nanoparticles, and in
Section 3 we focus on the so-called “benzyl alcohol route”,
which proved to be an extraordinarily general approach to a
large number of metal oxide nanoparticles and metal-oxidebased ordered organic–inorganic hybrid nanostructures.
2. Metal Oxide Nanoparticles
Early studies on nonaqueous sol–gel processes date back
to the middle of the 19th century, when the reactions between
various metal chlorides and alcohols were investigated.
Ebelmen found that silicon tetrachloride forms silica gels in
ethanol,[51] and Demarcay prepared the corresponding titanium chloride ethoxide in an analogous experiment.[52] A few
decades later, Dearing and Reid,[53] followed by Gerrard and
Woodhead[54] continued the work of Ebelmen on the nonaqueous preparation of silica gels. In the 1990s, research on
nonhydrolytic preparation routes to metal oxides in various
forms such as gels,[55–57] thin films,[58] or nanopowders[59–63] was
intensified. Interestingly, already some of these experiments
led to the formation of nanoparticles. Nevertheless, it was
probably the work on the synthesis of titanium oxide nanoparticles, published in 1999 by three independent groups,[64–66]
which provided the main stimulus to this research field. Apart
from the work by Trentler et al.,[64] none of these earlier
examples involved the use of surfactants as size- and shapecontrolling agents. Nowadays, the family of metal oxide
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less control over crystallite size and shape
broader size distributions
formation of agglomerates
restricted redispersibility
nanoparticles that have been prepared by nonaqueous and
surfactant-free processes has grown immensely and ranges
from simple binary metal oxides to more complex ternary and
multi-metal systems such as perovskites and spinels. Not all
the synthesis routes to the various metal oxide nanoparticles
are discussed here in detail. However, Table 2 (binary metal
oxides), Table 3 (ternary and multi metal oxides), and Table 4
(doped metal oxides) give a relatively exhaustive overview of
the metal oxide nanoparticles synthesized in organic solvents
in the absence of water, together with the precursors and the
solvents used, the chemical formation mechanism (if known),
and the final particle morphology (if well-defined). Only
liquid-phase processes are listed, that is, thermal decomposition reactions in air or in other gases are not considered. The
reaction pathway refers to the condensation step, that is, the
metal-oxygen-metal bond formation, and generally represents the main mechanism in cases, in which several parallel
pathways were found. Furthermore, one has to keep in mind
that some of the precursors are hydrated, which means that
the water molecules may be responsible for the hydrolysis and
condensation, rendering these approaches hydrolytic. Nevertheless, they were included in Tables 2–4 the condition that no
additional water was used in the reaction batch.
A closer look at the various precursors and solvents listed
in Tables 2–4 clearly shows that most of the synthesis routes
are not generally applicable, but can only be used for a few
oxides. The solvent benzyl alcohol is an exception. It plays an
outstanding role, reacting with many metal oxide precursors
including metal alkoxides, halides, acetylacetonates, or acetates, thus giving access to more than 35 different metal oxide
nanoparticles and oxide-based hybrid nanostructures. For
example, the reaction of metal alkoxides in benzyl alcohol
proved to be applicable to the synthesis of a large selection of
metal oxides such as V2O3,[106] Nb2O5,[106] Ta2O5,[80] HfO2,[80]
SnO2,[82] In2O3,[82] CeO2,[34, 35] ZrO2,[49, 50] NaNbO3,[33] NaTaO3,[33]
BaTiO3,[72, 120, 121]
LiNbO3,[120]
[120]
[121]
[121]
BaZrO3,
SrTiO3,
and (Ba,Sr)TiO3.
In addition to oxygen-containing solvents, also amines, in
particular benzylamine, and nitriles can be used as solvents
for the transformation of metal acetylacetonates into binary
and ternary metal oxides.[76, 83, 131] This approach is particularly
advantageous for economic reasons due to the low cost of
acetylacetonates compared to metal alkoxides, especially for
transition metals with low oxidation states (II or III). In fact,
these alkoxides are generally very expensive and not always
commercially available.
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Metal Oxide Nanostructures
Table 2: Binary metal oxide nanoparticles synthesized by surfactant-free nonaqueous sol–gel routes.[a]
Metal oxide
Precursors
Solvents
Shape
Reaction mechanism
Reference
alumina
aluminum
sec-butoxide
various aluminum
alkoxides
[Al(acac)3]
sec-butanol
–
[59]
toluene
–
formation of isobutene
and dibutyl ether
–
[60]
c-alumina
g-alumina
or AlOOH
CeO2
CeO2
CeO2
Co3O4
CoO
CoO
CoO
Cr2O3
Cu2O
CuO
Fe2O3
Fe2O3
Fe3O4
Fe2O3
Fe2O3
Fe3O4
or Fe2O3
Fe3O4
Ga2O3
HfO2
HfO2
In2O3
In2O3
In2O3
In2O3
In2O3
In2O3
La(OH)3
NiO
Mn2O3
MnO
or Mn3O4
MnO
or Mn3O4
Mn3O4
ReO3
SnO2
SnO2
SnO2
Ta2O5
Ta2O5
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
benzylamine, acetophenone, or dimethyl sulfoxide
[Ce(acac)3]
benzylamine
Ce(OiPr)3
benzyl alcohol
CeIII 2-ethylhexanoate n-butyl ether
CoCl2
benzyl alcohol, ethanol, or hexanol
benzyl ether
[Co(acac)2]
ethanol
Co(OAc)2
Co(OAc)2
benzyl alcohol
[Cr(acac)3]
1,4-butanediol
Cu(OEt)2
acetone
ethanol
Cu(OAc)2
[Fe(acac)3]
1,4-butanediol, 1-butanol, or toluene
[Fe(acac)3]
benzylamine
[Fe(acac)3]
benzyl alcohol
[Fe(acac)3]
1,4-butanediol or toluene
Fe(OC2H4OCH3)3
octadecene
Fe(OAc)2
ethanol, or octanol,
or ethanol/acetic acid
or Fe(OAc)3
[Fe(acac)2], Fe(OAc)2, benzyl alcohol
or [Fe(acac)3]
[Ga(acac)3]
benzylamine
Hf(OEt)4
benzyl alcohol
HfCl4
benzyl alcohol
In(OiPr)3
benzyl alcohol
[In(acac)3]
benzylamine
In(OiPr)3
acetophenone
[In(acac)3]
2-butanone
[In(acac)3]
acetophenone
[In(acac)3]
acetonitrile
La(OiPr)3 + KMnO4 benzyl alcohol + 2-butanol
[Ni(acac)2]
1,4-butanediol
[Mn(acac)3]
1,4-butanediol
[Mn(acac)2]
benzyl alcohol
or KMnO4
Mn(OAc)2
benzyl alcohol
or [Mn(acac)2]
Mn(OAc)2·4 H2O
KOH + ethanol
Re2O7(C4H8O2)x
toluene
Sn(OtBu)4
benzyl alcohol
SnCl4
benzyl alcohol
SnII 2-ethylhexanoate n-butyl ether
Ta(OEt)5
benzyl alcohol
TaCl5
benzyl alcohol
Ti(OtBu)4
p-xylene, benzene, toluene, or cyclohexane
Ti(OnBu)4
various alcohols
Ti(OiPr)4
various alcohols + formic acid, acetic
acid or oxalic acid
TiCl4
various alcohols and acetic acid
TiCl4
various alcohols
TiCl4
benzyl alcohol
Ti(OiPr)4
toluene
Ti(OiPr)4
various ketones
Ti(OiPr)4
various aldehydes
TiCl4
ethanol
Ti(OnBu)4
1,4-butanediol
Ti(OnBu)4
1,4-butanediol or toluene
Ti(OiPr)4
2-propanol + aniline
tetrabutyl titanate
n-butanol
Angew. Chem. Int. Ed. 2008, 47, 5292 – 5304
spherical (g-alumina)
and rods (boehmite)
rods
spherical
spherical
spherical
–
cubelike
–
spherical
–
spherical
spherical
spherical
spherical
spherical
spherical
spheres
C C bond cleavage
[67]
–
C C bond formation
–
–
–
ester elimination
–
–
–
ester elimination
–
C C bond cleavage
–
–
–
ester elimination
[35]
[34, 35]
[68]
[69]
[70]
[71]
[72]
[73]
[35]
[74]
[75]
[76]
[48]
[77]
[78]
[79]
–
–
[72]
spherical
ellipsoidal
spherical
cubelike
spherical
spherical
spherical
spherical
spherical
fibers
spherical
spherical
spherical
C C bond cleavage
ether elimination
–
–
C C bond cleavage
–
–
–
–
Aldol-type reactions
–
–
–
[76]
[80]
[81]
[82]
[34, 76]
[34]
[34]
[34]
[83]
[84]
[73]
[73]
[85]
–
–
[72]
cubelike
spherical
spherical
spherical
spherical
spherical
spherical
spherical
–
–
ether elimination
ether elimination
–
–
–
–
[86]
[87]
[82]
[88]
[68]
[80]
[81]
[89]
spherical
–
–
ester elimination
[65]
[66]
spherical or rods
spherical or rods
spherical
spherical
spherical
spherical
spherical
spherical
spherical
rods
spherical
ester elimination
ether elimination
alkyl halide elimination
–
Aldol-type reactions
Aldol-type reactions
–
–
–
ester elimination
ether elimination
[90]
[91]
[92–94]
[95]
[96]
[96]
[97]
[98]
[99]
[100]
[101]
2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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N. Pinna and M. Niederberger
Table 2: (Continued)
Metal oxide
Precursors
Solvents
Shape
Reaction mechanism
Reference
TiO2
TiO2 (rutile or
anatase)
V2O3
VO1.52(OH)0.77
WO3·H2O
W18O49
W18O49
WO3·H2O
ZnO
ZnO
ZnO
ZnO
ZnO
ZnO
TiCl4
TiCl4
benzyl alcohol or n-butanol
acetone and other ketones
spherical, slightly elongated
spherical or rods
–
Aldol condensation
[102, 103]
[104, 105]
VO(OiPr)3
VOCl3
WCl6
WCl6
W(OiPr)6
WCl6
Zn(OCEt3)2
Zn(OAc)2·2 H2O
[Zn(acac)2]
Zn(acac)2·x H2O
Zn(OAc)2
[Zn(acac)2]·x H2O
benzyl alcohol
benzyl alcohol
benzyl alcohol
ethanol
benzyl alcohol
4-tert-butylbenzyl alcohol
acetone
ethanol
1,4-butanediol
benzylamine
benzyl alcohol
acetonitrile
–
–
–
–
–
–
Aldol-type reactions
ester elimination
–
C C bond cleavage
ester elimination
–
[106]
[107]
[107]
[108]
[109]
[110]
[61]
[111]
[73]
[76]
[72, 112]
[83]
ZnO
ZnO
[Zn(acac)2]·x H2O
Zn(OAc)2·2 H2O
–
ester elimination
[72]
[113]
ZnO
Zn(OAc)2
various aspect ratios
–
[114]
ZnO
ZnO
ZrO2
ZrO2
[Zn(acac)2]
zinc metal
Zr(OiPr)4·HOiPr
Zr(OiPr)4·HOiPr
benzyl alcohol
1-pentanol, m-xylene, and p-toluene
sulfonic acid monohydrate
various alcohols, glycols, n-alkanes,
and aromatic compounds
dibenzyl ether
aliphatic alcohols
glycols or toluene
benzyl alcohol
–
ellipsoids
platelets
rods
wires
platelets
spherical
spherical
spherical
–
rods or wires
spherical and hexagonally
shaped mesocrystals
–
spherical
–
spherical or nanorods
–
spherical
–
–
–
ether elimination
[115]
[116]
[62, 117]
[49, 50]
[a] acac = acetylacetonate.
The large number of suitable metal oxide precursors and
organic solvents offers many combinations for potential
reaction systems. Taking into account that the metal oxide
precursor as well as the solvent strongly influences the size
and the shape of the final nanocrystals, the possibility to
choose and to vary the initial reaction mixture represents a
powerful tool for the synthetic chemist to tailor the morphological characteristics. In the case of indium oxide, for
example, a wide range of precursors and solvents were used
to obtain different nanoparticle sizes and shapes.[34] However,
one has to keep in mind that it is not yet possible to make any
predictions regarding the final particle morphology based
solely on the composition of the reaction system.
Another important advantage of nonaqueous sol–gel
processes in comparison to aqueous systems is the accessibility of ternary and multi-metal oxide nanoparticles (Table 3). The different reactivity of metal oxide precursors
towards a specific solvent complicates the synthesis of phasepure multi-metal oxides. In contrast to aqueous systems, in
which this problem is particularly pronounced, it is easier to
match the reactivity of the metal oxide precursors in nonaqueous medium. It turned out that the use of chemically
different precursors represents an elegant method for the
preparation of phase-pure multi-metal oxides with complex
compositions. Examples include the synthesis of indium tin
oxide nanoparticles from indium acetylacetonate and tin tertbutoxide in benzyl alcohol,[127] InNbO4, MnNb2O6, YNbO4
from the corresponding metal acetylacetonates and NbCl5 in
benzyl alcohol,[126] CaNb2O6 from calcium acetate and
niobium ethoxide in 1,4-butanediol,[63] or ZnM2O4 (M = Cr,
Fe, Co, Mn) from zinc acetate and the respective metal
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acetylacetonate in 1,4-butanediol.[73] Nonaqueous reaction
conditions seem to be particularly suitable for doping of
binary metal oxide nanoparticles as well, as shown by the
examples given in Table 4.
Although a large number of methodologies have been
reported for the nonaqueous synthesis of metal oxide nanoparticles (see Tables 2–4), the condensation steps, that is, the
formation of the metal-oxygen-metal bond as the basic
structural unit, can be summarized in only five distinct
pathways (Scheme 1):[32, 81] Alkyl halide elimination (reaction 1), ether elimination (reaction 2), condensation of carboxylate groups (ester and amide elimination) (reaction 3),
C C coupling of benzylic alcohols and alkoxide molecules
(reaction 4), and aldol/ketimine condensation (reaction 5). It
is worth highlighting again that surfactant-free routes are, due
to the small number of starting compounds, particularly
suitable for the study of the reaction pathways based on the
analysis of the organic by-products.[36] Nevertheless, exactly
the same mechanisms were later also found in analogous
surfactant-directed routes.[2]
Alkyl halide elimination, the condensation between metal
halides and metal alkoxides (formed by the reaction of metal
halides with alcohols) under release of an alkyl halide, is
shown in reaction 1. This condensation mechanism is exemplified by the reaction between titanium tetrachloride and
benzyl alcohol leading to anatase nanoparticles.[32, 135] Ether
elimination (reaction 2) leads to the formation of a M-O-M
bond upon condensation of two metal alkoxides under
elimination of an organic ether, as reported for hafnium
oxide nanoparticles.[80] The ester elimination process involves
the reaction between metal carboxylates and metal alkoxides
2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Metal Oxide Nanostructures
Table 3: Ternary and multi-metal oxide nanoparticles synthesized by surfactant-free nonaqueous sol–gel routes.
Metal oxide
Precursors
Solvents
Shape
Reaction mechanism Reference
BaSnO3
BaTiO3
BaTiO3
BaTiO3
BaTiO3
BaTiO3
BaZrO3
BaZrO3
(Ba,Sr)TiO3
CaNb2O6
CoFe2O4
CrNbO4
FeNbO3
InNbO4
indium tin oxide
La1 xAxMnO3
(A=Ca, Sr, Ba)
LiNbO3
LiNbO3
MnNb2O6
NaNbO3
NaTaO3
PbTiO3, Pb(Zr,Ti)O3
PbZrO3
RE3NbO7[a]
SrTiO3
SrTiO3
YNbO4
ZnM2O4
(M=Cr, Fe, Co, Mn)
ZnGa2O4
ZnNb2O6
Zr6Nb2O17
Ba + Sn(OtBu)4
Ba(OiPr)2 + Ti(OiPr)4
Ba + Ti(OiPr)4
Ba + Ti(OiPr)4
Ba + Ti(OiPr)4
Ba(OH)2·8 H2O + Ti(OiPr)4
Ba(OiPr)2 + Zr(OiPr)4·HOiPr
Ba + Zr(OiPr)4·HOiPr
Ba + Sr + Ti(OiPr)4
Ca(OAc)2 + Nb(OEt)5
Co(OAc)2 + Fe(OAc)3
[Cr(acac)3] + Nb(OnBu)5
[Fe(acac)3] + Nb(OnBu)5
[In(acac)3] + NbCl5
[In(acac)3] + Sn(OtBu)4
various
2-butanone
acetone
2-propanol + benzene
benzyl alcohol
acetophenone
2-methoxyethanol + ethanol
acetone
benzyl alcohol
benzyl alcohol
1,4-butanediol
diethyleneglycol
1,4-butanediol
1,4-butanediol
benzyl alcohol
benzyl alcohol
benzyl alcohol or acetophenone
spherical
spherical
–
spherical
spherical
spherical
–
slightly elongated
spherical
–
spherical
–
–
spherical
spherical
–
–
Aldol-type reactions
–
C C bond formation
Aldol-type reactions
–
–
–
–
–
–
–
–
ether elimination
–
–
[33]
[118]
[119]
[72, 120, 121]
[122]
[123]
[118]
[120]
[121]
[63]
[124]
[63]
[63]
[125, 126]
[127, 128]
[129]
Li(OAc) + Nb(OnBu)5
Li + Nb(OEt)5
[Mn(acac)3] + NbCl5
Na + Nb(OEt)5
NaOEt + Ta(OEt)5
[Pb(acac)2] + Ti(OiPr)4
and/or Zr(OiPr)4·HOiPr
RE(OAc)3 + Nb(OEt)5
Sr + Ti(OiPr)4
Sr(OiPr)2 + Ti(OiPr)4
[Y(acac)3]·x H2O + NbCl5
Zn(OAc)2 + [M(acac)3]
1,4-butanediol
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
2-butanone
–
–
–
spherical
–
spherical
–
–
–
–
–
–
[63]
[120]
[126]
[33]
[33]
[130]
1,4-butanediol
benzyl alcohol
acetone
benzyl alcohol
1,4-butanediol
–
spherical
–
spherical
spherical
–
–
–
–
–
[63]
[121]
[118]
[126]
[73]
[Zn(acac)2]·x H2O + [Ga(acac)3] benzylamine
Zn(OAc)2 + Nb(OnBu)5
1,4-butanediol
[Zr(acac)4] + Nb(OnBu)5
1,4-butanediol
spherical
–
spherical
–
–
–
[131]
[63]
[63]
[a] RE = rare-earth metal.
Table 4: Doped metal oxide nanoparticles synthesized by surfactant-free nonaqueous sol–gel routes.
Metal oxide
Precursors
Solvents
Shape
Reaction mechanism
Reference
Fe-doped TiO2
Co-doped ZnO
Co-doped and
Mn-doped ZnO
Mn-doped
ZrO2
Ti(OiPr)4 + Fe(OAc)2
Zn(OAc)2·2 H2O
Zn(OAc)2 + Co(OAc)2
Zn(OAc)2 + Mn(oleate)2
Zr(OiPr)4·HOiPr +
[Mn(acac)3] or Mn(OAc)2
ethanol and acetic acid
ethanol
benzyl alcohol or
benzyl alcohol + anisole
benzyl alcohol
spherical
various
nanorods and nanowires
ester elimination
–
–
[132]
[133]
[112]
spherical
–
[134]
or between metal carboxylates and alcohols (reaction 3). One
example is the reaction between zinc acetate and benzyl
alcohol for the preparation of zinc oxide nanoparticles.[72, 112]
Alkyl halide, ether and ester elimination are the most
common routes. However, due to the excellent catalytic
activity of the metal centers in the metal oxide precursor
species, peculiar and more complex organic reactions such as
C C bond formation between alkoxy groups were also
observed (reaction 4). The most prominent example is the
formation of BaTiO3 nanoparticles from metallic barium and
Ti(OiPr)4 in benzyl alcohol.[121] Whereas in the case of BaTiO3
the presence of a basic species was a prerequisite for C C
bond formation,[32, 135] transition metals with high Lewis
acidity, such as Nb, Y, and Ce, can directly catalyze this
Angew. Chem. Int. Ed. 2008, 47, 5292 – 5304
Guerbet-like reaction.[32, 35, 136] If ketones are used as solvents,
the release of oxygen usually involves aldol condensation,
where two carbonyl compounds react with each other under
elimination of water (reaction 5). The water molecules act as
oxygen supplying agent for the metal oxide formation.
Examples include the formation of ZnO,[61] BaTiO3,[118] and
TiO2[96, 137] in acetone. Further and more detailed information
about the chemical pathways to metal oxides can be found
elsewhere.[32, 138, 139]
Figure 1 presents a small selection of metal oxide nanoparticles synthesized in benzyl alcohol as solvent. One can
immediately see that the crystallite shapes range from spheres
(ZrO2, BaTiO3, indium tin oxide (ITO) in Figure 1 a, c, and d
respectively) and fibers (ZnO:Co in Figure 1 b) to wires
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using any additional surfactants. Other examples include
Nb2O5[106] and WO3·H2O[107] nanoplatelets, and ZnO[112] and
La(OH)[84] nanorods. Preliminary results showed that the
anisotropic crystallite growth is often induced by organic
species that formed in situ during the reaction course,[32, 42, 43]
and therefore the study of the organics in these reactions will
be prerequisite to a complete understanding of the formation
of nanoparticles.[36]
High-resolution transmission electron microscopy
(HRTEM) studies proved that in spite of the generally
moderate reaction temperatures most of the as-synthesized
nanoparticles are characterized by high crystallinity. Figure 2
Scheme 1. Selected condensation steps in nonaqueous sol–gel processes together with one example. 1) Alkyl halide elimination, 2) ether
elimination, 3) ester elimination, 4) C C bond formation between
benzylic alcohols and alkoxides, 5) aldol-like condensation reactions.
Figure 2. HRTEM images of a) part of a cubelike In2O3 nanoparticle,
b) a 2 nm-sized Sn0.95In0.05Ox nanoparticle, c) a 16 nm-sized Fe3O4
nanoparticle, and of d) part of a Nb2O5 nanoplatelet. Insets: Respective power spectra.
Figure 1. TEM images of selected nanoparticles: a) ZrO2, b) ZnO:Co,
c) BaTiO3, d) ITO, e) InNbO4, f) W18O49 [insets: part of one bundle
(lower right) and individual nanowires (upper left)].
(W18O49 in Figure 1 f), and also the size range covers one
nanometer to micrometers. But it is important to point out
that within the same reaction system the obtained nanoparticles are often rather uniform (see, for example, the
BaTiO3 nanoparticles). However, the InNbO4 nanoparticles
in Figure 1 e are less homogenous in size and shape. The
W18O49 nanowires in Figure 1 f already point to the possibility
of obtaining anisotropic crystallite morphologies without
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shows selected HRTEM images together with their respective
power spectrum. Figure 2 a depicts a part of a 20 nm-sized
In2O3 nanocrystal with a cubelike shape, well-defined edges,
and monocrystalline behavior as demonstrated by the power
spectrum as inset.[82] A similarly high crystallinity can be
observed for nanoparticles obtained by the “benzyl alcohol
route” with sizes as small as 2 nm, for example, Sn0.95In0.05Ox
(Figure 2 b).[140] Despite its particularly small size, this cubeshaped particle shows a perfect crystalline order. For larger
crystallites, such as the magnetite particles in Figure 2 c,[48] the
monocrystalline order extends over the whole particle to the
edge, resulting in sharp and well-defined facets. Figure 2 d
displays the [010] zone axis of a Nb2O5 orthorhombic
nanoplatelet.[106] Also here the crystalline order is remarkable, especially for an orthorhombic structure, which often
contains a large number of defects in the bulk phase.
3. Organic–Inorganic Hybrid Nanostructures Based
on Metal Oxides
Nonaqueous sol–gel reactions are not only suitable for the
formation of purely inorganic nanoparticles, but also for
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Metal Oxide Nanostructures
organic–inorganic hybrid nanostructures. These materials are
built up by spatially well-defined domains of an organic and
an inorganic component.[141] The combination of an organic
and inorganic part makes these structures multifunctional
with a large variety of physical properties and possible
applications, which do not only depend on the inorganic and
organic components, but also on the interface formed
between the two phases.[142] In general, the intercalated
inorganic species or nanobuilding blocks range from transition-metal oxo clusters to polyoxometalates and nanoparticles.[143–145]
The formation of organic–inorganic hybrid nanomaterials
in surfactant-free nonaqueous systems was only reported for
very few reaction systems. Also in this case, the “benzyl
alcohol route” plays an outstanding role, giving access to
various oxide-based hybrid nanostructures (Table 5).[151] A
notable feature of these processes is the fact that highly
ordered hybrid nanostructures are constructed in one-step,
governed either by an initially present organic constituent
(for example the solvent), or by an organic reaction product
formed during the reaction course. The organic species bind
strongly and irreversibly to the growing inorganic components, fulfilling several functions: 1) The selective capping of
specific crystal faces leads to a confinement of the inorganic
material and the formation of highly anisotropic shapes (e.g.
2D sheets); 2) the organic species serve as stabilizers and
prevent agglomeration of the inorganic nanocrystals; 3) the
organic species phase-separate from the liquid reaction
medium and, for example, through p–p interactions, assemble
the inorganic nanocrystals into larger superstructures, thereby
forming a stable hybrid material. It is astonishing that simple
reaction systems just consisting of a metal alkoxide and
benzyl alcohol (or derivatives thereof, for example, 4biphenylmethanol or 4-tert-butylbenzyl alcohol) are able to
form such complex and highly ordered lamellar structures.
Nevertheless, these hybrid nanostructures are built up
through quite complicated organic reaction mechanisms. In
the case of rare-earth (RE) oxides, the hybrid nanostructures
are characterized by very thin crystalline oxide layers of the
general formula RE2O3, which are regularly separated from
each other by organic layers of intercalated benzoate[136, 146] or
biphenolate[148] molecules. In fact, it turned out that oxidation
of benzyl alcohol to the acid is catalyzed on the surface of the
growing oxidic nanoclusters and therefore a self-confinement
effect takes place, which can explain the high regularity and
extraordinarily low thickness (0.6 nm) of the final oxide
layers. Interestingly, all rare-earth-based materials prepared
in benzyl alcohol exhibit common structural features such as a
similar interlamellar distance of about 1.8 nm (Figure 3 a and
b). TEM images of the hybrid nanostructures synthesized in
Figure 3. TEM images of a) samarium oxide– and b) neodymium
oxide–benzoate hybrid nanostructures, c) gadolinium oxide– and d)
neodymium oxide–biphenolate hybrid nanostructures.
4-biphenylmethanol are displayed in Figure 3 c and d. Similar
to the products obtained in benzyl alcohol, they also show a
typical lamellar structure. However, the interlamellar distance increases from 1.8 to 2.6 nm, because of the larger size
of the intercalated biphenolate molecules. The morphology is
also remarkably different. On the one hand, the gadolinium
oxide layers are less extended compared to the ones
synthesized in benzyl alcohol (Figure 3 c). On the other hand,
the neodymium oxide sheets extend over a larger area,
forming nanowires several mm long with the lamellar periodicity perpendicular to the long axis (Figure 3 d).
The preparation of tungsten oxide-based nanostructures
in benzyl alcohol represents another interesting reaction
system for the formation of organic–inorganic hybrids and
these experiments will be discussed in more details in the
following section.
Table 5: Ordered organic–inorganic hybrid nanostructures based on metal oxides.
Metal oxide
Precursors
Solvents
Structure
Reaction mechanism
Reference
Y2O3
Gd2O3
Nd2O3
Er2O3
Sm2O3
Gd2O3
Y2O3
TiO2
W18O49
W18O49
WO3·H2O
WO3·H2O
Y(OiPr)3
Gd(OiPr)3
Nd(OiPr)3
Er(OiPr)3
Sm(OiPr)3
Gd(OiPr)3
Y(OiPr)3
Ti(OiPr)4
W(OiPr)6
WCl6 + deferoxamine
WCl6
WCl6
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
benzyl alcohol
4-biphenylmethanol
4-biphenylmethanol
benzylamine
benzyl alcohol
benzyl alcohol
4-tert-butylbenzyl alcohol
benzyl alcohol + 4-tert-butylcatechol
lamellar
lamellar
lamellar
lamellar
lamellar
lamellar
lamellar
stacks of nanoplatelets
nanowire bundles
nanowire bundles
stacks of nanoplatelets
stacks of nanoplatelets
C
C
C
–
C
–
–
N
–
–
–
–
[136]
[146]
[146]
[147]
[146]
[148]
[148]
[149]
[109]
[150]
[110]
[110]
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C bond formation
C bond formation
C bond formation
C bond formation
C bond formation
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4. A Case Study: Tungsten Oxide Nanostructures
The morphological (and also structural) features of
tungsten oxide nanostructures were found to be particularly
sensitive to changes in the experimental conditions, which
makes them a model system for studying the influence of
precursor, solvent, and additive on the final product. When
tungsten chloride is allowed to react in anhydrous benzyl
alcohol, purely inorganic platelets of tungstite, several tens of
nanometers in side length, are obtained.[107, 150] Although
electron diffraction measurements indicate a single-crystalline nature of every platelet, HRTEM measurements (Figure 4 a) showed that they exhibit an internal composite
structure, which means that every platelet is formed by fusion
of a large number of small crystallites, just a few nanometers
in size.[150] The inner structure becomes visible as there is
some misalignment of the orientation of these primary
building blocks with respect to each other, leading to defects,
which are additionally indicated by diffuse spots in the power
spectrum. Also here, in spite of the simple starting solution
(precursor and solvent), a relatively complex crystal structure
is obtained, which raises the question about the underlying
crystallization pathway. The morphology and especially the
supercrystalline arrangement can be tuned through the
addition of a small quantity of specific ligands such as 4-tertbutylcatechol. Instead of individual particles, the assembler
molecules are able to arrange the nanoplatelets into rodlike
stacks (Figure 4 b),[110] yielding highly ordered organic–inorganic hybrid structures (similar to the ones found for the rareearth metals). These assembler-encoded nanoparticles can be
regarded, in analogy to polymer science, as “monomers” for
the “polymerization” towards organized one-, two-, or threedimensional inorganic mesostructures.[152] When deferoxamine mesylate is used as additive, long nanowires of W18O49
with a uniform diameter of about 1.3 nm were obtained in the
same reaction system. The aspect ratio was more than 500 and
the nanowires were organized into bundles (Figure 4 c).
Similar nanowires were also prepared without any additional
ligands, but then switching from tungsten chloride as precursor to tungsten isopropoxide was required.[109] The bundles
were held together by p–p interactions between benzaldehyde molecules (oxidation product of benzyl alcohol) adsorbed at their surface.[150] However, the bonds to the tungsten
oxide are much weaker than those between the carboxylate
species and the rare-earth oxides presented in Section 3.
Figure 4 d and e propose structural models for the tungsten
oxide nanowires and their assembly behavior resulting in the
hybrid nanostructure.
These examples illustrate some typical aspects of the
benzyl alcohol route. On the one hand, the reactions are quite
robust within one synthesis system, that is, as long as
precursor and solvent are kept unchanged, the synthesis is
well reproducible and leads to homogenous products. On the
other hand, the morphology of the final product strongly
depends on the precursor and solvent used, that is, metal
oxides with the same composition and crystal structure,
however obtained from different precursors and/or solvents,
are generally characterized by different crystallite sizes and
shapes. Although this feature provides a precious tool to
tailor the final particle morphology, it makes these synthesis
approaches, up to now, unpredictable. Although benzyl
alcohol plays an outstanding role in the preparation of a
large family of metal oxide nanoparticles, covering a broad
range of crystallite sizes and shapes, the systematic screening
of potential reaction systems is currently the only way to find
the proper synthesis protocol. Similar to surfactant-directed
approaches, the fundamental question of finding a relationship between a particular synthesis system and the final
particle morphology still awaits an answer, as exemplified by
the question, why in some cases purely inorganic nanoparticles form, and in other cases organic–inorganic hybrid
nanostructures. The main obstacle on the way to understanding the formation of metal oxide nanoparticles on a molecular
level is the fact that it is not enough to just consider the
components present in the initial reaction solution. On the
contrary, it is absolutely crucial to characterize all the organic
Figure 4. HRTEM images of a) part of a tungstite nanoplatelet with its power spectrum. Tungsten oxide hybrid nanostructures synthesized in the
presence of b) 4-tert-butylcatechol and c) deferoxamine mesylate as ligand. Schematic representation of d) the cross section of a 1.3 nm nanowire
inside one W18O49 unit cell oriented along the [010] direction, and e) the proposed model for the hybrid nanostructure. Parts a, c, d, and e were
reproduced from reference [150] with permission of the American Chemical Society.
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Metal Oxide Nanostructures
species that form during the reaction course, and to analyze
their potential interaction with the growing inorganic clusters.
As illustrated by the various tungsten oxide nanostructures in
Figure 4, already small amounts of low-molecular-weight
organic ligands are able to alter the morphological characteristics as well as the assembly behavior. And this observation is
certainly true for both initially present as well as in situ
formed coordinating species.
5. Metal Oxides for Applications: Perspectives
The good accessibility of the surface of metal oxide
nanoparticles prepared by surfactant-free routes should be
particularly advantageous for applications in (photo)catalysis
and gas sensor technology. Several studies proved that
surfactants adsorbed on the surface of titania strongly
influenced its photocatalytic activity.[153–155] Especially surfactants, such as sodium dodecylsulfate, that compete with the
pollutant for degradation on the surface of titania lower the
photocatalytic activity considerably.[153] Accordingly, metal
oxide nanoparticles synthesized in surfactant-free media
showed excellent photocatalytical properties. Titania nanoparticles prepared from TiCl4 and benzyl alcohol exhibited
higher activity than the standard Degussa P25 photocatalyst
for phenol degradation.[94] Although thermal treatment at
400 8C increased the photocatalytic activity to a maximum
due to the removal of organic residues on the nanoparticle
surface, the temperature is still low enough to keep the
nanocrystals small. In comparison, trioctylphosphine oxide, a
frequently used surfactant, starts to decompose at around
425 8C.[156] Particularly high photocatalytic activity in the
decomposition of acetic acid in aqueous solutions was also
achieved over titania nanoparticles synthesized from alkoxides in different alcohols.[65, 89] During the intense search for
photocatalysts active under visible light, InNbO4 nanoparticles of about 30 nm were recently prepared in benzyl
alcohol.[125] Their activity in the degradation of rhodamine B
in aqueous solution under visible light was found to be higher
than that of the standard photocatalyst P25 and bulk InNbO4
synthesized by the traditional high-temperature route.
In heterogeneous catalysis V2O3 made from vanadium
isopropoxide in benzyl alcohol is a good catalyst for the
oxidation of n-butane to give maleic anhydride.[157] The
peculiarity of this catalyst lies in its crystalline structure,
because the nanoparticle core consists of catalytically inactive
V2O3, whereas the surface is rather complex and contains
vanadium species of higher oxidation states (IV and V) that
are known to be responsible for the catalytic reactivity of
vanadium-based catalysts in partial oxidation reactions.
Resistive sensors based on metal oxide semiconductors
(MOSs) represent an important class of sensors due to their
simplicity, low cost, small size, and ability to be integrated in
electronic devices.[158] Applications of MOS sensors can be
found in many areas, such as in industry, environment, home
safety, biomedicine, automotive, and security. In gas sensing
applications surfactants are not only a problem because of the
required good contact between the nanoparticles constituting
the sensing layer and the electrode, but also for the long-term
Angew. Chem. Int. Ed. 2008, 47, 5292 – 5304
stability of the sensing device. High-temperature firing of the
sensor is necessary for the removal of the organic species and
the promotion of the electric conduction, which, however,
also induces a reduction of the volume of the sensing layer
and a subsequent decrease of the specific surface area of the
sensor due to particle growth and film densification. Hence,
significant advantages are expected to be obtained when
surfactant-free routes are used. Gas sensors fabricated by the
direct deposition of nanoparticles synthesized by the “benzyl
alcohol route” onto alumina substrates with gold contacts and
platinum heating elements were recently published.[31] In2O3,
SnO2, and mixed-phase nanoparticles showed good response
towards reducing and oxidizing gases, respectively.[82, 140] The
same In2O3 nanocrystals impregnated with 1 wt % of platinum
possess a unique high sensitivity to oxygen even at room
temperature.[159] Also tungsten oxide nanowires exhibit large
sensitivities towards NO2 and NH3.[109, 160] In view of these
results, surfactant-free approaches appear to be promising
routes for applications in catalysis and gas sensing. However,
any technological application will make it necessary to
produce these nanoparticles in large quantities. Surfactantfree nonaqueous routes are typically rather robust, which
means that slight changes in the reaction conditions do not
alter the structural and morphological features of the product.
Consequently, it is expected that up-scaling of these processes
should easily be possible. However, so far only the synthesis
of ZrO2 nanoparticles in 20 g quantities using the “benzyl
alcohol route” has been reported,[50] which is almost negligible in comparison to the flame synthesis, which produces
nanostructured materials such as carbon black, titania, fumed
silica, and alumina on an industrial scale in tons per day.[161, 162]
In comparison to flame-made materials, the costs for the
corresponding metal oxide nanoparticles prepared by nonaqueous routes are roughly 5–10 times larger. Therefore, it
can be expected that only complex metal oxides (doped,
ternary, or multi-metal oxides) that are difficult to obtain by
flame processes will find their way into technological
applications, especially in devices that require just a small
amount of nanoparticles or in high-value products. On the
other hand, new developments such as the extension of the
“benzyl alcohol route” to the microwave-assisted preparation
of binary and ternary metal oxide nanoparticles represents a
very fast and energy efficient methodology[72] and might
represent a more competitive alternative to gas-phase processes in the future.
6. Conclusion
In the last few years the number of synthetic approaches
to metal oxide nanoparticles reported in literature has
increased dramatically. But in spite of all these efforts,
generally valid concepts or basic underlying mechanistic
principles that would allow the development of a rational
synthesis strategy to inorganic nanomaterials in a predicted
way are not yet available. Nevertheless, these synthesis routes
gave access to a large collection of oxidic nanoparticles with a
wide range of compositions, monodisperse crystallite sizes,
sophisticated crystallite shapes, and with complex assembly
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N. Pinna and M. Niederberger
properties. However, there is no doubt that in comparison to
the extensive synthetic work, the in-depth investigation of the
physical and chemical properties has not yet reached the same
level. In our opinion, research on the synthesis of metal oxide
nanoparticles (or inorganic nanomaterials in general) should
mainly be motivated by the search for general concepts,
whereas the enhanced study of the properties is a key step on
the way to a faster implementation into technological
applications.
The nonaqueous and surfactant-free routes to metal oxide
nanoparticles presented in this Minireview offer the opportunity to find the appropriate synthesis method for a targeted
material with the desired properties. Due to their simplicity
and robustness, the use and exploitation of these routes is in
the reach of every scientist having basic chemical knowledge.
In spite of the immense progress in nanoparticle research,
the development of synthesis concepts based on a rational
strategy has remained a primary objective, and yet we are far
from achieving this goal. It is still impossible to prepare a
certain compound on the nanoscale with a desired composition, structure, size and shape, or even properties, intentionally and in a predicted way. One of the main reasons for this
major limitation is the fact that the role of the organic species
during the growth of the inorganic nanoparticles is not yet
understood on a molecular level. Whereas the size- and
shape-controlling effect of organic species, mainly surfactants,
has empirically been used for decades, the organic reaction
pathways, that is, the chemical transformation of the organic
components in the reaction mixture with reaction time, have
hardly been investigated. However, to gain new insights into
the crystallization and formation mechanisms of inorganic
nanoparticles, one has to study the interaction of all organic
species, initially added to the starting solution as well as in situ
formed during the reaction course, with the growing nuclei.
Only a detailed knowledge of which and how organic
compounds adsorb on the surface of the metal oxide nanoparticles makes it possible to go a step beyond simple trialand-error experiments toward the development of a rational
synthesis strategy for inorganic nanomaterials. It will be
absolutely essential to strengthen the use of in situ methods
for the study of nanoparticle growth under experimental
conditions, as was reported for the growth of barium
titanate,[163] tungstates,[164] or molybdenum oxide nanoparticles.[165] Further inspiration could be gained from the catalysis[166, 167] and gas sensing community,[168, 169] which broadly
apply operando techniques for the spectroscopic characterization of surface phenomena under real conditions.
Financial support from ETH Z,rich is acknowledged. This
work was partially supported by the European Network of
Excellence FAME.
Received: October 2, 2007
Published online: June 18, 2008
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