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Accepted Manuscript
Title: Synthesis of Au nanoparticle-decorated carbon nitride
nanorods with plasmon-enhanced photoabsorption and
photocatalytic activity for removing various pollutants from
water
Authors: Yali Chang, Zixiao Liu, Xiaofeng Shen, Bo Zhu,
Daniel K. Macharia, Zhigang Chen, Lisha Zhang
PII:
DOI:
Reference:
S0304-3894(17)30796-3
https://doi.org/10.1016/j.jhazmat.2017.10.040
HAZMAT 18945
To appear in:
Journal of Hazardous Materials
Received date:
Revised date:
Accepted date:
17-7-2017
17-10-2017
20-10-2017
Please cite this article as: Yali Chang, Zixiao Liu, Xiaofeng Shen, Bo Zhu,
Daniel K.Macharia, Zhigang Chen, Lisha Zhang, Synthesis of Au nanoparticledecorated carbon nitride nanorods with plasmon-enhanced photoabsorption and
photocatalytic activity for removing various pollutants from water, Journal of Hazardous
Materials https://doi.org/10.1016/j.jhazmat.2017.10.040
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Synthesis of Au nanoparticle-decorated carbon nitride nanorods with
plasmon-enhanced photoabsorption and photocatalytic activity for
removing various pollutants from water
Yali Changa,#, Zixiao Liua,#, Xiaofeng Shenb,#, Bo Zhua, Daniel K. Machariaa, Zhigang Chena
and Lisha Zhangb,*
a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College
of Materials Science and Engineering, Donghua University, Shanghai 201620, China
b
College of Environmental Science and Engineering, Donghua University, Shanghai 201620,
China
*Corresponding
author: E-mail address: lszhang@dhu.edu.cn (L.S. Zhang)
# These authors contributed equally to this work.
Graphical abstract
1
Highlights

The in-situ growth of Au nanoparticles on CN nanorods has been realized.

Au-CN exhibits a broad photoabsorption from UV to NIR with an edge at ~790 nm.

Au-CN has stronger photocurrent (~8.0 μA/cm2) than g-C3N4 (~2.9 μA/cm2) and CN
nanorods (~4.5 μA/cm2).

Au-CN exhibits higher photocatalytic activity than g-C3N4 and CN nanorods for
removing various pollutants from water.

The improved performances of Au-CN result from the plasmon effect of Au compared
with those of CN nanorods.
Abstract: Herein we have developed Au nanoparticle-decorated carbon nitride (Au-CN)
nanorods as novel and efficient photocatalysts. Au-CN with different Au/CN precursor molar
ratios (0.5%, 1% and 2%) have been prepared by a solvothermal-hydrothermal two-step
method, where CN nanorods have diameters of 20-30 nm and length of 0.5-1 μm while Au
nanoparticle have diameter of ~13 nm. Au-CN nanorods exhibit a broad photoabsorption from
ultraviolet to near-infrared with edge at ~790 nm, revealing an obvious red-shift compared
with g-C3N4 bulk (~460 nm), CN nanorods (~715 nm). Under visible-light irradiation,
1%Au-CN nanorods exhibit the highest photocatalytic activity, and they can degrade 98.2%
rhodamine B (RhB), 77.2% 4-chlorophenol (4-CP), 83.9% tetracycline (TC) and reduce 43.6%
hexavalent chromium (Cr(VI)) in 120 min, higher than those by pure CN nanorods (70.3%
RhB, 36.6% 4-CP, 54.6% TC, 23.1% Cr(VI)) and g-C3N4 bulk (31.5% RhB, 17.2% 4-CP,
36.9% TC, 11.8% Cr(VI)). Compared with CN nanorods, the obvious improvement of
photocatalytic activity of 1%Au-CN nanorods should be attributed to the plasmon-enhanced
photoabsorption and efficient separation of hole-electron pairs due to the introduction of Au
nanoparticles.
2
Keywords: carbon nitride nanorods; Au nanoparticles; surface plasmonic resonance;
visible-light; photocatalysis
1. Introduction
Environmental problems associated with toxic and harmful pollutants, such as
Rhodamine B (RhB), 4-chlorophenol (4-CP), tetracycline (TC) and hexavalent chromium
(Cr(VI)) in water have severe threat to human health[1]. Up to now, various treatment
methods for remediating water pollution have been explored, including biological methods,
physical methods and semiconductor photocatalysis[2]. Among these methods, semiconductor
photocatalysis has been attracting tremendous attention as a “green”, energy-saving and
efficient advanced-oxidation-process[3]. A prerequisite for the photocatalysis is to develop
semiconductor photocatalysts which have a wide photoabsorption range and high
photocatalytic activity. As one of the most typical photocatalysts, TiO2 has been widely
studied for the photocatalytic degradation of organic pollutants, but it is only active in the
ultraviolet (UV) or near-ultraviolet region (~4% of the solar light spectrum) due to its wide
band-gap (3.2 eV)[4]. To extend the utilization of solar light, it is necessary to develop
visible-light-driven (VLD) photocatalysts.
To date, a variety of VLD photocatalysts have been reported, such as metal oxides
(Bi2O3[5], Bi12O17Cl2[6]), sulfides (MoS2[7], and WS2[8]), nitrides (Ta3N5[9], BN[10], and
carbon nitride[11]). Among these VLD photocatalysts, carbon nitride (CN) has attracted a
great deal of attention due to its unique properties, such as low cost and excellent
photocatalytic activity[12-15]. To prepare carbon nitride, two methods have been chiefly
3
developed. One is the thermal polymerization of N-containing organic compound at high
temperature (500-800 °C). For example, with urea[16] or melamine[17] as the raw materials,
graphene-like carbon nitride (g-C3N4) bulk can be obtained by annealing at high temperature.
However, g-C3N4 bulk exhibits some drawbacks, such as low surface area (<10 m2 g-1[12])
resulting from high degree of polycondensation of monomers, narrow visible-light absorption
range (edge: ~460 nm[18]) due to wide band-gap (~2.7 eV). These drawbacks greatly limit its
practical applications. The other method is the hydrothermal polymerization of N-containing
organic compound in solution[19-21]. For example, Wang et al. synthesized carbon nitride
(CN) nanorods by hydrothermally treating cyanuric chloride and melamine in acetonitrile at
120-180 °C for 24-96 h[20]. CN nanorods exhibited a large surface area (~30 m2 g-1),
red-shifted visible-light absorption range (edge: ~650 nm) and the greatly improved
photocatalytic activity for degrading organic pollutant, compared with g-C3N4 bulk. However,
the practical applications of CN nanorods are still limited due to the unsatisfactory
photoabsorption range and photogenerated electron-hole separation efficiency. So, it is still
necessary to further optimize CN nanorods.
Recently, the formation of semiconductor heterojunctions has been demonstrated to be a
promising method to improve the photoabsorption and/or photocatalytic activity, as
summarized in our recent review[22]. To improve the photocatalytic performance of carbon
nitride, some carbon nitride-based heterojunctions have been well developed, including
C3N4-oxide (TiO2[23], ZnO[24]), C3N4-sulfide (CdS[25], MoS2[26]), C3N4-metal (Ag[27],
Au[28, 29]). Among these carbon nitride-based heterojunctions, C3N4-Au has drawn much
attention[28-33], since Au can extend light responses by localized surface plasmon resonance
(LSPR)[29] and act as trapping sites for electrons to promote photogenerated electron-hole
4
separations[31]. For example, Wang et al. reported that Au/g-C3N4 nanohybrids showed the
enhanced photocatalytic activities comparing to g-C3N4 for the reduction of p-nitrophenol to
p-aminophenol[28]. Sun et al. developed Au/g-C3N4 photocatalysts which exhibited
plasmon-enhanced visible-light absorption and excellent photocatalytic activity for the
degradation of methyl orange dye compared to pure g-C3N4[33]. However, those Au/g-C3N4
heterojunctions still exhibit the limited photoresponse range, due to the narrow
photoabsorption of g-C3N4 bulk.
As mentioned above, CN nanorods exhibit larger surface area and broader visible-light
absorption range than g-C3N4 bulk, and the coupling with Au nanoparticles can improve the
photoabsorption and photocatalytic activity. These features trigger our interests in developing
CN nanorods modified with Au nanoparticles. Herein, we designed and prepared Au
nanoparticles-decorated CN nanorods (Au-CN nanorods) by a solvothermal-hydrothermal
two-step method. Au-CN nanorods exhibited the enhanced photoabsorption, photocurrent and
photocatalytic activity. In addition, photocatalytic stability and mechanism were further
investigated.
2. Experiment
2.1 Synthesis of Au-CN nanorods
Materials and chemicals are shown in supporting information. CN nanorods were
prepared by solvothermally treating acetonitrile solution (60 mL) containing cyanuric chloride
(15 mmol) and melamine (7.5 mmol) at 180 °C for 96 h, according to the previous report[20].
The surface decoration of CN nanorods with Au nanoparticle was realized by a hydrothermal
reduction method as follow. CN nanorods (50 mg, ~0.54 mmol) were added to the aqueous
5
solution (60 mL) containing glucose (500 mg) and HAuCl4 with different amount (2.7 μmol,
5.4 μmol, or 10.8 μmol), and then the solution was stirred for 30 min. The above solution was
transferred to a 100 mL Teflon-lined autoclave, sealed, and hydrothermally treated at 120 °C
for 12 h. The solution was cooled to room temperature, and the final product was collected by
centrifugation, washed with water, and dried at 60 °C for 12 h. The samples with different
Au/CN precursor molar ratios of 0.5%, 1.0% and 2.0% were abbreviated as 0.5%Au-CN,
1%Au-CN and 2%Au-CN, respectively. For comparison, g-C3N4 bulk was also prepared by
directly thermal polymerization of melamine at 500 °C for 2 h and then deammoniation
treatment at 520 °C for 2 h [17]. The growth of Au on g-C3N4 nanosheets (molar ratios: 1.0%)
was similar the above process. The characterization process was shown in supporting
information.
2.2 Photoelectrochemical Analysis
The photocatalyst slurry (12.5 mg C3N4 powder, CN nanorods or Au-CN; mixed with 0.5
mL dimethylformamide ) was drop onto fluoride-tin oxide (FTO) glass whose side part was
previously protected by using scotch tape with the exposed area of 1 cm2. After air drying,
this photocatalyst-coated FTO glass was treated at 200 °C for 60 min to improve adhesion and
then used as the working electrode. The photoelectrochemical measurements were
investigated by a CHI 660D (Chenhua Instruments, Shanghai, China) electrochemical
workstation with a standard three-electrode system. A Pt plate and an Ag/AgCl (3 M KCl)
electrode were used as counter electrode and reference electrodes, respectively. Na2SO4
aqueous solution (0.2 M) was used as the electrolyte.
2.3 Measurements of photocatalytic activity
6
The photocatalytic activities of different photocatalysts (g-C3N4 bulk, 1%Au-g-C3N4
nanosheets, CN nanorods or Au-CN nanorods) were measured by degrading organic
pollutants and reducing heavy-metal ions. In a typical process, 20 mg of photocatalyst was
added into 100 mL aqueous solution containing rhodamine B (RhB, 5 mg L-1, pH=6.67),
4-chlorophenol (4-CP, 1 mg L-1, pH=6.62), tetracycline (TC, 20 mg L-1, pH=5.19) or
hexavalent chromium (Cr(VI), 20 mg L-1, pH=3.00, from K2Cr2O7 solution). Then, the
suspension was magnetically stirred for 60 min in the dark to ensure an absorption-desorption
equilibrium. After that, the above suspension was irradiated by a 300 W xenon lamp (Beijing
Perfect Light Co., Ltd., Beijing, China) with a cut-off filter (λ > 400 nm). During the
photocatalytic process, 3 mL suspension was collected in each 20 min and filtered by a 0.22
μm Millipore filter to get clear liquid. The filtrated RhB, TC or Cr(VI) solutions were
analyzed by using a UV1901PC spectrophotometer (Shanghai Yoke Instrument Co., Ltd.) by
measuring the intensities of the characteristic adsorption peak at 554 nm, 357 nm or 353 nm,
respectively. 4-CP concentrations were investigated by a high-performance liquid
chromatography (HPLC) with a C-18 reverse phase column (L=250 mm, id=4.6 mm and
particle size=5 mm) and a diode array detector (Agilent Technologies, 1100 series). The
mobile phase was methanol (80%) and water (20%), and the flow rate was 0.5 mL min-1. In
addition, the total organic carbon (TOC) test, photocatalytic stability, photo action spectra and
radical trapping experiments were carried out and shown in supporting information.
3. Results and discussion
3.1 Synthesis and characterization
7
CN nanorods were prepared by solvothermally treating cyanuric chloride and melamine
in acetonitrile solution at 180 °C for 96 h, according to the previous report[20]. Under these
conditions, CN frameworks could be formed due to the condensation of triazine tectons from
cyanuric chloride and melamine, accompanying the release of NH4Cl (Fig. 1a). The
morphology and size of CN sample were investigated by scanning electron microscope (SEM)
and transmission electron microscopy (TEM). Obviously, CN sample is composed of uniform
nanorods with diameters of 20-30 nm and lengths of 0.5-1 μm, and these nanorods have
smooth surface (Fig. 2a, b).
Subsequently, the surface decoration of CN nanorods by Au nanoparticles with different
precursor amount (0.5%, 1% and 2%) was carried out by hydrothermally treating CN
nanorods in aqueous solution of HAuCl4 and glucose at 120 °C for 12 h. In this reduction
process, Au3+ ions could be absorbed on the surface of CN nanorods, and then reduced by
glucose, resulting in in-situ growth of Au nanoparticles on CN nanorods (Fig. 1b). After the
reduction process, these Au-CN samples are still composed of nanorods (diameters: 20-30 nm;
lengths: 0.5-1 μm) whose surfaces are decorated with nanoparticles with average diameter of
~13 nm (Fig. S1). With the increase of Au/CN precursor ratio from 0.5% to 2%, the density of
nanoparticles goes up obviously, and the typical morphology of 1%Au-CN sample is shown
in Fig. 2c. HR-TEM image (the inset in Fig. 2c) reveals that the nanoparticle has an
interplanar spacing of 0.23 nm which is corresponding to the d-spacing value of (111) planes
of metallic Au (JCPDS Card No. 04-0784), suggesting the formation of Au nanoparticles.
With 1%Au-CN sample as the model, the elemental composition was analyzed by energy
dispersive spectroscopy (EDS) maps in the STEM mode (Fig. 3). HADDF-STEM image (Fig.
8
3a) reveals the presence of single nanorod whose surface is decorated with some nanoparticles.
Elemental mappings (Fig. 3b, c) confirm that elemental C and N are homogeneously
distributed among single nanorod, indicating that this nanorod should be CN compound. In
addition, elemental Au are distributed among these nanoparticles (Fig. 3d), suggesting that
nanoparticles should be Au.
The elemental composition and chemical status of 1%Au-CN sample as well as pure CN
nanorods were further investigated by X-ray photoelectron spectroscopy (XPS). The survey
spectrum (Fig. 4a) of CN nanorods exhibits two peaks at 284.8 eV and 399.1 eV, revealing
the presence of C and N elements. In addition, O1s peak at 531.8 eV can also be found, due to
the surface absorption and oxidation during the preparation process[34]. For 1%Au-CN
sample, besides the peaks of C1s, N1s, and O1s, there is an additional signal peak at 84.8 eV
(Au 4f) which should be associated with the presence of Au element. The atomic ratio of
Au/C/N/O is determined to be 0.89/39.5/51.22/8.39, which is very close to the precursor
A/CN molar ratio (1 mol%) and indicates that the weight ratio of Au in 1%Au-CN is ~11.7
wt%. To further analyze 1%Au-CN sample, the high-resolution spectra for C 1s, N 1s and Au
4f of 1%Au-CN nanorods are also depicted (Fig. 4b-d). C 1s spectrum (Fig. 4b) of 1%Au-CN
nanorods can be resolved into three peaks at 284.8 eV, 286.0 eV, and 288.4 eV, which can be
attributed to carbon nitride matrix group (C-C), the sp2-hybridized carbon atoms bonded to
nitrogen atom in the aromatic ring group (C-N-C) and the sp2-hybridized carbon atoms
bonded to three nitrogen atoms in the aromatic ring attached to the NH2 group (C-(N)3),
respectively[34]. N 1s spectrum (Fig. 4c) can be fitted into three peaks at 398.8 eV, 399.6 eV
and 400.5 eV, which should be attributed to the sp2-hybridized nitrogen atom (C-N=C),
9
sp3-hybridized nitrogen atom (N-(C)3 and N-H), respectively[35]. Importantly, Au 4f
spectrum shows two peaks positioned at 84.1 eV and 87.7 eV (Fig. 4d) which are
corresponding to Au 4f7/2 and Au 4f5/2, suggesting that Au is in metallic state[28]. These
results further confirm the existence of Au and CN in 1%Au-CN sample.
Subsequently, the phases of CN nanorods and different Au-CN samples were
investigated by XRD pattern (Fig. 5). For comparison, g-C3N4 sheet (Fig. S2a) was obtained
by directly one-step thermal polymerization of melamine, and 1%Au-g-C3N4 nanosheets were
prepared by the hydrothermal growth of Au nanoparticles on g-C3N4 nanosheet (Fig. S2b).
g-C3N4 sample exhibits two peaks located at 13.0° and 27.5°, which can be well indexed as
(100) and (002) planes of g-C3N4, respectively[36, 37]. Compared with g-C3N4, CN nanorods
only display one characteristic diffraction peak located at 27.5°, which corresponds to the
(002) plane of g-C3N4 due to the interlayer stacking of the conjugated aromatic systems[11].
After Au decoration, all Au-CN and 1%Au-g-C3N4 samples exhibit similar diffraction
patterns which include a peak at 27.5° for CN/g-C3N4 and four new diffraction peaks. These
new peaks are located at 38.2°, 44.4°, 64.6°, and 77.5°, which can be assigned to (111), (200),
(220) and (311) planes of metallic Au (JCPDS Card No. 04-0784). From 0.5%Au-CN to
2%Au-CN sample, the intensities of Au diffraction peaks goes up, indicating the increase of
crystallinity and/or content in the sample. Based on the above results, one can conclude that
Au-CN nanorods have been successfully prepared.
3.2 Optical and photoelectrochemical measurements
The optical properties of different samples (g-C3N4, 1%Au-g-C3N4 nanosheets, CN
nanorods, 0.5%Au-CN, 1%Au-CN and 2%Au-CN) were measured by UV-Vis-NIR diffuse
10
reflectance spectra (Fig. 6a). g-C3N4 exhibits a short-wavelength absorption with an edge at
~460 nm, which is similar to the previous report[18]. After the growth of Au, the absorption
edge of 1%Au-g-C3N4 nanosheets is red-shifted to ~712 nm, and a new absorption peak
appears at ~550 nm, which is a characteristic LSPR absorption of metallic Au
nanoparticles[29]. For CN nanorod sample, it exhibits a wide photoabsorption range from
ultraviolet (UV) to near infrared (NIR) region with an absorption edge at ~715 nm, indicating
a great red-shift compared with that (~460 nm) of pure g-C3N4. Importantly, after the
decoration of Au nanoparticles, all Au-CN (0.5%Au-CN, 1%Au-CN and 2%Au-CN) samples
have a similar diffuse reflectance spectrum, where a new absorption peak appears at ~550 nm
due to LSPR effect of Au nanoparticles. Furthermore, Au-CN samples have a very broad
photoabsorption range from UV to NIR with an edge at ~790 nm, indicating a great red-shift
compared with that of 1%Au-g-C3N4 nanosheets (~712 nm) and CN nanorods (~715 nm).
Thus, the replacement of g-C3N4 nanosheets with CN nanorods as well as the decoration of
Au nanoparticles result in the significantly broadened and stronger photoabsorption of Au-CN
samples, showing greater potential to absorb solar light for enhancing photocatalytic activity.
CN in Au-CN samples should have the similar band-gaps and band-edge positions
compared with those from pure CN nanorods. With pure CN nanorods and g-C3N4 as the
model, the band-gap can be calculated by using the following formula:[38]
(αhν) = A (hν - Eg) 2
(1)
where A is a constant related to the material, h refers to the Planck constant, ν is the light
frequency, Eg represents the band-gap energy, α is the absorption coefficient. The band-gap is
deterimined to be 2.68 eV for g-C3N4 and 1.65 eV for CN nanorods (Fig. 6b). In addition, the
11
band-edge positions of CN nanorods were investigated by electrochemical Mott-Schottky
plots in the dark (Fig.7a). The plots exhibit positive slope under various frequencies,
suggesting that CN nanorods are in n-type characteristic[39]. The linear extensions of these
curves intersect with each other at one point. The position of this intersection point can be
used to determine the flat-band potential, and it is estimated to be at -1.28 V vs. Ag/AgCl
(-1.08 V vs. NHE) for CN nanorods. Combined with the band-gap value (Fig. 6b), its valence
band (VB) potential is calculated to be 0.37 V vs. Ag/AgCl (0.57 V vs. NHE).
Photoelectrochemical experiments were carried out to study the photoresponse and
charge transfer rate. Fig. 7b displays the transient current-voltage responses of 1%Au-CN
nanorods under intermittent visible-light irradiation. Under light irradiation, the current
density jumps immediately. When the light is shuttered off, the current density falls back to
the original value. These facts prove an obvious photo-electric response of 1%Au-CN
nanorods. To compare the photoresponse properties of different samples (g-C3N4, CN
nanorods and 1%Au-CN nanorods), the i-t curves and EIS Nyquist plots were further
measured with a certain bias potential of 0.5 V. Fig. 7c shows the i-t curves for the samples.
Under same visible-light illuminations, the photocurrent density of 1%Au-CN nanorods can
reach ~8.0 μA/cm2, which is higher than that of pure g-C3N4 (~2.9 μA/cm2) and pure CN
nanorods (~4.5 μA/cm2). High photocurrent from 1%Au-CN nanorods results probably from
two reasons. On the one hand, 1%Au-CN nanorods have wider and stronger photoabsorption
ability, resulting in more photo-generated carriers. On the other hand, Au nanoparticles can
act as trap sites to capture photo-generated electrons before they recombine with holes,
leading to more effective separation of electron-hole pairs. To further confirm the rapid
electron transfer, Nyquist plots were utilized to estimate the electric conductivity of different
12
samples (Fig. 7d). Obviously, 1%Au-CN nanorods exhibit significantly smaller arc radius
than pure g-C3N4 and pure CN nanorods, suggesting that the photo-generated electrons can
transport faster in 1%Au-CN nanorods and thus the separation of electron-hole is more
effective. Based on the above results, one can conclude that Au decoration confers higher
photocurrent and lower charge-transfer resistance, which is beneficial for photocatalysis.
3.3 Photocatalytic performance
RhB is a widely used dye, and plenty of RhB wastewater has been discharged every day.
With RhB as a representative dye pollutant, the photocatalytic activity of Au-CN samples
(0.5%Au-CN, 1%Au-CN and 2%Au-CN) were investigated under visible-light irradiation
(Fig. S3). For comparison, the photocatalytic activity of g-C3N4, 1%Au-g-C3N4 and pure CN
nanorods were also investigated under the same conditions. When these photocatalysts are
added in RhB solution in the dark, they can adsorb only ~5% RhB molecules after 60min due
to the moderate surface areas (11~20 m2 g-1, as revealed in Fig. S4). Subsequently, the
photocatalytic reactions were carried out for another 120 min under visible-light irradiation
(Fig. S5, Fig. 8a). The blank test indicates that the direct photolysis of RhB is very difficult in
the absence of photocatalyst under visible-light irradiation. With pure g-C3N4, 1%Au-g-C3N4
or CN nanorods as photocatalyst, the degradation efficiency of RhB can just approach to
31.5%, 64.6% or 70.3% after 120 min of reaction, respectively. Importantly, when
0.5%Au-CN, 1%Au-CN and 2%Au-CN are used, the photodegradation efficiencies of RhB
reach respectively 94.3%, 98.2% and 87.7%, higher than that by g-C3N4, 1%Au-g-C3N4 or
CN nanorods. Obviously, 1%Au-CN exhibit the highest photocatalytic activity.
Subsequently, 1%Au-CN was used as the model of Au-CN samples to investigate the
removal of other organic pollutants. Colorless 4-CP is widely used in various industries
13
(pesticides, medicine and plastic), and 4-CP wastewater severely threatens human health due
to its high-toxicity. Under visible-light irradiation, 4-CP is very difficult to be degraded
without photocatalyst after 120 min (Fig. S6 and 8b). When pure g-C3N4 or CN nanorods are
used as photocatalysts, only 17.2% and 36.6% 4-CP can be degraded after 120 min.
Importantly, ~77.2% 4-CP can be degraded by 1%Au-CN nanorods after 120 min. In addition,
TC is a typical antibiotic, and TC wastewater lead to gastrointestinal irritation, diarrhea, renal
failure when it entered into aqueous environments. Under visible-light irradiation, ~83.9% TC
can be degraded by 1%Au-CN nanorods after 120 min (Fig. S7, Fig. 8c), remarkably higher
than that by pure g-C3N4 (36.9%) and CN (54.6%) nanorods. These facts confirm that
1%Au-CN can efficiently degrade various kinds of organic pollutant and exhibit the best
photocatalytic activity.
Beside organic pollutants, heavy-metal (such as Hg(Ⅱ), Pb(Ⅱ), Cr(VI)) elements are
widely presented in industrial wastewater, and they are considered as be mutagenic and
carcinogenic. With Cr(VI) as the model of heavy-metal pollutants, the photocatalytic
reduction of Cr(VI) was investigated by different photocatalysts (Fig. S8, Fig. 8d). Under
visible-light irradiation, ~43.6% Cr(VI) can be reduced by 1%Au-CN nanorods after 120 min,
higher than that by pure g-C3N4 (11.8%) and CN nanorods (23.1%). Furthermore, if
ammonium oxalate (AO, 0.4 mmol) as the hole-trapping agent is added into the Cr(VI)
wastewater, the reduction efficiency of Cr(VI) can be greatly enhanced to 95.2% by
1%Au-CN nanorods in 50 min (Fig. S9), obviously higher than that (~43.6% in 120 min) in
the absence of AO.
The mineralization of organic pollutants is the ultimate goal for wastewater treatment.
14
Herein, with RhB as a model of organic pollutants, its mineralization was investigated by
measuring total organic carbon (TOC) value during photocatalytic process mediated by
1%Au-CN (Fig. 9a). With the increase of irradiation time from 0 to 240 min, the TOC
concentration goes continuously down from 42.86 to 17.78 mg L-1. This fact indicates that
RhB is steadily degraded and its mineralization ratio is 58.5% at 240 min. Therefore, Au-CN
sample can efficiently degrade and mineralize organic pollutants under visible-light
irradiation.
The stability and recyclability of photocatalysts are critical issues for their practical
applications. Thus, the photocatalytic stability of 1%Au-CN nanorods was further
investigated by performing recycling tests (Fig. 9b). 1%Au-CN nanorods can degrade 98.2%
RhB after 120 min in the first cycle, and the efficiency still remains to be 88.7% after the
fourth cycle, revealing high photocatalytic activity and stability during the cycling tests. To
further confirm the stability, the phases and compositions of 1%Au-CN nanorods before and
after the cycling test were compared. Obviously, both 1%Au-CN samples exhibit the similar
XRD pattern (Fig. S10) and XPS spectra (Fig. S11, C1s, N1s and Au4f). Therefore, Au-CN
nanorods can be used as the efficient and stable photocatalyst.
3. 4 Photocatalytic mechanism
Compared with pure CN nanorods, 1%Au-CN sample has higher photocatalytic activity,
which should be attributed to the decoration of Au nanoparticles. It is well known that Au
nanoparticles may have two effects on the photocatalytic process. One is the fact that Au can
act as the electron acceptor to promote photogenerated electron-hole separations, which has
been well demonstrated in the previous reports[30, 31]. The other is that Au has strong
photoabsorption peak at ~550 nm due to LSPR effect (Fig. 6a), and then it can utilize
15
visible-light to produce hot-electrons that can inject to CB of semiconductor and then
participate photocatalytic reaction[40, 41]. To deduce the effects of Au nanoparticles on
photocatalytic activity, we used TC (without photoabsorption at >420 nm) as a model of
probe molecules and then investigated its degradation efficiency by CN nanorods and
1%Au-CN under the irradiation of monochromatic light with different wavelength (405, 450,
475, 500, 550, 580 or 635 nm) (Fig. 10). When monochromatic light with wavelength of 405
(or 450 nm) is used, CN nanorods and 1%Au-CN can degrade 83.1% and 85.3% (or 80.6%
and 84.4%) TC, respectively. In both photocatalysts, CN should be main component and it
can absorb the main 405/450 nm photons to generate electron-hole pairs. Thus, the slight
improvement (~3%) of degradation efficiency from CN to 1%Au-CN should result from Au
as the electron acceptor instead of photoabsorption agent. With the increase of light
wavelength from 450 to 635 nm, the degradation efficiency from CN goes down rapidly from
80.6% to 10.3%; while that from 1%Au-CN decreases moderately from 84.4% to 25.0%.
Especially, the degradation efficiency from CN and 1%Au-CN is respectively 36.6% and 67.2%
at 550 nm, with a very large difference (30.6%). Since CN has the significantly decreased
photoabsorption from 500 nm to 635 nm while Au nanoparticles have strong absorption peak
at 550 nm (Fig. 6), this obvious improvement (30.6%) of degradation efficiency should be
attributed to not only Au as the electron acceptor, but also Au as photoabsorption agent to
produce hot-electrons. To further analyze the effect of Au as photoabsorption agent, we used
808-nm laser to irradiate TC solution (20 mg/L, 100 mL) containing 500 mg pure CN
nanorods or 1%Au-CN (Fig. S12). No TC can be photocatalytically degraded by pure CN
nanorods, because CN nanorods cannot absorb 808-nm light. Interestingly, ~6.3% TC can be
degraded by 1%Au-CN, which should result from that Au nanoparticles in 1%Au-CN act as
16
photoabsorption agent for absorbing 808-nm laser and then producing hot-electrons to
degrade organic pollutants. Therefore, in photocatalytic process mediated by Au-CN, Au
should act as both the electron acceptor and photoabsorption agent.
The main active species in photocatalytic process are very important for deducing
photocatalytic mechanism. It is well known that benzoquinone (BQ), ammonium oxalate
(AO), silver nitrate (AgNO3) or isopropyl alcohol (IPA) has been used to capture superoxide
radical anions (·O2−), photogenerated holes (h+), electrons (e−) or hydroxyl free radicals (·OH),
respectively[42]. These trapping agents were added to RhB solution containing 1%Au-CN
nanorods, and the degradation efficiency of RhB were determined under visible-light
irradiation. With the addition of IPA or AgNO3, the degradation efficiency of RhB goes down
slowly from 98.2% to 80.7% or 87.2%, showing a weak suppression (Fig. 11a). Interestingly,
when BQ or AO is added, the degradation efficiency of RhB decreases rapidly from 98.2% to
21.9% or 33.7%, suggesting that both ·O2− and h+ have played an important role in the
photocatalytic process. At the same time, the degradation kinetic rate constants were also
calculated, and the constants declined from 0.03621 min-1 (no scavenger) to 0.01591 min-1
(AgNO3), 0.01299 min-1 (IPA), 0.00231 min-1 (AO) and 0.00141 min-1 (BQ) in the RhB
degradation tests (Fig. 11b). These results reveal that ·O2- radicals and h+ are the major active
species in the RhB degradation process by 1%Au-CN nanorods.
Based on the above results, one can conclude that there are two chief reasons for the
enhanced photocatalytic activity of Au-CN nanorods. One is mainly attributed to the strong
and broad photoabsorption ability of Au-CN nanorods, which is due to the broadened
photoabsorption of CN nanorods and LSPR effect from Au nanoparticles, as shown in Fig. 6.
17
Undoubtedly, this improved photoabsorption results in more efficiently utilization of solar
light and therefore production of more photogenerated carriers (Fig. 7c). The other is related
to LSPR-enhanced photocatalytic mechanism (Fig. 12). When Au-CN nanorods are irradiated
by visible-light, CN nanorods is excited to produce holes in the valence band (VB) and
electrons in the conduction band (CB). At the same time, the electrons on Au nanoparticles
surfaces are also excited from Fermi level (EF) to LSPR excited state, generating hot-electrons
with higher energy levels above CB of CN nanorods[43, 44]. Driven by the energy differences,
those excited hot-electrons of Au could easily jump across the Schottky barriers and inject
into CB of CN nanorods, which is similar to the previous report[40]. This injection process
will finally change the energy level equilibrium and form new E F below CB. Once the new EF
is developed, the photo-generated electrons at CB could be rapidly transferred to Au
nanoparticles[41, 45]. These trapped electrons by Au can reduce Cr (VI) to Cr (Ⅲ), or reduce
the surface-chemisorbed O2 to form ·O2- due to the fact that CB potential of CN nanorods
(-1.08 V vs. NHE) is lower than E(O2/·O2-) (-0.33 V vs. NHE)[46]. These produced ·O2- can
directly degrade organic pollutants in water. Meanwhile, the h+ in the VB of CN nanorods can
react with organic pollutants instead of oxidizing H2O to produce active species ·OH, which
can be explained by the fact that VB potential of CN nanorods (+0.57 V vs. NHE) is more
negative than the E(H2O/·OH) (+2.68 V vs. NHE)[47]. Therefore, the introduction of Au
nanoparticle greatly improve the photoabsorption, photocurrent and photocatalytic activity of
CN nanorods, making Au-CN nanorods as the promising, efficient and stable photocatalyst
for removing organic and heavy-metal pollutants in water.
4. Conclusion
18
In
summary,
Au-CN
nanorods
have
been
successfully
fabricated
by
a
solvothermal-hydrothermal two-step method. The optimum Au-CN show a greatly improved
photocatalytic activity for degrading organic pollutants (RhB, 4-CP, TC) and reducing
heavy-metal pollutants (Cr(VI)), compared with pure CN nanorods and g-C3N4 bulk. The
improved photocatalytic activity should result from the plasmon-enhanced photoabsorption
and rapid separation of photogenerated electro-hole pairs. Therefore, Au-CN nanorods have
great potential to be used as efficient and stable photocatalyst for removing organic and
heavy-metal pollutants in water. More importantly, this work provides an insight for the
design and development of photocatalysts with plasmon-enhanced photoabsorption and
photocatalytic activity.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of
China (Grant No. 21477019, 51473033 and 51773036), Program for Innovative Research
Team in University of Ministry of Education of China (IRT_16R13), Science and Technology
Commission of Shanghai Municipality (Grant No. 16JC1400700), Innovation Program of
Shanghai Municipal Education Commission (Grant No. 2017-01-07-00-03-E00055), the
Fundamental Research Funds for the Central Universities, and DHU Distinguished Young
Professor Program.
19
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Figure captions
Fig. 1 Schematic illustration of CN nanorods reaction (a) and in-situ growth of Au
nanoparticles on CN nanorods (b).
Fig. 2 SEM (a) and TEM (b) images of CN nanorods, TEM image (c) of 1%Au-CN nanorods
and histogram (d) of Au particle size distribution. The inset in c is HR-TEM image of Au
nanoparticle.
24
Fig. 3 HADDF-STEM image (a) and elemental mappings (b-d) of 1%Au-CN single nanorod.
Fig. 4 XPS survey spectra of CN nanorods and 1%Au-CN nanorods (a), and high-resolution
XPS spectra of C 1s (b), N 1s (c) and Au 4f (d) of 1%Au-CN nanorods.
25
Fig. 5 XRD patterns of g-C3N4, 1%Au-g-C3N4, CN nanorods, Au-CN samples (0.5%Au-CN,
1%Au-CN and 2%Au-CN) and standard XRD pattern of Au (JCPDS Card No. 04-0784).
Fig. 6 (a) UV-Vis-NIR diffuse reflectance spectra of g-C3N4, 1%Au-g-C3N4, CN nanorods and
Au-CN samples (0.5%Au-CN, 1%Au-CN and 2%Au-CN), (b) the plots of (αhν)1/2 versus the
energy (eV) of g-C3N4 and CN nanorods.
26
Fig. 7 (a) Mott-Schottky plots of CN nanorods at the selected frequencies of 800, 1000, 1500
and 2000 Hz, (b) photocurrent response plots of 1%Au-CN nanorods under different potential
from -0.8 to 1.0 V, (c) photocurrent response curves and (d) EIS Nyquist plots of g-C3N4, CN
nanorods and 1%Au-CN nanorods.
27
Fig. 8 The adsorption and degradation/reduction efficiency of (a) RhB (5 mg L-1), (b) 4-CP (1
mg L-1), (c) TC (20 mg L-1), (d) Cr(VI) (20 mg L-1) in aqueous solution (100 mL) versus the
exposure time under visible-light irradiation in the absence or presence of different
photocatalyst (20 mg).
Fig. 9 (a) TOC removal during RhB (50 mg L-1, 100 mL) degradation by 1%Au-CN (200 mg)
under visible-light irradiation. (b) Cycling photocatalytic test of RhB (5 mg L–1, 100 mL) by
1%Au-CN nanorods.
28
Fig. 10 The degradation efficiency of TC (5 mg L-1, 20 mL) by CN nanorods or 1%Au-CN
(10 mg) versus the wavelength of monochromatic light.
Fig. 11 (a) The degradation efficiency and (b) the rate kinetic constants of the photocatalytic
degradation of RhB (5 mg L-1, 100 mL) in the presence of AgNO3, IPA, AO or BQ.
Fig. 12 Schematic diagram of plasmon-enhanced photocatalytic mechanism.
29
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