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g-C3N4 layered heterostructures with controlled crystallinity towards superior photocatalytic degradation and H2 generation
Carbon 156 (2020) 488e498
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WOx/g-C3N4 layered heterostructures with controlled crystallinity towards superior photocatalytic degradation and H2 generation Xiao Zhang, Shuai He, San Ping Jiang* Fuels and Energy Technology Institute and Western Australia School of Mines: Minerals, Energy and Chemical Engineering, Curtin University, Perth, WA6845, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 July 2019 Received in revised form 29 August 2019 Accepted 29 September 2019 Available online 30 September 2019
The construction of g-C3N4 layered heterostructure is crucial to enhance charge carrier separation and expand the photo-adsorption in visible light region. Herein, amorphous and crystalline g-C3N4 nanosheets are produced by adjusting the thermal polycondensation process of precursors. WOx nanobelts are horizontally grown on ultrathin g-C3N4 nanosheets, resulting in the formation of WOx/g-C3N4 layered heterostructures rather than composites. The composition of WOx (WO3, WO2, W18O49) depends strongly on the preparation conditions (e.g. the amount of ascorbic acid). The compositions of WOx and crystallinity of geC3N4 affect the performance of the heterostructures with the best activity obtained on W18O49/g-C3N4 layered heterostructures. Amorphous W18O49/g-C3N4 layered heterostructures reveal enhanced photocatalytic performance (more than 4 times the performance of g-C3N4), while crystalline W18O49/g-C3N4 layered heterostructure exhibits superior hydrogen generation in visible light region (nearly 7 times higher compared with that of g-C3N4). The results indicate the promising potential of W18O49/g-C3N4 layered heterostructures as superior photocatalysts for degradation and H2 generation. © 2019 Elsevier Ltd. All rights reserved.
Keywords: WOx/g-C3N4 layered heterostructures Phootcatalysts RhB photodegradation Hydrogen evolution reaction Visible light region
1. Introduction Graphitic carbon nitride (g-C3N4) as a photocatalyst for the photocatalytic degradation of organic dyes and hydrogen generation via water splitting under visible light irradiation has stimulated tremendous interest due to its non-metallic composition, desirable bandgap (2.7 eV), and high stability in acidic/basic environment [1e4]. Bulk g-C3N4 prepared via conventional methods with a low specific surface area and irregular morphology shows a quick recombination of photogenerated carriers which limits the application [5e9]. Significant efforts have been done to improve the photocatalytic activity of g-C3N4 through element doping, morphology control, heterojunction formation. For example, enhanced photocatalytic performance has been reported for one dimensional (1D) g-C3N4 with different morphologies [10e14]. Two dimensional (2D) g-C3N4 materials have also attracted significant attention due to its high surface areas, increased active sites on the surface, excellent electron mobility. Especially, 2D g-C3N4 can be used as a support for charge carrier separation and also for surface material dispersion processes [15].
* Corresponding author. E-mail address: [email protected] (S.P. Jiang). https://doi.org/10.1016/j.carbon.2019.09.083 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
In order to inhibit the quick recombination rate of photogenerated carriers, coupling g-C3N4 nanosheets with other compounds have been considered a prevalent method [16]. For example, Yu’s group reported on a silver chromate (Ag2CrO4) as a promising photosensitizer due to its narrow band gap (~1.8 eV) which might favour the light absorption of g-C3N4 [16e18]. The composites of g-C3N4 restrain the recombination of photogenerated carriers and improve the photocatalytic performance [19e21]. The construction of layered-heterostructures provides the possibility to overcome poor quantum efficiency of g-C3N4 for H2 generation. Belt and layered second semiconductor materials are good candidates for creating g-C3N4 based layered-heterostructures. Tungsten oxides (WOx) as a chemically stable and promising photoanode material reveals tuneable bandgaps (2e3 eV) and high electron mobility [22e25]. WOx has been used to prepare the composites with photocatalytic materials to improve the catalytic performance in visible region [25]. WOx with different compositions and microstructures has been studied for application as electrocatalysts for water oxidation and Li-ion batteries [26,27]. Since tungsten possess variable oxidation states (e.g. IV, V, and VI), sub-stoichiometric tungsten oxides exhibit broad absorption in visible and infrared regions because of the presence of oxygen vacancies [28,29]. For example, Paik et al. reported a direct
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photocatalytic H2 evolution from sub-stoichiometric tungsten oxide (WOx) nanowires with the use of sacrificial alcohol [30]. Single layered g-C3N4 coupled with CsxWO3 nanorods was also reported to show significant high photocatalytic activity for H2 evolution and rhodamine (RhB)/phenol degradation in full solar spectrum [31]. Herein, we report for the first time the synthesis of WOx nanobelts horizontally grown on ultrathin g-C3N4 nanosheets with controlled crystallinity with superior photocatalytic activity. The nanobelts were grown and homogeneous distributed on ultrathin g-C3N4 nanosheets. Amorphous and crystalline g-C3N4 nanosheets were created by controlling the thermal polycondensation reactions of precursors. The results indicate that the layered heterostructures consisted of WOx and amorphous g-C3N4 reveal enhanced photocatalytic performance in visible light region while WOx nanobelts formed on crystalline g-C3N4 layered heterostructures show superior H2 generation reaction as compared with that of WO3/g-C3N4 and WO2/WO3/g-C3N4. 2. Experimental section 2.1. Preparation of g-C3N4 and WOx/g-C3N4 layered heterostructures Melamine, WCl6, ascorbic acid (AA), rhodamine (RhB), and triethanol amine (78%), tert-butanol, AgNO3, para-benzoquinone, Nafion solution, ethanol, Na2SO4, and H2SO4 were purchased from Sigma Aldrich and Sinopharm Chemical Reagent Co. Ltd. Chemicals were analytical-grade and were used without further purification. Amorphous and crystalline g-C3N4 nanosheets were prepared using a similar procedure published in our recent paper [32]. Typically, crystalline g-C3N4 was prepared using a slow heat-treatment procedure at 750 C and amorphous g-C3N4 was prepared via a fast heat-treatment procedure at 650 C. The as-prepared g-C3N4 sample was dispersed and sonicated in water. Finally, the suspension was precipitated and washed with water and ethanol for several times. The growth of WOx nanobelts on g-C3N4 nanosheets was carried out via hydrothermal synthesis. Typically, 0.06 g of WCl6 was dissolved in ethanol with 0.6 g of g-C3N4 followed by sonication for 5 min. The molar ratio of WCl6 and AA was adjusted to study the effect of AA on the composition of WOx formed on g-C3N4 nanosheets. After further stirring for 20 min, a uniform suspension was obtained. The suspension was then heat-treated at 220 C for 12 h. The resulting sample was washed for several times using ethanol. The amount of WCl6 was adjusted to control the ratio of W/g-C3N4. Table 1 gives the preparation conditions and properties of samples used in this study. 2.2. Material characterization The transmission electron microscopy (TEM) images, high-angle
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annular dark-field scanning TEM (HAADF-STEM) images, and corresponding element mapping of samples were recorded using a transmission electron microscope (FEI Titan G2 80-200 TEM/STEM with Chemi STEM Technology, FEI company, US). The UVevisible diffuse reflectance and absorption spectra of samples were obtained using a UVevisible spectrophotometer (U-4100, Hitachi). The X-ray diffraction (XRD) patterns of samples were taken on an Xray diffractometer (Bruker D8, Germany) with Cu Ka radiation source. The X-ray photoelectron spectroscopy (XPS) spectra of samples were recorded on Kratos Axis Ultra DLD spectrometer with a monochromatic AlKa (1486.6eV) X-rays operating at 150 W.
2.3. Photocatalytic and electrochemical characterization Photocatalytic H2 generation was measured using a connected system of Pyrex top-irradiation reaction vessel and a glass-closed gas circulation. 10 mg of sample and 100 ml of triethanolamine aqueous solution (triethanolamine:deionized water ¼ 1:9) were mixed followed by sonication (25 min) before light exposure. The system was evacuated to remove air before testing. With constant stirring of the sample solutions, the measurement was then done using a 300 W Xe lamp with a cutoff filter allowing l > 420 nm. Shimadzu GC-7920 gas chromatography (Argon as carrier gas) was used to measure the amount of H2 evolved. Photocatalytic degradation using RhB was carried out under visible light. RhB is one of the most commonly used dye for degradation studies. RhB solution (50 ml, 10 mg L/l) was used to mix with 30 mg of the samples followed by stirring in dark for 1 h in order to achieve equilibrium absorption state. Absorbance of RhB was then measured on a UVeVis spectrophotometer with 300 W Xe arc lamp and a UV-cut off filter allowing l 420 nm. Electrochemical performance was measured using CHI 660E electrochemical analyzer with a three-electrode system. Graphite rod and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. Catalyst ink (2 mg mL1) was prepared by mixing the as synthesized catalysts with a Nafion (5 wt%) mixture solution (Nafion:water:alcohol ¼ 0.025:4:1 in V/V), followed by pipetting 6 mL of the prepared catalysts ink onto a glassy carbon electrode with diameter of 3.0 mm. The catalyst loading was 0.17 mgcm2. Linear sweep voltammetry (LSV) curve was then measured in a 0.5 M H2SO4 solution at a scan rate of 5 mV s1. For the three-electrode system, with 0.1 mol/L of Na2SO4 aqueous solution acted as the electrolyte, a Pt plate was used as the counter electrode, Ag/AgCl electrode was used as the reference electrode, a thin film of the catalyst sample cast on the surface of indium tin oxide (ITO) glass was used as the working electrode. Electrochemical impedance spectroscopy (EIS) measurements were done using a 10 mV alternating current amplitude in the frequency ranges of 10 mHze100 kHz at open circuit potential. For the photocurrent response measurement, 300 W Xe lamps equipped with different wavelengths of filters was used as the light sources.
Table 1 Preparation conditions and properties of WOx/g-C3N4 composite samples. Samplea
g-C3N4
W/AA molar ratio
WOx
Preparation temperature for g-C3N4 ( C)
ACN-0 ACN-1 ACN-2 ACN-3 ACN-4 ACN-5 CCN-1 CCN-2
Amorphous Amorphous Amorphous Amorphous Amorphous Amorphous Crystalline Crystalline
1/0 1/1 1/2 1/10 2/1 10/1 1/1 1/2
WO3 W18O49 W18O49 W18O49 W18O49 WO2/WO3 W18O49 W18O49
650 650 650 650 650 650 750 750
a
ACN denotes amorphous g-C3N4 and CCN denotes crystalline g-C3N4.
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3. Results and discussion 3.1. Phase and microstructure of WOx/g-C3N4 layered heterostructures g-C3N4 nanosheets were obtained via a thermal polycondensation procedure of melamine at high temperatures. The crystallinity of g-C3N4 depends strongly on the distribution of heptazine units in the layered networks. Herein, crystalline g-C3N4 was synthesized by further thermal polymerization of bulk g-C3N4 prepared at a lower temperature with melamine as the raw material. Slow thermal polymerization process is crucial. The crystallinity of thin g-C3N4 nanosheets affects the band gap which determines the optic adsorption as discussed in the literature [32]. Fig. 1 shows the formation procedure of WOx/g-C3N4 layeredheterostructures. The WOx monomer was first deposited on the surface of g-C3N4 nanosheets followed by nucleation process. The nuclei grew horizontally along a specific crystal plane to form thin WOx nanobelts. The formation of WOx nanobelts from nucleation process has been confirmed by the SEM examinations of the samples with different reaction times of 1, 2, 3 and 5 h (Fig. S1, Supporting Information). With the increase of the reaction time, the nanobelt-shaped WOx started forming on the surface of g-C3N4, indicating the step-by-step growth process of WOx nanobelts on the surface of g-C3N4 as shown in Fig. 1. Fig. 2 shows the TEM images of CCN-1 and ACN-1 samples. The results indicate that WOx nanobelts were grown on ultrathin and layered g-C3N4 to form layered heterostructures. The WOx nanobelts with high degree of crystallinity were homogeneously distributed on crystalline g-C3N4 nanosheets (shown in Fig. 2bec). These nanobelts formed on crystalline g-C3N4 (CCN-1) appear to be more homogeneously distributed across the surface as compared with the ones on amorphous g-C3N4 (ACN-1). The interplanar distance between adjacent lattice planes of WOx nanobelts is 0.378 nm (Fig. 2c), coinciding well with the (010) lattice of W18O49, and that regular lattice fringes are observed in the crystalline g-C3N4 [33,34]. In contrast, the degree of crystallinity of WOx nanobelts decreases
when amorphous g-C3N4 was used. The lattice spacing measured is approximately 0.378 nm (Fig. 2f), indicating that the exposed crystal plane of the WOx nanobelts in ACN-1 is also (010) planes of W18O49. However, the lattice fringes are not well orientated, which may be related to the amorphous nature of g-C3N4 substrate. The nanobelts formed on amorphous g-C3N4 (ACN-1) are not homogeneously distributed and vary in a wide range of lengths as compared to that formed on crystalline g-C3N4 (CCN-1). This implies that the crystallinity of g-C3N4 affects the growth and morphology of the nanobelts [32]. Compared with amorphous surface, crystalline surface has a low energy which decreases nucleation and limits the growth of WOx nanobelts [35e37]. The slow growth process results in a WOx nanobelt with welldeveloped lattice fringes and homogeneous distribution, as shown in Fig. 2c. In contrast, surfaces of amorphous g-C3N4 have more defects, resulting in higher surface energy and increasing in WOx nucleation probabilities and growth rate. As shown in Fig. S2 (Supporting Information), nanobelts formed on amorphous gC3N4 (ACN-1) are relatively short in length and also appear to be aggregated as compared with that formed on crystalline g-C3N4 (CCN-1). The results indicate that the amorphous g-C3N4 favors a higher distribution of WOx nanobelts over crystalline g-C3N4. The growth of the nanobelts occurred on the edge of the nanosheets as well as within or between the nanosheets and the bended nanobelts were also observed on the edge of the nanosheets (Fig. S3, Supporting Information). The results indicate that the nanobelts and nanosheets may consist of few layers. Fig. 3 shows the XPS spectra (e.g. C1s, N1s, O1s, and W4f) of CCN-1 and ACN-1 samples. The peaks corresponding to CeC (sp2 bonded carbon), CeOH (epoxy/hydroxyls), OeC]O, CeN/C]O (carbonyls), and satellite C1s peak of the C1s spectra of samples are shown in Fig. 3a and e. The N1s spectra of samples in Fig. 3b and f shows the three peak positions corresponding to NeH (graphitic N), CeN]C (Pyridinic-N), N-(C)3 (Pyrrolic-N), and p excitations. Due to the difference of the C 1s spectra of the samples and the similarity of N 1s spectra, the C/N ratios of the samples are different. This is ascribed to the distillation of N element occurred at high
Fig. 1. Formation of WOx/g-C3N4 layered heterostructures.
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Fig. 2. TEM images of WOx/g-C3N4 samples with different magnification. (a, b, c) CCN-1 prepared using crystalline g-C3N4 with W/AA ratio of 1:1 and (d, e, f) ACN-1 prepared using amorphous g-C3N4 with W/AA ratio of 1:1.
temperatures [30]. The samples show very similar O1s, and W4f spectra in Fig. 3c, and d, 3g, and 3h, suggesting that the distribution state of W and O elements is similar. For sample CCN-1, peaks at 35.3 and 37.5 eV can be attributed to W4f7/2 and W4f5/2 of W6þ state, while the one at 34.0 and 36.3 eV corresponds to W5þ state (Fig. 3d) [34]. This indicates that the defects may endow W18O49 with specific performance. For the layered-heterostructures of W18O49 and amorphous g-C3N4 (ACN-1), the peak position corresponding to W6þ and W5þ state changed, suggesting the difference of bonding state of W18O49 on amorphous and crystalline g-C3N4. The elemental composition of the samples based on the XPS data are shown in Table S1 (Supporting Information). To confirm the formation of WOx, the HAADF-STEM images and corresponding element mapping of CCN-1 were obtained and the results are shown in Fig. 4. Similar result was also obtained for sample ACN-1 (Fig. S4, Supporting Information). The distribution of N and C is homogeneous and the distribution of W and O is in the area of W18O49 nanobelts. To study the formation of nanobelts, pure WOx samples were prepared without adding g-C3N4 under the same conditions. For the nanobelt formation and composition of WOx, AA plays a very important role. We found that WO3 nanoflowers were obtained in case of no AA addition and with the addition of AA, WO3 nanofibers instead of nano-flower were obtained (Fig. S5, Supporting Information). As a reducing agent, AA can be adsorbed on the surface of WOx crystals, working as a blocking agent to control growth rate and exposed crystal plane. Nevertheless, the exact reason for the formation of nanobeltshaped WOx on g-C3N4 is not clear at this stage. Since AA works as a reducing agent during reaction process, the composition of WOx strongly depends on the amount of AA. Fig. 5 shows the XRD patterns of composite samples prepared using different W/AA ratios on amorphous g-C3N4. Two XRD peaks
corresponding to the (100) and (002) facets of g-C3N4 were observed for all samples. The weak peak at 12.8 is caused by the periodic arrangement of the triazine in g-C3N4 and the higher intensity peak at 27.9 is related to the stacking of the conjugated aromatic system of carbon nitride [32]. Using a W/AA molar ratio of 10/1, a mixed phase of WO2 and WO3 was obtained (sample ACN-5). In the case of W6þ/AA ratio of 1:1, orthorhombic WO2 and hexagonal WO3 phases were obtained, whereas only hexagonal WO3 phase was obtained in the absence of AA in the reactor (Fig. S6, Supporting Information). When W6þ:AA ¼ 2:1, a strong diffraction peak at 23.08 , a characteristic peak of W18O49, appears (dotted line in Fig. 5). This indicates the formation of the highly crystalline phase of WOx. With the increase of AA content, e.g., W6þ:AA ratio ¼ 1:10, excess AA could adsorb on the surface of WOx crystalline phase and block the further growth and deposition of the WOx nanobelts. This appears to be supported by a decrease of the intensity of the diffraction peak at 23.08 with the increase of the AA content in the reaction. 3.2. Photocatalytic properties of WOx/g-C3N4 layered heterostructures Fig. 6 shows the UVeVis diffuse reflectance spectra and plots of (ahn)1/2 vs photon energy of WOx heterostructured samples with amorphous g-C3N4. The band gap of the samples was estimated by using the transformed Kubelka-Munk function as follow according to DRS plots [35e37]. 2
Aðhv EgÞn ¼ ahv
(1)
The band gaps of samples ACN-1, ACN-2, ACN-4, and ACN-5 were estimated to be 2.86, 2.82, 2.88, and 2.89 eV, respectively, which is lower than 2.90 eV measured on amorphous g-C3N4
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Fig. 3. C1s, N1s, O1s, W4f XPS spectra of (a, b, c, d) sample CCN-1 and (e, f, g, h) sample ACN-1.
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Fig. 4. HAADF-STEM image and N, C, W, and O mapping of sample CCN-1.
Fig. 5. XRD patterns of WOx/g-C3N4 composite samples using different amount of AA and amorphous g-C3N4.
(Fig. 6d). The decreased band gap confirms the formation of W18O49/g-C3N4 layered-heterostructure. As shown in Fig. 6a and b, the crystalline g-C3N4 has an advantage in the initial band gap energy and the band gap is 2.70 eV, lower than that of the amorphous g-C3N4. The band gap energy of the amorphous g-C3N4 is improved to a greater extent after the formation and deposition of WOx nanobelts with the absorption band edge closer to the blue region. Among the samples studied, ACN-1 and ACN-2 have
relatively low bandgap energy, indicating the absorption over a wider range. The photocurrent responses of samples under visible light irradiation were measured and the results are shown in Fig. 7. The photocurrent response value of amorphous g-C3N4 is very low. However, the formation of layered-heterostructures remarkably enhances the photocurrent response value. Especially, the photocurrent response value of sample ACN-1 exhibited the highest intensity compared with other samples. As the transient photocurrent is related to the conductivity of the materials, the higher photocurrent indicates the better conductivity of the material. EIS measurement is another effective way to show the charge separation and transfer efficiencies. Impedance plots of ACN-1 and ACN-2 electrodes with or without light irradiation were investigated (see Fig. 8a). Regardless of the presence or absence of irradiation, the slope of the EIS curve of ACN-1 is smaller than that of ACN-2, indicating that ACN-1 has a higher ion transport rate. With no light irradiation, sample ACN-1 shows a smaller impedance arc radius than that of sample ACN-2, indicating that sample ACN-1 has high efficiency of charge transfer. This may indicate that highly crystalline W18O49 phase improves the electrical conductivity of the layered heterostructures. In addition, the separation and transfer efficiency of photogenerated electron and hole pairs increased dramatically with light irradiation for ACN-1 and ACN-2 samples due to strong absorption of W18O49 in visible region. 3.3. Photocatalytic activities of WOx/g-C3N4 layered heterostructures The photocatalytic activity of amorphous WOx/g-C3N4 layered
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Fig. 6. (a) UV-VIS diffuse reflectance spectra and (b) plots of (ahn)1/2 vs photon energy of amorphous g-C3N4, ACN-1, crystalline g-C3N4 and CCN-1; (c) UV-VIS diffuse reflectance spectra and (d) plots of (ahn)1/2 vs photon energy of amorphous g-C3N4, ACN-1, ACN-2, ACN-4 and ACN-5.
Fig. 7. Photocurrent-time (i-t) curves of amorphous g-C3N4, ACN-1, ACN-2 and ACN-5 layered heterostructures under visible light irradiation.
heterostructure samples was examined on the photocatalytic degradation of RhB in aqueous solutions under visible light irradiation and the results are shown in Fig. 9. As shown in Fig. 9a, the photocatalytic performance of crystalline g-C3N4 is better than that of amorphous g-C3N4, however, ACN-1 shows a better photodegradation efficiency as compared to CCN-1. This indicates that the distribution and morphology of WOx nanobelts deposited on g-
C3N4 may have significant effect on the photocatalytic activity of layered heterostructures. WOx nanobelts on crystalline g-C3N4 (CCN-1) are longer on average (Fig. 2). On the other hand, WOx nanobelts on the surface of amorphous g-C3N4 (ACN-1) varied in a wide size rage (with variety of lengths), which could improve the light absorption capacity and increase the exposure of active site of the sample to a considerable extent. The degradation performance of pure WO3 modified C3N4 sample, prepared with no AA addition (ACN-0) is shown in Fig. 9c for comparison. Both g-C3N4 and W18O49 are typical photocatalytic materials under visible light [34,38,39]. As shown, less than 50% of RhB was degraded after 30 min reaction using ACN-0. This result confirms that the composition of WOx play an important role for enhancing the photocatalytic performance of amorphous g-C3N4. Among all samples, ACN-1 (W/AA molar ratio of 1/1) shows the best photocatalytic activity (Fig. 9c). However, as the W/AA ratio continuously decreases, the photocatalytic efficiency of the sample gradually decreases. To further demonstrate photocatalytic reaction kinetics of the samples, the photo degradation plots can be fitted using a pseudo first order reaction as follows [34,38,39].
lnðC = C0 Þ ¼ Kt
(2)
where C0 and Ct are initial and reaction concentrations with time and t is the irradiation time. When pseudo-first-order kinetics is used to simulate the photocatalytic process of WOx/g-C3N4 samples, the RhB decomposition tendency of the samples studied can be divided into two stages (Fig. 9b). The degradation rate constant
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Fig. 8. EIS curves of ACN-1 and ACN-2 with or without irradiation. Curves shown in (b) are the enlarged section of (a) in high frequency region.
Fig. 9. (a) Photocatalytic degradation performances and (b) degradation kinetic models of pure g-C3N4 (amorphous and crystalline), ACN-1 and CCN-1 for RhB degradation under visible light (420 nm) irradiation; (c) Photocatalytic degradation performances of ACN-0, ACN-1, ACN-2, ACN-3, ACN-4, ACN-5 and g-C3N4 (amorphous) and (d) degradation kinetic models of ACN-0, ACN-2, ACN-3, ACN-4 and ACN-5 for RhB under visible light (420 nm) irradiation.
(K1) of the sample generally has a large value within the first 10 min. At this stage, the adsorbed amount on the sample surface reaches saturation, and the reaction is mainly determined by the concentration of RhB. The degradation rate constant (K2) of samples has a relatively smaller value within the 10e30 min range. The
catalytic reaction at this stage is mainly determined by the dispersion of the sample in aqueous solution and the density of exposed surface active sites [40]. The degradation rate constants K1, K2 and R2 values of amorphous g-C3N4, ACN-1, crystalline g-C3N4 and CCN-1 are shown in Table 2 and those of ACN-0, ACN-2, ACN-3,
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Table 2 Degradation rate constants and R2 values of amorphous g-C3N4, ACN-1, crystalline gC3N4 and CCN-1. Sample
K1
R21
K2
R22
Amorphous g-C3N4 ACN-1 Crystalline g-C3N4 CCN-1
3.05 24.93 12.11 13.57
0.97 0.99 0.99 0.99
4.05 1.86 6.65 8.26
0.95 0.94 0.99 0.99
ACN-4 and ACN-5 are shown in Table S2 (Supporting Information). The R2 values of all the samples are higher than 0.9, demonstrating that the photocatalytic reaction kinetics of these samples follows the pseudo first-order kinetic model. As shown in Fig. 9d, ACN-1 shows the best photocatalytic activity. In the first 10 min, the reaction rate constant of ACN-1 is 8 times higher than that of amorphous g-C3N4 and twice higher as that of crystalline g-C3N4. This demonstrates that the formation of W18O49/g-C3N4 layeredheterostructures enhance the separation and transfer efficiency of photogenerated electron and hole pairs, thus increasing the photocatalytic efficiency. To further understand the photocatalytic reaction mechanism, cyclic stability test and active group capture experiment were carried out. As shown in Fig. 10a, sample ACN-1 exhibites stable photodegradation efficiency during the four cycles of the test. The degradation rate of RhB of ACN-1 remained the same. In addition, the SEM and XRD analysis of sample ACN-1 indicate that there is no visible change in the morphology and phase composition of the sample before and after the photodegradation tests (Fig. S7, Supporting Information). In order to identify the free radicals in the photodegradation reaction, tertiary butanol, AgNO3, triethanol amine, para-benzoquinone, were used as free radical capture agents for hydroxyl radicals ($OH), electronic (e), hole (hþ), and superoxide free radical (O 2 ), respectively [8,34] (Fig. 10b). The presence of AgNO3 and para-benzoquinone have no significant effect for the photocatalytic reaction, indicating that electronic (e) and superoxide free radical (O 2 ) are not involved in the photodegradation reaction of RhB on ACN-1. However, a significant decrease in photocatalytic performance was observed when triethanol amine and tert-butanol were added to the solution. This indicates the presence of free radicals of O 2 and $OH (or $H2O) formed by the reaction between the photogenerated e and hþ and the O2 and H2O molecules in the solution. These free radicals are strong oxidizing agent and react with RhB molecules close to or on the surface of ACN-1 photocatalyst. The photogenerated hþ plays a
leading role in the photocatalytic reaction in which WOx/g-C3N4 is involved. The efficiency of photodegradation of the current study is also compared with some of the most recently reported results in similar systems (see Table 3) [18,41e43]. It shows that the WOx/gC3N4 layered heterostructure prepared in this work show comparable and better catalytic efficiency and degradation efficiency, as compared with the similar tungsten oxide modified C3N4 photocatalysts reported in the literature. Hydrogen generation under visible light (420 nm) of the WOx/ g-C3N4 layered heterostructure samples were also measured and the results are shown in Fig. 11. A significant increasing trend in hydrogen evolution was observed for all samples. The best results were obtained on crystalline W18O49/g-C3N4 layered heterostructure (sample CCN-1) with the highest hydrogen generation activity among the four samples including crystalline g-C3N4, amorphous g-C3N4 and ACN-1 sample (Fig. 11a). The amount of H2 generated increased linearly with time. Average rate constants for H2 evolution for amorphous g-C3N4, ACN-1, crystalline g-C3N4 and CCN-1 under visible light (l > 420 nm) were obtained. As shown in Fig. 11a, the H2 evolution rate of CCN-1 is twice as higher as the gC3N4, indicating excellent efficiency of CCN-1 for photocatalytic H2 evolution. To evaluate the stability of CCN-1, cyclic experiments on hydrogen production were carried out. After four cycles, the hydrogen generation efficiency of sample CCN-1 remained almost unchanged (Fig. 11b). In fact, the hydrogen production of the CCN-1 increased slightly and after five cycles, the amount of hydrogen produced remained basically stable (Fig. S8, Supporting Information). This indicates the good conductivity of CCN-1 sample with good stability due to high crystallinity. The charge transfer process in W18O49/g-C3N4 heterostructures during H2 evolution reaction is illustrated in Fig. 12. According to the experiment, no hydrogen was produced when pure WOx was used as a photocatalyst. This is ascribed to the band gap of WOx nanobelts and its conduction band (CB) potential is more positive than the Hþ/H2 redox potential. After combining with crystalline gC3N4, the photocatalytic hydrogen production performance was significantly enhanced. Under the irradiation of visible light, W18O49 and g-C3N4 components are simultaneously excited. CCN-1 revealed enhanced hydrogen production activity. This is due to the fact that crystalline g-C3N4 with less structure defects causes slow growth of WOx nanobelts and thus leads to formation of high quality heterostructure with high carrier transfer efficiency. The high crystallinity and less internal defects may also promote electron migration at the interface in the H2 reduction reaction.
Fig. 10. (a) Cyclic stability test of photocatalytic degradation of RhB by ACN-1; (b) Photocatalytic performances with the addition of various free radical capture agents.
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Table 3 The experimental data of this paper in comparison with the recently published literature. References
Degradation time (min)
Degradation efficiency (%)
Composition
This work Xu et al., 2015 [18] Yao et al., 2019 [41] Jia et al., 2019 [42] Singh et al., 2019 [43]
30 90 60 30þ 160
100% 99% 100% 90% 97%
W18O49/g-C3N4 Ag2WO4/g-C3N4 WO3/g-C3N4 g-C3N4/WO3/NQDs WO3/g-C3N4
Fig. 11. (a) Photocatalytic H2 production performances of amorphous g-C3N4 (I), ACN-1 (II), crystalline g-C3N4 (III) and CCN-1 (IV). The inset shows average H2 evolution rate and (b) Photocatalytic H2 production cyclic test of CCN-1 under visible light (420 nm).
g-C3N4 nanosheets was much more homogeneous than those on amorphous g-C3N4 nanosheets. The layered-heterojunction of W18O49/g-C3N4 revealed enhanced electrochemical performance compared with WO3/g-C3N4 and WO2/WO3/g-C3N4 samples. In addition, the layered-heterostructures consisted of W18O49 and amorphous g-C3N4 revealed good photocatalytic performance (about 4 times higher comparing with g-C3N4), while the layerheterostructures of W18O49 and crystalline g-C3N4 revealed superior H2 evolution reaction under visible light. Acknowledgments This work was supported by the Australian Research Council under the Discovery Scheme (Project No. DP180100731, DP180100568). Appendix A. Supplementary data
Fig. 12. Illustration of charge transfer in W18O49/g-C3N4 heterostructures for H2 evolution reaction.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.09.083. References
4. Conclusions WOx nanobelts horizontally grown on ultrathin g-C3N4 nanosheets to create layered-heterojunctions have been synthesized for the first time, showing enhanced the photocatalytic performance. WOx nanobelts were grown along a specific crystal plane on amorphous and crystalline g-C3N4 nanosheets via a hydrothermal synthesis method using AA to control the morphology and composition of WOx. The composition of WOx (WO3, WO2, W18O49) strongly depended on the preparation conditions such as the molar ratio of W/AA. The distribution of W18O49 nanobelts on crystalline
[1] Y. Liu, J. Wang, P. Yang, Photochemical reactions of g-C3N4-based heterostructured composites in Rhodamine B degradation under visible light, RSC Adv. 6 (2016) 34334e34341. [2] K.S. Lakhi, D.-H. Park, K. Al-Bahily, W. Cha, B. Viswanathan, J.-H. Choy, A. Vinu, Mesoporous carbon nitrides: synthesis, functionalization, and applications, Chem. Soc. Rev. 46 (2017) 72e101. [3] Z. Chen, X. Yu, Q. Zhu, T. Fan, Q. Wu, L. Zhang, J. Li, W. Fang, X. Yi, Steam engraving optimization of graphitic carbon nitride with enhanced photocatalytic hydrogen evolution, Carbon 139 (2018) 189e194. [4] X. Yu, T. Fan, W. Chen, Z. Chen, Y. Dong, H. Fan, W. Fang, X. Yi, Self-hybridized coralloid graphitic carbon nitride deriving from deep eutectic solvent as effective visible light photocatalysts, Carbon 144 (2019) 649e658. [5] Y. Zhang, T. Mori, L. Niu, J. Ye, Non-covalent doping of graphitic carbon nitride polymer with graphene: controlled electronic structure and enhanced optoelectronic conversion, Energy Environ. Sci. 4 (2011) 4517e4521.
498
X. Zhang et al. / Carbon 156 (2020) 488e498
[6] X. Ma, Y. Lv, J. Xu, Y. Liu, R. Zhang, Y. Zhu, A strategy of enhancing the photoactivity of g-C3N4 via doping of nonmetal elements: a first-principles study, J. Phys. Chem. C 116 (2012) 23485e23493. [7] Y. Hong, C. Li, D. Li, Z. Fang, B. Luo, X. Yan, H. Shen, B. Mao, W. Shi, Precisely tunable thickness of graphitic carbon nitride nanosheets for visible-lightdriven photocatalytic hydrogen evolution, Nanoscale 9 (2017) 14103e14110. [8] Z. Liu, G. Wang, H.-S. Chen, P. Yang, An amorphous/crystalline g-C3N4 homojunction for visible light photocatalysis reactions with superior activity, Chem. Commun. 54 (2018) 4720e4723. [9] L. Shi, Z. Li, K. Marcus, G. Wang, K. Liang, W. Niu, Y. Yang, Integration of Au nanoparticles with a g-C3N4 based heterostructure: switching charge transfer from type-II to Z-scheme for enhanced visible light photocatalysis, Chem. Commun. 54 (2018) 3747e3750. [10] X. Bai, L. Wang, R. Zong, Y. Zhu, Photocatalytic activity enhanced via g-C3N4 nanoplates to nanorods, J. Phys. Chem. C 117 (2013) 9952e9961. [11] M. Tahir, C. Cao, N. Mahmood, F.K. Butt, A. Mahmood, F. Idrees, S. Hussain, M. Tanveer, Z. Ali, I. Aslam, Multifunctional g-C3N4 nanofibers: a templatefree fabrication and enhanced optical, electrochemical, and photocatalyst properties, ACS Appl. Mater. Interfaces 6 (2014) 1258e1265. [12] Y. Liu, X. Zhang, J. Wang, P. Yang, In situ growth of sulfide/g-C3N4 nanoheterostructures with an adjusted band gap toward enhanced visible photocatalysis, Phys. Chem. Chem. Phys. 18 (2016) 31513e31520. [13] Y. Liu, X. Zhang, J. Wang, P. Yang, Preparation of luminescent graphitic C3N4 NS and their composites with RGO for property controlling, RSC Adv. 6 (2016) 112581e112588. [14] B. Zhu, J. Zhang, C. Jiang, B. Cheng, J. Yu, First principle investigation of halogen-doped monolayer g-C3N4 photocatalyst, Appl. Catal. B Environ. 207 (2017) 27e34. [15] D. Huang, Z. Li, G. Zeng, C. Zhou, W. Xue, X. Gong, X. Yan, S. Chen, W. Wang, M. Cheng, Megamerger in photocatalytic field: 2D g-C3N4 nanosheets serve as support of 0D nanomaterials for improving photocatalytic performance, Appl. Catal. B Environ. 240 (2019) 153e173. [16] D. Xu, B. Cheng, W. Wang, C. Jiang, J. Yu, Ag2CrO4/g-C3N4/graphene oxide ternary nanocomposite Z-scheme photocatalyst with enhanced CO2 reduction activity, Appl. Catal. B Environ. 231 (2018) 368e380. [17] D. Xu, B. Cheng, S. Cao, J. Yu, Enhanced photocatalytic activity and stability of Z-scheme Ag2CrO4-GO composite photocatalysts for organic pollutant degradation, Appl. Catal. B Environ. 164 (2015) 380e388. [18] D. Xu, B. Cheng, J. Zhang, W. Wang, J. Yu, W. Ho, Photocatalytic activity of Ag2MO4 (M ¼ Cr, Mo, W) photocatalysts, J. Mater. Chem. 3 (2015) 20153e20166. [19] Y. He, L. Zhang, B. Teng, M. Fan, New application of Z-scheme Ag3PO4/g-C3N4 composite in converting CO2 to fuel, Environ. Sci. Technol. 49 (2015) 649e656. [20] W. Yu, D. Xu, T. Peng, Enhanced photocatalytic activity of g-C3N4 for selective CO2 reduction to CH3OH via facile coupling of ZnO: a direct Z-scheme mechanism, J. Mater. Chem. 3 (2015) 19936e19947. [21] B. Zhu, P. Xia, Y. Li, W. Ho, J. Yu, Fabrication and photocatalytic activity enhanced mechanism of direct Z-scheme g-C3N4/Ag2WO4 photocatalyst, Appl. Surf. Sci. 391 (2017) 175e183. come, M. Morales, C. Angeles Chavez, L.F. Ramírez-Verduzco, [22] M.A. Cortes-Ja pez-Salinas, J.A. Toledo-Antonio, WOx/TiO2 catalysts via titania nanoE. Lo tubes for the oxidation of dibenzothiophene, Chem. Mater. 19 (2007) 6605e6614. [23] F. Wang, C. Di Valentin, G. Pacchioni, Rational band gap engineering of WO3 photocatalyst for visible light water splitting, ChemCatChem 4 (2012) 476e478. [24] K. Wang, Y. Shi, Q. Dong, Y. Li, S. Wang, X. Yu, M. Wu, T. Ma, Low-temperature and solution-processed amorphous WOX as electron-selective layer for perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 755e759. [25] X.-H. Jiang, Q.-J. Xing, X.-B. Luo, F. Li, J.-P. Zou, S.-S. Liu, X. Li, X.-K. Wang, Simultaneous photoreduction of Uranium(VI) and photooxidation of Arsenic(III) in aqueous solution over g-C3N4/TiO2 heterostructured catalysts under
simulated sunlight irradiation, Appl. Catal. B Environ. 228 (2018) 29e38. [26] X. Shi, L. Cai, I.Y. Choi, M. Ma, K. Zhang, J. Zhao, J.K. Kim, J.K. Kim, X. Zheng, J.H. Park, Epitaxial growth of WO3 nanoneedles achieved using a facile flame surface treatment process engineering of hole transport and water oxidation reactivity, J. Mater. Chem. 6 (2018) 19542e19546. [27] C. Wang, Y. Zhao, L. Zhou, Y. Liu, W. Zhang, Z. Zhao, W.N. Hozzein, H.M.S. Alharbi, W. Li, D. Zhao, Mesoporous carbon matrix confinement synthesis of ultrasmall WO3 nanocrystals for lithium ion batteries, J. Mater. Chem. 6 (2018) 21550e21557. [28] Q. Mi, A. Zhanaidarova, B.S. Brunschwig, H.B. Gray, N.S. Lewis, A quantitative assessment of the competition between water and anion oxidation at WO3 photoanodes in acidic aqueous electrolytes, Energy Environ. Sci. 5 (2012) 5694e5700. [29] X. Feng, Y. Chen, Z. Qin, M. Wang, L. Guo, Facile fabrication of sandwich structured WO3 nanoplate Arrays for efficient photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 8 (2016) 18089e18096. [30] T. Paik, M. Cargnello, T.R. Gordon, S. Zhang, H. Yun, J.D. Lee, H.Y. Woo, S.J. Oh, C.R. Kagan, P. Fornasiero, C.B. Murray, Photocatalytic hydrogen evolution from substoichiometric colloidal WO3ex nanowires, ACS Energy Letters 3 (2018) 1904e1910. [31] A. Shi, H. Li, S. Yin, J. Zhang, Y. Wang, H2 Evolution over g-C3N4/CsxWO3 under NIR light, Appl. Catal. B Environ. 228 (2018) 75e86. [32] X. Zhang, J.-P. Veder, S. He, S.P. Jiang, Construction of 2D g-C3N4 lateral-like homostructures and their photo- and electro-catalytic activities, Chem. Commun. 55 (2019) 1233e1236. [33] K. Deng, Z. Hou, X. Deng, P. Yang, C. Li, J. Lin, Enhanced antitumor efficacy by 808 nm laser-induced synergistic photothermal and photodynamic therapy based on a indocyanine-green-attached W18O49 nanostructure, Adv. Funct. Mater. 25 (2015) 7280e7290. [34] C. Jia, X. Zhang, K. Matras-Postolek, B. Huang, P. Yang, Z-scheme reduced graphene oxide/TiO2-Bronze/W18O49 ternary heterostructure towards efficient full solar-spectrum photocatalysis, Carbon 139 (2018) 415e426. [35] Q. Han, C. Hu, F. Zhao, Z. Zhang, N. Chen, L. Qu, One-step preparation of iodinedoped graphitic carbon nitride nanosheets as efficient photocatalysts for visible light water splitting, J. Mater. Chem. 3 (2015) 4612e4619. [36] R. Ye, H. Fang, Y.-Z. Zheng, N. Li, Y. Wang, X. Tao, Fabrication of CoTiO3/gC3N4 hybrid photocatalysts with enhanced H2 evolution: Z-scheme photocatalytic mechanism insight, ACS Appl. Mater. Interfaces 8 (2016) 13879e13889. [37] G. Zhang, A. Savateev, Y. Zhao, L. Li, M. Antonietti, Advancing the n / p* electron transition of carbon nitride nanotubes for H2 photosynthesis, J. Mater. Chem. 5 (2017) 12723e12728. [38] X. Zhang, C. Jia, Y. Xue, P. Yang, Fabrication of rGO/g-C3N4 composites via electrostatic assembly towards charge separation control, RSC Adv. 7 (2017) 43888e43893. [39] Z. Jiang, X. Zhang, J. Wang, L. Chen, H.-S. Chen, P. Yang, Ultrastable g-C3N4 assemblies with high quantum yield and reversible photoluminescence, Chem. Commun. 54 (2018) 13519e13522. [40] H. Hu, J. Liu, Z. Xu, L. Zhang, B. Cheng, W. Ho, Hierarchical porous Ni/Co-LDH hollow dodecahedron with excellent adsorption property for Congo red and Cr(VI) ions, Appl. Surf. Sci. 478 (2019) 981e990. [41] J. Yao, M. Zhang, H. Yin, Y. Luo, X. Liu, Improved photocatalytic activity of WO3/C3N4: by constructing an anchoring morphology with a Z-scheme band structure, Solid State Sci. 95 (2019), 105926. [42] J. Jia, C. Jiang, X. Zhang, P. Li, J. Xiong, Z. Zhang, T. Wu, Y. Wang, Urea-modified carbon quantum dots as electron mediator decorated g-C3N4/WO3 with enhanced visible-light photocatalytic activity and mechanism insight, Appl. Surf. Sci. 495 (2019), 143524. [43] J. Singh, A. Arora, S. Basu, Synthesis of coral like WO3/g-C3N4 nanocomposites for the removal of hazardous dyes under visible light, J. Alloy. Comp. 808 (2019), 151734.
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Carbon journal homepage: www.elsevier.com/locate/carbon
WOx/g-C3N4 layered heterostructures with controlled crystallinity towards superior photocatalytic degradation and H2 generation Xiao Zhang, Shuai He, San Ping Jiang* Fuels and Energy Technology Institute and Western Australia School of Mines: Minerals, Energy and Chemical Engineering, Curtin University, Perth, WA6845, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 July 2019 Received in revised form 29 August 2019 Accepted 29 September 2019 Available online 30 September 2019
The construction of g-C3N4 layered heterostructure is crucial to enhance charge carrier separation and expand the photo-adsorption in visible light region. Herein, amorphous and crystalline g-C3N4 nanosheets are produced by adjusting the thermal polycondensation process of precursors. WOx nanobelts are horizontally grown on ultrathin g-C3N4 nanosheets, resulting in the formation of WOx/g-C3N4 layered heterostructures rather than composites. The composition of WOx (WO3, WO2, W18O49) depends strongly on the preparation conditions (e.g. the amount of ascorbic acid). The compositions of WOx and crystallinity of geC3N4 affect the performance of the heterostructures with the best activity obtained on W18O49/g-C3N4 layered heterostructures. Amorphous W18O49/g-C3N4 layered heterostructures reveal enhanced photocatalytic performance (more than 4 times the performance of g-C3N4), while crystalline W18O49/g-C3N4 layered heterostructure exhibits superior hydrogen generation in visible light region (nearly 7 times higher compared with that of g-C3N4). The results indicate the promising potential of W18O49/g-C3N4 layered heterostructures as superior photocatalysts for degradation and H2 generation. © 2019 Elsevier Ltd. All rights reserved.
Keywords: WOx/g-C3N4 layered heterostructures Phootcatalysts RhB photodegradation Hydrogen evolution reaction Visible light region
1. Introduction Graphitic carbon nitride (g-C3N4) as a photocatalyst for the photocatalytic degradation of organic dyes and hydrogen generation via water splitting under visible light irradiation has stimulated tremendous interest due to its non-metallic composition, desirable bandgap (2.7 eV), and high stability in acidic/basic environment [1e4]. Bulk g-C3N4 prepared via conventional methods with a low specific surface area and irregular morphology shows a quick recombination of photogenerated carriers which limits the application [5e9]. Significant efforts have been done to improve the photocatalytic activity of g-C3N4 through element doping, morphology control, heterojunction formation. For example, enhanced photocatalytic performance has been reported for one dimensional (1D) g-C3N4 with different morphologies [10e14]. Two dimensional (2D) g-C3N4 materials have also attracted significant attention due to its high surface areas, increased active sites on the surface, excellent electron mobility. Especially, 2D g-C3N4 can be used as a support for charge carrier separation and also for surface material dispersion processes [15].
* Corresponding author. E-mail address: [email protected] (S.P. Jiang). https://doi.org/10.1016/j.carbon.2019.09.083 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
In order to inhibit the quick recombination rate of photogenerated carriers, coupling g-C3N4 nanosheets with other compounds have been considered a prevalent method [16]. For example, Yu’s group reported on a silver chromate (Ag2CrO4) as a promising photosensitizer due to its narrow band gap (~1.8 eV) which might favour the light absorption of g-C3N4 [16e18]. The composites of g-C3N4 restrain the recombination of photogenerated carriers and improve the photocatalytic performance [19e21]. The construction of layered-heterostructures provides the possibility to overcome poor quantum efficiency of g-C3N4 for H2 generation. Belt and layered second semiconductor materials are good candidates for creating g-C3N4 based layered-heterostructures. Tungsten oxides (WOx) as a chemically stable and promising photoanode material reveals tuneable bandgaps (2e3 eV) and high electron mobility [22e25]. WOx has been used to prepare the composites with photocatalytic materials to improve the catalytic performance in visible region [25]. WOx with different compositions and microstructures has been studied for application as electrocatalysts for water oxidation and Li-ion batteries [26,27]. Since tungsten possess variable oxidation states (e.g. IV, V, and VI), sub-stoichiometric tungsten oxides exhibit broad absorption in visible and infrared regions because of the presence of oxygen vacancies [28,29]. For example, Paik et al. reported a direct
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photocatalytic H2 evolution from sub-stoichiometric tungsten oxide (WOx) nanowires with the use of sacrificial alcohol [30]. Single layered g-C3N4 coupled with CsxWO3 nanorods was also reported to show significant high photocatalytic activity for H2 evolution and rhodamine (RhB)/phenol degradation in full solar spectrum [31]. Herein, we report for the first time the synthesis of WOx nanobelts horizontally grown on ultrathin g-C3N4 nanosheets with controlled crystallinity with superior photocatalytic activity. The nanobelts were grown and homogeneous distributed on ultrathin g-C3N4 nanosheets. Amorphous and crystalline g-C3N4 nanosheets were created by controlling the thermal polycondensation reactions of precursors. The results indicate that the layered heterostructures consisted of WOx and amorphous g-C3N4 reveal enhanced photocatalytic performance in visible light region while WOx nanobelts formed on crystalline g-C3N4 layered heterostructures show superior H2 generation reaction as compared with that of WO3/g-C3N4 and WO2/WO3/g-C3N4. 2. Experimental section 2.1. Preparation of g-C3N4 and WOx/g-C3N4 layered heterostructures Melamine, WCl6, ascorbic acid (AA), rhodamine (RhB), and triethanol amine (78%), tert-butanol, AgNO3, para-benzoquinone, Nafion solution, ethanol, Na2SO4, and H2SO4 were purchased from Sigma Aldrich and Sinopharm Chemical Reagent Co. Ltd. Chemicals were analytical-grade and were used without further purification. Amorphous and crystalline g-C3N4 nanosheets were prepared using a similar procedure published in our recent paper [32]. Typically, crystalline g-C3N4 was prepared using a slow heat-treatment procedure at 750 C and amorphous g-C3N4 was prepared via a fast heat-treatment procedure at 650 C. The as-prepared g-C3N4 sample was dispersed and sonicated in water. Finally, the suspension was precipitated and washed with water and ethanol for several times. The growth of WOx nanobelts on g-C3N4 nanosheets was carried out via hydrothermal synthesis. Typically, 0.06 g of WCl6 was dissolved in ethanol with 0.6 g of g-C3N4 followed by sonication for 5 min. The molar ratio of WCl6 and AA was adjusted to study the effect of AA on the composition of WOx formed on g-C3N4 nanosheets. After further stirring for 20 min, a uniform suspension was obtained. The suspension was then heat-treated at 220 C for 12 h. The resulting sample was washed for several times using ethanol. The amount of WCl6 was adjusted to control the ratio of W/g-C3N4. Table 1 gives the preparation conditions and properties of samples used in this study. 2.2. Material characterization The transmission electron microscopy (TEM) images, high-angle
489
annular dark-field scanning TEM (HAADF-STEM) images, and corresponding element mapping of samples were recorded using a transmission electron microscope (FEI Titan G2 80-200 TEM/STEM with Chemi STEM Technology, FEI company, US). The UVevisible diffuse reflectance and absorption spectra of samples were obtained using a UVevisible spectrophotometer (U-4100, Hitachi). The X-ray diffraction (XRD) patterns of samples were taken on an Xray diffractometer (Bruker D8, Germany) with Cu Ka radiation source. The X-ray photoelectron spectroscopy (XPS) spectra of samples were recorded on Kratos Axis Ultra DLD spectrometer with a monochromatic AlKa (1486.6eV) X-rays operating at 150 W.
2.3. Photocatalytic and electrochemical characterization Photocatalytic H2 generation was measured using a connected system of Pyrex top-irradiation reaction vessel and a glass-closed gas circulation. 10 mg of sample and 100 ml of triethanolamine aqueous solution (triethanolamine:deionized water ¼ 1:9) were mixed followed by sonication (25 min) before light exposure. The system was evacuated to remove air before testing. With constant stirring of the sample solutions, the measurement was then done using a 300 W Xe lamp with a cutoff filter allowing l > 420 nm. Shimadzu GC-7920 gas chromatography (Argon as carrier gas) was used to measure the amount of H2 evolved. Photocatalytic degradation using RhB was carried out under visible light. RhB is one of the most commonly used dye for degradation studies. RhB solution (50 ml, 10 mg L/l) was used to mix with 30 mg of the samples followed by stirring in dark for 1 h in order to achieve equilibrium absorption state. Absorbance of RhB was then measured on a UVeVis spectrophotometer with 300 W Xe arc lamp and a UV-cut off filter allowing l 420 nm. Electrochemical performance was measured using CHI 660E electrochemical analyzer with a three-electrode system. Graphite rod and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. Catalyst ink (2 mg mL1) was prepared by mixing the as synthesized catalysts with a Nafion (5 wt%) mixture solution (Nafion:water:alcohol ¼ 0.025:4:1 in V/V), followed by pipetting 6 mL of the prepared catalysts ink onto a glassy carbon electrode with diameter of 3.0 mm. The catalyst loading was 0.17 mgcm2. Linear sweep voltammetry (LSV) curve was then measured in a 0.5 M H2SO4 solution at a scan rate of 5 mV s1. For the three-electrode system, with 0.1 mol/L of Na2SO4 aqueous solution acted as the electrolyte, a Pt plate was used as the counter electrode, Ag/AgCl electrode was used as the reference electrode, a thin film of the catalyst sample cast on the surface of indium tin oxide (ITO) glass was used as the working electrode. Electrochemical impedance spectroscopy (EIS) measurements were done using a 10 mV alternating current amplitude in the frequency ranges of 10 mHze100 kHz at open circuit potential. For the photocurrent response measurement, 300 W Xe lamps equipped with different wavelengths of filters was used as the light sources.
Table 1 Preparation conditions and properties of WOx/g-C3N4 composite samples. Samplea
g-C3N4
W/AA molar ratio
WOx
Preparation temperature for g-C3N4 ( C)
ACN-0 ACN-1 ACN-2 ACN-3 ACN-4 ACN-5 CCN-1 CCN-2
Amorphous Amorphous Amorphous Amorphous Amorphous Amorphous Crystalline Crystalline
1/0 1/1 1/2 1/10 2/1 10/1 1/1 1/2
WO3 W18O49 W18O49 W18O49 W18O49 WO2/WO3 W18O49 W18O49
650 650 650 650 650 650 750 750
a
ACN denotes amorphous g-C3N4 and CCN denotes crystalline g-C3N4.
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3. Results and discussion 3.1. Phase and microstructure of WOx/g-C3N4 layered heterostructures g-C3N4 nanosheets were obtained via a thermal polycondensation procedure of melamine at high temperatures. The crystallinity of g-C3N4 depends strongly on the distribution of heptazine units in the layered networks. Herein, crystalline g-C3N4 was synthesized by further thermal polymerization of bulk g-C3N4 prepared at a lower temperature with melamine as the raw material. Slow thermal polymerization process is crucial. The crystallinity of thin g-C3N4 nanosheets affects the band gap which determines the optic adsorption as discussed in the literature [32]. Fig. 1 shows the formation procedure of WOx/g-C3N4 layeredheterostructures. The WOx monomer was first deposited on the surface of g-C3N4 nanosheets followed by nucleation process. The nuclei grew horizontally along a specific crystal plane to form thin WOx nanobelts. The formation of WOx nanobelts from nucleation process has been confirmed by the SEM examinations of the samples with different reaction times of 1, 2, 3 and 5 h (Fig. S1, Supporting Information). With the increase of the reaction time, the nanobelt-shaped WOx started forming on the surface of g-C3N4, indicating the step-by-step growth process of WOx nanobelts on the surface of g-C3N4 as shown in Fig. 1. Fig. 2 shows the TEM images of CCN-1 and ACN-1 samples. The results indicate that WOx nanobelts were grown on ultrathin and layered g-C3N4 to form layered heterostructures. The WOx nanobelts with high degree of crystallinity were homogeneously distributed on crystalline g-C3N4 nanosheets (shown in Fig. 2bec). These nanobelts formed on crystalline g-C3N4 (CCN-1) appear to be more homogeneously distributed across the surface as compared with the ones on amorphous g-C3N4 (ACN-1). The interplanar distance between adjacent lattice planes of WOx nanobelts is 0.378 nm (Fig. 2c), coinciding well with the (010) lattice of W18O49, and that regular lattice fringes are observed in the crystalline g-C3N4 [33,34]. In contrast, the degree of crystallinity of WOx nanobelts decreases
when amorphous g-C3N4 was used. The lattice spacing measured is approximately 0.378 nm (Fig. 2f), indicating that the exposed crystal plane of the WOx nanobelts in ACN-1 is also (010) planes of W18O49. However, the lattice fringes are not well orientated, which may be related to the amorphous nature of g-C3N4 substrate. The nanobelts formed on amorphous g-C3N4 (ACN-1) are not homogeneously distributed and vary in a wide range of lengths as compared to that formed on crystalline g-C3N4 (CCN-1). This implies that the crystallinity of g-C3N4 affects the growth and morphology of the nanobelts [32]. Compared with amorphous surface, crystalline surface has a low energy which decreases nucleation and limits the growth of WOx nanobelts [35e37]. The slow growth process results in a WOx nanobelt with welldeveloped lattice fringes and homogeneous distribution, as shown in Fig. 2c. In contrast, surfaces of amorphous g-C3N4 have more defects, resulting in higher surface energy and increasing in WOx nucleation probabilities and growth rate. As shown in Fig. S2 (Supporting Information), nanobelts formed on amorphous gC3N4 (ACN-1) are relatively short in length and also appear to be aggregated as compared with that formed on crystalline g-C3N4 (CCN-1). The results indicate that the amorphous g-C3N4 favors a higher distribution of WOx nanobelts over crystalline g-C3N4. The growth of the nanobelts occurred on the edge of the nanosheets as well as within or between the nanosheets and the bended nanobelts were also observed on the edge of the nanosheets (Fig. S3, Supporting Information). The results indicate that the nanobelts and nanosheets may consist of few layers. Fig. 3 shows the XPS spectra (e.g. C1s, N1s, O1s, and W4f) of CCN-1 and ACN-1 samples. The peaks corresponding to CeC (sp2 bonded carbon), CeOH (epoxy/hydroxyls), OeC]O, CeN/C]O (carbonyls), and satellite C1s peak of the C1s spectra of samples are shown in Fig. 3a and e. The N1s spectra of samples in Fig. 3b and f shows the three peak positions corresponding to NeH (graphitic N), CeN]C (Pyridinic-N), N-(C)3 (Pyrrolic-N), and p excitations. Due to the difference of the C 1s spectra of the samples and the similarity of N 1s spectra, the C/N ratios of the samples are different. This is ascribed to the distillation of N element occurred at high
Fig. 1. Formation of WOx/g-C3N4 layered heterostructures.
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Fig. 2. TEM images of WOx/g-C3N4 samples with different magnification. (a, b, c) CCN-1 prepared using crystalline g-C3N4 with W/AA ratio of 1:1 and (d, e, f) ACN-1 prepared using amorphous g-C3N4 with W/AA ratio of 1:1.
temperatures [30]. The samples show very similar O1s, and W4f spectra in Fig. 3c, and d, 3g, and 3h, suggesting that the distribution state of W and O elements is similar. For sample CCN-1, peaks at 35.3 and 37.5 eV can be attributed to W4f7/2 and W4f5/2 of W6þ state, while the one at 34.0 and 36.3 eV corresponds to W5þ state (Fig. 3d) [34]. This indicates that the defects may endow W18O49 with specific performance. For the layered-heterostructures of W18O49 and amorphous g-C3N4 (ACN-1), the peak position corresponding to W6þ and W5þ state changed, suggesting the difference of bonding state of W18O49 on amorphous and crystalline g-C3N4. The elemental composition of the samples based on the XPS data are shown in Table S1 (Supporting Information). To confirm the formation of WOx, the HAADF-STEM images and corresponding element mapping of CCN-1 were obtained and the results are shown in Fig. 4. Similar result was also obtained for sample ACN-1 (Fig. S4, Supporting Information). The distribution of N and C is homogeneous and the distribution of W and O is in the area of W18O49 nanobelts. To study the formation of nanobelts, pure WOx samples were prepared without adding g-C3N4 under the same conditions. For the nanobelt formation and composition of WOx, AA plays a very important role. We found that WO3 nanoflowers were obtained in case of no AA addition and with the addition of AA, WO3 nanofibers instead of nano-flower were obtained (Fig. S5, Supporting Information). As a reducing agent, AA can be adsorbed on the surface of WOx crystals, working as a blocking agent to control growth rate and exposed crystal plane. Nevertheless, the exact reason for the formation of nanobeltshaped WOx on g-C3N4 is not clear at this stage. Since AA works as a reducing agent during reaction process, the composition of WOx strongly depends on the amount of AA. Fig. 5 shows the XRD patterns of composite samples prepared using different W/AA ratios on amorphous g-C3N4. Two XRD peaks
corresponding to the (100) and (002) facets of g-C3N4 were observed for all samples. The weak peak at 12.8 is caused by the periodic arrangement of the triazine in g-C3N4 and the higher intensity peak at 27.9 is related to the stacking of the conjugated aromatic system of carbon nitride [32]. Using a W/AA molar ratio of 10/1, a mixed phase of WO2 and WO3 was obtained (sample ACN-5). In the case of W6þ/AA ratio of 1:1, orthorhombic WO2 and hexagonal WO3 phases were obtained, whereas only hexagonal WO3 phase was obtained in the absence of AA in the reactor (Fig. S6, Supporting Information). When W6þ:AA ¼ 2:1, a strong diffraction peak at 23.08 , a characteristic peak of W18O49, appears (dotted line in Fig. 5). This indicates the formation of the highly crystalline phase of WOx. With the increase of AA content, e.g., W6þ:AA ratio ¼ 1:10, excess AA could adsorb on the surface of WOx crystalline phase and block the further growth and deposition of the WOx nanobelts. This appears to be supported by a decrease of the intensity of the diffraction peak at 23.08 with the increase of the AA content in the reaction. 3.2. Photocatalytic properties of WOx/g-C3N4 layered heterostructures Fig. 6 shows the UVeVis diffuse reflectance spectra and plots of (ahn)1/2 vs photon energy of WOx heterostructured samples with amorphous g-C3N4. The band gap of the samples was estimated by using the transformed Kubelka-Munk function as follow according to DRS plots [35e37]. 2
Aðhv EgÞn ¼ ahv
(1)
The band gaps of samples ACN-1, ACN-2, ACN-4, and ACN-5 were estimated to be 2.86, 2.82, 2.88, and 2.89 eV, respectively, which is lower than 2.90 eV measured on amorphous g-C3N4
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Fig. 3. C1s, N1s, O1s, W4f XPS spectra of (a, b, c, d) sample CCN-1 and (e, f, g, h) sample ACN-1.
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Fig. 4. HAADF-STEM image and N, C, W, and O mapping of sample CCN-1.
Fig. 5. XRD patterns of WOx/g-C3N4 composite samples using different amount of AA and amorphous g-C3N4.
(Fig. 6d). The decreased band gap confirms the formation of W18O49/g-C3N4 layered-heterostructure. As shown in Fig. 6a and b, the crystalline g-C3N4 has an advantage in the initial band gap energy and the band gap is 2.70 eV, lower than that of the amorphous g-C3N4. The band gap energy of the amorphous g-C3N4 is improved to a greater extent after the formation and deposition of WOx nanobelts with the absorption band edge closer to the blue region. Among the samples studied, ACN-1 and ACN-2 have
relatively low bandgap energy, indicating the absorption over a wider range. The photocurrent responses of samples under visible light irradiation were measured and the results are shown in Fig. 7. The photocurrent response value of amorphous g-C3N4 is very low. However, the formation of layered-heterostructures remarkably enhances the photocurrent response value. Especially, the photocurrent response value of sample ACN-1 exhibited the highest intensity compared with other samples. As the transient photocurrent is related to the conductivity of the materials, the higher photocurrent indicates the better conductivity of the material. EIS measurement is another effective way to show the charge separation and transfer efficiencies. Impedance plots of ACN-1 and ACN-2 electrodes with or without light irradiation were investigated (see Fig. 8a). Regardless of the presence or absence of irradiation, the slope of the EIS curve of ACN-1 is smaller than that of ACN-2, indicating that ACN-1 has a higher ion transport rate. With no light irradiation, sample ACN-1 shows a smaller impedance arc radius than that of sample ACN-2, indicating that sample ACN-1 has high efficiency of charge transfer. This may indicate that highly crystalline W18O49 phase improves the electrical conductivity of the layered heterostructures. In addition, the separation and transfer efficiency of photogenerated electron and hole pairs increased dramatically with light irradiation for ACN-1 and ACN-2 samples due to strong absorption of W18O49 in visible region. 3.3. Photocatalytic activities of WOx/g-C3N4 layered heterostructures The photocatalytic activity of amorphous WOx/g-C3N4 layered
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Fig. 6. (a) UV-VIS diffuse reflectance spectra and (b) plots of (ahn)1/2 vs photon energy of amorphous g-C3N4, ACN-1, crystalline g-C3N4 and CCN-1; (c) UV-VIS diffuse reflectance spectra and (d) plots of (ahn)1/2 vs photon energy of amorphous g-C3N4, ACN-1, ACN-2, ACN-4 and ACN-5.
Fig. 7. Photocurrent-time (i-t) curves of amorphous g-C3N4, ACN-1, ACN-2 and ACN-5 layered heterostructures under visible light irradiation.
heterostructure samples was examined on the photocatalytic degradation of RhB in aqueous solutions under visible light irradiation and the results are shown in Fig. 9. As shown in Fig. 9a, the photocatalytic performance of crystalline g-C3N4 is better than that of amorphous g-C3N4, however, ACN-1 shows a better photodegradation efficiency as compared to CCN-1. This indicates that the distribution and morphology of WOx nanobelts deposited on g-
C3N4 may have significant effect on the photocatalytic activity of layered heterostructures. WOx nanobelts on crystalline g-C3N4 (CCN-1) are longer on average (Fig. 2). On the other hand, WOx nanobelts on the surface of amorphous g-C3N4 (ACN-1) varied in a wide size rage (with variety of lengths), which could improve the light absorption capacity and increase the exposure of active site of the sample to a considerable extent. The degradation performance of pure WO3 modified C3N4 sample, prepared with no AA addition (ACN-0) is shown in Fig. 9c for comparison. Both g-C3N4 and W18O49 are typical photocatalytic materials under visible light [34,38,39]. As shown, less than 50% of RhB was degraded after 30 min reaction using ACN-0. This result confirms that the composition of WOx play an important role for enhancing the photocatalytic performance of amorphous g-C3N4. Among all samples, ACN-1 (W/AA molar ratio of 1/1) shows the best photocatalytic activity (Fig. 9c). However, as the W/AA ratio continuously decreases, the photocatalytic efficiency of the sample gradually decreases. To further demonstrate photocatalytic reaction kinetics of the samples, the photo degradation plots can be fitted using a pseudo first order reaction as follows [34,38,39].
lnðC = C0 Þ ¼ Kt
(2)
where C0 and Ct are initial and reaction concentrations with time and t is the irradiation time. When pseudo-first-order kinetics is used to simulate the photocatalytic process of WOx/g-C3N4 samples, the RhB decomposition tendency of the samples studied can be divided into two stages (Fig. 9b). The degradation rate constant
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Fig. 8. EIS curves of ACN-1 and ACN-2 with or without irradiation. Curves shown in (b) are the enlarged section of (a) in high frequency region.
Fig. 9. (a) Photocatalytic degradation performances and (b) degradation kinetic models of pure g-C3N4 (amorphous and crystalline), ACN-1 and CCN-1 for RhB degradation under visible light (420 nm) irradiation; (c) Photocatalytic degradation performances of ACN-0, ACN-1, ACN-2, ACN-3, ACN-4, ACN-5 and g-C3N4 (amorphous) and (d) degradation kinetic models of ACN-0, ACN-2, ACN-3, ACN-4 and ACN-5 for RhB under visible light (420 nm) irradiation.
(K1) of the sample generally has a large value within the first 10 min. At this stage, the adsorbed amount on the sample surface reaches saturation, and the reaction is mainly determined by the concentration of RhB. The degradation rate constant (K2) of samples has a relatively smaller value within the 10e30 min range. The
catalytic reaction at this stage is mainly determined by the dispersion of the sample in aqueous solution and the density of exposed surface active sites [40]. The degradation rate constants K1, K2 and R2 values of amorphous g-C3N4, ACN-1, crystalline g-C3N4 and CCN-1 are shown in Table 2 and those of ACN-0, ACN-2, ACN-3,
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Table 2 Degradation rate constants and R2 values of amorphous g-C3N4, ACN-1, crystalline gC3N4 and CCN-1. Sample
K1
R21
K2
R22
Amorphous g-C3N4 ACN-1 Crystalline g-C3N4 CCN-1
3.05 24.93 12.11 13.57
0.97 0.99 0.99 0.99
4.05 1.86 6.65 8.26
0.95 0.94 0.99 0.99
ACN-4 and ACN-5 are shown in Table S2 (Supporting Information). The R2 values of all the samples are higher than 0.9, demonstrating that the photocatalytic reaction kinetics of these samples follows the pseudo first-order kinetic model. As shown in Fig. 9d, ACN-1 shows the best photocatalytic activity. In the first 10 min, the reaction rate constant of ACN-1 is 8 times higher than that of amorphous g-C3N4 and twice higher as that of crystalline g-C3N4. This demonstrates that the formation of W18O49/g-C3N4 layeredheterostructures enhance the separation and transfer efficiency of photogenerated electron and hole pairs, thus increasing the photocatalytic efficiency. To further understand the photocatalytic reaction mechanism, cyclic stability test and active group capture experiment were carried out. As shown in Fig. 10a, sample ACN-1 exhibites stable photodegradation efficiency during the four cycles of the test. The degradation rate of RhB of ACN-1 remained the same. In addition, the SEM and XRD analysis of sample ACN-1 indicate that there is no visible change in the morphology and phase composition of the sample before and after the photodegradation tests (Fig. S7, Supporting Information). In order to identify the free radicals in the photodegradation reaction, tertiary butanol, AgNO3, triethanol amine, para-benzoquinone, were used as free radical capture agents for hydroxyl radicals ($OH), electronic (e), hole (hþ), and superoxide free radical (O 2 ), respectively [8,34] (Fig. 10b). The presence of AgNO3 and para-benzoquinone have no significant effect for the photocatalytic reaction, indicating that electronic (e) and superoxide free radical (O 2 ) are not involved in the photodegradation reaction of RhB on ACN-1. However, a significant decrease in photocatalytic performance was observed when triethanol amine and tert-butanol were added to the solution. This indicates the presence of free radicals of O 2 and $OH (or $H2O) formed by the reaction between the photogenerated e and hþ and the O2 and H2O molecules in the solution. These free radicals are strong oxidizing agent and react with RhB molecules close to or on the surface of ACN-1 photocatalyst. The photogenerated hþ plays a
leading role in the photocatalytic reaction in which WOx/g-C3N4 is involved. The efficiency of photodegradation of the current study is also compared with some of the most recently reported results in similar systems (see Table 3) [18,41e43]. It shows that the WOx/gC3N4 layered heterostructure prepared in this work show comparable and better catalytic efficiency and degradation efficiency, as compared with the similar tungsten oxide modified C3N4 photocatalysts reported in the literature. Hydrogen generation under visible light (420 nm) of the WOx/ g-C3N4 layered heterostructure samples were also measured and the results are shown in Fig. 11. A significant increasing trend in hydrogen evolution was observed for all samples. The best results were obtained on crystalline W18O49/g-C3N4 layered heterostructure (sample CCN-1) with the highest hydrogen generation activity among the four samples including crystalline g-C3N4, amorphous g-C3N4 and ACN-1 sample (Fig. 11a). The amount of H2 generated increased linearly with time. Average rate constants for H2 evolution for amorphous g-C3N4, ACN-1, crystalline g-C3N4 and CCN-1 under visible light (l > 420 nm) were obtained. As shown in Fig. 11a, the H2 evolution rate of CCN-1 is twice as higher as the gC3N4, indicating excellent efficiency of CCN-1 for photocatalytic H2 evolution. To evaluate the stability of CCN-1, cyclic experiments on hydrogen production were carried out. After four cycles, the hydrogen generation efficiency of sample CCN-1 remained almost unchanged (Fig. 11b). In fact, the hydrogen production of the CCN-1 increased slightly and after five cycles, the amount of hydrogen produced remained basically stable (Fig. S8, Supporting Information). This indicates the good conductivity of CCN-1 sample with good stability due to high crystallinity. The charge transfer process in W18O49/g-C3N4 heterostructures during H2 evolution reaction is illustrated in Fig. 12. According to the experiment, no hydrogen was produced when pure WOx was used as a photocatalyst. This is ascribed to the band gap of WOx nanobelts and its conduction band (CB) potential is more positive than the Hþ/H2 redox potential. After combining with crystalline gC3N4, the photocatalytic hydrogen production performance was significantly enhanced. Under the irradiation of visible light, W18O49 and g-C3N4 components are simultaneously excited. CCN-1 revealed enhanced hydrogen production activity. This is due to the fact that crystalline g-C3N4 with less structure defects causes slow growth of WOx nanobelts and thus leads to formation of high quality heterostructure with high carrier transfer efficiency. The high crystallinity and less internal defects may also promote electron migration at the interface in the H2 reduction reaction.
Fig. 10. (a) Cyclic stability test of photocatalytic degradation of RhB by ACN-1; (b) Photocatalytic performances with the addition of various free radical capture agents.
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Table 3 The experimental data of this paper in comparison with the recently published literature. References
Degradation time (min)
Degradation efficiency (%)
Composition
This work Xu et al., 2015 [18] Yao et al., 2019 [41] Jia et al., 2019 [42] Singh et al., 2019 [43]
30 90 60 30þ 160
100% 99% 100% 90% 97%
W18O49/g-C3N4 Ag2WO4/g-C3N4 WO3/g-C3N4 g-C3N4/WO3/NQDs WO3/g-C3N4
Fig. 11. (a) Photocatalytic H2 production performances of amorphous g-C3N4 (I), ACN-1 (II), crystalline g-C3N4 (III) and CCN-1 (IV). The inset shows average H2 evolution rate and (b) Photocatalytic H2 production cyclic test of CCN-1 under visible light (420 nm).
g-C3N4 nanosheets was much more homogeneous than those on amorphous g-C3N4 nanosheets. The layered-heterojunction of W18O49/g-C3N4 revealed enhanced electrochemical performance compared with WO3/g-C3N4 and WO2/WO3/g-C3N4 samples. In addition, the layered-heterostructures consisted of W18O49 and amorphous g-C3N4 revealed good photocatalytic performance (about 4 times higher comparing with g-C3N4), while the layerheterostructures of W18O49 and crystalline g-C3N4 revealed superior H2 evolution reaction under visible light. Acknowledgments This work was supported by the Australian Research Council under the Discovery Scheme (Project No. DP180100731, DP180100568). Appendix A. Supplementary data
Fig. 12. Illustration of charge transfer in W18O49/g-C3N4 heterostructures for H2 evolution reaction.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.09.083. References
4. Conclusions WOx nanobelts horizontally grown on ultrathin g-C3N4 nanosheets to create layered-heterojunctions have been synthesized for the first time, showing enhanced the photocatalytic performance. WOx nanobelts were grown along a specific crystal plane on amorphous and crystalline g-C3N4 nanosheets via a hydrothermal synthesis method using AA to control the morphology and composition of WOx. The composition of WOx (WO3, WO2, W18O49) strongly depended on the preparation conditions such as the molar ratio of W/AA. The distribution of W18O49 nanobelts on crystalline
[1] Y. Liu, J. Wang, P. Yang, Photochemical reactions of g-C3N4-based heterostructured composites in Rhodamine B degradation under visible light, RSC Adv. 6 (2016) 34334e34341. [2] K.S. Lakhi, D.-H. Park, K. Al-Bahily, W. Cha, B. Viswanathan, J.-H. Choy, A. Vinu, Mesoporous carbon nitrides: synthesis, functionalization, and applications, Chem. Soc. Rev. 46 (2017) 72e101. [3] Z. Chen, X. Yu, Q. Zhu, T. Fan, Q. Wu, L. Zhang, J. Li, W. Fang, X. Yi, Steam engraving optimization of graphitic carbon nitride with enhanced photocatalytic hydrogen evolution, Carbon 139 (2018) 189e194. [4] X. Yu, T. Fan, W. Chen, Z. Chen, Y. Dong, H. Fan, W. Fang, X. Yi, Self-hybridized coralloid graphitic carbon nitride deriving from deep eutectic solvent as effective visible light photocatalysts, Carbon 144 (2019) 649e658. [5] Y. Zhang, T. Mori, L. Niu, J. Ye, Non-covalent doping of graphitic carbon nitride polymer with graphene: controlled electronic structure and enhanced optoelectronic conversion, Energy Environ. Sci. 4 (2011) 4517e4521.
498
X. Zhang et al. / Carbon 156 (2020) 488e498
[6] X. Ma, Y. Lv, J. Xu, Y. Liu, R. Zhang, Y. Zhu, A strategy of enhancing the photoactivity of g-C3N4 via doping of nonmetal elements: a first-principles study, J. Phys. Chem. C 116 (2012) 23485e23493. [7] Y. Hong, C. Li, D. Li, Z. Fang, B. Luo, X. Yan, H. Shen, B. Mao, W. Shi, Precisely tunable thickness of graphitic carbon nitride nanosheets for visible-lightdriven photocatalytic hydrogen evolution, Nanoscale 9 (2017) 14103e14110. [8] Z. Liu, G. Wang, H.-S. Chen, P. Yang, An amorphous/crystalline g-C3N4 homojunction for visible light photocatalysis reactions with superior activity, Chem. Commun. 54 (2018) 4720e4723. [9] L. Shi, Z. Li, K. Marcus, G. Wang, K. Liang, W. Niu, Y. Yang, Integration of Au nanoparticles with a g-C3N4 based heterostructure: switching charge transfer from type-II to Z-scheme for enhanced visible light photocatalysis, Chem. Commun. 54 (2018) 3747e3750. [10] X. Bai, L. Wang, R. Zong, Y. Zhu, Photocatalytic activity enhanced via g-C3N4 nanoplates to nanorods, J. Phys. Chem. C 117 (2013) 9952e9961. [11] M. Tahir, C. Cao, N. Mahmood, F.K. Butt, A. Mahmood, F. Idrees, S. Hussain, M. Tanveer, Z. Ali, I. Aslam, Multifunctional g-C3N4 nanofibers: a templatefree fabrication and enhanced optical, electrochemical, and photocatalyst properties, ACS Appl. Mater. Interfaces 6 (2014) 1258e1265. [12] Y. Liu, X. Zhang, J. Wang, P. Yang, In situ growth of sulfide/g-C3N4 nanoheterostructures with an adjusted band gap toward enhanced visible photocatalysis, Phys. Chem. Chem. Phys. 18 (2016) 31513e31520. [13] Y. Liu, X. Zhang, J. Wang, P. Yang, Preparation of luminescent graphitic C3N4 NS and their composites with RGO for property controlling, RSC Adv. 6 (2016) 112581e112588. [14] B. Zhu, J. Zhang, C. Jiang, B. Cheng, J. Yu, First principle investigation of halogen-doped monolayer g-C3N4 photocatalyst, Appl. Catal. B Environ. 207 (2017) 27e34. [15] D. Huang, Z. Li, G. Zeng, C. Zhou, W. Xue, X. Gong, X. Yan, S. Chen, W. Wang, M. Cheng, Megamerger in photocatalytic field: 2D g-C3N4 nanosheets serve as support of 0D nanomaterials for improving photocatalytic performance, Appl. Catal. B Environ. 240 (2019) 153e173. [16] D. Xu, B. Cheng, W. Wang, C. Jiang, J. Yu, Ag2CrO4/g-C3N4/graphene oxide ternary nanocomposite Z-scheme photocatalyst with enhanced CO2 reduction activity, Appl. Catal. B Environ. 231 (2018) 368e380. [17] D. Xu, B. Cheng, S. Cao, J. Yu, Enhanced photocatalytic activity and stability of Z-scheme Ag2CrO4-GO composite photocatalysts for organic pollutant degradation, Appl. Catal. B Environ. 164 (2015) 380e388. [18] D. Xu, B. Cheng, J. Zhang, W. Wang, J. Yu, W. Ho, Photocatalytic activity of Ag2MO4 (M ¼ Cr, Mo, W) photocatalysts, J. Mater. Chem. 3 (2015) 20153e20166. [19] Y. He, L. Zhang, B. Teng, M. Fan, New application of Z-scheme Ag3PO4/g-C3N4 composite in converting CO2 to fuel, Environ. Sci. Technol. 49 (2015) 649e656. [20] W. Yu, D. Xu, T. Peng, Enhanced photocatalytic activity of g-C3N4 for selective CO2 reduction to CH3OH via facile coupling of ZnO: a direct Z-scheme mechanism, J. Mater. Chem. 3 (2015) 19936e19947. [21] B. Zhu, P. Xia, Y. Li, W. Ho, J. Yu, Fabrication and photocatalytic activity enhanced mechanism of direct Z-scheme g-C3N4/Ag2WO4 photocatalyst, Appl. Surf. Sci. 391 (2017) 175e183. come, M. Morales, C. Angeles Chavez, L.F. Ramírez-Verduzco, [22] M.A. Cortes-Ja pez-Salinas, J.A. Toledo-Antonio, WOx/TiO2 catalysts via titania nanoE. Lo tubes for the oxidation of dibenzothiophene, Chem. Mater. 19 (2007) 6605e6614. [23] F. Wang, C. Di Valentin, G. Pacchioni, Rational band gap engineering of WO3 photocatalyst for visible light water splitting, ChemCatChem 4 (2012) 476e478. [24] K. Wang, Y. Shi, Q. Dong, Y. Li, S. Wang, X. Yu, M. Wu, T. Ma, Low-temperature and solution-processed amorphous WOX as electron-selective layer for perovskite solar cells, J. Phys. Chem. Lett. 6 (2015) 755e759. [25] X.-H. Jiang, Q.-J. Xing, X.-B. Luo, F. Li, J.-P. Zou, S.-S. Liu, X. Li, X.-K. Wang, Simultaneous photoreduction of Uranium(VI) and photooxidation of Arsenic(III) in aqueous solution over g-C3N4/TiO2 heterostructured catalysts under
simulated sunlight irradiation, Appl. Catal. B Environ. 228 (2018) 29e38. [26] X. Shi, L. Cai, I.Y. Choi, M. Ma, K. Zhang, J. Zhao, J.K. Kim, J.K. Kim, X. Zheng, J.H. Park, Epitaxial growth of WO3 nanoneedles achieved using a facile flame surface treatment process engineering of hole transport and water oxidation reactivity, J. Mater. Chem. 6 (2018) 19542e19546. [27] C. Wang, Y. Zhao, L. Zhou, Y. Liu, W. Zhang, Z. Zhao, W.N. Hozzein, H.M.S. Alharbi, W. Li, D. Zhao, Mesoporous carbon matrix confinement synthesis of ultrasmall WO3 nanocrystals for lithium ion batteries, J. Mater. Chem. 6 (2018) 21550e21557. [28] Q. Mi, A. Zhanaidarova, B.S. Brunschwig, H.B. Gray, N.S. Lewis, A quantitative assessment of the competition between water and anion oxidation at WO3 photoanodes in acidic aqueous electrolytes, Energy Environ. Sci. 5 (2012) 5694e5700. [29] X. Feng, Y. Chen, Z. Qin, M. Wang, L. Guo, Facile fabrication of sandwich structured WO3 nanoplate Arrays for efficient photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 8 (2016) 18089e18096. [30] T. Paik, M. Cargnello, T.R. Gordon, S. Zhang, H. Yun, J.D. Lee, H.Y. Woo, S.J. Oh, C.R. Kagan, P. Fornasiero, C.B. Murray, Photocatalytic hydrogen evolution from substoichiometric colloidal WO3ex nanowires, ACS Energy Letters 3 (2018) 1904e1910. [31] A. Shi, H. Li, S. Yin, J. Zhang, Y. Wang, H2 Evolution over g-C3N4/CsxWO3 under NIR light, Appl. Catal. B Environ. 228 (2018) 75e86. [32] X. Zhang, J.-P. Veder, S. He, S.P. Jiang, Construction of 2D g-C3N4 lateral-like homostructures and their photo- and electro-catalytic activities, Chem. Commun. 55 (2019) 1233e1236. [33] K. Deng, Z. Hou, X. Deng, P. Yang, C. Li, J. Lin, Enhanced antitumor efficacy by 808 nm laser-induced synergistic photothermal and photodynamic therapy based on a indocyanine-green-attached W18O49 nanostructure, Adv. Funct. Mater. 25 (2015) 7280e7290. [34] C. Jia, X. Zhang, K. Matras-Postolek, B. Huang, P. Yang, Z-scheme reduced graphene oxide/TiO2-Bronze/W18O49 ternary heterostructure towards efficient full solar-spectrum photocatalysis, Carbon 139 (2018) 415e426. [35] Q. Han, C. Hu, F. Zhao, Z. Zhang, N. Chen, L. Qu, One-step preparation of iodinedoped graphitic carbon nitride nanosheets as efficient photocatalysts for visible light water splitting, J. Mater. Chem. 3 (2015) 4612e4619. [36] R. Ye, H. Fang, Y.-Z. Zheng, N. Li, Y. Wang, X. Tao, Fabrication of CoTiO3/gC3N4 hybrid photocatalysts with enhanced H2 evolution: Z-scheme photocatalytic mechanism insight, ACS Appl. Mater. Interfaces 8 (2016) 13879e13889. [37] G. Zhang, A. Savateev, Y. Zhao, L. Li, M. Antonietti, Advancing the n / p* electron transition of carbon nitride nanotubes for H2 photosynthesis, J. Mater. Chem. 5 (2017) 12723e12728. [38] X. Zhang, C. Jia, Y. Xue, P. Yang, Fabrication of rGO/g-C3N4 composites via electrostatic assembly towards charge separation control, RSC Adv. 7 (2017) 43888e43893. [39] Z. Jiang, X. Zhang, J. Wang, L. Chen, H.-S. Chen, P. Yang, Ultrastable g-C3N4 assemblies with high quantum yield and reversible photoluminescence, Chem. Commun. 54 (2018) 13519e13522. [40] H. Hu, J. Liu, Z. Xu, L. Zhang, B. Cheng, W. Ho, Hierarchical porous Ni/Co-LDH hollow dodecahedron with excellent adsorption property for Congo red and Cr(VI) ions, Appl. Surf. Sci. 478 (2019) 981e990. [41] J. Yao, M. Zhang, H. Yin, Y. Luo, X. Liu, Improved photocatalytic activity of WO3/C3N4: by constructing an anchoring morphology with a Z-scheme band structure, Solid State Sci. 95 (2019), 105926. [42] J. Jia, C. Jiang, X. Zhang, P. Li, J. Xiong, Z. Zhang, T. Wu, Y. Wang, Urea-modified carbon quantum dots as electron mediator decorated g-C3N4/WO3 with enhanced visible-light photocatalytic activity and mechanism insight, Appl. Surf. Sci. 495 (2019), 143524. [43] J. Singh, A. Arora, S. Basu, Synthesis of coral like WO3/g-C3N4 nanocomposites for the removal of hazardous dyes under visible light, J. Alloy. Comp. 808 (2019), 151734.