Combining Heterojunction Engineering with Surface Cocatalyst

Aug 13, 2017 - The fluorescence intensity and optical properties were tested on a fluorescence spectrophotometer (Hitachi, F-7000) and UV–vis spectr...
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Research Article pubs.acs.org/journal/ascecg

Combining Heterojunction Engineering with Surface Cocatalyst Modification To Synergistically Enhance the Photocatalytic Hydrogen Evolution Performance of Cadmium Sulfide Nanorods Peifang Wang, Tengfei Wu, Chao Wang, Jun Hou, Jin Qian, and Yanhui Ao* Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, No. 1 Xikang road, Nanjing 210098, China S Supporting Information *

ABSTRACT: Photocatalytic decomposition of water to hydrogen is an energy conversion process just like photosynthesis. Herein, for the first time, CoP-modified CdS/g-C3N4 composite nanorods were synthesized on the basis of the concept of combining heterojunction engineering with cocatalyst modification. The obtained CoP-CdS/g-C3N4 composites exhibit excellent photocatalytic activity and good photostability when applied as a photocatalyst for water reduction. The H2 production rate reaches up to 23 536 μmol g−1 h−1, which was about 14 times higher than that of pure CdS. Furthermore, the stability of the composite was obviously improved. The outstanding performance of the CoP-CdS/gC3N4 composites can be attributed to the following reasons: (1) Intimate contact between CdS and g-C3N4 can effectively promote the electron−hole pair spacial separation. (2) The introduction of CoP as cocatalyst on the CdS/g-C3N4 nanorods can further extract photogenerated electrons from CdS/g-C3N4 and lower the overpotential of H+ reduction. KEYWORDS: Water splitting, Visible light, Noble-metal-free cocatalyst, CdS, g-C3N4

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INTRODUCTION Since Honda and Fujishima discovered the phenomenon that water can decompose to generate hydrogen by titanium dioxide, harnessing the energy of the sun and its storage as chemical bonds in molecular hydrogen from water are considered the most promising approach to provide clean and renewable energy.1−6 Various kinds of semiconductorbased photocatalysts have been designed for efficient hydrogen generation such as sulfides and nitrides, metal oxides, perovskites, etc.7−11 Cadmium sulfide attracts more and more attention because of the small bandgap at around 2.4 eV and suitable band edge position.12−15 However, the photocatalytic activity of pure CdS is relative low, which restricts its practical application. Furthermore, CdS is unstable because of its photocorrosion induced by photogenerated holes. To overcome these points, enormous efforts have been made to improve the hydrogen yield and stability of CdS.16,17 In that respect, coupling with other semiconductors possessing wellpaired band structures is an efficient and facile strategy to enhance the photocatalytic activity of CdS. In 2009, Wang’s group reported that graphitic carbon nitride (g-C3N4), a cheap and nontoxic metal-free polymeric-like carbon nitride, has the ability to split water into hydrogen under visible light.18 The g-C3N4 was regarded as one of the best candidates for converting solar energy to chemical energy, © 2017 American Chemical Society

because of its accessible, nontoxic, highly stable, and excellent optical properties.19−21 The attempt to use g-C3N4 to modify other semiconductors can successfully improve their hydrogen yield.22−27 Recently, Zhang et al. reported that the synergic effect between g-C3N4 and CdS can effectively accelerate the charge separation and transfer corrosive holes from CdS to robust C3N4.28 Such a strategy is positive and it improves the hydrogen production rate compared with the corresponding individual component because of the charge separation and transfer holes from CdS to g-C3N4. However, the improvement is far from enough for practical applications. For a further acceleration of the photogenerated charge separation to achieve better hydrogen yield, the introduction of a cocatalyst to the photocatalytic system has been proven to be an efficient and facile strategy. Recently, transition-metal-based materials (e.g., CoP, Ni2P, MoSx, and Ni(OH)2) have been intensively studied because they not only have similar zerovalent metal features, but also have high abundance and low cost.29−33 Among the transition-metal-based materials, CoP exhibits an outstanding efficiency and good stability in the electrocatalytic hydrogen evolution process. For example, Cao Received: April 6, 2017 Revised: July 11, 2017 Published: August 13, 2017 7670

DOI: 10.1021/acssuschemeng.7b01043 ACS Sustainable Chem. Eng. 2017, 5, 7670−7677

Research Article

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Preparation of CoP-CdS/g-C3N4 Composites. A calculated amount of CoP was first dispersed in 30 mL of ethanol. After being ultrasonically treated for 30 min, 0.2 g of CdS/g-C3N4 was added to the above suspension and continued stirring for 20 min. Before the products were dried in a vacuum oven, the rotary evaporator was used to remove most of the ethanol. We used the same method to obtain the 1%, 3%, 5%, and 8% CoP-CdS/g-C3N4 composites. A similar synthesis procedure was applied for the synthesis of 5% CoP-CdS and 5% CoP-g-C3N4 with as-prepared precursor materials, separately. Characterizations. Transmission electron microscopy (TEM, JEOL, JEM-2100) was used for imaging. The crystalline structure of the as-prepared hybrids was examined by power X-ray diffraction (XRD) with a Rigaku/Smartlab diffractometer using Cu Kα radiation at 40 kV and 100 mA in the 2θ range 10−80° at a scanning rate of 20° min−1. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific ESCALAB 250 instrument with Al Kα source. The fluorescence intensity and optical properties were tested on a fluorescence spectrophotometer (Hitachi, F-7000) and UV−vis spectrophotometer (Shimadzu, UV-3600). Photoluminescence (PL) spectra were obtained using a Hitachi F-700 fluorescence spectrophotometer. The photocurrents were measured on an electrochemical analyzer (CHI660D) in a standard three-electrode system with the reference electrode of Ag/AgCl, the Pt plate working as the counter electrode, and the working electrode of corresponding sample films on FTO (fluorine-doped tin oxide). A 300 W Xe lamp provided the simulated visible light source. A 0.1 M Na2SO4 solution was used as the electrolyte. Photocatalytic Hydrogen Production. The hydrogen production experiments were carried out in a gas-closed system with 5 mg of photocatalyst power suspended in a 50 mL solution containing 0.35 M sodium sulfide and 0.25 M sodium sulfite. After the system was evacuated by vacuum pump, a 300 W Xe lamp with a cutoff filter (400 nm) shines on the photocatalyst to trigger the photocatalytic reaction. The evolved hydrogen was determined online with a gas chromatograph (GC-7900, TCD, N2 as a carrier gas) after 1 h of illumination, and the reaction lasted for 5 h. The apparent quantum efficiency (QE) was measured under the same photocatalytic reaction condition; here, 300 W Xe lamps with a 420, 450, 500, and 550 nm band-pass filter were used as light sources. The output intensity was measured using FLA4000 spectrometer firmware. The liquid level is ∼12 cm away from the lamp, and the illuminated area is 19.62 cm2. The QE was measured through the equation below:39,40

and co-workers discovered that the CoP can act as an efficient cocatalyst and greatly improve the rate of hydrogen production of CdS.34 Yi et al. also reported that the optimal loading amount of CoP on metal-free g-C3N4 can promote the separation of photogenerated electron−hole pairs and eventually achieve a high water-splitting property.27 Herein, we present a highly efficient photocatalytic system in which the CoP cocatalyst and g-C3N4 are proposed to modify CdS nanorods. The hydrogen production rate of the CoP-CdS/ g-C3N4 composite can reach up to 23 536 μmol g−1 h−1, which is 14 times higher than that of CdS. In addition, the composite exhibits high stability during 20 h of irradiation. The high photocatalytic activity and stability can be ascribed to the highly efficient separation of electron−hole pairs because of the formation of a heterojunction. In this system, the g-C3N4 wrapped on the CdS nanorod can accept holes generated from CdS. Furthermore, the CoP served as cocatalyst which can lower the overpotential of H+ reduction and further extract photogenerated electrons from CdS/g-C3N4.

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EXPERIMENTAL SECTION

Chemicals. Cadmium chloride (CdCl 2 ·2.5H 2 O), thiourea (NH2CSNH2), ethylenediamine, barbituric acid, dicyandiamide, methanol, sodium citrate, cobalt nitrate hexahydrate [Co(NO3)2· 6H 2 O], sodium hydroxide (NaOH), sodium hypophosphite (NaH2PO2·H2O), sodium sulfide (Na2S), sodium sulfite (Na2SO3), and ethanol were all agent-grade and used without further purification. Preparation of CdS. The CdS nanorods were prepared as the previous paper reported.35 Typically, 9.26 g of CdCl2·2.5H2O and 9.26 g of NH2CSNH2 were added into a 200 mL Teflon-lined autoclave containing 120 mL of ethylenediamine. Then, the autoclave was heated to 160 °C and maintained for 36 h. The products were collected and washed several times with deionized water and ethanol, separately, after the autoclave was cooled to room temperature. Preparation of g-C3N4. The g-C3N4 was prepared by a copolymerization method as reported previously.36 A 4 g portion of dicyandiamide and 0.2 g of barbituric acid were mixed in distilled water (20 mL) and stirred 15 min to form a homogeneous suspension. Then, the suspension was heated at 90 °C with stirring until all of the water was evaporated. Then, the white solid was transferred to the crucible and heated at 550 °C for 4 h under air. Then, the obtained yellowish bulk was ground and collected. Preparation of CdS/g-C3N4. The CdS/g-C3N4 composite nanorods were fabricated through a chemisorption and self-assembly method.28 An accurately weighed 0.06 g portion of g-C3N4 was mixed with 20 mL of methanol. Then, the above suspension was ultrasonicated for 1 h to reach a homogeneous suspension. A 1.5 g portion of CdS was then added in the suspension and continuously stirred for 24 h. Finally, the methanol was removed by rotary evaporator and dried in a vacuum oven at 60 °C. During the experimental process, g-C3N4 first adsorbs on the surface of CdS nanorods and then undergoes a rolling mechanism and process. Hence, the g-C3N4 could curl up and wrap around the CdS nanorods to form a core/shell structure.37,38 The photocatalytic activities of different weight contents of g-C3N4 (2%, 4%, and 8%) were determined, which were also prepared under the same conditions. Preparation of CoP Nanoparticles. CoP nanoparticles were prepared by the reported method.34 First, an excess of 0.5 M NaOH solution was added into the 100 mL solution containing 200 mg of Co(NO3)2 and 50 mg of sodium citrate. The precipitates were separated and dried in a vacuum oven for 8 h to obtain the Co(OH)2 precursor. Then, 0.05 g of Co(OH)2 and 0.25 g of NaH2PO2 were ground to form a uniform distribution. The mixture was annealed at 300 °C for 1 h at a heating rate of 2 °C min−1 in argon atmosphere. The obtained black solid was ground into a powder and washed with water and ethanol in turn for several times and dried in a vacuum oven.

number of reacted electrons × 100% number of incident photons number of evolved H 2 molecules × 2 = × 100% number of incident photons

QE (%) =

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RESULTS AND DISCUSSION Structures and Characterizations. Figure 1 displays the XRD patterns of the CdS, g-C3N4, CdS/g-C3N4, CoP, and CoP-CdS/g-C3N4 composites with different CoP contents. For pure g-C3N4, a major diffraction peak exists at 27.4°, corresponding to the (002) peaks of the graphitic phase.26 All diffraction peaks for pure CdS can be assigned to the wellcrystallized CdS (JCPDS card 65-3414).41 For a further, clear observation of the XRD diffraction pattern of pure CoP, the amplified XRD spectrum is shown in the inset of Figure 1. The XRD diffraction pattern of pure CoP shows distinct peaks at 2θ = 48.4°, corresponding to (211) planes of CoP (JCPDS card 29-0497),42 which may be attributed to the fact that the CoP has an amorphous structure. The peak at 40.9° corresponds to the phase of Co2P (JCPDS card 32-0306).27 In the course of phosphorization, unavoidably, the Co precursor partly agglomerates because of the high temperature. Therefore, this resulted 7671

DOI: 10.1021/acssuschemeng.7b01043 ACS Sustainable Chem. Eng. 2017, 5, 7670−7677

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The chemical compositions and element state of 5% CoPCdS/g-C3N4 are examined by X-ray photoelectron spectroscopy (XPS), and the obtained results are depicted in Figure 2. The Cd 3d spectrum (Figure 2a) reveals two peaks at 404.6 and 411.4 eV that belong to Cd 3d2/5 and Cd 3d3/2, respectively.45 The C 1s signal can be deconvoluted into three peaks at 284.8, 286.1, and 288.1 eV.46,47 The strongest peak at 284.8 eV can originate from the C−C bonds. The peak at 288.1 eV can probably be ascribed to carbon atoms bonded to three nitrogen atoms in the g-C3N4, and the peak at 286.1 eV belonged to the sp3-coordinated species from the defects on the g-C3N4 surface. The peak seated at 779.1 eV descends from the Co (2p3/2) species in the CoP42 (Figure 2c). The positive shift in comparison to metallic Co (778.1−778.2 eV) reveals that the cobalt is in the oxidation state.27 Finally, the peak appearing at 133.4 eV is attributed to the P 2p3/2 in the CoP.48 The above results prove that the CoP and g-C3N4 exist in the 5% CoPCdS/g-C3N4 composite. The transmission electron microscopy images of the prepared materials are shown in Figure 3. Figure 3a shows the TEM image of pure CdS. We can clearly observe that the CdS shows a nanorod-like morphology with good uniformity. From Figure 3b, it can be seen that the CdS nanorods are partly entwined with g-C3N4 to form an unclosed core−shell structure. The HRTEM image of CdS/g-C3N4 is shown in Figure 3c; the calculated lattice spacing of 0.339 nm in the core belongs to the (002) crystal plane of CdS.49 The shell region with lattice spacing of 0.321 nm corresponds to the (002) interlayer-stacking distance of g-C3N4.50 Contrary to the smooth surface of CdS and CdS/g-C3N4, small CoP particles

Figure 1. XRD pattern of CdS, g-C3N4, CoP, and CoP-CdS/g-C3N4 synthesized by loading different contents of CoP.

in uncompleted phosphorization and the formation of Co2P.43 Actually, the Co2P is also an excellent cocatalyst and could provide the active sites for hydrogen production, which is similar to CoP.44 The CoP-CdS/g-C3N4 composites show similar diffraction peaks with pure CdS. However, the peak intensity decreased regularly with the increasing CoP content. Furthermore, it can be seen that no significant difference is found in the XRD patterns after the introduction of g-C3N4 and CoP to the CdS nanorods, probably because the contents of gC3N4 and CoP are too low to be detected.

Figure 2. XPS spectra of (a) Cd 3d, (b) C 1s, (c) Co 2p, and (d) P 2p. 7672

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Figure 3. (a) TEM image of pure CdS nanorods (the inset is the corresponding HRTEM image). (b) TEM image of the CdS/g-C3N4 composite (the inset is the corresponding magnification TEM image). (c) HRTEM of the CdS/g-C3N4 composite. (d) TEM images of the 5% CoP-CdS/gC3N4 composite.

wavelength range 300−800 nm.52 After CoP is loaded onto the g-C3N4, an increase of UV−vis light absorption over the entire wavelength range is discovered, proving the better light absorption of CoP-CdS/g-C3N4, which may be beneficial to the improvement of photocatalytic H2 evolution activities. The photoluminescence (PL) intensity was obtained at the 320 nm excitation wavelength. As show in Figure 5, there is a strong

anchor on the CdS/g-C3N4 nanorods to form CoP-CdS/gC3N4 composites (Figure 3d). The light absorption properties of the as-prepared samples were examined by the UV−vis absorption spectra, and the obtained results are shown in Figure 4. Compared to pure CdS,

Figure 4. UV−vis diffuse reflectance spectra of CdS, CoP, g-C3N4, CdS/g-C3N4, and 5% CoP-CdS/g-C3N4.

Figure 5. Photoluminescence spectra of the as-prepared samples: gC3N4, CdS, CdS/g-C3N4, and 5% CoP-CdS/g-C3N4.

the g-C3N4 has a broader absorption range. The UV−vis absorption spectra show obvious visible light absorption in the region of 500−700 nm due to the carbon atoms introduced by barbituric acid, which change the electronic structure of gC3N4.51 Since the content of g-C3N4 wrapped on the CdS is relatively low, the optical absorption feature of CdS/g-C3N4 does not apparently change. The UV−vis diffuse reflectance spectra show that the CoP has strong absorption in the

emission peak of about 470 nm of the pure g-C3N4 sample. The CdS/g-C3N4 shows a similar PL spectrum but with weaker intensity compared with pure CdS. The quenched PL in the CdS/g-C3N4 composite indicates that the g-C3N4 coated on the CdS nanorods can help to separate the photogenerated carriers. The PL spectrum of CoP-CdS/g-C3N4 also shows a similar shape as the CdS/g-C3N4. The relatively low intensity indicated 7673

DOI: 10.1021/acssuschemeng.7b01043 ACS Sustainable Chem. Eng. 2017, 5, 7670−7677

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electrons and holes. Interestingly, coating the CdS nanorods with a small amount of g-C3N4 can substantially improve the photocatalytic H2 generation rate to the value of 2982 μmol h−1 g−1, which is about 1.8 times that of CdS (1668 μmol h−1 g−1). The hydrogen evolution of different contents of g-C3N4 coated on the CdS was also explored and shown in Figure S1 (Supporting Information). As can be seen, the optimal photocatalytic activity was achieved at 4 wt % g-C3N4 content. As the g-C3N4 content further increased to 8 wt %, the photocatalytic activity decreases rapidly probably because the dense and closed g-C3N4 shell prevents CdS from contacting the reaction solution. Furthermore, when loading a certain amount of CoP as cocatalysts, the photocatalytic hydrogen production performance increases with the increasing amount of CoP. The highest photocatalytic activity was achieved at 5% CoP content, and the hydrogen production rate reaches 23 536 μmol h−1 g−1, which is about 14 times that of CdS and 8 times that of CdS/g-C3N4. A further increase in the content of CoP to 8% caused the decreased performance. Thus, we can deduce that excess CoP will decrease the light absorption of CdS and then the amount of photogenerated electrons and holes. The 5% CoP-CdS/g-C3N4 nanocomposite was compared with 5% CoP-CdS and 5% CoP-g-C3N4 by testing them under the same experimental conditions. As shown in Figure S2 (Supporting Information), the hydrogen evolution rate for the 5% CoPCdS/g-C3N4 nanocomposite was much higher than those of CdS (1668 μmol h−1 g−1), 5% CoP-CdS (7406 μmol h−1 g−1), and 5% CoP-g-C3N4 (325 μmol h−1 g−1). Furthermore, the apparent quantum efficiencies of the 5% CoP-CdS/g-C3N4 were measured with the same light source with a 420, 450, 500, and 550 nm band-pass filter, separately. The apparent quantum yield with the 420, 450, 500, and 550 nm band-pass filter is calculated to be 38.5%, 35.4%, 13.5%, and 0%, respectively. For the irradiation wavelength at λ = 550 nm, the catalytic system showed 0% apparent quantum efficiency. Because the band gap of CoP is 1.7 eV, the maximum absorbance wavelength is located at 729 nm. On the basis of the above discussion, we can deduce that the CoP mainly acts cocatalytically in the CoPCdS/g-C3N4 photocatalytic system. In addition, the stability of CoP-CdS/g-C3N4 was tested via recycling experiments, and the results are shown in Figure 7b. The H2 production rate shows no decrease after 4 irradiation recycles of 20 h. The result indicates that the CoP-CdS/g-C3N4

that the electrons generated on the CdS/g-C3N4 preferentially transfer to the CoP cocatalyst, which reduces the rate of electron−hole recombination. The transient photocurrent responses of the pure g-C3N4, CdS, CdS/g-C3N4, and 5% CoP-CdS/g-C3N4 on FTO electrodes were analyzed to further study the charge separation in the light on−off process. As shown in Figure 6, the CdS/g-

Figure 6. Transient photocurrent response of the pure g-C3N4, CdS, CdS/g-C3N4, and 5% CoP-CdS/g-C3N4 under visible light irradiation.

C3N4 shows higher photocurrent than pure CdS and g-C3N4 probably because the design can efficiently promote the charge separation. In the same condition, the 5% CoP-CdS/g-C3N4 shows much higher photocurrent compared with CdS and CdS/g-C3N4. The experiment results are consistent with the above PL results, which further prove that the combining of CoP cocatalyst and g-C3N4 plays an important role in suppressing the recombination of the photogenerated charge carriers. Therefore, it can be predicted that the CoP-CdS/gC3N4 composites would exhibit higher photocatalytic activity than the corresponding individual component. Photocatalytic Hydrogen Generation Properties. The photocatalytic hydrogen evolution experiments of the asprepared samples including CoP-CdS/g-C3N4 with varying content of CoP along with pure g-C3N4 and CdS were evaluated under visible light irradiation. As shown in Figure 7a, pure g-C3N4 and CdS have a poor hydrogen generation performance probably because of a high recombination rate of

Figure 7. (a) Photocatalytic hydrogen rates over CoP-CdS/g-C3N4 with different contents of CoP, pure g-C3N4, CdS, and CdS/g-C3N4. (b) Recycling tests of 5% CoP-CdS/g-C3N4 composite and pure CdS. 7674

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Figure 8. Mechanism for photocatalytic hydrogen generation over CoP-CdS/g-C3N4.

between g-C3N4 and CdS can effectively promote the separation of the photogenerated electron and holes in the composite. The introduction of CoP as cocatalyst further accelerates the separation rate of the photogenerated electron− hole pairs and provides a catalytic active site for H2 production.

composites have good stability during the photocatalytic reaction. Instead of decreasing, we can clearly find that the H2 production rate slightly increased. This interesting phenomenon is probably because the H2 production reaction is an activated process, and the surface of the photocatalyst can absorb more photons with the increasing irradiation time.27 In addition, we also test the stability of CdS. After the 20 h of irradiation, the H2 yield is reduced to 76% of the original production yield value. The good stability property of CoPCdS/g-C3N4 composites may come from the good efficient charge transfer path resulting from the well-designed heterostructure.53 Photocatalytic Hydrogen Evolution Mechanism. We put forward a possible mechanism of the enhanced photocatalytic hydrogen generation based on the above research results. As depicted in Figure 8, the band edge position of gC3N4 and CdS make up the type II heterojunction; that is, the VB and CB (valence band and conduction band) of g-C3N4 are higher than those of CdS, a structure which is beneficial for the charge carrier separation. Specifically, the electrons emerged from the g-C3N4 can be quickly transferred to the conduction band of CdS nanorods, and the excited holes on CdS can move to the g-C3N4 at the same time. The efficient separation of photogenerated charge carriers benefited from the specially designed CdS/g-C3N4 composite. Since the CdS/g-C3N4 composite nanorods are not a closed system, the electrons on the CdS can migrate to the cocatalyst CoP. Meanwhile, the CoP also accepts the electrons transferred from the conduction band of the g-C3N4 which serves as a catalytic active site in the water-splitting reaction. Hence, the recombination of electron− hole pairs can be efficiently prevented, thus increasing the dynamics of photocatalytic hydrogen production. Therefore, we propose two main factors to explain why the CoP-CdS/g-C3N4 composite can achieve the higher hydrogen production: (1) The skillfully designed heterojunctions greatly promote the separation of electrons and holes. (2) CoP can lower H+ reduction overpotential and serve as a catalytic active site.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01043. Comparisons of the photocatalytic activities of the optimized photocatalyst with other catalysts (CdS, 5% CoP-CdS, and 5% CoP-g-C3N4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86 25 83787330. E-mail: [email protected]. ORCID

Yanhui Ao: 0000-0002-3665-9881 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for grants from National Science Funds for Creative Research Groups of China (51421006), Program for Changjiang Scholars and Innovative Research Team in University (IRT13061), National Science Fund for Distinguished Young Scholars (51225901), the National Science Fundation of China for Excellent Young Scholars (51422902), the Key Program of National Natural Science Foundation of China (41430751), the National Natural Science Foundation of China (51579073), Natural Science Foundation of Jiangsu Province (BK20141417), Fundamental Research Funds (2016B43814), and PAPD.

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CONCLUSIONS In conclusion, with the combination of heterojunction engineering and cocatalyst modification, a noble-metal-free composite is synthesized. The CoP-CdS/g-C3N4 hybrids exhibit a much more highly efficient photocatalytic generation ability compared with the pure g-C3N4, CdS, and CdS/g-C3N4 heterostructure. The best H2 generation rate (23 536 μmol h−1 g−1) is about 14 times higher than that of CdS (1668 μmol h−1 g−1) and 8 times higher than that of CdS/g-C3N4 (2982 μmol h−1g−1). The results demonstrated that the heterojunction

REFERENCES

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DOI: 10.1021/acssuschemeng.7b01043 ACS Sustainable Chem. Eng. 2017, 5, 7670−7677

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ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.7b01043 ACS Sustainable Chem. Eng. 2017, 5, 7670−7677

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DOI: 10.1021/acssuschemeng.7b01043 ACS Sustainable Chem. Eng. 2017, 5, 7670−7677