Fabrication of g-C₃N₄/Au/CdZnS Z-scheme photocatalyst to enhance photocatalysis performance

Abstract

In this study, a sandwich-structured C₃N₄/Au/CdZnS photocatalyst with Au nanoparticles sandwiched between C₃N₄ and CdZnS was fabricated. The results showed that there was a 6.3 mmol g⁻¹ increase of hydrogen production for C₃N₄/Au/CdZnS photocatalyst compared to C₃N₄/CdZnS in 4 h. The photoinduced I–t curve, electrochemical impedance spectrum (EIS), photogenerated electron lifetime and photoluminescence spectra showed that the photocatalytic ability of the composite was enhanced by anchoring Au as the electron transfer mediator between C₃N₄ and CdZnS through the Z-scheme charge-carrier transfer mechanism. Au functioned as an electron transition mediator, which could induce a combination between photogenerated electrons and holes in the CdZnS and C₃N₄, respectively. These photogenerated holes of CdZnS can be involved in the S²⁻/SO₃²⁻ oxidation process and the electrons from the C₃N₄ surface might be involved in the water to hydrogen reduction reaction, thus enhancing the photocatalysis performance of C₃N₄/Au/CdZnS photocatalyst.

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1. Introduction

In this century, humans are facing two challenges: energy shortage and environmental pollution, and the key method to resolve that challenge is to invest in better ways to explore clean energy.^1,2 Solar energy is considered as an inexhaustible renewable resource and photovoltaics is a method of converting solar radiation into chemical energy using semiconductors that split water into hydrogen and oxygen, which is regarded as one of the breakthrough approaches. Great progress has been made in photocatalysis technology but its industrialization still remains as a problem.^3,4

Bio-inspired by photosynthesis, Bard⁵ proposed the concept of artificial photosynthesis and designed a Z-scheme photocatalytic water splitting system, in which the energy required to drive each photocatalyst is reduced, allowing visible light to be utilized more efficiently than on conventional photocatalysis systems. Tada et al.⁶ proposed the concept of all-solid-state Z-scheme photocatalysts in a sandwich-structured CdS/Au/TiO₂ photocatalyst, where Au nanoparticles (NPs) were anchored between CdS and TiO₂. It was found that the photogenerated electrons in the conduction band (CB) of TiO₂ could transfer through the Au layer and recombine with the photogenerated holes left in the valance band (VB) of CdS. The reducing property of the photogenerated electrons was enhanced and the strongly oxidizing photogenerated holes produced by TiO₂ could be involved in the oxidation reaction of H₂O, thus dramatically improving the efficiency of redox stability in the photocatalysis reaction. In Li et al.’s study,⁷ Au nanoparticles were deposited in the nanorod array and then CdS was in situ deposited on Au, which formed a sandwich-structured TiO₂–Au–CdS photoanode. It was found that addition of Au nanoparticles increases the charge-transfer lifetime, reduces the trap-state Auger rate, suppresses the long-time scale back transfer, and partially compensates the negative effects of the surface trap states, thus improving the photoelectric conversion efficiency. Iwase⁸ took graphene oxide as an electron mediator and connected two semiconductors, Ru/SrTiO₃:Rh and BiVO₄, showing that graphene oxide could transfer photogenerated electrons as in a Z-scheme. Ye et al.⁹ studied Ag/AgX/BiOX photocatalytic activity and showed that Ag played a different role for Ag/AgX/BiOX photocatalysts. When Ag was present between AgX and BiOX, it functioned as a stronger photocatalyst than that on the AgX surface, contributing to a Z-scheme charge-carrier transfer mechanism of Ag sandwiched between AgX and BiOX.¹⁰

g-C₃N₄ is an analog of graphite polymer and studies have shown that g-C₃N₄ shows good photocatalytic property, which can be further improved by combination with other semiconductors^11–15 or by impurity doping.^16–23 The conduction band of g-C₃N₄ is more negative in potential while the valence band is negative, and the oxidation ability of the photogenerated holes is comparatively weak. If we fabricate the Z-scheme structure and induce photogenerated holes which can recombine with the electrons remaining in another semiconductor, the photocatalysis performance can be certainly improved. Based on this premise, the g-C₃N₄ photocatalyst material was fabricated and ultrasonically dispersed. Then Au nanoparticles were deposited on the surface of g-C₃N₄ as the electron transition mediator. Finally stable ZnCdS was applied on the surface of Ag to form a sandwich-structured Z-scheme g-C₃N₄–Au–CdZnS nanoparticles system.

2. Experimental section

2.1. Photocatalyst preparation

All reagents used in this study were analytical grade from Aladdin Industrial Corporation, China, and were used without any further purification. 10 g of dicyanodiamine was sintered at 550 °C with a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere for 3 h to prepare g-C₃N₄. 0.2 g g-C₃N₄ was sonicated in 40 mL deionized water and 10 mL methanol for 30 min. Then 2 wt% chloroauric acid (10 mg L⁻¹) was added and exposed under 300 W Xe for 5 h. The product was centrifuged and rinsed with water, and then dried at 60 °C for 24 h to obtain g-C₃N₄/Au. Finally, CdZnS was deposited on g-C₃N₄/Au by a bath deposition method. The prepared g-C₃N₄/Au was dispersed into a 80 mL solution containing 0.198 g Cd(CH₃CO₂)₂·2H₂O, 0.163 g Zn(CH₃COO)₂·2H₂O and 0.5 mL (NH₄)₂S, and the mixture was then transferred to a 100 mL stainless steel autoclave and heated to 140 °C for 24 h. The product g-C₃N₄/Au/CdZnS (CN/Au/CdZnS) was then obtained by drying at 60 °C for 24 h.

2.2. Characterization

The morphology of the prepared samples was analyzed using a scanning electron microscope (SEM, JSM-6700F; JEOL, Tokyo, Japan). X-Ray diffraction (XRD, D/MAX-2500/PC; Rigaku Co., Tokyo, Japan) was used to identify the crystalline structures of the series of samples. The morphology of the all-solid-state CN/Au/CdZnS was observed using a field emission transmission electron microscope (FE-HRTEM, Tecnai G2 F20, FEI Company, USA). The elemental composition and bonding information of the synthesized CN/Au/CdZnS three-phase material were analyzed using X-ray photoelectron spectroscopy (XPS, Axis Ultra, Kratos Analytical Ltd., England). A UV-visible diffuse reflectance spectrophotometer (U-41000; HITACHI, Tokyo, Japan) was used to study the optical absorption properties of the series of samples. The excited-state electron radiative decay of the prepared photocatalysts were characterized using a fluorescence spectrometer (PL, Fluoro Max-4, HORIBA Jobin Yvon, France).

2.3. Photocatalytic water splitting and dye degradation testing

In the photocatalytic water splitting test, 0.1 g photocatalyst was used and the solution was a mixture of 100 mL 0.25 M Na₂S and 0.35 M Na₂SO₃. Before the test, the photo-reaction system was subjected to vacuum until the pressure gauge was stable. An externally illuminating 150 W Xe lamp (PLS-SXE300, Beijing Changtuo Co., Ltd., China) was used as the white light source and visible light was achieved using a 420 nm cut-off filter to remove ultraviolet light. The distance between the light source and the solution that produced hydrogen was 10 cm. The testing temperature was maintained at 5 °C by a condensate water system.

2.4. Fabrication of photoelectrodes and the photoelectron-chemical performance test

20 mg photocatalyst was mixed with 0.5 mL of ethanol in an agate mortar, and the mixture was ground into a slurry. 50 μL of the slurry was spread evenly over the FTO glass (the exposed effective area was 1 cm × 1 cm). This was then dried in ambient air and heated at 100 °C for 1 h, and the conductive wire then connected to fabricate the photoelectrode.

A three-electrode system was used to measure the PEC performance. The photoelectrode, a large piece of platinum, and Ag/AgCl (saturated KCl) were used as the working, counter and reference electrodes, respectively. All tests were performed in 0.1 M Na₂SO₄ electrolyte by a CHI 660D electrochemical system (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). The photoinduced current density–time (I–t) curves was measured at a bias potential of 0 V (vs. Ag/AgCl). The electrochemical impedance spectroscopy measurements were carried out at a bias potential of 0 V and the frequency range was from 10⁵ to 10⁻¹ Hz.

3. Results and discussion

The XRD patterns of the composites are shown in Fig. 1. Curve a shows the XRD curve of C₃N₄ for which the peak at 2θ = 27.4 (002) corresponds to the interlayer stacking of C₃N_4. Curve b shows the XRD pattern of C₃N₄ composite, from which we can see the interlayer stacking peak of C₃N₄ and characteristic diffraction of Au,²⁴ indicating that the Au–C₃N₄ composite was formed. Curve c shows the characteristic diffraction peak of composite CN/CdZnS. From this curve, the characteristic diffraction peak of CdZnS was easily observed, while the peak of C₃N₄ at 27.4° (002) overlapped with that of CdZnS. Curve d shows the XRD pattern of the CN/Au/CdZnS triple-phase composite, which shows characteristic diffraction peaks of C₃N₄ and CdZnS. However, the characteristic diffraction peak of Au was not observed, this phenomenon may be due to covalent bonding between S²⁻ and Au, thus decreasing the degree of crystallinity of Au. Therefore, other techniques are required to further prove the morphology of Au composition.

Fig. 1 XRD patterns of different materials (a) CN; (b) CN/Au; (c) CN/CdZnS; (d) CN/Au/CdZnS; (e) CdZnS.

Fig. 2 shows SEM images of the composites. Fig. 2A is the microstructure of C₃N₄, from which we can observe lamellar structures of C₃N₄. Fig. 2B shows the microstructure of C₃N₄ after the CdZnS deposition. Fig. 2C is the microstructure of CN/Au which shows numerous large nanoparticles on the surface of the flake structure of C₃N₄, which is the result of Au deposition. Fig. 2D is the microstructure of CN/Au/CdZnS, which is similar to that of Fig. 2B, with numerous large nanoparticles scattered on the surface of the flake structure of C₃N₄. We can not identity the existence of Au or the microscopic distribution of CN/Au/CdZnS by the SEM result. Therefore EDS mapping and TEM are needed for further clarification.

Fig. 2 SEM images of (A) CN; (B) CN/CdZnS; (C) CN/Au; (D) CN/Au/CdZnS.

The EDS mapping results of CN/Au/CdZnS are shown in Fig. 3. We tested the elements distribution of an selected area of this sample with 5000 times magnification. The image indicated that the sample was formed by the elements N, C, Cd, Zn, S and Au.

Fig. 3 EDS mapping of CN/Au/CdZnS for different elements.

The CN/Au/CdZnS composite was characterized by X-ray photoelectron spectroscopy (XPS) in Fig. 4. Fig. 4A shows the XPS survey spectrum of the CN/Au/CdS composite, indicating the presence of N, C, Cd, Zn, S and Au in the composite, which is consistent with the SEM mapping results. As the XRD patterns (curve d) showed no diffraction peak of Au in CN/Au/CdZnS composite, we paid special attention to the high-resolution diffraction peak in the XPS survey. Fig. 4B shows the XPS spectrum of Au 4f with the characteristic peak at 90.0 eV, confirming the existence of Au in the composite.

Fig. 4 (A) Survey XPS of CN/Au/CdZnS; (B) Au 4f region.

The morphologies and the distribution of CN/Au/CdZnS three-phase material were analyzed using TEM (see Fig. 5). Fig. 5A shows the TEM image of CN/Au, from this result we can find that some Au nanoparticles are distributed on the surface of the CN layer. Low-resolution TEM of CN/Au/CdZnS (Fig. 5B) shows that particles with less than 50 nm diameter were observed directly on the large surface of C₃N₄, with darker and smaller particles depositing on it. The high-resolution TEM image (Fig. 5C) reveals that the Au nanoparticles were loaded between C₃N₄ and CdZnS, indicating that C₃N₄, Au, CdZnS three phase composites were not randomly distributed but arranged in an orderly manner in the CN/Au/CdZnS sandwich nanostructure. Because of the strong electropositivity of the Au, the S²⁺ could be trapped by Au easily, so the CdZnS selectively grew on Au nanoparticles.⁶

Fig. 5 TEM of (A) CN/Au; (B) low-resolution TEM of CN/Au/CdZnS; (C) high-resolution TEM of CN/Au/CdZnS.

To evaluate the photocatalytic performance of the composite, its photocatalytic ability to reduce water to hydrogen under visible light illumination was measured. Fig. 6A shows the curves of photocatalytic hydrogen production yields of various composites over 4 h. Curve a of Fig. 6A represents that of C₃N₄, which indicates that the hydrogen production from pure g-CNS was very low and was only 2.0 mmol g⁻¹. After the deposition of Au (curve b), the catalytic performance of the CN/Au was better than that of the pure C₃N₄, because of the photocatalytic ability of Au to reduce water to hydrogen.

Fig. 6 (A) Photocatalytic hydrogen evolution ability of CN, CN/Au, CdZnS, CN/CdZnS, CN/Au/CdZnS in aqueous solution containing 0.25 M Na₂S and 0.35 Na₂SO₃ under visible light illumination; (B) performance of CN/Au/CdZnS with evacuation every 4 h without renewing the sacrificial agents.

Curve c of Fig. 6A is the photocatalytic hydrogen production of pure CdZnS, showing that the generated hydrogen is 14.9 mmol g⁻¹, which was substantially improved compared to those of C₃N₄ because of the contribution of the expanded visible light absorption range of CdZnS. Curve d shows that the hydrogen yield using composite CN/CdZnS after 5 h light exposure reached 18.3 mmol g⁻¹ and its catalytic ability was improved compared to that of pure CdZnS. This was resulting from the heterojunction electric field between CdZnS and C₃N₄, which could dramatically improve the efficiency of photoelectron generation and hole separation.

Curve e shows the photocatalytic hydrogen generation curve using the composite CN/Au/CdZnS, indicating that the hydrogen yield reached 24.6 mmol g⁻¹ after 4 h exposure, higher than for composite CN/CdZnS. This demonstrates that when existing between the C₃N₄ and CdZnS, Au functioned as an electron transition system, which could induce a combination between photogenerated electrons and holes in CdZnS and C₃N₄, respectively. These photogenerated electrons of CdZnS can be involved in the S²⁻/SO₃²⁻ reduction process and the holes from the C₃N₄ surface might be involved in the water to hydrogen oxidation reaction, thus enhancing the catalytic ability. Fig. 6B shows the photocatalytic stability measurement of CN/Au/CdZnS. Every measurement cycle was for 4 h. After five cycles, the hydrogen production yield shows no decrease, confirming that the CN/Au/CdZnS photocatalyst was quite stable in the S²⁻/SO₃²⁻ system.

In order to study the role of Au between C₃N₄ and CdZnS, powders of composites CN/CdZnS and CN/Au/CdZnS were made into photoelectrodes and the photoinduced I–t and EIS data were tested in 0.1 M Na₂SO₄ solution. Fig. 7A shows the photoelectrode photoinduced I–t curves, which reflect the photoelectric conversion ability of the composites. Comparing the two I–t curves, we can see that when Au is anchored between C₃N₄ and CdZnS, its photogenerated current increased significantly, which shows that as the electron transfer mediator, Au can increase the separation efficiency of photo-induced electrons and holes as well as enhancing the transferability of those materials.

Fig. 7 (A) Photoinduced I–t curves and (B) EIS curves of CN/CdZnS and CN/Au/CdZnS electrodes.

To some extent, EIS curves can reflect the electron migration resistance in thin films. Fig. 7B shows the thin-film photoelectrode EIS result of CN/CdZnS and CN/Au/CdZnS. Comparing these two curves we can easily see that when is Au anchored between C₃N₄ and CdZnS, the EIS arc resistance declined obviously, demonstrating that the existence of Au can lower the electron transferability in the composite thin film, thus optimizing the transferability of photogenerated electrons between C₃N₄ and CdZnS, to attain a Z-scheme charge-carrier transfer mechanism.

For most semiconductors, photogenerated electrons and holes can bind after the excitation by incident light, leading to the transfer of energy to fluorescence. Binding efficiency and photoelectron lifetime can be measured by monitoring the fluorescence intensity and lifetime. For photocatalyst CN/Au/CdZnS, if Au can act as carrier of photogenerated electrons, it will inhibit the binding efficiency between photogenerated holes and electrons, resulting in an increased lifetime of the photoelectrons. Therefore photoluminescence (PL) spectra and the excited state electron radioactive decay lifetime of CN/CdZnS and CN/Au/CdZnS were measured to characterize the effect of Au in the CN/Au/CdZnS photocatalyst. Fig. 8A shows the photoluminescence spectra of CN/CdZnS and CN/Au/CdZnS, indicating strong fluorescence peaks in the 400–750 nm range for both catalysts. The fluorescence peak intensity of CN/Au/CdZnS was much weaker than that of CN/CdZnS, because Au inhibited the binding efficiency between photogenerated holes and electrons. Fig. 8B shows the excited-state electron radioactive decay lifetime of CN/CdZnS and CN/Au/CdZnS. The excited-state electron radioactive decay lifetime of CN/CdZnS and CN/Au/CdZnS were 16.8 and 18.8 ns, respectively, indicating that the incorporation of Au increased the photoelectron lifetime of the CN/Au/CdZnS photocatalyst, because of the contribution of Au as the electron transfer mediator.

Fig. 8 (A) PL spectra of CN/CdZnS and CN/Au/CdZnS powders; (B) excited-state electron radioactive decay of CN/CdZnS and CN/Au/CdZnS powders.

Fig. 9 presents the UV-vis diffuse reflection results of the as prepared photocatalysts. The absorption threshold of CdZnS was nearly 528 nm, corresponding to a 2.3 eV bandgap.²⁵ The absorption threshold of CN is approximately 460 nm, corresponding to a bandgap of 2.7 eV. However, the as-prepared CN/Au/CdZnS photocatalyst showed a similar absorption threshold as CdZnS, indicating that the CdZnS acts as the main light absorption material in the CN/Au/CdZnS composite.

Fig. 9 UV-vis diffuse reflection spectra of ZnCdS, CN and CN/Au/ZnCdS photocatalysts.

Fig. 10 presents the possible electron transfer mechanism of the g-C₃N₄/Au/CdZnS composite photocatalyst. When this photocatalyst is illuminated by light, the photogenerated holes in the VB of g-C₃N₄ will shift to the Au and annihilate with the electrons generated by CdZnS. So the photogenerated holes on CdZnS with strong oxidizing power will participate in the oxidation process of S²⁺. Meanwhile, the photogenerated electrons in g-C₃N₄ with strong reducing power, will react with the H⁺ for hydrogen evolution. Through this Z-scheme electrons transfer process, the redox reaction ability of photogenerated electrons and holes could be increased, at the same time, the recombination process of the separated electrons and holes could be inhabited, so that photocatalytic performance of the g-C₃N₄/Au/CdZnS composite photocatalyst will be improved.

Fig. 10 Proposed Z-scheme mechanism for electron transfer in g-C₃N₄/Au/CdZnS composite photocatalyst.

4. Conclusions

In this study, first the g-C₃N₄ photocatalyst was prepared, then Au nanoparticles and CdZnS nanoparticles were in situ deposited on the surface of g-C₃N₄, forming the sandwich nanostructure C₃N₄/Au/CdZnS photocatalyst. The C₃N₄/Au/CdZnS photocatalyst exhibited better performance for visible light photocatalytic hydrogen evolution, with 24.6 mmol g⁻¹ hydrogen yield in 4 h, representing an increase of 6.3 mmol g⁻¹ relative to C₃N₄/CdZnS. The photoinduced I–t curve, EIS and photoluminescence spectra showed that the photocatalytic ability of the composite was enhanced by anchoring Au as the electron transfer mediator between C₃N₄ and CdZnS through the Z-scheme charge-carrier transfer mechanism.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (No. 2014CB643304).

Notes and references

1. Z. Liu, X. Zhang, S. Nishimoto, T. Murakami and A. Fujishima, Environ. Sci. Technol., 2008, 42, 8547.
2. Y. Bu and Z. Chen, J. Power Sources, 2014, 272, 647.
3. X. Chen, S. Shen, L. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503.
4. A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253.
5. A. J. Bard and M. A. Fox, Acc. Chem. Res., 1995, 28, 141–145.
6. H. Tada, T. Mitsui, T. Kiyonaga, T. Akita and K. Tanaka, Nat. Mater., 2006, 5, 782.
7. J. T. Li, S. K. Cushing, P. Zheng, T. Senty, F. Meng, A. D. Bristow, A. Manivannan and N. Q. Wu, J. Am. Chem. Soc., 2014, 136, 8438.
8. A. Iwase, Y. H. Ng, Y. Ishiguro, A. Kudo and R. Amal, J. Am. Chem. Soc., 2011, 133, 11054.
9. L. Q. Ye, J. N. Liu, C. Q. Gong, L. H. Tian, T. Y. Peng and L. Zan, ACS Catal., 2012, 2, 1677.
10. X. Lin, J. Hou, S. Jiang, Z. Lin, M. Wang and G. Che, RSC Adv., 2015, 5, 104815.
11. X. Wang, X. Chen, A. Thomas, X. Fu and M. Antonietti, Adv. Mater., 2009, 21, 1609.
12. Y. Zhang, T. Mori, L. Niu and J. Ye, Energy Environ. Sci., 2011, 4, 4517.
13. Y. Z. Zhang, T. Mori, J. H. Ye and M. Antonietti, J. Am. Chem. Soc., 2010, 132, 6294.
14. S. C. Yan, Z. S. Li and Z. G. Zou, Langmuir, 2010, 26, 3894.
15. G. Liu, P. Niu, C. Sun, S. C. Smith, Z. Chen, G. Q. Lu and H. M. Cheng, J. Am. Chem. Soc., 2010, 132, 11642.
16. Y. Bu, Z. Chen, C. Feng and W. Li, RSC Adv., 2014, 4, 38124.
17. Y. He, J. Cai, T. Li, Y. Wu, Y. Yi, M. Luo and L. Zhao, Ind. Eng. Chem. Res., 2012, 51, 14729.
18. Y. Di, X. Wang, A. Thomas and M. Antonietti, ChemCatChem, 2010, 2, 834.
19. L. Ge, C. Han, J. Liu and Y. Li, Appl. Catal., A, 2011, 409–410, 215.
20. Y. Bu and Z. Chen, Electrochim. Acta, 2014, 144, 42.
21. S. C. Yan, S. B. Lv, Z. S. Li and Z. G. Zou, Dalton Trans., 2010, 39, 1488.
22. J. Dong, M. Wang, X. Li, L. Chen, Y. He and L. Sun, ChemSusChem, 2012, 5, 2133.
23. L. Zhang, D. Jing, X. She, H. Liu, D. Yang, Y. Lu and L. Guo, J. Mater. Chem. A, 2014, 2, 2071.
24. W. Li, C. Feng, S. Dai, J. Yue, F. Hua and H. Hou, Appl. Catal., B, 2015, 168–169, 465.
25. M. Huang, J. Yu, C. Deng, Y. Huang, M. Fan, B. Li, Z. Tong, F. Zhang and L. Dong, Appl. Surf. Sci., 2016, 365, 227.

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