Boosting visible-light photocatalytic hydrogen evolution with an

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Boosting visible-light photocatalytic hydrogen evolution with an efficient CuInS2/ZnIn2S4 2D/2D heterojunction Zhongjie Guan, Jingwen Pan, Qiuye Li, Guoqiang Li, and Jianjun Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06587 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

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ACS Sustainable Chemistry & Engineering

Boosting visible-light photocatalytic hydrogen evolution with an efficient CuInS2/ZnIn2S4 2D/2D heterojunction Zhongjie Guana,b,*, Jingwen Pana, Qiuye Lia,*,Guoqiang Lib,and Jianjun Yanga aNational

& Local Joint Engineering Research Center for Applied Technology of

Hybrid Nanomaterials, Henan University, Kaifeng 475004, China bSchool

of Physics & Electronics, Henan University, Kaifeng475004, China

* E-mail:[email protected]; [email protected]; Abstract: Developing photocatalysts with a high efficient charge separation efficiency remains a challenge in the solar hydrogen production. Herein, we devised and prepared a unique 2D/2D heterojunction of CuInS2/ZnIn2S4 nanosheets for solar hydrogen evolution. Structural characterizations reveal that the CuInS2/ZnIn2S4 2D/2D heterojunction with lattice match consists of the thin thickness of nanosheets and has a large interface contact area, boosting charges transfer and separation. Benefiting from the

favorable

2D/2D

heterojunction

structure,

the

CuInS2/ZnIn2S4

2D/2D

heterojunction photocatalyst with 5wt% CuInS2 yields the highest H2 evolution rate of 3430.2 μmol·g-1·h-1. In addition, the apparent quantum efficiency of 5%CuInS2/ZnIn2S4 2D/2D heterojunction reaches to 12.4% at 420 nm, which is high among the ZnIn2S4based 2D/2D heterojunctions. The enhanced photocatalytic H2 evolution comes from the boosting charge separation. This work demonstrates that a 2D/2D heterojunction provides a potential way for significantly improving the solar hydrogen production performance of ZnIn2S4. Keywords: CuInS2/ZnIn2S4 2D/2D heterojunction; Charge separation; Solar hydrogen production Introduction Solar hydrogen generation from water via semiconductor photocatalyst is a potential technology to resolve the energy crisis.1,2 Exploiting a high-efficient photocatalyst is critical due to its decisive role in the practical application of solar hydrogen generation.3-6 Among various photocatalysts, ZnIn2S4 has been widely researched owing to its nontoxicity and suitable conduction band position.7-9 However,

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pure ZnIn2S4 shows a poor solar hydrogen production performance due to the high charge recombination efficiency. To overcome these drawbacks, doping and coupling with other semiconductor photocatalyst are commonly employed in previous studies.1012

However, the heterojunction with a randomly designed morphological structure has

a limited ability for improving the charge separation efficiency. Therefore, an efficient strategy should be developed to further ameliorate the charge separation. Recently, the 2D/2D heterojunction consisting of a thin 2D layered structure exhibits a great advantage to improve the charge separation.13-15 The 2D/2D heterojunction usually possesses a small thickness of 2D layered structure and a large interface contact area, which will be benefited to the charges transfer from bulk-to-surface as well as heterojunction interface, thus improving photocatalytic activity. Lin et al reported that the g-C3N4/ZnIn2S4 2D/2D heterojunction showed higher solar hydrogen evolution activity than the g-C3N4/ZnIn2S4 2D/0D heterojunction due to a large interface contact area in 2D/2D heterojunction.16 Lately, Ni2P/ZnIn2S4 and MoS2/ZnIn2S4 2D/2D heterostructure were also constructed and exhibited much higher H2 evolution activity than pure ZnIn2S4.17,18 However, the lattice match between the 2D and 2D layer are seldom considered in previous studies. A low interface state density can be obtained in the 2D/2D heterojunction with lattice match, which is beneficial to interface charge transfer and separation.19 CuInS2 is a ternary chalcogenide p-type semiconductor and also employed for solar hydrogen generation in previous studies by reason of its high absorption coefficient and a suitable band gap of about 1.5 eV.20-22 Growing CuInS2 on ZnIn2S4 can form a p-n junction to accelerate the electron-hole pairs separation. What's more, compared with other coupling photocatalysts, the lattice between ZnIn2S4 and CuInS2 match well due to their analogous sulfides. To the best our knowledge, it is the first report about the CuInS2/ZnIn2S4 2D/2D heterojunction for solar hydrogen generation. Therefore, it is necessary to construct a CuInS2/ZnIn2S4 2D/2D heterojunction for ameliorating the fast charge recombination. In this study, the ultrathin CuInS2 nanosheets were grown in situ on the surface of ZnIn2S4 to form a 2D/2D heterojunction structure using a simple solvothermal method. The hydrogen evolution activity of photocatalysts was measured under visible-light


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irradiation using Na2S and Na2SO3 as the sacrificial reagents. Through optimizing the weight contents of CuInS2, the CuInS2/ZnIn2S4 2D/2D heterojunction composite shows much higher hydrogen evolution performance than that of pure ZnIn2S4. Moreover, the structure of CuInS2/ZnIn2S4 2D/2D heterojunction and mechanism of enhanced photocatalytic performance were also investigated in detail. Experimental Synthesis of ZnIn2S4 photocatalysts 1 mmol ZnCl2, 2 mmol InCl3·4H2O and 6 mmol thioacetamide were added into 60 ml H2O step and step under stirring condition. Then the above mixture solution was added into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was heated and kept at 220 °C for 24 h. The yellow powders were gathered by centrifugation after reaction and washed several times with deionized water and absolute ethanol. Finally, the ZnIn2S4 photocatalysts was dried at 60 °C for further use. Synthesis of CuInS2/ZnIn2S4 2D/2D heterojunction photocatalysts The CuInS2/ZnIn2S4 2D/2D heterojunction composite were constructed via a solvothermal method.23 Firstly, 0.062 mmol CuCl, 0.062 mmol InCl3·4H2O and 0.25 mmol thiourea were added into 60 ml ethylene glycol under stirring. Then 0.30 g ZnIn2S4 powdered was added and still stirred for 30 min. The above mixture solution was added into a 100 mL Teflon-lined stainless steel autoclave and kept at a temperature of 200 °C for 12 h. The powder photocatalysts were gathered after reaction and washed three times using distilled water and ethanol. Finally, the powders were dried in an vacuum oven at 60 °C for 12 h. For the purpose of achieving a strong contact between CuInS2 and ZnIn2S4, the powders were annealed at a temperature of 300 °C for 2 h under N2. After above steps, the CuInS2/ZnIn2S4 2D/2D heterojunction composite was achieved and named as 5%CuInS2, where 5% indicates the mass ratio of CuInS2 to ZnIn2S4. Other CuInS2/ZnIn2S4 2D/2D heterojunction composites with different weight contents of CuInS2 (10%, 15%, 20%) were also prepared using the above procedure. Pure CuInS2 was synthesized as a reference without adding the ZnIn2S4 through the same method. The detailed characterization information of structure, photoelectric and hydrogen



evolution performance for the photocatalysts are given in the Supporting Information. Results and discussion Characterization of photocatalysts

Figure 1. XRD patterns of ZnIn2S4, CuInS2 and CuInS2/ZnIn2S4 composites with different weight contents of CuInS2. The XRD patterns of ZnIn2S4, CuInS2 and CuInS2/ZnIn2S4 composites with different weight contents of CuInS2 are shown in Figure 1. The positions of several obvious diffraction peaks are consistent with the standard peaks of hexagonal ZnIn2S4 (JCPDS No. 65-2023), respectively.25 The diffraction peaks at 27.8, 46.2 and 54.6 can be indexed as the (112), (204) and (116) crystal plane of tetragonal CuInS2 (JCPDS No. 27-0159).23 In-situ growing CuInS2 on ZnIn2S4 using a solvothermal method does not affect the crystal structure of ZnIn2S4. For all CuInS2/ZnIn2S4 composite samples, the diffraction peaks of CuInS2 are not observed. This is mainly due to the main diffraction peaks of CuInS2 too closing to that of ZnIn2S4 and thus overlapping with each other. In the following, other highly sensitive characterization techniques were used to identify CuInS2 in the composite samples.


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Figure 2. UV-Vis DRS of ZnIn2S4, CuInS2 and CuInS2/ZnIn2S4 composites with different weight contents of CuInS2. Figure 2 shows the UV-Vis DRS of ZnIn2S4, CuInS2 and CuInS2/ZnIn2S4 composites with different weight contents of CuInS2. The absorption band edge of pure ZnIn2S4 locates at about 530 nm, corresponding to about 2.3 eV band gap (see Figure S1, Supporting Information). Pure CuInS2 exhibits a strong visible-light absorption in a wide range. The absorption edge of pure CuInS2 is at about 890 nm and the band gap value of CuInS2 is calculated to be about 1.40 eV (see Figure S1 and Figure S2, Supporting Information), which is close to previous studies.26,27 As can be seen from Figure 2, the absorption edges of all CuInS2/ZnIn2S4 composite samples are extended to about 800 nm. With the increase content of CuInS2, the visible-light absorption of CuInS2/ZnIn2S4 composites is gradually enhanced. The enhanced visible-light absorption for the CuInS2/ZnIn2S4 composite samples indicate the CuInS2 photocatalyst are loaded on the surface of ZnIn2S4. The morphologies of ZnIn2S4 and 5%CuInS2/ZnIn2S4 heterojunction composite were also investigated and the results are given in Figure 3. Ahierarchical flower-like architecture is formed for pure ZnIn2S4, which constitutes with the thin nanosheets (see Figure 3 (a) and (b)). The thickness of nanosheets is about 10-20 nm. In Figure 3 (e) and Figure S3 of the Supporting Information, the ultrathin nanosheets of CuInS2 were also successfully constructed using a simple solvothermal method. Some wrinkles at the edges of CuInS2 nanosheets are observed. In situ growing CuInS2 on



Figure 3. SEM images of ZnIn2S4 (a, b) and 5%CuInS2/ZnIn2S4 (c, d). TEM images of CuInS2 (e) and 5%CuInS2/ZnIn2S4 composite (g). HRTEM images of CuInS2 (f) and 5%CuInS2/ZnIn2S4 composite (h). the surface ZnIn2S4 has no effect for its hierarchical structure (see Figure 3 (c) and (d)). In Figure 3 (d), some CuInS2 nanosheets are observed. As shown in Figure 3 (g), a typical 2D/2D heterojunction forms between CuInS2 and ZnIn2S4. Some CuInS2 nanosheets uniformly disperse on the surface of ZnIn2S4. The CuInS2/ZnIn2S4 2D/2D heterojunction has a large interface contact area, which will generate a wide range of electric fields to accelerate the photo-generated charge separation. In addition, the small thickness of ZnIn2S4 and CuInS2 nanosheets can reduce the charge transfer distance from bulk to surface, which is conducive to the photo-generated charge separation. In Figure 3 (f), the lattice spacing of 0.32 nm is attributed to (102) plane of CuInS2. In Figure 3 (h), the lattice distance of 0.32 nm in light region is assigned to the (102) plane of ZnIn2S4. While the lattice spacing of 0.32 nm in dark area could come from the (102) plane of CuInS2 due to the similar morphology of CuInS2. The EDS element mapping of Zn, In, S and Cu for the 5%CuInS2/ZnIn2S4 composite sample was also investigated and the results are indicated in Figure S4 and Figure S5 of the Supporting Information. The presence of Cu element further indicates the CuInS2/ZnIn2S4 heterojunction is formed. A low interface state density can be obtained due to the lattice match between


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the CuInS2 and ZnIn2S4 layer, which can efficiently improve the interface charge transfer and separation.19 The morphology of 5%CuInS2/ZnIn2S4 composite with loading 2wt% Pt is shown in Figure S6 of the Supporting Information. The Pt nanoparticles are relatively uniform dispersed on the surface of CuInS2 and ZnIn2S4.

Figure 4. High-resolution XPS spectra of Zn 2p (a), In 3d (b), S 2p (c) and Cu 2p (d) for the 5%CuInS2/ZnIn2S4 sample. (e) XPS spectra of Zn 2p for the ZnIn2S4 and 5%CuInS2/ZnIn2S4 samples. Figure 4 shows the surface element chemical states of ZnIn2S4 and 5%CuInS2/ZnIn2S4 composite. In Figure 4 (a), the peaks at 1021.6 eV and 1044.7 eV are indexed to the Zn2p3/2 and Zn2p1/2, respectively, which can be assigned to the Zn2+ in ZnIn2S4.28 In Figure 4(b), the spectrum of In3d shows two binding energy peaks at 444.8 eV and 452.4 eV, which can be assigned to In3+ in CuInS2 and/or ZnIn2S4.29 From Figure 4 (c), the presence of 161.4 eV and 162.5 eV peaks are related to the binding energy of S2p, indicating the S2- in CuInS2 and/or ZnIn2S4.28,29 In Figure 4 (d), two binding energy peaks of Cu2p are observed at 931.7 eV and 951.7 eV, which are ascribed to the Cu+ in CuInS2.30 No satellite peaks of Cu2+ can be observed in the binding energy spectrum of Cu2p. As shown in Figure 4 (e), compared with pure ZnIn2S4, the binding energies of Zn2p for the 5%CuInS2/ZnIn2S4 composite sample show a slightly shift towards high binding energy. The result indicates that that there is a strong interfacial chemical interaction between CuInS2 and ZnIn2S4, promoting the



interface charge transfer.31,32 Based on above structural characterizations, it is clear that the CuInS2/ZnIn2S4 2D/2D heterojunction with a large interface contact area, lattice match and a strong interfacial interaction was successfully synthesized by a simple twostep hydrothermal method. Photocatalytic activity and stability

Figure 5. (a) The rates of H2 evolution for ZnIn2S4, CuInS2 and CuInS2/ZnIn2S4 composites with different weight contents of CuInS2 under visible light irradiation (λ > 420 nm). (b) Time courses of H2 production for ZnIn2S4, CuInS2 and CuInS2/ZnIn2S4 composites with different weight contents of CuInS2 under visible light irradiation (λ > 420 nm).

Figure 6. Cyclic stability of H2 production activity over the 5%CuInS2/ZnIn2S4 sample under visible light irradiation (λ > 420 nm). The rates of H2 production for ZnIn2S4, CuInS2 and CuInS2/ZnIn2S4 composites with different weight contents of CuInS2 are shown in Figure 5. In Figure 5 (a), bare ZnIn2S4 shows a low photocatalytic performance for hydrogen production on account


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of the serious photo-generated charge recombination. After combined with CuInS2, the hydrogen production rate of 5%CuInS2/ZnIn2S4 composite is significantly increased and reaches to the highest value of 3430.2 μmol·g-1·h-1, which is much higher than that of bare ZnIn2S4. The apparent quantum efficiency of 5%CuInS2/ZnIn2S4 2D/2D heterojunction reach to 12.4% at 420 nm, which is high among the ZnIn2S4-based 2D/2D heterojunctions (see Table S1, Supporting Information). The result suggests that the unique CuInS2/ZnIn2S4 2D/2D heterojunction with a large contact area, lattice match and thin nanosheets can effectively boost the charge separation efficiency of ZnIn2S4, which is confirmed by PL spectra in the following. The H2 evolution rate of CuInS2/ZnIn2S4 composites decrease with further increase the content of CuInS2 because of the low activity of CuInS2. Time courses of H2 production for ZnIn2S4, CuInS2 and CuInS2/ZnIn2S4 composites with different weight contents of CuInS2 are indicated in Figure 5 (b). The amount of produced H2 increased over reaction time for different samples. An important indicator for practical application is the stability of photocatalyst. Therefore, the cycle experiments for H2 evolution over the 5%CuInS2/ZnIn2S4 sample were carried out and the results are given in Figure 6. No decrease of hydrogen evolution performance is observed after three cycle tests. A comparison of the XPS spectra of the fresh and stable tested 5%CuInS2/ZnIn2S4 composite are shown in Figure S7 of the Supporting Information. After stable tested, the Cu2p, In3d and S2p core level spectra are the same with the fresh 5%CuInS2/ZnIn2S4 photocatalyst, implying the stability of CuInS2. Mechanism of enhanced performance of CuInS2/ZnIn2S4 2D/2D heterojunction photocatalyst PL spectra is an effective method to assess the performance of charge separation.33,34 The PL spectra of bare ZnIn2S4 and 5%CuInS2/ZnIn2S4 composite samples are shown in Figure 7 (a). The PL peak for pure CuInS2 locates at about 510 nm (see Figure S8, Supporting Information), which can be ascribed to a high level transition in CuInS2.21 A strong PL peak at about 533 nm was detected for pure ZnIn2S4, which origins from the band gap transition.35 The strong intensity of PL peak indicates the serious charge recombination in ZnIn2S4. The intensity of PL for ZnIn2S4 is



effectively quenched by constructing the CuInS2/ZnIn2S4 2D/2D heterojunction. The result suggests that the charge separation is improved via the 2D/2D heterojunction. The photocurrent curves of ZnIn2S4 and 5%CuInS2/ZnIn2S4 composite samples were also recorded to further reveal the charge transfer process and the results are shown in Figure 7 (b). The 5%CuInS2/ZnIn2S4 composite sample shows much higher photocurrent than pure ZnIn2S4, which confirms the enhanced charge transfer from ZnIn2S4 to CuInS2. Based on above analysis, it can be concluded that the reason of the improved photocatalytic hydrogen evolution is enhanced charge separation efficiency.

Figure 7. (a) PL spectra of pure ZnIn2S4 and 5%CuInS2/ZnIn2S4 composite samples. (b) Transient photocurrent responses of pure ZnIn2S4 and 5%CuInS2/ZnIn2S4 composite samples. The band positions of ZnIn2S4 and CuInS2 were measured by Mott-Schottky and valence-band XPS spectra (see Figure 8). From Figure 8 (a), ZnIn2S4 is an n-type semiconductor. While CuInS2 shows a p-type semiconductor character. The conductive types of ZnIn2S4 and CuInS2 are consistent with previous reports.36,37 The flat band positions of ZnIn2S4 and CuInS2 locate at -0.06 VRHE and 0.63 VRHE, respectively. As shown in Figure 8 (b), the distances between the Fermi energy and valence band maximum for ZnIn2S4 and CuInS2 are 1.30, and 0.05 V, respectively. The band gap of ZnIn2S4 and CuInS2 can be estimated to be 2.30 eV and 1.40 based on the DRS results, respectively (see Figure S1, Supporting Information). According to above datum, the conduction band positions of ZnIn2S4 and CuInS2 locate at -1.06 VRHE and -0.72 VRHE, respectively. The valence band positions of ZnIn2S4 and CuInS2 are 1.24 VRHE and 0.68


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VRHE, respectively. Therefore, the CuInS2/ZnIn2S4 composite forms an I-type heterojunction. A schematic diagram of energy band positions and the mechanism of chemistry reaction process for hydrogen evoluton over the CuInS2/ZnIn2S4 heterojunction composite were plotted in Figure 9. A schematic diagram of band bending in the CuInS2/ZnIn2S4 heterojunction interface is shown in Figure S9 of the Supporting Information. An energy barrier is formed in the conduction band of heterojunction interface, which may hinder the electrons transfer from the ZnIn2S4 to CuInS2 to some extent. Unlike the electrons, the photo-generated holes can easily transfer from the ZnIn2S4 to CuInS2. As a result, the charge recombination can be inhibited in a certain extent. During the photocatalytic H2 production process, the electrons in the CB of ZnIn2S4 reduced the protons into H2. At the same time, the accumulated holes in the VB of CuInS2 can be quickly consumed by the sacrificial reagents.

Figure 8. (a) Mott-Schottky curves of ZnIn2S4 and CuInS2. (b) Valence-band XPS spectra of ZnIn2S4 and CuInS2.



Figure 9. (a) A schematic diagram of energy band position for the CuInS2/ZnIn2S4 heterojunction. (b) Mechanism of reaction process for hydrogen evolution over the CuInS2/ZnIn2S4 heterojunction under visible light irradiation. Conclusion The CuInS2/ZnIn2S4 sheet-on-sheet structure were successfully synthesized by a simple solution chemical method. The CuInS2/ZnIn2S4 2D/2D heterojunction with a large interface contact area and a lattice match is benefited for photo-generated charge separation. After optimization, the 5wt%CuInS2/ZnIn2S4 2D/2D heterojunction shows much higher photocatalytic performance of hydrogen evolution than that of bare ZnIn2S4. The enhanced photocatalytic activity can be ascribed to the improved charge separation efficiency. This study provides a strategy to efficiently improve the charge separation of ZnIn2S4 by constructing a robust 2D/2D heterojunction. Supporting Information Additional experimental information and data. Notes The authors declare no competing financial interest. Acknowledgments The work was supported by the National Natural Science Foundation of China (51702087 and 21673066), the Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University. References (1) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor


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Graphical abstract The visible-light photocatalytic hydrogen production performance of ZnIn2S4 was remarkably enhanced by constructing a robust CuInS2/ZnIn2S4 2D/2D heterojunction.


Boosting visible-light photocatalytic hydrogen evolution with an

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