CuInS2 Nanosheet Heterostructure Films

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Energy, Environmental, and Catalysis Applications

Hierarchical SnS2/CuInS2 nanosheet heterostructure film decorated with C60 for remarkable photoelectrochemical water splitting Fangfang Zhang, Yajie Chen, Wei Zhou, Can Ren, Haijing Gao, and Guohui Tian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21222 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

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ACS Applied Materials & Interfaces

Hierarchical SnS2/CuInS2 Nanosheet Heterostructure Film Decorated with C60 for Remarkable Photoelectrochemical Water Splitting Fangfang Zhang, Yajie Chen*, Wei Zhou, Can Ren, Haijing Gao, Guohui Tian* Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's Republic of China, Heilongjiang University, Harbin 150080 P. R. China

Abstract Rational architectural design and catalyst components are beneficial to improve the photoelectrochemical performance. Herein, hierarchical SnS2/CuInS2 nanosheet heterostructure porous films were fabricated and decorated with C60 to form photocathodes for photoelectrochemical water reduction. Large-size CuInS2 nanosheet film was firstly grown on the transparent conducting glass to form substrate film. Then small-size SnS2 nanosheets were epitaxially grown on the double-side of CuInS2 nanosheets to form uniform hierarchical porous laminar film. The addition of C60 on the surface of SnS2/CuInS2 porous nanosheets effectively increased visible light absorption of composite photocathode. Photoluminescence spectroscopy and impedance spectroscopy analyses indicated that the formation of SnS2/CuInS2 heterojunction and decoration of C60 significantly increased photocurrent density by promoting the electron-hole separation and decreasing the resistance to the transport of charge carriers. The hierarchical SnS2/CuInS2 nanosheet heterostructure porous film containing multiscale nanosheets and pore configurations can enlarge surface area and enhance visible light utilization. These beneficial factors make the optimized -1ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C60 decorated SnS2/CuInS2 photocathode exhibit much higher photocathodic current (4.51 mA cm-2 at applied potential -0.45 V vs. RHE) and stability than the individual CuInS2 (2.58 mA cm-2) and SnS2 (1.92 mA cm-2) nanosheet film photocathodes. The study not only reveals the promise of C60 decorated hierarchical SnS2/CuInS2 nanosheet heterostructure porous film photocathodes for efficient solar energy harvesting and conversion, but also provides rational guidelines in designing high-efficiency photoelectrodes from earth abundant and low-cost materials allowing widely practical applications. Keywords: C60/SnS2/CuInS2; nanosheet heterostructure porous film; photocathode; photoelectrochemical water splitting; hydrogen evolution

1. Introduction Energy is an important material basis for human survival and development, and the sustained and rapid development of the world economy is inseparable from strong energy security.1 The fossil energy stored on the earth is limited, but solar energy is a clean and abundant renewable energy. Therefore, utilizing solar energy to produce clean and usable fuel energy is an effective strategy to resolve the energy crisis. 2,3 Among these solar conversion techniques, the production of hydrogen energy by photoelectrochemical (PEC) water splitting has attracted great attention because of its environmental friendliness and sustainable property.4,5 In PEC water splitting system, excellent photoelectrocatalysts play critical role in effectively enhancing slow-moving kinetic processes at low overpotentials.6-8 Pt is the most effective catalyst for hydrogen evolution reaction. Unfortunately, large-scale application is limited by the -2ACS Paragon Plus Environment

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scarcity and the high cost of Pt.9 Utilization efficient and inexpensive semiconductor materials to replace Pt in PEC water splitting is considered to be a promising alternative approach to boost cathodic current and enhance the hydrogen evolution efficiency.10,11 Predominantly TiO2 has been widely studied for PEC water splitting due to their high chemical stability, nontoxicity, and low cost. However, low visible light absorption restrict its extensive application.12,13 In order to utilize visible light, narrow band bap semiconductor materials have been widely studied, Copper oxides (Cu2O and CuO) are potential candidates for PEC water splitting because of their innocuity, low price, and suitable band gap. But their poor photostability in the electrolyte with time and low photocurrent limit their performance as photocathodes in PEC water splitting.14-16

Recent years, metal sulfides have been used in solar water splitting because of their abundance,

appropriate

optical

properties,

and

adequate

charge

transport

properties.17,18 Among these metal sulfides, SnS2 exhibits strong optical anisotropy and electronic conductivity, which make SnS2 more suitable for both photocatalytic and electrocatalytic research.19,20 Besides, some p-type I−III−VI ternary metal sulfides have been widely studied in PEC hydrogen production as the photocathode materials.21,22 For example, CuInS2 has attracted considerable attention because of its high visible light absorption ability and tunable band gap energy.23 The critical problems of metal sulfides photocathodes used in PEC water splitting are photocorrosion and fast recombination of photoinduced electron-hole pairs, leading to appreciable reduction of photocurrent.24 Therefore, effective strategies are required to -3ACS Paragon Plus Environment


improve the cathodic photocurrent and stability of photoelectrocatalysts in photoelectrochemical hydrogen generation. The formation of heterojunction can not only accelerate photoinduced charge transfer and separation but also enhance photostability.25-27 For example, stable In2S3 thin layer has been loaded on CuInS2 thin film to successfully enhance cathodic photocurrent and stability, confirming that constructing the band structure system with coupling layer of proper metal sulfide can improve the photocathode performance.28 Especially the coupled heterostructures establishing the epitaxial relation at the heterostructure interface could greatly facilitate the charge-separation process and offer much-improved charge transport compared to common nanocomposites.29,30 For example, Patra et al. controlled the heterogeneous nucleations and growth of CuInS2 on appropriate facets of Au seeds, and the unique 0D-2D Au-CuInS2 coupled structures exhibited remarkably improved PEC water reduction performance.31 Some SnS2-based heterostructure composites (e.g. SnS2/g-C3N4 and SnS2/MoS2) have been prepared and widely applied in the photocatalysis and battery through the synergistic effect of SnS2 and other components in the composites.32,33 For the photocorrosion problem of metal sulfides, previous studies have proved that loading a thin layer of inexpensive and environmentally carbon on the surface of metal sulfides can also effectively inhibit the photocorrosion process.34 As a famous carbon material, C60 has special delocalized conjugated structure, so C60 molecules possess the super-strong electron-withdrawing ability.35 Therefore, C60 can serve as an superior electron acceptor to promote the photogenerated charge separation.36 -4ACS Paragon Plus Environment

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Meanwhile, C60 molecules have been acted as stable protecting layer to inhibit the photocorrosion of semiconductors.37 Because of those advantages, it is highly desired to introduce C60 into the metal sulfides to enhance the activity and stability.38 Furthermore, thin film materials with special morphologies have gained enhanced PEC performance compared with their corresponding bulk counterparts.39,40 Among these morphologies and structures, 2D ultrathin nanosheets have become potential candidates in fields of PEC devices because of their unique structure, physical and chemical properties.41,42 The thin films engineered by 2D thin nanosheets can possess large reaction area, improved surface evolution kinetics, reduced carriers diffuse length and enhanced light absorption. Above these influencing factors provide useful guidelines in designing high-efficiency photocathodes for PEC water splitting. Synthesizing hierarchical porous nanosheet film substrate with high visible light harvesting, and then coupling appropriate semiconductor with nanosheet film substrate together with the surface modification via introducing amorphous-carbon layer to enhance stability and photoinduced charge transfer and separation rate. For this reason, considerably enhanced photoelectrochemical performance can be expected. Inspired by these discoveries, in this work, therefore, we fabricated C 60 decorated SnS2/CuInS2 photocathode for photoelectrochemical water splitting. As shown in Scheme S1, large-size CuInS2 nanosheets were firstly grown on the FTO glass to form hierarchical nanosheet porous film via the hydrothermal reaction. Then small-size SnS2 nanosheets epitaxially grew on the double-side surface of CuInS2 nanosheets -5ACS Paragon Plus Environment


during the hydrothermal process. The CuInS2 and SnS2 nanosheets in composite film have relatively matched band structures to form heterojunction. Meanwhile, C60 clusters were introduced to act as electron acceptor to promote charge separation and protect layer to inhibit the photocorrision of SnS2/CuInS2. The fabricated heterostructure composite films exhibited enhanced visible light absorption, increased catalytic active sites, and accelerated charge transport and separation. Thus the optimized C60 decorated hierarchical SnS2/CuInS2 nanosheet heterostructure porous film exhibited significantly enhanced photoelectrochemical hydrogen evolution activity and stability. 2. Experimental 2.1. Materials Thiourea (CH4N2S, 99.0%) was purchased from Kermel Chemical Reagent Co., Ltd (Tianjin, China). Indium nitrate (In(NO3)3∙4.5H2O, 99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Copper choride (CuCl2∙2H2O, 99.0%) was purchased from BASF Chemical Co., Ltd (Shanghai, China). Ethanol (C2H5OH, 99.7%) was purchased from Kermel Chemical Reagent Co., Ltd (Tianjin, China). Thioacetamide (C2H5NS, 99.0%) was purchased from the GuangFu Fine Chemical Industry Research Institute. Tin chloride (SnCl4∙5H2O) was purchased from the GuangFu Fine Chemical Industry Research Institute. 2.2. Preparation of CuInS2 nanosheet film Typically, 85.4 mg CuCl2∙2H2O, 191 mg In(NO3)3∙4.5H2O and 152 mg thiourea (CH4N2S) were added into 25 mL ethanol under strongly stirring for 30 min, -6ACS Paragon Plus Environment

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separately. Afterwards, the mixture was transferred to Teflon-lined autoclave and subsequently, the thoroughly cleaned fluorine-doped tin oxide (FTO) conductive glass (Asahi Glass, Japan) substrate (4 × 2 × 1 cm 3) was placed in a Teflon-lined stainless steel autoclave at an angle of about 45 o. The Teflon-lined stainless steel autoclave was sealed and heated to 180 oC and maintained for 16 h. After cooling to room temperature naturally, the as-prepared products were washed several times with ethanol and deionized water, and finally dried at 60 oC for 12 h. 2.3. Preparation of SnS2/CuInS2 nanosheet films In a typical synthesis of SnS2/CuInS2 films, 140.4 mg SnCl4∙5H2O and 60.4 mg C2H5NS were added separately into 25 mL ethanol under stirring intensively, then the mixture was transferred to Teflon-lined autoclaved and the as-prepared CuInS2 nanosheet film was immersed into the above solution and then heated 100 oC for 6 h. After cooling to room temperature naturally, the obtained SnS2/CuInS2 nanosheet films were washed with ethanol and then dried at 60 oC for 12 h. Similar experiments were also done by changing the initial amount of SnCl4∙5H2O (87.5 and 175.5 mg) and C2H5NS (37.5 and 75.5 mg) in the reaction process. The obtained films were named from low to high SnCl4∙5H2O and C2H5NS concentration as: SnS2-1/CuInS2, SnS2-1.6/CuInS2 and SnS2-2/CuInS2. The synthetic process was shown in the Scheme S1. 2.4. Preparation of C60-decorated SnS2/CuInS2 nanosheet films The C60-decorated SnS2/CuInS2 nanosheet films were synthesized as follows: The optimized SnS2/CuInS2 nanosheet film prepared in step 2.2 was immersed into 20 -7ACS Paragon Plus Environment


mL C60-toluene solution (30 mg L-1) with stirring for 20 min, and then died at 50 oC for 6 h. 2.5. Characterization The morphology and structure of obtained samples were characterized by scanning electron microscopy (SEM, Hitachi, S-4800) and the microstructure of samples were studied by transmission electron microscopy (TEM) on a JEOL 2100 TEM microscope operated at 200 kV. The X-ray diffraction (XRD) data of the final crystal phase information was tested by a Smartlab 9KW X-ray diffractometer using Cu Ka radiation (λ= 0.15405 nm, 40 kV, 100 mA). Raman spectra were collected from Horiba HR 800 spectrometer operated at a excitation wavelength of 457.9 nm. The optical properties of film was analyzed by UV-vis diffuse reﬂectance spectra (DRS) on the a UV-vis spectrophotometer (ShimadzuUV-2550). The chemical composition information of acquired product was detected by using X-ray photoelectron spectroscopy (XPS, Kratos-AXISULTRA DLD, Al Ka X-ray source). Photoluminescence (PL) was measured on a FL920 spectrofluorometer (Edinburgh Instruments) with the photoexcitation wavelength set at 350 nm. 2.6. Photoelectrochemical measurements PEC measurements were carried out on a BAS100B electrochemical workstation in a standard three-compartment cell containing 200 mL of 0.5 mol∙L−1 Na2SO4 solution as supporting electrolyte that was purged with nitrogen for 10 min prior to the measurement with different films as the working electrode, a graphite rod as the counter electrode, and a saturated Ag/AgCl as the reference electrode. Photocurrents -8ACS Paragon Plus Environment

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of the different samples were obtained under solar simulator lighting using a 300 W Xe lamp (100 mW cm-2) with a 1.5 AM filter. The potential was swept from -0.6 to +0.25 V (vs RHE) at a sweeping rate of 10 mV∙s−1. 3. Results and discussion 3.1. Morphology and structure characterization The pure CuInS2 and SnS2 films used in this study were grown on FTO glass substrates, respectively. As confirmed by XRD characterization (Figure 1A). The as-obtained CuInS2 and SnS2 were crystallized chalcopyrite CuInS2 (JCPDS card No. 27-0159) and hexagonal SnS2 (JCPDS card No. 23-0677), respectively. 43,44 In the XRD patterns of SnS2-1.6/CuInS 2 and C60/SnS2-1.6/CuInS 2, all the diffraction peaks can be assigned to chalcopyrite CuInS2 (JCPDS card No. 27-0159) and hexagonal SnS2 (JCPDS card No. 23-0677), indicating the formation of SnS2/CuInS2 composite. The samples were also characterized by Raman spectra (Figure 1B). The wide peak at about 300 cm −1 can be regarded as the integration of 292 and 307 cm−1, which are assigned to A1, E modes of the chalcopyrite phase of CuInS2.45 The main peak at 317 cm−1 and a weak peak at 204.5 cm -1 are assigned to the characteristic A 1g and Eg modes of SnS2.46 The Raman spectra of SnS2-1.6/CuInS 2

and

C60/SnS2-1.6/CuInS 2

possess

the

Raman

spectra

characteristics of both SnS2 and CuInS2, indicating the formation of SnS2/CuInS2 heterostructure

composite.

Meanwhile,

in

the

Raman

spectrum

of

C60/SnS2-1.6/CuInS2, the bands at 265, 489, 1415, and 1460 cm-1 can be assigned to the hg(1) 272 cm-1, ag(1), hg(7), hg(8) modes for the Raman-active vibration mode of C60, respectively, indicating the successful decoration of C60 on the SnS2-1.6/CuInS 2.47 The surface morphology and structure difference between CuInS2 and SnS2 films were revealed by SEM characterization. As shown in -9ACS Paragon Plus Environment


Figure 1C, D, the pristine CuInS2 film was made up of micrometer-size nanosheets with mutual cross-linking and smooth surface, and the thickness of nanosheets is about 50 nm. These cross-linked CuInS 2 nanosheets formed porous structure nanosheet film. For comparison, SnS2 nanosheets only have nanometer scale length, and the thickness of nanosheets is about 15 nm. These SnS 2 nanosheets form densely packed multilevel pore structure film (Figure 1E, F).

Figure 1. (A) and (B) are the XRD patterns and Raman spectra of the different samples scraped from the corresponding films, respectively. (C) and (D) are the SEM images of CuInS 2 films with different magnifications. (E) and (F) are the SEM images of SnS 2 films with different magnifications.

The morphology and structure of SnS2/CuInS2 films prepared from different -10ACS Paragon Plus Environment

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amounts of starting materials were also characterized by SEM. As shown in Figure 2A1-A3, the prepared large-size CuInS2 nanosheets act as supporting skeletons, and small-size SnS2 nanosheets then epitaxially grew on the double surface of CuInS2 nanosheets (SnS2-1.6/CuInS2). Moreover, the surface morphologies of the films have significant change with the change of the feed ratios of the starting materials. With the increase of feed ratio, the SnS2 nanosheets on the surface of CuInS2 skeletons became dense (SnS2-1.6/CuInS2). When the feed ratio reaches a high level, additional SnS2 nanosheets covered on the upside of SnS2 nanosheet film to form double-layer film (SnS2-2/CuInS2), which is also revealed by the cross-sectional SEM images in Figure 2A4-C4. The thicknesses of the SnS2-1/CuInS2 (Figure 2A4) and SnS2-1.6/CuInS2 (Figure 2B4) single-layer films are about 8 μm. The thickness (12 μm) of the SnS2-2/CuInS2 double-layer film is significantly increased compared with the single-layer films. Both the upper surface and cross-sectional SEM images showed porous structures consisting of uniformly distributed nanosheets. It is apparent that the formed SnS2-1.6/CuInS2 film can possess increased catalytic active sites and visible light absorption compared with SnS2-1/CuInS2 and SnS2-2/CuInS2 films, thus contributing

to

the

sufficient

contact

between

catalyst

and

electrolyte.

Energy-dispersive X-ray spectroscopy (EDS) elemental mapping images in Figure S1 further proved the presence of Sn, S, Cu, In, and C elements in the film, confirming the existence and uniform distribution of C60 on the surface of SnS2-1.6/CuInS2 film.

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Figure 2. SEM images of the as-obtained nanosheets arrays film with different magnifications: (A1-A4) SnS2-1/CuInS2, (B1-B4) SnS2-1.6/CuInS2, and (C1-C4) SnS2-2/CuInS2. Thereinto, A4, B4, and C4 are the corresponding cross-sectional SEM images.

The TEM characterization further confirmed the two-dimensional nanosheet structure of CuInS2 (Figure 3A) and SnS2 (Figure 3C). The HRTEM images in Figure 3B and 3D expressly show the lattice fringe of 0.166 nm and 0.163 nm, respectively, which match well with the (312) plane of the chalcopyrite phase CuInS2 and (103) plane of hexagonal SnS2, respectively.48,49 From the TEM image of the optimized C60/SnS2-1.6/CuInS2 in Figure 3E, some small nanosheets grown on the large-size -12ACS Paragon Plus Environment

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nanosheets can be observed. The corresponding HRTEM image (Figure 3F) indicates that nanoscale SnS2 nanosheets preferentially grew along the (312) crystal of large-size CuInS2 nanosheets and formed CuInS2-SnS2 heterojunction, which enable better charge separation and transport compared with individual CuInS2 and SnS2.

Figure 3. (A), (C), and (E) are the TEM images of CuInS2, SnS2, and C60/SnS2-1.6/CuInS2, respectively. (B), (D), and (F) are the corresponding HRTEM images, respectively.

The XPS technique was used to characterize the chemical states of Cu, In, S, and Sn elements of different samples. The survey XPS spectrum shown in Figure S2 indicates the existence of Cu, In, S, Sn, and C elements in the obtained C60/SnS2-1.6/CuInS2. In the Figure 4A, the Cu 2p orbit is split into two peaks loaded at 931.9 eV and 951.7 eV, which can be referred to Cu 2p3/2 and Cu 2p1/2 of Cu1+, respectively.50 In the In 3d XPS specta, the peaks at 444.6 eV and 452.1 eV (Figure 4B) can be accordingly assigned to In 3d5/2 and In 3d3/2, respectively.51 In Figure 4C, -13ACS Paragon Plus Environment


the two peaks at 162.0 eV and 163.3 eV in the high-resolution S 2p spectrum of SnS2 are consistent with the S2−. Meanwhile, the two peaks shift to the lower binding energy (161.6 eV and 162.8 eV) in the S 2p XPS spectrum of C60/SnS2-1.6/CuInS2. In Figure 4A, the presence of Sn peaks at 487.1 eV for Sn 3d5/2 and 495.5 eV for Sn 3d3/2 in the XPS spectrum represents the characteristic signals of Sn4+ in SnS2 (Figure 4D).52 But in the spectrum of C60/SnS2-1.6/CuInS2, the two peaks obviously shift to lower binding energies of 486.6 eV and 494.9 eV, respectively. The shifts of binding energies indicate the strong interaction between a CuInS2 and SnS2, which is favorable for the acceleration of the charge transport.

Figure 4. High resolution XPS spectra of (A) Cu 2p (B) In 3d (C) S 2p (D) Sn 3d of different samples.


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Figure 5. (A) Steady-state photoluminescence emission spectra, and (B) transient state photoluminescence decay spectra of different samples.

In order to understand the photogenerated charge separation in the PEC process, steady-state and time-resolved photoluminescence (PL) tests were carried out under the excitation wavelength of 325 nm at room temperature. Generally, the lower PL intensity indicates the lower recombination rate and the higher separation efficiency of

the

photo-generated

charge

carriers.

Figure

5A

shows

steady-state

photoluminescence decay spectra of different samples. The photoluminescence spectra of bare CuInS2 and SnS2 show a broad and strong emission peak around 520 nm, indicating there are serious electron and hole recombination in SnS2 and CuInS2. But the spectral intensity of the SnS2-1.6/CuInS2 was obviously reduced, implying the efficient charge separation originating from the heterojunction of CuInS2 and SnS2. After C60 decoration, the PL intensity of the C60/SnS2-1.6/CuInS2 was further reduced, which means C60 surface decoration has a positive effect on the charge separation and transport. Time-resolved PL spectra in Figure 5B showed that the photoinduced charge carriers lifetime of C60/SnS2-1.6/CuInS2 is obviously higher than that of -15ACS Paragon Plus Environment


individual SnS2 and CuInS2 (Table S1). The improved separation and transport efficiency of photoinduced charge carriers in the C60/SnS2-1.6/CuInS2 can remarkably promote the PEC activity. The optical absorption properties of different samples were studied by UV-vis absorption spectroscopy. As shown in Figure 6, CuInS2, SnS2, SnS2-1.6/CuInS2, and C60/SnS2-1.6/CuInS2 all exhibited strong absorption over the whole ultraviolet and visible light regions. Moreover, the photoresponse of C60/SnS2-1.6/CuInS2 in the visible light region is increased compared to that of CuInS2 and SnS2-1.6/CuInS2 due to the surface decoration of C60. The increased visible light absorption contributes to the enhancement of PEC activity.

Figure 6. UV-vis diffuse reflectance spectra of CuInS2, SnS2, SnS2-1.6/CuInS2, and C60/SnS2-1.6/CuInS2.

3.3. Photoelectrochemical performance To investigate the PEC activities of the samples, a three-electrode system was used in 0.5 M Na2SO4 electrolyte. The photocurrent-potential curves of CuInS2, SnS2, -16ACS Paragon Plus Environment

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SnS2-1.6/CuInS2, and C60/SnS2-1.6/CuInS2 were acquired via linear sweep voltammetry under both dark condition and visible light illumination, as shown in Figure 7. The single SnS2 and CuInS2 show relatively low current densities. But with the formation of SnS2/CuInS2 heterostruction and the introduction of C60, the dark currents were gradually increased (Figure 7A). Notably, under visible light irradiation, the photocurrents of different electrodes are much higher than the dark currents of the corresponding electrodes (Figure 7B). Similarly, the enhanced photocurrent response could be observed for the C60/SnS2-1.6/CuInS2 and SnS2-1.6/CuInS2 compared with individual CuInS2 and SnS2. The photocurrent densities of C60/SnS2-1.6/CuInS2 and SnS2-1.6/CuInS2 are 4.51 and 3.56 mA cm-2 under the -0.45 bias (vs RHE) condition, respectively, which are 1.74 and 1.37 times higher than that (2.58 mA cm-2) of individual CuInS2. The enhanced photocurrent can be ascribed to the enhanced visible light absorption and quick photoinduced charge transport and separation due to the introduction of SnS2 and C60. Meanwhile, the enhanced visible light utilization through multiple reflections of the hierarchical porous structure of the composite film also play a positive role in the enhanced photocurrent. Notably, the improved PEC activity come from the enhanced transport and separation rate of photoinduced charge carriers due to the formed SnS2/CuInS2 heterojunction as indicated in PL spectra (Figure 5). Moreover, the SnS2 nanosheets content in the SnS2/CuInS2 also influence the current densities under both dark and visible light irradiation conditions (Figure S3). With the increase of SnS2 nanosheets content, the PEC activity gradually increased and the optimized SnS2-1.6/CuInS2 showed the highest PEC activity, then -17ACS Paragon Plus Environment


the PEC activity decreased when double-layer film (SnS2-2/CuInS2) formed. It is because that when the dense SnS2 nanosheets covered the whole CuInS2 film, CuInS2 can not fully absorb visible light. So the yield of photoinduced electrons is decreased. Moreover, the carrier transport process in the double-layer film are also limited by the recombination in the bulk. For comparison, Pt wire was also used as counter electrode to replace graphite rod, their PEC activities had no obvious change (Figure S4). Moreover, the water contact angles in Figure S5 indicated the hydrophilicity of hierarchical C60/SnS2-1.6/CuInS2 nanosheet heterostructure porous film, which contributes to the intimate contact between catalyst and electrolyte, thus improving the PEC activity.

Figure 7. Photocurrent density-voltage curve of different electrode materials under dark (A) and visible light irradiation (B) conditions, transient photocurrent density-time curves (C) of the different electrodes at a constant applied voltage of -18ACS Paragon Plus Environment

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-0.45 V vs RHE, and (D) Linear sweep voltammograms and stability test at a constant applied voltage of -0.45 V vs RHE (inset) of C60/SnS2-1.6/CuInS2 electrode.

In order to study the PEC properties of different materials, transient photocurrent density-time curves of different photocathodes recorded under chopped light illumination at -0.45 V vs RHE were acquired (Figure 7C). These photocathodes also exhibit moderate dark current values. When visible light illuminates the photoelectrode, the photocurrent rises abruptly and then tends to be stable. After subtracting the dark current density, the C60/SnS2-1.6/CuInS2 shows photocurrent density of about 1.48 mA cm-2, which is higher than all of other photocathodes, indicating that the C60/SnS2-1.6/CuInS2 has better separation efficiency of photogenerated electron and hole. Obviously, the modification of SnS2/CuInS2 heterojunction and C60 has a promoting effect on the PEC properties of CuInS2. Moreover, the SnS2-1.6/CuInS2 film can possess the highest transient photocurrent density compared with SnS2-1/CuInS2 and SnS2-2/CuInS2 films, (Figure S6). The stability of different photoelectrodes was also investigated. As shown in Figure 7D, the photocurrent density of C60 decorated SnS2-1.6/CuInS2 after 15 recycles has no obvious decrease compared with that of the first recycle, and the photocurrent is relatively stable and constant for over 3 h under biasing potential of -0.45 V vs RHE (inset of Figure 7D). The results reveal the great promise of the C60/SnS2-1.6/CuInS2 in photoelectrochemical application. In this study, the three-dimensional hierarchical porous configuration and dual -19ACS Paragon Plus Environment


interfacial heterostructure of C60/SnS2-1.6/CuInS2 film exhibit remarkable influence on the PEC performance, and are expected to be responsible for the improved photocurrent in the photoelectrode. In order to confirm the thesis, we prepared C60/SnS2/CuInS2 solid film where SnS2 nanoparticle film loaded on the surface of CuInS2 solid film (Figure S7). The control PEC experiments showed that the prepared optimized hierarchical C60/SnS2-1.6/CuInS2 heterostructure porous film exhibited a better PEC performance than the C60/SnS2/CuInS2 bulk solid film (Figure S8). In PEC system, water splitting catalytic reactions occur on the surface of the photoelectrodes, so the morphology of electrode has a remarkable influence on the PEC catalytic reactions. For C60/SnS2-1.6/CuInS2 film, the stereo hierarchical porous nanosheet configuration can improve visible light utilization by realizing multiple visible light reflections of incident light via the irregular pores and form abundant heterostructure interfaces, which contribute to the enhancement of PEC activity.


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Figure 8. (A) and (B) are the EIS Nyquist plots of different samples in the dark (The inset in plot (A) is its equivalent circuit) and under visible light irradiation, respectively. (C) Mott–Schottky plots of pristine CuInS2 and C60/SnS2-1.6/CuInS2 films at a frequency of 5 kHz. (D) Incident photon to current conversion efficiency spectra of the different samples.

In order to determine the photocurrent is originated from the hydrogen evolution, the hydrogen production for the PEC was carried out under AM 1.5G (100 mW cm-2). Before the test, N2 is used to purge the electrolyzer to remove dissolved oxygen from the electrolyte.The hydrogen gas production can be seen at the photocathode with an applied bias of -0.45 V vs RHE. The produced hydrogen gas in a closed photoelectrochemical cell was collected and confirmed by a gas chromatograph equipped. Figure S9 shows the time course of the hydrogen gas, and the hydrogen gas amount increased linearly with reaction time. The C60/SnS2-1.6/CuInS2 photocathode possessed the hydrogen production rate of 127 μmol cm-2 at -0.45 V (vs RHE) under AM 1.5G for 6 h, which was about 20.03 and 4.37 times that of the pure SnS2 and CuInS2 photocathodes, respectively. The results revealed that the present PEC system can

significantly

promote

photo-generated

electrons

transfer

to

the

C60/SnS2-1.6/CuInS2 photocathode to produce hydrogen.

In order to evaluate the interface charge separation efficiency, electrochemical impedance spectroscopy (EIS) measurement was employed. Figure 8A, B show the -21ACS Paragon Plus Environment


EIS Nyquist plots of CuInS2, SnS2, SnS2-1.6/CuInS2, and C60/SnS2-1.6/CuInS2 under dark condition and visible light irradiation, respectively. The equivalent circuits were shown in inset of Figure 8A, B, where Rct is the charge transfer resistance, and Rs is the solution resistance. Both under dark condition and visible light irradiation, the Rct value of C60/SnS2-1.6/CuInS2 is much smaller than those of CuInS2, SnS2, and SnS2-1.6/CuInS2, which indicates a fast interfacial charge-transfer caused by the formation of SnS2/CuInS2 heterojunction and C60 decoration. Meanwhile, the radii of Nernst curves of the samples under visible light irradiation are smaller than those of the samples under dark condition, which indicates that visible light irradiation can promote effective separation of photogenerated electron-hole. Moreover, the EIS Nyquist plots in Figure S10 also showed that the optimized SnS2/CuInS2 ratio is more effective in promoting the separation of photogenerated electron-hole in the dark and especially under visible light irradiation. In order to further evaluate the interface charge transfer properties of the different electrodes, Mott-Schottky plots of different samples were measured. As shown in Figure 8C, the negative slope of the CuInS2 plot indicates its p-type property. Meanwhile, the lower slope of Mott-Schottky plot of C60/SnS2-1.6/CuInS2 denotes higher charge carrier concentration and more efficient charge transfer through cascade structure.53, 54 Moreover, the upwards shift of the Fermi level caused by the increased charge carrier concentration after the introduction of n-type SnS2 (Figure S11) and C60 could facilitate the charge separation at the interface of the heterostructure electrode and electrolyte. The accelerated transfer of photogenerated electrons from bulk to -22ACS Paragon Plus Environment

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surface of C60/SnS2-1.6/CuInS2 film was further confirmed by the Scanning Kelvin Probe (SKP) technique, which can exhibit relatively flat potential change and mensurate the interface electric field direction.55,56 The work functions of the different films in Figure S12 showed that the C60/SnS2-1.6/CuInS2 film possessed the lowest work function, indicating that the addition of SnS2 and C60 on the surface of CuInS2 film could greatly accelerate the escape of electrons from C60/SnS2-1.6/CuInS2 film surface and increase the chance of electrons to participate in hydrogen proton reduction and hydrogen evolution. Incident-photon-to-current-conversion efficiency (IPCE) tests were carried out to study the interplay between light absorption and PEC activity of the different photocathodes at -0.45 V vs. RHE (Figure 8D). It can be observed that both CuInS2 and SnS2 showed relative lower IPCE values than the SnS2-1.6/CuInS2 photocathode throughout the visible light wavelength region. Furthermore, the IPCE of SnS2-1.6/CuInS2 was further improved by the decoration of C60, and the IPCE value of C60/SnS2-1.6/CuInS2 reaches around 4-8% in the wavelength range from 400 to 600 nm. This indicates that the separation and transfer of photoexcited charge carriers are more efficient in C60/SnS2-1.6/CuInS2 under visible light irradiation. The integrated photocurrent density was also calculated by the IPCE data according to the equation reported previously.57 The integrated photocurrents for different electrodes (Figure S13) are similar to the measured photocurrent density values from photocurrent-voltage characterisation at -0.45 V vs RHE in Figure 7B.



Figure 9. Schematic diagram of the photoinduced charge-transfer process of the C60 decorated SnS2/CuInS2 photoelectrochemical system.

To understand the working principle of C60 decorated SnS2/CuInS2 photocathode system, a schematic diagram of the C60 decorated SnS2/CuInS2 was presented in Figure 9. When visible light is irradiated on the surface of C60 decorated SnS2/CuInS2 film, both SnS2 and CuInS2 absorb visible light, and electrons and holes are generated. Meanwhile, the C60 act as sensitizer to absorb visible light. Obviously, C60 decorated SnS2/CuInS2 heterostructure brings more efficient incident light utilization than when only CuInS2 is used as photocathode. The band gaps calculated from the absorption spectra of SnS2 and CuInS2 are 1.62 eV and 1.82 eV, respectively (Figure S14). The valence band positions of SnS2 and CuInS2 were 1.67 eV and 1.08 eV, respectively (Figure S15). Obviously, the conduction band and valence band of SnS2 are lower than those of CuInS2. The construction of this p-n heterojunction (SnS2/CuInS2) is -24ACS Paragon Plus Environment

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conducive to the transport of electronic carriers via their staggered band configuration. Electrons in conduction band of CuInS2 move toward the conduction band of SnS2, and then these electrons transferred to C60 to participate in adsorbing and reducing H+. Meanwhile, holes from the valence band of SnS2 migrate to that of CuInS2 because of the potential difference between SnS2 and CuInS2. Thus, C60 decorated SnS2/CuInS2 photocathode can realize efficient charge separation across the SnS2/CuInS2 heterojunction and improve its photoelectrochemical water reduction performance. In the configuration, surface decorated C60 also acts as passivation layer to protect SnS2/CuInS2 from oxidation, and thus making SnS2/CuInS2 photocathode exhibit a greatly improved photostability. 4. Conclusions In summary, we have designed and prepared C60 decorated SnS2/CuInS2 hierarchical porous heterostructure nanosheet photocathodes by growing SnS2 nanosheets on double-side of CuInS2 nanosheets decorated with C60 as electron acceptor and passivation layer for a photoelectrochemical hydrogen generation system. The obtained optimal C60 decorated SnS2/CuInS2 heterostructure nanosheet film is an efficient

photoelectrochemical

photocathode

with

good

photostability.

The

hierarchical porous heterostructure provides an enhanced surface area and abundant active sites, effective light capturing by light scattering effects, and rapid charge transfer by a directional transport path through the dual interfacial heterojuction. It is believed that this research can further promote the development of metal sulfide heterostructure films as non-noble metal photocathode materials for future -25ACS Paragon Plus Environment


photoelectrochemical hydrogen evolution and solar cell applications.

ASSOCIATED CONTENT Author Information *Corresponding author: [email protected], [email protected], Tel.: +86 451 8660 8781, Fax: +86 451 8667 3647 Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Natural Science Foundation of China (51772079, 51672073), Natural Science Foundation of Heilongjiang Province of China (B2017009), and Special Fund of Technological Innovation Talents in Harbin City (No. 2015RAQXJ003).

Supporting Information SEM images and elemental mappings, XPS spectra, and photoelectrochemical performance of the catalyst films under different conditions. Tables showing adsorption kinetics parameters, water contact angle photographs and other contents. This information is available free of charge via the Internet at http://pubs.acs.org/.

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(52) Zhang, X.; Zhang, P.; Wang, L. J.; Gao, H. Q.; Zhao, J. T.; Liang, C. H.; Hua, J. H.; Shao, G. S. Template-Oriented Synthesis of Monodispersed SnS2@SnO2 Hetero-Nanoflowers for Cr (VI) Photoreduction, Appl. Catal. B: Environ. 2016, 192, 17-25. (53) Chen, B. Y.; Fan, W. Q.; Mao, B. D.; Shen, H.; Shi, W. D. Enhanced Photoelectrochemical Water Oxidation Performance of a Hematite Photoanode by Decorating with Au–Pt Core–Shell Nanoparticles, Dalton Trans. 2017, 46, 16050-16057. (54) Rohloff, M.; Anke, B.; Zhang, S. Y.; Gernert, U.; Scheu, C.; Lercha, M.; Fischer, A. Mo-Doped BiVO4 Thin Films–High Photoelectrochemical Water Splitting Performance Achieved by a Tailored Structure and Morphology, Sustainable Energy Fuels 2017, 1, 1830-1846. (55) Kumar, A.; Mohanty, T. Electro-Optic Modulation Induced Enhancement in Photocatalytic Activity of N-Doped TiO2 Thin Films, J. Phys. Chem. C 2014, 118, 7130-7138. (56) Zhang, K. F.; Zhou, W.; Zhang, X. C.; Sun, B. J.; Wang, L.; Pan, K.; Jiang, B.J.; Tian, G. H.; Fu, H. G. Self-Floating Amphiphilic Black TiO2 Foams with 3D Macro-Mesoporous Architectures as Efficient Solar-Driven Photocatalysts, Appl. Catal. B: Environ. 2017, 206, 336-343. (57) Gurudayal; Chee, P. M.; Boix, P. P.; Ge, H.; Yanan, F.; Barber, J.; Wong, L. H. Core−Shell Hematite Nanorods: A Simple Method To Improve the Charge



Transfer in the Photoanode for Photoelectrochemical Water Splitting, ACS Appl. Mater. Interfaces, 2015, 7, 6852-6859.

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CuInS2 Nanosheet Heterostructure Films

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