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Enhanced Photocarrier Separation in Hierarchical Graphitic‑C3N4‑Supported CuInS2 for Noble-Metal-Free Z‑Scheme Photocatalytic Water Splitting Xiaoxue Li,† Keyu Xie,‡ Long Song,† Mengjia Zhao,† and Zhipan Zhang*,† †
Key Laboratory of Cluster Science, Ministry of Education of China; Beijing Key Laboratory of Photoelectric/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China ‡ State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China S Supporting Information *
ABSTRACT: The effective separation of photogenerated electrons and holes in photocatalysts is a prerequisite for efficient photocatalytic water splitting. CuInS2 (CIS) is a widely used light absorber that works properly in photovoltaics but only shows limited performance in solar-driven hydrogen evolution due to its intrinsically severe charge recombination. Here, we prepare hierarchical graphitic C3N4supported CuInS2 (denoted as GsC) by an in situ growth of CIS directly on exfoliated thin graphitic C3N4 nanosheets (g-C3N4 NS) and demonstrate efficient separation of photoinduced charge carriers in the GsC by forming the Z-scheme system for the first time in CIS-catalyzed water splitting. Under visible light illumination, the GsC features an enhanced hydrogen evolution rate up to 1290 μmol g−1 h−1, which is 3.3 and 6.1 times higher than that of g-C3N4 NS and bareCIS, respectively, thus setting a new performance benchmark for CIS-based watersplitting photocatalysts. KEYWORDS: Z-scheme, copper indium sulfide, graphitic carbon nitride, hydrogen generation, photocatalytic water splitting
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Kobayakawa et al. first studied CIS as the photocatalyst for hydrogen evolution under UV light and observed a rather low H2 generation rate (few μmol g−1 h−1), probably due to the severe charge recombination induced by lattice defects.10 One way to address this issue is to introduce cocatalysts of noblemetal species on the CIS to promote HER and accordingly reduce the chance of unwanted photocarrier recombination. Accordingly, Xie and co-workers prepared a Pt-loaded monodisperse hierarchical CIS architecture and showed a visible-light-driven H2 production rate of 84 μmol g−1 h−1 with Na2S and Na2SO3 as hole scavengers.11 In another work, Rumodified CIS quantum dots were synthesized and an enhanced H2 evolving rate of 522 μmol g−1 h−1 was demonstrated.12 More recently, Ye et al. developed an alloyed ZnS/CIS nanorods photocatalyst with Pt and Pd4S as cocatalysts and achieved a H2 generation rate of 225 μmol g−1 h−1.13 Despite all of these improvements, the use of noble-metal components suffers from their high material costs and should ideally be excluded from the system. However, attempts to replace noblemetal cocatalysts in CIS-based photocatalysts either by adopting low-cost transition metal compounds14 or building simple p−n junctions have so far failed to give competitive performance in photocatalytic HER.15,16
INTRODUCTION With the rapid depletion of fossil fuels and the environmental issues associated with their combustion, it is indispensable to develop clean and renewable energy systems to solve these problems. Photocatalytic splitting of water on semiconductorbased photocatalysts renders an attractive solution by directly converting the renewable solar energy into the clean fuel hydrogen.1−4 Generally, an ideal material for photocatalytic water splitting is expected to possess a suitable band gap to absorb a large portion of the solar spectrum and an appropriate conduction band (CB) potential negative enough to drive the hydrogen evolution reaction (HER).5 The apparent efficiency of the catalytic HER is literally determined by the harvesting of incident photons, the effective separation of light-induced charge carriers (electrons and holes), and the electron transfer from the CB of the photocatalyst to H+ in the electrolyte. As the recombination between photogenerated electrons and holes in the photocatalyst is deemed to be the dominant loss channel, the effective separation of the two is a prerequisite for efficient photocatalytic water splitting. For instance, the semiconducting CuInS2 (CIS) has been widely adopted as the light absorber in solar cells due to its high absorption coefficient and an optimal band gap of 1.5−1.9 eV. Starting from 3.62% reported in the late 1970s,6 the solar-to-electricity conversion efficiency of CISbased solar cells has well been improved to (16.5 ± 0.2)% during the past decade.7−9 On the contrary, CIS has only shown limited performance in solar-driven HER until now. © XXXX American Chemical Society
Received: April 29, 2017 Accepted: July 4, 2017 Published: July 4, 2017 A
DOI: 10.1021/acsami.7b06030 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
glycol intercalation approach under ultrasonication (Scheme 1a).31−33 The TEM images in Figure S3 and atomic force microscopy (AFM) height profiles in Figure S4 confirm the formation of thin g-C3N4 NS. The N2 adsorption−desorption experiment in Figure S5 shows that the specific surface area of g-C3N4 NS is over 18 times higher than that of the original gC3N4 spongy film (ca. 182 vs 10 m2 g−1), and such an improvement in the specific surface area is expected to expose more surface sites for the subsequent in situ growth of CIS. Precursors of CIS (see Experimental Section for details) were then added into the dispersion of g-C3N4 NS and hydrothermally treated to yield the GsC (Scheme 1b). For comparison, a control sample of CIS (denoted as bare-CIS) was also synthesized under the same condition but in the absence of g-C3N4 NS. Additionally, GsC samples with different g-C3N4-to-CIS mass ratios of 5:1, 2:1, 1:1, and 1:2 were prepared, and the sample of 2:1 ratio was found to feature the highest photocatalytic HER activity (vide infra). Therefore, unless otherwise specified, the GsC prepared with a 2:1 gC3N4-to-CIS mass ratio was used for different measurements and characterization. The GsC features a hierarchical flower-like architecture comprising intercrossed thin lamellae of a few hundred nanometers in length (SEM images in Figure 1a) and numerous nanoporous channels that permit sufficient impregnation of the electrolyte to the interior surface of the catalyst. While this morphology is not fundamentally different from that of the bare-CIS (Figure S6), topological profiles measured by the AFM in Figure 1b demonstrate that the thickness of a typical lamella in the GsC is 6.3 ± 0.8 nm, which is considerably larger than its counterpart in the bare-CIS (2.7 ± 0.7 nm) or the thickness of g-C3N4 NS (2.5 ± 0.5 nm, Figure S7). Additionally, as compared to the bare-CIS, the GsC has a much higher specific surface area (96.2 vs 17 m2 g−1) and more significant presence in micropores and mesopores (Figure S8), simultaneously allowing the formation of a larger catalyst/ electrolyte interface and a more facile release of H2 during the photocatalytic water splitting. The TEM image in Figure 1c shows g-C3N4 NS in the GsC remains to be amorphous, and the high-resolution TEM (HRTEM) image reveals the lattice fringes of 0.33 nm, corresponding to the interlayer distance between the (112) planes in the CIS (inset of Figure 1c). The energy-dispersive spectroscopy (EDS) elemental mapping images in Figure 1d demonstrate an even distribution of Cu and In elements throughout the GsC, suggesting a uniform growth of CIS. Additionally, the EDS compositional line scan across a typical laminar structure of the GsC clearly demonstrates that Cu and In signals from CIS keep steady in the left part of the scan but fall to the baseline exactly in the same place where the N signal from g-C3N4 appears and rises to peak. Such an opposite trend in elemental distribution suggests that the CIS layer only partly covers g-C3N4 NS and thus allows an easy access of the latter to hole scavengers in the electrolyte for sustainable photocatalytic hydrogen generation. The X-ray diffraction (XRD) pattern of the GsC (Figure S9) contains distinctive (112), (200), (204), and (116) diffraction peaks that can be well indexed to tetragonal CIS (PDF no. 27-0159),34 and two peaks at 26.7° and 12.1° stem from the periodic stacking of g-C3N4 layers and in-plane structural packing of nitrogen-linked heptazine units, respectively.35,36 Additionally, due to interactions between metal cations in the CIS and N sites in g-C3N4 NS, the high-resolution N 1s X-ray photoelectron spectroscopy (XPS) spectrum of the GsC (Figure
Alternatively, the separation efficiency of photogenerated electrons and holes can be improved by constructing an efficient Z-scheme system that completes the photocatalytic process in a two-step light excitation.17−23 A judicious selection of a second complementary semiconductor to CIS in the Zscheme scenario is therefore expected to render a concrete spatial separation of light-induced electrons from holes while maintaining the energy of the rest electrons and holes for corresponding photocathodic HER and hole scavenging, respectively.24,25 Unfortunately, to the best of our knowledge, there have been no reports demonstrating such a CIS-based Zscheme photocatalytic system for water splitting. Recently, exfoliated thin graphitic C3N4 nanosheets (g-C3N4 NS) have shown great promise in photocatalytic HER owing to their abundant reactive sites associated with the improved surface area and also the promoted charge separation by shortening the diffusion length of photoexcited electron−hole pairs within the decreased dimensions.26−30 Motivated by these results, we herein present a hierarchical g-C3N4-supported CuInS2 (GsC) by in situ hydrothermal growth of CIS on thin g-C3N4 NS toward Z-scheme photocatalytic water splitting. The GsC offers an intimate contact between CIS and g-C3N4 NS with strong couplings, suppressing the detrimental recombination of photogenerated carriers and resulting in a significant quenching in the photoluminescence (PL) of g-C3N4 NS to induce an over 6-fold increase in the PL lifetime. As the result, the GsC shows a high H2 production rate up to 1290 μmol g−1 h−1 under visible light irradiation (λ > 420 nm) with an outstanding stability in hours of operation, setting a new benchmark for visible-light-driven water splitting based on CIS photocatalysts.
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RESULTS AND DISCUSSION Scheme 1 summarizes the process of preparing the GsC. Briefly, a spongy g-C3N4 film was first obtained by directly Scheme 1. Schematic Illustration on the Preparation of the GsC for Z-Scheme Photocatalytic Water Splittinga
a (a) Exfoliated g-C3N4 NS. (b) Preparation of the GsC. (c) Cartoon showing the morphology of the GsC. (d) Z-Scheme photocatalytic water splitting in the GsC.
heating urea at 550 °C in Ar atmosphere, and the scanning electron microscopy (SEM, Figure S1) and transmission electron microscopy (TEM, Figure S2) images confirm its amorphous and multiholed nature. Subsequently, the spongy gC3N4 film was exfoliated to g-C3N4 NS by a reported ethylene B
DOI: 10.1021/acsami.7b06030 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. (a) SEM image and magnified view (inset) of the GsC. (b) AFM images of the GsC (upper left) and bare-CIS (bottom left), and corresponding height profiles along the lines shown in AFM images. (c) TEM and high-resolution TEM (inset) images of the GsC. (d) EDS elemental mapping images of the GsC (top and middle), and EDS composition line scan with elemental distributions along the line (bottom).
NHE, respectively. An energy band diagram for the GsC is thus depicted in Figure 2d, where energetically the VB of the CIS is capable to accept the photoinduced electrons from the CB of gC3N4 NS and the staggered alignment of energy levels here in the GsC can provide a Z-scheme system with the right band potential for photocatalytic water splitting. To rule out the energetically possible electron transfer from the CB of the CIS directly to the CB of g-C3N4 NS in the GsC, we performed the photodeposition of Pt nanoparticles to confirm the Z-scheme charge transfer pathway. As reported earlier, the reduction of Pt4+ in H2PtCl6 by photoinduced electrons under illumination would form Pt nanoparticles that are inclined to accumulate around electron-rich sites due to the electrophilic nature of Pt4+ species.37,41 As shown in the HRTEM image in Figure S13, the Pt nanoparticles are assembling around the CIS rather than staying randomly on the g-C3N4 part, implying that photoexcited electrons remain to reside on the CIS as predicted by the Z-scheme route instead of being transferred to g-C3N4 NS under illumination. To assess the photocatalytic activity of the current Z-scheme water-splitting system, the GsC was illuminated under visible light (λ > 420 nm) in aqueous solutions containing Na2S and Na2SO3 as hole scavengers but without any cocatalyst such as Pt and Ru. As shown in Figure 3a, continuous hydrogen evolution can be observed for 15 h without any noticeable
S10) lacks characteristic peaks associated with tertiary nitrogen N−(C)3 groups at 399.8 eV and positive charge localization in heterocycles at 404.3 eV when compared to that of g-C3N4 NS.31,36 Similarly, the FT-IR spectrum of the GSC shows the waning of stretching modes originated from CN heterocycles between 1230 and 1650 cm−1 and the N−H stretching vibration at ca. 3250 cm−1 (Figure S11),37 implying the strong coupling of electronic structures between CIS and g-C3N4 NS created during the in situ synthetic strategy. We then set out to determine the band structure of the GsC. For simplicity, only the optical band gaps of the bare-CIS and g-C3N4 NS are estimated by Tauc plots (Figure 2a) derived from the corresponding UV−vis spectra (Figure S12).22,23,38 As shown in Figure 2a, the extrapolated band gap values of the bare-CIS and g-C3N4 NS are 1.75 and 2.75 eV, respectively, both satisfying the preliminary requirement for a Z-scheme photocatalytic system, i.e., a band gap smaller than 3 eV.39,40 Mott−Schottky plots were employed to probe the CB levels of both semiconductors. As found in Figure 2b and 2c, the flatband potential of bare-CIS is around −1.86 eV vs the potential of normal hydrogen electrode (NHE) and considerably higher than that of g-C3N4 NS (−0.52 eV vs NHE). The valence band (VB) positions of the bare-CIS and g-C3N4 NS are therefore derived by the difference between the corresponding band gap values and the CB absolute values to be −0.11 and 2.23 eV vs C
DOI: 10.1021/acsami.7b06030 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. (a) Band gap energies of the GsC and the bare-CIS determined by Tauc plots. (b and c) Mott−Schottky plots of bare-CIS and g-C3N4 NS measured at frequencies 3.0, 2.0, and 1.0 kHz. (d) Electronic band structures of bare-CIS and g-C3N4 NS giving rise to a Z-scheme photocatalyst. CB, conduction band; VB, valence band.
the same trend as the absorption spectrum of the GsC, suggesting the catalytic performance of the GsC is mainly absorption limited. Photoelectrochemical techniques are further applied to study the improved separation of photocarriers and charge transfer kinetics between CIS and g-C3N4 NS in the GsC. In Figure 3c, when all samples were cast on the FTO electrode and illuminated under visible light, the GsC clearly shows a significantly higher photoelectric current density than those of the bare-CIS and g-C3N4 NS, proving its lowest recombination rate of photogenerated electrons and holes that has positively contributed to the highest hydrogen evolution rate as shown in Figure 3a. In addition, electrochemical impedance spectroscopy (EIS) is further used to study the kinetics of hydrogen evolution on these samples under light illumination. The Nyquist plots of EIS spectra are compared in Figure S17, and the fitting of these spectra is performed by using a simplified equivalent circuit proposed in the inset of Figure S17, where Rs, Rct, and CPE represent the series resistance of the electrode, the charge transfer resistance at the catalyst/electrolyte interface, and the corresponding constant phase element, respectively. Since significantly more electrons are excited to the CB during light illumination and these photogenerated electrons notably increase the exchange current density associated with electron transfers between the photocatalyst and protons in the electrolyte, the Rct value of a given sample is generally smaller under illumination than its counterpart measured in the dark, as is also found here. The GsC features the smallest charge transfer resistance (Rct) values in the dark, and more importantly, the dependence of Rct on the light intensity (Figure 3d) confirms that the GsC possesses the lowest Rct at any given light intensity, suggesting electrons in the GsC are more readily transferred to protons in the electrolyte to participate in the hydrogen evolution than those in the bareCIS or g-C3N4 NS.41,42 The photoluminescence (PL) of gC3N4 NS was drastically quenched in the GsC (Figure 3e), suggesting a substantially reduced charge recombination
catalyst deactivation when an intermittent evacuation was performed every 5 h. The average hydrogen evolution rate of the GsC reached 1290 μmol g−1 h−1, which was 6.1 and 3.3 times higher than that of the bare-CIS and g-C3N4 NS, respectively. Photocatalytic HER activities of different GsC samples prepared with g-C3N4-to-CIS mass ratios of 5:1, 1:1, to 1:2 were further measured (Figure S14), and in comparison, the GsC prepared with the g-C3N4-to-CuInS2 mass ratio of 2:1, is proven to have the best photocatalytic HER activity. In addition, a hydrogen generation rate of 1284 μmol g−1 h−1 is still observed after the GsC was recycled and stored in the dark for 30 days, where the SEM images in Figure S15 confirm that the morphology of the GsC remains unchanged and thus accounts for its decent stability. Overall, as compared in Figure 3b and more comprehensively in Table S1, the GsC is of superior activity to all previously reported CIS-based photocatalysts in visible-light-driven hydrogen evolution, thus setting a new performance benchmark even without using any noblemetal cocatalyst. Interestingly, the photocatalytic HER rate of the GsC is much higher than a simple sum of those of the bareCIS and g-C3N4 NS. This can be rationalized by the far more efficient separation of photoexcited electrons and holes in the GsC with the Z-scheme system. As proposed previously, when electrons in the VB of the bare-CIS or g-C3N4 NS are excited to CB with holes created in the VB at the same time, the photogenerated electron−hole pairs would fast recombine in the absence of a cocatalyst and eventually lead to a poor photocatalytic activity in water splitting. In contrast, the GsC features a Z-scheme photocatalytic system that involves the capture of light-induced electrons in the CB of g-C3N4 NS by the VB of CIS and thus spatially separates the remaining photogenerated electrons in the CB of CIS away from the holes in the VB of g-C3N4 NS. As a result, more electrons in the CIS survive and naturally produce more hydrogen over the same time period. The external quantum efficiency (EQE) of the GsC was measured at representative wavelengths between 400 and 600 nm (Figure S16), and it is found that the EQE follows D
DOI: 10.1021/acsami.7b06030 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. (a) Photocatalytic activities of (i) GsC, (ii) g-C3N4 NS, (iii) bare-CIS and stable hydrogen evolution under irradiation with visible light (λ > 420 nm). (b) Hydrogen evolution rates for the GsC in comparison with different reported CIS-based photocatalyst in water splitting. (c) Transient photocurrent densities of GsC, bare-CIS, and g-C3N4 NS obtained under visible-light illumination (λ > 420 nm) at 0.1 V vs Ag/AgCl. (d) Dependence of Rct on light intensities in EIS experiments. (e) PL spectra of the GsC, g-C3N4 NS, and bare-CIS with excitation wavelength of 370 nm. (f) Time-resolved PL spectra of GsC and g-C3N4 NS with an excitation wavelength of 404 nm.
hybrid catalyst utilizes charge transfer pathways in a Z-scheme system that substantially promote the separation of photogenerated electron and holes, thereby efficiently reducing the notoriously fast charge recombination normally found in CISbased photocatalysts to achieve a record hydrogen evolution rate for the material under visible light illuminations. The current work also shows that a judicious selection of complementary semiconductors at the nanoscale level can effectively manipulate the charge transfer kinetics of lightinduced carriers for a better utilization of absorbed photons, providing new insights in developing next-generation noblemetal-free photocatalysts for hydrogen generation.
because the PL essentially originates from the electron−hole recombination.43 Time-resolved transient PL decay spectra (Figure 3f) monitored at their maximum emission wavelength (425 nm for the GsC and 465 nm for g-C3N4 NS) reveal that all triexponentially fitted lifetimes of the GsC are longer than those of g-C3N4 NS, with average PL lifetimes of 13.08 ns for the GsC and 2.16 ns for g-C3N4 NS, respectively. The average PL lifetimes are generally viewed as a rough measure for estimating the separation efficiency of photogenerated charges, as the longer PL lifetimes provide more opportunities for free charges to participate in the desired HER process.44 The remarkably improved PL lifetime by a factor of 6 observed in the GsC indicates a far better separation of photogenerated electrons and holes due to the presence of the Z-scheme system, accounting for its greatly enhanced photocatalytic hydrogen generation.
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EXPERIMENTAL SECTION
Reagents. Cuprous chloride (CuCl), indium chloride tetrahydrate (InCl3·4H2O), urea, and thiourea were purchased from Aladdin. Ethylene glycol was purchased from Fuchen Chemical Reagents Factory (Tianjin, China). All materials were used as received without any further purification. Synthesis of the g-C3N4 Spongy Film. The g-C3N4 film was synthesized by directly heating 12 g of urea from room temperature to 550 °C in Ar atmosphere with a ramp rate of 1 °C min−1 and stabilized for 4 h before cooled to room temperature.
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CONCLUSION In conclusion, we presented a simple hydrothermal method to prepare a hierarchical g-C3N4-supported CuInS2 architecture as a highly efficient photocatalyst for visible-light-driven hydrogen evolution in the absence of any noble-metal cocatalyst. The E
DOI: 10.1021/acsami.7b06030 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces Synthesis of the GsC. The as-synthesized g-C3N4 spongy film was dispersed in 40 mL of ethylene glycol under ultrasonication for 2 h for exfoliation to thin g-C3N4 nanosheets. CuCl (0.2 mmol), InCl3·4H2O (0.2 mmol), and thiourea (0.08 mmol) were added and stirred for 1 h at room temperature before the mixture was transferred into a Teflonlined stainless steel autoclave of 50 mL capacity. After heating at 200 °C for 12 h, the autoclave was naturally cooled to room temperature. The product was collected by centrifuging and washed several times with water and ethanol before being finally dried under a vacuum oven at 60 °C for 4 h. To optimize the HER performance of the GsC, a range of samples was prepared with different g-C3N4-to-CIS mass ratios of 5:1, 2:1, 1:1, and 1:2. The GsC-2:1 was shown to feature the highest photocatalytic HER activity (see discussion), and unless otherwise specified, the GsC sample prepared with a g-C3N4-to-CIS mass ratio of 2:1 was used for different measurements and characterization. Photochemical Deposition of Pt on the GsC. The Pt photodeposition experiment was conducted by an impregnation method in the aqueous solution of H2PtCl6·6H2O. A 100 mg amount of the GsC powder was added to 30 mL of an aqueous solution containing the desired amount of H2PtCl6·6H2O (1.0 wt % Pt vs the GsC). After 10 min visible light irradiation (λ > 420 nm), the sample was centrifuged, rinsed by water and ethanol, and dried at 60 °C. The spatial distribution of Pt nanoparticles was investigated by HRTEM observation. Characterization. The SEM and TEM images were taken by fieldemission scanning electron microscopy (FE-SEM, JSM-7500F) and transmission electron microscopy (TEM, Hitachi 7650B). The topology of the samples was measured by atomic force microscopy (AFM, Veeco D3100). X-ray diffraction (XRD) patterns were obtained by using a Netherlands 1710 diffractometer with a Cu Kα irradiation source (λ = 1.54 Å). X-ray photoelectron spectroscopy (XPS) data were acquired by an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. Photoluminescence (PL) spectra were recorded on a PerkinElmer LS55 spectrofluorometer. The time-resolved PL spectra were measured on a Fluorolog-3 spectrofluorometer with an excitation wavelength of 460 nm. Fourier-transformed infrared (FT-IR) spectra were recorded on a Bruker VERTEX 700 spectrometer, and UV−vis diffuse reflectance spectra were recorded on an UV-3600 UV−vis-NIR spectrometer (Shimadzu). The N2 adsorption−desorption isotherms were measured on a NOVA2200e analyzer (Quantachrome, USA) with the specific surface area and pore size distribution obtained through the nonlocal density functional theory (NLDFT) and the Brunauer−Emmett−Teller (BET) methods. Photocatalytic Measurements. A 50 mg amount of photocatalyst was dispersed in a 100 mL of an aqueous solution containing 10 vol % triethanolamine, 0.25 M Na2S, and 0.2 M Na2SO3. Photocatalytic HER was carried out in a top-irradiation vessel connected to a gas-closed glass system (Beijing Perfect Light Technology Co., Ltd., Auto-Sampling Controller Labsolar-AI). The temperature of the reaction solution was carefully maintained below 6 °C during the whole experiment. The reactor was then sealed and evacuated several times to remove air before being irradiated under a 300 W Xe lamp (λ ≥ 420 nm). The amount of evolved H2 was analyzed by gas chromatography (GC-7900) with high-purity nitrogen carrier gas. Electrochemical Measurements. Mott−Schottky plots were conducted in a three-electrode cell by using a CHI760D electrochemical workstation (Chenhua Instruments, China). A platinum foil and the Ag/AgCl electrode (KCl, 0.1M) were used as the counter electrode and reference electrode, respectively. The working electrode was prepared through a clean fluoride−tin oxide (FTO) coated with the sample film. The aqueous solution of 0.5 M Na2SO4 was used as the electrolyte and purged with nitrogen gas for 30 min before measurement. For the electrochemical impedance measurement, the samplecoated FTO glass was used as the working electrode immersed in the same electrolyte used in the photocatalytic HER. The impedance spectra were measured at open-circuit condition in the dark or under
light illumination by using a CHI760D electrochemical workstation (Chenhua Instruments, China). The frequency range was 0.1−10 kHz, and the magnitude of the modulation signal was 10 mV. The obtained spectra were fitted with Z-View software (v2.8b, Scribner Associates Inc.) in terms of an appropriate equivalent circuit. For photocurrent measurements, the sample-coated FTO glass was used as the working electrode under intermittent light irradiation of 1 sun with the same electrolyte used in the photocatalytic HER, and photocurrent was recorded by chronoamperometry performed on a CHI760D electrochemical workstation (Chenhua Instruments, China). The thickness of the active photoelectrode was 27 μm.
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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/acsami.7b06030. SEM images and TEM images of spongy g-C3N4 film, gC3N4 NS, bare-CIS; AFM image, BET, XRD, XPS, and FI-IR spectra, and Nyquist plots of g-C3N4 NS, bare-CIS, and GsC (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Fax: 86-10-68918608; Tel: 86-10-68918608; E-mail: zhipan@ bit.edu.cn. ORCID
Zhipan Zhang: 0000-0003-4815-1468 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the fund from the State Key Laboratory of Solidification Processing in NWPU (SKLSP201601).
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REFERENCES
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DOI: 10.1021/acsami.7b06030 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
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DOI: 10.1021/acsami.7b06030 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
