Scalable Low-Band-Gap Sb2Se3 Thin-Film Photocathodes for

Nov 22, 2017 - (2) The constituent elements of Sb and Se in Sb2Se3 are relatively earth-abundant, inexpensive, and low-toxic.(48) (3) The Sb–Se phas...
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Scalable Low-Band-Gap Sb2Se3 Thin-Film Photocathodes for Efficient Visible−NearInfrared Solar Hydrogen Evolution Li Zhang,† Yanbo Li,‡ Changli Li,† Qiao Chen,† Zhen Zhen,† Xin Jiang,† Miao Zhong,§ Fuxiang Zhang,∥ and Hongwei Zhu*,† Downloaded via CALIFORNIA INST OF TECHNOLOGY on June 26, 2018 at 19:56:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China ‡ Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, China § Department of Electrical and Computer Engineering, University of Toronto, 35 St. George Street, Toronto, Ontario M5S 1A4, Canada ∥ State Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian 116023, China S Supporting Information *

ABSTRACT: A highly efficient low-band-gap (1.2−0.8 eV) photoelectrode is critical for accomplishing efficient conversion of visible−near-infrared sunlight into storable hydrogen. Herein, we report an Sb2Se3 polycrystalline thinfilm photocathode having a low band gap (1.2−1.1 eV) for efficient hydrogen evolution for wide solar-spectrum utilization. The photocathode was fabricated by a facile thermal evaporation of a single Sb2Se3 powder source onto the Mo-coated soda-lime glass substrate, followed by annealing under Se vapor and surface modification with an antiphotocorrosive CdS/TiO2 bilayer and Pt catalyst. The fabricated Sb2Se3(Se-annealed)/CdS/TiO2/Pt photocathode achieves a photocurrent density of ca. −8.6 mA cm−2 at 0 VRHE, an onset potential of ca. 0.43 VRHE, a stable photocurrent for over 10 h, and a significant photoresponse up to the near-infrared region (ca. 1040 nm) in near-neutral pH buffered solution (pH 6.5) under AM 1.5G simulated sunlight. The obtained photoelectrochemical performance is attributed to the reliable synthesis of a micrometer-sized Sb2Se3 (Se-annealed) thin film as photoabsorber and the successful construction of an appropriate p−n heterojunction at the electrode−liquid interface for effective charge separation. The demonstration of a low-band-gap and high-performance Sb2Se3 photocathode with facile fabrication might facilitate the development of cost-effective PEC devices for wide solarspectrum utilization. KEYWORDS: antimony selenide, photocatalysis, surface modification, thin films, water splitting ffective utilization of solar energy requires a reliable energy storage technique to tackle the dilute and intermittent nature of sunlight.1,2 Hydrogen, regarded as a storable and clean fuel, is potentially one of the most reliable energy carriers for a carbon-neutral and sustainable society if it can be produced from renewable energy resources such as solar energy.3−5 Photoelectrochemical (PEC) hydrogen production via water splitting, which employs semiconducting materials as photoelectrodes for the direct conversion of solar energy into storable hydrogen, represents one of the most promising but challenging techniques for sustainable solar energy storage.6−9 Among various types of PEC configurations,6,10 a stacked dual-electrode PEC cell, consisting of high-band-gap and low-

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band-gap photoelectrodes with an optimal band-gap combination of approximately 2.0−1.6 and 1.2−0.8 eV, respectively, which are suitable for their respective ultraviolet−visible and visible−near-infrared sunlight absorption, has been identified as one of the most attractive approaches to achieve a solar-tohydrogen (STH) conversion efficiency of around 20%.11,12 In recent years, impressive progress has been achieved on highband-gap photoelectrode materials, which include a multitude Received: October 23, 2017 Accepted: November 22, 2017 Published: November 22, 2017 12753

DOI: 10.1021/acsnano.7b07512 ACS Nano 2017, 11, 12753−12763

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Figure 1. (a) Schematic of fabrication process for CdS-modified Sb2Se3 thin films. (b−e) Optical images of the SLG/Mo substrate, asdeposited Sb2Se3, 450 °C annealed Sb2Se3, and CdS-modified 450 °C annealed Sb2Se3. (f−i) Top-view SEM images of the SLG/Mo substrate, as-deposited Sb2Se3, 450 °C annealed Sb2Se3, and CdS-modified 450 °C annealed Sb2Se3. (j, k) Cross-sectional SEM images of the asdeposited Sb2Se3 and CdS-modified 450 °C annealed Sb2Se3.

600 °C) and a high saturated vapor pressure (ca. 1200 Pa at 550 °C).50 These two characteristics enable the preparation of high-quality Sb2Se3 polycrystalline thin films at a relatively low temperature using thermal sublimation of a powder source, which is a facile and high-throughput deposition technique that has been successfully adopted in the well-developed CdTe photovoltaics.53,54 For instance, only ca. 10 g of Sb2Se3 powder source (assuming material utilization of 50−90%) is needed to make a 1 m2 Sb2Se3 thin film with a thickness of 1 μm. (5) As an intrinsically p-type semiconductor, Sb2Se3 has a conduction band minimum (CBM) located at −4.15 eV55 (i.e., −0.29 VNHE at pH = 0). Thus, it is suitable as a photocathode for hydrogen evolution. In addition, Sb2Se3 polycrystalline thin film as an emerging photoabsorber for solar cells has achieved a solar conversion efficiency of around 6%,47 which is an encouraging initial step toward the Shockley−Queisser theoretical maximum efficiency of around 30% (using a band gap of 1.2−1.0 eV).56 Therefore, it is promising to achieve an outstanding performance of the Sb2Se3 polycrystalline thin-film photocathode for hydrogen evolution. All these highly appealing merits encourage the systematic investigation of an Sb2Se3 polycrystalline thin film as a low-band-gap photocathode for solar-driven hydrogen evolution. However, to date, there is no attempt on an Sb2Se3 polycrystalline thin-film photocathode coupled with the appropriate p−n junction for efficient and stable solar hydrogen evolution in a benign neutral electrolyte (i.e., close to the naturally available seawater and rainwater), except that very few reports have disclosed the utilization of Sb2Se3 nanoneedles/nanowires as photocathodes for hydrogen evolution under a harsh acidic electrolyte.57,58 Herein, we demonstrated that the large-scale Sb 2 Se 3 polycrystalline thin-film photocathode, prepared by facile

of n-type semiconductors such as BiVO4,13,14 Fe2O3,15−17 TiO2,18,19 WO3,20,21 (Ca,Sr,Ba,La)(Ti,Ta,Nb)(O,N)3,22−24 and Ta3N5,25−27 as well as several p-type semiconductors such as GaInP2,28,29 (Ag,Cu)GaSe2,30,31 Cu(In,Ga)S2,32,33 Cu2(Zn,Ba)Sn(S,Se)4,34,35 and Cu2O.36−39 However, studies on ideal lowband-gap (1.2−0.8 eV) photoelectrode materials well-suited for a stacked dual-electrode PEC cell are almost limited to n-/pSi9,40−43 and p-Cu(In,Ga)Se2 (or its solid solution with ZnSe)44,45 owing to the scarcity of suitable low-band-gap photoelectrode materials. This situation, therefore, brings our attention to exploring new low-band-gap photoelectrodes, which ideally should be based on simple semiconducting materials with characteristics of inexpensive elements and facile production at large scale. In this regard, antimony selenide (Sb2Se3) belongs to V2−VI3 (V = As, Sb, Bi; VI = S, Se, Te) binary materials that have been attracting tremendous research interest owing to their potential applications in thermoelectric and photovoltaic areas,46−48 as well as Li- and Na-ion batteries.49 Sb2Se3 appears to be a promising candidate as a low-band-gap photoelectrode for a stacked dual-electrode cell because of the following advantageous features: (1) Sb2Se3 has an ideal low-band-gap value (1.2−1.0 eV) and a high optical absorption coefficient (around 105 cm−1 near the absorption onset),47,48 which permits a strong visible−near-infrared sunlight absorption. (2) The constituent elements of Sb and Se in Sb2Se3 are relatively earth-abundant, inexpensive, and low-toxic.48 (3) The Sb−Se phase diagram has only one simple binary compound,50 namely, the orthorhombic Sb2Se3, indicating the phase and defects of Sb2Se3 could be much easier to control than the typical multinary chalcogenides such as Cu(In,Ga)(S,Se)2 and Cu2ZnSn(S,Se)4.51,52 (4) Sb2Se3 has a low melting point (ca. 12754

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Figure 2. (a−c) Current−potential curves of bare Sb2Se3, Sb2Se3/Pt, and Sb2Se3/CdS/Pt electrodes under chopped illumination by AM 1.5G simulated sunlight. The Sb2Se3 samples were annealed at the different temperatures under an Ar atmosphere. (d−f) Current−time curves of 400 °C annealed bare Sb2Se3, Sb2Se3/Pt, and Sb2Se3/CdS/Pt electrodes at 0 VRHE under AM 1.5G simulated sunlight. A chopped illumination was applied to the bare Sb2Se3 for a better observation of the extremely low cathodic photocurrent. All measurements were carried out in near-neutral pH buffered solution (pH = 6.5, 0.5 M Na2SO4 buffered with 0.25 M Na2HPO4 and 0.25 M NaH2PO4).

typical fish-like grains, a prerequisite to avoid possible peelingoff of the Sb2Se3 thin films from the Mo substrate when annealing is performed. The as-deposited Sb2Se3 thin films (Figure 1g) were amorphous, whereas the 450 °C annealed Sb2Se3 sample (Figure 1h) was polycrystalline, consisting of micrometer-sized elongated grains that packed tightly to form the thin-film Sb2Se3. The observed pin-holes in Figure 1h were probably due to the slight re-evaporation of Sb2Se3 during the annealing process. The CdS-modified 450 °C annealed Sb2Se3 (Figure 1i) exhibited a homogeneous coverage by a CdS overlayer. The cross-sectional SEM image of as-deposited Sb2Se3 on the SLG/Mo substrate (Figure 1j) indicated that the Sb2Se3 densely covered the Mo substrate with a thickness of ca. 1.5 μm. The cross-sectional view of CdS-modified 450 °C annealed Sb2Se3 (Figure 1k) revealed the Sb2Se3 layer consisting of large columnar grains is uniformly covered by a CdS overlayer (ca. 80 nm). It is noteworthy that the obtained Sb2Se3 thin film with large columnar grains is benefited by the low melting temperature of Sb2Se3 (ca. 600 °C) and is a highly desirable microstructure for high-performance polycrystalline thin-film solar cells61 and photocathodes.62,63 PEC Properties of Sb2Se3-Based Electrodes. Figure 2 displays the PEC performance of Sb2Se3-based electrodes under AM 1.5G simulated sunlight illumination (Figure S1, Supporting Information). As shown in Figure 2a, the bare Sb2Se3 electrodes annealed at 350−450 °C under an Ar atmosphere exhibited cathodic photocurrents, whereas the bare Sb2Se3 annealed at 300 °C showed a weak anodic photocurrent, indicating the appropriate annealing temperature is required to obtain p-type Sb2Se3. Meanwhile, the observed low cathodic photocurrent on all of the p-type Sb2Se3 electrodes (350−450 °C) implies a poor PEC performance of bare Sb 2 Se 3 photocathodes, suggesting the indispensability of further

thermal evaporation of a single Sb2Se3 powder source coupled with suitable annealing under the Se vapor and rational surfaceengineering by the CdS/TiO2 bilayer and Pt catalyst, afforded a stable photocurrent density of ca. −8.6 mA cm−2 at 0 VRHE and an onset potential of ca. 0.43 eV in near-neutral pH buffered solution (pH 6.5) under simulated sunlight, thus enabling the fabricated Sb2Se3 photocathode to exhibit one of the highest PEC performances ever reported for Sb2Se3-based electrodes in water splitting.57,58 Moreover, the obtained PEC performance of the Sb2Se3 photocathode in this study was comparable to the state-of-the-art polycrystalline thin-film photocathodes based on earth-abundant binary photoabsorbers such as Cu2O,36−39 CuO,59 and WSe2.60 The highly efficient PEC performance combined with the straightforward, inexpensive, and scalable process demonstrated the great potential of low-band-gap Sb2Se3 in fabricating cost-effective PEC devices with a wide solar-spectrum utilization for water splitting.

RESULTS AND DISCUSSION Fabrication and Surface Modification of Sb2Se3 Thin Films. As shown in Figure 1a, the Sb2Se3 thin film was prepared by facile thermal evaporation of a single Sb2Se3 powder source onto a Mo-coated soda-lime glass (SLG/Mo) substrate, followed by annealing to obtain a high-quality polycrystalline thin film. The surface of the annealed-Sb2Se3 thin film was then modified with n-type CdS to form a p−n heterojunction. The SLG/Mo substrate (Figure 1b), asdeposited Sb2Se3 (Figure 1c), 450 °C annealed Sb2Se3 (Figure 1d), and CdS-modified 450 °C annealed Sb2Se3 (Figure 1e) were all homogeneous in appearance over a large area (≥20 cm2). This result indicated the employed process for Sb2Se3 thin films is straightforward, high-throughput, and potentially feasible to scale up. A scanning electron microscopy (SEM) image revealed that the sputtered Mo (Figure 1f) exhibited 12755

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Figure 3. (a, b) XRD patterns and Raman spectra of Sb2Se3 annealed at different temperatures. (c, d) EDX spectrum and Tauc plot of 400 °C annealed Sb2Se3. Inset corresponds to the transmittance spectrum. (e) Plots of Se/Sb ratio and band-gap values of Sb2Se3 as a function of the annealing temperature. (f−h) SEM images of Sb2Se3 annealed at 300, 350, and 400 °C under an Ar atmosphere.

can be readily produced by facile thermal evaporation of an Sb2Se3 powder source along with a moderate annealing. However, further enhancement on the photocurrent and stability is needed to demonstrate a PEC performance that can be comparable to the state-of-the-art and emerging binary photocathodes. Thus, further enhancement was pursued through annealing of Sb2Se3 in the presence of Se vapor and protective coating of the CdS surface by antiphotocorrosive TiO2, as discussed in the following paragraphs. Structural Analysis of Sb2Se3 Thin Films. Before the evaluation of PEC performance of Sb2Se3-based photocathodes with Sb2Se3 annealed in the presence of Se vapor, structural properties of the Ar-annealed Sb2Se3 thin films were studied. As shown in Figure 3a, the X-ray diffraction (XRD) patterns of asdeposited Sb2Se3 thin films did not show distinct diffraction peaks except for the peak associated with the Mo substrate, suggesting the as-deposited Sb2Se3 thin films were amorphous. All of the annealed samples showed strong diffraction patterns corresponding to the orthorhombic Sb2Se3 (PDF No. 150861), indicating thermal annealing is a requisite for the transition from amorphous to polycrystalline. In addition, all of the annealed Sb2Se3 samples exhibited desirable orientations for (211) and (221) planes rather than the undesirable (120) plane.47 The Raman spectra of all Sb2Se3 samples (Figure 3b) exhibited two peaks centered at around 189 and 252 cm−1,

improvement through rational engineering of the Sb2Se3/liquid interface. After the surface decoration of Pt nanoparticles as the hydrogen evolution catalyst (Sb 2Se3/Pt, Figure 2b), a significant enhancement in photocurrent was observed for all Sb2Se3 photocathodes. With inserting an n-type CdS layer between Sb2Se3 and Pt (Sb2Se3/CdS/Pt, Figure 2c), a drastic enhancement in photocurrent was further obtained owing to the formation of a p−n heterojunction between Sb2Se3 and CdS. In particular, the fabricated Sb2Se3/CdS/Pt photocathodes with Sb2Se3 annealed at 400 °C achieved the highest photocurrent density of ca. −2.3 mA cm−2 at 0 VRHE and an onset potential of ca. 0.35 VRHE (defined as the potential where a photocurrent density exceeds −0.05 mA cm−2, similarly hereinafter) in neutral electrolyte under AM 1.5G simulated sunlight. Notably, the time course of photocurrent for Sb2Se3 (Figure 2d) and Sb2Se3/Pt (Figure 2e) showed a stable cathodic photocurrent, whereas the Sb2Se3/CdS/Pt (Figure 2f) exhibited a gradual decay within 1 h, which could be attributed to the partial photocorrosion of CdS and the accompanying loss of Pt catalyst.63 The obtained promising PEC performance of the Sb2Se3/ CdS/Pt electrode highlights the great potential of Sb2Se3 as photocathodes for hydrogen evolution because large-scale Sb2Se3 polycrystalline thin films with micrometer-sized grains 12756

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Figure 4. (a) Optical image of Sb2Se3(Se). (b) SEM image of Sb2Se3(Se). (c) HRTEM image of Sb2Se3(Se). Inset is the SAED pattern. (d) STEM image of Sb2Se3(Se) and the corresponding elemental mappings of Sb and Se. (e, f) Top and cross-sectional SEM images of the Sb2Se3(Se)/CdS/TiO2 sample. (g, h) Typical AES spectrum and the corresponding elemental mappings of Ti and O on the surface of the Sb2Se3(Se)/CdS/TiO2 sample. (i) SIMS depth profile of the Sb2Se3(Se)/CdS/TiO2 sample.

exceeds 500 °C (not shown here) because of the low melting point of Sb2Se3. On the basis of these results, we speculated the observed significant deficiency of Se might reduce the PEC performance of Ar-annealed Sb2Se3 electrodes because a significant deficiency of Se could lead to the formation of a large amount of Se vacancies, which could in principle act as n-type donors (reducing the p-type conductivity of photoabsorbers) and recombination centers (trapping the photogenerated carriers) to deteriorate the device performance as previously encountered in the Ar-annealed Sb2Se3 thin-film solar cells.64 Annealing in the presence of selenium (or sulfur) vapor was demonstrated to be an effective strategy to reduce the Se (or S) vacancies and improve the solar conversion efficiency of Sb2Se3 (or SnS) thin-film solar cells.64,65 Since a significant Se deficiency was observed for Ar-annealed Sb2Se3 in this work, an additional Se was introduced into Sb2Se3 through annealing in the presence of Se vapor, which led to an obvious improvement of device performance, as demonstrated below. Enhanced PEC Performance of Sb2Se3 through SeAnnealing and TiO2-Coating: Structural Properties. The Sb2Se3 sample annealed at 400 °C under Se vapor is abbreviated as Sb2Se3(Se). The obtained Sb2Se3(Se) adhered firmly to the SLG/Mo substrate and was homogeneous over a large scale (Figure 4a) with a surface color similar to that of Arannealed Sb2Se3 (Figure 1d). The contact between Mo and the Sb2Se3(Se) thin film was measured to be ohmic (Figure S4, Supporting Information). The XRD pattern and Raman spectrum of Sb2Se3(Se) confirmed the high purity of the

corresponding to the characteristic Sb−Se and Sb−Sb vibrations of Sb2Se3,55 respectively, which further confirmed the presence of the desired Sb2Se3 phase. Figure 3c shows a typical energy-dispersive X-ray spectrum (EDX) of the 400 °C annealed Sb2Se3, in which the average atomic ratio (over five random areas) of Se/Sb was measured to be 58.6/41.4, implying a significant deficiency of Se. The direct optical band gap of 400 °C annealed Sb2Se3 was determined to be ca. 1.16 eV from optical transmittance results (Figure 3d). The Se/Sb ratio and optical band gap of Sb2Se3 as a function of annealing temperature are plotted in Figure 3e (the corresponding data are shown in Table S1 and Figure S2, Supporting Information). It turned out that all of the annealed Sb2Se3 samples had relatively lower Se/Sb ratios compared with the as-deposited one, suggesting a partial loss of Se during the annealing process. The determined optical band-gap values revealed that all of the annealed Sb2Se3 samples had a band-gap value of around 1.2−1.1 eV, much lower than that of the sample without annealing (ca. 1.4 eV). The SEM images of annealed Sb2Se3 (Figure 3f−h for 300− 400 °C, Figure 1h for 450 °C, and Figure S3 in the Supporting Information for additional low-magnification SEM images) showed a gradual grain growth with increasing annealing temperature. Apparently, more uniform and large grains were obtained for the sample with the annealing temperature of 400 °C, indicating only a moderate annealing temperature is suitable to achieve micrometer-sized grains for Sb 2 Se 3 polycrystalline thin films. It should be noted that a significant re-evaporation would be observed if the annealing temperature 12757

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Figure 5. (a) Current−potential curves of Sb2Se3(Se)/CdS/Pt and Sb2Se3/CdS/Pt electrodes under AM 1.5G simulated sunlight. Inset corresponds to the current−time curve of Sb2Se3(Se)/CdS/Pt at 0 VRHE. (b) Mott−Schottky plots of Sb2Se3(Se) and Sb2Se3. (c, d) Current− potential and current−time curves of the Sb2Se3(Se)/CdS/TiO2/Pt electrode under AM 1.5G simulated sunlight. Inset corresponds to the HC-STH. (e) GC analysis of Sb2Se3(Se)/CdS/TiO2/Pt at 0 VRHE under a 300 Xe lamp (ca. 350−780 nm). (f) IPCEs of the Sb2Se3(Se)/CdS/ TiO2/Pt electrode at 0 VRHE. (g) Schematic band alignment of the Sb2Se3(Se)/CdS/TiO2 heterojunction. (h) PEC performance of state-ofthe-art photocathodes made of emerging binary photoabsorbers. All electrochemical measurements were carried out in near-neutral pH buffered solution (pH = 6.5, 0.5 M Na2SO4 buffered with 0.25 M Na2HPO4 and 0.25 M NaH2PO4).

shown in Figure 4c, the high-resolution transmission electron microscopy (HRTEM) image of Sb2Se3(Se) and selected area electron diffraction (SAED) pattern indicated Sb2Se3(Se) films had a good crystalline quality, in which the measured interplanar distances of 2.521 and 2.753 Å corresponded to the (321) and (3−30) planes of the orthorhombic Sb2Se3 (PDF No. 15-0861), respectively. The STEM-EDX elemental mappings of Sb and Se (Figure 4d) exhibited a homogeneous distribution, further confirming the superb crystal quality of Sb2Se3(Se). The surface of obtained Sb2Se3(Se) was then modified by CdS and further coated by atomic layer deposition (ALD) of TiO2. The top and cross-sectional SEM images of the resulting Sb2Se3(Se)/CdS/TiO2 sample (Figure 4e and f) indicated that the large columnar-grained Sb 2Se 3(Se) underlayer was conformally coated by CdS (ca. 80 nm) and TiO2 (ca. 15 nm) overlayers. As shown in Figure 4g and h, the typical auger electron spectrum (AES) and corresponding elemental mappings of Ti and O further confirmed a complete coverage of TiO2 on the surface of Sb2Se3(Se)/CdS. The Sb2Se3(Se)/ CdS/TiO2 sample was further studied by time-of-flight secondary ion mass spectroscopy (TOF-SIMS) as shown in

Sb2Se3 phase (Figure S5, Supporting Information). The band gap of Sb2Se3(Se) was determined to be ca. 1.15 eV (Figure S6, Supporting Information), which is similar to that of Arannealed Sb2Se3 at 400 °C. As shown in Figure 4b (and Figure S7 in the Supporting Information for additional lowmagnification SEM images), the SEM image of Sb2Se3(Se) exhibited large rod-shaped grains packed randomly and densely to form the polycrystalline thin film. The grain size of Sb2Se3(Se) was larger than that of Ar-annealed Sb2Se3 at 400 °C (Figure 3h), suggesting that the treatment with Se vapor during annealing promotes the grain growth. It is worth noting that direct annealing of the metallic Sb thin films under the same Se vapor failed to obtain high-purity Sb2Se3 polycrystalline thin films in large scale (Figure S8, Supporting Information), indicating annealing of as-deposited Sb2Se3 in the presence of Se vapor is much easier than direct selenization of metallic Sb to form high-quality Sb2Se3 polycrystalline thin films. The average atomic composition (over five random areas) of Se/Sb was determined by EDX to be 59.7/40.3, which was very close to the 3/2 stoichiometry and thus implied a higher composition of Se for Sb2Se3(Se) than that of Arannealed Sb2Se3 (58.6/41.4) at the same temperature. As 12758

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then evaluated by integrating the measured IPCEs of Sb2Se3(Se)/CdS/TiO2/Pt over the spectrum of simulated sunlight. As a result, the integrated photocurrent density correlated very well with the observed photocurrent density of 8.6 mA cm−2 at 0 VRHE. Notably, a theoretical maximum photocurrent density of ca. 40 mA cm−2 was calculated (Figure S13, Supporting Information), indicating that there is still ample space for further improvement. To elucidate the dramatically enhanced PEC performance caused by CdS/TiO2 bilayer modification, we calculated the band alignment of the Sb2Se3(Se)/CdS/TiO2/Pt photocathode. As shown in Figure 5g, the valence band offset (VBO) of the Sb2Se3(Se)/CdS interface was measured by X-ray photoelectron spectroscopy (XPS) to be a cliff of 1.02 eV (the details are described in the Supporting Information and Figure S14). In contrast, the reported cliff VBO of 1.1 eV was used for the CdS/TiO2 interface.66 According to these two VBO values and the determined band-gap values of Sb2Se3(Se) (1.15 eV), CdS (2.42 eV, Figure S15), and TiO2 (3.32 eV, Figure S16), the conduction band offsets (CBO) of Sb2Se3(Se)/CdS and CdS/TiO2 interfaces were determined to be a spike of 0.25 eV and a cliff of 0.2 eV, respectively. The resulting band alignment of the Sb2Se3(Se)/CdS/TiO2 heterojunction exhibited a large cliff-like VBO (sequentially formed by CdS and TiO2) and a little spike-like CBO at the Sb2Se3(Se)/CdS interface. In such a scenario, the formed large cliff-like VBO creates a desirable energy barrier that effectively prevents the transfer of photogenerated holes in the valence band of Sb2Se3(Se) toward the electrode−liquid interface for recombination. As a result, more photogenerated holes move to the back contact and later to the counter electrode through an external circuit for oxygen evolution. Meanwhile, the formed little spike-like CBO (0.25 eV) at the Sb2Se3(Se)/CdS interface, which is within the optimal value of a high-quality p−n heterojunction (between a cliff of ca. 0.2 eV and a spike of 0.4 eV),67,68 could allow the selective transfer of photogenerated electrons in the conduction band of Sb2Se3(Se) to reach the electrode−liquid interface through the CdS/TiO2 bilayer for hydrogen evolution with the assistance of a Pt catalyst. In addition to forming a p− n heterojunction with desirable band alignment to promote charge separation and collection, the CdS/TiO2 bilayer provides a stable surface because of the antiphotocorrosive nature of the TiO2 overlayer.69 As plotted in Figure 5h, the PEC performance of the surfacemodified Se-annealed Sb2Se3 polycrystalline thin-film photocathode in this report (−8.6 mA cm−2 at 0 VRHE, 0.43 VRHE, 85% of photocurrent remained after 10 h) was higher than that of the previously reported Sb2Se3/TiO2/Pt nanoneedle photocathode (−2 mA cm−2 at 0 VRHE, 0.4 VRHE, 5% remained after 3 h).57 Furthermore, the PEC performance is comparable to the state-of-the-art photocathodes based on the emerging binary photoabsorbers, including Cu2O,36−39 CuO,59 WSe2,60 and ZnTe,70 as illustrated in Table S2. Although the present PEC performance of the Sb2Se3-based photocathode is still lower than those well-studied binary photocathodes based on the commercial materials such as InP71 and CdTe,72 further improvement of the Sb2Se3-based photocathode for costeffective PEC devices is highly promising because of its high theoretical efficiency (i.e., 30%) and capability for facile fabrication at large scale using inexpensive and nontoxic elements.

Figure 4i, which further corroborated the presence of a welldefined multilayer structure consisting of Sb2Se3(Se)/CdS/ TiO2 on the Mo substrate. Enhanced PEC Performance of Sb2Se3 through SeAnnealing and TiO2-Coating: PEC Properties. As shown in Figure 5a, the Sb2Se3(Se)/CdS/Pt electrode exhibited a photocurrent density of ca. −6.2 mA cm−2 at 0 VRHE and an onset potential of ca. 0.40 VRHE, which was a significant enhancement compared with the Ar-annealed Sb2Se3/CdS/Pt (−2.3 mA cm−2 at 0 VRHE, onset potential of 0.35 VRHE). In addition, the Sb2Se3(Se)/Pt electrode also exhibited an enhanced PEC performance compared with Sb2Se3/Pt (Figure S9, Supporting Information). Therefore, it is evident that annealing of Sb2Se3 in the presence of Se vapor could effectively improve the PEC performance of Sb2Se3. The Mott−Schottky plots (Figure 5b, details are presented in the Supporting Information) reveal that the Sb2Se3(Se) had a significantly higher acceptor density (NA, 4.7 × 1016 cm−3) than that of Arannealed Sb2Se3 (4.2 × 1015 cm−3), accompanied by a slightly more positive flat-band potential (Efb, shifted to 0.59 from 0.47 VRHE). The increased acceptor density of the Sb2Se3(Se) sample is likely attributed to the effective reduction of donorlike Se vacancies resulting from the additional Se incorporation during annealing under the Se vapor.64 Thus, the observed enhanced PEC performance of Sb2Se3(Se)-based electrodes might benefit from the successful passivation of Se vacancies. After further modification by inserting TiO2 between CdS and Pt, the resulting Sb2Se3(Se)/CdS/TiO2/Pt exhibited an obvious PEC enhancement (Figure 5c), showing a cathodic photocurrent density of ca. −8.6 mA cm−2 at 0 VRHE, an onset potential of ca. 0.43 VRHE, and a half-cell solar-to-hydrogen efficiency (HC-STH) of ca. 0.68% at ca. 0.2 VRHE in nearneutral pH buffered solution (pH 6.5) under AM 1.5G simulated sunlight. The average PEC performance of Sb2Se3(Se)/CdS/TiO2/Pt (over five samples) was ca. −8.8 mA cm−2 at 0 VRHE with an onset potential of ca. 0.41 VRHE (Figure S10, Supporting Information). It should be noted that the deposited Pt was nanoparticles with a particle size of ca. 40 nm (Figure S11, Supporting Information). Furthermore, the Sb2Se3(Se)/CdS/TiO2/Pt photocathode (Figure 5d) demonstrated a stable photocurrent for over 10 h (ca. 85% of initial photocurrent remained), which displayed a dramatically improved stability compared with Sb2Se3(Se)/CdS/Pt (inset of Figure 5a, where ca. 50% of the initial photocurrent remained after only 1 h). This result suggested that the highly efficient PEC performance could be achieved through the modification of the CdS/TiO2 bilayer. In addition, the gas products produced in a time course of photocurrent measurement (Figure S12, Supporting Information) were quantified by gas chromatography (GC). As shown in Figure 5e, the Faradaic efficiencies of hydrogen evolved from the Sb2Se3(Se)/CdS/ TiO2/Pt photocathode and oxygen evolved from the counter electrode were approximately 100%, indicating the observed stable photocurrent is indeed originated from the hydrogen evolution via water splitting. Figure 5f displays the wavelength dependence of incident photon-to-current conversion efficiencies (IPCEs) for the Sb2Se3(Se)/CdS/TiO2/Pt sample measured at 0 VRHE. The IPCEs of Sb2Se3(Se)/CdS/TiO2/Pt reached ca. 20−30% in the 440−800 nm range. It started to decrease gradually to nearly zero at ca. 1040 nm, a wavelength close to the expected spectral response of Sb2Se3(Se) according to the determined band gap of ca. 1.2 eV. A photocurrent density of ca. 8.3 mA cm−2 was 12759

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Atomic Layer Deposition of TiO2. ALD of TiO2 was performed at 150 °C using a Beneq TFS-200 system and employing titanium tetrachloride (TiCl4) and H2O as the Ti and O precursors, respectively. Each ALD cycle consisted of a 0.5 s pulse of TiCl4 and a 0.5 s pulse of H2O. A 5 s purging of N2 was performed between each precursor pulse to remove extra precursor materials. These conditions exhibited a growth rate per cycle of around 0.5 Å, as measured on the Si witness sample. The depositing thickness of TiO2 was fixed at around 15 nm by controlling the cycle numbers. Photoassisted Electrodeposition of Pt. Prior to electrochemical measurements, the sample was fabricated into an electrode by soldering a lead wire onto the Mo back contact and sealing the needless parts with epoxy. The photoassisted electrodeposition of Pt was controlled by a CHI660D electrochemical workstation under a three-electrode configuration. A specimen, a Pt wire, and a Ag/AgCl electrode (saturated with aqueous KCl solution) were employed as the working, counter, and reference electrodes, respectively. The measured potential (VAg/AgCl) was converted to the reversible hydrogen electrode potential (VRHE) according to the Nernst equation (VRHE = VAg/AgCl + 0.059 pH + 0.197). The deposition of Pt was performed in a solution containing 0.1 mM H2PtCl6·6H2O (99.9% metals basis, Alfa Aesar), 0.5 M Na2SO4 (99%, Alfa Aesar), 0.25 M Na2HPO4 (99+%, Acros), and 0.25 M NaH2PO4 (99%, Acros) with pH at 6.5 measured by a pH meter (FE-20K, Mettler Toledo). Each deposition was performed at a fixed potential of 0 VRHE under illumination (ca. 350−780 nm light, 300 W Xe lamp) with magnetic stirring and Ar purging. The deposition continued until the saturation of the cathodic photocurrent. Photoelectrochemical Measurements. The PEC measurements were carried out using three-electrode cell configuration (a Pt wire as the counter electrode, a Ag/AgCl electrode as the reference electrode) in near-neutral pH buffered solution (pH 6.5, containing 0.5 M Na2SO4, 0.25 M Na2HPO4, and 0.25 M NaH2PO4) with magnetic stirring and Ar purging. A commercial AM 1.5G solar simulator (92250A-1000, Newport Oriel) was employed as the simulated sunlight. A standard Si solar cell (P/N 91150 V, Newport Oriel) and a spectroradiometer (LS-100, EKO instruments) were applied to calibrate the solar simulator. The Mott−Schottky measurements were carried out using a 10 mV ac voltage signal at a frequency of 2 kHz. A gas chromatograph (GC 2014, Shimadzu) was employed for gas products analysis. A monochromator equipped with a 1000 W Xe lamp and a calibrated Si photodiode (FDS100-CAL, Thorlabs) was used to measure the incident-photon-to-current-conversion efficiency. Structural Characterization. The samples were characterized by a scanning electron microscope (MERLIN VP Compact, Carl Zeiss) combined with energy dispersive X-ray spectroscopy, X-ray diffraction (Smartlab, Rigaku), Raman spectroscopy (LabRAM HR Evolution, Horiba) under 532 nm excitation, transmittance spectroscopy (Cary 5000 UV−vis−NIR, Agilent Technologies), X-ray photoelectron spectroscopy (Escalab 250Xi, Thermo Scientific), Auger electron spectroscopy (PHI-710, ULVAC-PHI), transmission electron microscopy (JEM-2010, JEOL), and time-of-flight secondary ion mass spectroscopy (TOF.SIMS 5-100, ION-TOF GmbH) with a Cs+ beam for the depth sputtering (2 keV, 350 × 350 μm2) and a Bi+ beam for the analysis (30 keV, 80 × 80 μm2). For TOF-SIMS measurements, the intensities of the MCs+ clusters were analyzed, where M denotes the matrix components of the multilayered sample.

CONCLUSIONS In conclusion, we have developed a highly efficient low-bandgap Sb2Se3 polycrystalline thin-film photocathode for durable hydrogen evolution with a significant photoresponse up to the near-infrared region (ca. 1040 nm). Sb2Se3 polycrystalline thin films with micrometer-sized grains were fabricated in large scale by using thermal evaporation of a single Sb2Se3 powder source onto the Mo-coated SLG substrates followed by moderate annealing (400 °C) under an Ar or Se+Ar atmosphere. The PEC performance of Sb2Se3 polycrystalline thin-film electrodes was dramatically enhanced by the construction of a p−n heterojunction with an antiphotocorrosive CdS/TiO2 bilayer and surface decoration with Pt catalyst. The Se-annealed Sb2Se3(Se) electrodes with surface modification showed a significantly enhanced PEC performance compared with the Ar-annealed analogues likely because of the reduced recombination by Se vacancies. Benefited from the desirable large grains of Sb2Se3(Se) and antiphotocorrosive p−n heterojunction, the fabricated Sb2Se3(Se)/CdS/TiO2/Pt electrode achieved a cathodic photocurrent density of ca. −8.6 mA cm−2 at 0 VRHE with a photocurrent onset of ca. 0.43 VRHE and a stable photocurrent for over 10 h in neutral electrolyte under simulated sunlight. This PEC performance is close to the state-of-the-art thin-film photocathodes made of other earthabundant binary photoabsorbers, such as Cu2O, CuO, and WSe2. Overall, the results of this study demonstrate that an Sb2Se3-based electrode could be utilized as a promising lowband-gap photocathode for constructing standalone dualelectrode cells for cost-effective visible−near-infrared solar hydrogen production in large scale, although a further improvement in the photocurrent density and onset potential through additional interfacial engineering and careful optimization (including the thickness and post-treatment) is required. EXPERIMENTAL SECTION Thermal Deposition of Sb2Se3 Thin Films. Sb2Se3 thin films were deposited by facile thermal evaporation of a single Sb2Se3 (99.99%) powder source onto Mo-coated soda-lime glass substrates at a base pressure of around 10−4 Pa. The Sb2Se3 powder source was loaded into a molybdenum boat that was positioned around 20 cm away from the sample holder. The deposition rate was kept at approximately 2 nm s−1, and the deposition thickness was fixed at 1.5 μm monitored using a microcrystalline quartz balance. No intentional heating of the Mo substrates was applied during the deposition. The as-deposited Sb2Se3 thin films were then annealed in a horizontal quartz tube furnace at different temperature (300−450 °C) for 1 h under a flowing Ar atmosphere (500 sccm), followed by cooling to room temperature naturally. Some of the as-deposited Sb2Se3 thin films were annealed (400 °C, 1 h) under a Se+Ar atmosphere with Ar (500 sccm) as the carrier gas. The Se vapor was introduced by placing an excess amount of Se (99.99%, loaded in a quartz boat) at the furnace upstream (around 30 cm away from the samples) and kept around 330 °C during the annealing process. Chemical Bath Deposition (CBD) of CdS. Prior to CBD process, the surface of the sample was pretreated in a bath solution of 0.01 M Cd(CH3COO)2·2H2O (98%, Alfa Aesar) and 5 M NH4OH (Alfa Aesar, 28 wt %) at around 65 °C for 15 min. The CBD of CdS was performed by immersing the pretreated sample into a bath solution containing 0.01 M Cd(CH3COO)2, 0.5 M SC(NH2)2 (99%, Alfa Aesar), and 5 M NH4OH for 10 min, during which the bath temperature gradually increased from room temperature to around 65 °C. After CdS deposition, the sample was rinsed immediately by deionized water and then annealed at 200 °C for 1 h under an air atmosphere.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b07512. Additional experimental images (Figures S1−S16) and tables (Table S1, S2), as well as the detailed experimental methods (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. 12760

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Fuxiang Zhang: 0000-0002-7859-0616 Hongwei Zhu: 0000-0001-6484-3371 Notes

The authors declare no competing financial interest.

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DOI: 10.1021/acsnano.7b07512 ACS Nano 2017, 11, 12753−12763

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DOI: 10.1021/acsnano.7b07512 ACS Nano 2017, 11, 12753−12763