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Evident Enhancement of Photoelectrochemical Hydrogen Production by Electroless Deposition of MB (M = Ni, Co) Catalysts on Silicon Nanowire Arrays Yong Yang, Mei Wang, Peili Zhang, Weihan Wang, Hongxian Han, and Licheng Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09600 • Publication Date (Web): 20 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016

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ACS Applied Materials & Interfaces

Evident Enhancement of Photoelectrochemical Hydrogen Production by Electroless Deposition of M-B (M = Ni, Co) Catalysts on Silicon Nanowire Arrays Yong Yang,† Mei Wang,*,† Peili Zhang,† Weihan Wang,† Hongxian Han,‡ Licheng Sun,†,§ †

State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116024, P. R. China ‡ Dalian National Laboratory for Clean Energy & State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China § Department of Chemistry, KTH Royal Institute of Technology, Stockholm 10044, Sweden

ABSTRACT: Modification of p-type Si surface by active and stable earth abundant electrocatalysts is an effective strategy to improve the sluggish kinetics for the hydrogen evolution reaction (HER) at p-Si/electrolyte interface and to develop highly efficient and low-cost photocathodes for hydrogen production from water. To this end, Si nanowire (Si-NW) array has been loaded with highly efficient electrocatalysts, M-B (M = Ni, Co), by facile and quick electroless plating to build M-B catalyst-modified Si nanowire-array-textured photocathodes for water reduction to H2. Compared with the bare Si-NW array, composite Si-NWs/M-B arrays display evidently enhanced photoelectrochemical (PEC) performance. The onset potential (Vphon) of cathodic photocurrent is positively shifted by 530–540 mV to 0.44–0.45 V vs. RHE, and the short-circuit current density (Jsc) is up to 19.5 mA cm−2 in neutral buffer solution under simulated 1 sun illumination. Impressively, the half-cell photopower conversion efficiencies (hc) of the optimized Si-NWs/Co-B (2.53%) and Si-NWs/Ni-B (2.45%) are comparable to that of Si-NWs/Pt (2.46%). In terms of the large Jsc, Vphon, and hc values, as well as the high Faradaic efficiency, Si-NWs/M-B electrodes are among the top performing Si photocathodes which are modified with HER electrocatalysts but have no buried solid/solid junction. KEYWORDS: cobalt boride, nickel boride, silicon, photocathode, hydrogen evolution reaction, photoelectrochemical catalysis

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INTRODUCTION One of the most challenging scientific issues for chemists in the current century is to develop energy-efficient and cost-effective technologies for the conversion of solar energy to hydrogen by water splitting. A promising approach to solar water splitting is to utilize a series-connected photocathode and photoanode in a tandem configuration to form a dual light absorber photoelectrochemical (PEC) cell, which is an optimum compromise design of a one-pot photocatalytic system and a PV/electrolyzer apparatus. The computational analysis predicts that more than 22% solar-to-hydrogen (STH) conversion efficiency could be obtained by employing a PEC cell consisting of a photoanode and a photocathode,1 which are made of light-absorber materials with a band gap combination of 1.65–1.80 eV/1.00–1.20 eV and loaded with earth-abundant catalysts for the oxygen-evolution reaction (OER) and hydrogen-evolution reaction (HER), respectively. To this end, highly efficient (large photovoltage and high photocurrent), durable, and inexpensive photoanodes and photocathodes for each half reaction of water splitting should be individually developed in advance for the construction of a tandem PEC cell with high STH efficiency. Silicon with a band gap of 1.12 eV and theoretical maximum photocurrent density of 44 mA cm−2 is a promising candidate as a light-absorber for a photocathode used in a tandem PEC water splitting cell.2,3 Compared with other p-type semiconductors such as Cu2O, CdX (X = Se, Te), CuInX2 (X = S, Se), InP, GaInP2, and GaAs, the advantages of Si are that i) it is the second most abundant elements in the Earth's crust, ii) it can absorb a significant portion of the solar spectrum due to its narrow band gap, iii) it is much less expensive than the Ga-, In-, and Ge-containing semiconductors, iv) it is more environmentally benign than the Cd-, Pb-, and As-containing semiconductors, and v) the processing techniques for Si-based semiconductor materials are mature, economical, and practical. In recent decade, studies on Si-based photocathodes have attracted extensive attention.4 One of the approaches to 2 ACS Paragon Plus Environment

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improving the STH efficiency of Si-based photocathodes is to use Si nanowire (Si-NW) and microwire (Si-MW) texturing arrays because the one-dimensional morphology can orthogonalize directions of incident light absorption and charge-carrier collection, so as to provide long optical paths for more light trap and short transport distances for efficient photogenerated charge-carrier collection.5,6 In addition, the wire array-structured Si surface can reduce the reflectance of incident light, increase the ratio of internal surface area to geometric area, and therefore allow for a high catalyst loading onto a given projected area of a Si-based photoelectrode. However, as many semiconductor materials, the inherent drawback of pristine p-Si is the sluggish charge-transfer kinetics at p-Si/electrolyte interface, due to its small exchange current density (j0 = 10−5 mA cm−2) for proton reduction to hydrogen at zero overpotential and its low flat band potential in an aqueous electrolyte.7,8 Modification of a Si photocathode surface by highly active and stable electrocatalysts can efficiently improve the HER kinetics at the electrode/electrolyte interface. Early studies have demonstrated that the loading of Pt onto a Si photocathode could induce a significant enhancement of photocatalytic activity of the electrode.7–12 However, the low abundance and high cost of Pt limit its application in a large scale. In very recent years, various earth-abundant electrocatalysts have been coupled with p-Si photoelectrodes as low-cost alternatives to Pt, such as Ni-Mo,13–16 Mo3S4,17 MoSx,18–23 WSx,24 Mo2C,25 W2C,26 MoOxSy,27 MoS2Cl,28 Co-S,29 CoX2 (X = S, Se),30,31 FeP,32 CoP,33–35 Ni2P and Ni12P5,36 CoMoSx,37 and CoPS.38 For these Si/earth abundant catalyst composite photoelectrodes, the reported photovoltage onsets (Vphon) are in the range of 0.12 to 0.87 V vs. reversible hydrogen electrode (RHE; potentials reported in this paper are all versus RHE), and the short-circuit photocurrent densities (Jsc) vary from 5.5 to 35 mA cm−2 under illumination of 100 mW cm−2 light intensity. The STH efficiency of a HER half cell (ηhc) largely depends on the morphology and surface texture of Si, the buried solid/solid junction engineering, and 3 ACS Paragon Plus Environment


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the electrocatalyst used. The highest reported ηhc values are 3.13% for the Ni12P5 nanoparticle decorated Si wire-array photocathode36 and 5.5 % for the p-i-n hydrogenated amorphous Si electrode with a TiO2 protection layer and Ni-Mo catalyst modification.15 Very recently, higher STH efficiencies (7.3%–7.5%) were obtained for photocathodes which were fabricated by integrating a monolithic multijunction solar cell made of amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon with a Ni electrocatalyst layer.39,40 Considering that the STH efficiency of a PEC cell is determined by the photocurrent and photovoltage at the maximum power point, an optimal trade-off between photocurrent and photovoltage is important to obtain high STH efficiency in the work of photoelectrode engineering by morphology and texture altering, junction building, electrocatalyst grafting, and element doping. Recently we found that the electroless plated Ni-B film was a very active and stable electrocatalyst at low overpotentials for the HER in aqueous media of entire pH range.41 The overpotential required to reach a current density of 10 mA cm−2 is only 54 mV in 1.0 M pH 7 phosphate buffer solution (PBS) for the Ni-B electrode. Good electrocatalytic performance of Co-B in a pH range of 4–9 was also reported very recently.42 In view of the superior activity and good stability of M-B (M = Ni, Co) electrocatalysts for the HER, these metal borides are promising candidates for interfacing with semiconductors to construct efficient composite photocathodes for light-driven water reduction to hydrogen. In this work, we coupled M-B electrocatalysts with Si nanowire-array-textured photocathodes (Si-NWs/M-B) and evaluated the photoelectrochemical performance of Si-NWs/M-B electrodes in neutral electrolyte. These photocathodes displayed photocurrent onset potentials (Vphon) at 0.30–0.45 V (Here Vphon is defined as the potential at a photocurrent density of −1 mA cm−2.) and photocurrent densities of 14.5–19.5 mA cm−2 (Jsc) at a bias of 0 V under simulated 1 sun illumination. The half-cell STH efficiency (hc) is up to 2.45% for p-Si-NWs/Ni-B and 2.53% for p-Si-NWs/Co-B. The 4 ACS Paragon Plus Environment

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Jsc, Vphon, and hc values displayed by p-Si-NWs/M-B photocathodes are among the best performances of the earth-abundant HER electrocatalyst-modified Si photocathodes which have no buried solid/solid junction.

EXPERIMENTAL SECTION Materials. Silicon wafers were purchased from Hangzhou Bojing Science and Technology Limited Company. Chemicals HF, AgNO3 (99.999%), and HNO3 were used as received from Aladdin Industrial Corporation. Compounds NiCl2· 6H2O, CoCl2· 6H2O, glycine, and ethylenediamine were purchased from Tianjin Guangfu Technology Development Limited Company and H2O2 from Tianjin Bodi Chemical Reagent Limited Company. Other compounds NaOH, NaBH4, NaH2PO4· 2H2O, and K2HPO4· 3H2O were purchased from Sinopharm Chemical Reagent Limited Company. Aluminum grains (99.999%) were got from Zhongnuo Advanced Materials Science and Technology Limited Company. All commercially available chemicals were used directly without further purification. The water used for fabrication of Si electrodes and as electrolytes was deionized with a Millipore AFS-E system (18.2 MΩ-cm resistivity). Fabrication of Si-NW Array Photocathode. Si-NWs were fabricated by Ag+-assisted chemical etching of silicon wafer.43 A p-type Si(100) wafer (B-doped; 10-20 Ω-cm, 1 cm2) is degreased by rinsing with acetone, ethanol, and ultrapure deionized water (DIW, 18.2 MΩ-cm resistivity) in sequence. The degreased Si sample was immersed in a solution of H 2O2/H2SO4 (1:3, v/v) at 90 °C for 30 min and rinsed with DIW, and then immersed in 5% HF to remove surface oxide for 3 min. After that, the backside of Si sample was protected by adhesive tape and then Si electrode was etched in aqueous etchant containing AgNO3 (10 mM) and HF (4.5 M) for about 1 min and rinsed with DIW. Subsequently, Si sample was immersed in the solution containing H2O2 (0.3 M) and HF (4.5 M) for 30 min and rinsed with DIW. The 5 ACS Paragon Plus Environment


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residual Ag seeds on the surface of Si-NWs were removed by immersing the sample in 70% HNO3 for 30 min and rinsing with DIW. Finally, an Al layer of about 300 nm was deposited onto the backside of the sample by vacuum evaporate plating technology and the resulting sample was annealed in Ar at 450 °C for 5 min to form an Ohmic contact with p-type Si. Tinned Cu wire was electrically connected to the Al film by Ag conductive adhesive (SPI supplies, PA, USA) and the Cu wire was threaded into a glass tube. Non-conductive hysol epoxy was used to seal the entire substrates except the region where Si-NWs resided. Fabrication of Si-NWs/M-B (M = Ni, Co) Photocathodes. Ni-B nanoparticles (NPs) were deposited onto Si-NWs by electroless plating.41 The precooled aqueous solution of ethanediamine (20% w/w, 25 mL) was added in portion into the solution of NiCl2· 6H2O (3.00 g, 25 mL) with stirring, which was precooled to the temperature lower than 10 °C with an ice bath. This Ni2+ mixture was then combined with the NaOH solution (25 mL) containing NaBH4 (0.2 g) at pH 13.5. The electroless plating solution was adjusted to 100 mL at pH 13.5 by 10 M NaOH solution. After etched in HF (5%) for 3 min to remove oxide layer and rinsed with DIW, Si-NW electrode was immersed into the prepared solution at 90 °C for deposition of Ni-B NPs. The deposition was carried out for 1, 3, and 5 s, followed by thoroughly rinsing the sample with DIW. For deposition of Co-B NPs, the electroless plating solution containing CoCl2· 6H2O (1.00 g), glycine (4.50 g), and NaBH4 (0.40 g) was prepared as an essentially identical procedure for the deposition of Ni-B. The solution was adjusted to 100 mL at pH 12.5 by using 10 M NaOH. If the pH of the solution was higher than 12.5, precipitate was quickly generated in the solution. Silicon nanowire electrode was immersed into the prepared solution at 60 °C for deposition of Co-B NPs. When the temperature of the solution was higher than 60 °C, a considerable Co-B NPs precipitated in solution before Si sample was put in. The deposition was carried out for 5, 10 and 15 s, followed by thoroughly rinsing the sample with DIW. 6 ACS Paragon Plus Environment

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Fabrication of Si-NWs/Pt Photocathode. After Si-NW electrode was etched in 5% aqueous HF solution for 3 min, it was immersed in a solution containing 0.4 M HF and 1 mM K2PtCl6 for 3 min, and then rinsed with DIW and dried under N2. This deposition procedure was repeated for deposition of more Pt NPs. The highest catalytic activity was obtained from the Si-NWs/Pt electrode which was fabricated for a total deposition time of 12 min. Characterization of Materials. The morphology of as-fabricated electrodes was imaged by scanning electron microscopy (SEM) (NOVA NanoSEM 450 equipped with an energy dispersive X-ray system), transmission electron microscopy (TEM) (FEI Tecnai G2 F20 S-TWIN), and scanning TEM (STEM) (Tecnai G2 F30 S-Twin with an acceleration voltage of 300 kV). The composition of the deposited materials was characterized by energy dispersive X-ray (EDX) analysis and the chemical states of Ni, Co, and B in deposited nanoparticulate materials were measured by X-ray photoelectron spectroscopy (XPS) (Thermo VG ESCALAB250) using a monochromatized Al Kα small-spot source and a 500 μm concentric hemispherical energy analyzer. The loading amount and the atomic ratio of Ni, Co, and B on Si-NWs were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) (Perkin Elmer 2000 DV). Photoelectrochemical Measurements. Photoelectrochemical measurements were carried out in a three-electrode cell using an electrochemical workstation (CHI650E) with Si-NWs, Si-NWs/Ni-B, Si-NWs/Co-B, or Si-NWs/Pt photocathode as a working electrode, Ag/AgCl as a reference electrode, and Pt foil (1 cm2) as a counter electrode. Phosphate buffer solution (2 M) at pH 7 was used as electrolyte for measurements. A xenon lamp equipped with an AM 1.5G filter was adopted as the illumination source and the light intensity was calibrated to 100 mW cm−2 prior to each experiment. Polarization curves of the as-fabricated Si-NW electrodes were swept linearly from positive to negative at a scan rate of 50 mV s−1. All potentials reported in this paper were converted to the RHE reference scale using E(RHE) = E(Ag/AgCl) 7 ACS Paragon Plus Environment


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+ 0.20 V + 0.059pH V. Each J–E curve shown in the main text and in the Supporting Information (SI) has been repeated by at least three parallel measurements of individual samples that were fabricated under the same conditions. The hc of a photocathode was calculated according to the equation: ηhc = JmppVmpp/Pin, where Jmpp and Vmpp are the current density and applied potential at the maximum power point, and Pin is the power of incident illumination.2 The stability of as-fabricated electrodes was measured at a bias of 0 V in a homemade gastight three-electrode cell under simulated 1 sun illumination. During controlled-potential coulometry, the gas in the headspace of the cell was analyzed by CEAULIGHT GC-7920 gas chromatograph equipped with a 5 Å molecular sieve column (2 mm × 2 m). The amount of hydrogen evolved was determined by GC with the external standard method and the hydrogen dissolved in solution was neglected. Measurement of Electrochemical Impedance Spectra. Electrochemical impedance spectra of Si-NWs, Si-NWs/Ni-B (3 s), and Si-NWs/Co-B (10 s) were collected using an electrochemical workstation (CHI650E). The measurement was carried out under illumination at a bias of 0 V, with the sweeping of frequency from 100 kHz to 1 Hz and with a 10 mV AC dither. A xenon lamp equipped with an AM 1.5G filter was used to illuminate the PEC cell with a fixed light intensity of 100 mW cm−2. Capacitance Measurements. Capacitance measurements of Si-NWs, Si-NWs/Ni-B (3 s), and Si-NWs/Co-B (10 s) were performed as the potential was swept from 0.6 V to −0.2 V using an electrochemical workstation (Zennium/IM6) without illumination. On the basis of capacitance data, the flat potentials of Si-NWs, Si-NWs/Ni-B(3 s), and Si-NWs/Co-B(10 s) were calculated using the Mott-Schottky relationship:51 1/Csc2 = 2(Ea − Efb − kT/e)/(eεε0NdA2), where Csc is the space charge capacity, Ea is the applied potential, Efb is the flat band potential, k is the Boltzmann constant, T is the absolute temperature, e is the electron charge, Nd is the 8 ACS Paragon Plus Environment

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donor density, ε is the dielectric constant of semiconductor, ε0 is the electric permittivity of vacuum, and A is the geometric area of the photoelectrode.

RESULTS AND DISCUSSION Silicon NW arrays were fabricated by Ag+-assisted chemical etching of silicon wafer.43 M-B (M = Ni, Co) NPs were deposited onto the surface of Si-NWs by electroless plating method.41,42 The deposition was carried out within a few seconds, specifically, being 1, 3, and 5 s for deposition of Ni-B and 5, 10, 15 s for Co-B, to tune the loading of electrocatalyst. This simple, fast, cheap, and convenient procedure is easily scalable for decorating the surface of a semiconductor with an electrocatalyst. SEM images show that a dense array of Si-NWs is vertically aligned on the original Si substrate with an average length of 6.6 μm (Figure S1). ICP-OES, TEM, and EDX analyses (Figure S2) of the bare Si-NW array indicate that Ag seeds have been completely removed from Si photoelectrode after Si-NW array was immersed in 70% HNO3 and rinsed with DIW. The morphology of M-B modified Si-NW electrodes was characterized by SEM and TEM analyses. The cross-sectional SEM images (Figures 1a and e) indicate that Ni-B and Co-B NPs have been loaded on the surface of Si-NWs. Furthermore, TEM images of M-B decorated Si-NW arrays, which were peeled off from the surface of Si substrate, clearly show that discrete NPs have been uniformly deposited on the surface of Si-NWs from tip to base (Figures 1b and f). For many reported Si-NWs/ and Si-MWs/catalyst electrodes, the decoration of catalysts is only on the tip of Si-NWs and Si-MWs.9,24,31,33 By contrast, decoration of a highly efficient H2-evolution catalyst along the sidewall of Si-NWs can better activate the surface of Si-NWs and therefore facilitate the HER. TEM images of the selected areas of M-B decorated Si-NWs illustrate that Ni-B and Co-B NPs adhere densely but discretely on the surface of Si-NWs with the particle sizes of 7–50 nm for Ni-B NPs and 9 ACS Paragon Plus Environment


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70–190 nm for Co-B NPs (Figures 1c and g). Furthermore, STEM images show that both Ni-B and Co-B NPs are amorphous materials with the latter in snowflake shape (Figures 1d and h), and EDX analyses indicate that the deposited particles are composed of Ni and B or Co and B for Si-NWs/Ni-B and Si-NWs/Co-B electrodes (Figure S3), respectively. To estimate the amount of deposited Ni, Co, and B elements, the as-fabricated Si-NWs/Ni-B and Si-NWs/Co-B photoelectrodes were first immersed in aqua regia overnight, and then the deposited materials were removed from Si substrate by sonification and dissolved in aqua regia for ICP-OES analysis. SEM images and EDX spectra (Figures S4 and S5) of Si-NWs/Ni-B and Si-NWs/Co-B measured after sonification indicate that Si-NWs have been completely removed from Si substrate, and there is no Ni, Co, and B element on the surface of remaining substrate. ICP-OES analyses of the aqua regia solutions reveal that with extending the deposition period from 1 to 5 s under otherwise identical conditions, the approximate loading amount increases from 12 to 50 g cm−2 for Ni and 4.5 to 11 g cm−2 for B. Accordingly, the Ni-to-B atomic ratio in Ni-B particles varies from 1:2.1 to 1:1.2 (Table S1). Similar analyses show that the loading amounts of Co and B increase from 21 to 62 and 3 to 5.5 g cm−2, respectively, and the Co-to-B atomic ratio in Co-B particles varies from 1:0.8 to 1:0.5 when the electroless plating period was extended from 5 to 15 s (Table S2). Apparently, the electroless plating period has considerable influence on the loading amount, the M-to-B ratio, and the size of deposited M-B NPs, and as a result, on the PEC performance of Si-NWs/M-B photoelectrodes as will be discussed below.

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Figure 1. Cross-sectional SEM images of a) Si-NWs/Ni-B(3 s) and e) Si-NWs/Co-B(10 s). TEM images of b, c) Ni-B and f, g) Co-B decorated Si-NWs; STEM images of d) Ni-B and h) Co-B NPs.

The chemical states of Ni, Co, and B in Ni-B and Co-B NPs on the surface of Si-NWs were investigated by XPS (Figure S6). In the Ni 2p window (Figure 2a) of the X-ray photoelectron spectrum of Si-NWs/Ni-B(3 s), the peaks with binding energies of 852.7 and 870.1 eV are associated with the elemental nickel in Ni-B and the peak of 853.7 eV binding energy stems from the nickel in oxidized state.44,45 The Ni0/Ni2+ molar ratio calculated from the Ni 2p peaks is about 4.4:1. The peaks at 188.6 and 193.2 eV binding energies in the B 1s region (Figure 2b) are assigned respectively to the elemental boron in Ni-B and to the boron in oxidized state, most possibly in B2O3.46 Accordingly, the broad peak in the oxygen region was fitted with three contributions assigned to the O 1s core levels for nickel oxide at 531.7 eV, silicon oxide at 532.5 eV, and boron oxide at 533.3 eV (Figure S7a). These oxidized compounds on the surface of Si-NWs were formed on the exposure of the as-prepared electrode in air. Similarly, two peaks at 778.2 and 779.0 eV binding energies of the Co 2p3/2 level in the X-ray photoelectron spectrum of Si-NWs/Co-B(10 s) (Figure 2c) imply the presence of both 11 ACS Paragon Plus Environment


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elemental and oxidized cobalt in an approximate Co0/Co2+ molar ratio of 2.32:1. In the B 1s region (Figure 2d), the peaks with binding energies of 188.3 and 192.5 eV arise from the elemental boron in Co-B and the oxidized boron in B2O3, respectively. There are also three peaks in the O 1s region due to cobalt oxide, silicon oxide, and boron oxide (Figure S7b). Compared with that of pure B (187.1 eV),42 the binding energies of 188.6 and 188.3 eV for the elemental boron in Ni-B and Co-B NPs are positively shifted by 1.5 and 1.2 eV, respectively. On the contrary, small negative shifts (0.1–0.2 eV) of the binding energy peaks of the 2p3/2 core levels of metallic Ni and Co in M-B NPs were observed compared to that of pure Ni (852.8 eV) and Co (778.4 eV).36,42 The shifts of these binding energy peaks of elemental B, Ni, and Co are an indication of electron donation from B to M in the M-B NPs, which is consistent with the trend of electron movement reported for transition metal-rich borides (MBx, M = Ni, Co, x < 2) in amorphous state.47

Figure 2. XP spectra of a) Ni 2p and b) B 1s regions of Si-NWs/Ni-B electrode; c) Co 2p and d) B 1s regions of Si-NWs/Co-B electrode. 12 ACS Paragon Plus Environment

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The PEC H2-evolution performances of Si-NWs and M-B (M = Ni, Co) composite photocathodes were evaluated using a three-electrode set-up in phosphate buffer solution (PBS) at pH 7 under simulated 1 sun illumination (100 mW cm−2). Initially, linear sweep voltammograms (LSVs) were recorded in neutral PBS without illumination. The current rose at an onset of –0.21 V for Si-NWs/Ni-B and –0.15 V for Si-NWs/Co-B (Figure 3). The bare Si-NWs displayed a photocurrent onset potential at −0.09 V under illumination. Photocurrent density-potential (J–E) curves shown in Figure 3a reveal that the loading amount of Ni-B NPs on the surface of Si-NWs apparently influences the performance of photocathode (Table S1). With the deposition of Ni-B for only 1 s, the photocurrent density of 15.6 mA cm−2 was achieved at a bias of 0 V with an onset potential of 0.44 V. When the deposition time was extended to 3 s, the value of Jsc increased to 17.3 mA cm−2 with a decrease of Vphon to 0.35 V. In the meantime, the limiting photocurrent density (Jph) was enhanced from 16.7 to 22.5 mA cm−2. With further extension of the deposition period to 5 s, a considerable decrease in photocurrent was observed, possibly due to the fact that the larger mass of deposited catalyst caused an optical loss at the semiconductor surface. Similarly, J–E curves (Figure 1b) of Si-NWs/Co-B photocathodes show that the optimal deposition time is about 10 s, which gives a Jsc of 19.5 mA cm−2 with an onset potential of 0.45 V. Extending of the Co-B deposition period also caused small attenuation in both Jsc and Vphon (Table S2). Decrease of photocurrent with electrocatalysts excessively loaded on the surface of Si is a common phenomenon for Si/inorganic solid catalyst composite photocathodes,22,38 because the cumulation of catalysts on the surface of Si material could attenuate the incident light into Si-NWs and increase the possible charge recombination at the photoelectrode.13,27,31,34 These factors are also speculated as the possible reasons, at least partly, for the small decrease of photovoltage onset at high Ni-B or Co-B loading. The as-fabricated Si-NWs/Ni-B and Si-NWs/Co-B photoelectrodes 13 ACS Paragon Plus Environment


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exhibit similar optimal Vphon (0.44–0.45 V), which are positively shifted by 530–540 mV as compared to that of a bare Si-NW array electrode under the same conditions. Accordingly, large photovoltages of 600–650 mV were observed for Si-NWs/Ni-B and Si-NWs/Co-B electrodes under illumination compared to the corresponding current onset potential without illumination.

Figure 3. Polarization curves of a) Si-NWs/Ni-B and b) Si-NWs/Co-B photocathodes fabricated in varied deposition periods.

Platinum NPs were also supported on Si-NW array by electroless plating according to the literature procedure,13 to make a direct comparison with M-B modified Si-NW arrays. The Si-NWs/Pt that was fabricated by electroless plating of Pt NPs for 12 min displayed the best performance for the HER (Figure S8). The optimal Jsc, Vphon, and ηhc of Si-NWs/Pt are 21.2 mA cm−2, 0.38 V, and 2.46% (Jmpp = 12.68 mA cm−2, Vmpp = 0.194 V), respectively, under identical conditions as aforementioned for Si-NWs/M-B. Further extending the plating period of Pt NPs to 15 min resulted in a decrease of Vphon and ηhc for Si-NWs/Pt. A comparison of the performances of Si-NWs/Ni-B and Si-NWs/Co-B with that of Si-NWs/Pt is shown in Figure 4a and Table 1. Both Si-NWs/Ni-B(1s) and Si-NWs/Co-B(10 s) display more positive onset potentials than that displayed by Si-NWs/Pt. Impressively, the ηhc values of 2.45% (Jmpp = 14 ACS Paragon Plus Environment

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10.34 mA cm−2, Vmpp = 0.237 V) for Si-NWs/Ni-B(1 s) and 2.53% (Jmpp = 11.18 mA cm−2, Vmpp = 0.226 V) for Si-NWs/Co-B(10 s) are comparable to that of Si-NWs/Pt (2.46%) under the same test conditions. The reported Si-NW array photoelectrodes decorated with Co-P, Ni2P, or Ni12P5 also exhibit photocurrent at more positive onset potentials and display comparable or higher STH efficiencies than a Pt-modified Si-NW array.33,36 Previous studies have revealed that the selective loading of Pt NPs on the tip region of Si-NWs is a typical phenomenon for Si-NWs/Pt which is fabricated by electroless plating of Pt NPs.9,10 The same loading feature of Pt NPs was also observed from TEM images of our Si-NWs/Pt electrodes (Figure S9). In contrast, Ni-B and Co-B NPs are distributed from tip to base on the surface of each Si NW (Figures 1b and f). Such uniform distribution of highly active HER electrocatalysts activates the most part of the sidewall of each Si-NW and benefits the collection of photogenerated electrons in the radial direction, so that Si-NWs/M-B photoelectrodes displayed evidently enhanced performance for PEC H2 production. The excellent electrocatalytic property of Ni-B and Co-B catalysts in neutral solution plays an important role in the enhancement of PEC H2 evolution.41,42 The j–E curves (Figure S10) of Si-NWs/Ni-B, Si-NWs/Co-B, and Si-NWs/Pt measured in the dark show that these electrodes display similar overpotentials, and the electrocatalytic activity of Si-NWs/Co-B is even higher than that of Si-NWs/Pt in neutral buffer solution.

Table 1. Comparison of the performances of Si-NWs/M-B and Si-NWs/Pt photocathodes Photocathode

Vphon (V vs. RHE)

Jsc (mA cm−2)

Jph (mA cm−2)

ff

ηhc (%)

Bare Si-NWs

−0.09

−0.17

–

–

–

Si-NWs/Ni-B(1 s)

0.44

−15.6

−16.7

0.36

2.45

Si-NWs/Co-B(10 s)

0.45

−19.5

−20.0

0.29

2.53

Si-NWs/Pt(12 min)

0.38

−21.2

−22.1

0.30

2.46



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Among the previously reported Si-MW and Si-NW array photocathodes that are directly loaded with earth abundant electrocatalysts (Table S3), Si-NWs/Ni12P5 (2.97%),36 Si-NWs/Ni2P (3.13%),36 Si-NWs/FeP (2.64%),31 and Si-NWs/CoP (2.86%)33 exhibit higher ηhc values than Si-NWs/Ni-B and Si-NWs/Co-B. The values of ηhc, reported for other Si wire and pillar photocathodes loaded with CoSe2,31 Co-P,33,34 Ni-Mo,13,14 MoS2,18 MoS3,19 WS2 and WS3,24 as well as N-doped carbon nanodots and graphene quantum sheets (N-GQSs),48–50 vary in the range of 0.03%–2.29%. The values of Voc are about 0.14 V for Si-MWs/CoSe2,30 0.22 V for Si-MWs/MoOxSy,27 and 0.32 V for n+p-Si/CoS photocathodes in neutral solutions under similar test conditions.29 To our delight, the photovoltage onsets (0.44–0.45 V) displayed by Si-NWs/M-B electrodes are among the highest ones reported so far for the crystalline Si-based photocathodes that have no buried solid/solid junction while modified with non-noble metal-based HER electrocatalysts.

Figure 4. a) Comparison of polarization curves of Si-NWs, Si-NWs/Ni-B(1 s), Si-NWs/Co-B(10 s), and Si-NWs/Pt(12 min) photocathodes in pH 7 PBS under 100 mW cm−2 illumination. b) Photocurrent densities of Si-NWs/Ni-B(3 s) and Si-NWs/Co-B(10 s) measured at a bias of 0 V under illumination for 4 h.


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The stability of Si-NWs/Ni-B and Si-NWs/Co-B photocathodes in photoelectrochemical H2 production was evaluated by potentiostatic photoelectrolysis in pH 7 PBS at an applied potential of 0 V under 100 mW cm−2 illumination for 4 h. The initial photocurrent densities are 15.0 and 16.8 mA cm−2 for Si-NWs/Ni-B and Si-NWs/Co-B (Figure 4b), respectively. It was noticed that the cathodic photocurrent density slightly increased to 16.2 mA cm−2 for Si-NWs/Ni-B and to 17.6 mA cm−2 for Si-NWs/Co-B during the first 30 min of illumination. The slight increase of photocurrent density is most possibly due to the reduction of Ni2+ and Co2+ species on the surface of Si-NWs and/or the gradual escape of B2O3 from the surface of M-B NPs to enter the solution under photoelectrolysis conditions, making more M-B active sites expose to the electrolyte. In the continuous PEC reaction, the photocurrent of Si-NWs/Ni-B was almost maintained in the first 2 h and 85% of original photocurrent remained after 4 h of illumination, while the photocurrent of Si-NWs/Co-B decreased apparently after 1 h of photoelectrolysis and only 56% of original photocurrent remained after 4 h. SEM images (Figures S11a and S12a) measured after 4 h of PEC test reveal that Si-NWs of both Si-NWs/Ni-B and Si-NWs/Co-B electrodes have been apparently corroded during long time photoelectrolysis in neutral solutions, most likely due to oxidation of Si surface. Furthermore, TEM investigations (Figures S11b-d and S12b-d) show that many M-B nanoparticles still remain on the surface of Si-NWs and there is no considerable change in the size of M-B particles compared to those on Si-NW arrays before used (Figures 1c, d, g, h). The used Si-NWs/Ni-B and Si-NWs/Co-B electrodes display strong Ni/B and Co/B peaks, respectively, in EDX spectra (Figures S13 and S14). Consistent with these observations, ICP analyses of the resulting electrolytes demonstrate that part of M-B NPs fall off Si-NWs into the solution (Table S4). Additionally, after photoelectrolysis for 4 h, Si-NWs/Ni-B and Si-NWs/Co-B electrodes were dissolved in aqua regia, and ICP-OES analyses indicated that there were no detectable Pt ions in the resulting solution. This evidence excludes the 17 ACS Paragon Plus Environment


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contamination of Si-NWs/M-B electrodes by Pt from counter electrode. All together, the analyses after long-time photoelectrolysis reveal that the instability of these photocathodes is mainly caused by degradation of Si-NWs and fall-off of electrocatalyst under PEC test conditions. The amounts of evolved H2 determined by gas chromatographic analyses are almost consistent with half of the number of electrons passed through the circuit in 1 h of the PEC reaction,

giving

Faradaic

efficiencies

of

94%–95%

for

Si-NWs/Ni-B(3

s)

and

Si-NWs/Co-B(10 s) (Figure S15). The high Faradaic efficiency implies that the large area of Si-NWs/M-B surface is HER active, resulting from dense and uniform decoration of highly active Ni-B or Co-B electrocatalyst NPs on the surface of Si-NWs. To understand the evidently enhanced PEC performance of Si-NWs/M-B electrodes, electrochemical impedance spectroscopy (EIS) measurements of the bare Si-NW array, Si-NWs/Ni-B(3 s), and Si-NWs/Co-B(10 s) were performed in neutral electrolyte under 100 mW cm−2 illumination at a bias of 0 V. The bare Si-NW array displayed a large semicircle in the Nyquist plot (Figure 5a), corresponding to the large resistance of direct charge transfer between the semiconductor Si and electrolyte. In contrast, Si-NWs/Ni-B and Si-NWs/Co-B photocathodes each exhibit two small semicircles. The one in high frequency range is related to the charge transfer in the semiconductor depletion layer of Si-NWs/M-B and the other in low frequency range signifies the charge transfer of double layer at catalyst/electrolyte interface.31–37 The significantly diminished resistance at the photocathode/electrolyte interface of Si-NWs/M-B electrodes compared to that of the bare Si-NW array electrode indicates that the uniformly decorated Ni-B and Co-B NPs on the surface of Si-NWs act as effective mediators, which greatly accelerate the charge transfer kinetics between illuminated Si-NWs and electrolyte, leading to fast reduction of H+ to H2.


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Figure 5. a) Nyquist impedance plots of Si-NWs, Si-NWs/Ni-B(3 s), and Si-NWs/Co-B(10 s) photocathodes measured under illumination (inset: zoomed view of EIS spectra at small impedance region). b) Mott-Schottky plots from capacitance measurements as a function of potential under dark conditions.

To have an insight into the origin of difference in the onset potential of photocurrent for M-B modified Si-NW arrays and the bare Si-NW array, capacitance measurements of Si-NWs, Si-NWs/Ni-B(3 s), and Si-NWs/Co-B(10 s) electrodes were carried out in neutral electrolyte as the potential was swept from 0.55 V to −0.18 V without illumination. Mott-Schottky plots of Si-NWs, Si-NWs/Ni-B, and Si-NWs/Co-B electrodes are shown in Figure 5b. The flat band potential (Efb) of a semiconductor can be estimated by extrapolation of the linear fitting to a capacitance of zero according to Mott-Schottky equation,51 1/Csc2 = 2(Ea − Efb − kT/e)/(eεε0NdA2). By this method, the Efb of Si-NWs is 0.26 V, which is much more positive than its Vphon (−0.09 V). In contrast, the values of Efb for Si-NWs/Ni-B and Si-NWs/Co-B are 0.42 and 0.47 V, respectively, which are close to their open circuit potentials. The positive shift of Efb will lead to larger band bending (Eb) at the solid/electrolyte interface according to the equation of Eb = Ea – Efb (Ea is generally negative at cathodic side).49 As a consequence, charge separation of photogenerated electrons and holes is promoted and charge recombination is suppressed to a certain extent. In general, the band edge positions of 19 ACS Paragon Plus Environment


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positively shifted flat bands of Si-NWs/Ni-B and Si-NWs/Co-B are correlated to their large open circuit voltages and in another aspect, the enlarged band bending in the depletion layer of semiconductor also makes a contribution for the evidently enhanced photocurrent density of M-B modified Si-NW array photocathodes.

CONCLUSIONS In summary, the loading of M-B (M = Ni, Co) NPs onto the surface of Si-NW arrays by quick electroless deposition evidently improves the performance of Si-NW array for the PEC H2 production. The onset potential of photocurrent is shifted from −0.09 V for a bare Si-NW array to 0.44–0.45 V for optimized Si-NWs/M-B electrodes, and the short-circuit photocurrent density (Jsc) is enhanced from 0.16 mA cm−2 for a bare Si-NW array to 19.5 mA cm−2 for Si-NWs/M-B electrodes in neutral buffer solution under 100 mW cm−2 illumination. Additionally, the light limited photocurrent density (Jph) is up to 22.5 mA cm−2, corresponding to 51% of the theoretical limit of 44 mA cm−2 for Si with a bandgap of 1.12 eV. The half-cell photopower conversion efficiencies (hc) of 2.45%–2.53% for Si-NWs/Ni-B and Si-NWs/Co-B are comparable to that of the Pt-modified Si-NW array (2.46%) in neutral solution under identical measuring conditions. The enhanced PEC H2 production of M-B modified Si-NW arrays is originated from the excellent electrocatalytic property of Ni-B and Co-B catalysts in neutral solution. The uniformly distributed M-B NPs along the sidewall of each Si-NW in the array make a dominant contribution to the low resistance at the photocathode/electrolyte interface of Si-NWs/M-B as compared to that of the bare Si-NW array, which leads to high photocurrent response and the fast H+/H2 reduction. Considering the outstanding PEC performance, the facile fabrication procedure, and the mild reaction conditions, Si-NWs/M-B provides one more choice of promising photocathodes to be adopted for construction of integrated PEC cells. 20 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information Tables for summarization of catalytic data and ICP analysis results; SEM images of Si-NWs; TEM images, EDX historgrams, and XPS spectra of Ni-B and Co-B decorated Si-NWs; TEM image and polarization curves of Si-NWs/Pt; EDX historgrams, SEM and TEM images of Si-NWs/Ni-B and Si-NWs/Co-B electrodes after photoelectrolysis; current efficiency plots for calculation of Faradaic efficiency. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21373040, 21673028, 21120102036 and 21361130020), the Basic Research Program of China (No. 2014CB239402), the PhD Program Foundation of the Ministry of Education of China (No. 20130041110024), the Swedish Energy Agency, the Swedish Research Council, and the K & A Wallenberg Foundation.



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Earth-Abundant Materials Using Amorphous Silicon Photocathodes and Al/Ni Contacts as Hydrogen Evolutioncatalyst. Chem. Phys. Lett. 2015, 638, 25–30. (40) Urbain, F.; Smirnov, V.; Becker, J.-P.; Lambertz, A.; Yang, F.; Ziegler, J.; Kaiser, B.; Jaegermann, W.; Rau, U.; Finger, F. Multijunction Si Photocathodes with Tunable Photovoltages from 2.0 V to 2.8 V for Light Induced Water Splitting. Energy Environ. Sci. 2016, 9, 145–154. (41) Zhang, P.; Wang, M.; Yang, Y.; Yao, T.; Han, H.; Sun, L. Electroless Plated Ni–Bx Films as Highly Active Electrocatalysts for Hydrogen Production from Water Over a Wide pH Range. Nano Energy 2016, 19, 98–107. (42) Gupta, S.; Patel, N.; Miotello, A.; Kothari, D. C. Cobalt-Boride: An Efficient and Robust Electrocatalyst for Hydrogen Evolution Reaction. J. Power Sources 2015, 279, 620–625. (43) Huang, Z.; Geyer, N.; Werner, P.; Boor, J. de; Gosele, U. Metal-Assisted Chemical Etching of Silicon: A Review. Adv. Mater. 2011, 23, 285–308. (44) Li, H.; Dai, W.; Qiao, M. Preparation of the Ni-B Amorphous Alloys with Variable Boron Content and Its Correlation to the Hydrogenation Activity. Appl. Catal. A 2003, 238, 119–130. (45) Wang, W.-J.; Qiao, M.-H.; Li, H.-X.; Dai, W.-L.; Deng, J.-F. Study on the Deactivation of Amorphous NiB/SiO2 Catalyst During the Selective Hydrogenation of Cyclopentadiene to Cyclopentene. Appl. Catal. A 1998, 168, 151–157. (46) Ong, C. W.; Huang, H.; Zheng, B.; Kwok, R. W. M.; Hui, Y. Y.; Lau, W. M. X-ray Photoemission Spectroscopy of Nonmetallic Materials: Electronic Structures of Boron and BxOy. J. Appl. Phys. 2004, 95, 3527–3534. (47) Carenco, S.; Portehault, D.; Boissière, C.; Mèzailles, N.; Sanchez, C. Nanoscaled Metal Borides and Phosphides: Recent Developments and Perspectives. Chem. Rev. 2013, 113, 7981–8065. 27 ACS Paragon Plus Environment


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Nanowire-textured silicon photocathodes decorated with nanoparticulate metal borides were fabricated by quick and facile electroless plating. These photocathodes display the photovoltage onsets of 0.44–0.45 V vs. RHE and the short-circuit photocurrent density up to 19.5 mA cm−2 in neutral electrolyte under simulated 1 sun illumination. Impressively, the photopower conversion efficiencies of Si-NWs/Ni-B and Si-NWs/Co-B (2.45%–2.53%) are comparable to that of Si-NWs/Pt (2.46%) for photoelectrochemical H2 production under identical test conditions. 197x192mm (150 x 150 DPI)

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