High-Performance Silicon Photoanode Enhanced by Gold

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6262−6268

High-Performance Silicon Photoanode Enhanced by Gold Nanoparticles for Efficient Water Oxidation Wenting Hong,†,‡,∥ Qian Cai,†,∥ Rongcheng Ban,§ Xu He,† Chuanyong Jian,† Jing Li,† Jing Li,§ and Wei Liu*,† †

CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Department of Physics/Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, Fujian 361005, China S Supporting Information *

ABSTRACT: Ni catalyst is a low-cost catalyst for oxygen evolution reaction (OER) on silicon metal−insulator−semiconductor photoanode. We found that Au nanoparticles incorporated with Ni nanoparticles can enhance the OER activity and stability of Ni nanoparticles due to the local surface plasmon resonance (LSPR) effect of the Au nanoparticles. The efficiency of NiAu/TiO2/n-Si photoanode can be boosted at least three times under the illumination (100 mW/cm2) by LSPR effect of the Au nanoparticles. A small onset potential of 1.03 V versus reversible hydrogen electrode (overpotential, η0 = −0.20 V) and a current density of 18.80 mA/cm2 at 1.23 V versus reversible hydrogen electrode can be obtained. The NiAu/TiO2/n-Si photoanode exhibits a high saturation current density of 35 mA/cm2, which is greater than that of most of the state-of-the-art silicon photoanodes. KEYWORDS: oxygen evolution reaction (OER), silicon photoanode, local surface plasmon resonance (LSPR), gold nanoparticles, nickel nanoparticles

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INTRODUCTION Photoelectrochemical water splitting produces hydrogen to provide an attractive direction that can reduce the usage of fossil fuels and meet the demand of renewable energy.1−4 However, water splitting remains a challenge, especially for oxygen evolution reaction (OER) due to the relatively complex 4-electron reaction mechanism.5,6 Semiconductors, such as silicon (Si), can absorb photons to generate electron and hole, which can be utilized to lower the OER overpotential. Among various semiconductors, Si is an attractive photoanode material for OER due to its small band gap of ∼1.12 eV that allows efficient light absorption. Si is mainly integrated into the metal−insulator−semiconductor (MIS) device using metal catalyst and a thin insulator layer to reduce photocorrosion, as well as enhance the performance of water splitting (Figure 1a).7−9 On the other hand, for Si-based MIS photoanode, the metal catalyst is critical for the high-performance OER. IrO2 and RuO2 are known as the best OER catalysts. However, low abundance and excessive cost of Ir and Ru severely restrict their applications in large scale.10−13 Numerous efforts have been devoted to exploring the nonprecious metal alternatives. Nickel (Ni), an earth-abundant transition metal with a high work function of ∼5.15 eV, has attracted a lot of attention for © 2018 American Chemical Society

photocatalysis due to its corrosion resistance and reasonable stability. Ni catalyst has been widely used to protect the surface of Si and serve as an active material for water oxidation.14−17 However, Ni has a low OER activity and is not stable on the Sibased photoanode under extreme pH conditions.18 Hence, it is desirable to enhance the activity and stability of Ni to boost the OER performance under high pH condition. Over the past several years, the composition of plasmonic Au nanoparticles and hematite, TiO2 semiconductors have been reported to facilitate the photocatalytic process for OER via the localized surface plasmon resonance (LSPR) effect generated by plasmonic Au.19,20 Under the irradiation of incident light, the conduction electron density of Au nanoparticles distributes unevenly by the external oscillating photoelectric field. A highdensity electron can form an electron cloud, which is accumulated on one side of the Au nanoparticle. The electric field comes from the Coulomb attraction between electron cloud and nuclei of Au that causes the collective oscillation of electrons.21−23 Such a coherent collective oscillation in Received: November 3, 2017 Accepted: January 31, 2018 Published: January 31, 2018 6262

DOI: 10.1021/acsami.7b16749 ACS Appl. Mater. Interfaces 2018, 10, 6262−6268

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of working principle of metal/TiO2/n-Si photoanode. (b) Schematic illustration of plasmon oscillation on plasmonic Au nanoparticle. (c) Atomic force microscopy (AFM) image of NiAu/TiO2/n-Si photoanode. (d) High-resolution transmission-electron microscopy (HRTEM) image of the cross-section of NiAu/TiO2/n-Si photoanode.

Figure 2. X-ray photoelectron spectroscopy (XPS) spectra of (a) Ni 2p, (b) Au 4f. (c) The Normalized elemental depth profile of Ni and Au. (d) The possible deposited process of NiAu catalyst on the surface of TiO2/n-Si substrate.

response to the external oscillating electric field is known as an LSPR phenomenon, as shown in Figure 1b. Au nanoparticles can increase visible light absorption in the near-surface region of the semiconductor and reduce the diffusion distance of the photogenerated holes to the electrolyte.24 In this work, to utilize the LSPR effect, the plasmonic Au nanoparticles are

employed to enhance the activity of Ni-on-Si substrate (NiAu/ TiO2/n-Si photoanode). By incorporating with plasmonic Au nanoparticles, the efficiency of the photoanode can be significantly improved by more than three times and the photoanode retains ∼93% activity after 20 h of stability test. 6263


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Figure 3. (a) Linear sweep voltammetry (LSV) curves of various metal/TiO2/n-Si photoanodes. (b) The extracted of Cdl from different electrodes. (c) Incident-photon-conversion efficiency (IPCE). (d) UV−vis absorbance spectra of NiAu/TiO2/n-Si photoanode. (e) Transient photocurrents of metal/TiO2/n-Si photoanodes at a constant bias of 1.23 V versus RHE under illumination. (f) The correlations between ln D and time.

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EXPERIMENTAL SECTION

process, the XPS spectra after OER testing is collected as shown in Figure S2. The peak of metallic Ni vanishes after OER testing, confirming that an entire surface Ni oxidation occurs during the OER process, whereas the peak at 855.8 eV assigned to Ni(OH)2 phase further indicates a transformation from NiOx to Ni(OH)2. Ni(OH)2 is confirmed to be the primary active center on the NiAu/TiO2/n-Si photoanode.15,25 In Au 4f XPS spectra (Figure 2b), the peaks at 84.08 and 87.78 eV correspond to Au 4f7/2 and Au 4f5/2 orbitals, respectively. The binding energies are well defined in relation to the Au0 chemical state.26,27 To further estimate the distribution of Au and Ni nanoparticles, the depth profile of Au 4f and Ni 2p signals are collected at various etch times (Supporting Information S2 and Figure S3a,b). The normalized element distribution of Ni and Au as a function of etching times is shown in Figure 2c. The distribution of Ni and Au particles extracted from the XPS indicates the relatively low level of overlap caused by the deposition sequence of Au and Ni. Therefore, based on the information of AFM, TEM and the distribution of Ni and Au, the stack geometry of NiAu catalyst can be proposed in Figure 2d. The Au (yellow) and Ni (green) films are deposited on the top of TiO2/n-Si substrate in the form of nanoparticles. However, Au (2 nm) is not sufficient to cover the TiO2 layer, leaving the interspace among nanoparticles. When the Ni film is deposited subsequently, Ni nanoparticles would embed on the interspace or overlap the Au nanoparticles, making the thickness of NiAu layer around ∼2.8 nm. For simplicity, the Au and Ni nanoparticles are considered as nanospheres. Figure 3a shows the linear sweep voltammetry (LSV) curves of TiO2/n-Si photoanodes with different catalyst combinations including NiAu (2/2 nm), Ni (2 nm), Au (2 nm), and Ni (4 nm) under 100 mW/cm2 simulated air mass 1.5 illumination, as illustrated in the Supporting Information S3. Ni (2 nm) catalyst shows an OER activity with the onset potential of 1.10 V vs reversible hydrogen electrode (RHE) (overpotential, η0 =

The Si photoanodes were fabricated with the P-doped (100) n-type silicon wafers with a resistivity of ∼0.5 Ω/cm to study water oxidation. The silicon wafers were cleaned in the solution of H2SO4/H2O2 (3:1) for 20 min. The cleaned silicon wafers were then immersed in diluted hydrofluoric acid (HF/H2O = 1:9) solution to completely remove the native oxide layer and subsequently washed with deionized water. Following these steps, the TiO2 layers were deposited by atomic layer deposition (ALD) on the freshly etched Si substrates immediately at 300 °C with the tetrakis(dimethylamino)titanium as a Ti precursor and water vapor as an oxygen source. The deposition rate in our study was ∼0.8 Å/cycle and 25 cycles of ALD was performed, making the thickness of TiO2 up to 2 nm. Subsequently, the metal catalyst was deposited on top of the TiO2/n-Si layers via electron beam (e-beam) evaporation. Finally, Ti/Au (20/100 nm) films were deposited onto the backside of Si substrate by e-beam evaporation to form an Ohmic contact. In this work, four different catalysts included NiAu (2/2 nm), Ni (2 nm), Au (2 nm), and Ni (4 nm) were fabricated to form metal/ TiO2/n-Si photoanodes.

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RESULTS AND DISCUSSION Figure 1c shows the atomic force microscopy (AFM) image of the NiAu/TiO2/n-Si photoanode. The deposited metal catalyst forms a film by nanoparticles with the thickness of ∼2.8 nm (measured by the white line in Figure 1c) in the Supporting Information S1 and Figure S1. The roughness of the surface is ∼0.7 nm. The interface of NiAu/TiO2/n-Si photoanode is studied by the cross-sectional high-resolution transmission electron microscopy (HRTEM) as shown in Figure 1d. There is no distinct SiO2 layer between TiO2 and Si substrate. The thickness of TiO2 and NiAu layer is confirmed to be 1.97 and 2.79 nm, respectively. The composition of NiAu/TiO2/n-Si photoanode is analyzed by X-ray photoelectron spectroscopy (XPS). In Ni 2p XPS spectra (Figure 2a), the binding energy of 852.67, 854.30, and 856.08 eV are assigned to metallic Ni, NiOx, and Ni(OH)2, respectively. To confirm the surface active center in the OER 6264


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ACS Applied Materials & Interfaces −0.13 V) and photocurrent density of ∼8.62 mA/cm2 at 1.23 V versus RHE, whereas Au (2 nm) catalyst has the ignorable OER activity. However, after depositing 2 nm Ni on the Au/TiO2/nSi photoanode (NiAu/TiO2/n-Si photoanode), a sharp onset current density can be observed at a potential of ∼1.03 V versus RHE. The overpotential is reduced to ∼−0.20 V, whereas the photocurrent density is enhanced to ∼18.80 mA/cm2 at 1.23 V versus RHE. To date, the saturated current density is 35 mA/ cm2, which is higher than that of most of the state-of-the-art Si MIS photoanode reported in literature.28−31 The performance of NiAu catalyst is higher than that of most of the Si photoanodes, as summarized in Table S1. We also notice that when the thickness of Ni catalyst is increased to 4 nm, the onset potential degenerates to 1.18 V versus RHE (overpotential, η0 = −0.05 V) and the current density is greatly decreased to 3.90 mA/cm2 at 1.23 V versus RHE. The PEC efficiencies for solar conversion to O2 (SCOE) with different catalysts are evaluated via LSV measurement through the equation32,33 SCOE (%) =

(1.23 − Vapp) × Jph Pin

× 100%

the surface metal catalysts but also attributed to the significant influence of Au nanoparticles. Incident-photon-conversion efficiency (IPCE) as a function of visible-light wavelength for the photoanodes is shown in Figure 3c. The IPCE is calculated by the equation IPCE (%) =

1240 × Jph Pmono × λ

× 100%

(2)

where Jph is the photocurrent density (mA/cm2), Pmono is the intensity of the incident monochromatic light (mW/cm2), and λ is the wavelength of the monochromatic light. The IPCE spectra are collected under visible light with the wavelength range of 400−800 nm, as presented in Figure 3c. It is observed that NiAu/TiO2/Si photoanodes are active over the entire visible-light wavelength range, which is in good agreement with the band gap of Si (1.12 eV). In addition, because the light absorption of photoanodes is major from the n-Si due to the ultrathin metal and TiO2 layers, the NiAu/TiO2/n-Si and Ni (4 nm)/TiO2/n-Si photoanodes present a similar spectral response. But it is worth noting that the IPCE value for the NiAu/TiO2/n-Si photoanode exhibits a significant improvement in the range of 500−600 nm, which corresponds to the localized surface plasmon resonance (LSPR) band induced by Au nanoparticles. Figure 3d shows the UV−vis absorbance spectra of the NiAu/TiO2/n-Si photoanode to investigate the effect of NiAu catalyst further. The red curve in Figure 3d is measured by a UV−vis (near-infrared) spectrophotometer (Lambda950). The black curve in Figure 3d is simulated by finite-difference-time-domain (FDTD) under illumination. The detailed FDTD calculation method is described in the Supporting Information S4 and Figure S6. The dimensions of nanoparticles for the FDTD calculation are extracted from the AFM image (Figure 1c). As shown in Figure 3d, the measured absorption peak around 546 nm is comparable to the simulated peak of 542 nm, which corresponds to the LSPR band induced by Au nanoparticles. The absorption peak is also consistent with the evident enhancement of the IPCE at the visible-light wavelength, implying the LSPR effect on NiAu catalyst. Hence, the improved OER performance of the NiAu/TiO2/n-Si photoanode can be attributed to the LSPR effect. Figure 3e presents the chopped light transient photocurrent response measurements of the metal/TiO2/n-Si photoanodes during repeated on−off illumination periods at 1.23 V versus RHE to investigate the photogenerated carrier recombination during the term of charge transportation. When the light is switched on, a positive transient photocurrent is observed, indicating the recombination of the accumulation of the holes in the Si space charge layer with the photogenerated electrons. When the light is turned off, a negative transient is observed, which is associated with the recombination of free electrons with the accumulated holes.36 It is apparent that NiAu catalyst exhibits a weakest spike transient photocurrent if the light is switched on/off, suggesting the excellent interface quality with the lowest recombination rate of the electron−hole pairs among other catalysts. The charge recombination behavior can be quantitatively confirmed by a normalized parameter (D) using the equation37

(1)

where Vapp is the applied bias potential, Jph is the photocurrent density, and Pin is the power density of illumination (100 mW/ cm2). The PEC efficiency of NiAu (2/2 nm) catalyst is ∼0.778%, which is more than three times larger than that of Ni (2 nm) catalyst (∼0.247%), indicating the strong effect of Au on the performance of NiAu/TiO2/n-Si photoanode. The onset potential measured for n-Si photoanodes under illumination is also compared with the heavily doped p+-Si photoanode in the dark (purple curve in Figure 3a). The photovoltage for photoanode with NiAu catalyst is calculated to be 518 mV (Figure S4a), which is close to the best single-junction Si photoanode.12 The interface behavior of the metal/TiO2/n-Si photoanodes is also measured by electrochemical impedance spectroscopy (EIS), as shown in Figure S4b. The inset in Figure S4b shows an equivalent circuit model comprising a solution resistance (Rs) and the parallel combination of a constant phase element and a charge transfer resistance (Rct). Rct can be described by a semicircle in the Nyquist plots and is inversely proportional to the conductivity of photoanode. The least semicircle Nyquist plot for NiAu catalyst implies the best electrical conductivity among the four types of photoanodes, which is consistent with the photoelectrochemical activity shown in Figure 3a. The electrochemical active surface area (ECSA) is another key factor for the catalyst in the OER process, as it is well known that a larger ECSA is responsible for the enhanced catalytic activity. The ECSA can be estimated from the cyclic voltammetry curves in 1 M KOH solution (Figure S5). The linear slope of capacitive current versus scan rate, equivalent to the twice of the electrochemical double-layer capacitance (Cdl), is used to explain the ECSA, as shown in Figure 3b.34,35 The Ni/TiO2/n-Si and NiAu/TiO2/n-Si photoanodes have the similar value of Cdl (1.16 and 1.23 mF/cm2), which are much larger than that of Au/TiO2/n-Si photoanode (0.074 mF/cm2). The similar value of Cdl also indicates the similar number of active sites (Ni(OH)2) for the Ni/TiO2/n-Si and NiAu/TiO2/ n-Si photoanodes. However, NiAu/TiO2/n-Si photoanode has a much better OER performance shown in Figure 3a, implying that the improved OER performance is not solely ascribed to

D=

It − Ist Iin − Ist

(3)

where It, Ist, and Iin are the time-dependent, steady-state, and initial photocurrent, respectively, as shown in Figure S7. The 6265


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strongly localized fields are visualized between Au nanoparticles, implying the generation and separation of the electron−hole pairs. The localized electric fields observed near the Au−Ni interface as well as the Au−TiO2 interface may suggest the charge transfer. Therefore, a photocatalytic process for the enhancement of O2 evolution mechanism is proposed in Figure 5b. Under illumination, the incident photons are absorbed by Au particles through their LSPR excitation (I), whereas the hot electrons created by LSPR are injected into TiO2 and transferred to the Si substrate by an external positive voltage (II).23 In addition, the photogenerated holes on Au particles serve as effective electron trappers to capture electrons from the connected Ni nanoparticles, thereby increasing the number of holes on Ni particles (III). Finally, the OH− is oxidized by the abundant holes on the Ni nanoparticles, resulting in the formation of O2 (IV).38 This increased density of holes eventually boosts the OER performance of the NiAu/ TiO2/n-Si photoanode. Furthermore, the LSPR field can facilitate the charge separation and help reduce the recombination of minority carriers (holes), which can cause most of the incident light to be absorbed in a thin layer (∼10 nm) under the surface. In our work, the light coupled near the TiO2/n-Si interface reduces the diffusion distance of photogenerated carriers.39−41 The efficient separation of electron−hole pairs in n-Si benefits from the short hole-transport distance, which can decrease the recombination during the transfer process from n-Si tunneling through TiO2 to the surface of the metal catalyst, leading to the large τ shown in Figure 3f. Thus, most of the photogenerated holes from Si substrate can diffuse to the surface of the NiAu catalyst to oxidize water. On the other hand, it is known that the difference in electron affinity and work function induces the band bending in the space-charge region for n-Si and the formation of Schottky barrier φs at the interface. The NiAu bimetallic may decrease the Schottky barrier φ′s by the introduction of Au with a lower work function of ∼5.1 eV, and further, improve the contact of TiO2 with Ni, thereby favoring the transfer of photogenerated holes. The corresponding energy band diagram for the Ni/TiO2/n-Si and NiAu/ TiO2/n-Si photoanodes is provided in Figure S8 in the Supporting Information.

transient time constant (τ) is defined as the time when ln D = −1 in the normalized plots of ln D ∼ t, as shown in Figure 3f. τ reflects the behavior of charge recombination and lifetime of the charge carriers. The τ of NiAu/TiO2/n-Si, Ni (2 nm)/ TiO2/n-Si, and Ni (4 nm)/TiO2/n-Si photoanodes is estimated to be around 3.39, 0.38, and 0.15 s, respectively. Therefore, it can be concluded that NiAu catalyst can decrease the recombination of photogenerated electrons and holes at the interface and increase their lifetime. Additionally, the stability test of NiAu/TiO2/n-Si photoanode is measured to study the stability of NiAu catalyst due to the highly corrosive and oxidative reaction conditions. Figure 4

Figure 4. Stability tests of NiAu/TiO2/n-Si photoanode in 1.0 M KOH electrolyte.

shows the photocurrent density as a function of time (j−t) over a period of 20 h while being held at a constant external potential to get the current density around 11 mA/cm2 in a 1 M KOH solution. A low-power white-light LED is employed as the irradiation source for stability measurement to avoid the thermal effects. The photoanode remains ∼93% activity after 20 h of stability test. Based on the distribution of Ni and Au nanoparticles, the spatial distribution of plasmonic field for the NiAu/TiO2/n-Si photoanode is simulated by the FDTD solutions to investigate the LSPR effect, as shown in Figure 5a. The monochromatic incident light (546 nm) is along the z-direction (Supporting Information S4 and Figure S6). From the simulation, the

Figure 5. (a) Spatial distribution of electric field in the y−z plane for NiAu/TiO2/n-Si structure. (b) Schematic of near-field enhancement mechanism for O2 evolution on NiAu/TiO2/n-Si photoanode. 6266


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(5) Oliver-Tolentino, M. A.; Vázquez-Samperio, J.; Manzo-Robledo, A.; González-Huerta, R. D.; Flores-Moreno, J. L.; Ramírez-Rosales, D.; Guzmán-Vargas, A. An Approach to Understanding the Electrocatalytic Activity Enhancement by Superexchange Interaction toward OER in Alkaline Media of Ni-Fe LDH. J. Phys. Chem. C 2014, 118, 22432−22438. (6) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel-iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: the Role of Intentional and Incidental Iron Incorporation. J. Am. Chem. Soc. 2014, 136, 6744−6753. (7) Sun, K.; Shen, S.; Liang, Y.; Burrows, P. E.; Mao, S. S.; Wang, D. Enabling Silicon for Solar-Fuel Production. Chem. Rev. 2014, 114, 8662−8719. (8) Scheuermann, A. G.; Lawrence, J. P.; Kemp, K. W.; Ito, T.; Walsh, A.; Chidsey, C. E.; Hurley, P. K.; McIntyre, P. C. Design Principles for Maximizing Photovoltage in Metal-Oxide-Protected Water-Splitting Photoanodes. Nat. Mater. 2016, 15, 99−105. (9) Scheuermann, A. G.; Lawrence, J. P.; Meng, A. C.; Tang, K.; Hendricks, O. L.; Chidsey, C. E.; McIntyre, P. C. Titanium Oxide Crystallization and Interface Defect Passivation for High Performance Insulator-Protected Schottky Junction MIS Photoanodes. ACS Appl. Mater. Interfaces 2016, 8, 14596−14603. (10) Ouattara, L.; Fierro, S.; Frey, O.; Koudelka, M.; Comninellis, C. Electrochemical Comparison of IrO2 Prepared by Anodic Oxidation of Pure Iridium and IrO2 Prepared by Thermal Decomposition of H2IrCl6 Precursor Solution. J. Appl. Electrochem. 2009, 39, 1361−1367. (11) Tsuji, E.; Imanishi, A.; Fukui, K.-I.; Nakato, Y. Electrocatalytic Activity of Amorphous RuO2 Electrode for Oxygen Evolution in an Aqueous Solution. Electrochim. Acta 2011, 56, 2009−2016. (12) Chen, Y. W.; Prange, J. D.; Duhnen, S.; Park, Y.; Gunji, M.; Chidsey, C. E.; McIntyre, P. C. Atomic Layer-Deposited Tunnel Oxide Stabilizes Silicon Photoanodes for Water Oxidation. Nat. Mater. 2011, 10, 539−544. (13) Scheuermann, A. G.; Prange, J. D.; Gunji, M.; Chidsey, C. E. D.; McIntyre, P. C. Effects of Catalyst Material and Atomic Layer Deposited TiO2 Oxide Thickness on the Water Oxidation Performance of Metal-Insulator-Silicon Anodes. Energy Environ. Sci. 2013, 6, 2487−2496. (14) Sun, K.; Park, N.; Sun, Z.; Zhou, J.; Wang, J.; Pang, X.; Shen, S.; Noh, S. Y.; Jing, Y.; Jin, S.; Yu, P. K. L.; Wang, D. Nickel Oxide Functionalized Silicon for Efficient Photo-Oxidation of Water. Energy Environ. Sci. 2012, 5, 7872−7877. (15) Sun, K.; Shen, S.; Cheung, J. S.; Pang, X.; Park, N.; Zhou, J.; Hu, Y.; Sun, Z.; Noh, S. Y.; Riley, C. T.; Yu, P. K.; Jin, S.; Wang, D. Si Photoanode Protected by a Metal Modified ITO Layer with Ultrathin NiOx for Solar Water Oxidation. Phys. Chem. Chem. Phys. 2014, 16, 4612−4625. (16) Kenney, M. J.; Gong, M.; Li, Y. G.; Wu, J. Z.; Feng, J.; Lanza, M.; Dai, H. J. High-performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation. Science 2013, 342, 836− 840. (17) Hu, S.; Shaner, M. R.; Beardslee, J. A.; Lichterman, M.; Brunschwig, B. S.; Lewis, N. S. Amorphous TiO2 Coatings Stabilize Si, GaAs, and GaP Photoanodes for Efficient Water Oxidation. Science 2014, 344, 1005−1009. (18) Jaksic, M. M. Hypo-Hyper-H-Electronic Interactive Nature of Interionic Synergism in Catalysis and Electrocatalysis for Hydrogen Reactions. Int. J. Hydrogen Energy 2001, 26, 559−578. (19) Wang, X.; Peng, K. Q.; Hu, Y.; Zhang, F. Q.; Hu, B.; Li, L.; Wang, M.; Meng, X. M.; Lee, S. T. Silicon/Hematite Core/Shell Nanowire Array Decorated with Gold Nanoparticles for Unbiased Solar Water Oxidation. Nano Lett. 2014, 14, 18−23. (20) Pu, Y. C.; Wang, G.; Chang, K. D.; Ling, Y.; Lin, Y. K.; Fitzmorris, B. C.; Lu, X.; Tong, Y.; Zhang, J. Z.; Hsu, Y. J.; Li, Y.; et al. Au nanostructure-decorated TiO2 Nanowires Exhibiting Photoactivity Across Entire UV-Visible Region for Photoelectrochemical Water Splitting. Nano Lett. 2013, 13, 3817−3823. (21) Seh, Z. W.; Liu, S.; Low, M.; Zhang, S. Y.; Liu, Z.; Mlayah, A.; Han, M. Y. Janus Au-TiO2 Photocatalysts with Strong Localization of

CONCLUSIONS In conclusion, plasmonic Au nanoparticles incorporated with Ni nanoparticles are deposited onto TiO2/Si substrate as a photoanode for OER. With the LSPR effect introduced by Au nanoparticles, a small onset potential of 1.03 V versus RHE and a current density of 18.80 mA/cm2 at 1.23 V versus RHE are obtained, enabling the NiAu/TiO2/n-Si photoanode at an efficiency of up to 0.778%. The photoanode retains ∼93% activity after 20 h of stability test. NiAu/TiO2/n-Si photoanode exhibits a high saturation current density of 35 mA/cm2, which is greater than most of the state-of-the-art silicon photoanodes. Finally, our demonstration provides a simple fabrication method to prepare Si-based photoanode using the LSPR effect toward an efficient water splitting at low cost.

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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.7b16749. Detailed descriptions of the height of NiAu catalyst from AFM, XPS spectra of Ni 2p spectra for NiAu/TiO2/n-Si photoanode after OER operation, depth profile of Au 4f and Ni 2p signals, photoelectrochemical testing, FDTD solutions method, the calculation of the transient dynamics for normalized parameter (D), and the energy band diagram for Ni/TiO2/n-Si and NiAu/TiO2/n-Si photoanodes are presented in the Supporting Information (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wei Liu: 0000-0001-9925-063X Author Contributions ∥

W.H. and Q.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, the National Natural Science Foundation of China (No. 61674152), the Natural Science Foundation of Fujian Province of China (No. 2017H6022, 2017J01130), and Science and Technology Program of Xiamen City of China (No. 3502Z20161223).

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REFERENCES

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High-Performance Silicon Photoanode Enhanced by Gold

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