High-performance silicon photoanode enhanced by gold

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16749 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Applied Materials & Interfaces

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⊥, 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 KEYWORDS: Oxygen evolution reaction (OER), silicon photoanode, local surface plasmon resonance (LSPR), gold nanoparticles, nickel nanoparticles

ABSTRACT: Ni catalyst is a low-cost catalyst for oxygen evolution reaction (OER) on silicon metal-insulator-semiconductor (MIS) 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 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 Au nanoparticles. A small onset potential of 1.03 V (overpotential, η0 = -0.20 V) and a current density of 18.80 mA/cm2 at 1.23 V versus RHE can be obtained.

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NiAu/TiO2/n-Si photoanode exhibits a high saturation current density of 35 mA/cm2, which is greater than most of the the-state-of-the-art silicon photoanodes. Introduction Photoelectrochemical water splitting produces hydrogen providing an attracting direction which can reduce the usage of fossil fuels and meet the demand of renewable energy.1-4 However, water splitting remains the challenges, 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 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 the efficient light absorption. Si is mainly integrated into the 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 high-performance OER. IrO2 and RuO2 are known as the best OER catalysts. However, low abundance and the excessive cost of Ir and Ru severely restrict their applications in large-scale.10-13 Numerous efforts have been devoted to exploring the non-precious metal alternatives. Nickel (Ni), an earth-abundant transition metal with a high work function of ~5.15eV, has attracted a lot of attention for photocatalysis due to its corrosion resistance and reasonable stability. Ni catalyst has been widely used to protect the surface of Si and serve as active material for water oxidation.14-17 However, Ni has low OER activity and is not stable on Si-based 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


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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 high-density 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 coherent collective oscillation in response to the external oscillating electric field is known as 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, 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 remains ~93% activity after 20 hours of stability test. Experimental Section 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 minutes. 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, TiO2 layers were deposited by atomic layer deposition (ALD) on the fresh etched Si substrates immediately at 300 °C with the Tetrakis (dimethylamino) titanium (TDMAT) as Ti precursor and water vapor as an oxygen source. The deposition rate in our study was ~0.8 Å/cycle, 25 cycles of ALD was performed, making the thickness of TiO2 up to 2 nm. Subsequently, the metal catalyst is deposited on top of


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TiO2/n-Si layers via electron beam (e-beam) evaporation. Finally, Ti/Au (20 nm/100 nm) films are deposited onto the backside of Si substrate by e-beam evaporation to form an Ohmic contact. In this work, four different catalysts include NiAu (2 nm/2 nm), Ni (2 nm), Au (2 nm), and Ni (4 nm) were fabricated to form metal/TiO2/n-Si photoanodes. Results and Discussion

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 microscope (HRTEM) image of the cross-section of NiAu/TiO2/n-Si photoanode. Figure 1c shows the Atomic force microscopy (AFM) image of 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 Supplementary information S1 and Figure S1. The


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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 nm and 2.79 nm, respectively.

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. 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 eV, 854.30 eV, and 856.08 eV are assigned to metallic Ni, NiOx and Ni(OH)2, respectively. To confirm the surface active center in OER process, XPS spectra after OER testing is collected as shown in Figure S2. The peak of metallic Ni vanished after OER testing, confirming that an entire surface Ni oxidation occurred during the OER process, while the peak at 855.8 eV assigned to Ni(OH)2 phase further indicate a transformation from NiOx to Ni(OH)2. Ni(OH)2 is confirmed to be the


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primary active centers on NiAu/TiO2/n-Si photoanode.15, 25 In Au 4f XPS spectra (Figure 2b), the peaks at 84.08 eV and 87.78 eV are corresponding to Au 4f7/2 and Au 4f5/2 orbitals, respectively. The binding energies are well defined related 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 (Supplementary 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 XPS indicates the relatively low level of overlap caused by 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 were 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 interspace among nanoparticles. When the Ni film was 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 were considered as nanospheres. Figure 3a shows the linear sweep voltammetry (LSV) curves of TiO2/n-Si photoanodes with different catalyst combinations including NiAu (2 nm/2 nm), Ni (2 nm), Au (2 nm), and Ni (4 nm) under 100 mW/cm2 simulated air mass (AM) 1.5 illumination as illustrated in Supplementary information S3. Ni (2 nm) catalyst shows an OER activity with the onset potential of 1.10 V (overpotential, η0 = -0.13 V) and photocurrent density of ~8.62 mA/cm2 at 1.23 V versus reversible hydrogen electrode (RHE), while Au (2 nm) catalyst has the ignorable OER activity. However, after depositing 2 nm Ni on the Au/TiO2/n-Si photoanode (NiAu/TiO2/n-Si photoanode), a sharp onset current density can be observed at a potential of


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~1.03 V versus RHE. The overpotential is reduced to ~-0.20 V, while 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 most of the Si photoanodes as summarized in Table S1. We also noticed that when the thickness of Ni catalyst is increased to 4 nm, 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.

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-visible 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 lnD and time. The PEC efficiencies for solar conversion to O2 (SCOE) with different catalysts are evaluated via LSV measurement through the equation, 32, 33


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SCOE (%) =

(1.23 − Vapp ) × J ph Pin

×100%

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(1)

where Vapp is the applied bias potential, Jph is the photocurrent density, Pin is the power density of illumination (100 mW/cm2). The PEC efficiency of NiAu (2 nm/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 (curve purple 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. Inset of Figure S4b shows an equivalent circuit model comprising a solution resistance (Rs) and the parallel combination of a constant phase element (CPE) and a charge transfer resistance (Rct). Rct can be described by 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 type photoanodes, which is consistent with the photoelectrochemical activity shown in Figure 3a. The electrochemical active surface area (ECSA) is another key factor for catalyst in OER process, as it is well known that the larger ECSA is responsible for the enhanced catalytic activity. The ECSA can be estimated from the cyclic voltammetry (CV) 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 Ni/TiO2/n-Si and NiAu/TiO2/n-Si photoanodes have the similar value of Cdl (1.16 mF/cm2 and 1.23 mF/cm2), which are much larger than that of Au/TiO2/n-Si


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photoanode (0.074 mF/cm2). The similar value of Cdl also indicates the similar number of active sites (Ni(OH)2) for Ni/TiO2/n-Si and NiAu/TiO2/n-Si photoanodes. However, NiAu/TiO2/n-Si photoanode has much better OER performance shown in Figure 3a, implying that the improved OER performance is not solely ascribed to 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 equation: IPCE (%) =

1240 × J ph 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 were collected under visible light with the wavelength range of 400 nm to 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, since the light absorption of photoanodes is major from the n-Si due to the ultra-thin 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 NiAu/TiO2/n-Si photoanode exhibits significant improvement in the range of 500 nm to 600 nm, which corresponds to the localized surface plasmon resonance (LSPR) band induced by Au nanoparticles. Figure 3d shows the UV-visible absorbance spectra of 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 (NIR) 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 Supplementary information S4 and Figure S6. The dimensions


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of nanoparticles for 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 IPCE at the visible light wavelength, implying the LSPR effect on NiAu catalyst. Hence, the improved OER performance of 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 photo-generated 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 electron-hole pairs among other catalysts. The charge recombination behavior can be quantitatively confirmed by a normalized parameter (D) using the equation37: D=

I t − I st I in − I st

( 3)

where It, Ist and Iin are the time-dependent, steady-state and initial photocurrent, respectively, as shown in Figure S7. The transient time constant (τ) is defined as the time when lnD = -1 in the normalized plots of lnD ~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 are estimated to be around 3.39 s, 0.38 s, and 0.15 s. Therefore, it


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can be concluded that NiAu catalyst can decrease the recombination of photo-generated electrons and holes at the interface and increase their lifetime.

Figure 4. The stability tests of NiAu/TiO2/n-Si photoanode in 1.0 M KOH electrolyte. Additionally, 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 shows the photocurrent density as a function of time (j-t) over a period of 20 hours while being held at a constant external potential to get the current density around 11 mA/cm2 in 1M KOH solution. A low power white-light LED is employed as irradiation source for stability measurement to avoid the thermal effects. The photoanode remains ~93% activity after 20 hours of stability test.


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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. Based on the distribution of Ni and Au nanoparticles, the spatial distribution of plasmonic field for NiAu/TiO2/n-Si photoanode is simulated by FDTD solutions to investigate the LSPR effect, as shown in Figure 5a. The monochromatic incident light (546 nm) is along the z-direction (Supplementary information S4 and Figure S6). From the simulation, the strongly localized fields are visualized between Au nanoparticles, implying the generation and separation of 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), while the hot electrons created by LSPR are injected into TiO2 and transferred to Si substrate by external positive voltage (II)23. In addition, photo-generated 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


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Ni nanoparticles, resulting in the formation of O2 (IV).38 This increased density of holes eventually boosts the OER performance of NiAu/TiO2/n-Si photoanode. Furthermore, LSPR field can facilitate the charge separation and help to reduce the recombination of minority carriers (holes) that it 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, reducing the diffusion distance of photo-generated carriers.39-41 The efficient separation of electron-hole pairs in n-Si benefits from short hole-transport distance that 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 of 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. 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 Ni/TiO2/n-Si and NiAu/TiO2/n-Si photoanodes is provided in Figure S8 in the supplementary information. Conclusions In conclusion, plasmonic Au nanoparticles enhanced Ni-based photocatalyst of NiAu composite with TiO2 layer on Si substrate as a photoanode for OER is fabricated. With the LSPR effect introduced by Au nanoparticles, a small onset potential of 1.03 V 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 efficiency up to 0.778%. The photoanode remains ~93% activity after 20 hours of stability test.


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NiAu/TiO2/n-Si photoanode exhibits a high saturation current density of 35 mA/cm2, which is greater than most of the the-state-of-the-art silicon photoanodes. Finally, our demonstration provides a simple fabrication method to prepare Si-based photoanode using LSPR effect towards the efficient water splitting at low cost. ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge via the Internet at http://pubs.acs.org. 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/nSi photoanodes are presented in Supporting Information. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions W.T.H and Q.C contributed equally to this work. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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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). REFERENCES (1) Cai, Q.; Hong, W. T.; Jian, C. Y.; Li, J.; Liu, W. Impact of Silicon Resistivity on the Performance of Silicon Photoanode for Efficient Water Oxidation Reaction. ACS catal. 2017, 7, 3277-3283. (2) Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J.; Nocera, D. G. Wireless Solar Water Splitting using Silicon-based Semiconductors and Earth-Abundant Catalysts. Science 2011, 334, 645-648. (3) Cai, Q.; Hong, W. T.; Li, J.; Jian, C. Y.; Liu, W. Silicon Photoanode for Efficient Ethanol Oxidation under Alkaline Condition. RSC Adv. 2017, 7, 21809-21814. (4) Liu, R.; Zheng, Z.; Spurgeon, J.; Yang, X. Enhanced Photoelectrochemical Water-Splitting Performance of Semiconductors by Surface Passivation Layers. Energy. Environ. Sci. 2014, 7, 2504-2517. (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.


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High-performance silicon photoanode enhanced by gold

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