Enhancing the Performance of Si-Based Photocathodes for Solar

Subscriber access provided by AUT Library

Energy, Environmental, and Catalysis Applications

Enhancing the Performance of Si-Based Photocathodes for Solar Hydrogen Production in Alkaline Solution by Facilely Intercalating a Sandwich N-Doped Carbon Nanolayer to the Interface of Si and TiO 2

Xuran Sun, Jian Jiang, Yong Yang, Yu Shan, Lunlun Gong, and Mei Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03757 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Enhancing the Performance of Si-Based Photocathodes for Solar Hydrogen Production in Alkaline Solution by Facilely Intercalating a Sandwich N-Doped Carbon Nanolayer to the Interface of Si and TiO2 Xuran Sun, Jian Jiang, Yong Yang,# Yu Shan, Lunlun Gong, and Mei Wang* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China

ABSTRACT: Photoelectrochemical (PEC) water splitting is a promising but immensely challenging technology for sustainable production of hydrogen. To this end, highly active, durable, and inexpensive photocathodes that operate under the conditions compatible with those for photoanodes are desired. Herein, the Si-based composite photocathodes were constructed by coating the front surface of Si with an N-doped carbon nanolayer and then a TiO2 protective layer, followed by decorating the electrode surface with Ni and Ni–Mo catalysts. The carbon nanolayer, denoted as CPDA, was formed directly on the Si surface by in situ self-polymerization of dopamine and followed by carbonization of the polydopamine (PDA) coating. The performance of as-fabricated Si photocathodes with and without the CPDA layer were comparatively studied for PEC hydrogen evolution reaction (HER) in alkaline electrolyte, to evaluate the effect of the sandwich CPDA layer in between the Si substrate and the TiO2 layer on the photoelectrocatalytic behaviors of Si-based electrodes. The photocathodes containing the CPDA layer demonstrated lower electron transfer resistance, higher built-in photovoltage, and larger band bending relative to the analogous electrodes without the CPDA layer. Accordingly, the short-circuit photocurrents of Ni and Ni–Mo decorated photocathodes with the CPDA layer were enhanced by a factor of 2.8‒3.3, and their open-circuit photovoltages were enlarged by 0.14‒0.22 V, compared to those of corresponding electrodes without the CPDA layer in 1 M KOH under simulated 1 sun illumination. Moreover, the photocathodes with the CPDA layer also exhibited ________________________ # Present address: Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, China ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

improved stability for PEC HER in alkaline solutions, with faradaic efficiency of 90%‒97% in the initial hour.

KEYWORDS: carbon coating, hydrogen production, photocathode, photoelectrocatalysis, silicon, water reduction

INTRODUCTION The rapid depletion of limited fossil fuels and the excess emission of carbon dioxide have attracted growing concerns, and thus extensive efforts have been devoted to the development of clean and renewable energy in recent years. One of the promising approaches to this end is to convert solar energy to hydrogen by water splitting. For practical solar-hydrogen technologies toward large-scale application, many factors have to be considered, such as the solar-to-hydrogen (STH) conversion efficiency and durability of systems and devices, the cost and reserve abundance of materials used, the continuity and security of operations, and the scalability and environmental impact of hydrogen production processes. Taken all of these factors into account, photoelectrochemical (PEC) water splitting is an especially promising but immensely challenging technology for sustainable production of hydrogen.1‒3 For the assembly of energy-efficient and cost-effective PEC cells, highly active, durable, and inexpensive photoanodes and photocathodes for each half reaction of water splitting should be individually developed in advance. Silicon-based photocathodes have been extensively studied in the past decade because of the narrow band gap (1.12 eV) and high theoretical maximum photocurrent density (44 mA cm−2) of Si, as well as its low cost, earth abundance, and environmental benign property compared to Ga-, In-, Ge-, As-, Cd-, and Pb-containing semiconductor materials.4,5 However, some fatal shortcomings of pristine Si, such as the sluggish charge-transfer kinetics at Si/electrolyte interface and the poor stability of Si in an aqueous electrolyte, have thwarted the practical 2 ACS Paragon Plus Environment

Page 2 of 28

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


application of Si-based photoelectrodes in PEC water splitting. In recent years, many Si-based composite photocathodes have been reported by directly decorating the Si surface with different earth-abundant electrocatalysts,6‒9 which can improve the kinetics of photogenerated electron transport and enhance the rate of hydrogen evolution reaction (HER) at the electrode surface. Most of the reported Si-based photocathodes have been operated in acidic solutions. In order to eventually build efficient PEC cell for total water splitting, one of the important issues that has to be considered is that the conditions for oxygen evolution reaction (OER) at photoanodes and for HER at photocathodes must be mutually compatible. Unfortunately, the-state-of-the-art earth-abundant metal-based OER electrocatalysts display high activity and good stability only in basic solutions. To seek a solution to the incompatibility of operating conditions for Si-based photocathodes and for earth-abundant OER catalyst-decorated photoanodes, some efforts have been made to protect Si-based electrodes from corrosion in basic aqueous solutions by using a covering layer.10‒13 However, the stability of most TiO2-protected Si photocathodes is still shorter than 1 h in strong basic solutions.14−16 Very recently, a Si photocathode protected by crystalline TiO2 with graded oxygen defects was demonstrated to be stable during 100 h of photoelectrolysis in 1 M NaOH.17 The development of Si-based photocathodes that are highly active and stable in alkaline solutions still remains as a challenging task. Carbon materials have recently received more attention in the domain of developing Si-based photoelectrodes, because of their good chemical stability, high electrical conductivity, and nontoxic property.18‒23 It was reported that carbon materials decorated on Si photoelectrodes could act not only as corrosion resistant catalysts for HER or OER but also as an effective surface passivation layer of Si electrodes. In another aspect, the carbon material formed by carbonization of polydopamine (PDA), hereafter denoted as CPDA, has been found to be highly electroconductive and has been applied in lithium ion battery anodes.24,25 Moreover, the CPDA 3 ACS Paragon Plus Environment


layer has been demonstrated to be capable of enhancing the charge carrier transfer of graphitic carbon nitride (g-C3N4),26 and of improving the stability of the FeP electrode for photo- and electrocatalytic HER.27 Inspired by these results, we fabricated a series of Si-based photocathodes coated with a CPDA nanolayer. The performances of as-fabricated Si-based photocathodes, i.e., Si/CPDA, Si/CPDA/TiO2, Si/CPDA/TiO2/Ni, and Si/CPDA/TiO2/Ni–Mo, as well as their reference electrodes, Si, Si/TiO2, Si/TiO2/Ni, and Si/TiO2/Ni–Mo, were comparatively studied for PEC hydrogen production in alkaline electrolytes. The results obtained from PEC tests revealed that the sandwich N-doped carbon layer facilitated the interface electron transfer from Si to the electrolyte and also displayed an effect on the surface passivation of Si. As such, the earth-abundant composite photocathodes, Si/CPDA/TiO2/Ni and Si/CPDA/TiO2/Ni–Mo, exhibited an evidently enhanced performance compared to their analogous electrodes without the CPDA layer, in terms of open-circuit photovoltage (Voc), short-circuit photocurrent density (Jsc), and stability for PEC hydrogen production in alkaline electrolytes.

EXPERIMENTAL SECTION Materials. Micropyramidally textured and planar p-type silicon wafers with 10‒20 Ω·cm resistivity were purchased from Hangzhou Bojing Science and Technology Limited Company. Tetrakis(dimethylamino)titanium(IV) (TDMAT) was purchased from J&K Scientific Limited Company, Na2MoO4 from Tianjin Guangfu Technology Development Limited Company, and H2O2 from Tianjin Bodi Chemical Reagent Limited Company. Chemicals HF, dopamine (DA) hydrochloride, tris(hydroxymethyl)aminomethane (THAM), and Ni(NH2SO3)2·4H2O were purchased from Aladdin Industrial Corporation, and other compounds NaOH, KOH, and NH3·H2O from Sinopharm Chemical Reagent Limited Company. Ga-In eutectic alloy was purchased from Aldrich, Ag conductive adhesive from SPI supplies (PA, USA), and epoxy resin from Loctite Corporation (Loctite 9462). All commercially available chemicals were used 4 ACS Paragon Plus Environment

Page 4 of 28

Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


directly without further purification. The water used for electrode fabrication and (photo)electrochemical measurements was deionized with a Millipore AFS-E system (18.2 MΩ·cm resistivity). Instruments. Scanning electron microscopy (SEM) images were taken on a Regulus SU8220 instrument with an acceleration voltage of 5 kV, and SEM energy-dispersive X-ray (EDX) spectra were obtained at an acceleration voltage of 20 kV. Transmission electron microscopy (TEM) images were taken on an FEI Tecnai G2 F30 S-TWIN at an acceleration voltage of 300 kV. X-Ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo VG ESCALAB250 surface analysis system using a monochromatized Al Kα small-spot source and a 500 μm concentric hemispherical energy analyzer. Raman spectra were recorded on a Thermo Scientific DXR Raman spectrometer with 514 nm excitation wavelength and 1.0 mW laser power level. The loading amounts of Ni and Mo on the Si surface were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, Perkin Elmer 2000 DV). The thicknesses of carbon and TiO2 layers on planar Si wafers were measured by atomic force microscope (AFM) on a Dimension ICON instrument. Fabrication of Si/CPDA. A freshly HF-treated Si wafer with its backside covered by an adhesive tape was immersed in 30 mL of THAM (0.12 g, 0.01 M) buffer solution containing DA hydrochloride (1.48 g, 0.26 M). The mixture was gently stirred at 30 °C for 80 min. During this period, a uniform PDA coating was formed on the front surface of the Si wafer by in situ self-polymerization of DA.28 The PDA-coated Si electrode was washed with distilled water and dried at room temperature for about 2 h. After the adhesive tape was removed from the backside of the Si wafer, the electrode was put in a tube furnace and annealed at 500 °C for 2 h under Ar at an initial heating rate of 5 °C min−1. Under such conditions, the PDA coating was carbonized to form an N-doped carbon nanolayer on the front surface of the Si electrode.27 The thickness of the carbon layer was tuned by adjusting the DA in-situ polymerization time (60, 80, and 90 min) to 5 ACS Paragon Plus Environment


control the PDA-coating amount on the Si front surface. The CPDA-coated Si wafer was immersed in 5% HF for about 2 min after annealed, and immediately used for further treatments. Fabrication of Si/CPDA/TiO2. The TiO2 protective layer was deposited in a layer-by-layer fashion onto the front surface of the Si/CPDA electrode by using an atomic layer deposition (ALD) system. The deposition was performed by using successive pulses of TDMAT and deionized water as the precursors under a nitrogen carrier gas in a vacuum chamber. During the ALD process, the cycle of precursor pulses was operated with 400 ms for TDMAT followed by a 20 s N2 purge and 20 ms for water followed by a 30 s N2 purge. The deposition temperature was set at 120 °C. The film thickness could be tailed by controlling the number of ALD cycles, with a growth rate of about 1 Å per cycle. The Si/TiO2 reference electrode was fabricated by an essentially identical procedure. Fabrication of Si/CPDA/TiO2/Ni and Si/CPDA/TiO2/Ni–Mo. The Ni and Ni–Mo catalysts were electrodeposited on the surface of Si/CPDA/TiO2 from the H3BO3 electrolyte containing Ni(NH2SO3)2·4H2O or Ni(NH2SO3)2·4H2O and Na2MoO4, according to the previously reported methods.29 The metal catalysts were deposited at a constant current density of 20 mA cm−2 over 5 to 20 s for Ni and 10 to 20 s for Ni–Mo, to control the loading amount of catalysts. The corresponding reference electrodes, Si/TiO2/Ni and Si/TiO2/Ni–Mo, were fabricated by identical procedures. Formation of a Back Contact for Si Photocathodes. The backsides of all electrodes were first painted with a Ga–In eutectic alloy as an ohmic contact layer, and then the copper wire was connected to the Ga–In eutectic layer by using an Ag conductive adhesive. After drying in air, the epoxy resin was used on both sides of the electrode to insulate and protect the back contact of the electrode, except for the intended illumination area (geometric surface area ~ 1 cm2) of the Si front side. PEC Measurements. PEC experiments were carried out in a homemade three-electrode cell 6 ACS Paragon Plus Environment

Page 6 of 28

Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


using an electrochemical workstation (CHI 660 E) with the as-fabricated Si/CPDA, Si/CPDA/TiO2, Si/CPDA/TiO2/Ni, and Si/CPDA/TiO2/Ni–Mo as working electrodes, a Hg/HgO (1 M KOH) as the reference electrode, and a Pt foil (1 cm2) as the counter electrode. A KOH solution (1 M, pH 14) was used as electrolyte for all electro- and photoelectrochemical measurements. The experimentally measured potentials versus Hg/HgO were converted to the ones versus reversible hydrogen electrode (RHE) using the equation of ERHE = EHg/HgO + E0(Hg/HgO vs NHE) + 0.059pH, where E0(Hg/HgO vs NHE) is 0.098 V at 25 ℃. A xenon lamp equipped with an AM 1.5G filter was employed as the illumination source, and the light intensity was calibrated to 100 mW cm−2 prior to each experiment. Polarization curves of as-fabricated Si electrodes were swept linearly from positive to negative at a scan rate of 10 mV s−1. Each polarization curve and open-circuit voltage (OCV) shown in the main text and in the Supporting Information were repeated by at least three parallel measurements of individual samples. The stability of as-fabricated electrodes was measured at a bias of 0 or −0.1 V in a homemade gastight three-electrode cell under simulated 1 sun illumination. The amount of hydrogen evolved in the gaseous phase of the PEC cell was determined by using an AGILENT GC-7890A gas chromatograph equipped with a 5 Å molecular sieve column (2 mm × 2 m) with the external standard method, and the hydrogen dissolved in solution was neglected. Measurements of Electrochemical Impedances and Capacitances. Electrochemical impedance (EI) spectra and capacitances of as-prepared Si electrodes were measured on an electrochemical workstation (Zennium/IM6) as reported in our previous work.30 The EI spectra were performed at a bias of 0 V under illumination with a fixed light intensity of 100 mW cm−2, and the sweeping of frequency was from 100 kHz to 0.1 Hz with a 10 mV AC dither. Capacitance measurements were carried out as the potential was swept from 0.17 to −0.21 V for the bare Si electrode, 0.40 to −0.21 V for Si/TiO2, 0.35 to 0.05 V for Si/TiO2/Ni–Mo, 0.42 to 0.20 V for Si/TiO2/Ni, 0.47 to −0.10 V for Si/CPDA and Si/CPDA/TiO2, 0.56 to 0.35 V for Si/CPDA/TiO2/Ni, 7 ACS Paragon Plus Environment


and 0.56 to 0.18 V for Si/CPDA/TiO2/Ni–Mo without illumination at 1000 Hz with applied 10 mV sinusoidal perturbation. RESULTS AND DISCUSSION The photocathodes were constructed on micropyramidally textured Si wafers according to the procedures described in the Experimental Section (Figure S1). Compared to the commonly used drop-casting or spin-coating of a pre-synthesized carbon nanomaterial suspension on the surface of an electrode, the direct formation of a carbon layer on a photoelectrode by carbonization of uniformly coated PDA can provide a better contact interface between the carbon layer and the electrode surface. For comparison, the reference photocathodes, Si, Si/TiO2, Si/TiO2/Ni, and Si/TiO2/Ni–Mo without the CPDA layer, were also fabricated. Figures 1a and S2b clearly show that the Si surface is covered by a carbon layer. The TEM images (Figure 1e,f) of the carbon material exfoliated from the Si/CPDA electrode by sonification illustrate that the CPDA coating is constituted by multilayered carbon nanosheets, which cover the Si surface with uniformly distributed granule-shaped corrugations. The SEM images of Si/CPDA/TiO2 (Figures 1b and S2d) show the growth of the carbon nanosheet corrugations relative to those on the Si/CPDA surface due to the formation of an additional layer of TiO2. A comparison of the SEM images of Si/CPDA and Si/CPDA/TiO2 with those of the Si electrodes without the carbon layer (Figures 1a and S2b vs Figure S2a; Figures 1b and S2d vs Figure S2c) reveals that the insertion of a sandwich carbon layer between Si and TiO2 resultes in a rougher surface of the Si electrode. The deposited Ni catalyst is evenly distributed on the Si/CPDA/TiO2 and Si/TiO2 surfaces as nanoparticles with a size of 30 to 80 nm (Figures 1c and S2e,f). The SEM images of Si/CPDA/TiO2/Ni–Mo (Figures 1d and S2h) and Si/TiO2/Ni–Mo (Figure S2g) show that the surfaces of photocathodes are uniformly covered with the worm-shaped Ni–Mo catalyst. The EDX spectra (Figure S3) and elemental mappings (Figures S4 and S5) demonstrate the evenly distributed elements of Ni, Mo, Ti, O and C on the modified Si photocathodes. 8 ACS Paragon Plus Environment

Page 8 of 28

Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Aerial SEM images of (a) Si/CPDA, (b) Si/CPDA/TiO2, (c) Si/CPDA/TiO2/Ni, and (d) Si/CPDA/TiO2/Ni–Mo. (e,f) TEM images of the carbon material exfoliated from the Si/CPDA electrode.

To estimate the thicknesses of the carbon and TiO2 layers, planar Si (Sipl) wafers were used to fabricate Sipl/CPDA and Sipl/CPDA/TiO2 by similar protocols as described above. The AFM morphology images and the corresponding height profiles (Figure S6) suggest an untrathin layer of < 5 nm for the CPDA coating and a mean thickness of about 15 nm for the TiO2 layer. A comparison of UV–vis reflection spectra (Figure S7) of Si/CPDA and the bare Si indicates that the carbon layer has no effect on the visible light absorption of Si, and the slightly weakened reflectance of Si/CPDA in the UV region compared to that of the bare Si wafer is due to the absorption of the carbon layer. Covering the Si/CPDA surface with a TiO2 layer led to an apparent attenuation of the reflectance in the UV region, owing to the absorption of the TiO2 layer.

9 ACS Paragon Plus Environment


Page 10 of 28

Figure 2. High-resolution XP spectra of (a) C 1s, (b) N 1s, and (c) O 1s regions for the as-fabricated Si/CPDA electrode. (d) Raman scattering spectra of Si/CPDA, Si/CPDA/TiO2, and the bare Si electrode.

The chemical states of elements on the surfaces of Si/CPDA, Si/CPDA/TiO2/Ni, and Si/CPDA/TiO2/Ni–Mo were investigated by XPS. For Si/CPDA, the peaks with binding energies (BEs) of 284.1 and 284.6 eV in the C 1s region are originated from sp2 C−C and sp3 C−C structures, respectively (Figure 2a).20,25 The peaks with higher BEs of 285.5 and 286.0 eV correspond respectively to sp2 C−N and C−O structures,20 and the peak at 287.8 eV arises possibly from sp3 C−N structure. Correspondingly, the peaks at 399.9 and 400.7 eV in the N 1s region are assigned to pyrrolic and quaternary nitrogen (Figure 2b),18,25 indicative of the presence 10 ACS Paragon Plus Environment

Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of N-doping in the CPDA material. In the O 1s region (Figure 2c), the peak at 531.4 eV is assigned to sp3 C−O, and the small peak at 532.5 eV to silicon oxide, which manifests a slight oxidation of the Si surface for the as-fabricated Si/CPDA electrode. The XPS results indicate that the CPDA layer coated on the Si surface has a similar structure to that of the N-doped graphene.24 Moreover, the relative intensity ratio (ID/IG) of 0.82 for the D band at 1373 cm−1 and the G band at 1595 cm−1 in the Raman spectrum of Si/CPDA implies that the carbon layer is composed of defect-rich graphene (Figure 2d).31 The high-resolution XP spectrum of the Ni 2p region for Si/CPDA/TiO2/Ni (Figure S8) suggests that the deposited nickel nanoparticles are composed of both metallic Ni and Ni2+ species. For Si/CPDA/TiO2/Ni–Mo, the peaks at 228.0 and 231.2 eV BEs in the Mo 3d region (Figure S9a) correspond to 3d5/2 and 3d3/2 core levels of Mo0.32 The peaks with BEs of 852.6 and 870.0 eV in the Ni 2p region (Figure S9b), together with the satellite peaks at 858.2 and 876.2 eV, correspond to the Ni 2p3/2 and Ni 2p1/2 core levels of Ni0, while the small peaks at 855.2 and 872.2 eV BEs are attributed to the Ni2+ species. The loading amounts of Ni and Ni–Mo catalysts on the surfaces of Si/CPDA/TiO2 and Si/TiO2 were determined by ICP-OES analysis (Tables S1 and S2). It was found that the loading amount of catalyst, either Ni or Ni–Mo, was somewhat lower on the Si/CPDA/TiO2 electrode than on Si/TiO2 with the same deposition time. The PEC HER performances of Si/CPDA and Si/CPDA/TiO2 were studied and compared with those of the bare Si and Si/TiO2 without the carbon layer. Initially, the influence of the carbon layer thickness on the PEC behavior of Si electrodes was explored. Photocurrent density-potential (Jph‒V) curves showed that the photocurrent of Si/CPDA apparently increased compared with that of the bare Si and gradually went up when the PDA-coating time was extended from 60 to 80 min (Figure S10). Further extending the PDA-coating time to 90 min caused an apparent decrease in photocurrent. The Jph‒V curves for Si/CPDA/TiO2 photocathodes with different thicknesses of the TiO2 layer are shown in Figure S11. Higher photocurrent density was observed for the electrode with a TiO2 11 ACS Paragon Plus Environment


Page 12 of 28

layer of about 15 nm thickness as compared to that observed for the Si/CPDA/TiO2 electrode with a thinner (10 nm) or thicker (20 nm) TiO2 layer. In following studies, the CPDA layer was fabricated with a PDA-coating time of 80 min prior to carbonization, and the TiO2 layer of about 15 nm thickness was used as a protecting layer of the front outmost surface of Si electrodes. As shown in Figure 3a, the potential of 0.02 V required by the optimized Si/CPDA/TiO2 electrode to reach a photocurrent density of −0.1 mA cm−2 is positively shifted by 90 mV as compared to that for the Si/TiO2 electrode without the sandwich carbon layer. Accordingly, the photocurrent of Si/CPDA/TiO2 is increased to −2.92 mA cm−2 from −1.78 mA cm−2 for Si/TiO2 at −0.4 V under illumination. Although Si/CPDA displayed electrocatalytic activity when the applied potential was more negative than −0.5 V (Figure S12), the electrocatalytic activity of Si/CPDA/TiO2 was as low as that of Si/TiO2, indicating that the CPDA beneath the TiO2 layer could not exert its intrinsic catalytic activity, instead, it acted as a conductive layer in between Si and TiO2.


Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) J‒V curves, (b) Nyquist plots measured at a bias of 0 V under illumination (inset in b: Enlarged plots of high frequency region), (c) OCV measurements, and (d) Mott-Schottky plots under dark conditions for the optimized Si/CPDA and Si/CPDA/TiO2 photocathodes, as well as the Si and Si/TiO2 reference electrodes.

The influence of the CPDA-coating on the charge transfer resistance of Si-based electrodes was evaluated by electrochemical impedance spectroscopy (EIS) measurements at a bias of 0 V under illumination, and the impedance data were modelled using equivalent circuits as shown in Figure S13. The Nyquist impedance plots (Figure 3b) illustrate that the semicircle for Si/CPDA/TiO2 is significantly smaller than that for Si/TiO2. This evidence indicates that the CPDA layer does play a role of conducting layer to reduce the resistance for transport of photogenerated electrons from the Si electrode to the electrolyte. Besides, the lower impedance of Si/CPDA/TiO2 compared to 13 ACS Paragon Plus Environment


Page 14 of 28

that of Si/CPDA most likely results from the better hydrophilicity of TiO2 than that of CPDA, which improves the infiltration of electrolyte on the electrode surface and hence reduces the electron transfer resistance at the electrode/electrolyte interface. The changes in OCV in the dark and under illumination were comparatively studied for Si/CPDA, Si/CPDA/TiO2, and their reference electrodes, to explore the influence of the CPDA layer on the interfacial energetics of Si-based photocathodes at the solid/liquid interface (Figure 3c). Apparently, the decoration of the Si surface by the PDA-carbonized layer influenced both the Fermi level of the photocathode in the dark and the built-in photovoltage (Vph) of the electrode under illumination. The OCVs of 0.35 and 0.75 V were observed in the dark for the bare Si and the Si/CPDA photocathodes, respectively, indicating that the Fermi level pinning at the surface state of Si/CPDA has considerably moved down compared to that of the bare Si due to the change in the electronic state of the electrode front layer.15,19,20,33 The variation of potentials in OCVs in the dark and under illumination provides the Vph of a photocathode under open-circuit conditions. Figure S14 shows the changes in OCVs in the dark and under illumination for Si/CPDA electrodes with different thicknesses of the CPDA layer. The Vph value was increased with extending the PDA-coating time from 60 to 80 min, while further extending the duration of PDA coating to 90 min led to decrease of Vph. The Vph of 0.33 V obtained for the optimized Si/CPDA electrode was about 0.19 V lager than that for the bare Si electrode. Moreover, in the dark, the Fermi level of Si/CPDA/TiO2 moved down by 0.18 V compared to that of Si/TiO2, indicating the difference in the equilibrium surface potential at the outermost layers of Si/CPDA/TiO2 and Si/TiO2. The OCV of Si/CPDA/TiO2 changed from 0.76 V in the dark to 1.17 V under illumination, corresponding to a Vph of 0.41 V, which is apparently larger than that of Si/TiO2 (Vph = 0.28 V). Moreover, to understand the influence of the CPDA layer on the flat-band potential and band bending of the Si-based photocathodes, capacitance measurements of Si/CPDA and Si/CPDA/TiO2, as well as the bare Si and Si/TiO2 electrodes, were performed in 1 M KOH without illumination. 14 ACS Paragon Plus Environment

Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The corresponding Mott-Schottky plots are given in Figure 3d. The flat band potential (Vfb) of photoelectrodes can be estimated by extrapolation of the linear fitting to a capacitance of zero.19,34 Notably, the Vfb of Si/CPDA (0.33 V) is 0.20 V more positive than that of the bare Si electrode, and the Vfb of Si/CPDA/TiO2 (0.40 V) is 0.14 V more positive than that of Si/TiO2. These results are consistent with the OCV measurements. Although Si/CPDA and Si/CPDA/TiO2 exhibited large Vfb, their photocurrent onset potentials were much more negative than their Vfb values (Figure 3a), dominantly due to the low catalytic activity of the electrode surface. Therefore, the Si/CPDA/TiO2 electrode was further modified by Ni and Ni–Mo catalysts, and the PEC HER performances of Si/CPDA/TiO2/Ni and Si/CPDA/TiO2/Ni–Mo were studied in 1 M KOH electrolyte under illumination. First of all, the loading amounts of Ni and Ni–Mo catalysts were optimized respectively. Figure S15a shows that the photocurrent onset potential (Vphon, defined as the potential at a photocurrent density of −0.5 mA cm−2) of Si/CPDA/TiO2/Ni is positively shifted from 0.28 to 0.36 V with a slight decrease in photocurrent when the catalyst deposition time was extended from 5 to 15 s (Table S3). Further extending the deposition time to 20 s led to an apparent decrease in photocurrent, possibly due to the loss of incident light at the semiconductor surface caused by the excessive loading of catalyst as reported for Si/inorganic material catalyst composite photoelectrodes.29,35‒37 Among the Si/CPDA/TiO2/Ni–Mo electrodes prepared with varying depositing time of the Ni–Mo catalyst, the one fabricated with 15 s of Ni–Mo electrodeposition displayed the highest Jsc (−17.87 mA cm−2) and the most positive Vphon (0.35 V) (Figure S15b and Table S4). The control experiments showed that no photocurrent was observed with scanning up to −0.4 V for Si/CPDA/TiO2/Ni and Si/CPDA/TiO2/Ni–Mo without illumination. The photocatalytic performances of the optimized electrodes with and without the CPDA layer are compared in Figures 4a and S16. The potential of 0.13 V required by Si/CPDA/TiO2/Ni to reach the photocurrent density of 10 mA cm−2 is positively shifted by 0.22 V compared to the 15 ACS Paragon Plus Environment


Page 16 of 28

potential required by Si/TiO2/Ni. Accordingly, the half-cell photopower conversion efficiency (ηhc) achieved for Si/CPDA/TiO2/Ni (ηhc = 1.27%) is enhanced by an order of magnitude compared to that obtained for Si/TiO2/Ni (ηhc = 0.12%). As for Si/CPDA/TiO2/Ni–Mo, the potential of 0.08 V required to reach the photocurrent density of 10 mA cm−2 is shifted to the positive direction by 0.15 V compared to the potential required by Si/TiO2/Ni–Mo. The ηhc of 0.91% achieved for Si/CPDA/TiO2/Ni–Mo is 4.3 times higher than that obtained for the optimized Si/TiO2/Ni–Mo electrode. The performances of Si/CPDA/TiO2/Ni and Si/CPDA/TiO2/Ni–Mo with the Vphon of 0.35‒0.36 V, the Jsc of 15‒18 mA cm−2, and the saturation photocurrent (Jph) of 17‒36 mA cm−2 for PEC HER in 1 M KOH compare favorably with the previously reported Si/Ti/Ni photocathode, which displayed the Vphon of 0.3 V, the Jsc of 14‒18 mA cm−2, and the Jph of 25‒30 mA cm−2 in alkaline solution under illumination with a light intensity of 225 mW cm−2.10 Figure 4b shows that the semicircles for Si/CPDA/TiO2/Ni and Si/CPDA/TiO2/Ni–Mo are significantly smaller than that for Si/CPDA/TiO2 (Figure 3b), indicating that the decoration of the Si/CPDA/TiO2 surface with Ni and Ni–Mo catalysts dramatically reduces the charge transfer resistance at the semiconductor/electrolyte interface and boosts the kinetics of HER at the electrode surface. More importantly, the semicircles for Si/CPDA/TiO2/Ni and Si/CPDA/TiO2/Ni– Mo are also considerably smaller than those for Si/TiO2/Ni and Si/TiO2/Ni–Mo, indicating that the Si photocathodes with the sandwich carbon layer display a diminished charge transfer resistance compared to the analogous electrodes without the carbon layer. This evidence suggests that the CPDA coating in between the interface of Si and TiO2 could facilitate the transport of photogenerated electrons to the electrode surface to be used for HER. Similarly, it is reported that the direct interface between graphic carbon and TiO2 without an adhesive interlayer can facilitate electron-hole separation and charge transfer under photoexcitation.38


Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) J‒V curves, (b) Nyquist plots measured at a bias of 0 V under illumination, (c) OCV measurements, and (d) Mott-Schottky plots under dark conditions for the optimized Si/CPDA/TiO2/Ni and Si/CPDA/TiO2/Ni–Mo electrodes, as well as the Si/TiO2/Ni and Si/TiO2/Ni– Mo reference electrodes.

The OCVs of Si/CPDA/TiO2/Ni, Si/CPDA/TiO2/Ni–Mo, Si/TiO2/Ni, and Si/TiO2/Ni–Mo show that intercalating a CPDA layer to the interface of Si and TiO2 leads to apparently enlarged OCVs for Si-based composite photocathodes (Figure 4c). The Vfb values estimated from the Mott-Schottky plots (Figure 4d) are in accordance with the results of OCV measurements for these photocathodes. The Vfb values of Si/CPDA/TiO2/Ni (0.48 V) and Si/CPDA/TiO2/Ni–Mo (0.47 V) are much more positive than those of Si/TiO2/Ni (0.37 V) and Si/TiO2/Ni–Mo (0.28 V). According to the equation of Vb = Va – Vfb (Va is the applied potential, which is always negative 17 ACS Paragon Plus Environment


Page 18 of 28

for HER), the positive shift of Vfb should result in the enhanced band bending (Vb) for the CPDA-coated Si electrodes compared to those of the corresponding reference electrodes without the sandwich CPDA layer (Figure S17).13,33 Larger band bending in the depletion region of the semiconductor electrodes could promote faster charge separation of photogenerated electrons and holes, and suppress the charge recombination at the solid/liquid interface. Therefore, both the lower charge transfer resistance and the larger band bending contribute to the evidently enhanced PEC activity of the Si-based photocathodes with the CPDA layer relative to that of the analogous electrodes without the CPDA layer.

Figure 5. Comparison of long-term stability in 1 M KOH under illumination with (a) Si/CPDA vs the bare Si and (b) Si/CPDA/TiO2 vs Si/TiO2 at a bias of −0.1 V, and (c) Si/CPDA/TiO2/Ni vs Si/TiO2/Ni and (d) Si/CPDA/TiO2/Ni–Mo vs Si/TiO2/Ni–Mo at a bias of 0 V.


Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To explore the influence of the CPDA coating on the stability of Si photocathodes, chronoamperometric photoelectrolysis experiments were conducted at a bias of −0.1 V for Si/CPDA and Si/CPDA/TiO2, and at 0 V for Si/CPDA/TiO2/Ni and Si/CPDA/TiO2/Ni–Mo in 1 M KOH under illumination for 2 h. The results are shown in Figure 5, together with the Jph‒t curves obtained for the corresponding reference electrodes under the same conditions for comparison. The losses in photocurrent for Si/CPDA were about 4% over 1 h and 13% over 2 h (Figure 5a). By sharp contrast, the bare Si displayed the losses in photocurrent by 12% and 55% over 1 and 2 h, respectively. Likewise, the Si/CPDA/TiO2 electrode also exhibited a better stability than Si/TiO2, specifically, with only 2% loss in photocurrent for the former versus 8% loss for the latter over 2 h of photoelectrolysis (Figure 5b). These observations manifest that the CPDA coating acts not only as an electrically conductive layer to facilitate charge transfer, but also as an additional protecting layer to suppress corrosion of the Si surface in alkaline solution. The photocurrent of Si/CPDA/TiO2/Ni–Mo is almost maintained constant over the first hour of photoelectrolysis experiment. By contrast, the Si/TiO2/Ni–Mo reference electrode without the CPDA layer lost 31% of its initial photocurrent after 1 h of photoelectrolysis (Figure 5c). The 17% loss of photoelectrocatalytic activity for Si/CPDA/TiO2/Ni–Mo over 2 h of illumination is much smaller than that (45%) for Si/TiO2/Ni–Mo. Similarly, Si/CPDA/TiO2/Ni also exhibited an apparently improved stability compared to that of Si/TiO2/Ni (Figure 5d). The hydrogen concentration in the gaseous phase of the gas-tight PEC cell was determined by gas chromatography after 1 h of PEC HER. The amount of H2 produced is close to that calculated according to the number of electrons passed through the circuit during 1 h of the photoelectrolysis experiment. The faradaic efficiencies are 90% for Si/CPDA/TiO2/Ni and 97% for Si/CPDA/TiO2/Ni–Mo (Figure S18). Because Ni and Ni–Mo have been widely used as highly active and stable catalysts for electrolysis and photoelectrolysis of water in strong alkaline electrolytes,29,39 the decay of activity 19 ACS Paragon Plus Environment


Page 20 of 28

for these non-noble metal catalyst-modified Si photocathodes is mainly caused by the dropping of catalyst from the electrode surface to the electrolyte, which was evidenced by ICP-OES analysis (Table S5), and also caused by corrosion of the Si surface during long-term photoelectrolysis experiments in alkaline solutions. The stabilities of Si/CPDA/TiO2/Ni–Mo and Si/CPDA/TiO2/Ni are comparable to that of the a-Si/AZO(20 nm)/TiO2(100 nm)/Mo2C (AZO = aluminum-doped zinc oxide) photocathode, which displayed a stable photocurrent for PEC HER in 1 M KOH for 1 h before an apparent decrease of photocurrent.16 In comparison, the reported a-SiC/TiO2(25

nm)/Ni/Ni–Mo

photocathode showed a stable photocurrent within 40 min for photoelectrolysis of water in 1 M KOH at 0 V applied bias,15 and the Pt- and Ni-modified multijunction Si photocathodes displayed a stable photocurrent only for a period shorter than 150 s, followed by a sharp decrease of photocurrent.14

CONCLUSIONS In summary, the front surface of a pyramid-textured Si wafer was coated with an N-doped carbon nanolayer by a straightforward method, i.e., carbonization of an in-situ formed PDA coating on Si. Taking this one step further, the Si/CPDA/TiO2/Ni and Si/CPDA/TiO2/Ni–Mo photocathodes were fabricated by ALD of TiO2 and then electrodeposition of Ni or Ni–Mo catalyst. Comparative studies on the performances of analogous Si-based photocathodes with and without the CPDA coating revealed that the sandwich CPDA layer acted as an electroconductive layer to reduce the charge transfer resistance of phtotocathodes and hence to promote the forward transfer of photoexcited electrons. Besides, the CPDA layer also resulted in higher built-in photovoltage and larger band bending of Si electrodes relative to that of the corresponding reference electrodes without the CPDA layer. As such, Si/CPDA/TiO2/Ni and Si/CPDA/TiO2/Ni–Mo displayed evidently enhanced photocurrents and positively shifted onset potentials compared to Si/TiO2/Ni and Si/TiO2/Ni–Mo for PEC HER in alkaline electrolyte. Moreover, the sandwich carbon layer also 20 ACS Paragon Plus Environment

Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


played a role of the additional protecting layer for the Si electrode to improve the stability of Si-based

photocathodes

under

PEC

testing

conditions.

The

Si/CPDA/TiO2/Ni

and

Si/CPDA/TiO2/Ni–Mo photocathodes are more stable than their analogous electrodes without the CPDA layer for PEC HER in strong alkaline solution. This part of work is a proof of concept to show that intercalating of the CPDA layer into the interface of Si and TiO2 by on-site carbonization of an in situ formed PDA coating is an effective and facile approach to enhance the PEC activity and stability of Si-based photocathodes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxxxx. Additional SEM images of as-fabricated Si electrodes; EDX spectra and elemental maps; AFM images; UV–vis reflectance spectra; XPS spectra; LSVs and OCVs of Si/CPDA; additional polarization curves of Si/CPDA/TiO2, Si/CPDA/TiO2/Ni, and Si/CPDA/TiO2/Ni–Mo; tables for summarization of ICP-OES analysis results and important data for PEC HER with Si/CPDA/TiO2/Ni, Si/CPDA/TiO2/Ni–Mo, and their reference photocathodes as working electrodes; current efficiency plots for calculation of faradaic efficiency (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M.W.). ORCID Mei Wang: 0000-0002-5531-5056 Notes 21 ACS Paragon Plus Environment


Page 22 of 28

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We would like to thank the National Natural Science Foundation of China (Nos. 21673028 and 21373040) and the Basic Research Program of China (No. 2014CB239402) for financial support of this work.

REFERENCES (1) Ager, J. W.; Shaner, M. R.; Walczak, K. A.; Sharp, I. D.; Ardo, S. Experimental Demonstrations of Spontaneous, Solar-Driven Photoelectrochemical Water Splitting. Energy Environ. Sci. 2015, 8, 2811−2824. (2) Peerakiatkhajohn, P.; Yun, J.-H.; Wang, S.; Wang, L. Review of Recent Progress in Unassisted

Photoelectrochemical

Water

Splitting:

from

Material

Modification

to

Configuration Design. J. Photon. Energy 2016, 7, 012006−012027. (3) Kaneko, H.; Minegishi, T.; Domen, K. Recent Progress in the Surface Modification of Photoelectrodes toward Efficient and Stable Overall Water Splitting. Chem. Eur. J. 2018, 24, 5697−5706. (4) 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. (5) Thalluri, S. M.; Borme, J.; Xiong, D.; Xu, J.; Li, W.; Amorim, I.; Alpuim, P.; Gaspar, J.; Fonseca,

H.;

Qiao,

L.;

Liu,

L.

Highly-Ordered

Silicon

Nanowire

Arrays

for

Photoelectrochemical Hydrogen Evolution: an Investigation on the Effect of Wire Diameter, Length and Inter-Wire Spacing. Sustainable Energy Fuels 2018, 2, 978−982. (6) Zhang, D.; Shi, J.; Zi, W.; Wang, P.; Liu, S. Recent Advances in Photoelectrochemical Applications of Silicon Materials for Solar-to-Chemicals Conversion. ChemSusChem 2017, 22 ACS Paragon Plus Environment

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10, 4324−4341. (7) Yi, S.-S.; Zhang, X.-B.; Wulan, B.-R.; Yan, J.-M.; Jiang, Q. Non-Noble Metals Applied to Solar Water Splitting. Energy Environ. Sci. 2018, 11, 3128−3156. (8) Luo, Z.; Wang, T.; Gong, J. Single-Crystal Silicon-Based Electrodes for Unbiased Solar Water Splitting: Current Status and Prospects. Chem. Soc. Rev. 2019, 48, 2158−2181. (9) Thalluri, S. M.; Borme, J.; Yu, K.; Xu, J.; Amorim, I.; Gaspar, J.; Qiao, L.; Ferreira, P.; Alpuim, P.; Liu, L. Conformal and Continuous Deposition of Bifunctional Cobalt Phosphide Layers on p-Silicon Nanowire Arrays for Improved Solar Hydrogen Evolution. Nano Res. 2018, 11, 4823−4835. (10)Feng, J.; Gong, M.; Kenney, M. J.; Wu, J. Z.; Zhang, B.; Li, Y.; Dai, H. Nickel-Coated Silicon Photocathode for Water Splitting in Alkaline Electrolytes. Nano Res. 2015, 8, 1577−1583. (11)Bae, D.; Shayestehaminzadeh, S.; Thorsteinsson, E. B.; Pedersen, T.; Hansen, O.; Seger, B.; Vesborg, P. C. K.; Ólafsson, S.; Chorkendorff, I. Protection of Si Photocathode Using TiO2 Deposited by High Power Impulse Magnetron Sputtering for H2 Evolution in Alkaline Media. Solar Energy Materials & Solar Cells 2016, 144, 758−765. (12)Moehl, T.; Suh, J.; Severy, L.; Wick-Joliat, R.; Tilley, S. D. Investigation of (Leaky) ALD TiO2 Protection Layers for Water-Splitting Photoelectrodes. ACS Appl. Mater. Interfaces 2017, 9, 43614−43622. (13)Bae, D.; Seger, B.; Vesborg, P. C. K.; Hansen, O.; Chorkendorff, I. Strategies for Stable Water Splitting via Protected Photoelectrodes. Chem. Soc. Rev. 2017, 46, 1933−1954. (14)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 to 2.8 V for Light Induced Water Splitting. Energy Environ. Sci. 2016, 9, 145−154. 23 ACS Paragon Plus Environment


Page 24 of 28

(15)Digdaya, I. A.; Rodriguez, P. P.; Ma, M.; Adhyaksa, G. W. P.; Garnett, E. C.; Smets, A. H. M.; Smith, W. A. Engineering the Kinetics and Interfacial Energetics of Ni/Ni−Mo Catalyzed Amorphous Silicon Carbide Photocathodes in Alkaline Media. J. Mater. Chem. A 2016, 4, 6842−6852. (16)Morales-Guio, C. G.; Thorwarth, K.; Niesen, B.; Liardet, L.; Patscheider, J.; Ballif, C.; Hu, X. Solar Hydrogen Production by Amorphous Silicon Photocathodes Coated with a Magnetron Sputter Deposited Mo2C Catalyst. J. Am. Chem. Soc. 2015, 137, 7035−7038. (17)Zheng, J.; Lyu, Y.; Wang, R.; Xie, C.; Zhou, H.; Jiang, S. P.; Wang, S. Crystalline TiO2 Protective Layer with Graded Oxygen Defects for Efficient and Stable Silicon Based Photocathode. Nat. Commun. 2018, 9, 3572−3581. (18)Sim, U.; Yang, T.; Moon, J.; An, J.; Hwang, J.; Seo, J.; Lee, J.; Kim, K. Y.; Lee, J.; Han, S.; Hong, B. H.; Nam, K. T. N-Doped Monolayer Graphene Catalyst on Silicon Photocathode for Hydrogen Production. Energy Environ. Sci. 2013, 6, 3658−3664. (19)Sim, U.; Moon, J.; An, J.; Kang, J. H.; Jerng, S. E.; Moon, J.; Cho, S.; Hong, B. H.; Nam, K. T. N-Doped Graphene Quantum Sheets on Silicon Nanowire Photocathodes for Hydrogen Production. Energy Environ. Sci. 2015, 8, 1329−1338. (20)Chen, D.; Dai, S.; Su, X.; Xin, Y.; Zou, S.; Wang, X.; Kang, Z.; Shen, M. N-Doped Nanodots/np+-Si Photocathodes for Efficient Photoelectrochemical Hydrogen Generation. Chem. Commun. 2015, 51, 15340−15343. (21)Meng, H.; Fan, K.; Low, J.; Yu, J. Electrochemically Reduced Graphene Oxide on Silicon Nanowire Arrays for Enhanced Photoelectrochemical Hydrogen Evolution. Dalton Trans. 2016, 45, 13717−13725. (22)Wang, X.; Peng, K.-Q.; Pan, X.-J.; Chen, X.; Yang, Y.; Li, L.; Meng, X.-M.; Zhang, W.-J.; Lee, S.-T. High-Performance Silicon Nanowire Array Photoelectrochemical Solar Cells Through Surface Passivation and Modification. Angew. Chem. Int. Ed. 2011, 50, 9861−9865. 24 ACS Paragon Plus Environment

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23)Qu, Y.; Li, F.; Zhang, P.; Zhao, L.; Liu, J.; Song, X.; Gao, L. Enhanced Photoelectrochemical Performance and Stability of Si Nanowire Photocathode with Deposition of Hematite and Carbon. Appl. Surface Science 2019, 471, 528−536. (24)Kong, J.; Yee, W. A.; Yang, L.; Wei, Y.; Phua, S. L.; Ong, H. G.; Ang, J. M.; Li, X.; Lu, X. Highly Electrically Conductive Layered Carbon Derived from Polydopamine and its Functions in SnO2-Based Lithium Ion Battery Anodes. Chem. Commun. 2012, 48, 10316−10318. (25)Li, H.; Shen, L.; Yin, K.; Ji, J.; Wang, J.; Wang, X.; Zhang, X. Facile Synthesis of N-Doped Carbon-Coated Li4Ti5O12 Microspheres Using Polydopamine as a Carbon Source for High Rate Lithium Ion Batteries. J. Mater. Chem. A 2013, 1, 7270−7276. (26)He, F.; Chen, G.; Yu, Y.; Zhou, Y.; Zheng, Y.; Hao, S. The Synthesis of Condensed C-PDA−g-C3N4 Composites with Superior Photocatalytic Performance. Chem. Commun. 2015, 51, 6824−6827. (27)Chung, D. Y.; Jun, S. W.; Yoon, G.; Kim, H.; Yoo, J. M.; Lee, K.-S.; Kim, T.; Shin, H.; Sinha, A. K.; Kwon, S. G.; Kang, K.; Hyeon, T.; Sung, Y.-E. Large-Scale Synthesis of Carbon-Shell-Coated FeP Nanoparticles for Robust Hydrogen Evolution Reaction Electrocatalyst. J. Am. Chem. Soc. 2017, 139, 6669−6674. (28)Nam, H. J.; Kim, B.; Ko, M. J.; Jin, M.; Kim, J. M.; Jung, D.-Y. A New Mussel-Inspired Polydopamine Sensitizer for Dye-Sensitized Solar Cells: Controlled Synthesis and Charge Transfer. Chem. Eur. J. 2012, 18, 14000−14007. (29)McKone, J. R.; Warren, E. L.; Bierman, M. J.; Boettcher, S. W.; Brunschwig, B. S.; Lewis, N. S.; Gray, H. B. Evaluation of Pt, Ni, and Ni−Mo Electrocatalysts for Hydrogen Evolution on Crystalline Si Electrodes. Energy Environ. Sci. 2011, 4, 3573−3583. (30)Yang, Y.; Wang, M.; Zhang, P.; Wang, W.; Han, H.; Sun, L. Evident Enhancement of Photoelectrochemical Hydrogen Production by Electroless Deposition of M−B (M = Ni, Co) 25 ACS Paragon Plus Environment


Page 26 of 28

Catalysts on Silicon Nanowire Arrays. ACS Appl. Mater. Interfaces 2016, 8, 30143−30151. (31)Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B 2000, 61, 14095−14107. (32)Chen, Y.-Y.; Zhang, Y.; Zhang, X.; Tang, T.; Luo, H.; Niu, S.; Dai, Z.-H.; Wan, L.-J.; Hu, J.-S. Self-Templated Fabrication of MoNi4/MoO3-x Nanorod Arrays with Dual Active Components for Highly Efficient Hydrogen Evolution. Adv. Mater. 2017, 29, 1703311. (33)Digdaya, I. A.; Han, L.; Buijs,T. W. F.; Zeman, M.; Dam, B.; Smets, A. H. M.; Smith, W. A. Extracting Large Photovoltages from a-SiC Photocathodes with an Amorphous TiO2 Front Surface Field Layer for Solar Hydrogen Evolution. Energy Environ. Sci. 2015, 8, 1585−1593. (34)Fan, R.; Mao, J.; Yin, Z.; Jie, J.; Dong, W.; Fang, L.; Zheng, F.; Shen, M. Efficient and Stable Silicon Photocathodes Coated with Vertically Standing Nano-MoS2 Films for Solar Hydrogen Production. ACS Appl. Mater. Interfaces 2017, 9, 6123−6129. (35)Huang, Z.; Chen, Z.; Chen, Z.; Lv, C.; Meng, H.; Zhang, C. Ni12P5 Nanoparticles as an Efficient Catalyst for Hydrogen Generation via Electrolysis and Photoelectrolysis. ACS Nano 2014, 8, 8121−8129. (36)Kwon, K. C.; Choi, S.; Hong, K.; Moon, C. W.; Shim, Y.; Kim, D. H.; Kim, T.; Sohn, W.; Jeon, J.; Lee, C.; Nam, K. T.; Han, S.; Kim, S. Y.; Jang, H. W. Wafer-Scale Transferable Molybdenum Disulfide Thin-Film Catalysts for Photoelectrochemical Hydrogen Production. Energy Environ. Sci. 2016, 9, 2240−2248. (37)Cabán-Acevedo, M.; Stone, M. L.; Schmidt, J. R.; Thomas, J. G.; Ding, Q.; Chang, H.-C.; Tsai, M.-L.; He, J.-H.; Jin, S. Efficient Hydrogen Evolution Catalysis Using Ternary Pyrite-Type Cobalt Phosphosulphide. Nat. Mater. 2015, 14, 1245−1251. (38)Lee, W. J.; Lee, J. M.; Kochuveedu, S. T.; Han, T. H.; Jeong, H. Y.; Park, M.; Yun, J. M.; Kwon, J.; No, K.; Kim, D. H.; Kim, S. O. Biomineralized N-Doped CNT/TiO2 Core/Shell Nanowires for Visible Light Photocatalysis. ACS Nano 2012, 6, 935−943. 26 ACS Paragon Plus Environment

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39)Brown, D. E.; Martmood, M. N.; Turner, A. K.; Hall, S. M.; Fogartv, P. O. Low Overvoltage Electrocatalysts for Hydroge Evolving Electrodes. Int. J. Hydrogen Energy 1982, 7, 405−410.



153x81mm (150 x 150 DPI)

ACS Paragon Plus Environment

Page 28 of 28

Enhancing the Performance of Si-Based Photocathodes for Solar

Recommend Documents