Phosphorous-doped molybdenum disulfide anchored on silicon as an efficient catalyst for photoelectrochemical hydrogen generation

Journal Pre-proof Phosphorous-doped Molybdenum Disulfide Anchored on Silicon as an Efficient Catalyst for Photoelectrochemical Hydrogen Generation Chih-Jung Chen, Vediyappan Veeramani, Yi-Hsiu Wu, Anirudha Jena, Li-Chang Yin, Ho Chang, Shu-Fen Hu, Ru-Shi Liu

PII:

S0926-3373(19)31006-9

DOI:

https://doi.org/10.1016/j.apcatb.2019.118259

Reference:

APCATB 118259

To appear in:

Applied Catalysis B: Environmental

Received Date:

9 May 2019

Revised Date:

28 August 2019

Accepted Date:

3 October 2019

Please cite this article as: Chen C-Jung, Veeramani V, Wu Y-Hsiu, Jena A, Yin L-Chang, Chang H, Hu S-Fen, Liu R-Shi, Phosphorous-doped Molybdenum Disulfide Anchored on Silicon as an Efficient Catalyst for Photoelectrochemical Hydrogen Generation, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118259

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Phosphorous-doped Molybdenum Disulfide Anchored on Silicon as an Efficient Catalyst for Photoelectrochemical Hydrogen Generation.

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Chih-Jung Chena, Vediyappan Veeramania, Yi-Hsiu Wub, Anirudha Jenaa,c, Li-Chang Yin d,*, Ho Changc,*

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, Shu-Fen Hub,*, and Ru-Shi Liua,c,* Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

b

Department of Physics, National Taiwan Normal University, Taipei 11677, Taiwan

c

Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of

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Technology, Taipei 10608, Taiwan d

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Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang

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110016, China

Corresponding Authors:

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* E-mail: [email protected] (L. C. Yin) Tel: +86-24-83978238 * E-mail: [email protected] (H. Chang) Tel: +886-2-27712171 ext. 2063 * E-mail: [email protected] (S. F. Hu) Tel: +886-2-77346088 * E-mail: [email protected] (R. S. Liu) Tel: +886-2-33661169

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Graphical Abstract

Research Highlights

P-doped MoS2 was integrated on Si pyramids as photocathode for hydrogen evolution.

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The Efficiency was enhanced by exposing edges and activating basal planes of MoS2.

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MoS1.75P0.25/Si pyramids showed the optimal current density of −23.8 mA cm−2.

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MoS1.75P0.25/2 nm TiO2/Si electrode presented the current retention of 84.0%.

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ABSTRACT

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Herein, molybdenum disulfide (MoS2) integrated on Si pyramids was used as a co-catalyst to improve charge separation efficiency. Various quantities of phosphorus (P) heteroatoms were doped into MoS2 materials to boost catalytic performance. Raman and extended X-ray absorption fine structure spectra showed that the introduction of P dopants increased the number of exposed edges and sulfur vacancies that acted as hydrogen evolution reaction (HER) active sites on MoS2 and enhanced photoelectrochemical activity. Density functional theory calculations revealed that the HER inert basal plane of MoS 2 became

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catalytically active after P atoms doping. MoS1.75P0.25/Si pyramids presented the optimal onset potential

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of +0.29 V (vs. RHE) and current density −23.8 mA cm−2. A titanium dioxide (TiO2) layer was prepared through atomic layer deposition and served as a passivation layer that improved photocathode stability.

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The photocurrent retention of MoS1.75P0.25/10 nm TiO2/Si pyramids was 84.0% after 2 h of

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chronoamperometric measurement.

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KEYWORDS

Hydrogen Evolution Reaction, Molybdenum Disulfide, Phosphorous Atoms Doping, Exposed Mo-edges

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and S-vacancies, Activated Basal Plane

1. Introduction

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The average annual worldwide energy consumption rate was 15–17 TW in 2010[1,2] and is projected to reach 25–27 TW by 2050.[3] The heavy reliance on fossil fuels to meet energy requirements has resulted in carbon dioxide (CO2) emission rates of 30.4 Gt and contributes to climate change and global warming.[4] Therefore, sustainable and renewable energy sources must be developed to solve the problem of fossil fuel depletion. Hydrogen (H2) is an environmentally friendly energy source that can be easily stored with high energy density. Thus, it is a promising alternative for solving the energy crisis. At present, .

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however, 96% of H2 is produced from natural gas through steam methane reformation, which generates CO2 byproducts.[5] Consequently, photoelectrochemical (PEC) water splitting, a significant approach for converting intermittent solar energy into storable H2 carriers without greenhouse gas generation, is receiving increasing attention.[6] The efficiency and stability of electrode materials must be improved to enable the practical application of water photolysis through PEC methods.[7]

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Silicon (Si) is the most ideal candidate PEC light absorber because its conduction band edge of −0.46 V (vs. NHE) is more negative than the hydrogen evolution reaction (HER) potential. Moreover, it presents

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a band gap of 1.12 eV that is appropriate for absorbing a high fraction of incident solar energy (visible

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and near infrared illumination). The theoretical maximum current density of Si photocathodes reaches 44 mA cm−2 under AM1.5G solar irradiation.[8] Furthermore, Si materials can be adopted in the practical

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application of PEC given their ubiquity, low cost, and mature fabrication technologies. Nevertheless, pristine Si photocathodes have poor performances given that they have high Gibbs free energy for H 2

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adsorption as a result of the sluggish HER kinetics on their surfaces.[9] Thus, noble metals are widely applied as co-catalysts to improve the HER overpotential of Si photocathodes.[10-13] However, the

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scarcity of these precious metals restricts the large-scale development of Si photocathodes. Oxidation resulting from direct exposure to aqueous electrolytes is another challenge encountered in the use of Si photocathodes during PEC reaction. The generation of silicon dioxide (SiO2), an electrical insulator,

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contributes to photocatalytic degradation and even device failure.[10,14] Therefore, titanium oxide (TiO2) or aluminum oxide (Al2O3) materials are utilized as passivation layers to prevent the oxidation of the Si photoabsorber.[14-16]

In our previous studies, we assembled cobalt dichalcogenide derivatives (CoX2; X = S or Se) on Si photocathodes as co-catalyst materials for solar hydrogen production.[17,18] However, the work functions of CoX2 show limited negative energy levels (vs. vacuum level) with respect to the Fermi level of p-type . 4

Si (p-Si) material. Consequently, the Ohmic junction barrier generated between CoX2/Si interfaces after band alignment results in the low fill factors of PEC results.[17] In our subsequent work, we anchored amorphous phosphorous (P)-doped CoSx on Si photocathodes and obtained moderate fill factors.[19] We suggested that the chemical and electronic structures of CoSx catalysts were modified through heteroatom introduction. Thus, the suitable band alignment obtained by integrating HER electrocatalysts on light

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absorbers is crucial for efficiently driving PEC reactions. Molybdenum disulfide (MoS2) has been widely incorporated on Si photocathodes for PEC

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applications through molybdenum sulfidization,[5,20,21] ammonium tetramolybdate thermolysis,[22-25]

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sputtering,[26] chemical vapor deposition,[27] or atomic layer deposition (ALD)[28], because MoS2 showed the promising HER activity.[29,30] The MoS2 layer on p-Si induces a type-II heterojunction band

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alignment that improves charge separation efficiency.[23,28,31] MoS2 thin films consist of a twodimensional (2D)-layered structure wherein each layer is weakly bonded to adjacent layers through van

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der Waals interactions. Bulk MoS2 presents poor catalytic performance because its basal plane is HER inert.[32] The density functional theory (DFT) calculation results of previous works have suggested that

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the edge sites of MoS2 exhibit a near-optimal hydrogen adsorption free energy (GH*) of 0.08 eV with superior HER chemical reactivity compared with the basal plane (GH* = 2 eV).[33,34] Jaramillo and coworkers have experimentally elucidated that the HER activity of MoS2 electrocatalyst is proportional

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to exposed edge content.[35] Extensive effort has been devoted to developing engineering approaches for maximizing the amounts of edge sites in MoS2 structures.[36-38] Li and colleagues have demonstrated that sulfur (S) vacancies in MoS2 have a considerable effect on HER catalytic efficiency.[39] A previous study has

shown that repairing

S-vacancies in

MoS2

film

through immersion

in

(3-

mercaptopropyl)trimethoxysilane followed by annealing at 300°C dramatically decreases the electrocatalytic activity of the film.[40] This result indicates that the edge sites and S-vacancies of MoS2 .

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exert critical effects on HER performance.[41] Modifying the chemical and electronic states of MoS2 catalysts through doping is an important strategy for ameliorating inherent HER reactivity.[42] The HER inert basal plane of MoX2 (X = S or Se) becomes catalytically active through heteroatom doping.[43-45] The catalytic ability of MoS2 on Si photoabsorbers has been boosted through doping with various heteroatoms.[9,24,42,46,47]

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Herein, Si pyramids that were prepared through chemical wet etching were used as the photocathodes for solar hydrogen production. As shown in Fig. 1, Earth-abundant MoS2 co-catalyst was anchored on Si

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electrodes through drop casting and thermal combustion to enhance photoinduced carrier collection.

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Various quantities of P heteroatoms were doped into MoS2 to expose additional edges and S-vacancies as HER active sites. The basal plane of MoS2 was transformed into HER catalytically active sites through

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doping with P atoms. The optimal PEC performance was achieved by MoS1.75P0.25/Si pyramids, which presented the onset potential of +0.29 V (vs. RHE) and current density −23.8 mA cm−2 at 0 V (vs. RHE).

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Chronoamperometric stability was improved through the deposition of a protective TiO 2 thin film on the photocathode. The current density of MoS1.75P0.25/10 nm TiO2/Si pyramids after 2 h of durability

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measurement was maintained at 84.0% of the initial performance.

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Fig. 1. Schematic of MoS2-xPx/Si pyramids as photocathode materials for solar hydrogen evolution.

2. Experimental Section 2.1 Chemicals and materials.

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Boron-doped p-type silicon (Si) wafer (resistivity: 1–25 Ω cm) was purchased from Semiconductor

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Wafer. Isopropyl alcohol (C3H8O, 99.5%) was purchased from Echo Chemical. Potassium hydroxide (KOH, 85%) was purchased from Showa Chemical Industry. Ammonium tetrathiomolybdate

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[(NH4)2MoS4, 99.95%] and hexamethyldisilazane (HMDS, ≥ 98%) were purchased from Alfa Aesar. Sodium hypophosphite (NaH2PO2, 98-101%) was purchased from Sigma-Alorich. Methanol (CH3OH, ≥

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99.8%) was purchased from Honeywell. All chemicals were used as received and without further

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purification.

2.2 Characterization of materials.

The morphologies of materials were observed via field-emission scanning electron microscopy (JSM-6700F, JEOL). An X-ray diffraction analyzer (D2 PHASER, Bruker) with Cu Kα radiation (λ =

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1.54178 Å) was adopted to characterize the crystal phase and crystallinity of analytes. The oxidation states and components of samples were resolved through X-ray photoelectron spectroscopy (Theta Probe Spectrometer, Thermo Scientific) with Al Kα source (hν = 1486.68 eV). The Raman spectra of materials were collected by utilizing a Raman spectrometer (DXR Microscope, Thermo Scientific) equipped with a

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532 nm laser. All X-ray absorption spectroscopy measurements were performed at the 01C1 and 16A1 beamlines of the National Synchrotron Radiation Research Center (NSRRC), Hsinchu City, Taiwan.

2.3 Fabrication of Si pyramids.

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Boron-doped p-type Si wafer was polished to a thickness of 325 ± 5 μm. The polished Si wafer was cleaned in the wet bench via immersion in standard clean-1 (NH4OH:H2O2:H2O = 1:4:20) solution at 45°C

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for 10 min and buffer oxidation etch (NH4F:HF = 1:7) solution at room temperature for 60 s. The cleaned

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Si wafer was dried with a spin rinse dryer. An electron beam evaporator was used to deposit aluminum (Al) thin film on the back side of the Si wafer as a current collector. Subsequently, Si pyramids were

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prepared through wet chemical etching. Teflon tapes were used as masks and pasted on Si wafers to prevent exposing nontarget areas to the etchant. A beaker containing 190 mL of deionized (DI) water with

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potassium hydroxide (4.02 g) was heated on a hot plate at 90 °C for 15 min. Isopropyl alcohol (5 vol%) was subsequently added to the above aqueous solution as the etchant. The Si wafer was dipped into the

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heated solution for 30 min and washed with DI water. Finally, the as-prepared Si pyramids were dried under nitrogen gas flow.

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2.4 Fabrication of MoS2-xPx/Si photocathodes. MoS2 co-catalysts were decorated on Si pyramids through drop casting and thermal combustion.

(NH4)2MoS4 (0.5 mmol) was dispersed in a mixed solution of hexamethyldisilazane (HMDS; 20 μL) and methanol (10 mL) via ultrasonication for 15 min. The suspension (6 μL) was drop-casted on the surfaces of Si pyramids that had been immersed in hydrofluoric acid (~5 wt.%) for 30 s for the removal of native .

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oxides. The Si pyramids were subsequently placed at room temperature for solvent evaporation and then annealed at 400°C under N2/H2 (20%/80%) atmosphere for 2 h. The desired amounts of sodium hypophosphite (NaH2PO2) were applied as the precursor of phosphorous dopants for the synthesis of MoS2-xPx on Si pyramids. The precursor solution was prepared through the addition of (NH4)2MoS4 (0.5 mmol) and NaH2PO2 (0.0625, 0.125, 0.25, 0.5 mmol) into HMDS/methanol for synthesizing MoS2-xPx (x = 0.125, 0.25, 0.5, and 1). Finally, a copper wire was installed on the Al back electrode of the Si

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photocathode by using Ag silver paste. The electrode was then heated to dryness at 60°C in an oven.

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Epoxy glue was pasted on the nontarget exposed area of the photocathode to prevent dark current

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2.5 Fabrication of MoS2-xPx/TiO2/Si photocathodes.

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generation.

by

using

an

atomic

layer

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Titanium dioxide (TiO2) thin films were prepared as passivation layers on the surfaces of Si pyramids deposition

(ALD)

system

(Savannah

G2,

Ultratech)

with

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tetrakis(dimethylamino)titanium (TDMAT) and DI water as the precursors. Nitrogen gas served as the purging gas and was sparged at a flow rate of 20 sccm. The chamber was maintained at 120°C for the growth of TiO2 thin films. Each ALD cycle consisted of a 0.1 s pulse of TDMAT, 30 s of exposure to TDMAT, a 0.25 s pulse of DI water, and 30 s of exposure to DI water. Deposition cycles were repeated

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until the desired thickness of the TiO2 layer was obtained. MoS2-xPx materials were subsequently deposited on TiO2-modified Si pyramids, as illustrated in the section on the fabrication of MoS2-xPx/Si photocathodes.

2.6 Fabrication of MoS2-xPx/Ti electrodes. .

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Bare titanium (Ti) foils showed no hydrogen evolution reaction (HER) catalytic performance between the potential range of 0 and −0.4 V (vs. RHE) in linear sweep voltammogram (LSV) measurements.[48-50] Thus, Ti foils were applied as the substrates for decorating MoS2-xPx catalysts and evaluating the electrochemical HER activities. The method used to deposit MoS2-xPx materials on Ti foils was similar to that used to prepare MoS2-xPx/Si pyramids. Specific concentrations of (NH4)2MoS4 and NaH2PO2 were dispersed in HMDS/methanol as the precursor solution. The loading amount of the

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precursor suspension for drop casting on Ti substrate was increased to 60 μL. Finally, the precursor-

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decorated Ti foils were thermally combusted.

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2.7 Photoelectrochemical measurements.

A three-electrode cell was applied for the photoelectrochemical (PEC) characterizations of MoS2photocathodes, which served as the working electrodes. Silver/silver chloride electrode (Ag/AgCl;

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xPx/Si

SSCE) and platinum (Pt) plate functioned as the reference electrode and counter electrode, respectively.

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A xenon lamp equipped with an air mass 1.5 (AM 1.5) filter was employed as the solar simulator, and 0.5 M sulfuric acid aqueous solution (pH = 0.3) was used as the electrolyte. The potential of the Ag/AgCl electrode was converted to that of a reversible hydrogen electrode (RHE) by using an equation (ERHE = EAg/AgCl + 0.059 pH + 0.198). All photocatalytic tests were performed at 25°C and under simulated solar

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illumination with 100 mW cm−2 power density. The temperature was maintained at 25 °C by using a water circulation system. LSV measurements were scanned from +0.45 to −0.45 V (vs. RHE) at a sweeping rate of 20 mV s−1. Chronoamperometric durability was assessed at 0 V (vs. RHE). Electrochemical impedance spectroscopy (EIS) data were collected under solar simulation with the sweeping frequency from 100000

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to 500 Hz and the amplitude of 10 mV. Mott-Schottky plots were recorded at 1000 Hz under the dark

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condition. All PEC results were recorded with a potentiostat (PGSTAT302N, Metrohm Autolab).

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2.8 Electrochemical measurements. The three-electrode system configuration adopted in PEC tests was also adopted for electrochemical characterizations. Electrocatalytic LSV measurements were swept from +0.10 to −0.40 V (vs. RHE). CV characterizations were performed to identify the geometric double layer capacitance (Cdl) at non-Faradaic overpotentials [0.1~0.2 V (vs. RHE)] at various scan rates (10, 20, 40, 60, 80, and 100 mV s−1) to estimate

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the HER active surface area.

3.1 Materials preparation and characterizations

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3. Results and Discussion

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Herein, Si pyramids were prepared through wet chemical etching (see details in the experimental section) and used as the photoabsorber for solar hydrogen evolution, as shown in Fig. 2a. Si pyramids

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reduced the reflectance of the incident illumination and increased the surface area that could react with protons in the electrolyte given their planar structure. As illustrated in Fig. 2a, P-doped MoS2 (MoS2-xPx)

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was anchored on Si pyramids through drop casting and thermal combustion (see details in experimental section). Ammonium tetrathiomolybdate ([NH4]2MoS4) was utilized as a precursor for the synthesis of MoS2 materials. The thermal pyrolysis of (NH4)2MoS4 occurs at room temperature to 200°C in H2 atmosphere and is correlated with the release of ammonia (NH3) and hydrogen sulfide (H2S), as illustrated

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by the following reaction: (NH4)2MoS4 → 2 NH3 + H2S + MoS3.[22,51] The subsequent phase transformation (MoS3 + H2 → MoS2 + H2S) occurs at heating temperatures above 200°C.[22,51] MoS2xPx

materials acted as co-catalysts that accelerate hydrogen evolution reaction (HER) kinetics on the Si

photoabsorber. Scanning electron microscopy (SEM) was adopted to investigate the morphology of the as-prepared MoS2-xPx/Si materials. Potassium hydroxide (KOH) was used to etch the surface of the (100).

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oriented Si wafer for the introduction of microstructures via anisotropic etching [Si + 2 OH− + 2 H2O → SiO2(OH)22− + 2 H2]. The kinetics of Si etching in KOH was face-dependent, and the rate of Si etching on the {100} planes of Si was considerably higher than that on the {111} planes of Si. The cross-sectional SEM image of the bare Si photocathode shows the presence of pyramidal structures with heights of approximately 2–8 µm and widths of 5–12 µm (Fig. 2b). The side walls of pyramids were mainly composed of Si(111), whereas those of the flat areas remained Si(100). Fig. 2c–e show that the

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morphology of the Si pyramids did not considerably change after the deposition of MoS2-xPx co-catalysts.

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MoS2 materials were homogeneously distributed in the form of nanoparticles on the Si photocathode (Fig. 2c). However, MoS1.75P0.25 and MoSP co-catalysts preferentially aggregated on Si pyramids, as shown in

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Fig. 2d–e. This distribution pattern indicates that large particles were generated upon the doping of P

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heteroatoms into MoS2 materials. Energy dispersive X-ray (EDX) spectroscopy in Fig. S1 was conducted to resolve the composition of MoS2-xPx co-catalysts that were soaked in H2SO4 aqueous solution (30 s)

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for eliminating side products. The elemental analysis results of each MoS2-xPx sample were summarized in Table S1. The compositions of MoS2-xPx corresponded to the molar ratio of precursors applied in the

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synthesizing procedure.

X-ray diffraction (XRD) spectroscopy was used to investigate the crystal structure, crystallinity, and phase purity of MoS2-xPx/Si samples. Scanning the strongest peak of Si wafer at 2θ values of

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approximately 69° in XRD patterns was avoided. Nevertheless, Fig. 3a shows two negligible sharp diffraction peaks at 2θ = 32.5° and 61.3° that corresponded to the underlying Si pyramids. This result indicates that MoS2-xPx co-catalysts were amorphous structures or presented at loading amounts that were too low to be detected. Herein, MoP material was also synthesized according to a previous research via a temperature-programmed reduction of air-calcined precursor obtained from ammonium molybdate tetrahydrate [(NH4)6Mo7O24·4 H2O], diammonium phosphate [(NH4)2HPO4] and citric acid.[52] Fig. S2a .

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shows a XRD spectrum of pure MoP powder without impurities. Nevertheless, there were no diffraction peaks of MoP observed in the XRD profiles of MoS2-xPx/Si materials (Fig. 3a). X-ray photoelectron spectroscopy (XPS) was performed with the binding energy of Mo 3d, S 2p, and P 2p regions to investigate the chemical composition and oxidation state of MoS2-xPx materials. The deconvoluted Mo 3d spectrum of the MoS2 material in Fig. 3b shows two different oxidation states that corresponded to Mo4+ and Mo6+ cations. Binding energies at 229.5 and 232.9 eV originated from the Mo 3d5/2 and Mo 3d3/2 orbitals of

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MoS2, respectively.[53] Another doublet located at 233 and 236.1 eV was correlated to the Mo6+ of MoO3

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and originated from the surface oxidation of MoS2 under air exposure.[54] The XPS peaks corresponding to the Mo4+ and Mo6+ ions of MoS1.75P0.25 shifted to low energies. The doublet of the MoS1.75P0.25 co-

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catalyst at 229.4 and 232.6 eV were attributed to MoS2 component,[55] whereas two peaks at 232.8 and

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235.6 eV were ascribed to MoO3 oxidation.[56] Binding energies centered at 228.9 and 232.0 eV, respectively, were related to the Mo 3d5/2 and Mo 3d3/2 orbitals of Mo bonded to P heteroatoms (Mo–

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P).[57] The single peak at 226.4 eV was assigned to the S 2s of MoS2 and MoS1.75P0.25 materials. The energy shift of MoS2 component in MoS1.75P0.25 likely resulted from the doping of P atoms, which had

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lower electronegativity than the host S atoms. The MoS2 peaks that corresponded to the S 2p3/2 and S 2p1/2 orbitals of divalent sulfide ions (S2−) were observed at 162.4 and 163.5 eV, respectively, as shown in Fig. 3c.[58] The doublet originating from the S2− anions of MoS1.75P0.25 shifted to the low binding energy of 162.0 and 163.3 eV,[59] because of charge transfer from phosphorous to sulfur anions.[60] The broad

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peak at approximately 168.5 eV was attributed to sulfate (SO42−) side-products and indicates that MoS2 and MoS1.75P0.25 were susceptible to oxidation in air. Fig. 3d shows the P 2p XPS spectrum of MoS1.75P0.25 material. The XPS peaks at 130.0 and 130.8 eV corresponded to the 2p3/2 and 2p1/2 states of P dopants bonded to Mo cations (P–Mo), respectively.[57] The component with the binding energy of 133.6 eV was ascribed to P oxidation products, including phosphate (PO43−) or phosphorus pentoxide (P2O5).[61] All .

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these oxide by-products were removed upon exposure to aqueous H2SO4 electrolyte and did not contribute

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to photoelectrochemical (PEC) results.[62]

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Fig. 2. (a) Schematic of MoS2-xPx/Si pyramid preparation. Cross-sectional SEM images of (b) bare Si, (c) MoS2/Si, (d) MoS1.75P0.25/Si, and (e) MoSP/Si pyramids.

MoS2 is a 2D layered material with a hexagonally packed structure. Each layer consisted of an atomic plane of Mo that was sandwiched between two atomic planes of S in a trigonal prismatic arrangement.[63] .

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MoS2 layers were stacked along the c-axis through van der Waals interactions. Jin’s group prepared MoS2 catalysts by the thermal sulfidization of molybdenum pentachloride (MoCl5) at the temperature above 375°C.[42] This material presented characteristic Raman peaks associated with 2H-MoS2 phase. Besides, MoSxCly was fabricated during the synthesis temperature was reduced to 275°C. The specific Raman modes of 2H-MoS2 disappeared due to the amorphous structure of MoSxCly catalyst. Thus, Raman spectroscopy was applied to evaluate the crystallinity of MoS2-xPx materials, as shown in Fig. 4a. Two

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characteristic peaks were observed from MoS2-xPx co-catalysts at the wavenumber that was correlated

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with E12g and A1g Raman modes of 2H-MoS2. Nevertheless, the Raman peaks of MoS2-xPx showed broadening width and weak intensity as compared to MoS2 single crystal in a previous work[38], owning

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to the lower crystallinity and the presence of in-layer disorders or defects.[64,65] Fig. 4b shows that the

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vibrational direction E12g Raman mode, which is preferentially excited for the terrace-terminated structure, is within the basal plane of MoS2 layers. By contrast, the Mo–S phonon of the A1g mode is preferentially

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excited for the edge-terminated film because of polarization dependence, and its vibration is attributed to the out-of-plane displacements of Mo and S atoms along the c-axis.[38,66,67] The integrated area ratios

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(E12g/E12g + A1g) of Raman results were adopted to analyze the texture information of MoS2-xPx cocatalysts (Fig. 4c). The decrement in these ratios followed the order of MoS2 > MoSP > MoS1.75P0.25. This order indicates that the number of exposed edge sites increased through the doping of P heteroatoms in

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MoS2.

The lattice vibration with E12g and A1g modes was predicted to strengthen (blue shift) as the layer

number of MoS2 increased from single to bulk given the enhancement in effective restoring interactions between atoms through interlayer van der Waals forces.[64] The blue shift of the A1g mode with increasing layer number observed in previous studies is consistent with this expectation. By contrast, the red shift of the E12g peak implies that long-range Coulombic interlayer interactions in multilayer MoS2 may dominate .

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the variation in lattice vibration.[63,64] Herein, SEM images (Fig. 2c) showed that the secondary conformations of MoS2 were nanoparticles which uniformly attached on the surface of Si pyramids. Moreover, EDX elemental mapping profiles present a complementary result, as shown in Fig. S3. TEM characterizations in previous works also revealed that primary MoS2 nanosheets composed secondary particles which were observed by SEM measurements.[36,66,68] As mentioned above, the vibration frequencies of E12g and A1g Raman modes are strongly influenced by the thicknesses of MoS2 materials.

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Hence, Raman spectra were adopt to speculate the primary morphologies of MoS2-xPx co-catalysts. The

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layer numbers of primary nanosheets were estimated on the basis of wavenumber difference (A1g−E12g), as shown in Fig. 4c.[63-65] In the present work, the E12g and A1g Raman peaks of MoS2 were

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approximately at 381.7 and 405.8 cm−1, respectively. This frequency difference (24.10 cm−1)

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corresponded to a thickness of 3–5 layers as reported by previous work.[63-65] The slight increment in the wavenumber difference of MoS1.75P0.25 and MoSP samples to 25.07 and 25.08 cm−1 , respectively, was

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attributed to the increased layered thickness of P-doped MoS2 materials. The characteristic Raman peak of MoP approximately located at 2435 cm−1 (Fig. S2b), which was also reported in a previous

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literature.[69] However, this Raman mode was not investigated in MoS2-xPx samples (Fig. S4a). Besides, Bose et. al synthesized molybdenum sulphoselenophosphide (MoSxSeyPz) spheroids as HER electrocatalysts.[70] The observations of peak shifting in XPS profiles and intensity decrease of E12g (MoSe) mode in Raman spectra strongly supported the replacement of Se by P atoms. In this study, Fig. 3b-c

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show that the XPS peaks of MoS2 component in MoS1.75P0.25 shifted to a lower binding energy, as compared with pristine MoS2. The integrated area ratios of MoS1.75P0.25 and MoSP in Raman spectra (Fig. 4c) decreased with respect to bare MoS2, which ascribed to the decline of E12g peak intensity. Based on above results, we proposed that the formation of isolated MoP phase was excluded, and S atoms of MoS2xPx

.

was substituted via P dopants. 17

of ro -p re lP

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Fig. 3. (a) XRD spectra of MoS2/Si, MoS1.75P0.25/Si, and MoSP/Si photocathodes. High-resolution (b) Mo

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3d, (c) S 2p, and (d) P 2p XPS spectra of MoS2 and MoS1.75P0.25 co-catalysts.

.

18

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Fig. 4. (a) Raman spectra of MoS2, MoS1.75P0.25, and MoSP co-catalysts. (b) Schematic of Raman vibration

re

modes of MoS2. (c) Area ratio and wavenumber difference for Raman spectra of MoS2-xPx materials.

lP

3.2 Photoelectrochemical and electrochemical measurements

A three-electrode PEC cell was applied to evaluate the photocatalytic efficiencies of MoS 2-xPx/Si

ur na

materials in 0.5 M sulfuric acid (H2SO4) solution under simulated solar illumination (100 mW cm−2). The linear sweep voltammograms (LSVs) of bare Si pyramids did not show photoresponses at 0 V, as shown in Fig. 5a. The photocurrent was generated by applying a bias of approximately −0.20 V (vs. RHE), and

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the current density reached −4.19 mA cm−2 at −0.45 V (vs. RHE). The poor PEC performance of Si pyramids may be attributed to low HER kinetics and insulating oxide film formation during the exposure to the aqueous electrolyte. In this work, the onset potentials of (photo)electrochemical measurements were defined as the voltage at which the current density reaches −1 mA cm−2. The onset potential of pristine Si pyramids was −0.31 V (vs. RHE), but MoS2/Si photocathodes presented positively shifted turn-on voltage. The photocurrent at 0 V (vs. RHE) and the onset potential of MoS2/Si improved to −10.5 mA cm−2 and .

19

0.21 V (vs. RHE), respectively. P doping was adopted to increase the number of exposed edges and Svacancies that can act as HER active sites, thus enhancing the catalytic activity of MoS 2 materials. The PEC results of the P-doped MoS2/Si photocathode were superior to those of MoS2/Si pyramids, and MoS1.75P0.25/Si materials presented optimal performance. SEM images (Fig. 2c–e) presented that P-doped MoS2 enlarged the particle diameters on Si pyramids, as compared with MoS2. Large particle size of co-

of

catalysts deteriorated the catalytic properties or led to the disappearance of surface effects.[71] Nevertheless, P dopants in MoS2-xPx materials improved the inherent catalytic activities to compensate for

ro

the drawbacks of increased particle size in the present research. Fig. 5b shows that the optimized onset potential and current density [at 0 V (vs. RHE)] of MoS1.75P0.25/Si were +0.29 V (vs. RHE) and −23.8 mA

-p

cm−2, respectively. However, the introduction of additional P heteroatoms into MoS 2 (X ≥ 0.5) reduced

re

the photocatalytic efficiency of MoS2-xPx/Si photocathodes. The turn-on voltage and photocurrent at 0 V (vs. RHE) of MoSP/Si pyramids decreased to +0.24 V and −18.1 mA cm−2, respectively. We posited that

lP

excess P dopants in MoS2-xPx/Si (X ≥ 0.5) acted as defective sites that promoted the recombination of electron-hole pairs on photocathodes and reduced PEC activity by trapping the photogenerated carriers.

ur na

As shown in Fig. 5a, a saturated photocurrent was generated upon the deposition of co-catalysts on Si pyramids. This result is attributed to the limited migration rate of photoinduced carriers on Si pyramids or the restricted diffusion efficiency of redox couples in the electrolyte. The current density saturation of MoS2/Si was −18.8 mA cm−2 and was obtained by applying voltages of more than −0.2 V (vs. RHE). The

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photocurrent was saturated at approximately −0.1 V (vs. RHE) after the introduction of P dopants into MoS2 materials. The MoS1.75P0.25/Si photocathode presented the optimal saturated current of −26.2 mA cm−2 (Fig. 5b).

.

20

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Fig. 5. (a) Photoelectrochemical linear sweep voltammograms of bare Si and MoS 2-xPx/Si (x = 0, 0.125,

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0.25, 0.5, and 1) photocathodes. (b) Onset potential, photocurrent density [at 0 V (vs. RHE)], and saturated

re

-p

current of photocathode materials.

The reflectance spectra (Fig. S4b) showed that most of visible light was absorbed by bare Si

lP

pyramids, but the incident irradiation with the wavelength below 500 nm was slightly reflected. Besides, MoS2-xPx contributed to small variations on this reflectance characteristic of Si photocathode. MoS2-xPx

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samples were also deposited on the quartz substrates by using the same loading amount of precursor solution for the preparation of MoS2-xPx/Si photocathodes. The UV-Vis absorption spectra of MoS2xPx/quartz

was shown in Fig. S4c. We proposed that the small absorption discrepancy between MoS2-xPx

materials was not the dominant parameter influencing the difference in PEC activity (Fig. 5). Besides, the

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band gap of MoS2 was not severely changed via doping P heteroatoms. In this work, transient cathodic photocurrent measurements (Fig. S5a) were adopted for evaluating the charge recombination of photocathodes. As the incident light was activated, pristine Si electrode immediately generated overshoot current and subsequently decreased to steady-state photocurrent due to the recombination of electron-hole pairs.[72] However, no overshoot was investigated in the transient photocurrent characterizations of .

21

MoS2-xPx/Si, which indicated to the suppression of charge recombination process via the presence of cocatalyst materials. Moreover, as compared with bare Si, the onset potential of MoS2-xPx/Si photocathode revealed a significantly positive shift in the LSV test (Fig. 5b). We suggested that reducing the recombination of photoinduced carriers was achieved by introducing MoS2-xPx onto Si pyramids, because of improving the surface chemistry.[72]

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In our previous study, pyrite cobalt disulfide (CoS2) co-catalysts with various thicknesses were modified on Si microwires as photocathodes for solar hydrogen production.[17] Electrochemical

ro

impedance spectroscopy (EIS) presented that large charge transfer resistance was observed from CoS 2 to

-p

redox couples in the electrolyte, as thickness of co-catalysts was high. This result suggests that highly thick CoS2 severed as charge recombination centers that increased diffusion length of photoinduced

re

carriers form Si light-absorber to react with protons in the solution. Hence, excessively thick co-catalyst decorated on Si electrode contributed to poor PEC performance in the LSV measurement. SEM images

lP

(Fig. 2c) and EDX elemental mapping spectra (Fig. S3) showed that MoS2 co-catalysts with the diameter of 100-500 nm homogeneously distributed on Si pyramids. However, MoS1.75P0.25 and MoSP materials

xPx

ur na

formed the particles with larger sizes (1-2 μm). This indicated that phosphorus dopants caused the MoS2aggregation. Herein, EIS measurements recorded under an illumination intensity of 100 mW cm−2 in

a three-electrode system were applied to assess charge transfer kinetics of MoS2-xPx/Si photocathodes.

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The Nyquist plots were fitted according to the equivalent circuit models (Fig. S5b).[73] The equivalent series resistance (Rs) indicates the overall resistance ascribing to the electrode materials, electrical contacts, and the electrolyte. Q1 and Q2 presents the constant phase elements. R1 stands for the charge transfer resistance from underlying Si pyramids to outer co-catalysts, whereas R2 denotes the resistance at the interface between co-catalysts and redox couples in the electrolyte. The Warburg resistance (W) arising from the diffusion. The R1 and R2 values of MoS2/Si were 7.94 and 31.6 Ω cm2, respectively. As .

22

introducing phosphorus dopants into MoS2 materials, the charge transfer resistance reduced and the optimized charge transfer kinetic was achieved by MoS1.75P0.25 electrode. Both R1 and R2 decreased in the order of MoS2/Si > MoSP/Si > MoS1.75P0.25 (Fig. S5d). We suggested that P-doped MoS2/Si electrodes with superior charge transfer efficiencies to those of MoS2/Si pyramids, which contributed to the improved PEC performances in Fig. 5. Moreover, the specific phosphorus doping concentration into MoS2 optimized the electron-transfer rate from Si to the electrolyte and suppressed the surface recombination.

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This result also demonstrates that the disadvantages of large particle diameters by doping phosphorus

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heteroatoms were compensated due to the improved intrinsic catalytic properties of MoS2-xPx marterials.

-p

Mott-Schottky measurements of bare Si and MoS2-xPx/Si electrodes were performed under a dark condition. The flat band potential (Efb) was determined by extrapolating the capacitance of zero. Bare Si

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pyramids exhibited an Efb of 0.092 V (vs. RHE), as shown in Fig. S5c. However, the Efb of MoS2-xPx/Si photocathodes showed the positive shift (Fig. S5d), which was due to the band coupling and the Fermi

lP

level reforming in the type-II heterojunction constructed by Si and MoS2-xPx materials.[74] The Efb values of MoS1.75P0.25/Si and MoSP/Si electrodes were about 0.36 and 0.35 V (vs. RHE), respectively, (Fig. S5c).

ur na

Moreover, P-doped MoS2 showed a larger Efb relative to MoS2/Si photocathode [0.34 V (vs. RHE)]. The magnitude of band bending (Eb) in the semiconductor is determined by the applied potential (E) and Efb, according to the following equation: Eb = E − Efb.[75] Pristine Si and MoS2-xPx/Si were served as

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photocathodes for the proton reduction, so E was more negative as compared with Efb. Thus, MoS2-xPx/Si revealed a higher band bending than bare Si electrode, which was caused by the larger Efb value. The charge separation of photogenerated electron-hole pairs at the electrode/electrolyte interface was facilitated through the superior band banding of MoS2-xPx/Si.[76] The presence of solar HER overpotential on the photocathode contributes to a negative shift of onset potential from Efb.[27,77] The poor turn-on voltage of bare Si electrode (Fig. 5) ascribed to the formation of surface oxidation. This result causes not .

23

only an enlarged kinetic barrier for photoinduced carriers transferring from Si to the electrolyte[78], but also a significant difference (0.40 V) between onset potential and Efb. However, MoS2-xPx/Si presented the much smaller discrepancy, which suggested that the decoration of co-catalyst materials declined the catalytic overpotential and boosted the HER reaction rate on photocathode.[26,79] The MoS1.75P0.25/Si photocathode was subjected to PEC durability characterization under simulated

of

solar irradiation (100 mW cm−2). The chronoamperometry results provided in Fig. S6a show that the initial photocurrent of MoS1.75P0.25/Si pyramids was approximately −23.5 mA cm−2, which corresponded with

ro

the LSV result (Fig. 5). However, the current density of MoS1.75P0.25/Si declined dramatically to −14.4

-p

mA cm−2 after 1 h of stability measurement. The photocurrent of MoS1.75P0.25/Si decayed to −12.2 mA cm−2, and its current retention was almost 51.9% after 2 h of chronoamperometric testing. MoS2-xPx co-

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catalysts were deposited on Si pyramids through drop casting and thermal combustion method. This preparation technique caused MoS2-xPx to incompletely cover on the Si surface. This degradation was

lP

attributed to the generation of insulating SiO2 on uncovered Si surfaces under electrolyte exposure because MoS1.75P0.25 co-catalysts preferentially aggregated on Si pyramids (Fig. 2d). However, the current density

ur na

recovered to the initial state, after MoS1.75P0.25/Si electrode was carried out the chronoamperometric test for 1 h and subsequently immersed into a diluted HF solution for removing the SiO 2 layer that generated during the photocatalytic reaction (Fig. S6b), which excluded the MoS1.75P0.25 catalytic destruction

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causing the decayed stability. Thus, a TiO2 thin film prepared through atomic layer deposition (ALD) was applied as the passivation layer to attenuate oxidation on the Si photoabsorber. MoS1.75P0.25/1 nm TiO2/Si and MoS1.75P0.25/10 nm TiO2/Si pyramids showed low preliminary current densities of −20.7 and −15.6 mA cm−2, respectively, given that the TiO2 protective layer was HER inert (Fig. S6a).[14,80] Notably, photocurrent retention by MoS1.75P0.25/10 nm TiO2/Si improved to 84.0% after 2 h of durability characterization. We did not further increase the thickness of the TiO2 passivation layer to prevent .

24

sacrificing photoconversion efficiency. The thick protective thin film on Si pyramids extended the diffusion length of photoinduced carriers and even caused photocatalytic degradation instead of ameliorating stability.[19] Although depositing MoS1.75P0.25 co-catalyst onto other stable photocathode may realize an improved PEC durability, using Si electrode in this work ascribed to the highest theoretical current density under solar irradiation among all of materials. [8] Hence, if MoS2-xPx material fully covers on Si photocathode by a new fabrication method developed in the future, it is functionalized co-catalyst

of

and passivation layer for simultaneously ameliorating the photocatalytic activity and stability.

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We summarized photocatalytic performance of previous works related to Si photocathodes with

-p

molybdenum chalcogenide co-catalysts in Table S2. MoS1.75P0.25/Si photocathode presented ideal onset potential of +0.29 V (vs. RHE) and short circuit current (Jsc) of −23.8 mA cm−2, as compared with reported

re

literatures. Fan et al. applied the sputtering deposition to decorate vertically standing MoS2 nanostructures on Si pyramids, and achieved high Jsc of −35.6 mA cm−2.[26] The introduction of n+-Si emitter on p-Si

lP

electrode not only eliminated the influence of co-catalyst materials on the band bending, but also boosted the photovoltage due to a built-in depletion region at the semiconductor-electrolyte junction. Moreover,

ur na

the deposition of p+-Si layer on the back of n+p-Si photocathode facilitated the photoinduced carrier collection and improved the fill factor of device.[81] Hence, Alarawi and coworkers developed n+np+-Si heterojunctions modified with MoS2 co-catalyst, which showed promising onset potential of +0.50 V (vs.

Jo

RHE) and Jsc value of −36.3 mA cm−2.[25] It is worth to note that the present study provided another PEC increasing strategy of MoS2/Si electrode by doping heteroatoms into MoS2 to enhance the inherent catalytic performance, which exposed high quantities of edge sites, induced the formation of S-vacancies, and activated basal planes (discussed later in 3.3 Mechanism discussion). In addition, with respect to bare MoS2, phosphorus-doped samples showed larger particle diameters on Si pyramids (Fig. 2c-e). Thus, we believed that reducing the particle size, introducing nanostructures into MoS2-xPx co-catalyst[26] or .

25

fabricating heterojunctions on Si photoabsorber[81] can further ameliorate its PEC efficiency. Besides, chronoamperometric test (Fig. S6b) elucidated that the photocurrent decay ascribed to the Si oxidation instead of the catalytic degradation of MoS2-xPx materials. With respect to previous reports (Table S2), MoS1.75P0.25/10 nm TiO2/Si revealed a moderate photocatalytic stability for 2 h. However, decorating inter layers on Si surface[5] and conformal coating of MoS2-xPx co-catalysts via new preparation methods such as sputtering deposition[26], chemical vapor deposition[27], or atomic layer deposition[28] can enhance

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of

the durability of MoS2-xPx/Si photocathode in the future.

Fig. 6. Electrochemical (a) linear sweep voltammograms, (b) Tafel plots, and (c) double-layer capacitance (Cdl) of MoS2/Ti, MoS1.75P0.25/Ti, and MoSP/Ti electrodes. (d) Onset potential, Tafel slope, and doublelayer capacitance of MoS2-xPx electrocatalysts. .

26

In previous works, LSVs of pristine Ti foil showed no cathodic current density observed between 0 and −0.4 V (vs. RHE).[48-50] Hence, in the present study, Ti foil was functioned as the substrate for depositing MoS2-xPx materials as electrocatalysts and assessing the catalytic performance of electrochemical HER in H2SO4 (0.5 M) aqueous electrolyte. LSVs were acquired to study the electrocatalytic activity of MoS2/Ti, MoS1.75P0.25/Ti, and MoSP/Ti electrodes and are shown in Fig. 6a. The onset potential of MoS2/Ti was −0.24 V (vs. RHE) (Fig. 6d), and its current density was −10 mA cm−2

of

with applied bias at −0.39 V (vs. RHE). The overpotentials of MoS1.75P0.25/Ti and MoSP/Ti electrodes

ro

underwent anodic shifts to −0.13 and −0.19 V (vs. RHE), respectively, as shown in Fig. 6a. The currents of MoS1.75P0.25/Ti and MoSP/Ti reached −10 mA cm−2 at −0.23 and −0.27 V (vs. RHE), respectively.

-p

These results indicate that P dopants improved the HER electrocatalytic efficiencies of MoS 2 materials.

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The HER mechanism can be expressed by using classic two-electron transfer reactions: the discharging step of the Volmer reaction (H3O+ + e− → H* + H2O) followed by the desorption step of the Heyrovsky

lP

reaction (H* + H3O+ + e− → H2 + H2O) or the recombination step of the Tafel reaction (H* + H* → H2), where H* indicates hydrogen atoms that were adsorbed on active sites on the surfaces of the catalysts.

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The Volmer, Heyrovsky, or Tafel reaction served as the HER rate-determining step given that the Tafel slopes of the electrocatalyst were 116, 38, or 29 mV decade−1, respectively. The Tafel plots of MoS2/Ti, MoS1.75P0.25/Ti, and MoSP/Ti electrodes are presented in Fig. 6b. The Tafel slope of the MoS2 catalyst was 144 mV decade−1. This result suggests that the Volmer reaction determined the HER rate of MoS 2.

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The Tafel slopes of MoS1.75P0.25 and MoSP were reduced to 90.9 and 79.4 mV decade−1 upon P doping into MoS2. This result reveals that the HER rates of P-doped MoS2 electrocatalysts were decided by competition between discharging and desorption steps. Herein, the cyclic voltammograms (CVs) shown in Fig. S7a–c were acquired over the non-Faradaic potential range [0.1–0.2 V (vs. RHE)] to quantify capacitive current as a function of scan rate for calculating the double-layer capacitance (Cdl) of the .

27

electrode materials (Fig. 6c). The Cdl value functioned as an estimate of the effective electrochemically active surface area between the solid–liquid interface for driving HER. The Cdl value of MoS2/Ti was 2.24 mF cm−2, whereas that of MoS1.75P0.25/Ti and MoSP/Ti were improved to 11.5 and 6.69 mF cm−2, respectively, as shown in Fig. 6d. These results demonstrate that doping P atoms into MoS2 generated an increased number of HER active sites for enhancing PEC and EC efficiencies. We propose that the MoS1.75P0.25 material with the maximum amount of active sites contributed to the optimal catalytic

of

performance, as indicated in Fig. 5a and Fig. 6a and in correspondence with Raman results (Fig. 4).

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Some previous studies reported that bare MoS2 was served as photocatalyst for hydrogen

-p

production.[82,83] Thus, liner sweep voltammogram of MoS2-xPx/Ti was performed under dark and illuminated conditions, as shown in Fig. S7d. However, no photoresponse was detected from the MoS2samples. This result might ascribe to the low loading amount of MoS2-xPx material, which was

re

xPx

analogous as the ignorable photocatalytic contribution of using TiO2 passivation layer.[14,80] Thus, we

lP

suggested that Si pyramids and MoS2-xPx materials were in charge of photoabsorber and co-catalyst for harvesting incident light and accelerating HER kinetics, respectively, in the PEC measurements (Fig. 5).

ur na

Freestanding photocatalyst powders for solar water splitting are either suspended in a solution or fixed at a reactor bed. However, the separating techniques must be applied to this photocatalytically generated chemical fuels for the real application. The PEC cell is composed of photoanode and photocathode

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materials. Because oxidization and reduction reactions separately occur on different electrodes, using PEC cell is facile for collecting oxygen and hydrogen gas from anode and cathode, respectively, without the separation. In the present work, MoS2-xPx/Si samples were functioned as photocathode for solar hydrogen evolution. As discussed above, the incident irradiation was dominantly captured through Si pyramids, whereas the photogenerated holes in Si materials were unable to drive water oxidation reaction due to insufficiently positive valence band edge. Hence, single MoS2-xPx/Si photocathode could not directly .

28

proceed overall solar water splitting. The target of this study is developing highly efficient MoS 2-xPx/Si photocathode, which can assemble with photoanode as a tandem cell for unassisted water photoelectrolysis in the future.[84,85] Besides, integrating the materials, responsible for photocatalytic oxygen generation, on MoS2-xPx/Si electrode may also drive the entire water splitting reaction.[86,87]

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3.3 Mechanism discussion

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X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure

-p

(EXAFS) were conducted in fluorescence mode to analyze the chemical states and local structures of MoS2, MoS1.75P0.25, and MoSP co-catalysts. The pre- and post-edge absorption spectra of the MoS2-xPx

re

materials were normalized to 0 and 1, respectively. For removing the oxide by-products on the surface, MoS2-xPx samples were immersed into in H2SO4 aqueous solution for 30 s. One dominant absorption peak

lP

in S K-edge XANES spectra of MoS2-xPx (Fig. S8a) located at about 2470 eV, which was correlated to sulfide materials.[88-90] Moreover, no obvious sulfate peak (~2480 eV) was observed on H2SO4-treated

ur na

MoS2-xPx samples.[19,89] The P K-edge XANES spectra of MoS2-xPx was shown in Fig. S8b. The peak at approximately 2146 eV was in good agreement with that of spectroscopic features for metal phosphide (2140-2150 eV), while no peaks originated from phosphate material (~2153 eV) was investigated.[91,92]

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The absorption edges in Mo K-edge XANES spectra did not undergo chemical shifts upon the doping of P heteroatoms into MoS2 materials, as shown in Fig. 7a. XANES measurements demonstrated that P doping contributed to the absence of variation in the electronic states of Mo4+ cations in MoS2. Hence, we suggested that phosphorus doping concentration in MoS2-xPx was closed to the atomic percentage collected by using EDX characterizations, as shown in Table S1.

.

29

Fig. 7b presents the schematics of the basal plane, Mo-/S-edges, and S-vacancies of the MoS2-xPx structure. The coordination numbers of the most adjacent S(P) and Mo atoms around Mo atoms in the basal plane were both 6. The first and second coordination shells of Mo atoms at Mo-/S-edges were composed of six S(P) and four Mo neighbors. The S-vacancies of MoS2-xPx contributed to the reduction in the number of S(P) anions, which coordinated with Mo atoms. Hence, the coordination numbers of neighboring S(P) and Mo atoms in the first and second shells of Mo atoms in MoS2-xPx materials

of

approached low values as the number of exposed HER active edge sites and S-vacancies increased and

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boosted the catalytic efficiency. The radial distributions for the Fourier transform k3-weighted EXAFS signals of MoS2-xPx co-catalysts are shown in Fig. 7c. Two peaks at approximately 1.9 and 2.9 Å were

-p

associated with Mo–S(P) and Mo–Mo coordination shells, respectively. Furthermore, small peaks at 1.5

re

Å of MoS2 and at 1.3 Å of MoS1.75P0.25 (Fig. 7c) attributed to the low frequency noise which were frequently investigated in EXAFS results.[65,93-95] The EXAFS peak intensity was proportional to the

lP

coordination number of the neighbors surrounding X-ray absorbing atoms. MoS2 presented the highest peak intensity of Mo K-edge radial distribution for EXAFS signals, as shown in Fig. 7c. However, the

ur na

reduction in the coordination numbers of adjacent S and Mo atoms upon the doping of P heteroatoms into MoS2 co-catalysts indicates that the number of exposed edges and S-vacancies increased. This finding was complementary with Raman results (Fig. 4) and electrochemically active surface area estimated using CV measurements (Fig. 6c). MoS1.75P0.25 showed the optimal HER catalytic activity (Fig. 5 and Fig. 6a),

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owing to the lowest coordination values of the first and second shells of Mo atoms. Herein, MoP powder was also prepared for XANES and EXAFS analyses. The Mo K-edge absorption edge of MoP located at approximately 20000 eV (Fig. S9a), which is correspond to the previous work.[96] Fig. S9b also shows that two EXAFS peaks were observed at about 1.9 and 2.9 Å, relating to Mo–P and Mo–Mo coordination

.

30

shells, respectively.[96,97] Nevertheless, it is difficult to distinguish Mo-S and Mo-P bonds in MoS2-xPx

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lP

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of

materials (Fig. 7c), because the atomic number difference between S and P is barely equal to one.

Fig. 7. (a) Mo K-edge XANES spectra of MoS2, MoS1.75P0.25, and MoSP co-catalysts. (b) Schematic of the basal plane, Mo-/S-edges, and S-vacancies of MoS2-xPx. (c) Radial distribution of Fourier transform k3-weighted EXAFS signals of MoS2-xPx materials. .

31

Previous works have shown that Mo-edges and S-vacancies have ideal HER activities given that they possess suitable binding strength with protons [the hydrogen adsorption free energy (GH*) around 0 eV]. These characteristics have been extensively studied and well established on the basis of density functional theory (DFT) calculations.[33,39,98,99] Therefore, to explore the mechanism behind the enhancement in HER performance via doping P atoms into MoS2, DFT calculations were performed to investigate the

of

possible structural changes shown by MoS2 upon P doping and the proton adsorption behavior of corresponding P-doped MoS2 systems. Herein, pristine MoS2 and MoS1.75P0.25 (P-doped MoS2 with a P-

ro

doping concentration of 12.5%) were simulated by using Mo16S32 and Mo16S28P4 systems, respectively. MoS1.75P0.25 was selected for DFT calculations given its optimal catalytic efficiency. As shown in Fig.

-p

S10, the formation energy of S-vacancy (SV) on the basal plane was 2.64 eV for MoS2 and decreased to

re

2.42 or 2.26 eV for MoS1.75P0.25 in accordance with the S-vacancy located at the first or second coordination shell of the P dopant. The low positive formation energies caused by P doping indicate that

lP

S-vacancy formation had been facilitated and increased S-vacancy density. This result is well consistent with our experimental observation from EXAFS spectra (Fig. 7b–c). Raman characterizations (Fig. 4)

ur na

indicate that a considerable number of edges were exposed after P doping into MoS 2. Thus, we further evaluated the formation energies (Eform.) of unsaturated S atoms on Mo-edge in bare MoS2 and MoS1.75P0.25. As presented in Fig. S11, the Eform. values of Mo-edge were 3.61 and 2.51 eV/nm for MoS2 and MoS1.75P0.25, respectively. The lower positive Eform. of Mo-edge of P-doped MoS2 than that of pristine

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MoS2 suggests that an increased number of Mo-edges were exposed after P doping and is in good agreement with the Raman and EXAFS results. The hydrogen adsorption energy on the Mo-edge was further simulated to assess the influence of P doping on the HER performance of MoS2. As shown in Fig. S12, the GH* on the Mo-edge of bare MoS2 was 0.09 eV and was highly consistent with previously reported values calculated through DFT.[33,43,45] The negligible reduction in GH* value to 0.06 eV .

32

after P doping into MoS2 indicates that P-doped MoS2 retained the HER activities of the Mo-edge unchanged. Previous studies have shown that doping heteroatoms into molybdenum dichalcogenide materials cause the HER inert basal plane to become catalytically active.[43-45] We further calculated the GH* value for the basal plane of MoS2-xPx (x = 0, 0.125, 0.25, 0.5, and 1) with different P doping concentrations

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(6.25% for MoS1.875P0.125, 12.5% for MoS1.75P0.25, 25% for MoS1.5P0.5, and 50% for MoSP) to clarify this point. As shown Table S3, among all possible adsorption configurations, the calculated GH* for the basal

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planes of MoS1.875P0.125 and MoS1.75P0.25 were −0.08 and −0.13 eV when one proton bonded with the P

-p

dopant and another adsorbed on its nearest neighboring S atom, respectively. These results imply that the basal planes of the two systems have ideal HER activity. The calculated GH* values for other cases were

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either positively too large or negatively too low and were thus unsuitable for driving HER. Interestingly,

lP

the basal plane of MoS1.75P0.25 showed more active sites (two protons respectively binding with the P dopant and the nearest S neighbor) than that of MoS1.875P0.125. The corresponding active site ratio to the

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whole basal plane of MoS1.875P0.125 and MoS1.75P0.25 was estimated as 40% and 86%, respectively. The joint results of Raman and EXAFS characterizations with DFT calculations show that doping P atoms into MoS2 increased S-vacancy and Mo-edge density. The HER catalytic performance of Mo-edge on P-doped MoS2 was comparable with that of bare MoS2. The HER inert basal plane became catalytically active after

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the incorporation of P dopants into MoS2 material. The MoS1.75P0.25 catalyst with optimal amounts of Svacancy, Mo-edge, and active basal plane contributed to its optimized (photo)electrochemical efficiencies, as shown in Fig. 5 and 6a.

.

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4. Conclusions MoS2 co-catalysts were prepared on p-Si pyramids through drop casting and thermal combustion to accelerate the kinetics of photoinduced carriers for solar hydrogen production. P dopants were introduced into MoS2 materials to further improve photocatalytic efficiency. The joint results of Raman and EXAFS spectra with DFT calculations revealed that P-heteroatom doping improved catalytic activity by increasing the number of exposed edges and S-vacancies that acted as HER active surfaces in MoS2. The HER inert

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basal plane became catalytically active through the introduction of P dopants into MoS2. The optimal PEC

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performance was achieved by utilizing MoS1.75P0.25/Si pyramids, which showed a low onset potential of +0.29 V (vs. RHE) and a photocurrent density of −23.8 mA cm−2 at 0 V (vs. RHE). The PEC durability

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of photocathodes were enhanced through decoration with a TiO2 passivation layer prepared through ALD.

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The current density retention of MoS1.75P0.25/10 nm TiO2/Si pyramids was 84.0% after 2 h of

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chronoamperometric measurement.

Acknowledgment

The authors are grateful for the financial support from the Ministry of Science and Technology (Contract No. MOST 106-2112-M-003-007-MY3 and MOST 107-2113-M-002-008-MY3). The authors

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also thank the National Natural Science Fundation of China (NSFC No. 51472249 and 51972312) for financial support. The theoretical calculations in this work were performed on TianHe-1(A) at the National Supercomputer Center in Tianjin and Tianhe-2 at the National Supercomputer Center in Guangzhou.

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