Amorphous Phosphorus-Doped Cobalt Sulfide Modified on Silicon

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Amorphous Phosphorus-doped Cobalt Sulfide Modified on Silicon Pyramids for Efficient Solar Water Reduction Chih-Jung Chen, Chi-Wei Liu, Kai-Chih Yang, Lichang Yin, Da-Hua Wei, Shu-Fen Hu, and Ru-Shi Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14571 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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ACS Applied Materials & Interfaces

Amorphous Phosphorus-doped Cobalt Sulfide Modified on Silicon Pyramids for Efficient Solar Water Reduction Chih-Jung Chen†, Chi-Wei Liu§, Kai-Chih Yang‡, Li-Chang Yin‖, Da-Hua Wei*,§, Shu-Fen Hu*,‡, and Ru-Shi Liu*,†,§ † Department of Chemistry, National Taiwan University, Taipei 10617 (Taiwan) § Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei 10608 (Taiwan) ‡ Department of Physics, National Taiwan Normal University, Taipei 11677 (Taiwan) ‖Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016 (China) KEYWORDS: Water Reduction, Hydrogen Evolution Reaction, Co-catalyst, P-doped Cobalt Sulfide, Si Pyramids ABSTRACT: Cobalt sulfide (CoSx) functioned as a co-catalyst to accelerate the kinetics of photogenerated electrons on Si photocathode, leading to the enhancement of solar hydrogen evolution efficiency. By doping phosphorus heteroatoms, CoSx materials showed an improved catalytic activity because of superior surface area and quantity of active sites. Furthermore, increased vacancies in unoccupied electronic states were observed, as more phosphorus atoms doped into CoSx co-catalysts. Although these vacant sites improved the capability to accept photoinduced electrons from Si photoabsorber, chemisorption energy of atomic hydrogen on catalysts was the dominant factor affecting in photoelectrochemical performance. We suggested that Pdoped CoSx with appropriate doping quantities showed thermoneutral hydrogen adsorption. Excess phosphorus dopants in CoSx contributed to excessively strong adsorption with H atoms, causing the poor consecutive desorption ability of photocatalytic reaction. The optimal P-doped CoSx decorated Si photocathode showed photocurrent of −20.6 mA cm−2 at 0 V. Moreover, TiO2 thin film was deposited on Si photocathode as a passivation layer for improving the durability. The current density of 10 nm TiO2modified photocathode remained at approximately −13.3 mA cm−2 after 1 h of chronoamperometry.

Introduction Solar illumination being converted into chemical fuels has been regarded as a promising sustainable energy source to substitute for fossil fuels. Silicon (Si), which possesses a small band gap and negative conduction band position, appropriately functions as a photocathode for absorbing incident visible light to drive the hydrogen evolution reaction (HER). However, the poor kinetics of photoinduced carriers on pristine Si photocathode contributes to severe recombination. An applied bias is required to migrate photogenerated carriers to the Si surface for the reaction with redox couples in the electrolyte, thereby leading to low photoconversion efficiency. A variety of catalyst materials, including noble metals1-4, metallic alloys5,6, chalcogenides7-13 and phosphide14,15 have been developed as potential catalysts for decorating on Si photocathodes to accelerate their charge separation and improve photoelectrochemical performance. Nevertheless, the use of Si photocathodes still suffers from long-term solar hydrogen production because of the formation of an oxide layer during direct exposure in aqueous solutions, causing the degradation of photocatalytic activity. Therefore, numerous passivation layers have been adopted to protect the Si surface from oxidation formation.16-20 Recently, cobalt sulfide derivative materials, such as cattierite (CoS2; pyrite series)8,21 and jaipurite (CoS)22-24, have been widely applied in electrochemical hydrogen generation,

and even as an oxygen evolution electrocatalyst.25 The HER activity of cobalt sulfide catalysts is derived from the exposed active sites; thus, considerable effort has been devoted to developing morphological modifications with high fractions of active surface.21 Furthermore, decorating cobalt sulfide derivatives on the conductive support improves electrochemical performance.22,24 However, these engineering strategies for boosting the efficiency of cobalt sulfide have failed to enhance the intrinsic catalytic activity. By contrast, the introduction of dopants into the cobalt sulfide materials have resulted in the amelioration of the inherent HER efficiency. A phosphorusdoped pyrite CoS2 electrocatalyst synthesized by Jin’s and Wang’s groups revealed thermoneutral hydrogen adsorption for increasing photocatalytic activity.26,27 In previous works, amorphous cobalt sulfide (CoSx) prepared by electrodeposition method showed promising performance for hydrogen generation.7,28 The integration of this potential catalyst on the photoabsorber for solar water splitting is worth further observation. In the present study, Si pyramids were prepared by a chemical wet etching method as photocathodes for solar hydrogen evolution. Amorphous cobalt sulfide fabricated by thermal reduction annealing functioned as the co-catalyst for improving the photocatalytic reactivity of Si photocathodes (Figure 1). The amorphous structure of cocatalysts was favored to expose more active sites to drive hydrogen production. Furthermore, various quantities of

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phosphorous (P) heteroatoms were applied for doping in CoSx to optimize the free energy of hydrogen adsorption. Optimal photoelectrochemical performance of P-doped CoSx/Si electrode showed onset potential and photocurrent at 0 V of 0.14 V and −20.6 mA cm−2, respectively. Moreover, titanium oxide (TiO2) protective thin film was fabricated on Si surface as the passivation layer by an atomic layer deposition (ALD) system to enhance stability. The current density of 10 nm TiO2modified photocathode remained at approximately −13.3 mA cm−2 after 1 h of chronoamperometry.

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CoSx/Si photocathode was fabricated in an analogous preparation as CoSx|P/Si pyramids. Only 1.455 g of Co(NO3)2 and 0.7612 g CH4N2S were utilized in the precursor solution. Fabrication of CoSx|P/TiO2/Si photocathode TiO2 passivation layer on the surface of Si pyramids was prepared by using an atomic layer deposition (ALD) system (Savannah G2, Ultratech). Tetrakisdimethylamido titanium (TDMAT) and nitrogen gas (N2) functioned as the precursor and carrier gas, respectively. Temperature of ALD chamber was kept constant at 200 °C throughout the fabrication process. Moreover, N2 flow rate was kept at 20 sccm. For one cycle deposition, vaporized DI water and the precursor material were subsequently pulsed for 0.1 and 0.25 sec, respectively, and 30 sec elapsed before the residual unreacted gas was pumped out of the chamber. Deposition cycles were repeated until the desired thickness of TiO2 layer was achieved. Afterward, the CoSx|P co-catalyst was synthesized through an identical method of preparing CoSx|P/Si photocathode.

Fabrication of CoSx/Ti and CoSx |P/Ti electrodes Figure 1. Schematic of CoSx|P/TiO2/Si pyramids photocathode materials for solar hydrogen evolution.

as

Experimental section Fabrication of Si photocathode Si pyramids were fabricated as photocathodes by utilizing a chemical wet etching method. Boron-doped p-type (100)oriented Si wafers (resistivity: 1–25 Ω cm) were used for preparing Si pyramids. First, a solution of potassium hydroxide (KOH, 2 wt.%, 4.038 g) and isopropyl alcohol (IPA, 5 vol.%, 10 mL) was added to 190 mL of deionized (DI) water. This mixed precursor solution was heated to 85 °C through a hot plate. After being immersed in the solution for 30 min, the Si wafer was rinsed with DI water and blown dry under a nitrogen flow.

Fabrication of CoSx/Si and CoSx|P/Si photocathodes Amorphous CoSx|P co-catalyst was synthesized on the surface of a Si pyramid photoabsorber (see details in the supporting information) using the thermal annealing method. Sulfur/phosphorus (S/P) molar ratios of CoSx|P materials were tuned by respectively dissolving various amounts of thiourea (CH4N2S) and sodium hypophosphite (H2NaO2P) precursors into methanol and hexamethyldisilazane (HMDS) mixed solutions. With CoSx|P-1 as a paradigm, Co(NO3)2 (1.455 g), H2NaO2P (0.4399 g), and CH4N2S (0.3806 g) were dispersed in methanol/HMDS (100 mL/200 μL) through ultrasonic agitation for 10 min. Afterward, 6.25 μL of CoSx|P-1 precursor solution was drop-casted onto the Si pyramid substrate. When methanol totally evaporated, dried Si samples were subsequently heattreated at 600 °C under N2/H2 (80/20 sccm) atmosphere for 2 h.

For evaluating the electrocatalytic characteristics, CoSx and CoSx|P materials were fabricated on the conducting Ti foils, which were analogous to the preparation of CoSx/Si and CoSx|P/Si pyramids. However, the volumes of precursor solutions for depositing the catalysts on Ti substrates were increased to 62.5 μL.

(Photo)electrochemical measurement A three-electrode cell was adopted for the (photo)electrochemical measurements. CoSx/Si and CoSx|P/Si photocathodes or CoSx/Ti and CoSx|P/Ti electrodes functioned as the working electrodes. Besides, a Pt plate and a silver chloride (Ag/AgCl) electrode were respectively utilized as the counter and reference electrodes. All (photo)electrochemical characterizations were conducted in the 0.5 M sulfuric acid (H2SO4) aqueous electrolyte (pH = 0.3) and recorded using an electrochemical workstation (760D, CH Instruments). For photocatalytic evaluations of CoSx/Si and CoSx|P/Si pyramids, a 500 W Xe lamp (Gloria-X500A, Zolix) equipped with an AM 1.5 filter was applied as solar simulation, and the intensity of incident illumination was retained at 100 mW cm−2. Linear sweep voltammetry (LSV) and that of photocathodes were performed from +0.55 V to −0.45 V (vs. RHE) with a scanning rate of 20 mV s−1. Furthermore, chronoamperometry was carried out under the applied bias of 0 V (vs. RHE). For electrochemical characterizations, LSV measurements of CoSx/Ti and CoSx|P/Ti electrodes were swept from +0.25 V to −0.45 V (vs. RHE) at a scanning rate of 5 mV s−1. In addition, these results were modified by iR and background corrections. Cyclic voltammetry (CV) at various scanning rates were conducted at the HER non-Faradaic potential range [between +0.1 and +0.2 V (vs. RHE)]. Differences of current densities between anodic and cathodic sweeps were linearly fitted with various scanning rates. The


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slope of the fitted line was equal to twice the geometric Cdl value of electrocatalyst.

materials were selected for subsequent characterizations because their catalytic performances were low, optimized, and moderate, respectively.

Characterization of materials Morphological information of photocathodes was observed by utilizing field-emission scanning electron microscopy (SEM) (JSM-6700F, JEOL). An X-ray diffraction (XRD) analyzer (D2 PHASER, Bruker) with Cu Kα radiation (λ = 1.54178 Å) was applied to investigate the crystal structure and crystallinity of the analytic materials. Reflectance and Raman spectra were collected through UV-Vis absorption spectrometer (EVOLUTION 220, Thermo) and Raman spectrometer (DXR microscope, Thermo) equipped with a 532 nm laser. Results and Discussion For convenience, we abbreviated Si pyramids decorated by P-doped CoSx with various doping concentrations as ‘‘CoSx|P-T/Si’’, in which T represents the S/P molar ratio of precursors for synthesizing the co-catalyst materials. Linearsweep voltammograms (LSV) of photocathodes were performed in a three-electrode cell in 0.5 M sulfuric acid (H2SO4) aqueous solution under solar simulation. Figure 2a reveals that bare Si exhibits no photoresponse at 0 V. This result indicates the low kinetics of the photogenerated carriers of pristine Si, leading to severe recombination of electron–hole pairs. The photocurrent of Si pyramids was initially generated at approximately −0.20 V and its current density reached −4.19 mA cm−2 at −0.45 V. In our previous work, Si microwire array was fabricated as a photoabsorber for solar hydrogen evolution through photolithography and dry etching techniques.13 However, Si pyramids in the presented study were prepared by utilizing a chemical wet etching method which is more facile for the real application in the future. Besides, the bare Si pyramids showed a better photocurrent, as compared with microwire structures. In this work, onset potentials of (photo)electrochemical characterizations are defined as the voltage at which the current density reaches −10 mA cm−2. As shown in Figure 2a, with respect to bare Si pyramids, the cocatalyst-modified Si photocathode presented an anodically shifted turn-on voltage. These results demonstrated that CoSx and CoSx|P efficiently transferred photoinduced carriers of Si pyramids to react with redox couples in the electrolyte. In Figure 2b, the onset potential and current density at 0 V of CoSx /Si were −0.047 V and −6.16 mA cm−2, respectively. After phosphorus atoms were doped into CoSx, turn-on voltage and photocurrent were further improved, as shown in Figure 2a. Photoelectrochemical performance was optimized by CoSx|P7/Si photocathode. Its optimal onset potential was 0.14 V and the current density was enhanced to −20.6 mA cm−2 at 0 V. However, with further doping of phosphorus heteroatoms into CoSx (T ≤ 3), the photocatalytic activities of CoSx|P/Si reduced. CoSx|P-1/Si pyramids showed poor turn-on voltage and current density (at 0 V), that is, 0.023 V and −11.5 mA cm−2, respectively. Besides, saturated photocurrent plateaus were observed in the CoSx/Si and CoSx|P/Si pyramids until the applied bias exceeded −0.15 V. The maximum saturated current density was −25.4 mA cm−2, which was achieved by CoSx|P3/Si. To analyze the effects of phosphorus dopants in CoSx cocatalysts on efficiencies, CoSx, CoSx|P-7, and CoSx|P-1

Figure 2. (a) Linear sweep voltammograms of bare Si, CoSx/Si, and CoSx|P/Si photocathodes under solar illumination. (b) Onset potential, photocurrent density at 0 V vs. RHE, and saturated photocurrent density of photocathode materials. In our previous reports, crystalline pyrite cobalt dichalcogenides (CoX2, where X = S or Se) were functioned as co-catalysts on Si microwires for photoelectrochemical hydrogen evolution.19,20 However, the work function of CoX2 materials showed a relatively negative energy level work function (vs. vacuum level) compared with the Fermi level of p-Si photoabsorber.29 Therefore, the band alignment induced a barrier between Si and co-catalysts contributed to poor fill factors of LSV results. Jin’s group prepared crystallized pyrite (CoPS) on p-Si photocathode for solar hydrogen evolution.26 The formation of an ohmic junction, which was attributed to an accumulation band bending generated by the work function of CoPS, caused no photoresponse at 0 V (vs. RHE) of this CoPS/Si material. However, Figure 2a showed a moderate fill factors of CoSx/Si and CoSx|P/Si photocathodes. We suggested that introducing dopants in amorphous catalysts modified their chemical and electronic structures.10 Consequently, electrochemical impedance spectroscopy (EIS) was applied to evaluate charge transfer kinetics of photocathodes, as show in Figure S1. In contrast to our previous work, barely one semicircle was observed in the Nyquist plots of CoSx/Si and CoSx|P/Si materials.19 We proposed that the charge transfer resistance of Si/co-catalyst interface was low, which was also investigated by a previous study.11 Besides, Figure S1 presented that the diameter of semicircle decreased in the order CoSx|P7/Si < CoSx|P-1/Si < CoSx/Si. This result indicated that CoSx|P7 catalysts showed optimal efficiency for accelerating the electron-transfer rate from Si to the electrolyte. The scanning electron microscopy (SEM) images (Figure 3a and S2a) show



that the width and height of bare Si pyramids are approximately 5–15 and 2–10 µm, respectively. Figure 3b to 3d reveal that the CoSx and CoSx|P co-catalyst modification resulted in no morphological variations on the underlying Si pyramids. Figure 3b and S2b reveal that CoSx particles dominantly aggregated on the bottom of Si pyramids. Given that phosphorus atoms were doped in CoSx, the decoration of the co-catalyst became more homogeneously distributed on the Si surface, as shown in Figure 3c to 3d and S2c to S2d. Furthermore, EDX mapping characterizations (Figure S3) showed the complementary results with SEM observations. This indicated diffusion lengths of photogenerated carriers were long in the bare CoSx, contributing to reduced improvement in charge separation on the Si photoelectrode. Notably, CoSx|P-1 co-catalyst showed more uniform deposition on Si pyramids than CoSx|P-7, but the CoSx|P-7/Si photocathode showed superior performance (Figure 2). We suggested that the inherent catalytic activity of the CoSx|P-7 material was optimal to compensate for its morphological defects.

Figure 3. Cross-sectional SEM images of (a) bare Si, (b) CoSx/Si, (c) CoSx|P-7/Si, and (d) CoSx|P-1/Si pyramids. The X-ray diffraction (XRD) spectra were obtained to investigate the crystallization of photocathode materials in Figure 4a. The strongest peak of Si at 2θ = 69° in the XRD spectra was avoided to be scanned. Two sharp peaks approximately at 2θ = 33° and 62° correspond to the diffraction of Si pyramids. Standards of CoS (JCPDS 65-3418), CoS2 (JCPDS 41-1471), and CoPS (JCPDS 27-0139) were employed to analyze the components of photoelectrode materials. However, even enlarging the baselines of XRD spectra, the strongest (102) diffraction peak of CoS or (200) of CoS2 and CoPS cannot be observed in the XRD spectra of CoSx/Si and CoSx|P/Si pyramids. In our previous study, molybdenum sulfide (MoS2) co-catalysts were decorated on Si microwires by the analogous method.13 The characteristic signals of MoS2 were detectable through XRD measurement. This result indicated the low loading amount of CoSx and CoSx|P co-catalyst or the amorphous crystalline structure, which will be further discussed in Raman characterization. Figure 4b shows the ultraviolet– visible (UV-Vis) reflectance spectra of CoSx/Si and CoSx|P/Si photocathodes. Visible light was absorbed by bare Si pyramids, whereas incident illumination with reduced wavelength (< 500 nm) was reflected. As CoSx and CoSx|P co-catalysts were modified on Si, the reflectance of the irradiation with the reduced wavelength decreased. This result showed that the light

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harvest capability of Si photocathode was improved to induce more carriers by depositing co-catalyst materials. Notably, the low discrepancy of UV-Vis reflectance spectra between CoSx/Si and CoSx|P/Si pyramids demonstrated that the absorption ability is not the dominant factor affecting the difference in photoelectrochemical performance in Figure 2. The Raman spectra of CoSx/Si and CoSx|P/Si photocathodes are presented in Figure 4c. A sharp peak is observed at about Raman shift of 519.9 cm−1, which is the characteristic vibration mode of the underlying Si substrate. Furthermore, a weak and broad peak was investigated in CoSx/Si pyramids at approximately 340 cm−1, corresponding to the Raman modes of the amorphous cobalt sulfide material in the previous study.28 Moreover, no characteristic Raman peaks of CoSx|P/Si were investigated in Figure 4c. This results were also discovered from synthesizing the crystalline MoS2 and amorphous MoSxCly electrocatalysts by Jin’s group.10 We suggested that as phosphorus heteroatoms were doped in CoSx, CoSx|P became highly amorphous, exposing increased active sites to generate solar hydrogen production.28 Therefore, with respect to CoSx/Si, CoSx|P/Si photocathode materials presented superior photocatalytic enhancement, as shown in Figure 2. To analyze the mechanisms of photocathodes driving the hydrogen evolution reaction, CoSx and CoSx|P materials were prepared on Ti foil to function as electrocatalysts for electrochemical characterizations. Electrocatalytic LSV measurements were performed by the same three-electrode configuration as solar photoelectrolysis. Figure S4a shows that CoSx/Ti electrode initially generated cathodic currents under an applied bias of approximately −0.15 V. Onset potential was approximately −0.34 V, as shown in Figure S4d. After doping phosphorus atoms in CoSx, the onset potentials of CoSx|P-7/Ti and CoSx|P-1/Ti anodically shifted to −0.20 and −0.22 V, respectively. Electrochemical LSV results were interpreted by Tafel plot to resolve the rate-determining step (RDS) in hydrogen generation in Figure S4b. The classic two-electron reactions of the hydrogen evolution mechanism are described in the two steps, as follows: a discharging step of the Volmer reaction (H3O+ + e− → Hads + H2O) followed by a desorption step of the Heyrovsky reaction (Hads + H3O+ + e−→ H2 + H2O) or a recombination of the Tafel reaction (Hads + Hads → H2), where Hads indicates the hydrogen atom adsorbed at the active site on the catalyst surface. Moreover, the catalyst with the Tafel slope of 116, 38, or 29 mV decade−1 demonstrated that the RDS in the hydrogen production process was the Volmer, Heyrovsky, or Tafel reactions, respectively. Figure S4b presents the Tafel plots of CoSx/Ti and CoSx|P/Ti electrodes. The Tafel slopes of CoSx, CoSx|P-7, and CoSx|P-1 electrocatalysts were separately 108, 53.1, and 55.2 mV decade−1. These results indicated that the HER rate of CoSx was dominantly decided by the discharging step, whereas that of the CoSx|P materials was determined through the desorption step. Furthermore, LSV measurements of CoSx/Ti and CoSx|P-7/Ti electrodes were carried out under dark and illuminated conditions, as shown in Figure S5. No photoresponse was observed from CoSx and CoSx|P-7 materials. This was also observed in our previous work, as marcasite CoSe2 embeddedC3N4 was deposited on fluorine-doped tin oxide (FTO) substrate for hydrogen evolution. LSV results showed the nearly similar performance under irradiation and in dark. Consequently, we suggested that CoSx and CoSx|P materials were only functioned as co-catalysts for accelerating the kinetics of photoinduced electrons on Si pyramids. Cyclic voltammograms (CV) of


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CoSx/Ti and CoSx|P/Ti electrodes were recorded in the nonfaradic potential range (0.1–0.2 V) at various scan rates in Figure S6. CV measurements were adopted to probe the electrochemical double-layer capacitance (Cdl), which was proportional to the electrochemically active surface area on electrocatalyst materials. Furthermore, the Cdl values of CoSx/Ti and CoSx|P/Ti electrodes are summarized in Figure S4c and S4d. The Cdl of CoSx/Ti is approximately 0.461 mF cm−2. However, the Cdl values of CoSx|P-7/Ti and CoSx|P-1/Ti respectively

increased to 0.760 and 0.905 mF cm−2 because the strongly amorphous structure exposed increased active surface areas to efficiently undergo hydrogen evolution. Notably, more HER active sites were observed on the CoSx|P-1/Ti electrocatalyst than on CoSx|P-7/Ti, but the CoSx|P-7/Ti material showed better (photo)electrochemical performances, as seen in Figure 2 and S4a. These results demonstrated that CoSx|P-7 showed an outstanding intrinsic catalytic activity.

Figure 4. (a) XRD, (b) UV-Vis reflectance, and (c) Raman spectra of bare Si, CoSx/Si and CoSx|P/Si photocathodes. By utilizing SEM and electrochemical measurements, we realized that CoSx|P-1 showed higher surface area and quantity of active sites but lower catalytic performance relative to CoSx|P-7. The band structures of co-catalyst materials seriously affected the capability to accept photoinduced electrons from the photoabsorber. Increased available electronic states in the co-catalysts may result in more efficient photoelectrochemical performance. Therefore, X-ray absorption spectra (XAS) were obtained to resolve the electronic structures of the co-catalysts. In our previous study, theoretical calculation of pyrite CoS2 (cattierite) revealed that the unoccupied electronic states were composed of Co 3d and S 3p orbitals.19 Besides, Sunkara’s group used the computational method to show that the unoccupied band structures of iron sulfide (FeS) nanotubes with the same crystal structure as jaipurite (CoS) were constructed by Fe 3d and S 3p orbitals.30 Therefore, we proposed that the unoccupied states of the amorphous CoSx and CoSx|P materials in this work were composed of Co 3d and S 3p (P 3p) orbitals. Consequently, the Co L-edge and S K-edge (P K-edge) XAS were applied to observe the unoccupied electronic states of CoSx and CoSx|P co-catalyst materials, which involved the transition from the 2p→3d and 1s→3p orbitals, respectively. Two peaks in the Co L-edge spectra corresponded to L2 (~795 eV) and L3 (~780 eV) absorption, as shown in Figure S7a. The L3 absorption intensity of CoSx|P co-catalysts was stronger than that of CoSx. This result indicates increased vacant sites generated in 3d orbitals as doping phosphorus heteroatoms into CoSx materials. The S K-edge absorption (Figure S7b) of CoSx located at approximately 2470 eV was in good agreement with

that of sulfide materials.19,27 Except of sulfide absorption, another broad sulfate peak was observed in CoSx|P co-catalysts at ~2480 eV. This was also discovered from XAS results of pyrite cobalt phosphosulfide electrocatalysts prepared by Wang’s group.27 Figure S7d shows the P K-edge absorption of CoSx|P materials. Peaks at approximately 2145 and 2155 eV agreed with phosphide and phosphate materials, respectively.31 These results presented that the surface of CoSx|P co-catalysts are susceptible to oxidation in air. However, the oxide byproducts were removed upon exposure to H2SO4 aqueous electrolyte21, leading to no contribution to the photocatalytic activity. Therefore, solar-driven carriers from Si pyramids only injected into the available electronic states of sulfide and phosphide components of CoSx|P co-catalysts. The sulfide absorption intensity (Figure S7c) increased in the order CoSx|P1 > CoSx|P-7 > CoSx. Besides, the edge jump of phosphide peaks in CoSx|P-1 was stronger in P K-edge absorption compared with the CoSx|P-7 photocathode, as shown in Figure S7e. Therefore, we proposed that CoSx doping with more phosphorus atoms produced more unoccupied states, which could accept more photogenerated carriers from Si pyramids. However, CoSx|P-7/Si exhibited the optimized photoconversion efficiency, as shown in Figure 2, indicating that the electronic states of the co-catalyst materials were not the dominant parameters dedicated to the performance of solar hydrogen evolution.



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comparable to that of platinum (~ −0.07 eV), as shown in Figure S11c.32 In the case of jaipurite derivatives (CoS, CoP0.5S0.5, and CoP0.125S0.875), the five-coordinated Co sites of CoS were the most stable sites for atomic hydrogen adsorption; however, the calculated GH* was 0.04 eV, indicating an endothermic process for atomic hydrogen adsorption at these sites (Figure S14). In the case of CoP0.5S0.5, the four-coordinated P sites were highly thermoneutral sites for hydrogen adsorption, with GH* of −0.17 eV, showing a thermodynamically stable adsorption of hydrogen atoms, as shown in Figure S15. As for CoP0.125S0.875, the five-coordinated Co sites were highly stable sites for hydrogen adsorption, with GH* of −0.29 eV. Notably, after hydrogen atoms were adsorbed at these five-coordinated Co sites, subsequent atomic hydrogen was adsorbed at the adjacent four-coordinated S and P sites with improved GH* of −0.14 and −0.07 eV, respectively (Figure S16). The DFT results of cattierite and jaipurite derivatives showed that phosphorus dopants with appropriate quantities improved the free energy of atomic hydrogen adsorption on cobalt sulfide materials, contributing to enhanced HER efficiency. However, an excess amount of P heteroatom doping may cause an excessively strong bond between H atoms and the catalyst, leading to poor consecutive desorption ability and weak electrocatalytic activity. Therefore, we suggested that the CoSx|P-7 with suitable phosphorus doping concentration possessed the optimized adsorption energy, resulting in higher catalytic efficiency compared with CoSx|P-1.

Figure 5. Free energy diagram of atomic hydrogen adsorption on different sites of (a) cattierite derivatives (CoS2, CoPS, and CoP0.25S1.75) and (b) jaipurite derivatives (CoS, CoP0.5S0.5, and CoP0.125S0.875). Hydrogen evolution activity of the catalyst is strongly correlated with the chemisorption energy of atomic hydrogen on the surface.32 Therefore, density functional theory (DFT) was used to calculate the free energy for atomic hydrogen adsorption (GH*) of cattierite (CoS2) and jaipurite (CoS) for evaluating the HER performance of the amorphous CoSx cocatalyst (see computational method for details in the supporting information). To observe the effect of phosphorus heteroatom doping on the HER efficiency of CoSx material, cattierite and jaipurite derivatives (with S/P molar ratio ＝ 1 and 7) were constructed for theoretical calculations. The free energy diagram of atomic hydrogen adsorption is summarized in Figure 5. For the DFT results of cattierite derivative (CoS2, CoPS, and CoP0.25S1.75), the GH* for hydrogen adsorption at different sites on the {100} surfaces of CoS2 and CoPS were analogous to those in the previous work26 because the same calculation method and computational settings were applied in this study. In the case of CoP0.25S1.75, the calculated GH* (0.29 eV) for hydrogen adsorption at the Co site was in between those of CoS2 (0.33 eV) and CoPS (0.27 eV), as shown in Figure 5a. This result revealed that phosphorus doping in CoS2 caused more favorable hydrogen adsorption at Co sites (Figure S11a). Furthermore, Figure S11b shows that the open P sites of CoP0.25S1.75 with GH* of −0.34 eV indicate more spontaneous hydrogen adsorption compared with that of CoPS (−0.16 eV). Specifically, after hydrogen was adsorbed at P sites of CoP0.25S1.75, the GH* of adjacent Co sites (−0.05 eV) became

Figure 6. Chronoamperograms (at 0 V) of CoSx|P-7/Si photocathodes with various thicknesses of TiO2 modification under solar illumination. Here, CoSx|P-7/Si pyramids with optimal photoelectrochemical activities were selected to measure chronoamperograms at 0 V under solar irradiation to analyze the photocatalytic durability. Figure 6 shows the initial current density of CoSx|P-7/Si of approximately −21.0 mA cm−2, which was complementary with LSV results in Figure 2. However, after chronoamperometric measurement for 30 min, the photocurrent seriously decayed to approximately −11.7 mA cm−2. When the CoSx|P-7/Si photocathode was immersed in a 5 wt.% HF solution, the photoresponse returned to the initial state but presented poor stability for the following 30 min characterization. This result indicated that oxide layer generation on the Si surface is the dominant factor contributing to this degradation, thereby resulting in the enhanced recombination of electron–hole pairs because of exposure to the aqueous electrolyte. As observed from the SEM images in Figure 3c, the surface around Si pyramidal tips was not protected by the modification of the CoSx|P-7 co-catalyst. To


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decrease the SiO2 generation on Si photocathode in the electrolyte, various thicknesses of TiO2 thin films functioning as passivation layers were decorated on Si pyramids using ALD system. The TiO2 layer prepared through ALD has been reported to possess pinholes in the previous works.33,34 Besides, Li et al. prepared sandwich structure TiO2/Pt/Si nanowires as photocathode for solar hydrogen production.35 The photoinduced electrons from the underlying Si substrate concentrated on these imbedded Pt nanoparticles, and further tunneled through TiO2 layer to reduce the protons in the electrolyte. These results allowed incompletely covered Si pyramids to contact co-catalysts in the presented study. Moreover, TiO2 protective layer synthesized by ALD permitted the passage of tunneling current from Si photoabsorber to CoSx|P materials. The original current density of CoSx|P-7/5 nm TiO2/Si photocathode was approximately −19.3 mA cm−2, which was slightly lower than that of pristine CoSx|P-7/Si. This was attributed to HER inert TiO2 material deposition, which led to the partially sacrificial photoconversion efficiency.18,35 After a chronoamperometric test for 30 min, the photocurrent of CoSx|P-7/5 nm TiO2/Si remained at −19.3 mA cm−2. Besides, its current density remained at −13.6 mA cm−2 after 1 h measurement, as shown in Figure 6. As the thickness of TiO2 protective layer increased to 10 nm, the photocatalytic degradation rate of CoSx|P-7/10 nm TiO2/Si photocathode further decreased. The photocurrent during the 1 h of characterization was approximately −13.3 mA cm−2. Nevertheless, the 15 nm TiO2 thin film on the Si surface not only sacrificed considerable photocurrents but decreased its protective capability. We proposed that this result was caused by the excessive TiO2 layer that provided the long diffusion length for photoinduced minority carriers, thereby leading to high possibility for recombination and photooxidation. SEM-EDX and XRD characterizations were further conducted for evaluating the morphology, composition, and structure of CoSx|P co-catalysts after driving photoelectrochemical hydrogen generation on Si photocathode. Figure S8 showed that CoSx|P-7 materials kept the amorphous structure after the chronoamperometric measurement. Besides, elemental mapping in Figure S9a showed CoSx|P-7 co-catalyst maintained a homogeneous distribution on Si pyramids upon the completion of chronoamperometric characterization. Figure S9b and S9c revealed EDX spectra of CoSx|P-7/Si photocathode before and after the chronoamperometric test. The elemental compositions were summarized in Table S1. The cation/anion ratio of CoSx|P-7 co-catalyst increased upon finishing the chronoampermetric analysis. We speculated that this was attributed to sulfate and phosphate materials, generated on the surface during synthesis process or exposure in air, dissolved in the liquid electrolyte.21 This result was also observed in XAS spectra (Figure S7). Notably, the photoelectrochemcial performance degradation of CoSx|P-7/Si photocathode recovered to the initial activity as HF acid etched the SiO2 layer, generated from the chronoamperometric measurement, on Si pyramids. Thus, we suggested that CoSx|P-7 was a durable cocatalyst for solar hydrogen evolution. During electrochemical HER measurements, CV scans were typically applied to pretreat the surface of electrocatalysts.36 However, the dissolution and redeposition of Pt counter electrode were severe while its repetitive oxidation and reduction. Consequently, these electrocatalysts effectively activated after CV cycles and yielded the extraordinary performance.37 Nevertheless, CV pretreatment was not conducted for photoelectrochemical

characterizations in this presented work. Furthermore, Pt materials were the optimal co-catalyst for solar hydrogen generation. Chronoamperograms (Figure 6) showed not only no activation for the efficiency, but slight degradation even by depositing TiO2 protective layer. EDX spectra (Figure S9) also showed that no Pt contamination on photocathode, after the chronoamperometric measurement. Therefore, we suggested that no redeposition from Pt counter electrode occurred on photoelelctrode in this study. Conclusions In summary, we demonstrated that the amorphous Pdoped CoSx efficiently functions as the co-catalyst for accelerating the kinetics of photogenerated carriers on Si photocathodes. Doping of phosphorus heteroatoms into CoSx produced increased vacancies in the unoccupied electronic states to accept photoinduced electrons. However, excess phosphorus dopants in CoSx caused excessively strong adsorption with H atoms. The CoSx|P-7 photocathode with appropriate phosphorus doping quantities showed an optimized photocurrent of −20.6 mA cm−2 at 0 V. By depositing a passivation layer, CoSx|P-7/10 nm TiO2/Si photocathode stably retained a current density of −13.3 mA cm−2 for 1 h of chronoamperometry.

ASSOCIATED CONTENT Supporting Information. Supporting Information Available ： Computational method and results of DFT theoretical calculations for cattierite and jaipurite derivatives, Nyquist plots of CoSx/Si and CoSx|P/Si photocathodes, SEM images and EDX spectra of CoSx/Si and CoSx|P/Si pyramids, LSVs, Tafel plots and double-layer capacitance of CoSx/Ti and CoSx|P/Ti electrodes, XAS spectra of CoSx/Si and CoSx|P/Si pyramids, XRD spectra of CoSx|P/Si photocathode after the chronoamperometric measurement.

AUTHOR INFORMATION Corresponding Authors * E-mails：[email protected] & [email protected] & [email protected]

ACKNOWLEDGMENT The authors are grateful for the financial support from Ministry of Science and Technology (Contract Nos. MOST 106-2112-M-003007-MY3 and MOST 107-2113-M-002-008-MY3). L. C. Yin thanks the National Science Foundation of China (No. 52472249) for the financial support and the NSFC-GD joint special fund for supercomputer science. The theoretical calculations in this work were performed on TianHe-1 (A) and TianHe-2 at National Supercomputer Centre in Tianjin and Guangzhou, respectively.

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Amorphous Phosphorus-Doped Cobalt Sulfide Modified on Silicon

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