Engineering Heterogeneous NiS₂/NiS Cocatalysts with Progressive Electron Transfer from Planar p‐Si Photocathodes for Solar Hydrogen Evolution

Abstract

The sluggish transfer of electrons from a planar p‐type Si (p‐Si) semiconductor to a cocatalyst restricts the activity of photoelectrochemical (PEC) hydrogen evolution. To overcome such inefficiency, an elegant interphase of the semiconductor/cocatalyst is generally necessary. Hence, in this work, a NiS2/NiS heterojunction (NNH) is prepared in situ and applied to a planar p‐Si substrate as a cocatalyst to achieve progressive electron transfer. The NNH/Si photocathode exhibits an onset potential of +0.28 V versus reversible hydrogen electrode (VRHE) and a photocurrent density of 18.9 mA·cm−2 at 0 VRHE, as well as a 0.9% half‐cell solar‐to‐hydrogen efficiency, which is much superior compared with those of NiS2/Si and NiS/Si photocathodes. The enhanced performance for NNH/Si is attributed to the contact between the sectional n‐type semiconducting NNH and the planar p‐Si semiconductor through a p‐Si/n‐NiS/n‐NiS2 manner that functions as a local pn‐junction to promote electron transfer. Thus, the photogenerated electron is transferred from p‐Si to n‐NiS within NNH as the progressive medium, followed by to Ni2+ and/or S22− of the defect‐rich n‐NiS2 phase as the key active sites. This systematic work may pave the way for planar Si‐based PEC applications of heterogeneous metal sulfide cocatalysts through the progressive transfer of electrons.

High-Performance NiS2 Hollow Nanosphere Cathodes in Magnesium-Ion Batteries Enabled by Tunable Redox Chemistry

“…In the high-resolution Ni 2p spectra (Figure c), the binding energies at 871.2 and 853.7 eV are assigned to Ni 2p 1/2 and Ni 2p 3/2 of Ni 2+ , respectively . The peaks for 6-NiS 2 and 4-NiS 2 hollow nanospheres have higher binding energies, indicating the anionic-rich feature of 8-NiS 2 hollow nanospheres. , In terms of S 2p spectra (Figure d), the binding energies of 8-NiS 2 hollow nanospheres present at 161.7 and 162.8 eV are associated with (S 2 ) 2– , and the peaks at 163.8 and 163.9 eV are related to the existence of (S n ) 2– . − In comparison, the corresponding peaks of S 2p in 4-NiS 2 and 6-NiS 2 hollow nanospheres have been shifted to higher binding energies, corresponding with the trend seen on Ni 2p. To detect the anionic distribution in NiS 2 hollow nanospheres, XPS of Ni 2p was conducted with different etching times of 0, 120, 300, and 600 s. For 4-NiS 2 and 6-NiS 2 , a more noticeable shift in Ni 2p XPS spectra was observed with increasing etching time compared to 8-NiS 2 (Figure e–g).…”

Surface/Interface Engineering of Si-Based Photocathodes for Efficient Hydrogen Evolution

“…Our group recently synthesized an n-type semiconducting NiS 2 /NiS heterojunction (NNH) cocatalyst on planar p-Si. 81 Such NNH/Si photocathode showed a photocurrent of −18.9 mA cm −2 at 0 V RHE and an onset potential of 0.28 V RHE in 0.5 M H 2 SO 4 . The enhanced PEC-HER performance benefits from the heterogenetic NNH nanostructure assisted by a progressive transfer system to promote photogenerated electron transfer, in which electrons are transferred from p-Si to the NiS phase, and then to the NiS 2 phase (Figure 7a).…”

“…This result can be explained by NiS handing more metallicity and electronic structural stability during the catalytic reaction. Our group recently synthesized an n-type semiconducting NiS 2 /NiS heterojunction (NNH) cocatalyst on planar p-Si . Such NNH/Si photocathode showed a photocurrent of −18.9 mA cm –2 at 0 V RHE and an onset potential of 0.28 V RHE in 0.5 M H 2 SO 4 .…”

“…(a) Schematic illustration of NNH/Si photocathode with progressive electron transfer. Reprinted with permission from ref . Copyright 2021 Wiley-VCH.…”

Recent trends in photoelectrochemical water splitting: the role of cocatalysts

“…As a result, the Ni Ni-based hybrid materials are also promising cocatalysts. For instance, a NiS 2 /NiS heterojunction (NNH) was deposited on planar Si to achieve progressive electron transfer 71 . The NNH/Si photocathode delivered able PEC performance compared to both the NiS 2 /Si and NiS/Si photocathodes in 0.5 M H 2 SO 4 .…”