Conformal and Continuous Deposition of Bifunctional Cobalt Phosphide Layers on p-Silicon Photocathodes for Improved Solar Hydrogen Evolution

Abstract

p-Silicon (Si) has been extensively investigated in recent years as a promising photocathode for solar-driven hydrogen evolution. However, it remains challenging to fabricate Si photocathodes having both high photoelectrocatalytic activity and long-term operational stability through a simple and affordable approach. Herein, we report conformal and continuous deposition of a di-cobalt phosphide (Co2P) layer on lithography-patterned highly-ordered Si nanowire (SiNW) arrays via a cost-effective drop-casting method followed by a low-temperature phosphorization treatment [1]. The as-deposited Co2P layer consists of crystalline nanoparticles and has an intimate contact with SiNWs, forming a well-defined SiNW@Co2P core/shell nanostructure. The conformal and continuous Co2P layer endows dual functions: on the one hand, it serves as a highly-efficient catalyst capable of substantially improving the photoelectrocatalytic activity towards the hydrogen evolution reaction (HER); on the other hand, it can effectively passivate SiNWs to protect them from photo-oxidation, thus prolonging the lifetime of the electrode. As a consequence, when used for solar-driven hydrogen evolution, the SiNW@Co2P photocathode with an optimized Co2P layer thickness exhibits a high photocurrent density of -21.9 mA cm-2 at 0 V versus reversible hydrogen electrode (RHE) and excellent operational stability of up to 20 hours, outperforming many nanostructured silicon photocathodes reported in the literature. Furthermore, we fabricate inverted pyramid textured Si (SiIP) photocathodes coated with a conformal and continuous layer of Co2P [2], which show a photocurrent density as high as -35 mA cm-2 and can constantly split water up to 150 hours without obvious degradation. The combination of passivation and catalytic functions in a single continuous layer represents a promising strategy for designing high-performance semiconductor photoelectrodes for use in solar-driven water splitting, which may simplify the fabrication procedures and potentially reduce the production cost. Acknowledgement This work was funded by ERDF funds through the Portuguese Operational Programme for Competitiveness and Internationalization COMPETE 2020, and national funds through FCT – The Portuguese Foundation for Science and Technology, under the project “PTDC/CTM-ENE/2349/2014” (Grant Agreement No. 016660). The work is also partially funded by the Portugal-China Bilateral Collaborative Programme (FCT/21102/28/12/2016/S).