MOF-Derived Bifunctional Iron Oxide and Iron Phosphide Nanoarchitecture Photoelectrode for Neutral Water Splitting

Abstract

The rational design of effective and inexpensive electrocatalysts for solar-powered water splitting is under intense focus to overcome barriers during half-cell reactions. Towards addressing this issue, we designed and sequentially synthesized Fe2O3 and FeP nanoparticles encapsulated on a carbonaceous matrix nanoarchitecture (Fe2O3@C and FeP@C) from an Fe-based 1,4-benzenedicarboxylate framework (Fe-MIL-88B) through high-temperature pyrolysis followed by a solid-/gas-phase low-temperature phosphidation process. These nanoparticles were employed as bifunctional electrocatalysts deposited onto a Si photoelectrode assembly. The changes in morphological and electronic properties of the as-prepared catalysts were investigated after controlled heat treatment. As-synthesized Fe2O3@C/Si and FeP@C/Si were found to be superior bifunctional photoelectrodes in a neutral aqueous medium under simulated solar irradiation of 100 mWcm(-2). Fe2O3@C/Si exhibited high activity for the oxygen evolution reaction (OER), providing a photoanodic current density of 2.5 mAcm(-2) at 1.65 V (vs. RHE), which was driven by the type-II heterojunction model in the Fe2O3@C/Si system. In parallel, the FeP@C/Si materials exhibited noticeable hydrogen evolution reaction (HER) activity, generating 10 mAcm(-2) cathodic current at -0.07 V (vs. RHE). The varied performance could be attributed to the bulk size dependency of the crystalline Fe2O3 phase on the conductive sp(2)-hybridized carbon framework and an intrinsic synergetic effect in the FeP@C, which originates from electronic interactions between Fe and P with high porosity, and which permits easy diffusion of the electrolyte and efficient electron transfer during hydrogen generation.