Ultrathin black phosphorus as a pivotal hole extraction layer and oxidation evolution co-catalyst boosting solar water oxidation

Abstract

Inefficient minority carrier extraction and sluggish oxygen evolution reaction (OER) kinetics are two principal contradictions restricting photoelectrochemical water oxidation. Here, the aforementioned restrictions could be simultaneously alleviated by coupling ultrathin black phosphorus (BP) nanosheets, taking the monoclinic WO₃ nanoflake array as a prototype. Derived from the unequal work functions, a large built-in electric field was formed, and the surface hole extraction efficiency was improved from 41.7 to 70.9% at 1.23 V. The efficient carrier exchange, in turn, inhibited self-oxidation of the exfoliated BP nanosheets and resulted in fine durability during the 12 h of stability testing. Moreover, density functional theory calculations revealed that coordinatively unsaturated phosphorus atoms intensified affinity to O species intermediates and lowered the activation energy, thereby reducing the OER overpotential. Consequently, a low onset potential of 0.21 V and a high photocurrent density of 1.88 mA cm⁻² were achieved. This work paves a way to rationally design the harmonious coexistence of BP-based electrocatalysts and photoanodes to achieve enhanced solar energy conversion.