Optical resonance and charge transfer behavior of patterned WO3 microdisc arrays

Abstract

One- to three-dimensional alignments of semiconductors on the micro- or nanoscale have been achieved to tailor their opto-physicochemical properties and improve their photoelectrochemical (PEC) performance. Here, to the best of our knowledge, we report for the first time the fabrication of vertically aligned, well-ordered WO₃ microdisc arrays via an electrodeposition process on lithographically patterned indium tin oxide (ITO) substrates as well as their geometry-specific photoelectrochemical properties. The as-fabricated WO₃ microdisc arrays exhibit enhanced light absorption as well as facilitated charge separation, leading to significantly higher PEC performance than WO₃ films. A finite-difference time-domain simulation of a single WO₃ microdisc indicates that strong optical resonances occur particularly in the central part of the microdisc, leading to enhanced optical absorption. A time-resolved photoluminescence study further reveals that the average lifetime of charge carriers (τ) in a microdisc array is shorter than that in a film by ∼60%. The reductively deposited Au particles are localized on the side of the microdisc and ITO substrate, which suggests that the photogenerated electrons are transferred to the same location. In addition, the oxidative deposition of FeOOH particles on the top surface and side of a microdisc indicates hole transfer pathways at the same location. This downward transfer of electrons and upward transfer of holes lead to efficient charge separation, and the radial direction appears to be the most preferred shortcut for the carriers inside the bulk of a microdisc.