Harnessing and storing visible light using a heterojunction of WO₃ and CdS for sunlight-free catalysis

Abstract

CdS and WO₃ (CdS/WO₃) bilayer film electrodes are fabricated to harness solar visible light (λ > 420 nm) and store photogenerated electrons for possible use during periods of unavailable sunlight. The overall film thickness is approximately 50–60 μm, while the CdS underlayer is slightly thinner than WO₃ owing to a packing effect. The energetics of CdS and WO₃ determined by optical and electrochemical analyses enables cascaded electron transfer from CdS to WO₃. The open circuit potential (E_OCP) of CdS/WO₃ under visible light (approximately −0.35 V vs. SCE) is nearly maintained even in the absence of light, with a marginal decrease (∼0.15 V) in ∼20 h of darkness. Neither CdS nor WO₃ alone exhibits such behavior. The electron lifetimes (τ) of CdS and WO₃ are each less than 100 s, whereas coupling of the two increases τ to ∼2500 s at the E_OCP. In the absence of dissolved O₂, τ further increases, suggesting that O₂ is the primary electron acceptor. In spite of oxic conditions, CdS/WO₃ is capable of continuously reducing Cr⁶⁺ to Cr³⁺ and Ag⁺ to Ag⁰ after removal of visible light. The number of utilized (i.e., stored) electrons in the reductions of Cr⁶⁺ and Ag⁺ are estimated to be ∼1.08 × 10¹⁷ and ∼0.87 × 10¹⁷, respectively. The primary role of CdS is to be a visible-light absorber in the 420–565 nm wavelength range, transferring the photogenerated electrons to WO₃. The electrons stored in WO₃ are gradually released to electron acceptors with suitable redox potentials.