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

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.