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 (lambda > 420 nm) and store photogenerated electrons for possible use during periods of unavailable sunlight. The overall film thickness is approximately 50-60 mu m, 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 (similar to 0.15 V) in similar to 20 h of darkness. Neither CdS nor WO3 alone exhibits such behavior. The electron lifetimes (t) of CdS and WO3 are each less than 100 s, whereas coupling of the two increases tau to similar to 2500 s at the E-OCP. In the absence of dissolved O-2, tau further increases, suggesting that O-2 is the primary electron acceptor. In spite of oxic conditions, CdS/WO3 is capable of continuously reducing Cr6+ to Cr3+ and Ag+ to Ag-0 after removal of visible light. The number of utilized (i.e., stored) electrons in the reductions of Cr6+ and Ag+ are estimated to be similar to 1.08 x 10(17) and similar to 0.87 x 10(17), 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.