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

By Kim, Seonghun; Park, Yiseul; Kim, Wooyul; Park, Hyunwoong
Published in Photochem. Photobiol. Sci. The Royal Society of Chemistry 2016

Abstract

CdS and WO3 (CdS/WO3) bilayer film electrodes are fabricated to harness solar visible light ([small lambda] > 420 nm) and store photogenerated electrons for possible use during periods of unavailable sunlight. The overall film thickness is approximately 50-60 [small 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 (EOCP) 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]0.15 V) in [similar]20 h of darkness. Neither CdS nor WO3 alone exhibits such behavior. The electron lifetimes ([small tau]) of CdS and WO3 are each less than 100 s, whereas coupling of the two increases [small tau] to [similar]2500 s at the EOCP. In the absence of dissolved O2, [small tau] 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 [similar]1.08 [times] 1017 and [similar]0.87 [times] 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.

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