Solar Visible Light Storage on WO3-CdS Heterojunction
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CdS and WO3 (CdS/WO3) bilayer film electrodes are fabricated to harness solar visible light (l > 420 nm) and store the photogenerated electrons for possible use during the unavailable periods of sunlight. The overall film thickness is approximately 50 - 60 mm while CdS underlayer is slightly thinner compared to WO3 due to a packing effect. The energetics of CdS and WO3 determined by optical and electrochemical analyses indicates cascaded electron transfer from CdS to WO3. The open circuit potential (EOCP) of CdS/WO3 under visible light (ca. -0.35 V vs. SCE) is nearly maintained even upon light-off, with marginal decrease (~0.15 V) in the dark period over 20 h. Neither CdS nor WO3 exhibit such behavior. The electron lifetimes (t) of CdS or WO3 alone are less than 100 sec, whereas coupling of CdS and WO3 increases t to ~2,500 sec at EOCP. In the absence of dissolved O2, t is further increased, suggesting that O2 is the primary electron acceptor. In spite of oxic condition, CdS/WO3 is found to be capable of continuously reducing Cr6+ to Cr3+ and Ag+ to Ag0, respectively, over 6 h after visible light-off. 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 a visible light absorber in the wavelength range of 420 to ca. 565 nm, while transferring the photogenerated electrons to WO3. The electrons stored at WO3 are gradually released to electron acceptors with suitable redox potentials.
