Sunlight-harnessing and storing heterojunction TiO2/Al2O3/WO3 electrodes for night-time applications

Heterojunction TiO2/Al2O3/WO3 (TAW) photoelectrodes were fabricated to harvest sunlight and simultaneously store the photogenerated electrons for prolonged periods. The insertion of an Al2O3 (A) blocking layer between the TiO2 and WO3 layers enhances the open circuit potentials (OCPs) of TiO2/WO3 (TW) in 0.1 M Na2SO4 upon light-on (photo-charging). Surprisingly, Al2O3 increases the residence time of the OCPs at sufficiently negative potentials (<-0.3 V-SCE) by 4-16 times depending on the Al2O3 content, even after AM 1.5 G light-off (discharging). Such prolonged discharging periods are further verified by a comparison of the electron lifetimes, which shows that the photogenerated electrons at TAW survive 6 times longer than those at TW. When Cr6+ is spiked into the solution during the discharging period, the change in OCP is accelerated, and simultaneously Cr6+ is reduced to Cr3+. The number of electrons stored and utilized for Cr6+ reduction is estimated to be around 1.3 x 10(17), which is almost tripled by the insertion of Al2O3. The Al2O3 effect is similarly found in Ag+ reduction with 4 times more electrons, but is less pronounced for the reduction of methylene blue and polyoxometalates due to both their reduction potentials being so close to the WO3 conduction band level and re-oxidation pathway by dissolved oxygen. Al2O3 appears to effectively block the transfer of trapped electrons at WO3 to TiO2, making the electrons more available for the reduction of substrates. Various surface analyses (SEM, EDX, XPS, and UV-Vis) were completed to examine and compare single, binary, and ternary component electrodes (T, W, TA, TAW, etc.). On the basis of the surface and electrochemical study, the effects of Al2O3 on the charging and discharging processes are discussed in detail.