Role of WO3 Layers Electrodeposited on SnO2 Inverse Opal Skeletons in Photoelectrochemical Water Splitting

Tin dioxide (SnO2) inverse opals (IOs) were developed by a spin-coating-assisted sol-gel process. The SnO2 IOs exhibited a pore size of similar to 260 nm in the 370 nm sized polystyrene bead (PS) templates. Electrodes having a WO3 layer with a band gap (E-g) of 2.6 eV were electrodeposited on the SnO2 IOs under a constant potential (-0.47 V-Ag/AgCl), where the thickness of the WO3 layer depended on the applied charge amount for WO3 electrodeposition (200-800 mC/cm(2)). As a control sample, a pure WO3 IO film with the same thickness of similar to 3.1 mu m was also prepared by electrodeposition. The pore diameter of the SnO2 IO structure declined noticeably as the deposited charge amount of the WO3 layer increased from 200 to 800 mC/cm(2), leading to eventual coverage of the SnO2 IO structure with the WO3 (800 mC/cm(2)) layer. Moreover, X-ray diffraction analysis indicated a steady increase of the signal intensity of the monoclinic WO3 planes as the deposited charge amount of the WO3 layer increased, indirectly indicating an increased loading amount of the WO3 layer. However, the optimum photoelectrochemical (PEC) response was achieved with the SnO2/WO3 (600 mC/cm(2)) IO electrode, which exhibited the highest photocurrent density (J(sc)) of 2.8 mA/cm(2) under full-sun conditions and 0.91 mA/cm(2) under visible light, indicating that the enhancement of the J(sc) under visible light contributed significantly to the improvement of the total J(sc), compared with the values for the pure SnO2, SnO2/WO3 (200, 400, and 800 mC/cm(2)), and WO3 IO electrodes. Furthermore, the favorable cascading band alignment between the SnO2 and WO3 layers promoted rapid charge separation and charge transport through the conductive SnO2 IO skeleton. Therefore, the heterojunction, formed from the highly conductive SnO2 core layer and visible-light-absorbing WO3 shell layer, can boost the PEC activity by complementary combination of the unique advantages of the materials.