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

Tin dioxide (SnO 2 ) inverse opals (IOs) were developed by a spin-coating-assisted sol–gel process. The SnO 2 IOs exhibited a pore size of ∼260 nm in the 370 nm sized polystyrene bead (PS) templates. Electrodes having a WO 3 layer with a band gap ( E g ) of 2.6 eV were electrodeposited on the SnO 2 IOs under a constant potential (−0.47 V Ag/AgCl ), where the thickness of the WO 3 layer depended on the applied charge amount for WO 3 electrodeposition (200–800 mC/cm 2 ). As a control sample, a pure WO 3 IO film with the same thickness of ∼3.1 μm was also prepared by electrodeposition. The pore diameter of the SnO 2 IO structure declined noticeably as the deposited charge amount of the WO 3 layer increased from 200 to 800 mC/cm 2, leading to eventual coverage of the SnO 2 IO structure with the WO 3 (800 mC/cm 2 ) layer. Moreover, X-ray diffraction analysis indicated a steady increase of the signal intensity of the monoclinic WO 3 planes as the deposited charge amount of the WO 3 layer increased, indirectly indicating an increased loading amount of the WO 3 layer. However, the optimum photoelectrochemical (PEC) response was achieved with the SnO 2 /WO 3 (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 SnO 2, SnO 2 /WO 3 (200, 400, and 800 mC/cm 2 ), and WO 3 IO electrodes. Furthermore, the favorable cascading band alignment between the SnO 2 and WO 3 layers promoted rapid charge separation and charge transport through the conductive SnO 2 IO skeleton. Therefore, the heterojunction, formed from the highly conductive SnO 2 core layer and visible-light-absorbing WO 3 shell layer, can boost the PEC activity by complementary combination of the unique advantages of the materials.