Joint Effects of Photoactive TiO₂ and Fluoride-Doping on SnO₂ Inverse Opal Nanoarchitecture for Solar Water Splitting

Inverse opal (IO) films of tin dioxide (SnO2) were fabricated on polystyrene (PS) beads (diameter=350 nm (±20 nm) with a spin coating method. To compensate for the large band gap (Eg=3.8 eV), a thin TiO2 shell was deposited on the SnO2-IO films with atomic layer deposition (ALD), which produced shells with thicknesses of 10-40 nm. The morphological changes and crystalline properties of the SnO2 and TiO2-coated SnO2 (herein after referred to as TiO2/SnO2) IO films were investigated with field-emission scanning electron microscopy and X-ray diffraction, respectively. The photoelectrochemical (PEC) behavior of the samples was tested in a 0.1 M KOH solution under 1 sun illumination (100 mW/cm2 with an AM 1.5 filter). The highest PEC performance was obtained with the TiO2(10 nm)/SnO2 IO films, which produced a photocurrent density (Jsc) of 4.67 mA/cm2 at 0.5 V (vs NHE) and was sequentially followed by the TiO2(20 nm)/SnO2-IO, TiO2(30 nm)/SnO2-IO, TiO2 (40 nm)/SnO2-IO and SnO2 IO films. Overall, the thin TiO2 shell covered on the SnO2-IO core enhanced Jsc by 3 orders of magnitude, which in turn the PEC activity. This is mainly ascribed to the extremely low charge-transfer resistance (Rct) in the photoelectrode/electrolyte and at the TiO2/SnO2 interface, as well as the contribution of the photoactive TiO2 layer, which has an Eg of 3.2 eV. Moreover, to improve the electrical conductivity of the core SnO2 IO film, the films were doped with 10 mol % of F. The F- doped films were labeled as the FTO IO film. The Rct of the FTO-IO films decreased because of the improved electronic conductivity, enhancing the PEC performance of the TiO2(10 nm)/FTO-IO films by approximately 20%. The core-shell nanowire mesh nanoarchitecture is therefore suggested to provide an insight for designing the peculiar structure based on the material's properties and the engineering of their band gap energy for highly efficient PEC performance.