Photoelectrochemical water oxidation at electrophoretically deposited WO3 films as a function of crystal structure and morphology

• WO3 particles were synthesized using a solvothermal method.
• The crystal structure, morphology and texture of the WO3 material depend on experimental parameters.
• Films were electrophoretically deposited, and the photoelectrochemical properties were characterized.
• Monoclinic and an orthorhombic WO3 show better water oxidation capabilities than the hexagonal material.
• The photocurrent depends on the texture of the monoclinic material, and a morphology corresponding to platelets characterized by preferential (002) reflections give the best results.

Phase-pure WO3 materials with different crystal structures and morphologies are prepared using alcoholysis and partial hydrolysis under solvothermal conditions, and their promise for photoelectrochemical water oxidation is evaluated under simulated sunlight. The materials are obtained in the hexagonal, orthorhombic and monoclinic structures. Electrophoretic deposition under ambient conditions is used to prepare mechanically stable and well-adhered films on transparent conducting glass substrates. Photoelectrochemical characterization of the films illustrates that sufficient electrical interconnectivity is achieved; however, a heat treatment at 500 °C dramatically improves the photocurrent, which is related to the passivation of defects and traps of the WO3 materials. The orthorhombic and monoclinic materials, the latter depending on the morphology and texture, achieve the highest photocurrents corresponding to water oxidation, while the hexagonal material has the lowest photocurrents. The novel orthorhombic material prepared in this work shows good promise for water oxidation, also because of the low density of electron traps as inferred from experiments under modulated illumination using a chopper. The properties of the monoclinic material depend on the morphology, in particular the texture. The electrochemical properties for a rod morphology with texture defined by the (002) or (020) reflections are characterized by lower photocurrents and a higher trap density, while a square platelets type of morphology with a texture characterized by the (002) reflection results in the highest photocurrents. The results indicate that electrophoretically deposited films of WO3 materials with shapes corresponding to two or three dimensions have better photoelectrochemical properties than one dimensional shapes such as rods and wires.

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