H₂O Adsorption on WO₃ and WO_3-x (001) Surfaces

Area of Science:

• Materials Science
• Surface Chemistry
• Computational Chemistry

Background:

• Tungsten oxide (WO₃) is a semiconductor oxide with significant applications in photocatalysis.
• Understanding water-surface interactions is critical for optimizing WO₃-based photocatalytic systems.
• Surface defects, particularly oxygen vacancies, are known to influence the electronic and chemical properties of metal oxides.

Purpose of the Study:

• To investigate the adsorption and dissociation of water on clean and oxygen-deficient WO₃ (001) surfaces.
• To determine the preferred site and energy of oxygen vacancy formation on the WO₃ surface.
• To elucidate the impact of oxygen vacancies on water interaction and the resulting electronic structure of WO₃.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model water adsorption and dissociation.
• Calculations included the evaluation of oxygen vacancy formation energy on various surface configurations.
• The electronic structure changes due to oxygen vacancies and water interaction were analyzed.

Main Results:

• The preferred oxygen vacancy formation occurs at the terminal oxygen atom along the c-axis, with lower energy compared to the bulk.
• Oxygen vacancies induce a semiconductor-to-metal transition, consistent with experimental observations of n-type conductivity.
• Water primarily adsorbs in a molecular, undissociated form on Lewis acidic W sites, with moderate interaction energy.
• Oxygen vacancies minimally influence the water adsorption process.
• Water desorption from a hydroxylated surface is an efficient route to generate a reduced WO₃ surface.

Conclusions:

• The study provides a detailed atomistic understanding of water interaction with clean and defective WO₃ surfaces.
• Oxygen vacancies significantly alter the electronic properties of WO₃, leading to metallic behavior, but do not strongly affect water adsorption.
• Water desorption from hydroxylated surfaces offers a pathway for surface reduction with implications for photocatalysis and electronic device applications.