Mechanism for the Growth of Thin Films of WO3 and Bronzes from Suspensions of WO3 Nanoparticles

WO₃ is an oxide with semiconducting and electrocatalytic properties, of high interest in a number of applications. It may exist in different hydrates, like WO₃.2H₂O and WO₃.H₂O. In aqueous solution, it may be reduced reversibly to bronzes (WO₃H_x) and lower valence oxides (WO_3-x), exhibiting different physical and chemical properties, like colour, hydrophobicity, and conductivity (protonic and electronic). In combination with platinum, it forms a mixed compound, Pt-WO₃, with interesting possibilities as catalyst for fuel cell electrodes.

Thin films of WO₃ can be grown electrochemically from aqueous solutions containing dissolved W precursors. Another possibility is the growth from WO₃ suspensions formed from insoluble tungsten compounds. The electrochemical growth from WO₃ suspensions is a process that requires the diffusion and sticking of the suspended (nano-) particles on the electrode surface.

In this communication, the growth of WO₃ films and derived compounds, from aqueous suspensions of WO₃ is studied with the help of the quartz crystal microbalance (EQCM). WO₃ suspensions are prepared from acidic WO₄^2- solutions, and deposited on gold and Pt substrates. Film growth rates by stepping and cycling the electrode potential are analysed with EQCM.

Fig. 1 contains steady state deposition rate and electrode current on a Pt substrate. For potentials above 0.15 V exists just an attachment of WO₃ particles leading to a constant, low, growth rate. Film grown in this regime tends to come off the electrode surface after some minutes. Below 0.15 V, growth rates increases linearly up to -0.2 V. The same figure shows that the film growth rate has close resemblance with the logarithm of stationary reductive current density due to reduction of water (H⁺ + e^- --> 0.5H₂). The acceleration of film growth rate in parallel with water reduction appears to indicate that hydrogen reduced species on the Pt surface participate in the electrochemical reduction of WO₃ suspensions. Such possibility is probable in the light of the well known H-spillover mechanism from Pt to WO₃ phase. Reactions for film growth can be proposed in this way:

Pt + H⁺ + e^- --> Pt-H_sp

[WO₃]_sus + xPt-H_sp --> WO₃H_x + Pt

 

The growth mechanism is analysed on the light of EQCM results, thin films are characterized (SEM, XPS), and the stability of the films in aqueous solutions is also analysed in view of their possible application as an electrocatalyst component for fuel cell electrodes.

Acknowledgements

This work is financed by the project ELECTROFILM, MAT2011-27151 from Ministry of Science and Innovation of Spain.