Reversible Intercalation of Protons in Tungsten Oxide Films for Multi-Transduction Sensor Applications

Tungsten oxide films under electrochemical bias, can form tungsten oxide bronzes (M x WO 3 with M being small cations such as H + or Li + ) featuring high electronic conductivity and change in coloration to deep blue 1 . This doping process occurs through the reduction of W 6+ , as described by the following equation: WO 3 + x H + + xe - -> HxWO3 The bronze phase can also be formed through exposure of tungsten oxide to H 2 at high temperatures (> 200 o C) or in presence of catalysts 2 , as described by the following equation: WO 3 + x/2 H 2 -> H x WO 3 Because of the significant changes in both optical and electrical proprieties of the films, electrochemical control of proton doping can be used as a strategy for the development of ion-gated transistors 3,4 , hydrogen sensors and smart windows 5 . In this work, we report the use of thin films prepared by sol-gel synthesis using polyperoxotungstic acid (PPTA) as a precursor. We used three different types of substrates (conductive transparent fluorine-doped tin oxide (FTO) glass, Si/SiO2 patterned with Au source-drain electrodes and Interdigitated electrodes patterned on transparent glass) to study the changes of color and conductivity following the formation of the bronze in both electrochemical and dry environments. In electrochemical environment, we show that gating by ionic liquid in transistor configuration can be used to propose an iontronic sensor to detect H 2 and H 2 O 6 . In dry environment, we show that simultaneous monitoring of visible light transmittance and electrical conductivity can be used a double transducer for H 2 sensors 6 . 1 M. S. Whittingham, in Solid State Ionics , 2004, vol. 168, pp. 255–263. 2 C. C. Chan, W. C. Hsu, C. C. Chang and C. S. Hsu, Sensors and Actuators, B: Chemical , 2011, 157 , 504–509. 3 M. Barbosa, X. Meng, F. Soavi, C. Santato and M. O. Orlandi, J Mater Chem C Mater , 2018, 6 , 1980–1987. 4 G. V. de Oliveira Silva, A. Subramanian, X. Meng, S. Zhang, M. S. Barbosa, B. Baloukas, D. Chartrand, J. C. Gonzáles, M. O. Orlandi, F. Soavi, F. Cicoira and C. Santato, J Phys D Appl Phys , 5 G. Cai, J. Wang and P. S. Lee, Acc Chem Res , 2016, 49 , 1469–1476. 6 M. S. Barbosa, R. A. da Silva, C. Santato and M. O. Orlandi, Journal of Vacuum Science & Technology A , 2022, 40 , 013202.