The Effect of Pressurized Carbon Dioxide in Cathodic Deposition of Metal Oxide Films
Authors: Chang Tso-Fu Mark , Chen Chun-Yi , Lin Wei-Hao , Hsu Yung Jung , Sone Masato 
Published: February 2020

As an important class of materials with a multitude of potential applications, metal oxide thin films and methods for their preparation have gathered a lot of attention. The synthetic approaches include hydrothermal methods, cathodic deposition, anodic oxidation, sol-gel method, and spray pyrolysis. Among them, cathodic deposition offers a low-cost yet effective process for production of metal oxides with controllable morphology and crystal structure. For a typical cathodic deposition, hydroxyl ions are generated by polarizing the substrate cathodically, which then causes a concentration gradient of hydroxyl ions from the substrate surface to the bulk solution. Since the concentration of hydroxyl ions is locally high at the substrate surface, metal ions initially dissociated in the electrolyte would combine with the hydroxyl ions and subsequently produce metal hydroxides and/or oxides on the surface of the substrate. By employing cathodic deposition, thin films of metal oxides such as TiO2 and SnO2 can be directly deposited on conductive substrates with complicated surface geometry. Such a feasibility of obtaining TiO2 and SnO2 thin films has significant implications in the aspects of photoelectrochemical applications, H2O2 sensor, and anode material for Li-ion batteries.
However, the as-deposited TiO2 and SnO2 are usually amorphous and an additional heat treatment process, for example, annealing at 400 °C for 1 hr, is needed to obtain crystalline TiO2 and SnO2. The need of the post-heat treatment limits selection of the substrates in cathodic deposition of the metal oxides, which further hinders the applicability of the products. Therefore, it is practically significant if crystalline metal oxides can be obtained directly from cathodic deposition without the post-heat treatment. On the other hand, the effect of pressure on crystallization of metal oxides deposited from the solution phase is rarely investigated. In a previous study, grain size of the TiO2 cathodically deposited with a supercritical CO2 (sc-CO2) emulsified electrolyte (SCE) was found to be increased with an increase in the pressure, which a post-heat treatment at 400 °C for 1 hr was still needed to have crystalline TiO2 [1].
Supercritical CO2 (sc-CO2) is CO2 at temperature and pressure above its critical point. Sc-CO2is non-polar, therefore, the solubility of metal salts and electrical conductivity are both very low in sc-CO2. In order to conduct electrochemical reactions, a surfactant is used to form emulsified electrolyte. In this study, the effect of the sc-CO2 emulsified electrolyte (SCE) on proproperties, especially the crystallinity, of the TiO2 and SnO2 thin films deposited cathodically will be reported.
Details of the high pressure electrochemical apparatus can be found in a previous study [2]. CO2 with a minimum purity of 99.9% was used. The TiO2 electrolyte was composed of 0.47 M NaCl, 25 mM TiCl3, and 75 mM NaNO3. The SnO2 electrolyte was composed of 25 mM SnCl2 and 75 mM NaNO3. For cathodic deposition of both TiO2 and SnO2, 80 vol.% of the aqueous electrolyte was used. For the TiO2, a constant current density and temperaure of 2.5 A/dm2 and 40°C, respectively, were used. For the SnO2, a constant current density and temperaure of 2.5 A/dm2 and 40°C, respectively, were used, and the pressure was varied from atmospheric pressure to 35 MPa. For the SnO2, a constant current density of 1 A/dm2 was used. The tempreaure was varied from 70 to 100°C, and the pressure was vaired from atmospheric pressure to 25 MPa.
For both the TiO2 and SnO2 thin films cathodically deposited at elevated pressure, an improvement in the crystallinity with an increase in the pressure was observed from the X-ray diffraction analysis. The high-resolution transmission electron microscopy and selected-area electron diffraction confirmed that the TiO2 deposited at 35 MPa had the anatase crystal structure. The electrochemical impedance spectroscopy also showed an enhancement in the charge carrier dynamics at the TiO2 electrode/electrolyte interface with an increase in the pressure, which is a result of the improved crystallinity. The effect of pressure on crystallinity of the SnO2 was more obvious than that on the TiO2. Grain size of the SnO2 films estimated by the Scherrer equation was found to increase from 3.47 to 14.82 nm with an increase in the pressure from 5 to 25 MPa.

References

[1] T.F.M. Chang, W.H. Lin, Y.J. Hsu, T. Sato, M. Sone, Electrochem. Commun. 33 (2013) 68-71.
[2] T.F. M. Chang, M. Sone, A. Shibata, C. Ishiyama, Y. Higo, Electrochim. Acta, 55 (2010) 6469-6475.