A hydrogen-doped helium atmospheric pressure plasma jet (APPJ) is shown to be effective for the chemical reduction of metal oxides. Copper and tin oxide films (CuO and SnO2) show rapid (<2 seconds) and complete reduction to zero valence metal after exposure to the plasma jet, as revealed by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and Raman spectroscopy. After a total residence time of the plasma jet of 100 seconds, titanium oxide (TiO2) produced a surface decorated with Ti2+, Ti3+ and Ti4+ with proportions of 16, 38 and 46 atom%, respectively, as determined by XPS peak integration. Similarly, with tungsten oxide (WO3), after exposure for a few seconds, W5+ was produced, yielding a deep blue electrically conductive coating. The treatment of these oxide films by this dielectric radio frequency (RF) barrier discharge plasma jet provides a level of redox conversion not seen in any other technique, particularly for TiO2, especially with a comparable power input. The precise nature of the reduction is unclear; however, the involvement of free electrons may have an important role in the reduction process
