A gas reactor device includes a plurality of microcavities or microchannels defined at least partially within a thick metal oxide layer consisting essentially of defect free oxide. Electrodes are arranged with respect to the microcavities or microchannels to stimulate plasma generation therein upon application of suitable voltage. One or more or all of the electrodes are encapsulated within the thick metal oxide layer. A gas inlet is configured to receive feedstock gas into the plurality of microcavities or microchannels. An outlet is configured to outlet reactor product from the plurality of microcavities or microchannels. In an example preferred device, the feedstock gas is air or O2 and is converted by the plasma into ozone (O3). In another preferred device, the feedstock gas is an unwanted gas to be decomposed into a desired form. Gas reactor devices of the invention can, for example, decompose gases such as CO2, CH4, or NOx.