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Layers of ZnO micro-nano structures were deposited on quartz substrates and subsequently plasma treated in O2 and N2. It was found that exposure to O2 plasma enhanced gas response to ethanol vapor of the ZnO layers by a factor 8, while exposure to N2 plasma deteriorated the gas response. This enhancement of the gas response upon O2 plasma treatment was attributed to the formation of nanocrystallites of ZnO on the original ZnO micro-nano structures which form the gas sensing layers. The surface chemical state of the original sample was found to be not affected by the O2 plasma while it was modified by the N2 treatment.