In this work a cold atmospheric pressure plasma jet was mounted above a distilled water reservoir to study the influence of a water surface on reactive oxygen and nitrogen species (RONS) generation. The RONS, namely O 3 and NO 2 , were measured by multi-pass cell Fourier transform infrared absorption spectroscopy in the far-field of the jet. A shielding gas device was used to control the ambient environment of the effluent. The liquid surface has nearly no influence on the RONS dynamics as a function of shielding gas composition. It was found, however, that the interaction with the liquid surface increases the density of NO x by a factor of 2-3 due to local increase in H 2 O concentration through evaporation of water from the liquid surface. In contrast to the case of ambient humidity increase, this locally higher H 2 O density distinctly influences the ratio of water to oxygen and nitrogen which results in a higher generation of OH molecules.

The undoped and metal‐doped zinc oxide columnar films (ZnO:Sn, ZnO:Fe, ZnO:Ag, and ZnO:Cu) are covered with an ultra‐thin layer of SiO2 (10–20 nm). The electrical, UV, and volatile organic compounds (VOCs) sensing properties are evaluated under different ambient conditions for ≈7 months to investigate the impact of the top SiO2‐layer on the long‐term stability of samples. The obtained results show a high immunity of sensing properties of SiO2‐coated samples to humidity. Furthermore, gas sensing measurements show that the loss in response after 203 days is significantly lower for coated samples indicating higher stability of sensing performance. For ZnO:Fe the gas response is reduced by about 90% after 203 days, but for SiO2‐coated ZnO:Fe columnar films the gas response is slightly reduced by only 38%. The density functional theory (DFT) calculations show that water species bind strongly with the surface SiO2 layer atoms with a −0.129 e− charge transfer, which is, much higher compared to the interaction with ethanol and acetone. Calculations show strong binding of water species on the SiO2 layer indicating preferential absorption of water molecules on SiO2. The obtained results demonstrate an important role of the top SiO2 ultra‐thin layer in order to produce humidity‐tolerant sensitive devices.

Zinc oxide has widespread use in diverse applications due to its distinct properties. Many of these applications benefit from controlling the morphology on the nanoscale, where for example gas sensing is strongly enhanced for high surface-to-volume ratios. In this work the formation of novel ZnO nanobrushes by plasma etching treatment as a new approach is presented. The morphology and structure of the ZnO nanobrushes are studied in detail by transmission and scanning electron microscopy. It is revealed that ZnO nanobrush structures are fabricated by self-patterned preferential etching of ZnO microtetrapods in a hydrogen–acetylene plasma. The etching process was found to be most effective at 1% C2H2 admixture. Nanowire arrays are formed enabled by sidewall passivation due to a-C:H deposition. The nanobrush structures are further stabilized by simultaneous deposition of a SiO x layer from the opposite direction. Highly sensitive (gas response S = 148), selective, and fast (response time 15 s, recovery time 6 s) hydrogen sensors are fabricated from single nanobrushes. Single nanobrush sensors show enhanced sensing performance in increased gas response S of at least 10 times and improved response as well as recovery times when compared to nonporous single ZnO nanorod sensors due to the small diameters (≈50 nm) of the formed nanowires as well as the strongly enhanced surface-to-volume ratio of the nanobrushes by a factor of more than 10.