# Highly Sensitive and Selective H\({}_{2}\)S Chemical Sensor Based on ZnO Nanomaterial
## 2 Materials and Methods  
ZnO nanomaterial was prepared based on our previously reported method with a few modifications, namely the anodization time and the applied potential [55]. First, metallic zinc films were deposited on 2 mm \(\times\) 2 mm \(\times\) 0.75 mm alumina substrates by means of radio-frequency (13.56 MHz) magnetron sputtering. The sputtering target was metallic zinc (purity 99.99%, 101.6 mm diameter, 6 mm thick, CAS number: 7440-66-6, Materion Advanced Chemicals Inc., Milwaukee, USA). The sputtering power and the time were 75 W and 35 min, respectively. To improve the adhesion of metallic films, the temperature of the alumina substrates was kept at 300 \({}^{\circ}\)C during the sputtering process. To perform the crystalline analysis of the materials, metallic zinc films were deposited on silicon substrates using the aforementioned sputtering regimes. Then, the metallic films were electrochemically anodized in a Teflon cell using a two-electrode system. The electrolyte solution was 2 M oxalic acid dihydrate (C\({}_{2}\)H\({}_{2}\)O\({}_{4}\)\(\cdot\)2H\({}_{2}\)O, ACS reagent, \(\geq\)99.5%, CAS number: 6153-56-6, Sigma-Aldrich, Inc., Steimhem, Germany) containing ethanol. A platinum foil was used as the cathode and the applied voltage during the anodization procedure was 20 V. The anodic oxidation procedure was performed at room temperature (RT) for 20 min. The anodized material was zinc oxalate dihydrate (ZnC\({}_{2}\)O\({}_{4}\)\(\cdot\)2H\({}_{2}\)O). As-prepared materials were transformed to crystalline ZnO nanostructures by thermal decomposition process under a 50 vol% O\({}_{2}\)/50 vol% Ar atmosphere at 500 \({}^{\circ}\)C for 8 h. To perform the gas sensing measurements, platinum electrodes with interdigital geometry were deposited onto the surface of obtained structures by means of radio-frequency magnetron sputtering. To carry out the sensing tests at different temperatures, a platinum heater was deposited on the backside of the substrates.  
The crystal structure of the prepared material was studied with the X-ray diffraction spectroscopy (XRD) technique, using an Empyrean diffractometer (PANalytical, Almelo, The Netherlands) mounting a Cu-LFF (\(\lambda\) = 1.5406 A) tube operated at 40 kV and 40 mA. The morphologies of the materials were examined by means of a LEO 1525 scanning electron microscope (SEM) equipped with a field emission gun (Carl Zeiss SMT AG, Oberkochen, Germany).  
The gas sensing properties of the material were studied by a flow-through technique in a computer-controlled thermostatic test chamber. The detailed description of the experimental setup for the gas sensing measurements has been reported [56]. The carrier gas was humid synthetic air with a flow rate of 0.2 l/min and a relative humidity of 40%. Before the purging of analyte gas to the test chamber, the samples were stabilized for 10 h at each operating temperature. The conductance of samples was monitored by means of the volt-amperometric technique and the applied voltage was 1 V. The response (S) of structures was calculated according to the typical convention for the n-type semiconductor material: \(\mathrm{S=\frac{\left(G_{f}-G_{0}\right)}{G_{0}}=\frac{\Delta G}{G_{0}}}\), where \(G_{0}\) is the sample conductance in air and \(G_{f}\) is the sample conductance in the presence of analyte gas [16; 57].