Temperature-dependent gas sensor application of chromium oxide structure

Abstract

Optical, topographic, and structural characterization of chromium oxide thin film grown with the magnetron RF sputter technique was performed separately as pre-annealing and post-annealing in this study. Likewise, gas sensor measurement was made separately before and after annealing. The crystal structure of the films was analyzed by X-ray diffractometer in the θ-2θ Bragg Brentana configuration (CuK_α (λ = 1.5405 Å). The films were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM) in tapping mode, UV–Vis photospectroscopy, and X-ray photoelectron spectroscopy (XPS) with an Al anode used as the X-ray source. As-grown chromium oxide thin films were obtained at 450 °C with the RF magnetron sputter system with 350 nm thickness and 150 W power. The band gap of the annealed sample at 550 °C was 3.02 eV, while the band gap of the unannealed sample was 3.66 eV. An unknown chromium oxide crystal structure was seen in the unannealed sample. A rhombohedral Cr₂O₃ structure was seen in the annealed sample. Raman spectra of annealed Cr₂O₃ thin film on glass showed the best peak shape of the Cr₂O₃ A1g mode at 550 cm⁻¹. Raman spectra of unannealed (as-grown) CrO thin film on glass showed the best peak shape of the CrO A1g mode at 842 cm⁻¹. By fitting the XPS data of the chromium oxide structure annealed at 550 °C, peaks were observed at the values of 530.46–531 eV for O1s_1/2 and 574.04–578.45 eV for Cr2p_3/2. Finally, while the as-grown Cr₂O₃ structure in gas sensor measurements reacted to hydrogen gas at 200 °C and 300 °C, the chromium oxide structure annealed at 550 °C did not react to the gas at all three temperature values. As-grown Cr₂O₃ structure is a suitable material for a gas sensor, while the material annealed Cr₂O₃ structure is more resistant to corrosion.