Temperature-Dependent Spectroscopic Characterization and Computational Study of Tin Dioxide for Gas-Sensing Applications

Abstract

Powders and thin fi lms composed of tin dioxide (SnO2) are promising candidates for various high-impact applications. Despite the prevalence of the material in such studies, it remains crucial that commercially available materials meet the quality demands of the industries to which these materials would most benefi t. Regardless of a wide range of studies that have already been done to characterize SnO2 material, we still need an understanding of its thermal behavior, especially over a wide range of temperatures. Numerous characterizing techniques have been employed in the current study to assess the quality of various samples, such as SnO2 powder and thin films of thickness, 41 nm, 78 nm, 97 nm, 373 nm, and 908 nm, on a UV-Quartz (SiO2) substrate. Structural characterization techniques, such as X-ray Diffraction (XRD) spectroscopy assisted us in confi rming the rutile tetragonal structure of SnO2 powder and films and to calculate their lattice parameters, crystallite size, and interplanar distance. The other technique that we used extensively is Raman spectroscopy at different temperatures to fully understand SnO2 powder and film behaviors and test their stability in harsh thermal environments. SnO2 powder sample was tested at increasing temperatures from -193 C to 400 C where the Raman-active vibrational modes (i.e., A1g, B2g, and Eg) experience a wavenumber red-shift and linewidth broadening due to the anharmonicity effect. While increasing and then decreasing the temperature, the Raman-active vibrational modes of SnO2 powdered sample were reproducible. At high temperatures above 300 C, IR, and forbidden modes become more pronounced due to increased defect density or disorder in the material. While testing SnO2 lm samples of different thicknesses and comparing them with bare quartz, we observed two IR-active modes (E(1)u (TO) and E(3)u (LO)) and one forbidden mode (A2g) of SnO2 at the highest thickness of 908 nm film. When increasing the temperature above 200 C of the 908 nm film sample, the SnO2 features disappear and merge with the quartz (SiO2) features. However, when decreasing the temperature to low temperature (i.e., -193 C), a new SnO2 Raman-active peak and an IR-active peak appear that are associated with the A1g band and E(3)u (LO)) band, respectively. No overall trend of wavenumber shift and linewidth broadening were noted due to oxygen vacancies or contamination. Other optical characterization methods, such as Ultraviolet-visible (UV-VIS) spectroscopy, was used to calculate the band gap of SnO2 film samples, and Fourier-Transform Infrared (FT-IR) spectroscopy was employed to further investigate the IR-active modes of SnO2 powder and thin lms. While testing the SnO2 powder sample, A2u(TO) and B1u features are observed, which are associated with IR-active mode and forbidden mode, respectively. The SnO2 908 nm lm sample shows an IR-active peak (i.e., A2u(TO)) that is overlapped with quartz IR features. In order to investigate the electronic state and chemical composition of SnO2 powder land film, we utilized X-ray Photoelectron Spectroscopy (XPS), a highly surface-sensitive technique. The SnO2 covers the quartz substrate evenly in the film samples due to minimal silicon (Si) electrons relative to the tin (Sn) electrons, and SnO2 powder has less carbon (C) and nitrogen (N) groups than the film samples. Imaging techniques, such as Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS) analysis and Atomic Force Microscopy (AFM), were exploited to investigate SnO2 powder and film samples' surface morphology, chemical composition, and topography, respectively. We con firmed the reduction of the spherical grains' size when comparing powdered and films of SnO2 and the increase in surface roughness for the thicker film (908 nm) by 3.4 nm. Finally, we used Molecular Dynamics (MD) simulation to examine how well SnO2 can adsorb CO2 in a gas mixture of CO2 and N2 at various temperatures. We determined that CO2 is adsorbed more than N2 at temperatures ranging from -193 C to 400 C. However, the reduced adsorption energy of CO2 is highest at -133 C, which is due to N2 having a lower critical temperature than CO2.