Room temperature gas sensing characteristics of titanium dioxide nanostructures: effects of hydrochloric acid on the structure and magnetic properties
Tshabalala, Zamaswazi Portia
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Monitoring and detection of toxic and combustible gases such as methane in underground mining, carbon monoxide from burning coal in our homes and odourless gases such as nitrogen dioxide and sulphur dioxide in industries has become the subject of extensive scientific and technological research and this has been motivated by their harmful impact on the environment and human health. Early detection of these gases can help prevent fatal incidences such as fire, suffocation and death. Development of portable gas sensors with higher sensitivity and selectivity, fast response and recovery times, low detection limit and capability of operating at room temperature is one of the challenges facing researchers world-wide. Various materials such as semiconductor metal oxides (MOX), polymers, and carbon nanotubes have been used for gas sensing application. Among all the various materials, MOX such TiO2, ZnO, SnO2, Fe2O3, WO3 are the most preferred materials for gas sensing application due to their noticeable response to any change in electrical resistance when exposed to either reducing or oxidizing gas and also due to their unique properties such as high stability and easy to synthesize. However, among the range of MOX semiconductors mentioned above, TiO2 has emerged as the preferred MOX semiconductor for gas sensing application due to its remarkable features such as nontoxicity, biocompatibility, high photocatalytic activity and affordability. TiO2 occurs in three crystalline forms namely: anatase, rutile and brookite. Anatase and rutile polymorphs are widely studied for technological applications. Therefore, in this study, we investigated the gas sensing properties of TiO2 nanoparticles annealed at 450 C, and that annealed at various temperatures, as well as those doped with various concentrations of Mn. The undoped TiO2 nanoparticles were synthesized from P25 Degussa via a simple hydrothermal method in an aqueous solution of sodium hydroxide (NaOH). The samples were washed with distilled water (H2O) and different concentrations of hydrochloric acid (0.25, 0.5 and 1.0 M) which acted as the morphological controlling agent. TiO2 doped with various concentration of Mn2+ were washed using 1.0 M HCl. To investigate the effect of hydrochloric acid (HCl) as a washing agent on the structure, morphology, optical, magnetic and gas sensing properties, x-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) and Kinosistec gas sensor testing system were used for characterization. Microscopy analysis showed that the sizes of the pure TiO2 nanoparticles were reduced when increasing the HCl concentration indicating that the particle sizes could be easily tailored and tuned by adjusting the HCl concentration. Structural analyses revealed a phase transformation from a mixture of anatase and rutile phases to pure anatase phase at higher HCl concentration. The PL, XPS, EPR and BET analyses disclosed that the 1.0 M sample contained relatively high concentration of oxygen vacancy, Ti4+ and Ti3+ interstitial defects and it also had higher surface area which played an important role in transforming the sensing properties, resulting in higher sensing response, sensitivity and selectivity to NO2 at room temperature. Furthermore, the effect of thermal annealing was investigated on the structural and gas sensing properties of the pure TiO2 nanoparticles washed with H2O and HCl, and annealed at different temperatures (300, 450, 700 °C) in air. Surface morphology analyses revealed that the nanoparticles transformed to nanorods after annealing at 700 C. The results showed that the sensing properties are dependent on annealing temperature. The 1.0 M TiO2 nanostructures annealed at higher temperatures (700 C) revealed improved sensing response to CH4 gas at room temperature due to higher surface area of 180.51 m2g-1 and point defects related to Ti3+ observed from EPR and PL analyses. In addition, the 1.0 M TiO2 sensing material annealed at 700 C also revealed an interstitial defect states which played a vital role in modulating the sensing properties. To improve sensitivity, selectivity and stability of the gas sensing materials, various concentrations (1.0, 1.5, 2.0, 2.5 and 3.0 mol % denoted as S1, S2, S3, S4 and S5, respectively) of Mn-ions were loaded on the TiO2 particles. The nanoparticles were characterised in detail using various analytical techniques. XRD analysis showed that the structure of both pure and Mn-doped TiO2 was tetragonal and no peaks corresponding to Mn or impurities were observed. Raman spectroscopy revealed quenching and peak broadening due to lattice disorder with increasing concentration of Mn. Optical studies revealed that the Mn loaded TiO2 nanoparticles have enhanced UV-Vis emission and a broad shoulder at 540 nm denoting defects induced by substitution of Ti4+ ions by Mn2+. The XPS and the EPR results revealed the presence of Ti4+, Ti3+ and single ionised oxygen vacancies in both pure and Mn loaded nanoparticles. Additionally, a hyperfine split due to Mn2+ ferromagnetic ordering was observed confirming incorporation of Mn ions into the lattice. Gas sensing studies revealed that Mn2+ loaded TiO2 surface improved the NO2 and NH3 sensing performances in terms of response and selectivity. The S1 (1.0 mol. % Mn) demonstrated an improved sensitivity of approximaterly 85.39 ppm-1 at 20 ppm NH3 gas at room temperature. Our findings showed that, the thermal annealing and Mn doping improve the sensitivity and selectivity and stability of the gas sensing materials. The results also validated that our sensing materials are highly sensitive and selective to CH4, NO2 and NH3 at room temperature. The observed room temperature response in this work, suggests that these TiO2 nanostructures are possible candidates for gas sensing application in work places, mining sectors, etc. Moreover, the findings in this work give a possible solution to the issue of energy consumption of metal oxide gas sensors.