Mesoporous In 2 O 3 nanocubes were achieved in this present work following a transformation from hierarchical structures of mesoporous In(OH) 3 nanocubes synthesized hydrothermally. Appreciable control on the morphology of In(OH) 3 nanostructures was attained by optimizing the reactant concentration, ratio of structure directing agent in the reaction medium (water/PEG ratio), the reaction temperature, and time. The synthesized samples were characterized extensively by XRD, TG-DTA, micro-Raman, and UV-DRS. Surface area and pore size distribution were determined from N 2 adsorption and desorption isotherms. Morphological evaluations carried out using electron microscopy in scanning (FE-SEM) and transmission (TEM) mode not only provided information on the size and shape of the materials but also revealed the hierarchical assembly consisting of primary and secondary structures. Controlled studies as a function of various reaction parameters and the morphological evolution observed are rationally correlated to propose a plausible formation mechanism. Further, hydrogen gas sensing properties (sensitivity, sensor response, and recovery time) of the asprepared In 2 O 3 nanostructures (nanocubes, nanobricks, nanoflakes, and nanoparticles) were investigated to demonstrate the influence of morphology. Owing to the porous structures and large surface area, In 2 O 3 nanocubes exhibit superior sensitivity with short response/recovery times at concentrations as low as 100 ppb. Surface decoration with Pd nanoparticles activates these nanocubes, promoting excellent sensing response and selectivity toward hydrogen at room temperature.

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A representative physical model depicting the H2S-sensing mechanism in air and H2S environments. Selectivity of the NiOBNG sensor to different interfering gases.

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Hierarchical mesoporous InO nanocubes and nitrogen-doped reduced graphene oxide-indium oxide nanocube (In) composites were prepared for carbon monoxide (CO) sensing. The as-synthesized materials were systematically investigated by different characterization techniques such as field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, thermogravimetic analysis, X-ray photoelectron spectroscopy, micro-Raman, Fourier transform infrared spectroscopy, and photoluminesce analysis. The obtained results are consistent with each other. The CO-sensing characteristics of the InO nanocubes and In composites were examined at different operating temperatures (35 °C < T < 300 °C) and CO concentrations (1-1000 ppm). Owing to their large surface-to-volume ratio and porosity, the InO nanocubes exhibited a superior sensitivity with a detection limit of 1 ppm at 250 °C. Furthermore, to enhance the sensing characteristics and reduce the operating temperature, a composite of NrGO and InO nanocubes was fabricated. The incorporation of NrGO drastically improved the sensing performance of the InO nanocubes, showing an excellent sensitivity (S ∼ 3.6-5 ppm of CO at ∼35 °C) with appreciably fast response (Γ ∼ 22 s) and recovery (Γ ∼ 32 s) times. The sensing studies supported by the structural and morphological material characteristics lead to the plausible sensing mechanism proposed.