Undoped CeO₂ and 0.50–5.00 mol% Fe-doped CeO₂ nanoparticles were prepared by a homogeneous precipitation combined with homogeneous/impreganation method, and applied as photocatalyst films prepared by a doctor blade technique. The superior photocatalytic performances of the Fe-doped CeO₂ films, compared with undoped CeO₂ films, was ascribed mainly to a decrease in band gap energy and an increase in specific surface area of the material. The presence of Fe³⁺ as found from XPS analysis, may act as electron acceptor and/or hole donor, facilitating longer lived charge carrier separation in Fe-doped CeO₂ films as confirmed by photoluminescence spectroscopy. The 1.50 mol% Fe-doped CeO₂ film was found to be the optimal iron doping concentration for MO degradation in this study.

The present work describes the effects of water on Fe-doped nanoparticulate CeO₂, produced by flame spray pyrolysis, which is a critical environmental issue because CeO₂ is not stable in typical atmospheric conditions. It is hygroscopic and absorbs ~29 wt % water in the bulk when exposed to water vapor but, more importantly, it forms a hydrated and passivating surface layer when immersed in liquid water. In the latter case, CeO₂ initially undergoes direct and/or reductive dissolution, followed by the establishment of a passivating layer calculated to consist of ~69 mol % solid CeO₂·2H₂O and ~30 mol % gelled Ce(OH)₄. Under static flow conditions, a saturated boundary layer also forms but, under turbulent flow conditions, this is removed. While the passivating hydrated surface layer, which is coherent probably owing to the continuous Ce(OH)₄ gel, would be expected to eliminate the photoactivity, this does not occur. This apparent anomaly is explained by the calculation of (a) the thermodynamic stability diagrams for Ce and Fe; (b) the speciation diagrams for the Ce⁴⁺-H₂O, Ce³⁺-H₂O, Fe³⁺-H₂O, and Fe²⁺-H₂O systems; and (c) the Pourbaix diagrams for the Ce-H₂O and Fe-H₂O systems. Furthermore, consideration of the probable effects of the localized chemical and redox equilibria owing to the establishment of a very low pH (<0) at the liquid-solid interface also is important to the interpretation of the phenomena. These factors highlight the critical importance of the establishment of the passivating surface layer and its role in photocatalysis. A model for the mechanism of photocatalysis by the CeO₂ component of the hydrated phase CeO₂·2H₂O is proposed, explaining the observation of the retention of photocatalysis following the apparent alteration of the surface of CeO₂ upon hydration. The model involves the generation of charge carriers at the outer surface of the hydrated surface layer, followed by the formation of radicals, which decompose organic species that have diffused through the boundary layer, if present.

Hydrogen sensing of thick films of nanoparticles of pristine, 0.2, 1.0 and 2.0 atomic percentage of Pt concentration doped ZnO were investigated. ZnO nanoparticles doped with 0.2–2.0 at.% Pt were successfully produced in a single step by flame spray pyrolysis (FSP) technique using zinc naphthenate and platinum(II) acetylacetonate as precursors dissolved in xylene. The particle properties were analyzed by XRD, BET, SEM and TEM. Under the 5/5 (precursor/oxygen) flame condition, ZnO nanoparticles and nanorods were observed. The crystallite sizes of ZnO spheroidal and hexagonal particles were found to be ranging from 5 to 20 nm while ZnO nanorods were seen to be 5–20 nm wide and 20–40 nm long. ZnO nanoparticles paste composed of ethyl cellulose and terpineol as binder and solvent respectively was coated on Al₂O₃ substrate interdigitated with gold electrodes to form thin films by spin coating technique. The thin film morphology was analyzed by SEM technique. The gas sensing properties toward hydrogen (H₂) was found that the 0.2 at.% Pt/ZnO sensing film showed an optimum H₂ sensitivity of ~164 at hydrogen concentration in air of 1 volume% at 300 °C and a low hydrogen detection limit of 50 ppm at 300 °C operating temperature.

Copper oxide (CuO) nanoparticles were successfully synthesized by a thermal method. The CuO nanoparticles were further characterized by thermogravimetric analysis (TGA), differential thermal analysis (DTA), X‐ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X‐ray spectrometry (EDS), and high resolution transmission electron microscopy (HRTEM), respectively. The specific surface area (SSA_BET) of CuO nanoparticles was determined by nitrogen adsorption. The SSA_BET was found to be 99.67 m²/g (d_BET of 9.5 nm). The average diameter of the spherical CuO nanoparticles was approximately 6–9 nm.

Undoped TiO₂ and TiO₂ nanoparticles doped with 1–5 at.% Nb were successfully produced in a single step by flame spray pyrolysis (FSP). The phase and crystallite size were analyzed by XRD. The BET surface area (SSA_BET) of the nanoparticles was measured by nitrogen adsorption. The trend of SSA_BET on the doping samples increased and the BET equivalent particle diameter (d_BET) (rutile) increased with the higher Nb-doping concentrations while d_BET (anatase) remained the same. The morphology and accurate size of the primary particles were further investigated by high-resolution transmission electron microscopy (HRTEM). The crystallite sizes of undoped and Nb-doped TiO₂ spherical were in the range of 10–20 nm. The sensing films were prepared by spin coating technique. The mixing sample was spin-coated onto the Al₂O₃ substrates interdigitated with Au electrodes. The gas sensing of acetone (25–400 ppm) was studied at operating temperatures ranging from 300–400 °C in dry air, while the gas sensing of ethanol (50–1,000 ppm) was studied at operating temperatures ranging from 250–400 °C in dry air.