A relatively new approach to the design of photocatalytic and gas sensing materials is to use the shape-controlled nanocrystals with well-defined facets exposed to light or gas molecules. An abrupt increase in a number of papers on the synthesis and characterization of metal oxide semiconductors such as a TiO₂, α-Fe₂O₃, Cu₂O of low-dimensionality, applied to surface-controlled photocatalysis and gas sensing, has been recently observed. The aim of this paper is to review the work performed in this field of research. Here, the focus is on the mechanism and processes that affect the growth of nanocrystals, their morphological, electrical, and optical properties and finally their photocatalytic as well as gas sensing performance.

Thin-film n-n nanoheterostructures of SnO₂/TiO₂, highly sensitive to NO₂, were obtained in a two-step process: (i) magnetron sputtering, MS followed by (ii) Langmuir-Blodgett, L–B, technique. Thick (200 nm) SnO₂ base layers were deposited by MS and subsequently overcoated with a thin and discontinuous TiO₂ film by means of L–B. Rutile nanopowder spread over the ethanol/chloroform/water formed a suspension, which was used as a source in L–B method. The morphology, crystallographic and electronic properties of the prepared sensors were studied by scanning electron microscopy, SEM, X-ray diffraction, XRD in glancing incidence geometry, GID, X-ray photoemission spectroscopy, XPS, and uv-vis-nir spectrophotometry, respectively. It was found that amorphous SnO₂ films responded to relatively low concentrations of NO₂ of about 200 ppb. A change of more than two orders of magnitude in the electrical resistivity upon exposure to NO₂ was further enhanced in SnO₂/TiO₂ n-n nanoheterostructures. The best sensor responses R_NO2/R₀ were obtained at the lowest operating temperatures of about 120 °C, which is typical for nanomaterials. Response (recovery) times to 400 ppb NO₂ were determined as a function of the operating temperature and indicated a significant decrease from 62 (42) s at 123 °C to 12 (19) s at 385 °C A much smaller sensitivity to H₂ was observed, which might be advantageous for selective detection of nitrogen oxides. The influence of humidity on the NO₂ response was demonstrated to be significantly below 150 °C and systematically decreased upon increase in the operating temperature up to 400 °C.

This paper treats a problem of nonstoichiometry inTiO2−ythin films deposited by reactive sputtering at controlled sputtering rates. Ion beam techniques, Rutherford backscattering (RBS), and nuclear reaction analysis (NRA) along with X-ray photoelectron spectroscopy have been applied to determine a deviation from stoichiometryyin the bulk and at the surface ofTiO2−ylayers. The critical review of these experimental methods is given. Defect structure responsible for the electrical resistivity of rutileTiO2is discussed.

Titanium dioxide photoanodes for hydrogen generation suffer from a profound mismatch between the optical absorption of TiO₂ and the solar spectrum. To solve the problem of low solar-to-chemical efficiency, optically active materials are proposed. In this work, TiO₂ thin films containing erbium were deposited by radio frequency RF magnetron sputtering under ultrahigh vacuum conditions UHV. Morphology, structural, optical and electronic properties were studied. TiO₂:Er thin films are homogenous, with uniform distribution of Er ions and high transparency over the visible VIS range of the light spectrum. However, a profound 0.4 eV blue shift of the fundamental absorption edge with respect to undoped TiO₂ was observed, which can be attributed either to the size effect due to amorphization of TiO₂ host or to the onset of precipitation of Er₂Ti₂O₇ nanocrystals. Near-infrared NIR to VIS up-conversion is demonstrated upon excitation at 980 nm, while strong green photoluminescence at 525 and 550 nm occurs upon photon absorption at 488 nm.