This review addresses the importance of Zn for obtaining multifunctional materials with interesting properties by following certain preparation strategies: choosing the appropriate synthesis route, doping and co-doping of ZnO films to achieve conductive oxide materials with p- or n-type conductivity, and finally adding polymers in the oxide systems for piezoelectricity enhancement. We mainly followed the results of studies of the last ten years through chemical routes, especially by sol-gel and hydrothermal synthesis. Zinc is an essential element that has a special importance for developing multifunctional materials with various applications. ZnO can be used for the deposition of thin films or for obtaining mixed layers by combining ZnO with other oxides (ZnO-SnO₂, ZnO-CuO). Also, composite films can be achieved by mixing ZnO with polymers. It can be doped with metals (Li, Na, Mg, Al) or non-metals (B, N, P). Zn is easily incorporated in a matrix and therefore it can be used as a dopant for other oxidic materials, such as: ITO, CuO, BiFeO_3, and NiO. ZnO can be very useful as a seed layer, for good adherence of the main layer to the substrate, generating nucleation sites for nanowires growth. Thanks to its interesting properties, ZnO is a material with multiple applications in various fields: sensing technology, piezoelectric devices, transparent conductive oxides, solar cells, and photoluminescence applications. Its versatility is the main message of this review.

The aim of the present study was the development of Nb-doped ITO thin films for carbon monoxide (CO) sensing applications. The detection of CO is imperious because of its high toxicity, with long-term exposure having a negative impact on human health. Using a feasible sol–gel method, the doped ITO thin films were prepared at room temperature and deposited onto various substrates (Si, SiO₂/glass, and glass). The structural, morphological, and optical characterization was performed by the following techniques: X-ray diffractometry (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV/Vis/NIR spectroscopic ellipsometry (SE). The analysis revealed a crystalline structure and a low surface roughness of the doped ITO-based thin films. XTEM analysis (cross-sectional transmission electron microscopy) showed that the film has crystallites of the order of 5–10 nm and relatively large pores (around 3–5 nm in diameter). A transmittance value of 80% in the visible region and an optical band-gap energy of around 3.7 eV were found for dip-coated ITO/Nb films on SiO₂/glass and glass supports. The EDX measurements proved the presence of Nb in the ITO film in a molar ratio of 3.7%, close to the intended one (4%). Gas testing measurements were carried out on the ITO undoped and doped thin films deposited on glass substrate. The presence of Nb in the ITO matrix increases the electrical signal and the sensitivity to CO detection, leading to the highest response for 2000 ppm CO concentration at working temperature of 300 °C.

Nitrogen-doped ZnO (ZnO:N) thin films, deposited on Si(100) substrates by RF magnetron sputtering in a gas mixture of argon, oxygen, and nitrogen at different ratios followed by Rapid Thermal Annealing (RTA) at 400 °C and 550 °C, were studied in the present work. Raman and photoluminescence spectroscopic analyses showed that introduction of N into the ZnO matrix generated defects related to oxygen and zinc vacancies and interstitials. These defects were deep levels which contributed to the electron transport properties of the ZnO:N films, studied by analyzing the current–voltage characteristics of metal–insulator–semiconductor structures with ZnO:N films, measured at 298 and 77 K. At the appliedtechnological conditions of deposition and subsequent RTA at 400 °C n-type ZnO:N films were formed, while RTA at 550 °C transformed the n-ZnO:N films to p-ZnO:N ones. The charge transport in both types of ZnO:N films was carried out via deep levels in the ZnO energy gap. The density of the deep levels was in the order of 10¹⁹ cm⁻³. In the temperature range of 77–298 K, the electron transport mechanism in the ZnO:N films was predominantly intertrap tunneling, but thermally activated hopping also took place.

Wastewater treatment targeting reuse may limit water scarcity. Photocatalysis is an advanced oxidation process that may be employed in the removal of traces of organic pollutants, where the material choice is important. Titanium dioxide (TiO₂) is a highly efficient photocatalyst with good aqueous stability. TiO₂ powder has a high surface area, thus allowing good pollutant adsorption, but it is difficult to filter for reuse. Thin films have a significantly lower surface area but are easier to regenerate and reuse. In this paper, we report on obtaining sol-gel TiO₂ thin films on spherical beads (2 mm diameter) with high surface area and easy recovery from wastewater. The complex influence of the substrate morphology (etched up to 48 h in concentrated H₂SO₄), of the sol dilution with ethanol (1:0 or 1:1), and the number of layers (1 or 2) on the structure, morphology, chemical composition, and photocatalytic performance of the TiO₂ thin films is investigated. Etching the substrate for 2 h in H₂SO₄ leads to uniform, smooth surfaces on which crystalline, homogeneous TiO₂ thin films are grown. Films deposited using an undiluted sol are stable in water, with some surface reorganization of the TiO₂ aggregates occurring, while the films obtained using diluted sol are partially washed out. By increasing the film thickness through the deposition of a second layer, the roughness increases (from ~50 nm to ~100 nm), but this increase is not high enough to promote higher adsorption or overall photocatalytic efficiency in methylene blue photodegradation (both about 40% after 8 h of UV-Vis irradiation at 55 W/m²). The most promising thin film, deposited on spherical bead substrates (etched for 2 h in H₂SO₄) using the undiluted sol, with one layer, is highly crystalline, uniform, water-stable, and proves to have good photocatalytic activity.

ZnO and Al-doped ZnO (AZO) thin films were prepared using the sol–gel method and deposited on a Silicon (Si(100)) substrate using the dipping technique. The structure, morphology, thickness, optical constants in the spectral range 300–1700 nm, bandgap (E_g) and photoluminescence (PL) properties of the films were analyzed using X-ray diffractometry (XRD), X-ray fluorescence (XRF), atomic force microscopy (AFM), scanning electron microscopy (SEM), spectroscopic ellipsometry (SE), Raman analysis and PL spectroscopy. The results of the structure and morphology analyses showed that the thin films are polycrystalline with a hexagonal wurtzite structure, as well as continuous and homogeneous. The PL background and broader peaks observable in the Raman spectra of the AZO film and the slight increase in the optical band gap of the AZO thin film, compared to undoped ZnO, highlight the effect of defects introduced into the ZnO lattice and an increase in the charge carrier density in the AZO film. The PL emission spectra of the AZO thin film showed a strong UV line corresponding to near-band-edge ZnO emission along with weak green and red emission bands due to deep-level defects, attributed to the oxygen-occupied zinc vacancies (O_Zn lattice defects).