For the sake of people's health and the safety of the environment, more efforts should be directed towards the fabrication of gas sensors that can operate effectively at room temperature (RT). In this context, increased attention has been paid to developing gas sensors based on rare-earth (RE)-doped transparent conducting oxides (TCO). In this report, lanthanum-doped zinc oxide (La-doped ZnO) films were fabricated by sol–gel and spin-coating techniques. XRD analysis revealed the hexagonal structure of the ZnO films, with preferred growth along the (002) direction. The crystallite size was decreased from 33.21 to 26.41 nm with increasing La content to 4.0 at.%. The UV–vis–NIR indicating that the films are highly transparent (˃ 80%), La-doping increased the UV blocking ability of the films and narrowed the optical band gap (Eg) from 3.275 to 3.125 eV. Additionally, La-doping has influenced the refractive index of the samples. Gas sensing measurements were performed at ambient temperature (30 °C) and a relative humidity (RH) of 30%, employing different flow rates of carbon dioxide (CO₂) gas used synthetically with air. Among the evaluated sensors, the ZnO: 4.0 at.% La sensor exhibited the most significant gas response, with a value of 114.22%. This response was observed when the sensor was subjected to a flow rate of 200 SCCM of CO₂ gas. Additionally, the sensor revealed a response time of 24.4 s and a recovery time of 44 s. The exceptional performance exhibited by the sensor makes it very appropriate for a wide range of industrial applications. Additionally, we assessed the effect of humidity, selectivity, reusability, repeatability, detection limit, and limit of quantification.

Thin films of Cu₂O/CuO mixed phase have been deposited on pre-cleaned glass substrate by a spin-coating technique. The influence of Cr-doping, (Cr + S) co-doping and the number of coated layers on the structures and optical behaviors of the films were investigated by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman, and UV-Visible spectroscopies. From XRD, FTIR and Raman results; the films are composed of polycrystalline monoclinic CuO and cubic Cu₂O phases with crystallite sizes ranged from 10.05 to 23.08 nm. Increasing the thickness improves the films’ crystallinity and decreases the defects level in the films. Cr and S incorporation encourages the growth of CuO phase at the expense of Cu₂O one and affects the preferred growth direction. The doping with Cr and S blue shifted the B_g mode and multi-phonon transitions. The direct and indirect optical band gaps decreased from 2.25 eV and 1.60 eV to 2.10 eV and 1.20 eV by growing the number of deposited layers from 2 to 8 layers. The film sensitivity towards CO₂ at different gas flow rate was studied and compared with those of similar systems. Also; response time, recovery time, detection limit, and limit of quantification are estimated.

For a safe environment, harmful-gas sensors of low cost and high performance are essential. For CO₂ gas sensing applications, Ba-doped CuO thin films with 4 mol% and 6 mol% Ba were produced on glass substrates using the successive ionic layer adsorption and reaction approach. Utilizing various techniques, crystallographic structures, nanomorphologies, and elemental compositions were examined to assess the impact of doping on the characteristics of the films. According to the structural and morphological analyses, the nanocrystalline films consisted of irregularly shaped nanoparticles, which assembled to form a rough surface with unequal grain sizes. Because of its nanoporous nature, the CuO:6% Ba thin film exhibited the most substantial nanomorphological change and the highest gas sensing capability. At varied CO₂ gas flow rates, the maximum sensor response (9.4%) and R_air/R_CO2 ratio (1.12) at room temperature (RT = 30 °C) were observed at 100 SCCM. By optimizing the sensor’s operating temperature, the sensor response value reached 82.2% at 150 °C, which is approximately eight times the value at RT. Selectivity, reusability, repeatability, detection limit, and quantification limit were all tested. It shows excellent response and recovery times of 5.6 and 5.44 s. In comparison to prior literature, the improved sensor is suited for use in industrial applications. Graphical abstract

Gas sensing is of significant importance in a wide range of disciplines, including industrial safety and environmental monitoring. In this work, a low-cost SILAR (Successive Ionic Layer Adsorption and Reaction) technique was employed to fabricate pure CuO, Zn-doped CuO, and Na-doped CuO nanotextured films to efficiently detect CO₂ gas. The structures, morphologies, chemical composition, and optical properties of all films are characterized using different tools. All films exhibit a crystalline monoclinic phase (tenorite) structure. The average crystallite size of pure CuO was 83.5 nm, whereas the values for CuO/Zn and CuO/Na were 73.15 nm and 63.08 nm, respectively. Subsequently, the gas-sensing capabilities of these films were evaluated for the detection of CO₂ in terms of sensor response, selectivity, recovery time, response time, and limits of detection and quantification. The CuO/Na film offered the most pronounced sensitivity towards CO₂ gas, as evidenced by a sensor response of 12.8% at room temperature and a low limit of detection (LoD) of 2.36 SCCM. The response of this sensor increased to 64.5% as the operating temperature increased to 150 °C. This study thus revealed a brand-new CuO/Na nanostructured film as a highly effective and economically viable sensor for the detection of CO₂.

Obtaining H₂ energy from H₂O using the most abundant solar radiation is an outstanding approach to zero pollution. This work focuses on studying the effect of Co doping and calcination on the structure, morphology, and optical properties of spin-coated SnO₂ films as well as their photoelectrochemical (PEC) efficiency. The structures and morphologies of the films were investigated by XRD, AFM, and Raman spectra. The results confirmed the preparation of SnO₂ of the rutile phase, with crystallite sizes in the range of 18.4–29.2 nm. AFM showed the granular structure and smooth surfaces having limited roughness. UV-Vis spectroscopy showed that the absorption spectra depend on the calcination temperature and the Co content, and the films have optical bandgap (E_g) in the range of 3.67–3.93 eV. The prepared samples were applied for the PEC hydrogen generation after optimizing the sample doping ratio, using electrolyte (HCl, Na₂SO₄, NaOH), electrode reusability, applied temperature, and monochromatic illumination. Additionally, the electrode stability, thermodynamic parameters, conversion efficiency, number of hydrogen moles, and PEC impedance were evaluated and discussed, while the SnO₂ films were used as working electrodes and platinum sheet as an auxiliary or counter electrode (2-electrode system) and both were dipped in the electrolyte. The highest photocurrent (21.25 mA/cm²), number of hydrogen moles (20.4 mmol/h.cm²), incident photon-to-current change efficiency (6.892%@307 nm and +1 V), and the absorbed photon-to-current conversion efficiency (4.61% at ~500 nm and +1 V) were recorded for the 2.5% Co-doped SnO₂ photoanode that annealed at 673 K.