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<p>Zinc oxide (ZnO) thin films were synthesized on glass substrates using spray pyrolysis at 450&#xa0;°C and examined in both as-deposited and post-annealed states, with annealing carried out at 400&#xa0;°C in air. Comprehensive characterization was performed to investigate the impact of annealing on crystallinity, morphology, optical transparency, electrical transport, photocatalytic activity, and gas-sensing behavior. X-ray diffraction (XRD) confirmed the hexagonal wurtzite structure for all films, with annealed ZnO exhibiting sharper diffraction peaks, indicating improved crystallinity through enhanced structural ordering and defect relaxation rather than significant crystallite growth. SEM revealed a morphological transition from irregular nanograins to a more compact microstructure, while EDAX confirmed improved stoichiometric balance after annealing. Optical analysis showed transmittance above 80% in the visible region and a slight decrease in the optical band gap from 3.24 to 3.23&#xa0;eV, attributed to reduced defect density and enhanced structural ordering. Although the band-gap variation is small, it is consistent with the observed red-shift of the absorption edge and indicates subtle modifications in the electronic structure following annealing. Thermoelectric measurements based on the Seebeck coefficient revealed a decrease in carrier concentration after annealing, consistent with classical semiconductor transport models. The carrier concentration values were estimated using a non-degenerate semiconductor approximation. Photoconductivity studies (UV photoresponse and I–V characteristics) demonstrated enhanced charge generation and mobility in annealed films. A unified defect-mediated mechanism is proposed, in which annealing reduces carrier trapping and recombination centers, thereby improving charge transport, photoconductivity, and photocatalytic performance. Gas-sensing evaluation revealed that the as-deposited ZnO film exhibited higher sensitivity toward NO₂ due to its higher density of adsorption-active defects, whereas the annealed film demonstrated improved linearity, stability, selectivity, and humidity tolerance. Humidity-resistance measurements (50% RH) indicated good operational stability, while selectivity tests showed that both films responded strongly to NO₂ with negligible interference from H₂, CO, CH₄, and NH₃. Photocatalytic degradation of methylene blue (MB) showed superior performance for annealed ZnO (91.12%) compared with the as-deposited film (55.31%). The degradation kinetics followed a pseudo-first-order model, with the annealed sample exhibiting a significantly higher reaction rate constant, confirming the beneficial effect of annealing on photocatalytic efficiency. Scavenger tests identified •OH radicals as the dominant oxidative species responsible for dye degradation.</p>

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Effect of post-annealing on the structural, optical, thermoelectric, photocatalytic, and gas-sensing performances of ZnO thin films deposited by spray pyrolysis

  • 期刊论文

  • 发表日期：

    2026年8月5日

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  • Abdelkader Djelloul,
  • Sabrina Roguai

摘要

Zinc oxide (ZnO) thin films were synthesized on glass substrates using spray pyrolysis at 450 °C and examined in both as-deposited and post-annealed states, with annealing carried out at 400 °C in air. Comprehensive characterization was performed to investigate the impact of annealing on crystallinity, morphology, optical transparency, electrical transport, photocatalytic activity, and gas-sensing behavior. X-ray diffraction (XRD) confirmed the hexagonal wurtzite structure for all films, with annealed ZnO exhibiting sharper diffraction peaks, indicating improved crystallinity through enhanced structural ordering and defect relaxation rather than significant crystallite growth. SEM revealed a morphological transition from irregular nanograins to a more compact microstructure, while EDAX confirmed improved stoichiometric balance after annealing. Optical analysis showed transmittance above 80% in the visible region and a slight decrease in the optical band gap from 3.24 to 3.23 eV, attributed to reduced defect density and enhanced structural ordering. Although the band-gap variation is small, it is consistent with the observed red-shift of the absorption edge and indicates subtle modifications in the electronic structure following annealing. Thermoelectric measurements based on the Seebeck coefficient revealed a decrease in carrier concentration after annealing, consistent with classical semiconductor transport models. The carrier concentration values were estimated using a non-degenerate semiconductor approximation. Photoconductivity studies (UV photoresponse and I–V characteristics) demonstrated enhanced charge generation and mobility in annealed films. A unified defect-mediated mechanism is proposed, in which annealing reduces carrier trapping and recombination centers, thereby improving charge transport, photoconductivity, and photocatalytic performance. Gas-sensing evaluation revealed that the as-deposited ZnO film exhibited higher sensitivity toward NO₂ due to its higher density of adsorption-active defects, whereas the annealed film demonstrated improved linearity, stability, selectivity, and humidity tolerance. Humidity-resistance measurements (50% RH) indicated good operational stability, while selectivity tests showed that both films responded strongly to NO₂ with negligible interference from H₂, CO, CH₄, and NH₃. Photocatalytic degradation of methylene blue (MB) showed superior performance for annealed ZnO (91.12%) compared with the as-deposited film (55.31%). The degradation kinetics followed a pseudo-first-order model, with the annealed sample exhibiting a significantly higher reaction rate constant, confirming the beneficial effect of annealing on photocatalytic efficiency. Scavenger tests identified •OH radicals as the dominant oxidative species responsible for dye degradation.