Effect of Co Doping on the Physical Properties and Organic Pollutant Photodegradation Efficiency of ZnO Nanoparticles for Environmental Applications

This paper presents a comprehensive investigation of the synthesis and characterization of Zn1-xCoxO (0 ≤ x ≤ 0.05) nanopowders using a chemical co-precipitation approach. The structural, morphological, and vibrational properties of the resulting ZnO nanostructures were assessed through X-ray diffraction, scanning electronic microscopy, and Raman spectroscopy to examine the influence of cobalt doping. Remarkably, a notable congruence between the experimental results and the density functional theory (DFT) calculations for the Co-doped ZnO system was achieved. Structural analysis revealed well-crystallized hexagonal wurtzite structures across all samples. The SEM images demonstrated the formation of spherical nanoparticles in all the samples. The vibrational properties confirmed the formation of a hexagonal wurtzite structure, with an additional Raman peak corresponding to the F2g vibrational mode characteristic of the secondary phase of ZnCo2O4 observed at a 5% cobalt doping concentration. Furthermore, a theoretical examination of cobalt doping's impact on the elastic properties of ZnO demonstrated enhanced mechanical behavior, which improves stability, recyclability, and photocatalytic activity. The photocatalytic study of the synthesized compositions for methylene blue (MB) dye degradation over 100 min of UV light irradiation demonstrated that Co doping significantly improves photocatalytic degradation. The material's prolonged lifetime, reduced rate of photogenerated charge carrier recombination, and increased surface area were identified as pivotal factors accelerating the degradation process. Notably, the photocatalyst with a Zn0.99Co0.01O composition exhibited exceptional efficiency compared to that reported in the literature. It demonstrated high removal activity, achieving an efficiency of about 97% in a shorter degradation time. This study underscores the structural and photocatalytic advancements in the ZnO system, particularly at lower cobalt doping concentrations (1%). The developed photocatalyst exhibits promise for environmental applications owing to its superior photocatalytic performance.

Novel Mg- and Ga-doped ZnO/Li-Doped Graphene Oxide Transparent Electrode/Electron-Transporting Layer Combinations for High-Performance Thin-Film Solar Cells

Herein, a novel combination of Mg- and Ga-co-doped ZnO (MGZO)/Li-doped graphene oxide (LGO) transparent electrode (TE)/electron-transporting layer (ETL) has been applied for the first time in Cu₂ ZnSn(S,Se)₄ (CZTSSe) thin-film solar cells (TFSCs). MGZO has a wide optical spectrum with high transmittance compared to that with conventional Al-doped ZnO (AZO), enabling additional photon harvesting, and has a low electrical resistance that increases electron collection rate. These excellent optoelectronic properties significantly improved the short-circuit current density and fill factor of the TFSCs. Additionally, the solution-processable alternative LGO ETL prevented plasma-induced damage to chemical bath deposited cadmium sulfide (CdS) buffer, thereby enabling the maintenance of high-quality junctions using a thin CdS buffer layer (≈30 nm). Interfacial engineering with LGO improved the V_oc of the CZTSSe TFSCs from 466 to 502 mV. Furthermore, the tunable work function obtained through Li doping generated a more favorable band offset in CdS/LGO/MGZO interfaces, thereby, improving the electron collection. The MGZO/LGO TE/ETL combination achieved a power conversion efficiency of 10.67%, which is considerably higher than that of conventional AZO/intrinsic ZnO (8.33%).

Highly sensitive breath sensor based on sonochemically synthesized cobalt-doped zinc oxide spherical beads

In this study, we introduce cobalt (Co)-doped zinc oxide (ZnO) spherical beads (SBs), synthesized using a sonochemical process, and their utilization for an acetone sensor that can be applied to an exhalation diagnostic device. The sonochemically synthezied Co-doped ZnO SBs were polycrystalline phases with sizes of several hundred nanometers formed by the aggregation of ZnO nanocrystals. As the Co doping concentration increased, the amount of substitutionally doped Co²⁺ in the ZnO nanocrystals increased, and we observed that the fraction of Co³⁺ in the Co-doped ZnO SBs increased while the fraction of oxygen vacancies decreased. At an optimal Co-doping concentration of 2 wt%, the sensor operating temperature decreased from 300 to 250 °C, response to 1 ppm acetone improved from 3.3 to 7.9, and minimum acetone detection concentration was measured at 43 ppb (response, 1.75). These enhancements are attributed to the catalytic role of Co³⁺ in acetone oxidation. Finally, a sensor fabricated using 2 wt% Co-doped ZnO SBs was installed in a commercially available exhalation diagnostic device to successfully measure the concentration of acetone in 1 ml of exhaled air from a healthy adult, returning a value of 0.44 ppm.

Analysis of oxygen vacancy in Co-doped ZnO using the electron density distribution obtained using MEM

Oxygen vacancy (VO) strongly affects the properties of oxides. In this study, we used X-ray diffraction (XRD) to study changes in the VO concentration as a function of the Co-doping level of ZnO. Rietveld refinement yielded a different result from that determined via X-ray photoelectron spectroscopy (XPS), but additional maximum entropy method (MEM) analysis led it to compensate for the difference. VO tended to gradually decrease with increased Co doping, and ferromagnetic behavior was not observed regardless of the Co-doping concentration. MEM analysis demonstrated that reliable information related to the defects in the ZnO-based system can be obtained using X-ray diffraction alone.