ZnO and Ti-doped ZnO (Ti-ZnO) nanoparticles were synthesized using rapid combustion. The morphology of ZnO and Ti-ZnO featured nanoparticles within cluster-like structures. The ZnO and Ti-ZnO structures exhibited similar hexagonal wurtzite structures and crystal sizes. This behavior occurred because Zn²⁺ sites of the ZnO lattice were substituted by Ti⁴⁺ ions. The chemical structure characterization implied the major vibration of the ZnO structure. The physisorption analysis showed similar mesoporous and non-rigid aggregation structures for ZnO and Ti-ZnO using N₂ adsorption–desorption. However, Ti-ZnO demonstrated a specific surface area two times higher than that of ZnO. This was a major factor in improving the photocatalytic degradation of methylene blue (MB). The photocatalytic degradation analysis showed a kinetic degradation rate constant of 2.54 × 10⁻³ min⁻¹ for Ti-ZnO, which was almost 80% higher than that of ZnO (1.40 × 10⁻³ min⁻¹). The transformation mechanism of MB molecules into other products, including carbon dioxide, aldehyde, and sulfate ions, was also examined.

ZnO nanoparticles (NPs) were comparatively synthesized via precipitation and combustion techniques. The ZnO NPs synthesized via precipitation and combustion exhibited similar polycrystalline hexagonal wurtzite structures. The large crystal sizes of ZnO NPs were obtained from the ZnO precipitation in comparison with those from the ZnO combustion, while the particle sizes were in the same range. The functional analysis implied that the ZnO structures had surface defects. Moreover, absorbance measurement showed the same absorbance range in ultraviolet light. In the photocatalytic degradation of methylene blue, ZnO precipitation exhibited higher degradation performance than ZnO combustion. This was attributed to the larger crystal sizes of ZnO NPs, which provided an enduring carrier movement at semiconductor surfaces and reduced electron-hole recombination. Thus, the crystallinity of ZnO NPs can be considered an important factor in photocatalytic activity. Furthermore, precipitation is an interesting synthesizing method for preparing ZnO NPs with large crystal sizes.

The high crystallinity of graphene quantum dots-ZnO nanocomposites is considered to have a significant effect in improving the carrier lifetime for enhanced photocatalytic degradation. The graphene quantum dots-ZnO nanocomposites were synthesized by adding graphene quantum dots solution into starting precursors during the precipitation. Characterization was performed using various techniques. High crystallinity of graphene quantum dots-ZnO nanocomposites is obtained in terms of increased crystal size and decreased dislocation density. The improved crystallinity increases the carrier lifetime on the material surface for the functional improvement of photocatalytic material. Photocatalytic test of methylene blue and methyl orange was performed under UV irradiation. Degradation rate constant reaches the maximum value for both organic dyes for the appropriate preparing condition of graphene quantum dots-ZnO nanocomposites. The graphene quantum dots-ZnO nanocomposites were then applied to degrade commercial glyphosate herbicide contaminants for an agricultural wastewater treatment investigation. The investigation aims to demonstrate a facile useful way of herbicide contaminant reduction for the better health of farmers. The graphene quantum dots-ZnO nanocomposites show an enhancement of the photocatalytic process with improved degradation rate constant (23% increased) in comparison to pure ZnO. Therefore, this work demonstrates that graphene quantum dots-ZnO nanocomposites can be used as a photocatalytic material for degrading organic dyes and commercial herbicide contaminants owing to its low-cost and environmental-friendly properties.

Dye-sensitized solar cells (DSSCs) have drawn attention in recent years for their cost-efficient, green, and convenient means of harnessing solar power. The function of DSSCs is designed around the counter electrode (CE), which is traditionally composed of platinum (Pt) for its good catalytic activity and good electric conductivity. However, Pt is expensive and in short quantity, and extensive studies have gone in search of substitute materials. Among them, materials in the system of chalcogenides, such as sulfides, selenides, and tellurides, have drawn considerable attention as potential candidates. These materials have various unique advantages, such as their capability to modulate their electronic property, good catalytic activity, and good resistance to chemicals, and therefore have good potential for optimizing function of DSSCs. This review presents an in-depth overview of where the situation stands in chalcogenide-based CEs, critically evaluating their synthetic routes, electrocatalytic activity, and stability. We compare and contrast various synthetic routes, such as hydrothermal, solvothermal, electrodeposition, chemical vapor deposition, atomic layer deposition, and solution-based synthesis, employed to alter the nanostructure and topography of such materials. Sulfide- and selenide-based materials have displayed competing power conversion efficiencies and favorable charge transfer behavior, but tellurides have potential through their exceptionally good electric conductivity. Despite such breakthroughs, limitations such as corrosion by the electrolyte, phase instability, and scalability of the routes of fabrication persist and serious bottlenecks persist. This review suggests possible strategies such as doping, composite formation, and formation of the protective layer to overcome such limitations and to ensure cost-efficient, high-performance DSSCs.