Nanostructure Shape-Effects in ZnO heterogeneous photocatalysis

Strategies in photochemical alcohol oxidation on noble-metal free nanomaterials as heterogeneous catalysts

Using light absorbing semiconductors to harvest solar energy has long been considered as a promising approach for driving chemical reactions. Photochemical oxidation of alcohols catalyzed by noble-metal free nanomaterials is particularly attractive due to the bio-derivable nature of the substrate and the wide availability of such catalysts. Here, a number of strategies that could facilitate the progress are discussed, where it is concluded that the photochemical alcohol oxidation process can benefit from both mechanistic optimization of the electron-transfer steps and structural engineering of the light-absorbing nanomaterials.

A Review of Synthesis Methods, Modifications, and Mechanisms of ZnO/TiO2-Based Photocatalysts for Photodegradation of Contaminants

The environment being surrounded by accumulated durable waste organic compounds has become a critical crisis for human societies. Generally, organic effluents of industrial plants released into the water source and air are removed by some physical and chemical processes. Utilizing photocatalysts as cost-effective, accessible, thermally/mechanically stable, nontoxic, reusable, and powerful UV-absorber compounds creates a new gateway toward the removal of dissolved, suspended, and gaseous pollutants even in trace amounts. TiO2 and ZnO are two prevalent photocatalysts in the field of removing contaminants from wastewater and air. Structural modification of the photocatalysts with metals, nonmetals, metal ions, and other semiconductors reduces the band gap energy and agglomeration and increases the affinity toward organic compounds in the composite structures to expand their usability on an industrial scale. This increases the extent of light absorbance and improves the photocatalytic efficiency. Selecting a suitable synthesis method is necessary to prepare a target photocatalyst with distinct properties such as high specific surface area, numerous surface functional groups, and an appropriate crystalline phase. In this Review, significant parameters for the synthesis and modification of TiO2- and ZnO-based photocatalysts are discussed in detail. Several proposed mechanistic routes according to photocatalytic composite structures are provided. Some electrochemical analyses using charge carrier trapping agents and delayed recombination help to plot mechanistic routes according to the direction of photoexcited species (electron-hole pairs) and design more effective photocatalytic processes in terms of cost-effective photocatalysts, saving time and increasing productivity.

Chelated Ion-Exchange Strategy toward BiOCl Mesoporous Single-Crystalline Nanosheets for Boosting Photocatalytic Selective Aromatic Alcohols Oxidation

The photoresponse and photocatalytic efficiency of bismuth oxychloride (BiOCl) are greatly limited by rapid recombination of photogenerated carriers. The construction of porous single-crystal BiOCl photocatalyst can effectively alleviate this issue and provide accessible active sites. Herein, a facile chelated ion-exchange strategy is developed to synthesize BiOCl mesoporous single-crystalline nanosheets (BiOCl MSCN) using acetic acid and ammonia solution respectively as chelating agent and ionization promoter. The strong chelation between acetate ions and Bi³⁺ ions introduces acetate ions into the precipitated product to exchange with Cl^- ions, resulting in large lattice mismatch, strain release, and formation of void-like mesopores. The prepared BiOCl MSCN photocatalyst exhibits excellent catalytic performance with 99% conversion and 98% selectivity for oxidation of benzyl alcohol to benzaldehyde and superior general adaptability for various aromatic alcohols. The theoretical calculations and characterizations confirm that the superior performance is mainly attributed to the abundant oxygen vacancies, plenty of accessible adsorption/active sites and fast charge transport path without grain boundaries.

The interface design and properties enhancement of ZnO/cellulose composites: Branching fiber network to guide the assembly of ZnO flower

Using renewable biomass resources to regulate the growth and properties of catalysts is sustainable nanotechnology for achieving efficient photocatalysis and recycling. This work suggested a way to produce paper-based photocatalysts and resize the embedded zinc oxide (ZnO) flowers. The combination of experimental analysis and theoretical simulations demonstrated that small pores of the branching fiber network enhanced the interfacial interaction between ZnO flowers and cellulose fibers, thereby improving mechanical properties and optimizing flower structure. The interaction energy and electron density difference (EDD) simulation results demonstrated that the ZnO/cellulose interface structure shares significant attraction and charge transfer. Cellulose fibers ground for 20 cycles (CFG20) possessed dense branching fiber network and loaded with the smallest ZnO flowers, achieving a balance of strong mechanical properties and reaction efficiency. Remarkably, ZnO/CFG20 paper-based catalyst indicated strong photodegradation efficiency (100% for methyl orange, 100% for phenol, and 85.23% for aniline) and excellent reusability. This work will pave the way for the green regulation of catalysts.

Enhanced Photocatalytic Degradation of Methylene Blue Using Ti-Doped ZnO Nanoparticles Synthesized by Rapid Combustion

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.

Nanostructured photocatalysts for the abatement of contaminants by photocatalysis and photocatalytic ozonation: An overview

The water scarcity, the presence of different contaminants in the worldwide waters and wastewaters and their impacts should motivate their good elimination and water management. With this, photocatalysis and photocatalytic ozonation are strong solutions to obtain good quality reclaimed water, for different applications. Nanostructured supported photo-active catalysts, such as the TiO₂, WO₃ or ZnO can positively affect the performance of such technologies. Therefore, different semiconductors materials have been aroused the interest of the scientific community, mainly due to its functional properties as well as characteristics imposed by the different nanostructures. With this, this work overviews different works and perspective on the TiO₂ nanotubes and other semiconductors nanostructures, with the analysis of different works from 2001 to 2022. Aspects as the substrate effect, electrolyte nature, aspect ratio, electrolyte aging, and annealing treatment but also the effect of morphology, anodization time, applied voltage, temperature and viscosity are discussed. Modification of TiO₂ nanotubes is also presented in this paper. The main objective of this work is to present and discuss the key parameters and their effects on the anodization of different semiconductors, as well as the results obtained until today on the degradation of different contaminants by photocatalysis and photocatalytic ozonation, as well as their use on the treatment of real wastewater. TiO₂ nanotubes present unique properties and highly ordered configuration, which motivate their use on photo-driven technologies for the pollutant's abatement, even when compared to other nanostructures. However, photocatalysts with activity on the visible range and solar radiation, such as the WO₃, can present higher performance and can decrease operational costs, and must be an important source and a key to find efficient and cost-friendly solutions.

Halloysite-Zinc Oxide Nanocomposites as Potential Photocatalysts

The synthesis and structural characterization of synthetic zinc oxide and halloysite-based zinc oxide nanocomposites (with 2–28 m/m% ZnO content) are presented. The chemical precipitation of zinc hydroxide precursors and its subsequent drying at 80 °C yielded dominantly zinc oxide (zincite). Thermal treatment at 350 °C completely transformed the remaining precursor to ZnO without causing structural dehydroxylation of the halloysite support. The procedure yielded zinc oxide nanoparticles with 10–22 nm average size having quasi-spherical scale-like morphology. The specific surface area of the synthetic zinc oxide was found to be low (13 m2/g), which was significantly enhanced after nanocomposite preparation (27–47 m2/g). The photocatalytic activity of the prepared nanocomposites was probed by the degradation of a phenolic compound (4-nitrophenol) upon UV irradiation in liquid phase. Compared to their individual constituents, an increased activity of the nanocomposites was observed, while the SSA-normalized photocatalytic activity revealed a synergic effect in nanocomposites above 9 m/m% ZnO content. The nanocomposites were found to be stable at pH = 5.6, with a minor and major mobilization of zinc ions at pH = 12.4 and pH = 1.9, respectively. The toxicity of leachates in different pH environments by Vibrio fischeri bioluminescence indicated low toxicity for ZnO nanoparticles and insignificant toxicity for the nanocomposites. The enhanced photocatalytic activity together with the lower toxicity of the halloysite-ZnO nanocomposites highlight their application potential in water treatment.

Hydrazone-linked 2D porphyrinic covalent organic framework photocatalysis for visible light-driven aerobic oxidation of amines to imines

Covalent organic frameworks (COFs) have recently gained rising consideration for visible light photocatalysis. Their property could be accurately established with specific reactions in which the most investigated one turns out to be the aerobic oxidation of amines. In this contribution, a hydrazone-linked 2D (two-dimensional) porphyrinic COF, Por-DETH-COF, was assembled from 5,10,15,20-tetrakis(4-benzaldehyde)porphyrin (p-Por-CHO) and 2,5-diethoxyterephthalohydrazide (DETH) and its photocatalytic activity was duly appraised with the aerobic oxidation of amines. Thereby, the red light-driven selective oxidation of benzyl amines to imines was obtained in very high conversions and selectivities with ambient air as the oxidant. Importantly, the photocatalytic system exhibited remarkable compatibility of functional groups and extensive scope of benzyl amines. Notably, the Por-DETH-COF photocatalyst displayed outstanding recyclability after five successive cycles. This work suggests that 2D COFs could contribute a unique juncture for selective organic transformations by photocatalysis.

Preparation of LDO@TiO2 core-shell nanosheets for enhanced photocatalytic degradation of organic pollution

TiO₂-based nanosheet materials with a core-shell structure are expected to be one of the promising photocatalysts for the degradation of organic pollution. It is a challenge to synthesize TiO₂ by the desired nucleation and growth process, so most reported TiO₂ core-shell photocatalysts are prepared using TiO₂ as a core material. Layered double hydroxides (LDHs) are considered ideal platforms to grow TiO₂in situ and further serve as additional components to improve the separation of photogenerated charge carriers. In this work, we report the design and fabrication of anatase TiO₂-coated ZnAl-layered double oxide (LDO@TiO₂) nanosheets, which involve the in situ growth of TiO₂ on ZnAl-LDH followed by subsequent calcination treatment. The resulting LDO@TiO₂ photocatalyst yields typical core-shell nanosheet morphology with a mesoporous structure, exhibiting excellent photodegradation and mineralization efficiency for organic pollution.