Anisotropic ZnO nanostructures and their nanocomposites as an advanced platform for photocatalytic remediation

Photocatalytic reduction of Cr(VI) using newly synthesized black phosphorus/ZnO nanocomposites

Removal of high toxic Cr(VI) with solar plays an important role in improving water pollution, but is facing a dilemma of developing excellent photocatalysts with high conversion efficiency and low cost. Different from traditional nano-structuring, this work focuses on the interfacial hybridization by considering the intrinsic difference in bonding interaction. Herein, we intentionally make some layered black phosphorus (BP) sheets with Van der Waals interaction to bond with ZnO surfaces, in which some additional electron channels can be formed by this multilevel atomic hybridization to accelerate carrier transfer and separation. Compared to the pristine ZnO and BP nanosheets, the light absorption and carrier separation efficiency can be sharply enhanced by this particular electronic structure, which makes the Cr reduction performance enhanced about 7.1 times. Our findings suggest a new insight into accelerating Cr(VI) reduction by designing interfacial atom hybridization.

An in-depth study of integrating cascaded photocatalytic H2O2 generation and activation with solar-driven interfacial evaporation for in-situ organic contaminant remediation

Integrating cascaded photocatalytic H2O2 generation and subsequent activation of H2O2 (into ·OH radicals) with solar-driven interfacial evaporation techniques offers an effective and sustainable approach for in-situ treating water contaminated with organic substances. Unlike traditional water-dispersed catalysts, the interfacial evaporation approach presents unique challenges in photocatalytic reactions. We explored these dynamics using an AgI/PPy/MF interfacial photothermal set, achieving H2O2 production efficiency (approximately 1.53 mM/g/h) - three times higher than submerged counterparts. This efficiency is attributed to exceptional solar light absorption (about 95 %), a significant surface photothermal effect (raising temperatures by approximately 36 °C), and enhanced oxygen availability (38 times more than in water), all characteristic of the interfacial system. The in-situ activation of H2O2 into ·OH notably improves the degradation of organic pollutants, achieving up to 99 % removal efficiency. This comprehensive analysis highlights the potential of combining photocatalytic H2O2 processes with interfacial evaporation for efficiently purifying organically polluted water.

ZnO nanostructured matrix as nexus catalysts for the removal of emerging pollutants

Water pollution stands as a pressing global environmental concern, elevating the significance of innovative, dependable, and sustainable solutions. This study represents an extensive review of the use of photocatalytic zinc oxide nanoparticles (ZnO NPs) for the removal of emerging pollutants from water and wastewater. The study examines ZnO NPs' different preparation methods, including physical, chemical, and green synthesis, and emphasizes on advantages, disadvantages, preparation factors, and investigation methods for the structural and morphological properties. ZnO NPs demonstrate remarkable properties as photocatalysts; however, their small dimensions pose an issue, leading to potential post-use environmental losses. A strategy to overcome this challenge is scaling up ZnO NP matrices for enhanced stability and efficiency. The paper introduces novel ZnO NP composites, by incorporating supports like carbon and clay that serve as photocatalysts in the removal of emerging pollutants from water and wastewater. In essence, this research underscores the urgency of finding innovative, efficient, and eco-friendly solutions for the removal of emerging pollutants from wastewater and highlights the high removal efficiencies obtained when using ZnO NPs obtained from green synthesis as a photocatalyst. Future research should be developed on the cost-benefit analysis regarding the preparation methods, treatment processes, and value-added product regeneration efficiency.

3D Lanthanide Neodymium Porphyrin Metal-Organic Framework for Photocatalytic Oxidation of Styrene

A novel three-dimensional lanthanide porphyrin-based MOF (Nd-PMOFs) was synthesized by using tetracarboxyphenyl porphyrin as the ligand and the lanthanide Nd as the coordination metal. Its specific crystal structure information was obtained by single-crystal diffraction with the space group C2/c and the empirical formula C₇₂H₄₅N₆Nd₂O_15.25. This new Nd porphyrin-based MOF with an organic framework formed by a unique coordination method enables the effective separation of photogenerated electrons and holes under photoluminescence, giving it excellent photocatalytic property which could be verified by the characterization data. The photocatalytic performance was examined by taking tert-butyl hydroperoxide as the oxidant and Nd-PMOFs as the catalyst for photocatalytic oxidation of styrene to benzaldehyde with 91.4% conversion and 81.2% benzaldehyde selectivity under optimal reactions, which surpasses most of the results reported in the literature. Several styrenes with other substituents were screened to explore the general applicability of Nd-PMOF for photocatalysis of styrene, among which Nd-PMOFs also exhibited excellent photocatalytic performance. This work offers the possibility to apply lanthanide organometallic frameworks, which are widely used in fluorescent materials, to photocatalysis. In addition, it also provides a new method for the catalytic generation of benzaldehyde from styrene that is consistent with the needs of modern green development.

Enhanced photocatalytic activity of cubic ZnSn(OH)6 by in-situ partial phase transformation via rapid thermal annealing

In this study, a mixed phase ZnSn(OH)₆/ZnSnO₃ photocatalyst was synthesized by calcining ZHS nanostructures via rapid thermal annealing (RTA) process. The composition ratio of ZnSn(OH)₆/ZnSnO₃ was controlled by changing the duration of the RTA process. The obtained mixed-phase photocatalyst was characterized by X-ray diffraction, field emission scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, ultraviolet photoelectron spectroscopy, photoluminescence, and physisorption analysis. Results showed that ZnSn(OH)₆/ZnSnO₃ photocatalyst obtained by calcining ZHS at 300 °C for 20 s displayed the best photocatalytic performance under UVC light illumination. Under optimized reaction conditions, ZHS-20 (0.125 g) demonstrated nearly complete removal (>99%) of MO dye in 150 min. Scavenger study revealed the predominant role of OH^• in photocatalysis. The enhanced photocatalytic activity of the ZnSn(OH)₆/ZnSnO₃ composites was mainly ascribed to the photosensitization of ZHS by ZTO and effective electron-hole separation at the ZnSn(OH)₆/ZnSnO₃ heterojunction interface. It is expected that this study will provide new research input for the development of photocatalyst through thermal annealing-induced partial phase transformation.

Heavy Metal Removal from Aqueous Effluents by TiO2 and ZnO Nanomaterials

The presence of heavy metals in wastewater, such as Ni, Pb, Cd, V, Cr, and Cu, is a serious environmental problem. This kind of inorganic pollutant is not biodegradable for several years, and its harmful effect is cumulative. Recently, semiconductor nanomaterials based on metal oxides have gained interest due to their efficiency in the removal of heavy metals from contaminated water, by inducing photocatalytic ion reduction when they absorb light of the appropriate wavelength. The most commonly applied semiconductor oxides for these purposes are titanium oxide (TiO2), zinc oxide (ZnO), and binary nanomaterials composed of both types of oxides. The main purpose of this work is to critically analyse the existent literature concerning this topic focusing specially in the most important factors affecting the adsorption or photocatalytic capacities of this type of nanomaterials. In particular, photocatalytic activity is altered by various factors, such as proportion of polymorphs, synthesis method, surface area, concentration of defects and particle size, among others. After a survey of the actual literature, it was found that, although these metal oxides have low absorption capacity for visible light, it is possible to obtain an acceptable heavy metal reduction performance by sensitization with dyes, doping with metallic or nonmetallic atoms, introduction of defects, or the coupling of two or more semiconductors.

Zinc hydroxystannate/zinc-tin oxide heterojunctions for the UVC-assisted photocatalytic degradation of methyl orange and tetracycline

Partial phase modification of zinc hydroxystannate (ZHS) is an effective technique for improving its light absorption capacity. In this study, a zinc hydroxystannate/zinc-tin oxide (ZHS/ZTO) heterostructure was synthesized via chemical co-precipitation followed by annealing. The as-prepared heterostructure revealed cubic crystal morphology along with high-intensity diffraction peaks in the XRD pattern. The XPS analysis of ZHS/ZTO heterostructures demonstrated the presence of key elements (Zn, Sn, and O) in their most stable ionic forms. The photocatalytic degradation efficiencies of the prepared samples were tested against methyl orange (MO) and tetracycline (TC) in an aqueous medium under UVC (254 nm) radiation. Under optimized conditions, maximum degradation efficiencies of 99% for MO and 97% for TC were observed in 120 and 180 min, respectively. Further, the predominant role of OH˙ radicals in the photocatalytic removal of MO and TC was evident through scavenging experiments. 2nd order kinetic model was outperformed in simulating the degradation mechanism of both targets over 1st and zero-order kinetic models. Finally, a photocatalytic degradation mechanism is proposed based on the energy values estimated for the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) using UPS analysis.

Crystal-facet and microstructure engineering in ZnO for photocatalytic NO oxidation

Photocatalysis is believed to be an important way of reducing NO pollutant in air and the facet engineering of semiconducting oxides could enhance the efficiency of the photocatalysis. ZnO nanoparticles with different exposed crystalline facets were successfully synthesized using a hydrothermal method and their photocatalytic degradation towards NO was investigated. The crystals from ZnCl₂ precursor were hexagonal mesoporous ones with exposed (0002) facet, while those from zinc acetate were in the form of flakes or wheat ears with enhanced exposure of (101(-)1) facet. Calcination in air imparted an enhanced the textural coefficient of the orientated facets as well as the oxygen defects. The nanocrystals with enhanced (0002) facet and lower flat-band energy did better in photoelectrochemical water-oxidation than those with exposed (101(-)1) facet that showed superior photocatalytic activity (approaching 76.7 ± 0.6% under 365 nm photons) for NO oxidation. According to theoretical calculations, (101(-)1) facet with O termination showed much higher affinity to NO molecules than other configurations, and the oxygen vacancy in ZnO played an minor role in the photocatalytic oxidation of NO. A high quantum efficiency approaching 97.5 ± 1.4% under 275 nm photons was obtained for the ZnO crystals from zinc acetate with mixed (0002) and (101(-)1) facets. This research explores the special characteristics of ZnO with different exposed facets and is important for the future design of highly efficient photocatalyst for hazardous material removal.

Evaluation of photocatalytic performances of PEG and PVP capped zinc sulfide nanoparticles towards organic environmental pollutant in presence of sunlight

Advanced oxidation processes triggered by nanoscale materials are promising owing to the in-situ generation of reactive radicals that can degrade toxic organic pollutants. In the present study, zinc sulfide (ZnS) nanoparticles with polyethylene glycol-4000 (PEG-4000) and polyvinylpyrrolidone (PVP) cappings were prepared using the chemical precipitation method and characterized thoroughly. Optical and structural characteristics of the capped ZnS nanoparticles were investigated and compared with those of uncapped ZnS nanoparticles. Results showed that PVP and PEG capped ZnS nanoparticles exhibited smaller crystallite size of 1.42 and 1.5 nm, respectively, as compared to uncapped ZnS (1.93 nm). Consequently, band gap energies of capped ZnS nanoparticles were higher which enable them to work as solar photocatalyst. The photocatalytic performance of the PEG, PVP-capped, and uncapped ZnS nanoparticles were evaluated against methyl orange (MO) dye and showed 85%, 87%, and 80% dye removal efficiencies, respectively. Degradation rate constant derived using Langmuir-Hinshelwood model revealed faster degradation kinetics bycapped ZnS photocatalysts owing to broader light absorption range. A possible dye degradation mechanism based on the energy levels positions was proposed to explain the route of photocatalytic degradation of MO by ZnS materials. It was inferred that the generation of reactive oxygen species by photogenerated electron-hole pairs facilitate degradation of MO dye molecules under sunlight illumination. It is expected that this work will provide insights into the development of strategies employed to achieve enhanced photocatalysis by nanoscale materials through organic capping.

Fabrication of magnetic ZnO/ZnFe2O4/diatomite composites: improved photocatalytic efficiency under visible light irradiation

The magnetic recoverable ZnO/ZnFe2O4/diatomite (ZZFDT) composite was synthesized by hydrothermal-precipitation method. The structure, optical properties and magnetic properties of the composites were characterized by different analytical instruments. ZZFDT-1 is composed of cubic spinel, hexagonal wurtzite, tetrahedron structure. SEM and TEM showed that ZnO and ZnFe2O4 particles were loaded onto the surface of diatomite, and the particle size was uniform. In addition, ZZFDT-1 is a typical mesoporous material with a specific surface area of 65.3 m2/g and pore size of about 12 nm. The response range of ZZFDT-1 is extended to visible light, and the band gap is 1.5 eV. Moreover, the M-H hysteretic curves of ZZFDT-1 exhibited superparamagnetic properties. The photocatalytic activity of different samples was evaluated by the conversion rate of oxytetracycline (OTC) under visible light. ZZFDT-1 has the best photocatalytic activity and the conversion is up to 95%. Because of its magnetic nature, it can be easily separated from the solution. The results showed that the ZZFDT composite has good photocatalytic activity under visible light. After being reused six times, it still has good stability.

Cerium-, Europium- and Erbium-Modified ZnO and ZrO2 for Photocatalytic Water Treatment Applications: A Review

In the last decades photocatalysis has become one of the most employed technologies for the implementation of the so-called Advanced Oxidation Processes (AOPs) for the removal of harmful pollutants from wastewaters. The materials identified as the best photocatalysts are transition metal oxides, in which the band structure allows charge carrier separation upon solar irradiation. The photoinduced charge carrier can thus cause oxidative and reductive redox reactions at the surface, inducing the formation of the radical species able to initiate the AOPs. Despite the great advantages of this process (non-toxic, cheap and environmentally clean), the main drawback lies in the fact that the most efficient semiconductors are only able to absorb UV irradiation, which accounts for only 5% of the total solar irradiation at the Earth’s surface and not enough to generate the required amount of electron-hole pairs. On the other hand, many efforts have been devoted to the sensitization of wide band gap transition metal oxides to visible light, which represents a higher percentage (almost 45%) in the solar electromagnetic spectrum. Among all the strategies to sensitize transition metal oxides to visible irradiation, doping with lanthanides has been less explored. In this regard, lanthanides offer a unique electronic configuration, consisting in 4f orbitals shielded by a 5s5p external shell. This occurrence, coupled with the different occupation of the localized 4f orbitals would provide an astounding opportunity to tune these materials’ properties. In this review we will focus in depth on the modification of two promising photocatalytic transition metal oxides, namely ZnO and ZrO2, with cerium, europium and erbium atoms. The aim of the work is to provide a comprehensive overview of the influence of lanthanides on the structural, optical and electronic properties of the modified materials, emphasizing the effect of the different 4f orbital occupation in the three considered doping atoms. Moreover, a large portion of the discussion will be devoted to the structural-properties relationships evidencing the improved light absorption working mechanism of each system and the resulting enhanced photocatalytic performance in the abatement of contaminants in aqueous environments.

Crystalline ZnO Photocatalysts Prepared at Ambient Temperature: Influence of Morphology on p-Nitrophenol Degradation in Water

Since the Industrial Revolution, technological advances have generated enormous emissions of various pollutants affecting all ecosystems. The detection and degradation of pollutants has therefore become a critical issue. More than 59 different remediation technologies have already been developed, such as biological remediation, and physicochemical and electrochemical methods. Among these techniques, advanced oxidation processes (AOPs) have been popularized in the treatment of wastewater. The use of ZnO as a photocatalyst for water remediation has been developing fast in recent years. In this work, the goals are to produce ZnO photocatalysts with different morphologies, by using a green sol-gel process, and to study both the influence of the synthesis parameters on the resulting morphology, and the influence of these different morphologies on the photocatalytic activity, for the degradation of an organic pollutant in water. Multiple morphologies were produced (nanotubes, nanorods, nanospheres), with the same crystalline phase (wurtzite). The most important parameter controlling the shape and size was found to be pH. The photoactivity study on a model of pollutant degradation shows that the resulting activity is mainly governed by the specific surface area of the material. A comparison with a commercial TiO2 photocatalyst (Evonik P25) showed that the best ZnO produced with this green process can reach similar photoactivity without a calcination step.

P Doping Promotes the Spontaneous Visible-Light-Driven Photocatalytic Water Splitting in Isomorphic Type II GaSe/InS Heterostructure

The development and design of clean and efficient water splitting photocatalysts is important for the current situation of energy shortage and environmental pollution. A new type of isomorphic GaSe/InS heterostructure is constructed, and the optoelectronic properties were studied through first-principles calculations. The results show that GaSe/InS vdW heterostructure is a type II semiconductor with a band gap of 2.09 eV. However, through the analysis of the energy band edge position and Gibbs free energy change of water splitting, it is found that the GaSe/InS heterostructure is difficult to undergo overall water splitting. Therefore, nonmetallic element P doping is considered, the established P-doped GaSe/InS (P-GaSe/InS) heterostructure could maintain the type II band arrangement, and under acidic conditions, P-GaSe/InS heterostructure could spontaneously undergo overall water splitting thermodynamically. Furthermore, the low exciton binding energy of P-GaSe/InS heterostructure highlights better light absorption performance. Therefore, these findings indicate that P-GaSe/InS heterostructure is a promising photocatalyst in overall water splitting.

Photocatalytic Conversion of Methane: Recent Advancements and Prospects

Abundant and affordable methane is not only a high-quality fossil fuel, it is also a raw material for the synthesis of value-added chemicals. Solar-energy-driven conversion of methane offers a promising approach to directly transform methane to valuable energy sources under mild conditions, but remains a great challenge at present. In this Review, recent advances in the photocatalytic conversion of methane are systematically summarized. Insights into the construction of effective semiconductor-based photocatalysts from the perspective of light-absorption units and active centers are highlighted and discussed in detail. The performance of various photocatalysts in the conversion of methane is presented, with the photooxidation classified according to the oxidant systems. Lastly, challenges and future perspectives in the photocatalytic oxidation of methane are described.