Photocatalytic Water Oxidation on ZnO: A Review

Enhanced Photocatalytic Performance under Ultraviolet and Visible Light Illumination of ZnO Thin Films Prepared by Modified Sol-Gel Method

Semiconductor oxides are frequently used as active photocatalysts for the degradation of organic agents in water polluted by domestic industry. In this study, sol-gel ZnO thin films with a grain size in the range of 7.5-15.7 nm were prepared by applying a novel two-step drying procedure involving hot air treatment at 90-95 °C followed by conventional furnace drying at 140 °C. For comparison, layers were made by standard furnace drying. The effect of hot air treatment on the film surface morphology, transparency, and photocatalytic behavior during the degradation of Malachite Green azo dye in water under ultraviolet or visible light illumination is explored. The films treated with hot air demonstrate significantly better photocatalytic activity under ultraviolet irradiation than the furnace-dried films, which is comparable with the activity of unmodified ZnO nanocrystal powders. The achieved percentage of degradation is 78-82% under ultraviolet illumination and 85-90% under visible light illumination. Multiple usages of the hot air-treated films (up to six photocatalytic cycles) are demonstrated, indicating improved photo-corrosion resistance. The observed high photocatalytic activity and good photo-corrosion stability are related to the hot air treatment, which causes a reduction of oxygen vacancies and other defects and the formation of interstitial oxygen and/or zinc vacancies in the films.

Enhanced photocatalytic activity of green synthesized zinc oxide nanoparticles using low-cost plant extracts

Developing stable and highly efficient metal oxide photocatalysts remains a significant challenge in managing organic pollutants. In this study, zinc oxide nanoparticles (ZnO NPs) were successfully synthesized using various plant extracts, pomegranate (P.M), beetroot roots (B.S), and seder, along with a chemical process. The produced ZnO NPs were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-Vis), Field Emission Scanning Electron Microscope (FESEM), High-Resolution Transmission Electron Microscopy (HRTEM), and Surface Area. For all prepared samples, the results indicated that the composition of the plant extract affects several characteristics of the produced particles, such as their photocatalytic properties, energy bandgap (Eg), particle size, and the ratio of the two intensity (0 0 2) and (1 0 0) crystalline planes. The particle size of the produced NPs varies between 20 and 30 nm. To examine NPs' photocatalytic activity in the presence of UV light, Methyl Orange (MO) was utilized. The Eg of  ZnO synthesized by the chemical method was 3.16 e. V, whereas it was 2.84, 2.63, and 2.59 for P.M, Seder, and B.S extracts, respectively. The most effective ZnO NPs, synthesized using Beetroots, exhibited a degradation efficiency of 87 ± 0.5% with a kinetic rate constant of 0.007 min-1. The ratio of the two intensity (0 0 2) and (1 0 0) crystalline planes was also examined to determine a specific orientation in (0 0 2) that is linked to the production of oxygen vacancies in ZnO, which enhances their photocatalytic efficiency. Furthermore, the increase in photocatalytic effectiveness can be attributed to the improved light absorption by the inter-band gap states and effective charge transfer.

Double hydrogen bonding-induced compact H-type π-π stacking enhancing rapid carrier transfer in perylene diimide supramolecules achieving high oxygen evolution performance

Perylene diimides (PDI) are widely used in photocatalytic oxygen evolution due to their deep valence band potentials. Here, we report the synthesis of a unique supramolecular photocatalyst (designated s-PDI-P1) by introducing hydroxyl and carboxyl groups at the imide position of PDI. This modification allows the formation of intermolecular double hydrogen bond structures between the hydroxyl groups, oxygen atoms on the perylene cores and the carboxyl groups. The resulting double hydrogen bonding structures reduce lateral slip and promote the formation of supramolecular structures with H-type π-π stacking. In addition, the intermolecular hydrogen bonding interactions between the hydroxyl groups and the oxygen atoms on the perylene cores bring the PDI molecules closer together, enhancing the conjugation of the PDI supramolecules and facilitating the formation of ultrathin nanosheet-like structures. In this study, we successfully constructed ultrathin nanosheets of the supramolecular photocatalyst s-PDI-P1 with a compact H-type π-π stacking structure, which exhibited enhanced charge transfer capability, shorter charge migration distance, and achieved a high photocatalytic oxygen evolution rate of 3.23 mmolg-1h-1. These results highlight the potential of intermolecular double hydrogen bond structures to improve the separation and migration driving force of photogenerated charges, thus providing a novel design strategy for organic photocatalysts.

Development of cellulose/ZnO based bioplastics with enhanced gas barrier, UV-shielding effect and antibacterial activity

Development of renewable and biodegradable plastics with good properties, such as the gas barrier, UV-shielding, solvent resistance, and antibacterial activity, remains a challenge. Herein, cellulose/ZnO based bioplastics were fabricated by dissolving cellulose carbamate in an aqueous solution of NaOH/Zn(OH)42-, followed by coagulation in aqueous Na2SO4 solution, and subsequent hot-pressing. The carbamate groups detached from cellulose, and ZnO which transformed from cosolvent to nanofiller was uniformly immobilized in the cellulose matrix during the dissolution/regeneration process. The appropriate addition of ZnO (below 10.67 wt%) not only improved the mechanical properties but also enhanced the water and oxygen barrier properties of the material. Additionally, our cellulose/ZnO based bioplastic demonstrated excellent UV-blocking capabilities, increased water contact angle, and enhanced antibacterial activity against S. aureus and E. coli, deriving from the incorporation of ZnO nanoparticles. Furthermore, the material exhibited resistance to organic solvents such as acetone, THF, and toluene. Indeed, the herein developed cellulose/ZnO based bioplastic presents a promising candidate to replace petrochemical plastics in various applications, such as plastic toys, anti-UV guardrails, window shades, and oil storage containers, offering a combination of favorable mechanical, gas barrier, UV-blocking, antibacterial, and solvent-resistant properties.

Preparation of a polyaniline/ZnO-NPs composite for the visible-light-driven hydrogen generation

Compositions of ZnO nanoparticles and polyaniline, in the form of emeraldine salt, were manufactured as thin layers by using the spin-coating method. Then, the effect of polyaniline content on their photoelectrochemical characteristics was studied. Results indicate that all the samples are sensitive to light. Besides, with 0.30% of PANI, the composite sample demonstrates the highest photocurrent density; also, its photocurrent increment starts to increase at a voltage of ⁓ 1.23 V (vs. RHE), which is approximately in accordance with the theoretical potential of water electrolysis. Furthermore, since the rate of electron-hole recombination in this composite sample is the lowest, it possesses the highest photoelectrochemical efficiency. Main findings were analyzed with respect to UV-visible absorption and photoluminescence spectra as well as SEM micrographs of the samples and Raman spectral measurements. Besides, electrochemical impedance spectroscopy analysis was applied to both pure ZnO and the sample with the best response. Effects of drying temperature and layer thickness were also investigated.

The physical properties and photocatalytic activities of green synthesized ZnO nanostructures using different ginger extract concentrations

The green synthesis method which is aligned with the sustainable development goals (SDGs) theory, is proposed to synthesize ZnO nanoparticles using ginger extract to treat the acidic wastewater and acidic factory effluent as a current challenge and the effects of the concentration of extracts on the synthesized ZnO nanostructures are investigated. The results declare that the single-phase hexagonal ZnO is formed using ginger extract concentration of less than 25 mL and the crystallite size of green synthesized ZnO NPs increased with increasing the concentration of ginger extract. Also, the significant effects of ginger extract concentration on the morphology of nanoparticles (nanocone, nanoflakes, and flower-like) and the particle size are demonstrated. The low concentration of ginger extract leads to the formation of the ZnO nanoflakes, while the flower-like structure is gradually completed by increasing the concentration of the ginger extract. Furthermore, significant changes in the specific surface area (SSA) of the samples are observed (in the range of 6.1-27.7 m2/g) by the variation of ginger extract concentration and the best SSA is related to using 10 mL ginger extract. Also, the strong effect of using ginger extract on the reflectance spectra of the green synthesized ZnO NPs, especially in the UV region is proved. The indirect (direct) band gap energies of the ZnO samples are obtained in the range of 3.09-3.20 eV (3.32-3.38 eV). Furthermore, the photocatalytic activities of the samples for the degradation of methylene blue indicate the impressive effect of ginger extract concentration on the degradation efficiency of ZnO nanoparticles and it reaches up to 44% and 83% for ZnO NPs prepared using 5 mL ginger extract in a pH of 4.3 and 5.6, respectively. This study provided new insights into the fabrication and practical application of high-performance ZnO photocatalysts synthesized using ginger extract in degrading organic pollutants in an acidic solution.

Safe Disposal of Accident Wastewater in Chemical Industrial Parks Using Non-Thermal Plasma with ZnO-Fe3O4 Composites

Chemical wastewater has a high concentration of toxic and hazardous antibiotic pollutants, which not only devastates the ecological environment and disrupts the ecological balance, but also endangers human health. This research proposed a non-thermal plasma (NTP) combined with a ZnO-Fe3O4 nano-catalyst system to achieve the efficient degradation of ciprofloxacin (CIP) in chemical wastewater. Firstly, ZnO-Fe3O4 composite materials were prepared using hydrothermal method and characterized with scanning electron microscopy (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), etc. With the sole NTP, NTP/ZnO, and NTP/ZnO-Fe3O4 systems, the removal efficiency of CIP can reach 80.1%, 88.2%, and 99.6%, respectively. The optimal doping amount of Fe3O4 is 14%. Secondly, the capture agent experiment verified that ·OH, ·O2-, and 1O2 all have a certain effect on CIP degradation. Then, liquid chromatography-mass spectrometry (LC-MS) was used to detect the intermediate and speculate its degradation pathway, which mainly included hydroxyl addition, hydroxyl substitution, and piperazine ring destruction. After treatment with the NTP/ZnO-Fe3O4 system, the overall toxicity of the product was reduced. Finally, a cyclic experiment was conducted, and it was found that the prepared ZnO-Fe3O4 catalyst has good reusability. The NTP/ZnO-Fe3O4 was also applied in practical pharmaceutical wastewater treatment and has practical applicability.

Solar Photocatalytic Activity of Ba-Doped ZnO Nanoparticles: The Role of Surface Hydrophilicity

Bare zinc oxide (ZnO) and Ba-doped ZnO (BZO) samples were prepared by using a simple precipitation method. The effects of Barium doping on the structural, morphological, and optoelectronic properties, as well as on the physico-chemical features of the surface were investigated and correlated with the observed photocatalytic activity under natural solar irradiation. The incorporation of Ba2+ ions into the ZnO structure increased the surface area by ca. 14 times and enhanced the hydrophilicity with respect to the bare sample, as demonstrated by infrared spectroscopy and contact angle measurements. The surface hydrophilicity was correlated with the enhanced defectivity of the doped sample, as indicated by X-ray diffraction, Raman, and fluorescence spectroscopies. The resulting higher affinity with water was, for the first time, invoked as an important factor justifying the superior photocatalytic performance of BZO compared to the undoped one, in addition to the slightly higher separation of the photoproduced pairs, an effect that has already been reported in literature. In particular, observed kinetic constants values of 8∙10-3 and 11.3∙10-3 min-1 were determined for the ZnO and BZO samples, respectively, by assuming first order kinetics. Importantly, Ba doping suppressed photocorrosion and increased the stability of the BZO sample under irradiation, making it a promising photocatalyst for the abatement of toxic species.

Improvement of the photocatalytic activity of ZnO thin films doped with manganese

In the herein report, we synthesized ZnO thin films doped with manganese (Mn). We studied the impact of Mn doping loads (1 %, 3 %, 5 % wt.) on physicochemical properties of the compounds. Furthermore, we presented the photocatalytic efficiency in removal of methylene blue dye. The structural assay indicated ZnO conserve the wurtzite crystalline structure after dopant insertion. Furthermore, the crystalline size of catalysts was reduced after dopant incorporation. The SEM analysis showed a change in surface morphology after modification of ZnO thin films. Furthermore, Raman spectroscopy verified the Mn insertion inside the ZnO lattice. After the doping process, band gap was reduced by 16 %, in comparison to bare ZnO. After the photocatalytic test, the doped catalysts showed better performance than bare ZnO in removing MB. The best test showed a kinetics constant value of 2.9 × 10-3 min-1 after 120 min of visible irradiation. Finally, the Mn(5 %):ZnO thin film was suitable after five degradation cycles, and the degradation process efficiency was reduced by 32%.

Aluminum Cation Doping in Ruddlesden-Popper Sr2TiO4 Enables High-Performance Photocatalytic Hydrogen Evolution

Hydrogen (H2) is regarded as a promising and renewable energy carrier to achieve a sustainable future. Among the various H2 production routes, photocatalytic water splitting has received particular interest; it strongly relies on the optical and structural properties of photocatalysts such as their sunlight absorption capabilities, carrier transport properties, and amount of oxygen vacancy. Perovskite oxides have been widely investigated as photocatalysts for photocatalytic water splitting to produce H2 because of their distinct optical properties, tunable band gaps and excellent compositional/structural flexibility. Herein, an aluminum cation (Al3+) doping strategy is developed to enhance the photocatalytic performance of Ruddlesden-Popper (RP) Sr2TiO4 perovskite oxides for photocatalytic H2 production. After optimizing the Al3+ substitution concentration, Sr2Ti0.9Al0.1O4 exhibits a superior H2 evolution rate of 331 μmol h−1 g−1, which is ~3 times better than that of Sr2TiO4 under full-range light illumination, due to its enhanced light harvesting capabilities, facilitated charge transfer, and tailored band structure. This work presents a simple and useful Al3+ cation doping strategy to boost the photocatalytic performance of RP-phase perovskites for solar water splitting.

High Photocatalytic Oxygen Evolution via Strong Built-In Electric Field Induced by High Crystallinity of Perylene Imide Supramolecule

A highly crystalline perylene imide supramolecular photocatalyst (PDI-NH) is synthesized via imidazole solvent method. The catalyst shows a breakthrough oxygen evolution rate (40.6 mmol g^-1 h^-1 ) with apparent quantum yield of 10.4% at 400 nm, which is 1353 times higher than the low crystalline PDI-NH. The highly crystalline structure comes from the ordered self-assembly process in molten imidazole solvent via π-π stacking and hydrogen bonding. Further, the excellent performance ascribes to the robust built-in electric field induced by its high crystallinity, which greatly accelerates the charge separation and transfer. What is more, the PDI-NH is quite stable and can be reused over 50 h without performance attenuation. Briefly, the crystalline PDI-NH with strong built-in electric field throws light on photocatalytic oxygen evolution, showing a new perspective for the design of organic photocatalysts.

CeO2 nanoarray decorated Ce-doped ZnO nanowire photoanode for efficient hydrogen production with glycerol as a sacrificial agent

Photoelectrochemical (PEC) biomass oxidation by the substitution of an oxygen evolution reaction is considered a promising strategy for efficient hydrogen production.

Sustainable photoanode for water oxidation reactions: from metal-based to metal-free materials

Sunlight affords an inexhaustible and primary energy for Earth. A photoelectrochemical system can efficiently harvest solar energy and convert it into chemicals. However, sophisticated processes and expensive raw materials are...

Challenges and prospects about the graphene role in the design of photoelectrodes for sunlight-driven water splitting

Graphene and its derivatives have emerged as potential materials for several technological applications including sunlight-driven water splitting reactions. This review critically addresses the latest achievements concerning the use of graphene as a player in the design of hybrid-photoelectrodes for photoelectrochemical cells. Insights about the charge carrier dynamics of graphene-based photocatalysts which include metal oxides and non-metal oxide semiconductors are also discussed. The concepts underpinning the continued progress in the field of graphene/photoelectrodes, including different graphene structures, architecture as well as the possible mechanisms for hydrogen and oxygen reactions are also presented. Despite several reports having demonstrated the potential of graphene-based photocatalysts, the achieved performance remains far from the targeted benchmark efficiency for commercial application. This review also highlights the challenges and opportunities related to graphene application in photoelectrochemical cells for future directions in the field.

ZnO Materials as Effective Anodes for the Photoelectrochemical Regeneration of Enzymatically Active NAD. ACS APPLIED MATERIALS & INTERFACES 2021;13:10719-10727. [PMID: 33645209 DOI: 10.1021/acsami.0c20630]

This work reports the study of ZnO-based anodes for the photoelectrochemical regeneration of the oxidized form of nicotinamide adenine dinucleotide (NAD⁺). The latter is the most important coenzyme for dehydrogenases. However, the high costs of NAD⁺ limit the use of such enzymes at the industrial level. The influence of the ZnO morphologies (flower-like, porous film, and nanowires), showing different surface area and crystallinity, was studied. The detection of diluted solutions (0.1 mM) of the reduced form of the coenzyme (NADH) was accomplished by the flower-like and the porous films, whereas concentrations greater than 20 mM were needed for the detection of NADH with nanowire-shaped ZnO-based electrodes. The photocatalytic activity of ZnO was reduced at increasing concentrations of NAD⁺ because part of the ultraviolet irradiation was absorbed by the coenzyme, reducing the photons available for the ZnO material. The higher electrochemical surface area of the flower-like film makes it suitable for the regeneration reaction. The illumination of the electrodes led to a significant increase on the NAD⁺ regeneration with respect to both the electrochemical oxidation in dark and the only photochemical reaction. The tests with formate dehydrogenase demonstrated that 94% of the regenerated NAD⁺ was enzymatically active.

Visible Light-Driven Photocatalytic Rhodamine B Degradation Using CdS Nanorods

In this work, highly crystalline CdS nanorods (NRs) were successfully synthesized by a facile, one-step solvothermal method. The as-prepared CdS NRs powder was characterized by XRD, FESEM, Raman, PL, XPS, BET, and UV-visible techniques to evaluate the structural, morphological, and optical properties. The photocatalytic performance of the as-synthesized CdS NRs was investigated for the photodegradation of RhB dye under visible light irradiations. It has been found that CdS NRs show maximum RhB degradation efficiency of 88.4% in 120 min. The excellent photodegradation ability of the CdS NRs can be attributed to their rod-like structure together with their large surface area and surface state. The kinetic study indicated that the photodegradation process was best described by the pseudo-first-order kinetic model. The possible mechanism for the photodegradation of RhB dye over CdS NRs was proposed in this paper.

Synthesis, Characterization, and Photocatalytic Performance of ZnO–Graphene Nanocomposites: A Review

ZnO is an exciting material for photocatalysis applications due to its high activity, easy accessibility of raw materials, low production costs, and nontoxic. Several ZnO nano and microstructures can be obtained, such as nanoparticles, nanorods, micro flowers, microspheres, among others, depending on the preparation method and conditions. ZnO is a wide bandgap semiconductor presenting massive recombination of the generated charge carriers, limiting its photocatalytic efficiency and stability. It is common to mix it with metal, metal oxide, sulfides, polymers, and nanocarbon-based materials to improve its photocatalytic behavior. Therefore, ZnO–nanocarbon composites formation has been a viable alternative that leads to new, more active, and stable photocatalytic systems. Mainly, graphene is a well-known two-dimensional material, which could be an excellent candidate to hybridize with ZnO due to its excellent physical and chemical properties (e.g., high specific surface area, optical transmittance, and thermal conductivity, among others). This review analyses ZnO–graphene nanocomposites’ recent advances, addressing the synthesis methods and the resulting structural, morphological, optical, and electronic properties. Moreover, we examine the ZnO–graphene composites’ role in the photocatalytic degradation of organic/inorganic pollutants.

Improvement of Bi doping in ZnO nanocrystals by co-doping with Al: crystal geometry calculations and photocatalytic activity

In this work we demonstrate enhancement in visible-light photocatalytic activity (PCA) of ZnO nanoparticles (NPs) with minimal attenuation of visible light transmittance. This approach can benefit numerous optoelectronic and photocatalytic applications. ZnO NPs were p-n co-doped with Al and Bi to improve Bi doping into the ZnO crystal. Al- and/or Bi-doped ZnO was coprecipitated by ammonia from aqueous nitrate solutions of Zn²⁺, Al³⁺, and Bi³⁺, followed by microwave heating. Doping concentrations in Al- and Bi- doped ZnO (AZO and BZO) and Al/Bi co-doped ZnO (ABZO) were 1, 3, 5, and 7 mole %. The resulting NPs were characterized by XRD, TEM, EDS, BET, and UV-visible absorption. While EDS shows that almost all added Bi was incorporated into the ZnO, XRD analysis of BZO reveals formation of α-Bi₂O₃ as a secondary phase due to the poor Bi solubility in ZnO. Co-doping of Al with Bi suppressed α-Bi₂O₃ formation and increased Bi solubility in ZnO. XRD-based calculations of the lattice constants and deformation strain, stress, and energy all show insertion of Al and/or Bi into the crystal with different extents according to the dopants' solubilities into ZnO. AZO and BZO NPs had E _g lowered by 0.05-1.39 eV and 0.30-0.70 eV, respectively, relative to ZnO. On the other hand, ABZO had E _g reductions of only 0.01-0.20 eV due to formation of acceptor-donor complex through co-doping. ABZO gave higher PCA enhancements with respect to E _g reductions (Δk _photo/-ΔE _g) than either AZO and BZO, with values up to 370, 126, and 13 min^-1 eV^-1, respectively.

Dry Hydrogen Production in a Tandem Critical Raw Material-Free Water Photoelectrolysis Cell Using a Hydrophobic Gas-Diffusion Backing Layer

A photoelectrochemical tandem cell (PEC) based on a cathodic hydrophobic gas-diffusion backing layer was developed to produce dry hydrogen from solar driven water splitting. The cell consisted of low cost and non-critical raw materials (CRMs). A relatively high-energy gap (2.1 eV) hematite-based photoanode and a low energy gap (1.2 eV) cupric oxide photocathode were deposited on a fluorine-doped tin oxide glass (FTO) and a hydrophobic carbonaceous substrate, respectively. The cell was illuminated from the anode. The electrolyte separator consisted of a transparent hydrophilic anionic solid polymer membrane allowing higher wavelengths not absorbed by the photoanode to be transmitted to the photocathode. To enhance the oxygen evolution rate, a NiFeOX surface promoter was deposited on the anodic semiconductor surface. To investigate the role of the cathodic backing layer, waterproofing and electrical conductivity properties were studied. Two different porous carbonaceous gas diffusion layers were tested (Spectracarb® and Sigracet®). These were also subjected to additional hydrophobisation procedures. The Sigracet 35BC® showed appropriate ex-situ properties for various wettability grades and it was selected as a cathodic substrate for the PEC. The enthalpic and throughput efficiency characteristics were determined, and the results compared to a conventional FTO glass-based cathode substrate. A throughput efficiency of 2% was achieved for the cell based on the hydrophobic backing layer, under a voltage bias of about 0.6 V, compared to 1% for the conventional cell. For the best configuration, an endurance test was carried out under operative conditions. The cells were electrochemically characterised by linear polarisation tests and impedance spectroscopy measurements. X-Ray Diffraction (XRD) patterns and Scanning Electron Microscopy (SEM) micrographs were analysed to assess the structure and morphology of the investigated materials.

Tuning Photocatalytic Performance of Multilayer ZnO for Water Splitting by Biaxial Strain Composites

Novel two-dimensional (2D) materials have received extensive attention in the field of photocatalysis due to their unique properties. Traditional ZnO material with wurtzite structure transforms into a stable graphite-like structure that has the characteristics of 2D material when its thickness is less than a few atomic layers. In this work, using first-principles calculations, we investigated the potential of multilayer graphite-like ZnO as a photocatalyst for water splitting. The results showed that multilayer ZnO is a series of direct bandgap semiconductors, and their band edge positions all straddle the redox potential of water. Increasing with the number of layers, the bandgap of multilayer ZnO decreased from 3.20 eV for one layer to 2.21 eV for six layers, and visible light absorption capacity was significantly enhanced. Hence, multilayer ZnO was indeed promising for photocatalytic water splitting. Furthermore, suitable biaxial tensile strain could decrease the bandgap and maintain the stable graphite-like structure at a broader thickness range. In contrast, excessive biaxial tensile strain could change the redox capacity of multilayer ZnO and prevent it from catalyzing water splitting. Our theoretical results show that six-layer ZnO under 1% biaxial strain had direct bandgap of 2.07 eV and represents the most excellent photocatalytic performance among these multilayer ZnO materials.

Minimizing electron-hole pair recombination through band-gap engineering in novel ZnO-CeO2-rGO ternary nanocomposite for photoelectrochemical and photocatalytic applications

A novel ZnO-CeO₂-rGO (ZCG) ternary nanocomposite with varying ZnO/CeO₂ weight proportions was synthesized by a hydrothermal process for photoelectrochemical water splitting and photocatalytic application. XRD diffraction peaks of ZCG nanocomposites displayed the patterns of ZnO and CeO₂ nanoparticles, and SEM revealed irregular flake-like particles, which were uniformly decorated on the rGO matrix. Increase in the intensity ratio of D and G bands from Raman spectra revealed changes in oxygen bonding in the ZnO-rGO (ZG) and ZCG nanocomposites. The shift in the band edge positions and the decrease in the band gap with increase in the cerium oxide content in ZCG composites were observed from UV-Vis and Mott-Schottky plots. XPS results showed that Ce³⁺ fraction increased with an increase in the cerium oxide content in ZCG nanocomposites. The ZCG3 (85:15) nanocomposite exhibited decreased electron-hole recombination rate as evidenced from the photoluminescence and electrochemical impedance spectroscopy Nyquist plots. The characteristic frequency in Bode's plot shifted to a lower frequency for the ZCG3 electrode demonstrating low interfacial charge transfer resistance, and ZCG3 photoelectrode displayed a higher photocurrent density of 0.69 mA/cm² at 1.5 V compared with other photoelectrode. The optimized and highly efficient ZCG3 nanocomposite exhibited improved photocatalytic degradation of methylene blue (MB) with a reaction rate constant of 0.0201 min^-1. Combination of defects in the form of Ce³⁺ ion and surface oxygen vacancies coupled with rGO as the electron acceptor improved the charge carrier density and carrier transport in addition to the formation Schottky-type junction and the presence of an internal electric field.

A Review of Microwave Synthesis of Zinc Oxide Nanomaterials: Reactants, Process Parameters and Morphoslogies

Zinc oxide (ZnO) is a multifunctional material due to its exceptional physicochemical properties and broad usefulness. The special properties resulting from the reduction of the material size from the macro scale to the nano scale has made the application of ZnO nanomaterials (ZnO NMs) more popular in numerous consumer products. In recent years, particular attention has been drawn to the development of various methods of ZnO NMs synthesis, which above all meet the requirements of the green chemistry approach. The application of the microwave heating technology when obtaining ZnO NMs enables the development of new methods of syntheses, which are characterised by, among others, the possibility to control the properties, repeatability, reproducibility, short synthesis duration, low price, purity, and fulfilment of the eco-friendly approach criterion. The dynamic development of materials engineering is the reason why it is necessary to obtain ZnO NMs with strictly defined properties. The present review aims to discuss the state of the art regarding the microwave synthesis of undoped and doped ZnO NMs. The first part of the review presents the properties of ZnO and new applications of ZnO NMs. Subsequently, the properties of microwave heating are discussed and compared with conventional heating and areas of application are presented. The final part of the paper presents reactants, parameters of processes, and the morphology of products, with a division of the microwave synthesis of ZnO NMs into three primary groups, namely hydrothermal, solvothermal, and hybrid methods.

Comparative Study of ZnO Thin Films Doped with Transition Metals (Cu and Co) for Methylene Blue Photodegradation under Visible Irradiation

We synthesized and characterized both Co-doped ZnO (ZnO:Co) and Cu-doped ZnO (ZnO:Cu) thin films. The catalysts’ synthesis was carried out by the sol–gel method while the doctor blade technique was used for thin film deposition. The physicochemical characterization of the catalysts was carried out by Raman spectroscopy, scanning electron microscopy (SEM), X-ray diffraction, and diffuse reflectance measurements. The photocatalytic activity was studied under visible irradiation in aqueous solution, and kinetic parameters were determined by pseudo-first-order fitting. The Raman spectra results evinced the doping process and suggested the formation of heterojunctions for both dopants. The structural diffraction patterns indicated that the catalysts were polycrystalline and demonstrated the presence of a ZnO wurtzite crystalline phase. The SEM analysis showed that the morphological properties changed significantly, the micro-aggregates disappeared, and agglomeration was reduced after modification of ZnO. The ZnO optical bandgap (3.22 eV) reduced after the doping process, these being ZnO:Co (2.39 eV) and ZnO:Co (3.01 eV). Finally, the kinetic results of methylene blue photodegradation reached 62.6% for ZnO:Co thin films and 42.5% for ZnO:Cu thin films.

Geometry-on-demand fabrication of conductive microstructures by photoetching and application in hemostasis assessment

Photo-corrosion is a common phenomenon observed in the photocatalytic semiconductor materials, which can seriously harm the photoelectric properties and performances in the energy applications. However, in this paper, we demonstrated that the photo-corrosion effects can be used for the microfabrication of conductive structures on a photocatalytic film like zinc oxide (ZnO), named as "photoetching". Our results demonstrated that microstructures can be prepared within seconds with a precision at an order of tens of micrometers using our current devices. Different from the previous work, the etching process was achieved free of conducting layer under the ZnO film, avoiding the short-circuit of the conductive micro-patterns and enabling the use for the impedance sensing. We demonstrated the fabricated ZnO microelectrode pairs can work for the electrochemical impedance measurements like assessment of hemostasis integrated with a microfluidic chip. Compared to the noble metal microelectrodes, the ZnO conductive microelectrodes can be fabricated within seconds and the low costs make it possible as a disposable diagnostic device. Besides, the photoetching technique can be performed without a cleanroom reducing the technical barriers, possibly helpful for the low resources areas. We believe the simplicity of device, low costs and fast fabrication can be useful in the relevant fields such as biomedical and energy harvesting, especially for low resources areas.

The Nanosized Dye Adsorbents for Water Treatment

Clean water is a vital element for survival of any living creature and, thus, crucially important to achieve largely and economically for any nation worldwide. However, the astonishingly fast trend of industrialization and population growth and the arisen extensive water pollutions have challenged access to clean water across the world. In this regard, 1.6 million tons of dyes are annually consumed. Thereof, 10%–15% are wasted during use. To decolorize water streams, there is an urgent need for the advanced remediation approaches involving utilization of novel materials and technologies, which are cost and energy efficient. Nanomaterials, with their outstanding physicochemical properties, can potentially resolve the challenge of need to water treatment in a less energy demanding manner. In this review, a variety of the most recent (from 2015 onwards) opportunities arisen from nanomaterials in different dimensionalities, performances, and compositions for water decolorization is introduced and discussed. The state-of-the-art research studies are presented in a classified manner, particularly based on structural dimensionality, to better illustrate the current status of adsorption-based water decolorization using nanomaterials. Considering the introduction of many newly developed nano-adsorbents and their classification based on the dimensionality factor, which has never been employed for this sake in the related literature, a comprehensive review will be presented.

Oxygen-defective ZnO porous nanosheets modified by carbon dots to improve their visible-light photocatalytic activity and gain mechanistic insight

A carbon dots/oxygen-defective ZnO (COZ) porous nanosheet composite photocatalyst was prepared via a one-step liquid-phase wet chemistry method for the highly efficient visible-light photocatalytic degradation of tetracycline (TC).

Photocatalytic degradation of ibuprofen in water using TiO2 and ZnO under artificial UV and solar irradiation

The degradation of anti-inflammatory and antipyretic drug (Ibuprofen; IBP) has been described in this study by using photocatalytic-based advanced oxidation processes. The catalysts (TiO₂ and ZnO) were activated by irradiation of artificial UV lamp and solar rays for the generation of highly oxidizing species which resulted in the degradation of IBP to intermediates and finally to carbon dioxide and water. In solar reactor, quartz and borosilicate tubes were installed for absorption of required ultraviolet rays and curved chrome plates were used to reflect and concentrate rays on the tubes containing feed mixture. The liquid chromatography, Total organic carbon (TOC), and Chemical oxygen demand (COD) tests were employed to determine the degradation rates and demineralization of solution samples. At catalyst dosing of 1-1.5 g/L, TiO₂ -based experiments showed high degradation rate under acidic conditions. Similarly, for ZnO catalyst, 1 g/L dozing rate was found to be effective under neutral conditions (pH = 7.0). UV lamp-based photocatalysis had higher degradation rate as compared to that of solar reactor. Moreover, better absorption of solar rays by quartz tubes resulted in higher degradation than that in borosilicate tubes. For UV lamp photocatalysis, the TOC and COD reduction was higher. With improved catalyst doping and better solar reactor design, solar-based IBP degradation could be more promising than UV-based catalysis. PRACTITIONER POINTS: TiO₂ and ZnO were employed to generate oxidizing agents for comparative photocatalytic degradation. Degradation rate of Ibuprofen with TiO₂ was much higher compared to ZnO. Quartz material was found more effective as radiation absorbing material than borosilicate glass for solar photo catalysis. Influence of catalyst loading (TiO₂ and ZnO) and pH conditions on degradation rate and mineralization of IBP was examined. IBP is a carcinogenic and endocrine disrupting drug so its degradation in water can protect ecological and human life.

Controlling the growth of single crystal ZnO nanowires by tuning the atomic layer deposition parameters of the ZnO seed layer

Semiconducting nanowires (NWs) offer exciting prospects for a wide range of technological applications. The translation of NW science into technology requires reliable high quality large volume production. This study provides an in-depth investigation of the parameters using an atomic layer deposition system to grow zinc oxide (ZnO) seed layers followed by the chemical bath deposition (CBD) of ZnO NWs to demonstrate the low-cost production of uniform single crystal wurtzite phase ZnO NWs that is scalable to large area substrates. The seed layer texture and the morphology of the NWs grown were systematically investigated using atomic force microscopy as a function of the seed layer deposition parameters. It is shown that the NWs growth orientation can be controlled by tuning the seed layer deposition parameters while maintaining the same CBD conditions. Likewise, the diameters and the surface densities of the NWs varied from 23 to 56 nm and 40 to 327 NWs μm^-2, respectively. Significantly, the relationship between the seed layer structure and the NW density indicates a clear correlation between the density of seed layer surface features and the resulting surface NW density of NWs grown.

Insight into antibiotics removal: Exploring the photocatalytic performance of a Fe3O4/ZnO nanocomposite in a novel magnetic sequential batch reactor

The purpose of this research was the preparation and photocatalytic evaluation of a novel nanocomposite (NC) based on Fe₃O₄/ZnO, to eliminate four persistent antibiotics in surface waters: sulfamethoxazole, trimethoprim, erythromycin and roxithromycin. Prior to the operation of the photocatalytic reactor, the influence of pH (3-9), catalyst concentration (50-800 mg L^-1), oxidant dose (0-100 mg L^-1) and concentration of different targets (10-100 μg L^-1) on the catalytic efficiency was evaluated. The analysis of reaction kinetics showed that degradation processes of the four antibiotics followed a pseudo-first-order kinetic model. Antibiotics adsorption onto the nanocomposite surface depended on their electrostatic nature and played an important role when decreasing the initial concentration of antibiotics. In this context, kinetic rates were higher at lower initial levels of organic pollutants, which is a favourable effect from a practical application perspective. On the other hand, a synergistic effect of the available Fe in the nanocomposite was found, contributing to the oxidation of antibiotics by photo-Fenton as a secondary reaction. Then, a magnetic photocatalytic reactor was operated under optimal conditions. The enhanced photonic efficiency of Fe₃O₄/ZnO in the system, as well as the ease of the magnetic separation and catalyst reusability, indicate the viability of this reactor configuration.

The role of self-trapped excitons in polaronic recombination processes in lithium niobate

Transient absorption and photoluminescence are experimentally investigated in the polaronic reference system lithium niobate, LiNbO[Formula: see text] (LN), with the aim to refine the microscopic model of small polaron dynamics in materials with strong electron-phonon coupling. As a unique feature, our study is performed by using two different spectroscopic methods, in crystals with dopants enhancing photorefraction or damage resistance, and over a broad temperature range from 15-400 K. Although being self-consistent for particular experimental conditions, the hitherto used microscopic polaronic models reveal inconsistencies when applied to this larger data set. We show that comprehensive modeling is unlocked by the inclusion of an additional type of polaronic state with the following characteristics: (i) strongly temperature- and dopant-dependent relaxation times, (ii) an absorption feature in the blue-green spectral range, and (iii) a Kohlrausch-Williams-Watts decay shape with a temperature-dependent stretching factor [Formula: see text] showing a behavior contrary to that of small, strong-coupling polarons. The hypothesis of self-trapped excitons (STEs, i.e. bound electron-hole pairs strongly coupled to Nb[Formula: see text] and O[Formula: see text] within a niobium-oxygen octahedron) and their pinning on defects as the microscopic origin of these characteristics is supported by a spectroscopic linkage of photoluminescence at low (15 K) and elevated (300 K) temperatures and explains the long-lifetime components in transient absorption as due to pinned STEs.

Structural and Magnetic Properties of Co‒Mn Codoped ZnO Nanoparticles Obtained by Microwave Solvothermal Synthesis

Zinc oxide nanoparticles codoped with Co2+ and Mn2+ ions (Zn(1−x−y)MnxCoyO NPs) were obtained for the first time by microwave solvothermal synthesis. The nominal content of Co2+ and Mn2+ in Zn(1−x−y)MnxCoyO NPs was x = y = 0, 1, 5, 10 and 15 mol % (the amount of both ions was equal). The precursors were obtained by dissolving zinc acetate dihydrate, manganese (II) acetate tetrahydrate and cobalt (II) acetate tetrahydrate in ethylene glycol. The morphology, phase purity, lattice parameters, dopants content, skeleton density, specific surface area, average particle size, average crystallite size, crystallite size distribution and magnetic properties of NPs were determined. The real content of dopants was up to 25.0% for Mn2+ and 80.5% for Co2+ of the nominal content. The colour of the samples changed from white to dark olive green in line with the increasing doping level. Uniform spherical NPs with wurtzite structure were obtained. The average size of NPs decreased from 29 nm to 21 nm in line with the increase in the dopant content. Brillouin type paramagnetism and an antiferromagnetic interaction between the magnetic ions was found for all samples, except for that with 15 mol % doping level, where a small ferromagnetic contribution was found. A review of the preparation methods of Co2+ and Mn2+ codoped ZnO is presented.

Zinc oxide nano-enabled microfluidic reactor for water purification and its applicability to volatile organic compounds

This paper reports fast and efficient chemical decontamination of water within a tree-branched centimeter-scale microfluidic reactor. The microreactor integrates Zinc oxide nanowires (ZnO NWs) in situ grown acting as an efficient photocatalytic nanomaterial layer. Direct growth of ZnO NWs within the microfluidic chamber brings this photocatalytic medium at the very close vicinity of the water flow path, hence minimizing the required interaction time to produce efficient purification performance. We demonstrate a degradation efficiency of 95% in <5 s of residence time in one-pass only. According to our estimates, it becomes attainable using microfluidic reactors to produce decontamination of merely 1 l of water per day, typical of the human daily drinking water needs. To conduct our experiments, we have chosen a laboratory-scale case study as a seed for addressing the health concern of water contamination by volatile organic compounds (VOCs), which remain difficult to remove using alternative decontamination techniques, especially those involving water evaporation. The contaminated water sample contains mixture of five pollutants: Benzene; Toluene; Ethylbenzene; m–p Xylenes; and o-Xylene (BTEX) diluted in water at 10 p.p.m. concentration of each. Degradation was analytically monitored in a selective manner until it falls below 1 p.p.m. for each of the five pollutants, corresponding to the maximum contaminant level (MCL) established by the US Environmental Protection Agency (EPA). We also report on a preliminary study, investigating the nature of the chemical by-products after the photocatalytic VOCs degradation process.

Insights into the enhanced photoelectrochemical performance of hydrothermally controlled hematite nanostructures for proficient solar water oxidation

A controlled hydrothermal reaction time showed an improvement in the PEC performance of 1D α-Fe₂O₃ nanorods due to an optimum aspect ratio and Sn⁴⁺ diffusion.