Photocatalysis for Water Decomposition by RuO2-Dispersed ZnGa2O4 with d10 Configuration

Removal of cetirizine dihydrochloride from different matrices waters using Bi2O3/TiO2 photocatalyst under simulated solar irradiation: Kinetics, mechanism, and effect of environmental media

In this study, the photodegradation of cetirizine dihydrochloride (CET) by Bi2O3/TiO2 heterojunctions under simulated solar light irradiation (300-800nm) was examined in detail for the first time. A hydrothermal synthesis of the photocatalyst was carried out, and several analytical techniques were used to characterize the product. The resulting Bi2O3/TiO2 photocatalyst effectively removed CET from an aqueous solution. The Bi2O3/TiO2 (5.0%/95.0%) ratio exhibited the highest photocatalytic performance for CET degradation, degrading 75.85% of CET after 60 min of irradiation, with a high pseudo-first-order rate constant (kapp = 0.022 min-1; t1/2 = 31.50 min; natural pH). Moreover, TOC decreased by 40.45% after 420 min of irradiation. The Bi2O3/TiO2 photocatalyst has also been proven effective in degrading CET in different real aqueous matrices (Seawater (99.89%) > spring water (68.44%) > tap water (52.62%)), and the degradation under natural solar irradiation is more effective and faster than under artificial irradiation. Additionally, the Bi2O3/TiO2 photocatalyst demonstrated excellent photo-stability in a five-cycle photocatalytic experiment. The influence of various parameters showed that the removal of CET was heightened with a dose of 1 g/L of the Bi2O3/TiO2 and enhanced under acidic conditions (pH = 2.3). Moreover, the involvement of different reactive species was investigated by introducing diverse scavengers, revealing that hydroxyl radicals and photo-holes were the main reactive species involved in the CET photodegradation process over the Bi2O3/TiO2 photocatalyst. The primary photodegradation byproducts were identified using HPLC-MS analysis, and a possible mechanism was proposed.

Combined Analyses on Electronic Structure and Molecular Orbitals of d10 Bimetal Oxide In2Ge2O7 and Photocatalytic Performances for Overall Water Splitting and CO2 Reduction

Semiconducting photocatalytic overall water splitting and CO2 reduction are possible solutions to the emerging worldwide challenges of oil shortage and continual temperature increase, and the key is to develop an efficient photocatalyst. Most photocatalysts contain the d0, d10 or d10ns2 metals, and a guiding principle is desired to help to distinguish outstanding semiconductors. Here, the d10 bimetal oxide In2Ge2O7 was selected as the target. First, density functional theory (DFT) calculations point out that the nonbonding O 2p orbitals dominate the valence band maximum (VBM), and In 5s-O 2s and Ge 4s-O 2s antibonding orbitals are the major components of conduction band minimum (CBM). Moreover, the molecular orbitals were analyzed to consolidate the DFT calculations and make it more understandable for chemists. Due to the very small specific surface area (0.51 m2/g) and wide band gap (4.14 eV), as-prepared In2Ge2O7 did not exhibit any overall water splitting activity; nevertheless, when loading with 1 wt% cocatalyst (i.e., Pt, Pd), the surficial charge recombination can be greatly eliminated and the overall water splitting activity is significantly improved to 33.0(4) and 17.2(7) μmol/h for H2 and O2 generation, respectively. The apparent quantum yield (AQY) at 254 nm is 8.28%. This observation is proof that the inherent electronic structure of In2Ge2O7 is beneficial for the charge migration in bulk. Moreover, this catalyst also exhibits an observable CO2 reduction activity in pure water, which is a competition reaction with water splitting, anyway, the CH4 selectivity can be enhanced by loading Pd. This is a successful attempt to unravel the structure-property relationship by combining the analyses on electronic structure and molecular orbitals and is enlightening to further discover good candidates to photocatalysts.

Synthesis and characterization of ZnGa2O4 composites and its photocatalytic properties for energy applications

ZnGa₂O₄ nanocomposites have been widely used for photocatalytic degradation of industrial dyes. In this work, ZnGa₂O₄ was synthesized from zinc sulphate heptahydrate ZnSO₄^.10H₂O and Gallium (III) oxide (Ga₂O₃) by hydrothermal method. As prepared, ZnGa₂O₄ nanocomposites was used as a photocatalyst degradation of three organic dyes rhodamine-B, methylene blue, and methyl orange, under ultraviolet (UV) light irradiation. The ZnGa₂O₄ nanocomposites structure, morphology, size and optical properties were studied by X-ray diffraction (XRD), Fourier transform Raman spectroscopy (FT-Raman), scanning electron microscopy (SEM), Transmission electron microscopes (TEM) and photoluminescence spectra (PL). Moreover, the results explained the rate-controlling mechanisms of the dye degradation process followed by second-order kinetics. After 100 min of adsorption kinetic models, the decomposition of rhodamine-B (7.2 Ct mg/L, 5.2 Ct mg/L, and 4.1 Ct mg/L), methylene blue (42.8 qt mg/g, 44.8 qt mg/g, and 45.9 qt mg/g), and methyl orange (42.8 qe mg/g, 44.8 qe mg/g, and 45.9 qe mg/g) respectively. This investigation study offers a promising method to design more efficient ZnGa₂O₄ nanocomposites based photocatalytic degradation of industrial organic dyes.

Synthesis, characterization, and dye degradation photocatalytic activity of the nano-size copper iron binary oxide

Magnetic nano-size copper iron binary oxide is synthesized via a sol-gel method using copper and iron nitrates as precursors and citric acid, chicken egg white, and starch as stabilizers followed by annealing at 400 °C and 800 °C in air. The TG-DTG, XRD, FESEM, EDX, VSM, and FT-IR and UV-Vis DRS spectroscopy methods are used for thermal, structural, magnetic, and optoelectronic characterizations. Depending on the stabilizer and annealing temperature, pure CuFe₂O₄, (CuFe₂O₄,CuO) or (CuFe₂O₄,CuO,Fe₂O₃) phases are formed with nano-size particles of 20-65 nm, having optical band gaps in the range of 2.15-2.60 eV (577-477 nm). Photocatalytic activities of the synthesized nano-size copper iron binary oxide samples are examined for degradation of Nile Blue textile dye displayed first-cycle removal (from water solution) efficiencies of 86.7-93.3%. Considering usage of non-toxic metals and low-cost green stabilizers, good degradation performances, and easy/efficient (magnetic) recyclability, this nano-size catalyst is suggested for further optimization studies for industrial applications.

Hierarchical Ternary Sulfides as Effective Photocatalyst for Hydrogen Generation Through Water Splitting: A Review on the Performance of ZnIn2S4

One of the major aspects and advantages of solar energy conversion is the photocatalytic hydrogen generation using semiconductor materials for an eco-friendly technology. Designing a low-cost efficient material to overcome limited light absorption as well as rapid recombination of photogenerated charge carriers is essential to achieve considerable hydrogen generation. In recent years, sulfide based semiconductors have attracted scientific research interest due to their excellent solar response and narrow band gap. The present review focuses on the recent approaches in the development of hierarchical ternary sulfide based photocatalysts with a special focus on ZnIn2S4. We also observe how the electronic structure of ZnIn2S4 is beneficial for water splitting and the various strategies involved for improving the material efficiency for photocatalytic hydrogen generation. The review places emphasis on the latest advancement/new insights on ZnIn2S4 being used as an efficient material for hydrogen generation through photocatalytic water splitting. Recent progress on essential aspects which govern light absorption, charge separation and transport are also discussed in detail.

An Overview of the Photocatalytic Water Splitting over Suspended Particles

The conversion of solar to chemical energy is one of the central processes considered in the emerging renewable energy economy. Hydrogen production from water splitting over particulate semiconductor catalysts has often been proposed as a simple and a cost-effective method for large-scale production. In this review, we summarize the basic concepts of the overall water splitting (in the absence of sacrificial agents) using particulate photocatalysts, with a focus on their synthetic methods and the role of the so-called “co-catalysts”. Then, a focus is then given on improving light absorption in which the Z-scheme concept and the overall system efficiency are discussed. A section on reactor design and cost of the overall technology is given, where the possibility of the different technologies to be deployed at a commercial scale and the considerable challenges ahead are discussed. To date, the highest reported efficiency of any of these systems is at least one order of magnitude lower than that deserving consideration for practical applications.

The Improvement of Coralline-Like ZnGa2O4 by Cocatalysts for the Photocatalytic Degradation of Rhodamine B

To date, various methods have been used to synthesize ZnGa2O4 material to promote photodegradation performance. However, cocatalysts, which usually play a crucial role in the photocatalyst system, have not been studied extensively in photocatalytic degradation reactions. In this paper, ZnGa2O4 semiconducting material was synthesized by a traditional high-temperature solid-state reaction. The as-prepared powder was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet–visible diffused reflectance spectroscopy. The results indicate that the as-prepared sample is a highly crystallized granular sample with a bandgap of 4.44 eV and a uniform particle size distribution. Density functional theory (DFT) was utilized to calculate the electronic structure of ZnGa2O4. The valence bands and conduction bands were chiefly composed of O 2p atomic orbitals and the hybridization orbitals of Ga 4s4p and Zn4s4p, respectively. The photocatalytic performance was tested via the decomposition of rhodamine B (RhB) under the irradiation of ultraviolet light. Cu, Ag, Au, Ni, Pt, and Pd cocatalysts were loaded on the ZnGa2O4 photocatalyst by a photodeposition method. The relatively optimal cocatalyst of ZnGa2O4 in the photocatalytic degradation reaction is Au. Thereafter, the effect of loading different usage amounts of the Au cocatalyst for the photodegradation of the ZnGa2O4 photocatalyst was studied in detail. The experimental results displayed that the optimum photodegradation activity was confirmed with the 3 wt% Au/ZnGa2O4 sample, which was 14.1 times more than the unloaded photocatalyst. The maximum photocatalytic degradation ratio of RhB was 96.7%, with 180 min under ultraviolet light.

Glucose-Mediated Synthesis of Hierarchical Porous ZnGa2O4 Microspheres for Effective Photocatalytic Removal of Aromatic and Arsenic Pollutants

Porous ZnGa2O4 microspheres (P-ZGO) are synthesized by a facial glucose-mediated microwave hydrothermal method followed by annealing. The morphological, photoelectric and photocatalytic properties of the as-prepared P-ZGO sample are characterized in detail, and the results show that the P-ZGO photocatalyst has a good crystallinity, large species surface area, hierarchical mesoporosity, and distinguished photoelectric properties. Under 254 nm UV irradiation, the P-ZGO sample shows a much higher activity and stability than TiO2 in the photocatalytic degradation of gas-phase aromatic pollutants. The average conversion efficiencies of toluene and benzene over P-ZGO are ~56.6% and ~44.3%, and with corresponding mineralization rates of ~86.3% and ~65.2%, respectively. No remarkable deactivation of P-ZGO is observed in a 60 h heterogeneous photoreaction. Furthermore, the as-prepared P-ZGO sample also shows an excellent photocatalytic efficiency (up to 99.8%) for the liquid-phase As(III) removal from water. The distinguished photocatalytic performance of P-ZGO can be ascribed to its unique electronic structures and hierarchical morphologies. According to the results of our analysis, a possible mechanism is also proposed to elaborate the photocatalytic oxidation process in the pollutants/P-ZGO system.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Enhanced Photocatalytic Degradation of 2-Butanone Using Hybrid Nanostructures of Gallium Oxide and Reduced Graphene Oxide Under Ultraviolet-C Irradiation

Hybrid nanostructures made of gallium oxide (Ga2O3) and reduced graphene oxide (rGO) are synthesized using a facile hydrothermal process method, where the Ga2O3 nanostructures are well dispersed on the rGO surface. The formed Ga2O3-rGO hybrids are characterized via Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), a diffuse reflectance Ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer, Brunauer–Emmett–Teller (BET), and photoluminescence (PL). Gas chromatography mass spectrometry (GC-MS) was used for analyzing volatile organic compounds (VOCs). The photocatalytic activity of the hybrid nanostructures is evaluated via the degradation of the 2-butanone, representing the VOCs under 254-nm radiation in the atmosphere. That activity is then compared to that of the Ga2O3 and commercial TiO2-P25. The Ga2O3-rGO hybrid shows enhanced photocatalytic degradation of 2-butanone compared to Ga2O3 and TiO2-P25, which is attributed to the enhanced specific surface area. The results indicate that the Ga2O3-rGO hybrid could be a promising method of enhancing photocatalytic activity and thereby effectively degrading VOCs, including the 2-butanone.

Electroless deposition of RuO2-based nanoparticles for energy conversion applications

The core/shell VO_x·mH₂O/RuO₂·nH₂O, synthesized by simply immersing VOx·mH₂O into RuCl₃ solution, shows a better catalytic activity of ORR than RuO₂·nH₂O.

Cross-linked bond accelerated interfacial charge transfer in monolayer zinc indium sulfide (ZnIn2S4)/reduced graphene oxide (RGO) heterostructure for photocatalytic hydrogen production with mechanistic insight

By separating the photo-excited charge carriers, the cross-linked bonds enabled the monolayer ZnIn₂S₄/RGO heterostructure to produce more H₂.

Catalytically active and chemically inert CdIn2S4 coating on a CdS photoanode for efficient and stable water splitting

Cadmium sulfide was popularly utilized as a light harvesting material for photoelectrochemical (PEC) water splitting, however, the drawback of poor durability limits its practical application. Herein, we show that a catalytically active and chemically inert cadmium indium sulfide (CdIn₂S₄) can improve the stability and even photocurrent of a CdS photoelectrode.

Enhanced photocatalytic activities of single-crystalline ZnGa2O4 nanoprisms by the coexposed {111} and {110} facets

Facet-control has been a fascinating method for preparation of highly active photocatalysts. Herein, we report the hydrothermal preparation of single-crystalline ZnGa₂O₄ nanoprisms with the {111} and {110} facets coexposed. Studies of their photocatalytic performance have revealed that the as-formed nanoprisms exhibit significantly enhanced photocatalytic activities compared with ZnGa₂O₄ nanocubes with only the {100} facets exposed and nanosheets with the {110} facets exposed for H₂ evolution and contaminant degradation. Theoretical calculations combined with valence band XPS results indicate the formation of a surface heterojunction by the coexposed {111} and {110} facets, which promotes the photogenerated electron and hole migration to the {110} and {111} facets, respectively, and enhances the photocatalytic performance.

Development of non-oxide semiconductors as light harvesting materials in photocatalytic and photoelectrochemical water splitting

This perspective summarizes recent advances in the use of (oxy)nitrides and oxysulfides as light harvesting semiconductors for photocatalytic or photoelectrochemical water splitting.

Robust, double-shelled ZnGa2O4 hollow spheres for photocatalytic reduction of CO2 to methane

Robust, double-shelled ZnGa₂O₄ hollow spheres were fabricated for photocatalytic reduction of CO₂ to methane.

Ga-Promoted Photocatalytic H2 Production over Pt/ZnO Nanostructures

Photocatalytic H2 generation is investigated over a series of Ga-modified ZnO photocatalysts that were prepared by hydrothermal methods. It is found that the structural, textural, and optoelectronic properties remarkably depend on the Ga content. The photocatalytic activity is higher in samples with Ga content equal to or lower than 5.4 wt %, which are constituted by Zn1-xGaxO phases. Structural, textural, and optoelectronic characterization, combined with theoretical calculations, reveals the effect of Ga in the doped ZnO structures. Higher Ga incorporation leads to the formation of an additional ZnGa2O4 phase with spinel structure. The presence of such a phase is detrimental for the textural and optoelectronic properties of the photocatalysts, leading to a decrease in H2 production. When Pt is used as the cocatalyst, there is an increase of 1 order of magnitude in the activity with respect to the bare photocatalysts. This is a result of Pt acting as an electron scavenger, decreasing the electron-hole recombination rate and boosting the H2 evolution reaction.

Theoretical studies on the form and effect of N-doping in an ZnGa2O4 photocatalyst

Hybrid functional calculations were implemented to elucidate the origin of the improved visible light photocatalytic activity of N-ZnGa₂O₄.

Recent progress in oxynitride photocatalysts for visible-light-driven water splitting

Photocatalytic water splitting into hydrogen and oxygen is a method to directly convert light energy into storable chemical energy, and has received considerable attention for use in large-scale solar energy utilization. Particulate semiconductors are generally used as photocatalysts, and semiconductor properties such as bandgap, band positions, and photocarrier mobility can heavily impact photocatalytic performance. The design of active photocatalysts has been performed with the consideration of such semiconductor properties. Photocatalysts have a catalytic aspect in addition to a semiconductor one. The ability to control surface redox reactions in order to efficiently produce targeted reactants is also important for photocatalysts. Over the past few decades, various photocatalysts for water splitting have been developed, and a recent main concern has been the development of visible-light sensitive photocatalysts for water splitting. This review introduces the study of water-splitting photocatalysts, with a focus on recent progress in visible-light induced overall water splitting on oxynitride photocatalysts. Various strategies for designing efficient photocatalysts for water splitting are also discussed herein.

Copper(i) cysteine complexes: efficient earth-abundant oxidation co-catalysts for visible light-driven photocatalytic H2 production

Inspired by nature, a Cu(i)–cysteine complex serving as an oxidation co-catalyst to consume photo-induced holes has been developed for highly efficient visible light-driven photocatalytic H₂ production.

Improvement of photocatalytic oxidation activity on a WO3/TiO2 heterojunction composite photocatalyst with broad spectral response

WO₃/TiO₂ that can efficiently use ultraviolet and visible light simultaneously was designed and investigated for photocatalytic O₂ evolution.

An overview on visible light responsive metal oxide based photocatalysts for hydrogen energy production

The present review summarizes the recent development and challenges in visible light responsive metal oxide based photocatalysts for water splitting.

Suppression of photocatalysis and long-lasting luminescence in ZnGa2O4 by Cr3+ doping

Long-lasting luminescence intensity and photocatalytic activity are highly correlated; thus, we can develop photocatalytic activity via controlling luminescence properties.

Photocatalytic decomposition of humic acids in anoxic aqueous solutions producing hydrogen, oxygen and light hydrocarbons

Photocatalytic water splitting for hydrogen and oxygen production requires sacrificial electron donors, for example, organic compounds. Titanium dioxide catalysts doped with platinum, cobalt, tungsten, copper and iron were experimentally tested for the production of hydrogen, oxygen and low molecular weight hydrocarbons from aqueous solutions of humic substances (HS). Platinum-doped catalyst showed the best results in hydrogen generation, also producing methane, ethene and ethane, whereas the best oxygen production was exhibited by P25, followed by copper--and cobalt-containing photocatalysts. Iron-containing photocatalyst produced carbon monoxide as a major product. HS undergoing anoxic photocatalytic degradation produce hydrogen with minor hydrocarbons, and/or oxygen. It appears that better hydrogen yield is achieved when direct HS splitting takes place, as opposed to HS acting as electron donors for water splitting.

Photocatalytic CO(2) reduction using non-titanium metal oxides and sulfides

Titanium dioxide (TiO2 ) is by far the most widely used photocatalyst both for the degradation of pollutants and in the field of renewable energies for the production of solar fuels. However, TiO2 has strong limitations in CO2 reduction, particularly under visible light irradiation. The flat-band potential of electrons in the conduction band of TiO2 is lower than that required for CO2 reduction and, therefore, it seems appropriate to develop and validate materials other than TiO2 . In addition, the photoresponse of TiO2 requires photons of wavelengths in the UV range shorter than 380 nm and strategies to implement a visible-light photoresponse on TiO2 by doping have not been completely satisfactory particularly because of problems in reproducibility and stability of the materials. For these reasons, we focus in this Review on semiconductors other than TiO2 that show photocatalytic activity in CO2 reduction. Attention has been paid to the irradiation conditions to put the productivity data into context. The role of co-catalyst and heterojunctions to increase the efficiency of charge separation is also discussed. Our aim is to describe the state of the art in the field of photocatalytic CO2 reduction using materials other than TiO2 , trying to trigger further research in this area.

Photoassisted fabrication of zinc indium oxide/oxysulfide composite for enhanced photocatalytic H2 evolution under visible-light irradiation

A photoassisted approach has been developed to synthesize a zinc indium oxide (Zn5In2O8)/oxysulfide composite through in situ sulfuration of vacancy-rich Zn5In2O8. It was found that vacancies have a considerable impact on the formation of the composite. The composite exhibited an increased photocatalytic H2 evolution activity under visible-light irradiation, which probably resulted from the enhanced ability to separate photoinduced electrons and holes. The H2 evolution rate over the composite was about 17 times higher when using vacancy-rich rather than conventional Zn5In2O8. This study provides a new method of improving the activity of photocatalysts.

Efficient photocatalytic hydrogen production by platinum-loaded carbon-doped cadmium indate nanoparticles

Undoped and carbon doped cadmium indate (CdIn(2)O(4)) powders were synthesized using a sol-gel pyrolysis method and evaluated for hydrogen generation activity under UV-visible irradiation without the use of a sacrificial reagent. Each catalyst powder was loaded with a platinum cocatalyst in order to increase electron-hole pair separation and promote surface reactions. Carbon-doped indium oxide and cadmium oxide were also prepared and analyzed for comparison. UV-vis diffuse reflectance spectra indicate the band gap for C-CdIn(2)O(4) to be 2.3 eV. C-doped In(2)O(4) showed a hydrogen generation rate approximately double that of the undoped material. When compared to platinized TiO(2) in methanol, which was used as a control material, C-CdIn(2)O(4) showed a 4-fold increase in hydrogen production. The quantum efficiency of the material was calculated at different wavelength intervals and found to be 8.7% at 420-440 nm. The material was capable of hydrogen generation using visible light only and with good efficiency even at 510 nm.