Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications

Leveraging Persistent Homology Features for Accurate Defect Formation Energy Predictions via Graph Neural Networks

In machine-learning-assisted high-throughput defect studies, a defect-aware latent representation of the supercell structure is crucial for the accurate prediction of defect properties. The performance of current graph neural network (GNN) models is limited due to the fact that defect properties depend strongly on the local atomic configurations near the defect sites and due to the oversmoothing problem of GNN. Herein, we demonstrate that persistent homology features, which encode the topological information on the local chemical environment around each atomic site, can characterize the structural information on defects. Using the dataset containing a wide spectrum of O-based perovskites with all available vacancies as an example, we show that incorporating the persistent homology features, along with proper choices of graph pooling operations, significantly increases the prediction accuracy, with the MAE reduced by 55%. Those features can be easily integrated into the state-of-the-art GNN models, including the graph Transformer network and the equivariant neural network, and universally improve their performance. Besides, our model also overcomes the convergence issue with respect to the supercell size that was present in previous GNN models. Furthermore, using the datasets of defective BaTiO3 with multiple substitutions and multiple vacancies as examples, our GNN model can also predict the defect-defect interactions accurately. These results suggest that persistent homology features can effectively improve the performance of machine learning models and assist the accelerated discovery of functional defects for technological applications.

Recent advances in synthesis and application of Magnéli phase titanium oxides for energy storage and environmental remediation

High-temperature reduction of TiO2 causes the gradual formation of structural defects, leading to oxygen vacancy planar defects and giving rise to Magnéli phases, which are substoichiometric titanium oxides that follow the formula Ti n O2n-1, with 4 ≤ n ≤ 9. A high concentration of defects provides several possible configurations for Ti4+ and Ti3+ within the crystal, with the variation in charge ordered states changing the electronic structure of the material. The changes in crystal and electronic structures of Magnéli phases introduce unique properties absent in TiO2, facilitating their diverse applications. Their exceptional electrical conductivity, stability in harsh chemical environments and capability to generate hydroxyl radicals make them highly valuable in electrochemical applications. Additionally, their high specific capacity and corrosion resistance make them ideal for energy storage facilities. These properties, combined with excellent solar light absorption, have led to their widespread use in electrochemical, photochemical, photothermal, catalytic and energy storage applications. To provide a complete overview of the formation, properties, and environmental- and energy-related applications of Magnéli phase titanium suboxides, this review initially highlights the crystal structure and the physical, thermoelectrical and optical properties of these materials. The conventional and novel strategies developed to synthesise these materials are then discussed, along with potential approaches to overcome challenges associated with current issues and future low-energy fabrication methods. Finally, we provide a comprehensive overview of their applications across various fields, including environmental remediation, energy storage, and thermoelectric and optoelectronic technologies. We also discuss promising new directions for the use of Magnéli phase titanium suboxides and solutions to challenges in energy and environment-related applications, and provide guidance on how these materials can be developed and utilised to meet diverse research application needs. By making use of control measures to mitigate the potential hazards associated with their nanoparticles, Magnéli phases can be considered as versatile materials with potential for next generation energy needs.

Electron Beam Irradiation-Induced Defects Enhance Pt-TiO2 Photothermal Catalytic Degradation in PAEs: A Performance and Mechanism Study

Phthalic acid esters (PAEs), ubiquitous semi-volatile organic compounds (SVOCs) in indoor environments, pose adverse effects on human health. However, their degradation mechanisms and pathways remain unclear. Herein, we developed an efficient photothermal catalyst by introducing defects (oxygen vacancies, OVs) on TiO2 (P25) surfaces via electron beam irradiation technology with different irradiation doses (100, 300, 500, and 700 kGy). The TiO2 with defects was employed as a support to prepare Pt-TiO2 catalysts for the photothermal degradation of di (2-ethylhexyl) phthalate (DEMP) and dimethyl phthalate (DMP), two representative PAEs. TiO2 pre-treated with a 300 kGy irradiation dose supported the Pt catalyst (Pt-Ti-P-300) and presented the optimal catalytic performance for DEMP and DMP degradation. Characterization results confirmed that OVs were successfully introduced to the catalysts. Meanwhile, OVs induced by electron beam irradiation expanded the light absorption range and improved the generation and separation of photogenerated carriers, which significantly enhanced the catalytic activity of the catalysts for PAE degradation. Importantly, the degradation mechanism and pathway of DMP were further explored by using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and gas chromatography-mass spectrometry (GC-MS). These findings provide important insights into the electron beam irradiation-mediated regulation of catalysts and the photothermal catalytic removal of PAEs in indoor environments.

Photo(electro)catalytic Water Splitting for Hydrogen Production: Mechanism, Design, Optimization, and Economy

As an energy carrier characterized by its high energy density and eco-friendliness, hydrogen holds a pivotal position in energy transition. This paper elaborates on the scientific foundations and recent progress of photo- and electro-catalytic water splitting, including the corresponding mechanism, material design and optimization, and the economy of hydrogen production. It systematically reviews the research progress in photo(electro)catalytic materials, including oxides, sulfides, nitrides, noble metals, non-noble metal, and some novel photocatalysts and provides an in-depth analysis of strategies for optimizing these materials through material design, component adjustment, and surface modification. In particular, it is pointed out that nanostructure regulation, dimensional engineering, defect introduction, doping, alloying, and surface functionalization can remarkably improve the catalyst performance. The importance of adjusting reaction conditions, such as pH and the addition of sacrificial agents, to boost catalytic efficiency is also discussed, along with a comparison of the cost-effectiveness of different hydrogen production technologies. Despite the significant scientific advancements made in photo(electro)catalytic water splitting technology, this paper also highlights the challenges faced by this field, including the development of more efficient and stable photo(electro)catalysts, the improvement of system energy conversion efficiency, cost reduction, the promotion of technology industrialization, and addressing environmental issues.

Glycerol Adsorption on TiO2 Surfaces: A Systematic Periodic DFT Study

Conversion of glycerol to added-value products is desirable due to its surplus during biodiesel synthesis. TiO2 has been the most explored catalyst. We performed a systematic study of glycerol adsorption on anatase (101), anatase (001), and rutile (110) TiO2 at the Density Functional Theory level. We found several adsorption modes on these surfaces, with anatase (101) being the less reactive one, leading to adsorption energies between -0.8 and -0.4 eV, with all adsorptions molecular in nature. On the contrary, anatase (001) is the most reactive surface, leading to both molecular and dissociative adsorption modes, with energies ranging from -4 to -1 eV and undergoing severe surface reconstructions in some cases. Rutile (110) also shows both molecular and dissociative adsorptions, but it is less reactive than anatase (001). Surfaces with oxygen vacancies affects the adsorbed states and energies. The electronic structure analysis reveals that glycerol adsorption mainly affects the band gap of the material and not the individual contributions to the valence and conduction band. Bader charge analysis shows that strong adsorption modes on anatase (001) and rutile (110) are associated with large charge transfer from glycerol to the surface, while weak and molecular adsorption modes involve low charge transfer.

Investigation of the Photocatalytic Activity of Copper-Modified Commercial Titania (P25) in the Process of Carbon Dioxide Photoreduction

The photocatalytic reduction of CO2 to useful products is an area of active research because it shows a potential to be an efficient tool for mitigating climate change. This work investigated the modification of titania with copper(II) nitrate and its impact on improving the CO2 reduction efficiency in a gas-phase batch photoreactor under UV-Vis irradiation. The investigated photocatalysts were prepared by treating P25-copper(II) nitrate suspensions (with various Cu2+ concentrations), alkalized with ammonia water, in a microwave-assisted solvothermal reactor. The titania-based photocatalysts were characterized by SEM, EDS, ICP-OES, XRD and UV-Vis/DR methods. Textural properties were measured by the low-temperature nitrogen adsorption/desorption studies at 77 K. P25 photocatalysts modified with copper(II) nitrate used in the process of carbon dioxide reduction allowed for a higher efficiency both for the photocatalytic reduction of CO2 to CH4 and for the photocatalytic water decomposition to hydrogen as compared to a reference. Similarly, modified samples showed significantly higher selectivity towards methane in the CO2 conversion process than the unmodified sample (a change from 30% for a reference sample to 82% for the P25-R-Cu-0.1 sample after the 6 h process). It was found that smaller loadings of Cu are more beneficial for increasing the photocatalytic activity of a sample.

Tuning the metal-support interactions in titanium dioxide-supported palladium photocatalysts against toluene in air

The activity of supported noble metal (e.g., palladium (Pd)) catalysts is often governed by the combined effects of multiple factors (e.g., electronic and geometric properties of the support, surface chemistry of metal nanoparticles (NPs), and metal-support interactions). Pd/titanium dioxide (TiO2) catalyst has been developed as a highly efficient photocatalytic degradation (PCD) system against gaseous toluene based on high-temperature pretreatment (300 and 450 °C) in a mixed stream of hydrogen (H2) and (N2). The interaction of Pd NPs with TiO2 synergistically improves the PCD efficiency of toluene through the efficient adsorption and activation of toluene as well as molecular oxygen (O2) and water (H2O) for the facile generation of reactive oxygen species (ROS (e.g., superoxide anion (•O2-) and hydroxyl (•OH) radicals)). The PCD efficiency of the prepared sample against 5 ppm toluene (at 20% relative humidity (RH)) is 79.6% with the values of maximum reaction rate, quantum yield, space-time yield, and clean air delivery rate as 9.9 μmol g-1 h-1, 1.68E-03 molecules photon-1, 1.68E-02 molecules photon-1 g-1, and 4.8 L h-1, respectively. Based on this research, the PCD mechanism of gaseous toluene has been explored along with the dynamic behavior of O2 and H2O for ROS generation and their relative contribution to the PCD of toluene. As such, this research offers a perspective for designing advanced photocatalysts through surface defect engineering.

Investigation of structural, optical, antibacterial, and dielectric properties of sol-gel and biosynthesized TiO2 nanoparticles

This study explored the structural, optical, antibacterial, and dielectric properties of TiO2 nanoparticles synthesized using two distinct approaches: sol-gel and biosynthesis. Density functional tight binding (DFTB+) and density functional theory (DFT) calculations were employed alongside experimental techniques to gain a comprehensive understanding of the electronic-property relationships. Allium sativum peel extract was utilized for the biosynthesis method. X-ray diffraction (XRD) affirmed the anatase phase formation for both nanoparticles. Rietveld technique was employed for a detailed structural analysis. The FESEM analysis revealed the diminutive particle size of TiO2 nanoparticles with a comparable size distribution for both variants. However, the biosynthesized variant exhibited smaller average particle size (26.74 nm) than the sol-gel variant (32.22 nm). Optical studies showed an absorption redshift for the biosynthesized variant (352 nm) relative to the sol-gel variant (347 nm). The band gap energy is higher for the sol-gel variant (3.17 eV) compared to the biosynthesized variant (3.02 eV). The biosynthesized nanoparticles showed strong antibacterial activity, with inhibition zones of 15 mm against E. coli and S. flexneri bacteria. Dielectric analysis revealed that the sol-gel synthesized nanoparticles exhibited a higher dielectric permittivity of 27.80 and a lower dielectric loss of 0.37 at 1 kHz, compared to the biosynthesized nanoparticles, which showed a dielectric permittivity of 19.48 and a dielectric loss of 0.69.

Oxygen vacancy-mediated Mn2O3 catalyst with high efficiency and stability for toluene oxidation

Oxygen vacancy engineering in transition metal oxides is an effective strategy for improving catalytic performance. Herein, defect-enriched Mn2O3 catalysts were constructed by controlling the calcination temperature. The high content of oxygen vacancies and accompanying Mn4+ ions were generated in Mn2O3 catalysts calcined at low temperature, which could greatly improve the low-temperature reducibility and migration of surface oxygen species. DFT theoretical calculations further confirmed that molecular oxygen and toluene were easily adsorbed over defective α-Mn2O3 (222) facets with an energy of -0.29 and -0.48 eV, respectively, and corresponding OO bond length is stretched to 1.43 Å, resulting in the highly reactive oxygen species. Mn2O3-300 catalyst with abundant oxygen vacancies exhibited the highest specific reaction rate and lowest activation energy. Furthermore, the optimized catalyst possessed the outstanding stability, water tolerance and CO2 yield. In comparison with the fresh Mn2O3-300 catalyst, the physical structure and surface property of the used catalyst remained almost unchanged regardless of whether undergoing the stability test at consecutive catalytic runs as well as high temperature, and water resistance test. In situ DRIFTS spectra further elucidated that introducing the water vapor had little effect on the reaction intermediates, indicating the excellent durability of the defect-enriched catalyst.

Localized Electron Redistribution in Methanol Molecules over the Sea Urchin-like Tricobalt Tetroxide/Copper Oxide Nanostructures for Fast Hydrogen Release

Catalytic methanolysis of ammonia borane (NH3BH3) is a prospective technology in the field of hydrogen energy in which hydrogen production and hydrogen storage can be integrated together. The splitting of the O-H bond is identified as the rate-determining step (RDS) in this reaction. Thus, a deep understanding of the relationship between the electronic structure of the catalyst, especially the localized electron density of active sites, and the breaking behaviors of the O-H bond is of extreme importance for the rational design of robust catalysts for the reaction. In this work, sea urchin-like tricobalt tetroxide/copper oxide (Co3O4/CuO) nanostructures with rich oxygen vacancies (Ov) were fabricated by a simple synthetic route. In NH3BH3 methanolysis, the optimal Co3O4/CuO sample exhibited ultrahigh catalytic activity with a turnover frequency (TOF) of 87.5 min-1. Interestingly, when NH3BH3 methanolysis was carried out under visible-light illumination, the TOF further increased to 116.4 min-1, which is the highest TOF value among those of the noble-metal-free catalysts ever documented in the literature. Theoretical calculation results evidenced that the Cu site in the Co3O4/CuO sample was in charge of the adsorption and activation of methanol molecules. Both the Ov and visible-light illumination can help electrons on the Cu site flow to the adsorbed methanol molecule, thus leading to localized electron redistribution of the methanol molecule and the extension of the O-H bond. The cooperation of Ov and visible light makes splitting of the O-H bond easier, which is favorable for fast hydrogen release from NH3BH3 methanolysis. This study helps us to gain an insight into the influence of localized electron redistribution of methanol molecules on the RDS, which conduces to the rational design of highly effective nanocatalysts. Moreover, the coinduction strategy for localized electron redistribution with oxygen vacancy engineering and visible-light illumination opens up a route to boost catalytic activity in NH3BH3 methanolysis.

Recent progresses and challenges in colloidal quantum dot light-emitting diodes: a focus on electron transport layers with metal oxide nanoparticles and organic semiconductors

Colloidal quantum dots (QDs) are highly promising for display technologies due to their distinctive optical characteristics, such as tunable emission wavelengths, narrow emission spectra, and superb photoluminescence quantum yields. Over the last decade, both academic and industrial research have substantially advanced quantum dot light-emitting diode (QLED) technology, primarily through the development of higher-quality QDs and more refined device structures. A key element of these advancements includes progress in the electron transport layer (ETL) technology, with metal oxide (MO) nanoparticles (NPs) like ZnO and ZnMgO emerging as superior choices due to their robust performance. Nevertheless, scalability challenges, such as particle agglomeration and positive aging, have prompted research into organic semiconductors that match the performance of MO NPs. This review aims to provide a detailed examination and comprehensive understanding of recent advances and challenges in ETLs based on both MO NPs and organic semiconductors, guiding future commercialization efforts for QLEDs.

Superior Singlet Oxygen Electrosynthesis via Neighboring Dual Molecular Oxygen Coactivation for Selective Tetracycline Detoxification

Oxygen (O2) electroreduction offers a green approach for singlet oxygen (1O2) synthesis in wastewater contaminants detoxification. However, traditional single O2 activation on single-metal catalytic sites seriously suffers from the kinetically-unfavorable desorption of adsorbed superoxide species (•O2 -*/•OOH*). Here, we demonstrate a novel dual O2 coactivation pathway on shortened Fe1-OV-Ti sites for superior 1O2 electrosynthesis through a rapid disproportionate process between surface-confined •O2 -*/•OOH*. Theoretical calculations combined with in situ electrochemical spectroscopies demonstrated that the shortened distance between Fe single atom and adjacent unsaturated Ti atom facilitates the direct recombination of surface-confined Fe-•OOH and Ti-•OO- to yield 1O2, bypassing the formidable •O2 -*/•OOH* desorption process. Impressively, Fe1-OV-Ti could realize an excellent 1O2 electrosynthesis rate of 54.5 μmol L-1 min-1 with an outstanding 1O2 selectivity of 97.6 % under neutral condition, surpassing that of Fe1-O-Ti (27.1 μmol L-1 min-1, 91.7 %). Using tetracycline (TC) as a model pollutant, the resulting Fe1-OV-Ti electrode achieved nearly 100 % degradation in 120 min at -0.6 V, meanwhile preventing the generation of toxic intermediates. This study provides a new 1O2 electrosynthesis strategy by controlling the distance of adjacent catalytic sites for the coactivation of dual molecular oxygen.

Oxidative stress-modifying effects of TiO2nanoparticles with varying content of Ti3+(Ti2+) ions

Nanoparticles (NPs) with reactive oxygen species (ROS)-regulating ability have recently attracted great attention as promising agents for nanomedicine. In the present study, we have analyzed the effects of TiO2defect structure related to the presence of stoichiometric (Ti4+) and non-stoichiometric (Ti3+and Ti2+) titanium ions in the crystal lattice and TiO2NPs aggregation ability on H2O2- and tert-butyl hydroperoxide (tBOOH)-induced ROS production in L929 cells. Synthesized TiO2-A, TiO2-B, and TiO2-C NPs with varying Ti3+(Ti2+) content were characterized by x-ray powder diffraction, transmission electron microscopy, small-angle x-ray scattering, x-ray photoelectron spectroscopy, and optical spectroscopy methods. Given the role of ROS-mediated toxicity for metal oxide NPs, L929 cell viability and changes in the intracellular ROS levels in H2O2- and tBOOH-treated L929 cells incubated with TiO2NPs have been evaluated. Our research shows that both the amount of non-stoichiometric Ti3+and Ti2+ions in the crystal lattice of TiO2NPs and NPs aggregative behavior affect their catalytic activity, in particular, H2O2decomposition and, consequently, the efficiency of aggravating H2O2- and tBOOH-induced oxidative damage to L929 cells. TiO2-A NPs reveal the strongest H2O2decomposition activity aligning with their less pronounced additional effects on H2O2-treated L929 cells due to the highest amount of Ti3+(Ti2+) ions. TiO2-C NPs with smaller amounts of Ti3+ions and a tendency to aggregate in water solutions show lower antioxidant activity and, consequently, some elevation of the level of ROS in H2O2/tBOOH-treated L929 cells. Our findings suggest that synthesized TiO2NPs capable of enhancing ROS generation at concentrations non-toxic for normal cells, which should be further investigated to assess their possible application in nanomedicine as ROS-regulating pharmaceutical agents.

Enhanced photocatalytic performance of colored Ti2O3-Ti3O5-TiO2 heterostructure for the degradation of antibiotic ofloxacin and bactericidal effect

Removing emergent contaminants, such as pharmaceuticals, and inhibiting bacteria by photocatalysis represents an interesting alternative for water remediation. We report the effective preparation of colored powders containing Ti2O3, Ti3O5, and TiO2, by a simple thermal oxidation reaction of a Ti2O3 precursor from 400 °C to 800 °C. The material obtained at 500 °C (P500 sample) exhibited the highest photocatalytic performance under simulated solar light, reaching 54% degradation of antibiotic ofloxacin and a bacteria inactivation of 51% and 62% for Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), respectively. The superoxide anion radical was the main specie contributing to the photodegradation of ofloxacin, while the hydroxyl radical showed negligible effect. A synergy between the physicochemical properties of the phases in the P500 sample contributes to the electrons transfer, visible light absorption capability and generation of reactive oxygen species, resulting in its remarkable photoactivity. The comparison in terms of surface-specific activity revealed that the P500 sample is more efficient than commercially available TiO2 P25. This fact opens the option of using commercially available Ti2O3 and TiO2 P25 to obtain composites for promoting photoinduced reactions using natural solar light.

Ba2+-doping introduced piezoelectricity and efficient Ultrasound-Triggered bactericidal activity of brookite TiO2 nanorods

Exploring highly efficient ultrasound-triggered catalysts is pivotal for various areas. Herein, we presented that Ba2+ doped brookite TiO2 nanorod (TiO2: Ba) with polarization-induced charge separation is a candidate. The replacement of Ba2+ for Ti4+ not only induced significant lattice distortion to induce polarization but also created oxygen vacancy defects for facilitating the charge separation, leading to high-efficiency reactive oxygen species (ROS) evolution in the piezo-catalytic processes. Furthermore, the piezocatalytic ability to degrade dye wastewater demonstrates a rate constant of 0.172 min-1 and achieves a 100 % antibacterial rate at a low dose for eliminating E. coli. This study advances that doping can induce piezoelectricity and reveals that lattice distortion-induced polarization and vacancy defects engineering can improve ROS production, which might impact applications such as water disinfection and sonodynamic therapy.

Enhanced Photocatalytic Properties and Photoinduced Crystallization of TiO2-Fe2O3 Inverse Opals Fabricated by Atomic Layer Deposition

The use of solar energy for photocatalysis holds great potential for sustainable pollution reduction. Titanium dioxide (TiO2) is a benchmark material, effective under ultraviolet light but limited in visible light utilization, restricting its application in solar-driven photocatalysis. Previous studies have shown that semiconductor heterojunctions and nanostructuring can broaden the TiO2's photocatalytic spectral range. Semiconductor heterojunctions are interfaces formed between two different semiconductor materials that can be engineered. Especially, type II heterojunctions facilitate charge separation, and they can be obtained by combining TiO2 with, for example, iron(III) oxide (Fe2O3). Nanostructuring in the form of 3D inverse opals (IOs) demonstrated increased TiO2 light absorption efficiency of the material, by tailoring light-matter interactions through their photonic crystal structure and specifically their photonic stopband, which can give rise to a slow photon effect. Such effect is hypothesized to enhance the generation of free charges. This work focuses on the above-described effects simultaneously, through the synthesis of TiO2-Fe2O3 IOs via multilayer atomic layer deposition (ALD) and the characterization of their photocatalytic activities. Our results reveal that the complete functionalization of TiO2 IOs with Fe2O3 increases the photocatalytic activity through the slow photon effect and semiconductor heterojunction formation. We systematically explore the influence of Fe2O3 thickness on photocatalytic performance, and a maximum photocatalytic rate constant of 1.38 ± 0.09 h-1 is observed for a 252 nm template TiO2-Fe2O3 bilayer IO consisting of 16 nm TiO2 and 2 nm Fe2O3. Further tailoring the performance by overcoating with additional TiO2 layers enhances photoinduced crystallization and tunes photocatalytic properties. These findings highlight the potential of TiO2-Fe2O3 IOs for efficient water pollutant removal and the importance of precise nanostructuring and heterojunction engineering in advancing photocatalytic technologies.

Acid etching-induced nanocutting of LaNiO3 transplanting self-assembled photodiode array-like LaNiO3/N,P-RGO nanoreactor for efficient photocatalytic hydrogen evolution

Nanoscale graphene-semiconductor composite photocatalysts with fascinating properties in the photocatalytic hydrogen evolution have inspired numerous interests in broad research fields. The architectures with efficient light response and promoting charge separation at the interface between reduced graphene oxide (RGO) and semiconductor are critical, yet synthesizing them remains a formidable challenge. Herein, the photodiode array-like LaNiO3/N,P-RGO (LNO/N,P-RGO) nanoreactor was constructed using an innovative strategy of acid etching-induced nanocutting self-assembly. Ammonium dihydrogen phosphate working as both a nitrogen phosphorus co-dopant and an acid etching reagent, cuts perovskite LaNiO3 (LNO) nanoparticles into nanorods, which are bonded evenly on the nitrogen phosphorus co-doped reduced graphene oxide (N,P-RGO) to form an n-n semiconductor heterojunction LNO/N,P-RGO as a photodiode array-like nanoreactor via hydrothermal treatment. The photodiode array-like nanostructure exposes more active sites that are conducive to light absorption. The robust Ni-C and P-O bonds promote the narrowing of space-charge region at the interface by UV irradiation, thereby improving the transport of photogenerated carriers by visible light irradiation. The LNO/N,P-RGO nanoreactor exhibits excellent photocatalytic hydrogen evolution performance with a yield of up to 354 μmol g-1 h-1 under UV-visible light, which is 50 times higher than that of pure perovskite LNO, and it also displays favorable recycling stability.

Self-Modification of Defective TiO2 under Controlled H2/Ar Gas Environment and Dynamics of Photoinduced Surface Oxygen Vacancies

In recent years, defective TiO2 has caught considerable research attention because of its potential to overcome the limits of low visible light absorption and fast charge recombination present in pristine TiO2 photocatalysts. Among the different synthesis conditions for defective TiO2, ambient pressure hydrogenation with the addition of Ar as inert gas for safety purposes has been established as an easy method to realize the process. Whether the Ar gas might still influence the resulting photocatalytic properties and defective surface layer remains an open question. Here, we reveal that the gas flow ratio between H2 and Ar has a crucial impact on the defective structure as well as the photocatalyic activity of TiO2. In particular, transmission electron microscopy (TEM) in combination with electron energy loss spectroscopy (EELS) revealed a larger width of the defective surface layer when using a H2/Ar (50 %-50 %) gas mixture over pure H2. A possible reason could be the increase in dynamic viscosity of the gas mixture when Ar is added. Additionally, photoinduced enhanced Raman spectroscopy (PIERS) is implemented as a complementary approach to investigate the dynamics of the defective structures under ambient conditions which cannot be effortlessly realized by vacuum techniques like TEM.

A Comprehensive Review of Solar Photocatalysis & Photothermal Catalysis for Hydrogen Production from Biomass: from Material Characteristics to Engineering Application

Biomass photoreforming is a promising way of producing sustainable hydrogen thanks to the abundant sources of biomass feedstocks. Solar energy provides the heat and driven force to initial biomass oxidation coupled with H2 evolution. Currently, biomass photoreforming is still far from plant-scale applications due to the lower solar energy utilization efficiencies, the low H2 yield, and the lack of appropriate photoreactors. The production of H2 from photoreforming of native biomass and platform molecules was summarized and discussed with particular attention to the prospects of scaling up the catalysis technology for mass hydrogen production. The types of photoreforming, including photocatalysis and photothermal catalysis, were discussed, consequently considering the different requirements for photoreactors. We also reviewed the photoreactors that support biomass photoreforming. Numerical simulation methods were implemented for the solid-liquid two-phase flow and inter-particle radiative transfer involved in the reaction process. Developing concentrated photothermal catalytic flowed reactors is beneficial to scale-up catalytic hydrogen production from biomass.

Novel ternary nanocomposite (TiO2@Fe3O4-chitosan) system for nitrate removal from water: an adsorption cum photocatalytic approach

Nitrate pollution of water emerging from various anthropogenic activities has become a major environmental concern because of its deleterious effects on natural water resources. The present work deals with the synthesis of the ternary nanocomposite based on chitosan, iron oxide (Fe3O4), and titanium dioxide (TiO2) and its application for the removal of nitrates from model-contaminated water. Fe3O4 derived through a coprecipitation method was incorporated into the chitosan matrix which was fabricated in the form of beads. The wet gel beads were then successfully coated with sol-gel-derived silver-doped titanium dioxide sol followed by drying under suitable conditions to get the functional nanocomposite beads. The synthesized functional materials were further characterized for their structural, morphological, and textural features using X-ray diffraction analysis, physical property measurement (PPMS), Fourier transform infrared (FTIR) analysis, UV visible spectroscopy analysis (UV-vis), BET surface area analysis (BET), field emission scanning electron microscopic (FESEM), and transmission electron microscopy (TEM) analysis. The ternary nanocomposites were further used for the removal of nitrates via adsorption cum photocatalytic reduction technique from the model contaminated water when subjected to an adsorption study under dark conditions and photocatalytic study under UV/visible/sunlight for a definite time. Fe3O4 in the nanocomposite provides enhanced adsorption features whereas the functional coating of titanium dioxide aids in the removal of nitrates through the photocatalytic reduction technique. The functional beads containing 3% Fe3O4 in the wet gel form (CTA-F3) have excellent nitrate removal efficiency of ~ 97% via adsorption cum solar photocatalysis towards the removal of nitrate ions from 50 ppm nitrate solution, whereas the dried nanocomposite beads have got a nitrate removal efficiency of ~ 68% in 1 h from 100 ppm nitrate solution. Continuous flow adsorption cum photocatalytic study was performed further using the oven-dried functional beads in which flow rate and bed height were varied while maintaining the concentration of feed solution as constant. A nitrate removal efficiency of 65% and an adsorption capacity of 4.1 mgg-1 were obtained for the CTA-F3 beads in the continuous flow adsorption cum photocatalysis experiment for up to 5 h when using an inlet concentration of 100 ppm, bed height 12 cm, and flow rate 5.0 ml min-1. A representative fixed-bed column adsorption experiment conducted using CTA-F3 beads for the treatment of a real groundwater sample shows reasonable results for nitrate removal (71.7% efficiency) along with a significant removal rate for the other anions as well. Thus, the novel adsorbent/photocatalyst developed is suitable for the removal of nitrates from water due to the synergistic effect between Fe3O4, chitosan, and titanium dioxide.

Effect of Nitrogen Doping on the Photocatalytic Properties and Antibiofilm Efficacy of Reduced TiO2 Nanoparticles

Nanomaterial-mediated antibacterial photodynamic therapy (aPDT) emerges as a promising treatment against antibiotic-resistant bacterial biofilms. Specifically, titanium dioxide nanoparticles (TiO2 NPs) are being investigated as photosensitizers in aPDT to address biofilm related diseases. To enhance their photocatalytic performance in the visible spectral range for biomedical applications, various strategies have been adopted, including reduction of TiO2 NPs. However, despite improvements in visible-light photoactivity, reduced TiO2 NPs have yet to reach their expected performance primarily due to the instability of oxygen vacancies and their tendency to reoxidize easily. To address this, we present a two-step approach to fabricate highly visible-light active and stable TiO2 NP photocatalysts, involving nitrogen doping followed by a magnesium-assisted reductive annealing process. X-ray photoelectron spectroscopy analysis of the synthesized reduced nitrogen-doped TiO2 NPs (H:Mg-N-TiO2 NPs) reveals that the presence of nitrogen stabilizes oxygen vacancies and reduced Ti species, leading to increased production of reactive oxygen species under visible-light excitation. The improved aPDT efficiency translates to a 3-fold enhancement in the antibiofilm activity of nitrogen-doped compared to undoped reduced TiO2 NPs against both Gram-positive (Streptococcus mutans) and Gram-negative (Porphyromonas gingivalis, Fusobacterium nucleatum) oral pathogens. These results underscore the potential of H:Mg-N-TiO2 NPs in aPDT for combating bacterial biofilms effectively.

Recent progress in defect-engineered metal oxides for photocatalytic environmental remediation

Rapid industrial advancement over the last few decades has led to an alarming increase in pollution levels in the ecosystem. Among the primary pollutants, harmful organic dyes and pharmaceutical drugs are directly released by industries into the water bodies which serves as a major cause of environmental deterioration. This warns of a severe need to find some sustainable strategies to overcome these increasing levels of water pollution and eliminate the pollutants before being exposed to the environment. Photocatalysis is a well-established strategy in the field of pollutant degradation and various metal oxides have been proven to exhibit excellent physicochemical properties which makes them a potential candidate for environmental remediation. Further, with the aim of rapid industrialization of photocatalytic pollutant degradation technology, constant efforts have been made to increase the photocatalytic activity of various metal oxides. One such strategy is the introduction of defects into the lattice of the parent catalyst through doping or vacancy which plays a major role in enhancing the catalytic activity and achieving excellent degradation rates. This review provides a comprehensive analysis of defects and their role in altering the photocatalytic activity of the material. Various defect-rich metal oxides like binary oxides, perovskite oxides, and spinel oxides have been summarized for their application in pollutant degradation. Finally, a summary of existing research, followed by the existing challenges along with the potential countermeasures has been provided to pave a path for the future studies and industrialization of this promising field.

Synergistic effect of atomically dispersed Cu species and Ti-defects for boosting photocatalytic CO2 reduction over hierarchical TiO2

The photocatalytic water-mediated CO2 reduction reaction, which holds great promise for the conversion of CO2 into valuable chemicals, is often hindered by inefficient separation of photogenerated charges and a lack of suitable catalytic sites. Herein, we have developed a glycerol coordination assembly approach to precisely control the distribution of atomically dispersed Cu species by occupying Ti-defects and adjusting the ratio between Cu species and Ti-defects in a hierarchical TiO2. The optimal sample demonstrates a ∼4-fold improvement in CO2-to-CO conversion compared to normal TiO2 nanoparticles. The high activity could be attributed to the Ti defects, which enhance the photogenerated charge separation and simultaneously facilitate the adsorption of water molecules, thereby promoting the water oxidation reaction. Moreover, by means of in situ EPR and FTIR spectra, we have demonstrated that Cu species can effectively capture photogenerated electrons and facilitate the adsorption of CO2, so as to catalyze the reduction of CO2. This work provides a strategy for the construction of atomic-level synergistic catalytic sites and the utilization of in situ techniques to reveal the underlying mechanism.

Constructing extrinsic oxygen vacancy on the surface of photocatalyst as CO2 and electrons reservoirs to improve photocatalytic CO2 reduction activity

Constructing own oxygen vacancies in the photocatalysts is a very promising method to improve their photocatalytic CO2 reduction activity. However, some catalysts have excellent stabilities, making it difficult for them to construct their own oxygen vacancies. To simplify the above difficulty of stable photocatalysts, constructing extrinsic oxygen vacancies on their surface as a novel idea is proposed. Here, a stable TiO2 nanosheet is chosen as a research object, we uniformly deposited BiOCl quantum dots on their surface via a simple adsorption-deposition method. It is found that BiOCl quantum dots are able to simultaneously self-transform into defective BiOCl with many oxygen vacancies when the photocatalyst is performed photocatalytic CO2 reduction. These extrinsic oxygen vacancies can act as "CO2 and photo-generated electrons reservoirs" to improve CO2 capture and accelerate the separation of photogenerated electrons and holes. For the above reasons, the modified TiO2 showed obvious enhancement of photocatalytic CO2 reduction compared to pristine TiO2 and BiOCl. This work may open a new avenue to broaden the use of oxygen vacancies in the process of photocatalytic CO2 reduction.

A Bulk Oxygen Vacancy Dominating WO3-x Photocatalyst for Carbamazepine Degradation

Creating oxygen vacancy in tungsten trioxide (WO3) has been considered as an effective strategy to improve the photocatalytic performance for degrading organic pollutants. In this study, oxygen vacancies were introduced into WO3 by thermal treatment under Ar atmosphere and their proportion was changed by setting different treatment times. WO3-x samples show better photoelectric properties and photocatalytic degradation performance for carbamazepine (CBZ) than an oxygen-vacancy-free sample, and WO3-x with the optimal proportion of oxygen vacancies is obtained by thermal treatment for 3 h in 550 °C. Furthermore, it discovers that the surface oxygen vacancies on WO3-x would be recovered when it is exposed to air, resulting in a bulk oxygen vacancy dominating WO3-x (bulk-WO3-x). The bulk-WO3-x exhibited much higher degradation efficiency for CBZ than WO3-x with both surface and bulk oxygen vacancies. The mechanism study shows bulk-WO3-x mainly degrades the CBZ by producing OH radicals and superoxide radicals, while oxygen-vacancy-free sample mainly oxidizes the CBZ by the photoexcited hole, which requires the CBZ to be adsorbed on the surface for degradation. The radical generated by bulk-WO3-x exhibits stronger oxidizing capacity by migrating to the solution for CBZ degradation. In summary, the influence of oxygen vacancy on photocatalytic degradation performance depends on both the proportion and location distribution and could lie in the optimization of the photodegradation mechanism. The results of this study could potentially broaden our understanding of the role of oxygen vacancies and provide optimal directions and methods for oxygen vacancy regulation for photocatalysts.

Super-Assembled Multilayered Mesoporous TiO2 Nanorockets for Light-Powered Space-Confined Microfluidic Catalysis

In the field of sustainable chemistry, it is still a significant challenge to realize efficient light-powered space-confined catalysis and propulsion due to the limited solar absorption efficiency and the low mass and heat transfer efficiency. Here, novel semiconductor TiO2 nanorockets with asymmetric, hollow, mesoporous, and double-layer structures are successfully constructed through a facile interfacial superassembly strategy. The high concentration of defects and unique topological features improve light scattering and reduce the distance for charge migration and directed charge separation, resulting in enhanced light harvesting in the confined nanospace and resulting in enhanced catalysis and self-propulsion. The movement velocity of double-layered nanorockets can reach up to 10.5 μm s-1 under visible light, which is approximately 57 and 119% higher than that of asymmetric single-layered TiO2 and isotropic hollow TiO2 nanospheres, respectively. In addition, the double-layered nanorockets improve the degradation rate of the common pollutant methylene blue under sustainable visible light with a 247% rise of first-order rate constant compared to isotropic hollow TiO2 nanospheres. Furthermore, FEA simulations reveal and confirm the double-layered confined-space enhanced catalysis and self-propulsion mechanism.

Understanding and optimizing the sensitization of anatase titanium dioxide surface with hematite clusters

The presence of hematite (Fe2O3) clusters at low coverage on titanium dioxide (TiO2) surface has been observed to enhance photocatalytic activity, while excess loading of hematite is detrimental. We conduct a comprehensive density functional theory study of Fe2O3clusters adsorbed on the anatase TiO2(101) surface to investigate the effect of Fe2O3on TiO2. Our study shows that TiO2exhibits improved photocatalytic properties with hematite clusters at low coverage, as evidenced by a systematic study conducted by increasing the number of cluster adsorbates. The adsorption of the clusters generates impurity states in the band gap improving light absorption and consequently affecting the charge transfer dynamics. Furthermore, the presence of hematite clusters enhances the activity of TiO2in the hydrogen evolution reaction. The Fe valence mixing present in some clusters leads to a significant increase in H2evolution rate compared with the fixed +3 valence of Fe in hematite. We also investigate the effect of oxygen defects and find extensive modifications in the electronic properties and local magnetism of the TiO2-Fe2O3system, demonstrating the wide-ranging effect of oxygen defects in the combined system.

A comprehensive review of oxygen vacancy modified photocatalysts: synthesis, characterization, and applications

Photocatalytic technology is a novel approach that harnesses solar energy for efficient energy conversion and effective pollution abatement, representing a rapidly advancing field in recent years. The development and synthesis of high-performance semiconductor photocatalysts constitute the pivotal focal point. Oxygen vacancies, being intrinsic defects commonly found in metal oxides, are extensively present within the lattice of semiconductor photocatalytic materials exhibiting non-stoichiometric ratios. Consequently, they have garnered significant attention in the field of photocatalysis as an exceptionally effective means for modulating the performance of photocatalysts. This paper provides a comprehensive review on the concept, preparation, and characterization methods of oxygen vacancies, along with their diverse applications in nitrogen fixation, solar water splitting, CO2 photoreduction, pollutant degradation, and biomedicine. Currently, remarkable progress has been made in the synthesis of high-performance oxygen vacancy photocatalysts and the regulation of their catalytic performance. In the future, it will be imperative to develop more advanced in situ characterization techniques, conduct further investigations into the regulation and stabilization of oxygen vacancies in photocatalysts, and comprehensively comprehend the mechanism underlying the influence of oxygen vacancies on photocatalysis. The engineering of oxygen vacancies will assume a pivotal role in the realm of semiconductor photocatalysis.

The direct Z-scheme character and roles of S vacancy in BiOCl/Bi2S3-(001) heterostructures for superior photocatalytic activity: a hybrid density functional investigation

Given some current speculations and controversies regarding the type of BiOCl/Bi2S3-(001) heterostructure in experiments, it is of great importance to clarify these controversies and further explain the relevant experimental results. In this work, based on first-principles hybrid density functional calculations, it is verified that the BiOCl/Bi2S3-(001) heterostructure is a direct Z-scheme photocatalyst with high photo-generated carrier separation efficiency and strong redox ability that can react with O2 and OH- to produce photocatalytic active species of superoxide ions (˙O2-) and hydroxyl radicals (˙OH), respectively. This is consistent with the experimental findings and explains the excellent photocatalytic performance of the BiOCl/Bi2S3-(001) heterostructure in experiments. Besides, excitingly, it is found that the optical absorption, built-in electric field intensity, interlayer recombination probability, hydrogen evolution reaction ability, and the difference in electron-hole mobility are further enhanced via S vacancy introduction in BiOCl/Bi2S3-(001). Therefore, the significant roles of S vacancy in further improving the photocatalytic properties of the BiOCl/Bi2S3-(001) heterostructure are profoundly revealed. This work can provide valuable theoretical insights for designing the superior direct Z-scheme BiOCl/VS-Bi2S3-(001) heterostructure with promising photocatalytic properties.

Nanoflower-like NiCo2O4 Composite Graphene Oxide as a Bifunctional Catalyst for Zinc-Air Battery Cathode

Developing efficient bifunctional catalysts for nonprecious metal-based oxygen reduction (ORR) and oxygen evolution (OER) is crucial to enhance the practical application of zinc-air batteries. The study harnessed electrostatic forces to anchor the nanoflower-like NiCo2O4 onto graphene oxide, mitigating the poor inherent conductivity in NiCo2O4 as a transition metal oxide and preventing excessive agglomeration of the nanoflower-like structures during catalysis. Consequently, the resulting composite, NiCo2O4-GO/C, exhibited notably superior ORR and OER catalytic performance compared to pure nanoflower-like NiCo2O4. Notably, it excelled in OER catalytic activity of the OER relative to the precious metal RuO2. As a bifunctional catalyst for ORR and OER, NiCo2O4-GO/C displayed a potential difference of 0.88 V between the ORR half-wave potential and the OER potential at 10 mA·cm-2, significantly lower than the 1.08 V observed for pure flower-like NiCo2O4 and comparable to the 0.88 V exhibited by precious metal catalysts Pt/C + RuO2. The NiCo2O4-GO/C-based zinc-air battery demonstrated a discharge capacity of 817.3 mA h·g-1, surpassing that of precious metal-based zinc-air batteries. Moreover, charge-discharge cycling tests indicated the superior stability of the NiCo2O4-GO/C-based zinc-air battery compared to its precious metal-based counterparts.

Removal of sulfonylurea herbicides with g-C3N4-based photocatalysts: A review

The accumulation of agricultural chemicals in the environment has become a global concern, of which sulfonylurea herbicides (SUHs) constitute a significant category. Solar-driven photocatalysis is favored for removing organic pollutants due to its high efficiency and environmental friendliness. Graphite carbon nitride (g-C3N4)-based materials with superior catalytic activities and physicochemical stabilities are promising photocatalysts. This review describes the g-C3N4-based materials and their uses in the photocatalytic degradation of SUHs or other organic pollutants with similar structures. First, the fundamentals of g-C3N4-based materials and photocatalytic SUHs degradation are discussed to provide an in-depth understanding of the mechanism for the photocatalytic activity. The ability of different g-C3N4-based materials to photocatalytically degrade SUH-like structures is then discussed and summarized based on different modification strategies (morphology modulation, elemental doping, defect engineering, and heterojunction formations). Meanwhile, the effects of different environmental factors on the photocatalytic performance of g-C3N4-based materials are described. Finally, the major challenges and opportunities of g-C3N4-based materials for the photocatalytic degradation of SUHs are proposed. It is hoped that this review will show the feasibility of photocatalytic degradation of SUHs with g-C3N4-based materials.

Tunable structure of TiO2 deposited by DC magnetron sputtering to adsorb Cr (VI) and Fe (III) from water

TiO2 thin films with mixtures of the anatase, rutile, and brookite phases were deposited on glass substrates via magnetron sputtering. Based on XRD and Raman results, the TiO2-0.47 and TiO2-3.47 films principally contained the brookite phase, while the TiO2-1.27 and TiO2-2.13 films were primarily anatase. The capacities of the TiO2 films to adsorb heavy metals were tested with Cr(VI) and Fe(III) solutions, and the maximum Cr(VI) and Fe(III) adsorption capacities were realized with the TiO2-0.47 film (334.5 mg/g) and TiO2-3.47 film (271.3 mg/g), respectively. SEM‒EDS results revealed the presence of Cr and Fe on the surfaces of the films, thus corroborating the ability of the TiO2 films to adsorb and remove heavy metals. They are strong candidates for use in wastewater treatment plants.

Investigation of Interface Characteristics and Physisorption Mechanism in Quantum Dots/TiO2 Composite for Efficient and Sustainable Photoinduced Interfacial Electron Transfer

Owing to their superior stability compared to those of conventional molecular dyes, as well as their high UV-visible absorption capacity, which can be tuned to cover the majority of the solar spectrum through size adjustment, quantum dot (QD)/TiO2 composites are being actively investigated as photosensitizing components for diverse solar energy conversion systems. However, the conversion efficiencies and durabilities of QD/TiO2-based solar cells and photocatalytic systems are still inferior to those of conventional systems that employ organic/inorganic components as photosensitizers. This is because of the poor adsorption of QDs onto the TiO2 surface, resulting in insufficient interfacial interactions between the two. The mechanism underlying QD adsorption on the TiO2 surface and its relationship to the photosensitization process remain unclear. In this study, we established that the surface characteristics of the TiO2 semiconductor and the QDs (i.e., surface defects of the metal oxide and the surface structure of the QD core) directly affect the QD adsorption capacity by TiO2 and the interfacial interactions between the QDs and TiO2, which relates to the photosensitization process from the photoexcited QDs to TiO2 (QD* → TiO2). The interfacial interaction between the QDs and TiO2 is maximized when the shape/thickness-modulated triangular QDs are composited with defect-rich anatase TiO2. Comprehensive investigations through photodynamic analyses and surface evaluation using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and photocatalysis experiments collectively validate that tuning the surface properties of QDs and modulating the TiO2 defect concentration can synergistically amplify the interfacial interaction between the QDs and TiO2. This augmentation markedly improved the efficiency of photoinduced electron transfer from the photoexcited QDs to TiO2, resulting in significantly increased photocatalytic activity of the QD/TiO2 composite. This study provides the first in-depth characterization of the physical adhesion of QDs dispersed on a heterogeneous metal-oxide surface. Furthermore, the prepared QD/TiO2 composite exhibits exceptional adsorption stability, resisting QD detachment from the TiO2 surface over a wide pH range (pH = 2-12) in aqueous media as well as in nonaqueous solvents during two months of immersion. These findings can aid the development of practical QD-sensitized solar energy conversion systems that require the long-term stability of the photosensitizing unit.

Sonochemical regulation of oxygen vacancies for Bi2WO6 nanosheet-based photoanodes to promote photoelectrochemical performance

Integration of oxygen vacancies (Vo) into nanostructured semiconductor-based photocatalysts has been recognized as a promising strategy for enhancing the performance of photoelectrochemical (PEC) water splitting. However, precisely controlling the Vo concentration in photocatalysts via an effective and tunable approach remains challenging. Herein, a series of optimized bismuth tungstate (Bi2WO6) nanosheet-based photoanodes with varying concentrations of Vo were prepared by a sonochemical method with in situ cavitation detection, which enables accurate manipulation of the acoustic cavitation intensity applied to the surface of Bi2WO6 photoanodes in alkaline solution. Based on the analysis of the Vo concentration and sound field characteristics, the mechanism of sonochemical regulation of Vo in Bi2WO6 nanosheets was interpreted. Specifically, the increase in Vo concentration can be attributed to the enhancement of Bi-O bond dissociation. This enhancement is influenced not only by the intensified impact of shear force and the generation of active radicals by transient cavitation, but also by the accelerated diffusion of the reactant, a result of stable cavitation. By optimizing the transient and stable cavitation intensity, a Vo-rich Bi2WO6 photoanode was obtained without altering the microstructure of Bi2WO6 nanosheets. The presence of high concentration Vo facilitates the interfacial chemical reactivity and the transmission of photogenerated carriers, leading to the drastic promotion of the PEC water splitting performance. The transient photocurrent density of the Vo-rich Bi2WO6 photoanode reaches 69.2 μA cm-2 (1.23 V vs. RHE), 7.86 times that of the untreated Bi2WO6 photoanode. Additionally, the charge injection efficiency increases to 35.4%. This work provides a controllable and effective method for defect engineering of nanostructured semiconductor-based electrodes.

Photonic post-processing of a multi-material transparent conductive electrode architecture for optoelectronic device integration

Emerging flexible optoelectronic devices require multi-material processing capabilities to fully enable the use of temperature-sensitive substrates and materials. This report demonstrates how photonic sintering enables the processing of materials with very different properties. For example, charge carrier transport/blocking metal-oxides, and transparent conductive silver nanowire-based electrodes ought to be compatible with low-energy and high-throughput processing for integration onto flexible low-temperature substrates. Compared to traditional post-processing methods, we show a rapid fabrication route yielding highly-stable hybrid electrode architectures on polyethylene terephthalate (PET). This architecture consists of an interconnected silver nanowire network encapsulated with a thin crystalline photo-sensitive titanium dioxide (TiO2) coating, allowing both layers to be treated using independent photonic post-processing sintering steps. The first step sinters the nanowires, while the second completes the conversion of the top metal-oxide layer from amorphous to crystalline TiO2. This approach improves on the fabrication speed compared to oven processing, while delivering optical and electrical characteristics comparable to the state of the art. Optimized transparency values reach 85% with haze values down-to 7% at 550 nm, while maintaining a sheet resistance of 18.1 Ω sq.-1. However, this hybrid architecture provides a much stronger resilience to degradation, which we demonstrate through exposure to harsh plasma conditions. In summary, this study shows how carefully-optimized photonic curing post-processing can provide more-stable hybrid architectures while using a multi-material processing technique suitable for high-volume manufacturing on low-temperature substrates.

Tailoring of Visible to Near-Infrared Active 2D MXene with Defect-Enriched Titania-Based Heterojunction Photocatalyst for Green H2 Generation

A wide solar light absorption window and its utilization, long-term stability, and improved interfacial charge transfer are the keys to scalable and superior solar photocatalytic performance. Based on this objective, a noble metal-free composite photocatalyst is developed with conducting MXene (Ti3C2) and semiconducting cauliflower-shaped CdS and porous Cu2O. XPS, HRTEM, and ESR analyses of TiOy@Ti3C2 confirm the formation of enough defect-enriched TiOy (where y is < 2) on the surface of Ti3C2 during hydrothermal treatment, thus creating a third semiconducting site with enough oxygen vacancy. The final material, TiOy@Ti3C2/CdS/Cu2O, shows a broad absorption window from 300 to 2000 nm, covering the visible to near-infrared (NIR) range of the solar spectrum. Photocatalytic H2 generation activity is found to be 12.23 and 16.26 mmol g-1 h-1 in the binary (TiOy@Ti3C2/CdS) and tertiary composite (TiOy@Ti3C2/CdS/Cu2O), respectively, with good repeatability under visible-NIR light using lactic acid as the hole scavenger. A clear increase of efficiency by 1.53 mmol g-1 h-1 in the tertiary composite due to NIR light absorption supports the intrinsic upconversion of electrons, which will open a new prospective of solar light utilization. Decreased charge-transfer resistance from the EIS plot and a decrease in PL intensity established the improved interfacial charge separation in the tertiary composite. Compared to pure CdS, H2 generation efficiency is 29.6 times higher on the noble metal-free tertiary composite with an apparent quantum efficiency of 12.34%. Synergistic effect of defect-enriched TiOy formation, creation of proper dual p-n junction on a Ti3C2 sheet as supported by the Mott-Schottky plot, significant NIR light absorption, increased electron mobility, and charge transfer on the conductive Ti3C2 layer facilitate the drastically increased hydrogen evolution rate even after several cycles of repetition. Expectantly, the 2D MXene-based heterostructure with defect-enriched dual p-n junctions of desired interface engineering will facilitate scalable photocatalytic water splitting over a broad range of the solar spectrum.

Ni-Fe bimetallic catalysts with high dispersion supported by SBA-15 evaluated for the selective oxidation of benzyl alcohol to benzaldehyde

A wetness impregnation method was used to impregnate the substrate with a substantial quantity of oleic acid together with a metal precursor, leading to significantly dispersed Ni-Fe bimetallic catalysts based on mesoporous SBA-15. Using a wide variety of characterization methods, such as XRD, BET, and TEM Analysis, the physiochemical properties of the catalyst were determined. The addition of the metal does not have any effect on the structural characteristics of the SBA-15 catalyst, as validated by transmission electron microscopy (TEM), which shows that the prepared SBA-15 supported catalyst has a hexagonal mesoporous structure. The catalytic capabilities of the Ni-Fe-SBA-15 catalysts were evaluated in the conversion of BzOH using tert-butyl hydroperoxide (TBHP) as an oxidant and acetonitrile as a solvent. The Ni/Fe-SBA-15 (NFS-15) catalytic composition is the best of the developed catalysts, with a maximum conversion of 98% and a selectivity of 99%. In-depth investigations were conducted into the molar ratio of TBHP to BzOH, the dosage of the catalyst, the reaction rate, temperature, and solvent. The recycling investigations indicate that the synthesized Ni/Fe-SBA-15 (NFS-15) catalyst seems to be more durable up to seven successive cycles.

Amorphous titanium dioxide with abundant defects induced by incorporation of silicon dioxide: A potential non-radical activator of hydrogen peroxide

Catalysts based on titanium dioxide (TiO2) have demonstrated a significant potential for oxidizing intractable organic pollutants in heterogenous Fenton-like reactions, even in the absence of light irradiation. In this study, defective amorphous TiO2 enriched Ti3+ and oxygen vacancies (Ov) was synthesized by incorporation of silicon dioxide (SiO2) via a simple sol-gel method. Based on a systematic exploration, an optimal amount of SiO2 was found to be crucial in promoting the formation of Ov and Ti3+, which was achieved by maximizing the non-hexacoordinate structure (Ti4/5/7c) in amorphous TiO2. Furthermore, an unusual singlet oxygen (1O2) based non-radical mechanism was confirmed. It was proposed that the hydroxyl radicals (•OH) produced by Ti3+ active sites during hydrogen peroxide (H2O2) activation may adsorb on the TiO2 surface for extended periods, owing to their favorable interactions with the surface Ov and hydroxyl groups (-OH), thus facilitating their transformation to 1O2. Moreover, the optimized catalyst exhibited favorably catalytic performance across a broad pH range (3-11), making it a promising candidate for applications in rigorous environmental conditions.

A comprehensive review on the boosted effects of anion vacancy in the heterogeneous photocatalytic degradation, part I: Focus on sulfur, nitrogen, carbon, and halogen vacancies

The greenest environmental remediation way is the photocatalytic degradation of organic pollutants. However, limited photocatalytic applications are due to poor sunlight absorption and photogenerated charge carriers' recombination. These limitations can be overcome by introducing anion vacancy (AV) (O, S, N, C, and Halogen) defects in semiconductors that enhance light harvesting, facilitate charge separation, modulate electronic structure, and produce reactive radicals. In continuing part A of this review, in this part, we summarized the recent AVs' research, including S, N, C, and halogen vacancies on the boosted photocatalytic features of semiconductor materials, like metal oxides/sulfides, oxyhalides, and nitrides in detail. Also, we outline the recently developed AV designs for the photocatalytic degradation of organic pollutants. The AV creating and analysis methods and the recent photocatalytic applications and mechanisms of AV-mediated photocatalysts are reviewed. AV engineering photocatalysts' challenges and development prospects are illustrated to get a promising research direction.

CdSe Quantum Dots Bedecked on ZnO/TiO2/CuO Ternary Nanocomposite for Enhanced Photocatalytic and Photovoltaic Applications

Herein, we synthesized a CdSe quantum dots (QDs)-decorated ternary metal oxide nanocomposite of ZnO/TiO2/CuO through a simple hydrothermal method. The prepared nanocomposite exhibited monoclinic, hexagonal, and cubic phase structures in XRD (X-ray diffraction) analysis. UV-vis absorbance spectra showed the broad absorption spectrum. SEM (scanning electron microscopy) clearly showed the presence of nanoparticles and confirmed the elements through elemental mapping. TEM (transmission electron microscopy) confirmed the nanostructure of metal oxides decorated with QDs. The average particle size was 45 nm for metal oxides and 7 nm for QDs. XPS (X-ray photoelectron spectroscopy) also confirmed the surface elemental composition. The prepared nanocomposites were introduced as photoanodes in DSSCs (dye-sensitized solar cells) and as photocatalysts for industrial dye solution. Among these samples, CdSe@CuO/TiO2/ZnO showed an improved performance of PCE (photon conversion efficiency) of 3.68% in DSSC and 96% photocatalytic degradation efficiency. It showed a recycling efficiency of ∼92% after 4 cycles against methylene blue (MB) organic dye under visible light irradiation.

Hydrodeoxygenation of Oxygenates Derived from Biomass Pyrolysis Using Titanium Dioxide-Supported Cobalt Catalysts

Bio-oil upgrading to produce biofuels and chemicals has become an attractive topic over the past decade. However, the design of cost- and performance-effective catalysts for commercial-scale production remains a challenge. Herein, commercial titania (TiO2) was used as the support of cobalt (Co)-based catalysts (Co/TiO2) due to its low cost, high availability, and practicability for commercialization in the future. The Co/TiO2 catalysts were made with two different forms of TiO2 (anatase [TiO2-A] and rutile [TiO2-R]) and comparatively evaluated in the hydrodeoxygenation (HDO) of 4-propylguaicol (4PG), a lignin-derived model compound. Both Co/TiO2 catalysts promoted the HDO of 4PG following a similar pathway, but the Co/TiO2-R catalyst exhibited a higher activity in the early stages of the reaction due to the formation of abundant Ti3+ species, as detected by X-ray photoelectron spectroscopy (XPS) and hydrogen-temperature programed reduction (H2-TPR) analyses. On the other hand, the Co/TiO2-A catalyst possessed a higher acidity that enhanced propylcyclohexane production at prolonged reaction times. In terms of reusability, the Co/TiO2-A catalyst showed a higher stability (less Co leaching) and reusability compared to Co/TiO2-R, as confirmed by transmission electron microscopy (TEM) and inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses. The HDO of the real bio-oil derived from pyrolysis of Leucaena leucocephala revealed that the Co/TiO2-A catalyst could convert high oxygenated aromatics (methoxyphenols, dimethoxyphenols, and benzenediols) to phenols and enhanced the phenols content, hinting at its potential to produce green chemicals from bio-feedstock.

Multi-antibiotics removal under UV-A light using sol-gel prepared TiO2: Central composite design, effect of persulfate addition and degradation pathway study

The removal of three antibiotics i.e., metronidazole (MNZ), ciprofloxacin (CIP) and tetracycline (TET), from aqueous system via TiO2 photocatalysis under UV-A light was investigated. Photocatalyst(s) were prepared using sol-gel method under different calcination temperatures (400-800 °C) and water-alcohol ratio. The spherical shaped catalyst (mean particle size ∼ 61 nm) was characterized via FTIR, XRD, BET, SEM, Raman, XPS, UV-DRS, and Fluorometry, and point of zero charge was also determined (pHPZC ∼ 6.6). Batch photo-catalytic degradation studies have shown complete degradation of MNZ, CIP and TET after 50, 75 and 20 min with a TOC removal of 37%, 44% and 31%, respectively. The activity of sol-gel prepared TiO2 was comparatively higher than commercially available pure anatase TiO2 nanoparticles due to lesser mean particle size. The ratio of water to alcohol in the preparation of TiO2 catalyst was found to have significant effect on antibiotic removal. Moreover, persulfate (PS) addition of 0.1 g/L amplified the pseudo-first-order removal-rate constant by 2.75, 3.3 and 1.6 times for MNZ, CIP and TET, respectively. The higher initial pH values (8 and 10) have shown the best removal efficiency for all antibiotics. Subsequently, central composite design (CCD) experiments were conducted under multi-antibiotic conditions. Near complete removal of all antibiotics were observed within 120 min. Scavenging studies revealed that hydroxyl and superoxide radicals play major roles in photo-catalytic degradation of MNZ, CIP and TET. During photocatalysis, MNZ degradation was initiated by hydroxylation reaction, CIP by piperazine ring opening by hydroxyl attack and TET by multiple hydroxylation process. Overall, TiO2 showed good efficiency at degrading multiple antibiotics and has the potential for practical application on a larger scale.

Black TiO2 Photoanodes for Direct Methanol Photo Fuel Cells

Photocatalytic fuel cells (PFCs) are considered the next generation of energy converter devices, since they can harvest solar energy through relatively low-cost semiconductor material to convert the chemical energy of renewable fuels and oxidants directly into electricity. Here, we report black TiO2 nanoparticle (NP) photoanodes for simple single-compartment PFCs and microfluidic photo fuel cells (μPFCs) fed by methanol. We show that Ti3+ and oxygen vacancy (OV) defects at the TiO2 NPs are easily controlled by annealing in a NaBH4-containing atmosphere. This optimized noble-metal-free black TiO2 photoanode shows superior PFC performance for methanol oxidation and O2 reduction with a maximum power density (Pmax) ∼2000% higher compared to the undoped TiO2. At flow conditions, the black TiO2 photoanode showed a Pmax ∼90 times higher than the μFC equipped with regular TiO2 in the dark. The PFC and μPFC operate spontaneously with little activation polarization, and black TiO2 photoanodes are stable under light irradiation. The improved photoactivity of the black TiO2 photoanode is a consequence of the self-doping with Ti3+/OV defects, which significantly red-shifted the bandgap energy, induced intragap electronic states, and widened both the valence band and conduction band, enhancing the overall absorption of visible light and decreasing the interfacial charge transfer resistance.

The Role of Lattice Defects on the Optical Properties of TiO2 Nanotube Arrays for Synergistic Water Splitting

In this study, we report a facile one-step chemical method to synthesize reduced titanium dioxide (TiO2) nanotube arrays (NTAs) with point defects. Treatment with NaBH4 introduces oxygen vacancies (OVs) in the TiO2 lattice. Chemical analysis and optical studies indicate that the OV density can be significantly increased by changing reduction time treatment, leading to higher optical transmission of the TiO2 NTAs and retarded carrier recombination in the photoelectrochemical process. A cathodoluminescence (CL) study of reduced TiO2 (TiO2-x) NTAs revealed that OVs contribute significantly to the emission bands in the visible range. It was found that the TiO2 NTAs reduced for a longer duration exhibited a higher concentration of OVs. A typical CL spectrum of TiO2 was deconvoluted to four Gaussian components, assigned to F, F+, and Ti3+ centers. X-ray photoelectron spectroscopy measurements were used to support the change in the surface chemical bonding and electronic valence band position in TiO2. Electron paramagnetic resonance spectra confirmed the presence of OVs in the TiO2-x sample. The prepared TiO2-x NTAs show an enhanced photocurrent for water splitting due to pronounced light absorption in the visible region, enhanced electrical conductivity, and improved charge transportation.

Electrocatalytic Mechanism of Defect in Spinels for Water and Organics Oxidation

Spinels display promising electrocatalytic ability for oxygen evolution reaction (OER) and organics oxidation reaction because of flexible structure, tunable component, and multifold valence. Unfortunately, limited exposure of active sites, poor electronic conductivity, and low intrinsic ability make the electrocatalytic performance of spinels unsatisfactory. Defect engineering is an effective method to enhance the intrinsic ability of electrocatalysts. Herein, the recent advances in defect spinels for OER and organics electrooxidation are reviewed. The defect types that exist in spinels are first introduced. Then the catalytic mechanism and dynamic evolution of defect spinels during the electrochemical process are summarized in detail. Finally, the challenges of defect spinel electrocatalysts are brought up. This review aims to deepen the understanding about the role and evolution of defects in spinel for electrochemical water/organics oxidation and provide a significant reference for the design of efficient defect spinel electrocatalysts.

Tailoring defects in SrTiO3 by one step nanoarchitectonics for realizing photocatalytic nitrogen fixation in pure water

Surface contamination of materials by nitrogenous impurities is a major problem that can bias the quantification of ammonia in photocatalytic N₂ fixation reactions. In this work, SrTiO₃ nanocubes were prepared by using a nitrogenous precursor and engineered with Ti³⁺ sites and oxygen vacancy defects in a one-step solvothermal approach. It was observed that the synthesized materials were containing surface nitrogenous impurities and therefore a rigorous cleaning procedure was adopted to eliminate them to the best extent. The contribution of unavoidable surface impurities was deduced in the form of adventitious NH₃ by employing control experiments and a realistic photocatalytic NH₃ generation was achieved. It was found that pristine SrTiO₃ showed no photocatalytic activity, whereas one of the defected SrTiO₃ materials showed the highest NH₃ formation under natural sunlight in pure water, which was ascribed to the tuned defect sites, enhanced surface area and efficient separation of photogenerated charges. Based on the experimental results, a stringent protocol has been suggested for materials synthesis while working with nitrogenous precursors and for subsequent photocatalytic N₂ fixation experiments. Thus, the present study provides a simple and affordable procedure for catalyst synthesis for the studied application and expands the scope of perovskite oxide materials to fabricate efficient photocatalysts for sustainable NH₃ production.

Mechanistic study of the increased phototoxicity of titanium dioxide nanoparticles to Chlorella vulgaris in the presence of NOM eco-corona

Widespread applications and release of photoactive nanoparticles (NPs) such as titanium dioxide (TiO₂) into environmental matrices warrant mechanistic investigations addressing toxicity of NPs under environmentally relevant conditions. Accordingly, we investigated the effects of surface adsorbed natural organic matters (NOMs) such as humic acid, tannic acid and lignin on the band gap energy, abiotic reactive oxygen species (ROS) generation, surface chemistry and phototoxicity of TiO₂ NPs. Initially, a liquid assisted grinding method was optimized to produce TiO₂ NPs with a NOM layer of defined thickness for further analysis. Generally, adsorption of NOM reduced the band-gap energy of TiO₂ NPs from 3.08 eV to 0.56 eV with humic acid, 1.92 eV with tannic acid and 2.48 eV with lignin. Light activated ROS generation by TiO₂ NPs such as hydroxyl radicals, however, was reduced by 4, 2, 9 times in those coated with humic acid, tannic acid and lignin, respectively. This reduction in ROS despite decrease in band gap energy corroborated with the decreased surface oxygen vacancy (as revealed by X-ray Photoelectron Spectroscopy (XPS)) and quenching of ROS by surface adsorbed NOM. Despite the reduced ROS generation, the NOM-modified TiO₂ NPs exhibited an increased phototoxicity to Chlorella vulgaris in comparison to pristine TiO₂ NPs. Further analysis suggested that photoactivation of NOM modified TiO₂ NPs releases toxic degradation products. Findings from our studies thus provide mechanistic insight into the ecotoxic potential of NOM-modified TiO₂ NPs when exposed to light in the environment.

Efficient Photo-Assisted Thermal Selective Oxidation of Toluene Using N-Doped TiO2

Selective oxidation of toluene is a key reaction to produce high value-added products but remains a big challenge. In this study, we introduce a nitrogen-doped TiO₂ (N-TiO₂) catalyst to create more Ti³⁺ and oxygen vacancy (O_V), which act as active sites for selective oxidation of toluene via activating O₂ to superoxide radical (•O₂^-). Interestingly, the resulting N-TiO₂-2 exhibited an outstanding photo-assisted thermal performance with a product yield of 209.6 mmol·g_cat^-1 and a toluene conversion of 10960.0 μmol·g_cat^-1·h^-1, which are 1.6 and 1.8 times greater than those obtained under thermal catalysis. We showed that the enhanced performance under photo-assisted thermal catalysis was attributed to more active species generation by making full use of photogenerated carriers. Our work suggests a viewpoint to apply a noble-metal-free TiO₂ system in the selective oxidation of toluene under solvent-free conditions.

Oxygen-deficient TiO2-x interlayer enabling Li-rich Mn-based layered oxide cathodes with enhanced reversible capacity and cyclability

The unique anion redox mechanism of Li-rich Mn-based layered oxide (LMLO) cathodes endows them with a higher specific capacity compared with conventional cathodes. However, the irreversible anion redox reactions can cause structural degradation and sluggish electrochemical kinetics in the cathode, resulting in a poor electrochemical performance in the batteries. Thus, to address these issues, a single-sided conductive oxygen-deficient TiO2-x interlayer was applied on a commercial Celgard separator as a coating layer towards the LMLO cathode. After coating TiO2-x, the initial coulombic efficiency (ICE) of the cathode increased from 92.1% to 95.8%, the capacity retention improved from 84.2% to 91.7% after 100 cycles, and the rate performance of the cathode was significantly enhanced from 91.3 mA h g-1 to 203.9 mA h g-1 at 5C. Operando differential electrochemical mass spectroscopy (DEMS) showed that the coating layer could restrain the release of oxygen in the battery, especially from the initial formation process. The X-ray photoelectron spectroscopy (XPS) results demonstrated that the favorable oxygen absorption by the TiO2-x interlayer benefitted the suppression of side reactions and cathode structural evolution and favored the formation of a uniform cathode-electrolyte interphase on the LMLO cathode. This work provides an alternative path to address the issue of oxygen release in LMLO cathodes.

Optimized photodegradation of palm oil agroindustry waste effluent using multivalent manganese-modified black titanium dioxide

This article presents a methodological approach to use manganese (Mn³⁺Mn⁷⁺)-modified black titanium dioxide (Mn/BTiO₂) as a photocatalyst to optimize and improve visible-light-driven photodegradation of treated agro-industrial effluent (TPOME). A modified wet chemical process was used to prepare BTiO₂. The BTiO₂ was then wet impregnated with Mn and calcined at 300 °C for 1 h to produce Mn/BTiO₂. The activity of Mn/BTiO₂ was investigated in terms of photo-assisted elimination of chemical oxygen demand (COD), phenolic compounds (PCs), color, and total organic carbon (TOC). Using the design of experiments (DOE), the conditions of the photocatalytic process, including photocatalyst loading, Mn concentration, hydrogen peroxide (H₂O₂) dose, and irradiation time, were optimized. Under the optimum conditions (0.85 g/L photocatalyst loading, 0.048 mol/L H₂O₂ dose, 0.301 wt.% Mn concentration, and 204 min irradiation time) COD, PCs, color, and TOC removal efficiencies of 88.87%, 86.04%, 62.8%, and 84.66%, respectively, were obtained. Statistical analysis showed that the response variable's removal from TPOME estimation had high R² and low RMSE, MSE, MAD, MAE, and MAPE values, indicating high reliability. This study demonstrated the significant potential of the developed photocatalytic system for the treatment of waste effluent generated by the palm oil industry and other agro-industries, with the ability to simultaneously reduce a number of organic pollution indicators (OPIs).