Selective Photoreduction of Nitric Oxide to Nitrogen by Nanostructured TiO2Photocatalysts: Role of Oxygen Vacancies and Iron Dopant

Novel TiO2-Supported Gold Nanoflowers for Efficient Photocatalytic NOx Abatement

In this study, we pioneered the synthesis of nanoflower-shaped TiO2-supported Au photocatalysts and investigated their properties. Au nanoflowers (Au NFs) were prepared by a Na-citrate and hydroquinone-based preparation method, followed by wet impregnation of the derived Au NFs on the surface of TiO2 nanorods (TNR). A uniform and homogeneous distribution of Au NFs was observed in the TNR + NF(0.7) sample (lower Na-citrate concentration), while their distribution was heterogeneous in the TNR + NF(1.4) sample (higher Na-citrate concentration). The UV-Vis DR spectra revealed the size- and shape-dependent optical properties of the Au NFs, with the LSPR effect observed in the visible region. The solid-state EPR spectra showed the presence of Ti3+, oxygen vacancies and electron interactions with organic compounds on the catalyst surface. In the case of the TNR + NF(0.7) sample, high photocatalytic activity was observed in the H2-assisted reduction of NO2 to N2 at room temperature under visible-light illumination. In contrast, the TNR + NF(1.4) catalyst as well as the heat-treated samples showed no ability to reduce NO2 under visible light, indicating the presence of deformed Au NFs limiting the LSPR effect. These results emphasized the importance of the choice of synthesis method, as this could strongly influence the photocatalytic activity of the Au NFs.

Photocatalytic NO x removal and recovery: progress, challenges and future perspectives

The excessive production of nitrogen oxides (NO x ) from energy production, agricultural activities, transportation, and other human activities remains a pressing issue in atmospheric environment management. NO x serves both as a significant pollutant and a potential feedstock for energy carriers. Photocatalytic technology for NO x removal and recovery has received widespread attention and has experienced rapid development in recent years owing to its environmental friendliness, mild reaction conditions, and high efficiency. This review systematically summarizes the recent advances in photocatalytic removal, encompassing NO x oxidation removal (including single and synergistic removal and NO3 - decomposition), NO x reduction to N2, and the emergent NO x upcycling into green ammonia. Special focus is given to the molecular understanding of the interfacial nitrogen-associated reaction mechanisms and their regulation pathways. Finally, the status and the challenges of photocatalytic NO x removal and recovery are critically discussed and future outlooks are proposed for their potential practical application.

Biotemplated Fe/La-co-doped TiO2 for photocatalytic depth treatment of compressed leachate from refuse transfer station

Compressed leachate is a contaminated liquid containing various organic and inorganic pollutants produced in municipal refuse transfer stations, which pollute soil and groundwater, posing serious risks to the environment and human health. The Environmental Technology Co., Ltd. (Shenzhen, Guangdong Province, South China) treated compressed leachate obtained from a refuse transfer station. The chemical oxygen demand (COD) (641.2 mg/L) of treated compressed leachate did not meet the wastewater quality standards in China for discharge into municipal sewers (COD ≤ 500 mg/L) and the company's design discharge requirements (COD ≤ 400 mg/L). Therefore, their further in-depth treatment is necessary. To this end, waste tobacco leaves were used as the biotemplate herein, and Fe/La-co-doped TiO2 (xFe,yLa)-TTiO2(g) was synthesized using a solvothermal-assisted biotemplating method. The photocatalytic depth treatment of compressed leachate was performed under simulated solar light using the prepared catalysts. After (3Fe,3La)-TTiO2(g) treatment, the COD of the leachate decreased from 641.2 to 280.1 mg/L, and the COD removal rate was 1.2, 1.1, and 1.6 times higher than that of pure Fe-doped, La-doped and non-biological template TiO2, respectively. Characterization confirmed that the biological template endowed the catalyst with a unique morphology and high specific surface area. Its rich activity sites are conducive to enhancing the adsorption capacity of pollutants and providing an ideal place for photocatalytic reactions. Co-doping with iron and lanthanum ions altered the band structure of TiO2 and promoted the interconversion of Fe3+/Fe2+ and La3+/La2+ during photocatalysis. First-principles density functional theory simulations demonstrated that co-doping Fe and La in TiO2 created impurity levels that facilitated the transfer of photogenerated electrons. This study provides a new purification pathway for the depth treatment of compressed leachate.

Single-Atom Ni Supported on TiO2 for Catalyzing Hydrogen Storage in MgH2

As an efficient and clean energy carrier, hydrogen is expected to play a key role in future energy systems. However, hydrogen-storage technology must be safe with a high hydrogen-storage density, which is difficult to achieve. MgH2 is a promising solid-state hydrogen-storage material owing to its large hydrogen-storage capacity (7.6 wt %) and excellent reversibility, but its large-scale utilization is restricted by slow hydrogen-desorption kinetics. Although catalysts can improve the hydrogen-storage kinetics of MgH2, they reduce the hydrogen-storage capacity. Single-atom catalysts maximize the atom utilization ratio and the number of interfacial sites to boost the catalytic activity, while easy aggregation at high temperatures limits further application. Herein, we designed a single-atom Ni-loaded TiO2 catalyst with superior thermal stability and catalytic activity. The optimized 15wt%-Ni0.034@TiO2 catalyst reduced the onset dehydrogenation temperature of MgH2 to 200 °C. At 300 °C, the H2 released and absorbed 4.6 wt % within 5 min and 6.53 wt % within 10 s, respectively. The apparent activation energies of MgH2 dehydrogenation and hydrogenation were reduced to 64.35 and 35.17 kJ/mol of H2, respectively. Even after 100 cycles of hydrogenation and dehydrogenation, there was still a capacity retention rate of 97.26%. The superior catalytic effect is attributed to the highly synergistic catalytic activity of single-atom Ni, numerous oxygen vacancies, and multivalent Tix+ in the TiO2 support, in which the single-atom Ni plays the dominant role, accelerating electron transfer between Mg2+ and H- and weakening the Mg-H bonds. This work paves the way for superior hydrogen-storage materials for practical unitization and also extends the application of single-atom catalysis in high-temperature solid-state reactions.

Bandgap Narrowing of BaTiO3-Based Ferroelectric Oxides through Cobalt Doping for Photovoltaic Applications

Following the finding of power conversion efficiency above the Shockley-Queisser limit in BaTiO3 (BTO) crystals, ferroelectric oxides have attracted scientific interest in ferroelectric photovoltaics (FPV). However, since ferroelectric oxides have a huge bandgap (>3 eV), progress in this sector is constrained. This paper proposes and demonstrates a new ferroelectric BaTi1-xCoxO3 powder (0 ≤ x ≤ 0.08), abbreviated as BTCx, that exhibited a bandgap decrease with increased Co content. Notably, changing the composition from x = 0.0 to 0.08 caused the system to show a bandgap drop from 3.24 to 2.42 eV. The ideal design with x = 0.08 displayed an abnormal PV response. Raman spectroscopy measurements were used to investigate the cause of the bandgap decrease, and density functional theory was used to interpret the analyzed results. According to our findings, Co2+ doping and oxygen octahedral distortions enhance bandgap reduction. This research sheds light on how bandgap tuning developed and laid the way for investigating novel low-bandgap ferroelectric materials for developing next-generation photovoltaic applications.

Oxygen vacancies induced band gap narrowing for efficient visible-light response in carbon-doped TiO2

The band gap of rutile TiO2 has been narrowed, via the formation of oxygen vacancies (OVs) during heat treatment in carbon powder (cHT) with embedding TiO2 coatings. The narrowed band gap efficiently improves the visible light response of TiO2 coatings, to further enhance the visible-light-driven photocatalytic activity. The change in OVs during cHT has been studied by manipulation of cHT temperature and time. The effect of OVs on the band structure of nonstoichiometric TiO2-x has been further calculated by first-principles calculations. With raising the temperature, SEM images show that the nano-size fiber-like structure forms on the surface of TiO2 coatings, and the amount of the fiber-like structure significantly increases and their size changes from nano to micro under 800 °C, contributing to cause an increase in accessible surface area. The UV-Vis results reveal that the band gap of TiO2 has been narrowed during cHT, due to the formed oxygen vacancies. The XPS results further confirm that the formation of surface defects including OVs, and the XPS depth profile further shows the decreased relative amount of O whereas increased relative amount of carbon. Notably, after cHT for TiO2 coatings, the photocatalytic activity first increases then decreases with raising the temperature, achieving approximately 3 times at 850 °C. The first-principles calculation suggest that the OVs in TiO2 coatings with localized electrons could facilitate the band gap narrowing, further favoring to enhance the photocatalytic activity under visible light.

Semirigid Highly Conjugated Zirconium-Organic Framework for the Capture of Micropollutants and Solar-Light Photodegradation

Micro-organic pollutants, particularly organic dyes and personal care products (PPCPs), are widely present in wastewater, and thus pose a serious risk to human health. The capture and solar-light photodegradation of micro-organic pollutants are highly challenging tasks, which require the design and synthesis of microporous materials with specific structures. As we know, organic dyes and PPCPs can be absorbed via π-π* stacking. In this paper, an iron-based metal-organic framework (Fe-UiO-68-terNap) containing semirigid conjugated aromatic ligands is prepared for the capture and solar-light photodegradation of multiple water contaminants. UiO-68-terNap was synthesized based on ternaphthalene with π-π* stacking, which would increase the adsorption capacities of organic micropollutants in wastewater. Additionally, the formation of Fe-O-Zr enhances the charge-separation ability resulting in the successful degradation of micropollutants in 240 min. The novel material has been elucidated by single-crystal X-ray diffraction and Fe K-edge XANES, which provide key insights at a molecular level for the design of novel materials for the capture and photodegradation of organic micropollutants.

Tuning the Microstructures of ZnO To Enhance Photocatalytic NO Removal Performances

Effective removal of kinetically inert dilute nitrogen oxide (NO, ppb) without NO₂ emission is still a challenging topic in environmental pollution control. One effective approach to reducing the harm of NO is the construction of photocatalysts with diversified microstructures and atomic arrangements that could promote adsorption, activation, and complete removal of NO without yielding secondary pollution. Herein, microstructure regulations of ZnO photocatalysts were attempted by altering the reaction temperature and alkalinity in a unique ionic liquid-based solid-state synthesis and further investigated for the removal of dilute NO upon light irradiation. Microstructure observations indicated that as-tuned photocatalysts displayed unique nucleation, diverse morphologies (spherical nanoparticles, short and long nanorods), defect-related optical characteristics, and enhanced carrier separations. Such defect-related surface-interface aspects, especially V_o″-related defects of ZnO devoted them to the 4.16-fold enhanced NO removal and 2.76 magnitude order decreased NO₂ yields, respectively. Improved NO removal and toxic product inhabitation in as-tuned ZnO was disclosed by mechanistic exploitations. It was revealed that regulated microstructures, defect-related charge carrier separation, and strengthened surface interactions were beneficial to active species production and molecular oxygen activation in ZnO, subsequently contributing to the improved NO removal and simultaneous avoidance of NO₂ formation. This investigation shed light on the facile regulation of microstructures and the roles of surface chemistry in the oxidation of low concentration NO in the ppb level upon light illumination.

Highly effective Fe-doped TiO2nanoparticles for removal of toxic organic dyes under visible light illumination

This article addresses the synthesis of Fe³⁺doped TiO₂nanoparticles with variations of molar concentrations of Fe³⁺and their adequate use as potential photocatalysts for Photocatalysis applications. Synthesized photocatalysts were characterized thoroughly by different analytical techniques in terms of morphological, chemical, structural, crystalline, optical, electronic structure, surface area etc properties. The occurrence of red shift phenomenon of the energy band gap attributes to the transfer of charges and transition between the d electrons of dopant and conduction band (CB) or valence band (VB) of TiO₂. The doping of Fe³⁺ions generates more trap sites for charge carriers with the surface trap sites. Thorough experimental conclusions revealed that the Fe³⁺ions necessarily regulate the catalytic property of TiO₂nanomaterial. The obtained total degradation efficiency rate of Methylene Blue (MB) was 93.3% in the presence of 0.1 M Fe³⁺in the host material and for Malachite Green Oxalate the efficiency was 100% in the presence of 0.05 M and 0.1 M Fe³⁺in the host material. In both the cases the total visible light irradiation time was 90 min. The adsorption properties of the photocatalysts have been also performed in a dark for 90 min in the presence of MB dye. However, till now there are hardly reported photocatalysts which shows complete degradation of these toxic organic dyes by visible light driven photocatalysis. of potential values of valence and conduction band shows the production of active oxidizing species for hydrogen yield and the possible mechanism of the Schottky barrier has been proposed. A schematic diagram of visible light driven Photocatalysis has been pictured showing degradation activity of Fe³⁺-TiO₂catalysts sample.

Composite TiO2-based photocatalyst with enhanced performance

TiO₂ is the most studied photocatalyst because of its non-toxicity, chemical stability, and low cost. However, the problem of TiO₂ is its low activity in the visible region of the spectrum. In this study, we focused on the preparation of composite photocatalytic materials with altered light absorption properties. TiO₂ P25 and various metal oxides were mechanically joined by ball-milling and immobilized on glass plates. The prepared samples were evaluated based on their ability to degrade NO in gas phase. The formation of undesirable byproducts was also investigated. Four best performing composites were later chosen, characterized, and further evaluated under various conditions. According to their performance, the metal oxide additives can be divided into three groups. P25/Fe₂O₃ showed the most promising results-an increase in overall deNO_x activity under modified ISO conditions and altered selectivity (less NO₂ is formed) under both simulated outdoor and simulated indoor conditions. On the other hand, P25/V₂O₅ composite showed negligible photocatalytic activity. The intermediate group includes P25/WO₃ and P25/ZnO photocatalysts, whose performances are similar to those of pristine P25.

Fundamental Limit of Selectivity in Photocatalytic Denitrification over Titania

Although photocatalytic decomposition of NO (deNO) into N₂ and O₂ is low-cost and non-polluting, it has a low NO conversion efficiency. Establishing the activity and selectivity trend among active sites is an important base to explore and improve the deNO processes. Because the experimental performances are determined by the reaction rate, it is worthwhile to investigate the kinetic limiting steps calculated by comparative microkinetic modeling. We found that, without illumination, N₂ production is inactive over various TiO₂ surfaces/sites, but photogenerated holes can break the scaling relation of the dark condition by weakening O₂* adsorption, leading to a significant increase in deNO activity on defective titania surfaces. However, the low N₂ selectivity can be attributed to the small strength of N₂O adsorption. In contrast, the N₂ selectivity is enhanced in Ti-modified zeolite because of a stronger N₂O* adsorption. We demonstrate here that the reaction phase diagram analysis can clearly establish a global picture of reaction activity and selectivity over various catalytic sites. In combination with microkinetic modeling, it can effectively determine the kinetic limits, providing insights to improve the design of photocatalysts.

NH2-MIL-125(Ti) modified graphitic carbon nitride with carbon vacancy for efficient photocatalytic NO removal

This paper reports the application of NH₂-MIL-125(Ti) modified carbon nitride C_v-C₃N₄ with carbon vacancies in the removal of NO. We performed a series of characterizations of the complex and compared various ratios with the individual components. The results of UV spectrum analysis show that the composite's range of light absorption expanded due to the modification of Ti metal-organic framework. Furthermore, the results of photocurrent and electrical impedance indicate the compound has a better ability to generate and transfer electrons. The increase in the compound's NO removal efficiency (up to 63%) indicates that MOF has a positive effect on C_v-C₃N₄ modification-a good method for photocatalysis. Moreover, the compound can promote photocatalysis in a favorable direction.

Highly active Cs2SnCl6/C3N4 heterojunction photocatalysts operating via interfacial charge transfer mechanism

In this study, a novel lead-free perovskite heterojunction Cs₂SnCl₆/C₃N₄ composite was constructed and applied for photocatalytic NO purification. After design optimization, the Cs₂SnCl₆/C₃N₄ heterojunction exhibit excellent and stable photocatalytic NO purification ability under visible-light irradiation, which is significantly better than pristine Cs₂SnCl₆ and C₃N₄. Combined in-situ DRIFTS and electron spin resonance spin-trapping, the mechanism of Cs₂SnCl₆/C₃N₄ photocatalytic NO removal was revealed. Under visible-light irradiation, the photo-generated electrons on the conduction band of C₃N₄ would spontaneously migrate to the CB of Cs₂SnCl₆, leaving holes (h⁺) on the valence band of C₃N₄, contributing to efficiently segregated charge carriers and improved photocatalytic NO purification. Density functional theory calculations also revealed the directional electron transfer at the C₃N₄ and Cs₂SnCl₆ interface, in which the charge was migrated from C₃N₄ to Cs₂SnCl₆ induced by the internal electric field. This research sheds fresh light on the fabrication of Cs₂SnCl₆/C₃N₄ heterojunctions as well as its effective interfacial charge separation.

Insight into the surface property modification-enhanced C3N4 performance of photocatalytic nitrogen fixation

The surface properties of the catalyst have an important influence on the process of heterogeneous reactions. We modified g-C₃N₄ with dicarboxylic acids with different hydrophobicity. Through experiments, we found that the NH₃ yields of modified carbon nitrides can reach 267.89 μmol h^-1 g^-1 when only dissolved nitrogen is involved. But if both dissolved nitrogen and gaseous nitrogen are present in the reaction, the NH₃ yield can reach as high as 751.83 μmol h^-1 g^-1, demonstrating that the participation of dissolved nitrogen alone is not enough and gaseous nitrogen indeed promotes the reaction of photocatalytic nitrogen fixation. Meanwhile, the nitrogen fixation performance of the catalyst is positively correlated with its hydrophobicity, indicating that a reasonable adjustment of the catalysts' hydrophobicity can give them a certain wettability to activate water, while also providing a hydrophobic surface for insoluble gas-phase nitrogen adsorption. This provides new ideas and directions for the design of future heterogeneous reaction catalysts.

Synthesis of sodium titanates and their use in the photocatalytic degradation of NO

The synthesis and characterization of sodium titanates (ST), and their evaluation in the photocatalytic reduction of nitric oxide (NO) are described herein. The materials were synthesized by a hydrothermal route using 5 M NaOH as the mineralizer agent and a TiO₂ content of 0.06 mg/mL (expressed as the mass ratio of TiO₂/mL of NaOH), at 170 °C for 48 h, resulting in sodium tri- and hexa-titanates. A nanotubular morphology was observed for the ST, as proved by scanning electron microscopy (SEM); a subsequent heat-treatment at 400 °C allowed a complete transformation of sodium tri- to hexa-titanates and an increase in bandgap. The obtained ST were impregnated with Ag⁺ and Zn⁺ cations, ST-Ag and ST-Zn, respectively, to tune the materials' bandgap. XPS analysis of the ST-Ag materials showed evidence of metallic Ag, pointing to the formation of silver nanoparticles, whereas for ST-Zn oxide phases were mainly spotted. The materials were evaluated for the photocatalytic reduction of NO using a reactor fed with a continuous flow rate of NO, generated in situ at a flow of 280 mL/min using nitrogen and a 253-nm UV irradiation source. The photocatalytic tests showed that pristine ST (tri- and hexa-titanates) displayed better performance in the reduction of NO with respect to the impregnated samples (ST-Ag, ST-Zn). Maximum degradation efficiencies of 80% were achieved when 1 g of photocatalyst was used with a flow of 280 mL/min and a 253 nm UV lamp.

Synthesis, modification and application of titanium dioxide nanoparticles: a review

Titanium dioxide (TiO₂) has been heavily investigated owing to its low cost, benign nature and strong photocatalytic ability. Thus, TiO₂ has broad applications including photocatalysts, Li-ion batteries, solar cells, medical research and so on. However, the performance of TiO₂ is not satisfactory due to many factors such as the broad band gap (3.01 to 3.2 eV) and fast recombination of electron-hole pairs (10^-12 to 10^-11 s). Plenty of work has been undertaken to improve the properties, such as structural and dopant modifications, which broaden the applications of TiO₂. This review mainly discusses the aspects of TiO₂-modified nanoparticles including synthetic methods, modifications and applications.

Singlet Oxygen and Mobile Hydroxyl Radicals Co-operating on Gas-Solid Catalytic Reaction Interfaces for Deeply Oxidizing NOx

Learning from the important role of porphyrin-based chromophores in natural photosynthesis, a bionic photocatalytic system based on tetrakis (4-carboxyphenyl) porphyrin-coupled TiO₂ was designed for photo-induced treating low-concentration NO_x indoor gas (550 parts per billion), achieving a high NO removal rate of 91% and a long stability under visible-light (λ ≥ 420 nm) irradiation. Besides the great contribution of the conventional ^•O₂^- reactive species, a synergic effect between a singlet oxygen (¹O₂) and mobile hydroxyl radicals (^•OH_f) was first illustrated for removing NO_x indoor gas (¹O₂ + 2NO → 2NO₂, NO₂ + ^•OH_f → HNO₃), inhibiting the production of the byproducts of NO₂. This work is helpful for understanding the surface mechanism of photocatalytic NO_x oxidation and provides a new perspective for the development of highly efficient air purification systems.

Photocatalytic Air Purification Using Functional Polymeric Carbon Nitrides

The techniques for the production of the environment have received attention because of the increasing air pollution, which results in a negative impact on the living environment of mankind. Over the decades, burgeoning interest in polymeric carbon nitride (PCN) based photocatalysts for heterogeneous catalysis of air pollutants has been witnessed, which is improved by harvesting visible light, layered/defective structures, functional groups, suitable/adjustable band positions, and existing Lewis basic sites. PCN-based photocatalytic air purification can reduce the negative impacts of the emission of air pollutants and convert the undesirable and harmful materials into value-added or nontoxic, or low-toxic chemicals. However, based on previous reports, the systematic summary and analysis of PCN-based photocatalysts in the catalytic elimination of air pollutants have not been reported. The research progress of functional PCN-based composite materials as photocatalysts for the removal of air pollutants is reviewed here. The working mechanisms of each enhancement modification are elucidated and discussed on structures (nanostructure, molecular structue, and composite) regarding their effects on light-absorption/utilization, reactant adsorption, intermediate/product desorption, charge kinetics, and reactive oxygen species production. Perspectives related to further challenges and directions as well as design strategies of PCN-based photocatalysts in the heterogeneous catalysis of air pollutants are also provided.

Efficient visible light photocatalytic NO abatement over SrSn(OH)6 nanowires loaded with Ag/Ag2O cocatalyst

SrSn(OH)₆ (SSOH) possesses a high oxidation potential in the valence band (VB), which is suitable for photocatalytic oxidation removal of pollutants. However, the electrons in the VB of these catalysts are difficult to transition to the conduction band (CB) under visible light, which makes it difficult to utilize sunlight effectively. In this work, Ag/Ag₂O is loaded on the surface of SSOH nanowires, which stimulates the interfacial charge-transfer transition on SSOH. Compared with pure-phase SSOH, the NO abatement ratio of Ag/Ag₂O-SSOH under visible light irradiation is increased to 45.10%. The e^- in the VB of Ag₂O are excited into the CB under visible light, and are further transferred to the Ag to react with O₂ to produce superoxide radicals. The photo-excited e^- in the VB of SSOH enter into the VB of Ag₂O through interfacial charge-transfer transition to recombine with the photo-generated holes in the VB of Ag₂O, thereby leaving photo-generated holes in the VB of SSOH. The holes in the VB of SSOH have sufficient oxidizing ability to oxidize the adsorbed hydroxyl groups into hydroxyl radicals. This work provides a new perspective for photocatalytic removal of pollutants by wide band gap photocatalyst under visible light.

Shedding Light on the Role of Chemical Bond in Catalysis of Nitrogen Fixation

Ammonia (NH₃ ) and nitrates are essential for human society because of their widespread utilization for producing medicines, fibers, fertilizers, etc. In recent years, the development on nitrogen fixation under mild reaction conditions has attracted much attention. However, the very low conversion efficiency and ambiguous catalytic mechanism remain the major hurdles for the research of nitrogen fixation. This review aims to clarify the role of chemical bond in catalytic nitrogen fixation by summarizing and analyzing the recent development of nitrogen fixation research. In detail, the atomic-scale mechanism of nitrogen fixation reaction, the various methods to improve the nitrogen fixation performance, and the computational investigation of nitrogen fixation are discussed, all from a chemical bond perspective. It is hoped that this review could trigger more profound pondering and deeper exploration in the field of catalytic nitrogen fixation.

X-ray-facilitated redox cycling of nanozyme possessing peroxidase-mimicking activity for reactive oxygen species-enhanced cancer therapy

Nanomaterials with shifting or mixed redox states is one of the most common studied nanozyme with peroxidase-like activity for chemodynamic therapy (CDT), which can decompose hydrogen peroxide (H₂O₂) of tumor microenvironment into highly toxic reactive oxygen species (ROS) by a nano-catalytic way. However, most of them exhibit an insufficient catalytic efficiency due to their dependence on catalytic condition. Herein, a potential methodology is proposed to enhance their enzymatic activity by accelerating the redox cycling of these nanomaterials with shifting or mixed redox states in the presence of X-ray. In this study, the nanocomposite consisting of SnS₂ nanoplates and Fe₃O₄ quantum dots with shifting or mixed redox states (Fe²⁺/Fe³⁺) is used to explore the strategy. Under external X-ray irradiation, SnS₂ cofactor as electron donor can be triggered to transfer electrons to Fe₃O₄, which promotes the regeneration of Fe²⁺ sites on the surface of the Fe₃O₄. Consequently, the regenerated Fe²⁺ sites react with the overexpressed H₂O₂ to persistently generate ROS for enhanced tumor therapy. The designed nanocomposite displays the synergistic effects of radiotherapy and CDT. The strategy provides a new avenue for the development of artificial nanozymes with shifting or mixed redox states in precise cancer treatments based on X-ray-enhanced enzymatic efficacy.

Altering Hydrogenation Pathways in Photocatalytic Nitrogen Fixation by Tuning Local Electronic Structure of Oxygen Vacancy with Dopant

To avoid the energy-consuming step of direct N≡N bond cleavage, photocatalytic N₂ fixation undergoing the associative pathways has been developed for mild-condition operation. However, it is a fundamental yet challenging task to gain comprehensive understanding on how the associative pathways (i.e., alternating vs. distal) are influenced and altered by the fine structure of catalysts, which eventually holds the key to significantly promote the practical implementation. Herein, we introduce Fe dopants into TiO₂ nanofibers to stabilize oxygen vacancies and simultaneously tune their local electronic structure. The combination of in situ characterizations with first-principles simulations reveals that the modulation of local electronic structure by Fe dopants turns the hydrogenation of N₂ from associative alternating pathway to associative distal pathway. This work provides fresh hints for rationally controlling the reaction pathways toward efficient photocatalytic nitrogen fixation.

A Review of Photocatalytic Materials for Urban NOx Remediation

NOx is a pervasive pollutant in urban environments. This review assesses the current state of the art of photocatalytic oxidation materials, designed for the abatement of nitrogen oxides (NOx) in the urban environment, and typically, but not exclusively based on titanium dioxide (TiO2). Field trials with existing commercial materials, such as paints, asphalt and concrete, in a range of environments including street canyons, car parks, tunnels, highways and open streets, are considered in-depth. Lab studies containing the most recent developments in the photocatalytic materials are also summarised, as well as studies investigating the impact of physical parameters on their efficiency. It is concluded that this technology may be useful as a part of the measures used to lower urban air pollution levels, yielding ∼2% NOx removal in the immediate area around the surface, for optimised TiO2, in some cases, but is not capable of the reported high NOx removal efficiencies >20% in outdoor urban environments, and can in some cases lower air quality by releasing hazardous by-products. However, research into new material is ongoing. The reason for the mixed results in the studies reviewed, and massive range of removal efficiencies reported (from negligible and up to >80%) is mainly the large range of testing practices used. Before deployment in individual environments site-specific testing should be performed, and new standards for lab and field testing should be developed. The longevity of the materials and their potential for producing hazardous by-products should also be considered.

Jointed Synchronous Photocatalytic Oxidation and Chromate Reduction Enabled by the Defect Distribution upon BiVO4: Mechanism Insight and Toxicity Assessment

Exploring active and ecological materials for the restoration of complex pollution system is highly desired. This study presents a facile defect-tailoring strategy for combined pollutants purification with BiVO₄ photocatalysis in which the jointed synchronous reaction of oxidation and reduction is integrated instead of the sequential reaction in two individual systems. XPS and EPR reveal that BiVO₄ with a suitable oxygen vacancies (OVs) concentration and distribution exhibits superior photocatalytic activity under the coexistence of TC-HCl and Cr(VI) with Cr(VI) reduction efficiency increased by 71 times compared with the individual Cr(VI) system along with TC-HCl removal efficiency comparable to a single TC-HCl system. The mechanism of synchronous redox reactions mediated by surface OVs is revealed by comprehensive characterization together with reaction kinetic analysis, and the electronic band structure adjustment induced by the OVs variation is confirmed. Active species identification tests and intermediate product analysis confirm that singlet oxygen (¹O₂) accounts for the selective oxidation of TC-HCl, while electrons dominate the reduction of Cr(VI), under a coexistent environment. The influence of water quality parameters (e.g., pH, cations, anions, and organic substances) on the photocatalytic activity is investigated considering the complexity of the real aquatic environment. Importantly, toxicity assessment with Gram-negative strain E. coli as a model bacterium validates that the toxicity of the intermediates can be reduced to low or even ultralow levels. This work is dedicated to the mechanistic study of defect photocatalysis over BiVO₄ and provides a jointed synchronous reaction system for combined pollutant purification.

Surface Structure Engineering of Nanosheet-Assembled NiFe2O4 Fluffy Flowers for Gas Sensing

In this work, we present a strategy to improve the gas-sensing performance of NiFe₂O₄ via a controllable annealing Ni/Fe precursor to fluffy NiFe₂O₄ nanosheet flowers. X-ray diffraction (XRD), a scanning electron microscope (SEM), nitrogen adsorption–desorption measurements and X-ray photoelectron spectroscopy (XPS) were used to characterize the crystal structure, morphology, specific surface area and surface structure. The gas-sensing performance was tested and the results demonstrate that the response was strongly influenced by the specific surface area and surface structure. The resultant NiFe₂O₄ nanosheet flowers with a heating rate of 8 °C min⁻¹, which have a fluffier morphology and more oxygen vacancies in the surface, exhibited enhanced response and shortened response time toward ethanol. The easy approach facilitates the mass production of gas sensors based on bimetallic ferrites with high sensing performance via controlling the morphology and surface structure.

Atomic-Level and Modulated Interfaces of Photocatalyst Heterostructure Constructed by External Defect-Induced Strategy: A Critical Review

Despite the existence of numerous photocatalyst heterostructures, their separation efficiency and charge flow precision remain low due to the poor study on interfacial properties. The photocatalysts with confined defects can effectively control the photogenerated carrier migration, but the metastability of such defects considerably decreases the photocatalyst stability. Meanwhile, the introduction of defective region can increase the coordinative unsaturation and delocalize local electrons to promote their interactions with the molecules/ions in that region. The selective growth of modulated heterogeneous interface by defect-induced strategy may not only increase the stability of defective structures, but also enhance the migration of interfacial charges. Using this method, photocatalytic heterostructures with low contact resistances and intimate interfaces are constructed to achieve the optimal charge migration in terms of efficiency and accuracy. In this work, the point, linear, and planar heterogeneous interfaces and related defect engineering techniques are discussed. Particularly, it is focused on the external, defect-induced interfacial heterogeneities with various spatial and dimensional configurations, which exhibit modulated and controllable interfacial properties. Furthermore, the main aspects of fabricating photocatalyst heterostructures by the defect-induced strategy, including the i) controllable generation of defects, ii) advanced characterization methods, and iii) elaborate construction of the minimal interface, are described.

Surface structure-dependent photocatalytic O2 activation for pollutant removal with bismuth oxyhalides

The purification of water and air by semiconductor photocatalysis is a rapidly growing area for academic research and industrial innovation, featured with ambient removal of organic or inorganic pollutants by using solar light as the energy source and atmospheric O₂ as the green oxidant. Both charge transfer and energy transfer from excited photocatalysts can overcome the spin-forbidden nature of O₂. Layered bismuth oxyhalides are a new group of two-dimensional photocatalysts with an appealing geometric and surface structure that allows the dynamic and selective tuning of O₂ activation at the surface molecular level. In this Feature Article, we specifically summarize our recent progress in selective O₂ activation by engineering surface structures of bismuth oxyhalides. Then, we demonstrate selective photocatalytic O₂ activation of bismuth oxyhalides for environmental control, including water decontamination, volatile organic compound oxidation and nitrogen oxide removal, as well as selective catalytic oxidations. Challenges and opportunities regarding the design of photocatalysts with satisfactory performance for potential environmental control applications are also presented.

Mechanism study on TiO2 inducing O2- and OH radicals in O3/H2O2 system for high-efficiency NO oxidation

To achieve high NO oxidation efficiency, excessive O₃ must be used, which would lead to the high cost and escape of ozone. Herein, we adopted low cost and environmental-friendly TiO₂ as the catalyst of low concentration O₃ and H₂O₂ system for high-efficiency NO_x oxidation. The Ti sites on TiO₂ were the deprotonation sites of H₂O₂ and H₂O into Ti-OOH and Ti-OH species, respectively. We found that the surface of rutile phase TiO₂ had a low concentration Ti-OOH component but a large amount of Ti-OH after contacting with H₂O₂ solution, thus lots of ·OH and a few O₂^- radicals formed with introducing O₃ molecules. H₂O₂ solution induced the formation of a large amount of Ti-OOH and Ti-OH species on the anatase phase TiO₂ surface, thus lots of O₂^- generated in the O₃/H₂O₂ system. O₂^- and OH radicals could efficiently oxidize NO, in which O₂^- radicals could oxidize NO to NO₃^- in one step with high selectively. Therefore, anatase TiO₂ had better performance in NO_x oxidation than rutile phase TiO₂. The effect of temperature and SO₂ concentration on NO oxidation was also investigated, the results showed that TiO₂-A/O₃/H₂O₂ system promoted NO oxidation at a low temperature and a low concentration of SO₂.

The Use of Ultra-Small Fe3O4 Magnetic Nanoparticles for Hydrothermal Synthesis of Fe3+-Doped Titanate Nanotubes

A method of the hydrothermal synthesis of Fe³⁺-doped titanate nanotubes (TNT) is reported in which the ultra-small Fe₃O₄ nanoparticles are used as the sources of Fe³⁺ ions. The magnetic nanoparticles with a diameter of about 2 nm are added during the washing stage of the hydrothermal procedure. During washing, they gradually degrade and at the same time, the titanate product is transformed into nanotubes. The obtained nanotubes were characterized by structural and magnetic measurements. It was found that, depending on the value of the external magnetic field, they may show the property of room temperature ferromagnetism, paramagnetism or they may be diamagnetic. It was also shown that the modified TNTs have greater photocatalytic activity compared to unmodified TNTs.

TiO2 superstructures with oriented nanospaces: a strategy for efficient and selective photocatalysis

Highly ordered superstructures of semiconductor nanocrystals contain abundant nanometer-scale pores between the crystals; however, there have been difficulties in controlling the size and orientation of these nanospaces without the use of a template or a capping reagent. This constraint has affected their development and applications in potential fields including catalysis and optoelectronics adversely. In this study, we synthesized a rod-shaped TiO2 mesocrystal (TMC) having a length of a few hundreds of micrometers and comprising regularly ordered anatase TiO2 nanocrystals that form oriented nanospaces by exposed {001} facets. Finite-difference time-domain (FDTD) calculations of electric fields and in situ fluorescence imaging with a polarization sensitive dye on a single mesocrystal were performed to reveal anisotropic adsorption and excitation of the dyes. Furthermore, the photodegradation of the dyes was found to be more facilitated in nanospaces formed by the specific facets, as compared with the dyes randomly adsorbed on the outer surfaces. Consequently, the selectivity of photocatalytic reactions based on the molecular size and redox was enhanced by introducing the concept of oriented nanospace.

One-pot synthesis of Fe2O3/Y2O3@TNS for enhanced photocatalytic and adsorption properties

TiO₂-based nanosheets (TNS) modified with Fe₂O₃ and Y₂O₃ particles (Fe₂O₃/Y₂O₃@TNS), possessing a laminar structure with large specific surface area of 382 m² g⁻¹, were synthesized via a one-pot hydrothermal method.

Mg0.8Zn0.2O microspheres: preparation, characterization and application for degrading organic dyes

Photocatalytic degradation of organic pollutants in wastewater is one of the most promising strategies for environmental remediation, and photocatalysts as the prerequisite have received considerable attention.

Au nanocrystals decorated TiO2 nanotubes for photocatalytic nitrogen fixation into ammonia

Nitrogen photofixation has been successfully performed on TiO₂ nanotubes decorated with Au nanocrystals which clearly outperform commercial P25 under the same conditions.

3D graphene-based gel photocatalysts for environmental pollutants degradation

Enormous research interest is devoted to fabricating three-dimensional graphene-based gels (3D GBGs) toward improved conversion of solar energy by virtue of the intrinsic properties of single graphene and 3D porous structure characteristics. Here, this concise minireview is primarily focused on the recent progress on applications of 3D GBGs, including aerogels and hydrogels, in photocatalytic degradation of pollutants from water and air, such as organic pollutants, heavy metal ions, bacteria and gaseous pollutants. In particular, the preponderances of 3D GBG photocatalysts for environmental pollutants degradation have been elaborated. Furthermore, in addition to discussing opportunities offered by 3D GBG composite photocatalysts, we also describe the existing problems and the future direction of 3D GBG materials in this burgeoning research area. It is hoped that this review could spur multidisciplinary research interest for advancing the rational utilization of 3D GBGs for practical applications in environmental remediation.

Single-molecule and -particle probing crystal edge/corner as highly efficient photocatalytic sites on a single TiO2 particle

The exposed active sites of semiconductor catalysts are essential to the photocatalytic energy conversion efficiency. However, it is difficult to directly observe such active sites and understand the photogenerated electron/hole pairs' dynamics on a single catalyst particle. Here, we applied a quasi-total internal reflection fluorescence microscopy and laser-scanning confocal microscopy to identify the photocatalytic active sites at a single-molecule level and visualized the photogenerated hole-electron pair dynamics on a single TiO₂ particle, the most widely used photocatalyst. The experimental results and density functional theory calculations reveal that holes and electrons tend to reach and react at the same surface sites, i.e., crystal edge/corner, within a single anatase TiO₂ particle owing to the highly exposed (001) and (101) facets. The observation provides solid proof for the existence of the surface junction "edge or corner" on single TiO₂ particles. These findings also offer insights into the nature of the photocatalytic active sites and imply an activity-based strategy for rationally engineering catalysts for improved photocatalysis, which can be also applied for other catalytic materials.