Facet-dependent photocatalytic NO conversion pathways predetermined by adsorption activation patterns

Boosting photocatalytic NO oxidation mediated by high redox charge carriers from visible light-driven C3N4/UiO-67 S-scheme heterojunction photocatalyst

The construction of CN/UiO-67 (CNU) S-scheme heterojunction composites through in situ formation of UiO-67 on carbon nitride (C3N4) helps to address the limitations of carbon nitride (CN) in photocatalytic NO elimination. The optimized CNU3 demonstrates superior photocatalytic efficiency, which is attributed to electronic channels constructed by Zr-N bonds and S-scheme electron transport mechanism, effectively promoting the efficient separation of photogenerated charge carriers with high redox potentials. Density Functional Theory (DFT) calculations reveal redistributed electronic orbitals in CNU3, with progressive and continuous energy levels near the Fermi level, which bolsters electronic conduction. Comprehensive quenching experiments, Electron Paramagnetic Resonance (EPR), and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) analyses highlight a synergistic interplay of electrons, holes, and superoxide radicals in CNU3, inhibiting the generation of toxic nitrogen oxide intermediates and culminating in highly efficient photocatalytic NO oxidation. This study not only elucidates the mechanisms underpinning the enhanced performance of CNU3 heterojunctions but also offers new perspectives on the preparation and interfacial charge separation of heterojunction photocatalysts.

BiOI-SnO2 Heterojunction Design to Boost Visible-Light-Driven Photocatalytic NO Purification

The efficient, stable, and selective photocatalytic conversion of nitric oxide (NO) into harmless products such as nitrate (NO3-) is greatly desired but remains an enormous challenge. In this work, a series of BiOI/SnO2 heterojunctions (denoted as X%B-S, where X% is the mass portion of BiOI compared with the mass of SnO2) were synthesized for the efficient transformation of NO into harmless NO3-. The best performance was achieved by the 30%B-S catalyst, whose NO removal efficiency was 96.3% and 47.2% higher than that of 15%B-S and 75%B-S, respectively. Moreover, 30%B-S also exhibited good stability and recyclability. This enhanced performance was mainly caused by the heterojunction structure, which facilitated charge transport and electron-hole separation. Under visible light irradiation, the electrons gathered in SnO2 transformed O2 to ·O2- and ·OH, while the holes generated in BiOI oxidized H2O to produce ·OH. The abundantly generated ·OH, ·O2-, and 1O2 species effectively converted NO to NO- and NO2-, thus promoting the oxidation of NO to NO3-. Overall, the heterojunction formation between p-type BiOI and n-type SnO2 significantly reduced the recombination of photo-induced electron-hole pairs and promoted the photocatalytic activity. This work reveals the critical role of heterojunctions during photocatalytic degradation and provides some insight into NO removal.

Carbon nanodots with a controlled N structure by a solvothermal method for generation of reactive oxygen species under visible light

Carbon nanodots can function as photosensitizers that have the ability to generate reactive oxygen species such as singlet oxygen, hydroxy (OH) radicals, and superoxide ions. However, most of these can only be generated upon ultraviolet light excitation. Additionally, the mechanism of reactive oxygen species generation by carbon nanodots remains unclear. The development of carbon nanodots that can photosensitize under visible light irradiation is desirable for applications such as photodynamic therapy and pollutant decomposition under visible light. Here, we report novel carbon nanodot-based photosensitizers that generate reactive oxygen species under visible light; they were synthesized using a solvothermal method with two solvents (formamide and water) and amidol as the carbon source. Carbon nanodots from the solvothermal synthesis in formamide showed blue fluorescence, while those obtained in water showed green fluorescence. The photo-excited blue-fluorescent carbon nanodots produced OH radicals, superoxide ions, and singlet oxygen, and therefore could function as both type I and type II photosensitizers. In addition, photo-excited green-fluorescent carbon nanodots generated only singlet oxygen, therefore functioning as type II photosensitizers. It is proposed that the two photosensitizers have different origins of reactive oxygen species generation: the enrichment of graphitic N for blue-fluorescent carbon nanodots and molecular fluorophores for green-fluorescent carbon nanodots.

Typical layered structure bismuth-based photocatalysts for photocatalytic nitrogen oxides oxidation

Traditional NO_x treatment methods require external reducing reagents and harsh reaction conditions, which is not conducive to effectively eliminate NO_x at low concentration, especially at ppb levels. Fortunately, low concentration NO_x can be removed by photocatalytic oxidation under mild reaction conditions. Bismuth (Bi)-based photocatalysts with the layered structure have obtained considerable concerns of photocatalytic NO_x oxidation. This review focused on typical layered Bi-based photocatalysts (Bi₂WO₆, Bi₂O₂CO₃, BiOY (YCl, Br, and I), BiOIO₃, and BiOCOOH) with the structure of [Bi₂O₂]²⁺ layer for photocatalytic NO_x oxidation. The strategies (morphological control, defect engineering, heterostructure construction, etc.) to improve photocatalytic oxidation activity were summarized. Furthermore, the mechanism involving various free radicals (hydroxyl radical, superoxide radical, etc.) of photocatalytic oxidation of NO_x was proposed. In addition, the non-NO₂ selectivity was also illuminated. Lastly, the current drawbacks and further research directions for photocatalytic NO_x oxidation were elaborated. The development of photocatalysts with high photocatalytic activity, wide light absorption range, and non-NO₂ selectivity is the focus of future research. This review aims to provide a pandect and theoretical guidance for the practical application of photocatalytic oxidation of NO_x.

Switching on photocatalytic NO oxidation and proton reduction of NH2-MIL-125(Ti) by convenient linker defect engineering

Photocatalysis technology has been widely adopted to abate typical air pollutants. Nevertheless, developing photocatalysts aimed at improving photocatalytic efficiency is a challenge. Herein, the linker-defect NH₂-MIL-125(Ti) photocatalyst was synthesized through a convenient one-step heating-stirring method (just adjusting multiple temperatures) to firstly realize efficient photocatalytic performances of NO removal and hydrogen evolution. The optimal sample (named 65-NMIL) with a linker-defect content of 32.08% exhibited a NO removal ratio of 65.49%, which was 37.57% higher than that of pristine NH₂-MIL-125(Ti), and displayed better H₂-production activity. Through ESR, it was confirmed that 65-NMIL can generate more •O₂^- and •OH under visible light, and the radical trapping experiment further proved that •O₂^- played a more important role in photocatalytic activity. Moreover, the photocatalytic NO oxidation process was also monitored by in situ DRIFTS, it was found that the defective samples could promote the oxidation of NO and intermediates to the final product (NO₃^-). On the basis of the above-mentioned photocatalytic experimental results and characterization, a possible mechanism or pathway was proposed and illustrated. This work can provide a new strategy for the subsequent defect engineering for photocatalytic MOFs materials to further solve environmental and energy crises.

Ce-Doped W18O49 Nanowires for Tuning N2 Activation toward Direct Nitrate Photosynthesis

Nitrate acts as a fundamental raw material in modern industrial and agricultural fields. Recently, photocatalytic nitrogen oxidation into nitrate has been expected to be an alternative method to replace the industrial nitrate synthesis process, which encounters many challenges, i.e., huge energy consumption and greenhouse gas emission. We synthesized Ce-doped W₁₈O₄₉ nanowires (Ce-W₁₈O₄₉) to realize photocatalytic nitrogen oxidation into nitrate under mild conditions. The defect state generated by coupling of Ce³⁺ introduction and surface plasma state acts as an "electron trap" to restrain photogenerated electrons, so as to facilitate the separation of photogenerated electron-hole pairs and prolong their lifetime. W₁₈O₄₉ doped with 5 mol % Ce exhibited the highest yield of nitrate (319.97 μg g^-1 h^-1) without any sacrificial agent, which is about 5 times higher than that of pristine W₁₈O₄₉. This work provides new insight into achieving high-efficiency photocatalytic nitrate evolution activity from direct N₂ oxidation by controlling the energy band structure of photocatalysts.

Ambient Air Purification by Nanotechnologies: From Theory to Application

Air pollution has been a recurring problem in northern Chinese cities, and high concentrations of PM2.5 in winter have been a particular cause for concern. Secondary aerosols converted from precursor gases (i.e., nitrogen oxides and volatile organic compounds) evidently account for a large fraction of the PM2.5. Conventional control methods, such as dust removal, desulfurization, and denitrification, help reduce emissions from stationary combustion sources, but these measures have not led to decreases in haze events. Recent advances in nanomaterials and nanotechnology provide new opportunities for removing fine particles and gaseous pollutants from ambient air and reducing the impacts on human health. This review begins with overviews of air pollution and traditional abatement technologies, and then advances in ambient air purification by nanotechnologies, including filtration, adsorption, photocatalysis, and ambient-temperature catalysis are presented—from fundamental principles to applications. Current state-of-the-art developments in the use of nanomaterials for particle removal, gas adsorption, and catalysis are summarized, and practical applications of catalysis-based techniques for air purification by nanomaterials in indoor, semi-enclosed, and open spaces are highlighted. Finally, we propose future directions for the development of novel disinfectant nanomaterials and the construction of advanced air purification devices.

Optimized strategies for (BiO)2CO3 and its application in the environment

Photocatalysis is a new type of technology, which has been developed rapidly for solving environmental problems such as wastewater or air pollutants in recent years. Also, the effective performance and non-secondary pollution of photocatalytic technology attract much attention from researchers. As a "sillén" phase oxide, the (BiO)₂CO₃ (BOC) is a great potential photocatalyst attributing to composed of alternate Bi₂O₂²⁺ and CO₃^2- layers, which is a benefit for transportation of electrons. Besides, BOC has attracted much attention from researchers because of its excellent characters of non-toxic, environmentally friendly, and low-cost. However, BOC has a defect on wide band gap, which is limited for the usage of visible light, so a great number of published papers focus on the modifications of BOC to improve its photocatalytic efficiency. This article mainly summarizes the modifications of BOC and its application in the environment, guiding for designing BOC-based materials with high photocatalytic activity driven by light. Moreover, the research trend and prospect of BOC photocatalyst were briefly summarized, which could lay the foundation for forming a green and efficient BOC-based photocatalytic reaction system. Importantly, this review might provide a theoretical basis and guidance for further research in this field.

Doping and facet effects synergistically mediated interfacial reaction mechanism and selectivity in photocatalytic NO abatement

The surface atomic coordination and arrangement largely determine photocatalytic properties. Whereas, the intrinsic impact of surface microstructures on the reaction mechanism and pathway is still unclear. Herein, via constructing N-doped Bi₂O₂CO₃ photocatalysts with diverse exposed facets, (1 1 0) and (0 0 1) facet, we testify that the pivotal roles of crystal facet and doping effect on the intermediate production and reactivity for photocatalytic nitric oxide (NO) abatement. The photoreactivity of N-doped Bi₂O₂CO₃ is documented to be higher than that of the pure samples because of the enhanced light absorption and charge transfer. Further in situ probing experiments and theoretical calculations verify that the unique adsorption patterns and activated intermediates on the (1 1 0) facet facilitate the formation of final products and inhibit the generation of toxic NO₂ by-product in terms of thermodynamics. More importantly, we found that the selective and nonselective oxidation processes are emerged over (1 1 0) and (0 0 1) facets of Bi₂O₂CO₃, respectively.

Insight into facet-dependent photocatalytic H2O2 production on BiOCl nanosheets

The generation of H₂O₂ on BiOCl(001) originates from O₂ reduction, while there are two pathways on BiOCl(010).

Facet-selective charge separation in two-dimensional bismuth-based photocatalysts

In this review, we summarize recent advances in the facet design of bismuth-based single-crystal plates based on facet-dependent charge separation for photocatalytic reactions, and the manipulation of the spatial charge separation is highlighted.

Heterojunction interface of zinc oxide and zinc sulfide promoting reactive molecules activation and carrier separation toward efficient photocatalysis

Heterojunction photocatalysts, which can alleviate the low carrier separation efficiency and insufficient light absorption capacity of a single catalyst, have received widespread attention. However, the specific interfacial structure of the heterojunction and its effect on the photocatalytic reaction is still unclear. Herein, a battery of zinc oxide/zinc sulfide (ZnO@ZnS) heterojunction microspheres with different degrees of sulfuration were successfully constructed via a facile hydrothermal method. The as-prepared photocatalysts shown decent aerobic nitric oxide (NO) oxidation performance under visible light irradiation, and the results of various characterization techniques illustrated that the superior photoactivity could be ascribed to the spatial separation of photoinduced electron-hole pairs due to the synergy of the internal electric field and the band offset. More importantly, density functional theory (DFT) calculations revealed that the heterojunction interface can significantly promote the generation of reactive oxygen species (ROS) and NO⁺ reaction intermediates and thus accelerate the photocatalytic reaction. Finally, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technology was used to time-dependently monitor the NO oxidation process, revealing the photocatalytic mechanism. This work investigated the role of the heterojunction interface in the gas-phase catalytic reaction, broadening the practical application of the ZnO@ZnS heterojunction.

Unveiling the Mechanism of Polymer Grafting on Graphene for Functional Composites: The Behavior of Radicals

Polymer-graphene composites have attracted significant attention; however, their formation mechanisms are a focus of debate. This work tries to clarify how grafting occurs on graphene by electron spin resonance techniques. As a result, two pathways are found. One passes through the radicals formed by cleaving CO bonds on graphene are transferred to monomers, then grafting and polymerization proceed. Another mechanism passes through the oxy-radicals, which react with monomers in solution and finally react with carbon radicals on graphene. Based on the mechanism, various types of polymer-graphene composites are prepared, and applied to electrical conductive sheets, basic catalysts, and acidic catalysts.

Noble-metal-free cobaloxime coupled with metal-organic frameworks NH2-MIL-125: A novel bifunctional photocatalyst for photocatalytic NO removal and H2 evolution under visible light irradiation

The novel bifunctional NH₂-MIL-125/Co(dmgH)₂ composite catalysts with several different Co(dmgH)₂ contents that can simultaneously achieve photocatalytic NO removal and hydrogen production were first prepared by a simple and convenient method. The corresponding physical and chemical properties of the composite catalysts were characterized by SEM, XRD, ESR, in situ DRIFTS, etc. The characterization results indicated that the noble-metal-free Co(dmgH)₂, which was much cheaper and more available than most noble-metals such as Pt, could be an effective co-catalyst to accelerate the separation of photogenerated electron-hole pairs, further eventually enhancing the photocatalytic efficiency. Under visible-light irradiation for half an hour, the NO removal ratio of NH₂-MIL-125/Co(dmgH)₂ (3 wt%) increased by 22.7 % compared with the pristine NH₂-MIL-125 without Co(dmgH)₂ loading. In addition, it was found that Eosin Y dye-sensitized NH₂-MIL-125/Co(dmgH)₂ (3 wt%) was capable of promoting a hydrogen generation rate of 2195 μmol g^-1 h^-1 under visible light, which was 12.6 times greater than the original NH₂-MIL-125. This strategy was expected as an available way to fabricate noble-metal-free molecular complexes with metal-organic frameworks (MOFs) to enhance the photocatalytic NO removal and hydrogen production performance simultaneously.

In situ preparation of 2D MoS2 nanosheets vertically supported on TiO2/PVDF flexible fibers and their photocatalytic performance

Two-dimensional (2D) MoS₂ nanosheets vertically supported on TiO₂/PVDF flexible fibers have been successfully synthesized by combining electrospinning with a low temperature hydrothermal method without acid. The morphology of the 2D MoS₂ nanosheets could be controlled by adjusting the experimental parameters. The loaded 2D MoS₂ nanosheets can not only broaden the light capture range of TiO₂, but also greatly inhibit the recombination rate of photogenerated electron-hole pairs. Due to the synergistic effect between MoS₂ and TiO₂, the photocatalytic rate for levofloxacin hydrochloride is about 40 times higher than that for MoS₂ only. Recycle experiments have proved the stability and reusability of TiO₂/PVDF@2D MoS₂ nanosheets. The mechanism is investigated by quenching experiments. The results show that the superoxide anion radical (•O₂ ^-), the hydroxyl radical (•OH) and the hole (h⁺) all have contributions to photocatalysis. This work widens the range of materials to synthesize the composites of 2D MoS₂ nanosheets and provides a new and gentle method for preparing flexible large-scale heterostructures for environmental protection.

Influence of pH and Surface Orientation on the Photochemical Reactivity of SrTiO3

The photochemical reactivity of the SrTiO₃ surface is affected by the pH of the surrounding aqueous solution. Scanning electron microscopy and atomic force microscopy have been used to quantify the amount of silver that is photochemically reduced on the surfaces of (100), (110), and (111) oriented crystals as a function of pH. For all orientations, the reactivity increases from pH 3, reaches a maximum, and then decreases at higher pH. The pH associated with the maximum reactivity depends on the crystallographic orientation of the surface. The results indicate that the solution pH influences the charge on the SrTiO₃ surface. The amount of surface charge influences band bending within SrTiO₃, and the maximum reactivity is achieved at a surface charge where neither the photocathodic nor the photoanodic reaction limit the overall reaction rate.

Integration of plasmonic effect into MIL-125-NH2: An ultra-efficient photocatalyst for simultaneous removal of ternary system pollutants

Industrial effluents often contain mixed metal ions and dyes, and it is difficult to efficiently remove both types of contaminants simultaneously. Here, MIL-125-NH₂@Ag/AgCl composites were for the first time developed through a facile deposition-photoreduction method for simultaneously removing Cr(VI)/Rhodamine B (RhB)/Malachite Green (MG) ternary system pollutants under visible-light irradiation. The capacities of Cr(VI) reduction dramatically increased to 98.4% in the coexistence of RhB and MG compared to that of binary (Cr(VI)/RhB (69.6%) or Cr(VI)/MG (67.5%)) and single Cr(VI) (29%) systems. In the meantime, the degradation efficiencies of dyes especially RhB in the ternary system were also improved compared to that of their individual systems. On the grounds of all the experimental results, it can be concluded that the efficient light-harvesting and electrons migration in MIL-125-NH₂@Ag/AgCl and the synergistic effect of redox reactions between Cr(VI) and dyes hinder the recombination of photo-induced electron-hole pairs, which are responsible for their high photocatalytic activity to eliminate the mixed pollutants. This study provides a new route to construct high-performance photocatalysts for the practical treatment of wastewater containing mixed pollutants.

Controllable synthesis of a 3D ZnS@MoO3 heterojunction via a hydrothermal method towards efficient NO purification under visible light

Heterojunctions can deliver superior photocatalytic efficiency via modulating the surface-interface reaction, enhancing light absorption and hindering charge carrier recombination.

A facile synthesis of a highly efficient β-Bi2O3/Bi2O2CO3 heterojunction with enhanced photocatalytic NO oxidation under visible light

Photocatalytic oxidation mechanism of NO on β-Bi₂O₃/Bi₂O₂CO₃.

La-Doped ZnWO4 nanorods with enhanced photocatalytic activity for NO removal: effects of La doping and oxygen vacancies

La³⁺-Doped ZnWO₄ nanorods were prepared via a hydrothermal method for the photocatalytic NO removal under simulated solar light irradiation.