Crystal-facet and microstructure engineering in ZnO for photocatalytic NO oxidation

Synergistic optimization of charge carrier separation and transfer in ZnO through crystal facet engineering and piezoelectric effect

Rational regulation of photogenerated charge carrier separation and transfer is a key strategy for optimizing photocatalytic activity. Leveraging the synergistic effects of crystal facet engineering and the piezoelectric effect, a series of hexagonal Zinc oxide (ZnO) photocatalysts with varying exposure ratios of the {002} and {210} facets were successfully synthesized and employed under simultaneous excitation by simulated sunlight and ultrasound. As expected, compared to amorphous ZnO, the hexagonal ZnO samples demonstrated a significant enhancement in piezo-photocatalytic tetracycline hydrochloride degradation and polyethylene terephthalate reforming processes. In-depth investigations confirm that the pronounced piezo-photocatalytic performance of the hexagonal ZnO samples is attributed to the synergistic effect of the built-in electric field formed at the facet junctions and the polarization electric field generated by the piezoelectric effect, both of which significantly influence charge separation and carrier mobility. These findings offer new strategies for improving catalytic efficiency and advancing sustainable technologies through photocatalysis.

Photocatalysis of nanoparticles mediates the response of plants towards nitric oxide in air

Nitric oxide (NO), a signaling molecule involved in plant growth and metabolism, can be synthesized endogenously or assimilated from the atmosphere. Although NO can be oxidized to nitrate or NO2 by phototactically induced hot charge carriers over nanoparticles (NPs) under insolation, the specific role of NP-mediated NO transformation on foliage surfaces in stimulating plant enzymes remains unclear. In this research, Trident (T) and Willow (W) types of water spinach (Ipomoea aquatica Forsk) were employed to probe the physiological alteration by NO transformation and oxidants over photocatalytic NPs. The biomass of T was significantly enhanced by the 240-ppb NO stress or NPs, while W was immune to either condition. According to in-situ diffuse-reflectance-infrared-Fourier-transform spectroscopy, foliar ZnO or TiO2 (2 mg per plant) stimulated the oxidation of NO to NO3- and the production of •OH. NPs enhanced the activity of antioxidant enzymes and NR (nitrate reductase [NAD(P)H]) in T with low SOD (superoxide dismutase, 32 ± 7 Umin-1g-1), NR activity (reaching 188 ± 4 nmol h-1g-1) was positively correlated with a 46.39 % increase in biomass. Conversely, W, endowed with ample SOD (502 ± 30 Umin-1g-1) to offset the stress caused by NO or NPs, displayed negligible growth or NR alterations. Our findings indicate that a high [SOD] could counterbalance the oxidizing NO stress, while the photocatalytic NO conversion into nitrate could boost the NR production in plant with low [SOD] and under NO stress. This research contributes to understanding the impact of NPs application and plant responses to pollutant gas stress.

Facilitated uranium and organic removal and electricity production via a zinc oxide/carbon felt cathode equipped hybrid tandem photocatalytic fuel cell

The combined pollution by uranium and organic matter has posed a serious challenge in the treatment of radioactive wastewater, while traditional treatment methods suffered from the problems of poor treatment efficiency and difficult recycling. Therefore, this study developed a novel hybrid tandem photocatalytic fuel cell (HTPFC) system which decorated with a ZnO modified carbon felt (ZnO/CF) cathode. This HTPFC not only efficiently removed UO22+ and organic matter while generating electricity, but can also be quickly disassembled and reassembled. The ZnO loading greatly improved the electrochemical properties of CF and introduced abundant active sites, making the constructed HTPFC exhibited excellent applicability, reusability and practical application prospects. When UO22+ coexisted with various organics, such as p-nitrophenol, ciprofloxacin, atrazine, ibuprofen, sulfamethoxazole, and tetracycline hydrochloride (TCH), the organic matter removal efficiencies were ≥94.36 %, and the UO22+ removal efficiencies were ≥94.56 %, accompanied with a maximum power output density of ≥1.31 mW·cm-2. After five cycles, the UO22+ and TCH removal efficiency still remained 92.34 % and 89.75 %, respectively. Notably, over 98.0 % of each of UO22+ and TCH can be removed under the actual sunlight illumination, indicating that the fabricated HTPFC had a great practical application potential. This study may present a new approach for disposing the complex wastewater containing heavy metals and organic compounds.

Hollow flower-like WO3@TiO2 heterojunction microspheres for the photocatalytic degradation of rhodamine B and tetracycline

In the context of sustainable development, the utilization of semiconductor materials for the degradation of dyes, antibiotics, heavy metals, and pesticides in wastewater under visible light has emerged as a focal point of contemporary research. In this investigation, a WO3@TiO2 composite was synthesized via a solvothermal method, with the composite exhibiting a molar ratio of 5% WO3 to TiO2 precursors demonstrating optimal photocatalytic degradation performance. This material achieved complete degradation of 20 mg per L Rhodamine B (RhB) dye and tetracycline (TC) antibiotic within 30 min. Furthermore, the effects of initial pollutant concentration and solution pH on catalytic efficacy were systematically explored. The findings revealed that at RhB concentrations below 40 mg L-1, the degradation proceeded at an accelerated rate, with a rate constant exceeding 0.128 min-1. The catalyst exhibited robust performance across a broad pH range, attaining peak degradation efficiency at pH ≈ 3. The exceptional photocatalytic prowess of the WO3@TiO2 composite is predominantly attributable to its distinctive hollow microstructure, the intimate interfacial synergy between WO3 and TiO2, and the efficient separation of photogenerated electrons and holes facilitated by the type-II heterojunction architecture.

Resource utilization of electric arc furnace dust: Efficient wet desulfurization and valuable metal leaching kinetics investigation

Electric arc furnace dust (EAFD) is hazardous solid waste containing valuable metals such as iron (Fe) and zinc (Zn) in large quantities. This presents a considerable recycling issue. This study proposes an integrated and efficient approach to solid waste management. The experimental results demonstrate that the EAFD slurry effectively purifies SO2 while simultaneously recovering Zn resources. The major active components for desulfurization in EAFD were found to be inc oxide and ZnFe2O4, with depletion being the main cause of reduced desulfurization performance. The maximum Zn leaching rate reached 49.35%, and SO2 100% removal was achievable under ideal leaching conditions. A shrinkage kernel model analysis of the leaching kinetics showed that the initial apparent activation energy of the Zn leaching reaction was 15.51 kJ mol-1, suggesting that interfacial chemical reactions were in charge of the process. Based on the experimental and characterization results, a possible mechanism for metal ions leaching during EAFD slurry desulfurization was proposed. This study presents a novel approach for the resource utilization of hazardous solid waste EAFD, offering a sustainable solution for metal recovery and pollution control.

Enriching Oxidation Sites-Based Facets in Lead Chromate to Boost Photoelectrochemical Sensing Response

A key issue in photoelectrochemical applications is the modification of the behavior of photogenerated charge barriers. An effective strategy to improve the photoelectrochemical performance of semiconductor materials is to use the facet effect to promote spatial charge separation. In this work, three different morphologies of lead chromate (PbCrO4) crystals are prepared by a simple hydrothermal method that used ammonium fluoride as the structure-directing agents. Spatial separation of photogenerated electrons and holes is clearly demonstrated in the (012), (020), and (200) facets of PbCrO4 crystals. In situ photo-deposition experiments reveal that the oxidation and reduction sites are distributed on the anisotropic (012) and (020)/(200) facets of all the PbCrO4 crystals. PbCrO4 synthesized with a high Pb2+/F- ratio with maximum exposure of (012) facet has remarkably better performance in photoelectrochemical detection of ascorbic acid compared with PbCrO4 synthesized either without ammonium fluoride or with a low Pb2+/F- ratio. The photoelectrochemical detection performance correlates well with the surface photovoltage difference between the anisotropic facets. The study provides fundamental understanding of the facet-dependent activity of PbCrO4 crystals, which will be beneficial for advancing understanding of spatial charge separation in semiconductor-based photoelectrochemical applications.

Bismuth oxybromide nanosheets as an efficient photocatalyst for dye degradation

The contamination of water resources by organic pollutants presents significant environmental and health challenges. Therefore, it is urgent to develop highly efficient and green approach for treating organic water pollutants. Bismuth oxybromide (BiOBr) has gained attention due to its high photoactivity. In this work, we report a modification to improve its photocatalytic activity. BiOBr were prepared using a capping agent, benzene-1,3,5-tricarboxylic acid, to tune the morphology of the compound. The resulting BiOBr shows nanosheet morphology, which provides a high surface-to-volume ratio and a larger conduction band compared to bulk BiOBr. As a result, the BiOBr nanosheets show the highest efficiency for photodegradation of Rhodamine B, compared to benchmark TiO2 and bulk BiOBr catalysts.

Enhanced desulfurization performance of model fuel by Cu-ZnO/TiO2 heterostructure

A facile hydrothermal approach was employed to synthesize a novel Cu-ZnO/TiO2 Z-heterojunction with a high density of defects, which was then utilized for the oxidative desulfurization process, demonstrating excellent photodegradation performance. The results showed that by adjusting components such as Cu, ZnO, and TiO2, the removal efficiency of DBT reached 88.12% within a duration of 240 min. In the 5 repeated experiments, 7.5%Cu-ZnO/TiO2 still exhibited high stability and could be reused. The improved photocatalytic performance of the 7.5%Cu-ZnO/TiO2 composite can be attributed to its high light absorption capability and well-matched energy levels, which are due to the abundant presence of imperfections. The adoption of a Z-heterojunction has enabled efficient separation and transfer of photo-generated electrons and holes (e-/h+), thereby reducing the probability of charge carrier recombination.

NOx precipitation and valorization driven by photocatalysis and adsorption over red soil

Nitrogen oxides (NOx) emissions can cause air pollution that is harmful to human health, even producing serious ecological problems. Whether it is diluted in the air or not, the management and valorization of NOx from industrial emissions have been constrained by technology and finance. This study shows that red soil can be used as a photocatalyst to convert NOx into soil nitrate nitrogen (NO3--N) in the soil. The addition of zinc oxide (ZnO) and titanium dioxide (TiO2) onto the soil surface improves the photocatalytic precipitation efficiency of 1 ppm NO, approaching a removal efficiency of 77 % under ultraviolet (UV) light. The efficiency of red soil in precipitating NOx through adsorption exceeded that of photocatalysis at 100 ppm NOx (e.g. 16.02 % versus 7.70 % in 0.1-mm soil). Pot experiment reveals that the precipitated NO3--N promoted biomass of water spinach (Ipomoea aquatica Forsk). Additionally, adding ZnO or TiO2 also affects mineral nutrition. This demonstration of converting air pollutants into available nitrogen (N) for plant growth not only provides a new perspective on treatment and valorization for NOx but also sheds light on the transport of N in the air-soil-plant path.

Europium insertion into MgAl hydrotalcite-like compound to promote the photocatalytic oxidation of nitrogen oxides

Easy synthesis of efficient, non-toxic photocatalysts is a target to expand their potential applications. In this research, the role of Eu3+ doping in the non-toxic, affordable, and easily prepared MgAl hydrotalcite-like compounds (HTlcs) was explored in order to prepare visible light semiconductors. Eu doped MgAl-HTlcs (MA-xEu) samples were prepared using a simple coprecipitation method (water, room temperature and atmospheric pressure) and europium was successfully incorporated into MgAl HTlc frameworks at various concentrations, with x (Eu3+/M3+ percentage) ranging from 2 to 15. Due to the higher ionic radius and lower polarizability of Eu3+ cation, its presence in the metal hydroxide layer induces slight structural distortions, which eventually affect the growth of the particles. The specific surface area also increases with the Eu content. Moreover, the presence of Eu3+ 4f energy levels in the electronic structure enables the absorption of visible light in the doped MA-xEu samples and contributes to efficient electron-hole separation. The microstructural and electronic changes induced by the insertion of Eu enable the preparation of visible light MgAl-based HTlcs photocatalysts for air purification purposes. Specifically, the optimal HTlc photocatalyst showed improved NOx removal efficiency, ∼ 51% (UV-Vis) and 39% (visible light irradiation, 420 nm), with excellent selectivity (> 96 %), stability (> 7 h), and enhanced release of •O2- radicals. Such results demonstrate a simple way to design photocatalytic HTlcs suitable for air purification technologies.

Recent Advancements on Spin Engineering Strategies for Highly Efficient Electrocatalytic Oxygen Evolution Reactions

Oxygen evolution reaction (OER) is a widely employed half-electrode reaction in oxygen electrochemistry, in applications such as hydrogen evolution, carbon dioxide reduction, ammonia synthesis, and electrocatalytic hydrogenation. Unfortunately, its slow kinetics limits the commercialization of such applications. It is therefore highly imperative to develop highly robust electrocatalysts with high activity, long-term durability, and low noble-metal contents. Previously intensive efforts have been made to introduce the advancements on developing non-precious transition metal electrocatalysts and their OER mechanisms. Electronic structure tuning is one of the most effective and interesting ways to boost OER activity and spin angular momentum is an intrinsic property of the electron. Therefore, modulation on the spin states and the magnetic properties of the electrocatalyst enables the changes on energy associated with interacting electron clouds with radical absorbance, affecting the OER activity and stability. Given that few review efforts have been made on this topic, in this review, the-state-of-the-art research progress on spin-dependent effects in OER will be briefed. Spin engineering strategies, such as strain, crystal surface engineering, crystal doping, etc., will be introduced. The related mechanism for spin manipulation to boost OER activity will also be discussed. Finally, the challenges and prospects for the development of spin catalysis are presented. This review aims to highlight the significance of spin engineering in breaking the bottleneck of electrocatalysis and promoting the practical application of high-efficiency electrocatalysts.

Visible light photocatalytic NOx removal with suppressed poisonous NO2 byproduct generation over simply synthesized triangular silver nanoparticles coupled with tin dioxide

The treatment or conversion of air pollutants with a low generation of secondary toxic substances has become a hot topic in indoor air pollution abatement. Herein, we used triangle-shaped Ag nanoparticles coupled with SnO2 for efficient photocatalytic NO removal. Ag triangular nanoparticles (TNPs) were synthesized by the photoreduction method and SnO2 was coupled by a simple chemical impregnation process. The photocatalytic NO removal activity results show that the modification with Ag TNPs significantly boosted the removal performance up to 3.4 times higher than pristine SnO2. The underlying roles of Ag TNPs in NO removal activity improvement are due to some advantages of Ag TNPs. Moreover, the Ag TNPs contributed photogenerated holes as the main active species toward enhancing the NO oxidation reaction. In particular, the selectivity toward green products significantly improved from 52.78% (SnO2) to 86.99% (Ag TNPs/SnO2). The formation of reactive radicals under light irradiation was also verified by DMPO spin-trapping experiments. This work provides a potential candidate for visible-light photocatalytic NO removal with low toxic byproduct generation.

The effects of water, substrate, and intermediate adsorption on the photocatalytic decomposition of air pollutants over nano-TiO2 photocatalysts

The photocatalytic performance of nano-TiO2 photocatalysts in air pollutant degradation greatly depends on the adsorption of water, substrates, and intermediates. Especially under excessive humidity, substrate concentration, and intermediate concentration, the competitive adsorption of water, substrates, and intermediates can seriously inhibit the photocatalytic performance. In the past few years, extensive studies have been performed to investigate the influence of humidity, substrate concentration, and intermediates on the photocatalytic performance of TiO2, and significant advances have been made in the area. However, to the best of our knowledge, there is no review focusing on the effects of water, substrate, and intermediate adsorption to date. A comprehensive understanding of their mechanisms is key to overcoming the limited application of nano-TiO2 photocatalysts in the photocatalytic decomposition of air pollutants. In this review, the progress in experimental and theoretical fields, including a recent combination of photocatalytic experiments and adsorption and photocatalytic simulations by density functional theory (DFT), to explore the impact of adsorption of various reaction components on nano-TiO2 photocatalysts is comprehensively summarized. Additionally, the mechanism and broad perspective of the impact of their adsorption on the photocatalytic activity of TiO2 in air treatment are also critically discussed. Finally, several solutions are proposed to resolve the current problems related to environmental factors. In general, this review contributes a comprehensive perspective of water, substrate, and intermediate adsorption toward boosting the photocatalytic application of TiO2 nanomaterials.

Visible light photocatalysis: efficient Z-scheme LaFeO3/g-C3N4/ZnO photocatalyst for phenol degradation

In this work, a Z-scheme LaFeO3/g-C3N4/ZnO heterojunction photocatalyst with large specific surface (68.758 m2/g) and low cost (0.00035 times the cost of per gram of Au) was easily synthesized by glucose-assisted hydrothermal method. The structure, surface morphology, and optical properties of the photocatalyst were investigated. The constructed Z-scheme heterojunction catalysts can enhance the visible light absorption and carrier separation efficiency. Among these photocatalysts, the 10%-LaFeO3/g-C3N4/ZnO composite possesses the premium performance for efficient degrading 97.43% of phenol within 120 min. Even after 5 cycles, it still sustains an excellent photocatalytic stability (92.13% phenol degradation). According to the XPS surface states and the capture of active species on LaFeO3/g-C3N4/ZnO, the electrons would be transferred from ZnO and LaFeO3 to g-C3N4. In addition, ·OH plays an important role in photocatalytic reactions for phenol degradation. Thus, the proposed possible photocatalytic reaction mechanism of Z-scheme LaFeO3/g-C3N4/ZnO can provide a more economical and efficient conception for phenol degradation.

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.

Efficient electrochemical NO reduction to NH3 over metal-free g-C3N4 nanosheets and the role of interface microenvironment

The ever-increasing NO emission has caused severe environmental issues and adverse effects on human health. Electrocatalytic reduction is regarded as a win-win technology for NO treatment with value-added NH₃ generation, but the process is mainly relied on the metal-containing electrocatalysts. Here, we developed metal-free g-C₃N₄ nanosheets (deposited on carbon paper, named as CNNS/CP) for NH₃ synthesis from electrochemical NO reduction under ambient condition. The CNNS/CP electrode afforded excellent NH₃ yield rate of 15.1 μmol h^-1 cm^-2 (2180.1 mg g_cat^-1 h^-1) and Faradic efficiency (FE) of ∼41.5 % at - 0.8 and - 0.6 V_RHE, respectively, which were superior to the block g-C₃N₄ particles and comparable to the most of metal-containing catalysts. Moreover, through adjusting the interface microenvironment of CNNS/CP electrode by hydrophobic treatment, the abundant gas-liquid-solid triphasic interface improved NO mass transfer and availability, which enhanced NH₃ production and FE to about 30.7 μmol h^-1 cm^-2 (4424.2 mg g_cat^-1 h^-1) and 45.6 % at potential of - 0.8 V_RHE. This study opens a novel pathway to develop efficient metal-free electrocatalysts for NO electroreduction and highlights the importance of electrode interface microenvironment in electrocatalysis.

Hydrophilic polypyrrole and g-C3N4 co-decorated ZnO nanorod arrays for stable and efficient photoelectrochemical water splitting

Hydrophilic polypyrrole and g-C3N4 co-decorated ZnO nanorod arrays were synthesized for stable and efficient photoelectrochemical water splitting.