Green synthesis of boron and nitrogen co-doped TiO2 with rich B-N motifs as Lewis acid-base couples for the effective artificial CO2 photoreduction under simulated sunlight

Pd Nanoparticle-Modified BiOBr/CdS S-Scheme Photocatalyst for Enhanced Conversion of CO2

S-scheme heterojunction photocatalyst-coupled plasma-resonance effect can enhance the response range and absorption of light and charge transfer, and, at the same time, obtain strong redox ability, which is an effective way to improve CO2 conversion. In this work, plasma S-scheme heterojunctions of Pd/BiOBr/CdS with heterogeneous interfaces have been successfully constructed by a simple hydrothermal method. The possible reaction mechanism was proposed by in situ infrared, ultraviolet-visible spectroscopy (UV-vis), electron paramagnetic resonance (ESR), density functional theory (DFT), and electrochemical techniques. It was proved that the plasma S-scheme heterojunction can enhance the charge separation efficiency and improve the photocatalytic activity. When the loading ratio is Pd0.6-10%-BiOBr/CdS, it has the best performance, and the CO yield is 30.24 μmol/g, which is 15 and 30 times that of pure BiOBr and CdS, respectively. The results show that with the strong absorption of photon energy and the special electron transfer mode of S-scheme heterojunction, the charge can be effectively separated and transferred, and the photocatalytic activity is significantly improved. This study provides a useful strategy for charge transfer kinetics of plasma S-scheme heterojunction photocatalysts.

Enhancing photocatalytic CO2 reduction with TiO2-based materials: Strategies, mechanisms, challenges, and perspectives

The concentration of atmospheric CO2 has exceeded 400 ppm, surpassing its natural variability and raising concerns about uncontrollable shifts in the carbon cycle, leading to significant climate and environmental impacts. A promising method to balance carbon levels and mitigate atmospheric CO2 rise is through photocatalytic CO2 reduction. Titanium dioxide (TiO2), renowned for its affordability, stability, availability, and eco-friendliness, stands out as an exemplary catalyst in photocatalytic CO2 reduction. Various strategies have been proposed to modify TiO2 for photocatalytic CO2 reduction and improve catalytic activity and product selectivity. However, few studies have systematically summarized these strategies and analyzed their advantages, disadvantages, and current progress. Here, we comprehensively review recent advancements in TiO2 engineering, focusing on crystal engineering, interface design, and reactive site construction to enhance photocatalytic efficiency and product selectivity. We discuss how modifications in TiO2's optical characteristics, carrier migration, and active site design have led to varied and selective CO2 reduction products. These enhancements are thoroughly analyzed through experimental data and theoretical calculations. Additionally, we identify current challenges and suggest future research directions, emphasizing the role of TiO2-based materials in understanding photocatalytic CO2 reduction mechanisms and in designing effective catalysts. This review is expected to contribute to the global pursuit of carbon neutrality by providing foundational insights into the mechanisms of photocatalytic CO2 reduction with TiO2-based materials and guiding the development of efficient photocatalysts.

Adsorption and Visible Photocatalytic Synergistic Removal of a Cationic Dye with the Composite Material BiVO4/MgAl-LDHs

Adsorption and photocatalysis are effective in removing organic pollutants from wastewater. This study is based on the memory effects of MgAl-layered double hydroxides (MgAl-LDHs) after high-temperature calcination. By introducing bismuth vanadate (BiVO4) during the reformation of the layered structure via contact with water, a composite material BiVO4/MgAl-LDHs with enhanced adsorption and visible light catalytic performance was synthesized. The effects of the calcination temperature, ratio, initial methylene blue (MB) concentration, and catalyst dosage on the adsorption and photocatalytic performance were investigated. The BiVO4/MgAl-LDHs showed better photocatalytic performance than the pure BiVO4 and MgAl-LDHs. Under the optimal conditions, the proportion of MB adsorbed in 20 min was 66.1%, and the percentage of MB degraded during 100 min of photolysis was 92.4%. The composite photocatalyst showed good chemical stability and cyclability, and the adsorption-degradation rate was 86% after four cycles. Analyses of the adsorption and photocatalytic mechanisms for the composite material showed that synergistic adsorption and visible light photocatalysis contributed to the excellent catalytic performance of the BiVO4/MgAl-LDHs. A highly adsorbent photocatalytic composite material exhibiting outstanding performance was prepared via a simple, cost-effective, and environmentally friendly method, providing reference information for the removal of organic pollutants from liquids.

Design and preparation of highly active TiO2 photocatalysts by modulating their band structure

Titanium dioxide photocatalysts with high reduction potential and visible light response hold great promise in photochemical conversion. Here, we used a simple glycine doping method to synthesize novel N-TiO2@C photocatalysts with upward shifted conduction bands and narrowed band gaps as well as inhibited recombination of photoinduced electron-hole pairs. The N-TiO2@C photocatalysts exhibited higher visible light response and remarkably enhanced photocatalytic activity in the production of nicotinamide adenine dinucleotide (NADH) by photomediated reduction of NAD+ without any electron mediator. The yield of NADH was up to 70.3 % far greater than that of the undoped TiO2 (11.3 %), and it stabilized at ca. 60 % after 10 cycles. The viability of coupling NADH regeneration with enzymatic reaction (alcohol dehydrogenase) was established in aldehyde reduction where formaldehyde was specifically reduced to methanol. These findings shed new light on the modulation of the band structure of semiconductors and develop an electron mediator free strategy for NADH-dependent artificial photosynthesis through coupled photocatalytic and enzymatic approaches.

Solvent-controlled synthesis of Ti-based porphyrinic metal–organic frameworks for the selective photocatalytic oxidation of amines

The photocatalytic activity of metal-organic frameworks (MOFs) can be managed by the milieu of synthesis. Herein, N,N'-dimethylacetamide (DMA) and N,N'-diethylformamide (DEF) were employed as solvents for the synthesis of two Ti-based porphyrinic MOFs, namely Ti-PMOF-DMA and Ti-PMOF-DEF, from tetrabutyl orthotitanate and 4,4',4'',4'''-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid). Notably, both DMA and DEF were adsorbed onto the Ti-oxo clusters of the two MOFs to shape their properties. Ti-PMOF-DMA was observed with better optoelectronic response and charge transfer than Ti-PMOF-DEF. Moreover, Ti-PMOF-DMA owned a larger pore volume than Ti-PMOF-DEF, imparting more accessible sites to benzyl amines. Ti-PMOF-DMA exhibited better activity in selective photocatalytic aerobic oxidation of benzylamine than Ti-PMOF-DEF. Irradiated by red light-emitting diodes, outstanding results for selective conversion of  benzyl amines to imines over Ti-PMOF-DMA were attained. Superoxide radical anion, generated by the electron transfer from porphyrin via Ti-oxo clusters to dioxygen, turned out to be the primary reactive oxygen species. There was generality towards aerobic oxidation of amines to imines and considerable stability for Ti-PMOF-DMA. This work provides a new perspective on the altering MOFs to enhance photocatalytic organic transformations.

Photocatalytic Reduction of CO2 with N-Doped TiO2-Based Photocatalysts Obtained in One-Pot Supercritical Synthesis

The objective of this work was to analyze the effect of carbon support on the activity and selectivity of N-doped TiO₂ nanoparticles. Thus, N-doped TiO₂ and two types of composites, N-doped TiO₂/CNT and N-doped TiO₂/rGO, were prepared by a new environmentally friendly one-pot method. CNT and rGO were used as supports, triethylamine and urea as N doping agents, and titanium (IV) tetraisopropoxide and ethanol as Ti precursor and hydrolysis agent, respectively. The as-prepared photocatalysts exhibited enhanced photocatalytic performance compared to TiO₂ P25 commercial catalyst during the photoreduction of CO₂ with water vapor. It was imputed to the synergistic effect of N doping (reduction of semiconductor band gap energy) and carbon support (enlarging e⁻-h⁺ recombination time). The activity and selectivity of catalysts varied depending on the investigated material. Thus, whereas N-doped TiO₂ nanoparticles led to a gaseous mixture, where CH₄ formed the majority compared to CO, N-doped TiO₂/CNT and N-doped TiO₂/rGO composites almost exclusively generated CO. Regarding the activity of the catalysts, the highest production rates of CO (8 µmol/gTiO₂/h) and CH₄ (4 µmol/gTiO₂/h) were achieved with composite N¹/TiO₂/rGO and N¹/TiO₂ nanoparticles, respectively, where superscript represents the ratio mg N/g TiO₂. These rates are four times and almost forty times higher than the CO and CH₄ production rates observed with commercial TiO₂ P25.

Blue light-powered hydroxynaphthoic acid-titanium dioxide photocatalysis for the selective aerobic oxidation of amines

Solar photocatalysis is the key to resolve many environmental challenges but is usually hard to achieve over a metal oxide semiconductor. Therefore, assembling π-conjugated molecules onto semiconductors becomes an efficient approach to solar conversion via ligand-to-metal charge transfer. Here, a rational design of ligands for titanium dioxide (TiO₂) is presented to produce robust visible light photocatalysts. Three hydroxynaphthoic acids (HNAs) were selected as ligands by extending an extra benzene ring of salicylic acid (SA) at 3,4 or 4,5 or 5,6 positions. These ligands could regulate the performance of TiO₂ in which 2-hydroxy-1-naphthoic acid (2H1NA) endows the best outcome. In detail, blue light-powered cooperative photocatalysis of 2H1NA-TiO₂ with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 5 mol%) inaugurates the expeditious formation of imines by oxidation of amines with atmospheric oxygen (O₂). Interestingly, the increase of the O₂ pressure from 1 atm to 0.4 MPa promoted the selective oxidation of benzylamine but thereafter declined with a further boost to 0.6 MPa. Notably, an electron transfer between the oxidatively quenched 2H1NA-TiO₂ and TEMPO is established, offering a new pathway for environmental applications. This work presents a strategy in designing cutting-edge visible light photocatalysts via altering semiconductors with surface ligands.

Ultrasound-Assisted Synthesis of Nonmetal-Doped Titanium Dioxide Photocatalysts for Simultaneous H2 Production and Chemical Oxygen Demand Removal from Industrial Wastewater

A series of nonmetal-doped titanium dioxide (Nm _x /TiO₂, where x is the weight fraction of nonmetal elements) photocatalysts was prepared via ultrasonic-assisted impregnation for simultaneous hydrogen (H₂) production and chemical oxygen demand (COD) removal from industrial wastewater. Three types of Nm elements, carbon (C), silicon (Si), and phosphorus (P), were explored. The P₁/TiO₂ exhibited a higher photocatalytic activity for H₂ production and COD removal than the C₁/TiO₂ and Si₁/TiO₂ photocatalysts. Approximately 6.43 mmol/g photocatalyst of H₂ was produced, and around 26% COD removal was achieved at a P₁/TiO₂ loading of 4.0 g/L, a light intensity of 5.93 mW/cm², and a radiation time of 4 h. This is because the P₁/TiO₂ photocatalyst exhibited lower point of zero charge values and a more appropriate band position compared with other Nm _x /TiO₂ photocatalysts to produce H⁺, which can consequently form H₂, and reactive oxygen species (HO^· and O₂ ^{· -}), which serve as oxidizing agents to degrade the organic pollutants. Increasing the content of the P element doped into the TiO₂-based material up to 7.0% by weight enhanced the H₂ production and COD removal up to 8.34 mmol/g photocatalyst and 50.6%, respectively. This is attributed to the combined effect of the point of zero charge value and the S _BET of the prepared photocatalysts. The photocatalytic activity of the P₇/TiO₂ photocatalyst was still higher than the TiO₂-based material after the fourth use.

Oxygen vacancy-rich hierarchical BiOBr hollow microspheres with dramatic CO2 photoreduction activity

Conversion of carbon dioxide into useful chemicals has attracted great attention. However, the significant bottlenecks facing in the field are the poor conversion efficiency of CO₂ and low selectivity of products. Herein, hierarchical BiOBr hollow microspheres are fabricated by a solvothermal method using ethylene glycol (EG) as solvent in presence of polyvinyl pyrrolidone (PVP). The hollow BiOBr microspheres prepared at 120 °C exhibit the best performance for CO₂ photoreduction. The evolution rates of product CO and CH₄ are up to 88.1 µmol g^-1h^-1 and 5.8 µmol g^-1h^-1, which are 8.8 times and 5.8 times higher than that of plate-like BiOBr respectively. The hollow microspheres possess larger specific area and generate multiple reflections of light in the cavity, thus enhancing the utilization efficiency of light. The modulated electronic structure by oxygen vacancy (OVs) is beneficial to the transfer of photogenerated electrons and holes. Especially, the enriched charge density of BiOBr by OVs is conductive to the adsorption and activation of CO₂, which could lower the overall activation energy barrier of CO₂ photoreduction_. In summary, the synergistic effect of the hollow structure with OVs plays a vital role in boosting the photoreduction of CO₂ for BiOBr. This work provides a new opportunity for designing the high efficiency catalyst by morphology engineering with defects at the atomic level for CO₂ photoreduction.