Bandgap engineering via boron and sulphur doped carbon modified anatase TiO2: a visible light stimulated photocatalyst for photo-fixation of N2 and TCH degradation

Outstanding photocatalytic nitrogen fixation performance of TiO2 QDs modified by Bi2O3/NaBiS2 nanostructures upon simulated sunlight

Nowadays, a promising material for NH₃ production under mild and safe conditions using heterogeneous photocatalysts is very important. In this regard, Bi₂O₃ and NaBiS₂ nanoparticles were combined with TiO₂ quantum dots (QDs) through a facile hydrothermal process. The TiO₂ QDs/Bi₂O₃/NaBiS₂ nanocomposites displayed excellent performance in the photofixation of nitrogen upon simulated sunlight. The NH₃ generation rate constant over the optimum nanocomposite was 10.2 and 3.3-folds higher than TiO₂ (P25) and TiO₂ QDs photocatalysts, respectively. The spectroscopic and electrochemical studies affirmed more effective segregation and transfer of photo-induced charge carriers within ternary nanocomposite, due to the developing tandem n-n-p heterojunctions, which led to more lifetime of charges. Moreover, the impacts of solvent, pH, electron scavenger, and lake of nitrogen molecules on the NH₃ generation were investigated. Finally, it was concluded that the TiO₂ QDs/Bi₂O₃/NaBiS₂ nanocomposite, with appealing features of more activity, high stability, and a facile one-pot synthesis method, is a promising photocatalyst in nitrogen fixation technology.

Preparation of NaNbO3 microcube with abundant oxygen vacancies and its high photocatalytic N2 fixation activity in the help of Pt nanoparticles

In this study, the photocatalytic N₂ immobilization performance of NaNbO₃ is enhanced via oxygen vacancy introduction and Pt loading. The designed Pt-loaded NaNbO₃ with rich oxygen defects (Pt/O-NaNbO₃) is synthesized by combining ion-exchange and photodeposition methods. Characterization result indicates that the O-NaNbO₃ has hollow microcube morphology and higher surface area than NaNbO₃. The introduced oxygen defects greatly affect the energy band structure. The band gap is slightly narrowed and the conduction band is raised, allowing O-NaNbO₃ to generate electrons with strong reducibility. Moreover, the oxygen defects reduced the work function of NaNbO₃, leading to increased charge separation in the bulk phase. The loaded Pt nanoparticles can further increase the surface charge separation via the formed Schottky barriers between Pt and O-NaNbO₃, which was thought to be the primary cause of the increased photocatalytic activity. Additionally, the oxygen vacancies and metal Pt also contribute to the adsorption and activation of N₂. Under the combined effect of the above changes, Pt/O-NaNbO₃ presents much higher photoactivity than NaNbO₃. The optimized NH₃ production rate reaches 293.3 μmol/L g^-1h^-1 under simulated solar light, which is approximately 2.2 and 20.2 times higher than that of O-NaNbO₃ and NaNbO₃, respectively. This research offers a successful illustration of how to improve photocatalytic N₂ fixation and may shed some light on how to design and construct efficient photocatalysts by combining several techniques.

The role of biochar in the photocatalytic treatment of a mixture of Cr(VI) and phenol pollutants: Biochar as a carrier for transferring and storing electrons

Biochar (BC) is widely used to remove environmental pollutants due to its photocatalytic activity. However, the mechanism of BC in photocatalysis remains unclear. In this study, soybean straw biochar (D500), dewatered sludge biochar (S500) and TiO₂/BC composite catalysts were prepared to test their photocatalytic activity in the photocatalysis-dark reaction using phenol and Cr(VI) as the representative pollutants. D500 had a good graphitized structure, layered structure and more active sites, which led to good photocatalytic activity. Compared with D500, S500 did not have a similar structure, resulting in a lack of photocatalytic activity. In addition, the efficiency of Cr(VI) and phenol removal using D500/TiO₂ as a catalyst was higher than that obtained using D500 and TiO₂, respectively. TiO₂ coupled with D500 increased the generation of photoexcited electrons and reduced the recombination of e^--h⁺ pairs. The removal efficiency of TiO₂/D500 for Cr(VI) (80.4 %) and phenol (77.7 %) in the hybrid systems was higher than that of Cr(VI) and phenol in unitary systems. This difference was mainly attributed to the inhibition of e^--h⁺ pair recombination by phenol and Cr(VI), which function as electron quenchers and hole quenchers, respectively. Furthermore, D500 stored electrons under light and released these electrons under dark conditions. When D500 was combined with TiO₂, the electrons on the biochar activated the catalytic redox activity of TiO₂, thereby removing pollutants under dark conditions. Meanwhile, TiO₂/D500 also exhibited good reusability and stability. In summary, this study provides new insight into the role of biochar in photocatalysis.

Two-dimensional ultrathin metal-based nanosheets for photocatalytic CO2 conversion to solar fuels

Artificially simulated photosynthesis has created substantial curiosity as the majority of efforts in this arena have been aimed to upsurge solar fuel efficiencies for commercialization. The layered inorganic 2D nanosheets offer considerably higher tunability of their chemical surface, physicochemical properties and catalytic activity. Despites the intrinsic advantages of such metal-based materials viz., metal oxides, transition metal dichalcogenides, metal oxyhalides, metal organic frameworks, layered double hydroxide, MXene's, boron nitride, black phosphorous and perovskites, studies on such systems are limited for applications in photocatalytic CO₂ reduction. The role of metal-based layers for CO₂ conversion and new strategies such as surface modifications, defect generation and heterojunctions to optimize their functionalities are discussed in this review. Research prospects and technical challenges for future developments of layered 2D metal-based nanomaterials are critically discussed.

N-doping TiO2 hollow microspheres with abundant oxygen vacancies for highly photocatalytic nitrogen fixation

Photocatalytic fixation of nitrogen to ammonia (NH₃) is a green but low-efficiency technology due to the high recombination of photo-generated carriers and poor light absorption of photocatalysts. Generally, the adsorption capacity for N₂ and the band position of TiO₂ are responsible for bandgap, light-adsorption, and the separation of photocarriers. Therefore, they play crucial roles to improve catalytic activity. Herein, N-doping TiO₂ hollow microspheres (NTO-0.5) with oxygen vacancies were synthesized via a hydrothermal method using phenolic resin microsphere as a template. The obtained NTO-0.5 achieves an impressive ammonia yield of 80.09 μmol g_cat^-1h^-1. Oxygen vacancies of NTO-0.5 were confirmed by ESR, Raman, XPS, Zeta potential, and H₂O₂ treatment for reducing oxygen vacancies. The ammonia yield of NTO-0.5 decreases to 34.78 μmol g_cat^-1h^-1 after reducing oxygen vacancies by H₂O₂ treatment, which demonstrates the importance of oxygen vacancies. The oxygen vacancies narrow the bandgap from 3.18 eV to 2.83 eV and impede the recombination of photo-generated carriers. The hollow microspheres structure is conducive to light absorption and utilization. Therefore, the synergistic effect between the oxygen vacancies and the hollow microspheres structure boosts the efficiency of photocatalytic nitrogen fixation. After four cycles, the ammonia production yield still maintains at 76.52 μmol g_cat^-1h^-1, meaning high stability. This work provides a new insight into the construction of catalysts with oxygen vacancies to enhance photocatalytic nitrogen fixation performance.

A review on vertical and lateral heterostructures of semiconducting 2D-MoS2 with other 2D materials: a feasible perspective for energy conversion

Fossil fuels as a double-edged sword are essential to daily life. However, the depletion of fossil fuel reservoirs has increased the search for alternative renewable energy sources to procure a more sustainable society. Accordingly, energy production through water splitting, CO₂ reduction and N₂ reduction via photocatalytic and electrocatalytic pathways is being contemplated as a greener methodology with zero environmental pollution. Owing to their atomic-level thickness, two-dimensional (2D) semiconductor catalysts have triggered the reawakening of interest in the field of energy and environmental applications. Among them, following the unconventional properties of graphene, 2D MoS₂ has been widely investigated due to its outstanding optical and electronic properties. However, the photo/electrocatalytic performance of 2D-MoS₂ is still unsatisfactory due to its low charge carrier density. Recently, the development of 2D/2D heterojunctions has evoked interdisciplinary research fascination in the scientific community, which can mitigate the shortcomings associated with 2D-MoS₂. Following the recent research trends, the present review covers the recent findings and key aspects on the synthetic methods, fundamental properties and practical applications of semiconducting 2D-MoS₂ and its heterostructures with other 2D materials such as g-C₃N₄, graphene, CdS, TiO₂, MXene, black phosphorous, and boron nitride. Besides, this review details the viable application of these materials in the area of hydrogen energy production via the H₂O splitting reaction, N₂ fixation to NH₃ formation and CO₂ reduction to different value-added hydrocarbons and alcohol products through both photocatalysis and electrocatalysis. The crucial role of the interface together with the charge separation principle between two individual 2D structures towards achieving satisfactory activity for various applications is presented. Overall, the current studies provide a snapshot of the recent breakthroughs in the development of various 2D/2D-based catalysts in the field of energy production, delivering opportunities for future research.

Water-Induced Formation of Ni2 P-Ni12 P5 Interfaces with Superior Electrocatalytic Activity toward Hydrogen Evolution Reaction

The interface between two material phases typically exhibits unique electronic states distinct from their pure phases, thus, providing a very promising channel to construct catalysts with excellent activity and stability. Here, water-induced formation of Ni₂ P-Ni₁₂ P₅ through a one-step phosphorization of nickel foam (NF) is demonstrated for the first time. The abundant interfaces endow Ni₂ P-Ni₁₂ P₅ /NF with excellent electrocatalytic hydrogen evolution reaction (HER) activity in alkaline condition, with an overpotential of 76 mV at a current density of 10 mA cm^-2 and of 147 mV at a current density of 100 mA cm^-2 , and a Tafel slope of 68.0 mV dec^-1 . The Ni₂ P-Ni₁₂ P₅ /NF also exhibits better durability than Pt/C/NF during HER at relatively large overpotential. Density functional theory calculations show that the electronic states at the Ni₂ P-Ni₁₂ P₅ interface are greatly altered, which enables optimal hydrogen adsorption, accelerates the charge transfer kinetics, and thus enhances the HER electrocatalytic activity. Superior overall water-splitting performance is also obtained by combining Ni₂ P-Ni₁₂ P₅ /NF with NiFe-layered double hydroxide (LDH) oxygen evolution reaction (OER) catalyst. Overpotentials of the cell for achieving 10 mA cm^-2 are only 324 mV. This work provides a facile method for the preparation of interfaces between different nickel phosphide polymorphs toward HER.

C-,N- and S-Doped TiO2 Photocatalysts: A Review

This article presents an overview of the reports on the doping of TiO2 with carbon, nitrogen, and sulfur, including single, co-, and tri-doping. A comparison of the properties of the photocatalysts synthesized from various precursors of TiO2 and C, N, or S dopants is summarized. Selected methods of synthesis of the non-metal doped TiO2 are also described. Furthermore, the influence of the preparation conditions on the doping mode (interstitial or substitutional) with reference to various types of the modified TiO2 is summarized. The mechanisms of photocatalysis for the different modes of the non-metal doping are also discussed. Moreover, selected applications of the non-metal doped TiO2 photocatalysts are shown, including the removal of organic compounds from water/wastewater, air purification, production of hydrogen, lithium storage, inactivation of bacteria, or carbon dioxide reduction.