Controlling the Photocatalytic Activity and Benzylamine Photooxidation Selectivity of Bi2WO6 via Ion Substitution: Effects of Electronegativity

Doping or ion substitution is often used as an effective strategy to improve photocatalytic activities of several semiconductors. Most frequently, the dopants provide extra states to increase light absorption, alter the electronic structure, or lower the carrier recombination. This work focuses on ion substitution in Bi₂WO₆, where the dopants modify band-edge potentials of the catalysts. Specifically, we investigate how the electronegativity (EN) of the dopant could be used to tune the band-edge potentials and how such changes influence the photocatalytic mechanism. Compared to Te that has a lower EN, I lowers the band-edge potentials. While substitutions with both ions enhance Rh B photodegradation and benzylamine photooxidation, the modified band potentials of I-doped Bi₂WO₆ influence the benzylamine photooxidation pathway, resulting in higher selectivity. Additionally, substitution of I⁷⁺ in the Bi₂WO₆ lattice improves the morphologies, decreases the band-gap energy, and reduces the carrier recombination. As a result, I-doped Bi₂WO₆ shows almost 3 times higher %conversion while maintaining 100% selectivity in the oxidative coupling of benzylamine. The findings here signify the importance of the choices of dopants on the photocatalytic reactions and would benefit the design of other related materials for such applications.

Advanced catalytic CO2 hydrogenation on Ni/ZrO2 with light induced oxygen vacancy formation in photothermal conditions at medium-low temperatures

Selective CH4 formation from CO2 hydrogenation is an appealing yet challenging sunlight-driven or thermal-driven process due to low solar energy utilization efficiency or high energy input.

Nb2O5-Based Photocatalysts

Photocatalysis is one potential solution to the energy and environmental crisis and greatly relies on the development of the catalysts. Niobium pentoxide (Nb2O5), a typically nontoxic metal oxide, is eco-friendly and exhibits strong oxidation ability, and has attracted considerable attention from researchers. Furthermore, unique Lewis acid sites (LASs) and Brønsted acid sites (BASs) are observed on Nb2O5 prepared by different methods. Herein, the recent advances in the synthesis and application of Nb2O5-based photocatalysts, including the pure Nb2O5, doped Nb2O5, metal species supported on Nb2O5, and other composited Nb2O5 catalysts, are summarized. An overview is provided for the role of size and crystalline phase, unsaturated Nb sites and oxygen vacancies, LASs and BASs, dopants and surface metal species, and heterojunction structure on the Nb2O5-based catalysts in photocatalysis. Finally, the challenges are also presented, which are possibly overcome by integrating the synthetic methodology, developing novel photoelectric characterization techniques, and a profound understanding of the local structure of Nb2O5.

Promoting superoxide generation in Bi2WO6 by less electronegative substitution for enhanced photocatalytic performance: an example of Te doping

Doping with elements with lower electronegativity, like Te, shifts the band potentials of Bi2WO6 to the point that superoxide radical generation is feasible. As a result, an optimum of 2.5 at% Te doping improves the activity of Bi2WO6 by 48 times.

Facile fabrication of Fe-doped Nb2O5 nanofibers by an electrospinning process and their application in photocatalysis

It is of top priority to develop highly efficient visible-light photocatalysts to realize the practical applications of photocatalysis in industry. Niobium pentoxide (Nb2O5) is considered as a potentially attractive candidate for the visible-light-driven photodegradation of organic pollutants. In an effort to enhance its photocatalytic activity, Fe-doped Nb2O5 nanofibers with various Fe contents (the molar ratios of Fe to Nb were 0.005/1, 0.01/1, 0.03/1 or 0.05/1) were successfully prepared by an electrospinning method. The structural features, morphologies, and optical properties of the as-prepared samples were investigated. Photocatalytic activities of the samples were evaluated through degradation of Rhodamine B (RhB) under visible light irradiation. All the prepared Fe-doped Nb2O5 nanofibers exhibited much higher activities for degrading RhB solution than the pristine Nb2O5 nanofibers, and the maximum degradation yield of 98.4% was achieved with the nanofibers (Fe to Nb: 0.03/1) under visible light irradiation for 150 min. The photocatalytic degradation rate fitted a pseudo-first-order equation, and the rate constants of reactions with Fe-doped Nb2O5 nanofiber (the molar ratios of Fe to Nb were 0.03/1) or pure Nb2O5 nanofiber were 0.0282 min-1 and 0.0019 min-1, respectively. Doping Fe ions into the nanofibers enhanced the absorption within the visible-light range and reduced the photo-generated electron-hole pair recombination, and thus improved the photocatalytic activity. These attractive properties suggest that the Fe-doped Nb2O5 nanofibers have great potential for applications in the future to solve pollution issues.

Effects of different exchanging ions on the band structure and photocatalytic activity of defect pyrochlore oxide: a case study on KNbTeO6

The effects of different exchanging ions including Ag, Cu, and Sn on enhancing the photocatalytic activity of KNbTeO₆ are investigated by means of experiments and calculations.

Simple Synthesis of K₄Nb₆O17/C Nanosheets for High-Power Lithium-Ion Batteries with Good Stability

In this work, a series of two-dimensional (2D) large-size nanosheets were prepared through one-step exfoliation of the huge K₄Nb₆O17 crystals. The K₄Nb₆O17 nanosheets with the thickness of about 2 nm was used as the templates of dopamine polymerization and was then carbonized to form C-doped K₄Nb₆O17 nanosheets. More importantly, the C-doped K₄Nb₆O17 nanosheets exhibited excellent electrochemical performance with high specific capacity (381 mA h g-1 at 0.05 A g-1, 0.5⁻3.0 V vs. Li/Li⁺) and stable cyclability at high current density (remarkably, preserved a capacity of discharge approximately 90 mA h g-1 at 5 A g-1 after 1000 cycles). The good electrochemical performances of the C-doped K₄Nb₆O17 nanosheets can be attributed to the outstanding 2D structure and large specific surface, which afforded the short transport route for ion and electron. These noteworthy results demonstrated that the new 2D nanomaterials might be potential candidates for the high-performance, environmentally friendly, and low-cost electrochemical energy storage equipment.

Ambient electrochemical N2-to-NH3 fixation enabled by Nb2O5 nanowire array

A Nb₂O₅ nanowire array on carbon cloth (Nb₂O₅/CC) is efficient for ambient electrocatalytic N₂-to-NH₃ fixation with excellent selectivity in 0.1 M Na₂SO₄.

New Insights into the Electronic Structure and Photoelectrochemical Properties of Nitrogen-Doped HNb3O8 via a Combined in Situ Experimental and DFT Investigation

The nitrogen-doping approach has been intensively adopted to improve various properties of metal oxides, especially for adjusting the energy band structure and extending the photoresponse range of oxide photocatalysts. However, the nitrogen doping behavior is still unintelligible and complex due to the diversity of compositions and crystal structures. In this work, new insights into the electronic structure and photoelectrochemical (PEC) properties of nitrogen-doped HNb₃O₈ were presented. On the one hand, we utilized an in situ experimental strategy to ascertain the effect of nitrogen doping on the energy band and photoelectrochemical (PEC) properties of HNb₃O₈ and nitrogen-doped HNb₃O₈ (N-HNb₃O₈). Their energy band level, donor densities, and interfacial charge transfer properties were studied by Mott-Schottky plots and electrochemical impedance spectroscopy. After nitrogen doping, the conduction band position is unusually descended by 0.23 eV, the valance band position is raised by 0.51 eV, the donor density (N_d) is increased from 3.71 × 10²¹ to 6.46 × 10²¹ cm^-3, and interfacial charge transfer efficiency is reduced, though. On the other hand, density functional theoretical calculations were also conducted, so as to understand the electronic structures of HNb₃O₈ and N-HNb₃O₈. After nitrogen doping, the electronic structure is modified due to the upshift of the valance band edge consisting of hybrid N 2p and O 2p orbitals and the downshift of the conduction band edge consisting of the H 1s and Nb 4d orbitals. Furthermore, these insights into the behavior of nitrogen-doped semiconductors have important guiding significance toward their potential applications.

Facile preparation of protonated hexaniobate nanosheets and its enhanced photocatalytic activity

Exfoliated hexaniobate nanosheets E-H₂K₂Nb₆O_17-x (E-HKNO) with broad light absorption (up to 850 nm) and high adsorption properties were prepared via ion exchange and transient annealing processes with micron-size K₄Nb₆O₁₇ powders as the precursor. The as-prepared E-HKNO nanosheets show excellent visible light photodegradation performances when compared to degussa P25, which was evaluated in terms of degradation of Rhodamine B (Rh B). High adsorption and broad light absorption characteristics could be attributed to the exfoliation behavior and the reduction of surface Nb⁵⁺ to Nb⁴⁺, which was confirmed by x-ray photoelectron spectroscopy (XPS) and Raman spectra. From the Mott-Schottky analysis, the E-HKNO is an n-type semiconductor and has a higher flat band voltage (-0.46 V versus RHE at pH = 7), compared with K₄Nb₆O₁₇. In addition, the electrochemical impedance spectroscopy (EIS) indicates that the E-HKNO nanosheets have an increased semiconductor-electrolyte charge transfer resistance, which is not conducive to the separation of photogenerated carriers (e^--h⁺). Accordingly, a small amount of holes scavenger (EDTA) was added to improve the photodegradation performance of the E-HKNO, since the holes scavenger can inhibit the recombination of the photogenerated carriers. This work provides not only a facile method for the preparation of an efficient E-HKNO nanosheets photocatalyst, but also new insights for further enhancing the photodegradation performance by adding trace scavenger.