Reactivity of some vanadium oxides: An EPR and XRD study

Simultaneously Utilizing Excited Holes and Electrons for Piezoelectric-Enhanced Photoproduction of H2O2 from S-Scheme 2D S-Doped VOx/g-C3N4 Nanostructures

2D/2D step-scheme (S-scheme) piezo-photocatalysts for the production of fine chemicals, such as hydrogen peroxide (H2O2), have attracted significant attention of global scientists owing to the efficiency in utilizing surface piezoelectric effects from 2D materials to overcome rapid charge recombination in photocatalytic processes. In this research, we reported the fabrication of 2D S-doped VOx deposited on 2D g-C3N4 to produce H2O2 via the piezo-photocatalytic process with high production yields at 20.19 mmol g-1 h-1, which was 1.75 and 4.87 times higher than that from solely piezo-catalytic and photocatalytic H2O2 generation. The finding pointed out that adding sulfur (S) to VOx can help to improve the catalytic outcomes by modifying the electronic properties of pristine VOx. In addition, when coupled with g-C3N4, the presence of S limits the formation of graphene in the VOx/g-C3N4 composites, causing shielding effects and pushing the cascade reactions toward water generation in the materials. Besides, the research also sheds light on the charge transport between g-C3N4 and S-VOx under irradiation and how the composites work to trigger the formation of H2O2. The presence of S in the composite systems enhances charge transfer between two semiconductors by strengthening the internal electric fields (IEF) to drive electrons moving in one direction, as demonstrated by density functional theory (DFT) calculations. Moreover, the formation of H2O2 significantly relies on the reduction of oxygen to generate oxygenic radical species at the g-C3N4 sites. Meanwhile, S-VOx provides oxidative sites in the composites to oxidize water molecules to directly or indirectly generate H2O2 or O2, which will further participate in the reactions to produce the final products. This study confirms the validation of S-scheme piezo-photocatalysts, thus encouraging further research on developing heterojunction materials with high catalytic efficiency, which can be used in practical conditions.

Oxygen Vacancies on NH4 V4 O10 Accelerate Ion and Charge Transfer in Aqueous Zinc-Ion Batteries

Vanadium-based compounds are identified as promising cathode materials for aqueous zinc ion batteries due to their high specific capacity. However, the low intrinsic conductivity and sluggish Zn2+ diffusion kinetics seriously impede their further practical application. Here, oxygen vacancies on NH4 V4 O10 is reported as a high-performing cathode material for aqueous zinc ion batteries via a facile hydrothermal strategy. The introduction of oxygen vacancy accelerates the ion and charge transfer kinetics, reduces the diffusion barrier of zinc ions, and establishes a stable crystal structure during zinc ion (de-intercalation). As a result, the oxygen vacancy enriched NH4 V4 O10 exhibits a high specific capacity of ≈499 mA h g-1 at 0.2 A g-1 , an excellent rate capability of 296 mA h g-1 at 10 A g-1 and the specific capacity cycling stability with 95.1% retention at 5 A g-1 for 4000 cycles, superior to the NVO sample (186.4 mAh g-1 at 5 A g-1 , 66% capacity retention).

Weakly Polarized Organic Cation-Modified Hydrated Vanadium Oxides for High-Energy Efficiency Aqueous Zinc-Ion Batteries

Vanadium oxides, particularly hydrated forms like V2O5·nH2O (VOH), stand out as promising cathode candidates for aqueous zinc ion batteries due to their adjustable layered structure, unique electronic characteristics, and high theoretical capacities. However, challenges such as vanadium dissolution, sluggish Zn2+ diffusion kinetics, and low operating voltage still hinder their direct application. In this study, we present a novel vanadium oxide ([C6H6N(CH3)3]1.08V8O20·0.06H2O, TMPA-VOH), developed by pre-inserting trimethylphenylammonium (TMPA+) cations into VOH. The incorporation of weakly polarized organic cations capitalizes on both ionic pre-intercalation and molecular pre-intercalation effects, resulting in a phase and morphology transition, an expansion of the interlayer distance, extrusion of weakly bonded interlayer water, and a substantial increase in V4+ content. These modifications synergistically reduce the electrostatic interactions between Zn2+ and the V-O lattice, enhancing structural stability and reaction kinetics during cycling. As a result, TMPA-VOH achieves an elevated open circuit voltage and operation voltage, exhibits a large specific capacity (451 mAh g-1 at 0.1 A g-1) coupled with high energy efficiency (89%), the significantly-reduced battery polarization, and outstanding rate capability and cycling stability. The concept introduced in this study holds great promise for the development of high-performance oxide-based energy storage materials.

Electrochemically Induced Phase Transformation in Vanadium Oxide Boosts Zn-Ion Intercalation

Vanadium oxides are excellent cathode materials with large storage capacities for aqueous zinc-ion batteries, but their further development has been hampered by their low electronic conductivity and slow Zn2+ diffusion. Here, an electrochemically induced phase transformation strategy is proposed to mitigate and overcome these barriers. In situ X-ray diffraction analysis confirms the complete transformation of tunnel-like structural V6O13 into layered V5O12·6H2O during the initial electrochemical charging process. Theoretical calculations reveal that the phase transformation is crucial to reducing the Zn2+ migration energy barrier and facilitating fast charge storage kinetics. The calculated band structures indicate that the bandgap of V5O12·6H2O (0.0006 eV) is lower than that of V6O13 (0.5010 eV), which enhanced the excitation of charge carriers to the conduction band, favoring electron transfer in redox reactions. As a result, the transformed V5O12·6H2O delivers a high capacity of 609 mA h g-1 at 0.1 A g-1, superior rate performance (300 mA h g-1 at 20 A g-1), fast-charging capability (<7 min charging for 465 mA h g-1), and excellent cycling stability with a reversible capacity of 346 mA h g-1 at 5 A g-1 after 5000 cycles.

Vanadium-catalyzed Oxidative Conversion of Primary Aromatic Alcohols into Amides and Nitriles with Molecular Oxygen

Amides or nitriles are important building blocks because of the widespread occurrence in chemistry and biology. The development of green and efficient catalytic approaches to introduce nitrogen functionality is highly desired. Herein a vanadium-based material V-N-C-700 was prepared via a simple and convenient method, and employed for liquid-phase catalytic ammoxidation of alcohols with molecular oxygen. By using V-N-C-700/2-picolinic acid, primary aromatic alcohols was smoothly converted into the amides and nitriles in the presence of urea. The corresponding aldehydes are the key intermediates, and 2-picolinic acid could significantly enhance oxidation of alcohols into aldehydes. The amides were formed simultaneously along with nitriles, rather than only from nitriles via successive hydration. This work further expands non-noble metal catalysts for the preparation of amides and nitriles.

Intercalation Pseudocapacitive Zn2+ Storage with Hydrated Vanadium Dioxide toward Ultrahigh Rate Performance

The weak van der Waals interactions enable ion-intercalation-type hosts to be ideal pseudocapacitive materials for energy storage. Here, a methodology for the preparation of hydrated vanadium dioxide nanoribbon (HVO) with moderate transport pathways is proposed. Out of the ordinary, the intercalation pseudocapacitive reaction mechanism is discovered for HVO, which powers high-rate capacitive charge storage compared with the battery-type intercalation reaction. The main factor is that the defective crystalline structure provides suitable ambient spacing for rapidly accommodating and transporting cations. As a result, the HVO delivers a fast Zn²⁺ ion diffusion coefficient and a low Zn²⁺ diffusion barrier. The electrochemical results with intercalation pseudocapacitance demonstrate a high reversible capacity of 396 mAh g^-1 at 0.05 A g^-1 , and even maintain 88 mAh g^-1 at a high current density of 50 A g^-1 .

Impacts of Oxygen Vacancies on Zinc Ion Intercalation in VO2

The aqueous zinc ion battery has emerged as a promising alternative technology for large-scale energy storage due to its low cost, natural abundance, and high safety features. However, the sluggish kinetics stemming from the strong electrostatic interaction of divalent zinc ions in the host crystal structure is one of challenges for highly efficient energy storage. Oxygen vacancies (V_O^••), in the present work, lead to a larger tunnel structure along the b axis, which improves the reactive kinetics and enhances Zn-ion storage capability in VO₂ (B) cathode. DFT calculations further support that V_O^•• in VO₂ (B) result in a narrower bandgap and lower Zn ion diffusion energy barrier compared to those of pristine VO₂ (B). V_O^••-rich VO₂ (B) achieves a specific capacity of 375 mAh g^-1 at a current density of 100 mA g^-1 and long-term cyclic stability with retained specific capacity of 175 mAh g^-1 at 5 A g^-1 over 2000 cycles (85% capacity retention), higher than that of VO₂ (B) nanobelts (280 mAh g^-1 at 100 mA g^-1 and 120 mAh g^-1 at 5 A g^-1, 65% capacity retention).

Insights into the Exfoliation Process of V2O5·nH2O Nanosheet Formation Using Real-Time 51V NMR

Nanostructured hydrated vanadium oxides (V₂O₅·nH₂O) are actively being researched for applications in energy storage, catalysis, and gas sensors. Recently, a one-step exfoliation technique for fabricating V₂O₅·nH₂O nanosheets in aqueous media was reported; however, the underlying mechanism of exfoliation has been challenging to study. Herein, we followed the synthesis of V₂O₅·nH₂O nanosheets from the V₂O₅ and VO₂ precursors in real time using solution- and solid-state ⁵¹V NMR. Solution-state ⁵¹V NMR showed that the aqueous solution contained mostly the decavanadate anion [H₂V₁₀O₂₈]^4- and the hydrated dioxovanadate cation [VO₂·4H₂O]⁺, and during the exfoliation process, decavanadate was formed, while the amount of [VO₂·4H₂O]⁺ remained constant. The conversion of the solid precursor V₂O₅, which was monitored with solid-state ⁵¹V NMR, was initiated when VO₂ was in its monoclinic forms. The dried V₂O₅·nH₂O nanosheets were weakly paramagnetic because of a minor content of isolated V⁴⁺. Its solid-state ⁵¹V signal was less than 20% of V₂O₅ and arose from diamagnetic V⁴⁺ or V⁵⁺.This study demonstrates the use of real-time NMR techniques as a powerful analysis tool for the exfoliation of bulk materials into nanosheets. A deeper understanding of this process will pave the way to tailor these important materials.

Toxicity assessment and mechanistic investigation of engineered monoclinic VO2 nanoparticles

Growing interest in monoclinic VO2 nanoparticles (NPs) among consumers and the industries of window coatings, solar sensors etc. has brought particular attention to their safety concerns. The toxicity of this new class of nanomaterials in bacterial ecosystems has not yet been evaluated. The degree of crystallinity is a significant parameter that determines the performance of materials. However, the previously reported methods for toxicity assessment have neglected its influence. In this work, we systematically evaluated the toxicity of VO2 NPs with different degrees of crystallinity to four typical bacterial strains and studied the influence of physicochemical properties and aging treatment on their antibacterial effect. The results showed that the toxicity of VO2 nanoparticles was very low. Interestingly, we found that antibacterial properties of VO2 NPs were dependent on both bacterial strains and VO2 particle properties. In addition, the highly crystalline VO2 NPs were more toxic than normal and industrial VO2 particles. We attribute the crystallinity-related toxicity to the dissolved vanadium, the physical interactions between the bacteria and particles, and the generation of reactive oxygen species, as supported by our experimental results and theoretical calculation.

Surface-Enhanced Raman Scattering of the Bariandite Oxide Layer on a Vanadium Dioxide Crystal

Variable-temperature (25–100°C) Raman spectra of a crystal of commercial VO2 have revealed surface-enhanced Raman scattering (SERS) of the V=O stretching mode of the bariandite-like vanadium oxide species, V10O24·9H2O, that is formed on the surface of the crystal. Upon passing through the semiconductor-to-metal phase transition of VO2 at 68°C, there is an approximately three to five-fold increase in Raman intensity of the V=O stretching mode. This effect is reversible with hysteresis upon decreasing the temperature. The temperature dependence of the Raman spectra at temperatures below the transition suggest that even the semiconductor phase has some SERS effect.