Pd@Bi2Ru2O7/BiVO4 Z-Scheme Heterojunction Nanocomposite Photocatalyst for the Degradation of Trichloroethylene

Photocatalytic degradation of chlorinated persistent organic pollutants is a very challenging process due to the high redox potential of the C-Cl bond that requires wide band gap catalysts that are activated under UV light. Designing a Z-scheme heterojunction between visible light-activated metal oxides with compatible band gaps enables these redox potentials. Herein, we report the design of a pyrochlore/Aurivillius Z-scheme heterojunction to enhance the photocatalytic activity of BiVO4 for the degradation of trichloroethylene. We prepared Bi2Ru2O7/BiVO4 heterostructured photocatalysts by a controlled hydrothermal approach. Upon optimizing the Bi2Ru2O7 ratio to 1.0 wt %, the heterostructured photocatalyst demonstrated enhanced activity in the degradation of trichloroethylene (TCE) under simulated sunlight irradiation compared to bare BiVO4 and Bi2Ru2O7, respectively. Decorating the surface of the catalyst with palladium nanodomains to create the Pd@Bi2Ru2O7/BiVO4 nanocomposite showed a substantial increase in the photocatalytic degradation of TCE. The material characterization indicated that the architecture of the material provides a synergy of enhancing the redox potential of the photocatalyst and improving the charge carrier dynamics. Furthermore, the photoelectrochemical characterization confirmed that the dual heterojunctions in the Pd@Bi2Ru2O7/BiVO4 nanocomposite resulted in improved interfacial charge carrier transfer and enhanced the electron/hole separation efficiency compared to the nonpalladized catalysts. This work provides a promising approach for band gap engineering of visible light photocatalysts for the degradation of halogenated persistent organic pollutants.

Pyrochlore oxides as visible light-responsive photocatalysts

This perspective describes the use of pyrochlore oxides in photocatalysis with focus on the strategies to enhance their activity.

A Selective Synthesis of TaON Nanoparticles and Their Comparative Study of Photoelectrochemical Properties

A simplified ammonolysis method for synthesizing single phase TaON nanoparticles is presented and the resulting photoelectrochemical properties are compared and contrasted with as-synthesized Ta2O5 and Ta3N5. The protocol for partial nitridation of Ta2O5 (synthesis of TaON) offers a straightforward simplification over existing methods. Moreover, the present protocol offers extreme reproducibility and enhanced chemical safety. The morphological characterization of the as-synthesized photocatalysts indicate spherical nanoparticles with sizes 30, 40, and 30 nm Ta2O5, TaON, and Ta3N5 with the absorbance onset at ~320 nm, 580 nm, and 630 nm respectively. The photoactivity of the catalysts has been examined for the degradation of a representative cationic dye methylene blue (MB) using xenon light. Subsequent nitridation of Ta2O5 yields significant increment in the conversion (ζ: Ta2O5 < TaON < Ta3N5) mainly attributable to the defect-facilitated adsorption of MB on the catalyst surface and bandgap lowering of catalysts with Ta3N5 showing > 95% ζ for a lower (0.1 g) loading and with a lamp with lower Ultraviolet (UV) content. Improved Photoelectrochemical performance is noted after a series of chronoamperometry (J/t), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) measurements. Finally, stability experiments performed using recovered and treated photocatalyst show no loss of photoactivity, suggesting the photocatalysts can be successfully recycled.

Photocatalytic Properties of Bi2-xTi2O7-1.5x (x = 0, 0.5) Pyrochlores: Hybrid DFT Calculations and Experimental Study

The photocatalytic properties of Bi_2-xTi₂O_7-1.5x (x = 0, 0.5) pyrochlores are examined via ab initio calculations and experiments. A coprecipitation method is applied for the synthesis of nanopowder pyrochlores. The pyrochlore phase formation starts at 500 °C (Bi₂Ti₂O₇) and 550 °C (Bi_1.5Ti₂O_6.25). Nanopowders are found to be a metastable character of pyrochlore phases. The presence of bismuth and oxygen vacancies enhances the thermal stability of the Bi_1.5Ti₂O_6.25 phase in comparison with the Bi₂Ti₂O₇ phase. The estimated crystallite size is 30-40 nm with noticeable agglomerates of about 100-300 nm according to scanning electron microscopy (SEM) and with the formation of particles (510-580 nm) in the aqueous medium. The isoelectric points of the nanopowders seem to be shifted to the strongly acidic region, resulting in the formation of negative surface particle charges of -33 mV (Bi₂Ti₂O₇) and -27 mV (Bi_1.5Ti₂O_6.25) at pH 5.88 in distilled water. The specific surface area is 11.5 m²/g (Bi₂Ti₂O₇) and 12.00 m²/g (Bi_1.5Ti₂O_6.25). The use of the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional allows achieving an excellent agreement between theoretical and experimental structural parameters. The screened Coulomb hybrid HSE03 functional is the most appropriate for describing the optoelectronic properties. Bismuth titanate pyrochlores are wide-gap semiconductors with strong abilities to be active photocatalysts under visible irradiation. The optical E_g values for direct/indirect transition according to the experiment, 3.19/2.94 eV (x = 0) and 3.24/3.03 eV (x = 0.5), and the DFT/HSE03 calculations, 2.92/2.87 (x = 0) and 3.42/- eV (x = 0.5), are in the visible light region and are close. The calculated low effective masses of the charge carriers and suitable band edge positions confirm the ability of the pyrochlores to act as photocatalysts. The photocatalytic activity has been evaluated through the decomposition of rhodamine B under visible irradiation. Bi₂Ti₂O₇ shows the highest activity in comparison with Bi_1.5Ti₂O_6.25, which is in good agreement with theoretically predicted and experimentally revealed characteristics.

Photocatalytic removal using g-C3N4 quantum dots/Bi2Ti2O7 composites

In this work, a simple method to load of g-C₃N₄ quantum dots (CN QDs) onto Bi₂Ti₂O₇ (BTO) microsphere with the amount of CN QDs (3, 7, 10 and 15%). The photocatalyst was used for the treatment of water pollutants under visible-light illumination, which proved that CNBTO composites showed improved photocatalytic activity matched up to pure BTO. Reformation of BTO with CN QDs enhanced the light assimilation capacity, and promoted the isolation of photo-induced electron-hole pairs. The trapping experiments and ESR were announced the holes (h⁺) and superoxide oxide (O₂^-) played the key role, and the relative mechanism of the photocatalytic process was proposed. Meanwhile, the effects of CN QDs content, pH and initial pollutant concentration on the removal efficiency of ciprofloxacin (CIP) were studied. Results showed that the CN QDs loaded on BTO presented higher photocatalytic efficiency, and an optimum value for the dosage of photocatalytic in pH 8.0.

A Facile Method for the Preparation of Colored Bi₄Ti₃O12-x Nanosheets with Enhanced Visible-Light Photocatalytic Hydrogen Evolution Activity

Bi₄Ti₃O_12−x nanosheet photocatalysts with abundant oxygen vacancies are fabricated by a facile solid-state chemical reduction method for the first time. This method is simple in operation, has short reaction time, and can be conducted at mild temperatures (300~400 °C). The electron paramagnetic resonance, thermogravimetric analysis, X-ray photoelectron spectrometer, and positron annihilation lifetime spectra results indicate that oxygen vacancies are produced in Bi₄Ti₃O_12−x, and they can be adjusted by tuning the reduction reaction conditions. Control experiments show that the reduction time and temperature have great influences on the photocatalytic activities of Bi₄Ti₃O_12−x. The optimal Bi₄Ti₃O_12−x is the sample undergoing the reduction treatment at 350 °C for 60 min and it affords a hydrogen evolution rate of 129 μmol·g⁻¹·h⁻¹ under visible-light irradiation, which is about 3.4 times that of the pristine Bi₄Ti₃O₁₂. The Bi₄Ti₃O_12−x photocatalysts have good reusability and storage stability and can be used to decompose formaldehyde and formic acid for hydrogen production. The surface oxygen vacancies states result in the broadening of the valence band and the narrowing of the band gap. Such energy level structure variation helps promote the separation of photo-generated electron-hole pairs thus leading to enhancement in the visible-light photocatalytic hydrogen evolution. Meanwhile, the narrowing of the band gap leads to a broader visible light absorption of Bi₄Ti₃O_12−x.

Rare-Earth-Based MIS Type Core-Shell Nanospheres with Visible-Light-Driven Photocatalytic Activity through an Electron Hopping-Trapping Mechanism

A bilayered rare-earth-based metal-insulator-semiconductor, Dy₂O₃@SiO₂@ZnO core-shell nanospheres, was synthesized by a stepwise synthesis for enhanced visible photocatalytic activity. The prepared material was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, field-emission scanning electron microscopy, energy-dispersive spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller, and electron paramagnetic resonance techniques. Dy₂O₃@SiO₂@ZnO core-shell nanospheres were found be in a spherically arranged cauliflower-like morphology (40-60 nm). The high-resolution transmission electron microscopy analysis proved the core-shell morphology of the prepared material with a single Dy₂O₃ core and two shells comprising SiO₂ and ZnO. The material possessed a surface roughness of 4. 98 nm (2 × 2 μm area) and a band gap energy of 2.82 eV. The in situ generation of OH radicals was confirmed by electron paramagnetic resonance. Electron hopping through the SiO₂ layer from ZnO to Dy₂O₃ played a major role in trapping electrons in the f-shells of lanthanides, thus, preventing the recombination of electron-hole pair. X-ray photoelectron spectroscopy studies proved the band alignment of the material. Brunauer-Emmett-Teller analysis further showed the core-shell surface area was 14 m²/g. The visible photocatalytic activity was tested against 2,4-D (2,4-dichlorophenoxyacetic acid), an endocrine disruptor. The kinetic studies showed that the photocatalytic degradation process followed a pseudo-first-order pathway. The photocatalyst was found to be reusable even up to the third cycle.

Design and simple synthesis of composite Bi12TiO20/Bi4Ti3O12 with a good photocatalytic quantum efficiency and high production of photo-generated hydroxyl radicals

Hybrid Bi₁₂TiO₂₀/Bi₄Ti₃O₁₂ composites with a good photocatalytic quantum efficiency explained using a Z-scheme mechanism.

Encapsulating Bi2Ti2O7 (BTO) with reduced graphene oxide (RGO): an effective strategy to enhance photocatalytic and photoelectrocatalytic activity of BTO

Multimetal oxides (AxByOz) offer a higher degree of freedom compared to single metal oxides (AOx) in that these oxides facilitate (i) designing nanomaterials with greater stability, (ii) tuning of the optical bandgap, and (iii) promoting visible light absorption. However, all AxByOz materials such as pyrochlores (A2B2O7)--referred to here as band-gap engineered composite oxide nanomaterials or BECONs--are traditionally prone to severe charge recombination at their surface. To alleviate the charge recombination, an effective strategy is to employ reduced graphene oxide (RGO) as a charge separator. The BECON and the RGO with oppositely charged functional groups attached to them can be integrated at the interface by employing a simple electrostatic self-assembly approach. As a case study, the approach is demonstrated using the Pt-free pyrochlore bismuth titanate (BTO) with RGO, and the application of the composite is investigated for the first time. When tested as a photocatalyst toward hydrogen production, an increase of ∼ 250% using BTO in the presence of RGO was observed. Further, photoelectrochemical measurements indicate an enhancement of ∼ 130% in the photocurrent with RGO inclusion. These two results firmly establish the viability of the electrostatic approach and the inclusion of RGO. The merits of the RGO addition is identified as (i) the RGO-assisted improvement in the separation of the photogenerated charges of BTO, (ii) the enhanced utilization of the charges in a photocatalytic process, and (iii) the maintenance of the BTO/RGO structural integrity after repeated use (established through reusability analysis). The success of the self-assembly strategy presented here lays the foundation for developing other forms of BECONs, belonging to perovskites (ABO3), sillenite (A12BO20), or delafossite (ABO2) groups, hitherto written off due to limited or no photoelectrochemicalactivity.