Effective visible-excited charge separation in silicate-bridged ZnO/BiVO4 nanocomposite and its contribution to enhanced photocatalytic activity

In Situ Synthesis of Bi2MoO6/Bi2SiO5 Heterojunction for Efficient Degrading of Persistent Pollutants

Photocatalytic degradation is an environmentally friendly way to eliminate environmental pollution. Exploring a photocatalyst with high efficiency is essential. In the present study, we fabricated a Bi₂MoO₆/Bi₂SiO₅ heterojunction (BMOS) with intimate interfaces via a facile in situ synthesis method. The BMOS had much better photocatalytic performance than pure Bi₂MoO₆ and Bi₂SiO₅. The sample of BMOS-3 (3:1 molar ratio of Mo:Si) had the highest removal efficiency by the degradation of Rhodamine B (RhB) up to 75% and tetracycline (TC) up to 62% within 180 min. The increase in photocatalytic activity can be attributed to constructing high-energy electron orbitals in Bi₂MoO₆ to form a type II heterojunction, which increases the separation efficiencies of photogenerated carriers and transfer between the interface of Bi₂MoO₆ and Bi₂SiO₅. Moreover, electron spin resonance analysis and trapping experiments showed that the main active species were h⁺ and •O₂^- during photodegradation. BMOS-3 maintained a stable degradation capacity of 65% (RhB) and 49% (TC) after three stability experiments. This work offers a rational strategy to build Bi-based type II heterojunctions for the efficient photodegradation of persistent pollutants.

Photoelectrochemical Performance Improving Mechanism: Hybridization Appearing at the Energy Band of BiVO4 Photoanode by Doped Quantum Layers Modification

Surface passivation of the photoelectrode by wide bandgap semiconductor quantum layer is an important strategy to improve work stability and surface state inhibition. However, an inevitable energy barrier is generated during the quantum tunneling process of the photocarriers. To overcome this shortage, a tandem photo-generated hole transfer route is fabricated on BiVO₄ photoanode by doped dual-quantum layers modification, Ni-ZnO (5 nm) and Rh-SrTiO₃ (≈10 nm). Modulated photoelectrochemical (PEC), Scanning Kelvin Probe (SKP), and DFT calculation method results indicate that a tandem hole ohmic contact route is formed in the photoanode to reduce the quantum tunneling energy barrier, meanwhile, the photon absorption capacity of BiVO₄ is improved after doped quantum layers modification. Both a phenomenal attribute to the energy band hybridization between Ni, Rh 3d orbits in quantum layers with BiVO₄ photoanode. Then, the modified BiVO₄ photoanode achieves the recoded photocurrent density of 6.47 and 5.18 mA cm^-2 (Na₂ SO₃ electrolyte, V_RHE  = 1.23 V) under simulated sun light (100 mW cm^-2 AM 1.5 G) by xenon lamp illumination without and with UV composition cutting down to ≈5%, respectively. Generally, this work will highlight a potential application in the fields of PEC water splitting and photovoltaic conversion for various semiconductor nanomaterials.

Visible-Light-Driven Water Oxidation on Self-Assembled Metal-Free Organic@Carbon Junctions at Neutral pH

Sustainable water oxidation requires low-cost, stable, and efficient redox couples, photosensitizers, and catalysts. Here, we introduce the in situ self-assembly of metal-atom-free organic-based semiconductive structures on the surface of carbon supports. The resulting TTF/TTF^•+@carbon junction (TTF = tetrathiafulvalene) acts as an all-in-one highly stable redox-shuttle/photosensitizer/molecular-catalyst triad for the visible-light-driven water oxidation reaction (WOR) at neutral pH, eliminating the need for metallic or organometallic catalysts and sacrificial electron acceptors. A water/butyronitrile emulsion was used to physically separate the photoproducts of the WOR, H⁺ and TTF, allowing the extraction and subsequent reduction of protons in water, and the in situ electrochemical oxidation of TTF to TTF^•+ on carbon in butyronitrile by constant anode potential electrolysis. During 100 h, no decomposition of TTF was observed and O₂ was generated from the emulsion while H₂ was constantly produced in the aqueous phase. This work opens new perspectives for a new generation of metal-atom-free, low-cost, redox-driven water-splitting strategies.

Role of Interfacial Defects in Photoelectrochemical Properties of BiVO4 Coated on ZnO Nanodendrites: X-ray Spectroscopic and Microscopic Investigation

Synchrotron-based X-ray spectroscopic and microscopic techniques are used to identify the origin of enhancement of the photoelectrochemical (PEC) properties of BiVO₄ (BVO) that is coated on ZnO nanodendrites (hereafter referred to as BVO/ZnO). The atomic and electronic structures of core-shell BVO/ZnO nanodendrites have been well-characterized, and the heterojunction has been determined to favor the migration of charge carriers under the PEC condition. The variation of charge density between ZnO and BVO in core-shell BVO/ZnO nanodendrites with many unpaired O 2p-derived states at the interface forms interfacial oxygen defects and yields a band gap of approximately 2.6 eV in BVO/ZnO nanocomposites. Atomic structural distortions at the interface of BVO/ZnO nanodendrites, which support the fact that there are many interfacial oxygen defects, affect the O 2p-V 3d hybridization and reduce the crystal field energy 10Dq ∼2.1 eV. Such an interfacial atomic/electronic structure and band gap modulation increase the efficiency of absorption of solar light and electron-hole separation. This study provides evidence that the interfacial oxygen defects act as a trapping center and are critical for the charge transfer, retarding electron-hole recombination, and high absorption of visible light, which can result in favorable PEC properties of a nanostructured core-shell BVO/ZnO heterojunction. Insights into the local atomic and electronic structures of the BVO/ZnO heterojunction support the fabrication of semiconductor heterojunctions with optimal compositions and an optimal interface, which are sought to maximize solar light utilization and the transportation of charge carriers for PEC water splitting and related applications.

Flower-like Bi2SiO5/Bi4MoO9 heterostructures for enhanced photocatalytic degradation of ciprofloxacin

Bi₂SiO₅/Bi₄MoO₉ photocatalysts with heterostructures were successfully prepared using a one-pot solvothermal route. The effect of the molybdenum source on composite formation is discussed. Under ultraviolet light irradiation, the Bi₂SiO₅/Bi₄MoO₉ heterojunction photocatalyst exhibited higher photocatalytic performance than Bi₂SiO₅ and Bi₄MoO₉ towards the degradation of ciprofloxacin (CIP). This dramatically enhanced photoactivity can be ascribed to the construction of a heterojunction interface between Bi₂SiO₅ and Bi₄MoO₉, which not only suppresses the recombination of photoexcited charge carriers but also enhances light absorption. In addition, from a practical point of view, the the effect of initial CIP concentration and coexisting ions on the photodegradation process using as-prepared Bi₂SiO₅/Bi₄MoO₉ heterojunction photocatalysts was explored. Trapping experiments demonstrate that photoexcited holes and superoxide radicals are the main active species in the photodegradation of CIP over Bi₂SiO₅/Bi₄MoO₉ heterojunctions. Meanwhile, the conduction band and valence band potentials of Bi₂SiO₅ and Bi₄MoO₉ were measured by density functional theory calculation, diffuse reflectance spectroscopy and Mott-Schottky curves. A possible photocatalytic mechanism for CIP degradation over the Bi₂SiO₅/Bi₄MoO₉ heterojunction is proposed.

Bridge engineering in photocatalysis and photoelectrocatalysis

Solar driven photocatalysis and photoelectrocatalysis have emerged as promising strategies for clean, low-cost, and environmental-friendly production of renewable energy and removal of pollutants. There are three crucial steps for the photocatalytic and photoelectrochemical (PEC) processes: light absorption, charge separation and transportation, and surface catalytic reactions. While significant achievement has been made in developing multiple-component photocatalysts to optimize the three steps for improved solar-to-chemical energy conversion efficiency, it remains challenging when weak interfacial contact between components/particles hinders charge transfer, restricts electron-hole separation and lowers the structural stability of catalysts. Moreover, owing to the mismatch of energy bands, an undesirable charge transfer direction leads to an adverse consequence. To tackle these challenges, bridges are implemented to smoothen the interfacial charge transfer, improve the stability of catalysts, mediate the charge transfer directions and improve the photocatalytic/PEC performance. In this review, we present the advances in bridge engineering in photocatalytic/PEC systems. Starting with the definition and classifications of bridges, we summarize the architectures of the reported bridged photocatalysts. Then we systematically discuss the insight into the roles and fundamental mechanisms of bridges in various photocatalytic/PEC systems and their contributions to activity enhancement in various reactions. Finally, the challenges and perspectives of bridged photocatalysts are featured.

Highly Water-Stable Dye@Ln-MOFs for Sensitive and Selective Detection toward Antibiotics in Water

The host-guest composite, RhB@Tb-dcpcpt, is synthesized by trapping rhodamine B (RhB) into the channels of [Me₂NH₂][Tb₃(dcpcpt)₃(HCOO)]·DMF·15H₂O (Tb-dcpcpt, DMF = N,N'-dimethylformamide) via an ion-exchange process. The photophysical property of RhB@Tb-dcpcpt exhibits stable columinescence of RhB and Tb³⁺ ions in the whole excitation range of 300-390 nm, realizing an excitation-wavelength-independent yellow light emission. Powder X-ray diffraction and photoluminescence analysis illustrate the outstanding stabilities of RhB@Tb-dcpcpt on structural and photophysical properties in water medium. Subsequently, a bifunctional sensing process toward antibiotics is designed in terms of luminescent intensity and color. As a result, RhB@Tb-dcpcpt could realize sensitive and selective detection toward nitrofuran antibiotics (nitrofurazone and nitrofurantoin) via  luminescent quenching process and toward quinolone antibiotics (ciprofloxacin and norfloxacin) via luminescent color-changing process. Systematic analysis on the sensing mechanism reveals that photoinduced electron transfer and inner filter effect contribute to the realization of the sensing process.

ZnO Porous Nanosheets with Partial Surface Modification for Enhanced Charges Separation and High Photocatalytic Activity Under Solar Irradiation

ZnO porous nanosheets (PNSs) with partial surface modification were fabricated by means of depositing amorphous BiVO₄ on basic zinc carbonate nanosheets followed by calcining at 500 °C. At low levels of anchored amorphous BiVO₄, the surface of ZnO PNSs was partially evolved into Bi_3.9Zn_0.4V_1.7O_10.5 (BZVO). The measurements for photocurrent and photoluminescence demonstrate that partial-surface BZVO-modified ZnO PNSs (ZB_0.01) could significantly inhibit the recombination of photoinduced carriers. This should be ascribable to the driving from surface potential difference produced by non-junction part and vertical p-n BZVO/ZnO junction part on the surface of ZB_0.01. Furthermore, the photocatalytic efficiency in degradation of reactive brilliant red for ZB_0.01 under weak solar irradiation is about 8 times higher than that under strong visible-light illumination. The discussion regarding reasons for this enhancement demonstrates that each component in photocatalysts having rational valence-band maximum and conduction-band minimum energy levels is essential to obtain high-activity sunlight-driven catalysts.

A Self-Assembled Organic/Metal Junction for Water Photo-Oxidation

We report the in situ self-assembly of TTF, TTF^•+, and BF₄^- or PF₆^- into p-type semiconductors on the surface of Pt microparticles dispersed in water/acetonitrile mixtures. The visible light photoactivation of these self-assemblies leads to water oxidation forming O₂ and H⁺, with an efficiency of 100% with respect to the initial concentration of TTF^•+. TTF^•+ is then completely reduced to TTF upon photoreduction with water. The Pt microparticles act as floating microelectrodes whose Fermi level is imposed by the different redox species in solution; here predominantly TTF, TTF^•+, and HTTF⁺, which furthermore showed no signs of decomposition in solution.

Facile synthesis of hierarchically nanostructured bismuth vanadate: An efficient photocatalyst for degradation and detection of hexavalent chromium

Heterostructured nanomaterials can paid more significant attention in environmental safety for the detection and degradation/removal of hazardous toxic chemicals over a decay. Here, we report the preparation of hierarchically nanostructured shuriken like bismuth vanadate (BiVO₄) as a bifunctional catalyst for photocatalytic degradation and electrochemical detection of highly toxic hexavalent chromium (Cr(VI)) using the green deep eutectic solvent reline, which allows morphology control in one of the less energy-intensive routes. The SEM results showed a good dispersion of BiVO₄ catalyst and the HR-TEM revealed an average particle size of ca. 5-10 nm. As a result, the BiVO₄ exhibited good photocatalytic activity under UV-light about 95% reduction of Cr(VI) to Cr(III) was observed in 160 min. The recyclability of BiVO₄ catalyst exhibited an appreciable reusability and stability of the catalyst towards the photocatalytic reduction of Cr(VI). Also, the BiVO₄-modified screen printed carbon electrode (BiVO₄/SPCE) displayed an excellent electrochemical performance towards the electrochemical detection of Cr(VI). Besides, the BiVO₄/SPCE demonstrated tremendous electrocatalytic activity, lower linear range (0.01-264.5 μM), detection limit (0.0035 μM) and good storage stability towards the detection of Cr(VI). Importantly, the BiVO₄ modified electrode was also found to be a good recovery in water samples for practical applications.

BiVO4 Photoanode with Exposed (040) Facets for Enhanced Photoelectrochemical Performance

A BiVO₄ photoanode with exposed (040) facets was prepared to enhance its photoelectrochemical performance. The exposure of the (040) crystal planes of the BiVO₄ film was induced by adding NaCl to the precursor solution. The as-prepared BiVO₄ photoanode exhibits higher solar-light absorption and charge-separation efficiency compared to those of an anode prepared without adding NaCl. To our knowledge, the photocurrent density (1.26 mA cm^-2 at 1.23 V vs. RHE) of as-prepared BiVO₄ photoanode is the highest according to the reports for bare BiVO₄ films under simulated AM1.5G solar light, and the incident photon-to-current conversion efficiency is above 35% at 400 nm. The photoelectrochemical (PEC) water-splitting performance was also dramatically improved with a hydrogen evolution rate of 9.11 μmol cm^-2 h^-1, which is five times compared with the BiVO₄ photoanode prepared without NaCl (1.82 μmol cm^-2 h^-1). Intensity-modulated photocurrent spectroscopy and transient photocurrent measurements show a higher charge-carrier-transfer rate for this photoanode. These results demonstrate a promising approach for the development of high-performance BiVO₄ photoanodes which can be used for efficient PEC water splitting and degradation of organic pollutants.

3D WO3 /BiVO4 /Cobalt Phosphate Composites Inverse Opal Photoanode for Efficient Photoelectrochemical Water Splitting

A novel 3D WO₃ /BiVO₄ /cobalt phosphate composite inverse opal is designed for photoeletrochemical (PEC) water splitting, yielding a significantly improved PEC performance.

Comparison of sandwich and fingers-crossing type WO3/BiVO4 multilayer heterojunctions for photoelectrochemical water oxidation

Sandwich and fingers-crossing type WO₃/BiVO₄ multilayer heterojunctions were fabricated to investigate the influence of the junction structure on their photoelectrochemical performances.

Effect of the Si/TiO2/BiVO4 heterojunction on the onset potential of photocurrents for solar water oxidation

BiVO4 has been formed into heterojunctions with other metal oxide semiconductors to increase the efficiency for solar water oxidation. Here, we suggest that heterojunction photoanodes of Si and BiVO4 can also increase the efficiency of charge separation and reduce the onset potential of the photocurrent by utilizing the high conduction band edge potential of Si in a dual-absorber system. We found that a thin TiO2 interlayer is required in this structure to realize the suggested photocurrent density enhancement and shifts in onset potential. Si/TiO2/BiVO4 photoanodes showed 1.0 mA/cm(2) at 1.23 V versus the reversible hydrogen electrode (RHE) with 0.11 V (vs RHE) of onset potential, which were a 3.3-fold photocurrent density enhancement and a negative shift in onset potential of 300 mV compared to the performance of FTO/BiVO4 photoanodes.

Synthesis of silicate-bridged ZnO/g-C3N4 nanocomposites as efficient photocatalysts and its mechanism

The built silicate bridges are favorable for charge transfer and separation, which lead to the greatly enhanced photoactivities of ZnO/g-C₃N₄.

Improved photoactivity of TiO2–Fe2O3 nanocomposites for visible-light water splitting after phosphate bridging and its mechanism

The phosphate bridges built are favorable for charge transfer and separation, leading to a greatly-enhanced photoactivity for water splitting.