Fabrication of porous nanoflake BiMO x (M = W, V, and Mo) photoanodes via hydrothermal anion exchange

Revealing the Role of Electronic Doping for Developing Cocatalyst-Free Semiconducting Photocatalysts

Developing cocatalyst-free photocatalysts is highly desired because it could avoid the very slow interfacial electron transfer that makes photocatalytic photon utilization a dilemma. However, even in the optimal case, photocatalysts without the use of cocatalysts deliver comparable performance only for conventional construction. We demonstrate here that electronic doping not only provides catalytically active sites in cocatalyst-free photocatalysts but also plays certain additional roles. These electronic states can efficiently channel the trapped electrons to the semiconductor surface without suffering from time-consuming detrapping and can facilitate the extraction of photogenerated holes. These features endow our demonstrated tungsten-doped CdS with evident superiority in photocatalytic performance over conventional counterparts loaded with platinum cocatalysts.

The Deep Eutectic Solvent Precipitation Synthesis of Metastable Zn4V2O9

A precipitation method involving a deep eutectic solvent (DES)─a mixture of hydrogen bond donor and acceptor─is used to synthesize a ternary metal oxide. Without toxic reagents, precipitates consisting of Zn₃(OH)₂V₂O₇·nH₂O and Zn₅(OH)₆(CO₃)₂ are obtained by simply introducing deionized H₂O to the DES solution containing dissolved ZnO and V₂O₅. Manipulation of the synthetic conditions demonstrates high tunability in the size/morphology of the two-dimensional nanosheets precipitated during the dynamic equilibrium process. According to differential scanning calorimetry and high-temperature powder X-ray diffraction, Zn₃V₂O₈ and ZnO obtained by the annealing of the precipitate are intermediates in the reaction pathway toward metastable Zn₄V₂O₉. Intimate mixing of the metal precursors achieved by the precipitation method allows access to the metastable zinc-rich vanadate with unusually rapid heat treatment. The UV-vis and surface photovoltage spectra reveal the presence of sub-band gap states, stemming from the reduced vanadium (V⁴⁺) center. Photoelectrochemical measurements confirm weak photoanodic currents for water and methanol oxidation. For the first time, this work shows the synthesis of a metastable oxide with the DES-precipitation route and provides insight into the structure-property relationship of the zinc-rich vanadate.

Manipulating the Fate of Charge Carriers with Tungsten Concentration: Enhancing Photoelectrochemical Water Oxidation of Bi2 WO6

Bismuth tungstate (Bi₂ WO₆ ) thin film photoanode has exhibited an excellent photoelectrochemical (PEC) performance when the tungsten (W) concentration is increased during the fabrication. Plate-like Bi₂ WO₆ thin film with distinct particle sizes and surface area of different exposed facets are successfully prepared via hydrothermal reaction. The smaller particle size in conjunction with higher exposure extent of electron-dominated {010} crystal facet leads to a shorter electron transport pathway to the bulk surface, assuring a lower charge transfer resistance and thus minimal energy loss. In addition, it is proposed based on the results from conductive atomic force microscopy that higher W concentration plays a crucial role in facilitating the charge transport of the thin film. The "self-doped" of W in Bi₂ WO₆ will lead to the higher carrier density and improved conductivity. Thus, the variation in the W concentration during a synthesis can be served as a promising strategy for future W based photoanode design to achieve high photoactivity in water splitting application.

Highly efficient activation of peroxymonosulfate by Co3O4/Bi2WO6 p-n heterojunction composites for the degradation of ciprofloxacin under visible light irradiation

Photocatalytic technology assisted via peroxymonosulfate (PMS) has good potential in water treatment. In this study, the Co₃O₄/Bi₂WO₆ composite was constructed via an in-situ calcination process and used to activate PMS for the degradation of ciprofloxacin (CIP) under visible light irradiation. The obtained 5 wt% Co₃O₄/Bi₂WO₆(CBWO-2) can highly effectively remove 86.2% CIP within 5 min visible light irradiation in presence of PMS. The excellent degradation performance of Co₃O₄/Bi₂WO₆/PMS system can be attributed to the synergistic effect between p-n heterojunction and PMS activation. The conduction band and valence band deviation between Co₃O₄ and Bi₂WO₆ were calculated by XPS techniques. Besides, DFT calculations were performed to further confirm the internal structure between Co₃O₄ and Bi₂WO₆. This work not only provides an approach to fabricate heterostructures but also indicated that Co₃O₄/Bi₂WO₆/PMS/Vis system is a potential environment remediation alternative for the efficient removal of recalcitrant organic compounds from wastewaters.

Microwave-assisted synthesis of 3D Bi2MoO6 microspheres with oxygen vacancies for enhanced visible-light photocatalytic activity

Oxygen vacancies (OVs) defects in metal oxide-based photocatalysts play a crucial role in improving the charge carrier separation efficiencies to enhance the photocatalytic performances. In this work, OVs were introduced in 3D Bi₂MoO₆ microspheres through a facile and fast microwave-assisted method via the modulation of tetramethylethylenediamine (TMEDA). EPR, Raman and XPS results demonstrated that large amounts of oxygen vacancies were formed on the surface of BMO microspheres. The photocatalytic properties of the samples were studied by degradation of tetracycline (TC) under visible light. The optimal Bi₂MoO₆ with OVs exhibited optimum photocatalytic activity, and the degradation rate was 7.0 times higher than that of pristine Bi₂MoO₆. This enhancement can be attributed to the 3D structure furnishing more surface active sites and suitable OVs defects favoring the electron-hole separation. Moreover, the defective Bi₂MoO₆ microspheres exhibit high stability because the photocatalytic activity remains almost unchanged after 5 cycles, making them favorable for practical applications. Finally, a possible visible light photocatalysis mechanism for the degradation of TC was tentatively proposed.

Current progress in developing metal oxide nanoarrays-based photoanodes for photoelectrochemical water splitting

Solar energy driven photoelectrochemical (PEC) water splitting is a clean and powerful approach for renewable hydrogen production. The design and construction of metal oxide based nanoarray photoanodes is one of the promising strategies to make the continuous breakthroughs in solar to hydrogen conversion efficiency of PEC cells owing to their owned several advantages including enhanced reactive surface at the electrode/electrolyte interface, improved light absorption capability, increased charge separation efficiency and direct electron transport pathways. In this Review, we first introduce the structure, work principle and their relevant efficiency calculations of a PEC cell. We then give a summary of the state-of the-art research in the preparation strategies and growth mechanism for the metal oxide based nanoarrays, and some details about the performances of metal oxide based nanoarray photoanodes for PEC water splitting. Finally, we discuss key aspects which should be addressed in continued work on realizing high-efficiency metal oxide based nanoarray photoanodes for PEC solar water splitting systems.

Enhanced photoelectrochemical sensing based on novel synthesized Bi2S3@Bi2O3 nanosheet heterostructure for ultrasensitive determination of L-cysteine

The design of a low-cost and highly efficient photoactive heterojunction material for sensing is still a challenging issue. On the basis of the formation of sheet-like Bi₂O₃ via coating Bi₂S₃, a novel Bi₂O₃@Bi₂S₃ heterostructure is controllably synthesized via a facile water bath approach. The prepared Bi₂O₃@Bi₂S₃ nanosheets show a superior photoelectrochemical (PEC) performance for the detection of L-cysteine (L-Cys), and the photocurrent signal is three and four times higher than those of Bi₂S₃ and Bi₂O₃ under visible irradiation, respectively. Also, the heterostructure presents an outstanding linear range for the detection of L-Cys: 0.1-10,000 μM. In addition, the mechanism of improved PEC response of Bi₂O₃@Bi₂S₃ nanosheets is investigated according to the estimated energy band positions. Thus, the integration of the novel heterostructure and the photoelectrochemical technique demonstrates a rapid photocurrent response, showing a great effect on the performance of the sensing system and a new PEC method for highly selective and sensitive chemical detection. Graphical abstract.

Oxygen vacancy boosted photocatalytic decomposition of ciprofloxacin over Bi2MoO6: Oxygen vacancy engineering, biotoxicity evaluation and mechanism study

Herein, efficient visible light driven photocatalytic degradation of ciprofloxacin was realized over Bi₂MoO₆ with oxygen vacancies (OVs) which can be tunably introduced through a facile solvothermal method via the modulation of tetramethylethylenediamine (TMEDA). The optimal Bi₂MoO₆ with OVs possessed the highest CIP degradation rate of 1.799 mg min^-1 m^-1, about 8.4 times than that of the pristine Bi₂MoO₆. And more than half of CIP was mineralized in only 2 h. The biotoxicity of ciprofloxacin and its byproducts to E. coli K-12 and saccharomyces cerevisiae was thoroughly eliminated after 6 h's photocatalytic treatment. Characterization methods revealed the rich oxygen vacancies in Bi₂MoO₆ not only endowed it with broader visible light absorption and faster transfer of photogenerated carriers, but also provided abundant absorption sites of surface oxygen for efficient molecular oxygen activation. Correspondingly, plentiful active species were produced and participated in the photocatalytic process, thereby efficiently promoting the ciprofloxacin degradation. Based on the HPLC-MS analysis, a possible decomposition pathway of CIP was finally proposed with the first decomposition step of pipetazine ring oxidation and breakage. This work might open up new avenues for superior visible light driven photocatalysts design to deal with pharmaceutical compounds contamination via tunable OVs Engineering.

Adjusting the Reduction Potential of Electrons by Quantum Confinement for Selective Photoreduction of CO2 to Methanol

The production of CH₃ OH from the photocatalytic CO₂ reduction reaction (PCRR) presents a promising route for the clean utilization of renewable resources, but charge recombination, an unsatisfying stability and a poor selectivity limit its practical application. In this paper, we present the design and fabrication of 0D/2D materials with polymeric C₃ N₄ nanosheets and CdSe quantum dots (QDs) to enhance the separation and reduce the diffusion length of charge carriers. The rapid outflow of carriers also restrains self-corrosion and consequently enhances the stability. Furthermore, based on quantum confinement effects of the QDs, the energy of the electrons could be adjusted to a level that inhibits the hydrogen evolution reaction (HER, the main competitive reaction to PCRR) and improves the selectivity and activity for CH₃ OH production from the PCRR. The band structures of photocatalysts with various CdSe particle sizes were also investigated quantitatively to establish the relationship between the band energy and the photocatalytic performance.

Tunable syngas production from photocatalytic CO2 reduction with mitigated charge recombination driven by spatially separated cocatalysts

Photocatalytic CO2 reduction represents a sustainable route to generate syngas (the mixture of CO and H2), which is a key feedstock to produce liquid fuels in industry. Yet this reaction typically suffers from two limitations: unsuitable CO/H2 ratio and serious charge recombination. This paper describes the production of syngas from photocatalytic CO2 reduction with a tunable CO/H2 ratio via adjustment of the components and surface structure of CuPt alloys and construction of a TiO2 mesoporous hollow sphere with spatially separated cocatalysts to promote charge separation. Unlike previously reported cocatalyst-separated hollow structures, we firstly create a reductive outer surface that is suitable for the CO2 reduction reaction. A high evolution rate of 84.2 μmol h-1 g-1 for CO and a desirable CO/H2 ratio of 1 : 2 are achieved. The overall solar energy conversion yield is 0.108%, which is higher than those of traditional oxide and sulfide based catalysts (generally about 0.006-0.042%). Finally, density functional theory calculations and kinetic experiments by replacing H2O with D2O reveal that the enhanced activity is mainly determined by the reduction energy of CO* and can be affected by the stability of COOH*.

Preparation of a BiVO4 nanoporous photoanode based on peroxovanadate reduction and conversion for efficient photoelectrochemical performance

A unique, controllable and facile method based on peroxovanadate reduction and conversion to prepare BiVO₄ nanoporous films is presented. In this method, a slow and controllable reduction of peroxovanadate with ethanol was used, which was the crucial step to ensure the uniform deposition of V₂O₅·xH₂O on an F-doped tin oxide substrate, and subsequently the annealed V₂O₅·xH₂O film was converted to a BiVO₄ film by a simple impregnation method with Bi³⁺ under the oriented effect of polyethylene glycol. The converted BiVO₄ film possessed a single monoclinic scheelite structure and exhibited an optimal water splitting photocurrent density of 1.10 mA cm^-2 at 1.23 V vs. RHE in 0.1 M KH₂PO₄ (pH 7) under AM 1.5G illumination with an incident photon-to-current conversion efficiency of ∼22.4% at 400 nm using an annealed V₂O₅·xH₂O film deposited for 3 hours. The BiVO₄ film also showed excellent water splitting performance and degradation efficiency in the PEC degradation of methylene blue and tetracycline hydrochloride with a rate constant of 0.63 h^-1 and 0.21 h^-1, respectively.

Mediator- and co-catalyst-free direct Z-scheme composites of Bi2WO6-Cu3P for solar-water splitting

Exploring new single, active photocatalysts for solar-water splitting is highly desirable to expedite current research on solar-chemical energy conversion. In particular, Z-scheme-based composites (ZBCs) have attracted extensive attention due to their unique charge transfer pathway, broader redox range, and stronger redox power compared to conventional heterostructures. In the present report, we have for the first time explored Cu₃P, a new, single photocatalyst for solar-water splitting applications. Moreover, a novel ZBC system composed of Bi₂WO₆-Cu₃P was designed employing a simple method of ball-milling complexation. The synthesized materials were examined and further investigated through various microscopic, spectroscopic, and surface area characterization methods, which have confirmed the successful hybridization between Bi₂WO₆ and Cu₃P and the formation of a ZBC system that shows the ideal position of energy levels for solar-water splitting. Notably, the ZBC composed of Bi₂WO₆-Cu₃P is a mediator- and co-catalyst-free photocatalyst system. The improved photocatalytic efficiency obtained with this system compared to other ZBC systems assisted by mediators and co-catalysts establishes the critical importance of interfacial solid-solid contact and the well-balanced position of energy levels for solar-water splitting. The promising solar-water splitting under optimum composition conditions highlighted the relationship between effective charge separation and composition.

Fabrication of Pt/Cu3(PO4)2 ultrathin nanosheet heterostructure for photoelectrochemical microRNA sensing using novel G-wire-enhanced strategy

Herein, we focus on preparing a highly efficient photocatalytic material to construct a signal-on photoelectrochemical (PEC) sensing platform in view of the rigorous demand of accurate miRNA quantification. The well-dispersed Pt nanoclusters-coated copper phosphate ultrathin nanosheets (PtNCs/Cu₃(PO₄)₂NSs) were first successfully synthesized as a photoelectrode material. Because of the ultrathin two-dimensional lamellar structure of Cu₃(PO₄)₂NSs with a 1.3 nm thickness, as well as the homogeneous size and abundant PtNCs loaded on Cu₃(PO₄)₂NSs, the resultant PtNCs/Cu₃(PO₄)₂NSs were employed as a photoelectrode material for the first time and revealed outstanding photocatalytic activity in PEC sensing as a substrate. As a well-designed protocol, we realized accurate miRNA quantification via a novel signal amplification strategy based on G-wire superstructure exponentially ligating a signal probe, which possesses efficient and simple operation compared to the traditional amplification method. Moreover, the electron donor is generated in situ by lactate oxidase (Lox) labels catalyzing lactate for H₂O₂ production, boosting the efficient separation of electron-hole pairs for further signal amplification. Impressively, this PEC sensing platform is commendably utilized to determine miRNA-141 from prostate carcinoma cell line 22Rv1. This study, considering the excellent PtNCs/Cu₃(PO₄)₂NSs combined with G-wire superstructure for exponential signal amplification strategy, paves a new path in biosensing and clinical diagnosis.

The first synthesis of Bi self-doped Bi2MoO6–Bi2Mo3O12 composites and their excellent photocatalytic performance for selective oxidation of aromatic alkanes under visible light irradiation

Bi self-doped Bi₂MoO₆–Bi₂Mo₃O₁₂ composites were first synthesized by a one-step method and they showed excellent photocatalytic performance in the selective oxidation of aromatic alkanes under visible light irradiation.

Fabrication and behaviors of CdS on Bi2MoO6 thin film photoanodes

Most Bi-based photoelectrodes have suitable band gaps and can effectively promote hydrogen evolution from water splitting, but there are few studies up to now for simple preparation methods for Bi-based binary metal oxides as photoanodes.