Improving the Photo-Oxidative Performance of Bi2MoO6 by Harnessing the Synergy between Spatial Charge Separation and Rational Co-Catalyst Deposition

Manipulation of charge carrier flow in Bi4NbO8Cl nanoplate photocatalyst with metal loading

Separation of photoexcited charge carriers in semiconductors is important for efficient solar energy conversion and yet the control strategies and underlying mechanisms are not fully established. Although layered compounds have been widely studied as photocatalysts, spatial separation between oxidation and reduction reaction sites is a challenging issue due to the parallel flow of photoexcited carriers along the layers. Here we demonstrate orthogonal carrier flow in layered Bi4NbO8Cl by depositing a Rh cocatalyst at the edges of nanoplates, resulting in spatial charge separation and significant enhancement of the photocatalytic activity. Combined experimental and theoretical studies revealed that lighter photogenerated electrons, due to a greater in-plane dispersion of the conduction band (vs. valence band), can travel along the plane and are readily trapped by the cocatalyst, whereas the remaining holes hop perpendicular to the plane because of the anisotropic crystal geometry. Our results propose manipulating carrier flow via cocatalyst deposition to achieve desirable carrier dynamics for photocatalytic reactions in layered compounds.

pH-induced hydrothermal synthesis of Bi2WO6 nanoplates with controlled crystal facets for switching bifunctional photocatalytic water oxidation/reduction activity

Bifunctional photocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have attracted growing interest to understand the mechanisms governing different evolution reactions, and the bifunctional activity of a single type of crystalline photocatalyst has gained especial attention. We herein report the high photocatalytic OER and HER activities of Bi₂WO₆ nanoplates (BWO NPs) which are synthesized by a simple hydrothermal method, and the switchable OER and HER performances controlled by the pH value of the precursor solvent. In the pH range from 4 to 9, the thickness of BWO NPs along the [001] direction exhibits interesting dependence on the pH value, which decreases as the pH value increases. Correspondingly, the BWO NPs obtained at the pH value of 7 (BWO-7) show the highest photocatalytic OER activity, while the BWO NPs synthesized at the pH value of 9 (BWO-9) exhibit the highest photoactivity towards HER. The electronic band structure analysis indicates that the highest photocatalytic OER activity is related to the band alignment of the valence band maximum of Bi₂WO₆, which determines the efficient separation of photogenerated electrons and holes as well as the fast charge transfer kinetics. The crystal facet evolution resulting from thickness reduction promotes the exposure of {001} facets for HER and decreases the exposure of {100} and {010} facets for OER. This work provides new insights into the combined effects of crystal facets and electronic band structures on photocatalysis.

Synergism of carbon quantum dots and Au nanoparticles with Bi2MoO6 for activity enhanced photocatalytic oxidative degradation of phenol

Localized surface plasmon resonance (LSPR) offers an opportunity to enhance the efficiency of photocatalysis. However, the photocatalysts's plasmonic enhancement is still limited, as most metals/semiconductors depend on LSPR contribution of isolated metal nanoparticles. In the present work, carbon quantum dots (CQDs) and Au nanoparticles (NPs) were simultaneously assembled on the surface of a three-dimensional (3D) spherical Bi₂MoO₆ (BMO) nanostructure with surface oxygen vacancies (SOVs). The collective excitation of CQDs and Au NPs demonstrated an effective strategy to improve the utilization of up-conversion emission and plasmonic energy. The contribution of CQDs and Au NPs assembled on the surface of BMO (7 wt% CQDs/Au/BMO) realized a photocatalytic phenol degradation enhancement (apparent rate constants, k _app/min^-1) of 56.5, 9.5 and 3.9, and 2.2-fold increase compared to BMO, BMO-SOVs, Au/BMO and CQDs/BMO, respectively. The as-fabricated 7 wt% CQDs/Au/BMO exhibited the highest mineralization rate for phenol degradation with 72.4% TOC removal rate in 120 min. The excellent photocatalytic performance of CQDs/Au/BMO was attributed to the synergistic effect of CQDs, Au NPs and SOVs. The CQD up-conversion emission synergetically boosts Au NPs' LSPR significantly promoting the separation and migration of photogenerated electron (e^-)/hole (h⁺) pairs, which could improve the oxygen molecule activation process and thereby their ability to generate reactive oxygen species (ROS). The present work is a step forward to understand and construct similar photocatalysts using an entirely reasonable hypothesis of activity enhancement mechanism according to the active species capture experiments and band structure analysis.

Facet-selective charge separation in two-dimensional bismuth-based photocatalysts

In this review, we summarize recent advances in the facet design of bismuth-based single-crystal plates based on facet-dependent charge separation for photocatalytic reactions, and the manipulation of the spatial charge separation is highlighted.

Preparation of Pt/γ-Bi2MoO6 Photocatalysts and Their Performance in α-Alkylation Reaction under Visible Light Irradiation

Bi(NO3)3·5H2O and (NH4)6Mo7O24·4H2O were used as precursors to synthesize flaky γ-Bi2MoO6 samples by a hydrothermal method, and Pt/γ-Bi2MoO6 samples with different mass fractions were prepared by an NaBH4 reduction method. Alpha alkylation of benzyl alcohol and acetophenone with photocatalysts under visible light irradiation was performed, and the activity of 4 wt % Pt/γ-Bi2MoO6 (γ-Bi2MoO6 was prepared by a nitric acid method, pH = 9, and reaction temperature 180 °C) was the best. The photocatalytic reaction conditions were optimized by changing various kinds of variables, such as the type of catalyst, solvent, and base, and the amount of base, catalyst, and reactant. The optimal conditions for the organic reaction were 75 mg 4 wt % Pt/γ-Bi2MoO6, 6 mL n-heptane, 1.2 mmol NaOH, 1 mmol acetophenone, and 3 mmol benzyl alcohol. Under the optimal reaction conditions, the effects of different light wavelengths and light intensities on the reaction were measured, and the cycling ability of the photocatalyst was tested. After five cycles, the photochemical properties of the catalyst were relatively stable. Finally, the active substances were identified (such as electrons (e-), holes (h+), hydroxyl radicals (•OH), and superoxide radicals (•O2-).

Light-Induced Formation of MoOxSy Clusters on CdS Nanorods as Cocatalyst for Enhanced Hydrogen Evolution

Metal and metal-oxide particles are commonly photodeposited on photocatalysts by reduction and oxidation reactions, respectively, consuming charges that are generated under illumination. This study reveals that amorphous MoO_xS_y clusters can be easily photodeposited at the tips of CdS nanorods (NRs) by in situ photodeposition for the first time. The as-prepared MoO_xS_y-decorated CdS samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma (ICP) to determine the composition and the possible formation pathways of the amorphous MoO_xS_y clusters. The MoO_xS_y-tipped CdS samples exhibited better hydrogen evolution performance than pure CdS under visible-light illumination. The enhanced activity is attributed to the formation of intimate interfacial contact between CdS and the amorphous MoO_xS_y clusters, which facilitates the charge separation and transfer. Through time-resolved photoluminescence (TRPL) measurements, it was clearly observed that all MoO_xS_y-decorated CdS samples with different loadings of MoO_xS_y showed a faster PL decay when compared to pure CdS, resulting from the effective trapping of photogenerated electrons by the MoO_xS_y clusters. Kelvin probe force microscopy (KPFM) was further used to study the surface potentials of pure CdS NRs and MoO_xS_y-decorated CdS NRs. A higher surface potential on MoO_xS_y-decorated CdS NRs was observed in the dark, indicating that the loading of MoO_xS_y resulted in a lower surface work function compared to pure CdS NRs. This contributed to the effective electron trapping and separation, which was also reflected by the increased photoelectrochemical response. Thus, this study demonstrates the design and facile synthesis of MoO_xS_y-tipped CdS NRs photocatalysts for efficient solar hydrogen production.

Modulation of Bi2 MoO6 -Based Materials for Photocatalytic Water Splitting and Environmental Application: a Critical Review

Highly active photocatalysts driving chemical reactions are of paramount importance toward renewable energy substitutes and environmental protection. As a fascinating Aurivillius phase material, Bi₂ MoO₆ has been the hotspot in photocatalytic applications due to its visible light absorption, nontoxicity, low cost, and high chemical durability. However, pure Bi₂ MoO₆ suffers from low efficiency in separating photogenerated carriers, small surface area, and poor quantum yield, resulting in low photocatalytic activity. Various strategies, such as morphology control, doping/defect-introduction, metal deposition, semiconductor combination, and surface modification with conjugative π structures, have been systematically explored to improve the photocatalytic activity of Bi₂ MoO₆ . To accelerate further developments of Bi₂ MoO₆ in the field of photocatalysis, this comprehensive Review endeavors to summarize recent research progress for the construction of highly efficient Bi₂ MoO₆ -based photocatalysts. Furthermore, benefiting from the enhanced photocatalytic activity of Bi₂ MoO₆ -based materials, various photocatalytic applications including water splitting, pollutant removal, and disinfection of bacteria, were introduced and critically reviewed. Finally, the current challenges and prospects of Bi₂ MoO₆ are pointed out. This comprehensive Review is expected to consolidate the existing fundamental theories of photocatalysis and pave a novel avenue to rationally design highly efficient Bi₂ MoO₆ -based photocatalysts for environmental pollution control and green energy development.

Pt nanoparticles decorated heterostructured g-C3N4/Bi2MoO6 microplates with highly enhanced photocatalytic activities under visible light

Exploring an efficient and photostable heterostructured photocatalyst is a pivotal scientific topic for worldwide energy and environmental concerns. Herein, we reported that Pt decorated g-C₃N₄/Bi₂MoO₆ heterostructured composites with enhanced photocatalytic performance under visible light were simply synthesized by one-step hydrothermal method for methylene blue (MB) dye degradation. Results revealed that the synthetic Pt decorated g-C₃N₄/Bi₂MoO₆ composites with Bi₂MoO₆ contents of 20 wt.% (Pt@CN/20%BMO) presented the highest photocatalytic activity, exhibiting 7 and 18 times higher reactivity than the pure g-C₃N₄ and Bi₂MoO₆, respectively. Structural analyses showed that Bi₂MoO₆ microplates were anchored on the wrinkled flower-like g-C₃N₄ matrix with Pt decoration, leading to a large expansion of specific surface area from 10.79 m²/g for pure Bi₂MoO₆ to 46.09 m²/g for Pt@CN/20%BMO. In addition, the Pt@CN/20%BMO composites exhibited an improved absorption ability in the visible light region, presenting a promoted photocatalytic MB degradation. Quenching experiments were also conducted to provide solid evidences for the production of hydroxyl radicals (^•OH), electrons (e⁻), holes (h⁺) and superoxide radicals (^•O²⁻) during dye degradation. The findings in this critical work provide insights into the synthesis of heterostructured photocatalysts with the optimization of band gaps, light response and photocatalytic performance in wastewater remediation.