Preparation of Au-BiVO4 heterogeneous nanostructures as highly efficient visible-light photocatalysts

Advanced oxidation processes for the removal of natural organic matter from drinking water sources: A comprehensive review

Natural organic matter (NOM), a key component in aquatic environments, is a complex matrix of organic substances characterized by its fluctuating amounts in water and variable molecular and chemical properties, leading to various interaction schemes with the biogeosphere and hydrologic cycle. These factors, along with the increasing amounts of NOM in surface and ground waters, make the effort of removing naturally-occurring organics from drinking water supplies, and also from municipal wastewater effluents, a challenging task requiring the development of highly efficient and versatile water treatment technologies. Advanced oxidation processes (AOPs) received an increasing amount of attention from researchers around the world, especially during the last decade. The related processes were frequently reported to be among the most suitable water treatment technologies to remove NOM from drinking water supplies and mitigate the formation of disinfection by products (DBPs). Thus, the present work overviews recent research and development studies conducted on the application of AOPs to degrade NOM including UV and/or ozone-based applications, different Fenton processes and various heterogeneous catalytic and photocatalytic oxidative processes. Other non-conventional AOPs such as ultrasonication, ionizing radiation and plasma technologies were also reported. Furthermore, since AOPs are unlikely to achieve complete oxidation of NOM, integration schemes with other water treatment technologies were presented including membrane filtration, adsorption and others processes.

Visible-light-induced Ag/BiVO4 semiconductor with enhanced photocatalytic and antibacterial performance

An Ag-loaded BiVO₄ visible-light-driven photocatalyst was synthesized by the microwave hydrothermal method followed by photodeposition. The photocatalytic performance of the synthesized samples was evaluated on a mixed dye (methylene blue and rhodamine B), as well as bisphenol A in aqueous solution. Similarly, the disinfection activities of synthesized samples towards the Gram-negative Escherichia coli (E. coli) in a model cell were investigated under irradiation with visible light (λ ≥ 420 nm). The synthesized samples have monoclinic scheelite structure. Photocatalytic results showed that all Ag-loaded BiVO₄ samples exhibited greater degradation and a higher mineralization rate than the pure BiVO₄, probably due to the presence of surface plasmon absorption that arises due to the loading of Ag on the BiVO₄ surface. The optimum Ag loading of 5 wt% has the highest photocatalytic performance and greatest stability with pseudo-first-order rate constants of 0.031 min^-1 and 0.023 min^-1 for the degradation of methylene blue and rhodamine B respectively in a mixture with an equal volume and concentration of each dye. The photocatalytic degradation of bisphenol A reaches 76.2% with 5 wt% Ag-doped BiVO₄ within 180 min irradiation time. Similarly, the Ag-loaded BiVO₄ could completely inactivate E. coli cells within 30 min under visible light irradiation. The disruption of the cell membrane as well as degradation of protein and DNA exhibited constituted evidence for antibacterial activity towards E. coli. Moreover, the bactericidal mechanisms involved in the photocatalytic disinfection process were systematically investigated.

Controlled growth and optical response of a semi-hollow plasmonic nanocavity and ultrathin sulfide nanosheets on Au/Ag platelets

Herein, we report a strategy to construct a semi-hollow plasmonic nanocavity and grow ultrathin sulfide nanosheets inside. The competition and cooperation of Au deposition with Ag etching based on flat Ag nanoplates are proposed. For the establishment of the semi-hollow nanocavity, Au shells are grown on Ag nanoplates, which serve as a stable frame, followed by partial etching of the Ag nanoplates. By controlling the thickness of the initial Ag nanoplates or the injected amount of etchant, the nanocavity size is fine-tuned. Significantly, the remaining unetched Ag layers provide a flat platform for the growth of 2D ultrathin sulfides of Ag₂S and CdS inside the semi-hollow plasmonic nanocavity. Strong plasmon resonance and large local field enhancement are exhibited inside the plasmonic cavity where the ultrathin semiconductor sulfides are grown, indicating strong plasmon-exciton interactions in the hybrids. Furthermore, this synthetic approach is extended to grow other metal sulfides such as Bi₂S₃ and PbS. The combination of a flat plasmonic cavity with ultrathin semiconductor nanosheets in this study provides a new strategy for the development of unique plasmon-based hybrids with excellent optical properties.

Efficient photoelectrochemical water oxidation using a TiO2 nanosphere-decorated BiVO4 heterojunction photoanode

Constructing heterojunctions by coupling dissimilar semiconductors is a promising approach to boost charge separation and charge transfer in photoelectrochemical (PEC) water splitting. In this work, we fabricated a highly efficient TiO₂/BiVO₄ heterojunction photoanode for PEC water oxidation via a simple hydrothermal method. The resulting heterojunction photoanodes show enhanced PEC performance compared to the bare BiVO₄ due to the simultaneous improvements in charge separation and charge transfer. Under simulated sunlight illumination (AM 1.5G, 100 mW cm⁻²), a high photocurrent of 3.3 mA cm⁻² was obtained at 1.23 V (vs. the reversible hydrogen electrode (RHE)) in a neutral solution, which exceeds those attained by the previously reported TiO₂/BiVO₄ heterojunctions. When a molecular Co–cubane catalyst was immobilized onto the electrode, the performance of the TiO₂/BiVO₄ heterojunction photoanode can be further improved, achieving a higher photocurrent density of 4.6 mA cm⁻² at 1.23 V, an almost three-fold enhancement over that of the bare BiVO₄. These results engender a promising route to designing an efficient photoelectrode for PEC water splitting.

Constructing heterojunctions by coupling dissimilar semiconductors is a promising approach to boost charge separation and charge transfer in photoelectrochemical (PEC) water splitting.

Controllable synthesis of graphitic carbon nitride nanomaterials for solar energy conversion and environmental remediation: the road travelled and the way forward

This review discusses advances in the synthesis and design of g-C₃N₄-based nanomaterials and their various photocatalytic and photoredox applications.

Au nanoparticle-doped Co3O4–CoFe2O4@SiO2 as a catalyst for visible-light-driven water oxidation

Composites of low dielectric constant SiO₂, Au and metal oxide was obtained, it showed higher O₂ evolution performance due to enhancing the electron transfer rate.

Facile synthesis of Bi/BiVO4 composite ellipsoids with high photocatalytic activity

Bi/BiVO₄ composites with excellent photocatalytic performance were firstly fabricated via a simple hydrothermal method.

Using p-type PbS Quantum Dots to Quench Photocurrent of Fullerene-Au NP@MoS2 Composite Structure for Ultrasensitive Photoelectrochemical Detection of ATP

Ultrasensitive and rapid quantification of the universal energy currency adenosine triphosphate (ATP) is an extremely critical mission in clinical applications. In this work, a "signal-off" photoelectrochemical (PEC) biosensor was designed for ultrasensitive ATP detection based on a fullerene (C₆₀)-decorated Au nanoparticle@MoS₂ (C₆₀-Au NP@MoS₂) composite material as a signal indicator and a p-type PbS quantum dot (QD) as an efficient signal quencher. Modification of wide band gap C₆₀ with narrow band gap MoS₂ to form an ideal PEC signal indicator was proposed, which could significantly improve photocurrent conversion efficiency, leading to a desirable PEC signal. In the presence of p-type PbS QDs, the PEC signal of n-type C₆₀-Au NP@MoS₂ was effectively quenched because p-type PbS QDs could compete with C₆₀-Au NP@MoS₂ to consume light energy and electron donor. Besides, the conversion of a limited amount of target ATP into an amplified output PbS QD-labeled short DNA sequence (output S₁) was achieved via target-mediated aptazyme cycling amplification strategy, facilitating ultrasensitive ATP detection. The proposed signal-off PEC strategy exhibited a wide linear range from 1.00 × 10^-2 pM to 100 nM with a low detection limit of 3.30 fM. Importantly, this proposed strategy provides a promising platform to detect ATP at ultralow levels and has potential applications, including diagnosis of ATP-related diseases, monitoring of diseases progression and evaluation of prognosis.

Direct evidence of multichannel-improved charge-carrier mechanism for enhanced photocatalytic H2 evolution

In the field of photocatalysis, the high-charge recombination rate has been the big challenge to photocatalytic conversion efficiency. Here we demonstrate the direct evidence of multichannel-improved charge-carrier mechanism to facilitate electron-hole transfer for raising photocatalytic H₂ evolution activity. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and UV-Vis diffuse reflectance spectroscopy (DRS), were used to characterize the as-fabricated samples. The result shows that the present design of Au/Pt nanoparticles (NPs) decorated one-dimensional Z-scheme TiO₂/WO₃ heterostructure composite nanofibers have been fabricated, which even exhibited excellent light absorption in the visible region and greatly enhanced photocatalytic activities on H₂ generation comparing with pure TiO₂, TiO₂/WO₃ and Pt/WO₃/TiO₂ nanofibers. This greatpromotion is mainly on account of the photosynthetic heterojunction system, which include the surface plasmon resonance (SPR) of Au nanoparticles, low overpotential of Pt nanoparticles, and more importantly, the one-dimensional multichannel-improved charge-carrier photosynthetic heterojunction system with Pt as an electron collector and WO₃ as a hole collector. Transferring photoinduced electrons and holes at the same time, leading to effective charge separation was directly proved by ultraviolet photoelectron spectroscopy, electrochemical impedance spectroscopy, photocurrent analysis and incident photon-to-electron conversion spectrum.

Dominance of Plasmonic Resonant Energy Transfer over Direct Electron Transfer in Substantially Enhanced Water Oxidation Activity of BiVO4 by Shape-Controlled Au Nanoparticles

The performance of plasmonic Au nanostructure/metal oxide heterointerface shows great promise in enhancing photoactivity, due to its ability to confine light to the small volume inside the semiconductor and modify the interfacial electronic band structure. While the shape control of Au nanoparticles (NPs) is crucial for moderate bandgap semiconductors, because plasmonic resonance by interband excitations overlaps above the absorption edge of semiconductors, its critical role in water splitting is still not fully understood. Here, first, the plasmonic effects of shape-controlled Au NPs on bismuth vanadate (BiVO₄ ) are studied, and a largely enhanced photoactivity of BiVO₄ is reported by introducing the octahedral Au NPs. The octahedral Au NP/BiVO₄ achieves 2.4 mA cm^-2 at the 1.23 V versus reversible hydrogen electrode, which is the threefold enhancement compared to BiVO₄ . It is the highest value among the previously reported plasmonic Au NPs/BiVO₄ . Improved photoactivity is attributed to the localized surface plasmon resonance; direct electron transfer (DET), plasmonic resonant energy transfer (PRET). The PRET can be stressed over DET when considering the moderate bandgap semiconductor. Enhanced water oxidation induced by the shape-controlled Au NPs is applicable to moderate semiconductors, and shows a systematic study to explore new efficient plasmonic solar water splitting cells.

New Understanding on Photocatalytic Mechanism of Nitrogen-Doped Graphene Quantum Dots-Decorated BiVO4 Nanojunction Photocatalysts

Bismuth vanadate (BiVO4) is a promising candidate as a visible-light-driven photocatalyst in the aspect of practical applications. To investigate the origin of active species from BiVO4 and understand the influence of the variations of the photocatalytic process, comparative studies on zero-dimensional nitrogen-doped graphene quantum dot (NGQD)-decorated BiVO4 have been carried out for methylene blue photodegradation. It was found that the hydroxyl group-rich NGQD surface and the established heterojunction structure between NGQDs and BiVO4 were greatly beneficial for the conversion of the •OH radical. With NGQD decoration, the dominant oxidant species for NGQDs/BiVO4 were confirmed to be •OH and H2O2, rather than holes originating from the valence band of unmodified BiVO4. The synergistic photocatalytic mechanism with respect to the interfacial charge transport and the conversion of active species was proposed. The achievement of the controllable active species significantly altering the activity may be applied for different photocatalytic reactions.

Sputtering gold nanoparticles on nanoporous bismuth vanadate for sensitive and selective photoelectrochemical aptasensing of thrombin

An efficient photoelectrochemical aptasensor based on sputtering Au NP-modified nanoporous BiVO₄ was rationally designed and fabricated, and it exhibited excellent sensitivity and selectivity for the detection of thrombin with a low detection limit of 0.5 pM.

In situ coupling of Ti2O with rutile TiO2 as a core–shell structure and its photocatalysis performance

The bimodal oxidation behavior of TiH₂ generates a core–shell structure. The formation of Ti₂O promotes visible-light response.

Carbon dot and BiVO4 quantum dot composites for overall water splitting via a two-electron pathway

Carbon dot and BiVO₄ quantum dot composites (CDs/BiVO₄ QDs) show a significantly improved photocatalytic activity and high stability for overall water splitting. By using 5% CDs/BiVO₄ QDs as photocatalysts, the H₂ evolution of 0.92 μmol h^-1, was achieved under solar light irradiation without any cocatalysts or sacrificial reagents, which is about 4 times that of BiVO₄ QDs (0.21 μmol h^-1). Note that, for the CD/BiVO₄ QD catalyst, the produced H₂ and O₂ are approximately equal to the stoichiometric ratio (H₂ : O₂ = 1.80 : 1), while that of BiVO₄ QDs is nonstoichiometric (about only 0.06 : 1). In addition, the present water splitting occurs via a two-electron pathway and CDs and BiVO₄ QDs served as the reduction and oxidation reaction active sites, respectively.

Plasmon-enhanced nanoporous BiVO4 photoanodes for efficient photoelectrochemical water oxidation

Conversion of solar irradiation into chemical fuels such as hydrogen with the use of a photoelectrochemical (PEC) cell is an attractive strategy for green energy. The promising technique of incorporating metal nanoparticles (NPs) in the photoelectrodes is being explored to enhance the performance of the photoelectrodes. In this work, we developed Au-NPs-functionalized nanoporous BiVO4 photoanodes, and utilized the plasmonic effects of Au NPs to enhance the photoresponse. The plasmonic enhancement leads to an AM 1.5 photocurrent of 5.1 ± 0.1 mA cm(-2) at 1.23 V versus a reverse hydrogen electrode. We observed an enhancement of five times with respect to pristine BiVO4 in the photocurrent with long-term stability and high energy-conversion efficiency. The overall performance enhancement is attributed to the synergy between the nanoporous architecture of BiVO4 and the plasmonic effects of Au NPs. Our further study reveals that the commendable photoactivity arises from the different plasmonic effects and co-catalyst effects of Au NPs.

Visible/near-IR-light-driven TNFePc/BiOCl organic-inorganic heterostructures with enhanced photocatalytic activity

Although semiconductor photocatalysis has been reported for more than 40 years, the spectral response is still focused on the region of UV-Visible and it is seldom extended to more than 600 nm. In this work, visible/near-IR-light-driven 2,9,16,23-tetranitrophthalocyanine iron (FeTNPc)/bismuth oxychloride (BiOCl) organic-inorganic heterostructures have been synthesized by a two-step solvothermal method. The obtained products were characterized by X-ray diffraction, Fourier transform infrared spectra, scanning electron and transmission microscopy, energy dispersive X-ray spectrometer, UV-vis diffuse reflectance spectroscopy, nitrogen adsorption-desorption, and electrochemical measurements. The photocatalytic activity for the decomposition of methyl orange and bisphenol A solution can be significantly improved under visible/near-IR-light irradiation. Through detecting the main oxidative species by trapping experiments, the results show holes and ˙O2(-) radicals are majorly and minorly responsible for photodegradation respectively. What's more, the FeTNPc/BiOCl composite photocatalyst still retained the photocatalytic activity after three cycle measurements.

Fabrication of plasmonic Au/TiO2 nanofiber films with enhanced photocatalytic activities

TiO2 nanofiber films (TiO2 NFF) with visible light scattering ability were prepared using a hydrothermal method. Au nanoparticles (Au NPs) were then deposited on the surface of TiO2 film using a microwave-assisted chemical reduction process. An overlapped light-trapping phenomenon was observed in Au/TiO2 NFF due to the light-scattering nanostructures of TiO2 film and the localized surface plasmon resonance (LSPR) of Au NPs. The MB degradation over the composite films is much faster than that of pure TiO2 film. The enhanced photocatalytic activity is primarily attributed to the charge transfer property and the overlapped light-trapping nanostructures of Au/TiO2.

Hydrothermal synthesis of novel BiFeO3/BiVO4 heterojunctions with enhanced photocatalytic activities under visible light irradiation

BiFeO₃/BiVO₄ heterojunction photocatalysts were synthesized by a hydrothermal method.

Investigation of adsorption and photocatalytic activities of in situ cetyltrimethylammonium bromide-modified Bi/BiOCl heterojunction photocatalyst for organic contaminants removal

Bi/BiOCl heterojunction was prepared via a hydrothermal method, using cetyltrimethylammonium bromide (CTAB) as a stabilizing agent.

Preparation of Self-Assembled Spherical g-C3N4/tz-Bi(0.92)Gd(0.08)VO4 Heterojunctions and Their Mineralization Properties

A novel kind of spherical g-C3N4/tz-Bi0.92Gd0.08VO4 heterojuctions are prepared via a simple microwave hydrothermal method. The sphere self-assembled mechanism and the heterojunction photocatalytic mechanism are mainly studied in this article. The results show that the added g-C3N4 sheets first anchor on ms-BiVO4 surfaces and then polymerize to form the coating layers, generating steric effect which is competing with Gd(3+) induction effect to affect the crystal transformation (from ms-BiVO4 to tz-BiVO4) and its growth during those processes. Afterward, independent coated structures are further polymerized and assembled into g-C3N4/tz-Bi0.92Gd0.08VO4 spheres. Because ECB (-0.95 V) of g-C3N4 is more negative than ECB (-0.05 V) of tz-BiVO4, the photoelectrons of g-C3N4 can be transferred into tz-BiVO4 surfaces through the heterojunction structure, so as to promote the separation rate of electron-hole pairs. In general, the adding of g-C3N4 can introduce hydroxyl groups to catch the photoholes and can inject electrons to react with dissolved oxygen to boost the production of active groups. Depending on such an orderly cooperation, g-C3N4/tz-Bi0.92Gd0.08VO4 heterojunction catalysts exhibit high and stable mineralization properties.

Porous Au-Ag Alloy Particles Inlaid AgCl Membranes As Versatile Plasmonic Catalytic Interfaces with Simultaneous, in Situ SERS Monitoring

We present a novel porous Au-Ag alloy particles inlaid AgCl membrane as plasmonic catalytic interfaces with real-time, in situ surface-enhanced Raman spectroscopy (SERS) monitoring. The Au-Ag alloy particles inlaid AgCl membranes were obtained via a facile two-step, air-exposed, and room-temperature immersion reaction with appropriate annealing process. Owing to the designed integration of semiconductor component AgCl and noble metal Au-Ag particles, both the catalytic reduction and visible-light-driven photocatalytic activities toward organic contaminants were attained. Specifically, the efficiencies of about 94% of 4-nitrophenol (4-NP, 5 × 10(-5) M) reduction after 8 min of reaction, and degradation of rhodamine 6G (R6G, 10(-5) M) after 12 min of visible light irradiation were demonstrated. Moreover, efficiencies of above 85% of conversion of 4-NP to 4-aminophenol (4-AP) and 90% of R6G degradation were achieved as well after 6 cycles of reactions, by which robust recyclability was confirmed. Further, with distinct SERS signals generated simultaneously from the surfaces of Au-Ag particles under laser excitation, in situ SERS monitoring of the process of catalytic reactions with superior sensitivity and linearity has been realized. Overall, the capability of the Au-Ag particles inlaid AgCl membranes to provide SERS monitored catalytic and visible-light-driven photocatalytic conversion of organic pollutants, along with their mild and cost-effective fabrication method, would make sense for in-depth understanding of the mechanisms of (photo)catalytic reactions, and also future development of potable, multifunctional and integrated catalytic and sensing devices.

Polymeric photocatalysts based on graphitic carbon nitride

Semiconductor-based photocatalysis is considered to be an attractive way for solving the worldwide energy shortage and environmental pollution issues. Since the pioneering work in 2009 on graphitic carbon nitride (g-C3N4) for visible-light photocatalytic water splitting, g-C3N4 -based photocatalysis has become a very hot research topic. This review summarizes the recent progress regarding the design and preparation of g-C3N4 -based photocatalysts, including the fabrication and nanostructure design of pristine g-C3N4 , bandgap engineering through atomic-level doping and molecular-level modification, and the preparation of g-C3N4 -based semiconductor composites. Also, the photo-catalytic applications of g-C3N4 -based photocatalysts in the fields of water splitting, CO2 reduction, pollutant degradation, organic syntheses, and bacterial disinfection are reviewed, with emphasis on photocatalysis promoted by carbon materials, non-noble-metal cocatalysts, and Z-scheme heterojunctions. Finally, the concluding remarks are presented and some perspectives regarding the future development of g-C3N4 -based photocatalysts are highlighted.

Graphitic carbon nitride based nanocomposites: a review

Graphitic carbon nitride (g-C(3)N(4)), as an intriguing earth-abundant visible light photocatalyst, possesses a unique two-dimensional structure, excellent chemical stability and tunable electronic structure. Pure g-C(3)N(4) suffers from rapid recombination of photo-generated electron-hole pairs resulting in low photocatalytic activity. Because of the unique electronic structure, the g-C(3)N(4) could act as an eminent candidate for coupling with various functional materials to enhance the performance. According to the discrepancies in the photocatalytic mechanism and process, six primary systems of g-C(3)N(4)-based nanocomposites can be classified and summarized: namely, the g-C(3)N(4) based metal-free heterojunction, the g-C(3)N(4)/single metal oxide (metal sulfide) heterojunction, g-C(3)N(4)/composite oxide, the g-C(3)N(4)/halide heterojunction, g-C(3)N(4)/noble metal heterostructures, and the g-C(3)N(4) based complex system. Apart from the depiction of the fabrication methods, heterojunction structure and multifunctional application of the g-C(3)N(4)-based nanocomposites, we emphasize and elaborate on the underlying mechanisms in the photocatalytic activity enhancement of g-C(3)N(4)-based nanocomposites. The unique functions of the p-n junction (semiconductor/semiconductor heterostructures), the Schottky junction (metal/semiconductor heterostructures), the surface plasmon resonance (SPR) effect, photosensitization, superconductivity, etc. are utilized in the photocatalytic processes. Furthermore, the enhanced performance of g-C(3)N(4)-based nanocomposites has been widely employed in environmental and energetic applications such as photocatalytic degradation of pollutants, photocatalytic hydrogen generation, carbon dioxide reduction, disinfection, and supercapacitors. This critical review ends with a summary and some perspectives on the challenges and new directions in exploring g-C(3)N(4)-based advanced nanomaterials.

In situ formed Bi/BiOBrxI1−x heterojunction of hierarchical microspheres for efficient visible-light photocatalytic activity

Bi/BiOBr_xI_1−x heterojunction of hierarchical microspheres formed in situ with efficient visible-light photocatalytic activity were synthesized by a facile one-step solvothermal method.

BiVO4/MIL-101 composite having the synergistically enhanced visible light photocatalytic activity

The BiVO₄/MIL-101 composite and pure materials were characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, UV-vis diffuse reflectance absorption spectra and photoluminescence emission spectra.

One-dimensional Bi2O3 QD-decorated BiVO4 nanofibers: electrospinning synthesis, phase separation mechanism and enhanced photocatalytic performance

In this work, we design and successfully fabricate novel Bi₂O₃ quantum dot (QD)-decorated BiVO₄ nanofibers by electrospinning. Furthermore, it exhibits enhanced photocatalytic activities by promoting the separation of photoinduced electrons and holes.