Metal-Particle-Decorated ZnO Nanocrystals: Photocatalysis and Charge Dynamics

Double-Solvent-Induced Derivatization of Bi-MOF to Vacancy-Rich Bi4O5Br2: Toward Efficient Photocatalytic Degradation of Ciprofloxacin in Water and HCHO Gas

MOF-derived photocatalytic materials have potential in degrading ciprofloxacin (CIP) in water and HCHO gas pollutants. Novel derivatization means and defect regulation are effective techniques for improving the performance of MOF-derived photocatalysis. Vacancy-rich Bi4O5Br2 (MBO-x) were derived in one step from Bi-MOF (CAU-17) by a modified double-solvent method. MBO-50 produced more oxygen vacancies due to the combined effect of the CAU-17 precursor and double solvents. The photocatalytic performance of MBO was evaluated by degrading CIP and HCHO. Thanks to the favorable morphology and vacancy structure, MBO-50 demonstrated the best photocatalytic efficiency, with 97.0% removal of CIP (20 mg L-1) and 90.1% removal of HCHO (6.5 ppm) at 60 min of light irradiation. The EIS Nyquist measurement, transient photocurrent response, photoluminescence spectra, and the calculation of energy band information indicated that the vacancy sites can effectively capture photoexcited electrons during the charge transfer process, thus limiting the recombination of electrons and holes, improving the energy band structure, and making it easier to produce superoxide anion radical (·O2-) and to degrade CIP and HCHO. The improvement of photocatalytic performance of MBO-50 in HCHO degradation due to the bromine vacancy generation and filling mechanism was discussed in detail. This work provides a promising new idea for the modulation of MOF-derived photocatalytic materials.

Manipulation of interfacial charge dynamics for metal-organic frameworks toward advanced photocatalytic applications

Compared to other known materials, metal-organic frameworks (MOFs) have the highest surface area and the lowest densities; as a result, MOFs are advantageous in numerous technological applications, especially in the area of photocatalysis. Photocatalysis shows tantalizing potential to fulfill global energy demands, reduce greenhouse effects, and resolve environmental contamination problems. To exploit highly active photocatalysts, it is important to determine the fate of photoexcited charge carriers and identify the most decisive charge transfer pathway. Methods to modulate charge dynamics and manipulate carrier behaviors may pave a new avenue for the intelligent design of MOF-based photocatalysts for widespread applications. By summarizing the recent developments in the modulation of interfacial charge dynamics for MOF-based photocatalysts, this minireview can deliver inspiring insights to help researchers harness the merits of MOFs and create versatile photocatalytic systems.

Defects Healing of the ZnO Surface by Filling with Au Atom Catalysts for Efficient Photocatalytic H2 Production

Healed defects on photocatalysts surface and their interaction with plasmonic nanoparticles (NPs) have attracted attention in H2 production process. In this study, surface oxygen vacancy (Vo ) defects are created on ZnO (Vo -ZnO) NPs by directly pyrolyzing zeolitic imidazolate framework. The surface defects on Vo -ZnO provide active sites for the diffusion of single Au atoms and as nucleation sites for the formation of Au NPs by the in situ photodeposition process. The electronically healed surface defects by single Au atoms help in the formation of a heterojunction between the ZnO and plasmonic Au NPs. The formed Au/Vo -Au:ZnO-4 heterojunction prolongs photoelectron lifetimes and increases donor charge density. Therefore, the optimized photocatalysts of Au/Vo -Au:ZnO-4 has 21.28 times higher H2 production rate than the pristine Vo -ZnO under UV-visible light in 0.35 m Na2 SO4 and 0.25 m Na2 SO3 . However in 0.35 m Na2 S and 0.25 m Na2 SO3 , the H2 production rate is 25.84 mmole h-1 g-1 . Furthermore, Au/Vo -Au:ZnO-4 shows visible light activity by generating hot carries via induced surface plasmonic effects. It has 48.58 times higher H2 production rate than pristine Vo -ZnO. Therefore, this study infers new insight for defect healing mediated preparation of Au/Vo -Au:ZnO heterojunction for efficient photocatalytic H2 production.

Antibacterial and Photocatalytic Activity of ZnO/Au and ZnO/Ag Nanocomposites

The use of a combination of nanoparticles as antimicrobial agents can be one strategy to overcome the tendency of microbes to become resistant to antibiotic action. Also, the optimization of nano-photocatalysts to efficiently remove persistent pollutants from wastewater is a hot topic. In this study, two composites ZnO/Au (1% wt.) and ZnO/Ag (1% wt.) were synthesized by simple aqueous solution methods. The structure and morphology of the r nanocomposites were analyzed by structural and optical characterization methods. The formation of AuNPs and AgNPs in these experiments was also discussed. The antimicrobial properties of ZnO, ZnO/Au, and ZnO/Ag nanomaterials were investigated against Gram-negative bacteria (Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus). The results showed an increase of 80% in the antimicrobial activity of ZnO/Au against Pseudomonas aeruginosa compared with 30% in the case of ZnO/Ag. Similarly, in the case of the S. aureus strain tests, ZnO/Au increased the antimicrobial activity by 55% and ZnO/Ag by 33%. The photocatalytic tests indicated an improvement in the photocatalytic degradation of methylene blue (MB) under UV irradiation using ZnO/Au and ZnO/Ag nanocomposites compared to bare ZnO. The photocatalytic degradation efficiency of ZnO after 60 min of UV irradiation was ∼83%, while the addition of AuNPs enhanced the degradation rate to ∼95% (ZP2), and AgNP presence enhanced the efficiency to ∼98%. The introduction of noble metallic nanoparticles into the ZnO matrix proved to be an effective strategy to increase their antimicrobial activity against P. aeruginosa and S. aureus, and their photocatalytic activity was evaluated through the degradation of MB dye. Comparing the enhancing effects of Au and Ag, it was found that ZnO/Au was a better antimicrobial agent while ZnO/Ag was a more effective photocatalyst under UV irradiation.

Modulating Electronic Metal-Support Interactions to Boost Visible-Light-Driven Hydrolysis of Ammonia Borane: Nickel-Platinum Nanoparticles Supported on Phosphorus-Doped Titania

Ammonia borane (AB) is a promising material for chemical H2 storage owing to its high H2 density (up to 19.6 wt %). However, the development of an efficient catalyst for driving H2 evolution through AB hydrolysis remains challenging. Therefore, a visible-light-driven strategy for generating H2 through AB hydrolysis was implemented in this study using Ni-Pt nanoparticles supported on phosphorus-doped TiO2 (Ni-Pt/P-TiO2 ) as photocatalysts. Through surface engineering, P-TiO2 was prepared by phytic-acid-assisted phosphorization and then employed as an ideal support for immobilizing Ni-Pt nanoparticles via a facile co-reduction strategy. Under visible-light irradiation at 283 K, Ni40 Pt60 /P-TiO2 exhibited improved recyclability and a high turnover frequency of 967.8 molH 2 ${{_{{\rm H}{_{2}}}}}$  molPt -1  min-1 . Characterization experiments and density functional theory calculations indicated that the enhanced performance of Ni40 Pt60 /P-TiO2 originated from a combination of the Ni-Pt alloying effect, the Mott-Schottky junction at the metal-semiconductor interface, and strong metal-support interactions. These findings not only underscore the benefits of utilizing multipronged effects to construct highly active AB-hydrolyzing catalysts, but also pave a path toward designing high-performance catalysts by surface engineering to modulate the electronic metal-support interactions for other visible-light-induced reactions.

A 2D Dy-based metal-organic framework derived from benzothiadiazole: structure and photocatalytic properties

A 2D Dy(III) metal-organic layer (MOL 1) was synthesized under solvothermal conditions. Structural analysis suggests that the Dy(III) ions in each one-dimensional (1D) arrangement are evenly arranged in the form of broken lines. The 1D chains are linked to one another via ligands to form a 2D layer that generates a 2D surface with elongated apertures. The photocatalytic activity study suggests that MOL 1 exhibits good catalytic activity in flavonoids by the formation of an O₂˙^- radical as an intermediate. This is the first reported method of synthesizing flavonoids using chalcones.

Mechanistic investigation of cellulose regulating the morphology and photocatalytic activity of Al-doped ZnO

The morphology of metal oxide is a crucial factor for improving of catalysis properties. As a renewable and environmentally friendly biomass material, cellulose has been widely used to induce the morphology of semiconductors. The contributions of cellulose hydroxyl groups and spatial hindrance in tailoring Al doped ZnO (AZO) morphologies were investigated. The morphology of AZO could be gradually induced from flake-like to flower-like with the increase of cellulose hydroxyl content per unit volume. At the same time, the changes in spatial hindrance had no apparent effect on the morphology of AZO. So the cellulose hydroxyl groups that act to induce the in situ growth of AZO nanoparticles on cellulose substrates. The results further confirmed the strong interaction between cellulose hydroxyl groups and Zn²⁺. In addition, the photocatalytic activities of Al-doped ZnO/cellulose nanocomposites (AZOC) with different morphologies were evaluated by the degradation of bisphenol A (BPA). The high hydroxyl contents of cellulose substrates contributed to the growth of flower-like AZO with high light utilization and photocatalytic activity. This work proposed cleaner strategies to modify semiconductor morphologies for photocatalysis by regulating the content of cellulose hydroxyl contents.

Fast all-fiber ultraviolet photodetector based on an Ag-decorated ZnO micro-pillar

There are urgent demands of ultraviolet (UV) photodetectors with high sensitivity and fast response due to the wide application of ultraviolet light in the fields of medical treatment, space exploration, optical communication and semiconductor industry. The response speed of traditional ZnO-based UV photodetectors is always limited by the carrier mobility and electrical resistance caused by the external circuits. Utilizing the all-optical detection method may replace the complex circuit structure and effectively improve the response speed of photodetectors. Here, a fast-response fiber-optic UV photodetector is proposed, where a ZnO micro-pillar is fixed on the end face of a fiber-tip and acts as a Fabry-Pérot interferometer (FPI). Under the irradiation of UV light, the photo-generated carriers change the refractive index of the ZnO micro-pillar, leading to a redshift of the interference wavelengths of the ZnO FPI. To enhance this effect, a discontinuous Ag film with an island-like structure is coated on the surface of ZnO micro-pillars through magnetron sputtering, and therefore the sensitivity of the proposed device achieves to 1.13 nm/(W·cm^-2), which is 3.9 times higher than that of without Ag-decoration, due to the intensification of photo-carrier change with the help of the Schottky junction formed between Ag film and ZnO micro-pillar. Meanwhile, since the response speed of the proposed device is mainly determined by the temporal RI change of ZnO micro-pillar, the fiber-optic UV photodetector also shows very fast response with a rise time of 35 ns and a decay time of 40 µs. The demonstrated structure takes full advantage of optical fiber devices, exhibiting compactness, flexibility, fast response and immune to electromagnetic interference, which paves a new way for the next generation of photodetection devices.

Synthesis and applications of TiO2/ZnO hybrid nanostructures by ZnO deposition on TiO2 nanotubes using electrochemical processes

In recent years, TiO2/ZnO hybrid nanostructures have been attracting the interest of the scientific community due to their excellent photoelectrochemical properties. The main advantage of TiO2/ZnO hybrid nanostructures over other photocatalysts based on semiconductor materials lies in their ability to form heterojunctions in which the valence and conduction bands of both semiconductors are intercalated. This factor produces a decrease in the band gap and the recombination rate and an increase in the light absorption range. The aim of this review is to perform a revision of the main methods to synthesise TiO2/ZnO hybrid nanostructures by ZnO deposition on TiO2 nanotubes using electrochemical processes. Electrochemical synthesis methods provide an easy, fast, and highly efficient route to carry out the synthesis of nanostructures such as nanowires, nanorods, nanotubes, etc. They allow us to control the stoichiometry, thickness and structure mainly by controlling the voltage, time, temperature, composition of the electrolyte, and concentration of monomers. In addition, a study of the most promising applications for TiO2/ZnO hybrid nanostructures has been carried out. In this review, the applications of dye-sensitised solar cell, photoelectrocatalytic degradation of organic compounds, photoelectrochemical water splitting, gas sensors, and lithium-ion batteries have been highlighted.

Novel ultralong and photoactive Bi2Ti4O11/TiO2 heterojunction nanofibers toward efficient textile wastewater treatment

The elimination of dyes from textile wastewater with a lower carbon footprint is highly contingent on the design of green catalysts. Here, we innovatively developed ultralong one-dimensional Bi₂Ti₄O₁₁/TiO₂ heterojunction nanofibers via electrospinning so as to photocatalytically degrade dyes efficiently and sustainably through the utilisation of renewable solar irradiation. The heterostructured Bi₂Ti₄O₁₁/TiO₂ nanofibers exhibited desirable activity in the visible light region through the slight shift of the absorption edge to a longer wavelength. The Bi₂Ti₄O₁₁/TiO₂ nanofibers calcined at 550 °C had a lower optical band gap (3.08 eV) than that of the pure TiO₂ (3.32 eV), as evidenced by their higher photocatalytic degradation kinetics of a model dye (Acid Orange 7) (2.5 times greater than those of pure TiO₂). The enhanced visible light photocatalytic performance arose from the formation of both the Bi₂Ti₄O₁₁/TiO₂ heterojunction and the effective separation of photogenerated holes and electrons. The employment of ultralong Bi₂Ti₄O₁₁/TiO₂ heterojunction nanofibers for dye removal/decolourisation under visible light is an efficient, cost effective and sustainable solution, which will provide significant insights for practical textile wastewater treatment in view of practical engineering applications.

The elimination of dyes from textile wastewater with a lower carbon footprint is highly contingent on the design of green catalysts.

Original Approach to Synthesize TiO2/ZnO Hybrid Nanosponges Used as Photoanodes for Photoelectrochemical Applications

In the present work, TiO2/ZnO hybrid nanosponges have been synthesized for the first time. First, TiO2 nanosponges were obtained by anodization under hydrodynamic conditions in a glycerol/water/NH4F electrolyte. Next, in order to achieve the anatase phase of TiO2 and improve its photocatalytic behaviour, the samples were annealed at 450 °C for 1 h. Once the TiO2 nanosponges were synthesized, TiO2/ZnO hybrid nanosponges were obtained by electrodeposition of ZnO on TiO2 nanosponges using different temperatures, times, and concentrations of zinc nitrate (Zn(NO3)2). TiO2/ZnO hybrid nanosponges were used as photoanodes in photoelectrochemical water splitting tests. The results indicate that the photoelectrochemical response improves, in the studied range, by increasing the temperature and the Zn(NO3)2 concentration during the electrodeposition process, obtaining an increase in the photoelectrochemical response of 141% for the TiO2/ZnO hybrid nanosponges electrodeposited at 75 °C with 10 mM Zn(NO3)2 for 15 min. Furthermore, morphological, chemical, and structural characterization was performed by Field Emission Scanning Electron Microscopy (FE-SEM) with Energy Dispersive X-Ray spectroscopy (EDX), Raman Confocal Laser Spectroscopy, X-Ray Photoelectron Spectroscopy (XPS), and Grazing Incidence X-Ray Diffraction (GIXRD).

Physical and photocatalytic properties of sprayed Dy doped ZnO thin films under sunlight irradiation for degrading methylene blue

Dysprosium-doped zinc oxide (ZnO) thin films have been prepared through spray pyrolysis onto glass substrates. Cross-sections of the deposited thin films were assessed through Scanning Electron Microscopy (SEM), showing thicknesses between 200 and 300 nm. The thin film roughness was evaluated using the obtained images from the Atomic Force Microscope (AFM) micrographs. The crystallographic structure of the samples was analyzed by X-ray diffraction (XRD) revealing polycrystalline thin films. However, the slight shift towards a higher 2θ angle in Dy-doped ZnO films as compared to the pure ones indicates the incorporation of Dy³⁺ into the ZnO crystal lattice. The analysis of the oxidation state via X-ray photoelectron spectroscopy (XPS) confirms the incorporation of Dy ions in the ZnO matrix. Besides, UV-Vis-NIR spectrophotometry analysis and photoluminescence (PL) spectroscopy showed that bandgap energy values of ZnO decreased when dysprosium doping increased. Therefore, Dy doped ZnO thin films can be potentially used as a solar-light-driven photocatalyst. Among the different doping yields, the ZnO doped with 6% dysprosium provides the highest degradation rate for methylene blue (MB) under solar irradiation. Specifically, 9% of dye degradation was achieved under sunlight irradiation for 120 minutes.

Dysprosium-doped zinc oxide (ZnO) thin films prepared through spray pyrolysis show outstanding photocatalytic activity for the degradation of methylene blue.

Effort of ionic radius on doped style of silver and copper/zinc oxide nanorods for photodegradation of methylene blue

The homogeneous Cu-ZnO and heterogeneous Ag/ZnO nanorods were synthesised by controlling the ionic radius of Cu and Ag with lattice of ZnO. The structure of Cu-ZnO and Ag/ZnO was investigated by a series of methods. The study of the photoelectric properties suggested both the homogeneously and heterogeneously doped ZnO displayed an increased photocatalytic performance. It was worth noting that the Cu-ZnO and Ag/ZnO composites shown different degradation activities of methylene blue under the visible and UV light irradiation. The data suggested that the Cu species incorporated into the crystal lattice of ZnO nanorods could promote the utilisation of visible light, leading to 2.5- and 2.7-fold increase in the degradation rates compared with Ag/ZnO and ZnO nanorods. However, when UV light was employed, on the one hand, the significant increase of the activity for both Cu-ZnO and Ag/ZnO compared with ZnO was observed and on the other hand, the degradation rate of Cu-ZnO was close to that of Ag/ZnO. The investigation suggested that the metallic Cu in the ZnO crystal lattice could promote the separation of electrons and holes generated by ZnO under visible light. As for the UV light, both doping styles could favour the separation process. The present work could provide a further insight for the design of ZnO-based nanomaterials and development of their applications in environmental catalysis.

Yolk-shell nanostructures: synthesis, photocatalysis and interfacial charge dynamics

Solar energy has long been regarded as a promising alternative and sustainable energy source. In this regard, photocatalysts emerge as a versatile paradigm that can practically transform solar energy into chemical energy. At present, unsatisfactory conversion efficiency is a major obstacle to the widespread deployment of photocatalysis technology. Many structural engineering strategies have been proposed to address the issue of insufficient activity for semiconductor photocatalysts. Among them, creation of yolk-shell nanostructures which possess many beneficial features, such as large surface area, efficient light harvesting, homogeneous catalytic environment and enhanced molecular diffusion kinetics, has attracted particular attention. This review summarizes the developments that have been made for the preparation and photocatalytic applications of yolk-shell nanostructures. Additional focus is placed on the realization of interfacial charge dynamics and the possibility of achieving spatial separation of charge carriers for this unique nanoarchitecture as charge transfer is the most critical factor determining the overall photocatalytic efficiency. A future perspective that can facilitate the advancement of using yolk-shell nanostructures in sophisticated photocatalytic systems is also presented.

Enhanced charge separation in TiO2/nanocarbon hybrid photocatalysts through coupling with short carbon nanotubes

The interfacial contact between TiO₂ and graphitic carbon in a hybrid composite plays a critical role in electron transfer behavior, and in turn, its photocatalytic efficiency. Herein, we report a new approach for improving the interfacial contact and delaying charge carrier recombination in the hybrid by wrapping short single-wall carbon nanotubes (SWCNTs) on TiO₂ particles (100 nm) via a hydration-condensation technique. Short SWCNTs with an average length of 125 ± 90 nm were obtained from an ultrasonication-assisted cutting process of pristine SWCNTs (1–3 μm in length). In comparison to conventional TiO₂–SWCNT composites synthesized from long SWCNTs (1.2 ± 0.7 μm), TiO₂ wrapped with short SWCNTs showed longer lifetimes of photogenerated electrons and holes, as well as a superior photocatalytic activity in the gas-phase degradation of acetaldehyde. In addition, upon comparison with a TiO₂–nanographene “quasi-core–shell” structure, TiO₂-short SWCNT structures offer better electron-capturing efficiency and slightly higher photocatalytic performance, revealing the impact of the dimensions of graphitic structures on the interfacial transfer of electrons and light penetration to TiO₂. The engineering of the TiO₂–SWCNT structure is expected to benefit photocatalytic degradation of other volatile organic compounds, and provide alternative pathways to further improve the efficiency of other carbon-based photocatalysts.

The interfacial contact between TiO₂ and graphitic carbon in a hybrid composite plays a critical role in electron transfer behavior, and in turn, its photocatalytic efficiency.

Ultrathin Z-scheme 2D/2D N-doped HTiNbO5 nanosheets/g-C3N4 porous composites for efficient photocatalytic degradation and H2 generation under visible light

To realize highly efficient utilization of solar energy for solving problems of environmental pollution and energy shortage has attracted increasing attention. Herein, a two-step exfoliation-restacking process was employed to construct ultrathin Z-scheme two-dimensional (2D)/2D N-doped HTiNbO₅ nanosheets/g-C₃N₄ (RTCN) heterojunction composites with the increased specific surface areas, showing the enhanced photocatalytic performance for rhodamine B (RhB) degradation and hydrogen (H₂) generation under visible light irradiation. A 2D/2D heterojunction structure was formed between N-doped H⁺-restacked HTiNbO₅ nanosheets (N-RTNS) and g-C₃N₄, which was beneficial for the effectively spatial separation of photogenerated charge carriers. The improved photocatalytic activities may be attributed to the synergistic effects of the increased specific surface area, N-doping and 2D/2D heterostructure. The active species of holes (h⁺), hydroxyl (•OH) and superoxide (•O₂^-) radicals contributed to RhB photodegradation. A Z-scheme photocatalytic mechanism was proposed over RTCN-2 composite, showing dual advantages of the highly redox ability and efficient charge carrier separation.

Enhanced photocatalytic degradation of Rhodamine B, antibacterial and antioxidant activities of green synthesised ZnO/N doped carbon quantum dot nanocomposites

In order to explore an alternative photocatalyst for environmental remediation, we report a two-step process for the synthesis of zinc oxide/nitrogen doped carbon quantum dot nanocomposites (ZnO@NCQD NCs).

Rapid photocatalytic degradation of cationic organic dyes using Li-doped Ni/NiO nanocomposites and their electrochemical performance

This work reports novel bi-functional Li-doped Ni/NiO nanocomposites as potential candidates for energy storage and water treatment applications.

Pyrolytic synthesis of organosilane-functionalized carbon nanoparticles for enhanced photocatalytic degradation of methylene blue under visible light irradiation

The use of carbon-based nanomaterials as effective photocatalysts is an ideal alternative for environmental remediation. Here, (3-aminopropyl)triethoxysilane-functionalized carbon nanoparticles (SiCNPs) were prepared using a simple pyrolysis method with sodium citrate and urea as the precursors. The samples were characterized by X-ray diffraction, transmission electron microscopy, ultraviolet-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, and photo-electrochemical measures. The obtained SiCNPs-2.0 showed a better visible light response and more effective photocatalytic activity for degradation of methylene blue (MB) compared with pure CNPs. Under visible light irradiation, 98.8% of the MB was decomposed within 75 min when SiCNPs-2.0 was used as the photocatalyst. The high photocatalytic activities of SiCNPs-2.0 could be attributed to enhanced light absorption in the visible region, and improved photogenerated electron-hole separation efficiency. A possible photocatalytic mechanism for removal of MB over SiCNPs-2.0 was proposed based on active species trapping experiments. Recycling experiments showed that SiCNPs-2.0 had good stability during photocatalysis. This work provides a new easy method to synthesize carbon-based nanomaterials and to catalytically degrade organic pollutants in water under visible light irradiation.

ZnO UV Photodetectors Modified by Ag Nanoparticles Using All-Inkjet-Printing

To further improve the performance of all-inkjet-printing ZnO UV photodetector and maintain the advantages of inkjet printing technology, the inkjet printing Ag nanoparticles (NPs) were deposited on the inkjet printing ZnO UV photodetector for the first time. The inkjet printing Ag NPs can passivate the surface defects of ZnO and work as surface plasmons from the characterization of photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and finite difference time domain method (FDTD) simulation. The normalized detectivity (D*) of the Ag NP-modified detector reaches to 1.45 × 1010 Jones at 0.715 mW incident light power, which is higher than that of 5.72 × 109 Jones of the bare ZnO photodetector. The power-law relationship between the photocurrent and the incident light power of the Ag NP-modified ZnO detector is Ipc ∝ P2.34, which means the photocurrent is highly sensitive to the change of incident light power.

Plasmonic Ag-Decorated Few-Layer MoS2 Nanosheets Vertically Grown on Graphene for Efficient Photoelectrochemical Water Splitting

A controllable approach that combines surface plasmon resonance and two-dimensional (2D) graphene/MoS₂ heterojunction has not been implemented despite its potential for efficient photoelectrochemical (PEC) water splitting. In this study, plasmonic Ag-decorated 2D MoS₂ nanosheets were vertically grown on graphene substrates in a practical large-scale manner through metalorganic chemical vapor deposition of MoS₂ and thermal evaporation of Ag. The plasmonic Ag-decorated MoS₂ nanosheets on graphene yielded up to 10 times higher photo-to-dark current ratio than MoS₂ nanosheets on indium tin oxide. The significantly enhanced PEC activity could be attributed to the synergetic effects of SPR and favorable graphene/2D MoS₂ heterojunction. Plasmonic Ag nanoparticles not only increased visible-light and near-infrared absorption of 2D MoS₂, but also induced highly amplified local electric field intensity in 2D MoS₂. In addition, the vertically aligned 2D MoS₂ on graphene acted as a desirable heterostructure for efficient separation and transportation of photo-generated carriers. This study provides a promising path for exploiting the full potential of 2D MoS₂ for practical large-scale and efficient PEC water-splitting applications.

Au-Mediated Charge Transfer Process of Ternary Cu2O/Au/TiO2-NAs Nanoheterostructures for Improved Photoelectrochemical Performance

Based on a facile three-step preparation method, Cu2O/Au/TiO2-NAs ternary heterojunction nanocomposites have been successfully synthesized by electrodepositing a Cu2O layer on the surface of Au nanoparticles (NPs) decorated highly ordered TiO2 nanotube arrays (NAs). The structure, surface morphology, chemical composition, and optical and intrinsic defects properties of the as-prepared samples are characterized by transmission and scanning electron microscopy (TEM and SEM), X-ray diffraction (XRD), UV-vis light absorbance spectra, Raman scattering, and X-ray photoelectron spectroscopy (XPS). Simultaneously, the Cu2O/Au/TiO2-NAs ternary nanohybrids exhibited progressively improved photoelectrocatalytic (PEC) performance compared with the dual Cu2O/TiO2-NAs type-II nanoheterojunctions, confirming by the photocurrent density versus testing time curve (amperometric I-t curve), open-circuit potential versus testing time curve (V oc-t curve), and electrochemical impedance spectroscopy (EIS) measurements, which were mainly ascribed to the synergistic effect of reduced interfacial charge transfer resistance and boosted energetic charge carriers generation associated with embedding Au NPs. Furthermore, the self-consistent charge transfer mechanism of Z-scheme and interband transitions mediated with Au NPs for Cu2O/Au/TiO2-NAs triple nanocomposites is proposed, which was evaluated by nanosecond time-resolved transient photoluminescence (NTRT-PL) spectra excited by 266 and 400 nm, respectively. Following this scheme, UV-vis light photocatalytic activities of Cu2O/Au/TiO2-NAs ternary nanohybrids were elaborated toward photodegradation of methyl orange (MO) in aqueous solution, and the photodegradation rate of optimum triple nanocomplex was found to be 90%.

Synthesis and photocatalytic degradation activities of phosphorus containing ZnO microparticles under visible light irradiation for water treatment applications

A series of phosphorus containing ZnO (P-ZnO) photocatalysts with various percentages of phosphorus were successfully synthesized using the hydrothermal method. The structural, physical and optical properties of the obtained microparticles were investigated using diverse techniques such as X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible diffusion reflectance spectroscopy (UV-Vis DRS), photoluminescence (PL) spectroscopy, transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and N₂ adsorption-desorption analysis. The photocatalytic activities of the pure and P-ZnO samples were evaluated for the degradation of Rhodamine B (RhB) under visible light irradiation. The parameters such as pH, catalyst dosage, contaminant concentration and effect of persulfate as an oxidant were studied. It was found that the P-ZnO1.8% photocatalyst could destroy 99% of RhB (5 ppm) in 180 min at pH = 7; furthermore, it degraded ∼100% of 5 and 10 ppm of the RhB pollutant in 120 and 180 min, respectively, only by adding 0.01 g of persulfate into the reaction solution. To determine the photocatalytic mechanism, 2-propanol, benzoquinone and EDTA were used and it was indicated that hydroxyl radicals, superoxide ions and holes, all had major roles in the photocatalytic degradation but the hydroxyl radical effect was the most significant. The phenol degradation was also investigated using the P-ZnO1.8% optimum photocatalyst which could destroy 53% of the phenol (5 ppm) in 180 min. According to the reusability test, it was proved that after 5 cycles, the catalyst activity was not highly changed and it was potentially capable of pollutant degradation.

Fabrication of ZnO nanoparticles adorned nitrogen-doped carbon balls and their application in photodegradation of organic dyes

In the present study, a novel ZnO nanoparticles adorned nitrogen-doped carbon balls (ZnO@CBs) were successfully synthesized from polybenzoxazine and ZnO nanoparticles through a simple carbonization method. The typical wurtzite hexagonal zinc oxide phase in ZnO@CBs and degree of graphitization were revealed by the X-ray diffraction pattern. The field emission scanning electron microscopy confirmed that the synthesized carbon materials have well dispersed ball-like structure, wherein, the ZnO nanoparticles are distributed evenly on the carbon balls (CBs). The synthesized ZnO@CBs with different wt.% (20, 40, 60 and 80) and bare ZnO nanoparticles were investigated for methylene blue (MB) dye degradation experiment. The synthesized ZnO@CBs exhibited high activity in the degradation of MB. Among the different wt.% of ZnO@CBs, 60 wt.% of ZnO@CBs showed the highest MB degradation ratio (99%) with a fast degradation rate (1.65% min⁻¹) under the following optimum conditions: 20 mg of ZnO@CBs in 50 mL of MB solution at room temperature.

A Comparative Study of Microcystin-LR Degradation by UV-A, Solar and Visible Light Irradiation Using Bare and C/N/S-Modified Titania

In an endeavor to tackle environmental problems, the photodegradation of microcystin-LR (MC-LR), one of the most common and toxic cyanotoxins, produced by the cyanobacteria blooms, was examined using nanostructured TiO2 photocatalysts (anatase, brookite, anatase–brookite, and C/N/S co-modified anatase–brookite) under UV-A, solar and visible light irradiation. The tailoring of TiO2 properties to hinder the electron–hole recombination and improve MC-LR adsorption on TiO2 surface was achieved by altering the preparation pH value. The highest photocatalytic efficiency was 97% and 99% with degradation rate of 0.002 mmol L−1 min−1 and 0.0007 mmol L−1 min−1 under UV and solar irradiation, respectively, using a bare TiO2 photocatalyst prepared at pH 10 with anatase to brookite ratio of ca. 1:2.5. However, the bare TiO2 samples were hardly active under visible light irradiation (25%) due to a large band gap. Upon UV, solar and vis irradiation, the complete MC-LR degradation (100%) was obtained in the presence of C/N/S co-modified TiO2 with a degradation rate constant of 0.26 min−1, 0.11 min−1 and 0.04 min−1, respectively. It was proposed that the remarkable activity of co-modified TiO2 might originate from its mixed-phase composition, mesoporous structure, and non-metal co-modification.

AgCl-ZnAl Layered Double Hydroxides as Catalysts with Enhanced Photodegradation and Antibacterial Activities

Surface-modified ZnAl layered double hydroxides (LDHs) were prepared by reaction of AgNO3, with both ZnAlCl (LDH1) and ZnAlCO3 exchanged on the surface with chloride anions (LDH3). In this way, AgCl nanoparticles with crystalline domains ranging from 40 to 100 nm were grown on the LDH surface. An additional sample was prepared by partial reduction of silver to obtain Ag@AgCl-LDH (LDH2). The composites were tested as catalysts in Rhodamine B (RhB) degradation, wherein LDH2 showed complete cleavage of RhB after 45 min of irradiation versus 70 min needed in the presence of AgCl. This time decreased to 35 min for LDH1 and 15 min for LDH3, underlining the role of the AgCl dimensions and anion in the interlayer region. Studies on the reactive species involved in the degradation process revealed that, for all catalysts, O2·− was the main active species, while, to some extent, holes contribute to the activity of the LDH3. Finally, the composites showed high bactericidal activity, under irradiation, against Escherichia coli, comparable with that of Gentamicin, the positive control. A synergic effect of silver released from the composites and the production of reactive oxygen species was considered.

Fast and Efficient Sun Light Photocatalytic Activity of Au_ZnO Core-Shell Nanoparticles Prepared by a One-Pot Synthesis

Gold nanostructures absorb visible light and show localized surface plasmon resonance bands in the visible region. Semiconducting ZnO nanostructures are excellent for ultraviolet detection, thanks to their wide band gap, large free exciton binding energy, and high electron mobility. Therefore, the coupling of gold and ZnO nanostructures represents the best-suited way to boost photodetection. With the above perspective, we report on the high photocatalytic activity of some Au_ZnO core-shell nanoparticles (NPs) recently prepared by a one-pot synthesis in which a [zinc citrate]^- complex acted as the ZnO precursor, a reducing agent for Au³⁺, and a capping anion for the obtained Au NPs. The overall nanostructures proved to be Au(111) NPs surrounded by a thin layer of [zinc citrate]^- that evolved to Au_ZnO core-shell nanostructures. Worthy of note, with this photocatalyst, sun light efficiently decomposes a standard methylene blue solution according to ISO 10678:2010. We rationalized photodetection, reaction rate, and quantum efficiency.

Highly improved photocatalytic degradation of rhodamine B over Bi2Ga4-x Fe x O9 solid solutions under visible light irradiation

In this work, Bi2Ga4-x Fe x O9 (0 ≤ x ≤ 1.2) solid solutions were prepared via the traditional high-temperature solid-state reaction. The Le Bail fitting on the powder X-ray diffraction patterns shows that these solid solutions were successfully synthesized. Scanning electron microscopy showed that the Bi2Ga3.2Fe0.8O9 sample was composed of sub-micron particle crystallites. Energy dispersive spectroscopy analysis and X-ray photoelectron spectroscopy were used to identify that the Fe element is trivalent when doping into the crystal structure. Ultraviolet-visible diffused reflectance spectra suggested that the bandgap of Bi2Ga3.2Fe0.8O9 is narrower than that of the undoped Bi2Ga4O9 sample. Three strategies, including Fe3+ doping, addition of H2O2, and loading of the cocatalyst, were utilized to improve the photocatalytic degradation activity. The optimum photocatalytic performance was obtained over 2.5 wt% Cu/Bi2Ga3.2Fe0.8O9 sample in 20 ppm RhB aqueous solution (containing 1.5 mL H2O2) under visible light irradiation. Its photodegradation rate is 8.0 times that of Bi2Ga4O9 containing 0.5 mL H2O2. The 2.5 wt% Cu/Bi2Ga3.2Fe0.8O9 photocatalyst remained stable and active even after four cycles. Also, its photocatalytic conversion efficiency for RhB was nearly 100%, which was achieved in 3 hours. The photocatalytic mechanism indicated that ·OH and h+ played an important role in the photocatalytic degradation reaction.

Mechanistic Insights into Photodegradation of Organic Dyes Using Heterostructure Photocatalysts

Due to its low cost, environmentally friendly process, and lack of secondary contamination, the photodegradation of dyes is regarded as a promising technology for industrial wastewater treatment. This technology demonstrates the light-enhanced generation of charge carriers and reactive radicals that non-selectively degrade various organic dyes into water, CO2, and other organic compounds via direct photodegradation or a sensitization-mediated degradation process. The overall efficiency of the photocatalysis system is closely dependent upon operational parameters that govern the adsorption and photodegradation of dye molecules, including the initial dye concentration, pH of the solution, temperature of the reaction medium, and light intensity. Additionally, the charge-carrier properties of the photocatalyst strongly affect the generation of reactive species in the heterogeneous photodegradation and thereby dictate the photodegradation efficiency. Herein, this comprehensive review discusses the pseudo kinetics and mechanisms of the photodegradation reactions. The operational factors affecting the photodegradation of either cationic or anionic dye molecules, as well as the charge-carrier properties of the photocatalyst, are also fully explored. By further analyzing past works to clarify key active species for photodegradation reactions and optimal conditions, this review provides helpful guidelines that can be applied to foster the development of efficient photodegradation systems.

Control of electrical conductivity of highly stacked zinc oxide nanocrystals by ultraviolet treatment

Zinc oxide (ZnO) nanocrystals (NCs) were synthesized using a modified sol-gel method. Ultraviolet (UV) treatment was performed under various atmospheres on the highly stacked ZnO NCs. The prepared NCs were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, photoluminescence spectroscopy, and atomic force microscopy to investigate their structural, electrical, and electrochemical properties. Through these analyses, the effect of the UV treatment on the chemical and electrical characteristics of ZnO NCs was established. According to the analyses, the organic ligands in the NCs were decomposed, and the particles were densified. The mobility of UV-treated ZnO NCs thin films increased to 1.4 cm²/Vs, almost 2 orders higher than the UV untreated ZnO thin films. It was confirmed that the recombination from oxygen vacancies of ZnO could be controlled by UV irradiation. As decreased oxygen vacancies, the band gap of ZnO NCs was increased from 3.2 eV to 3.27 eV.

Effects of Au nanoparticles and ZnO morphology on the photocatalytic performance of Au doped ZnO/TiO2 films

Au doped ZnO nanocomposite films on TiO₂ seeding layer (AuZ/T) were fabricated by hydrothermal processing and their photocatalytic performance was investigated. It could be found that the AuZ/T with micrometer(μm)-sized, lying ZnO bulks revealed optimal photocatalytic performance toward methyl orange under simulated sunlight, whose apparent degradation rate constant K _app of 1.31 was about 20% higher compared to that of ZnO/TiO₂ and 3 times higher compared to that of ZnO. The Au nanoparticles, TiO₂ seeding layer and hydrothermal processing time imposed vital influence on the morphology of ZnO nanostructures, which played key roles in the formation of ZnO/TiO₂ heterojunction and charge transfer (CT) inside it, as demonstrated by kinetics of transient photoluminescence (PL) decaying. The incorporation of Au nanoparticles not only induced the variations of ZnO crystallinity and reduction of ZnO band gap (E _g), but also generated the Schottky heterojunction of metal-semiconductor, which would be beneficial to the CT inside nanocomposite films and separation of photo-generated electron-hole pairs, as verified by the remarkable PL suppression. The mechanism responsible for photocatalysis enhancement, which was resulted from the hybrid effects of Au nanoparticles and the ZnO morphology was discussed in details.

TiO2–Au composite nanofibers for photocatalytic hydrogen evolution

TiO₂-based materials for photocatalytic hydrogen (H₂) evolution have attracted much interest as a renewable approach for clean energy applications. TiO₂–Au composite nanofibers (NFs) with an average fiber diameter of ∼160 nm have been fabricated by electrospinning combined with calcination treatment. In situ reduced gold nanoparticles (NPs) with uniform size (∼10 nm) are found to disperse homogenously in the TiO₂ NF matrix. The TiO₂–Au composite NFs catalyst can significantly enhance the photocatalytic H₂ generation with an extremely high rate of 12 440 μmol g⁻¹ h⁻¹, corresponding to an adequate apparent quantum yield of 5.11% at 400 nm, which is 25 times and 10 times those of P25 (584 μmol g⁻¹ h⁻¹) and pure TiO₂ NFs (1254 μmol g⁻¹ h⁻¹), respectively. Furthermore, detailed studies indicate that the H₂ evolution efficiency of the TiO₂–Au composite NF catalyst is highly dependent on the gold content. This work provides a strategy to develop highly efficient catalysts for H₂ evolution.

The H₂ production rate of TiO₂–Au nanofibers is dramatically improved to 12 440 μmol g⁻¹ h⁻¹, 10 times that of pure TiO₂.

Atomic layer deposition with rotary reactor for uniform hetero-junction photocatalyst, g-C3N4@TiO2 core–shell structures

A heterojunction of TiO₂ grown on g-C₃N₄ particles is demonstrated using atomic layer deposition (ALD), equipped with a specifically designed rotary reactor for maintaining stable mechanical dispersion of g-C₃N₄ particles during ALD. The photocatalytic activity of the g-C₃N₄@ALD-TiO₂ core–shell composites was examined using the degradation of rhodamine B dye under visible light irradiation. The optimal composite with 5 ALD cycles of TiO₂ exhibited the highest photocatalytic reaction rate constant among the composites with a range of ALD cycles from 2 to 200 cycles, which was observed to be 3 times higher than that of pristine g-C₃N₄ and 2 times higher than that of g-C₃N₄@TiO₂ composite prepared using a simple impregnation method. The ALD-TiO₂ were well-dispersed on the g-C₃N₄ surface, while TiO₂ nanoparticles were agglomerated onto the g-C₃N₄ in the g-C₃N₄@TiO₂ composite prepared by the impregnation method. This created uniform and stable heterojunctions between the g-C₃N₄ and TiO₂, thus, enhancing the photocatalytic activity.

A heterojunction of TiO₂ grown on g-C₃N₄ particles is demonstrated using atomic layer deposition (ALD), equipped with a specifically designed rotary reactor for maintaining stable mechanical dispersion of g-C₃N₄ particles during ALD.

In Situ Synthesis of All-Solid-State Z-Scheme BiOBr0.3I0.7/Ag/AgI Photocatalysts with Enhanced Photocatalytic Activity Under Visible Light Irradiation

A series of novel visible light driven all-solid-state Z-scheme BiOBr_0.3I_0.7/Ag/AgI photocatalysts were synthesized by facile in situ precipitation and photo-reduction methods. Under visible light irradiation, the BiOBr_0.3I_0.7/Ag/AgI samples exhibited enhanced photocatalytic activity compared to BiOBr_0.3I_0.7 and AgI in the degradation of methyl orange (MO). The optimal ratio of added elemental Ag was 15%, which degraded 89% of MO within 20 min. The enhanced photocatalytic activity of BiOBr_0.3I_0.7/Ag/AgI can be ascribed to the efficient separation of photo-generated electron-hole pairs through a Z-scheme charge-carrier migration pathway, in which Ag nanoparticles act as electron mediators. The mechanism study indicated that ·O₂^- and h⁺ are active radicals for photocatalytic degradation and that a small amount of ·OH also participates in the photocatalytic degradation process.

Broad-Band High-Sensitivity ZnO Colloidal Quantum Dots/Self-Assembled Au Nanoantennas Heterostructures Photodetectors

Tunable plasmonic resonance induced by the collective oscillation of the electrons on metallic nanostructures can excellently enhance the light response of ZnO films, which provides an effective way to break through the limitation of the performance of ZnO photodetectors. Here, broad-band high-performance ZnO/Au heterostructures photodetectors with various morphologies of self-assembled Au nanoantennas are fabricated via a facile approach under the spin-coated ZnO colloidal quantum dots films. With a systematic control on growth condition, the self-assembled Au nanoantennas undergo a drastic evolution from the corrugated nanomounds to the island-like nanostructures, and the light absorption of the resulting ZnO/Au heterostructures correspondingly exhibits a strongly morphological dependence on the Au nanoantennas. Meanwhile, the photoresponse of the ZnO-based photodetectors is significantly improved throughout a wide spectrum between UV and visible regions owing to the enhanced light absorption induced by the localized surface plasmon resonance. As a result, the optimal switch ratio of the ZnO/Au heterostructures photodetector increases by 1 order to ∼1.13 × 10⁵ than that of the pristine ZnO one because of the obviously increased photocurrent ( I_ph) and comparable dark current, thus leading to ∼9.1 and ∼4.9 times increases in the photoresponsivity and the normalized detectivity. Meanwhile, the significant increases in the I_ph of ∼5.2 and ∼9.7 times are likewise observed with the ZnO/Au heterostructures under 530 nm and white-light illumination. This work can offer a handy and effective approach for the fabrication of ultrasensitive ZnO-based photodetectors within a broad-band wavelength by utilizing the Au plasmonic nanostructures.

"Dark Deposition" of Ag Nanoparticles on TiO2: Improvement of Electron Storage Capacity To Boost "Memory Catalysis" Activity

"Memory catalysis" (MC) studies have received appreciable attention recently because of the unique talent to retain the catalytic performance in the dark condition. However, the MC activity is still low owing to the relatively limited electron storage capacity of the present materials. Here, a TiO₂@Ag composite was synthesized by a "dark-deposition (DD)" method, which is based on the electron trap effect of TiO₂. Unlike traditional photodeposition (PD), an exploration of the morphology and chemical compositions of as-prepared samples shows that DD can inhibit the growth of Ag nanoparticles and the formation of Ag₂O, which greatly improve the electron storage capacity. We further demonstrated that the maximum electronic capacity was in the order of TiO₂@Ag-DD (1 μmol/mg) > TiO₂@Ag-PD (0.35 μmol/mg) > TiO₂ (0.11 μmol/mg). Moreover, the enhanced MC activity was confirmed by various degradation experiments. Especially, the use of TiO₂@Ag-DD as a round-the-clock catalyst for the degradation of multicomponent pollutants has also been achieved. This strategy opens a door for enhancing the MC activity and reveals that the coupling of photocatalysis and MC may provide a new opportunity for the continuous removal of pollutants in day and night. It also may be extended to other fields, such as energy storage and continuous disinfection.