WS2 and C-TiO2 Nanorods Acting as Effective Charge Separators on g-C3 N4 to Boost Visible-Light Activated Hydrogen Production from Seawater

Rationally designed Ti3C2/N, S-TiO2/g-C3N4 ternary heterostructure with spatial charge separation for enhanced photocatalytic hydrogen evolution

The charge separation and transfer are the major issues dominating the under-laying energy conversion mechanism for photocatalytic system. Construction of semiconductor-based heterojunction system considered to be viable option for boosting the spatial charge separation and transfer in the photocatalytic water splitting system. Here, we design a ternary heterojunction of Ti₃C₂/N, S-TiO₂/g-C₃N₄ by thermal annealing and ultrasonic assisted impregnation method having a well-designed n-n heterojunction and noble metal free Schottky junction for adequate hydrogen evolution. The optimal content of 4 wt% Ti₃C₂ on N, S-TiO₂/g-C₃N₄ (4-TC/NST/CN) exhibit the highest rate of hydrogen generation 495.06μ mol h^-1 which is 3.1, 4.1 and 1.6 fold higher than the pristine N, S doped-TiO₂, g-C₃N₄ and binary hybrid (N, S doped-TiO₂/g-C₃N₄) respectively, with 7% apparent conversion efficiency (ACE). The increment in the activity is described to the robust photogenerated carrier separation and double charge transfer channels because of the formation of dual heterojunction (n-n heterojunction and Schottky junction). XRD and Raman results revealed the occupancy of Ti₃C₂ in the heterojunction due to the strong interaction between Ti₃C_2, with N, S doped-TiO₂ and g-C₃N₄. The HRTEM analysis confirmed the formation of close interfacial junction between the Ti₃C₂, N, S doped-TiO₂ and g-C₃N₄. Moreover, the higher photocurrent, low PL intensity and lower impedance arc suggested the lower charge carrier recombination rate in 4-TC/NST/CN heterojunction. This work represents a significant development to establish a sound foundation for future design of MXene-based ternary hybrid system towards significant charge carrier separation and transfer for H₂ production activity.

Engineering of g-C3N4-based photocatalysts to enhance hydrogen evolution

The technology of photocatalytic hydrogen production that converts abundant yet intermittent solar energy into an environmentally friendly alternative energy source is an attractive strategy to mitigate the energy crisis and environmental pollution. Graphitic carbon nitride (g-C₃N₄), as a promising photocatalyst, has gradually received focus in the field of artificial photosynthesis due to its appealing optical property, high chemical stability and easy synthesis. However, the limited light absorption and massive recombination of photoinduced carriers have hindered the photocatalytic activity of bare g-C₃N₄. Therefore, from the perspective of theoretical calculations and experiments, many valid approaches have been applied to rationally design the photocatalyst and ameliorate the hydrogen production performance, such as element doping, defect engineering, morphology tuning, and semiconductor coupling. This review summarized the latest progress of g-C₃N₄-based photocatalysts from two perspectives, modification of pristine g-C₃N₄ and interfacial engineering design. It is expected to offer feasible suggestions for the fabrication of low-cost and high-efficiency photocatalysts and the photocatalytic mechanism analyses assisted by calculation in the near future. Finally, the prospects and challenges of this exciting research field are discussed.

Titanium Vacancies in TiO2 Nanofibers Enable Highly Efficient Photo-Driven Seawater Splitting

Photo-driven seawater splitting is considered as one of the most promising techniques for sustainable hydrogen production. However, the high salinity of seawater would deactivate catalysts and consumes the photogenerated carriers. Metal vacancies in metal oxide semiconductors are critical to directed electron transfer and high salinity resistance, thus desirable but remains a challenge. We demonstrate a facile controllable calcination approach to synthesize TiO 2 nanofibers with rich Ti-vacancies with excellent photo/electro performances and long-time stability in photo-driven seawater splitting, including photocatalysis and photoelectrocatalysis. Experimental measurements and theoretical calculations reveal the formation of titanium vacancies, as well as its unidirectional electron trap and superior H + adsorption ability for efficient charge transfer and corrosion resistance of seawater. Therefore, the characteristics and mechanism have been proposed at an atomic-/nanoscale to clarify the generation of titanium vacancies and the corresponding interfacial electron transfer.

Catalytic conversion of seawater to fuels: Eliminating N vacancies in g-C3N4 to promote photocatalytic hydrogen production

The use of solar energy to decompose seawater and produce hydrogen is of great significance in solving the energy crisis. Numerous studies have shown that vacancies can significantly improve photocatalytic activity due to their electron-rich nature. However, our recent research has shown that materials with vacancies are not suitable for photocatalytic reactions in seawater. In this study, g-C₃N₄ with rich N vacancies was selected as the research object, and urea was used as the precursor; in this system, the N vacancies in g-C₃N₄ could be effectively reduced by the addition of ZIF-8 (ZCNQx). The activity of ZCNQ40 was 5.6 times higher than that of g-C₃N₄ in fresh seawater, but only 3.1 times higher in freshwater. Based on the analysis of the experimental results, we believe that g-C₃N₄ has a limiting relationship between H+ adsorption catalysis and H₂ product desorption. In addition, seawater contains many heteroatoms that will also compete with proton (H+) reduction. The results of our study show that catalysts with vacancies are not necessarily suitable for catalytic reactions in seawater media. This research will stimulate new ideas for research into the conversion of solar energy to chemical energy in seawater media.

Copper Sulfide Nanoassemblies for Catalytic and Photoresponsive Eradication of Bacteria from Infected Wounds

Bovine serum albumin (BSA)-encapsulated copper sulfide nanocrystals (CuS NCs) were prepared by heating an alkaline solution containing copper ions and BSA without an additional sulfur source. At a high BSA concentration (0.8 mM), nanoassembly of the as-formed CuS NCs occurs to form BSA-CuS NCs as a result of the formation of BSA gel-like structures. In addition to their intrinsic photothermal properties, the BSA-CuS NCs possess rich surface vacancies and thus exhibit enzyme-like and photodynamic activities. Spontaneous generation of hydrogen peroxide (H₂O₂) led to the in situ formation of copper peroxide (CPO) nanodots on the BSA-CuS NCs to catalyze singlet oxygen radical generation. The antimicrobial response was enhanced by >60-fold upon NIR laser irradiation, which was ascribed to the combined effect of the photodynamic and photothermal inactivation of bacteria. Furthermore, BSA-CuS NCs were transdermally administered onto a methicillin-resistant Staphylococcus aureus-infected wound and eradicated >99% of bacteria in just 1 min under NIR illumination due to the additional peroxidase-like activity of BSA-CuS NCs, transforming H₂O₂ at the infection site into hydroxyl radicals and thus increasing the synergistic effect from photodynamic and photothermal treatment. The BSA-CuS NCs exhibited insignificant in vitro cytotoxicity and hemolysis and thus can serve as highly biocompatible bactericides in preclinical applications to effectively eradicate bacteria.

Nanoporous Ta3N5via electrochemical anodization followed by nitridation for solar water oxidation

Nanoporous tantalum nitride (Ta3N5) is a promising visible-light-driven photoanode for photoelectrochemical (PEC) water splitting with a narrow band gap of approximately 2.0 eV. It can utilize a large portion of the solar spectrum up to 600 nm to improve the activity of photooxidation reactions because of enhanced light scattering and an overall increase of the surface area with high light absorption and carrier collection. Herein, we synthesized a new n-type nanoporous tantalum nitride film on Ta foil by electrochemical anodization with a fluorinated electrolyte. Post-annealing in a nitrogen/ammonia mixture gas environment then transformed amorphous TaOx to crystalline Ta3N5. Effects of annealing temperature on the microstructure, optical properties, and PEC properties of samples were then investigated under changeable stoichiometry of Ta and N elements in the Ta-based nitride film. Results showed that the film annealed at 1000 °C showed high crystallinity, high visible light absorption, and a highly conductive interlayer between the substrates, resulting in the highest photocurrent density (JSC) of ∼0.25 mA cm-2 at 1.23 VRHE in PEC water splitting. In addition, depending on the annealing temperature, it is possible to engineer band alignment in the nanoporous Ta3N5 layer, allowing a beneficial charge transfer process.

Smart Assembly of Sulfide Heterojunction Photocatalysts with Well-Defined Interfaces for Direct Z-Scheme Water Splitting under Visible Light

A Z-scheme photocatalytic water-splitting system is an effective approach to integrate the advantages both hydrogen- and oxygen-evolution photocatalysts. The interfacial properties of the heterojunctions have a great influence on the efficiency through the crystal orientation and the charge kinetics. In this study, a general chemical vapor deposition process and a gentle cation-exchange method were combined to assemble Z-scheme photocatalysts between CdS and MnS. As a result of the well-defined heterojunction interfaces and distinctive structural benefits, without cocatalysts, the 1 D CdS/MnS hybrid photocatalyst exhibited a significantly increased photocatalytic H₂ evolution rate of 1595 μmol h^-1  g^-1 (apparent quantum efficiency of 22.6 % at λ=420 nm), which is over 10.5 times higher than that of CdS. Moreover, a better photocatalytic stability is demonstrated over particulate (42 h cycling measurement) and photoelectrochemical (3000 min continuous measurement) systems. Overall, this work provides a unique experimental insight into high-quality heterojunction interface design and new Z-scheme photocatalyst synthesis.

Recent Achievements in Development of TiO2-Based Composite Photocatalytic Materials for Solar Driven Water Purification and Water Splitting

Clean water and the increased use of renewable energy are considered to be two of the main goals in the effort to achieve a sustainable living environment. The fulfillment of these goals may include the use of solar-driven photocatalytic processes that are found to be quite effective in water purification, as well as hydrogen generation. H2 production by water splitting and photocatalytic degradation of organic pollutants in water both rely on the formation of electron/hole (e-/h+) pairs at a semiconducting material upon its excitation by light with sufficient photon energy. Most of the photocatalytic studies involve the use of TiO2 and well-suited model compounds, either as sacrificial agents or pollutants. However, the wider application of this technology requires the harvesting of a broader spectrum of solar irradiation and the suppression of the recombination of photogenerated charge carriers. These limitations can be overcome by the use of different strategies, among which the focus is put on the creation of heterojunctions with another narrow bandgap semiconductor, which can provide high response in the visible light region. In this review paper, we report the most recent advances in the application of TiO2 based heterojunction (semiconductor-semiconductor) composites for photocatalytic water treatment and water splitting. This review article is subdivided into two major parts, namely Photocatalytic water treatment and Photocatalytic water splitting, to give a thorough examination of all achieved progress. The first part provides an overview on photocatalytic degradation mechanism principles, followed by the most recent applications for photocatalytic degradation and mineralization of contaminants of emerging concern (CEC), such as pharmaceuticals and pesticides with a critical insight into removal mechanism, while the second part focuses on fabrication of TiO2-based heterojunctions with carbon-based materials, transition metal oxides, transition metal chalcogenides, and multiple composites that were made of three or more semiconductor materials for photocatalytic water splitting.

Enhancement of photocatalytic activity of g-C3N4 by hydrochloric acid treatment of melamine

Modified g-C₃N₄ samples (g-X, where X corresponds to the number of hours of acid treatment of the melamine) with outstanding photocatalytic performance were prepared by using hydrochloric acid-treated melamine as a precursor and calcining at 550 °C for 2 h. An x-ray diffractometer, field-emission scanning electron microscope, infrared spectrometer, N₂ adsorption-desorption test, x-ray photoelectron spectroscopy, and ultraviolet-visible diffuse-reflectance spectroscopy analysis were carried out to characterize the phase composition, microstructure, chemical structure, specific surface area (SSA), chemical states, elemental composition and optical properties of the samples, respectively. The photocatalytic performance of the samples was evaluated by degrading the Rhodamine B (RhB) aqueous solution. The results showed that the crystal structure and vibration bands of melamine changed due to the reaction with hydrochloric acid. The crystallinity and grain size of g-C₃N₄ in g-X (X = 1, 2, 4, 6, 8, 10) reduced, and the SSA values of g-X increased compared to that of the g-0 sample, which was synthesized from pristine melamine. The g-X samples exhibited excellent photocatalytic activity towards degradation of RhB compared to g-0. The photocatalytic activity of the g-X samples increased gradually as the acid treatment time of the melamine increased from 1 h to 2 h, and then decreased gradually with the extension of the acid treatment time. The rate constant (k) values of g-X are higher than that of g-0. g-2 presented the highest rate constant (k = 0.052 min^-1), which was 5.5 times higher than that of g-0. The improved photocatalytic activity of the g-X samples was attributed to the higher SSA value, the appearance of surface defects, the outstanding photo-carrier separation efficiency and stronger light harvesting ability of g-X, with the last two factors being more significant. Acid treatment of melamine is helpful in the preparation of high performance g-C₃N₄ photocatalyst, and the microstructure and photocatalytic performance of g-C₃N₄ were affected significantly by the acid treatment time.

Facile synthesis of g-C3N4(0.94)/CeO2(0.05)/Fe3O4(0.01) nanosheets for DFT supported visible photocatalysis of 2-Chlorophenol

Visible light active g-C3N4(0.94)/CeO2(0.05)/Fe3O4(0.01) ternary composite nanosheets were fabricated by facile co-precipitation routes. The density functional theory (DFT) computations investigated changes in geometry and electronic character of g-C3N4 with CeO2 and Fe3O4 addition. Chemical and surface characterizations were explored with XRD, XPS, SEM, TEM, PL, DRS and Raman measurements. DRS and PL spectroscopy evidenced the energy band gap tailoring from 2.68 eV for bulk g-C3N4 and 2.92 eV for CeO2 to 2.45 eV for the ternary nanocomposite. Efficient electron/hole pair separation, increase in red-ox species and high exploitation of solar spectrum due to band gap tailoring lead to higher degradation efficiency of g-C3N4(0.94)/CeO2(0.05)/Fe3O4(0.01). Superior sun light photocatalytic breakdown of 2-Chlorophenol was observed with g-C3N4 having CeO2 loading up to 5 wt%. In case of ternary nanocomposites deposition of 1 wt% Fe3O4 over g-C3N4/CeO2 binary composite not only showed increment in visible light catalysis as predicted by the DFT studies, but also facilitated magnetic recovery. The g-C3N4(0.94)/CeO2(0.05)/Fe3O4(0.01) nanosheets showed complete mineralization of 25 mg.L-1 2-CP(aq) within 180 min exposure to visible portion of sun light and retained its high activity for 3 consecutive reuse cycles. The free radical scavenging showed superoxide ions and holes played a significant role compared to hydroxyl free radicals while chromatographic studies helped establish the 2-CP degradation mechanism. The kinetics investigations revealed 2.55 and 4.04 times increased rate of reactions compared to pristine Fe3O4 and CeO2, showing highest rate constant value of 18.2 × 10-3 min-1 for the ternary nanocomposite. We present very persuasive results that can be beneficial for exploration of further potential of g-C3N4(0.94)/CeO2(0.05)/Fe3O4(0.01) in advance wastewater treatment systems.