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

A Schottky/Z-Scheme Hybrid for Augmented Photocatalytic H2 and H2O2 Production

The prodigious employment of fossil fuels to conquer the global energy demand is becoming a dreadful threat to the human society. This predicament is appealing for a potent photocatalyst that can generate alternate energy sources via solar to chemical energy conversion. With this interest, we have fabricated a ternary heterostructure of Ti3C2 nanosheet modified g-C3N4/Bi2O3 (MCNRBO) Z-scheme photocatalyst through self-assembly process. The morphological analysis clearly evidenced the close interfacial interaction between g-C3N4 nanorod, Bi2O3 and Ti3C2 nanosheets. The oxygen vacancy created on Bi2O3 surface, as suggested by XPS and EPR analysis, supported the Z-scheme heterojunction formation between g-C3N4 nanorod and Bi2O3 nanosheets. The collaborative effect of Z-scheme and Schottky junction significantly reduced charge transfer resistance promoting separation efficiency of excitons as indicated from PL and EIS analysis. The potential of MCNRBO towards photocatalytic application was investigated by H2O2 and H2 evolution reaction. A superior photocatalytic H2O2 and H2 production rate for MCNRBO is observed, which are respectively around 5 and 18 folds higher as compared to pristine CNR nanorod. The present work encourages for the development of a noble, eco-benign and immensely efficient dual heterojunction based photocatalyst, which can acts as saviour of human society from energy crisis.

Regulating the Morphology Modification To Prepare the High Charge Separation Efficiency and Visible Light Responsive Dual-Type-II B-CN/H-TiO2/BS-CN Heterojunction for Wastewater Treatment

For the conventional type-II heterojunction photocatalyst, their photocatalytic activity is affected by the limited separation efficiency of electron-hole pairs, exquisitely designed heterojunction photocatalysts are highly prospective materials for inducing charge transfer efficiently. Typically, enhancing the separation efficiency of electron-hole pairs in photocatalysts has been a formidable challenge. Here, the hollow mesoporous TiO2 (H-TiO2), the bulk g-C3N4 (B-CN), and g-C3N4 with bamboo shape (BS-CN) are prepared by simple processes. Among them, it is surprising to find that the band structure of g-C3N4 can be regulated and controlled by adjusting its structure. The B-CN/H-TiO2/BS-CN (CNTOCN) dual-type-II heterojunction photocatalyst and B-CN/H-TiO2 (CNTO) type-II heterojunction photocatalyst are designed to improve the separation efficiency of electron-hole pairs. The superiority of CNTOCN dual-type-II heterojunction photocatalyst is demonstrated by the photocatalysis experiment, the band structure analysis, and the photoelectric characterization. The results show that CNTOCN (0.8428 h-1) has much higher photocatalytic activity than H-TiO2 (0.0812 h-1), B-CN (0.3569 h-1), and CNTO (0.5934 h-1). The improvement of photocatalytic activity is ascribed to the establishment of the dual-type-II heterojunction charge transfer mechanism. This work presents an approach to designing efficient dual-type-II heterojunction photocatalysts for the sustainable conversion of solar energy to photodegrade dyes in dyeing wastewater.

Metainterface Heterostructure Enhances Sonodynamic Therapy for Disrupting Secondary Biofilms

Implant-related secondary infections are a challenging clinical problem. Sonodynamic therapy (SDT) strategies are promising for secondary biofilm infections by nonsurgical therapy. However, the inefficiency of SDT in existing acoustic sensitization systems limits its application. Therefore, we take inspiration from popular metamaterials and propose the design idea of a metainterface heterostructure to improve SDT efficiency. The metainterfacial heterostructure is defined as a periodic arrangement of heterointerface monoclonal cells that amplify the intrinsic properties of the heterointerface. Herein, we develop a TiO2/Ti2O3/vertical graphene metainterface heterostructure film on titanium implants. This metainterface heterostructure exhibits extraordinary sonodynamic and acoustic-to-thermal conversion effects under low-intensity ultrasound. The modulation mechanisms of the metainterface for electron accumulation and separation are revealed. The synergistic sonodynamic/mild sonothermal therapy disrupts biofilm infections (antibacterial rates: 99.99% for Staphylococcus aureus, 99.54% for Escherichia coli), and the osseointegration ability of implants is significantly improved in in vivo tests. Such a metainterface heterostructure film lays the foundation for the metainterface of manipulating electron transport to enhance the catalytic performance and holding promise for addressing secondary biofilm infections.

A H2O2 Oxidation Approach to Ti3C2/TiO2 for Efficient Photocatalytic Removal of Distinct Organic Pollutants in Water

To develop versatile photocatalysts for efficient degradation of distinct organic pollutants in water is a continuous pursuit in environment remediation. Herein, we directly oxidize Ti3C2 MXene with hydrogen peroxide to produce C-doped anatase TiO2 nanowires with aggregates maintaining a layered architecture of the MXene. The Ti3C2 MXene provides a titanium source for TiO2, a carbon source for in situ C-doping, and templates for nanowire aggregates. Under UV light illumination, the optimized Ti3C2/TiO2 exhibits a reaction rate constant 1.5 times that of the benchmark P25 TiO2 nanoparticles, toward photocatalytic degradations of trace phenol in water. The mechanism study suggests that photogenerated holes play key roles on the phenol degradation, either directly oxidizing phenol molecules or in an indirect way through oxidizing first the surface hydroxyl groups. The unreacted Ti3C2 MXene, although with trace amounts, is supposed to facilitate electron transfer, which inhibits charge recombination. The unique nanostructure of layered aggregates of nanowires, abundant surface oxygen vacancies arising from the carbon doping, and probably the Ti3C2/TiO2 heterojunction guarantee the high photocatalytic efficiency toward removals of organic pollutants in water. The photocatalyst also exhibits an activity superior to, or at least comparable to, the benchmark P25 TiO2 toward photodegradations for typical persistent organic pollutants of phenol, dye molecule of rhodamine B, antibiotic of tetracycline, pharmaceutical wastewater of ofloxacin, and pesticide of N,N-dimethylformamide, when evaluated in total organic carbon removal.

Self-healing of oxygen vacancies and phase transition-induced built-in electric field regulate H2 and H2O2 production

Oxygen vacancies in photocatalysts are an important parameter for improving their catalytic activities. However, this study found that oxygen vacancies do not always help with photocatalytic properties. MXene nanosheets were annealed at different temperatures to prepare nanosheets of titanium dioxide (TiO2) with anatase or rutile phase. Photo-induced self-healing of oxygen vacancies in MXene nanosheets annealed at 400 °C promoted the efficient photocatalytic evolution of hydrogen (H2) with a ∼ 6.8 mmol/g optimized yield. In contrast, the photo-induced increase of oxygen vacancies in MXene nanosheets annealed at 500 °C corresponded to an optimized ∼ 1.8 mmol/g/h yield of hydrogen peroxide (H2O2). Spectroscopic and electrochemical methods revealed that the built-in electric field between the anatase and rutile TiO2 facilitated the fast charge separation from the anatase to rutile TiO2. The photo-induced self-healing of oxygen vacancies and the phase transition-induced built-in electric field regulated the photocatalytic selectivity of H2 and H2O2 production.

Nanoflower-like graphitic carbon nitride aerogel: Artful cyanuric acid-controlled synthesis and enhanced photocatalytic hydrogen evolution activity

The previously reported studies on cyanuric acid-assembly strategy usually ignores the promoting function of cyanuric acid in the production of g-C3N4, limiting the development of molecular assembly strategies. In this study, a cyanuric acid-controlled synthesis strategy involving the pre-assembly of cyanuric acid with melamine and subsequent one-step calcination was developed to produce a three-dimensional (3D) nanoflower-like graphitic carbon nitride (g-C3N4) aerogel. Some cyanuric acid molecules underwent a polycondensation reaction with melamine during the pre-assembly process and finally polymerized into the g-C3N4 structure during subsequent calcination. Meanwhile, the remaining cyanuric acid molecules assembled with melamine via hydrogen-bond interactions and underwent incomplete decomposition during subsequent calcination, which not only promoted the production of 3D nanoflower-like aerogel structures, but also introduced the carbonyl (CO) and hydroxyl (-OH) groups onto the g-C3N4 surface, resulting in the successful generation of a 3D nanoflower-like oxygen-modified g-C3N4 aerogel. Moreover, the fabricated g-C3N4 aerogel exhibited a greatly enhanced H2 production rate (1573 μmol h-1 g-1), which is ∼ 6.6 times higher than that of bulk g-C3N4 (239 μmol h-1 g-1) owing to the synergistic promotion function of ultrathin nanoflower-like aerogel and oxygen modification structures. This strategy provides a theoretical basis for the development of highly efficient g-C3N4 photocatalysts via molecular assembly.

Deep Electronic State Regulation through Unidirectional Cascade Electron Transfer Induced by Dual Junction Boosting Electrocatalysis Performance

Unidirectional cascade electron transfer induced by multi-junctions is essential for deep electronic state regulation of the catalytic active sites, while this advanced concept has rarely been investigated in the field of electrocatalysis. In the present work, a dual junction heterostructure (FePc/L-R/CN) is designed by anchoring iron phthalocyanine (FePc)/MXene (L-Ti3 C2 -R, R═OH or F) heterojunction on g-C3 N4 nanosheet substrates for electrocatalysis. The unidirectional cascade electron transfer (g-C3 N4 → L-Ti3 C2 -R → FePc) induced by the dual junction of FePc/L-Ti3 C2 -R and L-Ti3 C2 -R/g-C3 N4 makes the Fe center electron-rich and therefore facilitates the adsorption of O2 in the oxygen reduction reaction (ORR). Moreover, the electron transfer between FePc and MXene is facilitated by the axial Fe─O coordination interaction of Fe with the OH in alkalized MXene nanosheets (L-Ti3 C2 -OH). As a result, FePc/L-OH/CN exhibits an impressive ORR activity with a half-wave potential (E1/2 ) of 0.92 V, which is superior over the catalysts with a single junction and the state-of-the-art Pt/C (E1/2 = 0.85 V). This work provides a broad idea for deep regulation of electronic state by the unidirectional cascade multi-step charge transfer and can be extended to other proton-coupled electron transfer processes.

Synergistic Promotion of Photocatalytic Degradation of Methyl Orange by Fluorine- and Silicon-Doped TiO2/AC Composite Material

The direct or indirect discharge of organic pollutants causes serious environmental problems and endangers human health. The high electron-hole recombination rate greatly limits the catalytic efficiency of traditional TiO2-based catalysts. Therefore, starting from low-cost activated carbon (AC), a photocatalyst (F-Si-TiO2/AC) comprising fluorine (F)- and silicon (Si)-doped TiO2 loaded on AC has been developed. F-Si-TiO2/AC has a porous structure. TiO2 nanoparticles were uniformly fixed on the surface or pores of AC, producing many catalytic sites. The band gap of F-Si-TiO2/AC is only 2.7 eV. In addition, F-Si-TiO2/AC exhibits an excellent adsorption capacity toward methyl orange (MO) (57%) in the dark after 60 min. Under the optimal preparation conditions, F-Si-TiO2/AC showed a significant photodegradation performance toward MO, reaching 97.7% after irradiation with visible light for 70 min. Even under the action of different anions and cations, its degradation efficiency is the lowest, at 64.0%, which has good prospects for practical application. At the same time, F-Si-TiO2/AC has long-term, stable, practical application potential and can be easily recovered from the solution. Therefore, this work provides new insights for the fabrication of low-cost, porous, activated, carbon-based photocatalysts, which can be used as high-performance photocatalysts for the degradation of organic pollutants.

MoSe2-NiSe dual co-catalysts modified g-C3N4 for enhanced photocatalytic H2 generation

Solar energy conversion into hydrogen (H₂) energy has attracted much attention. However, the low light utilization rate and fast carrier recombination of photocatalysts extremely limit the practical application of photocatalytic H₂ production. In this paper, MoSe₂-NiSe with abundant active sites and interfacial electronic structures as dual co-catalysts were assembled on g-C₃N₄ nanosheets (NSs) vis a solvothermal reaction process. MoSe₂-NiSe/g-C₃N₄ NSs composite exhibited improved light absorption and photoelectrochemical properties. The photocatalytic H₂ production rate of MoSe₂-NiSe/g-C₃N₄ composite achieved 2379.04 μmol·h^-1·g^-1, which is 99.25, 1.44, and 3.67 times those of pure g-C₃N₄ nanosheets (23.97 μmol·h^-1·g^-1), MoSe₂/C₃N₄ (1654.57 μmol·h^-1·g^-1), and NiSe/C₃N₄ (649.08 μmol·h^-1·g^-1), respectively. The apparent quantum efficiency (AQE) value of MoSe₂-NiSe/g-C₃N₄ achieved 4.07 % under light at λ = 370 nm. The corresponding characterization and experiments proved that 2D ultrathin g-C₃N₄ NSs with a large surface area and short charge-transfer distance could facilitate light scattering and the transport of photoexcited electrons. MoSe₂-NiSe, as a dual co-catalyst, showed strong electronic synergistic interaction between the interfaces, thus improving the conductivity and promoting the electron transfer process.

Enhanced pollutant photodegradation over nanoporous titanium-vanadium oxides with improved interfacial interactions

This study addressed the separation problem of colloidal catalytic powder from its solution and pore blockage of traditional metallic oxides by fabricating nanoporous composites of titanium (Ti)-vanadium (V) oxide via magnetron sputtering, electrochemical anodization, and annealing processes. The effect of V-deposited loading on the composite semiconductors was investigated by varying V sputtering power (20-250 W) to correlate their physicochemical properties to the photodegradation performance of methylene blue. The obtained semiconductors revealed circular and elliptical pores (14-23 nm) and formed different metallic and metallic oxide crystalline phases. Within the nanoporous composite layer, V ions substituted Ti⁴⁺, leading to Ti³⁺ formation accompanied by decreased band gap values and higher visible-light absorption. Thus, the band gap of TiO₂ was 3.15 eV, while that of Ti-V oxide with the maximum V content (at 250 W) was 2.47 eV. The interfacial separators between clusters in the mentioned composite created traps disrupting the charge carrier movements between crystallites, thereby decreasing the photoactivity. In contrast, the composite prepared with the minimum V content showed approximately 90% degradation efficiency under solar-simulated irradiation resulting from the homogeneous V dispersion and the lower recombination possibility, owing to its p-n heterojunction constituent. The nanoporous photocatalyst layers with their novel synthesis approach and outstanding performance can be applied in other environmental remediation applications.

Nanoarchitecture of a Ti3C2@TiO2 Hybrid for Photocatalytic Antibiotic Degradation and Hydrogen Evolution: Stability, Kinetics, and Mechanistic Insights

Designing of a visible-light-driven semiconductor-based heterojunction with suitable band alignment and well-defined interfacial contact is considered to be an effective strategy for the transformation of solar-to-chemical energy and environmental remediation. In this context, MXenes have received tremendous attention in the research community due to their merits of abundant derivatives, elemental composition, excellent metallic conductivity, and surface termination groups. Meanwhile, a facile synthetic strategy for MXene-derived TiO₂ nanocomposites with stable framework and higher photocatalytic activity under visible-light irradiation still remains a challenge for researchers. Herein, we report a novel synthetic strategy of preparing a two-dimensional Ti₃C₂@TiO₂ nanohybrid by a facile reflux method under acidic conditions. In this oxidation reaction, protonation of the hydroxyl terminal group of MXene creates Ti more electrophilic and susceptible to an oxidative nucleophilic addition reaction with the presence of both water and oxygen. The physicochemical properties of the nanohybrid Ti₃C₂@TiO₂ were verified by varieties of characterization techniques. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy analysis specifically elucidated the intimate interfacial interaction between Ti₃C₂ and TiO₂. The optimized Ti₃C₂@TiO₂-48 h photocatalyst exhibited the highest tetracycline hydrochloride (TCH, 90% in 90 min) degradation efficiency in comparison to pristine TiO₂ with a rate constant (k) of 0.02463 min^-1. The major contribution of ^•O₂^- and ^•OH radicals throughout photocatalytic TCH degradation was confirmed by the trapping experiment. Moreover, the photocatalyst showed the highest hydrogen generation rate of 140.8 μmol h^-1 along with an apparent conversion efficiency of 2.2%. The excellent photocatalytic activity of Ti₃C₂@TiO₂ originated from the superior electrical conductivity of cocatalyst Ti₃C₂, which facilitated spatial photogenerated e^-/h⁺ separation and transfer at the Ti₃C₂ MXene@TiO₂ interface. Overall, this research work will describe a promising protocol of designing MXene-derived photocatalysts toward efficient environmental remediation and wastewater treatment applications.

Potassium Poly(heptazine imide) Coupled with Ti3C2 MXene-Derived TiO2 as a Composite Photocatalyst for Efficient Pollutant Degradation

The photocatalytic degradation of pollutants is an effective and sustainable way to solve environmental problems, and the key is to develop an efficient, low-cost, and stable photocatalyst. Polymeric potassium poly(heptazine imide) (K-PHI), as a new member of the carbon nitride family, is a promising candidate but is characterized by a high charge recombination rate. To solve this problem, K-PHI was in-situ composited with MXene Ti₃C₂-derived TiO₂ to construct a type-II heterojunction. The morphology and structure of composite K-PHI/TiO₂ photocatalysts were characterized via different technologies, including TEM, XRD, FT-IR, XPS, and UV-vis reflectance spectra. Robust heterostructures and tight interactions between the two components of the composite were verified. Furthermore, the K-PHI/TiO₂ photocatalyst showed excellent activity for Rhodamine 6G removal under visible light illumination. When the weight percent of K-PHI in the original mixture of K-PHI and Ti₃C₂ was set to 10%, the prepared K-PHI/TiO₂ composite photocatalyst shows the highest photocatalytic degradation efficiency as high as 96.3%. The electron paramagnetic resonance characterization indicated that the^·OH radical is the active species accounting for the degradation of Rhodamine 6G.

Architecture and Kinetic Studies of Photocatalytic H2O2 Generation and H2 Evolution through Regulation of Spatial Charge Transfer via Z-Scheme Path over a (001) Facet Engineered TiO2@MXene/B-g-C3N4 Ternary Hybrid

Spatial charge separation and migration are the critical shortcomings dominating the core energy conversion corridors of photocatalytic systems. Here, a biomimetic multi-interfacial architecture providing strong coupled interaction and rapid charge transmission for photostable and competent photocatalytic H2O2 production and H2 evolution is proposed. The triple-hybrid all-solid-state Z-scheme system was formed with the (001) facet exposed TiO2 nanosheets derived from MXene layers and B-g-C3N4 nanosheets (M/(001)TiO2@BCN) through an electrostatic self-assembly strategy with intimate electronic interaction due to Ti orbital modulation and proper stacking among the hybrids. The metallic and highly conductive MXene layers act as solid state electron mediators in the Z-scheme heterojunction that promote electron-hole separation and migration efficiency. Specifically, the MTBCN-12.5 composite provides optimum yield of H2O2 up to 1480.1 μmol h-1 g-1 and a H2 evolution rate of 408.4 μmol h-1 (with ACE 6.7%), which are 4 and 20 fold greater than the pristine BCN, respectively. The enhanced photocatalytic performance is systematically identified by the increased surface area, higher cathodic and anodic current densities of -1.01 and 2.27 mA cm-2, delayed charge recombination as supported by PL and EIS measurement, and excellent photostability. The Z-scheme charge transfer mechanism is validated by time-resolved photoluminescence (TRPL) analysis, cyclic voltametric analysis, and the radical trapping experiment as detected by PL analysis. This research marks a substantial advancement and establishes the foundation for future design ideas in accelerating charge transfer.