In situ growth of TiO2 nanoparticles on nitrogen-doped Ti3C2 with isopropyl amine toward enhanced photocatalytic activity

In-situ assembling novel N-Ti3C2/BiOClxBr1-x composites with enhanced photocatalytic degradation and nitrogen reduction activity

Herein, a collection of novel N-Ti3C2/BiOClxBr1-x composites are fabricated via a simple in-situ sonochemical process. Not only the preparation method for N-Ti3C2 but also the photocatalytic system of N-Ti3C2/BiOClxBr1-x are firstly developed. Multiple characterizations jointly demonstrate the successful fabrication of the composites. Compared to that of BiOClxBr1-x, the maximum improvements of 1.16, 1.25 and 1.26 folds are severally confirmed for the photocatalytic degradation of levofloxacin, Rhodamine B, and methylene blue over N-Ti3C2/BiOClxBr1-x composites. In addition, through radicals trapping tests, the primary active species in photocatalytic degradation process are verified to be O2-. Moreover, N-Ti3C2/BiOClxBr1-x composites also exhibit 1.18 and 1.14 times enhancements for NH3 production compared with that of BiOClxBr1-x with or without the presence of methanol, respectively. In addition, the maximum improvements of photo-current and photo-potential for BiOClxBr1-x are 1.29 and 1.86 folds with the introduction of N-Ti3C2, respectively. The enhanced photocatalytic activity of N-Ti3C2/BiOClxBr1-x composites is owing to the heightened light absorption, increased specific surface area, and accelerated separation of photoinduced carriers. Additionally, the stable photocatalytic properties of N-Ti3C2/BiOClxBr1-x are confirmed by three photocatalytic recycle tests on pollutant degradation and nitrogen reduction combined with X-ray diffraction patterns before and after three recycles. This study suggests that N-Ti3C2 is an efficient ornamentation for boosting photocatalytic activity ofBiOClxBr1-x, which can also be expanded as a promising modifier for other semiconductors.

N-doped Ti3C2-reinforced porous g-C3N4 for photocatalytic contaminants degradation and nitrogen reduction

Herein, a series of N-doped Ti3C2/porous g-C3N4 composites are ultrasonically prepared from N-doped Ti3C2 and porous g-C3N4 under N2 atmosphere. The structure, morphology, and optical characteristics of the as-prepared composites are characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, etc. Moreover, photocatalytic measurements show that N-doped Ti3C2 is an excellent modifier for porous g-C3N4 to heighten its photocatalytic activity. Only 44.1% of rhodamine B can be degraded by the photocatalysis of pristine porous g-C3N4, while the photocatalytic degradation ratio of rhodamine B can reach up to 97.5% for the optimal N-doped Ti3C2 loading composites under visible light for 15 min. Moreover, the photocatalytic tests of N2 fixation confirm that the optimal composites show the highest production yield of NH4+ (11.8 μmol gcat-1 h-1), which is 2.11-folds more than that of porous g-C3N4 (5.6 μmol gcat-1 h-1). The reinforced photocatalytic properties are revealed to profit from the more photogenerated electrons and holes' separation, higher ability for light response, and more abundant active sites. This work develops the route for boosting the photocatalytic properties of porous g-C3N4 with N-doped Ti3C2.

Pillaring Behavior of Organic Molecules on MXene: Insights from Molecular Dynamics Simulations

Pillaring MXene with organic molecules is an effective approach to expand the interlayer spacing and increase the accessible surface area for enhanced performance in energy storage applications. Herein, molecular dynamics simulations are employed to explore the pillaring effect of six organic molecules on Ti3C2O2. The interlayer spacing and structural characteristics of MXene after the insertion of different organic molecules are examined, and the influence of the type and quantity of organic molecules on the pillared MXene structure is systematically investigated. The results demonstrate that the inserted molecules are influenced by interactions between MXene layers, resulting in a thinner morphology. Effective pillar support on MXene is achieved only when a specific quantity of organic molecules is inserted between the layers. Furthermore, different organic molecules occupy distinct surface areas on MXene when acting as pillars. Pillaring molecules with a Pi-conjugated ring structure require a larger surface area on MXene, whereas those with a branched structure occupy a smaller surface area. Additionally, organic molecules containing oxygen functional groups tend to aggregate due to hydrogen bonding, impeding their diffusion within MXene sheets. Considering the interlayer expansion of MXene, surface area occupation, and diffusion characteristics, the isopropylamine demonstrates the most favorable pillaring effect on MXene. These findings provide valuable insights into the design and application of pillared MXenes in energy storage and other applications. Further studies on the properties and applications of the optimized pillared MXene structures will be conducted in the future.

Photocatalytic Activity of the Oxidation Stabilized Ti3 C2 Tx MXene in Decomposing Methylene Blue, Bromocresol Green and Commercial Textile Dye

Two-dimensional MXenes are excellent photocatalysts. However, their low oxidation stability makes controlling photocatalytic processes challenging. For the first time, this work elucidates the influence of the oxidation stabilization of model 2D Ti3 C2 Tx MXene on its optical and photocatalytic properties. The delaminated MXene is synthesized via two well-established approaches: hydrofluoric acid/tetramethylammonium hydroxide (TMAOH-MXene) and minimum intensive layer delamination with hydrochloric acid/lithium fluoride (MILD-MXene) and then stabilized by L-ascorbic acid. Both MXenes at a minimal concentration of 32 mg L-1 show almost 100% effectiveness in the 180-min photocatalytic decomposition of 25 mg L-1 model methylene blue and bromocresol green dyes. Industrial viability is achieved by decomposing a commercial textile dye having 100 times higher concentration than that of model dyes. In such conditions, MILD-MXene is the most efficient due to less wide optical band gap than TMAOH-MXene. The MILD-MXene required only few seconds of UV light, simulated white light, or 500 nm (cyan) light irradiation to fully decompose the dye. The photocatalytic mechanism of action is associated with the interplay between surface dye adsorption and the reactive oxygen species generated by MXene under light irradiation. Importantly, both MXenes are successfully reused and retained approximately 70% of their activity.

Ti4+-dopamine/sodium alginate multicomponent complex derived N-doped TiO2@carbon nanocomposites for efficient removal of methylene blue

A one-pot route for the preparation of TiO₂@carbon nanocomposite from Ti⁴⁺/polysaccharide coordination complex has been developed and shown advantages in operation, cost, environment, etc. However, the photodegradation rate of methylene blue needs to be improved. N-doping has been proven as an efficient means to enhance photodegradation performance. Thus, the present study upgraded the TiO₂@carbon nanocomposite to N-doped TiO₂@carbon nanocomposite (N-TiO₂@C) from Ti⁴⁺-dopamine/sodium alginate multicomponent complex. The composites were characterized by FT-IR, XRD, XPS, UV-vis DRS, TG-DTA, and SEM-EDS. TiO₂ was a typical rutile phase, and the carboxyl groups existed on N-TiO₂@C. The photocatalyst consequently showed high removal efficiency of methylene blue (MB). The cycling experiment additionally indicated the high stability of N-TiO₂@C. The present work provided a novel route for preparing N-TiO₂@C. Moreover, it can be extended to prepare N-doped polyvalent metal oxides@carbon composites from all water-soluble polysaccharides such as cellulose derivatives, pectin, starch, and guar gum.

In situ oxidized TiO2/MXene ultrafiltration membrane with photocatalytic self-cleaning and antibacterial properties

Self-cleaning, antimicrobial ultrafiltration membranes are urgently needed to alleviate the low flux problems caused by membrane fouling in water treatment processes. In this study, in situ generated nano-TiO₂ MXene lamellar materials were synthesized and then 2D membranes were fabricated using vacuum filtration. The presence of nano TiO₂ particles as an interlayer support layer widened the interlayer channels, and also improved the membrane permeability. The TiO₂/MXene composite on the surface also showed an excellent photocatalytic property, resulting in enhanced self-cleaning properties and improved long-term membrane operational stability. The best overall performance of the TiO₂/MXene membrane at 0.24 mg cm^-2 loading was optimal, with 87.9% retention and 211.5 L m^-2 h^-1 bar^-1 flux at a filtration of 1.0 g L^-1 bovine serum albumin solution. Noticeably, the TiO₂/MXene membranes showed a very high flux recovery under UV irradiation with a flux recovery ratio (FRR) of 80% as compared to the non-photocatalytic MXene membranes. Moreover, the TiO₂/MXene membranes demonstrated over 95% resistance against E. coli. And the XDLVO theory also showed that the loading of TiO₂/MXene slowed down the fouling of the membrane surface by protein-based contaminants.

A dual S-scheme TiO2@In2Se3@Ag3PO4 heterojunction for efficient photocatalytic CO2 reduction

The excellent photoresponse of semiconductors enables them to be promising photocatalysts for CO₂ reduction, but practical application is hampered by fast recombination of photogenerated carriers, low CO₂ capture capacity and poor stability. Herein, mesoporous hollow nanospheres of a dual S-scheme titanium dioxide@indium selenide@silver phosphate (TiO₂@In₂Se₃@Ag₃PO₄) heterojunction with a large specific surface area are designed and synthesized. The products of photocatalytic CO₂ reduction are CH₄, CH₃OH and CO with yields of 3.98, 4.32 and 8.2 μmol g^-1 h^-1, respectively, and the photocatalysts exhibit excellent cycle performance. The excellent photocatalytic performance is attributed to the large specific surface area of the samples and the construction of dual S-scheme heterojunctions. The large specific surface area can provide sufficient active sites for photocatalytic activity. Simultaneously, the built-in electric field (IEF) in the dual S-scheme exposed to light can facilitate the migration of photogenerated electrons from the CB of the oxidation photocatalyst (OP) to the VB of the reduction semiconductor (RP), where they recombine with the photogenerated holes on the VB of the RP, leaving behind photogenerated carriers with high redox ability for photocatalytic activity. This work provides new insights into the mechanism and design of highly efficient heterojunction photocatalysts for CO₂ reduction.

Magnetic Ti3C2 MXene Nanomaterials for Doxorubicin Adsorption from Aqueous Solutions: Kinetic, Isotherms, and Thermodynamic Studies

In this work, the magnetic Ti₃C₂ MXene functionalized with β-cyclodextrin was prepared and characterized using scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectra, X-ray diffraction, X-ray photoelectron spectroscopy, vibrating sample magnetometry, and thermogravimetric analysis. The synthesized nanomaterial was used as an adsorbent to adsorb doxorubicin from aqueous solutions, and the experimental parameters that affected the adsorption efficiency were investigated. In addition, the adsorption characteristics including adsorption kinetics, adsorption isotherm, and thermodynamics were researched comprehensively. The adsorption kinetics of doxorubicin followed a pseudo-second-order kinetic model, which indicated that adsorption was the rate-limiting step, and the maximum adsorption capacity was 7.35 μg mg^-1 by shaking for 60 min at pH 7.0. The adsorption isotherm was well described using the Freundlich model, which implied that multilayer adsorption took place over the prepared nanomaterial for doxorubicin adsorption. The negative values of Gibbs free energy change (ΔG ⁰ < 0) demonstrated that doxorubicin adsorption was a spontaneous process. The positive values of entropy change (ΔS ⁰ > 0) implied that doxorubicin adsorption was an increasing random process. Enthalpy change values were positive (ΔH ⁰ > 0) and indicated that the adsorption of doxorubicin was endothermic. The adsorption percentage of doxorubicin remained in the range of 41.05-44.09%, and the relative standard deviation (RSD) based on the adsorption percentage through five replicate adsorption and desorption processes was 2.8%. These results indicated that the magnetic Ti₃C₂ MXene nanomaterials can be an effective adsorbent to adsorb DOX from aqueous solutions.

One-pot in-situ hydrothermal synthesis of ternary In2S3/Nb2O5/Nb2C Schottky/S-scheme integrated heterojunction for efficient photocatalytic hydrogen production

MXene-derived photocatalysts continue to fascinate the research community in developing photo-driven green and sustainable fuel production. However, the efficiency of MXene-derived photocatalyst is still low due to the wide bandgap and high recombination rate of photo-excited charge carriers. Here, we have synthesized the Nb₂C MXene-derived ternary photocatalyst via one-pot in-situ hydrothermal method for photocatalytic hydrogen (H₂) evolution. The partial oxidation of Nb₂C MXene into Nb₂O₅ nanorods and coupling with In₂S₃ nanoparticles via in-situ chemical anchoring were the key factors toward high efficiency and long-term stability during photocatalytic H₂ evolution. The optimized ternary photocatalyst composite manifested the highest H₂ evolution efficiency at 68.8 µmol g^-1 h^-1, which was 11 and 7.5 times higher than the Nb₂O₅/Nb₂C (NNC) and pure In₂S₃ photocatalyst, respectively. Moreover, the photocatalytic stability of the optimized ternary photocatalyst composite was analyzed for five consecutive cycles, and above 87% activity retention was observed even after the fifth cycle without any obvious decline. The separation efficiency of photoexcited charge carriers could be attributed to the synergic effect of the In₂S₃/Nb₂O₅ heterojunction and the redox reactions at different sites of the composite. More importantly, the participation of Schottky junction and S-scheme heterojunction charge transfer for the obtained novel ternary photocatalyst was evaluated via ultraviolet photoelectron spectroscopy (UPS) and electron paramagnetic resonance (EPR). This research will provide additional insight into the extended potential of MXene-derived ternary photocatalysts towards efficient H₂ production to meet future global energy demands.

Two-Dimensional MXene-Originated In Situ Nanosonosensitizer Generation for Augmented and Synergistic Sonodynamic Tumor Nanotherapy

Despite the merits of high tissue-penetrating depth, no ionizing radiation, and low cost, sonodynamic therapy (SDT) still suffers from a low quantum yield of reactive oxygen species (ROS), limited delivery efficiency, and potential toxicity of sonosensitizers. Different from the direct delivery of sonosensitizers into tumor tissue for SDT, this work reports the fabrication of two-dimensional (2D) nanosonosensitizers/nanocatalysts (Ti₃C₂/CuO₂@BSA) for the in situ generation of nanosonosensitizers by responding to the tumor microenvironment, achieving the high-performance and synergistic sonodynamic/chemodynamic tumor therapy. CuO₂ nanoparticle integration on 2D Ti₃C₂ MXene achieved in situ H₂O₂ generation in an acidic tumor microenvironment for oxidizing Ti₃C₂ to produce TiO₂ nanosonosensitizers, accompanied by the enhanced separation of electrons (e^-) and holes (h⁺) by the carbon matrix after oxidation, further augmenting the SDT efficacy. Ultrasound irradiation during the sonodynamic process also enhanced the Cu-initiated Fenton-like reaction to produce more ROS for synergizing the sonodynamic tumor therapy. The experimental results confirm and demonstrate the synergistic therapeutic effects of chemodynamic and sonodynamic nanotherapy both in vitro and in vivo. The antitumor mechanisms of synergistic chemodynamic and sonodynamic therapies are associated with the upregulation of oxidative phosphorylation, ROS generation, and apoptosis as demonstrated by RNA sequencing. This work thus provides a distinct paradigm of 2D MXene-originated in situ nanosonosensitizer generation for augmented and synergistic sonodynamic tumor nanotherapy.

Element-Doped Mxenes: Mechanism, Synthesis, and Applications

Heteroatom doping can endow MXenes with various new or improved electromagnetic, physicochemical, optical, and structural properties. This greatly extends the arsenal of MXenes materials and their potential for a spectrum of applications. This article comprehensively and critically discusses the syntheses, properties, and emerging applications of the growing family of heteroatom-doped MXenes materials. First, the doping strategies, synthesis methods, and theoretical simulations of high-performance MXenes materials are summarized. In order to achieve high-performance MXenes materials, the mechanism of atomic element doping from three aspects of lattice optimization, functional substitution, and interface modification is analyzed and summarized, aiming to provide clues for developing new and controllable synthetic routes. The mechanisms underlying their advantageous uses for energy storage, catalysis, sensors, environmental purification and biomedicine are highlighted. Finally, future opportunities and challenges for the study and application of multifunctional high-performance MXenes are presented. This work could open up new prospects for the development of high-performance MXenes.

Recent Advances in Oxidation Stable Chemistry of 2D MXenes

As an emerging star of 2D nanomaterials, 2D transition metal carbides and nitrides, named MXenes, present a large potential in various research areas owing to their intrinsic multilayer structure and intriguing physico-chemical properties. However, the fabrication and application of functional MXene-based devices still remain challenging as they are prone to oxidative degradation under ambient environment. Within this review, the preparation methods of MXenes focusing on the recent investigations on their thermal structure-stability relationships in inert, oxidizing, and aqueous environments are systematically introduced. Moreover, the key factors that affect the oxidation of MXenes, such as, atmosphere, temperature, composition, microstructure, and aqueous environment, are reviewed. Based on different scenarios, strategies for avoiding or delaying the oxidation of MXenes are proposed to encourage the utilization of MXenes in complicated environments, especially at high temperature. Furthermore, the chemistry of MXene-derived oxides is analyzed, which can offer perspectives on the further design and fabrication of novel 2D composites with the unique structures of MXenes being preserved.

A Literature Review of Modelling and Experimental Studies of Water Treatment by Adsorption Processes on Nanomaterials

A significant growth in the future demand for water resources is expected. Hence researchers have focused on finding new technologies to develop water filtration systems by using experimental and simulation methods. These developments were mainly on membrane-based separation technology, and photocatalytic degradation of organic pollutants which play an important role in wastewater treatment by means of adsorption technology. In this work, we provide valuable critical review of the latest experimental and simulation methods on wastewater treatment by adsorption on nanomaterials for the removal of pollutants. First, we review the wastewater treatment processes that were carried out using membranes and nanoparticles. These processes are highlighted and discussed in detail according to the rate of pollutant expulsion, the adsorption capacity, and the effect of adsorption on nanoscale surfaces. Then we review the role of the adsorption process in the photocatalytic degradation of pollutants in wastewater. We summarise the comparison based on decomposition ratios and degradation efficiency of pollutants. Therefore, the present article gives an evidence-based review of the rapid development of experimental and theoretical studies on wastewater treatment by adsorption processes. Lastly, the future direction of adsorption methods on water filtration processes is indicated.

Theoretical Design and Structural Modulation of a Surface-Functionalized Ti3C2Tx MXene-Based Heterojunction Electrocatalyst for a Li-Oxygen Battery

Two-dimensional MXene with high conductivity has metastable Ti atoms and inert functional groups on the surface, greatly limiting application in surface-related electrocatalytic reactions. A surface-functionalized nitrogen-doped two-dimensional TiO₂/Ti₃C₂T_x heterojunction (N-TiO₂/Ti₃C₂T_x) was fabricated theoretically, with high conductivity and optimized electrocatalytic active sites. Based on the conductive substrate of Ti₃C₂T_x, the heterojunction remained metallic and efficiently accelerated the transfer of Li⁺ and electrons in the electrode. More importantly, the precise regulation of active sites in the N-TiO₂/Ti₃C₂T_x heterojunction optimized the adsorption for LiO₂ and Li₂O₂, facilitating the sluggish kinetics with a lowest theoretical overpotential in both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Employed as an electrocatalyst in a Li-oxygen battery (Li-O₂ battery), it demonstrated a high specific capacity of 15 298 mAh g^-1 and a superior cyclability with more than 200 cycles at 500 mA g^-1, as well as the swiftly reduced overpotential. Furthermore, combined with the in situ differential electrochemical mass spectrometry, ex situ Raman spectra, and SEM tests, the N-TiO₂/Ti₃C₂T_x heterojunction electrode presented a superior stability and reduced side reaction along with the high performance toward the ORR and OER. It provides an efficient insight for the design of high-performance electrocatalysts for metal-oxygen batteries.

In-situ synthesis of N-doped TiO2 onto Ti3C2 MXene with enhanced photocatalytic activity in selective reduction of nitrate to N2

Photocatalysis exhibited promises in the reduction of nitrates into harmless dinitrogen. Herein, the synthesis of N-doped TiO2/Ti3C2 at low calcination temperature using NH4Cl and Ti3C2 was reported for the first...

Highly efficient piezoelectric field enhanced photocatalytic performance via in situ formation of BaTiO3 on Ti3C2Tx for phenolic compound degradation

Novel BaTiO3/Ti3C2Tx piezo-photocatalysts are fabricated via an in-situ solvothermal method. The synergistic effect of BaTiO3 and Ti3C2Tx increases piezo-photocatalytic activity.

Enhanced photocatalytic activities of a hierarchical ZnO/V2C MXene hybrid with a close coupling heterojunction for the degradation of methyl orange, phenol and methylene blue dye

A hierarchical ZnO/V2C MXene hybrid exhibited enhanced photocatalytic performance due to its close coupling heterojunction facilitating photo-generated carrier transfer.

Continuous photocatalysis via photo-charging and dark-discharging for sustainable environmental remediation: Performance, mechanism, and influencing factors

Continuous photocatalysis via photo-charging and dark-discharging presents a paradigm shift in conventional photocatalysis with the requirement of continuous illumination to maintain the catalytic activity. This is expected to meet the ever-increasing demand for sustainable development of energy and environment driven by natural day-night cycles. Substantial advances in continuous photocatalysis for various environmental applications under light-dark cycles have been witnessed during the last decade. However, there lacks a systematic and critical review on basic but important information of continuous photocatalysis for environmental remediation, challenging robust scientific progress of this technology towards potential practical use. Here, the general description of continuous photocatalysis involving energy storage mechanisms (hole and electron storage) and characterizations (electron storage behaviors, release behaviors and storage capacity) has been first introduced. Importantly, the remediation performance and mechanism of continuous photocatalysis for environmental applications are qualitatively and quantitatively demonstrated, including chemical pollutant oxidation and reduction, microbial pathogen inactivation, and multifunctional treatment. In addition, key factors influencing its remediation performance are analyzed, for the first time, from both operational and environmental views. The ample opportunities in the field of continuous photocatalysis for sustainable environmental remediation are also pointed out, calling for more efforts to fill current knowledge gaps in the future.