Recent progression in MXene-based catalysts for emerging photocatalytic applications of CO2 reduction and H2 production: A review

The development of advanced materials for efficient photocatalytic H2 production and CO2 reduction is highly recommended for addressing environmental issues and producing clean energy sources. Specifically, MXenes have emerged as two-dimensional (2D) materials extensively used as high-performance cocatalysts in photocatalyst systems owing to their outstanding features of structure and properties such as high conductivity, large specific surface area, and abundant active sites. Nevertheless, there is a lack of deep and systematic studies concerning the application of these emerging materials for CO2 reduction reaction (CRR) and H2 production (HER). This review first outlines the essential features of MXenes, encompassing the synthesis methods, composition, surface terminations, and electronic properties, which make them highly active as cocatalysts. It then examines the recent progress in MXene-based photocatalysts, emphasizing the synergy achieved by coupling MXenes as co-catalysts with semiconductors, utilizing MXenes as a support for the consistent growth of photocatalysts, leading to finely dispersed nanoparticles, and exploiting MXene as exceptional precursors for creating MXene/metal oxide photocomposite. The roles of engineering surface terminations of MXene cocatalysts, MXene quantum dots (QDs), and distinctive morphologies in MXenes-based photocatalyst systems to enhance photocatalytic activity for both HER and CRR have been explored both experimentally and theoretically using DFT calculations. Challenges and prospects for MXene-based photocatalysts are also addressed. Finally, suggestions for further research and development of effective and economical MXenes/semiconductors strategies are proposed. This comprehensive review article serves as a valuable reference for researchers for applying MXenes in photocatalysis.

Phosphorus-Doped Hollow Tubular g-C3N4 for Enhanced Photocatalytic CO2 Reduction

Photocatalytic CO2 reduction is a tactic for solving the environmental pollution caused by greenhouse gases. Herein, NH4H2PO4 was added as a phosphorus source in the process of the hydrothermal treatment of melamine for the first time, and phosphorus-doped hollow tubular g-C3N4 (x-P-HCN) was fabricated and used for photocatalytic CO2 reduction. Here, 1.0-P-HCN exhibited the largest CO production rate of 9.00 μmol·g-1·h-1, which was 10.22 times higher than that of bulk g-C3N4. After doping with phosphorus, the light absorption range, the CO2 adsorption capacity, and the specific surface area of the 1.0-P-HCN sample were greatly improved. In addition, the separation of photogenerated electron-hole pairs was enhanced. Furthermore, the phosphorus-doped g-C3N4 effectively activated the CO2 adsorbed on the surface of phosphorus-doped g-C3N4 photocatalysts, which greatly enhanced the CO production rate of photocatalytic CO2 reduction over that of g-C3N4.

Fermi-level shift, electron separation, and plasmon resonance change in Ag nanoparticle-decorated TiO2 under UV light illumination

Noble metal nanoparticles are widely used as co-catalysts for storing and separating electrons in semiconductor photocatalysis. Thus, evaluating this ability is important and meaningful to understand the photocatalytic mechanism. Employing Ag nanoparticles, the present study combined in situ photoconductance and theoretical analysis to evaluate the Fermi-level (E_F) shift in a UV-illuminated Ag/TiO₂ system under gaseous conditions. Based on this, the role of the Ag nanoparticles in storing and separating electrons was discussed. It was found that the E_F of Ag/TiO₂ is located deeper in the gap and a variation in temperature has less effect on the E_F of Ag/TiO₂ compared to the undecorated TiO₂. The analysis showed that ∼46 electrons can be stored in 10 nm Ag nanoparticles under our experimental conditions, which does not change with temperature. The electron traps in TiO₂ can affect the electron distribution in the TiO₂ and Ag nanoparticles. It was observed that the localized surface plasmon resonance (LSPR) of the Ag nanoparticles exhibited a blue-shift under UV light illumination, which is generally ascribed to the electron storage in the Ag nanoparticles. However, we showed that the blue-shift is not related to the electron storage in the Ag nanoparticles, and thus it cannot be used as an indicator for evaluating their electron-storage ability. The in situ XPS analysis also does not support that the LSPR blue shift is associated with the reduction in the Ag₂O layer and TiO₂.

2D VS2 @MXene Based Zinc Ion Batteries with SPANI-Contained Electrolyte Enables Dendrite-Free Anode for Stable Cycling

Regarded as one of the popular cathode materials in aqueous zinc ion batteries (ZIBs), VS₂ has unsatisfied cycling stability and relatively low capacity owing to its poor conductivity and low mechanical properties. To this regard, compositing VS₂ with high-conductive 2D transition metal carbide (MXene) has been an effective method recently. However, the Zn dendrite on the anode electrode derived from the uncontrollable sluggish migration of solvated Zn²⁺ /H₂ O ions seriously threatens the application safety of ZIB batteries. To effectively regulate the diffusion of zinc ions, in this work a conductive polymeric electrolyte of sulfonated polyaniline (SPANI) is added in the electrolyte solution. Under the Zn²⁺ /SPANI interactions confirmed by X-ray diffraction, Raman, and zeta potential experiments, the Zn²⁺ /H₂ O combination is weakened, and the deposition rate of Zn²⁺ is increased evaluated by the galvanostatic intermittent titration technique. Theoretical simulation shows that the electrostatic shielding by SPANI combining Zn^2- at the zinc/electrolyte interface has important contribution to the significant suppression of Zn dendrite. Accordingly, the fabricated VS₂ @MXene||ZnSO₄ +SPANI||Zn battery shows high capacity (368.0 mAh g^-1 at 0.1 A g^-1 ), which remains 96% after 5000 cyclic charge-discharge operations. This work develops an available strategic idea for suppressing growth of metallic dendrites to improve the ZIB performances.

Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

Suppressing the amount of carbon dioxide in the atmosphere is an essential measure toward addressing global warming. Specifically, the photocatalytic CO₂ reduction reaction (CRR) is an effective strategy because it affords the conversion of CO₂ into useful carbon feedstocks by using sunlight and water. However, the practical application of photocatalyst-promoting CRR (CRR photocatalysts) requires significant improvement of their conversion efficiency. Accordingly, extensive research is being conducted toward improving semiconductor photocatalysts, as well as cocatalysts that are loaded as active sites on the photocatalysts. In this review, we summarize recent research and development trends in the improvement of cocatalysts, which have a significant impact on the catalytic activity and selectivity of photocatalytic CRR. We expect that the advanced knowledge provided on the improvement of cocatalysts for CRR in this review will serve as a general guideline to accelerate the development of highly efficient CRR photocatalysts.

TiO2 Nanoparticle/Polyimide Nanocomposite for Ultrahigh-Temperature Energy Storage

With the development of electronic technology, there is an increasing demand for high-temperature dielectric energy storage devices based on polyimides for a wide range of applications. However, the current nanofillers/PI nanocomposites are used for energy harvesting at no more than 200 °C, which does not satisfy the applications in the oil and gas, aerospace, and power transmission industries that require an operating temperature of 250-300 °C. Therefore, we introduced a nanocomposite based on nonsolid TiO₂ nanoparticles and polyimide (PI) with high energy storage performance at an ultrahigh temperature of 300 °C. The synergy of excellent dielectric properties and a high breakdown strength endowed the nanocomposite with a low loading content of 1 wt% and a high energy storage density of 5.09 J cm^-3. Furthermore, we found that the nanocomposite could stably operate at 300 °C with an outstanding energy storage capability (2.20 J cm^-3). Additionally, finite element simulations demonstrated that the partially hollow nanostructures of the nanofillers avoided the evolution of breakdown paths, which optimized the breakdown strength and energy storage performance of the related nanocomposites. This paper provides an avenue to broaden the application areas of PI-based nanocomposites as ultrahigh-temperature energy-storage devices.

A Review on MXene Synthesis, Stability, and Photocatalytic Applications

Photocatalytic water splitting, CO₂ reduction, and pollutant degradation have emerged as promising strategies to remedy the existing environmental and energy crises. However, grafting of expensive and less abundant noble-metal cocatalysts on photocatalyst materials is a mandatory practice to achieve enhanced photocatalytic performance owing to the ability of the cocatalysts to extract electrons efficiently from the photocatalyst and enable rapid/enhanced catalytic reaction. Hence, developing highly efficient, inexpensive, and noble-metal-free cocatalysts composed of earth-abundant elements is considered as a noteworthy step toward considering photocatalysis as a more economical strategy. Recently, MXenes (two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides) have shown huge potential as alternatives for noble-metal cocatalysts. MXenes have several excellent properties, including atomically thin 2D morphology, metallic electrical conductivity, hydrophilic surface, and high specific surface area. In addition, they exhibit Gibbs free energy of intermediate H atom adsorption as close to zero and less than that of a commercial Pt-based cocatalyst, a Fermi level position above the H₂ generation potential, and an excellent ability to capture and activate CO₂ molecules. Therefore, there is a growing interest in MXene-based photocatalyst materials for various photocatalytic events. In this review, we focus on the recent advances in the synthesis of MXenes with 2D and 0D morphologies, the stability of MXenes, and MXene-based photocatalysts for H₂ evolution, CO₂ reduction, and pollutant degradation. The existing challenges and the possible future directions to enhance the photocatalytic performance of MXene-based photocatalysts are also discussed.

Boosted CO2 photoreduction performance on Ru-Ti3CN MXene-TiO2 photocatalyst synthesized by non-HF Lewis acidic etching method

Photocatalytic CO₂ reduction to produce value-added products is considered a promising solution to solve the global energy crisis and the greenhouse effect. In this study, Ti₃CN MXene was synthesized using a Lewis acidic etching method without the usage of toxic hydrofluoric acid (HF). Ti₃CN MXene was then used as a support for the in situ hydrothermal growth of TiO₂ and Ru nanoparticles. In the presence of 0.5 wt% Ru, Ru-Ti₃CN-TiO₂ shows CO and CH₄ production rates of 99.58 and 8.97 μmol/g, respectively, in 5 h under Xenon lamp irradiation, more than 20.5 and 9.3 times that of commercial P25. The enhancement in photocatalytic activity was attributed to the synergy between the in-situ growth of TiO₂ on Ti₃CN MXene and Ru nanoparticles. It was proven experimentally that Ti₃CN MXene can provide abundant pathways for electron transfer. The separation and transfer of the photo-induced charge were further increased with the help of Ru and Ti₃CN MXene, leaving more electrons to participate in the subsequent CO₂ reduction reaction. We believe that this work will encourage more attention to designing environment-friendly MXene-based photocatalysts for CO₂ photoreduction using the non-HF method.

Highly Sensitive Photothermal Fiber Sensor Based on MXene Device and Vernier Effect

A photothermal fiber sensor based on a microfiber knot resonator (MKR) and the Vernier effect is proposed and demonstrated. An MXene Ti₃C₂T_x nanosheet was deposited onto the ring of an MKR using an optical deposition method to prepare photothermal devices. An MXeneMKR and a bare MKR were used as the sensing part and reference part, respectively, of a Vernier-cascade system. The optical and photothermal properties of the bare MKR and the MXeneMKR were tested. Ti₃C₂T_x was applied to a photothermal fiber sensor for the first time. The experimental results showed that the modulation efficiency of the MXeneMKR was 0.02 nm/mW, and based on the Vernier effect, the modulation efficiency of the cascade system was 0.15 nm/mW. The sensitivity was amplified 7.5 times. Our all-fiber photothermal sensor has many advantages such as low cost, small size, and good system compatibility. Our sensor has broad application prospects in many fields. The proposed stable MKR device based on two-dimensional-material modification provides a new solution for improving the sensitivity of optical fiber sensors.

In Situ Self-Assembled ZIF-67/MIL-125-Derived Co3O4/TiO2 p-n Heterojunctions for Enhanced Photocatalytic CO2 Reduction

Photocatalytic CO₂ reduction to carbon fuels is regarded as an ideal and sustainable way to provide clean energy and solve environmental crisis. Herein, a p-n Co₃O₄/TiO₂ heterojunction photocatalyst was synthesized by one-step pyrolysis of self-assembly ZIF-67/MIL-125, which was used in photocatalytic CO₂ reduction for the first time. Co₃O₄ nanocages were highly dispersed on the surface of TiO₂ nanoplates with an intimate contact. The optimal Co₃O₄/TiO₂ exhibited a significantly enhanced CO evolution rate of 1256 μmol g^-1 h^-1 under simulated solar light, which was 2.4 times higher than that of pure Co₃O₄. The high photocatalytic performance of Co₃O₄/TiO₂ was attributed to its enriched active sites and formed p-n heterojunctions. According to the electrocatalytic measurements, the possible mechanism and photoinduced charge transfer process were discussed in detail. We believe that this research provides a facile and efficient approach to fabricate MOF-derived heterojunction photocatalysts for CO₂ reduction.

0D/2D Co0.85Se/TiO2 p–n heterojunction for enhanced photocatalytic H2 evolution

The H2 rate of 15%-Co0.85Se/TiO2 is 2312.5 μmol g−1 h−1, which is 10.3 times and 10.8 times higher than TiO2 and Co0.85Se. The enhanced activity is attributed to the higher electrochemically active surface area and the formation of p–n heterostructure.

Coke-resistant Ni-based bimetallic catalysts for the dry reforming of methane: effects of indium on the Ni/Al2O3 catalyst

In the quest for highly efficient coke-resistant catalysts for the dry reforming of methane (DRM) to produce syngas, a series of Ni–In/γ-Al2O3 catalysts with various Ni contents were prepared via a “two-solvent” method and used for the DRM reaction.

A surface-alkalinized Ti3C2 MXene as an efficient cocatalyst for enhanced photocatalytic CO2 reduction over ZnO

Surface alkalinized Ti₃C₂ MXene with high electronic conductivity and CO₂ adsorption/activation ability is used as an efficient co-catalyst for boosting the photocatalytic activity of ZnO for CO₂ reduction into hydrocarbon solar fuels.

Pyrochlore oxides as visible light-responsive photocatalysts

This perspective describes the use of pyrochlore oxides in photocatalysis with focus on the strategies to enhance their activity.

Photocatalytic CO2 reduction to CH4 on iron porphyrin supported on atomically thin defective titanium dioxide

The synergistic effect of OVs and FeTPP on 2D TiO2 improves the efficiency and selectivity of CO2 photoreduction to CH4.