Cocatalysts from types, preparation to applications in the field of photocatalysis

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.

Synchrotron Radiation Based X-ray Absorption Spectroscopy: Fundamentals and Applications in Photocatalysis

Photocatalysis is one of the most promising green technologies to utilize solar energy for clean energy achievement and environmental governance. There is a knotty problem to rational designing high-performance photocatalyst, which largely depends on an in-depth insight into their structure-activity relationships and complex photocatalytic reaction mechanisms. Synchrotron radiation based X-ray absorption spectroscopy (XAS) is an important characterization method for photocatlayst to offer the element-specific key geometric and electronic structural information at the atomic level, on this basis, time-resolved XAS technique has a huge impact on mechanistic understanding of photochemical reaction owing to their powerful ability to probe, in real-time, the electronic and geometric structures evolution within photocatalysis reactions. This review will focus on the fundamentals of XAS and their applications in photocatalysis. The detailed applications obtained from XAS is described through the following aspects: 1) identifying local structure of photocatalyst; 2) uncovering in situ structure and chemical state evolution during photocatalysis; 3) revealing the photoexcited process. We will provide an in depth understanding on how the XAS method can guide the rational design of highly efficient photocatalyst. Finally, a systematic summary of XAS and related significance is made and the research perspectives are suggested.

Evidence for an Interface of Hybrid Cocatalysts Favoring Photocatalytic Hydrogen Evolution Kinetics

Hybrid cocatalysts have great application potential for improving the photocatalytic hydrogen evolution performance of semiconductors. The interfaces between components of hybrid cocatalysts make a great contribution to the improvement, but the associated mechanisms remain unclear. Herein, we prepared and tested three comparative CdS-based photocatalysts with NiS, NiS/Ni9S8, and Ni9S8 as the cocatalysts separately. The emphasis is placed on investigating the effect of the NiS/Ni9S8 interfaces on the photocatalytic hydrogen evolution performance of CdS. NiS/Ni9S8 exhibits a higher ability than NiS and Ni9S8 in making CdS a more active photocatalyst for water splitting. It shows that NiS, NiS/Ni9S8, and Ni9S8 perform similarly in terms of promoting the charge transfer and separation of CdS based on steady-state and time-resolved photoluminescence studies. At the same time, the linear sweep voltammetry and electrochemical impedance spectroscopy tests combined with the density functional theory calculations reveal that the component interfaces of NiS/Ni9S8 enable us to lower the water splitting activation energy, the charge-transfer resistance from the cocatalyst to sacrificial agent, and hydrogen adsorption Gibbs free energy. It is evidenced from this work that component interfaces of hybrid cocatalysts play a vital role in accelerating the dynamics of hydrogen evolution reactions.

NiS/Cd0.6 Zn0.4 S Schottky Junction Bifunctional Photocatalyst for Sunlight-Driven Highly Selective Catalytic Oxidation of Vanillyl Alcohol Towards Vanillin Coupled with Hydrogen Evolution Reaction

Selective oxidation of biomass-based molecules to high-value chemicals in conjunction with hydrogen evolution reaction (HER) is an innovative photocatalysis strategy. The key challenge is to design bifunctional photocatalysts with suitable band structures, which can achieve highly efficient generation of high-value chemicals and hydrogen. Herein, NiS/Cd0.6 Zn0.4 S Schottky junction bifunctional catalysts are constructed for sunlight-driven catalytic vanillyl alcohol (VAL) selective oxidation towards vanillin (VN) coupling HER. At optimal conditions, the 8% NiS/Cd0.6 Zn0.4 S photocatalyst achieves high activity of VN production (3.75 mmol g-1 h-1 ) and HER (3.84 mmol g-1 h-1 ). It also exhibits remarkable VAL conversion (66.9%), VN yield (52.1%), and selectivity (77.8%). The photocatalytic oxidation of VAL proceeds a carbon-centered radical mechanism via the cleavage of αC-H bond. Experimental results and theoretical calculations show that NiS with metallic properties enhances the electron transfer capability. Importantly, a Ni-S-Cd "electron bridge" formed at the interface of NiS/Cd0.6 Zn0.4 S further improves the separation/transfer of electrone/h+ pairs and also furnishes HER active sites due to its smaller the |ΔGH* | value, thereby resulting in a remarkably HER activity. This work sheds new light on the selective catalytic oxidation VAL to VN coupling HER, with a new pathway towards achieving its efficient HER efficiency.

Recent developments in lead-free bismuth-based halide perovskite nanomaterials for heterogeneous photocatalysis under visible light

Halide perovskite materials, especially lead-based perovskites, have been widely used for optoelectronic and catalytic applications. However, the high toxicity of the lead element is a major concern that directs the research work toward lead-free halide perovskites, which could utilize bismuth as a promising candidate. Until now, the replacement of lead by bismuth in perovskites has been well studied by designing bismuth-based halide perovskite (BHP) nanomaterials with versatile physical-chemical properties, which are emerging in various application fields, especially heterogeneous photocatalysis. In this mini-review, we present a brief overview of recent progress in BHP nanomaterials for photocatalysis under visible light. The synthesis and physical-chemical properties of BHP nanomaterials have been comprehensively summarized, including zero-dimensional, two-dimensional nanostructures and hetero-architectures. Later, we introduce the photocatalytic applications of these novel BHP nanomaterials with visible-light response, improved charge separation/transport and unique catalytic sites. Due to advanced nano-morphologies, a well-designed electronic structure and an engineered surface chemical micro-environment, BHP nanomaterials demonstrate enhanced photocatalytic performance for hydrogen generation, CO₂ reduction, organic synthesis and pollutant removal. Finally, the challenges and future research directions of BHP nanomaterials for photocatalysis are discussed.

In situ self-exsolved ultrasmall Fe2P quantum dots from attapulgite nanofibers as superior cocatalysts for solar hydrogen evolution

Developing highly active, stable, and cost-efficient cocatalysts for photocatalytic H₂ evolution is pivotal in the area of renewable energy conversion. Herein, we present a straightforward, low-temperature phosphidation strategy for in situ exsolving doped Fe ions from natural attapulgite (ATP) nanofibers into a supported Fe₂P cocatalyst for the photocatalytic H₂ evolution reaction (HER). The resulting Fe₂P QDs/ATP features highly dispersed Fe₂P QDs with an average size of <2 nm and a strong interfacial interaction between self-exsolved Fe₂P QDs and the ATP substrate, thus providing ample and stable active sites for the photocatalytic HER. When employed as a cocatalyst, Fe₂P QDs/ATP exhibits superior catalytic activity and notable stability in a molecular system with low-cost xanthene dyes as the photosensitizer under visible light irradiation. More importantly, Fe₂P QDs/ATP can also efficiently and stably catalyze the photocatalytic HER when simply combined with various semiconductor photocatalysts (g-C₃N₄, TiO₂, and CdS). This strategy of exsolving transition metal ions from substrates is an effective yet simple approach for the development of highly active supported HER cocatalysts for renewable and clean energy conversion.

Cocatalysts for Photocatalytic Overall Water Splitting: A Mini Review

Photocatalyst overall water splitting is usually restricted by low carrier separation efficiency and a slow surface reaction rate. Cocatalysts provide a satisfactory solution to significantly improve photocatalytic performance. In this review, some recent advances in cocatalysts for photocatalytic overall water splitting are gathered and divided into groups. Firstly, the loading method of the cocatalyst is introduced. Then, the role of the cocatalyst applied for the photocatalytic overall water splitting process is further discussed. Finally, the key challenges and possible research directions of photocatalytic overall water splitting are proposed. This review is expected to promote research on the design of efficient cocatalysts in photocatalytic systems for overall water splitting.

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.

Molecular Cocatalyst of p-Mercaptophenylboronic Acid Boosts the Plasmon-Mediated Reduction of p-Nitrothiophenol

Localized surface plasmon resonance (LSPR) has been demonstrated to be highly effective in the initialization or acceleration of chemical reactions because of its unique optical properties. However, because of the ultrashort lifetime (fs to ps) of plasmon-generated hot carriers, the potential of LSPR in photochemical reactions has not been fully exploited. Herein, we demonstrate an acceleration of the plasmon-mediated reduction of p-nitrothiophenol (PNTP) molecules on the surface of silver nanoparticles (AgNPs) with in situ Raman spectroscopy. p-Mercaptophenylboronic acid (PMPBA) molecules coadsorbed on AgNP surfaces act as a molecular cocatalyst in the plasmon-mediated reaction, resulting in a boosting of the PNTP reduction. This boosting is attributed to the improved transfer and separation of the plasmon-generated hot carriers at the interface of the AgNPs and coadsorbed PMPBA molecules. Our finding provides a highly simple, cost-effective, and highly effective strategy to promote plasmonic photochemistry by introducing a molecular cocatalyst, and this strategy can be extended to promote various plasmon-mediated reactions.

Boosting photocatalytic CO2 reduction via Schottky junction with ZnCr layered double hydroxide nanoflakes aggregated on 2D Ti3C2Tx cocatalyst

Designing efficient photocatalysts is vital for the photoreduction of CO₂ to produce solar fuels, helping to alleviate issues of fossil fuel depletion and global warming. In this work, a novel ZnCr-LDH/Ti₃C₂T_x Schottky junction is successfully synthesized using an in situ coprecipitation method. ZnCr-LDH nanoflakes collectively grow on the surface of Ti₃C₂T_x MXene nanosheets. When using Ti₃C₂T_x MXene as a cocatalyst in the prepared heterojunction, the light absorption intensity, photo-induced electron separation and migration efficiency increase. As a result, the composite ZnCr-LDH/Ti₃C₂T_x results in significant improvement in the performance of photocatalytic CO₂ reduction under simulated solar irradiation. The optimized sample ZCTC25 has the highest photocatalytic CO₂ reduction rates of 122.45 μmol g^-1 CO and 19.95 μmol g^-1 CH₄ (after 6 h of irradiation). These values are approximately 2.65 times higher than those of pristine ZnCr-LDH. The product selectivity towards CO is 86%. This work provides a new method for the construction of novel 2D semiconductor photocatalysts and enriches the application of an unusual type of layered double hydroxides in the photoreduction of CO₂.

A MOF-derived hierarchical CoP@ZnIn2S4 photocatalyst for visible light-driven hydrogen evolution

A hierarchical CoP@ZnIn₂S₄ photocatalyst was prepared via a MOF-templated strategy. Owing to the unique composition and morphology that can facilitate the separation of photoexcited carriers, enhance light absorption and provide high surface area, CoP@ZnIn₂S₄ exhibited a H₂ evolution rate of 0.103 mmol h^-1 and remained stable over 24 hours.

Construction of a multidimensional CdS@MoS2 heterojunction for enhancing the activity and transfer efficiency of photogenerated carriers

A CdS@MoS2 composite photocatalyst exhibits excellent photocatalytic activity to replace noble metals by constructing a n–n heterojunction.

Multifunctional Chiral Three-Dimensional Phosphite Frameworks Showing Dielectric Anomaly and High Proton Conductivity

Chair 3D Co(II) phosphite frameworks have been prepared by the ionothermal method. It belongs to chiral space group P3₂21, and the whole framework can be topologically represented as a chiral 4-connected qtz net. It shows a multistep dielectric response arising from the reorientation of Me₂-DABCO in the chiral cavities. It can also serve as a pron conductor with high conductivity, 1.71 × 10^-3 S cm^-1, at room temperature, which is attributed to the formation of denser hydrogen-bonding networks providing efficient proton-transfer pathways.