A CsPbBr3 /TiO2 Composite for Visible-Light-Driven Photocatalytic Benzyl Alcohol Oxidation

Recent progress of metal halide perovskite materials in heterogeneous photocatalytic organic reactions

Photocatalytic technology is widely regarded as an important way to utilize solar energy and achieve carbon neutrality, which has attracted considerable attentions in various fields over the past decades. Metal halide perovskites (MHPs) are recognized as "superstar" materials due to their exceptional photoelectric properties, readily accessible and tunable structure, which made them intensively studied in solar cells, light-emitting diodes, and solar energy conversion fields. Since 2018, increased attention has been focused on applying the MHPs as a heterogeneous visible light photocatalyst in catalyzing organic synthesis reactions. In this review, we present an overview of photocatalytic technology and principles of heterogeneous photocatalysis before delving into the structural characteristics, stability, and classifications of MHPs. We then focus on recent developments of MHPs in photocatalyzing various organic synthesis reactions, such as oxidation, cyclization, C-C coupling etc., based on their classifications and reported reaction types. Finally, we discuss the main limitations and prospects regarding the application of metal halide perovskites in organic synthesis.

A Review of Synthesis Methods, Modifications, and Mechanisms of ZnO/TiO2-Based Photocatalysts for Photodegradation of Contaminants

The environment being surrounded by accumulated durable waste organic compounds has become a critical crisis for human societies. Generally, organic effluents of industrial plants released into the water source and air are removed by some physical and chemical processes. Utilizing photocatalysts as cost-effective, accessible, thermally/mechanically stable, nontoxic, reusable, and powerful UV-absorber compounds creates a new gateway toward the removal of dissolved, suspended, and gaseous pollutants even in trace amounts. TiO2 and ZnO are two prevalent photocatalysts in the field of removing contaminants from wastewater and air. Structural modification of the photocatalysts with metals, nonmetals, metal ions, and other semiconductors reduces the band gap energy and agglomeration and increases the affinity toward organic compounds in the composite structures to expand their usability on an industrial scale. This increases the extent of light absorbance and improves the photocatalytic efficiency. Selecting a suitable synthesis method is necessary to prepare a target photocatalyst with distinct properties such as high specific surface area, numerous surface functional groups, and an appropriate crystalline phase. In this Review, significant parameters for the synthesis and modification of TiO2- and ZnO-based photocatalysts are discussed in detail. Several proposed mechanistic routes according to photocatalytic composite structures are provided. Some electrochemical analyses using charge carrier trapping agents and delayed recombination help to plot mechanistic routes according to the direction of photoexcited species (electron-hole pairs) and design more effective photocatalytic processes in terms of cost-effective photocatalysts, saving time and increasing productivity.

Solar-Light-Driven Photocatalytic Oxidative Coupling of Phenol Derivatives over Bismuth-Based Porous Metal Halide Perovskites

The selective oxidative coupling of phenol derivatives, involving carbon-carbon (C-C) and carbon-oxygen (C-O) bond formation, has emerged as a critical approach in the synthesis of natural products. However, achieving precise control over the selectivity in coupling reactions of unsubstituted phenols utilizing solar light as the driving force remains a big challenge. In this study, we report a series of porous Cs3Bi2X9 (X=Cl, Br, I) photocatalysts with tailored band gaps and compositions engineered for efficient solar-light-driven oxidative phenol coupling. Notably, p-Cs3Bi2Br9 exhibited about 73 % selectivity for C-C coupling, displaying a high formation rate of 47.3 μmol gcat -1 h-1 under solar radiation. Furthermore, this approach enables control of the site-selectivity for phenol derivatives on Cs3Bi2X9, enhancing C-C coupling. The distinctive porous structure and appropriate band-edge positions of Cs3Bi2Br9 facilitated efficient charge separation, and surface interaction/activation of phenolic hydroxyl groups, resulting in the kinetically preferred formation of C-C over C-O bond. Mechanistic insights into the reaction pathway, supported by comprehensive control experiments, unveiled the crucial role of interfacial charge transfers and Lewis acid Bi sites in stabilizing phenolic intermediates, thereby directing the regioselectivity of diradical couplings and resulting in the formation of unsymmetrical biphenols.

Enhancing the Photocatalytic Activity of Halide Perovskite Cesium Bismuth Bromide/Hydrogen Titanate Heterostructures for Benzyl Alcohol Oxidation

Halide perovskite Cs3Bi2Br9 (CBB) has excellent potential in photocatalysis due to its promising light-harvesting properties. However, its photocatalytic performance might be limited due to the unfavorable charge carrier migration and water-induced properties, which limit the stability and photocatalytic performance. Therefore, we address this constraint in this work by synthesizing a stable halide perovskite heterojunction by introducing hydrogen titanate nanosheets (H2Ti3O7-NS, HTiO-NS). Optimizing the weight % (wt%) of CBB enables synthesizing the optimal CBB/HTiO-NS, CBHTNS heterostructure. The detailed morphology and structure characterization proved that the cubic shape of CBB is anchored on the HTiO-NS surface. The 30 wt% CBB/HTiO-NS-30 (CBHTNS-30) heterojunction showed the highest BnOH photooxidation performance with 98% conversion and 75% benzoic acid (BzA) selectivity at 2 h under blue light irradiation. Detailed optical and photoelectrochemical characterization showed that the incorporating CBB and HTiO-NS widened the range of the visible-light response and improved the ability to separate the photo-induced charge carriers. The presence of HTiO-NS has increased the oxidative properties, possibly by charge separation in the heterojunction, which facilitated the generation of superoxide and hydroxyl radicals. A possible reaction pathway for the photocatalytic oxidation of BnOH to BzH and BzA was also suggested. Furthermore, through scavenger experiments, we found that the photogenerated h+, e- and •O2- play an essential role in the BnOH photooxidation, while the •OH have a minor effect on the reaction. This work may provide a strategy for using HTiO-NS-based photocatalyst to enhance the charge carrier migration and photocatalytic performance of CBB.

Insights into interfacial mechanisms: CsPbBr3 nanocrystals as sustainable photocatalysts for primary amine oxidation

CsPbBr3 nanocrystals (NCs) employed as a photocatalyst resulted in efficient benzylamine oxidation under oxygen atmosphere. Improved reaction yields stem from favorable -NH2 functional group interactions on the NC surface, while additional interactions with -OMe or -SMe functional groups post-product formation result in lower yields. These insights into interfacial interactions and mechanistic aspects advance sustainable chemical transformations through cost-effective and recyclable CsPbBr3 NC-catalyzed primary amine oxidation.

Structural Modification Strategies, Interfacial Charge-Carrier Dynamics, and Solar Energy Conversion Applications of Organic-Inorganic Halide Perovskite Photocatalysts

Over the past few decades, organic-inorganic halide perovskites (OIHPs) as novel photocatalyst materials have attracted intensive attention for an impressive variety of photocatalytic applications due to their excellent photophysical (chemical) properties. Regarding practical application and future commercialization, the air-water stability and photocatalytic performance of OIHPs need to be further improved. Accordingly, studying modification strategies and interfacial interaction mechanisms is crucial. In this review, the current progress in the development and photocatalytic fundamentals of OIHPs is summarized. Furthermore, the structural modification strategies of OIHPs, including dimensionality control, heterojunction design, encapsulation techniques, and so on for the enhancement of charge-carrier transfer and the enlargement of long-term stability, are elucidated. Subsequently, the interfacial mechanisms and charge-carrier dynamics of OIHPs during the photocatalytic process are systematically specified and classified via diverse photophysical and electrochemical characterization methods, such as time-resolved photoluminescence measurements, ultrafast transient absorption spectroscopy, electrochemical impedance spectroscopy measurements, transient photocurrent densities, and so forth. Eventually, various photocatalytic applications of OIHPs, including hydrogen evolution, CO2 reduction, pollutant degradation, and photocatalytic conversion of organic matter.

Photocatalysis Based on Metal Halide Perovskites for Organic Chemical Transformations

Heterogeneous photocatalysts incorporating metal halide perovskites (MHPs) have garnered significant attention due to their remarkable attributes: strong visible-light absorption, tuneable band energy levels, rapid charge transfer, and defect tolerance. Additionally, the promising optical and electronic properties of MHP nanocrystals can be harnessed for photocatalytic applications through controlled crystal structure engineering, involving composition tuning via metal ion and halide ion variations, dimensional tuning, and surface chemistry modifications. Combination of perovskites with other materials can improve the photoinduced charge separation and charge transfer, building heterostructures with different band alignments, such as type-II, Z-scheme, and Schottky heterojunctions, which can fine-tune redox potentials of the perovskite for photocatalytic organic reactions. This review delves into the activation of organic molecules through charge and energy transfer mechanisms. The review further investigates the impact of crystal engineering on photocatalytic activity, spanning a diverse array of organic transformations, such as C-X bond formation (X = C, N, and O), [2 + 2] and [4 + 2] cycloadditions, substrate isomerization, and asymmetric catalysis. This study provides insights to propel the advancement of metal halide perovskite-based photocatalysts, thereby fostering innovation in organic chemical transformations.

Photocatalytic CO2 Reduction Coupled with Oxidation of Benzyl Alcohol over CsPbBr3@PANI Nanocomposites

Herein, we successfully prepare conductive polyaniline (PANI)-encapsulated CsPbBr3 perovskite nanocrystals (PNCs) that demonstrate much improved photocatalytic performance and stability toward the CO2 reduction reaction (CRR) coupled with oxidation of benzyl alcohol (BA) to benzaldehyde. Due to the acid-base interaction between CO2 and PANI, CO2 molecules are selectively adsorbed on PANI in the form of carbamate. As a result, the rate of production of CO (rCO) reaches 26.1 μmol g-1 h-1 with a selectivity of 98.1%, which is in good agreement with the rate of oxidation (∼27.0 μmol g-1 h-1) of BA. Such a high reduction/oxidation rate is enabled by the fast electron transfer (∼2.2 ps) from PNCs to PANI, as revealed by femtosecond transient absorption spectroscopy. Moreover, because of the benefit of the encapsulation of PANI, no significant decrease in rCO is observed in a 10 h CRR test. This work offers insight into how to simultaneously achieve improved photocatalytic performance and stability of CsPbX3 PNCs.

Encapsulation of CsPb2Br5 in TiO2 Microcrystals to Enhance Environmental Stability

All-inorganic lead halide perovskite has emerged as an attractive semiconducting material due to its unique optoelectronic properties. However, its poor environmental stability restricts its broad application. Here, a simple method for the fabrication of CsPb2Br5/TiO2 is investigated. The introduction of p-aminobenzoic acid, which has two functional groups, is critical for the capping of amorphous TiO2 on CsPb2Br5. After calcination at 300 °C, amorphous TiO2 crystallizes into the anatase phase. The CsPb2Br5/TiO2 NCs show high long-term stability in water and enhanced stability against ultraviolet radiation and heat treatment, owing to the chemical stability of TiO2. More importantly, photo-electrochemical characterizations indicate that the formation of TiO2 shells can increase the charge separation efficiency. Hence, CsPb2Br5/TiO2 exhibits improved photoelectric activity owing to the electrical conductivity of the TiO2 in water. This study provides a new route for the fabrication of optoelectronic devices and photocatalysts based on perovskite NCs in the aqueous phase. Furthermore, the present results demonstrate that CsPb2Br5/TiO2 NCs has considerable potential to be used as a photoelectric material in optical sensing and monitoring.

Radial Nano-Heterojunctions Consisting of CdS Nanorods Wrapped by 2D CN:PDI Polymer with Deep HOMO for Photo-Oxidative Water Splitting, Dye Degradation and Alcohol Oxidation

Solar energy harvesting using semiconductor photocatalysis offers an enticing solution to two of the biggest societal challenges, energy scarcity and environmental pollution. After decades of effort, no photocatalyst exists which can simultaneously meet the demand for excellent absorption, high quantum efficiency and photochemical resilience/durability. While CdS is an excellent photocatalyst for hydrogen evolution, pollutant degradation and organic synthesis, photocorrosion of CdS leads to the deactivation of the catalyst. Surface passivation of CdS with 2D graphitic carbon nitrides (CN) such as g-C₃N₄ and C₃N₅ has been shown to mitigate the photocorrosion problem but the poor oxidizing power of photogenerated holes in CN limits the utility of this approach for photooxidation reactions. We report the synthesis of exfoliated 2D nanosheets of a modified carbon nitride constituted of tris-s-triazine (C₆N₇) linked pyromellitic dianhydride polydiimide (CN:PDI) with a deep oxidative highest occupied molecular orbital (HOMO) position, which ensures sufficient oxidizing power for photogenerated holes in CN. The heterojunction formed by the wrapping of mono-/few layered CN:PDI on CdS nanorods (CdS/CN:PDI) was determined to be an excellent photocatalyst for oxidation reactions including photoelectrochemical water splitting, dye decolorization and the photocatalytic conversion of benzyl alcohol to benzaldehyde. Extensive structural characterization using HR-TEM, Raman, XPS, etc., confirmed wrapping of few-layered CN:PDI on CdS nanorods. The increased photoactivity in CdS/CN:PDI catalyst was ascribed to facile electron transfer from CdS to CN:PDI in comparison to CdS/g-C₃N₄, leading to an increased electron density on the surface of the photocatalyst to drive chemical reactions.

Exploring the Role of Short Chain Acids as Surface Ligands in Photoinduced Charge Transfer Dynamics from CsPbBr3 Perovskite Nanocrystals

The most commonly used surface capping ligands, like oleic acid and oleylamine, passivate the surface of perovskite nanocrystals (PNCs) to enhance their stability and optical properties. However, due to their inherent insulating nature, charge transport across the surface of the PNCs is hindered, limiting their application in devices. In this study, we have post-treatment CsPbBr₃ PNCs with short chain ligands benzoic acid (BA) and ascorbic acid (AA) and observed that both acid-treated PNCs show enhanced stability and optical properties. Still, BA-treated PNCs show the highest charge transport rate due to their conjugating nature. The photoelectrochemical measurements also show the most efficient electron flow across the surface of the PNC with BA-treated PNCs. A longer carrier lifetime and fast charge transfer make BA-treated PNCs ideal candidates for application in real-life devices.

Stabilization and Performance Enhancement Strategies for Halide Perovskite Photocatalysts

Solar-energy-powered photocatalytic fuel production and chemical synthesis are widely recognized as viable technological solutions for a sustainable energy future. However, the requirement of high-performance photocatalysts is a major bottleneck. Halide perovskites, a category of diversified semiconductor materials with suitable energy-band-enabled high-light-utilization efficiencies, exceptionally long charge-carrier-diffusion-length-facilitated charge transport, and readily tailorable compositional, structural, and morphological properties, have emerged as a new class of photocatalysts for efficient hydrogen evolution, CO₂ reduction, and various organic synthesis reactions. Despite the noticeable progress, the development of high-performance halide perovskite photocatalysts (HPPs) is still hindered by several key challenges: the strong ionic nature and high hydrolysis tendency induce instability and an unsatisfactory activity due to the need for a coactive component to realize redox processes. Herein, the recently developed advanced strategies to enhance the stability and photocatalytic activity of HPPs are comprehensively reviewed. The widely applicable stability enhancement strategies are first articulated, and the activity improvement strategies for fuel production and chemical synthesis are then explored. Finally, the challenges and future perspectives associated with the application of HPPs in efficient production of fuels and value-added chemicals are presented, indicating the irreplaceable role of the HPPs in the field of photocatalysis.

A Review on Halide Perovskite-Based Photocatalysts: Key Factors and Challenges

A growing number of research articles have been published on the use of halide perovskite materials for photocatalytic reactions. These articles extend these materials' great success from solar cells to photocatalytic technologies such as hydrogen production, CO₂ reduction, dye degradation, and organic synthesis. In the present review article, we first describe the background theory of photocatalysis, followed by a description on the properties of halide perovskites and their development for photocatalysis. We highlight key intrinsic factors influencing their photocatalytic performance, such as stability, electronic band structure, and sorption properties. We also discuss and shed light on key considerations and challenges for their development in photocatalysis, such as those related to reaction conditions, reactor design, presence of degradable organic species, and characterization, especially for CO₂ photocatalytic reduction. This review on halide perovskite photocatalysts will provide a better understanding for their rational design and development and contribute to their scientific and technological adoption in the wide field of photocatalytic solar devices.

Recent Progress in Halide Perovskite Nanocrystals for Photocatalytic Hydrogen Evolution

Due to its environmental cleanliness and high energy density, hydrogen has been deemed as a promising alternative to traditional fossil fuels. Photocatalytic water-splitting using semiconductor materials is a good prospect for hydrogen production in terms of renewable solar energy utilization. In recent years, halide perovskite nanocrystals (NCs) are emerging as a new class of fascinating nanomaterial for light harvesting and photocatalytic applications. This is due to their appealing optoelectronic properties, such as optimal band gaps, high absorption coefficient, high carrier mobility, long carrier diffusion length, etc. In this review, recent progress in halide perovskite NCs for photocatalytic hydrogen evolution is summarized. Emphasis is given to the current strategies that enhance the photocatalytic hydrogen production performance of halide perovskite NCs. Some scientific challenges and perspectives for halide perovskite photocatalysts are also proposed and discussed. It is anticipated that this review will provide valuable references for the future development of halide perovskite-based photocatalysts used in highly efficient hydrogen evolution.

Photocatalytic Deposition of Nanostructured CsPbBr3 Perovskite Quantum Dot Films on Mesoporous TiO2 and Their Enhanced Visible-Light Photodegradation Properties

Herein, we report the in situ photocatalytic deposition of cesium lead bromide (CsPbBr₃) perovskite quantum dots on mesoporous TiO₂-coated fluorine-doped tin oxide (FTO/TiO₂) electrodes. The mesoporous TiO₂ layer is used as a photocatalyst to promote the following: (1) the Pb deposition from a Pb²⁺ aqueous solution and (2) the in situ Pb conversion into CsPbBr₃ perovskite in the presence of a CsBr methanolic solution without any organic capping agent. Both steps are carried out under ultraviolet light irradiation under ambient conditions without any post-treatment. The obtained FTO/TiO₂/CsPbBr₃ film was characterized by UV-vis diffuse reflectance spectroscopy, X-ray diffraction, photoluminescence spectroscopy, scanning electron microscopy, and transmission electron microscopy. The FTO/TiO₂/CsPbBr₃ heterojunction exhibited enhanced visible-light photodegradation activity demonstrated for the oxidation of curcumin organic dye as a model system. The novel and simple approach to fabricating a supported photocatalyst represents a scalable general method to use semiconductors as a platform to incorporate different perovskites, either all-inorganic or hybrid, for optoelectronic applications. The perovskite deposition method mediated by the UV light at room temperature could be further applied to flexible and wearable solar power electronics.

2D/2D CsPbBr3/BiOCl Heterojunction with an S-Scheme Charge Transfer for Boosting the Photocatalytic Conversion of CO2

The rational design of a two-dimensional (2D)/2D "face-to-face" heterojunction photocatalyst is crucial for the mediation of interfacial charge transfer/separation. Herein, a unique 2D/2D step-scheme (S-scheme) photocatalyst of CsPbBr₃/BiOCl is constructed by the self-assembly of CsPbBr₃ and BiOCl nanosheets (NSs). Profiting from the effective interface contact and appropriate band structures between CsPbBr₃ and BiOCl NSs, a valid S-scheme heterojunction of CsPbBr₃/BiOCl is established. Density functional theory (DFT) calculations and a series of characterization techniques including X-ray photoelectron spectra (XPS), photoassisted Kelvin probe force microscopy (KPFM), and electron spin resonance (ESR) systematically corroborate the S-scheme charge-transfer mechanism between CsPbBr₃ and BiOCl. The formation of an S-scheme heterojunction endows the photocatalyst with boosted charge separation and retainment of the highest redox ability. As a result, the obtained 2D/2D CsPbBr₃/BiOCl S-scheme photocatalyst shows much superior CO₂-reduction performance to single CsPbBr₃ and BiOCl. This investigation provides new insights into the construction of novel S-scheme heterojunctions based on 2D/2D photocatalytic systems.

Metal Halide Perovskite Based Heterojunction Photocatalysts

With fascinating photophysical properties and a strong potential to utilize solar energy, metal halide perovskites (MHPs) have become a prominent feature within photocatalysis research. However, the effectiveness of single MHP photocatalysts is relatively poor. The introduction of a second component to form a heterojunction represents a well-established route to accelerate carrier migration and boost reaction rates, thus increasing the photoactivity. Recently, there have been several scientific advances related to the design of MHP-based heterojunction photocatalysts, including Schottky, type II, and Z-scheme heterojunctions. In this Review, we systematically discuss and critically appraise recent developments in MHP-based heterojunction photocatalysis. In addition, the techniques for identifying the type of active heterojunctions are evaluated and we conclude by briefly outlining the ongoing challenges and future directions for promising photocatalysts based on MHP heterojunctions.

Lead-Free Cs2 AgSbCl6 Double Perovskite Nanocrystals for Effective Visible-Light Photocatalytic C-C Coupling Reactions

Lead halide perovskite nanocrystals (NCs) have been regarded as a promising potential photocatalyst, owing to their high molar extinction coefficient, low economic cost, adjustable light absorption range, and ample surface active sites. However, the toxicity of lead and its inherent instability in water and polar solvents could hinder their wide application in the field of photocatalysis. Herein, with α-alkylation of aldehydes as a model reaction, C-C bond-forming is demonstrated in high yield by using lead-free double perovskite Cs₂ AgSbCl₆ NCs under visible light irradiation. Moreover, the photocatalytic performance is simply improved by rational control of the surface ligands and a reaction mechanism involving a radical intermediate is proposed. Although the stability requires further amelioration, the results indicate the enormous potential of lead-free double perovskite NC photocatalysts for organic synthesis and chemical transformations.

Reveal the Humidity Effect on the Phase Pure CsPbBr3 Single Crystals Formation at Room Temperature and Its Application for Ultrahigh Sensitive X-Ray Detector

Generally, growing phase pure CsPbBr₃ single crystals is challenging, and CsPb₂ Br₅ or Cs₄ PbBr₆ by-products are usually formed due to the different solubilities of CsBr and PbBr₂ in the single solvent. Herein, the growth of high-quality phase pure CsPbBr₃ perovskite single crystals at room temperature by a humidity controlled solvent evaporation method is reported first. Meanwhile, the room temperature phase transition process from three dimensional (3D) cubic CsPbBr₃ to two dimensional (2D) layered tetragonal CsPb₂ Br₅ and the detailed mechanism induced by humidity are revealed. Moreover, compared with the organic-inorganic perovskite, the prepared CsPbBr₃ single crystals are much more stable under high humidity, which satisfies the long-term working conditions of X-ray detectors. The X-ray detectors based on CsPbBr₃ single crystals show a high sensitivity and a low detection limit of 1.89 μGy_air s^-1 , all of which meet the needs of medical diagnosis.

Immobilizing CsPbBr3 perovskite nanocrystals on nanoporous carbon powder for visible-light-driven CO2 photoreduction

Perovskite CsPbBr₃ nanocrystals (NCs) have emerged as promising candidates for the photocatalytic reduction of CO₂. However, the CO₂ conversion efficiency of pristine CsPbBr₃ is still unsatisfactory, mainly due to severe radiative recombination, poor stability and low CO₂ capturing ability. Coincidentally, nanoporous carbon powder (NCP) has received tremendous attention for environmental remediation and renewable energy production. Herein, by immobilizing CsPbBr₃ NCs into the pores of NCP, a CsPbBr₃/NCP hybrid has been successfully constructed for the first time, which combined the advantages of CsPbBr₃ NCs and NCP. In comparison with pristine CsPbBr₃, the resultant hybrid photocatalyst exhibited much superior photocatalytic performance in CO₂ reduction, which could be attributed to the enhanced electron extraction and transfer between CsPbBr₃ NCs and NCP, higher CO₂ capturing ability and more active sites for CO₂ activation. Furthermore, the nanohybrid displayed remarkable reusability in photocatalytic CO₂ reduction. This study is anticipated to provide a new pathway for the design and fabrication of high-performance photocatalysts based on perovskites for solar-energy-conversion applications.

Effects of Crystallinity on the Photocatalytic Polymerization of 3,4-Ethylenedioxythiophene over CsPbBr3 Inverse Opals

Due to their high absorption coefficient and long carrier lifetime, halide perovskites are promising candidates for photocatalysts. For this study, the antisolvent crystallization protocol and the colloidal crystal templating approach were combined to fabricate the highly crystalline cesium lead bromide perovskite with inverse opal morphology (IO-CsPbBr3). Scanning electron microscopy and transmission electron microscope images demonstrate the three-dimensional well-ordered porous structures of the IO-CsPbBr3 and their single-crystalline features. The presented approach not only provides hierarchical porous structures but also enhances overall crystallinity. When used as catalysts to promote the polymerization of 2,2′,5′,2″-ter-3,4-ethylenedioxythiophene, the highly crystalline IO-CsPbBr3 exhibits a superior photocatalytic performance compared to its polycrystalline counterpart. Furthermore, the morphology and the crystalline structure of the highly crystalline IO-CsPbBr3 are well preserved under photocatalytic conditions. This novel approach enables the preparation of a halide perovskite inverse opal with high crystallinity.

Application of Metal Halide Perovskites as Photocatalysts in Organic Reactions

This review summarizes the current status of the application of metal halide perovskites (MHPs) as photocatalysts in organic syntheses/transformations. It is shown that the optimal and unique electronic properties of MHPs can be advantageously used in several reaction types providing pros with respect to traditional photocatalysts. While still being at infancy, such field of application of MHPs as effective photocatalysts will for sure become a central research topic in the forthcoming years, thanks also to their rich structural and chemical tunability, which may provide tailored materials for most of the envisaged organic reactions.

How to apply metal halide perovskites to photocatalysis: challenges and development

Semiconductor photocatalysts are widely used in environmental remediation and energy conversion processes that affect social development. These processes involve, for example, hydrogen production from water splitting, carbon dioxide reduction, pollutant degradation, and the conversion of raw organic chemical materials into high-value-added chemicals. Metal halide perovskites (MHPs) have become a new class of promising cheap and easy to manufacture candidate materials for use in photocatalytic semiconductors due to their advantages of high extinction coefficients, optimal band gaps, high photoluminescence quantum yields, and long electron-hole diffusion lengths. However, their unstable ion-bonded crystal structures (very low theoretical decomposition energy barriers) limit their widespread application. In this review, we introduce the physical properties of MHP materials suitable for photocatalysis, and MHP-based photocatalytic particle suspension systems, photoelectrode thin film systems, and photovoltaic-photo(electro)chemical systems. Then, numerous studies realizing efficient and stable photocatalytic water splitting, carbon dioxide reduction, organic conversion, and other reactions involving MHP materials were highlighted. In addition, we conducted rigorous analysis of the potential problems that could hinder progress in this new scientific research field, such as Pb element toxicity and material instability. Finally, we outline the potential opportunities and directions for photocatalysis research based on MHPs.

Boosting Photocatalytic Activity and Stability of Lead-Free Cs3Bi2Br9 Perovskite Nanocrystals via In Situ Growth on Monolayer 2D Ti3C2Tx MXene for C-H Bond Oxidation

Light-driven selective oxidation of saturated C-H bonds with molecular oxygen, as an alternative to conventional thermochemical catalysis, allows a sustainable and eco-friendly manner to convert solar energy into highly value-added oxygenates. However, the photocatalytic oxidation of hydrocarbons still remains a great challenge owing to the low efficiency in the separation and transfer of photogenerated charge of the currently available photocatalytic materials. Herein, we report a novel perovskite-based heterostructure photocatalyst, in which ligand- and lead-free all-inorganic perovskite Cs₃Bi₂Br₉ nanocrystals (NCs) with uniform crystal size and high crystallinity were homogeneously distributed on the surface of ultrathin two-dimensional (2D) monolayer Ti₃C₂T_x MXene nanosheets in an in situ growth manner. The resultant heterostructure featured with intimate interface between Cs₃Bi₂Br₉ NCs and Ti₃C₂T_x MXene and strong visible-light adsorption not only exhibits significant enhancement in the performance of photocatalytic oxidation of challenging aromatic and aliphatic alkanes under visible-light irradiation but also greatly improves the stability of Cs₃Bi₂Br₉ NCs under a reaction environment. Comprehensive characterizations reveal that the formation of an intimate interface between Cs₃Bi₂Br₉ NCs and highly conductive ultrathin 2D Ti₃C₂T_x MXene nanosheets via strong interaction markedly accelerates the separation and transfer efficiency of photogenerated electron-hole pairs and simultaneously suppresses their recombination, resulting in improved utilization of the excited charges, which account for the highly enhanced photocatalytic performance.

Surface modification for improving the photoredox activity of CsPbBr3 nanocrystals

In recent years inorganic lead halide perovskite nanocrystals (PNCs) have been used in photocatalytic reactions. The surface chemistry of the PNCs can play an important role in the excited state interactions and efficient charge transfer with redox molecules. In this work, we explore the impact of CsPbBr₃ nanocrystal surface modification on the excited state interactions with the electron acceptor benzoquinone (BQ) for three different ligand environments: as oleic acid/oleylamine (OA/OAm), oleic acid (OA)/trioctylphosphine (TOP), and oleic acid (OA)/oleylamine (OAm)/trioctylphosphine (TOP) ligands. Our finding concludes that amine-free PNCs (OA/TOP capped) exhibit the best excited state interactions with benzoquinone compared to the conventional oleylamine ligand environment. The photoinduced electron transfer (PET) rate constants were measured from PL-lifetime decay measurement. The amine-free PNCs show the highest PET which is 9 times higher than that of conventional ligand capped PNCs. These results highlight the impact of surface chemistry on the excited-state interactions of CsPbBr₃ NCs and in photocatalytic applications. More importantly, this work concludes that amine-free PNCs maintain a redox-active surface with a high photoinduced electron transfer rate which makes them an ideal candidate for photocatalytic applications.

Recent Progress in Designing Halide-Perovskite-Based System for the Photocatalytic Applications

The halide perovskite material has attracted vast attention as a versatile semiconductor in the past decade. With the unique advantages in physical and chemical properties, they have also shown great potential in photocatalytic applications. This review aims at the specific design principles triggered by the unique properties when employing halide-perovskite-based photocatalytic systems from the following perspectives: (I) Design of photoelectrocatalytic device structures including the n-i-p/p-i-n structure, photoelectrode device encapsulation, and electrolyte engineering. (II) The design of heterogeneous photocatalytic systems toward the hydrogen evolution reaction (HER) and CO2 reduction reaction, including the light management, surface/interface engineering, stability improvement, product selectivity engineering, and reaction system engineering. (III) The photocatalysts for the environmental application and organic synthesis. Based on the analyses, the review also suggests the prospective research for the future development of halide-perovskite-based photocatalytic systems.

Thickness-induced band-gap engineering in lead-free double perovskite Cs2AgBiBr6 for highly efficient photocatalysis

In recent years, two-dimensional (2D) lead-free double perovskites have been attracting much attention because of their unique performance in photovoltaic solar cells and photocatalysis. Nonetheless, how thickness affects the photoelectric properties of lead-free double perovskite remains unclear. In this work, by means of density functional theory (DFT) with a spin orbit coupling (SOC) effect, we have investigated the electronic and optical properties systemically, including band structures, carrier mobility, optical absorption spectra, exciton-binding energies, band edges alignment and molecule adsorption performance of Cs2AgBiBr6 with different thicknesses. The calculated results revealed the thickness-induced band gap and optical performance for Cs2AgBiBr6. It shows a low band gap and outstanding optical absorption of visible and ultraviolet light. When the thickness is reduced to a monolayer, Cs2AgBiBr6 moves from an indirect band gap to a direct band gap. Moreover, the carrier mobility of Cs2AgBiBr6 is excellent and the exciton-binding energy increases with the decreased thickness. Importantly, an analysis of molecule adsorption and band edge alignment indicates that Cs2AgBiBr6 is prone to H2O adsorption and H2 desorption theoretically, which is conducive to the photocatalytic water splitting for hydrogen generation and other photovatalytic reactions. Our work suggests that Cs2AgBiBr6 is a potential candidate as a solar cell or a photocatalyst, and we provide theoretical explorations into reducing the layers of lead-free double perovskite materials to 2D atomic thickness for a better photocatalytic application, which can serve as guidelines for the design of excellent photocatalysts.

Photocorrosion at Irradiated Perovskite/Electrolyte Interfaces

Metal-halide perovskites transformed optoelectronics research and development during the past decade. They have also gained a foothold in photocatalytic and photoelectrochemical processes recently, but their sensitivity to the most commonly applied solvents and electrolytes together with their susceptibility to photocorrosion hinders such applications. Understanding the elementary steps of photocorrosion of these materials can aid the endeavor of realizing stable devices. In this Perspective, we discuss both thermodynamic and kinetic aspects of photocorrosion processes occurring at the interface of perovskite photocatalysts and photoelectrodes with different electrolytes. We show how combined in situ and operando electrochemical techniques can reveal the underlying mechanisms. Finally, we also discuss emerging strategies to mitigate photocorrosion (such as surface protection, materials and electrolyte engineering, etc.).

Direct Z-Scheme Heterojunction of Semicoherent FAPbBr3/Bi2WO6 Interface for Photoredox Reaction with Large Driving Force

Metal halide perovskites with direct band gap and strong light absorption are promising materials for harvesting solar energy; however, their relatively narrow band gap limits their redox ability when used as a photocatalyst. Adding a second semiconductor component with the appropriate band structure offsets can generate a Z-scheme photocatalytic system, taking full advantage of the perovskite's intrinsic properties. In this work, we develop a direct Z-scheme photocatalyst based on formamidinium lead bromide and bismuth tungstate (FAPbBr₃/Bi₂WO₆) with strong redox ability for artificial solar-to-chemical energy conversion. With desirable band offsets and strong joint redox potential, the dual photocatalyst is shown to form a semicoherent heterointerface. Ultrafast transient infrared absorption studies employing selective excitation reveal synergetic photocarrier dynamics and demonstrate Z-scheme charge transfer mechanisms. Under simulated solar irradiation, a large driving force photoredox reaction (∼2.57 eV) of CO₂ reduction coupled with benzyl alcohol oxidation to benzaldehyde is achieved on the Z-scheme FAPbBr₃/Bi₂WO₆ photocatalyst, harnessing the full synergetic potential of the combined system.

Metal Halide Perovskite@Metal-Organic Framework Hybrids: Synthesis, Design, Properties, and Applications

Metal halide perovskites (MHPs) have excellent optoelectronic and photovoltaic applications because of their cost-effectiveness, tunable emission, high photoluminescence quantum yields, and excellent charge carrier properties. However, the potential applications of the entire MHP family are facing a major challenge arising from its weak resistance to moisture, polar solvents, temperature, and light exposure. A viable strategy to enhance the stability of MHPs could lie in their incorporation into a porous template. Metal-organic frameworks (MOFs) have outstanding properties, with a unique network of ordered/functional pores, which render them promising for functioning as such a template, accommodating a wide range of MHPs to the nanosized region, alongside minimizing particle aggregation and enhancing the stability of the entrapped species. This review highlights recent advances in design strategies, synthesis, characterization, and properties of various hybrids of MOFs with MHPs. Particular attention is paid to a critical review of the emergence of MHP@MOF for comprehensive studies of next-generation materials for various technological applications including sensors, photocatalysis, encryption/decryption, light-emitting diodes, and solar cells. Finally, by summarizing the state-of-the-art, some promising future applications of reported hybrids are proposed. Considering the inherent correlation and synergic functionalities of MHPs and MOFs, further advancement; new functional materials; and applications can be achieved through designing MHP@MOF hybrids.

Stereoselective C-C Oxidative Coupling Reactions Photocatalyzed by Zwitterionic Ligand Capped CsPbBr3 Perovskite Quantum Dots

Semiconductor quantum dots (QDs) have attracted tremendous attention in the field of photocatalysis, owing to their superior optoelectronic properties for photocatalytic reactions, including high absorption coefficients and long photogenerated carrier lifetimes. Herein, by choosing 2-(3,4-dimethoxyphenyl)-3-oxobutanenitrile as a model substrate, we demonstrate that the stereoselective (>99 %) C-C oxidative coupling reaction can be realized with a high product yield (99 %) using zwitterionic ligand capped CsPbBr₃ perovskite QDs under visible light illumination. The reaction can be generalized to different starting materials with various substituents on the phenyl ring and varied functional moieties, producing stereoselective dl-isomers. A radical mediated reaction pathway has been proposed. Our study provides a new way of stereoselective C-C oxidative coupling via a photocatalytic means using specially designed perovskite QDs.

Recent Progress in Engineering Metal Halide Perovskites for Efficient Visible-Light-Driven Photocatalysis

Artificial photosynthesis has attracted increasing attention due to recent environmental and energy concerns. Metal halide perovskites (MHPs) demonstrating excellent optoelectronic properties have currently emerged as novel and efficient photocatalytic materials. Herein, the structural features of MHPs that are responsible for the photoinduced charge separation and charge migration properties are briefly introduced, and then important and necessary photophysical and photochemical aspects of MHPs related to photoredox catalysis are summarized. Subsequently, the applications of MHPs for solar energy harvesting and photocatalytic conversion, including H₂ evolution, CO₂ reduction, degradation of organic pollutants, and photoredox organic synthesis, are extensively demonstrated, with a focus on strategies for improving the performance (e.g., selectivity, activity, stability, recyclability, and environmental compatibility) of these MHP-based photocatalytic systems. To conclude, existing challenges and prospects on the future development of MHP-based materials towards photoredox catalysis applications are detailed.

Direct Z-Scheme 0D/2D Heterojunction of CsPbBr3 Quantum Dots/Bi2WO6 Nanosheets for Efficient Photocatalytic CO2 Reduction

Photocatalytic CO₂ reduction is an appealing approach to convert solar energy into high value-added chemicals. All-inorganic CsPbBr₃ quantum dots (QDs) have emerged as a promising photocatalyst for reducing CO₂. However, pristine CsPbBr₃ has a low catalytic performance, mainly due to severe charge recombination. Herein, a 0D/2D heterojunction of CsPbBr₃ QDs/Bi₂WO₆ nanosheet (CPB/BWO) photocatalysts is fabricated for photocatalytic CO₂ reduction. The CPB/BWO photocatalyst achieves excellent photocatalytic performance: the total yield of CH₄/CO is 503 μmol g^-1, nearly 9.5 times higher than the pristine CsPbBr₃. The CPB/BWO heterojunction also exhibits much-improved stability during photocatalytic reactions. On the basis of various characterization techniques, our investigations verified a direct Z-scheme charge migration mechanism between CsPbBr₃ QDs and Bi₂WO₆ nanosheets. The improved photocatalytic performance is originated from the high spatial separation of photoexcited charge carriers in CPB/BWO, which can also preserve strong individual redox abilities of two components. This work reports an efficient direct Z-scheme heterojunction photocatalytic system based on metal halide perovskites. The novel strategy we proposed may bring up new opportunities for the development of metal halide perovskite photocatalysts with greatly enhanced activities.

Recent Progresses on Metal Halide Perovskite-Based Material as Potential Photocatalyst

Recent years have witnessed an incredibly high interest in perovskite-based materials. Among this class, metal halide perovskites (MHPs) have attracted a lot of attention due to their easy preparation and excellent opto-electronic properties, showing a remarkably fast development in a few decades, particularly in solar light-driven applications. The high extinction coefficients, the optimal band gaps, the high photoluminescence quantum yields and the long electron–hole diffusion lengths make MHPs promising candidates in several technologies. Currently, the researchers have been focusing their attention on MHPs-based solar cells, light-emitting diodes, photodetectors, lasers, X-ray detectors and luminescent solar concentrators. In our review, we firstly present a brief introduction on the recent discoveries and on the remarkable properties of metal halide perovskites, followed by a summary of some of their more traditional and representative applications. In particular, the core of this work was to examine the recent progresses of MHPs-based materials in photocatalytic applications. We summarize some recent developments of hybrid organic–inorganic and all-inorganic MHPs, recently used as photocatalysts for hydrogen evolution, carbon dioxide reduction, organic contaminant degradation and organic synthesis. Finally, the main limitations and the future potential of this new generation of materials have been discussed.

The Role of Surface Termination in Halide Perovskites for Efficient Photocatalytic Synthesis

Halide perovskites have received attention in the field of photocatalysis owing to their excellent optoelectronic properties. However, the semiconductor properties of halide perovskite surfaces and the influence on photocatalytic performance have not been systematically clarified. Now, the conversion of triose (such as 1,3-dihydroxyacetone (DHA)) is employed as a model reaction to explore the surface termination of MAPbI₃ . By rational design of the surface termination for MAPbI₃ , the production rate of butyl lactate is substantially improved to 7719 μg g^-1  cat. h^-1 under visible-light illumination. The MAI-terminated MAPbI₃ surface governs the photocatalytic performance. Specially, MAI-terminated surface is susceptible to iodide oxidation, which thus promotes the exposure of Pb^II as active sites for this photocatalysis process. Moreover, MAI-termination induces a p-doping effect near the surface for MAPbI₃ , which facilitates carrier transport and thus photosynthesis.

Photoelectrochemically active perovskite QDs/TiO2 inverse opal with enhanced photoluminescence intensity

Photoluminescence intensity of the perovskite QDs coupled with TiO₂ was decreased significantly owing to the electron transfer between them. Hererin, the composite of CsPb(Cl_0.4Br_0.6)₃ with TiO₂ inverse opal was fabricated and we have proved that the effect of scattering of TiO₂ inverse opal layer by layer under the incident excitation light for the enhancement of perovskite QDs photoluminescence intensity is far greater than the decrease of photoluminescence intensity caused by the electron transfer between QDs and TiO₂. Particularly, photoelectrochemical characterizations exhibit high charge separation effciency and fast response speed in water. This study opens new possibilities for optoelectronic and photo display applications of perovskites-based NCs.