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.

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.

Sr2NiWO6 Double Perovskite Oxide as a Novel Visible-Light-Responsive Water Oxidation Photocatalyst

Screening of stable visible-light-responsive water oxidation semiconductor photocatalysts is highly desired for the development of photocatalytic water splitting systems. Herein, a visible-light-absorbing Sr₂NiWO₆ double perovskite oxide photocatalyst was successfully prepared via a conventional solid-state reaction method. The intrinsic Sr₂NiWO₆ shows photocatalytic oxygen evaluation reaction (OER) activity of 60 μmol h^-1 g^-1, even without loading any cocatalysts. The DFT calculation indicates that the Ni species on the surface is the active site for the OER. The photocatalytic OER activity was further improved by loading Pt and RuO₂ dual redox cocatalysts on the surface of Sr₂NiWO₆ to achieve a photocatalytic OER activity of 420 μmol h^-1 g^-1, which corresponds to a remarkable apparent quantum efficiency (AQE) of 8.6% (λ ≈ 420 nm). The result indicates that Sr₂NiWO₆ is one of the best double perovskite oxide-based photocatalysts for the photocatalytic OER, and the activity is even comparable to the benchmark BiVO₄-based photocatalyst. The improvement of the photocatalytic OER activity is due to the provision of more active redox sites as well as the synergetic effect of the dual redox cocatalysts in facilitating charge separation and transfer. This work demonstrates that double perovskite oxides may serve as a novel class of efficient and stable oxide-based semiconductor photocatalysts for water splitting.

Density Functional Study of Cubic, Tetragonal, and Orthorhombic CsPbBr3 Perovskite

Cesium lead bromide (CsPbBr3) perovskite has recently gained significance owing to its rapidly increasing performance when used for light-emitting devices. In this study, we used density functional theory to determine the structural, electronic, and optical properties of the cubic, tetragonal, and orthorhombic temperature-dependent phases of CsPbBr3 perovskite using the full-potential linear augmented plane wave method. The electronic properties of CsPbBr3 perovskite have been investigated by evaluating their changes upon exerting spin-orbit coupling (SOC). The following exchange potentials were used: the local density approximation (LDA), Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA), Engel-Vosko GGA (EV-GGA), Perdew-Burke-Ernzerhof GGA revised for solids (PBEsol-GGA), modified Becke-Johnson GGA (mBJ-GGA), new modified Becke-Johnson GGA (nmBJ-GGA), and unmodified Becke-Johnson GGA (umBJ-GGA). Our band structure results indicated that the cubic, tetragonal, and orthorhombic phases have direct energy bandgaps. By including the SOC effect in the calculations, the bandgaps computed with mBJ-GGA and nmBJ-GGA were found to be in good agreement with the experimental results. Additionally, despite the large variations in their lattice constants, the three CsPbBr3 phases possessed similar optical properties. These results demonstrate a wide temperature range of operation for CsPbBr3.

Inorganic Halide Double Perovskites with Optoelectronic Properties Modulated by Sublattice Mixing

All-inorganic halide double perovskites have emerged as a promising class of materials that are potentially more stable and less toxic than lead-containing hybrid organic-inorganic perovskite optoelectronic materials. In this work, 311 cesium chloride double perovskites (Cs₂BB'Cl₆) were selected from a set of 903 compounds as likely being stable on the basis of a statistically learned tolerance factor (τ) for perovskite stability. First-principles calculations on these 311 double perovskites were then performed to assess their stability and identify candidates with band gaps appropriate for optoelectronic applications. We predict that 261 of the 311 Cs₂BB'Cl₆ compounds are likely synthesizable on the basis of a thermodynamic analysis of their decomposition to competing compounds (decomposition enthalpy <0.05 eV/atom). Of these 261 likely synthesizable compounds, 47 contain no toxic elements and have direct or nearly direct (within 100 meV) band gaps between 1 and 3 eV, as computed with hybrid density functional theory (HSE06). Within this set, we identify the triple-alkali perovskites Cs₂[Alk]⁺[TM]³⁺Cl₆, where Alk is a group 1 alkali cation and TM is a transition-metal cation, as a class of Cs₂BB'Cl₆ double perovskites with remarkable optical properties, including large and tunable exciton binding energies as computed by the GW-Bethe-Salpeter equation (GW-BSE) method. We attribute the unusual electronic structure of these compounds to the mixing of the Alk-Cl and TM-Cl sublattices, leading to materials with small band gaps, large exciton binding energies, and absorption spectra that are strongly influenced by the identity of the transition metal. The role of the double-perovskite structure in enabling these unique properties is probed through an analysis of the electronic structures and chemical bonding of these compounds in comparison with other transition-metal and alkali transition-metal halides.

Structural and electronic properties of NaTaO3 cubic nanowires

Sodium tantalate 1-D nanostructures show novel properties due to their edge confinement region, which may be relevant for distinct applications.

Effects of a Ni cocatalyst on the photocatalytic hydrogen evolution reaction of anatase TiO2 by first-principles calculations

Loading Ni_n clusters on anatase TiO₂ surfaces remarkably reduces the Gibbs free energy of the photocatalytic hydrogen evolution reaction.

Facile hydrothermal synthesis of CuO–Cu2O/GO nanocomposites for the photocatalytic degradation of organic dye and tetracycline pollutants

A bifunctional nanoscale photocatalyst was constructed by developing a one-step method for the in situ growth of CuO–Cu₂O nanoparticles on GO sheets, and the photocatalytic mechanism was inferred.

One-pot fabrication of 2D/2D HCa2Nb3O10/g-C3N4 type II heterojunctions towards enhanced photocatalytic H2 evolution under visible-light irradiation

Effective 2D-composite photocatalysts, HCa₂Nb₃O₁₀/g-C₃N₄, with enhanced photocatalytic activity are conveniently synthesized by a facile one-pot method.

Recent advances in synthesis and application of perovskite quantum dot based composites for photonics, electronics and sensors

In recent years, halide perovskite quantum dots (HP-QDs) based composites have been widely developed and used in various applications owing to their unique photonic, electronic and mechanical properties, as well as high stability to water, oxygen, heat and illumination. Remarkable efforts have been made in the synthesis and applications of these materials in photonics, electronics, sensors and other fields. Besides these topics, we also cover enhancement of optoelectronic properties as well as chemical, thermal and photostability of HP-QDs-based composites. We hope this review will promote both the development and applications of perovskite-based materials.

Tailoring of an unusual oxidation state in a lanthanum tantalum(IV) oxynitride via precursor microstructure design

Perovskite-type oxynitrides hold great potential for optical applications due to their excellent visible light absorption properties. However, only a limited number of such oxynitrides with modulated physical properties are available to date and therefore alternative fabrication strategies are needed to be developed. Here, we introduce such an alternative strategy involving a precursor microstructure controlled ammonolysis. This leads to the perovskite family member LaTa(IV)O2N containing unusual Ta4+ cations. The adjusted precursor microstructures as well as the ammonia concentration are the key parameters to precisely control the oxidation state and O:N ratio in LaTa(O,N)3. LaTa(IV)O2N has a bright red colour, an optical bandgap of 1.9 eV and a low (optically active) defect concentration. These unique characteristics make this material suitable for visible light-driven applications and the identified key parameters will set the terms for the targeted development of further promising perovskite family members.

Unveiling the efficiency of microwave-assisted hydrothermal treatment for the preparation of SrTiO3 mesocrystals

Material processing has become essential for the proper control, tuning and consequent application of the properties of micro/nanoparticles. In this case, we report herein the capability of the microwave-assisted hydrothermal (MAH) method to prepare the SrTiO3 compound, as a case study of inorganic compounds. Analyses conducted by X-ray diffraction, X-ray photoelectron and X-ray absorption spectroscopies confirmed that the MAH route enables the formation of pristine SrTiO3. The results indicated that the combination of thermal and non-thermal effects during the MAH treatment provides ideal conditions for an efficient and rapid synthesis of pristine SrTiO3 mesocrystals. Scanning electron microscopy images revealed a cube-like morphology (of ca. 1 μm) formed via a self-assembly process, influenced by the MAH time. Additionally, photoluminescence measurements revealed a broad blue emission related to intrinsic defects, which decreased with the MAH synthesis time.

Hollow Nanostructures for Photocatalysis: Advantages and Challenges

Photocatalysis for solar-driven reactions promises a bright future in addressing energy and environmental challenges. The performance of photocatalysis is highly dependent on the design of photocatalysts, which can be rationally tailored to achieve efficient light harvesting, promoted charge separation and transport, and accelerated surface reactions. Due to its unique feature, semiconductors with hollow structure offer many advantages in photocatalyst design including improved light scattering and harvesting, reduced distance for charge migration and directed charge separation, and abundant surface reactive sites of the shells. Herein, the relationship between hollow nanostructures and their photocatalytic performance are discussed. The advantages of hollow nanostructures are summarized as: 1) enhancement in the light harvesting through light scattering and slow photon effects; 2) suppression of charge recombination by reducing charge transfer distance and directing separation of charge carriers; and 3) acceleration of the surface reactions by increasing accessible surface areas for separating the redox reactions spatially. Toward the end of the review, some insights into the key challenges and perspectives of hollow structured photocatalysts are also discussed, with a good hope to shed light on further promoting the rapid progress of this dynamic research field.

Perovskite Structure Associated with Precious Metals: Influence on Heterogenous Catalytic Process

The use of perovskite-based materials and their derivatives can have an important role in the heterogeneous catalytic field based on photochemical processes. Photochemical reactions have a great potential to solve environmental damage issues. The presence of precious metals in the perovskite structure (i.e., Ag, Au, or Pt) may improve its efficiency significantly. The precious metal may comprise the perovskite lattice as well as form a heterostructure with it. The efficiency of catalytic materials is directly related to processing conditions. Based on this, this review will address the use of perovskite materials combined with precious metal as well as their processing methods for the use in catalyzed reactions.

Perovskite Oxide Based Electrodes for High-Performance Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is an attractive strategy for the large-scale production of renewable hydrogen from water. Developing cost-effective, active and stable semiconducting photoelectrodes is extremely important for achieving PEC water splitting with high solar-to-hydrogen efficiency. Perovskite oxides as a large family of semiconducting metal oxides are extensively investigated as electrodes in PEC water splitting owing to their abundance, high (photo)electrochemical stability, compositional and structural flexibility allowing the achievement of high electrocatalytic activity, superior sunlight absorption capability and precise control and tuning of band gaps and band edges. In this review, the research progress in the design, development, and application of perovskite oxides in PEC water splitting is summarized, with a special emphasis placed on understanding the relationship between the composition/structure and (photo)electrochemical activity.

Oxygen-Vacancy-Introduced BaSnO3- δ Photoanodes with Tunable Band Structures for Efficient Solar-Driven Water Splitting

To achieve excellent photoelectrochemical water-splitting activity, photoanode materials with high light absorption and good charge-separation efficiency are essential. One effective strategy for the production of materials satisfying these requirements is to adjust their band structure and corresponding bandgap energy by introducing oxygen vacancies. A simple chemical reduction method that can systematically generate oxygen vacancies in barium stannate (BaSnO₃ (BSO)) crystal is introduced, which thus allows for precise control of the bandgap energy. A BSO photoanode with optimum oxygen-vacancy concentration (8.7%) exhibits high light-absorption and good charge-separation capabilities. After deposition of FeOOH/NiOOH oxygen evolution cocatalysts on its surface, this photoanode shows a remarkable photocurrent density of 7.32 mA cm^-2 at a potential of 1.23 V versus a reversible hydrogen electrode under AM1.5G simulated sunlight. Moreover, a tandem device constructed with a perovskite solar cell exhibits an operating photocurrent density of 6.84 mA cm^-2 and stable gas production with an average solar-to-hydrogen conversion efficiency of 7.92% for 100 h, thus functioning as an outstanding unbiased water-splitting system.

Facile Chemical Synthesis and Potential Supercapattery Energy Storage Application of Hydrangea-type Bi2MoO6

Soft chemical synthesis is used to obtain a hydrangea-type bismuth molybdate (Bi2MoO6) supercapattery electrode that demonstrates considerable energy/power density and cycling life. Structure and morphology studies, initially, reveal a phase-pure polycrystalline and hydrangea-type surface appearance for Bi2MoO6, which upon testing in an electrochemical energy storage system displays supercapattery behavior, a combination of a supercapacitor and a battery. From the power law, an applied-potential-dependent charge storage mechanism is established for the Bi2MoO6 electrode material. A Trasatti plot evidences the presence of inner and outer surface charges. The hydrangea-type Bi2MoO6 electrode demonstrates a specific capacitance of 485 F g-1 at 5 A g-1 and a stability of 82% over 5000 cycles. An assembled symmetric supercapattery with a Bi2MoO6//Bi2MoO6 configuration demonstrates energy and power densities of 45.6 W h kg-1 and 989 W kg-1, respectively. A demonstration elucidating the lighting up of three light-emitting diodes, connected in series, by the symmetric supercapattery signifies the practical potentiality of the as-synthesized hydrangea-type Bi2MoO6 electrode in energy storage devices.

Solid-State, Low-Cost, and Green Synthesis and Robust Photochemical Hydrogen Evolution Performance of Ternary TiO2/MgTiO3/C Photocatalysts

Solar-driven photochemical hydrogen evolution is a promising route to sustainable hydrogen fuel production. Large-scale preparation of highly active photocatalysts using elementally abundant and less-expensive materials is urgently required for widespread practical application. Here, we report a highly efficient and low-cost TiO₂/MgTiO₃/C heterostructure photocatalyst for photochemical water splitting, which was synthesized on gram scale via a facile mechanochemical method. The heterostructure and carbon sensitization offer excellent photoconversion efficiency as well as good photostability. Under irradiation of one AM 1.5G sunlight, the optimal TiO₂/MgTiO₃/C photocatalyst can show a great solar-driven hydrogen evolution rate (33.3 mmol·h⁻¹·g⁻¹), which is much higher than the best yields ever reported for MgTiO₃-related photocatalysts or pure TiO₂ (P-25). We hope this work will attract more attention to inspire further work by others for the development of low-cost, efficient, and robust photocatalysts for producing hydrogen in artificial photosynthetic systems.

•

The synthetic process does not involve any organic solvents and hazardous by-products

•

The optimal sample shows a H₂ evolution rate of 33.3 mmol·h⁻¹·g⁻¹ under one sunlight

•

The optimal sample keeps a H₂ evolution rate of 1.46 mmol·h⁻¹·g⁻¹ under visible light

•

Uniform carbon layer was ingeniously deposited on the surface of photocatalysts

Sonophotocatalytic treatment of rhodamine B using visible-light-driven CeO2/Ag2CrO4 composite in a batch mode based on ribbon-like CeO2 nanofibers via electrospinning

CeO₂/Ag₂CrO₄ composite photocatalyst was successfully fabricated using electrospinning and calcination and chemical precipitation method based on CeO₂ ribbon-like fibers and characterized by field-emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX), X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS) and Fourier-transform infrared spectroscopy (FT-IR). The as-obtained CeO₂/Ag₂CrO₄ composite used photocatalytic performance in the sonophotodegradation of rhodamine B in aqueous solution under visible-light (LED) irradiation. DRS analysis illustrates that CeO₂/Ag₂CrO₄ composite exhibited enhanced absorption in the visible region-attributed CeO₂ nanofibers_. The effect of four effective parameters including initial concentration of rhodamine B (RhB), photocatalyst dosage, pH, and irradiation time was studied and optimized using central composite design. The kinetic studies confirmed ability of pseudo first-order reaction based on the Langmuir-Hinshelwood model for fitting empirical data, while its rate constant (k_obs), L-H rate constants (k_r), and L-H adsorption constants (K_A) were 0.0449 min^-1, 11.66 mg L^-1 min^-1 and 1.09E-3 mg L^-1, respectively. The enhanced photocatalytic activity could be ascribed to the ultrasound field and formation of a heterojunction system among CeO₂ and Ag₂CrO₄, which lead to a better mass transfer and higher efficiency of charge electron-hole separation, respectively.

Inkjet Printing of Sc-Doped TiO2 with Enhanced Photoactivity

Here we report the methodology for nanocomposite fabrication based on the inkjet printing technique. Doped TiO2 nanoparticles with Sc contents up to 10 wt.% were synthesized and adapted towards a facile fabrication of microscale structures and thin film printing. Implementation of the state-of-the-art low-temperature synthesis allowed to us successfully incorporate high concentrations of Sc3+ ions into the TiO2 lattice and improve the light absorption characteristics of the resulting materials. Without affecting the anatase structure substantially, Sc doping gave rise to an intensified absorbance capacity and provided the means for the efficient fabrication of Sc-TiO2 microarchitectures via the inkjet printing technique. The changes in the spectral and structural characteristics of the Sc-TiO2 composites were observed by Energy Dispersive X-Ray spectroscopy (EDX), X-ray diffraction (XRD), and UV-vis methods. The rheological parameters of the colloidal suspension based on the synthesized Sc-TiO2 nanoparticles were adapted for inkjet printing in terms of the optimal viscosity, morphology, and surface tension. The developed individual ink characteristics allowed us to produce a close coherence between the enhanced optical properties of the Sc-TiO2 prepared the sol–gel method and the inkjet-printed films. The introduced methodology features the possibility to inkjet-print doped and pure TiO2 robust films for potential large-scale fabrication.

Interstitial N-doped SrSnO3 perovskite: structural design, modification and photocatalytic degradation of dyes

An easy-to-manipulate, two-step, solid-state synthetic method was adopted to incorporate N element into the SrSnO₃ perovskite for structural modification, which improved its photocatalytic performance.

Effect of trap states on photocatalytic properties of boron-doped anatase TiO2 microspheres studied by time-resolved infrared spectroscopy

The photocatalytic hydrogen and oxygen evolution switching effect in the water splitting of two boron-doped anatase TiO₂ microspheres was elucidated from the viewpoint of trap states.

Effect of the oxygen coordination environment of Ca–Mn oxides on the catalytic performance of Pd supported catalysts for aerobic oxidation of 5-hydroxymethyl-2-furfural

Pd/CaMn₂O₄ provides ideal active sites for oxygen adsorption and desorption, resulting in the promoted charge transfer ability and catalytic activity.

Prussian blue derived Fe2N for efficiently improving the photocatalytic hydrogen evolution activity of g-C3N4 nanosheets

Prussian blue derived Fe₂N nanoparticles to efficiently improve the photocatalytic H₂-generation rate over pure g-C₃N₄ nanosheets.

In situ topotactic formation of 2D/2D direct Z-scheme Cu2S/Zn0.67Cd0.33S in-plane intergrowth nanosheet heterojunctions for enhanced photocatalytic hydrogen production

2D/2D direct Z-scheme Cu₂S/Zn_0.67Cd_0.33S in-plane intergrowth nanosheet heterojunction exhibited the high-performance hydrogen evolution activity.

Time-resolved Fluorescent Detection for Glucose Using a Complex of Luminescent Layered Titanates and Enzymes

Luminescent europium-doped layered titanates (Eu-TiOx) were synthesized and complexed with horseradish peroxidase (HRP) and glucose oxidase (GOx). The emission of a resultant Eu-TiOx/HRP/GOx complex decreased upon the addition of glucose in the presence of guaiacol. The emission decrease was dependent on the concentrations of glucose, and the detection limit for glucose was 3.1 μM. The proposed system would be promising as a new detection method for glucose.

Insight into the enhanced photocatalytic activity of SrTiO3 in the presence of a (Ni, V/Nb/Ta/Sb) pair

Increasing applications of SrTiO3 as a photocatalyst in recent times drive the development of various strategies through doping with foreign elements to improve its efficiency under sunlight. Motivated by the recent experimental observation of increased lifetime of photogenerated charge carriers due to codoping of Ta into Ni-doped SrTiO3 (R. Niishiro et al., Phys. Chem. Chem. Phys., 2005, 7, 2241-2245, and A. Yamakata et al., J. Phys. Chem. C, 2016, 120, 7997-8004), we systematically investigate the detailed electronic structure of Ni-doped SrTiO3 in the presence and absence of Ta. The present theoretical study reveals that Ni-doping reduces the effective band gap by introducing unoccupied Ni-3d states in the forbidden region, while addition of Ta passivates these states. Here, we have properly explained the fact that improved photoconversion efficiency can be achieved only when the proportion of Ta is double with respect to that of Ni. The defect formation energy for the 1 : 2 type (Ni, Ta)-codoped SrTiO3 is energetically more favourable than that of the 1 : 1 type variety. The present study also explored the possibility of using V, Nb, and Sb in place of Ta to aim at better utilization of visible light activity. Interestingly, we arrive at a conclusion that V and Nb may be better choices over experimentally reported Ta for achieving enhanced photocatalytic activity of Ni-doped SrTiO3 under visible light. Finally, applicability of all these codoped systems for the generation of hydrogen and oxygen through water splitting has been checked by inspecting their band edge levels with respect to water redox levels.