Metal-organic frameworks (MOFs)-based efficient heterogeneous photocatalysts: Synthesis,properties and its applications in photocatalytic hydrogen generation,CO2 reduction and photodegradation of organic dyes

Metal-organic frameworks (MOFs) are a new class of functional materials having porous structures that show extraordinary specific surface areas, and tunable surface chemistry; hence, they hold great potential as photocatalysts. This review describes the fundamentals of MOFs and possible new research directions in the area of heterogeneous MOFs that can provide enhanced photocatalytic performance, especially for hydrogen production, degradation of emerging organic pollutants, and CO₂ reduction. The role of MOFs as multifunctional photocatalysts for light-stimulated organic reactions through an effective combination of metal/ligand/guest-based photocatalysts is discussed. Recent literature is discussed critically on the design and selection of materials, with possible directions to improve their catalytic properties. Furthermore, this comprehensive review systematically discusses the current developments of various MOFs-based hybrid nanostructures as multifunctional photocatalysts from different points, including several synthetic methodologies, key features, photocatalytic mechanism, and various influencing parameters to enhance catalytic efficiency. The recent achievements are critically discussed in the designing and selection of MOFs-based functional materials, with directions to effectively improve their catalytic properties for various photocatalytic applications. The article also summarizes with challenges and future prospects for the cost-effective and large scale photocatalytic applications of MOFs-based heterostructured catalysts.     

A critical review on core/shell-based nanostructured photocatalysts for improved hydrogen generation

Photocatalytic water splitting into gaseous hydrogen and oxygen in the presence of semiconductor photocatalysts under a visible spectrum of solar irradiation is one of the most promising processes for plummeting energy demands and environmental pollution. Among the successful photocatalytic materials, the core/shell nanostructures show promising results owing to their fascinating morphology that protects the surface features of the core besides the effective separation of photo-excitons resulting in an enhanced rate of hydrogen production up to 162 mmol h⁻¹g⁻¹_cat, which is a notable highest value reported in the literature. In this review, we have focused on the basic characteristics of the core-shell structure-based semiconductor photocatalytic systems and their efficient water-splitting reactions under light irradiation. Comprehensive detail on various synthesis methods of core-shell nanostructures, shell thickness-dependent properties, charge-transfer reaction mechanisms, and photocatalytic stability are highlighted in this review. Core-shell nanostructured materials have been extensively used as a photocatalyst, co-catalyst, and by coupling with supporting materials to improve the apparent quantum efficiency up to 45.6%. Besides, important photocatalytic properties that influence the redox reactions i.e., effective exciton separation, the effect of different light sources/wavelengths, surface charge modeling, photocatalytic active sites, and turnover frequency (TOF) have also been focused on and extensively described. Finally, the present and future prospects of the core-shell nanostructured photocatalysts for solar energy conversion into green hydrogen production have been expounded.     

Recent advances in the design of semiconductor hollow microspheres for enhanced photocatalyticv water splitting

Photocatalytic water splitting is a promising method to produce clean and renewable energy, which provides an alternative solution to solve environmental and resource problems. New catalysts based on semiconductor nanoparticles have received increasing attention since they facilitate all the reactions needed for “artificial photosynthesis”. In recent decades, hollow microspheres have provided an ideal platform for efficient utilization. Scientists are working to understand the basic principles, band structures, and modification strategies of hollow microspheres to enhance photocatalytic performance. In this paper, the research progress of hollow microsphere photocatalysts in the field of water splitting is reviewed. In particular, the photocatalytic principles of hollow microspheres and the methods to improve the performance of semiconductor photocatalysts are discussed in depth. The structural advantages and defects of hollow microspheres and modification methods of semiconductor band structure are introduced. Finally, the remaining challenges are summarized, and some insights into new trends and improvement directions for hollow materials are provided. This review will provide new insights for understanding hollow microspheres and help researchers in related fields to have a deeper and more comprehensive understanding of hollow microspheres in photocatalytic water splitting.     

Review of photocatalytic water‐splitting methods for sustainable hydrogen production

This paper examines photocatalytic hydrogen production as a clean energy solution to address challenges of climate change and environmental sustainability. Advantages and disadvantages of various hydrogen production methods, with a particular emphasis on photocatalytic hydrogen production, are discussed in this paper. Social, environmental and economic aspects are taken into account while assessing selected production methods and types of photocatalysts. In the first part of this paper, various hydrogen production options are introduced and comparatively assessed. Then, solar‐based hydrogen production options are examined in a more detailed manner along with a comparative performance assessment. Next, photocatalytic hydrogen production options are introduced, photocatalysis mechanisms and principles are discussed and the main groups of photocatalysts, namely titanium oxide, cadmium sulfide, zinc oxide/sulfide and other metal oxide‐based photocatalyst groups, are introduced. After discussing recycling issues of photocatalysts, a comparative performance assessment is conducted based on hydrogen production processes (both per mass and surface area of photocatalysts), band gaps and quantum yields. The results show that among individual photocatalysts, on average, Au–CdS has the best performance when band gap, quantum yield and hydrogen production rates are considered. From this perspective, TiO₂–ZnO has the poorest performance. Among the photocatalyst groups, cadmium sulfides have the best average performance, while other metal oxides show the poorest rankings, on average. Copyright © 2016 John Wiley & Sons, Ltd.     

Metal-organic frameworks for CO2 photoreduction

Metal-organic frameworks (MOFs) have attracted much attention because of their large surface areas, tunable structures, and potential applications in many areas. In recent years, MOFs have shown much promise in CO₂ photoreduction. This review summarized recent research progresses in MOF-based photocatalysts for photocatalytic reduction of CO₂. Besides, it discussed strategies in rational design of MOF-based photocatalysts (functionalized pristine MOFs, MOF-photosensitizer, MOF-semiconductor, MOF-metal, and MOF-carbon materials composites) with enhanced performance on CO₂ reduction. Moreover, it explored challenges and outlook on using MOF-based photocatalysts for CO₂ reduction.     

A review of TiO2 nanostructured catalysts for sustainable H2 generation

Hydrogen is an attractive alternative to fossil fuels that addresses several environmental and energy shortage issues. Nano-sized TiO₂-based photocatalysts with unique structural and functional properties are the most extensively studied photocatalytic nanomaterials for hydrogen production and pollutant degradation. However, titania is hampered by a wide band gap, low utilization of solar light and a rapid recombination of electron/hole pairs. These issues limit its photocatalytic performance. In this review, we present the latest developments in the fabrication of different higher dimensional TiO₂ nanostructured materials that aim to address these inherent limitations to an otherwise very promising material. Specifically, we will look into critical engineering strategies to enlarge the active surface area, enhance visible light absorption and suppress the recombination of electrons/holes that benefit their photo/photoelectric-catalytic water splitting activity. Finally, the current challenges and perspectives for TiO₂ nanostructures are also discussed. Continuous efforts are necessary to endow TiO₂-based materials with novel advanced functionality and commercialization potential in the coming years.     

Graphitic carbon nitride heterojunction photocatalysts for solar hydrogen production

Photocatalytic hydrogen production is considered as an ideal approach to solve global energy crisis and environmental pollution. Graphitic carbon nitride (g-C₃N₄) has received extensive consideration due to its facile synthesis, stable physicochemical properties, and easy functionalization. However, the pristine g-C₃N₄ usually shows unsatisfactory photocatalytic activity due to the limited separation efficiency of photogenerated charge carriers. Generally, introducing semiconductors or co-catalysts to construct g–C₃N₄–based heterojunction photocatalysts is recognized as an effective method to solve this bottleneck. In this review, the advantages and characteristics of various types of g–C₃N₄–based heterojunction are analyzed. Subsequently, the recent progress of highly efficient g–C₃N₄–based heterojunction photocatalysts in the field of photocatalytic water splitting is emphatically introduced. Finally, a vision of future perspectives and challenges of g–C₃N₄–based heterojunction photocatalysts in hydrogen production are presented. Predictably, this timely review will provide valuable reference for the design of efficient heterojunctions towards photocatalytic water splitting and other photoredox reactions.     

Review of photoluminescence performance of nano-sized semiconductor materials and its relationships with photocatalytic activity

In this review, the photoluminescence (PL) performance and mechanism of nano-sized semiconductor materials, such as TiO₂ and ZnO, are introduced, together with their attributes and affecting factors. Moreover, the applications of PL spectra in environmental photocatalysis are discussed in detail, viz. the inherent relationships between the PL intensity and photocatalytic activity are revealed on the basis of PL attributes, demonstrating that the PL spectra can reflect some important information such as surface defects and oxygen vacancies, surface states, photo-induced charge carrier separation and recombination processes in nano-sized semiconductor materials. Thus, the PL spectrum can provide a firm foundation in theory for designing and synthesizing new semiconductor photocatalysts with high activity, as well as quickly evaluating the photocatalytic activity of semiconductor samples.     

Borate particulate photocatalysts for photocatalytic applications: A review

Borate is a kind of wide-bandgap semiconducting material with rich structure and variety, which is usually used in optical properties research. Its unique crystal structure has essential research value for expanding the applications of photocatalysis. This review summarizes the recent research progress of borate photocatalysis, including novel borate photocatalyst, borate-based composite, and borate glass photocatalyst. Furthermore, the energy and environmental photocatalysis applications of borate photocatalysts are discussed, such as overall water splitting reaction, water decomposition to generate hydrogen or oxygen, degradation of pollutants, and others. The strategy of the combination of theoretical calculation and experiment to explore new borate photocatalysts was also introduced. Finally, some developing directions and challenges in borate photocatalytic materials are summarized.     

Recent advances in covalent organic framework (COF) nanotextures with band engineering for stimulating solar hydrogen production: A comprehensive review

Due to the continuous consumption of fossil fuels, natural reserves are depleting and it has been earnest need for developing new sources of energy. Among the several solar energy conversion techniques, photocatalytic hydrogen (H₂) generation is regarded as one of the most promising routes. Till date, several metal-based semiconductor materials have been investigated, however, H₂ generation is not substantial with the notion of sustainable development. Current research trends show the growing interest in advanced and metal free photocatalyst materials such as covalent organic frameworks (COFs) due to their several benefits such as crystalline porous polymers with pre-designed architectures, large surface area, exceptional stability, and ease of molecular functionalization. By combining COFs with other functional materials, composites may be created that display unique characteristics that exceed those of the separate components. This work provides a comprehensive development on COFs as a photocatalysts and their composites/hybrids for photocatalytic hydrogen generation with a focus on visible-light irradiation. To reduce the dependency on novel metals and overcome the drawbacks of individual material, the creation of composite materials based on covalent-organic frameworks (COFs) are systematically discussed. In addition, advantages in terms of performance, stability, durability of composites/hybrids COFs for photocatalytic hydrogen production in reference to traditional catalysts are investigated. Different composites such as metals loading, morphological development, band engineering, and heterojunctions are systematically discussed. Finally, challenges and opportunities associated with constructing COF-based catalysts as future research prospective for chemistry and materials science are highlighted.     

High efficiency photocatalytic CO2 reduction realized by Ca2+ and HDMP group Co-modified graphitic carbon nitride

The development of highly efficient and visible-light responsive carbon nitride (CN) photocatalysts is desirable to address energy shortages and environmental pollution challenges. Herein, we synthesized novel 2-hydroxy-4,6-dimethylpyrimidine (HDMP) group and Ca²⁺ co-modified carbon nitride (CN) photocatalyst (CN-CAA) using a facile in situ copolymerization procedure employing urea and calcium acetylacetonate (CAA) as precursors. The HDMP group and Ca²⁺ co-modification contributed to increased electron density and modulated electronic structure, resulting in extended visible light harvesting and accelerated separation and migration of photoinduced charge carriers. Benefiting from the enhanced visible light utilization and improved photoexcited carriers separation and transportation, the CN-CAA exhibited significantly elevated visible-light-driven photocatalytic activity for CO₂ reduction. This work provided a new insight into the photocatalytic performance promotion of CN through molecular engineering and metal ions incorporation co-modification.     

Rational design and fabrication of surface tailored low dimensional Indium Gallium Nitride for photoelectrochemical water cleavage

Currently several type of energy sources exist in the modern world. The energy makes people's life more comfortable, easy, time savings, fast transformation of information and various modes of transmission. Because of large demand of energy, efforts on production of energy increases day by day which subsequently increase serious environmental concerns such as pollution and lack of existing natural resources. In this respect, several attempts have been proposed for new type of renewable and chemical energy systems to overcome the economic burden, global warming and environmental problems caused by the use of conventional fossil fuels. Hydrogen production via water splitting is a promising and ideal route for renewable energy using the most abundant resources of solar light and water. Cost effective photocatalyst for Photoelectrochemical (PEC) water splitting using semiconductor materials as light absorbers have been extensively studied due to their stability and simplicity. Over the past few decades, various metal oxide photocatalysts for water splitting have been developed and their photocatalytic application was studied under UV irradiation. Alternative semiconductor photocatalyst should harness solar energy in the visible light, one such semiconductor material is indium gallium nitride (InGaN), owing to its suitable and tunable energy band-gap, chemical resistance and notable photoelectrocatalytic activity. This review article is initiated with the brief introduction about the origin and methods of production of hydrogen gas from both renewable and nonrenewable energy sources. Multi-functional properties and applications of InGaN are described along with past and recent efforts of InGaN materials for hydrogen evolution by several investigators are provided in detail. In addition, future prospects and ways to improve the PEC performance of InGaN are presented at the end of this review.     

Organic conjugated polymers and polymer dots as photocatalysts for hydrogen production

Owing to the outstanding characteristics of tailorable electronic and optical properties, semiconducting polymers have attracted considerable attention in recent years. Among them, organic polymer dots process large breadth of potential synthetic diversity are the representative of photocatalysts for hydrogen production, which presents both an opportunity and a challenge. In this mini-review, first, the organic polymer photocatalysts were introduced. Then, recent reports on polymer dots which showed a superior photocatalytic activity and a robust stability under visible-light irradiation, for hydrogen production were summarized. Finally, challenges and outlook on using organic polymer dots-based photocatalysts from hydrogen production were discussed.     

A concise review: MXene-based photo catalytic and photo electrochemical water splitting reactions for the production of hydrogen

Since 2015, a number of breakthroughs in the generation of new MAX phases using specified double transition metals have made possible the synthesis of unique MXenes with significant chemical diversity and structural complexity, which are rare in 2D families. MXene and semiconductor hybrids are shown to be effective photocatalysts because to their unique interface features. For photocatalytic purposes, a Schottky heterojunctions may provide faster charge separation and a lower Schottky barrier. When it comes to photocatalytic and photo electrochemical applications, photocatalysts are predicted to be the greatest and most popular new photocatalysts. We discussed some of the semiconductor-based nanocomposites supported by MXenes, including photocatalytic and photo electrochemical water splitting. Next, we discussed some of the difficulties and opportunities that have arisen from working with MXenes to advance semiconductor-based photocatalysts. Scientists in related fields are noticed to prompt the development of novel photocatalysts based on semiconductors.     

Solar-based photoreduction of methyl orange using zeolite supported photocatalytic materials

A new class of novel photocatalysts has been prepared by supporting TiO₂ on the zeolite matrix by various routes of synthesis. Different transition metals like cobalt, nickel, and ruthenium have been incorporated in these photocatalysts, alongwith molybdenum based heteropolyacid (HPA) to improve the photocatalytic activity of these materials. Photoreduction of methyl orange under solar radiation was compared with photoreduction in presence of artificial visible light illumination to evaluate their photocatalytic activity. The quantity of methyl orange photoreduced by the cobalt containing photocatalyst was about 2.40 mg/g of TiO₂ under the influence of sunlight as compared to 4.111 mg/g of TiO₂ under artificial visible light illumination. However, the efficiency of the photocatalyst is high as compared to P25 TiO₂ under solar light (0.508 mg/g of TiO₂). The high photocatalytic activity of these materials is due to the synergistic effect of incorporation of transition metals in combination with TiO₂ and HPA supported by the zeolite matrix. These materials are being evaluated for photocatalytic water splitting.     

A review on graphitic carbon nitride (g-C3N4) – metal organic framework (MOF) heterostructured photocatalyst materials for photo(electro)chemical hydrogen evolution

Graphitic carbon nitride (g-C₃N₄)-based heterostructured photocatalysts have recently attracted significant attention for solar water splitting and photocatalytic hydrogen (H₂) evolution, because of their alterable physicochemical, optical and electrical properties, such as tunable band structure, ultrahigh specific surface area and controllable pore size, defect formation and active sites. On the other hand, metal-organic frameworks (MOFs) possess a favorable surface area, permanent porosity and adjustable structures that allow them to be suitable candidates for diverse applications. In this review, we therefore comprehensively discuss the structural properties of heterogeneous g-C₃N₄/MOF-based photocatalysts with a special emphasis on their photocatalytic performance regarding the mechanism of heterogeneous photocatalysis, including advantages, challenges and design considerations.     

CuInS2-based photocatalysts for photocatalytic hydrogen evolution via water splitting

The realization of efficient photocatalytic hydrogen evolution (PHE) significantly depends on the development of durable and effective semiconductor photocatalysts. Copper indium sulfide (CuInS₂) is an emerging ternary chalcogenide semiconductor material for solar-to-chemical energy application, because it possesses a suitable bandgap, environment-friendly elements, and a low melting point. CuInS₂-based semiconductor photocatalysts have been investigated for PHE via water splitting, but current PHE performance still has difficulty in meeting commercial application requirements and needs to be further improved. In this review, the basic semiconductor properties of CuInS₂, including its crystal and band structures, are introduced, and its PHE mechanism is discussed in detail. The PHE performance of CuInS₂-based photocatalysts is systematically discussed, with a focus on morphology, engineered structure, and heterojunction construction. Finally, issues and challenges currently encountered in the PHE application of CuInS₂-based photocatalysts and their possible solutions are presented.     

Nanostructured materials for the visible-light driven hydrogen evolution by water splitting: A review

The conversion of abundantly available photonic energy into useful chemical energy is considered to be a greener protocol for addressing the energy shortage. Recently, since most of the emphasis has been centralized on the semiconductor-based photocatalysis; the designing and fabrication of the novel semiconductor photocatalytic material is happening at a blistering rate. Recently, the nanostructured materials have attracted ever-growing research attention as photocatalytic material for hydrogen generation reaction by dissociation of water. Such photocatalytic nanomaterials are known to exhibit superior activity than their corresponding bulk counter-parts because of the improved interfacial charge separation and the broad surface area providing sufficient active sites. However, the improvement in the efficiency and selectivity towards hydrogen production reaction under solar or visible light radiation always remains a challenging assignment. In the present review, the segregation of the so far reported nanostructured photocatalysts into different categories, based on their dimensionality such as 0-D, 1-D and 2-D materials, is implemented. Furthermore, their synthetic route and the photocatalytic hydrogen evolving efficiencies are explored and briefly summarized. Moreover, the methodology of development of nanocomposite materials leading to the construction of heterojunctions including Type-I, Type-II, Type-III, Z-Scheme and S-Scheme system is also discussed. In addition, an in-depth investigation on the charge carrier's generation, separation and their transportation is also reviewed. Finally, the future perspectives regarding the designing of an efficient, stable and economic photoactive nano-architecture material for the efficient hydrogen production via photocatalytic dissociation of water are also pointed.     

Synthesis of carbon-doped SnO2 nanostructures for visible-light-driven photocatalytic hydrogen production from water splitting

Environmental issues: global warming, organic pollution, CO₂ emission, energy shortage, and fossil fuel depletion have become severe threats to the future development of humans. In this context, hydrogen production from water using solar light by photocatalytic/photoelectrochemical technologies, which results in zero CO₂ emission, has received considerable attention due to the abundance of solar radiation and water. Herein, a single-step thermal decomposition procedure to produce carbon-doped SnO₂ nanostructures (C–SnO₂) for photocatalytic applications is proposed. The visible-light-driven photocatalytic performance of the as-prepared materials is evaluated by photocatalytic hydrogen generation experiments. The bandgaps of the photocatalysts are determined by ultraviolet–visible diffused reflectance spectroscopy. The crystallinity, morphological features (size and shape), and chemical composition and elemental oxidation states of the samples are investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The proposed simple thermal decomposition method has significant potential for producing nanostructures for metal-free photocatalysis.     

Co-application of liquid phase plasma process for hydrogen production and degradation of acetaldehyde over NiTiO2 supported on porous materials

Photocatalytic decomposition of acetaldehyde-contained wastewater was assessed for the degradation of pollutants and the production of hydrogen. Liquid phase plasma was applied in the photoreaction as a light source. The evolution of hydrogen and acetaldehyde degradation were characterized by the photocatalytic decomposition system. Ni-loaded TiO₂ photocatalysts and various porous materials were introduced to the photocatalytic reaction. The photochemical decomposition by irradiation of the liquid phase plasma without photocatalysts produced some hydrogen evolution with the degradation of acetaldehyde, which was attributed to the decomposition of the reactant by active species generated by the irradiation of liquid phase plasma. The Ni loading on TiO₂ brought out an enhancement of acetaldehyde degradation and hydrogen evolution. In the photocatalysis of acetaldehyde-contained wastewater using the liquid phase plasma, hydrogen evolution was accelerated due to the additional hydrogen production by the photocatalytic decomposition of acetaldehyde. The porous materials could be used as an effective photocatalytic support. MCM-41 mesoporous material has acted as a highly efficient photocatalytic support to the TiO₂ photocatalyst.