Plasmonic hot electrons: Potential candidates for improved photocatalytic hydrogen production

The photocatalytic water splitting is an emerging way of solar to hydrogen conversion due to its cheap and clean nature. The plasmon excitations in metallic sub-wavelength nanostructures are improving the semi-conductor industry through broadening of absorption range, enhancing charge separation and surging optical density of states in coupled semi-conductor. Besides these traditional enhancement mechanisms, the semi-conductor can be externally charged by additional carriers generated from plasmon-decay in ultrafast time scales. The mechanisms behind the generation of these metallic hot electrons and their transfer to semiconductor for enhanced photocatalytic hydrogen production have been reviewed. Furthermore, the road barriers limiting the efficiency of photocatalytic water splitting and its possible solutions through coupling with plasmonic resonators are presented. The possible problems and the expected solutions are proposed with the help of reasonable literature providing perspective for future research. This research finds its importance not only in green energy but opto-electronic industry as well.     

等离激元效应促进的光催化分解水制氢

  目的  利用光催化分解水的方式直接将太阳能转化并存储为氢气的化学能,是发展清洁能源促进低碳经济的有效途径。文章围绕等离激元效应对光催化分解水制氢的促进机理进行综述,以推进其产业化应用。  方法  阐释了等离激元效应在光催化分解水反应过程中的微观机制,分析了等离激元粒子在增强太阳光的吸收能力、拓展吸收光谱的响应范围、促进光生电子空穴的分离、提升光生载流子的热力学能、以及提供光催化反应活性中心等方面发挥的重要作用。  结果  通过总结当前等离激元效应促进光催化分解水制氢的研究进展,浅析了目前存在的问题,并对该领域的未来发展趋势进行了展望。  结论  基于等离激元效应在太阳能生产燃料中的巨大应用潜力,不同学科背景的相关学者协同合作,对影响光催化剂效率和寿命的各项决定性因素积极研发攻关,将促进该技术领域获得重要突破。     

Chemical strategies in molybdenum based chalcogenides nanostructures for photocatalysis

Since the last two decades, plenty of environmental issues have risen up due to the damage which humans have caused to the planet for the sake of development. The continual ignorance of global climate change and the stalemate approach of major oil producing industries led to the catastrophic melting of glaciers in Arctic and Antarctica and very recently the highest mountain peak of Sweden have become 24 m shorter which is the evident outcome of climatic disturbance. The chaotic unbalancing in the reservoirs of natural resources is leading us to the several crisis which has a potential to affect the livelihood. Among the various techniques used for the development of sustainable energy, photocatalysis is regarded as one of the simplest technique which can yield enormous amount of energy by the utilization of solar energy for meeting the world's demand of an energy requirement and which can be exploited in the degradation of toxic pollutants i.e. organic as well as inorganic pollutants for environment remediation. Transition metal chalcogenides (TMCs) have a potential to get adsorb easily and be utilized for solving the energy-related problems. Large number of photocatalysts has been fabricated, among them Molybdenum (Mo) chalcogenides nanostructures, which also belong to the class of TMCs exhibit exceptional properties such as non-toxicity, low cost and structural flexibility which give them edge over the other materials. Furthermore, the tunable band gap of Mo chalcogenides nanostructures makes them the promising candidates for efficient hydrogen evolution via photocatalytic water splitting in the visible light illumination. This review deals with the photocatalytic applications of Mo based chalcogenides nanostructures in efficient hydrogen production via water splitting and degradation of dyes. It also discusses the recent developments in fabricating Molybdenum chalcogenides nanostructures, their role in the photocatalytic water splitting and discusses the efforts which have been made to improve their photocatalytic activity for extending their applications to the scalable point.     

Photocatalytic water splitting hydrogen production via environmental benign carbon based nanomaterials

Environmentally benign hydrogen production via photochemical and photoelectrochemical processes by water splitting using carbon-based nanomaterials utilizes sunlight as the source of energy. Owing to their large surface area, pore volume, chemical and thermal stability, and favorable morphology, the carbon-based nanomaterials are quite effective in photocatalytic water splitting. The present review elucidates the photocatalytic nature of carbon materials such as graphene, graphene oxide, carbon nanotubes, graphitic carbon nitride, and fullerenes as they have the tendency to narrow the band gap, allocate electrons, and act as semiconductors, co-catalysts, photosensitizers, and support materials. The production methods, advantages as well as shortcomings of carbon-based materials and their applications in hydrogen production are critically discussed.     

Pristine and Se-/In-doped TlAsS2 enhance the solar energy-driven water splitting for hydrogen generation

The enhanced photocatalytic performance of Se-/In-doped TlAsS₂ to generate hydrogen from water splitting is investigated based on the first-principle density functional theory calculation with meta-GGA + TPSS. Three structures, namely, pristine TlAsS₂ and substitutions of S with Se and Tl with In, are considered. Their geometrical lattices are fully optimized and their electronic and optical properties are calculated to evaluate the photocatalytic efficiency for hydrogen generation. Results show that the three structures can be used for solar energy photocatalysis to generate hydrogen from water splitting. Moreover, the Se- and In-doped atoms can strengthen the absorption coefficient within the visible light range. Therefore, these structures are promising catalysts for generating hydrogen from water splitting through solar energy photocatalysis.     

Photocatalytic hydrogen production: A solar energy conversion alternative?

Since the beginning of earth, ultra-violet rays from the sun have been splitting water molecules in the upper reaches of the atmosphere, producing hydrogen and oxygen. At the same time, photosynthesis has been sustaining life on earth by utilizing the longer wavelengths of sunlight to split carbon and water molecules forming carbohydrates and oxygen. Attempts are being made to imitate these processes for the purpose of harnessing solar energy for man's use.This paper presents a state-of-the-art review of photolysis of water by sunlight with consequent energy storage in the molecular bonds of hydrogen and oxygen. The photocatalytic solar energy conversion methods reviewed are classified according to the photosensitizer type. That includes catalysts such as compound salts, compound semi-conductor crystals and photosynthetic dyes. An assessment of these methods concludes that photolysis of water as a solar energy conversion process does not seem feasible at present or for the immediate future. On the other hand, efficiency estimates indicate that with research emphasis this unconventional energy conversion method may prove to be a long-term alternative in harnessing solar energy for man's use.     

A current perspective for photocatalysis towards the hydrogen production from biomass-derived organic substances and water

Recently, an increasing interest has been devoted to produce chemical energy – hydrogen (H₂) by converting sustainable sunlight energy via water splitting and reforming of renewable biomass-derived organic substances. These photocatalytic processes are very promising, sustainable, economic, and environment-friendly. Herein, this article gives a concise overview of photocatalysis to produce H₂ as solar fuel via two approaches: water splitting and reforming of biomass-derived organic substances. For the first approach – photocatalytic water splitting, there are two reaction types have been used, including photoelectrochemical (PEC) and photochemical (PC) cell reactions. For the second approach, biomass-derived oxygenated substrates could undergo selective photocatalytic reforming under renewable solar irradiation. Significant efforts to date have been made for photocatalysts design at the molecular level that can efficiently utilize solar energy and optimize the reaction conditions, including light irradiation, type of sacrificial reagents. Critical challenges, prospects, and the requirement to give more attention to photocatalysis for producing H₂ are also highlighted.     

Review on hydrogen production photocatalytically using carbon quantum dots: Future fuel

They are sometimes identified as zero-dimensional (0D) nanoparticles. These particles have gained much attention in water splitting into hydrogen and oxygen through photocatalytic conversion. CQDs act as semiconductor few nm sizes, due to very small size; their optical and electronic properties differ from larger particles. CQDs particle has high stability, mild toxicity besides conductivity. These particles are environmentally friendly due to low toxicity and also have excellent luminescence. Therefore they can be utilized as a potential source for the splitting of water photocatalytically. The parting of water into H₂ and O₂ will enable us to produce or collect hydrogen to be used as a future fuel. The review summarizes the efforts made by various researchers in the field of utilizing carbon quantum dabs for water splitting which may be further followed by future researchers for commercial-scale hydrogen production. Thus, the study concludes the methods for the production of CQDs and their utilization under sunlight by catalytically hydrogen gas production from water.     

Novel one- and two-dimensional InSbS3 semiconductors for photocatalytic water splitting: The role of edge electron states

Photocatalytic water splitting has become a promising technology to solve environmental pollution and energy shortage. Exploring stable and efficient photocatalysts are highly desired. Herein, we propose novel low-dimensional InSbS₃ semiconductors with good stability based on density functional theory. Such InSbS₃ structures could be obtained from their bulk crystal by suitable exfoliation methods. Our calculations indicate that two-dimensional (2D) and one-dimensional (1D) InSbS₃ nanostructures have moderate band gaps (2.54 and 1.97 eV, respectively) and suitable band edge alignments, which represents sufficient redox capacity for photocatalytic water splitting. 2D InSbS₃ monolayer possesses oxygen evolution reaction (OER) activity and 1D InSbS₃ single-nanochain possesses hydrogen evolution reaction (HER) activity under acidic conditions. Interestingly, two edge electron states can be introduced when the dimension of InSbS₃ is reduced from 2D to 1D and the new electron states can exist in arbitrary-width nanoribbons, which can effectively promote the process of HER. Moreover, InSbS₃ monolayer and single-nanochain also exhibit large solar-to-hydrogen efficiency, high carrier mobility, and excellent optical absorption properties, which can facilitate the process of photocatalytic reactions. Our findings can stimulate the synthesis and applications of low-dimensional InSbS₃ semiconductors for overall water splitting.     

Photocatalytic degradation and hydrogen evolution using bismuth tungstate based nanocomposites under visible light irradiation

In this work, Bi₂WO₆/PANI composites were synthesized for efficient removal of ciprofloxacin (CIP) from urban wastewater and production of hydrogen energy in the absence of sacrificial agents. The experimental study visualizes the formation of 2D based nanostructures and perceived that these nanostructures could provide more photocatalytic active-sites for removal of CIP and also increase the oxidation/reduction of water for hydrogen energy production. The PXRD showed excellent crystallinity/orthorhombic structure with crystallite size 10–23 nm. The Bi₂WO₆/PANI composites, compared to Bi₂WO₆, exhibited higher efficiency and stability for degradation of CIP and production of hydrogen energy. CIP was effectively degraded 98% by Bi₂WO₆/PANI (5%) and the effect of different parameters such as pH, catalyst-concentration, and effect of CIP-concentration was also analyzed. The hydrogen energy rate was 490.56 h⁻¹g⁻¹ by using Bi₂WO₆/PANI (5%). The improved photocatalytic performance of Bi₂WO₆/PANI composite was mainly ascribed to the unique hierarchical structures, harvesting extended absorption of visible light, higher surface area, and higher crystallinity. The current findings may provide new insights to fabricate nanomaterials for environmental and energy issues.     

Photocatalytic hydrogen generation by splitting of water from electrospun hybrid nanostructures

MWCNT-TiO₂ hybrid nanostructures are prepared using sol–gel and electrospinning followed by post annealing of as-spun nanofibers at 450 °C per 1 h in air. These hybrid nanostructures composed of MWCNTs varied from 0 to 20% (w/w) and are characterized by SEM, TEM, XRD, and FT-IR analysis. MWCNT-TiO₂ hybrid structures are utilized in commercially available Methylene blue (MB) dye degradation and found that 2% of MWCNT exhibit superior kinetic constant 6.379 × 10⁻³ min⁻¹ extracted. In addition, we demonstrate that the doping of MWCTs within TiO₂ leads to a significant enhancement of the UV–vis light assisted photocatalytic activity is optimized in comparison with higher (5, 10 and 20%) compositions. UV–vis assisted photocatalytic hydrogen is evolved by photoelectrolytic splitting of water by using MWCNT-TiO₂ hybrid nanostructures as electrode.     

Facile synthesis of noble-metal free polygonal Zn2TiO4 nanostructures for highly efficient photocatalytic hydrogen evolution under solar light irradiation

Designing of noble-metal free and morphologically controlled advanced photocatalysts for photocatalytic water splitting using solar light is of huge interest today. In the present work, novel polygonal Zn₂TiO₄ (ZTO) nanostructures have been synthesized by citricacid assisted solid state method for the first time and synthesized nanostructures were characterized by using various techniques like PXRD, UV-Vis-DRS, PL, FT-IR, BET, FE-SEM and TEM for their structural, optical, chemical, surface and morphological properties. The PXRD and UV-Vis-DRS analysis show the existence of cubic and tetragonal phases. FE-SEM and TEM results confirm the formation of polygonal ZTO nanostructures. Synthesised ZTO nanostructures have been potentially applied for solar light-driven photocatalytic hydrogen evaluation from water splitting and compare the photocatalytic activity with synthesized conventional Zn₂TiO₄ and commercially available TiO₂, ZnO photocatalysts. A high rate of 529 μmolh^?1g^?1 solar light-driven photocatalytic H₂ evolution has been achieved by using a small amount (5 mg) of polygonal Zn₂TiO₄ nanostructures from glycerol-water solution. The enhanced photocatalytic performance of the polygonal Zn₂TiO₄ nanostructures compare to conventional Zn₂TiO₄ under solar light irradiation is due to the large surface area and low recombination rate. However having the same bandgap, the polygonal Zn₂TiO₄ nanostructures have shown enhanced photocatalytic performance than that of commercially available TiO₂, ZnO photocatalysts.     

A review on integrated thermochemical hydrogen production from water

Hydrogen production from water splitting is considered one of the most environmentally friendly processes for replacing fossil fuels. Among the various technologies to produce hydrogen from water splitting, thermochemical cycles using chemical reagents have the advantage of scale up compared to other specific facilities or geological conditions required. According to thermochemical processes using chemical redox reactions, 2-, 3-, 4-step thermochemical water splitting cycles can generate hydrogen more efficiently due to reducing temperatures. Increasing the number of cycles or steps of thermochemical hydrogen production could reduce the required maximum temperature of the facility. In addition, recently developed hybrid thermochemical processes combined with electricity or solar energy have been studied on a large scale because of the reduced cost of hydrogen production. Additionally, hybrid thermochemical water splitting combined with renewable energy can result in not only reducing the cost, but also increasing hydrogen production efficiency in terms of energy. As for a green energy, hydrogen production from water splitting using sustainable and renewable energy is significant to protect biological environment and human health. Additionally, hybrid thermochemical water splitting is conducive to large scale hydrogen production. This paper reviews the multi-step and highly developed hybrid thermochemical technologies to produce hydrogen from water splitting based on recently published literature to understand current research achievements.     

Exploration of 1D-2D LaFeO3/RGO S-scheme heterojunction for photocatalytic water splitting

Sustainable energy innovation is spearheading the way to achieve decarbonisation through commercially viable and highly competitive renewable technologies for green hydrogen. Photocatalytic water splitting has received global attention, as it promotes the direct conversion of solar energy to chemical energy and hydrogen production. Lanthanum orthoferrite (LaFeO₃) has been selected due to its narrow bandgap perovskite-oxides (ABO₃) type nature, low cost and high chemical stability but it is limited with fast charge recombination. To circumvent its constraint of fast charge recombination, an efficient graphene-based nanocomposite has been prepared by employing reduced graphene oxide (RGO) nanosheets as charge separators for visible light driven photocatalytic water splitting. Here, we present a thorough physical and spectroscopic characterization of the Lanthanum orthoferrite/Reduced Graphene oxide (LaFeO₃/RGO) nanocomposites, and investigate its photocatalytic and photoelectrochemical performance. The photocurrent density of the nanocomposites demonstrated ∼21 times higher in comparison to pure LaFeO₃. The as-prepared nanocomposites have been successfully used as photocatalysts for H₂ generation through water reduction under visible light. A significant enhancement in H₂ generation has been recorded for nanocomposites (∼82 mmol g⁻¹ h⁻¹) as compared to that of bare LaFeO₃ (∼9 mmol g⁻¹ h⁻¹) which is among the highest values obtained using noble-metal-free graphene-based photocatalytic nanocomposites. This work offers a facile approach for fabricating highly efficient 1D-2D heterostructure for photocatalysis application.     

Recent trends in MXene/Metal chalcogenides for electro-/photocatalytic hydrogen evolution reactions

Hydrogen due to high energy density and ecologically benign characteristics can become an excellent energy carrier for a sustainable energy economy and to appease the energy demand of humankind. Moreover, cost-effective and long-lasting photocatalysts can make the hydrogen generating process more economical and suitable. Recently, MXene have become one of the most sought-after composite materials for photocatalytic hydrogen generation. However, the photocstalytic performance can be further enhanced by doping with other semiconductor materials. Transition metal chalcogenides (Transition metals = Cu, Co, Ni, Zn, Cd, Mo, W)/MXene composites and mixed transition metal chalcogenide/MXene nanocomposites have been extensively investigated for the photocatalytic hydrogen generation. These materials possess unique two-dimensional layered structure that ameliorates the photocatalytic water splitting performance by increasing the light adsorption even at low photon flux density. The 2D design assists in reducing the distance necessary to transverse charge carriers to the surface. Because the layered structure tends to trap electrons in the ultrathin layers, 2D materials have unusual optoelectronic properties. In-plane covalent bonding assisted the creation of various heterojunctions and heterostructures in these 2D materials. Water splitting and hydrogen production are aided by the high surface area of these 2D materials. Due to its diverse elemental composition, unique 2D structure, good photoelectronic characteristics, large surface area, and many surface terminations. The design and production of many types of materials used as catalysts for the hydrogen evolution process are discussed in this article.     

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.     

(In,Cu) Co-doped ZnS nanoparticles for photoelectrochemical hydrogen production

In and Cu co-doped ZnS nanoparticles were successfully synthesized in DI water and ethanol solvent by a sonochemical approach using citric acid as surfactants in aqueous medium. FESEM micrographs show that In and Cu co-doped ZnS crystallites have a rough surface nanostructure and the as-synthesized photocatalysts were tested for the photocatalytic hydrogen evolution from water splitting via the irradiation of simulated sunlight. Among In and Cu co-doped ZnS products, 4In4CuZnS photocatalyst can achieve the maximum hydrogen production rate (752.7 μmol h⁻¹ g⁻¹) in 360 min under simulated sunlight illumination. Meanwhile, we separated the hydrogen and oxygen cells using an ion exchange membrane. Both electrodes (working electrode and Pt electrode) are dipped into each cell containing an aqueous solution containing 0.1 M Na₂S at pH 3 to convert water into hydrogen and oxygen under solar irradiation. As expected, the photoelectrochemical water splitting cells could significantly improve the photocatalytic activity, where the 4In4CuZnS nanoparticles shows the photoelectrochemical performance with photocurrent density of 12.2 mA cm⁻² at 1.1 V and hydrogen evolution rate of 1189.4 μmol h⁻¹ g⁻¹.     

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.     

Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.     

Solar photocatalytic reactor performance for hydrogen production from incident ultraviolet radiation

Solar based hydrogen production is a promising alternative to methods based on fossil fuels, such as steam methane reforming (SMR) and coal gasification. A more economically viable way of producing hydrogen from water is under active investigation by many researchers, to convert solar energy to chemical energy with higher efficiency. In this paper, supramolecular complexes developed by Brewer (2006) for photocatalytic hydrogen production are examined, particularly for larger scale engineering reactors that can use visible light to dissolve the photocatalysts in water, causing the splitting of water molecules into hydrogen and hydroxyl ions. This paper analyzes and optimizes the system parameters associated with this system. A predictive model for the reactor is developed for a batch type photocatalytic reactor. Results are presented and discussed to evaluate how the system parameters affect the hydrogen production rate, and solar to hydrogen efficiency, using a monochromatic LED array and Rhodium based photocatalysts.