Recent advancements of layered double hydroxide heterojunction composites with engineering approach towards photocatalytic hydrogen production: A review

Photocatalytic hydrogen production has been considered as one of the most promising alternatives for providing clean, sustainable, and renewable energy sources. Tremendous investigation and efforts have been devoted to increase the efficiency of the solar to energy conversion of a photocatalyst. Layered double hydroxide (LDH) received scientific attention for its excellent compositional flexibility and controllable morphology, leading to the facile incorporation of the metal species into their layered structure. The unique multi-structure and the tunability of its band gap make LDH more prominent in the field of photocatalysis. This article highlights the recent developments in the fabrication of LDH-based photocatalyst nanocomposites and the engineering approaches for augmenting their photocatalytic hydrogen production efficiency. The thermodynamics and challenges in photocatalytic water splitting are deliberated to understand the pathways to construct efficient semiconductor photocatalysis system. The efficiency enhancement of LDH-based photocatalysts are comprehensively discussed by giving special attention to the heterojunction engineering of type I, type II, p-n junction, Z-scheme, S-scheme, and R-scheme. Fabrication of the hybrid LDH nanocomposites through band gap engineering and metal loading are summarised. The architectural and morphological tuning of LDH-based composite through the construction of the novel core-shell structure and layer-by-layer nanosheets are also demonstrated. Finally, the future recommendations are outlined to provide insights for their development in the photocatalysis field.     

Recent development in band engineering of binary semiconductor materials for solar driven photocatalytic hydrogen production

Photocatalytic hydrogen production from water splitting is a promising approach to develop sustainable renewable energy resources and limits the global warming simultaneously. Despite the significant efforts have been dedicated for the synthesis of semiconductor materials, key challenge persists is lower quantum efficiency of a photocatalyst due to charge carrier recombination and inability of utilizing full spectrum of solar light irradiation. In this review, recent developments in binary semiconductor materials and their application for photocatalytic water splitting toward hydrogen production are systematically discoursed. In the main stream, fundamentals and thermodynamic for photocatalytic water splitting and selection of photo-catalysts has been presented. Developments in the binary photocatalysts and their efficiency enhancements though surface sensitization, surface plasmon resonance (SPR) effect, Schoktty barrier and electrons mediation toward enhanced hydrogen production has been deliberated. Different modification approaches including band engineering, coupling of semiconductor catalysts, construction of heterojunction, Z-scheme formation and step-type photocatalytic systems are also discussed. The binary semiconductor materials such as TiO₂, g-C₃N₄, ZnO, ZnS, Fe₂O₃, CdS, WO₃, rGO, V₂O₅ and AgX (Cl, Br and I) are systematically disclosed. In addition, role of sacrificial reagents for efficient photocatalysis through reforming and hole-scavenger are elaborated. Finally, future perspectives for photocatalytic water splitting towards renewable hydrogen production have been suggested.     

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.     

Experimental investigation of a solar tower based photocatalytic hydrogen production system

In this paper, production of hydrogen from concentrated solar radiation is examined by a laboratory scale solar tower system that is capable of handling continuous flow photocatalysis. The system is built and studied under a solar simulator with an aiming area of 20 × 20 cm². The fraction of solar spectrum useful for water splitting depends on the energy band gap of the selected photocatalyst. Two types of nano-particulate photocatalysts are used in this work: ZnS (3.6 eV) and CdS (2.4 eV). The effect of light concentration on photocatalysis performance is studied using Alfa Aesar 99.99% pure grade, 325 mesh ZnS nano-particles. An improved quantum efficiency of 73% is obtained as compared to 45% with the same sample under non-concentrated light in a previous study. Only 1.1% of the energy of the solar radiation spectrum can be used by ZnS catalyst. A mixture of CdS and ZnS nano-particulate photocatalysts (both Alfa Aesar 99.99% pure grade, 325 mesh) is used to conduct a parametric study for a wider spectrum capture corresponding to 18% of the incident energy. Hydrogen production increases from 0.1 mmol/h to 0.21 mmol/h when the operating conditions are varied from 25 °C and 101 kPa to 40 °C and 21 kPa absolute pressures. Furthermore, the implementation of a continuous flow process results in an improvement in the energy efficiency by a factor of 5.5 over the batch process.     

A perspective on the fabrication of heterogeneous photocatalysts for enhanced hydrogen production

This perspective provides an insight to the possibility of adopting hydrogen as a key energy-carrier and fuel source, through Photocatalytic water splitting in the near future. The need of green and clean energy is increasing to overcome the growing demand of sustainable energy throughout globe, owing to CO₂ emission using fossil fuels. To generate highly efficient and cost-competitive hydrogen, the semiconductor based heterojunction nanomaterials have gained tremendous consideration as a promising way. Currently, the efficiency for hydrogen generation through UV–Vis active photocatalysts is relatively low. The key issues are found to be poor separation of photogenerated electron/hole, less surface area, and low absorption region of electromagnetic spectrum. Such issues arise due to inappropriate band edge potentials and large bandgap of present catalyst. A lot of schemes has been devoted to design and fabricate efficient photocatalysts for improved photocatalytic performance in recent years. However, it seems still a challenge and imperative to greatly comprehend the fundamental aspects, photocatalysis and transfer mechanisms for complete deployment of electron/hole pairs. Further, to produce hydrogen to a larger extent through photocatalytic water splitting, the photocatalyst has been modified through co-catalysts/dopants using numerous techniques including the Z-scheme, hybridization, crystallinity, morphology, tuning of band edge positions, reduction of the band gap, surface structure etc., such that these heterogeneous photocatalysts may have ability to absorb enough light in the UV-VIS-IR region. This type of heterogeneous photocatalysts has the ability to improve the rate of efficiency for hydrogen evolution through absorption of sufficient light of solar spectrum and enhance the separation of charge-carriers by inhibiting recombination of electron/hole pairs. We surmise that taking into account the aforesaid factors should support in scheming an efficient photocatalysts for hydrogen production through water splitting, eventually prompting technological developments in this field.     

Photocatalytic membrane reactors for hydrogen production from water

Hydrogen is considered today a promising environmental friendly energy carrier for the next future, since it produces no air pollutants or greenhouse gases when it burns in air, and it possesses high energy capacity. In the last decades great attention has been devoted to hydrogen production from water splitting by photocatalysis. This technology appears very attractive thanks to the possibility to work under mild conditions producing no harmful by-products with the possibility to use renewable solar energy. Besides, it can be combined with the technology of membrane separations making the so-called photocatalytic membrane reactors (PMRs) where the chemical reaction, the recovery of the photocatalyst and the separation of products and/or intermediates simultaneously occur. In this work the basic principles of photocatalytic hydrogen generation from water splitting are reported, giving particular attention on the use of modified photocatalysts able to work under visible light irradiation. Several devices to achieve the photocatalytic hydrogen generation are presented focusing on the possibility to obtain pure hydrogen employing membrane systems and visible light irradiation. Although many efforts are still necessary to improve the performance of the process, membrane photoreactors seem to be promising for hydrogen production by overall water splitting in a cost-effective and environmentally sustainable way.     

Indirect Z-scheme hydrogen production photocatalyst based on two-dimensional GeC/MoSi2N4 van der Waals heterostructures

The stacked two-dimensional materials with suitable band gap are crucial for photocatalytic hydrogen production. Here, using first-principles calculations, the GeC/MoSi₂N₄ heterojunction with a band gap of 1.80 eV is calculated thoroughly. The indirect band alignment of Z-scheme and high carrier mobility boost the separation of electron-hole pairs, allowing more electrons and holes participating in the reactions. Additionally, the band-edge potential perfectly satisfies the requirements for redox potential of water splitting. Furthermore, the Gibbs free energy (−0.552 eV) close to zero indicates the heterojunction can conduct HER exceedingly well, providing a guarantee for photocatalytic hydrogen production. Remarkably, the light absorption coefficient peak is about 1.39 × 10⁵ cm⁻¹ within the visible light range enables the heterojunction to absorb more visible light from the spectrum. In short, results demonstrate the GeC/MoSi₂N₄ heterojunction is a promising photocatalyst for visible light water splitting, which will pave the way for the development of water splitting hydrogen production.     

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.     

Hydrogen evolution from solar water splitting on nanostructured copper oxide photocathodes

Copper oxide has received much attention as a photocatalyst for solar water splitting, since it has a band gap of ～2.0?eV with favourable energy band positions for water cleavage; it is abundant and environmentally friendly. In this paper, we report the preparation of copper oxide thin films on copper substrate with different morphology by a solution route in large scales. The method was simple, low cost, and can be completed in the absence of any surfactant. Copper oxide films were characterised by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), Fourier transform-infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The XRD analysis and FT-IR analysis confirm the formation of copper oxide films. SEM images show gradual development of hierarchical structures of copper oxide with different morphology. The films were successfully tested as photocathodes in a photoelectrochemical cell, and their photocatalytic activity was evaluated by testing the solar water splitting. The photocatalytic activity of these films was found to be size-dependent and films with smaller dimensions demonstrated a significant increase in photocatalytic activities during solar water splitting process. Based on the above discussion, we believe that these samples are attractive candidates as a visible-light-driven photocatalyst.     

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.     

Visible light responsive photocatalysts developed by substitution with metal cations aiming at artificial photosynthesis

To solve resource, energy, and environmental issues, development of sustainable clean energy system is strongly required. In recent years, hydrogen has been paid much attention to as a clean energy. Solar hydrogen production by water splitting using a photocatalyst as artificial photosynthesis is a promising method to solve these issues. Efficient utilization of visible light comprised of solar light is essential for practical use. Three strategies, i.e., doping, control of valence band, and formation of solid solution are often utilized as the useful methods to develop visible light responsive photocatalysts. This mini-review introduces the recent work on visible-light-driven photocatalysts developed by substitution with metal cations of those strategies.     

A critical review in strategies to improve photocatalytic water splitting towards hydrogen production

Water splitting for hydrogen production under light irradiation is an ideal system to provide renewable energy sources and to reduce global warming effects. Even though significant efforts have been devoted to fabricate advanced nanocomposite materials, the main challenge persists, which is lower efficiency and selectivity towards H₂ evolution under solar energy. In this review, recent developments in photo-catalysts, fabrication of novel heterojunction constructions and factors influencing the photocatalytic process for dynamic H₂ production have been discussed. In the mainstream, recent developments in TiO₂ and g-C₃N₄ based photo-catalysts and their potential for H₂ production are extensively studied. The improvements have been classified as strategies to improve different factors of photocatalytic water splitting such as Z-scheme systems and influence of operating parameters such as band gap, morphology, temperature, light intensity, oxygen vacancies, pH, and sacrificial reagents. Moreover, thermodynamics for selective photocatalytic H₂ production are critically discussed. The advances in photo-reactors and their role to provide more light distribution and surface area contact between catalyst and light were systematically described. By applying the optimum operating parameters and new engineering approach on photoreactor, the efficiency of semiconductor photocatalysts for H₂ production can be enhanced. The future research and perspectives for photocatalytic water splitting were also suggested.     

Enhanced solar-driven water splitting by ZnO/CdTe heterostructure thin films-based photocatalysts

Solar-driven hydrogen production by water splitting using a photocatalyst is considered the most effective approach to produce hydrogen. Hydrogen is the most suitable renewable energy source. The efficiency of hydrogen production is still low. The efficiency of hydrogen production through photocatalysis can be enhanced by preparing a suitable and efficient photocatalyst. In this work, ZnO thin films were deposited on CdTe thin films at 600 °C, 650 °C, and 700 °C temperatures to form ZnO/CdTe heterostructure thin films by chemical vapor deposition (CVD) as photoelectrodes for water splitting. The photoelectrochemical properties showed that ZnO/CdTe heterostructure thin films have better photocurrent response compared to pristine ZnO and CdTe thin films. EIS results showed that the charge transfer at the electrode-electrolyte interface for ZnO/CdTe heterostructure thin films is much better than that of the pristine ZnO film. The ZnO/CdTe-700 °C heterostructure thin film has a 112-fold higher rate of photocatalytic hydrogen generation than pure ZnO.     

Solar hybrid photo-thermochemical sulfur-ammonia water-splitting cycle: Photocatalytic hydrogen production stage

One of the main limitations of existing solar thermochemical water-splitting cycles (WSC) are that they utilize only thermal component of the solar irradiation neglecting its photonic component. A new hybrid photo-thermochemical sulfur–ammonia (HySA) WSC developed at the Florida Solar Energy Center allows circumventing this shortcoming. In the HySA cycle, water splitting occurs by means of solar beam splitting which enables utilization of the quantum (UV–Vis) portion of the solar spectrum in the hydrogen production stage and the thermal (IR) portion in the oxygen production stage. Present work investigates the photocatalytic hydrogen production step using narrow band gap CdS and CdSZnS composite photocatalysts, and ammonium sulfite as an electron donor. The choice of the electron donor was determined by the considerations of its regenerability in the thermal stages of the HySA cycle. This article examines the impact of photocatalyst and cocatalyst loading, temperature, and light intensity on hydrogen production rates. Photocatalysts, cocatalysts and photoreaction products were analyzed by a number of materials characterization (XRD, SEM, TEM, EDS) and analytical (GC and IC) methods. The experimental data obtained provide guidance for the improved solar photoreactor design.     

Efficient solar hydrogen production by photocatalytic water splitting: From fundamental study to pilot demonstration

Photocatalytic water splitting with solar light is one of the most promising technologies for solar hydrogen production. From a systematic point of view, whether it is photocatalyst and reaction system development or the reactor-related design, the essentials could be summarized as: photon transfer limitations and mass transfer limitations (in the case of liquid phase reactions). Optimization of these two issues are therefore given special attention throughout our study. In this review, the state of the art for the research of photocatalytic hydrogen production, both outcomes and challenges in this field, were briefly reviewed. Research progress of our lab, from fundamental study of photocatalyst preparation to reactor configuration and pilot level demonstration, were introduced, showing the complete process of our effort for this technology to be economic viable in the near future. Our systematic and continuous study in this field lead to the development of a Compound Parabolic Concentrator (CPC) based photocatalytic hydrogen production solar rector for the first time. We have demonstrated the feasibility for efficient photocatalytic hydrogen production under direct solar light. The exiting challenges and difficulties for this technology to proceed from successful laboratory photocatalysis set-up up to an industrially relevant scale are also proposed. These issues have been the object of our research and would also be the direction of our study in future.     

Enhancing hydrogen evolution of water splitting under solar spectra using Au/TiO2 heterojunction photocatalysts

An effective improvement of hydrogen evolution from water splitting under solar light irradiation was investigated using quantum dots (QDs) compounds loaded onto a Au/TiO₂ photocatalyst. First, Au/TiO₂ was prepared by the deposition-precipitation method, and then sulfide QDs were loaded onto the as-prepared Au/TiO₂ by a hydrothermal method. QDs were loaded onto Au/TiO₂ to enhance the energy capture of visible light and near-infrared light of the solar spectrum. The results indicated that the as-prepared heterojunction photocatalysts absorbed the energy from the range of ultraviolet light to the near-infrared light region and effectively reduced the electron-hole pair recombination during the photocatalytic reaction. Using a hydrothermal temperature of 120 °C, the as-prepared (ZnS–PbS)/Au/TiO₂ photocatalyst had a PbS QDs particle size of 5 nm, exhibited an energy gap of 0.92 eV, and demonstrated the best hydrogen production rate. Additionally, after adding 20 wt % methanol as a sacrificial reagent to photocatalyze for 5 h, the hydrogen production rate reached 5011 μmol g⁻¹ h⁻¹.     

Photon-absorption-based explanation of ultrasonic-assisted solar photochemical splitting of water to improve hydrogen production

The ultrasonic-assisted solar photochemical splitting of water had been explored in recent years to enhance hydrogen production efficiency. In this study, a photon-absorption-based study was conducted to investigate the mechanism of the ultrasonic-assisted solar photochemical splitting of water. An elaborate test bench for temperature-controlled, ultrasonic-assisted solar photochemical water splitting was designed, set up, and tested. A comparison of the hydrogen production between the ultrasonic-assisted and conventional solar photochemical splitting of water was carried out. The effective nanoparticle size before and after ultrasonic vibration, as well as after solar photocatalysis, was analyzed. Furthermore, the spectral absorptivity of the nanofluids before and after ultrasonic vibration, as well as after solar photocatalysis, was investigated by both experimental and numerical methods. The investigation indicated that the improved particle dispersion in the solution prepared by ultrasonication allowed the absorbance of more incoming sunlight. The amount of hydrogen produced by the ultrasonic-assisted hydrogen production was 3.45 times that of conventional solar photochemical splitting of water without pre-ultrasonicated. Besides, an effective spectral absorptivity coefficient was proposed as a modified measure of spectral absorptivity. In addition, the optimal particle diameter was optimized using the Monte Carlo ray tracing method to identify the best light absorption performance.     

A critical review of metal-doped TiO2 and its structure–physical properties–photocatalytic activity relationship in hydrogen production

Hydrogen production is an effective way to replace the primary energy to provide renewable sources as well as preserve the environment. Significant efforts have been developed to increase the effectiveness of hydrogen production through many methods. However, the challenge is still on-going, which exhibits insufficient efficiency and weak selectivity toward hydrogen production. Photocatalysis is one of the best methods to produce hydrogen as well as sustained the environment. Here, modification of TiO₂ by metal doping photocatalyst is reviewed. The right conclusions only can be obtained if consistent data is used. So, in this review, the data used are only data generated from a research group, Bunsho Ohtani's research group of Hokkaido University, that used titania photocatalyst in the production of hydrogen. The photocatalytic activity of photocatalysts and their relationship with hydrogen production and the factors that affect hydrogen production are discussed critically using fuzzy graph and fuzzy logic modelling. Modification of TiO₂ photocatalyst and its application for the production of hydrogen are studied. The modification is designated as mono-, bi-, and trimetallic metal doping. Moreover, there is no clarification has been done on the factors that affect the photocatalytic activity in hydrogen production. Thus, the mathematical tool, which is the fuzzy logic controller (FLC) is introduced in photocatalysis to provide the future direction of the structure-physical properties-photocatalytic activity relationship of metal-doped TiO₂ photocatalyst. Au/TiO₂ is used as the photocatalyst model towards the production of hydrogen under UV light irradiation in the form of a fuzzy graph. It was found that the low amount of Au metal doping and high surface area are the dominant factors to obtain high-efficiency hydrogen production for Au/TiO₂ photocatalyst.     

Nature inspired artificial photosynthesis technologies for hydrogen production: Barriers and challenges

Hydrogen drives the big wheel of nature. Hydrogen nuclear fusion in the sun produces light and heat. Solar flux reaching the earth's surface in an hour is far more than global annual energy demand. Photosynthesis traps 100-TW solar energy annually into biomass on land at 0.1% efficiency that is about six times more than global yearly energy demand. All photosynthetic organisms (photoautotroph) annually convert 100-billion tons of carbon in the atmosphere into biomass. The rampant rise in energy demand requires to replicate natural photosynthesis process artificially to convert solar energy and Carbon dioxide (CO₂) in liquid and burnable gaseous fuels. Chemists, physicists and biologists are collaborating to develop suitable catalysts for artificial photosynthesis. There is a consensus the sun can fuel transport sector by hydrogen and power grid by photo-electricity. It is well in time to develop a full spectrum of solar technologies instead of keeping ourselves plugged to hydrocarbon honey. Photocathodes and catalysts can mediate water splitting using nature-inspired artificial photosynthesis. Economic hydrogen production can accomplish the grand energy transition from fossil fuels to sustainable and renewable energy sources. This paper reviews the recent advances in artificial photosynthesis technologies and presents our work on the microbial fuel cell for hydrogen production and points out technical barriers and operational challenges.