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Photocatalytic water splitting is a process that uses photocatalysis for the dissociation of water (H2O) into hydrogen (H
2) and oxygen (O
2). The inputs are light energy (photons), water, and a catalyst(s). The process is inspired by Photosynthesis, which converts water and carbon dioxide into oxygen and carbohydrates. Water splitting using solar radiation has not been commercialized. [1] Photocatalytic water splitting is done by dispersing photocatalyst particles in water or depositing them on a substrate, unlike Photoelectrochemical cell, which are assembled into a cell with a photoelectrode. [2] Hydrogen fuel production using water and light (photocatalytic water splitting), instead of petroleum, is an important renewable energy strategy.
Two mole of H2O is split into 1 mole O
2 and 2 mole H
2 using light in the process shown below.
A photon with an energy greater than 1.23 eV is needed to generate an electron–hole pairs, which react with water on the surface of the photocatalyst. The photocatalyst must have a bandgap large enough to split water; in practice, losses from material internal resistance and the overpotential of the water splitting reaction increase the required bandgap energy to 1.6–2.4 eV to drive water splitting. [2]
The process of water-splitting is a highly endothermic process (ΔH > 0). Water splitting occurs naturally in photosynthesis when the energy of four photons is absorbed and converted into chemical energy through a complex biochemical pathway (Dolai's or Kok's S-state diagrams). [3]
O–H bond homolysis in water requires energy of 6.5 - 6.9 eV (UV photon). [4] [5] Infrared light has sufficient energy to mediate water splitting because it technically has enough energy for the net reaction. However, it does not have enough energy to mediate the elementary reactions leading to the various intermediates involved in water splitting (this is why there is still water on Earth). Nature overcomes this challenge by absorbing four visible photons. In the laboratory, this challenge is typically overcome by coupling the hydrogen production reaction with a sacrificial reductant other than water. [6]
Materials used in photocatalytic water splitting fulfill the band requirements and typically have dopants and/or co-catalysts added to optimize their performance. A sample semiconductor with the proper band structure is titanium dioxide (TiO
2) and is typically used with a co-catalyst such as platinum (Pt) to increase the rate of H
2 production. [7] A major problem in photocatalytic water splitting is photocatalyst decomposition and corrosion. [7]
Photocatalysts must conform to several key principles in order to be considered effective at water splitting. A key principle is that H
2 and O
2 evolution should occur in a stoichiometric 2:1 ratio; significant deviation could be due to a flaw in the experimental setup and/or a side reaction, neither of which indicate a reliable photocatalyst for water splitting. The prime measure of photocatalyst effectiveness is quantum yield (QY), which is:
To assist in comparison, the rate of gas evolution can also be used. A photocatalyst that has a high quantum yield and gives a high rate of gas evolution is a better catalyst.
The other important factor for a photocatalyst is the range of light that is effective for operation. For example, a photocatalyst is more desirable to use visible photons than UV photons.
The efficiency of solar-to-hydrogen (STH) of photocatalytic water splitting, however, has remained very low.
A STH efficiency of 9.2% indium. [8]
NaTaO
3:La yielded the highest water splitting rate of photocatalysts without using sacrificial reagents. [7] This ultraviolet-based photocatalyst was reported to show water splitting rates of 9.7 mmol/h and a quantum yield of 56%. The nanostep structure of the material promotes water splitting as edges functioned as H
2 production sites and the grooves functioned as O
2 production sites. Addition of NiO particles as co-catalysts assisted in H
2 production; this step used an impregnation method with an aqueous solution of Ni(NO
3)
2•6H
2O and evaporated the solution in the presence of the photocatalyst. NaTaO
3 has a conduction band higher than that of NiO, so photo-generated electrons are more easily transferred to the conduction band of NiO for H
2 evolution. [9]
K
3Ta
3B
2O
12 is another catalyst solely activated by UV and above light. It does not have the performance or quantum yield of NaTaO
3:La. However, it can split water without the assistance of co-catalysts and gives a quantum yield of 6.5%, along with a water splitting rate of 1.21 mmol/h. This ability is due to the pillared structure of the photocatalyst, which involves TaO
6 pillars connected by BO
3 triangle units. Loading with NiO did not assist the photocatalyst due to the highly active H
2 evolution sites. [10]
(Ga
.82Zn
.18)(N
.82O
.18) had the highest quantum yield in visible light for visible light-based photocatalysts that do not utilize sacrificial reagents as of October 2008. [7] The photocatalyst featured a quantum yield of 5.9% and a water splitting rate of 0.4 mmol/h. Tuning the catalyst was done by increasing calcination temperatures for the final step in synthesizing the catalyst. Temperatures up to 600 °C helped to reduce the number of defects, while temperatures above 700 °C destroyed the local structure around zinc atoms and were thus undesirable. The treatment ultimately reduced the amount of surface Zn and O defects, which normally function as recombination sites, thus limiting photocatalytic activity. The catalyst was then loaded with Rh
2-yCr
yO
3 at a rate of 2.5 wt% Rh and 2 wt% Cr for better performance. [11]
Proton reduction catalysts based on earth-abundant elements [12] [13] carry out one side of the water-splitting half-reaction.
A mole of octahedral nickel(II) complex, [Ni(bztpen)]2+ (bztpen = N-benzyl-N,N’,N’-tris(pyridine-2-ylmethyl)ethylenediamine) produced 308,000 moles of hydrogen over 60 hours of electrolysis with an applied potential of -1.25 V vs. standard hydrogen electrode. [14]
Ru(II) with three 2,2'-bipyridine ligands is a common compound for photosensitization used for photocatalytic oxidative transformations like water splitting. However, the bipyridine degrades due to the strongly oxidative conditions which causes the concentration of Ru(bpy)32+ to diminish. Measurements of the degradation is difficult with UV-Vis spectroscopy but MALDI MS can be used instead. [15]
Cobalt-based photocatalysts have been reported, [16] including tris(bipyridine) cobalt(II), compounds of cobalt ligated to certain cyclic polyamines, and some cobaloximes.
In 2014 researchers announced an approach that connected a chromophore to part of a larger organic ring that surrounded a cobalt atom. The process is less efficient than a platinum catalyst although cobalt is less expensive, potentially reducing costs. The process uses one of two supramolecular assemblies based on Co(II)-templated coordination of Ru(bpy)+32 (bpy = 2,2′-bipyridyl) analogues as photosensitizers and electron donors to a cobaloxime macrocycle. The Co(II) centers of both assemblies are high spin, in contrast to most previously described cobaloximes. Transient absorption optical spectroscopies indicate that charge recombination occurs through multiple ligand states within the photosensitizer modules. [17] [18]
Bismuth vanadate is a visible-light-driven photocatalyst with a bandgap of 2.4 eV. [19] [20] BV have demonstrated efficiencies of 5.2% for flat thin films [21] [22] and 8.2% for core-shell WO3@BiVO4 nanorods with thin absorbers. [23] [24] [25]
Bismuth oxides are characterized by visible light absorption properties, just like vanadates. [26] [27]
Tungsten diselenide has photocatalytic properties that might be a key to more efficient electrolysis. [28]
Systems based on III-V semiconductors, such as InGaP, enable solar-to-hydrogen efficiencies of up to 14%. [29] Challenges include long-term stability and cost.
2-dimensional semiconductors such as MoS
2 are actively researched as potential photocatalysts. [30] [31]
An aluminum‐based metal-organic framework made from 2‐aminoterephthalate can be modified by incorporating Ni2+ cations into the pores through coordination with the amino groups. [32] Molybdenum disulfide
Organic semiconductor photocatalysts, in particular porous organic polymers (POPs), attracted attention due to their low cost, low toxicity, and tunable light absorption vs inorganic counterparts. [33] [34] [35] They display high porosity, low density, diverse composition, facile functionalization, high chemical/thermal stability, as well as high surface areas. [36] Efficient conversion of hydrophobic polymers into hydrophilic polymer nano-dots (Pdots) increased polymer-water interfacial contact, which significantly improved performance. [37] [38] [39]
Beweries, et al., developed a light-driven "closed cycle of water splitting using ansa-titanocene(III/IV) triflate complexes". [40]
An Indium gallium nitride (In x Ga1-x N) photocatalyst achieved a solar-to-hydrogen efficiency of 9.2% from pure water and concentrated sunlight. The effiency is due to the synergistic effects of promoting hydrogen–oxygen evolution and inhibiting recombination by operating at an optimal reaction temperature (~70 degrees C), powered by harvesting previously wasted infrared light. An STH efficiency of about 7% was realized from tap water and seawater and efficiency of 6.2% in a larger-scale system with a solar light capacity of 257 watts. [41]
Solid solutions Cd
1-xZn
xS with different Zn concentration (0.2 < x < 0.35) have been investigated in the production of hydrogen from aqueous solutions containing as sacrificial reagents under visible light. [42] Textural, structural and surface catalyst properties were determined by N
2 adsorption isotherms, UV–vis spectroscopy, SEM and XRD and related to the activity results in hydrogen production from water splitting under visible light. It was reported that the crystallinity and energy band structure of the Cd
1-xZn
xS solid solutions depend on their Zn atomic concentration. The hydrogen production rate increased gradually as Zn concentration on photocatalysts increased from 0.2 to 0.3. The subsequent increase in the Zn fraction up to 0.35 reduced production. Variation in photoactivity was analyzed for changes in crystallinity, level of the conduction band and light absorption ability of Cd
1-xZn
xS solid solutions derived from their Zn atomic concentration.
A "photoelectrochemical cell" is one of two distinct classes of device. The first produces electrical energy similarly to a dye-sensitized photovoltaic cell, which meets the standard definition of a photovoltaic cell. The second is a photoelectrolytic cell, that is, a device which uses light incident on a photosensitizer, semiconductor, or aqueous metal immersed in an electrolytic solution to directly cause a chemical reaction, for example to produce hydrogen via the electrolysis of water.
Photoelectrolysis of water, also known as photoelectrochemical water splitting, occurs in a photoelectrochemical cell when light is used as the energy source for the electrolysis of water, producing dihydrogen which can be used as a fuel. This process is one route to a "hydrogen economy", in which hydrogen fuel is produced efficiently and inexpensively from natural sources without using fossil fuels. In contrast, steam reforming usually or always uses a fossil fuel to obtain hydrogen. Photoelectrolysis is sometimes known colloquially as the hydrogen holy grail for its potential to yield a viable alternative to petroleum as a source of energy; such an energy source would supposedly come without the sociopolitically undesirable effects of extracting and using petroleum.
Artificial photosynthesis is a chemical process that biomimics the natural process of photosynthesis. The term artificial photosynthesis is used loosely, referring to any scheme for capturing and then storing energy from sunlight by producing a fuel, specifically a solar fuel. An advantage of artificial photosynthesis would be that the solar energy could converted and stored. By contrast, using photovoltaic cells, sunlight is converted into electricity and then converted again into chemical energy for storage, with some necessary losses of energy associated with the second conversion. The byproducts of these reactions are environmentally friendly. Artificially photosynthesized fuel would be a carbon-neutral source of energy, but it has never been demonstrated in any practical sense. The economics of artificial photosynthesis are noncompetitive.
Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen:
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a photocatalyst, the excited state of which "repeatedly interacts with the reaction partners forming reaction intermediates and regenerates itself after each cycle of such interactions." In many cases, the catalyst is a solid that upon irradiation with UV- or visible light generates electron–hole pairs that generate free radicals. Photocatalysts belong to three main groups; heterogeneous, homogeneous, and plasmonic antenna-reactor catalysts. The use of each catalysts depends on the preferred application and required catalysis reaction.
Photosensitizers are light absorbers that alter the course of a photochemical reaction. They usually are catalysts. They can function by many mechanisms, sometimes they donate an electron to the substrate, sometimes they abstract a hydrogen atom from the substrate. At the end of this process, the photosensitizer returns to its ground state, where it remains chemically intact, poised to absorb more light. One branch of chemistry which frequently utilizes photosensitizers is polymer chemistry, using photosensitizers in reactions such as photopolymerization, photocrosslinking, and photodegradation. Photosensitizers are also used to generate prolonged excited electronic states in organic molecules with uses in photocatalysis, photon upconversion and photodynamic therapy. Generally, photosensitizers absorb electromagnetic radiation consisting of infrared radiation, visible light radiation, and ultraviolet radiation and transfer absorbed energy into neighboring molecules. This absorption of light is made possible by photosensitizers' large de-localized π-systems, which lowers the energy of HOMO and LUMO orbitals to promote photoexcitation. While many photosensitizers are organic or organometallic compounds, there are also examples of using semiconductor quantum dots as photosensitizers.
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Photoelectrochemistry is a subfield of study within physical chemistry concerned with the interaction of light with electrochemical systems. It is an active domain of investigation. One of the pioneers of this field of electrochemistry was the German electrochemist Heinz Gerischer. The interest in this domain is high in the context of development of renewable energy conversion and storage technology.
Graphitic carbon nitride (g-C3N4) is a family of carbon nitride compounds with a general formula near to C3N4 (albeit typically with non-zero amounts of hydrogen) and two major substructures based on heptazine and poly(triazine imide) units which, depending on reaction conditions, exhibit different degrees of condensation, properties and reactivities.
A solar fuel is a synthetic fuel produced using solar energy, through photochemical, photobiological, electrochemical, or thermochemical methods. Sunlight is the primary energy source, with its radiant energy being transduced to chemical energy stored in bonds, typically by reducing protons to hydrogen, or carbon dioxide to organic compounds.
Bismuth vanadate is the inorganic compound with the formula BiVO4. It is a bright yellow solid. It is widely studied as visible light photo-catalyst with a narrow band gap of less than 2.4 eV. It is a representative of "complex inorganic colored pigments," or CICPs. More specifically, bismuth vanadate is a mixed-metal oxide. Bismuth vanadate is also known under the Colour Index International as C.I. Pigment Yellow 184. It occurs naturally as the rare minerals pucherite, clinobisvanite, and dreyerite.
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Photogeochemistry merges photochemistry and geochemistry into the study of light-induced chemical reactions that occur or may occur among natural components of Earth's surface. The first comprehensive review on the subject was published in 2017 by the chemist and soil scientist Timothy A Doane, but the term photogeochemistry appeared a few years earlier as a keyword in studies that described the role of light-induced mineral transformations in shaping the biogeochemistry of Earth; this indeed describes the core of photogeochemical study, although other facets may be admitted into the definition.
In chemistry, plasmonic catalysis is a type of catalysis that uses plasmons to increase the rate of a chemical reaction. A plasmonic catalyst is made up of a metal nanoparticle surface which generates localized surface plasmon resonances (LSPRs) when excited by light. These plasmon oscillations create an electron-rich region near the surface of the nanoparticle, which can be used to excite the electrons of nearby molecules.
Jinhua Ye is a Chinese chemist who is a professor at the National Institute for Materials Science in Tsukuba. Her research considers high-temperature superconductors for photocatalysis. She was elected Fellow of the Royal Society of Chemistry in 2016 and has been included in the Clarivate Analytics Highly Cited Researcher every year since then.
Maytal Caspary Toroker is an associate professor in the Department of Materials Science and Engineering at Technion-Israel Institute of Technology, Haifa, Israel. She is recognized for her significant contributions in the field of computational materials science, particularly in its applications to catalysis, charge transport, and energy conversion devices.
Solar reforming is the sunlight-driven conversion of diverse carbon waste resources into sustainable fuels and value-added chemicals. It encompasses a set of technologies operating under ambient and aqueous conditions, utilizing solar spectrum to generate maximum value. Solar reforming offers an attractive and unifying solution to address the contemporary challenges of climate change and environmental pollution by creating a sustainable circular network of waste upcycling, clean fuel generation and the consequent mitigation of greenhouse emissions.
Green photocatalysts are photocatalysts derived from environmentally friendly sources. They are synthesized from natural, renewable, and biological resources, such as plant extracts, biomass, or microorganisms, minimizing the use of toxic chemicals and reducing the environmental impact associated with conventional photocatalyst production.