Photocatalytic water splitting
Encyclopedia
Photocatalytic water splitting is the term for the production of hydrogen
(H2) and oxygen
(O2) from water
by directly utilizing the energy from light
. Hydrogen fuel
production has gained increasing attention as oil and other nonrenewable fuels become increasingly depleted and expensive. Methods such as photocatalytic water splitting are being investigated to produce hydrogen fuel, which burns cleanly and can be used in a hydrogen fuel cell. Water splitting
holds particular interest since it utilizes the inexpensive natural resource water. Photocatalytic water splitting has the simplicity of using a powder in solution and sunlight to produce H2 and O2 from water and can provide a clean, renewable energy source, without producing greenhouse gases or having many adverse effects on the atmosphere. Theoretically, only solar energy (photons), water and a catalyst are needed.
H2O is split into O2 and H2, the stoichiometric ratio of its byproducts are 2:1. When Hydrogen gas is burned a pale-blue flame is observed. The reaction is endothermic ((ΔH > 0) because most photon energies absorb and favorably complete oxidation reaction; it applies to the degradation of toxic organic compound in air and water, for example, TiO2 (mineral.) In contrast, the exothermic process (ΔH < 0) when photosynthesis has happened; the photon energy absorbed is converted into the chemical energy that provide for plants- this process usually happens in biological pathway of water splitting (KE involves). The production of hydrogen from water requires large amounts of energy, this makes it difficult to compete with existing energy generation using coal or natural gas since such energy is not eco-friendly.
Photocatalysts used in water splitting have several strict requirements. The redox potential needed to drive the reduction of hydrogen from water is 0.0ev Vs. Normal Hydrogen Electrode (NHE) at pH=0, and oxidation of oxygen from water is 1.23 ev vs. NHE (oxidation reaction) at pH=0. Therefore the minimum potential difference (voltage) needed to split water 1.23ev at pH=0. Since the minimum band gap for successful water splitting at pH=0 is 1.23 eV, corresponding to light of 1008 nm, the electrochemical requirements can theoretically reach down into infrared light, albeit with negligible catalytic activity. These values are true only for a completely reversible reaction at standard temperature and pressure (thermodynamics) (1 bar and 25 degrees celsius ).
Theoretically, infrared light has enough energy to split water into hydrogen and oxygen; however, this reaction is kinetically very slow because the wavelength is greater 380nm. The potential is less than 3.0eV to make the efficient use of solar. Water splitting can transfer charges, but not be able to corrosion for long term stability. Defects within crystalline photocatlysts can act as recombination sites ultimately lowering efficiency.
Materials used in photocatalytic water splitting fulfill the band requirements outlined previously and typically have dopants and/or co-catalysts added to optimize their performance. A sample semiconductor with the proper band structure is TiO2. However, due to the relatively positive conduction band
of TiO2, there is little driving force for H2 production, so TiO2 is typically used with a co-catalyst such as Pt to increase the rate of H2 production. It is routine to add co-catalysts to spur H2 evolution in most photocatalysts due to the conduction band placement. Most semiconductors with suitable band structures to split water absorb mostly UV light; in order to absorb visible light, it is necessary to narrow the band gap. Since the conduction band is fairly close to the reference potential for H2 formation, it is preferable to alter the valence band
to move it closer to the potential for
O2 formation, since there is a greater natural overpotential
.
Photocatalysts can suffer from catalyst decay and recombination under operating conditions. Catalyst decay becomes a problem when using a sulfide
-based photocatalyst such as CdS
, since the sulfide
in the catalyst is oxidized to elemental sulfur
at the same potentials used to split water. Thus, sulfide
-based photocatalysts are not viable without sacrificial reagents such as sodium sulfide
to replenish any sulfur lost, which effectively changes the main reaction to one of hydrogen evolution as opposed to water splitting. Recombination of the electron-hole pairs needed for photocatalysis can happen with any catalyst and is dependent on the defects and surface area of the catalyst; thus, a high degree of crystallinity is required to avoid recombination at the defects.
The conversion of solar energy to hydrogen by means of water splitting process is one of the most interesting ways to achieve clean and renewable energy systems. However if this process is assisted by photocatalysts suspended directly in water instead of using photovoltaic and an electrolytic system the reaction is in just one step, therefore it can be more efficient.
This quantity is a reliable determination of how effective a photocatalyst is; however, it can be misleading due to varying experimental conditions. To assist in comparison, the rate of gas evolution can also be used; this method is more problematic on its own because it is not normalized, but it can be useful for a rough comparison and is consistently reported in the literature. Overall, the best photocatalyst has a high quantum yield and gives a high rate of gas evolution.
The other important factor for a photocatalyst is the range of light absorbed; although UV-based photocatalysts will perform better per photon
than visible light-based photocatalysts due to the higher photon energy, far more visible light reaches the Earth's surface than UV light. Thus, a less efficient photocatalyst that absorbs visible light may ultimately be more useful than a more efficient photocatalyst absorbing solely UV light and above.
particles as cocatalysts assisted in H2 production; this step was done by using an impregnation method with an aqueous solution of Ni(NO3)2•6H2O and evaporating the solution in the presence of the photocatalyst. NaTaO3 has a conduction band higher than that of NiO
, so photogenerated electrons are more easily transferred to the conduction band of NiO
for H2 evolution.
did not assist the photocatalyst due to the highly active H2 evolution sites.
temperatures for the final step in synthesizing the catalyst. Temperatures up to 600°C helped to reduce the number of defects, although temperatures above 700°C destroyed the local structure around zinc atoms and was 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 Rh2-yCryO3 at a rate of 2.5 wt % Rh
and 2 wt% Cr
to yield the best performance.
For example, Pt/TiO2 (anatase phase) is an example catalyst to use in water splitting. These photocatalysts combine with thin NaOH aqueous layer to make a solution that can split water into H2 and O2. TiO2 only absorbs in the UV due to its large band gap(>3.0ev), it will perform better than most visible light photocatalyts because it does not photocorrode as easily. Most ceramic materials have large band gaps; this means they have stronger covalent bonds than other semiconductors with lower bandgaps- It will perform a little light in the UV than visible light-based photocatalysts because of higher photon energy (photo irradiation)
have been reported . Members are tris(bipyridine
) cobalt(II), compounds of cobalt ligated to certain cyclic polyamines and certain cobaloximes.
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
(H2) and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O2) from water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
by directly utilizing the energy from light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
. Hydrogen fuel
Hydrogen fuel
An ecologically-friendly fuel which uses electrochemical cells or combusts in internal engines to power vehicles and electric devices. It is also used in the propulsion of spacecraft and can potentially be mass produced and commercialized for passenger vehicles and aircraft.In a flame of pure...
production has gained increasing attention as oil and other nonrenewable fuels become increasingly depleted and expensive. Methods such as photocatalytic water splitting are being investigated to produce hydrogen fuel, which burns cleanly and can be used in a hydrogen fuel cell. Water splitting
Water splitting
Water splitting is the general term for a chemical reaction in which water is separated into oxygen and hydrogen. Efficient and economical water splitting would be a key technology component of a hydrogen economy. Various techniques for water splitting have been issued in water splitting patents in...
holds particular interest since it utilizes the inexpensive natural resource water. Photocatalytic water splitting has the simplicity of using a powder in solution and sunlight to produce H2 and O2 from water and can provide a clean, renewable energy source, without producing greenhouse gases or having many adverse effects on the atmosphere. Theoretically, only solar energy (photons), water and a catalyst are needed.
Concepts
H2O is split into O2 and H2, the stoichiometric ratio of its byproducts are 2:1. When Hydrogen gas is burned a pale-blue flame is observed. The reaction is endothermic ((ΔH > 0) because most photon energies absorb and favorably complete oxidation reaction; it applies to the degradation of toxic organic compound in air and water, for example, TiO2 (mineral.) In contrast, the exothermic process (ΔH < 0) when photosynthesis has happened; the photon energy absorbed is converted into the chemical energy that provide for plants- this process usually happens in biological pathway of water splitting (KE involves). The production of hydrogen from water requires large amounts of energy, this makes it difficult to compete with existing energy generation using coal or natural gas since such energy is not eco-friendly.
Photocatalysts used in water splitting have several strict requirements. The redox potential needed to drive the reduction of hydrogen from water is 0.0ev Vs. Normal Hydrogen Electrode (NHE) at pH=0, and oxidation of oxygen from water is 1.23 ev vs. NHE (oxidation reaction) at pH=0. Therefore the minimum potential difference (voltage) needed to split water 1.23ev at pH=0. Since the minimum band gap for successful water splitting at pH=0 is 1.23 eV, corresponding to light of 1008 nm, the electrochemical requirements can theoretically reach down into infrared light, albeit with negligible catalytic activity. These values are true only for a completely reversible reaction at standard temperature and pressure (thermodynamics) (1 bar and 25 degrees celsius ).
Theoretically, infrared light has enough energy to split water into hydrogen and oxygen; however, this reaction is kinetically very slow because the wavelength is greater 380nm. The potential is less than 3.0eV to make the efficient use of solar. Water splitting can transfer charges, but not be able to corrosion for long term stability. Defects within crystalline photocatlysts can act as recombination sites ultimately lowering efficiency.
Materials used in photocatalytic water splitting fulfill the band requirements outlined previously and typically have dopants and/or co-catalysts added to optimize their performance. A sample semiconductor with the proper band structure is TiO2. However, due to the relatively positive conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
of TiO2, there is little driving force for H2 production, so TiO2 is typically used with a co-catalyst such as Pt to increase the rate of H2 production. It is routine to add co-catalysts to spur H2 evolution in most photocatalysts due to the conduction band placement. Most semiconductors with suitable band structures to split water absorb mostly UV light; in order to absorb visible light, it is necessary to narrow the band gap. Since the conduction band is fairly close to the reference potential for H2 formation, it is preferable to alter the valence band
Valence band
In solids, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature....
to move it closer to the potential for
O2 formation, since there is a greater natural overpotential
Overpotential
Overpotential is an electrochemical term which refers to the potential difference between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency...
.
Photocatalysts can suffer from catalyst decay and recombination under operating conditions. Catalyst decay becomes a problem when using a sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
-based photocatalyst such as CdS
Cadmium sulfide
Cadmium sulfide is the inorganic compound with the formula CdS. Cadmium sulfide is a yellow solid. It occurs in nature with two different crystal structures as the rare minerals greenockite and hawleyite, but is more prevalent as an impurity substituent in the similarly structured zinc ores...
, since the sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
in the catalyst is oxidized to elemental sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
at the same potentials used to split water. Thus, sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
-based photocatalysts are not viable without sacrificial reagents such as sodium sulfide
Sodium sulfide
Sodium sulfide is the name used to refer to the chemical compound Na2S, but more commonly it refers to the hydrate Na2S·9H2O. Both are colorless water-soluble salts that give strongly alkaline solutions...
to replenish any sulfur lost, which effectively changes the main reaction to one of hydrogen evolution as opposed to water splitting. Recombination of the electron-hole pairs needed for photocatalysis can happen with any catalyst and is dependent on the defects and surface area of the catalyst; thus, a high degree of crystallinity is required to avoid recombination at the defects.
The conversion of solar energy to hydrogen by means of water splitting process is one of the most interesting ways to achieve clean and renewable energy systems. However if this process is assisted by photocatalysts suspended directly in water instead of using photovoltaic and an electrolytic system the reaction is in just one step, therefore it can be more efficient.
Method of evaluation
Photocatalysts must conform to several key principles in order to be considered effective at water splitting. A key principle is that H2 and O2 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, both of which do not indicate a reliable photocatalyst for water splitting. The prime measure of photocatalyst effectiveness is quantum yield (QY), which is:- QY (%) = (Number of reacted electrons) / (Number of incident photons) × 100%
This quantity is a reliable determination of how effective a photocatalyst is; however, it can be misleading due to varying experimental conditions. To assist in comparison, the rate of gas evolution can also be used; this method is more problematic on its own because it is not normalized, but it can be useful for a rough comparison and is consistently reported in the literature. Overall, the best photocatalyst has a high quantum yield and gives a high rate of gas evolution.
The other important factor for a photocatalyst is the range of light absorbed; although UV-based photocatalysts will perform better per photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
than visible light-based photocatalysts due to the higher photon energy, far more visible light reaches the Earth's surface than UV light. Thus, a less efficient photocatalyst that absorbs visible light may ultimately be more useful than a more efficient photocatalyst absorbing solely UV light and above.
NaTaO3:La
NaTaO3:La yields the highest water splitting rate of photocatalysts demonstrated as of October 2008 without using sacrificial reagents. This UV-based photocatalyst was shown to be highly effective with 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 H2 production sites and the grooves functioned as O2 production sites. Addition of NiONickel(II) oxide
Nickel oxide is the chemical compound with the formula NiO. It is notable as being the only well characterized oxide of nickel . The mineralogical form of NiO, bunsenite, is very rare. It is classified as a basic metal oxide...
particles as cocatalysts assisted in H2 production; this step was done by using an impregnation method with an aqueous solution of Ni(NO3)2•6H2O and evaporating the solution in the presence of the photocatalyst. NaTaO3 has a conduction band higher than that of NiO
Nickel(II) oxide
Nickel oxide is the chemical compound with the formula NiO. It is notable as being the only well characterized oxide of nickel . The mineralogical form of NiO, bunsenite, is very rare. It is classified as a basic metal oxide...
, so photogenerated electrons are more easily transferred to the conduction band of NiO
Nickel(II) oxide
Nickel oxide is the chemical compound with the formula NiO. It is notable as being the only well characterized oxide of nickel . The mineralogical form of NiO, bunsenite, is very rare. It is classified as a basic metal oxide...
for H2 evolution.
K3Ta3B2O12
K3Ta3B2O12, another catalyst activated by solely UV light and above, does not have the performance or quantum yield of NaTaO3:La. However, it does have the ability to split water without the assistance of cocatalysts 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 TaO6 pillars connected by BO3 triangle units. Loading with NiONickel(II) oxide
Nickel oxide is the chemical compound with the formula NiO. It is notable as being the only well characterized oxide of nickel . The mineralogical form of NiO, bunsenite, is very rare. It is classified as a basic metal oxide...
did not assist the photocatalyst due to the highly active H2 evolution sites.
(Ga.82Zn.18)(N.82O.18)
(Ga.82Zn.18)(N.82O.18) has the highest quantum yield in visible light for visible light-based photocatalysts that do not utilize sacrificial reagents as of October 2008. The photocatalyst gives a quantum yield of 5.9% along with a water splitting rate of 0.4 mmol/h. Tuning the catalyst was done by increasing calcinationCalcination
Calcination is a thermal treatment process applied to ores and other solid materials to bring about a thermal decomposition, phase transition, or removal of a volatile fraction. The calcination process normally takes place at temperatures below the melting point of the product materials...
temperatures for the final step in synthesizing the catalyst. Temperatures up to 600°C helped to reduce the number of defects, although temperatures above 700°C destroyed the local structure around zinc atoms and was thus undesirable. The treatment ultimately reduced the amount of surface Zn
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
and O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
defects, which normally function as recombination sites, thus limiting photocatalytic activity. The catalyst was then loaded with Rh2-yCryO3 at a rate of 2.5 wt % Rh
RH
RH, Rh, rH, or rh can stand for:* Riemann hypothesis, an important unsolved problem in mathematics* Ryan's Hope, a soap opera* Rhesus factor, a classification to describe blood types in humans* Rhodium, a chemical element...
and 2 wt% Cr
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
to yield the best performance.
Pt/TiO2
TiO2 is the best photocatalyst because it yields high quantum number as well as giving high rates of H2 gas evolution.For example, Pt/TiO2 (anatase phase) is an example catalyst to use in water splitting. These photocatalysts combine with thin NaOH aqueous layer to make a solution that can split water into H2 and O2. TiO2 only absorbs in the UV due to its large band gap(>3.0ev), it will perform better than most visible light photocatalyts because it does not photocorrode as easily. Most ceramic materials have large band gaps; this means they have stronger covalent bonds than other semiconductors with lower bandgaps- It will perform a little light in the UV than visible light-based photocatalysts because of higher photon energy (photo irradiation)
Cobalt based systems
Photocatalysts based on cobaltCobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
have been reported . Members are tris(bipyridine
Bipyridine
Bipyridines are a family of chemical compounds with the formula 2, which are formed by the coupling of two pyridine rings. Six isomers of bipyridine exist, but two isomers are prominent: 2,2'-bipyridine is a popular ligand in coordination chemistry and 4,4'-bipyridine is a precursor to the...
) cobalt(II), compounds of cobalt ligated to certain cyclic polyamines and certain cobaloximes.