Photocatalytic water splitting

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Photocatalytic water splitting is an artificial photosynthesis process using photocatalysis for the dissociation of water (H2O) into hydrogen (H
2
) and oxygen (O
2
). Only light energy (photons), water, and a catalyst(s) are needed, since this is what naturally occurs in natural photosynthetic oxygen production and CO2 fixation.[1][2][3][4]

Hydrogen fuel production using water and light (photocatalytic water splitting), instead of petroleum, is an important renewable energy strategy.

Concepts[edit]

When H2O is split into O
2
and H
2
in a stoichiometric ratio of 1:2

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).[5]

O–H bond homolysis in water requires energy of 6.5 - 6.9 eV (UV photon).[6][7] 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.[8]

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.[9] A major problem in photocatalytic water splitting is photocatalyst decomposition and corrosion.[9]

Method of evaluation[edit]

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:

QY (%) = (Photochemical reaction rate) / (Photon absorption rate) × 100%[9]

To assist in comparison, the rate of gas evolution can also be used. 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 that is effective for operation. For example, a photocatalyst is more desirable to use visible photons than UV photons.

Photocatalyst compounds[edit]

Cd
1-x
Zn
x
S
[edit]

Solid solutions Cd
1-x
Zn
x
S
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.[10] 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 irradiation. It was found that the crystallinity and energy band structure of the Cd
1-x
Zn
x
S
solid solutions depend on their Zn atomic concentration. The hydrogen production rate increased gradually when the Zn concentration on photocatalysts increased from 0.2 to 0.3. The subsequent increase in the Zn fraction up to 0.35 leads to lower hydrogen production. Variation in photoactivity is analyzed in terms of changes in crystallinity, level of the conduction band and light absorption ability of Cd
1-x
Zn
x
S
solid solutions derived from their Zn atomic concentration.

NaTaO
3
:La
[edit]

NaTaO
3
:La yields the highest water splitting rate of photocatalysts without using sacrificial reagents.[9] 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 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 was done by using an impregnation method with an aqueous solution of Ni(NO
3
)
2
•6H
2
O
and evaporating the solution in the presence of the photocatalyst. NaTaO
3
has a conduction band higher than that of NiO, so photogenerated electrons are more easily transferred to the conduction band of NiO for H
2
evolution.[11]

K
3
Ta
3
B
2
O
12
[edit]

K
3
Ta
3
B
2
O
12
, another catalyst activated by solely UV light and above, does not have the performance or quantum yield of NaTaO
3
:La. However, it can 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 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.[12]

(Ga
.82
Zn
.18
)(N
.82
O
.18
)
[edit]

(Ga
.82
Zn
.18
)(N
.82
O
.18
) has the highest quantum yield in visible light for visible light-based photocatalysts that do not utilize sacrificial reagents as of October 2008.[9] The photocatalyst gives a quantum yield of 5.9% and a water splitting rate of 0.4 mold/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, though 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-y
Cr
y
O
3
at a rate of 2.5 wt% Rh and 2 wt% Cr to yield the best performance.[13]

Molecular catalysts[edit]

Inexpensive proton reduction molecular catalysts based on earth-abundant elements are considered a promising alternative toward sustainable solar energy conversion.[14][15] (Note that these examples are not water-splitting catalysts. These only carry out one side of the water splitting half-reaction.)

One of the most efficient hydrogen evolution molecular electrocatalysts designed is the octahedral nickel(II) complex, [Ni(bztpen)]2+ (bztpen = N-benzyl-N,N’,N’-tris(pyridine-2-ylmethyl)ethylenediamine). A mole of [Ni(bztpen)]2+ produces 308,000 moles of hydrogen during 60 hours of electrolysis with an applied potential of -1.25 V vs. standard hydrogen electrode.[16]

Photocatalysts based on cobalt have been reported.[17] Members are tris(bipyridine) cobalt(II), compounds of cobalt ligated to certain cyclic polyamines, and certain 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 using a platinum catalyst; cobalt is less expensive, potentially reducing total 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) centres 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.[18][19]

Bismuth vanadate[edit]

Bismuth vanadate photocatalyst is a visible-light-driven photocatalyst with a bandgap of 2.4 eV.[20][21] based systems have demonstrated record solar-to-hydrogen (STH) conversion efficiencies of 5.2% for flat thin films[22][23] and 8.2% for core-shell WO3@BiVO4 nanorods with extremely thin absorber architecture.[24][25][26]

Bismuth oxides[edit]

Bismuth oxides are characterized by having visible light absorption properties, just like vanadates. For this reason, the mixture of bismuth with other transition metals is widely studied.[27][28]

Tungsten diselenide (WSe2)[edit]

Tungsten diselenide may have a role in future hydrogen fuel production, as a recent discovery in 2015 by scientists in Switzerland revealed that the compound's own photocatalytic properties might be a key to significantly more efficient electrolysis of water to produce hydrogen fuel.[29]

III-V semiconductor systems[edit]

Systems based on the material class of III-V semiconductors, such as InGaP, enable currently the highest solar-to-hydrogen efficiencies of up to 14%.[30] Long-term stability of these high-cost, high-efficiency systems does, however, remain an issue.

2D semiconductor systems[edit]

2-dimensional semiconductors are being actively researched as good candidates for photocatalysts in water splitting. Common 2D catalyst could be metal chalcogenide nanosheets such as Mo2S.[31][32]

Aluminum‐based metal-organic frameworks (MOF)[edit]

An aluminum‐based metal-organic framework (MOF) made from 2‐aminoterephthalate is a photocatalyst for oxygen evolution. This MOF can be modified by incorporating Ni2+ cations into the pores through coordination with the amino groups. The resulting MOF is an efficient photocatalyst for overall water splitting.[33]

Porous organic polymers (POPs)

Organic semiconductor photocatalysts, in particular porous organic polymers (POPs), have attracted significant attention due to the advantages over inorganic counterparts – their low cost, low toxicity, and tunable light absorption.[34][35][36] Apart from this, high porosity, low density, diverse composition, facile functionalization, high chemical/thermal stability, as well as high surface areas are making POPs ideal systems for converting solar energy to hydrogen, an environmentally friendly fuel.[37] By efficient conversion of hydrophobic polymers into hydrophilic polymer nano-dots (Pdots), polymer-water interfacial contact is therefore increased, which results in significantly improved photocatalytic performance of these materials.[38][39][40]

See also[edit]

References[edit]

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