User:Tha Stunna
Created the photocatalytic water splitting article; original submission is shown here for now.
Photocatalytic water splitting is the term for the production of H2 and 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. Water splitting holds particular interest since it utilizes the inexpensive natural resource water. Photocatalytic water splitting has the advantage of the simplicity of using a powder in solution and sunlight to produce H2 and O2 from water. Although the process is still undergoing research, it offers the potential to provide clean energy which can be stored more easily than electricity.
Concepts
Photocatalysts used in water splitting have several strict band requirements. The reduction reaction to form H2 occurs at 0 V, so the conduction band must be at a potential < 0 V to have any driving force for the reaction. The oxidation reaction to form O2 occurs at 1.23 V, so the valence band must be at a potential > 1.23 V to have any driving force for the reaction. However, the total band gap is limited due to the requirement of higher energy light to overcome larger band gaps. For visible light (wavelength > 380 nm) photocatalysis, the band gap must be ≤ 3.0 eV. Since the minimum band gap for successful water splitting is 1.23 eV, corresponding to light of 1100 nm, the electrochemical requirements can theoretically reach down into infrared light, albeit with negligible catalytic activity.
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.[1]
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.[1]
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%[1]
This quantity is a powerful 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 then 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.
Sample photocatalysts
NaTaO3:La
NaTaO3:La yields the highest water splitting rate of photocatalysts demonstrated as of October 2008 without using sacrifical reagents.[1] 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 NiO 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.[2]
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 NiO did not assist the photocatalyst due to the highly active H2 evolution sites.[3]
(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.[1] 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 calcination 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 [[oxygen|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.[4]
References
- ^ a b c d e A. Kudo, Y. Miseki, “Heterogeneous photocatalyst materials for water splitting” Chem. Soc. Rev., 38, 253-278 (2009).
- ^ H. Kato, K. Asakura, A. Kudo, “Highly Efficient Water Splitting into H and O over Lanthanum-Doped NaTaO Photocatalysts with High Crystallinity and Surface Nanostructure” J. Am. Chem. Soc., 125, 3082 (2003).
- ^ T. Kurihara, H. Okutomi, Y. Miseki, H. Kato, A. Kudo, “Highly Efficient Water Splitting over K3Ta3B2O12 Photocatalyst without Loading Cocatalyst” Chem. Lett., 35, 274 (2006).
- ^ K. Maeda, K. Teramura, K. Domen, “Effect of post-calcination on photocatalytic activity of (Ga1-xZnx)(N1-xOx) solid solution for overall water splitting under visible light” J. Catal., 254, 198 (2008).