Metal Oxide Photocatalysis

Metal oxide photocatalysis is based on the use of metal oxides (for example titanium dioxide) as light-activated catalysts in the destruction of organic and inorganic materials and in organic chemistry synthesis. In this article, we will be looking at the use of thee types of materials in the degradation of organic matter, which has applicability in environmental remediation (aqueous and air-borne) and self-cleaning surfaces. The technique is already widely used in commercial applications, but is still hampered by one significant limitation. These materials generally absorb primarily ultra-violet light, and research in recent years has been concentrating on developing visible-light active materials, with an emphasis on nano-particulate materials to maximize surface area. This article discusses the background to metal oxide photocatalysis, using titanium dioxide as the exemplar material, and looks at strategies being researched to enhance the photocatalytic efficiency.

Introduction

Titanium dioxide is a white powder, with titanium in oxidation state IV. Its d-electron configuration is therefore d0, and the white colour is explained by the lack of d-d or metal centred transitions. It exists in several polymorphs – two of interest here: anatase and rutile. As it is a semiconductor, its HOMO is termed a valence band and LUMO is termed a conduction band. Light absorption effectively results in a ligand to metal charge transfer, electrons from oxygen are transferred to the vacant titanium d-orbitals. For anatase (3.2 eV) and rutile (3.0 eV), this transition is in the UVA region, resulting in a sharp absorption band at 390 – 400 nm.

Looking more closely at the electronic processes, promotion of an electron to the conduction band, on irradiation by UV light, results in a ‘hole’ in the valence band – essentially a detriment of the electron density that was localised on that orbital, and usually assigned a positive charge to symbolize the loss of negative electron (of course negative and positive are just arbitrary notations). The hole is powerfully oxidizing – the orbital very much wants to retrieve electron density just lost after light irradiation. It can retrieve this simply by the electron in the conduction band recombining with the valence band – recombination is a sum of radiative (i.e. emission may be observed) and non-radiative processes. Based on the energy gap law, the fact that rutile energy levels are closer mean that the non-radiative process is more efficient, and hence recombination is more efficient. This is an important observation which we will return to shortly.

Alternative pathways to recombination are possible, and as you can guess, these result in the use of these materials as photocatalysts. The hole has the potential to oxidise water that may be on the surface of the material resulting in the formation of hydoxyl radicals. Hydroxyl radicals are themselves very powerful oxidisers, and can easily oxidise any organic species that happens to be nearby, ultimately to carbon dioxide and water. Meanwhile, upstairs in the conduction band, the electron has no hole to recombine with, since it has oxidised surface bound water. It quickly looks for an alternative to reduce, and rapidly reduces oxygen to form the superoxide anion. This can subsequently react with water to form, again, the hydroxyl radical. The processes are summarized below.

Top: Light of energy exceeding band gap results in charge separation, with electron reducing a donor (usually oxygen) and hole oxidising a donor (usually water); bottom: summary of processes occuring

Top: Light of energy exceeding band gap results in charge separation, with electron reducing a donor (usually oxygen) and hole oxidising a donor (usually water); bottom: summary of processes occuring. Image based on Bahnemann (2004).

At the level of the material’s surface, the requirements for efficient photocatalysis can be deduced from the electronic reactions – there should be surface bound water to allow for efficient oxidation; and the water should be aerated to provide oxygen to the solution. Additionally, the degradation of the pollutant by the catalyst requires for the pollutant to be adsorbed or very close to the surface of the material, and hence the greater the surface area of the material, the more pollutant can adsorb. Nanoparticulate materials are therefore preferred as they vastly increase the surface area (see DSSC post).

Pilkington self-cleaning glass is an example of use of this technology in a commercial application. A thin film of nanoparticulate titanium dioxide is coated onto panes of glass (it is so thin that it is transparent). The glass is in the normal course of events, acquiring dirt. The titanium dioxide on the glass, once exposed to sunlight, produces hydroxyl radicals which degrade any surface adsorbed dirt. Once washed down with rain, this decomposed dirt is removed and the glass is ready for another cycle. The same process is observed for any organic species – they react with the hydroxyl radical to ultimately form carbon dioxide and water.

Given that the materials work readily, it is a good time to detail the limitations. the primary limitation is that the materials absorb only UV light, so the activation by sunlight is completed by the 5% of sunlight that is in the UV region. A large amount of research has looked into ways to enhance the visible light activity of the materials. Another limitation is the fact that recombination is an efficient, competitive process, and given that this is a less efficient process with anatase, it is generally accepted that anatase is a preferred photocatalyst to rutile. Below, we will discuss approaches taken to both increase the visible light absorption capability and increase the efficiency of subsequent reactivity over the recombination process.

Moving to Visible Light Absorption Capability

Given the requirement for UV light activation of TiO2, researchers became interested in tuning the materials so that they would become activated by visible light (e.g. room light) for applications for indoor use or by solar light for outdoor use. Various approaches were considered, and in 2001, a Japanese chemist named Asahi working out of Toyota labs, published a paper in the journal Science on nitrogen doped titanium dioxide materials. Nitrogen doping produced what is commonly called yellow TiO2 (because of, unsurprisingly, its yellow colour!) which showed effective UV and visible light activity. While there is some debate around how the activity is increased, the N-doped TiO2 is shown to have a much greater absorbance in the visible region (extending from a sharp cut off at about 390 nm to a broad cut off at above 500 nm). This subsequently increased the amount of visible light activity the material could absorb, and hence meant that visible light-activated photocatalysis was achievable.

There has been some discussion in the literature on the mechanism on enhancement of nitrogen doping, and the mechanism described here is one put forward by Nakoto (2004) and Irie (2003), and counters Asahi’s original explanation that the N-doping narrowed the gap between the valence band and conduction band of titania. these researchers proposed that the introduction of nitrogen introduced new occupied (i.e. electron rich) orbitals in between the valence band (which are comprised primarily of O-2p orbitals) and conduction band (which are comprised primarily of Ti-3d orbitals). These N-2p orbitals acted as a step up for the electrons in the O-2p orbital, which once populated had now a much smaller jump to make to be promoted into the conduction band.Once this process occurs, electrons from the original valence band can migrate into the mid-band gap energy level, leaving a hole in the valence band, which reacts as described before.

N-doping as explained by Nakoto and Irie. Doping with nitrogen results in a mid-band gap energy level which reduces the energy gap required for charge separation

N-doping as explained by Nakoto and Irie. Doping with nitrogen results in a mid-band gap energy level which reduces the energy gap required for charge separation

Increasing efficiency by incorporation of metal nanoparticles

Given that charge separation requires a great deal of effort, a second theme of research (as well as increasing visible light activity) is to facilitate charge separation. One clever way of doing this is to incorporate noble metal nanoparticles such as silver or gold into the titanium dioxide material. As an example, incorporation of a small amount of silver (1 – 5%) results in increased efficiency in photocatalysis. Silver has a “Fermi level” or electron accepting region at an energy just below the conduction band. Therefore, after light absorption and charge separation, the electron in the conduction band can be effectively trapped by the silver, while the hole oxidises water and forms hydroxyl radicals, without the threat of recombination. Various researchers, including our own work, have shown that there is an optimum amount or “Goldilock’s zone” of silver to add – just enough is needed so that there are silver sites dispersed through the material to rapidly trap electrons, but that too much silver may cover the titanium dioxide and prevent light absorption. In addition, too much silver may mean that the silver acts as a recombination site itself – essentially it will form a bridge between an electron and a hole.

The emission of titanium dioxide (and of similar studies with zinc oxide) can be interpreted as a measure of the recombination efficiency. Studies examining the emission of these metal oxides have demonstrated that the emission intensity reduces on increasing amounts of silver – indicating that the silver is trapping electrons and reducing electron-hole recombination, as indicated in the diagram below.

Incorporation of silver nanoparticles facilitate longer charge separation by trapping photogenerated electrons

Incorporation of silver nanoparticles facilitate longer charge separation by trapping photogenerated electrons

Heterojunctions

A similar strategy to that described above, an a rapidly evolving area, is the idea of incorporating different semiconductors which have different conduction band energy levels. The strategy is as before, trap the electron so the hole has more time to react. A simple example is the anatase-rutile heterojunction. Rutile has a smaller band gap (by about 0.2 eV) to anatase, although their valence band levels are at similar energies. Therefore, in an analogous fashion to the situation with silver, above, charge separation in anatase, followed by electron injection into the rutile conduction band means that there is a hole in the valence band of anatase that can freely oxidise water. It is no coincidence that the industry standard photocatalyst, Degussa P25, has a 75:25 ratio of anatase:rutile (it also has a very small particle size).

Summary

Semiconductor photocatalysis is the utilisation of photogenerated strongly oxidising hydroxyl radicals, which can be applied to a wide range of scenarios, including organic degradation (for pollution remediation) and in organic synthesis. Light induced charge separation, followed by generation of hydroxyl radicals is in the normal course of event reliant on UV light, given the energy gap (band gap) of titanium dioxide. Strategies to enhance the photocatalytic activity include doping to reduce the energy required for charge separation and incorporation of nanoparticles to lengthen the period of charge separation. The size of the materials is also a factor, as for degradation of materials, the pollutant needs to be very near to or adsorbed onto the surface of the semiconductor, and nanoparticulate materials mean that a greater surface area can be exploited.

References

Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K. and Taga, Y., Visible-light photocatalysis in nitrogen-doped titanium oxides, Science, 2001, 294, 269 – 271. Asahi’s paper describing his results on N-TiO2. the work shows irradiation by UV-only and visible-only light, showing the enhancement by N-TiO2 with visible light source.

Bahnemann, D., Photocatalytic water treatment: solar energy applications, Solar Energy, 2004, 77, 445–459. Prof Bahnemann is one of Europe’s most active researchers in this field, and this very readable paper shows how the technology can and is used in solar decontamination technology.

Nakamura R, Tanaka T, and Nakato Y., Mechanism for visible light responses in anodic photocurrents at N-doped TiO2 film electrodes, J. Phys Chem. B., 2004, 108, 10617 – 10620. (See also Irie, H et al, J. Phys Chem. B., 2003, 107, 5483 – 5486). Papers explaining the origin of the hypothesis for the mid-gap energy levels introduced by nitrogen doping.

Seery, M. K., George, R., Floris, P. and Pillai, S. C., Silver doped titanium dioxide nanomaterials for enhanced visible light photocatalysis, J. Photochem. Photobiol A: Chemistry, 2007, 189(2-3), 258 – 263 and Georgekutty, R., Seery, M. K. and Pillai, S. C., A Highly Efficient Ag-ZnO Photocatalyst: Synthesis, Properties and Mechanism, J. Phys. Chem. C, 2008, 112(35), 13563 – 13570. these papers detail the incorporation of silver into titanium and zinc oxides respectively, including some consideration of mechanism.

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