Functional materials fuel photon power

Materials World magazine
,
1 Dec 2010
Zinc oxide nanorods grown using aqueous growth techniques at Queen Mary, University of London. The rods are ca 100-125nm across and 5,000nm long

Piezo or ferroelectric properties, are finding favour in photocatalysis and photovoltaic devices. Dr Steve Dunn, Senior Lecturer, at the Centre for Materials Research, Queen Mary, University of London, outlines the potential of these functional materials. 

There are a number of ways in which a photon supplied, preferably from sunlight, can be harnessed to provide energy. This energy can be in the form of electricity or a chemical compound or fuel. Perhaps the three most familiar are direct generation of an electric supply known as a photovoltaic or solar cell, the harnessing of the sunlight to produce heat and thereafter steam, such as in a heliostat, or the chemical conversion of a nonreactive species to a reactive one – photocatalysis. We are most familiar with this when applied to the conversion of CO2 into sugars in plants – photosynthesis. When recreated by humans, this is known as artificial photosynthesis.

Functional materials, such as those that exhibit piezo or ferroelectric properties, are now finding applications in photocatalysis and photovoltaic devices. While the performance of functional semiconductors – such as ferroelectrics – has yet to be fully determined, this approach is developing new understanding that could yet have a significant impact on current technology and, more importantly, society in general.

In the sunlight

There is a growing interest in the use of zinc oxide (ZnO) as the matrix material responsible for collecting photogenerated electrons. Zinc oxide is a material that exhibits a variety of properties that make it particularly suitable for this application. It is also piezoelectric. Another valuable feature is that it is relatively easy to make ZnO into nanorods. The morphology of a nanorod allows direct contact to the back electrode, so, in contrast to TiO2 nanopowders, there is reduced resistance for electron mobility.

Another type of photovoltaic device uses Bismuth ferrite (BiFeO3) as the complete photovoltaic. This material is sandwiched between a transparent bottom electrode and metallic top electrode to produce a homojunction photovoltaic. The cell relies on the distinct band bending associated with ferroelectric materials to generate the photovoltage. However, in the case of ZnO nanorods, the physical factors influencing device performance might not be the complete story.

A peculiarity of ZnO and all piezoelectric materials is that movement induces an electric dipole in the material. This is a source of an electric field that can be used to generate power and is the basis of a number of devices that convert vibrational energy into electrical energy. But as a ZnO nanorod vibrates, one side becomes slightly positively charged and the other slightly negatively charged. In the case of a piezoelectric material, the electric dipole can only form upon deformation of the material. In a ferroelectric material, there is a state where the material is deformed at an atomic level and is stable (up to a given temperature). These materials can sustain an internal dipole indefinitely.

Dramatic impact

The influence of this internal dipole on the material’s interaction with incoming photons is dramatic.

A traditional photocatalyst suffers from a number of drawbacks. The first is back reaction of products. When a photocatalyst reacts with a reactant the products are present on the material’s surface. So, as the products leave the surface, they can react to form the reactant. The second limitation is that photoexcited species – the electrons and holes formed in the semiconductor – can readily recombine to release heat without participating in any photochemistry.

Thirdly, these materials tend to have band gaps that mean they are either unstable under illumination or are only photoactive under ultraviolet irradiation. A ferroelectric material can directly address the first two points through the novel materials chemistry that is inherent in the crystallography of the structure. The dipole in each lattice of the crystal acts as a local field and effectively separates the electron-hole pairs. This has the distinct result of producing spatially determined photochemistry, as well as enhancing the separation of the photo-induced species.

Schematic of piezoelectric material developing an electric field when subjected to stress. The space charge layer is determined by the internal dipole due to the ferroelectric nature of the material interacting with mobile carriers in the material, and can be patterned into a given shape that is stable during photochemical interactions. The space charge layer is predominantly determined by the ferroelectric nature of the material and is not strongly affected by external interactions

There are now several examples where this has been harnessed to deliver some innovative and exciting photochemistry. A further benefit of these materials is that, although many are based on perovksite structures and exhibit properties, such as band gap of around 3.2 - 3.5eV that are very close to TiO2, there are a number of materials that are different and are proving to be useful in applications such as water purification, water splitting, the removal of heavy metals from waste streams, nanostructured patterning and artificial photosynthesis.

Bismuth ferrite is multiferoic, in that it exhibits both ferroelectric and magnetic properties. It is also photoactive with a band gap of around 2eV. This means that the material is photoactive in the visible region. Coating ferroelectric materials with TiO2 preserves the functionality of the underlying material, giving spatial photochemistry with exciton separation – but with the added advantage of being photostable under a TiO2 surface.

There has been significant research focusing on traditional semiconductor materials ranging from Si to TiO2 that has resulted in marginal, but interesting, improvements of catalysts performance. With the exception of photovoltaic systems, even with doping, nanostructuring and other materials modifications, these traditional materials are, as yet, incapable of performing at levels sufficient for commercialisation. 

Schematic band bending in a ferroelectric material

(a) Domains patterned into a ferroelectric substrate to
direct the photochemical REDOX chemistry of a metal salt (AgNO3) at sub
μm resolution (b) Resulting pattern of deposited Ag on the written
pattern showing good fidelity between written pattern and produced
pattern. The metal is only deposited in regions of interest showing high control of photochemical reaction (c) Limits to length scale of patterns with a 70nm Ag nanowire grown via photochemistry on a patterned ferroelectric
surface.

Images courtesy of Queen Mary, University of London

 

Further information

Centre for Materials Research, Queen Mary, University of London, 327 Mile End Road, London E1 4NS, UK. Tel: +44 (0)20 7882 5555. Email: s.dunn@qmul.ac.uk Dunn’s research is focused on developing an understanding of the photocatalytic and photovoltaic properties of functional materials.