Band alignment of light harvesting nanomaterials and metal oxides for photovoltaic and photocatalytic applications.

Lead Research Organisation: University of Manchester
Department Name: Materials

Abstract

Production of hydrogen using sunlight and a catalyst is potentially a way to reduce carbon emissions from the use of fossil fuels. However, in order to compete with fossil fuels the catalysts must be cheap to manufacture, sustainable and be robust. The white pigment, titanium dioxide was used to produce hydrogen by photoelectrocemically splitting water in the 1970s, but nearly 50 years later a stable system based on TiO2 still has not been realised. One reason for this is that TiO2 does not absorb in the visible region of the solar spectrum. In order to have a high efficiency with regards to sunlight we need a system which will absorb in the visible spectrum. There have been several approaches to this including functionalising metal oxides such as TiO2 and ZnO with dyes. In these systems the dye absorbs sunlight and an electron is injected into the titanium dioxide, leaving a hole in the dye which can react with the water to form hydrogen and oxygen. However these organic dyes tend to be unstable in the long term. An alternative is to use a inorganic sensitising nanomaterial. These have the dual advantages of increased stability and also that the band gap energy, which governs which wavelengths of light are absorbed, can be tuned to ensure optimum absorption.
The alignment of the bands of the oxide semiconductor and the functionalising nanomaterial is critical for water splitting. If the occupied bands of the sensitising nanomaterial overlap with the occupied valence band of the oxide then recombination of the electron and hole can occur between these two materials, preventing the hole from reacting with water. Similarly if the empty band of the nanomaterial, into which the electron is excited upon absorption of light, does not overlap with the empty band of the oxide then the excited electron cannot be injected into the oxide, and is then likely to recombine with the hole in the nanomaterial. This study will use high resolution spectroscopy, to determine the relative positions of the occupied and unoccupied bands in the two-dimensional (2-D) material, MoS2 and two metal oxides with the potential to be used in solar water splitting devices, ZnO and TiO2. By understanding where the bands lie relative to one another, we can design 2-D materials which have the ideal band structure both for light absorption and charge injection. The work will also allow us to study the stability the 2-D metal sulphide in the atmosphere, particularly with regards to the formation of sulphates by reaction with water or oxygen in the atmosphere.

Planned Impact

The proposed work will determine the electronic structure in the band gap region for titanium dioxide functionalised with two dimensional transition metal dichalcogenides (TDMCs). This interface is of interest as a potential water splitting system, to produce hydrogen using sunlight as the energy source to drive the reaction.
The design of efficient solar hydrogen generating catalysts or materials is of great importance since, unlike photovoltaics, the energy produced can be transported, and also used at night. Although hydrogen can be used directly as a fuel it can also be reacted with CO2 to form hydrocarbon fuels. Therefore the realisation of efficient solar fuel catalysts capable of water splitting has potentially huge societal impact, both in the UK and internationally. Any such devices will of course benefit industry - directly these materials can be designed and manufactured on a large scale which would benefit the UK economy, but the opportunity to produce low cost energy from sunlight would also help to reduce costs in all aspects of industry in the longer term.

The work proposed here is of a fundamental nature and as such is unlikely to lead to the production of these systems directly, but it will form part of a larger proposal between the PI, Co-I and others to investigate TDMCs as light harvesting systems for metal oxide and metal nitride semiconductors. The data will also be made available to the scientific community, including engineers and scientists who design solar water splitting systems, and will aid in their selection of materials.

The proposal will also assist in training of PhD students in the spectroscopic techniques we will use here, and ensure the UK maintains a body of highly-skilled people in complex electronic spectroscopy, and also in the use of synchrotron radiation.

Publications

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Description In sensitised solar cells the energy bands of the sensitiser must fulfil certain criteria. Firstly the separation of the highest occupied (HO) and lowest unoccupied (LU) bands must be capable of absorbing visible light. Second the alignment of the sensitiser HO and LU bands relative to the valence and conduction bands of the semiconductor substrate must ensure that charge transfers in the correct direction. 2-D (mono and few layered) SnS fulfils the first criterion and in this work we wished to determine whether the second one was also suitable for use of SnS as a sensitiser on metal oxide semiconductors. We were also interested in the relative alignments of bands between the two most common forms of titania (TiO2), rutile and anatase. Our work has shown that for ZnO and anatase and rutile TiO2 the energy band alignment of few layer SnS aligns well, with the HO band in the band gap of the oxides and the LU band lying just above the conduction band of the oxides. We also noted that for ZnO the number of oxygen vacancies at the surface leads to differences in band bending, but that the method of deposition of the SnS (i.e. by dropping SnS in suspension in ethanol under atmospheric pressure conditions) seems to result in healing of the O-vacancies and the removal of band bending.
Exploitation Route The data has been analysed and a manuscript is being drafted. We hope that the information will aid us or others in designing novel photovoltaics and hydrogen generation systems and potentially allow design of systems where organic molecules, metal particles or graphene will be used to enhance charge transfer, or charge trapping between the chalcogenide and the oxide/nitride. Myself and the Co-I will be submitting a full proposal to EPSRC on the subject in the coming year.
Sectors Electronics,Energy