Carbon Nanodots
Carbon nanodots (CNDs) have useful optoelectronic and chemical properties that have attracted great interest since their accidental discovery in 2004. They have numerous potential applications including directly replacing metal-based semiconductor quantum dots , where the significantly reduced toxicity would be of benefit in bio-imaging and applications where environmental leaching is possible. They have excellent solubility in a range of organic solvents and water, showing promise for integration into photovoltaic, photocatalytic and light emitting devices.
We are synthesizing these fluorescent CNDs from renewable precursors such as glucose, chitosan and a variety of biomass using the low temperature synthesis. The control of the fundamental properties through size and functional groups will further allow new devices to be developed with distinctive properties and open up new possible applications. For example, the broad absorption spectra of carbon dots and easy synthesis and processing from a biomass precursor makes them viable as low cost sensitizers for various semiconductor and organic solar cell applications. An area that we look at in more detail is the use of CNDs for the sensitisation and passivation of photocatalytic substrates such as TiO2.
Photocatalysis
With the accelerated industrial development and economic growth, environmental problems and energy issues have drawn increasing attentions in recently years. Developing renewable energies is critical to address the problem, as to replace the usage of fossil fuels and control the green-house gas emission. Among those, sunlight holds great promise as a sustainable, abundant and globally available energy supply over the longer term. At present, one strategy of capturing and storing solar energy is to harvest that energy with special designed “antenna” and use it as driving force to synthesise fuels and chemicals. This so called “antenna” is what we are trying to develop in our group, know by the term “photocatalyst”. The idea of photocatalyst is to use solar energy to drive thermodynamic uphill reaction to produce highly energetic chemical fuels (e. g. water splitting for generating H2) and to degrade hazardous organic compounds in industrial effluent (e. g. pollutant decomposition).
Figure 1: Major processes occurring on a semiconductor system following electronic excitation.
Semiconductors as photocatalyst have been undergoing increasing study since the 1970s, after Fujishima and Honda discovered the photocatalytic water splitting on TiO2 electrodes in 1972. In a typical semiconductor photocatalysis process, when exposed under super-band-gap irradiation, the catalyst can absorb photons to excite electrons on the valence band. The excitons will jump onto the conduction band and leave holes behind. The separated electrons and holes can then migrate to the surface, where the electrons and holes are involved in subsequent reduction and oxidation reactions based on the assigned mission. In the meantime, a large portion of electron-hole pairs would recombine and dissipate the solar energy into useless heat. This whole process can be diagrammed in Figure 1.
Figure 2: Band structure of CND-semiconductor heterojunctions
However, even though the theoretical efficiency is quite high, drawbacks in semiconductor systems, such as poor light absorption, high charge recombination rate, short carrier lifetime has hindered the practical application in real lift. In this regard, it is essential to develop efficient photocatalyst systems to meet the industrial need.
Carbon dots have been widely demonstrated to be good photosensitizer for some wide band gap semiconductor or photocatalyst itself due to the light harvesting ability, good electron donating/accepting property and easily accessible energy states. Properly constructed band structure between photocatalyst interfaces would build up junctions that can efficiently separate electrons and holes, prevent charge recombination and accelerate charge transfer. The mechanism is shown in Figure 2.