46786-AC10
Investigation on Novel Photoactive Carbon Materials for Their Unique Properties Relevant to Energy Conversion
We have been carrying out fundamental investigations on a newly discovered class of photoactive carbon nanomaterials. These are small particles of neat carbon, which upon surface functionalization by organic or polymeric molecules exhibit interesting optical and electronic properties such as strong photoluminescence and photoinduced electron transfer. Our emphases have been on the measurement and quantification of the photophysical and photochemical properties of these carbon nanomaterials toward an understanding of the related excited state processes and effects associated with the material composition and configuration; the probing and evaluation of the electron transfer characteristics and processes as relevant to the potential development of photochemical energy conversion technologies; and the exploration of strategies with respect to the modification of these carbon nanomaterials that may lead to significant improvements in their desired properties and performance. We have made progress and obtained valuable results in our investigations.
As reported before and further confirmed in this project, the structure of these photoactive carbon nanomaterials (dubbed “carbon dots”) is rather simple. Basically, the small carbon particle core is essential, and the particle surface is oxidized and then functionalized by organic molecules, which could be oligomeric or polymeric (though longer-chain molecules better for the solubility of the resulting dots). Among representative covalent functionalization schemes is the use of classical amidation chemistry, similar to what has been widely used in the chemical functionalization of carbon nanotubes at the oxidized surface defect sites. Extensive and careful experiments have been performed in this project, from which the results are generally in support of such a simple structure for the carbon dots. Specifically, the as-prepared carbon dots were carefully purified by repeated dialyses to completely remove all small molecule species. The resulting sample was thoroughly characterized by using NMR and other instrumental techniques (thermogravimetric analysis, for example) for the structural elucidation. In such a structural framework, the optical properties of carbon dots must be associated with the passivated carbon particle surface.
The photoinduced electron transfer behavior of carbon dots has been revisited in terms of fluorescence quenching experiments. The results suggest that the fluorescence emissions in carbon dots are quenched by either strong electron donor or acceptor molecules. For example, the fluorescence intensities of carbon dots in room-temperature solution are quenched by 2,4-dinitrotoluene (a strong electron acceptor) at or even exceeding the diffusion-controlled limit (the quenching rate constant of 1010 M-1s-1 for the linear portion of the Stern-Volmer plots at lower concentrations) and beyond (the upward curvature due to contributions from near-neighbor static quenching). Similar quenching behavior has been observed with strong electron donor molecules such as N,N-diethylaniline (DEA). These fluorescence quenching results are consistent with the disruption of the electron or hole trapping on the passvated carbon particle surface by the strong electron acceptor or donor molecules, respectively. Also consistent with such a mechanistic picture are the experimental results suggesting that the electron acceptors or donors must necessarily be strong in order for the significant fluorescence quenching effects. The observed dependence of fluorescence quenching on the electron donating or accepting ability of the quencher also serves to support the electron transfer quenching mechanism.
We have experimented with the coating of already formed carbon dots with silver metal to evaluate the effects on fluorescence properties. The carbon dots are soluble in aqueous solution, so that the coating (deposition) of silver onto the carbon core surface could be readily accomplished via photolysis (photoirradiation of the carbon dots in aqueous solution in the presence of silver cations). Interestingly, the observed fluorescence intensities of the carbon dots decreased with the coating (deposition) of more and more silver in the photolysis. Mechanistically, the results may be explained by the same quenching effects discussed above, namely that the coating of silver metal is probably equivalent to the surface adsorption of strong electron donor molecules.
The photoinduced electron transfer properties in carbon dots are particularly relevant to their potential uses in energy conservation. In order to explore the photoinduced electrochemical behavior of carbon dots, we have set up thin-film fabrication and photocurrent measurement capabilities, and we have performed preliminary experiments with these capabilities aimed at exploring the potential of carbon dots-based materials in photoconversion applications. We will continue the exploration both in house and through external collaborations.
Two graduate students participated in the project. One of them just graduated last month and has been employed by a major multinational company.
