UV-Assisted Photo-catalytic Reduction of Graphene Oxide
Citation: Graeme Williams, Brian Seger, Prashant V. Kamat UV-Assisted Photo-catalytic Reduction of Graphene Oxide.
Internet Archive Scholar (search for fulltext): UV-Assisted Photo-catalytic Reduction of Graphene Oxide
Download: http://pubs.acs.org/doi/abs/10.1021/nn800251f
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Summary
Carbon nano-structures such as carbon nano-tubes and graphene offer new opportunities to develop nano-composites with unusual electronic catalytic properties. Here, dispersion of catalyst particles on carbon nanostructures can provide new ways to increase the surface area and improve the catalyst performance of energy conversion devices. In maiden effort, Graeme Williams & Co-workers have succeeded in carrying out UV-induced photo-catalytic reduction of graphene oxide and maintaining well-separated graphene semiconductor composite sheets. In the present study, Williams et.al prepared a mixture of graphene oxide and colloidal TiO2 in ethanol and subjected it to steady state Ultra-Violet irradiation and absorption spectra of graphene oxide and TiO2 suspension in ethanol (a) before UV-irradiation, (b) after 5 min, and (c) after 15 min of UV irradiation shows a change in color of a 10 mM solution of TiO2 nano-particles with 0.5 mg/mL GO before and after UV irradiation for 2 h.
The change in color from light brown to dark brown to black can be seen as the reduction of GO proceeds. This color change has previously been suggested as partial restoration of the network within the carbon structure and has been witnessed through chemical reduction of the GO sheets. No significant changes in the absorption could be seen during UV-irradiation when TiO2 was excluded from the suspension. This observation establishes the mediating role of TiO2 in the reduction of GO. Since TiO2 particles carry surface charge in suspensions, they can be readily suspended in polar solvents. Thus, the TiO2-graphene composite becomes readily soluble in polar solvents.The photo catalytic method differs from the chemical reduction method of graphite oxide (e.g,reduction with hydrazine or with mild reducing agent sodium borohydride) that is usually employed to obtain graphene sheets from graphene oxide. The advantage of photocatalytic reduction is that it can be triggered on demand by tuning the UV-irradiation.
Here, Ti4+ centres serve as trapping sites for photo generated electrons. TiO2 suspensions turn blue during UV irradiation as the electrons accumulate within the TiO2 particles. These trapped electrons remain stable in deaerated solution, thus enabling quantitative extraction by the controlled addition of graphene oxide, shows the decrease in absorbance with incremental addition of deaerated graphene oxide suspension. It is important that the addition of the suspension with a micro syringe is done in the presence of nitrogen. Any trace contamination of air results in the scavenging of electrons by oxygen.
To probe the electron transfer between excited TiO2 and GO, Graeme Williams & Co-workers carried out nanosecond laser flash photolysis using 308 nm laser pulse excitation. The absorption of the trapped electron was monitored at 650 nm.The appearance of the absorption was prompt, indicating the completion of the trapping process within pulse duration. In the presence of GO the magnitude of the absorption decreased by 50% indicating the transfer of electrons to GO within the pulse duration. These results further confirm the role of TiO2 in transferring the electrons to Graphite oxide in the sub microsecond time scale. The films cast with graphene oxide TiO2 suspension prior to UV irradiation showed measured resistance of 233 Kohm. Two hours of irradiation of the suspension resulted in an order of magnitude decrease in the resistance, to an average value of 30.5 Kohm ,Since both samples (before and after UV reduction) contained same concentration of TiO2, the contribution of TiO2 to decreased resistivity is expected to be constant.
Overall decrease in resistance following the UV-exposure, further reduction in resistance is desirable if one is interested in the design of electronic devices. The present method offers a new and soft method of reduction as compared to the conventional approach of using elevated temperatures or strong reducing agents for reducing graphene oxides. The photo catalytic methodology not only provides an on-demand UV assisted reduction technique but also opens up new ways to obtain photoactive graphene-semiconductor composites.
