Supporting Information Decorating Graphene Sheets with Gold Nanoparticles Ryan Muszynski, Brian Seeger and, Prashant V. Kamat* Radiation Laboratory, Departments of Chemistry & Biochemistry and Chemical & Biomolecular Engineering University of Notre Dame, Notre Dame, IN 46556-0579
Methods and Procedures Materials: Microcrystaline grade graphite (APS 2-15 micron), octadecylamine and tetrachloroaurate were obtained from Alfa Aesar (Ward Hill, MA). The potassium chlorate was obtained from JT Baker (Phillipsburg, NJ). All other chemicals obtained from Fischer Scientific were analytical grade and were used as supplied. Preparation of Graphene Oxide: To functionalize and exfoliate the graphene layers from graphite we followed the graphite oxide preparation methodology established by Staudenmaier (Staudenmaier, L. Ber. Dtsch. Chem. Ges. 1898, 31, 1481). 2.5g of graphite was added to mixture of 44 ml of concentrated sulfuric acid and 29 ml of nitric acid in a hood. While stirring the graphite mixture, 23 g of potassium chlorate was slowly added to the graphite mixture over 30 minutes. Slow and controlled addition of potassium chlorate is essential to avoid any violent or explosive reactions. The suspension was stirred in the hood for 5 days as yellowish gas evolved from the mixture. The color of the mixture changed from yellow to blue. The mixture was diluted with 2L of water and then filtered through coarse filter paper. 250 ml of 5% HCl mixture was then washed over the filter cake. Next the graphene oxide was resuspended in water and centrifuged to remove any excess HCl. The washing cycle was repeated two more times. The remaining solid was dried at 353 K overnight and ground up with a mortar and pestle to a fine powder. Functionalization with Octadecylamine (ODA) to the graphene: Using a procedure developed by Haddon et al., we attached octadecylamine (ODA) to the graphene sheets. This procedure consisted of first reacting 200mg of graphene with 1ml of dimethylformamide and 12ml of SOCl 2 in a dry nitrogen atmosphere. This mixture was reacted at 70 C for 24 hours. After the completion of the reaction the excess SOCl 2 was removed by distillation. The remaining graphene was then heated to 120 C and 580mg of ODA was added. The ODA melted (melting point 52 C) and the reaction mixture was stirred for 4 days. After cooling the solution to room temperature, boiling ethanol was added to suspend the oda functionalized graphene. This solution was quickly filtered through 0.05 μm filter paper to remove dissolved (excess) ODA. The filter cake was washed with hot ethanol 3 more times to verify that all the unbound ODA was removed. The filter cake was then dispersed in
tetrahydrofuran (THF) to solubilize the ODA attached graphene. This solution was filtered using a coarse filter paper. The remaining filter cake, which consisted mostly of unreacted graphene, was discarded. Using a Rotovap THF was evaporated off from the filtrate. The isolated product, ODA functionalized graphene (graphene-oda) was stored and known amount was resuspended in THF as needed. Synthesis of gold nanoparticles in graphene-oda suspension: 2 ml of a 0.08 mg/ml aqueous solution of ODA-Graphene was transferred to a vial. 50 μl of an aqueous 0.5M NaBH 4 solution was added and stirred for 5 minutes. 50μl of 30mM of aqueous tetrachloroaurate solution was added to the solution. This experiment was repeated with graphene concentrations of 0.16, 0.32, 0.48, and 0.64 mg/ml as well as one experiment where no graphene was used. Transient Absorption Spectroscopy. Ultrafast (femtosecond) transient absorption experiments were conducted using a Clark-MXR 2010 laser system and an optical detection system provided by Ultrafast Systems (Helios). The source for the pump and probe pulses is the fundamental of the Clark laser system (775 nm, 1 mj/pulse, fwhm 130 fs, 1 khz repetition rate). 5% of the output is used to generate a white light continuum probe pulses. Prior to creating the white light probe, the fundamental is fed through a delay line providing an experimental time window of 1.6 ns with a maximum step resolution of 7 fs. The pump beam is attenuated to 5 μj/pulse with a spot size of 2 mm (diameter) at the sample where it is merged with the white light incident on the sample cell with an angle <10. After passing through the samples, the probe is focused onto a 200 μm core fiber connected to a CCD spectrograph (Ocean Optics, S2000-UV-vis (425-800 nm), or Sensors Unlimited, SU-LDV-512LDB (750-1200 nm)), enabling time-resolved spectra to be recorded. Typically, 5000 excitation pulses are averaged to obtain the transient spectrum at a set delay time. Kinetic traces at appropriate wavelengths are assembled from the time-resolved data. All measurements were conducted at room temperature. Microscopy. A 4500S Hitachi scanning electron microscope was used to look at the structure of the ODA-Graphene with and without gold particles. Both species were dropcast onto carbon fiber paper and then put under the microscope. Figure3A shows a plain ODA-Graphene while
Figure 3B shows an ODA-graphene particle where gold particles have been bonded onto the surface. Both images show individual graphene sheets. In some cases few stacked sheets could also be seen. Figure 3A and 3B SEM images of graphene sheets before and after modification with gold nanoparticles. The Gold-graphene solutions in which 0.08 mg/ml and 0.64 mg/ml of graphene was used were each looked at under a high resolution TEM as shown in Figure 4A and B, respectively. The 0.08mg/ml solution has an average gold particle size of 11.7nm (+/- 3.4nm) while the 0.64 mg/ml graphene solution mixture has a particle size of 5.9nm (+/- 1.8 nm). This indicates that increasing the amount of graphene relative to gold will decrease the gold particle size. These images were taken on a carbon grid, thus making it extremely hard to differentiate the graphene from background carbon. The SEM image however shows the gold particles are definitely embedded in the graphene. A B 40 10 nm 40 10 nm Figure 4. TEM images of gold nanoparticles stabilized in THF suspension of (A) 0.08 mg/ml and (B) 0.64 mg/ml of graphene.