Supporting Information

Transcription

1 Supporting Information Yao et al /pnas Fig. S1. In situ LEEM imaging of graphene growth via chemical vapor deposition (CVD) on Pt(111). The growth of graphene on Pt(111) via a CVD process was performed by exposing the bare Pt(111) to 10 7 Torr ethylene at around 950 K. The growth was monitored by in situ LEEM. A series of snapshots from the in situ LEEM imaging are displayed, showing nucleation of graphene islands, island coalescence, and formation of full monolayer graphene overlayer. Wrinkles form on the full monolayer graphene when the Gr/Pt(111) surface is cooled down to room temperature. The field of view (FoV) of all of the LEEM images is 20 μm. 1of5

2 Fig. S2. Pressure-dependent CO adsorption on bare Pt(111) surface. (A) CO PM-IRRAS; (B) CO TPD. Each spectrum was recorded after 10 min CO exposure at the indicated CO pressure followed by evacuation to UHV. PM-IRRAS spectra (A) show a strong on-top CO adsorption peak at 2,098 cm 1 and a weak bridged CO adsorption peak at 1,855 cm 1 upon exposure of Torr CO. Neither peak position nor peak intensity changes when exposing in higher CO pressure (up to Torr). TPD spectra (B) always show similar broad CO desorption peaks between 320 and 550 K, irrespective of the CO exposure pressure. Fig. S3. Models used for DFT calculations. 7 7 supercells were used for the DFT calculations on Pt(111) (A) and Gr/Pt(111) (B) surfaces. Initial states, transition states, and final states of reaction paths for oxidation between CO and O are shown on Pt(111) (I III) and Gr/Pt(111) (IV VI). Black balls: C; red balls: O. 2of5

3 Fig. S4. Graphene coverage effect on CO adsorption. CO TPD. Each spectrum was recorded after that the Pt(111), 0.5 ML Gr/Pt(111), 0.8 ML Gr/Pt(111), and 1.0 ML Gr/Pt(111) surfaces were exposed to Torr CO for 10 min at room temperature and then evacuated to UHV. CO uptake was only observed on bare Pt(111) and submonolayer graphene surfaces. No CO intercalation was detected by CO TPD on the 1 ML Gr/Pt(111) under Torr CO exposure condition. Fig. S5. PM-IRRAS studies in the CO desorption from the bare Pt(111) and 1 ML Gr/Pt(111) surfaces. CO PM-IRRAS of Pt(111) (A) and 1 ML Gr/Pt(111) (B) surfaces, which have been saturated with CO at room temperature and then annealed under UHV at the indicated temperatures. On the 1 ML Gr/CO/Pt(111) surface, CO almost desorbs when annealing to 400 K, whereas the complete CO desorption happens at 450 K. The results indicate that CO adsorption on Pt has been weakened due to the presence of the graphene overlayer. 3of5

4 Fig. S6. In situ ambient pressure XPS studies of CO adsorption on Pt(111). XPS O1s (A) and Pt 4f (B) spectra from a bare Pt(111) surface exposed to Torr and 0.1 Torr CO, respectively. The spectra from the bare Pt(111) surface are included as a comparison. The Pt(111) surface is saturated with 10 6 Torr CO, showing two O 1s peaks at and ev, due to CO adsorbed on top and bridged sites, respectively. With an increase of CO pressure to 0.1 Torr, the adsorbed CO signals increase slightly due to an increased saturation CO coverage under higher CO pressure on Pt(111). In 0.1 Torr CO atmosphere, an O 1s peak from gas phase CO is also detected at ev. XPS Pt 4f of Pt(111) was fitted with Pt4f 7/2 at and ev, due to the bulk and surface Pt atoms, respectively. After being saturated with Torr CO, the Pt 4f 7/2 component at ev from the surface Pt atoms has been strongly attenuated, and meanwhile two new features appear with Pt 4f 7/2 at and ev due to surface Pt atoms adsorbed with top and bridged CO, respectively. Fig. S7. In situ ambient pressure XPS studies of CO intercalation on Gr/Pt(111). (A) XPS C 1s spectra acquired from the 1 ML Gr/Pt(111) surface exposed to CO atmosphere from to 0.5 Torr. The CO-intercalated surface was annealed at 633 K in UHV and the C 1s spectrum acquired from the annealed surface was also included for comparison. (B) The change of the C 1s binding energy position as a function of CO pressure. With the increase of CO pressure from UHV to 0.5 Torr, the C 1s binding energy shifts to lower position by 0.2 ev. After annealing at 633 K in UHV, the C 1s binding energy resumes back to the original value. The decrease in the C 1s binding energy position after the CO intercalation indicates weakened interaction between graphene overlayers and Pt(111) substrate. 4of5

5 Fig. S8. In situ ambient pressure XPS studies of O 2 intercalation on Gr/Pt(111). XPS O 1s (A), Pt 4f (B), and C 1s (C) spectra from the 1 ML Gr/Pt(111) surface exposed to UHV, 0.1 Torr O 2 at room temperature, and 0.1 Torr O 2 at 373 K. Under 0.1 Torr O 2 at room temperature, an O 1s adsorption peak is detected at ev due to oxygen bonded to surface Pt atoms, which indicates that oxygen intercalation already takes place on the Gr/Pt(111) surface. At an elevated temperature (373 K), a large increase in the O 1s intensity at ev has been observed. Under the present O 2 pressure and temperature conditions, no change in the C 1s peak intensity indicates that the oxidation of the graphene overlayers is quite limited. With oxygen intercalation under the graphene overlayers, a new Pt 4f 7/2 peak at 71.5 ev due to surface Pt atoms bound to oxygen atoms appears with the cost of the Pt 4f 7/2 component at 70.9 ev from the Pt surface atoms. After the oxygen intercalation at 373 K, the surface Pt component of the Pt 4f 7/2 at 70.9 ev is completely extinct. The 0.2 ev negative binding energy shift of the C 1s spectra from the 1 ML Gr/Pt(111) surface after oxygen intercalation at 373 K indicates the decoupling of the graphene overlayers from the Pt surface due to the oxygen intercalation, which is similar to C 1s binding energy shift caused by the CO intercalation as shown in Fig. S7. Fig. S9. Postreaction characterization. (A) CO PM-IRRAS and (B) Auger electron spectroscopy (AES) of 1 ML Gr/Pt before and after CO oxidation characterization. CO PM-IRRAS were recorded under reaction condition of 20 Torr CO + 10 Torr O 2 at room temperature. AES were recorded under UHV conditions. After CO oxidation reaction, the reactor was evacuated to UHV, and the sample was transferred to the main chamber under vacuum for AES characterization. No CO adsorption on bare Pt surface (CO peak position at 2,098 cm 1 ) after reaction and almost identical AES spectra before and after CO oxidation reaction indicate the graphene overlayer is stable under the reaction conditions used in the current experiments. 5of5