Single-atom Catalysis Using Pt/Graphene Achieved through Atomic Layer Deposition

Transcription

1 Supplementary Information Single-atom Catalysis Using Pt/Graphene Achieved through Atomic Layer Deposition Shuhui Sun 1,2, Gaixia Zhang 1,2, Nicolas Gauquelin 3,4, Ning Chen 5, Jigang Zhou 5, Songlan Yang 5, Weifeng Chen 5, Xiangbo Meng 1, Dongsheng Geng 1, Mohammad N. Banis 1, Ruying Li 1, Siyu Ye 6, Shanna Knights 6, Gianluigi A. Botton 3,4,7, Tsun-Kong Sham 8 & Xueliang Sun 1 1 Department of Mechanical and Materials Engineering, University of Western Ontario, London N6A 5B9, Canada 2 Institut National de la Recherche Scientifique-Énergie, Matériaux et Télécommunications, Varennes, QC J3X 1S2, Canada 3 Canadian Centre for Electron Microscopy, McMaster University, Hamilton L8S 4M1, Canada 4 Brockhouse Institute for Materials Research, McMaster University, Hamilton L9S 4M1, Canada 5 Canadian Light Source, Saskatoon S7N 0X4, Canada 6 Ballard Power Systems Inc., 9000 Glenlyon Parkway, Burnaby, V5J 5J8, Canada 7 Department of Materials Science and Engineering, McMaster University, Hamilton L8S 4L7, Canada 8 Department of Chemistry, University of Western Ontario, London N6A 5B7, Canada 1

2 Preparation of graphene nanosheet. Natural graphite powder (1 g, 99.99%, Sigma- Aldrich) was first stirred in concentrated sulphuric acid (H 2 SO 4, 23 ml) followed by the addition of sodium nitrate (NaNO 3, 0.5 g) at room temperature. The stirring lasted for 16 h, after that the mixture was cooled down to 0 C. Potassium permanganate (K 2 MnO 4, 3 g) was then added into the solution. After 2 h, a green slurry was formed at around 35 C, which was then stirred for another 3 h. Then, H 2 O (46 ml) was slowly added into the solution at around 98 C. The suspension was kept at this temperature for 30 min before it was further diluted with another addition of water and hydrogen peroxide (H 2 O 2, 140 ml). The suspension was subsequently filtered and washed until reaching a neutral ph, then dried in a vacuum oven at 60 C to obtain graphite oxide (GO). To obtain graphene, the as-synthesized GO was heated at around 1050 C in Ar atmosphere for 30 s in a tube furnace. ALD apparatus. The schematic illustration of the apparatus used for ALD is shown in Fig. S1. It consists of a wall reactor with an outlet leading to a vacuum pump which can also prevent air from getting into the system. The inlet is connected to a system of valves which can control the delivery of purge gas and precursors. Since the Pt ALD temperature window is relatively narrow, it is important to ensure that the graphene support is at the right temperature during deposition. To increase the heat transfer from the stage to the graphene powder, hot wall conditions were replicated by placing graphene powder inside a machined stainless steel container with a perforated Al lid. Pt and Au XANES. The Pt L- 3,2 -edge white line intensity was analyzed using the Au metal L 3,2 -edge XANES as the background since Au metal exhibits no white line and has the same face center cubic structure as Pt metal. Both Au and Pt XANES were calibrated 2

3 relative to the edge threshold (E o, point of inflection of the rising edge jump) with the edge jump normalized to unity (atomic edge jump, for Pt unity edge jump at the L 3 and L 2 edge has a value of 2.5 x10 3 cm -1 and 1.13 x10 3 cm -1, respectively) 1. Thus the area under the difference curve in the white line region between Pt and Au, denoted A 3 and A 2 for the L 3 and L 2 edge respectively, will be proportional to the unoccupied densities of states of Pt 5d character for the samples of interest. Reference [1] McMaster, W. H., Kerr Del Grande, N., Mallett, J. & Hubbell, J. H. Compilation of X-ray Cross Sections ~NTI Services, Springfield, VA, (1969). 3

4 O 2 MeCpPtMe 3 Figure S1 Schematic illustration of ALD Pt process. 4

5 A B 200 nm Figure S2 SEM (A) and TEM (B) images of graphene. 5

6 A A' B B' C C' Figure S3 SEM (A, B, C) and TEM (A', B', C') images of Pt/GNS with 50, 100, and 150 ALD cycles, respectively. 6

7 A 2 nm -1 B 2 nm -1 C -1 Figure S4 HRTEM (A, B, C) images of Pt/GNS with 50, 100, and 150 ALD cycles, respectively. 7

8 A B Figure S5 Bright field TEM (A) and (B) HAADF-STEM image of 100 ALD cycles Pt on GNS. There is sharp contrast between the bright field TEM image and HAADF- STEM image. The HAADF-STEM (B) reveals that, in addition to the Pt nanoparticles shown in (A), there are numerous individual Pt atoms, as well as very small Pt clusters of size 1 nm that consisting only of a few atoms. 8

9 Normalized intensities Pt M Cu K Cu K Area1 (6.7nm) Area2 (4.5nm) Area3 (3nm) Area4 (2.2nm) Area5 (1.2nm) Area6 (single atoms) Pt L Energy (kev) Figure S6 EDX spectrum of Pt/GNS taken from different areas. 9

10 Current density/macm (a) ALD50Pt/GNS (b) ALD100Pt/GNS (c) ALD150Pt/GNS (d) Pt/C (ETEK) Times/s (a) (b) (c) (d) Figure S7 Chronoamperometry (CA) curves of various Pt catalysts recorded in solution of 1.0 M CH 3 OH M H 2 SO 4 at a constant potential of 0.6 V vs. RHE for 20 min: (a) ALD150Pt/GNS, (b) ALD100Pt/GNS, (c) ALD50Pt/GNS, (d) Pt/C. 10

11 a a' Counts Particle size (nm) 10 nm 4 nm b 500 b' 400 Counts Particle size (nm) 10 nm 4 nm Figure S8 HAADF-STEM images, at different magnifications, of ALD100Pt/graphene before (a, a') and after (b, b') the electrochemical stability test. 11

12 Figure S9 X-ray absorption studies. The normalized XANES spectra at Pt L 2 of samples (a) Pt foil (black line); (b) ALD50Pt/GNS (olive line); (c) ALD100Pt/GNS (magenta line); (d) ALD150Pt/GNS Pt/GNS (blue line). 12

13 2.0 Pt L Edge Jump Au L 3 Difference curve E -E o (ev) Figure S10 Pt L 3 -edge WL intensity by subtracting the corresponding XANES of that of the Au. 13