Surfactant-Directed Synthesis of Mesoporous Pd Films with Perpendicular Mesochannels as Efficient Electrocatalysts

Transcription

1 Supporting Information Surfactant-Directed Synthesis of Mesoporous Pd Films with Perpendicular Mesochannels as Efficient Electrocatalysts Cuiling Li, Bo Jiang,, Nobuyoshi Miyamoto, Jung Ho Kim, Victor Malgras, and Yusuke Yamauchi*, World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki , Japan. Faculty of Science and Engineering, Waseda University, Okubo, Shinjuku, Tokyo , Japan. Department of Life, Environment and Materials Science, Fukuoka Institute of Technology (FIT), Wajiro-Higashi, Higashi, Fukuoka , Japan Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong, Squires Way, North Wollongong, NSW 2500, Australia. S1

2 Experimental Chemicals: Sodium tetrachloropalladate (Na 2 PdCl 4 ), 0.5 M sulfuric acid, and formic acid were purchased from Nacalai. Co. Cetylmethylammonium chloride solution (25 wt% in water) was purchased from Sigma-Aldrich. All the chemicals were used without further purification. Preparation of mesoporous Pd films with perpendicular channels: The electrochemical fabrication of mesoporous Pd films was performed by using a CHI 842B electrochemical analyzer (CH Instrument, U.S.) with a standard three-electrode cell system, including an Ag/AgCl (saturated KCl) electrode as the reference electrode, a platinum wire as the counter electrode, and gold-coated silicon (Au-Si) substrate as the working electrode. The Au-Si substrate was set in an aqueous solution (9.0 ml) containing 0.5 ml of cetylmethylammonium chloride solution and 0.5 ml of 40 mm sodium tetrachloropalladate (Na 2 PdCl 4 ) in a water bath pre-heated at 50 ºC. Due to the strong interaction between [C 16 TA] + and [PdCl 4 ] 2-, the precipitate was formed rapidly after adding the Pd precursor to the surfactant solution. With increasing the bath temperature (over 50 ºC), the mixture becomes transparent. Therefore, here 50 C was selected in the present work. A constant potential of 0.0 V vs. Ag/AgCl was then used for electrodepositing the mesoporous Pd films with perpendicular channels. The obtained films were then soaked in ethanol for 24 h to thoroughly remove the surfactants, and then dried with nitrogen gas for further characterizations. The electrochemical surface area (ECSA) of the films was calculated according to the method proposed by Woods. [1] At a potential of about 1.50 V vs. RHE, a Pd oxide monolayer would form on the Pd surface. With the assumption of 424 μc cm -2 for the reduction of the Pd oxide, the ECSA of Pd can be calculated. The geometric area of the mesoporous Pd electrode used in this experiment was 0.3 cm 2. The nonporous Pd films were also prepared following the same procedure, but without adding CTAC. Characterization: Scanning electron microscope (SEM) images were obtained using a Hitachi HR-SEM SU8000 microscope at an accelerating voltage of 5 kv. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken using a JEOL JEM-2100F microscope at an accelerating voltage of 200 kv. Small angle X-ray scattering (SAXS) profiles were recorded by a Rigaku NANO-Viewer (Cu Kα radiation) with a camera length of 700 mm operated at 40 kv and 30 ma. Wide-angle film X-ray diffraction (XRD) profiles were recorded with a Rigaku SmartLab 3kW-BU diffractometer with monochromated Cu Kα radiation. Electrochemical analysis: Cyclic voltammograms (CVs) were performed with a CHI 842B electrochemical analyzer (CH Instrument, USA). A conventional three-electrode cell was used, including an Ag/AgCl electrode as a reference electrode, a platinum wire as a counter electrode, and the Pd films (mesoporous Pd, nonporous Pd or Pd black) as a working electrode. In the case of the commercially available Pd black (PdB) catalyst, a glassy carbon electrode (GCE) was used as the working electrode. Prior to the surface coating, the GCE was polished with 1.0 and 0.05 μm alumina powder, rinsed with deionized water and dried under a nitrogen gas flow. 5.0 μg of the PdB suspension was then coated on the surface of the GCE. After drying in atmospheric condition, a Nafion solution (5.0 μl, 0.5 wt%) was subsequently coated on the surface and S2

3 dried at room temperature before further electrochemical measurements. All potential values were referenced to an Ag/AgCl (saturated KCl) electrode. The formic acid oxidation reaction investigations were carried out in 0.5 M H 2 SO 4 solution containing 0.5 M formic acid. S3

4 Figure S1 Top-view SEM images of the obtained mesoporous Pd films with perpendicular mesochannels at (a) low and (b) high magnifications. S4

5 Figure S2. Cross-sectional HAADF-STEM image of the Pd film with vertically aligned mesochannels. Figure S3 High resolution TEM of the pore wall. The crystal distortions are marked by arrows. S5

6 Figure S4 Typical cross-sectional TEM image at low magnification highlighting the homogeneity of the film thickness. Figure S5 In-plane pore ordering of the perpendicular mesochannels in the mesoporous Pd film. (a) Bright-field TEM image, (b and d) low- and high-magnified HAADF-STEM images, and (c) the simulated in-plane pore ordering. The electron beam axis is parallel to the mesochannels. S6

7 Figure S6 (a-e) Cross-sectional SEM images of the obtained films with various deposition times. [(a) 1000 s, (b) 1500 s, (c) 2000 s, (d) 3600 s, and (e) 7200 s]. (f) Relationship between the film thickness and the deposition time. Figure S7 (a1, b1) Top surface SEM images and (a2, b2) 2D GI-SAXS patterns of the as-prepared Pd films with various thicknesses [(a) 270 nm and (b) 540 nm]. The deposition times are 3600 s and 7200 s, respectively. S7

8 Figure S8 SAXS patterns of (a) C 16 TAC solution and (b) C 16 TAC+Na 2 PdCl 4 mixture. The line (c) is the theoretical curve calculated based on the form factor of mono-disperse hard spheres with a radius of 2.2 nm and the structure factor of the dispersed spheres. The structure factor is based on Ornstein-Zernike and Percus-Yevick approximations [2] and is calculated with the parameters of hard-sphere interaction distance (Rhs = 5.9 nm) and the miceller volume fraction (f = 0.24). Figure S9 UV-vis spectra of various solutions (Na 2 PdCl 4 solution, C 16 TAC solution, C 16 TAC+Na 2 PdCl 4 mixture). The photographs of Na 2 PdCl 4 solution before and after mixing with C 16 TAC are shown in the inset. S8

9 Figure S10 Scheme illustrating the formation of mesoporous Pd films with perpendicular mesochannels obtained from dynamic transition of spherical micelle assemblies. S9

10 Figure S11 SEM and 2D GI-SAXS patterns of the mesoporous Pd films under various potentials [(a) 0.3 V (vs. Ag/AgCl), (b) 0.2 V (vs. Ag/AgCl), (c) 0.1 V (vs. Ag/AgCl), (d) 0.0 V (vs. Ag/AgCl), (e) -0.1 V (vs. Ag/AgCl), (f) -0.2 V (vs. Ag/AgCl), (g) -0.3 V (vs. Ag/AgCl), and (h) -0.5 V (vs. Ag/AgCl), respectively.]. S10

11 Figure S12 2D GI-SAXS patterns of the obtained Pd films electrodeposited from electrolytes containing surfactants at various concentrations [(a) 43 mm (Optimal concentration), (b) 215 mm, and (c) 430 mm, respectively.]. Figure S13 CV curves obtained in 0.5 M H 2 SO 4 by using nonporous Pd films with various deposition times (600 s, 1000 s, 1500 s, and 200 sec, respectively). S11

12 Table S1 Comparison of pore sizes of various compositions obtained by using the cetylmethylammonium chloride/bromide (C 16 TAC/C 16 TAB) as the pore directing agents. N. Sample Name Surfactant Pore Size (nm) Ref. 1 Mesoporous silica C 16 TAC Silicate/aluminosilicate mesoporous molecular sieves C 16 TAC Porous organosilicas C 16 TAC Mesoporous silica films C 16 TAB Au-nanocage@mSiO C 16 TAB Mesoporous aluminosilicate C 16 TAB Dual-compartment Janus mesoporous silica nanoparticles C 16 TAB Mesoporous silica C 16 TAB Vanadium silicates C 16 TAB Porous silica nanospheres C 16 TAB Ordered mesoporous silica nanoparticles C 16 TAB Mesoporous silica spheres C 16 TAB References [1] D. A. J. Rand, R. Woods, J. Electroanal. Chem. 1971, 31, 29. [2] K. Mortensen, J. Phys.: Condens. Matter 1996, 8, A103. [3] H. Yang, N. Coombs, G. A. Ozin, Nature 1997, 386, 692. [4] J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, J. L. Schlenker, J. Am. Chem. Soc. 1992, 114, [5] M. C. Burleigh, M. A. Markowitz, M. S. Spector, B. P. Gaber, Langmuir 2001, 17, [6] Z. Teng, G. Zheng, Y. Dou, W. Li, C.-Y. Mou, X. Zhang, A. M. Asiri, D. Zhao, Angew. Chem. Int. Ed. 2012, 51, [7] J. Yang, D. Shen, L. Zhou, W. Li, X. Li, C. Yao, R. Wang, A. M. El-Toni, F. Zhang, D. Zhao, Chem. Mater. 2013, 25, [8] Z. Zhao, Y. Han, F.-S. Xiao, S. Qiu, L. Zhu, R. Wang, Y. Yu, Z. Zhang, B. Zou, Y. Wang, H. Sun, D. Zhao, Y. Wei, J. Am. Chem. Soc. 2001, 123, [9] X. Li, L. Zhou, Y. Wei, A. M. El-Toni, F. Zhang, D. Zhao, J. Am. Chem. Soc. 2014, 136, [10] J. Yang, Y. Deng, Q. Wu, J. Zhou, H. Bao, Q. Li, F. Zhang, F. Li, B. Tu, D. Zhao, Langmuir 2010, 26, [11] V. Luca, D. J. MacLachlan, K. Morgan, Chem. Mater. 1997, 9, [12] Y. G. Jin, S. Z. Qiao, Z. P. Xu, J. C. D. da Costa, G. Q. Lu, J. Phys. Chem. C 2009, 113, [13] Y. S. Lin, C. P. Tsai, H. Y. Huang, C. T. Kuo, Y. Hung, D. M. Huang, Y. C. Chen, C. Y. Mou, Chem. Mater. 2005, 17, [14] R. I. Nooney, D. Thirunavukkarasu, Y. Chen, R. Josephs, A. E. Ostafin, Chem. Mater. 2002, 14, S12