Formation of HCP Rhodium as a Size Effect

Transcription

1 Supporting Information Formation of HCP Rhodium as a Size Effect Jinglu Huang, Zhi Li, Haohong Duan, Zhiying Cheng, Yadong Li, Jing Zhu, Rong Yu* J. Huang, Z. Cheng, Prof. J. Zhu, Prof. R. Yu National Center for Electron Microscopy in Beijing School of Materials Science and Engineering Key Laboratory of Advanced Materials of Ministry of Education of China State Key Laboratory of New Ceramics and Fine Processing Tsinghua University Beijing , P. R. China ryu@tsinghua.edu.cn Z. Li, H. Duan, Prof. Y. Li Department of Chemistry, Tsinghua University Beijing , China S1

2 Synthesis. In this study, the nanoparticles of Rh were prepared using two different ways. The first one is the electron-beam induced decomposition of the rhodium monolayers, which were prepared using PVP-capped method, as described in detail before. 1 The second way is direct synthesis of rhodium nanoparticles via the solvothermal reaction. Rh(acac) 3 (20 mg) was dissolved in oleylamine (10 ml) and the mixture was stirred for 5 minutes, then transferred in a balloon flask. The flask was sealed and maintained at 220 C for 0.5 hour, then added with a reductant, which was prepared by dissolving Tert-Butylamine borane (200 mg) in oleylamine (3 ml) and ultrasonic wave shaking for 5 minutes. The mixture in the flask was kept stirring till the temperature cooled down to 60 C and then immediately washed by ethyl alcohol to remove the OAM. Finally, the product was separated by centrifugation. The TEM samples were prepared by dissolving the product in cyclohexane and then ultrasonic wave shaking for 5 minutes. Structural characterization. High-resolution TEM investigations were performed on an FEI Titan aberration-corrected TEM, operated at 300 kv. The negative Cs imaging mode 2 was used, with the spherical aberration (Cs) set to -13 µm. The positions of atomic columns were determined by two-dimensional Gaussian fitting of image maxima using MacTempasX software. DFT calculations. First-principles calculations based on the density function theory (DFT) were performed to investigate the atomic and electronic structure of fcc and hcp rhodium by using the Vienna Ab initio Simulation Package (VASP) 3-4. The S2

3 projector augmented wave (PAW) 5 pseudopotentials and Perdew-Burke-Enzenhof (PBE) 6 exchange-correlation functional were used in the calculations. The semi-core 4p electrons were considered as valence. An energy cut-off of 300eV was chosen for the plane wave basis sets. We have studied eight types of low index surfaces, respectively were fcc(111), fcc(110), fcc(100), fcc(311), hcp(112 0), hcp(0001), hcp(101 0) and hcp(101 2). Slab models were built in order to satisfy the periodic boundary condition. The vacuum region between slabs were set to be about 15 Å. A mesh of were used in the k-point sampling in the Brillouin zone. Series of models with thicknesses from 2 Å to 2 nm were built for each type of surface. Structure relaxations were performed for each model by minimizing the Hellmann-Feynman force down to less than 0.01eV/Å. The energy difference between the last two electronic iterations were required to be less than 10-7 ev to ensure energy convergence. During the relaxations the stress tensors were calculated, the in-plane lattice parameters were allowed to change but the out-of-plane lattice parameters were fixed. After structure optimization, the local potentials in real space were calculated for each model. The work function was determined by the equation W = E vac -E Fermi. Where E vac is the potential at the center of the vacuum region, E Fermi is the calculated Fermi level. Aberration-corrected TEM images Additional aberration-corrected TEM data are shown in Fig. S1. Several percent of very small particles in the product of the solvothermal reaction can be identified as the S3

4 hcp structure. Most of the hcp particles show an anisotropic shape, with [0001] as the preferred growth direction. While the size of the hcp particles is around 2-3 nm in the short axis direction, it is 6-8 nm in the long axis direction. The small dimension in the directions normal to the [0001] direction would give very diffuse scattering for (010) reflection in XRD patterns. Fig. S1. Aberration-corrected TEM images of hcp particles in the product of the solvothermal reaction. The particles show a preferred growth direction [0001]. X-ray Diffraction. Kusada et al. 7-9 prepared very small ruthenium nanoparticles of pure fcc and hcp structures. Due to the small size, the characteristic peaks, (010) at 38 and (102) at 58, are weak even for pure hcp nanoparticles, in particular for the smallest particles 7. S4

5 Fig. S2 show a typical XRD pattern of the Rh nanoparticles of the solvothermal reaction. The XRD signals from the hcp Rh particles are very weak, due to the low percentage of the hcp structure and the small dimension of the hcp particles in the directions normal to the [0001] direction, leading to very diffuse scattering for the (010) reflection in XRD patterns. It remains a challenge to prepare pure hcp Rh particles, as demonstrated by Kusada et al. for Ru in Ref As suggested by Fig. 7 in the main text, a possible route for pure hcp Rh is to synthesize much smaller particles or clusters than reported in this work. Using molecular dynamics simulations, Chien et al. 10 showed that hcp clusters of 26 and 48 atoms can be the most stable structures, which are 1 nm or less in diameter. Fig. S2. A typical XRD pattern of the product of the solvothermal reaction. References: 1. Duan, H.; Yan, N.; Yu, R.; Chang, C. R.; Zhou, G.; Hu, H. S.; Rong, H.; Niu, Z.; Mao, J.; Asakura, H.; Tanaka, T.; Dyson, P. J.; Li, J.; Li, Y., Nat. Commun. 2014, 5, Jia, C. L.; Lentzen, M.; Urban, K., Science 2003, 299, S5

6 3. Kresse, G.; Furthmuller, J., Phys. Rev. B 1996, 54, Kresse, G.; Furthmuller, J., Comput. Mater. Sci. 1996, 6, Kresse, G.; Joubert, D., Phys. Rev. B 1999, 59, Perdew, J. P.; Burke, K.; Ernzerhof, M., Phys. Rev. Lett. 1996, 77, Kusada, K.; Kobayashi, H.; Yamamoto, T.; Matsumura, S.; Sumi, N.; Sato, K.; Nagaoka, K.; Kubota, Y.; Kitagawa, H., J. Am. Chem. Soc. 2013, 135, Song, C.; Sakata, O.; Kumara, L. S. R.; Kohara, S.; Yang, A.; Kusada, K.; Kobayashi, H.; Kitagawa, H., Sci. Rep. 2016, 6, Kusada, K.; Kitagawa, H., Adv. Mater. 2016, 28, Chien, C.-H.; Blaisten-Barojas, E.; Pederson, M. R., J. Chem. Phys. 2000, 112, S6