On the role of surface diffusion in determining the shape or morphology of noble-metal nanocrystals Xiaohu Xiaa,1, Shuifen Xiea,1, Maochang Liua,1, Hsin-Chieh Pengb, Ning Luc, Jinguo Wangc, Moon J. Kimc,d, and Younan Xiaa,b,e,2 aThe Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, GA 30332; bSchool of Chemistry and Biochemistry and eSchool of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332; cDepartment of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX 75080; and dDepartment of Nanobio Materials and Electronics, World Class University, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea Edited by Gabor A. Somorjai, University of California, Berkeley, CA, and approved March 19, 2013 (received for review December 18, 2012) Controlling the shape or morphology of metal nanocrystals is central a cubic seed is determined by the ratio between the rates for to the realization of their many applications in catalysis, plasmonics, atom deposition and surface diffusion (Vdeposition/Vdiffusion). When and electronics. In one of the approaches, the metal nanocrystals are Vdeposition/Vdiffusion >> 1, surface diffusion can be ignored and grown from seeds of certain crystallinity through the addition of thereby the growth will be largely confined to the corner sites along atomic species. In this case, manipulating the rates at which the the <111> directions, resulting in the formation of Pd octapods atomic species are added onto different crystallographic planes of (Fig. 1B, i). On the contrary, when Vdeposition/Vdiffusion << 1, the a seed has been actively explored to control the growth pattern of growth will be dominated by surface diffusion and be switched to a seed and thereby the shape or morphology taken by the final the <100> and <110> directions as most of the adatoms at the product. Upon deposition, however, the adsorbed atoms (adatoms) corners can quickly migrate to edges and side faces of a cubic seed, may not stay at the same sites where the depositions occur. Instead, promoting the formation of a cuboctahedron as the final product they can migrate to other sites on the seed owing to the involve- (Fig. 1B, iv). Similar arguments can also be applied to the situations ment of surface diffusion, and this could lead to unexpected where the ratios of Vdeposition/Vdiffusion are between these two deviations from a desired growth pathway. Herein, we demon- V V extremes. For example, when deposition/ diffusion is slightly larger CHEMISTRY strated that the growth pathway of a seed is indeed determined by than 1, a small portion of the adatoms at the corners will migrate the ratio between the rates for atom deposition and surface to the edges (which are relatively more active than the side faces diffusion. Our result suggests that surface diffusion needs to be – due to a lower coverage density for the Br ions) of a seed, leading taken into account when controlling the shape or morphology of to the formation of Pd concave nanocubes (Fig. 1B, ii). When metal nanocrystals. Vdeposition/Vdiffusion is slightly less than 1, some of the adatoms will stay at corners while the rest can diffuse to both edges and side seeded growth | shape control | noble metals faces of a seed. As a result, the final product will be an enlarged Pd nanocube with slight truncations at the corners (Fig. 1B, iii). urface diffusion is a general process that involves the motion of We conducted a set of experiments based on seed-mediated Sadsorbed atoms (adatoms), molecules, or atomic clusters on growth to validate the proposed mechanisms. The growth involved the surface of a solid material (1, 2). Over the past decades, it has the use of Pd nanocubes as seeds in an aqueous solution, with emerged as an important concept in many areas of surface science, L-ascorbic acid (AA), Na2PdCl4, and poly(vinyl pyrrolidone) (PVP) including catalysis, epitaxial growth, and electromigration of voids serving as the reductant, Pd precursor, and stabilizer, respectively. (3–7). Here, we demonstrated that surface diffusion also plays In a standard synthesis, an aqueous Na2PdCl4 solution was injected a pivotal role in determining the growth pathway of a seed and thus using a syringe pump into an aqueous suspension containing AA, fi the shape or morphology taken by the nal product in a solution- PVP, and Pd seeds that were hosted in a glass vial at room tem- phase synthesis of metal nanocrystals. Fig. 1 schematically illus- perature (∼22 °C) under magnetic stirring (see Materials and trates four possible pathways for the growth of a cubic seed. As Methods for experimental details). For this synthesis, Na2PdCl4 is a model system, we focused on Pd nanocubes with slight truncation supposed to be immediately reduced into Pd atoms by AA upon at corners and edges, together with six side faces passivated by – addition into the reaction solution due to the strong reduction chemisorbed Br ions. In the following discussion, we refer to them power of AA (14, 15). As such, the concentration of the newly as “Pd cubic seeds” for simplicity. We chose them as seeds for two formed Pd atoms in the reaction solution and thereby Vdeposition will major reasons: (i) they had a well-defined shape, together with be mainly determined by the injection rate for Na2PdCl4 solution a set of low-index facets on the surface (8, 9); and (ii) their side – that can be readily controlled through the use of a syringe pump. faces are blocked by Br ions to ensure selective deposition of Because surface diffusion is a thermally promoted process with its atoms onto the corner sites during seed-mediated growth (10–12). rate increasing with temperature (2), Vdiffusion can be adjusted by These two distinctive features allowed us to easily track the de- presetting the oil bath to a specific temperature. Collectively, the position of atoms and their surface diffusion during a growth roles played by V and V can be separated from each fi diffusion deposition process by analyzing the shape or morphology of the nal product. other for investigation by varying the reaction conditions. The newly formed Pd atoms resulting from the reduction of a Pd precursor are expected to deposit at the corners of a cubic seed – because the side faces are blocked by the chemisorbed Br ions Author contributions: X.X., S.X., and Y.X. designed research; X.X., S.X., and M.L. per- (Fig. 1A, 1). Upon deposition, there will be two different options formed research; N.L., J.W., and M.J.K. contributed new reagents/analytic tools; X.X., for these adatoms: staying at the corner sites or migrating to other S.X., M.L., H.-C.P., and Y.X. analyzed data; and X.X. and Y.X. wrote the paper. sites, including edges and side faces, through surface diffusion The authors declare no conflict of interest. (Fig. 1A, 2 and 3). It should be pointed out that only surface dif- This article is a PNAS Direct Submission. fusion was allowed here to move atoms from corners to edges and 1X.X., S.X., and M.L. contributed equally to this work. side faces of a seed during growth. Other mechanisms such as 2To whom correspondence should be addressed. E-mail: [email protected]. Ostwald ripening (13) were not considered because the side – This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. faces of a seed were blocked by Br ions. The growth pathway of 1073/pnas.1222109110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1222109110 PNAS Early Edition | 1of5 Downloaded by guest on September 25, 2021 respectively. The sizes (defined as the distance between two op- posite {100} facets of a nanocrystal as illustrated in Fig. S3)of these four different types of Pd nanocrystals were measured to be 15.5, 18, 20.5, and 23 nm, respectively. Therefore, the growth of 15-nm cubic seeds was mainly confined to the <111> directions at 0 °C (Fig. 2A) and almost no deposition was found along the <100> directions. When the reaction temperature was in- creased from 0 to 75 °C (Fig. 2D), however, the growth was switched to the <100> and <110> directions because the size of the cuboctahedrons (i.e., 23 nm) was consistent with that of a cuboc- tahedron produced through exclusive growth on the {100} and {110} facets of a 15-nm cubic seed (Fig. S4) (17, 18). These results imply that, for the current system, the critical reaction temper- atures for all of the adatoms to stay at corners sites or diffuse to the edges and side faces of a cubic seed are 0 and 75 °C, respectively. In the second set of experiments, we adjusted the injection rate for Na2PdCl4 solution while keeping the reaction temperature the same as the standard synthesis (22 °C). In these cases, the ratio of Vdeposition/Vdiffusion should increase as the injection rate for Na2PdCl4 solution increases because Vdiffusion is fixed. According to the proposed mechanisms (Fig. 1), it is not difficult to argue that Pd nanocrystals with shapes and sizes similar to those shown in Fig. 2 would also be obtained by decreasing the injection rate for Na2PdCl4 solution. As expected, Pd nanocrystals with shapes ranging from octapods to concave nanocubes, nanocubes with slight truncation at corners, and cuboctahedrons were indeed obtained when the injection rate for Na2PdCl4 solution was de- creased from 1.5 to 0.75, 0.5, and 0.25 mL/h, respectively (Fig. S5). These results again supported our proposed mechanisms. Inter- estingly, the Pd octapods shown in Fig.