Supporting Information. Controlling the Morphology of Rhodium Nanocrystals by Manipulating the Growth Kinetics with a Syringe Pump

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1 Supporting Information Controlling the Morphology of Rhodium Nanocrystals by Manipulating the Growth Kinetics with a Syringe Pump Hui Zhang,, Weiyang Li, Mingshang Jin, Jie Zeng, Taekyung Yu, Deren Yang, and Younan Xia *, Department of Biomedical Engineering, Washington University, St. Louis, Missouri State Key Laboratory of Silicon Materials, and Department of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang , P. R. China * To whom correspondence should be addressed. xia@biomed.wustl.edu Experimental section In a standard synthesis, 0.3 mmol L-ascorbic acid (AA, Sigma-Aldrich) and 0.9 mmol potassium bromide (KBr, Fisher) were co-dissolved in 7 ml of ethylene glycol (EG, J. T. Baker) hosted in a three-neck flask. The flask was then equipped with a reflux condenser and a Teflon-coated magnetic stir bar, preheated in air at 110 o C for 2 h, and then ramped to 140 o C. Meanwhile, 0.12 mmol of sodium hexachlororhodate(iii) (Na 3 RhCl 6, Aldrich,) and 1.2 mmol of poly(vinyl pyrrolidone) (PVP, M.W. 55,000, Aldrich) were separately dissolved in 6 ml of EG at room temperature. These two solutions were then injected simultaneously into the flask through a syringe pump with a designated injection mode, including slow plus slow, slow plus fast, fast plus slow, and fast plus fast. The slow and fast here correspond to injection rates of 4 ml/h and 60 ml/h, respectively. For the fast plus slow mode, 30 ml EG solution containing 0.6 mmol Na 3 RhCl 6 was needed to obtain the concave nanocubes, and the molar ratios of the reactants including Na 3 RhCl 6, AA, KBr, and PVP were kept the same as the standard synthesis. In order to understand the growth mechanisms, a 1

2 series of intermediate products were sampled from the reaction solution with glass pipets over the course of each synthesis. The nanocrystals were collected by centrifugation and washed with acetone and ethanol several times to remove EG and excess PVP. Transmission electron microscopy (TEM) images were taken using a Tecnai G2 Spirit Twin microscope operated at 120 kv (FEI, Hillsboro, OR). High-resolution TEM images were obtained using a field-emission 2100F microscope operated at 200 kv (JEOL, Tokyo, Japan). Scanning electron microscopy (SEM) images were captured with a Nova NanoSEM 230 field-emission microscope (FEI, Hillsboro, OR) operated at an accelerating voltage of 30 kv. Samples were prepared by dropping an ethanol suspension of the as-prepared nanocrystals onto carbon-coated copper grids (for TEM) or silicon substrates (for SEM) and dried under ambient conditions. 2

3 Figure S1. TEM images and size distribution of Rh nanocrystals that were obtained under a slow plus slow injection mode at different reaction times: (a) 0.25, (b) 1, (c) 20 h; and (d) a plot of the size as a function of reaction time. The slow here corresponds to an injection rate of 4 ml/h. The size was defined as the average edge length of the concave nanocubes. The insets show TEM images of the corresponding samples at a higher magnification. The arrows in Figure S1a correspond to normal cubes of Rh. The scale bars in the insets are 5 nm. 3

4 Figure S2. Size distributions of Rh nanocrystals that were obtained under a slow plus slow injection mode at different reaction times for the synthesis shown in Figure S1: (a) 0.1, (b) 0.25, (c) 0.5, (d) 1, (e) 2, (f) 4, (g) 8, and (h) 20 h. 4

5 Figure S3. TEM images of Rh nanocrystals that were obtained under fast plus slow injection mode after different amounts of the precursor had been injected in the nucleation step: (a) 0.9, (b) 0.5, (c) 0.3, and (d) 0.1 ml. The total volume of the precursor added was 6 ml for all syntheses. A less amount of the precursor used in the nucleation step resulted in the formation of fewer seeds and thus larger nanocrystals in the final product. The slow and fast here correspond to injection rates of 4 ml/h and 60 ml/h, respectively. The scale bars in the insets are 5 nm. 5

6 Figure S4. SEM and TEM images of the concave nanocubes of Rh shown in Figure 3: (a) A typical SEM image of the concave nanocubes, and (b-d) TEM images of an individual concave nanocube recorded from the [100], [110], and [111] zone axes, respectively. 6

7 Figure S5. TEM images of Rh nanocrystals that were obtained under slow plus slow injection mode at different molar ratios of KBr to Na 3 RhCl 6 : (a) 30:1, (b) 15:1, (c) 1:1, (d) 1:5, (e) 1: 20, and (f) 1:50, while all other parameters were kept the same as the standard synthesis. The slow here corresponds to an injection rate of 4 ml/h. 7

8 Figure S6. Schematic illustrations showing the morphological changes for Rh nanocrystals formed under four different injection modes. The formation of nanocrystals can be separated into two steps: nucleation and growth. The four injection modes can be classified into two categories according to the injection rate used for the growth step because this parameter plays the most critical role in determining the growth habit: (a) slow injection in the growth step, and (b) fast injection in the growth step. 8

9 Figure S7. Schematic illustration showing how the Rh atoms are added to the surface of a cubic seed of Rh with truncated corners and edges, in the presence of Br - as a capping agent for the {100} facets. The growth can be divided into three major processes including arrival, growth, and surface diffusion. 9