Deterministic Synthesis of Pd Nanocrystals Enclosed by High-Index Facets and Their Enhanced Activity toward Formic Acid Oxidation

Abstract

Noble-metal nanocrystals enclosed by high-index facets are of growing interest due to their enhanced catalytic performance in a variety of reactions. Herein, we report the deterministic synthesis of Pd nanocrystals encased by high-index facets by controlling the rate of deposition (V_deposition) relative to that of surface diffusion (V_diffusion). For octahedral seeds with truncated corners, a reduction rate (and thus deposition rate) faster than that of surface diffusion (i.e., V_deposition/V_diffusion > 1) led to the formation of concave trisoctahedra (TOH) with high-index facets. When the reduction was slowed down, in contrast, surface diffusion dominated the growth pathway. In the case of V_deposition/V_diffusion ≈ 1, truncated octahedra with enlarged sizes were produced. When the reduction rate was between these two extremes, we obtained concave tetrahexahedra (THH) without or with truncation. Similar growth patterns were also observed for the cuboctahedral seeds. When the Pd octahedra, concave TOH, and concave THH were tested for electrocatalyzing the formic acid oxidation (FAO) reaction, those with high-index facets were advantageous over the conventional Pd octahedra enclosed by {111} facets. This work not only contributes to the understanding of surface diffusion and its role in nanocrystal growth but also offers a general protocol for the synthesis of nanocrystals enclosed by high-index facets.

1

Effect of surface diffusion on the overgrowth of Pd seed. Schematics showing: (a) the pathways for adding Pd atoms onto the surface of a truncated octahedral Pd seed whose slightly truncated corners are capped with Br^–, and (b) morphological evolutions of the seed under four different experimental conditions defined by the rate of atom deposition relative to surface diffusion (V _deposition /V _diffusion ).

2

TEM (left panel) and SEM (right panel) images of the Pd nanocrystals obtained by adding different amounts of AA into the reaction solution: (a,e) concave TOH, in the case of 60 mg of AA; (b,f) concave THH in the case of 40 mg of AA; (c,g) concave THH with truncated {111} facets in the case of 30 mg of AA; and (d,h) truncated octahedra with slightly concave {100} facets in the case of 20 mg of AA. The scale bars in (d) and (h) apply to all other images in the same column, while the scale bar in the inset of (e) is 20 nm and applies to all other insets. The insets in (a–d) are models of the corresponding nanocrystals.

3

TEM images of the nanocrystals obtained at different reaction temperatures: (a) concave TOH with a rough surface when conducted at 20 °C; (b) concave TOH when conducted at 40 °C, with the surface being smoother than that in (a); (c) concave THH with smooth surface when conducted at 70 °C; and (d) truncated concave THH with smooth surface when conducted at 80 °C. The insets in (b–d) show models of the corresponding nanocrystals.

4

TEM images of products obtained using a standard procedure except for the use of different types of Pd seeds: (a,b) 13 and 21 nm Pd octahedra as the seeds and (c,d) truncated octahedra and concave THH obtained as the products when the 13 and 21 nm Pd octahedra were used as seeds, respectively. The scale bar in the inset of (a) is 10 nm and applies to all other insets.

5

TEM images of the products obtained using 16 nm cuboctahedra as the seeds and by adding different amounts of AA into the reaction solution: (a) the cuboctahedral seeds; (b) concave nanocubes when 60 mg of AA was added; (c) concave nanocubes with truncated corners when 40 mg of AA was added; and (d) truncated octahedra with slightly concave {100} facets when 20 mg of AA was added. The insets show SEM images of the corresponding nanocrystals. The scale bars in all of the insets are 20 nm.

6

Cyclic voltammograms taken from Pd catalysts based on octahedra (black), concave TOH (blue), and concave THH (red), respectively. (a) Cu_UPD curves were obtained in a N₂-saturated 0.05 M H₂SO₄ + 0.05 M CuSO₄ solution at a scan rate of 5 mV s^–1. The currents were normalized to the geometric area of the glassy carbon electrode (0.07065 cm²). (b) The polarization curves for FAO obtained in a N₂-saturated 0.1 M HClO₄ + 0.5 M HCOOH solution at a scan rate of 50 mV s^–1. The currents were normalized to those of their corresponding ECSAs.