Controlled synthesis of hierarchical tetrapod Pd nanocrystals and their enhanced electrocatalytic properties

Abstract

Well-defined highly branched hierarchical tetrapod Pd nanocrystals with mutually embedded metameres were successfully synthesized with Pd(acac)₂ as a precursor and CO as a reducing agent in the presence of SDS. SDS played an essential role in controlling the hierarchical tetrapod morphologies of the final products. The formation of the hierarchical Pd tetrapods was ascribed to the SDS-confined growth from the four tips along the <111> directions. The as-prepared hierarchical Pd nanostructures demonstrated outstanding electrocatalytic activity.

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Excellent control of noble metallic nanocrystals with uniform and well-defined morphologies has attracted tremendous attention for decades owing to their unique chemical and physical properties as well as their novel nanostructures. Since their properties are highly dependent upon their sizes, shapes and surface structures, a lot of synthetic strategies have been developed to tailor their morphologies. For the case of palladium, various nanostructures such as nanowires,¹ nanorods,² nanobars,^2b nanocubes,³ nanoplates,⁴ nanosheets,⁵ tetrahedra,⁶ octahedra,⁷ icosahedra,⁸ tetrahexahedra⁹ and nanocorolla¹⁰ as well as concave tetrahedra,¹¹ concave cubes,¹² hollow cubes,¹³ nanocages¹⁴ and multipod nanostructures¹⁵ have been generated. Among them, branched or multipod metallic nanocrystals with protruding arms have received particular interest due to their unique morphologies, more edges, tunable optical responses and enhanced catalytic performance on account of their large surface areas, multiple high-angle edges, and sharp corners. Tilley et al. first reported branched Pd nanocrystals with less than 40% tripods in a pressure reaction vessel.^15a Huang developed a simple method to create Pd tripods in aqueous solution in the presence of copper ions at ordinary pressure and demonstrated high catalytic activities for Sonogashira coupling reaction.^15f Recently, tetrapod Pd nanocrystals were prepared by CO or CO/H₂ reducing strategies and displayed high electrocatalytic activities.^15g,h Furthermore, all the obtained branched or multipod Pd nanocrystals were consisted of triangular or tetrahedral joints with simple rod-based or rhombohedral arms. To date, however, highly branched multipod Pd with orderly hierarchical nanostructures has not been synthesized except for hierarchical Rh nanostructures.¹⁶ So, the synthesis of Pd nanocrystals with unprecedented nanostructures such as highly branched, especially hierarchical multipod remains a significant challenge.

Herein, we demonstrate a facile synthesis of highly branched tetrapod-shaped Pd nanocrystals with arthrogenous arms, a novel class of hierarchical nanostructures. The co-adsorption of sodium dodecyl sulfate (SDS) and CO was crucial to the formation of the hierarchical tetrapod Pd nanocrystals. The as-prepared hierarchical Pd tetrapods exhibited highly enhanced electrocatalytic activity, which were 3-fold much stronger than those of commercial Pd black (ESI†).

Fig. 1 shows the representative TEM images of the as-prepared Pd nanocrystals. As can be seen, the resulting particles were present in highly branched tetrapod or tripod morphologies under TEM (Fig. 1a and b), displaying a special hierarchical nanostructure feature. It was noted that the hierarchical tetrapod-shaped nanostructures were the dominant morphological feature though accompanying with a few real tripods due to one arm cracking. Because of the orientation to lie flat onto the substrate using three of their four apexes, some particles were observed as tripod-like projections (Fig. 1b). Interestingly, each one of the tetrapod Pd nanoparticles was consisted of four arthrogenous arms which were jointed together by tetrahedral stretch, and each arm with an average length of approximately 98 nm as determined from the center to the tip was constructed with four metameres embedded mutually one after another. Fig. 1c shows the image of a partial arm which was clearly present with mutually embedded feature. Each one metamere was derived from rhombohedral segment and the proposed geometric model for one hierarchical tetrapod with vertical projection from one tip was shown as Fig. 1d. The HRTEM image (Fig. 1e and S1, ESI†) and the corresponding FFT pattern (Fig. 1f) showed lattice fringes with an interplanar spacing of 0.23 nm, which was indexed to (111) planes of face-centered cubic (fcc) Pd.

Fig. 1 (a) Typical TEM image and (b) enlarged TEM of the hierarchical tetrapod Pd nanocrystals prepared with a CO flow rate of 0.2 mL s⁻¹ at 100 °C for 3 h. (c) TEM image of a partial arm. (d) The geometric model of a hierarchical tetrapod. (e) High magnification TEM image of the top part of a stem. (f) High-resolution TEM image of the squared area indicated in (e). The bottom-right inset in (f) shows the corresponding FFT pattern.

The results implied that the formation of hierarchical arms may be ascribed to the growth from the four tips along the <111> axis directions. The exposed {111} surfaces in the as-prepared hierarchical tetrapod Pd nanocrystals were further verified by electrochemical CO stripping experiments. Only one CO electro-oxidation (CO_ox) peak at 0.928 V (versus SCE) was observed for the freshly-prepared hierarchical Pd tetrapods in 0.1 M H₂SO₄ solution without introducing any additional CO, which can be assigned to the CO stripping on Pd {111} facets (Fig. S2a, ESI†).¹⁷ Then this CO_ox peak disappeared in the following second scanning (Fig. S2b, ESI†). Moreover, one peak for CO electro-oxidation appeared again at 0.934 V (Fig. S2c, ESI†) for the CO-stripped Pd-modified electrode after CO dosing, which was still assigned to CO stripping on {111} facets. These results showed that CO was adsorbed only on {111} facets either for the freshly-prepared or those cleaned Pd nanocrystals.

XRD pattern of the as-prepared hierarchical Pd tetrapods shows five characteristic peaks at 40.55°, 46.85°, 68.46°, 82.43° and 86.94°, corresponding to the (111), (200), (220), (311) and (222) lattice planes, which were consistent with the standard diffraction pattern for face-centered cubic (fcc) Pd (JCPDS card no. 00-001-1201) (Fig. S3, ESI†). XPS measurement demonstrated the binding energy of Pd3d_5/2 and Pd3d_3/2 at 334.58 and 339.90 eV (Fig. S4, ESI†), respectively, referring to C1s = 284.6 eV, and the interval was 5.32 eV, which were coincident with the reference value (335.10 and 340.36 eV),¹⁸ indicating Pd⁰ with the as-prepared hierarchical Pd tetrapods.

The effect of SDS on the growth of the hierarchical tetrapod Pd nanocrystals was investigated. As shown in Fig. 2a, when the amount of SDS was halved, each hierarchical arm in an individual Pd particle was consisted of only two metameres, and the average length of a single branch arm was significantly reduced. While a double amount of SDS was used, simple small Pd tetrapods mixed with a few concave tetrahedral particles were generated (Fig. 2b). In addition, similar hierarchical Pd tetrapods constructed with two-metamere branches were obtained by using sodium dodecyl benzene sulfonate (SDBS) instead of SDS (Fig. 2c), while concave tetrahedral Pd nanocrystals were preferred with using Na₂SO₄ (Fig. 2d). Moreover, the absence of SDS led to the formation of irregular Pd nanosheets (Fig. S5, ESI†). These results suggested that the formation of hierarchical Pd tetrapods was dependent on the adsorption and confinement effect of SDS.

Fig. 2 TEM images of the Pd nanocrystals prepared with different amount of SDS or other sodium salts. (a) 73 mg of SDS; (b) 294 mg of SDS; (c) SDBS; (d) Na₂SO₄.

Furthermore, the hierarchical degree for each arm of the as-prepared Pd nanocrystals was dependent upon the reaction temperature. Under the same other conditions, Pd tetrapods with short two-metamere arms were generated at a lower temperature (80 °C) (Fig. 3a). Nevertheless, inhomogeneous hierarchical tetrapods accompanying with small concave tetrahedral Pd nanoparticles were obtained at a higher temperature (120 °C) (Fig. 3b). These observations revealed that the hierarchical morphological feature was related to concave tetrahedral nanostructure and the growth of hierarchical tetrapods might be accomplished by kinetic control.

Fig. 3 TEM images of Pd nanocrystals prepared at different CO flow rates and temperatures. (a) 0.2 mL s⁻¹, 80 °C; (b) 0.2 mL s⁻¹, 120 °C; (c) 0.1 mL s⁻¹, 100 °C; (d) 0.3 mL s⁻¹, 100 °C.

In addition, the flow rate of CO played an important role in the formation of uniform hierarchical Pd tetrapods. With a lower CO flow rate of 0.1 mL s⁻¹, the average arm length of the as-prepared hierarchical Pd tetrapods with more metameres decreased to about 68 nm (Fig. 3c). When CO was bubbled with a flow rate of 0.3 mL s⁻¹, inhomogeneous tetrapod-branched Pd nanoparticles with an average branch length of 60 nm were generated (Fig. 3d). This suggested that CO flow rate in a certain range determined the dimension and uniformity of the as-prepared Pd tetrapods, but it had little effect on their morphologies.

The morphological evolution of the as-prepared Pd nanoparticles with the reaction time was also investigated. It was found that Pd tetrapods with four-metamere arms were produced with an average arm length of about 56 nm in 15 min reaction (Fig. S6a, ESI†). These nanocrystals continued to grow in 180 min. The average arm length increased to 72 nm at 30 min, 85 nm at 60 min, 105 nm at 300 min (Fig. S6b–d, ESI†). Obviously, the hierarchical tetrapod feature readily appeared with jigsam-shaped arthrogenous arms at 15 min though they looked distinctly scrawny. With increasing the reaction time, the hierarchical arms became plump accompanying with the arm's extending. No significant increase in the average arm length was observed beyond 180 min, indicating that the optimum reaction time was 180 min. These results implied that the hierarchical feature of the as-prepared Pd tetrapods was independent on etching and the hierarchical arms were developed at the very beginning.

The above experimental evidences confirmed that SDS was the most crucial to the formation of highly branched hierarchical tetrapod Pd nanostructures under the condition of appropriate CO flow rate and temperature. According to the previous report in which Pd tetrapods with rhombohedral arms were generated by CO reduction in the presence of carboxylate,^15g we suggested that the formation of hierarchical tetrapods derived from the simple tetrapod seeds with primary monomeric rhombohedral pod structure, which were related to concave tetrahedral structure. In the initial stage of the reaction, Pd seeds with tetrapod arms were formed. As the reaction proceeded, the selective adsorption of SDS on the surfaces of the rhombohedral arms resulted in the outgrowth of arms from all the four tips of a tetrapod core. In the presence of appropriate amount of SDS, the four tips of the primary tetrapod were exposed and served as newly growing sites on which the skeletal nanocrystal grew again along <111> directions. As a result, the hierarchy for each arm in a tetrapod Pd nanostructure was generated with mutually embedded metameres. However, if insufficient SDS was used, the adsorption of SDS on the nucleus crystallite decreased, the continuous growth from the tips along <111> directions was relaxed, resulting in the formation of two-metamere arms (Fig. 2a). While an excess amount of SDS was added, most of the tips as growing sites were covered by SDS and thus hierarchical growth along <111> tips were suppressed, resulting in a great number of primary concave tetrahedral nanoparticles (Fig. 2b). Moreover, due to the size-dependent steric repulsion effects of benzene rings when SDBS was used instead of SDS, the adsorption of SDBS on the surfaces of the rhombohedral metameres decreased and hierarchical growth was blocked, so that the hierarchical tetrapod nanocrystals with two-metamere arms were dominant (Fig. 2c). While Na₂SO₄ was used, CO-confined growth was dominant due to the much weak adsorption of sulfate ions on Pd surfaces, leading to produce some concave tetrahedral Pd nanoparticles as well as irregular polyhedral particles (Fig. 2d).

Compared with normal polyhedral or simple branched nanocrystals, the hierarchical Pd tetrapods obtained here demonstrated a larger surface area to volume ratio as well as higher surface energy. The continuous growth preferentially at the newly formed tips along <111> directions, which facilitated hierarchical tetrapod nanostructure, required site-selective atomic deposition under kinetic control due to the highest surface energy at the tips.

Together with the effects of SDS, temperature, CO flow and reaction time, they implied that the selective adsorption of SDS molecules on the rhombohedral surfaces of the primary tetrapod-shaped Pd particles was favorable for site-selective atomic deposition, leading to SDS-confined growth of hierarchical tetrapod Pd nanostructures, while the growth confinement effect of CO was responsible for the generation of primary tetrapod-shaped Pd particles. Though it was favorable for the formation of tetrapod Pd nanostructures at lower temperature (80 °C) due to kinetic control, hierarchical growth from the tips was confined since a strong adsorption of SDS and CO slowed down Pd atomic addition on the growing sites, resulting in the formation of fewer metameres in branched arms (Fig. 3a). At a higher temperature (120 °C), SDS-confined effect was weakened due to thermodynamic control and CO-confined growth was dominant, leading to produce small concave tetrahedral Pd nanoparticles besides some inhomogeneous short-arm hierarchical tetrapods. So, an appropriate temperature was necessary for the enhanced hierarchical growth from the four vertices along <111> directions. Accordingly, the formation process of the hierarchical Pd tetrapods can be illustrated as the Scheme 1 (Fig. S7, ESI†). The morphology evolution developed from particles to concave tetrahedral, simple tetrapods, hierarchical tetrapods with two-metamere arms, and even highly branched hierarchical tetrapods with four-metamere arms with the reaction proceeding.

The catalysis activities of the as-prepared hierarchical tetrapod Pd nanocrystals were investigated by electrocatalytic oxidation of formic acid. Commercial Pd black was used as reference for comparison. Fig. 4 shows the cyclic voltammetry (CV) curves for the electro-oxidation of formic acid. The maximum current density was measured to be 12.52 mA cm⁻² on the hierarchical tetrapod Pd nanocrystals at 0.21 V (Fig. 4a), while it was 4.12 mA cm⁻² on the Pd black at 0.38 V (Fig. 4b). The activity of the hierarchical Pd tetrapods, though having a bigger mean size, was 3 times greater than that of Pd black. This may be ascribed to their more edges and corners, and even more terraces of the unique hierarchical Pd tetrapods. These results revealed that the as-prepared hierarchical tetrapod Pd nanocrystals with (111) surfaces demonstrated an outstanding electrocatalytic activity.

Fig. 4 CV curves for electro-oxidation of formic acid by the as-prepared hierarchical tetrapod Pd nanocrystals (a) and commercial Pd black (b). The formic acid oxidation was recorded in 0.5 M H₂SO₄ + 0.5 M HCOOH solution at a scan rate of 50 mV s⁻¹.

In summary, well-defined hierarchical tetrapod Pd nanocrystals with highly branched four-metamere arms were successfully synthesized with Pd(acac)₂ as a precursor, PVP as a stabilizer, CO as a reducing agent in the presence of SDS at 100 °C for 3 h under atmospheric pressure. The addition of SDS was crucial to the formation of the hierarchical tetrapod Pd nanostructures of the final products. The formation of the hierarchical Pd tetrapods was ascribed to the SDS-confined growth from the tips along <111> direction. The as-prepared new kind of branched hierarchical Pd nanostructures demonstrated highly enhanced electrocatalytic activity and would be a promising candidate for wide applications in catalysis and others.

Acknowledgements

This research was supported by the National Nature Science Foundation of China (Grant no. 21273289).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and characterization of hierarchical tetrapod Pd nanocrystals, experimental details, TEM images, CO stripping voltammetry, XRD, XPS and the Scheme for the formation process. See DOI: 10.1039/c3ra47487a

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