Metal nanocrystals supported on metal oxides often exhibit improved catalytic activity and selectivity as compared to unsupported ones,[1–4] which is known to arise from several factors, including the shape and size of metal nanocrystals, the metal oxidation state, and the support effect.[5–11] Early methods for preparing metal nanocrystal on metal oxide hybrid nanostructures mainly involved calcination of metal oxide powders impregnated with metal precursors at high temperatures (> 400 C).[12–17] However, the products from this approach often suffer from low content (typically less than 10 wt%) and poor dispersion of metal nanocrystals.[18–21] It remains a challenge to develop a simple and reliable route to the synthesis of nanostructures consisting of highly dispersed noble-metal nanocrystals supported on nanocrystalline oxides with high surface areas. Among various possible hybrid nanostructures, Pt nanocrystals supported on cerium oxides (CeO 2) are of particular interest owing to their wide applications in catalysis including watergas shift reaction,[22, 23] CO oxidation,[24] and selective hydrogenation,[25, 26] as well as gas sensing.[27] Recently, Yan and co-workers reported the synthesis of Pt/CeO 2 nanostructures consisting of 1–2 nm Pt nanocrystals (ca. 25–30 wt%) supported on sub-10 nm CeO 2 nanocrystals by introducing SiO 2 shells on Pt/CeO 2 particles to protect their aggregation during the calcination, but the procedure required multiple, tedious steps including the formation and removal of SiO 2 shells before and after calcination, respectively.[28] Here we report a simple, aqueous-phase route to the synthesis of Pt/CeO 2 hybrid nanostructures consisting of sub-3 nm Pt nanocrystals supported on octahedral CeO 2 nanocrystals with edge length of about 10 nm. In this approach, Pt/CeO 2 nanostructures were generated by in situ reduction of negatively charged PtCl 4 2− precursors adsorbed on the positively charged surface of 6-aminohexanoic acid (AHA)-stabilized CeO 2 nanocrystals through electrostatic attraction. Using this approach, we were able to routinely produce Pt/CeO 2 hybrid nanostructures with high content and good dispersion of Pt nanocrystals in high yields. The overall procedure is simple and readily scalable because it does not require hightemperature calcination or any other complicated steps. We also investigated the catalytic property of these Pt/CeO 2 hybrid nanostructures by employing the reduction of p-nitrophenol into p-aminophenol by NaBH 4 as a model reaction.

In this study, CeO 2 nanocrystals were synthesized by reacting cerium (III) nitrate with AHA in an aqueous solution, as reported previously.[29] Figure 1a shows a transmission electron microscopy (TEM) image of the as-synthesized CeO 2 nanocrystals. Since the contrast of a nanocrystal depends on its orientation relative to the electron beam, the randomly oriented CeO 2 nanocrystals on a TEM grid could show different contrasts in the TEM image.[30] They were uniform in both size and shape, and exhibited an octahedral morphology with edge length of roughly 10 nm. The powder X-ray diffraction (XRD) pattern taken from the CeO 2 octahedra confirmed their cubic fluorite structure (Figure S1a, Fm3 m, a= 5.411 Å, JCPDS Card No. 34–0394). In the FT-IR spectrum (Figure S1b), a distinct peak at 1720 cm− 1 can be assigned to the carboxylate ion (-COO−) rather than carboxylic acid (-COOH), which would have a broad peak between 2700 cm− 1 and 3500 cm− 1. Since the AHA-stabilized CeO 2 octahedra were positively charged (the zeta potential value was+ 48.4 mV at pH≈ 5.5, Table 1), it is not unreasonable to assume that the carboxylic acid …