Site-selective sulfurization of bromide-capped palladium nanocubes by polysulfide and the underlying mechanism

This article describes site-selective sulfurization of Pd nanocubes capped by a monolayer of chemisorbed Br ions. High-resolution transmission electron microscopy and high-angle annular dark-field scanning TEM observations showed that PdS was not formed until a certain quantity of polysulfide (S-x(2-)) ions had been introduced (300 mu l, or 18.8 ppm in the final reaction solution). Spot energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy analyses confirmed that the surface-chemisorbed Br ions were completely substituted by S-x(2-) ions before the initiation of the sulfurization reaction. In the presence of sufficient S-x(2-) ions (> 300 mu l or > 18.8 ppm), PdS phase was selectively developed from the highly active corners, which then moved to the edges and finally towards the center until the entire nanocube was converted into PdS. The resultant PdS was found to be amorphous by electron microscopy and powder x-ray diffraction measurements. The amorphous structure of PdS facilitated the penetration and diffusion of S-x(2-) species and thus acceleration of the reaction kinetics. As a result, the sulfurization of 13 nm Pd nanocubes was completed within a few minutes after the addition of adequate Na2Sx, leading to a much more severe poisoning effect, compared with other noble metals such as Ag, by sulfur.