Electrostatic Adsorption of Platinum onto Carbon Nanotubes and Nanofibers for Nanoparticle Synthesis

Abstract

Strong Electrostatic Adsorption (SEA) has been demonstrated as a simple, scientific method to prepare well dispersed Pt nanoparticles over typical forms of carbon: activated, black, and graphitic carbons. Many varieties of specialty carbons have been invented in the last few decades including multi-walled nanotubes, nanofibers, graphene nanoplatelets, etc. In this work, we explore whether SEA can be applied to these specialty carbons for the synthesis of Pt nanoparticles. Over a number of oxidized and unoxidized multiwalled nanotubes and nanofibers, the point of zero charge (PZC) was measured and the uptake of anionic Pt complexes (Pt hexachloride, [PtCl 6 ] 2− , and cationic Pt complexes (platinum tetraammine, [Pt(NH 3 ) 4 ] 2+ ) as functions of final pH were surveyed. Pt nanoparticles on the various supports were synthesized at the optimal pH and were characterized by x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The specialty carbons displayed volcano-shaped uptake curves typical of electrostatic adsorption for both Pt anions at low pH and Pt cations at high pH. However, the regimes of uptake often did not correspond to the measured PZC, probably due to surface impurities from the carbon manufacturing process. This renders the measured PZC of these specialty carbons unreliable for predicting anion and cation uptake. On the other hand, the anion and cation uptake curves provide an "effective" PZC and do indicate the optimal pH for the synthesis of ultrasmall nanoparticle synthesis. High resolution STEM imaging also showed that with SEA it is possible to disperse nanoparticles on the surface as well as the inner walls of the specialty carbons.

Stabilization of Ultrasmall Platinum Nanoparticles by Nitrogen-Doped Carbon: Implications for Catalysis and Electrocatalysis

“…22,55,56 The same leftward shift in broad XRD Pt peaks of air-exposed Pt/NC can actually be seen in two previous reports 47,50 (Figure 2 of each reference) but was not recognized as oxidation. Figure 2c accentuates the difference With careful background subtraction and fitting as developed in previous studies 23,55,57 and described in detail in the Supporting Information Figures S2 and S3, the peaks can be fitted as shown in Figure 2d. The fully reduced sample is fitted with Pt (111) and (200) fcc peaks with FWHMs, per the Scherrer equation, corresponding to a size of 0.98 nm.…”

“…With careful background subtraction and fitting as developed in previous studies ,, and described in detail in the Supporting Information Figures S2 and S3, the peaks can be fitted as shown in Figure d. The fully reduced sample is fitted with Pt (111) and (200) fcc peaks with FWHMs, per the Scherrer equation, corresponding to a size of 0.98 nm.…”

“…To enable future applications, it will often be critical to maximize the number of active catalytic sites per mass of Pt by minimizing the size of the nanoparticles . However, as the nanoparticles become ultrasmall (<2 nm), they are prone to oxidization under ambient conditions as demonstrated in recent XRD studies using synchrotron radiation and benchtop powder XRD with a high-sensitivity silicon strip detector. − While for some reactions this is beneficial, − for fuel cell reactions this can lead to a loss in reactivity, − and can eventually lead to sintering of the Pt phase into larger particles and a corresponding loss of catalytically active sites. − In either case, the effective application of these materials will be greatly advanced by the ability to control the oxidation and sintering of the nanoparticles by modifications of the carbon support.…”