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Fine-Tuning Porosity and Time-Resolved Observation of Nucleation and Growth of Single Platinum Nanoparticles Matthew W. Glasscott, and Jeffrey E. Dick ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00546 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019
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ACS Nano
Fine-Tuning Porosity and Time-Resolved Observation of Nucleation and Growth of Single Platinum Nanoparticles Matthew W. Glasscott and Jeffrey E. Dick* Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599 *To whom correspondence should be address: e-mail:
[email protected] Telephone: +1919-966-5229
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Abstract: Porous metal nanoparticles (NPs) are extremely important to a variety of applications; however, robust control over NP porosity is difficult to achieve. Here, we demonstrate control over NP porosity using nanodroplet-mediated electrodeposition by introducing glycerol into water droplets. Porosity approached zero under viscous conditions (>6 cP), and intermediate viscosities allowed the fine-tuning of NP porosity between 0 and 15%. This method also allowed for control over average pore radius (1 to 5 nm) and pore density (2 to 6 x 1015 pores/m2). Reduced mass transfer within water droplets was validated by studying single chloroplatinate-filled water droplet (droplet radius ~ 450 nm) collisions on a platinum ultramicroelectrode (UME, rUME = 5 m). Collision transient lifetimes in the i-t response increased with increasing viscosity, and the total charge per event was conserved. The change in shape was consistent with the nucleation and growth of a platinum NP within the droplet, which was confirmed by fitting transients to classical nucleation and growth theory for single centers as a function of overpotential. This analysis allowed electrokinetic growth and diffusion-controlled growth to be distinguished and semi-quantified at the single NP level. Keywords: electrocrystallization, electrodeposition, nanodroplet, nanoparticle, nucleation and growth, porosity
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ACS Nano
The economical production of fuel cell technologies necessitates maximizing the surface area of the catalyst of interest,
1,2
which can be achieved by using porous nanomaterials. Unfortunately,
only a few examples for the preparation of porous metal NPs exist in the literature,3-6 and the average pore size and density has proven difficult to control. We recently reported the electrodeposition of Pt NPs from water nanodroplets.7 In this experiment, nanodroplets are filled with a certain concentration of hexachloroplatinic acid and suspended in a 0.1 M tetrabutylammonium perchlorate (TBAP) and 1,2–dichloroethane (DCE) continuous phase by ultrasonication. When water droplets are incident on an electrode biased sufficiently negative to drive the reduction of chloroplatinate, porous Pt NPs form. We were able to quantify porosity and nanopore tortuosity using Focused Ion Beam nanoSlice Tomography (FIB-nST), an advanced charecterization technique developed by our laboratory.8 Breugelmans and co-workers reported the electrodeposition of porous Pt NPs using a double-pulse technique.5,6 In that paper, the authors operated under very high mass transfer conditions with a rotating disc electrode. These results led us to hypothesize that Pt NPs electrodeposited from water droplets are porous because of the increased mass transfer in atto- to zepto-liter volumes. This arises due to the dependence of mass transfer coefficient, mO, on the radius of the electrode, a, which has been estimated at
