Size Effect of Pt Nanoparticles Deposited By Plasma-Enhanced Atomic Layer Deposition on the Electrocatalytic Activity Toward the Methanol Oxidation Reaction

Platinum is the most important catalyst used for direct methanol fuel cells (DMFC) because its excellent electrocatalytic activity toward the methanol oxidation reaction (MOR).  Most of recent research on DMFC focuses on the size minimization of the Pt catalyst, which can reduce the use of the precious Pt catalyst and increase the electroactive surface area.  Moreover, nanosized Pt catalyst can significantly enhance the electrocatalytic activity toward MOR.  In this study, we used plasma-enhanced atomic layer deposition (PEALD) to deposit Pt nanoparticles of different sizes on the TiO₂substrate, and studied the electrocatalytic activity of Pt nanoparticles toward MOR as a function of the particle size.  

    Pt nanoparticles were deposited on the TiO₂ substrate by PEALD with a plasma power of 5W at 200^oC using (methylcyclopentadienyl) trimethyl platinum (MeCpPtMe₃) as the Pt precursor.  Due to the self-limiting growth kinetics of ALD reactions, the size of Pt nanoparticles can be controlled by the number of the PEALD cycle, which was varied from 8 to 50 cycles in this study.  The TiO₂ was the nature oxide layer on a 100 nm thick Ti thin film sputter-deposited on the Si(100) wafer. Transmission electron microscopy (TEM) reveals that Pt nanoparticles has a size in the range between 3 nm and 7 nm depending on the number of the PEALD cycle.  Cyclic voltammetry (CV) and CO stripping analysis were performed to study the electrocatalytic activity of the Pt/TiO₂ electrode toward MOR in acidic media.  The Pt/TiO₂ electrode prepared with 15 PEALD cycles has a Pt particle size distribution peaking at ~4 nm, and has the best electrochemical performance, including electrocatalytic activity and CO tolerance.  With the PEALD cycle number increased to 50, Pt nanoparticles trend to coalesce, and the electrode exhibits an electrochemical performance similar to a continuous Pt thin film.  The better electrocatalytic activity of Pt nanoparticles prepared with a smaller number of the PEALD cycle can be ascribed to the synergistic effect of the nanosized Pt catalyst and the electronic interaction between Pt nanoparticles and the TiO₂support.  A schematic model describing the role of Pt nanoparticle size in the synergistic effect will be presented at the meeting.  

Keywords: direct methanol fuel cells, DMFC, plasma-enhanced atomic layer deposition, PEALD, methanol oxidation reaction, Pt nanoparticles, TiO₂.