Platinum Layer Growth

Figure 1 shows AES data for the oxidized titanium surface before and after deposition of 30 X of platinum with the substrate held at 130 K. The platinum thickness was calculated from the attenuation of the oxygen AES signal assuming layered growth of the metal. From the spectra It Is clear that the platinum was sufficient to completely attenuate the underlaying features of the titanium oxide. 

After annealing at 325°C for 5 min, the Pt2Si layer, about 98 nm thick, was formed in all four initial specimens. During its growth for 10 min, the platinum layer, 115 nm thick, was consumed. Therefore, the first specimen transformed into the Pt2Si—Si(l 11) one. The layer of the PtSi compound... 

U. Bardi, A. Santucci, and G. Rovida. Study of the Growth Mechanism of Platinum Layers on the Na<) 7WO3 Single Crystal Surface. Surf. Sci. 162 337 (1985). 

As shown in Figs. 3(b) and (c), the deposition of a platinum layer of monoatomic height was initiated as soon as the potential was stepped to +0.70 V. The growth of the first layer seemed to be essentially in a 2D mode. The deposited platinum layer grew... 

Borup et al. [20] observed with aging tests at different relative gas humidities at 80°C an accelerated platinum particle growth. A decrease in hydrophobidty of the gas diffusion layer and a change in the water uptake of the membrane were also... 

The process of oxide-layer growth on platinum has been thoroughly investigated for smooth platinum surfaces in aqueous electrolytes and in the gas phase (Angerstein-Kozlowska et al. 1973 Conway et al. 1990 Conway and Jerkiewicz 1992 Harrington 1997). While Conway et al. (1990) proposed rapid diffusion of oxide species followed by a slow oxide turnover process, Harrington (1997) opted for slow formation of the oxide species foUowed by rapid diffusion of oxide species across the surface. The kinetics of surface oxide formation on fuel-cell-type platinum catalysts has also been studied. Paik et al. (2004) observed that surface oxide formation on a platinum electrode occurs rapidly under realistic operating conditions of... 

From these observations it can be concluded that the changes in catalyst properties in water under reductive circumstances especially at high pH are the result of two phenomena 1) Growth of the platinum particles by a process in which the crystallites become mobile 2) Coverage of the platinum surface. The latter can be caused either by coverage with carbonaceous products originating from the support or by disappearance of the mobile platinum particles in between the graphite layers. 

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