Transition metals, such as Pt, are important as catalysts in fuel cells. The cost of Pt requires that this metal be utilized as efficiently as possible. Previous studies have demonstrated that flat, continuous Pt surfaces have an activity for the fuel cell oxygen reduction reaction (ORR) per surface Pt atom that is 5-10 times larger than the activity of 3 nm Pt nanoparticles on carbon-supported electrocatalysts. Consequently, very thin continuous Pt layers are needed to achieve high activity per mass of Pt and favorable economics.

 

Pt has a high surface energy and does not readily wet low surface energy oxide and carbon material surfaces. Instead, Pt nanoclusters are formed during the nucleation of Pt ALD on these low surface energy supports. A continuous Pt film is possible only after the coalescence of the nanoclusters when the film thickness is >5 nm. One possible route to obtain a continuous and ultrathin Pt film is to deposit on an adhesion layer that has a higher surface energy than Pt. In this case, the Pt will wet the adhesion layer because the deposited Pt film will lower the surface energy. One material that has a higher surface energy than nearly all other metals, including Pt, is tungsten (W).

 

W ALD using WF6 and Si2H6 is known to nucleate and grow rapidly on Al2O3 ALD surfaces [1]. The surface chemistry of W ALD is able to form chemical bonds to the underlying Al2O3 substrate that overcome the surface energy differences. In this study, Pt ALD is performed using MeCpPtMe3 and H2 plasma on W ALD adhesion layers grown on Al2O3 ALD films. X-ray photoelectron (XPS) and x-ray reflectivity (XRR) studies have demonstrated that Pt ALD nucleates nearly immediately on the W ALD adhesion layers. The XPS and XRR data are consistent with a layer-by-layer growth model. Pt ALD films can be deposited that are 1 nm thick, conformal and continuous. Initial electrochemical measurements on these thin films have yielded a large proportion of the anticipated ORR activity benefit.

 

[1] R.A. Wind, F.H. Fabreguette, Z.A. Sechrist and S.M. George, “Nucleation Period, Surface Roughness and Oscillations in Mass Gain per Cycle during W Atomic Layer Deposition on Al2O3”, J. Appl. Phys. 105, 074309 (2009).