The Effectiveness of Sputtered PtCo Catalysts on TiO[sub 2] Nanotube Arrays for the Oxygen Reduction Reaction

Pt and PtCo binary catalyst layers were prepared on self-ordered nanotube (TONT) arrays with the length of and diameter of employing a dual-gun sputtering method. The sputtered Pt induced a slight increase of the wall thickness of TONT. With increasing Co content in the alloy catalyst layer, a slight reduction of pore diameter on the TONT was also found. Well-dispersed mixed nanophases of Pt and Co were observed along the inner wall of TONT, and crystalline properties from the crystalline plane of Pt, Co, and were identified by selective area electron diffraction. Among several compositions of additional Co on Pt, Co catalyst on Pt showed the best oxygen reduction reaction (ORR) activity with the positive shift of onset potential. This is attributed to Pt electronic modification by an alloying effect and the shortening of the Pt–Pt interatomic distance. However, the coverage from the physically deposited sputtering method is incomplete; thus, the uniformity of the catalyst layer could be improved along the inner wall of TONT. Moreover, the influence of the pore size for diffusion of oxygen molecules on the ORR was investigated by increasing the sputtering time for . The sample with the pore diameter of (sputtering time of ) showed the maximum ORR activity (maximum upshift of half-wave potential), while above that, the ORR activity started to be degraded, resulting from restricted diffusion of oxygen molecules and the reduced surface area of catalyst layer. In order to examine the effect on large surface area of TONT and pore diameter, a compact film prepared on a Ti substrate was compared for the catalyst layer. This compact support revealed significantly degraded ORR activity.