Electrochemical Decoration of Core Shell Metal Nano Particles over TiO2 Nanotubes for Oxygen Reduction Reaction
Authors: Santhosh Bukka , Umehara Yuhei , Higashimine Koichi , Vedarajan Raman , Matsumi Noriyoshi 
Published: February 2020

The key for fabrication of fuel cell and other energy storage devices lies in developing a highly efficient nano material catalyst for oxygen reduction reaction [1]. More importantly achieving a nano material catalyst without any sacrificial agent is a viable option because sacrificial agents themselves are harming environment. The electrochemical synthesis of metal nano particles replacing the traditional chemical methods would be a viable option because of its simplicity in operation and ease of control of the deposition conditions. Pioneering work of Fujishima and Honda in the year 1972 [2] triggered the usage of TiO2 semiconductor material as photocatalyst. TiO2 has edge over other semiconductors because it is cheap, nontoxic and environmental friendly [3] however, it lacks the efficiency because of its absorption in UV region, large band gap and charge-hole recombination. To enhance the absorption of TiO2, one of the methodologies is, decoration of plasmonic nano particles over TiO2 due to which absorption in visible light increased. At the same time platinum nanoparticles are the most used cathode electrode materials to catalyze the sluggish oxygen reduction reaction [4]. However, decoration of platinum over TiO2 induces a huge barrier for electron transfer at the interface of Pt/TiO2 [5]. Many research groups proposed a cocatalytic system with Au and Pt which can not only lower this barrier but also induces localised surface plasmon resonance. However, these materials follow traditional methods to design metal nano particles on TiO2. Further, synergistic effects of core shell platinum and gold nanoparticles over TiO2 has been less studied and elucidated. The present research work aims at achieving uniform decoration of bimetallic core shell nano particles over TiO2 by electrochemical reduction within a short span of 30 seconds. The prepared active material is a core shell nano structure of Pt-Au decorated over TNT which is evaluated for its catalytic activity and to understand the effects of bimetals over TiO2.
Titania nano tubes (TNT) were synthesized similar to a previously reported literature [6]. Then uniform distribution of metal nano particles was achieved by electrochemical deposition. In short, 0.01 wt% of aqueous metal precursors (H2PtCl6 and HAuCl4) were deposited on TNT by applying -2.0 V for 30 sec using computer controlled potentiostat in 0.5M NaCl aq. electrolyte with Ag/AgCl as reference electrode and platinum as counter electrode. Different core shell nano structures have been prepared such as AuPt@Pt-TNT (Pt as core and AuPt as shell) and Pt@Au-TNT (Au as core and Pt as shell) and characterized by TEM and elemental mapping which shows uniform distribution of core shell nano structures over entire 15 µm TiO2nano tube.

Electrochemical measurments: TNT decorated with core shell nano particles were studied for oxygen reduction reaction in oxygen saturated 0.5 M HClO4 aq. CV measurements were carried out using Pt wire as the counter electrode, Ag/AgCl as reference electrode and titanium chip decorated with metal nano particles as working electrode at 20 mV/s. The voltammogram of core shell nano particles showed very good oxygen reduction reaction at 0.45 V. This novel research revealed important insights about decoration of core shell nano metals over TiO2by electroreduction method and have important implications in design of highly active catalysts for oxygen reduction reaction.

References:

(1) Dai, L.; Xue, Y.; Qu, L.; Choi, H. J.; Baek, J.-B. Chem. Rev.
2015, 115, 4823–4892.
(2) Fujishima, A.; Honda, K. Nature,
1972, 238, 37–38.
(3) Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C. Chem. Rev.
2014, 114, 9987–10043.
(4) Oezaslan, M.; Hasché, F.; Strasser, P. J. Phys. Chem. Lett.
2013, 4, 3273–3291.
(5) Hung, S. F.; Yu, Y. C.; Suen, N.-T.; Tzeng, G. Q.; Tung, C. W.; Hsu, Y. Y.; Hsu, C. S.; Chang, C. K.; Chan, T. S.; Sheu, H. S. Chem. Commun.
2016, 52, 1567–1570.
(6) Vedarajan, R.; Ikeda, S.; Matsumi, N. Nanoscale Res. Lett.
2014, 9, 573-577.