Pt nanoparticles on TiO₂ with novel metal–semiconductor interface as highly efficient photocatalyst

Abstract

Pt nanoparticles were supported on the TiO₂ surface using a glow discharge modified impregnation method. The metal–semiconductor interface structure was observed by high-resolution TEM. The fringes of Pt[111] were nearly perpendicular to those of TiO₂[101] at the interface. Such alignment relationship resulted in a largely distorted interface allowing a close contact and enhanced interaction between the metal and the semiconductor. When used as photocatalyst, this Pt–TiO₂ interface could improve the photo-induced electron transfer from the semiconductor to the metal and promote the near-UV absorption. The photocatalyst with such interface exhibited higher activity for hydrogen generation from alcohol/water mixture than that with a less distorted interface.

Introduction

Heterogeneous metal-supported catalysts are extensively used to catalyze numerous reactions. In some cases the support itself is not active and acts as a channel of faster supply of adsorbed reactants to active metal. However, in most cases the support plays a direct role in catalytic reactions, which will not take place without the cooperation of the support and the metal. In this case the activity of catalyst is greatly dependent on the structure and property of metal–support interface because energy and mass is transferred and exchanged via this interface. Specifically, some reactions take place over active sites located at the metal–support interface [1].

Photocatalytic reactions have attracted increasing interests due to its promising potential in the utilization of solar energy for pollutants degradation and hydrogen generation [1], [2], [3], [4], [5], [6], [7], [8], [9]. Titanium dioxide is widely used as a photocatalyst because it is relatively highly efficient, cheap, non-toxic, chemically and biologically inert, and photostable. Noble metals are generally applied to modify TiO₂, among them Pt is the most active [6], [7], [8], [9]. The photocatalytic reaction over Pt/TiO₂ requires the cooperation of Pt and TiO₂. Firstly, the TiO₂ semiconductor is excited by band-gap irradiation to generate electron–hole pairs. Secondly, the photo-generated electrons in the conduction band of TiO₂ migrate to the Pt particles through the Pt–TiO₂ interfaces. Finally, the electrons are trapped by the Pt particles for reduction reaction and the holes remain in the valence band for oxidation reaction [1], [2], [3], [4], [5], [6], [7], [8], [9]. The bottleneck of the process is the low charge separation and transfer efficiency with intense electron–hole recombination. Therefore the second step is critical to determine the activity of Pt/TiO₂ and the metal–semiconductor interface is very important because it affects the charge separation and transfer. With this consideration, we believe that better metal–support interface would improve the photocatalytic performance of Pt/TiO₂. There are many investigations to improve the property of TiO₂ semiconductor for higher photocatalytic activity [2], [3], [4], [5], [6]. To our knowledge, however, there are few reports to promote the Pt–TiO₂ interface. The objective of the present work was to investigate the effect of catalyst preparation methods on the metal–semiconductor interface for the improvement in the photocatalytic activity of Pt/TiO₂. Photocatalytic hydrogen generation from ethanol solution was chosen to evaluate the photocatalytic activity because this representative process combined hydrogen generation and pollutants degradation. It was found that the Pt/TiO₂ with novel Pt–TiO₂ interface exhibits enhanced photocatalytic activity.