Synthesis And Photocatalytic Properties Of Noble Metal-Loaded TiO₂ Nanotube Arrays

Due to its unique structure-highly ordered, high specific surface area, good mechanical strength and close contact with the media, TiO2 nanotube arrays (TNTAs) can not only improve the efficiency of material to absorb sunlight, but also effectively promote the generation, separation and transfer of photoinduced carriers, and has been considered as a novel energy material with excellent photocatalytic properties. In recent years, TNTAs have shown potential application prospects in a wide range of fields in photocatalytic degradation of organic pollutants, dye-sensitized solar cells, antifouling and self-cleaning and so on, which could expectedly help solve the current serious energy and environmental problems. Thus, TNTAs have received extensive attention.In this work, TNTAs was firstly prepared by anodic oxidation, and the loading of Au/Pt nanoparticles or Au-Pt bimetalic nanoparticles on the surface of TNTAs was then obtained by using a photoreduction method, thus Au/TNTAs, Pt/TNTAs and Au-Pt/TNTAs were successfully prepared. The photocatalytic performance as well as the photocatalytic mechanisms of as-prepared noble metal-loaded TNTAs composite materials were also explored. The main contents are as follows:1. Preparation and characterizations of as-prepared TNTAsTNTAs were prepared by electrochemical anodic oxidation in two different organic electrolyte systems (ethylene glycol and glycerol) using Ti foil and Pt wire as the anode and cathode electrode, respectively. Experimental results showed that the TNTAs, which were prepared in the electrolyte of NH4F/ethylene glycol, were unstable and easily detached from the Ti substrate, thus making TNTAs unsuitable for subsequent use. On the other hand, when using the electrolyte of NH4F/glycerol, TNTAs could be preprared with a highly-stable structure and flat surface, which firmly contacted with the Ti substrate. The tube diameter of TNTAs was ca.110±10 nm, the tube wall was ca.15 nm, and the tube length was ca.1?m. Moreover, after thermal treatment, TNTAs could be transfered from amorphous to anatase, and showed enhanced UV-vis absorption abilities compared to commercial TiO2 (P25) nanoparticles.2. Preparation and photocatalytic properties of single metal-loaded TNTAs (Au/TNTAs or Pt/TNTAs)Au/TNTAs or Pt/TNTAs composite materials were prepared by photodeposition of Au or Pt nanoparticles on the surface of as-prepared TNTAs using HAUCL4.3H2O and H2PtCl6.6H2O as precursor solutions, respectively, and their structural morphologies and properties were examined by a series of testing measurements. Experimental results show that the successful loading of precious metal nanoparticles evenly on the surface of TNTAs could be obtained by the photoreduction method, and the deposited precious metal nanoparticles was contacted firmly with TNTAs. Meanwhile, the as-prepared Au/TNTAs or Pt/TNTAs showed excellent photocatalytic activities in the photodegradation of rhodamine B (RhB). The enhanced photocatalytic activities could be ascribed to the formation of Schottky barriers arised from the contact between the precious metal and TNTAs. This barrier could effectively act as electron traps to prevent the recombination of photogenerated electrons and holes within TNTAs, and thus delay the service life of the photogenerated electron-hole pairs, leading to the enhanced photocatalytic activities of TNTAs.3. Preparation and photocatalytic properties of bimetallic Au-Pt loaded TNTAs (Au-Pt/TNTAs)Au-Pt/TNTAs composite materials were successfully prepared by simultaneous photodeposition of Au and Pt nanoparticles on the surface TNTAs using HAUCL4.3H2O and H2PtCL6.6H2O as precursors. Experimental results demonstrated that about 2 nm Au-Pt nanoparticles were well deposited on the surface of TNTAs. Meanwhile, it was demonstrated from the photocatalytic measurements that the Au-Pt/TNTAs possessed much higher photocatalytic activities than the as-prepared TNTAs, Au/TNTAs or Pt/TNTAs. This significant enhancement could be ascribed to not only the formation of Schottky barrier between the loaded Au-Pt metals and TNTAs, but also the synergistic effect arised from the bimetallic metals (Au or Pt), which could improve the photoresponse of the materials to the UV-vis light, and faciliate the separation and transfer of photogenerated carriers, thus leading to the remarkable enhancement of photocatalytic activities.