Template-free formation of vertically oriented TiO2 nanorods with uniform distribution for organics-sensing application

https://doi.org/10.1016/j.jhazmat.2011.01.125Get rights and content

Abstract

High-density arrays of vertically oriented TiO2 nanorods with uniform distribution on Ti foil have been formed through template-free oxidation of Ti in hydrogen peroxide solutions. Subsequent thermal treatment was applied for growing mixed crystal structures to pursue higher performance. Morphology characterization using field emission scanning electron microscopy (FESEM) shows a nanorod diameter in the range of 20–50 nm with a length of 1.5 μm. X-ray diffraction (XRD) measurement demonstrates the crystallization of the TiO2 nanorods prior to thermal treatment and the formation of anatase and rutile mixed phase after thermal treatment. The mixed crystal TiO2 nanorods show a much higher performance than pure anatase in photoelectrochemical experiments. Steady-state photocurrent resulted from photocatalytic oxidation of organic compounds by TiO2 nanorods is employed as response signal in determination of the organics to yield a linear range of 0–1.1 mM for glucose. For other organics, an excellent linear relationship between the net steady-state photocurrent and the concentration of electrons transferred in exhaustive oxidation for these organics is obtained, which empowers the mixed crystal TiO2 nanorods to serve as versatile material in organics-sensing application.

Introduction

Titanium dioxide is a versatile material and has been investigated considerably due to its unique photoelectronic and photochemical properties [1]. As photocatalyst for water splitting and degradation of organics, TiO2 powder may have disadvantage in practical applications because of the difficulties in separating and recycling the fine particles [2]. TiO2 nanocrystalline film which immobilizes the semiconductor catalyst on a solid support will exhibit its advantage in this regard. A number of methods have been developed to prepare TiO2 thin films, for instance, electron-beam evaporation [3], chemical vapor deposition [4], [5], physical vapor deposition [6], dip-coating [7], [8], spin-coating [9], and screen-printing [10], [11]. The thin films prepared through those methods are usually composed of TiO2 nanoparticles, as illustrated in Fig. 1A. The photogenerated electrons transport across the nanoporous TiO2 layer via the TiO2 network formed by the interconnected nanoparticles. However, some of these particles may not be interconnected, which results in a limited number of effective electron pathways that have the shortest distance and lowest resistance [12]. Furthermore, the structural disorder at the contact between two crystalline nanoparticles leads to enhanced scattering of free electrons, thus reducing electron mobility [13]. Recently, the use of one-dimensional nanostructures instead of nanocrystalline films in photoelectrochemistry has been considered. TiO2 nanorods [14], nanotubes [15], [16], nanowires [17], [18] and nanocolumns [19] have been used in highly efficient photovoltaic devices and as photocatalysts for hydrogen generation and degradation of organic pollutants. These one-dimensional nanomaterials with ordered architecture (Fig. 1B) allow for an oriented and much shorter random walk path for photogenerated carriers. As a result, improved charge separation and charge transport can be achieved [13].

The formation of one-dimensional nanostructures has been realized using various techniques. Anodization of Ti foil is now widely used to prepare TiO2 nanotube arrays [15], but the corrosive etching process in the anodization takes place only when the toxic F is added. Although TiO2 nanotubes and nanorods were also fabricated by templating ZnO nanorod array film through sol–gel process [20] and liquid phase deposition [21], the formation and subsequent removal of the ZnO template using wet-chemical etching complicated these methods. Template-free formation of one-dimensional TiO2 nanorod arrays was achieved by oxidizing titanium substrate in an oxidation atmosphere at 850 °C [22], [23]. Besides, fabrication of TiO2 nanorod arrays on transparent conducting substrates was accomplished in solutions containing HCl without template [14], [18], [24]. However, the TiO2 synthesized by the two above mentioned methods was rutile phase due to the high temperature and strong acidic medium applied during respective synthesis. Among the three different crystal structures of TiO2, rutile, anatase, and brookite, rutile is the thermodynamically stable phase, whereas anatase has been generally accepted to have a higher photocatalytic activity than rutile owing to its anomalously large Born effective charge tensor as the result of the presence of enhanced Ti(3d states)-O(2p states) hybridization [25]. The photocatalytic activity of anatase TiO2 can be greatly improved by mixing rutile nanostructures due to the mixed crystal effect [26], [27]. Therefore, safe and simple formation of TiO2 nanorod arrays with anatase or anatase/rutile structure is significant for their application in photovoltaic devices and photocatalysis.

In recent years, TiO2 film composed of nanoparticles has been used as a sensor for water quality assessment based on the photocatalytic oxidation of organics [28]. Electrocatalysis of some novel electrodes, such as PbO2 modified electrode [29], RhO2/Ti electrode [30], copper electrode [31], [32], boron-doped diamond electrode [33], [34], have been used in determination of chemical oxygen demand (COD) based on the measurement of currents or charge generated from electrocatalytic oxidation of organic compounds. Photocatalytic oxidation approach, which utilizes TiO2 as photocatalyst, can offer superior oxidation ability for oxidizing a much wider spectrum of organic compounds [35]. Therefore, there is a promising application for TiO2 photocatalysis in determination of organics in aqueous solution by analyzing the photocurrents corresponded to the photocatalytic oxidation of the organics. However, the application of one-dimensional TiO2 nanostructures in this field was seldom reported.

In this paper, a simple and undefiled method was introduced to fabricate large-scale, uniformly distributed and vertically oriented TiO2 nanorod arrays on a titanium substrate using H2O2 solutions as the oxygen source in the oxidation of Ti at a low temperature. The prepared TiO2 was in anatase phase and mixed phase after thermal treatment. The TiO2 nanorod arrays with mixed phase were verified to have a higher photocatalytic activity, and were used as sensors to assemble photoelectrochemical cells for organics-sensing application in aqueous solutions.

Section snippets

Experimental

Formation and characterization of TiO2 nanorod arrays: Ti foil (purity 99.99%) was purchased from Shanghai Zhenming nonferrous metal material Co., Ltd. The other chemicals were analytical grade and used as received from Sinopharm Chemical Reagent Co., Ltd. Ti foil sized 2 cm × 2 cm × 0.1 mm was degreased by sonication in acetone, 2-propanol, and ethanol to remove the contamination, subsequently rinsed with ultrapure water (18.2 MΩ), and finally dried in a nitrogen stream. After rinsing, Ti foil was

Morphology and phase characterization of TiO2 nanorod arrays

After oxidizing with the 30 wt% H2O2 solution for 72 h at 80 °C, the Ti foil had lost its metallic luster and turned grayish white, while white precipitations appeared in the solution. The color of the solution also changed slowly from colorless to yellow after the oxidation began, and then faded to colorless again at the end of the reaction. The color came from the formation of the complex [TiO(H2O2)]2+ [37], and its disappearance resulted from the decomposition of the complex caused by forming

Conclusion

The present paper demonstrated high-density arrays of vertically oriented TiO2 nanorods on Ti foil by a simple and undefiled method via oxidizing Ti foil in H2O2 solution. This method was template-free, and the formed TiO2 nanorod arrays were in anatase phase prior to thermal treatment or mixed phase after thermal treatment. The photoelectrochemical experiments indicated that the TiO2 nanorod arrays in mixed phase presented higher photocatalytic activity than the anatase TiO2 nanorod arrays.

Acknowledgements

We gratefully acknowledge the financial support of Shanghai Municipal Education Commission (no. 07SG37), National Natural Science Foundation of China (no. 51072034), Shanghai Leading Academic Discipline Project (B603), the Cultivation Fund of the Key Scientific and Technical Innovation Project (no. 708039), and the Program of Introducing Talents of Discipline to Universities (no. 111-2-04).

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