Effects of geometric and crystal structures on the photoelectrical properties of highly ordered TiO 2 nanotube arrays

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The photoelectrical properties of highly ordered TiO2 nanotube (TNT) arrays have been systematically and quantitatively studied and found to be closely related to their geometric and crystal structures. The geometric characteristics, including the nanotube diameter and length, were modiﬁed by adjusting the anodization potentials and durations, while the crystal structure was modiﬁed by thermal annealing at different temperatures. The nanotube array samples with the mixed crystalline phases possess higher photoconversion efﬁciency than those with the single anatase or rutile phase. The optimal content of rutile phase is about twice of that of anatase phase. In terms of the inﬂuence of the geometric structure, the TNT arrays with larger inner diameters and longer tube lengths have better photoelectrical properties. A geometric roughness factor has been applied to describe the combinative effect of the geometric characteristics. The TNT sample with the geometric roughness factor of 125.32 shows the superior photoconversion efﬁciency of 13.2%. The underlying mechanism has also been discussed in detail. I. INTRODUCTION

Highly ordered TiO2 nanotube (TNT) arrays prepared by electrochemical anodization have been attracting attention from both fundamental research and applied perspectives, driven by their unique photoactivity in a broad range of applications, extending from photocatalytic degradation of organic materials and hydrogen production by water photoelectrolysis to gas sensors and solar cells.1,2 In contrast to random nanoparticle systems where slow carrier diffusion typically limits device performance,3 the precisely oriented nature of the crystalline (after annealing) TNT arrays makes them excellent electron percolation pathways for charge transfer between interfaces.4,5 Furthermore, the nanotube array architecture offers the ability to modify the absorption and propagation of light by precisely designing and controlling the architectural parameters including nanotube inner diameter, wall thickness, and tube length.6 However, the poor spectral response of TNT arrays as well as the relatively low efﬁciency of light utilization is still the main obstacle that limits their applications using solar energy.7 Besides their wide band gap, both the geometric and crystal structures play key roles in determining the light absorption, charge transfer, and photocatalytic performance. In general, electrochemical anodization of Ti metal is a relatively simple process to generate well-deﬁned TiO2 a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.38 J. Mater. Res., Vol. 27, No. 7, Apr 14, 2012

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nanotubular structure.8 Anodization variables, including electrolyte composition and pH values, anodization potential and duration, annealing ambient and temperature, can inﬂuence the TNT morphology and the photoelectrochemical performance.9–11 It has been established that the inner diameter is primarily determined by the anodization poten

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