Photocurrent Conversion in Anodized TiO2 Nanotube Arrays: Effect of

Article pubs.acs.org/JPCC

Photocurrent Conversion in Anodized TiO2 Nanotube Arrays: Effect of the Water Content in Anodizing Solutions Lok-kun Tsui,† Takayuki Homma,‡ and Giovanni Zangari*,† †

Department of Materials Science and Engineering, University of Virginia, 395 McCormick Rd., Charlottesville, Virginia 22904, United States ‡ Department of Applied Chemistry, Waseda University, Okubo, Shinjuku, Tokyo 169-8555, Japan S Supporting Information *

ABSTRACT: The TiO2 nanotube system exhibits properties of interest in photoelectrochemical water splitting for hydrogen production, including high surface area and vectorial charge transport along the nanotube length. Changes in the anodizing electrolyte chemistry provide the means to control nanotube morphology as well as their length and width. Despite the large amount of work available on nanotube synthesis, however, a thorough assessment of the effect of anodization conditions on the photoelectrochemical performance is still unavailable. In this paper, we characterize TiO2 nanotubes produced by varying the water content of the organic anodization electrolyte and investigate the influence of the electrolyte on their photoelectrochemical performance. We find that the photocurrent efficiency of the nanotubes is optimized by using an 11 vol % water:ethylene glycol ratio. We also demonstrate that a double-anodization technique produces a cleaner surface, resulting in higher photon-to-current conversion efficiencies of up to 30% at 350 nm. Raman spectroscopy, X-ray diffraction, and electrochemical impedance studies support the notion that the variation in crystallinity as a function of water content is the main factor in determining the photocurrent efficiency of the nanotube system.

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μmare formed from solvents containing as little as 2 vol % water;9−12 nanotubes as wide as 730 nm have also been grown in dimethyl sulfoxide solutions at high voltage.13 A range of nanotube morphologies has been demonstrated by varying the anodization potential or by tuning the water content of the electrolyte in which the nanotubes are synthesized. Berger et al. examined the morphology of nanotubes grown in electrolytes with different water content and determined the concentration of impurity elements such as C, F, and N as a function of water content.14 Electrochemical impedance studies were performed by Castro et al.15 and Ali Yahia et al.,16 who developed equivalent circuit models for nanotubes grown at different voltages.15,16 Kojima et al. devised models to study the rates of anodization for electrolytes containing varying amounts of NH 4 F and water and investigated the effect of anodization conditions on morphology and F content.17 Despite the considerable amount of studies focused on the morphology and composition of anodized TiO2 nanotubes, there is still a need to systematically determine the factors that influence the photoelectrochemical performance of such nanotubes, in order to enable optimization of the anodization variables. In this context, there is widespread interest in developing a synthesis route to produce highly uniform arrays of TiO2

INTRODUCTION The direct conversion of solar energy into chemical fuels potentially provides a versatile solution to the intermittent output of photovoltaic devices, allowing the storage of solar energy for later use. Notwithstanding the lack of practical technological solutions to its storage and transportation,1 hydrogen can simply be produced by photoelectrochemically induced water splitting in aqueous solutions.2,3 TiO2 nanotube array anodes are most promising for this application; besides being stable in most aqueous solutions under irradiation, they provide high surface area, a large density of charge carriers with high mobility, and one-dimensional charge transport.4 Titania nanotubes can be synthesized at low temperature and low energy intensity by anodization of Ti foils or films in solutions containing F− ions; a dynamic balance between the electrochemical formation of TiO2 and Ti dissolution by F− produces the self-aligned nanotube array structure. Electrochemically synthesized nanotubes of the first generation were grown from aqueous acidic HF solutions and were limited to ∼500 nm in length due to the aggressive etching of HF.5,6 Neutral aqueous solutions containing fluoride salts such as NaF or NH4F exhibit lower etch rates and allow formation of nanotubes up to 2.5 μm long.7, In contrast, the third generation of TiO2 nanotubes was produced in organic solvents including ethylene glycol, glycerol, or dimethyl sulfoxide. These organic solutions provide the most versatile method to produce TiO2 nanotubes, as they may access a wide range of tube diameters and lengths, as well as a variety of morphologies. The longest nanotubesup to 250 © 2013 American Chemical Society

Received: January 10, 2013 Revised: March 20, 2013 Published: March 21, 2013 6979

dx.doi.org/10.1021/jp400318n | J. Phys. Chem. C 2013, 117, 6979−6989

The Journal of Physical Chemistry C

Article

Table 1. Anodization Conditions and Dimensions forTiO2 Nanotubes Formed in Electrolytes with Varying Water Contenta water content (vol %)

water content (wt %)

NH4F content (wt %)

applied voltage (V)

anodization time (min)

length (μm)

pore diam (nm)

2 11 25 45

1.8 10 23 42

0.3 0.5 0.5 0.5

50 20 20 20

5 60 60 60

0.7 (s), 1.2 (d) 1.1 1.0 1.1