Improving the Photoelectrochemical Response of TiO2 Nanotubes

Jan 31, 2013 - Dejan Cibrev , Milena Jankulovska , Teresa Lana-Villarreal , and Roberto Gómez. ACS Applied Materials & Interfaces 2014 6 (13), 10304-...
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Improving the Photoelectrochemical Response of TiO2 Nanotubes upon Decoration with Quantum-Sized Anatase Nanowires Milena Jankulovska, Irene Barceló, Teresa Lana-Villarreal,* and Roberto Gómez Institut Universitari d’Electroquímica i Departament de Química Física, Universitat d’Alacant, Apartat 99, E-03080 Alacant, Spain S Supporting Information *

ABSTRACT: TiO2 nanotubes (NTs) have been widely used for a number of applications including solar cells, photo(electro)chromic devices, and photocatalysis. Their quasi-one-dimensional morphology has the advantage of a fast electron transport although they have a relatively reduced interfacial area compared with nanoparticulate films. In this study, vertically oriented, smooth TiO2 NT arrays fabricated by anodization are decorated with ultrathin anatase nanowires (NWs). This facile modification, performed by chemical bath deposition, allows to create an advantageous self-organized structure that exhibits remarkable properties. On one hand, the huge increase in the electroactive interfacial area induces an improvement by 1 order of magnitude in the charge accumulation capacity. On the other hand, the modified NT arrays display larger photocurrents for water and oxalic acid oxidation than bare NTs. Their particular morphology enables a fast transfer of photogenerated holes but also efficient mass and electron transport. The importance of a proper band energy alignment for electron transfer from the NWs to the NTs is evidenced by comparing the behavior of these electrodes with that of NTs modified with rutile NWs. The NT-NW self-organized architecture allows for a precise design and control of the interfacial surface area, providing a material with particularly attractive properties for the applications mentioned above.

1. INTRODUCTION An impressive effort has been made in the past decade to design synthetic routes that allow for the preparation of well-defined TiO2 nanomaterials. The possibility of tuning the nanoparticle (NP) size, shape, and crystalline structure is the first step toward a rational design and fabrication of optimized structures for their future applications in photocatalysis, solar cells, batteries, smart films, etc. In this context, one-dimensional structures including nanowires (NWs) and nanocolumns but also nanotubes (NTs) are particularly attractive, as they can offer large surface areas preserving (mass and charge) transport properties.1 TiO2 NTs have been prepared using different routes, such as sol−gel, hydrothermal, and sonochemical synthesis, chemical treatments of TiO2 NPs, anodization, and template-assisted routes.1−6 Among these methodologies, anodization has become particularly popular since it was reported by Zwilling et al.7 These authors showed that the oxidation of a titanium alloy in chromic acid electrolytes containing hydrofluoric acid generates vertically aligned NTs. Since then, TiO2 NTs with different sizes have been prepared in fluoride-based electrolytes by anodization of titanium.8−10 Nowadays, the pore size, wall thickness, and NT length can be tailored by adjusting electrolyte composition, water content, anodization voltage, and time. Either the as-grown NTs can be detached from the titanium foil as a membrane,11 or they can be strongly bound to the titanium surface, showing good mechanical properties. Their relatively large surface area (compared to the geometrical one) and high structural order combined with their superior © 2013 American Chemical Society

electronic properties make them an attractive architecture for the applications previously mentioned.1,6,12 Recently, a number of works have appeared showing the benefits of modifying the NT surface with different inorganic NPs. For example, the electrochromic properties of TiO2 NT films can be improved by depositing WO3 NPs13 and their electro- and photocatalytic performance by including Pt and Ag NPs.14,15 They can even harvest visible light by sensitization with chalcogenide quantum dots.16,17 On the other hand, the modification with metal oxide NPs has been described as a method to increase the active surface area. This increase can be achieved by enhancing the NT roughness through deposition of TiO2 NPs,18 NWs,19 or ZnO nanorods.20 In such a case, the adequate NT electronic properties are preserved, allowing to prepare more efficient dye sensitized solar cells and active photocatalysts.18−20 Another approach to increase the active surface area is to prepare TiO2 nanocomposites constituted by disordered nanoribbons or NWs on the top surface of the NT arrays. These hierarchical structures can be fabricated by splitting the NTs in a controlled anodization process.21 Recently, other works about quasi-one-dimensional TiO2 nanomaterials with a complex hierarchical structure such as NPs-NWs or NPs-nanorods have appeared. Such a kind of TiO2 nanocomposite has remarkable photocatalytic properties Received: November 8, 2012 Revised: January 29, 2013 Published: January 31, 2013 4024

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recorded using a scan rate of 20 mV/s. The current densities are given on the basis of the geometric electrode area. A 300 W Xe arc lamp (Ushio) equipped with a water filter was used for UV−vis irradiation. The incident light intensity was monitored with an optical power meter (Oriel model 70310) equipped with a bolometer (Ophir Optronics 71964), being in all cases around 800 mW cm−2.

due to the large surface area and enhanced electron transport rate.22,23 In the present study, titania NTs have been fabricated by anodization24 and subsequently decorated with ultrathin anatase NWs (∼2 nm in diameter) employing a simple chemical bath deposition (CBD) method.25 The photoelectrochemical properties of this hierarchically organized structure have been studied and compared with those of naked NTs and NTs decorated with rutile NWs. We have already demonstrated in a previous work that TiO2 NTs are a good substrate for the growth of rutile NWs.19 The CBD method employed led to a high rutile NW coverage inside the NTs but also at the outer part of the NT walls. Because of the well-organized structure, the transport of electrolyte ions into the underlying NTs was not sterically limited, presenting the corresponding electrodes superior photoelectrocatalytic properties in comparison with those made of bare NTs. The improvement was ascribed to an enhanced surface area while keeping the favorable transport properties typical of NT electrodes.19 The modification with anatase NWs reported here causes an additional enhancement, probably due to a proper band alignment and to a diminished density of electron traps.

3. RESULTS AND DISCUSSION As previously reported in the literature, the anodization process of a titanium foil in ethylene glycol in the presence of fluoride results in the generation of amorphous TiO2 NTs.26 In addition, during the extended anodization process a precipitate is generated on top of the NTs (possibly titanium oxyhydroxide) as a result of chemical etching. This precipitate probably has a deleterious effect in the subsequent NT decoration, as it could block the pores, preventing the growth of the NWs inside the structure. In order to eliminate this solid from the surface, ultrasonic cleaning was performed by immersing the samples in ethanol in an ultrasonic bath for only a few seconds prior to the thermal treatment. An analysis of the top surface of the NTs by SEM reveals that the top surface of the as-formed NTs is covered by an irregular precipitate (particularly for the samples anodized for long periods of time), while the cleaned samples show an open NT structure (see Supporting Information, Figure S1). The elimination of this amorphous deposit is a crucial step for the subsequent successful homogeneous deposition of any secondary material (see below). However, it should be mentioned that ultrasound can also induce a partial rupture of the NTs (depending on the ultrasound frequency and power). On the other hand, the as-formed TiO2 NTs are amorphous and there is a need of thermal treatment to obtain crystalline NTs.27 In this work, the samples were annealed at 450 °C for 1 h in air. The crystalline structure was investigated by Raman spectroscopy and X-ray Diffraction (XRD). Figure 1 shows the Raman spectrum of thermally treated TiO2 NTs and for the sake of comparison, the spectrum of a nanoporous film composed of commercial anatase NPs (Alfa Aesar). Characteristic modes appear in the Raman spectra (Figure 1a) at 145 and 640 cm−1 (Eg), 400 cm−1 (B1g), and 516 cm−1 (doublet A1g and B1g), which can be ascribed to the anatase crystal phase. Figure 1b shows the XRD pattern for TiO2 NTs after thermal annealing. As can be seen, the main features of the diffraction pattern also confirm the anatase structure ((101) peak at 25° and (211) peak at 62.8°). SEM was used to characterize the surface morphology, microstructure, and length of the thermally treated NTs. SEM images (Figure 2) show that the NTs are well-aligned and organized into uniform arrays. Independently of the anodization time, the TiO2 NTs are characterized by smooth walls with an inner diameter of ∼100 nm and a wall thickness of ∼15 nm. The length of the NTs varies with the anodization time, being ∼3 μm for 1 h and ∼12 μm for 5 h, as observed for different NT-array fabrication procedures. In general, the pore size and thickness depend on the anodization potential, while the length varies with the anodization time, up to a maximum value.27 The electrochemical characterization of TiO2 NT electrodes was performed in nitrogen-purged 0.1 M HClO4. As shown in Figure 3, the CVs show a well-defined accumulation region. Their symmetric shape indicates that there are no faradaic processes or ohmic resistances, even for the longest NTs. The

2. EXPERIMENTAL SECTION TiO2 NT electrodes were prepared by anodization in a twoelectrode cell as reported in ref 24. A titanium foil was used as an anode and a gold foil as a cathode (with a 2 cm separation between them). The anodization was conducted at room temperature, at a bias of 40 V, in 160 mL of ethylene glycol + 20 mL of dimethyl sulfoxide + 20 mL of H2O in the presence of 0.5 wt % NH4F. Anodization time was varied between 1 and 5 h as to obtain NTs with different lengths (from ∼3 to ∼12 μm). After anodization, the samples were rinsed with ethanol and sonicated for 10 s in an ultrasonic bath. This treatment allows to remove the solvent trapped inside the TiO2 NTs. Furthermore, it eliminates the precipitate generated on top of the NTs. Subsequently, the as-prepared TiO2 NTs were submitted to a thermal treatment for 1 h in air at 450 °C, which triggers the transformation of the originally amorphous phase into anatase. After annealing, the TiO2 NT electrodes were decorated with anatase NWs by CBD from an aqueous titanium fluoride (TiF4) solution.25 Concretely, the precursor solution was prepared by adding TiF4 (5 mM) to an aqueous solution of pH ∼ 1 containing hydrochloric acid. The NT electrodes were immersed into the precursor solution and kept at 60 °C for times ranging from 10 to 150 min. The morphology of the TiO 2 NT electrodes was characterized using scanning electron microscopy (SEM, Hitachi S-3000N) and transmission electron microscopy (TEM, JEOL, JEM-2010 equipped with a Mega View II camera (SIS)). The crystal structure was determined by X-ray diffraction (Seifert JSO-Debyeflex 2002) using the Cu Kα line and by Raman spectroscopy (LabRam spectrometer, HORIBA Jobin Yvon). The electrochemical experiments were performed in a threeelectrode cell fitted with a quartz window, using a computercontrolled Autolab PGSTAT 30 potentiostat. A Pt wire and a Ag/AgCl/KCl(sat) electrode were used as a counter and a reference electrode, respectively. A N2-purged 0.1 M HClO4 solution in ultrapure water (Millipore Elix 3) was used as working electrolyte. Cyclic voltammograms (CVs) were 4025

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Figure 1. (a) Raman spectrum of a TiO2 NT electrode after thermal annealing. For comparison, the Raman spectrum of a nanoporous electrode based on anatase commercial nanopowder (Alfa Aesar) is also included. (b) X-ray diffraction pattern for an annealed TiO2 NT electrode. (The asterisk indicates the peaks corresponding to anatase; the rest correspond to the titanium substrate.)

Figure 2. SEM images (top and cross-sectional view) of thermally annealed TiO2 NTs prepared using different anodization times: 1 h (a, b), 2 h (c, d), 3 h (e, f), 4 h (g, h), and 5 h (i, j).

current can be ascribed to a chemical capacitance that exponentially increases with the applied potential.28 This capacitive current is proportional to the electrochemical active surface area, as the accumulated charge should be compensated by the adsorption/insertion of cations.29,30 In agreement, the charge of the accumulation region increases with the anodization time. For the particular morphology of the NTs, where the pore diameter and wall thickness remain constant, the charge of the accumulation region (proportional to the interfacial area) can be directly correlated to the NT length (see Figure S2). Anatase NTs were used as substrates in a chemical bath at 60 °C containing TiF4, pH 1. According to the pioneering work of Yamabi and Imai,25 under such acidic conditions, Ti(IV) is present in solution mainly as Ti(OH)22+, and the following equilibrium is established: Ti(OH)2 2 + ⇄ TiO2 (s) + 2H+

Figure 3. CVs for TiO2 NTs in the dark in N2-purged 0.1 M HClO4. Scan rate: 20 mV s−1.

(1) 4026

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The selective adsorption of fluoride on specific facets parallel to the c-axis prevents the crystal growth perpendicular to this axis, growing TiO2 along the [001] direction. After this treatment, the smooth surface of the TiO2 NTs became comparatively rough. A TEM analysis shows that the nanotubular morphology is well retained, but a substantial deposit inside the NTs is generated (see Figures S3 and S4). Figure 4 shows TEM images of TiO2 NTs prior and after the

Figure 4. TEM images of TiO2 NTs (anodization time 1 h) prior to (a) and after decoration (b) with anatase NWs (CBD for 45 min).

CBD process. From a detailed inspection of the images, we can conclude that the CBD induces the growth of extremely thin NWs, about 2 nm in diameter, in agreement with previous results using FTO substrates.31 It is noteworthy that the apparent tube wall thickness increases after the hydrothermal treatment (see Figure S5). A careful analysis shows the presence of anatase NWs oriented close to perpendicularly to the inner NT surface, as well as at the outer part of the NT walls (Figure 5). During the CBD, heterogeneous nucleation at the NT surface takes place, growing TiO2 NWs directly on the NT walls. As the deposition proceeds, bundles of randomly oriented NWs are generated inside the NTs. If the CBD takes place for a very long time, it is also possible to grow a thick layer of NW bundles on top of the NT structure. This overlayer is undesirable as it could have a deleterious effect for the final application; it could act as a blocking layer, hampering the transport of solution species into the NTs. In order to optimize the final structure, CBD was performed for different times (ranging from 10 to 150 min) using NTs prepared by anodization for 1 h (about 3 μm in length). The corresponding CVs are shown in Figure 6a. It is evident that the charge of the accumulation region increases with the deposition time, indicating an increase in the active surface area. The symmetric CV profiles also indicate that there are no significant charge/electrolyte transport limitations in any of the electrodes. We have used the charge of the accumulation region (directly proportional to the active interfacial area) as a criterion to optimize the NW deposition. It is expected that once the NTs are filled with NWs, the increase in the active surface area slows down because the area where the NWs can be grown would be highly diminished (finally approaching the projected geometric area). In order to limit the margin of error caused by inhomogeneties related to small differences in the TiO2 NT morphology, the ratio of the charge integrated between −0.3 and −0.7 V (accumulation region) before and after decoration, QNT+NW/QNT, is calculated. This value is an indication of the

Figure 5. TEM images of TiO2 NTs (anodization time 1 h) decorated with anatase NWs (CBD for 45 min). Side (a) and top (b, c) views.

Figure 6. (a) CVs for TiO2 NTs (1 h anodization) decorated with different amounts of anatase NWs in N2-purged 0.1 M HClO4. Scan rate: 20 mV s−1. (b) Dependence of the QNT+NW/QNT ratio on the NW deposition time. The QNT+NW/QNT ratio was calculated from CVs measured under the same conditions as in (a).

relative increase in the active interfacial area. In Figure 6b, QNT+NW/QNT is plotted against the deposition time. As can be observed, this ratio first increases rapidly and then it tends to a 4027

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the accumulation region diminishes. Other authors have also pointed out the importance of having an open, well-defined top morphology,32 demonstrating that it significantly affects charge transfer and determines the overall efficiency in photovoltaic cells.33,34 The as-prepared hierarchical nanostructures, formed by onedimensional nano-objects in two different size regimes, show an increased capacity that can be exploited for practical applications such as lithium batteries, sensitized solar cells, or photo(electro)chromics. For the latter applications a transparent substrate is needed; although in this work the NTs have been grown on a Ti foil, it is also possible to transfer the NT array to an FTO substrate.11 In such a case, their active surface area can be increased in a subsequent step by decoration with rutile or anatase NWs, which have shown remarkable electrochromic properties.31 Further work is underway along these lines. On the other hand, for photoelectrochemical applications, useful nanoporours TiO2 layers should also have high surface areas to favor charge transfer to solution, simultaneously assuring that the distance that electrons need to travel within the nanostructure toward the back-contact is as short as possible to diminish recombination. To meet both goals, TiO2 structures should contain interconnected mesoscopic pores, which provide an enhanced interfacial contact between semiconductor and solution, while maintaining a continuous path for electron transport. It is expected that the NWs within the NTs will increase the surface area, shorten the path for charge transfer, and enhance surface chemistry, while the NT framework would form a direct transport path for the electrons, simultaneously allowing for efficient mass transport. In order to confirm these ideas, we have studied the photo(electro)catalytic properties of electrodes composed of NTs decorated with NWs. Figure 8 shows the CVs in the dark and under illumination for a TiO2 NT electrode (1 h anodization time) obtained prior and after its decoration with anatase NWs in nitrogen-purged 0.1 M HClO4. The deposition was carried out only for 5 min, but as it can be deduced from the charge accumulation region, it already induces a significant increase in the active surface area (of about three times). We have chosen this degree of modification in order to decorate the NT walls, without filling completely the NTs. Under illumination, for the naked NT electrode, a photocurrent is measured at potentials more positive than −0.1 V (vs Ag/AgCl/KCl(sat.)). The photocurrent, ascribed to water photooxidation, increases as the bias becomes more positive approximately in an inverted parabolic way, as expected for electrodes having a space charge region. The recombination of photoinduced electron−hole pairs is probably inhibited by the presence of an electric field inside the NTs.35,36 On the other hand, the photocurrent value after decoration indicates a slight improvement for water photooxidation efficiency. This could be related with a shift of the conduction band edge toward more negative electrode potentials (higher potential energies) due to an equilibration process between the NW and NT bands. Nevertheless, when the negative-going scan of the CV in the dark for the NTs is multiplied by a factor of 3, the resulting curve is rather similar to that corresponding to the NT array electrode modified with NWs. This is a good indication of the same energetic location for the surface/ conduction band states for both electrodes.

constant value. Concretely, for a NW deposition time of 45 min a plateau is reached. We can consider that, for such a deposition time, the NTs are already completely filled. (We have considered that the NWs inside the NT are electroactive on the basis of the large multiplication factor of the interfacial area. See Supporting Information for further details.) For longer times, the CBD would only induce the generation of NWs on top of the NT layer, generating an overlayer loosely attached to the underlying NTs, as this portion of the films does not appear to be electroactive. Once the deposition time was optimized, the length of the NTs was modified. In principle, the decoration with NWs by CBD can be performed independently of the NT length. However, for very long NTs, the diffusion of the TiO2 precursor to the NT bottom would be hindered due to mass transport limitations at the pores. In such a case, NW deposition would occur at the outer part of the NT array, being the solution filling the inner part depleted in Ti (IV). Therefore, it should be difficult to cover with NWs the inner part of the NTs. We have prepared electrodes using different NT lengths (with anodization times longer than 1 h) using the optimized CBD time (for NTs prepared by 1 h anodization). If the deposition of NWs is homogeneous along the NT, the ratio QNT+NW/QNT should be constant. Accordingly, Figure 7 shows

Figure 7. CVs for NT electrodes prepared with different anodization times and decorated with anatase NWs (deposition time 45 min). Working solution: N2-purged 0.1 M HClO4. Scan rate: 20 mV s−1.

an approximately constant QNT+NW/QNT ratio as the NTs become longer. Therefore, we can conclude that the CBD method employed here allows us to homogeneously decorate long NTs with anatase NWs (at least as long as ∼12 μm, corresponding to 5 h of anodization). This result clearly contrasts with similar experiments in which TiO2 NTs are decorated with rutile NWs, having virtually the same morphology.19 In such a case, QNT+NW/QNT diminishes as the length of the NTs increases, indicating a depletion of the rutile precursor at the bottom of the NTs during decoration. This result could be influenced by the morphology of the upper part of the NT film. In the present study, we have taken particular care to ensure the removal of the nonhomogeneous deposit on top of the as-prepared NTs, which was not the case in our precedent study with rutile NWs. Probably sonication also breaks the upper part of the NTs, giving rise to an open structure that facilitates the deposit. In fact, upon sonication, 4028

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tration cannot be restored in the time window of a CV at 20 mV s−1. Figure 9 shows the photocurrent transients at −0.1 V for water and oxalic acid photooxidation for bare NT electrodes

Figure 9. Photocurrent transients in the absence (dashed line) and in the presence (solid line) of 2 mM oxalic acid for a NT electrode with different amounts of anatase NWs. Supporting electrolyte: N2-purged 0.1 M HClO4. Applied potential: −0.1 V.

and NTs decorated with different amounts of anatase NWs. The photocurrent density for both water and oxalic acid oxidation increases upon NW deposition, showing the highest photocurrent for the sample composed of NTs completely filled with NWs (45 min deposition). This result clearly points to the importance of increasing the NT active surface area for improving photoanode performance. On the other hand, in all cases, including that of naked NTs, the transient in the presence of oxalic acid shows a spike. The photocurrent decays afterward until a plateau is reached. In principle, the photocurrent spike is directly proportional to the amount of oxalic acid adsorbed in the dark, whereas the photocurrent at the plateau is dictated by the amount of adsorbed molecules in the photostationary state.38,39 The fact that, in the presence of oxalic acid, the photocurrent does not drop to the value corresponding to water photooxidation indicates that the hierarchical structure is still a relatively open structure, with remarkable mass transport properties. The situation was similar for the previously studied hierarchically organized structures based on TiO2 NTs decorated with rutile NWs.19 To compare the best performance of both structures, we should consider that in the case of anatase NWs the interfacial area can be increased by a factor as large as 12, while in the case of rutile NWs it increases at most by a factor of 8. However, if we pay attention to the photocurrent developed under the same experimental conditions, it is multiplied by a factor of ∼3 for rutile decoration, while it increases by a factor of 10 for anatase for both water and oxalic acid oxidation. The additional enhancement for anatase NWs could be related to a larger photoactivity of anatase NWs compared with rutile NWs. However, work is underway to study the behavior under illumination of thin films based on anatase and rutile NWs, and the initial photoelectrochemical experiments for methanol and formic acid photooxidation show similar results for both NW structures. Therefore, the additional benefit of anatase NWs can be related to a more favorable stepwise energy band alignment. The anatase NW conduction band edge is shifted negatively by 0.2 V with respect to that of rutile.31 This could favor electron transfer toward anatase NTs, which in addition provides a

Figure 8. CVs for TiO2 NTs before and after decoration with anatase NWs (5 min CBD time) in the dark and under illumination in N2purged 0.1 M HClO4 in the absence (a) and in the presence (b) of 2 mM oxalic acid. Scan rate: 20 mV s−1. Inset: details of the CV negative-going scan in the dark for NTs before (multiplied by a factor of 3) and after decoration.

Interestingly, when investigating the photoelectrochemical properties of TiO2 (anatase and rutile) NWs directly deposited on conducting glass, we found out that, due to the tiny size of these nanostructures, they are not efficient for water photooxidation.37 The situation seems to be rather different when they are deposited on highly ordered NTs. Probably an enhanced electron extraction due to the presence of an electric field within the NTs inhibits recombination, increasing water photooxidation.19 In this work we have chosen oxalic acid as a model organic molecule in order to investigate the influence of the increased interfacial area of the hierarchically organized structures on their photoelectrochemical properties. This choice is based on the strong adsorption of oxalic acid on TiO2, which makes feasible its photooxidation through a direct hole transfer. Therefore, even small changes on the surface of the electrode will influence the photooxidation process.38,39 Figure 8b shows the CVs in the dark and under illumination for the TiO2 NT electrode after its decoration with anatase NWs (deposition time 5 min) in the presence/absence of 2 mM oxalic acid. An increase in the photocurrent is observed in the presence of oxalic acid, which can be ascribed to a diminution of recombination, combined with the current doubling effect.40 In fact, the largest difference in the CVs triggered by the presence of oxalic acid appears in the region between −0.3 and 0.2 V, i.e., in the region where recombination is favored. However, the fact that the forward and backward scans do not overlap probably indicates a certain limitation of mass transport due to the high illumination intensity employed in combination with a low concentration of oxalic acid. The question that arises is whether this limitation is mainly due to a fast consumption of adsorbed oxalate, whose surface concen4029

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region on the anodization time; TEM images of decorated NTs; discussion about the electroactivity of the NWs inside the NTs; CVs for a NT electrode prior and after decoration with anatase NWs in the presence/absence of oxalic acid; CVs for a NT electrode prior and after decoration with rutile NWs. This material is available free of charge via the Internet at http:// pubs.acs.org.

straightforward path for the electrons toward the substrate. On the other hand, in the case of rutile NWs, one should expect the generation of electronic traps at the boundaries between the NWs and the NTs due to a discontinuity of the crystalline structure.41 In fact, the CVs in the dark for NT electrodes decorated with rutile NWs show the presence of a quasireversible couple of peaks at potentials more positive than the charge accumulation region (see Figure S7). The presence of these monoenergetic trap states is associated with grain boundaries. This pair of peaks is virtually absent for anatase decorated NTs. However it should be mentioned that for anatase these electronic traps appear at the initial part of the accumulation region, making more difficult to clearly discern them (particularly in acidic media).41

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of the Spanish Ministry of Economy and Competitiveness through projects HOPE CSD2007-00007 (Consolider Ingenio 2010), PRIPIBIN-2011-0816, and MAT2009-14004 (Fondos FEDER). M. Jankulovska is grateful to the Spanish Government for the award of an FPI grant. We thank C. Almansa and V. López for performing TEM and SEM measurements, respectively.

4. CONCLUSIONS In this work, an anatase self-organized hierarchical structure has been fabricated by a simple two-step method that combines the electrochemical anodization of titanium foil and a subsequent chemical bath deposition. In this way, it has been possible to fabricate highly ordered, vertically oriented TiO2 nanotube arrays decorated with extremely thin anatase nanowires. The asprepared architecture has been characterized morphologically by TEM and SEM, structurally by Raman spectroscopy and XRD, and electrochemically by cyclic voltammetry and photocurrent transients. The results reveal an homogeneous growth of TiO2 nanowires, even for nanotubes as long as 12 μm. The decoration with anatase nanowires induces an impressive enhancement of the interfacial area of the tubular structure. As deduced from the cyclic voltammograms, the interfacial area can be multiplied by a factor of 12, without detecting any charge transport limitation ascribed to a blockage of the nanotubes by a massive nanowire deposition. A number of advanced applications such as Li-ion batteries, electrochromic devices, and sensors could benefit from the outstanding interfacial area and charge transport properties resulting from nanowire decoration. Other applications such as photo(electro)catalysis and water splitting could also take advantage of the enhanced contact area with the electrolyte. In fact, larger photocurrents have been measured for both water and oxalic acid photooxidation. The hierarchically organized structure conserves the tubular framework that provides a direct path for the electrons toward the substrate, hindering recombination. In addition, its relatively open structure triggers good mass transport properties. These results are in line with previous experiments performed with nanotubes decorated with rutile nanowires. However, the modification with anatase causes an additional enhancement in photoactivity, probably due to a proper band alignment and a diminished number of electron traps at the nanowire/nanotube boundaries. In conclusion, chemical bath processing to grow anatase nanowires may stand out as a simple and promising modification route to impart an enhanced interfacial area to quasi-one-dimensional large TiO2 nanotubes. This procedure illustrates the rational design of titania nanostructures optimized for a number of applications.

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AUTHOR INFORMATION

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REFERENCES

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Morphological effect of the ultrasonic bath treatment; dependence of the NT length and the charge of the accumulation 4030

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