Photoelectrochemical and photocatalytic activity of tungsten doped TiO2 nanotube layers in the near visible region

Electrochimica Acta 56 (2011) 10557–10561

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Photoelectrochemical and photocatalytic activity of tungsten doped TiO2 nanotube layers in the near visible region C. Das, I. Paramasivam, N. Liu, P. Schmuki ∗ Department of Materials Science, WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany

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Article history: Received 15 November 2010 Received in revised form 16 May 2011 Accepted 17 May 2011 Available online 26 May 2011 Keywords: TiO2 nanotubular layer TiW alloy Tungsten doping Photoelectrochemical measurements Photocatalysis Electrochemistry

a b s t r a c t In the present work we study the effect of WO3 doping on the photo-electrochemical behavior of selforganized TiO2 nanotube layers. Mixed oxide nanotubes were grown by anodization of Ti–W alloys containing 0.2 and 9% W, with a thicknesses of the oxide nanotube layers adjusted to about 1.1–1.2 ␮m. We show that by WO3 doping, the near visible photoresponse and photocatalytic performance can drastically be enhanced. While a content of 9 at% WO3 in photoresponse experiments is most beneficial, in long term experiments a higher efficiency is observed for the 0.2 at% W content. This is due to a gradual leaching of WO3 (dissolution into the electrolyte) for the higher WO3 content. This demonstrates that under optimized WO3 doping conditions a lasting visible light activation of TiO2 nanotubes can be achieved. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction In the last decade anodic TiO2 nanotubes (TiNTs) are one of the most widely studied semiconducting nanostructures due to a strikingly simple synthesis route [1] and wide ranging potential applications. TiO2 nanotubes are explored in virtually every field where TiO2 nanoparticles are already in use, such as dye sensitized solar cells [2,3], photo catalysis [4,5], self-cleaning material [6,7], photonic crystals [8], gas sensing devices [9,10], and biomaterials [11–13]. However, photonic or photoelectrochemical applications of TiO2 using solar light are generally hampered by the large bandgap (3.2 eV) that allows only for a use of approx. 7% of the solar light. Therefore, a large body of research work targets band-gap engineering of TiO2 in order to access also the visible light range [14], in particular for photocatalytic applications. Visible light activation of TiO2 nanotubes has been reported using treatments with carbon [15,16], ammonia [17], urea [18], or doping with Cr [19] or sulphur [20] etc. The performance of the tubes can also be increased by decoration with different nano materials, such as loading with Ag and Au or WO3 nanoparticles [21,22]. A most straightforward method to dope the oxide of the tube is the use of Ti alloys containing other desired metals in the electrochemical anodization process. This leads to formation of nanotubes consisting of the oxides of the alloying elements. Such mixed oxide

∗ Corresponding author. Tel.: +49 9131 852 7575; fax: +49 9131 852 7582. E-mail address: [email protected] (P. Schmuki). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.05.061

nanotubes can show a significantly enhanced photocatalytic performance compared with neat TiO2 nanotubes [23–25]. Recently we showed that the UV photocatalytic activity of TiO2 can most strongly be enhanced by WO3 doping [26]. In the present work we study the photocatalytic properties of the WO3 doped TiO2 in the visible range and characterize the photoelectrochemical behavior and stability of WO3 doped nanotube systems.

2. Experimental Substrates for anodic nanotube growth were titanium foils (thickness 0.1 mm, 99.6% purity), and Ti–W alloys of two different compositions 0.2 at% W and 9 at% W. Ti–0.2 at% W (Ti–0.2W) and Ti–9 at% W (Ti–9W) were obtained from Advent Materials and GKSS Forschungszentrum in Geesthacht, Germany, respectively. Prior to tube growth, samples were degreased by successive sonication with acetone, isopropanol and methanol, and rinsed with deionized (DI) water. Then the samples were dried in a nitrogen stream. To achieve a comparable thicknesses of ∼1.1–1.2 ␮m of the nanotube layers, the anodic tubes on Ti, Ti0.2W and Ti9W, were grown using an electrolyte of glycerol:water (50:50) + 0.27 M NH4 F at 20 V for 2 h (Ti), ethylene glycol + 0.2 M HF at 120 V for 12 min (Ti9W), and ethylene glycol + 0.2 M HF at 120 V for 10 min (Ti0.2W) respectively [27,28]. After anodization, the oxide tubes were immersed in ethanol for 24 h and dried in a nitrogen stream. To convert samples from amorphous to anatase, thermal annealing at 450 ◦ C was performed for 1 h in air.

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The morphology of the samples was characterized in a fieldemission scanning electron microscope (Hitachi FE-SEM S4800). Photocatalysis experiments were carried out by measuring the decay kinetics of Rhodomine-B at room temperature using near visible light of wavelength  = 405 nm (Dragon Laser MDL III405 9030014) in a dark room. For this, the concentration of active dye was measured in regular intervals of time at  = 554 nm using a UV/VIS spectrophotometer (Lambda XLS, Perkin Elmer). Rate constant for dye decay kx were obtained using Langmuir–Hinshelwood (mechanism) equation C = C0 exp(−kx t). The electrochemical cell used for all the photo current measurements had a quartz glass window and contained the three electrodes connected to a potentiostat Jaissle Modell 1002 T-NC. The counter and reference electrodes were platinum and Ag/AgCl (3 M KCl), respectively. For photoelectrochemical characterization we used illumination with a 150 W Xe arc lamp (LOT-Oriel Instruments, Stanford, CT) combined with a 1/8m monochromator (Oriel Cornerstone 7400). Photocurrent spectra were recorded at 500 mV and 1000 mV (vs. Ag/AgCl electrode).

3. Results and discussion Oxide nanotube layers were grown on the metallic substrates as explained in the experimental section. Fig. 1 shows SEM images of the self-organized TiO2 and TiW (Ti0.2W and Ti9W) mixed oxide nanotubular layers. The anodization conditions were adjusted to obtain comparable dimensions of nanotubular layers, i.e. a tube length of ∼1.2 ␮m (obtained from SEM cross sections) and diameters of ∼80–100 nm. In the case of the TiW alloys, as reported previously, a thin porous initiation layer is present on the top of nanotubular layer [28] as apparent in Fig. 1b and c. Fig. 2a shows the photocurrent spectra of TiO2 , Ti0.2W and Ti9W with an applied bias of 500 mV in a 0.1 M H2 SO4 electrolyte. It can be seen that the addition of tungsten lowers the photocurrent in the UV region compared with pure TiO2 nanotubes. However, for the W-containing samples the photocurrent increases in the near visible region. The higher the tungsten content, the higher is the visible response. From the inset of Fig. 2a, a replot of the data for an indirect band gap [29–31] yields bandgap energies (Eg ) of (Ti9W) ∼2.83 eV; (Ti0.2W) ∼3.01 eV; (TiNT) ∼3.03 eV. It is noteworthy that although only the Ti9W sample shows a distinct energy gap, also for the 0.2W sample a clear tail to lower energies is apparent. Fig. 2b shows photocurrent transients recorded using a laser of a wavelength of 405 nm in 0.1 M H2 SO4 at 500 mV and 1000 mV. Fig. 2c shows the potential dependence of the photocurrent obtained from the steady state current from each transients. In the range from 0 V to 1 V vs. Ag/AgCl, the trend of the photocurrent magnitude is the same. The photocurrent response increases clearly at the potential greater than 300 mV and it is the highest for the Ti–9W nanotubes whereas TiO2 shows only a very low response. It is remarkable, however, that the potential dependence of the response shows an exponential shape. This is in contrast to the case where super-band gap illumination is used (inset of Fig. 2c); i.e. for  > Eg (TiO2 ) a typical Gärtner-type of voltage dependence [Iph ∝ (U − Ufb )0.5 ] is obtained [32] which is also in line with earlier work on TiO2 nanotubes. The experimental behavior leads to high photocurrent for Ti9W at high bias, e.g. the photocurrent nearly tripled at 1000 mV applied bias (of Iph ∼ 11 ␮A) compared with 500 mV (Iph ∼ 3 ␮A). Such a characteristic has in the literature been ascribed to a Poole–Frenkel type of mechanism [33,34]. In this case trapping of electron or excitation to a trapped state is fast. The photocurrent there is determined by the escape probability of trapped electrons to the conduction band (i.e. to overcome the activation energy to the conduction band). An applied field lowers the activation energy in the field direction in an exponential manner

Fig. 1. SEM images of TiNT (a), Ti–0.2W oxide nanotubes (b) and Ti–9W oxide nanotubes (c). The inset shows the top views of the samples.

[35], while processes against the field become accordingly hampered, as schematically illustrated in Fig. 2d. Generally, tungsten oxide states are reported to lie within the band gap of TiO2 (some 0.2–0.8 eV below the conduction band of TiO2 ) [36]. Therefore an interpretation of interband states (or surface states with a corresponding energetic position), that can be filled with electrons (from the TiO2 valence band) upon excitation with the 405 nm (3.06 eV) laser, seems plausible. A Poole–Frankel behavior may thus indicate that the escape of electron to the TiO2 conduction band would determine the sub bandgap photocurrent. Disregarding some uncertainties of this mechanism the results show clearly a significant visible photoresponse. In order to test if this activation of photocurrent also activates the materials to be visible light photocatalysts, dye decay experiments were performed for all three samples using an excitation wavelength of 405 nm. Fig. 3a shows the decay of Rhodamine-B (RhB) dye in

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Fig. 2. Photocurrent spectra of TiO2 , Ti–0.2W and Ti–9W in 0.1 M H2 SO4 at 500 mV applied potential and inset represents the band gap evaluation of all the three samples (a), photocurrent transient of TiNT, Ti–0.2W and Ti–9W respectively at an applied potential of 500 mV and 1000 mV vs. Ag/AgCl (b), photocurrent with the potential from 0 mV to 1000 mV in 0.1 M H2 SO4 at a laser wavelength of 405 nm: the inset – represents the potential dependent photocurrent at 350 nm for TiNT and Ti9W in 0.1 M H2 SO4 (c). Schematic band gap diagram of TiO2 with illustrating a Poole–Frankel type of behavior indicated are the activation energies against the field EA and with the field E A (d).

0.1 M Na2 SO4 for the TiNT, Ti–0.2W and Ti–9W oxide nanotubes. Clearly, the Ti0.2W oxide nanotubes show the highest photocatalytic activity (kTi0.2W = −0.440 h−1 ) in comparison with Ti9 W (kTi9W = −0.143 h−1 ) and TiNT (kTiNT = −0.163 h−1 ). This illustrates that the tungsten oxides in the structure not only establish a visible photocurrent but also strongly enhance the visible photocatalytic activity. Nevertheless it is, at first sight, surprising that the highest photocatalytic activity is obtained for the 0.2% sample (i.e. not for the 9% sample that showed the highest photocurrent). The rea-

son for this effect becomes evident from long term photocurrent experiments as shown in Fig. 3b and c. For the Ti9W tubes the photocurrent signal is not stable over time but significantly decays from ∼3.5 ␮A to ∼1.0 ␮A in 0.1 M H2 SO4 electrolyte after approximately 40 min. In contrast, the Ti0.2W shows an almost constant photocurrent response with time. For the 0.1 M Na2 SO4 electrolyte a similar behavior is observed. In both electrolyte systems the photocurrent of the Ti–9W oxide nanotubes, over time, drops to values below the Ti–0.2W oxide nanotubes. This result can be explained by the

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Fig. 3. Photocatalytic degradation of Rhodamine-B (RhB) dye using a 405 nm wavelength laser (a), photocurrent response with respect to time with applied bias of 500 mV in 0.1 M H2 SO4 (b) and 0.1 M Na2 SO4 (c).

fact that WO3 oxides are not very stable in aqueous electrolytes but quite soluble [37–39], i.e. the higher content WO3 -oxides will have a strong tendency to loose WO3 by leaching. As a consequence of this leaching effect, in photocatalytic experiments, that run for a considerably long time, the activity of the Ti–9W oxide is strongly deteriorated. As the photocurrent measurements in cases such as Fig. 2b are usually performed on a much shorter time scale, they are much less affected by leaching.

Fig. 4. Photocurrent transient before and after CH3 OH addition with applied bias of 500 mV using 405 nm laser; for TiO2 nanotubes (a), Ti–0.2W oxide nanotube (b) and Ti–9W oxide nanotubes (c), photocatalytic degradation of Rhodamine-B (RhB) dye using 405 nm wavelength with 500 mV applied bias.

Accordingly at higher pH values an even faster decay of the photoresponse can be observed in line with the expectations [40], that accelerated leaching takes place at alkaline pH-values (for Ti–9W and even for Ti0.2W).

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This leaching effect observation is in line with earlier XPS works that indicated that anodiyation of Ti–W alloys leads to a composite oxide, where the tungsten phase remains essentially as WO3 [41]. Another interesting point in the context of TiO2 and WO3 electrodes is the addition of methanol of the electrolyte. Frequently, enhanced hole capturing [42] and current doubling [43,44] effects have been reported for methanol containing electrolytes. Fig. 4a–c shows photocurrent transients of TiNT, Ti–0.2W and Ti–9W oxide nanotubes for a 0.1 M H2 SO4 electrolyte with and without 2 M CH3 OH. Clearly, the photocurrent excited at 405 nm increases for the tungsten containing samples strongly in CH3 OH containing electrolyte. The highest photocurrent amplification is observed for the highest W content. As the photocatalytic effect on the oxide surface (without applied potential) basically could be attributed to a conduction band mechanism (e− ejection to the electrolyte and peroxide formation) or to a valence band mechanism (h+ ejection and OH radical formation). Some additional photocatalytic decay measurements were performed under applied anodic bias to separate the effect. Anodic bias is expected to accelerate the e− –h+ separation, lead to a increased transport of the holes to the surface (e− away from the surface) and thus to promote the h+ -pathway. Fig. 4d shows that in fact the application of a positive bias has no significant effect on the photocatalytic performance for the Ti9W samples and leads even to a deteriorating effect on the Ti0.2W material. This is in contrast to a strong accelerating effect if super band gap illumination (>3.2 eV) is used [45]. One may therefore deduce that while for the pure TiO2 nanotube electrodes and illumination with UV light a valence band mechanism is prevalent, the beneficial effect introduced by tungsten and visible light activation is related to near conduction band states and electron transfer to the electrolyte. Several approaches are discussed in literature to account for the observation [46,47,36] of a beneficial effect of WO3 addition to TiO2 nanoparticles. The strong effect of WO3 to the photocatalytic activity is mainly due to the enhanced hole-transfer kinetics to the electrolyte. This is in line with literature [26,48] which suggests the presence of WO3 in TiO2 to influence the recombination rate of the photogenerated electron–hole (e− –h+ ) pairs that may be either due to localized heterojunction formation [49] (due to mismatch of the TiO2 and WO3 band energies) or to the formation of activating surface species such as W(VI) states. The present work clearly favors the formation of specific surface features such as W(VI) states that act as mediators for charge transfer to the electrolyte. Nevertheless, under applied bias photocurrent amplification by methanol addition can be observed that is increasing with W content, indicating that under applied anodic potential indeed hole transfer is crucial and current multiplication reactions occur which are facilitated by WO3 in the oxide structure.

4. Conclusions In the present work, investigations of the visible photoresponse and the photocatalysis of WO3 doped titania nanotubes were performed. Clearly, WO3 addition leads to an enhanced photoresponse and a faster visible photocatalytic decomposition kinetics for organics. We ascribe the beneficial effect of WO3 to the photoelectrochemical response to a conduction band effect. We propose that these states can be filled by visible irradiation and are able to contribute to the photocurrent by field aided thermal excitation to the TiO2 conduction band. For the photocatalytic activity these states lead to a facilitated electron transfer kinetics to the

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