Electrical and Point Defect Properties of TiO2 Nanotubes Fabricated

ARTICLE pubs.acs.org/JPCC

Electrical and Point Defect Properties of TiO2 Nanotubes Fabricated by Electrochemical Anodization Ilie Hanzu, Thierry Djenizian, and Philippe Knauth* Aix-Marseille Universit
e—CNRS, UMR 6264: Laboratoire Chimie Provence, Centre Saint J
er^ome, F-13397 Marseille Cedex 20, France ABSTRACT:

Pure and N-doped titanium oxide nanotubes (TiO2nt) were manufactured by anodization of sputtered Ti thin films on a Si (100) substrate. Both solid state contacts and electrolyte contacts were used to investigate the properties of TiO2nt. Mott-Schottky analysis gave a flat-band potential Efb = -0.57 V vs Ag/AgCl for pure TiO2nt and Efb = -0.22 V vs Ag/AgCl for N-doped TiO2nt. The charge carrier density was ND = 6.7
1020 cm-3 for pure TiO2nt and ND = 3.9
1020 for N-doped TiO2nt. This corresponds to 1.1% oxide ion vacancies in pure TiO2nt. In nitrogen-doped TiO2nt, the decrease of donor density would correspond to 0.47% of vacancies being occupied by nitrogen acceptors. This investigation also allowed estimation of the apparent diffusion coefficient of Hþ in TiO2nt. Following the Randles-Sevcik method, the effective proton diffusion coefficient in TiO2nt is (2 ( 1) 3 10-11 cm2 s-1 while in N-doped TiO2nt it is (4 ( 1) 3 10-11 cm2 s-1. Using the Warburg diffusion element determined by electrochemical impedance spectroscopy, the proton diffusion coefficient is (2 ( 1) 3 10-11 cm2 s-1 for pure TiO2nt while for nitrogen-doped TiO2nt a value of (7 ( 3) 3 10-11 cm2 s-1 is found. These values are consistent with those of the Randles-Sevcik method.

’ INTRODUCTION Nanotubular titanium dioxide fabricated by anodization (TiO2nt) has recently gained significant attention as a material with increased versatility. Anodic self-assembled titania nanotubes (TiO2nt) have truly emerged as a functional material, and a wide spectrum of applications have already been well documented in the literature. TiO2 nanotubes show high photoelectrochemical performance under UV and visible light.1-5 They were successfully used to design dye-sensitized solar cells,6-10 to perform the photocleavage of water11-13 and were doped with nitrogen,14,15 carbon,16 and chromium,17 which modified the band gap and improved the photoelectrochemical response and other electronic properties.18,19 Titanium oxide nanotubes show ultrahigh hydrogen gas sensitivity20 with direct applications in gas sensors.21 The tubes also exhibit strong electrochromic behavior with respect to Hþ 22 and Liþ 23 insertion. Nanostructured titania24 has very recently re-emerged as a possible negative material in Li-ion batteries25 showing a high rate capability, competitive capacity, and good cycling behavior. Catalysis26 and photocatalysis,27,28 fuel cells,29,30 and biological and biomedical r 2011 American Chemical Society

applications31,32 are currently being pioneered in several research groups. It is possible to tune the nanotubes wetting properties,33,34 to adjust the tube geometry,35,36 and to produce different morphologies37 by changing the electrochemical parameters. As a function of the anodization conditions, diameters may vary between 30 and 200 nm while lengths can be adjusted between 0.2 and 1000 μm when working in nonaqueous electrolytes.38,39 More recently, it was shown that self-assembled titania nanotubes can be used as seed layers for the fabrication of vertical tin nanowires.40-42 Needless to say, such nanostructured materials with tunable aspect ratios are much desired for engineering purposes and development. The fact that self-assembled TiO2 nanotube arrays can be prepared on Si wafers43-45 constitutes another important asset of these materials opening the path toward on-chip integration of many of the applications mentioned above. Received: November 24, 2010 Revised: February 15, 2011 Published: March 14, 2011 5989

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The Journal of Physical Chemistry C However, very few of the fundamental properties and characteristics of these materials have been investigated, the research focusing so far rather on exploring possible applications than toward elucidating the intrinsic properties of self-assembled anodic TiO2 nanotubes. Electrical properties are among the most important features of any functional material. Titanium dioxide belongs to the category of wide band gap semiconductors. In pristine macroscopic form, it presents a band gap between 3 and 3.2 eV, depending on the crystal structure (anatase, rutile, brookite). However, the band gap of materials can also be tuned by adjusting the size of particles or microstructure of the solid. Since the energy of surface states is known to be quite different from the energy of bulk states, the band structure of the nanosized solid will consequently be different from the band structure of a large single crystal. The electric conductivity of a polycrystalline solid depends significantly on the ratio between the number of atoms at grain boundaries and the atoms in the grain core. The capital influence of point defects on the chemical and electrical properties of nanocrystalline titania has already been established.46-48 Amorphous nanotubular TiO2 can be prepared by various techniques, one of the most important being today electrochemical anodization. Improved knowledge concerning TiO2nt properties opens the way toward optimization of many of its potential technological applications, such as the recent new TiO2nt-based negative electrodes for Li-ion microbatteries.49-53

’ EXPERIMENTAL SECTION Mirror-polished Si wafers (SSP/E-Prime grade quality, SiTech Inc.) doped with boron (p-type dopant, F = 1-10 Ω cm) cut along the (100) crystallographic plane were cleaned with acetone, isopropanol, and methanol, in this order, for 15 min in an ultrasonic bath. The native oxide layer was then removed by dipping the Si wafers in 1 wt % HF aqueous solution for 30 s followed by rinsing with water and quick drying in a stream of Ar. The cleaned and etched Si substrates were immediately fit into the PVD (physical vapor deposition) chamber which was subsequently pumped down to around 10-6 mbar. A Ti thin layer was dc sputtered on the as-prepared Si substrate using a laboratory-made PVD device. A 99.9% Ti sputtering target was used, and an ultrapure Ar atmosphere was maintained inside the deposition chamber at a pressure of 8
10-4 mbar during deposition. When nitrogen doping of some samples was desired, N2 was injected into the sputtering chamber while controlling and setting the N2 partial pressure at 1.33
10-5mbar. Using a deposition plasma current of 150 mA, a 2 μm thick Ti deposit was obtained after 2 h of sputtering. The Ti-sputtered Si wafers were cleaved in smaller rectangular pieces using a diamond tip and then fitted into a special electrochemical cell schematically shown in Figure 1. The Ti thin film was anodized in potentiostatic regime by applying a voltage of 60 V for 30 min across the two-electrode cell using an EG&G Parstat 2273 potentiostat. No reference electrode was used for anodization. The working electrode (Ti thin film) was placed at 3 cm from the counter electrode (a large Pt foil). The anodization bath contained 96.7% glycerol, 1.3% NH4F, and 2% H2O. The bath was stirred for 24 h before the experiment in a sealed container using a magnetic stirrer in order to ensure perfect homogeneity of the bath. This procedure leads to the formation of a self-organized array of X-ray amorphous TiO2 nanotubes (80-100 nm in diameter, 3

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Figure 1. Schematic representation of the electrochemical cell used for anodization of pure and N-doped Ti thin films and for the subsequent electrochemical measurements in electrolyte solutions.

Figure 2. X-ray diffraction patterns obtained for TiO2nt layers at room temperature (R.T.) and after annealing in air at 450 °C/3 h. The reflections of anatase become observable after annealing.

μm long, with a wall thickness of 20 nm). After anodization the TiO2nt were treated with 0.1% HF for 90 s in order to remove any remaining initial compact layer formed during initial stages of anodization. For solid state measurements, the prepared TiO2nt samples were fitted again in the sputtering chamber that was vacuumed again down to 10-6 mbar. A laboratory made molybdenum target (99.7%) was used to sputter Mo contacts 2.5 mm in diameter through an adequate mask. The Mo contacts were further connected to the electrochemical instruments using a microelectronics-grade Au wire 25 μm in diameter (Goodfellow, U.K.). The electrical properties of TiO2nt in solid state and in solution were investigated by cyclic voltammetry (CV) and electrochemical impedance spectrocopy (EIS). The EIS measurements were 5990

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Figure 3. EDX spectrum of N-doped TiO2nt. Figure 5. Nyquist plots of TiO2nt in solid state for different bias values. All potentials were measured vs Mo counter electrode. The TiO2nt were around 3 μm long and 80-100 nm in diameter.

Figure 4. Cross-sectional SEM image showing the electrode configuration used to measure the electrical properties of TiO2nt in the solid state. The measurements were performed between the sputtered Mo top contact and the bottom Ti layer.

carried out using a Solartron SI 1287 Electrochemical Interface coupled with a SI 1260 Impedance Analyzer. The other electrochemical measurements were carried out using an EG&G Parstat 2273 potentiostat/galvanostat. A commercial Ag/AgCl reference electrode (Schott, Germany) was used. The structure and microstructure were investigated by scanning electron microscopy (SEM) using a Philips ESEM 130 scanning electron microscope and by X-ray diffraction with a Siemens D-5000 diffractometer using Cu KR radiation.

’ RESULTS The synthesized nanotubular titania is considered fully amorphous with respect to X-ray diffraction (Figure 2). However, some crystalline nanodomains may be present with a size smaller than the coherence length of X-ray radiation. It is possible to convert some of the TiO2nt into anatase phase by moderate annealing at 450 °C. An EDX (energy dispersive X-ray) spectrum (Figure 3) shows clearly the existence of nitrogen within TiO2 nanotubes, when

the titanium films are sputtered in the presence of nitrogen gas. However, for lighter nuclei (N, O, F) the spectrum is only qualitative. Solid State Electrical Measurements of TiO2nt. A cross section of a typical sample used for solid state electrical measurements is presented in Figure 4. The sputtered Mo top contact is of good quality, and while contacting very well the rim of the nanotubes, the Mo is not sputtered inside the nanotubes. The Ti/TiO2 contact is also expected to be of high quality, since the titania nanotubes were grown out of the Ti film. The small gap visible in Figure 4 at the level of the Ti/TiO2 interface is an artifact, which appears only when the sample is cut in order to be observed in cross section. The sample can be handled without excessive caution: for instance, the sample can be blown with compressed Ar without any peeling off or other damage to the TiO2 nanotubes. The cut samples were only used for SEM observations and not for electrical measurements. No reference electrode was used, hence establishing a classic two-electrode configuration. Characteristic impedance spectra at different bias values are shown in Figure 5. As can be seen in Figure 5, the interfacial impedance arc changes strongly with the applied bias. The conductivity was determined considering the void ratio of the nanotube. A porosity of 75% was estimated from the SEM micrographs. This corresponds to a situation in which the current passes only through 25% of the Mo contact area. The resistance can be obtained from the high frequency intercept with the real axis (R = 113 ( 4 Ω). This value might include contact resistances which cannot be deconvoluted inside the accessible frequency range and can therefore be considered an upper bound value. Using the equation σ ¼

11 l f RS

ð1Þ

where l is the TiO2nt length (3 μm), S the electrode contact area (Mo contact diameter: 2.5 mm), and f the correction factor, taking the presence of voids in TiO2nt into account (f = 0.25), 5991

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Figure 6. Bode plots (up, modulus; bottom, phase angle) of TiO2nt acquired at different bias values in aqueous electrolyte solution. All potentials were measured vs Ag/AgCl reference electrode.

one obtains a lower bound value of conductivity of 2.2
10-4 S cm-1. The relation between partial conductivity σi, charge carrier mobility ui, and charge carrier density Ni can be written as σ i ¼ qi ui Ni

Figure 7. Bode plots of TiO2nt doped with N acquired at different bias values. All potentials were measured vs Ag/AgCl reference electrode.

decreases by several orders of magnitude at high bias values where the TiO2nt is no longer a blocking electrode (Table 1). The impedance of the constant phase element (CPE) can be written as ZCPE ðωÞ ¼

ð2Þ

where qi is the elementary charge. Assuming predominant n-type conductivity and a typical electron mobility in nanostructured TiO2 (ui= 2
10-4 cm2 V-1 s-1),54 it is possible to estimate the charge carrier density in TiO2 nanotubes. Recent studies show that the low electron mobility in TiO2 nanotubes is not due to grain boundary scattering or disorder-induced localization, like in other nanomaterials, but results from exciton-like trap states.55 Using the electron mobility given above, we can estimate a lower bound value of the charge carrier density of about 2
1019 cm-3, but a more precise determination is needed. The investigation was continued by using an electrolyte solution to contact the TiO2nt rather than a metallic contact. Electrical Measurements in Aqueous Electrolyte Solution: Influence of Doping. Impedance plots of TiO2nt acquired in a carefully deaerated 1 M (NH4)2SO4 solution for pure and for nitrogen-doped samples are shown in Figures 6 and 7, respectively. Although it was not possible to fit the impedance data at all bias values using the same equivalent circuit, partial fits were carried out successfully at frequencies higher than 1 Hz which is considered sufficient for Mott-Schottky analysis.56,57 The equivalent circuit was chosen according to the work reported by the group of Sutter.58,59 It can be noticed that the behavior between 1 and 100 Hz gradually shifts from an initial capacitive behavior to a resistive one at high bias values (high cathodic potentials), a process hinted at by the gradual depression of the corresponding peak in the phase angle vs frequency diagram. This can be understood as a transition from a blocking electrode at low bias to a passing electrode at high bias where a Faradaic reaction takes place. Indeed, the charge transfer resistance (R2)

1 ðjωTÞP

ð3Þ

where P is the characteristic exponent and T the modulus. For P values sufficiently near to 1, the modulus can be considered equal to a capacitance, which can be used for Mott-Schottky analysis. The Mott-Schottky equation, which allows determination of charge carrier density (ND) and flat-band potential (Efb), can be expressed as ! ! 1 2 kT ¼ E - Efb ð4Þ C2 qεε0 ND q where the symbols have the following meaning: C, areal capacity of the space charge layer; q, elementary charge (1.6
10-19C); ε0, vacuum permittivity (8.85
10-14 F cm-1); ε, dielectric constant of the studied semiconductor; ND, concentration of donors in the studied semiconductor; E, applied external bias; Efb, flat-band potential; k, Boltzmann’s constant; T, absolute temperature. Mott-Schottky plots corresponding to pure and N-doped TiO2nt are shown in Figure 8. By extrapolating the linear parts of the Mott-Schottly plots, a value Efb = -0.57 V vs Ag/AgCl was found for TiO2nt while in the case of N-doped TiO2nt the value of the flat-band potential was Efb = -0.22 V vs Ag/AgCl. Concerning the charge carrier density, a value of ND = 6.7
1020 cm-3 was found for amorphous TiO2nt and ND = 3.9
1020 for N-doped TiO2nt. These values were calculated assuming a relative permittivity of TiO2nt ε = 100 according to literature.58 5992

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Table 1. Fitted Impedance Elements from the Impedance Spectra of TiO2nt and N-Doped TiO2nt equivalent circuit

sample type TiO2nt

TiO2nt, N-doped

bias (V vs Ag/AgCl)

R1 (Ω)

R2 (Ω)

CPE1-T (F) -5

CPE1-P

-0.1

7.4

5.00
10

3.21
10

0.93

-0.2

7.4

3.28
105

3.50
10-5

0.93

-0.3 -0.4

7.3 7.2

1.18
105 3.15
104

4.11
10-5 5.19
10-5

0.91 0.89

-0.5

7.0

9576

8.13
10-5

0.84

-0.6

6.5

1743

2.31
10-4

0.73

-0.7

6.4

90

2.71
10-4

0.70

-0.9

6.2

14

4.30
10-3

0.56

5

0

29

6.11
105

3.57
10-5

0.85

-0.1

28

4.16
105

4.89
10-5

0.80

-0.2 -0.3

26 25

1.70
105 1.38
105

9.93
10-5 2.72
10-4

0.72 0.64

-0.4

23

71

6.11
10-4

0.55

-0.5

21

12

2.61
10-3

0.53

Figure 8. Mott-Schottky plots obtained from data presented in Figure 6 and Figure 7. Both samples show n-type behavior with N-doped TiO2nt samples having a lower concentration of charge carriers because N acts as an acceptor.

Another important aspect of this study is to determine up to which maximum bias the Mott-Schottky analysis can be performed. Mott-Schottky conditions stipulate that the semiconductor electrode should be blocking, i.e., no interfacial charge transfer. However, it is known for some time that titania can accommodate various ionic species that have a small ionic radius such as protons or Li ions. Figure 9 and Figure 10 show typical CV curves in aqueous ammonium sulfate solution obtained with different sweep rates. The reduction current shows clearly two waves. An electrochemical reaction takes place at cathodic potentials lower than -0.5 V vs Ag/ AgCl. The most likely reaction ascribed to the first current wave/ peak is the insertion of protons within TiO2nt. At lower potentials the evolution of molecular hydrogen takes place at the electrode. The anodic current similarly presents two waves. The first wave seen when sweeping in anodic direction is due to the

Figure 9. Cyclic voltammograms in 1 M (NH4)2SO4 on an TiO2nt electrode at different scan rates. The blank experiment performed on Ti thin film shows that the contribution of metallic Ti to the overall current is minimal and can be neglected.

adsorbed hydrogen on the TiO2nt that has been generated during the cathodic sweep. Titania has a nonnegligible catalytic activity, and also the total surface of the nanotubes is much higher than the surface of the planar electrode. Both aspects should enhance this first oxidation wave. The second peak in the anodic direction is attributed to the proton deinsertion reaction from the TiO2nt. The electrochemical proton insertion can be written as xHþ þ TiO2 þ xe- f Hx TiO2 ð5Þ The electrons are injected into the conduction band corresponding to reduction of some Ti4þ to Ti3þ ions. For large insertion, TiO2nt becomes a degenerate semiconductor. In this case, the space charge layer will no longer be modified by the applied bias, since the Debye length is inversely proportional to 5993

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Figure 10. Cyclic voltammograms performed in 1 M (NH4)2SO4 on N-doped TiO2nt electrode at different scan rates. The blank experiment performed on Ti thin film shows that the contribution of metallic Ti to the overall current is minimal and can be neglected.

charge carrier density in the material. This change can be also observed in the impedance plots (Figure 6 and Figure 7), where below the flat-band potential the capacitive behavior at intermediate frequencies disappears, as the electrode/electrolyte interface is no longer blocking.

’ DISCUSSION The experimental results can be related to the point defect chemistry of titanium dioxide. First of all, titania tends to be oxygen-deficient at ambient temperature and pressure, and the material is thus a n-type semiconductor according to the following defect equilibrium, written in the Kr€oger-Vink nomenclature (eq 6): 1 0 OO ¼ V •• O þ O2 þ 2e 2

ð7Þ

This equation can be combined with eq 6 to obtain NO f N0 O þ h•

jpeak ¼ 2:687 3 105 n3=2 v1=2 D1=2 C

ð6Þ

Electrically charged oxide ion vacancies (V•• O) generated by the removal of oxygen from the lattice (i.e., a reduction process) are compensated by the injection of electrons in the conduction band. Electrons that are generated via this mechanism represent the majority charge carriers whose concentration can be measured in various ways, one of them being the Mott-Schottky experiment. Nitrogen doping decreases the charge carrier density, and as a consequence the conductivity of the N-doped sample is lower. This can be easily explained if we consider the following doping reaction, according to Kr€oger-Vink nomenclature: 1 0 • V •• O þ N2 f N O þ 3h 2

consistent with nitride ions on substitutional oxide sites. This reduction of electron density is clearly illustrated by the lower ND value determined in the case of nitrogen-doped TiO2nt and by the shift of the flat-band potential versus positive values, consistent with a displacement of the Fermi level from the conduction band edge versus the valence band edge. Reports of N-doped titania have also assigned other oxidation states to nitrogen, including N2-like states on either substitutional or interstitial sites with various bonding with the surrounding framework.60,61 However, the shift of the flat-band potential cannot be explained entirely by the variation of the charge carrier density (N D ). Typically, the value of the flat-band potential as a function of N D is given by a Nernst-type law. It can rapidly be deduced that the measured 0.25 V variation of the flat-band potential would correspond to a difference of about 4 orders of magnitude of N D between pure and doped samples. It is then obvious that the shift of the flat-band potential cannot be induced by N D variations alone. Another contribution is related to surface states, due to accumulated adsorbates, impurities, or defects, pinning the Fermi level and consequently the observed experimental value of the flatband potential. Assuming the density of TiO2nt is close to that of anatase (4 g cm-3) and using the previously determined donor concentration, the amount of oxygen vacancies in the nanotubes can be calculated: oxygen vacancies occupy 1.1% of the oxygen sites in the structure of pure TiO2nt. In N-doped TiO2nt, the decrease of donor density would correspond to 0.47% of vacancies occupied by N acceptors. This investigation also allows estimation of the apparent diffusion coefficient of Hþ in TiO2nt. For a diffusion-limited electrochemical process investigated by cyclic voltammetry, the relation between the value of the peak current density and the scan rate is known as the Randles-Sevcik equation62

ð8Þ

Nitride dopant ions on substitutional oxide sites are acceptors: they have an excess negative charge compensated by the creation of electron holes, because the bulk electroneutrality must be preserved. It is likely that nitrogen incorporated during Ti sputtering in N2 containing Ar atmosphere is in nitride form in TiO2nt: the determined charge carrier concentration is

ð9Þ

where the symbols have following meaning: n, number of exchanged electrons; v, scan rate (V s-1); D, diffusion coefficient (cm2 s-1); C, concentration of the analyte (mol L-1). The constant has unit 2.687
10-5 C mol-1 V-1/2 at 298 K. Although this equation was first demonstrated for diffusionlimited processes taking place in solution,62 Kulesza, Faulkner, and Cox demonstrated its applicability to proton diffusion in the solid state.63-66 Figure 11 shows the excellent linear relation obtained with our experimental data. The concentration of the redox centers (Ti4þ) in the solid TiO2nt was determined at 50 mol L-1, assuming again a TiO2nt density of 4 g cm-3. The determined effective proton diffusion coefficient in TiO2nt is (2 ( 1) 10-11 cm2 s-1 while in N-doped TiO2nt it is (4 ( 1) 10-11 cm2 s-1. At potentials below flat-band potential, the TiO2nt switch from blocking to passing behavior. A striking feature at these low potentials is that the exponent P of the constant phase element (CPE) nears a value of 0.5. This is consistent with the characteristic of a Warburg diffusion element, whose impedance expression can be written for a single charge carrier ZW ðωÞ ¼ ðjωTW Þ-0:5 5994

ð10Þ

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as shown above in eq 12. The bond dissociation energy is significantly lower for N-H bonds (340 kJ mol-1) than for O-H bonds (430 kJ mol-1)70 in accordance with the higher electronegativity of oxygen. Therefore, in a classical approach proton hopping in the presence of nitride ions might occur more easily than in the presence of oxide ions. This qualitative discussion might explain the tendency of proton diffusion coefficients between pure and N-doped TiO2nt.

Figure 11. Graphical representation of the peak current vs square root of the sweep rate used for determination of the proton diffusion coefficient in TiO2nt and in N-doped TiO2nt.

where TW is the modulus of the Warburg element, which has the following expression TW ¼

RT pffiffiffiffiffiffi 2D

n2 F 2 AC

ð11Þ

’ AUTHOR INFORMATION Corresponding Author

where the symbols have the following meaning: R, gas constant (J mol-1 s-1); F, Faraday’s constant (96 500 C mol-1); A, active electrode area (cm2); C, concentration of inserted species in the solid (mol cm-3). The concentration of inserted protons in TiO2nt can be calculated by integration of the current density peaks shown in Figure 9 with respect to time. Assuming that proton insertion occurs on oxide ion sites, as in proton-conducting oxides, according to eq 12 OO þ H• f OH•O

’ CONCLUSION The electrical and electrochemical properties of nanotubular titanium dioxide can be nicely described, assuming predominant n-type conductivity due to oxygen vacancy formation. The donor density in pure TiO2nt (6.7
1020 cm-3) corresponds to 1.1% of oxide ion vacancies. Nitrogen dopants behave as acceptors; the calculated donor density (3.9
1020 cm-3) corresponds to 0.5% nitrogen acceptors. The flat-band potential equals -0.57 V vs Ag/AgCl for pure TiO2nt and -0.22 V vs Ag/AgCl for N-doped samples. Proton insertion can be studied below flat-band potential: proton diffusion coefficients calculated from RandlesSevcik and Warburg equations are in good agreement, with an average value about 2
10-11 cm2 s-1 for pure TiO2nt and 5
10-11 cm2 s-1 for N-doped TiO2nt.

ð12Þ

this concentration corresponds to insertion in 12% of oxygen sites. This is equivalent to 24% of the total Ti4þ being reduced to Ti3þ, which is consistent with TiO2nt being a degenerated semiconductor. The developed electroactive surface of the TiO2nt electrode was estimated assuming the nanotubes are perfect cylinders 100 nm in diameter and 3 μm long. The estimated diffusion coefficient using the Warburg impedance is (2 ( 1) 10-11 cm2 s-1 for pure TiO2nt and (7 ( 3) 10-11 cm2 s-1 for N-doped TiO2nt. These values are consistent with those obtained by the RandlesSevcik method and comparable to those reported for some other oxide materials. For instance, in amorphous WO3 the effective proton diffusion coefficient is between 10-11 and 10-9 cm2 s-1.67 This material is known to present a particularly high proton insertion kinetics and is used for electrochromic devices.68 The enhancement of the proton diffusion coefficient in N-doped TiO2nt is an unexpected feature: based on ionic radius consideration, the presence of nitride ions in the lattice should hamper the movement of protons through the solid. Diffusion of protons through electrochromic oxides such as MoO369 occurs via an activated hopping process in which the proton is stabilized by the charge of oxygen in the solid, effectively forming OH•. Movement of protons then requires breaking O-H interactions

*E-mail [email protected]; tel. þ33 (0) 491 63 71 14; fax þ33 (0) 491 63 71 11.

’ ACKNOWLEDGMENT The authors thank Alain Tonetto and Roger Notonier of the Centre Commun de Microscopie de St. Charles for SEM images, Carine Chassigneux of Laboratoire Chimie Provence for kind assistance with Ti sputtering, and Alain Garnier of Laboratoire Chimie Provence for X-ray diffraction measurements. ’ REFERENCES (1) Ruan, C. M.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Sol. Energy Mater. Sol. Cells 2006, 90, 1283. (2) Shankar, K.; Paulose, M.; Mor, G. K.; Varghese, O. K.; Grimes, C. A. J. Phys. D: Appl. Phys. 2005, 38, 3543. (3) Chen, S. G.; Paulose, M.; Ruan, C.; Mor, G. K.; Varghese, O. K.; Kouzoudis, D.; Grimes, C. A. J. Photochem. Photobiol., A 2006, 177, 177. (4) Xie, Y. B. Electrochim. Acta 2006, 51, 3399. (5) Xie, Y. B.; Zhou, L. M.; Huang, H. T. Mater. Lett. 2006, 60, 3558. (6) Macak, J. M.; Tsuchiya, H.; Ghicov, A.; Schmuki, P. Electrochem. Commun. 2005, 7, 1133. (7) Mor, G. K.; Kim, S.; Paulose, M.; Varghese, O. K.; Shankar, K.; Basham, J.; Grimes, C. A. Nano Lett. 2009, 9, 4250. (8) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (9) Paulose, M.; Shankar, K.; Varghese, O. K.; Mor, G. K.; Hardin, B.; Grimes, C. A. Nanotechnology 2006, 17, 1446. (10) Hahn, R.; Stergiooulus, T.; Macak, J. M.; Tsoukleris, D.; Kontos, A. G.; Albu, S. P.; Kim, D.; Ghicov, A.; Kunze, J.; Falaras, P.; Schmuki, P. Phys. Status Solidi—Rapid Res. Lett. 2007, 1, 135. (11) Paulose, M.; Mor, G. K.; Varghese, O. K.; Shankar, K.; Grimes, C. A. J. Photochem. Photobiol., A 2006, 178, 8. (12) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191. (13) Park, J. H.; Park, O. O.; Kim, S. Appl. Phys. Lett. 2006, 89, 3. 5995

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