Nanotube Arrays - American Chemical Society

Article pubs.acs.org/JPCC

Density of Deep Trap States in Oriented TiO2 Nanotube Arrays Qian Zhang,†,‡ Verónica Celorrio,† Kieren Bradley,†,§,∥ Flurin Eisner,∥ David Cherns,∥ Wei Yan,‡ and David J. Fermín*,† †

School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, U.K. Department of Environmental Science & Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China § Bristol Centre for Functional Nanomaterials, Nanoscience and Quantum Information Building, Tyndall Avenue, Bristol BS8 1FD, U.K. ∥ School of Physics, HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, U.K. ‡

S Supporting Information *

ABSTRACT: Correlations between the population of deep trap states in an array of TiO2 nanotubes (NT) and the dynamic photocurrent responses under supra-band-gap illumination are investigated. Ordered arrays of TiO2 NT of 10 μm length, 125 nm inner diameter, and 12 nm wall thickness featuring strong anatase character were obtained by anodization of Ti in ethylene glycol solution containing NH4F. Cyclic voltammograms at pH 10 show the characteristic responses for nanostructured TiO2 electrodes, in particular a sharp cathodic peak as the electron density in the film increases. These responses are associated with the population of deep trap states with an average value of 5 × 104 electrons per NT. Dynamic photocurrent measurements clearly show that the characteristic rise time of the photocurrent increases as the potential is increased above the onset region for charging deep trap states. At potentials in which deep trap states are fully depopulated in the dark, photocurrent rise time approaches values just below 1 s, which is more than 3 orders of magnitude slower than the estimated RC time constant. The occupancy of the deep trap states under photostationary conditions is a fraction of the density of states estimated from voltammetric responses. These findings are discussed in the context of current views about trap states in high surface area TiO2 electrodes.

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INTRODUCTION Nanostructured TiO2 films are among the most versatile platforms for constructing excitonic photovoltaic systems, high surface area photoanodes, and sensing and display devices.1 The complex carrier transport properties of these materials are often identified as one of the key parameters in determining the efficiency of these systems. As the dimensions of the crystalline domains become smaller than the characteristic Debye length, carrier transport is essentially determined by diffusion. The effective carrier diffusion in nanostructured materials can be affected by the dynamic of trapping/detrapping at band-gap states.2 In the case of TiO2, there have been numerous efforts in identifying and quantifying carrier trap states employing a variety of techniques including photoluminescence,3−5 visible and IR spectroelectrochemistry,6−8 dynamic photoelectrochemical measurements,2,9−12 time-resolved THz spectroscopy,13 and impedance spectroscopy.14−20 Yet, uncovering the chemical nature of these states remains controversial. Electron trap states in nanostructured TiO2 films are described in terms of a density of states (DOS) which decreases exponentially from the conduction band edge, commonly referred to as the conduction band tail (CBT), as well as deep trap states exhibiting a narrow energy range.2,16,21−23 While the CBT is associated with disordered © 2014 American Chemical Society

semiconductor materials, deep trap states have been linked to low coordinated Ti sites. In electrochemical studies, the appearance of a sharp cathodic peak as the electrode Fermi energy is swept toward the conduction band edge has been linked to deep trap states.24,25 Gomez and co-workers have investigated these responses in TiO2 nanoparticle and nanotubes assemblies in various electrolyte solutions,26 concluding that deep trap states are mainly associated with grain boundaries. Recently, Halverson et al. have provided evidence that proton intercalation strongly affect the distribution of subband-gap states in nanoporous TiO2 films, leading to a decrease in the activation energy for electron trapping.11 Hole trap states are commonly associated with surface hydroxyl states, although recent studies suggest that these traps are linked to oxygen vacancies in the 101 plane.3 Hole trapping dynamics is crucial for rationalizing the performance of TiO2 for water splitting and oxidation of organic compounds.27 It has been shown that surface recombination at rutile single crystals during water photo-oxidation is effectively quenched in the presence of organic species such as formic acid and methanol,28 Received: May 23, 2014 Revised: July 8, 2014 Published: July 9, 2014 18207

dx.doi.org/10.1021/jp505091t | J. Phys. Chem. C 2014, 118, 18207−18213

The Journal of Physical Chemistry C

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RESULTS AND DISCUSSION Structural Analysis of the NT Arrays. Figure 1 shows a set of SEM images corresponding to the annealed TiO2 NT

suggesting that fast hole capture via hydroxyl states leads to the photo-oxidation of the organic species. Indeed, methanol is commonly employed as hole scavenger in photoelectrochemical studies of TiO2 photoanodes.29 In this work, we investigate dynamic photoelectrochemical responses originating from photoexcitation of ordered arrays of TiO2 nanotubes (NT) as a function of the potential bias in an aqueous solution. NT arrays are generated by Ti anodization in NH4F containing ethylene glycol (EG) solutions, followed by annealing in air. Voltammetric responses of the NT arrays at pH 10 exhibit the characteristic sharp cathodic peak at potentials close to the onset of electron accumulation in the conduction band. Our data show a clear correlation between the electron occupation at these deep trap states and the photocurrent rising time associated with the photo-oxidation of SO32−, a powerful hole scavenger.

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Article

EXPERIMENTAL SECTION

Preparation and Characterization of TiO2 NT Arrays. Prior to anodization, 0.25 mm thick titanium foil (SigmaAldrich, >99.7% purity) pieces were cleaned by successive 15 min sonication steps in acetone, ethanol, and Milli-Q water, followed by drying with a stream of high-purity Ar. The anodization was conducted based on the method developed by Grimes and co-workers,30 employing a two-electrode configuration in EG (≥99%, Sigma-Aldrich) containing 0.3 wt % NH4F (≥98%, ACS reagent) and 3 vol % Milli-Q water. A constant potential of 60 V was applied for 1 h at room temperature. After anodization, the as-prepared samples were thoroughly rinsed with Milli-Q water to remove surface debris and dried with high-purity argon. As-grown NT arrays were annealed in air at 500 °C for 2 h with a ramp rate of 1 °C min−1. The morphological analysis of the NTs was conducted employing a JEOL field emission gun SEM 6330 and a JEOL 120 kV 1200 Mk1 TEM microscope. X-ray diffraction was carried out on Bruker-AXS D8 Advance equipment using Cu Kα source (λ = 0.154 16 nm). Electrochemical Studies. Electrochemical and photoelectrochemical experiments were performed in a single compartment cell provided with a quartz window to minimize attenuation of the UV light source. A platinum foil and a Ag/ AgCl (3 M KCl) were used as secondary and reference electrodes, respectively. For electrochemical experiments in the dark, working electrodes with a geometric surface area of 0.071 cm2 were employed. Photocurrent studies were recorded in electrodes with a 1.0 cm2 geometric surface area. Cyclic voltammetry and electrochemical impedance spectroscopy were recorded with an Ivium Compactstat. A 375 ± 10 nm InGaN LED (Thorlabs) was employed as the light source for all photoelectrochemical experiments. The photon flux impinging on the electrochemical cell was estimated using a calibrated Si photodiode (Newport Corporation). Corrections for reflection losses from the cell window and electrode surface were not considered in these studies. Dynamic photocurrent responses were measured under a square wave light perturbation. In order to examine the effect of the light perturbation frequency, the photocurrent amplitude was measured with a home-built potentiostat and a lock-in amplifier (Stanford Research Systems SR830).

Figure 1. Scanning electron microscopy images a cross section (a), top (b), and bottom (c) of TiO2 NT arrays obtained by anodization in NH4F containing ethylene glycol electrolyte at 60 V for 2 h.

arrays after the annealing step. The cross-sectional image (Figure 1a) shows well-aligned NT arrays with a length of 10 ± 2 μm. At the top of the NT arrays, a relatively narrow distribution of inner tube diameter of 125 ± 5 nm (Figure 1b) and wall thickness of 12 ± 2 nm can be observed. Figure 1c shows a cross section of the array closer to the Ti substrate, indicating that the inner diameter decreases toward the bottom of NT. The geometry of the NT arrays is consistent with previous reports employing similar anodization conditions.30,31 XRD analysis of the NT arrays as grown and after annealing in air at 500 °C are displayed in Figure 2. The fact that only diffraction peaks associated with Ti are observed in the asgrown samples indicates that the anodized material is essentially amorphous. After annealing, the NTs exhibit a strong anatase character with peaks associated with the 101, 004, and 200 planes. The broadening of the XRD peaks suggests that NTs are composed of small crystalline domains. Using the Scherer equation as a first approximation, average crystallite size of approximately 25 nm can be estimated from the 101 peak. Further evidence of the nanocrystalline nature of the NT arrays can be obtained from the TEM images in Figure 3. The dark field measurement in Figure 3a shows a strong contrast in segments of the NT, indicating the presence of strongly 18208



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Figure 2. XRD patterns of as-grown NT arrays and after annealing in air at 500 °C for 2 h. The most prominent peaks of the annealed material can be assigned to the 101, 004, and 200 planes of anatase.

diffracting single crystalline domains. High-resolution images of a wall segment (Figure 3b,c) show crystalline domains in the range of 25−50 nm. The average d-spacing estimated from the lattice fringes in Figure 3c is 3.52 ± 0.07 Å, consistent with the anatase 101 plane. This analysis confirms that each individual NT consists of a mosaic of nanoscopic anatase crystallites, rather than extended single crystal structures. Charging Deep Trapping States by Potential Bias. Figure 4a displays characteristic cyclic voltammograms of NT arrays at various scan rates in Ar-saturated 0.1 M Na2SO3 electrolyte at pH 10. As the potential is swept toward negative values, a strong increase of the capacitive current can be observed along with a distinctive peak located at around −0.3 V. The peak position shifts to slightly more negative potentials as the scan rate is increased. A significantly broader anodic peak is observed in the reversed scan in the same potential range. According to a recent communication by Bertoluzzi et al., these characteristic voltammetric features can be rationalized in terms of filling of narrow deep trap states (sharp cathodic peak) and a relatively slow depopulation of these states (broader anodic peak).15 Indeed, fast trapping/detrapping kinetics involving deep states would manifest itself by symmetric anodic and cathodic peak currents whose position would be little affected by the potential scan rate. It should also be mentioned that no current responses are observed at positive potential, indicating the presence of a strong blocking layer preventing the oxidation of SO32− in solution. The dependence of the charge (Qtrap) associated with the cathodic peak at −0.3 V as a function of the scan rate (ν) is shown in Figure 4b. In the calculation of Qtrap, we performed a linear subtraction of the broader background current response. Based on this approach, Figure 4b shows a slight decrease in Qtrap with increasing scan rate. As mentioned above, the sharp cathodic response is consistent with the population of deep trap states in the milliseconds time scale, which is in agreement with the main conclusions drawn by Gomez and co-workers.26 This description also relates well with the analysis recently described by Bisquert and co-workers, who pointed out that deconvolution of responses associated with deep and shallow trap states is not trivial as the demarcation level has a dynamic nature.16 As discussed further below, the population of these states has a profound effect on dynamic photocurrent responses under periodic light perturbation. Dynamic Photocurrent Responses. Photocurrent−potential curves under periodic square wave light perturbation

Figure 3. TEM images of NT: dark field image highlighting crystalline segments of NT (a) and high-resolution images of a NT segment (b). Zooming into the area highlighted by the red box in (b) allowed imaging an individual nanocrystallite (c). The characteristic spacing of the lattice fringes in the crystalline domain is consistent with the 101 plane of bulk anatase.

(375 nm) of the NT arrays are shown in Figure 5. Figure 5a contrasts the photocurrent density (normalized by the geometrical surface area) in the presence of Na2SO4 and Na2SO3 at various photon fluxes. The photocurrent onset potential for the oxidation of SO32− is located at potentials significantly more negative than in the case of water photooxidation (SO42− containing electrolyte). This behavior indicates that SO32− acts as a strong hole scavenger, minimizing recombination losses at the NT surface. This result is consistent with previous studies at n-CdS single crystal32 and BiVO4 electrodes.33 The other striking feature in Figure 5a is the appearance of a photocurrent peak at −0.25 V in SO32− containing solutions. For water photo-oxidation, this peak is observed at potentials above 0.5 V. The decrease in the photocurrent amplitude at 17 Hz light perturbation cannot be 18209



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Figure 4. Cyclic voltammograms NT arrays at various scan rates in 0.1 M Na2SO3 at pH 10 (a). The current responses are normalized by the geometric surface area of the electrodes. Charge density associated with the cathodic peak at −0.3 V (Qtrap) as a function of scan rate (b). Charge density was obtained by integrating the peak current using a linear subtraction of the background current. As discussed in the text, the charge density is related to the DOS of electron deep trap levels.

Figure 5. Photocurrent−potential curves recorded under square wave perturbation at 17 Hz and lock-in detection in the presence of 0.1 M Na2SO4 (black open symbols) or 0.1 M Na2SO3 (red solid symbols) at various photon fluxes (a). Photocurrent−voltage curves in 0.1 M Na2SO3 with a photon flux of 1.35 × 1015 cm−2 s−1 and various perturbation frequencies (b).

rationalized in terms of surface recombination processes, considering that the electron density in the NT decreases as the electrode potential is swept toward positive potentials. In order to focus on the dynamics of trap population, the rest of the discussion will concentrate on photocurrent responses in the presence of SO32−. Photocurrent−potential curves as a function of the light perturbation frequency in SO32− solutions are illustrated in Figure 5b. A clear increase of the photocurrent amplitude at more positive potentials can be observed as the frequency decreases. The frequency dependence of the photocurrent amplitude at positive potentials is indicative of a slow response of the photoelectrode upon illumination. Interestingly, the onset potential for the slow dynamic photoresponses overlaps with the potential range in which deep trap states are depopulated (Figure 4a). Figure 6a displays a set of photocurrent transients in 0.1 M Na2SO3 solution at various electrode potentials. The photocurrent magnitude after 5 s increases with increasing applied potential up to −0.1 V, where it reaches saturation (photostationary state). In the context of conventional semiconductor/ electrolyte interfaces, photocurrent saturation indicates that surface recombination is essentially negligible. The photocurrent magnitude in the saturation region is linearly dependent on the photon flux (results not shown), which is also consistent with an efficient hole capture by the SO32− at the NT/ electrolyte interface. The decrease of the photocurrent magnitude at more negative potential reflects an increase in the probability of recombination events as the electron density in the NT increases.

Figure 6b shows the photocurrent transients normalized by the steady state photocurrent. A rather interesting trend can be observed with regards to the photocurrent time constant in the on and off transients. The time constant sharply decreases between −0.5 and −0.3 V, followed by a noticeable increase up to potentials above 0 V. As discussed below, the increase of the photocurrent rise time constant at positive potentials cannot be explained in terms of RC time constant of the cell, as the phenomenological capacitance sharply decreases in this potential range. Considering that surface recombination is minimized by fast hole scavenging involving SO32−, the dynamic photocurrent responses can be rationalized in terms of electron diffusion to the back contact and the kinetics of trapping/detrapping from deep states. The rest of the discussion will focus on the latter parameter and how this is affected by the applied potential. Figure 7a contrasts the observed photocurrent rise time constant (τon) with the RC time constant as a function of the applied potential. τon was estimated by adjusting the “on” photocurrent transient to the general expression

jph = g[1 − e−t / τon]

(1)

where g corresponds to the steady state photocurrent. On the other hand, the RC time constant was estimated from electrochemical impedance spectra recorded at various potentials (see Supporting Information, Figure S1). The impedance spectra exhibit the characteristic transmission line behavior as described in the literature.34−36 From the high 18210



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Figure 7. Phenomenological photocurrent rise (τon) and RC time constants for the NT array as a function of the applied potential in 0.1 M Na2SO3 and a photon flux of 1.35 × 1015 cm−2 s−1 (a). Charge associated with depopulation of deep trap states (Qph,detrap) obtained by integration of the photocurrent transient up switching off illumination (b).

Figure 6. Photocurrent transient measurements at various electrode potentials in 0.1 M Na2SO3 and a photon flux of 1.35 × 1015 cm−2 s−1 (a). Photocurrent transients normalized by the steady state photocurrent density (g), highlighting the effect of bias potential on the rise time (b).

required in order to effectively deconvolute both processes. This could be performed using intensity modulated photocurrent spectroscopy and photoelectrochemical impedance spectroscopy. These approaches will be discussed in a separate work. The effect of introducing constant bias illumination on transient photocurrent responses further illustrates the role of deep trap states (see Supporting Information, Figure S2). The photocurrent rise time at 0.0 V systematically increases as the bias light, superimposed to the square wave perturbation, increases. On the other hand, no significant changes on τon are observed with bias illumination at potentials below −0.3 V. These trends illustrate the interplay between light and potential bias on the occupancy of deep trap states and how this manifests itself on the dynamic photoresponses. At potentials in which the occupancy of deep trap states in the dark is low, the photocurrent dynamics is essentially controlled by the trapping and detrapping rates as well as the rate of carrier generation. In the case of slow collection on minority carriers at the surface, e.g. photogeneration of oxygen, recombination losses will also play a key role in the photocurrent dynamics. However, there is no clear evidence of surface recombination in the case of SO32−, particularly at very positive potentials. The excess charge stored at deep trap levels under photostationary conditions (Qph,derap) is shown in Figure 7b as a function of the potential bias. This charge is estimated from integration of photocurrent relaxation during the “off” transients. As the Fermi level is decreased below the deep trap level (positive potentials), Qph,derap reaches a saturation value close to 3 × 10−5 C cm−2. This charge is approximately

frequency limit of the impedance data, the uncompensated resistance (R) was estimated to be 20 Ω. The sharp increase in the RC time constant with decreasing applied potentials is determined by the effective capacitance estimated from the low frequency limit of the impedance spectra. This behavior is also consistent with the potential dependent on the so-called chemical capacitance, which contains contributions from shallow and deep trap levels. In the absence of electron trapping events, the parameter τon should approach the RC time constant of the cell. As shown in Figure 7a, this trend is observed toward negative potentials. As the potential is increased above 0.0 V, τon becomes more than 3 orders of magnitude larger than the RC time constant. Again, the divergence in the potential dependence of τon and RC intensifies at the onset potential for depopulation of deep trap states. Based on the previous observation, it could be postulated that τon at positive potentials is linked to the rate constant of electron trapping at deep states. The time constant for electron trapping rate saturates at potentials at which the states are effectively depopulated in the dark. This is consistent with the plateau in the photocurrent amplitude at a given frequency of perturbation at potentials above 0.0 V (see Figure 5). Similar time constants for the detrapping process can be obtained from the “off” transients. However, a close examination of the transient responses reveals that describing the “on” and “off” transients in terms of a single time constant is a crude approximation. In the range in which the RC and trapping time constants are of the same order a more accurate approach is 18211



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voltammetric data in the dark (Figure 4b) and the average NT number density (8 × 109 cm−2), it can be estimated that up to 5 × 104 electrons can be stored in deep trap states in a single NT at negative bias. Under the conditions investigated in this report, approximately 50% of these states are occupied once the photocurrent reaches steady state conditions under illumination. We believe this number may vary depending on the illumination level, the presence electron acceptor species in solution (e.g., O2), and the possibility of electron−hole recombination. Finally, these studies illustrate the effectiveness of dynamic measurements employing periodic light perturbation and lockin detection in identifying the presence of deep trap states. In the presence of a strong hole scavenger (SO32−), photocurrent−potential curves are characterized by a maximum in the range in which the population of trap states is dependent on the applied potential. At positive potential (fully depleted trap states), the photocurrent amplitude strongly decreases with increasing frequency of light perturbation. We have recently reported a similar phenomenological behavior in ZnO nanocrystalline rods.38 As illustrated in Figure 4, this approach requires efficient collection of minority carriers at the semiconductor surface, minimizing the dynamic effects associated with surface recombination.28

50% of the total charge estimated from integration of the voltammetric responses associated with the deep trap levels (see Figure 4b). This observation suggests that the electron occupancy in the deep traps under constant illumination is determined not only by the position of the Fermi level but also by the carrier generation rate and the rate constants of electron trapping and detrapping. Jennings and co-workers have formulated an elegant model incorporating these issues in the context of dye-sensitized TiO2 NT solar cells.12 Finally, an issue not explicitly discussed in this work is the energy difference between deep trap states and the conduction band edge of the NT. From the seminal work by Rothenberger et al.,8 the onset potential for spectroscopic features associated with conduction band electrons in anatase nanoparticles was estimated close to −1.0 V vs Ag/AgCl. Jankulovska et al. concluded that the potential associated with the conduction band edge of anatase nanowires is between −1.2 and −1.3 V,37 although a long band tail was also proposed extending by 0.5 eV into the band gap. In our studies, the photocurrent onset potential in the presence of SO32− strongly suggests that the conduction band edge potential is located in the range of −0.6 V vs Ag/AgCl. Consequently, the deep trap state is centered between 0.3 and 0.4 eV below the conduction band edge. The cyclic voltammograms in Figure 4 are characterized by that the broadening of the capacitive current occurs at potentials more positive than the peak associated with the deep trap levels. These responses do suggest a degree of overlap between the states associated with the CBT and the deep trap level. In this respect, it could be argued that the complex electronic structure in the vicinity of the conduction band edge energy is responsible for the apparent dispersion in the values reported for the conduction band edge. This complex picture is likely to be correlated to the level of crystallinity and the properties of nanoscale grain boundaries.

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ASSOCIATED CONTENT

S Supporting Information *

Impedance spectra at various electrode potentials and transient photocurrent measurements at 0.0 V and various bias illuminations (Figures S1 and S2, respectively). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +44 117 9288981; e-mail [email protected] (D.J.F.).

CONCLUSIONS These studies highlight a clear correlation between the population deep trap states in aligned TiO2 NTs with photocurrent responses under band-gap illumination in the presence of strong hole acceptors in solution. The electron population in the deep trap levels can be tuned by the applied potential, manifesting itself by a sharp cathodic peak at potentials close to the onset of electron accumulation in the film. In the potential range where deep trap levels are fully depopulated the phenomenological photocurrent time constant (τon) is several orders of magnitude slower than the RC time constant of the cell as obtained from impedance measurements. These slow photocurrent responses are associated with the rate of filling of deep trap levels upon illumination. In the particular conditions investigated, τon can reach values as slow as 1 s. Bertoluzzi et al. have recently estimated values of the order of 25 ms in assemblies of highly crystalline submicrometer anatase particles employing fast scan voltammetric measurements.16 On the other hand, depopulation of trap states in the second time scale have been observed in dye-sensitized nanoparticle solar cells.2 These observations seem to confirm that the origin of the deep trap levels is strongly correlated with the microstructure of the TiO2 film. Indeed, our high-resolution TEM images show that the NTs are essentially composed of small nanocrystalline domains. Another important observation highlighted in these studies is the fraction of trap levels populated under photostationary conditions. Considering the charge densities obtained from

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Q.Z. acknowledges the support by the Chinese Scholarship Council, and V.C. is indebted to the Royal Society and the UK National Academy for the financial support via the Newton Fellowship programme. K.B. and D.J.F. are grateful for the support from the Bristol Centre for Functional Nanomaterials (EPSRC Centre for Doctoral Training).

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REFERENCES

(1) Kamat, P. V. TiO2 Nanostructures: Recent Physical Chemistry Advances. J. Phys. Chem. C 2012, 116, 11849−11851. (2) Peter, L. M. The Gratzel Cell: Where Next? J. Phys. Chem. Lett. 2011, 2, 1861−1867. (3) Mercado, C. C.; Knorr, F. J.; McHale, J. L.; Usmani, S. M.; Ichimura, A. S.; Saraf, L. V. Location of Hole and Electron Traps on Nanocrystalline Anatase TiO2. J. Phys. Chem. C 2012, 116, 10796− 10804. (4) Mercado, C. C.; Knorr, F. J.; McHale, J. L. Observation of Charge Transport in Single Titanium Dioxide Nanotubes by Micro-Photoluminescence Imaging and Spectroscopy. ACS Nano 2012, 6, 7270− 7280. (5) Knorr, F. J.; McHale, J. L. Spectroelectrochemical Photoluminescence of Trap States of Nanocrystalline TiO2 in Aqueous Media. J. Phys. Chem. C 2013, 117, 13654−13662. 18212



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(25) Wang, H.; He, J.; Boschloo, G.; Lindström, H.; Hagfeldt, A.; Lindquist, S.-E. Electrochemical Investigation of Traps in a Nanostructured TiO2 Film. J. Phys. Chem. B 2001, 105, 2529−2533. (26) Jankulovska, M.; Berger, T.; Wong, S. S.; Gómez, R.; LanaVillarreal, T. Trap States in TiO2 Films Made of Nanowires, Nanotubes or Nanoparticles: An Electrochemical Study. ChemPhysChem 2012, 13, 3008−3017. (27) Bertoluzzi, L.; Bisquert, J. Equivalent Circuit of Electrons and Holes in Thin Semiconductor Films for Photoelectrochemical Water Splitting Applications. J. Phys. Chem. Lett. 2012, 3, 2517−2522. (28) Fermin, D. J.; Ponomarev, E. A.; Peter, L. M. Dynamics of Photo-Processes at the n-TiO2 Aqueous Electrolyte Interface. In Photoelectrochemistry; Electrochemical Society Proceedings Vol. 97-20; Rajeshwar, K., et al., Eds.; The Electrochemical Society: Pennington, NJ, 1997; pp 62−71. (29) Lynch, R. P.; Ghicov, A.; Schmuki, P. A Photo-Electrochemical Investigation of Self-Organized TiO2 Nanotubes. J. Electrochem. Soc. 2010, 157, G76−G84. (30) Prakasam, H. E.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. A New Benchmark for TiO2 Nanotube Array Growth by Anodization. J. Phys. Chem. C 2007, 111, 7235−7241. (31) Shankar, K.; Basham, J. I.; Allam, N. K.; Varghese, O. K.; Mor, G. K.; Feng, X.; Paulose, M.; Seabold, J. A.; Choi, K.-S.; Grimes, C. A. Recent Advances in the Use of TiO2 Nanotube and Nanowire Arrays for Oxidative Photoelectrochemistry. J. Phys. Chem. C 2009, 113, 6327−6359. (32) Fermín, D. J.; Ponomarev, E. A.; Peter, L. M. A kinetic Study of CdS Photocorrosion by Intensity Modulated Photocurrent and Photoelectrochemical Impedance Spectroscopy. J. Electroanal. Chem. 1999, 473, 192−203. (33) Kim, T. W.; Choi, K. S. Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990−994. (34) Bisquert, J. Chemical Capacitance of Nanostructured Semiconductors: Its Origin and Significance for Nanocomposite Solar Cells. Phys. Chem. Chem. Phys. 2003, 5, 5360−5364. (35) Fabregat-Santiago, F.; Bisquert, J.; Garcia-Belmonte, G.; Boschloo, G.; Hagfeldt, A. Influence of Electrolyte in Transport and Recombination in Dye-Sensitized Solar Cells Studied by Impedance Spectroscopy. Sol. Energy Mater. Sol. Cells 2005, 87, 117−131. (36) Fabregat-Santiago, F.; Mora-Sero, I.; Garcia-Belmonte, G.; Bisquert, J. Cyclic Voltammetry Studies of Nanoporous Semiconductors. Capacitive and Reactive Properties of Nanocrystalline TiO2 Electrodes in Aqueous Electrolyte. J. Phys. Chem. B 2003, 107, 758−768. (37) Jankulovska, M.; Berger, T.; Lana-Villarreal, T.; Gómez, R. A Comparison of Quantum-Sized Anatase and Rutile Nanowire Thin Films: Devising Differences in the Electronic Structure from Photoelectrochemical Measurements. Electrochim. Acta 2012, 62, 172−180. (38) Bunzli, C.; Parker, D.; Bradley, K.; Fermin, D. J. Deep Surface Trap States at ZnO Nanorods Arrays. In MRS Spring Meeting Proceedings, 2014; Eickhoff M., Ed., in press (MRSS14-1672-G13-04).

(6) Berger, T.; Anta, J. A. IR-Spectrophotoelectrochemical Characterization of Mesoporous Semiconductor Films. Anal. Chem. 2012, 84, 3053−3057. (7) Berger, T.; Anta, J. A.; Morales-Florez, V. Spectroscopic Properties of Electrochemically Populated Electronic States in Nanostructured TiO2 Films: Anatase versus Rutile. Phys. Chem. Chem. Phys. 2013, 15, 13790−13795. (8) Rothenberger, G.; Fitzmaurice, D.; Gratzel, M. Spectroscopy of Cconduction Band Electrons in Transparent Metal Oxide Semiconductor Films: Optical Determination of the Flat Band Potential of Colloidal Titanium Dioxide Films. J. Phys. Chem. 1992, 96, 5983− 5986. (9) Cao, F.; Oskam, G.; Meyer, G. J.; Searson, P. C. Electron Transport in Porous Nanocrystalline TiO2 Photoelectrochemical Cells. J. Phys. Chem. 1996, 100, 17021−17027. (10) Dloczik, L.; Ileperuma, O.; Lauermann, I.; Peter, L. M.; Ponomarev, E. A.; Redmond, G.; Shaw, N. J.; Uhlendorf, I. Dynamic Response of Dye-Sensitized Nanocrystalline Solar Cells: Characterization by Intensity-Modulated Photocurrent Spectroscopy. J. Phys. Chem. B 1997, 101, 10281−10289. (11) Halverson, A. F.; Zhu, K.; Erslev, P. T.; Kim, J. Y.; Neale, N. R.; Frank, A. J. Perturbation of the Electron Transport Mechanism by Proton Intercalation in Nanoporous TiO2 Films. Nano Lett. 2012, 12, 2112−2116. (12) Jennings, J. R.; Ghicov, A.; Peter, L. M.; Schmuki, P.; Walker, A. B. Dye-Sensitized Solar Cells Based on Oriented TiO2 Nanotube Arrays: Transport, Trapping, and Transfer of Electrons. J. Am. Chem. Soc. 2008, 130, 13364−13372. (13) Richter, C.; Schmuttenmaer, C. A. Exciton-Like Trap States Limit Electron Mobility in TiO2 Nanotubes. Nat. Nanotechnol. 2010, 5, 769−772. (14) Bisquert, J. Influence of the Boundaries in the Impedance of Porous Film Electrodes. Phys. Chem. Chem. Phys. 2000, 2, 4185−4192. (15) Bertoluzzi, L.; Badia-Bou, L.; Fabregat-Santiago, F.; Gimenez, S.; Bisquert, J. Interpretation of Cyclic Voltammetry Measurements of Thin Semiconductor Films for Solar Fuel Applications. J. Phys. Chem. Lett. 2013, 4, 1334−1339. (16) Bertoluzzi, L.; Herraiz-Cardona, I.; Gottesman, R.; Zaban, A.; Bisquert, J. Relaxation of Electron Carriers in the Density of States of Nanocrystalline TiO2. J. Phys. Chem. Lett. 2014, 5, 689−694. (17) Fabregat-Santiago, F.; Mora-Seró, I.; Garcia-Belmonte, G.; Bisquert, J. Cyclic Voltammetry Studies of Nanoporous Semiconductors. Capacitive and Reactive Properties of Nanocrystalline TiO2 Electrodes in Aqueous Electrolyte. J. Phys. Chem. B 2003, 107, 758−768. (18) Taveira, L. V.; Sagüeś , A. A.; Macak, J. M.; Schmuki, P. Impedance Behavior of TiO2 Nanotubes Formed by Anodization in NaF Electrolytes. J. Electrochem. Soc. 2008, 155, C293−C302. (19) Pu, P.; Cachet, H.; Sutter, E. M. M. Electrochemical Impedance Spectroscopy to Study Photo-Induced Effects on Self-Organized TiO2 Nanotube Arrays. Electrochim. Acta 2010, 55, 5938−5946. (20) Gimenez, S.; Dunn, H. K.; Rodenas, P.; Fabregat-Santiago, F.; Miralles, S. G.; Barea, E. M.; Trevisan, R.; Guerrero, A.; Bisquert, J. Carrier Density and Interfacial Kinetics of Mesoporous TiO2 in Aqueous Electrolyte Determined by Impedance Spectroscopy. J. Electroanal. Chem. 2012, 668, 119−125. (21) Nelson, J. Continuous-Time Random-Walk Model of Electron Transport in Nanocrystalline TiO2 Electrodes. Phys. Rev. B 1999, 59, 15374−15380. (22) Berger, T.; Monllor-Satoca, D.; Jankulovska, M.; Lana-Villarreal, T.; Gómez, R. The Electrochemistry of Nanostructured Titanium Dioxide Electrodes. ChemPhysChem 2012, 13, 2824−2875. (23) Bisquert, J.; Cendula, P.; Bertoluzzi, L.; Gimenez, S. Energy Diagram of Semiconductor/Electrolyte Junctions. J. Phys. Chem. Lett. 2013, 5, 205−207. (24) Boschloo, G.; Fitzmaurice, D. Spectroelectrochemical Investigation of Surface States in Nanostructured TiO2 Electrodes. J. Phys. Chem. B 1999, 103, 2228−2231. 18213


Nanotube Arrays - American Chemical Society

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