Electronic structure of TiO2 nanotube arrays from X

View Online / Journal Homepage / Table of Contents for this issue

PAPER

www.rsc.org/materials | Journal of Materials Chemistry

Electronic structure of TiO2 nanotube arrays from X-ray absorption near edge structure studies J. G. Zhou,a H. T. Fang,b J. M. Maley,d M. W. Murphy,c J. Y. Peter Ko,c J. N. Cutler,a R. Sammynaiken,d T. K. Sham,*c M. Liue and F. Lie

Downloaded by Institute of Metals Research, CAS on 28 April 2012 Published on 24 July 2009 on http://pubs.rsc.org | doi:10.1039/B909225K

Received 11th May 2009, Accepted 26th June 2009 First published as an Advance Article on the web 24th July 2009 DOI: 10.1039/b909225k We report an X-ray absorption near edge structure (XANES) investigation of several TiO2 nanotube arrays, including the as-prepared nanotube arrays from electrochemical anodic oxidation of Ti foil (as-prepared ATNTA), as-prepared nanotube arrays after annealing at 580 C (annealed ATNTA) and annealed ATNTA after electrochemical intercalation with Li (Li-intercalated ATNTA). XANES at the O K-edge and Ti L3,2 and K edges shows distinctly different spectral features for the as-prepared and the annealed ATNTA, characteristic of amorphous and anatase structures, respectively. Intercalation of Li into annealed ATNTA induces a surprising, yet spectroscopically unmistakable, anatase to rutile transition. XANES at the Li K-edge clearly shows ionic features of Li in ATNTA. The charge relocation from Ti 3d to O 2p at the conduction band in TiO2 was also observed when Li ions were intercalated into annealed ATNTA albeit no noticeable reduction of Ti4+ to Ti 3+ was observed. The O K-edge shows a distinctly enhanced feature in the multiple scattering regime, indicating a close to linear O–Li–O arrangement in Li-intercalated ATNTA. These results show bonding changes between Ti and O resulting from the interaction of Li ions in the TiO2 lattices. Such bonding variation has also been supported by X-ray excited optical luminescence (XEOL), which suggests Li+-defect interactions. The implications of these results are discussed.

I.

Introduction

Nanostructured TiO2, such as nanoparticles, nanotubes and nanowires, have been demonstrated as important functional materials in environmental applications,1 energy fields (solar cells and Li ion batteries),2,3 and electrochromic devices.4 TiO2 nanotube arrays were made through electrochemical oxidation of a Ti foil, henceforth denoted ATNTA (anodic TiO2 nanotube arrays). The unique configuration of TiO2 nanotube arrays on Ti foil5,6 is extremely desirable for practical applications, such as photoassisted water splitting,7,8 dye-sensitized solar cells,9,10 photodegradation,11 field emitters,12 biosensors,13 gas sensors,14–16 cathodic protection,17 etc. ATNTA can be used directly as an ideal electrode, in which Ti foil is the current collector and TiO2 serves as the active material. We have synthesized ATNTA and studied its applications in Li ion batteries18 and hybrid electrochemical capacitors,19 and find ATNTA has merits as an anode material in Li ion rechargeable energy storage devices in terms of high rate capability. Additional insights into the electronic structure of as-prepared ATNTA, annealed ATNTA and Li-intercalated ATNTA will improve the understanding of this material, a Canadian Light Source Inc, University of Saskatchewan, Saskatoon, Canada S7N 0X4 b School of Material Science and Engineering, Harbin Institute of Technology, 150001 Harbin, China c Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7. E-mail: [email protected] d Saskatchewan Structural Sciences Centre, University of Saskatchewan, Canada e Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China

6804 | J. Mater. Chem., 2009, 19, 6804–6809

which will facilitate engineering design. Thus it is crucial to evaluate the structure and electronic properties of ATNTA so that the optimum structures for specific applications can be realized. X-Ray absorption near edge structure (XANES) is a spectroscopic technique using synchrotron radiation. It follows the dipole selection rules. Since the transition-matrix elements involve the core level and the unoccupied electronic states with Dl ¼ 1 (l: angular momentum) character, XANES is thus an element, site, and local structure specific method in mapping the unoccupied partial densities of states. Therefore XANES is very useful in studying electronic structure under favorable conditions, such as low z elements or shallow core levels, and the surface chemistry of many nanomaterials.20–25 With the absorption of X-ray photons, light emitting material can convert the X-ray energy it absorbs into optical photons via a radiative de-excitation channel. A spectroscopic technique that tracks this X-ray photon-in, optical photon-out process is known as X-ray excited optical luminescence (XEOL).22 XEOL can be used to evaluate the nature and distribution of defects in nanomaterials. Here we report the XANES and XEOL studies of ATNTA and Li-doped ATNTA at the Ti L3,2, O K and Li K-edges for further understanding the electronic structure changes in ATNTA associated with the annealing treatment as well as the effects of Li intercalation.

II. Experimental procedure ATNTA was prepared by anodic oxidation of a Ti metal foil5 and is henceforth referred to as ‘‘as-prepared ATNTA’’. The This journal is ª The Royal Society of Chemistry 2009

View Online

spectrometer. Raman spectroscopy measurements were conducted at Saskatchewan Structural Science Center using a Renishaw System 2000 for additional structural confirmation.

III.

Results and discussion

a. Structure and bonding of ATNTA from XANES at the Ti L3,2, O K and Ti K-edges before and after annealing


Fig. 1 XRD spectra of as-prepared ATNTA and annealed ATNTA.

as-prepared ATNTA after rinsing with de-ionized water was annealed in air at 580 C for 3 h to attain the crystalline state and is henceforth called ‘‘annealed ATNTA’’. X-Ray powder diffraction (XRD) of the as-prepared ATNTA shows the diffraction pattern of the substrate metal while XRD for the annealed ATNTA shows a dominant anatase pattern with some rutile features as shown in Fig. 1. The intercalation of Li ions into annealed ATNTA was conducted in an electrochemical cell where Li foil was the counter electrode, 1M LiClO4 in ethylene/ dimethyl carbonate (1:1 in volume) was the electrolyte and annealed ATNTA was discharged at 1A/g to 1.0 V (vs Li+/Li) to reach full intercalation. The ATNTA thus obtained is henceforth denoted ‘‘Li-ATNTA’’. SEM images of annealed ATNTA in Fig. 2 show that TiO2 nanotubes thus obtained are vertically oriented and have a diameter of 60 nm and a length of 1.0 mm. The details about the synthesis and characterization of ATNTA have been reported elsewhere.18 XANES spectra were obtained at the Canadian Light Source (CLS) on the spherical grating monochromator (SGM) beamline (6E/E: 104) for the Ti L-edge and the O K-edge and on the plane grating monochromator (PGM) beamline (6E/E: 104) for the Li K-edge. XANES spectra were recorded in the surface sensitive total electron yield (TEY) using specimen current and bulk sensitive fluorescence yield (FY) using a multi-channel plate detector. Data were normalized to the incident photon flux. After background correction, the spectra at the Ti L3,2-edge and O K-edge were normalized to the edge jump, the difference in absorption coefficient just below and at a flat region above the edge (485 eV and 575 eV for Ti and O, respectively). XEOL was conducted on SGM with photon energy at the O K-edge (532 eV). XEOL spectra were recorded using an Ocean Optics QE 65000

Fig. 2 SEM of annealed ATNTA showing the morphology of the oriented TiO2 nanotubes, (a) top view and (b) side view.

This journal is ª The Royal Society of Chemistry 2009

The Ti L3,2-edges of the as-prepared and annealed ATNTAs are shown in Fig. 3a. Previously reported theoretically simulated XANES26 for TiO2 in an octahedral environment and experimental results27 on bulk samples of rutile and anatase (the two common crystal structures of TiO2 resulting from a tetragonal distortion from Oh symmetry) are shown in Fig. 3(b). Ti2O3 (Ti3+ model compound) is also shown in Fig. 3(b) for comparison. We can see from Fig. 3a and b that for all TiO2 XANES, there are a couple of weak pre-edge shoulder peaks, denoted s1 and s2, followed by two series of sharp resonances denoted by a1, a2 and b1 b2, for the apparent L3 and L2 edges, respectively. Before we proceed further, it is useful to note that due to the small spin– orbit coupling in the 2p shell of 3d transition metals in general, and in the early elements such as Ti in particular, together with a sizable crystal field when the metal ion forms a compound, the Ti L3,2-edge XANES results from transitions from 2p3/2 and 2p1/2 to the final state of Ti 3d5/2,3/2 character (Dl ¼ 1, Dj ¼ 0, 1) are modified markedly by the combined spin–orbit and crystal field

Fig. 3 (a) Ti L32-edge XANES of as-prepared ATNTA and annealed ATNTA. (b) Theoretical Ti L32-edge XANES of TiO2 in an Oh environment (ref. 26) and model compounds rutile and anatase (ref. 27).

J. Mater. Chem., 2009, 19, 6804–6809 | 6805


View Online

interactions. Therefore the L3 and L2 features cannot be straightforwardly interpreted by the dipole selection rules and have to be looked at collectively including spin–orbit interaction and crystal field splitting since the eigenstates are no longer truly atomic in nature. Returning to Fig.3b, the splitting within each edge into multiple peaks is due to spin orbital and crystal effects.26 For example, the relative intensities of the L3-edge doublets a1 and a2 (similar for L2-edge) and their energy separation are related to the strength of the crystal field (10 Dq). In general, the larger the crystal field, the more intense the a1 peak relative to a2, the larger the separation.26 For a tetragonal distortion, the local symmetry at the Ti site is reduced to D4h and D2d for rutile and anatase, respectively. The result of this distortion on the XANES is to further split the second peak of the L3 resonance a2 into a20 and a200 .26,27 Of particular interest is the relative intensities of the doublet a20 and a200 ; i.e. that a20 > a200 in anatase and a20 < a200 in rutile. These characteristic intensity features are used to assign the dominant structure in ATNTA in this work. From Fig. 3a, we see that the Ti L3,2-edge XANES of annealed ATNTA exhibits an intensity pattern of a20 > a200 similar to that of anatase (Fig. 3b), we can then determine the dominant structure of the annealed ATNTA as anatase with confidence;26,27 in the XANES of the as-prepared ATNTA nearly all absorption features are blurred relative to the crystalline samples. These features reflect the highly disordered and amorphous character in the as-prepared ATNTA. However, the lack of a20 and a200 splitting indicates that there is no significant tetragonal distortion locally. The O K-edge shown in Fig. 4 probes the O 2p projected unoccupied density of states in the conduction band. Due to hybridization between O 2p and Ti 3d or Ti 4sp, the spectrum features: 1) transitions into the O 2p–Ti 3d hybridized bands which split into peaks a and b because of the crystal field, and 2) transitions into O 2p–Ti 4sp hybridized bands (peaks c and d).28,29 Again, the anatase character28,29 is observed in the O K-edge spectrum of annealed ATNTA. The higher energy resonance peak e is also only observed in annealed ATNTA, which again reflects the crystalline nature of the annealed sample (long range order). Interestingly, despite the smoothed out features, some local order remains in the as-prepared (amorphous) ATNTA, as was also seen in the Ti L3,2-edge XANES.

Careful examination of the Ti L3,2-edge XANES in Fig. 3a shows that the intensity of peak a1 is lower than that of a2 in the as-prepared ATNTA, while the trend is opposite in the annealed ATNTA. This suggests a weaker crystal field or more undercoordinated Ti atoms (for instance, four-fold coordinated Ti ion with elongated axial oxygen ligands)26 in the as-prepared ATNTA than in the annealed ATNTA; this is consistent with the observation that the separation between a1 and a2 is smaller than the separation between a1 and the centroid of a20 and a200 , since theory shows that the relative intensity (a1/a2) increases with the crystal field (10 Dq), as does the energy separation between a1 and a2 (a1 and a2 merge into one peak in the absence of a crystal field).26 Similar observations can be made in the O K-edge XANES in Fig. 4 where the XANES for the as-prepared sample appears like a much broadened XANES for the annealed ATNTA. This observation indicates that despite the dominant amorphous features some local order remains at both Ti and O sites in the as-prepared ATNTA though longer range order is clearly absent (lack of peak e in the as-prepared sample).

Fig. 4 O K-edge XANES of as-prepared ATNTA and annealed ATNTA.

Fig. 5 Li K-edge XANES of (a) Li-intercalated ATNTA and (b) Li2CO3 and LiCoO2.

6806 | J. Mater. Chem., 2009, 19, 6804–6809

b. Structure and bonding of Li intercalated annealed ATNTA from Li, Ti and O XANES Fig. 5a and b shows the Li K-edge XANES of ATNTA after Li-intercalation and the XANES of LiCoO2 (Li is well characterized to be in the ionic state30) and Li2CO3 in both FY and TEY, respectively. We can see from Fig. 5a that the Li-intercalated ATNTA exhibits the characteristic sharp features at



View Online

62 eV for ionic lithium although the TEY (at pre-edges at 59 eV and 60 eV) indicates that some surface Li2CO330 formed during storage in the ambient. The more intense feature at 62 eV observed in FY than in TEY also indicates that Li+ doping takes place throughout the ‘‘bulk’’. The effect of Li intercalation has been tracked by comparing the XANES at the Ti L3,2-edge and O K-edge between annealed ATNTA and Li-intercalated ATNTA in Fig. 6. Along with the Li ion intercalation into the interstitial sites in anatase TiO2, electrons are doped into the lattice (to the empty conduction bands) to balance the charge introduced by Li ion electrochemical intercalation so that electro-neutrality is maintained.31 Such a process is expected to alter the electronic structure in the annealed ATNTA (anatase TiO2). Ti L3,2-edge and O K-edge spectra in Fig. 6a and b respectively, clearly exhibit the modification of electronic structure when Li ions were intercalated into the annealed ATNTA. From Fig. 6a, several observations are noted and conclusions can be drawn from the Ti L3,2-edge XANES: first, the increase of the white line in Li intercalated ATNTA shows a DOS increases in the conduction band which indicates the charges are withdrawn from Ti 3d projected orbital considering the polycrystalline nature of annealed ATNTA (assuming no prevailing symmetry arguments for a randomly oriented sample); secondly, other than peaks of a20 and a200 , all resonances including the preedge peaks and the transition into the Ti 4sp32 (magnified in inset) remain unchanged after intercalation. The alteration in

peak intensity ratio of a20 /a200 indicates a different Ti coordination shell33 in ATNTA distorted by Li intercalation. A comparison of the XANES of Li-intercalated ATNTA in Fig. 6a with those of anatase and rutile in Fig. 3b reveals that the structure after Li intercalation is dominantly rutile, a perhaps surprising result. Although the conventional wisdom is that Li+ prefers to intercalate into anatase rather than rutile as anatase has a lower density, this is not necessary the case for ATNTA with a large surface area and thin tube wall of less than 20 nm. In fact, high Li electroactivity of nano-rutile has recently been reported.34 Another interesting observation is that electron doping does not reduce Ti4+ into Ti3+ at a detectable level. Reduction to Ti3+ almost certainly involves the localization of charge in Ti 3d orbitals35 and will shift the absorption peaks to lower photon energy.26 Therefore it can be concluded that Li intercalation under the conditions of the present study only alters the interaction between Ti and O ions in ATNTA, not their individual charge states. Such alteration can be elucidated by the change in O K-edge XANES after Li ion intercalation as shown in Fig. 6b where the O K-edge XANES for annealed ATNTA before and after Li intercalation, together with the corresponding XANES for rutile and anatase (inset) are shown.36 The XANES for ATNTA after Li intercalation exhibits a reduction in O 2p–Ti 3d bands, which implies charge relocation from Ti 3d to O 2p (consistent with the more intense white line at the corresponding XANES at the Ti L-edge in Fig. 6(a)). Such charge relocation infers the possibility that Li intercalation into ATNTA is making the Ti–O bonds more ionic, which agrees with the conductivity decrease in lithiated anatase TiO2.37 Finally, it is interesting to note that there is a much enhanced feature in the O K-edge XANES for the Li-intercalated ANTNA at 541 eV (peak c). This feature may be attributed to the result of a focusing effect due to the presence of Li in the outgoing path of the O photoelectron wave in a nearly linear O–Li–O arrangement.35 The Li–O interaction can also be associated with XEOL as shown in Fig. 7 where we see that XEOL of the annealed ANTNA has two luminescence bands centered at 500 nm and 800 nm without the expected near band gap transition (380 nm38). The absence of a near band gap excitonic transition is associated with the presence of effective non-radiative and radiative decay channels at longer wavelengths. The green and red luminescences are

Fig. 6 Ti L3,2 (a) and O K-edge (b) XANES of annealed ATNTA and Li intercalated ATNTA (inset, ref. 36).

Fig. 7 XEOL of as-prepared and Li-intercalated ATNTA with excitation at 532 eV.


J. Mater. Chem., 2009, 19, 6804–6809 | 6807

View Online

Acknowledgements We thank Dr L. Zuin, Dr R. Blyth and Mr T. Regier at CLS for their technique assistance. The work at the University of Western Ontario was supported by NSERC, CFI, OIT and CRC (TKS). Research at HIT was supported by National Science Foundation of China (Grant No. 5060211 and 50872026). H. T. Fang also acknowledges the support of a Senior Visiting Fellowship from the Centre for Chemical Physics at UWO.


References

Fig. 8 Raman of annealed and Li-intercalated ATNTA, rutile features (R) become significant after intercalation, although some anatase features (A) remain.

tentatively assigned to bulk and surface defects, respectively. XEOL is obviously changed markedly when Li is intercalated into annealed ATNTA as can be seen in Fig. 7 where luminescence at 500 nm is quenched entirely and the red emission is blue shifted (750 nm). The details of this shift remain to be unraveled but it is almost certainly associated with the interaction of Li+ with oxygen vacancies. We have also obtained Raman results on the annealed and Li-intercalated ATNTA as shown in Fig. 8. It is evident from Fig. 8 that upon Li intercalation, the anatase dominant ATNTA shows rutile features. The residual anatase signal detected in Raman is most likely due to the different sampling depths between XANES (angle dependent XANES at the Ti L-edge shows similar a20 /a200 ratios in all Ti L-edge XANES in both TEY and FY) and Raman.

IV Summary of results and their implications Ti L3,2-edge and O K-edge XANES have been used to study ANTNA, a TiO2 nanotube array prepared electrochemically from a Ti metal foil. We found from the analysis of the XANES results using established theory and XANES of model TiO2 rutile and anatase structures that the as-prepared ATNTA is dominantly amorphous. Upon annealing at 580 C, ATNTA crystallizes into a dominantly anatase structure, as revealed from the signature of the a20 /a200 intensity ratio at the Ti L3-edge. The intercalation of Li into the annealed ATNTA results in the appearance of a rutile phase, as its Ti L3-edge shows the unmistakable rutile signature in its a20 /a200 intensity ratio while the corresponding O K-edge shows the focusing effect of interstitial Li. The presence of Li in the Li-intercalated ATNTA was confirmed by Li K-edge XANES. Li intercalation into ANTNA modifies the surroundings of the Ti atoms and facilitates charge redistribution from Ti 3d to O 2p (making the bonding of Ti–O more ionic). The bonding variation before and after Li intercalation was also consistent with XEOL observation involving interaction of Li with defects. The intercalation of Li ions in the present study does not result in detectable Ti3+ states in ATNTA. 6808 | J. Mater. Chem., 2009, 19, 6804–6809

1 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269. 2 C. H. Jiang, I. Honma, T. Kudo and H. S. Zhou, J. Power Sources, 2007, 166, 239. 3 A. R. Armstrong, G. Armstrong, J. Canales and P. G. Bruce, Adv. Mater., 2005, 17(7), 862. 4 R. Hahn, A. Ghicov, H. Tsuchiya, J. M Macak, A. G. Mu~ noz and P. Schmuki, Phys. Stat. Sol. A, 2007, 204, 1281. 5 J. M. Macak, H. Tsuchiya, L. Taveira, S. Aldabergerova and P. Schmuki, Angew. Chem. Int. Ed., 2005, 44, 7463. 6 W. J. Lee, M. Alhoshan and W. H. Smyrl, J. Electrochem. Soc., 2006, 153, B499. 7 G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese and C. A. Grimes, Nano Lett., 2005, 5, 191. 8 J. K. J. H. Park, J. H. S. Kim and A. J. Bard, Nano Lett., 2006, 6, 24. 9 J. M. Macak, H. Tsuchiya, A. Ghicov and P. Schmuki, Electrochem. Commun, 2005, 7, 1133. 10 G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese and C. A. Grimes, Nano Lett., 2006, 6, 215. 11 Y. K. Lai, L. Sun, Y. C. Chen, H. F. Zhuang, C. J. Lin and J. W. Chin, J. Electrochem. Soc., 2006, 153, D123. 12 G. Liu, F. Li, D. W. Wang, D. M. Tang, C. Liu, X. L. Ma, G. Q. Lu and H. M. Cheng, Nanotechnology, 2008, 19, 025606. 13 S. Q. Liu and A. C. Chen, Langmuir, 2005, 21, 8409. 14 O. K. Varghese, D. W. Gong, M. Paulose, K. G. Ong, E. C. Dickey and C. A. Grimes, Adv. Mater, 2005, 15, 624. 15 Y. Y. Zhang, W. Y. Fu, H. B. Yang, Q. Qi, Y. Zeng, T. Zhang, R. X. Ge and G. T. Zou, Appl. Surf. Sci., 2008, 254, 5545. 16 H. F. Lu, F. Li, G. Liu, Z. G. Chen, D. W. Wang, H. T. Fang, G. Q. Liu, Z. H. Jiang and H. M. Cheng, Nanotechnology, 2007, 19, 405504. 17 H. Y. Li, X. D. Bai, Y. H. Ling, J. Li, D. L. Zhang and J. S. Wang, Electrochem. Solid-State Lett., 2006, 9, B28–B31. 18 H. T. Fang, M. Liu, D. W. Wang, T. Sun, D. S. Guan, F. Li, J. G. Zhou, T. K. Sham and H. M. Cheng, Nanotechnology, 2009, 20, 225701. 19 D. W. Wang, H. T. Fang, F. Li, Z. G. Chen, Q. S. Zhong, G. Q. Lu and H. M. Cheng, Adv. Funct. Mater, 2008, 18, 3787. 20 Y. H. Tang, T. K. Sham, Y. F. Hu, C. S. Lee and S. T. Lee, Chem. Phys. Lett, 2002, 366, 636. 21 Y. H. Tang, P. Zhang, P. S. Kim, T. K. Sham, Y. F. Hu, X. H. Sun, N. B. Wong, Fung, Y. F. Zheng, C. S. Lee and S. T. Lee, Appl. Phys. Lett., 2001, 79, 3773. 22 X. T. Zhou, F. Heigl, M. W. Murphy, T. K. Sham, T. Regier, I. Coulthard and R. I. R. Blyth, Appl. Phys. Lett., 2006, 89, 213109; T. K. Sham and R. A. Rosenberg, ChemPhysChem, 2007, 2557; T. K. Sham, Int. J. Nanotechnology, 2008, 5, 1194. 23 J. G. Zhou, X. T. Zhou, X. H. Sun, M. Murphy, F. Heigl, T. K. Sham and Z. F. Ding, Can. J. Chem., 2007, 85, 756. 24 X. T. Zhou, J. G. Zhou, M. W. Murphy, J. Y. P. Ko, F. Heigl, T. Regier, R. I. R. Blyth and T. K. Sham, J. Chem. Phys., 2008, 128, 144703. 25 J. G. Zhou, X. T. Zhou, X. H. Sun, R. Y. Li, M. Murphy, Z. F. Ding, X. L. Sun and T. K. Sham, Chem. Phys. Lett., 2007, 437, 229. 26 G. S. Henderson, X. Liu and M. E. Fleet, Phys. Chem. Minerals, 2002, 29, 32. 27 F. M. F. de Groot, J. C. Fuggle, B. T. Thole and G. A. Sawatzky, Phys. Re. B, 1990, 41, 928.


View Online

34 35 36 37 38

S. A. Gammon and L. J. Terminello, Phys. Rev. B, 2004, 69, 245102. S. Y. Hu, L. Kienle, Y. G. Guo and J. Maier, Adv. Mater, 2006, 18, 1421. M. V. Koudriachova and S. De Leeuw, Phys. Rev. B, 2004, 69, 054106. Z. Y. Wu, G. Ouvrard, P. Gressier and C. R. Natoli, Phys. Rev. B, 1997, 16, 10382. R. Van de Krol, A. Goossens and E. A. Meulenkamp, J., Appl Phys, 2001, 90, 2235. Y. Liu and R. O. Claus, J. Am. Chem. Soc., 1997, 119, 5273.


28 S. J. Stewart, M. Fernandez-Garcia, C. Belver, B. S. Mun and F. G. Requejo, J. Phys. Chem. B, 2006, 110, 16482. 29 R. Bryson, H. Sauer, W. Engel, J. M. Thomas, E. Zeitler, N. Kosugi and H. Kuroda, J. Phys. Conden. Matter, 1989, 1, 797. 30 H. Kobayashi, S. Emura, Y. Arachi and K. Tasumi, J. Power Sources, 2007, 174, 774. 31 J. W. Xu, C. H. Jia, B. Cao and W. F. Zhang, Electrochem. Acta, 2007, 52, 8044. 32 G. van der Lann, Phys. Rev., 1990, B 41(17), 12366. 33 S. O. Kucheyev, T. van Buuren, T. F. Baumann, J. H. Satcher, Jr., T. M. Willey, R. W. Meulenberg, T. E. Felter, J. F. Poco,


J. Mater. Chem., 2009, 19, 6804–6809 | 6809