- Email: [email protected]
ethylene glycol electrolyte containing LiOH or KOH as photoanode for dye-sensitized solar cell
Journal of Photochemistry and Photobiology A: Chemistry 343 (2017) 33–39
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited paper
TiO2 nanotube arrays formation in fluoride/ethylene glycol electrolyte containing LiOH or KOH as photoanode for dye-sensitized solar cell Nyein Nyeina,b , Wai Kian Tanc , Go Kawamurab , Atsunori Matsudab , Zainovia Lockmana,* a Green Electronics NanoMaterials Group, School of Materials and Mineral Resources Engineering Campus, Universiti Sains Malaysia, 14300, Nibong Tebal, Penang, Malaysia b Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, Toyohashi, 441-8580, Aichi, Japan c Center for International Education, Toyohashi University of Technology, Aichi, Toyohashi, 441-8580, Japan
A R T I C L E I N F O
Article history: Received 12 January 2017 Received in revised form 6 April 2017 Accepted 12 April 2017 Available online 18 April 2017 Keywords: Anodisation TiO2 nanotube arrays Dye-sensitized solar cells
A B S T R A C T
The formation of TiO2 nanotube arrays by anodisation of Ti in fluoride/ethylene glycol (EG) electrolyte added to it lithium hydroxide (LiOH) or potassium hydroxide (KOH) at 60 V for 60 min was investigated. Highly ordered 15 mm long TiO2 nanotubes (TNTs) were produced in LiOH/fluoride/EG and 18 mm long TNTs were obtained in KOH/fluoride/EG. The as-anodized TNTs in these alkaline electrolytes were found to be crystalline and obviously annealing increases the crystallinity. Annealed TNTs were used as a photoanode in a backside illuminated dye-sensitized solar cell (DSSC). Photoconversion efficiency, h of 3% was achieved for sample with photoanode prepared in KOH/fluoride/EG higher than DSSC with TNTs photoanode fabricated in LiOH/fluoride/EG. © 2017 Published by Elsevier B.V.
1. Introduction Dye-sensitized solar cell (DSSC) requires a photoanode onto which electrons are transferred from the sensitized dyes to its surface for the generation of photocurrent. To date, mesoporous TiO2 with high surface area has been used as the photoanode [1]. However, mesoporous TiO2 has some disadvantages such as high recombination of carriers and poor charge collection efficiency [2]. To overcome this problem, several studies have been attempted in fabricating DSSCs with one-dimensional (1-D) TiO2 nanostructures such as nanowires [3], nanorods [4] and nanotubes [5,6]. Among all these nanostructures, 1-D TiO2 nanotube arrays fabricated by anodisation method have attracted great attention because the nanotubes can be made vertically aligned on a substrate. Such morphology can facilitate fast transport of electrons to the back contact [6,7], producing higher photocurrent despite not as high as expected. Moreover, high surface area nanotubes provide better dye/semiconductor interaction. In order to further enhance the efficiency of the cell; morphology, length, and crystallinity of the TNTs need to be optimized [8,9]. Often high aspect ratio and crystalline TNTs, especially in anatase phase are desired. To fabricate nanotubes with high aspect ratio by anodic process,
* Corresponding author. E-mail address: [email protected] (Z. Lockman). http://dx.doi.org/10.1016/j.jphotochem.2017.04.015 1010-6030/© 2017 Published by Elsevier B.V.
anodisation parameters have been routinely studied; particularly the choice of electrolyte, its pH [10] and the presence of water (or peroxide) in the electrolyte [11]. Voltage and time are equally important parameters that also influence the morphologies and the dimensions of nanotubes [12]. One important electrolyte ingredient for nanotubes formation is fluoride salt. In organic electrolyte, NH4F is commonly used and generally, if 0.1 wt% NH4F is added to the electrolyte, compact film is produced. On the other hand, porous, random oxide is produced if 0.5 wt% is used [13]. For the success in nanotubes formation, the wt% of NH4F has been kept within 0.1–0.5 wt%. However, apart from fluoride content, the pH of the electrolyte is also an important factor in determining the success of nanotubes formation. For instance, buffered electrolyte at pH 7 but with excessive NH4F will still facilitate the fabrication of nanotubes even though the wall of tubes are not very smooth [14]. In this present work, electrolyte with pH 9 and rather high amount of NH4F (0.7 wt%) were experimented. LiOH or KOH was added as to increase the pH of the electrolyte. We postulated that higher pH electrolyte can suppress surface etching of the nanotubes, and with an excessive amount of NH4F, chemical etching within the nanotubes at the bottom part can be made more vigorous and thus long nanotubes can be produced at a faster rate. Herein comparisons of the characteristics of anodized Ti in LiOH/fluoride/EG and KOH/fluoride/EG were made. Crystallinity of the TNTs formed was also studied and
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annealed TNTs were assembled in a DSSC as a photoanode. To date, despite several attempts on the use of TNTs as photoanode for a DSSC, most of the results show rather low conversion efficiency, h. Typically, 15–20 mm long TNTs result in h of around 0.5–2.0% [15,16] for back-side illuminated DSSC. Here higher h was recorded perhaps due to better crystallinity of the TNTs, allowing for faster electron transport as well as good adherence of the film to the substrate and incorporation of foreign species within the oxide lattice. 2. Experimental section Titanium foils (0.127 mm thickness, 99.7% purity, Stream Chemicals, USA) were degreased by sonicating in acetone, isopropanol, ethanol and deionized (DI). Electrochemical anodisation was carried out in a two-electrode cell, with Ti as the anode and a platinum rod as the counter electrode at 60 V for 60 min at room temperature. The distance between the Ti foil and Pt electrode was kept at 3 cm. 100 ml ethylene glycol (EG) added to it NH4F (0.7 wt%) and 1 M LiOH or KOH (5 wt%) were used as the electrolyte. After anodisation was completed, the samples were sonicated in acetone then rinsed in deionized (DI). Annealing was done at 450 C for 3 h to crystalize anatase TNTs. The morphologies of anodized Ti were observed by a field emission scanning electron microscope (SEM, Zeiss SUPRA 35 VP, Germany). X-ray diffraction (XRD, Bruker D8 Advance), Raman Spectrometer (Jasco NRS-3100, Japan) and High Resolution Transmission Electron Microscope (HRTEM, Hitachi H-800 and JEOL JEM-2100F, Japan) were used for crystallinity and phases study. Chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS), PHI Quantera SXM scanning X-ray microprobe (ULVAC-Phi, Inc., Japan) using an Al cathode (hn = 1486.6 eV) under a base pressure of 5 10 9 Torr. 100 W power X-ray power, 26 eV pass energy with 45 take-off angle were used. The core level spectra of Ti 2p, O ls and C ls were recorded by high-resolution scans. Smart Soft-XPS spectra were deconvoluted with GaussianLorentzian curve-fitting technique using the MultiPak software 9.0 (ULVAC- Phi, Inc, Japan). The binding energy of the XPS spectra was calibrated using C 1s (284.6 eV). Light absorption properties of the samples were investigated using V-670 ultraviolet/visible spectrophotometer (UV–vis, Jasco, Japan). To investigate the use of the anodized foil as photoanode in a DSSC, anodized Ti was immersed in 0.3 mM N719 dye (Sigma, Aldrich, USA) with acetonitrile/tert-butanol (volume ratio 1:1) for 24 h. Then, the foil was rinsed with ethanol to remove excess dyes
on its surface. Counter electrode used was platinum sputtered ITO (E-1030 ionic sputter, Hitachi, Japan) glass. Electrolyte for the DSSC was composed of 0.03 M di-iodine, 0.6 M 1–2-dimethyl-3-propylimidazolium iodide, 0.1 M lithium iodide and 0.5 M 4-tertbutylpyridine in acetonitrile. 3. Results and discussion Impedance responses of LiOH/fluoride/EG, KOH/fluoride/EG, and H2O/fluoride/EG electrolytes were done to investigate the conductivity of the electrolytes (Fig. 1(a)). The conductivity of H2O/ fluoride/EG electrolyte is 1.8 mS cm 1 and with the addition of LiOH and KOH the conductivity increases to 1.9 mS cm 1 and 2 mS cm 1, respectively. The slight increase of conductivity may be due to the presence of OH in the electrolyte. It is worth to mention here that LiOH did not completely dissolve in EG compared to KOH and conductivity measurement was done before the anodisation process was conducted i.e. the electrolyte was free from Ti4+ ions. Time-dependent anodisation current behavior in the three electrolytes: LiOH/fluoride/EG, KOH/fluoride/EG, and H2O/fluoride/EG shown in Fig. 1 (b) are rather similar. This indicates the same fundamental anodic reactions as generally described by localized dissolution model and diffusion-limited anodic growth at the pore bottom are followed by the three samples. The movement of ionic species; O2 and OH , passing through the barrier layer at the pore bottom is believed to govern the growth process. Nonetheless, the magnitude of the current density is different in different electrolytes i.e. H2O/fluoride/EG displays higher current density compared to the other two electrolytes. High current density can be related to the dissolution process whereby dissolution is thought to be faster in H2O/fluoride/EG. The addition of LiOH or KOH into the electrolyte results in the decrease of the current density hence dissolution of TNTs in such electrolyte is much suppressed. Dissolution is pH dependent whereby less dissolution is expected in alkaline electrolytes. Fig. 2 shows surface and cross-section SEM micrographs of TNTs formed in fluoride/EG electrolyte containing (a) LiOH added EG (LiOH/fluoride/EG) and (b) KOH added EG (KOH/fluoride/EG) at 60 V for 60 min. As a comparison, Ti anodized in H2O added EG (H2O/fluoride/EG) electrolyte was also fabricated and as observed, shorter TNTs that are 10 mm long were produced in this electrolyte (Fig. 1c). The addition of LiOH (Fig. 2a) or KOH (Fig. 2b) significantly increased the lengths to 15 mm and 18 mm, respectively. However, the diameter of the nanotubes does not change very much with the use of these electrolytes (170 nm). Diameter of TNTs fabricated in
Fig. 1. (a) Complex impedance spectra of different electrolytes (b) Current density-time curve for anodized Titanium at 60 V for 1 h.
N. Nyein et al. / Journal of Photochemistry and Photobiology A: Chemistry 343 (2017) 33–39
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Fig. 2. SEM images of TNTs formed in (a) LiOH/fluoride/EG (b) KOH/fluoride/EG and (c) H2O/fluoride/EG electrolytes at 60 V for 60 min.
H2O/fluoride/EG is around 150 nm. The adherence of the anodic film for this sample is also poorer compared to the other two samples. The significant role of electrolyte in TNTs formation has been established [14] whereby the electrolyte must first support oxide formation, and it has to contain adequate fluoride ions for porosification process to initiate. Electrolyte pH influences the chemical dissolution process and to achieve long TNTs, dissolution at the bottom of the pores must be encouraged while a rather protective environment is required at the surface to prevent excessive chemical etching. Excessive chemical etching results in short TNTs. In here, the addition of LiOH or KOH was observed to increase the pH of the electrolyte to 9. Since the pH is high chemical dissolution at the surface of the nanotubes is reduced, hence long nanotubes were produced. The addition of K+ or Li+ in the electrolyte can also affect the conductivity of the bulk electrolyte which also effects the growth behavior of the nanotubes. TEM images of TNTs formed in fluoride/EG electrolyte containing LiOH, KOH and H2O are shown in Fig. 3. From Fig. 3a (i) and Fig. 3b (i), the walls of the TNTs made in LiOH/fluoride/EG and KOH/fluoride/EG are seen smoother than those fabricated in H2O/fluoride/EG (Fig. 3c (i)). Lattice fringes in Fig. 3a–c (ii), with
spacing of 0.35 nm, are revealed indicating crystals of (101) anatase, in agreement with the XRD patterns of the samples (Fig. 4b). The crystallographic structures were further confirmed by SAED as shown in Fig. 3 (a, b and c (iii)). The diffraction rings can be indexed to (101), (004), (200), (105) and (204) planes of anatase, indicating the polycrystalline nature of TNTs. XRD patterns of as-anodized TNTs made in LiOH/fluoride/EG, KOH/fluoride/EG, and H2O/fluoride/EG reveal that the TNTs are amorphous (Fig. 4a). Only titanium peaks are detected. After annealing at 450 C the TNTs are transformed to anatase (Fig. 4b). Diffraction peaks are indexed at 25.4 , 37.8 , 48.5 , 54.0 , 55.2 , 68.9 , and 75.0 corresponding to anatase crystalline planes of (101), (004), (200), (105), (211), (204), (116) and (215), respectively. Fig. 5 shows the Raman spectra of the samples anodized in (a) H2O-, (b) LiOH-, and (c) KOH-added fluoride/EG electrolyte. As can be seen, the as-anodized TNTs in H2O/fluoride/EG are amorphous. However, anatase peaks are obvious for TNTs formed in LiOH/ fluoride/EG and KOH/fluoride/EG electrolyte. Anatase TiO2 with space group (I41/amd) has six Raman active modes: Eg = 144, D19 4h 197 and 639 cm 1, B1g = 399 and 519 cm 1 and A1g + B1g = 513 cm 1. The presence of anatase Eg active modes at 140 cm 1 and 600 cm 1 and B1g or A1g + B1g at 400 cm 1 and 500 cm 1 are really obvious for both alkaline electrolyte anodized samples indicating crystalline
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Fig. 3. TEM and HRTEM images and the SAED patterns of TNTs formed in (a) LiOH/fluoride/EG (b) KOH/fluoride/EG, and (c) H2O/fluoride/EG electrolytes at 60 V for 60 min.
Fig. 4. XRD patterns of (a) as-anodized and (b) annealed TNTs formed in H2O/fluoride/EG, LiOH/fluoride/EG, and KOH/fluoride/EG electrolytes at 60 V for 60 min. Annealing was at 450 C for 3 h.
nature of the oxide even without heat treatment. In anodisation process, amorphous-to-crystalline transformation within an anodic film can be associated with oxygen generation [17]. Therefore, in alkaline electrolyte gas generation is likely considering more
OH ions exist in the solution. Localize breakdown within the oxide can then occur inducing local field enhancement and joule heating which also promote the nucleation of anatase TiO2. Often this occurs at the interface between the oxide and metal [17].
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Fig. 5. Raman spectra of as-anodized TNTs formed in H2O/fluoride/EG, LiOH/fluoride/EG, and KOH/fluoride/EG electrolytes at 60 V for 60 min (a) as-anodized (without annealing) and (b) after annealing at 450 C for 3 h.
Crystallization of amorphous titania in the TNTs can also arise through a rearrangement of the TiO62–octahedral units in amorphous TiO2. As explained by Yanagisawa et al. [18] anatase is composed of TiO6 octahedra and to obtain anatase TiO2, rearrangement of the TiO62 octahedral units in the amorphous oxide is required. The phase transition is catalyzed by water, which adsorbs on the surface of TiO2. Here, the same is thought to occur, but with the aid of surface adsorbed OH . OH forms bridges between the Ti-O octahedral units that share a common vertex using two lone pairs of electrons in oxygen [18,19]. The two octahedral will then be interconnected to a single side, forming anatase crystals once dehydration occurs as illustrated in Fig. 6. For all annealed samples as seen in Fig. 5b, Raman peaks at 144 cm 1, 395 cm 1, 514 cm 1 and 633 cm 1 are enhanced indicating the growth of anatase crystals within the TNTs. The anatase peaks of TNTs formed in LiOH/fluoride/EG and KOH/ fluoride/EG are narrower and sharper than that of TNTs formed in H2O/fluoride/EG indicating better crystallinity for these samples. Elemental composition and surface chemical state of the fabricated TNTs were investigated using XPS. Fig. 7a shows the full XPS spectra of the TNTs formed in H2O/fluoride/EG, LiOH/ fluoride/EG, and KOH/fluoride/EG electrolytes. Dominant peaks of Ti 2p, O 1s, and C 1s are observed in all samples. Fig. 7b shows the Ti 2p spectra for the anodized Ti foils. The Ti 2p peaks located at 458.8 eV and 465.5 eV correspond to the Ti 2p3/2 and Ti 2p1/2 of Ti4+ in TiO2. The O 1s spectra for the samples are shown in Fig. 7c. Here, O 1 s peak at 530 eV is observed which is due to the presence of oxygen atoms in TiO2. Fig. 7d shows the C 1 s spectra of the samples. C 1 s peak is detected at 285 eV which is attributed to Ti C O bonds of carbonate species in the sample originating from the residual carbon or carbonate species absorbed on the
surface of TNTs or carbon insertion within TNTs. The resulting atomic concentration of the components is shown in Table 1. Carbon contents in TNTs formed in LiOH/fluoride/EG and KOH/ fluoride/EG electrolytes are higher than that of the TNTs formed in H2O/fluoride/EG electrolyte. Insertion of carbon in TNTs can induce unintentional doping effect which may improve the electronic properties of TNTs. The absorption spectra (Fig. 8) shows that the TNTs formed in KOH/fluoride/EG electrolyte exhibit better absorption than the LiOH/fluoride/EG sample. In fact, the absorbance of TNTs made in H2O/fluoride/EG electrolyte is similar to TNTs prepared in LiOH/ fluoride/EG. Perhaps longer nanotube lengths, better degree of anatase crystallinity and more carbon insertion of TNTs made in KOH/fluoride/EG electrolyte have caused the enhancement of light absorption [19]. Adsorption of TNTs loaded with dyes occurs at the visible region of the spectrum, again, with higher absorbance for TNTs made in KOH/fluoride/EG. Fig. 9 shows the photocurrent–voltage (I–V) characteristics of backside illuminated DSSCs using the fabricated TNTs. The obtained results are shown in Table 2 and are compared with the results from other published articles for photoanode utilizing TNTs of similar length. DSSC assembled with TNTs formed in KOH/ fluoride/EG electrolyte produced in this work has the highest h of 3.0% and short circuit current of 8.6 mA/cm2. However, the FF for the cell (0.56) is similar to other reported cells and no substantial differences in the photovoltage are observed as well. The rather low FF for DSSC assembled by anodic TNTs could be due to the presence of a defective oxide layer between the TNTs and the substrate. As anodisation is a field dependent process, incorporation of foreign species to the anodic film is inevitable. Moreover, at the interface between the oxide and the Ti, more insulating
Fig. 6. The formation of anatase TNTs during anodisation in alkaline electrolytes.
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Fig. 7. XPS spectra of TNTs formed in LiOH/fluoride/EG, KOH/fluoride/EG and H2O/fluoride/EG electrolytes at 60 V for 60 min (a) full spectra (b) Ti 2p (c) O 1s and (d) C 1s spectra.
Table 1 Elemental and chemical composition of the fabricated TNTs. Atomic% of elements
LiOH
KOH
H2O
Ti 2p O 1s C 1s
25.5 57.6 16.9
26 55 19
25 56 13
Fig. 8. Absorption spectra of TNTs formed in LiOH/fluoride/EG, KOH/fluoride/EG, and H2O/fluoride/EG electrolytes at 60 V for 1 h.
rutile TiO2 may have formed and interfered with electron transport to the back contact of the cell. The low FF is therefore attributed to a larger series (sheet) resistance at the TNTs|Ti interface. Diameters of TNTs used as photoanode in DSSC can also be compared in Table 2. As seen, TNTs made in LiOH/fluoride/EG and KOH/fluoride/EG electrolytes are larger (170 nm) than other samples. Larger diameter TNTs may have a positive effect to the performance of the DSSC whereby dye penetration inside the nanotube channels may perhaps improved as capillary effect is
Fig. 9. I V characteristics of DSSCs using fabricated TNTs in LiOH/fluoride/EG, KOH/fluoride/EG and H2O/fluoride/EG electrolytes (60 V for 60 min).
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Table 2 Summary of current–voltage (I–V) curves of TNTs based back-side illuminated DSSCs. Electrolyte (anodisation voltage and time)
Length (mm)
Diameter (nm)
Jsc (mA/cm2)
Voc (V)
FF
h (%)
Refs.
EG/NH4F/H2O (60 V for 12 h) EG/DMSO (60 V for 18 h) EG/NH4F/HF (120 V for 2 h) EG/HF (1, 2 and 3 h)
18 10
100 150
7.2 3.2
0.65 0.65
0.45 0.55
2.1 1.2
[15]
18 10 20 30 15 18 10
150 150
3.1 7.8 9.6 10.4 8.1 8.6 7.1
0.63 0.65 0.66 0.64 0.63 0.64 0.64
0.62 0.46 0.45 0.43 0.55 0.56 0.53
2.6 2.3 2.8 2.8 2.7 3.0 2.3
[16] [20]
EG/NH4F/LiOH (60 V for 60 min) EG/NH4F/KOH (60 V for 60 min) EG/NH4F/H2O (60 V for 60 min)
170 170 150
negligible. To compare between the performances of DSSC assembled from the three sets of TNTs made in this work, it can be concluded that the longest TNTs have the highest photocurrent and conversion efficiency (i.e. TNTs made in KOH/fluoride/EG). Increase in tube length increases the surface area of the photoanode resulting in more dye absorption leading to better light harvesting. Longer TNTs are also known to scatter light contributing to the better performance DSSC. Moreover, anodic film from KOH/fluoride/EG bath has a good adherence to the Ti substrate thus the transfer of electrons to the back contact is more efficient. There is also a possibility of carbon insertion influencing the absorption of light within the TNTs themselves for electron-hole pairs generation which may contribute to the free carriers. 4. Conclusion In this study, TNTs were produced in LiOH or KOH in fluoride/EG electrolyte solution. The formed TNTs are highly ordered, have uniform surface and well-separated. The TNTs are also crystalline in their as-anodised condition. Annealing improved obviously the crystallinity of the TNTs. Annealed TNTs were then used as a photoanode in a DSSC. 18 mm TNTs (fabricated in KOH in fluoride/ EG) exhibited the highest h of 3% than 15 mm (2.7%) and 10 mm (2.3%). Longer TNTs have higher surface area hence more sites for dye adsorption. With the well crystalline nature, electron transport is also expected to be much improved producing high photocurrent under solar radiation. However, the FF and photovoltage are not as high perhaps due to high resistance barrier layer at the oxide|metal interface. Acknowledgements ASEAN University Network for Science and Engineering Education Development Network (AUN/SEED-Net) Project under
This work
Grant No. 304/6050294 and Japan International Cooperation Agency (JICA). Nanomaterials development is partly supported by Research University Grant (USM-TUT Collaboration)1001/ PBAHAN/870048. References [1] B. O’regan, M. Grfitzeli, Nature 353 (1991) 737–740. [2] B.-X. Lei, J.-Y. Liao, R. Zhang, J. Wang, C.-Y. Su, D.B. Kuang, J. Phys. Chem. C 114 (2010) 15228–15233. [3] J.-Y. Liao, B.-X. Lei, H.-Y. Chen, D.-B. Kuang, C.-Y. Su, Energy Environ. Sci. 5 (2012) 5750–5757. [4] K. Fujihara, A. Kumar, R. Jose, S. Ramakrishna, S. Uchida, Nanotechnology 18 (2007) 365709. [5] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano. Lett. 7 (2007) 69. [6] L.-L. Li, C.-Y. Tsai, H.-P. Wu, C.C.- Chen, E.W.-G. Diau, J. Mater. Chem. 20 (2010) 2753–2758. [7] G. Kawamura, H. Ohmi, W.K. Tan, Z. Lockman, H. Muto, A. Matsuda, Nanoscale. Res. Lett. 10 (2015) 1–6. [8] H.M.A. Javed, W. Que, Z. He, J. Nanosci. Nanotech. 14 (2014) 1085–1098. [9] N.-G. Park, J.V.D. Lagemaat, A.J. Frank, J. Phys. Chem. B 104 (2000) 8989–8994. [10] S. Sreekantan, Z. Lockman, R. Hazan, M. Tasbihi, L.K. Tong, A.R. Mohamed, J. Alloy Compd. 485 (2009) 478–483. [11] S. Sreekantan, L.C. Wei, Z. Lockman, J. Electrochem. Soc. 58 (2011) 397–402. [12] Z. Lockman, S. Sreekantan, S. Ismail, L. Schmidt-mende, L.J. MacManusDriscoll, J. Alloy Compd. 503 (2010) 359–364. [13] P. Roy, S. Berger, P. Schmuki, Angew. Chem. Int. Ed. 50 (2011) 2904–2939. [14] Z. Lockman, S. Ismail, S. Sreekantan, L. Schmidt-mende, J.L. MacManusDriscoll, Nanotecnology 21 (2010) 5601. [15] H.Y. Hwang, A.A. Prabu, D.Y. Kim, K.J. Kim, Sol. Energy 85 (2011) 1551–1559. [16] J.R. Jennings, A. Ghicov, L.M. Peter, P. Schimuki, A.B. Walker, J. Am. Chem. Soc. 130 (2008) 13364–13372. [17] H. Habazaki, M. Uozumi, H. Konno, K. Shimizu, P. Skeldon, G.E. Thompson, Crystallization of anodic titania on titanium and its alloys, Corr. Sci. 45 (2003) 2063–2073. [18] K. Yanagisawa, J. Ovenstone, J. Phys. Chem. B 103 (1999) 7781–7787. [19] N.K. Allam, C.A. Grimes, Langmuir 25 (2009) 7234–7240. [20] X. Hu, T. Zhang, Z. Jin, J. Zhang, W. Xu, J. Yan, W. Xu, J. Yan, J. Zhang, L. Zhang, Y. Wu, Mater. Lett. 62 (2008) 4579–4581.
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited paper
TiO2 nanotube arrays formation in fluoride/ethylene glycol electrolyte containing LiOH or KOH as photoanode for dye-sensitized solar cell Nyein Nyeina,b , Wai Kian Tanc , Go Kawamurab , Atsunori Matsudab , Zainovia Lockmana,* a Green Electronics NanoMaterials Group, School of Materials and Mineral Resources Engineering Campus, Universiti Sains Malaysia, 14300, Nibong Tebal, Penang, Malaysia b Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, Toyohashi, 441-8580, Aichi, Japan c Center for International Education, Toyohashi University of Technology, Aichi, Toyohashi, 441-8580, Japan
A R T I C L E I N F O
Article history: Received 12 January 2017 Received in revised form 6 April 2017 Accepted 12 April 2017 Available online 18 April 2017 Keywords: Anodisation TiO2 nanotube arrays Dye-sensitized solar cells
A B S T R A C T
The formation of TiO2 nanotube arrays by anodisation of Ti in fluoride/ethylene glycol (EG) electrolyte added to it lithium hydroxide (LiOH) or potassium hydroxide (KOH) at 60 V for 60 min was investigated. Highly ordered 15 mm long TiO2 nanotubes (TNTs) were produced in LiOH/fluoride/EG and 18 mm long TNTs were obtained in KOH/fluoride/EG. The as-anodized TNTs in these alkaline electrolytes were found to be crystalline and obviously annealing increases the crystallinity. Annealed TNTs were used as a photoanode in a backside illuminated dye-sensitized solar cell (DSSC). Photoconversion efficiency, h of 3% was achieved for sample with photoanode prepared in KOH/fluoride/EG higher than DSSC with TNTs photoanode fabricated in LiOH/fluoride/EG. © 2017 Published by Elsevier B.V.
1. Introduction Dye-sensitized solar cell (DSSC) requires a photoanode onto which electrons are transferred from the sensitized dyes to its surface for the generation of photocurrent. To date, mesoporous TiO2 with high surface area has been used as the photoanode [1]. However, mesoporous TiO2 has some disadvantages such as high recombination of carriers and poor charge collection efficiency [2]. To overcome this problem, several studies have been attempted in fabricating DSSCs with one-dimensional (1-D) TiO2 nanostructures such as nanowires [3], nanorods [4] and nanotubes [5,6]. Among all these nanostructures, 1-D TiO2 nanotube arrays fabricated by anodisation method have attracted great attention because the nanotubes can be made vertically aligned on a substrate. Such morphology can facilitate fast transport of electrons to the back contact [6,7], producing higher photocurrent despite not as high as expected. Moreover, high surface area nanotubes provide better dye/semiconductor interaction. In order to further enhance the efficiency of the cell; morphology, length, and crystallinity of the TNTs need to be optimized [8,9]. Often high aspect ratio and crystalline TNTs, especially in anatase phase are desired. To fabricate nanotubes with high aspect ratio by anodic process,
* Corresponding author. E-mail address: [email protected] (Z. Lockman). http://dx.doi.org/10.1016/j.jphotochem.2017.04.015 1010-6030/© 2017 Published by Elsevier B.V.
anodisation parameters have been routinely studied; particularly the choice of electrolyte, its pH [10] and the presence of water (or peroxide) in the electrolyte [11]. Voltage and time are equally important parameters that also influence the morphologies and the dimensions of nanotubes [12]. One important electrolyte ingredient for nanotubes formation is fluoride salt. In organic electrolyte, NH4F is commonly used and generally, if 0.1 wt% NH4F is added to the electrolyte, compact film is produced. On the other hand, porous, random oxide is produced if 0.5 wt% is used [13]. For the success in nanotubes formation, the wt% of NH4F has been kept within 0.1–0.5 wt%. However, apart from fluoride content, the pH of the electrolyte is also an important factor in determining the success of nanotubes formation. For instance, buffered electrolyte at pH 7 but with excessive NH4F will still facilitate the fabrication of nanotubes even though the wall of tubes are not very smooth [14]. In this present work, electrolyte with pH 9 and rather high amount of NH4F (0.7 wt%) were experimented. LiOH or KOH was added as to increase the pH of the electrolyte. We postulated that higher pH electrolyte can suppress surface etching of the nanotubes, and with an excessive amount of NH4F, chemical etching within the nanotubes at the bottom part can be made more vigorous and thus long nanotubes can be produced at a faster rate. Herein comparisons of the characteristics of anodized Ti in LiOH/fluoride/EG and KOH/fluoride/EG were made. Crystallinity of the TNTs formed was also studied and
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annealed TNTs were assembled in a DSSC as a photoanode. To date, despite several attempts on the use of TNTs as photoanode for a DSSC, most of the results show rather low conversion efficiency, h. Typically, 15–20 mm long TNTs result in h of around 0.5–2.0% [15,16] for back-side illuminated DSSC. Here higher h was recorded perhaps due to better crystallinity of the TNTs, allowing for faster electron transport as well as good adherence of the film to the substrate and incorporation of foreign species within the oxide lattice. 2. Experimental section Titanium foils (0.127 mm thickness, 99.7% purity, Stream Chemicals, USA) were degreased by sonicating in acetone, isopropanol, ethanol and deionized (DI). Electrochemical anodisation was carried out in a two-electrode cell, with Ti as the anode and a platinum rod as the counter electrode at 60 V for 60 min at room temperature. The distance between the Ti foil and Pt electrode was kept at 3 cm. 100 ml ethylene glycol (EG) added to it NH4F (0.7 wt%) and 1 M LiOH or KOH (5 wt%) were used as the electrolyte. After anodisation was completed, the samples were sonicated in acetone then rinsed in deionized (DI). Annealing was done at 450 C for 3 h to crystalize anatase TNTs. The morphologies of anodized Ti were observed by a field emission scanning electron microscope (SEM, Zeiss SUPRA 35 VP, Germany). X-ray diffraction (XRD, Bruker D8 Advance), Raman Spectrometer (Jasco NRS-3100, Japan) and High Resolution Transmission Electron Microscope (HRTEM, Hitachi H-800 and JEOL JEM-2100F, Japan) were used for crystallinity and phases study. Chemical composition was analyzed by X-ray photoelectron spectroscopy (XPS), PHI Quantera SXM scanning X-ray microprobe (ULVAC-Phi, Inc., Japan) using an Al cathode (hn = 1486.6 eV) under a base pressure of 5 10 9 Torr. 100 W power X-ray power, 26 eV pass energy with 45 take-off angle were used. The core level spectra of Ti 2p, O ls and C ls were recorded by high-resolution scans. Smart Soft-XPS spectra were deconvoluted with GaussianLorentzian curve-fitting technique using the MultiPak software 9.0 (ULVAC- Phi, Inc, Japan). The binding energy of the XPS spectra was calibrated using C 1s (284.6 eV). Light absorption properties of the samples were investigated using V-670 ultraviolet/visible spectrophotometer (UV–vis, Jasco, Japan). To investigate the use of the anodized foil as photoanode in a DSSC, anodized Ti was immersed in 0.3 mM N719 dye (Sigma, Aldrich, USA) with acetonitrile/tert-butanol (volume ratio 1:1) for 24 h. Then, the foil was rinsed with ethanol to remove excess dyes
on its surface. Counter electrode used was platinum sputtered ITO (E-1030 ionic sputter, Hitachi, Japan) glass. Electrolyte for the DSSC was composed of 0.03 M di-iodine, 0.6 M 1–2-dimethyl-3-propylimidazolium iodide, 0.1 M lithium iodide and 0.5 M 4-tertbutylpyridine in acetonitrile. 3. Results and discussion Impedance responses of LiOH/fluoride/EG, KOH/fluoride/EG, and H2O/fluoride/EG electrolytes were done to investigate the conductivity of the electrolytes (Fig. 1(a)). The conductivity of H2O/ fluoride/EG electrolyte is 1.8 mS cm 1 and with the addition of LiOH and KOH the conductivity increases to 1.9 mS cm 1 and 2 mS cm 1, respectively. The slight increase of conductivity may be due to the presence of OH in the electrolyte. It is worth to mention here that LiOH did not completely dissolve in EG compared to KOH and conductivity measurement was done before the anodisation process was conducted i.e. the electrolyte was free from Ti4+ ions. Time-dependent anodisation current behavior in the three electrolytes: LiOH/fluoride/EG, KOH/fluoride/EG, and H2O/fluoride/EG shown in Fig. 1 (b) are rather similar. This indicates the same fundamental anodic reactions as generally described by localized dissolution model and diffusion-limited anodic growth at the pore bottom are followed by the three samples. The movement of ionic species; O2 and OH , passing through the barrier layer at the pore bottom is believed to govern the growth process. Nonetheless, the magnitude of the current density is different in different electrolytes i.e. H2O/fluoride/EG displays higher current density compared to the other two electrolytes. High current density can be related to the dissolution process whereby dissolution is thought to be faster in H2O/fluoride/EG. The addition of LiOH or KOH into the electrolyte results in the decrease of the current density hence dissolution of TNTs in such electrolyte is much suppressed. Dissolution is pH dependent whereby less dissolution is expected in alkaline electrolytes. Fig. 2 shows surface and cross-section SEM micrographs of TNTs formed in fluoride/EG electrolyte containing (a) LiOH added EG (LiOH/fluoride/EG) and (b) KOH added EG (KOH/fluoride/EG) at 60 V for 60 min. As a comparison, Ti anodized in H2O added EG (H2O/fluoride/EG) electrolyte was also fabricated and as observed, shorter TNTs that are 10 mm long were produced in this electrolyte (Fig. 1c). The addition of LiOH (Fig. 2a) or KOH (Fig. 2b) significantly increased the lengths to 15 mm and 18 mm, respectively. However, the diameter of the nanotubes does not change very much with the use of these electrolytes (170 nm). Diameter of TNTs fabricated in
Fig. 1. (a) Complex impedance spectra of different electrolytes (b) Current density-time curve for anodized Titanium at 60 V for 1 h.
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Fig. 2. SEM images of TNTs formed in (a) LiOH/fluoride/EG (b) KOH/fluoride/EG and (c) H2O/fluoride/EG electrolytes at 60 V for 60 min.
H2O/fluoride/EG is around 150 nm. The adherence of the anodic film for this sample is also poorer compared to the other two samples. The significant role of electrolyte in TNTs formation has been established [14] whereby the electrolyte must first support oxide formation, and it has to contain adequate fluoride ions for porosification process to initiate. Electrolyte pH influences the chemical dissolution process and to achieve long TNTs, dissolution at the bottom of the pores must be encouraged while a rather protective environment is required at the surface to prevent excessive chemical etching. Excessive chemical etching results in short TNTs. In here, the addition of LiOH or KOH was observed to increase the pH of the electrolyte to 9. Since the pH is high chemical dissolution at the surface of the nanotubes is reduced, hence long nanotubes were produced. The addition of K+ or Li+ in the electrolyte can also affect the conductivity of the bulk electrolyte which also effects the growth behavior of the nanotubes. TEM images of TNTs formed in fluoride/EG electrolyte containing LiOH, KOH and H2O are shown in Fig. 3. From Fig. 3a (i) and Fig. 3b (i), the walls of the TNTs made in LiOH/fluoride/EG and KOH/fluoride/EG are seen smoother than those fabricated in H2O/fluoride/EG (Fig. 3c (i)). Lattice fringes in Fig. 3a–c (ii), with
spacing of 0.35 nm, are revealed indicating crystals of (101) anatase, in agreement with the XRD patterns of the samples (Fig. 4b). The crystallographic structures were further confirmed by SAED as shown in Fig. 3 (a, b and c (iii)). The diffraction rings can be indexed to (101), (004), (200), (105) and (204) planes of anatase, indicating the polycrystalline nature of TNTs. XRD patterns of as-anodized TNTs made in LiOH/fluoride/EG, KOH/fluoride/EG, and H2O/fluoride/EG reveal that the TNTs are amorphous (Fig. 4a). Only titanium peaks are detected. After annealing at 450 C the TNTs are transformed to anatase (Fig. 4b). Diffraction peaks are indexed at 25.4 , 37.8 , 48.5 , 54.0 , 55.2 , 68.9 , and 75.0 corresponding to anatase crystalline planes of (101), (004), (200), (105), (211), (204), (116) and (215), respectively. Fig. 5 shows the Raman spectra of the samples anodized in (a) H2O-, (b) LiOH-, and (c) KOH-added fluoride/EG electrolyte. As can be seen, the as-anodized TNTs in H2O/fluoride/EG are amorphous. However, anatase peaks are obvious for TNTs formed in LiOH/ fluoride/EG and KOH/fluoride/EG electrolyte. Anatase TiO2 with space group (I41/amd) has six Raman active modes: Eg = 144, D19 4h 197 and 639 cm 1, B1g = 399 and 519 cm 1 and A1g + B1g = 513 cm 1. The presence of anatase Eg active modes at 140 cm 1 and 600 cm 1 and B1g or A1g + B1g at 400 cm 1 and 500 cm 1 are really obvious for both alkaline electrolyte anodized samples indicating crystalline
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Fig. 3. TEM and HRTEM images and the SAED patterns of TNTs formed in (a) LiOH/fluoride/EG (b) KOH/fluoride/EG, and (c) H2O/fluoride/EG electrolytes at 60 V for 60 min.
Fig. 4. XRD patterns of (a) as-anodized and (b) annealed TNTs formed in H2O/fluoride/EG, LiOH/fluoride/EG, and KOH/fluoride/EG electrolytes at 60 V for 60 min. Annealing was at 450 C for 3 h.
nature of the oxide even without heat treatment. In anodisation process, amorphous-to-crystalline transformation within an anodic film can be associated with oxygen generation [17]. Therefore, in alkaline electrolyte gas generation is likely considering more
OH ions exist in the solution. Localize breakdown within the oxide can then occur inducing local field enhancement and joule heating which also promote the nucleation of anatase TiO2. Often this occurs at the interface between the oxide and metal [17].
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Fig. 5. Raman spectra of as-anodized TNTs formed in H2O/fluoride/EG, LiOH/fluoride/EG, and KOH/fluoride/EG electrolytes at 60 V for 60 min (a) as-anodized (without annealing) and (b) after annealing at 450 C for 3 h.
Crystallization of amorphous titania in the TNTs can also arise through a rearrangement of the TiO62–octahedral units in amorphous TiO2. As explained by Yanagisawa et al. [18] anatase is composed of TiO6 octahedra and to obtain anatase TiO2, rearrangement of the TiO62 octahedral units in the amorphous oxide is required. The phase transition is catalyzed by water, which adsorbs on the surface of TiO2. Here, the same is thought to occur, but with the aid of surface adsorbed OH . OH forms bridges between the Ti-O octahedral units that share a common vertex using two lone pairs of electrons in oxygen [18,19]. The two octahedral will then be interconnected to a single side, forming anatase crystals once dehydration occurs as illustrated in Fig. 6. For all annealed samples as seen in Fig. 5b, Raman peaks at 144 cm 1, 395 cm 1, 514 cm 1 and 633 cm 1 are enhanced indicating the growth of anatase crystals within the TNTs. The anatase peaks of TNTs formed in LiOH/fluoride/EG and KOH/ fluoride/EG are narrower and sharper than that of TNTs formed in H2O/fluoride/EG indicating better crystallinity for these samples. Elemental composition and surface chemical state of the fabricated TNTs were investigated using XPS. Fig. 7a shows the full XPS spectra of the TNTs formed in H2O/fluoride/EG, LiOH/ fluoride/EG, and KOH/fluoride/EG electrolytes. Dominant peaks of Ti 2p, O 1s, and C 1s are observed in all samples. Fig. 7b shows the Ti 2p spectra for the anodized Ti foils. The Ti 2p peaks located at 458.8 eV and 465.5 eV correspond to the Ti 2p3/2 and Ti 2p1/2 of Ti4+ in TiO2. The O 1s spectra for the samples are shown in Fig. 7c. Here, O 1 s peak at 530 eV is observed which is due to the presence of oxygen atoms in TiO2. Fig. 7d shows the C 1 s spectra of the samples. C 1 s peak is detected at 285 eV which is attributed to Ti C O bonds of carbonate species in the sample originating from the residual carbon or carbonate species absorbed on the
surface of TNTs or carbon insertion within TNTs. The resulting atomic concentration of the components is shown in Table 1. Carbon contents in TNTs formed in LiOH/fluoride/EG and KOH/ fluoride/EG electrolytes are higher than that of the TNTs formed in H2O/fluoride/EG electrolyte. Insertion of carbon in TNTs can induce unintentional doping effect which may improve the electronic properties of TNTs. The absorption spectra (Fig. 8) shows that the TNTs formed in KOH/fluoride/EG electrolyte exhibit better absorption than the LiOH/fluoride/EG sample. In fact, the absorbance of TNTs made in H2O/fluoride/EG electrolyte is similar to TNTs prepared in LiOH/ fluoride/EG. Perhaps longer nanotube lengths, better degree of anatase crystallinity and more carbon insertion of TNTs made in KOH/fluoride/EG electrolyte have caused the enhancement of light absorption [19]. Adsorption of TNTs loaded with dyes occurs at the visible region of the spectrum, again, with higher absorbance for TNTs made in KOH/fluoride/EG. Fig. 9 shows the photocurrent–voltage (I–V) characteristics of backside illuminated DSSCs using the fabricated TNTs. The obtained results are shown in Table 2 and are compared with the results from other published articles for photoanode utilizing TNTs of similar length. DSSC assembled with TNTs formed in KOH/ fluoride/EG electrolyte produced in this work has the highest h of 3.0% and short circuit current of 8.6 mA/cm2. However, the FF for the cell (0.56) is similar to other reported cells and no substantial differences in the photovoltage are observed as well. The rather low FF for DSSC assembled by anodic TNTs could be due to the presence of a defective oxide layer between the TNTs and the substrate. As anodisation is a field dependent process, incorporation of foreign species to the anodic film is inevitable. Moreover, at the interface between the oxide and the Ti, more insulating
Fig. 6. The formation of anatase TNTs during anodisation in alkaline electrolytes.
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Fig. 7. XPS spectra of TNTs formed in LiOH/fluoride/EG, KOH/fluoride/EG and H2O/fluoride/EG electrolytes at 60 V for 60 min (a) full spectra (b) Ti 2p (c) O 1s and (d) C 1s spectra.
Table 1 Elemental and chemical composition of the fabricated TNTs. Atomic% of elements
LiOH
KOH
H2O
Ti 2p O 1s C 1s
25.5 57.6 16.9
26 55 19
25 56 13
Fig. 8. Absorption spectra of TNTs formed in LiOH/fluoride/EG, KOH/fluoride/EG, and H2O/fluoride/EG electrolytes at 60 V for 1 h.
rutile TiO2 may have formed and interfered with electron transport to the back contact of the cell. The low FF is therefore attributed to a larger series (sheet) resistance at the TNTs|Ti interface. Diameters of TNTs used as photoanode in DSSC can also be compared in Table 2. As seen, TNTs made in LiOH/fluoride/EG and KOH/fluoride/EG electrolytes are larger (170 nm) than other samples. Larger diameter TNTs may have a positive effect to the performance of the DSSC whereby dye penetration inside the nanotube channels may perhaps improved as capillary effect is
Fig. 9. I V characteristics of DSSCs using fabricated TNTs in LiOH/fluoride/EG, KOH/fluoride/EG and H2O/fluoride/EG electrolytes (60 V for 60 min).
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Table 2 Summary of current–voltage (I–V) curves of TNTs based back-side illuminated DSSCs. Electrolyte (anodisation voltage and time)
Length (mm)
Diameter (nm)
Jsc (mA/cm2)
Voc (V)
FF
h (%)
Refs.
EG/NH4F/H2O (60 V for 12 h) EG/DMSO (60 V for 18 h) EG/NH4F/HF (120 V for 2 h) EG/HF (1, 2 and 3 h)
18 10
100 150
7.2 3.2
0.65 0.65
0.45 0.55
2.1 1.2
[15]
18 10 20 30 15 18 10
150 150
3.1 7.8 9.6 10.4 8.1 8.6 7.1
0.63 0.65 0.66 0.64 0.63 0.64 0.64
0.62 0.46 0.45 0.43 0.55 0.56 0.53
2.6 2.3 2.8 2.8 2.7 3.0 2.3
[16] [20]
EG/NH4F/LiOH (60 V for 60 min) EG/NH4F/KOH (60 V for 60 min) EG/NH4F/H2O (60 V for 60 min)
170 170 150
negligible. To compare between the performances of DSSC assembled from the three sets of TNTs made in this work, it can be concluded that the longest TNTs have the highest photocurrent and conversion efficiency (i.e. TNTs made in KOH/fluoride/EG). Increase in tube length increases the surface area of the photoanode resulting in more dye absorption leading to better light harvesting. Longer TNTs are also known to scatter light contributing to the better performance DSSC. Moreover, anodic film from KOH/fluoride/EG bath has a good adherence to the Ti substrate thus the transfer of electrons to the back contact is more efficient. There is also a possibility of carbon insertion influencing the absorption of light within the TNTs themselves for electron-hole pairs generation which may contribute to the free carriers. 4. Conclusion In this study, TNTs were produced in LiOH or KOH in fluoride/EG electrolyte solution. The formed TNTs are highly ordered, have uniform surface and well-separated. The TNTs are also crystalline in their as-anodised condition. Annealing improved obviously the crystallinity of the TNTs. Annealed TNTs were then used as a photoanode in a DSSC. 18 mm TNTs (fabricated in KOH in fluoride/ EG) exhibited the highest h of 3% than 15 mm (2.7%) and 10 mm (2.3%). Longer TNTs have higher surface area hence more sites for dye adsorption. With the well crystalline nature, electron transport is also expected to be much improved producing high photocurrent under solar radiation. However, the FF and photovoltage are not as high perhaps due to high resistance barrier layer at the oxide|metal interface. Acknowledgements ASEAN University Network for Science and Engineering Education Development Network (AUN/SEED-Net) Project under
This work
Grant No. 304/6050294 and Japan International Cooperation Agency (JICA). Nanomaterials development is partly supported by Research University Grant (USM-TUT Collaboration)1001/ PBAHAN/870048. References [1] B. O’regan, M. Grfitzeli, Nature 353 (1991) 737–740. [2] B.-X. Lei, J.-Y. Liao, R. Zhang, J. Wang, C.-Y. Su, D.B. Kuang, J. Phys. Chem. C 114 (2010) 15228–15233. [3] J.-Y. Liao, B.-X. Lei, H.-Y. Chen, D.-B. Kuang, C.-Y. Su, Energy Environ. Sci. 5 (2012) 5750–5757. [4] K. Fujihara, A. Kumar, R. Jose, S. Ramakrishna, S. Uchida, Nanotechnology 18 (2007) 365709. [5] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano. Lett. 7 (2007) 69. [6] L.-L. Li, C.-Y. Tsai, H.-P. Wu, C.C.- Chen, E.W.-G. Diau, J. Mater. Chem. 20 (2010) 2753–2758. [7] G. Kawamura, H. Ohmi, W.K. Tan, Z. Lockman, H. Muto, A. Matsuda, Nanoscale. Res. Lett. 10 (2015) 1–6. [8] H.M.A. Javed, W. Que, Z. He, J. Nanosci. Nanotech. 14 (2014) 1085–1098. [9] N.-G. Park, J.V.D. Lagemaat, A.J. Frank, J. Phys. Chem. B 104 (2000) 8989–8994. [10] S. Sreekantan, Z. Lockman, R. Hazan, M. Tasbihi, L.K. Tong, A.R. Mohamed, J. Alloy Compd. 485 (2009) 478–483. [11] S. Sreekantan, L.C. Wei, Z. Lockman, J. Electrochem. Soc. 58 (2011) 397–402. [12] Z. Lockman, S. Sreekantan, S. Ismail, L. Schmidt-mende, L.J. MacManusDriscoll, J. Alloy Compd. 503 (2010) 359–364. [13] P. Roy, S. Berger, P. Schmuki, Angew. Chem. Int. Ed. 50 (2011) 2904–2939. [14] Z. Lockman, S. Ismail, S. Sreekantan, L. Schmidt-mende, J.L. MacManusDriscoll, Nanotecnology 21 (2010) 5601. [15] H.Y. Hwang, A.A. Prabu, D.Y. Kim, K.J. Kim, Sol. Energy 85 (2011) 1551–1559. [16] J.R. Jennings, A. Ghicov, L.M. Peter, P. Schimuki, A.B. Walker, J. Am. Chem. Soc. 130 (2008) 13364–13372. [17] H. Habazaki, M. Uozumi, H. Konno, K. Shimizu, P. Skeldon, G.E. Thompson, Crystallization of anodic titania on titanium and its alloys, Corr. Sci. 45 (2003) 2063–2073. [18] K. Yanagisawa, J. Ovenstone, J. Phys. Chem. B 103 (1999) 7781–7787. [19] N.K. Allam, C.A. Grimes, Langmuir 25 (2009) 7234–7240. [20] X. Hu, T. Zhang, Z. Jin, J. Zhang, W. Xu, J. Yan, W. Xu, J. Yan, J. Zhang, L. Zhang, Y. Wu, Mater. Lett. 62 (2008) 4579–4581.