Chapter 1 INTRODUCTION AND LITERATURE REVIEW XXIII

Transcription

1 Chapter 1 INTRODUCTION AND LITERATURE REVIEW XXIII

2 CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW I. INTRODUCTION Nanotubes are of great interest for a wide array of technological and biological applications, especially due to their high surface-to-volume ratios and size dependent properties. The innovations of carbon nanotubes [1] with their variety of interesting properties have stimulated the quest for the synthesis of nanotubular structures of other materials. One such widely acclaimed material that was developed through a simple process of electrochemical oxidation or anodization is the nanotubular array of titanium dioxide (TiO 2 ); commonly called as titania nanotube arrays or TNAs, developed on metallic titanium substrates or ITO, FTO etc. Several recent studies have indicated that titania nanotubes have improved properties compared to any other form of titania for application in solar cells [2-4], gas sensors [5, 6], photocatalysis [7, 8], photoelectrochemical hydrogen generation [9, 10], orthopedic implants [11, 12], etc. Titania nanotubes have been prepared by a variety of techniques including sol gel method [13], electrophoretic deposition [14], hydrothermal [15] and anodization [16, 17]. Amongst the various methods of processing titania nanotubes, anodization of metallic titanium resulting in vertically aligned TiO 2 nanotubular arrays is a simple and cost-effective electrochemical process. Gong and co-workers [18] pioneered the synthesis of the first generation of vertically oriented titania nanotube arrays extending up to 500 nm length, by electrochemical oxidation of titanium in aqueous HF electrolyte. It is well known that the properties and performance of TiO 2 are dependent partly on its crystallinity and morphology. Various applications of these nanotube arrays, such as dye sensitized

3 solar cells (DSSC) and water photo electrolysis, demand high aspect ratio nanotubes [19, 20]. Literature reports also suggest that photo-induced processes are strongly dependent on the fabrication conditions and morphology of the nanotubes [21, 22]. The aligned porosity, crystallinity and oriented nature of the nanotube arrays also make them attractive electron percolation pathways for vectorial charge transfer between interfaces [23]. Chen et.al has reported a power conversion efficiency of ~ 7% for TiO 2 nanotube based dye sensitized solar cells (DSSC) at laboratory scale [24]. In order to enhance this efficiency further, a substantial improvement in dimensional and morphological features of the nanotubes during anodization process is essential. Alterations in nanotubular dimensions and spacing are an important attribute that enables the use of such nanostructures for a variety of applications. The nanotube diameter, length, spatial density, wall thickness, surface area, etc. are parameters that when altered can influence biological, optical and chemical properties of the surface. Subtle differences in the above parameters can influence the use of these tubular structures in applications as diverse as solar cells to biomedical implants. This thesis work is an attempt to maneuver the processing conditions and electrolyte chemistry in order to control the morphological parameters of the nanotubes. In this thesis, various methods adopted to obtain morphologically modified nanotubular structures of TiO 2 through alterations in the anodization conditions are elaborated. In addition, the choice of different electrolyte systems are also addressed which can play a role in morphological control. An attempt to evaluate the parameters that regulate the dimensions of TNAs in HF-based aqueous and non-aqueous electrolytes is also made. The use of a multi-step anodization process in fabricating ordered honeycomb titania structures which can aid improvements in solar cell performance is also explored. II. LITERATURE REVIEW 1.1 An overview of TiO 2 Nanostructures Nanostructured materials have gained importance in a variety of applications in recent years due to their significantly enhanced physico-chemical properties. Amrita Centre for Nanosciences and Molecular Medicine 2

4 Concurrently, an exponential growth of research activities is noticeable in nanoscience and technology over the past few decades [25-27]. Novel physical and chemical properties emerge when the size of the material becomes smaller and smaller, and down to the nanometer scale. The specific surface area and surface-tovolume ratio increase dramatically as the size of a material decreases [28]. The same governs all types of nanomaterials, whether they are metallic, semiconductor, ceramic or inorganic. Amongst their unique properties, the movement of electrons and holes in semiconductor nanomaterials is primarily governed by quantum confinement [29]. The transport properties in such materials are related to phonons, the kinetics of which is largely affected by the size and geometry of the materials [30]. Functional metal oxide nanostructures are of considerable interest due to their novel properties, as illustrated by the electron emission of zinc oxide nanowires [31], luminescence exhibited by europium-doped nanoporous titania [32], high ethanol sensitivity of nanocrystalline tin oxide [33], etc. Titanium dioxide, TiO 2, is an n-type indirect semiconductor material with electronic band gap larger than 3.0 ev which absorbs light primarily in the UV region. Scientists and technologists have shown great interest in this material due to its chemical and physical properties as well as its stability [34]. This material is known to exist in several forms, among them the most abundant being anatase, rutile and brookite [35]. Titanium dioxide is largely used as a white pigment in paints and food as well as ultraviolet (UV) ray protection in cosmetic products. In 1972, Fujishima and Honda discovered the phenomenon of photo catalytic splitting of water on a TiO 2 electrode under ultraviolet light [36]. Since then, enormous efforts have been devoted to the research on TiO 2 material, which has led to many promising applications in areas ranging from photovoltaics and photocatalysis to photo/electrochromics and sensors. These applications can be roughly divided into energy and environmental categories, many of which depend not only on the properties of the TiO 2 material itself, but also on the interactions of TiO 2 materials with the environment. The invention of carbon nanotubes by Iijima in 1991 [1] motivated the synthesis of nanotubular structures of other materials. In order to take advantage of nanotubes Amrita Centre for Nanosciences and Molecular Medicine 3

5 for various device applications, it is essential to orient nanotubes on substrates and generate ordered arrays. Nanotubular materials based on transition metal oxides form an important category owing to the wide range of outstanding properties they offer [37]. Amongst all transition metal oxides, TiO 2 is the most studied material as it exhibits a broad range of functional properties. In addition to the nanotubes, TiO 2 can be developed into various other one-dimensional morphologies such as nanorods, nanowires, etc [38-40] Methods of Fabrication of TiO 2 nanostructures One-dimensional nanostructures of titania have been prepared by a wide variety of techniques including sol gel method [13], electrophoretic deposition [14], hydrothermal [15] and anodization [41, 42]. Amongst the one-dimensional structures of TiO 2, nanotubes have gained utmost prominence owing to their far-fledged applications in diverse fields such as supercapacitors [43, 44], biofiltration membranes [45], photoelectrochemical cells for solar generation of hydrogen [46, 47], etc. For this reason, the fabrication of aligned TiO 2 nanotubes has grabbed tremendous attention in recent years. Researchers have explored various techniques for fabricating homogenous arrays of titania nanotubes on various substrates including metallic titanium, indium tin oxide, fluorine tin oxide, etc. Although anodization, the process of electrochemical oxidation of titanium is a widely acclaimed, simple and cost-effective strategy for developing vertically aligned TiO 2 nanotubular arrays, titania nanotubes have also been prepared by a variety of other methods including the templating growth [48], hydrothermal process [49, 50] and seeded growth [51]. The templating method is relatively complicated as it needs to introduce and remove the template. The hydrothermal process can produce proton-containing separated crystalline nanotubes formed by rolling Ti O mono-sheets of H 2 Ti 3 O 7 [52]. In the seeded growth, orientation of the nanotubes is random. Amrita Centre for Nanosciences and Molecular Medicine 4

6 1.2 Anodization Amongst the various methods that have evolved for creating vertically oriented nanotubular structures, anodization is a technique based on electrochemical oxidation which can be applied both at the laboratory scale as well as at an industrial scale. Anodization or anodic oxidation is a well-established electrochemical method for surface modification of valve metals such as Ti, Hf, Ta, W, Nb, Zr, etc., yielding a protective oxide layer [49]. Highly ordered vertically oriented titania nanotube arrays (TNAs) have been fabricated via anodization of metallic titanium [53]. Using this technique, micro-porous oxide films have been developed as a uniform layer on the metallic substrate, aiding several applications. This technique ensures a high specific surface area, scaling up and alteration in the dimensions and morphologies of nanotubes. Furthermore, as-synthesized amorphous titania nanotube arrays (TNAs) can be transformed to a crystalline TiO 2 form, increasing the mechanical strength and extending the potential application of this material [54 59]. Additionally, nanotubes produced by this method are well aligned and closely packed. Anodization of metals (eg: Titanium) is conducted using a two-electrode electrochemical cell with the metal as the anode and platinum as cathode, at a constant potential [60]. These experiments are commonly carried out by stirring the electrolyte which helps to reduce the double layer thickness at the metal/electrolyte interface, and ensures uniform local current density and temperature over the anodic surface [61]. When a constant voltage or current is applied between the anode and cathode, electrode reactions (oxidation and reduction) in combination with fielddriven ion diffusion (Figure 1.1) lead to the formation of an oxide layer on the anode surface. Anodization process can be performed either at constant voltage (potentiostatic) or constant current (galvanostatic process). If the applied voltage exceeds the dielectric breakdown limit of the oxide, the oxide will no longer be resistive to prevent further current flow and oxide growth, which will lead to more Amrita Centre for Nanosciences and Molecular Medicine 5

7 gas evolution and sparking. This technique is, thus, also known as Anodic Spark Deposition (ASD) or Micro-Arc Oxidation (MAO). It has been reported that the breakdown potentials for H 3 PO 4 and H 2 SO 4 are typically 80 and 100 V respectively [60, 62]. Below the breakdown limit, the anodic oxide film formed is relatively thin and non-porous using the above mentioned electrolytes. Figure 1.1: Representation of an electrochemical cell in which metallic Ti is anodized. Fabrication variables include temperature, voltage, ph and electrolyte composition [Grimes et al, Springer, 2009] Anodization Parameters The anodization process can be influenced by many factors such as anodization voltage, nature of substances composing the electrolyte (acidic nature, aqueous or non-aqueous solvent, nature of cations, addition of other compounds), ph, duration and temperature. The resulting oxide film properties (such as degree of nanometer roughness, morphology, chemistry, etc.) after anodization vary over a wide range according to different anodization process parameters. Acids such as phosphoric acid (H 3 PO 4 ), sulfuric acid (H 2 SO 4 ), acetic acid (CH 3 COOH), etc., neutral salts, and alkaline solutions are the conventionally used electrolytes for anodization. Usually, H 3 PO 4 and H 2 SO 4 were used to produce thick (tens of microns) oxide layers at high voltages. These electrolytes were used to produce conventional micro-porous Amrita Centre for Nanosciences and Molecular Medicine 6

8 structures, whereas fluoride solutions were found in recent years to have the ability of producing titania nanotubular structures [63, 64]. In fluoride containing electrolytes, deviations from the typical valve metal anodization electrochemistry occur. This is due to the fact that the formed oxide [Eq.1.2a] is now significantly chemically attacked dissolved and the formation of soluble fluoride complexes takes place according to following equations exemplified with Ti as the metal: Ti 2H O TiO H e [1.2a] 4 2 Ti 6F [ TiF6 ] [1.2b] 2 TiO2 6F 4H [ TiF6 ] 2H 2O [1.2c] These equations are used later in this thesis to explain the formation chemistry of titania nanotubes. In the absence of an electric field, these reactions account for the dissolution of the already formed anodic TiO 2 immersed in the fluoride-containing solution, or etching of the Ti substrate (under dissolved TiO 2 ) via dissolution of Ti 4+ species. In the presence of an electric field, an anodic oxide is formed (Eq. 1.2a) and it is simultaneously dissolved (Eq. 1.2c). Due to these reasons, both processes are in dynamic equlibrium [65]. 1.3 Anodization of metals and metallic alloys A major advantage of anodization is the feasibility to tune the size and shape of nanotubular arrays on various kinds of materials, viz., metals and metal alloy substrates, to obtain desired dimensions, meeting the demands of specific applications by means of controlled anodic oxidation. It can be classified as one of the simplest, cheapest and most straight-forward approaches to develop ordered porous nanostructures under right conditions. Recent advances in the understanding of a self-ordering growth by anodic oxidation have made this technique the method of choice for developing strictly Amrita Centre for Nanosciences and Molecular Medicine 7

9 ordered nanotube arrays of metals such as Ti, Al, etc. [66]. Anodization of metallic Ti generating TiO 2 nanotubes resembles the classical example of electrochemical selfordering as in the anodization of Al to form ordered porous alumina (Al 2 O 3 ) and that of Silicon forming porous Si structures. Figure 1.2 depicts examples of the selfordered structures of alumina and silicon. The high degree of ordering, lateral homogeneity and regularity of the anodized structures is clearly evident from the microscopic images shown in Figure 1.2 [67, 68]. It has been known for decades that porous oxide layers on aluminium can be grown by anodization typically in acidic electrolytes, while anodization in neutral electrolytes leads to a compact oxide layer [69-71]. In 1995, Masuda and Fukuda [72] showed that upon adjusting the anodization conditions to an optimum, an oxide containing highly ordered pore arrays could be grown. Such ordered alumina structures have been proposed, for example, to be used as a photonic crystal structure [73] or as a template for the deposition of other materials [74, 75]. The concept of growing these nanotube arrays was successively applied to other valve metals, such as Hf, Ta, W, Nb, Zr, as well as alloys such as TiNb, TiZr, TiAl etc. Figure 1.2: Scanning electron microscope (SEM) images of (a) self-organized porous alumina [adapted from Wagemaker et al, J. Phys. Chem. B, 2004,] and (b) porous silicon [adapted from Albu et al, Nano Lett., 2007] formed by electrochemical anodization. The images show side views of the layers, the insets shows magnified views (from the marked areas). Amrita Centre for Nanosciences and Molecular Medicine 8

10 1.3.1 Anodization of valve metals In recent years, there have been reports on the formation of self-ordered nanoporous and nanotube arrays by anodic oxidation of other valve metals such as Zr [76-79] Nb [80-83], W [84-86], Ta [87-89] and Hf [90]. In all these works, fluoride anion containing electrolytes were used for achieving controlled anodization of metals under anodic bias applied for several hours, leading to the growth of selforganized oxide layers on their surfaces. In contrast to alumina, anodization of Ti, Hf and Zr lead to formation of metal oxide nanotubes (hollow cylinders) perpendicular to the substrate and separated from each other by a gap [91, 92]. In the other cases (W, Nb, Ta), anodization leads to the growth of porous oxide layers [93]. Such kind of nanotubular arrays on Zr, Nb, W, Ta and Hf have found applications in the field of oxygen sensor [94] fuel cells [95, 96] etc. Figure 1.3: The number of articles published on valve metal oxide nanopore/nanotube layers formed by electrochemical anodization on different valve metals (Adapted from Ghicov et al, Chem. Commun., 2009). Amrita Centre for Nanosciences and Molecular Medicine 9

11 1.3.2 Anodization of metallic alloys Although all the valve metals find use in multitudes of applications, metallic titanium has scored the highest in this list, with the technique of anodization being the most widely explored method of preparing nanotubes. This is clearly exemplified from the publication statistics on TiO 2 nanotube layers formed by electrochemical oxidation (Figure 1.3). Apart from pure metallic titanium, the technique of anodization has also been extended to generating nanotubular features on various metallic alloys. Surface modification of metals such as Ti and its alloys have emerged as a potential strategy in improving the biocompatibility of biomedical implants [60]. Several attempts have been made to improve the surface properties of titanium-based implants to enhance initial bone bonding. Various surface modification techniques include sand-blasting, acid etching, plasma spraying, etc have been investigated [97-99]. However, anodization has evolved to be a simple, cost-effective and ideal approach to nano surface modification of titanium based alloys. The growth of nanotubes on various alloys increases the potential functionality of the tubes (e.g. incorporation of doping species in the nanostructure). Such nanotube layers can also be applied as surface coatings on various technical alloys. Using the same approach as for Ti, ie., controlled anodization in dilute fluoride electrolytes, nanotube layers have been successfully grown on intermetallic compounds such as TiAl [100], binary alloys such as TiNb [101], TiZr [102, 103] or complex biomedical alloys such as Ti 6 A l7 Nb [104] and Ti 29 Nb 13 Ta 4.6Zr [105, 106]. It was shown that the range of achievable diameters and lengths of TiO 2 -based nanotubes can be significantly expanded, if a binary Ti Nb alloy, rather than pure Ti, is used as a substrate. For anodic nanotubes formed on TiZr alloys [107] the morphological character of the oxide nanotubes are between those of titanium oxide and zirconium oxide nanotubes. The nanotubes have a straight and smooth morphology with a diameter ranging from 15 to 470 nm and a length up to 21 µm depending on the terminal anodization potential (i.e. they show a largely expanded structural flexibility compared to Amrita Centre for Nanosciences and Molecular Medicine 10

12 nanotubes formed on the individual elements). The alloy Ti 29 Nb 13 Ta 4.6Zr has been developed by Niinomi et al. for biomedical applications, in order to reduce the elastic modulus of titanium alloys to the level of living bone [108]. Although numerous reports are available on the modification of metallic alloys using the technique of anodization, such surface modified alloys find little translational use in industry and till date the major thrust has been on the anodization of metallic titanium itself. 1.4 Fabrication of Titanium dioxide (TiO 2 ) Nanotube Arrays by Anodization In 1991, Zwilling and co-workers [109] reported the porous surface of titania films electrochemically formed in fluorinated electrolyte by titanium anodization. Figure 1.4: TiO 2 nanotube array morphologies achieved by Ti anodization. The upper images are the architecture resulting from Ti anodization in an ethylene glycol bath; the lower image is the architecture resulting from Ti anodization in an aqueous KF bath (Adapted from Grimes et al, Springer, 2009). Amrita Centre for Nanosciences and Molecular Medicine 11

13 A decade later, Gong and co-workers [18] first reported formation of uniform titania nanotube arrays via anodic oxidation of titanium in hydrofluoric (HF) electrolyte. Ever since Grimes group reported titania porous structure formation by anodizing titanium foils in a fluoride contained aqueous electrolyte, extensive efforts have been made to fabricate TiO 2 nanostructures by anodization. Titania nanotube arrays (TNAs) not only provide a larger surface but also are beneficial for electron transport. Thus, TNAs are an excellent module for use in optical and electronic devices, such as dye sensitized solar cells [ ] photo catalysis [114] and sensors [ ]. In all of these applications, the dimensions of the TNAs, such as the outer diameter and the length of the nanotubes, have profound effects on their performance. Several researchers have focused on the precise control and variations in nanotube morphology [118], length, pore size and wall thickness [119]. Illustrative TiO 2 nanotube array morphologies grown by Ti anodization are shown in Figure 1.4. These vertically oriented nanotubes are firmly attached to the substrate with an open top and the bottom closed by a barrier layer of metal oxide. Electrolytes have great influence on the formation and control over the structural properties of the TNAs. The use of different electrolytic solvents permits control of the architecture from well-separated, stand-alone nanotubes, to densely packed arrays. The wall thickness of typical nanotube ranges from 5 to 30 nm, the pore size from 20 to 350 nm, and the length from 0.2 to 1000 μm. The aspect ratio, defined as the nanotube length- to-outer diameter ratio, can be controlled from about 10 to approximately 20,000 by selection of appropriate anodization variables [120]. 1.5 Influence of electrolytes in anodization of Titanium Great improvements have been made in the modulation of the titania nanotube morphologies including the nanotube length, pore size, and nanotube ordering in different electrolytes by varying the anodization conditions. Electrolytes govern both the formation and dissolution rates of the oxide layers and subsequently have a significant effect on the morphology of the resultant titania nanotubular structures. To Amrita Centre for Nanosciences and Molecular Medicine 12

14 date, highly ordered TNAs with tunable pore size and nanotube length have been achieved in both aqueous and non-aqueous electrolytes, either fluoride containing or fluoride free [121]. In 1999, Zwilling and co-workers achieved self-organized porous TiO 2 by anodizing a Ti-based alloy in an acidic, fluoride-based electrolyte [109, 121]. In 2001, Gong and co-workers fabricated self-organized, highly uniform TiO 2 nanotube arrays by anodizing Ti in an aqueous dilute hydrofluoric acid (HF) electrolyte [18]. Maximum nanotube lengths in this first synthesis generation were approximately 500 nm. Second generation nanotubes evolved, with typical lengths of ~ 7 μm through appropriate control of the electrolyte ph which helped to reduce the chemical dissolution of TiO 2 during anodization [122, 123]. The electrolyte ph should necessarily be high, but in the acidic range. Subsequently, the third-generation TiO 2 nanotube arrays with lengths of up to approximately 1000 μm were fabricated using non-aqueous, polar organic electrolytes such as formamide, dimethylsulfoxide (DMSO), ethylene glycol (EG) or diethylene glycol (DEG) [124, 125]. Grimes et al have suggested that the fourth generation nanotubes would be non-fluoride-based Fabrication of TNAs using aqueous electrolytes: The first generation HF-Based electrolytes In 2001, TNAs were firstly prepared by Gong et al. [18] in aqueous 0.5 wt% HF solution at room temperature using anodizing voltages from 3 to 20 V. At low anodization voltages, the film morphology is similar to that of porous (sponge-like) alumina [126], with typical pore sizes from 15 to 30 nm. Increasing the voltage to 10V removes the particulate appearance and replaces it with discrete, hollow, cylindrical tube-like features. The nanotube structure is lost at anodizing voltages greater than 23V and replaced by a sponge-like, randomly porous structure. Adding acetic acid to 0.5% HF electrolyte in 1:7 ratio resulted in more mechanically robust nanotubes without changing their shape or size [117, 127]. Amrita Centre for Nanosciences and Molecular Medicine 13

15 A constant anodization voltage produces straight nanotubes. Sweeping the anodization voltage produces tapered, conical shaped nanotubes. In 0.5% hydrofluoric acid solution, ph 1.0, and with an applied voltage between 10 and 23 V, nanotubes of well-defined shape are obtained, with their length and diameter proportional to the applied voltage. The anodization voltage was ramped up from 10 to 23 V at rates varying from 0.43 to 2.6 V/min to achieve conical nanotubes with continuously increasing pore size from top to bottom [128]. Figure 1.5 shows the FESEM images of tapered nanotubes obtained by controlling the ramping voltage. Figure 1.5: FESEM cross-sectional views of tapered nanotubes obtained: (a) by ramping the anodization voltage from 10 to 23 V over a 30 min period, 0.43 V/min, then holding the voltage at 23 V for 10 min, (b) by initially anodizing the sample at 10 V for 20 min then increasing the voltage at 1.0 V/min to 23 V then held constant at 23 V for 2 min, (c) straight nanotubes obtained by applying a constant 23 V for 45 min. Here, d denotes diameter of the conical apex, and D diameter of conical base (Mor et al, J. Mater. Res, 2003). Ruan et al. [129] later found that the surface morphology of the nanotubes anodized in an electrolyte containing 1% HF and 2.5% HNO 3 at 20 V for 4 h showed a uniform, clean, regular nanotube structure with a length of about 400 nm. However, anodization in 0.5 M H 3 BO % HNO 3 +1% HF at 20 V for 4h led to a nanotube length of about 560 nm with a large degree of pore irregularity. Anodization of Ti at constant 10 V for 2 h in 1M H 3 PO 4 with fluoride ion concentrations between wt%, yielded self-organized nanotube structures with 50 nm pore diameters. Optimized conditions for self-organized pore formation occurred at 0.3 wt% HF in 1M H 3 PO 4, with both tube length and pore diameter being potential dependent. Using an electrolyte with ph of 4.5, consisting of 1M H 3 PO 4 + 1M NaOH wt% HF Amrita Centre for Nanosciences and Molecular Medicine 14

16 [130], nanotube arrays with a diameter of 100 ± 10 nm, length 1 ± 0.1 μm, and tube wall thickness of 10 ± 2 nm were achieved by anodizing Ti at 20 V for 2 h [131]. Using the same electrolyte, Premchand et al. obtained self-organized nanotube arrays of TiO 2 by anodizing sputter-deposited titanium films 1.9 μm thick on p-type Si (100) substrates [132]. An average tube length of 560 nm, diameter ~80 nm and wall thicknesses ~20 nm was achieved. Yu and co-workers fabricated nanoporous/nanotube array films by anodizing sputter-deposited Ti films in wt% HF between 3 and 20 V [133]. Thus, the first generation of titania nanotube arrays fabricated by anodization using an aqueous HF-based electrolyte could be grown only to a maximum length of about 500 nm, despite varying the concentration of electrolytes. Hence, it was generally concluded that to increase the tubular length further using an HF-based electrolyte is extremely difficult. Although fluoride containing solution can help to dissolve TiO 2, the strong acidity of HF solution results in a faster dissolution of the formed TiO 2 nanotubes, which prevents the further growth of nanotubes. Hence, attempts are ongoing to improve the nanotube array length for utilizing these structures for varied applications Fabrication of TNAs using Buffered electrolytes: The second generation The insufficiency of the electrolyte conditions in generating long titania nanotubes with controllable dimensions led to the second generation electrolytes. Cai and co-workers first fabricated nanotube arrays several microns long using either KF or NaF electrolytes at variable ph [134, 135]. A summary is presented in Table 1.1. The desired ph was attained by adding NaOH, sulfuric acid (ph 1 2), sodium hydrogen sulfate, or citric acid (ph ) to the electrolyte before adding KF. For a fixed F - concentration of 0.1 mol/l and 1 mol/l H 2 SO 4 medium, the potential window for nanotube formation was between 10 and 25 V (Samples 01 to 08); outside this potential range no nanotubes were formed [134]. Amrita Centre for Nanosciences and Molecular Medicine 15

17 Thus, it is clearly evident that electrolyte ph affects both the behavior of the electrochemical etch and the chemical dissolution due to the hydrolysis of titanium ions. With increasing ph, the hydrolysis rate increases and this, in turn, slows the rate of chemical dissolution as shown in Figure 1.6 (Samples 10 and 13 for 10 V, 12 and 17 for 25 V) and Table 1.1, longer nanotubes were formed at higher ph. Table 1.1: Electrolyte ph and composition, anodization conditions, and size of the resulting nanotubes (Cai et al, J. Mater. Res,2005). To achieve high-aspect-ratio growth of TiO 2 nanotubes, Macak and coworkers sought to adjust the dissolution rate of TiO 2 by localized acidification at the Amrita Centre for Nanosciences and Molecular Medicine 16

18 pore bottom while a protective environment was maintained along the pore walls and at the pore mouth [ ]. Researchers have tried several modifications of process parameters to tune TNAs by adjusting the electrolyte s ph. A couple of reports are mentioned below. Taviera and co-workers performed anodization of Ti using a constant current density of ma/cm 2 in 1M (NH 4 ) 2 SO wt% NH 4 F electrolyte resulting in a tubular structure [139]. Samples anodized at 1 ma/cm 2 resulted in 950 nm thick tubular layer, with a tube diameter ranging from 60 to 90 nm. Macak et al anodized titanium foil at a constant potential of 20 V after a potential ramp of 0.1 V/s in 1M Figure 1.6: Lateral view of the nanotubes formed in different ph solutions (ph>1). The anodization conditions for each sample are listed in Table 1.1. Inset of Samples 10 and 12 show variation of pore size with anodization potential for a ph 2.8 electrolyte (Mor et al, J. Mater. Res,2003). Na 2 SO 4 electrolyte with 0.5 wt% NaF for 6 h resulting in a highly ordered porous TiO 2 layer of a maximum 2.4 μm thickness and typical pore diameters of 100 nm [139]. However, in 1M (NH 4 )H 2 PO 4 containing 0.5 wt% NH 4 F, TiO 2 nanotubes formed at 20 V had a maximum length of 4 μm with a pore diameter of 100 nm after Amrita Centre for Nanosciences and Molecular Medicine 17

19 40 h of anodization [131, 141, 142]. DC magnetron sputtering of titanium films, less than a micron in thickness, on silicon substrates were anodized in 1M Na 2 SO M citric acid with the addition of 0.4 wt % NaF at potential range from 10 to 20 V [143, 144]. At 20 V, the length and tube outer diameter of the arrays were ~ 458 and~ 100 nm. Recently, Lockman et al [145] reported that length of 3 μm TNAs can be prepared by anodization in neutral electrolyte consisting of 1M Na 2 SO wt% NH 4 F with ph ~ 7 for 30 min. Hence, upon using ph modifiers such as KF, NaF or NH 4 F based solution as an electrolyte, the dissolution of titanium oxide in the solution decreases dramatically, and the thickness of anodized films can be significantly increased to several micrometers [146]. The acidity of such electrolytes was usually tuned by adding HF, H 2 SO 4 or Na 2 SO 4. At increased ph values, the hydrolysis content increases and that, in turn, produces a significant amount of hydrous titanic oxide as a precipitate on the nanotube surface. The ideal ph range for the formation of longer nanotubes appears to be between three and five; with lower ph values producing shorter but clean nanotubes and higher ph values yielding longer tubes with unwanted precipitates. However, alkaline solutions are not favorable for self-organized nanotube formation [109, 147] Fabrication of TNAs using polar organic electrolytes: The third generation In third generation of electrolytes, various polar organic electrolytes including formamide, dimethyl sulfoxide (DMSO) and ethylene glycol (EG) have been utilized to fabricate nanotube arrays of significantly extended length, i.e., several tens of microns. The key to successfully growing long nanotube arrays is to keep the water content of the anodization bath below 5%. In the case of organic electrolytes, the reduced availability of oxygen reduces the oxide formation compared to aqueous electrolytes [148]. At the same time, reduced water content decreases the chemical dissolution of the oxide in fluorine-containing electrolytes thus aiding the formation Amrita Centre for Nanosciences and Molecular Medicine 18

20 of longer nanotubes. Illustrative formamide electrolyte compositions include formamide/n-methylformamide (NMF) containing 1 5 wt% of deionized water and wt% NH 4 F [149, 150]. Figure 1.7 represents the SEM images of 70 μm long TNAs formed in formamide (FA) based electrolyte at constant potential of 35 V for 48 h. The average outer diameter of the nanotubes is ~180 nm, wall-thickness ~24 nm, with an aspect ratio of ~390. Nanotubes with smaller pore diameter, but approximately 10% longer were obtained using a NMF electrolyte. Lower anodization potentials generally result in shorter nanotubes with smaller diameters. The increase in nanotube length with anodization voltage is attributed to the increased driving force for ionic transport through the barrier layer at the bottom of the pore, in turn resulting in faster movement of the Ti/TiO 2 interface into the Ti metal. As seen for the aqueous and buffered aqueous electrolytes, pore diameter increases with anodization voltage. Figure 1.7: FESEM images of TiO 2 nanotubes grown in FA based electrolyte at 35V for 48h showing: (a) Cross-section at lower magnification, (b) Cross-section at high magnification and (c) Top surface image (Paulose et al, J. Phys. Chem. B, 2006). Shankar and co-workers [149] used formamide-water mixtures containing fluoride ions to study the effect of five different cationic species on TiO 2 nanotube array formation. The choice of cations is a significant factor both in nanotube growth rate and its length. Under similar conditions, electrolytes containing the cation tetrabutylammonium (from tetrabutylammonium fluoride Bu 4 NF) produced the longest nanotubes of ~ 94 μm. In 2005, Ruan et al [151] reported synthesis of titania nanotube arrays by Ti anodization in fluorinated dimethyl sulfoxide (DMSO) and ethanol mixture. The Amrita Centre for Nanosciences and Molecular Medicine 19

21 addition of ethanol limits the nanotube array length to ~ 2.5 μm. Nanotube arrays of extended length from DMSO electrolytes were reported by Paulose and co-workers in 2006 [124]. Here, metallic Ti was anodized at different DC voltages (10 to 70 V) in a two-electrode electrochemical cell using an electrolyte containing DMSO and HF by varying the HF concentration from 1 to 6% for 20 to 90h. Voltage affects parameters such as pore size, wall thickness, and length of the nanotubes, as shown in Table 1.2. After anodizing at 40 V, the wall thickness was in the range of 8 17 nm, with the variation attributed to the fluctuations in dissolution of the titanium metal. Table 1.2: Effect of anodization voltage on the morphology of the resulting TiO 2 nanotube arrays, for a 70 h 2% HF-DMSO anodization [Yoriya et al, J. Phys. Chem. C, 2007]. Figure 1.8 shows FESEM images of samples prepared in the 2.0% HF- DMSO solution and anodized at 40 V (images a, c,e) and 60 V (images b, d,f) for 70 h [100, 152, 153]. Anodization parameters such as voltage, HF concentration, electrolyte water content, and anodization time play a key role in determining the nanotube length of the resulting nanotubes. A maximum length was achieved at 60 V, with a drastic decrease in length observed for anodization at 70 V and a complete dissolution of the Ti sample at 80 V after 24h. DMSO is a protophilic solvent that accepts a proton from HF which in turn reduces its activity. DMSO modifies the space charge region in the pores and thereby avoids lateral etching. This leads to a steady pore growth and decreased chemical etching of the nanotube walls. Due to the addition of water to the electrolyte, field-assisted etching of titanium at the pore bottom occurs. Water also assists chemical dissolution opening the pore mouth, resulting in thinner nanotube walls. Addition of water to the electrolyte greatly improved adhesion between the nanotubes and underlying oxide barrier layer, reducing a problem common to nanotube arrays synthesized in DMSO electrolytes. Amrita Centre for Nanosciences and Molecular Medicine 20

22 Anodization duration also affects the top-surface features of nanotube arrays. Anodization times less than 40 h usually produces tubes clogged with surface debris, ie., the solution precipitate. Longer anodization times result in more titanium dissolution into the electrolyte, which leads to higher electrolyte conductivity that in turn helps to prevent precipitation from occurring on the top surface of the nanotubes. Figure 1.8: FESEM images of a TiO 2 nanotube array sample grown from a 2.0% HF- DMSO electrolyte for 70 h at: (a, c,e) 40 V and (b, d, f) 60 V. Top surface images are seen in (a, b), cross sectional images in (c, d), and tube bottom view images, removed from the underlying barrier layer, are seen in (e, f) (Paulose et al, J. Phys. Chem. B, 2006, Yoriya et al, J. Phys. Chem. C, 2007). Using ethylene glycol (EG) electrolytes, TiO 2 nanotube arrays were prepared at anodization voltages ranging from 20 to 65 V, and the contents of NH 4 F and water ranging from 0.1 to 0.5 wt% and 1 to 4% respectively [131, 154, 155]. The current-time behaviors during fixed-potential anodization of Ti in ethylene glycol electrolytes are broadly similar. All exhibit initially stable or nearstable anodization current followed by a rapidly decreasing current until the current Amrita Centre for Nanosciences and Molecular Medicine 21

23 reaches a well-defined plateau and then slowly decreases. During the initial high current period gas evolution at the anode is observable. Since gas evolution requires electronic charge transfer, this is indicative that electronic conduction dominates early in the process. After initial formation of an insulating oxide layer the current drops rapidly, in this region electronic conduction decreases blocked by the formed oxide and ionic conduction through the TiO 2 increases. Once the oxide layer is completed over the entire exposed anodic surface, electronic conduction through the TiO 2 barrier layer becomes negligible and all conduction is ionic. So far, most of the titania nanotubes are prepared in fluoride electrolytes, in which the best pore ordering and control of the morphology have been achieved. Titania nanotubes could also be obtained by anodizing titanium in fluoride-free electrolytes Fabrication of TNAs using Non-Fluoride based electrolytes: The fourth generation In 2005, Nakayama et al. reported the formation of TiO 2 nanotube bundles, i.e. clumps of tubes rather than arrays, by anodization in a perchloric acid solution [156]. In 2007, Richter et al. synthesized similar nanotube bundles in 0.5M oxalic acid, 0.5M formic acid and 0.05M sulfuric acid with M NH 4 Cl added in each case. The nanotubes were found to contain about 20 % carbon [157] with less than 5% chlorine, suggesting that the role of chlorine in tube formation was mostly catalytic. Chloride-based anodization has an advantage over the techniques mentioned above as it can grow long titania nanotubes (up to 60 μm) in a very short time. The growth rate is over 1000 times faster than that in fluoride media. For example, anodization in a chloride-based electrolyte for 10 min may lead to a film of 60 μm in thickness. A similar experiment carried out in a fluoride-based electrolyte for 17 h only produced a layer of 17 μm in thickness. The chloride ions could perhaps play a similar role to that of fluoride ions during the anodization of titanium. However, when a chloride electrolyte is used, the dissolution rates of TiO 2 in a strong electric field (at pore base) and in a very weak field (at pore mouth) seem to be significantly Amrita Centre for Nanosciences and Molecular Medicine 22

24 different. This allows a fast growth of long nanotubes. Hahn et al. formed bundles of high aspect ratio TiO 2 nanotubes with 10 nm tube walls using anodization of Ti foil under high voltage conditions in perchlorate electrolytes [158]. Recently, it was demonstrated that nanotubes could be grown in an aqueous H 2 SO 4 solution alone [159]. Barrier type anodic titanium oxide films can be grown in a phosphate buffer solution with the ph adjusted by adding NaH 2 PO 4 and Na 2 HPO 4 [160]. In addition, porous anodic titanium oxide films have also been successfully produced in fluoridefree hot phosphate/glycerol electrolyte [161]. 1.6 Formation chemistry and self-ordering of nanotubes It is generally accepted that the mechanism of formation of TiO 2 nanotubes resembles that of the anodic aluminium oxide nanopores [ ]. At the anode (and using the example of Ti) under sufficiently high voltage, the key processes are: (i) Ti 0 is oxidized to Ti 4+ at the metal oxide interface and the Ti 4+ ions migrate (under the applied field) outwards, (ii) O 2- ions are (under the applied field) incorporated in the film and migrate towards the metal oxide interface. Depending on the transfer numbers in the oxide lattice for O 2- in comparison with the metal cation, an oxide film will tend to grow at the outer or inner oxide interface. As the system is under a constant applied voltage, the field within the oxide is progressively reduced by the increasing oxide thickness; the process is hence continuously slowing down and finally self-limiting. In other words, the decreasing field strength leads to the growth of a compact oxide layer with a finite thickness that essentially is proportional to the applied voltage [167]. An anodic film grows when the surface of titanium is oxidized with oxide anionic flow driven by a high electric field. The majority of oxide ions are from dissociation of water (when a water-containing electrolyte is used) and field assisted dissolution of oxide [120]. Therefore, at the electrolyte/oxide interface, we can expect H 2 O H OH... (1) OH O 2 H... (2) Amrita Centre for Nanosciences and Molecular Medicine 23

25 4 2 2 Ti O TiO... (3) The cations (H + ) move towards the cathode and oxide anions (O 2- and OH - ) migrate through the oxide layer, forming TiO 2 and titanium hydroxide at the oxide/ti interface, 2 Ti 2O TiO2 4 e... (4) Ti xoh Ti ) ( OH x xe... (5) Where x (1 to 4) is yet to be determined. The formation of hydroxide is proposed based on the observation of the double layer wall of the titania nanotubes [168]. When a relatively strong acidic F-containing electrolyte is used, part of oxide ions would move to the solution as water [169], 2 TiO2 6F 4H [ TiF6 ] 2H 2O... (6) Or form oxygen bubbles at the anode 2O 2 O2 4e... (7) Nanotube formation in fluoride ion bearing electrolytes occurs as a result of the interplay between three simultaneously occurring processes, namely the field assisted oxidation of Ti metal to form titanium dioxide, the field assisted dissolution of Ti metal ions in the electrolyte, and the chemical dissolution of Ti and TiO 2 due to etching by fluoride ions, which is substantially enhanced by the presence of H + ions. The following equations represent the oxide layer formation (1, 2) and dissolution (3, 4). 2 2 H 2 O O 4H 4e (1) Ti O 2 TiO 2 (2) 2 TiF H O TiO2 6F 4H (3) Ti 6F TiF (4) Amrita Centre for Nanosciences and Molecular Medicine 24

26 For titanium, in contrast to aluminium, a low ph is not sufficient to create porous oxide layers (a low ph is not sufficient to solvatize Ti 4+ ) and under almost all anodization conditions compact oxide layers are obtained. Only in the presence of fluoride ions does the situation become different. A key feature of the fluoride ions is that they are able to form water-soluble [TiF 6 ] 2- complexes. The complex formation aids the prevention of Ti-oxide layer formation at the tube bottom, but this also leads to mild but permanent chemical dissolution of the formed TiO 2. Moreover, due to the small ionic radius, fluorides are able to enter the growing TiO 2 lattice and get transported by the applied field through the oxide (competing with O 2- ) to the metal oxide interface [170]. The current time curve for electrolyte containing fluorides deviates from the classical high-field growth (Figure 1.9). Figure 1.9: Characteristic current transients for Ti anodization with and without fluorides in the electrolyte [65]. After an initial exponential decay (phase I) the current increases again (phase II) with a time lag that is shorter, the higher the fluoride concentration. Then, the current reaches a quasi-steady state (phase III). This steady state current increases with increasing fluoride concentration [171]. The initial drastic current drop is due to the formation of a compact oxide film at the beginning stage which elevates the resistance and reduces current densities. In Amrita Centre for Nanosciences and Molecular Medicine 25

27 the next stage, the surface is locally activated and pores start to grow randomly (II). Due to the pore growth the active area increases and the current increases. After some time, many pores have initiated and individual pores start interfering with each other. This is followed by a competition between the growing pores, and the current densities start to decrease again, until a stable, ordered pore growth has been established (III). The current densities show periodical fluctuations. The fluctuations are closely related to the growth and dissolution of the oxide films. The periodicity of current oscillations during the anodization process can be correlated with the regularly spaced ripples at the sidewalls of the tubes. With organic electrolytes, the donation of oxygen is more difficult in comparison to water and results in a reduced tendency to form oxide. The reduction in water content allows formation of thinner or lower quality barrier layers through which ionic transport may be enhanced, due to the faster movement of the Ti/TiO 2 interface into the titanium substrate giving rise to a longer tube [172]. The key factor controlling tube diameter is the anodization voltage [173, 174]. At higher anodization voltages, the driving force for ionic transport through the barrier layer at the bottom of the pore is greater, resulting in a faster movement of the Ti/TiO 2 interface into the titanium metal surface. This in turn leads to longer nanotubes at higher anodization voltages [128, 175]. The type and concentration of electrolytes, for a given applied potential, have to be appropriately selected to obtain better pore diameter control of the nanotubes. This restriction is imposed by the conductivity and ph value of the electrolyte, which in turn determines the size of the pores. 1.7 Morphology control of TiO 2 nanotubes In several applications demanding the use of TiO 2 nanotube arrays specifically, dye sensitized solar cells (DSSCs) and hydrogen storage, a smooth topography and orderly arrangement of the nanotubes are desirable [ ]. The demand for a strict periodicity as in a photonic crystal [179] entails very good uniformity of the TiO 2 nanotubes in an array. In this regard, morphology of TiO 2 Amrita Centre for Nanosciences and Molecular Medicine 26

28 nanotube arrays, e.g., smoothness of the layer top and orderliness of nanotubes, still remains to be further improved. So far, several effective approaches for fabricating highly ordered TiO 2 nanostructures have been developed, mainly including ion track lithography, [180, 181] atomic layer deposition (ALD), [182], two step anodization [183] and self-organization [184, 185]. Among them, the two-step anodization of titanium is the most convenient and economical method [186]. Titania nanotubes with various morphologies different from the typical nanotube arrays can be obtained by altering the anodizing parameters and processes. Alterations in the anodization voltage in a specific manner have yielded titania nanostructures as varied as bamboo-type nanotubes, [187] branched tubes, intertubes and multilayer nanotubes [188, 189], nanolace [187], etc. Examples of such structures are shown in Figure Bamboo-tubes as in Figure 1.10a can be obtained by voltage pulsing between two levels [188]. TiO 2 nanolace sheets (Figure 1.10b) could be obtained by extending alternating anodization with a sequence of 50 s at 120 V and 600 s at 0 V. The authors ascribed the formation of this morphology to the different etching rates by fluoride containing electrolyte [189]. The walls of nanotubes formed at 120 V periods are severely etched during the 0 V periods, and the reinforced compact parts could well survive. Repeating such anodization sequences results in a stack of the nanolace sheets. Figure 1.10c shows double layer titania nanotubes prepared by an increase of anodizing voltage from 15 V to 30 V in ethylene glycol electrolyte containing 0.3 wt% NH 4 F and 2 vol% H 2 O [190]. It has been found that the pore size of nanotubes is a function of anodizing voltage. When the voltage is suddenly dropped to a much lower value, the pore size would have to be reduced considerably. A large nanotube may be split into two or more small nanotubes, resulting in some hierarchically branched nanotubes [191]. If the growth is entirely interrupted and the system is re-anodized after a sufficient time, then tube insertion as in Figure 1.10d occurs. Amrita Centre for Nanosciences and Molecular Medicine 27

29 Figure 1.10: SEM micrographs from bamboo type nanotubes (a) (crosssection), (b) nanolace (top), (c) branched-type nanotubes (cross-section) (figure reprinted with permission from Albu et al.164 Copyright (2008) Wiley-VCH) and (d) formation of intertubes (bottom) after applying pulsing potential for different times. 1.8 Two- step anodization process Two-step anodization process of titanium involves several steps. This is mainly used for improving the quality (orderliness) of nanotubes. The common factors found by scientists are usage of aged electrolytes and substrates with patterned imprints. Mostly, all the anodization processes are carried out potentiostatically. Li et al. [192] fabricated highly ordered nanotubes and some novel nanostructures of titania (TiO 2 ) with a two-step anodization method. In the first anodization step, a well ordered imprint was created on the metallic Ti substrate in a selected electrolyte system (eg: ethylene glycol containing 0.25 wt% NH 4 F and 1 v% H 2 O). This provided well-ordered imprints that served as a template for the further growth of nanotubes. After removal of the resulting oxide by ultrasonication, the second step of anodization was carried out in aged electrolytes. Aging of the electrolyte during the second step was found critical in modulating the morphological variations. The structures produced by second-step anodization appreciably outperformed the conventional nanotubes in terms of size uniformity and arrangement orderliness. For lower anodization voltages in the second step, lotus root-shaped TiO 2 nanostructure (Figure 1.11a) resulted. When the second anodization step was further divided into two stages, double-layered (Figure 1.11b) nanotube arrays containing two distinctly separate parts, viz., a bamboo-shaped upper one and smooth-walled lower one were formed. These results demonstrate the effectiveness and Amrita Centre for Nanosciences and Molecular Medicine 28

30 controllability of the two-step anodization method in producing high-quality TiO 2 nanotubes. b Figure 1.11: Represents FESEM images of (a) Lotus-root-shaped nanostructure obtained under a 30 V anodizing voltage in the second-step anodization, (b) Double-layered TiO 2 nanotube arrays (Li et al, J. Phys. Chem. C, 2009). A major work exploiting the use of multi-step anodization has been recently carried out by Ye et.al [193] who used the hierarchical nanotubular titania structures developed by two-step anodization and further modified through hydrothermal treatment for fabricating DSSC. The first step anodization is carried out in ethylene glycol containing NH 4 F at 50V for 3 hours. The resulting nanotube film was then removed by ultrasonication, leaving behind hexagon-like footprints on the Ti surface. A second anodization was then performed under the same condition for 2 hours to produce well-aligned TiO 2 nanotubes in which a layer of mesopores was formed on the top. All these results demonstrated the feasibility of applying two-step anodization process in generating morphologically tunable nanostructure arrays. Such structures would have potential applications in the field of solar cells, photonic crystals, and hydrogen storage. 1.9 Applications of Titania Nanotubes Certain specific properties of TiO 2 such as its semi conductive nature or the specific ion intercalation properties find use in varieties of applications ranging from photocatalysis, self-cleaning and anti-fogging; to its utility for solar cells, batteries, Amrita Centre for Nanosciences and Molecular Medicine 29

31 electrochromic devices, etc [167]. Several recent studies have shown that titania nanotubes have better properties compared to many other forms of titania for applications in photocatalysis [194] gas sensors, [195, 196, 106] photoelectrolysis, [197, 198] and photovoltaics [ ]. In many of these applications, the defined geometry of the nanotube layer can bring specific advantages. For example, the specific tube geometry allows the fabrication of flow-through membranes with size selective (filtering) properties or a much defined reaction path (defined retentiontimes). Furthermore, a whole range of applications is based on use of TiO 2 in biomedical devices where the tubular nature may be exploited for drug delivery, or situations wherein the periodic nanoscale size influence its interaction with biological tissues [167, 1120] DSSCs using TiO 2 nanotubes One of the widest uses of TiO 2 in functional applications is in dye-sensitized solar cells (DSSC), which is actually based on an original idea of Fujishima and Honda [202]. For making TiO 2 electrodes active under visible light by dye sensitization, the work of Clark and Sutin [203] where a Ru-complex was used as a dye was adopted. In 1985 Desilvestro et al. [204] showed that the light to electricity conversion efficiency of dye sensitized TiO 2 can be drastically enhanced if the latter is used in nanoparticulate form. Since 1991, dye-sensitized nanoparticulate TiO 2 has been used in Grätzel-type solar cells [205]. The record efficiency reported using this construct was 12.3% [205]. However, the slow transport of charge carriers in DSSCs based on TiO 2 nanoparticles limited the photoconversion efficiency because of the relatively high probability for recombination. Amongst the materials being developed for DSSCs, TiO 2 remains the most promising candidate because of its high efficiency, chemical inertness, photostability and low cost [207]. One of the key advantages of using anodically grown TiO 2 nanotube layers is the fact that its crystalline structure as well as morphology can be adjusted over a wide range. Recently, several studies demonstrated that tube length Amrita Centre for Nanosciences and Molecular Medicine 30

32 and radius are crucial parameters for optimal light conversion efficiency [ ]. However, apart from purely geometrical effects of the tubes, morphology of the outermost surface is particularly crucial. Overall, for contemporary TiO 2 nanotube based solar cells, the critical factor is the available dye-covered surface area on the tube walls [213, 214]. Dye-loading is not only found to depend on the geometrical area (tube length, diameter), but also on the annealing temperature of nanotubular layer. Figure 1.12 illustrates a typical dye solar cell structure based on titania nanotube arrays illuminated from the backside. Light enters the cell through the transparent conducting glass. The dye absorbs enough energy from photons and an electron is excited from the molecular ground state to an excited state, from where an electron can be injected into the conduction band of TiO 2. This then diffuses to the anode. Meanwhile, the dye molecule can regain its missing electron when it travels back to the electrolyte from the external circuit. The pore size and tube length of nanotubes have a significant effect on the performance of nanotube based DSSCs. Zhu et al. [213] found that the DSSC conversion efficiency was in direct proportion to the nanotubular length. When nanotube length increased from 1.9 to 5.7 μm, the photocurrent efficiency increased from 1.7 to 3%. The photoconversion efficiency of DSSCs fabricated using TNAs has been recently improved to over 6% [215, 216]. A typical TiCl 4 treatment by immersing the tubular film into the TiCl 4 aqueous solution, followed by appropriate rinsing and annealing, could increase the roughness of the tube walls and thus improve the cell performance through an increased effective surface area for dye adsorption [2-4]. Ye et.al [193] have recently demonstrated that using a combination of two-step anodization with hydrothermal process, generating vertically oriented TiO 2 nanoscopic tubes with large number of nanoparticles on the surface, led to increased dye loading and a subsequent high power conversion efficiency of 7.75%. Amrita Centre for Nanosciences and Molecular Medicine 31

33 Figure 1.12: Schematic diagram of backside illuminated dye solar cell structures [10] Photo catalysis and water photo electrolysis As mentioned earlier, TiO 2 has excellent photo catalytic properties based on the fact that the valence band position allows photo generated holes to react with the surrounding water to form OH radicals. These in turn decompose essentially any organic material in the surrounding to H 2 O and CO 2 [170]. Eversince Fujishima and Honda [36] found in 1972 that anatase TiO 2 could decompose water into hydrogen and oxygen under light illumination, TiO 2 has been widely applied for photo electrochemical water splitting. The optimum band gap of an effective semiconductor for water photo electrolysis was reported to be in the range of ev. [217]. TiO 2 has a much wider band gap of about 3 ev, the photo catalytic activation of which falls in the ultraviolet light range, which accounts for only a small fraction (8%) of sun s energy. It is demanded to extend the photo response of TiO 2 into the visible spectral range, which accounts for about 45% of sun s energy [218]. Recently it has been Amrita Centre for Nanosciences and Molecular Medicine 32