One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices

One-dimensional (1D) TiO 2 nanostructures (e.g., nanotubes, nanobelts, nanowires, and nanorods) have been considered to be very attractive candidates for various applications including photocatalytic degradation of pollutants, photocatalytic CO 2 reduction into energy fuels, water splitting, solar cells, supercapacitors, and lithium-ion batteries. More importantly, the dimensionality associated with zero-dimensional TiO 2 nanostructures gives unique physical properties, including a high aspect ratio structure, chemical sta -bility, excellent electronic or ionic charge transfer, and a specific interface effect. This chapter elaborates on crystal structure and properties, preparation techniques, strate gies for improving photocatalytic activity of 1D-TiO 2 nanostructure and its applications. Amongst all preparation techniques, the influence of experimental parameters on mor phologies of 1D-TiO 2 nanostructure using hydro/solvothermal method is extensively explained. Furthermore, some critical engineering strategies to enhance the properties of 1D-TiO 2 nanostructures like increasing the surface area, extending the light absorption, and efficient separation of electrons/holes that advantage their potential applications are described. Moreover, a brief summary of their environmental and energy applications is provided. on the development of sustainable environmental remediation and energy tech nologies based the photocatalytic process by driving solar light as renewable source of energy.


Introduction
The wide applications of titanium dioxide, such as pigments, sunscreen, paints, and various commercialize applications correspond to its profound optical refractive ability, chemical stability, and low toxicity. Furthermore, TiO 2 has been extensively investigated as one of the most promising materials in photovoltaic devices, photocatalysis, photodegradation, energy storage, and electrochromic devices. This is being established when the photoelectrolysis of water into H 2 and O 2 was reported under UV irradiation of the solar spectrum over the surface of TiO 2 photoelectrode in the photoelectrochemical (PEC) cell by Fujishima and Honda. In recent years, 1D-TiO 2 nanostructures have been significantly studied due to its distinctive advantages with regard to less charge carrier recombination rate and unique physical and chemical properties [1,2]. In general, 1D nanostructures are well-known for their high aspect ratio in which at least one of its dimensions should be in the range of 1-100 nm regardless of their shapes such as rod, wire, belt or tube. The properties of one-dimensional single crystalline Ti-O-based nanomaterials are comparable with titania nanoparticles not only inherit almost all typical features of the nanoparticles, but also illustrate some new properties and improved performance in specific area [1]. It can afford direct transport pathways for charge carriers, decouple the direction for light absorption, and charge carrier transportation. Moreover, 1D semiconductor nanostructures are able to facilitate the light absorption and scattering, which is beneficial for photocatalytic reactions.
However, wide applications of 1D-TiO 2 nanostructures are diminished due to absorption of small portion of solar spectrum, relatively low specific surface area, and presence of single phase that pose certain limitations for it performance [3]. Several published research works have illustrated that well designed 1D-TiO 2 nanostructures play a significant role to overcome the above-mentioned problem, preserve the intrinsic characteristics of TiO 2 , and finally the material with some new properties. In recent years, breakthroughs have continually been made in the preparation, modification, and applications of 1D-TiO 2 nanomaterials. In this chapter, we would like to address the crystal structure of TiO 2 and various synthesis process of 1D-TiO 2 nanostructured materials. Then, we will look into some critical structure engineering strategies that give 1D-TiO 2 nanostructured materials excellent properties for various applications.

Figure 1a
reveals that tetragonal structure of anatase is the result of corner (vertices) sharing octahedron which form (001) planes. In rutile structure, sharing edges of octahedrons at (001) planes give the tetragonal structure as shown in Figure 1b. While an orthorhombic structure of Brookite is the result of sharing both edges and corners of octahedrons [5,6]. Rutile is the most stable crystal phase of TiO 2 while anatase is a metastable phase, which can be transformed to thermodynamically stable rutile over calcination temperature exceeding ~600°C [7,8].
It is worth to mention, amongst all three different phases of TiO 2 , anatase is well-known phase for better photocatalytic performance in various applications, including photocatalytic degradation of organic dye molecules, photocatalytic water splitting and dye-sensitized solar cells compared to other phases [9,10]. The photocatalytic reaction usually accelerates when a semiconductor interacts with light with enough energy (or a certain wavelength). Two simultaneously reactions occur, including oxidation of dissociative adsorbed H 2 O by photogenerated holes and reduction of an electron acceptor by photoexcited electrons. In the photocatalysis process, light energy greater than the bandgap energy is required to transfer photoexcited electrons to the conduction band of semiconductors. Figure 2 shows that the absorption of photons with enough energy (λ ≤ 390 nm for anatase TiO 2 ) transfer photoexcited electron to the conduction band ( e ¯ CB ) and leave a positive hole behind in the valence band ( h VB + ) [4,11]. Many parameters such as size, specific surface area, pore volume, pore structure, crystalline phase, and the exposed surface facets can significantly influence the photocatalytic performance of TiO 2 [2,10]. It is well known that the photocatalytic applications of TiO 2 are based on the nanoscale materials due to the quantum confinement effects in nanoparticles. Therefore, this effect could change the electron and hole transport behavior and shifts the electronic band structures [7,12]. However, they also show unavoidable disadvantages such as fast recombination rate of electron and holes, slow charge carriers transfer and high recycling cost [3]. It is noteworthy to mention that effective charge separation can be considered as the most important factor to determine the photocatalytic activities. Construction of one-dimensional nanostructure of TiO 2 photocatalyst like nanowire, nanorod, and nanotube also facilitates charge transportation and promotes charge separation efficiency [1]. The one-dimensional titanium oxide with a large surface area can be used as catalytic carrier and be beneficial in absorbing degradation products [13]. One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices http://dx.doi.org/10.5772/intechopen.72976

Synthesis of 1D-TiO 2 nanostructures
Several methods such as hydrothermal, vapor deposition, sol-gel, and electrospinning, etc. were applied to synthesis various morphologies of 1D-TiO 2 nanostructures like nanotubes, nanorods, nanowires, nanobelts, nanosheets and nanofiber. In this section, we provide the comprehensive information related to hydrothermal method, which is widely used for manufacturing of small particles in the ceramic industry using aqueous or non-aqueous solution.

Hydrothermal
Hydrothermal is one of the most common methods to synthesis 1D-TiO 2 nanostructure due to simple setup, facile operation, and desirable results. Ever since Kasuga et al. [12] in 1988 showed the first evidence that oxide nanotubes can be obtained easily via chemical treatment, without the need for molds for replication or templates, much research has been carried out on the formation of 1D-TiO 2 nanostructures. In a typical hydrothermal synthesis, TiO 2 or its precursors are dissolved in a concentrated aqueous acidic or alkaline solution and is implemented stainless steel at elevated temperature and pressures [13]. In the former method, the reactants are usually titanium salts with hydrochloric acid and the reaction normally leads to the formation of TiO 2 nanorods. In the latter method, the reactants are TiO 2 nanoparticles and sodium hydroxide solution, which dissolution-recrystallization is always involved in this process and the products include nanotubes, nanowires, and nanobelts. Figure 3 shows the various morphology of synthesized 1D-TiO 2 nanostructure with hydrothermal method [13][14][15][16]. Although the synthesis process seems simple, the preparation parameters including the choice of TiO 2 precursors, the hydrothermal condition (temperature, the concentration of reactants and hydrothermal duration), and post washing procedures play important role in the crystal structures and physicochemical properties of 1D-TiO 2 nanostructures [17]. The choice of initial raw materials such as anatase, rutile, brookite, and amorphous TiO 2 may affect the morphology of the resultant 1D-TiO 2 nanostructures but no systematic data is available. Yuan and Su [18] reported the effect of various TiO 2 precursors on the morphology of produced 1D-TiO 2 nanostructures. Crystalline anatase or rutile or commercial P-25 as the raw materials formed titanium oxide nanotubes with diameter 10 nm in the range of reaction temperature of 100-160°C as illustrated in Figure 4a [18]. In addition, the surface area of product also affected by raw materials as surface areas of the produced nanotubes from commercial P-25 powder was higher than lab-made anatase TiO 2 . It noteworthy to mention that no nanotubes were identified when amorphous TiO 2 powders were the precursor with similar hydrothermal treatment in the presence of NaOH. Figure 4b shows that the product morphology was non-tubular needle-shaped fibers morphology in the presence of NaOH with concentration of 5-15 mol/l at the hydrothermal temperature range of 100-160°C. In addition, Nian et al. [19] synthesized anatase TiO 2 nanorods with a specific crystal-elongation direction through hydrothermal treatment of titanate nanotube suspensions under an acidic environment in the absence of surfactants or templates. They suggested that the transformation of the tube to rode is a result of local shrinkage of the tube walls to form anatase crystallites and the subsequent oriented attachment of the crystallites. Furthermore, the hydrothermal temperature strongly controls the morphologies of products. In addition, the increasing of hydrothermal temperature improves yield, length, and degree of crystallinity of nanotubes. The yield of nanotubes increased with the hydrothermal temperature when the temperature was in the range of 100-150°C. The experimental results showed that hydrothermal treatment at below 100°C is not effective to transfer TiO 2 particles and a large amount of residual TiO 2 particles can be found in the product. In another perspective, increasing temperature could facilitate unidirectional crystal growth, leading to different morphologies of 1D-TiO 2 nanostructures. As Yuan et al. reported that crystalline or amorphous TiO 2 powder mainly was transferred to nanoribbons with very high yields (almost 100%) when hydrothermal temperature was in the range of 180-250°C with the NaOH concentration of 5-15 mol/l as shown in Figure 4c [18]. Moreover, the hydrothermal treatment duration has a strong effect on the morphological structure of the synthesized product; it also plays a major rule in the conversion of the nanotube structure into nanoribbons as reported by Elsanousi et al. [20]. Figure 5 shows the effect of hydrothermal treatment duration (5-72 h) at the fixed temperature of 180°C on the morphology of the titanate nanotubes and nanoribbons. The hollow nanotubes were found out with an outer diameter of about 10 nm at treatment duration of 5 and 20 h. While further treatment duration up to 72 h was caused bundles of nanoribbons with widths ranging from 50 to 500 nm and lengths up to several tens of micrometers. Additionally, the experimental results illustrate that there are critical conditions depending on both the treatment duration and varying temperatures (120-195°C), possibly due to a critical pressure, which is needed to be reached so that the transformation process takes place. The concentration and type of alkaline solution also play an important role in the hydrothermal process. The increasing concentration of NaOH can accelerate the hydrothermal reaction owing to the enhancement of Ti (IV) dissolution and exfoliation rates of the precursors. High yield of nanotubes with the maximum surface area of 350 m 2 /g can be synthesized with the NaOH concentration between 10 and 15 mol/l. The yield of nanotubes is very low when the NaOH concentration is lower than 5 M or as high as 20 M [18]. Bavykin et al. [21] investigated the influence of the binary NaOH/KOH aqueous mixture used in the hydrothermal process on the morphology of 1D-TiO 2 nanostructures. All observed nanostructures, including nanosheets, nanotubes, nanofibers, and nanoparticles, have been mapped over a wide range of compositions (from pure NaOH to pure KOH) and temperatures (from 50 to 110°C). The hydrothermal process is a cost-effective method with good dispersibility and high purity illustrating the great potential for formation of 1D-TiO 2 nanostructures. However, we cannot ignore its limitations, which diminish its wide applications. For instance, slow reaction kinetics result in long reaction, limited length of the nanotubes, and non-uniformed nanotube for large-scale application.
The coupling hydrothermal treatment with microwave heating, ultrasonication and a rotating autoclave on the reaction mixture can reduce the shortcomings of hydrothermal technique [17,22].

Solvothermal method
Solvothermal method facilitates the synthesis of nanometer-sized crystalline TiO 2 powder at relatively low temperatures. Solvothermal reactions are similar to hydrothermal method while a non-aqueous solvent reacts under conditions of high pressure and mild temperature. This method shows promise for developing nanotechnologies. Organic solvents during solvothermal synthesis control the properties of products, corresponding to the structure. The various physical and chemical properties of selected solvent such as reactivity, the polarity, coordinating ability of the solvent, etc. affect the morphology and the crystallization of the final products. Furthermore, the influence of other reaction parameters such as temperature, stirring conditions, and co-solvent (water-ethanol, water-ethylene glycol) on the morphologies of the synthesized nanostructures (nanotubes, nanorods, nanowires, and nanoribbons), as well as their growth mechanism, have been explored [23]. Chen et al. [24] reported the  preparation of a single-layer polycrystalline anatase TiO 2 (SLP TiO 2 ) nanosheets Figure 6a with a porous structure through a simple solvothermal method by employing, rod-like titanyl sulfate, as the starting material, in the presence of glycerol, followed by a calcination process.
The structure and morphology were found to be dependent on the experimental conditions such as solvothermal reaction time, morphology of titanyl sulfate, and solvent type. Que et al. [25] successfully synthesized the nitrogen-fluorine co-doped TiO 2 nanobelts (Figure 6b) with anatase phase structure by the solvothermal method, which employs amorphous titania microspheres as the precursor. Results demonstrate a significantly enhanced photocatalytic degradation of methyl orange compared to commercial TiO 2 . Zhao and his co-workers [26] reported the synthesis of TiO 2 nanorod arrays (TNRs) directly on FTO glass (Figure 6c) through the solvothermal method, and thermal treatments. The results show that the crystal structure does not change due to thermal treatment. However, the surface morphology appears to change significantly from a thin amorphous layer to tiny crystallite spheres. All of these changes lead to a 39% improvement in the photoelectric conversion efficiency for the nanorod-based photoanode in dye-sensitized solar cells (DSSCs). These findings might be useful in photoelectrical applications of the solvothermal method.

Other synthesis method
Sol-gel method is another solution-based growth technique, offering several major advantages for mass production of nanomaterials including low-cost, simple processing, and good scalability. This technique is typically conducted through two steps; sol preparation, including mixing the precursors such as metal organic compounds or inorganic metal salts through vigorous stirring to complete hydrolysis and polymerization reaction and gel preparation by removing solvent and converting the sol to a three-dimensional network [27]. This technique is mainly useful for synthesizing oxide ceramic nanomaterials from hydrolyzing titanium precursors. Through sol-gel process, TiO 2 NPs can be aligned following their crystal orientations and form NWs. For example, Rodríguez-Reyes et al. prepared nanocrystalline TiO 2 wires (Figure 7a-c) by the sol-gel method, using titanium isopropoxide (TIP) and acetic acid as a TiO 2 sol modifier in alcohol solvent showed to be a successful synthesis route of Ti─O─Ti inorganic network with controlled properties [28]. The apparent 1-D morphology of TiO 2 -related nanowires was thermally stable from 400 to 600°C, showing a similar diameter (about 76 nm); however, crystallite size increases with respect to temperature from 13 to 75 nm.
Vapor deposition method has been developed to high degree of crystallinity 1D-TiO 2 nanostructures (usually single crystal TiO 2 ) and it can be classified in chemical vapor deposition (CVD) and physical vapor deposition (PVD) [27]. Chen et al. [29] grown well-aligned densely-packed rutile TiO 2 nanorod via metal-organic chemical vapor deposition (MOCVD) (Figure 8a), using titanium-tetraisopropoxide (TTIP, Ti(OC 3 H 7 ) 4 ) as a source reagent at a deposition temperature of 550°C and under an oxygen pressure of 1.5 and 5 mbar, respectively. The rutile TiO 2 nanorods (Figure 8b) were grown with a very high density and exhibited uniform height. However, this method requires expensive equipment and the cost is too high for mass production.
In addition, nanofibers in different forms, such as core-shell hollow and porous nanofibers are produced with electrospinning method as one of the most conventional methods [30]. These structures of nanofibers can be utilized for new applications such as ultra-filtration, fuel cells, membranes, tissue engineering, catalysis and hydrogen storage. Electrospinning provides  One-Dimensional Titanium Dioxide and Its Application for Photovoltaic Devices http://dx.doi.org/10.5772/intechopen.72976 a straightforward electrohydrodynamical mechanism to produce fibers with diameters less than 100 nm, even up to 5 nm. Under the influence of an electric field, a pendant droplet of the polymer solution at the spinneret is deformed into a conical shape. The post heat treatment is usually needed to remove the solvent and solidify the fiber structures. The viscosity, conductivity, and applied solvents, as well as the conformation and molecular weight of the polymer limit the electrospun ability of a polymer solution. Some polymers are not spinnable because of limited solubility in a proper solvent for electrospinning, having proper polar characteristics [31]. In addition, electrospinning is an efficient method for mass production however; the high resistance caused by the polycrystalline characteristics of the product nanofibers limits its applications. Figure 9 shows that synthesized anatase nanofibers using electrospinning technique by Li and Xia [30]. They injected ethanol solution including poly(vinyl pyrrolidone) (PVP) and titanium tetraisopropoxide through a needle under a strong electrical field. The final product was the composite nanofibers with lengths up to several centimeters, consisting of PVP and amorphous TiO 2 and followed by calcination process at 500°C. The average diameter of nanofibers was varied from 20 to 200 nm owing to changing a number of parameters like ratio between PVP and titanium tetraisopropoxide, their concentrations in the alcohol solution, the strength of the electric field, and the feeding rate of the precursor solution.

Strategies for improving TiO 2 nanostructured photoactivity
As mentioned in the previous sections, 1D-TiO 2 has a wide range of applications, however in this book chapter, we intend to address only the photocatalytic applications including photocatalytic degradation and photocatalytic solar hydrogen production. Nanostructure materials need to meet some requirements for photocatalytic application; (i) should possess a sufficiently large active surface area, (ii) have a broad light absorption band to utilize the full range of solar spectrum, and (iii) should be an effective charge carrier separation to transfer more electron and hole to the interface of the electrode/electrolyte. Nevertheless, there is no Titanium Dioxide -Material for a Sustainable Environment single material, which can match all above criteria [1,6,32,33]. Here, we reviewed the developed strategies on bandgap engineering of titania to extend light absorption into visible light through doping metal and non-metal ions and compositing with another semiconductor for synergic absorption and charge separation for enhanced utilization of solar energy [34].

Doping with metal and non-metal ions
Incorporation selective doping of metal ion into 1D-TiO 2 nanostructured materials has been proven an efficient route to improve visible light absorption with hindered charge carrier recombination rate. The presence of transition metal ion in the structure of 1D-TiO 2 nanostructured materials increases the formation of Ti 3+ ions, leading to improve photocatalytic activity, owing to the existence of more oxygen defects, which facilitate the efficient adsorption of oxygen on the titania surface. In addition, the substitution of metal ions into the TiO 2 induces visible light absorption because of introducing intraband state close to the CB or VB edge and charge transfer transition between the d electrons of the dopant and the CB (or VB) of TiO 2 nanostructures [34,35]. Wang et al. [36] investigated the effect of Fe, Mn and Co as dopants on the photoelectrochemical cell performance of TiO 2 nanorods. The maximum photocurrent density of 2.92 mA/cm 2 at 0.25 V vs. Ag/AgCl for Fe/TiO 2 , which is five times higher than that of undoped TiO 2 confirmed the presence of Fe into TiO 2 is the most favorable metal to improve the photocatalytic activity of TiO 2 compared to others. Figure 10a shows that the photocurrent density of Fe-TiO 2 is as high as 0.96 mA/cm 2 at 0.25 V vs. Ag/AgCl under visible light illumination (>420 nm). Incident-photon-to-current-conversion (IPCE) efficiency (up to 18%) measurements reveal that the Fe-TiO 2 nanorod sample significantly improves the photoresponse not only in the UV region but also in the visible light region, as illustrated in Figure 10b.  Moreover, doping non-metal atoms such as nitrogen, sulfur carbon, boron and iodine can extend the absorbance to the visible region and improve the stability of materials through the simple and effective method [37]. For instance, TiO 2 /graphene composites were synthesized by Xing et al. [38] through hydrothermal method by decorating Ti 3+ self-doped TiO 2 nanorods on boron-doped graphene sheets, in which NaBH 4 acted as reducing agent and sources of boron dopant on graphene. The produced TiO 2 nanorods had the length of 200 nm with exposed (100) and (010) facets as shown in Figure 11a.
The loading of TiO 2 nanorods on graphene sheets was characterized by TEM (Figure 11b), and confirmed TiO 2 nanoparticles were covalent bonded to GO, forming a composite favoring the separation of electron-hole pairs (Figure 11c). All of the composites tested exhibited improved photocatalytic activities as measured by the degradation of methylene blue and phenol under visible light irradiation. This better photocatalytic activity was attributed to the synergistic effect between Ti 3+ self-doped TiO 2 and boron-doped graphene.

Coupling TiO 2 with semiconductors
The fabrication, design, and tailoring of coupling other semiconductors like CdS, Cu 2 O, CdSe, WO 3 , etc. with 1D-TiO 2 nanostructures to achieve better charge carrier separation in a light energy conversion system have received significant attentions. The charge transfer from one semiconductor to another with suitable band edge positions is thermodynamically favorable to increase the lifetime of the charge carriers thus promoting the interfacial charge transfer and catalytic efficiency [34,39]. Zhu et al. [39] deposited CdS on TiO 2 nanotube arrays (TNTAs) by successive ionic layer adsorption and reaction (SILAR) method for visible-light-driven hydrogen production and organic compound degradation. CdS has narrow bandgap (∼2.40 eV) and relatively high visible absorption coefficient of CdS enables its highly desirable use in solar applications. Coupling CdS with TiO 2 can hamper electron-hole recombination that dominating the charge separation. Figure 10 shows the schematic diagram of charge transfer in CdS-TNTAs photocatalyst for visible-light photocatalysis. Therefore, this charge transfer can accelerate the separation of charge carriers and enhance the visible-light response and photocatalytic activity for H 2 generation and Rh B degradation of CdS-TNTAs.

Applications of 1D-TiO 2 nanostructures
TiO 2 nanostructured materials are widely used as photocatalysts due to its high oxidation and reduction ability. 1D-TiO 2 nanostructures have been paid much attention to photocatalytic degradation of pollutants, photocatalytic CO 2 reduction into energy fuels, photocatalytic water splitting, solar cells, supercapacitors and lithium batteries. This section is dedicated to the application of 1D-TiO 2 nanostructures in photocatalytic water splitting and dye sensitized solar cells (Figure 12).

Dye sensitized solar cell
The first sensitization of large bandgap energy toward the visible region was reported in 1972 with ZnO semiconductor with the photoconversion efficiency of 1-2.5%. Gratzel et al. reported a breakthrough in the efficiency over 7% in 1991 using large surface area nano-crystalline TiO 2 thin film, sensitizing with ruthenium complex. They explained that high surface area of TiO 2 helps to better absorb and attach dye on the surface of TiO 2 thin film [40,41]. Figure 13a and b displays a schematic presentation of DSSC and its operation principle. It includes nano-crystalline TiO 2 thin film as a working electrode (WE) or photoanode with a monolayer of sensitizer in contact with iodide/tri-iodide redox electrolyte, which is sandwiched by second conductive glass covered with platinum as a counter electrode (CE). The most efficient DSSC had the highly mesoporous of anatase phase of TiO 2 which was coated on the surface of FTO (F-doped tin oxide) glass substrate with thickness 5-20 μm and covered by a monolayer of sensitizer. To overcome the drawback of TiO 2 nanoparticles, a 1D-TiO 2 nanostructure is often applied as photoanode to enhance electron transfer ability. Figure 14a shows a novel TiO 2 nanoparticles and TiO 2 nanotube (TNP/TNA) multilayer photoelectrode via a layer-by-layer assembly process to improve the DSSC performance as reported by Yang et al. [42]. The fabricated DSSC with multilayer photoelectrode has higher efficiency than the single-layer or bare DSSCs. The TNP/TNA four-layer photoelectrode provided a large surface area for dye adsorption with the highest photocurrent density (Figure 14b) and maximum photoconversion efficiency of 7.22% because of effective electron transport.

Photocatalytic solar hydrogen production
Energy dense fossil fuels are non-renewable source and the most coveted fuel that have ever been discovered, burning fossil fuels release such significant amount of greenhouse gases in  the atmosphere as major threats to the environment and human health. Hydrogen has been established as a clean energy carrier in many applications such as automotive, domestic heating, aircrafts and stationary power generation. Utilizing solar energy to split water into H 2 and O 2 in photoelectrochemical (PEC) cell is in fact one of the most promising technologies for hydrogen production. It would be interesting to combine solar energy and water in PEC cell to produce truly renewable and low environmental impact fuel on both large and small scale. A photocatalyst is the core of this system with some requirements like an optimal bandgap energy approximately 2 eV and a sufficient negative CB position [6]. Up to date, there has not been found any photocatalyst that meets all requirements for hydrogen production in the PEC cell. Amongst different metal oxide photocatalysts, TiO 2 is an attractive n-type photocatalyst in terms of hydrogen production regardless of its limitations [43,44]. The vertically oriented 1D-TiO 2 nanostructure are promising materials for photocatalytic solar hydrogen production due to their impressive vectorial 1D-channel pathways for fast electron transport in the axial direction. Shinde et al. [45] synthesized nanocomposite heterojunction photoanode involving   CdS nanoflowers (NFs) and one-dimensional TiO 2 nanotube (TNT) arrays. An anodization method was employed for fabrication of TiO 2 nanotube (TNT) arrays and CdS NFs was decorated on the surface of TNT using hydrothermal method as shown in Figure 15a-d. As-grown CdS-NF/TiO 2 -NT array photoanode exhibited a 5.5-fold photocurrent enhancement in a polysulfide electrolyte compared to the pristine TiO 2 NT photoanode. Annealing of TiO 2 NTs as well as CdS NFs led to further improvement in the photocurrent owing to greater crystallinity, significantly higher visible light photon absorption and improved interface properties between CdS and TiO 2 . The better photocatalytic performance of CdS-NF/TiO 2 -NT was attributed to effective absorption of the visible light photons, leading to the photo-generation of electron-hole pairs and greater charge carrier separation as shown in Figure 15e.

Conclusion
Over recent decades, the tremendous efforts have been paid to develop TiO 2 nanomaterials and large amount of information provided on their synthesis, modification, and applications. The one-dimensional TiO 2 nanostructures have been comprehensively studied due to its distinctive advantages with regard to less charge carrier recombination rates and unique physical and chemical properties. In this chapter, we first explain the structural features of the three TiO 2 polymorphs that have been seen in TiO 2 nanostructures. Then, the various synthesis processes of one-dimensional TiO 2 nanostructures were discussed. Hydro/solvothermal method and the effect of the experimental parameters on the formation of various morphologies and properties of 1D-TiO 2 nanostructures were thoroughly reviewed. In addition, we studied some strategies on bandgap engineering of titania to improve optical properties and charge carrier separation and transfer to the surface of photocatalyst. Finally, the applications of 1D-TiO 2 nanostructures in photocatalytic water splitting and dye-sensitized solar cells have been reported with regard to their specific structure and properties. Therefore, these data favorable for further investigation on the development of sustainable environmental remediation and energy technologies based the photocatalytic process by driving solar light as renewable source of energy.