Titanium dioxide (TiO2) nanostructures with different crystal structures and various morphologies were synthesized by hydrothermal process to utilize them in photocatalytic and photovoltaic applications.
The photocatalytic ability of TiO2 depends on its crystallinity, crystal structure, grain size, surface properties, morphology and composition.
Titanium Dioxide (TiO2)
Titanium (Ti) is one of the mostly available material in the earth crust (ninth). The common raw materials are rutile, ilmenite and leucoxene. Ti may have been in the oxide form of TiO, Ti2O3, TiO2, Ti3O5, Ti4O7; TinO2n−1 where, n ranges between 3– 9. The most stable form is TiO2 (Titanium (IV) oxide or titania) which is used in cosmetics, drugs, pigment, paper, and semiconductor. The reason of application of TiO2 as a semiconductor is due to its many advantages like low cost, wide abundancy, nontoxicity, biocompatibility, stability, proper band gap energy and photocatalytic activity. After discovery of water splitting by Fujishima and Honda, TiO2 was mostly studied for photocatalytic applications. The photocatalytic ability of TiO2 depends on its crystallinity, crystal structure, grain size, surface properties, morphology and composition.
TiO2 has three main crystal structure which are anatase, rutile and brookite as seen in Figure 2.4. The structural properties of the phases calculated by Pseudopotential Hartree-Fock model and some physical propeties are tabulated in Table 1. It has also other synthetic forms in monoclinic, tetragonal and orthorhombic structures. Also it has five high pressure forms which are α-PbO2-like, baddeleyite-like, cotunnite-like, orthorhombic and cubic phase.
Rutile is the most stable form of TiO2. Anatase phase of TiO2 is a metastable one, which is widely used for photocatalytic applications due to its wide band gap, surface properties, proper structure for electron diffusion. In general, majority of TiO2 synthesis processes result in rutile formation. It is mostly used in high temperature conditions such as gas sensors.
Brookite is another metastable phase of TiO2 between anatase and rutile. It is generally ignored due to difficulty of synthesis. Ti+4 atoms are coordinated to six oxygen atoms to form TiO6 octahedra in all three forms of TiO2. Anatase phase is formed by corner sharing octahedras, rutile is formed by edge sharing of octahedras, and brookite is formed by both edge and corner sharing of octahedras. Some physical property classification of anatase and rutile are shown in Table 2.
Table 2.Physical properties of anatase and rutile.
|Molecular Weight (g/mol)||79.88||79.88|
|Melting point (oC)||1825||1825|
|Boiling point (oC)||2500~3000||2500~3000|
|Light absorption (nm)||<380||<415|
TiO2 could be recognized as an insulator material due to its wide band gap energy (3 eV for rutile, 3.2 eV for anatase). However, TiO2 becomes in oxygen deficiency when it is equilibrated under low oxygen atmosphere. Thus, it becomes an n-type semiconductor with free electrons as charge carriers. The defects in oxygen deficient form could be caused by both intrinsic and extrinsic types which are controlled by experimental conditions and foreign anions and/or cations, respectively. The deviation from stoichiometry in TiO2-x depends on x, which may be attributed to be a function of oxygen partial pressure P(O2) as:
where, mx signifies the characteristic of defect.
The reduced charge regime is mainly a function of Ti+3 interstitials, which are compensated by electrons. The strongly reduced part is due to doubly ionized oxygen vacancies and reduced regime is dominated by ionic charge compensation. The oxidized part could be achieved by high oxygen partial pressure. All these defects have a significant importance on electrical properties of TiO2. The valance band corresponds to O related states and the conduction band corresponds to Ti related states. Anatase possesses a narrower 3d band compared to rutile, which is the result of localization of Ti 3d states due to large Ti-Ti atomic distances in this polymorph.
In photocatalytic process, an electron excites to the conduction band and leaves a hole in valance band under light illumination. Electron illumination is based on proper band gap that is light energy greater than band gap energy of semiconductor excites electron from valance band to conduction band. Relatively larger band gap energy of TiO2 makes it active only under UV illumination (<387 nm for anatase and 410 nm for rutile).
After excitation, formed charge carriers (electron and hole) can be trapped by Ti3+ and oxygen defect sites and/or recombination occurs. Separated charge carriers diffuse to the surface and initiates photocatalytic reactions. Holes are trapped by surface adsorbed H2O and oxidizes H2O to form H+ and OH– radicals that are very powerful oxidants. These strong oxidants oxidize adsorbed organic species. Consequently, CO2 and H2O are being formed. If the electrons are trapped by the adsorbed oxygen molecule reducing O2 to O2–, forms peroxide radicals like OOH– and H2O2 by reacting with H+.
Photocatalytic and photovoltaic properties of TiO2 highly depend on its synthesis method that has a direct influence on electronic configuration, crystal structure, and morphology. TiO2 suitable for optoelectronic applications can be synthesized by several methods like sol-gel, solvothermal, hydrothermal, atomic layer deposition (ALD), chemical vapor deposition (CVD), micro emulsion, micelle and inverse micelle processes.
One-dimensional TiO2 or titanate related nanomaterials with high morphological specificity, such as nanotubes, nanosheets, nanoribbons, nanowires, nanofibers and nanorods, have attracted considerable attention due to their interesting chemical and physicochemical properties. The high interest to the one dimensional materials was initiated by first carbon nanotube discovery in the early 1990s which is promising for many applications due to their excellent mechanical, optical, electrical and/or chemical properties. Nevertheless, materials other than C are much easier to synthesize due to diverse chemistry. The 1D structures may be used in a wide range of applications such as medical purpose, electrochemistry, environmental purification, gas sensors
A time line showing briefly the historical background of TiO2 related nanotubular structures can be seen in Figure 2.
The first TiO2-based nanotubes was reported by Hoyer in 1996 via an electrochemical deposition using naturally occurring porous aluminum oxide. Up to now 1D TiO2 nanostructures were synthesized using three different methods which are chemical template synthesis, electrochemical anodic oxidation method and alkaline hydrothermal treatment. The preparation of TiO2 1D structure by chemical templating usually involves controlled sol-gel hydrolysis of Ti compounds in the presence of templating agents followed by polymerization of TiO2 in the self-assembled template or deposition of TiO2 on the surface of the template.
The procedure ends by removing templating agent and calcination. Although the template assisted method attracted much attention in the early 2000s which makes possible to prepare numerous materials with a regular and controlled morphology by adjusting the template morphology, this method is mostly disadvantageous because of the high cost of template material separating.
In 2001, Grimes and co-workers  reported the self-organized TiO2 nanotube arrays by direct anodization of Ti foil in a H2O-HF solution at room temperature. The nanotubes were oriented in the same direction. The thickness of the film was only 200 nm. One end is open while the other end in contact with foil was always closed. However, nanotubes cannot be separated from each other and should be calcined for crystalline material. In 1998, Kasuga and co-workers first reported a simple method for the preparation of TiO2 nanotubes by hydrothermal process using precursor TiO2 in a strong alkaline solution (KOH or NaOH) at high temperature for a long time followed by a washing step with water or acidic solutions.
In a typical process, several grams of TiO2 can be converted to any 1D structure at temperatures in the range 110-150 oC . It has been shown that any TiO2 structure (anatase, rutile, brookite, amorphous forms) can be transformed into 1D structure. There are a large number of crystal modifications after hydrothermal treatment. Several titanate structures achieved after hydrothermal treatment and/or post treatments were summarized by Bavykin et al. These structures are given in Table 3.
Table 3 Phase variations using base assisted hydrothermal process, crystal structure and some of the diffraction angles.
Layered titanate nanostructures are promising not only because of the advantage of easy and cheap production, but also because of the intriguing hydrated structure and morphological changes induced by surface chemistry. However, there are several problems for determining the crystal structure of alkaline hydrothermal product. The most important problem is instability. Structure easily transforms by washing with distilled water, acidic treatment, and calcination. The other one is small crystals cause small coherent area which results in broadening of the peaks forming reflections in the XRD data.
Furthermore, wrapping or spreading along a certain crystallographic axis during the formation of 1D structure, results in widening of peaks making assignment difficult. Initially, Kasuga and co-workers suggested that their product is Peng and co-workers proposed that the crystal structure of titanate nanotubes corresponded to the layered trititanic acid (H2Ti3O7) with a monoclinic crystal structure. A schematic showing the crystal structure of monoclinic trititanic acid in a TiO6 edge-sharing octahedron representation is shown in Figure 2 ; the three different projections corresponding to crystallographic axes.
A nanotubular morphology of layered trititanic acid may be formed by obtained by rolling several (100) planes around axis  or . It has been propose that rolling of the plane occurs around the  axis such that the axis of the nanotube is parallel to the b-axis of monoclinic H2Ti3O7 . Wu et al. have proposed that rolling of the (100) plane could occur around axis . In both cases, the walls of the nanotubes consist of several layers, typically separated by 0.72 nm. The structure of each layer corresponds to the structure of the (100) plane of monoclinic titanates, which is a set of closely packed TiO6, edge-sharing octahedral.
Figure 2 Typical XRD pattern of Na2Ti3O7 and (b) Crystal structure of Na titanate