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TiO2 Nanoparticles: Properties and Applications

Ozioma U. Akakuru1, Zubair M. Iqbal1,2*, and Aiguo Wu1

1Cixi Institute of Biomedical Engineering, CAS Key Laboratory of Magnetic Materials and Devices, & Key Laboratory of Additive Manufacturing Materials of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, No. 1219 Zhongguan West Road, Ningbo 315201, P.R. China

2School of Materials Science and Engineering, Zhejiang Sci-Tech University, No.2 Road of Xiasha, Hangzhou, 310018, P.R. China

1.1 Introduction

The important discovery of ultraviolet (UV) light-mediated water splitting on titanium dioxide (TiO2) surface by Fujishima and Honda [1] has promoted immense research on other applications of TiO2 nanoparticles, particularly in nanobiotechnology and nanomedicine [2]. Undoubtedly, the past few decades have witnessed an exponential growth in nanoscience and nanotechnology research activities [3-5]. Nanoparticles have gained immense interest for both academic and industrial applications owing to the fact that the use of these nanoparticles with other materials has improved scientific discoveries and breakthroughs [6]. These advances have been aided by the fact that new chemical and physical properties of materials emerge on reduction of their sizes to the nanometer range and varying their shapes [7]. It has been reported that the specific surface area and the ratio of the surface to the volume of nanomaterials dramatically increase as their sizes decrease [8]. Interestingly, high surface area, a consequence of small particle size, is of benefit to TiO2 nanoparticles, as the surface area-dependent interaction/reaction of TiO2 nanoparticles devices and those of the contact media basically occurs at the surfaces or interfaces [7].

Interestingly, TiO2 nanoparticles have been extensively studied owing to their low production costs, mechanical and chemical stabilities, thin film transparency, bio- and chemical inertness, hydrophilicity, high light conversion efficiency, and corrosion resistance [9,10]. Based on these exceptional properties, TiO2 nanoparticles have vast array of applications that include nanomedicine, nanobiotechnology, solar and electrochemical cells, wastewater treatment, food, soil remediation, gas sensing, cosmetics, plastics, paint and paper productions, hydrogen fuel generation, antiseptics and antibacterial compositions, self-cleaning devices, and printing inks [10,11]. Extensive research has been recently conducted on the nanomedical application of TiO2 nanoparticles in the domains of cancer therapy and imaging. These recent progresses in nanomedicine do not only depend on the TiO2 nanoparticles themselves but also on their functionalization with other inorganic or organic compounds [12]. Specifically, TiO2 nanoparticles combined with magnetic nanoparticles have been used as magnetic resonance imaging (MRI) contrast agents and inorganic photosensitizers for photodynamic therapy [13], nanocarriers in chemotherapy [12], and in the recently discovered hydrogenated black TiO2 nanoparticles as efficient cancer photothermal therapeutic agents [14]. Their cancer therapeutic efficacy has been linked to their good biocompatibility, low cytotoxicity, and unique photocatalytic properties [15].

Some researchers recently opined that there are many other new applications for TiO2 that are either under way or presently in pilot production [16]. These new applications may be uncovered sooner than expected, due to the US FDA approval of TiO2 to be freely incorporated into numerous domestic products (dental pastes, non-parenteral medicines, tablets, and oral capsules), thereby dramatically increasing the production and availability of TiO2 for various applications [17]. This chapter will therefore discuss the properties, synthesis, and applications of TiO2 nanoparticles with respect to nanobiotechnology and nanomedicine.

1.2 Properties of TiO2 Nanoparticles

The fascinating properties of TiO2 nanoparticles have dramatically enhanced their applications in various aspects [18,19]. For instance, their high light-conversion efficiencies have been exploited for the fabrication of energy devices [20]. Their chemical stability, thin film transparency, and low production costs are responsible for their utility as photocatalysts for various environmental remediation strategies such as wastewater treatment, air pollution, and soil viability improvement [9]. Recently, TiO2 nanoparticles were applied for cancer photothermal therapy (PTT), exploiting their non-radiative recombination ability [14]. In the following subsections, specific properties of TiO2 nanoparticles will be discussed in more detail.

1.2.1 Crystal Properties

Nanocrystalline TiO2 exists in three major polymorphic forms, which include rutile, anatase, and brookite, based on the conditions of fabrication and post fabrication heat treatment [21]. The fourth polymorphic form TiO2(B) is quite uncommon [22]. Aside these four polymorphs, some researchers have reported the successful synthesis of two high-pressure phases from that of rutile: the TiO2(II) that has the PbO2-like structure [23] and the TiO2(H) that structure looks more like a hollandite [24].

Both the anatase and rutile phases possess tetragonal crystal structures even though they do not belong to the same phase groups, while brookite has an orthorhombic structure and the uncommon TiO2(B) phase is monoclinic [22,25,26]. As shown in Figure 1.1, the distortion of anatase phase octahedral structure is slightly larger than that of rutile [27]. It has been reported that even though the rutile phase is less stable than the anatase phase at 0 K, corresponding energy difference between these phases is rather small (about 2-10 kJ/mol). With respect to solar cell application, anatase phase TiO2 is chosen over other phases as a result of its low density, high electron mobility, and low dielectric constant [22]. It is also attractive that in an anatase crystal, the reactivity of its (101) facets is much lower than that of its (001) facets [28].

Figure 1.1 The crystal structures of anatase, rutile, brookite TiO2 phases.

Source: Adapted with permission from Dambournet et al. 2010 [27]. Copyright American Chemical Society.

As a consequence of the anatase phase low density, it easily undergoes transition to the rutile phase at high temperatures (usually around 450-1200 °C) [29]. This observed transformation is not only temperature dependent but is also affected by some other factors such as dopant concentration, initial phase, and particle size [30]. It has also been observed that both the brookite and anatase phases usually transform to the rutile phase at pre-determined particle sizes, wherein the rutile phase gains higher stability against the anatase phase at particles sizes that are greater than 14 nm [31]. Additionally, it has been reported that whenever the rutile phase is formed, it grows quicker in comparison to the anatase phase [22]. The crystal properties of TiO2 are summarized in Table 1.1.

Table 1.1 The crystal properties of TiO2 [20-22].

1.2.2 Optical Properties

The extensive use of TiO2 nanoparticles in optical devices are attributed to their excellent mechanical durability, high transparency in the visible region, and chemical stability in aqueous medium [18]. Several other structural parameters such as phase composition, band gap, crystalline quality, size distribution, morphology, porosity, and particle size have been reported to influence the optical activities of TiO2 nanoparticles [32]. Remarkably, decreasing the particle size of TiO2 nanoparticles from 200 nm to smaller materials of about 10 nm or less changes the optical properties of these nanoparticles from opaque to transparent in...