1. Introduction

Titanium oxide is one of naturally occurring mineral and it was first discovered in 1795. The commercial production of titanium oxide was started way back in 1920s[1]. It was found one of safest material to humans hence, widely used for various applications. The disposal of titanium dioxide or titanium-based materials to the terrestrial environment is causing no health hazard that attracts further its utilization in variety of products. It is used as a white pigment as it does not absorb light in visible region[2]. The present status showed that the titanium dioxide is used for pigment, paints, sunscreens, ointments, toothpaste, solar panels, vehicle mirror coatings, catalyst and in environmental remediation including air, water and soil etc[3-11]. The titanium dioxide occurred in nature to its three different crystalline forms, viz., the anatase (tetragonal), rutile (tetragonal) and brookite (orthorhombic) having rutile as most abundant in nature. The rutile is a stable mineral phase whereas the anatase and brookite are metastable phase and transformed into the rutile at higher temperature[12].

1.1. Crystal structure of titanium dioxide

The structures of titanium dioxide mineral phases are widely studied. The basic parameters are compiled in table 1. It is to be noted that the two structure of titanium dioxide, i.e., rutile and anatase are having a chain of TiO6 octahedra. In this each Ti⁴⁺ is surrounded with octahedron of six O^2-. It is further showed that the crystal structures of rutile and anatase phases differ in distortion of each octahedron and the assembly arrangement of the octahedron chains. Moreover, slight orthorhombic distortion obtained in the octahedron of rutile whereas significant distortion is observed with the anatase phase. The anatase shows higher distance of Ti-Ti atoms whereas it possesses shorter distance of Ti-O than rutile phase. In the rutile structure, each octahedron is in contact with 10 neighbour octahedrons (two sharing edge oxygen pairs and eight sharing corner oxygen atoms), while, in the anatase structure, each octahedron is in contact with eight neighbours (four sharing an edge and four sharing a corner). These differences in lattice structures has caused different mass densities and electronic band structures between the two forms of TiO₂[2,13]. On the other hand, rutile phase is found most stable at ambient pressure and temperature in macroscopic sizes whereas anatase phase is more stable in nanoscopic sizes[14-16]. Furthermore, the melting temperature of rutile is reported to be 1,825 °C whereas the anatase phase irreversibly transforms to rutile at ca. 500 °C.

Table 1. The Physical Parameters of Different Mineral Phases of Titanium Dioxide

Figure 1. Lattice structures of rutile and anatase TiO₂[13].

Similarly, the brookite possesses orthorhombic crystal structure and the unit cell was demonstrated by the space group Pbca[12]. The structure of brookite is having octahedra contained with titanium atom occupied at its centre and oxygen atoms to its corners (Figure 2). The octahedra share edges and corners with each other to such an extent as to give the crystal structure with correct chemical composition. The octahedra are distorted and present the oxygen atoms in two different positions[17,18]. The bond lengths between the titanium and oxygen atoms are all different.

Figure 2. Representative octahedron of the crystalline structure of brookite[17].

2. Applications of Titanium Dioxide

It was mentioned categorically that energy, water, food, environment and poverty are the main issues of mankind on this planet[19,20]. Among this the energy is regarded as one of the most important parameters for the humanities followed by the water. This is important because of significant depletion with increasing demand of fresh and clean water resources[21]. Further, it was pointed that the issues relating to fresh and clean water further associated with the environmental issues as well due to the elevated level of contaminants of water bodies and at places it reached alarming[22].

The titanium dioxide is widely employed in photo-chemical reactions which is basically divided into two ways: (i) it includes the photo-in-duced redox reactions of adsorbed substances and (ii) the photo-in-duced hydrophilic conversion of titanium oxide itself[23]. Further, the advent of advanced analytical methods and use of newer and advanced materials gained better understanding of phenomenon as well the future scope of subject. The present review, therefore critically analyzes the development of advanced materials based on the titanium dioxide in the remediation of water contaminated with variety of pollutants using safer and cleaner options. It further includes the potential threat of emerging pollutants in aquatic environments and role of advanced oxidation process in the remediation using the titanium oxide or titanium based advanced materials. Further, the future scope of the study is demonstrated for energy efficient viable technology.

2.1. Semiconductor titanium dioxide

Photo induced catalytic reactions are primarily conducted by employing the heterogeneous photocatalysts having moderately wide energy gap between the conduction band (CB) and the valence band (VB). The separation between the CB and VB is known as the band-gap energy (E_gap). The absorption of photon energy of certain wavelengths by a semiconductor promotes electron transfer from the VB to the CB, leaving vacancies (or holes) in the VB. The photo-generated electron/hole (e^-/h⁺) pairs promotes greatly the reduction and oxidation of species adsorbed at the surface of the semiconducting materials and induces oxidative degradation of species in solution through radical induced photo catalytic reactions[24,25]. Therefore, the titanium oxide which is a useful semiconducting material having the band gap energy of anatase, rutile and brookite is 3.21, 3.0 and 3.13 eV, respectively[26]. The band energy suggested that the titanium dioxide, in all its mineral phases, are active in the ultra violet irradiation. Therefore, under the solar radiations the flaking of paints and the degradation of fabrics contained with titanium dioxide is readily observed[27]. During early days, the titanium dioxide is termed as ‘photosensitizer’ because it was observed that by absorption of UV light a photo bleaching of dyes take place on the surface of titanium dioxide[28].

It was the Fujishima and Honda (1969)[29] who applied the n-type of rutile titanium dioxide semiconductor electrode along with a platinum black counter electrode. The electrochemical photolysis of water was conducted and the detailed photochemical reactions were demonstrated as:

\(\mathrm{TiO}_{2}+\mathrm{h} v \rightarrow \mathrm{e}^{-}+\mathrm{h}^{+}\)        (1)

(at the TiO₂ electrode)

\(2 \mathrm{H}_{2} \mathrm{O}+4 \mathrm{h}^{+} \rightarrow \mathrm{O}_{2}+4 \mathrm{H}^{+}\)       (2)

(at the Pt electrode)

The overall reaction was:

\(2 \mathrm{H}_{2} \mathrm{O}+4 \mathrm{h} v \rightarrow \mathrm{O}_{2}+2 \mathrm{H}_{2}\)      (3)

The photo induced reactions were conducted utilizing the photon wavelength (415 nm) of shorter than the band gap of rutile titanium dioxide (3.0 eV). This concluded that by using the titanium dioxide and photon of required energy, the water is possibly decomposed without applying an external potential. This was the pioneering studies using the semiconductor titanium dioxide for photochemical applications which eventually paves the ways for researchers to explore wider applications of titanium dioxide in diverse area of studies. The literature survey reveals that numerous studies were conducted and several useful reviews are appeared in literature[2,23,30-41]. Although a wide range of review articles are published for titanium dioxide and its various possible applications. However, a scanty of reviews are published in the area of modified or nanocomposite of titanium dioxide and its applications in environmental studies[36,38,40]. Therefore, the present review paper is emphasizing and critically analysing the role of titanium dioxide and its nanocomposites for various environmental applications.

The titanium dioxide semiconductor is having wide technological implications and it is one of benchmark semiconductor for photocatalysis however, the wider applications are limited because of two major drawbacks (i) Rapid combination of photo generated electron-hole pairs makes reasonably low quantum yields and (ii) the high energy band gap i.e., 3.21, 3.0 and 3.13 for anatase, rutile and brookite, respectively (iii) the light absorption for anatase is only 4~5% of solar spectrum (iv) agglomeration, low surface area and less absorbability restricts the environmental applications of titanium dioxide[26]. Therefore, this requires UV light of irradiation and the natural solar light contains max. 5% of UV light. Therefore, researchers have introduced various methods to enhance the spatial separation of the photogenerated charge carriers and to provide visible light responsive so as to utilize efficiently the solar light[42,43]. Hence, to improve the catalytic activity using the visible light, the dopant elements are introduced with the titanium dioxide. The dopants are of non-metals (e.g., N, P, S etc.) or the metallic impurities (transition metals, Zn, V, Cu, or lanthanides etc.) or even the noble metals (e.g., Ag, Au etc.). The mechanism and behaviour of catalytic activity was greatly assessed and studied. It was also reported that an ideal morphology and high crystallinity of titanium dioxide provides suitable optical response to visible light itself. The hydrothermal method gains the well-defined and morphology and high crystallinity with high purity of semicon ductor[44-48].

2.2. Non-metal doping with TiO₂

The nitrogen was doped with titanium dioxide powder using the urea as precursor materials. Thermally, N was doped with the titanium dioxide and calcined at 550 °C[49].

Grinding, ∆

\(\mathrm{TiO}_{2}+\mathrm{x} / 2 \mathrm{H}_{2} \mathrm{N}-\mathrm{CO}-\mathrm{NH}_{2} \rightarrow \mathrm{TiO}_{2-\mathrm{x}} \mathrm{N}_{\mathrm{x}}+\mathrm{xH}_{2} \mathrm{O}+\mathrm{x} / 2 \mathrm{CO}\)      (4)

The powder XRD data showed that the 2 q values of (1 0 1) plane of anatase TiO₂ was shifted slightly to higher values for N doped sam-ples and similarly, it showed red shift in the visible region. Moreover, the presence of N was confirmed by the XPS spectra that showed peaks around 399 eV which was due to the anionic N-making linkage with the O-Ti-N[49,50]. The catalyst is efficient in the degradation of phenol using the solar light. It was categorically mentioned that the band gap of titanium dioxide was shortened by introducing the 2p states of nitrogen and oxygen[51]. Other studies showed that the nitrogen doping provides unique band above the valence band and the UV light would cause excitation from both bands while visible light could only excite the higher lying band. They reported that this effect would not be seen if the N 2p band overlapped with the valence band forming a single higher lying band[52]. It was pointed that higher the photocatalytic activity of N-doped titanium dioxide catalysts in the visible region is possibly due to higher content of nitrogen, small particle size, high specific surface area and naturally the small bandgap energy[54].

On the other hand, the doping of fluorine with titanium dioxide, Ti³⁺is formed by reduction of Ti⁴⁺. The impurities level located at deeper potential than the CB of titanium dioxide[55,56]. The doing of carbon takes place both at anionic and cationic sites if titanium dioxide which forms various types of impurity level between its band gap[51,54,57]. This eventually, decreases the excitation energy to enable it in the visible region. In a line, composite type of photocatalyst F-TiO₂/N, C-TiO₂obtained by a simple physical mixing of the F doped TiO₂ (anatase) and N and C doped TiO₂ (anatase). The materials were characterized by the XPS analysis. Further, the material showed fairly a good efficiency in the photocatalytic decomposition of NO_x using the visible light irradiation[58]. It was interesting to observe that the e^-/h⁺ recombination was significantly suppressed due to the charge transfer reactions that greatly occurs between two types of semiconductors having different band structures. Further, the e^-/h⁺ transfers at the interface of composite material by two different path ways: (i) double-charge transfer mechanism. In this case the photoexcited e^- in CB of semiconductor B transfer to CB of semiconductor A, and photoexcited holes in VB of semiconductor A transfer to VB of semiconductor B. This is due to the e^-/h⁺ they readily accumulate in CB of semiconductor A and VB of semiconductor B, respectively. This results a suppression of recombination of excited electrons and holes. and (ii) Z scheme mechanism where the photoexcited e^- in CB of semiconductor A transfer to VB of semiconductor B, and combine with photoexcited holes in VB of semiconductor B. Consequent upon e^-/h⁺ are separated and accumulated in CB of semiconductor B and VB of semiconductor A, respectively (cf Figure 3).

Figure 3. Schematic diagram of electron-hole separation at the interface of composite, (a) double-charge transfer mechanism and (b) Z scheme mechanism[58].

The phosphorous doped titanium dioxide is synthesized by the hydrothermal process and the nanowires (NWs) are obtained at moderate temperature whereas it is annealed at 550 °C resulting the nanoflakes (NFs) of titanium dioxide. The precursor materials are employed as Ti(IV)isopropoxide and phosphoric acid[44]. Further, the absorption edge of Ti-NWs shows blue shift whereas the Ti-NFs displays red-shift compared to their precursor samples (cf Figure 4). This was due to the change in morphology and crystallinity of the samples. It is further noted that no absorption peak was observed in the visible region.

Figure 4. UV-visible diffuse reflectance absorption spectra of Ti-NWs, Ti-NFs and the precursor samples. The inset shows the enlarged portion of the absorption edge[44].

The calcination process could also improve the crystallinity and hence the morphology of titanium dioxide. It is also pointed that high calcination temperatures affects the agglomeration of titanium dioxide particles[59-60], phase transformation[61,62] and the loss of the nitrogen dopant in the catalyst[63]. Therefore, this weakens the charge separation resulting with low photocatalytic efficiency. The textile is coated with the N-doped titanium dioxide and the textile shows an enhanced tensile strength. This is suggested that the surface of the functionalized textiles with N-TiO₂ nanoparticle is relatively rougher and the SEM images indicated that the nanoparticles of TiO₂ is visible on the surface. The roughness of textile surface shows enhanced friction hence; provides stronger interlocking which exhibits higher tensile strength[64].

The thermal N doping with titanium dioxide, often conducted with urea, however, the urea under pyrolysis produces the polymeric γ-C₃N₄and showed fairly good photolytic properties[65-67]. Therefore, the γ- C₃N₄-TiO₂ is studied and interpreted the structural aspects of catalyst and photocatalytic behavior[68]. The SEM and TEM analytical images are indicated that the TiO₂ nanoparticles (30~50 nm) and ultrathin two-dimensional (2-D) γ-C₃N₄ nanoflake having length of 50~ 200 nm are dispersed with the material. Further, TiO₂ nanoparticles are evenly distributed on the nanoflake surface, and free γ-C₃N₄ nanoflakes are appeared occasionally on the surface (cf Figure 5)[68].

Figure 5. The hybridization between TiO₂ and g-C3N4 can be clearly observed in high-resolution TEM[68].

2.3. Metal cations doping with titanium dioxide

Metal cations doping with the titanium dioxide network yields useful structural changes hence; the photocatalytic activity is significantly changed. Rare earth elements are widely utilized in the doping studies because of their tendency of forming the complexes through the f-orbitals and functional groups of pollutants and aggregating onto the sur-face of catalyst. Additionally, it was noted that the rare earth ions doped titanium dioxide showed redshift in UV-Visible spectra[69]. Several rare earth metals (La, Ce, Pr, Nd, Eu, or Gd) are doped with the sepiolite clay/TiO₂ to obtain the nanocomposite material[70]. The sol-gel process was adopted to obtain the nanocomposite material. The dopant ions are confirmed with the XPS analysis (Figure 6) and the photo luminescence spectra (PL) is also shown in Figure 7. The PL spectra indicated two PL peaks around 395 and 468 nm which are due to band edge free excitons and the binding excitons[71]. It was suggested that titanium dioxide nanoparticles aggregated on the surface of sepiolite. This tends to allow free movement of electrons, resulting easier formation of excitons from bind electrons and formation of an exciton energy level near the bottom of the conduction band. The RE³⁺ doped samples exhibit the lower intensity than the undoped sample, and the intensity of Eu-TiO₂/Sep is the lowest among all samples. Lower the fluorescence intensity, the lower recombination of the photogenerated electron-hole pairs. This facilitates for an enhanced photocatalytic activity. Therefore, these results suggested that Eu-TiO₂/Sep had showed excellent photocatalytic ability among these RE³⁺ doped samples. The DRS analysis of these samples showed that the band gap energy was eventually remain same or increased slightly (except Gd doped TiO₂/Sep), however, the photocatalytic activity of these solids is significantly increased at least for the degradation of Orange G using the visible light radiations. It was previously noted that the Ti³⁺ doped titanium dioxide mesocrystals caused to increase the charge separation of the photo-induced electron-hole pairs for visible light photocatalytic hydrogen production[72-74]. The mesocrystalline titanium dioxide photocatalyst was synthesized by the introduction of oxygen vacancy defects for efficient visible light photocatalysis[75]. Two phase vapor hydrolysis was conducted to obtain the Ti³⁺-TiO₂ mesocrystals were obtained for visible light. The DRS spectra showed that the band gap was varied from 2.9 to 3.13 eV. The material employed for efficient splitting of H₂O for H₂ production in presence of methanol. The mechanism indicated that with the illumination of visible light, the photo-induced electrons from the conduction band of titanium dioxide transfers long ways for H₂O reduction at active sites that restricts the recombination with holes. Whereas the holes are efficiently removed by the methanol. It was due to the interactions between Ti³⁺ species and special mesocrystal structures which significantly enhances the visible light photocatalytic activity of Ti³⁺-TiO₂[76]. On the other hand, the microwave hydrothermal method was adopted to fabricate the TiO₂/Sepiolite nanocomposites doped rare earth ions (La, Ce, Pr, Nd, Eu or Gd). The structure of nanocomposites is dependent to the size of the doped lanthanides. The XRD data indicated that nanocomposite materials are predominantly with anatase in phase except undoped composite and Gd-doped composite where a small reflection of rutile phase occurred indicating the formation of different crystalline phase of titania[77]. In a line the Nd-doped TiO₂-C aerogels are synthesized by the combined sol-gel and impregnation process. The TEM images of the solids indicated that Nd-doped TiO₂-C aerogel possesses uniform nano crystalline structures having the particle sizes 5~10 nm (cf Figure 8)[78]. Graphene having its unique electron properties was successfully introduced with TiO₂ in order to enhance catalytic behavior of semiconductor catalyst[79]. Additionally, it was reported that graphene supports in accepting and transporting the electrons which is useful to improve the recombination time in the semiconductor[80]. The TiO₂ was prepared by the sol gel process and graphene was in situ introduced in the solution mixture. The XRD and Raman data were confirmed that the semiconductor is having different structures i.e., 57.6 wt% anatase, 42.4 wt% brookite[81]. It was further reported that the material is useful in the abatement and remediation of NO₂ in atmospheric samples.

Figure 6. High-resolution XPS spectra of (a) Ti 2p, (b) RE 3 d (4 d), and (c) O 1s for RE-TiO2/Sep samples[70].

Figure 7. PL spectra of RE-TiO₂/Sep nanocomposites prepared from different rare-earth sources and the reference un-doped TiO₂/Sep[70].

Figure 8. TEM images of the samples (a: TiO₂-C-Nd%-1, b: TiO₂-C-Nd%-2, c: TiO₂-C-Nd%-3, d: TiO₂-C-Nd%-4, e: TiO₂-C-Nd%-5, f: TiO₂-C-Nd%-6)[78].

It was noted that atomic layer deposition (ALD) technique demonstrates the digital control of the deposited material by the sequential self-limiting surface reactions[82]. Moreover, ALD provides accurate deposits at sub nanoscale particles on the nanoparticles substrates or even within the pores[83]. A uniform surface was obtained using the ALD technique and interestingly the original specific surface area or pore structure was unaltered and favorably controlled sub nanoparticles composition. Moreover, the atomic-level thickness was suitably achieved[84-86]. Copper was deposited on the surface of TiO₂ by the ALD process[87]. The simple process was adopted as the Cu²⁺ was introduced within the titanium dioxide nanoparticles which was sonicated and annealed at 500 °C. Further, the powder CuO/TiO₂ was reduced to the Cu/TiO₂ at 300 °C using the 10% H₂/N₂ flow. The sample was again purged with N₂ to remove adhered H₂ at 650 °C.

2.4. LSPR or SPR effect with Noble Metal (NPs) doped TiO₂

The Surface Plasmon Resonance (SPR) or Localized Surface Plasmon Resonance (LSPR) phenomena is basically due to the resonance of incident electromagnetic radiations with collective oscillations in noble metal (NPs)/dielectric interfaces[88-90]. These oscillations cause a strong electromagnetic field with absorption bands at a certain and characteristic wavelength[91], which are sensitive to change the refractive index of surrounding medium[92-94]. The noble metals viz., Ag, Au, Pt etc. show strong localized surface plasmon resonance effect which enables the material to be utilized in visible light. The localized surface plasmon occurs due to the strong electromagnetic field confinement near the doped noble metal nanoparticles (NPs)[95]. A small fraction of evanescent field penetrates within the dielectric[95]. This is even lower than 10 nm[96,97] and translating in an increase of sensitivity for refractive index changes near the NPs surface[96,98-100]. The easiest way of observing the LSPR absorption band is by the transmitted electromagnetic radiations, known as transmission LSPR (T-LSPR)[101-103]. This is easily detected by a standard spectrophotometer[101,104]. It was demonstrated that doped noble metals (NPs) with TiO₂ network possesses high Schotty barriers within the metals. Hence, this traps efficiently the excited electrons and facilitating the electron hole separation and promoting electron transfer process[105-110]. The other reports indicated that Au NPs stores the electrons generated in titanium dioxide by the UV light excitation and the quasi-Fermi level (quasi-EF) of Au (NPs) was shifted upward due to the electron interfacial transfer (IT) from titanium dioxide to the Au (NPs)[111]. The reduced Ag was deposited with the commercial titanium dioxide (DP25) using the photo deposition method[105]. The solid Ag-DP25 semiconductor showed characteristic plasmon broad absorption peak within the wavelength range 500~800 nm (Figures 9 & 10) in addition to the typical DP25 absorption profile. In a line a synergistic photocatalytic effect was observed by the deposition of Ag with N-doped titanium dioxide[112]. The photoluminescence studies confirmed the electron-hole recombination inhibited significantly in presence of Ag deposition on titanium dioxide and TiO_2-xNx surfaces. Moreover, the Ag-TiO_2-xNx showed enhanced visible-light photo catalytic activity compared to the N-doped or even bare titanium dioxide. The effect of solvents i.e., distilled water and Zamzam water (Zamzam water was claimed to have the most beautiful crystal molecule than other water)[113] was assessed in the structural and photocatalytic activity of Au (NPs) deposited and F-doped titanium dioxide[114]. The results indicated that the band gap estimated for the catalysts Au-TiO₂ was estimated to be 2.78 eV and 2.89 eV respectively for the distilled and Zamzam water which was significantly decreased from the 3.08 eV obtained for pristine titanium dioxide. Further, the Raman spectra showed that the F-modified samples were predominantly possessed with anatase phase and further the blank or Au-deposited samples were having the mixed phases i.e., anatase and rutile (Figure 11).

Figure 9. Plot of UV-vis absorbance spectra for Ag-DP25 and DP25 photocatalysts[105].

Figure 10. Plot of % reflectance versus wavelength for Ag-DP25 and DP25 photocatalysts[105].

Figure 11. Raman spectrum of (a) Commercial Anatase, (b) F-P-TiO₂ (HF), (c) Au(AS)/F-P-TiO₂ (HF), (d) P-TiO₂, (e) Au(AS)-P-TiO₂[114].

Nanocomposite Au/TiO₂ thin films were grown by reactive DC magnetron sputtering method onto the boron doped glass. The Au/TiO₂ thin films are having low thickness (~100 nm), and Au content is close to 13 at.% while it is annealed at 600 °C. This possessed with well-defined LSPR absorption band[115]. It was further observed that the temperature of annealing affected the optical transmittance spectra of the thin films. With an annealing temperature of 200 °C, the interference fringes almost disappeared. It was noted that at this temperature (200 °C), the NPs are significantly small and could not show strong localized absorption effect. However, the transmittance was increased to some extent. Further increasing the annealing temperature at 400 °C, the LSPR absorption band is clearly observed in the transmittance spectra (T-LSPR) (cf Figure 12).

Figure 12. Evolution of transmittance in the as-deposited samples and in annealed TiO₂. (a) Using different deposition times. (b) With different Au pellet area (including the TiO₂ sample without Au). (c) Samples deposited with different target current densities[115].

The plasmonic activity of the Au/TiO₂ photocatalyst largely depends upon the crystal form of titanium dioxide and the size of Au Nanoparticles[116]. Au/rutile (R)-TiO₂ possesses longer lifetime of the localized surface plasmon resonance (LSPR)-excited state than Au/ana-tase (A)-TiO₂ because of more effective decoupling between the LSPR and inter-band transition mode[117]. Similarly, the Au/R-TiO₂ showed enhanced plasmonic photocatalytic activity than Au/A-TiO₂[117-119], for amine oxidation[120], and for water oxidation[121]. On the other hand, the dependence of the Au/TiO₂ plasmonic photocatalytic activity on the Au particle size was affected by the action mechanisms, which was classified into the hot-electron transfer mechanism[122] and near-filed enhancement mechanism[123]. The hot electron heating mainly works at extremely-high light intensity[32]. The effect of electric field amplification[124], i.e., electron transfer from Au → TiO₂ was a key factor for LSPR-induced catalytic reactions using Au/TiO₂. This was primarily used for degradation of organic pollutants, hydrogen production, water oxidation etc.[125-133]. An interesting study showed that the mean particle size of Au (NPs) loaded with TiO₂ (rutile) was varied from 2.1 to 14.3 and the LSPR in the visible region was recorded[134]. The results are depicted in Figure 13. It is evident from the figure 13 that increase in mean particle size, the LSPR remarkably intensifies and its peak showed redshifts from 545 nm at d = 2.1 nm to 614 nm at a d = 14.3 nm[135]. Hydrothermal method was adopted to obtain the Au/TiO₂ (rutile) nanorod arrays. The characteristics of the Au/TiO₂ showed in Figure 14. The FE-SEM images showed that TiO₂was composed with nanorods having smooth surface which were grown on the FTO (fluorine doped tin oxide) glass surface along the direction of [0 0 1] of regular tetragons. The Au (NPs) (average size Ca. 7.5 nm) are uniformly-distributed on the titanium dioxide surface. Further, TEM images indicated that the Au (NPs) are intimately bonded with the TiO₂ nanorods. This, perhaps, favor the electron transfer between titanium dioxide and Au (NPs). Further, the UV-vis diffusion spectra of TiO₂ and Au/TiO₂ samples showed a sharp increase of absorption near 400 nm which is due to the intrinsic electron transition of TiO₂ from the VB to the CB. The Au/TiO₂ sample showed additional LSPR absorption around 550 nm[136]. Similarly, the thin film sample of Au/TiO₂ was obtained by simple dip coating method showed enhanced plasmonic effect and an enhanced photocatalytic behavior [137,138]. The TEM analysis indicated that Au (NPs) are ranged within 25~30 nm with an interplanar distance of 0.25 nm (cf Figure 15). The Au (NPs) are evenly dispersed within the titanium dioxide nanopillars and are not aggregated to form bulk gold[137]. The SERS (surface enhanced Raman scattering) has shown the ternary TiO₂-gold nanoparticle-graphene oxide nanocomposite materials[139]. SERS properties of materials are mainly attributed to the strong electromagnetic field that produced by localized surface plasmon resonance (LSPR) of noble metals (Au, Ag). The ternary material TiO₂-gold nanoparticle-graphene oxide nanocomposite was synthesized by the two-step hydrothermal process and the material was found as recyclable. The scanning electron microscopy (SEM) images showed uneven surface structure of solid which is predicted due to the co-existence of TiO₂ and Au nanoparticles on rGO. Whereas the TEM images of TiO₂-Au-rGO nanocomposites showed the surface of wrinkled rGO which is as sembled by different sizes of nanoparticles distributed homogeneously. The SAED pattern indicated the characteristic rings for (211), (201) and (110) planes of anatase TiO₂ and the (220) and (111) planes of Au (cf Figure 16)[139].

Figure 13. (a) TEM image of Au/R-TiO₂ (d = 2.1 nm). (b) UV–Vis absorption spectra of Au/R-TiO₂ with varying mean particle size of Au (NPs)[134].

Figure 14. (A) FE-SEM image of surface morphology of Au/TiO₂ (the insets show size distribution of Au NPs); (B) TEM image of Au/TiO₂ sample; (C) HR-TEM image of one Au/TiO₂ NR; (D) UV-vis diffused absorption spectra of TiO₂ and Au/TiO₂ samples[135].

Figure 15. (a) SEM image, (b) TEM image, and (c) TEM elemental mapping of the nanocomposite Au NPs/TiO₂[137].

Figure 16. (a) SEM image of the TiO₂-Au-rGO nanocomposite. (b) Element mapping and EDX analysis of the TiO₂-Au-rGO nanocomposite. (c) TEM image of the TiO₂-AurGO nanocomposite. (d) SAED pattern of the TiO₂-Au-rGO nanocomposite[139].

Chitosan a natural biopolymer is recently employed as a templating agent in the preparation of titanium dioxide under the non-hydrolytic process. The chitosan enabled to crystallize the TiO₂ at moderate temperature of ca. 200 °C, giving rise to ~10 nm TiO₂ nanoparticles[140]. A single-phase rutile TiO₂ nanoparticles are synthesized using the chitin and TiCl4 as precursor materials[141]. The material was derived at low temperature, i.e., 80 °C and the optical band gap and the average crystallite size was found to be 3.32 eV and 2.95 nm, respectively. Further, the plasmonic nanocomposite chitosan/Ag/TiO₂ thin film was obtained by the hydrothermal process. The catalyst showed enhanced plasmonic behavior with an enhanced catalytic activity for the degradation of several organic contaminants viz., o-toluidine, salicylic acid and 4-aminomethyl benzoic acid[142].

2.5. Microbial studies

The use of nanoparticles in the antimicrobial activity is taking momentum in recent time because of their enhanced physical and chemical reactivities including as ion release, high surface area, sorption capacity and production of reactive oxygen species (ROS) viz., hydroxyl radicals (⋅OH) and superoxide anions (O₂^-)[143,144]. The ROS are primarily generated by the photoexcitation of semiconductor titanium dioxide under the UV illumination. Titanium dioxide nanoparticles or nanostructures are widely employed as a photocatalyst and widely used in self-cleaning of surface coatings, light-emitting diodes, solar cells, disinfectant sprays, sporting goods, environmental remediation, water purification and topical sunscreens UV radiations (λ < 400 nm)[145]. In a line, the water disinfection is primarily performed using the pure cultures of bacteria added to distilled or synthetic water[146]. However, few reports intended the photocatalytic inactivation of consortia of bacteria which is present in real water[147-149] or synthetic water prepared as per the guidelines of WHO[150]. The ROS are highly reactive and when contacted with bacterial cell components, e.g., lipids and DNA caused to cell death through with its oxidizing capability[151]. The titanium dioxide nanoparticles are capable of killing a wide range of microorganisms e.g., gram-positive and gram-negative bacteria, including the endospores, as well as fungi, algae, protozoa, and viruses in aqueous medium[152]. As mentioned previously, the noble metals doped titanium dioxide found to be suitable catalyst at visible light illumination. Hence, the Au was doped with titanium dioxide and shown efficient in the inactivation of several bacteria[153-155]. Interestingly, antimicrobial effect of titanium dioxide modified with Au in darkness was also recently reported[156,157]. An interesting study was conducted to assess the antimicrobial activity of the printed composite materials viz., TiO₂/SiO₂ and TiO₂/SiO₂/Au thin films under UVA-LED and solar illuminations. The results showed that the antimicrobial activity of printed coatings was enhanced by increasing the thickness of thin films (under UVA-LED radiation). This was showed that the TiO₂/SiO₂ and TiO₂/SiO₂/Au films, both had caused to enhance the inactivation by 113% (5 layers) for Total coliforms as compared to photolysis. It was further observed that E. coli was found most sen-sitive among these microorganisms. The difference of E. coli inactivation rate with TiO₂/SiO₂ and TiO₂/SiO₂/Au was almost negligible (cf Table 2)[158]. Further, since no release of TiO₂ was observed from the printed materials enhances the applicability of materials in the disinfection process. Similarly, the pure and Fe doped TiO₂ thin films on the glass substrate was assessed in the antimicrobial tests using the E. coli and S. aureus using the UV light illumination[159]. The International protocol was adopted for the antibacterial activity[160]. The Figure 18 and 19 showed the photocatalytic inactivation of S. aureus and E. coli, respectively using the synthesized catalyst in the dark and UV light. It was noted that the doped iron ions have caused for 100% inactivation against E. coli (6 V%) and against 97% against the S. aureus (3 V%) under UV illumination for 1 h.

Table 2. Kinetic Rate Constants and Statistical Parameters Obtained by Fitting of Experimental Data in Log-lineal Model[158]

Figure 17. International test method for evaluating the antibacterial activity JIS Z2801[160].

Figure 18. S. aureus surviving colonies for pure and doped TiO₂ thin films (A) in the dark at 37 ℃, 24 h. (B) under UV light at room temperature for 1 h. (0) Glass slides (control), (1) Pure TiO₂, (2) 3 V% Fe-TiO₂, (3) 4 V% Fe-TiO₂, (4) 5 V% Fe-TiO₂ and (5) 6 V% Fe-TiO₂[159].

Figure 19. E. coli surviving colonies for pure and doped TiO₂ thin films (A) in the dark at 37 ℃, 24 h. (B) under UV light at room temperature for 1 h. (0) Glass slides (control), (1) Pure TiO₂, (2) 3 V% Fe-TiO₂, (3) 4 V% Fe-TiO₂, (4) 5 V% Fe-TiO₂ and (5) 6 V% Fe-TiO₂[159].

The impact of nanoparticles in the biological system is an area of interest and a detailed study, in particular, the TiO₂ nanoparticles is important to demonstrate its impact in natural aquatic environment. Therefore, it was categorically demonstrated that in culture medium or aquatic environment the structures of nanoparticles that aggregates and agglomerates comprising primary nanoparticles are known as NOAAs (nano-objects, and their aggregates and agglomerates greater than 100 nm)[161]. Therefore, an interesting study shown the biological and ecological impact of TiO₂-NOAAs in the oxidative stress towards the yeast. effects of TiO₂-NOAAs on yeast under UV irradiation. The DNA microarray analysis showed that the yeast cells which are prefer-entially sorbed onto the TiO₂-NOAA under UV irradiation suffered oxidative stress. However, the quantitative PCR data indicated that the oxidative stress is due to the UV-irradiation rather the TiO₂. Further, study indicated that TiO₂-NOAAs without UV irradiation damaged the membranes of yeast cells, which induces yeast cells to synthesize glycogen and trehalose[162].

3. Remediation of Aquatic Environment

3.1. Pesticides/fungicides/herbicides and related pollutants

The widespread use of synthetic pesticides or herbicides in variety of crops resulted an enhanced level of contamination of the environment. Therefore, there is an emergent need of pest resistance to these products since the presence of pesticides residues in food causes severely the illicit effects to human and wildlife health[163]. In a line, imazapyr, is having chemical structure of imidazolinone, is a non-selective herbicide and prevents plant outgrowth interfering with cell growth and DNA synthesis[164]. The residues of imazapyr penetrates deep into the groundwater resulting long term ecological pollution with continued use[165]. The European Union regulatory limit of herbicides or pesticides in the drinking water is given to 0.1 μg/L[166]. The BASF Company is widely producing the fungicides Scala^® which targets the pathogens viz., gray mold and powdery mildew in a variety of crops that includes apples, oranges, strawberries, grape vines etc. The active substance is used Pyrimethanil in this pesticide. The Pyrimethanil is toxic to aquatic life and the organs of several animals[167]. It is even classified as Group C “possible human carcinogen” by the PubChem open chemistry database[168]. Therefore, the TiO₂ nanoparticles are supported with the β-SiC foams and the material is employed in the photocatalytic treatment of Scala^® a commercial fungicide along with the Pyrimethanil under the UV radiations. It was found that at the end of 4 hrs of irradiation, 88% of Pyrimethanil and 74% of Scala was removed having 58% of Pyrimethanil and 47% of Scala mineralization[169].

Similarly, the mesoporous In₂O₃-TiO₂ nanocomposites was successfully synthesized through sol gel approach in the presence of F127 triblock copolymer template. The In₂O₃ was varied in the preparation of nanocomposite materials. The TEM images of these solids showed that 0.1 and 1 wt% In₂O₃-TiO₂ nanocomposites possessed with nano-cubes (Figure 20 (a & b) and along with rhombohedra of TiO₂ nanoparticles. Figure 20(e) illustrated the HRTEM of a 1% In₂O₃-TiO₂ nanocomposite showed a homogenous In₂O₃ dispersion occurred within the TiO₂network. Moreover, 3% In₂O₃-TiO₂ nanocomposite showed conglomeration of TiO₂ nanoparticles. Further, HRTEM images of the synthesized rutile and anatase lattices were observed in Figure 20 (d~f). The materials were employed in the photocatalytic degradation of imazapyr and the results indicated that the highest photonic efficiency values of 13.5% were obtained for the 0.1% In₂O₃-TiO₂ nanocomposite in comparison to the 12% for mesoporous TiO₂ and 10.8% for commercial UV-100. Further, the degradation reactions were proposed as:

\(\mathrm{In}_{2} \mathrm{O}_{3}+\mathrm{h} v \rightarrow \mathrm{In}_{2} \mathrm{O}_{3}\left(\mathrm{CBe}^{-}+\mathrm{VBh}^{+}\right)\)      (5)

\(\mathrm{Im}_{2} \mathrm{O}_{3}\left(\mathrm{e}^{-}\right)+\mathrm{TiO}_{2} \rightarrow \mathrm{Im}_{2} \mathrm{O}_{3}+\mathrm{TiO}_{2}\left(\mathrm{e}^{-}\right)\)      (6)

\(\mathrm{TiO}_{2}\left(\mathrm{e}^{-}\right)+\mathrm{O}_{2} \rightarrow \mathrm{TiO}_{2}+\mathrm{O}_{2}^{\cdot-}\)      (7)

\(\mathrm{O}_{2}^{\cdot\ -}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{HO}_{2}^{\cdot}+^{-}\mathrm{OH}\)       (8)

\(\mathrm{HO}_{2}^{\cdot}+\mathrm{O}_{2}^{\cdot \ -}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{H}_{2} \mathrm{O}_{2}+\mathrm{O}_{2}+^{-}\mathrm{OH}\)       (9)

\(2 \mathrm{HO}_{2} \ ^\cdot \rightarrow \mathrm{H}_{2} \mathrm{O}_{2}+\mathrm{O}_{2}\)      (10)

\(\mathrm{H}_{2} \mathrm{O}_{2}+\mathrm{h}v \rightarrow 2 \cdot \mathrm{OH}\)       (11)

\(\mathrm{H}_{2} \mathrm{O}_{2}+\mathrm{e}^{-} \rightarrow \cdot \mathrm{OH}+{ }^{-} \mathrm{OH}\)      (12)

\(\cdot \mathrm{OH}+ Imazapyr \rightarrow mineralized \ products\)      (13)

\({ln_2O_3(h^+)} +lmazapyr \rightarrow {mineralized \ products}\)       (14)

Therefore, heterostructure In₂O₃-TiO₂ nanocomposites are greatly promoted the separation of photogenerated charge carriers and caused for enhanced degradation of Imazapyr[170].

Figure 20. TEM images of low and high magnification mesoporous In₂O₃-TiO₂ nanocomposites at 0.5% In₂O₃ (a and b), 2% In₂O₃ (d and e) and 5% In₂O₃ (g and h). HRTEM image of In₂O₃-TiO₂ nanocomposites at 0.5% In₂O₃ (c), 2% In₂O₃ (f) and 5% In₂O₃ (i). The insets at (c), (f) and (i) show the SAED patterns for the anatase phase[170].

3.2. Remediation of micropollutants

The micro-pollutants are the organic or inorganic compounds and are present in the environment at low level ranged from ng/L to mg/L [171,172]. It was reported that the occurrence of these micro-pollutants in the aquatic environment is one of serious environmental concerns [173-175]. These micro-pollutants are not degraded completely by the existing Waste Water Treatment Plants and often escape through and enter into the aquatic environment. The micro-pollutants are mostly of pharmaceuticals including the antibiotics, endocrine disrupting chemicals, personal care products etc. Similarly, the priority substances (PSs) are having contaminants of emerging concern (CECs) viz., human pathogens and antibiotic resistance genes (ARGs) known to be issue of concern affecting greatly the drinking water supplies. Hence, the wastewater treatment plants (WWTPs) are found to be the hot spots of these contaminations[176,177]. Therefore, in order to provide clean and safe water having sustainable water management is known to be the United Nations sustainable development goals (https://sustainablede velopment.un.org/sdgs). Therefore, the micropollutants have received a serious challenge for water and wastewater treatment technologies. The degradation of these recalcitrant micro-pollutants is attracted greater attention in recent past and the role of advanced oxidation process paves the way for the efficient and effective removal of these pollutants. However, several challenges are encountered, e.g., the complete mineralization of target pollutants, the formation of reactive by-products which are, sometime, more toxic than the parent or target pollutant, the required efficiency etc.

The titanium dioxide or the Nanocomposite of titanium dioxide are widely employed in photocatalytic degradation of micro-pollutants. Ti³⁺self-doped TiO₂/graphene nanocomposite was utilized in the degradation of several micropollutants viz., bisphenol A, acetaminophen and sulfamethoxazole using the persulfate and visible-light illumination [178]. The material TiO_2-x/rGO showed the band gap value of 2.79 eV and it is 0.36, 0.28, 0.22 eV narrower than that of pristine TiO₂, TiO₂/rGO, and TiO_2-x, respectively (cf Figure 21). This implied that the TiO_2-x/rGO nanocomposite was due to the formation of Ti3⁺ impurity levels[179-181] and the chemical bonding between TiO_2-x and the spe-cific sites of rGO[182-185]. Further, the photocatalytic activity of TiO_2-x/rGO-PS-Vis showed that almost 100% of acetaminophen was degraded within 40 min of operation having the reaction rate constant 0.452 min^-1. On the other hand, the removal efficiencies of phenol and sulfamethoxazole are found to be Ca 77% and 52%. Additionally, a high value of mineralization efficiencies was achieved i.e., 60, 53 and 26%, respectively for phenol, acetaminophen and sulfamethoxazole [178]. The antibiotic viz., ciprofloxacin, norfloxacin and anti-inflammatory drug ibuprofen were chosen to be degraded using the reusable Fe₃O₄/SiO₂/TiO₂ particles under UV-radiations[186]. Some of the micropollutants and its photocatalytic degradation using variety of cata-lysts are summarized in Table 3.

Table 3. The Micropollutants and Its Photocatalytic Degradation Using Variety of Catalysts

Figure 21. (A) degradation rate and (B) removal efficiency and mineralization efficiency of several emerging micropollutants by TiO_2-x/rGO-PS-Vis process (Reaction conditions: micropollutant concentration = 10 mg/L, photocatalyst dosage = 1.0 g/L, PS = 2.0 mM, light intensity = 2,000 ± 10 w/m²)[178].

4. Conclusions

The role of titanium dioxide nanoparticles or nanostructure materials are widely utilized in the diverse area of research including the photocatalyst for self-cleaning of surface coatings, light-emitting diodes, solar cells, disinfectant sprays, sporting goods, environmental remediation, water purification, topical sunscreens UV radiations etc. The performance and efficiency of the materials are widely dependent on the size of particles, morphology, mineral phase and nano material structural arrangements. Therefore, the present review delivers these aspects of nano structured titanium dioxide. Further, the limitations of the semiconductor TiO₂ in various applications is due to enhanced recombination rate of e^-/h⁺ and wide band gap energy in order to harness the solar radiations in visible range. The doping of noble nano metals, rare earth and non-metals in the synthesis of efficient catalyst was demonstrated. Further, the design of efficient photo catalyst or hybrid materials of TiO₂ is elaborated using the template synthesis route. The applications of nanocomposites in the remediation of variety of micro-pollutants including the pharmaceuticals, personal care products, pesticides or endocrine disrupting chemicals is reviewed critically. These studies indicated that the use of thin films in the remediation process is found to be efficient and showed enhanced applicability in the multiple/reuse of the materials. Further, the mechanistic aspects in the photo-catalytic degradation processes are need to be studied to pin point the degradative route using the fast techniques. Moreover, the newer hybrid nanocomposites are to be explored in the efficient use of titanium dioxide for the varied applications.