TiO₂ Nanostructures by Sol-Gel Processing

Abstract

This book chapter discusses the versatile sol-gel processing technique that has been used to synthesize the nanostructures of titanium dioxide (TiO2) and their different morphologies. The sol-gel syntheses of different nanostructures of TiO2, namely TiO2 nanoparticles, nanocrystalline thin film, nanorods, nanofibers, nanowires, nanotubes, aerogels, and opals are described. These nanostructures have been characterized by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) whose images clearly depict the formation of the nanostructures. Some of the morphologies of nano-TiO2 such as nanorods, nanotubes, nanofibers, nanowires, have been synthesized by sol-gel process in combination with spin-coating, dip-coating, template, surfactant, diblock polymer, micelles, polystyrene. In comparison to the bulk TiO2, presence of porous and nanocrystalline morphologies has played a role in enhancing the performance in applications such as photovoltaics, photocatalysis, photocatalytic water-splitting, H2 storage, gas sensors, photochromic, opto-electronic, and electrochromic devices. The chapter concludes with challenges and practical concerns in using the sol-gel process to produce thin films of complex oxides, porous nanostructures, solid nanorods, nanotubes, which need to be addressed in future research efforts.

1. Introduction

Titanium dioxide (aka titania, TiO₂) was discovered by English Mineralogist William Gregor (1791) in black magnetic sand in Cornwall. Few years later, it was independently isolated from the mineral Rutile and named by German scientist M.H. Klaproth in 1795. It was commercially produced during 1920s, as pigment [1]. Subsequently, it was used in sunscreens [2], paints [3], ointments, toothpaste [4], and so on.

Titanium dioxide is a colorless, opaque, chemically inert, non-toxic, and a semiconducting material that shows photocatalytic activity upon exposure to the light of energy beyond its bandgap value (3–3.2 eV). It shows absorption only in UV region. TiO₂ occurs in nature as three crystalline forms: Rutile, Anatase, and Brookite. Rutile phase shows absorption at slightly higher wavelength than the anatase form and the latter has been studied for its optical properties.

Ever since Fujishima—Honda effect of splitting of water using a TiO₂ electrode under UV light was discovered in 1972 [5, 6, 7], research efforts on TiO₂ have gained momentum in the past decades, especially in areas of photocatalysis, photovoltaics, photoelectrochemical cells, environmental pollution control, and sensors [8, 9, 10, 11].

There has been enormous research activity on nanoscience and nanotechnology in the past decades. When a bulk material is brought down to smaller size, and further to nanoscale, there has been a paradigm shift in the material’s physical and chemical properties. The nanomaterials surface area increases with decreasing size of the nanomaterial. Ability of these nanomaterials to transport electrons/holes faster in presence of light makes it attractive for photocatalytic/PV applications [12, 13, 14, 15]. With the advent of nanoscience and nanotechnology, breakthroughs were made in the approaches to syntheses/modifications of TiO₂ nanomaterials. Thus, we could obtain newer nanomaterials with different morphologies such as nanoparticles, nanocrystalline films, nanorods, nanotubes, aerogels, opals, nanowires, as well as mesoporous, and photonic structures. These new nanomaterials exhibit optical, structural, electronic, and thermal properties that are largely determined by their size and shape [16].

Enormous surface area of the nanocrystalline TiO₂ compared to the bulk titania, is critical to applications in adsorption, catalysis, sensing, H₂ storage, photovoltaics, and wastewater remediation. Nanocrystalline titania has been widely investigated for its photocatalytic activity that was used to mitigate water pollution by means of either (i) removal of pollutants by adsorption, or (ii) photodegradation of organic dyes/drugs in industrial and domestic wastewater.

The bandgap of TiO₂ bulk phase lies in the UV region (3.0 eV for rutile phase and 3.2 eV for anatase phase). You may recall that the bandgap of a semiconductor is the energy difference between higher-energy levels called Conduction Band (CB) and the lower-lying Valence Band (VB). As the particle size decreases, energy levels tend to become more discrete, thereby increasing the bandgap of the nanomaterial. Such a widening of the bandgap in nanomaterials has been attributed to the quantum confinement effect [17, 18]. Sakai et al. [19] found that the lower edge of the conduction band for the TiO₂ nanosheet was approximately 0.1 V higher, while the upper edge of the valence band was 0.5 V lower than that of the bulk (anatase) TiO₂. Thus, the nanocrystalline titania is transparent to the visible light since it has a wide bandgap (4–4.5 eV). In a PV device, absorption of UV (high energy) light by TiO₂ promotes electrons to the excited state which must be transported quickly to the cathode through a load (an electric lamp) to complete the circuit. This promotion of electrons in TiO₂ can be done using visible light (i.e., using less energy) by enhancing its visible absorption edge. It is done by adsorbing a colored dye molecule that shows maximum absorption in visible region. The adsorbed dye molecule produces photogenerated electrons on absorption of light of less energy (visible light) compared to a pure TiO₂ electrode. Thus-generated electrons can either be used to generate electric current in a PV device or can be used in a photochromic glasses, or in a photoelectrochromic displays, or can be involved in the photodegradation of pollutants in water. This chapter presents only the synthetic details pertaining to the different TiO₂ nanostructures, particularly by sol-gel processing and some combination techniques where applicable. Discussion on those TiO₂ nanostructures that are formed by other synthetic techniques is beyond the scope of the chapter.

2. Synthetic methods for TiO₂ nanostructures

Several wet chemical methods are available for the syntheses of nanomaterials that include sol-gel processing, The choice of the synthetic technique largely depends on the nanostructures and morphologies of the materials that are desired. However, sol-gel process is so versatile a technique that it offers a possibility of further processing of the sol, using an appropriate material/template to obtain the desired nanostructures (Figure 1) which include thin films, nanofiber, nanowire, nanotube, nanorod, aerogel, opal, microspheres, nanocrystal, doped nanoparticles, etc.

Figure 1.

Representative nanostructures of titanium dioxide.

2.1 Sol-gel method

Sol-gel processing has evolved as a powerful, yet versatile technique for the syntheses of inorganic materials such as glasses and ceramics [20, 21, 22]. It is a wet chemical process in which soluble metal alkoxide or nitrate precursor material is hydrolyzed to form a colloidal dispersion called sol. Subsequently, the sol undergoes polycondensation and forms infinite network of particles called gel. This is followed by aging which completes polymerization and removal of solvent converts the liquid sol into solid gel.

Further processing of the sol-gel by spin-coating or dip-coating on a substrate yields thin films. The sol is a low-viscosity liquid and with time, colloidal particles bond to form 3-dimensional network called gel. During gelation, viscosity increases sharply. With appropriate control of the viscosity of the sol, it can be spun into nanofibers, or can be grown into nanorods or nanotubes by passing the sol through the pores of a pre-formed template (membranes of alumina or polycarbonate (PC) with pores).

The sol-gel process has advantages over traditional methods: (i) high-purity materials can be synthesized at lower temperature; (ii) homogeneous multicomponent systems such as doped materials can be prepared by mixing appropriate precursor solutions; (iii) sol-gel process has been used to prepare nanostructures with different morphologies of the desired semiconductor and other inorganic materials; (iv) metal oxides and ceramics are chemically and thermally stable and this precludes the use of conventional methods (PVD, CVD) to synthesize nanostructures of the metal oxides.

Sol-gel steps (Figure 2) are described below:

Figure 2.

Steps involved in sol-gel processing. Adapted from the book entitled “Introductory Chapter: A brief Semblance of Sol-gel Method in Research” by GV Aguilar. IntechOpen. 2018. DOI: 10.5772/intechopen.82487.

Sol-gel monoliths are made, in general, by three approaches: (method 1) gelation of solution of colloidal powders; (method 2) hydrolysis and polycondensation of metal alkoxide or nitrate precursors, followed by hypercritical drying of gels; and (method 3) hydrolysis and polycondensation of alkoxide precursors followed by aging and drying under ambient conditions.

After gelation, the pore liquid is removed from the gel network by hypercritical drying, the network does not collapse, and a low-density Aerogel is formed.

Processing steps of sol-gel are: (i) mixing; (ii) casting; (iii) gelation; (iv) aging; (v) drying; (vi) dehydration or chemical stabilization; (vii) densification.

1. Mixing: the liquid Ti-alkoxide precursor is hydrolyzed by mixing in water at a pH in which it is not precipitated. This reaction yields Ti(OH)₄ which undergoes condensation forming ≡Ti∙O∙Ti≡ bonds. Further polycondensation brings about additional linkage of ≡Ti∙OH tetrahedra and this eventually, forms ∙Ti∙O∙Ti∙ network. Water and alcohol formed in the condensation reactions remain in the pores of the network. Hydrolysis and polycondensation reactions continue to occur, and this results in the formation of interconnected ∙Ti∙O∙Ti∙ bonds in solution, that is, Sol is formed. The size of the sol particles depend on pH and the ratio [H₂O]/[Ti(OR)₄].

2. Casting: since the sol is a low viscous liquid, it can be cast into a mold, or it can be drawn into wires, rods, tubes, fibers, …

3. Gelation: with time, the colloidal particles get interconnected to form a 3-D network, called Gel. With gelation, viscosity of the sol increases sharply and with a proper control of viscosity, fibers can be spun.

4. Aging: aging of the gel (called Syneresis) refers to maintaining the cast object, for hours to days, completely immersed in liquid. During aging, polycondensation continues, the thickness of interparticle necks increases, which lowers the porosity and increases the strength of the gel. The strength of the gel increases with aging time.

5. Drying: the liquid in the pores of the gel is removed during drying. For the smaller pores (<20 nm), gel starts cracking due to high surface energy. This cracking can be prevented by minimizing the liquid surface energy either by hypercritical drying (method 2), or by using surfactants or by eliminating smaller pores (method 1), or by obtaining monodisperse pores by controlling rates of hydrolysis and condensation (method 3).

6. Dehydration: ultraporous, stable solid is obtained by the removal of surface (Ti∙O) bonds. Thus, the obtained stable, strong, and porous gel is optically transparent which finds applications as optical components, when impregnated with optically active Fluors, dyes or nonlinear polymers [23, 24].

7. Densification: Heating the porous gel at high temperatures, removes the pores and causes densification to occur. For example, alkoxide-derived silica gels have been densified at temperature as low as 1000°C [25], and these gels possess superior purity and homogeneity, compared to other commercial processes.

Discussion on the sol-gel syntheses of different nanostructures of Titania such as Nanocrystalline thin films, nanoparticles, aerogels, opals, nanotubes, nanofibers, nanowires, microspheres, and nanorods, is presented in following sections.

2.1.1 Synthesis of titania thin films

Titania nanostructures have widely found applications in various fields which include gas sensing, photocatalysis, water purification, protein separation, Dye-sensitized solar cells (DSSC), solid-state DSSC, and perovskite solar cells (PSC) [26, 27, 28, 29, 30, 31]. These solar cell devices could not achieve photon-to-current efficiency or the device stability, when compared with the conventional Si-based solar cells. However, the fabrication of Si-based solar cells involves expensive and energy-intensive processes. Therefore, large-scale fabrication of the present-day PV devices has been largely determined by the key factors: cost- and energy-efficiency. In this regard, research on the devices that were utilizing titania nanostructures have gained significant attention [32].

Earlier, Spray deposition was found to be a promising method to produce thin films for the PV devices [33]. Among the wet chemical synthetic methods of titania nanostructures, polymer (polystyrene-b-poly (ethene oxide)) template-assisted sol-gel process in combination with spray deposition has been found to be powerful tool that allows one to control the nanoscale morphology (Figure 3) of the titania films with unique properties [34, 35]. Typical porosity values of the mesoporous titania thin films are 66 ± 2%. The amphiphilic diblock copolymer polystyrene-b-poly(ethylene oxide) (PS-b-PEO) was used as a structure directing template. A typical synthesis involves dissolving of PS-b-PEO (20 mg) initially in a good solvent (toluene, 6.7 ml), followed by adding 1-butanol (bad solvent, 2.68 ml). This was performed to induce micelle formation, and the precursor titanium(IV) isopropoxide (TTIP) (73.8 μl) was selectively incorporated into the PEO phase. This is followed by 30 min stirring. To analyze the role of HCl on the final nanostructure formation, two solutions, one solution containing 121 μl of 6 M HCl (called WHCl), and other without HCl (called NHCl) were used in the spray solution. Further the solutions (WHCl and NHCl) were stirred for 20 h (500 rpm) at ambient conditions. The remote-controlled spray gun was used to spray the solution on a silicon substrate, at a pressure (N₂ carrier gas) of 1.5 bar. After this film was deposited, polymer template was removed by calcination at 550°C for 3 h. The films formed in presence of HCl were of anatase phase and had small and uniform pores, while those without HCl, were polydisperse. The presence of HCl is said to promote hydrolysis over condensation reactions.

Figure 3.

(a, c) Surface topography (AFM) of titania thin films and (b, d) SEM images of titania thin films before and after polymer template removal. Reprinted with permission from Ref. [31]. © 2018 American Chemical Society.

2.1.2 Titania aerogels

Hydrolysis of precursors which are metal salts or metal alkoxides, followed by colloidal dispersion of solid precursors called sol. Further heat treatment results in complete polymerization and removal of solvent or water yields infinite network called solid gel phase. A wet gel is obtained by casting the sol into a mold. Further heating and drying removes the solvent to yield a highly porous, less-dense Aerogel.

Titania is a material of choice for catalytic applications, as it has high surface area and shows strong metal-support interactions. The titania samples prepared earlier by traditional methods, have reported low-to-moderate surface area. The most widely used Degussa P-25 TiO₂, was prepared by flame hydrolysis of titanium tetrachloride and is reported to have surface area of about 50 m²/g [36]. The Glidden TiO₂ that was prepared by the hydrolysis of titanium isopropylate & subsequent firing at 500°C, has a surface area of about 90 m²/g [37]. Some samples prepared by base hydrolysis of titanous chloride at different pH values, were dried in air at 110–120°C. These samples were reported to show surface area over 200 m²/g [38].

Titania aerogels have been considered as promising photocatalysts. Ko et al. [39] have synthesized Titania aerogels by sol-gel process by controlled hydrolysis of methanolic solution of titanium-n-butoxide in presence of water and nitric acid. Subsequent removal of water was done by drying with supercritical CO₂. In a typical synthesis, TiO₂ gel was prepared by a simple sol-gel process using titanium-n-butoxide (TIB) as precursor in ethanol, deionized water, and hydrochloric acid mixture. TIB was dissolved in 40 ml alcohol in a dry glove box. To this, solution containing 10 ml alcohol, nitric acid and DI water. Concentrations of TIB, water and acid were kept at 0.625 mmol of TIB/ml of alcohol, 4 mol of water/mol of TIB, and 0.125 mol of nitric acid/mol of TIB, respectively. The solution was stirred to obtain a clear gel which was allowed to age for 2 h. Then it was extracted in an autoclave with supercritical CO₂ at flow rate of 24.6 L/h, a temperature of 70°C, and a pressure of 2.07 × 107 Pa (3000 psi).

The conventional solvent removal method of drying the aerogel collapses the porous network due to tension at the liquid-vapor interface. Water and nitric acid contents in the hydrolysis reactions were varied in the process to achieve titania aerogels (Figure 4) with maximum surface area. Thus obtained TiO₂ aerogel had a BET surface area exceeding 200 m²/g, after calcination at 500°C for 2 h. This titania sample contains mesopores of 2–10 nm size and was of pure anatase form, which was shown by the Raman spectral bands at 441 cm⁻¹.

Figure 4.

SEM image of TiO₂ Aerogel. Reprinted with permission from Ref. [40]. © 1995 American Chemical Society.

The commercial TiO₂ (Degussa P-25) is a mixture of anatase and rutile phases, as shown by their XRD & Raman spectra. The anatase form of titania is of more interest in catalysis applications than the rutile form, because of the higher surface area of anatase than the rutile (7 vs. 200 m²/g). Compared to Degussa P-25, the titania aerogel prepared by the sol-gel process [40], had a surface area four times larger and was of pure anatase form. Tomkiewicz et al. [40] found a correlation of morphology of the TiO₂ aerogels with its catalytic activity. It was prepared by sol-gel process by dissolving titanium isopropoxide precursor in absolute ethanol and then mixing it with ethanol, DI water and nitric acid at concentrations ((Ti/ethanol/H₂O/HNO₃ = 1:20:3:0.08 ratio), followed by aging of the gel in alcohol for few days to weeks. The gel was then dried with CO₂ at its supercritical point (35°C and 1200 psi). This yielded aerogel with low density (0.5 g/cc) and high porosity (85%).

Dagan and Tomkiewicz [41] prepared titania aerogels using sol-gel process and supercritical drying (Ti/ethanol/H₂O/HNO₃ = 1:20:3:0.08 ratio). The aerogels had a surface area of 600 m²/g and 85% porosity, compared to the Degussa P-25. Photodegradation of salicylic acid using titania aerogel photocatalyst under near-UV light (2 h) was found to be 10 times much faster than the Degussa sample.

2.1.3 Titania opals

The word opal refers to a gemstone which exists in nature. Chemically, it is hydrated amorphous silica with water content varying between 6% and 10%. As for the titania nanostructures, opal has come to mean the shape of the nanostructure. The ordered arrays of TiO₂ opals (Figure 5) were prepared using opal gel template-assisted sol-gel process under uniaxial compression at ambient temperature [42].

Figure 5.

SEM images of: (a) the inverse polystyrene opal. (b) The hydrogel opal after freeze-drying. (c) The gel/titania composite opal without compressing the opal gel template during the sol-gel process. (Inset) image of the sample after calcined at 450°C for 3 h. (d–f) (main panel) oblate titania opal materials after calcined at 450°C for 3 h, subject to compression degree R of (d) 20%, (e) 35%, and (f) 50%. Reprinted with permission from Ref. [42]. © 2003 Royal Society of Chemistry.

Typically, silica opal was used as template to synthesize polystyrene (PS) inverse opal. An aqueous HF (40%) solution was applied to remove the silica template. Monomer solutions containing dimethylacrylamide, acrylic acid, and methylene bis-acrylamide (wt% ratio: 1:1:0.02) was prepared in aqueous ethanol (4:7 w/w) with a 30 wt% limit on total monomer content. Ethanol is preferred here to enable the diffusion of monomer solution into the inverse opal PS. Then, 1 wt% of the initiator 2,2′-azobisisobutyronitrile (AIBN) was added to the monomer solution to initiate free radical polymerization at 60°C for 3 h. The inverse opal PS template was removed by Soxhlet extraction, to obtain Opal gel. The opal gels with different properties can be obtained by modifying the monomer solution, hole sizes, and stacking structures of the inverse opal template.

Then, opal gel template was placed into large quantity of tetrabutyl titanate (TBT) at ambient temperature for 24 h. Thus, the swollen opal gel was immersed into water-ethanol mixture (1:1 w/w) for 5 h. During this time, TiO₂ sol-gel process begins to complete the formation of opal structure of the gel. Subsequent calcination yields TiO₂ opal with distinctive spherical contours.

3. Modifications of TiO₂ nanomaterials

TiO₂ nanomaterials have bandgap greater than 3 eV and so they are transparent to visible light. TiO₂ nanomaterials have found applications such as photocatalysis, photovoltaics, Sensing, electrochromics, photochromics, UV protection, photo-induced water-splitting that are largely dependent on its optical absorption characteristics. TiO₂ nanomaterials absorb in UV (higher energy) region because of their wide bandgap, and this limits the performance of the nanomaterials. Improving the performance of the TiO₂ nanomaterials is to shift its absorption from UV to visible region, that is, the nanomaterials perform better by using less amount of energy. This can be done by: (i) Doping the nanomaterials with suitable metal ion which can narrow the electronic bandgap and alter its optical properties; (ii) adsorbing a colored inorganic/organic compound on to the nanomaterial, i.e., sensitization of TiO₂ nanomaterial can improve its visible absorption edge; (iii) coupling the electrons in the Conduction band of metal nanoparticle surface with those in the conduction band of TiO₂ nanomaterial in a metal–TiO₂ nanocomposite. By doping or sensitization, the visible-absorbing and more active Titania nanomaterials have been obtained as evidenced by their utility in environment (photocatalysis, sensing) and energy (photovoltaics, water-splitting, photo-/electro-chromics, H₂ storage) fields for a sustainable development.

3.1 Synthesis of doped TiO₂ nanomaterials

Some organic compounds such as nitro-stilbene derivatives were found to show non-linear optical (NLO) activity. Sol-gel process is a wet chemical synthetic route that allows for the incorporation of optically active organic molecule into the inorganic metal-oxide glass matrix to obtain a doped gel with specific optical properties. The inorganic metal-oxide glass was found suitable for stabilizing the NLO materials because of higher thermal stability of the metal-oxide glass, compared to polymer [43]. The sol-gel process was used to fabricate titania films doped with NLO material, for use in electro-optic devices [44]. In a typical sol-gel process, precursor solution was prepared by mixing HCl/H₂O with isopropanol (PrOH) solution of tetraisopropoxytitanate (TPOT) containing the NLO materials with vigorous stirring at ambient temperature, at a molar ratio of 1:1:0.5:1.88 for TPOT:PrOH:HCl:H₂O. Final solution was used to spin-coat to obtain 1-micron-thick titania film on Indium Tin Oxide (ITO) glass substrate.

3.2 Synthesis of metal-doped titania nanomaterials

The TiO₂ nanoparticles were doped with 21 different metal ions by means of sol-gel process and this made a significant impact on photoreactivity, charge-recombination rate and interfacial electron-transfer rates, in a TiO₂-nanomaterial-based photovoltaic device, where photon-to-current efficiency is largely dependent on these factors [45].

Li et al. [46] found that La³⁺-doping of TiO₂ by sol-gel process, could impart thermal stability, prevent phase transformation, reduce nanoparticle size, and increase Ti³⁺-content on the surface.

The dopant Nd³⁺ ion (1.5 at%) in the TiO₂ nanoparticle, introduces energy level into the bandgap of the nanomaterial, to be the new LUMO. Thus, the dopant brings down the bandgap by 0.55 eV [47]. This new LUMO level brings down the energy of the bandgap, and this can shift the absorption onset of TiO₂-nanomaterials from UV to visible region, thereby altering its optical properties [48].

This bandgap narrowing results in a red-shift in the absorption of metal-doped TiO₂. With increase in atomic number of the metal-ion dopant, an energy level that is formed shifts the localized level to lower energy [49].

Pt-doped titania thin films were synthesized using sol-gel process, followed by dc magnetron sputtering for the Pt film deposition. Substrate sapphire wafer was cleaned in ultrasonication bath in sequence, in acetone, isopropanol and deionized water for 15 min and subsequently dried at 90°C for 5 min. The titania thin films were first prepared by sol-gel process by mixing 3.8 ml of ethanol with 0.7 g of Triton-X-100 under ambient conditions for 3 min. To this mixture, 0.68 ml of acetic acid and 0.36 ml of titanium (IV) isopropoxide were added. Using this sol, spin-coating process was repeated three times with a speed of 3000 rpm and short intermediate heating at 550°C. The titania thin films were then annealed at temperatures from 600 to 1000°C at the rate of 100°C/15 min. Then, a Pt thin film (20 nm thick) was deposited using dc magnetron sputtering. Ti-alkoxide was formed in situ by the esterification of alcohol by acid, and the alkoxide was hydrolyzed in presence of the non-ionic surfactant Triton X-100 to organize the material structure and well-defined nanophases. Annealing of the thin films at 600–800°C yielded anatase phase with smaller grains (15–28 nm), while the higher temperature (900–1000°C) annealing gave rise to rutile phase with larger grains (100–130 nm), high surface roughness and reduced bandgap energy (2.8 eV, compared to anatase 3.4 eV). Higher temperature annealed thin films showed higher sensitivity (10³–10⁴) to hydrogen gas that has been attributed to increased roughness and a greater number of adsorption sites [50].

3.3 Synthesis of TiO₂-nanoparticle-shelled microspheres

New, ‘Open Mouth’, Hollow TiO₂-nanoparticle-shelled (OMHTNPS) light-driven microcleaners were obtained via low-cost, high-throughput, facile sol-gel method, with the subsequent removal of carbon microspheres by a simple sintering process [51]. The shells of the prepared OMHTNPS microcleaners mainly contain 20-nm anatase TiO₂ nanoparticle. The carbon microspheres (CMSs) and TiO₂-coated CMSs (TiO₂@CMS) were prepared as reported [52]. These OMHTNPS showed 98% efficiency in the photodegradation of the Rhodamine B dye.

Synthesis of Janus micromotors: TiO₂ microspheres were prepared by the solvent extraction/evaporation method using tetrabutyl titanate as a precursor. Briefly, 1.0 ml of tetrabutyl titanate was dissolved in 40.0 ml of ethanol and incubated at room temperature for 3 h; then, TiO₂ microspheres were collected by centrifugation at 7000 rpm for 5 min and washed repeatedly with ethanol and ultrapure water (18.2 MΩ cm), three times each, then dried in air at room temperature. TiO₂ (anatase) microsphere is obtained after annealing for 2 h at 400°C. The X-ray diffraction (XRD) pattern reveals that the TiO₂ microspheres have a good anatase phase. For the TiO₂–Au light-driven Janus micromotor, TiO₂ microspheres (1.0 μm mean diameter) are used as the base particles. TiO₂ particles (10.0 μg) were first dispersed in 150.0 μl of ethanol. The sample was then spread onto glass slides and dried uniformly to form particle monolayers. The particles were sputter coated with a thin gold and nickel layer using a Sputter Coater for 3 cycles with 60 s per cycle. The metal layer thickness was found to be 40 nm, as measured by the Profilometer. For the TiO₂–Ni–Au magnetic Janus motors, TiO₂ particle monolayers were prepared as in the method above. A 40 nm layer of Au followed by a 10 nm layer of Ni were sequentially deposited on half of the particles by Sputter Coater. The TiO₂ microspheres were coated with Al₂O₃ layer using ultrahigh Vacuum Magnetron Sputter Coater. The micromotors (Figure 6) were subsequently released from the glass slides via pipet pumping and dispersed into double distilled water. The polystyrene–Au Janus microsphere as a control was fabricated with the same method using polystyrene microsphere. These light-driven, precisely controllable, and highly efficient TiO₂-based photocatalytic Janus micromotors offer possibilities in designing such light-driven nanomachines for a range of applications from nanofabrication [53] to environmental remediation [54].

Figure 6.

Catalytic scheme, SEM and EDX images of Au–TiO₂ micromotor. (A) Schematic of Catalytic TiO₂–Au Janus Micromotors powered by UV Light in water. (B) SEM image of a spherical TiO₂–Au micromotor. (C—E) The corresponding EDX images for Ti, Au, O, respectively. Scale bar, 0.5 μm. (F) Tracking lines illustrating the distances traveled by three micromotors in pure water over 1 s. Scale bar, 10 μm. Reprinted with permission from Ref. [53]. © 2016 American Chemical Society.

3.4 Titania-hybrid photonic crystals

Titania-hybrid photonic crystals are high dielectric lattices that find applications in light control for waveguiding and lighting devices, photocatalysis, photovoltaics, and sensing. Sol-gel processing used for their fabrication allows for the alternated spin-casting of high and low refractive index polymer solutions or the sol of titania particles and subsequent sintering. This solution-processing method has attracted interest owing to simpler structures, ease of fabrication, efficient scale-up, low-cost processing, and offers the product, flexibility, and permeability [55]. First Titania sol was prepared by adding 10 ml of polyacrylic acid (PAA) in butanol and catalytic amount of HCl (100 μl) to 10 ml of titanium butoxide precursor solution. The organic & inorganic components in the deposited films can be varied using different concentrations of titania precursor and PAA in the initial solution. The resultant solution is hydrolyzed by stirring at room temperature for 2 h, when a transparent sol is formed. Thin films and distributed Bragg reflectors (DBRs) were obtained by spin-coating of the sol and of the polymer solutions, at a speed of 5400–12,000 rotations per minute. The Ti-Hybrid was heated subsequently at 80°C, while the Si-Hybrid heated at 300°C. Multilayers were grown by spin-coating of alternated high (Ti-Hy) and low (Si-Hy, PMMA) refractive index media with alternating layer of PMMA, to form a DBR.

3.5 Template-assisted sol-gel syntheses

3.5.1 AAM-template-assisted synthesis of TiO₂ nanotubes

The TiO₂ nanotubes (Figure 7) were synthesized by sol-gel process in combination with Anodic Alumina Membrane (AAM) used as a template [56].

Figure 7.

SEM image of titania nanotubes using AAM template. Reprinted with permission from Ref. [56]. © 2005 American Chemical Society.

In a typical synthesis, a thin layer of TiO₂ sol is drawn into the pores of the AAM under vacuum. The TiO₂ sol was prepared by sol-gel process using Titanium tetraisopropoxide (TTIP) as Ti precursor. The TTIP solution was prepared by mixing TTIP with isopropanol and 2,4-pentanedione. After dipping the AAM template into this solution, the entire solution was drawn through the pores of AAM under vacuum. The Titania nanotubes were obtained after dissolving the membrane in 6 M NaOH solution for several minutes [57].

3.5.2 ZnO-nanorod as template

Zinc oxide nanorod array on a glass substrate was used as a template to fabricate TiO₂ nanotubes by sol-gel method [58]. By this method, TiO₂ sol was prepared first and the ZnO nanorod template was dip-coated by immersing in the sol and taken out at a slow speed, dried at 100°C for 10 min, further heated in air at 550°C for 1 h, to obtain ZnO/TiO₂ nanorod arrays. The template was removed by immersing the ZnO/TiO₂ nanorod arrays into dilute HCl solution. The TiO₂ nanotubes have a length of 1.5 micron, with inner diameter of 100–120 nm that is characteristic of the ZnO nanorod template. To get a well-aligned TiO₂ nanotube array, an optional dip-coating cycle (2–3 cycles) can be adopted.

3.5.3 Template-based sol-gel electrophoretic deposition

Electrophoresis is a type of motion of charged particles in a colloidal system or a sol, in response to the application of external electric field. When a charged particle is in motion, the solvent part tightly bound to the particle will move with it, whereas the counter-ions diffuse in the opposite direction. The electrophoretic deposition technique uses such an oriented motion of charged particles to grow films or monoliths by enriching the solid particles from a sol (prepared by sol-gel process) onto the surface of an electrode.

Limmer et al. [59] combined sol-gel synthesis and electrophoretic deposition in the growth of nanorods of various oxides including complex oxides such as barium titanate. Similar procedure was used to grow nanorods (Figure 8) of TiO₂. In a typical process, conventional sol-gel processing was used to prepare TiO₂ sols. By maintaining appropriate pH, electrostatically stabilized, nanoparticles dispersed uniformly in solvent were obtained with desired stoichiometric composition [60]. When an external electric field is applied, these nanoparticles move and deposit on the cathode or anode, depending on the zeta potential (surface charge) of the nanoparticles. Using radiation track-etched polycarbonate membranes with an electric field of 1.5 V/cm, nanowires with diameters of 40–175 nm and a length of 10 microns corresponding to the thickness of the membrane. By this method, many complex oxides (BaTiO₃, Sr₂Nb₂O₇) and inorganic-organic hybrids with desired composition, have been synthesized [61].

Figure 8.

SEM images of TiO₂ nanorods grown in a PC membrane with 200 nm diameter pores by sol—gel electrophoretic deposition. (A) Lower magnification image, showing that the rods are aligned and grown over a large area. Scale bar, 1 μm. (B) Higher magnification image of the nanorods. Examination of broken rods seen here shows that they are solid and dense. Scale bar, 1 μm. Reprinted with permission from Ref. [60]. © 2002 John Wiley and Sons.

3.5.4 Template-based electrochemical sol-gel deposition

Single crystal TiO₂ nanowires were synthesized by this sol-gel deposition method [62]. The electrolyte solution was prepared according to the work of Natarajan and Nogami [63]. First, Titanium powder was dissolved in a mixture of H₂O₂ and ammonia solution, then the excess H₂O₂ and ammonia were decomposed by heating the solution on a hot plate and, consequently, a yellow-colored gel was obtained. By dissolving the yellow gel in 4 M H₂SO₄, a red-colored solution formed and the red-colored solution was used as the stock solution for further electrodeposition. A certain amount of KNO₃ was added to the stock solution (about 145 mM), and the pH adjusted to 2–3 by using ammonia solution. The resultant solution was used as electrolyte in the electrodeposition process.

Electrodeposition was carried out at room temperature (20–25°C) using a three-electrode potentiostatic system which comprises SCE (reference) electrode, 2 cm × 1.5 cm Pt plate as counter-electrode and a small piece of AAO template with Au substrate as working electrode. The porous side of the working electrode was exposed to the electrolyte. The templates with pore diameters of 50, 22, and 20 nm were used in the fabrication. The deposition was carried out under potentiostatic conditions at −0.9 to 1.2 V. As a result, nanorods of amorphous TiO₂ gel formed. Subsequent heat treatment at 450°C for 24 h in air, yielded nanowires of single crystal TiO₂ with anatase (Figure 9) structure (diameters of 10, 20, and 40 nm and lengths of 2–10 μm).

Figure 9.

SEM of single-crystal TiO₂ nanowires. Reprinted with permission from Ref. [62]. © 2002 American Chemical Society.

The electrophoretic sol-gel method failed to synthesize nanorods of diameter less than 50 nm. Compared to the electrophoretic sol-gel process, the electrochemical sol-gel deposition technique has advantages: (i) It could readily achieve nanowires of diameter less than 20 nm, as templates with very small pores (<20 nm) can be used; (ii) lengths of nanowires can be controlled by varying deposition time and potential of the working electrode; and (iii) high local pH at the AAO pores causes hydrolysis, gelation, and aging processes. This forms a more compact gel structure with higher packing density, less shrinkage and less cracking.

3.6 Syntheses of TiO₂ nanoparticles

Titania nanoparticles of different sizes and shapes were obtained by sol-gel process involving the precursor TTIP under appropriate reaction conditions. Sugimoto et al. [64, 65, 66, 67, 68] developed the synthesis process by series of studies. The synthesis process consists of preparing a stock solution of titanium source (0.5 M Ti), by mixing the precursor TTIP with triethanolamine (TEOA) {[TTIP]/[TEOA] = 1:2} and water. The stock solution is diluted with shape controller solution (Amine) and then aged at 100°C for 1 day and at 140°C for 3 days. The pH of the solution is varied from 0.6 to 12, by adding HClO₄ or NaOH. With increase in pH, yield of the nanoparticles decreases to 9% (pH 12). This suggests that varying the pH had significantly decreased the nucleation rate of Anatase TiO₂, by reducing the concentration of the precursor. Amines used in the process include ethylene diamine, diethylene triamine, triethylene tetraamine, trimethylene diamine, TEOA. These amines function as shape controller as well as surfactants.

3.7 Inorganic sensitization

3.7.1 Sensitization by narrow bandgap semiconductors

Narrow bandgap semiconductors have been used as sensitizers to increase the visible absorption edge of the titania nanomaterials that have wide bandgap. This shifts the optical absorption to visible region so that these nanomaterials generate photocurrent with less energy. The inorganic semiconductor-sensitized TiO₂ nanostructures have been prepared usually by the sol-gel process [69, 70, 71, 72, 73].

Semiconductor PbS-sensitized TiO₂ nanocrystalline system has enabled quicker injection of photogenerated electrons from the PbS into the TiO₂ nanomaterial and generated strong photocurrent using visible light [74]. The nanocrystalline TiO₂ films were prepared using standard sol-gel techniques. First colloidal TiO₂ solutions were prepared from titanium isopropoxide (30 ml) and isopropanol (10 ml) in water (500 ml). Nitric acid was added to adjust the pH to 1. The organic components were evaporated by boiling the solution for 12 h and crystallization of TiO₂ particles resulted. The colloidal solution was then spin coated on glass substrates, provided with evaporated Cr contacts in planar geometry for the conductivity measurements. The freshly deposited films were heated for 5 min at 450°C. Coating and drying were repeated several times to get a film of desired thickness of 1 mm. At the end of this process, the samples were baked at 450°C for 30 min. The films consist of anatase crystallites (40–60 nm diameter), and are structurally stable up to 650°C. The internal surface area is 400 times larger than the projected area. The conductivity of the films is typically below ~10⁻⁹ (Ω cm)⁻¹ at room temperature. PbS clusters adsorbed on the internal surface of the TiO₂ clusters were then prepared by dipping the TiO₂ films into concentrated lead acetate solution and subsequently precipitating the adsorbed Pb²⁺ with a solution containing sodium sulfide. The PbS colloidal particles of about 30 Å diameter were obtained. Larger particles were formed by repeating the dip-coating process several times. The residual water in the films was removed by heating to 200°C at reduced pressure.

Figure 10 shows the photo-action spectra for the bare TiO₂ and the TiO₂–PbS films obtained after 1, 2, and 5 coatings of PbS. In the bare TiO₂ film, the onset of photoconduction is found to occur at 380 nm, corresponding to the 3.2 eV band gap of anatase (bulk) TiO₂ [75]. In the films coated with PbS, the TiO₂ response vanishes due to the high absorption of the PbS clusters in this wavelength region. Instead, a broad band due to the PbS emerges in the visible region with a maximum around 500 nm. Optical transmission spectra of the three TiO₂–PbS films (1, 2, and 5 coatings) were compared which showed a red-shift of the absorption, with increase in the number of PbS coatings, as shown by the absorption edge at 1.6 eV, 1.37 eV and 1.24 eV, respectively. This is indicative of efficient sensitization of TiO₂ by PbS. This has been related to the average PbS cluster sizes of 28, 35 and 40 Å. The clusters of size below 25 Å have been found to be more efficient sensitizers that gave rise to better photoconduction response with increase in light intensity.

Figure 10.

Photo-action spectra of TiO₂ film with multiple coatings of PbS nanocrystallites. Reprinted from Ref. [75], with permission of AIP Publishing.

Fitzmaurice et al. [76] found rapid electron injection into the TiO₂ electrode was possible with AgI-sensitization, as evidenced by the enhanced lifetime (>100 μs) of electron–hole pairs.

Typical synthesis involves the preparation of TiO₂ sol and AgI sol separately and mixing them later. TiO₂ sol was made at acidic pH (3.3) by the hydrolysis of TiCl₄. This TiO₂ sol was made alkaline (pH 11.4) by rapidly mixing it with required amount of sodium hydroxide. Aqueous AgI sol was made by rapid mixing of appropriate concentrations of AgNO₃ and KI solutions in presence of PVA stabilizer of concentration 0.002–0.2%.

The AgI–TiO₂ sandwich colloids were made by precipitation of AgI on the surface of TiO₂ particles. Silver nitrate solution (5 × 10⁻⁴ M) was mixed with alkaline solution of TiO₂ (1 g/L). After 20 s, the resultant solution was rapidly mixed with equivalent amount of KI. The solution was allowed to age for 12 min prior to use.

Wide bandgap semiconductor particles (TiO₂) with low-lying conduction band are combined with the narrow bandgap semiconductor (AgI, CdS) particles with high-lying conduction band. Upon illumination, electrons are transferred from AgI into the conduction band of Titania, while the holes remain with narrow bandgap semiconductor. This results in efficient electron transfer to the wide bandgap particles and minimizing the charge-recombination. This sensitization can potentially enhance the performance of AgI-sensitized TiO₂ nanostructures in photovoltaic cells, photocatalysis, new generation display monitors, non-linear optics [77].

Vogel et al. [78] extensively studied the sensitization of TiO₂ by different semiconductors CdS, Ag₂S, Bi₂S₃, Sb₂S₃ and found that efficient charge separation and photostability of TiO₂ could be achieved by surface modification of the titania nanostructures by such semiconductors. The relative positions of energy levels at the CdS–TiO₂ interface could be optimized for efficient charge separation, using the size quantization effect. Figure 11 shows the plot of photocurrent quantum yields (IPCE) for four differently treated TiO₂–PbS electrodes versus the illumination time (λ = 460 nm, 8 mW cm⁻²). Curve 1 refers to a decrease of high initial IPCE (65%) value in first few minutes of illumination for opaque microporous TiO₂ electrode. Curve 2 shows a clear improvement in photostability of the transparent PbS-sensitized TiO₂ electrode. Additional coating of CdS resulted in the increased IPCE value (74%). Further illumination for 4 days yielded the curve 4 and the photostability of the electrode is strongly enhanced.

Figure 11.

Photocurrent quantum yields for differently treated PbS–TiO₂ electrodes as a function of the illumination time with X = 460 nm and p = 8 mW cm⁻². Curve 1: one coating with PbS on a microporous TiO₂ substrate. Curves 2–4: one coating with PbS on a nanoporous TiO₂ substrate. Curve 2: as prepared. Curve 3: after deposition of a thin TiO₂ layer. Curve 4: after one additional coating with CdS. Reprinted with permission from Ref. [78]. © 1994 American Chemical Society.

The energy levels of sensitizer–substrate junction can be tailored by varying the energy levels of the sensitizer taking advantage of the size quantization effect and keeping the energy level of the substrate constant. As the particle size of sensitizer approaches that of the bulk, its lowest edge of the conduction band lies below that of the TiO₂ and electron transfer from the sensitizer to TiO₂ cannot occur. With CdS semiconductor nanoparticles as sensitizer, electron transfer from the excited CdS into the Titania electrode occurred only when the particle size of the CdS was sufficiently larger than 2 nm, suggesting the role of quantum size effects in the charge-transfer process. For a wide bandgap substrate like TiO₂, optimum particle size of the CdS for high photocurrent quantum yield has been found to be 4–5 nm.

The sensitization of the TiO₂ films with CdSe semiconductor nanoparticles shifted the absorption to visible region of the electromagnetic spectrum. Upon irradiation in visible region, the CdSe–TiO₂ composite photoanode in a photoelectrochemical cell showed IPCE of 12% due to rapid electron injection from the CdSe into the TiO₂ [79].

3.7.2 Sensitization by metal nanoparticles

Nanoporous TiO₂ films were loaded with Au nanoparticles and the Au nanoparticles were photoexcited due to plasmon resonance. Then charge separation occurred by the transfer of electrons from the Au nanoparticles into the TiO₂ film and by the electron transfer from donor in solution to the Au nanoparticles [80]. Similar loading of Au/Ag nanoparticles into the TiO₂ film was potentially useful in applications such as Photovoltaics, Plasmon sensors, photocatalysis [81]. Upon UV-light illumination, the TiO₂ nanorods sensitized with Au/Ag nanoparticles were found to sustain higher degree of conduction band electrons, compared to pure TiO₂ [82].

3.7.3 Sol-gel deposition of 2D array of Au/Ge nanodots on patterned TiO₂

Self-assembled Inorganic NanoPatterns (INPs) on crystalline silicon wafers as templated surfaces have been explored for the formation of Au and Ge nanoparticles. The substrates were prepared by sol-gel liquid deposition and evaporation induced self-assembly (EISA) of a hybrid solution composed of block copolymer micelles and TiO₂ inorganic metal oxide precursor. This resulted in a single layer of hexagonally arranged micelles surrounded by the inorganic precursors. After condensation and block copolymer decomposition by heat treatment, the final thin metal oxide layer bears uniform nanoperforations with controlled spacing (100–510 nm) and height (5–15 nm) according to the length of the block copolymer in use. Such arrays of self-organized metal nanodots on the TiO₂ nanopatterns have been studied for their optoelectronic applications [83]. In a typical synthesis, Sol-gel initial solutions are composed of TiCl₄:EtOH:H₂O:PB_12.5-b-PEO₁₅ (molar ratio = 1:40:7:1.5 × 10⁻³). PB_12.5-b-PEO₁₅ refers to polybutadiene-block-poly(ethylene oxide) with blocks of 12,500 and 15,000 g mol⁻¹. In the case of PB_5.5-b-PEO₃₀, the molar ratio is 10⁻³. The solution is divided in two parts: in part A, PB-b-PEO is dissolved in 2/3 of the ethanol and water; part B contains TiCl₄ and the remaining ethanol. The solutions are aged for 2 h at 70°C, and then part A is slowly cooled to room temperature in ∼30 min. Finally, both parts are mixed before use. Films are deposited on cleaned silicon wafer by dip coating at a temperature of 40°C and a relative humidity below 20%, using a withdrawal speed in the range of 1–3 mm s⁻¹ to obtain a film thickness of <10 nm, corresponding to a monolayer of INPs (Figure 12). Additional SEM characterization can be performed to assess that only a monolayer is deposited and to modify the withdrawal speed if not. The resulting film is then annealed at 450°C for 30 min. Substrates, previously dip-coated to obtain self-assembled perforations, are immersed into a diluted hydrofluoric acid (HF) solution of 1.17 mol⁻¹ for 20 s to remove the native silicon oxide at the bottom of the perforations and reveal the silicon surface without damaging the INPs. Open perforations of 28 ± 4 nm in diameter are obtained with accessibility of the substrate surface. Immediately after HF treatment, the INP substrates are placed under vacuum. Gold is deposited by sputtering at room temperature (P = 4 × 10⁻⁶ mbar).

Figure 12.

(a) Scheme depicting the process to obtain organized metallic nanodots. A monolayer of micelles embedded in a titania gel is first deposited on a silicon substrate. After annealing, TiO₂ INPs are formed revealing the bare silicon. Under the appropriate conditions, a single nanodot per perforation is obtained. (b) SEM images of the TiO₂ INPs network after annealing for (left) large perforations of 20 nm (PB_12.5-b-PEO₁₅), (right) small perforations of 12 nm (PB_5.5-b-PEO₃₀). (c) SEM images of nanodots hexagonally arranged in TiO₂ INPs: (left) nanocrystalline Ge nanodots and (right) Au nanodots. Reprinted with permission from Ref. [83]. © 2019 American Chemical Society.

Using block copolymer–micelles-assisted sol-gel deposition of TiO₂ on Si and thermal annealing, the substrates with INPs featuring hexagonally positioned perforations homogeneously sized and spaced, were prepared. These templates are used to selectively form individual nanodots in each perforation featuring typical size of 28 ± 5 nm for the Au nanodots.

4. Performance of TiO₂ nanomaterials

H₂ storage: TiO₂ nanotubes were found to store H₂ gas up to ~2 wt% at room temperature and a pressure of 6 MPa (at atomic ratio of H/TiO₂ of 1.6), compared to a much lower hydrogen concentration of 0.8 wt% for the bulk TiO₂. Of this 2 wt% of the adsorbed hydrogen, only 75% could be released at lower pressure, while the remaining 25% tend to be retained owing to chemisorption. Only a part (13%) of the chemisorbed hydrogen was completely released from the Nanotubes after heating at 70°C [84]. Bavykin et al. [85] found that the H₂ gas was adsorbed between the layers of multilayered walls of the Titania nanotubes in the temperature range −195°C to −200°C at 0–6 bar pressure.

Electrode in DSSC: DSSC with electrodes made of TiO₂ nanotubes (10-nm diameter & 30–300 nm long) showed an efficiency of 4.88% and short-circuit current density more than twice that showed by the device made of Degussa P-25 TiO₂ nanoparticle electrodes, under AM 1.5 illumination [86].

Ohsaki et al. [87] found that better efficiency of solar cells fabricated using TiO₂ nanotube electrodes was due to increase in electron density in TiO₂ nanotube electrodes, compared to the bulk TiO₂ (P-25) electrodes.

Grimes et al. [88] fabricated DSSC with TiO₂ nanotubes (46-nm pore diameter, 17-nm wall thickness, 360-nm long) showed a photocurrent efficiency of 2.9%, which was attributed to superior electron lifetimes and electron percolation, compared to TiO₂ nanoparticle system.

Water-splitting: Br ⁻/Cl⁻ doped nanocrystalline TiO₂ electrodes were reported to shift the absorption edge to the visible region and showed better efficiency of water splitting than pure TiO₂ [89]. Nickel-doped TiO₂ photocatalyst was found to generate hydrogen gas at nearly 125.6 l mol/h compared to 81.2 l mol/h for pure P-25 TiO₂ [90].

Yang et al. [91] found that TiO₂ nanotubes treated with H₂SO₄ solutions showed photocatalytic activity on degradation of acid orange II in the following order: TiO₂ nanotubes treated with 1.0 mol/L H₂SO₄ solution > TiO₂ nanotubes treated with 0.2 mol/L H₂SO₄ solution > untreated TiO₂ nanotubes > TiO₂ nanoparticles, since TiO₂ nanotubes treated with H₂SO₄ were composed of smaller particles and had higher specific surface areas.

Electrochromic displays/windows: Electrochromism is the ability of a material to change color upon oxidation or reduction. The TiO₂ nanomaterials have been widely investigated for applications in electrochromic windows and displays. Electrochromic windows will darken upon application of a small voltage, while it will become transparent to visible light/solar light on reversing the voltage. A smart window can regulate the entry of light/energy through it in such a way that the need for air-conditioning the room decreases. Nanocrystalline structure of the TiO₂ film makes possible 100–1000-fold amplification compared to a flat TiO₂ surface. An electrochromophore molecule (adsorbed on to the nanocrystalline Titania electrode) switches color on applying a small voltage. High conductivity of nanocrystalline nature of the electrode, fast electron exchange with the electrochromophore, optical amplification by the porous structure and fast charge compensation by the ions in the contacting liquid, make nanocrystalline TiO₂ electrodes, highly attractive components of the electrochromic devices. These electrodes can be fabricated using sol-gel process followed by spin-coating to obtain a film of desired thickness [92, 93].

4.1 Conclusions

There have been continuous research efforts on the syntheses of TiO₂ nanomaterials in the past decades, owing to its attractive properties found critical to a wide range of applications such as photovoltaics, photoelectrochemical cells, photocatalysis, environmental/wastewater remediation, photo−/electro-chromics, opto-electronics, NL optics, flexible electronics, H₂ storage, and gas sensors, to name a few. There has been continuous research on the syntheses and modifications of similar non-magnetic metal oxide nanostructures [94]. The progress in synthesizing the technologically important TiO₂ with newer nanostructures and better properties, could not have been possible without the underlying research efforts in instrumentation as well. The sol-gel processing that was used earlier for the syntheses of metal oxide nanoparticles, has progressed to developing the TiO₂ nanomaterials with different morphologies such as microspheres, aerogels, opals, nanotubes, nanorods and nanowires. This progress was made possible by the sol-gel process with assistance of template, surfactants, micelles, NLO-active material, spin-coating, dip-coating, electrophoretic deposition, polystyrene, and diblock polymer. Further, sensitization of TiO₂ nanomaterials was made possible in sol-gel process using metal ion dopants, metal nanoparticles, narrow bandgap semiconductors, and organic dyes, depending on the type of sensitization required.

There are some practical concerns/challenges such as precise control of deposition at single atomic level, and growing best quality films, in fabricating metal-oxide thin films by the sol-gel process. There is a possibility of losing the porosity of the films during high-temperature sintering. But the porosity of the nanostructure is important for applications such as Catalysis, sensors for organic/biocomponent, electrodes in solar cells. Syntheses of crystalline phase of complex metal oxides, without the high-temperature sintering, needs to be addressed. Template-assisted sol-gel processing for the nanorod/nanotubes requires complete filling of the template/pores by sol and enrichment of solid inside the pores. Difficulty in ensuring the complete filling of the template pores needs to be addressed. There is a steady and continuous progress in the research on TiO₂ nanomaterials which will continue to impact the research on energy and environmental remediation fields. This continuing research on the titania nanostructures may possibly shed light on the synthetic process modifications needed to address these concerns and issues, without resorting to expensive instrumentation.

Acknowledgments

I thank my family for their immense support during the preparation of the chapter, and Lord Almighty for giving me the inner strength and clarity in this endeavor. Last but not the least, I thank our esteemed publisher, M/s IntechOpen Limited for offering me this authorship/opportunity and the continuous support.

Additional information

ORCID ID: 0009-0008-8249-6412.