TiO2 Nanostructures by Sol-Gel Processing

*Srinivasa Raghavan*

## **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.

**Keywords:** sol-gel process, TiO2 nanostructures, sol-gel films, electrophoretic deposition, template-filling, nanorods, aerogels, opals, nanotubes, nanowires

## **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 [5–7], research efforts on TiO2 have gained momentum in the past decades, especially in areas of photocatalysis, photovoltaics, photoelectrochemical cells, environmental pollution control, and sensors [8–11].

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 [12–15]. 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.

## **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 [20–22]. 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

**Figure 1.** *Representative nanostructures of titanium dioxide.*

**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.*

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:

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.

i.*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].


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, Dyesensitized solar cells (DSSC), solid-state DSSC, and perovskite solar cells (PSC) [26–31]. 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 m<sup>2</sup> /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 m<sup>2</sup> /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.

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 m<sup>2</sup> /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 m<sup>2</sup> /g and 85% porosity, compared to the Degussa P-25.

**Figure 4.** *SEM image of TiO2 Aerogel. Reprinted with permission from Ref. [40]. © 1995 American Chemical Society.*

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 templateassisted sol-gel process under uniaxial compression at ambient temperature [42].

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.

*TiO2 Nanostructures by Sol-Gel Processing DOI: http://dx.doi.org/10.5772/intechopen.111440*

#### **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.*

## **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 electrooptic 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].

#### *TiO2 Nanostructures by Sol-Gel Processing DOI: http://dx.doi.org/10.5772/intechopen.111440*

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, spincoating 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].

## **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

### **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.*

*TiO2 Nanostructures by Sol-Gel Processing DOI: http://dx.doi.org/10.5772/intechopen.111440*

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].

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/

#### **Figure 7.**

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

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 wellaligned 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

### **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.*

## *TiO2 Nanostructures by Sol-Gel Processing DOI: http://dx.doi.org/10.5772/intechopen.111440*

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].

## *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. [64–68] 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 [69–73].

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.

#### *TiO2 Nanostructures by Sol-Gel Processing DOI: http://dx.doi.org/10.5772/intechopen.111440*

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.

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.

#### **Figure 10.**

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

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.

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

#### **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.*

#### *TiO2 Nanostructures by Sol-Gel Processing DOI: http://dx.doi.org/10.5772/intechopen.111440*

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:PB12.5-b-PEO15 (molar ratio = 1:40:7:1.5 × 10−3). PB12.5-b-PEO15 refers to polybutadiene-block-poly(ethylene oxide) with blocks of 12,500 and 15,000 g mol−1. In the case of PB5.5-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

#### **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 (PB12.5-b-PEO15), (right) small perforations of 12 nm (PB5.5-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.*

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).

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**

*H*2 *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 hightemperature 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.

## **Acknowledgements**

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.

## **Author details**

Srinivasa Raghavan Ramakrishna Mission Vivekananda College, Chennai, India

\*Address all correspondence to: srirag11@gmail.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Pfaff G, Reynders P. Angle-dependent Optical Effects Deriving from Submicron Structures of Films and Pigments. Chemical Reviews. 1999;**99**:1963

[2] Salvador A, Pascual-Marti MC, Adell JR, Requeni A, March JG. Analytical Methodologies for Atomic Spectrometric determination of Metallic oxides in UV Sunscreen Creams. Journal of Pharmaceutical and Biomedical Analysis. 2000;**22**:301

[3] Braun JH, Baidins A, Marganski RE. TiO2 Pigment Technology: A Review. Progress in Organic Coating. 1992;**20**:105

[4] Yuan SA, Chen WH, Hu SS. Fabrication of TiO2 Nanoparticles/Surfactant Polymer Complex Film on Glassy Carbon Electrode and its Application to Sensing Trace Dopamine. Materials Science and Engineering: C. 2005;**25**:479

[5] Fujishima A, Honda K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature. 1972;**37**:238

[6] Fujishima A, Rao TN, Tryk DA. Titanium Dioxide Photocatalysis. Journal of Photochemistry and Photobiology C Photochemistry Reviews. 2000;**1**:1

[7] Tryk DA, Fujishima A, Honda K. Recent Topics in Photoelectrochemistry: Achievements and Future Prospects. Electrochimica Acta. 2000;**45**:2363

[8] Grätzel M. Photoelectrochemical Cells. Nature. 2001;**414**:338

[9] Hagfeldt A, Grätzel M. Light-induced Redox Reactions in Nanocrystalline Systems. Chemical Reviews. 1995;**95**:49

[10] Linsebigler AL, Lu G, Yates JT Jr. Photocatalysis on TiO2 Surfaces: Principles Mechanisms and Selected Results. Chemical Reviews. 1995;**95**:735

[11] Millis A, Le Hunte S. An Overview of Semiconductor Photocatalysis. Journal of Photochemical and Photobiology. A. 1997;**108**:1

[12] Alivisatos AP. Perspectives on the Physical Chemistry of Semiconductor Nanocrystals. The Journal of Physical Chemistry. 1996;**100**:13226

[13] Alivisatos AP. Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science. 1996;**271**:933

[14] Burda C, Chen X, Narayanan R, El-Sayed MA. Chemistry and Properties of Nanocrystals of Different Shapes. Chemical Reviews. 2005;**105**:1025

[15] Murray CB, Kagan CR, Bawendi MG. Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystals Assemblies. Annual Review of Materials Science. 2000;**30**:545

[16] Roduner E. Nanoscopic Materials: Size-dependent Phenomena. Cambridge: Royal Society of Chemistry; 2006

[17] Kormann C, Bahnemann DW, Hoffmann MR. Preparation and Characterization of Quantum-Size Titanium Dioxide. The Journal of Physical Chemistry. 1988;**92**:5196

[18] Anpo M, Shima T, Kodama S, Kubokawa Y. Photocatalytic Hydrogenation of Propyne with Water on Small-Particle Titania: Size Quantization Effects and Reaction Intermediates. The Journal of Physical Chemistry. 1987;**91**:4305

[19] Sakai N, Ebina Y, Takada K, Sasaki T. Electronic Band Structure of Titania Semiconductor Nanosheets Revealed by Electrochemical and Photoelectrochemical Studies. Journal of the American Chemical Society. 2004;**126**:5851

[20] Brinker CJ, Scherer GW. Sol–Gel Science. NY: Academic Press; 1990

[21] Hench LL, West JK. Sol–Gel Process. Chemical Reviews. 1990;**90**:33

[22] Aegerter MA, Mehrotra RC, Oehme I, Reisfeld R, Sakka S, Wolfbeis O, et al. Optical and Electronic Phenomena in Sol—Gel glasses and Modern Applications. Vol. 85. Berlin: Springer; 1996

[23] Hench LL, Wang SH, Nogues JL. In: Gunshor RL, editor. Multifunctional Materials. Vol. 878. Bellingham, WA: SPIE; 1988. p. 76

[24] Hench LL, Wilson MJR, Balaban C, Nogues JL. Sol–Gel Processing of Large Silica Optics. In: Proceedings of 4th International Conference on Ultrastructure Processing of Ceramics, Glasses, and Composites. Tucson, AZ; 1989

[25] Klein LC, Garvey GJ. In: Hench LL, Ulrich DR, editors. In: Ultrastructure Processing of Ceramics, Glasses and Composites. New York: Wiley; 1984. p. 88

[26] Szeifert JM, Fattakhova-Rohlfing D, Georgiadou D, Kalousek V, Rathousky J, Kuang D, et al. "Brick and Mortar" Strategy for the Formation of Highly Crystalline Mesoporous Titania Films from Nanocrystalline Building Blocks. Chemistry of Materials. 2009;**21**:1260-1265

[27] Gratzel M, Bach U, Lupo D, Comte P, Moser JE, Weissortel F, et al. Solid-State Dye- Sensitized Mesoporous TiO2 Solar Cells with High Photon−To−Electron Conversion efficiencies. Nature. 1998;**395**:583-585

[28] Birkefeld LD, Azad AM, Akbar SA. Carbon Monoxide and Hydrogen Detection by Anatase Modification of Titanium Dioxide. Journal of the American Ceramic Society. 1992;**75**:2964-2968

[29] Terzian R. Photocatalyzed Mineralization of Cresols in Aqueous Media with Irradiated Titania. Journal of Catalysis. 1991;**128**:352-365

[30] Wang C, Yin L, Zhang L, Qi Y, Lun N, Liu N. Large Scale Synthesis and Gas-Sensing Properties of Anatase TiO2 Three−Dimensional Hierarchical Nanostructures. Langmuir. 2010;**26**:12841-12848

[31] Hohn N, Schlosser SJ, Bießmann L, Song L, Grott S, Xia S, et al. Impact of Catalytic Additive on Spray Deposited and Nanoporous Titania Thin Films Observed via *in Situ* X-Ray Scattering: Implications for Enhanced Photovoltaics. ACS Applied Nanomaterials. 2018;**1**:4227-4235

[32] Green MA, Hishikawa Y, Warta W, Dunlop ED, Levi DH, Hohl-Ebinger J, et al. Solar Cell Efficiency Tables (version 50). Progress in Photovoltaics. 2017;**25**:668-676

[33] Ares AE. Editor. IntechOpen: Thin Films; 2021. DOI: 10.5772/ intechopen.87838

[34] Roth SV, Santoro G, Risch JFH, Yu S, Schwartzkopf M, Boese T, et al. Patterned Diblock Co-Polymer Thin Films as Templates for Advanced Anisotropic Metal Nanostructures. ACS Applied Materials & Interfaces. 2015;**7**:12470-12477

[35] Peinemann K-V, Abetz V, Simon PFW. Asymmetric Superstructure Formed in a Block Copolymer Via Phase Separation. Nature Materials. 2007;**6**:992-996

[36] Tauster SJ, Fung SC, Garten RL. Strong Metal-Support Interactions. Group *TiO2 Nanostructures by Sol-Gel Processing DOI: http://dx.doi.org/10.5772/intechopen.111440*

8 Noble Metals Supported on Titanium Dioxide. Journal of the American Chemical Society. 1978;**100**:170-175

[37] Shastri AG, Datye AK, Schwank J. Gold-Titania Interactions: Temperature Dependence of Surface Area and Crystallinity of TiO2 and Gold Dispersion. Journal of Catalysis. 1984;**87**:265

[38] Ragai J, Sing KSW, Mikhail R. Origin of Porosity in Titania Gels. I. Microporous and Mesoporous Gels prepared from Titanous Chloride and Ammonia. Journal of Chemical Technology and Biotechnology. 1980;**30**:1

[39] Campbell LK, Na BK, Ko EI. Synthesis and Characterization of Titania Aerogels. Chemistry of Materials. 1992;**4**(6):1329-1333

[40] Zhu Z, Tsung LY, Tomkiewicz M. Morphology of TiO2 Aerogels. 1. Electron Microscopy. The Journal of Physical Chemistry. 1995;**99**:15945

[41] Dagan G, Tomkiewicz M. Preparation and Characterization of TiO2 Aerogels for Use as Photocatalysts. Journal of Non-Crystalline Solids. 1994;**175**:294

[42] Ji L, Rong J, Yang Z. Opal Gel Templated Synthesis of Oblate Titania Opals. Chemical Communications. 2003;**1080**

[43] Avnir D, Levy D, Reisfeld R. The Nature of the Silica Cage as Reflected by Spectral Changes and Enhanced Photostability of Trapped Rhodamine 6G. The Journal of Physical Chemistry. 1984;**88**:5956-5959

[44] Nosaka Y, Tohriiwa N, Kobayashi T, Fujii N. Two Dimensionally Poled Sol– Gel Processing of Titania Film doped with Organic Compounds for Nonlinear Optical Activity. Chemistry of Materials. 1993;**5**:930-932

[45] Choi W, Termin A, Hoffmann MR. The Role of Metal Ion Dopants in Quantumsized TiO2: Correlation between Photoreactivity and Charge Carrier Recombination Dynamics. The Journal of Physical Chemistry. 1994;**98**:13669

[46] Li FB, Li XZ, Hou MF. Photocatalytic Degradation of 2-Mercaptobenzothiazole in Aqueous La3+--TiO2 Suspension for Odor Control. Applied Catalysis B: Environmental. 2004;**48**:185-194

[47] Li W, Wang Y, Lin H, Shah SI, Huang CP, Doren DJ, et al. Bandgap Tailoring of Nd3<sup>+</sup> −Doped TiO2 Nanoparticles. Applied Physics Letters. 2003;**83**:4143

[48] Chen X, Lou Y, Dayal S, Qiu X, Krolicki R, Burda C, et al. Doped Semiconductor Nanomaterials. Nanosci. Nanotechnol. 2005;**5**:1408

[49] Umebayashi T, Yamaki T, Itoh H, Asai K. Analysis of Electronic Structures of 3d Transition Metal-Doped TiO2 based on Band Calculations. Journal of Physics and Chemistry of Solids. 2002;**63**:1909

[50] Haidrya AA, Puskelova J, Plecenik T, Durina P, Gregus J, Truchly M, et al. Characterization and Hydrogen Gas Sensing Properties of TiO2 Thin Films Prepared by Sol–Gel Method. Applied Surface Science. 2012;**259**:270

[51] Zheng C, Lin J, Song X, Gan Q, Lin X. TiO2-Nanoparticle-Shelled Light-Driven Microcleaner for Fast and Highly Efficient Degradation of Organic Pollutants. ACS Applied Nano Materials. 2022;**5**:16573

[52] Lin J, Tao Y, Liu J, Zheng C, Song X, Dai P, et al. TiO2 @ Carbon Microsphere Core-Shell Micromotors for Photocatalytic Water Remediation. Optical Materials. 2022;**124**:111989

[53] Dong R, Zhang Q, Gao W, Pei A, Ren B. Nanomotor lithography. ACS Nano. 2016;**10**:839

[54] Moo JG, Pumera M. Chemical Energy Powered Nano/micro/macromotors and the Environment. Chemistry--A European Journal. 2015;**21**:58-72

[55] Bertucci S, Megahd H, Dodero A, Fiorito S, Di Stasio F, Patrini M, et al. Mild Sol–Gel Conditions and High Dielectric Contrast: A Facile Processing toward Large-scale Hybrid Photonic Crystals for Sensing and Photocatalysis. ACS Applied Materials & Interfaces. 2022;**14**:19806-19817

[56] Chen Y, Crittenden JC, Hackney S, Sutter L, Hand DW. Preparation of a Novel TiO2-based p-n Junction Nanotube Photocatalyst. Environmental Science & Technology. 2005;**39**:1201

[57] Lee S, Jeon C, Park Y. Fabrication of TiO2 Tubules by Template Synthesis and Hydrolysis with Water Vapor. Chemistry of Materials. 2004;**16**:4292

[58] Qiu JJ, Yu WD, Gao XD, Li XM. Sol–Gel Assisted ZnO Nanorod Array Template to Synthesize TiO2 Nanotube Arrays. Nanotechnology. 2006;**17**:4695

[59] Limmer SJ, Seraji S, Forbess MJ, Wu Y, Chou TP, Nguyen C, et al. Electrophoretic Growth of Lead Zirconate Titanate Nanorods. Advanced Materials. 2001;**13**:1269

[60] Limmer SJ, Seraji S, Forbess MJ, Wu Y, Chou TP, Nguyen C, et al. Template-Based Growth of Various Oxide Nanorods by Sol–Gel Electrophoresis. Advanced Functional Materials. 2002;**12**:59

[61] Cao GZ. Growth of Oxide Nanorod Arrays through Sol Electrophoretic Deposition. The Journal of Physical Chemistry. B. 2004;**108**:19921

[62] Miao Z, Xu D, Ouyang J, Guo G, Zhao Z, Tang Y. Electrochemically Induced Sol–Gel Preparation of Single−Crystalline TiO2 Nanowires. Nano Letters. 2002;**2**:717

[63] Natarajan C, Nogami G. Cathodic Electrodeposition of Nanocrystalline Titanium Dioxide Thin Films. Journal of the Electrochemical Society. 1996;**143**(5):1547

[64] Sugimoto T, Okada K, Itoh H. Synthesis of Uniform Spindle-Type Titania Particles by the Gel-Sol Method. Journal of Colloid and Interface Science. 1997;**193**:140

[65] Sugimoto T, Zhou X. Synthesis of Uniform Anatase TiO2 Nanoparticles by The Gel-Sol Method 2. Adsorption of OH<sup>−</sup> Ions to Ti(OH)4 Gel and TiO2 Particles. Journal of Colloid and Interface Science. 2002;**252**:347

[66] Sugimoto T, Zhou X, Muramatsu A. Synthesis of Uniform Anatase TiO2 Nanoparticles by The Gel−Sol Method 1. Solution Chemistry of Ti(OH)(4-n)+ (n) Complexes. Journal of Colloid Interface Science. 2002;**252**:339

[67] Sugimoto T, Zhou X, Muramatsu A. Synthesis of Uniform Anatase TiO2 Nanoparticles by The Gel-Sol Method. 4. Shape Control. Journal of Colloid and Interface Science. 2003;**259**:53

[68] Sugimoto T, Zhou X, Muramatsu A. Synthesis of Uniform Anatase TiO2 Nanoparticles by The Gel-Sol Method. 3. Formation Process and Size Control. Journal of Colloid and Interface Science. 2003;**259**:43

[69] Uekawa N, Kajiwara J, Kakegawa K, Sasaki Y. Low Temperature Synthesis and Characterization of Porous Anatase TiO2 Nanoparticles. Journal of Colloid and Interface Science. 2002;**250**:285

### *TiO2 Nanostructures by Sol-Gel Processing DOI: http://dx.doi.org/10.5772/intechopen.111440*

[70] Fujii H, Inata K, Ohtaki M, Eguchi K, Arai H. Synthesis of CdS/TiO2 Nanocomposite via TiO2 coating on CdS Nanoparticle by Compartmentalized Hydrolysis of Ti alkoxide. Journal of Materials Science. 2001;**36**:527

[71] Matsumoto H, Matsunaga T, Sakata T, Mori H, Yoneyama H. Size Dependent Fluorescence Quenching of CdS Nanocrystals Caused by TiO2 Colloids as a Potential-Variable Quencher. Langmuir. 1995;**11**:4283

[72] Qian X, Qin D, Bai Y, Li T, Tang X, Wang E, et al. Photosensitization of TiO2 Nanoparticulate Thin Film Electrodes by CdS Nanoparticles. Journal of Solid State Electrochemistry. 2001;**5**:562

[73] Shen Q, Arae D, Toyoda T. Photosensitization of Nanostructured TiO2 with CdSe Quantum Dots: Effects of Microstructure and Electron Transport in TiO2 Substrates. Journal of Photochemical and Photobiology. A. 2004;**164**:75

[74] Spanhel L, Weller H, Henglein A. Photochemistry of Semiconductor Colloids. 22. Electron Ejection from Illuminated CdS into attached Titanium and Zinc Oxide particles. Journal of the American Chemical Society. 1987;**109**:6632

[75] Hoyer P, Koenenkamp R. Photoconduction in Porous TiO2 Sensitized by PbS Quantum Dots. Applied Physics Letters. 1995;**66**:349

[76] Fitzmaurice D, Frei H, Rabani J. Time-Resolved Optical Study on the Charge carrier Dynamics in a TiO2/ AgI Sandwich Colloid. The Journal of Physical Chemistry. 1995;**99**:9176

[77] Desilvestro J, Gratzel M, Kavan L, Moser J, Augustynski J. Highly Efficient Sensitization of Titanium Dioxide. Journal of the American Chemical Society. 1985;**107**:2988

[78] Vogel R, Hoyer P, Weller H. Quantum-sized PbS, CdS, Ag2S, Sb2S3, and Bi2S3 Particles as Sensitizers for Various Nanoporous wide-Bandgap Semiconductors. The Journal of Physical Chemistry. 1994;**98**:3183

[79] Robel I, Subramanian V, Kuno M, Kamat PV. Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO2 Films. Journal of the American Chemical Society. 2006;**128**:2385

[80] Tian Y, Tatsuma T. Mechanisms and Applications of Plasmon-induced Charge Separation at TiO2 Films Loaded with Gold Nanoparticles. Journal of the American Chemical Society. 2005;**127**:7632

[81] Tian Y, Tatsuma T. Plasmoninduced Photoelectrochemistry at Metal Nanoparticles Supported on Nanoporous TiO2. Chemical Communications. 2004;**1810**

[82] Cozzoli PD, Curri ML, Agostiano A. Efficient Charge Storage in Photoexcited TiO2 Nanorod-Noble Metal Nanoparticle Composite Systems. Chemical Communications. 2005;**3186**

[83] Bottein T, Bouabdellaoui M, Jean-Benoît C, Favre L, David T, Putero M, et al. Large Scale Self-Organization of 2D Hexagonal Ge and Au Nanodots on patterned TiO2. ACS Applied Nano Materials. 2019;**2**:2026-2035

[84] Lim SH, Luo J, Zhong Z, Ji W, Lin J. Room Temperature Hydrogen Uptake by TiO2 Nanotubes. Inorganic Chemistry. 2005;**44**:4124

[85] Bavykin DV, Lapkin AA, Plucinski PK, Friedrich JM, Walsh FC. Reversible Storage of Molecular Hydrogen by Sorption into Multilayered TiO2 nanotubes. The Journal of Physical Chemistry. B. 2005;**109**:19422

[86] Adachi M, Murata Y, Okada L, Yoshikawa S. Formation of Titania Nanotubes and Applications for Dyesensitized Solar Cells. Journal of the Electrochemical Society. 2003;**150**:G488

[87] Ohsaki Y, Masaki N, Kitamara T, Wada Y, Okamoto T, Sekino T, et al. Dye-Sensitized TiO2 Nanotube Solar Cells: Fabrication and Electronic Characterization. Physical Chemistry Chemical Physics. 2005;**7**:4157

[88] Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA. Use of Highly-Ordered TiO2 Nanotube Arrays in Dye-Sensitized Solar Cells. NanoLett. 2006;**6**:215

[89] Luo H, Takata T, Lee Y, Zhao J, Domen K, Yan Y. Photocatalytic Activity Enhancing for Titanium Dioxide by Co-Doping with Bromine and Chlorine. Chemistry of Materials. 2004;**16**:846

[90] Jing D, Zhang Y, Guo L. Study on the synthesis of Ni-Doped Mesoporous TiO2 and its Photocatalytic Activity for Hydrogen Evolution in Aqueous Methanol Solution. Chemical Physics Letters. 2005;**415**:74

[91] Yang SG, Quan X, Li XY, Fang N, Zhang N, Zhao HM. J. Environ. Sci. Vol. 17. (Beijing, China); 2005. p. 290

[92] Bonhote P, Gogniat E, Campus F, Walder L, Grätzel M, Nanocrystalline Electrochromic Displays. Displays 1999; 20: 137

[93] Bonhote P, Gogniat E, Gra¨tzel M, Ashrit PV. Novel Electrochromic Devices Based on Complimentary Nanocrystalline TiO2 and WO3 Thin Films. Thin Solid Films. 1999;**350**:269 [94] Singh JP, Singh V, Sharma A, Pandey G, Chae KH, Lee S. Approaches to Synthesize MgO Nanostructures for Diverse Applications. Heliyon. 2020;**6**:e04882

## **Chapter 4**

## Synthesis of Some Functional Oxides and Their Composites Using Sol-Gel Method

*Arafa Hassen, Adel M. El Sayed, Azza Al-Ghamdi and Mohamed Shaban*

## **Abstract**

Two main approaches for nanomaterials fabrication are the top-down and the bottom-up methods. The first is limited to mechanical grinding, thermal evaporation, ion sputtering, arc discharge, pulsed laser ablation, and other physical and chemical vapor deposition. These routes are costly, consume higher energy, and require complex technology such as ultrahigh vacuum. The bottom-up methods refer to the production of complex nanostructured materials from atoms and molecules. This approach is relatively simple and low in cost. However, it requires a good knowledge of the optical properties of the particles and their modifications when the particles are integrated with nanostructures. One of the widest bottom-up methods is the sol-gel. It involves a solution or sol (single-phase liquid) that undergoes a sol-gel transition (stable suspension of colloidal particles). In this chapter, we throw light on the history of sol-gel, its advantages, and limitations, operating this method for the production of different types of nanomaterials in the form of powders or thin films. In addition, some applications of the sol-gel-derived nanosized materials will be discussed.

**Keywords:** sol-gel preparation, metal oxide nanomaterials, characterization, oxides, sol-gel chemistry

## **1. Introduction**

Nanosized material, a material with at least one dimension limited to ˂100 nm (A nanometer is 10<sup>9</sup> of a meter.), displays unique and unexpected physicochemical properties. This behavior of nanomaterials arises from the large surface area to volume ratio and the quantum confinement effect that can be defined as the reduction of the band structure of the material into discrete quantum levels and the emerging of new energies for the electrons, resulting from the limited size of its particle, also known as the "size-effect." **Figure 1** shows that the surface atoms/volume ratio increases exponentially with decreasing particle size. Increasing the surface of the material increases its reactivity and photoelectrochemical performance. The accumulation of information on nanosized materials resulted in or emerged two branches, "Nanoscience" and "Nanotechnology." The former focuses on the preparation and

#### **Figure 1.** *The surface atoms/volume ratio (the determined surface-to-bulk atomic ratio) [1].*

characterization of the nanomaterials and the fundamental study of their properties, whereas the latter is related to designing and using structures and devices based on these nanosized materials in different applications [2, 3].

The literature survey revealed that the physical and chemical properties of nanosized materials as well as the particles' morphology (0D, 1D, 2D, ...), also depend on the preparation method and preparative parameters and conditions. With the continuous headway of nanotechnology, there are several methods or techniques for preparing nanosized materials which can be classified into two main branches; topdown and bottom-up; the top-down methods are based on breaking down large pieces/particles of the material to convert it to the required nanostructures. The "bottom-up" methods are based on assembling single atoms/molecules (in solutions or gas phase) into larger nanostructures. We will discuss the details of one of the bottomup methods in this chapter, named the sol-gel.

## **2. Sol-gel chemistry**

As a phenomenon, the sol-gel transition was discovered and explored by Ebelmen in 1846 by observing the slow transformation of silicic esters, in the presence of moisture, to hydrated silica and the spontaneous gelation when the alkoxide was placed in contact with the atmosphere [4]. However, the interest in the sol-gel method began in 1980 and received a continuous and increased interest exponentially until today, and we expect a growing interest during the current decade, as shown in **Figure 2**.

A sol is defined as a colloidal system in which the dispersion medium is a liquid, and the dispersed phase is a polymerized molecule or fine particles, where the particle/molecular size should be in the range of 1 nm – 1 μm. A gel is a continuous solid network that supports the continuous liquid phase [5]. In the typical sol-gel process, consecutive steps are the sol formation through hydrolysis, the sol-gel transition (gel state), the gel drying, and conversion into a calcined material, as shown in **Figure 3**. The chelating agent binds tightly with the metal ions to prevent the formation of

*Synthesis of Some Functional Oxides and Their Composites Using Sol-Gel Method DOI: http://dx.doi.org/10.5772/intechopen.111384*

**Figure 2.**

*Number of publications/decade utilizing the sol-gel route (Scopus database), # is the expected number [Scopus database].*

**Figure 3.** *Steps of sol-gel chemistry for powder and thin film formation [6, 7].*

aggregations. The sol-gel chemistry begins with mixing the precursor (acetate, nitrate, or chloride) with the solvent. If water is the solvent, the sol-gel is hydrolytic but named nonhydrolytic sol-gel in the case of using an organic solvent such as ethanol [6]. The solution prepared by the sol-gel chemistry is used cooperatively with coating techniques such as spray, dip, and spin coating. For thin film deposition, the chelating agent has the role of stabilizer to prevent the metal ions to be precipitated or agglomerated. The spin-coated films will form in nanoparticulate layers, as will be discussed.

According to Brinker and others, the sol-gel method is a technology where the solution containing the precursor solid materials evolves gradually to form a networked gel comprising both the liquid and solid phases. The precursors react with each other in the common solvent to form a colloidal suspension (sol). This sol undergoes a hydrolysis reaction that could be represented as *M OR* ð Þ*<sup>n</sup>* þ *nH*2*O* ! *M OH* ð Þ*<sup>n</sup>* þ *nROH* and condensation reaction: *M OH* ð Þ*<sup>n</sup>* ! *MO*0*:*5*<sup>n</sup>* þ ð Þ 0*:*5*n H*2*O* to form a continuous liquid network (gel) [4, 7]. Once the gel is formed, it can be coated and dried to form thin films, as will be discussed, or be further dried, and densified at higher temperatures to fine powder, depending on the application [5], see **Figure 3**.

The sol-gel approach became one of the key technologies of the twenty-first century owing to low-energy consumption, reproducibility, eco-friendly, simplicity, lowcost, and pollution-free. In addition, it allows the combination of inorganic/organic materials in a single-phase and yields an organic/inorganic hybrid coating which attracted great attention owing to their high compatibility, good adhesion to the substrate, and corrosion resistance. Moreover, the sol-gel technology is represented in low-temperature requirements, repeatability, and controllability. In addition, it is possible to tune the intrinsic properties and the elemental chemical composition of the material. The final product of the sol-gel reaction can be controlled by precursors, pH, processing time, and molar ratios between the reacting agents. Löbmann revealed that the sol-gel route could yield various topologies; porous λ/4 films, dense interference layers, and arrays of antireflective structures (called moth-eye). These topologies can be used for antireflective coatings for architectural glazing, the display industry, solar energy conversion, and ophthalmic lenses [8]. Controlling the structure of sol-gel prepared film could yield highly selective gas sensors [9]. Chen et al. [10] studied the effect of pH value (1–10) on the corrosion protection ability of the sol-gel coatings. The highest condensation degree occurred at pH 4, resulting in a compact and stable 3D sol-gel network of high crosslinking density, and this provided highly effective corrosion protection.

It was also found that the photocatalytic properties of the sol-gel prepared TiO2 nanopowder depend mainly on the sol composition, where the addition of water, HCl, and diethanolamine as well as the type of alcohol as solvent (ethanol, propanol, and butanol) were found to greatly affect the photocatalytic activity of the powder toward bromophenol blue dye removal [11]. Luo et al. [12] studied some of the variables related to the sol-gel preparation of CaO as a high-performance sorbent and they concluded that the molar ratio of H2O:Ca2+ had a minor effect on the CO2 sorption performance of the CaO, and the optimal molar ratio was 80:1. The optimal molar ratio of citric acid: calcium nitrate optimal molar ratio is 1:1, and adding an excess of citric acid led to more gaseous products. In addition, when the pH 3, the sol-gel structure was destroyed, and the optimal pH value was 2, where the best performance of CaO sorbent was achieved. A. C.-Soria et al. [13] fabricated Fe3C/few-layered graphene core/shell nanoparticles, with potential magnetic properties, embedded in a carbon matrix by a modified two-step surfactant sol-gel method, where the hydrolysis, polycondensation, and drying took place in a one-pot. Hashjin et al. [14] tuned up the sol-gel technique for preparing high-durable superhydrophobic coatings. The prepared layers are useful for anti-icing, self-cleaning, and anti-bacterial applications, in the energy and photovoltaic devices, textile and coating industry, construction, and aerospace industry.

Sol-gel technique, among various solution methods, is found to be more suitable for metal oxide thin films and nanopowder. Controlling the conditions of preparation, nanoparticles of controlled shape/morphology, control stoichiometry, size, textural, surface characteristics, purity, and high quality can be obtained. Besides, uncomplicated ideas can be executed via this technique for more recent and advanced technological applications [6]. In the following section, some examples of the sol-gel derived nanostructures will be mentioned with their characterization and some related applications. The data presented here are based on our experimental results. It would be better to throw light on some selected materials that were prepared using the sol-gel method.

## **3. Sol-gel preparation of NiO, CdO, SnO2, and PbO and their nanocomposites**

### **3.1 Experimental (preparation and characterization techniques)**

The precursor materials used for NiO, CdO, SnO2, and PbO preparation were: NiCl26H2O of molecular weight (MW = 237.7), supplied by Schorlau, Spin, Cd(NO3)2 MW = 236.42, supplied by Nova Oleochem Limited, SnCl2.2H2O, MW = 225.63, from Merck, and CH3COO)2Pb.3H2O, MW = 279.33, Adwik, Egypt, were used to prepare 0.7 M solutions by dissolving the required mass of each salt in 100 ml double-distilled water. To each solution, 8.825 g of oxalic acid (C2H2O4), as a chelating agent, was added under stirring at 60°C for 1 h. The obtained solutions were maintained in an oven at 80–90°C for 20 h to evaporate the excess water above the precipitate. The solutions were then cooled to room temperature and aged for 24 at room temperature (RT). Finally, the four gel was calcined at 400°C for 2 h to obtain the nanopowders: NiO, CdO, SnO2, and PbO nanoparticles (NP). The characterization of these nanometal oxides will be discussed.

The identifying of the crystalline phase and samples purity was done by recording XRD spectra using the PANalytical X'Pert PRO diffractometer, with Cu K*<sup>α</sup>* radiation of wavelength λ = 1.541 Ǻ, and scan in the range of 2θ = 5.0–80°. High-resolution transmission electron microscopy (HR-TEM) of model JEM, 2100, Jeol, Japan, was used to check the particle size and morphology of the prepared materials. For polymer nanocomposite films, the surface morphology was evaluated using field emissionscanning electron microscopy FE-SEM (Carl ZEISS Sigma 500 VP). In addition, the UV–vis spectra in the wavelength range of 200–1600 nm were recorded on a Shimadzu spectrophotometer (UV-3600 UV-Vis–NIR) with an accuracy of 0.2 nm.

#### **3.2 Results and discussion**

The crystallite size and phase identification of NiO, CdO, SnO2, and PbO were examined by XRD, shown in **Figure 4**, and the shape and particle morphology was studied by HR-TEM, as shown in **Figure 5**. **Figure 4** shows the XRD pattern of NiO; the sharp peaks indicate that a good crystallite material was grown by the sol-gel technique. The diffraction peaks at 2θ = 37.14, 43.13, and 62.89° are indexed for the crystal planes (111), (200), and (220) of NiO of rhombohedral [*fcc* (face-centered cubic) with a lattice constant *a* = 4.175 Å], in agreement with JCPDS No. 44–1159. Scherrer's formula (*C*<sup>s</sup> = 0.9*λ*Cu/*β*1*=*<sup>2</sup> *cos θ*) was utilized to calculate the crystallite size (*C*s), where *β*1*=*<sup>2</sup> is the full width at half maximum intensity. Considering the main detected peaks, the average *C*<sup>s</sup> was ≈ 28 nm.

In the XRD pattern of the sol-gel prepared CdO nanoparticles, all of the detected diffraction peaks are indexed to the cubic phase of CdO with a lattice parameter a = 4.69483 Å. The peaks at *2*θ ≈ 33.07°, 38.39°, 55.38°, 66°, and 69.38° are assigned to the (110), (200), (220), (311), and (222) crystal plans, respectively, according to JCPDS file No. 75–0592. This confirms the formation of CdO with excellent crystallinity and high purity, as no secondary phases were observed in the pattern of the CdO nanoparticles. The *C*<sup>s</sup> of the CdO nanoparticles were found to be in the range of 66.4–73.2 nm with an average of 70.18 nm. The pattern of SnO2 is also shown in **Figure 4**, where all the diffraction peaks with Miller indices of (110), (101), (200), (211), (220), (310), (112), and (301) are of the tetragonal (rutile) crystalline phase of

**Figure 4.** *(a-d) XRD patterns of the sol-gel-derived NiO, CdO, SnO2, and PbO nanoparticles.*

SnO2 according to JCPDS No. 72–1147. The (110) plane exhibits the highest intensity and presents the least surface energy and is the most thermodynamically and electrostatically stable [15]. The lattice parameters *a*, *b*, and *c* of the tetragonal SnO2 phase are determined from the formula: <sup>1</sup> *<sup>d</sup>*<sup>2</sup> <sup>¼</sup> *<sup>h</sup>*<sup>2</sup> <sup>þ</sup>*k*<sup>2</sup> ð Þ *<sup>a</sup>*<sup>2</sup> <sup>þ</sup> *<sup>l</sup>* 2 *<sup>c</sup>*2, and the calculated values were *a* = *b* = 4.473 Å and *c* = 3.189 Å. The *C*<sup>s</sup> values are in the range of 17.93–47.06 nm with an average size of 30.2 nm. In the case of lead monoxide, the XRD pattern is a mixture of *α-* and *β*-PbO. The diffraction peaks of the orthorhombic *β*-PbO are at 2θ = 29.06<sup>o</sup> , 30.35<sup>o</sup> , 32.59<sup>o</sup> , and 53.17o , with *a* = 5.88 Å, *c* = 4.74 Å, according to JCPDS card No. 77–1971. The other peaks are assigned to the tetragonal *α*-PbO, with *a* = 3.97 Å, *c* = 5.024 Å, in agreement with JCPDS card No. 85–1739. The *C*<sup>s</sup> of PbO is in the range of 24.4–113.4 nm with an average of 58.6 nm.

The HR-TEM image of NiO shows an average particle size of NiO in the range of 24.85–34.10 nm, which is smaller than that reported for NiO prepared from the thermal decomposition of Ni(OH)2 at 600°C [16]. TEM image of the CdO shows that CdO nanoparticles are well-defined and their size is in the range of 52–116 nm with an average particle size of 72 nm. Besides, the image for the SnO2 shows that SnO2 grains are segregated together and form agglomerates or clusters of primary crystallites. Most of the observed particles are tetragonal in shape. The average particle size measured by HR-TEM is �41 nm. Finally, the TEM image of the PbO formed as nanoparticles of sizes from tens of nm to <100 nm, with an average of about 59 nm, which is consistent with the XRD results.

Nickel oxide (NiO) is an interesting ceramic material with reasonable photostability and thermal stability, high melting at 1955°C, and a refractive index of ≈ 2.2. In addition, NiO is a *p*-type semiconductor with a wide optical bandgap (= 3.4– 4 eV) [17, 18]. When the sol-gel prepared NiO was incorporated at 0.5 and 1.0 wt% into a polymer matrix composed of carboxymethyl cellulose–polyvinyl pyrrolidone (CMC–PVP) blend, the reflectivity and refractive index of the blend dramatically changed, as shown in **Figure 6**. For the pure blend, R*%* is in the range of 3–5 and decreases with increasing the incident wavelength. However, this behavior converted to a bell-like shape, and the R*%* increased to 6.5–13% after 1.0 wt% NiO doping. Similarly, the refractive index (*n*) value of the blend changed from 1.45 to 1.59 in the

**Figure 6.** *Influence of NiO on the reflection R% (a), and refractive index n, (b) of a polymer blend.*

visible region and increased significantly to 2.233 after the NiO loading. This illustrates that NiO increased the packing density of the blend, and these nanoparticles act as scattering centers to increase the dispersion of light and increase the reflection and reflectivity of the matrix [19]. Therefore, NiO/blend are suitable material for coatings, lenses, and for engineering, and optoelectronic applications [20].

Cadmium oxide (CdO) is a promising II–VI compound that has *n*-type semiconductivity, resistivity in the order of 10�<sup>2</sup> –10�<sup>4</sup> Ω cm [21], high optical transmittance in the visible region, and a refractive index of 2.49 [22]. The direct (indirect) band gap is in the range of 2.2–2.5 eV (1.36–1.98 eV) [23]. Therefore, CdO has been used for catalytic and sensing applications and in some optoelectronic devices [23]. The sol-gel prepared CdO when mixed with PVC polymer resulted in decreasing the transmittance of the polymer from 89% to the range of 70–84% and shrinking its band gap from 5.12 eV to 4.96 eV, as shown in **Figure 7**. This result may reflect the important applications of the CdO/PVC nanocomposites in optical and/or electrical devices [24].

Tin oxide (SnO2) is also a transparent conducting oxide that exhibits outstanding electrical and optical properties. Its wide band gap (≈3.68 eV), high exciton binding energy (130 meV), high transmittance in the visible region of the spectra, high *n*-type conductivity (10<sup>2</sup> –10<sup>3</sup> Ω�<sup>1</sup> .cm�<sup>1</sup> ) at ambient temperature, nontoxicity, thermal stability, chemical sensitivity, and the low-cost makes SnO2 the best choice for the biomedical applications, gas sensing, photo-catalysis, solar cells, and future optoelectronic devices [25, 26]. In this chapter, we have used the sol-gel prepared SnO2 nanoparticles as nanofillers to modify the optical properties of a ternary blend composed of carboxymethyl cellulose–polyethylene glycol–polyvinyl alcohol (CMC–PEG– PVA). As shown in **Figure 8**. The *E*<sup>g</sup> value of the pure blend and SnO2/CMC–PEG– PVA nanocomposite were calculated from the absorption spectra (*Abs*.) of the samples by using Tuac' relation: ð Þ *<sup>α</sup>h<sup>υ</sup> <sup>r</sup>* <sup>¼</sup> *M h<sup>υ</sup>* � *Eg* , where *hν* is the energy of incident photons and *r* = 1/2 and 2 for the indirect and direct allowed transitions, respectively, and *<sup>α</sup>* <sup>¼</sup> <sup>2</sup>*:*<sup>303</sup> *Abs:* film thickness is the absorption coefficient.

**Figure 7.** *Effect of 1.0 wt% CdO on the transmittance (a), and optical gap (b) of PVC polymer.*

*Synthesis of Some Functional Oxides and Their Composites Using Sol-Gel Method DOI: http://dx.doi.org/10.5772/intechopen.111384*

**Figure 8.**

*Influence of SnO2 nanoparticles (at 1.0 wt% loading) on the direct (a) and indirect (b) optical gap and Urbach energy (c) of CMC-PEG-PVA ternary blend.*

As shown in **Figure 8**, both the direct and indirect transitions for the polymeric films are possible. This is evidenced by the linear relationship of both (*αhν*) <sup>2</sup> and (*αhν*) 0.5 on *hν* at higher photon energies. Extra-plotting the straight-line portions of the curves to zero absorption gives the *E*<sup>g</sup> values: direct *E*<sup>g</sup> = 5.28 and 5.04 eV and the indirect *E*<sup>g</sup> = 4.55 and 4.20 eV for the pure blend and 1.0 wt% SnO2/blend nanocomposite, respectively. Introducing the SnO2 nanoparticles induce energy levels inside the band gap of the blend matrix, resulting in the shrinking of *E*g. Similarly, doping with *α*-Fe2O3 nanorods at 1.0 wt*%* decreased *E*<sup>g</sup> of PVA–PEG from 5.28 to 4.83 eV [27]. The Urbach energy (*EU*), which is the width of the exponential absorp-

tion edge, can be calculated using the following equation [28]: *<sup>α</sup>* <sup>¼</sup> *<sup>α</sup><sup>o</sup>* exp *<sup>h</sup>ν*�*Ec EU* h �,

where *Ec* and *α<sup>o</sup>* are constants. The dependence of *ln* (*α*) on *hυ* for the films is shown in **Figure 8(c)**. The straight lines suggest that the absorption is according to the quadratic relation for inter-band transition, which satisfies the Urbach rule. The value of *E*<sup>U</sup> is taken as the reciprocal of the slope of the lines (*EU =* (d(ln*α*)*/*d*hυ*) �1 ), and its value was found to be 0.607 eV and 0.962 eV for pure and 1.0 wt% SnO2 loaded film, respectively. Thus *EU* changes inversely with *E*g. Increasing the *E*<sup>U</sup> is attributed to the disorder increase inside the material after SnO2 nanoparticles incorporation, resulting in the tailing in the valence and conduction bands.

Lead oxides exist with a variety of oxidation states; *α-* and *β-*PbO, *α-* and *β-*PbO2, Pb2O3, and Pb3O4 [29]. Among them, lead monoxide (PbO) is considered a transparent conducting oxide that has a high dielectric constant *ε'* = 25.9 [30] and a direct transition band gap of 1.96 eV for *α*-PbO [29, 31]. It is in use in a variety of applications, such as paints, pastes for a new lead acid battery, pigments, gas sensors, network modifiers in luminescent glassy materials, and nanodevices [31, 32]. Moreover, it can be used to increase the dielectric contestant and ac conductivity of polymeric

**Figure 9.** *Dielectric constant (a, b) and ac conductivity (c, d) of pure PVC and 1.0 wt % PbO-doped PVC.*

materials such as PVC, as shown in **Figure 9**. Incorporation of PbO nanoparticles increases the interfacial polarization due to the heterogeneous structure inside PbO/ PVC nanocomposites. Many conductive three-dimensional networks could be formed throughout the nanocomposite, assisting the charge carriers to hop from conducting clusters to neighbors and therefore increases the conductivity of the material [33].

## **4. Preparation of nanosized hematite with different sizes and morphology**

Controlling the morphology of the material at the nanosize is the key to broadening its industrial applications. Here we will describe tuning the microstructure and morphology of the nanosized hematite (*α*-Fe2O3) by varying the oxalic acid (chelating agent) molar ratio from 0.0 to 2.0 and the annealing temperature in the range 350–750° C. *α*-Fe2O3 is a direct band gap (*Eg* = 2.0–2.2 eV), an *n*-type semiconductor that can absorb about 40*%* of the solar spectrum. It has several advantages, such as its abundance, high thermal stability, nontoxic, high resistance to corrosion, melting point of 1350°C, and a high specific capacitance of 3623 F/g. Therefore, *α*-Fe2O3 is widely used in several technological fields, including rechargeable Li-ion batteries, recording devices, catalysis, biomedical, optical devices, solar cells, and gas sensors [34–37]. The literature survey displays that The *α*-Fe2O3 in the form of 0 D and 1D nanostructures have grown increasing interest. For example, *α*-Fe2O3 *<sup>δ</sup>* nanoparticles showed enhanced electrochemical performance and cycling stability as anode materials for Li-ion batteries [38]. Moreover, hematite of plate-like morphology, displayed a significant hysteretic behavior at ambient temperature with saturation magnetization *MS* = 2.15 emu/g and a coercivity *HC* = 1140 Oe, remanent magnetization *Mr* = 0.125 emu/g [39].

#### **4.1 The preparation and measurements**

A nanopowder of *α*-Fe2O3 was prepared by a template-free sol-gel method as follows; 18.92 g of FeCl3.6H2O (MW = 270.3), supplied by Nova Oleochem Limited, was dissolved in 100 ml double distilled by the magnetic stirring for 1.0 h. Different amounts of oxalic acid were added to the solution to maintain the molar ratios of

*Synthesis of Some Functional Oxides and Their Composites Using Sol-Gel Method DOI: http://dx.doi.org/10.5772/intechopen.111384*

oxalic acid/FeCl3.6H2O at 0.0:1.0, 0.5:1.0, 1.0:1.0, and 2.0:1.0. These samples named M = 0, M = 0.5, M = 1.0, and M = 2.0, respectively. The solution aged for about 15 h at the ambient temperature, then the excess water was thermally evaporated at 95°C for 3 h. The samples were calcined at 550°C in the air for 2.0 h. The sample M = 1.0 was calcined at different temperatures in the range of 350�950°C, for 2 h. The crystal structural analysis and size and morphology of the prepared powders have been carried out by X-ray diffraction (using PANalytical's X'Pert PRO) and the HR-TEM (JEM 2100, Jeol).

#### **4.2 Results and discussion**

**Figure 10** shows XRD patterns of the prepared materials (M = 0–2.0). All the diffraction peaks belong to the hexagonal structure of hematite according to JCPDS card no. 04–015-7029. No peaks related to any other FeO phases not detected. The strong peaks indicate the good crystallization of samples. The lattice parameters *a* and *<sup>c</sup>* and the volume V <sup>¼</sup> ffiffi 3 p <sup>2</sup> *a*<sup>2</sup>*c* of the hexagonal cell are calculated from the following relation: <sup>1</sup> *<sup>d</sup>*<sup>2</sup> <sup>¼</sup> <sup>4</sup> 3 *h*2 þ*k*<sup>2</sup> þ*hk a*2 � � <sup>þ</sup> *<sup>l</sup>* 2 *<sup>c</sup>*2, where (*hkl*) is Miller's indices. Increasing M from 0.0 to 2.0 resulted in shifting the main peaks: (012), (104), and (110) to lower 2θ values, as shown in the inset of **Figure 10**.

**Figure 10.** *XRD patterns of the sol-gel prepared hematite at different oxalic/hematite ratios.*

This indicates increasing the *d*-spacing values of the material, where Bragg's law states that *d* = *nλ/*2 *sinθ*. The V value is increased from 299.62 Å3 to 302.34 Å3 with an increasing M value from 0.0 to 2.0. The average *C*<sup>s</sup> were calculated considering the main reflections: (012), (104), (110), and (116) reflections. *C*<sup>s</sup> was decreased from 95.7 to 61.1 nm with increasing M, which illustrates the role of oxalic acid as a chelating agent to slow down the nucleation rate and encourage the hinder the particles' agglomerations.

**Figure 11** shows the XRD patterns of M = 1.0 sample calcined at 350–750°C. At 350°C, the thermal energy delivered to the material is sufficient to remove all of the organic molecules and the full oxidation of Fe into Fe2O3. Calcination temperatures higher than 350°C lead to an increase in the cell parameters and the diffraction peaks intensity. The *C*<sup>s</sup> were increased from 51 to 73 nm., denoting the enhancement of the crystallinity with annealing. **Figure 12** shows the HR-TEM analysis of the sol-gel prepared hematite nanopowders at chelating agent/hematite molar ratios in the range of 0.0–2.0. As seen, the *α*-Fe2O3 nanocrystals aggregated more compactly at M = 0.0, a decrease of exposed surfaces of *α*-Fe2O3 nanocrystals is expected. Increasing M to 1.0 changes the morphology of the *α*-Fe2O3 structure to take the form of nanorods. However, increasing in M value to reach 2.0, these nanorods converted completely to nanoparticles. Therefore, the chelating agent ratio is a vital parameter in the nano synthesis process by sol-gel technique toward controlling the shape and morphology of *α*-Fe2O3 nanostructures. **Figure 13** displays the morphology of the sample M = 1.0 after calcination at different temperatures (350–750°C). The 350°C gives hematite nanopowder with nanorod/nanoneedle structure. The formation of the nanorods becomes clearer with increasing the calcination temperature to 750°C. The observed nanorods have average diameters of 11.38 nm and different lengths. HR-TEM analysis illustrates that the shape/morphology of the obtained nanopowders is strongly influenced by the experimental conditions. Dissolving the FeCl3. 6H2O salt with oxalic acid at ambient temperature produces an iron oxalate (FeC2O4.2H2O) solution. Increasing M value from 0.0 to 2.0 may change the pH value of the solution. The crystal structure of the intermediate phase (FeC2O4.2H2O) [40], the pH value of the

**Figure 11.** *XRD patterns of the sol-gel prepared hematite at different calcination temperatures.*

*Synthesis of Some Functional Oxides and Their Composites Using Sol-Gel Method DOI: http://dx.doi.org/10.5772/intechopen.111384*

**Figure 12.** *(a-d) HR-TEM images for the sol-gel prepared hematite at different oxalic/hematite ratios.*

**Figure 13.** *(a-c) HR-TEM images for the sol-gel prepared hematite at different calcination temperatures.*

solution, the aging time, and the calcination temperature and annealing rate may affect the shape/morphology of the obtained structure. Therefore, further investigations and studies on these parameters are required to understand the mechanism that leads to the growth of these different morphologies [41]. **Figure 14** depicts E-SEM of some prepared hexaferrites.

## **5. Ferrites and hexaferrites**

The sol-gel method was also used to prepare different ceramics, for instance, as reported earlier [42, 43]. In which, M-type hexagonal ferrites Ba1-*x*Sr*x*Fe12O19, where

**Figure 14.** *(a-c) FE-SEM images of some prepared hexaferrites.*

*x* ranged from 0 to 0.75, were synthesized [42]. The as-prepared ceramics were characterized by different techniques. The samples were single-phase based on the XRD and neutron diffractions. The average particle size of these hexaferrites was in the range of nanometers. Besides, the properties of the Sr0.5Ba0.5RE0.6Fe11.4O19, where RE = La, Yb, Sm, Gd, Er, Eu, and Dy were reported [43]. It was emphasized that the quality of the samples was good based on the optimum use of the sol-gel method. **Figure 14** depicts the field emission-scanning electron microscope (FE-SEM) images of some prepared ceramic samples.

## **6. Rare earth oxides and titanium oxide-based perovskites**

Lanthanum oxide (La2O3) is a rare earth sesquioxide that is optically active with a wide bandgap energy range of 4.3–5.4 eV. The ultrafine La2O3 NP has attractive properties for automobile exhaust-gas convectors, optical filters, and catalysis, as a strengthening agent in structural and ceramic materials, in high κ gate dielectric materials [44]. On the other hand, the Y2O3 is an interesting host material for solidstate lasers, high-temperature refractories (melting point of 2430°C), and infrared ceramics, owing to the distinctive *4f* electronic configuration, corrosion resistivity, and thermal stability [45]. On the other hand, TiO2 is a semiconductor with *E*<sup>g</sup> ≈ 3.21 eV. Owing to its odorless, high melting (1843°C) and boiling (2972°C) points and high transmittance in the visible region, TiO2 is used for various applications such as paint, sunscreen, and antireflective coating, food coloring, photocatalyst, photovoltaic in solar cells, and optical filter [46, 47]. TiO2 has two crystalline forms; anatase and rutile. The first form is more chemically reactive and favorable in the industry [48]. Moreover, titanium oxide-based perovskites, such as NaTiO3 (sodium titanate),

attract increasing interest in PV cells and Li <sup>+</sup> batteries, as well as in biomedical and biochemical applications. This material has a band gap in the range of 2.2–2.5 eV and has a high cation-exchange capacity, thermal stability, and fast adsorption ability [49, 50].

### **6.1 Experimental**

To prepare La2O3 nanoparticles (NP), 27.85 g of LaCl3.7H2O (*M*<sup>W</sup> = 371.37, Schorlau) were dissolved in 100 m*l* of distilled water (DW), and about 9.5 g of oxalic acid was added, and the solution was stirred at 55–60°C for 1.0 h. This sol was held at 90°C for 20 h, then cooled to room temperature (RT) and aged for one day at RT. After that, it was calcined at about 500°C for 2.0 h. To prepare Y2O3, about 9.6 g of Y (NO3)36H2O (*M*<sup>W</sup> = 383.01 g/mol, supplied by PubChem USA) was dissolved in 50 ml distilled water by stirring 20 min. Then 3.15 g of oxalic acid was added to this solution, and the stirring continued for 6 h. The solution was held at 90°C to get rid of the excess water. The sol was turned to gel by aging for 12 h at RT, and then calcined at 400°C to obtain the nanopowders. For TiO2 preparation, 24 mL of Ti [OCH (CH3)2]4 (*M*<sup>W</sup> = 284.22, from Sigma) was added to 60 mL of 2-propanol under stirring for 30 min, and several water drops were added for the hydrolysis process, and the stirring continued for 6 h. The sol was aged for 12 h at RT, and then calcined at 400°C to obtain the nanopowders. The TiO2 was used to prepare NaTiO3 nanofibers, where 2.0 g of TiO2 powder was mixed with 200 ml NaOH solution (10 M) by sonication for 1 h. The white solution was poured into a Teflon-lined autoclave at 45–50°C for 24 h. The NaTiO3 precipitate was then washed several times with double-distilled water and then dried at 100°C for 48 h.

#### **6.2 Results and discussion**

**Figure 15(a)** displays the XRD pattern of the sol-gel-prepared La2O3. The sharp XRD peaks indicate the good crystallinity of the materials. The peaks at 2θ = 13.17, 26.48, 29.57, 40.11, 46.40, 51.55, 54.66, and 61.42° arise from the (001), (100), (011), (012), (110), (103), (112), and (202) crystal planes of La2O3 of hexagonal structure, consistent with the JCPDS no. 04–006-5083. The *C*<sup>s</sup> was calculated considering the most intense peaks; the average value of *C*<sup>s</sup> is ≈ 29.15 nm. The morphology was studied by the HR-TEM, as seen in the inset of the figure.

La2O3 has spherical NP morphology with a particle size of 30 nm, which is consistent with XRD results. Dal et al. [51] fabricated La2O3 NP of size 12.4 nm by sintering the La2O3 microparticles at 1250°C for 48 hrs with grinding for more than 3 hrs. This illustrates that our sol-gel process is low-cost in time and energy. **Figure 15(b)** displays the XRD peaks of the Y2O3, which are indexed as (211), (222), (400), (411), (332), (431), (440), (532), (622), (444), (552), and (800), corresponding to yttria (Y2O3) of body-centered cubic structured, according to JCPDS 043–1036. No other phases are detected in this spectrum, indicating that all the Y(NO3)3 entirely transformed into Y2O3 of single-phase after calcination at 400°C. The crystallite size of Y2O3 is 24.7 nm. The inset of **Figure 15(b)** shows the powder morphology, where the particle sizes of Y2O3 look like nanosheets (Ns), which are allocated with each other to be bigger than the calculated crystallite size from XRD. Similarly, Y2O3 Np of *Cs* = 8.7–27.8 nm) was prepared by the Pechini sol-gel method, and particle size measured by FE-SEM was in the range of 40–50 nm [52].

**Figure 15.** *(a-d) XRD patterns of La2O3, Y2O3,TiO2, and NaTiO3. The insets show the HR-TEM analysis of each material.*

The XRD pattern of TiO2, **Figure 15(c)**, consists of the following peaks; 2θ = 25.32°, 36.96°, 37.78°, 48.08°, 53.9°, 55.05°, 62.68°, 68.77°, 70.26°, and 75.05°. These reflections correspond to Miller's indices of (101), (103), (004), (200), (105), (211), (204), (116), (220), and (215), respectively, as mentioned above their XRD peaks. This result confirms the formation of anatase TiO2 of lattice parameter *a* = 3.21 Å, which is consistent with the JCPDS card no. 21–1272. In addition, the inset of this figure shows an FE-SEM image of the sol-gel prepared TiO2 that shows nanoparticle morphology with an average particle size of 31.52 nm. This result is consistent with the estimated crystallite size by XRD. **Figure 15(d)** shows the XRD spectrum of the sol-gel/hydrothermally prepared NaTiO3. The peaks at 8.71o , 24.25o , 28.47<sup>o</sup> , and 48.37<sup>o</sup> are consistent with the JCPDS card no. 47–0124, confirming the high purity of the material [49]. The inset of this figure shows the HR-TEM image. NaTiO3 displays nanofibers-like morphology, with diameters in the range of 7.7–21.5 nm and lengths up to 79 nm.

*Synthesis of Some Functional Oxides and Their Composites Using Sol-Gel Method DOI: http://dx.doi.org/10.5772/intechopen.111384*

#### **Figure 16.**

*(a-c) SEM images for PVAc/PMMA blended and the blend loaded with Y2O3 nanosheets and TiO2 nanoparticles.*

When Y2O3 or TiO2 are introduced to PVAc/PMMA blend to make polymer nanocomposites, the morphology of this sol-gel prepared nanopowder did not change. **Figure 16(a**–**c)** shows the SEM image of PVAc/PMMA blend loaded with 1.0 wt% nanofillers. The un-doped blend surface exhibits a networked structure or wavy-like like the pool surface, **Figure 16(a)**. The fillers are distributed homogeneously and maintain their morphology, where Y2O3 distribute as nanosheets, **Figure 16(b)**, while TiO2 is like small spheres of agglomerated particles, **Figure 16(c)**.

Moreover, differently prepared nanofillers were prepared to load with polymers and to get nanocomposites for suitable applications [24, 53–55]. The dielectric permittivity of poly(methyl methacrylate, PMMA, significantly increased while the dielectric loss remained almost low due to the doping with CuO/Co3O4 nanoparticles [53]. The semiconducting properties of poly(vinyl acetate)/poly(methyl methacrylate), P(VAc/MMA), were enhanced by adding TiO2 nanoparticles and Y2O3 nanosheets to be used for some device applications [54]. The Zn0.95N0.05O (ZNO) nanoparticles loaded with polyvinyl chloride (PVC) affected the optical as well as the dielectric properties of the pristine sample [55]. **Figure 17** represents the pronounced change in the absorbance of the polymeric materials by adding the ZNO and TiO2 nanoparticles that were prepared using the sol-gel method.

#### **Figure 17.**

*The absorbance of PVAc/PMMA) loaded with TiO2 nanoparticles (a), and PVC loaded with 5 wt.% ZNO nanoparticles (b).*

## **7. Limitations exist for sol-Gel processing**


## **8. Conclusion and outlook**

The simplicity of the synthesizing process makes the sol-gel one of the most popular options in the coating industry and is projected to have a wide range of applications, with continued expansion. Recent progress in several applications was made by using the solgel technology, including the anti-reflection for solar cells, coating protection of aircraft, and cotton fabrics for flame retardant. In addition, the sol-gel process offers the possibility of large-area deposition, compared to vacuum-based-deposition processes, with the possible control of the microstructure and density of the films for improving the coloring efficiency and storage capacity of electrochromic films. Other advantages can be listed as:

	- 1.More research on the effect of several preparative parameters in the sol-gel synthesis on the final structure of products and their physicochemical properties should be carried out. These parameters (solution molarities and concentration, pH, temperature, aging and preannealing/annealing times, and drying conditions).
	- 2.Utilizing the sol-gel technology for CO2 capture within the developed materials (carbon-capture materials) will be a promising research area for a safe environment.

## **Acknowledgements**

This work is funded by the Deputyship of Research & Innovation, Ministry of Education in Saudi Arabia, through project number) 904/1443). In addition, the authors would like to express their appreciation for the support provided by the Islamic University of Madinah.

## **Declaration (conflict of interest)**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Therefore, there are no interests to declare toward any financial interests/ personal relationships that may be considered potential competing interests.

## **Author details**

Arafa Hassen<sup>1</sup> \*, Adel M. El Sayed1 \*, Azza Al-Ghamdi2,3 and Mohamed Shaban4,5

1 Faculty of Science, Physics Department, Fayoum University, El-Fayoum, Egypt

2 Department of Chemistry, College of Science, Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia

3 Basic and Applied Scientific Research Center (BASRC), Renewable and Sustainable Energy Unit, Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia

4 Faculty of Science, Department of Physics, Islamic University of Madinah, Al-Madinah Al-Munawarah, Saudi Arabia

5 Faculty of Science, Physics Department, Nanophotonics and Applications (NPA) Lab, Beni-Suef University, Beni-Suef, Egypt

\*Address all correspondence to: ash02@fayoum.edu.eg and ams06@fayoum.edu.eg

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Klabunde KJ, Stark J, Koper O, Mohs C, Park DG, Decker S, et al. Nanocrystals as stoichiometric reagents with unique surface chemistry. Journal of Physical Chemistry. 1996;**100**(30): 12142-12153. DOI: 10.1021/jp960224x

[2] Nair PAK, Vasconcelos WL, Paine K, Holley JC. A review on applications of sol-gel science in cement. Construction and Building Materials. 2021;**291**:123065. DOI: 10.1016/j.conbuildmat.2021.123065

[3] Zanurin A, Johari NA, Alias J, Ayu HM, Redzuan N, Izman S. Research progress of sol-gel ceramic coating: A review. Materials Today: Proceedings. 2022;**48**:1849-1854. DOI: 10.1016/j. matpr.2021.09.203

[4] Ozer N, Lampert CM. Electrochromic characterization of sol-gel deposited coatings. Solar Energy Materials and Solar Cells. 1998;**54**:147-156. DOI: 10.1016/S0927-0248(98)00065-8

[5] Borlaf M, Moreno R. Colloidal sol-gel: A powerful low-temperature aqueous synthesis route of nanosized powders and suspensions. Open Ceramics. 2021;**8**: 100200. DOI: 10.1016/j.oceram. 2021.100200

[6] Simon SM, George G, Sajna MS, Prakashan VP, Jose TA, Vasudevan P, et al. Recent advancements in multifunctional applications of sol-gel derived polymer incorporated TiO2- ZrO2 composite coatings: A comprehensive review. Applied Surface ScienceAdvances. 2021;**6**:100173. DOI: 10.1016/j.apsadv.2021.100173

[7] Eltoum MSA, Nasr RMO, Omer HMA. Preparation and characterization of CuO Nano particles using sol-gel method and its application as CuO/Al2O3 supported catalyst.

American Journal of Nano Research and Applications. 2020;**8**(2):16-21. DOI: 10.11648/j.nano.20200802.11

[8] Löbmann P. Antireflective coatings by sol-gel processing: Commercial products and future perspectives. Journal of Sol-Gel Science and Technology. 2017;**83**:291-295. DOI: 10.1007/s10971-017-4408-x

[9] Surekha P, Varshney AD, Jerusha E, Pant B, Rajesh AS. Optical applications of sol-gel nano-composites. Materials Today: Proceedings. 2022;**62**:2034-2037. DOI: 10.1016/j.matpr.2022.02.429

[10] Chen C, Yu M, Zhan Z, Ge Y, Sun Z, Liu J. Effect of pH on the structure and corrosion protection properties of sol-gel coatings. Corrosion Science. 2023;**212**: 110955. DOI: 10.1016/j. corsci.2022.110955

[11] Szołdra P, Frąc M, Pichór W. Effect of sol composition on the properties of TiO2 powders obtained by the sol-gel method. Powder Technology. 2021;**387**: 261-269. DOI: 10.1016/j.powtec. 2021.04.037

[12] Luo T, Luo C, Shi Z, Li X, Wu F, Zhang L. Optimization of sol-gel combustion synthesis for calcium looping CO2 sorbents, part I: Effects of sol-gel preparation and combustion conditions. Separation and Purification Technology. 2022;**292**:121081. DOI: 10.1016/j.seppur.2022.121081

[13] Soria AC, Sلnchez JL, Miralles CG, Varela M, Navarro E, Gonalez C, et al. Novel one-pot sol-gel synthesis route of Fe3C/few-layered graphene core/shell nanoparticles embedded in a carbon matrix. Journal of Alloys and Compounds. 2022;**902**:163662. DOI: 10.1016/j.jallcom.2022.163662

*Synthesis of Some Functional Oxides and Their Composites Using Sol-Gel Method DOI: http://dx.doi.org/10.5772/intechopen.111384*

[14] Hashjin RR, Ranjbar Z, Yari H, Momen G. Tuning up sol-gel process to achieve highly durable superhydrophobic coating. Surfaces and Interfaces. 2022;**33**:102282. DOI: 10.1016/ j.surfin.2022.102282

[15] Kesim MT, Durucan C. Indium tin oxide thin films elaborated by sol-gel routes: The effect of oxalic acid addition on optoelectronic properties. Thin Solid Films. 2013;**545**:56-63. DOI: 10.1016/j. tsf.2013.07.031

[16] El-Kemary M, Nagy N, El-Mehasseb I. Nickel oxide nanoparticles: Synthesis and spectral studies of interactions with glucose. Materials Science in Semiconductor Processing. 2013;**16**: 1747-1752. DOI: 10.1016/j.mssp. 2013.05.018

[17] El Sayed AM. Opto-structural and surface properties of silkworm-like nickel oxide thin films. Materials Research Express. 2019;**6**:116423. DOI: 10.1088/2053-1591/ab4663

[18] El Sayed AM. Exploring the morphology, optical and electrical properties of nickel oxide thin films under lead and iridium doping. Physica B. 2021;**600**:412601. DOI: 10.1016/j.physb.2020.412601

[19] El Sayed AM, Mohamad ADM. Synthesis, structural, thermal, optical and dielectric properties of chitosan biopolymer; influence of PVP and *α*-Fe2O3 Nanorods. Journal of Polymer Research. 2018;**25**:175. DOI: 10.1007/ s10965-018-1571-x

[20] El Sayed AM, Saber S. Structural, optical analysis, and Poole–Frenkel emission in NiO/CMC–PVP: Bionanocomposites for optoelectronic applications. Journal of Physics and Chemistry of Solids. 2022;**163**:110590. DOI: 10.1016/j.jpcs.2022.110590

[21] Colak H, Turkoglu O. Structural and electrical studies of Cu-doped CdO prepared by solid state reaction. Materials Science in Semiconductor Processing. 2013;**16**:712-717. DOI: 10.1016/j.mssp.2012.12.013

[22] Bazargan AM, Fateminia SMA, Ganji ME, Bahrevar MA. Electrospinning preparation and characterization of cadmium oxide nanofibers. Chemical Engineering Journal. 2009;**155**:523-527. DOI: 10.1016/j.cej.2009.08.004

[23] Tadjarodi A, Imani M. Synthesis and characterization of CdO nanocrystalline structure by mechanochemical method. Materials Letters. 2011;**65**:1025-1027. DOI: 10.1016/j.matlet.2010.12.054

[24] El Sayed AM, El-Sayed S, Morsi WM, Mahrous S, Hassen A. Synthesis, characterization, optical and dielectric properties of polyvinyl chloride/cadmium oxide nanocomposite films. Polymer Composites. 2014;**35**: 1842-1851. DOI: 10.1002/pc.22839

[25] Okabayashi J, Kono S, Yamada Y, Nomura K. Magnetic and electronic properties of Fe and Ni codoped SnO2. Journal of Applied Physics. 2012;**112**: 073917. DOI: 10.1063/1.4754454

[26] Marikkannan M, Vishnukanthan V, Vijayshankar A, Mayandi J, Pearce JM. A novel synthesis of tin oxide thin films by the sol-gel process for optoelectronic applications. AIP Advances. 2015;**5**: 027122. DOI: 10.1063/1.4909542

[27] El Sayed AM, Morsi WM. α-Fe2O3/ (PVA + PEG) nanocomposite films; synthesis, optical, and dielectric characterizations. Journal of Materials Science. 2014;**49**:5378-5387. DOI: 10.1007/s10853-014-8245-9

[28] El Fewaty NH, El Sayed AM, Hafez RS. Synthesis, structural and optical properties of tin oxide nanoparticles and its CMC/PEG–PVA nanocomposite films. Polymer Science Series A. 2016;**58**:1004-1016. DOI: 10.1134/S0965545X16060055

[29] Li S, Yang W, Chen M, Gao J, Kang J, Qi Y. Preparation of PbO nanoparticles by microwave irradiation and their application to Pb(II)-selective electrode based on cellulose acetate. Materials Chemistry and Physics. 2005;**90**: 262-269. DOI: 10.1016/j.matchemphys. 2004.02.022

[30] Lu BT, Luo JL, Lu YC. Passivity degradation of nuclear steam generator tubing alloy induced by Pb contamination at high temperature. Journal of Nuclear Materials. 2012;**429**: 305-314. DOI: 10.1016/j.jnucmat. 2012.06.021

[31] Kumaravel R, Krishnakumar V, Ramamurthi K, Elangovan E, Thirumavalavan M. Deposition of (CdO)1x(PbO)x thin films by spray pyrolysis technique and their characterization. Thin Solid Films. 2007; **515**:4061-4065. DOI: 10.1016/j. tsf.2006.10.128

[32] Motlagh MMK, Mahmoudabad MK. Synthesis and characterization of lead oxide nano-powders by sol-gel method. Journal of Sol-Gel Science and Technology. 2011;**59**:106-110. DOI: 10.1007/s10971-011-2467-y

[33] El Sayed AM, Morsi WM. Dielectric relaxation and optical properties of polyvinyl chloride/Lead monoxide nanocomposites. Polymer Composites. 2013;**34**:2031-2039. DOI: 10.1002/ pc.22611

[34] Khataee A, Gholami P, Vahid B. Catalytic performance of hematite nanostructures prepared by N2 glow discharge plasma in heterogeneous

Fenton-like process for acid red 17 degradation. Journal of Industrial and Engineering Chemistry. 2017;**50**:86-95. DOI: 10.1016/j.jiec.2017.01.035

[35] Qayyum HA, Al-Kuhaili MF, Durrani SMA. Investigation of fundamental and high order optical transitions in *α*-Fe2O3 thin films using surface barrier electroreflectance. Superlattices and Microstructures. 2017; **110**:98-107. DOI: 10.1016/j.spmi. 2017.08.057

[36] Alqasem B, Yahya N, Qureshi S, Irfan M, Rehman ZU, Soleimani H. The enhancement of the magnetic properties of *α*-Fe2O3 nanocatalyst using an external magnetic field for the production of green ammonia. Materials Science and Engineering B. 2017;**217**: 49-62. DOI: 10.1016/j.mseb.2016.12.002

[37] Ramya SIS, Mahadevan CK. Preparation and structural, optical, magnetic, and electrical characterization of Mn2+/Co2+/Cu2+ doped hematite nanocrystals. Journal of Solid State Chemistry. 2014;**211**:37-50. DOI: 10.1016/j.jssc.2013.11.022

[38] Zeng P, Zhao Y, Lin Y, Wang X, Li J, Wang W, et al. Enhancement of electrochemical performance by the oxygen vacancies in hematite as anode material for lithium-ion batteries. Nanoscale Research Letters. 2017;**12**:13. DOI: 10.1186/s11671-016-1783-0

[39] Tadić M, Čitaković N, Panjanc M, Stojanović Z, Marković D, Spasojević V. Synthesis, morphology, microstructure and magnetic properties of hematite submicron particles. Journal of Alloy Compounds. 2011;**509**:7639-7644. DOI: 10.1016/j.jallcom.2011.04.117

[40] Wang D, Wang Q, Wang T. Controlled synthesis of mesoporous hematite nanostructures and their

*Synthesis of Some Functional Oxides and Their Composites Using Sol-Gel Method DOI: http://dx.doi.org/10.5772/intechopen.111384*

application as electrochemical capacitor electrodes. Nanotechnology. 2011;**22**: 135604. DOI: 10.1088/0957-4484/22/13/ 135604

[41] El Sayed AM. Influence of the preparative parameters on the microstructural, and some physical properties of hematite nanopowder. Material and Research Express. 2018;**5**: 025025. DOI: 10.1088/2053-1591/aaad36

[42] El-Sayed S, Hashhash A, Refai HS, Rutkauskas AV, Baleidy WS, Lis ON, et al. The detailed studies of the structural and magnetic properties of hexaferrites Ba1-xSrxFe12O19 for 0.0 ≤ *x* ≤0.75. Journal of Materials Science: Materials in Electronics. 2021;**32**:10977. DOI: 10.1007/s10854-021-05757-1

[43] Hashhasha A, Hassen A, Baleidy WS, Refaia HS. Impact of rareearth ions on the physical properties of hexaferrites Ba0.5Sr0.5RE0.6Fe11.4O19, (RE = La, Yb, Sm, Gd, Er, Eu, and Dy). Journal of Alloys and Compounds. 2021; **873**:159812. DOI: 10.1016/j. jallcom.2021.159812

[44] Khanjani S, Morsali A. Synthesis and characterization of lanthanum oxide nanoparticles from thermolysis of nanostructured supramolecular compound. Journal of Molecular Liquids. 2010;**153**:129-132. DOI: 10.1016/j. molliq.2010.01.010

[45] Whiffen RMK, Bregiroux D, Viana B. Nanostructured Y2O3 ceramics elaborated by spark plasma sintering of nanopowder synthesized by PEG assisted combustion method: The influence of precursor morphological characteristics. Ceramics International. 2017;**43**:15834-15841. DOI: 10.1016/j. ceramint.2017.08.153

[46] Bardak T, Tankut AN, Tankut N, Sozen E, Aydemir D. The effect of nanoTiO2 and SiO2 on bonding strength and structural properties of poly (vinyl acetate) composites. Measurement. 2016;**93**:80-85. DOI: 10.1016/j. measurement.2016.07.004

[47] Bsiri N, Zrir MA, Bardaoui A, Bouaїcha M. Morphological, structural and ellipsometric investigations of Cr doped TiO2 thin films prepared by solgel and spin coating. Ceramics International. 2016;**42**:10599-10607. DOI: 10.1016/j.ceramint.2016.03.145

[48] Shi H, Magaye R, Castranova V, Zhao J. Titanium dioxide nanoparticles: A review of current toxicological data. Particle and Fibre Toxicology. 2013;**10**: 15. DOI: 10.1186/1743-8977-10-15

[49] El Sayed AM, El-Gamal S. Synthesis, optical, and electrical properties of starch/chitosan/NaTiO3 bionanocomposites modified with ErCl3. Physica Scripta. 2022;**97**:015805. DOI: 10.1088/1402-4896/ac40da

[50] El Sayed AM. Aspects of structural, optical properties, and relaxation in (BiFeO3 or NaTiO3)–PMMA: Hybrid films for dielectric applications. Journal of Physics and Chemistry of Solids. 2021; **148**:109767. DOI: 10.1016/j.jpcs. 2020.109767

[51] Dal ND, Chavda NN, Madhad PH, Kumar R, Bhammar NA, Udeshi B, et al. Structural and electrical properties of pure and doped lanthanum oxide. International Journal of Modern Physics B. 2021;**35**(20):2150210. DOI: 10.1142/S0217979221502106

[52] Hajizadeh-Oghaz M, Razavi RS, Barekat M, Naderi M, Malekzadeh S, Rezazadeh M. Synthesis and characterization of Y2O3 nanoparticles by sol-gel process for transparent ceramics applications. Journal of Sol-Gel Science and Technology. 2016;**78**:

682-691. DOI: 10.1007/s10971-016- 3986-3

[53] El-Sayed S, El Sayed AM. Preparation and characterization of CuO/Co3O4/poly(methyl methacrylate) nanocomposites for optical and dielectric applications. Journal of Materials Science: Materials in Electronics. 2021; **32**:13719. DOI: 10.1007/s10854-021- 05949-9

[54] El-Sayed S, El Sayed AM. Influence of the sol-gel-derived Nano-sized TiO2 and Y2O3 in improving the optical and electric properties of P(VAc/MMA). Brazilian Journal of Physics. 2021;**51**: 1584. DOI: 10.1007/s13538-021-00979-4

[55] Abdel-Baset T, El-Sayed S. The effect of Zn0.95Ni0.05O nanoparticles on the physical properties of polyvinyl chloride. Polymer Bulletin. 2022;**79**:2915. DOI: 10.1007/s00289-021-03614-z

Section 3
