TiO2 for Photocatalysis and Energy Conversion: A Detailed Overview of the Synthesis, Applications, Challenges, Advances and Prospects for Sustainable Development

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TiO₂ for Photocatalysis and Energy Conversion: A Detailed Overview of the Synthesis, Applications, Challenges, Advances and Prospects for Sustainable Development

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Dilshod Nematov^*

,Anushervon Ashurov,Mufazzala Umarzoda

Dilshod Nematov^*

,Anushervon Ashurov,Mufazzala Umarzoda

This version is not peer-reviewed

This preprints belongs to the Topic

Advanced Energy Harvesting Technology

Submitted:

22 August 2024

Posted:

26 August 2024

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Abstract

The term ‘photocatalysis’ has recently gained high popularity, and various products using photocatalytic functions have been commercialized. Of all the materials that may be used as photocatalysts, TiO₂ is virtually the only one that is now and most likely will remain appropriate for industrial application. Water and air purification systems, sterilization, hydrogen evolution, self-cleaning surfaces, and photoelectrochemical conversion are just a few of the products and applications in the environmental and energy domains that make extensive use of TiO₂ photocatalysis. This is due to the fact that TiO₂ has the lowest cost, most stability, and most effective photoactivity. Furthermore, history attests to its safety for both people and the environment because it has been used as a white pigment since antiquity. This review discusses some important aspects and issues concerning different synthesis methods and their influence on the structure and properties of TiO₂, as well as the concept of photocatalysis based on it as a promising biocompatible functional material that has been widely used in recent years. The advantages of TiO₂ applications in various fields of science and technology are discussed, including environmental protection, photocatalysis including self-cleaning surfaces, water and air purification systems, hydrogen liberation, photovoltaic energy, cancer diagnosis and therapy, coatings and dental products, etc. Information on the structure and properties of TiO₂ phases, as well as modern methods of synthesis of functional materials based on them are presented. This is followed by a detailed review of the basic principles of TiO₂ photocatalysis with a brief introduction to the modern concept of TiO₂ photocatalysis. Finally, the challenges and prospects of TiO₂ photocatalysis for various applications are briefly discussed. Recent advances in the fundamental understanding of TiO₂ photocatalysis at the atomic-molecular level are highlighted, and important recent advances in TiO₂ photocatalysis are summarized in terms of the design and engineering of new materials.

Keywords:

Subject: Physical Sciences - Condensed Matter Physics

1. Introduction

Fossil fuel usage is rising quickly due to the expansion of human society and unchecked industrial progress. The development of clean, safe, and sustainable energy technologies is therefore one of the most pressing issues facing humanity today, especially for researchers in related physical and chemical sciences. As a result, the world’s population may face environmental pollution (e.g., toxic emissions and industrial wastes) and a lack of resources for renewable energy sources ]. Solar energy is easily turned into chemical and electrical energy, making it one of the most significant clean and renewable energy sources on Earth [5]. One way to make pure hydrogen (H₂) is to split solar water (H₂O) [6,7]. To address the global energy and environmental issues, solar-powered clean energy solutions have to be extensively researched and implemented. Of all the energy technologies, one of the most significant developments in this regard is photocatalysis, which uses solar energy to regulate energy and chemical reactions [8,9,10,11].

Back in 1901, the Italian chemist Giacomo Chamizian conducted experiments on the systematic study of the effect of light on chemical reactions [12,13]. However, photocatalyst was not used in these experiments. Only in 1911, the key word “photocatalysis” appeared in scientific literature for the first time [14,15]. Then it was reported that ZnO was a photocatalyst for the bleaching of Prussian blue and other reactions, including the reduction of Ag⁺ to Ag [16]. In 1932, TiO₂ and Nb₂O₅ were found to be active in the photocatalytic reduction of AuCl₃ and AgNO₃ to Au and Ag, respectively [17]. Later, in 1938, titanium dioxide was first used as a photosensitizer for the bleaching of O₂ dyes [18], but at that time, researchers did not pay much attention to photocatalysis due to the lack of practical applications. In the early 1970s, the “oil crisis” and the environmental impact of industrial plants led scientists to search for alternative and renewable energy sources [19,20,21,22]. Just since that time, several pioneering works in this direction have been reported. In 1968, it was discovered that O₂ is formed on TiO₂ in an electrolytic cell under ultraviolet (UV) light [22,23]. In 1972, Fujishima and Honda [22] experimentally showed that photoelectrochemical cleavage of H₂O to produce O₂ on TiO₂ and H₂ on platinum black dye electrode and can be achieved by irradiating TiO₂ electrode with UV light. The photocatalytic splitting of H₂O on TiO₂ powder to form H₂ and O₂ with a molar ratio of 2:1 in an argon atmosphere was also reported in 1977 [24,25]. A little later, it became known that methanol (CH₃OH), as a sacrificial reagent, can significantly enhance the photocatalytic production of H₂ from CH₃OH-H₂O mixture [26]. In addition, photocatalytic reduction of CO₂ using different types of semiconductors as photocatalysts has been reported [27,28,29,30]. These pioneering works have shown that photocatalysis will be used in many fields in the future, and tremendous research attention has been paid to similar reactions using titanium dioxide as a photocatalyst in recent years. Besides TiO₂, various other materials such as Ta₂O₅, Ta₃N₅, SrTiO₃, Ag₃PO₄, BiVO₄, MoS₂, WSe_2, LaTiO₃, SrTaO₂N, CdS, TaON, RuO₂, Nb₂O₅ and their nanoparticles, have been applied directly to enable the utilization of solar energy for various photocatalytic reactions [31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58]. Due to the rapid increase in the use of heterogeneous photocatalysis, the literature describing this field has been summarized in numerous publications that not only review the development of photocatalysts and the characterization of photocatalytic processes, but also point out the challenges and opportunities encountered in heterogeneous photocatalysis [59,60,61,62,63,64,65,66,67,68].

Among various photocatalysts, titanium dioxide, as the most widely used “promising” photocatalyst, has been widely used in heterogeneous photocatalysis because of its chemical stability, non-toxicity and low cost [68,69,70,71,72,73,74,75,76,77,78,79]. In the past two decades, heterogeneous photocatalysis of titanium dioxide has spread very rapidly, undergoing various energy and environmental challenges such as direct solar splitting of H₂O into H₂ and decomposition of pollutants [80]. Although much progress has been made in heterogeneous TiO₂ photocatalysis, much remains unknown, which poses an interesting challenge not only to engineers but also to basic research scientists. Generally, a typical TiO₂ photocatalytic reaction contains many fundamental processes, including charge carrier formation, separation, relaxation, trapping, transport, recombination and transport, and bond breaking/forming, which need to be thoroughly investigated. Only when all these fundamental processes are clearly identified can a better understanding of TiO₂ photocatalysis be achieved [81,82,83], which is vital for the development of new photocatalysts and the characterization of new photocatalytic processes.

Ideally, investigating the fundamental processes of TiO₂ photocatalysis under real environmental conditions (e.g., in solution) is the best way to gain a deep understanding of TiO₂ photocatalysis. At present, the processes of charge carrier formation, separation, relaxation, capture, recombination and transport, as well as the stages of single charge transfer in TiO₂ photocatalysis have been systematically investigated by various methods using appropriate electron acceptors or electron donors under real environmental conditions [84,85,86,87]. However, in TiO₂ photocatalysis, the whole reaction is usually completed by several charge transfer and bond breaking/ bond formation steps. In addition, most of the reactants are neither good electron acceptors nor good electron donors. This makes it very difficult to identify mechanistic studies of TiO₂ photocatalysis under real environmental conditions. To achieve this goal, a number of surface science methods (including desorption-oriented methods, spectroscopy and electron spectroscopy) have been used to investigate the photocatalytic reactions on single crystal TiO₂ surfaces to provide important information for TiO₂ photocatalysis and gain an in-depth understanding of TiO₂ photocatalysis [88,89,90,91]. Therefore, fundamental studies of photocatalytic reactions on various TiO₂ single crystal surfaces (rutile and anatase) have been mainly carried out in recent years. Due to the chemical stability and ease of preparation of rutile TiO₂ (110), this surface has become one of the most widely studied surfaces to gain a fundamental understanding of adsorption, thermal and photochemical reactions of adsorbates on TiO₂ surfaces [92,93,94,95,96,97,98]. Using scanning tunneling microscopy (STM) and other ensemble averaging techniques (electron/photon/desorption spectroscopy), the adsorption, diffusion and reaction of various adsorbates in TiO₂ photocatalysis can be clearly identified at the molecular level on TiO₂ model surfaces [99,100,101,102,103,104,105,106], which can provide important information for understanding the photocatalysis of TiO₂ under real environmental conditions. As a result, a considerable amount of research has focused on the fundamental processes of TiO₂ photocatalysis, and many studies in this area have been summarized in a large number of reviews [32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,107,108,109,110,111,112,113,114,115,116]. It is generally believed that the bond breaking/forming stages in TiO₂ photocatalysis are induced by photogenerated electrons/holes [117,118,119,120,121].

However, a complete understanding of how photogenerated charge carriers contribute to bond formation/breaking in TiO₂ photocatalysis is still lacking, which is crucial to unravel the nature of TiO₂ photocatalysis. Fundamental studies of these processes using surface science techniques are needed to achieve a thorough understanding of these processes. In this review, we aim to provide a comprehensive overview of TiO₂ photocatalysis and its applications.

2. Titanium Dioxide, Synthesis, Characterization and Properties

2.1. Brief Introduction and Applications of TiO2

Titanium dioxide, also known as titanium (IV) oxide, is the inorganic compound derived from titanium with the chemical formula TiO₂. TiO₂ as a white inorganic powder, has been industrially produced and used in a wide variety of industries for more than a century [122,123,124,125,126,127]. Due to its non-toxic properties, light scattering ability, UV resistance and ultra-white color, titanium dioxide is used in a thousand different products and products, because it enhances the whiteness, brightness and attractiveness of products. In nature, titanium dioxide is found in the form of the minerals rutile, anatase and brukite. TiO₂ production is steadily increasing worldwide, as the demand for it is constantly growing. Titanium dioxide is extracted in several countries from titanium-containing raw materials, mainly for this purpose ilmenite ore is used, TiO₂ content in which reaches 60% [128]. It should be noted that only 5% of raw materials are used to produce pure titanium, and the rest is used to produce its oxides [129].

In terms of properties, TiO₂ is a versatile semiconductor material that has attracted considerable attention in recent years due to its potential in photocatalytic applications and environmental sensing. Due to its large surface area, chemical stability, unique electronic and optical properties, TiO₂ has been investigated for the determination of various volatile organic compounds (VOCs) in air and water [131,132]. TiO₂-based sensors have been developed for the detection of various VOCs, including benzene, toluene, xylene, and formaldehyde, with high sensitivity and selectivity. TiO₂-based sensors are also used to detect gases such as nitrogen dioxide and ozone, which are major air pollutants. In addition, the application of TiO₂ photocatalysis to decompose organic pollutants in water and air makes it a promising material for environmental remediation [133,134,135].

Having a specific gravity of 3.9-4.2, a melting point of 1854 °C, and a hardness of 5.5-6.5, titanium oxide (TiO₂) is soluble in hot sulfuric and hydrofluoric acids, but insoluble in water, organic acids, and dilute alkaline solutions [136,137]. Large band gap in TiO₂ semiconductor. Its anatas structure, located in the ultraviolet region of the spectrum (100-400 nm), has an optical gap of 3.2 eV and an absorption edge of 388 nm [138]. In the visible region of the spectrum (400-700 nm), rutile has a lower optical slit of 3.02 eV with an absorption edge of 410 nm, while the go-kita phase absorbs light in the near visible region with an energy slit of 2.96 eV [139]. Titanium dioxide thin films absorb ultraviolet light well but show good transparency in the visible wavelength range [139]. At λ = 550 nm, the rutile phase of titanium oxide thin films has an index of refraction of 2.8 and the anatase phase has an index of refraction of 2.49. TiO₂ used in industry is shown in Figure 1 [140,141,142]. With a high dielectric coefficient of 80-110 for the rutile phase and 50-60 for the anatase phase, titanium oxide (TiO₂) is electrically insulating. Its electrical resistance to breakdown voltage is 4 kVmm-1. Compared to semiconductors such as SnO₂, which has a resistivity of 30 Ohm-cm, the surface electrical resistivity of thin films for anatase and rutile structures is 1012 Ohm-cm, which is a high resistivity [143].

Due to its high dielectric coefficient, this material has several applications including anti-reflection surfaces and metal-oxide-semiconductor field-effect transistors as a dielectric gate material [145]. An electron is excited from the valence band to the conduction band of a semiconductor when a photon with energy equal to or greater than the gate energy (greater than 3 eV for titanium oxide (TiO2)) hits the semiconductor [146]. As a result, an electron-hole pair is formed. These charge carriers can react with an electron donor or acceptor, becoming trapped, or recombine with radiation and dissipate energy as heat when indirect gap semiconductors (or direct gap semiconductors) are used [147]. Due to its excellent advantages, titanium dioxide-based materials are attracting more and more attention every day, and hundreds of studies are devoted to their properties and applications every year. However, despite numerous works in this field, many aspects and problems still remain insufficiently studied and relevant. This statement highlights the need for an in-depth study of TiO₂ and emphasizes the significance of our review article.

2.2. Crystal Structures of TiO₂

TiO₂ belongs to the class of transition metal oxides and has several modifications: anatase, rutile, brookite, TiO₂ (B), TiO₂ (II), and TiO₂ (H) [148,149,150,151,152]. It is worth noting that the first three are widely distributed in nature. TiO₂ (B), with monoclinic structure is also found in nature, but rarely. TiO₂ (II) with the structure of PbO₂ and TiO₂ (H) with the structure of hollandite were obtained artificially from rutile under high-pressure conditions. In this review, only the main three crystal structures, anatase, rutile, and brukite, are discussed and their characteristics are summarized in Table 1 [150,151,152,153].

Each of these crystal structures has a unique arrangement of atoms and lattice parameters, but the basis of the crystal structure of these polymorphic modifications are TiO₆ octahedrons (see Figure 2). The octahedrons are arranged in such a way that they may share common vertices or edges. In anatase there are 4 common edges per octahedron, in rutile there are 2 [148]. Anatase has a tetragonal lattice with space group L4/amd and rutile P4/mnm, while the space group number of anatase and rutile is 141 and 136, respectively. This is the reason for the difference of their characteristics. Titanium dioxide with the structure of brukite belongs to the rhombic crystal system, with space group Pbca (space group number 29). In brukite, each octahedron shares common edges with two neighboring octahedrons, and they have a shorter length than the others. The unit cell consists of 8 TiO₂ units and is formed from TiO₆ octahedra (see Figure 2 (b)). Brukite has a more complex unit cell structure, larger volume, and is also the least dense of the 3 considered forms and is not often used for experimental studies [148].

2.3. Electronic Properties and Band Structure

The electronic structure of titanium dioxide has been well studied using various approaches [155,156,157]. The TiO₂ phases described above often have O 2p and Ti 3d states near their band edges, respectively, in their valence bands (VB) and conduction bands (CB). A large bandgap makes stoichiometric TiO₂ an excellent electrical insulator [158]. However, all of the TiO₂ crystalline materials contain point defects, such as Ovs, interstitial titanium ions (Ti³⁺), and substituted ions. Within crystalline materials, interstitial ions and vacancies are intrinsic imperfections that can have a substantial impact on the materials’ electrical conduction, mass transport, and catalytic properties. New electronic states, referred to as defect states, are introduced by point defects into the TiO₂ bandgap (Figure 3). The phases and surface features of TiO₂ influence the locations of defect states in the bandgap. R-TiO₂ (110) defect states, for instance, are situated roughly 0.8–1.0 eV below the CB edge [159]. On the other hand, A-TiO₂ (101) defect states are situated approximately 0.4-1.1 eV below the CB edge [160].

Photoemission experiments [162] report no band bending on the stoichiometric defect-free TiO₂ surfaces (e.g., R-TiO₂(110) and R-TiO₂(100)). TiO₂ is actually an n-type semiconductor due to point defects that can occur in the bulk or on the surface. When surface lattice oxygen atoms are removed during the preparation process, unpaired electrons (in the Ti 3d orbitals) remain on the surfaces, which is how defects on TiO₂ surfaces often appear as Ovs [163]. This is one of the bandgap’s defect states’ sources. As seen in Figure 4, the extra electrons given up by Ovs (which function as donor states) will collect in the near-surface region and cause a downward bending of the band [164]. In practical scenarios, charge transfer between the surface and adsorbates will occur when electron-rich TiO₂ surfaces adsorb different adsorbates. This could potentially change the direction of the band bend and further influence the chemistry of the TiO₂ surface. The valence band of TiO₂ is formed by external p - electrons of oxygen, and the bottom of the conduction band is predominantly formed by excited titanium ions [156]. Of particular importance for the electronic properties of titanium dioxide is the presence of partially reduced titanium (Ti³⁺), the level of which is located ~ 0.2 - 0.8 eV below the conduction band [157] and acts as donors. The presence of Ti³⁺ determines in many cases the conductivity of TiO₂. The resistivity of unalloyed anatase and rutile are in the range of 10⁴ - 10⁷ Ω-cm, but when Ti³⁺ is formed, it decreases to 10^-1 Ω∙cm for anatase and 10² Ω∙cm for rutile [165]. With the nanotube structure of TiO₂, its conductivity is important because it determines the efficiency with which electrons can be transferred along the length of the nanotubes. Thus, the electronic properties of TiO₂ are mainly determined by the crystal structure and the presence of Ti³⁺. Titanium dioxide has an inherent bend in the space charge region at the semiconductor/electrolyte interface, which is characteristic of a group of semiconductors. This bend is formed spontaneously on the surface and has a steeper bend in anatase than in rutile [166].

In TiO₂ with anatase structure, the process of hole capture by the surface dominates, since the spatial charge separation is achieved due to the transfer of photogenerated holes to the particle surface through a steep upward bend of the zones. In this case, in the rutile phase there is a bulk recombination of electrons and holes, and only holes generated very close to the surface are transferred to the surface. It is known that the concentration of charge carriers determines the depth of the bulk charge region [167]. The presence of impurities in the TiO₂ structure can contribute to an increase or decrease in the concentration of electrons and holes. Therefore, the presence of impurity compounds has a significant effect on the depth of the volume charge region bending and the photocatalytic activity of TiO₂.

2.4. Optical Properties of TiO₂

TiO₂ belongs to semiconductors with a wide bandgap width. According to literature data, the forbidden band width for anatase structure is 3.2 eV, brukite is 3.3 eV, and rutile is 3.0 eV [156]. Figure 5 shows the absorption spectrum of TiO₂ with anatase structure [168].

As can be seen from Figure 5, the absorption spectrum of titanium dioxide is limited to the ultraviolet region of solar radiation. Consequently, pure TiO₂ exhibits photocatalytic activity only when irradiated with ultraviolet light whose wavelength is less than 400nm. In the solar spectrum, the share of ultraviolet light does not exceed 7% [169]. To utilize the energy of visible radiation, it is necessary to expand the absorption spectrum of TiO₂. This would allow the utilization of solar radiation for photocatalytic processes. In the last 5 years, there have been many studies based on the idea of extending the wavelength range of photoactivation of TiO₂ in the visible light region and improving the efficiency of solar energy utilization. Activation and optimization of TiO₂ by visible light can be done by implanting metal ions, doping non-metallic atoms or sensitizing TiO₂ with dyes. Therefore, many efforts have been directed to overcome the drawbacks and optimize the absorptivity and photocatalytic properties of TiO₂, especially in the visible light range. Among them, ionic and cationic doping with the formation of structure defects is an effective approach [170,171]. The possibility of using these approaches is precisely due to the fact that the energy levels of such defects can be located in the forbidden zone, so that the absorption of light by defects (impurity absorption) is also possible in the longer wavelength region [171]. In many works, it has been shown that modified TiO₂ has a higher efficiency compared to pure TiO₂, but the conversion efficiency depends on many other parameters, such as the concentration of the doping additive, the energy levels of the additive in the titanium dioxide lattice, the distribution of doping atoms in the unit cell, etc. [169,170,171,172,173,174]. According to the literature [173], the presence of metal ion in the titanium oxide matrix significantly affects the absorption spectrum, the recombination rate of charge carriers and their dynamics, on the surface. At doping with metal ions often observed bathochromic effect, due to the emergence of additional levels in the forbidden zone. Doping of ions with stable configuration, such as Fe³⁺, Gd³⁺, Ru³⁺ and Os³⁺ leads to the narrowing of the forbidden zone or the appearance of additional energy levels [174], which improve charge transfer to the semiconductor surface and contribute to the absorption of visible light, improving their optoelectronic properties.

In most cases, titanium dioxide doped with nonmetallic elements in the anionic site shows an increase in absorption in the ultraviolet-visible part of the spectrum. For example, N-doped TiO₂ has recently been shown to be more active than pure TiO₂ (Figure 6) [175].

The study shows that doping nitrogen at the titanium position shifts the absorption edge to lower energies and increases the absorption capacity of the material in the visible region by reducing the forbidden band width [176,177]. Moreover, it is shown that doping nitrogen in the oxygen position is highly optimal due to comparable atom size, small ionization energy, and stability as well as shallow donor and acceptor levels in the conduction band and valence band due to oxygen vacancy [177]. It has also been reported that TiO₂ (anatase and rutile) containing nitrogen in the interstitials of the crystal lattice has isolated impurity levels in the forbidden zone [178].

3. Efficient Methods of TiO₂ Synthesis

3.1. Hydrothermal Method of TiO₂ Preparation

TiO₂ can be obtained by high-temperature hydrolysis of various precursors directly in the autoclave or by hydrothermal treatment [179]. For example, nanosized TiO₂ powders are obtained by adding a 0.5M solution of titanium butylate in isopropanol to deionized water ([H₂O]/[Ti]=50). Then peptization is carried out at 70°C for 1 hour in the presence of hydroxycitetraalkylammonium [180]. This method is also widely used for the synthesis of monodisperse TiO₂ nanoparticles. In [181], TiO₂ nanorods were obtained by hydrothermal treatment of a dilute TiCl₄ solution in the temperature range of 333–423 °K with a synthesis duration of 12 h. In [181], TiO₂ nanotubes were synthesized by hydrothermal treatment of commercial photocatalyst P25 in a 10 M aqueous NaOH solution at 130°C and 24 h of synthesis.

The glycolate-oxo-peroxo-titanium (IV) complex was subjected to hydrothermal treatment in [182] to obtain titanium dioxide. In an ice bath at room conditions, 20 mmol of titanium metal powder was dissolved in a combination of 40 ml of hydrogen peroxide solution and 10 ml of ammonia solution. After two hours, all of the titanium powder dissolved and a yellow solution containing the peroxo-titanium complex was formed. Then 30 mmol of glycolic acid was immediately added and the mixture was heated to 353 K to promote complexation and remove excess hydrogen peroxide and ammonia until it formed an orange gel. To obtain an aqueous solution of the ammonium salt of the glycolate-oxo-peroxo-titanium (IV) complex, it was dissolved in distilled water. Using 2, 4, 8, 12 and 16 cm³ of stock solution and the required amount of distilled water, 40 cm3 of titanium solutions with Ti = 12.5, 25.0, 50.0, 50.0, 75.0 and 100 mM concentration were created in the next step. After the solution was placed in a 50 cm3 jar, it was sealed with a stainless steel shroud and heated in an oven for 1 to 168 hours at 473 K. The autoclaves were then allowed to cool to ambient temperature Centrifugation was used to separate the precipitate formed, which was then washed thoroughly three times with deionised water. After drying overnight at 353 K in an oven, the sample was extracted. The particle sizes of TiO₂ polymorphs in the samples that were extracted from the Ti complex solution with a concentration of 50.0 mM as a function of the procedure duration are shown in Table 2.

The authors of the study [183] used the hydrothermal method to efficiently grow TiO₂ nanorods. To reduce hydrolysis and condensation, an equal volume of acetylacetone was combined with tetrabutyl orthotitanate. The mixture was then gradually stirred for five minutes at room temperature by adding 40 milliliters of water. Thirty milliliters of 28-30% aqueous ammonia solution was gradually added to the mixture dropwise with continuous stirring. The solution was then transferred to a 250 mL stainless steel autoclave and immersed in a silicone oil bath. The precursor solution was then heated to 170 °C and stirred continuously at this temperature for twenty-four hours. The autoclave was then spontaneously cooled down to ambient temperature. The final product was repeatedly thoroughly purified with aqueous HCl, 2-propanol and water. It was then dried for 12 h at 120 °C. Finally, the collected samples were incinerated for one hour in a high temperature furnace at 450 °C. X-ray examination of the samples showed that they contained a large number of anatase nanorods with an average pore width of 3.1 nm and a specific surface area of about 34.82 m2/g. To enhance the charge transfer ability, the authors of another work [184] obtained TiO₂ nanorods/nanoparticles using hydrothermal method. The obtained nanoparticles have a specific surface area of 84.83 m2/g and a pore diameter of 5.7 nm.

Using surfactant-assisted hydrothermal technology, TiO₂ with different morphologies including nanosheets, nanorods, nanotubes and nanoflowers were produced by adjusting the pH during the preparation process. The experimental results showed that the pH value is critical for controlling the shape of the generated TiO₂, as it can change the adsorption potential of surfactant on the surface of TiO₂ and its charge state in solution. The experimental protocols are summarised below: A solution resistant to hydrolysis at room temperature was created by mixing titanium isopropoxide and TEOA in the following ratio: TTIP: TEOA = 1:2. DI water was added to create solution A. Dodecanediamine was mixed with DI water to create solution B, which acts as a shape adjuster. Next, solution A and solution B were mixed. The pH of the mixed solution was adjusted by adding HClO₄ or NaOH. It was then placed in a 100 mL Teflon autoclave and incubated at 100°C for 24 h, after which the temperature was raised to 140°C for 72 h for nucleation and growth of TiO₂ particles. TiO2 nanospheres with particle sizes ranging from 30 to 60 nm were formed under acidic conditions at pH = 5.6. When the pH was increased, most of the prepared TiO₂ particles transformed from ellipsoids to nanorods. When the pH of the solution was changed to a value greater than 11, TiO₂ nanoflowers were formed. In the process of creating these structures, TiO2 nanosheets were initially formed and then folded to form the final structures at different pH values [185].

Hollow TiO₂ nanospheres can be obtained by hydrothermal method with the addition of aggressive chemicals, and they usually have large surface area and low density [185,186,187]. For example, the hydrothermal method was used to obtain porous hollow TiO₂ aggregates with a BET surface area of 168 m2/g and an average pore size of 12 nm. Ti(SO₄)₂ and NH₄F were dissolved in DI water for Liu preparation, then the resulting mixture was stirred and placed in a Teflon-lined autoclave. The hydrothermal synthesis was carried out in an electric furnace for six hours at 160°C. The appearance of the products is shown in Figure 7 (a-d). Despite having a greater estimated bandgap of 3.36 eV than P25 (~ 3.18 eV), the porous TiO₂ products exhibited double the activity of P25 when it came to the photodegradation of Rhodamine B [187]. The HF produced by NH₄F during the hydrothermal process is responsible for the production of porous, hollow TiO₂ aggregates. HF, a chemical etchant that is corrosive, will erode the inside of TiO₂ to create TiF4, which will ultimately result in hollow nanospheres. Using a similar hydrothermal technique, their subsequent investigation employed the metallic Ti powder as reactants together with a specific quantity of NH₄F and H₂O₂ (30 wt.%). With a diameter of about 1 μm, a shell thickness of 150 nm and a cavity size of about 600 nm, the obtained anatase TiO₂ resembled hollow spheres. The critical factors for the creation of TiO₂ hollow spheres from metallic Ti powders were reaction time, NH₄F concentration and H₂O₂ concentration. The H₂O₂ served as both an oxidant and a bubble generator, creating O2 bubbles that served as the aggregation centre when combined with Ti particles. After that, as shown in Figure 7(e, f) [186], TiO₂ nanoparticles gradually aggregated at the gas-liquid interface to form hollow TiO₂ spheres.

The hydrothermal method can be used to create rutile-brucite TiO₂ nanocomposite by replacing NaOH with HCl. The obtained TiO₂ was used for photooxidation of methyl blue as well as for photocatalytic conversion of CO₂. In [186], titanium chloride was combined with 2 mol/L HCl to obtain a clear solution. The solution was then combined with 1.0 mL of 5 V/V% Triton X-100 in ethanol. After transfer to a flask, the prepared solution was refluxed for 22 h at 100°C. The final rutile-brucite nanocomposite structures were obtained by centrifuging the obtained products, washing them in water, drying them at room temperature and then calcination at 500°C [187].

3.2. Solvothermal Synthesis of TiO₂

The solvothermal and hydrothermal methods are practically identical except that the solvothermal method uses a non-aqueous solvent. The temperature of the solvothermal process can be much higher because some organic solvents have higher boiling points [188]. Usually, the size, shape and crystal structure of the obtained TiO₂ nanoparticles can be well controlled using the solvothermal method. The method is a versatile way to synthesize various nanoparticles with narrow size and dispersity distributions. Using the solvothermal method, TiO₂ nanoparticles with a characteristic size of less than 5 nm can be obtained [189].

The alcohol solvothermal method was first applied to obtain TiO₂ by Kang et al. [190]. In Kang’s study, 1,4-butanediol was used as a solvent and titanium isopropoxide was used as a source of TiO₂. The 1,4-butanediol was mixed with titanium isopropoxide at 300°C for 50 min to facilitate the synthesis. The obtained TiO₂ powder was cleaned repeatedly with acetone and then left to dry for five hours at 100°C without calcination. The obtained anatase TiO₂ nanoparticles of size 20-50 nm were highly hydrophilic and were much more efficient in photocatalytic decomposition of chloroform than TiO₂ nanoparticles obtained by sol-gel method [190]. Subsequently, Nam and Han used the obtained TiO₂ for photodegradation of methyl orange and investigated the effect of several alcoholic solvents under the same conditions [191]. They got ready in the following ways: Titanium isopropoxide (0.1 mol) was added to glycerol, 1-butanol, and 1,4-butanediol, in that order. The combinations were heated to 300°C and kept there for an hour under auto-generated pressure. The findings demonstrated that the kinds of solvents employed during the reaction had a significant impact on the physical characteristics of the produced TiO₂, including crystal size, shape, and structure. Figure 8 (a-c) displays the SEM pictures of the TiO₂ that they produced [191].

The authors of [192] synthesized anatase TiO₂ nanosheets with pronounced {001} facets using an alcohol solvothermal technique. This procedure is a common method for fabricating nanosheets and can provide insight into subsequent methods. In a typical experiment, the pH was set to 1.8 and sufficient TiF₄ was added to a combination of hydrochloric acid and DI water to create a TiF₄ solution. Then 14.5 mL of the above aqueous solution of TiF₄, 13.38 mL of 2-propanol, and 0.5 mL of HF were placed in a Teflon-coated stainless steel autoclave. The autoclave was kept at 180°C for 5.5-44 h in an electric oven. Monocrystalline anatase TiO₂ nanosheets were obtained by centrifugation after the reaction, then washed three times with DI water and dried under vacuum overnight. Heat treatment at 600°C for 90 min was used to remove fluorine from the surface of anatase TiO₂. They showed that 2-propanol and HF promoted the isotropic development of single-crystalline TiO₂ nanosheets and that 2-propanol could enhance the stabilizing effect associated with fluorine adsorption on the (001) surface using first-order theoretical calculations. Figure 8 (d, e) shows the SEM images of the anatase TiO₂ nanosheets. TiO₂ nanosheets have the potential to remove organic pollutants by photocatalysis because they can produce five times more oxidative hydroxyl radicals (·OH) than P25 upon irradiation [193].

Huang et al. [193] prepared highly crystalline TiO₂ hollow spheres by alcohol solvothermal method without corrosion additives and shape controllers at 350°C, as shown in Figure 9. Although the surface area of the TiO₂ hollow spheres was only 28.2 m2/g by BET, they possessed good ability for photocatalytic degradation of methyl orange. Titanium n-butoxide (TNB) and ethanol (EtOH) were used for their synthesis. A mixed solution of TNB and EtOH was prepared by slowly adding TNB to EtOH with a certain volume ratio.

After stirring for 30 min, the mixture was transferred to a stainless steel autoclave for the solvothermal reaction at 350 °C for 4 h. The precipitate was then washed three times with anhydrous ethanol and dried overnight. The precipitate was then washed three times with anhydrous ethanol and dried overnight. The prepared anatase TiO₂ spheres consisted of nanoparticles with an average diameter of 30.2 nm. According to their observations, pure TNB can decompose into TiO₂ nanoparticles at temperatures above 350°C to form 1-butene as in the reaction Ti(OBu)₄ →TiO₂+CH₃CH₂CHCHCHCH, which can serve as bubble templates for growing hollow TiO₂ microspheres from nanoparticles [195].

With LA serving as a suitable coordination surfactant to promote the anisotropic crystal growth of TiO₂, nanorods would be formed. If controlling NH₄HCO₃ and LA in a desired mole ratio, TiO₂ with different morphologies could be acquired. Figure 10 are the TEM images of the TiO₂ nanoparticles and nanorods prepared. Adding a small amount of metal chloride or nitrate into the mixed solution of NH₄HCO₃, LA, triethylamine, cyclohexane and Ti(OBu)₄, metal-doped (Fe^{3 +}, Co^{2 +}, Sn^{4 +}, Ni^{2 +}) TiO₂ nanocrystallines can be obtained [196].

Atomically thin anatase TiO₂ wires with a diameter of 4.5 Å were prepared by the carboxylic acid solvothermal process. These diameter-tunable, ultrathin TiO₂ nanowires were obtained as a result of optimized reaction temperature and reaction time. In Yang’s synthesis [196], a certain amount of oleic acid and cyclohexane were mixed, and then Ti(OBu)₄ was added dropwise to the mixed solution. The resulting solution was heated to 150°C for 25 hr in a Teflon-lined stainless steel autoclave. The precipitation was extracted with an excess of ethanol. The titanium complex precursor was then redispersed in a mixture of octadecene, oleic acid, and oleylamine. The solution was heated to 180°C under stirring and maintained at that temperature for 1 hr to acquire N-doped TiO₂. Their research results suggested that a uniform mixture of oleic acid and oleylamine solution favors self-assembly of the wires. In addition, an increase in reaction temperature mainly increased the wire diameter, whereas prolongation of the precursor treatment time mainly caused an increase in the wire length. Their characterization data indicated that N-doping originated from the oxidative coupling of oleylamine on the surfaces of the atomically thin wires, forming the N–O–Ti surface structures. UV–vis absorption spectra indicated that the light absorption edge was 257 nm for the atomically thin TiO₂ wires, whereas it shifted to 600 nm with N doping. Figure 10 (e) is a TEM image of the N-TiO₂ obtained [197]. To further investigate the capping roles of oleic acid and oleylamine in the solvothermal system, Dinh found out that oleic acid and oleylamine had different binding strengths in controlling the growth of TiO₂ nanoparticles. By varying the ratio of them, TiO₂ with different shapes such as spherical, dog-bone, truncated and elongated rhombic was prepared. The thus-obtained TiO₂ was ascertained to be an excellent support for the synthesis of metal/TiO₂ photocatalyst in which metal clusters could be uniformly deposited on the surface of TiO₂ [198].

Thermal decomposition of titanium alkoxides by the solvothermal reaction in inert organic solvents, such as toluene and acetone, can produce crystallized TiO₂ nanoparticles [199,200]. In Praserthdam’s research, nanocrystalline TiO₂ was prepared by toluene using the solvothermal method. In their synthesis, titanium n-butoxide was used as the starting material and was suspended in toluene in a test tube, which was then placed in a 300 mL autoclave. The autoclave was purged by nitrogen, after which it was heated up to 300°C and was held at 300°C for 2 hr before cooling down to room temperature. The obtained TiO₂ was washed by methanol several times and quenched in air at 77 K. The as-prepared anatase powders were of spherical shape with a size of 8–15 nm. The quenching in their research contributed to the formation of Ti³⁺ surface defects due to the thermal shock effect and promoted the photocatalytic ethylene decomposition ability of TiO₂. Figure 10 (f) displays a TEM image of the TiO₂ as prepared [199].

The preparation of TiO₂ microspheres is usually promoted by the addition of surfactants, while [201,202,203,204] prepared TiO₂ microspheres (Figure 11) with the solvothermal method in acetone without surfactants. Acetone itself might serve as a shape controller, but it hasn’t been confirmed yet. In their preparation, a certain volume of TiCl₄ was added dropwise to acetone under vigorous stirring at 0°C. The concentration of TiCl₄ was adjusted to 0.3 mol/L. This mixed solution was transferred into an autoclave afterwards and was maintained at 120°C for 12 hr in an oven for solvothermal treatment. After reaction, the mixture was cooled to room temperature naturally. The resulting precipitates were filtered and thoroughly washed with excessive acetone and then dried at 120°C for 12 hr under vacuum. The precipitate was calcined under 500°C for 5 hr in air. The diameter of the layered microspheric anatase TiO₂ was about 3 μm with each layer around 300 [204]. On the contrary, the solvothermal preparation of TiO₂ in Chen’s work also used acetone as a solvent, but TiO₂ nanoparticles were formed instead of microspheres.

3.3. Sol-Gel Method of TiO₂ Production

Nanoscale TiO₂ particles are synthesized by sol-gel method using hydrolysis of titanium precursors [205]. Titanium alkoxide or titanium tetrachloride are used as precursor. At the first stage of sol-gel process hydrolysis of titanium (IV) precursor is carried out with subsequent polycondensation, which leads to the formation of colloidal solution -sol of hydroxide particles, the size of which does not exceed several tens of nanometers. Low water content (low level of hydrolysis) and excess of titanium alkoxide in the reaction mixture contribute to the development of Ti-O-Ti bond chains. Chain formation leads to the formation of a three-dimensional polymer skeleton with a near-ordered degree. The high rate of hydrolysis promotes the formation of Ti(OH)₄, which interrupts the development of the Ti-O-Ti skeleton. The presence of a large number of Ti-OH groups and insufficient development of the three-dimensional polymer skeleton leads to loose packing of particles [205,206,207].

The use of low processing temperatures (<100°C) and molecular level composition uniformity makes the sol-gel technique very promising for the synthesis and manufacture of inorganic and organic-inorganic hybrid nanomaterials, as compared to the previously stated approaches [207]. The sol-gel process makes it simple to adjust the size and form of the particles. The sol-gel technique, which is commonly used to create TiO2 materials, generates fine, spherical powders of uniform size and typically starts with an acid-catalyzed stage involving titanium (IV) alkoxides [208,209]. The ability to mold the resultant material into desired shapes, such as fiber, film, and monodispersed powder, is one of the most appealing aspects of the sol-gel process. Figure 12 illustrates how a sol-gel technique, as proposed by Mehrotra and Singh [208], applies a number of variables and phases to regulate the final morphology.

Metal oxides and metal chlorides are common precursors. The compound M─O─R, where M is metal, O is oxygen, and R is an alkyl group, is a metal alkoxide. The M─O bond is polarised, making it vulnerable to nucleophilic attack. Hydrolysis is a process in which an alkoxide in the presence of water undergoes a nucleophilic substitution reaction in which hydroxyl groups from water replace alkoxy groups (OR). Condensation is a process in which metal hydroxide groups combine with each other to form a hydrated metal-oxide network in which tiny nuclei are eventually formed.

The reactivity of metal alkoxides used in the sol-gel process needs to be controlled to obtain sols and gels with desired properties. This can be done by adding chelating ligands such as β-diketones, carboxylic acids or other complex ligands, or by using modifiers. To improve the control of the hydrolysis-condensation process in sol-gel fabrication, modifiers react with alkoxides to form new molecular precursors. These new precursors reduce functionality and reactivity, inhibit condensation and induce smaller species. The potential of acetylacetone to improve sol-gel processing of metal alkoxides was studied by Livage et al. in 1988 [209]. Susceptibility to hydrolysis is reduced when modifiers change the number of M-OR bonds available for hydrolysis. Since β-diketones are surface capping reagents and polymerisation fixatives, their use reduces nuclearity and results in the formation of fine particles. Acetic acid and other carboxylate ligands mainly act as bridging chelating ligands.

The sol-gel method has several advantages, which include [210]: (I) low-temperature preparation; (II) easy and efficient control of particle size, shape and properties; (III) improve the homogeneity of the raw materials; (IV) increase the purity of the starting material; and (IV) create the structure and properties of the material by appropriate choice of precursor.

3.4. Sonochemical and Microwave-Assisted Methods of TiO₂ Synthesis

The sonochemical strategy has been applied to deliver exceptionally photoactive TiO₂ nanoparticles by the hydrolysis of titanium tetraisopropoxide (TTIP) in unadulterated water or in an ethanol/water blend under ultrasonic radiation [211]. Acoustic cavitation, or the formation, growth, and collapse of bubbles within a liquid medium, is the basis for sonochemistry. Heat (~5000 K) and high tensions (~1000 atm) are delivered by cavitational breakdown [212]. Microwaves, which are electromagnetic waves with wavelengths ranging from 1 mm to 1 m and frequencies ranging from 0.3 to 300 GHz, are utilized in microwave-assisted techniques. As indicated by Zhu and Chen [213], microwave warming includes two principal systems to be specific dipolar polarization and ionic conduction. Any materials that contain versatile electric charges, for example, polar particles or leading particles are by and large intensity by microwaves. When polar molecules attempt to align themselves with the rapidly shifting alternating electric field in the microwave, they generate heat through rotation, friction, and collision. Assuming particles are available in arrangement, they will travel through the arrangement and continually taking an alternate route in view of the direction of the electric field bringing about nearby temperature climb because of erosion and impact [214].

Microwave warming is as an elective intensity hotspot for quick warming with more limited response time and higher response rate, selectivity and yield when contrasted with the regular warming techniques [213]. Pulsed microwave heating and continuous microwave heating are the two types of microwave heating. In 1995, Jacob et al. came up with two models for how microwaves increase reaction rates. The main instrument expects to be that, albeit the response time is vigorously abbreviated for a microwave-instigated response, the energy or component of the compound response isn’t modified suggesting that the upgrade of the response rate is because of the warm warming impact [215]. The second proposed system makes a supposition that there are “nonthermal microwave impacts” notwithstanding the warm impacts thus the impacts of microwave light in substance responses are because of both warm impacts and nonthermal impacts [216]. The nonthermal impacts are because of direct communication of microwaves with specific particles in the response medium.

Microwave radiations can likewise be applied to deliver different TiO₂ nanomaterials [217]. This method has the advantage of rapid heat transfer and selective heating for industrial processing. This procedure gives uniform conveyance of energy inside the example, better reproducibility and amazing control of exploratory boundaries. When compared to the several hours required for the conventional methods of forced hydrolysis at high temperatures (195°C), the colloidal TiO₂ nanoparticles can be prepared in a short amount of time (within 5–60 minutes) [218]. TiO₂ nanotubes which are unassuming and multi-walled with breadths of 8-12 nm and lengths somewhere in the range of 200 and 1000 nm were additionally pre-arranged utilizing this technique [219]. TiO₂ nanoparticles in the anatase stage were ready by Baldassari et al. [220] utilizing microwave-helped hydrolysis of titanium tetrachloride (TiCl₄) in a weaken acidic watery medium. Under microwave-hydrothermal conditions, they discovered that the product nearly crystallized within 30 minutes. Because the sulfate prevented brookite from crystallizing, they used H₂SO₄ as the acid to produce a pure anatase phase. In another review, they likewise pre-arranged TiO₂ nanoparticles in the rutile stage from TiCl₄ by a microwave-aqueous cycle at various temperatures somewhere in the range of 100 and 160°C for 5-120 min [220]. The morphology and size of the subsequent nanoparticles can be fluctuated by changing the hour of response, microwave power and reactant fixation.

In [221], titanium slags were converted into rutile TiO₂ powder by microwave activation (Figure 13). Then, the effects of the Na₂CO₃ additive on the calcined product’s surface functional groups, crystallinity, phase transformation, and surface microstructure were examined. The following is the makeup of titanium slag: 9.72% Fe, 5.87% Al₂O₃, 5.23% SiO₂, 1.23% MgO, 1.81% CaO, 75.34% TiO₂, and other trace elements, such S and P.Using a planetary ball mill (model: QM-3SP4), the material was first processed into a powder for 180 minutes in order to improve the specific surface area of the slag. The obtained titanium slag sample, weighing 100 g, was then equally divided into five pieces, each of which was mixed with Na₂CO₃ in an agate mortar for ten minutes.

For the mixes, the mass ratios of Na₂CO₃ to titanium slag were 0.2, 0.3, 0.4, 0.5, and 0.6, respectively. After that, the mixture was put in a corundum crucible and heated to 850°C for 30 minutes using a 1 kW microwave heating power in a microwave box reactor. With the use of a magnetic stirrer, 10 g of calcined slag was leached for 4 hours at 92–95°C using 20% HCl (mass ratio of liquid/solid: 4:1). Following three rounds of water washing, the residue from the leaching process was collected and put in a corundum crucible for high-temperature annealing in a microwave box reactor set at 900°C for 60 minutes with a 1 kW microwave heating power. The calcined product was then cooled and put to use in an analysis. The findings demonstrated that the ideal mass ratio of Na₂CO₃ was 0.4, at which point the average size of the crystallites was 43.5 nm and the rutile TiO₂ crystallinity attained its maximum value of 99.21 percent.

3.5. Synthesis of TiO₂ by Oxidation Method

These methods involve the oxidation of titanium metal using oxidants or anodization. Anodization of titanium sheet under a voltage between 10 and 20 V in 0.5% hydrogen fluoride leads to the formation of aligned TiO₂ nanotubes whose diameter is controlled by varying the applied voltage [212]. In another study, crystallized TiO₂ nanotubes were obtained when anodized titanium plate was heat treated at 500°C for 6 h in an oxygen environment [211]. Direct oxidation of the titanium metal with hydrogen peroxide has also been found to lead to the formation of TiO₂ nanorods. The TiO₂ can be obtained by placing a cleaned Ti metal plate in a 50 mL solution of 30 wt% H₂O₂ at 353 K for 72 h [222]. Formation of crystalline TiO₂ occurs via mechanism of dissolution precipitation and this phase can be controlled by addition of NaX (X = F⁻, Cl⁻, SO₄²⁻) inorganic salts. Addition of Na₂SO₄ and NaF results in the formation of anatase phase and when rutile phase is needed, NaCl can be added during dissolution precipitation [223].

Acetone, pure oxygen and a mixture of oxygen and argon can be used as sources of oxygen for oxidation of titanium metal. Acetone is a good source of oxygen and when used at high temperatures, it results in nanorods which are well aligned and highly dense. Use of pure oxygen or a mixture of oxygen and argon results in crystal grain films and morphology of the nanoparticles can be controlled by the diffusion competition of oxygen and titanium [223,224].

3.6. Synthesis of TiO₂ by Chemical Vapor Deposition (CVD)

In these methods, materials in the vapor state are condensed to form a solid phase material. The process is normally carried out in a vacuum chamber and if a chemical reaction takes place, it is called chemical vapor deposition (CVD) and physical vapor deposition (PVD) if no reaction occurs. Examples of CVD include electrostatic spray hydrolysis, diffusion flame pyrolysis, thermal plasma pyrolysis, ultrasonic spray pyrolysis, laser-induced pyrolysis and ultrasonic-assisted hydrolysis. TiO₂ films with grain size less than 30 nm and TiO₂ nanoparticles with sizes less than 10 nm were synthesized by pyrolysis of titanium tetraisopropoxide (TTIP) in a helium/oxygen atmosphere [225]. Thermal plasma synthesis [226] and spray pyrolysis [227] have been used in some studies but they are complex, capital and energy-intensive and the properties of the powder are not easy to control.

Chemical vapor deposition (CVD) is a method of producing thin films or powders by means of high-temperature decomposition reactions and/or gas-vapor interactions. is a method of producing thin films or powders by high-temperature decomposition reactions and/or interactions of gaseous precursors on a substrate (producing films) or in a reactor volume (producing powders) [228]. To date, it has been established that the nature of the substrate affects the size and distribution of crystals in TiO₂ films [229]. This method is used to obtain TiO₂ at 300-750°C, using titanium tetraisopropoxide as a precursor [230].

3.7. Green Synthesis of TiO₂

An ecologically benign substitute for the chemical method of creating nanomaterials is the “green” synthesis of TiO₂ functional materials. Using biological agents including bacteria, fungus, actinomycetes, yeast, and plants, the biological technique offers a multitude of resources for the production of nanoparticles [231,232]. The pace at which metal ions are reduced with the aid of biological agents is substantially quicker than it is because of the surrounding pressure and temperature. Significant advancements in “green” synthesis techniques for the creation of many nanoparticles have resulted from the extraction of TiO₂ from plant extracts [233]. Green tea extract was used by the authors in study [234] to create mesoporous TiO₂ nanoparticles using the sol-gel technique. A mixture of 9 ml of titanium isopropoxide and 60 ml of isopropanol was stirred continuously with a magnetic stirrer at room temperature for 1 hour. Then green tea extract was added in various ratios (0.5, 1, and 1.5 g in 30 ml of distilled water) and stirred slowly for 3 hours in order to obtain a colloidal solution. It was found that the pH of the solution was 6.0 during the TiO₂ nanoparticles synthesis. The resulting sol was kept at rest for 10 hours to obtain a gel. Then the gel was filtered, dried at 110°C for 3 hours and calcined at 500°C for 10 hours. The calcined samples were designated as NTG0.5, NTG1, and NTG1.5, which corresponded to mass ratios in samples 1:0.06, 1:0.12, and 1:0.18 TiO₂:GTE (extract), respectively. TiO₂ nanoparticles prepared without extract were monitored and were designated as NT. The mild, non-toxic, and inexpensive green tea extract, which hass active organic components, limited agglomeration, and promoted the growth of TiO₂ nanoparticles. In work [235], the authors have synthesized titanium dioxide nanoparticles by an improved hydrothermal method using Morinda citrifolia leaf extract. 50 ml of M. citrifolia leaf extract was added to 0.1 M TiCl₄ solution. The solution was transferred to a 100 ml stainless steel autoclave at 120 °C for 8 hours and then cooled to room temperature. A white suspension was obtained, which was centrifuged at 5,000 rpm/min for 10 minutes to remove unreacted chemicals. The resulting suspension was filtered and washed several times with deionized water and ethanol. The filtered suspension was dried in an oven at 100°C for 5 hours. Titanium hydroxide was calcined at 400°C for 4 hours in a muffle furnace, resulting in quasi-microspheres of TiO₂ nanoparticles. X-ray diffraction patterns showed the presence of rutile phase TiO₂ and confirmed an average crystallites size of 10 nm. The authors [236] synthesized TiO₂ nanoparticles by the hydrothermal method using Aloe Veragel for use as a photocatalyst in the degradation of picric acid. Aloe Verawas peeled and the gel was washed seven times under running water. 10 ml of the gel was added to 100 ml of deionized water and stirred for 1 hour. To this aqueous solution was added dropwise 0.1M titanium (IV) isopropoxide. The reaction mixture was stirred continuously for one hour at 20°C. The solution was kept in an autoclave at a temperature of 180 °C for 4 hours. Then the solution was heated on a hot plate at a temperature of 80 °C. The resulting product was ground and calcined in a muffle furnace at a temperature of 500°C for 5 hours. The size of the synthesized TiO2 nanoparticles ranged from 6 to 13 nm. In a study [237], titanium dioxide nanoparticles were efficiently synthesized using aqueous extracts of Parthenium hysterophorus leaves by microwave irradiation. The collected leaves were washed with distilled water to remove dust particles and contaminants. About 20 g of leaves were weighed and crushed into small pieces with a mortar and pestle. The samples were added to 100 ml of distilled water and boiled for 10 minutes at 60 °C in a microwave oven. After boiling, the extract left to cool at room temperature.

3.8. Electrodeposition and Ionic Liquid-Assisted Methods

Electrodeposition is a plating process in which ions in a solution migrate under the influence of an electric field (electrophoresis) and are deposited onto an electrode. In a normal process, components containing one or more dissolved metal salts are immersed in electrolytes, and the metallic ions are attracted to the cathode to be deposited. Electrodeposition is easily controlled, can produce tight coating with uniform thickness and is able to coat complex fabricated objects [238,239]. Electrodeposition of TiO₂ nanoparticles onto multiwalled carbon nanotube arrays was conducted in an electrolyte consisting of 3 mol/L KCl solution, 10 mmol/L H₂O₂, and 10 mmol/L Ti(SO₄)₂. Multiwalled carbon nanotube arrays were used as the working electrode, an Ag/AgCl electrode as the reference electrode, and Pt as the counter electrode. The working potential was − 0.10 V and the deposition time was 30 min [240].

Ionic liquids refer to salts in the liquid state. Actually, when heated to a high temperature, almost all salts can become ionic liquids. The ionic liquid referred to here is a kind of salt that is in liquid states at low temperatures (< 100°C) or even room temperature. The ion size of these liquids is usually large and poorly coordinated, resulting in a low bounding force and a loose structure, thus forming a liquid rather than a solid at a relatively low temperature. Ionic liquids have many merits, they exhibit excellent thermal stability, powerful solubility, good electrical conducting ability, low viscosity, and have almost no vapor pressure. Usually, the low temperature ionic liquids have at least an organic cation (such as methylimidazolium and pyridinium ions) and an inorganic canion (such as halide, tetrafluoro-borate, and hexafluoro-phosphoric ions). Despite the fact that the ionic liquid is usually poisonous, it has found its way into the research of pharmaceuticals, gas treatment, cellulose processing, solar thermal energy, etc., and is recently used to modify the preparation process of TiO₂ [241,242,243].

Different binary ionic liquids were applied to synthesize TiO₂ hollow spheres. It was found out that the shape, size and crystallinity were different by varying the binary ionic liquid composition, which is a result of different interface interactions. In a typical process, 3.6 mL of binary ionic liquids were mixed with 0.4 mL of anhydrous toluene solution containing 0.2 mol/L titanium isopropoxide. 6 mL methanol was then added and centrifuged. The final mesoporous TiO₂ was obtained after filtration and calcination at 500°C. They tested all the binary mixtures of six different ionic liquids, which were 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF₆]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([Hmim][PF₆]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF₄]), 1-octyl-3-methylimidazolium hexafluorophosphate ([Omim][PF₆]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim][BF₄]), and 1-octyl-3-methylimidazolium tetrafluoroborate ([Omim][BF₄]) and discovered that [Bmim][BF₄] + [Omim][PF₆] mixtures were the most effective group and would make anatase TiO₂ with a surface area of about 100 m²/g after calcination [244]. Ionic liquid-assisted hydrothermal synthesis was reported by using 3-carboxymethyl-1-methylimidazoliurn bisulfate ([CMIM][HSO₄]), titanium isopropoxide, concentrated HCl and H₂O to fabricate rutile TiO₂ nanorod films. In a typical process, 0.5 mL titanium isopropoxide was added to a mixed solution of DI water, concentrated HCl and 0.5 mL [CMIM][HSO₄]. The resulting transparent mixture was then transferred to a hydrothermal autoclave with a piece of glass immersed in the solution. The autoclave was heated to 180°C for 3 hr. When ionic liquids weren’t used in the process, the diameter of TiO₂ was about 250 nm, but when [CMIM][HSO₄] was used, the diameter of TiO₂ nanorods decreased to 62 nm. This was not only due to the effective prevention of the gathering of the nanoparticles by the surfactant-acted ionic liquids, but also because of the extended hydrogen bonding and ionic strength which favored the formation of small crystals. Figure 14 (a) is an FESEM image of TiO₂ nanorods in Mali’s work [245].

3.9. Synthesis of Nanoscale and Thin Film Structures of TiO₂

It is obvious that the use of nanosized TiO₂ particles leads to a significant increase in its photocatalytic activity. An undoubted advantage of nanoparticles compared to microparticles is the greater probability of charge release on the catalyst surface. Due to the fact that the penetration depth of UV light of TiO₂ particles is limited (~100 nm), only the outer surface is active [246]. Figure 14 (b) shows a diagram of light absorption by TiO₂ nano- and microparticles.

As can be seen from Figure 14 (b), reducing the size of the photocatalyst particles to nanoscale values promotes light absorption by the entire volume of particles. In this regard, the use of TiO₂ in heterogeneous photocatalysis processes is associated with the need to obtain nanosized particles. To date, TiO₂ nanoparticles are obtained with various morphologies, mainly nanotubes, nanowires, nanorods and mesoporous structures [247].

Thus, the properties and applications of titanium oxide (TiO₂) nanostructures largely depend on the particle size, structure, effective surface area, and surface properties. Since these properties are in turn influenced with synthesis methods, in this section we will have an overview of different methods of synthesis of nanoparticles Titanium oxide (TiO₂) thin films [248,249,250]. Chemical reactions for the synthesis of substances can take place in gaseous, liquid or solid forms. The rate of penetration of reactants in the gas or liquid phase is several times faster than in the solid phase. Therefore, the synthesis methods for titanium dioxide-based nanostructures are mainly divided into liquid-phase synthesis and vapour-phase synthesis [251,252,253]. Moreover, nanostructured TiO₂ particles can be obtained by oxidation of metallic titanium using various chemical oxidants [254]. TiO₂ nanorods were obtained by this method (by oxidation of a titanium metal plate with hydrogen peroxide). In [255], the authors showed that anodic oxidation of titanium in a fluoride-containing electrolyte allows obtaining nanostructured coatings consisting of TiO₂ tubes, the properties of which can be controlled by varying the oxidation conditions. However, the authors showed that such coatings with a nanotube length of more than 1 μm have low adhesion, and their application is still very limited. Then the authors in [256] proved that poor adhesion is due to the low packing density of the nanotubes (Figure 14 (c, d)).

According to modern concepts, the growth model of TiO₂ nanotubes during potentiostatic anodizing consists of several stages, and the reactions responsible for the formation of porous aluminum oxide and TiO₂ nanotubes [254,255,256,257] are identical. Despite the similarity of the processes occurring during the anodization of titanium and aluminum, the morphology of the resulting oxides differs greatly. For example, during the anodization of aluminum, a mesoporous structure is formed, whereas during the anodization of titanium, both mesoporous and nanotubular structures can be obtained [257]. At the same time, the use of porous carriers active only under the influence of UV light, inside the pores of which there are particles activated by light with a wavelength of 400–700 nm, seems very promising. Such an approach will make it possible to use both visible and UV radiation in photocatalytic processes.

4. Concept of Photocatalysis Using TiO₂

In 1967, while Professor Fujishima and his colleague Honda [22,237,258] were experimenting, they encountered a strange phenomenon. They observed that when TiO₂ and Pt electrodes were placed in water, the formed circuit is capable of decomposing water into oxygen and hydrogen without an external electricity input, only when exposed to light [259,260]. Following this phenomenon, Honda discovered that Titanium oxide (TiO₂) has strong oxidizing properties, and focused his subsequent studies on the effect of this valuable substance on environmental phenomena such as sterilization, disinfection and pollution removal [261,262,263,264,265,266,267]. This important discovery, known as the photocatalytic or Honda-Fujishima effect, resulted in antibacterial products, being one of the most advanced tools for disinfection of spaces and one of the main branches studied in the materials industry [264,265,266,267].

TiO₂ photocatalysis is a photon-driven reaction process [214], starting with a photoadsorption event on the surface or bulk of TiO₂ (Figure 15). When TiO₂ adsorbs photons with energies greater than or equal to its band gap (Eg), electrons in the filled VB are excited to the vacant CBs, leaving holes in the VB.

The photocatalysts TiO₂ exhibits light absorption property and redox ability based on the position of the VB and CB band edges and the value of Eg. Essentially, the photocatalytic properties of TiO₂ are based on the generation of electron-hole pairs within the semiconductor particle by electromagnetic radiation. These pairs then interact with adsorbed molecules through redox reactions once they reach the surface of the TiO₂ particle. In this case, a part of the electrons and holes can be subjected to recombination in the volume or on the surface of TiO₂ (Figure 16). For effective photocatalytic processes, it is necessary that redox reactions involving electron-hole pairs be more efficient than recombination processes. It is known that for most reactions, titanium dioxide in the anatase phase state exhibits higher activity than other polymorphic modifications [269]. It has been suggested that the high photoreactivity of anatase is due to the higher location of the Fermi level, which reduces the ability to absorb oxygen and increases the degree of hydroxylation (i.e., the number of hydroxyl groups on the surface) [270]. In general, semiconductor photocatalysis is considered, from this point of view, as a multi-step process, which is illustrated in Figure 15. Such a process is initiated by the photoexcitation with electromagnetic radiation equal to or exceeding Eg (1), the separation of the charge carrier pairs (2), the diffusion of e−/h+ species within the material towards the surface, and the surface charge transfer for the reduction of adsorbed electron acceptors (3), and the oxidation of adsorbed electron donors (4), respectively [268]. Accordingly, the photo-induced electrons and holes should migrate to reach the surface of the material and react with adsorbed chemical species via surface charge transfer. Therefore, the Eg of a semiconductor is the minimum thermodynamic requirement for photocatalysis [270].

One of the main limitations of semiconductor photocatalysis is the recombination of the photogenerated charge carriers, dissipating the absorbed energy as heat and affecting negatively the lifetime of the electrons and holes [268]. This undesired recombination occurred either indirectly, i.e., via surface defects (5), or directly, i.e., by band-to-band recombination (6). Such phenomena are highly reliant on the crystal structure of the semiconductor. To enhance effectively the redox reactions while minimizing recombination, the photogenerated charge carriers must migrate to the liquid junction through the solid and should react with adsorbed species directly at the semiconductor surface [270]. In the literature, there are data on reactions in which both crystalline phases show the same activity [271], and there are also data on the higher activity of rutile [272]. In addition, there are works in which the authors claim that a mixture of anatase (70–75%) and rutile (30–25%) is a more active photocatalyst than pure anatase [273]. The discrepancy in results may be due to various factors such as specific surface area, pore size, crystallite size, method of preparation, or the form in which the activity is expressed. A commercial titanium dioxide photocatalyst P25 (Evonik Industrials, Germany), consisting of an amorphous phase and an 80/20 mixture of anatase and rutile, shows higher activity in some reactions than pure crystalline phases [274]. The enhanced activity of the catalyst results from the efficient separation of charge carriers due to the multiphase nature of the particles [275]. Another commercial TiO₂ photocatalyst is Hombikat UV 100 (“Sachtlebem”, Germany), which consists only of anatase and has high activity due to the high rate of interfacial electron transfer. For example, it is known that the deposition of platinum on the surface of TiO₂ can lead to both an increase and a decrease in activity [275]. In most cases, this is explained by different ways of depositing Pt particles.

5. Growing Interest in the Application of TiO₂ Photocatalysis

In recent years, photocatalytic processes have found increasing applications in a variety of fields. Heterogeneous photocatalysis has already established itself as an inexpensive and sustainable technology for purifying water and air from a variety of hazardous pollutants, including organic substances and heavy metals [276]. The most active development of this technology is in Japan, USA, India and China. Figure 17 shows the number of scientific publications devoted to this field from 1967 to 2024, where it can be seen that in the last decade the interest in this field is steadily increasing [277]. According to the statistics on publications in the National Library of Medicine (PubMed) from 1967 to 2000, from 3 to 7 publications were published annually, but from 2001 to 2013 and 2014 to 2024 there is a high growth of interest in research in the field of photocatalysis.

The growing interest in this field is due to the fact that, unlike other processes such as reverse osmosis, nanofiltration, and ultrafiltration, photocatalysis is a cheap and potentially “stand-alone” water purification technology. At the same time, titanium dioxide, as a low-cost photocatalytic material, is prominent among other solid materials. In fact, according to Figure 18, the number of scientific publications dealing with titanium dioxide-based photocatalysis accounts for the bulk of publications in the field of photocatalysis. A total of 127 out of the total number (930) articles on photocatalysis have been published on the topic of titanium dioxide photocatalysis in the first trimester of 2024, of which titanium dioxide can be noted as the most sought-after material in this field of photocatalysis.

The use of sunlight or ultraviolet radiation makes photocatalytic purification technology inexpensive, environmentally friendly, and possible to use worldwide. The use of photocatalysis processes requires minimal equipment and is suitable for developing countries as well as remote sites without access to electricity. TiO₂ photocatalysis has been successfully used in many developing countries for fresh water disinfection and decolorization [278]. Future environmental purification technologies also involve the use of TiO₂ as a photocatalyst capable of utilizing organic pollutants from water, air, and other media due to the formation of free OH radicals during the transformation reaction [278]. It is known that titanium oxide dust particles can act bactericidally [279] and even inhibit cancer cells [280] under the influence of UV light. Those or other properties of titanium oxide depend on its structural and morphological features and chemical varieties. This in turn depends on the technological conditions of synthesis and surface modifications. Specific examples of photocatalysis applications are given below.

5.1. Purification of Water and Air from Organic Pollutants

Recently, considerable attention has been paid to the use of photocatalysis for the oxidation of organic pollutants in wastewater, in particular because of its ability to oxidize almost any toxic organic substance to CO₂ and H₂O [281]. The essence of the photocatalytic process of oxidation of organic compounds is as follows: under the action of light energy, electron-hole pairs are formed in TiO₂ particles. Holes, when coming to the surface of the particle, interact with the electron donor in solution or with hydroxyl ions to form strong oxidizing agents such as hydroxyl or superoxide radicals. In turn, the conduction electrons reaching the surface of TiO₂ interact with oxygen [282], which leads to the formation of superoxide anion radical, electron can interact with organic substances that can act as electron acceptors. The formation of such particles makes the TiO₂ surface a very strong oxidizer, which allows the mineralization of harmful substances by their photocatalytic oxidation to H₂O and CO₂. Figure 19 shows the scheme of formation of such oxidants on the surface of TiO₂ under the action of light energy.

Redox potentials (relative to the standard hydrogen electrode) for a number of the strongest oxidizing agents are presented in Table 3.

As can be seen from Table 3, the HO^•- particle is a very strong oxidizing agent (standard redox potential of 2.8 V), which allows the oxidation of most organic substances to CO₂ and H₂O. The processes occurring on the TiO₂ surface as a result of the oxidation of organic pollutants can be expressed by the following reactions:

\begin{array}{l} {T i O}_{2} + h v \to T i O_{2} (e_{C B}^{-} + h_{V B}^{+}) \\ {T i O}_{2} (h_{V B}^{+}) + H_{2} O \to T i O_{2} + H^{+} + {O H}^{•} \\ {T i O}_{2} (h_{V B}^{+}) + {O H}^{-} \to T i O_{2} + {O H}^{•} \\ {T i O}_{2} (e_{C B}^{-}) + O_{2} \to T i O_{2} + O_{2}^{-} \\ O_{2}^{-} + {O H}^{.} \to H O_{2}^{-} \\ O r g a n i c s + {O H}^{•} \to O x i d a t i o n p r o d u c t s \\ O r g a n i c s + h_{V B}^{+} \to O x i d a t i o n p r o d u c t s \\ O r g a n i c s + e_{C B}^{-} \to O x i d a t i o n p r o d u c t s \end{array}

Photocatalytic water and air purification systems with artificial UV radiation have been on the market for several years, while solar photocatalytic purification plants are at the stage of demonstration and pilot projects. In addition, the combination of photocatalysis and membrane treatment can reduce the fouling of the filter membrane and thus significantly increase the efficiency of water treatment. Nitrogen oxides in tunnels are a serious problem during the summer months, especially in dense and large cities with high traffic levels. Japan and a number of European countries are actively using cement with TiO₂ additives for tunnel construction. The installation of ultraviolet lamps in such tunnels allows controlling exhaust gas emissions, mainly NO and NO₂ [283].

5.2. TiO₂ and Water Photolysis

Hydrogen is an environmentally friendly source of energy. Currently, hydrogen is produced from various primary sources such as natural gas, fuel oil, methanol, biomass, and coal [284,285,286,287,288,289]. Among these sources, photolysis of water, using solar energy, has attracted the most attention because of its potential. Direct photocatalytic using sunlight is the most efficient way to produce hydrogen because it avoids energy losses in the process of electricity transmission. Interest in this process intensified after the work of Japanese scientists Haunda and Fujishima in 1972 [286], who were able to obtain hydrogen by irradiating aqueous suspension of TiO₂ with UV light. From the thermodynamic point of view, the reaction of water photolysis proceeds with a significant positive change in the Gibbs free energy (ΔG^o = 237.2 kJ/mol, 1.23 eV per electron) [287]. For water splitting, the absorbed photon energy must overcome this energy. From the electrochemical point of view, the process of water decomposition into hydrogen and oxygen is stepwise and two electrons take part in it. To overcome the Gibbs free energy, a semiconductor photocatalyst must have a forbidden zone width with an energy greater than 1.23 eV. In addition, the conduction band must be located higher than the water reduction potential, and the valence band ceiling must be located lower than the water oxidation potential.

5.3. Treatment of Water from Inorganic Compounds

In addition to organic compounds, wastewater contains a wide range of inorganic compounds that are sensitive to photochemical transformations on the catalyst surface. Bromate, chlorate, azide, halide ions, and oxidazote can be photodegraded on the TiO₂ surface. Metal salts such as AgNO₃, HgCl, and organometallic compounds (e.g., CH₃HgCl) can be removed from water [289,290,291].The content of metals in water, such as mercury (Hg), chromium (Cr), and lead (Pb), is very hazardous to human health. Removal of these toxic metals is a fundamental task in obtaining quality water. With the help of heterogeneous photocatalysis, it is possible to remove heavy metals from wastewater through their reduction by TiO₂. The authors of [292,293] demonstrated the possibility of photoreduction of metals from industrial wastewater, such as gold (Au), platinum (Pt), and silver (Ag).

5.4. Medical Applications of TiO₂

Hospitals and medical equipment are the only places where climate-controlled treatment and disinfection are required. However, in other environments where plants and animals grow and reproduce, making them susceptible to contamination, microbes and pests must be removed from surfaces, water and air [294]. A basic method of eradicating these microbes must be developed because microbial, viral and fungal structures are becoming increasingly resistant to the use of chemical disinfectants, and these chemicals themselves are dangerous and lethal to human life [295]. DNA damage can affect any biological structure. Therefore, one effective method of getting rid of them is to create proteins that disrupt DNA cell division cycles [296]. On the other hand, the increase in oxygen gas pressure is harmful and deadly for all types of life [297]. Thus, titanium surfaces, along with their photocatalytic properties, produce oxygen free radicals and create an oxygen poisoning for microorganisms [298]. Japanese scientists have found that adding TiO₂ to fabrics helps create an antibactericidal material. Thus, gowns sewn from this fabric can be treated with UV for disinfection after working hours [299]. In their works [299,300], the authors managed to find an application of the photocatalysis effect of TiO₂ nanoparticles in photodynamic therapy to destroy cancer cells. The researchers managed to remove cancerous tumors from the colon in mice using photo-catalytic oxidation [300]. TiO₂ nanoparticles were injected at the site of the cancerous tumor, and illumination was performed using a fiber optic cable. As a result, the light produced reactive oxygen species that oxidized the tumor cells.

Cancer is considered one of the greatest challenges for modern medicine. Despite the continuous development of modern cancer therapies, the first line of therapy is surgical removal of the tumour and/or radiation therapy. Chemotherapy is usually an adjunctive therapy, but its use is limited by many factors [301,302]. First of all, chemotherapeutic drugs are extremely toxic to rapidly proliferating, both cancerous and healthy, tissues of the human body. Therefore, new delivery systems are still being sought to increase the tissue specificity of therapy and reduce systemic effects. Many chemotherapeutic drugs are also ineffective due to the multidrug resistance (MDR) mechanism exhibited by cancer cells, which is related to the overexpression of some members of the ABC efflux transporter superfamily, which perceive the drug as a poison and eliminate it from the matrix. Currently, one of the most frequently studied chemotherapeutic drugs is doxorubicin (DOX). Although it offers many advantages in the therapy of various cancers, the use of doxorubicin is associated with side effects, among which cardiotoxicity is the most severe and dangerous [302,303]. The use of nanoparticles and the combination of chemotherapy with photodynamic or photothermal therapy could be a potential solution to both problems (Figure 20). Titanium nanoparticles have significant advantages in this field, providing efficient delivery of drug molecules, hence better pharmacokinetics, and targeted delivery.

The potential application of one-dimensional TiO₂ whiskers for drug delivery in cancer therapy and their synergistic effect on the internalization and accumulation of daunorubicin in SMMC-7721 cells were first investigated by Li et al. [304]. In addition, conjugation of doxorubicin (DOX) to TiO₂ nanoparticles was reported to induce a synergistic response in breast cancer cell lines [305]. For blood-contacting medical materials, it is important to minimize the tendency of their surface to adsorb blood proteins and enhance blood coagulability to reduce the risk of thrombosis. Titanium oxides are known to be highly compatible with blood, so they are often proposed as coatings for blood-contacting implants. The conducted study of the interaction of blood plasma proteins with titanium oxides will allow a better understanding of the nature of the interaction of foreign surfaces with blood, as well as their integration into tissues, starting from the first meeting with blood during surgery and then in the subsequent process of wound healing. Research in this area is being carried out at an accelerating pace and in recent years, optimization of the properties of titanium oxide nanoparticles for photodynamic therapy with excitation by UV or visible light after doping with organic photosensitizers has been developing.

Applications of titanium dioxide in medicine are going further than the design of drug delivery systems or applications as vehicles for chemotherapeutics. Titanium Oxide Nanoparticles have been applied in pharmacy, especially in pharmaceutical chemistry and technology, as well as medicine, including growing areas related to dentistry and surgery [306].

5.5. Photocatalytic Reduction of CO₂

The process of direct conversion of CO₂ and water vapour into hydrocarbon fuel using sunlight is aimed at solving two main global problems: ecological - reduction of CO₂ content in the atmosphere and energetic - obtaining high-energy fuel using the energy of sunlight. In [307], the process of CO₂ reduction by H₂O vapor on the surface of various semiconductor compounds (in the form of TiO₂, ZnO, CdS, SiC, and WO₃ powders) under xenon lamp illumination was studied. American scientists have developed a composite photocatalyst that acts selectively with respect to the process of reducing CO₂ to CO under the influence of visible light. The main components of this photocatalyst are an enzyme (catalyst), sensitizer, and nanoparticles of titanium dioxide [308]. The enzyme included in the composite photocatalyst catalyzes exclusively the two-electron reduction of CO₂ to CO, bypassing the undesirable one-electron radical pathway. In contrast to other TiO₂-based systems, the investigated system is characterized by selectivity and allows obtaining the target product without impurities.

The greatest success in this direction was achieved by American scientists, who, as a photocatalyst, used TiO₂ nanotubes obtained by anodic oxidation of titanium modified with Cu or Pt nanoparticles [309]. The total productivity for all hydrocarbons obtained in the laboratory was 160 μL/hour per gram of nanotubes. These results are 20 times higher than all previously described methods, but they are still too low for direct practical application. At present, there is an extensive search for new photocatalysts for CO₂ reduction, but despite the abundance of different photocatalysts, the main preference is still given to TiO₂, the surface of which is doped with metal nanoparticles [310]. Thus, water photolysis and carbon dioxide reduction are among the promising areas that have both scientific and applied significance for mankind.

6. Current Challenges

Despite intensive research over the past decades, the desired photocatalytic efficiency of TiO₂ has not yet been achieved at a level suitable for large-scale practical applications. For example, a survey of more than 80 major global water treatment companies showed that only about 4% have utilized photocatalysis on a pilot scale [311]. This is a clear sign of lack of patronage of the technology. If TiO₂ is chosen for the application of such technology, the effect of anions on the lifetime of TiO₂ reactor layers cannot be adequately addressed until the electronic, structural and optical properties of TiO₂ nanoparticles and their interactions in aqueous media are addressed in detail ciently, both theoretically and experimentally. The approach will focus on modifying the model of research methods and presentation and discussion of results. The style of research methods and discussion in reported cases of TiO₂ deactivation in aqueous suspensions does not reflect differences in the lattices or phase stability of TiO₂, while these differences (in lattices) play an important role in computational and experimental analysis. Early thermodynamic analyses of the phase stability of nanocrystalline anatase and rutile were performed by Zhang and Banfield [312]. They concluded that when the particle size is reduced to 14 nm, the structure of anatase becomes more stable than that of rutile. Penn and Banfield [313] indicated that anatase clusters dominate on 101 surfaces while rutile clusters usually dominate on 110 surfaces according to the results of Ramamurthy, Vanderbilt and King-Smith calculations [314]. The reason for the change in phase stability is the higher free energy of rutile arising from the energetics of the dominant surface facet types in such small particles [315]. These minute details should be taken into account in future modeling and experiments.

The behavior in volume also deserves consideration. In the case of bulk quantities, rutile is the most stable crystalline structure of TiO₂. For nanoparticles, however, the anatase structure becomes more favorable for medical applications [315]. This has been observed experimentally, for example, in the case of thin layers of TiO₂ [316]. The driving force behind this phase transformation is the surface energetics of the different surfaces of TiO₂. A nanoparticle is a closed object bounded by surface boundaries and the surface-to-volume ratio increases dramatically compared to larger particles, and the various surface energies begin to play an increasingly important role in particle energetics.

In addition, reports on the deactivation of TiO₂ in aqueous media often do not specify the reaction conditions (except pH and opacity), despite the fact that photoreactions are sensitive to reaction conditions. The phenomenon of photo-induced hydrophilic effect and its conditions are often ignored, and it is a well-known phenomenon in surface science research, describing a process in which UV irradiation in air causes water droplets to wet the surface of a TiO₂ film, resulting in a decrease in contact angle over time. Although temperature is also a factor when molecules or atoms come into contact with a surface, numerous analyses of temperature dependence, especially in gas-TiO₂ photocatalysis, have been published in Temperature Programmed Desorption (TPD) [317] as it provides information on how reaction temperature affects the binding energy of the adsorbate and surface, but its influence is often ignored when discussing aqueous photodegradation. Among the various materials that have been studied for photocatalytic CO₂ reduction, titanium dioxide is of great interest because of its high photocatalytic activity, high physical and chemical stability, non-toxicity and low cost [318,319]. But also in this area, there are two major problems faced by TiO₂-based photocatalysts for conventional applications. Thus, the large forbidden band width (3.3 eV), which limits its photoabsorption to ultraviolet (UV) light only, and the rapid recombination of electron-hole pairs that limits the photocatalytic efficiency negatively affect the yield fraction in CO₂ photoreduction. Regarding the problem of medical applications, studies have shown that exposure to TiO₂ can reduce cell viability and increase caspase-3 levels, indicating the possibility of inducing cell cytotoxicity and apoptosis. One study found that only particles smaller than 100 nm at doses of 50 and 100 g/mL could induce apoptosis of Caco-2 cells [320], which prompts a comprehensive investigation into the cause and solution of this problem. The toxicity of TiO₂ depends on factors such as particle size, shape, exposure scenario, dose, crystal structure and surface charge. It is crucial to thoroughly investigate the combined cytotoxic effect of doped TiO₂. TiO₂ is a common pigment in the plastic industry that may pose a threat to aquatic life due to the potential leaching of TiO₂ and other toxic plastic additives during environmental weathering of large pigmented plastic wastes [321]. In the former case, the surface functions of pigmented TiO₂ or TiO₂ nanoparticles differ between the two particle types and within each size class. In the latter case, the exposure pathway for human or environmental exposure differs between studies, with results showing that particle efficacy and biological model sensitivity differ between published reports. Although the knowledge base on industrially significant titanium dioxide and its environmental health effects is large, the fine details of the physicochemical properties of the particles combined with the design of the toxicology study add notable differences in the types and extent of adverse health effects observed [321]. However, despite the existing barriers, the development and understanding of a highly efficient TiO₂-based photocatalyst will continue, so the challenges of large-scale application of TiO₂ for water disinfection, cancer eradication, and carbon dioxide reduction are only a matter of time. Further work in this area should focus on optimizing the properties of this material by tuning and engineering its structure.

6. Opportunities

To date, most of the fundamental processes in TiO₂ photocatalysis can be studied on model TiO₂ surfaces by ensemble-averaged experimental methods, including STM, TPD, PSD, IRAS, etc. However, detailed dynamical information on the transient structure of adsorbates, reaction intermediates, bond breaking/formation, and changes in the electronic structures of adsorbates cannot yet be obtained using these methods. Fortunately, the development of ultrafast time-resolved methods gives us the ability to follow these dynamic processes in the photocatalysis of TiO₂. For example, the dynamic processes of bond breaking and bond formation during CO desorption and CO oxidation on Ru (0001) have been successfully tracked by Nilsson and co-workers [322] using time-resolved femtosecond emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) techniques. In addition, time-resolved UPS and 2PPE techniques have been successfully utilized to monitor electron transfer dynamics and valence band changes associated with electron-mediated surface reactions [314,315,316,317,318,319,320,321,322]. In addition, time-resolved SFG (TD-SFG) can monitor the transition structures of adsorbates in photocatalysis by tracking molecular vibrations. With the advent of new methodology and high-knockdown technology, it may provide opportunities to further deepen the understanding of TiO₂ photocatalysis and shed light on the fundamental photochemical mechanisms and basic principles of TiO₂ photocatalysis.

6. Summary and Future Perspectives

Photocatalysis is an environmentally friendly technological process in which solar energy is converted into useful chemical reactions. In claiming the most efficient photocatalytic activity of TiO₂, challenges and limitations arise, which are mainly related to the synthesis methods of functionalized titanium dioxide-based materials. Therefore, researchers aim to propose and/or modify various methods to overcome the existing barriers in TiO₂ photocatalysis applications and thereby enhance the further development of photocatalytic applications using titanium dioxide. In this review, the known methods for the synthesis of titanium dioxide are reviewed and summarized, and the advantages and challenges related to the properties and morphology of titanium dioxide-based functional materials obtained by these methods are discussed. Great importance in this context is given to the use of nanoparticles, which improve the expected results by modifying the size, shape and physicochemical properties of the particles. It is revealed that poor recombination of charge carriers is one of the main limitations in the photocatalytic process involving titanium dioxide, which can be solved by doping, metals and non-metallic modifiers that have the ability to suppress the recombination of photogenerated electrons and holes, providing charge carrier separation. At the same time, it can improve the photoenergy capture by narrowing the forbidden band of TiO₂. The applications of TiO₂ photocatalysis are based on environmental decontamination, biomedical, pharmaceutical and energy applications. Thus, TiO₂ photocatalysis plays a crucial role in achieving higher technological development while maintaining a balance with environmental sustainability, which is in line with some of the main goals of the UN sustainable development strategy.

Consideration of future perspectives is important to fill the gaps in the development process of TiO₂-based photocatalysis, which requires gradual development towards practical applications in different scenarios. Future research should focus on theoretical (DFT, machine learning predictions) and experimental studies based on the electron capture mechanism, to study in detail and precisely modify the process to achieve higher photocatalytic efficiency and extend the lifetime of photoexcited electrons and holes in TiO₂.

Acknowledgements

Not applicable.

Conflicts of Interest

The author declared no conflicts of interest.

Abbreviation List

TiO₂, titanium dioxide

BG, bandgap

CB, conduction band

VB, valence band

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Figure 1. Current applications and potential future use of TiO₂ [144].

Figure 2. Crystal structure of TiO₂: anatase (a), brukite (b), rutile (c) during thermal treatment, anatase and brukite change to rutile at temperatures of 400-1000 °C and ~750 °C, respectively [154].

Figure 3. Schematic diagram of the electronic structure of TiO₂ [161].

Figure 4. Scheme of surface band bending on a clean TiO₂ surface under vacuum conditions, induced by the donor-like surface defect states (●, electron; ○, hole).

Figure 5. Absorption spectrum of TiO₂ with anatase structure [168].

Figure 6. UV-vis absorption spectra of TiO₂ and N-TiO₂ [175].

Figure 7. (a-c) FESEM and (d) TEM images of the porous, hollow TiO₂ aggregates prepared by the hydrothermal treatment at 160°C for 6 hr [187]; (e, f) SEM overviews of the TiO₂ hollow spheres synthesized by the hydrothermal method at 423 K for 10 hr [186]. The scale bars for (a-d) are 5 mm, 500 nm, 100 nm, and 500 nm, respectively.

Figure 8. (a-c) SEM images for the TiO₂ prepared by the solvothermal method in various alcohol solutions [191]: (a) 300°C, butanol, (b) 300°C, 1,4-butanediol, and (c) 300°C, glycerol; (d, e) SEM images of the anatase TiO2 nanosheets synthesized with a reaction time of 11 hr [192].

Figure 9. SEM images of the TiO₂ samples made by the solvothermal method [195]: (a) a whole sphere, (b) magnified image of the selected part in (a), (c) a broken hollow sphere, (d) magnified image of the selected part in (c).

Figure 10. TEM images of TiO₂ nanoparticles (a, b) and TiO₂ nanorods (c, d) [196]; (e) TEM image of atomically thin N-TiO₂ wires grown at 180°C for 1 hr [197]; (f) TEM image of as-prepared [199].

Figure 11. TEM images of the anatase samples treated at 120°C (a), and calcined at 500°C for 5 hr (b) with different magnifications [201,202,203,204].

Figure 12. Different sol-gel process steps to control the final morphology of the product [209].

Figure 13. Scheme of the microwave synthesis of rutile TiO2 [221].

Figure 14. (a) An FESEM image of TiO₂ nanorods in Mali’s work [245]; (b) absorption of light quanta in TiO₂ micro- and nanoparticles [190]; Microphotographs of the TiO₂ coating obtained by anodizing titanium in a fluoride-containing electrolyte: (c)-surface, (d)-slip [256].

Figure 15. Schematic diagram of a typical excitation of electrons from the filled TiO₂ valence band to the unoccupied conduction band via band-to-band excitation.

Figure 16. Pathways of the photogenerated charge carriers in a semiconductor photocatalyst [270].

Figure 17. Annual statistics of scientific publications on photocatalysis.

Figure 18. Comparison of the total number of scientific publications on photocatalysis with those on titanium dioxide-based photocatalysis.

Figure 19. Scheme of formation of ОН^•, and O^2- particles on the surface of TiO₂ under the action of light.

Figure 20. Simplified mechanism of titanium (IV) oxide as drug delivery vehicle [302].

Table 1. Characteristics of the crystalline structure of TiO_2.

Parameter	Anatase	Rutile	Brookite
Crystal structure	Tetragonal	Tetragonal	Rhombic
Lattice parameters, nm	а= 0.3784 с= 0.9515	а= 0.45936 с= 0.29587	а= 0.9184 b = 0.5447 c = 0.5154
Density, g/cm³	3.79	4.13	3.99
Space group	L4/amd	P4/mnm	Pbca
Number of units in cell	2	2	4
O-Ti-O bond angle	77.7º, 92.6º	81.2º, 90.0º	77.0º -105º
Ti-O bond length, nm	0.1937(4) 0.1965(2)	0.1949(4) 0.1980(2)	0.187-0.204

Table 2. Particle sizes of TiO₂ [182].

Processing time (hours)	Crystal size, nm
Processing time (hours)	Anatase	Brookite	Rutile
2	4.9	7.7	29.1
3	5.7	9.7	34.8
12	8.6	14.0	51.7
24	10.0	15.6	57.4
72	12.9	19.8	63.7
168	16.6	23.5	67.2

Table 3. Oxidizing potentials of various chemical agents.

Oxidizing agent	Oxidizing potential, V
Hydroxyl radical, HO^•	+2.80
Electron hole in valence band, h⁺	+2.70
Ozone, O₃	+2.07
Hydrogen peroxide, H₂O₂	+1.78
Permanganate ion, (MnO4)^2-	+1.70
Chlorine dioxide, ClO₂	+ 1.15
Chlorine, Cl₂	+1.40
Oxygen, O₂	+1.20
Superoxide radical, O^2-	-0.33
Electron in the conduction band, e^–	-0.50

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TiO2 for Photocatalysis and Energy Conversion: A Detailed Overview of the Synthesis, Applications, Challenges, Advances and Prospects for Sustainable Development

Abstract

1. Introduction

2. Titanium Dioxide, Synthesis, Characterization and Properties

2.1. Brief Introduction and Applications of TiO2

2.2. Crystal Structures of TiO2

2.3. Electronic Properties and Band Structure

2.4. Optical Properties of TiO2

3. Efficient Methods of TiO2 Synthesis

3.1. Hydrothermal Method of TiO2 Preparation

3.2. Solvothermal Synthesis of TiO2

3.3. Sol-Gel Method of TiO2 Production

3.4. Sonochemical and Microwave-Assisted Methods of TiO2 Synthesis

3.5. Synthesis of TiO2 by Oxidation Method

3.6. Synthesis of TiO2 by Chemical Vapor Deposition (CVD)

3.7. Green Synthesis of TiO2