Titanium Dioxide: From Engineering to Applications

Abstract

Titanium dioxide (TiO₂) nanomaterials have garnered extensive scientific interest since 1972 and have been widely used in many areas, such as sustainable energy generation and the removal of environmental pollutants. Although TiO₂ possesses the desired performance in utilizing ultraviolet light, its overall solar activity is still very limited because of a wide bandgap (3.0–3.2 eV) that cannot make use of visible light or light of longer wavelength. This phenomenon is a deficiency for TiO₂ with respect to its potential application in visible light photocatalysis and photoelectrochemical devices, as well as photovoltaics and sensors. The high overpotential, sluggish migration, and rapid recombination of photogenerated electron/hole pairs are crucial factors that restrict further application of TiO₂. Recently, a broad range of research efforts has been devoted to enhancing the optical and electrical properties of TiO₂, resulting in improved photocatalytic activity. This review mainly outlines state-of-the-art modification strategies in optimizing the photocatalytic performance of TiO₂, including the introduction of intrinsic defects and foreign species into the TiO₂ lattice, morphology and crystal facet control, and the development of unique mesocrystal structures. The band structures, electronic properties, and chemical features of the modified TiO₂ nanomaterials are clarified in detail along with details regarding their photocatalytic performance and various applications.

1. Introduction

Over the past several decades, the increasing severe energy shortages and environmental pollution have caused great concern worldwide. To achieve sustainable development of society, there is an urgent need to explore environmentally friendly technologies applicable to pollutant recovery and clean energy supplies. In the long-term, solar energy is an inexhaustible source of renewable energy; therefore, developing technologies and materials to enhance solar energy utilization is central to both energy security and environmental stewardship. In 1972, Fujishima and Honda first published a study for producing hydrogen on titanium dioxide (TiO₂) photoelectrodes under ultraviolet light illumination, which garnered worldwide attention [1,2]. From then on, semiconductor photocatalysis has been considered one of the most promising pathways to address both hydrogen production and pollution abatement. Photocatalysis can be widely used anywhere in the world, providing natural solar light or artificial indoor illumination is available [3].

Semiconductor materials are often used as photocatalysts [4]. According to band energy theory, the discontinuous band structure of semiconductors is composed of low energy valence bands filled with electrons, high-energy conduction bands, and band gaps. When the energy of the incident photons equals or exceeds the bandgap, the photoexcitation of electron–hole pairs and the consequential photocatalytic redox reaction take place [5]. The photocatalytic process mainly involves the steps of generation, separation, recombination, and surface capture of photogenerated electrons and hole pairs. Photochemical reactions occur on the surface of a solid catalyst, which includes two half-reaction oxidation reactions of photogenerated holes and reduction reactions of photogenerated electrons [6]. The specific process that occurs in semiconductors is described in Figure 1. During this process, a large proportion of charge carriers (e⁻/h⁺ pairs) recombine quickly at the surface and interior of the bulk material, leading to the dissipation of absorbed energy in the form of light (photon generation) or heat (lattice vibration). Therefore, these charge carriers cannot participate in the subsequent photocatalytic reactions, which is detrimental to the whole process [7].

The electrons and holes that successfully migrate to the surface of the semiconductor without recombining can be involved in the reduction and oxidation reactions, respectively, which are the bases for photodegradation of organic pollutants and photocatalytic water splitting to produce H₂ [8]. As excellent oxidizers, the photogenerated holes can mineralize organic pollutants directly. In addition, the holes can also form hydroxyl radicals (•OH) with strong oxidizing properties. Photoexcited electrons, on the other hand, can produce superoxide radicals (O₂•⁻) and •OH. These free radicals and e⁻/h⁺ pairs are highly reactive and can induce a series of redox reactions. In addition, with respect to water splitting, photogenerated electrons can be captured by H+ in water to generate hydrogen, while holes will oxidize H₂O to form O₂ [9,10,11].

In general, to increase the activity of photocatalysts and utilize visible light more effectively, several requirements need to be satisfied. First, the light absorption process determines the amount of excited charges, which means that more charge carriers are likely to be accumulated on the surface if more light can be absorbed by the photocatalyst. Additionally, considering that ultraviolet (UV) light occupies less than 4% of sunlight’s emission spectrum, while visible light accounts for approximately 40%, a smaller bandgap is necessary for a semiconductor to absorb solar energy across a broad range of spectra. Therefore, improving the optical absorption properties has become a common purpose for photocatalyst design to enhance their overall activity [12]. In addition, the position of conduction bands (CBs) and valence bands (VBs) is critical, which are responsible for the production of active species, such as •OH, HO₂•, H₂O₂, and O₂•⁻. Furthermore, the photogenerated electrons and holes should be transported and separated efficiently in the photocatalyst because the fast recombination of charge carriers will otherwise result in low reactivity. Finally, the as-prepared photocatalytic materials and their modification processes should be environmentally friendly and economical [13].

Since 1972, TiO₂ has been intensively investigated due to its thermal and chemical stability, superhydrophilicity, low toxicity, and natural geologic abundance. Compared with other semiconductor materials, TiO₂ is of ubiquitous interest across many research fields and for many applications [14], such as photodegradation of pollutants and hazardous materials, photolysis (splitting) of water to yield H₂, artificial photosynthesis, etc. Nevertheless, the poor visible light absorption and fast electron–hole recombination, as well as the sluggish transfer kinetics of the charge carriers to the surrounding media, considerably limit the photocatalytic activities of TiO₂. Hence, during the past few decades, much effort has been devoted to overcoming these problems by, for example, reducing e⁻/h⁺ pair recombination and improving the optical absorption properties by energy band regulation, morphology control, and the construction of heterogeneous junctions [15].

In this review, we mainly focus on the regulation of the electronic structure and modification of the micromorphology of TiO₂ nanomaterials to achieve property enhancements that could be applicable to a variety of potential applications.

2. Energy Band Engineering of TiO₂

The absorption of incident light and redox potential of TiO₂ mainly depend on its energy band configuration [16]. To utilize solar energy more effectively, it is necessary to explore and develop longwave-light-sensitive TiO₂ photocatalysts with excellent performance on the basis of energy band engineering [17]. A better understanding of the electronic structure of TiO₂ is important for band gap modification. The molecular orbital bonding energy diagram in Figure 2 clearly shows the fundamental features of anatase TiO₂ [18]. The chemical bonding of anatase TiO₂ can be deconstructed into Ti, e.g., Ti t_2g (d_yz, d_xz, and d_xy), O p_σ (in the Ti₃O cluster plane), and O p_π (out of the Ti₃O cluster plane). The upper valence bands include three main regions: the σ bonding, which is located at the bottom, is the most stable bond type, and arises from the hybridization of Ti, e.g., O p_σ; the hybridization of the O p_π and Ti d_yz (or d_xz) orbitals constitutes the middle energy region of π bonding; and the higher energy region in the top of the valence bands, which is dominated by the O p_π orbitals. The conduction band is composed of Ti 3d and 4s, and the bottom of the conduction bands is composed of the isolated Ti d_xy orbitals [19,20]. For the purpose of narrowing the bandgap of TiO₂, three basic approaches of adjusting the VBs or CBs or the continuous modification of the VBs and CBs of the anatase are shown in Figure 3.

2.1. Doping of TiO₂

To extend the visible light response of TiO₂ and improve its photocatalytic activities, various modification strategies, such as dye sensitization, impurity or intrinsic doping or semiconductor coupling, have been developed [21,22,23]. Among them, introducing impurity ions into the TiO₂ crystal lattice to substitute the host anions and/or cations has earned much attention in the past decade.

By means of physical or chemical methods, researchers have been able to introduce a variety of ions into the TiO₂ matrix, where they change the band structure of TiO₂ by inducing impurity states within the bandgap [2], as shown in Figure 4. In general, ion doping contributes to the improved activities of TiO₂ in three ways: (1) by narrowing the bandgap and promoting the adsorption of the main region of the solar spectrum, such as doping with N, S, C, B, etc. [24,25]; (2) by improving the conductivity of TiO₂ and the mobility of charge carriers, the increased charge traps can reduce bulk recombination and separate photogenerated electrons and holes more efficiently (e.g., Zn, Fe, and Y) [26]; and (3) by altering the conduction band position of TiO₂ with certain metal ion dopants, such as Zr⁴⁺, Nb⁵⁺, and W⁶⁺, which further affects the carrier transfer properties [27].

TiO₂ doping can be doped with a variety of metal ions, including transition metal and rare earth metal ions. For transition metal dopants, such as Fe, Mn, V, Cu, and Cr, both delocalized and localized impurity states will be created within the band gap of TiO₂ along the crystal field splitting of metal 3d orbitals [28,29,30]. Mizushima et al. determined impurity levels of 1.9 to 3.0 eV below CBM by doping V, Cr, Mn, and Fe based on a large number of experimental results, and they suggested that cation vacancies may lead to these impurity states [31]. An early work by Borgarello et al. in 1982 reported that Cr³⁺-doped TiO₂ nanoparticles (investigated for properties of photocatalytic hydrogen evolution) exhibit excellent absorption of visible light in the range of 400 to 550 nm. They believed that the 3d electrons of Cr³⁺ were excited into the conduction band of TiO₂, thus inducing a visible light response [32]. Doping TiO₂ with certain earth rare metal ions represents another promising method to prolong the recombination time of charge carriers and improve their separation efficiency. The 4f electrons in most rare earth elements can give rise to the formation of a multielectron configuration, which acts as a shallow trap for photogenerated electrons and holes [33]. Furthermore, the use of rare earth metal ion dopants in TiO₂ tends to facilitate the utilization of solar light from ultraviolet to infrared light regions. Li et al. prepared a series of Ce-doped TiO₂ nanoparticles by the sol–gel method. The characterization results showed that Ce ions entered the TiO₂ matrix at Ti sites, leading to the formation of impurity states, as shown in Figure 5. In addition, enhanced separation of the photogenerated charge carriers was also realized due to the coexistence of Ce³⁺ and Ce⁴⁺ dopant ions [34].

Anandan et al. studied the photodegradation of monocrotophos under visible light irradiation with La-doped TiO₂. They associated rapid mineralization with the enhanced separation of electrons and holes by doping La³⁺ into the TiO₂ matrix, which subsequently generated a large number of •OH radicals along with the trapping of excess holes at the surface [35]. In contrast, based on the density functional theory calculation method, Sun et al. worked extensively on the changes of the electronic structure and the photocatalytic activity of TiO₂ after introducing substitutional La dopants. Their calculations demonstrate that the enhanced visible light absorption of La–TiO₂ mainly arises from adsorbed La on the TiO₂ surface rather than from substitutional La doping [36]. Notably, not all kinds of dopants give rise to positive consequences. Chio et al. systematically studied 21 kinds of metal ion-doped TiO₂ materials and their application with respect to various photocatalytic reactions [37]. The results associated with model reactions for the photocatalytic reduction of carbon tetrachloride and the photodegradation of chloroform indicated that only the doping of certain ions, such as Fe³⁺, Ru³⁺, Re⁵⁺, V⁴⁺, and Mo⁵⁺, increased reactivity. In addition, the study demonstrated that optimizing the content and placement of the dopant ions content play a positive role in affecting photocatalytic activity. Despite the robust photoactivity of certain metal ion-doped TiO₂ catalysts, some inevitable problems remain and need to be considered. The metal-doped nanomaterials have been shown to suffer from unstable optical properties and thermal instability, in addition to the need to use expensive ion implantation equipment to produce these enhanced materials [38]. Furthermore, the localized d-electron state formed in the band gap of TiO₂ may become the recombination center of photogenerated electron–hole pairs, thereby leading to a decline in the photocatalytic activity.

Recently, the non-metal doping of nitrogen (N), sulfur (S), carbon (C), fluorine (F), iodine (I), and phosphorus (P) has been extensively studied due to their relatively high photostability and photoelectric properties [39]. However, in comparison to metal-doped TiO₂, the role of the non-metal dopants as recombination centers of charge carriers might be minimized. By replacing the oxygen atoms in the TiO₂ lattice, the non-metal elements can significantly narrow the bandgap and thereby improve the visible light response of TiO₂. In addition, impurity states can be formed near the valence band edge alone with non-metal doping, as displayed in Figure 6. Instead of acting as recombination centers, these occupied levels can be regarded as shallow traps that effectively separate photogenerated electron–hole pairs [40].

In 2001, Asahi et al. first published research on N-doped TiO₂ nanomaterials, which initiated a wave of studies related to non-metal-doped photocatalysts [41]. In a similar work, Zhao et al. reported highly active N-doped TiO₂ nanotubes for CO₂ reduction. Despite the tubular structure with a large surface area providing more surface active sites, the N dopants contributed more to the improved photocatalytic activity. It was found that a redshift of the light absorption and a color center were achieved with N-doped TiO₂ nanotubes because N atoms can substitute for the lattice O atoms of TiO₂, thereby reducing its bandgap and resulting in a ~4 times higher visible light photocatalytic CO₂ reduction activity in comparison to pure TiO₂ nanotubes [42]. Irie et al. prepared C-doped TiO₂ nanoparticles by oxidizing TiC powder, and the efficiency of decomposing gaseous isopropanol under visible light was significantly improved [43]. S-doped anatase TiO₂ with a high surface area was obtained by Li et al. They treated pure TiO₂ using a supercritical strategy and used the materials for methylene blue degradation under visible light irradiation. S atoms with large diameters are difficult to dope into the TiO₂ lattice, but X-ray photoelectron spectroscopy (XPS) detected the existence of S–Ti–O bonds, which introduced lattice defects, acting as shallow traps for electrons and reducing carrier recombination [44]. Li et al. mixed HIO₃ with tetrabutyl titanate and hydrolyzed the samples directly to obtain I-doped TiO₂, which significantly boosted its visible light performance [45].

Although various non-metal ions are used for doping modification of TiO₂, N doping is still one of the most widely used methods to modify the electronic structure and to extend light absorption to the visible range [46]. However, researchers have not yet come to a complete agreement regarding the mechanisms associated with the N doping enhancements. In the literature, it is not difficult to find studies stating that it is not only the dopant concentration but also the dopant location in the TiO₂ lattice (surface or bulk, substitutional, and interstitial) that ultimately determines the photocatalytic properties [17,47]. In the case of N-doped TiO₂ nanomaterials, some researchers believe that only the substitution of O²⁻ by N³⁻ with high dopant concentrations can elevate the valence band edge, bringing about the desired band gap narrowing [48,49]. However, others suggest that the doping of N will induce oxygen vacancies in TiO₂ and that the enhanced visible light adsorption is associated with the local state induced in the band gap, rather than the generally believed theory that the introduction of N into the TiO₂ lattice can reduce its band gap, as shown in Figure 7 [50].

As another widely studied non-metal-doped TiO₂, F-doped TiO₂ also shows promising potential for photocatalytic applications. Zhang et al. obtained F-doped TiO₂ mesocrystals through the topological transformation of TiOF₂ precursors. An in situ characterization technique was adopted to detect the doping process. The results showed that the doping of F was accompanied by the formation of oxygen defects, which ensured a higher visible light response [51]. Park et al. added sodium fluoride to aqueous TiO₂ suspensions to obtain surface fluorinated TiO₂, and a series of characterizations showed that neither an improvement in crystallinity nor a redshift of the band edge was achieved, but the photocatalytic oxidation of phenol and Acid Orange was considerably enhanced. They attributed such photocatalytic improvement to fluorine surface modification, which enhances free •OH radical-mediated oxidation pathways [19]. Similar to the doping of N, the reason for the observed high performance upon F doping is still undetermined. Some studies suggest that instead of entering the TiO₂ lattice, fluorine ions adsorbed on the surface of TiO₂ can increase the wettability and surface acidity, which is beneficial to the adsorptivity and e⁻/h⁺ separation of the oxide [20]. Other researchers hold the opinion that a tail state in the band gap of TiO₂ is formed by F doping, which favors the more efficient utilization of incident light. Recently, an increasing number of studies proposed that a charge compensation effect induced by F doping brings about the formation of a certain amount of oxygen vacancies and Ti³⁺ in TiO₂, resulting in the enhanced absorption of visible light [52,53]. Although the principle of F doping is not very clear, the proper doping level of F can effectively improve the activity of TiO₂.

2.2. Intrinsic Defect Formation

In 2011, a black TiO₂ with a narrowed bandgap (approximately 1.5 eV) and fabricated by hydrogenation reduction was reported to achieve absorption of full spectrum sunlight and improved photocatalytic activity [54]. Unsurprisingly, this discovery has aroused worldwide scientific interest and paved the way towards intrinsic defect modification. Creating intrinsic defects in the TiO₂ lattice is a kind of self-structural modification that includes surface disorder layers, Ti³⁺/oxygen vacancy self-doping, formation of surface Ti–OH, and incorporation of doped-Consequentially, considerable changes in surface properties and electronic and crystal structures are often achieved in this process [55,56,57]. Furthermore, studies in terms of defect engineered TiO₂ have confirmed that these intrinsic defects are emerging as a promising attribute for improving the separation of electrons and holes, outperforming, in some cases, other kinds of modified TiO₂ nanomaterials [58].

Since the study by Chen et al., various methods have been developed to induce defects in TiO₂, including direct reduction of TiO₂; that is, the currently reported H₂, Al, Na, Mg, NaBH₄, hydrides, imidazoles, etc. can effectively transfer modify pure TiO₂ nanomaterials into their defect engineered counterparts under certain conditions [59,60]. In addition, electrochemical reduction and high-energy particle bombardment (such as photon beam and H₂ plasma or electron beam) are widely used to induce TiO₂ defects. Partial oxidation from low-valence-state Ti species such as TiH₂, TiO, TiCl₃, TiN, and even Ti foil represents another promising approach, fulfilling the needs for highly active TiO_2−x photocatalysts [61]. Liu et al. prepared rice-shaped Ti³⁺ self-doped TiO_2−x nanoparticles through mild hydrothermal treatment of TiH₂ in H₂O₂ aqueous solution, and proposed a unique “surface oxide-interface diffusion–redox mechanism” (as shown in Figure 8) to explain the formation process of TiO_2−x [62]. The defect types and their formation mechanism in TiO_2−x are closely related to the preparation methods. Generally, the Ti–H bond is present only in hydrogen-reduced TiO_2−x, while the surface disorder layer causes severe damage to the TiO₂ structure. Thus, relatively strong reduction conditions are required, such as high temperature/pressure hydrogen reduction, aluminothermic reduction, hydrogen plasma treatment, etc. Surface Ti–OH, Ti³⁺, and oxygen vacancies commonly exist in most defective TiO₂ nanostructures [63].

The dominant mechanism involved in improving photocatalytic performance by inducing intrinsic defects into TiO₂ can be explained, both experimentally and theoretically, to be the regulation of the band structure of TiO₂ and boosted charge separation and transport. For black TiO₂, band tail states and shallow dopant states can be formed to reduce its band gap and further increase its optical absorption properties. Chen et al. observed a disordered surface layer in black TiO₂ nanocrystals after a hydrogenation treatment, as shown in Figure 9. From the high-resolution transmission electron microscopy (HRTEM) spectra, it can be readily observed that the straight lattice fringes are bent at the edge of the particles, and the lattice spacing is no longer uniform, indicating that the hydrotreated black TiO₂ nanoparticles possess a “crystal-disordered” core–shell structure. Such a disordered layer is believed to facilitate the introduction of the tail state at the top of the valence band and the bottom of the conduction band, consequently yielding a redshift of the light absorption [54]. Moreover, because the disorder layer exhibits a set of properties that are distinct from those of their crystalline counterparts, rapid charge separation could be realized when the amorphous layer closely contacts crystalline TiO₂. The lattice distortions tend to blueshift the VBM while having less impact on CBM. Therefore, the photogenerated holes accumulate in the thin disordered shell and participate in the photocatalytic reactions immediately; electrons are widely spread in both the shell and core regions. This result highlights the strong synergistic effect on charge transfer between the crystalline and disordered parts [64].

For Ti³⁺/oxygen vacancy incorporation and H-doping in reduced TiO_2−x, the hybridization of Ti-3d, O-2p and H-1s orbitals results in the mid-gap states formation below the CBM and the Fermi level’s upshift [65,66]. The extra electrons in either Ti³⁺ or oxygen vacancies are inclined to occupy the empty states of Ti ions, forming new Ti 3d bands below the CBM. With a further increase in defect concentration, the 3d band shifts deeper and finally results in multiple bands in the CBM. Moreover, the existence of multiple mid-gap states as well as the associated derivate (surface Ti–OH) can also function as extra carrier trap sites or carrier scavengers to prolong the lifetime of electrons and holes [67]. The high concentration of electron donors will greatly improve the conductivity of materials and promote the transfer of carriers [68]. Wang et al. treated pure white TiO₂ with hydrogen plasma to fabricate H-doped black TiO₂ for photodegradation of methyl orange under visible light irradiation. The as-prepared samples showed a degradation rate 2.5 times that of the white counterpart [69]. Sinhamahapatra et al. reported a novel controlled magnesiothermic reduction to synthesize reduced TiO_2−x under 5% H₂/Ar atmosphere [70]. During this process, the band position and band gap, surface defects and oxygen vacancies can be well regulated to maximize the optical adsorption in the visible and infrared regions and minimize the charge recombination centers. As shown in Figure 10, a new controlled magnesium thermal reduction method to synthesize and reduce black TiO₂ under 5% H₂/Ar atmosphere. The material has the best band gap and band position, oxygen vacancy, surface defect, and charge recombination center, and the optical absorption in visible and infrared regions is improved obviously. These synergistic effects enable the defective TiO_2−x with Pt as a co-catalyst to produce H₂ at a rate of 43 mmol h⁻¹ g⁻¹ under the full solar wavelength light illumination, superior to other reported photocatalysts for hydrogen production.

To date, numerous strategies, either common or uncommon, have been developed to introduce various kinds of dopants or defects into the TiO₂ matrix. However, considering its highly stable nature, most methods are rigorous and energy-consuming, and are contrary to the sustainable and environmentally friendly development criteria. Therefore, an increasing number of studies are dedicated to seek convenient, economical, energy efficient, and environmentally friendly methods for the structural modification of TiO₂ [71]. In our recent studies, we developed a facile photoreduction strategy to induce intrinsic defects into anatase TiO₂ to modulate its band structure, thereby extending the absorption of incident light to the visible region. As shown in Figure 11, the band gap was narrowed to 2.7 eV, and the color changed to earth yellow after the photoreduction treatment. NH₄TiOF₃ mesocrystals were adopted as precursors, which can release fluorine and nitrogen ions during the topological transformation process. Thus, non-metal ion doping (i.e., F and N ions) was also achieved simultaneously, further improving the transport and separation of photogenerated charge carriers. The as-prepared NF–TiO_2−x exhibited excellent photocatalytic degradation and photoelectrochemical efficiency under visible light irradiation compared to pristine TiO₂ [72,73].

3. Morphology Modification

It is well known that the photocatalytic performance of semiconductors is closely related to their structural and morphological characteristics at the nanoscale, including their size, dimensionality, pore structure and volume, specific surface area, exposed surface facets, and crystalline phase content [74]. During the past few decades, numerous promising structure engineering strategies have been developed to fabricate highly active photocatalysts with the desired morphology and structure. Among them, particular emphasis has been placed on controlling and optimizing the structural dimensionality of a given semiconductor to improve its photocatalytic efficiency.

Zero-dimensional TiO₂ nanospheres are the most widely studied TiO₂-based materials because of their high specific surface area and attractive pore structures [75,76,77]. Figure 12 shows a classic ripening approach to synthesize hollow nanospheres [75]. As photocatalytic reactions take place on the surface of the photocatalyst, TiO₂ nanoparticles with smaller sizes are inclined to provide more reactive sites, resulting in better photocatalytic performance. Moreover, due to the quantum size effect, the photogenerated electrons and holes in the bulk regions are able to migrate to the surface of TiO₂ nanoparticles via shorter distances, thereby considerably reducing the carrier quench rate [78]. TiO₂ nanospheres are also good candidates as light captors, and their structural features enable as much light as possible to access the interior, resulting in amazing light harvesting capabilities. However, it should be mentioned that the diffusion length of photogenerated electrons and holes must be longer than the particle size to avoid the recombination of the dominant carriers on the surface of the photocatalyst, which is very important for achieving efficient charge carrier dynamics [79].

One-dimensional (1D) nanostructures, including nanotube (NT), nanorod (NR), nanobelt (NB), and nanowire (NW), have become a popular research topic in recent years. They have been extensively studied because of their distinct optical, electronic and chemical properties. Despite some similar features with nanoparticles, such as quantum confinement effects and large surface area, 1D nanomaterials possess many unique properties, which are hard for other categories of structured materials to achieve. For example, 1D nanostructures restrict the migration of electrons and protons by allowing the lateral confinement of electrons/protons and guide their transport in the axial direction [80,81]. Furthermore, excellent flexibility and mechanical properties enable them to be easily used and recycled. In this regard, 1D TiO₂-ordered nanostructures are promising not only for constructing highly active photocatalytic systems but also for building blocks for various (photo)electrochemical devices, such as batteries, fuel cells, solar cells, and photoelectrochemical cells. To further optimize the photocatalytic reactivity of 1D TiO₂ nanomaterials, one can precisely regulate the aspect ratio (the ratio of length to diameter) or modify these 1D nanostructures with novel strategies to accelerate electron transport and separation processes, as well as to enhance the capture of incident light; TiO₂ nanotubes are examples of these materials [82]. Through the electrochemical anodization process, it is possible to precisely control the tube crystal structure (anatase, rutile, or amorphous) and tube geometry (diameter and length), as shown in Figure 13a, or direct the tube arrangements to obtain a defined tube-to-tube interspace (Figure 13b). For the sake of extending the scope of application, constructing flow through membranes with TiO₂ nanotubes is a good choice (Figure 13c). Other modifications for minimizing charge carrier annihilation and boosting light harvesting are illustrated in Figure 13d–i, ranging from self-decoration to surface alterations to energy band engineering.

TiO₂ nanosheets, nanoflakes, and thin films consist of titania-based two-dimensional nanomaterials, which have flat surfaces and high aspect ratios. The lateral size of some nanomaterials is controllable, ranging from the sub-micrometer or even nanometer level to several tens of micrometers with thicknesses of 1–10 nm. Such structures provide TiO₂ nanomaterials with several unique characteristics, such as excellent adhesion to substrates, low turbidity and high smoothness [83]. Furthermore, when exposed to UV light irradiation, TiO₂ 2D nanomaterials exhibit superhydrophilicity, which leads to a variety of potential applications, such as self-cleaning coatings and electrodes in photoelectronic devices [84]. Notably, considering that photocatalytic reactions always occur on the surface of catalysts, the exposed crystal facets are of great importance in determining the photocatalytic performance. Accordingly, developing TiO₂ crystals with different active facets is highly desirable in many applications. In general, TiO₂ nanocrystals have three basic low-index exposed facets—{101}, {001}, and {010}—with surface energy relationships of {001}, 0.90 J m⁻² > {100}, 0.53 J m⁻² > {101}, 0.44 J m⁻² [85,86]. Therefore, as the most thermodynamically stable facets, the {001} crystal facet is dominant among most anatase TiO₂ nanomaterials, reducing the overall surface energy of the material. In 2008, Yang et al. first reported TiO₂ single crystals with 47% highly active {001} facets exposed to HF as capping agents [87]. This work has attracted considerable global attention. Since then, TiO₂ with various ratios of exposed {001} facets have been successfully fabricated [88]. Meanwhile, other active planes, such as {010}, {111}, and {110}, have also been reported and widely used in water splitting, solar cells, artificial light synthesis and other fields, as shown in Figure 14 [89]. Zheng et al. obtained {001} facet-oriented anatase by facile heat treatment of a tetrabutyl titanate, absolute ethanol, and HF mixture. Such a material with 85% {001} facets exhibited much higher photocatalytic activity in comparison to commercial P25 materials [90].

During the process of photocatalytic reactions, oxidation predominantly occurs in the {001} facets, while reduction occurs in the {101} crystal plane of TiO₂ because the {101} facet (with relatively low surface energy) tends to attract more electrons. Electron holes subsequently accumulate in the {001} plane, facilitating the space separation of electron–hole pairs [91]. In addition, Ti atoms of the {001} plane exist mainly in the form of 5-coordination, which can provide more active sites that more readily attract free reactant molecules than the {101} plane. Thus, when a certain proportion of {001} crystal facets are exposed, the photocatalytic activity increases rapidly. Nevertheless, it is not always the case that a higher {001} crystal face exposure ratio results in improved catalytic performance. Studies have reported that the photocatalytic activity is compromised when the proportion of {001} facets exceeds 71% [89]. In addition, faceted TiO₂ photocatalysts suffer from weak visible light utilization due to their large band gap. Hence, the modification of the electronic structure of faceted TiO₂ to fully utilize sunlight and promote the migration and separation of electron/hole pairs is highly desirable. Wang et al. prepared Ti³⁺ self-doped TiO₂ mesoporous nanosheets dominated by {001} facets with supercritical technology. They associated the extended region of incident light absorption with the introduction of Ti³⁺ [91]. Using an ionic liquid as a surface control agent, Biplab et al. synthesized microporous TiO₂ nanocrystals with exposed {001} facets. After depositing Pt on the surface, the hydrogen production rate in visible irradiation was greatly improved [92].

A three-dimensional TiO₂ hierarchical structure based on intrinsic shape-dependent properties has been the central focus of many recent studies. Designed and fabricated 3D TiO₂ nanomaterials commonly incorporate interconnected structures, hollow structures and hierarchical superstructures constructed from small dimensional building blocks [93]. Most of these novel structures include larger spatial dimensions and more varied morphologies. The high surface-to-volume ratio provides a more efficient diffusion path for reactant molecules, enabling the contaminant molecules to enter the framework of the photocatalyst for efficient purification, separation, and storage. In addition, the unique optical characteristic is of particular interest because many of these architectures have distinctive physicochemical properties favorable for incident light utilization. For example, when light is irradiated onto the surface of the TiO₂ hierarchical structure, photons are scattered multiple times, so the probability of the catalyst absorbing photons is increased; this phenomenon is known as the “trapping effect” and is illustrated in Figure 15 [94].

The hollow structure TiO₂ nanomaterials have attracted considerable attention due to their amazing light harvesting ability, low density, and large specific surface area. The hollow structure, on the one hand, is capable of providing a large amount of space to accommodate more reactant molecules, thereby increasing the effective contact between the catalyst and the reactants. On the other hand, incident light inside the cavity can undergo multiple reflections to capture more light, as shown in Figure 16 [95]. Kondo et al. obtained TiO₂ hollow nanospheres through hydrothermal and calcination processes with polymer polyethylene cationic balls as templates. The as-prepared photocatalyst had more favorable activity than its commercial counterparts with respect to decomposing isopropanol [96]. In the following work, an ultrathin TiO₂ shell-like structure was prepared in a similar manner with a shell thickness of approximately 5 nm. The morphology of the TiO₂ hollow materials prepared by the hard template method is relatively uniform, and the composition and thickness of the shells are adjustable. However, the preparation process is complicated and requires multiple execution steps to be realized. Moreover, the hollow structure may be destroyed when the template is removed. Therefore, alternative strategies, including soft templates and non-template methods, have played an increasingly important role in the development of hollow structure TiO₂ nanomaterials in recent years.

Li et al. prepared hollow TiO₂ nanospheres with high photocatalytic activity by a template-free process. The increased catalytic activity is mainly due to the multiple reflections of incident light inside the TiO₂ sphere, which extends the optical path [97]. Multichannel TiO₂ hollow nanofibers were constructed by Zhao et al. for degrading gaseous acetaldehyde, and the specific surface area of this material increased rapidly as the number of channels increased. They proposed that the multichannel hollow structures induced both an inner trap effect on gaseous molecules and a multiple-reflection effect on incident light, which were the main reasons for the improved photocatalytic activity of TiO₂ hollow fibers [98]. Shang et al. synthesized submicron-sized TiO₂ hollow spheres from a mixture of TiCl₄, alcohols, and acetone by a template-free solvothermal method. Control of the sphere size was achieved by adjusting the ratio of ethanol to acetone. Based on a series of characterizations, they suggested a possible formation mechanism for the hollow structure: the tiny anatase phase TiO₂ nanoparticles with poor crystallinity form through a hydrolysis reaction, due to the very high surface energy, and then quickly aggregate to form spheres. The increased water promotes the crystallinity of particles in the spherical shell, while the internal particles dissolve and migrate to the spherical shell, leading to the formation of highly crystalline TiO₂ hollow spheres [99]. An intriguing work carried out by Kang et al. to establish hierarchical anatase TiO₂ nanocubes with hollow structures has been reported recently. Instead of seeking complicated templates or surfactants, they directly converted NH₄TiOF₃ mesocrystals to hollow spiny TiO₂ with a high specific area and photodegradation activity [73].

4. TiO₂ Mesocrystals

It is widely accepted that for TiO₂-based photocatalytic materials, large crystallites result in high structural coherence, which benefits the transfer and separation of electron–hole pair, while the availability of plentiful reaction sites is dependent on obtaining large specific surface areas. However, producing a structure that simultaneously satisfies the requirements of large crystallites and high surface area is extremely challenging. Fortunately, the advent of mesocrystals is a promising material that may meet the challenge [100]. Mesocrystals were first proposed by Cölfen and Antonietti in 2005, and since then have received increased attention [101]. Different from the classical single crystals in which the crystal lattice of the entire sample is continuous with no grain boundaries and polycrystals whose units do not have the same orientation, mesocrystals are a new kind of superstructure material that follow a nonclassical crystallization process involving crystallographically ordered assemblies of nanocrystal building blocks. The relevant formation mechanisms of TiO₂ mesocrystals reported thus far mainly include topotactic transformation, mineral bridges, nanoparticle alignment with organic matrices, physical ordering, space constraints, and self-similar growth [100]. Different methods may give rise to different structures and morphologies, but the as-prepared TiO₂ mesocrystals are usually single-crystal-like structures with high porosity, surface area, and crystallinity; they are considered periodically hierarchical structures that are similar to sophisticated biominerals. All of these features pave the way for a wide range of applications, such as catalysis and energy storage and conversion [102].

Fabrication and modification strategies for TiO₂ mesocrystals have developed rapidly in recent years. Due to the similar structure between NH₄TiOF₃ and TiO₂, preparing TiO₂ mesocrystals through topotactic transformation from NH₄TiOF₃ represents an innovative process. As illustrated in Figure 17, the critical parameters in the {001} facets of both NH₄TiOF₃ and TiO₂ are quite similar, with an average lattice mismatch of 0.02%. The position of titanium atoms in the {001} plane of TiO₂ is similar to NH₄TiOF₃, but in NH₄TiOF₃, these are separated by ammonium ions in a lamellar structure. Hence, it is reasonable to use NH₄TiOF₃ as a starting material, transforming it into TiO₂ mesocrystals by thermal decomposition or aqueous hydrolysis with H₃BO₃ [71].

Based on this mechanism, Majima et al. performed extensive studies on tailoring TiO₂ mesocrystals with versatile structures and morphologies, as well as postmodifications to further improve their photocatalytic efficiency. For example, to investigate the anisotropic electron flow in different facets and to maximize their separation during the photocatalytic reaction, Zhang et al. controllably synthesized a specific facet-dominated TiO₂ superstructure with NH₄F as an orientation-directing agent [103]. Under UV light irradiation, mesocrystals with different facet ratios showed different reactivity orders in the photooxidation of 4-chlorophenol, i.e., {001} > {101} (by 1.7 times), and photoreduction, i.e., {101} > {001} (by 2–3 times).

Moreover, constructing the composite of MoS₂ and TiO₂ mesocrystals, as well as the co-catalyst selective modification on TiO₂, also showed the desired separation of photogenerated charge carriers during the hydrogen evolution reaction [104]. In terms of extending the absorption of incident light to the visible region, Zhang et al. tried doping or codoping non-mental elements into the TiO₂ matrix to examine the effects on its electronic structure and band gap. An in situ fluorine-doped TiO₂ superstructure was recently realized. F doping into TiO₂ mesocrystals for the incorporation of active color centers facilitates visible light harvesting and accelerates charge separation for hydrogen generation [51]. They further introduced nitrogen and fluorine codopants into {001} facet-oriented TiO₂ mesocrystals during topochemical transformation for photoreduction of Cr(VI) under visible light illumination. The extended optical light absorption could be attributed to doped nitrogen, which introduces the isolated mid-gap state. The high yield of hydroxyl radicals and preferential adsorption are correlated with fluorine doping, as confirmed by the comparison between untreated TiO₂ with TiO₂ washed in NaOH aqueous solution. The synergistic effect on charge separation and trapping was suggested through a femtosecond time-resolved diffused reflectance (TDR) measurement [105]. As shown in Figure 18, the g-C₃N₄ nanosheet/TiO₂ mesocrystal metal-free composite was successfully constructed by Elbanna et al. [106]. The as-prepared sample exhibited an excellent hydrogen evolution rate under visible light irradiation without any noble metal co-catalyst. Then, they further broadened the light capture of the TiO₂ mesocrystals to include near-infrared regions. Au nanorods (NRs) with various aspect ratios were loaded onto the surface of TiO₂ by the ligand exchange method. Different aspect ratios resulted in different incident light absorption and photogenerated electron transfer. The highest photocatalytic activity of Au NRs and TMC composites reached 924 μmol h⁻¹ g⁻¹ under visible-near-infrared (NIR) light irradiation [107].

Considering the aforementioned merits of mesocrystal nanomaterials, we recently tried different approaches to further improve the optical absorption properties of TiO₂ mesocrystals, in addition to their enhanced transfer and separation properties. Oxygen vacancies and N dopants were successfully introduced into the TiO₂ lattice with a facile low temperature calcination process [108], as shown in Figure 19. NH₄TiOF₃ mesocrystal nanocubes were used as precursors in our system, and topological transformation from NH₄TiOF₃ to TiO₂ facilitated the release and doping of nitrogen. Oxygen vacancies were also readily produced in the inert heating atmosphere. The significantly improved photodegradation and photoelectrochemical performance under visible light irradiation may be associated with the unique structure of mesocrystals as well as the introduction of foreign and intrinsic defects.

5. Separation of Charges

Since metals and metal oxides have different working functions, resulting in the formation of a Schottky potential barrier, an effective modification method is to deposit precious metals (Ag, Au, or Pt) on the surface of metal oxides.

Choi et al. presented Ag/TiO₂ by a photodeposition method [109]. Due to the different transfer rates of interface charges between electrons and holes to redox species in water, excessive charges can accumulate on photocatalysts [110,111]. By depositing Ag, which can provide a temporary home for excessive electrons, the composite utilized the electron storage capacity to promote the separation of electrons and holes to reduce Cr(VI) in the following dark period. Li et al. prepared a sandwich structure with CdS-Au-TiO₂ on a fluorine-doped tin oxide (FTO) substrate [112]. In this composite structure, Au nanoparticles not only acted as an electronic relay between CdS quantum dots (QDs) and TiO₂ to increase charge separation occurring on a long-time scale but also served as a plasma photosensitizer that prolonged the photoconversion to improve the absorption range of light. The rate of charge transfer and reverse transfer depends on the relative energy of the hot plasma electrons to the Schottky barrier [112]. The PEC performance is represented in Figure 20.

Precious metal deposition can greatly improve the performance of catalysts, but the scarcity of precious metals dramatically limits this modification method and makes it difficult to achieve industrial-scale production. In this case, the search for an inexpensive and efficient doped composite has also attracted much attention. Carbon, abundant on earth, has good electrical conductivity, and its combination with TiO₂ can result in excellent photocatalytic performance. Wang et al. demonstrated TiO₂–carbon nanoparticles by the sol–gel method and then synthesized core–shell-structured TiO₂ and amorphous carbon [113]. This unique morphology and structure result in the modified TiO₂ sample exhibiting enhanced responsiveness and excellent photocatalytic activity. Due to the rapid charge transfer in the carbon shell, both the carrier separation efficiency and the photodegradation of pollutants in water is improved. The reduced TiO₂ is also more efficient in the production of H₂ due to its correct edge position.

6. Application of TiO₂ Nanomaterials

Over the past several years, semiconductors, especially titanium dioxide, have been widely used as photocatalysts. It is well known that there are three main steps associated with the photocatalysis process: (1) generation of electrons and holes after the absorption of photons; (2) separation and migration of the charge; and (3) transition of the charge and reaction between the carriers and the reagent. To date, TiO₂ has been mainly applied in the areas of environmental conservation, new energy resources, and so on. In this section, we will focus on recent progress in these photocatalytic applications of TiO₂.

6.1. Applications in the Environment

6.1.1. Degradation of Aqueous Pollutants

Industrial development is often accompanied by pollution of the environment, especially water. Photocatalytic water treatment using heterogeneous semiconductors under visible light is considered an eco-friendly technology. Photocatalysis involves the generation of large numbers of electrons and holes on the surface of TiO₂ after the absorption of photons; the photogenerated holes have considerable oxidizing capacity and can degrade almost all organic contaminants including carbon dioxide (CO₂). However, due to its own deficiencies, such as a wide bandgap and fast recombination of electrons and holes, TiO₂ cannot make full use of sunlight to remove the pollutants in water. Wang et al. reported hydrogenation by TiO₂ nanosheets with exposed {001} facets maintained by the formation of Ti–H bonds [114]. By annealing the fine-sized pristine hydrothermal product under a high-pressure hydrogen atmosphere, the hydrogenation of F-modified anatase TiO₂ nanosheets (with exposed high percentages of {001} facets) was achieved. Under UV–Vis and visible light irradiation, this material decomposed methylene blue (MB) faster than P25 and pristine TiO₂, as shown in Figure 21.

Plodinec et al. applied black TiO₂ nanotube arrays with Ag nanoparticles, which promoted hydrogenation for the degradation of salicylic acid [115]. The photocatalyst can degrade salicylic acid effectively, and its photocatalytic performance far exceeds that of TiO₂ nanotubes and commercial TiO₂ P25 (the reference material used for the modeling of photocatalytic processes). Ling et al. prepared TiO₂ nanoparticles (with diameters of 10–23 nm) that exhibited photocatalytic activity [116]. The initial degradation rate of phenol by a TiO₂ nanocatalyst was 6 times higher than that achieved with H₂O₂ alone, and the addition of H₂O₂ to TiO₂ can increase the initial concentration of hydroxyl radicals and accelerate the degradation rate. Hao et al. developed a TiO₂/WO₃/GO nanocomposite (via a hydrothermal synthesis), which presented excellent optical absorbance and displayed excellent photocatalytic activity for the degradation of bisphenol A [117].

In addition to the oxidizing capacity, the photogenerated electrons on TiO₂ have strong reducing capacity to remove pollutants, such as Cd(II), Hg(II), As(V), and Cr(VI), from water; these cations can be reduced into less toxic metallic or ion states. Dusadee et al. fabricated a titania-decorated reduced graphene oxide (TiO₂∙rGO) nanocomposite via a hydrothermal process [110]. Studies on reducing the toxic Cr⁶⁺ (hexavalent chromium) ion toxicity using the titanium dioxide x/rGO numerical control have found that photocatalytic reduction of toxic Cr⁶⁺ generally increases with the increase in x. In addition, since rGO accelerates electron transport, the combination of photoexcited electrons and holes decreases leads to an increased duration of photocatalytic activity [118]. TiO₂ has facilitated many pollutant degradation processes such as the reduction of nitrate, the degradation of acid fuchsin, the decomposition of acetaldehyde, and the dechlorination of CCl₄ [119,120,121,122]. Due to the continued proliferation of environment pollutants, TiO₂ and other nanostructured materials should be vigorously developed in the future to improve the degradation of pollutants by photocatalysis.

6.1.2. Degradation of Air Pollutants

Just as industrial and technological developments can result in water pollution, so too can the atmosphere be adversely impacted by toxic pollutants that are emitted from chemical manufacturing plants, power plants, industrial facilities, transportation technologies, etc. Air pollution impacts the health of the global environment and the array of species that live within it, and new techniques are sought to reduce harmful airborne emissions. Highly efficient oxidation and reduction during photocatalysis are considered to be an effective method to degrade inorganic and organic air pollutants to improve air quality [123,124,125]. Similarly, TiO₂ is considered the most promising photocatalyst. Kakeru et al. prepared TiO₂ nanoparticles with palladium sub-nanoclusters (<1 nm) using the flame aerosol technique [126]. Under sunlight, these materials can remove NOx at approximately 3 to 7 times the rate of commercial TiO₂ (P25, Evonik) (without Pd). Natércia et al. prepared new composite materials of TiO₂ (P25) and N-doped carbon quantum dots (P25/NCQD) by a hydrothermal method, which was first used as the photooxidation catalyst of NO under the irradiation of ultraviolet and visible light [127]. The experiment showed that the conversion rate of the P25/NCQD composite material (27.0%) was more than twice that of P25 (10%) without modification, and the selectivity in visible light increased from 37.4% to 49.3%. The photocatalytic performance of the composite material in the UV region was also better than that of P25. Zeng et al. reported a H₂ reduction strategy to produce H–TiO₂ materials (with enhanced oxygen vacancy concentrations and distributions) that can promote formaldehyde decomposition in the dark [128]. Research of TiO₂-based photocatalysts has also been conducted to facilitate removal of tetrachloroethylene [129], acetone [130], benzene [131], phenol [73], etc. from the atmosphere.

6.2. Applications in Energy

6.2.1. Photocatalytic Hydrogen Generation

With the extensive use of nonrenewable fossil fuels, mankind is facing an unprecedented energy crisis. The photogenerated electrons on TiO₂ have strong reducing capacity, enabling hydrogen production from the photocatalytic splitting of water. Moreover, hydrogen combustion produces only water and no harmful emissions, and therefore its potential as a truly clean energy source has received considerable attention since it was discovered. Zou et al. reported a self-modified TiO₂ material with paramagnetic oxygen vacancies [132]. For the synthesis of V_o-TiO₂ (V_o: denotes a paramagnetic oxygen vacancy), they chose a porous amorphous TiO₂ material as a precursor that possessed a high surface area of 543 m² g⁻¹. The precursor was calcined in the presence of imidazole and hydrochloric acid at an elevated temperature in air to obtain the V_o-TiO₂ material [132]. The V_o-TiO₂ sample (for H₂ evolution from water) used methanol as a sacrificial reagent under visible light (≥400 nm) at room temperature, and the H₂ production rate was approximately 115 μmol h⁻¹ g⁻¹, which is substantially higher than that achieved with V_o-Ti³⁺-TiO₂ (32 μmol h⁻¹ g⁻¹). Zhou et al. introduced an ordered mesoporous black TiO₂ material that utilized a thermally stable and high surface area mesoporous TiO₂ as the hydrogenation precursor for treatment at 500 °C [133]. The samples possessed a relatively high surface area of 124 m² g⁻¹ and exhibited a photo response that extended from ultraviolet to visible light. As shown in Figure 22, the ordered mesoporous black TiO₂ material exhibits a high solar-driven hydrogen production rate (136.2 μmol h⁻¹), which is almost twice as high as that of pristine mesoporous TiO₂ (76.6 μmol h⁻¹). Zhong et al. constructed a covalently bonded oxidized graphitic C₃N₄/TiO₂ heterostructure that markedly increased the visible light photocatalytic activity for H₂ evolution by nearly a factor of approximately 6.1 compared to a simple physical mixture of TiO₂ nanosheets and O-g-C₃N₄ [134].

6.2.2. Photocatalytic CO₂ Reduction into Energy Fuels

In addition to reducing water to hydrogen, the photogenerated electrons on TiO₂ are capable of generating valuable solar energy fuels, such as CH₄, HCO₂H, CH₂O, CH₃OH, and CO₂, which are considered highly viable energy sources that can alleviate the problems associated with the production of greenhouse gases from the combustion of fossil fuels. Slamet et al. prepared Cu-doped TiO₂ through an improved impregnation method for photocatalytic CO₂ reduction [135]. Both the distribution of copper on the catalyst surface and the grain size of copper–titania catalysts (crystallite size of approximately 23 nm) were uniform, and it was determined that Cu doping can greatly enhance the photocatalytic performance of TiO₂ with respect to CO₂ reduction. Liu et al. found that copper-loaded titania photocatalysts, prepared via a one-pot, sol–gel synthesis method, comprised highly dispersed copper and that CO₂ photoreduction exhibited a strong volcano dependence on Cu loading, which reflected the transition from 2-dimensional CuO_x nanostructures to 3-dimensional crystallites; optimum CH₄ production was observed for 0.03 wt.% Cu/TiO₂ [136].

6.2.3. Solar Batteries

Since semiconductors absorb photons to produce photonic carriers and the photonic carriers move and separate at the same time, electric energy can be obtained through charge transport. TiO₂ can also be applied to dye-sensitized solar cells, Li-ion batteries, Na-ion batteries, and supercapacitors. Liu et al. synthesized a spring-like Ti@TiO₂ nanowire array wire that could be used as a photoanode in dye-sensitized solar cells; this configuration exhibited a conversion efficiency maintenance rate of more than 95.95% [137]. Another study reported the use of anatase TiO₂ nanotubes on rutile TiO₂ nanorod arrays as photoanodes in quantum dot-sensitized solar cells, which have a small thickness of 1 μm and an excellent solar energy conversion efficiency of approximately 1.04%; this is almost 2.7 times higher than the conversion efficiencies measured for solar cells using the original TiO₂ nanorod array photoanodes, as shown in Figure 23 [138]. Chen et al. implemented a C@TiO₂ nanocomposite as the anode material for lithium-ion batteries, which utilize the esterification of ethylene glycol with acetic acid in the presence of potassium chloride. Li-ion batteries utilizing the C@TiO₂ nanocomposite anode exhibited excellent rate performance and specific capacity (237 mA h⁻¹ g⁻¹), and a coulomb efficiency (CE) of approximately 100% after 100 cycles [139]. Su et al. synthesized anatase TiO₂ via a template approach for use as the anode in Na-ion batteries; use of the template-synthesized TiO₂ resulted in better battery performance in comparison to that achieved when amorphous and rutile TiO₂ was used as the anode material. Compared to other crystalline phases of titanium dioxide, anatase titanium dioxide produced the highest capacity, 295 mA h⁻¹ g⁻¹, in the second cycle, tested at a current density of 20 mA g⁻¹ [140]. Kim et al. developed a black-colored TiO₂ nanotube array synthesized by electrochemical self-doping of an amorphous TiO₂ nanotube array and N₂ annealing; the material exhibited good stability, high capacitance, and electrocatalytic performance, and is an excellent material for supercapacitors and oxide anodes [141].

6.2.4. Supercapacitors

Yang et al. developed a hybrid material, covalently coupled ultrafine H–TiO₂ nanocrystals/nitrogen-doped graphene, via the hydrothermal route [142]. Due to the strong interaction between H–TiO₂ nanocrystals and NG plates, the high structural stability of the H–TiO₂ nanocrystal aggregation is inhibited. At the same time, the NG matrix plays the role of electron conductor and mechanical skeleton, imparting good stability and electrochemical activity on most of the well-dispersed ultrafine H-TiO₂ nanocrystals [142]. The material exhibited a high reversible specific capacity of 385.2 F g⁻¹ at 1 A g⁻¹ and excellent cycling stability with 98.8% capacity retention. Parthiban et al. reported a blue titanium oxide (B-TiO2) nanostructure that was applied via a one-pot hydrothermal route and hydrothermal oxidation [143]. The B–TiO₂ nanostructure indicated excellent cycling stability with approximately 90.2% capacitance retention after 10,000 charge–discharge cycles.

6.3. Other Applications

6.3.1. Antibacterial and Wound Healing

It is generally believed that electron–hole pairs formed under light illumination, such as •O²⁻ and •OH, not only destroy all chemical contaminants but also kill microorganisms. Liu et al. proposed a TiO₂/Ag₂O heterostructure (produced by a facile in situ precipitation route) to enhance antibacterial activities [144]. Yu et al. synthesized a TiO₂/BTO/Au heterostructured nanorod arrays (exhibiting piezophototronic and plasmonic effects) by using a simple process that combined hydrothermal and PVD methods. This material can be used as an antibacterial coating for efficient light driven in vitro/in vivo sterilization and wound healing [145].

6.3.2. Drug Delivery Carriers

TiO₂ has the advantages of nontoxicity, stability, biocompatibility, and natural abundance. The preparation of TiO₂ with a high specific surface area can be advantageous in drug delivery carrier applications. Johan et al. controlled the kinetics of drug delivery from mesoporous titania thin films via surface energy and pore size control [146]. Different pore sizes ranging from 3.4 nm to 7.2 nm were achieved by the use of different structural guiding templates and expansive agents. In addition, by attaching dimethyl silane to the pore wall, the surface energy of the pore wall could be altered. The results indicated that the pore size and surface energy had significant effects on the adsorption and release kinetics of alendronate [146]. Biki et al. designed silica-supported mesoporous titania nanoparticles (MTN) coated with hyaluronic acid to cure breast cancer by effectively delivering doxorubicin (DOX) to the cancer cells [147]. Guo et al. deposited (onto the surface of MTN) hyaluronic acid and cyclic pentapeptide (ADH-1), which target CD44-overexpressing tumor cells and selectively inhibit the function of N-cadherin, respectively, to overcome the drug resistance of tumors [148].

Recently, Nakayama et al. found that H₂O₂-treated TiO₂ can enhance the ability to produce reactive oxygen species (ROS) in response to X-ray irradiation [149]. As shown in Figure 24, the atomic packing factor (APF) intensity indicated that hydroxyl radical production in the TiOx (H₂O₂-treated TiO₂) nanoparticles increased in a radiation dose-dependent manner in comparison to that of the non-H₂O₂-treated TiO₂ nanoparticles. This behavior allows H₂O₂-treated TiO₂ nanoparticles to act as potential agents for enhancing the effects of radiation in the treatment of pancreatic cancer. Dai et al. designed and synthesized a novel nanodrug delivery system for the synergistic treatment of lung cancer [150]. They loaded DOX onto H₂O₂-treated TiO₂ nanosheets. In this way, chemotherapy and radiotherapy were combined effectively for the synergistic therapy of cancers.

7. Conclusions

As discussed in this review article, TiO₂-based nanomaterials with wide band gaps have advantages associated with natural geologic abundance, nontoxicity and stability but they also exhibit inherent deficiencies and limitations related to ineffective visible light responses and other photocatalytic properties. The present review aimed to summarize key studies related to the marked enhancement of the photocatalytic performance of TiO₂ by analyzing its electrical structure and photocatalytic reaction process. We have highlighted TiO₂ photocatalysts with well-defined electrical and structure design, as well as tailored facets, dimensions, and remarkable morphologies, which are promising with respect to enhancing the photocatalytic properties of TiO₂. All works presented in this review has enabled the authors to obtain an in-depth understanding of the TiO₂ photocatalytic process, and the critical design of TiO₂ nanostructures with enhanced light absorption, high surface area, desired photostability, and charge carrier dynamics. We hope that this review will guide the future development of more robust TiO₂-based photocatalysts for large-scale applications.

Finally, photocatalysis technology is one of the most active research fields in the world in recent years. However, photocatalysis technologies based on TiO₂ semiconductor still suffer from several key scientific and technological problems, such as low solar energy utilization rate, inferior quantum yield, and difficult recovery, which greatly restricts its wide application in industry. The fundamental solution to improve solar energy absorption is energy band engineering, designing and regulating the bandgap to optimize the harvesting of incident photons. Narrow bandgap and direct semiconductor are more likely to make use of low energy light, but they are restricted by very high electron and hole recombination rate and the incompatible band-edge position. High quantum yield is inevitable for an idea photocatalysis in practical solar engineering, but it cannot be achieved simply doping or inducing intrinsic defects. More works are needed to do to search high quantum yield. All of the above problems depend on the deepening of basic research. Although at present, photocatalysis technology is still a long way from large-scale production and application, its huge potential excellent performance provides a good way for our development. In the near future, with the breakthrough of these key issues, the practical application of nano-photocatalytic materials will certainly be realized to improve our environment, provide cleaner energy, and bring more convenience to our daily life.

Author Contributions

X.K. and S.L. collected references, prepared figures, and wrote the original draft of the manuscript, they contributed equally to this work; Z.D. and Y.H. collected references and analyzed the data; X.S. gave valuable advice; Z.T. acted as a project director and contributed to subsequent revisions. All authors agreed to the final version of the paper.

Funding

This research was funded by the National Natural Science Foundation of China grant number 21571028, 21601027, the Fundamental Research Funds for the Central Universities grant number DUT16TD19, DUT17LK33, DUT18LK28 and the Education Department of the Liaoning Province of China grant number LT2015007.

Conflicts of Interest

The authors declare no conflicts of interest.

Figures

Figure 1. Photocatalytic process in semiconductor.

Figure 2. (a) Total and projected densities of states (DOS) of the anatase TiO2 structure and (b) molecular orbital bonding structure for anatase TiO2 [18]. Copyright 2004 The American Physical Society.

Figure 3. Three schemes of the band gap modifications of TiO2 match the solar spectrum: (a) a higher shift in valence band maximum (VBM); (b) a lower shift in conduction band minimum (CBM); and (c) continuous modification of both VBM and CBM.

Figure 4. TiO2 nanoparticles with different doping elements [2]. Copyright 2014 American Chemical Society.

Figure 5. Band energy structure and charge transfer [34]. Copyright 2017 American Chemical Society.

Figure 6. Comparison of atomic p levels among anions. The band gap of TiO2 is formed between the O 2pπ and Ti 3d states [39]. Copyright 2014 American Chemical Society.

Figure 7. (a) Diffuse reflectance spectra of the anatase TiO2 nanobelts before and after heat treatment in ammonia gas flow at different temperatures and (b) the band structure of N-doped-TiO2 under visible and UV light irradiation [50]. Copyright © 2009 American Chemical Society.

Figure 8. (A) Schematic of the formation mechanisms for the rice-shaped Ti3+ self-doped TiO2−x nanoparticles. (B,C) The interface diffusion–redox diagram. The green arrows indicate ion diffusion [62]. Copyrighted 2014 The Royal Society of Chemistry.

Figure 9. (A) Schematic illustration of the structure and electronic DOS of a semiconductor in the form of a disorder-engineered nanocrystal with dopant incorporation. (B) A photo comparing unmodified white and disorder-engineered black TiO2 nanocrystals. (C,D) HRTEM images of TiO2 nanocrystals before and after hydrogenation, respectively [54]. Copyright 2011 American Association for the Advancement of Science.

Figure 10. (a) H2 generation profile, (b) rate (rH2) of hydrogen generation for different samples, and (c) the stability study of the sample BT-0.5 under the full solar wavelength range of light [70]. Copyright 2015 The Royal Society of Chemistry.

Figure 11. (a) UV–Vis diffuse reflectance spectra and (b) Tauc plot for band gap determination [73]. Copyright 2018 Springer Nature Publishing AG.

Figure 12. (A) Schematic illustration (cross-sectional views) of the ripening process and two types (i and ii) of hollow structures. Evolution (TEM images) of TiO2 nanospheres synthesized with 30 mL of TiF4 (1.33 mM) at 180 °C with different reaction times:  (B) 2 h (scale bar = 200 nm), (C) 20 h (scale bar = 200 nm), and (D) 50 h (scale bar = 500 nm) [75]. Copyright 2004 American Chemical Society.

Figure 13. Schematic drawing of (a,b) formation and (c−i) modification of anodic nanotube arrays (as discussed in the text) [82]. Copyright 2017 American Chemical Society.

Figure 14. Summary of main shapes and applications (i.e., lithium ion batteries, photocatalytic hydrogen evolution, photodegradation, and solar cells) of anatase, rutile, and brookite TiO2 crystals with their surfaces consisting of different Facets [89]. Copyright 2014 American Chemical Society.

Figure 15. Schematic diagram of the reflecting and scattering effects in hierarchical microspheres [94]. Copyright 2014 The Royal Society of Chemistry.

Figure 16. Comparison of photocatalytic activities of titania spheres with solid, sphere-in-sphere, and hollow structures [95]. Copyright 2007 American Chemical Society.

Figure 17. Illustration of the oriented transformation of NH4TiOF3 mesophyte to TiO2 (anatase) mesocrystal [71]. Copyright 2008 American Chemical Society.

Figure 18. Representative scheme of electron injection and movement in g-C3N4 NS (31 wt %)/TMC during visible-light irradiation [106]. Copyright 2017 American Chemical Society.

Figure 19. Schematic representation of the synthesis of TiOx nanosheets. X-ray powder diffraction (XRD) pattern of (a) NH4TiOF3 and (b) N/TiO2−x. SEM images of (c,e) NH4TiOF3 and (d,f) N/TiO2−x [108]. Copyright 2019 The Royal Society of Chemistry.

Figure 20. (a) Electron relay effect of Au nanoparticles, facilitating the charge transfer from CdS QDs to TiO2 nanorods under the irradiation of incident solar light with a wavelength <525 nm. (b) Plasmonic energy transfer from the excited Au nanoparticles to TiO2 through hot electron transfer under the irradiation of incident solar light with a wavelength >525 nm. CB = conduction band, VB = valence band, EF = Fermi energy level, and Φb = Schottky barrier [112]. Copyright 2014 American Chemical Society.

Figure 21. Photocatalytic decomposition of MB (a) and •OH generation measurement (b) of TiO2 and TiO2–H under UV–Vis light irradiation. Schematic illustration (c) of the hydrogenation effect on the structural change in TiO2 and TiO2–H [114]. Copyright 2012 The Royal Society of Chemistry.

Figure 22. Photocatalytic hydrogen evolution of ordered mesoporous black TiO2 (a) and pristine ordered mesoporous TiO2 materials (b). (A) Cycling tests of photocatalytic hydrogen generation under AM 1.5 and visible light irradiation. (B) The photocatalytic hydrogen evolution rates under single-wavelength light and the corresponding QE. The inset enlarges the QE of single-wavelength light at 420 and 520 nm [133]. Copyright 2014 American Chemical Society.

Figure 23. (a) Electron lifetime as a function of Voc for TiO2 NRA and H–TiO2 NRA electrodes with various reaction times. (b) Recombination resistance (Rrec) of the QDSCs made from TiO2 NRAs and H-TiO2 NRAs at various forward biases in the dark. (c) Transient photovoltage responses of CdS–TiO2 NRAs and CdS–H-TiO2 NRAs. The wavelength of the laser pulse was 532 nm. Inset: schematic setup of TPV measurements. (d) Schematic configuration for our device showing the interfacial charge transfer and recombination processes [138]. Copyright 2015 The Royal Society of Chemistry.

Figure 24. ROS production by the TiOxNPs, PAA-TiOxNPs, and TiO2 NPs under X-ray irradiation. (A) Atomic packing factor (APF) intensity indicating that hydroxyl radical production in the TiOxNPs and the PAA-TiOxNPs increased in a radiation dose-dependent manner, but that of the TiO2 NPs did not. Irradiated radiation doses were 0, 5, 10, and 30 Gy. Data are shown as the mean ± SD from 5 independent experiments. (B) Production and scavenging of ROS by 1 mM vitamin C (Vit. C) or 1 mM glutathione (GSH). Histograms show the mean ± SD calculated from 5 independent experiments. (C) Hydrogen peroxide production from the TiOxNPs under X-ray irradiation [149]. Copyright 2016 Springer Nature Switzerland AG.