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Review

Morphology- and Crystalline Composition-Governed Activity of Titania-Based Photocatalysts: Overview and Perspective

1
Institute for Catalysis (ICAT), Hokkaido University, N21 W10, Sapporo 001-0021, Japan
2
Institute of Chemical Technology and Engineering, Faculty of Chemical Technology, Poznan University of Technology, 60-965 Poznan, Poland
3
Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, China
4
Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan
5
Nanomaterials and Nanotechnology Department, Central Metallurgical Research and Development Institute (CMRDI), P.O. Box 87 Helwan, Cairo 11421, Egypt
*
Author to whom correspondence should be addressed.
Catalysts 2019, 9(12), 1054; https://doi.org/10.3390/catal9121054
Submission received: 16 November 2019 / Revised: 5 December 2019 / Accepted: 7 December 2019 / Published: 11 December 2019

Abstract

:
Titania photocatalysts have been intensively examined for both mechanism study and possible commercial applications for more than 30 years. Although various reports have already been published on titania, including comprehensive review papers, the morphology-governed activity, especially for novel nanostructures, has not been reviewed recently. Therefore, this paper presents novel, attractive, and prospective titania photocatalysts, including zero-, one-, two-, and three-dimensional titania structures. The 1D, 2D, and 3D titania structures have been mainly designed for possible applications, e.g., (i) continuous use without the necessity of particulate titania separation, (ii) efficient light harvesting (e.g., inverse opals), (iii) enhanced activity (fast charge carriers’ separation, e.g., 1D nanoplates and 2D nanotubes). It should be pointed out that these structures might be also useful for mechanism investigation, e.g., (i) 3D titania aerogels with gold either incorporated inside the 3D network or supported in the porosity, and (ii) titania mesocrystals with gold deposited either on basal or lateral surfaces, for the clarification of plasmonic photocatalysis. Moreover, 0D nanostructures of special composition and morphology, e.g., magnetic(core)–titania(shell), mixed-phase titania (anatase/rutile/brookite), and faceted titania NPs have been presented, due to their exceptional properties, including easy separation in the magnetic field, high activity, and mechanism clarification, respectively. Although anatase has been usually thought as the most active phase of titania, the co-existence of other crystalline phases accelerates the photocatalytic activity significantly, and thus mixed-phase titania (e.g., famous P25) exhibits high photocatalytic activity for both oxidation and reduction reactions. It is believed that this review might be useful for the architecture design of novel nanomaterials for broad and diverse applications, including environmental purification, energy conversion, synthesis and preparation of “intelligent” surfaces with self-cleaning, antifogging, and antiseptic properties.

Graphical Abstract

1. Introduction

Titania (titanium(IV) oxide, TiO2) is one of the most famous and widely studied photocatalysts. Titania is a naturally occurring oxide of titanium, including anatase, rutile, brookite, TiO2(B), TiO2(H), TiO2(R), and akaogiite (new-found and exceedingly rare [1]) polymorphs. However, in nature, various mixtures of titania could be found, e.g., ilmenite (a black iron–titanium oxide with a chemical composition of FeTiO3, which is commonly used for titania synthesis via the sulfate method), rather than its pure crystals. Titania has been used extensively in our daily life, i.e., as a pigment, in the textile and paper industry, food, drugs, cosmetics, and as photocatalysts.
The application of titania for photocatalysis is probably the most important for future human development, since titania might be used to solve (at least in part) three out of 10 of humanity’s top problems (problems proposed by Professor Smaley [2]), i.e., energy, environment, and water. Under irradiation with energy equal or larger than its bandgap, titania is excited with electron transfer from valence band (VB) to conduction band (CB), and thus formed charge carriers (electrons in CB and holes in VB) might either recombine or migrate to the surface, where they can initiate redox reactions. Accordingly, solar energy conversion into electricity or/and fuels (including “artificial photosynthesis”), water treatment, and environmental purification, which are mainly by formed reactive oxygen species (ROS), are initiated by photogenerated charge carriers (electrons and holes).
The unique photocatalytic properties of titania are not only impacted by its intrinsic physicochemical characteristics, but also by the morphological and structural properties of titania particles, and it is particularly important to consider them to design an efficient and applicable photocatalysts. Understanding the correlation between the mentioned properties and photocatalytic activity is still unsatisfactory, which contributes to the slow development of technologies based on photocatalysis. The vision of highly selective photocatalysts arising from the potential achievements in the considered issue would reach a crucial milestone in the titania application, including environmental purification, energy conversion, synthesis, and “intelligent” surfaces with self-cleaning, antifogging, and antiseptic properties.
A huge number of various reports on titania photocatalysts have been published annually (Figure 1a) for different applications, including several insightful review papers, e.g., Hashimoto et al. [3], Zaleska [4], Kowalski et al. [5], and Tsang et al. [6]. Despite these papers, there is still a lack of comprehensive review focusing on the influence of the morphology and the structure on the photocatalytic performance. Therefore, in this paper, special emphasis will be put on the novel trends in the architecture design of titania and titania-based photocatalysts, such as faceted titania, inverse opal, microspheres, aerogels, core–shells, and mixed-phase structures.

2. Titania Polymorphs and Mixed-Phase Samples

2.1. Photocatalytic Activity of Titania Polymorphs

Two main polymorphs of titania, i.e., anatase and rutile (Table 1), have been extensively investigated both for photocatalysis and other applications (pigments, sunscreen creams), and recently, the high photocatalytic activities of brookite have been also reported [7,8]. Anatase, rutile, and TiO2(H) belong to tetragonal system, brookite, and TiO2(R) to orthorhombic, whereas TiO2(B) and akaogiite to monoclinic one (akaogiite will not be discussed in this paper, since its photoactivity has not been investigated yet.). Rutile is the most abundant and the most thermodynamically stable polymorph of TiO2; it is widely used in pigments and cosmetic products, mainly due to its high refractive index. In contrast, anatase and brookite are metastable, and thus thermal treatment results in their phase transition into rutile.
Anatase has been commonly reported as the most photocatalytically active form of titania; thus, it has been mainly investigated and used for many photocatalytic reactions, as shown in Figure 1b. Various reasons for its higher activity than other phases (including amorphous titania) have been proposed, such as a wider bandgap (and thus high oxidation and reduction ability), lower content of defects, higher content of hydroxyl groups on the surface, higher mobility of charges (i.e., mainly electrons), lower content of deep electron traps (ETs; and thus a lower rate of charge carriers’ recombination). It should be also mentioned that usually anatase has a much larger specific surface area than rutile (higher temperatures of rutile preparation results in nanoparticles (NPs)’ sintering and aggregation), and thus a larger content of adsorption sites for reacting molecules on its surface has been also proposed as another reason for its high photocatalytic efficiency.
The comparison of 35 titania photocatalysts showed that the photocatalytic efficiency did not depend only on the titania properties (specific surface area, polymorphic composition, defects’ content, crystallite, and particle sizes), but also on the kind of photocatalytic reaction [9]. Accordingly, it has been proposed that (i) large particle sizes result in efficient oxygen evolution, (ii) large specific surface areas (small crystallite and particle sizes) result in methanol dehydrogenation, (iii) anatase presence results in the oxidative decomposition of acetic acid, (iv) rutile presence and defects’ content result in the oxidative decomposition of acetaldehyde, and (v) rutile presence results in the synthesis of pipecolinic acid. Moreover, it should be pointed that mixed-phase rather than single-phase titania photocatalysts have been used in photocatalysis (as pure phase samples are rather rare), and thus the impact of the minor phase on the overall activity cannot be omitted. Interestingly, “phase-mixed” titania samples have been proposed as more active than single-phase samples [10,11,12,13,14], as discussed in Section 2.2.

2.2. Photocatalytic Activity of Mixed-Phase Titania

Different kinds of mixed-phase titania have already been prepared by various methods (sol–gel, sonochemical, hydrothermal, etc.) from different titania precursors (titanium isopropoxide, titanium butoxide, TiCl4, etc.) for diverse applications (degradation of organic pollutants, water splitting, reduction of CO2, etc.), as shown in Table 2.
Titania P25, known also as Degussa P25, Evonik P25, and Aeroxide P25, is probably the most famous mixed-phase titania, due to its extremely high photocatalytic activity in various photocatalytic reactions. For example, P25 was used for the decomposition of organic [44,45,46,47,48,49,50] and inorganic [51] compounds present in water and wastewater, the degradation of gas-phase pollutants [52,53,54,55,56], the purification of indoor environments (e.g., allergens’ removal [57]), inactivation of microorganisms [58,59,60,61,62,63], self-cleaning surfaces [64,65,66] and solar energy conversion [67]. The number of cited papers using “P25”and “photocatalysis” as keywords has exceeded 10,000 per year, as shown in Figure 2.
P25 is a white powder with fine NPs (ca. 30 nm) and hydrophilic nature, and thus its surface is coated with hydroxyl groups. Fine NPs form aggregates, in which primary NPs are loosely bound together. The density and specific surface area of P25 reached ca. 3.9 g cm−3 and ca. 50 m2 g−1, respectively [68,69]. The different composition of P25 might be found in the literature, i.e., 70%–85% anatase and 15%–30% rutile, and the content of the amorphous phase is usually not even considered [70,71,72]. It was reported that small crystals of anatase (ca. 25 nm) surrounded the large crystals of rutile [73,74,75]. Although P25 is commonly used as “standard” titania photocatalyst for activity comparison between different titania samples [69,74,76,77,78,79,80], the phase composition might vary between each P25 sample. Even P25 powders sampled from the same container possessed different content of anatase, rutile, and amorphous phase, i.e., 73%–85%, 13%–17% and 0%–13%, respectively [68,69], which is not surprising, since the composition depends on the flame conditions (and even the location of formed particles in the flame; P25 is produced by gas-phase flame synthesis). Therefore, for reliable comparison among samples, the homogenization of P25 powder has been proposed, resulting in the preparation of almost uniform sample, containing 78 ± 0.3% anatase, 14 ± 0.1% rutile, and 8 ± 0.8% amorphous phase [68].
Many explanations and possible reasons for the high photocatalytic activity of P25 and other mixed-phase titania have been proposed. The most popular one deals with the charge carriers’ transfer between different phases, resulting in the inhibition of charge carriers’ recombination, which is commonly named the “synergistic effect” [74,81,82]. Although usually electrons’ transfer from anatase to rutile has been proposed, due to the more negative CB of anatase than that of rutile [83,84,85,86,87,88,89,90], some reports have claimed an opposite direction of charges’ migration, e.g., (i) holes from an anatase core to a rutile overlayer and opposite electron transfer [82], and (ii) electrons from rutile to lattice trapping sites in anatase [75,81,91,92]. Mi and Weng have proposed five possible band alignments in mixed-phase (anatase/rutile) titania, as shown in Figure 3 [93]. Interestingly, type V (CBs being aligned) has been suggested as the most probable structure, which is based on transient infrared absorption-excitation energy scanning spectroscopy data. Therefore, the direction of electron migration must be controlled by dynamic factors, such as particle size, the presence of electron/hole scavengers on either the surface of anatase or rutile, or both.
Other proposed reasons for the high photocatalytic activity of P25 include the presence of amorphous phase and/or impurities (e.g., iron cations) [94], its low content of trapping sites (slow charge carriers’ recombination) [71], and its “excellent” surface properties (small NPs, large specific surface area, high crystallinity). However, it should be pointed out that P25 does not have the best surface properties among various titania photocatalysts, but it does have one of the best photocatalytic activities. Accordingly, titania photocatalysts with much better properties (larger specific surface area, smaller crystallites) than P25 showed lower photocatalytic activities. In 1991, Bickley et al. proposed that the high activity of P25 (and other mixed-phase titania photocatalysts) was caused by the enhancement in “the magnitude of the space-charge potential”, as a result of the interface between different phases and localized electronic states in amorphous titania [82].
Although it is unclear who first used the term “synergy” (or “synergistic effect”) between anatase and rutile to explain the high activity of P25, many reports have used it interchangeably for an interparticle electron transfer (IPCT) between polymorphs in P25 and other mixed-phase titania photocatalysts [86,95,96,97,98]. However, commonly, no experimental evidence has been shown, and “synergy” has been only speculated based on the results showing a higher activity of mixed-phase titania than that of the single phase one. Moreover, in many cases, the activity of P25 (or other mixed-phase samples) was only slightly higher than that of the most active phase (usually anatase, depending on the reaction system), but not higher than the simple sum of its parts (activity of anatase plus activity of rutile—“synergy”). Therefore, even the term synergy has been used incorrectly in some reports.
Fortunately, some reports have tried to confirm a “synergistic effect” and/or IPCT for mixed-phase titania, mainly by the simple physical methods, such as mixing and thermal treatment. The phase mixing might be performed either in an aqueous suspension of different titania polymorphs or by grinding (It should be pointed that grinding might result in a slight change of surface properties, e.g., a decrease in particle size by abrasion). For example, Ohno et al. proved synergy between anatase and rutile for the photocatalytic oxidation of naphthalene for samples prepared by two methods, i.e., grinding (different ratios of anatase to rutile) and thermal treatment (influencing anatase-to-rutile phase transition) [11]. The synergetic effect was found in both kinds of samples (Figure 4), and IPCT was proposed as its main reason. Interestingly, the highest activity was obtained for mixed-phase titania, which was prepared by grinding with similar composition to that of P25 (Figure 4a). Therefore, it was suggested that similar “synergy” could be responsible for high activity by P25, i.e., electron transfer between anatase and rutile, resulting in a prolonged lifetime of charge carriers.
It is obvious that metastable anatase and brookite convert irreversibly into rutile upon heating. Therefore, during thermal treatment, the mixture of anatase and rutile can be easily obtained, e.g., anatase particles could be loaded with thermally formed rutile. For example, Zhang et al. proposed the synergy between anatase and rutile in the anatase-rutile mixed-phase sample, prepared by the calcination of anatase obtained by the precipitation method [99]. However, in that study, La2O3 was loaded on the surface of titania also; thus, a possible synergy between anatase and rutile could not be confirmed directly. Moreover, it should be pointed that thermal treatment might change not only the composition of samples (anatase/rutile ratio), but also all other properties, e.g., crystallite/particle size, specific surface area, morphology, defects’ content. In some studies, “synergy” in P25 was suggested in comparison to anatase and rutile samples with quite different properties than those in P25, and usually these samples did not originate from P25 samples. Additionally, some activity tests have been carried out in the presence of loaded co-catalysts, and thus the properties of co-catalysts, which could be decisive for the overall activity, should be also considered.
Although IPCT between anatase and rutile in P25 was suggested by electron paramagnetic resonance (EPR) spectroscopy [75,81], the reference samples of anatase and rutile did not originate from P25, and thus it was impossible to decide unequivocally that IPCT was responsible for the high activity of P25. Accordingly, another approach was proposed by Ohtani, i.e., an isolation of rutile and anatase from P25 [68,69,100]. The successful isolation of crystalline phases by chemical methods, which was proposed first by Ohno et al. [101] and Ohtani et al. [100] for rutile and anatase isolation, respectively, was confirmed by various methods, e.g., XRD and diffuse reflectance spectroscopy (DRS), as shown in Figure 5 [68]. The photocatalytic activity data revealed another possible reason for the high activity by P25, i.e., the high intrinsic activity of anatase and rutile for different reactions. For example, anatase exhibited much higher activity than rutile and P25 for the oxidative decomposition of acetic acid and acetaldehyde, whereas rutile was the most active for oxygen liberation [69] and the dehydrogenation of methanol [68].

3. Nanostructured Titania: Zero-, One-, Two-, and Three-Dimensional Structures

Titania and other nanomaterials could be classified according to their dimension confinement in nanoscale (usually defined as less than 100 nm) measurements, and thus 0-dimensional (0D) might be referred to NPs, and nanospheres, whereas 1D and 2D have one and two dimensions larger than the nanoscale size, e.g., nanotubes and nanoplates, respectively. The structures with repeated bulk arrays could be designed as 3D titania, where none of their dimensions are confined to the nanoscale, except for their inner structure, e.g., composed of fine NPs, nanovoids, etc. Based on this concept, various types of titania nanostructures have been designed and reported. The most interesting, novel, and perspective examples are shortly presented in this chapter.

3.1. Zero-Dimensional Titania

Titania NPs (already presented in Section 2), nanospheres, nanorods, and nanorice NPs have been commonly used for photocatalytic reactions. Here, two kinds of nanostructures are shortly described, i.e., core–shells and faceted NPs.

3.1.1. Core–Shell Titania Composites

Core–shell titania nanostructures belong to titania composites, i.e., titania and other materials as mixed-phase composites. There are two main reasons for the preparation and usage of core–shells in photocatalysis, i.e., (i) reusability and (ii) enhanced activity. Although nanoparticulate titania is highly recommended for application, due to high activity (large specific surface area), the separation of the suspended photocatalyst might be costly (ultrafiltration). Therefore, photocatalyst supports, e.g., steel plates, molecular sieves, glass plates, Raschig rings, zeolites, activated carbon, and silica [102,103,104,105] have been applied. Glass microspheres recovered from fly ash were also used as titania support, and interestingly, higher activity was found for phenol degradation on microsphere/titania, despite it having a much lower surface area than that on particulate titania samples [106,107]. It was proposed that the composition of microspheres (the presence of Fe2O3) could influence the overall activity, i.e., the possible transfer of charges (both electrons and holes) from titania to Fe2O3 hindering charge carriers’ recombination. However, it should be pointed out that there is a risk that titania support might be mechanically destroyed during photocatalytic tests (pumps, stirrers, etc.), as has been observed for these microspheres [106,108]. Therefore, a mechanically stable core has been proposed, such as magnetic Fe3O4, which additionally allows the fast and easy separation of photocatalysts after reactions in the magnetic field [109,110,111]. However, the photoreaction between a core and a shell might result in the instability of the photocatalyst (e.g., dissolution of iron), and thus an insulator interlayer has been applied by Zielinska-Jurek et al. between a magnetic core and titania shell, as shown in Figure 6 [110].

3.1.2. Faceted Titania

The concept of titania synthesis as crystals with specific facets has been a hot research topic, especially since 2008 when Yang et al. prepared anatase crystals with a large percentage of reactive {001} facets [112]. Earlier in 1998, Vittadini et al. theoretically predicted the exceptional dissociative adsorption of water on an anatase {001} surface [113]. It is known that the reactivity of photocatalysts is strictly connected with a surface atomic arrangement and coordination, which determines the adsorption of reactant molecules, surface transfer between photoexcited electrons and reactant molecules, and finally the desorption of product molecules. The arrangement of crystal facets with different orientations changes the surface atomic arrangement and coordination, influencing the photocatalytic activity [114]. Pertaining to this, crystal facets engineering might be the solution for how to improve the usually poor selectivity of heterogeneous photocatalytic transformations by the minimization of dispersity of surface atomic structures [115].
Taking into consideration the surface energies of crystal facets, it is possible to estimate the equilibrium morphology of each titania polymorph using the Wulff construction theory [114], as shown for anatase, rutile, and brookite in Figure 7a–c. Accordingly, the equilibrium anatase shape is a slightly truncated bypiramid, which is enclosed with eight isosceles trapezoidal surfaces of {101} and two {001} facets located on the top/bottom of the crystal. The percentage of {001} facets in this equilibrium crystal is 6%. The surface energy of {001} (0.90 J m−2) is higher than for {101} (0.44 J m−2) [116,117]. Among titania crystals, anatase faceted particles have been most intensively studied. It is possible to find research works where anatase crystals with low-index facets, such as {101}, {001}, {010}, and {110}, and high-index facets such as {103}, {105}, {107}, {201}, {401}, {301}, and {106} have been prepared. In the case of rutile crystals, the facets of {110}, {011}, {001}, and {111} have been identified, whereas for brookite, the {100}, {001}, {101}, {111}, {210}, and {201} facets have been achieved [117]. The main crystal shapes of anatase, rutile, and brookite are presented in Figure 7d.
Faceted TiO2 crystals can be prepared by different methods, such as the wet-chemistry route (hydrothermal, solvothermal) and gas oxidation route. The wet-chemistry approach commonly involves the application of fluorine-containing species (e.g., hydrogen fluoride, ammonium fluoride, titanium(IV) fluoride, and tetrafluoroborate ion) as morphology-controlling agents to achieve the resultant faceted particles [112,120]. Other compounds, including polyvinyl alcohol [121] and carbonate ions, are generated during the decomposition of urea [122], and sodium chloride and/or sodium perchlorate [123] have also been used to control the shape. Dinh et al. used oleic acid (OA) and oleyl amine (OM) as capping agents with titanium n-butoxide (TBu) as the titania precursor. Various shapes of titania-faceted particles (bar, dog-bone, rhombic, elongated rombic, truncated rombic) were obtained by controlling the ratio of TBu:OA:OM and the reaction temperature (Figure 8) [124]. However, the disadvantage of this approach is the strong adsorption of shape-control reagents (e.g., fluoride ions) on the crystal surface or their doping into the lattice, which may result in a decrease in photocatalytic activity [121]. Moreover, at high temperatures, fluorine compounds generate toxic and corrosive products.
The solution for these problems might be the development of methods based on gas-phase reactions [78,125]. The adjustment of one of the main parameters of gas-phase synthesis of decahedral anatase particles (DAP), e.g., concentration of titania precursor—TiCl4 vapors, resulted in the morphological changes of the faceted particles (Figure 9).
One of the most important properties resulting from the presence of different predominant crystal facets is the photocatalytic activity that is connected with synergistic effects of different facets. The majority of studies reported that the photocatalytic activity of anatase crystals in the reaction of oxidation of organic compounds increased with an increase in the percentage of {001} facets [122]. It has been proposed that the coexistence of different crystal facets, e.g., {101} and {001}, might result in enhanced activity due to the synergistic effects between different facets. First, in the early studies, Ohno et al. reported the selective photodeposition of Pt and PbO2 particles on different facets of anatase and rutile crystals [73]. They postulated the existence of oxidation and reduction sites on {001} and {101} facets of anatase, and {011} and {110} facets of rutile, respectively. In the next research, Murakami et al. prepared DAP with a different aspect ratio between {001} and {101} facets (Figure 10a–c) [121]. They observed that the photocatalytic activity for the decomposition of acetaldehyde increased with an increase in the ratio of {101}/{001} exposed crystal facets, which might indicate that the spatial separation of redox sites requires the existence of different types of crystal facets. In another study, Murakami et al. analyzed the dependence of photocatalytic activity on the aspect ratio of shape-controlled rutile nanorods (nanorods—1D nanostructure) [123]. The photocatalytic activity for acetaldehyde decomposition increased with the decreasing aspect ratio, which was not due to an increase in the number of adsorption sites, but rather to a more optimal ratio of the surface areas of oxidation and reduction sites (Figure 10d) [13]. The significant study confirming facet-dependent reactions has been performed by Tachikawa et al., in which selective oxidation on {001} and reduction on {101} facets have been proved by single-molecule imaging and kinetic analysis [126]. It should be pointed that both the selective deposition of metal/oxides (e.g., [73]) and an appearance of reaction products on selective facets (e.g., [126]) do not unequivocally confirm whether that particular reaction happens on that facet. For example, Kobayashi et al. has shown that the adsorption of noble metals depends on the surface charges and pH value of the reaction mixture [127].
Although DAP particles have shown extremely high photocatalytic activity for both oxidation and reduction reactions (even higher than famous P25) [78], single-facet titania seems to be very attractive for mechanism study. For example, octahedral anatase particles (OAP) with eight {101} facets showed very high photocatalytic activity for the oxidation of acetic acid [128,129]. Although, various reports (as also shown in this review) have claimed that faceted morphology was beneficial for photocatalytic activity, the direct proof was not presented until the study by Wei et al. [128,129]. In that study, OAP-containing samples, which differed only by morphology (same specific surface area, crystalline size, crystallinity, composition, etc.), i.e., the content of faceted particles in the final product, were prepared by hydrothermal–ultrasonic reaction. Indeed, it was proven that faceted morphology was beneficial, since with an increase in the content of faceted NPs, activity increased. Moreover, it was proposed that faceted NPs contained a larger content of shallow than deep electron traps (ETs), which facilitated electrons’ migration instead of their permanent trapping, as assumed by photoacoustic spectroscopy (total content of ETs) and time-resolved microwave conductivity (electron mobility and lifetime; shallow ETs) [128].
The development of titania crystals with exposed facets provides an important platform for the rational design and production of efficient photocatalytic materials, including facet-dependent properties, and improving the performance in various applications. However, there are still unclear issues mostly regarding the mechanisms responsible for the unique photocatalytic properties of titania crystals with different facets. Despite that, it is thought that facet-dependent reaction selectivity is undoubtedly the most promising research direction in the further development of the efficient photocatalytic materials with high application potential.

3.2. One-Dimensional Titania

Titania nanotubes, nanowires, 1D-oriented nanopores, and other 1D nanostructures (such as nanorods) have been extensively investigated for various applications, including solar cells, photocatalysis, batteries, filtration membranes, biomedical use, and as templates for the synthesis of other nanostructures [5,130,131,132,133]. Kasuga et al. were probably the first who prepared titania nanotubes (TNTs) by the chemical method [134,135], whereas Imai et al. obtained TNTs in porous alumina membrane by a deposition technique [136]. Since then, huge effort has been put toward the preparation of highly uniform and well-oriented TNTs. The most famous and popular is probably the preparation of TNTs by the electrochemical method, i.e., the anodization of titanium film, which was started by Grimes and Schmuki [137,138,139]. It should be pointed that different morphologies could be obtained by changing the anodization conditions, e.g., TNTs, as well as the mesoporous and nanoporous structures, as shown in Figure 11. Recently, a very interesting approach has been proposed by the Zaleska-Medynska group, i.e., the anodization of titanium alloys to prepare modified titania by a one-step reaction, e.g., TiO2/Ag2O/Ag [132] and TiO2/CuxOy [131] from Ti/Ag and Ti/Cu alloys, respectively.
Although 1D nanostructures are very interesting, this chapter does not discuss them in detail since many review reports on TNTs (including also other 1D titania structures) have already been published, e.g., review papers on TNTs [5,140,141,142,143] and modified TNTs toward vis-response [144]. Here, only TNTs photonic crystals (PCs) are presented as novel and not well-explored titania nanostructures.
The designing of PCs, periodic optical nanostructures that affect the motion of photons, has been identified as an efficient method to enhance light harvesting, i.e., by using longer wavelengths (vis and IR). The uniqueness of PCs arises from the consistent spatial periodicity of the refractive index (n) in the well-ordered structure, which prevents the electromagnetic wave pathway at certain frequencies, resulting in the formation of the stop band and the photonic bandgap (PBG) [145]. The presence of PBG results in the formation of “slow photons” at both the red and blue edges of PBG, whereby the photons are scattered and reflected, causing the reduction of photons’ group velocity [145,146], which might be utilized for light absorption enhancement in titania-based photocatalysts.
Although PCs are usually designed as 3D nanostructures (e.g., opal and inverse opal), which are also discussed in this review (Section 3.4.2), TNT-PCs have been also reported [147,148]. TNT-PCs have been prepared by applying basic electrochemical anodization methods by alternating current and voltage, e.g., anodic oxidation [147] or two-step anodization [149,150,151]. The preparation techniques are critical for titania-based PCs, e.g., the roughness of the Ti foil before anodization has affected the light absorption properties, which are directly related to the periodic modulation properties [147,150]. The smoothness of the material surface after anodization would reflect the unique formation of PCs, which was observed by the appearance of various shiny colors (iridescence), as illustrated in Figure 12a [150]. TNT-PCs have been designed and explored in terms of the enhancement of the light harvesting. Many parameters of their preparation have been investigated, especially in consideration of the correlation between the photocatalytic activity and the PBG effects. For example, bamboo-like TNT-PC arrays with different nodes’ lengths were designed by adjusting and maintaining the applied voltage, as shown in Figure 12b–c [147]. Interestingly, it was found that PBG was bathochromic shifted with the shortening of TNT. Moreover, Chiarello et al. reported that the PBG position was directly related to the diameter of TNT, the distance between each nanotube, and their thickness [148]. It was concluded that the PBG positions shifted to longer wavelengths with an increase in the diameter of TNTs, resulting in an enhanced photogeneration of hydrogen.

3.3. Two-Dimensional Titania

It is probable that 2D titania nanostructures, such as nanofilms, nanoplates, nanosheets, and nanobelts are the least examined for photocatalysis among all others, especially comparing with a huge number of studies on particulate titania (0D), titania nanotubes (1D), and 3D titania nanostructures. The investigations on 2D nanostructures have been intensively growing in recent years, especially for solar energy conversion, e.g., with C3N4, MoS2, and graphene-like materials (graphene, graphene oxide, reduced graphene oxide), due to their relatively large surface area, fast transfer of charge carriers to the photocatalyst surface, and thus low recombination of charge carriers, and thus high photocatalytic activity. Although various materials have been prepared as 2D nanostructures, 2D titania nanostructures have been rarely prepared and reported. Of course, films of titania have been intensively studied and commercially applied (self-cleaning, antifogging, and antiseptic materials), but these are not 2D “nano” structures.
Although well-organized titania nanostructures might be easily obtained by a convenient hydrothermal method, such as nanowires (1D), nanotubes (1D), and faceted NPs (0D), other methods are used for the preparation of 2D titania nanostructures since titania films are usually deposited on some supports, such as glass, polyvinyl chloride, and quartz. Therefore, chemical vapor deposition (CVD) [152], plasma-enhanced chemical vapor deposition (PECVD) [153], dc-sputtering [154], anodization (exemplary shown in Figure 11 as “mesopores”) [155] and successive-ionic-layer-adsorption-and-reaction (SILAR) [156] methods have been proposed for the preparation of 2D titania photocatalysts. However, hydrothermal methods have been also used for the preparation of titania nanoplates (shown in Figure 13) for application as sensors to detect toxic gases [157].
An interesting study has been conducted by Deng et al. for amorphous titania nanofilms, prepared by the SILAR method from titanium isopropoxide [152]. Surprisingly, the authors obtained higher activity for formaldehyde mineralization on amorphous titania film than that on anatase film. It was concluded that abundant hydroxyl groups and a large surface area, allowing an efficient adsorption of formaldehyde, were responsible for its high photocatalytic activity. Moreover, it was proposed that rapid electron transfer along CB (>TiIIIOH)—which allocated terminal hydroxyl groups and minimized transfer length from bulk to surface (nanofilm layer)—resulted in a hindered recombination of charge carriers.
It should be pointed that similar to magnetic core–shell NPs (Section 3.1.1.) and TNT (1D), 2D titania materials have been mainly designed for possible commercial application, considering their continuous use. Therefore, 2D titania nanostructures have been tested for gas and liquid phases’ purification, e.g., (i) formaldehyde mineralization in the gas phase [152], (ii) the degradation of 2-(phosphomethylamino)acetic acid and methylene blue, and the inactivation of Micrococcus lutes bacteria in liquid phase [156]. All these studies concluded that titania nanofilms were highly recommended for commercial applications, e.g., using films inside photocatalytic reactors (covering of wall or lamp surfaces) for environmental remediation and disinfection purposes.
The various composites of titania with 2D nanostructures of other materials (titania/graphene, titania/graphene oxide, titania/reduced graphene oxide, and titania/C3N4) have been also investigated. Although these 2D nanostructures have been indicated as probably the most prospective for commercial applications, they are not discussed in this review, since well-written review papers have already been published, e.g., [158,159].

3.4. Three-Dimensional Titania

Three-dimensional titania structures, e.g., mesoporous structures, aerogels, opals, and inverse opals have been designed and prepared for two main purposes, i.e., possible commercial applications (to avoid the expensive recovery of particulate titania) and for mechanism study. Here, three kinds of 3D titania structures are shortly presented, i.e., aerogels, inverse opal photonic crystals (IO-PCs), and mesocrystals.

3.4.1. Titania Aerogels

Titania aerogels are very interesting nano/microstructures, due to their low density, porosity, high pore volume, and large specific surface area [160,161,162,163]. Although they are commonly composed of NPs of titania, the resultant aerogel structure is much larger, and thus titania aerogels might be classified as 3D structures. Titania aerogels has been proposed as efficient photocatalysts with activities similar to those by P25 [162]. An interesting approach was used by Pietron and coworkers to study the mechanism of plasmonic photocatalysis. Plasmonic photocatalysis, heterogeneous photocatalysis under vis irradiation on a wide bandgap semiconductor (usually titania) modified with plasmonic NPs, such as gold, silver, and copper (metals with localized surface plasmon resonance at vis range of solar spectrum), has been a hot topic recently [164]. Although plasmonic photocatalysts showed activity at broad irradiation ranges for various reactions, the mechanism has not been clarified yet. Two main mechanisms of plasmonic photocatalysis have been considered, i.e., energy and charge (usually electron) transfer. Accordingly, two kinds of titania aerogels have been prepared, i.e., with 5-nm gold NPs incorporated inside the 3D titania aerogel network (3D Au–TiO2) or not (Au/TiO2), as shown in Figure 14 [165,166]. Interestingly, it was found that 3D Au–TiO2 possessed much higher activity than that prepared by the simple deposition of gold on the surface of titania aerogel (Au/TiO2), suggesting that direct contact (interface) between gold and titania was crucial. Although authors discussed the possibilities of both mechanisms, the key factor of morphology (gold position inside the aerogel, resulting in a large interface of gold–titania) could suggest that the charge transfer mechanism, i.e., “hot” electron transfer from gold to the CB of titania, has been the predominant one in the case of gold–titania plasmonic photocatalysts under vis irradiation.

3.4.2. Inverse Opal Photonic Crystals (IO-PCs)

The 3D titania PCs (1D PCs in Section 3.2) have been intensively studied in recent years, due to the many advantages of the obtained meso/microporous structures, such as efficient mass transport, large surface area, and tunable porosity [167,168,169]. Inverse opal PCs (IO-PCs) comprise the exact replica structure obtained by using the opal as a template, as shown in Figure 15a. IO-PCs structures are usually obtained after undergoing three-step synthesis, namely (1) the self-assembly of monodispersed colloidal particles to form an opal structure, (2) infiltration of the precursor (here titania precursor), and finally (3) the removal of opal, which acts as a sacrificial template for the IO structures. All these steps are critical and need to be carried out by considering the optimum synthesis conditions to prevent defects and cracks, which could affect the final quality of the titania IO-PCs [170,171,172]. Different types of colloidal particles, e.g., silica (SiO2), polystyrene (PS), and polymethyl methacrylate (PMMA) have been used for IO formation. It should be pointed that the monodispersity of particles (<5% variation in the size/shape [171,173]) is necessary to form opal. The variety in the sizes and shapes of particles would result in disordered arrangement, which affects the quality of opal and IO structures. In contrast, some reports have claimed that polydispersed large SiO2 particles could also form close-packed arrays under negative pressure [174,175]. However, it should be pointed that for optical applications of opal and IO-PCs, monodispersed colloidal particles are the most recommended. The assembly process might be carried out by a few methods, i.e., vertical capillary deposition [176,177,178], spin coating [179], gravitational sedimentation [180], and sedimentation assisted by centrifugation [181]. Post-thermal treatment has been suggested as a good method to provide better mechanical stability and to form a neck between the particles [182]. Probably, the most difficult step is to obtain the stable IO structure, i.e., efficient titania infiltration/impregnation. Various approaches have been proposed for the IO structure, including drop casting and capillary force [178,183], vacuum infiltration [178,183], atomic layer deposition (ALD) [184,185], spin-coating [186], and chemical vapor deposition (CVD) [187]. Finally, the crystallization of titania is performed, which results in 15%–30% shrinkage of the structure, causing the pore size reduction [188,189,190,191]. The final step is to remove the template, which might be done by calcination (for polymers) [177,178,183,192] and chemical treatment with a concentrated base, sodium hydroxide, or hydrofluoric acid (for silica) [193]. In order to enhance light harvesting and/or the resultant properties and photocatalytic activities, modifications with metals, e.g., gold [179,191], silver [193], nickel [186,194], and semiconductors, e.g., cadmium selenide, cadmium sulfide quantum dots (QDs) [184,195], and zinc oxide NPs [196], have been proposed.
The PBG of IO-PCs could be tuned by various approaches, including the nanovoid diameter (as shown in Figure 15b,c) and the angle of the light irradiation. It was found that the PBG was hypsochromic shifted with an increase in the calcination temperature, resulting in a decrease in the nanovoids’ diameter [190]. The photocatalytic activities of IO-PCs have been investigated under both UV [149,197] and vis [186,191,193] irradiation, as summarized in Table 3. The most important property of IO structures is tunable nanovoid dimensions, resulting in the scattering and reflection of light. Many reports have reported how the nanovoid diameter could be related to the PBG, and thus the resultant photocatalytic activity. For example, the reduced velocity of photons and hindered recombination of charge carriers were the most evident for titania IO with the largest voids of 610 nm [198]. In contrast, the smallest voids in CdS-QD-sensitized titania IO gave the highest photoelectrochemical enhancements [184]. Hence, although the nanovoid diameter plays a vital role in PBG, the resultant photocatalytic activity might differ depending on various factors, including the specific surface area, porosity, and various co-modifications.
Apart from the nanovoid diameter, the slow photon effect, affected by the angle of irradiation source [183,188], would influence the overall photocatalytic activity. For example, PBG was blue-shifted with an increase in the irradiation angle [183,188,190]. Kim and Choi found that the degradation of stearic acid on IO with 247-nm voids was two times higher under irradiation at 40° than at different irradiation angles [147]. In contrast, Jovic et al. did not find the direct correlation between the voids’ sizes and the photocatalytic activity [183]. However, the overlapping between the bandgap of titania and the red-edge wavelength of PBG gave rise to the slow photon effect, and thus enhanced activity [183,188]. Similar correlation was found by Rahul et al. on the overlapping of the red-edge of PBG and the bandgap of titania by titania IO film with 215-nm voids [199], resulting in the complete decomposition of RhB dye in only 2 min. The importance of IO structures was confirmed in research on the comparison between original titania IOs and slightly destroyed structures, e.g., by ultrasonication [197] and grinding [189], where the undestroyed IOs structure was the most efficient.
Moreover, heterostructure engineering, e.g., the coupling of titania IO-PCs with other semiconductors and modifications with noble metals, has been shown to have a positive effect on the overall activity. Although various reasons have been proposed, including the separation of charge carriers, enhanced light harvesting, synergy, and a higher specific surface area, it is still unclear what properties are the most recommended for specific application. Considering photocatalytic reactions, it seems that the modification of titania IO-PCs with NPs/composites with the ability to absorb photons at or near PBG would result in photocatalytic materials of extremely high efficiency. Moreover, such materials might be very helpful for mechanism study, e.g., plasmonic photocatalysis (as discussed in Section 3.4.1). For example, the preparation of titania IO containing gold NPs (in each void) of the sizes corresponding to LSPR (localized surface plasmon resonance) of gold, the same or different to PBG of titania IO (λLSPR = λPBG, λLSPR > λPBG and λLSPR < λPBG), might enable finding (i) the direct correlation between the photoresponse and the properties (e.g., by action spectrum analysis), (ii) the difference between electron and energy transfer mechanism, and (iii) the key factor of photocatalytic activity.

3.4.3. Titania Mesocrystals

Mesocrystals are a quite new and fascinating nanostructure, which is composed of aligned nanocrystals [204,205,206]. The mesocrystal structure might be synthesized from the primary nanocrystals by self-assembly-based crystallization (non-classical crystallization, as shown in Figure 16 [204]) and a template method (e.g., polyethylene template [207]). For example, Wu et al. confirmed that rutile titania mesocrystals have been formed by the rapid crystallization by particle attachment (CPA), i.e., non-classical crystallization [208]. The mesocrystal structure has been considered as a potential functional material, combining both the properties of NPs (e.g., the shape, specific surface area, photocatalytic activity) and “advanced super structure” (e.g., increased crystallinity, improved charge transfer). Titania mesocrystals have been proposed for various applications, e.g., photocatalytic purification, solar energy conversion, and energy storage. Thanks to their 3D structure, titania mesocrystals can be also used for mechanism clarifications, such as titania aerogels (Section 3.4.1). For example, Bian et al. prepared titania mesocrystals with gold deposited either on basal or lateral surfaces of mesocrystals and on titania NPs [209]. They found that the deposition of gold on the basal structure resulted in enhanced photocatalytic activity, since “hot” electrons could easily migrate through the titania nanocrystal network to the edges of the plate-like mesocrystals, where they were temporary stored for further reactions. Due to this anisotropic electron flow, which hindered the recombination of charge carriers (back electron transfer to gold (Au→TiO2→Au) has been considered as the main reason for the low activity of plasmonic photocatalysts), this structure was characterized by long electron lifetime, and thus enhanced photocatalytic activity.

4. Summary and Conclusions

A large number of studies on the crystalline composition- and morphology-governed photocatalytic activity of titania-based materials have been extensively carried out until now. The advances performed in research in this field during the last decade are immensely significant and noteworthy. In this review, the state-of-the-art has been presented on the correlation between the structural/morphological aspects and TiO2 photocatalytic activity with a special emphasis directed on the new trends in the architecture design of titania and titania-based photocatalysts, such as faceted titania, inverse-opal structures, aerogels and core–shells.
First, the role of titania polymorphs for the photocatalytic activity has been discussed, including the exceptional importance of mixed-phase titania photocatalysts. In the literature, one can find many attempts to obtain such mixed-phase materials. They are applicable to the different types of photocatalytic reactions, such as the oxidation of organic pollutants, hydrogen generation, or CO2 conversion. Among them, a special role is played by commercially available P25, which is recognized as a reference material that is applied in various types of reactions due to its unique photocatalytic efficiencies. However, an explicit explanation of the phenomenon of the “synergistic effect” responsible for the high activity of mixed-phase TiO2 materials is still a challenge.
In the further part of the review, plenty of nanostructured titania materials have been discussed. The formal division of these materials into zero-, one-, two-, and three-dimensional structures has been performed and shortly summarized in Table 4. Among 0D titania nanostructures, the preparation of core–shell composites gives the possibility of incorporating additional functions to the photocatalytic titania-based materials, such as magnetic properties resulting from the presence of magnetic core, and thus enabling easy separation in the magnetic field. In the case of another 0D-structural approach: faceted titania particles, the role of different crystal facets, and the synergistic effect between them have been emphasized to clarify the high application potential of these materials, including the concept of facet-dependent reaction selectivity.
In the discussion of 1D and 3D nanostructures, the exceptional role of photonic nanocrystals in the form of titania nanotubes and 3D structures has been shown. The exceptional property of IO-PCs is the possibility of PBG tuning. Therefore, the modification of titania IO-PCs with other NPs (e.g., gold) or composites might introduce the ability of photoabsorption at or near the photonic bandgap, resulting in exceptionally efficient photocatalytic materials. Other promising 3D nanostructures can be recognized as titania aerogels with unique properties, such as low porosity, high pore volume, and large specific surface area, which are especially efficient as plasmonic photocatalysts with high visible-light activity. Two-dimensional (2D) structures (e.g., nanofilms, nanoplates, nanosheets), which are less often described in the literature, have been also considered in this review. Their application potential can be connected with environmental remediation and disinfection purposes (nanofilms).
The proper selection of the type of titania nanostructure together with considering the heterostructure engineering (e.g., coupling) or titania modification is the complex strategy to design the photocatalytic materials, which are oriented for the specific reaction systems. Despite the wide discussion about the mechanisms and reasons responsible for the specific properties of different morphological titania structures, it is still unclear which properties are the most recommended for the specified application. However, this review might be useful for the architecture design of novel titania-based nanomaterials for broad and diverse applications.

Funding

This research was financially supported by “Yugo-Sohatsu Kenkyu” for an Integrated Research Consortium on Chemical Sciences (IRCCS) project from Ministry of Education and Culture, Sport, Science and Technology-Japan (MEXT), and National Natural Science Foundation of China (NSFC; contract No. 51802087).

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

0D—zero-dimensional; 1D—one-dimensional; 2D—two-dimensional; 3D—three-dimensional; CB—conduction band; CVD—chemical vapor deposition; DAP—decahedral anatase particles; DRS—diffuse reflection spectrum; DSSCs—dye-sensitized solar cells; EISA—evaporation-induced self-assembly; EPR—electron spin resonance spectroscopy; ET(s)—electron trap(s); (g)—gas phase; IO—inverse opal; IO-PCs—inverse opal photonic crystals; IPCT—interparticle charge transfer; NP(s)—nanoparticle(s); OAP—octahedral anatase particles; P25—commercial titania with high photocatalytic activity from Evonik (known also as Degussa P25 and Aeroxide P25); PBG—photonic bandgap; PC(s)—photonic crystal(s); PECVD—plasma enhanced chemical vapor deposition; PMMA—polymethyl methacrylate; PS—polystyrene; QD(s)—quantum dot(s); ROS—reactive oxygen species; SILAR—successive-ionic-layer-adsorption-and-reaction; VB—valence band; TBu—titanium butoxide; TIP—titanium isopropoxide; TNT(s)—titania nanotube(s); TNT-PCs—titania nanotube-photonic crystals; vis—visible light.

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Figure 1. Number of papers published annually on: (a) titania (searched in Web of Science using “titania” or “TiO2” or “titanium dioxide” or “titanium(IV) oxide”) and (b) titania polymorphs: anatase (○), rutile (□), and brookite (◊) (searched in Web of Science using “anatase”, “rutile”, and “brookite”, respectively).
Figure 1. Number of papers published annually on: (a) titania (searched in Web of Science using “titania” or “TiO2” or “titanium dioxide” or “titanium(IV) oxide”) and (b) titania polymorphs: anatase (○), rutile (□), and brookite (◊) (searched in Web of Science using “anatase”, “rutile”, and “brookite”, respectively).
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Figure 2. Number of cited papers published annually on: P25 photocatalysis (searched in Web of Science using: “P25” and “photocatalysis”; November 2019).
Figure 2. Number of cited papers published annually on: P25 photocatalysis (searched in Web of Science using: “P25” and “photocatalysis”; November 2019).
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Figure 3. Schematic diagram of possible band alignments between anatase and rutile. Reprinted with permission from [93]. Copyright 2015 Creative Commons Attribution.
Figure 3. Schematic diagram of possible band alignments between anatase and rutile. Reprinted with permission from [93]. Copyright 2015 Creative Commons Attribution.
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Figure 4. Synergism between anatase and rutile for: (a) grinded titania samples (synergism observed only for fine anatase ST-01 mixed with rutile NS-51 (■), but not for two anatase samples: ST-01 and ST-41 (▲)) and (b) anatase sample thermally treated (synergism observed for mixed-phase titania obtained at 700–900 °C containing 27%–96.8% rutile; Copyright 2013 Elsevier.
Figure 4. Synergism between anatase and rutile for: (a) grinded titania samples (synergism observed only for fine anatase ST-01 mixed with rutile NS-51 (■), but not for two anatase samples: ST-01 and ST-41 (▲)) and (b) anatase sample thermally treated (synergism observed for mixed-phase titania obtained at 700–900 °C containing 27%–96.8% rutile; Copyright 2013 Elsevier.
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Figure 5. The properties of isolated samples and P25: (a) XRD patterns and (b) DRS spectra for anatase (ANA, red), rutile (RUT, blue) and P25 (HomoP25, black; P25 sample was first homogenized). Copyright 2018 Elsevier.
Figure 5. The properties of isolated samples and P25: (a) XRD patterns and (b) DRS spectra for anatase (ANA, red), rutile (RUT, blue) and P25 (HomoP25, black; P25 sample was first homogenized). Copyright 2018 Elsevier.
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Figure 6. Core–interlayer–shell NPs composed of Fe3O4 (core), SiO2 (interlayer), and TiO2 (shell): (a) Nanostructure scheme; and (b) Mapping of Fe3O4, SiO2, and TiO2 in Fe3O4/SiO2/TiO2) photocatalyst prepared by the microemulsion method (Fe–red, Si–green, and Ti–blue); adapted from [110]. Copyright 2017 Creative Commons Attribution.
Figure 6. Core–interlayer–shell NPs composed of Fe3O4 (core), SiO2 (interlayer), and TiO2 (shell): (a) Nanostructure scheme; and (b) Mapping of Fe3O4, SiO2, and TiO2 in Fe3O4/SiO2/TiO2) photocatalyst prepared by the microemulsion method (Fe–red, Si–green, and Ti–blue); adapted from [110]. Copyright 2017 Creative Commons Attribution.
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Figure 7. Equilibrium crystal shape of: (a) anatase (adapted with permission from [117]. Copyright 2011 American Physical Society); (b) rutile (adapted with permission from [118]. Copyright 1994 American Physical Society); (c) brookite (adapted with permission from [119]. Copyright 2007 American Physical Society); (d) Main crystal shapes of titania (adapted with permission from [117]. Copyright 2014 American Chemical Society).
Figure 7. Equilibrium crystal shape of: (a) anatase (adapted with permission from [117]. Copyright 2011 American Physical Society); (b) rutile (adapted with permission from [118]. Copyright 1994 American Physical Society); (c) brookite (adapted with permission from [119]. Copyright 2007 American Physical Society); (d) Main crystal shapes of titania (adapted with permission from [117]. Copyright 2014 American Chemical Society).
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Figure 8. Shape-controlled preparation of faceted titania particles; adapted with permission from [124]. Copyright 2009 American Chemical Society.
Figure 8. Shape-controlled preparation of faceted titania particles; adapted with permission from [124]. Copyright 2009 American Chemical Society.
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Figure 9. SEM images of decahedral anatase particles (DAP) samples prepared at different TiCl4 vapor concentrations: (a) 0.34 vol%; (b) 0.86 vol%; (c) 1.30 vol%, and (d) 2.13 vol%; adapted with permission from [78]. Copyright 2016 Elsevier.
Figure 9. SEM images of decahedral anatase particles (DAP) samples prepared at different TiCl4 vapor concentrations: (a) 0.34 vol%; (b) 0.86 vol%; (c) 1.30 vol%, and (d) 2.13 vol%; adapted with permission from [78]. Copyright 2016 Elsevier.
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Figure 10. Schematic images of: (ac) spatial separation of redox sites on anatase with exposed crystal facets: (a) DAP with a larger surface area of oxidation sites and smaller area of reduction sites; (b) DAP with a smaller surface area of oxidation sites and larger area of reduction sites; (c) stacked-structure particle with larger surface area of both oxidation and reduction sites (adapted with permission from [121]; Copyright 2009 American Chemical Society); (d) dependence of aspect ratio of shaped-controlled rutile rods on the surface area ratio of oxidation to reduction sites (adapted with permission from [123]; Copyright 2011 American Chemical Society).
Figure 10. Schematic images of: (ac) spatial separation of redox sites on anatase with exposed crystal facets: (a) DAP with a larger surface area of oxidation sites and smaller area of reduction sites; (b) DAP with a smaller surface area of oxidation sites and larger area of reduction sites; (c) stacked-structure particle with larger surface area of both oxidation and reduction sites (adapted with permission from [121]; Copyright 2009 American Chemical Society); (d) dependence of aspect ratio of shaped-controlled rutile rods on the surface area ratio of oxidation to reduction sites (adapted with permission from [123]; Copyright 2011 American Chemical Society).
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Figure 11. Different morphologies of metal oxides obtained by metal anodization, adapted with permission from [5]. Copyright 2013 Elsevier.
Figure 11. Different morphologies of metal oxides obtained by metal anodization, adapted with permission from [5]. Copyright 2013 Elsevier.
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Figure 12. Characteristics of titania nanotubes photonic crystals (TNT-PCs): (a) UV-vis-NIR diffuse reflectance absorption spectra, the inset presents a digital photograph of the TNT-PCs; adapted with permission from [150]. Copyright 2014 Copyright Clearance Center, Inc.; (b) FE-SEM image and (c) diffraction spectrum of TNT-PCs arrays (prepared TiO2 with different node lengths); adapted with permission from [147]. Copyright 2017 Elsevier.
Figure 12. Characteristics of titania nanotubes photonic crystals (TNT-PCs): (a) UV-vis-NIR diffuse reflectance absorption spectra, the inset presents a digital photograph of the TNT-PCs; adapted with permission from [150]. Copyright 2014 Copyright Clearance Center, Inc.; (b) FE-SEM image and (c) diffraction spectrum of TNT-PCs arrays (prepared TiO2 with different node lengths); adapted with permission from [147]. Copyright 2017 Elsevier.
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Figure 13. Electron microscopy images of titania nanoplates: (a,b) SEM and (c) TEM images; insets presenting (a) nanoplate shapes; (b) a crystal model of single crystalline anatase; adapted with permission from (after formatting) [157]. Copyright 2016 Elsevier.
Figure 13. Electron microscopy images of titania nanoplates: (a,b) SEM and (c) TEM images; insets presenting (a) nanoplate shapes; (b) a crystal model of single crystalline anatase; adapted with permission from (after formatting) [157]. Copyright 2016 Elsevier.
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Figure 14. Schemes showing titania aerogels (blue) with gold NPs (yellow dots): (a) incorporated inside the titania network (3D Au–TiO2); and (b) deposited on the surface of titania aerogels (Au/TiO2); drawn based on references [165,166].
Figure 14. Schemes showing titania aerogels (blue) with gold NPs (yellow dots): (a) incorporated inside the titania network (3D Au–TiO2); and (b) deposited on the surface of titania aerogels (Au/TiO2); drawn based on references [165,166].
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Figure 15. Characteristics of opal (polystyrene (PS) spheres) and inverse opal (IO) titania structures: (a) FESEM images of PS opal with: (i) 200-nm and (ii) 250-nm sphere sizes, and replicated titania IO films with: (iii) 180-nm and (iv) 207-nm voids; reflection spectra of: (b) PS templates and (c) titania IOs; adapted with permission from [178]. Copyright 2019 Creative Commons Attribution.
Figure 15. Characteristics of opal (polystyrene (PS) spheres) and inverse opal (IO) titania structures: (a) FESEM images of PS opal with: (i) 200-nm and (ii) 250-nm sphere sizes, and replicated titania IO films with: (iii) 180-nm and (iv) 207-nm voids; reflection spectra of: (b) PS templates and (c) titania IOs; adapted with permission from [178]. Copyright 2019 Creative Commons Attribution.
Catalysts 09 01054 g015
Figure 16. Schematic drawings of mesocrystal formation (red route) and classical crystal growth (blue route) [204]. Copyright 2012 American Chemical Society.
Figure 16. Schematic drawings of mesocrystal formation (red route) and classical crystal growth (blue route) [204]. Copyright 2012 American Chemical Society.
Catalysts 09 01054 g016
Table 1. Properties of titania polymorphs.
Table 1. Properties of titania polymorphs.
RutileAnataseBrookite
atomic spacing (Å) aa = 4.596
c = 2.958
a = 3.793
c = 9.510
a = 5.456
b = 9.182
c = 5.143
axial ratiosa:c = 1:0.64388a:c = 1:2.50725a:b:c = 0.5942:1:0.5601
molecule/unit cell a248
density (g cm−3) a4.253.884.12
crystal system atetragonal (ditetragonal dipyramidal)tetragonal (ditetragonal dipyramidal)orthorombic (dipyramidal)
bandgap (eV)3.02 d3.23 d3.14 e
absorption edge (nm)calculated b410384395
measured c413 f388 f395 e
a—data from webmineral.com; b—calculated from bandgap energy; c—experimentally estimated from absorption spectra; d–f—exemplary data (might slightly differ depending on properties): d—[15]; e —[16]; f —[17].
Table 2. Mixed-phase TiO2 photocatalysts: synthesis method, specific surface area, and applications.
Table 2. Mixed-phase TiO2 photocatalysts: synthesis method, specific surface area, and applications.
Ti PrecursorMethodCrystalline Structure aBET bApplicationRef.
TIPsol–gelA(21%–86%)
R(14%–79%)
50–94degradation of MB,
phenol, and 4ClP
[18]
TIPsol–gelA(60%–83%)/B(17%–40%)115conversion of CO2[19]
TIPsol–gelA(76.2%–84.6%)/B(12.8%–3.8%)/R(0%–2.6%)48–276degradation of MB[20]
TIPsol–gelA(53%–96%)/R(4%–47%)62–90-[21]
TIPsol–gel (+ US)A(79.6%–91.6%)/B(9.4%–20.4)75.5oxidation of acetone[22]
TIPsonochemicalA(49.5%–60%)/B(40%–50.5%)112–172n-pentane oxidation[23]
TIPEISAA(29%–95%)/R(5%–71%)145–161generation of H2[24]
TIP/HClEISAA(36%–70%)
R(19%–61%)/B(3%–25%)
96.1–77.8water splitting[25]
TBusol-gelA(65.5%–80.1%)/R(17.7%–34%)134–169CH3OH oxidation[26]
TBusol-gelA(18.1%–73.3%/R(16.8%–52.7%)35–165phenol degradation[27]
TBusol-gel and H2 plasmaA/B271.8–427.5degradation of phenol, RhB and RB5[28]
TBuhydrothermalA(72%)/R(28%)54.1lithium-ion batteries[29]
TBuhydrothermalA/B129–174HCHO degradation[30]
TBuhydrothermalA(64.9%)/R(35.1%)260MB degradation, H2 evolution[31]
TBuhydrothermal self-assemblyA(32–61.4
R(0%–20%)/B’(0%–48%)
104.3–129.9reduction of nitrate[32]
TBucoordination- self-assemblyA(40%–77%)
R(23%–60%)
63.6–78.4water splitting[33]
TBuelectrospinningA(94.6%)
R(5.4%)
27.2H2 generation and RhB degradation[34]
TBu/TiCl4EISAA(73%–98%)/R(2%–27%)92–135phenol degradation[35]
TBu/TiCl4EISAA(64%–91%)/R(9%–30%)90–118water splitting[25]
TBu/TiCl4EISAA(74%–92%)/R(8%–26%)73H2 generation[36]
K2Ti2O5soft-chemistry template-freeA/B’129degradation of CHCl3 and H2 generation[37]
Ti2(SO4)3hydrothermalA(64%–87%)/B(13%–36%)30–72IBU degradation[38]
Ti2(SO4)3hydrothermalC/N-A/B20.5–66MC-LR degradation[39]
Ti2(SO4)3hydrothermalA(61.8%)
B(38.2%)
62.3IBU, 4AAP, CYN, and MC-LR degradation[40,41,42,43]
Crystalline structure a—A: anatase, B: brookite, R: rutile, B’: TiO2(B); BET b—specific surface area (m2 g−1); TIP—titanium isopropoxide; TBu—titanium butoxide; US—ultrasounds; EISA—Evaporation-induced self-assembly (EISA); RhB—Rhodamine B; MB—methylene blue; 4ClP—4-chloro phenol; RB5—reactive black 5; MC-LR—microcystin-LR; CYN—cylindrospermopsin; IBU—ibuprofen; 4AAP—4-amino antipyrine.
Table 3. Titania IO-PCs: preparation method, applications and important findings.
Table 3. Titania IO-PCs: preparation method, applications and important findings.
IO-PCsOpal/TemplateInfiltration MethodPhotocatalytic TestsFindingsRef.
IO filmPMMACFSA degr. at 400 nm (0–50°)PBG max = 490 nm
most effective at 40°
[192]
IO filmsPSCFMB degr.; λ = 450 nm
CH3CHO(g) degr. at 365 nm
high activity [188]
IO filmsPSALDIO as photoanode (DSSCs)PCE of 2.22%[185]
IO filmsPSCFRhB degr. at 250800 nm95% degr. at 0° [190]
IO filmsPSSWRhB degr. λ = 320–800 nmVSDA[200]
LP-IO filmPMMACFMB degr. at 400 nmhigh film stability[177]
IO film with LCPSSW-vacuum-PBGmax = 452 and 528 nm
switchable PCs
[178]
IO powd. and filmsPMMAvacuum
CF
ethanol(g) oxidation
λmax = 365 nm
VSDA[183]
IO powd.PSCFMB degr. λ >400 nmVSDA[198]
IO powd.PMMAvacuumphenol degr. λmax = 365 nmVSDA[189]
Au/ IO powd.PMMACFwater splitting (H2)
under UV and solar
Au/IO—H2 generation[201]
Au/IO filmPSCFPEC water splittingAu/st-IO with high PD[179]
Ag/IO filmSiO2dip coatingMB degr. λ = 254 nm
and λ = 400–760 nm
Ag-enhanced activity[193]
CdSe/IOPSdip coatingSERS for MO
PEC measurement
CdSe-IO as SERS substrate with a high sensitivity[195]
CdS/IO filmPSALDPEC; solarCdS/IO with high PD[184]
Ni/
IO film
PSspin coatingSERS for 4-MBA
MO degr. λ >400 nm
Ni/IO for SERS with high detection ability[186]
BIO/Ni filmPSSW and CFCO evolution; λ >400 nmhigh rate of CO generation[194]
Au/F/IO filmPSSW and CFH2 evolution; λ >400 nmhigh rate of H2 generation[202]
(N-F)/IO filmPSSW aided by CFRhB degr.; solarPBG max = 336 and 850 nm
100% RhB degr.
[199]
IO/AuPSLPD2,4-DCP degr.; λ >420 nmVSDA[191]
H/IO powderPSforced impregn.H2 evolution; solarVSDA[203]
ZnO/IO filmPSSWMO degr.; λ = 320–800 nmVSDA[196]
2,4-DCP—2,4-dichlorophenol; 4-MBA—4-mercaptobenzoic acid; ALD—atomic layer deposition; Au—gold NPs; Au/F/IO—gold-loaded fluorinated IO; BIO—black titania inverse opal; CF—capillary force; degr.—degradation; DSSCs—dye-sensitized solar cells; FTiO2-IO: fluorinated titania inverse opal; g—gas phase; H—hydrogenated; IO—titania inverse opals; IO-PCs: inverse opal photonic crystals. LC—liquid crystal; LP—liquid precursor; LPD—liquid phase deposition; MB—methylene blue; MO—methyl orange; (N-F)—nitrogen and fluorine co-doped; NP—nanoparticle; PBG—photonic bandgap; PCE—power conversion efficiency; PEC—photoelectrochemical; PD—photocurrent density; PMMA—polymethylmethacrylate; PCs—Photonic crystals; PS—polystyrene; RhB—rhodamine B; SA—stearic acid; SERS—surface-enhanced Raman scattering; solar—experiments under solar irradiation; st—surface-textured; SW—sandwich; VSDA—void-size-dependent activity.
Table 4. Structure, synthesis, and exemplary applications of titania materials with different morphology. CVD: chemical vapor deposition, PECVD: plasma-enhanced chemical vapor deposition, SILAR: successive-ionic-layer-adsorption-and-reaction.
Table 4. Structure, synthesis, and exemplary applications of titania materials with different morphology. CVD: chemical vapor deposition, PECVD: plasma-enhanced chemical vapor deposition, SILAR: successive-ionic-layer-adsorption-and-reaction.
DimensionMorphologyPreparationSchemesApplicationsReferences
0Dnanorice, core–shells, NPs faceted particlessol–gel, hydrothermal, solvothermal, gas-phase Catalysts 09 01054 i001antifogging, antiseptic, self-cleaning surfaces, photocatalysts [78,112,113,114,115,116,117,118,119,120,121,122]
1Dnanotubes, nanowires, nanorodsanodization, hydrothermal Catalysts 09 01054 i002solar cells, batteries, photocatalysts [131,132,133,134,135,136,137,138,139,140,141,142,143,144,145]
2Dnanoplates, nanosheets, nanolayershydrothermal, solvothermal, CVD, PECVD, SILAR Catalysts 09 01054 i003sensors for toxic gas, photocatalysts, solar cells[150,151,152,153,154,155,210]
3Daerogels, inverse-opal photonic crystals, mesocrystalsSol–gel, template, self-assembling, hydrothermal Catalysts 09 01054 i004photocatalysts, solar cells, energy storage,[158,159,160,161], [163,164], [168,169,170], [202,203,204]

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Wang, K.; Janczarek, M.; Wei, Z.; Raja-Mogan, T.; Endo-Kimura, M.; Khedr, T.M.; Ohtani, B.; Kowalska, E. Morphology- and Crystalline Composition-Governed Activity of Titania-Based Photocatalysts: Overview and Perspective. Catalysts 2019, 9, 1054. https://doi.org/10.3390/catal9121054

AMA Style

Wang K, Janczarek M, Wei Z, Raja-Mogan T, Endo-Kimura M, Khedr TM, Ohtani B, Kowalska E. Morphology- and Crystalline Composition-Governed Activity of Titania-Based Photocatalysts: Overview and Perspective. Catalysts. 2019; 9(12):1054. https://doi.org/10.3390/catal9121054

Chicago/Turabian Style

Wang, Kunlei, Marcin Janczarek, Zhishun Wei, Tharishinny Raja-Mogan, Maya Endo-Kimura, Tamer M. Khedr, Bunsho Ohtani, and Ewa Kowalska. 2019. "Morphology- and Crystalline Composition-Governed Activity of Titania-Based Photocatalysts: Overview and Perspective" Catalysts 9, no. 12: 1054. https://doi.org/10.3390/catal9121054

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