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

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.

The Influence of The Light-Activated Titania P25 on Human Breast Cancer Cells

“…However, according to the producer, a supplied product does not occur in this form, but as tightly bonded aggregates formed during production [16], as also observed in this study (Figure 1a,b). P25 is a light-sensitive semiconductor of excellent photocatalytic activity [16,17,27,29,33]. When titania absorbs light with an energy equal or larger than its bandgap, i.e., ca.…”

“…Similarly, various literature reports have shown both extremely high and property-unreasonable activity of P25 titania. Therefore, it has been proposed that charge carriers' transfer between two titania polymorphs (anatase/rutile) could be responsible for the high activity of P25 [24][25][26][27], as well for other mixed-phase titania samples, e.g., anatase/brookite [27,28]. The co-existence of two phases with high intrinsic activities in different reaction systems could also be responsible for high activity of P25, i.e., anatase in photo-oxidation and rutile in photo-reduction reactions, as suggested for single-phase samples (pure anatase and pure rutile) isolated from the original P25 photocatalyst [16,29].…”

“…The co-existence of two phases with high intrinsic activities in different reaction systems could also be responsible for high activity of P25, i.e., anatase in photo-oxidation and rutile in photo-reduction reactions, as suggested for single-phase samples (pure anatase and pure rutile) isolated from the original P25 photocatalyst [16,29]. Accordingly, P25 has been widely used for various photocatalytic studies, mainly because of high activity, but also as a "standard" [27] during measurements of photocatalytic activities of various samples in different laboratories (it should be noticed that reliable comparison of photocatalysts is still challenging, even with apparent (not true) quantum yield estimations [30][31][32]). However, it should be pointed that the composition of P25 might slightly differ between samples, and even samples taken from the same container could show divergence, e.g., the content of anatase, rutile and noncrystal part varies from 76% to 80%, 13% to 15% and 6% to 11%, respectively [29,33].…”

Preparation and Characterization of Defective TiO2. The Effect of the Reaction Environment on Titanium Vacancies Formation

“…It results from the synergic effect of anatase and rutile [47], and it is in agreement with the previously described shifting of the absorbance spectrum maximum towards higher wavelengths (see in Figure 2). The TBT-TiO 2 sample (anatase with a minority of brookite) showed negligible photocatalytic activity in the visible light range [48][49][50].…”

Morphology-Governed Performance of Plasmonic Photocatalysts

“…The recombination results in an obvious decrease in photocatalytic efficiency, reaching quantum yields much lower than expected 100%. Accordingly, various methods have been proposed to limit this recombination [6], including properties' improvements, e.g., higher crystallinity, less defects' content, perfect crystal morphology (faceted particles) [7][8][9][10][11], larger content of shallow than deep electron traps (ETs) [12]), and various modifications, e.g., doping [13][14][15], surface modifications [16][17][18][19], heterojunction formation (coupled semiconductors) [20][21][22]. One of the most efficient methods to inhibit the charge carriers' recombination is the surface modification of photocatalyst with deposits of noble metals, because noble metals work as an electrons' scavenger ( Figure 1b) because of larger work function of metals than the electron affinity of oxide semiconductors, as first found for platinum-modified titania almost half century ago by Bard [23].…”