Layered TiO2 Composed of Anatase Nanosheets with Exposed {001} Facets: Facile Synthesis and Enhanced Photocatalytic Activity

Photocatalysis and Li-Ion Battery Applications of {001} Faceted Anatase TiO2-Based Composites

Anatase TiO2 are the most widely used photocatalysts because of their unique electronic, optical and catalytic properties. Surface chemistry plays a very important role in the various applications of anatase TiO2 especially in the catalysis, photocatalysis, energy conversion and energy storage. Control of the surface structure by crystal facet engineering has become an important strategy for tuning and optimizing the physicochemical properties of TiO2. For anatase TiO2, the {001} crystal facets are the most reactive because they exhibit unique surface characteristics such as visible light responsiveness, dissociative adsorption, efficient charge separation capabilities and photocatalytic selectivity. In this review, a concise survey of the literature in the field of {001} dominated anatase TiO2 crystals and their composites is presented. To begin, the existing strategies for the synthesis of {001} dominated anatase TiO2 and their composites are discussed. These synthesis strategies include both fluorine-mediated and fluorine-free synthesis routes. Then, a detailed account of the effect of {001} facets on the physicochemical properties of TiO2 and their composites are reviewed, with a particular focus on photocatalysis and Li-ion batteries applications. Finally, an outlook is given on future strategies discussing the remaining challenges for the development of {001} dominated TiO2 nanomaterials and their potential applications.

Surface engraving engineering of polyhedral photocatalysts

Surface engraving engineering of polyhedral photocatalysts is overviewed based on synthetic strategies and engraved surface-related photocatalytic mechanisms. Some challenges and perspectives are also proposed.

Hexafluorotitanic acid-assisted synthesis of large-sized, ultrathin titania nanosheets as multifunctional and high-performance photocatalysts

A fast cyclohexanol solvothermal pathway was developed to prepare highly dispersed anatase titania (TiO₂) nanosheets with an edge length of ∼800 nm and a thickness of ∼6 nm. Under the synergistic control of hexafluorotitanic acid and cyclohexanol, the exposed 001 facets reached ∼98.5% on the TiO₂ nanosheets obtained by the treatment at 180 °C for 150 min. Moreover, it was found that the phase transformation and structural development drastically occurred during the solvothermal treatment. When used as a catalyst for photodegradation of rhodamine B, the TiO₂ nanosheets exhibited a good recycling stability and a much higher photocatalytic degradation rate (nearly 99% degradation within 2.5 h) than the commercial P25 (93.6%). After being uniformly loaded with 4 wt.% of the Pt nanoparticles, the TiO₂ nanosheets displayed a photocatalytic hydrogen production rate of 2.239 mmol g^-1 h^-1 under simulated solar light, which was much higher than the pristine TiO₂ nanosheets (0.045 mmol g^-1 h^-1) as well as most of the reported TiO₂-based photocatalysts. The remarkable photocatalytic activity and good stability of the TiO₂ nanosheets with highly exposed 001 facets would make them find potential applications in both water treatment and hydrogen generation from water splitting under solar light.

Mesocrystals for photocatalysis: a comprehensive review on synthesis engineering and functional modifications

Mesocrystals are a new class of superstructures that are generally made of crystallographically highly ordered nanoparticles and could function as intermediates in a non-classical particle-mediated aggregation process. In the past decades, extensive research interest has been focused on the structural and morphogenetic aspects, as well as the growth mechanisms, of mesocrystals. Unique physicochemical properties including high surface area and ordered porosity provide new opportunities for potential applications. In particular, the oriented interfaces in mesocrystals are considered to be beneficial for effective photogenerated charge transfer, which is a promising photocatalytic candidate for promoting charge carrier separation. Only recently, remarkable advances have been reported with a special focus on TiO2 mesocrystal photocatalysts. However, there is still no comprehensive overview on various mesocrystal photocatalysts and their functional modifications. In this review, different kinds of mesocrystal photocatalysts, such as TiO2 (anatase), TiO2 (rutile), ZnO, CuO, Ta2O5, BiVO4, BaZrO3, SrTiO3, NaTaO3, Nb3O7(OH), In2O3-x (OH) y , and AgIn(WO4)2, are highlighted based on the synthesis engineering, functional modifications (including hybridization and doping), and typical structure-related photocatalytic mechanisms. Several current challenges and crucial issues of mesocrystal-based photocatalysts that need to be addressed in future studies are also given.

Citrate/F− assisted phase control synthesis of TiO2 nanostructures and their photocatalytic properties

A novel and controlled sol-hydrothermal synthesis method of nano-TiO₂ assisted by citrate/F⁻ has been developed. The photocatalytic activity of the synthesized TiO₂ was superior to the commercially available TiO₂.

Sandwichlike magnesium silicate/reduced graphene oxide nanocomposite for enhanced Pb²⁺ and methylene blue adsorption

A sandwichlike magnesium silicate/reduced graphene oxide nanocomposite (MgSi/RGO) with high adsorption efficiency of organic dye and lead ion was synthesized by a hydrothermal approach. MgSi nanopetals were formed in situ on both sides of RGO sheets. The nanocomposite with good dispersion of nanopetals exhibits a high specific surface area of 450 m(2)/g and a good mass transportation property. Compared to MgSi and RGO, the mechanical stability and adsorption capacity of the nanocomposite is significantly improved due to the synergistic effect. The maximum adsorption capacities for methylene blue and lead ion are 433 and 416 mg/g, respectively.

DSSC anchoring groups: a surface dependent decision

Electrodes in dye sensitised solar cells are typically nanocrystalline anatase TiO2 with a majority (1 0 1) surface exposed. Generally the sensitising dye employs a carboxylic anchoring moiety through which it adheres to the TiO₂ surface. Recent interest in exploiting the properties of differing TiO₂ electrode morphologies, such as rutile nanorods exposing the (1 1 0) surface and anatase electrodes with high percentages of the (0 0 1) surface exposed, begs the question of whether this anchoring strategy is best, irrespective of the majority surface exposed. Here we address this question by presenting density functional theory calculations contrasting the binding properties of two promising anchoring groups, phosphonic acid and boronic acid, to that of carboxylic acid. Anchor-electrode interactions are studied for the prototypical anatase (1 0 1) surface, along with the anatase (0 0 1) and rutile (1 1 0) surfaces. Finally the effect of using these alternative anchoring groups to bind a typical coumarin dye (NKX-2311) to these TiO₂ substrates is examined. Significant differences in the binding properties are found depending on both the anchor and surface, illustrating that the choice of anchor is necessarily dependent upon the surface exposed in the electrode. In particular the boronic acid is found to show the potential to be an excellent anchor choice for electrodes exposing the anatase (0 0 1) surface.

Highly reactive {001} facets of TiO2-based composites: synthesis, formation mechanism and characterization

Titanium dioxide (TiO2) is one of the most widely investigated metal oxides due to its extraordinary surface, electronic and catalytic properties. However, the large band gap of TiO2 and massive recombination of photogenerated electron-hole pairs limit its photocatalytic and photovoltaic efficiency. Therefore, increasing research attention is now being directed towards engineering the surface structure of TiO2 at the most fundamental and atomic level namely morphological control of {001} facets in the range of microscale and nanoscale to fine-tune its physicochemical properties, which could ultimately lead to the optimization of its selectivity and reactivity. The synthesis of {001}-faceted TiO2 is currently one of the most active interdisciplinary research areas and demonstrations of catalytic enhancement are abundant. Modifications such as metal and non-metal doping have also been extensively studied to extend its band gap to the visible light region. This steady progress has demonstrated that TiO2-based composites with {001} facets are playing and will continue to play an indispensable role in the environmental remediation and in the search for clean and renewable energy technologies. This review encompasses the state-of-the-art research activities and latest advancements in the design of highly reactive {001} facet-dominated TiO2via various strategies, including hydrothermal/solvothermal, high temperature gas phase reactions and non-hydrolytic alcoholysis methods. The stabilization of {001} facets using fluorine-containing species and fluorine-free capping agents is also critically discussed in this review. To overcome the large band gap of TiO2 and rapid recombination of photogenerated charge carriers, modifications are carried out to manipulate its electronic band structure, including transition metal doping, noble metal doping, non-metal doping and incorporating graphene as a two-dimensional (2D) catalyst support. The advancements made in these aspects are thoroughly examined, with additional insights related to the charge transfer events for each strategy of the modified-TiO2 composites. Finally, we offer a summary and some invigorating perspectives on the major challenges and new research directions for future exploitation in this emerging frontier, which we hope will advance us to rationally harness the outstanding structural and electronic properties of {001} facets for various environmental and energy-related applications.

Principles and mechanisms of photocatalytic dye degradation on TiO2based photocatalysts: a comparative overview

Pictorial representation of all possible dye degradation reaction in UV light initiated indirect dye degradation mechanism. This mechanism is practically more important over visible light initiated direct mechanism.

Blue phase-mixed anatase–rutile TiO2 heterogeneous junction composites: surface defect-induced reconstruction by F ion and superior lithium-storage properties

The surface defect reconstruction of blue phase-mixed anatase–rutile TiO₂ heterogeneous junction composites plays a crucial role in the enhancement of lithium-ion batteries.

Mesocrystals as a class of multifunctional materials

Mesocrystals that consist of crystallographically aligned individual building blocks and controlled level of porosity in between exhibit unique structures and multifunctional behavior.