Anatase TiO2Mesocrystals Enclosed by (001) and (101) Facets: Synergistic Effects between Ti3+and Facets for Their Photocatalytic Performance

Remarkable Enhancement of Photocatalytic Activity of High-Energy TiO2 Nanocrystals for NO Oxidation through Surface Defluorination

The superior photocatalytic activity of TiO2 nanocrystals with exposed high-energy (001) facets, achieved through the use of hydrofluoric acid as a shape-directing reagent, is widely reported. However, in this study, we report for the first time the detrimental effect of surface fluorination on the photoreactivity of high-energy faceted TiO2 nanocrystals towards NO oxidation (resulting in a NO removal rate of only 5.9%). This study aims to overcome this limitation by exploring surface defluorination as an effective strategy to enhance the photocatalytic oxidation of NO on TiO2 nanocrystals enclosed with (001) facets. We found that surface defluorination, achieved through either NaOH washing (resulting in an improved NO removal rate of 23.2%) or calcination (yielding an enhanced NO removal rate of 52%), leads to a large increase in the photocatalytic oxidation of NO on TiO2 nanocrystals with enclosed (001) facets. Defluorination processes stimulate charge separation, effectively retarding recombination and significantly promoting the production of reactive oxygen species, including superoxide radicals (·O2-), singlet oxygen (1O2), and hydroxyl radicals (·OH). Both in situ diffuse reflectance infrared Fourier-transform spectroscopy and density functional theory calculations confirm the higher adsorption of NO after defluorination, thus facilitating the oxidation of NO on TiO2 nanocrystals.

Synthesis and Photocatalytic Activity of TiO2/CdS Nanocomposites with Co-Exposed Anatase Highly Reactive Facets

In this work, TiO2/CdS nanocomposites with co-exposed {101}/[111]-facets (NH4F-TiO2/CdS), {101}/{010} facets (FMA-TiO2/CdS), and {101}/{010}/[111]-facets (HF-TiO2/CdS and Urea-TiO2/CdS) were successfully synthesized through a one-pot solvothermal method by using [Ti4O9]2- colloidal solution containing CdS crystals as the precursor. The crystal structure, morphology, specific surface area, pore size distribution, separation, and recombination of photogenerated electrons/holes of the TiO2/CdS nanocomposites were characterized. The photocatalytic activity and cycling performance of the TiO2/CdS nanocomposites were also investigated. The results showed that as-prepared FMA-TiO2/CdS with co-exposed {101}/{010} facets exhibited the highest photocatalytic activity in the process of photocatalytic degradation of methyl orange (MO), and its degradation efficiency was 88.4%. The rate constants of FMA-TiO2/CdS was 0.0167 min-1, which was 55.7, 4.0, 3.7, 3.5, 3.3, and 1.9 times of No catalyst, CdS, HF-TiO2/CdS, NH4F-TiO2/CdS, CM-TiO2, Urea-TiO2/CdS, respectively. The highest photocatalytic activity of FMA-TiO2/CdS could be attributed to the synergistic effects of the largest surface energy, co-exposed {101}/{010} facets, the lowest photoluminescence intensity, lower charge-transfer resistance, and a higher charge-transfer efficiency.

Facile synthesis of rGO-supported AgI-TiO2 mesocrystals with enhanced visible light photocatalytic activity

Facile synthesis of novel rGO-supported AgI decorated TiO₂ mesocrystals (AgI-TMCs-rGO) and their photocatalytic activity under visible light irradiation are reported. The catalysts were prepared by combining the hydrothermal reaction process and in situ deposition-precipitation method. The structural features and chemical compositions of the prepared catalysts were investigated by HRTEM (high resolution transmission electron microscopy), XRD (x-ray powder diffraction), Raman spectra, XPS (x-ray photoelectron spectroscopy), UV-vis DRS (UV-vis diffuse reflectance spectra), PLS (photoluminescence spectra), and N₂ physisorption measurements. The AgI-TMCs-rGO catalysts featured a large surface area, high adsorption capacity, enhanced photo-induced charge separation and strong absorbance of visible light. The intimate contact of various components and the fast transfer of charge carriers effectively suppresses photolysis of AgI and favors the structural stability of AgI-TMCs-rGO. The photocatalytic activity of the catalysts was evaluated by degrading Rhodamine B (RhB) in an aqueous solution under visible light irradiation. The highest photocatalytic activity was observed in the sample 20%Ag-TMCs-rGO, attributed to the synergistic effects of a lower band gap and a lowerrecombination rate of the charge carriers, along with the larger surface area of the fabricated sample.

Facet-Dependent Interfacial Charge Transfer in TiO2/Nitrogen-Doped Graphene Quantum Dots Heterojunctions for Visible-Light Driven Photocatalysis

Interfacial charge transfer is crucial in the efficient conversion of solar energy into fuels and electricity. In this paper, heterojunction composites were fabricated, comprised of anatase TiO2 with different percentages of exposed {101} and {001} facets and nitrogen-doped quantum dots (NGQDs) to enhance the transfer efficiency of photo-excited charge carriers. The photocatalytic performances of all samples were evaluated for RhB degradation under visible light irradiation, and the hybrid containing TiO2 with 56% {001} facets demonstrated the best photocatalytic activity. The excellent photoactivity of TiO2/NGQDs was owed to the synergistic effects of the following factors: (i) The unique chemical features of NGQDs endowed NGQDs with high electronic conductivities and provided its direct contact with the TiO2 surface via forming Ti–O–C chemical bonds. (ii) The co-exposed {101} and {001} facets were beneficial for the separation and transfer of charge carriers in anatase TiO2. (iii) The donor-acceptor interaction between NGQDs and electron-rich {101} facets of TiO2 could remarkably enhance the photocurrent, thus hindering the charge carriers recombination rate. Extensive characterization of their physiochemical properties further showed the synergistic effect of facet-manipulated electron-hole separation in TiO2 and donor-acceptor interaction in graphene quantum dots (GQDs)/TiO2 on photocatalytic activity.

Surface-Controlled Photocatalysis and Chemical Sensing of TiO2, α-Fe2O3, and Cu2O Nanocrystals

A relatively new approach to the design of photocatalytic and gas sensing materials is to use the shape-controlled nanocrystals with well-defined facets exposed to light or gas molecules. An abrupt increase in a number of papers on the synthesis and characterization of metal oxide semiconductors such as a TiO2, α-Fe2O3, Cu2O of low-dimensionality, applied to surface-controlled photocatalysis and gas sensing, has been recently observed. The aim of this paper is to review the work performed in this field of research. Here, the focus is on the mechanism and processes that affect the growth of nanocrystals, their morphological, electrical, and optical properties and finally their photocatalytic as well as gas sensing performance.

Oxygen-Deficient Dumbbell-Shaped Anatase TiO2-x Mesocrystals with Nearly 100 % Exposed {101} Facets: Synthesis, Growth Mechanism, and Photocatalytic Performance

The development of hierarchical TiO₂ superstructures with new morphologies and intriguing photoelectric properties for utilizing solar energy is known to be an effective approach to alleviate the serious problems of environmental pollution. Herein, unique oxygen-deficient dumbbell-shaped anatase TiO_2-x mesocrystals (DTMCs) enclosed by nearly 100 % {101} facets were readily synthesized by mesoscale transformation in TiCl₃ /acetic acid (HAc) mixed solution, followed by calcination under vacuum. These mesocrystals exhibited much higher photoreactivity toward removing the model pollutants methyl orange and Cr^VI than truncated tetragonal bipyramidal anatase nanocrystals (TNCs), anatase mesocrystals built from truncated tetragonal bipyramidal anatase nanocrystals (TTMCs), and anatase mesocrystals constructed by anatase nanocrystals with nearly 100 % exposed {101} facets (TMCs), revealing that both the oxidation and reduction abilities of anatase TiO₂ were simultaneously enhanced upon fabricating an oxygen-deficient mesocrystalline architecture with about 100 % exposed {101} facets. Further characterization illustrated that such an enhancement of photoreactivity was mainly due to the strengthened light absorption, boosted charge carrier separation, and nearly 100 % exposed {101} facets of the oxygen-deficient dumbbell-shaped anatase mesocrystals. This work will be useful for guiding the synthesis of oxygen-deficient ordered superstructures of metal oxides with desired morphologies and exposed facets for promising applications in environmental remediation.

Titanium Dioxide (TiO2) Mesocrystals: Synthesis, Growth Mechanisms and Photocatalytic Properties

Hierarchical TiO2 superstructures with desired architectures and intriguing physico-chemical properties are considered to be one of the most promising candidates for solving the serious issues related to global energy exhaustion as well as environmental deterioration via the well-known photocatalytic process. In particular, TiO2 mesocrystals, which are built from TiO2 nanocrystal building blocks in the same crystallographical orientation, have attracted intensive research interest in the area of photocatalysis owing to their distinctive structural properties such as high crystallinity, high specific surface area, and single-crystal-like nature. The deeper understanding of TiO2 mesocrystals-based photocatalysis is beneficial for developing new types of photocatalytic materials with multiple functionalities. In this paper, a comprehensive review of the recent advances toward fabricating and modifying TiO2 mesocrystals is provided, with special focus on the underlying mesocrystallization mechanism and controlling rules. The potential applications of as-synthesized TiO2 mesocrystals in photocatalysis are then discussed to shed light on the structure–performance relationships, thus guiding the development of highly efficient TiO2 mesocrystal-based photocatalysts for certain applications. Finally, the prospects of future research on TiO2 mesocrystals in photocatalysis are briefly highlighted.

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.

Hydrogen production from glycerol reforming: conventional and green production

The use of biomass to produce transportation and related fuels is of increasing interest. In the traditional approach of converting oils and fats to fuels, transesterification processes yield a very large coproduction of glycerol. Initially, this coproduct was largely ignored and then considered as a useful feedstock for conversion to various chemicals. However, because of the intrinsic large production, any chemical feedstock role would consume only a fraction of the glycerol produced, so other options had to be considered. The reforming of glycerol was examined for syngas production, but more recently the use of photocatalytic decomposition to hydrogen (H2) is of major concern and several approaches have been proposed. The subject of this review is this greener photocatalytic route, especially involving the use of solar energy and visible light. Several different catalyst designs are considered, together with a very wide range of secured rates of H2 production spanning several orders of magnitude, depending on the catalytic system and the process conditions employed. H2 production is especially high when used in glycerol-water mixtures.

Anatase TiO2 Mesocrystals: Green Synthesis, In Situ Conversion to Porous Single Crystals, and Self-Doping Ti3+ for Enhanced Visible Light Driven Photocatalytic Removal of NO

Mesocrystals are of great interest for a wide range of applications owing to their unique structural features and properties. The realization of well-defined metal oxide mesocrystals through a facile and green synthetic approach still remains a great challenge. Here, a novel synthesis strategy is reported for the production of spindle-shaped anatase TiO₂ mesocrystals with a single-crystal-like structure, which was simply achieved through the one-step hydrolysis reaction of TiCl₃ in the green and recyclable media polyethylene glycol (PEG-400) without any additives. Such anatase mesocrystals were constructed from small nanocrystal subunits (≈1.5-4.5 nm in diameter) and formed through oriented aggregation of the nanocrystals pre-formed in the reaction system. Owing to their novel structural characteristics, the as-synthesized anatase mesocrystals could be easily fused in situ into porous single crystals by annealing in air. More significantly, after being annealed in vacuum, Ti³⁺ sites could be easily induced in the anatase crystal lattice, resulting in the formation of Ti³⁺ self-doped anatase mesocrystals. The thus-transformed mesocrystals exhibited enhanced visible light activity towards the photocatalytic oxidation of nitric oxide (NO) to NO₃^- , which could be largely attributed to their intrinsic Ti³⁺ self-doped nature, as well as high crystallinity and high porosity of the mesocrystalline architecture.

Preparation of annular TiO2 nanoparticles constructed by high-energy surfaces and enhanced visible-light photocatalytic activity

Annular TiO₂ nanoparticles were synthesized through a combined solvothermal and etching process.

Strong interactions between Au nanoparticles and TiO2 mesocrystal: highly selective photocatalytic reduction of nitroarenes

Au/TiO2 mesocrystals (Au/TMCs) were synthesized via a template-free solvothermal approach, followed with in situ photoreduction of gold chloride tetrahydrate for deposition of Au nanoparticles (AuNPs). The microstructure and components of the photocatalysts were characterized with various techniques. The results indicated that the AuNPs selectively anchored on the (101) facet of TiO2 mesocrystals (TMCs). More interestingly, the strong interactions occurred between AuNP and TMC support, due to the formation of a close Schottky heterointerface, combined with Ti-O-Au chemical bonds partially formed. Unexpectedly, it was found that the AuNPs typically aligned with their [111] direction, parallelling to the [101] direction of TMC in many cases. Au/TMCs exhibited high selectivity and activity for the transformation of nitroarenes to corresponding azoxyarenes, which could be attributed to not only the plasmonic effect of AuNPs and the unique superstructure of TMC, but also the highly strong interaction between AuNPs and TMCs through the close Schottky heterointerface.

Exploring the important role of nanocrystals orientation in TiO₂ superstructure on photocatalytic performances

Efficient charge separation has been widely accepted as one of the important factors responsible for the photocatalytic water splitting, organic oxidation, and solar cell, etc. TiO2 mesocrystal is a superstructure which could largely enhance charge separation, where TiO2 nanocrystals with parallel crystallographic alignment assemble in a form of oriented aggregation. Here, the intercrystal misorientation in TiO2 superstructure was first concerned and evaluated on the influence of photocatalytic efficiency. Our results showed that the intercrystal misorientation in TiO2 superstructures had a harmful effect on the charge separation efficiency.

Faceted titania nanocrystals doped with indium oxide nanoclusters as a superior candidate for sacrificial hydrogen evolution without any noble-metal cocatalyst under solar irradiation

Development of unique nanoheterostructures consisting of indium oxide nanoclusters like species doped on the TiO2 nanocrystals surfaces with {101} and {001} exposed facets, resulted in unprecedented sacrificial hydrogen production (5.3 mmol h(-1) g(-1)) from water using methanol as a sacrificial agent, under visible light LED source and AM 1.5G solar simulator (10.3 mmol h(-1) g(-1)), which is the highest H2 production rate ever reported for titania based photocatalysts, without using any noble metal cocatalyst. X-ray photoelectron spectroscopy (XPS) analysis of the nanostructures reveals the presence of Ti-O-In and In-O-In like species on the surface of nanostructures. Electron energy-loss spectroscopy (EELS) elemental mapping and EDX spectroscopy techniques combined with transmission electron microscope evidenced the existence of nanoheterostructures. XPS, EELS, EDX, and HAADF-STEM tools collectively suggest the presence of indium oxide nanoclusters like species on the surface of TiO2 nanostructures. These indium oxide nanocluster doped TiO2 (In2O3/T{001}) single crystals with {101} and {001} exposed facets exhibited 1.3 times higher visible light photocatalytic H2 production than indium oxide nanocluster doped TiO2 nanocrystals with only {101}facets (In2O3/T{101}) exposed. The remarkable photocatalytic activity of the obtained nanoheterostructures is attributed to the combined synergetic effect of indium oxide nanoclusters interacting with the titania surface, enhanced visible light response, high crystallinity, and unique structural features.

Template-free synthesis of core-shell TiO2 microspheres covered with high-energy {116}-facet-exposed N-doped nanosheets and enhanced photocatalytic activity under visible light

Core-shell TiO2 microspheres possess a unique structure and interesting properties, and therefore, they have received much attention. The high-energy facets of TiO2 also are being widely studied for the high photocatalytic activities they are associated with. However, the synthesis of the core-shell structure is difficult to achieve and requires multiple-steps and/or is expensive. Hydrofluoric acid (HF), which is highly corrosive, is usually used in the controlling high-energy facet production. Therefore, it is still a significant challenge to develop low-temperature, template-free, shape-controlled, and relative green self-assembly routes for the formation of core-shell-structured TiO2 microspheres with high-energy facets. Here, we report a template- and hydrofluoric acid free solvothermal self-assembly approach to synthesize core-shell TiO2 microspheres covered with high-energy {116}-facet-exposed nanosheets, an approach in which 1,4-butanediamine plays a key role in the formation of nanosheets with exposed {116} facets and the doping of nitrogen in situ. In the structure, nanoparticle aggregates and nanosheets with {116} high-energy facets exposed act as core and shell, respectively. The photocatalytic activity for degradation of 2,4,6-tribromophenol and Rhodamine B under visible irradiation and UV/Vis irradiation has been examined, and improved photocatalytic activity under visible light owing to the hierarchical core-shell structure, {116}-plane-oriented nanosheets, in situ N doping, and large surface areas has been found.

Facet-dependent photocatalytic properties of TiO(2) -based composites for energy conversion and environmental remediation

Titanium dioxide (TiO2 ) is one of the most widely investigated metal oxides because of its extraordinary surface, electronic, and photocatalytic properties. However, the large band gap of TiO2 and the considerable recombination of photogenerated electron-hole pairs limit its photocatalytic efficiency. Therefore, research attention is being increasingly directed towards engineering the surface structure of TiO2 on the atomic level (namely morphological control of {001} facets on the micro- and nanoscale) to fine-tune its physicochemical properties; this could ultimately lead to the optimization of selectivity and reactivity. This Review encompasses the fundamental principles to enhance the photocatalytic activity by using highly reactive {001}-faceted TiO2 -based composites. The current progress of such composites, with particular emphasis on the photodegradation of pollutants and photocatalytic water splitting for hydrogen generation, is also discussed. The progresses made are thoroughly examined for achieving remarkable photocatalytic performances, with additional insights with regard to charge transfer. Finally, a summary and some perspectives on the challenges and new research directions for future exploitation in this emerging frontier are provided, which hopefully would allow for harnessing the outstanding structural and electronic properties of {001} facets for various energy- and environmental-related applications.

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.

K2TiO(C2O4)2-mediated synthesis of rutile TiO2 mesocrystals and their ability to assist photodegradation of sulfosalicylic acid in water

Flower-like rutile TiO₂ mesocrystals were synthesized by a K₂TiO(C₂O₄)₂-mediated low temperature solution route under the atmospheric pressure, which exhibited high photocatalytic activity because of the large specific surface area as well as the high charge separation rate inherent from the single crystal nature.

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.

Self-modification of titanium dioxide materials by Ti3+ and/or oxygen vacancies: new insights into defect chemistry of metal oxides

In this review, we highlight the recent research efforts towards understanding the defect chemistry of titanium dioxide. Particular attention is paid to the synthesis of self-modified TiO₂ materials with Ti³⁺/oxygen vacancies and the favorable effects of these defects on the properties and applications of the obtained materials.