Photogenerated defects in shape-controlled TiO2 anatase nanocrystals: a probe to evaluate the role of crystal facets in photocatalytic processes

Water effect on the band edges of anatase TiO2 surfaces: A theoretical study on charge migration across surface heterojunctions and facet-dependent photoactivity

The charge migration mechanism across the surface heterojunction constructed on an anatase TiO2 nanocrystal is still under debate. To solve this longstanding question, we present a systematic study of the band edges (vs. standard hydrogen electrode, SHE) of aqueous TiO2 interfaces with anatase (101), (100) and (001) surfaces, using a combination of density functional theory-based molecular dynamics (DFTMD) and efficient computational SHE (cSHE) methods. Our calculations show that the conduction band minimum (CBM) of the (101) surface is lower than that of (001) and (100) surfaces, which is thermodynamically favorable for electrons migrating to the (101) surface through the surface heterojunction, while the hole preferentially accumulates on the (100) surface due to its highest valence band minimum (VBM). In addition, we qualitatively explore the facet-dependent photocatalytic activity of anatase TiO2. Due to the possession of both the beneficial atomic structure (with 100% undercoordinated Ti5c atoms at the surface) and electronic structure (more strongly oxidizing holes in the VBM and efficient electron-hole spatial separation separation), the (001) surface exhibits the most efficient photocatalytic performance for water oxidation. Furthermore, it is confirmed that the use of simplified theoretical models neglecting the detailed atomic structures of water at the aqueous interface is inadequate to predict the band alignment of semiconductors relative to water redox potentials, so that it may result in substantial errors in evaluating the photocatalytic performance of materials to be used for water splitting.

Unveiling the Role of PEO-Capped TiO2 Nanofiller in Stabilizing the Anode Interface in Lithium Metal Batteries

Lithium metal batteries (LMBs) will be a breakthrough in automotive applications, but they require the development of next-generation solid-state electrolytes (SSEs) to stabilize the anode interface. Polymer-in-ceramic PEO/TiO₂ nanocomposite SSEs show outstanding properties, allowing unprecedented LMBs durability and self-healing capabilities. However, the mechanism underlying the inhibition/delay of dendrite growth is not well understood. In fact, the inorganic phase could act as both a chemical and a mechanical barrier to dendrite propagation. Combining advanced in situ and ex situ experimental techniques, we demonstrate that oligo(ethylene oxide)-capped TiO₂, although chemically inert toward lithium metal, imparts SSE with mechanical and dynamical properties particularly favorable for application. The self-healing characteristics are due to the interplay between mechanical robustness and high local polymer mobility which promotes the disruption of the electric continuity of the lithium dendrites (razor effect).

Oleic acid/oleylamine ligand pair: a versatile combination in the synthesis of colloidal nanoparticles

A variety of colloidal chemical approaches has been developed in the last few decades for the controlled synthesis of nanostructured materials in either water or organic solvents. Besides the precursors, the solvents, reducing agents, and the choice of surfactants are crucial for tuning the composition, morphology and other properties of the resulting nanoparticles. The ligands employed include thiols, amines, carboxylic acids, phosphines and phosphine oxides. Generally, adding a single ligand to the reaction mixture is not always adequate to yield the desired features. In this review, we discuss in detail the role of the oleic acid/oleylamine ligand pair in the chemical synthesis of nanoparticles. The combined use of these ligands belonging to two different categories of molecules aims to control the size and shape of nanoparticles and prevent their aggregation, not only during their synthesis but also after their dispersion in a carrier solvent. We show how the different binding strengths of these two molecules and their distinct binding modes on specific facets affect the reaction kinetics toward the production of nanostructures with tailored characteristics. Additional functions, such as the reducing function, are also noted, especially for oleylamine. Sometimes, the carboxylic acid will react with the alkylamine to form an acid-base complex, which may serve as a binary capping agent and reductant; however, its reducing capacity may range from lower to much lower than that of oleylamine. The types of nanoparticles synthesized in the simultaneous presence of oleic acid and oleylamine and discussed herein include metal oxides, metal chalcogenides, metals, bimetallic structures, perovskites, upconversion particles and rare earth-based materials. Diverse morphologies, ranging from spherical nanoparticles to anisotropic, core-shell and hetero-structured configurations are presented. Finally, the relation between tuning the resulting surface and volume nanoparticle properties and the relevant applications is highlighted.

Visible-Light-Active Black TiO2 Nanoparticles with Efficient Photocatalytic Performance for Degradation of Pharmaceuticals

Special attention has recently been paid to surface-defective titanium dioxide and black TiO₂ with advanced optical, electrical, and photocatalytic properties. Synthesis of these materials for photodegradation and mineralization of persistent organic pollutants in water, especially under visible radiation, presents interest from scientific and application points of view. Chemical reduction by heating a TiO₂ and NaBH₄ mixture at 350 °C successfully introduced Ti³⁺ defects and oxygen vacancies at the surface of TiO₂, with an increase in the photocatalytic degradation of amoxicillin—an antibiotic that is present in wastewater due to its intense use in human and animal medicine. Three TiO₂ samples were prepared at different annealing temperatures to control the ratio between anatase and rutile and were subjected to chemical reduction. Electron paramagnetic resonance investigations showed that the formation of surface Ti³⁺ defects in a high concentration occurred mainly in the anatase sample annealed at 400 °C, contributing to the bandgap reduction from 3.32 eV to 2.92 eV. The reduced band gap enhances visible light absorption and the efficiency of photocatalysis. The nanoparticles of ~90 m²/g specific surface area and 12 nm average size exhibit ~100% efficiency in the degradation of amoxicillin under simulated solar irradiation compared with pristine TiO₂. Mineralization of amoxicillin and by-products was over 75% after 48 h irradiation for the anatase sample, where the Ti³⁺ defects were present in a higher concentration at the catalyst’s surface.

Advances in facet-dependent photocatalytic properties of BiOCl catalyst for environmental remediation

Bismuth chloride oxide (BiOCl) is a typical V-VI-VII ternary oxide material, which is one of the widely studied metal oxides due to its unique surface, electronic and photocatalytic properties. However, the broad bandgap and the large number of photogenerated electron-hole pair complexes of BiOCl limit its photocatalytic efficiency. Since the photocatalytic performance of BiOCl is highly dependent on its exposed crystallographic facets, research attention has increasingly focused on the different structures and properties possessed by different crystallographic facets of BiOCl. This article reviews the basic principles of using different crystalline surfaces of BiOCl materials to enhance photocatalytic activity, summarizes the applications of BiOCl single-crystal catalysts and composite catalysts in the environmental field, and provides an outlook on the challenges and new research directions for future development in this emerging frontier area. It is hoped that the crystalline surface-related photocatalysis of BiOCl can be used to provide new guidance for the rational design of novel catalysts for various energy and environment-related applications.

Facet-Control versus Co-Catalyst-Control in Photocatalytic H2 Evolution from Anatase TiO2 Nanocrystals

Titanium dioxide (TiO2 ) and, in particular, its anatase polymorph, is widely studied for photocatalytic H2 production. In the present work, we examine the importance of reactive facets of anatase crystallites on the photocatalytic H2 evolution from aqueous methanol solutions. For this, we synthesized anatase TiO2 nanocrystals with a large amount of either {001} facets, that is, nanosheets, or {101} facets, that is, octahedral nanocubes, and examined their photocatalytic H2 evolution and then repeated this procedure with samples where Pt co-catalyst is present on all facets. Octahedral nanocubes with abundant {101} facets produce >4 times more H2 than nanosheets enriched in {001} facets if the reaction is carried out under co-catalyst-free conditions. For samples that carry Pt co-catalyst on both {001} and {101} facets, faceting loses entirely its significance. This demonstrates that the beneficial role of faceting, namely the introduction of {101} facets that act as electron transfer mediator is relevant only for co-catalyst-free TiO2 surfaces.

Electron and Energy Transfer Mechanisms: The Double Nature of TiO2 Heterogeneous Photocatalysis

Photocatalytic chemical transformations in the presence of irradiated TiO₂ are generally considered in terms of interfacial electron transfer. However, more elusive energy-transfer-driven reactions have been also hypothesized to occur, mainly on the basis of the indirect evidence of detected reaction products whose existence could not be justified simply by electron transfer. Unlike in homogeneous and colloidal systems, where energy transfer mechanisms have been investigated deeply for several organic syntheses, understanding of similar processes in heterogeneous systems is at only a nascent level. However, this gap of knowledge can be filled by considering the important achievements of synthetic heterogeneous photocatalysis, which bring the field closer to industrial exploitation. The present manuscript summarizes the main findings of previous literature reports and, also on the basis of some novel experimental evidences, tentatively proposes that the energy transfer in TiO₂ photocatalysis could possess a Förster-like nature.

Highly Ordered Inverse Opal Structures Synthesized from Shape-Controlled Nanocrystal Building Blocks

Three-dimensional ordered porous materials known as inverse opal films (IOFs) were synthesized using nanocrystals with precisely defined morphologies. Comprehensive theoretical and experimental studies of the volume fraction ratio and electrostatic interactions between nanocrystals and polystyrene templating particles enabled the formation of highly ordered crack-free photonic structures. The synthetic strategy was first demonstrated using titanium dioxide (TiO₂ ) nanocrystals of different shapes and then generalized to assemble nanocrystals of other functional materials, such as indium tin oxide and zinc-doped ferrite. Tunable photocatalytic activity of the TiO₂ IOFs, modulated through the choice of the shape of TiO₂ nanocrystals in conjunction with selecting desired macroscopic features of the IOF, was further explored. In particular, enhanced activity is observed for crack-free, highly ordered IOFs whose photonic properties can improve light absorption via the slow light effect. This study opens new opportunities in designing multi-length-scale porous nanoarchitectures having enhanced performance in a variety of applications.

Humic acid mediated toxicity of faceted TiO2 nanocrystals to Daphnia magna

Nano-bio interface is of great importance in dictating the interaction between the nanomaterials and biological system and thus the toxicity to aquatic organisms. Herein, two specific faceted TiO₂ nanocrystals, {101} and {001} facet, were exposed to Daphnia magna to explore facet-dependent toxicological responses in aquatic environment. Due to the different influences on oxidative stress process, the half-maximal effective concentration (EC₅₀) value of {001} TiO₂ (1.27 g L^-1) to D. magna was less than that of {101} TiO₂ (1.68 g L^-1). Suwannee river humic acid (SRHA) could significantly reduce the oxidative stress responses of TiO₂ nanocrystals and thus alleviate their toxicities to D. magna in aquatic environment. The protective effect of SRHA against TiO₂ toxicity exhibited a facet-dependent manner. Compared to {101} TiO₂, a more obvious detoxification effect was observed for {001} TiO₂. The high SRHA concentration could endow both faceted TiO₂ nanocrystals with a similar toxicity due to the formation of SRHA-corona on TiO₂ surface. This facet-affected toxicity of nanomaterials in aquatic environment would provide us new insights in predicting the exposure risk of nanomaterials in nature waters.

Abiotic oxidation of arsenite in natural and engineered systems: Mechanisms and related controversies over the last two decades (1999-2020)

Abiotic oxidation of toxic As(III) to As(V) is being deemed as a necessary step for the overall arsenic decontamination in both natural and engineered systems. Direct oxidation of As(III) by chemical oxidants, such as ozone, permanganate, ferrate, chlorine and chloramine, or naturally occurring minerals like Mn, Fe oxides, seems straightforward. Both O₂ and H₂O₂ are ineffective for arsenite oxidation, but they can be activated by reducing substances like Fe²⁺, Fe⁰ to increase the oxidation rates. Photo-induced oxidation of As(III) has been demonstrated effective in Fe complexes or minerals, NO₃^-/NO₂^-, dissolved organic matter (DOM), peroxygens and TiO₂ systems. Although a variety of oxidation methods have been developed over the past two decades, there remain many scientific and technical challenges that must be overcome before the rapid progress in basic knowledge can be translated into environmental benefits. To better understand the trends in the existing data and to identify the knowledge gaps, this review describes in detail the complicated mechanisms for As(III) oxidation by various methods and emphasizes on the conflicting data and explanation. Some prevailing concerns and challenges in the sphere of As(III) oxidation are also pointed out so as to appeal to researchers for further investigations.

Charge Carrier Processes and Optical Properties in TiO2 and TiO2-Based Heterojunction Photocatalysts: A Review

Photocatalysis based technologies have a key role in addressing important challenges of the ecological transition, such as environment remediation and conversion of renewable energies. Photocatalysts can in fact be used in hydrogen (H₂) production (e.g., via water splitting or photo-reforming of organic substrates), CO₂ reduction, pollution mitigation and water or air remediation via oxidation (photodegradation) of pollutants. Titanium dioxide (TiO₂) is a “benchmark” photocatalyst, thanks to many favorable characteristics. We here review the basic knowledge on the charge carrier processes that define the optical and photophysical properties of intrinsic TiO₂. We describe the main characteristics and advantages of TiO₂ as photocatalyst, followed by a summary of historical facts about its application. Next, the dynamics of photogenerated electrons and holes is reviewed, including energy levels and trapping states, charge separation and charge recombination. A section on optical absorption and optical properties follows, including a discussion on TiO₂ photoluminescence and on the effect of molecular oxygen (O₂) on radiative recombination. We next summarize the elementary photocatalytic processes in aqueous solution, including the photogeneration of reactive oxygen species (ROS) and the hydrogen evolution reaction. We pinpoint the TiO₂ limitations and possible ways to overcome them by discussing some of the “hottest” research trends toward solar hydrogen production, which are classified in two categories: (1) approaches based on the use of engineered TiO₂ without any cocatalysts. Discussed topics are highly-reduced “black TiO₂”, grey and colored TiO₂, surface-engineered anatase nanocrystals; (2) strategies based on heterojunction photocatalysts, where TiO₂ is electronically coupled with a different material acting as cocatalyst or as sensitizer. Examples discussed include TiO₂ composites or heterostructures with metals (e.g., Pt-TiO₂, Au-TiO₂), with other metal oxides (e.g., Cu₂O, NiO, etc.), direct Z-scheme heterojunctions with g-C₃N₄ (graphitic carbon nitride) and dye-sensitized TiO₂.

Site Sensitivity of Interfacial Charge Transfer and Photocatalytic Efficiency in Photocatalysis: Methanol Oxidation on Anatase TiO2 Nanocrystals

Photocatalytic oxidation of methanol on various anatase TiO₂ nanocrystals was studied by in situ and time-resolved characterizations and DFT calculations. Surface site and resulting surface adsorbates affect the surface band bending/bulk-to-surface charge migration processes and interfacial electronic structure/interfacial charge transfer processes. TiO₂ nanocrystals predominantly enclosed by the {001} facets expose a high density of reactive fourfold-coordinated Ti sites (Ti_4c ) at which CH₃ OH molecules dissociate to form the CH₃ O adsorbate (CH₃ O(a)_Ti4c ). CH₃ O(a)_Ti4c localized density of states are almost at the valence band maximum of TiO₂ surface, facilitating the interfacial hole transfer process; CH₃ O(a)_Ti4c with a high coverage promotes upward surface band bending, facilitating bulk-to-surface hole migration. CH₃ O(a)_Ti4c exhibits the highest photocatalytic oxidation rate constant. TiO₂ nanocrystals enclosed by the {001} facets are most active in photocatalytic methanol oxidation.

Probing the formation and optical properties of Ti3+–TiO2 with (001) exposed crystal facet by ethanol-assisted fluorination

(001)-faceted TiO₂ with Ti³⁺ defects that are exclusively embedded in the bulk lattice near the surface was synthesized.

Toward Informed Design of Nanomaterials: A Mechanistic Analysis of Structure-Property-Function Relationships for Faceted Nanoscale Metal Oxides

Nanoscale metal oxides (NMOs) have found wide-scale applicability in a variety of environmental fields, particularly catalysis, gas sensing, and sorption. Facet engineering, or controlled exposure of a particular crystal plane, has been established as an advantageous approach to enabling enhanced functionality of NMOs. However, the underlying mechanisms that give rise to this improved performance are often not systematically examined, leading to an insufficient understanding of NMO facet reactivity. This critical review details the unique electronic and structural characteristics of commonly studied NMO facets and further correlates these characteristics to the principal mechanisms that govern performance in various catalytic, gas sensing, and contaminant removal applications. General trends of facet-dependent behavior are established for each of the NMO compositions, and selected case studies for extensions of facet-dependent behavior, such as mixed metals, mixed-metal oxides, and mixed facets, are discussed. Key conclusions about facet reactivity, confounding variables that tend to obfuscate them, and opportunities to deepen structure-property-function understanding are detailed to encourage rational, informed design of NMOs for the intended application.

Simultaneous photocatalytic degradation of ibuprofen and H2 evolution over Au/sheaf-like TiO2 mesocrystals

Considerable effort has been devoted to the efficient degradation of pharmaceuticals and personal care products (PPCPs), while the chemical energy in these processes has been widely overlooked. In this study, we demonstrated the simultaneous hydrogen production and ibuprofen degradation through heterogeneous photocatalysis. By anchoring Au nanoparticles (NPs) on the (101) surface of sheaf-like TiO₂ mesocrystals with [001] orientation, efficient charge separation is achieved, which is essential for the photocatalytic redox reactions. XPS analysis showed that the binding energies of Ti 2p and O 1s indicated no shift after Au addition. Peaks observed at 81.8 and 85.5 eV due to Au 4f_7/2 and Au 4f_5/2 of metallic gold on the surface of Au/meso-TiO₂, confirmed the formation of Au NPs. The as-synthesized anatase TiO₂ mesocrystals are composed of small nanocrystals with a size of 8 nm and exhibit the uniform sheaf-like morphology along [001] orientation. As expected, the 1 wt% Au/TiO₂ mesocrystals shows the largest photocurrent density, highest H₂-evolution rate, and fastest photodegradation rate of ibuprofen under simulated sunlight irradiation among all the studied catalyst. Furthermore, the effect of solution pH, common anions (Cl^-, NO₃^-, and SO₄^2-) and cations (Na⁺, K⁺, and Ca²⁺) on photocatalytic H₂ evolution and degradation of ibuprofen were individually investigated and discussed. A mechanism for the simultaneous photocatalytic hydrogen generation and degradation of ibuprofen has also been proposed. This work opens up new opportunities for the development of energy efficient techniques for PPCPs degradation.

Effect of polyethylene glycol on crystal growth and photocatalytic activity of anatase TiO2 single crystals

In order to evaluate the effect of polyethylene glycol (PEG) on the growth of TiO₂ crystals, anatase TiO₂ crystals with different morphologies and structures were synthesized by controlling the content and type of PEG in a solvothermal system. Then, their morphology and structure were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Characterization results show that hydrofluoric acid can promote the formation of high activity (001) facets. Experiments on the effect of PEG on crystal growth show that the low molecular weight PEG (PEG400) can accelerate crystal differentiation and relieve the agglomeration of crystals in the presence of hydrofluoric acid. Besides, according to the experimental results, we found that PEG400 can reduce the agglomeration size and number of TiO₂ polycrystalline particles. Research on the photocatalytic activity proposed that the independence of single crystal and the integrity of (001) facets are the critical factors in advanced oxidation reaction. The resultant anatase TiO₂ single crystals could produce more hydroxyl radicals (˙OH) in the photocatalytic system, which exhibited remarkable photocatalytic performance for the degradation of Acid Red B.

Anatase TiO₂ crystals with different structures were synthesized. Experiments on the effect of polyethylene glycol show that the low molecular weight PEG (PEG400) can accelerate crystal differentiation and relieve the agglomeration of crystals.

TiO2 superstructures with oriented nanospaces: a strategy for efficient and selective photocatalysis

Highly ordered superstructures of semiconductor nanocrystals contain abundant nanometer-scale pores between the crystals; however, there have been difficulties in controlling the size and orientation of these nanospaces without the use of a template or a capping reagent. This constraint has affected their development and applications in potential fields including catalysis and optoelectronics adversely. In this study, we synthesized a rod-shaped TiO2 mesocrystal (TMC) having a length of a few hundreds of micrometers and comprising regularly ordered anatase TiO2 nanocrystals that form oriented nanospaces by exposed {001} facets. Finite-difference time-domain (FDTD) calculations of electric fields and in situ fluorescence imaging with a polarization sensitive dye on a single mesocrystal were performed to reveal anisotropic adsorption and excitation of the dyes. Furthermore, the photodegradation of the dyes was found to be more facilitated in nanospaces formed by the specific facets, as compared with the dyes randomly adsorbed on the outer surfaces. Consequently, the selectivity of photocatalytic reactions based on the molecular size and redox was enhanced by introducing the concept of oriented nanospace.

Core-Shell Heterostructured and Visible-Light-Driven Titanoniobate/TiO2 Composite for Boosting Photodegradation Performance

Herein, we report a one-dimensional (1D) S-doped K3Ti5NbO14@TiO2 (STNT) core-shell heterostructured composite with an enhanced photocatalytic degradation activity under visible light, which was prepared by a simple reassembly-calcination method using thiourea as the S source. The anisotropically shaped rods are favorable for the rapid transport of photogenerated charge carriers. The substitution of Ti4+ by S6+ is primarily incorporated into the lattice of the TiO2 shell so as to create an intra-band-gap state below the conduction band (CB) position, giving rise to Ti-O-S bonds and thus the visible light response. The presence of electron-deficient S atoms is of benefit to the decreased recombination rate of photogenerated electrons and holes by capturing electrons (e-). Meanwhile, a tight close interface between K3Ti5NbO14 and TiO2 was formed to achieve a nano-heterojunction structure, leading to the fostered separation of its interfacial photogenerated electrons and holes. The visible-light-driven photocatalytic degradation of methylene blue (MB) by STNT composites is higher than that by pure K3Ti5NbO14, owing to the synergistic effects of S doping and heterojunction. A possible photocatalytic mechanism was proposed with a reasonable discussion. This work may provide an insight into constructing highly efficient core-shell photocatalysts used toward sustainable environmental remediation and resource shortages.

β-SnWO4 with Morphology-Controlled Synthesis and Facet-Depending Photocatalysis

Faceted β-SnWO₄ microcrystals are prepared with different morphologies including tetrahedra, truncated tetrahedra, truncated octahedra, and short-spiked and long-spiked spikecubes. All of these morphologies are prepared with comparable experimental conditions via microwave-assisted synthesis of high-boiling alcohols (the so-called polyol method). The decisive parameters for controlled formation of one or the other morphology of faceted β-SnWO₄ microcrystals are studied and discussed, including microwave-assisted heating, Sn(OH)₂ as the Sn²⁺ reservoir, the temperature of particle nucleation, the temperature of particle growth, and the concentration of the starting materials. Morphology and crystallinity are characterized by scanning electron microscopy, X-ray powder diffraction, UV-vis, and Fourier-transform infrared spectroscopy. Finally, the photocatalytic properties of all obtained faceted microcrystals-tetrahedra, truncated tetrahedra, truncated octahedra, cubes, and short-spiked and long-spiked spikecubes-are exemplarily compared with regard to the photocatalytic decomposition of rhodamine B and the influence of the respective surface crystal planes.

3D Yolk@Shell TiO2- x/LDH Architecture: Tailored Structure for Visible Light CO2 Conversion

CO₂ photoconversion into hydrocarbon solar fuels by engineered semiconductors is considered as a feasible plan to address global energy requirements in times of global warming. In this regard, three-dimensional yolk@shell hydrogenated TiO₂/Co-Al layered double hydroxide (3D Y@S TiO_{2- x}/LDH) architecture was successfully assembled by sequential solvothermal, hydrogen treatment, and hydrothermal preparation steps. This architecture revealed a high efficiency for the photoreduction of CO₂ to solar fuels, without a noble metal cocatalyst. The time-dependent experiment indicated that the production of CH₃OH was almost selective until 2 h (up to 251 μmol/g_cat. h), whereas CH₄ was produced gradually by increasing the time of reaction to 12 h (up to 63 μmol/g_cat. h). This significant efficiency can be ascribed to the engineering of 3D Y@S TiO_{2- x}/LDH architecture with considerable CO₂ sorption ability in mesoporous yolk@shell structure and LDH interlayer spaces. Also, oxygen vacancies in TiO_{2- x} could provide excess sites for sorption, activation, and conversion of CO₂. Furthermore, the generated Ti³⁺ ions in the Y@S TiO₂ structure as well as connecting of structure with LDH plates can facilitate the charge separation and decrease the band gap of nanoarchitecture to the visible region.

N-Doped K3Ti5NbO14@TiO2 Core-Shell Structure for Enhanced Visible-Light-Driven Photocatalytic Activity in Environmental Remediation

A novel N-doped K3Ti5NbO14@TiO2 (NTNT) core-shell heterojunction photocatalyst was synthesized by firstly mixing titanium isopropoxide and K3Ti5NbO14 nanobelt, and then calcinating at 500 °C in air using urea as the nitrogen source. The samples were analyzed by X-ray diffraction pattern (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-Vis absorption spectroscopy and X-ray photoelectron spectroscopic (XPS) spectra. Anatase TiO2 nanoparticles were closely deposited on the surface of K3Ti5NbO14 nanobelt to form a nanoscale heterojunction structure favorable for the separation of photogenerated charge carriers. Meanwhile, the nitrogen atoms were mainly doped in the crystal lattices of TiO2, resulting in the increased light harvesting ability to visible light region. The photocatalytic performance was evaluated by the degradation of methylene blue (MB) under visible light irradiation. The enhanced photocatalytic activity of NTNT was ascribed to the combined effects of morphology engineering, N doping and the formation of heterojunction. A possible photocatalytic mechanism was proposed based on the experimental results.

Atomic Substitution Enabled Synthesis of Vacancy-Rich Two-Dimensional Black TiO2- x Nanoflakes for High-Performance Rechargeable Magnesium Batteries

Rechargeable magnesium (Mg) batteries assembled with dendrite-free, safe, and earth-abundant metal Mg anodes potentially have the advantages of high theoretical specific capacity and energy density. Nevertheless, owing to the large polarity of divalent Mg²⁺ ions, the insertion of Mg²⁺ into electrode materials suffers from sluggish kinetics, which seriously limit the performance of Mg batteries. Herein, we demonstrate an atomic substitution strategy for the controlled preparation of ultrathin black TiO_{2- x} (B-TiO_{2- x}) nanoflakes with rich oxygen vacancies (OVs) and porosity by utilizing ultrathin 2D TiS₂ nanoflakes as precursors. We find out that the presence of OVs in B-TiO_{2- x} electrode material can greatly improve the electrochemical performances of rechargeable Mg batteries. Both experimental results and density functional theory simulations confirm that the introduction of OVs can remarkably enhance the electrical conductivity and increase the number of active sites for Mg²⁺ ion storage. The vacancy-rich B-TiO_{2- x} nanoflakes exhibit high reversible capacity and good capacity retention after long-term cycling at large current densities. It is hoped that this work can provide valuable insights and inspirations on the defect engineering of electrode materials for rechargeable magnesium batteries.

Unveiling the Role of Defects on Oxygen Activation and Photodegradation of Organic Pollutants

The status of defects of TiO₂ are of fundamental importance in determining its physicochemical properties. Here we report a simple chemical deposition method for controllable synthesis of defective anatase TiO₂ nanocrystals under various calcination atmospheres. XPS and ESR analysis reveals that both the oxygen vacancies ( V_O) and the trivalent titanium (Ti³⁺) defects exist in TiO₂ after N₂ treatment (N-TiO₂). Meanwhile, mainly V_O defects can be obtained in TiO₂ with air calcination (A-TiO₂). ESR spectra for reactive oxygen species determination, clearly show that the visible light catalytic activity is mainly caused by the efficient activation of oxygen molecules to •O^2- species for A-TiO₂, which play an important role in hindering the accumulation of intermediates during p-chlorophenol (4-CP) photodegradation process. However, the oxygen molecules cannot be activated for N-TiO₂ even with superior visible light absorption and thus the photogenerated electron are reductant, which participated in the transformation of BQ to HQ via electron shuttle mechanism. Moreover, A-TiO₂ exhibits higher separation efficiency of photogenerated carriers than that of N-TiO₂, showing the critical role of V_O with a suitable concentration in transferring photogenerated charges.

Synthesis of cosmetic grade TiO2-SiO2 core-shell powder from mechanically milled TiO2 nanopowder for commercial mass production

TiO₂ nanoparticles as an active sunscreen ingredient generate reactive oxygen species (ROS) upon UVA irradiation which is cytotoxic, genotoxic and potential to damage the DNA. The health concern and potential risks from TiO₂ can be mitigated by shielding the particles through the suitable coating. Considering the advantages of SiO₂, SiO₂ coated TiO₂ nanoparticles can be a potential material which can replace TiO₂ for thickening, whitening, lubricating, and sunscreen ingredient in cosmetics. This article reports the synthesis of cosmetic grade TiO₂-SiO₂ core-shell nanopowder from mechanically milled TiO₂ nanopowder for commercial mass production. From commercial TiO₂ nanopowder was fabricated through size reduction by nanoset milling. Followed by the fabricated TiO₂ nanopowder coated with SiO₂ through sol-gel technique. A suitable optimum condition was explored for cosmetic grade TiO₂-SiO₂ core-shell nanopowder. Various physical properties and optical properties were analyzed. Synthesized of cosmetic grade TiO₂-SiO₂ core-shell nanopowder found to be at 100 nm size, with a homogeneous SiO₂ coating having UVA protection factor 39 and sun protection factor (SPF) is 42. From the size, safety, and SPF perspective it can be an excellent cosmetic grade powder and from process simplicity perspective it can be commercially viable.

Step-by-Step Growth of HKUST-1 on Functionalized TiO2 Surface: An Efficient Material for CO2 Capture and Solar Photoreduction

The present study reports on a simple preparation strategy of a hybrid catalyst, TiO2/HKUST-1, containing TiO2 anatase nanoparticles (NPs) with tailored morphology and photocatalytic activity coupled with a porous metal-organic framework (MOF), namely HKUST-1, as an advanced material for the CO2 photocatalytic reduction. In detail, TiO2/HKUST-1 catalyst was prepared via an easy slow-diffusion method combined with a step-by-step self-assembly at room temperature. The growth of crystalline HKUST-1 onto titania surface was achieved by functionalizing TiO2 nanocrystals, with phosphoesanoic acid (PHA), namely TiO2-PHA, which provides an intimate contact between MOF and TiO2. The presence of a crystalline and porous shell of HKUST-1 on the TiO2 surfaces was assessed by a combination of analytical and spectroscopic techniques. TiO2/HKUST-1 nanocomposite showed a significant efficiency in reducing CO2 to CH4 under solar light irradiation, much higher than those of the single components. The role of MOF to improve the photoreduction process under visible light was evidenced and attributed either to the relevant amount of CO2 captured into the HKUST-1 porous architecture or to the hybrid structure of the material, which affords enhanced visible light absorption and allows an effective electron injection from TiO2-PHA to HKUST-1, responsible for the photochemical reduction of CO2.