Core-shell structured titanium dioxide nanomaterials for solar energy utilization

Stimuli-Responsive Ti-Organic Gels and Aerogels Derived from Ti-Oxo Clusters: Hierarchical Porosity and Photocatalytic Activity

Herein, we report titanium-organic gels (TOGs) as new Ti-oxo-based materials that exhibit stimuli-responsive sol-gel transformations and hierarchical porosity upon the removal of solvent molecules. Heating a solution of Ti-oxo clusters and pyromellitic acid as a tetratopic ligand produces TOGs that readily become sols by applying physical stimuli such as shaking or vortexing under ambient conditions. Porous titanium-organic aerogels (TOAs) were obtained by the CO₂ supercritical point drying (CPD) of the TOGs, and their porous structures were characterized by N₂ adsorption and desorption isotherm measurements. These TOAs, based on the Ti-oxo clusters, possess hierarchical micro-, meso-, and macropores. Furthermore, accompanying the prominent photochromic phenomena, reduction of Ti⁴⁺ to Ti³⁺ was observed upon UV irradiation. The TOAs were successfully applied in the adsorption and photocatalytic degradation of several dye molecules. This research introduces a versatile method for preparing stimuli-responsive and porous Ti-oxo-based photocatalysts.

Spherical Mesoporous Materials from Single to Multilevel Architectures

Mesoporous materials with various structures have attracted considerable attention due to their distinctive properties such as large pore sizes, high surface areas, tunable pore structures, and controllable framework compositions. Among them, spherical mesoporous materials (SMMs) are of great interest owing to the unique spherical shape, which show the closed packing nature and lowest surface energy. The open mesopores and short channels of SMMs not only increase the density of high accessible active sites but also facilitate the mass diffusion with short length. These characteristics are particularly useful for applications in catalysis, adsorption, energy storage and conversion, biomedicine, and so on. In addition, the creation of a spherical shape is conformable to the law of natural selection because objects in nature tend to minimize energy, while the sphere is one of the most perfect matter structures. Therefore, the design and synthesis of SMMs are very important from both fundamental and technological viewpoints. Compared to the simple single-level, SMMs with more complex multilevel structures inevitably bring unusual mechanical, electrical, and optical properties, which are highly desired for practical applications. For example, the construction of core-shell structured SMMs has inspired great attention as they can combine multiple components into one functional unit, exhibiting ameliorated or new physicochemical properties, which cannot be obtained from the isolated one. The presence of a hollow cavity in the yolk-shell structure allows sufficient exposure of the core while maintaining the protective ability of the shell, which is conducive to retaining the distance-dependent properties of the core. Multishelled hollow structures consisting of two or more mesoporous shells are expected to show superior activities in various applications compared to their bulk counterparts because more active interfaces and unique compartmentation environments can be provided. Therefore, SMMs from single to multilevel structure represent a class of advanced nanostructured materials with unique structures and fascinating properties. In this Account, we highlight the progresses on the synthesis and applications of SMMs from single to multilevel architectures. The synthetic strategies have been summarized and categorized into (i) the modified Stöber method, (ii) the hydrothermal strategy, (iii) the biphase stratification approach, (iv) the nanoemulsion assembly method, (v) the evaporation induced aggregating assembly (EIAA) method, and (vi) the confined self-assembly strategy. Special emphasis is placed on the synthetic principles and underlying mechanisms for precise control of SMMs over the particle sizes, pore sizes, pore structures and functionalities as well as different levels of architectures. Moreover, the implementation performances in catalysis, drug delivery, and energy related fields have been highlighted. Finally, the opportunities and challenges for the future development of SMMs in terms of synthesis and applications are proposed.

Electron Localization and Transport in SnO2/TiO2 Mesoporous Thin Films: Evidence for a SnO2/SnxTi1-xO2/TiO2 Structure

A study of SnO₂/TiO₂ core/shell films was undertaken to investigate the influences of shell thickness and post deposition sintering on electron localization and transport properties. Electrochemical reduction of the materials resulted in the appearance of a broad visible-near IR absorbance that provided insights into the electronic state(s) within the core/shell structures. As the shell thickness was increased from 0.5 to 5 nm, evidence for the presence of a Sn_xTi_1-xO₂ interfacial state emerged that was physically located between the core and the shell. The lifetime of photoinjected electrons increased with the shell thickness. Electron transport occurred through the SnO₂ core; however, when materials with shell thicknesses ≥2 nm were annealed at 450 °C, a new electron transport pathway through the shell was evident. The data indicate that these materials are best described as SnO₂/Sn_xTi_1-xO₂/TiO₂ where electrons preferentially localize in a Sn_xTi_1-xO₂ interfacial state and transport through SnO₂ and annealed TiO₂ (if present). The implications of these results for applications in solar energy conversion are discussed.

N-Doped Yellow TiO2 Hollow Sphere-Mediated Visible-Light-Driven Efficient Esterification of Alcohol and N-Hydroxyimides to Active Esters

Herein we report a simple synthetic protocol for N-doped yellow TiO₂ (N-TiO₂ ) hollow spheres as an efficient visible-light-active photocatalyst using aqueous titanium peroxocarbonate complex (TPCC) solution as precursor and NH₄ OH. In the developed strategy, the ammonium ion of TPCC and NH₄ OH acts as nitrogen source and structure-directing agent. The synthesized N-TiO₂ hollow spheres are capable of promoting the synthesis of active esters of N-hydroxyimide and alcohol through simultaneous selective oxidation of alcohol to aldehyde followed by cross-dehydrogenative coupling (CDC) under ambient conditions upon irradiation of visible light. It is possible to develop a novel and cost-effective one-pot strategy for the synthesis of important esters and amides on gram scale using the developed strategy. The catalytic activity of N-TiO₂ hollow spheres is much superior to that of other reported N-TiO₂ samples as well as TiO₂ with varying morphology.

Electromagnetic energy in multilayered spherical particles

We obtain exact analytic expressions for (i) the electromagnetic energy radial density within and outside a multilayered sphere and (ii) the total electromagnetic energy stored within its core and each of its shells. Explicit expressions for the special cases of lossless core and shell are also provided. The general solution is based on the compact recursive transfer-matrix method, and its validity includes also magnetic media. The theory is illustrated on examples of electric field enhancement within various metallo-dielectric silica-gold multilayered spheres. The user-friendly MATLAB code, which includes the theoretical treatment, is available as a supplement to the paper.

Synthesis and Electrochemical Energy Storage Applications of Micro/Nanostructured Spherical Materials

Micro/nanostructured spherical materials have been widely explored for electrochemical energy storage due to their exceptional properties, which have also been summarized based on electrode type and material composition. The increased complexity of spherical structures has increased the feasibility of modulating their properties, thereby improving their performance compared with simple spherical structures. This paper comprehensively reviews the synthesis and electrochemical energy storage applications of micro/nanostructured spherical materials. After a brief classification, the concepts and syntheses of micro/nanostructured spherical materials are described in detail, which include hollow, core-shelled, yolk-shelled, double-shelled, and multi-shelled spheres. We then introduce strategies classified into hard-, soft-, and self-templating methods for synthesis of these spherical structures, and also include the concepts of synthetic methodologies. Thereafter, we discuss their applications as electrode materials for lithium-ion batteries and supercapacitors, and sulfur hosts for lithium–sulfur batteries. The superiority of multi-shelled hollow micro/nanospheres for electrochemical energy storage applications is particularly summarized. Subsequently, we conclude this review by presenting the challenges, development, highlights, and future directions of the micro/nanostructured spherical materials for electrochemical energy storage.

Bismuth-Based Photocatalysts for Solar Photocatalytic Carbon Dioxide Conversion

Photocatalytic CO₂ conversion into solar fuels is an effective means for simultaneously solving both the greenhouse effect and energy crisis. In the past ten years, bismuth-based photocatalysts for environmental remediation have experienced a golden period of development. However, solar photocatalytic CO₂ conversion has only been developed over the past five years and, until now, no reviews have been published on bismuth-based photocatalysts for the photocatalytic conversion of CO₂ . For the first time, solar photocatalytic CO₂ conversion systems are reviewed herein. Synthetic methods and photocatalytic CO₂ performances of bismuth-based photocatalysts, including Sillén-structured BiOX (X=Cl, Br, I); Aurivillius-structured Bi₂ MO₆ (M=Mo, W); and Scheelite-structured BiVO₄ , Bi₂ S₃ , BiYO₃ , and BiOIO₃ , are summarized. In addition, activity-enhancing strategies for this photocatalyst family, including oxygen vacancies, bismuth-rich strategy, facet control, conventional type II heterojunction, Z-scheme heterojunction, and cocatalyst deposition, are reviewed. Finally, the main mechanistic research methods, such as in situ FTIR spectroscopy and theoretical calculations, are presented. Challenges and research trends reported in studies of bismuth-based photocatalysts for photocatalytic CO₂ conversion are discussed and summarized.

Electronic Properties of Realistic Anatase TiO2 Nanoparticles from G0W0 Calculations on a Gaussian and Plane Waves Scheme

The electronic properties of realistic (TiO₂)_n nanoparticles (NPs) with cuboctahedral and bipyramidal morphologies are investigated within the many-body perturbation theory (MBPT) G₀W₀ approximation using PBE and hybrid PBEx (12.5% Fock contribution) functionals as starting points. The use of a Gaussian and plane waves (GPW) scheme reduces the usual O⁴ computational time required in this type of calculation close to O³ and thus allows considering explicitly NPs with n up to 165. The analysis of the Kohn-Sham energy orbitals and quasiparticle (QP) energies shows that the optical energy gap (O_gap), the electronic energy gap (E_gap), and the exciton binding energy (ΔE_ex) values decrease with increasing TiO₂ NP size, in agreement with previous work. However, while bipyramidal NPs appear to reach the scalable regime already for n = 84, cuboctahedral NPs reach this regime only above n = 151. Relevant correlations are found and reported that will allow one to predict these electronic properties at the G₀W₀ level in even much larger NPs where these calculations are unaffordable. The present work provides a feasible and practical way to approach the electronic properties of rather large TiO₂ NPs and thus constitutes a further step in the study of realistic nanoparticles of semiconducting oxides.

A facile fabrication strategy for anisotropic photonic crystals using deformable spherical nanoparticles

A 2D anisotropic photonic crystal (APC) of bowl-shaped nanoparticles has been fabricated using deformable spherical nanoparticles. The prepared 2D isotropic photonic crystal (IPC) of spherical nanoparticles is transformed into a 2D APC by a chemical etching process, in which the interiors of the spherical nanoparticles are preferentially dissolved to eventually form a bowl-like morphology. Due to the accurate and controllable deformability of the spherical nanoparticles, the arrangement and orientations of the bowl-shaped nanoparticles are highly ordered and uniform. The morphology, optical properties and surface wettability of the 2D APC are all distinct from those of the prepared 2D IPC. This facile strategy provides an easy and low-cost way to fabricate highly ordered and uniform APCs.

Achieving Secondary Dispersion of Modified Nanoparticles by Hot-Stretching to Enhance Dielectric and Mechanical Properties of Polyarylene Ether Nitrile Composites

Enhanced dielectric and mechanical properties of polyarylene ether nitrile (PEN) are obtained through secondary dispersion of polyaniline functionalized barium titanate (PANI-f-BT) by hot-stretching. PANI-f-BT nanoparticles with different PANI content are successfully prepared via in-situ aniline polymerization technology. The transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopic instrument (XPS) and Thermogravimetric analysis (TGA) results confirm that the PANI layers uniformly enclose on the surface of BaTiO₃ nanoparticles. These nanoparticles are used as functional fillers to compound with PEN (PEN/PANI-f-BT) for studying its effect on the mechanical and dielectric performance of the obtained composites. In addition, the nanocomposites are uniaxial hot-stretched by 50% and 100% at 280 °C to obtain the oriented nanocomposite films. The results exhibit that the PANI-f-BT nanoparticles present good compatibility and dispersion in the PEN matrix, and the hot-stretching endows the second dispersion of PANI-f-BT in PEN resulting in enhanced mechanical properties, crystallinity and permittivity-temperature stability of the nanocomposites. The excellent performances of the nanocomposites indicate that a new approach for preparing high-temperature-resistant dielectric films is provided.

Ultrafine Bi3TaO7 Nanodot-Decorated V, N Codoped TiO2 Nanoblocks for Visible-Light Photocatalytic Activity: Interfacial Effect and Mechanism Insight

Bi₃TaO₇ is a potential photocatalyst because of its high chemical stability, defective fluorite-type structure, and superior mobility of photoinduced holes. However, few studies have focused on the interfacial effects of Bi₃TaO₇-based photocatalysts. In this work, 0D Bi₃TaO₇ nanodot-hybridized 3D V and N codoped TiO₂ nanoblock (B/VNT) composites were first synthesized for the photocatalytic removal of oxytetracycline hydrochloride, 2,4,6-trichlorophenol, and tetrabromobisphenol A. The fabricated B/VNT had a photocatalytic performance superior to that of pristine components, and probable degradation pathways were proposed according to the primary intermediates identified by a gas chromatography-mass spectrometer. Interestingly, on B/VNT, the transfer of interfacial electrons was observed from V/N-TiO₂ to Bi₃TaO₇, and the formed built-in electronic field led to a direct Z-scheme structure, rather than type II, as confirmed by the generated ^•OH and ^•O₂^- radicals and band structures from the density functional theory calculation. Therefore, the strong interfacial electronic interaction on the B/VNT was significant, which drove faster photogenerated charge transfer, more visible-light adsorption, and active ^•OH and ^•O₂^- generation, thus improving the photocatalytic activity.

Construction of CdS/MoS2 heterojunction from core-shell MoS2@Cd-MOF for efficient photocatalytic hydrogen evolution

A new synthetic method was used to develop a CdS/MoS2 heterojunction. Cd-MOF was coated onto the surface of MoS2 flowers to construct a core-shell MoS2@Cd-MOF. Thioacetamide was used as a sulfur source to sulfurize the MoS2@Cd-MOF to form a CdS/MoS2 heterojunction. Since the Cd2+ ions were highly ordered and separated by the organic ligands of the Cd-MOF shell, the as-synthesized CdS/MoS2 heterojunction possessed a large surface area and intimate contact at the heterogeneous interface with a uniform loading of CdS nanoparticles on the MoS2 flowers. Consequently, the CdS/MoS2 heterojunction exhibited a significantly enhanced photocatalytic H2 evolution rate of average 5587 μmol h-1 g-1 under UV-visible light irradiation.

Recent Advances in the Use of Black TiO2 for Production of Hydrogen and Other Solar Fuels

Black TiO₂ has emerged as one of the most promising photocatalysts recently discovered. The reason behind its catalytic activity is considered to be due to the presence of defects and Ti³⁺ species at the surface of black TiO₂ nanostructures, which are crucial for its diverse applications. Moreover, disordered/crystalline surface layers and bulk regions have been identified and appear to influence the intrinsic properties of the material. Here, we present the latest studies on the use of black TiO₂ for metal free hydrogen production, as well as for CO₂ photoreduction and N₂ photofixation. After highlighting the structure/property relations, we conclude with some critical questions and suggest further topics of research in order to better understand the underlying mechanisms of light absorption in black TiO₂ , especially towards solar fuels production.

Pt/POMs/TiO2 composite nanofibers with an enhanced visible-light photocatalytic performance for environmental remediation

Pt/PMo₁₂/TiO₂ composite nanofibers have been prepared and exhibit a highly efficient visible-light photocatalytic performance for removing methyl orange, tetracycline, Bisphenol A and Cr(vi).

Ultrafast spectroscopic study of plasmon-induced hot electron transfer under NIR excitation in Au triangular nanoprism/g-C3N4 for photocatalytic H2 production

Ultrafast spectroscopy reveals plasmon-induced hot electron transfer under NIR excitation in Au triangular nanoprism/g-C₃N₄ for photocatalytic H₂ evolution.