Wenshou Wang

Porous TiO2/C Nanocomposite Shells As a High-Performance Anode Material for Lithium-Ion Batteries

Porous TiO2/C nanocomposite shells with high capacity, excellent cycle stability, and rate performance have been prepared. The synthesis involves coating colloidal TiO2 nanoshells with a resorcinol-formaldehyde (RF) layer with controllable thickness through a sol-gel-like process, and calcining the composites at 700 °C in an inert atmosphere to induce crystallization from amorphous TiO2 to anatase and simultaneous carbonization from RF to carbon. The cross-linked RF polymer contributes to the high stability of the shell morphology and the porous nature of the shells. A strong dependence of the capacity on the amount of incorporated carbon has been revealed, allowing the optimization of the electrode structure for high-rate cell performance.

Epitaxial Growth of Shape-Controlled Bi2Te3−Te Heterogeneous Nanostructures

A one-pot solution process has been devised to synthesize colloidal Bi2Te3-Te heterogeneous nanostructures (HNs) that comprise Bi2Te3 nanoplates and Te nanorods. By controlling the reaction kinetics, the reaction of TeO3(2-) and Bi(3+) in the presence of hydrazine first produces uniform Te nanorods and then grows Bi2Te3 nanoplates on the tips and surfaces of these Te nanorods, forming various shapes including nails, barbells, syringes, and accordions. The specific topological arrangement realized arises from the peculiar anisotropic reactivity of the first formed Te nanorods, whose tips are subsequently exploited to seed the heterogeneous nucleation of Bi2Te3 as enabled by the similar crystal structure and the small lattice mismatch between Te and Bi2Te3. Three important processes, heterogeneous nucleation of Bi2Te3 on the tips and/or surface of Te nanorods, homogeneous nucleation of Bi2Te3, and the direct reaction of a Bi precursor and Te nanorods to form hollow structures via the Kirkendall Effect, occur under various conditions. The manipulation of these processes provides a robust means for the fine shape control of Bi2Te3-Te HNs. It is envisioned that the tailored synthesis of Bi2Te3-Te HNs may promise unique opportunities for producing thermoelectric materials with greatly enhanced performance.

Photocatalytic colour switching of redox dyes for ink-free light-printable rewritable paper

The invention of paper as writing materials has greatly contributed to the development and spread of civilization. However, its large-scale production and usage have also brought significant environment and sustainability problems to modern society. To reduce paper production and consumption, it is highly desirable to develop alternative rewritable media that can be used multiple times. Herein we report the fabrication of a rewritable paper based on colour switching of commercial redox dyes using titanium oxide-assisted photocatalytic reactions. The resulting paper does not require additional inks and can be efficiently printed using ultraviolet light and erased by heating over 20 cycles without significant loss in contrast and resolution. The legibility of prints can retain over several days. We believe this rewritable paper represents an attractive alternative to regular paper in meeting the increasing global needs for sustainability and environmental protection.

Nanocrystalline TiO2-Catalyzed Photoreversible Color Switching

We report a novel photoreversible color switching system based on the photocatalytic activity of TiO2 nanocrystals and the redox-driven color switching property of methylene blue (MB). This system rapidly changes from blue to colorless under UV irradiation and recovers its original blue color under visible light irradiation. We have identified four major competing reactions that contribute to the photoreversible switching, among which two are dominant: the decoloration process is mainly driven by the reduction of MB to leuco MB by photogenerated electrons from TiO2 nanocrystals under UV irradiation, and the recoloration process operates by the TiO2-induced self-catalyzed oxidation of LMB under visible irradiation. Compared with the conventional color switching systems based on photoisomerization of chromophores, our system has not only low toxicity but also significantly improved switching rate and cycling performance. It is envisioned that this photoreversible system may promise unique opportunities for many light-driven actuating or color switching applications.

Enhanced Photoreversible Color Switching of Redox Dyes Catalyzed by Barium-Doped TiO2 Nanocrystals†

Colloidal barium-doped TiO2 nanocrystals have been developed that enable the highly reversible light-responsive color switching of redox dyes with excellent cycling performance and high switching rates. Oxygen vacancies resulting from the Ba doping serve as effective sacrificial electron donors (SEDs) to scavenge the holes photogenerated in TiO2 nanocrystals under UV irradiation and subsequently promote the reduction of methylene blue to its colorless leuco form. Effective color switching can therefore be realized without relying on external SEDs, thus greatly increasing the number of switching cycles. Bau2005doping can also accelerate the recoloration under visible-light irradiation by shifting the absorption edge of TiO2 nanocrystals to a shorter wavelength. Such a system can be further casted into a solid film to produce a rewritable paper on which letters and patters can be repeatedly printed using UV light and then erased by heating; this process can be repeated for many cycles and does not require additional inks.

Photocatalytic Color Switching of Transition Metal Hexacyanometalate Nanoparticles for High-Performance Light-Printable Rewritable Paper

Developing efficient photoreversible color switching systems for constructing rewritable paper is of significant practical interest owing to the potential environmental benefits including forest conservation, pollution reduction, and resource sustainability. Here we report that the color change associated with the redox chemistry of nanoparticles of Prussian blue and its analogues could be integrated with the photocatalytic activity of TiO2 nanoparticles to construct a class of new photoreversible color switching systems, which can be conveniently utilized for fabricating ink-free, light printable rewritable paper with various working colors. The current system also addresses the phase separation issue of the previous organic dye-based color switching system so that it can be conveniently applied to the surface of conventional paper to produce an ink-free light printable rewritable paper that has the same feel and appearance as the conventional paper. With its additional advantages such as excellent scalability and outstanding rewriting performance (reversibility >80 times, legible time >5 days, and resolution >5 μm), this novel system can serve as an eco-friendly alternative to regular paper in meeting the increasing global needs for environment protection and resource sustainability.

Photocatalytic removal of hexavalent chromium by newly designed and highly reductive TiO2 nanocrystals

Hexavalent chromium Cr(VI), a highly toxic oxyanion, widely occurs in drinking water supplies. This study designed and synthesized a new type of highly reductive TiO2 nanocrystals for photochemical Cr(VI) removal, via the thermal hydrolysis of TiCl4 in the presence of diethylene glycol (DEG). Surface analyses and hydroxyl radical measurements suggested that DEG was chemically bonded on TiO2 surface that resulted in an internal hole-scavenging effect and a high electron-releasing capacity, making it advantageous to conventional TiO2 materials. Upon UV irradiation, the synthesized TiO2 photocatalyst exhibited fast Cr(VI) reduction kinetics in diverse water chemical conditions. Fast elimination of Cr(VI) was achieved on a time scale of seconds in drinking water matrices. The removal of Cr(VI) by reductive TiO2 exhibited a three-stage kinetic behavior: an initial fast-reaction phase, a lag phase resulting from surface precipitation of Cr(OH)3(s), and a final reaction phase due to surface regeneration from oxidation-reduction induced ripening process. The lag phase disappeared in acidic conditions that prevented the formation of Cr(OH)3(s). The catalyst exhibited extremely high electron-releasing capacity that can be reused for multiple cycles of Cr(VI) removal in drinking water treatment.

Photocatalytic Self-Doped SnO2-x Nanocrystals Drive Visible-Light-Responsive Color Switching

Visible light-responsive reversible color switching systems are attractive to many applications since visible light has superior penetration and causes far less damages to organic molecules than UV. Here we report that self-doping of Sn2+ in SnO2-x nanocrystals red shifts their absorption to visible region and simultaneously produces oxygen vacancies, which can effectively scavenge photogenerated holes and thus enable color switching of redox dyes using visible light. Wavelength selective switching can also be achieved by coupling the photocatalytic activity of the SnO2-x NCs with the color switching kinetics of different redox dyes. The fast light response enables the further fabrication of a solid film that can be repeatedly written freehand using a visible laser pen or projection printing through a photomask. This discovery represents a big step forward towards practical applications, especially in areas with concerns on the safety and photodamage issues of UV light.Visible-light-responsive reversible color-switching systems are attractive to many applications because visible light has superior penetration and causes far less damage to organic molecules than UV. Herein, we report that self-doping of SnO2-x nanocrystals with Sn2+ red-shifts their absorption to the visible region and simultaneously produces oxygen vacancies, which can effectively scavenge photogenerated holes and thus enable the color switching of redox dyes using visible light. Wavelength-selective switching can also be achieved by coupling the photocatalytic activity of the SnO2-x NCs with the color-switching kinetics of different redox dyes. The fast light response enables the further fabrication of a solid film that can be repeatedly written on using a visible laser pen or projection printing through a photomask. This discovery represents a big step forward towards practical applications, especially in areas in which safety issues and photodamage by UV light are of concern.

Reversible Assembly and Dynamic Plasmonic Tuning of Ag Nanoparticles Enabled by Limited Ligand Protection

Dynamic manipulation of optical properties through the reversible assembly of plasmonic nanoparticles offers great opportunities for practical applications in many fields. The previous success, however, has been limited to Au nanoparticles. Reversible assembly and plasmonic tuning of Ag nanoparticles (AgNPs) have remained a significant challenge due to difficulty in finding an appropriate surface agent that can effectively stabilize the particle surface and control their interactions. Here, we overcome the challenge by developing a limited-ligand-protection (LLP) strategy for introducing poly(acrylic acid) with precisely controlled coverage to the AgNP surface to not only sufficiently stabilize the nanoparticles but also enable effective control over the surface charge and particle interaction through pH variation. The as-synthesized AgNPs can be reversibly assembled and disassembled and accordingly display broadly tunable coupling of plasmonic properties. Compared to the Au-based system, the success in the reversible assembly of AgNPs represents a significant step toward practical applications such as colorimetric pressure sensing because they offer many advantages, including broader spectral tuning range, higher color contrast, a one-pot process, and low materials and production cost. This work also highlights LLP as a new avenue for controlling the interparticle forces, their reversible assembly, and dynamic coupling of physical properties.

Fast-Response Flexible Photochromic Gels for Self-Erasing Rewritable Media and Colorimetric Oxygen Indicator Applications

Photoreversible color switching that can change colors with fast response and high stability is urgently desired in color-on-demand applications. Yet, developing such materials has long been a significant challenge. In this work, a strategy based on the integration of TiO2 nanoparticle (NP) photocatalytic color switching of redox dyes and poly(vinyl alcohol) gel matrix could produce robust and flexible photochromic gels (FPGs) that exhibit fast light-responsive time and high photoreversible stability. Benefited by the soft network structures and monomeric form of redox dyes in the FPG maintained by poly(vinyl alcohol) and ethylene glycol molecules, as well as enhanced photoreductive activity of TiO2 NPs modified by both surface ligands and oxygen vacancies, the FPG exhibits long photoreversible switching cycles (≥50 times), decoloration in a short period of less than 8 s upon UV illumination, and recoloration in 16 min in ambient air and rapidly in 140 s upon near-infrared light illumination. Consequently, the excellent photoreversible color switching of the FPGs is highly applicable as both self-erasing rewritable media and colorimetric oxygen indicators. We believe that the current systems represent a big step forward toward practical applications, such as time-sensitive information storage, colorimetric oxygen sensor, and potentially many other technologies.