UV/NIR-Light-Triggered Rapid and Reversible Color Switching for Rewritable Smart Fabrics

On-demand green/red light-responsive self-doped SnO2 nanoparticles for single/multi-color transitioning fabrics

Semiconductor/redox-based dual light-induced color switching systems (LCSs) with a visible light response at different wavelengths are highly sought after for efficient redox reactions. In this work, Sn2+ self-doped SnO2 has been designed as nanophotocatalysts for preparing visible light-responsive inks/fabrics with single/multi-color abilities. The self-doping of SnO2 nanoparticles results in the formation of oxygen vacancies due to charge compensation effects leading to electron-driven photoreduction and photooxidation of LSC inks. By mixing SnO2-x nanoparticles dispersions with specific redox-sensitive dyes can lead to the creation of well-designed sets of visible light-responsive semiconductor-driven LCS systems with both single-color (RGB) and multi-color (violet and green) changes. The exposure of LCS inks to green (550 nm) light culminates in the rapid photoreduction of the inks to decolorized state, while red (660 nm) light initiates the photooxidation in air. The combination of the LCS inks with -OH-rich polymers can be coated on the hydrophobic surface of the layered fabric to produce photo-responsive fabrics with single/multi-color response. The interaction of green light with the semiconductor-driven LCS systems allows the remote photo-printing of different images/letters on the LCS fabrics. Spontaneous erasure can be achieved by red light with high stability and repeatability (>35 cycles). The research in this paper provides new perspectives and insights for the development of new color-changing materials with potential applications as light-activated sensors and display units.

Photochromism of TiO2 Nanoparticles Doped with Molybdenum Ions: Neutral Color Change from Colorless to Black via Gray

Transition metal oxide nanoparticles have been extensively studied for the development of smart windows which are expected to be a promising technology to save energy in buildings. However, most of them turn blue under UV irradiation. Since the blue coloration affects the color of objects through the windows, the development of materials with a neutral color which hardly disturbs the view is more beneficial. In this work, we prepared a colorless-transparent TiO₂ colloidal solution containing Mo⁶⁺ ions (Mo-TiO₂), which turns black via gray in a nitrogen atmosphere under UV irradiation. An absorption peak was observed at 535 nm, which increased with an increase in the UV irradiation time and reached a constant value (Abs_max). The Abs_max value increased linearly with an increase in the Mo doping amount. We demonstrated that the photochromic behavior of the Mo-TiO₂ nanoparticles is completely different from that of pure MoO₃ or the mixture of MoO₃ and TiO₂. In addition, we performed a kinetic study to elucidate the mechanism and found that the coloration rate at the initial stage decreased with an increase in Mo doping amount. Based on the kinetic analysis, the following results are obtained: a color center is formed at a deeper energy level than the Mo dopant level; the number of the color center depends on the Mo doping amount, and the color center traps the photogenerated electrons more rapidly when it is isolated. The black color was bleached by purging the air in the solution. In particular, the gray state which is observed as a transient color is promising for the development of smart windows to shield the sunlight while allowing a clear and undistorted view.

Dual pH-/Photo-Responsive Color Switching Systems for Dynamic Rewritable Paper

Smart color switching materials that can change color with a fast response and a high reversibility have attracted increasing attention in color-on-demand applications. However, most of them can only respond to a single stimulus from their external environment, which dramatically limits their broad applications. To address this problem, we report a new strategy in developing a dual pH-/photo-responsive color switching system by coupling the pH-dependent and redox-driven color switchable neutral red (NR) with photoreductive TiO_2-x nanoparticles. The biodegradable TiO_2-x nanoparticles/NR/agarose gel film shows a rapid color switching between yellow and red upon stimulation with acidic/basic vapors in more than 20 cycles because of the protonation and deprotonation process of NR. Moreover, the film shows interesting photoreversible color switching properties under both acidic and basic conditions, including a fast response time and a high reversibility. Taking advantage of the excellent dual pH-/photo-responsive color switching properties, we demonstrated the potential applications of the TiO_2-x nanoparticles/NR/agarose gel film in dynamic rewritable paper, in which the created patterns by photo-printing produce dynamic color changing upon applying an acidic or a basic vapor. We believe that the result will enable a new path for the development of dual- and even multi-responsive color switching systems, broadening their new applications.

Synthesis of MnO2-Ag Nanojunctions with Plasmon-Enhanced Photocatalytic and Photothermal Effects for Constructing Rewritable Mono-/Multi-Color Fabrics

Semiconductor-mediated photoreversible color switching systems (PCSSs) have great potential to replace traditional photochromic materials, and the key is to obtain semiconductors with unique photocatalytic and photothermal features. Herein, we have developed MnO₂-Ag nanojunctions with plasmon-enhanced photocatalytic and photothermal effects for PCSSs. MnO₂-Ag nanojunctions are solvothermally synthesized with Mn(CH₃COO)₃, KMnO₄, and AgNO₃ in diethylene glycol as precursors, and they are composed of MnO₂ nanoparticles (∼30 nm) that are decorated by Ag nanodots (∼6 nm). The presence of Ag confers an enhanced visible photoabsorption with a narrow band gap for MnO₂ (E_g = 1.82 eV) and a weak/broad photoabsorption tail (∼875 nm) compared to that of pure MnO₂ (2.45 eV, ∼625 nm). By coupling MnO₂-Ag nanojunctions with various redox dyes, some PCSS inks can be obtained, and especially, the inks containing hydroxyethyl cellulose could be used to prepare rewritable fabrics. When inks and fabrics are irradiated by 475 nm light, rapid discoloration can occur, resulting from the photocatalytic reduction of the dye. Contrarily, the irradiation of 808 nm light promotes the rapid recoloration since Ag nanodots with plasmonic effects in the nanojunctions can absorb light to generate heat, which facilitates the oxidization of leuco dyes in air. Consequently, remote printing of figures was attained on the rewritable fabrics via 475 nm light illumination, and then, the erasure was performed by 808 nm light illumination in an O₂ atmosphere, with high reversibility and cycling stability. Therefore, MnO₂-Ag nanojunctions have tremendous promise for rewritable media, and the introduction of metal-semiconductor junctions as a nanophotocatalyst offers new insights for PCSSs.

Polyoxometalate-Containing Supramolecular Gels

Supramolecular gels are important soft materials with various applications, which are fabricated through hydrogen bonding, π-π stacking, electrostatic or host-guest interactions. Introducing functional groups, especially inorganic components, is an efficient strategy to obtain gels with robust architecture and high performance. Polyoxometalates (POMs), as a class of negatively-charged clusters, have defined structures and multiple interaction sites, resulting in their potential as building blocks for constructing POM-containing supramolecular gels. The introduction of POMs into gels not only provides strong driving forces for the formation of gels due to the characteristics of charged cluster and oxygen-rich surface, but also brings new properties sourcing from unique electronic structures of POMs. Though many POM-containing gels have been reported, a comprehensive review is still absent. Herein, the concept of POM-containing gels is discussed, following with the design strategies and driving forces. To better understand the results in the literature, detailed examples, which are classified into several categories based on the types of organic components, are presented to illustrate the gelation process and gel structures. Moreover, applications of POM-containing gels in energy chemistry, sustainable chemistry and other aspects are also reviewed, as well as the future developments of this field. This article is protected by copyright. All rights reserved.

Multi-Component Collaborative Step-by-Step Coloring Strategy to Achieve High-Performance Light-Responsive Color-Switching

Light-responsive color-switching materials (LCMs) are long-lasting hot fields. However, non-ideal comprehensive performance (such as color contrast and retention time cannot be combined, unsatisfactory repeatability, and non-automated coloring mode) significantly hinder their development toward high-end products. Herein, the development of LCMs that exhibit long retention time, good color contrast, repeatability, and the property of automatic coloring is reported. The realization of this goal stems from the adoption of a bio-inspired multi-component collaborative step-by-step coloring strategy. Under this strategy, a conventional one-step photochromic process is divided into a "light+heat" controlled multi-step process for the fabrication of the desired LCMs. The obtained LCMs can effectively resist the long-troubled ambient-light interference and avoid its inherent yellow background, thereby achieving the longest retention time and good repeatability. Multiple colors are generated and ultra-fast imaging compatible with the laser-printing technology is also realized. The application potential of the materials in short-term reusable identity cards, absorptive readers, billboards, and shelf labels is demonstrated. The results reported herein can potentially help in developing and designing various high-performance, switchable materials that can be used for the production of high-end products.

Achieving UV and X-ray Dual Photochromism in a Metal-Organic Hybrid via Structural Modulation

Rational design and synthesis of new photochromic sensors have been active research areas of inquiry, particularly on how to predict and tailor their properties and functionalities. Herein, two thulium 2,2':6',2''-terpyridine-4'-carboxylate (TPC)-functionalized metal-organic hybrids, Tm(TPC)₂(HCOO)(H₂O) (TmTPC-1) and Tm(TPC)(HCOO)₂ (TmTPC-2) with different photochromic response behaviors, have been successfully prepared, allowing for straightforward investigations of the structure-property correlation. Single-crystal X-ray diffraction and electron paramagnetic resonance analyses revealed that the incorporation of a unique dangling decorating TPC unit in TmTPC-1 offers a shorter and more accessible π-π interaction pathway between the adjacent TPC moieties than that in TmTPC-2. Such a structural feature leads to the production of radical species via a photoinduced intermolecular electron-transfer (IeMCT) process upon UV or X-ray irradiation, which ultimately endows TmTPC-1 with a rather unusual UV and X-ray dual photochromism. A linear relationship between the change of UV-vis absorbance intensity and X-ray dose was established, making TmTPC-1 a promising dosimeter for X-ray radiation with an extremely high energy threshold (30 kGy). To advance the development for real-world application, we have fabricated polyvinylidene fluoride (PVDF) membranes incorporating TmTPC-1 for functioning either as a UV imager or as an X-ray radiation indicator. Lastly, TmTPC-1 exhibits high thermal stability (up to 400 °C) and radioresistance (at least 900 kGy), and also excellent reversibility of photochromic transformation (at least 5 cycles).

Integration of Conductive Materials with Textile Structures, an Overview

In the last three decades, the development of new kinds of textiles, so-called smart and interactive textiles, has continued unabated. Smart textile materials and their applications are set to drastically boom as the demand for these textiles has been increasing by the emergence of new fibers, new fabrics, and innovative processing technologies. Moreover, people are eagerly demanding washable, flexible, lightweight, and robust e-textiles. These features depend on the properties of the starting material, the post-treatment, and the integration techniques. In this work, a comprehensive review has been conducted on the integration techniques of conductive materials in and onto a textile structure. The review showed that an e-textile can be developed by applying a conductive component on the surface of a textile substrate via plating, printing, coating, and other surface techniques, or by producing a textile substrate from metals and inherently conductive polymers via the creation of fibers and construction of yarns and fabrics with these. In addition, conductive filament fibers or yarns can be also integrated into conventional textile substrates during the fabrication like braiding, weaving, and knitting or as a post-fabrication of the textile fabric via embroidering. Additionally, layer-by-layer 3D printing of the entire smart textile components is possible, and the concept of 4D could play a significant role in advancing the status of smart textiles to a new level.

Blue/red light-triggered reversible color switching based on CeO2-x nanodots for constructing rewritable smart fabrics

Photoreversible color switching systems (PCSSs) have attracted increasing attention in various applications, but in most PCSSs the discoloration process usually relies on harmful UV light as a stimulus and the recoloration requires high temperature. To solve these problems, we have designed and prepared CeO2-x nanodots as novel photocatalytic components in PCSSs that respond to two kinds of visible light. CeO2-x nanodots are prepared by a solvothermal reaction with l-ascorbic acid as the reducing agent. CeO2-x nanodots with a size of ∼2 nm have a high concentration of oxygen vacancies, which confers a broadened photoabsorption with an edge at 500 nm, as well as a weak photoabsorption tail in the visible region (500-800 nm). To realize the color switching, both the CeO2-x/Dye/H2O solution and CeO2-x/dye/hydroxyethyl cellulose (HEC)-coated fabrics have been prepared. Under blue (450 nm) light irradiation, both the solution and fabric show a rapid discoloration in 30 s and 150 s, respectively, due to the efficient photocatalytic reduction of the redox dye by CeO2-x. Conversely, red (630 nm) light irradiation with air confers a rapid recoloration in 35 s for the solution and 200 s for the fabric, resulting from CeO2-x-mediated self-catalyzed oxidation. In particular, the required images and letters can be remotely printed on CeO2-x/Dye/HEC-coated T-shirts with a 450 nm laser pen, and then erased with 630 nm light, with high reversibility and stability. Therefore, the present CeO2-x/Dye/HEC PCSSs have great potential to construct rewritable smart fabrics for various applications.

Highly Flexible Multilayered e-Skins for Thermal-Magnetic-Mechanical Triple Sensors and Intelligent Grippers

This work reports a novel triple-functional electronic skin (e-skin) which shows both wonderful thermal-magnetic-mechanical sensing performance and interesting magnetic actuation behavior. The flexible e-skin comprises thermo-sensitive, magnetic, and conductive tri-components, and their sensitive characteristics under 5-70 °C, 0-1200 mT, and 0.1-5.1 MΩ are studied, respectively. Owing to the unique piezoresistive characteristic and magnetorheological effect, the e-skin exhibits a rapid response time (38 ms) to the external stimuli. The assembled e-skin with the triple-layer structure can act as a functional sensor to monitor various human motions, magnetic fields, and environmental temperatures. Based on this e-skin, an intelligent magneto-active gripper is further developed, and it can be used to grasp and transport targets by the actuated force of magnetic field under various working conditions. Importantly, the multi-functional sensing capability endows the gripper with real-time deformation and ambient temperature perception characteristics. As a result, because of the ideal multi-field coupling sensing and magnetic active features, this e-skin shows a wide prospect in wearable electronics, man-machine interactions, and intelligent transport systems.

Photoreductive BiOCl Ultrathin Nanosheets for Highly Efficient Photocatalytic Color Switching

The reversible photocatalytic color switching systems (PCSSs) driven by semiconductor nanoparticles have attracted considerable attention because of their wide applications. However, the developed semiconductor nanoparticles with photoreductive activity are mainly limited to TiO₂-based photocatalysts, which greatly hinder their broad applications. Here we report a cocapping ligand-assisted strategy for the development of photoreductive BiOCl ultrathin nanosheets with abundant oxygen vacancies. Both the cocapping ligands and oxygen vacancies in BiOCl ultrathin nanosheets act as sacrificial electron donors to efficiently scavenge the photogenerated holes, endowing the BiOCl ultrathin nanosheets high photoreductive activity and thus enabling the photocatalytic color switching of redox dyes, such as methylene blue (MB) and neutral red. By successfully integrating the BiOCl ultrathin nanosheet/MB/H₂O color switching system with poly(vinyl alcohol) hydrogel to fabricate a twistable gel film and simultaneously solving the dye-leaching issue of the gel film in a water environment, we further demonstrate its application in a colorimetric oxygen indicator for food packaging, exhibiting high sensitivity to monitor oxygen leakage by the naked eye. We believe the work opens a new avenue for designing photoreductive semiconductor nanomaterials to enrich the PCSSs and their applications.