Iridescent Films Research Articles

The preparation of cellulose nanocrystal/1,3‐butylene glycol composite structural color films and humidity‐responsive

AbstractCellulose nanocrystals (CNCs) can self‐assemble in an aqueous solution to form a chiral nematic liquid crystal structure, which is maintained in the subsequent solid iridescent film and presents a unique structural color. In this study, in order to explore the effect of ultrasound on the optical properties of CNC, CNC films with a chiral nematic helix structure and different structural colors were prepared by ultrasonic‐assisted dispersion and natural static evaporation. Results showed that prolonging the ultrasonic duration significantly increased layer pitches and red‐shifted the color of CNC films, likely due to increased steric hindrance and a larger electric double layer. On this basis, CNC/1,3‐butylene glycol (BG) composite films were prepared with uniform and structural color. Results showed that BG was incorporated into CNCs most likely through hydrogen bonds. Moreover, the presence of BG increased the layer pitches and red‐shifted the structural color of CNC/BG composite films, which indicates that structural color of the film has a good stability response to humidity. On the whole, our work provides a new point of view in understanding the role of ultrasound and small‐molecule additives in the preparation of CNC‐based films.

Iridescent Polymeric Film with Tunable Color Responses to Ultra-Trace Seb

Iridescent Polymeric Film with Tunable Color Responses to Ultra-Trace Seb

On an iridescent film of carbon nitride grown on an aluminum sheet and composed of overlapped oak-leaf shaped nanoparticles

An iridescent carbon nitrogen film g-C3N4 was formed by chemical vapor deposition (CVD) on an aluminum sheet starting from melamine cyanurate as a precursor at a temperature of 600 °C. The film was easily detached and thoroughly characterized using various techniques, including X-ray powder diffraction, UV − Vis and infrared spectroscopies, and field emission scanning microscopy. The material exhibited a particular layered microstructure consisting of stacks of nano-/micro-sized oak-leaf-shaped particles. The UV − Vis spectrum exhibited two defined bands corresponding to localized electrons; however, the apparent direct bandgap was 3.1 eV, similar to that of bulk g-C3N4. The IR spectrum was similar to that of bulk material, with differences in band intensities of the bending zone of tri-s-triazine units, in the range of 900 − 800 cm−1. The 2 D unfolding of the material with the formation of a thin layer composed of oak-leaf-like particles led to structural iridescent colors. For this reason, applications in optoelectronics and photovoltaic cells based on thin films prepared by the reported synthesis route may be envisaged, taking advantage of the properties of structural colors.

Ultrasonication pretreatment assisted rapid co-assembly of cellulose nanocrystal and metal ion for multifunctional application

Co-assembly of metal ion and cellulose nanocrystals (CNC) is a promising strategy to fabricate novel iridescent CNC materials with advanced applications. By combining ultrasonication pretreatment and vacuum-assisted self-assembly (VASA) technique, a facile and rapid strategy is proposed to prepare the Mn2+-doped carboxylated CNC (C-CNC) iridescent films with multifunctional application. The ultrasonication pretreatment temporarily disassembles the aggregates of C-CNC nanorods caused by the electrostatic interaction between negative charged C-CNC and Mn2+. The subsequent VASA process accelerates the self-assembly of chiral liquid crystals prior to the re-agglomeration of C-CNC by the bridge effect of Mn2+. Furthermore, the as-prepared Mn2+/CNC film exhibits a rapid and visible color change in ammonia atmosphere along with the formation of MnO2. The reversible change can be realized by the stimulation of reducing agent. The derived MnO2/C-CNC composite film displays efficient removal of methylene blue dye in aqueous solution by both of adsorption and degradation procedure.

Chiral Cellulose Nanocrystal Humidity-Responsive Iridescent Films with Glucan for Tuned Iridescence and Reinforced Mechanics.

The fabrication of biomimetic photonic materials with environmental stimuli-responsive functions from entirely biobased materials is becoming increasingly challenging with the growing demand for biodegradable materials. Herein, the effect of glucan with different molecular weights on the mechanical performance and tunable structural color of iridescent CNC composite films was investigated. The existence of glucan did not influence the self-assembly performance of CNCs, but rather led to an improvement in the mechanical performance, enabling cholesteric CNC composite films with an adjustable structural color. Simultaneously, the iridescent films showed a conspicuous redshift and enlarged initial pitch without obstruction of the chiral structure. In response to environmental humidity, the structural colors of the iridescent composite films can be changed by regulating their chiral nematic structure. In particular, the films demonstrate a reversible structural color change between blue and red at RH between 50 and 98%. The resulting biobased iridescent composite films have potential applications in decorative coating, optical and humidity sensing, and anticounterfeiting.

Shear-Coated Linear Birefringent and Chiral Cellulose Nanocrystal Films Prepared from Non-Sonicated Suspensions with Different Storage Time.

Nanocelluloses are very attractive materials for creating structured films with unique optical properties using different preparation techniques. Evaporation-induced self-assembly of cellulose nanocrystals (CNC) aqueous suspensions produces iridescent films with selective circular Bragg reflection. Blade coating of sonicated CNC suspensions leads to birefringent CNC films. In this work, fabrication of both birefringent and chiral films from non-sonicated CNC suspensions using a shear-coating method is studied. Polarization optical microscopy and steady-state viscosity profiles show that non-sonicated CNC suspensions (concentration of 6.5 wt%) evolve with storage time from a gel-like shear-thinning fluid to a mixture of isotropic and chiral nematic liquid crystalline phases. Shear-coated films prepared from non-sonicated fresh CNC suspensions are birefringent, whereas films prepared from suspensions stored several weeks show reflection of left-handed polarized light. Quantification of linear and circular birefringence as well circular dichroism in the films is achieved by using a Mueller matrix formalism.

Thermal annealing of iridescent cellulose nanocrystal films

The properties of chiral nematic and iridescent cellulose nanocrystal films with different monovalent cations (CNC-X) obtained through evaporation-induced self-assembly (EISA) can be modified by a variety of external stimuli. Here, we study the transformations of their optical and structural properties when the films are thermally annealed at 200 °C and 240 °C for up to 2 days. The chiral nematic structure of the most thermally stable films is not destroyed even after extensive heating due to the thermochemical stability of the cellulose backbone and the presence of surface alkali counterions, which suppress catalysis of early stage degradation. Despite the resilience of the cholesteric structure and the overall integrity of heated CNC-X films, thermal annealing is often accompanied by reduction of iridescence, birefringence, and transparency, as well as formation of degradation products. The versatility, sustainability, and stability of CNC-X films highlight their potential as temperature indicators and photonic devices.

Photoluminescence-enhanced cholesteric films: Coassembling copper nanoclusters with cellulose nanocrystals

Iridescent and luminescent composite films were fabricated through a coassembly strategy, in which glutathione-stabilized copper nanoclusters (GSH-CuNCs) were incorporated into chiral nematic structures of a cellulose nanocrystal (CNC) film. Through variations in the helical pitch, these composite films exhibited broadband reflection. The fluorescence emission spectrum of the composite film exhibited peaks at 439 and 600 nm, corresponding to crystallization-induced emission from CNCs and assembly-induced emission from CuNCs. The enhanced luminescence and prolonged lifetime of the composite film were attributed to the confinement effect of solid layers and attendant intermolecular interactions. By tuning the reaction time, temperature, and pH of the solution, the emission color and intensity of the CuNCs could be changed. At appropriate GSH and Cu2+ concentrations, the chiral organization of GSH-CuNCs enabled the composite CNC film to exhibit right-handed chiral fluorescence with an asymmetry factor of −0.16. Luminescent composite films were employed to fabricate LEDs with custom colors and patterns.

Biomimetic Mesoporous Cobalt Ferrite/Carbon Nanoflake Helices for Freestanding Lithium‐Ion Battery Anodes

AbstractStructural biomimicry is a fascinating concept to explore hierarchically organized nanomaterials for mechanical structures, catalysis, sensing, and energy storage applications. Here we report the fabrication of biomimetic mesoporous cobalt ferrite/carbon nanoflake materials with helical morphologies and evaluate their electrochemical properties as free‐standing lithium‐ion battery (LIB) anodes. Iridescent chiral nematic mesoporous chitosan films obtained from crab shells were combined with binary metallic ions to afford helical cobalt ferrite/chitosan membranes. The cobalt ferrite/chitosan composites were thermally converted to cobalt ferrite/carbon replicas with hybrid nanoflakes arranged in a twisted Bouligand‐type mesoporous network. The structure of the materials was probed by electron microscopy, powder X‐ray diffraction, and Raman spectroscopy. We directly used these freestanding cobalt ferrite/carbon films as binder‐ and additive‐free LIB anodes, where they showed a first discharge capacity of 862 mAh g−1 (at 100 mA g−1), which faded during subsequent charge‐discharge cycles. Our work demonstrates a new potential use of chiroptical chitosan membranes to develop energy storage materials, a process that may be extended to other metal‐oxide based components.

Co-assembling Polysaccharide Nanocrystals and Nanofibers for Robust Chiral Iridescent Films.

Assembling robust chiral biopolymer structures without compromising vivid optical iridescence is a grand challenge for biocomposite materials. Herein, we report a hierarchical nanocellulose nanostructure with a helicoidal organization co-assembled from chiral rigid cellulose nanocrystals (CNCs) and longer nanofibers isolated from the hydrolyzed wood pulp. This resulting highly iridescent chiral nanocellulose material is much tougher than traditional chiral CNC films. We found that the mixed nanocellulose are composed of needle-like nanocrystals and very long (up to 800 nm) flexible cellulose nanofibers (CNFs). Large-scale molecular simulation indicates that enhanced dynamic hydrogen bonding with labile networking facilitates mechanical reinforcement, owing to increased nanocrystal length, the co-assembly of nanofibrils in mixed bundles, and interchain entanglements. This study provides a novel strategy to transform the wood pulp residues into high-value-added photonic-bound polysaccharide materials. These hierarchical biomaterials can overcome the conflicting trends in designing balanced mechanical and optical performance of chiral biofilms and their conversion to robust chiral photonic materials with enhanced performance.

Light- and Humidity-Responsive Chiral Nematic Photonic Crystal Films Based on Cellulose Nanocrystals.

Light- and humidity-responsive chiral nematic photonic crystal (PC) films containing cellulose nanocrystals (CNCs) were fabricated. A photoactive polymer with hydrophilic groups, poly-(3,3'-benzophenone-4,4'-dicarboxylic acid dicarboxylate polyethylene glycol) ester, was coassembled with CNCs to form flexible iridescent films with a tunable chiral nematic order. In the coassembly process, the intermolecular hydrogen bonds of CNCs were weakened, which facilitated the fine regulation of the chiral PC nanostructure. The PC films displayed sensitive responses to both light and humidity. With increasing humidity from 30 to 100%, the chiral nematic helix pitch increased from 328 to 422 nm. The color of the PC films changed from blue to green, yellow, orange, and dark red with increasing relative humidity. Over 15 min of light irradiation, the absorption intensity of the films increased gradually. The light and humidity responses of the films were reversible. The films maintained their variable cholesteric liquid crystal texture and helical lamellar structure after light irradiation at different humidities. These PC films are expected to be useful in intelligent coatings and 3D printing.

Rapid fabrication of iridescent alumina films supported on an aluminium substrate by high voltage anodization

Porous alumina photonic crystals supported on an aluminium substrate were successfully fabricated by applying a high voltage for a short period of time in an oxalic acid solution. These films show brilliant structural colors, and the highest applied voltage was 250 V. The experimental results indicate that a photonic crystal supported on an aluminium substrate shows a uniform structural colour when a voltage is increased to 150 V and, with oxidation time increasing, the colour gradually turns blue. A reasonable explanation was given for this phenomenon through a theoretical analysis combined with microstructure measurement. In this study, the growth process and colour generation mechanism of porous anodic alumina under a linear anodizing voltage were discussed and analyzed in detail. These films may have great potential in colour display, decoration, anti-counterfeiting, sensing and other fields.

Fabrication of environmental humidity-responsive iridescent films with cellulose nanocrystal/polyols

Herein, we fabricated flexible and humidity-sensitive composite films employing cellulose nanocrystal (CNC) and polyols, i.e., glycerol (G), xylitol (X) and sorbitol (S). The effects of polyols with different molecular weights on the structure, optical properties, mechanical strength and humidity response of the composite films were investigated. Notably, the CNC-S film exhibited obvious reversible colour changes from light green to red upon a relative humidity (RH) change from 30 % to 95 %. Moreover, it was found that the composite films had a large colour-change range, good reversibility (>10 cycles), and excellent stability (>10 weeks). Overall, the results demonstrated that the CNC-S composite film can be used as a functional material for the preparation of flexible humidity sensors for the detection of environmental humidity changes in agriculture, industry, and other fields.

Fabrication and characterization of photonic cellulose nanocrystal films with structural colors covering full visible light

Chiral nematic cellulose nanocrystal (CNC) photonic films with structure color were prepared by regulating the acid-to-pulp ratio (8:1, 10:1, and 12:1) and the hydrolysis temperature (45 °C and 60 °C). CNC prepared with a lower acid-to-pulp ratio and hydrolysis temperature obtained a higher aspect ratio, which made it easy for CNC to contact each other to produce chiral nematic structures and present iridescent film. The as-prepared CNC iridescent films can reflect natural light with a wavelength in the range of 360–695 nm. Consequently, the CNC films showed structural color of blue to orange, dependent on the acid hydrolysis conditions. The strategy presented herein paves the way for the simple and cost-efficient preparation of photonic materials with structural colors.

Dual Response of Photonic Films with Chiral Nematic Cellulose Nanocrystals: Humidity and Formaldehyde.

Manipulating functional stimuli-responsive materials has been a hot topic in the research of smart sensors and anticounterfeiting encryption. Here, a novel functional chiral nematic cellulose nanocrystal (CNC) film showing dual responsiveness to humidity and formaldehyde gas was fabricated. The chiral nematic CNC iridescent film could respond to environmental humidity and formaldehyde gas changes by reversible motion. Interestingly, the humidity sensitivity of the CNC iridescent film could be gated by exposing the film to formaldehyde gas. At the same time, the formaldehyde-responsive behavior is strongly affected by the relative humidity (RH), and the response range could be tuned by changing the RH over a wide range. Importantly, the formaldehyde-induced color change could be altered from invisible to visible by the naked eye when the film was exposed to a humid environment. The mechanism of this dual response of the CNC iridescent film is ascribed to the synergistic effect of cooperation and competition between water and formaldehyde molecules by constructing physical cross-linking networks by hydrogen bonds among water, formaldehyde, and CNCs. Furthermore, the "RH-concentration of formaldehyde gas-color" ternary colorimetric system was simulated, which is thought to endow the CNC iridescent film with great potential to act as a sensor in the convenient visible detection of gaseous formaldehyde. Furthermore, this work provided a promising strategy to design multi-gas-sensitive devices with convenient detection, good stability, and excellent reversibility.

Recent Progress in Cellulose Nanocrystal Alignment and Its Applications.

Cellulose nanocrystal (CNC) research has addressed increased attention in the bioprocessing community, especially the utility of these materials in different fields, including biomedical and materials applications. In this review, the different preparation methods and properties of CNC materials will be initially briefly discussed. The latter focuses special attention on the alignment of CNCs and its applications. Different methods to induce CNC alignment, including self-assembly, shear-based alignment, magnetic and electric field alignment, spin-coating, dip-coating, etc., are reviewed in detail, followed by the different characterization techniques to quantify the alignments of CNCs. The current and potential applications of aligned CNCs, such as fillers to enhance the mechanical and thermal properties of biocomposites, photonic materials to deal with structural colors and enhancement in optical properties of iridescent films, and reduced wettability for high-resolution printing, are further discussed to provide more insights into this promising research direction. We hope that this review will shine light on CNC alignment and also open the door to different types of applications.

Iridescent Cellulose Nanocrystal Films Modified with Hydroxypropyl Cellulose.

The introduction of polymers into a chiral nematic cellulose nanocrystal (CNC) matrix allows for the tuning of optical and mechanical properties, enabling the development of responsive photonic materials. In this study, we explored the incorporation of hydroxypropyl cellulose (HPC) into a CNC film prepared by slow evaporation. In the composite CNC/HPC thin films, the CNCs adopt a chiral nematic structure, which can selectively reflect certain wavelengths of light to yield a colored film. The color could be tuned across the visible spectrum by changing the concentration or molecular weight of the HPC. Importantly, the composite films were more flexible than pure CNC films with up to a ten-fold increase in elasticity and a decrease in stiffness and tensile strength of up to six times and four times, respectively. Surface modification of the films with methacrylate groups increased the hydrophobicity of the films, and therefore, the water stability of these materials was also improved.

LA CHIMICA E I BENI CULTURALI, UN CONNUBIO VINCENTE: STUDIO TECNOLOGICO DI CERAMICHE E VETRI ATTRAVERSO I SECOLI

In the field of the researches concerning the characterization, conservation and restoration of cultural heritage, as well as the objects manufacturing techniques, it is essential to use diagnostic methods that are referred to as archaeometric techniques. Archaeometry (which measures what is ancient) deals with the scientific study, carried out by laboratory analyses, of ancient materials and objects with the aim to deep the knowledge of historical, archaeological, artistic and technological aspects. In the field of the study of ceramic artefacts, the case of Italian Renaissance ceramics decorated with the luster technique is particularly interesting; luster is an iridescent metallic film of gold or ruby red color obtained from silver and copper in the form nanoparticles using a very sophisticated technique that testifies to the extremely advanced knowledge possessed by the ancient potters.

Improving Homogeneity of Iridescent Cellulose Nanocrystal Films by Surfactant-Assisted Spreading Self-Assembly

Herein, photonic crystal (PC) films of cellulose nanocrystals (CNCs) with good color evenness were fabricated via drop-casting of a mixture droplet composed of CNC suspension, nonionic surfactant (poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) or zonyl), and a small amount of dimethyl sulfoxide. Thanks to the good fusion and rearrangement of CNC tactoids in CNC/surfactant droplets under enhanced shear force in spreading and the elimination of the coffee-ring effect, the produced iridescent films exhibit long-range ordered structures and uniform color in most parts of the films. Moreover, the affinity of zonyl to gold nanoparticles assists the formation of CNC/Zonyl/Au hybrid films with plasmonic chiral optical property and improving conductivity. This approach demonstrates an attractive and economic pathway to fabricate CNC PC films that will have broad applications in photonic sensors, optoelectronics materials, or responsive actuators.

Color-spectrum-broadened ductile cellulose films for vapor-pH-responsive colorimetric sensors

We introduce a smart colorimetric sensor platform, in which iridescent chiral nematic films composed of oxidized cellulose nanocrystals (OxCNCs) exhibit pH-responsive color changes. To provide dried OxCNC films with the controlled reflection of circularly polarized lights at specific wavelengths, we manipulated the half-pitch distance from 145 to 270nm by adjusting the surface charges of OxCNCs during 2,2,6,6-tetramethylpiperidine-1-oxyl-radical-mediated oxidation. Utilizing the controlled electrostatic repulsion generated by the difference in the surface charge of OxCNCs, the reflection wavelengths of the films could be tuned across a broad color spectrum. In addition, we improved flexibility or ductility of OxCNC composite films by coassembly with a hygroscopic polymer, polyethylene glycol, while maintaining their iridescent colors. We finally demonstrated that the OxCNC composite films could show reversible color changes in response to vapors of aqueous solutions with different pH values, thus enabling the development of a vapor-pH-responsive colorimetric sensor technology.