Nanocrystal Composite Films Research Articles

Preparation of CO2‐based waterborne polyurethane/cellulose nanocrystal composite films and research of their radiative cooling properties

AbstractConventional colored radiative cooling (ColPRC) has a single color and limited reflection of visible light bands, which affects its radiative cooling effect, and the introduction of multiple photonic crystal structures can improve its radiative cooling performance. Therefore, in this study, a composite film was established by cross‐linking cellulose nanocrystals (CNC) with CO2‐based waterborne polyurethane (WPU) to obtain multistage structural colors. X‐ray diffraction pattern, thermogravimetric analysis, scanning electron microscopy, spectral analysis, and so on were used to examine the structure and properties of the films. The results showed that with the increase of CNC content, the color shifted blue, the crystallinity increased, the flame retardancy increased, but the thermal stability decreased; the addition of WPU assists in resisting moisture; the reflectivity of the multistage structured films is improved in the VIS–NIR range compared with the monochromatic films (an average increase of 5%), which helps to reduce solar irradiation in the visible wavelengths; high emissivity (≈90%) in the atmospheric window, which facilitates deep‐space heat exchange. The maximum temperature difference of the multistage structural color film can reach 6°C under strong solar irradiation. This research can be applied in photovoltaic devices or outdoor thermal management to provide new ways to address human energy efficiency.

Graphene and graphene–cellulose nanocrystal composite films for sustainable anodes in biophotovoltaic devices

This work demonstrates the innovative utilisation of sheer exfoliated few-layer graphene and graphene–CNC films as eco-friendly electrodes for efficient current extraction from cyanobacteria, promising advancements in biophotovoltaic applications.

Research on deep eutectic solvents for the construction of humidity-responsive cellulose nanocrystal composite films

Photonic crystal materials based on cellulose nanocrystals (CNC), which are environmentally responsive and green, have attracted widespread attention. To overcome the brittleness of CNC films, many researchers have explored functional additives to improve their performance. In this study, a new green deep eutectic solvents (DESs) and an amino acid-based natural deep eutectic solvents (NADESs) were introduced into CNC suspensions for the first time, and hydroxyl-rich small molecules (glycerol, sorbitol) and polymers (polyvinyl alcohol, polyethylene glycol) were coassembled with the DESs and NADESs to form three-component composite films. The CNC/G/NADESs-Arg three-component film reversibly changed color from blue to crimson as the relative humidity rose from 35 % to 100 %; additionally, the elongation at break increased to 3.05 %, and the Young's modulus decreased to 4.52 GPa. The hydrogen bond network structure provided by trace amounts of the DESs or NADESs not only improved the mechanical properties of the composite films but also increased their water absorption capacities without destroying their optical activities. This allows for the development of more stable CNC films and creates potential for biological applications in the future.

Halloysite nanotubes enhance the mechanical properties and thermal stability of iridescent cellulose nanocrystal films.

Films of cellulose nanocrystals (CNCs) with chiral nematic organization can show vivid iridescence that arises from their hierarchical structure. Unfortunately, the brittleness of the films limits their potential applications. In this paper, we investigate the incorporation of halloysite nanotubes (HNTs) into CNC films to prepare organic-inorganic composite films with enhanced mechanical properties, while preserving the chiral nematic structure and brilliant iridescence. The hybrid composite films containing 10 wt% HNTs are more elastic than pristine CNC films, with a 1.3-fold increase in tensile strength and a 1.6-fold increase in maximum strain. As well, the incorporation of HNTs slightly improves the thermal stability of the composite films. These materials mimic the hybrid composite structures of crab shells, leading to enhanced mechanical properties and thermal stability of CNC films while maintaining iridescence.

Nanocavity-encapsulated perovskite nanocrystals with enhanced luminescence by Mie resonance

Perovskite nanocrystals (PeNCs) have been regarded as one of the most promising candidate materials for next-generation display technology due to their remarkable optical performance. However, addressing the limitations of inherent instability and low luminescence efficiency of PeNC films is still a huge obstacle in its commercialization process. Herein, the low-cost and large-area perovskite nanocrystal composite films (NCFs) based on hexagonally distributed TiO2-based metasurface (inspired by the surface structure of butterfly wings) in combination with PeNCs encapsulated in PMMA matrix were fabricated. The photoluminescence intensity of perovskite NCFs can be enhanced by a maximum of 3.89 times based on the Purcell effect by tailoring to match the emission peaks of their PeNCs in various bands using the flexibility of metasurface design. The encapsulation of PMMA polymer and TiO2 nanocavities also effectively enhanced the stability of PeNCs. Benefiting from these favorable characteristics, perovskite NCFs are applied to liquid crystal displays (LCDs) as color conversion layers, with color gamut reaching 122% of the NTSC standard and 180% of the commercial LCD screen, respectively, demonstrating that perovskite NCFs possess enormous potential in the high-end display fields. Furthermore, this metasurface fabrication process avoids complicated, time-consuming, and expensive process flow, laying the foundation for further commercialization beyond the laboratory.

UV-vis-IR Broad Spectral Photodetectors Based on VO2-ZnO Nanocrystal Films.

As a narrow band semiconductor at room temperature and a metallic material above ∼68 °C, functional VO2 films are widely investigated for smart windows, whereas their potential for ultraviolet–visible–infrared (UV–vis–IR) broad spectral photodetectors has not been efficiently studied. In this report, photodetectors based on VO2–ZnO nanocrystal composite films were prepared by nanocrystal-mist (NC-mist) deposition. An enhanced photodetection switching ratio was achieved covering the ultraviolet to infrared wavelength. Due to the synergetic effect of nanosize, surface, phase transition, percolation threshold, and the band structure of the heterojunction, the transfer and transport of photogenerated carriers modulate the device performance. This study probes new chances of applying VO2-semiconductor-based nanocomposites for broad spectral photodetectors.

Facile adjustment on cellulose nanocrystals composite films with glycerol and benzyl acrylate copolymer for enhanced UV shielding property

In the present work, cellulose nanocrystals (CNCs) composite films with suitable applicable capabilities were prepared by facilely incorporating glycerol (Gly) and poly(benzyl acrylate) (PBA). Chemical and morphological variations during the fabrication of the films were systematically characterized. The properties of modified CNCs composite films including UV blocking ability, mechanical strength and thermal properties were characterized to assess their applicable potentials. As a result, the composite films have good UV shielding property in UVC (220–280 nm) region and UVB (280–320 nm) region. The shielding performance of the modified film in the ultraviolet absorption region reached 92.77% to 95.49% respectively, without damaging the original chiral nematic structure of the films. Along with the modification, BACNC film improved the mechanical properties, presenting the tensile strength 16 times higher compared to pure CNCs film. The nanocomposite films proposed in this work showed promising potentials in broad fields, such as food preservation, medical protection, and surface coating applications.

Reduced graphene oxide/cellulose nanocrystal composite films with high specific capacitance and tensile strength

Due to the environmental degradation and energy depletion, the strategy for fabricating high-performance supercapacitor electrode materials based on graphene and nanocellulose has received great attention. Herein, an environmentally friendly reduced graphene oxide (RGO)/cellulose nanocrystal (CNC) composite conductive film was prepared using L-ascorbic acid (L-AA) as the reductant of graphene oxide (GO). Based on chemical structure analysis, L-AA was proved to be an effective reductant to remove oxygen containing groups of GO. Through microstructure observation, a unique stacking structure of CNC and RGO was observed, which could be largely attributed to the hydrogen bond interaction. Furthermore, the effect of CNC amount on the performance of RGO/CNC composite films was also systematically investigated. Particularly, the addition of CNC was found to exert a positive effect on the tensile strength, which might be mainly due to a mass of hydrogen bonds between the CNCs. Meanwhile, the RGO/CNC composite conductive film featured ideal electrical double-layer capacitive (EDLC) behavior, exhibiting a gravity specific capacitance of 222.5 F/g and tensile strength of 32.17 MPa at 20 wt% CNC content. Therefore, the RGO/CNC composite conductive films may hold great promise for environmentally friendly electrode materials of supercapacitors and flexible electrical devices.

Optimizing the performance of the near-infrared (NIR) photothermal conversion via modulating the domain size of the chiral nematic phase in co-assembled cellulose nanocrystal composite films

The self-assembled liquid crystal nanostructure of CNCs provides an effective phase-segregation for PEDOT, thus improving the NIR photothermal performance of CNCs/PEDOT:PSS/OS/TA composites significantly.

Bio-inspired non-iridescent structural coloration enabled by self-assembled cellulose nanocrystal composite films with balanced ordered/disordered arrays

Cellulose nanocrystal (CNC) is an interesting polysaccharide nanomaterial that can self-assemble into a chiral nematic liquid crystalline structure in aqueous solution, producing brightly structural color upon selectively reflection of visible light. However, till now, only iridescent structural colors can be created by such colloidal array, making it impossible to be used as colorimetric sensors. Here, inspired by longhorn beetle having short-range ordered but long-range disordered microstructures, it is demonstrated to develop low-/non-angle-dependent structural-color CNC composite films through interference against the long-range orders as a result of introducing positively charged poly (dimethyl diallyl ammonium chloride) (PDDA). By exploring balanced ordered/disordered structures, vivid angle-independent structural colors are realized, enabling accurate cognition for environmental signals, which is crucial to promising applications in sensing, anti-counterfeit, and decoration technology. • Low-/non-angle-dependent structural-color CNC composite films are fabricated. • Balanced ordered/disordered arrays are manipulated by co-assembly with PDDA. • The composite films can be used as colorimetric sensor.

Modulating transparency and colour of cellulose nanocrystal composite films by varying polymer molecular weight

HypothesisCellulose nanocrystals (CNC) can produce photonic composite films that selectively reflect light based on their periodic cholesteric structure. The hypothesis of this research is that by incorporating water-soluble polymer, photonic properties of CNC composite film can be designed by manipulating the polymer molecular weight. ExperimentalFlexible free-standing composite films of five different poly (ethylene glycol) (PEG) molecular weights were prepared via air drying under a controlled environment, and characterised by reflectance UV–vis spectrometer, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Films with each molecular weight were investigated over a concentration range. FindingsThe colour and transmission haze of the composite films was modified by varying both the PEG molecular weight and concentration. Depending on the molecular weight, the films were able to reflect light from the UV region (242 nm) across the visible spectrum to the near-infrared region (832 nm). Different trends in variation of the reflected light based on the molecular weight was found with increasing PEG concentration and was explained by weak depletion interactions occurring between CNC and PEG, which was reduced with increasing PEG molecular weight.

Preparation of collagen/cellulose nanocrystals composite films and their potential applications in corneal repair.

As the main component of the natural cornea, collagen (COL) has been widely applied to the construction of corneal repair materials. However, the applications of collagen are limited due to its poor mechanical properties. Cellulose nanocrystals (CNCs) possess excellent mechanical properties, optical transparency and good biocompatibility. Therefore, in this study, we attempted to introduce cellulose nanocrystals into collagen-based films to obtain corneal repair materials with a high strength. CNCs were incorporated at 1, 3, 5, 7 and 10 wt%. The physical properties of these composite films were characterized, and in vitro cell-based analyses were also performed. The COL/CNC films possessed better mechanic properties, and the introduction of CNCs did not affect the water content and light transmittance. The COL/CNC films demonstrated good biocompatibility toward rabbit corneal epithelial cells and keratocytes in vitro. Moreover, the collagen films with appropriate ration of CNCs effectively induced the migration of corneal epithelial cells and inhibited the myofibroblast differentiation of keratocytes. A collagen film with 7 wt% CNCs displayed the best combination of physical properties and biological performance in vitro among all the films. This study describes a nonchemical cross-linking method to enhance the mechanical properties of collagen for use in corneal repair materials and highlights potential application in corneal tissue engineering.

Bioinspired Hydrophobic Cellulose Nanocrystal Composite Films as Organic-Solvent-Responsive Structural-Color Rewritable Papers.

Lots of beetles, moths, and birds in the natural world present stunning unique structural colors as well as excellent hydrophobic performances. Herein, a novel bioinspired variable structural-color film with organic-solvent responsiveness and surface hydrophobicity was fabricated. Cellulose nanocrystals (CNCs) provided structural color with left-handed helicity. PEG-PPG-PEG triblock copolymers (PPPTCs) were blended with CNCs, giving rise to the organic-solvent-responsive structural color and wider red-shift window of the reflectance peak. The color of the film could be regulated repeatedly under the stimulus of cyclohexanone with an obvious red shift up to 107 nm, corresponding to a macroscopic color change from blue to yellow. Low-surface-energy compound hexadecyltrimethoxysilane (HDTMS) was covalently grafted on the surface in a one-step method to introduce hydrophobicity, successfully preventing the effect of water on the ordered nanostructure. Based on the bionics principle, the as-prepared CNC/PPPTC nanocomposite films with variable structural colors and hydrophobicity are beneficial to their prospective applications in display screens, rewritable hydrophobic structural-color-changing paper, biomimetic sensors, and so forth.

Preparation of polyimide/BaTiO3‐nanocrystal hybrid thin films and their dielectric property

High dielectric constant is highly desirable in capacitors and memory devices. In this work, oleic acid (OA)‐capped BaTiO3 nanocrystals were synthesized by a two‐phase approach. Polyimide (PI)/BaTiO3‐nanocrystal composite thin films with high dielectric constant have been successfully fabricated. The morphologies and dielectric properties of the hybrid films were exploited. The results showed that BaTiO3 nanocrystals can be uniformly dispersed in the PI thin films owing to the surface modification of OA‐capped BaTiO3 nanocrystals. It was found that the dielectric constant of composite film varies with the volume fraction of BaTiO3 nanocrystals and sintering temperatures and reaches a maximum value of 44.1, which is around 13 times higher than that of pristine PI thin film (3.2). These results demonstrated that PI/BaTiO3‐nanocrystal composite films have considerable application potential in microelectronic fields.

Flexible and Tailorable Alkylviologen/Cellulose Nanocrystals Composite Films for Sustainable Applications in Electrochromic Devices

AbstractThe new, sustainable, and pro‐environmental electrochromic devices (ECDs) have attracted tremendous attention due to the current issues regarding energy sources consumption and environmental pollution. A free‐standing composite film, consisting of cellulose nanocrystals (CNCs) as biological matrix, viologen, and KCl, was prepared by a casting method. Such a composite film is flexible and can be tailored into various shapes for patterned applications. The viologen/KCl/CNCs film was sandwiched between two indium‐tin oxide (ITO) glasses to construct viologen‐based ECDs. The performance of the viologen‐based ECDs did not depend on the organic solvent. ECDs based on four kinds of viologens with different substituent groups exhibited distinct coloration response. Besides the nature of the substituent groups attached to the bipyridine rings, the presence of CNCs is a crucial factor that affects the electrochromic performance of ECDs. The dimethyl‐substituted viologen ECD has a fast response time (10 s for bleaching and 7.3 s for coloring). And for the dibenzyl‐substituted viologen, a low voltage of −2.2 V can drive the coloration of its ECDs. The diethyl‐substituted viologen ECD exhibits 30 % transmittance variation and good coloring/bleaching stability. The ability to be tailored and degradability of the composite film provides a feasible strategy for the sustainability of electrochromic displays.

Dense Ge nanocrystals embedded in TiO2 with exponentially increased photoconduction by field effect

Si and Ge nanocrystals in oxides are of a large interest for photo-effect applications due to the fine-tuning of the optical bandgap by quantum confinement in nanocrystals. In this work, dense Ge nanocrystals suitable for enhanced photoconduction were fabricated from 60% Ge in TiO2 amorphous layers by low temperature rapid thermal annealing at 550 °C. An exponential increase of the photocurrent with the applied voltage was observed in coplanar structure of Ge nanocrystals composite films deposited on oxidized Si wafers. The behaviour was explained by field effect control of the Fermi level at the Ge nanocrystals-TiO2 layer/substrate interfaces. The blue-shift of the absorption gap from bulk Ge value to 1.14 eV was evidenced in both photocurrent spectra and optical reflection-transmission experiments, in good agreement with quantum confinement induced bandgap broadening in Ge nanocrystal with sizes of about 5 nm as found from HRTEM and XRD investigations. A nonmonotonic spectral dependence of the refractive index is associated to the Ge nanocrystals formation. The nanocrystal morphology is also in good agreement with the Coulomb gap hopping mechanism of T–1/2 -type explaining the temperature dependence of the dark conduction.

Chemical cross-linked polyvinyl alcohol/cellulose nanocrystal composite films with high structural stability by spraying Fenton reagent as initiator

Cross-linked polyvinyl alcohol (PVA) composite films with high structural stability were prepared by free radical copolymerization between cellulose nanocrystal (CNC) and maleic anhydride (MAH) modified PVA through spraying Fenton free radical as initiator. The influence of chemical cross-linked and physical network structure on mechanical, thermal and water absorption properties of the composite films were investigated. Compared to PVA and PVA/CNC composite film, significant improvements in the mechanical, thermal and water uptake properties of the cross-linked composite film were found. The tensile strength of the cross-linked composite film was enhanced from 23.1MPa (neat PVA film) and 32.6MPa (PVA/CNC-10%) to 42.5MPa, and the maximum thermal degradation temperature was increased from 266.8°C and 281.2°C to 366.7°C (cross-linked composite film). Besides, the water absorption was reduced from 385.9% and 220.6% to 175.7% for cross-linked composite film. It indicates that compared with physical network structure in PVA/CNC composite film, the multiple cross-linked networks showed excellent thermal stability, resistance of water swelling and structural stability at the same CNC loading level. Thus, the PVA/CNC composite film with the multiple cross-linked network shows greater property reinforcements.

Solution-processed field-effect transistors based on polyfluorene –cesium lead halide nanocrystals composite films with small hysteresis of output and transfer characteristics

We have designed and investigated electrical and optical properties of solution-processed organic field-effect transistors (OFETs) based on conjugated polymer PFO and perovskite –cesium lead halide nanocrystals (CsPbI3) composite films. It was shown that OFETs based on PFO:CsPbI3 films exhibit current-voltage (I-V) characteristics of OFETs with dominant hole transport and saturation current behavior at temperatures 200–300 K. It was found that PFO:CsPbI3 OFETs have a negligible hysteresis of output and transfer characteristics especially at temperatures below 250 K. The values of the hole mobility estimated from I-Vs of PFO:CsPbI3 OFETs were found to be ∼2.4 10−1 cm2/Vs and ∼1.9 10−1 cm2/Vs in saturation and low fields regimes respectively at 300 K; the hole mobility dropped down to ∼6 10−3 cm2/Vs and 2.8 10−3 cm2/Vs respectively at 200 K, and then down to 5.5 10−5 cm2/Vs at 100 K (in low field regime), which is characteristic of hopping conduction. The effect of sensitivity to light and light-emitting effect were found under application of negative source-drain and gate pulse voltages to PFO:CsPbI3 OFETs at 300 K. The mechanism of charge carrier transport in OFETs based on PFO:CsPbI3 hybrid films is discussed.

Structuring of cellulose nanocrystal composite films on shape memory polymers

Event Abstract Back to Event Structuring of cellulose nanocrystal composite films on shape memory polymers Urooj Gill1*, Travis Sutherland1 and Jose Moran-Mirabal1 1 McMaster University, Chemistry & Chemical Biology, Canada Introduction: Cellulose nanocrystals (CNCs) have been targeted as environmentally friendly dopants for thin films and bulk materials due to their biocompatibility, biodegradability, and high tensile strength[1]. Biocomposite films incorporating CNCs have been extensively studied; however, challenges remain in determining the mechanical properties of thin films in a humidity-independent manner. One route to access this information is to compressively buckle biocomposite films on a PDMS substrate, and then measure the periodicity of the resulting structures to extract a Young’s modulus. If a sufficiently large compressive stress is applied to a thin film, it undergoes wrinkling that results in micro/nanostructured surfaces, which are of interest due to their enhanced surface area and potential anti-microbial properties[2]. Here, we show that CNC-loaded biocomposite films formed through layer-by-layer (LbL) deposition can be irreversibly structured via compressive stress, which is generated by thermal shrinking of a shape-memory polymer (SMP). By characterizing the morphology of the structured surfaces, we determine the Young’s modulus of the CNC biocomposite films in a humidity-independent manner. We also show the application of the structured surfaces as platforms for the growth of murine RAW 264.7 cells. Materials and Methods: Biocomposite films were fabricated on polystyrene (PS) substrates through LbL deposition of alternating layers of CNCs and xyloglucan (XG) or polyethyleneimine (PEI). Films were assessed using white light interferometry (WLIM) and variable angle spectroscopic ellipsometry (VASE), for surface roughness and thickness. LbL films were structured by heat shrinking of PS at 135 °C.[2] Structured films were assessed using WLIM, SEM, and 2D fast Fourier transform (2D FFT) analysis, for roughness, morphology, and periodicity. 2D FFT analyzed the edges of wrinkled films to determine their periodicity. To calculate the Young’s modulus, the film thickness was plotted against the structure wavelength. Unstructured and structured films were then used as substrates to grow murine RAW 264.7 cells. Following fixation, cells were imaged with epifluorescence microscopy by using the dyes 4′,6-diamidino-2-phenylindole (DAPI) and phalloidin-AlexaFluor 488 to stain the nucleus and actin filaments. Results and Discussion: WLIM characterization of unstructured LbL films showed that despite changes in thickness, surface roughness was constant at 5.09 + 0.07 nm, while VASE analysis showed that film thickness increased linearly with the number of layers deposited onto the substrates. At the same number of layers, CNC-PEI films were thicker than CNC-XG films. After structuring, WLIM, SEM, and 2D FFT analysis showed that thicker films had higher roughness, larger structures, and higher wavelength values – this was also seen for CNC-PEI films compared to CNC-XG films, at the same number of layers. Young’s moduli indicated that CNC-XG films were stiffer than CNC-PEI films, with values of 39 + 1 GPa and 32 + 1 GPa. These biocomposite films were then used as a support for cell growth. Epifluorescence microscopy indicated that RAW 264.7 cells grew similarly on both unstructured and structured films, where they exhibited both round and “fried egg” morphologies. Conclusion: We continue to explore this structuring approach as a reliable and humidity-independent way of determining the Young’s modulus of CNC biocomposite thin films. Thus far, we demonstrate that biocomposite structures incorporating CNCs can be fabricated on PS substrates in a reproducible and stable manner. The structured surfaces remain intact under varying conditions including elevated temperatures, changes in relative humidity, following lifting-off of the structured films from the PS substrate, and culturing of cells onto the structured films. Using these structures as a platform for cell growth is highly desirable due to the biocompatibility and anionic charge of CNCs, and thus, these structures can be further explored for potential applications in areas such as in vitro studies.

Heavily doped silicon and nickel silicide nanocrystal composite films with enhanced thermoelectric efficiency

In this study, we have developed highly efficient thermoelectric materials based on p-type and n-type composite films of Si nanocrystals and Ni silicide nanocrystals. The heavy doping of the films with boron or phosphorus and thermal annealing of the films caused formation of nanocrystals with high electrical conductivities, low thermal conductivities, and high Seebeck coefficients, consequently leading to the high dimensionless figures of merit (ZT). For the p-type (B-doped) and n-type (p-doped) films, ZT is 0.13 and 0.06, respectively, which were much higher than that of bulk Si (<0.01) at RT and nanostructured bulk Si.