Flexible composite film with artificial opal photonic crystals for efficient all-day passive radiative cooling

All-day passive radiative cooling has recently attracted broader attention for its potential as a viable energy technology. Although tremendous progress has been achieved, the design and fabrication of low-cost high-efficiency radiators for all-day passive radiative cooling remains a challenge. Herein, we report a new type of flexible composite radiator film with built-in artificial opal-like structures for all-day passive radiative cooling. Using artificial opal structure concepts, the proposed polydimethylsiloxane (PDMS) radiator film with embedded polystyrene (PS) microsphere photonic crystals exhibits a sufficiently high solar reflectance of ∼92.7% when in a direct sunlight region, and a thermal emittance of ∼93.6% within the atmospheric window. Without the need for traditional reflectors like silver or aluminum foils, this composite film realizes subambient temperature reduction of ∼4.8 °C in direct sunlight and ∼8.5 °C during the night. This work provides a new fabrication approach for the low-cost production of structural polymer films for high performance and potential real word applications.

Fabrication of inverse opal photonic gel sensors on flexible substrates by transfer process

We demonstrate a platform technology for transferring opal films and photonic gel films to flexible substrates. The conventional fabrication procedure for inverse opal photonic gel (IOPG) sensors comprises three major steps: 1) the self-assembly of polystyrene μ-spheres to an opal template film within a channel between the top and bottom substrates, 2) infiltration and photo-polymerisation of the monomer mixture, and 3) etching of the opal template. Owing to the low processing yield of the first step, it is difficult to fabricate multiple sensor arrays on a single substrate. In this study, an opal film is formed between two substrates with different surface polarities, and the film is separated by disassembling the two substrates. The opal film on a medium polar substrate is covered using a flexible polyethylene terephthalate (PET) film, and opal-templated photo-polymerisation is performed. Finally, the photonic gel with the opal template is transferred to the PET film, and the opal template is etched out. Using the platform technique, the fabrications of pH-responsive IOPG and temperature-responsive IOPG sensors on PET films are respectively demonstrated. In addition, the IOPG containing the copolymer of acrylamide and 3-acrylamidophenylboronic acid was found to be responsive to glucose at physiological pH. All three sensors were fabricated using the same transfer method, differing only in the composition of monomer mixtures, and they all showed excellent sensitivity and repeatability on PET substrates. Due to the advantageous feature of the transfer method, dual sensors of pH-responsive IOPG and temperature-responsive IOPG were sequentially fabricated on a single PET film.

Investigation of Polystyrene-Based Microspheres from Different Copolymers and Their Structural Color Coatings on Wood Surface

Six kinds of polystyrene (PSt)-based colloidal microspheres were synthesized by adding acrylic acid (AA), methyl methacrylate (MMA), and butyl acrylate (BA) as comonomers in styrene emulsion polymerization. The structurally colored coatings on a wood surface were self-assembled by thermally assisted gravity deposition of these microspheres. Chemical compositions and structures of microspheres and morphological characteristics of microspheres and structural color coatings, as well as optical properties of coatings and their generated structural colors, were studied. Pure PSt microspheres had a smooth surface and uniform structure, while microspheres of copolymers had core–shell morphologies and a rough surface. Only poly(styrene-acrylic acid) (P(St-AA)) microspheres had good monodispersity and the resulting coating had a well-ordered photonic crystal structure. However, other kinds of microspheres could form short ranges of ordered amorphous photonic crystal structures and they displayed structural colors. Both the reflectivity of coatings to visible light and structural colors varied with microsphere size and self-assembly temperature.

Colorimetric Humidity Sensor Using Inverse Opal Photonic Gel in Hydrophilic Ionic Liquid

We demonstrate a fast response colorimetric humidity sensor using a crosslinked poly(2-hydroxyethyl methacrylate) (PHEMA) in the form of inverse opal photonic gel (IOPG) soaked in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM⁺][BF₄−]), a non-volatile hydrophilic room temperature ionic liquid (IL). An evaporative colloidal assembly enabled the fabrication of highly crystalline opal template, and a subsequent photopolymerization of PHEMA followed by solvent-etching and final soaking in IL produced a humidity-responsive IOPG showing highly reflective structural color by Bragg diffraction. Three IOPG sensors with different crosslinking density were fabricated on a single chip, where a lightly crosslinked IOPG exhibited the color change response over entire visible spectrum with respect to the humidity changes from 0 to 80% RH. As the water content increased in IL, thermodynamic interactions between PHEMA and [BMIM⁺][BF₄−] became more favorable, to show a red-shifted structural color owing to a longitudinal swelling of IOPG. Highly porous IO structure enabled fast humidity-sensing kinetics with the response times of ~1 min for both swelling and deswelling. Temperature-dependent swelling of PHEMA in [BMIM⁺][BF₄−] revealed that the current system follows an upper critical solution temperature (UCST) behavior with the diffraction wavelength change as small as 1% at the temperature changes from 10 °C to 30 °C.

Low-Power All-Organic Electrophoretic Display Using Self-Assembled Charged Poly( t-butyl methacrylate) Microspheres in Isoparaffinic Fluid

The increasing demands for display devices with low power consumption and outdoor readability have stimulated comprehensive research into full-color reflective displays that employ color-tunable photonic crystal technologies. Although the recently developed crystalline colloidal arrays (CCAs) of the charged microspheres have shown the outstanding color tunability, the practical application is limited because the use of highly polar liquid medium such as water is required to maintain the charges on the surface of microsphere, whereas it is not suitable for long-term use in an electric field. Herein, a self-assembled CCA from charged poly( t-butyl methacrylate) microspheres was successfully fabricated, which was stabilized by the charged inverse micelles of sodium di-2-ethylhexyl-sulfosuccinate in a nonpolar isoparaffinic fluid. A charged all-organic CCA was found to exhibit full-color tunability with a 1000-fold reduction in the power consumption (∼6 μW cm^-2) under a direct current voltage bias of 4 V in comparison to that in an aqueous system, which is a promising feature for a low-power-consumption display device.

Tunable Temperature Response of a Thermochromic Photonic Gel Sensor Containing N-Isopropylacrylamide and 4-Acryloyilmorpholine

In this study, thermochromic photonic gels were fabricated using 2-hydroxyethyl methacrylate (HEMA) as a hydrogel building block, and 4-Acryloyl morpholine (ACMO) and N-isopropylacrylamide (NIPAAM) as thermoresponsive monomers with different critical solution temperature behaviors. Rapid photopolymerization of opal-templated monomer mixtures of varying ACMO contents formed five individual thermochromic inverse opal photonic gels integrated on a single substrate. With temperature variation from 10 °C to 80 °C, the changes in reflective colors and reflectance spectra of the respective thermochromic gels were noted, and λ_peak changes were plotted. Because NIPAAM exhibits a lower critical solution temperature (LCST) at 33 °C, the NIPAAM-only gel showed a steep slope for dλ_peak/dT below 40 °C, whereas the slope became flatter at high temperatures. As the ACMO content increased in the thermochromic gel, the curve of dλ_peak/dT turned out to be gradual within the investigated temperature range, exhibiting the entire visible range of colors. The incorporation of ACMO in NIPAAM-based thermochromic gels therefore enabled a better control of color changes at a relatively high-temperature regime compared to a NIPAAM-only gel. In addition, ACMO-containing thermochromic gels exhibited a smaller hysteresis of λ_peak for the heating and cooling cycle.

Simultaneous Nitrogen Doping and Pore Generation in Thermo-Insulating Graphene Films via Colloidal Templating

We report a simple method for preparing highly efficient thermoelectric materials through the fabrication of nitrogen-doped reduced graphene oxide (GO) with a porous structure. The samples were produced by thermal annealing of GO/nitrogen-rich polystyrene (N-PS) particle composite films using a colloidal templating method. N-PS particles served as a nitrogen dopant source for the nitrogen-doped thermally reduced graphene oxide (TrGO) as well as sacrificial particles for the porous structure. The S values of the porous TrGO films were negative, indicating that the samples were transformed into n-type materials. Their porous structures simultaneously resulted in materials with high σ values and low in-plane κ values by providing numerous air cavities for phonon scattering and destruction of the anisotropic structure, maintaining an interconnected structure for an electron transport path. Thus, the porous TrGO films exhibited enhanced power factors and low κ values. The highest ZT value of 1.39 × 10^-4 was attained for a porous TrGO film annealed at 1100 °C, which was 1200 times higher than that of a nonporous TrGO film. This study emphasizes that an isotropic orientation of two-dimensional materials has a significant effect on the suppression of in-plane κ, leading to their enhanced thermoelectric performance.

Synthesis of Poly(styrene-acrylates-acrylic acid) Microspheres and Their Chemical Composition towards Colloidal Crystal Films

In this paper, polystyrene colloidal microspheres have been prepared using hexyl acrylate (HA), ethylhexyl acrylate (EHA), isooctyl acrylate (IOA), butyl acrylate (BA), or isobutyl acrylate (IBA) as comonomers. Microspheres with diameters from 212 to 332 nm and with a polystyrene content of 65–78% were prepared. The particles prepared in this work do not present the typical core-shell structure; as a consequence, DSC analysis showed that the microspheres exhibited only one Tg. TEM images show that the particles with comonomer content below ~30% were spherical and regular. Microspheres containing comonomer between 21 to 25% produced the less brittle films showing very iridescent colors. The films prepared from microspheres containing hexyl, ethylhexyl, and isooctyl acrylate as comonomers are firmly attached to the substrate due to their adhesive properties. The large decrease of the fragility observed in these films makes them much more attractive materials in sensing applications.

Non-aqueous microgel particles: synthesis, properties and applications

Microgels are cross-linked polymer latex particles that can form stable colloidal dispersions. Their typical sizes range from 10 to 1000 nm and they can swell in response to their external environment (pH, temperature and solvency). This swelling behaviour is central to many potential applications for microgels. The existing literature is dominated by studies of the properties of aqueous microgel dispersions. In contrast, this review focusses on the development of microgel particles in non-aqueous systems, looking at the challenges of studying these particles as well as their swelling behaviour. The five main mechanisms of producing microgel particles will be discussed and examples of materials used for microgels that swell in non-aqueous solvents will be given. Finally some examples of applications for non-aqueous microgels are given.

Mesoporous carbon-TiO₂ beads with nanotextured surfaces as photoanodes in dye-sensitized solar cells

Mesoporous TiO2 and carbon-TiO2 beads with highly roughened surfaces at the nanoscale were prepared by using triblock copolymer P123 simultaneously as template and carbon source in combination with colloid self-assembly. In addition, their role as modifier of the photoanode in the efficiency enhancement of dye-sensitized solar cells is discussed. Hierarchically organized TiO2 networks can provide fast electron transport paths, and ordered mesopores can enhance light scattering as well as facilitate infiltration of the electrolyte. It was found that there is an optimum loading level of mesoporous TiO2 and carbon-TiO2 beads, that is, 1.0 and 0.5 wt%, with respect to the control P25 TiO2 nanoparticles, respectively, for maximizing the photovoltaic performance. An increase in the photovoltaic performance by up to 21.45% was achieved by incorporation of mesoporous carbon-TiO2 beads into the conventional photoanode of DSSCs owing to enhanced charge transport and collection effects.

Direct current electric field assembly of colloidal crystals displaying reversible structural color

We report the application of low-voltage direct current (dc) electric fields to self-assemble close-packed colloidal crystals in nonaqueous solvents from colloidal spheres that vary in size from as large as 1.2 μm to as small as 0.1 μm. The assemblies are created rapidly (∼2 min) from an initially low volume fraction colloidal particle suspension using a simple capacitor-like electric field device that applies a steady dc electric voltage. Confocal microscopy is used to observe the ordering that is produced by the assembly method. This spatial evidence for ordering is consistent with the 6-fold diffraction patterns identified by light scattering. Red, green, and blue structural color is observed for the ordered assemblies of colloids with diameters of 0.50, 0.40, and 0.29 μm, respectively, consistent with spectroscopic measurements of reflectance. The diffraction and spectrophotometry results were found to be consistent with the theoretical Bragg's scattering expected for closed-packed crystals. By switching the dc electric field from on to off, we demonstrate reversibility of the structural color response on times scales ∼60 s. The dc electric field assembly method therefore represents a simple method to produce reversible structural color in colloidal soft matter.

Optically pumped distributed feedback dye lasing with slide-coated TiO₂ inverse-opal slab as Bragg reflector

We demonstrate an optical amplification of organic dye within a TiO2 inverse-opal (IO) distributed feedback (DFB) reflector prepared by a slide-coating method. Highly reflective TiO2 IO film was fabricated by slide coating the binary aqueous dispersions of polystyrene microspheres and charge-stabilized TiO2 nanoparticles on a glass slide and subsequently removing the polymer-opal template. TiO2 IO film was infiltrated, in turn, with the solutions of DCM, a fluorescent dye in various solvents with different indices of refraction. Optical pumping by frequency-doubled Nd:YAG laser resulted in amplified spontaneous emission in each dye solution. In accordance with the semi-empirical simulation by the FDTD method, DCM in ethanol showed the best emission/stopband matching for the TiO2 IO film used in this study. Therefore, photo excitation of a DCM/ethanol cavity showed a single-mode DFB lasing at 640 nm wavelength at moderate pump energy.