Metal-free, flexible infrared electrochromic devices were fabricated using PEDOT:Tos as both the electrochromic and the electrode material, enabling modulation of the devices' infrared signature and their effective temperature as seen by a thermal camera. In addition to evaluating the practical suitability of these devices, we report a detailed investigation of the thin film spectroscopic properties.

Energy harvesting from solar light employing nanostructured materials offer an economic way to resolve energy and environmental issues. We have developed an efficient light harvesting heterostructure based on poly(diphenylbutadiyne) (PDPB) nanofibers and ZnO nanoparticles (NPs)viaa solution phase synthetic route. ZnO NPs (~20 nm) were homogeneously loaded onto the PDPB nanofibers as evident from several analytical and spectroscopic techniques. The photoinduced electron transfer from PDPB nanofibers to ZnO NPs has been confirmed by steady state and picosecond-resolved photoluminescence studies. The co-sensitization for multiple photon harvesting (with different energies) at the heterojunction has been achievedviaa systematic extension of conjugation from monomeric to polymeric diphenyl butadiyne moiety in the proximity of the ZnO NPs. On the other hand, energy transfer from the surface defects of ZnO NPs (~5 nm) to PDPB nanofibers through Förster Resonance Energy Transfer (FRET) confirms the close proximity with molecular resolution. The manifestation of efficient charge separation has been realized with ~5 fold increase in photocatalytic degradation of organic pollutants in comparison to polymer nanofibers counterpart under visible light irradiation. Our results provide a novel approach for the development of nanoheterojunctions for efficient light harvesting which will be helpful in designing future solar devices.

Materials that provide independent control of infrared thermal radiation and haze in the visible could benefit many areas and applications, including clothing, packaging and optoelectronics. Here, we study this possibility for a metamaterial composite paper based on cellulose nanofibrils (CNF) and silicon dioxide (SiO₂) microparticles with infrared (IR) Fröhlich phonon resonances. This CNF-SiO₂ composite shows outstanding transparency in the whole visible wavelength range, with the option of controlling light diffusion and haze from almost zero to 90% by varying the SiO₂ microparticle concentration. We further show that the transparent metamaterial paper could maintain near-perfect thermal emittance in the atmospheric IR window, as attributed to strong IR absorption of both the nanocellulose and the resonant SiO₂ microparticles. The high IR emissivity and low visible absorption make the paper suitable for passive radiative cooling and we demonstrate cooling of the paper to 3 °C below ambient air temperature by exposing it to the sky.

Technological hurdles that still prevent the commercialization of fuel cell technologies necessitate designing low-cost, efficient and non-precious metals. These could serve as alternatives to high-cost Pt-based materials. Herein, a facile and effective microwave-assisted route has been developed to synthesize structurally uniform and electrochemically active pure and transition metal-doped manganese oxide nanoballs (Mn₂O₃ NBs) for fuel cell applications. The average diameter of pure and doped Mn₂O₃ NBs was found to be ~610 nm and ~650 nm, respectively, as estimated using transmission electron microscopy (TEM). The nanoparticles possess a good degree of crystallinity as evident from the lattice fringes in high-resolution transmission electron microscopy (HRTEM). The cubic crystal phase was ascertained using X-ray diffraction (XRD). The energy dispersive spectroscopic (EDS) elemental mapping confirms the formation of copper-doped Mn₂O₃ NBs. The experimental parameter using trioctylphosphine oxide (TOPO) as the chelating agent to control the nanostructure growth has been adequately addressed using scanning electron microscopy (SEM). The solid NBs were formed by the self-assembly of very small Mn₂O₃ nanoparticles as evident from the SEM image. Moreover, the concentration of TOPO was found to be the key factor whose subtle variation can effectively control the size of the as-prepared Mn₂O₃ NBs. The cyclic voltammetry and galvanostatic charge/discharge studies demonstrated enhanced electrochemical performance for copper-doped Mn₂O₃ NBs which is supported by a 5.2 times higher electrochemically active surface area (EASA) in comparison with pure Mn₂O₃ NBs. Electrochemical investigations indicate that both pure and copper-doped Mn₂O₃ NBs exhibit a bifunctional catalytic activity toward the four-electron electrochemical reduction as well the evolution of oxygen in alkaline media. Copper doping in Mn₂O₃ NBs revealed its pronounced impact on the electrocatalytic activity with a high current density for the electrochemical oxygen reduction and evolution reaction. The synthetic approach provides a general platform for fabricating well-defined porous metal oxide nanostructures with prospective applications as low-cost catalysts for alkaline fuel cells.

Manganese oxides are well known applied materials including their use as efficient catalysts for various environmental applications. Multiple oxidation states and their change due to various experimental conditions are concluded to be responsible for their multifaceted functionality. Here we demonstrate that the interaction of a small organic ligand with one of the oxide varieties induces completely new optical properties and functionalities (photocatalysis). We have synthesized Mn₂O₃ microspheres via a hydrothermal route and characterized them using scanning electron microscopy (SEM), X-ray diffraction (XRD) and elemental mapping (EDAX). When the microspheres are allowed to interact with the biologically important small ligand citrate, nanometer-sized surface functionalized Mn₂O₃ (NPs) are formed. Raman and Fourier transformed infrared spectroscopy confirm the covalent attachment of the citrate ligand to the dangling bond of Mn at the material surface. While cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) analysis confirm multiple surface charge states after the citrate functionalization of the Mn₂O₃ NPs, new optical properties of the surface engineered nanomaterials in terms of absorption and emission emerge consequently. The engineered material offers a novel photocatalytic functionality to the model water contaminant methylene blue (MB). The effect of doping other metal ions including Fe³⁺ and Cu²⁺ on the optical and catalytic properties is also investigated. In order to prepare a prototype for potential environmental application of water decontamination, we have synthesized and duly functionalized the material on the extended surface of a stainless steel metal mesh (size 2 cm × 1.5 cm, pore size 150 μm × 200 μm). We demonstrate that the functionalized mesh always works as a “physical” filter of suspended particulates. However, it works as a “chemical” filter (photocatalyst) for the potential water soluble contaminant (MB) in the presence of solar light.

Certain bird species have evolved spectacular colors that arise from organized nanostructures of melanin. Its high refractive index (~1.8) and broadband absorptive properties enable vivid structural colors, yet that are non-susceptible to photo-bleaching. Mimicking natural melanin structural coloration could enable several important applications, in particular for non-iridescent systems with colors that are independent of incidence angle. Here, we address this by forming melanin photonic crystal microdomes by inkjet printing. Owing to their curved nature, the microdomes exhibit non-iridescent vivid structural coloration, tunable throughout the visible range via the size of the nanoparticles. Large area arrays (> 1 cm2) of high-quality photonic microdomes could be printed on both rigid and flexible substrates, including on cellulose-based paper. Combined with scalable fabrication and non-toxicity of melanin, the presented photonic microdomes with non-iridescent structural coloration may find use in a variety of applications, including sensing, displays and anti-counterfeit holograms.