Interface Catalysts of Ni3Fe1 Layered Double Hydroxide and Titanium Carbide for High-Performance Water Oxidation in Alkaline and Natural Conditions.

The electrocatalytic oxygen evolution reaction (OER) is important for many renewable energy technologies. Developing cost-effective electrocatalysts with high performance remains a great challenge. Here, we successfully demonstrate our novel interface catalyst comprised of Ni3Fe1-based layered double hydroxides (Ni3Fe1-LDH) vertically immobilized on a two-dimensional MXene (Ti3C2Tx) surface. The Ni3Fe1-LDH/Ti3C2Tx yielded an anodic OER current of 100 mA cm-2 at 0.28 V versus reversible hydrogen electrode (RHE), nearly 74 times lower than that of the pristine Ni3Fe1-LDH. Furthermore, the Ni3Fe1-LDH/Ti3C2Tx catalyst requires an overpotential of only 0.31 V versus RHE to deliver an industrial-level current density as high as 1000 mA cm-2. Such excellent OER activity was attributed to the synergistic interface effect between Ni3Fe1-LDH and Ti3C2Tx. Density functional theory (DFT) results further reveal that the Ti3C2Tx support can efficiently accelerate the electron extraction from Ni3Fe1-LDH and tailor the electronic structure of catalytic sites, resulting in enhanced OER performance.

Ag nanoplatelets as efficient photosensitizers for TiO2 nanorods.

The lifetime for injecting hot electrons generated in Ag nanoplatelets to nearby TiO2 nanorods was measured with ultrafast transient IR absorption to be 13.1 ± 1.5 fs, which is comparable to values previously reported for much smaller spherical Ag nanoparticles. Although it was shown that the injection rate decreases as the particle size increases, this observation can be explained by the facts that (1) the platelet has a much larger surface to bulk ratio and (2) the platelet affords a much larger surface area for direct contact with the semiconductor. These two factors facilitate strong Ag-TiO2 coupling (as indicated by the observed broadened surface plasmon resonance band of Ag) and can explain why Ag nanoplatelets have been found to be more efficient than much smaller Ag nanoparticles as photosensitizers for photocatalytic functions. The fast injection rate, together with a stronger optical absorption in comparison with Au and dye molecules, make Ag nanoplatelets a preferred photosensitizer for wide bandgap semiconductors.

Interface Catalysis of Nickel Molybdenum (NiMo) Alloys on Two-Dimensional (2D) MXene for Enhanced Hydrogen Electrochemistry.

Development of efficient bifunctional nonprecious metallic electrocatalysts for hydrogen electrochemistry in alkaline solution is of importance to enable commercialization of a low-cost alkaline hydrogen fuel cell and water electrolyzer, but it is very challenging. Two-dimensional (2D) MXene-based electrocatalysts hold tremendous potential for the applications of hydrogen fuel cell and water electrolyzer. Here, we successfully immobilized transition-metal-based NiMo nanoparticles (NPs) on 2D Ti3C2Tx (Tx: surface terminations, such as O, OH, or F) surfaces by a wet chemical method. Our results demonstrate that the NiMo NPs are monodispersed on Ti3C2Tx with surface functionalization. These monodisperse NPs resulted in superior hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) activities in an alkaline media. The NiMo NPs/Ti3C2Tx in 1.0 M KOH yielded an HER current of -10 mA cm-2 at -0.044 V vs reversible hydrogen electrode (RHE), nearly 232 mV smaller than that of the parent NiMo NPs. The NiMo NPs/Ti3C2Tx produced an HOR current density of 1.5 mA cm-2 at 0.1 V vs RHE. Density functional theory (DFT) results further reveal that Ti3C2Tx support can facilitate the charge transfer to metallic NPs and tailor the electronic structure of catalytic sites, resulting in optimized adsorption free energies of H* species for hydrogen electrochemistry. This work provides a facile and universal strategy in the development of 2D Ti3C2Tx with nonprecious metals for low-cost bifunctional hydrogen electrocatalysts.

Hollow aluminum microspheres with high mass extinction coefficients in the long wave infrared.

Previous electromagnetic computations of multilayered dielectric/metallic spheres identified the ideal dimensions and composition for achieving optimized mass extinction coefficients (m2/g). A hollow metallic sphere, with a thin metallic shell, is one such example of a spherical structure that can theoretically achieve high mass extinction coefficients in the long wave infrared (LWIR) region (8-12 µm). To this end, we endeavored to demonstrate a cost-effective and scalable manufacturing approach for synthesizing and experimentally validating the mass extinction coefficients of hollow metallic spheres. Specifically, we detail a novel approach for fabricating hollow aluminum spheres using radio frequency (RF) magnetron sputter deposition. Sacrificial high-density polyethylene polymer microspheres were used as substrates for the deposition of thin layers of aluminum. The core shell structures were subsequently thermally processed to form the hollow micron sized aluminum shells. The mass extinction coefficients of the hollow aluminum spheres were subsequently measured and compared to computational results. A strong agreement between experimental and theoretical predictions was observed. Finally, the LWIR mass extinction coefficients of the hollow spheres were compared to high aspect ratio brass flakes, a common pigment used for LWIR attenuation, and other materials and geometries that are used for LWIR filtering applications. This comparison of both performance and availability revealed that the fabricated hollow aluminum spheres exhibited competitive LWIR properties using a more scalable and cost-effective manufacturing approach.

Iterative design of multilayered dielectric microspheres with tunable transparency windows.

Suspensions of microparticles dispersed in air or liquids are useful for designing media with desirable optical extinction properties within the visible or infrared spectrum. We describe here a numerical iterative optimization algorithm used to design multilayered concentric dielectric spheres with prescribed optical scattering properties. Our method integrates a computationally efficient rigorous electromagnetic solver, based on Mie theory, within an optimization loop to determine specific particle configurations that best meet a desired optical response. In particular, we show that this method can be used to design all-dielectric spherical particles that possess narrow tunable transparency windows while removing any angular dependency on the optical response.

Nanophotonic particle simulation and inverse design using artificial neural networks.

We propose a method to use artificial neural networks to approximate light scattering by multilayer nanoparticles. We find that the network needs to be trained on only a small sampling of the data to approximate the simulation to high precision. Once the neural network is trained, it can simulate such optical processes orders of magnitude faster than conventional simulations. Furthermore, the trained neural network can be used to solve nanophotonic inverse design problems by using back propagation, where the gradient is analytical, not numerical.

Cavity-based aluminum nanohole arrays with tunable infrared resonances.

This work details the successful computational design, fabrication, and characterization of a cavity-based aluminum nanohole array. The designs incorporate arrays of aluminum nanoholes that are patterned on a dielectric-coated (SiO2 or ZnSe) aluminum base mirror plane. This architecture provided a means of exploring the coupling of the localized resonances, exhibited by the aluminum nanohole array, with the cavity resonance that is generated within the dielectric spacer layer, which resides between the base plane mirror and the nanohole array. Rigorous coupled wave analysis (RCWA) was first used to computationally design the structures. Next, a range of lithographic techniques, including photolithography, E-beam lithography, and nanosphere lithography, were used to fabricate the structures. Finally, infrared spectroscopy and scanning electron microscopy (SEM) were used to characterize the spectral and structural properties of the multilayered devices, respectively. The overall goal of this study was to demonstrate our ability to design and fabricate aluminum-based structures with tunable resonances throughout the infrared region, i.e. from the short-wave through longwave infrared regions of the electromagnetic spectrum (1.5 -12 µm).

Effects of Molecular Structure and Solvent Polarity on Adsorption of Carboxylic Anchoring Dyes onto TiO2 Particles in Aprotic Solvents.

Interactions of molecules with the surface of TiO2 particles are of fundamental and technological importance. One example is that the adsorption density and energy of the dye molecules on TiO2 particles affect the efficiency of dye-sensitized solar cells (DSSC). In this work, we present measurements characterizing the adsorption of the two isomers, para-ethyl red (p-ER) and ortho-ethyl red (o-ER), of a dye molecule potentially applicable for DSSC onto TiO2 particles by second harmonic scattering (SHS). It is found that while at the wavelengths used here o-ER has a much bigger molecular hyperpolarizability, p-ER exhibits strong SHS responses but o-ER gives no detectable SHS when the dyes are added to the TiO2 colloids, respectively. This observation indicates that o-ER does not adsorb onto TiO2, likely due to steric hindrance. Furthermore, we investigate how solvents affect the surface adsorption strength and density of p-ER onto TiO2 in four aprotic solvents with varying polarity. The absolute magnitude of the adsorption free energy was found to increase with the specific solvation energy that represents the ability of accepting electrons and solvent polarity. It is likely that resolvation of the solvent molecules displaced by the adsorption of the dye molecule at the surface in stronger electron-accepting and more polar solvents results in a larger adsorption free energy for the dye adsorption.

Fundamental limits to optical response in absorptive systems.

At visible and infrared frequencies, metals show tantalizing promise for strong subwavelength resonances, but material loss typically dampens the response. We derive fundamental limits to the optical response of absorptive systems, bounding the largest enhancements possible given intrinsic material losses. Through basic conservation-of-energy principles, we derive geometry-independent limits to per-volume absorption and scattering rates, and to local-density-of-states enhancements that represent the power radiated or expended by a dipole near a material body. We provide examples of structures that approach our absorption and scattering limits at any frequency; by contrast, we find that common "antenna" structures fall far short of our radiative LDOS bounds, suggesting the possibility for significant further improvement. Underlying the limits is a simple metric, |χ|2/Im χ for a material with susceptibility χ, that enables broad technological evaluation of lossy materials across optical frequencies.

Fano resonance in a gold nanosphere with a J-aggregate coating.

We present a facile method to induce J-aggregate formation on gold nanospheres in colloidal solution using polyvinylsulfate. The nanoparticle J-aggregate complex results in an absorption spectrum with a split lineshape due to plasmon-exciton coupling, i.e. via the formation of upper and lower plexcitonic branches. The use of nanoparticles with different plasmon resonances alters the position of the upper plexcitonic band while the lower band remains at the same wavelength. This splitting is investigated theoretically, and shown analytically to arise from Fano resonance between the plasmon of the gold nanoparticles and exciton of the J-aggregates. A theoretical simulation of a J-aggregate coated and uncoated gold nanosphere produces an absorption spectrum that shows good agreement with the experimentally measured spectra.

Coherent plasmon-exciton coupling in silver platelet-J-aggregate nanocomposites.

Hybrid nanostructures that couple plasmon and exciton resonances generate hybridized energy states, called plexcitons, which may result in unusual light-matter interactions. We report the formation of a transparency dip in the visible spectra of colloidal suspensions containing silver nanoplatelets and a cyanine dye, 1,1'-diethyl-2,2'-cyanine iodide (PIC). PIC was electrostatically adsorbed onto the surface of silver nanoplatelet core particles, forming an outer J-aggregate shell. This core-shell architecture provided a framework for coupling the plasmon resonance of the silver nanoplatelet core with the exciton resonance of the J-aggregate shell. The sizes and aspect ratios of the silver nanoplatelets were controlled to ensure the overlap of the plasmon and exciton resonances. As a measure of the plasmon-exciton coupling strength in the system, the experimentally observed transparency dips correspond to a Rabi splitting energy of 207 meV, among the highest reported for colloidal nanoparticles. The optical properties of the silver platelet-J-aggregate nanocomposites were supported numerically and analytically by the boundary-element method and temporal coupled-mode theory, respectively. Our theoretical predictions and experimental results confirm the presence of a transparency dip for the silver nanoplatelet core J-aggregate shell structures. Additionally, the numerical and analytical calculations indicate that the observed transparencies are dominated by the coupling of absorptive resonances, as opposed to the coupling of scattering resonances. Hence, we describe the suppressed extinction in this study as an induced transparency rather than a Fano resonance.

Theoretical criteria for scattering dark states in nanostructured particles.

Nanostructures with multiple resonances can exhibit a suppressed or even completely eliminated scattering of light, called a scattering dark state. We describe this phenomenon with a general treatment of light scattering from a multiresonant nanostructure that is spherical or nonspherical but subwavelength in size. With multiple resonances in the same channel (i.e., same angular momentum and polarization), coherent interference always leads to scattering dark states in the low-absorption limit, regardless of the system details. The coupling between resonances is inevitable and can be interpreted as arising from far-field or near-field. This is a realization of coupled-resonator-induced transparency in the context of light scattering, which is related to but different from Fano resonances. Explicit examples are given to illustrate these concepts.

Transparent displays enabled by resonant nanoparticle scattering.

The ability to display graphics and texts on a transparent screen can enable many useful applications. Here we create a transparent display by projecting monochromatic images onto a transparent medium embedded with nanoparticles that selectively scatter light at the projected wavelength. We describe the optimal design of such nanoparticles, and experimentally demonstrate this concept with a blue-color transparent display made of silver nanoparticles in a polymer matrix. This approach has attractive features including simplicity, wide viewing angle, scalability to large sizes and low cost.

Layer-by-layer self-assembly of plexcitonic nanoparticles.

Colloidal suspensions of multilayer nanoparticles composed of a silver core, a polyelectrolyte spacer layer (inner shell), and a J-aggregate cyanine dye outer shell have been prepared for the first time. Absorption properties of the colloid were measured in the visible region. This multilayer architecture served as a framework for examining the coupling of the localized surface plasmon resonance exhibited by the silver core with the molecular exciton exhibited by the J-aggregate outer shell. The polyelectrolyte spacer layer promotes the formation of an excitonic J-aggregate while serving as a means of controlling the plasmon-exciton (i.e. plexciton) coupling strength through changing the distance between the core and the shell. An analytical expression based on Mie Theory and the Transfer Matrix Method was obtained for describing the optical response of these multilayered nanostructures. Computational and experimental results indicate that the absorption wavelength of the J-aggregate form of the dye is dependent on both the distance of the dye layer from the silver core and the degree of dye aggregation.

Optimization of broadband optical response of multilayer nanospheres.

We propose an optimization-based theoretical approach to tailor the optical response of silver/silica multilayer nanospheres over the visible spectrum. We show that the structure that provides the largest cross-section per volume/mass, averaged over a wide frequency range, is the silver coated silica sphere. We also show how properly chosen mixture of several species of different nanospheres can have an even larger minimal cross-section per volume/mass over the entire visible spectrum.

The determination of carbon dioxide concentration using atmospheric pressure ionization mass spectrometry/isotopic dilution and errors in concentration measurements caused by dryers.

An atmospheric pressure ionization mass spectrometry/isotopically labeled standard (APIMS/ILS) method has been developed for the determination of carbon dioxide (CO(2)) concentration. Descriptions of the instrumental components, the ionization chemistry, and the statistics associated with the analytical method are provided. This method represents an alternative to the nondispersive infrared (NDIR) technique, which is currently used in the atmospheric community to determine atmospheric CO(2) concentrations. The APIMS/ILS and NDIR methods exhibit a decreased sensitivity for CO(2) in the presence of water vapor. Therefore, dryers such as a nafion dryer are used to remove water before detection. The APIMS/ILS method measures mixing ratios and demonstrates linearity and range in the presence or absence of a dryer. The NDIR technique, on the other hand, measures molar concentrations. The second half of this paper describes errors in molar concentration measurements that are caused by drying. An equation describing the errors was derived from the ideal gas law, the conservation of mass, and Dalton's Law. The purpose of this derivation was to quantify errors in the NDIR technique that are caused by drying. Laboratory experiments were conducted to verify the errors created solely by the dryer in CO(2) concentration measurements post-dryer. The laboratory experiments verified the theoretically predicted errors in the derived equations. There are numerous references in the literature that describe the use of a dryer in conjunction with the NDIR technique. However, these references do not address the errors that are caused by drying.