Iridium-Based Selective Emitters for Thermophotovoltaic Applications.

The long-term operation of refractory-metal-based metamaterials is crucial for applications such as thermophotovoltaics. The metamaterials based on refractory metals like W, Mo, Ta, Nb, and Re fail primarily by oxidation. Here, the use of the noble metal Ir is proposed, which is stable to oxidation and has optical properties comparable to gold. The thermal endurance of Ir in a 3-layer-system, consisting of HfO2 /Ir/HfO2 , by performing annealing experiments up to 1240 °C in a pressure range from 2 × 10-6  mbar to 1 bar, is demonstrated. The Ir layer shows no oxidation in a vacuum and inert gas atmosphere. At temperatures above 1100 °C, the Ir layer starts to agglomerate due to the degradation of the confining HfO2 layers. An in situ X-ray diffraction experimental comparison between 1D multilayered Ir/HfO2 and W/HfO2 selective emitters annealed at 1000 °C, 2 × 10-6  mbar, over 100 h, confirms oxidation stability of Ir while W multilayers gradually disappear. The results of this work show that W-based metamaterials are not long-term stable even at 1000 °C. However, the oxidation resistance of Ir can be leveraged for refractory plasmonic metamaterials, such as selective emitters in thermophotovoltaic systems with strong suppression of long wavelength radiation.

Wafer-Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle-Assisted Chemical Etching and Pseudomorphic Thermal Oxidation.

Many biological materials exhibit a multiscale porosity with small, mostly nanoscale pores as well as large, macroscopic capillaries to simultaneously achieve optimized mass transport capabilities and lightweight structures with large inner surfaces. Realizing such a hierarchical porosity in artificial materials necessitates often sophisticated and expensive top-down processing that limits scalability. Here, an approach that combines self-organized porosity based on metal-assisted chemical etching (MACE) with photolithographically induced macroporosity for the synthesis of single-crystalline silicon with a bimodal pore-size distribution is presented, i.e., hexagonally arranged cylindrical macropores with 1 µm diameter separated by walls that are traversed by pores 60 nm across. The MACE process is mainly guided by a metal-catalyzed reduction-oxidation reaction, where silver nanoparticles (AgNPs) serve as the catalyst. In this process, the AgNPs act as self-propelled particles that are constantly removing silicon along their trajectories. High-resolution X-ray imaging and electron tomography reveal a resulting large open porosity and inner surface for potential applications in high-performance energy storage, harvesting and conversion or for on-chip sensorics and actuorics. Finally, the hierarchically porous silicon membranes can be transformed structure-conserving by thermal oxidation into hierarchically porous amorphous silica, a material that could be of particular interest for opto-fluidic and (bio-)photonic applications due to its multiscale artificial vascularization.

Adsorption and Inactivation of SARS-CoV-2 on the Surface of Anatase TiO2(101).

We investigated the adsorption of severe acute respiratory syndrome corona virus 2 (SARS-CoV-2), the virus responsible for the current pandemic, on the surface of the model catalyst TiO2(101) using atomic force microscopy, transmission electron microscopy, fluorescence microscopy, and X-ray photoelectron spectroscopy, accompanied by density functional theory calculations. Three different methods were employed to inactivate the virus after it was loaded on the surface of TiO2(101): (i) ethanol, (ii) thermal, and (iii) UV treatments. Microscopic studies demonstrate that the denatured spike proteins and other proteins in the virus structure readsorb on the surface of TiO2 under thermal and UV treatments. The interaction of the virus with the surface of TiO2 was different for the thermally and UV treated samples compared to the sample inactivated via ethanol treatment. AFM and TEM results on the UV-treated sample suggested that the adsorbed viral particles undergo damage and photocatalytic oxidation at the surface of TiO2(101) which can affect the structural proteins of SARS-CoV-2 and denature the spike proteins in 30 min. The role of Pd nanoparticles (NPs) was investigated in the interaction between SARS-CoV-2 and TiO2(101). The presence of Pd NPs enhanced the adsorption of the virus due to the possible interaction of the spike protein with the NPs. This study is the first investigation of the interaction of SARS-CoV-2 with the surface of single crystalline TiO2(101) as a potential candidate for virus deactivation applications. Clarification of the interaction of the virus with the surface of semiconductor oxides will aid in obtaining a deeper understanding of the chemical processes involved in photoinactivation of microorganisms, which is important for the design of effective photocatalysts for air purification and self-cleaning materials.

Strengthening Engineered Nanocrystal Three-Dimensional Superlattices via Ligand Conformation and Reactivity.

Nanocrystal assembly into ordered structures provides mesostructural functional materials with a precise control that starts at the atomic scale. However, the lack of understanding on the self-assembly itself plus the poor structural integrity of the resulting supercrystalline materials still limits their application into engineered materials and devices. Surface functionalization of the nanobuilding blocks with organic ligands can be used not only as a means to control the interparticle interactions during self-assembly but also as a reactive platform to further strengthen the final material via ligand cross-linking. Here, we explore the influence of the ligands on superlattice formation and during cross-linking via thermal annealing. We elucidate the effect of the surface functionalization on the nanostructure during self-assembly and show how the ligand-promoted superlattice changes subsequently alter the cross-linking behavior. By gaining further insights on the chemical species derived from the thermally activated cross-linking and its effect in the overall mechanical response, we identify an oxidative radical polymerization as the main mechanism responsible for the ligand cross-linking. In the cascade of reactions occurring during the surface-ligands polymerization, the nanocrystal core material plays a catalytic role, being strongly affected by the anchoring group of the surface ligands. Ultimately, we demonstrate how the found mechanistic insights can be used to adjust the mechanical and nanostructural properties of the obtained nanocomposites. These results enable engineering supercrystalline nanocomposites with improved cohesion while preserving their characteristic nanostructure, which is required to achieve the collective properties for broad functional applications.

Structural degradation of tungsten sandwiched in hafnia layers determined by in-situ XRD up to 1520 °C.

The high-temperature stability of thermal emitters is one of the critical properties of thermophotovoltaic (TPV) systems to obtain high radiative power and conversion efficiencies. W and HfO2 are ideal due to their high melting points and low vapor pressures. At high temperatures and given vacuum conditions, W is prone to oxidation resulting in instantaneous sublimation of volatile W oxides. Herein, we present a detailed in-situ XRD analysis of the morphological changes of a 3-layer-system: HfO2/W/HfO2 layers, in a high-temperature environment, up to 1520 °C. These samples were annealed between 300 °C and 1520 °C for 6 h, 20 h, and 40 h at a vacuum pressure below 3 × 10-6 mbar using an in-situ high-temperature X-ray diffractometer, which allows investigation of crucial alterations in HfO2 and W layers. HfO2 exhibits polymorphic behavior, phase transformations and anisotropy of thermal expansion leads to formation of voids above 800 °C. These voids serve as transport channels for the residual O2 present in the annealing chamber to access W, react with it and form volatile tungsten oxides. An activation energy of 1.2 eV is calculated. This study clarifies the limits for the operation of W-HfO2 spectrally selective emitters for TPV in high-temperature applications.

Giant electrochemical actuation in a nanoporous silicon-polypyrrole hybrid material.

The absence of piezoelectricity in silicon makes direct electromechanical applications of this mainstream semiconductor impossible. Integrated electrical control of the silicon mechanics, however, would open up new perspectives for on-chip actuorics. Here, we combine wafer-scale nanoporosity in single-crystalline silicon with polymerization of an artificial muscle material inside pore space to synthesize a composite that shows macroscopic electrostrain in aqueous electrolyte. The voltage-strain coupling is three orders of magnitude larger than the best-performing ceramics in terms of piezoelectric actuation. We trace this huge electroactuation to the concerted action of 100 billions of nanopores per square centimeter cross section and to potential-dependent pressures of up to 150 atmospheres at the single-pore scale. The exceptionally small operation voltages (0.4 to 0.9 volts), along with the sustainable and biocompatible base materials, make this hybrid promising for bioactuator applications.

Modulating the Mechanical Properties of Supercrystalline Nanocomposite Materials via Solvent-Ligand Interactions.

Supercrystalline nanocomposite materials with micromechanical properties approaching those of nacre or similar structural biomaterials can be produced by self-assembly of organically modified nanoparticles and further strengthened by cross-linking. The strengthening of these nanocomposites is controlled via thermal treatment, which promotes the formation of covalent bonds between interdigitated ligands on the nanoparticle surface. In this work, it is shown how the extent of the mechanical properties enhancement can be controlled by the solvent used during the self-assembly step. We find that the resulting mechanical properties correlate with the Hansen solubility parameters of the solvents and ligands used for the supercrystal assembly: the hardness and elastic modulus decrease as the Hansen solubility parameter of the solvent approaches the Hansen solubility parameter of the ligands that stabilize the nanoparticles. Moreover, it is shown that self-assembled supercrystals that are subsequently uniaxially pressed can deform up to 6 %. The extent of this deformation is also closely related to the solvent used during the self-assembly step. These results indicate that the conformation and arrangement of the organic ligands on the nanoparticle surface not only control the self-assembly itself but also influence the mechanical properties of the resulting supercrystalline material. The Hansen solubility parameters may therefore serve as a tool to predict what solvents and ligands should be used to obtain supercrystalline materials with good mechanical properties.

Alumina-Doped Zirconia Submicro-Particles: Synthesis, Thermal Stability, and Microstructural Characterization.

Zirconia nanoceramics are interesting materials for numerous high-temperature applications. Because their beneficial properties are mainly governed by the crystal and microstructure, it is essential to understand and control these features. The use of co-stabilizing agents in the sol-gel synthesis of zirconia submicro-particles should provide an effective tool for adjusting the particles' size and shape. Furthermore, alumina-doping is expected to enhance the particles' size and shape persistence at high temperatures, similar to what is observed in corresponding bulk ceramics. Dispersed alumina should inhibit grain growth by forming diffusion barriers, additionally impeding the martensitic phase transformation in zirconia grains. Here, alumina-doped zirconia particles with sphere-like shape and average diameters of ∼ 300 n m were synthesized using a modified sol-gel route employing icosanoic acid and hydroxypropyl cellulose as stabilizing agents. The particles were annealed at temperatures between 800 and 1200 ∘ C and characterized by electron microscopy, elemental analysis, and X-ray diffraction. Complementary elemental analyses confirmed the precise control over the alumina content (0-50 mol%) in the final product. Annealed alumina-doped particles showed more pronounced shape persistence after annealing at 1000 ∘ C than undoped particles. Quantitative phase analyses revealed an increased stabilization of the tetragonal/cubic zirconia phase and a reduced grain growth with increasing alumina content. Elemental mapping indicated pronounced alumina segregation near the grain boundaries during annealing.

Metamaterial emitter for thermophotovoltaics stable up to 1400 °C.

High temperature stable selective emitters can significantly increase efficiency and radiative power in thermophotovoltaic (TPV) systems. However, optical properties of structured emitters reported so far degrade at temperatures approaching 1200 °C due to various degradation mechanisms. We have realized a 1D structured emitter based on a sputtered W-HfO2 layered metamaterial and demonstrated desired band edge spectral properties at 1400 °C. To the best of our knowledge the temperature of 1400 °C is the highest reported for a structured emitter, so far. The spatial confinement and absence of edges stabilizes the W-HfO2 multilayer system to temperatures unprecedented for other nanoscaled W-structures. Only when this confinement is broken W starts to show the well-known self-diffusion behavior transforming to spherical shaped W-islands. We further show that the oxidation of W by atmospheric oxygen could be prevented by reducing the vacuum pressure below 10-5 mbar. When oxidation is mitigated we observe that the 20 nm spatially confined W films survive temperatures up to 1400 °C. The demonstrated thermal stability is limited by grain growth in HfO2, which leads to a rupture of the W-layers, thus, to a degradation of the multilayer system at 1450 °C.

Hierarchical supercrystalline nanocomposites through the self-assembly of organically-modified ceramic nanoparticles.

Biomaterials often display outstanding combinations of mechanical properties thanks to their hierarchical structuring, which occurs through a dynamically and biologically controlled growth and self-assembly of their main constituents, typically mineral and protein. However, it is still challenging to obtain this ordered multiscale structural organization in synthetic 3D-nanocomposite materials. Herein, we report a new bottom-up approach for the synthesis of macroscale hierarchical nanocomposite materials in a single step. By controlling the content of organic phase during the self-assembly of monodisperse organically-modified nanoparticles (iron oxide with oleyl phosphate), either purely supercrystalline or hierarchically structured supercrystalline nanocomposite materials are obtained. Beyond a critical concentration of organic phase, a hierarchical material is consistently formed. In such a hierarchical material, individual organically-modified ceramic nanoparticles (Level 0) self-assemble into supercrystals in face-centered cubic superlattices (Level 1), which in turn form granules of up to hundreds of micrometers (Level 2). These micrometric granules are the constituents of the final mm-sized material. This approach demonstrates that the local concentration of organic phase and nano-building blocks during self-assembly controls the final material's microstructure, and thus enables the fine-tuning of inorganic-organic nanocomposites' mechanical behavior, paving the way towards the design of novel high-performance structural materials.

Synthesis and thermal stability of ZrO2@SiO2 core-shell submicron particles.

ZrO2@SiO2 core-shell submicron particles are promising candidates for the development of advanced optical materials. Here, submicron zirconia particles were synthesized using a modified sol-gel method and pre-calcined at 400 °C. Silica shells were grown on these particles (average size: ∼270 nm) with well-defined thicknesses (26 to 61 nm) using a seeded-growth Stöber approach. To study the thermal stability of bare ZrO2 cores and ZrO2@SiO2 core-shell particles they were calcined at 450 to 1200 °C. After heat treatments, the particles were characterized by SEM, TEM, STEM, cross-sectional EDX mapping, and XRD. The non-encapsulated, bare ZrO2 particles predominantly transitioned to the tetragonal phase after pre-calcination at 400 °C. Increasing the temperature to 600 °C transformed them to monoclinic. Finally, grain coarsening destroyed the spheroidal particle shape after heating to 800 °C. In striking contrast, SiO2-encapsulation significantly inhibited grain growth and the t → m transition progressed considerably only after heating to 1000 °C, whereupon the particle shape, with a smooth silica shell, remained stable. Particle disintegration was observed after heating to 1200 °C. Thus, ZrO2@SiO2 core-shell particles are suited for high-temperature applications up to ∼1000 °C. Different mechanisms are considered to explain the markedly enhanced stability of ZrO2@SiO2 core-shell particles.

Dataset of ptychographic X-ray computed tomography of inverse opal photonic crystals produced by atomic layer deposition.

This data article describes the detailed parameters for synthesizing mullite inverse opal photonic crystals via Atomic Layer Deposition (ALD), as well as the detailed image analysis routine used to interpret the data obtained by the measurement of such photonic crystals, before and after the heat treatment, via Ptychographic X-ray Computed Tomography (PXCT). The data presented in this article are related to the research article by Furlan and co-authors entitled "Photonic materials for high-temperature applications: Synthesis and characterization by X-ray ptychographic tomography" (Furlan et al., 2018). The data include detailed information about the ALD super-cycle process to generate the ternary oxides inside a photonic crystal template, the raw data from supporting characterization techniques, as well as the full dataset obtained from PXCT. All the data herein described is publicly available in a Mendeley Data archive "Dataset of synthesis and characterization by PXCT of ALD-based mullite inverse opal photonic crystals" located at https://data.mendeley.com/datasets/zn49dsk7x6/1 for any academic, educational, or research purposes.

Exceptionally strong, stiff and hard hybrid material based on an elastomer and isotropically shaped ceramic nanoparticles.

In this work the fabrication of hard, stiff and strong nanocomposites based on polybutadiene and iron oxide nanoparticles is presented. The nanocomposites are fabricated via a general concept for mechanically superior nanocomposites not based on the brick and mortar structure, thus on globular nanoparticles with nanosized organic shells. For the fabrication of the composites oleic acid functionalized iron oxide nanoparticles are decorated via ligand exchange with an α,ω-polybutadiene dicarboxylic acid. The functionalized particles were processed at 145 °C. Since polybutadiene contains double bonds the nanocomposites obtained a crosslinked structure which was enhanced by the presence of oxygen or sulfur. It was found that the crosslinking and filler percolation yields high elastic moduli of approximately 12-20 GPa and hardness of 15-18 GPa, although the polymer volume fraction is up to 40%. We attribute our results to a catalytically enhanced crosslinking reaction of the polymer chains induced by oxygen or sulfur and to the microstructure of the nanocomposite.

Semiordered Hierarchical Metallic Network for Fast and Large Charge-Induced Strain.

Nanoporous metallic actuators for artificial muscle applications are distinguished by combining the low operating voltage, which is otherwise reserved for polymer-based actuators with interesting values of strain amplitude, strength, and stiffness that are comparable of those of piezoceramics. We report a nanoporous metal actuator with enhanced strain amplitude and accelerated switching. Our 3D macroscopic metallic muscle has semiordered and hierarchical nanoporous structure, in which µm-sized tubes align perpendicular with the sample surface, while nm-sized ligaments consist of the tube walls. This nanoarchitecture combines channels for fast ion transportation with large surface area for charge storage and strain generation. The result is a record reversible strain amplitude of 1.59% with a strain rate of 8.83 × 10-6 s-1 in the field of metallic based actuators. A passive hydroxide layer is self-grown on the metal surface, which not only contributes a supercapacitive layer, but also stabilizes the nanoporous structure against coarsening, which guarantees sustainable actuation beyond ten-thousand cycles.

Organically linked iron oxide nanoparticle supercrystals with exceptional isotropic mechanical properties.

It is commonly accepted that the combination of the anisotropic shape and nanoscale dimensions of the mineral constituents of natural biological composites underlies their superior mechanical properties when compared to those of their rather weak mineral and organic constituents. Here, we show that the self-assembly of nearly spherical iron oxide nanoparticles in supercrystals linked together by a thermally induced crosslinking reaction of oleic acid molecules leads to a nanocomposite with exceptional bending modulus of 114 GPa, hardness of up to 4 GPa and strength of up to 630 MPa. By using a nanomechanical model, we determined that these exceptional mechanical properties are dominated by the covalent backbone of the linked organic molecules. Because oleic acid has been broadly used as nanoparticle ligand, our crosslinking approach should be applicable to a large variety of nanoparticle systems.