Thin, Conformal, and Continuous SnO₂ Coatings on Three‐Dimensional Biosilica Templates through Hydroxy‐Group Amplification and Layer‐By‐Layer Alkoxide Deposition

Abstract

Appreciable effort is underway to develop robust protocols for synthesizing functional nanostructured assemblies. Desired characteristics for such protocols include precise control of structure (down to the nanoscale), versatile control of chemistry (for tailored functionality), and massively parallel assembly (for large-scale manufacturing). The precise, versatile, and scalable fabrication of functional nanostructured assemblies, particularly those with intricate 3D morphologies, remains a significant challenge for nanotechnology.Nature provides spectacular examples of precise and highly replicable 3D self-assembly of inorganic materials on the micrometer to nanometer scales.[1] Diatoms, for example, generate nanostructured silica microshells (frustules) with thousands of species-specific morphologies.[1d] Sustained reproduction of a particular diatom species can yield enormous numbers of frustules with similar 3D morphologies.[1e]Such intricate, genetically precise, and massively parallel 3D self-assembly under ambient conditions lies well beyond the current capabilities of synthetic micro-and nanofabrication. However, to utilize the precision and massive parallelism of such biological assembly for a variety of nanostructured devices, the silica-frustule chemistry needs to be altered for desired electronic, optical, chemical, or other properties.While gas-solid reactions [2a-c] and gas-phase deposition [2d,e] have been used to alter the chemistries of bioclastic structures, these approaches require volatile reactants orprecursors. An alternate range of chemistries may be accessed with liquid-phase deposition methods. For example, Kunitake, et al. [3] have used a layer-by-layer surface sol-gel process to deposit conformal oxide-bearing coatings on hydroxy-or carboxy-bearing surfaces (for example, latex spheres [3b,c] or cellulose [3d,e] ). However, this surface sol-gel process has not been used to generate thin, conformal, and continuous functional oxide coatings on intricate 3D biomineral structures.Herein, we demonstrate, for the first time, how the intricate nanostructured silica valves of diatom frustules may be coated with a thin (50 nm), conformal, and continuous layer of a functional oxide (SnO 2 ) through dendritic amplification of hydroxy groups on the silica surfaces and then use of an automated surface sol-gel process. A device built from such SnO 2 -coated diatom frustule valves acts as a sensitive detector for NO gas.Initial experiments produced patchy, discontinuous oxide coatings, which we attribute to a low density of surface hydroxy groups available to initiate the sol-gel reaction on the silica valves. Therefore, it was necessary to increase the number of accessible hydroxy functionalities through a series of chemical reactions (Scheme 1). The diatom silica was first subjected to an oxidizing RCA-1 cleaning solution and then treated with (3-aminopropyl)triethoxysilane to functionalize the surfaces with amine groups. The surfaces were then exposed, in alternating fashion, to solutions of po...

Porous Electrode Materials for Lithium‐Ion Batteries – How to Prepare Them and What Makes Them Special

“…SnO 2 coatings required amplifi cation of surface hydroxyl groups on silica diatoms by a thin interlayer of acrylate functionalized with glucosamine, for improved adhesion of the conformal SnO 2 coating. [ 109 ] The silica scaffold can then be removed by etching with HF or hot NaOH or KOH solution. This was shown, for example in the case of porous carbon templated by diatomaceous earth, in a preparation involving sucrose and sulfuric acid as the carbon source.…”

A Scalable Chemical Approach for the Synthesis of a Highly Tolerant and Efficient Oil Absorbent

“…The random arrangement of granulard omains provided the appropriate hierarchical topography for attaining superhydrophobicity.M ore importantly,t he granulard omains deposited on the MF consists of residual chemicalr eactivity-which was furtherc onfirmed through standard FTIR analysis [45][46][47] as shown in Figure 1E.T he peaks at 1410 cm À1 and 1735 cm À1 , which are the characteristicsignatures for the symmetric deformation of CÀHb ond for b carbon of the vinyl group and the carbonyls tretching of the ester linkage, revealed the existence of residual acrylate groupsi nt he deposited nanocomplexes. This residual chemical reactivity allowed post covalentm odification of the nanocomplex coated MF with selected alkylamine to attain essential low surface energy in the material.A fter the post chemical modification of the residual acrylate moieties with octadecylamine (ODA), the IR peak intensity at 1410 cm À1 significantly depleted with respect to the normalized carbonyl stretching peak at 1735 cm À1 ,w hichu nambiguously indicated the successful post covalent modification of the material through1 ,4-conjugate addition reaction.…”

Layer‐By‐Layer Dendritic Growth of Hyperbranched Thin Films for Surface Sol–Gel Syntheses of Conformal, Functional, Nanocrystalline Oxide Coatings on Complex 3D (Bio)silica Templates

“…The hydroxyl-rich diatom frustules were then subjected to an automated LBL surface sol-gel process [45] to decorate the frustules with a thin, iron oxide-bearing coating. In a typical surface sol-gel cycle, the surface modified diatom frustules were immersed in Fe(III) isopropoxide (0.01 M, Alfa Aesar) solution in anhydrous 2-propanol (99.8þ%, Acros Organics) for 10 min followed by vacuum filtration.…”

“…However, we have recently shown that the formation of a continuous surface sol-gelderived SnO 2 coating on diatom frustules required enrichment of the surface hydroxyl groups present on the native frustule surfaces. [45] In such prior work, surface hydroxyl amplification was accomplished through a straightforward, two-component, sequential, LBL-type method for fabricating hyperbranched multilayers that involved the repetitive Michael addition of polyfunctional amine-containing species to polyfunctional acrylates, followed by the final binding of a hydroxyl-rich amine (D-glucosamine). Subsequent use of the surface sol-gel process then yielded a thin ( 50 nm), conformal coating of nanocrystalline SnO 2 on diatom frustules that enabled a single diatom frustule to be used as a minimally-invasive, rapid, and sensitive NO gas detector.…”