Biomimetically Controlled Formation of Nanotextured Silica/Titania Films on Arbitrary Substrates and Their Tunable Surface Function

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Constructing ceramic thin film on solid surfaces through biomimetic processing was demonstrated early in last decade by aqueous-phase precipitation of ceramics to a functionalized surface via crystal nucleation and growth, and finally forming a ceramic thin-film pathway. This technique was widely utilized in the surface modification and pattern formation of inorganic components on polymeric substrates because the straightforward process of functionalizing the surface by physical and chemical etching routes can yield a modified surface of intimation environment of inorganic deposition. However, this approach is often limited to crystallizable inorganic materials, and is not in favor of controlling the hierarchical structures of the inorganic thin film via programmable routes. It should be say that much can be learned from biosilica for the construction of biomimetic ceramic thin film. Diatoms and sponges have extremely exquisite structures that consist of silica and a small quantity of organic matrix. Research into diatoms and marine sponges has shed light into silica formation by biomimetic pathways. For example, the organic matrix isolated from different species of biosilica revealed that the biosilica formation is controlled by species-specific gene products, such as long-chain polyamines, polypeptides, and proteins. Although the detailed role of the organic matrix and the mechanism of formation of biosilica are unclear, the derivatives from poly(propyleneimine) (PPI, [NHCH2CH2CH2]) presented in the differently shaped biosilica patterns may potentially play key roles in constructing specific silica structures. Similar to the derivatives of PPI in nature, synthetic linear poly(ethyleneimine) (LPEI), possessing only secondary amine units (NHCH2CH2) in its backbone, is a candidate for silica-deposition matrix. Usually, LPEI forms different crystalline structures when associated with water in certain molar ratios (NHCH2CH2)/nH2O (n1⁄4 0.5, 1.0, 1.5, 2.0). We have found that when the crystalline aggregate self-organized from cooling a hot aqueous solution of LPEI is used as a matrix for inducing silica formation, it simultaneously plays three key roles: as a scaffold in directing silica deposition, as a template in controlling silica morphology, and as a catalyst in inducing the reaction in aqueous media. Complex-structured microsized silica composed of nanoelementary units has been developed by designing the primary structure of LPEI by association with organic acids, by complexation with metal cations, and so on. Recent progress of biosilica-inspired control for nanostructured silica (SiO2) and titania (TiO2) materials, which are mediated by polyamines, polypeptides, and natural proteins, showed promising prospect in directing the desired ceramic structures, and afforded great achievements in designing hierarchically structured silica powders. However, only a few reports have been published on the construction of silica or titania-based nanostructured surfaces on substrates via the biosilica-inspired route. To the best of our knowledge, there is no report describing the formation of highly refined nanotextured silica/titania films on arbitrary substrates with arbitrary shapes via a simple but highly efficient process. A LPEI-mediated silica-deposition pathway, which seems logically programmable, is capable of expanding towards the silica/titania film architecture on any solids surface. In this report, we present our two-step strategy (Fig. 1) for constructing silica/titania thin films on arbitrarily shaped substrates covered by a layer of LPEI. Unlike previous approaches, which rely on functionalized surfaces and aqueousmedia-reaction deposition, we start with first preparing the LPEI-covered layer on arbitrarily shaped substrates (such as flat, curved, tubular glass, plastic, metal, etc.) via wetting the substrates with a hot aqueous solution of LPEI. Following, a silica or titania film is formed by placing the LPEI-covered material into the inorganic-source solutions (for details, see Supporting Information). This pathway produces refined nanotextured SiO2 and TiO2 films with complex morphologies on the substrates. First, via a two-step strategy, we fabricated silica films on the inner wall of soda-lime glass tubes (6 O 100mm) using LP505 [(NHCH2CH2)505] and silica sources such as tetramethoxysilane (TMOS), MS51 (5-mer of tetramethoxysilane), and water glass (sodium silicate). Experimental details are described in the Supporting Information. As shown in Figure 2a, the glass tubes with thin silica films on their inner walls became opaque. When small glass pieces collected from breaking the tubes were subjected to scanning electron microscopy (SEM) examination, we noticed that, surprisingly, the inner surface is covered by vertically arranged silica nanoblades forming grass-like structures (nanograss, see Figs. 2b–f and S1, Supporting Information). This nanograss structure is obtained from all the silica sources,