Surfactant‐Mediated Generation of Iso‐Oriented Dense and Mesoporous Crystalline Metal‐Oxide Layers

Abstract

The fabrication of thin layers of a transition-metal oxide with 3D oriented crystallites has substantial importance for industrial applications, for example, semiconductor technology. [1,2] Chemical vapor deposition [3] and molecular beam epitaxy [4] onto single-crystalline substrates are widely used fabrication techniques but they suffer from elaborate preparation setups. Most of the currently applied concepts to generate layers of oriented crystals involve the deposition of molecular species from the gas phase [5] (pulsed laser deposition, molecular beam epitaxy) or the liquid phase [6] (liquid phase epitaxy)on suitable single-crystalline surfaces. In these processes the crystals grow in a direction that is dictated by the lattice planes of the substrate exposed to the surface, and the substrate has to match the lattice parameter of the oxide (epitaxy). Solution-processing (SP) routes have been proposed as a versatile and facile alternative, especially for transition-metal oxides applying sol-gel reactions, because of the evident advantages of "beaker" chemistry and liquid-coating processing. [7][8][9] However, up until now SP has required single-crystalline substrates to achieve oriented crystallization, which represents a substantial limitation and complication. Therefore, it is not commonly expected and it has not been reported yet that crack-free layers of transition-metal oxides with a high degree of crystallographic orientation can be formed from aqueous solutions on arbitrary substrates.[10] Furthermore, the recently introduced class of mesoporous metal oxides could solely be prepared with a random distribution of nanocrystals within the pore walls. Here, we report a general, straightforward surfactant-mediated methodology for the preparation of dense and even well-ordered mesoporous films of transition-metal oxides with crystallographically oriented crystals (MoO 3 , Nb 2 O 5 , Ta 2 O 5 , V 2 O 5 , perovskites etc.) on noncrystalline substrates such as glass. In order to understand the underlying principles for this surprising finding, we performed a systematic study on a broad series of metal oxides (TiO 2 , CeO 2 , SnO 2 , etc.) to understand the relationship between the crystallographic orientation and the unit-cell characteristics of the different oxides. Interestingly, our studies revealed that this effect generally occurs for metal oxides with certain anisotropic unit-cell features, which is exemplified here in detail for MoO 3 and Nb 2 O 5 . It is justified to term this new surfactantmediated crystallization phenomenon "soft epitaxy", following the previously introduced polymer-controlled nucleation and crystallization of different inorganic salts as powders with various macroscopic 3D crystal structures. [11] As key element, our straightforward soft-epitactic process uses a surfactantmediated oriented nucleation from surfaces, starting from aqueous-ethanolic or water-free solutions, depending on the respective oxide. In essence, the approach involves dip-coating polar, wettable substrates (e.g.,...

Facile and General Synthesis of Thermally Stable Ordered Mesoporous Rare-Earth Oxide Ceramic Thin Films with Uniform Mid-Size to Large-Size Pores and Strong Crystalline Texture

“…[22] This is due, in part, to the fact that the synthesis of non-silicate oxides with both a well defined pore-solid architecture and highly crystalline walls itself is still challenging. One of the reasons for this is the lack of appropriate polymer SDAs.…”

“…In particular, we take advantage of the superior templating properties of poly(ethylene-co-butylene)-block-poly(ethylene oxide) -referred to as KLE [22,25,27,31] -and polyisobutylene-block-poly(ethylene oxide) -referred to as PIB x -b-PEO y [32] -diblock copolymers to achieve thermally stable, high quality nanomaterials with tailorable pore and grain sizes. Overall, it seems that the unique combination of nanoscale porosity with crystalline texture might offer the possibility to broaden the scope of application for these oxide ceramics.…”

Ordered Mesoporous Thin Films of Rutile TiO2 Nanocrystals Mixed with Amorphous Ta2O5

“…Brezesinski et al have recently reported the oriented growth of oxides assisted by surfactants. [18] These insights can explain the fact that oriented growth of rutile is observed for the mesoporous film, while no remarkable oriented growth of titania can be discerned in the nonporous film ( Figure 3). [18] Notably, the onset of crystallization (observable by WAXS) is higher than the decomposition temperature.…”

“…[18] These insights can explain the fact that oriented growth of rutile is observed for the mesoporous film, while no remarkable oriented growth of titania can be discerned in the nonporous film ( Figure 3). [18] Notably, the onset of crystallization (observable by WAXS) is higher than the decomposition temperature. We speculate that at lower temperatures oriented crystallization occurs at the interface to www.chemphyschem.org the substrate, which is not detectable by WAXS due to the very low quantities of material at the interface.…”

“…The fabrication of pure Ta 2 O 5 films follows a previously described recipe. [18] Film Characterization: AFM investigations were conducted using a Nanoscope III instrument (Digital Instruments) in the tapping mode. WAXS signals were collected in a Bruker D8 diffractometer with an accelerating voltage of 40 kV and a current of 40 mA, using Cu K a radiation.…”