Self‐Assembly and Crystallization Behavior of Mesoporous, Crystalline HfO<sub>2</sub> Thin Films: A Model System for the Generation of Mesostructured Transition‐Metal Oxides

Brezesinski, Torsten; Smarsly, Bernd M.; Iimura, Ken–ichi; Grosso, David; Boissière, Cédric; Amenitsch, Heinz; Antonietti, Markus; Sánchez, Clément

doi:10.1002/smll.200500024

Cited by 72 publications

(64 citation statements)

References 26 publications

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“…The crystallization of TiO 2 /KLE films takes place at 500°C, i.e., after the complete dehydration of the framework, [12,23] and provides a pure-phase (anatase), highly crystalline material without any X-ray-detectable traces of rutile or brookite ( Fig. 3a), in good agreement with electron diffraction data.…”

Section: Characterization Of the Mesostructure And Crystallinity Of Tsupporting

confidence: 71%

Highly Organized Mesoporous TiO₂ Films with Controlled Crystallinity: A Li‐Insertion Study

Fattakhova‐Rohlfing

Wark

Brezesinski

et al. 2006

Adv Funct Materials

157

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A study of electrochemical Li insertion combined with structural and textural analysis enabled the identification and quantification of individual crystalline and amorphous phases in mesoporous TiO2 films prepared by the evaporation‐induced self‐assembly procedure. It was found that the properties of the amphiphilic block copolymers used as templates, namely those of a novel poly(ethylene‐co‐butylene)‐b‐poly(ethylene oxide) polymer (KLE) and commercial Pluronic P123 (HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H), decisively influence the physicochemical properties of the resulting films. The KLE‐templated films possess a 3D cubic mesoporous structure and are practically amorphous when calcined at temperatures below 450 °C, but treatment at 550–700 °C provides a pure‐phase (anatase), fully crystalline material with intact mesoporous architecture. The electrochemically determined fraction of crystalline anatase increases from 85 to 100 % for films calcined at 550 and 700 °C, respectively. In contrast, the films prepared using Pluronic P123, which also show a 3D cubic pore arrangement, exhibit almost 50 % crystallinity even at a calcination temperature of 400 °C, and their transformation into a fully crystalline material is accompanied by collapse of the mesoporous texture. Therefore, our study revealed the significance of using suitable block‐copolymer templates for the generation of mesoporous metal oxide films. Coupling of both electrochemical and X‐ray diffraction methods has shown to be highly advisable for the correct interpretation of structure properties, in particular the crystallinity, of such sol–gel derived films.

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Section: Characterization Of the Mesostructure And Crystallinity Of Tsupporting

confidence: 71%

Highly Organized Mesoporous TiO₂ Films with Controlled Crystallinity: A Li‐Insertion Study

Fattakhova‐Rohlfing

Wark

Brezesinski

et al. 2006

Adv Funct Materials

157

180

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“…8 nm. [14,17,18] XRD experiments revealed that the crystallographic orientation of the nanocrystals was maintained even in the presence of mesopores, and the degree of preferred orientation only slightly decreased in comparison to the nonporous films prepared with low amounts of surfactant. Such mesoporous metal oxides with the nanocrystals being highly oriented with respect to the substrate have not been reported to date.…”

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confidence: 88%

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

et al. 2006

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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.,...

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“…An important prerequisite for the formation of a stable mesoporous ordered network is the complete solidification/condensation of the inorganic framework. [14] A separate condensation step at 100-300°C, performed at a very slow heating rate, is therefore indispensable to ensure the stability of the mesoscopic structure throughout crystallization. The evolution of the mesostructure was monitored by temperature-dependent 2D small-angle X-ray scattering (2D-SAXS) measurements.…”

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confidence: 99%

“…[13] Using these block copolymers in the EISA process and subsequent annealing provides various crystalline metal oxides with well-defined mesopore architectures. [13,14] Based on the EISA conceptual methodology in combination with KLE polymers as texture directors, we have developed an EISA procedure for the preparation of crystalline and transparent ITO layers with well-ordered, accessible mesoporosity and good electric conductivity. These layers were then used as hosts to incorporate and anchor guest molecules (dyes and ferrocene species) and as transparent electrodes at the same time.…”

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confidence: 99%

Transparent Conducting Films of Indium Tin Oxide with 3D Mesopore Architecture

et al. 2006

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Transparent conducting oxides (TCOs) with a well-defined mesoporous framework are of considerable interest for applications in optoelectronic, electrochromic, and electroluminescent devices, solar cells, and sensors. It is the unique combination of transparency, high conductivity, well-defined 3D mesoporosity and high surface area that allows the incorporation of optoelectroactive species, facilitates electron transport to these centers, and efficiently harvests the electron-induced optical response or, vice versa, the photon-induced electron flow. These features would allow and facilitate the further development of dye-sensitized solar cells (DSSCs). However, such conducting transparent frameworks with nanometer-scale periodicity have not been accessible yet, being available solely as nonporous and macroporous layers or powders. [1][2][3] The generation of indium tin oxide (ITO) with well-ordered accessible mesoporosity and high conductivity has not been accomplished yet, partly because of the necessity of high crystallinity, which is indispensable for good electrical conductivity but inconsistent with organized 3D mesoporosity. Recently, the preparation of such mesoporous material was attempted by deposition of ITO onto porous glass substrates, providing composites with an inhomogeneous distribution of ITO. [4,5] Generally, sol-gel techniques are more advantageous for the preparation as they are applicable to various metal oxides and different types of substrates, require mild preparation conditions and, most importantly, allow the control of porosity by appropriate structure-directing agents such as self-organized surfactant micelles from block copolymers. Only a few template-assisted sol-gel syntheses using ITO have been reported, providing powders with quite high resistivity [6] or materials with irregular porosity resulting from the loose packing of crystallites. [7,8] Hence, subsequent functionalization of such material is significantly impeded. Mesoporous powders and macroporous films of undoped indium oxide were produced by sol-gel processes, but the resistivity of such material is usually too high for practical applications. [3,9] In general, the synthesis of films is aggravated by the rather poor ability of indium compounds to form stable sols and the difficulty in controlling the crystal growth of ITO. Using evaporation-induced self-assembly (EISA), various metal oxides (in particular, TiO 2[10]) have recently been obtained as crystalline thin films with ordered 3D mesoporosity. However, this technique has not been applied to certain metal oxides, particularly ITO, because of the breakdown of the mesostructure upon crystallization. The need of suitable block-copolymer templates was identified as one of the key features in the synthesis of mesoporous films with high crystallinity. [11] Owing to a new type of block copolymers with vastly improved structure-directing properties (poly(ethylene-co-butylene)-b-poly(ethylene oxides), referred to as "KLEs"), [12] we have overcome the collapse of the mesoscopi...

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Self‐Assembly and Crystallization Behavior of Mesoporous, Crystalline HfO₂ Thin Films: A Model System for the Generation of Mesostructured Transition‐Metal Oxides

Cited by 72 publications

References 26 publications

Highly Organized Mesoporous TiO₂ Films with Controlled Crystallinity: A Li‐Insertion Study

Highly Organized Mesoporous TiO₂ Films with Controlled Crystallinity: A Li‐Insertion Study

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

Transparent Conducting Films of Indium Tin Oxide with 3D Mesopore Architecture

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Self‐Assembly and Crystallization Behavior of Mesoporous, Crystalline HfO2 Thin Films: A Model System for the Generation of Mesostructured Transition‐Metal Oxides

Cited by 72 publications

References 26 publications

Highly Organized Mesoporous TiO2 Films with Controlled Crystallinity: A Li‐Insertion Study

Highly Organized Mesoporous TiO2 Films with Controlled Crystallinity: A Li‐Insertion Study

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

Transparent Conducting Films of Indium Tin Oxide with 3D Mesopore Architecture

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Self‐Assembly and Crystallization Behavior of Mesoporous, Crystalline HfO₂ Thin Films: A Model System for the Generation of Mesostructured Transition‐Metal Oxides

Highly Organized Mesoporous TiO₂ Films with Controlled Crystallinity: A Li‐Insertion Study

Highly Organized Mesoporous TiO₂ Films with Controlled Crystallinity: A Li‐Insertion Study