Florian J. Heiligtag

Template-free co-assembly of preformed Au and TiO2 nanoparticles into multicomponent 3D aerogels

The self-assembly properties of titania nanoparticles, which are strong enough to bridge several lengths scales, make it possible to use them as building blocks for aerogels. Controlled by their surface functionalization, the nanoparticles undergo an oriented attachment process during gelation, building up a 3-dimensional macroporous network. The presence of other nanoparticles in the initial reaction mixture results in the formation of multicomponent aerogels, offering a flexible tool to vary the composition of the gels and thus the chemical properties. Exemplarily this is shown by the co-assembly of titania with gold nanoparticles, leading to a Au–TiO2 composite aerogel with enhanced photocatalytic activity under visible light.

Improved Nonaqueous Synthesis of TiO2 for Dye-Sensitized Solar Cells

Nonaqueous synthesis routes have emerged as a powerful platform for directly obtaining diverse metal oxide nanoparticles with high crystallinity and tunable compositions. The benzyl alcohol (BA) route, for example, has been applied toward dozens of oxides including binary, ternary, and even more complex multimetal systems. Here we compare anatase nanoparticles made from the BA route with the traditional hydrothermal route. XPS measurements indicated that the BA route resulted in more reduced Ti states, corresponding to additional oxygen vacancies. These defects resulted in additional trap states, slower recombination, and slower charge transport. The performance of BA anatase was improved by incorporating niobium intended to suppress oxygen vacancies. The higher performance Nb-containing films were post-treated to yield a 7.96% power conversion efficiency (AM 1.5), similar to the state-of-the-art hydrolytic TiO2 in the same configuration.

Assembly of BaTiO3 nanocrystals into macroscopic aerogel monoliths with high surface area.

Aerogels with their low density and high surface area are fascinating materials. However, their advantageous morphology is still far from being fully exploited owing to their limited compositional variety and low crystallinity. Replacing the sol-gel process by a particle-based assembly route is a powerful alternative to expand the accessible functionalities of aerogels. A strategy is presented for the controlled destabilization of concentrated dispersions of BaTiO3 nanoparticles, resulting in the assembly of the fully crystalline building blocks into cylindrically shaped monolithic gels, thereby combining the inherent properties of ternary oxides with the highly porous microstructure of aerogels. The obtained aerogels showed an unprecedentedly high surface area of over 300 m(2)  g(-1).

Anatase–silica composite aerogels: a nanoparticle-based approach

Herein we present the synthesis of anatase–silica aerogels based on the controlled gelation of preformed nanoparticle mixtures. The monolithic aerogels with macroscopic dimensions show large specific surface areas, and high and uniform porosities. The major advantage of such a particle-based approach is the great flexibility in pre-defining the compositional and structural features of the final aerogels before the gelation process by fine-tuning the properties of the titania and silica building blocks (e.g., size, composition and crystallinity) and their relative ratio in the dispersion. Specific surface functionalization enables control over the interaction between the nanoparticles and thus over their distribution in the aerogel. Positively charged titania nanoparticles are co-assembled with negatively charged Stoeber particles, resulting in a binary aerogel with a crystalline anatase and amorphous silica framework directly after supercritical drying without any calcination step. Titania–silica aerogels combine the photocatalytic activity of the anatase nanoparticles with the extensive silica chemistry available for silica surface functionalization.

Liquid-phase deposition of ferroelectrically switchable nanoparticle-based BaTiO3 films of macroscopically controlled thickness

BaTiO3 films are extensively used in many electrical devices, because they offer remarkable dielectric and ferroelectric properties. Here, we demonstrate a powerful, nanoparticle-based deposition route towards BaTiO3 films with systematic thickness control over a wide range up to several microns. The unusual control over the film thickness with the maintenance of crack free nanostructures, phase and ferroelectric properties of the BaTiO3 films allows us to fabricate various future devices of different thicknesses by a single deposition method. For this, films are deposited from stable dispersions of BaTiO3 nanocrystals, synthesized via an efficient microwave-assisted non-aqueous sol–gel approach. Crack-free films of controlled thickness are obtained by a carefully elaborated, alternating process of spin-coating and intermediate drying. According to X-ray diffraction and confocal Raman microscopy, the final, sintered films consist of BaTiO3 nanocrystals of about 20 nm in a hexagonal–tetragonal phase mixture. The nanoparticulate films display outstanding optical characteristics exceeding 90% transparency above 500 nm and a band gap of 3.5 eV. The latter, band gap, is larger than the classic bulk materials band gap of 3.2 eV, indicating a more electrically insulating nature of the films. Piezoresponse force microscopy gives evidence for potent ferroelectric switching. This newly accessible film processing route with wide film thickness tuning allows for desired ferroelectric response with the advantage of a wide film thickness to implicate building blocks for various applications e.g. ferroelectric random access memory devices, microelectromechanical system devices or Bragg reflectors.

Multifunctional microparticles with uniform magnetic coatings and tunable surface chemistry

Microplatelets and fibers that can be manipulated using external magnetic fields find potential applications as miniaturized probes, micromirrors in optical switches, remotely actuated micromixers and tunable reinforcements in composite materials. Controlling the surface chemistry of such microparticles is often crucial to enable full exploitation of their mechanical, optical and sensorial functions. Here, we report a simple and versatile procedure to directly magnetize and chemically modify the surface of inorganic microplatelets and polymer fibers of inherently non-magnetic compositions. As opposed to other magnetization approaches, the proposed non-aqueous sol–gel route enables the formation of a dense and homogeneous coating of superparamagnetic iron oxide nanoparticles (SPIONs) on the surface of the microparticles. Such coating provides a suitable platform for the direct chemical functionalization of the microparticles using catechol-based ligands displaying high affinity towards iron oxide surfaces. By adsorbing for example nitrodopamine palmitate (ND-PA) on the surface of hydrophilic magnetite-coated alumina platelets (Fe3O4@Al2O3) we can render them sufficiently surface active to generate magnetically responsive Pickering emulsions. We also show that microplatelets and fibers coated with a uniform iron oxide layer can be easily manipulated using low magnetic fields despite their intrinsic non-magnetic nature. These examples illustrate the potential of the proposed approach in generating functional, magnetically responsive microprobes and building blocks for several emerging applications.

Colloidal Routes to Macroscopic Monoliths of Porous Titania and Copper

Nanoparticles with their size and shape-dependent properties are the ideal building blocks for the fabrication of new materials with tailor-made functionalities.[1] However, the assembly of such nanoscale constituents to macroscopic materials requires subtle control over their arrangement in three dimensions and over several orders of length scales.[2] Based on two examples, this Highlight presents the preparation of macroscopic materials by liquid-phase chemistry using preformed metal oxide particles as building blocks. In the first case, surface-functionalized titanium dioxide nanoparticles of just a few nanometers in diameter are connected by oriented attachment into a three-dimensional aerogel monolith of macroscopic size (Fig. 1).[3] The gelation of the anatase nanocrystals is guided by the controlled destabilization of the {001} crystal facets. Such aerogels offer a large variety of interesting properties: They are extremely light, fully crystalline, translucent, highly porous, and exhibit a large surface area of about 550 m/g. The use of preformed nanoparticles with defined compositions and properties bears the unique advantage that different types of nanoscale building blocks can be co-assembled, resulting in multicomponent aerogels not accessible by any other technique.[3] The second example includes the electroless, but catalyst-free deposition of metallic copper, either as freestanding submicrometer-thin foil (Fig. 2a) or supported on a polymer substrate, which then can be processed into a line pattern for flexible electronics (Fig. 2b).[4] Deposition of the copper on spherical ZnO particles as building block/template gives access to copper capsules after removal of the template (Fig. 2c). Compaction and shaping of these capsules finally results in mechanically stable copper foam monoliths (Fig. 2d).[5] Pore size and shape are predetermined by the morphological characteristics of the metal oxide building block acting as template.