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Self-assembly of colloidal nanoparticles enables the easy building of assembly units into higher-order structures and the bottom-up preparation of functional materials. Nickel phosphides represent an important group of catalysts for hydrogen evolution reaction (HER) from water splitting. In this paper, the preparation of porous nickel phosphide superparticles and their HER efficiencies are reported. Ni and Ni 2 P nanoparticles are self-assembled into binary superparticles via an oil-in-water emulsion method. After annealing and acid etching, the as-prepared Ni-Ni 2 P binary superparticles change into porous nickel phosphide superparticles. The porosity and crystalline phase of the superparticles can be tuned by adjusting the ratio of Ni and Ni 2 P nanoparticles. The resulting porous superparticles are effective in driving HER under acidic conditions, and the modulation of porosity and phase further optimize the electrochemical performance. The prepared Ni 3 P porous superparticles not only possess a significantly enhanced specific surface area compared to solid Ni-Ni 2 P superparticles but also exhibit an excellent HER efficiency. The calculations based on the density functional theories show that the (110) crystal facet exhibits a relatively lower Gibbs free energy of hydrogen adsorption. This work provides a self-assembly approach for the construction of porous metal phosphide nanomaterials with tunable crystalline phase and porosity., (© 2024 Wiley‐VCH GmbH.)

The practical applications of resorcinol formaldehyde resin (RFR) aerogels are prevented by their poor mechanical properties. Herein, a facile template-directed method is reported to produce macroscopic free-standing cobalt silicate (CS)@RFR core-shell nanobelt aerogels that display superelastic behavior and outstanding thermal insulating and fire-resistant capability. The synthesis relies on the polymerization of RFR on pre-formed CS nanobelts which leads to in situ formation of hydrogel monoliths that can be transformed to corresponding aerogels by a freeze-drying method. The composite nanobelt aerogel can withstand a compressive load of more than 4000 times of its own weight and fully recover after the removal of the weight. It can also sustain 1000 compressive cycles with 6.9% plastic deformation and 91.8% of the maximum stress remaining, with a constant energy loss coefficient as low as 0.16, at the set strain of 30%. The extraordinary mechanical properties are believed to be associated with the structural flexibility of the nanobelts and the RFR-reinforced joints between the crosslinked nanobelts. These inorganic-organic composite aerogels also show good thermal insulation and excellent fire-proof capability. This work provides an effective strategy for fabricating superelastic RFR-based aerogels which show promising applications in fields such as thermal insulation, energy storage, and catalyst support., (© 2023 Wiley-VCH GmbH.)

The development of efficient and affordable electrode materials is crucial for clean energy storage systems, which are considered a promising strategy for addressing energy crises and environmental issues. Metal phosphorous chalcogenides (MPX 3 ) are a fascinating class of two-dimensional materials with a tunable layered structure and high ion conductivity, making them particularly attractive for energy storage applications. This review article aims to comprehensively summarize the latest research progress on MPX 3 materials, with a focus on their preparation methods and modulation strategies. Additionally, the diverse applications of these novel materials in alkali metal ion batteries, metal-air batteries, and all-solid-state batteries are highlighted. Finally, the challenges and opportunities of MPX 3 materials are presented to inspire their better potential in energy storage applications. This review provides valuable insights into the promising future of MPX 3 materials in clean energy storage systems., (© 2023 Wiley-VCH GmbH.)

Rechargeable batteries play an integral role toward carbon neutrality. Environmentally sustainable batteries should consider the trade-offs between material renewability, processability, thermo-mechanical and electrochemical performance, as well as transiency. To address this dilemma, we follow circular economy principles to fabricate fungal chitin nanofibril (ChNF) gel polymer electrolytes (GPEs) for zinc-ion batteries. These biocolloids are physically entangled into hierarchical hydrogels with specific surface areas of 49.5 m 2 ·g -1 . Ionic conductivities of 54.1 mS·cm -1 and a Zn 2+ transference number of 0.468 are reached, outperforming conventional non-renewable/non-biodegradable glass microfibre separator-liquid electrolyte pairs. Enabled by its mechanically elastic properties and large water uptake, a stable Zn electrodeposition in symmetric Zn|Zn configuration with a lifespan above 600 h at 9.5 mA·cm -2 is obtained. At 100 mA·g -1 , the discharge capacity of Zn/α-MnO 2 full cells increases above 500 cycles when replacing glass microfiber separators with ChNF GPEs, while the rate performance remains comparable to glass microfiber separators. To make the battery completely transient, the metallic current collectors are replaced by biodegradable polyester/carbon black composites undergoing degradation in water at 70 °C. This work demonstrates the applicability of bio-based materials to fabricate green and electrochemically competitive batteries with potential applications in sustainable portable electronics, or biomedicine., (© 2023 The Authors. Small published by Wiley-VCH GmbH.)

Transient batteries are expected to lessen the inherent environmental impact of traditional batteries that rely on toxic and critical raw materials. This work presents the bottom-up design of a fully transient Zn-ion battery (ZIB) made of nontoxic and earth-abundant elements, including a novel hydrogel electrolyte prepared by cross-linking agarose and carboxymethyl cellulose. Facilitated by a high ionic conductivity and a high positive zinc-ion species transference number, the optimized hydrogel electrolyte enables stable cycling of the Zn anode with a lifespan extending over 8500 h for 0.25 mA cm -2 - 0.25 mAh cm -2 . On pairing with a biocompatible organic polydopamine-based cathode, the full cell ZIB delivers a capacity of 196 mAh g -1 after 1000 cycles at a current density of 0.5 A g -1 and a capacity of 110 mAh g -1 after 10 000 cycles at a current density of 1 A g -1 . A transient ZIB with a biodegradable agarose casing displays an open circuit voltage of 1.123 V and provides a specific capacity of 157 mAh g -1 after 200 cycles at a current density of 50 mA g -1 . After completing its service life, the battery can disintegrate under composting conditions., (© 2022 The Authors. Small published by Wiley-VCH GmbH.)

Sodium ion batteries (NIBs) based on earth-abundant materials offer efficient, safe, and environmentally sustainable solutions for a decarbonized society. However, to compete with mature energy storage technologies such as lithium ion batteries, further progress is needed, particularly regarding the energy density and operational lifetime. Considering these aspects as well as a circular economy perspective, the authors use biodegradable cellulose nanoparticles for the preparation of a gel polymer electrolyte that offers a high liquid electrolyte uptake of 2985%, an ionic conductivity of 2.32 mS cm -1 , and a Na + transference number of 0.637. A balanced ratio of mechanically rigid cellulose nanocrystals and flexible cellulose nanofibers results in a mesoporous hierarchical structure that ensures close contact with metallic Na. This architecture offers stable Na plating/stripping at current densities up to ±500 µA cm -2 , outperforming conventional fossil-based NIBs containing separator-liquid electrolytes. Paired with an environmentally sustainable and economically attractive Na 2 Fe 2 (SO 4 ) 3 cathode, the battery reaches an energy density of 240 Wh kg -1 , delivering 69.7 mAh g -1 after 50 cycles at a rate of 1C. In comparison, Celgard in liquid electrolyte delivers only 0.6 mAh g -1 at C/4. Such gel polymer electrolytes may open up new opportunities for sustainable energy storage systems beyond lithium ion batteries., (© 2022 The Authors. Small published by Wiley-VCH GmbH.)

Photocatalysis has the potential to make a major technological contribution to solving pressing environmental and energy problems. There are many strategies for improving photocatalysts, such as tuning the composition to optimize visible light absorption, charge separation, and surface chemistry, ensuring high crystallinity, and controlling particle size and shape to increase overall surface area and exploit the reactivity of individual crystal facets. These processes mainly affect the nanoscale and are therefore summarized as nanostructuring. In comparison, microstructuring is performed on a larger size scale and is mainly concerned with particle assembly and thin film preparation. Interestingly, most structuring efforts stop at this point, and there are very few examples of geometry optimization on a millimeter or even centimeter scale. However, the recent work on nanoparticle-based aerogel monoliths has shown that this size range also offers great potential for improving the photocatalytic performance of materials, especially when the macroscopic geometry of the monolith is matched to the design of the photoreactor. This review article is dedicated to this aspect and addresses some issues and open questions that arise when working with macroscopically large photocatalysts. Guidelines are provided that could help develop novel and efficient photocatalysts with a truly 3D architecture., (© 2022 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Monolithic aerogels composed of crystalline nanoparticles enable photocatalysis in three dimensions, but they suffer from low mechanical stability and it is difficult to produce them with complex geometries. Here, an approach to control the geometry of the photocatalysts to optimize their photocatalytic performance by introducing carefully designed 3D printed polymeric scaffolds into the aerogel monoliths is reported. This allows to systematically study and improve fundamental parameters in gas phase photocatalysis, such as the gas flow through and the ultraviolet light penetration into the aerogel and to customize its geometric shape to a continuous gas flow reactor. Using photocatalytic methanol reforming as a model reaction, it is shown that the optimization of these parameters leads to an increase of the hydrogen production rate by a factor of three from 400 to 1200 µmol g -1 h -1 . The rigid scaffolds also enhance the mechanical stability of the aerogels, lowering the number of rejects during synthesis and facilitating handling during operation. The combination of nanoparticle-based aerogels with 3D printed polymeric scaffolds opens up new opportunities to tailor the geometry of the photocatalysts for the photocatalytic reaction and for the reactor to maximize overall performance without necessarily changing the material composition., (© 2021 The Authors. Small published by Wiley-VCH GmbH.)

Transient technology seeks the development of materials, devices, or systems that undergo controlled degradation processes after a stable operation period, leaving behind harmless residues. To enable externally powered fully transient devices operating for longer periods compared to passive devices, transient batteries are needed. Albeit transient batteries are initially intended for biomedical applications, they represent an effective solution to circumvent the current contaminant leakage into the environment. Transient technology enables a more efficient recycling as it enhances material retrieval rates, limiting both human and environmental exposures to the hazardous pollutants present in conventional batteries. Little efforts are focused to catalog and understand the degradation characteristics of transient batteries. As the energy field is a property-driven science, not only electrochemical performance but also their degradation behavior plays a pivotal role in defining the specific end-use applications. The state-of-the-art transient batteries are critically reviewed with special emphasis on the degradation mechanisms, transiency time, and biocompatibility of the released degradation products. The potential of transient batteries to change the current paradigm that considers batteries as harmful waste is highlighted. Overall, transient batteries are ready for takeoff and hold a promising future to be a frontrunner in the uptake of circular economy concepts., Competing Interests: The authors declare no conflict of interest., (© 2021 The Authors. Advanced Science published by Wiley‐VCH GmbH.)

A solid-state lithium-ion battery, in which all components (current collector, anode and cathode, electrolyte, and packaging) are stretchable, is introduced, giving rise to a battery design with mechanical properties that are compliant with flexible electronic devices and elastic wearable systems. By depositing Ag microflakes as a conductive layer on a stretchable carbon-polymer composite, a current collector with a low sheet resistance of ≈2.7 Ω □ -1 at 100% strain is obtained. Stretchable electrodes are fabricated by integrating active materials with the elastic current collector. A polyacrylamide-"water-in-salt" electrolyte is developed, offering high ionic conductivity of 10 -3 to 10 -2 S cm -1 at room temperature and outstanding stretchability up to ≈300% of its original length. Finally, all these components are assembled into a solid-state lithium-ion full cell in thin-film configuration. Thanks to the deformable individual components, the full cell functions when stretched, bent, or even twisted. For example, after stretching the battery to 50%, a reversible capacity of 28 mAh g -1 and an average energy density of 20 Wh kg -1 can still be obtained after 50 cycles at 120 mA g -1 , confirming the functionality of the battery under extreme mechanical stress., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The synthesis of metal oxide nanoparticles in organic solvents, so-called nonaqueous (or nonhydrolytic) processes represent powerful alternatives to aqueous approaches and have become an independent research field. 10 Years ago, when we published our first review on organic reaction pathways in nonaqueous sol-gel approaches, the number of examples was relatively limited. Nowadays, it is almost impossible to provide an exhaustive overview. Here we review the development of the last few years, without neglecting pioneering examples, which help to follow the historical development. The importance of a profound understanding of mechanistic aspects of nanoparticle crystallization and formation mechanisms can't be overestimated, when it comes to the design of rational synthesis concepts under minimization of trial-and-error experiments. The main reason for the progress in mechanistic understanding lies in the availability of characterization tools that make it possible to monitor chemical reactions from the dissolution of the precursor to the nucleation and growth of the nanoparticles, by ex situ methods involving sampling after different reaction times, but more and more also by in situ studies. After a short introduction to experimental aspects of nonaqueous sol-gel routes to metal oxide nanoparticles, we provide an overview of the main and basic organic reaction pathways in these approaches. Afterwards, we summarize the main characterization methods to study formation mechanisms, and then we discuss in great depth the chemical formation mechanisms of many different types of metal oxide nanoparticles. The review concludes with a paragraph on selected crystallization mechanisms reported for nonaqueous systems and a few illustrative examples of nonaqueous sol-gel concepts applied to surface chemistry., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Nanocomposites represent an interesting class of materials because their applications are of multidisciplinary importance. Herein, interactions at the interface of heterostructures, leading to superior performance and sometimes to synergistic interactions, are highlighted. Although the types of junctions seem to be similar in different fields, the requirements for their function is device-dependent and thus no generally valid recipe can be utilized. Nonetheless, the overlap between research fields unveils the unique opportunity to combine the different knowledge and expertise as an inspiration to designing new composites for photoelectrochemical water splitting, photocatalysis, chemoresistive sensors, batteries, and composite multiferroics. A non-comprehensive overview is provided with focus on selected examples to highlight the concepts behind these nanojunctions., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

WO 3 photoanodes with remarkable photocurrent densities are presented. These photoanodes were prepared from three different commercially available WO 3 nanopowders. Doctor blading of the nanopowders followed by a short annealing in air led to nanostructured films. The best photoanode showed a photocurrent density of 3.5 mA cm -2 at 1.23 V vs. RHE in 1 m CH 3 SO 3 H under AM 1.5 G illumination (100 mW cm -2 ), surpassing values reported so far for bare WO 3 photoanodes. The study also showed that the photocurrent was strongly dependent on the electrolyte, indicating oxidation of the electrolyte rather than of water. Oxygen evolution measurements performed in different electrolytes revealed that the amounts of oxygen were highly dependent on the electrolyte. By comparing the photocurrent values in the different electrolytes with the amount of evolving oxygen, it was found that the electrolyte producing the highest photocurrent was the electrolyte with the lowest oxygen evolution. Stability measurements showed that the more oxygen is produced, the less stable is the photoanode. These results clearly underline the difficulty to correlate the photocurrent values with oxygen evolution, drawing the attention to one of the major limitations of photoelectrochemical water splitting., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Alignment of nanowires over a large area of flat and patterned substrates is a prerequisite to use their collective properties in devices such as gas sensors. In this work, uniform single-crystalline ultrathin W18 O49 nanowires with diameters less than 2 nm and aspect ratios larger than 100 have been synthesized, and, despite their flexibility, assembled into thin films with high orientational order over a macroscopic area by the Langmuir-Blodgett technique. Alignment of the tungsten oxide nanowires was also possible on top of sensor substrates equipped with electrodes. Such sensor devices were found to exhibit outstanding sensitivity to H2 at room temperature., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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)., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The fundamental mission of materials science is the description of matter over all length scales. In this review, we apply this concept to particle research. Based on metal oxides, we show that every size range offers its specific features, and every size range had its era, when it was in the center of the research activities. In the first part of the review, we discuss on three metal oxides as examples, how and why the research focus changed its targeted size regime from the micrometer to the nanometer scale and back to the macroscopic world. Next, we present the distinct advantages of using nanoparticles over micrometer-sized particles in selected devices and we point out how such a shift in the size regime opens up new research directions. Finally, we exemplify the methods to introduce nanoparticles into macroscopic objects to make functional ceramics., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Macroscopic monoliths of copper foams have been prepared by a template-assisted wet-chemical process. The method offers subtle control over the pore size and size distribution, density and macroscopic size and shape of the metal foam. Uniaxial compression tests revealed different deformation behavior depending on the relative density. Non-vacuum-based and low-temperature routes are attractive for the cost-effective production of metal foams., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A time-dependent study on the formation of LiFePO₄ with olivine-type structure is presented. The material is synthesized through a non-aqueous route in benzyl alcohol assisted by microwave radiation. The LiFePO₄ forms with an anisotropic morphology of microscale stick-like particles. The detailed structure of these particles and their evolution with reaction time is revealed by transmission electron microscopy; a 3D reconstruction of a particle by electron tomography provides insight into the formation mechanism of these sticks. Without applying a thermal post-annealing treatment or a carbon coating, the electrochemical behavior of the LiFePO₄ microsticks is assessed for the preparation of cathodes in lithium-ion batteries., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Insight into the complex chemical mechanism for the formation of tungstite nanoparticles obtained by the reaction of tungsten hexachloride with benzyl alcohol is presented herein. The organic and inorganic species involved in the formation of the nanoparticles were studied by time-dependent gas chromatography and X-ray diffraction as well as by time-resolved in situ X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy. Principal component analysis revealed two intermediates, which were identified as WCl(4) and WOCl(4) by using linear combination analysis. Quick-scanning extended X-ray absorption fine structure spectroscopy enabled the time-dependent evolution of the starting compound, the intermediates and the product to be monitored over the full reaction period. The reaction starts with fast chlorine substitution and partial reduction during the dissolution of the tungsten hexachloride in benzyl alcohol followed by the generation of intermediates with W=O double bonds and finally the construction of the W-O-W network of the tungstite structure., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A detailed study is presented on the formation of MoO(2) nanoparticles from the dissolution of the precursor to the final rodlike product, with a focus on the exploration of the inorganic reaction occurring ahead of the nucleation step, and interplay between size and crystal structure of MoO(2). In situ X-ray absorption spectroscopy experiments show that the crystallization and the growth process of MoO(2) nanorods is initiated by rapid reduction of the MoO(2) Cl(2) precursor in benzyl alcohol and acetophenone. This reaction triggers the nucleation of 2 nm MoO(2) particles with spherical shape and hexagonal crystal structure. The transformation from spheres into rods emerges as a complex process driven by oriented attachment. High-resolution transmission electron microscopy and X-ray diffraction results provide evidence that the 2 nm particles first aggregate into 5-20 nm-large oriented assemblies. The increase in particle size induces the phase transition from hexagonal to the less symmetrical monoclinic crystal structure, and finally the transformation into rods. Is it shown that electrodes for lithium-ion batteries based on MoO(2) nanorods have a long-term cycling life. The specific discharge capacity even after 200 cycles at a discharge rate of 1 C is about 300 Ah kg(-1) ., (Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Hard and brittle monolithic tungsten oxide-polybenzylene nanohybrids can be obtained in one step by reacting tungsten iso-propoxide with benzyl alcohol. In a first step, crystalline tungsten oxide W(18)O(49) nanowires with a diameter of about 1.5 nm form via ether elimination reaction. Subsequently, the large residue of the benzyl alcohol is transformed to dibenzyl ether, which then polymerizes to polybenzylene, incorporating the nanoparticles into the forming polymer. The catalytic effect of the tungsten oxide nanowires on the quantitative formation of polybenzylene is proven by reacting them in different concentrations and at varying temperatures either with benzyl alcohol or with dibenzyl ether. Complete polymerization of benzyl alcohol is achieved within just 30 min by using a particle-to-monomer molar ratio of 1:115 at 160 degrees C. Lower reaction temperatures (100-130 degrees C) or higher ratios (1:340 and 1:680) prolong the reaction time to several hours. Further studies show that the tungsten oxide nanoparticles are able to completely polymerize various other alcohols with an aryl methanol group.

A general nonaqueous route for the synthesis of phase-pure transition-metal niobate (InNbO(4), MnNb(2)O(6), and YNbO(4)) nanocrystals was developed based on the one-pot solvothermal reaction of niobium chloride and the corresponding transition-metal acetylacetonates in benzyl alcohol at 200 degrees C. All samples were carefully characterized by XRD, TEM, HRTEM, and energy-dispersive X-ray (EDX) analysis. The crystallization mechanism of these niobate nanocrystals points to a two-step pathway. First, metal hydroxide crystals and amorphous niobium oxide are formed. Second, metal niobate nanocrystals are generated from the intermediates by a dissolution-recrystallization mechanism. The reaction mechanisms, that is, the processes responsible for the oxygen supply for oxide formation, were found to be rather complex and involve niobium-mediated ether elimination as the main pathway, accompanied by solvolysis of the acetylacetonate ligands and benzylation reactions.

Surfactant-free nonaqueous (and/or nonhydrolytic) sol-gel routes constitute one of the most versatile and powerful synthesis methodologies for nanocrystalline metal oxides with high compositional homogeneity and purity. Although the synthesis protocols are particularly simple, involving only metal oxide precursors and common organic solvents, the obtained uniform nanocrystals exhibit an immense variety of sizes and shapes. The small number of reactants in these routes enables the study of the chemical mechanisms involved in metal oxide formation. Nonhydrolytic routes to inorganic nanomaterials that used surfactants as size- and shape-controlling agents have been discussed recently. This Minireview supplements this topic by discussing surfactant-free processes, which have become a valuable alternative to surfactant-assisted as well as to traditional aqueous sol-gel chemistry routes.

We present a multigram scale, one-step nonaqueous synthesis route to monodisperse, highly crystalline ZrO(2) nanoparticles. The nanoparticles can be stabilized in nonpolar solvents via a simple functionalization procedure using only minute amounts of organic stabilizers. Their great potential in materials applications is demonstrated by the fabrication of organic-inorganic nanocomposites that can be selectively photopolymerized to inscribe extremely effective and volume holographic gratings with the highest refractive index contrast (n(1) of up to 0.024) achieved so far.

The crystallization pathway of indium tin oxide nanoparticles during solvothermal synthesis in benzyl alcohol was investigated. The synthesis was stopped after different reaction times in the range of 1-24 h, and the structural and morphological characteristics of the products were analyzed by powder X-ray diffraction, electron microscopy, and UV/Vis spectroscopy. Interestingly, the crystallization mechanism does not proceed along a simple nucleation and growth pathway, but involves a two-step process. In the first 12 h an intermediate phase is formed, which consists of nanocrystallites 3-6 nm in size stabilized by an organic matrix. In this organic-inorganic network the nanoparticles are aligned into superstructures, but without any crystallographic orientation. After 12 h the intermediate phase is abruptly transformed into indium tin oxide nanoparticles with the bixbyite structure and with crystallite sizes of about 10-12 nm. This step is accompanied by the disappearance of the organic phase and the loss of the superstructure.

Nonaqueous-solution routes to metal oxide nanoparticles are a valuable alternative to the known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and ability to control the crystal growth without the use of surfactants. In the first part of the review we give a detailed overview of the various solution routes to metal oxides in organic solvents, with a strong focus on surfactant-free processes. In most of these synthesis approaches, the organic solvent plays the role of the reactant that provides the oxygen for the metal oxide, controls the crystal growth, influences particle shape, and, in some cases, also determines the assembly behavior. We have a closer look at the following reaction systems in this order: 1) metal halides in alcohols, 2) metal alkoxides, acetates, and acetylacetonates in alcohols, 3) metal alkoxides in ketones, and 4) metal acetylacetonates in benzylamine. All these systems offer some peculiarities with respect to each other, providing many possibilities to control and tailor the particle size and shape, as well as the surface and assembly properties. In the second part we present general mechanistic principles for aqueous and nonaqueous sol-gel processes, followed by the discussion of reaction pathways relevant for nanoparticle formation in organic solvents. Depending on the system several mechanisms have been postulated: 1) alkyl halide elimination, 2) elimination of organic ethers, 3) ester elimination, 4) C--C bond formation between benzylic alcohols and alkoxides, 5) ketimine and aldol-like condensation reactions, 6) oxidation of metal nanoparticles, and 7) thermal decomposition methods.

Nanoparticle powders composed of surface-functionalized anatase crystals with diameters of about 3 nm self-organize into different structures upon redispersion in water. The assembly is directed by a small amount of a low-molecular-weight functional ligand (the "assembler") adsorbed on the surface of the nanoparticles. The ligand functionality determines the anisotropy of the resulting structures. Multidentate ligands, such as trizma ((HOCH(2))(3)CNH(2)) and serinol ((HOCH(2))(2)CNH(2)), with a chargeable terminal group preferentially induce the formation of anisotropic nanostructures several hundreds of nanometers in total length, whereas all the other investigated ligands (ethanolamine H(2)N(CH(2))(2)OH, glycine hydroxamate H(2)NCH(2)CONHOH, dopamine (OH)(2)C(6)H(3)(CH(2))(2)NH(3)Cl, tris (HOCH(2))(3)CCH(3)) mainly lead to uncontrolled agglomeration. Experimental data suggests that the anisotropic assembly is a consequence of the water-promoted desorption of the organic ligands from the {001} faces of the crystalline building blocks together with the dissociative adsorption of water on these crystal faces. Both processes induce the preferred attachment of the titania nanoparticles along the [001] direction. The use of polydentate and charged ligands to functionalize the surface of nanoparticles thus provides a versatile tool to control their arrangement on the nanoscale.

A nonaqueous synthetic route for the preparation of a regular crystalline yttria mesostructure is presented. The reaction between yttrium alkoxides and benzyl alcohol results in the formation of a highly ordered lamellar nanocomposite consisting of yttria layers with a confined thickness of about 0.6 nm, separated from each other by organic layers of intercalated benzoate molecules. Doping with europium leads to strong red luminescence. The nanostructure formation proceeds via two reactions. A C--C bond formation occurs between benzyl alcohol and the isopropanolate ligand. At the same time, yttrium oxide catalyzes two low-temperature hydride-transfer reactions to form benzoic acid and toluene from benzyl alcohol via benzaldehyde, thus limiting the growth of the thickness of the lamellae.