Cristina Giordano

Synthesis of Mo and W carbide and nitride nanoparticles via a simple "urea glass" route.

A simple, inexpensive, and versatile route for the synthesis of metal nitrides and carbides (such as Mo2N, Mo2C, W2N and WC) nanoparticles was set up. For the first time, metal carbides were obtained using urea as carbon-source. MoCl5 and WCl4 are in a first step contacted with alcohols and an appropriate amount of urea to form a polymer-like, glassy phase, which acts as the starting product for further conversions. Just by heating this phase it was possible to prepare either molybdenum and tungsten nitrides or carbides simply by changing the metal precursor/urea molar ratio. In this procedure, urea plays a double role as a nitrogen/carbon source and stabilizing agent (necessary for the nanoparticle dispersion). Molybdenum and tungsten nitride and carbides synthesized are almost pure and highly crystalline. Sizes estimated by WAXS range around 20 and 4 nm in diameter for Mo and W nitrides or carbides, respectively. The specific surface area was found between 10 and 80 m2/g, depending on the metal and the initial ratio of metal precursor to urea.

A Facile Molten-Salt Route to Graphene Synthesis

Efficient synthetic routes are continuously pursued for graphene in order to implement its applications in different areas. However, direct conversion of simple monomers to graphene through polymerization in a scalable manner remains a major challenge for chemists. Herein, a molten-salt (MS) route for the synthesis of carbon nanostructures and graphene by controlled carbonization of glucose in molten metal chloride is reported. In this process, carbohydrate undergoes polymerization in the presence of strongly interacting ionic species, which leads to nanoporous carbon with amorphous nature and adjustable pore size. At a low precursor concentration, the process converts the sugar molecules (glucose) to rather pure few-layer graphenes. The MS-derived graphenes are strongly hydrophobic and exhibit remarkable selectivity and capacity for absorption of organics. The methodology described may open up a new avenue towards the synthesis and manipulation of carbon materials in liquid media.

Synthesis of Highly Stable, Water‐Dispersible Copper Nanoparticles as Catalysts for Nitrobenzene Reduction

We report an aqueous-phase synthetic route to copper nanoparticles (CuNPs) using a copper-surfactant complex and tests of their catalytic efficiency for a simple nitrophenol reduction reaction under atmospheric conditions. Highly stable, water-dispersed CuNPs were obtained with the aid of polyacrylic acid (PAA), but not with other dispersants like surfactants or polymethacrylic acid (PMAA). The diameter of the CuNPs could be controlled in the range of approximately 30-85 nm by modifying the ratio of the metal precursor to PAA. The catalytic reduction of p-nitrophenol to p-aminophenol takes place at the surface of CuNPs at room temperature and was accurately monitored by UV/Vis spectroscopy. The catalytic efficiency was found to be remarkably high for these PAA-capped CuNPs, given the fact that at the same time PAA is efficiently preventing their oxidation as well. The activity was found to increase as the size of the CuNPs decreased. It can therefore be concluded that the synthesized CuNPs are catalytically highly efficient in spite of the presence of a protective PAA coating, which provides them with a long shelf life and thereby enhances the application potential of these CuNPs.

One pot route to sponge-like Fe3N nanostructures

Iron nitride (Fe3N) is a promising material to replace scarce and costly noble metals in many catalytic applications. Here, we report the synthesis of Fe3N nanostructures by a simple sol–gel based route. This aqueous, one-pot method based on a self-expanding polypeptide foam represents a breakthrough in Fe3N nanostructure synthesis, which previously has only been achieved through ammonolysis. Through extensive X-ray diffraction and compositional analysis, a formation mechanism is proposed, based on in-situ nitridation by the decomposing gel matrix. The Fe3N nanoparticle sponge is shown to be a promising catalyst for ammonia decomposition, an easy process for CO2-free hydrogen supply and off-gas treatment.

From Paper to Structured Carbon Electrodes by Inkjet Printing

Electronics undoubtedly provide the foundation of information technology, uniting efforts from a wide range of scientific disciplines. Generations of materials scientists have strived to develop new processes to meet the demand for more advanced materials while providing enhanced processability and making microelectronics even more affordable. However, microelectronics processing is highly integrated, and the process chain is not readily accessible. Furthermore, there is a trend towards simpler approaches, both to reduce the price and environmental impact and to allow more flexible layout and smaller numbers of units (down to single, customdesigned units), thus accelerating development processes and leading to new applications. Functional printing techniques have been identified as the most promising approach for this type of electronics. To date, processes are often difficult to scale up and, most importantly, they almost exclusively use thermally unstable precursors (usually the support). This has hampered the integration of printing techniques with well-established high-temperature processing techniques, a key requirement for future electronics. Herein we describe a solution to these problems, without sacrificing simplicity, by using a paper support and catalytic ink as two “reactants” to generate functional carbon/ceramic arrays and three-dimensional structures. This simple “beyond the lab” process with off-the-shelf equipment is suitable for the largescale and high-temperature production of materials with applicationas as electrodes and catalysts. The printing of chemically active substances has spurred key advances in several fields including colloid science and biopharmacy, in the construction of microstructured functional materials by means of sol–gel chemistry, in the manufacture of functional coatings, and in ionogel-based flexible electronics. This approach has already been used to generate a wide variety of materials and properties. Although some work has been done in the field of carbon conductors, it relied on the dispersal and printing of preformed carbon structures rather than an in situ process. Furthermore none of these methods addressed the application to the existing industry-standard high-temperature processing used in microelectronics manufacture. In a first step, we filled the cartridges of a commercial inkjet printer (see Figure S1 in the Supporting Information) with a metal catalyst precursor and printed defined two-dimensional, lateral patterns on clean cellulose paper. The resolution here is controlled by the printing process and above all by the paper structure. In a possible second step, this paper can be shaped, processed, or simply folded to a desired threedimensional structure. Final thermal conversion of this structure then leads to the reaction of the catalytic ink with the paper, thus creating in this case conductive structures of iron carbide in graphitic carbon. The precision with which the spatial conformation of a 3D paper object can be retained was shown by folding a paper crane (see Figure 1). We used a sheet of filter paper, folded it into an origami crane, soaked it with the catalytic ink (see below), and then calcined it under inert atmosphere. Finally, the calcined crane was coated with copper to demonstrate the homogeneity of the final structure and the high degree of processability that is possible with this system. Similarly, the catalytic carbonization of thin tissue paper demonstrates the flexibility and durability of the final material for improved handling and broader applications (see Figure 1E and Figure S4 in the Supporting Information). All experiments were repeated in parallel with powdered microcrystalline cellulose instead of filter paper (using the same ink but simply soaking the cellulose powder) in order to establish whether this process can be scaled up to larger quantities and whether the actual fiber form of the cellulose is required, for example, as a structural template. This scaling was possible, and the cellulose powder proved to be a good model system. A catalytic ink of iron(III) nitrate (14 g) in water (15 mL) was transferred into an empty ink cartridge with a syringe. The cartridge was placed back into the printer and could be used directly. For the sake of simplicity and to exclude the possibility that additives in printer paper influence the product formation, we mounted a piece of pure, cellulose filter paper (laboratory grade) on a sheet of standard A4 printer paper (see Figure S2 in the Supporting Information). The printed area (Figure 2A) was cut out and placed between two slides of quartz glass to prevent wrinkling during subsequent carbonization. Conversion to Fe3C, graphitic carbon, and amorphous carbon was achieved by a single heating step at 10 K min 1 to 800 8C under N2 flow followed immediately by cooling to room temperature (Figure 2B). The experiments were also performed with a heating rate of 2 K min 1 with no difference in the produced products. The final product shrinks to approximately 60% of the initial dimensions on the centimeter or micrometer scale (see Figure S3 in the Supporting Information). The microscopic [*] S. Glatzel, Dr. C. Giordano Colloid Chemistry, Max Planck Institute of Colloids and Interfaces Am M hlenberg 1, 14476 Golm (Germany) E-mail: cristina.giordano@mpikg.mpg.de

Iron Carbide: An Ancient Advanced Material

Iron carbide ranks amongst the oldest materials known to mankind. As a matter of fact, the combination of iron and carbon was discovered even before the pure metal and what ancient cultures named “iron” was, in reality, an iron/iron carbide composite. The presence of 6.7 wt% C in Fe 3 C in fact changes its properties dramatically: iron carbide is ceramiclike in mechanical behavior and chemically much more inert than pure iron. The so-called “meteorite iron” is rich in Cohenite, cannot be forged, and is apparently “noble” (does not corrode, even on long time scales and in contact with oxygen and water). The presence of Fe 3 C was confi rmed, for example, in ancient Damascene steel, [ 1 ] a 2500-year-old material largely used for swords and daggers due to its very special properties (e.g., superior hardness and lightness), which originates in the modern view from reinforcing the metallic iron with ceramic nanofi llers, more specifi cally iron carbide nanofi bers enwrapped in carbon nanotubes. All of these properties, mechanical and magnetic, as well as the chemical inertness, plus the property that iron is rather sustainable and nontoxic, can open new interest in this material in the form of nanostructures, either as pure iron carbide or in combination with second-phase carbon. Until now, nanosized iron carbide has been mainly observed as a side product in the synthesis of carbon structures, where metallic iron is used as a catalyst, for example, in chemical vapor deposition (CVD) [ 2 ] and pyrolysis processes [ 3 ] during the synthesis of carbon nanotubes. At a time when the literature presents countless procedures for the production of a plethora of nanoparticles and nanostructures (ranging from physical to chemical approaches, in water or solventless, by using a hard template or soft matter), it is surprising that a synthetic pathway to produce basic Fe 3 C nanoparticles in a reproducible, simple, and fast manner is still missing. Iron carbide nanoparticles would indeed be suitable for a variety of applications, from biomedicine (e.g., as a magnetically guided transporter for drugs [ 4 ] or as a contrast agent for magnetic resonance imaging [ 5 ] ) to electronics (e.g.,

Sponge‐like Nickel and Nickel Nitride Structures for Catalytic Applications

A safe and simple method to fabricate air-stable nickel nitride and nickel embedded in carbon and nitrogen matrix, with high surface area for catalytic applications, is presented. The new synthesis employs molten inorganic salts as the reaction media. The use of salt melt opens new possibilities for safe, simple, and cheap synthesis of metal nitrides and metals for energy-related applications.

Biomimetic Oxygen Activation by MoS2/Ta3N5 Nanocomposites for Selective Aerobic Oxidation

The selective oxidation of petroleum-based feedstocks to useful functionalized chemicals is an important family of chemical transformations. Of these transformations, the selective oxidation of alcohols, alkenes, amines, and sulfides are among the most challenging reactions in green chemistry. There is significant interest in the design of new, costeffective, and environmentally friendly heterogeneous catalysts that use molecular oxygen (O2) under mild conditions, to avoid the use of a large excess of toxic and expensive stoichiometric metal oxidants. Although a number of catalysts based on novel metals and transition-metal oxides have been introduced, the precise design of catalysts with well-defined behaviors that depend on surface properties and electron features is still desired. Such catalysts are significant not only for use with multifunctional substrates, but also for insightful studies of catalytic mechanisms. These challenges are expected to be met through facet engineering and component control at the catalyst surface and in the active sites on the level of nanochemistry. Crystal-facet engineering has been successfully introduced to exploit novel metal nanocatalysts with high-surface-energy planes. This approach has led to high activity and selectivity in oxidation catalysis. However, it is difficult to control facet growth in metal-oxide catalysts with lowsymmetry crystal structures owing to the complexity of their structures. On the other hand, the ability to effectively vary the surface properties and electronic features of metal oxides by doping with other elements of different electronegativity, such as N, P, and S, enables new strategies for catalyst design. For example, the introduction of N into metal oxides can increase the energy of the HOMO orbital and narrow the band gap to thus enhance the catalytic activity, although controlled nitridation is difficult by current synthetic strategies. Recently, we proposed Caand SiO2-assisted urea methods for the controlled nitridation of transition metals. Remarkably, we discovered tunable oxidation ability associated with tailored nitridation, namely, improved activity and tunable selectivity for alkene epoxidation on TaON and Ta3N5 nanoparticles (NPs) with H2O2. This discovery opens up opportunities to develop superior tantalum-based catalysts with well-defined properties, especially for reactions involving cheap O2 as the oxidant. Access to such catalysts is needed to enable the important factors for catalytic turnover and selectivity to be uncovered. However, the absence of O2 activation in such (oxy)nitrides synthesized so far seriously limits further exploration. Biomimetic studies point to a new way to develop catalysts by learning from nature. In nature, the active center of nitrogenase enzymes contains metal atoms usually bound to sulfur, such as active Mo S and Fe S clusters. In nitrogen fixation, Mo S and Fe S sites activate inert N2 to react with H, with the generation of NH3 and H2. [11,12] This process inspired the use of MoSx for electroand photoelectrocatalytic H2 evolution based on electron transfer from MoS2 to H . The close energy potentials of E(H/H2)= 0 V and E(O2/CO2)= 0.16 V versus the normal hydrogen electrode suggest that MoSx could be used as a biomimetic O2-activation reagent to exploit bifunctional tantalum-based nanocatalysts for aerobic oxidation reactions. Herein, we describe the development of a new MoS2/ Ta3N5 catalyst in which Ta3N5 NPs are integrated with ultrathin MoS2 layers on the nanoscale by a hydrothermal method. The MoS2 nanolayers act as a biomimetic O2activation reagent in the MoS2/Ta3N5 NPs, which showed high activity and selectivity in the aerobic oxidation of alcohols as a result of the synergistic effect betweenMoS2 and Ta3N5. The MoS2/Ta3N5 NPs were also active in the aerobic oxidation of alkenes, amines, and sulfides. The different activities observed for these different substrates imply the potential use of this catalyst with multifunctional substrates. For example, high selectivity for hydroxy-group oxidation (> 90%) was observed in the oxidation of unsaturated alcohols. Well-defined Ta3N5 NPs of approximately 20 nm in diameter were prepared by our previously reported SiO2assisted urea method (see Figure S1 in the Supporting Information). Hydrothermal treatment of the Ta3N5 NPs with varying amounts of ammonium heptamolybdate (AHM) and thiourea at 180 8C for 20 h (see the Supporting Information) gave a series of MoS2/Ta3N5 nanocomposites that varied in their MoS2 content. The color of the composites changed from red to black as the MoS2 content increased (Figure 1a; see also Figure S2 in the Supporting Information). Inductively coupled plasma analysis and CHNS elemental analysis were used to determine the Mo and S content, respectively. The [*] Dr. Q. S. Gao, Dr. C. Giordano, Prof. Dr. M. Antonietti Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Research Campus Golm 14424 Potsdam (Germany) E-mail: qingsheng.gao@mpikg.mpg.de

Synthesis of Early-Transition-Metal Carbide and Nitride Nanoparticles through the Urea Route and Their Use as Alkylation Catalysts

The use of urea as either a carbon or a nitrogen source enabled the synthesis of various early-transition-metal nitride and carbide nanoparticles (TiN, NbN, Mo(2)N, W(2)N, NbC(x)N(1-x), Mo(2)C and WC). The ability of these particles to promote alkylation reactions with alcohols was tested on benzyl alcohol and acetophenone at 150 degrees C for 20 h in xylene. Group IV and V ceramics proved to be able to catalyse the formation of 1,3-diphenyl propenone, whereas group VI ceramics showed a tendency to promote the Friedel-Crafts-type reaction of benzyl alcohol on xylene (the solvent). TiN featured the highest activity for the alkylation of ketones and was further tested for more difficult alkylations. Group VI ceramics were further investigated as catalysts for the Friedel-Crafts-type alkylation of aromatics with activated alcohols. Interestingly, even hexanol could be effectively used for these reactions.

Preparation of organic–inorganic hybrid Fe–MoOx/polyaniline nanorods as efficient catalysts for alkene epoxidation

Novel Fe-MoO(x)/polyaniline nanorods were fabricated via in situ polymerization of Mo(3)O(10)(C(6)H(5)NH(3))(2)·2H(2)O nanowires, in which interface reactions remarkably influenced the morphology of products; and the nanorods showed high performance in cyclooctene epoxidation due to the organic-inorganic hybrid structure and Fe(3+) additive.