The molecular path to inorganic materials – Zinc oxide and beyond

The molecular path to inorganic materials – Zinc oxide and beyond

Inorganica Chimica Acta 363 (2010) 4148–4157 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/lo...

1MB Sizes 40 Downloads 121 Views

Inorganica Chimica Acta 363 (2010) 4148–4157

Contents lists available at ScienceDirect

Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

Review

The molecular path to inorganic materials – Zinc oxide and beyond Sebastian Polarz *, Carlos Lizandara Pueyo, Michael Krumm Department of Chemistry, University Konstanz, 78457 Konstanz, Germany

a r t i c l e

i n f o

Article history: Available online 13 August 2010 Dedicated to Achim Muller Keywords: Precursor chemistry Metal oxide materials Nanomaterials

a b s t r a c t In the current article, we present a concept for the synthesis of complex nanoscaled materials. The synthetic strategy involves a stepwise assembly of materials starting from special molecular precursors possessing multiple information. Therefore, the article focuses on a strong pervasion of inorganic materials chemistry, solid-state chemistry and molecular chemistry. The concept introduced is finally highlighted by examples from our current research in the field of zinc oxide materials. Ó 2010 Elsevier B.V. All rights reserved.

Prof. Polarz was born in Bielefeld/Germany in 1974. He received his Diploma-degree from the University of Bielefeld in 1999 working together with Prof. A. Mueller on polyoxometalate chemistry. For PhD, he moved to the Max-Planck Institute for Colloids in Interfaces where has worked in the group of Prof. M. Antonietti. After a one-year Post-Doc together with Prof. G. A. Ozin/Toronto, Canada Prof. Polarz returned to Germany in 2003 starting his academic career as an Emmy-Noether research group leader first at the Ruhr-University Bochum and then at the Technical University Berlin. In 2007 he received a call to a W-3 professorship for functional inorganic materials at the University of Konstanz. Prof. Polarz research interests involve the areas ’mesoporous materials’, ’precursor chemistry’, ’self-assembly’, ’metal oxide semiconductors’ and ’heterogeneous catalysis’.

Carlos Lizandara-Pueyo was born in Barcelona (Spain) in 1982. He received his Diploma in chemistry from the University of Barcelona in 2007. At the same year he went to the University of Konstanz to undertake a doctoral thesis under the supervision of Prof. Dr. Sebastian Polarz. His topic is the synthesis of novel materials by a control over the shape of Zinc Oxide nanoparticles.

* Corresponding author. Tel.: +49 7531 884415; fax: +49 7531 884406. E-mail address: [email protected] (S. Polarz). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.06.053

S. Polarz et al. / Inorganica Chimica Acta 363 (2010) 4148–4157

4149

Michael Krumm was born in Singen Htwl. (Germany) in 1984. He received his Diploma in chemistry from the University of Konstanz in 2009. He then remained in Konstanz to do his doctor’s degree with Prof. Dr. Sebastian Polarz working on new ways to synthesize functional metal-chalkogenidmaterials from molecular precursors.

Contents 1. 2. 3. 4. 5.

6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical architectonics for materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organometallic clusters with metal oxo-cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZnO materials from organometallic oxo-cluster compounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Increasing the precursor complexity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Organometallic amphiphiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. A library of heterobimetallic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Much of the activity in materials chemistry during the last decade originated from the realisation that material properties do not only depend on composition but that various new features emerge as soon as the morphology of the respective materials scales at nanodimensions. Two general approaches are well established for the synthesis of nanoscaled, and in particular nanostructured materials, the latter ones being characterized by a mutual orientation of the nanoobjects to each other: The ‘bottom-up’ approach and the ‘top-down’ approach [1]. Both methods have their advantages and disadvantages. For the current contribution, only one important difference between the two methods should be mentioned. In ‘top-down’ one starts already from a material having the desired composition and structure. Because the indicated material has to be provided prior to the experiment, the ‘top-down’ method is exposed to one problem which is also common in classical solid-state chemistry. To obtain particular inorganic phases by traditional solid-state methods, due to the high activation energies for diffusion in the solid-state, high temperatures are applied [2]. The high temperatures prohibit that solids far from thermodynamic equilibrium can be prepared. However, it is obvious that solids either metastable in composition or crystal structure or both would potentially possess new, unexpected and exiting properties [2]. Therefore, it was recognized relatively early in solid-state chemistry that molecular routes to solids might solve the problems mentioned above because extreme conditions can be circumvented and mass transport is much easier [3–11]. The field of soft-chemistry emerged [12]. Presumably the most prominent example for soft-chemistry is the sol–gel process [13]. Most molecules represent metastable points on the global energy hypersurface. Thus, it is reasonable that their transformation to metastable materials is feasible [2]. In addition, many examples from the field of ‘single-source precursor chemistry’ have proven that it is possible to determine the composition of the final material already on the molecular scale [14–21,11]. Therefore, to a certain extend, plan-

4149 4149 4150 4153 4155 4155 4155 4156 4156 4156

ning the synthesis of a desired, extended material can become planing the structure of the molecular source.

2. Chemical architectonics for materials The development of a synthetic protocol addressing the issues mentioned above requires to be more refined than the basic idea of the ‘bottom-up’ approach. Such a synthetic concept might be called ‘chemical architectonics for materials’ in accordance to the definition of architectonics already made by ancient greek philosophers: architectonics is the craft to prepare from a given substance a material possessing function. It should also be mentioned that articles related on ‘‘chemical tectonics” have been published by others [22–26]. The latter articles can be seen as the antecessors to the ideas presented here [22–24], with the difference that we emphasize much more the potential of molecular precursors. The approach described in the current paper represents a consequent combination of the high state of development in the fields of inorganic molecular synthesis and materials chemistry. The ‘chemical architectonics for materials’ concept is schematically depicted in Fig. 1 and will be documented by concrete examples from the research of others and our own in the following chapters. At the starting point stand special molecular precursors which contain an inorganic core shielded by appropriate leaving groups. The size and structure of the inorganic core is given by the stoichiometry of the desired material. Ultimately it represents the smallest possible cut-out which unambiguously resembles the lattice structure of the target material. As the current article focuses on metal oxide materials, the mentioned inorganic core will be a metal oxide cluster (Fig. 1, molecular scale) [27]. The methods for preparing all sorts of metal-oxo-cluster compounds are well established [28–34]. Compared to the number of known molecular, multinuclear chalcogenide cluster compounds it is surprising that they have been used for the synthesis of complex inorganic materials relatively seldom.

4150

S. Polarz et al. / Inorganica Chimica Acta 363 (2010) 4148–4157

Fig. 1. Graphical representation of the synthetic concept for materials architectonics. Well established methods from molecular chemistry (Syi) allow for the preparation of special molecular precursors. These precursors play the role of first order tectons (TI) because they contain a central inorganic core which in an ideal case represents the smallest possible cut-out from a designated (potentially metastable) solid-state material. A nanoparticle can be generated via directed peel away of the organic shell (Syii). Ideally, the nanoparticle contains the same structural than the inorganic core at the molecular stage. This idea is indicated in the figure by the dark cube located beneath the translucent surface of the nanoparticle. The nanoparticle is already a functional entity. One of its various functions is to act as a new tecton (TII) for the assembly (Syiii) of an ensemble of nanoparticles to a nanostructured material (TIII) indicated by the solid nanoparticle inside the translucent nanostructure. Finally, a virtual energy diagram (white line) is correlated to the respective dimensions in the synthesis.

Sufficient chances for the ‘‘molecularization of solids” exist if the electron-density distribution of the respective section of the solidstate structure is alike to the inner core of the potential molecular precursor [35]. In case the differences are too large it is likely that structural information gets lost in the course of the materials synthesis due to restructuring processes. Only if the differences are small it may be possible to identify reaction pathways (denoted Syii in Fig. 1) for which a maximum of bonding motives are expected to stay intact [36]. The activation energies for the re-organization of metastable states regarding composition are expected to be significantly higher than those connected to crystal structure changes. The latter type of materials (polymorphs) will therefore be much more difficult to achieve. Consequently, the activation energy for the transition from the molecular precursor (‘‘the first order tecton TI”, Fig. 1) to the desired inorganic solid-state material, for instance a nanoparticle (‘‘the second-order tecton”) has to be smaller than the activation energy for the mentioned restructuring processes. Regarding the latter point, not only the bonding energies in the central inorganic core of the molecular source can be regarded as crucial but also the type of the organic shell surrounding the precursor. Therefore, although much higher synthetic effort (Syi, Fig. 1) is required for their preparation, organometallic compounds seem to be a good choice for TI (Fig. 1). An important argument is that most of them are relative labile regarding oxidation (. . .leading to oxides which are the focus of the current article) and also react readily with water (. . .also leading to oxides).

tures. The size and complexity of the molecular fragment is influenced by several parameters: – The nature and number of the organic groups. For metal-oxo fragments RM(Ol2)x with more than one bridging oxygen atoms (x > 1) either molecular or polymeric structures can be obtained depending on the steric requirements of the organic group (Scheme 1). Organic substituents with high steric demand facilitate the formation of molecular rather than polymeric structures. For x P 4, respectively for a high number of cross linking points, it becomes increasingly difficult to obtain molecular structures. However, also the number of organic blocking points have a significant effect. A nice example are the two phenyl-stabilised organoantimony-oxoclusters shown

3. Organometallic clusters with metal oxo-cores If one starts from a given metal oxide section and one substitutes the symmetry equivalent positions of the designated coordination polyhedron by an organic substituent, cross linking at this particular position is hindered or even interrupted. Already this simple substitution can lead to the formation of molecular struc-

Scheme 1. Influence of steric requirements on the molecular structures of organometallic metal-oxo clusters. Molecular structures (left) and polymeric structures (right), x being the number of bridging O atoms at the MR unit.

S. Polarz et al. / Inorganica Chimica Acta 363 (2010) 4148–4157

Scheme 2. Influence of the number of substituents.

in Scheme 2 [37]. The increase of blocking positions from two to three leads to a decrease in nuclearity of the oxo-clusters from four to two [38]. – An additional factor is the number of cross-linking sites that are blocked by the organic ligand. While in most cases methylgroups block only one site, cyclo-pentadienyl (Cp) is capable of blocking up to three sites (g5-coordination). A metal center

4151

which prefers octahedral coordination in the related solid-state structure can be reduced to an unit for which only three crosslinking sites are used. – The binding mode at the remaining oxygen atoms is of importance as well. The coordination geometry of the O2 ion is remarkably ambivalent. Terminal ([email protected]), g2, g3 and g4 oxygen centers are found very frequently (see Scheme 3). Much more seldomly in molecular structures one finds g5 and g6 oxygen centers. – The bonding angle (O–M–O) determines the ‘‘curvature” of the ‘‘OMx” building block. Typically, the O–M–O angle can be expected to be significantly smaller than 180° depending on how strong the metal orbitals interact with the lonepairs at the oxygen atoms. However, bonding angles smaller than 180° lead unherently to the formation of rings and cages of different symmetry (Scheme 4). – The ability of the metal to form [email protected] double bonds and its coordination geometry also influence the molecular structure of the oxo-cluster (see Scheme 3f–h).

Scheme 3. Influence of the binding mode of the oxygen atoms in organometallic metal-oxo clusters. The coordination mode is shown to the left and one illustrative example from the literature to the right [39–41].

4152

S. Polarz et al. / Inorganica Chimica Acta 363 (2010) 4148–4157

Scheme 4. Influence of the bonding angle OMO illustrated at some structures from the literature [42–44].

– Finally, for a given precursor core type (for instance a heterocubane) it is desirable to have different compositions at hand. Molecular compounds with heterocubane structure which should be discussed here as a representative case are known very long for many different metal ions and different bridging groups [45–48,49]. The focus of research during the last decade shifted to late transition metals. Many new heterocubanes could be prepared [50–54]. One motivation for the demand for such compounds is the observation of molecular magnetism and the finding of magnetochirality [55–59]. Building blocks of heterocubanes which contain transition metals with unpaired electrons represent interesting model systems for magnetic

interactions in multiple spin systems [60]. In the life sciences these complexes are relevant because of the presence of multinuclear active sites in metalloproteins as in redox active proteins like nitrogenase [61]. Molecular M4O4 oxo clusters containing organometallic entities have been reported only seldom (M = Zn, Cd, Hg, Au, Pt, In, Ga, Al, Be) [62–74]. A methylmagnesium alkoxycluster with a Mg4O4 core was synthesized very recently [75]. In the material sciences metal oxo complexes with heterocubane structure are of great interest as single source precursors (see also below). An excellent example is the exchange of the highly toxic and moisture sensitive dimethylcadmium with methylcadmium aminoalkoxides MeCd(dmae)

S. Polarz et al. / Inorganica Chimica Acta 363 (2010) 4148–4157

Fig. 2. The two ways for transforming zinc-alkyl-alkoxy heterocubane clusters into nanoscaled zinc oxide. A HRTEM image and a PXRD pattern or the resulting ZnO are shown as well.

which forms a heterocubane structure with a Cd4O4 core. The latter heterocubane can be used in the synthesis of CdO-based TCOs (transparent conducting oxides) [76].

4. ZnO materials from organometallic oxo-cluster compounds The synthesis of ZnO materials derived by organozinc precursors has been described in several papers [77–87]. The high reactivity of these molecular sources has tremendous advantages im

4153

comparison to the classical ZnO precursors due to the possibility to control nucleation and growth of the zinc oxide crystallites at relatively mild conditions [88]. Several good example have been described by Chaudret et al. in a number of extraordinary papers [81]. ZnO nanoparticles with different sizes and shapes were synthesized via the oxidation of dicyclohexylzinc in air in the presence of ligands and/or surfactants. Moreover, synthesis of several interesting composite materials have been reported [89,87]. The crystallization originating from organozinc compounds leads at first to the formation of zinc-oxo-species [82]. Thus, it appears to be promising to use metal-oxo clusters directly as single source precursors for the synthesis of zinc oxide [90–93]. Furthermore, the possibility to start the nucleation with zinc-oxo-clusters allowed to extract more information about the formation of Zn–O– Zn species and its mechanisms [91]. This facilitates a fine tuning of the properties of the resulting materials [93,94]. From the above mentioned organometallic oxo-clusters we selected one and attempted to apply the concept of materials synthesis as described before (Fig. 1). The type of clusters we selected were zinc-oxo-clusters possessing a heterocubane structure [R4Zn4O4R0 4] (see for instance Fig. 2). Due to their central ‘Zn4O4’ core they can be seen as a representative of zinc oxide on the molecular scale. Indeed, the treatment of the mentioned potential precursors either at elevated temperatures (T > 150 °C) or with water leads to the formation of zinc oxide as can be clearly proven by PXRD (Fig. 2) [95]. The resulting ZnO-materials contain a large number of nanosized crystallites (depending on conditions between 3 and 20 nm in size). These particles are highly agglomerated to larger aggregates which sometimes have astonishing shapes like the necklace of chains of spheres containing the nanocrystallites shown in Fig. 3a [96]. The latter materials can be categorized as so-called nanopowders [97]. Because the organometallic zinc-oxo-clusters used as precursors represent a metastable point on the energy hypersurface (Fig. 1), the nanopowders are very different from perfect zinc oxide not only due to the nanoscale characteristics. It is important to note that under the described

Fig. 3. The collection of different ZnO materials that can be prepared from the class of zinc-containing, organometallic heterocubanes: Nanopowders (a) [95], mesoporous ZnO (b) [101], particles supported by mesoporous silica matrices (c) [89], size-selected, ultra pure nanoparticles (d) [96], colloids (e) [102], and thin films (f) [103].

4154

S. Polarz et al. / Inorganica Chimica Acta 363 (2010) 4148–4157

conditions (solid precursor ? ZnO) the transformation process is kinetically and not thermodynamically controlled [95]. Under kinetically controlled preparation conditions a ZnO is obtained which is likely to possess various defects, most importantly impurity atoms like carbon and oxygen deficiency sites. It was found that such defects can strongly alter chemical and physical properties of the ZnO-materials [98,99,95]. For instance, oxygen vacancies represent the active sites of the catalytic transformation of CO with H2 to methanol over ZnO. It could be clearly shown that ZnO materials possessing a higher density of oxygen vacancies are catalytically more active regarding methanol production [99]. The latter aspect points to the fact the prepared ZnO represents not only a new material but that such materials can also serve as model system to investigate important processes. In addition, it could be shown that the density of oxygen vacancies formed, among other parameters, depends on the character of the organic leaving groups bound to the inorganic ‘Zn4O4’ core of the precursor [100,95]. For instance, the isopropoxy-ligand in [MeZnOisoPr]4 in contrast to other groups facilitates the elimination of acetone, thus, leaving an oxygen-deficient ZnO material behind. More refined morphologies and materials compared to the mentioned nanopowders can be prepared in several ways (Fig. 3). Due to their molecular character, the ZnO-precursors are characterized by high solubilities in organic solvents and significant volatility despite the high molecular weight. Both properties can be further tuned by alternative organic groups shielding the inorganic ‘Zn4O4’-core. The latter two properties allow the preparation of ZnO-materials from the liquid phase and also from the gasphase (chemical vapour synthesis, CVS). The application of colloid chemistry to the process (‘[RZnOR’]4 ? ZnO’) enables the preparation of colloidal dispersions of ZnO. For instance, the reaction of the heterocubanes dis-

solved in an organic solvent with water in the presence of a suitable amphiphile affords stable colloidal solutions with colloids 12 nm in size. However, the transfer of the process to gas-phase chemistry is even more powerful [96]. First, the precursors are evaporated and react then at elevated temperatures (T = 300–900 °C) to an aerosol of ZnO nanoparticles. Still in the aerosol, the particles can be charged, selected according to their weight (for details see [96]) and be post-sintered afterwards. Due to the highly defined conditions, and because even at high temperatures, due to the high dilution in the aerosol, still nanoscaled, non-agglomerated, size-selected ZnO particles of extraordinary purity and high crystallinity can be obtained (Fig. 3d). The particles prepared by the CVS process can then be further deposited on any imaginable substrate. For instance their assembly on a sensor-grid circuit enabled to achieve the more active (compared to SnO-sensors and other ZnO-sensors) and more durable ZnO-sensors [98]. It is even possible to bubble the particle aerosols from CVS into solvents. If the solvent contains amphiphiles, the amphiphiles instantaneously bind to the surface of the nanoparticles, stabilize them which then leads to the formation of colloids [102]. Due to the possibility in CVS to prepare sizeselected particles, the combination of colloid chemistry and CVS is unique. One of the advantages of the mentioned precursor system is that it contains the high amount of 50% ‘‘molecularly hidden” ZnO. Therefore, the transformation to ZnO is associated only with a relatively small loss of volume. However, because the precursors mentioned so far are crystalline solids, for the synthesis of more complex materials the precursors have to be dispersed first in an appropriate medium (gas or a solvent). While fine for the preparation of nanoparticle, dilution can make it very difficult to assemble nanostructured materials as indicated in Fig. 1 (TIII).

Fig. 4. Illustration of the self-assembly process of the new organometallic amphiphile [MeZnOPEG400]2 from the molecular to the micrometer length-scale, and a polarization microscopy image of the lyotropic phase and a photo indicating the optical properties of the gels.

S. Polarz et al. / Inorganica Chimica Acta 363 (2010) 4148–4157

4155

Fig. 5. The three alternatives to increase the chemical complexity of the zinc-oxo heterocubane precursor system. Exchange of one zinc (a), exchange of the methyl-group attached to zinc (b) and exchange of the alkoxy-group (c). [M] a metal containing fragment.

Again, the modification of the organic shell surrounding the ‘Zn4O4’ core solves the problem. The heterocubane [MeZnO– CH2CH2–O–Me]4 is actually liquid at room temperature. The liquid property of the precursor allows the infiltration of the undiluted precursor into practically every porous system. For instance, its infiltration and succeeding transformation in mesoporous silica materials results in size-selected ZnO-particles supported by the silica pores (Fig. 3c) [104,89]. The confined character of the particles inside the pores influenced strongly the catalytic performance regarding the aforementioned catalytic preparation of methanol. Highly active catalysts and interesting model systems could be obtained [104,89]. If instead of mesoporous silica an alternative, porous material is used which can be removed afterwards under conditions where ZnO is stable, a route towards the formation of nanoporous ZnO-materials can be accessed (Fig. 3b) [101]. A mesoporous carbon matrix fulfils the mentioned criteria. Consequently, the liquid [MeZnO–CH2CH2–O–Me]4 can be infiltrated into a mesoporous carbon matrix. The formation of ZnO is followed by the removal of the carbon by calcination [101]. The walls of the porous carbon as a template then become the pores of the resulting ZnO material. From the application of different templates ZnO materials with internal surface area of up to 202 m2/g and pore sizes ranging 3–9 nm can be obtained [101]. 5. Increasing the precursor complexity It was demonstrated in the paragraphs above that both, the inorganic core and the organic shell of the special organometallic precursor system described in the current paper are ‘‘set screws” to influence the properties of the final ZnO materials. Therefore, the question arises if even more information for the materials domain can be encoded on the molecular scale. Obviously, either the complexity can be increased regarding the organic shell (Section 5.1) order the inorganic core (Section 5.2). 5.1. Organometallic amphiphiles All of the above mentioned precursor systems contain relatively simple organic groups and are at the same time symmetric molecules. Therefore, we came up with the idea if it is possible to prepare organometallic-oxo-clusters which are less symmetric and if

at least one organic group can be significantly different to the others. As a consequence one could eventually obtain a molecule with amphiphilic properties. An amphiphile is a dipolar compound which contains two molecular parts possessing different solvent compatibilities. The classical example for amphiphiles are surfactants where the water-compatible head group (for instance ammonium) is attached to a long, hydrophobic chain (typically an alkane). The presented alkyl-alkoxy-zinc clusters are shielded by methyl- and alkoxy-groups (see Fig. 3), thus, represents an entity with hydrophobic character. In order to obtain an amphiphile it would be necessary to attach a long hydrophilic chain (like poly ethyleneglycol; PEG) to the oxo-cluster core. Unfortunately, it was not possible to attach one PEG chain to a heterocubane building block. Instead, a dimeric compound [MeZnOPEG]2 could be synthesized resembling a double-tailed surfactant (Fig. 4) [105]. Interestingly, this new organometallic amphiphile self-assembles over several lengthscales. First, the dimers form a lamellar phase via a microphase separation of the hydrophobic oxo-cluster entities and the polar PEG domains indicated by polarization microscopy, SAXS and TEM [105]. In contact to apolar solvents like toluene swelling of the hydrophobic domains and further structuring occurs. Surprisingly, relatively monodisperse spheres form which we interpret multilammelar in structure (Fig. 4). These spheres than agglomerate to a packing which due to the dimension of the single spheres (200 nm in size) behaves like an optical grating diffracting light in the visible. Due to this diffraction, although the system does not contain any chromophores, the resulting toluene containing gel is blue in color. In comparison to traditional amphiphiles, the discussed system is different because it right away contains a functional entity which can be transformed into the respective material (ZnO). One possibility is to use the PEG-chains as a placeholder for generating open porosity. Indeed, a bimodal, macro-nano-porous ZnO could be generated directly from the nanostructured gel while the morphology of the ZnO was directly influenced by the morphology of the [MeZnOPEG]2 phase [105]. 5.2. A library of heterobimetallic compounds The second alternative to increase the complexity of the information encoded to the mentioned molecular precursor sys-

4156

S. Polarz et al. / Inorganica Chimica Acta 363 (2010) 4148–4157

tem is to alter the composition of the inorganic core. Three alternatives exist to equip the precursor with an additional metaltype. One or more zinc atoms can be exchanged by different metals. Interestingly, it is possible to prepare a whole library of compounds with all possible combinations and permutations with M = Co, Ni, Mn, Fe, Li, Na, K [46,102,106]. The central core has then the composition ‘Zn4xMxO4’. For instance, a precursor containing the core ‘NiZn3O4’ could be prepared and isolated [46]. Under certain conditions [46], the transformation of this particular precursor leads to a nanopowder material Ni0.25Zn0.75O, meaning that the composition at the molecular scale determines the composition of the resulting solid. The mentioned composition is special because Ni2+ prefers octahedral coordination by oxygen (in NiO) and Zn2+ prefers tetrahedral coordination (in ZnO). Under normal circumstances the composition Ni0.25Zn0.75O would phase separate into a Zn-rich NiO phase and a Ni-rich ZnO phase. However, because the metastable composition is pre-organized on the molecular scale it is possible to prevent the expected phase-separation as long as the experimental conditions used during materials synthesis do not exceed the necessary activation energy for the restructuring process (compare to Fig. 1). Alternatively, the methyl groups attached to Zn in [MeZnOR]4 can be exchanged by an isolobal fragment containing a metal (Fig. 5b). The pseudo-halogen [Mn(CO)5]-fragment is isolobal to – CH3, therefore, a bimetallic heterocubane cluster [(CO)5MnZnOR]4 can be prepared and used for the synthesis of various ZnMnO3 materials [102]. Again, the composition of the inorganic core of the molecular precursor determines the composition of the final material obtained as nanopowders or colloids. Last but not least, it is also possible to use other groups than alkoxy-functions (Fig. 5c). While alkyl-alkoxyzinc heterocubanes can be prepared by the direct reaction of dimethylzinc with an equimolar amount of an alkylalcohol, the use of silanols affords in full analogy oxo-clusters with the composition [MeZnOSiR3]4 [107]. Then, because the inorganic core of the precursor contains the elements Zn, Si and O, it can be expected that zinc-silicate materials can be prepared applying all the methods described in Section 4 of this article. This is indeed the case as we showed by the preparation of zinc-silicate particles using CVS [108]. 6. Conclusions It was shown that multinuclear oxo-clusters of zinc are valuable molecular precursors for the preparation of various nanoscaled materials. The advantage of such molecular sources is that many options exist to tune the properties of the precursors and to deposit information about the materials domain at the molecular scale. The modification of the organic shell and the inorganic core of the precursors allow to influence important key properties of materials like composition, self-organization and defect formation. Furthermore, the molecular features enable to apply the materials synthesis at moderate conditions in order to perpetuate metastable characteristics. Synthetic approaches involving the solid-state, the liquid state and even the gas-phase can be applied. Acknowledgements

[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60]

SP thanks Prof. Driess for the cooperation in the various studies highlighted in the current article.

[61] [62] [63]

References

[64]

[1] G.A. Ozin, A.C. Arsenault, Nanochemistry: A Chemical Approach to Nanomaterials, 2005.

[65] [66]

M. Jansen, Angew. Chem. 114 (2002) 3896. R.C. Mehrotra, Inorg. Chim. Acta, Rev. 1 (1967) 99. R.C. Mehrotra, Adv. Inorg. Chem. Radiochem. 26 (1983) 269. R.C. Mehrotra, J. Non-Cryst. Solids 100 (1988) 1. D.C. Bradley, Chem. Rev. (Washington, DC, United States) 89 (1989) 1317. R.C. Mehrotra, J. Sol–Gel Sci. Technol. 2 (1994) 1. R.C. Mehrotra, A. Singh, Prog. Inorg. Chem. 46 (1997) 239. R.C. Mehrotra, A. Singh, M. Bhagat, J. Godhwani, J. Sol–Gel Sci. Technol. 13 (1998) 45. N.Y. Turova, Russ. Chem. Rev. 73 (2004) 1041. L.G. Hubert-Pfalzgraf, Coord. Chem. Rev. 180 (1998) 967. M. Veith, J. Chem. Soc., Dalton Trans. (2002) 2405. C.J. Brinker, G.W. Scherer, Sol–Gel Science. The Physics and Chemistry of Sol– Gel Processing, first ed., Academic Press Inc., London, 1990. A.H. Cowley, R.A. Jones, Angew. Chem. Int. Ed. (In English) 28 (1989) 1208. A.A. Wernberg, G. Braunstein, G. Pazpujalt, H.J. Gysling, T.N. Blanton, Appl. Phys. Lett. 63 (1993) 331. A.H. Cowley, R.A. Jones, Polyhedron 13 (1994) 1149. C.J. Carmalt, A.H. Cowley, R.D. Culp, R.A. Jones, Y.M. Sun, B. Fitts, S. Whaley, H.W. Roesky, Inorg. Chem. 36 (1997) 3108. S. Mathur, M. Veith, H. Shen, S. Hufner, in: Metastable, Mechanically Alloyed and Nanocrystalline Materials, 2002, pp. 341–346. M. Veith, S. Mathur, N. Lecerf, V. Huch, T. Decker, H.P. Beck, W. Eiser, R. Haberkorn, J. Sol–Gel Sci. Technol. 17 (2000) 145. L.G. Hubert-Pfalzgraf, J. Mater. Chem. 14 (2004) 3113. L.G. Hubert-Pfalzgraf, Inorg. Chem. Commun. 6 (2003) 102. M. Henry, Encyclopedia Nanosci. Nanotechnol. 5 (2004) 743. S. Mann, G.A. Ozin, Nature 382 (1996) 313. S. Mann, Nature 365 (1993) 499. A. Mueller, D. Fenske, P. Koegerler, Curr. Opin. Solid State Mater. Sci. 4 (1999) 141. C. Sanchez, G. Soler-Illia, F. Ribot, T. Lalot, C.R. Mayer, V. Cabuil, Chem. Mater. 13 (2001) 3061. C.N.R. Rao, Ann. Rev. Phys. Chem. 40 (1989) 291. M. Driess, H. Noeth, Editors Molecular Clusters of the Main Group Elements, 2004. E.L. Muetterties, J. Organomet. Chem. 200 (1980) 177. P. Braunstein, Perspect. Coord. Chem. (1992) 67. A. Proust, R. Villanneau, R. Delmont, V. Artero, P. Gouzerh, Polyoxometalate Chem. (2001) 55. M.G. Humphrey, M.P. Cifuentes, Organomet. Chem. 29 (2001) 289. M.G. Humphrey, M.P. Cifuentes, Organomet. Chem. 32 (2005) 214. H. Schnoeckel, Dalton Trans. (2005) 3131. M. Henry, Coord. Chem. Rev. 180 (1998) 1109. A. Müller, H. Reuter, S. Dillinger, Angew. Chem. Int. Ed. 34 (1995) 2328. D.B. Sowerby, M.J. Begley, P.L. Millington, J. Chem. Soc.-Chem. Commun. (1984) 896. J. Bordner, G.O. Doak, T.S. Everett, J. Am. Chem. Soc. 108 (1986) 4206. K. Umakoshi, K. Isobe, J. Organomet. Chem. 395 (1990) 47. F. Bottomley, D.F. Drummond, D.E. Paez, P.S. White, J. Chem. Soc.-Chem. Commun. (1986) 1752. M. Driess, K. Merz, S. Rell, Eur. J. Inorg. Chem. (2000) 2517. H.J. Breuning, M.A. Mohammed, K.H.Z. Ebert, Naturforsch. 49b (1994) 877. J.J. Vittal, Polyhedron 15 (1996) 1585. F. Heshmatpour, S. Wocadlo, W. Massa, K. Dehnicke, F. Bottomley, R.W. Day, Zeitschrift Fur Naturforschung Sect. B-J. Chem. Sci. 49 (1994) 827. T. Shiga, H. Oshio, Sci. Technol. Adv. Mater. 6 (2005) 565. S. Polarz, A. Orlov, M. Van den Berg, M. Driess, Angew. Chem. Int. Ed. 44 (2005) 7892. K. Isobe, Organomet. News (1995) 6. M.I. Yanovskaya, E.P. Turevskaya, V.G. Kessler, I.E. Obvintseva, N.Y. Turova, Integrat. Ferroelectrics 1 (1992) 343. E. Weiss, H. Alsdorf, H. Kuhr, Angew. Chem.-Int. Ed. 6 (1967) 801. S. Krieck, H. Gorls, M. Westerhausen, J. Organomet. Chem. 694 (2009) 2204. T. Oldag, H.L. Keller, Zeitschrift Fur Anorganische Und Allgemeine Chemie 632 (2006) 1267. C. Borgmann, C. Limberg, S. Cunskis, P. Kircher, Eur. J. Inorg. Chem. (2001) 349. W. Uhl, M. Pohlmann, Chem. Commun. (1998) 451. G. Aromi, A.S. Batsanov, P. Christian, M. Helliwell, A. Parkin, S. Parsons, A.A. Smith, G.A. Timco, R.E.P. Winpenny, Chem.-Eur. J. 9 (2003) 5142. O. Kahn, Mol. Magnetism, 1993. R.E.P. Winpenny, Advances in Inorganic Chemistry, vol. 52, 2001, pp. 1–111. A. Burkhardt, E.T. Spielberg, H. Gorls, W. Plass, Inorg. Chem. 47 (2008) 2485. G. Rikken, E. Raupach, Nature 390 (1997) 493. G. Rikken, E. Raupach, Nature 405 (2000) 932. S. Diewald, Y. Lan, R. Clerac, A.K. Powell, C. Feldmann, Zeitschrift Fur Anorganische Und Allgemeine Chemie 634 (2008) 1880. R.H. Holm, P. Kennepohl, E.I. Solomon, Chem. Rev. 96 (1996) 2239. A.M. Arif, A.A. Barron, Polyhedron 7 (1988) 2091. K.C.K. Swamy, C.G. Schmid, R.O. Day, R.R. Holmes, J. Am. Chem. Soc. 110 (1988) 7067. K.C.K. Swamy, S. Nagabrahmanandachari, Phosphorus, Sulfur Silicon Relat. Elem. 65 (1992) 9. W. Uhl, M. Pohlmann, Chem. Commun. (Cambridge) (1998) 451. A.B. Charette, C. Molinaro, C. Brochu, J. Am. Chem. Soc. 123 (2001) 12160.

S. Polarz et al. / Inorganica Chimica Acta 363 (2010) 4148–4157 [67] C.J. Barden, P. Charbonneau, H.F. Schaefer III, Organometallics 21 (2002) 3605. [68] S. Jana, T. Pape, N.W. Mitzel, Zeitschrift fuer Naturforschung, B Chem. Sci. 62 (2007) 1339. [69] A.L. Johnson, N. Hollingsworth, G. Kociok-Kohn, K.C. Molloy, Inorg. Chem. (Washington, DC, United States) 47 (2008) 9706. [70] G.L. Morgan, R.D. Renniek, C.C. Soong, Inorg. Chem. (Washington, DC, United States) 5 (1966) 372. [71] G.E. Coates, A.H. Fishwick, J. Chem. Soc. Sect. A, Inorg. Phys. Theor. (1968) 640. [72] H. Schmidbaur, M. Bergfeld, F. Schindler, Zeitschrift fuer Anorganische und Allgemeine Chemie 363 (1968) 73. [73] S.J. Harris, R.S. Tobias, Inorg. Chem. 8 (1969) 2259. [74] R.S. Tobias, C.E. Rice, W. Beck, B. Purucker, K. Bartel, Inorg. Chim. Acta 35 (1979) 11. [75] S. Heitz, Y. Aksu, C. Merschjann, M. Driess, Chem. Mater. 22 (2010) 1376–1385. [76] A.L. Johnson, N. Hollingsworth, G. Kociok-Kohn, K.C. Molloy, Inorg. Chem. 47 (2008) 9706. [77] S. Nicolay, S. Fay, C. Ballif, Cryst. Growth Des. 9 (2009) 4957. [78] D.J.D. Moet, L.J.A. Koster, B. de Boer, P.W.M. Blom, Chem. Mater. 19 (2007) 5856. [79] T. Aoki, Y. Shimizu, A. Miyake, A. Nakamura, Y. Nakanishi, Y. Hatanaka, Phys. Status Solidi B-Basic Res. 229 (2002) 911. [80] F. Rataboul, C. Nayral, M.J. Casanove, A. Maisonnat, B. Chaudret, J. Organomet. Chem. 643 (2002) 307. [81] M. Monge, M.L. Kahn, A. Maisonnat, B. Chaudret, Angew. Chem.-Int. Ed. 42 (2003) 5321. [82] M.L. Kahn, M. Monge, V. Colliere, F. Senocq, A. Maisonnat, B. Chaudret, Adv. Funct. Mater. 15 (2005) 458. [83] M.L. Kahn, M. Monge, E. Snoeck, A. Maisonnat, B. Chaudret, Small 1 (2005) 221. [84] M.L. Kahn, T. Cardinal, B. Bousquet, M. Monge, V. Jubera, B. Chaudret, Chem. Phys. Chem. 7 (2006) 2392. [85] M.L. Kahn, A. Glaria, C. Pages, M. Monge, L. Saint Macary, A. Maisonnat, B. Chaudret, J. Mater. Chem. 19 (2009) 4044. [86] C. Pages, Y. Coppel, M.L. Kahn, A. Maisonnat, B. Chaudret, Chem. Phys. Chem. 10 (2009) 2334. [87] F. Schroder, S. Hermes, H. Parala, T. Hikov, M. Muhler, R.A. Fischer, J. Mater. Chem. 16 (2006) 3565.

4157

[88] C. Lizandara-Pueyo, M. van den Berg, A. de Toni, T. Goes, S.J. Polarz, Am. Chem. Soc. 130 (2008) 16601. [89] S. Polarz, F. Neues, M. van den Berg, W. Grünert, L. Khodeir, J. Am. Chem. Soc. 127 (2005) 12028. [90] N. Pinna, G. Garnweitner, M. Antonietti, M. Niederberger, J. Am. Chem. Soc. 127 (2005) 5608. [91] I. Bilecka, P. Elser, M. Niederberger, ACS Nano 3 (2009) 467. [92] I. Bilecka, I. Djerdj, M. Niederberger, Chem. Commun. (2008) 886. [93] M. Cao, I. Djerdj, M. Antonietti, M. Niederberger, Chem. Mater. 19 (2007) 5830. [94] Z. Zhang, D.C. Leitch, M. Lu, B.O. Patrick, L.L. Schafer, Chem.-Eur. J. 13 (2007) 2012. [95] V. Ischenko, S. Polarz, D. Grote, V. Stavarache, K. Fink, M. Driess, Adv. Funct. Mater. 15 (2005) 1945. [96] S. Polarz, A. Roy, M. Merz, S. Halm, D. Schröder, L. Scheider, G. Bacher, F.E. Kruis, M. Driess, Small 1 (2005) 540. [97] J. Hambrock, S. Rabe, K. Merz, A. Birkner, A. Wohlfart, R.A. Fischer, M. Driess, J. Mater. Chem. 13 (2003) 1731. [98] S. Polarz, A. Roy, M. Lehmann, M. Driess, F.E. Kruis, A. Hoffmann, P. Zimmer, Adv. Funct. Mater. 17 (2007) 1385. [99] S. Polarz, J. Strunk, V. Ischenko, M. van den Berg, O. Hinrichsen, M. Muhler, M. Driess, Angew. Chem. 118 (2006) 3031. [100] D. Schroeder, H. Schwarz, S. Polarz, M. Driess, Phys. Chem. Chem. Phys. 7 (2005) 1049. [101] S. Polarz, A. Orlov, F. Schüth, A.H. Lu, Chem. Eur. J. 13 (2007) 592. [102] A. Orlov, A. Roy, M. Lehmann, M. Driess, S. Polarz, J. Am. Chem. Soc. 129 (2007) 371. [103] S. Polarz, A. Orlov, A. Hoffmann, M.R. Wagner, C. Rauch, R. Kirste, W. Gehlhoff, Y. Aksu, M. Driess, M.W.E. van den Berg, M. Lehmann, Chem. Mater. 21 (2009) 3889. [104] M. Van den Berg, S. Polarz, O.P. Tkachenko, K.V. Klementiev, M. Bandyopadhyay, L. Khodeir, H. Gies, M. Muhler, W. Grünert, J. Catal. 241 (2006) 446. [105] S. Polarz, R. Regenspurger, J. Hartmann, Angew. Chem., Int. Ed. 46 (2007) 2426. [106] K. Merz, S. Block, R. Schoenen, M. Driess, Dalton Trans. (2003) 3365. [107] K. Merz, H.M. Hu, S. Rell, M. Driess, Eur. J. Inorg. Chem. (2003) 51. [108] A. Roy, S. Polarz, S. Rabe, B. Rellinghaus, H. Zahres, F.E. Kruis, M. Driess, Chem.-Eur. J. 10 (2004) 1565.