HIGHLIGHT

www.rsc.org/materials | Journal of Materials Chemistry

Aerogels from metal chalcogenides and their emerging unique properties Santanu Bag,a Indika U. Arachchigea and Mercouri G. Kanatzidis*ab DOI: 10.1039/b804011g

Metal-oxide and carbon gels and aerogels have been known for a long time. These aerogels have always been of strong interest because of their intriguing properties and potential diverse technological impact. Thanks to new sol--gel chemistry, chalcogenide nanoparticle and cluster-based aerogels (chalcogels) are the latest aerogels to come to the field and are now gaining notoriety because of properties not available in conventional aerogels. In this article, we highlight the recent synthetic advances in chalcogenide aerogels and their emerging unique properties and discuss some potential applications.

Introduction Aerogels are low-density porous materials with high surface-to-mass ratios, high porosity and large open pores of all sizes. They have a lot of empty space. Such properties are best appreciated in catalysis, particle and protein immobilization, sensing and separations technologies.1 The extremely light polymeric structure (most often silica) in combination with chemical diversity in framework composition offers diverse properties such as high optical transparency, high absorption, improved mechanical stability and outstandingly low thermal and acoustic conductivity.2 Gels are constructed by the random agglomeration of

a Department of Chemistry, Northwestern University, Evanston, IL 60208-3113, USA b Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA. E-mail: [email protected]

primary nanoparticles via a hydrolysis-condensation (sol--gel) reaction, giving a three-dimensional covalently bonded network. This kind of ‘‘solventtemplated’’ procedure produces wet gels (hydrogels or alcogels) of which >95% of their volume is filled by liquid. Replacing the liquid with air without destroying the pore structure generates dry sponge-like aerogels. Dried gels can be obtained as powder, monoliths, fibers and granules. The diversity in chemical composition of primary nanoparticles offers the possibility for construction of purely inorganic and organic polymers as well as organic--inorganic hybrid aerogel systems. Silica aerogels and a number of their different modifications are the most widely studied. A number of other main group and transition metal oxide counterparts have been developed and used for specific applications.3 These materials generally have very wide energy gaps which limits their ability to absorb visible

light without further modification with dopants. On the other hand, non-oxidic aerogels are not well known and only recently have become the subject of investigation.4--6 The long studied carbon aerogels7a are also non-oxidic materials but their synthetic chemistry is limited to only one element. This Highlight will focus on the recent discoveries of chalcogenide aerogels which have broadened the field of aerogel research by opening up new possibilities for observing new phenomena and achieving new applications.

Metal chalcogenide aerogels Kistler7b was the first to report aerogels made of metal-oxide gels which are generally prepared by the hydrolysis of metal precursors and their subsequent condensation reactions. Kistler predicted that the family of aerogels could be limitless because as long as a gel of a given

Santanu Bag is a graduate student at Northwestern University, Evanston. He received his Bachelor degree in Chemistry from Presidency College, Kolkata in 2002 and his Masters degree in Chemistry from Indian Institute of Technology, Kanpur in 2004. He is currently working on the sol--gel synthesis of novel porous chalcogenides for the development of new functional materials. Santanu Bag

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Mercouri G Kanatzidis received his PhD degree in chemistry from the University of Iowa in 1984. He is currently a Charles E. and Emma H. Morrison Professor in Chemistry at Northwestern University. His research interests include metal chalcogenide chemistry, new thermoelectric materials, the synthetic design of framework solids, intermetallic phases and nanocomposite materials. Mercouri G: Kanatzidis

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substance could be prepared, an aerogel could then be produced from it. The problem in going beyond oxides comes from the generally ‘‘un-cooperative’’ chemistry of the systems of interest. Hydrolysis for example, generally does not work with chalcogenides and as a result the stabilization of gels is tricky. Nevertheless, the preparation of chalcogenide gels is possible. It has been achieved by three different synthetic routes: thiolysis,8,9 nanoparticle condensation10,11 and metathesis reactions between soluble chalcogenide clusters with linking metal ions.5

Aerogels from the thiolysis route Thiolysis of alkoxides, thiolates, silylamides and metal alkyl precursors in non-aerobic conditions in the presence of hydrogen sulfide have been shown to yield metal sulfide gels or precipitates. In this approach, formation of a gel structure or a bulk precipitate depends on the reaction kinetics of thiolysis and condensation. Successful tuning of the reaction kinetics was shown to produce a number of amorphous or poorly crystalline gels (LaSx,12 WSx,13 ZnS14 and GeSx15--18). However, the use of thiolysis routes for the production of chalcogenide gel structures is limited to certain metal sulfides, due to the scarcity of suitable precursors, as well as difficulties in handling and syntheses. Recently, Brock and coworkers reported the successful transformation of GeS2 gels made by thiolysis to aerogels.19 These GeS2 aerogels showed an enormous enhancement of the Brunauer--Emmett--Teller (BET) surface area from 478 m2 g
1 to 755 m2 g
1 compared to the originally made GeS2 xerogel and thus further validated Kistler’s postulate7b ‘‘the ability to form an aerogel is a general property of a gel’’.

transparent or opaque wet gels. In this approach, gelation relies on the kinetics of the surface thiolate oxidation, in which active sites for the nanoparticle condensation become available. If excess oxidant is present in the sol, too many active sites are generated on the nanoparticle surface, leading to precipitation, whereas, if too little oxidant is present in the sol, nanoparticles are completely passivated by the thiolates leading to a stable sol. Gacoin et al suggest that at a minimum concentration of oxidant/thiolate (Xmin) no gel is formed whereas above Xmin, gelation occurs.20 Hence, the porosity and the density of a resulting gel network can be fine tuned by varying the amount of oxidant. The generality of this route to prepare a number of chalcogenide gels and aerogels based on nanoparticles of ZnS, CdS, CdSe and PbS was described by Brock and coworkers.4,23--27 Gelation can be induced by a variety of oxidants (H2O2, tetranitromethane and even photo oxidation). Nanoparticles prepared by different synthetic methods have been successfully transformed into wet gels and aerogels.24 That an oxidant is required to cause gelation suggests that the linking of the nanoparticles randomly in three-

dimensions might occur via Se--Se bonds. However, detailed mechanistic study is underway. The obtained metal chalcogenide aerogels are monolithic, consist of a three-dimensional random assembly of colloidal nanoparticles and are morphologically similar to silica aerogels prepared by base catalysis (Fig. 1). These aerogels exhibit bulk densities as low as 0.07--0.35 g cm
3 and are meso to macroporous with BET surface areas up to 250 m2 g
1 (600 m2 g
1 silica equivalence). Even though the nanocrystals are locked into random three-dimensional architectures, the size dependent energetic features are mostly retained as a result of the low dimensionality (low-density) of the network [Fig. 1(b)]. For example, nanocrystals of CdSe when interconnected in a colloidal wet gel network retain their optical band gap.24 However, with increasing dimensionality (density) from wet gels to aerogels to xerogels the band gap of the linked nanoparticles shows a red shift, but compared to bulk CdSe itself the band gap values remain significantly blue shifted (Fig. 2).28 Thus the quantum confined optical features are a characteristic of these chalcogenide gels and are governed by the bulk densities of the networks.

Nanoparticle based aerogels Formation of wet CdS gels by a two step nanoparticle formation and condensation route was initially reported by Gacoin et al.10,11,20--22 In these syntheses, stable CdS nanoparticle sols were prepared by dispersing pre-formed thiolate-capped CdS nanoparticles in acetone. In a second step, partial removal of the thiolate groups from the nanoparticle surface using a chemical oxidant (H2O2) results in

Fig. 1 (a) Transmission electron micrograph of a CdSe aerogel prepared by a nanoparticle condensation route showing the meso to macro porous interconnected network of colloidal nanoparticles. (b) Optical absorption spectra of chalcogenide gels prepared by a nanoparticle condensation route. (c) A photograph of CdS wet gel (centre) and a xerogel (left) prepared by bench top drying and an aerogel (right) dried under supercritical conditions. Part a is reproduced with permission from 2006 American Chemical Society (adapted from ref. 24) and parts b and c are reproduced with permission from 2005 American Association for the Advancement of Science (adapted from ref. 4).

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Fig. 2 An illustration of the effect of network density on the optical properties of gel structures composed of CdSe nanoparticles. Adapted from ref. 26.

The band gap of the linked nanoparticles can be further tuned by heating, resulting in a systematic red shift in the absorption onset with increasing annealing temperature (100--300  C).4,23 This red shift is accompanied by a crystallite size growth and a phase change from cubic to hexagonal for CdS aerogels. Prepared by this approach, CdSe aerogels show weak band-edge emission from the linked nanoparticles. The weak emission is attributed to the residual thiolate ligands acting as traps for the electron--hole recombination.24 When the thiolate ligands are exchanged with pyridine, in the wet gel stage, the band-edge emission is considerably enhanced. Chalcogenide gel formation via nanoparticle condensation is an exciting and emerging field of non-oxidic sol--gel chemistry. This chemical route is general and ensures the production of a particulate gel by separating the nanoparticle formation from the condensation event. The properties of the resulting aerogels combine the characteristic features of nanoparticles (photoluminescence, quantum confinement) as well as the physical state of bulk materials (monoliths, thin films). Hence, the nanoparticulate gels prepared by this route can be used for applications that are insufficiently addressed by the bulk materials or individual quantum dots. The porosity and three-dimensional connectivity of nanoparticles in an aerogel network makes them ideal for experimentation in photo-catalysis, sensing and solar cell 3630 | J. Mater. Chem., 2008, 18, 3628--3632

applications. It should also be possible to make ternary or quaternary composite aerogel materials by simultaneously inducing gelation in a mixture of nanoparticles. The ability to make chalcogenide aerogels using binary II--VI and IV--VI quantum dots provides the means to carry out the nanoparticle condensation in a larger number of chalcogenide systems including metals, elemental and compound semiconductors as well as insulators.

Random network aerogels (chalcogels) The vast majority of metal chalcogenide frameworks are crystalline. Random networks, however, are also known and they make up a relatively large class of materials, the chalcogenide glasses. Therefore, randomness in a metal chalcogenide network is well known and creating it in solution so that a gel can be stabilized should also be possible. Recently, simple metathesis chemistry

was shown to serve as a versatile tool in making metal chalcogenide gels and aerogels.5 Chalcogenide clusters such as [M4Q10]4
, [M2Q6]4
and [MQ4]4
(M ¼ main group metals such as Ge and Sn; Q ¼ chalcogen atom) can be used as building blocks which can bind to metal ions such as Pt2+ to assemble random polymeric networks (chalcogels). Hydrogels generated by metathesis reactions were reported previously where cyanide groups bridge between two metal centres.29 Typically, those materials were formed in a reaction between a tetrachlorometalate (e.g. PdCl42
) salt and a transition metal cyanometalate [e.g. Co(CN)63
] salt. Due to the metal-cyanide framework, those were named ‘‘cyanogels’’. By analogy to the naming of cyanogels, gels obtained from metal chalcogenide building blocks are named ‘‘chalcogels’’ based on all chalcogenide frameworks. The generic structures of the anions used for chalcogel formation are shown in Fig. 3. A number of nuclear magnetic resonance (NMR) studies coupled with mass spectroscopic investigations showed that these units are stable in water often in equilibrium with one another.30,31 The important finding in this chemistry was that by linking the chalcogenide units with metal ion bridges in water leads to gelation and not precipitation. The reason for this is not well understood. One could speculate that the multi-dentate nature of the chalcogenide clusters which can form bonds with metal ions in all space directions, and the slow ligand substitution kinetics of the platinum complexes, combine to stabilize a continuous random open Pt/M/Q network unable to crystallize and fall out of solution. The schematic diagram proposed for chalcogel formation using platinum as the linking metal is shown in Fig. 4. An example of the actual gelation process is shown in Fig. 5. After mixing stoichiometric amounts of aqueous

Fig. 3 Molecular building blocks used for the preparation of chalcogels. M ¼ Ge, Sn; Q ¼ S, Se.

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Fig. 4 Diagram showing gel and aerogel formation.

Fig. 5 Polymerization of the precursor sol ([Ge4S10]4
+ Pt2+) during gelation is shown as a change in color with time.

[Ge4S10]4
and [PtCl4]4
solutions, the light pinkish sol gradually polymerizes with concomitant change in color over time and finally becomes a hard gel (see Fig. 5). A variety of different chalcogels can be obtained. After supercritical drying of the gels the formed aerogels have densities of 0.05 to 0.23 g cm
3 and very high surface areas; see the monolithic aerogel in Fig. 6. Depending upon the starting building block the surface area of the formed chalcogel could be changed, from 327 m2 g
1 to 117 m2 g
1 when the [Ge4Se10]4
or [SnS4]4
unit was used, respectively. The silica equivalence areas are 1674 and 1580 m2 g
1, respectively. BET surface areas for silica aerogels

range from 100 to 1600 m2 g
1. Nitrogen adsorption--desorption isotherms of these chalcogels exhibit a type-IV adsorption branch with a combination of H1 and

Fig. 6 Chalcogel made from a [Ge4S10]4
building unit linked with Pt2+. (a) Monolithic chalcogel before and (b) after supercritical drying shows very little volume loss. Reproduced with permission from ref. 5. Copyright 2007 American Association for the Advancement of Science.

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H3 hysteresis loops, characteristic of randomly interconnected mesopores (2 < d < 50 nm, where d is pore diameter). They exhibit sharp uptake in the low pressure region and absence of saturation in the adsorption branch consistent with presence of both micro (<2 nm) and macropores (>50 nm). The chalcogels can be synthesized with tunable optical band gap structures. The band gap transition of such a system strongly depends on the metal ion (Ge or Sn) and chalcogenide (S or Se) bridges onto the cluster and linking metal ion. Band gap energies spanning from infrared to visible regions can be obtained. Photoexcitation of these visible light absorbing networks could create reactive sites capable of interacting with incoming guest species. The presence of soft chalcogenide atoms defining the porous surface of chalcogels makes them strong absorbers of heavy metal ions. Oxide based zeolites and mesoporous silicates can effectively capture calcium or magnesium ions from water but they have no affinity for heavy metal ions such as Hg2+ or Pb2+. Surface modification by functionalized thiolate ligands32 or self assembled monolayer formation33 is necessary to make the silicas useful for heavy metal ion binding. In chalcogels, the entire chalcogenide

Fig. 7 Schematic illustration of heavy metal ion removal by a chalcogel.

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network functions as a ‘‘sticky’’ surface for heavy metal ion adsorption. Sulfur, for example, has a well known affinity for soft Lewis acids such as Hg2+, Cd2+ and Pb2+. So as these ions enter the chalcogel they inevitably form very strong M--S bonds. This is a classic case of Pearson’s hard/soft acid--base principle34 in action. When a hard ion and a soft ion such as Zn2+ and Hg2+ enter the pore space of Pt2(Ge4S10), the Zn2+ makes it through the material but Hg2+ is captured. A schematic diagram for heavy metal adsorption by a chalcogel is shown in Fig. 7. The high affinity to heavy metals could be useful in cleaning up contaminated water where heavy metal concentrations can be lowered to below drinking safety level. Of course, if this avenue were to be pursued inexpensive elements for the chalcogel will have to be developed. One issue that needs to be resolved is the nature of the Hg2+ ion absorption process, which in all likelihood is an ion exchange process. Analysis of the solution indicates that Pt ions are not involved. Relevant investigations regarding this issue are in progress.

Future prospects The class of aerogels has now expanded to include non-oxidic systems made either of nanocrystalline particles or random covalent amorphous chalcogenide frameworks. The unique porous structure and chemical diversity in framework composition in chalcogenide aerogels are of advantage in catalysis, adsorption and separation technologies. The generality of metathesis chemistry to give chalcogenide gels and the ability to use a variety of chalcogenido clusters and linking metals forecasts a much wider range of nonoxidic aerogels still to come. The chemistry associated with preparing chalcogels is, in principle at least, generalizable to other classes of materials such as nitrides, pnictides, halides, etc. As Kistler7b suggested more than eighty years ago, more classes of inorganic compounds are

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expected to join the aerogel family if they could be made into gels. High internal surface area, tunable optical properties and the presence of catalytic active sites, such as Pt, make these systems promising for future investigations in photocatalytic, heavy metal absorption and gas separation. The solution-based gelation process allows for preparing thin films expanding their potential as membranes as well as chemosensor and supercapacitor devices. Is research in aerogels all about nothing? In a way, yes. It is about the art and science of generating empty space. We need to increase our scientific understanding of the empty space in the pore system as well as the surface and bulk structures that create it and the interactions of external species with it. The expanding chemical diversity of aerogels will give more spectroscopic and experimental handles to study and understand these important issues.

Acknowledgements The authors thank the National Science Foundation (NSF) for financial support.

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