Three-dimensional frameworks of gallium selenide supertetrahedral clusters.

Dense chalcogenides of Ga, In, and Tl have been extensively studied because of their intriguing stoichiometry and because many of them are semiconductors, photoconductors, or light emitters. On the other hand, while crystalline porous oxides have been known for a long time, the development of crystalline porous chalcogenides, especially selenides and tellurides, is still in its early stages. Compared to porous oxides, which are usually insulators, crystalline porous chalcogenides can have substantially higher electrical conductivity, which in combination with uniform porosity could lead to new applications in areas such as shapeand sizeselective sensors and high-surface-area photocatalysts and photoelectrodes. One promising approach for the creation of crystalline porous chalcogenides is the directed assembly of chalcogenide clusters into three-dimensional (3D) frameworks. Among the most common chalcogenide clusters are a series of supertetrahedral clusters denoted as Tn. These clusters are regular tetrahedrally shaped fragments of the cubic ZnS-type lattice. Here, n is the number of metal sites on each edge of the cluster (Figure 1). The large size of the supertetrahedral clusters usually leads to a highly open framework, particularly when the interpenetration of multiple sublattices is avoided. Substantial success has recently been achieved with sulfides, but the progress with selenides is much slower. Not only are there few 3D open-framework selenides, the types of supertetrahedral selenide clusters are also limited. For example, although T5 sulfide clusters with as many as 35 metal atoms (e.g., Cu5In30S56 17 ) are known, the largest supertetrahedral selenide cluster prior to this work has only ten metal atoms (T3, In10Se20 10 ). Larger clusters are desirable because they can be useful for studying the quantum size effect and can also serve as building blocks for constructing crystalline porous frameworks. The gallium selenides are particularly challenging. Thus far, neither T3 (e.g., Ga10Se20 10 ) nor T4 (e.g., Zn4Ga16Se35 14 ) clusters are known. To our knowledge, no 3D gallium selenide open frameworks were known prior to this work. Here we report a series of 3D gallium selenide superlattices built up of supertetrahedral T3 and T4 clusters. Three 3D framework types (denoted as OCF-1ZnGaSe, OCF6GaSe, and OCF-13GaSe, respectively; OCF= organically directed chalcogenide framework) were realized (Table 1). Similar to the synthesis of zeolites, the preparation of chalcogenide open frameworks is performed in an alkaline environment under hydrothermal conditions at 200 8C or below. However, unlike the synthesis of zeolites, the selfassembly process in the chalcogenide system is usually preceded by redox reactions because one or more starting materials (e.g., S, Se, In) are in the elemental form. One of the most important advances reported here is the synthesis of the selenide T4 cluster Zn4Ga16Se35 14 in OCF1ZnGaSe. This is the largest selenide supertetrahedral cluster made so far. Prior to this work, the largest selenide supertetrahedral clusters were the indium selenide T3 cluster In10Se20 10 and ligand-terminated T3 clusters such as [(CH3)4N]4[Cd10Se4(SPh)16]. [13,17] The increase in the cluster size from T3 to T4 represents a significant advance towards the synthesis of large selenide supertetrahedral clusters such as T5 (M35X56) or T6 (M56X84). [7] Because of their uniform size and well-defined chemical composition, these supertetrahedral clusters could provide a unique opportunity for the study of quantum-confinement effects. Surprisingly, all three gallium selenide phases reported here are noncentrosymmetric, and none of them consists of two or more of the interpenetrating lattices that are common for these types of materials. This structural feature is also in contrast with the corresponding Ga–S system, in which all Figure 1. Polyhedral diagrams of T3 and T4 supertetrahedral clusters. The metal site is located at the center of each small red tetrahedron.