Synthesis of Fullerene‐like Cs2O Nanoparticles by Concentrated Sunlight

Abstract

While the first reported fullerenes and nanotube structures were composed of carbon, it was soon recognized that a plethora of comparable inorganic candidates should also exist. Because nanoparticles of compounds with a layered (2D) structure are unstable against folding, they should be able to form nanotubes and closed-cage inorganic fullerenelike (IF) structures. The first such nanostructures were identified in WS2 [4] and MoS2. [5] A rich assortment of IF nanostructures have been synthesized, and are finding practical uses in tribology, photonics, batteries, and catalysis. Among inorganic molecules that can achieve fullerene-like nanostructures, cesium oxide (Cs2O) [7] occupies a doubly prominent status. First, it is unique among known binary-alkali compounds in possessing a layered 3R-Cs2O anti-CdCl2 structure (powder diffraction file (PDF) no. 09-0104), where 3R denotes a unit cell composed of three molecular layers with rhombohedral symmetry. This suggests the likelihood of forming closed polyhedra or tubular shapes. The affiliated absence of dangling bonds should result in an intrinsically low reactivity of fullerene-like Cs2O nanoparticles. Second, overlayers of Cs2O reduce the work function of various photonic devices. This property renders films of such materials particularly useful for a multitude of applications in photoemissive systems, for example, photocathodes, negative-electron-affinity devices, image intensifiers, discharge lamps, television cameras, lasers, and catalytic converters. Unfortunately, Cs2O is extremely reactive in the ambient atmosphere, so its production and handling require high vacuum and very pure inert conditions. The dearth of chemical stability of Cs2O translates into problematic and expensive manufacturing and handling, which limits its technological scope and device lifetime. These realities motivate the quest for relatively uncomplicated highyield syntheses for chemically stable IF-Cs2O. Recently, the first synthesis of IF-Cs2O nanoparticles was described, based on continuous laser ablation of pure 3R-Cs2O powder in evacuated quartz ampoules. [11] The minute quantities generated encumbered both optimization of the procedure as well as detailed studies of material structure and properties. The expense of laser photons further militates against economic large-scale viability for this process. These considerations prompted the search for an alternative practical pathway to prepare IF-Cs2O. In the present work, experimental results for the exploitation of highly concentrated solar radiation (ultrabright incoherent light) toward that end are described. The pyrolytic solar ablation of the 3R-Cs2O precursor, in the absence of any other reagents, yielded considerable amounts of closed-cage (IF) nanoparticles as confirmed by transmission electron microscopy (TEM) and highresolution TEM (HRTEM). The solar-driven synthesis of IF-Cs2O was performed directly in evacuated quartz ampoules that contained 3R-Cs2O crystallites (see Experimental), under continuous irradiation with a concentrated solar power of 2.0–7.7 W and periods ranging from 30 to 840 s. A schematic of the solar ablation system and a photograph of the irradiation platform used in this work are presented in Figure 1a and b respectively. A transmissive optical fiber channeled high-flux sunlight from an outdoor minidish concentrator to the indoor lab bench. The fiber tip was held in close contact with the ampoule’s quartz wall. The corresponding flux values at the distal fiber tip were 2.5–9.8 W mm. (Flux values incident on the Cs2O C O M M U N IC A IO N