Impact of synthesis conditions on the morphology and crystal structure of tungsten nitride nanomaterials

Abstract

Nanocrystalline tungsten nitride (WN_x) aggregates and nanosheets are synthesized with a new alkylamine-based synthesis strategy for potential applications in nanoelectronics and catalysis. These applications preferentially require crystalline materials with controlled morphology, which has been rarely demonstrated for WN_x nanomaterials in the past. In the synthesis approach presented in this work, the morphology of nanoscale WN_x is controlled by long-chained amines that form lyotropic or lamellar phases depending on the surfactant concentration. The structural and chemical properties of the WN_x nanomaterials are studied in detail using different electron microscopic techniques in combination with electron spectroscopic analyses. Material synthesis and sample preparation for transmission electron microscopy (TEM) were performed in an argon atmosphere (Schlenk line and glovebox). The samples were inserted into the electron microscope via an air-tight TEM transfer holder to protect the material from hydrolysis and oxidation. From the lyotropic phase nanocrystalline WN_x aggregates were obtained, which consist of 2.4 ± 0.8 nm small crystallites of the cubic WN_x phase with a composition of WN_0.7. The lamellar phase with a higher surfactant concentration yields WN_x nanosheets with lateral dimensions up to 500 nm and a mean thickness of 2.1 ± 1.1 nm. The nanosheets are N rich with a composition WN_1.7–3.7 and occur in the hexagonal crystal structure. The nanosheets are often stacked on top of one another with frequent rotations of 4–6° around the hexagonal c axis, thereby forming commensurate interface structures between nanosheets. High stacking-fault densities and signs of nanotwins can be repeatedly observed in WN_x nanosheets.