Raman study on the pressure-induced phase transformation of nanographite at room temperature

Abstract

Pressure-induced phase transformation of graphite at room temperature has attracted considerable interest for understanding the mechanism and the pathways of its structural transformation. The transformation can be influenced by the crystallites or nanographite domains in graphite samples with crystal defects. However, few systematic studies have been conducted on the effects of nanographite on transformation. In this study, we use carbon nanowalls (CNWs) composed of nanographite domains with different sizes to present a Raman study on the phase transformation of nanographite under compression and decompression at room temperature. The pressure-induced phase transformation is clearly observed in CNWs as sp2 to sp3 transformation in graphite at room temperature. The compressed phase returns to the initial phase via decompression; however, transformation under compression and decompression exhibits a large hysteresis. The transition pressure under compression decreases with a decrease in the size of the nanographite domains. The smaller the domain size, the easier it is for the phase transition to occur. The phase transition can be attributed to the sp2 to sp3 transformation induced by sliding graphene layers in nanographite. The domain boundaries in CNWs can serve as a nucleation site of sp3 formation in phase transition and as a buffer for shrinking or expanding nanographite associated with phase transition. Our findings can provide important insights into understanding the phase transition of nanographite and for synthesizing new materials.