Perspectives on the Growth of High Edge Density Carbon Nanostructures: Transitions from Vertically Oriented Graphene Nanosheets to Graphenated Carbon Nanotubes

Abstract

Insights into the growth of high edge density carbon nanostructures were achieved by a systematic parametric study of plasma-enhanced chemical vapor deposition (PECVD). Such structures are important for electrode performance in a variety of applications such as supercapacitors, neural stimulation, and electrocatalysis. A morphological trend was observed as a function of temperature whereby graphenated carbon nanotubes (g-CNTs) emerged as an intermediate structure between carbon nanotubes (CNTs) at lower temperatures and vertically oriented carbon nanosheets (CNS), composed of few-layered graphene, at higher temperatures. This is the first time that three distinct morphologies and dimensionalities of carbon nanostructures (i.e., 1D CNTs, 2D CNSs, and 3D g-CNTs) have been synthesized in the same reaction chamber by varying only a single parameter (temperature). A design of experiments (DOE) approach was utilized to understand the range of growth permitted in a microwave PECVD reactor, with a focus on identifying graphenated carbon nanotube growth within the process space. Factors studied in the experimental design included temperature, gas ratio, catalyst thickness, pretreatment time, and deposition time. This procedure facilitates predicting and modeling high edge density carbon nanostructure characteristics under a complete range of growth conditions that yields various morphologies of nanoscale carbon. Aside from the morphological trends influenced by temperature, a relationship between deposition temperature and specific capacitance emerged from the DOE study. Transmission electron microscopy was also used to understand the morphology and microstructure of the various high edge density structures. From these results, a new graphene foliate formation mechanism is proposed for synthesis of g-CNTs in a single deposition process.

Flipping growth orientation of nanographitic structures by plasma enhanced chemical vapor deposition

“…Since then, several groups have explored the growth of VGNs on catalytic as well as non-catalytic metal / dielectric surfaces using various plasma based techniques such as microwave PECVD, dc plasma discharge, ICP-plasma and thermal plasma jet systems. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] In an excellent review, Bo et al 11 summarized the influence of various key process parameters on the synthesis and growth model for NGSs on various types of substrates. In spite of several existing reports, herein we note that, still there is no unique theory that could unveil the atomistic growth mechanism and give a prescription to optimize the process parameters for a given plasma source.…”

Role of nanocrystalline domain size on the electrochemical double-layer capacitance of high edge density carbon nanostructures

“…2(a) contains the highest density and the thinnest nanosheets of all the samples studied, indicating that the crystalline domain size is expected to be the smallest. TEM reported elsewhere [29] has shown that such structures terminate in few-layered graphene, down to two or three layers. The CNS structure in Fig.…”

“…It is intriguing that the nanocrystalline domain sizes are so similar for these samples, which could imply that FLG may nucleate at the sites of previously existing defects in support of the proposed nucleation and growth model for graphene foliates. [29,33] Interpretation of the nanocrystalline domain size calculation…”

“…Possible mechanisms for this formation have been discussed elsewhere. [28,29] In contrast to the hightemperature conditions of g-CNT growth, which results in a high foliate density but poor nanotube alignment, vertically aligned graphenated carbon nanotubes (VA g-CNTs) were achieved by reducing deposition temperature to 900°C and extending deposition time to 30 min. Hierarchical structures of few-layered graphene and carbon nanotubes were created by further extending the deposition time, hereafter referred to as carbon nanosheets on carbon nanotubes or CNS-on-CNTs.…”

“…Data from a previously reported design of experiments (DOE) study of MPECVD growth [29] was used to compare the I D /I G ratio and specific capacitance of a variety of carbon nanostructures from CNTs to highly defective amorphous carbon (a-C), thus extending the range of nanostructured carbons beyond those depicted in Fig. 2.…”