Vertically aligned carbon nanostructures using ECR plasma and their applications

Abstract

Vertically aligned carbon nanostructures are a type of carbon nanomaterial in which nanostructures are aligned lengthwise perpendicular to the plane of the substrate. In this thesis work, two vertically aligned carbon nanostructure are studied both of which are synthesized by plasma enhanced chemical vapour deposition (PECVD) using electron cyclotron resonance (ECR) plasma system. The first material, which is studied in the first part of the thesis, is a thin film of vertically aligned graphene nanosheets (VAGNs). In VAGNs, few layers of graphene nanosheets are aligned vertically to the substrate while being interconnected to each other giving the material a porous and a three-dimensional surface morphology with graphene nanosheets having very good electrical contact with the substrate. Electrochemical performance study of VAGNs thin films deposited on copper current collectors has been done as the anode of lithium ion battery. The effect of post-deposition plasma treatment on the electrochemical performance of the VAGNs thin films is then studied. Post-deposition argon plasma treatment has shown an improvement in the delithiation specific capacity of the material due to modification of the surface morphology from 43.9 μAh cm-2μm-1 to 68.1 μAh cm-2μm-1. The performance of the VAGNs thin films as an electrode of the electrochemical capacitor is also studied. The material deposited on stainless steel showed a specific capacitance of 0.98 mF cm-2 at the current rate of 1 mA (0.88 mA cm-2) for the VAGNs. As the material has a porous textured surface morphology, a study of the change in wettability of this hydrophobic material as a result of a change in the structure and the morphology is undertaken. VAGNs thin films showed a tunable hydrophobicity via the surface geometry of the material. It showed a maximum contact angle for water of 135o and exhibited an ideal Cassie mode of wetting. In the second part of the thesis work, synthesis of a new type of carbon nanostructure is reported in which individual nanostructures are tree-like in morphology and each tree-like nanostructure is vertically aligned on the substrate. Each nanostructure has a multiwalled carbon nanotube like “tree- trunk” with carbon films attached to it like “branches” in a tree. The nanostructure has the advantage of having a more exposed surface area along with all the interesting properties of well-studied vertically aligned carbon nanotubes. To exploit the extremely high surface area of interaction and the good electrical contact of the nanostructured thin films, the films are studied as the anode of the lithium-ion battery and as an electrode of electrochemical capacitors. In the lithium-ion battery, it has shown a specific capacitance of about 56 μAh cm−2μm−1 at a current of 10 μA (8.85 μA cm-2) and for electrochemical capacitors, it has shown a specific capacitance of 0.55 mF cm-2 at a current of 1 mA (0.88 mA cm-2). As the material has two-scale roughness, the material is studied for hydrophobicity and it has exhibited a maximum static contact angle with water of 165o and a completely non-sticking nature. This class of aligned carbon materials has a regular geometrical pattern with a surface morphology which can be controlled, both via deposition parameters and post-deposition treatments and have its properties directly related to their surface geometry. The high surface area with good electrical contact and the textured patterning gives these types of materials applicability in different fields successfully.