Applications of high surface area carbon nanomaterials

Nufer, Sebastian (2019) Applications of high surface area carbon nanomaterials. Doctoral thesis (PhD), University of Sussex.

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In recent years carbon nanomaterials have shown great potential in various applications; among them gas sensing and supercapacitor electrodes. Devices reported in the literature, which are built to exploit the properties of the nanomaterials are often based on expensive multistep processes. As nanomaterials start to be commercially available, the remaining challenge is to find scalable fabrication methods to take advantage of the nanomaterial characteristics. In this thesis, manufacturing techniques and materials are presented which enable technically and commercially interesting gas sensors based on nanomaterials with interdigitated electrodes.Graphene flake dispersions are readily available on the market in large quantities. Currently most available materials are actually multilayer graphene rather than single pristine sheets of carbon atoms. Flake sizes are typically below 1 µm preventing the readout of individual sheets. Nevertheless these small graphene sheets can be made to form a percolating film when dispensed and compressed in a L-S trough. Such films can be transferred to readout electrodes which have been pre-patterned by laser ablation. This simple three step process creates a sensor which is read-out by resistance measurement.

In this work, sensors have been manufactured, with this device architecture, showing performance compatible with personal safety applications. Their detection limit of 1 ppm for NH3 with a response time of 90 seconds competes with current commercial systems. Investigation into the sensing mechanism of these percolating films reinforces the scalability of the fabrication method. Using reducing (NH3) and oxidising (acetone) gases we determined the origin of the sensor’s response. KPFM and Raman analysis show basal plane doping of the graphene sheets in opposite
direction when exposed to the reducing and the oxidising agent. The resistance of the film increases in both cases when exposed to the gases. This leads to the conclusion that the dominating sensing mechanism in the percolating graphene film takes place at the contacts of the flakes in the film. This sensing mechanism allows the use of even relatively low quality graphene flakes to make effective devices. Defects in the basal plane play a negligible role within the films as the edge properties are responsible for the adsorption.

Another, less well-investigated nanomaterial with gas sensing properties is carbon nanofoam (CNF). CNF is formed in a diffusion limited aggregation process where individual carbon clusters form elongated networks with a ”web”-like appearance. Synthesis of CNF and deposition onto readout electrodes in a single step is made possible by using a pulsed laser. The CNF is formed during the interaction between the laser and a precursor material, positioned close to the electrode onto which the CNF is to be deposited. This direct deposition method makes a film with good adhesion and uniformity. Metal functionalization using physical vapor deposition can be applied to the CNF in order to modify its sensing properties. Preliminary gas sensing studies, exposing both CNF and metallised CNFs to a changing humidity environment show responses of +70 % for CNF and -30 % for metallized CNF when the humidity is decreased by 17%. This is among the highest reported response of carbon nanomaterials. The reaction of the CNF to dry air changing environment only takes 13 seconds to reach steady state while the functionalised material takes 32 seconds. The metallization of the CNF follows the percolation law as the foam acts as a scaffold for the sputtered metal particles. The sign of the response when exposed to a dry environment inverts when the percolation threshold is crossed. At the threshold the sensitivity to a change in humidity is suppressed. As water is a major interferent for carbon nanomaterials in gas sensing devices it is important to find ways to suppress the response of a device towards water.

CNF has also been used to make supercapacitors and their behaviour has been characterised.

Characterising the CNF in supercapacitor electrodes reveals a large influence of the gravimetric capacitance on the mechanical properties of the foam. Supercapacitors were made using directly-deposited CNF (as above) and also by transfer through a water sub-phase. The CNF lifts from the substrate when immersed into water and can then be picked up with another substrate. This transfer not only induces a change in morphology but also introduces a compressional stress. These effects more than double the gravimetric capacitance from 17 Fg−1 to 42 Fg−1. The fragility of the CNF that this reveals indicates the necessity of a one-step deposition process as the properties of the CNF are easily changed if mechanical force is applied. Nevertheless the transfer enhances the specific capacitance greatly.

In summary, laser-deposition technology and a combination of laser ablation and L-S deposition allows scalable fabrication of gas sensor devices based on carbon nanomaterials. Similar devices show interesting supercapacitive properties but they do not, so far, approach the state of the art as defined by gravimetric capacitance.

Item Type: Thesis (Doctoral)
Schools and Departments: School of Mathematical and Physical Sciences > Physics and Astronomy
Subjects: T Technology > T Technology (General) > T0174.7 Nanotechnology
Depositing User: Library Cataloguing
Date Deposited: 31 May 2019 13:04
Last Modified: 31 May 2019 13:04

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