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Title: Graphene oxide material interfaces in electronics, energy and environmental membranes
Author: Smith, Christopher T.
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2016
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The wonder material graphene promises to revolutionise countless applications as a result of its remarkable properties. However, an inability to process graphene in aqueous solution inhibits its potential for use in mass produced practical applications. This thesis investigates the use of graphene oxide (GO), a solution processable graphene based material produced from graphite in various applications. GO can be used to produce a graphene like material, reduced graphene oxide (rGO) which is also investigated. The scale up of GO is examined, showing no discernible difference to the final product with increasing batch sizes. By varying the concentration of oxidisers in the synthesis method, a series of GO materials with differing degrees of disorder and solution process ability are produced and characterised. Using this series of GO materials, a model is proposed to explain the effect of the oxidation process on the GO flakes. Using Raman spectroscopy, a decrease in average sp2 cluster size to approximately 1.2 nm is shown. This causes an increase in internal stress and structural disorder resulting in the broadening of the characteristic D and G peaks from 47 cm-1 and 26 cm-1 to 118 ± 6 cm-1 and 72 ± 5 cm-1, respectively. Thermogravimetric analysis (TGA) results confirm this increase in disorder, showing a decrease in the thermal decomposition temperature in air from 700oC to 450oC as oxygen in the atmosphere preferentially target sites of disorder. This analysis is used to determine the disorder present in a range of rGO samples, to determine the best material for use in various applications. The disorder present in GO also isolates sp2 clusters, resulting in an increase in the band gap ranging from 0.02eV to 3.4 eV, while the reduction methods tested restore conjugation between isolated regions, causing the band gap to drop significantly. GO is used as a hole transport layer in high efficiency organic photovoltaic (OPV) devices, producing power conversion efficiencies (PCE) of approximately 5% using the polymer blend Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]: Phenyl-C70-butyric acid methyl ester (PCDTBT:PC70BM) as the active layer. This represents an increase of 90% (+ 2.4% PCE) over devices without a hole transport layer, and results in similar efficiencies to devices using the standard material PEDOT:PSS. Additionally, due to the chemical stability of GO, the shelf lifetime of GO OPV devices is improved by 62% (+ 3200 hours) when compared with a reference PEDOT:PSS device. ii Using a low temperature (< 250oC) simultaneous spray coating chemical reduction method; GO films are sprayed and chemically reduced on a surface using vitamin C, while the conductivity is monitored in real time. This allows for a conductivity increase of 5 orders of magnitude, resulting in thin films with 16.68kΩ/□ sheet resistance and 66.8% transmission (measured at 550nm). This conductivity ratio, i.e. the electrical conductivity divided by the optical conductivity (ςDC/ςOp), of 0.05 for the devices is comparable with other rGO based conducting networks reported, produced without the need for high temperatures, treated substrates or toxic reducing agents, making it practical for use on flexible plastic substrates. Importantly, Raman analysis of the thin films suggests that the conductivity of the rGO thin film is only partially limited by the disorder of the individual rGO flakes, with a significant proportion of the resistance originating from another source, i.e. flake to flake junctions. GO materials are tested as environmental membranes for the adsorption of the textile dye Rhodamine B (RhB) absorbing as high as 106.5 mg per gram of GO adsorbent. Initial results suggest a link between the interlayer distance of GO based materials and their ability to quickly adsorb the dye. It is shown that by using a partially oxidised GO material, it is possible to adsorb the dye quickly (approx. 60 - 100 mg of dye adsorbed per gram of GO in 60 minutes), while minimising the GO left in solution (below 10 ppm stable in solution after 52 hours) reducing the likelihood of causing contamination. Furthermore, rGO based porous sponges are synthesised and, using SEM and X-ray CT, shown to be porous throughout, reducing the likelihood of contamination further because of the hydrophobicity associated with rGO materials. Additionally, a hybrid rGO based material is synthesised, which contains iron nanoparticles of approximately 25 nm in diameter, encased in an iron oxide shell and impregnated in rGO sheets. This material (Fe-rGO) is shown to be magnetic, and is used in both OPV applications and for environmental adsorption. Finally, Fe-rGO porous sponges are produced, which could be revolutionary for use in environmental remediation. The magnetic properties allow for the adsorbing Fe-rGO to be removed from solution after adsorption, allowing 99% of the RhB dye to be recovered through elution in ethanol.
Supervisor: Silva, S. R. ; Stolojan, V. Sponsor: Defence, Science and Technology Laboratory (DSTL)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available