A study of the characteristics of graphene oxide films irradiated by an Nd:YVO₄ nanosecond pulse laser to form reduced graphene oxide

Saul, Jonathan Matthew

Abstract

In recent years considerable research focus has been directed to graphene like materials that display properties that are similar to the excellent physical, chemical and mechanical characteristics of graphene. In this respect, one major research area is the reduction of graphene oxide (GO) and especially the laser reduction of graphene oxide.

The main aim of this thesis was to develop, construct and test an experimental system to create laser reduced graphene oxide films, and to classify the degree of reduction by measured changes in the surface characteristics, conductivity and resistance, and chemical composition of the surface. The objective was to achieve laser reduced graphene oxide with optimum high-quality graphene like features.

A system was developed using an Nd:YVO₄ laser with wavelength 1064nm and was successfully used to create reduced graphene oxide with the findings compared, with some success, to theoretical predictions and those of other researchers.

Many researchers have completed major experimental programmes that involve time consuming, expensive and repetitive tests where systematic and incremental changes are made to a large number of variables to establish the optimum characteristics of the degree of reduction. To avoid such repeat testing a primary objective of the thesis was to develop a system that yielded the optimum information from a single test. Such an experimental setup was successfully developed, and the resultant pattern of reduced graphene oxide was termed a z-scan.

The z-scan pattern was created by a range of fluence values which ranged from below the threshold value to create reduction through to those that caused full ablation of the surface. Good agreement was observed between the experimental results and the predicted pattern of the damage threshold which was observed at a fluence of 13.8 mJcm⁻². To better understand the quality of the reduced GO at points over the surface of the z-scan a further series of tests with large area irradiated surfaces at different fluence values were completed.

The roughness of these irradiated large area samples was independent of fluence but was enhanced by an order of magnitude when compared to that measured for GO. The conductivity of the irradiated sample was shown to increase with increase in laser fluence in an almost linear way.

Raman spectra demonstrated the usual D, G and 2D peaks and, in line with other researchers the reduction was interpreted by the ID/IG and I2D/IG ratios. As fluence was increased the relationship between the ID/IG ratio had an overall downward trend in the range 1.21 to 0.75 with a flattening of the relationship at 0.75 corresponding to a fluence circa 35 mJcm⁻². Examination of the I2D/IG ratio showed that there was a gradual increase from a value of 0.23 to 0.31 but in this case with a saturation value at a fluence circa 32 mJcm⁻². Hence it was concluded that a fluence in the range 32 mJcm⁻² to 35 mJcm⁻² resulted in the optimum reduction of the GO. Similarly, the XPS results recorded a significant change in the carbon and oxygen species in the GO and rGO. The oxygen content was shown to reduce significantly at a fluence of ~35 mJcm⁻² with a flattening off at higher values of fluence to a constant value. A corresponding increase in the C-C carbon occurred and the carbon/oxygen peak ratio increased significantly in the range 29.1 mJcm⁻² to 36.5 mJcm⁻² with an overall increase from 3.5 at a fluence of 21.8 mJcm⁻² to 13 at a fluence of 46.0. These results confirm that at a fluence in the range 30 – 35 mJcm⁻² the degree of reduction is sufficient to transform the carbon and oxygen species with the reduced GO having graphene like characteristics.

Temperature changes were also identified as a major factor to influence the reduction process and a temperature model, based on the theory of Yakovlev et al., 2019, has been described. The model predicted significantly high values at the centre of the laser beam but there was a rapid reduction in temperature in a radial direction away from the centre. Good agreement was observed between the predicted temperature and the boundary of reduced GO of the z-scan which was shown to occur at a temperature of circa 210°C.

Tests were also completed to establish the wettability of the irradiated surface and it was observed that the wettability contact angle increased with increase in laser fluence. Values increased in an almost linear way, but values increased from 46° for GO to a maximum value of 84° for the rGO. These values compare well with the results of other researcher who have used similar irradiated surfaces. As the surface roughness was independent of fluence the increases in contact angle were attributed to the changes in the chemistry of the surface. Based on the outputs of the surface chemistry analysis for optimum reduction at a fluence in the range 32 mJcm⁻² to 36 mJcm⁻² the corresponding contact angle was circa 80°.

In summary it was concluded that the experimental facility successfully met the overall aim of the thesis and provided an efficient and reliable one step technique to establish the optimum range of laser fluence to create good quality rGO, supported by physical, electrical and chemical results.

Citation

Saul, J. M. (2019). A study of the characteristics of graphene oxide films irradiated by an Nd:YVO₄ nanosecond pulse laser to form reduced graphene oxide. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4222440

• Thesis Type: Thesis
• Deposit Date: Jun 19, 2020
• Publicly Available Date: Feb 23, 2023
• Keywords: Physics
• Public URL: https://hull-repository.worktribe.com/output/4222440
• Additional Information: School of Mathematics and Physical Sciences, The University of Hull
• Award Date: May 1, 2019