Graphene based thin films for photocatalytic and optoelectronics applications

Abstract

The composite materials of metals, metal oxides or polymers with different forms of graphene like pristine graphene (GN), graphene oxide (GO) and reduced graphene oxide (rGO) have attracted tremendous attention worldwide due to their potential applications in various fields such as optical, photochemical, electrical, electrochemical, and environmental applications. However, besides the importance of this noble material in energy and environmental sector, there are still few reports on the fabrication of GO and rGO thin films through the combination of chemical and physical routes, which are spin coating and direct current radio frequency sputtering. The detailed characterization analysis and further investigation on the applications of GO and rGO thin based films fabricated by aforementioned methodology has poorly been reported. Therefore, this study focused on the synthesis of GO, rGO and their composites with zinc oxide (ZnO), copper (Cu), nickel oxide (NiO) by the combination of spin coating, low temperature thermal annealing and direct current radio frequency (DC/RF) sputtering technique for wastewater treatment and optoelectronics applications. The 2-chlorophenol (CPs) was selected as a model pollutant due to its high cytotoxic, mutagenic carcinogenic properties threating the living beings. Initially, GO was synthesized by modified Hummers’ method and afterwards, spin coating technique was used to deposit the GO thin films on glass substrate. The thin films deposited at different GO concentrations (1.6, 3.2, and 4.8 mg/mL) were qualitatively analyzed and further optimized having unique structural, optical, chemical states for further applications in environment and optics. The band gap of thin films was dependent on GO concentration which were 2.98, 2.86, and 2.71 eV for 1.6, 3.2 and 4.8 mg/mL of GO, respectively. The refractive indices were in the range of 1.35 to 1.58 depending on the GO concentration. The optimized GO thin films were further reduced to rGO by low temperature reduction. The reduction was done at different temperatures from room temperature to 300 °C. The band gap decreased from 4.10 to 2.41 eV with the formation of conjugated sp2 network. The increase in sp2 network from 36.57 to 68.71 % while the decrement of sp3 from 32.06 to 18.56% in rGO thin films also evidently proved the restoration of GN like properties in it. The optimized rGO thin films were further used to fabricate their composites with ZnO through DC/RF. The degradation efficiency (2-CP) increased from 44 to 74%. Further, GO/rGO nano composites thin films with NiO and Cu were synthesized to obtain their uniform thin films by controlling the deposition parameters of direct current radio frequency sputtering technique. The sp2 hybridization in rGO caused more dielectric loss as compared to GO because of its conductive path ways in Cu-ZnO. GO and rGO did not affect the preferred structural orientation of Cu-ZnO, and NiO. Sp2 network of GO/rGO also assisted their desirability for 2-CP which ultimately led to their degradation. The efficiency Cu-ZnO/rGO was 75% while in case of NiO composites it was found to be from 46 to 77 % depending on the NiO particle size. In summary, the physical and chemical properties of GO and rGO were improved in their thin film composites with ZnO, NiO and Cu which enhanced their capability as photo-catalyst which could be highly useful for building next generation devices in the field of optoelectronics and waste-water treatment.