GRAnPH® Nanotech, in collaboration with CENER (Spanish National Renewable Energy Center), have developed continuous graphene oxide (GO) and reduced graphene oxide (rGO) thin films over a variety of substrates by means of two different industrial deposition techniques: spin - coating and spray coating.
Graphene, an individual sheet of sp2-hybridized carbon atoms, has attracted increasing interest due to its unique electronic, mechanical and thermal properties, making it interesting for a wide variety of applications. However, the introduction of graphene, graphene oxide and reduced graphene oxide films in industry is limited nowadays due to its elevated production cost.
Spin coating is a film deposition technique already introduced in a variety of industries such as microelectronics and coating technologies, whose benefits include accurate thickness control, high deposition uniformity, fast process time and low equipment cost. However, substrates size and shape are the principal drawbacks to implement the technique at industrial scale. Spray coating is a deposition technique widely used in industry for multiple applications due to its fast process time, low equipment cost and versatility. As a drawback, the technique provides lower thickness control compared with spin-coating. The deposition of GO films by means of these techniques would represent a plausible alternative for the industrial application of graphene with low production costs.
Scientists at GRAnPH® Nanotech have developed an industrial process to obtain single sheets of GO based on the Hummers method. The GO sheets are stable in water suspension at several concentrations for months. Using that suspension, scientists at the Spanish National Renewable Energy Center (CENER) have deposited GO by means of spin coating and spray coating over a wide variety of substrates, with two main objectives: to completely characterize the GO material and its suspensions developed at GRAnPH® Nanotech facility; and to develop a process capable to provide a continuous and homogeneous film onto different materials surfaces.
GO suspensions have been deposited onto opaque substrates, such us silicon dioxide and silicon nitride deposited onto a silicon wafer; transparent materials like glass and different plastic materials. While the spray coating process can be easily applied to any kind of surface (regarding material properties and shape), spin coating process requires an individual optimization depending upon the material surface nature.
The GO deposits have been morphologically characterized by means of a wide variety of techniques, such as optical microscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Additionally, GO deposits on opaque substrates have been optically characterized by means of reflectivity measurements, and deposits on transparent substrates by means of transmittance measurements. GO thin films have been electrically characterized by means of a four point probe technique.
Continuous GO thin film deposited by spin-coating on a silicon dioxide/silicon substrate. SEM image (left), AFM image and profiles (right)
As it was expected, GO thin films have a poor conductivity, values higher than 109 Ω/sq were measured. On the other hand, transmittance values measured on the transparent substrates were in the range of 83%.
Most of the graphene applications require high conductivity, and in some specific cases high transmittance too. In order to decrease the sheet resistance of the GO thin films obtained by spray or spin coating techniques, several treatments were tested to obtain rGO thin films.
rGO thin films were characterized using the same techniques as for GO, transmittance and sheet resistance values were used as control parameters in the process development. Sheet resistance values as low as ~ 104Ω/sq have been obtained combined with a transmittance value of 86%. Depending upon the specific application, both sheet resistance and transmittance can be tuned separately.
The results obtained in the joint research show a very high potential for the industrial application of rGO thin films deposited by spin coating or spray coating onto multiple substrates using a GO suspension in water.
Top image shows Single layer GRAnPH® platelet, source: GRAnPH® Nanotech
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Guest writer, GRAnPH® Nanotech