Chapter 5 Synthesis of graphene
5.4. Methods of reducing graphene oxide
Figure 5-11 Yellow-brown appearance of GO solution.
During the washing process, it was observed that the synthesised GO was more black than brown in colour. Prior to centrifugation of the GO suspended in ethanol during the last washing step, visible shiny, floating particles were observed in the mixture. This suggested that unreacted graphite was present in the mixture due to the appearance of the floating particles. This under- oxidation of the material could have been caused by the explosive reaction. The resulting GO solid produced using this method was far darker in colour than those from the other four methods, as can be seen in Figure 5-12 below. The dark, mostly black appearance of the GO produced using this method indicated that the oxidation level of this product was fairly low [230] in comparison to the other four methods.
Figure 5-12 GO obtained via the Huang method after the last ethanol wash.
5.4.1. Reduction by sodium borohydride
The first approach towards reducing GO was the chemical approach. This approach is highly beneficial as it enables the graphene to be deposited onto any suitable substrate from solution [45]. As mentioned in Chapter 3, GO has successfully been reduced via hydrazine hydrate [63, 64] as well as anhydrous hydrazine [36] and sodium borohydride (NaBH4) [74].
While the products resulting from the reduction of GO via hydrazine have extensively been explored and characterised [45, 63, 64], Yang et al. [74] reduced GO through the use of a different reducing agent, sodium borohydride. It has been found that NaBH4 is the more effective reducing agent for GO since the resulting graphene produced using this method has a sheet resistance of 59 kΩ. sq-1 whereas that produced using hydrazine has a sheet resistance of 780 kΩ. sq-1† [70].
Another advantage of this reducing agent over the commonly used hydrazine is that NaBH4 is a nontoxic, noncorrosive and cheaper alternative [74] while hydrazine is toxic and potentially explosive [232, 233]. Another advantage of using NaBH4 over hydrazine is that the experimental procedure is simpler in terms of its execution since the reduction reaction is conducted under ambient conditions [74] whereas the GO solution containing hydrazine must be heated to 100 °C for the reduction reaction to occur [63]. Yang et al. [74] reported that the graphene produced via NaBH4 exhibited low electrical resistance and a wrinkled appearance thus resulting in a smaller average flake size. The small flake size is a disadvantage of reduction via NaBH4 as graphene with a large surface area is desirable for applications such as supercapacitors.
The method for reducing the GO via NaBH4 was as follows: a 1 mg/ml solution of GO and distilled water was prepared and ultrasonicated for 30 minutes. Note that GO does not refer to any of the previously discussed methods in particular as this method of reduction was carried out on each of the five GO samples. This beaker was then placed in a fume cupboard and 4.56 g of NaBH4 was added to the GO solution and left to stir at room temperature for 30 minutes. It was observed that the solution immediately became much darker in colour and bubbled aggressively upon the addition of the NaBH4. This colour change is depicted in Figure 5-13 below which shows the solution of distilled water and GO from the Tour method and the resulting black solution following the reduction reaction. After this period of time, the solution was ultrasonicated for 30 minutes. When the solution was left standing overnight to ensure the completion of the reaction, it was noted that a black precipitate had formed. This is an indication that the reduction of GO was successful since the removal of its surface functionality results in the formation of the hydrophobic, reduced GO [74]. Another ~100 ml of distilled water was added to the solution to wash any residual NaBH4 out from the solution. This solution was then filtered using a vacuum
† Note that these units have been expressed as kΩ. sq-1 rather than in kΩ so as not to represent an ambiguous quantity. The units of kΩ are often associated with the bulk resistance of a material while the units of kΩ. sq-1 indicate the sheet resistance of a thin film [45] [231].
filter. Nylon membrane filters with a pore size of 0.45 μm and diameter of 47 mm were used.
These filters were used since the particle size of the graphene is in the order of micrometres [234].
Also, the nylon membrane filters allow the solid to be more easily removed when compared to filter paper, as shown in Figure 5-13 (c). After the filtering of the solution, the rotary pump of the vacuum filter was left running for ~3 hours to facilitate drying of the graphene.
a b c
Figure 5-13 Colour change from the yellow-brown-colour of the GO solution (a) to the black colour following the reduction reaction via NaBH4 (b) and the resulting filtered graphene (c).
5.4.2. Reduction via ascorbic acid
The need for the reduction via an alternative route was inspired by difficulties experienced during electrospinning. The primary concern of using NaBH4 as the reducing agent was that it reacted unfavourably with the distilled water present in the electrospinning solution of aqueous PVA. Rapid bubbling was observed as a result of the reaction between the reducing agent and the distilled water [74] which caused the PVA solution to congeal thus rendering it unsuitable for electrospinning. This was a consequence of the inability to entirely remove the NaBH4 from the solution. Hence a reducing agent which did not have an adverse reaction when exposed to water was investigated which led to the method of reducing GO via ascorbic acid. This is discussed further in Chapter 6 while the method shall be detailed here since it pertains to the reduction of graphene which is the focus of this section.
Ascorbic acid, otherwise more commonly referred to as vitamin C, is a naturally occurring antioxidant which is vital for the growth and repair of various tissues in living organisms and as such, it is nontoxic and completely water soluble [232, 233]. Due to these properties of ascorbic acid, it is the more environmentally friendly method of reducing GO when compared to both hydrazine and NaBH4. The solvent used for this method is distilled water and the reduction occurs under aggressive stirring or ultrasonication over a period of several hours after which time, Zhang et al. [233] showed that majority of the oxygen-containing functional groups had been removed.
The effectiveness of ascorbic acid as a reducing agent for GO has been found to be comparable to that of hydrazine [232].
The method for reducing the GO via ascorbic acid was as follows: a 1 mg/ml solution of GO and distilled water was prepared and ultrasonicated for 30 minutes. Ascorbic acid (1.02 g) was weighed and added to the GO solution. There was no colour change observed immediately
after immediate addition of the ascorbic acid although the powder was completely dissolved upon stirring. The solution of GO and ascorbic acid was then ultrasonicated for 6 hours. The water bath of the ultrasonicator was periodically changed to maintain the temperature of the water at room temperature. The progressive colour change of the GO solution is depicted in Figure 5-14. The GO used was from the Tour method (the reason why this GO was used is explained in Section 5.5 as well as in Chapter 7). The solution was homogenously black in colour after 90 minutes as seen in Figure 5-14 (d). Barely any further colour change was discernible after this time. The solution was then filtered, and the vacuum filter remained on for an hour to facilitate drying of the solid.
a b
c d
Figure 5-14 GO solution from the Tour method (a); GO solution after addition of ascorbic acid (b); solution after 30 minutes ultrasonication (c); and solution after 90 minutes ultrasonication (d).
5.4.3. Reduction via thermal treatment
The last method of reducing GO was through thermal treatment at elevated temperatures (> 900 °C) [146]. While the previous two reduction methods involved the use of chemicals to achieve reduction, a nonchemical route was investigated to determine its efficiency when compared to the chemical routes. It was found by Wu et al. [146] that thermal treatment is in fact more effective at reducing GO than the chemical routes. The GO is placed in a high temperature furnace and subjected to temperatures of ~1000 °C which induces thermal exfoliation and reduction [70]. The layers of individual sheets constituting the GO are separated through the pressure of the Argon gas which is developed between the layers as a result of the high temperature [235]. It has been reported that the electronic properties of the graphene are significantly restored, however, the conductivity of the material does suffer considerably as a result of the defects introduced using this method of reduction [70].
A box was designed to house the GO flakes while in the furnace to protect the sample from contamination during heating (Figure 5-15). The box was made from a single sheet of molybdenum because this metal has an extremely high melting point of 2617 °C [236]. The method for reducing the GO via thermal treatment was then as follows: a film of GO was produced by sonicating 0.5 g of GO (produced via the modified Hummers’ method with additional KMnO4) in ~ 50 ml of distilled water for 30 minutes and filtering the solution using a nylon membrane filter (the reason why this GO was used is explained in Section 5.5 as well as in Chapter 7). The obtained GO film was placed within the box which was then placed within the vacuum furnace.
The box and its contents were heated at a rate of 300 °C per hour up to 1000 °C at which it was maintained for approximately 1 hour. The flow rate of the Argon gas was 4.8 L/hr. The furnace was then switched off and the GO contained within its housing was allowed to cool down naturally to room temperature within the furnace overnight. The GO film before and after thermal treatment is depicted in Figure 5-16. The GO films shrink as a result of their exposure to the heat.
The obtained reduced material is far lighter and more crinkled in appearance than the GO films, as expected. The reduced films were grey in colour as opposed to the brown GO films (Figure 5-16) and the mass of the films decreased following reduction.
Figure 5-15 Molybdenum box to house the GO samples in the furnace.
a b
Figure 5-16 GO films before (a) and after (b) thermal treatment.
The Energy-Dispersive X-Ray Spectroscopy (EDX) of the graphene using the SEM produced following the method as detailed above revealed that there was a significant amount of oxygen remaining in the sample which indicated that a large proportion of functional groups had not been removed (this is discussed in greater detail in Chapter 7). This is the result of the relatively short time at which the GO was subjected to a 1000 °C environment thereby resulting in insufficient deoxygenation of the sample. This may also be a result of the environment within the furnace which may not have been completely oxygen-free. When the time at 1000 °C was increased to 3 hours and GO film was broken into smaller flakes to facilitate heating of the material. A reduction of the oxygen content was achieved. This is discussed in Chapter 7.