In the present study, we report on an efficient method for large-area photoreduction of graphene oxide flexible films. The laser-based reduction can be carried out in situ and can be tuned to attain the properties required. A systematic study has been conducted to evaluate the variation of the degree of reduction with the actual reduction temperature, which is measured using an infrared thermal camera. Local reduction temperature is varied up to 350 °C, and the degree of reduction is measured using the C/O ratio. The C/O ratio is increased from 2:1 for graphene oxide to 10:1 for reduced graphene oxide. This high degree of reduction is observed at low temperatures, and also in a short period of time. Thermal conductivity properties calculated using the temperature distribution shows the in-plane thermal conductivities of graphene oxide and reduced graphene oxide are a few orders of magnitude lower than single layer graphene. This can be attributed to oxygen-defect scattering, and also due to the heat conduction through the thickness of the sample by way of contact between adjacent flakes. This photoreduction method provides a way for roll-to-roll scalable production of graphene-based flexible films.
Graphene oxide has been demonstrated to be a good precursor material for fa- brication of graphene-based materials in large quantities due to its low-cost and easily scalable synthesis process. The oxygen-containing functional groups in graphene oxide need to be removed to enable it for use in a wide range of appli- cations. Various thermal and chemical methods are commonly used for reduction of graphene oxide. These require the use of toxic chemicals such as hydra- zine, or thermal treatment at high temperatures, resulting in hazardous conditions, which are unsuitable for certain applications. To overcome the use of such hazardous means of reduction, unconventional means of heat application have been reported using a variety of light sources such as lasers and solar radiation. Photoreduction is a mild method, which is very appealing for microelectronics since a pattern of reduced graphene oxide can be formed on graphene oxide films.
Photoreduction can be achieved by two means―a photothermal process or a photochemical process. In a photothermal process, the generation of heat on exposure to high intensity light causes deoxidation of graphene oxide. In a photochemical process, a sacrificial compound is used for the reduction of graphene oxide. The UV-Vis absorption spectra show high absorption at UV and low visible wavelengths [
A variety of photochemical methods for the reduction of graphene oxide have been developed using UV-irradiation in the presence of various gases and cata- lysts. One of the first reports on photochemical reduction of graphene oxide was reported in 2011 by Kamat and group at the University of Notre Dame [
In a photothermal reduction process, the energy of the photons is converted to thermal energy, which increases the local temperature of the sample, resulting in reduction. Reduction of graphene oxide was reported by Matsumoto et al. using UV radiation in H2 or N2 at room temperature, without the use of a photocatalyst [
In addition to lasers as a means for photothermal reduction, novel light sources such as flash lamp and solar radiation have also been reported in the li- terature for graphene oxide reduction. Cote et al. reported using a xenon flash lamp to reduce graphene oxide at ambient conditions [
Although numerous studies have been reported on using lasers for reduction of graphene oxide, a systematic study on the heat generated in the sample and the resulting rise in temperature has never been reported. The temperature distribution in the graphene oxide films and the corresponding degree of reduction can be useful in tuning the required properties of the reduced graphene oxide film obtained. In this study, the temperature distribution on the surface of the graphene oxide film when subject to a laser is captured using an infrared thermal camera. The degree of reduction, measured by the change in carbon-to-oxygen ratio, is quantified with respect to the power density and the surface temperature of the graphene oxide film. The change in conductivity is also measured using a Keithley 2400 sourcemeter.
Graphene has been shown to have superior thermal conductivity of up to 5000 W/m-K, and has hence been of great interest to the scientific community to solve the severe problem of heat dissipation in microelectronics. The continuous shrinkage and increase in the number of electronic components in microelectronics has led to a thermal management problem. Due to the high thermal conductivities of carbon-based nanoscale materials such as carbon nanotubes and graphene, they have been considered as the next generation of thermal interface materials. Carbon nanotubes were used as fillers in polymeric matrices and used as thermal interface materials. However, the interface resistance at the interface between the nanotubes and the polymer limited the thermal conductivity of the composite [
The use of graphene in polymeric composites has also been reported for thermal management [
Multi-layer graphene-based flexible films can be promising candidates for thermal management due to the high thermal conductivity in the direction parallel to the graphene flakes, and low thermal conductivity in the thickness direction. Thermal conductivity of graphene has been shown to decrease with increasing number of graphene layers [
Although numerous studies have been reported in the literature on the thermal transport properties of pristine graphene and graphene-based composites using numerical and experimental methods, the studies on thermal properties of graphene oxide have been very few and far-between, with considerable differences observed in the results. And since graphene oxide has been receiving a lot of attention lately as a promising precursor for graphene-based materials, it is important to characterize the thermal properties of this material. In this study, a transient heating method using a laser is used to calculate the in-plane thermal properties of graphene oxide flexible films. The temperature on the surface of the film is monitored using an infrared thermal camera.
Graphene oxide (GO) was synthesized using the Hummers’ method [
To prepare samples for photoreduction, the graphene oxide films fabricated previously were suspended between two islands, which do not conduct heat, to avoid the conduction of heat from the sample. For photoreduction, a 532 nm Millennia CW laser manufactured by Spectra Physics was used. The laser has a
beam spot of 1 mm and a maximum power of 5 W. To obtain a line heat source on the sample, the laser beam was passed through a beam expander and a cylindrical lens, prior to impinging on the graphene oxide film. The thermal signature was captured using a SC6000 infrared thermal camera by FLIR Systems. The camera was connected to a laptop using an ethernet cable, and data was recorded using the FLIR ExaminIR software. A schematic of the setup is shown in
For the experiments, the laser power was varied from 65 mW to 400 mW, which correspond to average power densities of 3.25 W/cm2 to 20 W/cm2. Data was recorded at a sampling rate of 500 Hz. At each power level, the following procedure was used: the data recording was started, the laser was turned on for ~10 seconds, and data recording was stopped ~10 seconds after the laser is turned off to get a transient temperature response in the sample. The transient temperature data is useful to characterize the thermal properties of the sample. The reduction of the sample was characterized using energy-dispersive X-ray spectroscopy (EDS) in a Hitachi SU8030 scanning electron microscope and the electrical characterization was conducted using a 2-probe method and a Keithley 2400 sourcemeter.
At 15 W/cm2, the slope increases to 48.7 until the power density reaches 17 W/cm2. At 17.5 W/cm2, the rise in temperature is sudden and exponential, which is beyond the calibrated range of the thermal camera. Trusovas et al. conducted a COMSOL simulation to show that the temperature on the surface of a graphene oxide sample increased to beyond 1000˚C when the pulse energy exceeded 0.3 µJ [
Up to a power density of 17 W/cm2, a controlled reduction of the graphene oxide film was observed, without exfoliation or ablation. The reduction can be quantified by measuring the change in carbon-to-oxygen ratio in the reduced sample with respect to the graphene oxide film. The carbon-to-oxygen ratio was calculated by measuring the atomic weight percentages in the reduced films using energy dispersive X-ray spectroscopy in a scanning electron microscope.
From a combination of
The conductivity of each sample was also measured using a 2-probe method with a Keithley 2400 sourcemeter. To estimate the increase in conductivity, the values were normalized to that of graphene oxide before reduction.
The conductivity increases rapidly with increasing reduction temperature. Very minimal reduction is observed below 75˚C, and hence laser power density was adjusted to a range where reduction could be observed. A change in conductivity of almost five orders of magnitude is observed for samples reduced at ~350˚C.
For power densities beyond 17 W/cm2, exfoliation of the sample is observed. As mentioned earlier, a COMSOL simulation shows that the local temperature increases to beyond 1000˚C, which causes a tremendous build-up of internal pressure due to the presence of oxygen-containing functional groups. Since no external pressure is used to counter the internal pressure, the film exfoliates into reduced graphene oxide flakes. The escape of gases can be seen in the sequence of images in
In a previous study, the reduction of graphene oxide was demonstrated using stress confinement [
temperature required for effective reduction is lower, as is seen from
Using laser-based photoreduction, it has been observed that at power densities less than 17 W/cm2, controlled and effective reduction of graphene oxide films can be achieved without exfoliation into small flakes. The use of compression platens reduces the available pathways for oxygen functional groups to escape the graphene oxide film. In this laser-based reduction technique, since no stress confinement is used, the oxygen-based functional groups are free to escape through the thickness of the film, as well as laterally. Since more oxygen leaves the film due to the higher number of pathways, a higher carbon-to-oxygen ratio is observed at a similar temperature.
There are various advantages to using this method for reduction of graphene oxide films. This method of reduction can be used to tune the degree of reduction required to obtain the required properties. Since a line heat source is used, a larger area can be reduced, which is suitable for roll-to-roll and scalable fabrication of reduced graphene oxide films. It can also be used to reduce only a part of the film, while keeping other parts unreduced. Also, the reduction is achieved in a few seconds, making it a fast, energy efficient, and safe process.
The thermal properties of graphene oxide films can be calculated by using the variation of temperature at various points on the film against time.
The line-source transient heat conduction method is one of the most widely used to calculate the thermal conductivities of materials. For small distances away from the heating source, the variation of temperature with time is given by Equation (1), where
The above method requires the use of the average heat input per unit length, which cannot be accurately calculated using the current setup. Parker et al. described another method for determining the thermal diffusivity, heat capacity, and thermal conductivity of materials, which does not require the knowledge of the heat input [
The thermal conductivities for layered graphene oxide films are considerably lower than those for single layer graphene flakes. This can be due to the availability of more phonon modes for scattering, as well as the conductivity through the thickness of the sample. However, the values obtained in this study are close to those reported in the literature. Yu et al. reported thermal conductivity values of 3.91 W/m-K for graphene oxide using a nanoflash technique [
The thermal conductivities of reduced graphene oxide films were calculated using a similar method. The films were first reduced at different temperatures, and were then subjected to a laser flash at low power density. The low power density prevents further reduction of the sample, but does give the transient temperature response data, which is used to calculate the thermal conductivity. As seen from
An efficient method for large-area photoreduction of graphene oxide flexible films is reported in this article. The reduction can be carried out in situ and can be tuned to attain the properties required. For the first time, a systematic study has been conducted to evaluate the variation of the degree of reduction with the actual reduction temperature, which is measured using an infrared thermal camera. A high degree of reduction is observed at low temperatures, and also in a short period of time as opposed to previous studies reported in the literature. The temperature distribution data obtained using the thermal camera is also used to calculate the thermal conductivity of graphene oxide films. In-plane thermal conductivities of graphene oxide and reduced graphene oxide are a few orders of magnitude lower than single layer graphene. This can be attributed to
oxygen-defect scattering, and also due to the heat conduction through the thickness of the sample by way of contact between adjacent flakes. This photoreduction method provides a way for roll-to-roll scalable production of graphene-based flexible films.
This work was supported in part by the National Science Foundation through grant OISE-0730259, and in part by the Department of Transportation through the Infrastructure Technology Institute at Northwestern University. This work made use of Central Facilities supported by the MRSEC program of the National Science Foundation at the Northwestern University Materials Research Science and Engineering Center. The SEM and EDX work was performed in the EPIC facility of the NUANCE Center at Northwestern University. NUANCE Center is supported by NSF-NSEC, NSF-MRSEC, Keck Foundation, the State of Illinois, and Northwestern University.
Naik, G. and Krishnaswamy, S. (2017) Photoreduction and Thermal Properties of Graphene-Based Flexible Films. Graphene, 6, 27-40. https://doi.org/10.4236/graphene.2017.62003