If you have read our Grossman Group (MIT) review (click here to see the review) or a more recent update “Thinnest Solar Cell – Grossman Group (MIT)” (click here to see the update),you know how important graphene is for Prof. Grossman and his team.
Graphene itself may lack of important electrical properties but the addition of oxygen helps compensate that.That is why graphene oxide (GO) is a very promising low cost solution for applications like solar energy cells. The problem with the introduction of oxygen to graphene using currently available methods is either losing a good portion of the oxygen concentration or the requirement of very expensive equipment/techniques to process the solutions.
In a recent paper published in Nature Chemistry, “Scalable Enhancement of Graphene Oxide Properties by Thermally Driven Phase Separation” (click here to read the paper), the Grossman Group collaborates with other scientists from MIT as well as from UC Berkeley to provide a cheap & efficient annealing process to replace the inefficient and/or expensive methods currently used for GO structures.
The paper starts with indicating how the commonly used Hummers method results in the loss of oxygen, much needed part of GO for improved electrical properties. Next, the paper explains that the alternative methods to avoid the loss of oxygen, at least to a degree that could be considered success, require expensive equipment, complicated procedures, and annealing at 750 C. This pretty much restricts the use of these methods in large scale.
In order to explain the new process, the team starts with creating GO suspensions from synthetic graphite powder. When the GO suspensions were thermally annealed and monitored for nine days in two different temperatures (to monitor the effect of kinetic energy, experiments were conducted at 50 C and 80 C separately), the results revealed slightly improved light absorption as seen in the following graph.
When the team applies the same process to fry-dried GO samples prepared with the phase separation approach they have created, the light absorption increases dramatically, especially at 80 C as seen in the graph below.
Consistent with the improved light absorption, the fry-dried GO samples also show much less resistance compared to as-synthesized GO, as seen in the graph below. Here the resistance reduction is up to four orders of magnitude when the experiment is done in 80 C.
To summarize these results, the increased light absorption comes from the fact that the annealed fry-dried GO samples collect photons in visible wavelength whereas as-synthesized GO samples do not. This results in 38% increase in photon collection. Similarly, the new annealing process keeps the oxygen concentration intact but also groups oxygen atoms and carbon atoms separate. This increases the electrical properties of the structure and lowers the resistance by four orders of magnitude. The team uses Auger electron spectroscopy (AES) to show this change in the structure as seen below.
As a conclusion, we are extremely impressed with the results Grossman Group provides in this paper. One of the most promising solar energy cell material, graphene, is now more promising then ever. We would like to also praise the team for dedicating this paper to the memory of MIT campus police officer Sean Collier.