Light-Initiated Hydrogenation of Graphene is Low Cost, Efficient
UPPSALA and GOTHENBURG, Sweden, Nov. 4, 2016 — A study has demonstrated that graphene reacts with formic acid in a water solution upon irradiation of visible light. During the reaction, formic acid acts as masked hydrogen and a material is produced, adding extensive amounts of hydrogen to the graphene. Additionally, it was found that the irradiation of aromatic compounds such as formic acid and benzene caused the compounds to reach electronically excited states.
Aromatic chemical compounds have an inherently high stability and often they are not easy to degrade. However, the high stability of aromatic compounds (explained by Hückel´s "4n+2" rule) exists only when the compounds are in their electronic ground states. According to Baird’s rule, compounds that are aromatic in the ground state become antiaromatic and reactive in the excited state. Researchers can now use this rule to describe various behaviors of aromatic compounds when irradiated.
This image shows a new efficient and low-cost method for hydrogenation of graphene with visible light. Courtesy of Joakim Bergman, AstraZeneca.
Researchers at Uppsala University and AstraZeneca utilized Baird’s rule to show that benzene and several polycyclic aromatic hydrocarbons (PAHs) will undergo metal-free photochemical (hydro)silylations and transfer-hydrogenations at mild conditions, with the highest yield for naphthalene (photosilylation: 21 percent). First, they studied the addition of hydrosilanes to benzenes, naphthalene and gradually larger polycyclic aromatic hydrocarbons. Despite the fact that it was not possible to explain if, and how, Baird's rule could be applied to graphene (an essentially infinitely large polycyclic aromatic hydrocarbon), the team explored graphene chemistry and found a very efficient addition reaction when using formic acid.
Research revealed that CVD-graphene on SiO2 was efficiently transfer-photohydrogenated by using formic acid and water mixtures together with white light or solar irradiation under metal-free conditions.
"The reaction is convenient and cheap, and hydrogenated graphene may be applied within areas such as hydrogen storage. Additionally, upon functionalization of graphene one can open a band gap and this fact is of high relevance for electronics applications," said professor Henrik Ottosson of Uppsala University.
"It has become more common to apply light-initiated reactions during the development of new molecules in our drug research programs. We challenge ourselves to continuously develop more efficient and environmentally friendly chemical methods. The recent progress we have seen in photochemistry, highlighted by the results herein, will increase our opportunities to access chemistry that no one thought possible a few years ago. In addition, graphene based materials have exceptional inherent properties. There is a wealth of possible applications that could result in the next biomedical revolution," said Joakim Bergman, Innovative Medicines and Early Development Biotech Unit, AstraZeneca Gothenburg.
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