High quality reduced graphene oxide flakes by fast kinetically controlled and clean indirect UV-induced radical reduction

Abstract

This work highlights a surprisingly simple and kinetically controlled highly efficient indirect method for the production of high quality reduced graphene oxide (rGO) flakes via UV irradiation of aqueous dispersions of graphene oxide (GO), in which the GO is not excited directly. While the direct photoexcitation of aqueous GO (when GO is the only light-absorbing component) takes several hours of reaction time at ambient temperature (4 h) leading only to a partial GO reduction, the addition of small amounts of isopropanol and acetone (2% and 1%) leads to a dramatically shortened reaction time by more than two orders of magnitude (2 min) and a very efficient and soft reduction of graphene oxide. This method avoids the formation of non-volatile species and in turn contamination of the produced rGO and it is based on the highly efficient generation of reducing carbon centered isopropanol radicals via the reaction of triplet acetone with isopropanol. While the direct photolysis of GO dispersions easily leads to degradation of the carbon lattice of GO and thus to a relatively low electric conductivity of the films of flakes, our indirect photoreduction of GO instead largely avoids the formation of defects, keeping the carbon lattice intact.Mechanisms of the direct and indirect photoreduction of GO have been elucidated and compared.Raman spectroscopy, XPS and conductivity measurements prove the efficiency of the indirect photoreduction in comparison with the state-of-the-art reduction method for GO (hydriodic acid/trifluoroacetic acid). The rapid reduction times and water solvent containing only small amounts of isopropanol and acetone may allow easy process up-scaling for technical applications and low-energy consumption.

Shedding light on the soft and efficient free radical induced reduction of graphene oxide: hidden mechanisms and energetics

“…In this study different GO samples were efficiently reduced with photochemically generated (CH 3 ) 2 Cc(OH). 22 The obtained rGO from this approach showed conductivity values up to 5500 S m À1 , which indicates the efficiency of the method. Moreover, it is extremely simple to realize and up-scale the reduction method.…”

“…Among all methods of free radical generation radiation chemistry provides the best choice to generate radicals of dened structure in a wide range of solvents and temperatures. Recent studies [17][18][19][20][21] provided solid evidence for the feasibility of the reduction of GO in aqueous dispersion using radiation-or photochemical methods.…”

“…Moreover, it is extremely simple to realize and up-scale the reduction method. 22 Unfortunately, the mechanism and kinetic details of the reduction processes remained elusive. Information on molecular mechanisms, kinetics, and energetics are needed, however, to understand why the GO reduction employing free radicals leads to high-quality rGO, containing only a very small amount of residual functional groups, 17,22 as well as for any serious up-scaling attempts.…”

“…22 Unfortunately, the mechanism and kinetic details of the reduction processes remained elusive. Information on molecular mechanisms, kinetics, and energetics are needed, however, to understand why the GO reduction employing free radicals leads to high-quality rGO, containing only a very small amount of residual functional groups, 17,22 as well as for any serious up-scaling attempts. Considering up-scaling as a longer term perspective, precise knowledge of mechanisms and rate constants is required for drawing mass and heat balances as well as selecting the right photo reactor type etc.…”

“…All of them known in the literature for their general feasibility to reduce GO efficiently. [17][18][19][20][21][22][23] All selected reducing radicals are easy and exclusively accessible in aqueous GO dispersions using pulse radiolysis, which allows for monitoring the reactions of the radicals with GO spectroscopically and kinetically. This in mind, we performed a combined spectroscopic and kinetic study based on pulse radiolysis complemented by quantum chemical calculations.…”