Effect of Microwave Treatment in a High Pressure Microwave Reactor on Graphene Oxide Reduction Process-TEM, XRD, Raman, IR and Surface Electron Spectroscopic Studies.

Beata Lesiak,László Kövér,Grzegorz Trykowski,Artur Małolepszy,Neha Rangam,József Tóth,Stanisław Biniak,Leszek Stobiński

doi:10.3390/ma14195728

Abstract

Reduced graphene oxide (rGO) was prepared by chemical reduction of graphene oxide (GO) (with a modified Hummers method) in aqueous solutions of hydrazine (N2H4), formaldehyde (CH2O), formic acid (HCO2H) accompanied by a microwave treatment at 250 °C (MWT) by a high pressure microwave reactor (HPMWR) at 55 bar. The substrates and received products were investigated by TEM, XRD, Raman and IR spectroscopies, XPS, XAES and REELS. MWT assisted reduction using different agents resulted in rGOs of a large number of vacancy defects, smaller than at GO surface C sp3 defects, oxygen groups and interstitial water, interlayer distance and diameter of stacking nanostructures (flakes). The average number of flake layers obtained from XRD and REELS was consistent, being the smallest for CH2O and then increasing for HCO2H and N2H4. The number of layers in rGOs increases with decreasing content of vacancy, C sp3 defects, oxygen groups, water and flake diameter. MWT conditions facilitate formation of vacancies and additional hydroxyl, carbonyl and carboxyl groups at these vacancies, provide no remarkable modification of flake diameter, what results in more competitive penetration of reducing agent between the interstitial sites than via vacancies. MWT reduction of GO using a weak reducing agent (CH2O) provided rGO of 8 layers thickness.

Highlights

Nowadays, a large number of methods of graphene production using “top-to-bottom”methods and “bottom-to-top” methods are being applied [1,2,3]
The samples prepared under microwave treatment at ◦C (MWT) conditions were compared to samples already reported elsewhere, i.e., graphene oxide (GO) and GO-exp reduced using hydrazine in 100 mL water suspension of GO (0.6 wt%) mixed with 50 mL of 50% water solution of hydrazine hydrate boiled under reflux for 30 min, GO suspension treated for 15 min in microwaves at power at 400 W [22], GO reduced using formaldehyde prepared in 100 mL water suspension of GO (0.6 wt%) with 50 mL 1M water solution of
Differences were observed in the carbon K line (C_K-edge), where for the sample rGOCH2 O there is a double band at 289 and 318 eV, while for the Reduced graphene oxide (rGO)-CH2 O-MWT sample a single band at 289 eV confirming its higher structural order

Summary

Introduction

A large number of methods of graphene production using “top-to-bottom”methods (chemical synthesis, mechanical, chemical and electrochemical exfoliation) and “bottom-to-top” methods (chemical vapor deposition, epitaxial growth, rapid thermal annealing, biomass pyrolysis) are being applied [1,2,3]. A large-scale mass production and commercialization of graphene synthesis would require the effective, low-cost, energy and time, environment friendly chemical procedures. Mass production of graphene proceeds from exfoliated graphite intercalation compounds (GICs). The GIC, as a complex material, in which elements or molecules are intercalated between graphite layers, is produced using electrochemical and or chemical methods. The exfoliation of GIC results from the separation of graphene layers due to rapid evaporation of intercalates by a heating process, using a microwave heating. The exfoliation proceeds during electrochemical and/or chemical procedures, and in different solvents, being accompanied by a microwave irradiation. The microwave energy using a microwave absorbent is transformed efficiently into heat This microwave heating is applied as a fast-heating process and leads to a larger surface area and volume than in the conventional heating technique. The mechanism of microwave heating is different than the conventional heating mechanism due to the propagation of heating front from the core to the surface, resulting in a larger core temperature and penetration of microwaves and depends on the sample size, frequency and power of radiation, as well as exposure time and reaction environment [5]

Methods

Results

Conclusion