Heterostructured Hybrid rGO@α-MnO2/rGO@δ-MnO2 Nanoflower: An Efficient Catalyst for Aerobic Solvent-Free N-Alkylation Reactions and Energy Storage Material

A new reduced graphene oxide (rGO) based bi-phasic crystal of MnO2, namely alpha-MnO2 nanorods and delta-MnO2 nanoflakes containing heterostructured hybrid nanoflower rGO@alpha-MnO2/rGO@delta-MnO2 has been fabricated through a facile hydrothermal method followed by annealing treatment. The successful synthesis of the hybrid material was studied by XRD, Raman, BET, FESEM with EDX, FTIR and TEM analyses. An efficient N-alkylation reaction of substituted aromatic amines with aromatic alcohols was carried out under solvent-free aerobic conditions in the presence of catalytic amount of rGO@alpha-MnO2/rGO@delta-MnO2. The catalyst shows excellent activity in terms of high yields (up to 98%), short reaction time (10h) along with a simple work-up process. The spent material can be regenerated several times without causing any serious decrease in catalytic activity. Moreover, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and cyclic stability techniques were executed to evaluate the performances of rGO@alpha-MnO2, rGO@delta-MnO2, and rGO@alpha-MnO2/rGO@delta-MnO2 as energy storage materials. Among all those materials, rGO@alpha-MnO2/rGO@delta-MnO2 exhibited a proficient specific capacitance, C-S (267F/g at 1A/g) along with excellent cycling ability (similar to 83% retention up to 10000 cycles). The superb electrochemical performance of rGO@alpha-MnO2/rGO@delta-MnO2 might be ascribed to the combination of bi-phasic alpha-MnO2 and delta-MnO2 with rGO sheets, resulting in a flower-like structure.