Observation of bright green luminescence in an Eu²⁺ complexed graphene oxide composite through reduction of Eu³⁺

Abstract

Red light harvesting with Eu 3+ graphene complexes are well known in the literature. But embedding Eu 2+ in a graphene oxide matrix is a difficult proposition and could be of paramount interest because of emission tunability. Herein, we report the observation of highly visible green luminescence for Eu 2+ which occurs due to the transition between the 4f 7 (8S 7/2 ) ground state and the 4f 6 5d excited state with nanosecond time decay through reduction of Eu 3+ using a simple chemical approach. This newly formed luminescence complex may be used as a major potential candidate for application in optoelectronics and nanobiotechnology.Recently graphene has drawn much attention due to its fascinating optical and electronic properties for application in opto-electronic devices. 1-4 The optical properties of graphene can be tuned using several parameters, such as appropriate functionalization or decoration of nanoparticles or introducing a rare earth element onto a graphene sheet. 5-11 On the other hand, among the various rare earth elements the europium ion is an excellent candidate for the development of red luminescent graphene. Recently, Gupta et al. 12 reported an Eu 3+ complexed red luminescent graphene nanosheet through a solid state reaction. Alternatively, hydrothermally derived luminescent Eu-complexed reduced graphene oxide (RGO) macro-assemblies were successfully prepared by Wang et al. 13 Moreover, preparation of Eu(III)-coupled graphene oxide through a covalent interaction between the carboxylic group of GO and the amine group of phenanthroline derivatives has also been reported. 14 At the same time, a non-covalent approach was employed to synthesize bright red luminescent rare earth complexes functionalized with GO by Cao et al. 15 A successful chemical route attempt was undertaken to develop Eu-acac-phen nanoparticle functionalized GO by Wang et al. 16 As it is, to date most of the research has remained restricted towards the development of an Eu 3+ -GO complex as a potential red light emitter. Limited work has been done with Eu 2+ complexed materials wherein green to yellow broadband luminescence can be expected due to the transition between the 4f 6 5d excited state and the 4f 7 (8S 7/2 ) ground state. In this regard, many techniques have been employed over the years in which host material with Eu 3+ is irradiated in presence of X-rays, g rays, 17 N 2 /H 2 , H 2 , 18 solution combustion 19 for the reduction of Eu 3+ to Eu 2+ in various inorganic hosts. Reduction of Eu 3+ to Eu 2+ is also possible in a normal air atmosphere and a carbon reducing atmosphere as reported by Peng et al. [20][21][22][23] Dai et al., 24 Xie et al. 25 etc. All the previous reduction approaches either need a special atmosphere or high temperature or are very cumbersome to conduct. Therefore, chemical reduction of Eu 3+ to Eu 2+ has been found to draw keen interest in this era. In a recent endeavour Kim et al. 26 demonstrated a facile chemical method using oleic acid and hexadecylamine for the reduction of Eu 3+ to Eu 2+ emb...