New methods of synthesis and varied properties of carbon quantum dots with high nitrogen content
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Doping of a heteroatom such as nitrogen in carbon nanomaterials provides a means to tailor their electronic properties and chemical reactivities. In this article, we present simple methods to synthesize carbon quantum dots (CQDs) with high nitrogen doping content (18–22%), involving the reaction of glucose in the presence of urea under hydrothermal conditions or by microwave irradiation. The N-doped carbon quantum dots (N-CQDs) show high aqueous solubility and tunable photoluminescence (PL). Interaction of N-CQDs with exfoliated graphene or dimethylaniline quenches PL. Interaction of N-doped as well as undoped C-dots with electron-donating tetrathiafulvalene and electron-withdrawing tetracyanoethylene has been examined. The intense blue PL of CQDs has been exploited to produce white light by mixing the CQDs with yellow light emitting ZnO nanoparticles or graphene oxide. The N-doped CQDs exhibit superior photocatalytic activity compared to pristine CQDs.
I. INTRODUCTION 1,2
Among the novel nanocarbons of recent origin, carbon quantum dots (CQDs) constitute a new class of zerodimensional nanostructures with a diameter of ;10 nm. CQDs are stable, biocompatible, and exhibit characteristic photoluminescence (PL). They are easy to functionalize and are superior in some respects to the traditional semiconductor quantum dots.3–6 Doping in semiconductor materials is generally done to introduce defects which down the carrier lifetime and device performance. Introduction of a high level of defects would affect the distribution of energy levels within the band gap, by acting as recombination centers.5,6 Doping of nanocarbons with heteroatoms such as nitrogen allows one to alter their electronic, chemical, and surface properties.7–9 For example, N-doped carbon nanotubes show highly effective electrocatalytic activities for the oxygen reduction reaction.10,11 Similarly, N-doped graphene exhibits a band gap and may have applications in semiconducting devices,12 electrocatalysis,13 supercapacitors,14,15 and electrochemical biosensing.16 There are a few reports on the synthesis methods of N-doped carbon quantum dots (N-CQDs) such as hydrothermal method,17,18 ultrasonication,19 and microwave (MW) treatment20 by using an apparent carbon source in the presence of a nitrogen precursor such as ammonia or amines. Pyrolysis of nitrogencontaining carbon precursors has also been used to obtain N-CQDs.21 Nitrogen-doped graphene quantum dots have
II. EXPERIMENTAL A. Synthesis
Method 1: glucose (0.1 g) and urea (1 g) were dissolved in 5 mL distilled water. The solution was kept in Teflon-lined autoclave of 23-mL capacity and heated at 150 °C for 1 h.
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.295 J. Mater. Res., Vol. 29, No. 3, Feb 14, 2014
http://journals.cambridge.org
been synthesized by solution chemistry as well as by electrochemical means.22 The N/C atomic ratio in the CQDs reported so far was in the range of 2–18%, most of them with a low content of less than 10%. In search of a s
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