Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie Impact of butanol and ammonium fluoride on synthesizing and optical properties of N-doped-carbon dots Fatemeh Sadat Talatori a , Shohreh Fatemi a,* , Amideddin Nouralishahi b a School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box: 11365-4563, Tehran, Iran b Caspian Faculty of Engineering, College of Engineering, University of Tehran, P.O. Box 43841-119, Rezvanshahr, Iran ARTICLE INFO Keywords: Carbon dots Synthesis Absorption edge Photoluminescence Ammonium fluoride N and F doped CDs ABSTRACT In the present study, nitrogen doped carbon dots (N -CDs) were synthesized by thermal reaction of citric acid and monoehanolamine (MEA) under reflux condition. The impacts of butanol, as the co-solvent, and ammonium fluoride, as the source of fluorine and nitrogen, were investigated on the optical properties of synthesized CDs. A two-level full factorial design was considered for the experiments to analyze the effect of three factors; citric acid (CA) content (0.6–1.9 mol/l), ammonium fluoride (AF) content (1.1–4.5 mol/l) and volume ratio of butanol (butanol/monoetanolamine: 0–1). The physicochemical properties of synthesized CDs were characterized through, FTIR, Raman, TEM, UV–Visible spectroscopy, Photoluminescence, XPS and HRTEM analysis. FTIR results revealed C]C and C–N groups in synthesized CDs. D-band and G-band were observed in Raman spectra that proved CDs formation and AF caused more defects in CDs' structure. TEM images showed that in the presence of butanol, smaller CDs were formed with a relative wide size distribution. Absorption edge as a sign of CDs' band gap and absorption width as a sign for CDs' uniformity, were estimated from absorption spectra in UV–Visible spectroscopy. The absorption results revealed the significant effect of ammonium fluoride and citric acid on absorption edge, in such a way that it was approached to 650 nm at the maximum level of citric acid and ammonium fluoride, in this study. In addition, excitation – dependent emission were observed in synthesized CDs through PL analysis. Finally, for optimum CDs with highest absorption edge, HRTEM image indicated very small CDs (under 3 nm) and XPS results confirmed N and F doping in CDs' structure. 1. Introduction In recent years, fluorescent carbon dots (CDs) as a new type of ‘zero- dimensional’ carbon nanomaterials, have been receiving attention due to their tunable surface functionalities, good aqueous solubility, che- mical stability, biocompatibility, ease of preparation, highly tunable light absorption and low toxicity. By these properties, they have been widely used in photonics, biotechnology, chemical and biological sen- sing, and solar energy [1–3]. CDs are generally consistent with an amorphous carbon framework, which contains carbon cores with sp 2 structure (C]C) and surface functional groups [4]. Doping is the major pathway to control the electronic structure and optical properties of CDs by substituting het- eroatoms into the graphitic carbon frameworks [5,6]. Nitrogen is a very common doping element for CDs and various nitrogen-containing mo- lecules, such as primary amines, have been used to synthesize CDs. During the synthesis of CDs in the presence of amine molecules, they serve as N-doping and surface passivation agent [5,7]. Guo et al. employed citric acid by hydrothermal method without any dopants, to synthesize CDs. The absorption edge of the prepared CDs reached to 450 nm with excitation-dependent emission behavior [8]. Dong et al. used ethanolamine as both carbon and nitrogen con- taining precursor and synthesized CDs through a thermal reaction. Absorption edge of resultant CDs reached to 400 nm with excitation- dependent emission behavior [1]. Schneider et al. synthesized N-CDs based on citric acid and three different nitrogen containing precursors; ethylenediamine, hexamethylenetetramine and triethanolamine through hydrothermal method, and the absorption edge of the syn- thesized CDs reached to 550, 500 and 500 nm, respectively [9]. Deng et al. synthesized N-doped CDs using biomass tar and ethylenediamine through hydrothermal rout that synthesized CDs showed excitation- dependent emission behavior with absorption edge of 420 nm [10]. Hu et al. synthesized N- doped CDs using monoethanolamine (MEA) as nitrogen and carbon containing precursor with different carbon pre- cursors of citric acid, glucose, ascorbic acid and L-Systin. The results revealed that better CDs by structure and optical properties were https://doi.org/10.1016/j.solidstatesciences.2019.105988 Received 1 April 2019; Received in revised form 26 July 2019; Accepted 30 August 2019 * Corresponding author. E-mail address: shfatemi@ut.ac.ir (S. Fatemi). Solid State Sciences 97 (2019) 105988 Available online 31 August 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved. T