Elsevier

Applied Surface Science

Volume 507, 30 March 2020, 145027
Applied Surface Science

Short Communication
Synthesis and modification of pristine and nitrogen-doped carbon dots by combining template pyrolysis and oxidation

https://doi.org/10.1016/j.apsusc.2019.145027Get rights and content

Highlights

  • Few-layer graphene nanoflakes of 15–40 nm have been synthesized by template pyrolysis.

  • Oxidation by HNO3 led to the formation of carbon dots.

  • Correlations between pyrolysis conditions and properties of carbon dots.

  • Carbon dots of different colors from green to orange.

  • Surface modifications of carbon dots by n-buthylamine.

Abstract

The template pyrolysis synthesis, as a bottom-up approach, and oxidation, as a top-down approach, were combined to produce pristine and nitrogen doped carbon dots. Varying the precursor type, synthesis temperature and oxidation time the color of carbon dots was tuned from green to orange because of the bandgap changes and formation of interband states. TEM, Raman, and XPS data revealed the graphene core of carbon dots with the shell of carboxyl and hydroxyl functionalities. The bulk and surface N-doping of the carbon dots affected their photoluminescence in a different way because of the opposite effect on the π-electron system: the core modification led to a blue shift, while the surface amination resulted in a red shift. A blue shift was also observed for the undoped dots with increasing the synthesis temperature, while more prolonged oxidation led to a red shift. Excitation with different wavelengths revealed the inhomogeneity of photoluminescence sites in nitrogen-doped carbon dots.

Introduction

Carbon dots (CDs) are one of the youngest members of the family of carbon nanomaterials. Typically, CDs consist of a carbon core of less than 10–20 nm in diameter covered with functional groups. Water solubility, bleaching resistance, and nontoxicity distinguish them from semiconductor quantum dots [1]. Depending on the core structure, CDs can be divided onto graphene quantum dots (GQDs) with the 2D-layered graphene core, carbon quantum dots (CQDs) with the spherical layered core, and carbon nanodots (CNDs) with the amorphous core [2]. Among them GQDs stand out for their sp2-conjugated structure, unique electronic properties, complex bandgap structure, catalytic action, and high thermal conductivity, but their scalable production is still an issue [2], [3].

Tunable photoluminescent properties of CDs are important for their application in bio-imaging, light‐emitting diodes, full‐color displays [4], [5], [6]. Different synthetic approaches are applied to obtain CDs of different colors. In the case of bottom-up strategy a variation of the precursor type or careful separation of the CD mixture by column chromatography are used. The color of graphene-based photoluminescent materials obtained by a top-down approach can be tuned by varying the size or functionalization degree of sp2-domains [6], [7], [8], [9]. Therefore, it should be useful to combine the top-down and bottom-up approaches to develop a powerful and scalable technology for the synthesis of multi-color CDs with the desirable properties.

The template pyrolysis of hydrocarbons is an effective and scalable way to produce undoped and doped graphene nanoflakes (GNFs) of 10–100 nm in diameter [10]. This approach is flexible to synthesize GNF particles of different size, porosity, defectiveness, doping degree, etc. Oxidation of the resulted material immobilizes oxygen-containing functional groups on the surface of particles, making their structure similar to GQDs. In this work, we adopt the template pyrolysis synthesis of GNFs to the production of CDs. The developed technique combines both a bottom-up stage in which GNFs of 20 nm in size are grown on the surface of the MgO template and a top-down stage in which the grown GNFs are split into CDs by oxidative treatment. The selection of the precursor type and pyrolysis conditions affects the core structure of the finally produced CDs, which allows tuning their bandgap. Moreover, the top-down synthesis of CDs usually requires a long harsh treatment (24 h or even several days) [6], [9], [11]. But in our case 3 h of nitric acid treatment was enough, because the size of the treated GNFs was close to the optimal size of CDs and they contained many edge carbon atoms labile to oxidation. In addition, we used two techniques to produce N-doped CDs: the in-situ doping during CVD growth of GNFs and post-synthesis amine functionalization of CDs. These techniques allowed introducing N-atoms either into the core or on the surface of CDs.

Section snippets

Experimental

The template synthesis of GNFs was described in our previous work [10]. The MgO template was synthesized by long-term (2 days) precipitation of magnesium oxalate followed by calcination at 500 °C. To obtain GNFs, hexane or acetonitrile in a nitrogen gas flow was passed for 15 min over the template placed in a quartz tubular bed reactor heated to 700, 800 or 900 °C. The synthesized materials were washed via refluxing with 1:1 HCl/water solution and filtered with distilled water using a 0.2 µm

Results and discussion

TEM images (Fig. 2) showed that unoxidized GNFs were of about 20 nm in diameter and contained 4–7 graphene layers. The N-doped GNFs were more defective than the pristine ones, which is typical for doped graphene-based materials [12].

Produced CDs were characterized by TEM, SEM, XPS, and Raman spectroscopy (Fig. 3). According to the XPS analysis all the CD samples contained about 30 at.% of oxygen regardless of the precursor type, synthesis temperature, and oxidation time (Fig. 3a). At the same

Conclusions

We have demonstrated that the scalable template pyrolysis synthesis of GNFs followed by their oxidation allows producing pristine and N-doped graphene quantum dots. Their color can be tuned from green to orange by varying synthesis and post-treatment conditions. The proposed technique stands out among others by the possibility to tune the defectiveness and doping degree of graphene core of CDs at the CVD stage, while the small size of the synthesized GNFs allows using short oxidation time.

The

CRediT authorship contribution statement

Sergei Chernyak: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing, Funding acquisition. Angelina Podgornova: Investigation, Methodology. Sergey Dorofeev: Investigation. Sergey Maksimov: Investigation. Konstantin Maslakov: Investigation, Writing - original draft, Writing - review & editing. Serguei Savilov: Supervision, Resources, Writing - review & editing. Valery Lunin: Supervision, Writing - review & editing, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank Dr. O.Y. Isaikina for Raman spectroscopy study. This work was supported by the Russian Science Foundation under grant No. 18–73–00061. The authors acknowledge support from “Nanochemistry and Nanomaterials” MSU Equipment Center acting under Lomonosov Moscow State University Program of Development.

References (27)

  • H. Ding et al.

    Full-color light-emitting carbon dots with a surface-state-controlled luminescence mechanism

    ACS Nano.

    (2016)
  • K. Jiang et al.

    Red, green and blue luminescence by carbon dots: full-color emission tuning and multicolor cellular imaging

    Angew. Chem. Int. Ed.

    (2015)
  • L. Bao et al.

    Photoluminescence-tunable carbon nanodots: surface-state energy-gap tuning

    Adv. Mater.

    (2015)
  • Cited by (37)

    • A review of carbon dots in synthesis strategy

      2024, Coordination Chemistry Reviews
    View all citing articles on Scopus
    View full text