Synthesis and Surface Engineering of Carbon Quantum Dots by Electrical Discharges Generated Inside and in Contact with Liquid
Authors: Tarasenka Natalie , Nevar Alena , Nedelko Mikhail , Tarasenko Nikolai 
Published: July 2021

In this paper electrical discharge plasma in liquid was used for synthesis of carbon NPs. In addition, targeted changes of the NPs surface properties could be selectively achieved via treatment of as-prepared NPs by a gas-liquid interfacial discharge produced in contact with the colloidal solution. For synthesis a high-voltage spark discharge was ignited in the solution between two graphite electrodes immersed in water. The discharge in contact with liquid was generated between a stainless-steel capillary electrode served as cathode and the surface of the liquid. The discharge was ignited by applying the high voltage of 3.6 kV using a dc power supply with argon flowing through the capillary tube. The discharge current was kept constant in the range of 1 - 5 mA [1, 2].
The composition, morphology and optical properties of the synthesized and plasma treated NPs were studied by TEM, EDX, PL, FTIR, Raman and ultraviolet–visible spectroscopy The NPs obtained by the spark discharge in water looked like isolated spherical NPs with the average diameter about 3 nm as it followed from the TEM analysis. The HRTEM images of separate particles revealed that particles are crystalline and the measured interplanar distances were found to be 2.06 Å that correspond to the reflections between (111) planes in carbon having diamond-like structure. The SAED analysis further proved the formation of diamond-like structures with admixture of orthorhombic carbon phases, while the rings corresponding to graphitic structure were absent in the SAED patterns.
For the further studies of the prepared C NPs Raman and XRD techniques were used. The Raman spectrum contained two distinct peaks: narrow G-band near 1578 cm-1 attributable to the ordered graphite and the broadened D-band at 1349 cm-1 which is associated with disodered carbon and the presence of a carbon sp3 defects in the prepared C-NPs. It should be noted that the D-band is rather broad thus covering the range 1310-1320 cm-1, typical to the diamond and orthorhombic carbon structures.
Analogically, the results of the X-ray diffraction studies exhibited the broad peak in the region 20-40˚, so-called halo, indicated a presence of small particles with various carbon phases in the sample. Nevertheless, the diamond-like phases, observed in TEM measurements most probably are presented in the sample but only as very small particles that are produced through the condensation mechanism. It is known that NPs in electrical discharge are formed through two competing mechanisms namely via the condensation of the evaporated electrode material that are atoms, ions and small clusters with their subsequent growth into the NPs; and in result the destroying of the electrodes with the formation of the particles of much larger sizes. For the Raman studies, the colloid was deposited onto the substrate, and thus larger and better crystallized particles with graphite structure formed by the second route were dominant that can explain the contradiction of Raman and TEM results.
To modify the properties of NPs, the colloidal solution of C NPs synthesized by spark discharge was treated by reactive non-equilibrium gas-liquid discharge plasma contacting with the solution. The results of these studies showed that different surface chemistry activated in different solutions could be due to different reactions induced by plasma electrons at the plasma–solution interface compared to plasma–water one. Changing the surface groups is believed to be effective in improvement of photoluminescence properties of the synthesized C-dots.
Thus, the EDL technique has been demonstrated to be suitable for the preparation of carbon quantum dots with sizes 2-5 nm and narrow size distribution that are the part of the requirements for practical applications. In addition, a treatment of NPs by reactive non-equilibrium gas-liquid discharge plasma contacting with a colloidal solution can offer great opportunities for the NPs surface engineering and an improvement of the photoluminescent properties. Carbon NPs with size less than 10 nm – so called quantum dots due to their excellent biocompatibility, chemical inertness, and size-selective photoluminescence properties may find increasing applications in modern biotechnology as more suitable alternatives to the traditional semiconductor quantum dots.
The work was supported by the National Academy of Sciences of Belarus under project Convergence 2.4.06.
[1] A. Nevar, N. Tarasenka, M. Nedelko, N. Tarasenko Carbon Lett. – 2020 https://doi.org/10.1007/s42823-020-00147-9.
[2] V.S. Burakov, V.V Kiris, M.I. Nedelko, N.N. Tarasenka, A.A. Nevar, N. V. Tarasenko, Eur. Phys. J. Appl. Phys., 79 10801 (2017)