Simultaneous synthesis of carbon quantum dots and porous carbon using one step hydrothermal method for supercapacitor applications

The present work reports a one-step hydrothermal carbonization process used to simultaneously produce carbon quantum dots and porous carbon. The synthesized materials were characterized by using a Fourier transform infrared spectrometer (FT-IR) to study the chemical interactions, the optical properties were studied with the help of UV-visible absorption spectroscopy, and the morphology analysis of CQDs and porous carbon was examined using FESEM, in which spherical morphology confirms the formation of CQDs, while XRD confirms the amorphous nature of the porous carbon. The application of CQDs and porous materials is evaluated as a supercapacitor material by using the cyclic voltammetry technique. The specific capacitance of porous carbon was 161.33 Fg−1 at 0.01mVs−1 scan rate. This makes CQDs and porous carbon a promising candidate for applications in energy storage devices such as supercapacitors.


Introduction
Carbon Quantum Dots are the zero-dimensional nanomaterials that are most attracted to extensively researched materials having a tremendous application in various fields such as energy storage, bio-sensors, catalysts, drug delivery, and bio-imaging due to their outstanding properties like the plain synthetic route, unique structures of graphitic cores, amorphous shells, and various organic groups, and special merits including large surface area surface states, good dispersion in solvents, superior conductivity, and low cost of production over carbon nanomaterials such as graphene and carbon nanotubes [1].Porous carbon, in contrast to CQDs, has been studied for a long time.As an excellent adsorbent, it possesses a very large specific surface area, a high pore volume, and multiple active sites for doping, which have broad applications in pollutant adsorption, supercapacitors, hydrogen storage, and catalysis [2].Researchers have been able to tailor the properties of porous carbon to meet specific application requirements.For example, by optimising the pore size and surface chemistry, porous carbon can be designed to selectively adsorb certain pollutants or enhance catalytic activity.Additionally, advancements in carbon doping techniques have further expanded the range of applications for porous carbon materials [3].The current work focuses on the one-step hydrothermal carbonization process used for the simultaneous production of carbon quantum dots and porous carbon.The prepared materials were carefully investigated with the help of different characterization techniques.These materials exhibited excellent electrochemical performance, with high specific capacitance and good cycling stability.Furthermore, the porous structure of the carbon material provided a large surface area, enhancing the accessibility of electrolyte ions and improving overall energy storage capacity.Overall, our findings emphasise the possibility of employing carbon quantum dots and porous carbon as improved electrode materials in different energy storage devices [4].

Synthesis of CQDs and Porous carbon
CQDs and porous carbon were synthesised using citric acid and urea through a one-step hydrothermal carbonization method.In a typical procedure, 1 g of citric acid and 0.5 g of urea were added to 20 ml of DI water and stirred for about 30 minutes to get a clear suction.Then, the resulting solution was transferred to a 50 ml Teflon-lined autoclave, and the autoclave was heated at 150 o C for 18 hours using a hot air oven.After being cooled down to room temperature, the resultant brown aqueous solution is a carbon quantum dot, and the final filtrate product is dried at 60 o C for 12 hours to obtain porous carbon.

Characterization
Chemical compositions were studied using the Bruker Alpha ATR FT-IR spectrometer, optical behaviour was studied using the double beam computer interface Perkin Elmer Lambda 350 UV-Visible Spectrometer, structural analysis was carried out using the Rigaku X-ray diffractometer (XRD), surface morphology was examined using Sigma Zeiss Field Emission Scanning Electron Microscopy (FESEM), and electrochemical performance was carried out using a PC-based CHI 660E electrochemical workstation.

FT-IR spectra and UV-Visible
The FTIR spectra of CQDs in Figure 1(a) show that the peaks at 1043, 1211, and 1387 cm -1 are assigned to the C-H, C-N, and C=N vibration bonds [5].The two principle characteristic peaks at 1532 and 1671 cm -1 are observed for both CQDs and porous carbon belonging to the C=C and C=O stretching vibrations, which confirm the formation of porous carbon [6].The peak at 2343 cm -1 is due to atmospheric CO2, and the additional peaks at 3588, 3790, and 3860 cm -1 confirm the presence of O-H groups on the surface of CQDs and porous carbon.These functional groups enhance the hydrophilicity and stability of blue luminescent CQDs in an aqueous system [7].

XRD analysis
Figure 2(a) shows the XRD spectra of CQDs.CQDs show a broad characteristic intense peak at 19.6 0 with interlayer spacing of 0.45 nm attributed to the (002) hkl crystal plane, which is higher than that of graphitic lattice d-spacing.This confirms the poor crystalline nature of CQDs [10].

FESEM Image analysis
Figure 3 (a) depicts the FESEM morphology of CQDs.Without the addition of any surface passivating agent, the CQDs nucleate, develop, and self-assemble into tiny spheres under hydrothermal carbonization conditions.This spherical morphology verifies the successful synthesis of carbon quantum dots [12].The morphology of porous carbon is shown in Figure 3 (b, c, and d).The porous carbon possessed continuous sponge-and rod-like structural networks, which improved the structural integrity and connectivity of electron transport [13].The interconnected pore structure observed from all SEM images of porous carbon provides a large specific surface area for energy storage applications [14].

Electrochemical performances
Cyclic voltammetry is a powerful technique used to study the electrochemical behaviour of materials.By measuring the current response as a function of applied voltage, it can reveal valuable insights into the oxidation and reduction processes occurring at the material's surface [15].The electrochemical behaviour of CQDs and porous carbon is measured using three electrode configurations with potential windows ranging from -2 to 2 V and -0.8 to 0.8 V, respectively, as shown in Figure 4 (a, & b).It can be observed that CV curves retain an identical shape without any distortion, and the current under the CV curve mechanism increases with increasing scan rate from 10 to 50 mVs -1 [16][17].This implies that the electrode surface electrochemical response is independent of the scan rate.Additionally, the increasing current indicates that the reaction is becoming more efficient at higher scan rates.Furthermore, the FESEM images provide visual evidence of the porous carbon's large surface area, confirming its potential for enhanced performance in electrochemical processes.This increase in surface area allows for more active sites for electrochemical reactions to occur, resulting in a higher electrochemical activity of porous carbon than that of CQDs [18].
The specific capacitance of porous carbon is calculated by using the below equation [19].

𝐶𝑠𝑝 = ∫ 𝐼𝑑𝑉 𝑚𝑋∆𝑉𝑋𝜗
Where, Csp is specific capacitance, I is current response, ∆V is the voltage window, ϑ is scan rate and m is mass of the deposited electrode material.
The calculated specific capacitance of porous carbon is 161.33 Fg -1 at 10 mVs -1 scan rate.This high specific capacitance indicates that porous carbon has a high ability to store electrical charge.This makes it a promising material for applications in energy storage devices such as supercapacitors [4].

Conclusion
In this work, CQDs and porous carbon are successfully prepared simultaneously by the hydrothermal carbonization method.The prepared materials are confirmed by FTIR, UV-visible, XRD, and FESEM characterization techniques.The cyclic voltammetry study suggests that CQDs and porous carbon could be promising candidates for applications in energy storage.Additionally, the low scan rate suggests that the material's capacitance remains stable even at slower charging and discharging rates, further highlighting its potential for long-term energy storage solutions.

Figure 3 :
Figure 3: FESEM images of a) TCQDs and Porous carbon (b, c and d)