Carbon Quantum dots doped Chitosan/HPMC nano composites and their Functional, Structural, Morphological, Dielectric and Tensile properties

. Carbon Quantum Dots (CQDs) were prepared from modified hydrothermal method using Citric acid and ethane diamine as a source material. The synthesized CQDs were characterized by UV-Vis spectroscopy, Fluorescence Studies (FL), Atomic force microscopy (AFM). The particle size is conformed from Dynamic Light Scattering (DLS) analysis. The synthesized CQDs were doped in polymer blend with various weight percentages and the further characterizations were carried out for X-Ray Diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), dielectric properties, and tensile properties at room temperature and higher temperatures with the help of Universal Testing Mission (UTM). The dielectric behavior of Chitosan/HPMC/CQDs polymer nanocomposites (PNCs) were studied in the range of 50 Hz to 5 MHz frequency and the temperature ranging from 30-100 °C, using LCR meter. The dielectric constant ( ε'), dielectric loss ( ε̋ ') are found with different wt% of CQDs in the PNCs.


Introduction
Carbon quantum dots (CQDs), also called carbon dots (CDs), were a kind of zero-dimensional nanomaterial with size less than 10 nm, which were first discovered in 2004.As a sort of environmental-friendly carbon nanomaterial [1].Carbon dots (CDs) have gained much noteworthy globally from the start of this century, and recently, it has blossomed in every division of applied sciences.Due to excellent physical and chemical possessions including biocompatibility, CDs have extensive range of applications in drug transfer, photo catalysis, explosive finding, chemical identifying, food care, bio imaging, energy conversion, etc. [2].CDs have ability to distribute finely in water, they are nontoxic and biocompatible [3] due to small in size.
Citric acid is one of the most widely used sources for fabrication of highly fluorescent C-dots [4,5] due to its perfect stoichiometric ratio of C, H, and O (1 : 2 : 1).In the earlier, diverse methods have been used for the creation of C-dots, including 17, 18 electrochemical oxidation [6] laser ablation, [7, 8] arc discharge, [9] and microwave irradiation [10].However, hydrothermal carbonization has brought great spread over prevailing physical methods, which is due to its easiness and creation of CDs with superior quantum fruitage.Hydrothermal carbonization is a basic, environmental pleasant synthesis of C-dot in aqueous medium.
Decomposable polymeric materials are disposed and can be used numerous times before being decomposed.Chitosan (CS) degrades in environment naturally [11].CS is a polycationic found in nature which is, less-toxic, eco-friendly, less allergenic, bio polysaccharide derived from chitin found in nature [12,13].CS has a unique assembly, has many properties, rich in functionalities and extensive variability of uses in commercial and bio medical grounds [14].CS is prepared from the exoskeletons of crustaceans (shrimps and crabs), which are wastes like shells, heads and tails that nearly has 50% of the body mass [15,16].These unused wastes could be altered into valued products involving protein, chitin [17,18].Hydroxypropyl methylcellulose (HPMC) is bio polymer, derivative of cellulose [19].It is non-toxic, hydrophilic, that decomposes naturally.It has good mechanical properties.CS and HPMC and their blends have good film forming capacity [20].Blending of polymers lead to new materials having improved physicochemical and mechanical properties.
In the past U. B. Singh et.al have worked in Cd-Se quantum dots and 6CHBT liquid crystals composites and studied the dielectric properties [21].Köysal, O et.al has analyzed the Dielectric Performance of Cd-Te Quantum Dots Doped Nematic Liquid Crystals [22] and coated on an ITO glass and could achieve dielectric constant of 10 at 100 Hz with dielectric loss of 17.
Also Nor Aliya Hamizi et.al studied Optical Properties of Cd-Se Quantum Dots via Non-TOP based Route and calculated the dielectric parameters [23], and found that refractive index and absorbance are dependent on the real and imaginary parts of the dielectric constant correspondingly.Similarly Chen, T. et.al has studied the dielectric properties of graphene quantum dot-cobalt ferrite-poly (vinylidene fluoride) ternary composites prepared by solution casting method.They have achieved high dielectric constant and low loss at very low frequencies [24].Ahmed, R.M. et.al studied PNCs dielectric and electrical properties with quantum dots nanofiller.They followed solution casting method and the dielectric constant and the dielectric loss improved at low frequency and reduced at high frequency [25].
PNCs are amazing class of materials that have distinct and modified (mostly improved) physical characteristics [26,27].The nature of the polymers can be altered and enhanced by the nano-additives owing to their huge specific area, small sizes, quantum confinement effects and strong interfacial interactions [28].Quantum dots (QDs) are inorganic particles that have small size (less than 10 nm).The energy band gap lies between the valance and conduction band, in the case of a bulk semiconductor, it is constant one.Remarkably, a quantum confinement effect is grown by dropping the semiconductor dimension to be smaller than Bohr radius that leads in increasing the energy band gap and also changes the absorption wavelength [29].Owing to its exceptional optical and electronic properties, QDs are used in various potential requirements.In the present work we have synthesized quantum dots by hydro thermal method and studied its tuning effect with the blend (CS/HPMC) on dielectric, mechanical properties.

Materials
CDs have prepared by hydrothermal method, Citric acid and ethane diamine are purchased from Avra chemicals.CS prepared from shrimp shells is purchased from HI media laboratories Pvt Ltd with molecular weight of 3800-20000 Daltons.HPMC is purchased from Alfa Aesar.NaOH, HCl and acetic acid are used from SDF Chem-limited Distilled water is used thought this experiment.

Synthesis of CDs
CDs are synthesized through the simple hydrothermal method according to the previous literature Wang et.al., 2017 was reported.Citric acid (1.05 g) dissolved in 10 ml of water and then followed by addition of 0.335 ml of ethane diamine, and then autoclaved in a Teflon container and heating hot air oven at 200°C for 5h.After cooling to room temperature, the solution was centrifuged at 10,000 rpm for 20 min, to remove large particles and carefully supernatant solution of C-dots was collected and then dried overnight at 80°C to obtain a CDs powder.

UV-vis and PL-spectroscopy
Optical absorption peak of the CDs is detected in the UV region with a maximum absorption about 340 nm (Fig. 3a).This is ascribed to the n-π* transition of the C=O band and π -π * transition of the conjugated C=C band, classic signature of carbon dots is emission wavelength and size reliant photo luminescent performance [30].PL is most interesting performances of CDs [31].The synthesized CDs in water solution display blue luminescence on UV radiation.From the PL spectra of the CDs (Fig. 3b) it was clear that the PL intensity is dependent on the concentration of the carbon dots.The intensity of the PL spectra sharply increases with decreasing concentration of carbon dots.This may be due to decreasing interactions among the different polar groups at low concentrations.
The PL spectra of carbon dots with variation of excitation wavelength (340-420 nm) A strong PL emission peak located at 450 nm was observed with an excitation wavelength of 360 nm.The emission peak was also shifted to a higher wavelength with the increase of the excitation wavelength, which is shown clearly in Fig. 3b.

X-ray diffraction studies of CDs
The XRD profiles in Fig. 4 illustrates that the CDs have single broad diffraction peaks positioned about 22°, which is ascribed to the (002) lattice spacing of carbon-based materials with amorphous property [32].

Dynamic light scattering (DLS)
The DLS measurement of the NP dispersion is carried out by means of HORIBA SZ-100 nanoparticle analyzer at VIT University, Vellore, Tamil Nadu, India, with temperature set at 25.2 °C, through a scattering angle of 90° using dispersion medium viscosity 0.888 mPa•S.The average diameter of CDs is found to be 8 nm.

Synthesis of CS/HPMC/CDs
In the CS/HPMC/CDs preparation, primarily HPMC is liquefied in distilled water by stirring for 2h.Acetic acid and water are taken in chosen ratio (2 ml acetic acid 18 ml water is used) and CS was dissolved after 3h stirring.The above polymer solutions HPMC and CS were mixed and stirred for 1h.CDs alone is sonicated in water for 30 minutes then this dispersed solution is mixed into the dissolved CS/HPMC blend.CS/HPMC/CDs solution is stirred at RT for 2 h to get even dispersion.The following solution is casted for 6 h at 70 C in a Teflon Petridis.The casing is performed with a low cost homemade oven which can be varied from RT to 98 °C in 4 steps, as shown in the

AFM Studies of CS/HPMC/CDs
AFM is a fairly novel technique used for the surface study of polymers and their compounds.It is skilled of producing pictures of a non-conducting polymer surface without any chemical tint or etching [41].Fig. 10 signifies the AFM pictures of prepared PNCs, studied in tapping mode.Table 2 provides the values of arithmetic mean roughness (Sa) besides root mean square roughness (Sq).The roughness values (Sa) for pure CS is 22.94 nm.The roughness values (both Sa and Sq) are increasing gradually as amount of CDs increasing up to sample 'd' with weight percentage (0.2), than decrease as further loading of CDs.The roughness value is least (15.65 nm) for the sample e with 0.4 wt%.The variation in roughness with diverse wt% of CS/HPMC/CDs shows the good compatibility and fine mixing of the PNCs.

Dielectric studies of CS/HPMC/CDs
Dielectric property of CS/HPMC/CDs is hardly studied and the dielectric property of flexible CS/HPMC/CDs composites is exhilarating due to less cost, bio-degradable and easily process able.It has numerous uses and are used in sensors, actuators, fuel cells, and capacitors.11(a-b).From the dielectric studies, the value ε' of all the samples, attain a limiting constant value at upper frequencies, since dipoles do not turn in the direction of applied A.C electric field.At lesser frequencies dielectric constant rises remarkably, similarly at high temperatures, signifying the electrode polarization and space charge polarization effects have arisen illustrating non-Debye dependence [45,46].From the dielectric constant Fig. 11(b) it is seen that the value is greatest (157) for sample CS/HPMC/CDs 22, 77.80, 0.2 wt% at 50 Hz.But it has (Fig. 12) relatively high loss value (2.12).From the dielectric plots it is seen that sample with wt% CS/HPMC/CDs 22, 77.95, 0.05 has more dielectric (90) constant and low loss (5) compared to pure CS at RT for 50 Hz

Mechanical properties of CS/HPMC/CDs composites
The mechanical properties of polymers relay on their chain length, amount of crystallinity and cross linkage [47] and is a significant property for various uses.The tensile behavior for the PNCs (CS/HPMC/CDs) is found at room temperature (27°C) Fig. 13(a) and compared the properties at 35 °C (Fig. 13b), by 20X10 mm films through a scan speed of 1 mm/min.Fig. 14 (a, b) shows the home made set up for studying temperature effect on PNCs, and its schematic diagram.The tensile plots illustrates two different segments one elastic and other plastic sections.The segment that is straight is identified as elastic region.From the Table 3 it is comprehended that, factors such as proportionality limit, Young's Modulus, Ultimate strength are gradually increasing as wt % CDs increasing.From the relatively enhanced compared to pure CS.While these tensile parameters are smallest for CS/HPMC/CDs PSs at 35 °C for same wt%.Also Fig. (15) shows the area under stress-strain graph ((toughness (energy to break)) is utmost for CS/HPMC/CDs with 22/77.20/0.80wt% and smallest for pure CS.Moreover from the tensile graphs it is seen that the elastic nature of PNCs decreases as temperature increases and it is highest for the sample with wt.% CS/HPMC/CDs 22/77.60/0.40 and least for CS/HPMC/CDs 22/77.80/0.20 wt% at room temperature, the dielectric constant for the above sample is more compared to the pure CS.

Conclusion
CDs of 10 nm are successfully synthesized by simple hydrothermal method.The prepared CDs are further analyzed by XRD, FTIR, UV-vis, DLS studies.The bio-polymer composites of CS/HPMC/CDs are prepared by solution casting method taking CDs with different weight ratios that are found to be light weight and flexible.The dielectric properties as a function of frequency and temperature, mechanical properties at temperatures 27 C and 35 C have been tested for the first time.Out of all samples from the dielectric constant plots the value is greatest (157) for CDs 0.2 wt% at 50 Hz.The effect of temperature on the tensile properties also has been studies.It is seen that proportionality limit, Young's modulus and ultimate strength increases with increase in temperature whereas Toughness (energy to break) decreases.The proportionality limit and Young's modulus are greatly enhanced for the PNCs that have also greatest dielectric constant.Ultimately the films made by only 0.2 wt% of fillers can be used as a dielectric membrane in capacitors due to good dielectric and enhanced mechanical properties.

Fig. 1 Fig. 1
Fig. 1 Synthesis protocol for the preparation of CDs hydro thermal method Fig. 3 (a) UV-studies of synthesized CDs (b) Fluorescence studies of CDs at different wavelengths

,Fig. 5
Fig. 5 Particle size determinations by DLS of CDs

Fig. 6
Fig. 6 Represents the protocol for the preparation CS/HPMC/CDs PNCs

FTIR
spectroscopy affords information about the chemical assembly of composite material.FTIR spectra of Chitosan/HPMC/CDs with different compositions are shown in Fig.(9).The peaks 2918 cm -1 and 2837 cm -1 are associated to symmetric and asymmetric stretching vibrations of chitosan polymer[33].Peak at 1372 cm -1 are due to stretching vibrations of amide -III group in chitosan[34].A peak seen at 1454 cm -1 is due to the CH2 bending vibration of chitosan[35,36].From the FTIR spectra the peak in the region from 3,660 to 3,100 cm - 1 is related to the -OH stretching vibrations of the HPMC.A peak observed at 1,662 cm -1 reveal the -C-O of the 6-carbon cyclic ring of polymer HPMC[37].A peak raised at 1028 cm -1 belongs to the C-N stretching vibration of the acetyl group[38].A broad peak seen at 3462 cm -1 is due to the presences of O-H vibrations of carboxylic group[39].A peak seen at 1643 cm -1 is because of C=O stretching vibrations of carbonyl group because of carbon quantum dots.Two peaks seen at 1192 cm -1 and 1051 cm -1 are due to the presence of C-O, C-N stretching vibrations of C-dots respectively[40].

Fig. 13 Fig. 15 Fig. 14
Fig. 13 Represents the stress strain graph of CS/HPMC/CDs at (a) room temperature (27 °C) and at (b) at 35 °C with different wt%

Table . 3
it is seen that all the tensile parameters are

Table - 3
Represents tensile properties of PNCs at RT and at 35 °C