The synthesized hybrid composite is characterized using XRD, FTIR, Raman, SEM, Contact angle measurement, BET, CV, GCD, and EIS. The FTIR spectrum has been recorded in the range 4000 cm− 1 to 400 cm− 1 for all samples (Fig. 2). The peaks at 3426 cm− 1 and 1633 cm− 1 are attributed respectively to stretching and bending vibrations of hydroxyl groups[31]. The peaks at 2920 cm− 1 and 2861 cm− 1 corresponds to symmetric and antisymmetric vibrations of CH2. The peak at 1388 cm− 1 is the bending vibrations of C-OH functional groups. The peak at 1101 cm− 1 suggesting the reduction of carbonyl groups due to the formation of hydroxyl groups. The peak at 1046 cm− 1 corresponds to C-O stretching vibrations. The signature peak, 1720 cm− 1 of graphene oxide is not seen here (See figure S1 in the supporting information for comparison). This supports the formation of RGO hydrothermally. The absorbance peak at 1581 cm− 1 indicates the skeletal vibration of graphene sheets [32],[31],[33]. The peaks at 686 cm− 1 and 573 cm− 1 corresponds to stretching vibrations of Te-O bonds. The intensity of oxygen-containing groups decreased suggesting the reduction of graphene oxide in the composite.
FTIR data of ZTO shows different peaks like 3436 cm− 1 for O-H stretching vibration. 1648 cm− 1 is Zn-Te vibration and 1009 cm− 1 correspond to Zn-O stretching vibration. 1581 cm− 1 and 1376 cm− 1 related to C-C and C-N vibrations.1124 cm− 1 gives C-O vibration and 723 cm− 1 and 576 cm− 1 related to stretching vibrations of Te-O bonds[19].
The structural analysis of the material is obtained from the X-ray diffraction spectrum. The XRD pattern of ZTO, ZTR, and ARGO are shown in Fig. 3. The X-ray diffraction pattern matches with JCPDS Card no. 72-1283, corresponding to the planes of monoclinic Zn2Te3O8 in the space group C2/c. Notably, there are no signature peaks representing RGO are present in the ZTR due to the very low diffraction intensity of reduced graphene oxide compared to the crystalline ZTO. Furthermore, the XRD analysis of both ZTO and ZTR hybrid shows no additional peaks, indicating the absence of any other crystalline impurity[32, 33]. The XRD data of GO given in figure S2.
Figure 4 shows the Raman spectra of ARGO, ZTR, and ZTO compounds. The D and G bands are observed at 1308 cm− 1 and 1585 cm− 1 respectively for ZTR and ARGO. The intensity of the D band in ZTR is increased compared with ARGO, which corresponds to the presence of more sp2 domains during reduction[31]. The structural disorder is determined by calculating the intensity ratio between D and G bands. The ID/IG ratio of ZTR nanocomposite is 1.21, which is much higher than that observed for graphene oxide (See Figure S3a in SI) indicating the removal of oxygen functionalities and a partially ordered crystal structure of reduced graphene oxide sheets[34]. Raman results confirm the hydrothermal reduction of graphene oxide. The zincospiroffite nanoparticles are successfully deposited onto the reduced graphene oxide sheets in the solvothermal treatment at 200 oC in ZTR1, ZTR, and ZTR4 (figure S3b in SI). However, the ZTO sample shows no D or G bands, instead an intense peak corresponds to Te-O vibrations[35] is seen.
The morphological studies of ARGO, ZTO, and ZTR are shown in Fig. 5. The surface morphology of the three samples is different with respect to each other. Figure 5 (a) depicts ARGO's folded surface, which has a layer structure with irregular, and ultrathin paper-like morphology similar to graphene sheets. This indicates that the hydrothermal method[36] is successful in reducing GO (fig. S4a-b) to RGO. As a result of the folded layers and overlapped structure, the ARGO sample has a large surface area. The SEM image of the ZTO in Fig. 5 (b) shows an irregularly shaped sphere-like structure that is uniformly distributed. In the case of ZTR, the particles are uniformly distributed across the surface during the one-pot solvothermal technique, as shown in Fig. 5 (c). Furthermore, when compared to the ZTO system, an overlapped layer-like formation is visible during ZTR synthesis and is also observed in ZTR1 and ZTR4 (fig. S4c-d). The addition of graphene oxide during the synthesis improves the sheet-like formation of the ZTR, making it more porous than ZTO. Because of the larger surface area of the ARGO doubles the surface area of the ZTR. This sample is more electrochemically active than the other two.
The BET measurement is used for analyzing the porous nature of the synthesized material. Figure 6 shows the N2 adsorption–desorption results of the sample ZTO, ZTR, and ARGO. The analysis reveals it as a type IV hysteresis nature[37], which indicates the mesoporous behaviour. The specific surface area of ZTO, ARGO, and ZTR are respectively 6.17 m2 g− 1, 242.57 m2 g− 1, and 15.09 m2 g− 1. As a result of the hybrid synthesis process, ZTR has twice the surface area of ZTO. The zincospiroffite was distributed over the graphene sheets while using GO to increase the surface area and give rise to the doubled porosity. The hybrid material ZTR's high specific surface area will help to improve the electron transfer and mass transport during the electrochemical process. This is a good criterion for the material, which has a higher performance-specific capacitance. The BJH (Barrett-Joyner-Halenda) pore size distribution curve gives the mean pore diameter of ZTR as 4.29 nm and ZTO as 5.25 nm, which corresponds to mesopore characteristics. The p/po values of ZTO, ZTR, and RGO are 0.0161 cm3 g− 1, 0.0081 cm3 g− 1 and 0.2281 cm3 g− 1 respectively. The ZTR1 and ZTR4 hybrid details are discussed in SI Fig S5 and Table S1.
Measurement of the contact angle is one of the useful techniques to know how the electrode surface interacts with the electrolyte as well as the chemical nature of the surface. In the supercapacitor application, surface wettability is a primary evaluation technique. The surface science of the prepared material is critical to its storage application and efficiency39,40. The shape of the liquid droplets on the surface of the material as a result of the pressure difference created between the air and liquid interface is analyzed. Supercapacitance performance is improved due to the improved surface wettability of the liquid electrolyte.
According to the preliminary evaluation of the electrode surface that was subjected to CA measurement, the electrode-electrolyte interaction was good, and improved supercapacitor performance is anticipated39. Figure 7 (a) to (c) shows the results of the contact angle measurement of the composite samples, ARGO, ZTO, and ZTR. The obtained contact angle values for ARGO, ZTO, and ZTR are respectively 44.3°, 33.6o and 28.6o. There is a lower contact angle in ZTR than in the other two samples, ensuring that it is highly wettable. Generally, a hydrophilic surface has a contact angle less than 90o, and a hydrophobic surface has a contact angle greater than 90o. This indicates that all of our prepared samples' ARGO (44.3°), ZTO (33.6o), and ZTR (28.6°) are hydrophilic and better for the electrode-electrolyte interaction[38].
To evaluate the electrochemical and capacitive performance of the obtained hybrid materials CV (Cyclic Voltammetry) and GCD (Galvanostatic Charge Discharge) tests were carried out at different scan rates and current densities. The CV curve in Fig. 8 (a) ARGO shows double-layer capacitance, while 8 (b) ZTO and 8 (c) ZTR show pseudocapacitive behaviour with excellent charge diffusion at the electrode-electrolyte interface. As the scan rate increased from lower to higher, the curve shape was well maintained without any deviation, showing good performance from the supercapacitors. Increasing scan rates from 10 mV/s to 500 mV/s shifts the cathodic and anodic peak potentials in opposite directions. Comparing the CV and GCD profiles (figS6a and b) of ZTR1, ZTR, and ZTR4, ZTR shows better storage and is taken for further analysis. The specific capacitance value of ZTR is 513.51 F/g for 10 mV/s, which is excellent for the telluride material compared to other metal chalcogenides like CoS (41.36 F/g)[4] and CoS2 (348 F/g)[39]. The specific capacitance obtained for ZTR at scan rates 20 mV/s, 50 mV/s, 100 mV/s and 500 mV/s are respectively 304.88 F/g, 117.71 F/g, 70.28 F/g and 19.66 F/g for a potential window of -1V to 1V. ZTO-specific capacitance ranges between 42.29 F/g and 2.42 F/g, with a potential window of -1.2 V to 0.7 V, with a scan rate ranging from 10 to 500 mV/s. The specific capacitance obtained for ARGO is between 183.24 F/g for 10 mV/s and 14.70 F/g for 500 mV/s over a potential window of 0.8 V to -1.3 V. According to the improved specific capacitance of ZTR, the in-situ incorporation of RGO in ZTO provides a synergetic effect between the two compounds. ZTR hybrid is capable of working in a wide range of scan rates ranging from 10 mV/s to 500 mV/s. The more efficient storage property is due to the excellent oxidation and reduction reaction and good rate performance during the lower scan rate to the higher scan rate. As a result of the effective utilization of the higher surface area during intercalation-deintercalation, a high specific capacitance can be achieved while lowering the scan rate. A decrease in specific capacitance during a higher scan rate indicates the good rate capability of the synthesized material[37]. The redox process of hybrid in KNO3 takes place via intercalation-deintercalation of K+ ions in the electrolytic medium[19]. The following equations describe the expected mechanism (5)-(6),
The specific capacitance was gradually decreased with an increase in scan rate. The nanocomposite ZTR integrates the advantages of each component in the material or the synergic effect of both faradaic and non-faradaic components helps to increase the specific capacitance. Figure 8 (d) shows the charge-discharge curve of ZTR hybrid material over various current densities like 1 A/g to 10 A/g. The initial dip in the plot conveys some restricted electron transfer due to electron series resistance. The linear variation deals with the double-layer capacitance and slope part generated from the pseudocapacitive behaviour of the material. The distribution of specific capacitance with respect to the scan rate of the samples is shown in Fig. 9 (a). The lower scan rate provides higher specific capacitance due to better electrode-electrolyte contact.
Electrochemical impedance spectroscopy is one of the perfect tools used to analyze charge transfer resistance, phase angle, surface of the material, and electrode-electrolyte interface[19]. The higher frequency region in Fig. 9 (b) shows a smaller semi-circle compared to others, indicating less hindrance during the migration to the surface of the material and that imparts better specific capacitance for ZTR[40]. Considering all the parameters, ZTR shows a better performance due to enough electrode-electrolyte contact and better to-and-fro migration of K+ ions than other two compounds. Figure 9 (c) is the energy density variation at different current densities and is maximum at lower current density. The energy density and power density obtained for ZTR is 83.33 Wh/kg and 0.99 kW/kg. The continuous cycling experiment (up to 5000 cycles) under constant current of 10 A/g establishes the long-term cyclic stability of the hybrid ZTR electrode. Figure 9 (d) shows high-capacity retention of 99% up to 5000 cycles and Table 1 compares the specific capacitance and cyclic stability of other reported compounds with that of the ZTR electrode.
Table 1
Compares the specific capacitance, cyclic stability, and retention of the other metal telluride-based electrode material
Material
|
Specific capacitance
|
Cyclic stability
|
References
|
La2Te3
|
469 F/g
|
74% at 1000 cycles
|
[19]
|
NiTe
|
618 F/g
|
75% at 5000 cycles
|
[40]
|
CoTe
|
183 F/g
|
85% at 10000 cycles
|
[37]
|
SmTe3
|
144 F/g
|
69.3% at 1000 cycles
|
[41]
|
ZTR
|
600 F/g
|
99% at 5000 cycles
|
This work
|