A 2.45 GHz microwave electron cyclotron resonance facility was designed for ion beam implantation and ECR non-resonance and resonance plasma diagnosis at the central university of Punjab Bathinda. The ECR plasma cavity design is carried so that its radial ports are utilized as multifunctional ports. As previously reported, the ECR plasma cavity can occupy a higher electric field norm inside it (Swaroop et al., 2021). Therefore, non-resonance plasma production is carried out without permanent magnets for the plasma treatment of the materialistic samples. A 1.2 kW magnetron system produces a 2.45 GHz microwave, and a series of waveguide sections transport the microwave from the magnetron to the circular plasma cavity. A close view of plasma diagnosis facilities utilized for the plasma treatment of material samples is shown in Figure 1.

Since the microwave ion source is very sophisticated for transporting the microwave from magnetron to plasma cavity. Here it is also necessary that the cavity becomes the resonance cavity; therefore, any external probe’s presence might disturb the plasma’s production inside the circular cavity. Therefore, to keep in mind the above problem, a Teflon mesh is utilized to hold the sample at the center of the plasma cavity since Teflon is transparent for the microwave.

Titanium dioxide (TiO₂, 98%) and potassium hydroxide (KOH, 98%) were purchased from M/s Lobaa Chemie. Activated carbon (AC), Potassium Bromide (KBr, 99%) was purchased from M/s Sigma Aldrich. All samples were used without any further purification.

A fine powder of TiO₂ at 2%AC composite sample has been prepared after 1 hour of grinding, and the respective powder form is collected inside the wiles. The prepared sample of ' X ’ m g was taken and added 10% of KBr as a binder. Further, by using a hydraulic pressure unit number of pallets are prepared. A detailed pictorial view of the procedure is shown in Figure 2.

The plasma treatment for the prepared pallets has been carried out at ambient pressure conditions for 30 s, 60 s, 80 s, and 100 s. After the plasma treatment, a respective sample of different treatment times has collected and grinded for fine powder form. The prepared fined powder is an electrode material for cyclic voltammetry study, as shown in Figure 2.

The electronic properties of the obtained product have been quantified using UV-visible spectroscopy (Shimadzu UV-2450) over 190–900 nm. The vibrational spectroscopic analysis was carried out using Fourier transform infrared spectroscopy (Tensor 27) over 600–4,000 cm⁻¹. The study of crystalline phases was carried out using powder x-ray diffraction using (PANalytical X’pert) with Cu K_α 1.54 Å X-rays at 2ϴ = 10–90. The surface morphologies were examined using field emission scanning electron microscopy (FESEM, Merlin Compact), including energy-dispersive X-ray spectroscopy (EDS).

To study the effect of plasma treatment on super-capacitive performance, plasma untreated and treated sample was utilized for electrode testing. We used circular nickel foam with a radius of 0.4 cm as a current collector. The electrochemical active area of electrode is 0.5024 cm⁻². The collected samples were mixed with carbon black (12%) and KBr (8%) as a binder substance. A homogeneous slurry was prepared using Methylpyrrolidone (NMP) solution and coated over nickel form and dried at 60°C for 12 h. Here it is noted that the loading mass over elctrode is ∼ 3 mg. A similar process is followed for all the collected samples having different treatment times and kept mass loading constant. For electrochemical studies, a Whatman paper was used as a separator and 6M KOH (potassium hydroxide) as an electrolyte. Then, the electrochemical testing of electrodes was performed using the CHI-760 Electrochemical workstation within two potential ranges −0.5 to 0.5 V and 0 to 1. Here, we measured various parameters such as cyclic voltammetry (CV), galvanostatic charge-discharge, and impedance. All measurements were carried out at room temperature.

The optical properties analysis of plasma treated and untreated samples were carried out using UV-Visible spectroscopy technique in the range of 200–800 nm. From the UV spectrum, the absorbance peak for activated carbon (AC) emerges approximately at 195 nm. The untreated sample observed two absorbance peaks in the 200 and 400–700 nm range. Here sharp peak at 200 nm is attributed to AC (Qiao et al., 2010), and a broad peak in the range of 400–700 nm corresponds to TiO₂ transitions (Altaf et al., 2021). On the other hand, in the case of the UV spectrum of plasma treated samples for 30 s, 60 s, 80 s, and 100 s, the broad peak at ∼ 550 nm is shifted towards the lower wavelength region with increasing the plasma treatment time. The blue shift (shift towards low wavelength) in peak shows that the size of the nanoparticle is decreased after plasma treatment, as shown in Figure 3A.

Further, a Tauc equation determines the respective band gap of the untreated and plasma-treated samples, as shown in Figure 3B. From the literature, we found that TiO₂ has a ∼3.2 eV band gap (Makuła et al., 2018), and potassium bromide has a 7.2 band gap value (Arveson et al., 2018). Figure 3B shows that the untreated samples’ band gap lies below 6 eV (∼5.94), which is higher than TiO₂ and below from KBr. With the increase in the plasma treatment time, the band gap value of samples is 5.77, 5.75, 5.717, and 5.79 for 30 s, 60 s, 80 s, and 100 s, respectively. This signifies that the 80-s treatment time shows a lower band gap value than other treatment times. Furthermore, a 37.5% decrease in band gap is found from untreated samples compared to the 80-s plasma-treated samples.

FTIR studies have been carried out in the range of 500–3,500 cm⁻¹ to characterize the material’s functional group, as shown in Figure 4. The FTIR spectrum of manufactured activated carbon has three vibration bands between 800 and 1700 cm⁻¹. The vibration between 750 and 900 cm⁻¹ is attributed to C = C bands, the broad vibrations band between the 900–1,300 cm⁻¹ corresponds to C-O bands, and 1,500–1700 cm⁻¹ shows the C = C group bands. All peaks are well matched with the previous literature (Tran et al., 2017). TiO₂ has a sharp vibration band of ∼500–900 cm⁻¹, which is attributed to the bending vibration of O-Ti-O bands. Also, some small vibration bands appear in the range of 1,100–1,400 cm⁻¹, which are assigned to the C-O vibration bond. The slight vibration band between 2,800 and 3,000 shows the stretching band of the C-H bond. The small intensity of hydroxyl peak ∼3,200 cm⁻¹ shows little moisture absorbed by the TiO₂ powder sample. All peaks are well matched with the reported literature (Praveen et al., 2014).

Further, the FTIR spectrum of untreated composite material of 2%AC at TiO₂ with 10 wt% KBr binder. It was observed that due to the small doping of AC into TiO₂, the intensity of the vibration band for activation carbon is very low compared to the intensity of TiO₂ bands. In the untreated composite sample, the intensity of hydroxyl vibration is increased because of the presence of KBr, which easily captures the moisture at room temperature. In the FTIR spectrum of the plasma treated sample, we observed that the vibration band of O-Ti-O is shifted towards high wavenumber and symmetric nature of peak change to asymmetric. It suggested that the symmetry between O-Ti-O bonds exists in the untreated sample. However, as we increase the plasma treatment time, asymmetric stretching occurs in the O-Ti and Ti-O groups of O-Ti-O bonds.

This asymmetric nature is favorable for enhancing electrochemical performance (Zeng et al., 2011). Moreover, we noted that up to a 60-s plasma treatment sample has a sharp vibration band of 500–900 cm⁻¹, but for an 80-s plasma treatment sample, this vibration became wider, and after that, for 100 s, we again saw less broaden vibration band. This suggested that in the 80-s sample, there exists a high order of disorder than in another sample. Along this, with plasma treatment, we found that the intensity of the C-O band at 1,100–1,400 cm⁻¹ is decreased. Moreover, a new peak appears in the range 1,650–1750 cm^−1, attributed to C = O bands. Further, at ∼ 2,800 cm⁻¹ wavenumber, the intensity of the CH band firstly decreases at 30 s and again increases with increased plasma treatment time. This shows that there is stretching between the CH bands during plasma treatment. However, the intensity of the hydroxyl peak is decreased with plasma treatment.

The crystal structure of untreated and plasma-treated composite samples has been investigated using the X-ray diffractometry technique, and corresponding XRD results are shown in Figure 5. The XRD pattern for AC is well-matched with the literature (Xie et al., 2014), and all XRD diffraction peaks for TiO₂ was well-matched with the three different X’pert high score pdf reference number: 01-089-5010, 00-002-0406, 01-073-1764. This reference pattern shows that TiO₂ powder has two types of crystal structure; one is tetragonal with lattice parameter: a = b = 3.776 Å and c = 9.486   Å , and the other is cubic with lattice a = b = c = 4.17 Å. The percentage of Tetragonal structure dominates over cubic structure and found the bare TiO₂ has anatase phase. In the case of the untreated sample, the presence of a low quantity of activated carbon has no significant effect on the XRD patterns, i.e., crystal structure. It signifies that the XRD pattern of the untreated sample is almost similar to that of pristine TiO₂; in addition, different peaks appear at ∼ 27.5°, which contributes to the presence of KBr. XRD of KBr was well-matched with the X’pert high score pdf reference number 00-001-0695 and showed a cubic structure ( a = b = c = 3.285 Å) of KBr is involved.

On the other hand, as we go towards the XRD pattern of the 30 s to 60 s, all peaks are slightly shifted to higher two ϴ values, as represented in the zoomed portion. The peak at ∼ 27° split into doublet peak in which one is shown for cubic KBr presence, and another small peak shows the rutile TiO_2, which matched with X’pert high score pdf reference number 01-076-0649. Moreover, with plasma treatment, the overall structure of TiO₂ remains a combination of tetragonal and cubic structures. Further, as we go towards the XRD patterns of the 80-s plasma treatment sample, the anatase phase obtains for TiO_2, which has lattice parameter of the order, a = b = 3.783   Å   &   c = 9.51   Å , and KBr has a cubic structure with a = b = c = 6.578   Å . For 100-s plasma treatment, lattice papermaker in the tetragonal structure becomes = b = 3.789   Å   &   c = 9.54   Å . From the 80 s and 100 s, XRD peaks are shifted to lower 2ϴ, indicating the extension in the lattice parameter values. Compared to the untreated sample, we observed that with plasma treatment, there is a change in bond length of TiO₂ and KBr material, but the overall crystal structure remains the same, i.e., tetragonal + cubic.

Further, the degree of crystallinity of untreated and plasma-treated samples has been determined using the relation: %   c r y s t a l l i n i t y = A r e a   u n d e r   t h e   i n t e n s e   p e a k t o t a l   a r e a   t h e   c u r v e × 100

The determined crystallinity is shown in Figure 6A, where the untreated sample has a higher degree of crystallinity than 70%. With increasing the plasma treatment, crystallinity of the sample first decreases like 68%, 62%, and 52% for 30 s, 60 s, and 80 s plasma treated, respectively, then crystallinity factor increases for 100 s up to 64%. Furthermore, the percentage degree of amorphous has been evaluated, as shown in Figure 6B. It was found that the 80-s sample had a higher degree of amorphous (∼50%) than untreated and other plasma treatment samples.

To examine the effect of plasma treatment, the surface morphology of the sample material has been determined with the help of the FESEM technique with a working distance of ∼5.5 nm at an operating voltage of 20 kV. Figure 7A shows the FESEM images of the untreated sample. Figures 7B–F shows the FESEM images of microwave plasma treatment samples for 30 s, 60 s, 80 s, 100 s, and 120 s, respectively. A nanoparticle formation has been observed in all collectively FESEM images, although no specific change is honored when we simultaneously compare it with any next upcoming treatment time.

However, if we compare the untreated and 120-s microwave plasma treated samples, it is observed that with an increase in the plasma treatment time, the degree of agglomeration is decreased, and nanoparticles become dispersed over the surface. However, there is no change in morphological shape. It is also hard to detect the typical roughness of the untreated and treated samples. The energy dispersive X-ray elemental mapping for the 80-s plasma treated sample is shown in Figures 7G–K. It confirms the elemental composition are Ti, O, K, Br, and C and shows the uniform distribution of all elements over a carbon tape substrate.

The electrodes were prepared for the untreated, and microwave plasma treated samples to perform the electrochemical study. The present study is conducted for the two electrode configurations in a 6M KOH aqueous solution. Detailed cyclic-voltammetric (CV) profiles for the two-electrode system were obtained at different scan rates from 10 mV/s to 100 mV/s in a −0.5 V–0.5 V potential window, as shown in Figure 8.

From all collective CV profiles, it is observed that the area under the curve shows a gradual change for the microwave plasma treated samples having a period: of 30, 60, 80, and 100 s compared to the untreated TiO₂. The critical area under the curve for untreated and microwave plasma treated samples in different colors is in Figure 8. It is also found that microwave plasma treatment time at 80 s has a higher critical area under the curve than the comparison to the other sample at different plasma treatment times (0 s, 30 s, 60 s, and 100 s). This implies that 80 s microwave plasma treated sample has better capacitive performance than the other samples. Similarly, the respective plots are also obtained for the galvanostatic charge-discharge (GCD), as shown in Figure 9. GCD were performed at different current density from 0.2 mA/cm² to 1.4 mA/cm². It is observed that the charging and discharging time decreased with the increase in current density. As we know from the literature, the longer the discharging time, the higher the electrode’s capacitance will be. A comparative study of the GCD curve at 1 mA/cm² current density for all samples is shown in Figure 9F.

The discharge time for untreated and treated samples are 5.49 s, 5.54 s, 7 s, 24.45 s, and 7.42 s for 0 s, the 30 s, 60 s, 80 s, and 100 s microwave plasma treatment, respectively. At 1 mA/cm² current density, the electrode-potential drop (IR drop) for 0 s, 30 s, 60 s, 80 s, and 100 s treated sample are 171 mV, 181 mV, 162 mV, 104 mV, and 154 mV, respectively.

At 80 s, the plasma-treated sample possesses excellent symmetry and has a higher discharging time and minimum IR drop, which is beneficial for higher capacitance than other treated samples. Furthermore, electrochemical activity has been evaluated using CV and GCD by determining the specific capacitance. The specific capacitance/areal capacitance (C_A) from the CV plot is determined by relation (Wang et al., 2012): C A = A r e a   u n d e r   t h e   c u r v e v ∗ ∆ V ∗ S here, ' ∆ V ' is the potential window measured in volt, “ v ” is the scan rate measured in “mV/s,” and “S” is the surface area of the working electrode measured in cm². The specific capacitance from the GCD curve is calculated using the formula (Wang et al., 2012): C A = I ∗ ∆ t ∆ V ∗ S Where I/S is the current density (mA/cm²), and ∆ t is the discharging time measured in seconds. Areal specific capacitance (C_A) based on scan rate from 10 to 100 mV/s is shown in Figure 10A. It is observed that the specific capacitance shows decreasing behavior with the increase in scan rate. Here the untreated sample has the areal capacitance of approximately 15.4 mF/cm² at 10 mV/s. Further increase in plasma treatment time as 30 s and 60 s, we observe a gradual increase in areal capacitance, approximate 18 mF/cm^2, and 20 mF/cm^2, respectively. As we reach the time at 80 s, the C_A values enhanced approximately up to 20.826 at 10 mV/s, as shown in Figure 10A. Further, at 100 s of microwave plasma treatment, TiO₂ shows the lowest value of areal capacitance of 15.37 mF/cm² at 10 mV/s near the untreated sample.

Similarly, the areal capacitance behavior is also determined from the GCD curve at a current density of 0.2–1.4 mA/cm^2, as shown in Figure 10B. For untreated sample the C_A approximate ∼225 mF/cm² at 0.2 mA/cm² current density. As the plasma treatment time increases, the areal capacitance becomes 85 mF/cm², 81 mF/cm², and 230 mF/cm² for 30, 60, and 80 s respectively.

At 80 s microwave plasma treatment areal capacitance has maximum value which is approximate 24.59 mF/cm² at 1 mA/cm² current density and 230 mF/cm² at 0.2 mA/cm² current density. Further increasing plasma treatment time at 100 s, a decrement has been observed in areal capacitance up to 181 mF/cm². Therefore, CV and GCD analysis observed that the electrochemical performance starts showing decreasing behaviors as we go further plasma treatment time beyond 80 s. Maximum anodic and cathodic current at different scan rates is shown in Figure 10C. With increasing the scan rate from 10 to 100 mV/s, the anodic peak current shifted towards a higher potential, and vice versa in the case of the cathodic peak for all samples. This nature of the electrode shows excellent reversibility. It also shows the 80 s having a higher current than untreated and other plasma-treated samples. According to the charge storage mechanism, capacitance is mainly divided into two categories: the first surface-controlled capacitance and the second is diffusion-controlled capacitance. The capacitive mechanism is recognized using relation I = a v^b, where ‘I' is the current density, ' v' is the scan rate, and ‘a' and “b” are the constants. Figure 10D shows a plot between log(i) and log(v), by linear fitting of log(i) vs. log(v), b value calculated using power law expression such as: i = a. v^b. From literature, if “b”∼0.5, capacitance controlled by diffusive process, but for b = 1, revealed the capacitive contribution. All untreated and treated samples have a ‘b' value nearly equal to 0.5. It means intercalation -diffusive mechanism dominates over capacitive action for charge storage.

Further, the contribution of each individually capacitive and diffusion-controlled capacitance in charge storage is determined using the below relation. Generally, total current at fixed voltage termed as i(V) is a combination of capacitive current and diffusion-controlled current termed as K 1 v and K 2 √ v respectively shown in relation (Iqbal et al., 2020). i ( V ) = K 1 v + K 2 √ v Where v is the scan rate (mV/s), K₁ and K₂ are constants. By linear fitting the plot between i / √ v and √ v , we calculated the K₁ and K₂ values. Using K₁ and K₂, one can find the amount of capacitive contribution and diffusion-controlled capacitance. The capacitive and diffusion contribution in untreated and 80-s microwave plasma treated samples is shown in Figures 11A,B at different scan rates. Untreated sample has % diffusion and capacitive contribution as 93 (7), 87 (12), 85 (15), 81 (19) and 80 (20) at 10, 30, 50, 80 and 100 mV/s scan rates. Similarly, 80 s plasma treated sample has % diffusion and capacitive contribution as 90 (10), 84 (16), 80 (20), 76 (24) and 74 (26) at 10, 30, 50, 80 and 100 mV/s scan rates. It shows the maximum amount of areal capacitance that occurred from the diffusion-controlled process for untreated and 80-s plasma-treated samples. This data is commensurate with the b value determined from power law expression.

Further, the electrochemical changes process is recognized by electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy was performed at open circuit voltage with an AC amplitude of 0 mV in the frequency range from 0.01 Hz to 10⁵ Hz. Figure 11C represents the Nyquist plot of untreated and plasma-treated samples. The fitting of EIS spectrum was carried using demo version of ZSimpWin 3.20d software. The analogue fitted circuit shown in inset of Figure 11C. The main area of interest is high-frequency regions.

A non zero intercept of Nyquist plot on real axis shows the series bulk resistance (R_b). It also found that the 80 s have semicircles with a minor diameter. Further, diameter of small arc shows charge transfer resistance (R_ct) which include the charge movement on the surface of electrode. From the Nyquist plot, the bulk resistance (R_b) is determined by the intercept value of the high-frequency region of the Nyquist plot to the x-axis. Figure 11D shows the bulk and charge transfer resistance. It is clear 80s sample has the lowest charge transfer resistance. These impedance results strongly resemble the CV and GCD analysis presented above. C_dl is the double layer capacitance. The parallel combination of R_ct and C_dl is responsible for the small semicircle features. A straight line represents the capacitive action on electrode. The constant phase element (CPE) and Warburg constant (W) in the fitted circuit shows the diffusion disturbance at interface.

The areal energy density and power density determined using the formula E A = C ∆ V 2 2 ∗   3600   a n d P A = E d i s c h a r g e   t i m e × 3600 where C is the areal-specific capacitance (mF/cm²), ∆ V is a potential window (V), E_A is the areal energy density (mW. h/cm²), and P_A is the areal power density (mW/cm²). Further at 0.6, 1.0 and 1.4 mA/cm² current densities; the calculated energy and power density has been shown in Table 1.

From the above discussion, we found that untreated samples have a higher degree of agglomeration, which limits their electrochemical performance. As we carried out plasma treatment on TiO₂ with 30 s and 60 s, agglomeration decreased, and active sites were increased. As a result, C_sp is increased. At 80 s treated, the sample has the best combination of optimism roughness and a low degree of agglomeration, providing an excellent electrochemical performance. With further increase in plasma treatment time, the roughness and agglomeration are decreased as confirmed by XRD and amorphous percentage parameter; consequently, a fall in capacitance occurred. At last, we analyzed that plasma treatment synergistically affects the TiO₂ nanoparticle and improves their electrochemical performance. Moreover, a long-lasting plasma treatment may destroy the electrochemical performance.