Growth mechanism of 2D heterostructures of polypyrrole grown on TiO₂ nanoribbons for high-performance supercapacitors†

Abeer Enaiet Allah*^ab and Fatma Mohamed^abc
aDepartment of Chemistry, Faculty of Science, Beni-Suef University, 62514 Beni-Suef, Egypt. E-mail: abeer.abdelaal@science.bsu.edu.eg
^bMaterials Science Lab, Chemistry Department, Faculty of Science, Beni-Suef University, Beni-Suef 62514, Egypt
^cNanophotonics and Applications Lab, Faculty of Science, Beni-Suef University, Beni-Suef 62514, Egypt

First published on 3rd September 2024

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1 Introduction

The ability to pattern two-dimensional (2D) nanomaterials with unique structural aspects and well-defined architectures is highly attractive to materials science researchers for energy-storage applications.^1–3 Two-dimensional geometries have unique properties including high aspect ratios, electronic conductivity, and large surface area; however, directly patterning them with a regular structure is still a major challenge.^4–6 In the past decade, there has been a gradual shift in research design from materials synthesis to materials engineering and surface modification for practical applications.^7–10 Inspired by the impressive progress of metal oxides, metal oxide-like 2D nanosheets and their composites have been widely investigated.^11–13 Among them, titanium oxide nanosheets are exceptionally rich in versatile properties such as availability, environmental friendliness, low cost, and electronic properties. Therefore, they show great potential in catalysis, electronics, energy storage, and conversion applications.¹³ Indeed, 2D titanium oxide nanosheets with high stability, flexibility, and a large specific surface area are exquisite building backbones for constructing architectures.^14–16 The advantage of using 2D TiO₂ nanosheets is that both faces of this material can be utilized for the uniform growth of functional composites such as 2D titanium oxide nanosheets/conducting polymers with unique structures for charge storage.¹¹ Conducting polymers with intrinsic properties exhibit high electron affinity, good electrical conductivity, and redox behavior, which provide different electronic and electrochemical properties.^17,18 Particularly, polypyrrole (PPy) is considered one of the pioneering conducting polymers in electrochemistry due to its various promising properties such as ease of synthesis, flexibility, high electrical conductivity, stability, and redox behavior, which have been considered benchmarks for electrode materials used in energy storage applications.^7,17,19–21

Recent research has been devoted to designing 2D nanostructures with rational architectures that combine conducting polymers and metal oxide nanostructures as electrodes, which can remarkably improve the electrochemical performance of electrode materials. Such composites combine the inherent advantages of each building block and also offer remarkable new properties as a result of the synergistic combination of two different materials. Notably, 2D TiO₂ not only provides a large surface area for the growth and immobilization of conducting polymers such as PPy but also enables strong coordination bond formation between the metal centers of TiO₂ and nitrogen atoms in conducting polymers such as polyaniline (PANI) or polypyrrole (PPy), which support the controllable growth of 2D conducting polymers onto the 2D TiO₂ nanosheet surfaces. Furthermore, the transition metal centers of TiO₂ and the N atoms of PPy endow the composite with additional pseudocapacitance.²²

Supercapacitors have received significant attention in memory backup, hybrid electric vehicles, customer electronics, cell phone towers, communication, and transportation systems due to their superior power delivery, long-life durability, and high cycle stability.^23–26

In the context of active materials of electrodes and charge storage mechanisms, supercapacitors can be classified into two categories. The first one is electrochemical double-layer capacitors (EDLCs), which store charge electrostatically by forming a double layer of electrolyte ions on the active materials. However, their low energy densities constrain their practical application. The other is pseudocapacitors with high capacitance derived from faradaic redox reactions on the surface of active materials. Nevertheless, pseudocapacitors often suffer from weak stability during cycling owing to redox reactions. Increasing the stability and power density and extending the cycling durability have become areas of deep research focus to meet the demands of high-power delivery/uptake electronics.^26,27 Controlling the material pattern and structure is crucial for supercapacitor electrodes to boost their energy and power density by exposing both surfaces of 2D titanium oxide nanosheet/PPy. Additionally, PPy conductivity and uniform particle size are vital factors for improving the specific capacitance and the rate performance of electrodes in supercapacitors.²⁸

Herein, an innovative design of heterostructures composed of 2D titanium oxide nanosheets and PPy is presented for the first time. The 2D titanium oxide nanoribbons act as a template skeleton for the growth of polypyrrole by interfacial polymerization. When the newly designed heterostructure is used as an electrode material for supercapacitor applications, it exhibits an attractive specific capacitance of 280 F g⁻¹ at 0.5 A g⁻¹, excellent rate capability, and impressive cycling stability.

2 Materials and methods

2.1. Synthesis of pure TiO₂ nanoribbons

Typically, a homogeneous solution of TiO₂ was obtained by dispersing 4 gm of TiO₂ commercial powder in 400 mL of 10 M NaOH for 30 min. Then, hydrothermal treatment was performed at 170 °C for 24 h. After filtration and washing with deionized water, a multilayered TiO₂ nanosheet was obtained. Finally, the nanopowder was dried overnight and annealed at 500 °C for 3 h.

2.2. Synthesis of 2D PPy/titanium oxide nanosheets

2D TiO₂ was dispersed in 20 mL chloroform containing 0.05 M pyrrole by ultrasonic treatment for 1 h. Then, another solution of 0.05 M potassium persulfate in 20 mL hydrochloric acid was sonicated for 1 h and added to the above suspension. Next, the suspension was left for 2 h until a black-brown precipitate was formed at the interface of the immiscible solutions. During the formation of that precipitate, multiple samples were withdrawn every 30 mn for monitoring the growth of PPy on the TiO₂ surface by TEM. After separation and washing, the precipitate was dried for 24 hours at room temperature.

2.3. Characterization of the prepared nanostructures

The morphology of the samples was characterized using a high-resolution transmission electron microscope (HR-TEM, JEOL-2010F). The wide-angle X-ray diffraction (XRD) patterns were acquired on an XRD (Philips X'Pert Pro MRD). Finally, Fourier Transform Infrared Spectroscopy (FTIR) was performed using a Shimadzu-FTIR-340-Jasco spectrometer to show the important functional groups of the samples.

2.4. Electrode preparation

The as-prepared 2D TiO₂ and 2D PPy/TiO₂ samples were ground before preparing the inks. In a typical procedure, 2.0 mg of sample was dispersed in 400 μL of isopropanol/water mixed solvent (with a volume ratio of 1/2) containing 20 μL of 5.0 wt% Nafion solution and sonicated for at least 30 min to form a homogeneous ink. Next, 200 μL of the obtained suspension was added dropwise onto flexible graphite paper (thickness: 1 mm) with an area of 1 × 1 cm² and dried at 60 °C. All electrochemical measurements were carried out on a CHI 660E instrument. In a three-electrode system, the as-prepared working electrodes of 2D TiO₂ and 2D PPy/TiO₂ samples were investigated by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) in 1 M H₂SO₄ electrolyte, with a platinum wire as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode, by using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD). The specific capacitance of the as-made electrodes was calculated from GCD curves. The test of long-term stability for 2D PPy/TiO₂ was conducted by cycling it between 0.0 and 0.8 V vs. Ag/AgCl in 1 M H₂SO₄ at a scan rate of 100 mV s⁻¹.

For two electrode system measurements, two identical flexible graphite papers (thickness: 1 mm) with an area of 1 × 1 cm² were used in 1 M H₂SO₄. 200 μL of previously prepared ink with a mass loading of 1 mg was drop-cast on each graphite sheet.

The gravimetric specific capacitance (C_g, F g⁻¹) was calculated using the following:

	(1)

where I is the discharge current (A), t is the discharge time (s), m is the mass of active material (g), and ΔV is the potential change during the discharge process (V).

Energy density and power density can be calculated according to the following equations.

	(2)

	(3)

3 Results and discussion

X-ray diffraction can be used to determine the crystallinity. PPy is a widely used polymer compound that exhibits conductivity. The structure is characterized by a conjugated long chain. Consequently, a longer molecular chain results in a larger number of electrons and a more orderly arrangement of the molecular chain, thus enhancing the conductivity of electrons. These peaks are commonly found at precise 2θ angles. The XRD pattern of PPy exhibits a high degree of crystallinity, as depicted in Fig. 1. The presence of strong hydrogen bonding and electrostatic interactions between phenolic and amine groups in the polymer chain is indicated by the distinct peaks seen at 6.46°, 23°, and 28.21° in Fig. 1.²⁹

The XRD pattern of titanium nanosheets shows the formation of mixed phases from the rutile and anatase phases of TiO₂, as shown in Fig. 1, according to JCPDS numbers 88-1175 and 84-1286. The XRD pattern of 2D titanium oxide nanosheets shows distinct strong diffraction peaks at 11.3°, 27°, 36°, 47°, and 58.7°, which are attributed to rutile TiO₂. In addition, the diffraction peaks at 25° and 48° correspond to the presence of TiO₂ in the anatase phase.³⁰ In contrast to TiO₂, 2D titanium oxide nanosheet/PPy shows a mixed phase between TiO₂ and PPy, which enhances the crystallinity of the 2D PPy/TiO₂ hybrid. The small hump that appeared in the PPy polymer matrix at 2θ between 17°–19° shifted to a higher value of 21° and became narrow due to the interaction between the TiO₂ and polymer matrix.³¹ The peak located at 29° in the hybrid is attributed to the combination of a peak at 29.7° of TiO₂ and a peak at 29° for the polymer, which is probably due to the interaction between the pyrrole units and TiO₂, i.e., the polymerization of pyrrole is followed by the adsorption of pyrrole on TiO₂.³² In addition, Fourier-transform infrared (FTIR) spectroscopy was commonly employed to examine the chemical bonds of PPy. The FTIR peaks of PPy in Fig. 2 exhibited a feature at 1330 cm⁻¹, corresponding to in-plane C–H bending vibrations. Additionally, a peak at 1193 cm⁻¹ was found, indicating C–N–C stretching vibrations. The C–C asymmetrical and symmetrical stretching vibrations were detected at 1560 and 1477 cm⁻¹, respectively. The existence of additional functions, including N–H/O–H, C–O, and CO, was also verified using the characteristic stretching vibrations at 3370, 1115, and 1633 cm⁻¹, respectively. The band at 850 cm⁻¹ corresponds to C–H band oscillation.³³ From the infrared spectrum of chemically synthesized TiO₂, the sharp absorption band around 500 cm⁻¹ was attributed to the Ti–O and Ti–O–Ti stretching vibrations of TiO₂.³⁴ Other bands were observed at 1400 and 1620 cm⁻¹, which are assigned to Ti–O, Ti–OH, and O–H stretching, which were formed through strong hydrogen bonding. Moreover, there is a noticeable broad peak around 3500 cm⁻¹, which is ascribed to the OH function group.^29,33,35,36

The FTIR spectrum of the 2D PPy/TiO₂ is illustrated in Fig. 2, and all the characteristic peaks of PPy can be observed. The N–H stretching vibration peak is observed at 3413 cm⁻¹. The broadening of the peak located at 2500–3600 cm⁻¹ demonstrates the presence of OH on the surface of adsorbed water.³⁷ The chemical interaction between PPy and TiO₂ could be explained by the low intensities of the peaks in the range of 2500–3600 cm⁻¹, with the position shifted to a lower wavenumber.³⁸

The peaks at 1590 cm⁻¹ and 1495 cm⁻¹ correspond to CC, and the peak at 1460 is ascribed to C–N stretching vibration in the pyrrole ring. The deformation of N–H and the vibration of C–H is indicated by two peaks at 1024 cm⁻¹ and 1134 cm⁻¹, respectively; the peaks that appear below 740 cm⁻¹ correspond to the characteristic peaks of TiO₂. The band at 750 cm⁻¹ is assigned to C–H band oscillation. The split peak of C–H plane-bending vibration appears at 1024, confirming the strong interaction between the PPy and nanocrystalline 2D TiO₂.³² Based on the TEM observation, the formation mechanism of the 2D PPy/TiO₂ composite is depicted schematically in Fig. 3. As for bare TiO₂, a 1D nanoribbon is regarded as a typical intrinsic building unit obtained in the initial stage, as previously reported (Fig. 3A).³⁹ Subsequently, the early stages of the chemical oxidative polymerization of pyrrole on the TiO₂ nanoribbon surface in the presence of APS led to the heterogeneous nucleation of the monomer and the formation of the oriented conjugated structure of TiO₂ side by side (Fig. 3B).³⁵

The dispersibility and hydrophilicity of TiO₂ nanoribbons in an aqueous solution would ensure ample active sites, thereby facilitating good adsorption of monomers onto their surface. Obliviously, the strong coordination and interactions between the Ti metal centers of TiO₂ and the N groups of pyrrole monomers can be seen as dark patches anchored on the TiO₂ surface, which act as the nucleus for the initiation of polymerization, as shown in Fig. 3C. The linear chain structure and expanded nature of the PPy chains enable them to stimulate their self-assembly in horizontal arrays on the surface of conjugated TiO₂, and this will significantly enhance the crystalline features in good agreement with the XRD result (Fig. 3D and E). Therefore, the aggregation of PPy nanoparticles on the TiO₂ scaffold is not observed in the composites. Interestingly, at the terminal stage of polymerization, the assembled arrays of PPy/TiO₂ transformed into 2D PPy successive layers around the TiO₂ backbone, as confirmed by TEM direct observation (Fig. 3F, G and S1†).

The TEM description in Fig. 4a revealed the flat consecutive layers of PPy on the TiO₂ surface in a 2D porous structure. Further analysis confirmed the lateral close-packing arrangement of PPy into TiO₂ surfaces in a distinct architecture (Fig. 4b and c). Close examination of the 2D PPy/TiO₂ composite presents a porous structure, which can be seen clearly in the TEM image in Fig. 4d. Such a porous structure will not only confer diffusion pathways for the electrolyte ions but also provide plentiful networks for electron transfer.

The high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image reveals a porous matrix with a 2D structure of PPy on the surface of TiO₂ (Fig. 4e and S2†). In addition, the homogeneous growth of PPy on the TiO₂ surface is further recognized by the corresponding elemental mapping (Fig. 4f–i), in which Ti, C, N, and O elements are distributed homogeneously over the TiO₂ surface, which is in agreement with TEM results and EDX results, as shown in Fig. S3.†

Supercapacitors have received specific attention due to their good safety, long life cycle, high stability, and large power density. Generally, materials with high porosity, 2D structures, and unique electrical conductivity are indispensable for achieving high capacitance and rate capability. Owing to its unique structural advantages, high electrical conductivity, abundant pores, and layer-by-layer structure, the heterostructure of 2D PPy/TiO₂ is expected to be a promising electrode material for practical supercapacitors. To show the superiority of 2D PPy/TiO and to confirm the synergistic effect between TiO₂ and PPy, the electrochemical performances of bare TiO₂ nanosheets and 2D PPy/TiO₂ heterostructures were tested in a three-electrode system using a 1 M H₂SO₄ solution as the electrolyte. Fig. 5a compares the cyclic voltammetry (CV) curves of pure TiO₂ and 2D PPy/TiO₂ electrodes at a scan rate of 5 mV s⁻¹. The CV curves show a rectangular shape, which is typical for double-layer charge storage and is considered an indication of effective polarization at the surface of the electrode. Furthermore, a pair of redox peaks at 0.4/0.5 V indicates a pseudocapacitive behavior, which is due to the partial oxidation of Ti and the pseudo-capacitive behavior of TiO₂.^40,41 Notably, the CV curve of 2D PPy/TiO₂ at 5 mV s⁻¹ showed improved capacitive performance and a larger integrated area compared to that of pristine TiO₂ (Fig. 5a), indicating better capacitive properties. Interestingly, at a high scan rate of 300 mV s⁻¹, the CV profiles of both samples display a leaf-like shape with a large area appearing due to the rapid movement of electrolyte ions. This behavior indicates its highly capacitive nature with a significant ion response and high-rate capability. This result could be attributed to the rapid movement of electrolyte ions at a high scan rate. This case displays the process of double-layer behavior (Fig. 5b).^42–45 Different scan rates of 5–200 mV s⁻¹ in Fig. S4† reveal the good capacitive nature of samples.

The galvanostatic charge/discharge (GCD) curves of the TiO₂ and 2D PPy/TiO₂ electrodes are shown in Fig. 5c, d, and S5a–d†. The GCD curves of 2D PPy/TiO₂ electrodes at different current densities display triangular shapes with symmetric sides, confirming the high capacitive reversibility.^24,46

The specific capacitances were estimated using the GCD curves (Fig. 5e) of both samples (TiO₂ and 2D PPy/TiO₂). Clearly, at 0.5 A g⁻¹, TiO₂ and 2D PPy/TiO₂ have a specific capacitance of 128 F g⁻¹ and 280 F g⁻¹, respectively (Fig. 5c). At a current density of 2 A g⁻¹, 2D PPy/TiO₂ displayed a higher specific capacitance of 252.5 F g⁻¹ than TiO₂ with a capacitance of 102.5 F g⁻¹ (Fig. S5†). To the best of our knowledge, this value is also higher than those for all the reported TiO₂/polymer composites measured in an aqueous electrolyte using a three-electrode system (Table 1). The capacitances of TiO₂ and 2D PPy/TiO₂ at various current densities from 0.5 to 10 Ag⁻¹ are also calculated based on GCD (Fig. 5c, d, and S15a–d†). As expected for 2D PPy/TiO₂, the capacitance value of 2D PPy/TiO₂ surpasses that of pristine TiO₂ at any current density. Moreover, even at a relatively high current density of 10 A g⁻¹, the capacitance value of PPy/TiO₂ is as high as 150 F g⁻¹, displaying a remarkable rate of capacitance retention (54%). Such significant capacitance retention is an indication of easy charge propagation and quick ion transport within the electrode.^22,40,41,52

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To further demonstrate the superiority of 2D PPy/TiO₂, its long-term stability was investigated by CV at 100 mV s⁻¹ (Fig. 5d). The capacitance gradually increased until the capacitance retention achieved 110% after 10.000 cycles, possibly due to the activation process of the PPy/TiO₂ electrode by deep successive wetting of the electrolyte through the pores of the material. This result indicates good electrode stability.⁵³ Further, TEM observation for 2D PPy/TiO₂ after the stability test reveals the maintenance of the morphological structure with small protrusion due to the preparation of the electrode with Nafion and the impregnation of electrolyte through the electrode material matrix, as shown in Fig. S6.†

To further demonstrate the facilitated ion-diffusion kinetics within 2D PPy/TiO₂, electrochemical impedance spectroscopy (EIS) characteristic measurements were carried out for TiO₂ and 2D PPy/TiO₂. Typical Nyquist plots obtained in the frequency range from 100 kHz to 0.01 Hz are shown in Fig. 6. They consist of a small arc in the high-frequency region and a line in the medium to low-frequency region. The semicircle in the high-frequency range is related to the resistance of interfacial charge transfer between the electrode and electrolyte R_ct due to the intrinsic resistance of the active material and the ionic resistance of the electrolyte.⁵⁴ The metallic nature of TiO₂ endows it with low R_ct, and a further decrease in the R_ct of the 2D PPy/TiO₂ heterostructure confirms the good interfacial contact between the TiO₂ substrates and the 2D PPy layers, good charge transfer properties, and improved electrochemical activity.⁷ Notably, the 2D PPy/TiO₂ electrode shows a shorter and steeper gradient than the pure TiO₂ electrode in the low-frequency region, implying faster ion diffusion capability.⁵⁵

To further investigate the performance of pure TiO₂ and 2D PPy/TiO₂ as electrode materials for supercapacitors, electrochemical measurements were also conducted using a symmetric cell. Fig. 7a and b show the CV curves of the symmetrical supercapacitor at 5 mV s⁻¹ in the potential window of 0–0.8 V. The CV curves of both samples maintain an optimal rectangular shape with a redox peak, the same as shown in the three-electrode system measurements with improved behavior for 2D PPy/TiO₂ compared to TiO_2. At different high scan rates as shown in Fig. 7c and d, the CV profiles of both samples display a leaf-like shape due to the rapid movement of electrolyte ions at high scan rates.

The charge discharge curves of pure TiO₂ and 2D PPy/TiO₂ in Fig. 8 exhibited a symmetrical triangular shape, suggesting good electrochemical capacitive behavior and excellent reversibility of the charge/discharge cycle due to the contribution of the pseudocapacitive and EDLC behavior of the electrode materials owing to the presence of TiO₂ and 2D PPy. Clearly, increasing current density causes a decrease in charging/discharging time and capacitance. This behavior is attributed to the requirement of less time for the attainment of the same potential difference across the two electrodes at a higher current density value, indicating superior rate capability.^55,56 Obviously, 2D PPy/TiO₂ achieved a higher specific capacitance of 182 F g⁻¹ as compared to 85 F g⁻¹ for bare TiO₂ at 0.5 A g⁻¹. The increment of specific capacitance from 85 F g⁻¹ in TiO₂ to 182 F g⁻¹ in 2D PPy/TiO₂, which is nearly two times, could be ascribed to the synergistic combination of electric double-layer capacitance and pseudocapacitance.

As previously discussed for 2D PPy/TiO₂, the huge 2D exposed surface of a layered structure in an open layer-by-layer structure, with redox-active sites, contributed to pseudocapacitance, so the charge/discharge kinetics is qualitatively evaluated according to the following equations:

i = aνb	(4)

log(i) = log(a) + blog(v)	(5)

where i and v are the current density and scan rate, while a and b are constants, respectively. Generally, the charge storage process depends on the value of the exponential index b. b = 1 or close to 1 indicates that electric double-layer capacitance, which controls electrochemical behavior, is a surface-confined process with quick reaction kinetics. When b is close to 0.5, pseudocapacitance with slow reaction kinetics is the predominant electrochemical behavior, indicating a diffusion-controlled reaction mechanism. Fig. 9a presents the relationship between log(i) and log(ν). The calculated b values for TiO₂ and 2D PPy/TiO₂ ranged from 0.61 to 0.63 in the charging process and from 0.56 to 0.58 in the discharging process, which confirms that the electrochemical behavior is dominated by pseudocapacitance^56–58

The percentage contributions of both electric double-layer capacitance (C_EDL) and pseudocapacitance (C_PC) to the total specific capacitance were calculated using the Trasatti method. Fig. 9b displays the plotting of the inverse of specific capacitance (1/C) against the square root of the scan rate. According to eqn (6), the Y-intercept of the linearly fitted line in the low scan rate region (Fig. S7†) provided the inverse of the total specific capacitance (C_T) of the electrode materials. Moreover, Fig. 9C plots the specific capacitance (C) versus the inverse square root of the scan rate in order to calculate C_EDL. The value of the y-intercept obtained by plotting the fitted line in a high scan rate region (Fig. S8†) represents C_EDL according to the following equations.

	(6)

	(7)

The contribution of pseudocapacitance (C_PC) was calculated by subtracting C_EDL from C_T, where k₁ and k₂ are arbitrary constants. The percentage contributions of C_EDL and C_PC for TiO₂ and 2D PPy/TiO₂ are shown in Fig. 9d. Clearly, TiO₂ has 74% C_PC and 26% C_EDL, while 2D PPy/TiO₂ has 97.5% C_PC and 2.5% C_EDL. Furthermore, after the growth of 2D PPy on the surface of TiO₂, the C_PC contribution increases to 97.5% due to improved titanium dioxide's electrochemical activity with the addition of highly conductive PPy that accelerates and improves the electron transfer between the conductive state and semiconductive state and facilitates the redox transition of TiO₂. For this reason, the synergy between 2D PPy and TiO₂ nanoparticles increases specific capacitance.⁵⁹

To assess the activity of the prepared electrodes in the practical application of supercapacitor devices, the power density (P_s) and energy density (E_s) of electrode materials were calculated and compared as shown in the Ragone plots of TiO₂ and 2D PPy/TiO₂ in Fig. 10. The plot shows that 2D PPy/TiO₂ achieved a higher energy density as compared to bare TiO₂. 2D PPy/TiO₂ displays an energy density of 18.5 Wh kg⁻¹ and a power density of 351.5 W kg⁻¹ at a current density of 0.5 A g⁻¹, which is 2.3 times higher than that of TiO₂ (13.125 Wh kg⁻¹) at the same current density. These results reflect the synergistic effect of the combination of 2D PPy sheets on the metallic surface of TiO₂. Furthermore, it is outstanding in comparison with previous literature.⁵⁶

4 Conclusion

Two-dimensional polypyrrole PPy on TiO₂ nanoribbons was fabricated, forming an oriented 2D layer. The structural characterization of 2D PPy/TiO₂ was performed by TEM. The nucleation and growth mechanisms of PPy nanosheets involved homogeneous nucleation in bulk solution and heterogeneous nucleation on TiO₂ nanosheets. 2D PPy/TiO₂ displays superior electrochemical properties for supercapacitor applications. The path for electron transfer and ion transport is shortened by a special skeleton structure with a consistent distribution of layers with various constituents. The 2D PPy/TiO₂ electrode has a maximum specific capacitance of 280 F g⁻¹ at 0.5 A g⁻¹ with good rate capability and cycling stability. Therefore, this strategy will be recommended for enhancing polymerization-based patterning toward supercapacitor applications.

Data availability

All data generated or analyzed during this study are included in this published article (and its ESI† file)

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4na00121d

This journal is © The Royal Society of Chemistry 2024