Controllable synthesis of MnO2/polyaniline nanocomposite and its electrochemical capacitive property

Polyaniline (PANI) and MnO2/PANI composites are simply fabricated by one-step interfacial polymerization. The morphologies and components of MnO2/PANI composites are modulated by changing the pH of the solution. Formation procedure and capacitive property of the products are investigated by XRD, FTIR, TEM, and electrochemical techniques. We demonstrate that MnO2 as an intermedia material plays a key role in the formation of sample structures. The MnO2/PANI composites exhibit good cycling stability as well as a high capacitance close to 207 F g−1. Samples fabricated with the facile one-step method are also expected to be adopted in other field such as catalysis, lithium ion battery, and biosensor.


Background
As a sort of classic conducting materials, polyaniline (PANI) possesses good conductivity with specific organic characters that metal cannot match, which has attracted a lot of attentions for its wide applications in capacitance, sensors, ultrafast nonvolatile memory devices, and chemical catalysis [1][2][3][4][5]. MnO 2 has been widely studied as a promising environmentally benign transition metal oxide for sensor, catalyst, lithium battery, and electrochemical capacitor [6][7][8][9]. In the quest for superior performance, aniline has been polymerized in combining with other materials to obtain a promising performance, in which PANI/graphene, PANI/carbon nanotube, PANI/Au electrode, and others have been successfully synthesized [1,2,10]. Meanwhile, hybrid composites consisted of MnO 2 , and other materials have also been fabricated to improve their behaviors in battery or supercapacitor [11][12][13][14][15]. In particular, the structures of MnO 2 /PANI have been constructed with different methods, and the synergistic effect of MnO 2 and PANI has been demonstrated in supercapacitor and catalysis toward H 2 O 2 oxidation or organic dyes [16][17][18][19][20].
Considering the catalyst-size dependent reaction selectivity and agglomeration involved in nanostructures and specific nanoscale architecture, the big challenge for high-efficiency and outstanding features is still the controllable synthesis of uniform structures [21,22].
With respect to PANI synthesis, chemical and physical methods have been recommended [5,[23][24][25][26][27][28][29][30][31], in which the facile interfacial polymerization is a highly flexible approach without any templates [3,23,24]. The oxidant and reducing agent are separated in the aqueous and organic solutions, while the redox reaction can occur at the interface. As far as the products are removed into the bulk solution, new polymerization can happen at the interface while secondary growth of PANI are prevented, in which both the shape and size of the products can be controlled. In addition, synthesis of MnO 2 via reducing the compounds containing MnO 4 − and MnO 4 2− has been extensively used due to its simpleness and low cost. During that procedure, the pH of KMnO 4 solution plays a critical role in the intermediate oxidation state and finally the products: At a high pH, MnO 2 is the main product while Mn 2+ is the final product at a low pH.
Recently, due to the depleting of fossil fuels and the severe environmental problems caused by burning fossil fuels, supercapacitors with large-power density and longtime cycling have attracted attentions of many researchers [25,26]. As low-cost and easily obtained materials, the capacitive properties of MnO 2 , PANI, and MnO 2 /PANI composites have been widely studied [27][28][29].
In this work, we utilize the above mechanism to deliberately synthesize a series of MnO 2 /PANI composites with controllable morphology and uniform size by means of the interfacial polymerization and adjusting the pH of solutions. In the synthesis, monomer aniline and KMnO 4 are used as reducing agent in organic solution and oxidant in aqueous solution, respectively. PANI and MnO 2 /PANI are prone to diffusing into the aqueous phase because they are hydrophilic in the doped salt forms [3,23,24]. In the composite, PANI is expected to allow uniform MnO 2 particle dispersion and convenient electron transfer. In the present study, the formation mechanism and the electrochemical capacitive performance of the composites have been investigated.

Preparation of MnO 2 /PANI
Aniline was firstly distilled under reduced pressure. Then, 0.6 mL of aniline was dissolved in benzene (10 mL) solution, while 0.2 g of KMnO 4 was dissolved in the solution (20 mL) with 1 M ClO 4 − as the doping anion (we used HClO 4 as the source of ClO 4 − ). The organic solution was added into aqueous solutions slowly, and the mixture was kept overnight until the reactions conducted completely. The products were then washed with ultrapure water and centrifuged twice to remove residual benzene and KMnO 4 . Finally, the products were dried in the air for the latter use.

Preparation of the electrode
The composites were mixed with acetylene black (15 wt.%) and dispersed in 0.5 mL of anhydrous ethanol solution by sonication for 5 min. The mixtures were then cast onto a polished glassy carbon electrode and fasten with 2 μL of nafion ethanol solution (1% V/V). The electrodes were dried in the air for latter testing.
A CHI 760C electrochemical workstation (CHI Instruments, Austin, TX, USA) was used to collect electrochemical data. All electrochemical experiments were conducted in a three-electrode cell, in which a 1.5×1.5 cm 2 Pt plate was used as the counter electrode and a saturated calomel electrode was selected as the reference electrode.

Results and discussion
The schematic of MnO 2 /PANI fabrication procedure is shown in Figure 1. The reaction commences at the interface of the two solutions immediately as the aniline solution is carefully spread onto the aqueous solution of KMnO 4 . The interfacial polymerization does not terminate until KMnO 4 or aniline is consumed completely. The products diffuse into the aqueous solution spontaneously due to the doping procedure of the polymers and hydrophilic property of hydrate MnO 2 . The color of the products in different solutions (a to e: 1, 0.5, 0.2, 0.1, and 0 M HClO 4 , respectively, as shown in the inset of Figure 1) turns from green to brown. This color evolvement is attributed to the different components of composites accompanying with the change of PANI-doping degree. The SEM and TEM images, FTIR spectra, and XRD patterns were employed to investigate the components and the formation of the products.
PANI nanofibers form at the interface between the organic and 1 MHClO 4 aqueous solution. As observed in the SEM image ( Figure 2), the diameter and length of the nanofibers are around 100 to 200 nm and over 1 μm, respectively. Additionally, it reveals that the nanofibers are twisted and networks are formed by random interconnection, which agrees with the previous reports [3,23,24]. To indicate the evolvement of the samples' morphologies with the changing of acid concentrations, the TEM images of MnO 2 /PANI fabricated at different acid concentrations are collected in Figure 3. As shown in Figure 3A, PANI nanowires synthesized in 1 M HClO 4 solution is consistent with the SEM result in Figure 2. When the interfacial polymerization is carried out using 0.5 M HClO 4 ( Figure 3B), the conventional nanowire almost disappears. On the contrary, interconnected agglomerating chains appear. In addition, a number of hollow spheres can be observed. Interestingly, when the acid concentration decreases to 0.2 M ( Figure 3C), a larger portion of hollow spheres is observed. However, the portion of hollow spheres is decreasing with the decrease of the acid concentrations in the range of 0.1 and 0 M HClO 4 (shown in Figure 3D,E,F). In this way, we can modulate the sample structures easily by adjusting the pH of the aqueous solution.
An explanation in the procedure of composite fabrication is proposed in our work. Firstly, aniline monomers are polymerized only at the interface of the organic and aqueous phases, so that hydrophilic nanofibers can be separated from the interface and diffuse into the aqueous solution, which prevent the secondary growth and provide space for new nanofiber growing. Additionally, MnO 2 , as an oxidative regent for PANI polymerization, is used as sacrificial materials in forming various PANI structures [31,32]. According to the change of the morphologies (nanofibers, hollow spheres, and solid particles), it is reasonable to assume that the appearance of the intermediate of MnO 2 is a critical role in the formation of hollow spheres. As illustrated in Equations 1 and 2, for the low-acid concentration (0.5, 0.2, and 0.1 M), there is not enough H + at the interface to resolve the intermediate of MnO 2 because of the rapid H + consumption in the reaction (Equation 2). In the meantime, the resolution of MnO 2 restarts while the composite removes from the interface. The consequential reducing reaction of MnO 2 follows Equation 3 [33]: In the acid solution of lower concentrations (0.1 and 0 M HClO 4 ), MnO 2 appears both at the interface and the bulk solution, which caused a little portion of or no hollow spheres to obtain. In our study, it is thought that large amount of MnO 2 /PANI composites can be obtained at low-acid concentration, and the MnO 2 nanoparticles are wrapped by PANI.  Figure S1). The band near 1,303 cm −1 is becoming weaker from curves g to a in Figure 4, which suggests that the doping degree of PANI is changing with the acid concentration. The characteristic bands of curves a, b, and c in Figure 4 shifted right compared with the others, which is ascribed to the effect of MnO 2 on PANI. It demonstrates that

some special interaction exists between MnO 2 and PANI.
Due to the ordered and metallic-like property, conducting polymers possess particular crystallinity and orientation. As shown in the XRD patterns in Figure 5A, there are no identified peaks appeared for the products synthesized in low-acid concentrations (curves a to e: 0.1 M NaOH, and 0, 0.02, 0.05, and 0.1 M HClO 4 , respectively), which indicates the products are amorphous. For the products obtained at 0.2 (curve f ), 0.5 (curve g), and 1 M HClO 4 (curve h), two intense XRD peaks 2θ≈20 and 25°are observed corresponding to pure PANI according to previous literature [2]. All above results confirm that the crystallized PANI can be formed at higher acid concentrations in this work.
To further analyze the components at different acid concentrations, the samples were treated at 500°C (at which MnO x is crystallizing and PANI will be burned). The products obtained at 1, 0.5, and 0.2 M HClO 4 were burned out with no solids left, which indicates that there is no MnO 2 generating at such acid concentrations. Contrary to higher acid concentration, the solid residue of the products obtained at 0.1, 0.05, 0.02, and 0 M HClO 4 turned black. The FTIR spectra of the heattreated composites fabricated in 0.1, 0.05, 0.02, and 0 M HClO 4 were measured (Additional file 1: Figure S2). The characteristic FTIR spectra bands of PANI vanish after heat treatment, which confirms that PANI has been pyrolyzed after heat treatment.
The XRD patterns of the samples after heat treatment are shown in Figure 5B. The XRD patterns of the composite obtained in 0 (curve a) and 0.02 M HClO 4 (curve b) can be indexed to α-MnO 2 crystal structures [34]. Meanwhile, different XRD peaks are observed in Figure 5B (curves c and d), indicating the heat-treated product obtained in 0.1 M HClO 4 is Mn 2 O 3 and the heat-treated product obtained in 0.05 M HClO 4 are MnO 2 and Mn 2 O 3 . The results show that for as-prepared samples, Mn 2 O 3 phase is increasing with acid concentration. It is reported that the phase of manganese oxides is changing with temperature, and MnO 2 may transform to suboxide Mn 2 O 3 at 500°C to 900°C [33,[35][36][37][38]. The reductive matters such as CH 3 OH, CH 4 , and CO were studied as reductions for the phase transforming of MnO 2 to Mn 2 O 3 , and the mechanism was also suggested [34,39]. Therefore, we assume that the reductive matters generated during PANI decomposition procedure assists the transformation of MnO 2 to Mn 2 O 3 . Additionally, the aggravating degree of phase transforming of the heat-treated samples could be attributed to the increasing proportion of PANI in the composites. All the above results indicate that the MnO 2 generated in the polymerization of PANI process at low-acid concentration has a great effect on the formation of the hollow structure at higher acid concentrations as an intermediate.
In this work, the electrochemical performance of the composite was evaluated. The capacitance of MnO 2 is generated by the charge transferring among multivalent Mn element (Mn 2+ , Mn 3+ , Mn 4+ , and Mn 6+ ) [35], while PANI endures doping/dedoping companying with the redox process of PANI: Cyclic voltammetry (CV) curves of the composites are shown in Figure 6A. CV curves of as-prepared PANI nanofibers/MnO 2 crystallines are comparable with pure PANI and MnO 2 , respectively. The rectangle-like shape of CV curve suggests that MnO 2 /PANI fabricated in 0.02 M HClO 4 has an ideal capacitive characterization. Additionally, the rectangle-like shape potential region of MnO 2 /PANI (curve c) is relatively larger compared with that of the crystallized MnO 2 (curve e) and PANI (curve a). The capacitance C CP can be estimated according to the equation: C CP = (Q a + Q c )/(2 × ΔV), where Q a , Q c , and ΔV are indicative of the anodic and cathodic charges of CV and the potential region of CV, respectively. The  capacitances of the samples in curves a to e are 80, 45, 207, 143, and 46 F g -1 , respectively. The capacitance of MnO 2 /PANI (curve c) is larger than that of PANI (curve a) and MnO 2 (curve e). The extended ideal capacitive potential region and larger capacitance of MnO 2 /PANI composite are possibly due to the synergistic effect between the core of MnO 2 and the shell of PANI [32,35,40].
The charge-discharge curves of MnO 2 /PANI fabricated in 0.02 M HClO 4 were measured at various current densities (shown in Figure 6B). The E-t plots show symmetry, which indicate the reversible charge-discharge process of the MnO 2 /PANI composite. The specific capacitance of the sample can be calculated via the equation: C CP = i/|dE/ dt|, where |dE/dt| is estimated from the slope of the discharging curves. The capacitance of the composite at 2, 1, 0.5, 0.3, and 0.2 mA cm −2 achieves 159, 161, 170, 174, and 168 F g −1 , respectively. Additionally, the discrepancy of the largest composite capacitance values estimated from discharging and CV curves is lower than 20%, which suggests the high credibility of both techniques.
The stabilities of the samples were tested with 100 CV scan cycles (Additional file 1: Figure S3). After 100 cycles, the CV curves of PANI change obviously and the capacitances decreased largely (Additional file 1: Figure S3 A, B). However, with the increase of MnO 2 , the CV curves change a little and even no capacitance decrease is observed (as shown in Additional file 1: Figure S3 C,D,E). Compared with PANI samples obtained at higher acid concentration, MnO 2 /PANI nanocomposites possess noticeable capacitive stability. To investigate the long-term stability of as-prepared MnO 2 /PANI nanocomposites, the chargedischarge test of 1,000 cycles was conducted at 1 mA cm −2 in 0.1 M HClO 4 . As shown in Figure 6C (first 20 cycles are shown for clearly observation), the E-t plots are symmetric in shape and have almost no change during the long-term test. From Figure 6D, it can be seen that the discrepancy of capacitance of MnO 2 /PANI during 2,000-cycle test is lower than 5%, and there is no evident capacitance decrease after 1,000 cycles. The stability of the MnO 2 /PANI composite is thought due to the protection of the shield-surrounded PANI and uniform dispersion of MnO 2 particles, whereby avoiding severe particles conglomeration involved in the charge-discharge process [35,36]. The facile synthesis and ideal electrochemical capacitive performance will probably give the composites a promising prospect in the application of supercapacitors.

Conclusions
A series of samples including MnO 2 /PANI composites and PANI nanofibers were successfully synthesized by the facile interfacial polymerization. Controllable and uniform synthesis of the composites has been realized by simply adjusting the pH of the solution. The effect of acid concentration and the related mechanism of the formation of the products are investigated. We demonstrate that the intermediate of MnO 2 plays a key role in forming the hollow structures of PANI. The capacitance of the composite achieves 207 F g −1 , and the results suggest that the MnO 2 /PANI composites show superior performance over pure PANI or MnO 2 .