Intercalation of Nanoscale Multiferroic Spacers between the Two-Dimensional Interlayers of MXene.

We report a facile synthesis of MXene/bismuth ferrite (Ti3C2T x @ BiFeO3) nanocomposites using hydrazine hydrate induced with the hydrothermal method. Preparation of the composite with hydrazine hydrate prevents the formation of Bi25FeO39 inside the matrix. The 25 wt % loading of BiFeO3 acts as spacers for the multilayer restacking of MXene flakes by retaining the (002) plane. This promotes large dc conductivity with interfacial and defect dipole polarization mechanisms inside the composite material. The thinner M-H loop also indicates the minimal magnetic loss inside the composites. The facile synthesis strategy provides outstanding properties in Ti3C2T x @ BiFeO3 composites as two-dimensional multiferroic materials.

Introduction

The large-scale production of electric vehicles and wearable electronic devices is primarily integrated with the advanced technologies related to the efficient storage of electrical energies.[1,2] The higher cyclability and energy density make the electrochemical capacitors (ECs) reliable for such applications.[3] The charge storage mechanism and electrical properties of the electrode materials intrinsically determine the overall electrochemical properties of ECs.[4] Some materials with superior mechanical and electrical properties and the surface area largely control the EC’s intrinsic electrode properties.[5−7] MXenes, one of the evolving 2D classes of materials offers all these electrochemical properties for the better performance of the EC.[8−10] The synthesis of MXenes is performed through selective etching techniques from the MAX phase (MAX).[11] Here, M is a transitional metal ion such as Ti, A is a group XIII element such as Al, X is a carbonitride/carbide/nitride, and n = 1, 2, or 3.[12] After the selective etching of the A layer, the MXene compositions are generally presented as MXT,[13] where T is the surface functional group such as F, O, OH, etc.

Very soon after discovering MXene, it grows to nearly 30 different compositions of transitional metal ions’ carbonitrides/carbides/nitrides.[14] They mainly find applications in various fields of catalysis, water purification, sensors, optoelectronics, EMI shielding, and energy storage.[15−19] Among all these fields, the MXene family is mainly popular for energy storage applications.[20] Other than the properties such as superior mechanical and electrical properties and surface area, it also has functional groups and high conductivities.[21−23] So far, the use of MXene as an energy storage material for ECs reported higher capacitance.[24]

Generally, the conventional double-layer capacitors build electrical charge at the interface of the electrolytes and electrode materials. This process hindered the limit by which we could enhance the energy density of the capacitors. But the etching of the MAX powder leads to the formation of multilayers in MXene. Hence, integrating room-temperature multiferroics such as bismuth ferrite (BFO) as a spacer between the 2D MXene interlayers leads to a comprehensive capacitive charge storing mechanism.[25] In this mechanism, the presence of the dielectric bismuth ferrite material enhances the interaction of ions between the MXene layers, known as capacitive intercalations.[26] But so far, reported integration of BFO into Ti3C2T MXene used solvothermal techniques with heavy agglomerations.[27] Hence, the effort to optimize the synthesis technique for Ti3C2T @ BiFeO3 using the facile hydrothermal method is still lagging.

Herein, we synthesized a Ti3C2T @ BiFeO3 composite with the hydrothermal technique. We chose hydrazine hydrate as a reducing agent because it helps intercalate the BiFeO3 phase between the 2D layers of Ti3C2T. We further discuss the impact of replacing hydrazine hydrate with ammonia on the fabrication of the ferrite phase between the interlayers.

Experiments

Materials

Ti3AlC2 (titanium aluminum carbide) was purchased from Nanochemazone. Bi(NO3)3 · 5H2O (bismuth nitrate pentahydrate), Fe(NO3)3 · 9H2O (iron nitrate nanohydrate), KOH (potassium hydroxide), N2H4 · H2O (hydrazine hydrate), NH3 · H2O (ammonia), HNO3 (nitric acid), HF (hydrofluoric acid), and C2H5OH (ethanol) were supplied by Alpha Chemika. No further purification methods were used after that.

Synthesis of MXene Ti3C2T

Initially, 4 g of MAX phase was slowly mixed with 200 mL of HF and kept in a polypyrrole container in a fume hood, avoiding any exothermic heating. After that, the mixture was kept at a temperature of 35°C for 72 h with gentle stirring with a polypyrrole rod at an interval of 12 h. The black powders were rinsed several times with demineralized water and finally washed with C2H5OH until they reached pH 6 using a centrifuge machine.

Synthesis of the Composites

In situ preparation of the Ti3C2T @ BiFeO3 composite was carried out by the hydrothermal technique. In general, 1.0244 g of Bi(NO3)3 · 5H2O and 0.8533 g of Fe(NO3)3 · 9H2O were dispersed into 20 mL of demineralized water with 4 M KOH until the pH reached 11. In another container, 1.54 g of Ti3C2T powders were mixed with N2H4 · H2O until the pH reached 11. Both the mixtures were transferred into a Teflon container slowly with continuous stirring. Finally, the hydrothermal reaction was carried out for 8 h at 220°C. Thereafter, the brown powders were cleaned with demineralized water and vacuum-desiccated for 12 h at a temperature of 60°C to obtain the Ti3C2T @ BiFeO3 composite. The same procedure was also followed for the Ti3C2T @ Bi25FeO39 composite; only N2H4 · H2O was replaced by NH3 · H2O in this case. For both cases, stoichiometric formulations were carried out to reach 25 wt % of ferrite phase loading within the MXene structure.

Composite Characterizations

X-ray diffraction (XRD) was performed with a Bruker D2 model diffractometer (CuKα, 1.5418 Å). The FTIR spectra were obtained from a spectrophotometer (PerkinElmer 2000) up to 4000 cm–1. The morphology of the composite was determined with a Hitachi SU1510 scanning electron microscope. In diffuse reflection mode, the optical bandgap was deduced by a UV/Vis/NIR spectrometer (PerkinElmer Lambda 750). Magnetic measurements were taken at room temperature with a Quantum Design MPMS-XL EverCool SQUID for −16 kOe to 16 kOe field. The impedance and dielectric characteristics were found with a Hioki IM3536 LCR meter. The detailed process of the formation of the 2D structure and the integration of BiFeO3 and Bi25FeO39 with Ti3C2T is presented in Figure .

Figure 1

Demonstration of the fabrication process of the laminated structure and integration of the composite system (color online).

Results and Discussion

XRD and Reaction Mechanism

The powder XRD profile of pure BiFeO3, the composite Ti3C2T @ BiFeO3, the composite Ti3C2T @ Bi25FeO39, MXene Ti3C2T, and pure Ti3AlC2 is shown in Figure . The HF etching completely removes the (104) plane reflection of Ti3AlC2 due to aluminum layers, as it completely disappears from the XRD profiles of Ti3C2T.[28] It also resulted in a broadening and leftward shift from 9.5° to 8.9° of the (002) plane peak due to etching. The d spacing calculation with Bragg’s law indicates the available spacing of 0.93 nm between the 2D layers of MXene.[29,30] The lattice constant a = 0.56 nm of BiFeO3 corresponds to JCPDS 020-0169 with a rhombohedral R3c space group of symmetries.[31] The retention of the (002) peak for Ti3C2T @ BiFeO3 indicates the successful interfacial interactions between the two materials. The higher intense peaks in the composite belong to BiFeO3 in addition to Ti3C2T peaks.[32] The characteristics peaks related to the BiFeO3 phase appeared at (012), (104), (202), (024), (122), (208), (217), and (312). We may propose that hydrazine hydrate helps transfer the metal nitrates into metal hydroxide and oxide ions, which are then combined to form the BiFeO3 phase:

Figure 2

XRD for Ti3C2T @ Bi25FeO39, the composite Ti3C2T @ BiFeO3, pure BiFeO3, MXene Ti3C2T, and the Ti3AlC2 MAX phase (color online).

On the contrary, the use of another reducing agent like ammonia may initiate a different mechanism for the integration of the metal hydroxide and oxide ions as:

This might result in unsuccessfully induced orientations of the Bi25FeO39 phase within the available d space of 0.93 nm, as its lattice constant is a = 1.02 nm for the cubic I23 space group (JCPDS 046-0416).[33] Hence, the characteristic (002) plane reflection completely disappears in the composite structures due to new interfacial interactions in the Ti3C2T @ Bi25FeO39 composite. The gradual appearance of the Ti3C2(OH)2 peaks at 25.5° was also noticed in the Ti3C2T @ Bi25FeO39 composite.[34] The absence of the MXene peaks at 8.9° coming from (002) for the Ti3C2T @ Bi25FeO39 composite indicates the aggregation of the Bi25FeO39 phase on the MXene flakes.[35]

SEM

Figure presents micrographs obtained for all the samples. As can be noticed, the HF etching resulted in multilayer nanosheets of Ti3C2T due to the removal of aluminum layers from the dense, compact structures of Ti3AlC2 shown in Figure b. The interlayer spacing in Ti3C2T resulted in the (002) peak in the XRD profiles. The integration of BiFeO3 retained the sheet structure of Ti3C2T, but it became thick and non-uniform as shown in Figure d. This may be the different orientation of the rhombohedral BiFeO3 Bravais lattice in between the 2D MXene layers. But the use of ammonia while integrating the Bi25FeO39 phase in the Ti3C2T structure resulted in the aggregation of the particles all around the 2D structures as shown in Figure c. Maybe that resulted in the absence of the (002) peak in the XRD profiles of Ti3C2T @ Bi25FeO39 composites.[36]

Figure 3

SEM micrographs for (a) Ti3AlC2, (b) Ti3C2T, (c) Ti3C2T @ Bi25FeO39, and (d) Ti3C2T @ BiFeO3 composites.

FTIR Analysis

The vibrational spectra mainly originate from the hydroxyl group obtained from the FTIR characterization shown in Figure for the Ti3C2T @ Bi25FeO39 composite. The vibration (νO – H) from out-of-plane stretching and bending of the hydroxyl group originated at 3470 and 1695 m–1, respectively.[37] The similar nature of the FTIR spectra between Ti3C2T and Ti3C2T @ BiFeO3 indicates the absence of any oxidized phase in the composite structure.[38]

Figure 4

FTIR spectra for Ti3C2T, Ti3C2T @ BiFeO3, and Ti3C2T @ Bi25FeO39 obtained through the hydrothermal reaction (color online).

Bandgap Studies

Bandgap determination was performed using the reflectance spectra shown in Figure a. The reflectance of light within the 3000–8000 Å range shows a continuous pattern for Ti3C2T and Ti3C2T @ BiFeO3 samples, whereas the reflectance edge is visible for only Ti3C2T @ Bi25FeO39. MXene has superior conductivity to metals. Hence, the possible overlapping of the conduction and valence bands resulted in a zero bandgap for MXene,[39] whereas the fabrication of BiFeO3 with a small quantity retained the original multilayer sheet-like structure of MXene in the Ti3C2T @ BiFeO3 composite, which behaves similarly to the MXene powders. Not only that, the restacking in the Ti3C2T @ BiFeO3 composite does not affect the overall surface reflectance of the multilayer flakes.

Figure 5

(a) Reflectance spectra of Ti3C2T, Ti3C2T @ BiFeO3, and Ti3C2T @ Bi25FeO39 samples and (b) Tauc’s plot of Ti3C2T @ Bi25FeO39 (color online).

The strong aggregation of the Bi25FeO39 particles strongly affects the interaction of the Ti3C2T flakes with light, although the same 25 wt % anharmonic oscillations originating from the dipoles of the Fe, Bi, and O ions resulted in a separation between the two Morse curve states in Ti3C2T @ Bi25FeO39 composites. The direct bandgap is determined by Tauc’s plot with the Kubelka–Munk equation Eg = hν – [F(R( ∞ ))hν]2 shown in Figure b.[40] The narrow bandgap energy for the Ti3C2T @ Bi25FeO39 composites was determined to be 2.2 eV.

Capacitive and EIS Properties

The room-temperature variation of complex permittivity (ϵ′ and ϵ″) with frequency is presented in Figure . The frequency dispersion due to 25 wt % ferrite phase loading on MXene can be observed for both the composites. But the higher value of the complex permittivity (ϵ′ and ϵ″) was observed at lower frequencies for the BiFeO3 phase fabrication on MXene. These directly indicate the better storage and dissipation capabilities of the electrical energies by the Ti3C2T @ BiFeO3 composites. The different dielectric properties between the two composites indicate a different dielectric relaxation mechanism within the composites.[41] This nature relates to the interfacial nature of the MXene sheets and the existence of the rhombohedral lattice structure of the BiFeO3 nanoparticles. The interfacial nature of the MXene sheets resulted in the accumulation of electrons and ions in alternative layers, resulting in interfacial polarization. On the other hand, the rhombohedral lattice resulted in orientational relaxation inside the Ti3C2T @ BiFeO3 composites due to the spontaneous dipole moment with numerous defects in the BiFeO3 crystals causing permanent defect dipoles.[42] These defect dipoles aligned themselves to the applied electric field when the frequencies were low, resulting in the orientational polarization. At higher frequency, due to the misalignment owing to the effect of inertia, permittivity decreases. In general, the dielectric nature of the composites can be described by Debye and Maxwell–Wagner (M–W) mechanisms[43]whereas the first part of eq is the same as the Debye mechanism of eq . The fitting of the dielectric data for both the composites is presented in Figure b. It can be visibly noticed that the Ti3C2T @ Bi25FeO39 composites satisfy the Debye mechanism, whereas the Ti3C2T @ BiFeO3 composites obey the M–W mechanism. The dc conductivity (σ) determined from the fitting was found to be 1.30 × 1011 S/m Ti3C2T @ BiFeO3 composites. The strong agglomeration of the Bi25FeO39 phase affected the interfacial polarization due to the absence of the spontaneous dipole moments in the cubic lattice. These factors resulted in the Debye relaxation mechanism inside the Ti3C2T @ Bi25FeO39 composites.

Figure 6

(a) Real part of permittivity with frequency for Ti3C2T @ BiFeO3 and Ti3C2T @ Bi25FeO39 composites. (b) The dielectric data was fitted with the Maxwell–Wagner model. The pink line shows fitting results with the Debye model (color online).

The resistance of the composites was further calculated using the Nyquist plots shown in Figure . A single semicircular half-arc with different radii was observed for the composites. The higher half-arc radius for the Ti3C2T @ Bi25FeO39 composite indicates higher interface resistance, whereas the smaller half-arc radius of the Ti3C2T @ BiFeO3 composite indicates the easy charge transfer process between the interlayers.[44] Not only that, at higher frequencies, it offers the minimum real impedance and maximum imaginary impedance. The nature reverses with the decrease in frequency, indicating the capacitance behavior at room temperature.

Figure 7

Nyquist plot of the (a) Ti3C2T @ Bi25FeO39 and (b) Ti3C2T @ BiFeO3 composites with an equivalent circuit (inset) (color online).

This shift of the intra- and interlayer effects on the charge carriers can be understood with the equivalent circuit of the impedance data shown in the inset of Figure b, and the impedance of the circuit can be written as Rl + RilZCPE, where Rl is the intralayer resistance, Ril is the interlayer resistance, and ZCPE is the constant phase element. The obtained parameters with the fitting results are presented in Table . The interlayer resistance for the Ti3C2T @ Bi25FeO39 composite was found to be in GΩ due to the agglomeration of the Bi25FeO39 phase, whereas the interlayer resistance for the Ti3C2T @ BiFeO3 composite is the same as that for the 2D Ti3C2T as reported previously.[45] Hence, the successful fabrication of BiFeO3 in the 2D Ti3C2T resulted in a single relaxation mechanism, non-exponential variation, and decreased interlayer barrier on the mobility of charge carriers.

Table 1

Electrical Parameters Obtained from EIS Studies

parameter	Ti3C2Tx @ Bi25FeO39	Ti3C2Tx @ BiFeO3
Rl (Ω)	44	45
Ril (Ω)	2.506 × 108	60
T (Ω s–1) × 10–10	6.654	1.130
p	0.70	0.99

Magnetic Properties

The magnetic hysteresis loop for the Ti3C2T @ Bi25FeO39 and Ti3C2T @ BiFeO3 composites is presented in Figure at room temperature with a maximum of 16,000 Oe of the applied magnetic field. It is evident from the graph that the composite shows a weak magnetic behavior and the maximum of 16,000 Oe field is not sufficient to saturate the magnetization. The magnetization in this regard is expressed with the formula in terms of Bohr magneton (μB).[46] The obtained values of the saturation magnetization (Ms) were 0.0036 and 0.0031 μB for the Ti3C2T @ Bi25FeO39 and Ti3C2T @ BiFeO3 composites with respect to the ferromagnetic element like iron with a value of 5 μB. Both the composites show an equal coercivity of 119 Oe, but the Ti3C2T @ Bi25FeO39 composite has higher retentivity, as shown in the inset of Figure . Hence, a thinner M-H loop in Ti3C2T @ Bi25FeO39 with retained weak magnetic behavior indicates a lower magnetic loss inside the composites.

Figure 8

Room-temperature magnetic hysteresis loop for the Ti3C2T @ Bi25FeO39 and Ti3C2T @ BiFeO3 composites. The inset shows the retentivity and coercivity of the composites (color online).

Conclusions

The engineering of the 2D Ti3C2T structures with interconnected multilayers with nanometer spacing is crucial for capacitor applications. The better electrical conductivity plays a vital role in the transport of electrons within this interconnected network of 2D layers. Hence, the hybridization of the multiferroic BiFeO3 with spontaneous polarization due to the rhombohedral structure with MXene can be a critical step in fabricating superior capacitor materials. The main challenge associated with it is to stop agglomeration and provide mechanical support for the restacking of the flakes by finding a balance between the multiferroic spacers inside the 2D materials.

The design and synthesis of the composite material with an appropriate reducing agent and balanced wt % may lead to the desired morphology and performances using the electrostatic force of attraction of the ions. The use of ammonia as a reducing agent while hybridizing the multiferroic structure with the 2D Ti3C2T structure using the hydrothermal method may lead to severe agglomeration with the Ti3C2T @ Bi25FeO39 phase. The electrostatic interactions and neutralization of the −ve hydroxyl ion vibration by the +ve metal ions were observed with the redshift of the FTIR spectra with a 2.2 eV narrow bandgap energy in Ti3C2T @ Bi25FeO39 agglomerated samples.

At the same time, the use of N2H4 · H2O as a reducing agent while hybridizing the multiferroic structure with the 2D Ti3C2T structure leads to a Ti3C2T @ BiFeO3 composite as a spacer between the multilayers. The fabrication of 25 wt % BiFeO3 with MXene retained the original multilayer sheet-like structure and metallic behavior observed from SEM and UV–Vis analysis. The dielectric analysis indicates a dc conductivity of 1.30 × 1011 S/m magnitude with the presence of both interfacial and defect dipole polarization mechanisms. Moreover, the thin M-H loop means a minimal magnetic loss inside the composites. Hence, this straightforward, inexpensive, and quick method of preparation with superior properties may be applied to 2D multiferroic materials.