Effect of surfactants on the morphology of ferroelectric crystals grown from MXene

As one of the latest additions to the two-dimensional (2D) family, 2D transition metal carbides or carbonitrides (MXenes) have gained considerable attention in the last decade due to their unique properties. In general, 2D MXenes are fabricated via selective etching of the A-atomic layer in transition metal ternary carbide and/or nitride (MAX phase) precursor, where M is an early transition metal element (e.g., Ti, Nb, V, and Zr), A represents an A group element (e.g., Al, Zn, Si, and Au), and X stands for C and/or N. The general formula of 2D MXenes is M_n+1X_nT_x (n = 1–3), where T_x represents surface terminations (e.g., hydroxyl, oxygen, or fluorine).^1,2 MXenes show promise in numerous applications, such as electrochemical energy storage,^3–6 electromagnetic interference (EMI) shielding,^7–9 water purification,¹⁰ gas- and biosensors,^11,12 photothermal therapy,^13,14 and so on,¹⁵ due to their unique 2D layered structure, high metallic conductivity (>7000 Scm⁻¹), and hydrophilic, photothermal, and large electrochemically active surface. However, their potential in electronic applications has not been sufficiently explored.

In previous studies by the present authors, plate-like KNbO₃ ferroelectric crystals were synthesized using Nb₂C MXene and KOH as reactants and sodium dodecyl sulfate (SDS) as the surfactant,¹⁶ proving the versatility of MXenes in optoelectronic applications. During the hydrothermal process, the authors found that the use of surfactants has a significant influence on the morphology of the final product, i.e., KNbO₃ crystals. Several reports revealed that as an industrial surfactant, SDS plays a vital role in the hydrothermal synthesis of nanocrystals.^17–19 Ohmura et al. reported the influence of the SDS surfactant on the crystal growth dynamics of methane hydrate formed at the gas–liquid interface.¹⁷ Yu et al. demonstrated that SDS has a dominant effect on the morphology of NaNbO₃ nanostructures in a hydrothermal reaction.¹⁸ In the present study, the effects of the SDS surfactant on the morphologies of MXene-derived KNbO₃ crystals are examined. In particular, we demonstrate the shaping effect of the SDS surfactant and propose a mechanism based on repulsion between the negative surface functional and electronegative groups generated after dissolving SDS in distilled water. To verify the proposed model, we investigate the shaping effect of another surfactant, benzyltriethylammonium chloride (TEBAC), which can generate electropositive groups after dissolving in distilled water.

Figure 1(a) shows the crystal structure of the Nb₂AlC MAX phase, which was synthesized using niobium, aluminum, and graphite commercial powders as precursors. The Al atomic layer of the dense Nb₂AlC MAX phase was etched out via following a reported protocol,²⁰ and bulk Nb₂C MXene with a layered crystal structure was obtained [Fig. 1(b)]. Figure 1(c) shows the crystal structure of hydrothermally synthesized plate-like KNbO₃ crystals using Nb₂C MXene powder as a precursor.

The scanning electron microscopy (SEM) image of the Nb₂AlC MAX phase shows dense bulk morphology [Fig. 2(a)]. The atomic arrangement of the Nb₂AlC MAX phase is well illustrated by the scanning transmission electron microscopy (STEM) image acquired using the beam along the [1-100] zone axis [Fig. 2(b)]. Two layers of brighter spots (the Al atomic layers) were staggered with two adjacent layers of darker spots (Nb atomic layers); meanwhile, C was invisible because of its low contrast compared with the heavier Nb and Al atoms. The distance between two adjacent Al atomic layers was 8.5 Å, which is consistent with the previous report.²¹ The x-ray diffraction (XRD) pattern of the Nb₂AlC MAX phase presented characteristic peaks at 2θ values of ∼13° (002), ∼26° (004), and ∼40° (006), indicating the formation of a typical 211 MAX phase [shown in Fig. 2(c)]. To obtain Nb₂C MXene, the Al atomic layer was selectively etched out from the Nb₂AlC MAX phase. Figure 2(d) shows an accordion-like morphology of Nb₂C MXene after HF acid treatment. The STEM image of Nb₂C MXene taken along the [0001] zone axis [Fig. 2(e)] confirmed the hexagonal symmetry of Nb atoms. The XRD pattern of the Nb₂AlC MAX phase [Fig. 2(f)] shows that the (002) characteristic peak shifts to a lower value (2θ = 9°), indicating an obvious expansion of d-spacing due to the removal of Al atoms.

To check the effect of the surfactant on the morphology of MXene-derived ferroelectric crystals, hydrothermal synthesis of KNbO₃ crystals was performed by changing the amount of the surface surfactant. Figure 3(a) schematically illustrates the chemical structure of the SDS surfactant used in this hydrothermal process. Without the addition of the SDS surfactant, the final KNbO₃ product presented an irregular morphology [Fig. 3(b)], indicating that no shaping effect occurred during the entire hydrothermal process due to the absence of the surface surfactant. By adding SDS in moderate concentration (1 g), the increased shaping effect resulted in cube-like crystals [Fig. 3(c)]. Moreover, upon adding sufficient SDS (4.33 g) surfactant into the reaction mixture, the final KNbO₃ product presents a plate-like morphology [Fig. 3(d)]. These results indicate that the concentration of the SDS surfactant plays a predominant role in determining the morphology of MXene-derived KNbO₃ crystals. Figure 3(e) shows the atomic force microscopy (AFM) image of a KNbO₃ crystal with several atomic layers. The transmission electron microscopy (TEM) image further confirms the plate-like morphology of MXene-derived KNbO₃ single crystals [Fig. 3(g)]. The orthorhombic crystal structure of KNbO₃ crystals is verified using the selected area electron diffraction (SAED) pattern taken along the [001] direction (inset i), and the XRD pattern further confirmed the same [Fig. 3(f)]. According to these results, we propose a mechanism based on the repulsion between negative functional groups on the MXene surface and electronegative groups generated after dissolving SDS in distilled water. According to previous reports,^12,22,23 several negative functional groups (e.g., –F, –O, and –OH) exist that bind to Nb atoms in the Nb₂C MXene obtained using an aqueous solution method, which was confirmed by XPS analysis (Fig. S2). Meanwhile, the electronegative group, C₁₂H₂₅SO₄⁻, was formed after dissolving SDS in distilled water. After surrounding MXene using enough C₁₂H₂₅SO₄⁻ groups, mutual repulsion between surface binding-negative functional groups and C₁₂H₂₅SO₄⁻ prevents Nb₂C MXene flake aggregation during the entire hydrothermal reaction, which ensures that the nucleation and growth of plate-like KNbO₃ crystals get initiated on the MXene flakes. Furthermore, after 12 h of hydrothermal reaction, intermediate products were obtained; these were analyzed to investigate the reaction procedure of this hydrothermal process. The intermediate XRD pattern shows the existence of the NbO, Nb₂O₃, NbO₂, Nb₂O₅, and KNbO₃ phases (Fig. S1a), which indicates that Nb₂C MXene sequentially undergoes oxidation and alkalization to form the final products. Figure S1b presents the optical image of hexagonal NbO₂ and monoclinic Nb₂O₅ flakes. These results demonstrate that niobium oxides inherit the sheet-like character of MXene flakes.

To verify the mechanism proposed herein, we investigated the shaping effect of another surfactant, benzyltriethylammonium chloride (TEBAC), on the morphology of MXene-derived KNbO₃ crystals. Figure 4(a) illustrates the chemical structure of TEBAC, and Fig. 4(b) shows the chemical structure of electropositive C₆H₅N(CH₃)₃⁺ groups obtained after dissolving TEBAC in distilled water. Undergoing a 48-h hydrothermal reaction using Nb₂C MXene and KOH as precursors and TEBAC as the surfactant, the final KNbO₃ product presents typical bulk morphology [Fig. 4(c)], and Fig. 4(d) shows the XRD pattern of the as-synthesized bulk KNbO₃ crystals, indicating that electropositive C₆H₅N(CH₃)₃⁺ groups accelerate the aggregation process of MXene flakes because of its charge attraction with negative functional groups on the MXene surface. Thus, we can conclude that the hypothesis of electronegative groups can explain the shaping effect of SDS on the morphology of MXene-derived KNbO₃ crystals.

In summary, we proved that by loading a high concentration of SDS, the repulsion between electronegative C₁₂H₂₅SO₄⁻ groups and negative functional groups on the MXene surface prevents the aggregation of MXene flakes, which ensures the formation of plate-like KNbO₃ crystals after the hydrothermal process. In comparison, the final MXene-derived KNbO₃ product using TEBAC as a surfactant presented bulk morphology, indicating that the existence of electropositive C₆H₅N(CH₃)₃⁺ groups prompts the settlement of MXene flakes because of their charge attraction with surface-negative functional groups. Finally, we conclude that the electronegative C₁₂H₂₅SO₄⁻ groups play a vital role in determining the morphology of MXene-derived ferroelectric crystals.

See the supplementary material for the additional results.

We acknowledge funding support by Nanchang University (NCU) (Grant No. 1001-600513) and King Abdullah University of Science and Technology (KAUST) (Award No. OSR-2016-CRG5-2977). The authors would also like to acknowledge the Imaging and Characterization Laboratory at Harbin Institute of Technology (HIT) for their assistance.