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

In this study, we demonstrate the effects of surfactants on the morphology of Nb2C MXene-derived ferroelectric KNbO3 crystals. We synthesized plate-like KNbO3 crystals using Nb2C MXene and KOH as niobium and potassium sources, respectively, and sodium dodecyl sulfate as the surfactant. In addition, we conducted the same hydrothermal process for Nb2C MXene and KOH as niobium and potassium sources, respectively, and benzyltriethylammonium chloride as a surfactant; we found that the final product exhibits bulk morphology. We show that the morphology of MXene-derived KNbO3 crystals is strongly related to the interaction between the MXene flakes and the functional groups of the surfactant during the hydrothermal process. 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 Mn+1XnTx (n = 1–3), where Tx 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.