Formation processes, size changes, and properties of nanosheets derived from exfoliation of soft layered inorganic–organic composites

Exfoliation is a general route to obtain two-dimensional (2D) nanomaterials. A variety of methods have been developed for controlled exfoliation of layered materials based on stacking via van der Waals interaction, such as graphite and transition-metal dichalcogenides. On the other hand, rigid layered materials consisting of inorganic layers and interlayer metal ions stacked via electrostatic interaction, such as transition-metal oxides and clays, have a limited number of exfoliation methods. Here we studied a new exfoliation route through formation of soft layered composites. Intercalation of guest organic molecules changed rigid inorganic layered compounds into soft layered composites with stacking via van der Waals interaction. The soft layered inorganic–organic composites were exfoliated into surface-modified nanosheets in organic media. The layered composites showed swelling with dispersion in organic media. The time-course analyses suggest that the layered composites were simultaneously exfoliated in the vertical direction and fractured in the lateral direction. Thinner and smaller nanosheets were obtained with an increase in the exfoliation time. Although the resultant nanosheets gradually aggregated in the colloidal liquid, the original dispersion state was recovered with sonication for 5 min at room temperature. This exfoliation route using soft layered composites can be used in the synthesis and application of a variety of 2D nanomaterials.

The resultant C14-TiO2 was centrifuged, washed with purified water and ethanol, and then dried at room temperature.
Exfoliation of the layered C14-TiO2. The C14-TiO2 powder, typically 30 mg, was dispersed in 12 cm 3 of toluene at 60 °C under magnetic stirring around 300 rpm for certain period (1-120 h). The dispersion liquid was then filtered to remove the unexfoliated powder (pore size of the filter: 2.0 μm). The resultant supernatant was used as the colloidal liquid containing the C14-TiO2 nanosheets. Exfoliation of the C14-MnO2 was performed by the similar method according to our previous work. 29 Characterization. The particle-size distribution of the C14-TiO2 nanosheets was measured by dynamic light scattering (DLS, Otsuka Electronics, ELSZ-2000ZS). The colloidal liquid was casted on a cleaned silicon substrate for atomic force microscopy (AFM, Shimadzu SPM-9700HT) observation and dropped on a collodion membrane for transmission electron microscopy (TEM, FEI Tecnai) observation operated at 120 kV. The C14-TiO2 nanosheet was collected by filtration of the dispersion liquid using a filter with pore size 0.1 μm to measure the yield. The crystal structure and interlayer distance were analyzed by X-ray diffraction (XRD, Bruker D8 Advance) with Cu-Kα radiation. The organic content of the layered C14-TiO2 and nanosheets was measured by thermogravimetry (TG) analysis (Seiko, TG-DTA 7000) in air atmosphere. The morphology of the layered C14-TiO2 was observed by field-emission scanning A series of this dry-wet repetitive experiment were performed by the same sample in the order of (i) to (vi). The peak position was reversibly changed with immersion in toluene and drying.
The dried and wet states showed the interlayer distance around d0 = 2.9 nm (the open circles) and d0 = 3.8 nm (the filed circles), respectively. The results indicate the swelling of the original layered C14-TiO2 with toluene. Moreover, the swelling state was achieved with immersion in toluene even for 10 min at room temperature (the pattern (vi) in Fig. S1).
P. S5  The chemical formula of the C14-TiO2 was estimated to be H0.7-x(C14-NH2)xTi1.825O4·yH2O according to our previous report. R1,29,34 The x and y in the formula were calculated by TG analysis. These values were calculated to be x = 0.49, y = 1.11 for the original layered C14-TiO2 (i), x = 0.55, y = 0.75 for the nanosheets after exfoliation in toluene at 60 °C for 3 days (ii), x = 0.53, y = 0.67 for the nanosheets after exfoliation in toluene at 60 °C for 5 days (iii). The contents of the C14-NH2 were not changed even after the exfoliation. The results indicate that the surface C14-NH2 was not removed from the nanosheets in toluene.

Fig. S4. Schematic models of the C14-TiO2 monolayers (a,b) and bilayer (c).
According to our previous reports, both the sides of the resultant nanosheets were modified by the alkylamine (Fig. S4a). 29 The thickness of the monolayered C14-TiO2 was estimated to be 1.5-2.0 nm by AFM analysis in our previous report (Fig. S4a). 29 The thickness of 1.5 nm corresponds to the tilted angle of the alkyl chain to the layer (θ) 12.3 °. The thickness of the monolayer (tm / nm) is represented by (eq. S1) depending on θ (Fig. S4a), where the thickness of the bare titanate monolayer is assumed to be 0.7 nm according to the previous report and the molecular length of C14-NH2 is calculated to be 1.87 nm. 12,39 tm = 0.7 + 2 × 1.87 sin θ … (eq. S1) If the alkyl chains are arranged perpendicular to the layer surface (θ = 90 °), the maximum thickness of the monolayer is estimated to be around 4.5 nm (Fig. S4b). Therefore, the thickness of the monolayer is defined as 1.50-4.50 nm in the present work. The thickness of the bilayer (tb) structure is calculated to be 4.42-7.42 nm by (eq. S2) on the assumption that the interlayer space (d0) is the same as that of the layered C14-TiO2 (d0 = 2.92 nm) and the tm is regarded as the sum of t1 and t2 in Fig. S4c.
In this manner, the thickness of the N-layered nanosheets (tN) is calculated by (eq. S3).
Therefore, the thickness of the few-layer, namely N = 2-5, is defined as 4.50-16.5 nm. The nanosheets thicker than 16.5 nm, namely N > 5, is regarded as multi-layer.
The summarized data were shown in Fig. 8 in the main text.  Table S1, the materials with numbers 1-11,13,14 and 12,15-20 correspond to the layered compounds consisting of van der Waals interaction and electrostatic interaction, respectively.