*3.3.1. Atomically thick Co3O4 nanosheets through topotactic oxidation transformation*

The schematic illustration of the preparation of ultrathin Co3O<sup>4</sup> nanosheets is shown in **Figure 8**. As illustrated in **Figure 8a**, the thickness of (001)-oriented few layered α-Co(OH)2 is

**Figure 8.** (a) Schematic illustration of the preparation of ATCNs. (b) Schematic model of the top-view illustrating the topochemical transformation from α-Co(OH)2 (001) plane to Co3O4 (111) plane. (c–e) TEM image, HRTEM image and the corresponding fast Fourier transform pattern of ATCNs. (f and g) Tapping model AFM image and corresponding height profile of as-obtained nanosheets deposited on a silicon substrate. (h) Photograph of a colloidal suspension of the ATCNs displaying the Tyndall effect. (i) XRD pattern of ATCNs-based film displaying highly preferred [111] orien‐ tation [38].

closer to the thickness of (111)-oriented Co3O4 nanosheets. More specifically, the discrepancies between α-Co(OH)2 {100} (*d*=2.76 Å) and Co3O4 {220} (*d*=2.84 Å) are calculated to be below 3%. Moreover, Co2+ atomic arrangement in α-Co(OH)2 (001) plane and Co3O4 (111) plane is very closer (**Figure 8b**). These features enable atomically thick Co3O4 nanosheets to preserve the thickness of the precursor possible.

Atomically thick Co3O4 nanosheets were prepared by this method, as confirmed by TEM, AMF and XRD. From TEM image, one could clearly see ultrathin nanosheets with lateral size of about 400 nm with the near transparency of the sheets (**Figure 8c**). HRTEM image, and the corresponding fast Fourier transform (FFT) pattern in **Figure 8d** and **e** for the as-obtained product depicts its [111] preferential orientation. The thickness of the as-synthesized Co3O4 nanosheets is also evaluated by tapping-mode atomic force microscopy (AFM). AFM image and the corresponding height distribution (**Figure 8f, g**) show that the measured height is about 1.5 nm. Microscopically, the colloidal suspension of the product displays the Tyndall phenomenon (**Figure 8h**), so that the formation of homogeneous ultrathin nanosheets in ethanol can be inferred. As shown in **Figure 8i**, XRD pattern of the atomically thick nanosheetbased films can be readily indexed to be pure Co3O4 (JCPDS no. 42-1467) and also displays the highly preferred [111] orientation, which is corresponding to the result of HRTEM. All of the results undoubtedly confirm that Co3O4 nanosheets with ultrathin thickness have been successfully synthesized. The oriented growth of ATCNs is attributed to α-Co(OH)2-to-Co3O4 topochemical conversion with the relationship of [001] α-Co(OH)2 and [111] Co3O4.
