Protons and hydroxide ions show different preferences in the etching of CPs crystals. In the cases of Prussian Blue analogues (PBAs), H
+ ions prefer the faces, while OH
– ions prefer the corners. For instance, when Ni-Fe PBA cubes were etched by HCl (0.5 mol/L), the H
+ ions preferentially attacked the central area of the faces of a cubic crystal, leading to a faster dissociation of the faces than the edges [
24]. As a result, the initial Ni-Fe PBA cubes were converted into Ni-Fe PBA concave nanocubes at first, before becoming monocrystalline nanoframes. The morphological evolution is illustrated in
Fig. 2. When ammonia is used as the etchant, it provides OH
– ions [
25]. Nai
et al. reported the etching of Ni-Fe PBA cubes by using ammonia (28%–30%) at room temperature [
26]. Nanocages with open frameworks and hollow interiors were obtained after etching. By means of Fourier transform infrared (FT-IR) spectra and time-dependent transmission electron microscopy (TEM) analysis, the Ni
II-N≡C-Fe
III and Ni
III-N≡C-Fe
II species are found to coexist within the Ni-Fe PBA crystals. The Ni
II-N≡C-Fe
III species are located mainly at the center, while the Ni
II-N≡C-Fe
II is located near the surface of the crystals. Time-dependent observations indicated that the etching occurred preferentially with the Ni
II-N≡C-Fe
III species rather than Ni
II-N≡C-Fe
II due to the stronger affinity of the Fe
III species to the -CN- group (
Fig. 3). Based on the same etchant, Han
et al. fabricated Ni-Co PBA nanocages which were constructed of pyramidal walls [
27]. The key point is that there were richer number of defects in the center of the Ni-Co PBA crystals than in the external parts, because dissolution-recrystallization can reduce defects in the external part of the crystals. The parts with more defects could be removed faster than the other parts, resulting in preferential hollowing of the interior cores. The relevant TEM image shows the unique pyramid-like walls of the obtained nanocages. Furthermore, Feng
et al. converted Co-Co PBAs microcubes into microframes by using ammonia (28%–30%) as the etchant [
28]. This anisotropic etching process was ascribed to the different reactivity of corner and plane surfaces of PBA microcubes because of complexation between NH
3 and Co
2+ from Co-Co PBAs. The average thickness of the obtained microframes was 80 nm.