The products was firstly investigated by X-ray diffraction technique to determine its phase. As shown in Figure 1a, the diffraction peaks are well accordance with the characterization of the PDF card numbered 13-0356, demonstrating the products are CoSn(OH)6. SEM image in Figure 1b indicates the CoSn(OH)6 products have dense cube-like shape with edge length of ca. 120 nm. After etching in alkali, the cube-like shape is retained from Figure 1c, while a hollow structure is obtained based on the phenomenon of some cubes missing a face. TEM image of Figure 1d further demonstrates the hollow structure of CoSn(OH)6 nanocubes. On the other hand, Figure 1d reveals the individual nanocubes are not thorough hollow, which could be attributed to the non-uniform etching reaction. EDS of hollow CoSn(OH)6 nanocubes insert in Figure 1d indicates the presence of Co and Sn elements, further proving the formation of CoSn(OH)6.
Another evidence of the formation of hollow structure is specific surface area analyzed from N2 isotherms. As shown in Figure 2, N2 isotherms of both samples present the feature of mixed type I/IV curves classified by the international union of pure and applied chemistry (IUPAC). From the both curves, two samples have micropores testified by clear uptake distributed over low pressure region, and mesopores proving by a hysteresis loop sprinkled throughout high pressure region. The area of the hysteresis of hollow CoSn(OH)6 nanocubes compared to its dense structure indicates the increase of mesopores after etching procedure. The pore size distribution inserted further confirmed the changes. As presented, the non-uniform mesopores in dense CoSn(OH)6 nanocubes becomes the uniform distribution concentrated in 30 nm, demonstrating the formation of hollow structure again.
The electrochemical capability of the samples was tested by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) technique. The CVs of dense- and hollow- CoSn(OH)6 nanocubes in Figure 3a display a couple of redox peaks, which is related to the transition of Co2+/Co3+. The volt-ampere response current of hollow CoSn(OH)6 nanocubes is clear larger than that of dense CoSn(OH)6 nanocubes, suggesting the improved charge-store ability of these nanocubes after being hollowed, which could be related to the large surface area and more active sites provided by hollow structure. GCD results further prove the conclusion. As shown in Figure 3b, the non-linear shape of the two curves indicates pseudocapacitance behavior of the nanocubes, according with the CV feature. The longer time span of discharge process for hollow CoSn(OH)6 nanocubes than another implies high charge-storage quantity. After calculated, the specific capacity of hollow CoSn(OH)6 nanocubes is 42.7 µA h cm-2, larger than that of dense CoSn(OH)6 nanocubes (31.99 µA h cm-2).
The rate ability is one of the indicators to evaluate the advantages of pseudocapacitor materials. GCD curves at different current densities were used to test the rate ability of these nanocubes. As displayed in Figure 3c, GCD curves of hollow CoSn(OH)6 nanocubes retain the similar shape all the way, and the discharge time decreases with increased current density. The corresponding specific capacity was calculated and plotted in Figure 3d. For comparison, the relation of the specific capacity vs current density of dense CoSn(OH)6 nanocubes was also shown here. The specific capacity of hollow CoSn(OH)6 nanocubes is 16.17 µA h cm-2 , then its retention is 38% as current density increases 4 folds. While, the retention of dense CoSn(OH)6 nanocubes is 41.7% under the same change of current density. The results indicate hollow structure did not improve the rate ability of these nanocubes, perhaps, which could be ascribed to unimproved electric conductivity. Thus, enhancing electric conductivity of the electrode would be another strategy for CoSn(OH)6 nanocubes applying in the pseudocapacitors.