Fig.1 FE-SEM images of the precursors synthesized at 160℃ for 2 h (a), 3 h (b), 4 h (c), 5 h (d), 6 h (e)[48]

Fig.2 Evolution process of the family of multishelled ZnO hollow microspheres. Transmission electron microscopy images of carbonaceous microspheres after immersion in zinc nitrate solutions before heating (room temperature) and after heating at different temperatures (400℃, 400℃ for 30 min, 420℃, 440℃, 460℃, 480℃ and 500℃). 3 and 5 mol/L zinc nitrate solutions were used in samples Ⅰ, Ⅱ, Ⅲ and samples Ⅳ, Ⅴ, Ⅵ, Ⅶ, Ⅷ, Ⅸ, respectively. The diameters of carbonaceous microspheres used in samples Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ and Ⅵ are 3 μm, and those used in samples Ⅶ, Ⅷ, and Ⅸ are 4 μm. The temperature in the fast and medium heating modes is directly increased to 500℃ at 2 and 1℃/min respectively, while the temperature in the slow heating mode is increased to 500℃ at 1℃/minwith 30 min holding at 400℃. The scale bars are 1 μm for the samples from Ⅰ to Ⅵ and 1.3 μm for samples from Ⅶ to Ⅸ in the first column. All the scale bars in the second column are 0.5 μm, while the scale bars in the third and higher columns are all 0.3 μm(a). Illustration of the formation of multishelled ZnO hollow microspheres through different heating processes(b) [57]

Fig.3 SEM images of the three types of calcined Co3O4 and their structure models: Co3O4 cubes[(a)~(c)], Co3O4 truncated octahedra[(d)—(f)], and Co3O4 octahedra[(g)—(i)] [58]

Fig.4 Schematic illustration of the formation of the precursor FeOOH[62]

Fig.5 Time-dependent evolution of ZnO nanocrystals to ZnO hollow spheres with double-yolk egg structure: 1 h (a), 12 h (b), 24 h (c). The corresponding schematic graphs of the evolution process (d) [64]

Fig.6 Overall flowchart for fabrication of BHC-TiO2[(a)—(d)]; Corresponding FESEM[(e)—(h)], BSE-SEM [(i)—(l)] and TEM images[(m)—(p)] of the BHC-TiO2 fabrication procedure. The white arrows indicate the cracked hollow cubic structures. The scale bars are 1 μm [(e)—(h)] and 500 nm[(i)—(p)][66]

Fig.7 Schematic illustration of the process of MIL-53(Fe) growth and morphology of their corresponding iron oxides (a) and the formation process of Fe2O3-2 and Fe2O3-6 (b) [67]

Fig.8 SEM pictures of ZnO crystals grown under laser power density of 9.55 kW/cm2 (a), 15.92 kW/cm2 (b), 22.29 kW/cm2 (c), 28.66 kW/cm2 (d) in 200 kHz for 5 min. Schematic illustration of crystal growth in kinetically controlled (left direction) and thermodynamically controlled (right direction) processes (e). All scale bars represent 500 nm in length[68]

Fig.9 Effect of Tdep on TC(220), (311), (111), (400) and (511) of Co3O4 films (a), growth mechanism of (111) planes (b), enlarged FESEM image (c) and schematic of nanowall structure of Co3O4 film prepared at 773 K(d) [69]

Fig.10 SEM and TEM images of samples prepared at different temperatures: S-600[(a),(b)], S-750[(c),(d)], S-850[(e),(f)] and S-950[(g),(h)][70]

Fig.11 Additional Ce3+ sites derived from high-temperature reduction favoring adsorption of the reactant and intermediate[75]

Fig.12 TEM and high-resolution TEM (HRTEM) images of CeO2-R[(a)，(b)], CeO2-P[(c)，(d)], CeO2-C[(e)，(f)], SEM(g) and HRTEM(h) images of CeO2-O. The insets schematically illustrate the crystal planes exposed on the CeO2-R, CeO2-P, CeO2-C and CeO2-O[78]

Fig.13 Schematic CO oxidation pathways catalyzed by Au/CeO2(100) (a) and Au/CeO2(111) (b) with 8 or 4 adsorbed CO molecules show that CO oxidation occurs at the Au-CeO2 interface, although an additional O—C—O formation step [(b), step②] is required for Au/CeO2(111) [86]

Fig.14 TEM images [(ⅰ),(ⅱ)], HRTEM image (ⅲ), IFFT image (ⅳ) of the α-Fe2O3-THB sample(a). TEM images [(ⅰ),(ⅱ)], HRTEM image (ⅲ), IFFT image (ⅳ) of α-Fe2O3-QC sample (b). TEM image (ⅰ), SAED pattern (ⅱ), and schematic illustration (ⅲ) of α-Fe2O3-HS sample(c)[87]

Fig.15 CO selectivity of Pd/ZnO-N (a) and Pd/ZnO-P (b) with Pd loading amount under methanol conversion of 30%—40% [88]