Transmission electron microscopy is a powerful tool to directly image crystal structures. Not only that, it is often used to reveal crystal size and morphology, crystal orientation, crystal defects, surface structures, superstructures, etc. However, due to the 2D nature of TEM images, it is easy to make mistakes when we try to recover a 3D structure from them. Scanning electron microscopy is able to provide information on the particle size, morphology and surface topography. However, obtaining information on crystallinity of particles using SEM is difficult. In this microreview article, some practical cases of transmission and scanning electron microscopy investigations of inorganic crystals are reviewed. Commonly occurring uncertainties, imperfection and misunderstandings are discussed.

RHO zeolitic imidazolate framework (ZIF), Zn_1.33(O.OH)_0.33(nim)_1.167(pur), crystals with a rhombic dodecahedral morphology were synthesized by a solvothermal process. The growth of the crystals was studied over time using scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) and Brunauer–Emmett–Teller (BET) analyses, and a reversed crystal growth mechanism was revealed. Initially, precursor materials joined together to form disordered aggregates, which then underwent surface recrystallization forming a core–shell structure, in which a disordered core is encased in a layer of denser, less porous crystal. When the growth continued, the shell became less and less porous, until it was a layer of true single crystal. The crystallization then extended from the surface to the core over a six-week period until, eventually, true single crystals were formed.

The discovery of reversed crystal growth routes in zeolite analcime and zeolite A implies that crystal growth does not always follow the classic theory established 100 years ago. Aggregation of nanoparticles may dominate in the early stages of crystal growth, followed by surface crystallization, and then extension from surface to core of the disordered aggregates. A perfect polyhedral morphology can be developed in a thin surface crystalline layer of a particle with a disordered core. Evidence of such a novel crystal growth phenomenon can be also found in many other materials. This article highlights the recent achievements in this topic, which might have a significant impact on crystal engineering, materials science, and mineralogy.

Ordered mesoporous rutile and anatase TiO₂ samples are prepared using mesoporous silica SBA-15 as template and freshly synthesized titanium nitrate and titanium chloride solutions as precursors. The rutile material formed from the nitrate solution is monocrystalline and contains minimal amounts of Si with a Si:Ti ratio of 0.031(4), whereas the anatase material formed from the chloride solution comprises nanocrystals and contains a higher content of Si with a Si:Ti ratio of 0.18(3). It is found that control of temperature and selection of Ti-containing precursor play important roles in determining the crystal phase and crystallinity. A possible formation mechanism of porous crystalline TiO₂ is suggested. Characterization of these porous materials is performed by XRD, HRTEM, and nitrogen adsorption/desorption. SBA-15-templated mesoporous rutile TiO₂ exhibits a higher Li ion insertion capability than KIT-6-templated TiO₂ due to its larger surface area. Likewise mesoporous anatase TiO₂:SiO₂ composite has a better photoactivity than bulk TiO₂ or TiO₂-loaded SBA-15 for bleaching methylene blue.