Perovskite refers to a kind of ceramic oxide with cubic crystal shape and its molecular formula is ABO3. This kind of oxide was first discovered as a calcium titanate (CaTiO3) compound in perovskite (hence its name). The perovskite compounds are mainly orthorhombic, cubic, rhombohedral, tetragonal, monoclinic and triclinic. A-site ions are usually rare earth or alkaline earth metal elements with large ion radius, and they coordinate with 12 oxygen to form the densest cubic packing, which mainly plays a role in stabilizing perovskite structure. The B-site ions are generally elements with a small ionic radius (usually transition metal elements, such as Mn, Co, Fe, etc.), and they coordinate with 6 oxygens and occupie the center of the octahedron in the cubic close-packing. Because of the variability of the valence state of element B, it usually becomes the main component that determines many properties of perovskite materials.
Figure 1. Perovskite structure
- Catalysis: Compared with simple oxides, perovskite structure can make some elements exist in abnormal valence state and have non-stoichiometric oxygen, or can make the active metals exist in a mixed-valence state to give the solids some special properties. Because of their stable crystal structure, unique electromagnetic properties and high redox, hydrogenolysis, isomerization, electrocatalysis and other activities, these compounds have great potential for development in the fields of environmental protection and industrial catalysis. For example, some researchers have conducted a systematic study of perovskite-type oxides containing rare earth and cobalt, and found that perovskite-type composite oxides are effective catalysts for reducing the SO2 content in flue gas, so it is possible to use perovskite oxides instead of precious metals as catalysts for automobile exhaust gas purification. After that, through in-depth study of rare-earth perovskite-type catalysts, it was found that cobalt and manganate containing rare earth elements showed high catalytic activity in complete oxidation.
Figure 2. Manganese oxides with perovskite structure
- Solar cells: Perovskite is a class of material with a specific crystal structure, and can contain any number of elements. The metal elements used in perovskite solar cells are generally lead and tin. In perovskite solar cells, A ion usually refers to organic cations, and the most commonly used is CH3NH3+, B ions refer to metal cations, mainly Pb2+ and Sn2+. Perovskite solar cells with mesoporous structure are generally composed of FTO conductive glass, TiO2 dense layer, TiO2 mesoporous layer, perovskite layer, HTM layer and metal electrode. The application of perovskite materials in solar cells not only has obvious advantages in conversion efficiency, but also has a relatively simple manufacturing process. Perovskite solar cells also have the potential to combine with silicon panels to produce series cells with an efficiency of 30% or more.
Figure 3. Perovskite solar cell structure
The preparation methods of perovskite compounds mainly include the traditional high-temperature solid state method (ceramic process), sol-gel method, hydrothermal synthesis method, high energy ball milling method and precipitation method. In addition, there are vapor deposition method, supercritical drying method, microemulsion method and self-propagating high-temperature combustion synthesis method.
- Voorhoeve R J H, Johnson D W, Remeika Jr, et al. (1977). "Perovskite Oxides: Materials".Science in Catalysis Science 195-827.
- Panich N M, Pirogova G N, Korosteleva R I, et al. (1999) "Oxidation of CO and Hydrocarbons over Perovskite-Type Complex Oxides".Russian Chemical Bulletin. 48, 694-697.