Oxide in a broad sense refers to a compound composed of oxygen and another chemical element, such as carbon dioxide (CO2), calcium oxide (CaO), carbon monoxide (CO), etc. But the compound formed by the combination of oxygen and more electronegative fluorine is generally called fluoride. Most oxides have high thermal stability, especially oxides of group IIA and IVB elements, such as Li2O, Na2O, B2O3, Al2O3, SiO2, etc. There are also less thermally unstable oxides, such as halogen oxides, N2O5, Ag2O, HgO, etc. All oxides are pure substance.
Due to the advantages of rich variety, simple preparation method and different physical and chemical properties, oxides are widely used in many fields such as organic chemistry, biomedicine, and environment.
- Organic chemistry: Many oxides are often used in the field of organic chemistry due to their excellent redox ability and various crystal forms. SnO2 has a small particle size, a large specific surface area, and a band gap of 3.6 eV. It is an n-type semiconductor material. SnO2 has good chemical stability and photocatalytic activity, and is inexpensive, easily obtained, non-toxic and harmless, and is currently recognized as the most researched and developed photocatalytic material. However, pure SnO2 as a photocatalyst has its own defects. Modifying SnO2 by doping can effectively improve its photocatalytic efficiency. For example, by co-precipitation method, CuO and SnO2 are combined, and NaOH solution and acetic acid are used to adjust the pH. The combined SnO2 exhibits excellent photocatalytic ability and can be used in hydrogen production. In addition, oxides of some metals (such as Fe, Ge, Go, Cu, etc.) have good catalytic properties and can catalyze some types of redox reactions such as CO reduction of NO.
Figure 1. Reduction of NO catalyzed by Cu/CeO2-Fe2O3
- Biomedicine: Superparamagnetic iron oxide nanoparticles (SPIONs) are widely used for magnetic separation due to their stability, superparamagnetic properties, and biocompatibility. Under the external magnetic field, SPION is magnetized and enriched. Once the magnetic field is removed, it can be re-dispersed in the solution with recyclability. At present, SPION has been applied to the separation of various biological samples such as cells, proteins and nucleic acids (DNA, RNA). In addition, SPION can be accurately directed to target cells in the body under the guidance of an external magnetic field. Therefore, SPION drug carriers can be used in targeted therapy.
Figure 2. Schematic diagram of the process of SPION separating and detecting ovarian cancer cells from blood
- Environment: Oxides are widely used in the environmental field. As a porous transition metal oxide, MnOx has a large specific surface area, rich hydroxyl groups on the surface, and many crystal defects in the structure. MnOx is prone to generate a large number of electron holes and has a strong binding force with metal ions. It has strong absorption and enrichment ability for certain organic substances and heavy metals, and is used as a better adsorbent. In the environmental field, MnOx is widely used in the adsorption of atmospheric pollutants and the removal of heavy metals in water treatment.
Figure 3. Principle of MnOx catalytic oxidation of CO
The oxide can be divided into metal oxide and non-metal oxide according to whether the element composed of oxygen element is a metal element. The oxides can be divided into ionic oxides and covalent oxides according to the type of bonding or constituent particles. According to the difference of acidity and alkalinity, oxides can be divided into acidic oxides, basic oxides, amphoteric oxides and neutral oxides.
- Acidic oxides: oxides dissolved in water as acidic solutions are called acidic oxides, such as P4O10 and Sb2O5.
- Alkaline oxides: oxides dissolved in water as an alkaline solution are called alkaline oxides, such as Fe2O and CaO.
- Amphoteric oxides: oxides that react with strong acids to show alkalinity and react with strong bases to show acidity are called amphoteric oxides, such as ZnO.
- Neutral oxides: oxides that neither react with acids nor alkalis are called neutral oxides, such as CO and N2O.
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