Zinc oxide (ZnO) nanostructures are structures with at least one dimension on the nanometre scale, composed predominantly of zinc oxide. They may be combined with other composite substances to change the chemistry, structure or function of the nanostructures in order to be used in various technologies. Many different nanostructures can be synthesised from ZnO using relatively inexpensive and simple procedures. ZnO is a semiconductor material with a wide band gap energy of 3.3eV and has the potential to be widely used on the nanoscale. ZnO nanostructures have found uses in environmental, technological and biomedical purposes including ultrafast optical functions, dye-sensitised solar cells, lithium-ion batteries, biosensors, nanolasers and supercapacitors. Research is ongoing to synthesise more productive and successful nanostructures from ZnO and other composites. ZnO nanostructures is a rapidly growing research field, with over 5000 papers published during 2014–2019.
Synthesis ZnO creates one of the most diverse range of nanostructures, and there is a great amount of research on different synthesis routes of various ZnO nanostructures. The most common methods to synthesise ZnO structures is using chemical vapor deposition (CVD), which is best used to form nanowires and comb or tree-like structures.
Chemical vapor deposition (CVD) In vapor deposition processes, zinc and oxygen are transported in gaseous form and react with each other, creating ZnO nanostructures. Other vapor molecules or solid and liquid catalysts can also be involved in the reaction, which affect the properties of the resultant nanostructure . To directly create ZnO nanostructures, one can decompose zinc oxide at high temperatures where it splits into zinc and oxygen ions and when cooled it forms various nanostructures, including complex structures such as nanobelts and nanorings. Alternatively, zinc powder can be transported through oxygen vapor which react to form nanostructures . Other vapours such as nitrous oxide or carbon oxides can be used by themselves or in combination. These methods are known as vapor-solid (VS) processes due to their reactants states. VS processes can create a variety of ZnO nanostructures but their morphology and properties are highly dependent on the reactants and reaction conditions such as the temperature and vapor partial pressures. Vapor deposition processes can also use catalysts to assist the growth of nanostructures. These are known as vapor-liquid-solid (VLS) processes, and use a catalytic liquid alloy phase as an extra step in nanostructure synthesis to accelerate growth. The liquid alloy, which includes zinc, is attached to nucleated seeds made usually of gold or silica. The alloy absorbs the oxygen vapor and saturates, facilitating a chemical reaction between zinc and oxygen. The nanostructure develops as the ZnO solidifies and grows outwards from the gold seed. This reaction can be highly controlled to produce more complex nanostructures by modifying the size and arrangement of gold seeds, and of the alloys and vapor constituents.
Aqueous solution growth A large variety of ZnO nanostructures can also be synthesised by growth in an aqueous solution, which is desirable due to its simplicity and low processing temperature. A ZnO seed layer is used to begin uniform growth and to ensure nanowires are oriented. A solution of catalysts and molecules containing zinc and oxygen are reacted and nanostructures grow from the seed layer. An example of such a reaction involves hydrolysing ZnO(NO3)2 (zinc nitrate) and the decomposition of hexamethyltetramine (HMT) to form ZnO. Altering the growth solution and its concentration, temperature and structure of the seed layer can change the morphology of the synthesised nanostructures. Nanorods, aligned nanowire arrays, flower-like and disc like nanowires and nanobelt arrays, along with other nanostructures, can all be created in aqueous solutions by varying the growth solution.
Electrodeposition Another method to synthesise ZnO nanostructures is electrodeposition, which uses electric current to facilitate chemical reactions and deposition on electrodes. Its low temperature and ability to create precise thickness structures makes it a cost-effective and environmentally friendly method. Structured nanocolumnar crystals, porous films, thin films and aligned wires have been synthesised in this way. The quality and size of these structures depends on substrates, current density, deposition time and temperature. The bandgap energy is also dependent on these parameters, since it is dependent not only on the material but also its size due to the nanoscale effect on the band structure.
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