Q.The particle size of ZnO material is 30 nm. Based on the dimension it is classified as

Nanoscience is the study of structures and materials on the scale of nanometres ($1\,\text{nm}=10^{-9}\,\text{m}$); nanotechnology is the associated technology of designing, producing, characterizing and applying nano-structured materials. A solid whose particle size is under $100\,\text{nm}$ is a nano solid; above that it is a bulk solid -- the two can share identical chemical composition (e.g. ZnO exists in both forms) yet differ strikingly in mechanical, electrical, optical and magnetic properties, governed at the nanoscale by quantum confinement and surface effects.

Nature got there first. Nanoscale structures existed in living things long before laboratory nanoscience: a single strand of DNA is ~3 nm wide; a Morpho butterfly's wing-scale nanostructures produce structural (non-pigment) colour; peacock-feather iridescence comes from ~tens-of-nm photonic-crystal structures; a parrotfish's fluorapatite-fibre nanostructure gives its teeth extreme durability; and a lotus leaf's surface nanostructure gives it a self-cleaning property. Laboratories deliberately mimic each of these (tuning nanoparticle size for colour, engineering water-repellent nano-paints, designing ultra-durable synthetic materials from the parrotfish blueprint, etc.).

Synthesis follows two general strategies: the top-down approach (break bulk solids down into nano sizes, e.g. ball milling, sol-gel, lithography) and the bottom-up approach (assemble atoms/molecules up into nanostructures, e.g. plasma etching, chemical vapour deposition) -- the two meet from opposite ends at the same nanoparticle size range.