2. Top-down approach refers to slicing or
successive cutting of a bulk material to
get nano-sized particle
Bottom-up approach refers to the build-
up of material from the bottom: atom
by atom, molecule by molecule
Atom-by-atom deposition leads to the
formation of self-assembly of atoms
and clusters.
These clusters come together to form
self-assembled monolayers on the
surface of the substrate
Dr.P.Bharathi, AOP/FT 2
3. Real-time examples
Computer chips can be made “Top-down” - building
something by starting with a larger component and
carving away material (e.g. like a sculpture).
Metal nanowires are made bottom-up building something
by assembling smaller components (e.g. like building a
car engine)
Dr.P.Bharathi, AOP/FT 3
4. Builders versus Chiselers: a Historical View
Chiseling: Start with a relatively
uniform, large piece of material --
selectively shape/remove material to
define the desired object.
Building: Start with small building
blocks; combine and arrange them to
form interesting assemblies
Dr.P.Bharathi, AOP/FT 4
5.
6. Coprecipitation
Coprecipitation reactions involve the simultaneous
occurrence of nucleation, growth, coarsening, and/or agglomeration
processes.
Coprecipitation reactions exhibit the following characteristics:
(i) The products are generally insoluble species formed under conditions
of high supersaturation. (ii) Nucleation is a key step, and a large number
of small particles will be formed. (iii) Secondary processes, such as
Ostwald ripening and aggregation, dramatically affect the size,
morphology, and properties of the products. (iv) The supersaturation
conditions necessary to induce precipitation are usually the result of a
chemical reaction.
xAy+(aq) + yBx-(aq) AxBy(s)
Typical coprecipitation synthetic methods: (i) metals formed from
aqueous solutions, by reduction from nonaqueous solutions,
electrochemical reduction, and decomposition of metallorganic precursors;
(ii) oxides formed from aqueous and nonaqueous solutions; (iii) metal
chalconides formed by reactions of molecular precursors; (iV)
microwave/sonication-assisted coprecipitation.
7. Example: Precipitation of ZnS nanoparticles from a solution
containing thioacetamide and zinc acetate
Thioacetamide is used as a sulfide source.
Zn2+ + S2- ZnS
Murray C.B. et al., Annu. Rev. Mater. Sci. 2000, 30, 545.
