Zeolites as Templates: continued Synthesis: Most zeolite syntheses are performed by using template-synthesis not to be confused with using the materials to template nanostructures templates are often surfactants e.g., tetraalkyl ammonium ions different template --different zeolite difficult to control 28-1
Template molecule that "directs" assembly of zeolite is often removed thermally: decomposes to ammonia and other simple gas phase molecules e.g.,... leaves behind open structure for template use. AFM -- where are the pores? 28-2
Zeolite ZSM 5 used as growth medium for nanotubes nanotube zeolite film prepares a supported catalyst to assist growth 28-3
Macroporous Template Materials Most popular method of forming macroscopic porous materials is to use close-packed spherical beads e.g., latex beads 28-4
Method: 1. synthesize beads by an emulsion polymerization 2. pack beads into an array -- e.g., dip-coating 3. charge the inner space of beads with silicate,titania,or aluminate 4. treat at high temperature to remove polymer - 500 C 5. this forms a matrix (ceramic) another example: gold 28-5
Also colloids can be used: here CVD is used to deposit onto template template then removed thermally 28-6
LAST TOPIC: CORE SHELL STRUCTURES AND OXIDES Schedule: This topic then use remaining time to get labs finished and get ready for final exam Core shell structures can be epitaxial or core and shell can have different structures: One can be crystalline - the other amorphous CORE nanoparticle SHELL 28-7
Also: physical properties of core and shell can be very different e.g., one can be a conductor one can be "metallic" the other not... Many methods have been developed: coating vapour phase deposition self assembly metal oxide metal polymer oxide -polymer systems nanoparticles assembled on surfaces epitaxial core -shell 28-8
One Possible application needs more work... - New High Density Batteries The material contains a highly crystalline LiFePO4 core with a size of about 20-40 nm and a semi-graphitic carbon shell with a thickness of about 1-2 nm. The method devised to produce the material can be extended to the preparation of other electrode materials for lithium-ion batteries, such as Li 4 Ti 5 O 12 /carbon and Mn 3 O 4 /carbon composites. A Li-ion battery is currently the smallest and lightest way to store as much rechargeable electrical energy as possible. However, formulations less susceptible to thermal override and thus safer demonstrate a comparatively sluggish performance. The current strategy is to shorten the existing route within the electrode material. Although Lithium iron phosphate is an attractive cathode material because of its low raw material cost and stability it has a low energy density and electronic conductivity. 28-9
Application : Displays Size tunable fluorescence emission from CdSe/ZnS core shell nanocrystals CdSe quantum dots see NANO 2100 28-10
Epitaxial Core Shell Nanostructures First we look at semiconductor-semiconductor materials Semiconductors: beyond this course (See NANO 2100) Briefly: These are the materials we use in chips etc... Bands shown on left contain many MO's very close together (see CHEM 2060) 28-11
Semiconductor particles have different properties (optical and electronic) to bulk semiconductors. In NANO 2100 you will meet Quantum Size Effects Growth of one semiconductor on another in the nano regime requires close control WHY? Growth must ONLY occur on the nanoparticle No secondary growth 28-12
So: Concentration of Growth species must be controlled so that supersaturation is not high enough for nucleation but is high enough for growth. Two approaches Drop-wise addition of growth precursor into reaction mixture containg cores Vary temperature (and thus relative rates of lateral and secondary growth) Also solubility depends on surface curvature (beyond this course) example: CdS/CdSe: 3nm core: 140C 3.5nm core: 160C 4nm core 180 C 5.5 core 220C 28-13
Synthesis ZnS-capped/CdSe CdSe core: synthesis of CdE (E = S, Se, Te) uses techniques used by Murray and Stiegerwald Cd source is dimethyl cadmium (Cd (CH 3 ) 2 ) trioctylphosphine selenide is Se source TOPSe SOLVENT: Important choice: needs to be of correct polarity and not react with reactants!!!! 28-14
Small nanoparticles like these are often unstable: so solvent can act as a capping agent Solvent used is tri-octyl phosphine oxide (TOPO) and tri-octyl phosphine oxide (TOPO) Air sensitive reaction : so done in an inert atmosphere Also see lab manual for another method -- can you find any more? 28-15
ZnS Shell Zn/S solution: bis trimethyl silyl sulfide and dimethyl zinc coats 0.6nm layer of ZnS on Shell 28-16