Page 379 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

364 Environmental Applications of Nanomaterials

Nanomaterial templating
Nanomaterials can also be used as “molds” or templates for membranes.
In this case, the initial nanomaterials are no longer present in the mem-
brane structure, but rather have imparted their structure to another
material. Nanoparticles can be deposited on a substrate to form the ini-
tial template for a porous solid that will take on the mirror image of the
initial deposit [52].
Particle size, stability, and/or depositional trajectories can be controlled
to engineer template morphology and yield membranes with a desired
structure. When particles deposit on a surface following very predictable
trajectories, such as occurs when their transport is dominated by grav-
ity, an electrical field, or laminar flow, their depositional trajectory is said
to be ballistic. The templates formed by depositing particles on the sur-
face via ballistic trajectories tend to be compact. In contrast, when par-
ticles follow more random, diffusive trajectories to deposition, they tend
to form more open deposits that resemble dendrites or objects resem-
bling small trees. Particle surface chemistry can also be altered to con-
trol template morphology. When particle-particle interactions are
favorable (“sticky” particles), the resulting template tends to be dendritic.
When particles are not sticky, compact deposits form. Electrostatic forces,
dispersion forces, and capillary forces are likely to dominate particle
stickiness during template formation. For example, if templates are
formed by dip-coating a substrate in a suspension of particles and evap-
orating the suspending fluid, the deformation of the liquid-fluid interface
due to trapped colloidal particles gives rise to capillary forces exerted on
the particles. These forces are usually attractive [53]. Thus, by control-
ling solution chemistry (for example, ionic strength) and the conditions
of template drying, the morphology of the template can be controlled.
The self-assembly technique by capillary forces provides precise control
of the thickness of the film through sphere size and concentration in solu-
tion. Control of template morphology is illustrated in SEM images of
deposits of silica nanoparticles of 244 nm average diameter (Figure 9.6).

Figure 9.6 Templates of silica particles with an average diameter of 244 nm formed from
suspensions (a) ethanol (nonsticky) and (b) an aqueous solution 1.5 M NaCl (sticky).