Environmental Engineering Reference
In-Depth Information
or.to.fragment,.losing.some.of.the.characteristics.responsible.for.their.activity..For.
TEM,.nonconductive.materials.must.be.coated.with.a.conducting.material.such.as.
graphite,.potentially.obscuring.critical.features..On.most.available.instruments,.the.
sample.must.be.at.high.vacuum.during.analysis,.and.results.for.nanoparticles.with.
volatile.components,.such.as.hydrated.salts.or.oxides,.may.not.be.representative.for.
the.material.as.it.exists.outside.the.vacuum.
Environmental. SEM. (ESEM). instruments. have. been. developed. recently. that.
utilize.differential.pressure.zones..These.do.allow.analyses.with.the.sample.at.pres-
sures. closer. to. atmospheric,. and. ESEM. instrumentation. also. can. be. modiied. to.
allow.imaging.of.nanoparticles.while.in.suspension.in.water.or.other.liquid.media..
Condensation,. evaporation,. and. transport. of. water. inside. carbon. nanotubes. have.
been.monitored
in situ
with.ESEM.[11]..Bogner.et.al..[12].report.the.analyses.of.gold.
and. silica. nanoparticles. and. carbon. nanotubes. dispersed. in. water. using. this. tech-
nique,. which. they. have. named. “wet. scanning. transmission. electron. microscopy,”.
(wet.STEM).
...
scanning Probe Microscopy (sPM)
Scanning. probe. microscopy. (SPM),. a. relatively. newer. tool,. provides. a. true. three-
dimensional.surface.image..SPM.includes.a.variety.of.different.techniques,.includ
-
ing. atomic. force. microscopy. (AFM). and. scanning. tunneling. microscopy. (STM),.
which. have. proven. useful. for. imaging. and. measuring. materials. at. the. nanoscale..
SPM.techniques.are.based.on.a.mechanical.survey.of.the.surface.of.an.object.or.par-
ticle..A.very.ine.tip.mounted.on.a.cantilever.scans.over.the.surface.of.interest,.fol-
lowing.the.surface.proile..Interactions.between.the.tip.and.the.surface.delect.the.tip.
as.it.follows.the.surface.proile..The.movement.of.the.tip.in.response.to.the.interac-
tion.can.be.monitored.with.a.laser.relected.from.the.cantilever.to.a.photodiode.array.
(Figure.5.2)..STM.monitors.the.weak.electrical.current.induced.as.the.tip.is.held.a.
set.distance.from.the.surface..STM,.under.some.conditions,.can.provide.chemical.
composition.information.for.the.surface..With.AFM,.the.tip.responds.to.mechanical.
contact.forces.as.well.as.atom-level.interactions.between.the.tip.and.surface.(such.as.
chemical.bonding.forces,.van.der.Waals.forces,.or.electrostatic.forces).
Since. their. development. in. the. late. 1980s,. both. techniques. have. found. wide.
application. for. nanotechnology. materials. development,. as. illustrated. by. the.
characterization. of. fullerene. particles. in. Figure.5.3.. AFM. also. holds. promise. for.
environmental.applications..AFM.can.be.operated.at.ambient.pressure.and.can.char-
acterize.a.wide.range.of.particle.sizes.in.the.same.scan,.from.1.nm.to.8.µm.(microm-
eter).. It. can. analyze. particles. on. a. solid. substrate. at. atmospheric. pressure. or. in. a.
liquid.medium.such.as.water..It.has.been.used.to.characterize.the.morphology.and.
size.distribution.of.nanometer-sized.environmental.aerosol.particles.collected.from.
ambient. air,. as. well. as. for. engineered. TiO
2
. nanoparticles. [14].. The. size. distribu-
tion.and.morphology.of.natural.aquatic.colloids,.which.play.important.roles.in.con-
taminant.binding,.transport,.and.bioavailability,.also.have.been.characterized.with.
AFM.after.their.absorption.onto.a.mica.substrate.[15-17]..A.detailed.discussion.of.
AFM.is.provided.in.the.review.article.by.Burleson.et.al..[3];.further.information.on.
