CHM 326 Discovery Lab:

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CHM 326 Discovery Lab Silver Nanoparticle
Films Synthesis and Characterization Departme
nt of Chemistry December 2002
Katie Groom, Eugene Kwan, Alioska Orozco
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Ag Colloid Synthesis

• Phase Transfer Synthesis
• aqueous silver nitrate is reduced by sodium
  borohydride
• a phase transfer catalyst
• tetraoctylammonium bromide
• used to transfer silver nanoparticles into
  toluene layer
• upon addition of reductant, large number of
  nuclei form
• newly reduced silver forms on nuclei to form
  spherical particles
• synthesis is sensitive to cleanliness all
  glassware was cleaned in 31 HClHNO3

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Transmission Electron Microscopy (TEM) of Colloid
False Color Micrograph of Ag Colloid
Ellipse Fitting to Nanoparticles

• electron micrograph shows dark (red) regions
  where electron density is high colloid drop-cast
• grainy background is a polymer matrix raw image
  is 1024x1024 8-bit grayscale ? false colored
• grayscale is 0-bit thresholded and fitted with
  NIST software (ImageJ) to ellipses
• size distribution is based on the average of the
  major and minor axes approximates spheres
• particles adopt roughly spherical shape to
  minimize surface energy

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Determining Ag Colloid Size
Nanoparticle UV-vis Spectrum
Nanoparticle Size Distribution
plasmon resonance broad due to polydisperse size
distribution

• size distribution is left-truncated only
  particles where S/N gt 3 are shown
• indicates highly polydisperse colloid
  apparently there are many tiny particles left
• most particles are small roughly half of
  resolved particles are between 4 and 5 nm
• black squares indicate the normalized integral
  of the corresponding size bin
• two dimensional particle density 3.3 x 1014
  particles/m2
• synthesis needs optimization!

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1H and 13C NMR Characterization of Colloid
300 MHz Proton Spectrum
75 MHz Carbon Spectrum

• both spectra indicate the presence of
  tetraoctylammonium bromide (TOAB)
• supports hypothesis that colloid is surrounded
  by TOAB micelles
• spectra taken in deuterated toluene, methanol

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Cyclic Voltammetry (CV) Experimental Setup
Electrochemical Setup
Triangular Voltage Sweep

• current is monitored as a function of potential
• potential is monitored between working electrode
  and reference electrode
• small current passes between working and counter
  electrode

Resulting Profile Sample CV

• to examine solutions, different electrode
  surfaces, potentials, and electrolytes can be
  used
• - to examine surfaces, easily reversible
  electrochemical redox couples are used as probes

peaks show redox reactions
Potential (vs. Ag/AgCl, V)
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Cyclic Voltammetry of Ag Colloid

• colloid solution probed by CV
• Pt working electrode used
• small additions of silver colloid cause shift in
  old peak positions and the appearance of new
  peaks
• new peaks probably due to redox behavior of TOAB
• note increased TOAB concentration and increased
  uncorrected cell resistance with successive
  colloid additions

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Layer by Layer Assembly of Films

• contaminants and physisorbed particles were
  removed from slides with thorough rinsing this
  ensured successful monolayer deposition
• - to obtain optimum deposition, slides were
  immersed in the Ag colloid for 24 hours for each
  layer
• following immersion in the Ag colloid, layer
  formation was monitored by
• UV-vis spectroscopy
• - slides were initially yellow in color,
  progressing to a purple appearance as more layers
  were added

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UV-vis Spectroscopy Monitoring Layer Formation
Ethanedithiol Linker on ITO
Ethanedithiol Linker on Glass

• as more layers are added, absorbance maximum
  increases
• corresponding to an increase in the amount of
  material that is present on the slides

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UV-vis Spectroscopy Monitoring Layer Formation
Octanedithiol Linker on ITO
Octanedithiol Linker on Glass

• compare peak positions with ethanedithiol
  linkers
• octanedithiol linked slides are considerably
  blue-shifted compared to ethanedithol slides

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Monitoring Layer Formation Absorbance Maximum
Ethanedithiol Linker on ITO
Ethanedithiol Linker on Glass

• - an increase in the absorbance maximum
  corresponds to an increase in the amount of
  material after each successive layer
• as more layers are added, the peak red-shifts,
  indicating an increase in inter-nanoparticle
  coupling
• absorbances for each layer roughly follow Beers
  Law constant amounts are added per layer
• nonzero intercept indicates new material may end
  up partially being deposited in the previous
  layer
• particle films are disordered

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Slide Preparation for Cyclic Voltammetry (CV)
Electrochemical Slide Preparation

• redox chemistry occurs at small window
• window is small to ensure that mass transport is
  rate limiting
• epoxy is insulating
• alligator clip punches through layers to ITO
  coat
• current travels down ITO and through film
• slide is used as the working electrode in a CV
  setup
• positive feedback iR compensation is required to
  correct the film resistance
• corrected resistances are approximately metallic
• uncorrected cell resistance can be a measure of
  the slide conductivity

Cyclic Voltammetry Setup
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CV Characterization of Octanedithiol Layers

• hydroquinone, a well-known reversible redox
  couple, used as an electrochemical probe to study
  surface
• black CV is on Pt red CV is on octanedithiol
  slide
• note change in peak position and intensity
• quasi-reversible profile is consistent with a
  reversible redox couple on a metallic surface
• iR compensation required

• Acknowledgements
• procedures, TEM images, and general help Paul
  Trudeau - use of CV Andrei Yudin
• - lab space Al-Amin Dhirani, Dan Mathers
  - NMR Tim Burrow
• miscellaneous Jordan Dinglasan, Dan Mathers,
  Chem Store Staff