CHM 326 Discovery Lab: Silver Nanoparticle Films: Synthesis and Characterization Department of Chemistry December 2002 Katie Groom, Eugene Kwan, Alioska Orozco

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 3:1 HCl:HNO 3

Transmission Electron Microscopy (TEM) of Colloid False Color Micrograph of Ag ColloidEllipse 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 1024x 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

Determining Ag Colloid Size Nanoparticle UV-vis SpectrumNanoparticle Size Distribution plasmon resonance: broad due to polydisperse size distribution - size distribution is left-truncated: only particles where S/N > ~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 particles/m 2 - synthesis needs optimization!

1 H and 13 C NMR Characterization of Colloid - both spectra indicate the presence of tetraoctylammonium bromide (TOAB) - supports hypothesis that colloid is surrounded by TOAB micelles - spectra taken in deuterated toluene, methanol 300 MHz Proton Spectrum75 MHz Carbon Spectrum

Current (mA) reduction oxidation Potential vs. (V) Cyclic Voltammetry (CV) Experimental Setup Potential (vs. Ag/AgCl, V) peaks show redox reactions Electrochemical SetupTriangular 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

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

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

UV-vis Spectroscopy: Monitoring Layer Formation - as more layers are added, absorbance maximum increases - corresponding to an increase in the amount of material that is present on the slides Ethanedithiol Linker on ITO Ethanedithiol Linker on Glass

Octanedithiol Linker on ITOOctanedithiol Linker on Glass UV-vis Spectroscopy: Monitoring Layer Formation - compare peak positions with ethanedithiol linkers - octanedithiol linked slides are considerably blue-shifted compared to ethanedithol slides

Monitoring Layer Formation: Absorbance Maximum - 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 Beer’s 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 Ethanedithiol Linker on ITOEthanedithiol Linker on Glass

Slide Preparation for Cyclic Voltammetry (CV) - 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 Electrochemical Slide Preparation Cyclic Voltammetry Setup

CV Characterization of Octanedithiol Layers 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 - 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