CHM 326 Discovery Lab:

Silver Nanoparticle Films: Synthesis and Characterization

Department of ChemistryDecember 2002

Katie Groom, Eugene Kwan, Alioska Orozco

1.0543 g of TOABin 50 mL toluene

0.1750 g AgNO3 in 25 mL deionized water

stir for 20 minutesorganic phase

aqueous phase

isolate and centrifugeorganic phase

0.0427 g NaBH4

in 25 mL deionizedwater

2) Work up with:0.1 M H2SO4

1 M Na2CO3

deionized water

1) Stir for 2 hours

Ag colloid

Scheme 1: Preparation of Ag colloid

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:HNO3

N+

Br-

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

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200

5

10

15

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65

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

55%

60%

65%

Particle Counts % of Total Counts

Ag Nanoparticle Size Distribution

Pe

rce

nta

ge o

f T

otal

Par

ticl

es

Nu

mbe

r o

f P

arti

cle

s

Particle Size (nm)

300 350 400 450 500 550 600 650 700 750 800

-0.2

0.0

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2.8

3.0 416

solvent: toluene

Ag Colloid UV-Vis Spectrum

Ab

sorb

anc

e (a

.u.)

Wavelength (nm)

Determining Ag Colloid SizeNanoparticle 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 1014 particles/m2 - synthesis needs optimization!

8 7 6 5 4 3 2 1 0

0.0

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0.4

0.6

0.8

1.0

1.2

1.4

1.6

toluene ArH toluene CH3

399.11 MHz, SW=6000 Hz, 160 transientsApodization: Exponential LB (resolution enhance)Linear Prediction (2x), Zero F ill (2x)

Proton NMR of Ag Nanoparticles

nanoparticles

tetraoctylammoniumbromide

Inte

nsi

ty (a

.u.)

(ppm)

140 120 100 80 60 40 20 0

0

1

2

3

4

5

6

7

75.45 MHz, SW=18798 Hz, 750 transientsApodization: Exponential LB (S/N enhance)Linear Prediction (2x), Zero Fill (2x)

Carbon-13 NMR of Ag Nanoparticles

(d8)-toluene

tetraoctylammoniumbromide

nanoparticles

Inte

nsity

(a.u

.)

(ppm)

1H and 13C 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 Spectrum 75 MHz Carbon Spectrum

Cur

rent

(m

A)

reduction

oxidation

Potential vs. (V)

Cyclic Voltammetry (CV) Experimental Setup

Time (s)

Potential (V)

triangular sweep vs. reference electrode potential

Potential (vs. Ag/AgCl, V)

peaks show redox reactions

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

Ag,Ag/Cl referenceelectrode

Pt working electrode

Pt counterelectrode

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

-0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0 -2.2-2

0

2

4

6

8

10

12

14

increasing amounts ofag colloid added tocv sample solution

Conditions:Solvent: 3:1 toluene:acetonitrileSupp. Electrolyte: Bu

4NBF

4

Working Electrode: PtCounter Electrode: PtReference Electrode: Ag/AgClScan Speed: 250 mV/secScans: 5 eachiR Compensation: ActiveDegassing: N

2 purge, 60 sec

a few dropsCell Resistance: 3075

a few more dropsCell Resistance: 3075

CV: Effect of Colloid Additions+1.0 mLCell Resistance: 3673

+0.5 mLCell Resistance: 3344

Curr

ent (

x 10

-5 A

)

Voltage (V)

-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

into

1/2 inch x 3/4 inch ITO slide

heat solution at 80 degrees for 20

minutes

place slide into 110 degree oven

for10 minutes

silanized ITO slide

25 mL beaker containing a 2% (vol.)

solution of 3-aminopropyl-diethoxysilane in toluene

into

20 mL vial containingconcentrated Ag colloid

solution for 24 hours

Silanized slidewith first layer of Ag

colloid

Scheme 2: Silanization of ITO slide

Silanized slidewith first layer of Agcolloid 20 mL vial containing

0.5mM solution of dithiolsolution in absolute

ethanol for 10 minutes

into Return slide to vialcontaining Ag solution

Result: Assemblyof layer 2

Note: Additional layers formed by the same process shown above.

Scheme 3: Layer by layer assembly of molecularly linked Ag nanoparticles

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

300 400 500 600 700 8000.00

0.05

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0.55

0.60

0.65

layer 5

layer 4

layer 3

layer 2

layer 1

UV-Vis Spectra of Layer-by-Layer Growth on ITO (Ethanedithiol)

Ab

sorb

an

ce (

a.u

.)

Wavelength (nm)

300 350 400 450 500 550 600 650 700 750 800-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

layer 5

layer 4

layer 3

layer 2layer 1

UV-Vis Spectra of Layer-by-Layer Growth on Glass (Ethanedithol)

Ab

so

rba

nce

(a

.u.)

Wavelength (nm)

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

300 400 500 600 700 800

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layer 5

layer 4

layer 3

layer 2

layer 1

UV-Vis Spectra of Layer-by-Layer Growth on ITO (Octanedithiol)

Ab

sorb

an

ce

(a.u

.)

Wavelength (nm)

300 350 400 450 500 550 600 650 700 750 800-0.1

0.0

0.1

0.2

0.3

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0.7

layer 5

layer 4

layer 3

layer 2

layer 1

UV-Vis Spectra of Layer-by-Layer Growth on Glass (Octanedithiol)

Ab

sorb

an

ce

(a.u

.)

Wavelength (nm)

Octanedithiol Linker on ITO Octanedithiol 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

1 2 3 4 50.05

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0.75

0.80

0.85Slide Growth Monitoring - Absorbance Maxima

Ab

sor

ban

ce

Ma

xim

um (a

.u.)

Number of Immersions (Layers)

Slide 1 - ITO/Ethanedithiol Slide 2 - ITO/Ethanedithiol Slide 3 - ITO/Octanedithiol Slide 4 - ITO/Octanedithiol Slide 5 - Glass/Ethanedithiol Slide 6 - Glass/Octanedithiol

1 2 3 4 5360

380

400

420

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460

480

500

520

540

560Slide Growth Monitoring - Redshif ting

aggregation

Loc

atio

n o

f A

bs

orb

an

ce

Ma

xim

um

(n

m)

Number of Immersions (Layers)

Slide 1 - ITO/Ethanedithiol Slide 2 - ITO/Ethanedithiol Slide 3 - ITO/Octanedithiol Slide 4 - ITO/Octanedithiol Slide 5 - Glass/Ethanedithiol Slide 6 - Glass/Octanedithiol

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 ITO Ethanedithiol Linker on Glass

Ag,Ag/Cl referenceelectrode

Ag counterelectrode

slide

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

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8

Work ing Electrode: Octanedithiol Layers on ITOCell Resistance: 193 , Scan Speed: 500 mV/sec

Working Electrode: ITO on glassCell Resistance: 190 Scan Speed: 100 mV/sec

Conditions:Solvent: 0.1 M aqueous Na

2SO

4

Redox Probe: 1 mM hydroquinoneCounter Electrode: PtReference Electrode: Ag/AgCliR Compensation: ActiveDegassing: N

2 purge, 60 sec

and Self-Assembled Layers (Octanedithiol)CV: Comparison of Hydroquinone Redox on ITO

Vo

ltag

e (x

10-4

A)

Voltage (V)

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