Synthesis of Silver Nanoparticles via 

the Chemical Reduction Method

Caleb M. Wigham, Dr. Jeffrey J. Richards

1
Introduction

• Printable Conductive Coatings

oHigh electrical and thermal conductivity

2

https://en.wikipedia.org/wiki/Plasmonic_solar_cell#/media/File:PSC_using_Metal_Nanoparticles.png

https://www.medgadget.com/2009/01/flexible_electronics_might_signal_new_era_of_embeddable_medical_devices.html

https://stonybrook.digication.com/michael_santana/Final_Paper_Antimicrobial_Effects_of_Silver_Nanopa

• Antimicrobial Coatings

o Bactericidal properties

• Photovoltaics and Sensors 

oStrong Interactions with Light
Emerging Applications

3

Richards, J. J.; Scherbarth, A. D.; Wagner, N. J.; Butler, P. D. ACS Appl. Mater. Interfaces 2016, 8 (36), 24089–24096.

• Controllable surface chemistry of conducting 

particles can lead to electrical storage 

applications such as flow batteries

Electrical Percolation

• Insulating particles with conductive surface layers have been shown

to have excellent conductivity under the electrical percolation

threshold

• Charges can be sent over distances without direct physical contact

between particles

• Goal of this project is to study the behavior of conducting particles

with insulating surface layers
Project Goals

4

• Produce conductive particle solutions with insulating layers to promote colloidal 

stability

• Control surface chemistry and production of nanoparticles by exploring effects of 

varying reaction conditions and stabilizing methods

• Characterize physical properties by means of ultraviolet-visible spectroscopy (UVVIS), transmission electron microscopy (TEM), and dynamic light scattering 

(DLS)
What are Silver Nanoparticles?

5

• Small silver particles on a length scale of 10 – 30 nm (4000x thinner than a human hair!)

• Typically spherical or elliptical in shape, though cubes and diamonds are possible

• When suspended, dilute particles appear yellow, turning reddish brown as concentration increases

TEM of silver nanospheres 10-20 

nm in diameter

Stable Solutions of 1mM (left) and 

5mM (right) silver nanoparticles
Surface Plasmon Resonance

6

Surface Plasmon Resonance: Interaction of free electrons confined in a metallic nanoparticle with light

• Optical Absorption Spectroscopy

• Intensity of Light (I)

Absorption = ln ( I/Io)

• Wavelength gives size comparing ability, not actual size

• Color indicates particle stability

Sample

0

0.2

0.4

0.6

0.8

1

300 400 500 600

ABS

Wavelength (nm)

Detector

Io I

Mulfinger, L.; Solomon, S. D.; Bahadory, M.; Jeyarajasingam, A. V.; Rutkowsky, S. A.; Boritz, C. J. Chem. Educ. 2007, 84 (2), 322.

Stable, Monodispersed Stable, Shape Evolution (polydispersed) Aggregated (Unstable)
Reduction of Silver Nitrate by Sodium Borohydride

7

• 2mM NaBH4 prepared in ice-bath

• 1mM AgNO3 added dropwise

• Addition time of 3 minutes

And the result.... 

AgNO3 + NaBH4(excess) + 4H2O(excess)  Agm + NaNO3 + B(OH)3 + 4H2 + NaOH

7

90 Minutes
What Keeps the Particles Stable?

8

Ag

NP

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

Ag

NP

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

Ag

NP

BH4

-

BH4

- BH4

-

Ag

NP

Ag

NP Ag

NP

BH4

-

BH4

- BH4

-

BH4

-

Ag

NP

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

Ag

NP

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

Ag

NP

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

Maximum Time of Stability from Borohydride Repulsion alone: 90 minutes

Something else needed to be added to extend life of solution

BH4

-

 
Addition of a Stabilizing Agent

9

Ag

NP

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

+ 

Polyvinylpyrrolidone (PVP) is a water-soluble commonly used for stabilizing metal nanoparticles. 

PVP is used for its ability to be removed from 

silver without changing surface chemistry

(PVP)

Left: Unstable Solution without PVP after 90 minutes

Right: Stable Solution with PVP after 3 weeks

Jia, C.-J.; Schüth, F. Physical Chemistry Chemical Physics2011, 13(7), 2457.
Exploring the Effects of PVP Molecular Weight on Particle Stability

10

0 hrs

18 hrs

10 kDa 29 kDa 55 kDa

Increasing Stability

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

300 350 400 450 500 550 600

ABS

Wavelength (nm)

10 kDa

29 kDa

55 kDa

Decreasing MW

Due to the fast kinetics of this reduction reaction, high molecular weight PVP stabilizes the silver nanoparticles 

better than lower molecular weights.

Molar Ratios of PVP:Ag Constant, varying MW
Process Diagram

11

Increasing PVP:Ag molar ratio alone can not stabilize high 

concentration solutions.

0

2

4

6

8

10

12

14

16

18

0 20 40 60

PVP:Ag Molar Ratio

mM AgNO3

Silver 

Sediment 

Layer

1mM

6 PVP:Ag

5mM

2 PVP:Ag

50mM

10 PVP:Ag
Time Dependence of Shape Evolution

390

395

400

405

410

415

420

10 100 1000

Peak Wavelength (nm)

Time (seconds)

After Reaction

+18 hours

0

0.5

1

1.5

2

2.5

10 100 1000

Max ABS

Time (seconds)

During Reaction

After Reaction

Stopping the reaction right after complete addition of 

silver nitrate can yield desired results

During Reaction

12
Reason for Shape Evolution

Ag

NP

2Ag+ + 2OH- Ag2O + H2O

 Ag

NP

Ag2O

Ag

NP

Ag

+ NP  Ag

NP

Ag

NP Ag

NP

+ OHBH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4

-

BH4 BH - 4

-

BH4

-

13
Reason for Shape Evolution

Shape Evolved, 10mM Silver Nanoparticle Solutions

14

Silver 

Particles

Silver 

Oxide 

Bridges

Silver 

Particles

Silver 

Oxide 

Bridges
Process Diagram

1 5 10 25 50

16 9.61

10 8.8 9.46 9.77 9.9 9.99

6 8.64 9.32 9.64 9.95 10.04

2 8.86 9.39 9.71 9.91 10.06

0.5 8.61 9.45 9.78 9.98 10.09

pH at Time 18 hrs (roughly)

PVP:Ag Molar Ratio

Silver Nitrate Concentration (mM)

0

2

4

6

8

10

12

14

16

18

0 20 40 60

PVP:Ag Molar Ratio

mM AgNO3

pH has an inverse correlation with particle stability

Increasing pH

15
Quenching Reaction by pH Adjustment

16

Decreasing pH increases monodispersity at the 

expense of particle concentration

Basic (pH 9-10) Neutral (pH 6-7)

Acidic (pH 4-5) Very Acidic (pH < 3)

0

0.2

0.4

0.6

0.8

1

300 350 400 450 500 550 600

ABS

Wavelength (nm)

Basic Solution

Neutral Solution

Acidic Solution
Conclusions

17

• Ionic repulsion can not provide stability, a stabilizing agent such as PVP is required

• Decreasing pH increases monodispersity at the expense of particle concentration

• Weak acidity (pH 6-7) will dissolve only Ag2O, leaving a stable, monodispersed silver nanoparticle solution.

Ag

NP

Ag

NP Ag

NP

0

0.2

0.4

0.6

0.8

1

300 400 500 600

ABS

Wavelength (nm)

Basic Solution

Stable monodispersed 10mM 

silver nanoparticle solution

Reason for shape evolution over time
Future Work

• Push past known concentration limits for production of silver nanoparticles

• Extract particles from water and redisperse in various solvents

• Test the electrical conductivity of these particles suspended in solution at the onset of 

electrical percolation

18
Acknowledgements

• Jeffrey Richards - Mentor

• Julie Borchers, Joe Dura – NCNR SURF Directors

• John Riley

• Mikala Shremshock

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