Synthesis of Silver Nanoparticles Via the Chemical Reduction Method Caleb M

Synthesis of Silver Nanoparticles via the Chemical Reduction Method Caleb M. Wigham, Dr. Jeffrey J. Richards

1 Introduction

• Printable Conductive Coatings o High electrical and thermal conductivity

• Antimicrobial Coatings o Bactericidal properties

• Photovoltaics and Sensors oStrong Interactions with Light

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

• Controllable surface chemistry of conducting particles can lead to electrical storage applications such as flow batteries

Richards, J. J.; Scherbarth, A. D.; Wagner, N. J.; Butler, P. D. ACS Appl. Mater. Interfaces 2016, 8 (36), 24089–24096. 3 Project Goals

• 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 (UV- VIS), transmission electron microscopy (TEM), and dynamic light scattering (DLS)

4 What are Silver Nanoparticles? • 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 Stable Solutions of 1mM (left) and nm in diameter 5mM (right) silver nanoparticles

5 Surface Plasmon Resonance

Surface Plasmon Resonance: Interaction of free electrons confined in a metallic nanoparticle with light • Optical Absorption Spectroscopy 1 0.8 Io I 0.6 Sample Detector ABS 0.4 • Intensity of Light (I) 0.2 Absorption = ln ( I/Io) 0 300 400 500 600 • Wavelength gives size comparing ability, not actual size Wavelength (nm)

• Color indicates particle stability

Stable, Monodispersed Stable, Shape Evolution (polydispersed) Aggregated (Unstable) Mulfinger, L.; Solomon, S. D.; Bahadory, M.; Jeyarajasingam, A. V.; Rutkowsky, S. A.; Boritz, C. J. Chem. Educ. 2007, 84 (2), 322. 6 Reduction of Silver Nitrate by Sodium Borohydride

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

• 2mM NaBH4 prepared in ice-bath

• 1mM AgNO3 added dropwise • Addition time of 3 minutes

And the result…. 90 Minutes

77 What Keeps the Particles Stable?

- - BH4 BH4 BH - - 4 BH4 - BH4 - - - - - BH4 Ag BH4 BH BH4 BH4 - 4 - - BH4 - BH - NP BH4 BH4 4 BH4 - Ag - - BH4 BH4 BH4 - - NP - Ag BH - BH4 BH - BH4 4 BH4 - 4 - - Ag BH4 BH4 BH NP - BH - 4 NP Ag BH - BH4 4 - Ag 4 - Ag BH4 NP - - -  BH4  NP - BH BH4 BH4 - 4 BH4 NP - BH4 BH - BH4 4 BH - Ag BH - 4 - BH - 4 BH - Ag - BH4 - NP 4 4 BH4 BH4 - NP - - BH4 BH4 - BH4 BH4 - BH - BH4 - 4 BH4

Maximum Time of Stability from Borohydride Repulsion alone: 90 minutes

Something else needed to be added to extend life of solution

8 Addition of a Stabilizing Agent

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

(PVP)

- - BH4 BH4 - - BH4 BH4 - - BH4 Ag BH4 NP -  PVP is used for its ability to be removed from BH - BH4 + 4 - - BH4 - BH4 silver without changing surface chemistry BH4

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. 9 Exploring the Effects of PVP Molecular Weight on Particle Stability 1 Molar Ratios of PVP:Ag Constant, varying MW 0.9 10 kDa 29 kDa 55 kDa 0.8 0.7 10 kDa 0.6 29 kDa 0 hrs 55 kDa 0.5 ABS 0.4 Decreasing MW 0.3 0.2 18 hrs 0.1 0 300 350 400 450 500 550 600 Wavelength (nm) Increasing Stability Due to the fast kinetics of this reduction reaction, high molecular weight PVP stabilizes the silver nanoparticles better than lower molecular weights.

10 Process Diagram

1mM 18 6 PVP:Ag 16 14 12 10 5mM 8 2 PVP:Ag 6

PVP:Ag PVP:Ag Molar Ratio 4 2 0 0 20 40 60 50mM mM AgNO3 10 PVP:Ag Silver Increasing PVP:Ag molar ratio alone can not stabilize high Sediment concentration solutions. Layer

11 Time Dependence of Shape Evolution

420 2.5

415 2 410 After Reaction During Reaction 1.5 405 1 After Reaction

400 Max ABS During Reaction 395 0.5 Peak Wavelength (nm) Peak Wavelength 390 0 10 100 1000 10 100 1000 Time (seconds) Time (seconds)

+18 hours Stopping the reaction right after complete addition of silver nitrate can yield desired results

12 Reason for Shape Evolution

+ - 2Ag + 2OH Ag2O + H2O

- - Ag2O BH4 BH4 - - BH4 BH4 Ag - Ag - BH4 - BH4 - OH  NP NP BH4 + - BH4 - BH4 BH - - 4 BH4

Ag Ag Ag Ag NP Ag NP NP +  NP NP

13 Reason for Shape Evolution

Shape Evolved, 10mM Silver Nanoparticle Solutions

Silver Oxide Bridges Silver Silver Particles Oxide Bridges

Silver Particles

14 Process Diagram

Increasing pH 18 16 14 Silver Nitrate Concentration (mM) 12 pH at Time 18 hrs (roughly) 1 5 10 25 50 10 16 9.61 8 10 8.8 9.46 9.77 9.9 9.99 6 MolarRatio 6 8.64 9.32 9.64 9.95 10.04 2 8.86 9.39 9.71 9.91 10.06

PVP:Ag PVP:Ag Molar Ratio 4 0.5 8.61 9.45 9.78 9.98 10.09 2 PVP:Ag 0 0 20 40 60 mM AgNO3 pH has an inverse correlation with particle stability

15 Quenching Reaction by pH Adjustment

Basic Solution 0.8 Neutral Solution Acidic Solution 0.6 ABS 0.4 Basic (pH 9-10) Neutral (pH 6-7)

0.2

0 300 350 400 450 500 550 600 Wavelength (nm) Decreasing pH increases monodispersity at the expense of particle concentration Acidic (pH 4-5) Very Acidic (pH < 3)

16 Conclusions

• 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.

0.8 Basic Solution

Ag 0.6 Ag NP Ag ABS NP NP 0.4 0.2

0 Reason for shape evolution over time 300 400 500 600 Stable monodispersed 10mM Wavelength (nm) silver nanoparticle solution

17 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