Synthesis and Characterisation of Gold-Silver nanoparticles

Abstract

The preparation of gold-silver nano particles by reduction of gold and silver ions using sodium citrate is reported here. The U-V visible absorption spectra of each sample were measured. The absorption peak showed that the particle present is bimetallic. Particle size analysis of the nanoparticles was done using Malvern Zetasizer instrument. The reaction of the solution with NaCl proved that gold nanoparticle can be used as sensors

Introduction

Since the concept of nanoparticles was known, the possible applications for gold and silver nano particles have been the interest of many researchers [1]. The techniques based on nano-particles have been gaining importance in the research field from the past few years. The change of molecular,electronic and optical properties with the change in particle size has been the key interest [1]. Metal nanoparticles have mainly been studied because of their unique optical properties .Nanoparticles of the alkali metals and the noble metals copper, silver, and gold have a broad absorption band in the visible region of the electromagnetic spectrum. [2]

Gold nanoparticles are usually made from gold complex by the reduction of these complexes using sodium citrate. The formation of Plasmon resonance band contributes to the optical properties of the colloid particles. [1]. Solutions of gold metal nanoparticles show a very intense color, which is absent for the bulk material as well as for the atoms. [2].

Experimenal

As stated in the lab manuel under experiment 6. No changes were made.

Results and Discussion

Observations

The initial gold complex solution was observed to be colourless at first and after heating the colour slowly changed to reddish brown.

Colour changes observed for nanoparticle solution with different composition are:-

Volume of Gold soln./ml Volume of Silver soln./ml Observation after heating

2.00 0.50 Colourless to reddish brown

1.75 0.75 Colourless to dark orange

1.50 1.00 Colourless to orange

0.75 1.75 Colourless to yellow

0.00 2.50 Colourless to pale yellow

In this experiment HAuCl4 was reduced with the addition of sodium citrate. Charged gold particles were reduced to neutral gold atoms. Due to the formation of gold nano particles by this reduction the colourless (pale yellow) gold solution became reddish brown in colour.

In the second part of the experiment the gold-silver nano particles were produced by the reduction of gold and silver by using sodium citrate. The colour changes of the solutions at different compositions were noticed. At different composition of the bimetallic nanoparticle, a new energy level will be formed and at each new energy level light of different wavelength will be absorbed thus contributing to the formation of different coloured complexes.

Visible spectra analysis

Solution Mole fraction of Au

Concentration of Au

Absorbance Amax

Wavelengthλmax

Molar extinction co-efficient

1 1 0.0000625 0.997 518.5 15952

2 0.8 0.00005 1.216 507.524320

3 0.7 0.00004375 1.065 503 24342.86

4 0.6 0.0000375 1.067 503 28453.33

5 0.3 0.00001875 0.796 502.5 42453.33

6 0 0 0.306 420.5 0

Calculations

Mole fraction of Au = mol of Au/(mol of Au + mol of Ag)

Molar extinction co-efficient =

Amax = ε CL

Where Amax is the maximum absorbance, ε is the molar extinction co-efficient

C is the molar concentration of Au and

L is the light path length (1 cm)

0 0.2 0.4 0.6 0.8 1 1.20

100

200

300

400

500

600f(x) = 670.213178294574 xR² = 0.792820021883506

Wavelength of absorbtion as a func-tion of mole fraction od Au

Wavelength vs molar coef-ficientLinear (Wavelength vs mo-lar coefficient)

Mole fraction of Au

Wav

elen

ght m

ax in

nm

Fig. 1 Graph plotted with maximum wave length of absorption against mole fraction

0 0.2 0.4 0.6 0.8 1 1.20

50001000015000200002500030000350004000045000

f(x) = 31882.1705426356 xR² = 0.647469895789157

Molar extinction co-efficient versus mole fraction

Molar extinction co-effi-cient versus mole fractionLinear (Molar extinction co-efficient versus mole fraction)

Mole fraction of Au

Mol

ar co

-effi

cient

Fig.2 Graph plotted with Molar extinction co-efficient against mole fraction

Molar extinction co-efficient of pure gold is 4330M-1cm-1 where as the value calculated from this experiment is 15952 which is a higher value. Errors caused during the experiment has caused this much variation in the results.

Particle size analysis

Mole fraction of Au Particle size in nm

1 13.62

0.3 51.34

0 29.75

0 0.2 0.4 0.6 0.8 1 1.20

10

20

30

40

50

60 Praticle size analysis

Series2

Mole fraction

Prac

ticle

size

in n

m

Fig:-3 Graph plotted with particle size in nm against mole fraction

The average particle size for the gold nano particle from this experiment is 34.22nm. Systematic variation with the composition is observed in the result.

Gold nano particle as sensor

Colour changes were observed when NaCl solution was added to the gold nanoparticle solution. When 1M NaCl was added to the gold nanoparticle solution, the colour of the solution changed from reddish brown to light grey colour. When saturated NaCl was added to the solution the colour changed from reddish brown to dark grey.

The addition of sodium chloride to the nanoparticle solution will destabilise the nanoparticle present and the gold particle combine together to form large particle. Thus a different wavelength of light will be absorbed by this particle and changes colour.

The colour changes observed during the addition of NaCl give evidence that gold nano particle cam be used as sensor.

Questions

1. The colloid particle of the gold and siver solution is surrounded by a double ionic layer. The negatively charged citrate ions from the sodium citrate accumulate over the nano particles.

These ions act as a charge stabiliser and introduce a surface energy which repeals the particles and prevent them from forming larger molecules. The repulsion force provided by this shield is greater than that of the force needed for the particles to combine together, thus preventing the gold nanoparticles from forming large particle.

2. In this method, the sodium citrate acted as a reducing agent and it helped in the formation of gold nanoparticles. But as sodium citrate is a weaker reducing agent, it is not strong enough to reduce silver ions.

Uniform silver nano particles can be obtained through the reduction of silver ions by e ethanol at a temperature of 80°C to 100°C under atmospheric conditions.[3]

3. The optical absorption spectra help to confirm that a bimetallic particle is present instead of gold and silver particle. In the absorption spectra there is only one absorption band (Plasmon resonance band) which shows that there is only one particle present. If there was two particles present in the mixture then there should be two absorption bands showing the presence of two different particles.

4. The particle size of gold, silver and gold-silver nanoparticle was measured using Malvern zetsizer. It measures the average size of the particle in the solution by dynamic light scattering. In dynamic light scattering, when light hits small particles the light scatters in all directions so long as the particles are small compared to the wavelength. This scattered light then undergoes either constructive or destructive interference by the surrounding particles. The shift in light frequency is related to the size of particle.

5. When 1M NaCl was added to the gold nanoparticle solution, the colour of the solution changed from reddish brown to light grey colour. When saturated NaCl was added to the solution the colour changed from reddish brown to dark grey. Sodium chloride has increased the concentration concentration in the solution. The repulsive force formed by citrate inos were removed when NaCl was added. So the gold nanoparticles combine together and form larger molecules which result in the absorption of light of a different wavelength and thus contributing a different colour.

Conclusion

Bimetallic alloy of gold and silver nanoparticle was obtained by the reduction of gold and silver ions by using sodium citrate. The absorption band showed just one Plasmon resonance band which confirmed that the particle prepared is bimetallic and not a mixture of gold and silver nanoparticles. The results obtained by this experiment were bit high when compared to the literature values. The application of gold nanoparticle as sensor was also obtained through the experiment. The colour changes at different parts of the experiment are discussed.

Reference

[1] Synthesis and Characterisation of Gold-Silver Nanoparticles –Experiment 6 Lab manuel

[2] “Alloy Formation of Gold-Silver Nanoparticles and the Dependence of the Plasmon Absorption on Their Composition.” S. Link, Z. L. Wang,and M. A. El-Sayed.

[3]“Preparation of Silver Nanoparticles and Their Characterization” R. Das, S. S. Nath, D. Chakdar, G. Gope, R. Bhattacharjee