Synthesis of Gold Nanoparticles by Citrate Method to Improve Light Absorption in Photoelectrodes

Minh Tran1, Rebekah De Penning2, Madeline Turner1, and Sonal Padalkar1,3

AcknowledgementsIowa State University: College of EngineeringIowa State University Foundation

1x 5x4x2x 1x 5x4x3x

1x 5x4x3x2x 1x 5x4x3x2x

500 nm 500 nm

1. Department of Mechanical Engineering, 3. Microelectronics Research Center; Iowa State University, Ames, IA 500112. Department of Engineering, Dordt College, Sioux Center, IA 51250

Motivation

Gold nanoparticles (Au NPs) enhance light absorption in photovoltaics by

Surface plasmon resonance (SPR)Scattering centers

One specific example is shown below

Schematic diagram showing Au NPs drop cast onto surface of Si pn junction diode, resulting in enhanced absorption and photocurrent

Objectives

Study the influence of: Citrate concentration represented as Na3Ct /

HAuCl4 ratio Temperatureon Au NP size, size distribution, morphology, and reaction kinetics

Experimental Details

Observations Images of the reaction

mixture, which changed color after citrate addition

t1 and t2 are noted as nucleation and growth periods, respectively.

increaseincr

ease

This table shows the pH values momentarily after citrate addition.

pH increases with increasing citrate concentration

Effect of Citrate Concentration Reaction Kinetics

[1]. Atwater, H. A.; Polman, A. Nature Materials 2010, 9, 205-213.

50 nm

60oC 80oC 100oC

60oC 80oC 100oC

Transmission Electron Microscopy (TEM) images of Au NPs synthesized using Na3Ct / HAuCl4 ratios of 1.50:1 (upper row) and 4.33:1 (lower row) at different temperatures. At lower ratios, Au NPs appear elongated. At higher ratios, they appear faceted or quasi-spherical. The red

arrows point to those of faceted shape.

Indexed Selected Area Electron Diffraction (SAED) pattern of Au NPs synthesized at 80oC and a Na3Ct / HAuCl4 ratio of 1.50, indicatingPolycrystalline nature of Au NPsFace-centered cubic structure of Au NPs

In low citrate concentration regime (0.25-2), particle sizes are large and decrease abruptly with increasing citrate ratio.

In high citrate concentration regime (>2), particle sizes are small and essentially stable with varying citrate ratios

This graph, constructed by Au NP syntheses at 100oC using ratios from 0.25 to 14.15, agrees with literature

Effect of Temperature

As temperature increases, SPR wavelength and FWHM decrease very slightlyAt every temperature, SPR wavelength and FWHM decrease substantially

from low to high concentration regime. Large FWHM implies polydispersity

d ≈ 17 nm

d ≈ 53 nm

The trends for nucleation (t1) and growth (t2) times are similar in both low and high citrate concentration regimes

Reaction kinetics increases exponentially with increasing temperature for both t1 and t2

Higher citrate concentration results in slower reaction kinetics

Logarithmic plot of reaction time vs. T for Na3Ct / HAuCl4 ratios of 1.50:1 and 4.33:1

The Chemical ReactionsDissolution of HAuCl4 in water: HAuCl4 + H2O H3O+ + AuCl4

- (1)Hydrolysis of AuCl4

-: AuCl4

- + H2O [AuCl3(OH)]- + H+ + Cl- (2)AuCl4

- + 2H2O [AuCl2(OH)2]- + 2H+ + 2Cl- (3)Oxidation of citrate to dicarboxy acetone: C6H5O7

3- C5H4O52- + CO2 + H+ + 2e- (4)

Reduction of Au(III) to Au(I): AuCl4

- + 2e- AuCl2- + 2Cl- (5)

[AuCl3(OH)]- + 2e- AuCl2- + Cl- + OH- (6)

[AuCl2(OH)2]- + 2e- AuCl2- + 2OH- (7)

Disproportionation of Au(I) to metallic Au: 3AuCl2

- 2Au0 + AuCl4- + 2Cl- (8)

The Formation Mechanism

Future DirectionsDeposition of Au NPs on shape-controlled ZnO nanostructures in

our lab for photocatalytic and solar cell applicationsDeposition of Au NPs on metallic or dielectric colloids to form

nanostructures with Au shell

Conclusions Size and morphology of Au NPs strongly depend on Na3Ct /

HAuCl4 ratio, but only slightly with temperature Reaction kinetics is affected by temperature and citrate ratio

The appearance of SPR peak indicates that Au NPs start to form

The blue-shift and narrowing of the peak indicate that Au NPs grow to their mature size

Experimental steps

t is the total time starting at citrate addition and ending 30 minutes after the reaction mixture turns red

Size distributions of Au NPs synthesized at 100oC for Na3Ct / HAuCl4 of 1.50:1 (left) and 4.33:1 (right). The sizes of elongated particles are determined by their widths

N = 257 particles

N = 132 particles

[2]. Schaadt, D. M.; Feng, B.; Yu, E. T. Appl. Phys. Lett. 2005, 86.