Synthesis of Gold Nanoparticles by Citrate Method to Improve Light Absorption in Photoelectrodes Minh Tran 1 , Rebekah De Penning 2 , Madeline Turner 1 , and Sonal Padalkar 1,3 Acknowledgements Iowa State University: College of Engineering Iowa State University Foundation 1x 5x 4x 2x 1x 5x 4x 3x 1x 5x 4x 3x 2x 1x 5x 4x 3x 2x 500 nm 500 nm 1. Department of Mechanical Engineering, 3. Microelectronics Research Center; Iowa State University, Ames, IA 50011 2. 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 Na 3 Ct / HAuCl 4 ratio Temperature on Au NP size, size distribution, morphology, and reaction kinetics xperimental Details Observation s Images of the reaction mixture, which changed color after citrate addition t 1 and t 2 are noted as nucleation and growth periods, respectively. i n c r e a s e increa se This table shows the pH values momentarily after citrate addition. pH increases with increasing citrate concentratio Effect of Citrate Concentration Reaction Kinetics [1]. Atwater, H. A.; Polman, A. Nature Materials 2010, 9, 205-213. 50 nm 60 o C 80 o C 100 o C 60 o C 80 o C 100 o C Transmission Electron Microscopy (TEM) images of Au NPs synthesized using Na 3 Ct / HAuCl 4 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 80 o C and a Na 3 Ct / HAuCl 4 ratio of 1.50, indicating Polycrystalline nature of Au NPs Face-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 100 o C using ratios from 0.25 to 14.15, agrees with literature Effect of Temperature As temperature increases, SPR wavelength and FWHM decrease very slightly At 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 (t 1 ) and growth (t 2 ) times are similar in both low and high citrate concentration regimes Reaction kinetics increases exponentially with increasing temperature for both t 1 and t 2 Higher citrate concentration results in slower reaction kinetics Logarithmic plot of reaction time vs. T for Na 3 Ct / HAuCl 4 ratios of 1.50:1 and 4.33:1 The Chemical Reactions Dissolution of HAuCl 4 in water: HAuCl 4 + H 2 O H 3 O + + AuCl 4 - (1) Hydrolysis of AuCl 4 - : AuCl 4 - + H 2 O [AuCl 3 (OH)] - + H + + Cl - (2) AuCl 4 - + 2H 2 O [AuCl 2 (OH) 2 ] - + 2H + + 2Cl - (3) Oxidation of citrate to dicarboxy acetone: C 6 H 5 O 7 3- C 5 H 4 O 5 2- + CO 2 + H + + 2e - (4) Reduction of Au(III) to Au(I): AuCl 4 - + 2e - AuCl 2 - + 2Cl - (5) [AuCl 3 (OH)] - + 2e - AuCl 2 - + Cl - + OH - (6) [AuCl 2 (OH) 2 ] - + 2e - AuCl 2 - + 2OH - (7) Disproportionation of Au(I) to metallic Au: 3AuCl 2 - 2Au 0 + AuCl 4 - + 2Cl - (8) The Formation Mechanism Future Directions Deposition of Au NPs on shape-controlled ZnO nanostructures in our lab for photocatalytic and solar cell applications Deposition of Au NPs on metallic or dielectric colloids to form nanostructures with Au shell Conclusions Size and morphology of Au NPs strongly depend on Na 3 Ct / HAuCl 4 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 100 o C for Na 3 Ct / HAuCl 4 of 1.50:1 (left) and 4.33:1 (right). The sizes of elongated particles are determined by their widths N = 257 particle s N = 132 particle s [2]. Schaadt, D. M.; Feng, B.; Yu, E. T. Appl. Phys. Lett. 2005, 86.