Nano Res 95 Electronic Supplementary Material Charge Transfer and Retention in Directly Coupled Au CdSe Nanohybrids Bo Gao 1, Yue Lin 1, Sijie Wei 1, Jie Zeng 1,2, Yuan Liao 1, Liuguo Chen 1, David Goldfeld 2, Xiaoping Wang 1, Yi Luo 1, Zhenchao Dong 1 ( ), and Jianguo Hou 1 ( ) 1 Hefei National Laboratory for Physical Sciences at the Microscale University of Science and Technology of China, Hefei, Anhui 230026, China 2 Department of Biomedical Engineering, Washington University, St. Louis, MO 63130, USA Supporting information to DOI 10.1007/s12274-011-0188-8 1. Figures Figure S-1 AFM topograph (a) and phase image (b) of the Au CdSe nanohybrids on a native oxide covered Si substrate. The NH size obtained from the AFM image is slightly larger than the results of the TEM images ( 7 nm) due to the tip convolution effect Figure S-2 Schematic illustration of the two-electron reduction reaction where MB + dye molecules are reduced to neutral MBH [S-1]. The hydrogen ions can come from the reaction product of the hole scavenging reaction of ethanol molecules shown in Fig. S-3 Figure S-3 Schematic chemical equation of ethanol acting as a hole scavenger [S-2] Address correspondence to Zhenchao Dong, zcdong@ustc.edu.cn; Jianguo Hou, jghou@ustc.edu.cn
96 Nano Res Figure S-4 Another set of photocatalytic experiments of MB + dye in (a) the isolated CdSe QD solution and (b) the solution of the 1:1 Au/CdSe mixture as a contrast to that with the Au CdSe NH solution (Fig. 5), showing the negligible variation in absorption behavior for both samples as a function of the pre-irradiation time. These data were used to plot the corresponding curves in Fig. 5(c) Figure S-5 Influence of the number of capping agents around a Au NP on the absorption spectra of MB + in the NH solution for (a) the Au CdSe NH sample reported in the text and (b) another Au CdSe NH sample that was diluted from an identical solution to that in (a). Sample (a) was precipitated using methanol and dissolved in toluene once while sample (b) was precipitated using methanol and dissolved in toluene three times, which means that some of the DDA capping agents originally attached to the Au NP surface will be dissolved in the solvent. As a result, the number of capping agents around any given Au NP is smaller in sample (b) in comparison to that of sample (a). The absorbance spectra as a function of the standing time indicate that the MB + molecules can still be reduced by the charge on the Au NPs for a standing time of 90 min for sample (a) with more capping agent, while for sample (b) with less capping agent, the charge on the Au NPs has essentially leaked out after a standing time of 90 min and essentially no reduction of MB + molecules is observed. (c) Schematic illustration of DDA Au NP connections through the Au N bonding (usually, there are more than 100 DDA agents around a Au NP, but for simplicity, only one Au DDA connection is shown) 2. Methods Ⅰ Estimation of interparticle spacing The average distance between isolated metal NPs and isolated semiconductor QDs in a mixture can be estimated on the basis of the particle concentration in the solution and the size of the nanoparticles synthesized. For the 1:1 Au/CdSe mixture, the concentration of the Au 3+ source (from HAuCl 4 ) in toluene is 25 mmol/l (denoted
Nano Res 97 as C Au ) while the concentration of the Cd 2+ source in toluene is 20 mmol/l (denoted as C Cd ). According to high-resolution TEM images, the particle size is 6 nm in diameter for the Au NP and 4 nm in diameter for CdSe QDs. Assuming both nanostructures are spherical in shape, their volume can be estimated as follows: V Au 4 R Au3 /3 113 nm 3, V CdSe 4 R CdSe3 /3 34 nm 3 Since the atomic volume of Au is approximately 10.2 cm 3 /mol [S-3], i.e., 0.017 nm 3 /atom, the number of Au atoms per Au NP can be estimated as N Au-per NP V Au /0.017 6647 Similarly, the unit volume of a CdSe QD is approximately 0.056 nm 3 /atom assuming a ZnO-like structure [S-4], and the number of Cd atoms per CdSe QD can be estimated as N Cd-per QD V CdSe /0.056 607 Therefore, for a volume of 1.655 ml of the Au NP solution (denoted as V Au-NP ) used in the experiment, the number of Au NPs can be estimated as N Au-NP C Au V Au-NP Na/N Au-per NP 25 10 3 1.655 10 3 6.02 10 23 /6647 3.8 10 15 For a volume of 0.2 ml of the CdSe QD solution (denoted as V QD ) used in the experiment, the number of CdSe QDs can be estimated as N QD C Cd V QD Na/N Cd-per QD 20 10 3 0.2 10 3 6.02 10 23 /607 3.9 10 15 The Au/CdSe mixture solution thus prepared gives a 1:1 ratio of the Au NPs to the CdSe QDs. For a total volume of 1.855 ml of such mixture solution, we can have the particle concentration as follows for both Au NPs and CdSe QDs: C Au-NP C CdSe-QD 2.1 10 15 particles/ml. To estimate the average interparticle distance between the Au NPs and CdSe QDs, we assume that these two nanostructures arrange themselves in the mixed solution in a simple cubic lattice similar to CsCl [S-5]. With this assumption, each unit cell will consist of one Au NP and one CdSe QD, with each of them surrounded by eight different neighbors located at the cube corners. The volume of the cell is 4.8 10 5 nm 3 via V c 1/C Au-NP. Thus, the average center-to-center distance between isolated CdSe QDs and isolated Au NPs can be estimated via d inter ( 3/2) (V c 1/3 ), i.e., d inter 67 nm. Note that this is the average interparticle distance in the mixed solution, and the particles might get closer when moving according to Brownian motion. Ⅱ Analysis of quenching yield and quenching rate The quenching yield (Q q ) and quenching rate (k Au ) reported in Table 1 were estimated according to the following procedures. Since this additional part of quenching is caused by the presence of Au NPs, a rough assumption was made that the quenching due to the Au NPs opens up an additional nonradiative decay channel (k Au ) for excited CdSe QDs while the other two decay (k r ) and nonradiative (k nr ) channels remain relatively constant. An averaged lifetime was also used during the estimation. Thus, we can have 1 CdSe + knr (1)
98 Nano Res 1 NH + knr+ kau (2) here CdSe refers to the average lifetime of CdSe QDs and NH to the average lifetime of the Au CdSe NHs. Therefore, the quenching rate can be estimated by 1 1 k Au (3) NH Under the above assumptions, the PL quantum yield for CdSe QDs (denoted as Q CdSe ) and Au CdSe NHs (denoted as Q NH ) can be calculated as follows: CdSe Q Q CdSe NH k + k r nr k + k + k r nr Au (4) (5) Thus, the quenching yield of the Au CdSe NHs in comparison to the CdSe QDs can be described by: Q q QNH 1 1 1 Q k + k + k k + k NH CdSe r nr Au r nr CdSe (6) Similar procedures can be used to estimate the quenching effect of the Au NPs on the Au/CdSe mixture solution via Eqs. (3) and (6), the results are also listed in Table 1. It is worth noting that the quenching yield estimated for the 1:1 Au/CdSe mixture from the transient spectra is 49%, in good agreement with the intensity variation observed in the steady-state spectra shown in Fig. 3(b). In addition, the estimated quenching rate for the Au CdSe NHs (i.e., k Au 0.161 ns 1 ) is much faster than that of the Au/CdSe mixture (i.e., 0.034 ns 1 ), which leads to the much stronger quenching observed for the NHs. Ⅲ Estimation of the amount of charge transfer during light irradiation in terms of the reduction of MB + absorption intensity Since the reduction of MB + to MBH is a two-electron reaction (schematically shown in Fig. S-1), one can estimate the average number of electrons transferred per NH (N e-pernh ) according to the number of MB + molecules that are reduced (denoted as N MB+reduced ) [S-6]. Namely, the number of electrons absorbed by MB + (denoted as N e-mb+ ) can be calculated according to the charge balance via N e-mb+ N MB + reduced 2, in which N MB+reduced can be estimated from the observed percentage of the MB + absorption reduction (P abs ) multiplied by the number of MB + molecules originally in solution (N MB+ ). That is, N MB + reduced P abs N MB +. Assume that the number of electrons absorbed by MB + (N e-mb+ ) comes solely from the charge transfer from the CdSe QDs to the AuNPs in the NH solution (N e-nh ), i.e., N e-mb+ N e-nh. Thus, N e-pernh N e-nh /N NH N e-mb+ /N NH N MB + reduced 2/N NH, in which N NH is the number of Au CdSe NHs in the solution. Based on the absorption spectral data in Fig. 5(c) for the sample containing 1 ml of MB + (3 10 6 mol/l, see text) and 2 ml of NH solution (1 10 8 mol/l, see text)
Nano Res 99 and after a pre-irradiation for 45 min, Pabs is found experimentally to be 46%. Therefore, N MB+reduced P abs N MB+ 0.46 3 10 6 mol/l 1 ml 1.38 10 9 mol, and thus, the average number of electrons transferred to one Au NP is: N e-pernh N MB+reduced 2/N NH 1.38 10-9 mol 2/(1 10 8 mol/l 2 ml) 138. Ⅳ Estimation of the number of electrons transferred to a Au NP via single electron charging mechanism One possible mechanism for storage of electrons at the Au NPs is via single electron charging, through which electrons are transferred to the Au NP one by one until the Fermi level of Au NPs is aligned with the conduction band of CdSe QDs [S-6, S-7]. For a Au NP with a radius of R, the self-capacitance of the Au NP can be defined as C 4 ε 0 ε s R [S-8], in which the vacuum permittivity ε 0 8.85 10 12 F/m and the relative dielectric constant surrounding the Au NP is approximated to that of ethanol, i.e., ε s ε ethanol 25.7 [S-9]. where R 1.5 nm and C 4.3 10 18 F. Once an electron is added onto the Au NP, the potential energy of the Au NP will be increased by V e/c 37 mv. (On the other hand, the charging energy is defined as E C e 2 /2C, and equals 18 mev in the present case.) Since the potential difference between the conduction band of CdSe QDs and the Fermi level of Au NPs (i.e., E E CdSe Au CB EF ) is about 0.5 V, the number of electrons that can be charged onto the Au NP via single electron charging mechanism can be estimated via N SET E/ V, i.e., ~14 electrons. Ⅴ Estimation of the number of DDA capping agents around a Au NP and related charge storage The observed dependence of the charge retention time on the DDA concentration (Fig. S-5) suggests the possibility that the charge is stored at the Au DDA interface, probably via Au N bonds with the charge location shifted toward the N atom. In this case, the charge retention time will depend on the number of Au N bonds and thus the number of DDA surface capping agents around any single Au NP. For a Au NP with a diameter of 3 nm, the number of the surface Au atoms can be estimated as follows through the assumption that the Au NP (with a radius of R NP 1.5 nm) consists of a core NP (with a radius of R core ) and a surface single-atom layer, whose thickness can be estimated using the Au Au distance ( 0.25 nm) [S-10]. Thus, R core R NP d Au Au 1.5 0.25 1.25 nm. Using a similar procedure used in Method I for estimating the number of Au atoms inside a Au NP, the total number of Au atoms for the whole Au NP is 831, while the number of Au atoms for the core NP is 481. Therefore, the number of surface Au atoms on the Au NP is 350. Due to the steric repulsion, probably not every Au atom on the surface will be attached to a DDA agent, and therefore if one assumes that only one half of the surface Au atoms are attached, there will still be around 175 DDA molecules attached to a single Au NP, which corresponds to an additional charge retention capability up to about 175 electrons. ESM References [S-1] Hallock, A. J.; Berman, E. S. F.; Zare, R. N. Ultrafast kinetic measurements of the reduction of methylene blue. J. Am. Chem. Soc. 2003, 125, 1158 1159. [S-2] Muller, B. R.; Majoni, S.; Meissner, D.; Memming, R. Photocatalytic oxidation of ethanol on micrometer- and nanometer-sized semiconductor particles. J. Photochem. Photobiol. A-Chem. 2002, 151, 253 265. [S-3] Helmenstine, A. M. Gold Facts. http://chemistry.about.com/od/elementfacts/a/gold.htm. [S-4] Puzder A.; Williamson, A. J.; Zaitseva, N.; Galli G. The effect of organic ligand binding on the growth of CdSe nanoparticles probed by ab initio calculations. Nano Lett. 2004, 4, 2361 2365.
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