Anisotropic Chemical Reactivity of Gold Spheroids and Nanorods

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Anisotropic Chemical Reactivity of Gold Spheroids and Nanorods Nikhil R. Jana,† Latha Gearheart, Sherine O. Obare, and Catherine J. Murphy* Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208 Received September 19, 2001. In Final Form: November 16, 2001 We observed anisotropic chemical reactivity of gold spheroids during cyanide dissolution and reaction with persulfate. Cyanide dissolution of 2-5 aspect ratio spheroids (with 12-30 nm short axis) starts at the ends of the long axis and leads to intermediate lower aspect ratio spheroids and spheres. Persulfate converts the spheroids into spheres. In contrast, cyanide dissolution of 18 aspect ratio nanorods (with 16 nm short axis) starts simultaneously at many sites and does not change the nanorod length throughout dissolution. Spheroids are thermally less stable than rods and slowly convert to spheres with time, and thus anisotropic chemical reactivity is presumably due to their lower stability.

Introduction Metal nanoparticles show size- and shape-dependent physical and chemical properties.1,2 With decreasing particle size, the particle reactivity toward oxidation increases.1a,b Thus noble metals become very reactive in the nanosize range.3 Most of the previous work regarding nanoparticles deals with those having spherical morphology. However, nonspherical nanoparticles should have different, perhaps more interesting chemical properties. Room temperature annealing of gold nanoparticles on a NaCl substrate showed that cubic gold particles spontaneously evaporated but particles with spherical morphologies remain unchanged.4 El-Sayed et al. showed that gold nanorods of aspect ratio 2-7, prepared in the presence of surfactants, have unstable {110} facets.5 Under the influence of laser pulse or heat, those nanorods transformed into short rods or spheres.6-8 During those reshaping processes, the unstable {110} facets of nanorods converted into stable {100} and {111} facets.9 The cyanide leaching of gold is widely used for its mineral processing and often the presence of impurities strongly influences the gold dissolution processes.10 Cyanide dissolution of spherical gold colloids in the presence * To whom correspondence should be addressed: [email protected]. † Present address: R R R Mahavidyalaya, Hooghly-712406, WB, India. (1) (a) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (b) Belloni, J. Curr. Opin. Colloid Interface Sci. 1996, 1, 184. (c) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 81, 1647. (d) Claus, P.; Bruckner, A.; Mohr, C.; Hofmeister. H. J. Am. Chem. Soc. 2000, 122, 11430. (2) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (3) (a) Pal, T.; Sau, T. K.; Jana, N. R. Langmuir 1997, 13, 1481. (b) Pal, T.; Jana, N. R.; Sau, T. K., Corros. Sci. 1997, 39, 981. (4) Joseyacaman, M.; Mikiyoshida, M. Phys. Rev. B 1992, 46, 1198. (5) Wang, Z. L. Gao, R. P.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 5417. (6) Mohamed, M. B.; Ismail, K. Z.; Link, S. El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 9370. (7) Link. S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 6152. (8) Chang, S. S.; Shih, C. W.; Chen, C. D.; Lai, W. C.; Wang, C. R. C. Langmuir 1999, 15, 701. (9) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 7867. (10) (a) Jeffrey, M. I.; Ritchie, I. M. J. Electrochem. Soc. 2001, 148, D29. (b) Jeffrey, M. I.; Ritchie, I. M. J. Electrochem. Soc. 2000, 147, 3272. (c) Deschenes, G.; Lastra, R.; Brown, J. R.; Jin, S.; May, O.; Ghali, E. Miner. Eng. 2000, 13, 1263. (d) Tshilombo, A. F.; Sandenbergh, R. F. Hydrometallurgy 2000, 60, 55.

of oxygen results in a uniform decrease in the particle diameter.11 Electrochemical dissolution of gold in the presence of cyanide showed that the dissolution started at defect sites at a negative electrode potential, but at higher overpotentials, etching occurred rapidly and uniformly.12 Anisotropic dissolution of gold {111} electrodes has been reported for a very positive potential in the presence of chloride and perchloric acid.13 In this work we investigated the cyanide dissolution of gold spheroids and nanorods in H2O under ambient conditions using dissolved oxygen as the oxidizing agent. We also compared the thermal stability of those particles and reactivity toward persulfate, which is a stronger oxidizing agent than oxygen. Our results indicate that nanoparticle shape strongly influences its stability and chemical reactivity. Experimental Section Synthesis. HAuCl4‚3H2O (Sigma), KCN (Fisher), (NH4)2S2O8 (Sigma), cetyltrimethylammonium bromide (CTAB) (Aldrich), and ascorbic acid (Aldrich) were used as received. Gold spheroids with aspect ratios between 2 and 5 and nanorods with aspect ratio 18 (all having short axis 12-30 nm), were prepared according to our seed-mediated growth procedures.14,15 In these preparations, citrate is the capping agent for the seeds; CTAB is the capping agent for the spheroids and nanorods grown from the seeds. Shapes were tuned from cylindrical to needlelike by varying the seed to metal salt concentration ratios and using additives such as silver salts, cyclohexane, and acetone in the presence of aqueous CTAB.14 After preparation, excess surfactant present in the particle solution was removed by centrifuging the solution at 14 000 rpm for 10 min. During centrifugation, particles were precipitated and the colorless supernatant containing surfactant was then removed. The particles precipitated in this way were completely redispersed in deionized water, and the gold concentration in [Au] was 2.5 × 10-4 M. Next, seven clean test tubes (labeled a-g) were each filled with 1 mL of gold spheroid solution. Varying volumes of stock potassium cyanide solution (10 mM) were then added to each set to make final cyanide concentrations of 0.0 M (a), 10-5 M (b), 10-4 M (c), 2.5 × 10-4 M (d), 5 × 10-4 M (e), 10-3 M (f), and 2.5 × 10-3 M (g). Finally, all (11) McCarthy, A. J.; Coleman, R. G.; Nicol, M. J. J. Electrochem. Soc. 1998, 145, 408. (12) Zamborini, F. P.; Crooks, R. M. Langmuir 1997, 13, 122. (13) Ye, S.; Ishibashi, C.; Uosaki, K. Langmuir 1999, 15, 807. (14) Jana, N. R.; Gearheart, L.; Murphy, C. J. Adv. Mater. 2001, 13, 1389. (15) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065.

10.1021/la0114530 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/10/2002

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Figure 1. UV-vis spectra of 2.5 ( 0.5 aspect ratio gold spheroids with 17 nm short axis (2.5 × 10-4 M in terms of gold) after 1 h of the addition of various amounts of cyanide solution: a, 0.0 M; b, 10-5 M; c, 10-4 M; d, 2.5 × 10-4 M; e, 5 × 10-4 M; f, 10-3 M; g, 2.5 × 10-3 M sodium cyanide. the solutions were vortexed for 60 min to complete the dissolution reaction. Depending on the cyanide concentration, the violet color of particle dispersions turned colorless (in f and g), changed to red (in e), or did not change appreciably (in b and c), within 10-15 min of adding cyanide solution. The rate of dissolution was slower for the sets with lower cyanide concentrations, and these were allowed to react for 1 h. The final colors of each solution remained stable for at least 1 month, and no precipitation of gold particles was observed. Similar to the spheroid experiments, seven sets (a′-g′) of 0.01 wt % gold nanorod solutions were prepared and mixed with the same amounts of cyanide solutions and vortexed for 60 min. The effect of persulfate on the gold spheroids and nanorods was also tested. The procedures for these experiments were similar to the cyanide dissolution experiments, except that freshly prepared aqueous ammonium persulfate solution (22.8 mg of (NH4)2S2O8 dissolved in 100 mL of distilled water) was used instead of cyanide. The final concentration of persulfate in the reaction mixture was varied from 1 to 1000 µM. The spectral and microscopic measurements were carried out after 24 h of mixing due to the slower rate of persulfate reaction compared to cyanide. Instrumentation. The particle size was measured using a JEOL JEM-100CXII transmission electron microscope (TEM) operating at 100 kV. Sizing was enabled using an AMT Kodak Megaplus digital camera and software. For TEM sample preparation, 1-2 µL of the nanoparticle solution was placed on a carboncoated copper grid and allowed to dry at room temperature. At least 100-150 particles were counted from each sample for particle size and shape measurements. Electronic absorption spectra of nanoparticle solutions and kinetic measurements were performed with a CARY 500 Scan UV-Vis-NIR spectrophotometer. Surface-enhanced Raman scattering (SERS) measurements were performed using a Detection Limit Solution 633 instrument utilizing a filtered fiber-optic probe equipped with a microscope objective to focus radiation from a 633 nm helium neon laser onto the sample and collect the 180° Raman backscatter. Laser power at the sample was 25 mW, and a 10 s integration time was used for each measurement. Spectral acquisition and processing were enabled with DLSPEC and GRAMS/32 (Galactic Industries) software. Cyanide SERS. The concentrations of NaCl and the CTAB micellar surfactant were varied in order to achieve optimum conditions for the cyanide SERS signal. For higher concentrations of CTAB (>10-3 M), no cyanide SERS signal was observed even after 2-3 days, for any salt concentration from 0.01 to 1.0 M NaCl. However, when CTAB concentrations were lowered (10-3 M, the SERS intensity decreased with time as the particles dissolved. The CNSERS indicates that CN- adsorbs on the particle surface (16) Hesse, E.; Creighton, J. A. Chem. Phys. Lett. 1999, 303, 101.

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Figure 4. Successive UV-vis spectra of gold plasmon bands during cyanide dissolution of set “f”: (1) before cyanide addition; (2) after 20 s of cyanide addition; (3-10) after 50 s intervals from 2. The inset shows the TEM of intermediate short spheroids corresponding to spectrum 3 isolated by rapidly centrifuging the particles from reaction mixture.

Figure 5. TEM of gold nanorods before (a) and 24 h after cyanide treatment (b).

at 10-5-10-4 M CN- concentrations, although no appreciable particle dissolution occurs at such low CNconcentration as evident from the unchanged absorption spectra. Effect of Persulfate. The stability of gold spheroids of aspect ratio 2-5 in the presence of persulfate was studied over a 7-day period. Whereas 20-50 nm size gold spheres did not appear to react with persulfate (based on no changes in the absorbance spectrum or morphology according to TEM), the spheroids reacted slowly and converted to spheres within 24 h. Various aspect ratio spheroids all produced similar results. Figure 6a shows the UV-vis spectra of spheroids with an aspect ratio of 2.9 ( 0.4 (30 nm short axis) according to TEM (Figure 7a). These spheroids were prepared at a lower gold seed concentration with the addition of silver ions (to induce a needlelike shape).14 The spheroids are characterized by a longitudinal plasmon band at 790 nm and transverse band at 535 nm. In the presence of 10-6 M persulfate, the long wavelength band blue shifted by ∼100 nm with no change of the transverse band (Figure 6b), and the TEM showed that the spheroids of aspect ratio ∼2 decreased

Figure 6. Influence of persulfate on the UV-vis spectra of 2.5 × 10-4 M 2.9 ( 0.4 aspect ratio gold spheroids with 30 nm short axis: a, 0.0 M; b, 10-6 M; c, 10-5 M; d, 10-4 M ammonium persulfate. The spectra were taken 24 h after the addition of persulfate.

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Figure 7. TEM of gold spheroids before and 24 h after persulfate treatment: a, no persulfate; b, 10-6 M persulfate; c, 10-5 M persulfate.

primarily in length but not width (Figure 7b). At 10-5 M persulfate concentration, the long wavelength band disappeared within 3-4 h, but the 535 nm band remained unchanged (Figure 6c). The TEM images showed that all spheroids converted to faceted spheres (Figure 7c). With a further increase in persulfate concentration, the position of the plasmon band did not change even after 24 h, indicating spheres remain inactive in the presence of persulfate (Figure 6d). Similar experiments with 18 aspect ratio nanorods showed that they did not react with persulfate. We did not observe any SERS spectra of persulfate with any of the gold nanoparticle samples. Effect of Heat. Spheroids with aspect ratio ∼2 were unstable and converted to spheres at room temperature within 5-6 h of preparation; however, their stability increased as the aspect ratio increased. Removal of the surfactant template (by centrifugation) or heating the particle solution further decreased the spheroid stability. For example, spheroids with aspect ratio of 2 (with ∼20 nm short axis) showed a transverse plasmon band at 520 nm and a longitudinal plasmon band at 635 nm, but within 24 h of preparation, the longitudinal band red shifted and disappeared (Supporting Information, Figure 2). TEM images indicated that those spheroids converted to spheres. Spheroids with 2.5 aspect ratio were stable for 2-3 weeks even after removing the surfactant but converted to spheres upon boiling the solution for 15 min (Supporting Information, Figures 3 and 4). Nanorods of aspect ratio 18 were stable and did not convert to other shapes even after 1 h of boiling the particle solutions. (We note that solutions of gold spheres, aspect ratio 1, are typically stable to boiling water conditions.)

Discussion Gold spheres and nanorods do not react with persulfate within 1 week, although according to the reduction potential (taken from Lange’s Handbook of Chemistry) of gold (E° of Au3+/Au0 is +0.85 V vs NHE in the presence of Br- from CTAB) and persulfate (E° is +2.0 V vs NHE for S2O82-/SO42-), oxidation is favorable. However, gold spheroids reacted with persulfate and were converted to spheres, suggesting that the reactivity is due to the needlelike edges of the spheroids. In the presence of persulfate, gold atoms present at the spheroid edges possibly oxidized and dissolved according to the following reaction:

2Au + S2O82- a 2Au+ + SO42-

(1)

As a result, spheroids transform into spheres. In cyanide dissolution, oxygen acts as the oxidizing agent of gold(0), and cyanide acts as complexing agent of gold ions. The overall reaction can be written as10

4Au + 8CN- + O2 + 2H2O a 4Au(CN)2- + 4OH(2) with peroxide forming as an intermediate. The mechanism of dissolution can be considered the result of two constituent half-cell reactions: the anodic oxidation of gold (E° of Au+/Au0 is -0.6 V vs NHE in the presence of CN-) and cathodic reduction of oxygen (E° is + 0.69 V vs NHE for O2/H2O2).10a,11,17 From the reduction potential point of view, the dissolution reaction should be thermodynami-

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cally favorable. However, the overall rate is controlled by the anodic half-reaction that has three successive steps10a

Au + CN- f AuCN-ads

(3)

AuCN-ads f AuCNads + e-

(4)

AuCNads + CN- f Au(CN)2-

(5)

where AuCN-ads is the AuCN- adsorbed to the gold surface. It is presumed that formation of protective films of adsorbed AuCN on the gold surface (that may be polymeric in nature where the CN acts as bidentate ligand) slows the dissolution rate.10a There is also evidence of AuCN adlayer formation from scanning tunneling microscopic experiments.18 This anodic process is strongly catalyzed by the presence of trace impurities such as lead, silver, etc., and possibly these impurities disturb the formation of protective AuCN films.10 For nanosize particles, the total surface area exposed to the reaction medium is very high. This can result in a significant alteration of the redox properties of the nanoparticles.1a For example, Henglein et al. showed that the adsorption of electron-rich ions such as CN-, S2-, and I- onto silver nanoparticle surfaces increased the particle reactivity toward oxygen.19 They proposed that adsorption of such ions onto the particle surface increased the particle’s Fermi potential, leading to the increased oxidation tendency of the particle.19 A similar increased oxidation tendency of silver particles3a in the presence of BH4- and of gold particles3b in the presence of I- or SCNwas also observed. In the present case, CN- may have a similar effect of increasing the oxidation tendency of gold particles after their adsorption onto the gold particle surface. For 2.5 × 10-4 M Au0, dissolution was not initiated at