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Langmuir 2006, 22, 736-741
Role of Halide Ions and Temperature on the Morphology of Biologically Synthesized Gold Nanotriangles Akhilesh Rai,† Amit Singh,† Absar Ahmad,‡ and Murali Sastry*,§ Nanoscience Group, Materials Chemistry DiVision and Biochemical Sciences DiVision, National Chemical Laboratory, Pune 411 008, India, Tata Chemicals Limited, Leela Business Park, Andheri-Kurla Road, Andheri, Mumbai 400059, India ReceiVed July 28, 2005. In Final Form: NoVember 5, 2005 In this paper, we demonstrate the effect of halide ions on the formation of biogenically prepared gold nanotriangles using the leaf extract of lemongrass (Cymbopogon flexuosus) plant. We have also studied the effect of halide ions on the morphology of biogenic nanotriangles. It has been shown that iodide ions have a greater propensity to transform flat gold nanotriangles into circular disklike structures as compared to other halide ions. The study also suggests that the presence of Cl- ions during the synthesis promotes the growth of nanotriangles, whereas the presence of I- ions distorts the nanotriangle morphology and induces the formation of aggregated spherical nanoparticles. The change in the morphology of gold nanotriangles has been explained in terms of the ability of the halide ions to stabilize or inhibit the formation of (111) faces to form [111] oriented gold nanotriangles. Last, we have also shown that the temperature is an important parameter for controlling the aspect ratio and the relative amounts of gold nanotriangles and spherical particles. The results show that, by varying the temperature of reaction condition, the shape, size, and optical properties of anisotropic nanoparticles can be fine-tuned.
Introduction
* To whom correspondence should be addressed. E-mail: msastry@ tatachemicals.com. † Nanoscience Group, Materials Chemistry Division, National Chemical Laboratory. ‡ Biochemical Sciences Division, National Chemical Laboratory. § Tata Chemicals Limited.
nanoparticles exhibit interesting optical properties when compared to their spherical counterparts. For example, flat disks and nanotriangles of silver and gold exhibit two (or more) prominent absorption bands; a low wavelength transverse surface plasmon (SP) absorption band (out of plane SP vibration) and an often more intense longitudinal absorption band at longer wavelengths (in-plane SP vibration component) similar to that observed for nanorods.1 We have recently reported an eco-friendly and efficient synthesis of highly anisotropic gold nanotriangles using lemongrass leaf extract as a natural reducing and shape-controlling agent.12 The broad in-plane SP band of the gold nanotriangles was observed to extend well into the near-infrared (NIR) region of the electromagnetic spectrum.12,13 The absorption maximum of the in-plane SP band of the gold nanotriangles can be tuned by controlling their size by simply varying the concentration of lemongrass extract in the reaction medium.13 Gold nanotriangles with strong NIR absorption are expected to have application in cancer hyperthermia14 and in architectural applications such as infrared absorbing optical coatings.13 Unlike in the case of gold and silver nanorods where the mechanism leading to their formation is fairly well understood,15
(1) (a) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (b) Mulvaney, P. Langmuir 1996, 12, 788-800. (c) Link, S.; Mohamed. M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (d) Zong, R.-L.; Zhou, J.; Li, Q.; Du, B.; Li, B.; Fu, M.; Qi, X.-W.; Li, L.-T. J. Phys. Chem. B 2004, 108, 16173. (2) McConnell, W. P.; Novak, J. P.; Brousseau, L. C., III.; Fuierer, R. R.; Tenent, R. C.; Feldheim, D. L. J. Phys. Chem. B 2000, 104, 8925. (3) Liu, C.; Zhang, Z. J. Chem. Mater. 2001, 13, 2092. (4) Moreno-Manas, M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638. (5) (a) Jana, N. R.; Gearheart, C.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (b) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (c) Kim, J.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (6) (a) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (b) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (c) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew. Chem. 2004, 43, 3759. (7) Hao, E.; Kelly, L.; Hupp, J. T.; Schatz, G. C. J. Am. Chem. Soc. 2002, 124, 15182. (8) Sun, Y.; Mayers, B.; Xia, Y. Nano Lett. 2003, 3, 675. (9) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955. (b) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736. (c) Sun, Y.; Mayers, B.; Xia, Y. AdV. Mater. 2003, 15, 641. (10) (a) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (b) Hao, E.; Bailey, R. C.; Schartz, G. C.; Hupp, J. T.; Li, S. Nano Lett. 2004, 4, 327.
(11) (a) Shao, Y.; Jin, Y.; Dong, S. Chem. Commun. 2004, 1104. (b) Malikova, N.; Pastoriza-Santos, I.; Schierhorn, M.; Kotov, N. A.; Liz-Marzan, L. M. Langmuir 2002, 18, 3694. (c) Sarma, T. K.; Chattopadhyay, A. Langmuir 2004, 20, 35203524. (d) Jin, R.; Cao.; Y. W.; Mirkin, C. A.; Kelly, K. A.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (e) Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487. (12) Shankar, S. S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad, A.; Sastry, M. Nat. Mater. 2004, 3, 482-488. (13) Shankar, S. S.; Rai, A.; Ahmad, A.; Sastry, M. Chem. Mater. 2005, 17, 566. (14) (a) Loo, C.; Lin, A.; Hirsch, L.; Lee, M. H.; Barton, J.; Halas, N.; West, J.; Drezek, R. Technol. Cancer Res. Treat. 2004, 3, 30. (b) Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J.; West, J. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 135491. (c) Nanospectra biosciences, Inc. Huston Texas, (www.nanospectra.coma) Simpson, C. R.; Kohl, M.; Essenpreis, M.; Cope, M. Phys. Med. Bio. 1998, 43, 24652478. (d) Anderson, R. R.; Parrish, J. A. J. InVest. Dermatol. 1981, 77, 13. (15) (a) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 7, 771. (b) Gao, J.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19, 9065. (c) Wei, G.; Zhou, H.; Liu, Z.; Song, Y.; Wang, Li.; Sun, L.; Li, Z. J. Phys. Chem. B 2005, 109, 8738. (d) Zhang, S.-H.; Jiang, Z.-Y.; Xie, Z.-H.; Xu, X.; Huang, R.-B.; Zheng, L.-S. J. Phys. Chem. B 2005, 109, 9416.
In recent years, one of the major challenges in materials research has been the development of experimental recipes for the systematic control of the shape of inorganic nanoparticles. Control over the shape and size of metallic nanoparticles enables tuning of their optical,1 electronic,2 magnetic,3 and catalytic4 properties. A number of shapes ranging from rods5 to cubes,6 disks,7 belts,8 wires,9 and mono-/bi-/tri-/tetrapod metallic nanostructures10 may be routinely obtained by solution based methods. However, most of these methods require improvement in yield, purity, and monodispersity of the anisotropic structures that are formed. Insofar as gold is concerned, many reports deal with the synthesis of spherical nanoparticles by chemical methods but relatively few reports have appeared on the synthesis of metallic nanostructures of triangular morphology.11 Such high aspect ratio
10.1021/la052055q CCC: $33.50 © 2006 American Chemical Society Published on Web 12/16/2005
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factors contributing to the formation of nanotriangles/nanoprisms are not well understood. Recent reports have highlighted the role of inorganic species or ions in controlling the shape of nanoparticles. Pileni et al. have show that copper nanocrystals of rod and cubelike morphology can be produced in Cu(AOT)2isooctane-water reverse micelles in the presence of different salts. Even though the shape of the micellar template does not change significantly due to the presence of different salts, nanocrystal growth was found to be a strong function of the salt used.16 Xia et al. have reported an elegant polyol-based approach for the large-scale synthesis of silver nanostructures of tetrahedral and truncated cubic morphology.17 They have recently observed that trace amounts of chloride and oxygen (from air) promote the selective etching and dissolution of twinned nuclei of silver, which thereby leads to the growth of nanoscale cubes and tetrahedra of silver.18 Thus, it is clear that halide ions have a pronounced effect on the nucleation and growth of metallic nanocrystals. As briefly mentioned earlier, we have recently demonstrated that large concentrations of gold nanotriangles can be synthesized by an eco-friendly process employing lemongrass extract as a reducing and shape-directing agent.12 As part of our ongoing investigation into the growth mechanism of gold nanotriangles using this plant extract, we present herein our study on the effect of reaction temperature and halide ions present during and after the biosynthesis of gold nanotriangles using lemongrass leaf extract on the morphology of the nanoparticles. Our principle observations are that Cl- and Br- ions present during the synthesis promote the growth of large concentrations of gold nanotriangles (in that order) while I- ions completely distort the morphology and lead to the formation of aggregated spherical and hollow nanoparticles. Bromide and iodide ions are known to be capable of replacing chemisorbed Cl- ions on the gold nanoparticle surfaces16 and could thereby potentially control the rate of growth of the nuclei and the morphology of the product synthesized by lemongrass extract. Significantly, we have also observed that addition of various halide ions to solutions of already prepared gold nanotriangles using lemongrass leaf extract alters their morphology. The temperature of the reaction medium is a critical factor that determines the nature of nanoparticles formed; at higher temperatures, the percentage of gold nanotriangles relative to spherical particles is significantly reduced, whereas lower reaction temperatures promote nanotriangle formation. Presented below are details of the investigation. Experimental Section All chemicals (KI, KCl, KBr, and KF; HAuCl4; and sodium borohydride) were obtained from Aldrich Chemicals and were used as received. All of the glassware was cleaned with soap solution followed by aqua-regia and rinsed with deionized water prior to experiments. The lemongrass extract used in all experiments described herein was prepared from 100 g of thoroughly washed lemongrass leaves that were finely cut and boiled in 100 mL sterile distilled water for 5 min. 8 mL of this leaf extract was added to 100 mL of 10-3 M aqueous HAuCl4 solution for reduction. UV-vis-NIR spectra were recorded after completion of the reaction, which typically required 12 h. The gold nanoparticle solution thus obtained was purified by repeated centrifugation at 1000 rpm for 10 min followed by redispersion of the pellet in deionized water. After 4 cycles of this (16) (a) Filankembo, A.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 5865. (b) Filankembo, A.; Giorgio, S.; Lisiecki, I.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 7492. (17) Wiley, B.; Herricks, T.; Sun.; Y.; Xia, Y. Nano Lett. 2004, 4, 1733. (18) Im, S. H.; Lee, Y. T.; Wiely, B.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 2154.
Langmuir, Vol. 22, No. 2, 2006 737 treatment, the percentage of gold nanotriangles relative to spherical particles increased to ca. 90% and this solution was then used for further experimentation. To study the effect of halide ions on the morphology of nanotriangles postsynthesis, 1 mL of 10-2 M aqueous solutions of KF, KCl, KBr, and KI were added to 9 mL of purified triangles so that the final concentration of halide ions in the solution was 10-3 M. These solutions were then allowed to stand for 24 h at room temperature, following which they were analyzed by a number of techniques as detailed below. The effect of halide ions on the morphology of the nanotriangles during growth was investigated by reacting 1 mL of 10-2 M aqueous halide ion solution (KF, KCl, KBr, and KI) with 9 mL of 10-3 M aqueous HAuCl4 solution and 0.8 mL of lemongrass extract. The role of temperature on the morphology of the biologically prepared gold nanoparticles was studied by carrying out the reaction of 0.8 mL of lemongrass leaf extract with 10 mL of 10-3 M HAuCl4 solution at 40, 50, 60, 70, and 80 °C for 5 h. UV-vis-NIR spectra for all gold nanoparticle solutions were measured using a JASCO dualbeam spectrophotometer (model V-570) operated at a resolution of 1 nm. Transmission electron microscopy (TEM) measurements of the gold nanoparticles synthesized using lemongrass leaf extract under different conditions were carried out on a JEOL model 1200EX instrument operated at an accelerating voltage at 80 kV. Samples for the TEM measurements were prepared by drop coating the nanoparticles onto carbon-coated copper TEM grids placed on a clean Teflon piece. A chemical analysis of the gold nanotriangles after postsynthesis exposure to aqueous solutions of KF, KCl, KBr, and KI was carried out by X-ray photoelectron spectroscopy (XPS). The gold nanoparticle samples were subjected to 4 cycles of centrifugation and redispersion in deionized water to remove uncoordinated halide ions. Nanoparticulate films for XPS were cast from solution onto Si (111) wafers and subjected to analysis on a VG MicroTech ESCA 3000 instrument at a pressure better than 1 × 10-9 Torr. The different core levels were recorded with un-monochromatized Mg KR radiation (photon energy, 1253.6 eV) at pass energy of 50 eV and electron takeoff angle (angle between electron emission direction and surface plane) of 60°. The overall resolution of measurement in XPS is thus ∼1 eV. The core level binding energies (BEs) were aligned with reference to the C 1s BE of 285 eV. The spectra have been background corrected using the Shirley algorithm19 prior to curve resolution. For all the XPS measurements, the gold nanoparticle solution after halide ion treatment was subjected to 4 cycles of centrifugation and redispersion in deionized water to free the nanoparticles from any unbound ions/ molecules present in the initial solution.
Results and Discussion Figure 1A shows the UV-vis-NIR spectra of gold nanoparticles produced by the reduction of aqueous gold ions by lemongrass leaf extract both in the absence (curve 1, nanoparticle solution without purification) and presence of 10-3 M KF, KCl, KBr, and KI (curves 2-5, respectively); these spectra were recorded after completion of the reaction (typically 12 h). The UV-vis-NIR spectrum from the gold nanoparticle solution synthesized in the absence of additional halide ions (curve 1) clearly shows two absorption bands; one at ca. 538 nm, which is attributed to excitation of the transverse component of the SPR (out of plane vibration component), whereas the second, higher wavelength component appears at ca. 1035 nm in the NIR region of the electromagnetic spectrum and is assigned to the longitudinal component of the SPR (in plane vibration component). The presence of two well-separated absorption bands is a characteristic feature of anisotropic nanoparticles and occurs in structures such as nanorods and nanowires as well.1 That the nanoparticles are indeed anisotropic in shape is seen by the TEM image recorded from the particles in this reaction (Figure 1B); (19) Shirley, D. A. Phys. ReV. B 1972, 5, 4709.
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Figure 1. (A) UV-vis-NIR spectra of gold nanoparticles produced in the absence (curve 1) and presence of 10-3 M aqueous KF, KCl, KBr, and KI (curves 2-5, respectively) by the reduction of aqueous gold ions by lemongrass extract. (B-F) Representative TEM images of gold nanoparticles obtained by the reduction of aqueous gold ions by lemongrass extract in the absence (B) and presence of KF (C), KCl (D), KBr (E), and KI (F). The inset in C shows the histogram of population of gold nanotriangles in the presence of different halide ions. The insets in E and F are higher magnification images of the gold nanoparticles in the main image.
a large number of flat triangular gold nanoparticles are seen together with spherical particles, as described in our earlier publications.12,13 The reaction conditions for synthesis of gold nanotriangles were controlled in ordered to obtain nanoparticles with longitudinal SPR ∼ 1000 nm so that any variation in the peak position could be conveniently monitored. It is interesting to see that the optical properties of the gold nanoparticles synthesized using lemongrass extract in the presence of halide ions (curves 2-5) are rather different from that observed in the previous case (curve 1). The longitudinal component of SPR band of gold nanoparticles synthesized in the presence of 10-3 M KF appears to blue shift by 25 nm (curve 2, Figure 1A) relative to that of the nanotriangles synthesized in the absence of halide ions (curve 1, Figure 1A). Along with this small blue shift, it is observed that the net absorption of both the SPR bands has decreased. A similar synthesis of gold nanoparticles in the presence of KCl led to a red shift of the longitudinal SPR by 80 nm (curve 3, Figure 1A), whereas a more pronounced change was observed for the UV-vis-NIR spectrum of gold nanoparticles synthesized in the presence of KBr (curve 4, Figure 1A). In this latter experiment, the position of the longitudinal SPR band appears to red shift beyond 1400 nm and is accompanied by a large increase in the transverse component of the SPR band. Drastic changes were also observed in the spectrum of gold nanoparticles synthesized in the presence of KI wherein a single absorption band at ca. 554 nm is observed with little evidence of a peak in the NIR region (curve 5, Figure 1A). In a recent report, we had experimentally demonstrated the dependence of
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the position of the longitudinal SPR band on the edge length of the gold nanotriangles/hexagons synthesized using lemongrass extract.13 In light of this earlier study, the observed changes in the optical absorption spectra of gold nanoparticles synthesized in the presence and absence of halide ions can be attributed to a variation in the size and shape of the gold nanoparticles formed. As briefly discussed above, the gold nanoparticles synthesized in the absence of halide ions (Figure 1B) are a mixture of triangular and spherical morphologies. Figure 1C-F show representative TEM images recorded from gold nanoparticles synthesized using lemongrass extract in the presence of KF, KCl, KBr, and KI, respectively. As suggested by the UV-vis-NIR studies, the gold nanoparticles synthesized in the presence of KF and KCl (Figure 1C,D) resemble those synthesized in the absence of these additives (Figure 1B) and show a large percentage of well faceted gold nanotriangles/truncated nanotriangles. The gold nanoparticles synthesized by the lemongrass extract possibly have sugar derivative molecules on their surface13 along with chloride ions contributed from the chloroaurate ions during the reduction process. F- ions are not known to not strongly interact with the surface of gold20 and, thus, would explain why this halide does not alter the shape and size of the gold nanotriangles to any noticeable extent. On the other hand, Cl- ions have the ability to chemisorb on the surface of gold20,21 and possibly compete with the biomolecules of lemongrass leaf extract in binding to the gold nanoparticle surface. Cl- ions are known to form a hexagonal closed packed adlayer on the Au (111) surface.21 The formation of flat [111] oriented gold nanotriangles/truncated triangles is possibly a result of the cooperative effect of the Clions and the biomolecules of lemongrass leaf extract. The observed changes in the UV-vis-NIR spectra could thus be a result of the variation in the relative concentration of the competing anions and the surface-active biomolecules, which causes the changes in size and shape of gold nanoparticles. An increase in Cl- ion concentration therefore leads to a red shift of the longitudinal SPR band indicating a size distribution of gold nanoparticles with larger size nanotriangles/truncated nanotriangles. Br- ions also are known to chemisorb onto the surface of gold, indeed more so than Cl- ions.20 Br- ions too form a hexagonal packed adlayer on the Au (111) surface, although with a greater mismatch with the underlying Au (111) lattice22 relative to the Cl- ion adlayer. Together with the UV-vis-NIR spectra (curve 4, Figure 1A) and the corresponding TEM images (Figure 1E) of gold nanoparticles synthesized using lemongrass leaf extract in the presence of Br- ions, it is observed that the percentage of the spherical particles has increased (increased transverse SPR band in curve 4) along with a marginal increase in the size of the flat gold nanostructures. The inset of Figure 1E shows a higher magnification image of gold nanotriangles synthesized in the presence of Br- ions; it is observed that there is little change in the overall morphology and it is still predominantly triangular. The most drastic changes in the UV-vis-NIR spectra were observed in the case where the gold nanoparticles were grown in the presence of I- ions (curve 5, Figure 1A). This change is mirrored in the TEM images of the nanoparticles grown in the I- experiment (Figure 1F) which clearly shows the presence of large aggregates of spherical gold nanoparticles with a complete absence of triangular nanoparticles. Thus, it is clear that KI strongly suppresses the growth of gold nanotriangles. The inset of Figure 1F shows a high magnification image of aggregated (20) Gao, P.; Weaver, M. J. J. Phys. Chem. 1986, 90, 4057. (21) Magnussen, O. M.; Ocko, B. M.; Adzic, R. R.; Wang, J. X. Phys. ReV. B 1995, 51, 5510. (22) Magnussen, O. M.; Ocko, B. M.; Wang. J. X.; Adzic, R. R. J. Phys. Chem. 1996, 100, 5500.
Biologically Synthesized Gold Nanotriangles
spherical gold nanoparticles, which is consistent with the corresponding UV-vis-NIR spectrum (curve 5, Figure 1A), where only a single intense absorption band centered at ca. 554 nm is observed. Cheng et al have also observed similar aggregated gold structures in their study on the effect of KI on the morphology of citrate reduced gold nanoparticles.23 The complete morphological change in gold nanoparticles in the presence of I- is due to the largest mismatch of the I- adlayer with the Au (111) lattice plane24 relative to the other halide ions. This leads to significant interfacial strain that clearly does not favor formation of gold nanoparticles with extended (111) faces as required in the triangular morphology. The inset in Figure 1C shows the population of gold nanotriangles when chloroaurate ions were reduced using lemongrass extract in the presence of different halide ions. The population of gold nanotriangles is drastically decreased from KF to KI due to the difference in the chemisorption ability and mismatch behavior of different halide ions with gold nanoparticles. Thus, the number of gold nanotriangles is decreased from 45% in the control to 4% in the presence of I- ions, while with F-, Cl-, and Br- ions, intermediate gold nanotriangle populations are observed. This result is consistent with the UVvis-NIR data and TEM results for the corresponding halide ions (Figure 1). It is thus clear that halide ions, in particular I- ions, exert a major influence over the evolution of the shape in gold nanocrystals. Iodide ions have a strong affinity to chemisorb on gold surfaces and thus have the ability to induce surface reconstruction on the adsorbed faces.25 It would thus be instructive to ask a question as to whether these halide ions could induce morphological changes upon addition to preformed gold nanotriangles and whether surface effects associated with adlayer formation would propagate through to the bulk. Figure 2A shows the UV-vis-NIR spectra of purified gold nanotriangles treated with different halide ions after complete reaction with gold nanoparticles (typically 24 h). Curve 1 in Figure 2A corresponds to a spectrum recorded from a purified biologically produced gold nanotriangles, which were then used for reaction with the different halide ions. This curve shows a transverse SPR band at ca. 540 nm and a longitudinal SPR band at ca. 1290 nm. The decrease in the transverse SPR band in comparison with the corresponding band in the spectrum recorded from the as-prepared (and therefore, not purified) gold nanotriangles (curve 1, Figure 1A) clearly indicates removal of spherical nanoparticles from the purified nanotriangle solution. Curves 2-4 in Figure 2A correspond to the UV-vis-NIR spectra recorded from gold nanotriangles posttreated with 10-3 M KF, KCl, and KBr, respectively; these spectra show no discernible shift in the position of the longitudinal SPR band. The addition of 10-3 M KI, however, appears to induce a detectable change in the UV-vis-NIR spectrum of the nanoparticles (curve 5) and the longitudinal SPR is observed to blue shift (curve 5, Figure 2A). TEM images (Figure 2B) of the purified lemongrass extract reduced gold nanotriangles/truncated triangles are seen to have well faceted structures with smooth edges. These flat structures do not seem to undergo any noticeable change after treatment with KF and KCl solutions (Figure 2, panels C and D, respectively) in agreement with the UV-vis-NIR spectroscopy inference. Though no significant change was observed in the UV-visNIR spectrum on treatment with KBr solution, minor variations are noticeable in the TEM measurements (Figure 2E). It is observed that the edges of most of the nanotriangles now appear corrugated with their overall triangular/truncated triangular (23) Cheng, W.; Dong, S.; Wang, E. Angew. Chem., Int. Ed. 2003, 42, 449. (24) Ocko, B. M.; Watson, G. M.; Wang, J. J. Phys. Chem. 1994, 98, 897. (25) Gao, X.; Weaver, M. J. J. Phys. Chem. 1993, 97, 8685.
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Figure 2. (A) UV-vis-NIR spectra of purified gold nanotriangles (curve 1) after postsynthesis treatment with 10-3 M aqueous KF, KCl, KBr, and KI solutions (curves 2-5, respectively). Representing TEM images recorded from purified gold nanotriangles before (B) and after treatment with 10-3 M aqueous KF (C), KCl (D), KBr (E), and KI solutions (F). The insets in E and F are higher magnification images of the gold nanoparticles in the main image.
structure still intact in case of a majority of particles. The inset of Figure 2E shows a higher magnification image of the irregular, wavy edges of the gold nanoparticles after treatment with KBr in much greater detail. Treatment of gold nanotriangles/truncated triangles with KI is observed to have a much greater influence on the morphology of the particles and a majority of the particles are now transformed into circular and disk shaped structures (Figure 2F). This shape transformation is seen in greater detail in the higher magnification image of some of the particles shown in the inset of Figure 2F. The explanation for the shape changes in the biologically prepared gold nanotriangles observed in the case of posttreatment with Br- and I- ions follows the lines put forward in the earlier studies (Figure 1, growth of the triangles in the presence of the halide ions). Although Cl- ions do possess an affinity for Au (111) surfaces, the crystallography of this adlayer is commensurate with the underlying packing of the gold atoms.20,21 This does not induce interfacial strain, and hence, little morphology change is observed. F- ions do not bind efficiently to Au (111) surfaces;20 on the other hand, Br- and I- ions bind strongly to the Au (111) surface and induce interfacial strains (this being larger for Iions than Br- ions).22-24 It is likely that this strain is relieved via transformation of the perfect, triangular structure into a structure that possesses corrugated edges and thus has more defects. This is at best a tentative explanation and further experiments are required to clarify this point. The presence/absence of the different halide ions on the surface of the biologically synthesized gold nanotriangles after postsynthesis exposure to aqueous solutions of KF, KCl, KBr, and KI after thorough washing was investigated by XPS measurements. In the case of gold nanotriangles exposed to KF, the F1s core
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Figure 4. Representative TEM images of gold nanotriangles synthesized using lemongrass plant extract at (A) 40, (B) 50, (C) 60, (D) 70, and (E) 80 °C. (F) Histogram of percentage of gold nanotriangles (light hatches) and spherical nanoparticles (dark hatches) produced by lemongrass reduction of gold ions at various temperatures.
Figure 3. (A) UV-vis-NIR spectra of biologically prepared gold nanoparticles at different reaction temperatures; curves 1-6 correspond to gold solutions prepared at RT, 40, 50, 60, 70, and 80 °C, respectively. (B) Average size of gold nanotriangles (edge-lengths) biologically synthesized at different temperatures; these edge-lengths have been determined from the TEM images shown in Figure 4.
level signal was below the detection limits of the instrument used (Supporting Information, Figure S1A). This clearly indicates that fluoride ions are not bound to the surface of the nanoparticles and, thus, do not play a role in the morphological alternation of gold nanotriangles. Clearly F- ions are unable to displace surface bound Cl- ions on the triangles that arise as a byproduct of the reduction of AuCl4- ions by lemongrass extract. In the case of nanotriangles exposed to KCl, a detectable Cl 2p core level signal was observed (Supporting Information, Figure S1B) as would be expected. In the case of gold nanoparticles reacted with aqueous KBr and KI, strong Br 3d and I 3d signals are observed (Supporting Information, Figure S1, panels C and D, respectively) clearly revealing their role as strong surface-active agents. Bromide and iodide ions may thus be implicated in the gold nanoparticle morphology variation and concomitant changes in the optical properties observed earlier (Figure 2). The Au 4f core level signal in lemongrass extract reduced gold nanoparticles before and after exposure to the different halide ions showed the presence of two chemically distinct spin-orbit pairs centered at 84 and 85.7 eV binding energy that are attributed to Au0 and Au+ species, respectively, from the core metal and surface bound ionic species (data not shown).13 Besides halide ions and order of addition of reagents in the synthesis procedure, the reaction temperature was found to be an important parameter in modulating the morphology of the gold nanoparticles formed by the reduction of AuCl4- ions by lemongrass extract. Figure 3A shows the UV-vis-NIR spectra recorded after complete reduction of gold ions using lemongrass leaf extract at different temperatures; curves 1-6 correspond to spectra of nanoparticles prepared at room temperature (RT), 40, 50, 60, 70, and 80 °C, respectively. A comparison of the UVvis-NIR spectra indicates that, as the temperature increases, the in-plane (longitudinal) SPR band shifts monotonically to lower wavelengths from 1080 nm at RT (curve 1) to ca. 800 nm at 80 °C (curve 6). Concomitantly, there is an increase in intensity of the out-of-plane (transverse) SPR band. From data presented above and in earlier work from this group,13 the shift to lower
wavelengths of the in-plane SPR band is symptomatic of a reduction in the edge-length of the triangles or, in other words, to a decrease in the particle aspect ratio. Furthermore, the increase in the out-of-plane SPR band indicates the presence of an increasing percentage of spherical particles relative to triangles as the reaction temperature is increased. That this is indeed the case is seen in the TEM images recorded from the nanoparticles synthesized at different temperatures (Figure 4A-E, representative images recorded from nanoparticles synthesized at 40, 50, 60, 70, and 80 °C, respectively). An analysis of the edgelengths of the gold nanotriangles in the overall population shows that the average triangle size decreases with increasing temperature (Figure 3B) as inferred from the UV-vis-NIR spectra (Figure 3A). The percentage contribution of gold triangles and spheres was also estimated from the images shown in Figure 4A-E and other similar images and the data obtained is shown in the histogram plot in Figure 4F. It can be seen from this plot that with increasing temperature, the percentage of triangles goes down and is, as expected, accompanied by an increase in the number of spherical gold particles, again in agreement with the UV-vis-NIR data of Figure 3A. Increasing the reaction temperature during reduction of HAuCl4 solution using lemongrass extract results mainly in an increase in the rate of reduction of gold ions; this in turn leads to an enhanced nucleation rate and a higher population of spherical nanoparticles. Furthermore, due to the higher rate of reduction, most of the chloroaurate ions are consumed in the formation of nuclei, and thus, the secondary reduction process on the surface of the preformed nuclei is stalled. Consequently, we obtain a much higher population of spherical nanoparticles in comparison with triangular ones when the temperature of the reaction system is high (Figure 4F). The formation of the gold nanotriangles is a kinetically driven process and is a result of aggregation and rearrangement of smaller size particles, which act as a nuclei for further growth into anisotropic, triangular structures.26 We believe that the low rate of reduction of metal ions at lower temperatures might possibly facilitate the oriented growth of nuclei and thus should promote the formation of anisotropic nanoparticles.27 Thus, simple variation in the temperature of the reaction medium during reduction of gold ions by lemongrass plant extract enables tailoring of the size of the triangular nanoparticles and, thus, their optical properties. (26) (a) Chiang, Y.-S.; Turkevich, J. J. Colloid Sci. 1963, 18, 772. (b) Engelbrecht, J.; Synman, H. Gold Bull. 1983, 16, 66. (27) Shankar, S. S.; Bhargava, S.; Sastry, M. J. Nanosci. Nanotechnol. 2005, 5, 1721.
Biologically Synthesized Gold Nanotriangles
In summary, it has been shown that the presence of halide ions either during the growth of gold nanoparticles by the reduction of aqueous AuCl4- ions using lemongrass leaf extract or after the synthesis of the particles significantly affects the morphology of the particles formed. Although F- ions do not bind to the gold particle surface, Cl-, Br-, and I- ions bind strongly to the (111) surface of gold and, by virtue of the mismatch between the halide adlayer and the (111) surface, alter the morphology of the particles formed; Cl- ions promote the formation of nanotriangles, whereas Br- and I- ions lead to distortion of the triangular morphology in that order. The solution reaction temperature of gold nanoparticles synthesized using lemongrass extract (without halide ions) also plays a crucial role in determining the particle size. With increasing temperature, the gold nanotriangles are smaller and also of reduced percentage relative to spherical particles. The ability to tailor the size/morphology of gold
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nanoparticles and thus their optical properties by simple variation in experimental conditions by a fully green chemistry approach could be important in applications such as hyperthermia and architectural optical coatings. Acknowledgment. A.R. and A.S. thank the Council of Scientific and Industrial Research (CSIR), Govt. of India, for financial assistance. This work was supported by a network grant on Custom Tailored Special Materials to M.S. from the CSIR and is gratefully acknowledged. Supporting Information Available: X-ray photoelectron core level spectra recorded from biologically prepared gold nanoparticles reacted with different halides (S1, one page). This material is available free of charge via the Internet at http://pubs.acs.org. LA052055Q
