Synthesis of Palladium Nanoparticles by Reaction of Filamentous

(PdCl2°) has been investigated at 25-100 °C for up to 28 days. We report that the release of organic materials from the cyanobacteria promoted the p...
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Synthesis of Palladium Nanoparticles by Reaction of Filamentous Cyanobacterial Biomass with a Palladium(II) Chloride Complex Maggy F. Lengke,* Michael E. Fleet, and Gordon Southam Department of Earth Sciences, UniVersity of Western Ontario, London, Ontario, N6A 5B7, Canada ReceiVed April 28, 2007. In Final Form: June 8, 2007 The interaction of cyanobacterial biomass (Plectonema boryanum UTEX 485) with aqueous palladium(II) chloride (PdCl2°) has been investigated at 25-100 °C for up to 28 days. We report that the release of organic materials from the cyanobacteria promoted the precipitation of Pd(0) as crystalline spherical and elongate nanoparticles (e30 nm), both in solution and as dispersed and encrusted nanoparticles on cyanobacterial cells. In contrast, under abiotic conditions at 100 °C, palladium hydride (PdHx) was the principal palladium phase precipitated, with only minor amounts of palladium metal.

Introduction The unique chemical, optical, electronic, and magnetic properties of metal nanoparticles has led to a growing interest in their synthesis. Metal nanoparticles have a high surface-tovolume ratio due to their small size and high proportion of edges and corners, and therefore are useful in applications such as catalysis,1,2 biosensing,3,4 biological labeling,5 electronics,6 optical devices,6 and controlled drug delivery.7 In particular, the widespread use of nanoparticles of platinum group metals in industrial and automotive catalysts is increasing, because they have high corrosion resistance and are stable to oxidation at high temperatures.8 The automobile catalysts in North America employ a combination of platinum, palladium, and rhodium to oxidize carbon monoxide and hydrocarbons to carbon dioxide and water, and to reduce nitrogen oxides to nitrogen gas. Currently, there are no acceptable substitutes for the platinum group metals in the automobile catalyst application. In addition, palladium nanoparticles have also been known as an effective catalyst for in situ remediation of trichloroethene (TCE) and carbon tetrachloride (CT) from contaminated groundwater.9,10 As a result, there is a growing interest in the recovery of the platinum group metals from industrial wastes, but the recycling technology lags behind demand.11 The synthesis of palladium nanoparticles of different sizes, ranging from 1 to tens of nanometers, and shapes, including spherical, icosahedral, dodecadahedral, and octahedral, has been conducted using chemical reduction,12,13 electrochemical deposi* E-mail: [email protected] or [email protected]. Present address: Geomega Inc., 2995 Baseline Road, Suite 202, Boulder, Colorado 80303, USA. (1) Lewis, L. N. Chem. ReV. 1993, 93, 2693. (2) Moreno-Manas, M.; Pleixats, R. Acc. Chem. Res. 2003, 36, 638. (3) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature (London) 1996, 382, 607. (4) Han, M.; Gao, X.; Su, J. Z.; Nie, S. Nat. Biotechnol. 2001, 19, 631. (5) Nicewarner-Pena, S. R.; Freeman, R. G.; Reiss, B. D.; He, L.; Pena, D. J.; Walton, I. D.; Cromer, R.; Keating, C. D.; Natan, M. J. Science 2001, 294, 137. (6) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (7) Rao, C. N. R.; Cheetham, A. K. J. Mater. Chem. 2001, 11, 2887. (8) Lloyd, S. M.; Lave, L. B.; Matthews, H. S. EnViron. Sci. Technol. 2005, 39, 1384. (9) Li, T.; Farrell, J. EnViron. Sci. Technol. 2000, 34, 173. (10) Nutt, M. O.; Hughes, J. B.; Wong, M. S. EnViron. Sci. Technol. 2005, 39, 1346. (11) Yong, P.; Rowson, N. A.; Farr, J. P. G.; Harris, I. R.; Macaskie, L. E. J. Chem. Technol. Biotechnol. 2002, 77, 593. (12) Schmid, G. Chem. ReV. 1992, 92, 1709.

tion,14,15 and sonochemical reduction16-18 in aqueous solutions. The usual synthetic route to prepare palladium nanoparticles involves the reduction of a palladium salt (usually a halide) in solution by citrate, hydrogen, or sodium borohydride.12,19 To maintain the formation of stable palladium nanoparticles, stabilizers such as polymers, polyelectrolytes, and stabilizing ligands have been used.20,21 Other methods, including photochemical, radiolytic reduction, and metal evaporation methods, have also been applied to produce palladium nanoparticles.22-24 Biological processes have recently been considered as possible methods for the synthesis of nanoparticles, especially the development of “green” synthetic approaches for nanoparticles. The biosynthesis of palladium nanoparticles of different sizes, ranging from 0.6 to tens of nanometers has been conducted using bacteria in the pH range 2-7.11,25-29 The formation of extracellular and intracellular palladium nanoparticles by bacteria (DesulfoVibrio desulfuricans, DesulfoVibrio fructosiVorans, DesulfoVibrio Vulgaris, Shewanella oneidensis, and Bacillus sphaericus JG-A12) has been demonstrated by reacting the cells with palladium(II) chloride (Na2PdCl4, Pd(NH3)4Cl2).11,25-30 The suggested mechanisms for bioreduction of palladium by Des(13) Bonnemann, H.; Brijoux, W.; Brinkmann, R.; Fretzen, R.; Joussen, Th.; Koeppler, R.; Korall, B.; Neiteler, P.; Richter, J. J. Mol. Catal. 1994, 86, 129. (14) Reetz, M. T.; Quaiser, S. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 2240. (15) Li, F.; Zhang, B.; Dong, S.; Wang, E. Electrochim. Acta 1997, 42, 2563. (16) Mizukoshi, Y.; Okitsu, K.; Maeda, Y.; Yamamoto, T. A.; Oshima, R.; Nagata, Y. J. Phys. Chem. B 1997, 101, 7033. (17) Dhas, N. A.; Gedanken, A. J. Mater. Chem. 1998, 8, 445. (18) Nemamcha, A.; Rehspringer, J.-L.; Khatmi, D. J. Phys. Chem. B 2006, 110, 383. (19) Schmid, G.; Harms, M.; Malm, J.-O.; Bovin, J.-O.; van Ruitenbeck, J.; Zandbergen, H. W.; Fu, W. T. J. Am. Chem. Soc. 1993, 115, 2046. (20) Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116, 7401. (21) Chen, S.; Huang, K.; Stearns, J. A. Chem. Mater. 2000, 12, 540. (22) Cardenas-Trivino, G.; Klabunde, K. J.; Dale, E. B. Langmuir 1987, 3, 986. (23) Yonezawa, Y.; Sato, T.; Kuroda, S.; Kuge, K. J. Chem. Soc., Faraday Trans. 1991, 87, 1905. (24) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1992, 96, 10419. (25) Lloyd, J. R.; Yong, P.; Macaskie, L. E. Appl. EnViron. Microbiol. 1998, 64, 4607. (26) de Vargas, I.; Macaskie, L. E.; Guibal, E. J. Chem. Technol. Biotechnol. 2004, 79, 49. (27) de Windt, W.; Aelterman, P.; Verstraete, W. EnViron. Microbiol. 2005, 7, 314. (28) de Windt, W.; Boon, N.; van den Bulcke, J.; Rubberecht, L.; Prata, F.; Mast, J.; Hennebel, T.; Verstraete, W. Antonie Van Leeuwenhoek 2006, 90, 377. (29) Pollmann, K.; Merroun, M.; Raff, J.; Hennig, C.; Selenska-Pobell, S. Lett. Appl. Microbiol. 2006, 43, 39. (30) Baxter-Plant, V. S.; Mikheenko, I. P.; Macaskie, L. E. Biodegradation 2003, 14, 83.

10.1021/la7012446 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007

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ulfoVibrio desulfuricans were mediated by the metabolic activity, possibly via formate hydrogenlyase reactions.11 In this study, the formation of palladium nanoparticles was investigated by reacting palladium(II) chloride with the filamentous cyanobacterium Plectonema boryanum (strain UTEX 485). This organism was chosen in this study because cyanobacteria, known as blue-green bacteria, form one of the largest and most important groups of bacteria on earth and are commonly found in natural environments. Various species of cyanobacteria and algae have been known to adsorp and take up heavy metal ions.31-33 The carboxyl groups on dead algae (algal biomass) bind to various metal ions,34 while intracellular polyphosphates as well as extracellular polysaccharide appear to participate in metal sequestration by live algae.35,36 Although previous studies on metal removal by cyanobacteria and algae have been conducted for bioremediation and biorecovery purposes, this approach could also be extended for the development and application of nanoparticles. Therefore, the goals of this study were to investigate the formation of palladium nanoparticles by reaction of aqueous PdCl2° with cyanobacterial biomass and to understand the effect of complex organic/inorganic reactions on the shape and size of palladium nanoparticles. Experimental Section Cyanobacteria Cultures. The filamentous cyanobacterium, Plectonema boryanum strain UTEX 485 (obtained from the culture collection at the University of Texas at Austin, Texas), was grown in batch cultures in BG-11 medium and buffered with 10 mM HEPES at a control temperature of 29 °C under ambient CO2 conditions. Further details on culture preparation are given in Lengke et al.37 Cyanobacterial and Abiotic Experiments. To initiate the experiments, 5 mL of aqueous PdCl2° solution (∼550 mg/L; prepared from PdCl2; Alfa Aesar Company, Ward Hill, Massachusetts) was added to 5 mL of cyanobacteria culture (∼8 mg dry weight). The experiments were conducted at 25, 60, and 100 °C for up to 28 days after the addition of palladium solution. The range of temperatures from 25 to 100 °C was chosen to investigate the changes in morphology of palladium nanoparticles with temperature. The experiments were maintained in the dark due to the instability of palladium solution exposed to light. For experiments at 25, 60, and 100 °C, reactants were contained in sealed borosilicate vials in temperature-regulated laboratory ovens (Blue M Electric Company and Fisher Isotemp Oven, respectively). pH, Eh, total palladium, and cyanobacteria population were measured with time. All experiments were conducted in duplicate. Abiotic control experiments consisting of ∼550 mg/L palladium were conducted using PdCl2° solution without the presence of cyanobacteria. Bacterial Viability and Total Bacterial Counts. The effects of PdCl2° on the cyanobacteria were monitored during the course of the cyanobacterial experiments by determining bacterial viability using the LIVE/DEAD BacLight bacterial viability kit (L-7007, Molecular Probes, Inc., Eugene, Oregon). When viewed by a fluorescence microscope, viable bacteria stained with the LIVE/ DEAD BacLight reagents appear green, whereas dead bacteria appear red. The total number of bacteria in the cultures was determined by the direct counting method using a Petroff-Hauser counting chamber and a phase contrast light microscope (Nikon Labophot microscope). (31) Gadd, G. M. Experientia 1990, 46, 834. (32) Kuyucak, N.; Volesky, B. Biosorption of HeaVy Metals; Volesky, B., Ed.; CRC Press: Boca Raton, Florida, 1990; pp 173-198. (33) Bender, J.; Gould, J. P.; Vatcharapijarn, Y.; Young, J. S.; Phillips, P. Water EnViron. Res. 1994, 66, 679. (34) Gardea-Torresdey, J. L.; Becker-Hapak, M. K.; Hosea, J. M.; Darnall, D. W. EnViron. Sci. Technol. 1990, 24, 1372. (35) Kaplan, D.; Christiaen, D.; Arad, S. M. Appl. EnViron. Microbiol. 1987, 53, 2953. (36) Zhang, W.; Majidi, V. EnViron. Sci. Technol. 1994, 28, 1577. (37) Lengke, M. F.; Fleet, M. E.; Southam, G. Langmuir 2006, 22, 7318.

Figure 1. Variation of total soluble Pd, pH, and Eh with time for cyanobacterial and abiotic experiments with PdCl2° solution at 25 to 100 °C; C and A represent cyanobacterial and abiotic experiments, respectively. Chemical Analyses. The pH and Eh were monitored using a Denver Instrument Basic pH/ORP/temperature meter. The pH electrode was calibrated using buffer solutions 4, 7, and 10 with analytical uncertainties in measurements of pH of (0.05 pH unit. The Eh was measured using an ORP electrode and calibrated using ZoBell’s solution.38 Total palladium concentrations were measured over the course of the experiments with a Perkin-Elmer 3300-DV inductively coupled plasma optical emission spectrometer (ICPOES). The uncertainty in measured palladium was e5%, with detection limits of 0.1 mg/L. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). Unstained whole sample mounts of cyanobacteria and palladium nanoparticles from the experiments were examined with a Phillips CM-10 transmission electron microscope (TEM) operated at 80 kV. The whole mounts were prepared by floating Formvar carbon-coated 200 mesh copper grids on a drop of sample for several minutes to allow the bacteria and any fine-grained particles to attach to the grid. The mounts were then washed with distilled, deionized water and allowed to air-dry. Selected area electron diffraction (SAED) patterns of the precipitated solids were obtained by TEM from unstained whole mounts to ascertain the crystallinity of solids. The precipitated particles of palladium and reacted cyanobacteria cells were also examined using a LEO 1540 XB focus ion beam scanning electron microscope equipped with an energy dispersive spectrometer (FIB-SEMEDS). All samples for FIB-SEM-EDS were carbon-coated before analysis. X-ray Photoelectron Spectroscopy (XPS). Reaction products were analyzed by XPS to investigate the oxidation state of palladium and sulfur. All samples were dried in the vacuum chamber of the XPS. A modified Surface Science Laboratories SSX-100 spectrometer, with a monochromatized Al KR X-ray source and an analytical chamber with a base pressure of 10-9 Torr, was used to collect XPS spectra. The spectrometer work function was adjusted (38) Nordstrom, D. K. Geochim. Cosmochim. Acta 1977, 41, 1835.

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Figure 2. TEM micrographs of cyanobacterial cells incubated in the presence of PdCl2° at 60 °C and 28 days: (A) palladium nanoparticles precipitated on the cyanobacterial surface; (B) spherical nanoparticles of palladium precipitated in solution. Scale bars in A and B are 200 and 20 nm, respectively. to 335.1 ( 0.05 eV [with peak full-width half-maximum (fwhm) of 1.68 ( 0.05 eV] for the Pd 3d5/2 peak. The energy dispersion was set to give an energy difference of 857.5 ( 0.1 eV between the Cu 2p3/2 and Cu 3p1/2 lines of metallic Cu. Internal referencing of spectra was made to the C 1s peak of graphite-like C at 285.00 eV to compensate for charging effects in the samples. Survey scans were recorded using a 600 µm spot size and a fixed pass energy of 160 eV, whereas narrow scan spectra were recorded using a 300 µm X-ray spot size and a fixed pass energy of 50 eV. Integrated photoelectron intensities were corrected for the Scofield cross sections and the inelastic mean free path for kinetic energy. Spinorbit doublet components for S 2p3/2 and S 2p1/2 are separated by 1.18 eV with a peak/area ratio of 2:1, and for Pd 3d5/2 and Pd 3d3/2 by 5.26 eV with a peak/area ratio of ∼1.5:1.39,40 Reference binding energies for Pd 3d and S 2p species were taken from Table 1 (in Supporting Information)39-55 and Knipe and Fleet,56 respectively. X-ray Diffraction (XRD). XRD data were collected with Co KR radiation using a Rigaku Totaflex RU-200B equipped with rotatory anode. Data collection was made in the step scan mode over the 2θ range 2-82°, at 45 kV and 160 mA. (39) Kumar, G.; Blackburn, J. R.; Albridge, R. G.; Moddeman, W. E.; Jones, M. M. Inorg. Chem. 1972, 11, 296. (40) Vedrine, J. C.; Dufaux, M.; Naccache, C.; Imelik, B. Faraday Trans. 1 1978, 74, 101. (41) Bozon-Verduraz, F.; Omar, A.; Escard, J.; Pontvianne, B. J. Catal. 1978, 53, 126. (42) Wehner, P. S.; Tustin, G. C.; Gustafson, B. L. J. Catal. 1984, 88, 246. (43) Fleisch, T. H.; Hicks, R. F.; Bell, A. T. J. Catal. 1984, 87, 398. (44) Narayana, M.; Michalik, J.; Contarini, S.; Kevan, L. J. Phys. Chem. 1985, 89, 3895. (45) Briggs, D.; Seah, M. P. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, 2nd ed.; Wiley: Chichester, U.K., 1996. (46) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin Elmer Corporation: Norwalk, CT, 1992. (47) Lin, W.; Wiegand, B. C.; Nuzzo, R. G.; Girolami, G. S. J. Am. Chem. Soc. 1996, 118, 5977. (48) Hierso, J.-C.; satto, C.; Feurer, R.; Kalck, P. Chem. Mater. 1996, 8, 2481. (49) Dhas, N. A.; Cohen, H.; Gedanken, A. J. Phys. Chem. B 1997, 101, 6834. (50) Schildenberger, M.; Prins, R.; Bonetti, Y. C. J. Phys. Chem. B 2000, 104, 3250. (51) Guimaraes, A. L.; Dieguez, C.; Schmal, M. J. Phys. Chem. B 2003, 107, 4311. (52) Bera, D.; Kuiry, S. C.; Seal, S. J. Phys. Chem. B 2004, 108, 556. (53) Yuranov, I.; Kiwi-Minsker, L.; Buffat, P.; Renken, A. Chem. Mater. 2004, 16, 760. (54) Niklewski, A.; Strunskus, T.; Witte, G.; Woll, C. Chem. Mater. 2005, 17, 861. (55) Zhuo, G.; Lu, M.; Yang, Z.; Tian, F.; Zhou, Y.; Zhang, A. Cryst. Growth Des. 2007, 7, 187. (56) Knipe, S. W.; Fleet, M. E. Can. Mineral. 1997, 35, 1485.

Results Cyanobacterial and Abiotic Experiments. Changes in Pd concentration, pH, and Eh during the course of the experiments are shown in Figure 1. On addition of PdCl2° solution to the cyanobacterial culture, the pH values were stable at 1.9, while the Eh values decreased from 0.80 to 0.60 V. All cyanobacteria were killed within several hours at all temperatures investigated (25-100 °C), probably due to the low pH and high temperatures. The optimal temperature for cyanobacteria growth is 29 °C, and they would not survive the elevated temperatures presently investigated (60-100 °C). The soluble palladium concentrations significantly decreased within 7 days at 25-100 °C, and then slightly decreased by about ∼30-50 ppm after 7 days (Figure 1). A significant increase in the precipitation of palladium occurred over the interval from 25 to 100 °C. There was no significant variation between duplicate experiments. A brown solid was macroscopic evidence of palladium precipitation. In abiotic experiments using PdCl2° solution, total soluble palladium concentrations were relatively constant at 25 °C but decreased with time at 60 and 100 °C (Figure 1). The precipitation of palladium was observed as a brown solid at 100 °C. Eh values slightly decreased about 0.1 V at 25 to 100 °C. TEM and SEM. TEM and SEM observations on products of cyanobacteria-PdCl2° experiments at 25-100 °C are presented in Figures 2 and 3. At 25 °C, the addition of PdCl2° to the cyanobacteria caused the precipitation of spherical palladium nanoparticles at cell surfaces. Spherical and elongate palladium nanoparticles were also precipitated in solutions with diameter ranging from 1 to 20 nm. TEM-SAED of the particles in the 28 day experiment revealed powder ring diffraction patterns that were consistent with crystalline palladium metal, and SEM-EDS showed the occurrence of palladium with traces of sulfur, phosphorus, and chloride. Sulfur and phosphorus were from the cyanobacteria cells, whereas chloride was trace of the palladium solution used (PdCl2°). At 60 °C, palladium nanoparticles were deposited at cell surfaces and were associated with organic material derived from the cyanobacterial cells (Figure 2A). In solution, spherical palladium nanoparticles with size ranging from 1 to 20 nm were observed (Figure 2B). TEM-SAED of the nanoparticles revealed powder ring diffraction patterns consistent with nanocrystalline palladium. SEM-EDS showed the occurrence of palladium with traces of sulfur and chloride. At 100 °C, the cyanobacterial cells were encrusted with palladium nanoparticles,

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Figure 3. (A) A SEM micrograph of precipitated palladium encrusted on the filaments of cyanobacterial cells at 100 °C and 28 days; (B) TEM micrograph of spherical nanoparticles of palladium in solution; (C) TEM-SAED diffraction powder ring pattern consistent with very fine grained palladium; d-spacings of 0.224, 0.194, 0.137, and 0.117 nm correspond to reflections 111, 002, 022, and 113, respectively; (D) SEM-EDS spectrum for area A. Scale bars in A and B are 1 µm and 40 nm, respectively.

Figure 4. (A) SEM micrograph of nanoparticles of palladium hydride and palladium metal in the abiotic PdCl2° systems at 100 °C and 28 days; (B) SEM-EDS spectrum for area A. Scale bar in A is are 1 µm.

and separation of some filaments into their constituent cells was observed (Figure 3A). Spherical palladium nanoparticles with diameter ranging from 2 to 30 nm were also observed within the cells and in solution (Figure 3B). TEM-SAED of the nanoparticles revealed powder ring diffraction patterns (Figure 3C) consistent

with nanocrystalline palladium. SEM-EDS showed the occurrence of palladium with traces of sulfur (Figure 3D). The size of palladium nanoparticles was not studied systematically, because variation in size with temperature was much less than the variation within a single product.

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Figure 6. Powder X-ray diffraction scans of fine-scale reaction products from experiments on reduction of aqueous Pd(II) and Pt(IV) chlorides: (A) palladium, abiotic conditions, 100 °C, 28 day; (B) palladium, with Plectonema boryanum, 60 °C, 28 day; (C) platinum, abiotic conditions, 180 °C, 1 day. Note that, in the absence of cyanobacteria, Pd(II) chloride at pH 1.9 reduces to palladium hydride (PdHx, with x ) 0.32) and minor palladium metal.

Figure 5. Narrow-region Pd 3d XPS spectra from the reaction products of (A) cyanobacteria-PdCl2° experiments after 28 days and 25 °C, showing subordinate Pd(II) and dominant Pd(0); (B) cyanobacteria-PdCl2° experiments after 28 days and at 60 °C, showing Pd(0) and Pd(II); (C) cyanobacteria-PdCl2° experiments after 28 days and at 100 °C, showing dominant peaks for Pd(0) and subordinate Pd(II); (D) the abiotic PdCl2° experiments at 100 °C and 28 days, showing dominant Pd(0).

In abiotic experiments, PdCl2° was stable in aqueous solution at 25 °C for 28 days, whereas nanoparticles of a palladium phase were precipitated at 60 and 100 °C; however, the precipitation of palladium alloy at 100 °C was particularly obvious (Figure 4A), and spherical particles with size