aerobic synthesis of palladium nanoparticles

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Rev.Adv.Mater.Sci. 27(2011) 31-42 Aerobic synthesis of palladium nanoparticles

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AEROBIC SYNTHESIS OF PALLADIUM NANOPARTICLES PFP HG O1

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Received: June 27, 2010 Abstract. In this paper, we present the results of the synthesis of palladium nanoparticles (NPs) in different solvents with different reduction methods, in order to study the solvent both as a stabilizer and as a dispersant in the colloid, without any inert atmospheres, additional protective molecules or special treatments, to transform the nanoparticles into zero-valent NPs. In this particular case, dimethylsulfoxide (DMSO), dimethylformamide (DMF), ethylene glycol (EG), ethanol (EtOH), and water (H2O) are the solvents employed, all under aerobic conditions. The reduction methods include the solvent itself, photoreduction, chemical reduction with either sodium borohydride or sodium citrate, and (sonochemical) ultrasonic irradiation.

1. INTRODUCTION Palladium can be used in many applications. Apart from being one of the most versatile metals for promoting or catalyzing reactions [1], it is used in the area of palladium-nanoparticle catalysis, where it is considered one of the most promising solutions towards efficient reactions under mild, environmentally benign conditions in the context of Green Chemistry [2]. Other applications that have been tested in several fields are electronic and photonic devices, telecommunications, sensors [3-5] and the use of palladium as an auxiliary in organic reactions [6-8], among others. Consequently, the study of the different ways in which palladium nanoparticles can be obtained has become more and more interesting, with monodisperse sizes and well-defined morphologies [9-17], and with the use of numerous types of stabilizers as capping agents, some of which include block copolymers, [18-21] dendrimers [22-25], polymers [26-28], etc. Researchers have

found that the type of stabilizer that is used to cap the nanoparticles affects their stability [29]. Much effort has been devoted to producing Pd nanostructures, with catalytic studies involving either homogeneous or heterogeneous systems (NPs supported on oxides such as silica, alumina, or other metal oxides, and forms of carbon supports, including carbon nanotubes) [2]. There are many studies on Pd NP synthesis employing solvents and reducing agents. However, all these cases use auxiliary molecules to protect the NPs from growing and agglomerating, and taking precautions to work under inert atmospheres is quite common in this type of studies. For example, water (H2O), which is the most common of the solvents studied in the present work, has a long list of around 468 papers in the literature, where the protective molecules employed include functionalized Poly(ethylene glycol). There are more than 7 papers on dimethylsulfoxide (DMSO), 2 publications on dimethylformamide (DMF), 135 papers on ethanol

Corresponding author: Rocio Redon, e-mail: [email protected] y) ( (6Ug R TVUHe fUj8V e Vc8 % Ae U%

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(EtOH), 80 papers on ethylene glycol (EG), over 50 papers on sodium borohydride, and 9 papers on sodium citrate, the latter two being the most employed Pd(II) reducing agents in literature on palladium. Nevertheless, we use them, as mentioned before, in the presence of molecular oxygen. In general, a lot has been learned about these materials over the last few decades [30-59]. The synthesis of Pd nanoparticles is usually conducted under inert atmospheres such as argon or nitrogen, and with the use of molecules to protect the nanoparticle surface of oxide formation. Nevertheless, these procedures are expensive considering the addition of extra molecules and the complexity of the oxygen-free process. Thus, in this paper, we report the synthesis and characterization of palladium nanoparticles with no protective molecules other than the solvent, and in an aerobic atmosphere. These nanoparticles were prepared by reducing Pd(II) in 5 different solvents, which can work both as protective molecules and as Pd(II)-reducing agents, and in the presence of other reducing agents such as photoreduction, ultrasonic irradiation, and additional chemicals such as sodium citrate and sodium borohydride. Palladium NPs obtained in this way were analyzed for composition, shape and size, depending on the reducing conditions used. Although there have been many studies related to this matter, the syntheses of these metallic nanoparticles were carried out under inert atmospheres such as nitrogen or argon. In this particular study, we are not using any protective molecules or inert atmospheres.

2. EXPERIMENTAL 2.1. Materials The following were purchased from Sigma-Aldrich and used without further purification: sodium borohydride (99%), sodium citrate (99%) ethylene glycol (99.8%), anhydrous ethanol (99.5%), dimethylsulfoxide (99%), N,N-dimethylformamide (98%), acetonitrile (99%) and PdCl2 (99%). Tri-distilled water (99.9%) was bought from Hycel. Complex [PdCl2(CNCH3)2] was synthesized by the literature methods [60].

-BM JUJABT N KM F Kand V.M. Ugalde-Saldivar Pd(II)

solvent additional reduce agent

PdNPs

Scheme 1. General schematization for the synthesis of Pd(0) NPs.

by deposition of a drop of the colloidal dispersion onto 200 mesh Cu grids coated with a carbon/collodion layer. High-resolution transmission electron microscopy (HRTEM) micrographs were obtained on a JEOL 2000F microscope operating at 200 kV, using the same sample preparation as in TEM experiments. The particle size distribution was determined using a digitalized amplified micrograph. All solutions were prepared with ethylene glycol as solvent at 10-3 M concentration. Solutions were measured by using ethylene glycol as a blank at 24 to 48-hour intervals after preparation. A derivative method was used to perform a quantitative analysis of the species formed, in order to determine the absorbance values of each of the absorption maximums observed in the region of 300 to 500 nm. For this purpose, it was necessary to obtain the derivatives of the different spectra for further analysis. The X-ray diffraction pattern was measured using a Siemens D5000 diffractometer with CuK radiation ( = 1.,+ -n% 9BHDUZ d aVc d Z dhVc V VRd fc VU in solution on a CytoViva high-resolution condenser and images were obtained using the CytoViva Hyperspectral Imaging System.

2.3. Synthesis Preparation of colloidal Pd NPs. 200 mL of [PdCl2(CNCH3)2] 10-3 M solution was prepared in the corresponding solvent under aerobic conditions. A 2x10-4 M concentration was obtained after dilution (5 mL of 10-3 M in 25 mL of the corresponding solvent). Then, a reduction method (excess of NaBH4 0.1616 mmol- or Na3C6H5O7.2H2O -0.1700 mmol; photoreduction with 256 nm UV-lamp or 20 kHz ultrasonic irradiation) was applied to each solution, which was then stirred while the formation of Pd NPs was monitored by UV-vis absorption spectra, taking one spectrum every 5 minutes for 1 hour.

2.4. Electrochemical studies 2.2. Instruments UV-visible absorption spectra, in colloidal dispersion, were obtained using the Ocean Optics USB200 miniature fibreglass optical spectrometer. Transmission electron micrographs (TEM) were obtained on a JEOL 1200EXII microscope, operating at 60kV,

General procedure for electrochemical measurements. Electrochemical experiments were carried out using a typical three-electrode cell. The vitreous graphite electrode (area: 7.1 mm2) was used as the working electrode, a platinum wire as the counter electrode, and an Ag-AgBr/Bu4NBr0.1M in

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Table 1. Average sizes and their standard deviations of all Pd-NPs based on TEM or HRTEM analysis 6g Vc RXVUZ R Ve Vc | % (% Reduction method Photorreduction NaBH4(nm) ( = 275nm)(nm)

Solvent

Ultrasonic Irradiation (kHz)(nm)

Ethylene glycol H 2O Dimethylfor-mamide

2.4(29.3w) 3.1(87.53w) 2.7

2.6 2.0(23.12w) immeasurable

Dimethylsulf-oxide

2.1 (w1 )(/| % (,i )0% *|(% *0 sph1 (0*|( (% ) 6.2

)*% 00| % .*

(/%0| % +0

*+% 0,|(%0

3.2

2.5

2.5

Ethanol

Na3C6H5O7 (nm)

2.5(51.62w) 1.9(19.96w) 1.3 nm(31.29w) 1.9 nm 50/2 nm

Pure solvent(nm)

2.6 25/2.5 Starting material Starting material

Starting material

w

obtained from X-ray diffraction pattern. *obtained with CytoViva image. Particles size that form the 25 nm ones. w : wires sph: spheres

ethylene glycol as a pseudo reference. The voltammograms were obtained on a PAR263-A potentiostat-galvanostat. All voltammograms begin at an open circuit potential and are obtained in both anodic and cathodic sweeps. The employed supporting electrolyte is a 0.1M Bu4PF6 in either DMSO or DMF, and a saturated solution in ethylene glycol. In order to report the potentials used according to the IUPAC convention, voltammograms were obtained for approximately 10-3 M solution of ferrocene (Fc) in a supporting electrolyte. The half-wave potentials were estimated from E1/2 = (Eap + Ecp)/2, where Eap and Ecp are the anodic and cathodic peak potentials, respectively.

3. RESULTS AND DISCUSSION 3.1. Synthesis method Concentration is a key factor in the colloidal synthesis of nanoparticles, because if the initial palladium salt concentration is greater than 10-3 M, the particles begin to agglomerate and precipitate out of the dispersion, but if the concentration is 10-4 M or less, then the dispersion is homogeneous and stable for months. Depending on the different reduction methods and solvents employed in the synthesis of palladium nanoparticles (Scheme 1), shape and size varied as expected. Therefore, when ultra-

Fig. 1. TEM and HRTEM micrographs of palladium nanoparticles in different solvents with ultrasonic irradiation (US) as an additional reducing agent: (a) EG, (b) DMSO, (c) H2O, (d) DMF, and (e) EtOH.

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Fig. 2. TEM micrographs of palladium(0) nanoparticles in different reduction media: (a) H2O/ NaBH 4 , (b) EG/US, (c) EG/NaBH 4 , (d) EG/ Na3C6H5O7, and (e) DMF/citrate.

Fig. 4. TEM micrographs of palladium oxide nanoparticles in water with different additional reducing agents: (a) without any reducing agent, (b) with photoirradiation, and (c) with ultrasonic irradiation.

Fig. 3. HRTEM micrographs of palladium(0) nanoparticles in different reduction media; (a) EtOH/ US, (b) EtOH/citrate, (c) EtOH/UV, (d) EtOH/NaBH4, and (e) EG/ Na3C6H5O7.

sonic irradiation was used as an additional reduction method, the particles obtained were the largest (Table 1) and the most irregular, with plenty of unsaturated sites (Fig. 1) - an excellent characteristic for their use as catalysts-. On the other hand, chemical reduction, either with sodium borohydride or with sodium citrate, results in the smallest nanoparticles (Table 1), generally of spherical shape (Figs. 2 and 3). When water is used as a solvent and the final product is PdO nanoparticles, the particles tend to

form wires constituted of small clusters that are formed by smaller nanoparticles (Fig. 4). Finally, when the final product is Pd(0), the geometry is nearly spherical (Figs. 2 and 3). When water is used as a solvent, the particles obtained are of PdO except in the case of sodium borohydride, where Pd(0) was obtained. On the contrary, when ethanol or ethylene glycol are used, the obtained particles are of Pd(0) in all cases, including when no additional reducing agent but the solvent is used, which implies that the solvent is working as a reducing agent and as a stabilizer, even with the presence of molecular oxygen. The solvent reactions that can be achieved are shown in Scheme 2 [61-67]. With DMF and DMSO, Pd(II) is always obtained in the dispersion in addition to Pd(0), except in the case where the solvent is used without any further reduction methods, where the starting material is the only product observed. Based on these results, it can be said that the reduction method plays an important role in the size and shape of palladium nanoparticles, and that it can

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Aerobic synthesis of palladium nanoparticles H2 O

+

2H + O

CH3 CH2 OH

HOCH2 CH2 OH

HCON(CH3 )2

2-

[2] +

CH3 CHO+ 2H + 2 e

CH3 COOCCH3

[4]

-

[5]

(CH3 )2 SO 2 + (CH3 )2 S

[6]

+

NaBH4 + 2H2 O

[3]

H2 O

(CH3 )2 NCOOH+ 2H + 2 e 2(CH3 )2 SO

-

NaBO 2 + 4 H2

[7]

NaBH4 + 4 HOCH2 CH2 OH NaB(OCH2 CH2 OH)4 + 4 H2

[8]

NaOOCCH2 COH(COONa) CH2 COONa+H2 O NaOOCCH2 CH(OH) CH2 COONa+ NaHCO 3

[9]

Scheme 2. Possible oxidation reactions taking place with the different solvents. [2] Water. The O2- specie is the one which reacts with Pd(II) after water; [3] EtOH; [4] EG; [5] DMF; [6] DMSO; [7] NaBH4 in water; [8] NaBH4 in EG; [9] Sodium citrate. be designed by choosing the appropriate method for the desired purpose.

3.2. UV-visible electronic absorption spectroscopy The palladium clusters were synthesized in solution using the solvent both as a reducing agent and as a stabilizer. In a typical reaction (Scheme 1), the Pd(II) salt precursor was dissolved and left to react for a certain amount of time under aerobic conditions, recording the absorption spectrum every 5 minutes for 1 hr. Reaction progress was monitored by UV-visible spectrometry (Fig. 5). We observed some bulk metal precipitation at the end of the reaction, which gave way to the optical precipitation that can be observed in the spectra obtained for the different methods and solvents, especially with ethylene glycol as solvent and/or NaBH4 as additional reducing agent. As the reaction proceeded, Pd(II) ions were reduced to Pd(0) atoms and grew further to form clusters. This transition decreased the absorbance at 426 nm, which is the main observation in the different UV absorption spectra obtained; in some cases, the appearance of a shoulder at around 380 nm (assigned to palladium nanoclusters [68]) was observed.

In spite of observing such variations in the absorption intensity of both signals, a precise estimation of these changes cannot be made due to baseline shifts caused by the formation of solid particles in the colloidal suspension, as was observed previously in our laboratory [69]. Thus, it was necessary to determine each spectrum derivative at different times and calculate the difference between regions in the derivative spectrum, as we did in previous works. We established both delta ( ) values, which correspond to the amount of Pd(0) nanoparticles and to the amount of Pd(II) in solution after 9 days of reaction. When such values were compared, it was possible to conclude that: a) Pd(0) nanoparticles dispersed in all solvents reached a maximum after nine days, after which a decline is observed, because the clusters are big enough to precipitate during this time. Additionally, the amount of these clusters also increases with time, and the amount of Pd NPs in dispersion decreases until disappearing, as occurs with EG/NaBH 4 , EG/ Na3C6H5O7, EG/US, EtOH/NaBH4, H2O/NaBH4 and DMSO/NaBH4; b) Pd(II) in solution is reduced to Pd(0) with ethylene glycol or ethanol. Thus, disappearance of Pd (II) in solution occurs from the beginning until nearly the end of the observation period, as in EtOH/UV, EtOH/ Na3C6H5O7 or DMF/ NaBH4 (Fig. 6). This was confirmed through a timebased electrochemical study of the same solution, which proves that the amount of Pd(II) was almost completely reduced on observation day 9 (vide infra).

3.3. Microscopy studies Based on the different zones analyzed in TEM experiments, it can be observed that the particles tend to orientate themselves in the form of chains containing Pd nanoparticles distributed along these chains (especially when H2O is employed), and PdO is obtained (Fig. 4). The palladium oxide structure might help to form these chains through intermolecular interactions like van der Waals, through the palladium and oxygen of neighbouring structures, or through hydrogen bonds from the solvent, as represented in Fig. 7. Other examples are found with EG and citrate as an additional reducing agent, and EG without any additional reducing agents, where Pd(0) is obtained. As in the case of Yang et al., the proposed mechanism for the formation of this shortrange ordered linear Pd NP chains is the scaffolding method [70-74], with the solvent used as scaffold for the adsorption of Pd(II). The ion-absorbed EG templates can then transform into a linear as-

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-BM JUJABT N KM F Kand V.M. Ugalde-Saldivar

Fig. 5. Final UV-visible spectrum of each reduction method; initial ( x x CR7=4 ~ ~ f] e c Rd Z T irradiation ( d UZ f TZ e c Re V ~ ~ aY eZ c c RUZ Re Z xxx R U e YV d ] gV eZ e dV] W ( xx xxZe YVUZ W W Vc Ve d e fUZ VUd ] g Ve dR : (b) H2O, (c) DMF, (d) DMSO, (e) EtOH.

sembly of Pd NP chains by reduction reaction (Scheme 3). In the rest of the solvents, we found that the different synthesis procedures give differVe T] fde VcdZ kVdhYZ TYc R XVW c (% *| % , Z H2O, with sodium borohydride as an additional reUfTZXRXV ee *+% 0,|(% ( Z 9BHD hZ e Y sodium citrate as an additional reducing agent (Table 1).

On the other hand, in a Z-contrast analysis of a sample with borohydride as an additional reducing agent in ethanol as solvent (Fig. 8), there is evidence of a NP cover structure [2, 75-78], which supports the idea of ions covering the NPs when a chemical redox reaction takes place in colloidal nanoparticle synthesis. The presence of chloride or other anions near the NP surface was demonstrated

Aerobic synthesis of palladium nanoparticles

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Fig. 6. Comparison of delta ( Abs) values based on the amount of Pd(0) nanoparticles and the amount of Pd(II) in solution after 9 days of reaction. Absorbance change at 320 nm assigned to Pd(0) NPs and 425 nm assigned to dispersed Pd(II).

Scheme 3. Schematic representation of the scaffolding mechanism so as to form Pd NP chains: dissolution of [PdCl2(CH3CN)2] in ethylene glycol; deposition of palladium salt in the solvent chains; attachment of palladium ions via ion exchange reaction, and metal reduction. Adapted from reference [71].

by Finke, who showed that the order of stabilization of Ir NPs by anions followed the trend polyoxometallate > citrate > polyacrylate ~ chloride [75]. As the particles were immersed in this structure, it was necessary to apply energy in order to clearly locate the nanoparticles, obtaining some Z RXVd WCEdhZ e YR Rg Vc RXVdZ kV W-% )| % (, nm.

3.4. Powder X-Ray diffraction The powder X-ray diffraction pattern (Fig. 9) reveals the formation of a nanocrystalline product, consistent with a cubic face-centred structure for palla-

Fig. 7. Possible intermolecular interactions through the (a) oxygen of neighbouring PdO structures; (b) hydrogen bond from the interaction between the solvent (H2O) and oxygen from neighbouring PdO structures.

dium, and a tetragonal body-centred structure for palladium oxide. All diffraction peaks can be perfectly indexed to the Pd(0), PdO, or [PdCl2(CNCH3)2] structures (JCPDS Cards 88-2335 and 88-2434, respectively). In the case of the starting material, an X-ray diffractogram was recorded before the synthesis of the nanoparticles and compared with the one obtained from the resulting NPs. Particle average size was calculated using the Debye Scherer formula, Dhkl = 0.89 /( cos B), where is the Cu Mc RjhRg V] V Xe Y (% ,+ -n B is the Bragg diffraction angle, and is the peak width at half-maximum. Average sizes were larger than the ones obtained by TEM or HRTEM images, which is expected, since the determination is an average; furthermore, the measured particles were already pre-

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-BM JUJABT N KM F Kand V.M. Ugalde-Saldivar

Fig. 8. Z contrast image of NPs obtained from the reduction of Pd(II) with NaBH4 in ethanol as solvent. The arrows pointing at the NPs cover the structure, which had to be beamed in order to obtain the NP HRTEM Z RXV W ;Z X% ,U% IYZ dW Z Xfc VR] d dY hdRc Vac VdV e Re Z RURae VUW c e YV] Z e Vc Re fc VP )Q W:] VTe cde Vc Z TÅ (i.e., electrostatic with the halide anions located between the positively charged NP surface and the tetraetoxiboride anions) stabilization of metal NPs obtained by reduction of a metal chloride salt in the presence of a tetra-etoxi cation (Yi-type reaction between ethanol and borohydride [66] Sch. 2, Eq. [8]). The presence of chloride or other anions near the NP surface was demonstrated. Finke showed that the order of stabilization of Ir NPs by anions followed the trend: polyoxometallate > citrate > polyacrylate ~ chloride [75].

Fig. 9. Examples of the 3 different XRD patterns obtained from the as-prepared particles; in EG with NaBH4 as additional reducing agent, obtaining Pd(0); in H2O with US, obtaining PdO and with sodium citrate as additional reducing agent, where no reaction proceeded and only [PdCl2(CH3CN)2], the starting material was determined.

cipitated in contrast with the TEM or HRTEM results, which come from particles still in dispersion. However, the broadening of the diffraction peaks indicates that the product is composed of small Pd nanoparticles. On the other hand, the particles that could be measured by X-ray diffraction were those that showed chain formations, thus, these results might be revealing a preferential orientation of nanocrystals along a single direction. Average crystallite size was calculated applying the peak broadV ZX Ve Y U fdZXe YVT] RddZ TR]HTYVc c VcLRc

ren equation over the (111), (200), and (220) reflections for Pd(0), and (101), (110), and (112) reflections for PdO.

3.5. Redox studies Pd(II) and ethylene glycol in ethylene glycol. In order to observe the oxide-reduction processes involved in the synthesis, we carried out a voltammetric study of DMSO, DMF, and EG as solvents, which will be discussed later in this paper.

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Aerobic synthesis of palladium nanoparticles

Table 2. Voltammetric potential for the Pd(II) reduction process; oxidation and reduction processes for Hand H+, respectively, resulting from the borohydride study in the different solvents. Solvent

Ecp (V | Fc+ - Fc) PdII + 2ePd0

Eap (V | Fc+ - Fc) 2HH2 + 2e-

Ecp (V | Fc+ - Fc) 2H + 2eH2

EG DMSO DMF

-0.24 -0.92 -0.90

-* 0.18 0.37

-* -3.06 -2.82

* In EG, there are no NaBH4 oxide-reduction signals, but instead there is a modification of the anodic limit potential (Eal = 1.01 V) and the cathodic limit potential (Ecl = -1.24 V). These modifications correspond to hydrogen and ethylene glycolate obtained from the reaction between ethylene glycol and NaBH4.

Fig. 10. Typical cyclic voltammograms in Bu4PF6 0.1M DMF solutions of a) Pd(II) 1.11 x 10-3 M, b) freshly prepared equivalent molar mixture of Pd(II) and sodium citrate, and c) equivalent molar mixture of Pd(II) and sodium citrate after 2 days of mixture.

The characterizations of Pd(II) obtained in EG, DMSO and DMF are similar to those obtained previously [69]. Two oxide-reduction processes were observed, one of Pd(II) reduction to Pd(0) (PdII + 2ePd0), (Table 2), and a second one corresponding to oxidation from the Pd(0) added to the electrode to Pd(II) (Pd0 PdII + 2e-). In the characterization of citrate, we found that it was non-electroactive in all solvents employed, while borohydride exhibited two reactions; one related to the formation of hydrogen in a hydride (H-), (2HH2 + 2e-), and a second one related to the reduction of H+ from donor hydronium species like H2O (Table 2.) In the studies, based on the reaction between Pd(II) and citrate in a 1:1 ratio, it was possible to

Fig. 11. Typical cyclic voltammograms in Bu4PF6 0.1M DMSO solutions of a) Pd(II) 1.11x10-3 M, b) freshly prepared equivalent molar mixture of Pd(II) and sodium borohydride, c) mixture of Pd(II) and sodium borohydride in a 1:2 ratio and d) mixture of Pd(II) and sodium borohydride in a 1:2 ratio after 2 days of mixture.

observe that the Pd(II) reduction process (Ic in Fig. 10) depends on the solvent, being faster in ethylene glycol (6 h.) then in DMF (2 days), and slowest in DMSO (5 days). The voltammogram graphs are included in the supplementary material (Figs. S1); also, Fig. 10 shows an example of Pd(II) and Pd(II)+ sodium citrate (Na3C6H5O7) in DMF. On the other hand, when borohydride is the reducing agent, employed in the same 1:1 (NaBH4:Pd(II)) ratio, the disappearance of the palladium(II) reduction signal (Ic in Figure 11) is almost immediate, independent of the solvent used. Again, the corresponding voltammogram graphics are included in the supplementary material (Figs.

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S2 and S3), and Figure 11 shows an example of Pd(II) and Pd(II)+ sodium borohydride (NaBH4) in DMSO.

4. SUMMARY In conclusion, we have prepared palladium nanoparticles in different solvents with additional reducing agents under aerobic conditions without any inert atmospheres, additional protective molecules or special treatments successfully. The solvents worked both as protective molecules to avoid nanoparticle agglomeration and as reducing agents, thus playing an important role in the size and shape of palladium nanoparticles and can be designed just by choosing the proper one for the desired purpose. The solvent and counter-ions from the resulting reaction worked both as templates and as stabilizers, in particular ethylene glycol, which facilitates the formation of palladium nanoparticle chains. When PdO is obtained in water, the nanoparticles tend to form chains containing smaller nanoparticles in their bodies. Ethylene glycol and EtOH are oxidationsusceptible with Pd(II). During the process, Pd(II) is reduced to Pd(0). Since a slower redox process occurs between Pd(II) and citrate than between Pd(II) and the rest of the reducing agents employed, obtained nanoparticles are smaller in the presence of citrate.

ACKNOWLEDGEMENTS The authors are grateful to CytoViva, Inc. for CytoViva high-resolution images. Financial support for this research by PAPIIT (IN106405, IN101308) and PUNTA is gratefully acknowledged.

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SUPPLEMENTARY MATERIAL

Fig. S1. Typical cyclic voltammograms in Bu4PF6 0.1 M DMSO solutions of an equivalent molar mixture of Pd(II) and sodium citrate in support electrolyte (SE) a) freshly prepared, b) after 2 days and b) after 5 days.

Fig. S2. Typical cyclic voltammograms in Bu4PF6 0.1 M DMF solutions of a) Pd(II) 1.11x 10-3 M in support electrolyte (SE), b) freshly prepared equivalent molar mixture of Pd(II) and sodium borohydride support electrolyte (SE) at E- = -2.50 V and c) at E- = -1.44.

Fig. S3. Typical cyclic voltammograms in Bu4PF6 0.1 M EG solutions of a) Pd(II) 1.24 x 10-3 M in support electrolyte (SE) and b) a mixture of Pd(II) 1.24 x 10-3 M and 5.46 x 10-3 M of sodium borohydride in support electrolyte (SE).

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