aerobic synthesis of palladium nanoparticles · aerobic synthesis.of7palladium-nanoparticles 31 y)...

31Aerobic synthesis of palladium nanoparticles

© 2011 Advanced Study Center Co% Ltd%

Rev.Adv.Mater.Sci. 27(2011) 31-42

Corresponding author: Rocio Redon, e-mail: [email protected]

AEROBIC SYNTHESIS OF PALLADIUM NANOPARTICLES

Rocío Redón1 Samantha K. Rendón-Lara1 Ana L. Fernández-Osorio1

and V. M. Ugalde-Saldivar3

1Centro de Ciencias Ap]icadas y Desarro]]o Tecno]ógico, Universidad Naciona] Autónoma de México,Cd% Universitaria A%P% 70-186, C%P% 04510, Coyoacán, México D% F%, México

2FES-Cuautit]án, Universidad Naciona] Autónoma de México, Edo% de Mexico, México3Universidad Naciona] Autónoma de México, Facu]tad de Química, Mexico City 04510, D%F% Mexico

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 astabilizer and as a dispersant in the colloid, without any inert atmospheres, additional protectivemolecules or special treatments, to transform the nanoparticles into zero-valent NPs. In thisparticular case, dimethylsulfoxide (DMSO), dimethylformamide (DMF), ethylene glycol (EG), etha-nol (EtOH), and water (H2O) are the solvents employed, all under aerobic conditions. The reduc-tion methods include the solvent itself, photoreduction, chemical reduction with either sodiumborohydride or sodium citrate, and (sonochemical) ultrasonic irradiation.

1. INTRODUCTION

Palladium can be used in many applications. Apartfrom being one of the most versatile metals for pro-moting or catalyzing reactions [1], it is used in thearea of palladium-nanoparticle catalysis, where it isconsidered one of the most promising solutions to-wards efficient reactions under mild, environmen-tally benign conditions in the context of Green Chem-istry [2]. Other applications that have been testedin several fields are electronic and photonic devices,telecommunications, sensors [3-5] and the use ofpalladium as an auxiliary in organic reactions [6-8],among others. Consequently, the study of the dif-ferent ways in which palladium nanoparticles canbe obtained has become more and more interest-ing, with monodisperse sizes and well-defined mor-phologies [9-17], and with the use of numeroustypes of stabilizers as capping agents, some ofwhich include block copolymers, [18-21] dendrimers[22-25], polymers [26-28], etc. Researchers have

found that the type of stabilizer that is used to capthe nanoparticles affects their stability [29]. Mucheffort has been devoted to producing Pdnanostructures, with catalytic studies involving ei-ther homogeneous or heterogeneous systems (NPssupported on oxides such as silica, alumina, orother metal oxides, and forms of carbon supports,including carbon nanotubes) [2]. There are manystudies on Pd NP synthesis employing solvents andreducing agents. However, all these cases use aux-iliary molecules to protect the NPs from growingand agglomerating, and taking precautions to workunder inert atmospheres is quite common in thistype of studies. For example, water (H2O), which isthe most common of the solvents studied in thepresent work, has a long list of around 468 papersin the literature, where the protective moleculesemployed include functionalized Poly(ethylene gly-col). There are more than 7 papers on dimethyl-sulfoxide (DMSO), 2 publications ondimethylformamide (DMF), 135 papers on ethanol

32 R. Redón, S.K. Rendón Lara, A.L. Fernández Osorioand V.M. Ugalde-Saldivar

(EtOH), 80 papers on ethylene glycol (EG), over 50papers on sodium borohydride, and 9 papers onsodium citrate, the latter two being the most em-ployed Pd(II) reducing agents in literature on palla-dium. Nevertheless, we use them, as mentionedbefore, in the presence of molecular oxygen. In gen-eral, a lot has been learned about these materialsover the last few decades [30-59].

The synthesis of Pd nanoparticles is usuallyconducted under inert atmospheres such as argonor nitrogen, and with the use of molecules to pro-tect the nanoparticle surface of oxide formation.Nevertheless, these procedures are expensive con-sidering the addition of extra molecules and thecomplexity of the oxygen-free process. Thus, in thispaper, we report the synthesis and characterizationof palladium nanoparticles with no protective mol-ecules other than the solvent, and in an aerobic at-mosphere. These nanoparticles were prepared byreducing Pd(II) in 5 different solvents, which can workboth as protective molecules and as Pd(II)-reducingagents, and in the presence of other reducing agentssuch as photoreduction, ultrasonic irradiation, andadditional chemicals such as sodium citrate andsodium borohydride. Palladium NPs obtained in thisway were analyzed for composition, shape and size,depending on the reducing conditions used. Al-though there have been many studies related to thismatter, the syntheses of these metallicnanoparticles were carried out under inert atmo-spheres such as nitrogen or argon. In this particularstudy, we are not using any protective molecules orinert atmospheres.

2. EXPERIMENTAL

2.1. Materials

The following were purchased from Sigma-Aldrichand used without further purification: sodium boro-hydride (99%), sodium citrate (99%) ethylene gly-col (99.8%), anhydrous ethanol (99.5%), dimethyl-sulfoxide (99%), N,N-dimethylformamide (98%),acetonitrile (99%) and PdCl2 (99%). Tri-distilledwater (99.9%) was bought from Hycel. Complex[PdCl2(CNCH3)2] was synthesized by the literaturemethods [60].

2.2. Instruments

UV-visible absorption spectra, in colloidal dispersion,were obtained using the Ocean Optics USB200miniature fibreglass optical spectrometer. Transmis-sion electron micrographs (TEM) were obtained ona JEOL 1200EXII microscope, operating at 60kV,

by deposition of a drop of the colloidal dispersiononto 200 mesh Cu grids coated with a carbon/collo-dion layer. High-resolution transmission electronmicroscopy (HRTEM) micrographs were obtainedon a JEOL 2000F microscope operating at 200 kV,using the same sample preparation as in TEM ex-periments. The particle size distribution was deter-mined using a digitalized amplified micrograph. Allsolutions were prepared with ethylene glycol assolvent at 10-3 M concentration. Solutions weremeasured by using ethylene glycol as a blank at 24to 48-hour intervals after preparation. A derivativemethod was used to perform a quantitative analysisof the species formed, in order to determine theabsorbance values of each of the absorption maxi-mums observed in the region of 300 to 500 nm. Forthis purpose, it was necessary to obtain the deriva-tives of the different spectra for further analysis. TheX-ray diffraction pattern was measured using a Si-emens D5000 diffractometer with CuK radiation( = 1.5406 Å % DMSO dispersions were measuredin solution on a CytoViva high-resolution condenserand images were obtained using the CytoVivaHyperspectral Imaging System.

2.3. Synthesis

Preparation of colloidal Pd NPs. 200 mL of[PdCl2(CNCH3)2] 10-3 M solution was prepared in thecorresponding solvent under aerobic conditions. A2x10-4 M concentration was obtained after dilution(5 mL of 10-3 M in 25 mL of the corresponding sol-vent). 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 ul-trasonic irradiation) was applied to each solution,which was then stirred while the formation of PdNPs was monitored by UV-vis absorption spectra,taking one spectrum every 5 minutes for 1 hour.

2.4. Electrochemical studies

General procedure for electrochemical mea-surements. Electrochemical experiments werecarried out using a typical three-electrode cell. Thevitreous graphite electrode (area: 7.1 mm2) was usedas the working electrode, a platinum wire as thecounter electrode, and an Ag-AgBr/Bu4NBr0.1M in

solvent

additionalreduce agent

Pd(II) PdNPs

Scheme 1. General schematization for the synthe-sis of Pd(0) NPs.


Reduction methodSolvent Ultrasonic Photorreduction NaBH4(nm) Na3C6H5O7 Pure

Irradiation ( = 275nm)(nm) (nm) solvent(nm)(kHz)(nm)

Ethylene glycol 2.4(29.3†) 2.6 2.5(51.62†) 1.9(19.96†) 2.6H2O 3.1(87.53†) 2.0(23.12†) 1.3 nm(31.29†) 1.9 nm 25/2.5‡

Dimethylfor-mamide 2.7 immeasurable - 50/2 nm Starting material

Dimethylsulf-oxide 2.1 23%99|0%73* 18%09 |0%49* 34%95|1%09* Starting (w:218|0%15 x material

29%3|1%39, sph:193|11%2*

Ethanol 6.2 3.2 2.5 2.5 Starting material

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

Table 1. Average sizes and their standard deviations of all Pd-NPs based on TEM or HRTEM analysis(Average diameter/nm, |0%1 %

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

ethylene glycol as a pseudo reference. Thevoltammograms were obtained on a PAR263-Apotentiostat-galvanostat. All voltammograms beginat an open circuit potential and are obtained in bothanodic and cathodic sweeps. The employed sup-porting electrolyte is a 0.1M Bu4PF6 in either DMSOor DMF, and a saturated solution in ethylene gly-col. In order to report the potentials used accordingto the IUPAC convention, voltammograms were ob-tained for approximately 10-3 M solution of ferrocene(Fc) in a supporting electrolyte. The half-wave po-tentials were estimated from E1/2 = (Eap + Ecp)/2, whereEap and Ecp are the anodic and cathodic peak po-tentials, respectively.

3. RESULTS AND DISCUSSION

3.1. Synthesis method

Concentration is a key factor in the colloidal syn-thesis of nanoparticles, because if the initial palla-dium salt concentration is greater than 10-3 M, theparticles begin to agglomerate and precipitate outof the dispersion, but if the concentration is 10-4 Mor less, then the dispersion is homogeneous andstable for months. Depending on the different re-duction methods and solvents employed in the syn-thesis of palladium nanoparticles (Scheme 1), shapeand size varied as expected. Therefore, when ultra-


Fig. 2. TEM micrographs of palladium(0)nanoparticles in different reduction media: (a) H2O/NaBH4, (b) EG/US, (c) EG/NaBH4, (d) EG/Na3C6H5O7, and (e) DMF/citrate.

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.

Fig. 4. TEM micrographs of palladium oxidenanoparticles in water with different additional re-ducing agents: (a) without any reducing agent, (b)with photoirradiation, and (c) with ultrasonic irradia-tion.

sonic irradiation was used as an additional reduc-tion method, the particles obtained were the largest(Table 1) and the most irregular, with plenty of un-saturated sites (Fig. 1) - an excellent characteristicfor their use as catalysts-. On the other hand, chemi-cal reduction, either with sodium borohydride or withsodium citrate, results in the smallest nanoparticles(Table 1), generally of spherical shape (Figs. 2 and3). When water is used as a solvent and the finalproduct is PdO nanoparticles, the particles tend to

form wires constituted of small clusters that areformed by smaller nanoparticles (Fig. 4). Finally,when the final product is Pd(0), the geometry isnearly spherical (Figs. 2 and 3).

When water is used as a solvent, the particlesobtained are of PdO except in the case of sodiumborohydride, where Pd(0) was obtained. On the con-trary, when ethanol or ethylene glycol are used, theobtained particles are of Pd(0) in all cases, includ-ing when no additional reducing agent but the sol-vent is used, which implies that the solvent is work-ing as a reducing agent and as a stabilizer, evenwith the presence of molecular oxygen. The solventreactions that can be achieved are shown in Scheme2 [61-67]. With DMF and DMSO, Pd(II) is alwaysobtained in the dispersion in addition to Pd(0), ex-cept in the case where the solvent is used withoutany further reduction methods, where the startingmaterial is the only product observed. Based onthese results, it can be said that the reductionmethod plays an important role in the size andshape of palladium nanoparticles, and that it can


+ 2-

2H O 2H + O [2]

+ -

3 2 3CH CH OH CH CHO+ 2H + 2 e [3]

2 2 3 3HOCH CH OH CH COOCCH [4]

3 2 2

+ -

3 2

HCON(CH ) H O

(CH ) NCOOH+ 2H + 2e [5]

3 2 3 2 2 3 22(CH ) SO (CH ) SO + (CH ) S [6]

4 2 2 2NaBH + 2H O NaBO + 4H [7]

4 2 2

2 2 4 2

NaBH + 4HOCH CH OH

NaB(OCH CH OH) + 4H [8]

2 2 2

2 2 3

NaOOCCH COH(COONa)CH COONa+H O

NaOOCCH CH(OH)CH COONa+ NaHCO [9]

Scheme 2. Possible oxidation reactions taking placewith the different solvents. [2] Water. The O2- specieis the one which reacts with Pd(II) after water; [3]EtOH; [4] EG; [5] DMF; [6] DMSO; [7] NaBH4 inwater; [8] NaBH4 in EG; [9] Sodium citrate.

be designed by choosing the appropriate methodfor the desired purpose.

3.2. UV-visible electronic absorptionspectroscopy

The palladium clusters were synthesized in solu-tion using the solvent both as a reducing agent andas a stabilizer. In a typical reaction (Scheme 1), thePd(II) salt precursor was dissolved and left to reactfor a certain amount of time under aerobic condi-tions, recording the absorption spectrum every 5minutes for 1 hr. Reaction progress was monitoredby UV-visible spectrometry (Fig. 5). We observedsome bulk metal precipitation at the end of the re-action, which gave way to the optical precipitationthat can be observed in the spectra obtained for thedifferent methods and solvents, especially with eth-ylene glycol as solvent and/or NaBH4 as additionalreducing agent. As the reaction proceeded, Pd(II)ions were reduced to Pd(0) atoms and grew furtherto form clusters. This transition decreased the ab-sorbance at 426 nm, which is the main observationin the different UV absorption spectra obtained; insome cases, the appearance of a shoulder at around380 nm (assigned to palladium nanoclusters [68])was observed.

In spite of observing such variations in the ab-sorption intensity of both signals, a precise estima-tion of these changes cannot be made due tobaseline shifts caused by the formation of solid par-ticles in the colloidal suspension, as was observedpreviously in our laboratory [69]. Thus, it was nec-essary to determine each spectrum derivative atdifferent times and calculate the difference betweenregions in the derivative spectrum, as we did in pre-vious works. We established both delta ( ) values,which correspond to the amount of Pd(0)nanoparticles and to the amount of Pd(II) in solutionafter 9 days of reaction. When such values werecompared, it was possible to conclude that: a) Pd(0)nanoparticles dispersed in all solvents reached amaximum after nine days, after which a decline isobserved, because the clusters are big enough toprecipitate during this time. Additionally, the amountof these clusters also increases with time, and theamount of Pd NPs in dispersion decreases until dis-appearing, as occurs with EG/NaBH4, EG/Na3C6H5O7, EG/US, EtOH/NaBH4, H2O/NaBH4 andDMSO/NaBH4; b) Pd(II) in solution is reduced toPd(0) with ethylene glycol or ethanol. Thus, disap-pearance of Pd (II) in solution occurs from the be-ginning until nearly the end of the observation pe-riod, as in EtOH/UV, EtOH/ Na3C6H5O7 or DMF/NaBH4 (Fig. 6). This was confirmed through a time-based electrochemical study of the same solution,which proves that the amount of Pd(II) was almostcompletely reduced on observation day 9 (vide in-fra).

3.3. Microscopy studies

Based on the different zones analyzed in TEM ex-periments, it can be observed that the particles tendto orientate themselves in the form of chains con-taining Pd nanoparticles distributed along thesechains (especially when H2O is employed), and PdOis obtained (Fig. 4). The palladium oxide structuremight help to form these chains through intermo-lecular interactions like van der Waals, through thepalladium and oxygen of neighbouring structures,or through hydrogen bonds from the solvent, as rep-resented in Fig. 7. Other examples are found withEG and citrate as an additional reducing agent, andEG without any additional reducing agents, wherePd(0) is obtained. As in the case of Yang et al., theproposed mechanism for the formation of this short-range ordered linear Pd NP chains is the scaffold-ing method [70-74], with the solvent used as scaf-fold for the adsorption of Pd(II). The ion-absorbedEG templates can then transform into a linear as-


Fig. 5. Final UV-visible spectrum of each reductionmethod; initial (— • — • , NaBH4 (~ ~ , u]trasonicirradiation (— — , sodium citrate (~ ~ ,photoirradiation (• • • and the so]vent itse]f(— • • — • • in the different studied so]vents (a EG,(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 foundthat the different synthesis procedures give differ-ent c]uster sizes, which range from 1%3 | 0%5 nm inH2O, with sodium borohydride as an additional re-ducing agent, to 34%95 | 1%1 nm in DMSO withsodium citrate as an additional reducing agent(Table 1).

On the other hand, in a Z-contrast analysis of asample with borohydride as an additional reducingagent in ethanol as solvent (Fig. 8), there is evi-dence of a NP cover structure [2, 75-78], which sup-ports the idea of ions covering the NPs when achemical redox reaction takes place in colloidalnanoparticle synthesis. The presence of chloride orother anions near the NP surface was demonstrated


Fig. 6. Comparison of delta ( Abs) values basedon the amount of Pd(0) nanoparticles and theamount 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).

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

by Finke, who showed that the order of stabilizationof Ir NPs by anions followed the trendpolyoxometallate > citrate > polyacrylate ~ chlo-ride [75]. As the particles were immersed in thisstructure, it was necessary to apply energy in orderto clearly locate the nanoparticles, obtaining someimages of NPs with an average size of 6%2 | 0%15nm.

3.4. Powder X-Ray diffraction

The powder X-ray diffraction pattern (Fig. 9) revealsthe formation of a nanocrystalline product, consis-tent with a cubic face-centred structure for palla-

dium, and a tetragonal body-centred structure forpalladium oxide. All diffraction peaks can be per-fectly 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 syn-thesis of the nanoparticles and compared with theone obtained from the resulting NPs. Particle aver-age size was calculated using the Debye Schererformula, Dhkl = 0.89 /( cos B), where is the CuX-ray wave]ength (1%5406 Å , B is the Bragg diffrac-tion angle, and is the peak width at half-maxi-mum. Average sizes were larger than the ones ob-tained by TEM or HRTEM images, which is ex-pected, since the determination is an average; fur-thermore, the measured particles were already pre-

Scheme 3. Schematic representation of the scaf-folding 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 re-action, and metal reduction. Adapted from reference[71].


Fig. 8. Z contrast image of NPs obtained from the reduction of Pd(II) with NaBH4 in ethanol as solvent. Thearrows pointing at the NPs cover the structure, which had to be beamed in order to obtain the NP HRTEMimage of Fig% 5d% This figure a]so shows a representation, adapted from the ]iterature [2], of “E]ectrostericÅ(i.e., electrostatic with the halide anions located between the positively charged NP surface and thetetraetoxiboride anions) stabilization of metal NPs obtained by reduction of a metal chloride salt in thepresence of a tetra-etoxi cation (Yi-type reaction between ethanol and borohydride [66] Sch. 2, Eq. [8]). Thepresence of chloride or other anions near the NP surface was demonstrated. Finke showed that the order ofstabilization of Ir NPs by anions followed the trend: polyoxometallate > citrate > polyacrylate ~ chloride[75].

Fig. 9. Examples of the 3 different XRD patternsobtained from the as-prepared particles; in EG withNaBH4 as additional reducing agent, obtaining Pd(0);in H2O with US, obtaining PdO and with sodiumcitrate as additional reducing agent, where no reac-tion proceeded and only [PdCl2(CH3CN)2], the start-ing material was determined.

cipitated in contrast with the TEM or HRTEM re-sults, which come from particles still in dispersion.However, the broadening of the diffraction peaks in-dicates that the product is composed of small Pdnanoparticles. On the other hand, the particles thatcould be measured by X-ray diffraction were thosethat showed chain formations, thus, these resultsmight be revealing a preferential orientation ofnanocrystals along a single direction. Average crys-tallite size was calculated applying the peak broad-ening method, using the c]assica] Scherrer–War-

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

3.5. Redox studies

Pd(II) and ethylene glycol in ethylene glycol.In order to observe the oxide-reduction processesinvolved in the synthesis, we carried out avoltammetric study of DMSO, DMF, and EG as sol-vents, which will be discussed later in this paper.


Solvent Ecp (V | Fc+ - Fc) Eap (V | Fc+ - Fc) Ecp (V | Fc+ - Fc) PdII + 2e- Pd0 2H- H2 + 2e- 2H + 2e- H2

EG -0.24 -* -*DMSO -0.92 0.18 -3.06DMF -0.90 0.37 -2.82

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

Table 2. Voltammetric potential for the Pd(II) reduction process; oxidation and reduction processes for H-

and H+, respectively, resulting from the borohydride study in the different solvents.

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 mixtureof Pd(II) and sodium citrate after 2 days of mixture.

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) andsodium borohydride in a 1:2 ratio and d) mixture ofPd(II) and sodium borohydride in a 1:2 ratio after 2days of mixture.

The characterizations of Pd(II) obtained in EG,DMSO and DMF are similar to those obtained pre-viously [69]. Two oxide-reduction processes wereobserved, one of Pd(II) reduction to Pd(0) (PdII + 2e-

Pd0), (Table 2), and a second one correspondingto oxidation from the Pd(0) added to the electrodeto Pd(II) (Pd0 PdII + 2e-).

In the characterization of citrate, we found that itwas non-electroactive in all solvents employed, whileborohydride exhibited two reactions; one related tothe formation of hydrogen in a hydride (H-), (2H-

H2 + 2e-), and a second one related to the reductionof H+ from donor hydronium species like H2O (Table2.)

In the studies, based on the reaction betweenPd(II) and citrate in a 1:1 ratio, it was possible to

observe that the Pd(II) reduction process (Ic in Fig.10) depends on the solvent, being faster in ethyleneglycol (6 h.) then in DMF (2 days), and slowest inDMSO (5 days). The voltammogram graphs are in-cluded 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 re-ducing agent, employed in the same 1:1(NaBH4:Pd(II)) ratio, the disappearance of thepalladium(II) reduction signal (Ic in Figure 11) is al-most immediate, independent of the solvent used.Again, the corresponding voltammogram graphicsare included in the supplementary material (Figs.


S2 and S3), and Figure 11 shows an example ofPd(II) and Pd(II)+ sodium borohydride (NaBH4) inDMSO.

4. SUMMARY

In conclusion, we have prepared palladiumnanoparticles in different solvents with additionalreducing agents under aerobic conditions withoutany inert atmospheres, additional protective mol-ecules or special treatments successfully. The sol-vents worked both as protective molecules to avoidnanoparticle agglomeration and as reducing agents,thus playing an important role in the size and shapeof palladium nanoparticles and can be designed justby choosing the proper one for the desired purpose.The solvent and counter-ions from the resulting re-action worked both as templates and as stabiliz-ers, in particular ethylene glycol, which facilitatesthe formation of palladium nanoparticle chains. WhenPdO is obtained in water, the nanoparticles tend toform chains containing smaller nanoparticles in theirbodies. Ethylene glycol and EtOH are oxidation-susceptible with Pd(II). During the process, Pd(II) isreduced to Pd(0). Since a slower redox processoccurs between Pd(II) and citrate than between Pd(II)and the rest of the reducing agents employed, ob-tained nanoparticles are smaller in the presence ofcitrate.

ACKNOWLEDGEMENTS

The authors are grateful to CytoViva, Inc. forCytoViva high-resolution images. Financial supportfor 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 mix-ture of Pd(II) and sodium citrate in support electro-lyte (SE) a) freshly prepared, b) after 2 days and b)after 5 days.


0.1 M DMF solutions of a) Pd(II) 1.11x 10-3 M insupport electrolyte (SE), b) freshly prepared equiva-lent molar mixture of Pd(II) and sodium borohydridesupport electrolyte (SE) at E- = -2.50 V and c) atE- = -1.44.


0.1 M EG solutions of a) Pd(II) 1.24 x 10-3 M insupport electrolyte (SE) and b) a mixture of Pd(II)1.24 x 10-3 M and 5.46 x 10-3 M of sodium borohy-dride in support electrolyte (SE).

aerobic synthesis of palladium nanoparticles · aerobic synthesis.of7palladium-nanoparticles 31 y)...

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