This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014 New J. Chem.

Cite this:DOI: 10.1039/c3nj00894k

Green and easily scalable microwave synthesisof noble metal nanosols (Au, Ag, Cu, Pd) usableas catalysts

M. Blosi,*a S. Albonetti,b S. Ortelli,ab A. L. Costa,a L. Ortolanic and M. Dondia

A green synthesis process was developed for the production of PVP-coated noble metal nanoparticles in the

form of stable nanosols. Water is the environmentally benign solvent; glucose serves as a mild, renewable

and non-toxic reducing agent and microwave irradiation is an effective and fast heating technique. The same

green process has been optimized to obtain several metal nanoparticles (Au, Ag, Cu, Pd), and therefore

encourages the easy preparation of bimetallic nanostructures. Nanosols were characterized by dynamic

light scattering DLS, HR-TEM, UV-Vis spectroscopy, XRD and ICP-AES. The total reaction yield for all the

samples was assessed, the prepared nanoparticles were spherical shaped with an average diameter

ranging from 3 to 20 nm. Nanosols with excellent stability over several months, achieved even for high

solid contents, were prepared. Additionally, it is shown that all of the synthesized nanoparticles can act

as effective catalysts for the reduction of 4-nitrophenol (4-NP) in the presence of NaBH4 (which is

otherwise unfeasible without a metal catalyst). This reduction was spectrophotocally followed and the

rate constants were determined by measuring the change in absorbance at 400 nm (the wavelength

typical of 4-NP) as a function of time. The following ranking of decreasing efficiency of the catalyst was

found: Pd 4 Au 4 Ag 4 Cu.

1. Introduction

During recent years there has been great interest directedtowards the synthesis of metal nanoparticles in order to exploretheir special properties, which are distinctly different fromtheir bulk counterparts. It has been mainly noble metals whichare exploited, despite their high cost, due to their optical,1

electrical and catalytic properties,2 so meeting the growinginterest for their application versatility not only from a scientificpoint of view, but from an industrial prospect too is important.

The integration of green chemistry principles into nano-technology and nanoscience has attracted much attention overthe past decade, aiming for the design of more sustainablesynthesis processes.3–5 The use of cheap, nontoxic chemicals,environmentally benign solvents and renewable materials aresome of the essential issues in the nanomaterials science fieldin light of the ‘‘green’’ synthetic strategy for industrial scalemanufacture.6 It is well known that the reaction medium and

the chelating and reducing agents are three key factors for thesynthesis and stabilization of metal nanoparticles; these factorsshould be comprehensively considered from an economic andgreen chemistry perspective. Most of the synthetic routesreported in the literature, even if scalable for large production,are based on organic solvents thus implying a complex environ-mental path to the industrial production.7–12 To date, someexpensive and/or toxic chelating agents (thiols, oleic acid,hexadecylamine, trioctylphosphine oxide)13–15 have been employedto prepare metal nanoparticles in organic solvents making thesesynthesis processes less promising for a subsequent industrialscale up. Moreover, concerning the reducing agents, a large bodyof literature16–19 proposed strong and hazardous reducing agents,such as hydrazine, sodium borohydride (NaBH4) and dimethylformamide (DMF), which are highly reactive and present potentialenvironmental and biological risks.

In this work, we report an environmentally-friendly approachwhich is easily transferable to the production of different noblemetal nanoparticles (Au, Ag, Cu and Pd) in the form of stablenanosols even at high solid loading.20 Water is an environmentallybenign solvent, glucose serves as a mild, renewable and non-toxicreducing agent and polyvinylpyrrolidone (PVP) is a water-soluble,cheap and non-toxic chelating additive. The same synthesisapproach has been optimized for the synthesis of each metal,allowing for the simple preparation of bimetallic structures.21–23

a ISTEC-CNR, Institute of Science and Technology for Ceramics,

National Research Council, Via Granarolo 64, 48018, Faenza, Italy.

E-mail: magda.blosi@istec.cnr.it; Fax: +39 054646381; Tel: +39 0546699718b Department of Industrial Chemistry ‘‘Toso Montanari’’, INSTM,

Research Unit of Bologna, Italyc IMM-CNR, Institute for Microelectronics and Microsystems, Via Piero Gobetti 10,

40129 Bologna, Italy

Received (in Porto Alegre, Brazil)6th August 2013,Accepted 24th October 2013

DOI: 10.1039/c3nj00894k

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Furthermore, the reaction yield, never considered in the literature,was assessed for each sample.

Microwave irradiation, also suitable for large scale produc-tion,24,25 was used as it enables homogeneous heating of thetreated volumes and rapid achievement of the desired tempera-tures. In fact, by using a microwave continuous flow plant, thetypical criticisms of batch microwave heating for large produc-tion, such as the difficulty of microwave penetration into largesolvent volumes or the need for a large microwave chamber,can be easily overcome, so representing a good starting pointfor process intensification.

Finally, the catalytic performances of all the prepared nano-particles were assessed by the hydrogenation of 4-nitrophenol(4-NP) to 4-aminophenol (4-AP) in the presence of NaBH4, aprobe reaction widely used to test the catalytic activity of metalnanoparticles.26,27

2. Experimental2.1. Synthesis of nanosols

The following analytic grade reagents were used: HAuCl4,CuSO4�5H2O, NaOH, AgNO3, PdCl2, polyvinylpyrrolidone PVP(Mwa 29 000), 4-nitrophenol, NaBH4, all from Sigma-Aldrich(Sigma-Aldrich, USA), and D(+)glucose (Merck, DE).

Metallic nanoparticles were prepared following a patentedprocedure,20 which provides stable nanosols by using a micro-wave assisted eco-friendly method.

Au colloids. PVP (0.35 g) and glucose (from 0.20 to 0.40 g)were mixed in a round bottom flask containing water (80 ml)and the solution was microwave heated at ambient pressureto a temperature ranging from 70 to 90 1C (heating rate of30 1C min�1). At this temperature NaOH aqueous solution(0.88 M) and HAuCl4 (10 ml, 0.11 M) were added to the flaskand stirred for 2.5 min. After the reaction, a red, stable suspen-sion of gold nanoparticles with a solid loading of 0.2 wt% wasobtained. In order to optimize the system in terms of stability,particle size and reaction yield, the following synthesis para-meters were evaluated: temperature, glucose/metal and NaOH/metal molar ratios (Table 1).

Ag colloids. Ag colloids were obtained through the reductionof AgNO3 in the presence of PVP. First, 3.13 g of PVP weredissolved in 90 ml of distilled water, then 0.28 g of NaOH and0.90 g of D(+)glucose were added to the solution. When all thereagents were dissolved, the solution was heated at a tempera-ture of 70 1C and 10 ml of an aqueous AgNO3 solution (0.5 M)was injected and the reaction was allowed to proceed for 3 minutes.In order to optimize the reaction, the key molar ratios of glucose/AgNO3 and NaOH/AgNO3 were tuned.

By adding AgNO3, the solution immediately turned brown incolour with intense yellow shades. Highly stable suspensionswith a solid loading of 0.5 wt% were initially achieved and, byusing the optimized conditions, the solid loading was furtherincreased to 4 wt%.

Cu colloids. Cu colloids were obtained through the reduction ofCuSO4�5H2O. In a typical experiment, PVP (5.66 g), glucose (7.2 g)and NaOH (1.2 or 2.4 g) were mixed in a round bottom flaskcontaining water (180 ml) and the solution was then microwaveheated at ambient pressure to 100 1C (heating rate of 30 1C min�1).At this temperature, the CuSO4�5H2O solution (20 ml, 0.165 M) wasadded and the temperature was maintained for a time rangingfrom 10 to 40 minutes. In order to achieve the pure metallic phase,both the NaOH/metal molar ratio and the reaction time wereevaluated (Table 2). A red sol with a solid loading of 0.1 wt%and a time stability of 24 hours was obtained.

Pd colloids. Pd colloids were synthesized through thereduction of PdCl2. The experiments were performed followingthe same procedure used for gold colloids. In the Pd case, onlythe NaOH/metal molar ratio was adjusted during the synthesisoptimization procedure.

2.2. Apparatus

The microwave system used is a MicroSYNTHplus (Milestone, IT),whose reaction chamber is provided with magnetic stirring,a reflux system and an optical fiber temperature controller. Themicrowave power is generated by 2 � 800 W magnetrons with afrequency of 2.45 GHz. In order to respect the scheduledheating ramp, the power is continuously supplied and auto-matically modulated by software; for each ramp only themaximum deliverable power can be imposed.

Table 1 Characteristics of the Au and Pd synthesized samples. All thesamples were prepared with a solid loading of 0.2 wt%a

SampleTsynth

(1C)nGluc/Mem+

nNaOH/Mem+

+DLS

(nm) PDIlmax

(nm)Yield(%)

Au01 70 1 2 — — — —Au02 90 1 2 64 0.3 — —Au03 90 2 2 48 0.3 549 76.5Au04 90 2 4 27 0.6 538 99.9Au05 90 2 8 17 0.2 521 100

Pd01 90 2 4 36 0.4 — —Pd02 90 2 8 23 0.3 — 100Pd03 90 2 12 88 0.2 — —

a Tsynth: synthesis temperature; nGluc/Mem+: glucose/metal molar ratio;nNaOH/Mem+: NaOH/metal molar ratio; +DLS: mean diameter by dynamiclight scattering; PDI: polydispersion index; lmax: maximum wavelength oflight absorbance.

Table 2 Characteristics of Ag synthesized samplesa

SampleConc Ag0

(wt%)nNaOH/nAg+

nGluc/nAg+

+DLS

(nm) PDIlmax

(nm)Yield(%)

Ag01 0.1 1.0 1.0 100 0.1 432Ag02 0.1 1.5 1.0 67 0.1 409 99.8Ag03 0.1 2.0 1.0 60 0.4 403Ag04 0.1 7.0 1.0 38 0.5 400

Ag05 0.1 1.5 0 150 0.1 —Ag06 0.1 1.5 0.5 114 0.1 423Ag07 0.1 1.5 2.0 70 0.2 410

Ag08 0.5 1.5 1.0 44 0.2 410 99.5Ag09 1 1.5 1.0 27 0.2 413 99.0Ag10 2 1.5 1.0 42 0.5 407Ag11 4 1.5 1.0 87 0.3 413

a Abbreviations as in Table 1.

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2.3. Analytical characterization

Metallic nanosols were characterized by optical spectroscopy(UV-Vis), dynamic light scattering (DLS), X-ray diffraction (XRD),inductively coupled plasma-atomic emission spectrometry (ICP-AES)and transmission electron microscopy (TEM). UV-Vis extinctionspectra were measured with a Lambda 35 spectrophotometer(Perkin Elmer, UK), using a quartz cuvette as sample-holder.Samples for UV-Vis spectroscopy were prepared by diluting theas prepared colloidal suspension with water in order to load thesame metal concentration for every sample into the cuvette.

Dynamic light scattering (DLS) was used to monitor thehydrodynamic diameter and the particle size distribution of thesuspensions. Measurements were carried out by a Nano S(Malvern, UK) working at a fixed angle of 1731. Samples wereproperly diluted with water and poured in a polystyrene cuvettebefore measurement. The hydrodynamic diameter includes thecoordination sphere and the species adsorbed on the particlesurface such as stabilizers, surfactants and so forth. DLSanalysis also provides a polydispersion index parameter (PDI),ranging from 0 to 1, quantifying the colloidal dispersion degree;for PDI below 0.2 a sol can be considered as monodispersed.

Diffraction patterns were collected for the synthesized samplesdripped on a glass slide and dried at 100 1C for 15 minutes.Analyses were performed by the Bruker D8 Advance diffractometer(Germany) operating in y/2y configuration, with a LynxEye detector(20–801 2y range, 0.02 stepsize, 16 s time-per-step equivalent).The quantitative phase composition was performed by usingthe RIR method.

Unreacted metal cations were detected by ICP-AES in orderto infer the reaction yield. For this purpose, 50 ml of the assynthesized colloid was poured into a semi-permeable osmoticmembrane (Visking tube), which was submerged in a de-ionizedwater bath. Osmotic pressure caused the exchange of unreactedcations into the external water and the water entrance into thetube. After one hour, equilibrium was attained and the externalliquid underwent ICP (Liberty 200, Varian, Australia) quantita-tive analyses (Au line 267.595 Å, Ag line 328.068 Å, Cu line324.754 Å, Pd line 340.458 Å, plasma power 1.20 kW, plasma flow15 L min�1, sample pump rate 15 rpm).

The observations of particle size distribution and morphologywere made using a transmission electron microscope (Tecnai F20,FEI, The Netherlands). Nanoparticles were dispersed on a standardcopper grid for TEM by dripping the prepared solutions, whichwere dried in air and then treated at 100 1C for 5 minutes. Imageanalysis was performed on more than 100 particles for each systemto calculate the particle size distribution shown as a histogram,average diameter and standard deviation.

2.4. Catalytic characterization

The catalytic reduction of 4-nitrophenol by NaBH4 was studiedat room temperature (25 1C) in a standard quartz cuvette with a1 cm path length and about 3 ml volume. The samples preparedwere properly diluted with distilled water in order to achievea metal concentration of 1.1 � 10�2 mM. Thus, 10 ml of thediluted suspensions were mixed with 5 ml of a 4-nitrophenol

solution (9.0 � 10�2 mM) and 1 ml of a NaBH4 aqueoussolution (0.72 M). An aliquot of the solution was poured intothe quartz cuvette and the absorption spectra were collected bya Lambda 35 spectrophotometer (Perkin Elmer, USA) in therange between 250 and 500 nm. The rate constants of thereduction process were determined by measuring the change inabsorbance at 400 nm, corresponding to the 4-nitrophenolateion, as a function of time. Although the 4-NP solution absorbsat 317 nm, a second peak appears at 400 nm after the additionof the NaBH4 solution, forming the 4-nitrophenolate ion. Thereaction is of the second order (1), however, as the concentrationof NaBH4 is in a large excess with respect to the concentration ofthe reagent (4-NP), the reduction rate can be regarded asindependent of NaBH4, changing the system to a pseudo firstorder reaction (2). Therefore, the rate constants of the reactioncan be determined by measuring the change in absorbance at400 nm, the wavelength typical of 4-NP in alkaline conditions,as a function of time.

n = k � [4-NP] � [NaBH4] second order (1)

n = K � [NP] pseudo first order (2)

3. Results and discussion3.1. Synthesis and characterization of the sols

The syntheses of metals were developed by improving somefundamental parameters: the NaOH/metal and glucose/metalratios, the temperature and the reaction time. Although theglucose is a very weak reducing agent28 and is able somehowto reduce the considered metals, its reducing power can bedrastically enhanced in alkaline conditions.29 The alkaline environ-ment, in fact, promotes the dehydrogenation of the a-protonactivating the opening of the glucose ring and the followingoxidation of glucose to gluconic acid. For this purpose, the bestmatching of the two molar ratios, NaOH/metal and glucose/metal, is considered to be a key parameter for the reaction,ensuring a total reaction yield. Furthermore, in order tobetter control the growth mechanism and ensure the forma-tion of small particles with a narrow particle size distribution,a fast nucleation, where the greatest amount of the pre-cursor is simultaneously converted into the final product,is preferred.30

3.1.1. Gold. The synthesis of Au nanosols was optimized bytuning of the temperature, the glucose/Au and NaOH/Au molarratios. The features of the prepared gold samples are sum-marized in Table 1. Samples Au01 and Au02 were obtained attwo different temperatures, 70 and 90 1C, respectively. While at70 1C the reduction of the salt did not start, keeping the typical

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colour of the precursor solution, at 90 1C the reaction occurredand the typical red wine colour was observed.

The samples Au03, Au04 and Au05, all prepared at 90 1C,showed the influence of the molar ratios of glucose/Au andNaOH/Au. Following the reaction (3), sample Au02 was syn-thesized with a defect of glucose and sample Au03 with anexcess of glucose.

2Au3+ + 3C6H12O6 + 6OH� - 2Au0 + 3C6H12O7 + 3H2O(3)

The particle size is lower for sample Au03, which was preparedwith an excess of the reducing agent, probably because thereduction was faster and the growth step limited. Therefore,working with a slight excess of glucose improved the reducingpower of the system and a smaller particle size was achieved.However, the reaction yield assessed for sample Au03 was notcomplete, and a stronger reducing character of the system isnecessary. Since the reduction potential of the glucose can beimproved by tuning the pH towards alkaline values,28 increasingthe NaOH/Au ratio can result in a complete and fast reduction ofthe precursor to metal. In fact for samples Au04 and Au05,prepared with increasing NaOH/Au ratios, the reaction yieldincreased to 100%, while the mean diameters decreased fromabout 50 nm for Au03 to 20 nm for sample Au05. Low poly-dispersion indexes point out the uniform size distribution of theparticles, except in sample Au04 where a bimodal curve appeared(Fig. 1). As a consequence, the NaOH/Au ratio was set to 8,corresponding to a large excess with respect to stoichiometry (3)and resulting in a pH as high as 10.

Fig. 2 shows a TEM micrograph of the optimized sample Au05.The particles exhibit a spherical shape and a mean diameter of7 � 3 nm. No amorphous phase was detected. As reportedelsewhere31–34 the mismatch between TEM and DLS size isdue to various reasons. Firstly, the laser scattering techniquemeasures the hydrodynamic diameter inclusive of PVP andcoordinated molecules. Furthermore, since dispersed particlescan aggregate locally, the coarsest sizes in the DLS distributionconsist of agglomerates which strongly influence the mean size,resulting in a larger average primary particle size calculated

from the TEM micrographs. XRD spectra prove the presence ofthe metallic phase with the peak broadening typical of nanometriccrystallites (Fig. 3). NaCl, formed as a synthesis by-product,can be easily removed by washing the suspension throughultrafiltration.

UV-Vis spectra (Fig. 4) indicate that the typical surfaceresonance peak of gold nanoparticles is at around 520 nm.Increasing the amount of NaOH, a blue shift was observed andthe wavelength ranged from 549 nm for Au03, to 538 nm for Au04,and 521 nm for Au05. For sample Au03, prepared with the

Fig. 1 DLS particle size distribution for gold samples synthesized usingdifferent NaOH/metal ratios.

Fig. 2 TEM micrograph of the Au sample (left) and a histogram of themean particle diameter (right).

Fig. 3 XRD patterns of Au and Ag nanosols (Au05 and Ag02).

Fig. 4 UV-Vis spectra for Au samples synthesized with different NaOH/metal ratios (in brackets).

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lowest NaOH/Au molar ratio, two peaks at a higher wavelengthwere observed, probably due to the presence of coarser particles.In fact, for Au03, the reduction reaction was incomplete and thenucleation was too slow, thus providing aggregates.

Over time, the stability of all the synthesized gold sampleswas very good, as no precipitation occurred after several months.This is a key property to scale up the reaction to a continuousflow production plant.18 For gold nanosols the optimized sampleis Au05, characterized by the following experimental conditions:nGluc/Me: 2, nNaOH/Me: 8, temperature: 90 1C.

3.1.2. Silver. The parameters optimized for silver nanosolswere NaOH/Ag and glucose/Ag molar ratios. Optimum matchingbetween these ratios allowed us to obtain a stable nanosol witha homogeneous particle size distribution.

Ag+ + 12C6H12O6 + 1

2H2O - Ag + 12C6H11O7

� + 32H+ (4)

Table 2 outlines the synthesis parameters and characteriza-tion of the silver nanosols. Samples from Ag01 to Ag04 wereobtained with different NaOH/metal ratios and a fixed amountof glucose. The NaOH allows increasing of the reduction rate,not only because it enhances the reductive power of theglucose, but also because it generates a lot of Ag2O nuclei, aninstable compound which rapidly transforms into metallic Ag.As discussed by Chou and co-workers,28 this Ag2O precipitatemight serve as the nuclei for the subsequent reduction to Ag0,so lowering the energy barrier of formation. For this reason, thehigher amount of base, the smaller the particle diameter.Furthermore, it was observed that over a ratio of 1.5 the PDIvalues increased and the sols exhibited a slight opalescence,probably due to the formation of aggregates, which although notdetected by DLS, produced a precipitate during the 24–72 hoursafter synthesis. A stable sol is achieved only if equilibriumbetween the nucleation and growth phenomena is reached,like that for sample Ag02, with a NaOH/Ag ratio of 1.5. In fact,if the reduction rate is too slow, like in sample Ag01, only afew nuclei are generated and the growth step is favored,resulting in a particle size increase. On the contrary, if thereduction rate is too fast, a large fraction of small particles isformed, but the polydispersion increases because numerousnuclei are formed in multiple steps, thus promoting the aggre-gation phenomena.

UV-Vis spectra (Fig. 5) performed on the sols exhibit thesurface plasmon resonance band typical of nanosilver at400–410 nm. As expected, a higher amount of NaOH, for examplein sample Ag04, induces a blue shift due to the increasingamount of smaller particles, as observed by DLS. However, themeasured absorbance was lower probably because a lower numberof particles remained in suspension.

In order to evaluate the influence of the reducing agent,the glucose/metal molar ratio was changed from 0 to 2 and theNaOH/metal ratio was maintained at 1.5. Obviously, no reductionoccurred without glucose in sample Ag05 and for sample Ag2Oprecipitation occurred immediately. Likewise for the NaOHconcentration, the right amount of glucose corresponds tonucleation and growth equilibrium. The addition of the stoichio-metric amount of glucose in sample Ag06 promotes, if compared

with sample Ag02 obtained with an excess of the reducer,a clear particle size coarsening further confirmed by the redshift of the plasmon resonance peak (Fig. 5). In contrast,samples Ag02 and Ag04, prepared with a slight or a large excessof glucose, respectively, present very similar results, but withthe formation of marked opalescence. Sample Ag04 showed amarked opalescence, resulting in the clear lowering of theabsorbance band and followed by precipitation after a week.The only sample characterized by an optimal size distributiontogether with an outstanding time stability is sample Ag02,which has an average particle size of 20 � 7 nm with a sphericalshape (Fig. 6). Only silver is present in the XRD patterns (Fig. 3)with broad peaks due to the small crystallites, estimated to be10 nm. On the whole, the synthesis parameters used for Ag02has the best balance between reduction, nucleation and growthphenomena. Finally, in order to increase the solid loading ofthe sols, samples Ag08, Ag09, Ag10 and Ag11 were preparedat higher concentrations, from 0.5 to 4 wt%, exploiting theoptimized parameters of Ag02. The concentrated sols werecompletely comparable to sample Ag02, in terms of particlesize distribution, optical properties and stability over time(Table 2). Sample Ag11, in which the stability was maintainedbut larger particles, due to the very high Ag concentration wereproduced, is an exception. Reaction yield, tested for the optimizedsamples, is higher than 98%, proving that the reduction reactionwas almost complete.

Fig. 5 UV-Vis spectra for silver samples synthesized with different NaOH/metal and glucose/metal molar ratios.

Fig. 6 TEM micrograph (left) and a histogram of the mean diameter(20 � 7 nm) of sample Ag02 (right).

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For silver nanosols the optimized sample is Ag02, preparedwith the following experimental parameters: nGluc/Me: 1,nNaOH/Me: 1.5, temperature: 70 1C.

3.1.3. Copper. The main issue in synthesizing Cu nano-particles using weak reducing agents is achieving the totalreduction to the metallic phase, avoiding any stabilization ofthe intermediate oxide phases. In fact, all the performed trialsindicate that the reaction proceeds through the formation ofCu2O and only with strong reducing conditions can this oxidebe completely converted to metal.

Cu2+ - Cu2O - Cu0 (5)

From preliminary tests it was found that the temperaturenecessary to enable such a reduction of metal is 90 1C and theglucose/metal ratio needs to be set at 12, corresponding to avery large excess of the reducing agent. As a matter of fact, forlower temperatures or lower glucose/metal ratios, the prevailingphase was always Cu2O.

The effect of the NaOH/Cu ratio was explored for samplesCu01 and Cu03 (Table 3), whose difference in metal yield wasevident, proving that a higher NaOH/Cu molar ratio wasrequired in order to foster the reduction of the salt. Further-more, the reaction time was increased and the transformationfrom oxide to metal was promoted; samples in Table 3 weretreated for a time from 1 to 40 minutes and their phaseevolution is shown in Fig. 7. By increasing the reaction time,metal formation was observed with a high yield after 40 minutesof reaction. The XRD patterns, repeated after 3 months on sampleCu05 stored in the colloidal form, demonstrate that the oxide

phase only raised by one percent (from 1% to 2%) proving thatCu sols are highly stable even in aqueous media. On the otherhand, the particle size measured by DLS drastically increased inthe same period of time, and the occurrence of a thin oxide layerformed on the surface of the nanoparticles is observed (Fig. 8),which prevents further oxidation of the particle core. On theother hand, these systems showed a strong tendency to aggre-gate, as precipitation occurred within 24 hours after synthesisand for all samples the mean diameters are larger than 200 nm(Table 3). Furthermore, the particle size distributions areGaussian, therefore a progressive growth in size and polydispersityis observed for longer reaction times (Fig. 9); as expected, thepermanence at the synthesis temperature promotes the particleaggregation. TEM observations of sample Cu05 (Fig. 10) show aspherical shape of the primary particles with a narrow andhomogeneous distribution (mean diameter about 3 � 1 nm).In this case, TEM results were very different from the DLS data,

Table 3 Characteristics of Cu synthesized samples. All the samples wereprepared with a solid loading of 0.1 wt%a

SampleTime(min)

nNaOH/Cu2+

Cu phase(wt%)

+DLS(nm) PDI

Yield(%)

Cu01 10 9 0.7 — — —Cu02 1 18 9 258 0.8 —Cu03 10 18 40 215 0.2 98.8Cu04 20 18 59 236 0.3 —Cu05 40 18 99 314 0.4 98.8Cu05-t 40 18 98 670 0.6 —

a Abbreviations as in Table 1.

Fig. 7 XRD patterns of copper samples synthesized with increasing reac-tion time.

Fig. 8 Mechanism of copper formation during synthesis and its surfaceoxidation during storage.

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due to the strong aggregation phenomena occurring in thenanosols. For these samples, the DLS results are only reliable forthe aggregation phenomena and not for particle size. Concern-ing the reaction efficiency in terms of yield, data already indicatea total yield after 10 minutes for sample Cu03, even if at this timethe most formed phase was the oxide. This fact further provesthat the reduction to metal involves the formation of oxide;in the first minutes all the precursors turn into Cu2O, then themetallic phase is gradually obtained after longer reaction times.

For copper the optimized conditions are the following:nGluc/Me: 12, nNaOH/Me: 18, temperature: 70 1C and reactiontime of 40 minutes.

3.1.4. Palladium. Three different samples of palladiumnanosols were prepared by tuning the NaOH/metal molar ratio(Table 1). Sample Pd02 is the best in terms of stability over timeand particle size distribution, thus fitting the nucleation andgrowth equilibrium. In fact, for the lowest value of NaOH/metalratio the typical peak of PdO was detected by XRD (Fig. 11),indicating that the system had not completely reacted; on theother hand, the highest NaOH/Pd ratio produced an undesiredcoarsening of the mean diameter of the Pd nanoparticles. Theoptimal experimental conditions for Pd nanosols are the sameused for Au: nGluc/Me: 2, nNaOH/Me: 8, temperature: 90 1C.

3.2. Catalytic activity

The catalytic performance of the synthesized particles, for thereduction of 4-NP to 4-AP, was tested as a model reaction with

an excess amount of NaBH4. Accordingly, the reduction ratescan be regarded as being independent of the concentrationof NaBH4.35

This reaction is particularly easy to follow because only theproduct 4-aminophenol (4-AP) is formed and the extent of thereaction can be determined by measuring the change in UV-Visabsorbance at 400 nm. Without the catalyst, the solution is verystable and the absorption remains unchanged confirming theabsence, during our tests, of non-catalytic reactions.

All the synthesized nanoparticles acted as effective catalystsfor the hydrogenation of 4-nitrophenol in the presence of NaBH4.In Fig. 12 two typical degradation spectra of 4-nitrophenolate arereported for Au (a) and Ag (b). Adding the synthesized sols to thereaction mixture causes a gradual decoloration of the solutionand ultimately a bleaching of the yellow colour, due to theformation of 4-AP, which presents no peaks in the visible regionand only has a weak absorption of violet wavelength stemmingfrom its peak at 300 nm.

The results are summarized in Table 4 in terms of the kineticconstant, conversion and turn over frequency (TOF, defined as

Fig. 9 DLS particle size distribution for copper samples obtained usingdifferent synthesis times.

Fig. 10 TEM micrograph (left) and a histogram of the mean particlediameter (right) of sample Cu05.

Fig. 11 XRD patterns of the palladium samples. (D = Pd, � = PdO, * = NaCl.)

Fig. 12 Typical evolution of UV-Vis spectra as a function of time duringthe hydrogenation of 4-NP using (a) Au and (b) Ag catalysts. Conditions: Auor Ag = 6.9 � 10�6 M; 4-NP = 2.8 � 10�5 M; NaBH4 = 4.5 � 10�2 M.

Table 4 Catalytic performances of the optimized nanosolsa

Sample IT (s) K � 10�2 (s�1) Conversion (%) TOF � 10�2 (s�1)

Au05 330 1.70 100 1.30Ag02 720 0.21 89 0.67Cu05 240 0.20 96 0.26Pd02b 22 1.95 99 26.00

a IT: induction time; K: kinetic constant, TOF: turn over frequency (s�1).b The catalytic activity of Pd nanosols were measured with a lowerPd/4-NP ratio.

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the number of moles of reduced 4-NP per mole of catalystper second). As reported elsewhere36–38 the reaction does not startimmediately after the addition of the catalyst, but only after anoticeable time lag, i.e., the induction time (IT). This may beascribed to several reasons: a thin oxide coating on the particlesurface or the presence of oxygen dissolved in water. In order tounderstand the main origin of the detected IT, different experi-mental settings on gold nanoparticles were tested, by changingthe order of the reagents added to the catalytic reaction.

The induction time, as reported in Table 4, corresponds tothe standard addition order, which is NaBH4, 4-NP and thenthe catalyst. The order was then modified to: NaBH4, catalystand 4-NP. Using this order, the NaBH4 should reduce thesurface oxide layer, if present, on the metal nanoparticles.Anyway, the results show only a partial decrease of the IT forgold, from 330 to 275 s, proving that the presence of surfaceoxidation is not the main reason for the reaction delay. Instead,the IT was only completely reduced for all the prepared samplesby bubbling nitrogen for 1 minute before the addition of NaBH4

and adding the reagents in the following order: catalyst, 4-NPand finally NaBH4. Potentially, this order allowed for theremoval of the oxygen dissolved in the solution which wouldinvolve NaBH4 in a competitive reaction with 4-NP.39

The hydrogenation reaction can be divided into two mainparts; firstly, the decomposition of NaBH4 on the surface ofnanoparticles, producing H-atoms, which in turn are availableto the 4-NP molecules and finally, the addition of protons to the4-NP and the simultaneous removal of its oxygen, following oneof the typical mechanisms of heterogeneous reactions such asthe Eley–Rideal or the Langmuir–Hinshelwood.38

Here, metal nanoparticles act as a H carrier, adsorbing hydro-gen from NaBH4 and releasing it during the reduction of 4-NP. Asreported by other groups working on polymer supported nano-particles,40 our tests confirm the best activity of the palladium sol,probably due to its intrinsic characteristics of efficient hydrogenrelay.41–43 In fact, due to the high reactivity of the Pd sol, the testswere performed by lowering the concentration of the catalyst, tobetter follow the reaction. For this reason, the k value for Pd is notdirectly comparable with the others. In any case, a comparison ofcatalysts can be carried out on the basis of the turnover frequency;the TOF value of palladium is one and two orders of magnitudehigher than those of gold and copper, respectively.

According to our experiments, the observed ranking in thehydrogenation of 4-NP is: Pd 4 Au 4 Ag 4 Cu which is inagreement with the literature.40,44 All these metals are commonlyexploited for their catalytic properties, nevertheless, Pd and Au areused the most for hydrogenation reactions. On the other hand,silver and copper exhibit the worst reactivity for the reactioninvestigated here. However, considering that particle size also affectsthe catalytic activity of the nanoparticles, any direct comparison ofdifferent samples, which do not have the same mean diameter isdifficult. Although the particle size may have an effect on the finalcatalytic performance, the fact that Cu nanoparticles have a smallerdiameter (by TEM) clearly indicates that the main influence on thecatalytic activity stems from the intrinsic properties of each metal,such as electronic structure and affinity toward reagents.

4. Conclusions

A simple and versatile green method was developed to producedifferent noble metals, and the same procedure was success-fully applied for preparing Au, Ag, Cu and Pd nanosols with atotal reaction yield. The high versatility degree, together with itseco-friendly characteristics, make this process an ideal candi-date for industrial scale up.

Optimum control of reduction, nucleation and growthphenomena was achieved for each material throughout an extensiveoptimization process, which was achieved by tuning of the synthesisparameters and widely supported by UV-Vis, DLS, TEM and XRDanalyses. Optimal matching of NaOH/metal and glucose/metaltogether with time and temperature was confirmed to be key forthe control of the products in terms of size, time stability andreaction yield, which were assessed and maximized for each sample.

Except for Cu nanoparticles, the prepared samples showedan excellent stability over time, even at very high solid loading,as shown by the Ag sample with a concentration of 4 wt%.

Furthermore, the prepared particles acted as effective catalystsin the reduction of 4-NP to 4-AP; all the materials showed a typicalinduction time (IT). Complete reduction was achieved by changingthe order of reagent addition and by bubbling nitrogen in thesolvent before adding the NaBH4. Concerning the catalytic activity,significant differences were observed among the samples; particu-larly the ranking for the hydrogenation of 4-NP which was observedto be: Pd 4 Au 4 Ag 4 Cu. The best catalytic performance wasobserved for Pd, probably due to its intrinsic characteristic efficienthydrogen relay, while silver and copper showed the worst reactivity.However, comparison among the different samples is very complexbecause several aspects can affect the reactivity such as: the particlesize, the particle shape, the presence of defects on the surface andthe intrinsic properties typical of each metal, linked for exampleto the electronic structure. For these reasons the preparation ofbimetallic structures, which is now ongoing using the sameprocedure, may result in an interesting way to create catalystswith synergetic properties and unexpected behaviors.

Acknowledgements

The research leading to these results received funding fromColorobbia S. p. A. Research Centre, from the EuropeanCommunity’s Seventh Framework Programme (FP7/2007–2013)under the Grant Agreement no. 280716 and from FondazioneBanca del Monte and Cassa di Risparmio of Faenza.

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