Anales AFA Vol. 33 Nro. 4 (Enero 2023 - Marzo 2023) 103-111

https://doi.org/10.31527/analesafa.2022.33.4.103

Materia Condensada

FORMACIÓN DE NANOPARTÍCULAS DE PALADIO POR EL MÉTODO DE LOS

POLIOLES: INFLUENCIA DE LAS CONDICIONES ALCALINAS

FORMATION OF PALLADIUM NANOPARTICLES BY THE POLYOL METHOD:

INFLUENCE OF ALKALINE CONDITIONS

J. F. Sánchez M. 1, H. A. Ritacco 1, M. D. Sánchez *1

1 Instituto de Física del Sur (IFISUR), Departamento de Física, Universidad Nacional del Sur

(UNS), CONICET. Av. L. N. Alem 1253, B8000CPB - Bahía Blanca, Argentina.

Autor para correspondencia: * msanchez@uns.edu.ar

Recibido: 13/06/2022; Aceptado: 01/07/2022

ISSN 1850-1168 (online)

Resumen

Se estudió el efecto del hidróxido de sodio (NaOH) sobre el tamaño de nanopartículas de paladio (Pd)

obtenidas por la ruta del poliol simple. Las nanopartículas se sintetizaron a temperatura ambiente

utilizando cloruro de paladio (II) (PdCl2) y NaOH disuelto en etilenglicol (EG) como promotores de

la reacción de reducción. No se utilizaron agentes protectores ni estabilizadores. Monitoreamos la

cinética de reacción y el crecimiento de las nanopartículas por espectroscopía UV-vis y su

cristalinidad por difracción de rayos X (XRD) en función de la concentración de NaOH. El tamaño

de los cristalitos se evaluó a partir del patrón de difracción. Encontramos que el crecimiento de

nanopartículas está fuertemente influenciado por la relación molar NaOH:Pd. Se obtuvieron tamaños

de cristalitos de 2 a 24 nm para proporciones molares de 1 a 33. A concentraciones más bajas de

NaOH, se encontró que el proceso de nucleación y crecimiento de las nanopartículas estaba controlado

por la reducción de los precursores de iones Pd. A concentraciones más altas, la reducción intermedia

de las especies de Pd-Cl-OH determina la tasa de crecimiento de las nanopartículas que da como

resultado la formación de nanopartículas de tamaño final más pequeño.

Palabras clave: nanopartículas de paladio, hidróxido de sodio, método de polioles, coloides de

paladio.

Abstract

The effect of sodium hydroxide (NaOH) on the size of palladium (Pd) nanoparticles obtained by the

simple polyol route was studied. Nanoparticles were synthesized at room temperature using

palladium(II) chloride (PdCl2) and NaOH dissolved in ethylene glycol (EG) as reduction reaction

promoters. No protective agents or stabilizers were used. We monitored the reaction kinetics and the

growth of the nanoparticles by UV-vis spectroscopy and their crystallinity by powder X-ray

diffraction (XRD) as a function of NaOH concentration. Crystallite size was evaluated from the

diffraction pattern. We found that nanoparticle growth is strongly influenced by the NaOH : Pd molar

ratio. Crystallite sizes from 2 to 24 nm were obtained for molar ratios ranging from 1 to 33. At lower

concentrations of NaOH, the nucleation and growth process of the nanoparticles were found to be

controlled by the reduction of the Pd ion precursors. At higher concentrations, the intermediate

reduction of Pd-Cl-OH species determines the nanoparticle growth rate resulting in the formation of

the smallest final size nanoparticles.

Keywords: Palladium nanoparticles, Sodium hydroxide, Polyol method, Palladium colloids.

1. INTRODUCTION

The production of nanoparticles from different materials has been a topic of wide interest for

several years. The improvement of techniques, precursors and particle size control have been the main

objectives of previous works [1]. Among the different synthesis techniques, the most popular has been

the polyol method. Presented as a simple, versatile and easy to implement method [2], it has been

successfully applied to a variety of metals [3]. In particular, palladium (Pd) nanoparticles have

received much attention, given their multiple applications, especially in catalysis [4, 5] and

biomedicine [6]. Ethylene glycol (EG) has been commonly used as a solvent and reducing agent in

the production of Pd nanoparticles in a polyol medium [7]. The decomposition of EG produces a

reduction, and the new metallic phase initiates the nucleation and growth that will produce

nanoparticles [8]. However, the reducing power of EG is limited, hence increasing the system

temperature is used to bolster oxidation [9]. The addition of promoting compounds such as hydrazine

[10] or sodium borohydride (NaBH4) [11] is a common practice to facilitate the decomposition of EG

and shorten reaction times. In this way, it is possible to increase the synthesis reaction rate, but it

becomes more difficult to control stabilization and material dispersion in these conditions [12, 13].

Size could potentially be tuned by careful control of temperature, but high temperatures have been

associated with irregular particle growth [14]. Because of these difficulties, the addition of promoters

and surfactants for aggregation control during synthesis has also become a common practice [15].

Although the results of such products are promising, it has been found that chemical residues of

surfactants were chemically adsorbed on the surface of the nanoparticles, potentially compromising

the effectiveness of the produced material for catalytic applications [16]. Wang et al.[17] showed that

it is possible to obtain stabilized noble metal nanoparticles with good dispersion using a protector-

free polyol method. Arora et al. [18] demonstrated that even polyol can act as a stabilizer and that by

controlling solely the reaction conditions it is possible to synthesize monodispersed Pd nanoparticles.

Quinson et al. [19] obtained nanoparticles in surfactantfree EG and studied size control with sodium

hydroxide (NaOH) as a promoter. These studies used the combined effects of promoters and

temperature control to obtain small particle sizes. However, Chen et al. [20] explored the possibility

of using only NaOH to promote nanoparticle synthesis. Since alkaline conditions promote the

reduction of noble metals, it is accepted that the role of NaOH is that of a pH regulator [12].

Nevertheless, it has been observed that hydroxide ions might have an additional catalytic function

[21], affecting the reaction kinetics and therefore the particle size [12, 22]. The amount of hydroxide

(OH−) employed is then expected to become a key element in size controlled colloidal synthesis [19,

23]. The aim of the present study was to analyze the influence of hydroxide concentration on the

formation kinetics of Pd nanoparticles by the simple polyol method. No stabilizers were used, and

temperature was kept at room temperature to discard possible effects on the synthesis and particle

size, apart from the OH− concentration. We followed the evolution of the metallic precursor reduction

and nanoparticle growth with UV-vis absorbance spectroscopy. Powder X-ray diffraction (XRD) was

used to study the crystallinity, morphology and particle size evolution as a function of NaOH

concentration. In this context, we found that at lower OH− concentration both nucleation and grow

process are regulated by the reduction of the Pd precursor given that the induction times are shorter.

As a consequence, few nuclei are created resulting in the formation of larger final size nanoparticles.

The increase in the OH− concentration leads to an increase in the induction time as a result of the

formation of Pd-Cl-OH intermediate species. The dissolution of this species regulates the formation

of PdO by increasing the number of nuclei and giving the smallest final size nanoparticle,

independently of the subsequent growth processes that could take place.

2. METHODS

Materials

Palladium(II) chloride (PdCl2, 99.99%) metal precursor from Spex Industries, Inc. USA, Sodium

hydroxide (Na(OH), ≥99%) from Merck, KGaA, Germany and Ethylene Glycol anhydrous (EG,

99.8%) from Sigma-Aldrich, Co. USA were used in the synthesis. Acetaldehyde (99.5%), from

Honeywell Riedel-de HaënTM, USA, was also used as a standard of comparison. All reagents used

were analytical grade, stored at room temperature and used without additional purification procedures.

Synthesis of Pd nanoparticles

We obtained Pd nanoparticles by the well-known polyol method [20]. A stock solution of PdCl2

(10 mM) in EG was prepared with the addition of concentrated hydrochloric acid (∼ 1 µ l HCl : 1 ml

EG) to enhance the PdCl2. By taking different volumes of stock solution and adding NaOH solution

in EG up to a total volume of 5 ml, samples with [NaOH]/[Pd] molar ratios (R) from 1 to 33 were

obtained. The NaOH solution was prepared by diluting a known amount of the base in EG at room

temperature. Before use, all glassware was carefully and thoroughly cleaned and dried.

Mixing was carried out at room temperature and under vigorous stirring. Several minutes after

mixing the reagents, a black colloid deposit containing the nanoparticle materials was observed. To

extract the nanoparticles, the colloid was washed five times with acetone (1:5 vol) in a sonication

(330 W, 50 Hz)-centrifugation (10000 rpm, 10 min) sequence. At each wash, the supernatant was

removed and replaced with fresh acetone. The final material was redispersed in alcohol.

Characterization

Ultraviolet visible absorption spectroscopy (UV-vis).

Synthesis product formation and reaction evolution were followed by UV-vis spectroscopy.

Reactions were carried out in 1 cm quartz cells (1 cm optical pathway) in an Ocean Optics USB2000

spectrophotometer. To ensure a complete and uniform mixture of the reagents, a magnetic stirrer was

used during the spectrum collections. The measurement started when the NaOH solution was added,

at a spectrum measuring interval of 0.1 sec from the 180 to 800 nm wavelength. Further measurements

of the final solution were performed to determine the amount of remaining Pd. In these measurements,

2.5 ml samples were centrifuged (15000 rpm, 10 min) to remove solids, and the supernatant was

analyzed as described above. An acetaldehyde/EG mixture was also analyzed as a reference standard.

Transmission electron microscopy (TEM).

Samples were prepared using droplets of the colloid. These droplets were washed with acetone

and redispersed in methanol. A drop of this suspension was deposited on a carbon-coated copper

microscopy grid (400 mesh). Grids were dried, and the excess liquid was removed with tissue paper.

The grids thus prepared were analyzed using a JEOL-100CX II TEM at 100 kv and a FEI-HR TEM

(TalosTM F200X) at 200 kV.

X-ray diffraction (XRD).

The crystalline structure of the samples was analyzed by powder XRD using a Panaytical

Empyrean III diffractometer with a CuKα (1.5418 Å) radiation. Diffractograms were obtained in the

interval 30 ≤ 2θ ≤ 90, with a step size of 0.02◦ and a speed of 1 sec/step. Samples were prepared by

drying a few milligrams of the solid material obtained from the colloid after washing.

3. RESULTS AND DISCUSSION

Formation of Pd nanoparticles

The polyol method employed started with the dissolution of the Pd salt and progressed through

the commonly accepted reactions depicted in Fig. 1. In the particular case of PdCl2, acidification of

the medium is required for complete dissolution. The late addition of NaOH solution would enhance

EG oxidation into acetaldehyde, reducing Pd and producing 2,3-butanedione as main oxidation

product.

The reaction progress can be observed by the naked eye. The initial PdCl2/EG solution (bright

orange) changes color according to the concentration of the added NaOH. A black dispersion, typical

of reduced Pd colloids, can be observed as the NaOH solution is added. This first Pd reduction is fast

FIG. 1: Reduction reaction of metallic ions by ethylene glycol oxidation.

and precedes the nucleation and growth process of nanoparticles. The Pd/EG solutions analyzed by

UV-vis showed two well-defined peaks at 330 and 437 nm. The intensity of these peaks is directly

correlated with the Pd2+ concentration [24], so their attenuation indicates the reduction of Pd2+ and,

therefore, the formation of metallic palladium (Pd0). Thus, we monitored the reaction progress by

following the peak with the highest intensity. In this wavelength interval, no interferences from EG

or NaOH/EG solution are expected.

In preliminary experiments, we observed that the Pd reduction and nanoparticle production were

possible in the absence of NaOH. However, the reaction rate at room temperature was very slow. The

UV-vis spectra from these Pd/EG solutions show different features in comparison to a freshly

prepared solution; however, after several weeks no significant amount of sedimented Pd0 was

obtained.

The spectra (between 190 - 540 nm) of the supernatant solutions at different NaOH concentrations

are shown in Fig. 2. From the PdCl2/EG 10 mM reference solution, the minimum Pd concentration

would be reached at R ≈ 4. A precipitate of clustering particles that sediment and leave a translucent

supernatant can be obtained at these concentrations, which implies that all available Pd is reduced and

the Pd2+ peak should not be present. Instead of this peak, two well-defined signals are detected at 220

and 282 nm in agreement with those obtained from the NaOH/EG solution at equivalent

concentrations. As the NaOH concentration increases, the intensity of the peaks increases; the signals

may correspond to by-products formed during the synthesis, probably an acetaldehyde. Analysis of

the EG/acetaldehyde mixture prepared with pure reagents showed peaks at the same wavelengths,

which suggests the presence of remaining acetaldehyde not consumed during the reaction. In this

scenario, the action of NaOH would be mainly catalytic (providing OH− ions), increasing the

production of acetyls by EG dehydration [25, 26], according to the first part of the reaction scheme

presented in Fig. 1. The resulting high concentration of acetyls increases the possibility of reduction

of Pd ions and the consequent increase in conversion. There is little evidence in the literature

confirming the presence of acetaldehyde, as it is an unstable compound [27] that might oxidize into

other species easily. In spite of this, Chen et al. [20] identified -CHO groups by infrared spectrometry

in a Tollens reaction and Joseyphus et al. [28] detected acetaldehyde using UV vis with a 2,4-

dinitrophenyl hydrazine test solution. The formation of dioxane, glycolaldehyde and carboxylic acid

has also been suggested, specially at high temperature [23, 29]. A complete discussion on the presence

and quantification of the reaction by-products is beyond the scope of the present study, and further

research is needed.

Crystalline structure, particle size and morphology

The crystalline structure, average particle size and morphology of the obtained material were

analyzed by XRD and TEM. Fig. 3 shows the diffraction patterns for different NaOH concentrations.

Planes (111), (200), (220), (311) and (222) at Bragg angles 2θ of 40.2◦, 46.6◦, 68.0◦, 82.0◦ and 86.5◦,

respectively, are identified by comparison with the Crystallography Open Database, COD file

1534921, indicating that the samples are composed of Pd0 in a face-centered cubic (fcc) structure.

These results are in good agreement with those reported by Long et al. [30] for Pd nanoparticles in

EG. In some samples, we observed the presence of sodium oxalate (Na2C2O4), which could be

removed after the purification process. Although the formation of Pd(OH)2 has been suggested [20],

this compound was not detected in our samples.

TEM images showed that the material is formed by irregularly shaped aggregates, as seen in Fig.

4 for the case of R = 10, which is representative of the overall samples. Nevertheless, it can be assumed

that these clusters are formed by smaller particles, although it is difficult to make a proper estimation

of the particle size mainly due to the overlapping caused by the aggregation itself. These aggregates

FIG. 2: UV-vis absorbance (left scale) and remaining Pd concentration (right scale) of Pd-EG solutions for

different R (0-10, as indicated): after completion of the reaction (black solid lines); without NaOH and stored

for four weeks (red dashed double dotted line); fresh acetaldehyde-EG solution (blue dashed dotted line); and

fresh NaOH-EG solution for a similar concentration of R = 4 (green short dashed line). Conditions: [Pd]0 =

10 mM, VTotal = 5 ml, room temperature.

FIG. 3: XRD patterns of Pd nanoparticles for different R, as indicated.

were already described in the literature and attributed to the self-assembling nature of oxalate dianion

[31]. In this context, XRD data were also used to calculate the crystalline domain size to assess the

effect of NaOH concentration on the size of the nanoparticles. The average crystallite size can be

estimated from the width of the powder diffraction peaks. As can be seen, the initially narrow XRD

peaks increase in width as the NaOH concentration increases (Fig. 3). This correlation may suggest a

direct relationship between nanoparticle size and NaOH/Pd ratio, even when it is difficult to rule out

that the nanoparticles may consist of more than one crystallite.

Fig. 5 shows the average crystallite size as a function of R, obtained from the Williamson-Hall

plot [32, 33]. The average size decreases rapidly from 24 to ∼4 nm for R < 6, remaining an

approximately constant size of about 2.5 nm for higher NaOH concentrations. This behavior is in

good agreement with those published elsewhere [20, 34].

To obtain more insight about nanoparticle morphology, we evaluated the crystallite sizes for each

diffraction peak by applying the Scherrer equation [32]. The results for different R values are plotted

in Fig. 6. As expected, the average size for each plane increases as the proportion of the Pd precursor

decreases. Furthermore, a noticeable relative growth of the average crystallite size is observed for

planes (111) and (222), while a decrease is expected according to the Bragg angle increase. This result

suggests that the crystallite has a preferential growth in the direction of plane {111} [31, 35, 36].

However, for R less than 3, the differences between the sizes obtained for the plane (111) and the

ones obtained for the plane (222) become larger as R decreases; for values greater than 3, the

difference

FIG. 4: TEM image of Pd nanoparticle aggregates obtained for R = 10.

FIG. 5: Average size of Pd nanoparticles from XRD analysis.

becomes negligible. Therefore, it can be assumed that the nanoparticles have a nonspherical shape

and that their anisotropy increases as R increases.

FIG. 6: Average crystallite size of Pd nanoparticles for different R obtained from XRD analysis by applying

the Scherer equation.

Nanoparticle formation process

The kinetic study of nanoparticle formation was performed by in situ UV-vis data analysis of the

reaction, without stabilizers. The omission of stabilizing agents allowed the analysis of the process

without diffusional or mass transfer limitations that might have originated by the use of these

substances. In this way, the maximum formation speed of the nanoparticles can be determined [37].

Fig. 7 shows the absorbance at 437 nm as a function of time for different R values. This process occurs

differently in each case, and the plotted curves vary according to the time interval and the NaOH

concentration used. The identified stages are in agreement with typical nanoparticle formation

mechanisms [38, 39]. The reduction reaction starts at the time tad when NaOH is added to the

PdCl2/EG reference solution. Thereafter, the absorbance sharply drops due to a combination of

dilution and Pd reduction effects. At this stage, seed nuclei appear and would eventually develop into

nanoparticles. Following this stage, the absorbance increases owing to the dispersion produced by the

nuclei growth. This growth continues until the ripening phase is reached, at which point Pd2+ ion

concentration is minimum and larger particles would continue to grow at the expense of smaller ones.

This particle coarsening has been studied in the formation of metallic nanoparticles [40, 41]. As the

reagent concentration increased, the time difference between nucleation and growth was greater and

maturation was clearly observable. Under these conditions, the results are similar to those reported by

Tojo et al. [42] in obtaining metallic particles in a confined environment. When the amount of reagent

is increased, the formation of nuclei is favored and the amount of Pd+ decreases. However, if the

reducing agent is not strong enough, unstable nuclei will form, redissolve and condense on the surface

of more stable ones. For this reason, the total number of particles decreases causing a dispersion

reduction, which eventually leads to an absorbance drop. Finally, the absorbance signal becomes

noisy due to the effect of continuous aggregation and precipitation of larger particles. In all

experiments, a rapid reduction step was observed, with the minimum absorbance being recorded a

few seconds after the start of the reaction. However, the reaction was not completed in all cases. At

low NaOH concentrations, this variation is approximately 45%, and increases to 93% at the maximum

concentration. It has been reported that reduction processes would have a direct effect on the

formation of nuclei, and therefore also on the final nanoparticle sizes [43].

FIG. 7: Evolution of the absorption for reactions with different concentration of NaOH as a function of time

at room temperature, [Pd]0 = 10mM, Vtotal = 5 ml.

To study these processes, the first step was addressed using the slope of the curve (absorbance

versus time) at the beginning of the reaction. Experimental data for the different reactions are

presented in Fig. 8. Initial rate (starting slope) values along with the R concentrations values are

presented in Fig. 9a. and the final average nanoparticle size along with initial rates in Fig. 9b. These

results suggest the existence of two underlying mechanisms that would affect nanoparticle size in

relation to the NaOH concentration. At lower concentration (R = 0 to 2) the reduction rates increase

from 0.5 s−1 up to 2.4 s−1 and the average particle size drops from about 18 nm up to ∼ 10 nm. For

higher values of R, the reduction rate decreases to a minimum of 0.7 s−1, which was not altered with

further increases of R. In agreement with this behaviour, the particle size decreases to a minimum of

around 3 nm for the highest R values tested (Fig. 9b).

Once the absorbance minimum was reached, an induction period became apparent, which precedes

an increase in the UV-vis signal (see Fig. 7). We defined this induction period (ti) as the time in which

absorbance does not vary, that is, from – dA/dt ≈ 0 until the rate of increase is continuous and greater

than 0.2 s−1 (which is the minimum absorbance change that can be detected by our equipment).

FIG. 8: Absorbance (λ = 437 nm) vs. time of the initial region of the Pd2+ reduction curves at different R.

FIG. 9: (a) Initial rate for different R ratios. (b) Particle size function of the initial rate, (R value is indicated).

Fig. 10 shows how ti increases with R and a linear correlation from R = 2. The maximum value of

ti was 20 min, achievable at R = 33, after which a barely perceptible (∼ 0.2 s−1) increase in absorbance

occurs, probably due to the noise produced by the aggregation of the nanoparticles. These results

indicate that as the NaOH concentration increases, ti also increases, which is indicative of the

formation of small-sized nanoparticle.

At the end of the induction period, the absorbance increase rate is different for each case. For R

< 3, ti is very short and the experimental data can be represented by the first-order kinetic law, dA/dt

= k A, where k is the kinetic constant. A plot of ln A against time would correspond to a straight line

(see Fig. 11a), being the slope the rate constant. A similar experimental behavior was observed by Cai

[44] in silver nanoparticle formation kinetics and by Sau [45] in a seed particle-mediated synthesis.

When R is increased (3, 4), the reduction increases, a longer induction time is recorded and finally a

rapid increase in absorbance is observed. At this point, the nucleation and growth stages begin to be

distinguished separately. For higher R (6, 8), the induction time continues to increase and the growth

kinetics can be fitted using a sigmoidal curve, suggesting an autocatalytic reaction. If this autocatalytic

stage exists, then ln [a / (1-a)] will change linearly with time, where a = Af /At and At and Af are the

absorbance at t and final times, respectively [46, 47]. This is plotted in Fig. 11b; the growth is actually

autocatalytic, which becomes less pronounced with the increase of R. At high R values, the growth

rate and the slope of the curve are so low that the absorbance versus time could be described by a

straight line (linear growth, Fig. 7).

FIG. 10: (a) Induction time for different R ratios. (b) Particle size function of the induction time, (R is

indicated).

FIG. 11: (a) Absorbance as a function of reaction time (first-order growth). (b) Plots of ln(a/(1-a) as a function

of reaction time (autocatalytic growth).

Influence of NaOH concentration

The different behaviors seen in Fig. 7 could be explained on the basis of the initial NaOH

concentration. This concentration would determine the reduction reaction rate and the following

nucleation and growth steps. Initially, when R < 2, the reaction rate is low. The concentration of Pd

ions remains high given the weak reduction character of the solution, thus few nuclei will form. Then,

the first particles formed act as catalysts, attracting precursor ions to their surface, where they would

be subsequently reduced [48-50]. The low OH− concentration delays the reduction process, and

therefore could increase the adsorption of precursor ions to the newly formed nuclei. Furthermore,

the absence of capping agents or other protective agents allows easy access of ions to the surface of

the catalytic nuclei. In addition, since there is a high material flow to the surface, both reduction and

growth occur simultaneously, speeding up the increase in the average size of the newly formed

particles. Reported studies have proved that high concentrations of a strong reducing agent promote

fast initial reduction and the formation of a large number of small nuclei. In other words, the average

particle size decreases as the concentration of the reducing agent increases. In the present study. we

found that the initial rate decreases as the amount of hydroxide increases (Fig. 10a). This relationship

could be indicative of a deceleration of the reduction process; consequently, the nanoparticles formed

will tend to grow. However, no larger nanoparticles were observed. To study this phenomenon, we

followed the evolution of the UV-vis spectra of each reaction as we increased the NaOH concentration

from R = 0 to 33. Fig. 12a shows the absorbance spectra corresponding to zero induction time, ti, for

each reaction. As the reaction progresses from the NaOH addition (tad) to the beginning of the

induction period, ti, for R > 2 the intensity of the 437 nm peak decreases, and also shifts to lower

wavelengths as shown in 12b for the extreme case of R = 33. Initially, this occurs by nonisosbestic

behavior, implying that the starting Pd (Pd2+) is transformed into an intermediate ionic complex [51]

(possibly of the Pd-Cl- OH type), which will eventually decompose and reduce to Pd0. The speciation

of Pd has been researched [52, 53], and published results show that the species distribution is highly

pH dependent. Under acidic conditions, only [PdCl4]−2 type ions are present [54]; as pH increases,

these species gradually disappear and start forming complexes of [PdClm(OH)n]2−-type composition

that end up being the predominant species [55]. Mechanisms for the formation of these complexes, in

which NaOH is used as the reducing agent, have been proposed in the polyol reduction process [56].

In the present study, the dissolution of Pd chloride was performed in acidic medium (pH = 2), so the

predominant complexes would be tetrachloropalladates. Thus, the observed variations in the UV-vis

are due to the decrease in the concentration of these compounds. For higher R values, along with the

reduction, the formation of chloro-hydroxo complexes occurs, showing that absorption bands are

lower wavelengths. The UV-vis signal variation then corresponds to both the reduction of Pd

[PdCl4]−2 and the formation of a new complex, decreasing the intensity and shifting the signal,

respectively. This behavior is pronounced as the concentration of OH− ions increases, so the initial

reaction rate is lower [57]. The formation of intermediates also limits growth, decreasing the average

particle size. The strong reducing character of the solution (R > 3) creates many small nuclei, which

do not have the ability of attracting precursor ions to trigger autocatalytic surface growth, as these

would not be available. The intermediates compete for these ions and become a regulated supply store

of Pd2+ [58].

FIG. 12: (a) Variation of the characteristic Pd absorption bands for different R when the induction time is

equal to zero. (b) Maximum absorbance peak evolution for R = 33.

The slow dissolution of the intermediates would control the eventual Pd reduction [28], governing

the formation of new nuclei and the increase in size of the nanoparticles [56]. This behavior will

produce a lower growth rate, as indicated by longer induction times, and a steeper slope in the growth

curve as R increases.

These observations indicate that NaOH not only regulates the pH in the EG nanoparticle synthesis

reaction. It Initially acts by promoting the EG oxidation, which will increase the rate of the reaction,

therefore reducing the average size of the formed nuclei. At the same time, the remaining hydroxides

could interact with metal ions, capturing them from the solution and preventing them from

participating in the autocatalytic growth step. The process involves a simultaneous reduction of Pd

ions and the formation of intermediate species, which are determining factors in the control of the

reduction reaction and the size of the nanoparticles.

4. CONCLUSIONS

Palladium nanoparticles were obtained by NaOH-promoted EG reduction at room temperature.

The nanoparticle formation process was analyzed by UV-vis spectroscopy, and the influence of the

NaOH concentration was studied. A typical three-stage mechanism (reduction, nucleation and

autocatalytic growth) was observed in the formation of the nanoparticles. The reaction involves the

formation of an intermediate Pd species (Pd-Cl-OH), which decomposes into Pd0. The dependence of

the average particle size on the initial OH concentrations is attributed to the formation of this

compound, which competes for the available Pd2+ ions and thus modifies the growth kinetics. Hence,

the hydroxide acts as a catalyst that promotes EG decomposition and simultaneously as a regulator of

the amount of Pd ions available for reduction. The overall effect observed is a control of the average

nanoparticle size, which is in the range of 24 to 4 nm as R increases to a value between 4 and 6, and

then stabilizes around 3 nm for higher values of R. As we show by means of XRD analysis, the

resulting nanoparticles have a preferential growth along the plane {111} direction; therefore, they are

nonspherical in shape with an increasing anisotropy as R decrease. Our results show that for this

synthesis process increasing the concentration of NaOH lengthens the induction times, resulting in

the generation of more nuclei, which determines the smallest final particle size independently of

subsequent growth processes.

12 
Acknowledgements 
This work was supported by the Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT), 
Argentina  [PICT-2019-3185];  the  Consejo  Nacional  de  Investigaciones  Científicas  y  Técnicas 
(CONICET), Argentina [PIP GI n◦ 11220200101754CO]; and Universidad Nacional del Sur (UNS), 
Argentina [PGIUNS 24/F080 and 24/F082]. 
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