On the role of Pb 0 atoms on the nucleation and growth of PbSe and PbTe nanoparticles

RESEARCH PAPER

On the role of Pb0 atoms on the nucleation and growthof PbSe and PbTe nanoparticles

Domingo I. Garcia-Gutierrez • Lina M. De Leon-Covian •

Diana F. Garcia-Gutierrez • M. Trevino-Gonzalez •

M. A. Garza-Navarro • S. Sepulveda-Guzman

Received: 5 November 2012 / Accepted: 28 March 2013

� Springer Science+Business Media Dordrecht 2013

Abstract In this contribution, a nucleation and

growth mechanism of PbSe and PbTe nanoparticles

are proposed. The formation and growth of PbSe and

PbTe nanoparticles during their reaction synthesis were

studied and followed using transmission electron

microscopy, and their related techniques. In the synthe-

sis method, trioctylphosphine-selenide and telluride

were used as the chalcogen precursors, while lead oleate

was employed as the lead precursor. Different synthesis

conditions were tested to assess the effect of varying the

reaction time, lead to chalcogen ratio, reaction temper-

ature, and lead oleate concentration. The synthesized

nanoparticles were characterized by means of electron

diffraction, energy dispersive X-ray spectroscopy,

scanning transmission electron microscopy, and elec-

tron energy loss spectroscopy, to obtain information

related to their morphology, crystal structure, and

composition. The experimental results suggest that the

growth of the lead chalcogenide nanoparticles greatly

relies on the reduction of Pb2? ions to Pb0 atoms at early

reaction times; this reduction of the lead precursor is

evidenced by the formation of Pb nanoparticles with

sizes between 1 and 3 nm under certain synthesis

conditions. These Pb nanoparticles gradually disappear

as the reaction progresses, suggesting that the reduced

Pb0 atoms are able to contribute to the growth of the

PbSe and PbTe nanoparticles, reaching sizes between 8

and 18 nm. The current results contribute to a better

understanding of the nucleation and growth mecha-

nisms of lead chalcogenide nanoparticles, which will

enable the definition of more efficient synthesis routes of

these types of nanostructures.

Keywords Lead chalcogenides � PbSe � PbTe �Nanoparticles

Introduction

The study of lead chalcogenide nanostructures has

gained a lot of attention in recent years, due to their

possible applications in the field of energy generation

from alternate sources (Yan et al. 2008; Tai et al. 2008;

Yang et al. 2010; Wang et al. 2011a, b; Huynh et al.

2002; Cui et al. 2006; Choi et al. 2009; Koleilat et al.

2008; Johnston et al. 2008). This interest has been

emphasized by its low cost of production, and the

D. I. Garcia-Gutierrez � L. M. De Leon-Covian �D. F. Garcia-Gutierrez � M. A. Garza-Navarro �S. Sepulveda-Guzman

Facultad de Ingenierıa Mecanica y Electrica, FIME,

Universidad Autonoma de Nuevo Leon, UANL, Av.

Universidad S/N, Cd. Universitaria, C.P 66450 San

Nicolas de los Garza, N L, Mexico

D. I. Garcia-Gutierrez (&) � L. M. De Leon-Covian �D. F. Garcia-Gutierrez � M. Trevino-Gonzalez �M. A. Garza-Navarro � S. Sepulveda-Guzman

Centro de Innovacion, Investigacion y Desarrollo en

Ingenierıa y Tecnologıa, CIIDIT, Universidad Autonoma

de Nuevo Leon, UANL, Km. 10 de la nueva carretera al

Aeropuerto Internacional de Monterrey, PIIT Monterrey,

C.P. 66600 Apodaca, N L, Mexico

e-mail: [email protected]

123

J Nanopart Res (2013) 15:1620

DOI 10.1007/s11051-013-1620-7

flexibility offered by different synthesis routes (Yu et al.

2004; Murphy et al. 2006; Wise 2000). Several chemical

approaches have been proposed for the synthesis of lead

chalcogenide nanostructures, which have proven to be

effective on the control of its final size and shape. The

reported experimental evidence suggests that, depend-

ing on the synthesis parameters such as reaction

temperature, reaction time, molar ratio between the

lead precursor and the capping agent, molar ratio

between the lead precursor and the chalcogen (Te and

Se) precursor, reaction media, capping agent, among

others, it is possible to produce a wide variety of shapes

and morphologies: spherical nanoparticles (Yu et al.

2004; Murphy et al. 2006; Du et al. 2002; Lu et al. 2004),

cubic nanoparticles (Lu et al. 2004; Lifshitz et al. 2003;

Achimovicova et al. 2009; Kerner et al. 2001; Mokari

et al. 2007), nanorods (Murphy et al. 2006; Ziqubu et al.

2010), nanowires (Lifshitz et al. 2003; Cho et al. 2005),

cuboctahedral nanoparticles, octahedral nanoparticles

(Mokari et al. 2007), star-shaped nanoparticles (Houte-

pen et al. 2006), flower-shaped nanoparticles (Zhang

et al. 2007), etc. The control on the morphology, crystal

structure, and composition is strongly related to the

nucleation and growth mechanisms governing the

reaction synthesis. Nevertheless, the nucleation and

growth mechanisms of these chalcogenide systems are

not entirely understood. The information available in the

literature regarding the nucleation and growth mecha-

nisms in the lead chalcogenide systems is not extensive

enough to offer a full understanding of the pathways

governing these mechanisms. As a consequence, the

information used as a reference has been based,

primarily, in the experimental observations and

accepted nucleation and growth mechanisms in the

cadmium chalcogenide systems, due to their similarities

and to the fact that these systems have been more

extensively studied.

One of the recently accepted nucleation and growth

mechanism for cadmium selenide nanocrystals pro-

poses that phosphine selenides and anhydrous cadmium

octadecylphosphonate undergo a slow chemical reac-

tion (Owen et al. 2010). This reaction involves the

cleavage of the phosphorous chalcogenide double bond

(TOP=Se), proceeds by the nucleophilic attack of the

phosphonate or oleate on a (TOP=Se)—Cd (Liu et al.

2007), generating the initial CdSe species that will be

continuously liberated to the reaction medium, causing

supersaturation, nucleation, and nanocrystal growth

(Owen et al. 2010). Another recent study reports a

similar mechanism for the nucleation and growth of

PbSe nanoparticles (Evans et al. 2010). In this study, the

authors propose a similar reaction between the lead

oleate and a complex between a secondary phosphine,

present in the reaction, and the selenium. They propose

that this reaction provides the PbSe species that will

generate the nuclei, and will allow the growth of the

PbSe quantum dots (Evans et al. 2010). In this same

study, the authors report that the oxidation of a

secondary phosphine reduces the Pb2? ions on the lead

oleate to Pb0, after several hours of reaction and at high

temperatures (140 �C), and that this metallic product

seems to be inert to commonly used selenium sources,

hence, they believe, it is unlikely involved in the

quantum dot formation and growth.

In the current contribution, the formation and

growth mechanism of PbSe and PbTe nanoparticles

during their reaction synthesis were studied. Several

reaction times were assessed, considering different

synthesis conditions, in order to follow the nucleation

and growth processes of the synthesized PbTe and

PbSe nanoparticles, using transmission electron

microscopy and their related techniques. Based on

our experimental evidence, we propose a novel

nucleation and growth mechanism for the lead chal-

cogenide nanoparticles, which can be used as a basis to

improve the synthesis of these kinds of nanoparticles

systems, following analogous chemical approaches.

Experimental section

Materials

Lead oxide (PbO, 99.999 %), oleic acid (OA, 90 %),

selenium powder (99.5 %, 100 mesh), tellurium

powder (99.5 %, 100 mesh), trioctylphosphine (TOP,

90 %), 1-octadecene (ODE, 90 %), anhydrous tetra-

chloroethylene (TCE, 99 %), anhydrous hexane,

anhydrous chloroform, anhydrous ethanol, and anhy-

drous acetone were purchased from Sigma–Aldrich;

whereas dissolvent as anhydrous methanol was pur-

chased from J.T. Baker. All reagents were used as

received without further purification.

Synthesis of PbSe nanoparticles

A 1 M trioctylphosphine-selenium (TOP-Se) solution

was prepared by adding 1.974 g of Se to a 25-ml

of 12 J Nanopart Res (2013) 15:1620

123

volumetric flask and it was diluted to the mark with

TOP, the mixture was heated at 50 �C and stirred until

it became completely transparent, and then it was

stirred at room temperature overnight and stored under

N2 atmosphere. The lead oleate precursor was prepared

by mixing PbO, oleic acid (OA), and 1-octadecene

(ODE) and heated at 170 �C for 30 min, under stirring.

Typically, for the synthesis of the PbSe nanoparticles

the temperature of the lead oleate precursor was

lowered to 150 �C and the TOP-Se solution was

quickly injected; immediately after adding the TOP-

Se, the PbSe nanoparticles began to form. Aliquots

were extracted at different reaction times: 1, 2, 5, 8, 15,

30, and 60 min. Once the selected reaction time was

reached, the reaction was quenched by adding anhy-

drous chloroform and placing the reaction in a cold

water bath. Several runs were carried out varying the

Pb:Se precursor’s ratio at 1:2, 3:1, 1:1, and 1:3. The

lead concentration in the lead precursor was also varied

at 0.40, 0.48, 0.32, and 0.104 M for Pb:Se precursor’s

ratio of 2:1, 3:1, 1:1, and 1:3, respectively. ODE was

used as the solvent, and the reaction volume was 50 ml.

The lead oleate preparation and the PbSe nanoparticles

synthesis were carried out in a N2 atmosphere.

Synthesis of PbTe nanoparticles

PbTe nanoparticles were synthesized in a three-neck-

round-bottom flask equipped with a condenser and a

hot plate with stirring. 1.56 g of lead oxide was added

to a mixture of 6.2 ml of OA and 18.8 ml of ODE,

giving a lead oleate with a molarity of 0.26 M; another

lead oleate solution was prepared with a molarity of

0.065 M. This mixture was heated to 170 �C and

stirred in a N2 atmosphere for 30 min; within this time,

the solution turned colorless, indicating the formation

of lead oleate. The trioctylphosphine-tellurium (TOP-

Te) precursor was prepared by adding 3.2 g of

tellurium powder to 50 ml of TOP. This solution was

stirred, in a N2 atmosphere at room temperature for

20 h. Afterwards, the solution was heated to 150 �C for

20 min. Once the temperature had stabilized, the TOP-

Te was quickly injected to the solution of lead oleate;

immediately after adding the TOP-Te, the PbTe

nanoparticles began to form. Two different reaction

temperatures were studied, 130 �C and 170 �C. Ali-

quots were extracted at different reaction times: 1, 2, 5,

8, 15, 30, and 60 min. Once the selected reaction time

was reached, the reaction was quenched by the addition

of anhydrous chloroform and by placing the reaction in

a cold water bath. Four experiments were carried out

varying the Pb:Te precursor’s ratio at 1.5:1, 3:1, 1:1,

and 1:3. For the Pb:1.5/Te:1 case, 10 ml of the TOP-Te

solution was quickly added to the lead oleate solution;

in the Pb:3/Te:1 case, 4.33 ml of the TOP-Te solution

was quickly added to the lead oleate; while for the Pb:1/

Te:1 case, 13 ml was added; and for the Pb:1/Te:3 case,

39 ml of the TOP-Te solution was added. Once the

selected reaction time was reached, the reaction was

placed in a cold water bath and anhydrous hexane was

added to the reaction. The PbTe nanoparticles synthe-

sis were carried out in a N2 atmosphere.

After the reaction was finished, the nanoparticles

were extracted from the reaction media by means of

centrifugation. The extraction was carried out using

anhydrous methanol and then precipitating the nano-

particles with anhydrous acetone. The precipitated

nanoparticles were redispersed in anhydrous hexane

(for PbTe) or anhydrous chloroform (for PbSe), as the

case; and the centrifugation and extraction process

were repeated several times.

Characterization

The synthesized nanoparticles were characterized by

means of transmission electron microscopy (TEM)

with the use of a FEI TEM Titan G2 80-300 operated at

300 kV, with scanning transmission electron micros-

copy (STEM) capabilities and equipped with a High

Angle Annular Dark Field (HAADF) detector from

Fishione, a Bright Field STEM detector from Gatan,

an Annular Dark Field STEM detector from Gatan,

and an EDAX energy dispersive X-ray spectroscopy

(EDXS) detector. Electron Energy Loss Spectroscopy

(EELS) spectra were acquired in a TEM JEOL JEM-

ARM 200F. The TEM samples were prepared casting

one drop of a solution containing the nanoparticles on

a carbon-coated copper grid.

Results and discussion

Morphological characteristics

In order to assess the variation of the morphological

characteristics of the synthesized nanoparticles sys-

tems, first we analyzed the samples at different

reaction times. Figure 1 resumes the morphological

J Nanopart Res (2013) 15:1620 of 12

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characteristics of the PbSe nanoparticles synthesized

with a precursors’ ratio of Pb:1/Se:1. Figure 1a shows,

at short reaction times (1 min), the size of the

nanoparticles can be grouped in two populations,

one composed of nanoparticles with sizes between 1

and 3 nm, and another with sizes between 5 and 7 nm.

After 2 min, these populations could still be observed,

although the population of the smaller nanoparticles

decreases in number, whereas the one of larger

nanoparticles increases (see Fig. 1b). After 8 min of

reaction, the population of smaller nanoparticles

almost disappears, and the resultant population seems

to be composed only of nanoparticles with sizes

between 6 and 8 nm (see Fig. 1c). At 15 min of

reaction, the population of the smaller nanoparticles

cannot be observed anymore; instead, the observed

nanoparticles display an average size of 10.6 nm (see

Fig. 1d). Moreover, after 30 and 60 min of reaction, it

can be observed that the particle size increases to 11

and 11.44 nm, respectively, and its morphology

changes from spherical to cubic, along with the

formation of nanoparticles agglomerates (see Fig. 1e, f).

It is worthwhile to mention that analogous features were

observed in both nanoparticles systems, PbSe and PbTe, at

all Pb/Se and Pb/Te precursors’ ratios.

On the other hand, the variation on the molar ratio

between the lead and the chalcogen precursors was

also considered to elucidate its influence on the final

characteristic of the synthesized nanoparticles. Fig-

ure 2 shows the TEM images obtained from nanopar-

ticles synthesized with molar ratios of Pb:1/Se:1, Pb:1/

Se:3, and Pb:3/Se:1, using 15 min as the reaction time.

As it can be observed, the nanoparticles synthesized

with a molar ratio of Pb:1/Se:1 have an average

particle size of 9.57 nm (see Fig. 2b, e), whereas those

prepared at a molar ratio of Pb:1/Se:3 have a size of

8.28 nm (see Fig. 2a, d). Moreover, in both cases, the

absence of populations of smaller nanoparticles can be

noticed. It is worthwhile to mention that for reactions

with excess of chalcogen precursor, or even with a

molar ratio of one between the precursors, only the

population of larger nanoparticles can be notice on

both synthesized systems, PbSe and PbTe. However,

Fig. 2c, f shows, in the cases when the lead precursor

was used in excess (i.e., Pb:3/Se:1), the synthesized

nanoparticles can be grouped in two populations

again; one of small nanoparticles, with sizes between 1

and 3 nm, and another with an average size of 8.8 nm,

even at reaction times of 15 min.

Next, we proceeded to evaluate the effect of the

reaction temperature on the morphology of the syn-

thesized nanoparticle systems. As an example, Fig. 3

resumes the morphological characteristic of the PbTe

nanoparticles synthesized at 130 and 170 �C, with a

molar ratio of Pb:1.5/Te:1, using a lead oleate solution

with concentration of 0.26 M, and considering 10 min

as reaction time. As it can be notice, at 130 �C the

population of smaller nanoparticles is not observed,

instead there is only a population composed of cubic

nanoparticles with an average size of 13 nm (see

Fig. 3a, c). Nonetheless, at 170 �C there can be

observed two populations of nanoparticles, one com-

posed of nanoparticles with size between 1 and 3 nm,

and the other composed of cubic nanoparticles with an

average size of 18 nm.

Finally, we evaluated the effect of the concentration

of lead on the lead precursor on the synthesis of the

nanoparticles. We synthesized PbTe nanoparticles at a

reaction temperature of 130 �C, considering 2 min of

reaction time and using a molar ratio of Pb:1/Te:1.

Figure 4a, c shows, after 2 min of reaction with a lead

concentration on the lead precursor of 0.065 M, the

population of smaller nanoparticles is not visible at all,

although there can be observed nanoparticles that have

an average size of 8 nm, as well as a narrow particle

size distribution. However, as it can be observed in

Fig. 4b, d, using a lead precursor with a higher

concentration of lead (0.26 M) leads to the appearance

of the aforementioned smaller particles population,

along with nanoparticles with analogous characteris-

tics to those observed in the previous experiment,

when a lead precursor with a lower concentration of

lead was used (see Fig. 4a, c).

From these results are safe to argue that at short

reaction times and using higher concentration of lead

on the reaction leads to the appearance of two different

particle size populations. However, in order to eluci-

date the crystalline characteristics and the composition

of the observed nanoparticles populations, we pro-

ceeded to analyze the synthesized samples by means

of HAADF-STEM, EDXS, EELS, and SAED

techniques.

Crystalline characteristics and composition

Figure 5 shows a HAADF-STEM image, correspond-

ing EDXS spectrum, as well as the SAED pattern

obtained from the sample prepared with molar ratio of


123

Pb:1/Se:3, synthesized with a reaction time of 15 min.

As it can be notice, there is a group of nanoparticles

enclosed by a red square (see Fig. 5a), whose compo-

sition can be attributed to lead-selenide (see Fig. 5b).

This assumption is confirmed by the SAED pattern

obtained from this zone (enclosed by the red square).

Herein is possible to identify reflections related to the

families of planes {111}, {200}, {220}, {222}, {400},

and {422} of the FCC crystalline structure reported for

PbSe (see JCPDS card #: 06-0356).

Figure 6 summarizes the crystalline characteristic

and composition of the sample prepared with a molar

ratio Pb:1/Te:1, using 15 min of reaction time. From

the observed X-ray signals, there can be notice that the

composition of the group of particles, selected in

Fig. 6a, can be related to lead-telluride (see Fig. 6b).

Moreover, the SAED pattern obtained from this zone

reveals that their crystalline structure is congruent

with that reported for the FCC atomic arrangement in

PbTe crystals (see JCPDS card #: 38-1435); from this

pattern, one can identify reflections related to the

family planes {200}, {220}, {222}, {400}, {420}, and

{422} of this crystalline structure (see Fig. 6c). It is

worthwhile mentioning that, regardless of the synthe-

sis conditions, the PbSe and PbTe nanoparticles

always presented the FCC crystal structure.

On the other hand, Fig. 7 shows the studies on the

population of smaller nanoparticles observed in the

synthesized lead chalcogenide systems. Analyzing the

sample prepared with a molar ratio of Pb:1/Te:1, after

1 min of reaction at 130 �C, it can be notice that the

X-ray signals on the EDXS spectrum, obtained from

the zone indicated in Fig. 7a, are mainly related to the

characteristic radiation emitted by Pb atoms (see

Fig. 7b). Moreover, the SAED pattern, obtained from

a similar zone, suggests that the crystalline structure of

these nanoparticles is congruent with that reported for

lead (see JCPDS card #: 44-0872). In this pattern,

reflections attributable to family planes {100}, {101},

and {103} of its hexagonal symmetry can be observed

(see Fig. 7c; Table 1). In order to perform a more

thorough analytical study trying to determine the

Fig. 1 TEM images of the nanoparticles synthesized with a

precursors’ ratio of Pb:1/Se:1, at a reaction temperature of

150 �C, and with a molar ratio between the oleic acid and the Pb

of 2.5, at the following reaction times: a 1 min, b 2 min,

c 8 min, d 15 min, d 30 min, and e 60 min


123

composition of this population of smaller nanoparti-

cles, we performed an EELS analysis, on similar

regions of this sample, trying to detect the character-

istic M4,5 EELS signal of Te, but no measurable peaks

attributable to such signal could be observed in the

acquired spectra.

This evidence suggests that the small nanoparticles

that were observed in the synthesized lead chalcogen-

ide systems are Pb nanoparticles. Furthermore, as it

was previously described, the appearance of these

nanoparticles occur at (1) short reaction times (1 min),

regardless of the other synthesis parameters (see

Fig. 1); (2) longer reaction times (even at 15 min)

with high concentration of lead precursor in the

reaction (see Fig. 2); (3) long reaction times (10 min)

and high reaction temperature (170 �C) (see Fig. 3);

and (4) short reaction times (2 min) with high

concentration of lead in the lead precursor (see Fig. 4).

However, when a low concentration of lead

precursor was used to synthesize whichever of the

lead chalcogen systems, the population of small

nanoparticles almost disappears after 8 min of reac-

tion. Nonetheless, at high concentration of lead

precursor, even after 15 min of reaction, the preva-

lence of a large number of small nanoparticles was

observed. These observations imply that a large

availability of lead precursor leads to the formation

of higher numbers of Pb nanoparticles, which seem to

disappear as the reaction progresses. Moreover, the

fact that they prevail even after 10 min, at a high

reaction temperature, when low concentration of lead

precursor is used, points to that fact that their

formation is thermally favored.

Nucleation and growth mechanism

As it was presented, our experimental results suggest

the reduction of Pb2? ions, from lead oleate, into Pb0

atoms at early reaction times, suggested by the

presence of Pb nanoparticles. In addition, their

Fig. 2 TEM images of the nanoparticles synthesized with a

reaction temperature of 150 �C, a reaction time of 15 min, with

a molar ratio between the oleic acid and the Pb of 2.5, and the

following ratios for the lead and selenium precursors: a and

d Pb:1/Se:3; b and e Pb:1/Se:1; and c and f Pb:3/Se:1


123

formation and subsequent disappearance, as well as

the remarkable change on the sizes and morphology of

the PbSe and PbTe nanoparticles, as reaction pro-

gresses, imply that the reduced Pb0 atoms actually

contribute to the growth of the particles in both

systems. As has been reported elsewhere, the reduc-

tion of Pb2? ions into Pb0 atoms could be promoted by

the thermal oxidation of secondary phosphine (diphen-

ylphosphine, DPP) in the TOP reagent that we used in

our reaction (Evans et al. 2010). Nonetheless, in the

mentioned report, the reduction of Pb2? ions through

such mechanism requires several hours of reaction,

and occurs after the formation of the lead chalcogenide

nanoparticles, hence making unlikely their contribu-

tion to the formation and growth of PbSe and PbTe

nanoparticles. However, in the present study, Pb

nanoparticles were clearly observed at very short

reaction times, and it was also observed how these Pb

nanoparticles disappeared as the reaction time

increased. It is proposed that the observed Pb nano-

particles are not stable, since we believe they are

nucleating and re-dissolving in the reaction as time

elapses, sometimes the reduced Pb0 atoms contribute

to the formation of the lead nanoparticles, while in

other cases contribute to the growth of the lead

chalcogenide nanoparticles; but once the reaction is

stopped, the Pb nanoparticles that were formed at that

time remain in the final product. At the end of the

reaction, once all of the reagents have been consumed,

all of the Pb0 atoms are only forming part of the

chalcogenide nanoparticles. The time it takes for the

Pb nanoparticles to stop forming depends on the

concentration of lead on the reaction, as it was

observed on the experimental results, at higher

concentration of lead on the reaction, the Pb nanopar-

ticles can still be observed at longer reaction times

Fig. 3 TEM images of the nanoparticles synthesized with a molar ratio between the precursors of Pb:1.5/Te:1, a reaction time of

10 min, with a molar ratio between the oleic acid and the Pb of 3 and a reaction temperature of a and c 130 �C and b and d 170 �C


123

(15 min); and at lower concentration of lead on the

reaction, the Pb nanoparticles stop to be observed at

shorter reaction times (2 min).

In an attempt to get a more detailed insight on the

manner those Pb0 atoms contributes to the growth of

the lead chalcogenide nanoparticles, elemental line

Fig. 4 TEM images of the nanoparticles synthesized with a molar ratio between the precursors of Pb:1/Te:1, a reaction temperature of

130 �C, a reaction time of 2 min, and a molarity of Pb in the lead precursor of a and c 0.065 M and b and d 0.26 M

Fig. 5 a HAADF-STEM image of a group of PbSe nanopar-

ticles analyzed; the red square marks the area analyzed for the

EDXS study and the SAED study. b EDXS spectrum acquired

for the analyzed nanoparticles. c SAED pattern recorded from

the group of nanoparticles analyzed. (Color figure online)


123

profile analyses were performed on individual nano-

particles. Figure 8b shows the EDXS line profile

performed on a PbTe nanoparticle that is shown in

Fig. 8a; the red line observed in the image corresponds

to the region analyzed during the experiment. In the

elemental profiles, it can be seen that the Pb signal

starts to appear slightly before the Te signal does, and

it also ends slightly after. It can also be seen that the Pb

signal is slightly higher in both edges of the nanopar-

ticle, and closer to the center is comparable to that

from Te atoms. These results suggest a higher

concentration of lead atoms in the surface of the

nanoparticles. It is worthwhile mentioning that this

tendency was observed in both lead chalcogenide

nanoparticles systems. The presence of high concen-

tration of Pb atoms on the lead chalcogenide

Fig. 6 a HAADF-STEM image of a group of PbTe nanopar-

ticles analyzed; the red square marks the area analyzed for the

EDXS study and the SAED study. b EDXS spectrum acquired

for the analyzed nanoparticles. c SAED pattern recorded from

the group of nanoparticles studied. (Color figure online)

Fig. 7 a HAADF-STEM

image showing an area with

a high concentration of Pb

nanoparticles, the redrectangle shows the area

analyzed for the EDXS

analysis, EELS analysis, and

SAED study. b EDXS

spectrum showing a clear Pb

signal, but no clear Te signal

can be detected. c SAED

pattern obtained from a

group of Pb nanoparticles.

d EELS spectrum from the

analyzed region, no clear Te

M4,5 peak can be detected.

The sample studied

corresponds to the

nanoparticles synthesized

with a molar ratio between

the precursors of Pb:1/Te:1,

a reaction temperature of

130 �C, and a reaction time

of 1 min. (Color figure

online)


123

nanoparticles has been previously reported by other

research groups (Moreels et al. 2008), and supports our

hypothesis about the contribution of Pb0 atoms on the

growth of the synthesized lead chalcogenide nanopar-

ticles, PbSe and PbTe. However, more detailed studies

based on EELS are underway to obtain a better insight

on the surface composition.

Based on the experimental evidence observed in the

current study, a formation and growth mechanism for

lead chalcogenide nanoparticles are proposed in

Fig. 9. It is proposed that the first units of PbX

(X=Se or Te) are formed by the mechanism described

by Evans et al. 2010, where a cleavage of the

phosphorous chalcogenide double bond (DPP=Se)

and proceeds by the nucleophilic attack of the oleate

on a (TOP=X)—Pb2?, generate the initial PbX species

(see Fig. 9a). However, a parallel process to the

formation of these PbX units or nuclei, is the reduction

of the Pb2? ions to Pb0 atoms by the oxidation of the

secondary phosphines, DPP, present in the reaction

(see Fig. 9b). These Pb0 atoms end up forming the Pb

nanoparticles that can be seen at early reaction times

and in the reactions with the excess of Pb. In addition,

another event that is taking place simultaneously is the

detachment of the X atoms from the ternary and/or

secondary phosphines, to end up in the reaction media

as X0 atoms. We have observed Te crystals (not

shown) in some of the reactions studied, in particular

in the reaction case Pb:1/Te:3, which suggest that not

all of the Te is interacting with the phosphines, leaving

some Te atoms available to contribute to the growth of

the nanoparticles along with the Pb0 reduced atoms.

The similarities observed in the results of both systems

suggest the availability of Se atoms in the reaction

media to contribute to the growth of the PbSe

nanoparticles along with the Pb0 atoms, just like in

the PbTe system. The Pb0 and the X0 atoms available

in the reaction media start to deposit on the surface of

the PbX nuclei previously formed, along with the PbX

species formed by the mechanism proposed by Evans

et al. 2010, contributing to the growth of the PbX

nanoparticles (see Fig. 9c). Figure 1 showed, it can be

seen that the concentration of Pb nanoparticles is high,

and the concentration of PbX nanoparticles is low at

short reaction times, but as the reaction time increases,

the amount of Pb nanoparticles observed decreases

considerably, and even at very high reaction times no

Pb nanoparticles can be observed, while the amount of

PbX nanoparticles increases considerably as the

reaction time increases. This observation supports

the argument previously stated that the Pb0 reduced

atoms and the X0 atoms start to deposit on the surface

of the previously formed PbX nuclei, and contribute to

the growth of the PbX nanoparticles. Finally, the

surface of the nanoparticles ends up with an excess of

lead, as it is suggested by the result presented in Fig. 8.

Table 1 Interplanar spacings measured on the electron dif-

fraction pattern acquired from the region shown in Fig. 7a

Electron diffraction

pattern

JCPDS 44-0872 Pb

Interplanar

spacing (A)

Interplanar

spacing (A)

Atomic

plane (hkl)

3.01 3.02 100

2.69 2.66 101

2.09 2.11

1.79 1.75 110

1.54 1.59 103

Interplanar spacings from JCPDS card # 44-0872

Fig. 8 a HAADF-STEM

image showing one PbTe

nanoparticle analyzed. The

red line indicates the region

where the linescan study

was performed, going from

left to right. b EDXS line

profiles for the Pb Ma and Te

La signals. The Pb Ma signal

can be observe to start and

finish, slightly before and

after the Ta La signal. (Color

figure online)


123

Nonetheless, the lead signal observed on the surface of

the PbX nanoparticles could be related to the presence

of a capping layer at this surface. This capping layer on

the surface of the nanoparticles could have its origin

on the oxidation of the Pb0 atoms deposited on the last

layer of the nanoparticles, which are oxidized by the

oleic acid found in the reaction media, forming a lead

oleate capping layer. More detailed analyses are

underway to get a better insight on the capping layer

nature.

Conclusions

In conclusion, the variation of several synthesis

parameters, such as reaction time, lead to chalcogen

ratio, reaction temperature, and lead oleate concen-

tration, promoted the formation of two different

populations of nanoparticles, differentiated by their

size. The population of larger nanoparticles was the

one related to the chalcogenide nanoparticles (PbSe

and PbTe), whereas the population of smaller nano-

particles, with a size between 1 and 3 nm, was related

to the presence of Pb nanoparticles, as suggested by

EDXS, EELS, and electron diffraction results. The

observation of these Pb nanoparticles points to the

reduction of the Pb2? ions to Pb0 metallic atoms at

early stages of the reaction, and it was also observed

how these Pb0 atoms contribute to the growth of the

PbSe and PbTe nanoparticles. Based on the observa-

tions made, a growth mechanism for these types of

nanoparticles is proposed, where the first species of

PbX are formed by the routes currently accepted in the

literature, although, in a parallel process, the Pb2? ions

are reduced by the oxidation of a secondary phosphine

(DPP), present in the original TOP. Once the Pb0

atoms are formed, they are able to contribute to the

growth of the PbSe and PbTe nanoparticles; and the

lead chalcogenide nanoparticles end up with an excess

of Pb on its surface, as suggested by EDXS results.

Fig. 9 Growth mechanism proposed for the PbTe and PbSe nanoparticles, based on the experimental observations presented


123

These results allow us to elucidate in better detail the

mechanisms of nucleation and growth of lead chalco-

genide nanoparticles, which will contribute to a better

understanding of the processes involved in the forma-

tion of this type of nanoparticles, the role played by the

different reagents in the reaction and eventually will

translate into synthesis methods more efficient, with

better control on the results obtained.

Acknowledgments This study was supported by the Mexican

Secretary of Education (SEP), PROMEP program, through the

project ‘‘Apoyo a la Incorporacion de Nuevos PTC’’, Project

Number PROMEP/103.5/10/3889, and by CONACYT Mexico

through Project Numbers 148391 and 154303. The authors also

thank the International Center for Nanotechnology and Advanced

Materials, ICNAM, at the University of Texas at San Antonio, for

the support provided for the use of their Advanced Microscopy

Center. LMDC and DFGG thank and acknowledge the financial

support received from CONACYT Mexico.

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On the role of Pb 0 atoms on the nucleation and growth of PbSe and PbTe nanoparticles

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