Physica E 43 (2011) 1826–1832

Contents lists available at ScienceDirect

Physica E

1386-94

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physe

Polymer-assisted crystallization of low-dimensional lead sulfide particles

Nicoleta Preda a,n, Edina Rusen b, Monica Enculescu a, Elena Matei a, Bogdan Marculescu b,Ionut Enculescu a

a National Institute of Materials Physics, Multifunctional Materials and Structures Laboratory, Bucharest, P.O. Box MG-7, R-77125, Romaniab University Politehnica Bucharest, Department of Polymer Science, Calea Victoriei no. 71101, Bucharest, Romania

a r t i c l e i n f o

Article history:

Received 31 January 2011

Received in revised form

9 June 2011

Accepted 16 June 2011Available online 29 June 2011

77/$ - see front matter & 2011 Elsevier B.V. A

016/j.physe.2011.06.019

esponding author. Tel.:þ40 21 3690170x156

ail address: nicol@infim.ro (N. Preda).

a b s t r a c t

PbS micro- and nanoparticles were synthesized by a simple precipitation reaction of lead nitrate with

thioacetamide in hydrosoluble polymer water solutions. The effects of four water soluble polymers:

polyacrylamide (PAM), polyvinyl alcohol (PVA), polyethylene glycol (PEG) and poly-N-vinyl pyrrolidone

(PVP) on the PbS crystallites morphology and structural properties were investigated by scanning

electron microscopy (SEM) and X-ray diffraction (XRD). It was found that for the PbS particles obtained

in the PVA, PEG and PVP, the (2 0 0) diffraction peak of the nanocrystals becomes dominant. The highest

texture in the [2 0 0] direction was observed for the crystallites obtained in the presence of PVP.

Polydisperse PbS particles with cubic morphology and size ranging from 100 nm to several microns are

obtained in the case of PAM and PEG. Monodisperse cubic PbS crystallites with an average size of

200 nm are formed in the presence of PVA and PVP.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

The accuracy in designing inorganic nanoparticles with tai-lored morphology has become a key step in nanotechnology.Thus, in the last few years a number of synthesis procedures weredeveloped in order to control the size, shape and surface mor-phology of semiconductor nanoparticles [1–5]. Those parametershave significant effects on their chemical and physical properties.According to the literature, novel properties of nanoparticlesappear not only due to their small size, but also due to theiruniform size distribution.

Among the inorganic semiconductors, lead sulfide (PbS) is animportant IV–VI semiconductor with rather small band gap(0.41 eV at room temperature) and relatively large excitationBohr radius (18 nm) [6]. The band gap of PbS crystallites is verysensitive to finite size effects, shifting from near-infrared tovisible region as the characteristic dimension decreases. Also,the shape of the particles is important for its specific properties.Such nanoscaled particles show promising properties useful forapplications in optoelectronic devices such as light-emittingdiodes, infrared detectors, infrared sensors, optical switches, solarcells, etc. [7–10].

Various methods were used for the synthesis of PbS particlesin sub-micrometer/nanometer dimensions including solvother-mal [1,2], hydrothermal [11,12], thermal decomposition [13],

ll rights reserved.

; fax: þ40 21 3690177.

microwave irradiation [14], sonochemical [15], template-assisted[16], etc. Different morphologies were obtained such as rods,belts, flowers, ribbons, cubes, dendrites, stars, wires, multipods orother geometries. It is well known that a key role in controllingthe morphology of the PbS micro- and nanoparticles is played byorganic additives such as surfactants [5,17–20] or polymers[15,21–25]. The surfactants inevitably cause contamination ofthe particles because they are difficult to be washed completelyaffecting the material properties and its applications.

The polymers can provide steric stabilization to the colloidalparticles and therefore reduce the possibility of coagulationeffects. For example, polymers containing either sulfonate [23],sulfonic acid [24] or carboxylate [25] groups in their side chainsand that are able to coordinate with Pb2þ ions provide a size-limiting effect for grown particles. On the other hand, thepolymers can exert a direct influence on the semiconductingcrystallite morphology by selective adsorption on some specificcrystalline facets during the crystal growth process. Therefore,polymer-mediated crystallization is a novel, general and promis-ing technique for crystal morphology design at ambient condi-tions with a great potential for fabrication of semiconductornanoparticles with interesting and useful physical properties.

The main problem is to produce uniform-shaped PbS nano-particles with a narrow size distribution. Usually, chemical pre-paration involves precipitation of a solid phase from solution,which includes nucleation and growth of particles in the solution.Control of the nucleation and the following growth are the keyfactors for the synthesis of monodisperse nanoparticles. If thegrowth mechanism and process are well understood, there is

N. Preda et al. / Physica E 43 (2011) 1826–1832 1827

a better chance to prepare inorganic nanoparticles with expectedmorphology at high yields. Thus, the development of a facile,effective and polymer-mediated crystallization approach for thecontrolled preparation of PbS particles is highly desirable for thedetailed investigation of its growth behavior and potential large-scale production.

The aim of this paper is to present a simple route forsynthesizing PbS micro- and nanoparticles via precipitation reac-tion of lead nitrate with thioacetamide in hydrosoluble polymerwater solutions. Water-soluble nonionic polymers are often usedas additives (‘‘capping reagents’’) in nanostructures fabricationbecause they can induce morphological evolution of inorganicnanocrystals [26].

In this context, we focus our study on the influence of differenthydrosoluble polymers (polyacrylamide, polyvinyl alcohol, poly-ethylene glycol and poly-N-vinyl pyrrolidone) on the morphologyand structural properties of PbS crystallites. Furthermore, becausethe PbS particles can be synthesized by the chemical reactionbetween a lead salt and a sulfur source in the presence ofa hydrosoluble polymer [27] or during the polymerization of anorganic monomer [28], we discuss the influence of the synthesismethod on the semiconductor particles size. For the compa-rative study we chose as monomer acrylamide because it is awater soluble compound. This means that the reaction environ-ment is compatible with the two reactants, lead nitrate andthioacetamide, used as sources for the PbS chemical synthesis.The structural properties and the morphology of the PbS crystal-lites were studied using X-ray diffraction (XRD) and scanningelectron microscopy (SEM).

2. Experimental

The PbS crystallites were prepared by the chemical reactionbetween lead nitrate (Merck) and thioacetamide (Aldrich). In thefirst step, water solution of 0.2 g Pb(NO3)2 in 50 ml was prepared.This solution was placed in thermostatic bath under continuousstirring and 0.065 g CH3CSNH2 was added when it reached 80 1C.The appearance of the black color is the first evidence of theformation of PbS crystallites. After 12 h the black powder wascollected through centrifugation, washed several times withdistilled water and dried on filter paper at room temperature.

Next we evaluated the influence of the synthesis approach(either the addition of the reactants during the polymerizationreaction of a monomer or the mixture of the reactants in thepresence of a polymer aqueous solution) on the morphology andstructural properties of semiconductor particles.

In the second step, we synthesized PbS particles during thepolymerization reaction of acrylamide, a water soluble monomer.This procedure using acrylamide (Merck) as monomer and 2,20-azobisisobutyronitrile (Fluka) as initiator is described further.Acrylamide (AM) was recrystallized from chloroform; 2,20-azobi-sisobutyronitrile (AIBN) was purified by recrystallization frommethanol and dried in vacuum at room temperature. The recipeconsists in 50 ml distilled water, 2.35 g AM and 0.0313 g AIBN.Also, 0.2 g Pb(NO3)2 was added to the aqueous solution contain-ing the monomer and the initiator. All reactants were dosed intothe flask and were deoxygenated by nitrogen bubbling for 10 min.The solution was well stirred and heated at 80 1C. As thepolymerization began, 0.065 g CH3CSNH2 was added and theheating continued. The formation of semiconductor particleswas indicated by the appearance of a black color in the reactionmixture. After 12 h, the mixture was precipitated in methanolto give a final black product. The powder was separated byfiltration on filter paper and then washed with water in order toremove polyacrylamide. In this way, the size and morphology of

the PbS crystallites obtained after the polymer removal could beinvestigated.

In the final step, for the synthesis of PbS crystallites in thepresence of water soluble polymers we used: polyacrylamide(PAM; Mw 10000), polyvinyl alcohol (PVA; Mw 13000–23000),polyethylene glycol (PEG; Mw 8500–11500) and poly-N-vinylpyrrolidone (PVP; Mw 29000) purchased from Aldrich. Maintain-ing constant all the other reactions parameters we synthesizedPbS particles using different concentrations of hydrosolublepolymers: polyacrylamide (0.01% and 0.04%), polyvinyl alcohol(0.01% and 0.02%), polyethylene glycol (0.01%, 0.02% and 0.04%)and poly-N-vinyl pyrrolidone (0.01% and 0.02%). The as-preparedpowders were separated by centrifugation. Then the blackprecipitates were washed with distilled water and centrifugedseveral times in order to dissolve the hydrosoluble polymer andfinally dried in air at room temperature.

The morphology of the powder samples were evaluatedusing a Zeiss Evo 50 XVP scanning electron microscope (SEM).The samples were sputtered with a thin layer of gold prior toimaging. The crystalline phase of the powder samples wasidentified by X-ray diffraction (XRD) on a Bruker AXS D8 Advanceinstrument with Cu Ka radiation (l¼0.154 nm). The source wasoperated at 40 kV and 40 mA and the Kb radiation was eliminatedusing a nickel filter.

3. Results and discussion

The formation of PbS crystallites in the absence or in thepresence of hydrosoluble polymer could be described as follows.The sulfur source used in this work, thioacetamide, hydrolyzes inthe aqueous solution and releases H2S gradually according to thefollowing reaction:

CH3CSNH2(aq)þ2H2O-H2S(aq)þCH3COO�(aq)þNH4þ(aq) (1)

Upon hydrolysis, H2S slowly forms S2� ions, which combinewith Pb2þ ions forming PbS crystallites. The slow release of S2�

anions significantly affects the kinetic growth of PbS particles.

Pb2þ(aq)þH2S(aq)-PbSkþ2Hþ(aq) (2)

Generally, the synthesis of colloid crystallites includes twoprocesses: nucleation and growth. Usually, fresh PbS crystallitesare unstable in the solution and have a tendency to aggregate andgrow into large PbS crystals until they finally become stable. Forspecific growth conditions, the nucleation and the growth rates ofdifferent crystal facets determine the overall structure andmorphology [29]. A water soluble polymer present during thesynthesis of PbS crystallites could act as surface modifier or stericstabilizer. Thus, the size of semiconducting particles is restrictedwhen the steric hindrance caused by the polymer chains inhibitthe growth of seed PbS particles formed in the nucleation stage.Also, when the surface of PbS crystallites is partially capped bypolymer their growth in some direction may be restrained, whichfinally leads to anisotropic growth towards one-dimensionaldirection. The preferential adsorption of polymers from aqueoussolutions onto certain crystal facets directs the growth of crystalinto various shapes by controlling the growth rates along differ-ent crystal axes. Surface energies associated with the crystal-lographic planes are usually different. For crystalline PbS the{1 1 1} facets containing either Pb or S have a higher surfaceenergy than those of {1 0 0} facets, which contain mixed Pb and S.Consequently, the electron density at the {1 0 0} facets is insuffi-cient to attract polymers. The polymers preferential adsorption onthe {1 1 1} facets in the case of the PbS could explain why mostlycubic geometries are usually obtained for this semiconductor.

Fig. 1. Powder XRD patterns of PbS crystallites prepared from lead nitrate and

thioacetamide: in the absence of hydrosoluble polymers (a) during the polymer-

ization of AM (as synthesized (b) and after washing with water (c)) and in the

presence of hydrosoluble polymers at different concentrations: PAM (0.01%—d;

0.04%—e), PVA (0.01%—f; 0.02%—g), PEG (0.01%—h; 0.02%—i; 0.04%—j) and PVP

(0.01%—k; 0.02%—l).

N. Preda et al. / Physica E 43 (2011) 1826–18321828

It has to be mentioned that the shape of a face-centered cubic(fcc) crystal, as PbS crystal, was mainly determined by the ratio ofthe growth rate in the [1 0 0] direction to the growth rate in the[1 1 1] direction [30].

In the case of synthesis of PbS crystallites during the poly-merization of AM the reaction mechanism may imply the follow-ing steps. In the beginning the radicals resulted from thedissociation of AIBN initiate the polymerization of AM. Whenthe sulfur source is added to the mixture, the hydrolysis ofthioacetamide leads to the formation of PbS nanoparticles simul-taneously with the polymerization reaction. Therefore, the semi-conductor nanoparticles could be homogeneously well dispersedin the PAM matrix.

The XRD patterns of the synthesized powders are shown in theFig. 1. The diffractograms show main peaks at 2y: 26.01; 30.01,43.01, 51.01, 53.51, 62.51, 69.01, 71.01 and 79.01, which correspondto the Miller index of the reflecting planes for (1 1 1), (2 0 0),(2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 0) and (4 2 2). All thediffraction peaks in the spectra are analogous to the literaturepattern of face-centered cubic phase PbS (galena) powder (JCPDSfile no. 5-592), confirming the formation of PbS particles. Thestrong and sharp diffraction peaks suggest that the as-obtainedproducts are well crystallized. No obvious impurity phase isobserved. It has to be noticed that for the PbS samples synthe-sized in PVA (curves f and g), the widths of diffraction peaks arelarger than those observed for the PbS crystallites synthesized inthe absence of polymer suggesting that the particle size becomesslightly smaller. Also, it has to be noticed in Fig. 1 that theintensity of (2 0 0) diffraction peak of the semiconductor crystal-lites increases for the PbS samples prepared in PVA and PEG whenthe polymer concentration increases. This experimental fact isobserved especially in the case of PbS crystallites obtained in thePVP (curves k and l). In other words, the intensity distribution ofall X-ray diffraction peaks for these samples deviates drasticallyfrom what is characteristic to the PbS crystalline powder pro-duced in the absence of polymers (curve a). This increase of theintensity diffraction peak is a consequence of the growth direc-tion. Thus, the intensity of the (2 0 0) diffraction peak is muchhigher than that of the (1 1 1) diffraction peak, suggesting ahigher growth rate on the {1 0 0} facets in comparison to the{1 1 1} facets. Also, it appears that the PbS crystallites have apreferred (1 0 0) lattice plane orientation parallel to the substratesurface. So, we can conclude that the PbS samples obtained in thepresence of PVP (curves k and l) are highly textured in the [2 0 0]direction. Another observation from Fig. 1 is that the intensitydistribution of the diffraction peaks depends on the polymerconcentration used to prepare the PbS particles, especially inthe case of PEG and PVP. For example, the intensity ratio (I200/I111)of the first two peaks in the XRD pattern increases with increasingpolymer concentration. A large intensity ratio (I200/I111) is corre-lated with a high degree of orientation of the (1 0 0) lattice planeand vice-versa. Therefore the above result demonstrates that the(1 0 0) lattice planes of the PbS particles prepared in PEG or PVPbecome better oriented as the concentration of polymer increases.The experimental results shown in Fig. 1 suggest that althoughPAM, PVA, PEG and PVP are water soluble polymers, there seemsto be a significant difference among their capping abilities on thePbS crystalline facets.

The morphology of all synthesized samples were directlyobserved by SEM (Figs. 2–7). Generally, micro- and nanoparticleswith cubic morphology can be found in all cases.

The low-dimensional PbS particles obtained under typicalconditions without the addition of polymer are mainly polydis-perse cubic particles with size ranging from 100 nm to severalmicrons (Fig. 2). Observing the morphological aspects, it can beseen that cubes have clear edges and clean faces.

The SEM image of the powder synthesized by the addition ofthe reactants during the polymerization reaction of AM presentedin Fig. 3 (top) reveals large irregular-shaped particles. Thesemiconductor particles could not be observed due to the factthat they are embedded in the polymer matrix. For irrefutable

Fig. 2. SEM images of PbS particles produced in the absence of additives.

Fig. 3. SEM images of PbS particles obtained during the acrylamide polymeriza-

tion: as synthesized (top) and after washing with water (bottom).

Fig. 4. SEM images of PbS particles synthesized in the presence of PAM: 0.01%

(top) and 0.04% (bottom).

N. Preda et al. / Physica E 43 (2011) 1826–1832 1829

proof that the PbS crystallites are formed in the PAM matrix, thepowder was washed with water in order to remove the polymer.Thus, the size and morphology of the PbS crystallites could beinvestigated. The SEM images of the powder obtained after thePAM dissolution are observed in Fig. 3 (bottom). Polydisperse

cubic particles with sizes ranging from 100 nm to several micronsare obtained. Also, the formation of PbS during the polymeriza-tion of AM is certified by the X-ray pattern of the sample (Fig. 1curve b).

Fig. 4 shows the SEM images of the PbS crystallites obtained attwo PAM concentrations. Comparing with the PbS particlessynthesized during the polymerization of AM (Fig. 3 bottom), inthis case the PbS cubes with size ranging from 100 nm to 1 mmwere obtained for both concentrations (Fig. 4). However, increaseof the amount of polymer results in the increase of the number ofPbS cubes with lower sizes (Fig. 4 bottom). A plausible explana-tion for this result could be linked to the increase of the viscosityin the reaction system, which might cause a slower diffusion ofions or growing units. The experiments show that for bothconcentrations, the adsorption of PAM on the PbS facets doesnot provid a uniform size distribution.

In Fig. 5 the SEM images of PbS particles prepared in thepresence of PVA are observed. The particles are quasi-monodis-persed and irregular, with rugged surfaces and an average size of200 nm. Due to the oxygen atoms from the polymeric chain, PVAmay have stronger interaction with the side facets of PbS crystal-lites, therefore it can inhibit the growth of certain facets by heavycapping. In our opinion, PVA wraps the entire surface of PbScrystallites, which can inhibit the growth in all directions. In thisway quasi-monodispersed semiconductor particles are obtained.

Fig. 5. SEM images of PbS particles synthesized in the presence of PVA: 0.01%

(top) and 0.02% (bottom).

Fig. 6. SEM images of PbS particles synthesized in the presence of PEG: 0.01%

(top), 0.02% (middle) and 0.04% (bottom).

N. Preda et al. / Physica E 43 (2011) 1826–18321830

The SEM images of the PbS particles synthesized in thepresence of PEG are shown in the Fig. 6. It appears that theamount of PEG used plays an important role on the morphology ofPbS nanostructures. We consider that the strong affinity of theoxygen atoms of PEG for Pb2þ ions place them near PEG chains,which creates a nucleation domain for the subsequent reactionbetween Pb2þ and S2� ions. Furthermore, PEG may createconnections between PbS particles into ‘‘flocs’’ through a bridgingflocculation mechanism. When the PEG amount increased, thePbS particles with sizes ranging from 100 nm to 1mm andirregular shape were formed (Fig. 6 bottom). The (1 1 1) planeof PbS crystal possesses the highest surface energy and theadsorption of suitable additives, such as PEG on this plane maybe higher than for the other crystalline planes. Although there isno direct evidence for PEG selective absorption abilities ondifferent crystal facets, we can presume that PEG tends to attachto the (1 1 1) plane and suppresses the growth rate of this facet.Thus, PbS particles covered by PEG on the {1 1 1} facets arepreferentially produced. Fig. 6 shows clearly the influence of thePEG amount on morphologies of the PbS crystallites. When theamount of polymer is small, polydisperse crystalline cubes wereobtained. The increase of polymer amount results in a flocculationmechanism. When the amount of PEG becomes larger, irregular-shaped particles were observed. This may be due to the stronginteraction between the oxygen atoms of the PEG and thecrystallizing PbS.

In the presence of PVP, PbS cubic or rectangular parallelepipedshaped particles with sizes ranging from 100 nm to 1 mm areobtained (Fig. 7). The increase of the PVP amount leads to acomplete elimination of the PbS particles with parallelepipedshape and produced only fine PbS uniform smooth particles withcubic shape having average length of about 200–300 nm (Fig. 7bottom). The perfect cubic PbS particles, which were observedwhen the amount of PVP increases, suggests a coverage of the

Fig. 7. SEM images of PbS particle synthesized in the presence of PVP: 0.01% (top)

and 0.02% (bottom).

randomly oriented

PbS particles

PAM, PEG

PVA

PVP

Pb2+ + S2-

textured

PbS particles

Fig. 8. Schematic illustration of the formation of PbS structures in the presence of

hydrosoluble polymers.

N. Preda et al. / Physica E 43 (2011) 1826–1832 1831

(1 1 1) crystal planes by polymer molecules. The predominantabsorption of PVP on the (1 1 1) planes may be due to theelectrostatic interactions, which originate from the lone pair ofoxygen atoms on the carbonyl group of pyrrolidone moiety. Also,a steric hindrance effect due to the pyrrolidone ring can beimplied to the size limiting of PbS particles. Thus, PVP directsthe crystal growth producing the perfect cubic geometries.Corresponding XRD patterns (Fig. 1 curves k and l) confirm thehighly structured growth of PbS particles.

A simple schematic illustration regarding the growth processof PbS crystallites in the presence of water soluble polymers ispresented in Fig. 8. As one observes from SEM and X-ray

diffraction measurements when using PAM, PVA and PEG thePbS particles are agglomerated with random crystalline orienta-tion, while in the case of PVP the semiconducting particles areisolated. As a result, for the PVP mediated growth we observed atexture of the synthesized PbS crystallites (Fig. 1 curve l) con-firmed also by SEM evaluation (Fig. 7 bottom).

These results conclude that the capping ability of a watersoluble polymer (PAM, PVA, PEG or PVP) significantly controls thecrystal growth of PbS micro- and nanoparticles.

4. Conclusions

Summarizing, we demonstrated that hydrosoluble polymers(PAM, PEG, PVA and PVP) can act as highly effective additive forthe preparation of PbS micro- and nanoparticles with almostuniform size and shape. This method does not require complexapparatus or reagents, and it is fast and seedless. It was found thatthe polymer type and its concentration plays an important role inthe morphology and structural properties of the PbS crystallites.

The polymer effect on the growth direction was confirmed byXRD and SEM measurements. The sharp and intense XRD diffrac-tion peaks prove that the PbS low-dimensional particles are wellcrystallized in cubic structure. For the PbS crystallites obtained inthe PVA, PEG and PVP, the (2 0 0) diffraction peak of the semi-conductor particles becomes dominant. The highest texture for PbSparticles is observed in the case of PVP. From the four polymersused PAM and PEG produce polydisperse PbS particles with sizeranging from 100 nm to several microns while PVA and PVP lead tomonodisperse PbS particles with an average size of 200 nm.

It is expected that the information about how to manipulatethe morphology of PbS crystallites using water soluble polymercan be easily extended to similar sulfide semiconductors particles.

Acknowledgements

This work was supported by CNCSIS-UEFISCSU, Project num-ber PNII-RU code 59/2010 (Contract no. 18/28.07.2010).

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