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Colloids and Surfaces A: Physicochem. Eng. Aspects 345 (2009) 82–87

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

ffect of morphology and concentration on capping ability of surfactant inhape controlled synthesis of PbS nano- and micro-crystals

ulbir Singh, Aleisha A. McLachlan, D. Gerrard Marangoni ∗

ept. of Chemistry, St Francis Xavier University, P.O. Box 5000, 1 West Street, Antigonish, NS B2G 2W5, Canada

r t i c l e i n f o

rticle history:eceived 23 December 2008eceived in revised form 12 April 2009ccepted 16 April 2009vailable online 24 April 2009

a b s t r a c t

Micro- and nano-crystals of PbS with different morphologies have been successfully prepared at 80 ◦Cunder aqueous phase conditions by using cationic Gemini/dimeric (m-s-m) surfactants as capping agents.The effect of Gemini spacer length and concentration has been evaluated to make PbS crystal synthesismore selective. Well-defined PbS nano- and micro-structures including dendrites, nanocubes, and trun-cated nanocubes were selectively synthesized by varying the spacer length of Gemini surfactants. The

eywords:ead sulfideemini surfactantselective synthesiscanning electron microscopy

shape and size controlled synthesis of PbS crystals was obtained due to preferential adsorption of Geminisurfactants. To provide a comparison between monomeric and dimeric surfactants, PbS crystals have alsobeen synthesized using DTAB (a conventional monomeric counterpart of Gemini surfactant) as a cappingagent.

© 2009 Elsevier B.V. All rights reserved.

ransmission electron microscopy-ray diffraction

. Introduction

The production of nanocrystalline materials and the determi-ation of their properties have attracted a great deal of attention

n the recent literature [1–4] due to their potential applications inwide range of emerging devices [5–7]. In many cases, the prop-

rties of the nanoscale materials (e.g., the mechanical, chemical,ptical, electrical, electro-optical and magneto-optical propertiesf these materials) are different from those of the bulk materialsnd are dependent on the particle size [2]. Architectural controlf nanosized materials with well-defined shapes is critical for theuccess of “bottom–up” approaches toward future nanodevices.n recent years, there have been an increasing number of studiesn novel nanomaterials with defined shapes such as, nanocubes8], nanorods [9–11], nanowires [12–18], nanofibers [19], nanobelts20], nanotubes [21–24], and dendrites [8,25,26]. It should be notedhat we are still limited in our ability to understand and predict thenal structure of many nanomaterials. The possibility of program-ing the system to yield building blocks with desired shape and

rystallinity is still an area of active research.

Semiconductor nanocrystals are very important materials for

he fabrication of nanobuilding blocks such as nanodevices [27].ead sulfide (PbS) is a narrow gap semiconductor with a cubic rockalt structure, that has been gaining more attention in the literature.

∗ Corresponding author. Tel.: +1 902 867 2324; fax: +1 902 867 2414.E-mail address: gmarango@stfx.ca (D.G. Marangoni).

927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2009.04.033

As a result, an extensive amount of work on its preparation, proper-ties and applications is being carried out [28]. Generally lead sulfideis used in nanodevices such as light emitting diodes [29] single-electron transistors [30], and infrared detectors [31]. PbS quantumdots with stable efficient luminescence in the near-IR spectralrange has a wide range of potential applications including com-munication systems, biology imaging, and infrared photodetectors.PbS nanocrystals with various morphologies, such as spheres,cubes, rods, tubes, wires, truncated octahedrons, and dendriteshave been made under different experimental conditions [32–34].Due to the soft-template effect, reproducibility, and facile aqueousphase conditions, surfactant-assisted synthesis of semiconduc-tor nanoparticles (NP) is gaining popularity. So far, very little isunderstood about the effect of surfactant morphology and concen-tration on the shape of nano/micro-crystals of PbS. In fact, manyof the surfactant assisted nanoparticle syntheses reported in theliterature have been carried out in conventional single-headed,single-tailed surfactants. Surfactants consisting of two hydrophilicand hydrophobic groups are called Gemini or dimeric surfactants;they have a structure that can be viewed as two monomeric sur-factant units connected at or near the head groups by a spacer[35–37]. This kind of architecture provides solution properties thatare dependent upon the nature and size of the spacer [38,39].

Over the past 15 years, Gemini surfactants have been gainingmore research interest over conventional surfactants, as they areconsidered superior to corresponding conventional, monomericsurfactants in a number of aspects. Primarily, they have much lowercritical micelle concentrations (cmc) values and superior wetting

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The formation of PbS particles has been studied at low surfactantconcentration (below the cmc value of surfactant) and at high sur-factant concentration (above the cmc value). The same conditionshave been applied using DTAB as a capping agent in order to providea comparison between dimeric and monomeric surfactants.

K. Singh et al. / Colloids and Surfaces A:

roperties. Although the soft templating effect of short spacer Gem-ni surfactants has been investigated in the literature, to the bestf our knowledge there is no report in the literature indicatinghe effect of Gemini spacer on the shape of PbS crystals. In theresent study, we have selected a series of cationic Gemini surfac-ants with varying spacer length for the synthesis of PbS nano- and

icro-crystals. The effect of surfactant concentration and spacerength on the size and shape of the PbS particles has been evalu-ted using microscopy techniques. It is shown that the spacer lengthnd the concentration of the Gemini surfactant profoundly influ-nce the size and morphology of the nanocrystals. These results arexplained in terms of the capping effects of the Gemini surfactantn the nanocrystals.

. Experimental

.1. Materials

Acetic acid (AC), lead acetate, and thioacetamide (TAA) were pur-hased from ACP Montreal, BDH Laboratory, and EMD Chemical Inc.,espectively. All these chemicals were used as received withouturther purification. The m-s-m type cationic Gemini surfactantswhere m and s refers to the number of carbon atoms in the alkylhain and spacer, respectively)

N,N′-Bis(dimethyldodecyl)-1,6-hexanediammonium dibromide(12-6-12)N,N′-Bis(dimethyldodecyl)-1,8-octanediammonium dibromide(12-8-12)N,N′-Bis(dimethyldodecyl)-1,10-decanediammonium dibromide(12-10-12)N,N′-Bis(dimethyldodecyl)-1,12-dodecanediammonium dibro-mide (12-12-12)

ere prepared according to the method reported elsewhere[40,41].hese surfactants were purified by repeated crystallizations. 12--12 and 12-10-12 were recrystallized from acetone; however,he least quantity of ethanol was used to solubilize the surfac-ant in a hot saturated solution of surfactant and acetone. 12-6-12nd 12-12-12 were recrystallized from pure acetonitrile and ace-one, respectively. Dodecyltrimethylammonium bromide (DTAB)as purchased from Sigma–Aldrich and was recrystallized from an

cetone/ethanol mixture before use.

.2. Preparation of PbS crystals

PbS microcrystals (MC) and nanocrystals (NC) were synthesizedccording to the method reported by Bakshi et al. [26]. In a typicalrocedure, 30 mL of distilled water was placed in a round-bottomedlass flask. 5 mL of aqueous surfactant solution was added to theask under constant stirring. 4 mL of 1 M aqueous acetic acid wasdded to the flask, followed by the addition of 2 mL of 0.5 M aque-us lead acetate and 2 mL of aqueous 0.5 M thioacetamide underonstant stirring at room temperature. After the mixing was com-leted, the reaction mixture was kept in an oil bath at a temperaturef 80 ◦C for 48 h under static conditions. This led to the formation ofblack colloidal solution, indicating the formation of PbS NC. The

eaction is given below.

CH CSNH (aq) 2H O (l) + Pb(CH COO) (aq)

3 2 2 3 2

→ CH3COONH4 (aq) + PbS (s) + 2CH3COOH (aq) (1)

The PbS was collected and washed twice with water followedy multiple washings with methanol.

ochem. Eng. Aspects 345 (2009) 82–87 83

2.3. Characterization of samples

UV–vis spectra of aqueous solutions were acquired by using aUV spectrophotometer (CARY 100 Scan from Varian) in the wave-length range of 200–800 nm. The X-ray diffraction (XRD) patternswere recorded on an X’Pert pro Diffractometer (PANanalytical Co.)using Co-K� irradiation (45 kV, 40 mA). The shape and size of PbScrystals were characterized by transmission electron microscopy(TEM) and scanning electron microscopy (SEM). For TEM studiesthe samples were prepared by mounting a drop of solution on a car-bon coated Cu grid and allowing it to dry in air followed by stainingwith Uranyl acetate. The samples were observed with the help of aPhilips transmission electron microscope operating at 100 kV. SEMstudies have been carried out with a scanning microscope from JeolLtd. (JSM-5300). Prior to analysis, samples were air dried and thengold coating was applied with a sputter coater (Polaran, SC502).

3. Results

Fig. 1a depicts a representative XRD spectrum for a PbS particlesynthesized in this study. The peaks are very prominent and canbe indexed according to the rock salt structured PbS crystal havingan Fm3m space group [34]. The strong, sharp peaks suggest a well-defined crystallized product. Fig. 1b shows the UV–vis absorptionspectrum for a PbS particle; there is absorption in the band rangefrom 450 to 200 nm, which is typical for PbS [28].

Fig. 1. X-ray powder diffraction pattern (a) and UV–vis spectrum (b) for PbS crystalssynthesized using 12-12-12 and 12-6-12, respectively.

84 K. Singh et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 345 (2009) 82–87

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ig. 2. TEM micrograph of PbS crystals in the presence of low surfactant concentrarystals in the presence of low surfactant concentration of 12-6-12 (e), 12-8-12 (f), 1

.1. Low surfactant concentration

The TEM and SEM images for the low concentration (0.232 mM)emini surfactant systems are shown in Fig. 2 (with successive

ncrease in spacer from top to bottom). One can see that for the 12--12 system (Fig. 2a), star shape particles of 1.803 ± 0.1 �m in sizere formed. Increase in spacer from 12-6-12 to 12-8-12 results inhe formation of five-armed dendrites along with four-armed starsaverage leaf span of 1.130 ± 0.1 �m, Fig. 2b). As the spacer lengthpproaches 10 methylene units, for 12-10-12, there is formation ofarge structures along with some truncated cubes having averageizes of 1.41 �m and 192 nm, respectively (Fig. 2c). Finally, whenhe spacer length is increased to 12 methylene units, for 12-12-12,here is formation of NC that are roughly cubic in shape (Fig. 2d).

he SEM images depicting all of these shapes and also showing theelative abundance of the various species are shown in Fig. 2e–h.areful examination of Fig. 2 reveals that under identical conditionsf synthesis, an increase in Gemini spacer length results in the for-ation of more ordered cubic geometry over the leafy structures

f 12-6-12 (a), 12-8-12 (b), 12-10-12 (c), and 12-12-12 (d). SEM micrograph of PbS12 (g), and 12-12-12 (h).

and, further more, the particle size also decreases from the �m tonm range.

3.2. High surfactant concentration

The TEM and SEM images for the high concentration (1.5 mM)Gemini surfactant systems are shown in Fig. 3. From the TEM images(Fig. 3a–d), one can see the existence of hexagonal as well as somediamond shaped PbS nanocrystals for the various kinds of Geminisurfactants used in the synthesis. Cubic and octahedral geome-try can be extracted from a hexagon and a diamond, respectively,as shown in the inset of Fig. 3b. These cubes and octahedronsare clearly visible in the SEM images for the Gemini surfactantshaving the shortest and the longest spacer lengths (i.e. 12-6-12

and 12-12-12, Fig. 3e and f), with a relative abundance of cubes.It is evident that under high surfactant concentration conditions,the effect of spacer length is diminished and all systems resultin the formation of nanostructures with size ranging from 160 to215 nm.

K. Singh et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 345 (2009) 82–87 85

Fig. 3. TEM micrograph of PbS crystals in the presence of high surfactant concentration of 12-6-12 (a), 12-8-12 (b), 12-10-12 (c), and 12-12-12 (d). Inset in the panel (b)s ograph1

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hows the line-guided view for cube and octahedron. High magnification SEM micr2-12-12 (f).

.3. Studies with DTAB

To provide a comparison between Gemini and conventionalurfactants as capping agents in PbS synthesis, identical experi-ents have been carried out using DTAB, which is a conventionalonomeric counterpart of the m-s-m type cationic Gemini sur-

actants. The results for UV, TEM and SEM studies are shown inig. 4. It has been observed that under low surfactant conditions,TAB results in the formation of stars with predominantly fourrms having a wing span of approximately 2.5–3.0 �m. This obser-ation is consistent with the studies carried out by Bakshi et al.26]. The results for high surfactant concentrations were even morenteresting. During the course of the reaction, a brick red emul-ion was formed instead of black emulsion (as observed for Geminind, low DTAB concentration conditions). This red coloration stayednaffected even when the reaction time was increased to 72 h. Aignificant difference can be seen in the UV spectrum of the prod-ct from the red and black emulsion (Fig. 4a). The TEM results for

he product from the red emulsion are shown in Fig. 4b. Framesc and 4d are high magnification SEM images of the final productfter 48 and 72 h, respectively. All of the images from Frames 4b–4dhow the formation of cubic and octahedral crystals with an aver-ge size of 75–100 nm. To gain better insight into the formation of

of PbS crystals in the presence of high surfactant concentration of 12-6-12 (e) and

the red emulsion we have carried out the reaction for DTAB usingdouble the amount of the sulfur course (TAA). Once again, the redcoloration was observed in the beginning; however, after 48 h thered color was no longer apparent in the reaction mixture and cubeswith a size range from 145 to 230 nm were observed under TEMstudies (supplementary Fig. S1).

4. Discussion

The results from the various studies indicate that Gemini sur-factants are efficient capping agents and their concentration andspacer length have a direct effect on the shape of PbS crystals.

4.1. Spacer effect

The surfactant assisted synthesis of PbS crystals has alreadybeen studied from various aspects. Zhou et al. [8] have studied theinfluence of initial temperature, the molar ratio of Pb(AC)2/TAA,

and duration of reaction, while Bakshi et al. [26] have studied theeffect of hydrophobicity of twin tails over the conventional sur-factants. To make a precise comparison between conventional andtwin tail Gemini surfactants, it becomes important to study theeffect of spacer length of the Gemini surfactants on all desirable

86 K. Singh et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 345 (2009) 82–87

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ig. 4. (a) UV–vis spectrum for PbS crystals. (b) TEM micrograph of PbS crystals in thf PbS crystals in the presence of high surfactant concentration DATB for reaction ti

roperties such as the control of morphology of PbS crystals. Theequirement arises from the fact that colloidal properties of Gem-ni surfactants are more interesting primarily due to the spacer42]. Due to high hydrophobicity and morphology control, Gem-ni surfactants are considered to be better shape-directing agentsver conventional surfactants. The Gemini surfactants used in theresent study are strong ionic surfactants. According to Bakshi etl. [26] and Zhou et al. [8], these surfactants are expected to pref-rentially adsorb at the {1 1 1} planes of the PbS particles due tohe presence of a greater charge driven by high atomic density. Asas observed previously, this would leave the low atomic density

1 0 0} facets poorly capped or uncapped with surfactant molecules,ence, remaining available for further nucleation. Consequently,referential growth at the {1 0 0} facets occurs in comparison tohat occurring at the {1 1 1} facets; in other words, the formation ofix-arm dendrites, would result. According to Wang et al. [34], theour-arm and five-arm dendrites may be the result of the melt-offf six-arm dendrites. Hence, the crystalline dendrite would haveconsiderably increased surface energy compared with the equi-

ibrium shape of the crystal and is, therefore, thermodynamicallynstable. Hence, it is also possible that the origin of the dendrites

n these Gemini surfactant systems is also related to the kineticsf crystal growth, as indicated previously [34]. The capping abilityf a shape-directing surfactant is also related to its surface adsorp-ion behavior, which is in turn dependent on the spacer length ofemini surfactants. The adsorption behavior of Gemini surfactantst the air/water interface has been studied previously [38,43]. Itas been concluded that spacer folding occurs for Gemini surfac-ants with longer spacers resulting in the surfactant occupying less

urface area at the air/water interface. However, results presentedn this study indicate that the Gemini spacer is adsorbing flatlyt the water/PbS interface resulting in the tendency of the surfac-ant to cap all available surfaces for maximum coverage. Therefore,

great variety of crystal surfaces are fully developed leading to

nce of high surfactant concentration for DTAB. High magnification SEM micrograph48 h (c) and 72 h (d).

the formation of large particles, as in the case of 12-10-12. Theformation of cubes, in the case of 12-12-12, can be attributed tothe quenching of the reaction product without the evolution ofother morphologies (mainly due to high hydrophobicity). There-fore, it can be readily concluded that an increase in spacer lengthresults in the Gemini surfactant having less selectivity for facet{1 1 1} over {1 0 0}. As a result the formation of cubic type crystalsover the ferns/dendrites is favored. The effect of the spacer lengthon the size and shape of the PbS nanoparticles is diagrammed inSupplementary Fig. S2.

4.2. Concentration effect

It is clearly evident in Figs. 3 and 4 that at surfactant concen-trations above the cmc the capping behavior of all surfactants isthe same regardless of spacer length and surfactant morphology.Crystals with high symmetry (no fern or dendrites), e.g. cubes andoctahedrals, were generated under high surfactant concentrationseven when DTAB, a monomeric counterpart was used as a cappingagent. The formation of high symmetry crystals can be explained onthe basis of the capping ability of surfactants and the existence ofmicelles in the aqueous phase. The capping ability of any surfactantis related to its adsorption; under high concentration conditionssurfactant molecules try to adsorb at all available surfaces regard-less of the atomic density and as a result, surfactant selectivityis lost. Furthermore, the existence of micelles in solution leads tothe controlled release of reactants and the partial solubilization ofthe final product. In other words, micelles affect the overall kinet-ics of the reaction. This effect is considered to be responsible for

the formation of higher symmetry nanocrystals over dendrites andthe refinement of the overall synthesis. These observations becomemore apparent from the modified reaction for DTAB high concentra-tion where external kinetic control leads to the formation of cubeswithout the existence of a red color emulsion.

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K. Singh et al. / Colloids and Surfaces A:

. Conclusions

In this study, we demonstrated the shape controlled synthesisf PbS micro- and nano-crystals using Gemini surfactants as cap-ing agents. Capping ability of Gemini surfactants has been studiednder the effect of spacer length and surfactant concentration. Fromhe results for the various studies, it can be readily concluded thatn increase in spacer length results in the Gemini surfactant hav-ng less selectivity for facet {1 1 1} over {1 0 0}. As a result, theormation of cubic/octahedral type crystals over the fern/dendriterystals is favored. It is also clearly evident that at surfactant con-entrations above the cmc the capping behavior of all surfactantss the same regardless of spacer length and surfactant morphology.rystals with high symmetry (no fern or dendrites), e.g. cubes andctahedrals, were generated under high surfactant concentrationven with DTAB, a monomeric counterpart. It is further concludedhat cost effectiveness and surfactant uses can be readily adjustedy making an intelligent selection of capping agent under preciseoncentration conditions. Based on the systematic studies on thehape control, this approach is expected to be employed for thehape controlled synthesis of other fcc structural semiconductoraterials.

cknowledgements

The financial support of NSERC (Discovery Grant, D.G.M.,esearch Capacity Grant, StFX), the Atlantic Innovation Fund, andt. F.X. University are greatly appreciated.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.colsurfa.2009.04.033.

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