European Polymer Journal 49 (2013) 3530–3538

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Contents lists available at ScienceDirect

European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Macromolecular Nanotechnology

CdS and Cd carboxylate nanoclusters dispersed in polymermatrix produced by a freeze drying method

0014-3057/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.eurpolymj.2013.08.017

⇑ Corresponding author. Tel.: +1 848 445 3817.E-mail address: mhara@rutgers.edu (M. Hara).

Chonggang Wu a, Thomas J. Emge b, Frederic Cosandey c, Masanori Hara a,⇑a Department of Chemical and Biochemical Engineering, Rutgers University, Piscataway, New Jersey 08854-8058, United Statesb Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey 08854-8087, United Statesc Department of Materials Science and Engineering, Rutgers University, Piscataway, New Jersey 08854-8058, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 June 2013Received in revised form 13 August 2013Accepted 17 August 2013Available online 26 August 2013

Keywords:CdS nanoclustersCadmium carboxylate nanoclustersPMMA ionomerPMMA

Cadmium sulfide (CdS) nanoclusters were prepared by a freeze drying method from twotypes of cadmium carboxylates. One was cadmium methacrylates that were part of poly(-methyl methacrylate) (PMMA) ionomer. The other was cadmium acetates that were dis-persed in PMMA. X-ray diffraction was mainly used to study the formation and the sizeof nanoclusters. The size of CdS made from the ionomer was 0.9 nm, whereas that fromthe composite of cadmium acetate and PMMA was 2 nm. This was consistent with the sizedifference of the precursors of CdS: i.e., Cd carboxylate nanoclusters (ionic aggregates)were smaller in the ionomer than in the PMMA mixture, because ionic groups in the iono-mer were constrained due to their connectivity to backbone chains and thus forming smal-ler ionic aggregates. Once stabilized, however, CdS nanocluster sizes were unchangeddespite thermal treatments at up to 220 �C for 24 h for both systems. Structural transfor-mations from a freeze dried cadmium carboxylate powder, to a CdS-containing powder,and to a heat-treated CdS-containing sample are speculated for both types of systems.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Semiconductor nanoclusters (quantum dots, QDs) of 1–10 nm have been the subject of active investigation duringthe past 20 years [1–6]; and, QDs have found various appli-cations, such as biological imaging, photovoltaic devices,and light-emitting devices [5]. Due to their extremelysmall size, QDs exhibit electronic and optical propertiesthat are very different from those of bulk semiconductors.As the size of the semiconductor clusters is reduced to theexciton Bohr radius, its electronic properties begin tochange, a phenomenon referred to as the quantum con-finement effect [6]. The confinement effect appears as ashift to shorter wavelengths in the absorption spectrum(i.e., a blue shift) and thus changes the color of semicon-ductors, which reflects a change in the band gap. Sincesmall particles are thermodynamically less stable than lar-

ger particles are, preparation of kinetically stable (frozen)nanoclusters in a proper media is desirable for many appli-cations [7,8]. One of the widely used methods to freeze themeta-stable structure and to prevent further aggregationof nanoclusters is use of polymers [1,7]. When nanoclus-ters are mixed with a proper polymer, polymer chainscan be attached to nanoclusters and stabilize them bydeveloping repulsive forces between the nanoclusters(i.e., steric stabilization) [1,9,10]. Thus, various polymershave been used as a protecting (stabilizing) agent. Amongthem, ionomers are considered unique.

Ionomers are long-chain polymers that contain ionicgroups of less than 10–15 mol%, usually occurring asside-chain substituents attached to nonionic backbonechains. It is well established that self-assembly of ionicgroups in ionomers leads to the formation of nm-sizedionic aggregates in the nonionic polymer matrix [11].Despite random incorporation of ionic groups into polymerchains in random ionomers, resulting ionic aggregates arenearly monodisperse in size due to the constraints

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provided by polymer chains. This is similar to the effect ofconstraints provided by polymer chains to stabilize colloi-dal particles [10,11]. Since ionic aggregates can containdivalent metallic ions, these ionic aggregates can be con-verted to semiconductor nanoclusters by proper reactions[12–17]. For example, when passing H2S gas through theionomer that contains Pb2+ as counterions, ionic aggregatesthat contain lead ions are converted to PbS nanoclusters[13]. These clusters are surrounded and stabilized by poly-mer chains from the ionomer. Thus, an ionomer can func-tion as both a precursor and a protecting agent to formQDs. Samples are made either by precipitating the sul-fide/polymer composite or by solution casting as a thinfilm in which sulfide particles are dispersed. In another ap-proach, we reported a freeze drying (powder) method bywhich porous polymer matrix containing PbS was pro-duced [17]. During freeze drying, ionomer solution struc-ture is frozen and solvent molecules are removed bysublimation; thus, porous structure made of ionomer isformed. When this porous powder is subject to reactionwith H2S gas, lead ions are converted to PbS. This powdermay be used directly, for example as a catalyst, or powdercan be fabricated into 3D samples (e.g., sheets) by com-pression molding the powder for a short time (e.g.,5 min). The polymer chains surrounding PbS can minimizeaggregation of PbS. Also, powder can be dissolved in a sol-vent which cannot dissolve PbS. Thus, the freeze dryingmethod has advantages over other processing methods.

Although the freeze drying method is used to producethe semiconductor nanoclusters from an ionomer, weknow very little about the structural changes occurring inthe process. Generally, reports on nanoclusters made fromionomers are mostly concerned with a final product, semi-conductor nanocluster (e.g., PbS); and, little attention hasbeen paid to their precursors (e.g., Pb carboxylates)[12,13,17,18]. Since final product (ODs) is controlled by aprocessing method, understanding structural changes dur-ing the processing is important to control the structure andproperties of QDs. In this work, instead of studying struc-ture and properties of ODs, we focus our study on the pro-cessing methods and structural changes occurring duringformation of QDs. We prepared CdS nanoclusters fromCd-salt PMMA ionomers using the freeze drying method.In the ionomer, ionic aggregates are connected to backbonechains by strong covalent bonds; thus, ionic aggregates areconstrained to form small aggregates by polymer chains.This is in contrast to the colloidal system where polymerchains are attached by weaker secondary bonds, and lessconstraints are provided. To understand the effect of cova-lent attachment of ions to backbone chains, CdS nanoclus-ters were also made from a composite of cadmium acetateand PMMA, where ions are not covalently attached to poly-mer chains. Thus, we can study the confinement effect pro-vided by polymer chains to ionic aggregation and thusnanocluster formation, which is unique for ionomers. Be-cause PMMA ionomers containing divalent metal ionswere found to be more manageable than polystyrene (PS)ionomers in organic solvents [17], we chose to use PMMAionomers in this work.

2. Experimental section

2.1. Materials and preparation of samples

PMMA acid copolymer, poly(methyl methacrylate-co-methacrylic acid) (Mw = 1.0 � 105), and PMMA(Mw = 1.0 � 105) were purchased from Polysciences. Theacid (ionic) content was 12 mol%, determined by conducto-metric titration. The characterization of the polymers weredescribed elsewhere [19]. Cadmium acetate (Cd(OAc)2)(hydrate) and hydrogen sulfide (H2S) gas were purchasedfrom Sigma–Aldrich. PMMA acid copolymer was dissolvedin benzene/methanol (9/1 v/v) and acid groups were neu-tralized with an excess amount of cadmium acetate (5% ex-cess as calculated) to ensure complete neutralization.Cadmium acetate was assumed to be dihydrate(Cd(OAc)2)�2H2O, which has the maximum hydration level,for calculation. The solution was then poured into a largevolume of methanol to precipitate the ionomer (Cd salt).The ionomer sample was collected by filtration andwashed with methanol several times to remove the excesscadmium acetate. The ionomer sample was then dissolvedin benzene/methanol (9/1 v/v) and freeze dried.

The freeze-dried ionomer powder was reacted withH2S gas to produce CdS nanoclusters. Cd-salt ionomersample was placed in a Claisen flask that was fitted witha gas inlet and an outlet. The outlet was connected to aballoon to maintain the flask under a positive H2S blanket.The Cd-salt ionomer was maintained under a positive gasblanket for 1 day during which the color changed frompure white to light yellow. To remove the excess H2S,the flask was opened to atmosphere in a well-ventilatedfume hood for about 3 h. Further removal of the gas wascarried out in a vacuum oven at room temperature for an-other 12 h.

For comparison, CdS nanoclusters that were dispersedin a PMMA matrix were prepared. PMMA and cadmiumacetate were dissolved separately in benzene/methanol(9/1 v/v) and the cadmium acetate solution was addeddrop-wise to the PMMA solution under stirring. Afterfreeze drying the solution, the resultant powder was re-acted with H2S gas in a similar manner as for Cd-salt iono-mers. The weight fraction of Cd2+ ions (relative to polymerchains) in the cadmium acetate/PMMA composite waschosen to be the same as that in the Cd-salt ionomer. Pow-der samples were heat treated by vacuum drying at hightemperatures (up to 250 �C). Sheet specimens were pre-pared by melt pressing in a Waback compression moldingmachine. Dry powder sample was placed between twopieces of Teflon-coated aluminum foil (McMaster Carr)and melt pressed between two preheated flat metal plates.

2.2. Experimental procedures

Wide-angle X-ray scattering (WAXS) measurementswere conducted by using an area detector (Bruker, HiStar)and a rotating anode X-ray generator (Enraf-Nonius,FR571) with a graphite monochromator (Cu Ka;

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k = 1.542 Å), operating at 40 kV and 60 mA. A sheet speci-men was mounted directly onto a sample holder. A powderspecimen was placed in a glass capillary with 1 mm diam-eter and mounted onto a sample holder. All the data werecollected at room temperature for 30 min. CdS nanoclus-ters were also studied by transmission electron micros-copy (TEM) using a Topcon 002B, operating at anaccelerating voltage of 200 kV. The CdS/PMMA compositewas dissolved in dichloroethane and the CdS/PMMA acidcopolymer composite was dissolved in dimethylformam-ide, respectively. These solutions were deposited on car-bon-coated TEM grids, and after evaporation of thesolvent, pictures were taken. The CdS nanoclusters wereimaged by dark-field technique [20] using the portion ofthe first (111) ring from CdS.

3. Results and discussion

3.1. Processing and size determination of nanoclusters

Two types of CdS/polymer composites were made andcompared in this study. One was made from cadmium ace-tates dispersed in PMMA: when reacted with H2S, Cd ionswere converted to CdS in the PMMA matrix. The other wasmade from the Cd-salt ionomer: when reacted with H2S,Cd ions were converted to CdS and carboxylate ions wereconverted to carboxylic acids (i.e., methacrylic acids). Thus,CdS nanoclusters were dispersed in the matrix of thePMMA acid copolymer (see Scheme 1). We showed thatfreeze drying of an ionomer solution produced an ionomerpowder sample with high porosity, which was then reactedwith H2S gas to produce sulfide [17]. In this work, we ap-plied the freeze drying method not only to the Cd-saltionomer but also to the Cd acetate in PMMA to makeCdS. The processing procedures are summarized in Fig. 1.

To study the formation and the size of nanoclusters,WAXS was mainly used. WAXS patterns of nanoclusterswere obtained by subtracting the contribution of the ma-trix polymer from that of the composite (i.e., nanoclustersand the matrix polymer). The positions of three peaks ob-served for both PMMA and the PMMA acid copolymer (amajor peak at �13� and two minor peaks at �30� and�39�) are in good agreement with literature values foramorphous PMMA [21,22]. After subtraction, these peaksdisappeared. A similar method of background subtractionwas used for samples that contained very small metalnanoclusters (e.g., Pd) in a silica matrix: broad low inten-

Scheme 1. Chemical reaction for prepara

sity peaks, arising from the metal nanoclusters, wereclearly delineated by subtraction [23]. For these nanoclus-ters, the smaller the crystallites, the broader the peaks;and, the average diameter, D, of nanoclusters can be deter-mined by using the Scherrer equation [24,25].

D ¼ kkB cos hB

ð1Þ

where k is the wavelength of the X-ray (0.154 nm), 2hB isthe peak (Bragg) angle, B is the angular width at the half-height of the peak (at 2hB), and k is the Scherrer constant(k = 0.9 is used in this work [25]). Originally developedfor determining the grain size of crystals, now the equationis widely used to determine the size of nanoclusters[7,26,27]. We used the 1st peak, which is stronger thanother peaks, to determine the average size of crystallites.However, even when the smaller 2nd peak was used (forexample in Fig. 2), the similar value of D was obtainedwithin ±5%.

3.2. Heat-treated CdS nanoclusters

Fig. 2 compares the WAXS profiles of CdS nanoclustersthat were made from the Cd(OAc)2/PMMA composite(Fig. 2a) and from the PMMA ionomer (Fig. 2b). In Fig. 2a,WAXS pattern is characteristic of cubic CdS crystals (zincblende structure): a (111) peak at ca. 26�, a (220) peakat ca. 44�, and a (311) peak at ca. 50� [18]. CdS size is2.0 nm determined by using Eq. (1) for the first peak. TheWAXS peaks corresponding to the hexagonal structure thatis generally observed for bulk CdS are not noted [28]. InFig. 2b, the scattering profile is similar to Fig. 2a exceptthat peaks become broader and smaller as compared withFig. 2a, reflecting the smaller size of CdS made from theionomer. CdS size is 0.9 nm determined by using Eq. (1)for the first peak. Fig. 2b also shows that the 3rd (311)peak disappears, which may reflect less ordered structureof the smaller CdS clusters. We calculated the number ofions per nanoclusters by assuming that the shape ofnanoclusters is spherical for simplicity. Also, we used thefacts that CdS takes a zinc sulfide type FCC structure andthat ionic radii of Cd2+ and S2� are 0.097 nm and0.184 nm, respectively [29]. The calculation shows thatthere are 61 Cd2+S2� pairs for the CdS clusters in PMMAand 6 Cd2+S2� pairs for the CdS clusters in PMMA acidcopolymer. For the latter, because of a small number ofions per cluster, many ions are present on the cluster

tion of CdS from PMMA ionomer.

Fig. 3. Dark-field TEM images of (heat-treated) CdS nanoclusters madefrom: (a) cadmium acetate/PMMA composite; (b) Cd-salt PMMA ionomer.

Fig. 2. Typical WAXS patterns of the (heat-treated) CdS nanoclusters made from: (a) cadmium acetate/PMMA composite; (b) Cd-salt PMMA ionomer.

Fig. 1. A processing procedure used in this work.

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surface, disturbing the ordered structure as observed.Overall, the most notable difference between the two typesof composites is the smaller size of CdS in the PMMA acidcopolymer than in the PMMA: the size is reduced to lessthan half (2.0 nm vs. 0.9 nm) and the volume (and thenumber of ions per cluster) is less than 10%.

When nanoclusters are very small, it is sometimes diffi-cult to apply WAXS to determine the accurate size [25]. Toconfirm the size difference obtained by WAXS for the twotypes of composites, we also applied TEM (dark filed imag-ing) for the CdS nanoclusters. Fig. 3a is for CdS in PMMAand Fig. 3b is for CdS in PMMA acid copolymer. Dark-fieldimages have higher contrast than bright field images, andbright regions in the image reflect only the ordered areas[20,30]. Thus, CdS nanoclusters are seen as bright areas.Clearly, particle sizes are larger in Fig. 3a than those inFig. 3b: size ranges of ca. 1–1.4 nm and 0.5–0.7 nm, respec-tively, were determined by using an electron beam inten-sity profile for many particles imaged. The valuesobtained by TEM are often smaller than those by WAXS,because WAXS reflects bigger particles more, while TEMreflects all particles when enough particles are observed

[31]. Despite smaller values than those obtained by X-raydiffraction, diameter values are consistent, indicating thatthe size of CdS in the acid copolymer is about half the sizeof CdS in PMMA. Also, a well-defined electron diffractionpattern was observed in the area from which picture ofFig. 3a was taken, but more diffuse diffraction patternwas observed in the area from which picture of Fig. 3bwas taken (not shown), indicating the formation of moreordered structure for CdS in PMMA and less ordered

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structure for very small CdS made from the ionomer. Thisis consistent with the WAXS result as already described.Because the sizes of small particles determined by WAXSwere reasonable by comparing with TEM results, we usedWAXS exclusively to study other samples that containedlarger particles than these small CdS. Certainly, compara-tive use of WAXS for nanoclusters in this work iswarranted.

The size difference between the two types of compos-ites is also noted by sample colors, which reflect thechange in the band gap arising from the size change ofCdS. CdS nanoclusters in PMMA are deep yellow/orange(Fig. 4a), whereas those from the PMMA ionomer are paleyellow (Fig. 4b). For comparison, bulk CdS is orange(Fig. 4c). The color is considered as a combined color ofthe lights that are reflected by CdS nanoclusters [32].When CdS nanocluster becomes larger, the band gap be-comes smaller [12] and lights of lower energies are ab-sorbed. Therefore, smaller CdS nanoclusters are paleyellow, because only a light with the highest energy (vio-let) can be absorbed and thus a complementary color ofviolet (i.e., yellow) is observed. When nanoclusters becomelarger, a light of lower energy (blue) can be absorbed inaddition to a violet light, and now the complementary

Fig. 4. Colors of (a) (heat-treated) CdS in PMMA; (b) (heat-treated) CdSfrom PMMA ionomer; (c) bulk CdS. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of thisarticle.)

color of blue (i.e., orange) is added to yellow, thus colorturns yellow–orange [32]. The difference in color clearlyreflects the difference in CdS size between the two typesof composites, as demonstrated by WAXS and TEM.

3.3. Non-heat-treated CdS nanoclusters (made from freezedried powder)

Before applying heat treatments, CdS powders (Fig. 1c)were made from precursors (cadmium carboxylates)(Fig. 1b) that were made by freeze drying. The WAXS re-sults of these CdS powders are compared in Fig. 5. CdSnanoclusters in PMMA show a clear WAXS pattern(Fig. 5a), reflecting the FCC CdS structure. The CdS size is2.0 nm determined from the first peak. In contrast, CdSnanoclusters in the PMMA acid copolymer show no scat-tering profile (Fig. 5b). This indicates formation of verysmall CdS nanoclusters that are made from the PMMAionomer. The result is consistent with that obtained forPbS nanoclusters that were made from a polystyrene iono-mer: i.e., a cast film developed no WAXS peaks, whereasthe film developed a WAXS pattern upon heating due tothe formation of larger PbS clusters [17].

For these powder samples, the CdS nanoclusters inPMMA are yellow/orange (Fig. 6a), whereas those fromthe PMMA ionomer are white (Fig. 6b), again reflectingthe size difference. The color of CdS in PMMA is same be-fore and after heat treatment, which reflect the same sizeof CdS clusters, 2.0 nm determined by WAXS as explainedabove. In contrast, CdS nanoclusters of a very small sizeare made from the PMMA ionomer. In this case, the bandgap is too large and no lights can be absorbed, thus the col-or of reflected light is a mixture of all colors, i.e., white. Thisis in agreement with the report that very small CdS nanocl-usters are obtained as a white powder when precipitatedfrom aqueous CdS colloidal solution [1].

3.4. Precursors of CdS (Cd carboxylates)

The results by WAXS, TEM, and sample color observa-tion all indicate the size difference of CdS between thetwo types of composites. It is not unreasonable to expectthat the size difference described above about CdS nanocl-usters may start from the size difference between cad-mium carboxylates (i.e., the precursors of CdS) (Fig. 1b).Thus, WAXS profiles are compared for the precursors:Fig. 7a is for cadmium acetate in PMMA and Fig. 7b is forCd carboxylates (methacrylates) in the Cd-salt ionomer.Fig. 7a shows a WAXS pattern with three peaks at ca.10�, ca. 25�, and ca. 45�: D = 2.8 nm is obtained by usingEq. (1) for the first peak. In contrast, Fig. 7b shows noWAXS pattern; i.e., size of the Cd carboxylates in the Cd-salt ionomer is too small to be determined by WAXS. Thisis because covalent connection of ionic groups to backbonechains makes the ionic aggregates small and because ionicaggregates in the ionomer solution are further destroyedby methanol that is added to benzene [33]; and, solutionstructure is maintained and reflected in the structure ofthe freeze dried sample [34].

It is well established that, by increasing the size of crys-tallites, WAXS peaks become sharper and the number of

Fig. 5. WAXS patterns of the CdS nanocluster powders prior to heat treatment: (a) CdS in PMMA; (b) CdS from PMMA ionomer.

Fig. 6. Colors of freeze dried samples: (a) CdS in PMMA; (b) CdS fromPMMA ionomer. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

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peaks is increased [35]. Thus, we applied heat treatment tothe samples of Fig. 7, because heating the sample makesnanoclusters larger and should develop more peaks (seeFig. 8). From Fig. 8a, D = 8.6 nm is determined from the firstpeak for the heated cadmium acetate/PMMA sample. Thisis compared with D = 2.8 nm for the unheated sample(Fig. 7a). Also, Fig. 8a shows more peaks than Fig. 7a. Thepeak positions are consistent with those of the WAXS pat-tern of the crystalline Cd(OAc)2(III) structure reported byAkhmetov et al. [36]. A WAXS pattern also appears by heat-ing the Cd-salt ionomer sample (Fig. 8b) in contrast to apatternless result for the unheated sample (Fig. 7b). This

Fig. 7. WAXS patterns of freeze dried non-heat-treated samples: (a) cadmiumcarboxylates in the Cd-salt PMMA ionomer.

may be again due to the formation of larger nanoclusters(Cd methacrylates) upon heating (D = 1.9 nm from the firstpeak) [37]. This change is similar to the one observed forthe CdS/PMMA acid copolymer composite: no peaks forthe unheated sample (Fig. 5b) but peaks of D = 0.9 nm forthe heated sample (Fig. 2b). Note that we carried out heat-ing experiments (results in Fig. 8) to help the assessment ofstructure which was made using WAXS data of Fig. 7; but,we did not use heated samples for making CdS nanoclus-ters (we used freeze dried samples to make CdS nanoclus-ters by the freeze drying method).

3.5. Heat treatments of CdS

Once CdS clusters are thermally stabilized, both PMMAand PMMA acid copolymer seem to work as an effectivestabilizer for CdS nanoclusters, because further heat treat-ments either by vacuum heating (Fig. 1d) or compressionmolding (Fig. 1e) at high temperatures do not cause furtheraggregation. For example, the size of CdS formed in PMMAby reacting with H2S a freeze-dried composite of cadmiumacetate and PMMA was 2.0 nm (Fig. 9-1). When this sam-ple was compression molded (Fig. 9-2 and -3) or vacuumdried (Fig. 9-4 and -5), WAXS patterns were similar andCdS size remained 2.0 ± 0.2 nm. In the PMMA acidcopolymer, an initially patternless profile (Fig. 10b-1) is

acetate nanoclusters in the cadmium acetate/PMMA composites; (b) Cd

Fig. 8. WAXS patterns of heat-treated samples: (a) cadmium acetate nanoclusters in the cadmium acetate/PMMA composites, compression molded at190 �C for 6 min; (b) Cd carboxylates in the Cd-salt PMMA ionomer, compression molded at 220 �C for 30 min.

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transformed to a WAXS pattern upon heating for as shortas 10 min (Fig. 10b-2) and then unchanged: CdS size was0.9 ± 0.1 nm for compression molded samples (Fig. 10a)and 0.8 ± 0.2 nm for vacuum dried samples (Fig. 10b).These seem to be the stable sizes for CdS nanoclusters (sur-rounded by the PMMA acid copolymer) under the givenheat treatment condition. Compression molded samplesshowed more consistent data, probably due to more effi-cient heat transfer during thermal treatment. Nevertheless,prolonged thermal treatments (up to 24 h at 220 �C) didnot induce further aggregation of CdS nanoclusters for both

Fig. 9. WAXS patterns of the CdS nanoclusters made from cadmiumacetate/PMMA composites: (1) 0 min (freeze dried and H2S treated); (2)compression molded at 190 �C for 6 min; (3) compression molded at250 �C for 6 min; (4) vacuum heated at 190 �C for 24 h; (5) vacuumheated at 220 �C for 24 h.

systems. Thus, both PMMA and PMMA acid copolymerwork as an effective stabilizer for CdS nanoclusters. SinceCdS nanoclusters are made of ionic groups, interactions be-tween the ionic groups (Cd2+ S2�) on the surface of CdSnanoclusters and the dipoles of –COOCH3 groups in poly-mer chains can prevent further aggregation of CdSnanoclusters.

3.6. Structural transformations

While a complete picture is yet to be obtained, struc-tural changes occurring during the processing are specu-lated for two types of systems based on the dataobtained in this work. In the composite of cadmium ace-tate and PMMA (Fig. 11a), ionic aggregates made of cad-mium acetate are formed in solution. Because, insolution, ions are not connected to polymer chains viacovalent bonds, large ionic aggregates are formed, whichare surrounded by PMMA chains. This condition is similarto that of colloids stabilized by polymer chains. This struc-ture may be maintained during freeze drying [38]. Whenthe freeze dried powder of cadmium acetate/PMMA wasreacted with H2S gas, acetate ions are replaced with smal-ler sulfide ions (S2�) to form CdS; thus, nanoclusters be-come smaller (2.8 nm for cadmium acetate ? 2 nm forCdS). Because of the structure surrounded (stabilized) byPMMA chains, heating does not cause further aggregation.Thus, the CdS made from a freeze dried sample maintainsthe size during heat treatments.

In contrast, for Cd-salt PMMA ionomer (Fig. 11b), verysmall ionic aggregates are formed, because covalentattachment of ions to the polymer chains makes ionicaggregates very small. This is due to the constraining effectby polymer chains well known for ionomers [11]. In addi-tion, there may be some unaggregated Cd2+ ions in solu-tion, because methanol is known to weaken ionicaggregates in ionomer solution [33]. When reacted withH2S, Cd2+ ions are converted to CdS. However, strong con-nection of ions to polymer chains is now lost; and, these

Fig. 11. A schematic figure showing the structural changes occurring during processing for: (a) cadmium acetate/PMMA composite; (b) Cd-salt PMMAionomer.

Fig. 10. WAXS patterns of the CdS nanoclusters made from Cd-salt PMMA ionomer: (a) compression molded at 220 �C for (1) 6 min, (2) 30 min, (3) 2 h, (4)8 h; (b) vacuum heated at 220 �C for (1) 0 min (freeze dried and H2S treated), (2) 10 min, (3) 20 min, (4) 30 min, (5) 24 h.

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largely unaggregated ions are simply dispersed in the poly-mer matrix. Upon heating for 10 min (cf. Fig. 10b-2), aggre-gation of CdS occurs until the size reaches an optimumvalue (0.8 nm), which is still smaller than CdS in thePMMA; and, the size of these aggregates surrounded (sta-bilized) by PMMA acid copolymer chains remains the sameupon further heating (250 �C for 24 h). The aggregation ofsulfides and resulting formation of very small clusters havebeen demonstrated for a PbS/PS-ionomer system (Fig. 1 of[16]), which is similar to the mechanism described abovefor CdS/PMMA-ionomer system.

4. Conclusions

By using a freeze drying method, we have made CdSnanoclusters from the Cd-salt PMMA ionomer and fromthe cadmium acetate/PMMA composite. The CdS nanoclus-ters are smaller when they are made from the ionomerthan when they are made from cadmium acetate dispersedin PMMA: the size is less than half (0.9 nm vs. 2.0 nm) andthe volume is less than 10%. This is consistent with theobservation on the precursors of CdS:Cd carboxylates inthe Cd-salt ionomer are smaller than cadmium acetates

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in PMMA. Ionic groups (ionic aggregates) are constrainedto form smaller sized nanoclusters due to the connectivityto backbone chains in the ionomer solution. This size dif-ference is translated into the size difference in the CdSnanocluster powder. Once stabilized, the size of CdSnanoclusters remains constant despite prolonged heating(for 24 h at 220 �C) both in PMMA and in PMMA acidcopolymer possibly due to interactions between the ionicgroups (Cd2+ S2�) of CdS and the dipoles of carboxylategroups. We speculated structural transformations from afreeze dried cadmium carboxylate powder, to a CdS-con-taining powder, and to a heat-treated CdS-containing sam-ple. Wide-angle X-ray scattering pattern and the color ofCdS quantum dots that are made from the PMMA ionomerremain the same at room temperature for 6 months, indi-cating no further aggregation of CdS nanoclusters. Thesesamples may be used as a good catalyst for gas-phase reac-tions, since CdS nanoclusters have a huge surface area andare dispersed in a highly porous polymer matrix (due tofreeze drying).

Acknowledgments

Acknowledgment is made to the Donors of the Ameri-can Chemical Society Petroleum Research Fund for supportof this research.

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