Growth dynamics of polyamidoamine dendrimer encapsulated CdS nanoparticles

Journal of Crystal Growth 361 (2012) 108–113

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Growth dynamics of polyamidoamine dendrimer encapsulated CdS nanoparticles P.F. Me´ndez a, S. Sepulveda b, J. Manrı´quez a, F.J. Rodrı´guez a, E. Bustos a, A. Rodrı´guez a, Luis A. Godı´nez a,n a b

´n y Desarrollo Tecnolo ´gico en Electroquı´mica. Parque Tecnolo ´gico Qro Sanfandila, P.O. Box 76703, Pedro Escobedo, Quere´taro, Me ´xico Centro de Investigacio ´n, Investigacio ´n y Desarrollo en Ingenierı´a y Tecnologı´a, Universidad Auto ´noma de Nuevo Leo ´n, Nuevo Leo ´n, Me´xico Centro de Innovacio

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 August 2012 Received in revised form 24 August 2012 Accepted 4 September 2012 Communicated by T. Nishinaga Available online 12 September 2012

CdS nanoparticles (CdS-nps) were synthesized employing charged and neutral polyamidoamide (PAMAM) dendrimers (generation G3.5, G4.0 and G4.0-OH with 64 –COO  , –NH3þ and –OH peripheral groups, respectively) as templates. The morphological characterization of CdS-nps was carried out using TEM measurements that showed spherical nanoparticles characterized by average diameter values between 3.6 and 4.7 nm. Using UV–vis experiments, CdS-nps formation was evaluated using a diffusion controlled growth model where the S2 diffusion coefficient throughout the external and internal functional groups of dendrimers was found to be an important parameter. In this way, the results indicate that nanoparticle formation is primarily affected by the charged nature of the external functional groups of the dendrimer template. & 2012 Elsevier B.V. All rights reserved.

Keywords: A2. Growth models from solutions B1. Nanomaterials B1. Polyamidoamine dendrimers B2. Semiconducting cadmium compounds

1. Introduction Due to the strong dependence of the optoelectronic properties of semiconductor nanoparticles on their size and shape, research on synthetic protocols has attracted the attention of many research groups around the world [1–4]. CdS is for instance a type II–VI semiconductor widely employed in photocatalysis [5–6], sensor design [7–8], non linear optic materials [9–10], optoelectronic devices [11–12] and photovoltaic applications [13–15]. While the most popular routes to obtain CdS nanoparticles (CdS-nps) require surface modifiers that influence nanoparticle growth such as cetyltrimethylammonium bromide (CTAB) [16–17] or bis (2-ethylhexyl)sulfosuccionate (AOT) [18–19], other synthetic protocols employ polymeric matrixes. In this regard, Chin et al. [20] reported the synthesis of CdS-polystyrene nanoparticles which show a broad UV–vis response, therefore suggesting a wide range of nanoparticle sizes. Another interesting type of compounds that have been used for the synthesis of CdS are the hyper-ramified polymers also known as dendrimers [21–22]. These materials are characterized by chemical structures with cavities and molecular surface environments that define endoand exo-receptor properties that can be used to selectively retain guest molecules. In this way, dendrimer complex formation can

n

Corresponding author. Tel.: þ52 442 2116006; fax: þ 52 442 2116007. E-mail address: [email protected] (L.A. Godı´nez).

0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.09.006

be promoted by different specific interactions such as electrostatic forces, solvophobic effects, steric confinement, and other types of driving interactions (Van der Waals, hydrogen bonding, etc). Many examples of dendrimer-based host–guest complexes have been reported, and among these, the cation–PAMAM (polyamidoamine) dendrimer complex (using for example Cu2 þ , Pd2 þ , Pt2 þ , Ni2 þ and Ru3 þ ) [23–27] stands out as a classic example of a nanoparticle synthesis template. In this approach, the dendrimer molecule works as a nano-reactor where a chemical reduction of the complexed cation can be carried out. In a similar fashion, the dendrimer molecule can also be used to encapsulate a cation and promote its chemical reaction with another species that, being in solution, can diffuse through the hyper-ramified molecule and react inside the dendritic structure. Specifically it has been shown that the synthesis of CdS semiconductor nanoparticles can be carried out inside a PAMAM dendrimer molecule and in this way, control of the size and the photoluminescent properties of the resulting CdS quantum dots can be achieved [22]. Published reports [28] indicate that dendrimer-encapsulated CdS can be prepared by two methods [29]. In the first approach, Cd2 þ and S2– salts are added to an aqueous or methanolic PAMAM dendrimer solution yielding a mixture of intradendrimer (templated) and interdendrimer particles. An alternative, higheryield method, relies on the sequential addition of small aliquots of Cd2 þ and S2– to alcoholic dendrimer solutions. In this regard, Luo et al. [30] have reported the mechanism of CdS nanoparticle formation in the presence of PAMAM dendrimer generation G4.0, where cadmium ions coordinate with amide groups inside the

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dendrimer structure, and eventually react with diffusing sulfide ions. On the other hand, Lemon and Crooks [31] have reported the formation of CdS in the presence of hydroxyl terminated PAMAM dendrimers of different generation finding that, as expected, the size of the dendrimer-confined CdS-nps is strongly dependent on dendritic generation (G). However, the effects of the dendrimer chemical structure on the growth dynamics of the CdS-nps have not been, to the best of our knowledge, reported. Therefore, in this work, the growth dynamics of CdS-nps were studied using S2  solutions containing previously prepared PAMAM dendrimer –Cd2 þ complexes (Cd2 þ /PAMAM) bearing 64 external –COO  , –NH3þ or –OH functional groups.

2. Materials and methods PAMAM dendrimers generations G3.5 (bearing 64 –COOH groups), G4.0 (64 –NH2 groups) and G4.0-OH (having 64 peripheral –OH groups) were purchased from Aldrich and employed to synthesize CdS-nps at 25 1C as follows. First, Cd2 þ /PAMAM complexes were obtained in water– MeOH mixtures prepared from pure MeOH (J.T. Baker, HPLC grade), adjusted to pH 7.5 7 0.2 with aqueous 0.1 M  HCl. The PAMAM–Cd2 þ solutions were maintained under stirring for 24 h using 1 mM  PAMAM dendrimer and 5 mM Cd(NO3)2 (J.T. Baker, 99%) for dendrimer G3.5 and 10 mM for the other two dendrimers (G4.0 and G4.0-OH). The ionization state of the external functional groups at this pH conditions can be predicted from the pKa values of the peripheral groups of the PAMAM dendrimers that in aqueous solution correspond to 4.5, 9.23 and 4 14, for G3.5, G4.0 and G4.0-OH, respectively [32]. Although the synthesis of the CdS-nps was carried out in a methanol–water mixture (87–13%), studies of Opitz and Wagner [33] suggest that in MeOH, the superficial amines of G4.0 dendrimer are protonated at the working pH value (NH3þ ). Furthermore, the pKa of the carboxylic groups of G3.5 increases to 6.8 (80–20 MeOH/H2O%vv) as reported by Ivanovic et al. [34], and therefore, we considered that the carboxylic groups in the G3.5 dendrimer are unprotonated (–COO  ). Meanwhile the G4.0-OH dendrimer remains neutral (–OH). Once the Cd2 þ / PAMAM complexes were prepared, 126 mL of each solution were diluted to 1.75 mL with pure MeOH contained in a 4.5 mL-quartz UV–vis cuvette and mixed with a methanolic Na2S  9H2O (Karal, 98%) solution under stirring. CdS-nps growth dynamics were then followed in situ by recording UV–vis absorption spectra of the cuvette content. In this way, the spectral data was obtained using an Ocean Optics USB2000 þ spectrophotometer controlled by the SpectraSuite software v6.0 previously installed in a PC. On the other hand, fluorescence measurements were performed with a Fluorolog 3 spectrophotometer, Horiba Jobin Yvon. Finally, the morphologic analyses of freshly prepared CdS-nps were performed from the data of a Transmission Electron Microscope (TEM) FEI TITAN 80–300 operated with an accelerating voltage of 300 kV.

3. Results and discussion 3.1. Structural characterization Fig. 1 shows the TEM images of CdS-nps synthesized in the presence of the three PAMAM dendrimers studied. The microscope images confirmed that spherical CdS-nps were present in all the samples and that their size distribution was nearly monodisperse. The size analysis (histograms not shown) from

Fig. 1. TEM images of CdS-nps synthesized in the presence of: (a) G3.5, (b) G4.0 and (c) G4.0-OH PAMAM dendrimers. The inset of Fig. 1a and b correspond to the electron diffraction patterns (SAED) an the circle in 1a, 1b and 1c, to one CdS nanoparticle.

Fig. 1 indicates that the diameter interval of CdS-nps fell between 3.6 and 4.7 nm. For dendrimers G3.5 and G4.0 the CdS-nps average values were from 3.6 nm to 4.0 nm and the largest nanoparticle size was obtained in the presence of G4.0-OH with an average size of 4.7 nm. Consistent with the difference in average nanoparticle size, while CdS-nps prepared in the presence of dendrimers G3.5 and G4.0 showed good crystallinity, CdS-nps obtained inside of G4.0-OH did not allow obtaining the electron diffraction patterns (SAED). This can be explained by considering that, quick nanocrystal formation inside neutral dendrimer molecules does not favor ordered crystalline structures, as opposed to what happens in the presence of charged dendrimer templates. Consistent with the interplanar distance calculated from the SAED of the insets in Fig. 1, CdS-nps obtained in the presence of G3.5 and G4.0 dendrimers, show a hexagonal structure.

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3.2. Optical characterization Fig. 2 shows the temporal evolution of the UV–vis absorption spectra related to CdS-nps formation inside PAMAM dendrimers G3.5, G4.0 and G4.0-OH. The evolutions of the spectra obtained with the three hyper-ramified molecules reveal a red-shift of the wavelength onset of the dendrimer-confined CdS-nps, thus confirming that nanoparticle size increases as a function of time. The

Fig. 3. Fluorescence spectra for CdS-nps: G3.5 (—), G4.0 (—) and G4.0-OH (y).

CdS-nps mean radius was computed by feeding the optical bandgap to the well-known Brus equation [35]. The plots of nanoparticle growth vs. time will be discussed later. Fig. 3 on the other hand, shows the fluorescence response of CdS-nps after 1 h of preparation time. Inspection of Fig. 3 shows that there is a blue-shift on the emission wavelength of the dendrimer confined CdS-nps (502 nm for G3.5, 522 nm for G4.0 and 531 nm for G4.0-OH) when compared to that of bulk CdS (650 nm) [36]. Consistent with their similar average nanoparticle size, the emission wavelength and intensity of CdS-nps formed in the presence of the two charged dendrimers, G3.5 and G4.0, were found to be similar. In contrast, larger CdS-nps prepared within the G4.0-OH dendrimer showed both, smaller fluorescence intensity and emission wavelength shift due to their larger size. 3.3. CdS-nps growth kinetics

Fig. 2. Temporal evolution of UV–vis absorption spectra obtained for CdS-nps formation in the presence of (a) G3.5, (b) G4.0 and (c) G4.0-OH PAMAM dendrimers.

Different models for diffusion controlled nanoparticle growth have been reported. The Ostwald ripening model, for example, proposes a growth mechanism where small particles dissolve and the monomer thereby released is consumed by larger particles [37]. Lifshitz and Slyozov and Wagner, on the other hand, developed the LWS theory [38] that describes the ensemble of particles during Ostwald ripening and while it can be used in the stationary regime during colloidal formation, it is not suitable for the earlier transient stages of particle growth. Also, Sugimoto [39] suggested a theoretical model in which the size dependent growth rate of nanoparticles was obtained by considering the Gibbs–Thomson equation. In this context, Talapin et al. [40] modified the model proposed by Sugimoto, considering the particle size dependence on the activation energies of the growth and dissolution processes. The model, therefore, involves the Gibbs–Thomson effect and the magnitude of capillary length. Recently, other diffusion-controlled models have been reported. Xie et al. [41] for example, developed a diffusion-controlled particle growth model in which the contribution of the flux of solute passing through a spherical surface follows the law of Fick. Su et al. [42] also developed a model that involves both; bulk diffusion and surface reaction process. In this context, two considerations were made for the treatment of the data obtained in this work (a) Cd2 þ only exists inside the dendrimer template in the Cd2 þ –PAMAM complex form and (b) The growth process is controlled by mass transport of the counterion S2  across the region of the internal structure of

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the PAMAM dendrimer (zone I) and across the peripheral dendrimer interphase (zone II) (see Fig. 4). Under these assumptions, in this paper a diffusion-controlled nanoparticle growth approach was employed for a better understanding of the dendrimer effects on the CdS-nps formation (see Eq. (1)) [41–42]. t¼

"

1 6Dab

2

In

2

a2 þ abr þ b r 2 ðabrÞ2

# pffiffiffi a þ2br þc 2 3 arctan pffiffiffi 3

ð1Þ

3

where a3 ¼ V m ðC 0S2 C eq2 Þ, b ¼ 4=3pN0 , C 0S2 is the initial conS centration of S2  , C eq2 the equilibrium concentration of S2  , Vm S the molar volume, No the site density, t, time, D, the S2  diffusion coefficient, r the nanoparticle radius and C an integration constant. In this way, CdS-nps radii vs. time plots were constructed for semiconductor particle growth inside the three dendrimers surveyed (see Fig. 5). Analysis of the corresponding data in Fig. 5, reveals that CdS-nps sizes increase quickly until approximately 125 s for the charged dendrimers, G3.5 and G4.0, and 250 s for the neutral G4.0-OH polymer (zone I) followed by a second region in the graph above these times (zone II) where CdS-nps growth is slower. Inspection of Fig. 5 shows a clear dependence of the CdSnps growth process, on the nature of the peripheral functional group of the dendrimer used as a template. In this way, inspection of longer times in Fig. 5 (zone II) reveals that while the use of carboxylated- and aminated-dendrimers (G3.5 and G4.0, respectively) results in CdS-nps radius sizes of about 1.99 nm, hydroxyl

Fig. 5. Computational fitting (solid lines) of the experimental data for CdS-nps synthesis using: (J) G3.5 (n) G4.0 or (n) G4.0-OH PAMAM dendrimers as templates.

Table 1 Fitted values of parameters for zone I. Dendrimer

DI (nm2/s)

a

b (nm  1)

Correlataion coefficient R2

G3.5 G4.0 G4.0-OH

8.61  10  10 8.97  10  10 8.85  10  10

126.02 130.37 120.36

61.69 64.68 58.59

0.97 0.99 0.98

Table 2 Fitted values of parameters for zone II. n rII is the maximum radius of CdS-nps during the growth process in zone II given by rII ¼ a/b (zone II).

Fig. 4. Representation of the starburst dendrimer zones in which CdS-nps are prepared.

Dendrimer

DII (nm2/s)

a

b (nm  1)

rIIn (nm)

Correlation coefficient R2

G3.5 G4.0 G4.0-OH

9.99  10  10 2.58  10  11 1.93  10  6

130.23 141.28 11.99

64.60 67.30 5.34

2.02 2.09 2.24

0.99 0.94 0.98

terminated-dendrimers (G4.0-OH) can host larger CdS-nps close to 2.13 nm at 3000 s of synthesis time. In order to rationalize the different growth kinetics promoted by the template dendrimers surveyed, the diffusion coefficients of S2  ions (DI and DII) across the two different dendrimer structure zones depicted in Fig. 4 were estimated using the model described by Eq. (1). The computational fitting in the three cases (G3.5, G4.0 and G4.0-OH) indicated an adequate correlation to the behavior predicted by Eq. (1) (see solid lines in Fig. 5 and rightend columns of Tables 1 and 2). In this way, the fitted parameters in Table 1 show that DI values for zone I are essentially independent of the dendrimer type confirming that at the initial stage of CdS-nps synthesis, nanoparticle growth is characterized by diffusion across the internal structure of the dendrimer molecule. On the other hand, it is important to highlight that although the diffusion coefficients are the same inside the three dendrimers surveyed, CdS nucleation time and size are different. These differences affect the growth rate and can be explained by considering that the concentration gradient of S2  is, in fact, affected by the charge of the external dendrimer groups, yielding a change in the S2 flux across the dendrimer internal structure (zone I).

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As opposed to the description of the process occurring in zone I, the diffusion coefficients of S2  in zone II are expected to be different and strongly dependent on the presence of charged functional groups in the dendrimer molecule. Consistent with this picture, the values of DII (see Table 2) reveal that while the charged dendrimers, G3.5 and G4.0, imposes a strong influence on the S2  diffusion coefficients across the external functional groups of the dendrimers, the uncharged G4.0-OH PAMAM polymer is characterized by a substantially larger DII and therefore a faster diffusion growth control; a fact that explains the rapid growing and the larger CdS-nps obtained when this template is employed. These results are also consistent with the morphological and spectroscopic studies previously discussed.

4. Conclusions The fitting of the spectroscopic experimental data to the growth model of nanoparticles controlled by diffusion and the study of images obtained from TEM experiments of CdS-nps formed inside charged and uncharged PAMAM dendrimers allowed us to understand the effect of the exterior and interior nature of the dendrimer template on the growth formation process. In this way, our results indicate that the predominant factor in nanoparticle growth kinetics is the presence of charge in the peripheral functional groups of the dendrimer template. The resulting electrostatic interaction between the reacting anion and the template surface structure was therefore found to affect not only the size of the CdS-nps formed but also their crystalline properties. Further studies with different dendrimers, in different media, and with a controlled surface charge density of the polymeric template, will certainly give further insight into the growth mechanism of semiconductor nanoparticles, thus allowing a fine control of their size and their associated physicochemical properties.

Acknowledgements The authors wish to thank the Mexican Council for Science and Technology (CONACyT, projects 83894 and 106000) for financial support of this work and Professor L. Ortiz-Frade and T. Griffith a Collaborator in CIDETEQ under the sponsorship of the US Peace CorpsPeace for their valuable comments. P.M also acknowledges CONACyT for a graduate fellowship (grant 104180).

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