Communication pubs.acs.org/cm
Special Role for Zinc Stearate and Octadecene in the Synthesis of Luminescent ZnSe Nanocrystals Mateusz Banski,*,† Mohammad Afzaal,‡ Mohammad A. Malik,§ Artur Podhorodecki,† Jan Misiewicz,† and Paul O’Brien*,§ †
Department of Experimental Physics, Wroclaw University of Technology, Wroclaw 50-370, Poland Center of Research Excellence in Renewable Energy, King Fahd University of Petroleum and Minerals, PO Box 1292, Dhahran 31261, Saudi Arabia § The School of Materials and The School of Chemistry, The University of Manchester, Manchester M13 9PL, United Kingdom ‡
S Supporting Information *
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stabilizing ions for the NC surface. The reduction of elemental Se by ODE has been identified as the likely rate-limiting step for the formation of monodispersed NCs.17 In related cases of cadmium sulfide and cadmium selenide NCs, chemical complexities involved during their preparation have only recently been understood.17−19 X-ray powder diffraction (XRD) confirmed the formation of crystalline ZnSe nanoparticles, with the cubic zinc-blende (ZB) structure (Supporting Information Figure S1). No peaks for other crystalline forms were observed.20 The line width of the ⟨220⟩ reflection peak was analyzed using the Scherrer equation, and the average diameters of ZnSe NCs were ca. 6, 4.5, and 5 nm for [ZnSt2]:[Se] molar ratios of 0.5:1, 1:1, and 1:0.5, respectively. Transmission electron microscope (TEM) showed nearly spherical and highly crystalline particles as shown in Figure 1. The concentration of ZnSt2 strongly influenced the NC morphology. The samples prepared with stoichiometric [ZnSt2]:[Se] are regular shaped and are close to monodispersed. In other samples some agglomeration of NCs is clearly seen in the TEM images. The particles from TEM images (Figure 1) have average diameters of 5.8, 5.4, and 5.6 nm for [ZnSt2]:[Se] ratios 0.5:1, 1:1, and 1:0.5, respectively.
he last two decades have seen emergence of three main solution based routes1 for the preparation of semiconductor nanocrystals (NCs): hot injection, chemical reduction, and heating up methods. In many initial protocols trioctylphosphine oxide (TOPO)2 is considered as an obligatory reactant. However, after detailed investigations it was realized that TOPO plays the role of a solvent and transports necessary phosphorus containing compounds for the processing of NCs.3−5 Further development of NCs synthesis successfully approached phosphine-free protocols, which are based on coordinating oleic acid (OA) and noncoordinating 1octadecene (ODE).6 However, their role in the NCs synthesis is still yet to be fully understood. Indeed, luminescent zinc selenide (ZnSe) NCs using tributylphosphine (TBP) or trioctylphosphine (TOP) for generating reactive selenium (Se) species has been well documented.7,8 Simultaneously, the activation of zinc stearate (ZnSt2) by alkylamines is reported to be essential for the synthesis of NCs.7,9 It is widely accepted that TOP and TBP are not ideal, and therefore, low-cost and more environmental friendly Se routes have been put forward.10 Some of the reactants studied for ZnSe NCs include bis(trimethylsilyl)selenium,11 selenium dioxide (dispersed in ODE),12 and Se (dispersed in paraffin oil13 or ODE14). In all of these cases the hot-injection method is preferred. Zhang and co-workers have used an injection technique to prepare ZnSe NCs, by reacting zinc nonanoate and hydrogen selenide (H2Se) in the presence of octylamine (OcA) and oleylamine (OLA).15 However, the exact role of amines remains unclear. Recently, Peng’s group presented a comprehensive report about the Se-ODE suspension emphasizing its advantages in a hot-injection method.16 This confirms results of Jasieniak et al., who synthesized high quality CdSe NCs by hot-injection of SeODE suspension to cadmium oleate solution.5 The authors believe without providing any evidence that crystalline powdered selenium thermally dissolves into its various allotropes (polymeric Se) which act as selenium precursor. In this communication, for the first time we present a detailed investigation of this reaction and propose a mechanism for the formation of ZnSe NCs. The reaction involves a simple synthetic approach to luminescent ZnSe NCs, which essentially involves the heating of ZnSt2, Se, and ODE at 290 °C. The role of ZnSt2 is to provide both zinc and limited numbers of © XXXX American Chemical Society
Figure 1. TEM (a−c) and corresponding size distributions (d−f) of ZnSe nanocrystals prepared with [ZnSt2]:[Se] molar ratios of 0.5:1, 1:1, and 1:0.5, respectively. Inset in (a−c) shows corresponding highresolution images of ZnSe nanocrystals. Received: January 19, 2015 Revised: May 17, 2015
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DOI: 10.1021/acs.chemmater.5b00347 Chem. Mater. XXXX, XXX, XXX−XXX
Communication
Chemistry of Materials Energy dispersive X-ray analysis shows that the final products differ from the ideal stoichiometry (Supporting Information Figure S2). The NCs grown with an excess of Se, i.e., a [ZnSt2]: [Se] molar ratio of 0.5:1, have a final composition close to stoichiometric. The other samples have Zn to Se ratios that increase with the [ZnSt2]:[Se] molar ratio. The amount of Zn in all samples equals or exceeds that of Se, which may imply that delivery of selenium ions is limited in the growth process (as discussed later). The excess zinc covers the NC surface; thus, Zn rich cationic facets can be expected to be formed. The ZnSe NCs were studied by Fourier transform infrared spectroscopy (FTIR) (Supporting Information Figure S3). In all samples, the presence of a long aliphatic chain is shown by methylene (CH2) vibration at ∼1460 cm−1. There are no bands at ∼1710 cm−1 as associated with carboxylate. Two bands appear at 1538 and 1398 cm−1, which are associated with the asymmetric and symmetric stretching vibrations of carboxylate anions, respectively. This observation confirms that the negatively charged carboxylate (COO−) is the surface passivating ligands for the NCs. The 1538 cm−1 stretch for zinc rich samples corresponds well to that of zinc stearate. The band is noticeably shifted toward higher wavenumber (1549 cm−1) for NCs with a stoichiometric [ZnSt2]:[Se] molar ratio. The observed shift suggests that the carboxylate ligands are coordinated to ZnSe facets and result in less stable solutions (as shown in Figure 1). Controlled nucleation and subsequent particle growth is crucial for the successful synthesis of NCs. The concentrations of precursors as well as the number of available ligands during growth are both critical parameters since they determine the number of available monomers and reactivity. Precise control is an important challenge in the processing of high quality NCs.21 Yu and Peng correlated the role of long alkene acids concentration with monomer reactivity.22 They found that the higher the ligand concentration, the lower the monomer reactivity. If the OA concentration was too high, the resulting NCs were broadly dispersed in size, probably due to the lack of discrete nucleation. If the concentration of ligand is only just sufficient to provide surface stabilization, the reactivity of the system increases. Rapid nucleation can lead to a large number of smaller, homogeneous particles.21 In our protocol the capping ligand is stearate anions delivered as ZnSt2. The Zn2+ ions should exhibit their maximum reactivity in the synthesis due to the limited concentration of coordinating ligands in the solution. The UV−vis and photoluminescence (PL) spectra were recorded for all samples during the course of reaction (Figure 2). The first set of NCs prepared with the [ZnSt2]:[Se] molar ratio (1:2) results in rapid growth of NCs (first absorption band 361 nm after 1 min of synthesis). When the growth time is prolonged the absorption peaks broadened significantly. It implies that higher Se concentration leads to large NCs and results in nonhomogenous growth. In contrast, samples prepared with stoichiometric [ZnSt2]:[Se] (1:1), or with excess [ZnSt2]:[Se] ratio (2:1), show a wider band gap after 1 min, and the position of absorption peaks stabilizes at ∼405 nm after 15 min. In both cases, a narrowing of the absorption as well as emission peaks is observed with increasing NCs size that is followed by a slight increase in PL fwhm (Supporting Information Figure S4) as the synthesis time is increased to more than 15 min. On the basis of these results we propose that, unless Se is used in excess, the stearate anions efficiently stabilize the growth of ZnSe NCs.
Figure 2. UV−vis absorption and photoluminescence spectra collected during the growth of ZnSe nanocrystals. The samples were taken at 1, 2, 4, 6, 9, 15, 30, and 60 min and were diluted to similar absorption intensity for the measurements.
Good surface stabilization by ligands is needed to retain colloid stability (temporal stability of ZnSe NCs is presented in Supporting Information Figure S5) and band edge emission from the NCs. In the Se rich samples, no emission is observed. For other samples, intensive excitonic emission bands are present, and their blue-shift due to quantum confinement is clearly visible (Figure 2). These results confirm that despite the limited number of ligands available in the reaction, the stabilization of ZnSe NCs surface by stearate anions is sufficient to obtain emitting NCs with a PL quantum yield of ∼8% (Supporting Information Figure S6). The evolution of UV−vis absorption peak positions for the samples are presented in Figure 2. By using absorption data and the equation proposed by Mochizuki et al.23, size of NCs generated during the reaction were calculated. A continuous increase in NC diameter from 2.9 to 6.2 nm was observed over 15 min of reaction. Slow growth is often considered as an advantage as it can enable precise size control.7 The reduction of Se is a critical part of the experiment as it releases the selenide containing species for NC formation.24 Zou et al. reported a mechanism for CdSe in which the first step involved the reduction of Se to H2Se and the oxidation of long alkane chains to alkene chains.25 This is followed by reaction between cadmium oleate and dissolved H2Se to form CdSe NCs. Shen et al. proposed a mechanism in which Se dissolved in ODE is held to act as a reducing agent and forms an active complex before the growth of ZnSe NCs.14 Both of the above studies involved an injection based technique. In our case, we propose that dissolution of Se in ODE results in the formation of a Se−ODE complex which subsequently breaks down to provide active Se ions for ZnSe NCs nucleation and growth (Scheme 1). To verify the findings, 1.24 g (15.5 mmol) of selenium powder was added to 7.8 g (31.0 mmol) of octadecene, and the reaction mixture was heated under nitrogen at 210 °C for 12 h with stirring. The color of the mixture changed from black to dark orange after about 20 min of reaction. The final product after 12 h of heating was found to be viscous dark brown. There was no black precipitate. The dark brown product was characterized by UV/vis spectroscopy, FTIR, mass spectroscopy, and 1H and 13C NMR spectroscopy. The UV/vis spectra were recorded (Figure 3a), and an increased absorption accompanied by a color change is clearly visible for ODE-Se solution. Figure 3b shows an IR spectrum of the resulting ODE−Se complex. Characteristic peaks of the ODE vinyl group CH, the out-of-plane bending, are located B
DOI: 10.1021/acs.chemmater.5b00347 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
presence of carbon disulfide.26 Indeed, selenophene and its derivatives have been extensively explored in synthetic chemistry.27 In summary, a robust synthesis of luminescent ZnSe NCs is presented by heating ZnSt2, ODE, and Se in a single flask. The results suggest that zinc precursor provides both zinc ions and a limited number of surface passivating ligands for the NC surface. One important part of the process is the coupling reaction between ODE and Se to generate a tetrahydroselenophene derivative which acts as an efficient selenium precursor for the nucleation and growth of ZnSe NCs.
Scheme 1. Possible Reaction between Two Molecules of Octadecene with One Selenium Atom Generating Activated Selenium Precursor and Its Involvement with the Synthesis of ZnSe Nanocrystals
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details: XRD, EDX, mass spectrum, 1H and 13C NMR, photoluminescence quantum yield, and absorption and photoluminescence temporal stability. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b00347.
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AUTHOR INFORMATION
Corresponding Authors
*(M. Banski) E-mail:
[email protected]. *(P. O’Brien) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.B. would like to acknowledge The Iuventus Plus program (no. IP2011 001271) and Foundation for Polish Science (FNP) “Start” for the financial support. This work was supported by a grant from The National Center for Research and Development (Lider/13/14/L-2/10/NCBiR/2011). The FT-IR spectra were measured thanks to National Science Centre Grant No. DEC-2011/03/D/ST3/02640. M.A. wishes to acknowledge NSTIP strategic technologies program number (12-ENE320404), Saudi Arabia, for the financial support.
Figure 3. (a) Absorption spectra of octadecene and selenium solution when heated at 210 °C for 12 h. Inset shows photos of (i) octadecene, (ii) octadecene + selenium for 1 min at 210 °C, and (iii) octadecene + selenium for 12 h at 210 °C. (b) FTIR spectra of resulting octadecene and selenium mixture at 210 °C for 1 min and 12 h.
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at 909 and 992 cm−1, the CHCH2 stretch is located at 1642 cm−1, and the CH stretch is located at 3078 cm−1 before heating. However, after heating at 210 °C, vinyl bands disappeared and bands at 965 and 1721 cm−1 appeared. This clearly indicates that ODE has reacted with Se while no change was observed when pure ODE was heated. We proposed that, during the heating process, two ODE molecules undergo a coupling reaction with one Se atom to form a selenophene ring. The mass spectrum (Supporting Information Figure S7) of the resulting dark brown product showed a molecular ion peak at m/z = 584 corresponding to ODE−Se−ODE while other peaks appear at m/z 504 (ODEODE) and 331 (ODE-Se). The final proof of 2,5dihexadecyltetrahydroselenophene formation was provided by nuclear magnetic resonance (1H NMR and 13C NMR) spectra as presented and discussed in Supporting Information Figures S8−S10. On the basis of the above observations, we believe that in situ generated 2,5-dihexadecyltetrahydroselenophene acts as a suitable selenium precursor and reacts with zinc stearate to yield bright ZnSe NCs as depicted in Scheme 1. A similar cyclic selenium product (1,3-thiaselenole-2-thione) was produced by the reaction of metallic selenium with sodium acetylide in the
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