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glycerol two-phase systems
Colloids and Surfaces A 581 (2019) 123812
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Preparation and growth mechanism of CdS quantum dots in octadecene/ glycerol two-phase systems
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Ya Di , Kunling Lu, Yaling Tian, Yan Liu, Yunwang Zhao, Yue Zheng The First Hospital in Qinhuangdao Affiliated to Hebei Medical University, Qinhuangdao 066004, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Growth mechanism CdS quantum dots Octadecene/glycerol two-phase systems One-step synthesis
Two-phase synthesis is an advantageous alternative to the traditional synthetic method, due to its less toxicity, controllable, mild synthetic conditions and easy large-scale synthesis. However, meeting novel synthesis, the conventional trial-and-error approach could not provide a clear understanding. We herein report synthesis and mechanism investigation of CdS quantum dots in octadecene/glycerol two-phase system. The effects of different reaction parameters and conditions including reaction temperature, reaction time, reactant concentrations, and synthesis routes (one-step and two-step approach) on both nucleation and particle growth were investigated. It was found that the synthesis course was a growth dominated process depending on both CdS(monomer) and CdS (nuclei), and controlled by the interface of ODE/glycerol. The present work provided a new and clear understanding about two-phase system synthesis on semiconductor quantum dots, noble metal nanocrystals and some alloy nanomaterials.
1. Introduction Semiconductor quantum dots (QDs) have attracted considerable attentions due to their superior properties, including high quantum yield, broad excitation spectrum, narrow and size-dependent emission [1–4]. Therefore, extensive efforts have been devoted to synthesis of
QDs so that their size, shape and surface properties can be effectively controlled [5–8]. The two-phase synthesis is an advantageous alternative to the traditional synthetic method, including organic (organometallic) phase and aqueous phase, because this synthetic strategy is less toxicity, controllable, mild synthetic conditions and easy largescale synthesis [6,7,9,10]. Advanced synthesis has made it possible to
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Corresponding author. E-mail addresses: [email protected] (Y. Di), [email protected] (K. Lu), [email protected] (Y. Tian), [email protected] (Y. Liu), [email protected] (Y. Zhao), [email protected] (Y. Zheng). https://doi.org/10.1016/j.colsurfa.2019.123812 Received 17 April 2019; Received in revised form 10 July 2019; Accepted 15 August 2019 Available online 16 August 2019 0927-7757/ © 2019 Published by Elsevier B.V.
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growth of QDs. That is a broad reaction temperature range resulted from the high boiling point both ODE and glycerol (from room temperature to well above 100 °C under the normal pressure), environmentally benign and almost nontoxic, and using in an anhydrous condition. Moreover, we attempted to elucidate the key parameters including time, temperature, synthetic route and mole ratio of precursors influencing on the preparation of final QDs and proposed a more in-depth outlook applicable towards the optimization of synthesis. Our research would provide a new and clear understanding about twophase system synthesis on semiconductor QDs, noble metal nanocrystals and some alloy nanomaterials.
control nanomaterials with designed functionalities. However, meeting novel two-phase systems, the conventional trial-and-error approach could not provide a clear understanding on the formation and growth of QDs. Especially, multiple intermediate states are usually involved from atoms to nanoclusters, to nanoparticles, to mesocrystals, and then to bulk crystals in this kind of syntheses, which bring more challenges. Magic-sized quantum dots (MSQDs), as an important and unique crystal type, often transiently or sustainably existed in reaction process in preparation of QDs, which consisted with the certain number of atoms and amazing properties. For example, three pyramidal [Cd35Se20(O2CPh)30(H2N–C4H9)30], [Cd56Se35(O2CPh)42(H2N–C4H9)42] and [Cd84Se56(O2CPh)56(H2N–C4H9)56] MSQDs, were synthesized exhibiting the zinc blende structure [11]. [Cd32Se14(SePh)36(PPh3)4] was reported with a mixed structure composed of both zinc blende and wurtzite [12]. These discrete energy structures of MSQDs also showed the strong size-dependent quantum confinement as the regular QDs. And, the ground state was split into many subtle structure states due to strong quantum confinement. Based on research of MSQDs, numerous studies have investigated the growth process of QDs in two-phase systems although it seemed that many theories were contradictory [13–16]. Yu et al. [17] reported a nucleation-growth mechanism for the formation of CdS QDs in a liquid-paraffin/glycerol biphasic system and discussed the implication of MSQDs as medium in formation of CdS QDs. They thought the process from MSQDs into regular one in a liquidparaffin/glycerol would be based on particle-particle aggregation, deposition of monomers and oriented attachment of MSQDs. Pan et al. [16] proposed a novel model of two-phase mechanism with lengthy nucleation and growth periods for preparing monodisperse nanocrystals by slowing nucleation and growth rates, increasing the surface tension and changing the polarity of capping agent or solvent. As we known, the classical growth behaviours of nanoparticles in colloid chemistry theory were described in the process of nucleation and growth by LaMer [18,19] and Ostwald [20,21]. The process of nucleation and growth was divided into three stages: (I) The concentration of free monomers in solution increased rapidly; (II) “burst nucleation” generated from the free monomer in solution, which led to the significantly concentration reduction of the monomers; (III) nucleation growth occurring due to the diffusion of the monomers in the solution. Subsequently, Ostwald [20] described the growth mechanism of nanoparticles by the Gibbs-Thomson relation based on the solubility change of NPs depending on their sizes. The smaller particles, due to their high solubility and surface energy, were likely to dissolve and grow into the larger ones in turn. Lee et al. [22] introduced a digestive ripening theory, which made an inverse of Ostwald ripening. That is, the smaller particles grow with the consumption of the larger ones. This process of smaller particles growth and dissolution in solution was also controlled by the surface energy of the particle. However, during the novel MSQDs media two-phase synthesis, the MSQDs seemed to be involved in the total process. The mechanism of nonclassical nucleation and growth of QDs could not be explained by these theories. Moreover, as we known, some problems must be considered in MSQDs media twophase nucleation and growth of QDs including the lower particle crystallinity and wider size distribution, volatility and toxicity in organic/water systems. And, water-based synthesis cannot be used for high-temperature (above100 °C) synthesis under the normal pressure. These challenges in two-phase synthesis hindered seriously the research and application of QDs. Meanwhile, the influence of the reaction parameters and conditions (including temperature, reaction time, mole ratio of precursors, one-step or two-step route) for MSQD synthesis and the effect of these parameters on the MSQDs-mediated formation of regular QDs in the two- phase system are rather limited. Herein, we systematically investigated the nucleation and growth process of CdS QDs in ODE/ glycerol two-phase system using optical spectra, especially UV absorption spectrum. ODE/glycerol, as a longchain hydrocarbons and polyols two-phase systems provided some advantages for investigation of MSQDs media two-phase nucleation and
2. Experimental procedure 2.1. Materials and methods Cadmium oxide (CdO, ≥99%), was purchased from Aladdin Industrial Corporation (Shanghai, China). oleic acid (OA, AR), 1-octadecene (ODE, 90%), thiourea (AR) and glycerol (AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All the chemicals were analytical reagent grade without further purification. 2.2. Apparatus The structural and optical properties of CdS QDs were characterized and studied by TEM, and optical spectroscopy. UV–vis absorption spectroscopy and fluorescence spectrum measurements were recorded on a UV-2550 (Shimadzu, Kyoto, Japan) and F-7000 (Hitachi, Tokyo, Japan). Transmission electron microscopic (TEM) was obtained by a HT7700 transmission electron microscope (Hitachi, Tokyo, Japan) 2.3. One-step synthesis of CdS QDs Particularly, we simplify the traditional synthesis process based on our group’s previous report [23]. In this case, the CdS QDs were synthesized via one-pot route in the two-phase liquid/liquid system. The CdS QDs were synthesized by sequential addition of 0.512 g (4 mmol) of CdO, 0.038 g (0.5 mmol) of thiourea, 10 mL of glycerol, 7 mL of ODE and 3 mL of oleic acid in a three-neck flask at room temperature. The mixture was stirred and heated under an N2 atmosphere. To monitor the growth of the quantum dots, the temporal evolution of the absorption and emission spectra were recorded from all aliquots of the assynthesized quantum dots. All the samples for the measurements were as-prepared and without any further purification. 2.4. Two-step synthesis of CdS QDs The CdS QDs were synthesized by following a modified version of the process of Yu [12]. 0.32 g (2.5 mmol) of CdO, 5 ml of OA, and 7.5 ml of ODE were added to a three-neck flask under vigorous stirring, and the mixture was heated to 140 ℃ for at least half an hour. An N2 flow was needed in this process including the natural cooling step of the mixture solutions. Subsequently, 5 ml (about 0.2 mol/L) of the cadmium stock solution was diluted to 10 ml with ODE. And 0.019 g (0.25 mmol) of thiourea was dissolved in 10 ml of glycerol as the Sprecursors. The two solutions were then transferred into a reaction flask heated to 140 ℃ under the magnetic stirring and nitrogen atmosphere conditions, in which the molar ratio of Cd/S is 8:1. Aliquots were taken for optical measurements. 3. Results and discussion Compared with homogeneous one-phase system, the reactions in two-phase synthesis of QDs were different. Generally, molecular precursors were usually separated within the organic and the aqueous phase due to their hydrophobic or hydrophilic characters. Near the 2
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Fig. 1. Temporal evolution of the absorption spectra and PL spectra of CdS QDs via one-step synthesis (A, C) and two-step synthesis (B, D) recorded at various reaction times, (Cd/S = 8:1, 140 ℃,0–4 h).
liquid-liquid interface, crystal nuclei being stabilized by capping agent were generated, which would either grow into regular quantum dots or be consumed via a ripening process during the reaction. To further understand the two-phase system, particularly in two-step synthesis, we examined the effects of synthetic route and reaction time, reactant concentrations, reaction temperature on the nucleation and growth of CdS QDs.
the peak intensity of CdS315 MSQDs (or CdS315 (nuclei), ultra-small QDs) and CdS397 MSQDs changed slightly (CdS315 MSQDs, CdS315 (nuclei) and CdS397 MSQDs meant that their absorption peaks fixed at 315 nm, 315 nm and 397 nm). However, in the traditional discontinuous growth of QDs, the intensity of peaks at longer wavelengths increased frequently based on the decrease of absorption peaks intensity at short wavelengths.
3.1. Effect of synthetic route
D = (−6.6521 × 10−8) λ3 + (−1.9557 × 10−4) λ2 − (−9.2352 × 10−2) λ + (13.29)
Fig. 1 showed the evolution of absorption (A), (C) and emission spectra (B), (D) of CdS QDs with time via one-step and two-step synthesis, respectively. The molar ratio of Cd/S was 8:1, and the reaction time was counted after the system temperature reached to 140 ℃ under agitation. The samples were measured at different time intervals up to 4 h. The temporal evolution of all absorption spectra exhibited a discontinuous growth as shown in Fig. 1(A). For continuous growth, the first exciton peak in the absorption spectrum of QDs would continuously move to long wavelengths. But for these syntheses of CdS MSQDs via one-step route, the sharp absorption peaks at 315 and 397 nm with the fixed position were exhibited for a long reaction time over 240 min. It meant that two families of CdS MSQDs co-exist. The size of CdS MSQDs being estimated by the empirical formula as Eq. (1) [24] were around 1.5 nm and 3.3 nm. The emission peaks CdS MSQDs were shown in Fig. 1(B), which shifted from 414 to 418 nm. The full width at half-maximum (FWHM) varied from 23 to 26 nm when the reaction time changed from 5 to 240 min. The nonresonant stokes shift (NRSS) of as-synthesized CdS MSQDs was about 21 nm, which indicated that the CdS MSQDs were with a well-defined structure (Fig. 2(A)) The TEM image also showed that the particle size of CdS MSQDs were concentrated at about 3 nm which was close to the calculation based on the absorption spectrum and the obtained MSQDs were nearly spherical with desirable monodisperses (Fig. 2(B), (C)). It was worth noting that
(1)
where D (nm) is the diameter of a given CdS nanocrystal sample and λ (nm) is the absorption peak position of the corresponding sample. Compared with the one-step route, two-step synthesis of CdS QDs exhibited different development in UV–vis absorption and PL spectrum as shown in Fig. 1(C) and (D). Although, in the obtained samples, the CdS315 (nuclei) still existed in the solution, the CdS397 MSQDs as in the one-step synthesis did not appear again. Another distinct absorption peaks which represented the regular CdS QDs were constructed. The absorption peaks shifted from 379 nm to 403 nm with the time increasing, which meant the diameters of CdS QDs varying from about 2.8 nm to 3.5 nm as Fig. 3 shown. The same evolution of PL emission spectra was shown in Fig. 1(D) and the sharp emissions moved to the higher wavelength with time lasting from 401 nm to 426 nm and the FWHM varied about from 24 to 27 nm. In the two-step synthesis, the precursor, such as cadmium stock solution, was synthesized as the preferred step and then to react to format the CdS QDs at ODE/glycerol interface, which provided a relatively faster reaction speed in comparison with the one-step synthesis. Until all the chemicals dissolved gradually into formatting precursor, the precursor began to construct the CdS QDs in the one-step synthesis, thus, the reaction speed was more moderate so that the CdS MSQDs (or CdS (nuclei)) would exist in solutions in all reaction procedure. 3
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Fig. 2. UV–vis and PL spectrum of CdS QDs (A), TEM images (B) and particle size distribution histogram (C) via one-step synthesis.
and glycerol in two-phase system, would create more barriers and made an opportunity for slower reaction. With the reaction proceeded, the supplied quantity of the Cd precursor gradually exceeded the exhausting quantity, which resulted in the accumulation of Cd precursors as well as CdS monomers. There is a point, where the monomer concentration exceeded the critical value of nucleation and the new QDs would nucleate. If the nucleation effectively led to the higher supersaturation drop, far below the critical concentration for nucleation, monomers were mainly used to satisfy the growth of new QDs because of their large chemical potentials, provided by its small size. Thus, the particle size of preformed QDs was constant during that period similar to the case for the feeding molar ratios of 1:1 and 2:1 (Fig. 4). If the monomer concentration was kept at a higher level, the growth and nucleation of all the QDs must occur at the same time as observed in the feeding molar ratios of 4:1 and 8:1. In addition, this two-phase one-step synthesis showed distinctly different growth kinetics from the
3.2. Effect of Cd/S molar ratio The feed molar of Cd/S ratio was the important parameter so that it could potentially manipulate the nucleation and growth of quantum dots. To study the influence of molar ratio (Cd/S) on characters of CdS QDs, a series of experiments on Cd/S including 1:1, 2:1, 4:1, 8:1 were performed at 140 °C for 5 min in the ODE/glycerol system via one-step route as shown in Fig. 4. At molar ratios of 4:1 and 8:1, the nucleation and growth of CdS QDs simultaneously occurred. That was, while the absorption peaks of CdS RQDs existed, the ones of CdS315 (nuclei) also appeared. However, at feed molar ratios of 1:1 and 2:1, the nucleation period lasted a shorter procedure. Therefore, only distinct absorption peaks of CdS RQDs appeared at 408 and 426 nm, respectively. An interesting phenomenon was that the diameter of CdS QDs decreased with the improvement of molar ratios between Cd2+ and S2− as the blue shift of the absorption peaks implied. The interface between ODE
Fig. 3. TEM images and particle size of CdS QDs via two-step synthesis whose absorption peaks was 379 nm ((A), (B)) and 403 nm ((C), (D)) (Cd/S = 8:1, 140 ℃). 4
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the higher temperatures. Significantly, magic-sized QDs with the first absorption peak at 315 nm formed at 120 ℃ as the only style in solution. With the increase of reaction temperature, such as 140 ℃, 160 ℃ and 180 ℃, when CdS315 (nuclei) existed in the solution, meanwhile, another absorption peak appeared at 382, 388 and 408 nm respectively, which represented the regular CdS QDs. Interestedly, the intensity of the absorption peak at 315 nm increased with temperature ascending instead of degradation and disappearance with the absorption peaks’ red shift of QDs as the many references discussed [28,29]. In our investigated, CdS315 (nuclei) could be stable in the higher temperature, including 140 ℃, 160 ℃, 180 ℃. As some research had proved that nanoclusters with magic numbers which is thermodynamically stable and with local minima chemical potentials because of the closed-shell configurations [28,30,31]. Due to their extremely small sizes, the MSQDs formed under a relatively high chemical potential and they could be stable at relatively high monomer concentrations because the higher temperature would accelerate the form of monomer. We also found that in the relatively low monomer concentration (lower temperature, such as 120 ℃, or the growth process of QDs) the MSQDs also presented. Under lower monomer concentration in the solution, by the “tunneling” of interface, that the CdS(nuclei) grew into regular QDs through the lower thermodynamic barrier and decompose to monomers thorough the high barrier. And the different temperature would experience different growth traces as shown in Fig. 6. The CdS nuclei/ MSQDs would appear and exist in the specific stages, such as 140 ℃ and 160 ℃. However, for another conditions, CdS QDs exhibited the characters of regular QDs growth with the evolution of time and PL spectra also showed irregular transformation. For example, in 160 ℃ and 180 ℃, multiple PL peaks appeared and the intensity gradually increased with the time. Furthermore, the regular CdS QDs exhibited a temperature-depended growth process via the two-step synthesis as shown in Fig. 5(B), which demonstrated that the nuclei had grown into the larger particle with the temperature rising. As the previous jobs, in this two-phase system, the monomer concentration has a critical saturation value, M0 [25]. When monomer concentration was beyond M0, the MSQDs would form, however, it would dissolve back when the monomer concentration was lower than the ‘critical value’. The MSQDs served as a reservoir for monomers condensation and dissolution because these processes happened at a higher speed in this two-step synthesis than one-step synthesis [29,32]. All these discussions implied that the nucleation and growth of CdS QDs in the octadecene/glycerol two-phase systems by one-step synthesis via a nonclassical route.
Fig. 4. Temporal evolution of the absorption spectra of CdS QDs sampled at molar ratio of Cd/S (samples: 1:1,2:1,4:1,8:1) via one-step synthesis at 140℃.
traditional synthesis of QDs. Alivisatos A. P. et al. [25] proposed that the precursor concentration was inversely proportional to the average particle radius as the following equations:
Q0 Vm = v *
n∞ =
dn + vn dt
Q0 Vm v
(2)
(3)
where Q0 was the supply rate of monomer, Vm was the molar volume of the solid, v was the growth rate of a nucleus, and n∞ represents the particle molar density. They inferred that nucleation rate v* was proportional to the production of monomers. If kept the growth rate v constant, the particle number was mainly dependent on Q0 which was proportional to the precursor concentration. For example, in hot injection reactions, increasing the concentration of one precursor would result in the increase of the nucleus number and the smaller particle size. However, in twophase synthesis, the size of regular CdS QDs decreased with increase of the precursor concentration during the feeding molar ratios of Cd/S from 1:1 to 8:1. We thought that a high feed molar ratio improved the usage efficiency of scarce component atom and gave rise to a relatively quick nucleation number. Moreover, the Cd atoms were abundant at a higher Cd/S molar ratio, resulting in the increasing amount of Cd(OA)2 molecules. The nucleus surface was covered by the excess Cd(OA)2 or Cd(OA)+ molecules through electrostatic interactions, which prevented the aggregation and growth of the nuclei. [26,27]. Thus, when the regular CdS QDs grew in solution, the CdS (nuclei) also existed simultaneously.
3.4. Nucleation and growth mechanism of CdS QDs in the ODE/glycerol two-phase system via one-step synthesis Based on the experimental data, the possible nucleation and growth mechanism of CdS QDs in ODE/glycerol two-phase system via one-pot route was proposed, detailed in Scheme 1, which comprised of two pathways from reaction precursors via Cd-S (soluble) and CdS (monomer) to RQDs (top), as well as via Cd-S (soluble) to MSQDs (bottom). As Scheme 1, due to the difference of solubility, when all chemicals were loaded in flask together, through agitating vigorously, CdO would get the opportunity to collide and react with OA, leading to the form of Cd precursor and release of Cd2+ in ODE. S precursor, which resulted from CN2H4S, dissolved into glycerol and met with Cd precursor to achieve Cd-S (soluble) near the interface between ODE and glycerol, which was the “liquidlike” aggregates and about 1 nm in size [33]. Then, the CdS (monomer) could creat with the reaction progress. Meanwhile, CdS315 (nuclei) were obtained at interface of ODE/glycerol. Once oleic acid capped Cd-S (soluble), CdS (monomer) or CdS(nuclei) formed, which was capped with identical capping agent, they would go into ODE phase, and then returned to the ODE/glycerol interface to grow continuously. This two-phase growth process would make the reaction rate slowly because of the diffusion resistance for both nucleation and growth at the organic-water interfaceThus, there was
3.3. Effect of temperature Temperature can also influence the synthesis of CdS QDs. Especially, the temperature can affect the reaction rates, the saturation level of the precursor, reaction equilibrium. Because the collision of the precursor molecule and reaction speed increased in higher temperature. So, the reactivity of the precursors and monomer concentration were influenced directly by temperature. Therefore, the temperature would control the final size of QDs, composition, and optical property. Fig. 5(a) showed the absorption spectra evolution of CdS QDs at different temperature when the Cd/S molar ratio is 4:1 via one-step route at 30 min. During the synthesis, the CdS315 (nuclei) was seemed to possess more stability, which existed in all reaction process without growing into regular QDs. The MSQDs/QDs mixture form generated in 5
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Fig. 5. Temporal evolution of the absorption spectra of CdS QDsrecorded at various reaction temperature (samples:120, 140,160,180 ℃) via one-step synthesis(A) and two-step synthesis(B), (Cd/S = 8:1).
certain reaction parameters and conditions including the reaction temperature, reaction time, mole ratio of precursors, and synthesis route (one-step or two-step), one or both of CdS(nuclei) and CdS MSQDs would be existed in the solution through all the reaction process. On the other hand, if the CdS monomers played a major role, one or both of CdS(nuclei) and CdS RQDs would exhibit. As a result, the nucleation and growth of CdS QDs in the ODE/glycerol two-phase system mainly relied on the ratio between CdS (monomer) and CdS (nuclei) which was analogous to discussion on the dependence of extinction coefficients absorbances [28,34–37]. Compared with onephase synthesis, the reactions in two-phase system, occured much slower, which would give more opportunities to monitor nucleation and growth of QDs.
enough time to make ultra-small size CdS QDs. The formation and consumption of CdS (monomer), CdS(nuclei) were almost at the same instant which was different from the traditional two-step synthesis procedure or “hot-injection” synthesis. Thus, the feature equilibria were built based on CdS (monomer), CdS(nuclei). As a reservoir, the concentration of them was controlled to the saturation value, which was limitation concentration of both CdS (monomer) and CdS(nuclei) condensing to CdS MSQDs and CdS QDs. As we known, in the classical Ostwald ripening process, the growth of larger nanocrystals came from consuming small ones. Thus, CdS(nuclei) was hard to grow into the final nanocrystals becuase of the majority of nuclei were consumed. In fact, the two-phased system provided a slow reaction process. When CdS (monomer) and CdS(nuclei) were presented, if formation or consumption of CdS315 (nuclei) was dominant in growth process under the
Fig. 6. Temporal evolution of the absorption spectra and PL spectra of CdS QDs via one-step synthesis with time recorded at various reaction temperature((A),120 ℃; (B),140 ℃; (C),160 ℃; (D),180 ℃; Cd/S = 8:1). 6
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Scheme 1. Procedures of nucleation and growth of CdS quantum dots in the octadecene/glycerol two-phase systems via one-step synthesis.
4. Conclusion
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In conclusion, we have proposed a nonclassical nucleation and growth mechanism associated with one-step synthesis for preparing CdS QDs in the ODE/glycerol two-phase system by investigating the different reaction parameters including the reaction temperature, reaction time, mole ratio of precursors, and synthesis route (one-step or two-step). The synthesis course was a growth dominated process of both CdS (monomer) and CdS(nuclei), and controlled by the interface of ODE/glycerol. If formation or consumption of CdS(nuclei) was dominant in growth process one or both of CdS(nuclei) and CdS MSQDs would be in the solution through the reaction process. However, if the CdS monomers played a major role, one or both of CdS(nuclei) and CdS RQDs would be the main content. The nucleation and growth of CdS QDs in the octadecene/glycerol two-phase systems via one-step synthesis exhibited nonclassical characters. Our research would provide a new and clear understanding about two-phase system synthesis and pushed the progress for the synthesis of QDs, noble metal nanocrystals and some alloy nanomaterials. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This research was supported by the Key Research and Development Program of Qin Huang Dao (201602A110). References [1] Q. Nie, L. Yang, C. Cao, Y. Zeng, G. Wang, C. Wang, S. Lin, Interface optimization of ZnO nanorod/CdS quantum dots heterostructure by a facile two-step low-temperature thermal treatment for improved photoelectrochemical water splitting, Chem. Eng. J. 325 (2017) 151–159. [2] Y. Wang, S. Ge, L. Zhang, J. Yu, M. Yan, J. Huang, Visible photoelectrochemical sensing platform by in situ generated CdS quantum dots decorated branched-TiO2 nanorods equipped with Prussian blue electrochromic display, Biosens. Bioelectron. 89 (2017) 859–865. [3] K. Wu, Y.S. Park, J. Lim, V.I. Klimov, Towards zero-threshold optical gain using charged semiconductor quantum dots, Nat. Nanotechnol. 12 (2017) 1140. [4] J. Wang, X. Wang, H. Tang, Z. Gao, S. He, J. Li, S. Han, Ultrasensitive electrochemical detection of tumor cells based on multiple layer CdS quantum dotsfunctionalized polystyrene microspheres and graphene oxide – polyaniline composite, Biosens. Bioelectron. 100 (2018) 1–7. [5] J. Wang, X. Wang, H. Tang, Z. Gao, S. He, R. Niu, Y. Zheng, S. Han, A ratiometric magnesium sensor using DNAzyme-templated CdTe quantum dots and Cy5, Sens. Actuators B Chem. 272 (2018) 146–150. [6] J. Wang, X. Wang, H. Tang, Z. Gao, S. He, D. Ke, Y. Zheng, S. Han, Facile synthesis and properties of CdSe quantum dots in a novel two-phase liquid/liquid system, Opt. Mater. 72 (2017) 737–742. [7] J. Wang, K. Guo, D. Ke, S. Han, Synthesis and mechanism study of CdS quantum dots in two-phase liquid/liquid interfaces via one-pot route, Chem. Phys. Lett. 618 (2015) 11–13.
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[34] D. Battaglia, X. Peng, Formation of high quality InP and InAs nanocrystals in a noncoordinating solvent, Nano Lett. 2 (2002) 1027–1030. [35] X. Peng, Mechanisms for the shape-control and shape-evolution of colloidal semiconductor nanocrystals, Adv. Mater. 15 (2003) 459–463. [36] X. Peng, L. Manna, W. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos, Shape control of CdSe nanocrystals, Nature 404 (2000) 59. [37] Z.A. Peng, X. Peng, Mechanisms of the shape evolution of CdSe nanocrystals, J. Am. Chem. Soc. 123 (2001) 1389–1395.
families of magic-sized CdTe quantum dots with bright bandgap photoemission, Chem. Eng. J. 215–216 (2013) 23–28. [32] Z. Hens, R.K. Čapek, Size tuning at full yield in the synthesis of colloidal semiconductor nanocrystals, reaction simulations and experimental verification, Coord. Chem. Rev. 263–264 (2014) 217–228. [33] M. Liu, K. Wang, L. Wang, S. Han, H. Fan, N. Rowell, J.A. Ripmeester, R. Renoud, F. Bian, J. Zeng, K. Yu, Probing intermediates of the induction period prior to nucleation and growth of semiconductor quantum dots, Nat. Commun. 8 (2017) 15467.
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Preparation and growth mechanism of CdS quantum dots in octadecene/ glycerol two-phase systems
T
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Ya Di , Kunling Lu, Yaling Tian, Yan Liu, Yunwang Zhao, Yue Zheng The First Hospital in Qinhuangdao Affiliated to Hebei Medical University, Qinhuangdao 066004, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Growth mechanism CdS quantum dots Octadecene/glycerol two-phase systems One-step synthesis
Two-phase synthesis is an advantageous alternative to the traditional synthetic method, due to its less toxicity, controllable, mild synthetic conditions and easy large-scale synthesis. However, meeting novel synthesis, the conventional trial-and-error approach could not provide a clear understanding. We herein report synthesis and mechanism investigation of CdS quantum dots in octadecene/glycerol two-phase system. The effects of different reaction parameters and conditions including reaction temperature, reaction time, reactant concentrations, and synthesis routes (one-step and two-step approach) on both nucleation and particle growth were investigated. It was found that the synthesis course was a growth dominated process depending on both CdS(monomer) and CdS (nuclei), and controlled by the interface of ODE/glycerol. The present work provided a new and clear understanding about two-phase system synthesis on semiconductor quantum dots, noble metal nanocrystals and some alloy nanomaterials.
1. Introduction Semiconductor quantum dots (QDs) have attracted considerable attentions due to their superior properties, including high quantum yield, broad excitation spectrum, narrow and size-dependent emission [1–4]. Therefore, extensive efforts have been devoted to synthesis of
QDs so that their size, shape and surface properties can be effectively controlled [5–8]. The two-phase synthesis is an advantageous alternative to the traditional synthetic method, including organic (organometallic) phase and aqueous phase, because this synthetic strategy is less toxicity, controllable, mild synthetic conditions and easy largescale synthesis [6,7,9,10]. Advanced synthesis has made it possible to
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Corresponding author. E-mail addresses: [email protected] (Y. Di), [email protected] (K. Lu), [email protected] (Y. Tian), [email protected] (Y. Liu), [email protected] (Y. Zhao), [email protected] (Y. Zheng). https://doi.org/10.1016/j.colsurfa.2019.123812 Received 17 April 2019; Received in revised form 10 July 2019; Accepted 15 August 2019 Available online 16 August 2019 0927-7757/ © 2019 Published by Elsevier B.V.
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growth of QDs. That is a broad reaction temperature range resulted from the high boiling point both ODE and glycerol (from room temperature to well above 100 °C under the normal pressure), environmentally benign and almost nontoxic, and using in an anhydrous condition. Moreover, we attempted to elucidate the key parameters including time, temperature, synthetic route and mole ratio of precursors influencing on the preparation of final QDs and proposed a more in-depth outlook applicable towards the optimization of synthesis. Our research would provide a new and clear understanding about twophase system synthesis on semiconductor QDs, noble metal nanocrystals and some alloy nanomaterials.
control nanomaterials with designed functionalities. However, meeting novel two-phase systems, the conventional trial-and-error approach could not provide a clear understanding on the formation and growth of QDs. Especially, multiple intermediate states are usually involved from atoms to nanoclusters, to nanoparticles, to mesocrystals, and then to bulk crystals in this kind of syntheses, which bring more challenges. Magic-sized quantum dots (MSQDs), as an important and unique crystal type, often transiently or sustainably existed in reaction process in preparation of QDs, which consisted with the certain number of atoms and amazing properties. For example, three pyramidal [Cd35Se20(O2CPh)30(H2N–C4H9)30], [Cd56Se35(O2CPh)42(H2N–C4H9)42] and [Cd84Se56(O2CPh)56(H2N–C4H9)56] MSQDs, were synthesized exhibiting the zinc blende structure [11]. [Cd32Se14(SePh)36(PPh3)4] was reported with a mixed structure composed of both zinc blende and wurtzite [12]. These discrete energy structures of MSQDs also showed the strong size-dependent quantum confinement as the regular QDs. And, the ground state was split into many subtle structure states due to strong quantum confinement. Based on research of MSQDs, numerous studies have investigated the growth process of QDs in two-phase systems although it seemed that many theories were contradictory [13–16]. Yu et al. [17] reported a nucleation-growth mechanism for the formation of CdS QDs in a liquid-paraffin/glycerol biphasic system and discussed the implication of MSQDs as medium in formation of CdS QDs. They thought the process from MSQDs into regular one in a liquidparaffin/glycerol would be based on particle-particle aggregation, deposition of monomers and oriented attachment of MSQDs. Pan et al. [16] proposed a novel model of two-phase mechanism with lengthy nucleation and growth periods for preparing monodisperse nanocrystals by slowing nucleation and growth rates, increasing the surface tension and changing the polarity of capping agent or solvent. As we known, the classical growth behaviours of nanoparticles in colloid chemistry theory were described in the process of nucleation and growth by LaMer [18,19] and Ostwald [20,21]. The process of nucleation and growth was divided into three stages: (I) The concentration of free monomers in solution increased rapidly; (II) “burst nucleation” generated from the free monomer in solution, which led to the significantly concentration reduction of the monomers; (III) nucleation growth occurring due to the diffusion of the monomers in the solution. Subsequently, Ostwald [20] described the growth mechanism of nanoparticles by the Gibbs-Thomson relation based on the solubility change of NPs depending on their sizes. The smaller particles, due to their high solubility and surface energy, were likely to dissolve and grow into the larger ones in turn. Lee et al. [22] introduced a digestive ripening theory, which made an inverse of Ostwald ripening. That is, the smaller particles grow with the consumption of the larger ones. This process of smaller particles growth and dissolution in solution was also controlled by the surface energy of the particle. However, during the novel MSQDs media two-phase synthesis, the MSQDs seemed to be involved in the total process. The mechanism of nonclassical nucleation and growth of QDs could not be explained by these theories. Moreover, as we known, some problems must be considered in MSQDs media twophase nucleation and growth of QDs including the lower particle crystallinity and wider size distribution, volatility and toxicity in organic/water systems. And, water-based synthesis cannot be used for high-temperature (above100 °C) synthesis under the normal pressure. These challenges in two-phase synthesis hindered seriously the research and application of QDs. Meanwhile, the influence of the reaction parameters and conditions (including temperature, reaction time, mole ratio of precursors, one-step or two-step route) for MSQD synthesis and the effect of these parameters on the MSQDs-mediated formation of regular QDs in the two- phase system are rather limited. Herein, we systematically investigated the nucleation and growth process of CdS QDs in ODE/ glycerol two-phase system using optical spectra, especially UV absorption spectrum. ODE/glycerol, as a longchain hydrocarbons and polyols two-phase systems provided some advantages for investigation of MSQDs media two-phase nucleation and
2. Experimental procedure 2.1. Materials and methods Cadmium oxide (CdO, ≥99%), was purchased from Aladdin Industrial Corporation (Shanghai, China). oleic acid (OA, AR), 1-octadecene (ODE, 90%), thiourea (AR) and glycerol (AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All the chemicals were analytical reagent grade without further purification. 2.2. Apparatus The structural and optical properties of CdS QDs were characterized and studied by TEM, and optical spectroscopy. UV–vis absorption spectroscopy and fluorescence spectrum measurements were recorded on a UV-2550 (Shimadzu, Kyoto, Japan) and F-7000 (Hitachi, Tokyo, Japan). Transmission electron microscopic (TEM) was obtained by a HT7700 transmission electron microscope (Hitachi, Tokyo, Japan) 2.3. One-step synthesis of CdS QDs Particularly, we simplify the traditional synthesis process based on our group’s previous report [23]. In this case, the CdS QDs were synthesized via one-pot route in the two-phase liquid/liquid system. The CdS QDs were synthesized by sequential addition of 0.512 g (4 mmol) of CdO, 0.038 g (0.5 mmol) of thiourea, 10 mL of glycerol, 7 mL of ODE and 3 mL of oleic acid in a three-neck flask at room temperature. The mixture was stirred and heated under an N2 atmosphere. To monitor the growth of the quantum dots, the temporal evolution of the absorption and emission spectra were recorded from all aliquots of the assynthesized quantum dots. All the samples for the measurements were as-prepared and without any further purification. 2.4. Two-step synthesis of CdS QDs The CdS QDs were synthesized by following a modified version of the process of Yu [12]. 0.32 g (2.5 mmol) of CdO, 5 ml of OA, and 7.5 ml of ODE were added to a three-neck flask under vigorous stirring, and the mixture was heated to 140 ℃ for at least half an hour. An N2 flow was needed in this process including the natural cooling step of the mixture solutions. Subsequently, 5 ml (about 0.2 mol/L) of the cadmium stock solution was diluted to 10 ml with ODE. And 0.019 g (0.25 mmol) of thiourea was dissolved in 10 ml of glycerol as the Sprecursors. The two solutions were then transferred into a reaction flask heated to 140 ℃ under the magnetic stirring and nitrogen atmosphere conditions, in which the molar ratio of Cd/S is 8:1. Aliquots were taken for optical measurements. 3. Results and discussion Compared with homogeneous one-phase system, the reactions in two-phase synthesis of QDs were different. Generally, molecular precursors were usually separated within the organic and the aqueous phase due to their hydrophobic or hydrophilic characters. Near the 2
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Fig. 1. Temporal evolution of the absorption spectra and PL spectra of CdS QDs via one-step synthesis (A, C) and two-step synthesis (B, D) recorded at various reaction times, (Cd/S = 8:1, 140 ℃,0–4 h).
liquid-liquid interface, crystal nuclei being stabilized by capping agent were generated, which would either grow into regular quantum dots or be consumed via a ripening process during the reaction. To further understand the two-phase system, particularly in two-step synthesis, we examined the effects of synthetic route and reaction time, reactant concentrations, reaction temperature on the nucleation and growth of CdS QDs.
the peak intensity of CdS315 MSQDs (or CdS315 (nuclei), ultra-small QDs) and CdS397 MSQDs changed slightly (CdS315 MSQDs, CdS315 (nuclei) and CdS397 MSQDs meant that their absorption peaks fixed at 315 nm, 315 nm and 397 nm). However, in the traditional discontinuous growth of QDs, the intensity of peaks at longer wavelengths increased frequently based on the decrease of absorption peaks intensity at short wavelengths.
3.1. Effect of synthetic route
D = (−6.6521 × 10−8) λ3 + (−1.9557 × 10−4) λ2 − (−9.2352 × 10−2) λ + (13.29)
Fig. 1 showed the evolution of absorption (A), (C) and emission spectra (B), (D) of CdS QDs with time via one-step and two-step synthesis, respectively. The molar ratio of Cd/S was 8:1, and the reaction time was counted after the system temperature reached to 140 ℃ under agitation. The samples were measured at different time intervals up to 4 h. The temporal evolution of all absorption spectra exhibited a discontinuous growth as shown in Fig. 1(A). For continuous growth, the first exciton peak in the absorption spectrum of QDs would continuously move to long wavelengths. But for these syntheses of CdS MSQDs via one-step route, the sharp absorption peaks at 315 and 397 nm with the fixed position were exhibited for a long reaction time over 240 min. It meant that two families of CdS MSQDs co-exist. The size of CdS MSQDs being estimated by the empirical formula as Eq. (1) [24] were around 1.5 nm and 3.3 nm. The emission peaks CdS MSQDs were shown in Fig. 1(B), which shifted from 414 to 418 nm. The full width at half-maximum (FWHM) varied from 23 to 26 nm when the reaction time changed from 5 to 240 min. The nonresonant stokes shift (NRSS) of as-synthesized CdS MSQDs was about 21 nm, which indicated that the CdS MSQDs were with a well-defined structure (Fig. 2(A)) The TEM image also showed that the particle size of CdS MSQDs were concentrated at about 3 nm which was close to the calculation based on the absorption spectrum and the obtained MSQDs were nearly spherical with desirable monodisperses (Fig. 2(B), (C)). It was worth noting that
(1)
where D (nm) is the diameter of a given CdS nanocrystal sample and λ (nm) is the absorption peak position of the corresponding sample. Compared with the one-step route, two-step synthesis of CdS QDs exhibited different development in UV–vis absorption and PL spectrum as shown in Fig. 1(C) and (D). Although, in the obtained samples, the CdS315 (nuclei) still existed in the solution, the CdS397 MSQDs as in the one-step synthesis did not appear again. Another distinct absorption peaks which represented the regular CdS QDs were constructed. The absorption peaks shifted from 379 nm to 403 nm with the time increasing, which meant the diameters of CdS QDs varying from about 2.8 nm to 3.5 nm as Fig. 3 shown. The same evolution of PL emission spectra was shown in Fig. 1(D) and the sharp emissions moved to the higher wavelength with time lasting from 401 nm to 426 nm and the FWHM varied about from 24 to 27 nm. In the two-step synthesis, the precursor, such as cadmium stock solution, was synthesized as the preferred step and then to react to format the CdS QDs at ODE/glycerol interface, which provided a relatively faster reaction speed in comparison with the one-step synthesis. Until all the chemicals dissolved gradually into formatting precursor, the precursor began to construct the CdS QDs in the one-step synthesis, thus, the reaction speed was more moderate so that the CdS MSQDs (or CdS (nuclei)) would exist in solutions in all reaction procedure. 3
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Fig. 2. UV–vis and PL spectrum of CdS QDs (A), TEM images (B) and particle size distribution histogram (C) via one-step synthesis.
and glycerol in two-phase system, would create more barriers and made an opportunity for slower reaction. With the reaction proceeded, the supplied quantity of the Cd precursor gradually exceeded the exhausting quantity, which resulted in the accumulation of Cd precursors as well as CdS monomers. There is a point, where the monomer concentration exceeded the critical value of nucleation and the new QDs would nucleate. If the nucleation effectively led to the higher supersaturation drop, far below the critical concentration for nucleation, monomers were mainly used to satisfy the growth of new QDs because of their large chemical potentials, provided by its small size. Thus, the particle size of preformed QDs was constant during that period similar to the case for the feeding molar ratios of 1:1 and 2:1 (Fig. 4). If the monomer concentration was kept at a higher level, the growth and nucleation of all the QDs must occur at the same time as observed in the feeding molar ratios of 4:1 and 8:1. In addition, this two-phase one-step synthesis showed distinctly different growth kinetics from the
3.2. Effect of Cd/S molar ratio The feed molar of Cd/S ratio was the important parameter so that it could potentially manipulate the nucleation and growth of quantum dots. To study the influence of molar ratio (Cd/S) on characters of CdS QDs, a series of experiments on Cd/S including 1:1, 2:1, 4:1, 8:1 were performed at 140 °C for 5 min in the ODE/glycerol system via one-step route as shown in Fig. 4. At molar ratios of 4:1 and 8:1, the nucleation and growth of CdS QDs simultaneously occurred. That was, while the absorption peaks of CdS RQDs existed, the ones of CdS315 (nuclei) also appeared. However, at feed molar ratios of 1:1 and 2:1, the nucleation period lasted a shorter procedure. Therefore, only distinct absorption peaks of CdS RQDs appeared at 408 and 426 nm, respectively. An interesting phenomenon was that the diameter of CdS QDs decreased with the improvement of molar ratios between Cd2+ and S2− as the blue shift of the absorption peaks implied. The interface between ODE
Fig. 3. TEM images and particle size of CdS QDs via two-step synthesis whose absorption peaks was 379 nm ((A), (B)) and 403 nm ((C), (D)) (Cd/S = 8:1, 140 ℃). 4
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the higher temperatures. Significantly, magic-sized QDs with the first absorption peak at 315 nm formed at 120 ℃ as the only style in solution. With the increase of reaction temperature, such as 140 ℃, 160 ℃ and 180 ℃, when CdS315 (nuclei) existed in the solution, meanwhile, another absorption peak appeared at 382, 388 and 408 nm respectively, which represented the regular CdS QDs. Interestedly, the intensity of the absorption peak at 315 nm increased with temperature ascending instead of degradation and disappearance with the absorption peaks’ red shift of QDs as the many references discussed [28,29]. In our investigated, CdS315 (nuclei) could be stable in the higher temperature, including 140 ℃, 160 ℃, 180 ℃. As some research had proved that nanoclusters with magic numbers which is thermodynamically stable and with local minima chemical potentials because of the closed-shell configurations [28,30,31]. Due to their extremely small sizes, the MSQDs formed under a relatively high chemical potential and they could be stable at relatively high monomer concentrations because the higher temperature would accelerate the form of monomer. We also found that in the relatively low monomer concentration (lower temperature, such as 120 ℃, or the growth process of QDs) the MSQDs also presented. Under lower monomer concentration in the solution, by the “tunneling” of interface, that the CdS(nuclei) grew into regular QDs through the lower thermodynamic barrier and decompose to monomers thorough the high barrier. And the different temperature would experience different growth traces as shown in Fig. 6. The CdS nuclei/ MSQDs would appear and exist in the specific stages, such as 140 ℃ and 160 ℃. However, for another conditions, CdS QDs exhibited the characters of regular QDs growth with the evolution of time and PL spectra also showed irregular transformation. For example, in 160 ℃ and 180 ℃, multiple PL peaks appeared and the intensity gradually increased with the time. Furthermore, the regular CdS QDs exhibited a temperature-depended growth process via the two-step synthesis as shown in Fig. 5(B), which demonstrated that the nuclei had grown into the larger particle with the temperature rising. As the previous jobs, in this two-phase system, the monomer concentration has a critical saturation value, M0 [25]. When monomer concentration was beyond M0, the MSQDs would form, however, it would dissolve back when the monomer concentration was lower than the ‘critical value’. The MSQDs served as a reservoir for monomers condensation and dissolution because these processes happened at a higher speed in this two-step synthesis than one-step synthesis [29,32]. All these discussions implied that the nucleation and growth of CdS QDs in the octadecene/glycerol two-phase systems by one-step synthesis via a nonclassical route.
Fig. 4. Temporal evolution of the absorption spectra of CdS QDs sampled at molar ratio of Cd/S (samples: 1:1,2:1,4:1,8:1) via one-step synthesis at 140℃.
traditional synthesis of QDs. Alivisatos A. P. et al. [25] proposed that the precursor concentration was inversely proportional to the average particle radius as the following equations:
Q0 Vm = v *
n∞ =
dn + vn dt
Q0 Vm v
(2)
(3)
where Q0 was the supply rate of monomer, Vm was the molar volume of the solid, v was the growth rate of a nucleus, and n∞ represents the particle molar density. They inferred that nucleation rate v* was proportional to the production of monomers. If kept the growth rate v constant, the particle number was mainly dependent on Q0 which was proportional to the precursor concentration. For example, in hot injection reactions, increasing the concentration of one precursor would result in the increase of the nucleus number and the smaller particle size. However, in twophase synthesis, the size of regular CdS QDs decreased with increase of the precursor concentration during the feeding molar ratios of Cd/S from 1:1 to 8:1. We thought that a high feed molar ratio improved the usage efficiency of scarce component atom and gave rise to a relatively quick nucleation number. Moreover, the Cd atoms were abundant at a higher Cd/S molar ratio, resulting in the increasing amount of Cd(OA)2 molecules. The nucleus surface was covered by the excess Cd(OA)2 or Cd(OA)+ molecules through electrostatic interactions, which prevented the aggregation and growth of the nuclei. [26,27]. Thus, when the regular CdS QDs grew in solution, the CdS (nuclei) also existed simultaneously.
3.4. Nucleation and growth mechanism of CdS QDs in the ODE/glycerol two-phase system via one-step synthesis Based on the experimental data, the possible nucleation and growth mechanism of CdS QDs in ODE/glycerol two-phase system via one-pot route was proposed, detailed in Scheme 1, which comprised of two pathways from reaction precursors via Cd-S (soluble) and CdS (monomer) to RQDs (top), as well as via Cd-S (soluble) to MSQDs (bottom). As Scheme 1, due to the difference of solubility, when all chemicals were loaded in flask together, through agitating vigorously, CdO would get the opportunity to collide and react with OA, leading to the form of Cd precursor and release of Cd2+ in ODE. S precursor, which resulted from CN2H4S, dissolved into glycerol and met with Cd precursor to achieve Cd-S (soluble) near the interface between ODE and glycerol, which was the “liquidlike” aggregates and about 1 nm in size [33]. Then, the CdS (monomer) could creat with the reaction progress. Meanwhile, CdS315 (nuclei) were obtained at interface of ODE/glycerol. Once oleic acid capped Cd-S (soluble), CdS (monomer) or CdS(nuclei) formed, which was capped with identical capping agent, they would go into ODE phase, and then returned to the ODE/glycerol interface to grow continuously. This two-phase growth process would make the reaction rate slowly because of the diffusion resistance for both nucleation and growth at the organic-water interfaceThus, there was
3.3. Effect of temperature Temperature can also influence the synthesis of CdS QDs. Especially, the temperature can affect the reaction rates, the saturation level of the precursor, reaction equilibrium. Because the collision of the precursor molecule and reaction speed increased in higher temperature. So, the reactivity of the precursors and monomer concentration were influenced directly by temperature. Therefore, the temperature would control the final size of QDs, composition, and optical property. Fig. 5(a) showed the absorption spectra evolution of CdS QDs at different temperature when the Cd/S molar ratio is 4:1 via one-step route at 30 min. During the synthesis, the CdS315 (nuclei) was seemed to possess more stability, which existed in all reaction process without growing into regular QDs. The MSQDs/QDs mixture form generated in 5
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Fig. 5. Temporal evolution of the absorption spectra of CdS QDsrecorded at various reaction temperature (samples:120, 140,160,180 ℃) via one-step synthesis(A) and two-step synthesis(B), (Cd/S = 8:1).
certain reaction parameters and conditions including the reaction temperature, reaction time, mole ratio of precursors, and synthesis route (one-step or two-step), one or both of CdS(nuclei) and CdS MSQDs would be existed in the solution through all the reaction process. On the other hand, if the CdS monomers played a major role, one or both of CdS(nuclei) and CdS RQDs would exhibit. As a result, the nucleation and growth of CdS QDs in the ODE/glycerol two-phase system mainly relied on the ratio between CdS (monomer) and CdS (nuclei) which was analogous to discussion on the dependence of extinction coefficients absorbances [28,34–37]. Compared with onephase synthesis, the reactions in two-phase system, occured much slower, which would give more opportunities to monitor nucleation and growth of QDs.
enough time to make ultra-small size CdS QDs. The formation and consumption of CdS (monomer), CdS(nuclei) were almost at the same instant which was different from the traditional two-step synthesis procedure or “hot-injection” synthesis. Thus, the feature equilibria were built based on CdS (monomer), CdS(nuclei). As a reservoir, the concentration of them was controlled to the saturation value, which was limitation concentration of both CdS (monomer) and CdS(nuclei) condensing to CdS MSQDs and CdS QDs. As we known, in the classical Ostwald ripening process, the growth of larger nanocrystals came from consuming small ones. Thus, CdS(nuclei) was hard to grow into the final nanocrystals becuase of the majority of nuclei were consumed. In fact, the two-phased system provided a slow reaction process. When CdS (monomer) and CdS(nuclei) were presented, if formation or consumption of CdS315 (nuclei) was dominant in growth process under the
Fig. 6. Temporal evolution of the absorption spectra and PL spectra of CdS QDs via one-step synthesis with time recorded at various reaction temperature((A),120 ℃; (B),140 ℃; (C),160 ℃; (D),180 ℃; Cd/S = 8:1). 6
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Scheme 1. Procedures of nucleation and growth of CdS quantum dots in the octadecene/glycerol two-phase systems via one-step synthesis.
4. Conclusion
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In conclusion, we have proposed a nonclassical nucleation and growth mechanism associated with one-step synthesis for preparing CdS QDs in the ODE/glycerol two-phase system by investigating the different reaction parameters including the reaction temperature, reaction time, mole ratio of precursors, and synthesis route (one-step or two-step). The synthesis course was a growth dominated process of both CdS (monomer) and CdS(nuclei), and controlled by the interface of ODE/glycerol. If formation or consumption of CdS(nuclei) was dominant in growth process one or both of CdS(nuclei) and CdS MSQDs would be in the solution through the reaction process. However, if the CdS monomers played a major role, one or both of CdS(nuclei) and CdS RQDs would be the main content. The nucleation and growth of CdS QDs in the octadecene/glycerol two-phase systems via one-step synthesis exhibited nonclassical characters. Our research would provide a new and clear understanding about two-phase system synthesis and pushed the progress for the synthesis of QDs, noble metal nanocrystals and some alloy nanomaterials. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This research was supported by the Key Research and Development Program of Qin Huang Dao (201602A110). References [1] Q. Nie, L. Yang, C. Cao, Y. Zeng, G. Wang, C. Wang, S. Lin, Interface optimization of ZnO nanorod/CdS quantum dots heterostructure by a facile two-step low-temperature thermal treatment for improved photoelectrochemical water splitting, Chem. Eng. J. 325 (2017) 151–159. [2] Y. Wang, S. Ge, L. Zhang, J. Yu, M. Yan, J. Huang, Visible photoelectrochemical sensing platform by in situ generated CdS quantum dots decorated branched-TiO2 nanorods equipped with Prussian blue electrochromic display, Biosens. Bioelectron. 89 (2017) 859–865. [3] K. Wu, Y.S. Park, J. Lim, V.I. Klimov, Towards zero-threshold optical gain using charged semiconductor quantum dots, Nat. Nanotechnol. 12 (2017) 1140. [4] J. Wang, X. Wang, H. Tang, Z. Gao, S. He, J. Li, S. Han, Ultrasensitive electrochemical detection of tumor cells based on multiple layer CdS quantum dotsfunctionalized polystyrene microspheres and graphene oxide – polyaniline composite, Biosens. Bioelectron. 100 (2018) 1–7. [5] J. Wang, X. Wang, H. Tang, Z. Gao, S. He, R. Niu, Y. Zheng, S. Han, A ratiometric magnesium sensor using DNAzyme-templated CdTe quantum dots and Cy5, Sens. Actuators B Chem. 272 (2018) 146–150. [6] J. Wang, X. Wang, H. Tang, Z. Gao, S. He, D. Ke, Y. Zheng, S. Han, Facile synthesis and properties of CdSe quantum dots in a novel two-phase liquid/liquid system, Opt. Mater. 72 (2017) 737–742. [7] J. Wang, K. Guo, D. Ke, S. Han, Synthesis and mechanism study of CdS quantum dots in two-phase liquid/liquid interfaces via one-pot route, Chem. Phys. Lett. 618 (2015) 11–13.
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