ARTICLE pubs.acs.org/JPCC
Synthesis and Characterization of ZnSe Nanocrystals by W/O Reverse Microemulsion Method: The Effect of Cosurfactant Lin Yang, Ruishi Xie, Lingyun Liu, Dingquan Xiao, and Jianguo Zhu* College of Materials Science and Engineering, Sichuan University, Chengdu 610064, People’s Republic of China
bS Supporting Information ABSTRACT: ZnSe nanocrystals with a cubic zinc blende crystal structure were synthesized by reverse microemulsions of water/Triton X-100/2-propanol/cyclohexane. The phase formation, state of particle size distribution, morphological characteristics, composition, and optical properties of the obtained nanocrystals were investigated by means of X-ray diffraction, PSD, transmission electronic microscope, Fourier transform infrared, UVvisible spectroscopy, and photoluminescence. All the measurement results showed that the particle size of ZnSe nanocrystals is governed by the amount of cosurfactant (2-propanol) besides the molar ratio of water to surfactant in the reverse micelles system. The presence of cosurfactant favors the modulation of the strength of surfactant film and the exchange dynamics of micelles. ZnSe nanocrystal synthesized by using an appropriate amount of 2-propanol possesses a narrow particle size distribution and a good crystallinity. A growth mechanism involving the possible formation of nanoparticles based on the effect of cosurfactant in microemulsion has been explored in this paper.
’ INTRODUCTION The preparation and characterization of IIVI semiconductor nanocrystals have attracted much attention recently due to their great potential in optoelectronic applications in the fields of lightemitting diodes (LEDs), solar cells, sensors, and optical recording materials.110 As one of the important Zn-based IIVI semiconductors, zinc selenide (ZnSe) has been considered to be an applicable material for optoelectronic devices due to its wide direct band gap (2.67 eV) and large exciton binding energy (21 meV).1113 Moreover, ZnSe is also a promising material for windows, lenses, output couplers, beam expanders, biomedical labels, and optically controlled switching, due to its low absorptivity at infrared wavelength and giant photosensitivity. It is desirable to fabricate nanocrystals that indicate high photoluminescence quantum yields (PLQYs), narrow size distribution, and especially size- and shape-tunable optical properties.1418 Consequently, considerable efforts are continuously devoted to the development of synthetic methods. To date, there are many methods of synthesizing monodispersed nanoparticles, such as coprecipitation,1921 hydrothermal synthesis,13,22,23 reverse micelles,2429 solgel method,30,31 and spray pyrolysis.32 Among these methods mentioned above, water-in-oil (W/O) microemulsion (i.e., reverse micelles) technique is one of the most recognized methods due to its several advantages, e.g., soft chemistry, demanding no extreme pressure or temperature control, easy to handle, and requiring no special or expensive equipment. The principal reason for using microemulsion in the preparation of ultrafine materials is that it is a powerful method for controlling the particle size and corresponding size distribution.33 In general, W/O microemulsion is an isotropic, r 2011 American Chemical Society
thermodynamically stable dispersion of nanometer-sized water droplets in a continuous oil medium. This is compartmentalized by a monolayer of surfactant and cosurfactant molecules localized at water/oil interfaces. The droplets undergo Brownian motion and continuously collide with each other, leading to the dynamic exchange of reacting species through an open water channel formed across the surfactant films. This dynamic process ensures a homogeneous repartition of the reactants among the water droplets, at nanoscale level, of hydrophilic and hydrophobic domains, resulting in the formation of monodispersed nanoparticles of controlled characteristics.34 Nevertheless, very few reports35,36 of the preparation of ZnSe nanocrystal by W/O microemulsion method could be found generally. In our study, a novel synthetic medium has been exploited, ZnSe nanocrystals were synthesized by a W/O reverse microemulsion method using cyclohexane as oil phase, Triton X-100 as surfactant, and 2-propanol as cosurfactant, respectively. It was found that the particle size of ZnSe nanocrystals formed in W/O microemulsions is directly influenced by two parameters: the molar ratio of water to surfactant (W0) and the molar ratio of cosurfactant to surfactant (P0). The effect of the amount of 2-propanol (denoted as P0) on the formation and characteristics of ZnSe nanocrystals has been mainly discussed. Meanwhile, the nanocrystals exhibit size-dependent shift in the absorption and photoluminescence emission spectra, which have been analyzed in terms of a quantum confinement model. Received: May 24, 2011 Revised: August 17, 2011 Published: August 19, 2011 19507
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’ EXPERIMENTAL SECTION a. Chemicals. Zinc acetate dehydrate (Zn(AC)2 3 2H2O, AR, 99.0%), sodium borohydride (NaBH4, AR, 97.0%), and selenium powder (Se, 3N, 99.9%) were used as starting materials. Cyclohexane (C6H12, AR, 99.5%), Triton X-100 (C8H17C6H4(OCH2CH2)10OH, CP), and 2-propanol ((CH3)2CHOH, AR, 99.7%) were used as received. Ultrapure deionized water was used for the preparation of all aqueous solutions. b. Synthesis of ZnSe Nanocrystals. The synthesis of ZnSe nanocrystals was based on the reaction of zinc acetate with sodium hydroselenide (NaHSe) by microemulsion method. All reactions were carried out in oxygen-free water degassed by nitrogen. First, NaHSe aqueous solution was freshly prepared by dissolving Se powder in NaBH4 solution under nitrogen atmosphere at room temperature for about 20 min, while the aqueous solution of Zn(AC)2 was prepared by dissolving Zn(AC)2 3 2H2O in deionized water under continuous stirring to form a clear solution. Then, two types of microemulsions were prepared separately. Each microemulsion was composed of Triton X-100 as surfactant, 2-propanol as cosurfactant, cyclohexane as a continuous oil phase and reactant solution as aqueous phase. While the reactant concentration was kept constant at 0.2 M, the molar ratio of water to surfactant (W0 = [H2O]/[Triton X-100]) was varied in the range of 48120, and the molar ratios of cosurfactant to surfactant were P0 = [2propanol]/[Triton X-100] = 0, 6, 17, and 26, respectively. After mixing these two dispersed microemulsions, a turbid yellow solution was obtained, indicating the presence of ZnSe quasinanospheres. Then the resulting mixture was heated to 90 °C and continuously stirred using a magnetic agitator for 5 h. The resultant yellow powders were centrifuged at 8000 rpm for 5 min, washed with deionized water and anhydrous ethanol for several times until no segregation of white surfactant was observed at the top of centrifuge tube, and finally dried at 50 °C under vacuum. c. Characterizations. The crystallographic information on the obtained ZnSe nanocrystals was established by X-ray diffraction (XRD, D/max-rA model, using nickel-filtered Cu Kα radiation). Particle size distribution (PSD) was measured with a dynamic light scattering (DLS) particle size analyzer (Horiba, LB-550). The morphology and dispersion state of ZnSe nanocrystal were examined using transmission electronic microscope (TEM, JEM2010). Its corresponding size distribution histogram was determined by measuring the sizes of 100 nanoparticles. The composition of the ZnSe nanocrystal was investigated by Fourier transform infrared spectroscopy (FT-IR, Perkin-Elmer, Spectrum one). The UVvis adsorption spectra were performed on a Shimadzu 2100 UVvisible spectrophotometer in the wavelength range of 300800 nm. Fluorescence spectra were obtained with an F-7000 FL spectrophotometer with a 450 W xenon lamp as excitation source. Excitation wavelengths were set at 340 nm. All the optical measurements were performed at room temperature under ambient conditions. The samples were diluted with deionized water and placed directly in quartz cuvettes (1 cm path length) for characterization without any size sorting. The quantum yields (QYs) of ZnSe nanocrystals were estimated according to the procedure described in ref 18 using quinine sulfate in 0.05 M H2SO4 solution as a reference standard (QY = 55%).
’ RESULTS AND DISCUSSION In the present study, the amount of cosurfactant (2-propanol) was varied by the P0 value. The X-ray diffraction patterns of ZnSe
Figure 1. X-ray diffraction patterns of ZnSe nanocrystals synthesized at different W0 values as a function of P0.
Figure 2. Average particle sizes of ZnSe nanocrystals synthesized at different W0 values as a function of P0.
nanocrystals synthesized at different W0 values as a function of P0 are shown in Figure 1. All of the diffraction peaks can be indexed as cubic zinc blende structure of ZnSe with lattice parameter a = 5.670 Å (JCPDS Card No. 80-0021), which is in good agreement with the values reported in the literature.13,23,32 Decreasing the P0 value 26 from to 0 shows an increase of diffraction peak intensity and a decrease of diffraction peak fullwidth at half-maximum (fwhm) due to the grain growth. No characteristic peaks corresponding to impurities are found, showing the high purity of these samples. Careful inspection of Figure 1, panels a and b, also indicates that there remain much more amorphous particles in the ZnSe nanocrystals synthesized at W0 = 120. Some amorphous particles begin to crystallize, and the crystallinity gradually strengthens with decreasing the P0 value. As a comparison, ZnSe nanocrystals synthesized at W0 = 48 possess better crystallinity. According to Scherrer’s equation,29 the average particle sizes calculated from XRD patterns of ZnSe nanocrystals are shown in Figure 2, where the error bars indicate the mean deviation. Similar trends are observed for ZnSe nanocrystals synthesized at the two different values of W0, whatever W0 was used, the crystal size gradually decreases with the increase of P0 value. The particle sizes of ZnSe nanocrystals are relatively larger in the case of without the addition of 2-propanol (P0 = 0). It implies that, in this synthesis situation, cosurfactant could play a role in controlling the crystal size. Moreover, from the data reported in Figure 2, we note that at the same values of P0, the particle size of ZnSe nanocrystals synthesized at W0 = 48 is smaller than that of ZnSe 19508
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Figure 4. TEM of as-synthesized ZnSe nanocrystal with W0 = 48 and P0 = 26.
Figure 3. Particle size distribution (PSD) patterns of ZnSe nanocrystals synthesized at different P0 values.
nanocrystals synthesized at W0 = 120, for the reason that a lower value of W0 signifies a smaller water core of droplets, which favors for the formation of smaller nanoparticles. Taking ZnSe nanocrystals synthesized at W0 = 48 with different amounts of 2-propanol for example, Figure 3 shows their state of particle size distribution (PSD) measured by DLS particle size analyzer. The nanocrystals prepared after centrifugation were ultrasonically dispersed in anhydrous ethanol for 20 min and then dropped into quartz cuvettes for the DLS measurements. Owing to the agglomeration of nanoparticles, it is crucial to choose a better condition to get the well-dispersed ZnSe nanocrystals. After a careful evaluation of a series of measurement experiments, we chose anhydrous ethanol as dispersant and the ultrasonication time of 20 min. It was found to be the optimal conditions of the PSD measurements using DSL particle size analyzer, which is reliable for determining the PSD and average sizes of nanocrystals. These were also confirmed by the results of our previous experiments performed by repeated measurements and comparison with TEM. From Figure 3a, we can see that the PSD of ZnSe nanocrystal synthesized at P0 = 0 is the widest, so there is a great variation in particle size, raging from 2.8 to 14.5 nm. The larger particles of g5.6 nm constitute a large percentage and possibly result from agglomeration. In contrast, ZnSe nanocrystals with a relatively narrow PSD and the predominant sizes around 5 and 4 nm were prepared by using the 2-propanol amount of P0 = 6 and 17, as shown in Figure 3. panels b and c, respectively. When the 2-propanol amount of P0 = 26 is employed in synthesis, the corresponding ZnSe nanocrystal has an average particle diameter of 3.23 nm, close to the datum in Figure 2 (3 nm), and possesses a narrowest PSD, ranging from 2.86 to 3.76 nm, as shown in Figure 3d, which is also found to be similar to the size distribution obtained from TEM image after a careful comparison (Figure S1 in the Supporting Information). To sum up, it is relevant to observe that at the same value of W0, ZnSe nanocrystal synthesized in a ternary microemulsion (P0 = 0) presents a much wider PSD, and the nanocrystals synthesized in quaternary microemulsions (P0 6¼ 0) at larger 2-propanol amount are smaller and have the narrower PSD as compared to those synthesized at lower P0 values. With the increase of P0 value, the descending trend in particle size is comparable to that calculated from XRD patterns (Supporting Information, Figure S1).
Figure 5. FT-IR spectrum of as-synthesized ZnSe nanocrystal with W0 = 48 and P0 = 26.
In Figure 4, the morphology and nanostructure of ZnSe nanocrystal synthesized at W0 = 48 and P0 = 26 are characterized by TEM. The sample is dispersed in ethanol by ultrasonic treatment (15 min), dipped onto a copper grid, and then characterized by TEM. Figure 4a presents an overview TEM image of the sample, it can be seen that the ZnSe nanocrystal exhibits a better dispersity and relatively uniform spherical shape. As shown by the size distribution histogram in the insert of Figure 4a, its average diameter is 3.18 ( 0.03 nm, which is the most accurate diameter and quite comparable to the result calculated by XRD pattern and that estimated from PSD (Figure 3d). The overlapping of particles in some areas of the image is due to the TEM sample preparation technique. It can be further noted that, as shown in Figure 4b, the lattice spacing of the ZnSe nanocrystal is about 0.32 nm, corresponding to the (111) plane of a cubic zinc blende ZnSe structure, which is agreement with the XRD result. The regular existence of the distinct lattice planes inside the nanocrystal suggests the most nanoparticles have a good crystalline structure with no defects. To understand absorption of organic component (Triton X-100 and 2-propanol) on the surface of ZnSe nanocrystals, FT-IR measurement on as-prepared ZnSe nanocrystal synthesized at W0 = 48 and P0 = 26 was carried out as shown in Figure 5. The broad absorption peak at 3427 cm1 owing to the stretching vibration of hydroxyl groups (OH) is clearly observed. The stretching vibration at 2921 and 2848 cm1 are assigned to the asymmetric and symmetric CH2 stretch, respectively. The intense stretch at 1635 cm1 is derived from the benzenoid group of Triton X-100.37 The OH in-plane and out-of-plane bands appear at 1456 and 920 cm1, respectively.32 The existence of a 19509
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Figure 6. UVvis absorption spectra of ZnSe nanocrystals synthesized at different W0 values as a function of P0.
Figure 7. PL spectra of ZnSe nanocrystals synthesized at different W0 values as a function of P0, obtained using an excitation wavelength of 340 nm.
remarkable absorption of 2-propanol on the surface of ZnSe nanoparticles, acting as a capping agent could be confirmed by the consideration that the presence of residual 2-propanol makes the nanocrystals soluble in ethanol, giving a clear yellow solution from which nanoparticles can not be easily separated by centrifugation in our experiments. The cosurfactant acting as a capping agent38 can effectively prevent the particle agglomeration and produce ZnSe nanocrystals with enhanced stability in air. It is also worth mentioning that using a proper amount of alcohol as cosurfactant would prevent from phase separation behaviors between water and oil, favoring the formation and stability of water-in-oil microemulsions.39 In Figure 6, ZnSe nanocrystals synthesized in different conditions were monitored by UVvis absorption spectroscopy. The spectra show the evidence of quantum confinement effect, indicating the formation of nanometer-sized ZnSe particles. Both the position and the height of main shoulder in absorption spectrum are related to the size of semiconductor particles. As shown in Figure 6, panels a and b, the absorption spectra show a noticeable red shift with the decrease of P0 value, indicative of the capability of controlling the particle size of ZnSe nanocrystals by varying the amount of 2-propanol as well. In the case of W0 = 48, the resulting ZnSe nanocrystals synthesized at P0 = 26 and 17 are relatively smaller and narrowly distributed, which can be also demonstrated by their comparatively shorter absorption wavelengths and sharper absorption peaks. At P0 = 0, it should be noted that the distinguishable absorption peak of ZnSe nanocrystal observed here do not commonly occur because of a weaker quantum confinement effect. For the ZnSe nanocrystals synthesized at W0 = 120, similar results are obtained where a distinct absorption peak is shifted to longer wavelength and attenuated with decreasing P0 value, as shown in Figure 6(b). The PL spectra of ZnSe nanocrystals synthesized in different conditions are shown in Figure 7. It is observed that the PL spectra show a noticeable blue shift in the wavelength of bandedge luminescence with the increase of P0 value. Figure 7a shows that as P0 value increases, the band-edge luminescence is blueshifted from 426, 410, and 398 to 386 nm. The considerable shift observed in the PL spectra as a function of the amount of 2-propanol confirms that a significant role of cosurfactant in controlling the particle size of ZnSe nanocrystals by locating at water/oil interfaces has indeed to be invoked. It might be worthwhile to note here that the increase of P0 value leads to an increase in the emission intensity of ZnSe nanocrystals synthesized at W0 = 48, which is contrary to that of
ZnSe nanocrystals synthesized at W0 = 120. We consider the main reason for these interesting phenomena may be that at the lower molar ratio of water to surfactant (W0 = 48), the descending trend in particle size with the increase of P0 value is more obvious (comparing with the case of W0 = 120 as shown in Figure 2). The decrease in the particle size could increase the radiative transition rate, resulting in enhanced band-edge luminescence intensity due to quantum confinement and surface effect. Thus the as-prepared ZnSe nanocrystals show an increase of QYs from 7% up to 24% with increasing P0 value from 0 to 26 (Figure S2 in the Supporting Information), similar to that reported in the previous literature.4,18 In contrast, at the higher molar ratio of water to surfactant (W0 = 120), according to the XRD patterns shown in Figure 1b, with the increase of P0 value, an increase in the amount of amorphous particles would cause defect states in ZnSe nanocrystals, which quench the luminescence, thus leading to a significant deterioration in the band-edge luminescence intensity. The QYs are found to be generally low (less than 10%). It is also instructive to note that all the ZnSe nanocrystals have separation between the absorption and bandedge emission peaks (Stokes shift). The emission spectra have perfect Gaussian shapes, which clearly indicate the pure bandedge emission without any significant trap-state emission. The smaller overlap area (Supporting Information, Figure S3) between the absorption and emission spectra would be favorable for optoelectronic applications.32 Semiconductor nanocrystal with a particle radius significantly smaller than the exciton Bohr radius shows strong size-dependent optical properties due to the quantum confinement effect. In this regime there is a progressive increase in band gap energy with decreasing the particle size. Moreover, because of a high surface area to volume ratio and high surface energy, as the nanocrystal becomes smaller, the number of uncoordinated atoms on the surface would increase, and the bond length would shrink spontaneously, allowing the strain induced by the lattice mismatch to be distributed over a large fraction of the constituent atoms. It is known that the lattice strain increases with decreasing the particle size.40 Consequently it was also found that the band gap energy of semiconductor nanocrystals increases with the decrease of particle size due to the imperfect coordination and strain in the self-equilibrium state.41 Thereby, it is natural to pursue whether lattice strain would have an effect on the blue shift in band edge in our experiments. The band gap energy (Eg) of ZnSe nanocrystals was estimated from the absorption spectra. The theoretical band gap energy can be calculated by 19510
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Figure 8. Relationship between the band gap energy and the particle size of ZnSe nanocrystals.
Brus equation as follows:8 h2 1 1 Eg ðrÞ ¼ Eg ðbulkÞ þ 2 þ mh 8r me
!
1:8e2 εr
ð1Þ
By considering the bulk band gap (2.67 eV), effective masses (me* = 0.157me and mh* = 0.64me), and dielectric constant (ε = 8.7), as well as substituting the nanoparticle diameter d for radius r, this equation is simplified as Eg ðdÞ ¼ 2:67 þ
11:9 0:596 eV d2 d
ð2Þ
The relationship between the band gap energy and the particle size of ZnSe nanocrystals measured from DLS is shown in Figure 8. The band gap energies of the as-prepared ZnSe nanocrystals can be estimated to be shifted toward higher energy as compared to the 2.67 eV (corresponding to 465 nm) band gap of the bulk material. This is accordant with the previous work32,4244 that ZnSe nanocrystals exhibit size-dependent absorption and photoluminescence in the wavelength range of 300450 nm. The measured values estimated from UVvis spectra are shown as solid circles with the error bars on the x and y axes reflecting the mean deviation of particle size and band gap energy, respectively. The theoretical values calculated from eq 2 are represented by the solid line, which implies the quantum confinement effect. A slight discrepancy between experimental and calculated data for the band gap energy is found, as shown in Figure 8. Because the experimental error could also explain the discrepancy between theory and experiment, the effect of sizedependent lattice strain on the band gap energy is fairly minor. So we consider that the large blue shifts in the absorption and photoluminescence emission spectra are primarily caused by quantum confinement of an electron hole pair (exciton), which is the main basic contribution to the variation in band gap energy, although there may have been an additional contribution from the size-dependent lattice strain. In general, for a given reactant concentration, particle nucleation can be enhanced by increasing the exchange rate of micelles. At high exchange rates, many reactants form active monomeric reacting species at once and grow simultaneously up to the critical size necessary to form nuclei, so there are only a very few reacting species that can be use for growth, resulting in the smaller particles. On the contrary, at low exchange rates, only a small fraction of reactants which are converted into monomeric reacting species at early reaction time can grow into nuclei. Then,
these nuclei adsorb all the others of small monomeric reacting species through subsequently dynamic exchanges and then grow into the larger particles.45 For surfactant film with a higher strength (a lower deformability or a stronger attachment to droplets), open water channels are less likely to occur for reactants to pass through, leading to a low exchange rate, whereas a lower strength of surfactant film indicates a high exchange rate. In summary, the lower strength of surfactant film (a highly flexible surfactant film) implies a lower steric hindrance to the droplet exchange of reacting species45 and, hence, leads to the formation of more nuclei and smaller final particles. In our experiments, an appropriate amount of 2-propanol is favorable to get stable water-in-oil microemulsions and desired nanocrystal characteristics. The plain presence of the surfactant (Triton X-100) makes the interfacial film more compact. The addition of cosurfactant (2-propanol) to ternary microemulsion is known to reduce the strength of surfactant film by affecting the compactness of the film and its stability, and is generally considered responsible for increasing the exchange rate of micelles, thus allowing the exchange of reacting species.38 So reactants are almost exhausted at early stage of the nucleation process, inhibiting both growths by autocatalysis and ripening, and consequently leading to the ZnSe nanocrystals synthesized in quaternary microemulsions (P0 6¼ 0) being smaller than those synthesized in ternary microemulsions (P0 = 0). Therefore, it can be found that in this study, the higher addition of 2-propanol could lead to the formation of much smaller ZnSe nanocrystals due to the lower strength of surfactant film. A successful synthetic strategy for high-quality nanocrystals should provide a wide range of desired particle sizes. We observe that increasing the amount of cosurfactant results in relatively smaller nanoparticles. Through this increment, size-tuned ZnSe nanocrystals can be synthesized effectively.
’ CONCLUSIONS ZnSe nanocrystals with a cubic zinc blende crystal structure were synthesized by water-in-oil reverse microemulsion method. Experimental results indicate that the average particle size is governed by the molar ratio of cosurfactant to surfactant (P0) besides the molar ratio of water to surfactant (W0) in the reverse micelles system. ZnSe nanocrystal synthesized in a ternary microemulsion (P0 = 0) presents a larger crystal size and a much wider PSD, and the nanocrystals synthesized in quaternary microemulsions (P0 6¼ 0) at larger 2-propanol amount are smaller and have the narrower PSD as compared to those synthesized at lower P0 values. Meanwhile, with the increase of P0 value from 0 to 26, a systematic blue shift was observed for both excitonic absorption peaks and band-edge emission peaks of the resulting ZnSe nanocrystals. The significant blue shift provides a clear evidence for the role of cosurfactant in controlling the particle size of ZnSe nanocrystals. It is concluded that the addition of cosurfactant to ternary microemulsion can reduce the strength of surfactant film by affecting the compactness of the film and its stability, and is generally considered responsible for increasing the exchange rate of micelles, thus reactants are almost exhausted at early stage of the nucleation process, inhibiting both growths by autocatalysis and ripening. Therefore, it can be found that in this study, the higher addition of 2-propanol could lead to the formation of much smaller ZnSe nanocrystals due to the lower strength of surfactant film. Moreover, the cosurfactant could act as a capping agent, which can effectively prevent the particle agglomeration and produce ZnSe 19511
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’ ASSOCIATED CONTENT
bS
Supporting Information. Supporting results mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86 28 85412415. Fax: +86 28 85416050. E-mail: nic0400@ scu.edu.cn.
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’ NOTE ADDED AFTER ASAP PUBLICATION This manuscript was originally posted to the Web on August 19, 2011, with an error to the Experimental Section. The corrected version was reposted on September 19, 2011.
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