Growth and Phase-Transformation Mechanisms of Nanocrystalline

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Growth and Phase-Transformation Mechanisms of Nanocrystalline CdS in Na2S Solution Yansong Xiong,† Jing Zhang,† Feng Huang,† Guoqiang Ren,† Weizhen Liu,† Dongsong Li,† Chen Wang,‡ and Zhang Lin*,† State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, People’s Republic of China, National Center for NanoScience and Technology, Beijing 100080, People’s Republic of China ReceiVed: February 25, 2008; ReVised Manuscript ReceiVed: April 10, 2008

As-synthesized 3.5-nm sphalerite CdS was hydrothermally coarsened in a series of Na2S solutions at 150 °C, aiming to understand the effect of the Na2S additive on the growth and phase-transition mechanisms. X-ray diffraction (XRD) data indicate that, with increasing Na2S concentration, both the growth rate and the ratio of wurtzite CdS increase. Moreover, high-resolution transmission electron microscopy (HRTEM) data reveal that, in contrast to the regular surface nucleation mode in aqueous solution, the new wurtzite phase of CdS in Na2S solution appears in the interior regions of crystal, and the interfacial nucleation mode greatly promotes the generation of the wurtzite-type structure. The oriented-attachment (OA) mechanism during growth was found to contribute to the unique mode of phase transformation. We propose that the strong interfacial adsorption effect of Na2S promotes the growth of CdS via the OA mechanism and the OA-mediated phase transformation. Introduction Nanoscale semiconductors are of particular interest because of their unique electronic and optoelectronic properties and many potential applications, such as light-emitting diodes,1,2 solar cells,3 nonlinear optical materials,4 electronic and optoelectronic devices,5 biological labeling,6,7 and so forth. In particular, as an important group II-VI semiconductor material, CdS has been extensively studied because of its large value of Eg, which allows light emission between blue and red wavelengths.8 Recently, syntheses of various kinds of CdS nanocrystals were reported.9–11 It was suggested that the size, morphology, and structure of nanocrystallites are very important factors for determining the optical and electronic properties of CdS nanocrystals.9–11 At the same time, with the wide distribution of nanomaterials in the environment, increased interest has focused on effective methods for the treatment of hazardous nanomaterials. Based on an idea of artifically accelerating the reaction and growth of nanomaterials, the nanowaste can be effectively transformed into bulk material with low adsorption affinity.12 Thus, investigations related to the phase transformations and rapid crystal growth of specific hazardous nanomaterials under selective mineralizers are greatly needed. CdS usually exists in two phases: sphalerite (cubic) and wurtzite (hexagonal).8,13,14 The bulk wurtzite structure is the thermodynamically stable phase under normal conditions (such as room temperature and atmospheric pressure).15,16 The intrinsic energy difference between these two phases of CdS is very small,17 so the controlled growth of CdS nanocrystals in only one phase at a time is difficult to achieve.18,19 Some studies have found that sphalerite CdS nanoparticles exist in smaller sizes and that, with the growth of the particles, the wurtzite structure appears.8,20 Therefore, it was suggested that the phase * To whom correspondence should be addressed. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ National Center for NanoScience and Technology.

stability between the sphalerite and wurtzite structures of CdS was size-dependent. In addition, nanocrystal growth and phase transitions also depend highly on the solution environment, nanocrystal surface state, and aggregation of the particles. Relevant studies have been reported for the ZnS and Ti2O systems revealing that the phase-transition pathway varies with the nanocrystal surface state.21–23 A few studies related to the growth and phase transition of nanocrystalline CdS have also been reported. For example, when CdS nanoparticles are annealed in Ar + S2 at normal pressure, the phase transition from sphalerite to wurtzite is temperature-dependent.24,25 In addition, under hydrothermal conditions, the addition of HCl or Cd(NO3)2 can promote the crystal growth and phase transformation of CdS nanocrystals.26 However, to date, the relationships between the interface effect of solution and the phase-transition pathway and rate of CdS nanocrystals have not been elucidated. In this work, assynthesized CdS nanocrystals were hydrothermally treated at 150 °C in different concentrations of Na2S solution, in order to understand the interface effects of Na2S solution on the growth and phase-transition mechanisms. We found that, under the interface adsorption effects of Na2S, an interfacial nucleation and phase-transition mode occurred that obviously increased the rate of the phase transition from sphalerite to wurtzite. Experimental Section The primary CdS nanoparticles were synthesized in aqueous solution without any surfactant in order to avoid the influence of other absorbents. Briefly, an aqueous solution of 0.15 M sodium sulfide was dropped into 0.15 M cadmium chloride aqueous solution. The mixture was stirred vigorously to obtain homogeneity, aged for 30 min, and rinsed with distilled water several times until Cl- was below the detection limit by 0.1 M AgNO3 solution. The precipitates were dried and ground into a powder for the subsequent experiments. Separately, 0.12 g of as-synthesized nanocrystalline CdS, 10 mL of H2O, and either 0.5, 1.0, or 2.0 M Na2S were mixed in

10.1021/jp801628e CCC: $40.75  2008 American Chemical Society Published on Web 06/04/2008

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Figure 1. Analysis of the as-synthesized CdS: (a) TEM image and EDS, (b) XRD pattern, and (c) HRTEM image.

a Teflon-lined stainless steel autoclave of 23-mL capacity. The autoclaves were sealed and heated at 150 °C. At the appropriate time interval, the autoclave containers were removed and quenched to room temperature. The precipitates were collected and washed with deionized water. X-ray diffraction (XRD) was used to identify the crystal structures, average particle sizes, and phase compositions of the initial and coarsened samples. Diffraction data were recorded using a PANalytical X’Pert PRO diffractometer with Cu KR radiation (40 kV, 40 mA) in the continuous scanning mode. The 2θ scanning range was from 15° to 60° in steps of 0.02° with a collection time of 57 s per step. The average crystallite size was calculated from the peak broadening using the Scherrer equation. The phase composition of a sample can be calculated from the integrated intensities of the wurtzite (100) peak (2θ ) 24.84°) and the overlapping sphalerite (111) and wurtzite (002) peaks (2θ ) 26.53°). If the intensity ratio of wurtzite (100) to the overlapping peak is R, then the weight fraction of wurtzite (Fw) can be calculated as

Fw )

R k - (k k - 1)R ′

(1)

where k′) 0.825 represents the intensity ratio of the wurtzite (002) peak to the wurtzite (100) peak and k ) 0.341 is the intensity ratio of the wurtzite (100) peak to the sphalerite (111) peak. High-resolution transmission electron microscopy (HRTEM) was used to determine the morphology and detailed microstructure and phase identification of individual particles. Samples were prepared for HRTEM study by dispersing the CdS powder onto 200-mesh carbon-coated copper grids. HRTEM analyses were performed using a JEOL JEM2010 HRTEM instrument at 200 kV. Results Analysis of the As-Synthesized CdS. Figure 1a shows a representative TEM image of as-synthesized CdS particles. The energy dispersive X-ray spectroscopy (EDS) pattern of these particles (inset) shows that the d spacings are 0.337, 0.206, and

0.175 nm, characteristic of the (111), (220), and (311) planes, respectively, of sphalerite CdS. Judging from these results and the XRD pattern in Figure 1b, we concluded that the assynthesized CdS was the sphalerite phase. According to the peak broadening of the XRD spectra and the Scherrer formula, we calculated the mean diameter of the initial particles as about 3.5 ( 0.3 nm. The HRTEM image of an individual nanoparticle also confirmed the size, morphology, and phase structures of the initial particles (Figure 1c). Analysis of the Phase Transition and Particle Growth. Figure 2a-d shows time-series XRD patterns of CdS particles upon hydrothermal treatment at 150 °C, with concentrations of Na2S of 0, 0.5, 1.0, and 2.0 M, respectively. The patterns show that ,for each concentration of Na2S, as the reaction time increases, two reflection peaks grow at 2θ ) 24.8° and 28.2°, which belong to the (100) and (101) planes, respectively, of wurtzite CdS. In addition, as the coarsening time increases, the line width of the XRD pattern decreases, indicating that the size of CdS nanocrystal continues to grow as the phase transformation occurs. Using eq 1, the content of wurtzite phase can be calculated from the XRD patterns. As shown in Figure 3, the proportion of wurtzite CdS increases not only with increasing growth time, but also with increasing concentration of Na2S. Moreover, when coarsening is performed in concentrated Na2S solution (2 M), the content of wurtzite CdS is obviously higher than that in aqueous solution. HRTEM-Based Microstructural Analysis. To evaluate the distribution of sphalerite and wurtzite CdS and to identify their microstructures, typical samples were examined by HRTEM. As illustrated in Figure 4a, for hydrothermal treatment without Na2S, the newly formed wurtzite regions can only be found capping the surface of the particles. Also, most of the particles were round, which is characteristic of Ostwald-ripening (OR) mechanism of crystal growth (as in Figure 4a). With the addition of Na2S, as revealed in Figure 4b-d, surface-capping wurtzite regions could still be seen. Moreover, we found that an increasing percentage of wurtzite regions formed in the interior of large particles. As the concentration of Na2S solution increased, the wurtzite domains in the interior of particles increased. Together with the appearance of wurtzite regions, the growth of CdS nanoparticles shows evidence of the oriented-attachment (OA) mechanism (as in Figure 4b-d). Therefore, according to the above analysis, we clearly found that the presence of Na2S increased the ratio of wurtzite CdS, induced more wurtzite phase in the interior region of large particles, and promoted the OA-based growth of CdS nanoparticles. Discussion Generally, the kinetic barriers for phase transformations of very small nanoparticles are much smaller than those of the counterpart bulk materials.27 Therefore, it is viewed that phase transformations from a metastable state to a stable state are much easier for nanoparticles. In the view of thermodynamics, there is a critical particle size of phase transition23,28,29 above which the metastable phase will completely transform into the thermodynamically stable phase. However, this phenomenon is rarely observed. As in our system, a critical size for the phase transformation was not found, and the percentage of wurtzite structural kept increasing with the particle growth. Obviously, it is interesting to see that the kinetic factors often dominate the phase transition. Mode of Phase Transition. Our experiments revealed that the CdS nanocrystals transformed from sphalerite to wurtzite

Growth and Phase Transformation of CdS in Na2S Solution

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Figure 2. Time-series XRD patterns of CdS particles upon hydrothermal treatment at 150 °C for 2, 4, 12, and 28 h in (a) 0, (b) 0.5, (c) 1.0, and (d) 2.0 M Na2S solution.

Figure 3. XRD-determined wurtzite proportion vs time at 150 °C. (The concentration of Na2S was varied from 0 to 2 M).

during crystal growth. To understand how this process occurs, we investigated the distributions of sphalerite and wurtzite CdS and the microstructure present by HRTEM. On the basis of these results combined with the characteristics of nucleation and phase transformation found in previous studies,30–32 we propose three possible nucleation modes of wurtzite as follows: Mode I, Sphalerite Phase DissolWed and Wurtzite Phase Nucleated. As shown in Figure 5a, primary sphalerite CdS particles dissolve and give rise to nuclei of the wurtzite phase, and then these nuclei develop into wurtzite CdS particles in solution. If nucleated in this way, CdS particles that contain only wurtzite can be produced. However, after careful and extensive examination of the CdS samples by HRTEM, we could not find any such wurtzite CdS particles. Thus, it can be concluded that the formation of wurtzite phase in CdS nanoparticles does not occur in this way. Mode II, Wurtzite Phase Nucleated on the Surface of Sphalerite Particle. Dangling bonds on the surface of nanoparticles lead to a higher chemical potential for surface atoms

Figure 4. Typical HRTEM images of CdS particles after hydrothermal treatment at 150 °C: (a) in deionized water for 12 h, (b) in 0.5 M Na2S solution for12 h, (c) in 1.0 M Na2S solution for 4 h, and (d) in 2.0 M Na2S solution for 4 h. In the illustrated diagrams at right, the positions of wurtzite (Wur.), sphalerite (Sph.), twin (T), and stacking fault (SF) are demonstrated.

than for interior atoms,33 and thus the activity of surface atoms is much higher.30 Wurtzite nuclei could form on the surface of the sphalerite CdS nanoparticles. As shown in Figure 5b, if crystal growth occurred in this way, a crystal could grow into particles with two different microstructures. One possibility is that the wurtzite regions rapidly propagate to the entire particle

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Figure 5. Model of nucleation: (a) sphalerite CdS dissolves in solution and forms wurtzite nuclei, (b) wurtzite forms by surface nucleation, and (c) wurtzite forms by interfacial nucleation.

and then grow into large wurtzite particles (Figure 5b1). Alternatively, if the wurtzite nuclei cannot propagate to the entire particle rapidly, particles capped with wurtzite regions could be observed (Figure 5b2). After careful HRTEM examination of many particles, only the latter situation was found (Figure 4a-d). Therefore, we conclude that surface nucleation contributes to the phase transition of CdS when coarsened in 0-2.0 M Na2S solution and that a kinetic barrier hinders the wurtzite nuclei from propagating to the entire particle rapidly. Mode III, Wurtzite Nucleated Wia Interfacial Interaction of Particles. This type of mode is usually related to either interfacial nucleation31 or the growth pathways of particles.32 It is well-known that crystal growth often occurs via the OR mechanism in solution.34 Recently, studies revealed that nanocrystals could sometimes grow by the OA mechanism, which could produce defects such as edge dislocations, stacking faults, and twins.34,35 As atoms in these defects have much higher chemical potentials, nucleation at these defects becomes possible.30 If the wurtzite nuclei nucleated via interfacial interaction, as shown in Figure 5c, the wurtzite nuclei should form in the interior of particles. Afterward, the crystal growth could produce two kinds of microstructures as revealed in Figure 5c1 and Figure 5c2. In the first case, the wurtzite nuclei could rapidly propagate to the entire particle and then develop into large wurtzite particles. In the second case, if the wurtzite nuclei could not propagate rapidly enough, wurtzite regions would form in the interior of the particles. From our HRTEM images (Figure 4c,d), we found that, when particles were coarsened in 0.5-2.0 M Na2S solution, wurtzite CdS actually existed in the interior of particles. In contrast, in a control experiment (coarsened in 0 M Na2S), such structures were not found. Furthermore, when coarsening was performed in 2.0 M Na2S for 4 h, as shown in Figure 4d, evidence of OA-based growth of CdS was found. Therefore, on the basis of these observations, we suggest that, with increasing Na2S concentration, OA-based growth of CdS was facilitated, and wurtzite could easily exist in the joint part between two nanoparticles. A kinetic barrier prevents the

complete propagation of the wurtzite phase to the whole crystal, so a phenomenon as in Figure 5c2 was observed. In summary, mode I was not found in our systems. During coarsening in aqueous solution, only mode II contributed to the phase transition of CdS. However, during coarsening in Na2S solution, modes II and III occurred together, and as the concentration of Na2S increased, the interfacial nucleation of wurtzite contributed to an increasing extent. Effects of Na2S and OA-Mediated Phase Transition. As discussed above, during the coarsening of CdS nanoparticles in H2O, phase transition occurs by surface nucleation. In contrast, in Na2S solution, another phase-transition pathway is available, and this unique interfacial nucleation mode promotes the rate and ratio of phase transition (as shown in Figure 3). In particular, when coarsening was performed in concentrated Na2S solution (such as 2 M), the phase-transition rate of the CdS nanoparticles was greatly enhanced. Previous investigations of growth kinetics revealed that surface adsorption could influence the mechanism in the initial period of nanocrystal growth.36–39 When capped with easily desorbed ligands (such as in H2O), nanoparticles grow through a combination of the OA and OR mechanisms.40 In contrast, under strong surface adsorption of ligands (such as in concentrated NaOH), the dissolution speed of the nanoparticles is decreased or the collision between nanoparticles is promoted,36 both of which will increase the probability of the OA mechanism.37 In our experiments, we also found that, with increasing concentration of Na2S solution, the number of round nanoparticles growing by the OR mechanism decreased, and the number of particles with many stacking faults or twins growing by the OA mechanism increased. Thus, we propose that the strong interaction of Na2S with CdS nanocrystals41 leads to the growth of CdS via the OA mechanism. Theoretically, when two pure sphalerite particles grow by the OA mechanism, only three possible structures can be generated: defect-free sphalerite particles, twins, and stacking faults.40 It is reported that a stacking fault results in three or

Growth and Phase Transformation of CdS in Na2S Solution

Figure 6. XRD patterns of CdS particles after hydrothermal treatment for 4 h at 150 °C in (a) 2.0 M Na2S, (b) 2.0 M Na2SO4, and (c) H2O.

four layers taking on the 2H structure for sphalerite nanoparticles, and a twins also could result in three layers taking on the 2H structure.8,30,40 These twins or stacking faults from OAbased growth can act as new wurtzite nuclei in the interior of particles and then induce an OA-mediated internal phase transformation as described by mode III. According to the above analysis, we conclude that the strong interfacial adsorption effect of Na2S promotes the unique mode of internal phase transformation. To further confirm the effect of Na2S, we used Na2SO4 solution in place of Na2S solution and observed the coarsening of CdS nanoparticles. As shown in Figure 6, the as-synthesized CdS nanocrystals were hydrothermally treated in (a) 2.0 M Na2S, (b) 2.0 M Na2SO4, and (c) H2O at 150 °C. As a result, we found that, when Na2S was replaced by Na2SO4, the SO42- ions could not induce an obvious phase transformation. The XRD pattern of the sample coarsened in Na2SO4 solution was similar to that of the sample coarsened in H2O. Conclusions The effect of Na2S on the growth and phase transition of CdS nanoparticles was investigated. Our study illustrates that the effect of Na2S could promote the phase transition and particle growth of CdS nanocrystals. Surface-mediated and OA-mediated phase transformations were observed in this system. We suggest that the strong interaction between the Na2S solution and the CdS nanocrystals is crucial to induce the phase-transition mode of interfacial nucleation. Acknowledgment. Financial support for this study was provided by the CAS Foundation (KJCX1.YW.07), Outstanding Youth Fund (40772034), NNSF of China (20501020, 20501021), YIF (2007F3120) of Fujian Province, Fund of Fujian Key Laboratory of Nanomaterials (2006L2005), and the CAS Knowledge Innovation Project (KJCX2.YW.W01). References and Notes (1) Colovin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (2) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800. (3) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (4) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302.

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