Shape Control of PbS Crystals under Microwave Irradiation Zheng-Ping Qiao,* Yong Zhang, Liang-Tian Zhou, and Qucuo Xire MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, P. R. China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2394–2396
ReceiVed December 18, 2006; ReVised Manuscript ReceiVed September 17, 2007
ABSTRACT: The growth of PbS crystal was kinetically controllable by adjusting the concentration of the anion surfactant sodium dodecylbenzenesulfonate (SDBS) and the acidity of the solution under microwave irradiation. SDBS favored stacking in the (111) plane but blocked that along the  zone axis. Adjusting the ratio of SDBS and HNO3 led to the formation of hexapod, concave, and truncated octahedron PbS. In recent years, researchers have focused on the controllable growth of crystals since the shape, or growth direction, of crystals can affect their properties.1 Furthermore, how to confine selective crystal planes is also important in design of one dimensional crystals.2 It is well known that the shape of the crystal is determined by both thermodynamic and kinetic factors. The equilibrium morphology always has the lowest surface energy from the thermodynamic view. But, by changing kinetic factors, the equilibrium morphology can be changed.3 In addition, microwave (MW) irradiation has been recently utilized to synthesize new material, its merits being low energy consumption and high efficiency.4 It is too fast, however, to control the shape of a crystal, especially without a template.5 Therefore, it is of interest to investigate the factors that affect the growth of crystals via MW irradiation.6 PbS was synthesized from reaction between lead(II) acetate trihydrate (Pb(CH3CO2)2 · 3H2O) and thioacetamide CH3–CS–NH2 (TAM). TAM decomposes to H2S, which can release S2- under microwave irradiation.7 The power of microwave (Sumsung, Spectra 900W) is 254 W for 10 min. A typical powder X-ray diffraction (PXRD) pattern of the product (Figure S1, Supporting Information) shows that cubic (Fm3m) PbS (JCPDF card 5592) was obtained. It can be deduced that the growth of the product will be affected by the acidity of the system since the release of S2- will be affected by acidity and the concentration of S2- can instinctively be expected to affect the growth of PbS. Different concentrations of HNO3 aqueous were first used as a reaction media to investigate the parameter of acidity. Pb(CH3CO2)2 · 3H2O (2 mmol) was dissolved in different concentrations of HNO3 aqueous (10 mL), that is, 0.25, 0.5, 1, and 1.5 mol · L-1 for samples 1–4, respectively. TAM (2 mmol) was added after several minutes. The systems were microwaved under 254 W for 10 min. SEM images of samples 1–4 are shown in Figure 1a–d, respectively. The morphology of products 1–4 was cuboid. The crystal size increased with increasing acidity. Based on the Bravais law, (100), (010), and (001) of cubic PbS with lowest lattice area will appear in the final crystal. The equilibrium structure of 1–4 is cuboid, which is coincidental with our results. This indicates that the acidity of solution had no effect on the final shape of the PbS crystal. With the increase of acidity, the release of S2- decreased. As a result, nucleation was decreased. The slow release of S2- allowed enough time for PbS to form the thermodynamically stable structure. The crystal grew larger in keeping with Ostwald ripening. The shape of the crystals undertook no change indicating that acidity of the solution only affected the size. * E-mail: [email protected]
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Figure 1. SEM images of samples 1–4 prepared from Pb(CH3CO2)2 · 3H2O (2 mmol), TAM (2 mmol), and different concentrations of HNO3 aqueous (10 mL): (a) 0.25; (b) 0.5; (c) 1; (d) 1.5 mol · L-1 (bar equals 1 µm).
Capping molecules can tailor the surface energy by capping the surface of nuclei and thus control the shape of the crystals. The anion surfactant sodium dodecylbenzenesulfonate (SDBS) was used as a capping molecule in this work. It was reported that the –OSO3in sodium dodecyl sulfate reacted with Pb2+ to form (C12H25–OSO3)2Pb.8 The –SO3- in SDBS was expected to react with Pb2+ to form (C12H25–C6H5–SO3)2Pb and subsequently affect morphology. At first, only surfactant SDBS (6 × 10-4 mol · L-1) was added into the system, that is, Pb(CH3CO2)2 · 3H2O (2 mmol) and TAM (2 mmol) in 10 mL water, to observe the action of SDBS on the growth of PbS crystal. It was microwaved under 254 W for 10 min. The product was named sample 5. A typical PXRD (Figure S1, Supporting Information) shows that sample 5 was pure PbS. Its SEM and TEM images and a selected electron diffraction pattern are shown in Figure 2. Sample 5 was hexagonal thin flakes with some fragments. The selected diffraction pattern was a  zone axis diffraction pattern and indicates that the hexagonal thin flake crystals developed within two-dimensions of the (111) plane. The (111) surface typically has a higher surface energy than the (100). However, atoms undertook an in-plane (111) stack in this sample. Obviously, quasi-2-dimensional crystals were obtained by the introduction of SDBS. This is reasonable because Na+ in SDBS was substituted by Pb2+. The more atoms in the plane, the more SDBS molecules bind. The atom density of lead in (111) is higher than that in (100). Sequentially the surface energy of the (111) plane
10.1021/cg060923r CCC: $37.00 2007 American Chemical Society Published on Web 11/17/2007
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Figure 2. SEM (a) and TEM (b) images and selected diffraction pattern (c) of sample 5 prepared from system of Pb(CH3CO2)2 · 3H2O (2 mmol) and TAM (2 mmol) in 10 mL aqueous solution in the presence of SDBS (6 × 10-4 mol · L-1).
Figure 4. SEM and TEM images and selected electron diffraction patterns of sample 9 prepared from Pb(CH3CO2)2 · 3H2O (2 mmol), TAM (2 mmol), SDBS (6 × 10-4 mol · L-1), and HNO3 (0.5 mol · L-1). The total volume of solution was 10 mL.
Figure 3. SEM images of samples prepared from a system of Pb(CH3CO2)2 · 3H2O (2 mmol) and TAM (2 mmol) in 10 mL aqueous solution in the presence of HNO3 (1 mol · L-1) and SDBS in different concentrations: (a, b) 6 × 10-4 mol · L-1 (sample 6); (c, d) 1.2 × 10-3 mol · L-1 (sample 7); (e, f) 2.4 × 10-3 mol · L-1 (sample 8). Panels a, c, and e are the large scale SEM images of samples 6, 7, and 8, respectively.
the  zone axis, and the (111) stack followed up normally without capping reagent. After addition of SDBS (sample 6), the stack along the  zone axis was stereoblocked, but the in-plane (111) stack was developed. In sample 7, the stack along the  zone axis was stereoblocked, and the in-plane stack was further developed while the concentration of SDBS was increased. As a result of competition between the stack along the  zone axis and the stack in the (111) plane, the shape of crystal changed from concave octahedron to truncated octahedron. The growth was affected by SDBS and HNO3 jointly. The final morphologies were the result of competition. For sample 8, the concentrations of SDBS and HNO3 were high. The ultimate shape of crystal was a mixture, which indicated no collaboration between SDBS and HNO3. At the end, PbS with pure hexapod morphology was successfully designed. The strategy was a completely stereoblocked the stack along the  zone axis and a fully developed the stack along the  zone axis. Based on the results above, lower acidity with a relatively higher concentration of SDBS should be added into system. SDBS at 6 × 10-4 mol · L-1 and HNO3 at 0.5 mol · L-1 were selected. The product was named sample 9. The morphology shown from SEM image (Figure 4) was hexapod. The TEM image and selected electron diffraction patterns are shown in the Figure 4 inset. The electron diffraction patterns show the  zone axis diffraction. Each pod was developed along the  zone axis from topology analysis. The process is shown in Figure S3, Supporting Information. All of selected diffractions in the center and pods of the crystal are similar. Every  zone axis diffraction pattern is matched well to the growth direction of their corresponding pods. This indicated that the hexapod crystal is single crystal. In summary, PbS samples with controllable morphologies were prepared under MW irradiation. HNO3 favored formation of cuboid PbS; the size of crystal increased with the increase of acidity. SDBS favored stacking in the (111) plane but blocked the stack along the  zone axis. Cooperation of SDBS and HNO3 led to hexapod and concave and truncated octahedron PbS by adjustment of their ratio.
was selectively lowered; the stack of (111) plane was quick. But stack along the  zone axis was stereoblocked simultaneously. Atoms stacked within the plane. For further research, similar experiments were done in the presence of SDBS and HNO3 to investigate their cooperation. We kept the acidity of system at 1 mol · L-1 HNO3 and changed the concentration of SDBS to 6 × 10-4, 1.2 × 10-3, and 2.4 × 10-3 mol · L-1 for samples 6, 7, and 8, respectively. Their morphologies, as shown in Figure 3a–f, were concave octahedron, truncated octahedron, and a mixture of hexapod and truncated cube, respectively. Compared with samples 1–4 (0 mol · L-1 SDBS), in samples 6 (6 × 10-4 mol · L-1 SDBS) and 7 (2.4 × 10-3 mol · L-1 SDBS), with the increase of SDBS, the in-plane (111) stack of atoms went quickly. In samples 1–4, the crystal developed along
Acknowledgment. We thank NSFC (Grant No. 20401016) and the Science & Technology Bureau of Guangdong Province (Grant No. 04009720). Supporting Information Available: Details of the experimental methods, typical PXRD of product, additional discussion of samples 1–4, a large scale SEM image of sample 5, and selected electron diffraction pattern analyses. This material is available free of charge via the Internet at http://pubs.acs.org.
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