Triangular PbS Nano-Pyramids, Square Nanoplates, and Nanorods

Jun 26, 2008 - Synopsis. Triangular nanopyramids, square nanoplates, and nanorods of PbS were formed at the air−water interface under Langmuir monol...
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Triangular PbS Nano-Pyramids, Square Nanoplates, and Nanorods Formed at the Air/Water Interface Chang-Wei Wang,† Hong-Guo Liu,*,† Xiang-Tao Bai,† Qingbin Xue,† Xiao Chen,† Yong-Ill Lee,‡ Jingcheng Hao,† and Jianzhuang Jiang†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2660–2664

Key Laboratory for Colloid and Interface Chemistry of Education Ministry, Shandong UniVersity, Jinan 250100, China, and Department of Chemistry, Changwon National UniVersity, Changwon 641-773, Korea ReceiVed April 26, 2007; ReVised Manuscript ReceiVed February 8, 2008

ABSTRACT: PbS nanocrystals with different shapes were synthesized at the air/PbCl2 aqueous solution interface via reaction between Pb2+ and H2S gas under poly(9-vinylcarbazole) (PVK) thin films and characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), electron diffractometry (ED), scanning electron microscopy (SEM), and UV-vis spectroscopy. It was found that the shapes of the nanocrystals depend on the experimental conditions, such as temperature, subphase concentration, and H2S amount. Triangular nanopyramids and nanoplates/nanorods were formed at ca. 20 and 30 °C, respectively. The (111) plane of the nanopyramid is parallel to the water surface, and the other three sides are (100), (010), and (001) faces, respectively. For the rectangular nanoplates and nanorods, the (001) planes are parallel to the water surface. The formation of the nanopyramids should be related to lattice matching between the two-dimensional arrays of nitrogen atoms in PVK Langmuir monolayers at 20 °C with the (111) crystal face of PbS, while the formation of nanoplates and nanorods at 30 °C would be ascribed to the variation of the Langmuir monolayer structure. In addition, the shapes of nanoplates/nanorods changed with the subphase concentrations and H2S volumes.

1. Introduction PbS is an important direct band gap semiconductor material with a rather small bulk band gap of 0.41 eV at 300 K and a relatively large exciton Bohr radius of 18 nm.1 PbS nanostructures has aroused much attention recently due to their unique properties arising from the quantum confinement effects. For example, the band gap can be widen to a few electronvolts from the bulk value;2,3 the absorption threshold and the luminescent characteristics cover both visible and near-infrared regions;4,5 and the nanoparticles show exceptional third-order nonlinear optical property.2 As a consequence, they have a technological potential in the fields of optical switches,2 solar cells,6 light emitting diodes,7 Pb2+ ion-selective sensors,8 and IR detectors,9 etc. In order to study the size-dependent properties, a great deal of work has been devoted to the synthesis of spherical and polyhedral PbS nanocrystals with tunable size in diverse methods, including wet chemical reactions in solutions,10 microemulsions11 and vesicles,12 and solid/gas reactions between thin solid films containing Pb2+ ions, such as Langmuir-Blodgett multilayers,13 self-assembled monolayers,14 layer-by-layer films,15 and casting films16 with H2S gas. On the other hand, the properties of semiconductor nanoparticles depend not only on their size, but also on their shapes. Recently, PbS nanoparticles with different shapes, such as nanocubes,17 hollow nanospheres,18 star-shaped nanocrystals,19 nanowires,20 nanorods,21 and nanotubes,22 were synthesized in solutions in the presence of surfactants or with the help of hard templates. It can be seen that most previous efforts on PbS shape anisotropy have been concentrated on one-dimensional (1D) and star-shaped nanostructures. Herein, we focus on the controlled preparation of PbS triangular nanopyramids and square nano* To whom correspondence may be addressed. E-mail: [email protected]. Tel: +86-531-88362805. Fax: +86-531-88564750. † Shandong University. ‡ Changwon National University.

plates/nanorods by using a Langmuir monolayer technique. Only a few reports on semiconductor nanopyramids have been published very recently.23 The Langmuir monolayer technique has been applied to synthesize inorganic nanoparticles extensively since the early 1990s. For example, triangular PbS nanoparticles were prepared at the air/water interface via reactions between Pb2+ ions in the subphase and H2S in the gaseous phase templated by condensed arachidic acid Langmuir monolayers,24 or via a modified procedure by injecting Na2S aqueous solution to the subphase.25 In addition, oriented triangular PbS nanoparticle 1D arrays were fabricated by a onestep process at the air/water interface templated by a linear polymer monolayer.26 In this paper, supramolecular structures formed by a conjugated polymer PVK at the air/water interface were used as templates to synthesize PbS nanoparticles. It was found that not only triangular nanopyramids but also square nanoplates and nanorods were formed at the air/water interface under different conditions.

2. Experimental Procedures 2.1. Chemicals. Poly(9-vinylcarbazole) (PVK, Mw: 81800, Mn: 48800) was purchased from Aldrich and used as received. PbCl2 (>99.0%) was purchased from Shanghai Chemical Plant. 2.2. Formation of PbS Nanoparticles. PbCl2 aqueous solution with concentrations of 1 × 10-3, 1 × 10-4 and 1 × 10-5 mol · L-1 were used as subphases, and PVK chloroform solution with the concentration of 0.046 mg · mL-1 was used as spreading solution. A beaker with the inner diameter of 5.7 cm containing 20 mL of the subphase solution was placed in a container, and then 65 µL of the PVK chloroform solution was spread onto the subphase surface by using a microsyringe. After evaporation of chloroform for 15 min, H2S gas was produced by the reaction of H2SO4 with Na2S, and then the container was sealed. The volume of the produced H2S gas was controlled to be 0.02, 0.05, 0.1, 0.2, 0.35, 0.5, and 1.0 mL, respectively, by controlling the amount of Na2S. After 1 h, the products formed at the air/water interface were transferred onto Formvar-covered or carbon-coated 230-mesh copper grids and hydrophobic quartz slides by the Langmuir-Scha¨fer method

10.1021/cg070398b CCC: $40.75  2008 American Chemical Society Published on Web 06/26/2008

PbS Nanocrystals

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Figure 2. HRTEM images of the triangular nanopyramid.

Figure 1. TEM (a, b, d) and SEM (c) micrographs of PbS triangular nanopyramids formed at the air/water interface at 20 °C. The concentration of PbCl2 aqueous solution is 1 × 10-4 mol · L-1, and the volumes of H2S gas are 0.02, 0.10, 0.1, and 1.0 mL for the images of a-d, respectively. for further characterization. The experiments were carried out at around 20 and 30 °C, respectively. 2.3. Characterization. The samples were characterized by transmission electron microscopy (TEM, JEM-100CXII), selected-area electron diffractometry (SAED), high-resolution transmission electron microscopy (HRTEM, GEOL-2010), scanning electron microscopy (SEM, JEOL JSM-7600), and UV-vis spectroscopy (U4100, Hitachi), respectively.

3. Results and Discussion 3.1. PbS Nanopyramids. When the reactions were carried out at around 20 °C with the subphase concentration of 1 × 10-4 mol · L-1, PbS triangular nanoparticles were produced, as revealed by TEM and SEM micrographs shown in Figure 1. Most nanoparticles are equilateral triangular ones. The side length first increases and then deceases with increasing H2S amounts. The average side lengths of the nanoparticles are 47, 66, and 47 nm when 0.02, 0.1, and 1.0 mL of H2S were used, respectively. The triangular nanoparticles look like nanopyramids under TEM, because the contrast of the nanoparticle image is different for different parts. A typical pyramid can be seen from Figure 1b, as labeled by a square. The nanopyramid structure was further confirmed by SEM observation (Figure 1c). The corresponding SAED pattern shows clear diffraction spots, indicating the crystalline of the nanoparticles. It should be noted that six nearly symmetrical elongated spots appear in the ring of (220), indicating the preferentially oriented nanoparticles at the interfaces with [111] zone axis nearly parallel to the incident electron beam. Figure 2 gives the HRTEM image of the nanopyramids. Hexagonal close packed lattice structure appears. The fringes are separated by 0.21 nm, corresponding to the interplanar distance between {220} facets. It is well-known that the [111] HRTEM image of the perfect fcc crystal is built by 3 sets of the {220} spacing with a 6-fold symmertry.27 The result suggests the formation of a perfect fcc PbS crystal with the (111) plane perpendicular to the incident electron beam. In addition to the equilateral triangular nanopyramids whose (111) plane is parallel to the substrate surface, there are a few isosceles right-angle triangles whose (100) plane is parallel to the substrate surface, as shown in Figure 3. This indicates that the nanopyramids are tetrahedrons surrounded by (111), (100),

Figure 3. TEM images of the equilateral triangular and right-angled triangular nanopyramids.

(010), and (001) faces. It is believed that these triangles are formed at the air/water interface with their (111) plane parallel to the interface and turned over when transferred onto the solid substrate. The formation of the triangular nanoparticles should be attributed to the template effect of PVK thin films due to the interaction between the nitrogen atoms in PVK chains and Pb atoms in the nanoparticles. If the two-dimensional (2D) arrays consisting of nitrogen atoms can match the lattice of Pb atoms in the (111) face, the PbS nanocrystals would grow with the (111) face parallel to the air/water interface, leading to the formation of triangular nanoparticles. It was reported that annealed PVK have two kinds of crystals, that is, isotactic 3/1 helix and syndiotactic 2/1 helix.28,29 Molecular models show that these two configurations result in 7.4 and 5.2 Å chain periodicity, respectively, and the planar pendant groups lie close to and parallel to one another, resulting in π-π attraction between the pendant groups. Fendler,24 Yao,25 and Jiang14b illuminated the formation of triangular PbS nanoparticles under an arachidic acid Langmuir monolayer and on a mercaptoundecanoic acid self-assembled monolayer by lattice matching between the Pb atom arrays in the (111) face and the 2D arrays of carboxylic acid groups in Langmuir and self-assembled monolayers, respectively. The analysis showed that the carboxylic groups formed perfect 2D hexagonal arrays with a distance between the adjacent groups of 4.90 and 5.00 Å, respectively. Further analysis indicated that this kind of 2D array matches the Pb atom lattice in the (111) face perfectly. These analyses indicate that 2D hexagonal arrays with the parameter of around 5 Å can induce the formation of the PbS (111) face. Herein, if the polymer takes the 2/1 syndiotactic configuration, the PVK molecules may adopt two kinds of orientations, as shown in Scheme 1a,c, resulting in two kinds of 2D arrays, as shown in Scheme 1b,d. The distance between the adjacent carbazole groups in the same side of the chain would be ca. 5 Å. Electron diffraction experiment indicates that the thin film of PVK formed at the air/water interface is amorphous, so it is possible to tune the distance between the adjacent chains to form

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Scheme 1. (a-d) 2D Arrays of Carbazyl Groups Formed at the air/water Interface

Figure 4. UV-vis spectra of the formed PbS nanopyramid films under different conditions and the pure PVK film.

hexagonal arrays with the parameter of ca. 5 Å, which can act as a template to induce the formation of the (111) plane of the triangular nanopyramids. It was said that the earliest report on the formation of PbS nanoparticles appeared in 1948 by passing H2S gas over an aqueous solution of lead acetate.26,30 The (001) planes of the formed PbS nanoparticles are parallel to the interface. We have repeated this experiment and found the formation of larger cuboids and cubes. This means that PbS tends to form hexahedrons around {100} facets in the aqueous solutions without any surfactant. In the present case, there are just Pb2+ and Cl- ions in the subphase. After the (111) face was produced at the interface, the nanoparticles grew under the thin films to form another three (100), (010), and (001) faces, resulting in the formation of triangular pyramids. Of course, in the presence of surfactants, octahedral nanoparticles around eight {111} facets can be synthesized, such as fcc PbSe nanoparticles.31 The size of the nanopyramids changes with the H2S volumes. This should be related to the nucleation and growth process of the particles. It can be calculated based on the amounts of H2S gas and Pb2+ ions used that the molar ratios of S2-/Pb2+ increase from 0.45 to 22.3 by increasing the volumes of H2S from 0.02 to 1.0 mL. With the smallest molar ratio, fewer nuclei formed, and the amount of PbS used for growth is less, too, resulting in smaller particles. With increasing the molar ratios, not much more nuclei are formed, and the amounts of PbS species for growth are efficient, leading to formation of larger particles. However, when the molar ratio became larger than 10, a great deal of nuclei formed at the initial stage of the reaction, the amount used for growth became less, leading to formation of smaller particles. The reaction time of 1 h is the same in all the experiments. Figure 4 shows the UV-vis spectra of the samples. All the curves show similar characteristics. Strong excitonic absorption band and a shoulder appear at ca. 340-350 and 610 nm, respectively. Furthermore, slight differences exist between the curves. For example, the absorption onset locates at ca. 1600, 2400, and 1600 nm for the samples prepared with the H2S volumes of 0.02, 0.1, and 1 mL, respectively. Because the size of the nanopyramids is larger than the exciton Bohr radius of 18 nm, a quantum confinement effect is not expected. However, the absorption onset and the excitonic

absorption band shown in Figure 4 shifted to the blue region compared with the onset of bulk PbS of 3020 nm, indicating the existence of the quantum confinement effect. This should be attributed to the smaller tips and edges of the nanopyramids.19e It was reported that the optical properties of nonspherical nanocrystals are controlled by the lowest dimension of the particles,32 and the nanocrystals show a positiondependent quantum size effect.33 There are several reports on the UV-vis spectra of PbS nanoparticles. Although isotropic spheres of PbS with a size of less than 18 nm showed regular red-shifted excitonic absorption peaks from the visible to infrared region with increasing particle size,10c,f,34 most spectra show featureless profiles without excitonic peaks. The disappearance of the excitonic absorption peak was attributed to the existence of surface defect sites that can trap the electron-hole pair generated by light. The strong interaction between the exciton and the trapped electron-hole pair would bleach the exciton absorption.10g,21a In our experiment we observed strong excitonic absorption bands at 330-340 nm and shoulders at ca. 600 nm, indicating that there is no or less surface defects in the nanopyramids. It was reported that the shoulder at 600 nm is a peak actually, but is hidden under the tail of the intense band at 300 nm.35 In addition, it is wellknown that the absorption onset shifts to the red region with increasing particle size.10g,h,16a,21a So the change trend of the absorption onset of the nanopyramids reflects the size change of the thin tips and edges of the nanoparticles with different sizes. 3.2. PbS Nanosquares and Nanorods. When the reactions were carried out at 30 °C, square and rectangular nanoplates and nanorods instead of nanopyramids appeared under various conditions. The morphologies are shown in Figure 5. When the subphase with a higher concentration of 1 × 10-3 mol · L-1 was used, nanocubes were formed. Clear diffraction rings appear in the corresponding ED pattern, which can be indexed to (111), (200), (220), (311), (400), (420), etc. of fcc PbS. It should be noted that the rings of (111) and (311) are much weaker than those of (200), (220), (400), and (420). When the subphase with the concentration of 1 × 10-4 mol · L-1 was used, square nanoplates were formed. The corresponding ED pattern gives symmetrical elongated square diffraction spots that can be indexed to (200) and (220), respectively, indicating that the formed nanoparticles arrange at the interface with a low degree of orientation. When the subphase concentration decreased further to 1 × 10-5 mol · L-1, nanoparticles with different shapes were produced, which depends on the amount of H2S. When a smaller amount of H2S (0.05 mL) was used, nanocuboids and cubes were produced; when the H2S volume increased to

PbS Nanocrystals

Figure 5. TEM micrographs of PbS nanoplates and nanorods formed at the air/water interface at 30 °C with the subphase concentrations of 1 × 10-3 (a, b), 1 × 10-4 (c, d), and 1 × 10-5 mol · L-1 (e-g) with different H2S volumes of 0.05 (a, c, e), 0.10 (b, d, f), and 0.50 mL (g).

Figure 6. SEM micrographs of the formed PbS nanoplates and nanorods.

0.10-0.35 mL, nanorods with a width of ca. 10-20 nm and length of ca. 200 nm were produced, whereas when the H2S volume increased further to 0.5 mL, thin square nanoplates appeared. The side length of the largest nanoplate is more than 100 nm; the corresponding ED pattern gives clear diffraction spots of (200) and (220) faces, indicating the single crystalline nature of the formed nanostructures. Compared with the square plates formed with a subphase concentration of 1 × 10-4 mol · L-1, the square plates shown in Figure 5g seem thinner, as indicated by the star-like fringes on the plate surface, which may arise from the bending of the thin plate.36 The position of the electron diffraction spots also confirms the bending of the plate. The nanostructures were further characterized by SEM, as shown in Figure 6. It can be seen that the thickness of the square plates with a side length of ca. 80 nm is 22 nm, confirming the formation of plates. It can be also found that there are grooves along the nanorods. Figure 7 shows the HRTEM images of the nanoplates and nanorods. Cross lattice pattern appears in the micrographs. The interplanar distance was measured to be 0.30 nm, which coincides with that of (200) planes of fcc PbS, indicating the formation of a single crystalline nanosquare and nanorod with the (001) faces parallel to the air/water interface. Furthermore,

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Figure 7. HRTEM micrographs of the formed PbS nanoplate and nanorod.

we can see clearly from the image of the nanorod that a groove with the width of several nanometers exists in the (001) face. As discussed above, PVK may take syndiotactic configuration to form an ordered hexagonal array of N-carbazole groups with a distance between the nitrogen atoms of ca. 5 Å at 20 °C. With an increase in the temperature, the amorphous thin film would expand, leading to an enlargement of the distance between the nitrogen atoms on neighbor chains. So the hexagonal array of the N-carbazole groups would be destroyed and the triangular nanoparticles would not be produced. It is possible that the PVK structure just acts as a planar adsorber to induce the formation of the square nanoparticles. The subphase concentration has a great influence on the morphologies of the nanoparticles. As can be seen from Figure 5a,c,e, when the same H2S volume of 0.05 mL was used, the formed PbS nanoparticles show distinct morphologies on the surfaces of the PbCl2 aqueous solutions with different concentrations. With the highest concentration of 1 × 10-3 mol · L-1, the reaction proceeds rapidly at the interface due to the higher Pb2+ concentration in the interfacial phase, leading to rapid growth of the nanoparticles. It is well-known that when the species are generated at a sufficiently high rate, the final product will have no choice but to take the thermodynamically favored shapes. So PbS nanocubes were formed. When the concentration is decreased to 1 × 10-4 mol · L-1, the growth rate falls off due to the decrease of Pb2+ concentration in the interfacial phase, and the PbS species used for growth would be produced mainly at the interface, resulting in the formation of anisotropic nanosquares. When the lowest concentration of 1 × 10-5 mol · L-1 was used, fewer nuclei were formed at the early stage of the reaction, and most of the PbS species used for growth would be generated in the subphase due to the lower surface concentration of Pb2+ and relative higher dispersion rate of H2S due to the high molar ratio of S2-/Pb2+ of 11. The particles grow to the subphase, resulting in the formation of the nanocubes and nanocuboids. In addition, the volume of H2S gas greatly affects the PbS morphologies. When the Pb2+ concentration of 1 × 10-5

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mol · L-1 was used, cubes and cuboids, nanorods, and square thin nanoplates were produced in turn with increasing H2S amounts. When 0.05 mL of H2S was introduced, as discussed above, the nuclei grow to form regular cubes and cuboids. By increasing the volume of H2S gas, although more H2S gas would penetrate into the subphase, more and more nuclei formed at the interface at the early stage of the reaction. It is possible that the growth and attachment of the nuclei result in the formation of the nanorods and nanoplates.

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4. Conclusion PbS nanoparticles with different shapes of triangular pyramid, square plate, cube, cuboid and rod were produced under thin films of PVK at the air/water interface via reactions under different experimental conditions. The formation of these nanostructures can be related to templating effect, nucleation, and growth process. The shape and the size of the nanoparticles can be tuned by varying the experimental parameters. Acknowledgment. The authors acknowledge the financial support of Education Ministry of China, and NSFC (20304006 and 20428101).

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