Organometallic-Route Synthesis, Controllable Growth, Mechanism

Feb 21, 2014 - ABSTRACT: Lead selenide (PbSe) nanostructures with well-defined star-shaped morphology are successfully fabricated via a...
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Organometallic-Route Synthesis, Controllable Growth, Mechanism Investigation, and Surface Feature of PbSe Nanostructures with Tunable Shapes Genrong Shao,†,‡ Guihuan Chen,†,‡ Weilai Yang,† Tao Ding,†,‡ Jian Zuo,† and Qing Yang*,†,‡,§ †

Hefei National Laboratory of Physical Sciences at the Microscale, ‡Department of Chemistry, and §Laboratory of Nanomaterials for Energy Conversion, University of Science and Technology of China (USTC), Hefei 230026, Anhui, People’s Republic of China S Supporting Information *

ABSTRACT: Lead selenide (PbSe) nanostructures with well-defined star-shaped morphology are successfully fabricated via a facile organometallic synthetic route from the reaction of tetraphenyl lead (Ph4Pb) with triphenylphosphine selenide (Ph3PSe) in dibenzylamine (DBA) with the assistance of oleic acid (OA) and oleylamine (OAm) at 220 °C for 30 min. The structure and shape of the nanocrystals are investigated by techniques of XRD, SEM, TEM, HRTEM, SAED, and EDX, and it is interesting that the obtained PbSe nanostars present Pb-rich features, although the PbSe nanostars are still in typical rock salt phase. Experimental investigations and ATR-FTIR studies demonstrate that the media of DBA, OA, and OAm with an order OA > DAB > OAm play important roles in the growth of the PbSe nanostars with well-defined shapes because the media not only serve as solvents but capping materials. The synergetic effects of the media are also favorable for the growth of PbSe nanocrystals with the well-defined star-shaped morphologies in the current reaction system. Meanwhile, varied PbSe nanostructures with cubic, sidecut cubic, and octahedral shapes can be fabricated by regulating the relevant reaction conditions, and all of these nanostructures prepared in the procedures demonstrate Pb-rich features due to the selective capping effects of the media to the exposed Pb(II) ions. It is confirmed that the specific shape and geometry of the nanostructures can be tuned by controlling the exposed crystal surfaces and/or the corresponding compositions via the variation of reaction conditions in the media.

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INTRODUCTION Lead selenide (PbSe) has attracted great attention in the fields of near-infrared lasers1 due to its narrow band gap (0.28 eV). PbSe has been also proven to be an extremely promising material for energy conversions.2−5 Research in PbSe nanocrystals continues to pursue the development of novelty applications and access to fundamental principles, which will undoubtedly provide the intriguing possibility to control the synthesis of PbSe nanostructures with desired shapes and thus regulate the unique shape-dependent properties. Up to now, the syntheses in addition to growth mechanism of nanostructures including PbSe have been intensively investigated.1,6−10 Many geometrical PbSe nanostructures, such as cubes,11−13 zigzag,3,14 one-dimensional (1D) nanocrystals via the growth regime of oriented attachment,3,15−17 and star-shaped, 18−20 have been fabricated by varied techniques. It is noted that most of them are synthesized in © 2014 American Chemical Society

solutions, especially in the media of trioctylphosphine (TOP) 3,21,22 in addition to trioctylphosphine oxide (TOPO)23 and/or oleic acid (OA)17,22,24 because these media play important roles in the formation of the nanostructures with different morphologies due to their selective absorption onto the crystal facets. For example, Murray and co-workers reported the synthesis of PbSe nanocrystals from the reaction of Pb-oleate and TOP-Se in diphenylether (Ph2O) with TOP via a rapid injection process, and obtained the nanocrystals with the size tuning from 3.5 to 15 nm in diameter.25 Meanwhile, in addition to nanowires with undulated, zigzag, straight, and star-shaped branched shapes, the PbSe nanocrystals have been further tuned with quasiReceived: November 5, 2013 Revised: January 6, 2014 Published: February 21, 2014 2863

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nanocrystals were also achieved in the media (6 mL) of DBA (5.75 mL) and OA (0.25 mL) without employment of OAm. Synthesis of Side-Cut Cube, Cubic, Eight-Armed, Smooth Surface, and Rough Surface Octahedral PbSe Nanocrystals. PbSe nanostructures with side-cut cube, cubic, eight-armed, smooth surface, and rough surface octahedral shapes were also achieved via procedures similar to those of the nanostars except for the variation of reaction conditions. For the synthesis of side-cut cubic PbSe nanocrystals, 0.25 mmol of Ph3PSe and 0.25 mmol of Ph4Pb were added into 100 mL three-necked flask containing 6 mL of OAm and magnetically stirred under a flow of argon. After being stirred for 30 min at 150 °C, the stock solution was heated to 220 °C at the rate of 10 °C/min and reacted at this temperature for another 30 min. The mixture then was cooled to room temperature naturally. The preparation of cubic PbSe nanocrystals was performed in 5 mL of DBA with addition of 1 mL of OAm. Eight-armed and smooth surface octahedral PbSe nanocrystals were prepared in pure DBA and OA, respectively, whereas the rough surface octahedral PbSe nanocrystals were produced in a media of 5.75 mL of OAm and 0.25 mL of OA. All of the experiments performed in the current work were carried out under argon atmosphere. The as-synthesized PbSe nanocrystals were precipitated from the reaction mixture with 5 mL of toluene and separated via centrifugation (9000 rpm, 4 min, and three times) and then dried at 60 °C in air for further characterizations. Characterization. The samples were characterized by different analytic techniques. The morphologies of the products were observed by scanning electron microscopy (SEM, JSM-6700F) and transmission electron microscopy (TEM, Hitachi H-7650). The microstructures of the as-prepared samples were determined by X-ray powder diffraction (XRD, on a Philips X’pert PRO X-ray diffractometer, Cu Kα, λ = 1.54182 Å), high-resolution TEM (HRTEM, a JEOL-2010 transmission electron microscope), and electron diffraction (ED). Meanwhile, the compositions of the samples were investigated by energy dispersive X-ray spectroscopy (EDX, OXFORD INCA system). The surface structures of the samples were determined by attenuated total reflection Fourier transformed infrared (ATR-FTIR) spectroscopy (Prestige-21, SHIMADZU).

spherical, cubic, octahedral, and star-shaped morphologies through a careful adjustment of media including squalane, Ph2O, and/or octyl ether with stabilizing agents of OA, TOP, and/or hexadecylamine (HDA), as well as variation of mole ratio of feeding stock, reaction temperatures, and times. There is no doubt that the reaction media play a considerably important role in the growth of the PbSe nanocrystals with specific geometric shapes.3 Subsequently, following the main procedures of Murray’s group,3 Houtepen et al. also find that the media can influence the growth of the nanocrystals with different shapes tuning from quasi-spheres to octahedral and even to star-shape by slightly mediating the reaction conditions, for example, using trace acetic acid.24 Such controlling of media strategies to nanostructures has been also applied intensively in varied groups.3,26−29 However, the nature behind the crystal growth with specific morphologies has not been approached sufficiently so far. Further studies are needed to address some fundamental questions. More specifically, what are the state and structure of the media, and how do media and media absorption modulate the crystal growth? In this research, we have successfully developed an alternative solution approach, a facile organometallic synthetic route, for the growth of well-defined star-shaped PbSe nanocrystals. In this route, the reaction of tetraphenyl lead (Ph4Pb) and triphenylphosphine selenide (Ph3PSe) was done using a new organo-amine of dibenzylamine (DBA) as reaction media with the addition of oleic acid (OA) and oleylamine (OAm) at 220 °C. It is evident that the obtained PbSe nanostars exhibit Pb-rich features, even though the nanostars are still in typical rock salt phase. To reveal mechanisms, we examined the effects of the media and the growth process of the nanostructures using attenuated total reflection Fourier transformed infrared (ATR-FTIR) spectroscopy. It is found that the media of DAB, OA, and OAm not only serve as solvents but capping materials. The synergetic effects of the three media of DAB, OA, and OAm favor the growth of PbSe nanocrystals with well-defined star-shaped morphologies in the current reaction system. Also, varied PbSe nanostructures with cubic, side-cut cubic, and octahedral shapes can be tuned in the procedures by changing the reaction conditions, especially the variation of the media. Results from this study may provide an insight into the growth mechanism and aid in achieving remarkable different morphologies through selectively using the solvents and capping materials.

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RESULTS AND DISCUSSION The PbSe nanocrystals were prepared by reaction of Ph4Pb and Ph3PSe at 220 °C for 30 min in DBA (5.2 mL) with addition of OA (0.4 mL) and OAm (0.4 mL). The X-ray diffraction (XRD) pattern (Figure S1, Supporting Information) shows that the asprepared samples are in rock salt structure (JCPDS card, no. 78-1902). The relative intensity of {111} over {100} (or {200}) crystal facets is 0.34, which is somewhat larger than the ratio of 0.32 between {111} over {100} (or {200}) in the standard JCPDS card, demonstrating the nanocrystals perhaps having preferred crystallographic orientation. In the pattern, there are no impurities observed, suggesting the formation of pure PbSe nanocrystals with high degree of crystallinity from the present organometallic synthetic route facilely. Figure 1A shows a low-magnification SEM image for the asprepared products, and it is found that they present uniform star-shaped PbSe nanocrystals with 150−200 nm diameter. A corresponding enlarged SEM image (Figure 1B) suggests that they exhibit well-defined star-shaped geometry with six symmetrical arms grown along ⟨100⟩ directions. These sixarm stars look like concave octahedrons in that they have an octahedral profile in a geometrical view except that the surfaces and edges of the octahedron are concaved selectively. The arms are determined as square pyramids in profile of which four side surfaces of the arms converge to the top point symmetrically. At the same time, the four side surfaces look like concaved notches along the surface bisector through the top point (tip). The TEM image (Figure 1C) clearly shows a two-dimensional

EXPERIMENTAL SECTION

Marerials. Dibenzylamine (DBA, analytical grade, 99%) and tetraphenyl lead (Ph 4 Pb) were purchased from Alfa Aesar. Triphenylphosphine selenide (Ph3PSe) was purchased from TCI. Oleylamine (OAm) was purchased from Aldrich, and oleic acid (OA) was purchased from Shanghai Chemical Reagents Co. China. All of the chemicals were used as obtained without further purification. Synthesis of Typical Star-Shaped PbSe Nanocrystals. In a typical procedure, 0.25 mmol of Ph3PSe and 0.25 mmol of Ph4Pb were added into 100 mL three-necked flask containing 5.2 mL of DBA, 0.4 mL of OA, and 0.4 mL of OAm, and magnetically stirred under a flow of argon. The mixture was heated to 150 °C at the rate of 10 °C/min and kept at this temperature for 30 min to remove the moisture and oxygen. It was then heated to 220 °C at the rate of 10 °C/min and kept at this temperature for 30 min. The resulting black mixture was cooled to room temperature naturally, and 5 mL of toluene was added into reaction system. The precipitate was separated via centrifugation (9000 rpm) and washed with toluene three times, and then dried at 60 °C in air for further characterizations. The star-shaped PbSe 2864

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they still possess the architecture with six arms from different directions. Differing from the ⟨111⟩ projection, Figure 2B and C presents varied geometric configurations for the star-shaped PbSe nanocrystals projected along ⟨110⟩ and ⟨100⟩ directions when they are obtained by two and one of their six arms standing on the grid, respectively. Figure 2D−F presents the corresponding ED patterns for the nanostructures demonstrated in Figure 2A−C, in which the diffraction spots can be accordingly indexed as the ones from [111] to [110] and to [100] zone axes, respectively. Figure 2G−I gives their corresponding schematic atom stacking models, which show the views of cubic rock salt phase from different observation directions. As compared to ⟨110⟩ and ⟨100⟩ projections, ⟨111⟩ projection has three arms standing on the substrate and possesses the lowest center of gravity. Therefore, it is the most stable state of these star-shaped architectures and can be seen frequently (Figures 1C, 2A). In the current organometallic route, PbSe can be synthesized from the reaction between Ph4Pb and Ph3PSe in a wide range of reaction parameters based on the experimental investigations. The formation of PbSe commonly resulted from direct action of elemental selenium released from organometallic precursor of Ph3PSe with Ph4Pb in the elevated reaction temperatures above 200 °C, although the decomposition of individual precursor (Ph4Pb with melting point 225 °C (ref 30) or Ph3PSe with melting point 188−189 °C) involved in the same procedures is not obviously observed. On the basis of organometallic and coordination chemistry, Ph3PSe may be easily prepared via a treatment of PPh3 with red (α-monoclinic) selenium,31 and it is expected that elemental selenium first releases from Ph3PSe during the initial step of the reaction due to the decomposition of Ph3PSe at the relative high temperatures. Second, the released elemental selenium acts with Ph4Pb (Pb) to form PbSe thermodynamically because the Gibbs free energy of formation of PbSe has a large negative number (ΔfG° = −101.7 kJ mol−1).32 To reveal the growth mechanism of the nanostars and shape evolution of the nanostructures, many additional experiments have been performed by varying the reaction conditions, and the detailed SEM images of the obtained samples are shown in Figure 3. Figure 3A demonstrates the nanostructured samples obtained in pure DBA (6.0 mL) without addition of OA and

Figure 1. (A) Low-, (B) high-magnified SEM, (C) TEM, and (D) high-resolution TEM images of the star-shaped PbSe nanocrystals obtained from reaction of Ph4Pb with Ph3PSe at 220 °C for 30 min in DBA (5.2 mL) with assistance of OAm (0.4 mL) and OA (0.4 mL). Inset in (D) shows the electron diffraction (ED) pattern obtained from an arm of a nanostar.

(2D) projection view along their [111] zone axis, from which their six branches appear to have slightly sharp tips, consistent with the SEM observations. The lattice d-spacing is calculated to be 2.16 Å in the HRTEM image from a single arm of a typical star as shown in Figure 1D, which is well consistent with the {220} lattice planes (d = 2.159 Å). The inset in Figure 1D shows their corresponding electron diffraction (ED) pattern, which can be indexed as ⟨111⟩ zone axis of cubic PbSe nanostructure, further confirming its single-crystal nature. EDX measurements permit one to determine the composition of the nanostructures (Supporting Information Figure S2), which are composed of Pb and Se at the ratio of 55:45 (1.222). The structure and geometry of the star-shaped PbSe nanocrystals were further checked and determined by TEM and ED. Figure 2A shows a typical TEM image for the starshaped PbSe nanostructure of which three arms stand on the copper TEM grid during the TEM imaging, and the nanostructure demonstrates a typical planar hexagonal star shape due to the projection along ⟨111⟩ direction. Viewing along different projected directions, some nanocrystals do not appear as the planar hexagonal star-shaped morphology, but

Figure 3. SEM images for the nanostructures obtained at 220 °C in (A) pure DBA (6.0 mL) for 30 min, (B) pure OA (6.0 mL) for 30 min (with magnified image in inset), (C) pure DBA (5.5 mL) for 30 min with addition of OA (0.5 mL) for another 30 min, and (D) pure OA (5.5 mL) for 30 min with addition of DBA (0.5 mL) for another 30 min.

Figure 2. (A−C) TEM images and (D−F) ED patterns of different projections of a single typical star-shaped PbSe nanocrystal with zone axis of (D) ⟨111⟩, (E) ⟨110⟩, and (F) ⟨100⟩, respectively. (G−I) Schematic models show their corresponding views from different observation directions, respectively. 2865

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OAm but keeping other conditions constant. These nanocrystals show another kind of star-shape with eight arms growing along ⟨111⟩ directions, which are different from those along ⟨100⟩ directions in Figures 1 and 2. The size of the eight-arm stars is about 600 nm in diameter, which is much larger than that of the six-arm stars prepared in DBA (5.2 mL) with addition of OA (0.4 mL) and OAm (0.4 mL) as seen in Figure 1. Geometrically, the eight-arm stars have a cubic profile, with concave surfaces and edges, rather than the concave octahedron profile in the six-arm stars, shown in Figures 1, 2A−C. Figure 3B is the image for the samples prepared in pure OA (6.0 mL), and the resulting nanostructures form octahedral shapes with smooth surfaces; see magnified SEM image in inset in Figure 3B and the corresponding TEM images as shown in Supporting Information Figure S3. The size of the nano-octahedrons is about 40 nm in diameter, which is much smaller than that of the nanostructures prepared in DBA (Figure 3A). As compared to the two samples prepared in the two individual media (Figure 3A,B), we can find that the nanostructures in DBA illustrate a concave profile of a cube, while the ones in OA show a convex geometry of an octahedron. The two kinds of nanostructures obtained in DBA and OA represent two different types of features for the stars: concave and octahedral, respectively. It is worth mentioning that the sizes of the six-arm stars fall in the dimensions between these two kinds of the nanostructures prepared in DBA and OA, although there are considerable differences between the dimensions for the two kinds of nanostructures prepared in DBA and OA, respectively. To investigate the effects of DBA and OA, OA and DBA are added into the reaction systems of Figure 3A and B, respectively, for another 30 min. The final produced nanostructures are all similar to the shapes of six-arm stars (Figure 3C and D) and those shown in Figure 1. This result suggests that the formation of the six-arm star-shaped nanostructures mainly resulted from the cooperative effect of DBA and OA. Meanwhile, many more additional experimental investigations can also confirm the synergetic effect of DBA and OA that leads to the growth of the nanostructures with six-armstar shapes, even though these obtained nanostructures demonstrate somewhat a variation in the degree of concave or convex, as seen in Supporting Information Figure S4. The growth process investigation is highly desirable to access the growth mechanism of the PbSe nanostructures, which will help to provide a useful tool to control the growth of the PbSe nanostructures with tunable shapes. Figure 4A shows the SEM image for the PbSe nanostructures achieved in DBA (5.75 mL) with OA (0.25 mL) in the absence of OAm. They also show six-arm star-shaped nanostructures, similar to those in the ternary solutions of DBA−OA−OAm (Figures 1, 2). In addition, the sizes of the six-arm stars prepared under the two different conditions are almost the same. The performed experiment in Figure 4A provides a simple procedure for the growth of the six-arm stars (Figure 1). With addition of OAm (1 mL) in DBA (5 mL) in the absence of OA, cubic PbSe nanocrystals are obtained (Figure 4B), suggesting the geometry can be tuned in the reaction system selectively. When DBA is completely replaced by OAm (6 mL) without adding any other additives, the PbSe nanocrystals present side-cut cubic shapes with convex geometries (Figure 4C), which just show slight variation of cubes with spherical profile. The experiments confirm that the concave feature of the nanostars resulted from the use of DBA, instead of OAm and/or OA. Meanwhile, uniform octahedral PbSe nanocrystals (Figure 4D) are

Figure 4. PbSe nanocrystals with different morphologies of (A) star shape in DBA (5.75 mL) with OA (0.25 mL), (B) cube in DBA (5 mL) with OAm (1 mL), (C) side-cut cube in pure OAm (6 mL), and (D) rough surface octahedron (clearly as seen in magnified image in inset) in OAm (5.75 mL) with OA (0.25 mL), respectively.

fabricated via using OAm (5.75 mL) as solvent with addition of a small amount of OA (0.25 mL). The size of the octahedrons is 400 nm in diameter, which is larger than that obtained in pure OA (Figure 3B). More interestingly, the surfaces of the nano-octahedrons are not smooth, and it is found that there are many pyramid-like tips embedded in the surfaces in the magnified SEM image, as seen in the inset of Figure 4D. In the procedures, it is found that the shapes of the PbSe nanocrystals can be manipulated by the variation of media as demonstrated in the above investigations, and it is also found that the media play important roles in the shape evolution of the PbSe nanostructures. For easy understanding, the chemical structure of the media of DBA, OA, and OAm is provided (Supporting Information Figure S5). Typically, OA, as a longchain fatty acid, is extremely necessary for the formation of the star-shaped architectures in DBA no matter whether OAm is involved or not (Figures 1, 4A). As compared to the six-arm stars, the ones obtained with adding OAm show higher symmetrical and sharper arms (Figure 1) than those grown without OAm (Figure 4A). OAm seems to enhance the development of the nanostars with uniform shapes, while the omission of OAm does not limit and hamper the growth of PbSe nanostructures with star shapes. So, it can be deduced that the effect of OAm to PbSe is relatively weak in general. However, without adding OA in DBA, there are not any nanostars obtained except for the nanostructures of cubes (Figure 4B) or side-cut cubes in spherical profile (Figure 4C). The results determine that OA is favorable for the stability of {111} crystal surfaces, but OAm shows less selections because the planes of {100}, {110}, and even {111} can be observed in the image, although the {100} planes demonstrate a high percentage in the nanostructures (Figure 4C). The formation of the octahedral nanostructures in OAm with OA (Figure 3D) confirms that OA would stabilize the growth of PbSe nanostructures with exposed {111} crystal planes, consistent with the results in pure OA (Figure 3B). As for DBA, it absorbs on crystals selectively and leads to the nanostructures to form concave surfaces, although the detailed absorbed crystal planes switch considerably under different conditions (Figures 1, 3A, 4A). It is confirmed that the synergistic effects of DBA and OA lead to the formation of the six-arm stars on the basis of the 2866

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positive Pb2+ or a negative Se2− charged ion are active facets that do not appear in a PbSe crystal without surface modifications. However, in the pure OA system, the achieved nanostructures show octahedral shapes (Figure 3B). This result confirms the fact that OA abides with the {111} planes of the nanostructures and passivates the {111} planes to lower their surface free energy. In detail, when oleic acid (OA) is employed as the media, OA or mainly oleate roots (OA−) would act with Pb(II) ions intensively in the system. Although the thermodynamic data of lead oleate (Pb(II)OA2) are not documented in literature, it can be confirmed that its Gibbs energy must be at least about several dozen kilojoules per mole (kJ mol−1) due to the intensive interaction of Pb(II) with OA− on the basis of chemistry principle. The energy is close/ equivalent to the level of PbSe Gibbs energy, and there will be an equilibrium (PbSe + 2OA− → Pb(II)−(OA−) + Se2−) between PbSe and OA (or OA−/Pb(II)OA2) in the system. With the employment of OA, the surface of PbSe will be selectively dissolved and reconstructed accordingly in the balance of crystal and solution thermodynamics in addition to growth kinetics. In other words, the acidity of OA leads to OA− competing with Se2− to weaken the interactions between Pb(II) and Se2−. Also, in the Pb(II)-exposed {111} planes of rock salt PbSe crystals, the percentage of Pb atom located in {111} planes is 100%, while in both {100} and {110} planes the percentage is only 50% (but {100} planes have a higher density of nodes than {110} ones). OA has trends to act with Pb(II) ions in all planes of {111}, {110}, and {100}, while OA (or OA−) will be favorable to abide on positive-changed Pb(II)exposed {111} planes to grow convex octahedral PbSe crystals from cubic nuclei in the media of OA due to solution thermodynamics and crystal growth kinetics. That is to say, the intensive interaction of OA with Pb(II) leads to the active Pb(II)-exposed {111} facets shifting to the passivated states with biding the fatty hydrophobic tails,34 which results in the convex octahedrons in the final growth step (Figure 3B, Supporting Information Figures S3 and S6C). Such phenomena of growth of convex octahedrons have been also reported in previous work.12,17,22,24 In the media of OAm with a small amount of OA, the nanostructures do also demonstrate octahedral shapes in profile except for somewhat rough surfaces (Figure 4D). On the contrary, the nanostructures obtained in pure OAm just show cubic shape with some sides partially cut (Figure 4C), and such shapes show less variation of cubes, as a thermodynamic state for the shape of PbSe crystal. So, it can be revealed that OA gives priority to OAm for the shape controlling of the PbSe nanostructures. At the same time, in pure DBA, the nanocrystals show cubic shapes with {100} planes being concaved (labeled by an arrow in Figure 3A) besides another kind of star-shapes with eight arms grown along ⟨111⟩ directions (Figure 3A) that are different from those in Figures 1 and 4A. These experiments provide much more direct experimental evidence to confirm that OA abides on the Pb(II)-exposed {111} planes leading to the formation of PbSe nanocrystals with octahedral shapes (Figures 3B, 4D) mainly due to the intensive electrostatic interaction between Pb(II) and oleate ion (OA−), and DBA absorbs on {100} planes and even on some other planes including {110}, etc. The nanostars with eight ⟨111⟩-direction arms (Figure 3A) can be considered as cubes with both {100} and {110} planes concaved due to the selective absorption of DBA on {100} and {110} planes in the

above experimental investigations (Figure 3C,D and Supporting Information Figure S4). In the growth process of PbSe, it includes two steps as usual: the initial nucleation step and the subsequent growth step. Generally, the growth of crystal is the balance of thermodynamics and kinetics of the reaction system when thermodynamics is feasible. The system thermodynamics involves two parts from both crystals (composed of bulk and surface) and solutions (varied depending on media selected). The influence of solution/media on crystal growth resulted from their interaction on the surface of crystals (precisely the interface between crystal and solution). The variation of different solutions of crystal surface leads to tunable surface free energy of PbSe crystals accordingly throughout the growth process and even in the very beginning of nucleation step, and such variation in thermodynamics would further govern the growth kinetic process with varied specific shapes. This is why we could tune the PbSe crystal shapes in the current route with variation of reaction media. As we know, a large negative value of Gibbs energy of PbSe (ref 32) favors formation of PbSe thermodynamically.33 For PbSe with rock salt structure, the crystals will be present in cubic shapes, exposed with {100} facets (electroneutrality balance planes), thermodynamically, due to the charge balance between Pb(II) ions and negative Se ions within {100} planes. So, in the initial stage, the crystal thermodynamics would force the as-formed PbSe crystal nuclei to keep an approximately cubic shape (Scheme 1) even in chemical solvent at relatively Scheme 1. Schematic Illustration Depicting the Formation of Different Morphologies of PbSe Nanostructures under Varied Reaction Conditions

high temperature. Such phenomena for the nucleation of PbSe nanocrystals in the early stage have been convinced by previous investigations in noncoordinating (commonly nonpolar or weak-polar) solvents (see example of phenyl ether in the report by J. A. Hollingsworth and et al.)1,10 and even coordinating solvents.15,20,22 Actually, the samples show cubes or cubic profiles with some variation for the reaction performed in the early stage (for 5 min at 220 °C, Supporting Information Figure S6B,D) in the present route except for the employment of strong coordination solvent of OA (Supporting Information Figure S6A,C). All of these results confirm that the nucleation (formation in early stage) of PbSe in a reaction system is mainly favored and dominated/governed by crystal thermodynamics. As a result, PbSe immerses in cubic shapes in a free growth process thermodynamically due to the charge balance between Pb(II) ions and negative Se ions in {100} crystal planes, and at the same time, the {111} planes with either a 2867

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Figure 5. ATR-FTIR spectra for the PbSe nanocrystals prepared in (A) DBA (5.75 mL) with OA (0.25 mL) as seen in Figure 4A, (B) pure OA (6 mL) in Figure 3B, and (C) OAm (5.75 mL) and OA (0.25 mL) in Figure 4D, and for the media of DBA, OA, and OAm from (D) to (F), respectively.

that the use of DBA has a trend toward the PbSe nanostructures with concaved geometries whether OA is used in the reaction system or not. The dramatic shape changes of the nanostars from eight arms (Figure 3A) to six ones (Figure 4A) show that the {111} facets of the nanostructures switch from an active status to a passive one due to addition of OA in DBA. Crystallographically, if a facet of a crystal is active, it grows fast, and subsequently it would disappear with the proceeding of the crystal growth. On the contrary, the facet will reserve if it is capped by capping materials. In pure DBA (without adding OA), the {111} planes are the active crystal facets that favorably grow along the ⟨111⟩ directions to form eight-arm stars (Figure 3A), whereas with the addition of OA in DBA, the eight {111} facets are passivated considerably due to intensive interaction of OA with Pb(II), and the nanostructures exist as nanostars with six arms grown along ⟨100⟩ direction (Figure 4A). The above investigations suggest that OA demonstrates more intensive interaction with PbSe than DBA does because, for the geometrical shapes of the PbSe nanocrystals, the use of OA switches the nanostructures from the profiles of cubes to those of octahedrons. The order of OA > DBA to PbSe can be further confirmed by attenuated total

growth process in the pure DBA reaction system, and, as a result, the absorption and passivation of {100} and {110} facets with DBA would lead to the growth of the PbSe nanostructures along eight ⟨111⟩ directions from cubic nucleus because a few {100}-concaved nanocubes in relative small size are also observed in Figure 3A (labeled by arrow). The small-size nanocubes with {100}-facet concaved can be considered as an early stage or a not-well developed form of the nanostars with eight ⟨111⟩-direction arms. However, with the addition of a small amount of OA (0.25 mL) in DBA (5.75 mL), the shape of the nanostructures changes dramatically from eight-arm stars (Figure 3A) to six-arm stars (Figure 4A). As noted before, the eight-arm stars have a cubic profile in the geometrical view except that the surfaces and sides of the cube are concaved selectively due to the selective absorption of DBA. Similarly, the six-arm stars have an octahedral shape in profile with the octahedron concaved by DBA subjected to the intensive effects of OA. So, for the nanostars with six ⟨100⟩-direction arms (Figures 1, 4A), they are equivalent to a geometry of octahedron with eight {111} and even in addition to twelve {110} planes concaved under the conditions in DBA with adding OA. The results not only confirm that OA has a more intensive interaction with PbSe than DBA does, but also imply 2868

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Figure 6. (A−C) TEM images of the star-shaped PbSe nanocrystals obtained at 220 °C for 30 min in DBA with different amounts of OA and OAm: (A) 4.8 mL of DBA with 0.4 mL of OA and 0.8 mL of OAm, (B) 4.8 mL of DBA with 0.8 mL of OA and 0.4 mL of OAm, (C) 4.4 mL of DBA with 0.8 mL of OA and 0.8 mL of OAm, and (D−F) TEM images of the star-shaped PbSe prepared at 220 °C for 30 min in the media of DBA with different amounts of OA: (D) 5.2 mL of DBA with 0.8 mL of OA, (E) 4.5 mL of DBA with 1.5 mL of OA, and (F) 3.5 mL of DBA with 2.5 mL of OA, respectively. The scale bars are 300 nm.

3342 cm−1 are observed for CO in OA and N−H stretch in OAm, respectively. The weak absorption of the primary amine group (3342 cm−1) in OAm implies its weak effect to PbSe. As compared to the spectra in the presence of two amines (Figure 5A and C), the varied stretching absorption of amine groups detemines that DBA has a more intensive interaction with the nanocrystals than does OAm. Although the action of OAm to PbSe is relatively weak as compared to that of OA, the use of OAm does improve the quality of the six-arm nanostars from the media in DBA with OA (Figure 4A) to that in DBA with both OA and OAm (Figure 1). So, we are convinced that there is a synergistic effect of OA, DBA, and OAm that can promote the growth of the hexagonal pod-like nanostars with welldefined shapes besides the specific selective absorptions of an individual pure media among DBA (Figure 5D), OA (Figure 5E), and OAm (Figure 5F). These IR investigations reveal the state and structure of the media that can modulate the crystal growth via selective absorptions. Meanwhile, the electrostatic force between OA (oleic group) and Pb(II) is more intensive than the interaction between two amines and Pb(II). As noted, amines have a relative weak interaction to Pb(II) ions as compared to OA because the surface absorption of amines on crystals is mainly attributed to intermolecular forces or weak coordination effects between the crystal surface and organoamine molecules. The value of absorption energy is commonly equivalent to several kilojoules to a dozen kilojoules per mole, and the value is much lower than those between Pb(II) and Se2− and/or OA−. So, OA is actually prior to DBA and OAm in shape controlling of the PbSe nanostructures. As compared to the two amines (Supporting Information Figure S5), DBA is a secondary amine with two benzyl groups, while OAm is a primary amine with a long carbon-chain group.12,37 The benzyl group is an electron-donating group rather than an electron-withdrawing group, although it has a phenyl group. As a result, DBA would have a relatively intensive effect on PbSe as compared to OAm, which is consistent with the ATR-FTIR detection. Meanwhile, DBA with two benzyl groups may also demonstrate more intensive steric hindrance effects on PbSe as compared to OAm with a linear one. Now let us suppose that DBA absorbs on and covers the crystal surfaces

reflection Fourier transform infrared (ATR-FTIR) spectroscopic investigations (Figure 5). Figure 5A−C shows the ATR-FTIR spectra for the nanostars (Figure 4A) obtained in DBA (5.75 mL) with OA (0.25 mL), octahedrons (Figure 3B) in pure OA (6 mL), and rough octahedrons (Figure 4D) in OAm (5.75 mL) with OA (0.25 mL), respectively. In Figure 5A, it is found that the bands in the region 3000−2850 cm−1 are assigned to the C−H stretching modes and the band at 1456 cm−1 is due to the C−H bending mode.35 Also, the absorption at 1738 cm−1 associated with the CO bond stretching mode is obviously observed in the spectrum. These measurements demonstrate that the assynthesized PbSe nanocrystals in DBA with OA (Figure 4A) are capped with OA because that is similar to the status of the PbSe nanostructures (Figure 5B) that are obtained in the media of pure OA (Figure 3B). In the spectra, the only characteristic absorption band at 3322 cm−1 ascribed to −NH stretching mode in pure DBA (Figure 5D) is not obviously observed, while the peak at 1496 cm−1 due to the −NH scissoring mode is detected except for the absorption that is considerably reduced. The weak bands at 3029 and 3068 cm−1 assigned to the C−H stretching modes of benzyl group in DBA molecules are detected in the spectrum (Figure 5A). These detections suggest that DBA has an interaction with the nanostructures, and it actually serves as a capping material besides solvent in the samples (Figure 3A). Accordingly, the interaction would favor the growth of the nanostructures with tunable morphologies. Such phenomenon has been also observed in another system for the controlled growth of varied nanostructures capped by coordinating media.36 The ATRFTIR spectroscopic investigations offer a molecular-level evidence to support the proposed order of OA > DBA to PbSe. Now look back at the two kinds of star-shaped nanostructures, it can be deduced geometrically that the switch of the six-arm stars with cubic profile (Figure 3A) to the ones with octahedral profile (Figures 1, 4A) is mainly due to the intensive effect of OA prior to DBA based on the IR studies (Figure 5A,B). Figure 5C shows the ATR-FTIR spectrum for the PbSe samples obtained in OAm (5.75 mL) with OA (0.25 mL). It is found that the absorption of both OA and OAm can be detected in the spectrum because the bands at 1735 and 2869

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evenly at the initial step; due to the large steric barrier of DBA, the leaking spaces with the edge and especially the corner are larger than those with the facets. That is to say, the mass transfer at the corner areas has a relative large transferring flux as compared to that at facets and even edges in the growth process. So, the use of DBA leads to direct concave of crystals on surface relevant to the steric barrier, and it looks like the facets are passivated by the media selectively with the growth proceedings. Yet for OAm with almost linear structure, the selective influence on crystal growth is relatively weak. The analysis not only supports the order of DAB > OAm, but also gives a reasonable explanation for the star-shaped with concave shapes in addition to the growth mechanism yielding to the shape control of the crystals in the route. So, the order OA > DBA > OAm in addition to ATR-FTIR investigations provide a valuable hint to reveal the growth mechanism for the PbSe nanostars and understand the controllable growth of the PbSe nanostructures with tunable shapes. Furthermore, the order of the media is also favorable for the understanding of the nanostructures with varied sizes and tunable textures. In addition, quantity of the media, reaction temperature, and reaction time are also investigated in the procedures. In general, the media play an important role in the growth of the nanostructures, but it is also found that the quantities of DBA, OA, and OAm are not quite so infective to the growth and the shape of the PbSe nanostructures because the shape of the nanostructured stars changes slightly in geometry with the variation of the mole ratio and quantity of the media. As seen in the TEM images for the PbSe six-arm stars obtained at 220 °C for 30 min, the shapes of the nanostructures do not change considerably in DBA with different amounts of OA and OAm (Figure 6A−C), and also similar phenomena can be observed in DBA with OA (Figure 6D−F). It can be concluded that the solvent molecules of DBA, OA, and OAm would mainly act with PbSe via monolayer-molecule absorption because the quantity exceeding the critical value of such absorption is of no use for the surface modification of the nanostructures, and such a proposal can also be supported by the analysis of ATR-FTIR investigations (Figure 5). Meanwhile, the influence of reaction temperature and time is also checked, and it is found that there are no obvious changes in a wide range from 220 to 260 °C (Supporting Information Figure S7). It is of no doubt that the variation of the mole ratio and quantity of the media could lead to the change of the texture and the shape evolutions of the nanostructures. These investigations confirm that the shapes of the PbSe nanostructures can be manipulated in the current system via the variation of OA, DBA, and OAm base on the understanding of such solvents of surfactants involved in the media. In short, OA coordinates Pb(II) on {111} planes to form PbSe with octahedral profile, and as for the molecule of DBA with high symmetrical features between two benzyl groups and intensive steric barrier, the absorption of DBA molecules on PbSe nanocrystals results in the formation of concave shapes symmetrically for the nanostructures. In cooperating with other media of OA and/or OAm, DBA would realize the growth of the nanostars with well-defined shapes and the shape evolutions of other nanostructures rationally in the current procedures. In the syntheses, it is found that there are some relations between the shape and the surface features of the obtained nanostructures.2,5,36 Figure 7 is the corresponding XRD pattern for the nanostructures with shapes of cube (Figure 4B), eightarm star (Figure 3A), side-cut cube (Figure 4C), star-shaped

Figure 7. XRD patterns of the different morphologies of the PbSe nanostructures.

(Figure 4A), smooth surface octahedron (Figure 3B), and rough surface octahedron (Figure 4D), and they are all in the rock salt phase. The relative intensity of {111} over {100} crystal facets is 0.17, 0.26, 0.27, 0.34, 0.35, and 0.48 for the nanostructures in the shapes, respectively, and shows a successive increase in turn, indicating a prominence of {111} facets for the nanostructures with respect to different morphologies even if they are still maintained in the rock salt phase from cubes to octahedrons. The crystals in other shapes are between the two extremes of cubes and octahedrons, and their molar ratio and relative intensity will be in the range of two extremes. So, we could assess Pb/Se ratio relevant to the measured relative intensity between {111} and {100} planes for the crystals with different shapes. Meanwhile, the compositions of the PbSe nanostructures with different morphologies are determined by EDX measurements, as shown in Supporting Information Figure S8. It is found that the atomic ratios of Pb over Se are 1.16, 1.18, 1.21, 1.22, 1.22, and 1.24 for the nanocrystals with typical cube, eight-arm star, side-cut cube, star-shaped, smooth surface octahedron, and rough surface octahedron shapes, respectively, and they all present Pb-rich features, consistent with the six-arm stars prepared in DBA with OA and OAm (Figure 1, Supporting Information Figure S1). Interestingly, they display strong positive response to the areas of the exposed {111} crystalline facets of the nanostructures. Figure 8 is a chart sketching out the molar ratios of Pb/Se versus the relative diffraction intensity of {111} over {200} planes for the six nanocrystals with different morphologies. As we know, generally, the relative diffraction intensity of the {111} over {200} facets reflects the relative explosion of {111} crystal planes in nanostructures. So, in the current investigations, the increase of the molar ratios of Pb/Se is correlated with the area increase of {111} planes for the rock salt PbSe, and in fact the increase of Pb/Se from 1.16 to 1.24 leads to the increase of relative diffraction intensity of their {111}/{200} faces from 0.17 to 0.48 (Figure 8) and vice versa. These results reveal that the shape and exposed surface structure of the PbSe nanocrystals highly correspond with the compositions of the nanocrystals (Supporting Information Table S1), and in turn the specific shape and geometry of the nanostructures can be tuned by controlling the exposed crystal surfaces and/or the corresponding compositions through the variation of reaction conditions.

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CONCLUSIONS A facile organometallic synthetic route has been developed for the preparation of well-defined star-shaped PbSe nanostruc2870

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with experiments of ATR-FTIR detection and valuable discussions in this Article.

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Figure 8. Pb/Se ratios obtained by EDX measurements corresponding to their integrated peaks intensity of {111}/{200} faces of the products with these different morphologies. Peaks intensities of {111}/{200} faces are 0.17 (cube), 0.26 (eight-arm star), 0.27 (sidecut cube), 0.34 (star-shape), 0.35 (smooth surface octahedron), and 0.48 (rough surface octahedron), respectively. The increase in Pb/Se ratios indicates a clear trend that the products with highly intensive {111} facets are to be the most Pb-rich nanostructures.

tures in DBA with the addition of OA and OAm at 220 °C for 30 min. The growth processes of the nanostructures are carefully investigated, and it is found that the reaction media of DBA, OA, and OAm play crucial roles in the formation of welldefined six-arm stars. Typically, OA coordinates Pb(II) on {111} planes to form PbSe with octahedral profile, and DBA, with two benzyl groups and intensive steric barrier, leads to the formation of PbSe nanostructures in symmetrically concave geometries. In cooperating with other media of OA and/or OAm, DBA would realize the growth of the nanostars with well-defined shapes and the shape evolution of other nanostructures rationally in the current procedures with the understanding of the synergetic effects of the media. Meanwhile, the Pb-rich and surface characters with their specific shapes are also discussed in the current investigations, and these findings would provide a valuable guidance for the controlled syntheses of other nanostructures with novel architectures for potential applicants.

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ASSOCIATED CONTENT

* Supporting Information S

Materials of XRD pattern and EDX spectra of the PbSe nanostars, chemical structure of media and corresponding FTIR spectra, and SEM and TEM images of the PbSe samples with other morphologies. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-551-63600243. Fax: +86-551-63606266. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2010CB934700, 2012CB922001), the National Nature Science Foundation of China (21071136, 51271173), and the Research Fund for the Doctoral Program of Higher Education of China (no. 20103402110033). Professor Shuji Ye of USTC is also acknowledged for assisting 2871

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