Shape Evolution of Lead Telluride and Selenide Nanostructures Under Different Hydrothermal Synthesis Conditions

B. Poudel,∗ W. Z. Wang, D. Z. Wang, J. Y. Huang, and Z. F. Ren∗ Department of Physics, Boston College, Chestnut Hill, MA 02467, USA

Face-open nanoboxes of lead telluride and selenide have been synthesized by a simple hydrothermal method. Nano- and microcrystals of various morphologies, including microﬂowers, semi- microﬂowers, cubic nanoparticles, etc., have also been observed at different synthesis conditions. Temperature, time, and concentrationsDelivered of various by reactants Ingenta play to: a major role in controlling the morphology and shape evolution of the product.Boston This College simple synthesis technique for the growth of various nano- and microstructures opensIP : a 136.167.55.151 new route to prepare hierarchical structures of a variety of binary semiconducting materials in a large quantity. A possible growth mechanism of such nano- and microstructures has been proposed.Fri, 19 May 2006 17:35:30

Keywords: Lead Telluride and Lead Selenide, Nanoboxes, Hydrothermal Method.

RESEARCH ARTICLE 1. INTRODUCTION 2. EXPERIMENTAL DETAILS

Binary IV–VI semiconducting compounds form a very Both PbTe and PbSe with various morphologies were pre- interesting class of materials because of their potential pared by a simple hydrothermal method.Starting materials applications in thermoelectric, electronic, and opto- including Polyethylene Glycol (PEG, molecular weight: electronic devices due to their unique thermal and electri- 20,000), Sodium Hydroxide (NaOH), Sodium Tellurite 1 2 cal properties. Among the IV–VI class of materials, lead (NaTeO3), Sodium Selenite (NaSeO3), Lead Acetate Tri- telluride (PbTe) and selenide (PbSe) have attracted great hydrate (PbAc), and Hydrazine Hydrate were purchased attention for their applications in thermoelectric power from Aldrich and/or Alfa-Aesar.In a typical synthesis of generations.Both of these materials have small band PbTe face-open nanoboxes, 50 mg of PEG and 2.4 g of gap and large exciton Bohr radius, which allows size- NaOH were added to 50 mL of de-ionized water.After quantization effect to be observed easily even for relatively a few minutes of stirring, 1 mMol each of NaTeO3 and large crystal size.After the earlier reports on hierarchi- PbAc were added to the solution and stirred until the cal nanostructures of ZnO3 and MgO,4 many reports have reactants were dissolved completely.10 mL of Hydrazine been published on hierarchical structures of various mate- Hydrate was finally added to the solution and transferred rials such as SiGe, BiSe, and metal oxides, with mor- into a Teflon-lined autoclave.The sealed vessel was kept phologies including nanobelts, multipods, nanotrees, and in a furnace at a temperature of 100 C for 10 hours and microflowers5–18 during the last a couple of years.Also, then cooled down to room temperature.The product was there are some reports on flower-like structures obtained washed several times with water.The morphology of the with lead sulphide (PbS).19–25 Here, we report a few inter- samples was examined using a Scanning Electron Micro- esting nano- and micro-structures of PbTe and PbSe that scope (SEM, JEOL-6340F) and a Transmission Electron were synthesized by a simple hydrothermal method, which Microscope (TEM, JEOL-2010F), while the atomic com- may find their application in thermoelectric and other inno- position was determined using an Energy Dispersive X-ray vative technologies.This technique could be applied to Analysis (EDX) spectrometer attached to the TEM.The prepare various similar nanostructures of other semicon- phase of an individual nano- or micro-crystals was studied ducting materials. using Selected Area Electron Diffraction (SAED) while the phase of the bulk powder was analyzed by X-ray diffraction (XRD, Bruker-AXS, G8 GAADS) using Cu- ∗Authors to whom correspondence should be addressed. K radiation.

1050 J. Nanosci. Nanotechnol. 2006, Vol. 6, No. 4 1533-4880/2006/6/1050/004 doi:10.1166/jnn.2006.163 Poudel et al. Lead Telluride and Selenide Nanostructures Under Different Hydrothermal Synthesis Conditions

For the preparation of PbSe microﬂowers, a similar pro- 626 (200) cedure was followed with NaTeO3 replaced by NaSeO3 as the Se source and at a lower reaction temperature, 50 C.

3. RESULTS AND DISCUSSION (220) Figures 1a and 1b are typical SEM images of the PbTe nanoboxes synthesized at 100 C, which show that such Intensity (a.u.) nanoboxes have face-openings in all the six faces, named (222) face-open nanobox hereafter.Also, it is clearly seen that (111) (400) such face-open nanoboxes have layer by layer structure. (311) The overall sizes of such face-open nanoboxes vary from 0 about a hundred nanometers to a micrometer, however, the 21 30 40 50 60 individual layer thickness is much smaller, a few tens of 2-Theta (deg.) nanometers.Figures 1c and 1d, low magnification SEM Fig. 2. XRD pattern of the face-open nanoboxes shown in Figure 1. EERHARTICLE RESEARCH and TEM images respectively, show the abundance of such The pattern of the standard face centered cubic PbTe phase has been structure.Figure 1e is the high resolution TEM image of shown as vertical lines for comparison. part of the face-open nanoboxes to show the high crystallinity.SAED pattern in the inset of Figure 1eDelivered shows that by Ingenta(Fig.2) of to: the as-prepared product shows that all the peaks the face-open nanoboxes are single crystals.XRD patternBoston Collegecould be indexed to a pure PbTe face centered cubic struc- IP : 136.167.55.151ture.Standard pattern of PbTe has been shown as the ver- Fri, 19 May 2006tical 17:35:30 lines for comparison. The morphology of the product changes with the change in hydrothermal treatment temperature.For example, when the treatment temperature is increased to 125 C, the faces of the product were more open forming semi-microflowers as shown in Figures 3a and 3b, viewed along different ori- entations.When the hydrothermal treatment temperature is further increased to 160 C, a flower-like morphology as shown in Figure 3c was observed.Figure 3d is the low magnification SEM image showing the abundance of the morphology.SEM (Fig.3e) and TEM (Fig.3f) images of similar microflowers viewed along a corner clearly demon- strate that the microflowers have 8 corners and 4-fold in-plane symmetry.The corners grow along 111 direc- tion and the planes on {100}.EDX studies of an individual microflower show that atomic percentage ratio of Pb and Te is close to 50:50 verifying the PbTe composition.All the microflowers are identical in shape but differ slightly in size.However, when the temperature was increased to 185 C, the microflower size and shape doesn’t change much as shown in SEM picture (Fig.3g), but secondary growth on the tips of microflowers was obvious.Figure 3h is the low magnification SEM picture which shows a large number of identical microflowers.The result is very reproducible and can also be produced in large scale (gram quantities) in high yields, more than 95%. However, when a lower temperature (85 C) was used, most of the products are nanocubes as shown in Figure 4a. High resolution TEM picture of a typical nanocube Fig. 1. SEM and TEM images of PbTe nanoboxes synthesized at (Fig.4b) shows that the cubes are highly crystallized and 100 C.(a) and (b) High magnification SEM images showing the typical their size varies from about 10 to 50 nm with an average morphologies, (c) and (d) low magnification SEM and TEM images dimension about 30 nm.The yield of the product is low respectively, showing the abundance, and (e) high magnification TEM image showing high crystallinity.Inset in (e) is the SAED pattern show- at such a low temperature.However, it can be improved ing its single crystalline nature. by increasing the concentration of sodium hydroxide,

J. Nanosci. Nanotechnol. 6, 1050–1053, 2006 1051 Lead Telluride and Selenide Nanostructures Under Different Hydrothermal Synthesis Conditions Poudel et al.

Fig. 4. SEM and TEM images.(a) SEM image of solid PbTe nanocubes synthesized at 85 C and (b) high resolution TEM image of nanocubes to illustrate the high crystallinity.

Very similar morphologies of PbSe were observed at lower temperatures when the Te source was replaced with

NaSeO3.In particular, face-open nanoboxes of PbSe were observed at 50 C and solid nanocubes having dimensions about 30 nm were observed close to room temperature. Delivered by IngentaBecause to: of the cubic symmetry of PbTe and PbSe, they Boston Collegeprefer to grow into a cubic shape once a particle grows to IP : 136.167.55.151a certain minimum size when there are no constraints.This Fri, 19 May 2006idea 17:35:30 has been verified via a reaction at low temperature where they don’t have sufficient kinetics to grow and form small cubic nanoparticles, as shown in Figure 4a. When surfactant was not used, there were no face- RESEARCH ARTICLE open nanoboxes observed at 100 C, instead we observed solid cubic nanoparticles as shown in Figure 5a.When 50% alcohol was used as a solvent, which essentially reduces the size of the micelles, microflowers composed of smaller nanosegments were observed.At a higher temperature, 160 C, without the use of the surfactant, we observed solid cubes having larger sizes as shown in Figure 5b.This implies the important role of surfactant PEG to form the hierarchical structures.On the basis of these observations and a large number of other experi- ments, we propose a growth mechanism of such nano- Fig. 3. SEM and TEM images of PbTe micro-structures.(a) and (b) are SEM images of typical semi-microflowers synthesized at 125 C show- and micro-structures.When the temperature is low enough, ing the morphologies from different angles.(c) and (d) are high and low because of stabilization effect of the surfactant and the pre- magnification SEM images of the product obtained at 160 C showing a ferred growth of PbTe and PbSe in cubic symmetry seeded typical microflower and the abundance, respectively while (e) and (f) are by the reduced lead, we observe nanocubes.At higher SEM and TEM images of microflowers observed from a corner, respec- temperatures, the PbTe molecules first grow to the criti- tively.(g) and (h) are SEM images of semi-microflowers synthesized at 185 C showing the morphology and abundance, respectively. cal size determined by specific surface energy and Gibbs free energy26 that are dependent of reaction conditions. hydrazine hydrate and a longer reaction time, typically Because of the ability of PEG surfactant to prevent the 48 hours. nanoparticles from approaching the surface due to its steric These observations show that there is a continuous stabilization effect,27 these cubic particles can act as nuclei transformation of morphologies from nanocubes to face- particles to allow growing a secondary layer from their open nanoboxes to semi-microflowers and finally to corners.The secondary layer can start to grow only from microflowers with increasing synthesis temperature.All the corners of cubic nuclei particles where the surfactant the above results are very reproducible and similar results concentration is less and surface energy is higher com- were obtained with different sources of Te, for example pared to the faces.There will be no growth in the center

TeCl4.However, with the use of TeCl 4, the structures were of the face where the surfactant concentration is higher not as clean as we observed with the use of NaTeO3 and and hence not allowing particles to diffuse through them. many dendrite structures were also observed under SEM When the kinetics of the reaction is stronger compared in the samples prepared at higher temperatures. to the repulsive force due to surfactant chains, secondary

1052 J. Nanosci. Nanotechnol. 6, 1050–1053, 2006 Poudel et al. Lead Telluride and Selenide Nanostructures Under Different Hydrothermal Synthesis Conditions

Acknowledgments: The work was partially supported by DOE DE-FG02-00ER45805, NSF NIRT 0304506, and Intel.

References and Notes

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Received: 18 December 2005.Accepted: 13 January 2006.

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