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Growth of PbTe nanorods controlled by polymerized tellurium anions and metal(II) amides via composite-hydroxide-mediated approach
Materials Research Bulletin 44 (2009) 1846–1849
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Growth of PbTe nanorods controlled by polymerized tellurium anions and metal(II) amides via composite-hydroxide-mediated approach Buyong Wan a,b, Chenguo Hu a,*, Hong Liu c, Yufeng Xiong d, Feiyun Li a, Yi Xi a, Xiaoshan He a a
Department of Applied Physics, Chongqing University, 174 Shapingba Street, Chongqing 400044, PR China College of Physics and Information Technology, Chongqing Normal University, Chongqing 400047, PR China c State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China d National Center for Nanoscience and Technology, Beijing 100080, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 January 2009 Received in revised form 15 May 2009 Accepted 28 May 2009 Available online 6 June 2009
The pure face-centered-cubic PbTe nanorods have been synthesized by the composite-hydroxidemediated approach using hydrazine as a reducing agent. The method is based on reaction among reactants in the melts of potassium hydroxide and sodium hydroxide eutectic at 170–220 8C and normal atmosphere without using any organic dispersant or surface-capping agent. Scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy were used to characterize the structure, morphology and composition of the samples. The diameters of nanorods are almost fixed, while the lengths can be tunable under different growth time and temperatures. The growth mechanism of PbTe nanorods is investigated via UV–vis absorption, demonstrating that polymerized tellurium anions and metal(II) amides in the hydrazine hydroxide melts could control the crystallization and growth process of PbTe nanostructures. The band gap of assynthesized PbTe nanorods has been calculated based on UV–vis–NIR optical diffuse reflectance spectra data. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis D. Diffusion
1. Introduction Nanoscale one-dimensional semiconductive materials have attracted much attention due to their importance in fundamental and potential applications in nanodevices [1–3]. Especially, onedimensional (1D) thermoelectric nanomaterials are of great interest for the construction of high performance thermoelectric (TE) devices. Theoretical calculations indicate that improvement in TE efficiency can be achieved as the diameter of the 1D structure approaches a few nanometers [4,5]. PbTe and PbTe-based solid solutions are promising thermoelectric materials in the intermediate range of temperature (500–900 K) because of their high thermoelectric figure of the merit (ZT), high melting point, good chemical stability, low vapor pressure and good chemical strength [6–9]. In the past decades, thermoelectric properties have been studied in a large number of works [7,10]. At present, well-known investigations are PbTe-based quantum-well [8,11] and quantumdot [12] superlattices in which a significant increase in thermoelectric figure of merit in comparison with bulk crystals has been observed. Besides, lead telluride (PbTe) is a good candidate
* Corresponding author. Tel.: +86 23 65104741; fax: +86 23 65111245. E-mail addresses: [email protected], [email protected] (C.G. Hu). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.05.016
material for photodetectors in the mid- and far-infrared bands and mid-infrared quantum-well laser diode because of its high quantum efficiency, low noise level at working temperature and ability to tune peak wavelength by adjusting alloy composition [13–15]. All these stimulate further studies of low-dimensional structures based on PbTe. Up to now, many methods have been developed to synthesize crystalline PbTe, including ultrasonic synthesis [16], sonoelectrochemistry [17], hydrothermal crystallization [18], microemulsion [19], solvothermal synthesis [20], etc. Recently, 1D PbTe structures have been synthesized [21–23] via two-step process or in an organic solvent. Herein, we have developed an approach for synthesis of PbTe nanorods which has the advantages of one-step, easy scale-up and low cost. The growth mechanism of PbTe nanorods is investigated via analysis of the different reactions by UV–vis spectra. 2. Experimental details PbTe nanorods were synthesized by the CHM approach [24–26]. An amount of 9 g of mixed hydroxides (NaOH:KOH = 51.5:48.5) was placed in a 25 ml Teflon vessel, 0.5 mmol of Te powder with 3 ml hydrazine (as reducing agent) were put into the Teflon vessel. Then the Teflon vessel was oscillated in ultrasonic water bath at
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3. Results and discussion
Fig. 1. XRD spectra of the PbTe nanorods synthesized under different conditions.
room temperature. When the color of the liquid in the vessel became purplish red, 0.5 mmol Pb(NO3)2 was added in it. Then the vessel was sealed and put into a furnace preheated to 170–220 8C. After reacting for 12–48 h, the vessel was taken out and cooled naturally down to room temperature. The black solid product obtained was washed using deionized water for several times, and diluted HCl (pH 3.0) to remove possible hydroxides adsorbed on the product. The clean samples were obtained after being washed twice with deionized water again. The morphology and size of the synthesized samples are characterized at 20 kV by a field-electron scanning electron microscopy (Nova 400 Nano SEM) and at 200 kV by a transmission electron microscopy (TEM, TECNAI20, Philips). Energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometer (XRD) with Cu Ka radiation (l = 1.5418 A´˚ ), in step of 48 with a count time of 1 min were used to investigate the crystal phase and chemical composition. An UV–vis–NIR Spectrophotometer (Hitachi U-4100) was used to examine the different reaction process and measure the optical properties of the products.
In order to investigate the size change under different conditions, experiments were systematically done at temperatures of 170, 200 and 220 8C and with growth time of 12, 24 and 48 h, respectively. Fig. 1 shows the XRD patterns in the 2u range from 238 to 708 of the as-synthesized PbTe samples. All peaks on the XRD spectra are perfectly indexed as the pure face-centered-cubic phase of PbTe with lattice constant a = 6.443 A´˚ , which is consistent with the standard value (a = 6.454 A´˚ ) of bulk face-centered-cubic ¯ PbTe (Fm3m ð2 2 5Þ, JCPDS: 781905). Based on the Scherrer equation, the crystallite size of a sample is inversely proportional to the full-width-half-maximum (FWHM), indicating that a broader peak represents smaller crystallite size. It is noticed that the samples prepared at higher temperatures or for longer time exhibit narrowed peaks, which means the crystallite sizes of the products become large. In addition, the intensity of peak become stronger with the increase of growth time, indicating the crystallization of PbTe nanorods is better. The morphology of the obtained PbTe products is characterized by scanning electron microscopy (SEM). Fig. 2a–d gives the image of PbTe synthesized in different conditions. In these images, the diameter of nanorods are of 50–100 nm, and do not obviously change with varying temperature and time, while the typical lengths of nanorods are of 200–400, 300–500, 400–700 and 500– 1000 nm at 170 8C for 12 h, 200 8C for 12 h, 200 8C for 24 h and 200 8C for 48 h, respectively, as are shown in Fig. 2a–e, indicating that the length of the nanorods increase distinctly with the increase of temperature and time. EDS measurements in the selected area of Fig. 2d show that the elements in the nanorods are Pb and Te only and the atomic ratio of Pb and Te is close to 1:1 (Fig. 2f) (Si represents the peak of substrate). Fig. 3a and b shows typical TEM images of the as-synthesized PbTe nanorods and the diffraction pattern in the selected area of Fig. 3a, which matches the XRD spectra in Fig. 1. In the hydroxide melts, an obvious understanding of the reaction is the disproportionation reaction of the chalcogen occurred in the alkaline condition [27]. That is: 3E þ 6OH ! 2E2 þ EO3 2 þ H2 OðEdenoteschalcogenÞ
Fig. 2. SEM images of PbTe nanorods synthesized at (a) 170 8C for 12 h, (b) 200 8C for 12 h, (c) 200 8C for 24 h and (d) 200 8C for 48 h. (e) Enlarged image of 200 8C for 48 h, (f) EDS spectrum in the white frame of (e) showing the presence of Pb and Te, the Si signal is from the substrate.
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Fig. 3. (a) TEM image and (b) electron diffraction of the PbTe nanorods prepared at 200 8C for 48 h.
Whereas, different chalcogens have different reaction process owing to the increase in metallic property of S, Se and Te. Through the electrode potential analysis, the disproportionation reaction to Te is less likely to occur (E0 2 ¼ 0:57 V, E0Te=Te2 ¼ 1:143 V). TeO3 =Te Parkin and co-workers [28] have carefully studied the dissolution process of the sulfur in liquid ammonia system, as well as the reaction of some sulfide, and demonstrated that there are a series of sulfur–ammonia anions and sulfur polyanions in the liquid sulfur system such as S2N, S3N, S3, S42 and so on. These ions either have strong oxidative property or release the sulfurous sources such as S2. According to the analysis, we believe that there are polymerized anions such as Te4N5, Te42 and Te22 anion in hydrazine alkaline melts, which form the Te or Te–N rings like S8, as polytelluride anions of the form Tex2 (x = 2–6) with rings or chains and a number of cationic polytelluride species exist in basic, polar solvents (e.g. NH3 or ethylenediamine) by electrochemical synthesis [29,30]. To validate the reaction process in hydrazine–KOH/NaOH system, the absorption spectra of the different reaction solutions were investigated. Te powder is put in the NaOH/KOH aqueous solution or hydrazine hydrate solution. And these solutions are oscillated in an ultrasonic water bath for 30 min. The absorption spectra of the solutions are examined by UV–vis–NIR Spectrophotometer (Hitachi U-4100). In Fig. 4, there are great differences in absorption in different solutions. The absorption spectrum of the NaOH/KOH aqueous solutions is shown in the curve (a) of Fig. 4. There are only two weak features centered at about 240 and 370 nm. When Te powder was put in the NaOH/KOH aqueous
Fig. 4. UV–vis absorption spectra of (a) NaOH/KOH aqueous, (b) NaOH/KOH + Te, and (c) NaOH/KOH + N2H4 + Te solutions.
solution, the color of the solution became gradually flat-red, the absorption spectrum in the curve (b) shows there are two peaks centered at 520 and 250 nm, which might correspond to the absorption peak of tellurium polyanions Te22, Te32, Te42 [31,32]. Telluride can also disproportionate in solution (two Te22 anions can each lose an electron and combine to form the Te42 species) [30]. When 3 ml N2H4H2O is put in the vessel in which the Te powder and NaOH/KOH are pre-put, the color of the solution becomes red rapidly and finally deep purplish red under the ultrasonic vibration. The curve (c) in Fig. 4, the peaks centered at 520 nm and from 260 to 330 nm may be the absorption peaks of the tellurium polyanions and tellurium–nitrogen anions, and the peak intensity in NaOH/KOH hydrazine solutions is stronger than that in NaOH/KOH aqueous solutions. So, the forming process of PbTe can be inferred as follows: under attack of nucleophilic hydrazine, tellurium or tellurium–nitrogen rings have been opened, forming Te chain. And Te–Te bond is broken as the reaction continues, which leads to the degradation of Te chain and constant release of the active tellurium source Te2 [33]. At the same time, dissolution of Pb(NO3)2 in hydrazine hydrate melts results in the formation of the metal(II) amides [29], Pb(NH2)2 or Pb(NH3)2+, metal(II) amides rupture and release the cation Pb2+ with the increase of temperature and reaction time. Subsequently, Pb2+ and Te2 in the hydroxide melts fall together to form the initial nuclei of PbTe. The factors affecting the crystal growth involve kinetics and crystallography. It has been concluded by Murphy [34] that the selected absorption of molecules and ions in solution to different crystal faces directs the growth of nanoparticles into various shapes by controlling the growth rates along different crystal axes. Wang [35] suggested that the shape of a fcc nanocrystal is to be determined mainly by the ratio of the growth rate in h1 0 0i to that in h1 1 1i. Then, with respect to the structure of fcc PbTe, we speculate that these structures are formed in an analogous process. The different rates of adsorption and deadsorption of hydrazine molecules on the different planes of PbTe nuclei would kinetically influence the growth rates of these planes. So, in the hydroxide melts, the hydrazine not only plays a role as reducing agent and the complexing reagent to form a complex, but also control the anisotropic growth of PbTe into nanorod shape. The whole process can be described diagrammatically in Scheme 1. The polymerized anions and metal(II) amides serve to reduce the speed of reaction because the Te rings and Pb(NH2)2 greatly decrease the free Te2 or Pb2+ concentration in the solution. It is well-known that a slow reaction rate is favorable for crystallization and separating the growth step from the nucleation step [36]. It is also noted that the mixed hydroxide melts and ultrasonic process play an important role in controlling nucleation and growth of PbTe nanorods. The large viscosity of the hydroxide melts also reduces the speed of reaction and avoids agglomeration. This property is an important factor in obtaining well-dispersed nanostructures from the reaction without using a surface-capping material. Besides, in the ultrasonic process, as acoustic cavitation provides a unique interaction of energy, the element telluriums are fully deoxidized to polymerized anions by hydrazine (N2H4) and disperse uniformly in alkaline hydrazine, which is also very important in the formation of uniform nanorods. We have done experiments without the ultrasonic process for synthesizing PbTe products, and nanocrystals of irregular shapes such as rods, particles and flakes, as well as the impurities such as element Te or Pb have been obtained in the products. And the sizes of the products are not uniform, range from a few nanometers to a few micrometers. Possibly, the Te cannot be deoxidized simultaneously to polymerized tellurium anions by hydrazine only in the hydroxide melts, so the released Te2 ions start to connect with Pb2+ ions locally to grow into variform and varisize nanocrystals. The ultrasonic process enables Te to react fully with N2H4 and
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Scheme 1. Illustration of the formation process of PbTe nanorods.
separation of the growth step from the nucleation step. So the PbTe nuclei grow along a specific direction to form the nanorods. The band gap of as-synthesized PbTe nanorods is about 0.326 eV, which is obtained from the optical diffuse reflectance spectra can analysis. We believe that these cubic phase PbTe nanorods are promising building units for future TE nanodevices, and nanopolycrystalline materials. Acknowledgments This work has been funded by the NSFC (20741006), The Science and Technology Research Project of Chongqing Municipal Education Commission of China (KJ080819). Thanks to Prof. Xia Lin of Chongqing University for the English language editing. References
Fig. 5. Optical diffuse reflectance spectra (UV–vis–NIR) of the PbTe nanorods synthesized at 200 8C 24 h.
polymerized tellurium anions to distribute uniformly, and consequently results in forming the single structure and uniform size of PbTe nanorods in the viscous hydroxide melts. Based on the procedure described in the literature [37], the optical diffuse reflectance spectra can be used to estimate the band gap of the nanostructures. Because the individual particle size is much less than the thickness of the sample layer made from nanoparticles, an ideal diffuse reflectance with constant scattering coefficient could be expected. The Kubelka–Munk function, which is the ratio of the absorption (a) to scattering factors (S), is used to plot the absorbance. Experimental results for the absorption coefficients for PbTe nanorods synthesized at 200 8C 24 h are shown in Fig. 5, in which the K–M function (a/S) is plotted against photon energy (hn). The optical band gap is obtained by extrapolation to zero of the linear part of the curve, which is about 0.326 eV. Uniform crystalline PbTe nanorods with diameter of about 50–100 nm, close to its average Bohr exciton radius of 46 nm, show a certain quantum confinement effect. The blue shift of the nanorods compared to bulk PbTe (Eg = 0.31 eV) can be observed. 4. Conclusions
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
We have successfully achieved the synthesis of highly pure PbTe nanorods with the diameter of 50–100 nm and length of hundreds of nanometers via a novel composite-hydroxidemediated approach at temperature of 170–200 8C. It is strongly suggested that the polymerized tellurium anions and metal(II) amides (Pb(NH2)2) in the hydrazine hydroxide melts control the growth process of the PbTe nanorods. The Te rings and Pb(NH2)2 greatly decrease the concentration of free ions in the solution through slowly releasing the Te2 or Pb2+ ions, and reduce the speed of reaction, and consequently benefit to crystallization and
[27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Growth of PbTe nanorods controlled by polymerized tellurium anions and metal(II) amides via composite-hydroxide-mediated approach Buyong Wan a,b, Chenguo Hu a,*, Hong Liu c, Yufeng Xiong d, Feiyun Li a, Yi Xi a, Xiaoshan He a a
Department of Applied Physics, Chongqing University, 174 Shapingba Street, Chongqing 400044, PR China College of Physics and Information Technology, Chongqing Normal University, Chongqing 400047, PR China c State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China d National Center for Nanoscience and Technology, Beijing 100080, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 January 2009 Received in revised form 15 May 2009 Accepted 28 May 2009 Available online 6 June 2009
The pure face-centered-cubic PbTe nanorods have been synthesized by the composite-hydroxidemediated approach using hydrazine as a reducing agent. The method is based on reaction among reactants in the melts of potassium hydroxide and sodium hydroxide eutectic at 170–220 8C and normal atmosphere without using any organic dispersant or surface-capping agent. Scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy were used to characterize the structure, morphology and composition of the samples. The diameters of nanorods are almost fixed, while the lengths can be tunable under different growth time and temperatures. The growth mechanism of PbTe nanorods is investigated via UV–vis absorption, demonstrating that polymerized tellurium anions and metal(II) amides in the hydrazine hydroxide melts could control the crystallization and growth process of PbTe nanostructures. The band gap of assynthesized PbTe nanorods has been calculated based on UV–vis–NIR optical diffuse reflectance spectra data. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis D. Diffusion
1. Introduction Nanoscale one-dimensional semiconductive materials have attracted much attention due to their importance in fundamental and potential applications in nanodevices [1–3]. Especially, onedimensional (1D) thermoelectric nanomaterials are of great interest for the construction of high performance thermoelectric (TE) devices. Theoretical calculations indicate that improvement in TE efficiency can be achieved as the diameter of the 1D structure approaches a few nanometers [4,5]. PbTe and PbTe-based solid solutions are promising thermoelectric materials in the intermediate range of temperature (500–900 K) because of their high thermoelectric figure of the merit (ZT), high melting point, good chemical stability, low vapor pressure and good chemical strength [6–9]. In the past decades, thermoelectric properties have been studied in a large number of works [7,10]. At present, well-known investigations are PbTe-based quantum-well [8,11] and quantumdot [12] superlattices in which a significant increase in thermoelectric figure of merit in comparison with bulk crystals has been observed. Besides, lead telluride (PbTe) is a good candidate
* Corresponding author. Tel.: +86 23 65104741; fax: +86 23 65111245. E-mail addresses: [email protected], [email protected] (C.G. Hu). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.05.016
material for photodetectors in the mid- and far-infrared bands and mid-infrared quantum-well laser diode because of its high quantum efficiency, low noise level at working temperature and ability to tune peak wavelength by adjusting alloy composition [13–15]. All these stimulate further studies of low-dimensional structures based on PbTe. Up to now, many methods have been developed to synthesize crystalline PbTe, including ultrasonic synthesis [16], sonoelectrochemistry [17], hydrothermal crystallization [18], microemulsion [19], solvothermal synthesis [20], etc. Recently, 1D PbTe structures have been synthesized [21–23] via two-step process or in an organic solvent. Herein, we have developed an approach for synthesis of PbTe nanorods which has the advantages of one-step, easy scale-up and low cost. The growth mechanism of PbTe nanorods is investigated via analysis of the different reactions by UV–vis spectra. 2. Experimental details PbTe nanorods were synthesized by the CHM approach [24–26]. An amount of 9 g of mixed hydroxides (NaOH:KOH = 51.5:48.5) was placed in a 25 ml Teflon vessel, 0.5 mmol of Te powder with 3 ml hydrazine (as reducing agent) were put into the Teflon vessel. Then the Teflon vessel was oscillated in ultrasonic water bath at
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3. Results and discussion
Fig. 1. XRD spectra of the PbTe nanorods synthesized under different conditions.
room temperature. When the color of the liquid in the vessel became purplish red, 0.5 mmol Pb(NO3)2 was added in it. Then the vessel was sealed and put into a furnace preheated to 170–220 8C. After reacting for 12–48 h, the vessel was taken out and cooled naturally down to room temperature. The black solid product obtained was washed using deionized water for several times, and diluted HCl (pH 3.0) to remove possible hydroxides adsorbed on the product. The clean samples were obtained after being washed twice with deionized water again. The morphology and size of the synthesized samples are characterized at 20 kV by a field-electron scanning electron microscopy (Nova 400 Nano SEM) and at 200 kV by a transmission electron microscopy (TEM, TECNAI20, Philips). Energy dispersive X-ray spectroscopy (EDS) and X-ray diffractometer (XRD) with Cu Ka radiation (l = 1.5418 A´˚ ), in step of 48 with a count time of 1 min were used to investigate the crystal phase and chemical composition. An UV–vis–NIR Spectrophotometer (Hitachi U-4100) was used to examine the different reaction process and measure the optical properties of the products.
In order to investigate the size change under different conditions, experiments were systematically done at temperatures of 170, 200 and 220 8C and with growth time of 12, 24 and 48 h, respectively. Fig. 1 shows the XRD patterns in the 2u range from 238 to 708 of the as-synthesized PbTe samples. All peaks on the XRD spectra are perfectly indexed as the pure face-centered-cubic phase of PbTe with lattice constant a = 6.443 A´˚ , which is consistent with the standard value (a = 6.454 A´˚ ) of bulk face-centered-cubic ¯ PbTe (Fm3m ð2 2 5Þ, JCPDS: 781905). Based on the Scherrer equation, the crystallite size of a sample is inversely proportional to the full-width-half-maximum (FWHM), indicating that a broader peak represents smaller crystallite size. It is noticed that the samples prepared at higher temperatures or for longer time exhibit narrowed peaks, which means the crystallite sizes of the products become large. In addition, the intensity of peak become stronger with the increase of growth time, indicating the crystallization of PbTe nanorods is better. The morphology of the obtained PbTe products is characterized by scanning electron microscopy (SEM). Fig. 2a–d gives the image of PbTe synthesized in different conditions. In these images, the diameter of nanorods are of 50–100 nm, and do not obviously change with varying temperature and time, while the typical lengths of nanorods are of 200–400, 300–500, 400–700 and 500– 1000 nm at 170 8C for 12 h, 200 8C for 12 h, 200 8C for 24 h and 200 8C for 48 h, respectively, as are shown in Fig. 2a–e, indicating that the length of the nanorods increase distinctly with the increase of temperature and time. EDS measurements in the selected area of Fig. 2d show that the elements in the nanorods are Pb and Te only and the atomic ratio of Pb and Te is close to 1:1 (Fig. 2f) (Si represents the peak of substrate). Fig. 3a and b shows typical TEM images of the as-synthesized PbTe nanorods and the diffraction pattern in the selected area of Fig. 3a, which matches the XRD spectra in Fig. 1. In the hydroxide melts, an obvious understanding of the reaction is the disproportionation reaction of the chalcogen occurred in the alkaline condition [27]. That is: 3E þ 6OH ! 2E2 þ EO3 2 þ H2 OðEdenoteschalcogenÞ
Fig. 2. SEM images of PbTe nanorods synthesized at (a) 170 8C for 12 h, (b) 200 8C for 12 h, (c) 200 8C for 24 h and (d) 200 8C for 48 h. (e) Enlarged image of 200 8C for 48 h, (f) EDS spectrum in the white frame of (e) showing the presence of Pb and Te, the Si signal is from the substrate.
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Fig. 3. (a) TEM image and (b) electron diffraction of the PbTe nanorods prepared at 200 8C for 48 h.
Whereas, different chalcogens have different reaction process owing to the increase in metallic property of S, Se and Te. Through the electrode potential analysis, the disproportionation reaction to Te is less likely to occur (E0 2 ¼ 0:57 V, E0Te=Te2 ¼ 1:143 V). TeO3 =Te Parkin and co-workers [28] have carefully studied the dissolution process of the sulfur in liquid ammonia system, as well as the reaction of some sulfide, and demonstrated that there are a series of sulfur–ammonia anions and sulfur polyanions in the liquid sulfur system such as S2N, S3N, S3, S42 and so on. These ions either have strong oxidative property or release the sulfurous sources such as S2. According to the analysis, we believe that there are polymerized anions such as Te4N5, Te42 and Te22 anion in hydrazine alkaline melts, which form the Te or Te–N rings like S8, as polytelluride anions of the form Tex2 (x = 2–6) with rings or chains and a number of cationic polytelluride species exist in basic, polar solvents (e.g. NH3 or ethylenediamine) by electrochemical synthesis [29,30]. To validate the reaction process in hydrazine–KOH/NaOH system, the absorption spectra of the different reaction solutions were investigated. Te powder is put in the NaOH/KOH aqueous solution or hydrazine hydrate solution. And these solutions are oscillated in an ultrasonic water bath for 30 min. The absorption spectra of the solutions are examined by UV–vis–NIR Spectrophotometer (Hitachi U-4100). In Fig. 4, there are great differences in absorption in different solutions. The absorption spectrum of the NaOH/KOH aqueous solutions is shown in the curve (a) of Fig. 4. There are only two weak features centered at about 240 and 370 nm. When Te powder was put in the NaOH/KOH aqueous
Fig. 4. UV–vis absorption spectra of (a) NaOH/KOH aqueous, (b) NaOH/KOH + Te, and (c) NaOH/KOH + N2H4 + Te solutions.
solution, the color of the solution became gradually flat-red, the absorption spectrum in the curve (b) shows there are two peaks centered at 520 and 250 nm, which might correspond to the absorption peak of tellurium polyanions Te22, Te32, Te42 [31,32]. Telluride can also disproportionate in solution (two Te22 anions can each lose an electron and combine to form the Te42 species) [30]. When 3 ml N2H4H2O is put in the vessel in which the Te powder and NaOH/KOH are pre-put, the color of the solution becomes red rapidly and finally deep purplish red under the ultrasonic vibration. The curve (c) in Fig. 4, the peaks centered at 520 nm and from 260 to 330 nm may be the absorption peaks of the tellurium polyanions and tellurium–nitrogen anions, and the peak intensity in NaOH/KOH hydrazine solutions is stronger than that in NaOH/KOH aqueous solutions. So, the forming process of PbTe can be inferred as follows: under attack of nucleophilic hydrazine, tellurium or tellurium–nitrogen rings have been opened, forming Te chain. And Te–Te bond is broken as the reaction continues, which leads to the degradation of Te chain and constant release of the active tellurium source Te2 [33]. At the same time, dissolution of Pb(NO3)2 in hydrazine hydrate melts results in the formation of the metal(II) amides [29], Pb(NH2)2 or Pb(NH3)2+, metal(II) amides rupture and release the cation Pb2+ with the increase of temperature and reaction time. Subsequently, Pb2+ and Te2 in the hydroxide melts fall together to form the initial nuclei of PbTe. The factors affecting the crystal growth involve kinetics and crystallography. It has been concluded by Murphy [34] that the selected absorption of molecules and ions in solution to different crystal faces directs the growth of nanoparticles into various shapes by controlling the growth rates along different crystal axes. Wang [35] suggested that the shape of a fcc nanocrystal is to be determined mainly by the ratio of the growth rate in h1 0 0i to that in h1 1 1i. Then, with respect to the structure of fcc PbTe, we speculate that these structures are formed in an analogous process. The different rates of adsorption and deadsorption of hydrazine molecules on the different planes of PbTe nuclei would kinetically influence the growth rates of these planes. So, in the hydroxide melts, the hydrazine not only plays a role as reducing agent and the complexing reagent to form a complex, but also control the anisotropic growth of PbTe into nanorod shape. The whole process can be described diagrammatically in Scheme 1. The polymerized anions and metal(II) amides serve to reduce the speed of reaction because the Te rings and Pb(NH2)2 greatly decrease the free Te2 or Pb2+ concentration in the solution. It is well-known that a slow reaction rate is favorable for crystallization and separating the growth step from the nucleation step [36]. It is also noted that the mixed hydroxide melts and ultrasonic process play an important role in controlling nucleation and growth of PbTe nanorods. The large viscosity of the hydroxide melts also reduces the speed of reaction and avoids agglomeration. This property is an important factor in obtaining well-dispersed nanostructures from the reaction without using a surface-capping material. Besides, in the ultrasonic process, as acoustic cavitation provides a unique interaction of energy, the element telluriums are fully deoxidized to polymerized anions by hydrazine (N2H4) and disperse uniformly in alkaline hydrazine, which is also very important in the formation of uniform nanorods. We have done experiments without the ultrasonic process for synthesizing PbTe products, and nanocrystals of irregular shapes such as rods, particles and flakes, as well as the impurities such as element Te or Pb have been obtained in the products. And the sizes of the products are not uniform, range from a few nanometers to a few micrometers. Possibly, the Te cannot be deoxidized simultaneously to polymerized tellurium anions by hydrazine only in the hydroxide melts, so the released Te2 ions start to connect with Pb2+ ions locally to grow into variform and varisize nanocrystals. The ultrasonic process enables Te to react fully with N2H4 and
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Scheme 1. Illustration of the formation process of PbTe nanorods.
separation of the growth step from the nucleation step. So the PbTe nuclei grow along a specific direction to form the nanorods. The band gap of as-synthesized PbTe nanorods is about 0.326 eV, which is obtained from the optical diffuse reflectance spectra can analysis. We believe that these cubic phase PbTe nanorods are promising building units for future TE nanodevices, and nanopolycrystalline materials. Acknowledgments This work has been funded by the NSFC (20741006), The Science and Technology Research Project of Chongqing Municipal Education Commission of China (KJ080819). Thanks to Prof. Xia Lin of Chongqing University for the English language editing. References
Fig. 5. Optical diffuse reflectance spectra (UV–vis–NIR) of the PbTe nanorods synthesized at 200 8C 24 h.
polymerized tellurium anions to distribute uniformly, and consequently results in forming the single structure and uniform size of PbTe nanorods in the viscous hydroxide melts. Based on the procedure described in the literature [37], the optical diffuse reflectance spectra can be used to estimate the band gap of the nanostructures. Because the individual particle size is much less than the thickness of the sample layer made from nanoparticles, an ideal diffuse reflectance with constant scattering coefficient could be expected. The Kubelka–Munk function, which is the ratio of the absorption (a) to scattering factors (S), is used to plot the absorbance. Experimental results for the absorption coefficients for PbTe nanorods synthesized at 200 8C 24 h are shown in Fig. 5, in which the K–M function (a/S) is plotted against photon energy (hn). The optical band gap is obtained by extrapolation to zero of the linear part of the curve, which is about 0.326 eV. Uniform crystalline PbTe nanorods with diameter of about 50–100 nm, close to its average Bohr exciton radius of 46 nm, show a certain quantum confinement effect. The blue shift of the nanorods compared to bulk PbTe (Eg = 0.31 eV) can be observed. 4. Conclusions
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
We have successfully achieved the synthesis of highly pure PbTe nanorods with the diameter of 50–100 nm and length of hundreds of nanometers via a novel composite-hydroxidemediated approach at temperature of 170–200 8C. It is strongly suggested that the polymerized tellurium anions and metal(II) amides (Pb(NH2)2) in the hydrazine hydroxide melts control the growth process of the PbTe nanorods. The Te rings and Pb(NH2)2 greatly decrease the concentration of free ions in the solution through slowly releasing the Te2 or Pb2+ ions, and reduce the speed of reaction, and consequently benefit to crystallization and
[27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
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