- Email: [email protected]
LnxPb1−xTe(Ln:Nd3+,Yb3+) nanomaterials: Synthesis, characterization, physical properties, and optical properties
Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
LnxPb1 xTe(Ln:Nd3+,Yb3+) nanomaterials: Synthesis, characterization, physical properties, and optical properties Younes Hanifehpour a,*, Nazanin Hamnabard a, Anita Veettikkunnu Chandran a, Sang Woo Joo a,*, Bong-ki Min b a b
School of Mechanical Engineering, WCU Nano Research Center, Yeungnam University, Gyeongsan 712-749, Republic of Korea Center for Research Facilities, Yeungnam University, Gyeongsan 712-749, Republic of Korea
A R T I C L E I N F O
Article history: Received 29 July 2014 Received in revised form 16 September 2014 Accepted 27 September 2014 Available online 5 October 2014 Keywords: PbTe Nanomaterial Blue shift Hydrothermal Electrical conductivity
A B S T R A C T
LnxPb1 xTe(Ln:Nd3+,Yb3+) nanomaterials were synthesized using the facile hydrothermal method. The prepared samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). XRD analysis demonstrated that PbTe phase is cubic. By increasing the concentration of Ln3+ ions, the DRS spectra of PbTe shows blue shifts instead of red shifts due to bonding changes. XPS analysis of doped PbTe samples confirms the incorporation of Yb and Nd into the lattice. The electrical conductivity of Lndoped PbTe is shown to be higher than that of pure PbTe, and increases with temperature. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Thermoelectric (TE) materials are attracting increasing attention in the fields of energy harvesting and heat-toelectricity energy conversion [1]. Much attention has been paid to the IV–VI lead chalcogenide for applications in solar cells, TE devices, telecommunications, field effect transistors (FETs), photodetectors, and photovoltaics. The lead chalcogenide, compared to many traditional II–VI and III–V materials, has smaller band gaps and a larger Bohr radius [2]. PbTe, as a member of the lead chalcogenide family, is a preferred thermoelectric material, and was one of the first materials studied by Ioffe and his colleagues in the middle of the last century [3]. This compound has attracted significant attention due to its small band gap (0.31 eV at 300 K), high dielectric constant, high mobility and face-centered cubic structure (FCC). Compared to other semiconductor materials, the quantum size effect can be observed in large structures as a result of the large exciton Bohr radius of PbTe (46 nm) [4–6].
* Corresponding authors. Tel.: +82 538101456. E-mail addresses: [email protected], [email protected] (Y. Hanifehpour), [email protected] (S.W. Joo).
The optical properties and thermoelectric efficiency of PbTe depend, in order, on the deep defect electronic states in the neighborhood of the band gap, and on larger values of the dimensionless figure of merit ZT [7]. ZT is given by ZT = S2T/rk, where S, r, T, and k are the Seebeck coefficient, electrical resistivity, absolute temperature, and thermal conductivity, respectively [8– 10]. Therefore, it is necessary to maximize the value of ZT in order to large Seebeck coefficient, low electrical resistivity, and low thermal conductivity of the PbTe material [11]. Rare earth-substituted nanomaterials with various compositions have become increasingly vital in diverse areas such as novel photocatalysts, luminescent devices, light-emitting displays, biological labeling, and imaging [12–18]. This is due to the introduction of dopant levels within the bandgap and modification of the band structure. Various investigations have been conducted of PbTe doped with a lanthanide with the goal of enhancing the thermoelectric and optical properties [19–21]. There is no documentation in the literature of doping PbTe with ytterbium (Yb) and neodymium (Nd) as lanthanide ions. The incorporation of large electropositive ions such as lanthanides atoms (i.e., Yb and Nd) into PbTe could be considered, in order to affect its electronic, electrical, and physical properties. In the present work, we considered the preparation, and the structural, electrical, and optical properties of Yb+3 and Nd+3 co-doped with lead tellouride using a co-reduction method at a hydrothermal condition.
http://dx.doi.org/10.1016/j.jiec.2014.09.038 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
252
200
200
220 222 311
331
422
220 222 311
111
X=0.1
Intensity
111
420
400
400
420 331
422
X=0.08 Intensity
X=0.08 X=0.06
X=0.06
X=0.04
X=0.04
X=0.02
X=0.02
X=0 20
30
40
50 2θ/Degree
60
X=0.1
70
X=0 80
20
30
40
50 2θ/degree
60
70
80
Fig. 1. XRD pattern of prepared PbTe and Nd-doped PbTe nanoparticles. Fig. 2. XRD pattern of as-prepared PbTe and Yb-doped PbTe nanoparticles.
Experimental details Materials All chemicals used in this study were of analytical grade and were used without further purification. Lead (II) nitrate (Pb(NO3)2
99%) was purchased from Kanto Chemical Company (Japan). Sodium hydroxide (NaOH 98%); sodium tellurite (Na2TeO3 100 mesh 99%); ytterbium (III) acetate hydrate (Yb(C2H3O2)3xH2O H2O 99.99%); Nd(CH3CO2)3xH2O Neodymium (III) acetate hydrate; and sodium borohydride (NaBH4 98%) were purchased from Sigma–Aldrich.
Fig. 3. (a) Structure of rock salt-type NaCl of the host lattice PbTe. For example, Nd or Yb is presented as a substitution impurity ion into the cation sublattice.
Fig. 4. SEM images of PbTe at two different magnifications.
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
Synthesis of PbTe, Yb, and Nd-doped PbTe samples PbTe, Yb, and Nd-doped PbTe nanoparticles were prepared with variable Yb and Nd mole fractions (0–10 mol%). In a typical experiment, stoichiometric Na2TeO3 powders (1 mmol), Pb(NO3)2 (1 mmol), Nd(CH3CO2)3 (1 mmol), Yb(CH3CO2)3 (1 mmol), and
253
NaOH (0.1 g) were added to 30 mL of distilled water in a beaker, and the mixture was dispersed to form a deposition by constant stirring. Subsequently, 0.5 g of NaBH4 was placed in a beaker. The color of the solution changed to purple/black after being stirred for 10 min. The mixture was transferred into a 50 mL stainless Teflonlined autoclave, maintained at 240 8C for 24 h, and then cooled to
Fig. 5. SEM images of (a) Nd0.04Pb0.98Te, (b) Nd0.1Pb0.9Te, (c) Yb.04Pb0.98Te, and (d) Yb0.1Pb0.9Te nanoparticles synthesized at 180 8C for 24 h.
Fig. 6. TEM images of (a), (b) PbTe, (c) Yb0.06Pb0.94Te, and (d) Nd0.06Pb0.94Te.
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
254
room temperature naturally. The final product was washed with distilled water and absolute alcohol several times to remove residual impurities, and then dried at 40 8C for 24 h. The final black powders were thus obtained. 1.1. Characterization instruments The X-ray diffraction (XRD) diffraction patterns of the obtained products were recorded on a diffractometer (D8 Advance, Bruker, Germany) with monochromatic high intensity Cu Ka radiation (l = 1.5406 A˚), an accelerating voltage of 40 kV, and an emission current of 30 mA. A scanning electron microscope (SEM) (S-4200,
Hitachi, Japan) was used to observe the surface state and morphology of the prepared nanoparticles. An high resolution transmission electron microscope (HRTEM) image and selected area electron diffraction (SAED) pattern were recorded using Cs-corrected high resolution transmission electron microscopy (TEM) (JEM-2200FS, JEOL, Japan) operated at 200 kV. The TEM sample was prepared by placing a drop of ethanol suspension of samples (PbTe and doped PbTe), dispersed with an ultrasonic bath, onto a carbon-coated copper grid and allowing it to dry. The absorption spectra were recorded with a UV–vis spectrophotometer (Varian Cary 3 Bio, Australia). Chemical compositions and the chemical states of the samples were
Fig. 7. Particle size histograms of (a) PbTe, (b) Nd0.04Pb0.98Te, and (c) Yb0.04Pb0.98Te nanoparticles.
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
255
Fig. 8. UV/vis absorption measurements for (a) Nd-doped PbTe. (b) Yb-doped PbTe with different molar concentrations of dopant.
analyzed using X-ray photoelectron spectroscopy (XPS) using a spectrometer (K-ALPHA, Thermo Scientific, U.K.). The four-point probe method was used to measure the electrical and thermoelectrical resistivity of the samples. A small oven was used to control the variation of temperature of the samples from room temperature to about 200 8C (maximum). A small chip (1 mm in thickness and 7 mm in length) was used for this analysis.
2. Results and discussion Figs. 1 and 2 show the XRD patterns of pure PbTe, Nd1 xPbxTe, and Yb1 xPbxTe nanoparticles. The strong and sharp peaks suggest that the samples are well crystallized. All the diffraction peaks can be assigned to the cubic phase of PbTe (JCPDF: 78-1905) as shown in Fig. 3 [22]. No peaks corresponding to other impurities of the Yb2O3 or Nd2O3 phase were observed in the pattern, indicating that the Yb and Nd were completely doped into the PbTe crystal lattice. From the X-ray diagram, we note that there was no change in the peak positions for the doped compounds. This shows that the pure and doped-PbTe systems have the same crystal symmetry. Therefore, all these samples show the similar basic XRD pattern. Fig. 4 shows SEM images of PbTe at two different magnifications using the hythrothermal method, and indicates that the sample is composed of homogenous nanocubes. Many nanocuboids were easily found. The mean size of the nanocuboids is 50 nm. Fig. 5 shows SEM images of the Yb-doped and Nd-doped PbTe samples, respectively. The figure shows uniform nanometer-scale particles with good size distribution in all cases. The assynthesized product exhibits cubic morphology with a narrow size distribution of 20–60 nm. The SEM images confirm that the doping of Yb and Nd into the structure of PbTe did not change the morphology of the PbTe nanoparticles (see supplementary data).
The SEM images of doped samples indicate that the samples are homogeneous with Yb or Nd dopants substituting for Pb sites in the PbTe compound. The samples do not contain any other dopant dominating phases. The shapes and sizes of the undoped and Nd and Yb-doped PbTe nanoparticles (1%) were analyzed using the TEM images shown in Fig. 6. The pure and Nd and Yb-doped PbTe nanoparticles both appeared similar sphere with good crystallinity confirming SEM results, respectively. The particles are irregular in shape, but are agglomerated in the case of the PbTe (Fig. 6(a)), and Yb, and Nd-doped PbTe samples (Figs. 6(c) and (d), respectively). For further investigation, size distribution histograms of these nano-particles were prepared using the manual microstructure distance measurement program. Fig. 7(a)–(c) shows the distribution size histograms of as-prepared samples. In all four cases, the particles are uniform and have a narrow range of distribution. The UV–vis diffuse reflectance spectra were used to evaluate photophysical properties of the as-synthesized nanoparticles. The absorption of light by the semiconductor nanoparticles depends on the gap between the conduction and valence band, and the impurity concentration in the host lattice [23]. The DRS spectra of
Table 1 Absorption edges for LnxPb1 xTe compounds. Material
Eg (eV)
PbTe Nd0.02Pb0.98Te Nd0.06Pb0.094Te Nd0.1Pb0.9Te Yb0.02Pb0.98Te Yb0.06Pb0.94Te Yb0.1Pb0.9Te
1.621 1.643 1.677 1.727 1.646 1.668 1.700
256
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
pure PbTe, Nd-doped PbTe, and Yb-doped PbTe are illustrated in Fig. 8(a) and (b). The reflectance characteristics of the doped PbTe sample were quite similar to that of the undoped sample. We note that all
samples showed a strong photoabsorption in the visible light range. There is a blue shift in the absorbance spectra of Ln-doped PbTe in comparison to PbTe, which was not expected for doped materials. The electronic properties of lead telluride could be
Fig. 9. XPS spectra of (a,b) Nd-doped and (c,d)Yb-doped PbTe compound.
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
257
of the UV–vis diffuse reflectance spectrum. Band gap energy is presented in Table 1. The band gap shows an increase with increased dopant concentration from 0.02 to 0.1 M. XPS analysis of 1% Nd-doped and Yb-doped PbTe nanoparticles was carried out to confirm the incorporation of Nd and Yb into the PbTe lattice, and to identify the oxidation state of Nd and Yb in the PbTe lattice. The XPS spectra of 1% Nd-doped and Yb-doped PbTe nanoparticles are shown in Fig. 9(a) and (b), respectively. The energy scale was calibrated with the C 1s peak of the carbon contamination at 286.22 eV. The peak near 540 eV is due to the specific species such as carbonate, adsorbed O2, and H2O [25]. Fig. 9(b) and (d) shows that weak peaks located at 982.59 and 189.52 eV, respectively, exist in the XPS spectrum of Ln-doped PbTe nanoparticles attributed to Nd3+ and Yb3+, respectively. The four probe method was used to measure the electrical and thermoelectrical resistivities of the samples. Fig. 10(a) shows the electrical resistivity of Yb and Nd-doped PbTe nanomaterials. The electrical resistivity measured at room temperature for pure PbTe was on the order of 0.09 10 2 Vm. The minimum value of electrical resistivity for Yb3+-doped compounds is 0.009 10 2 Vm and 0.007 10 2 Vm for Nd3+-doped PbTe. Fig. 10(b) shows the temperature dependence of the electrical resistivity for Ln-doped PbTe between 290 and 340 K, wherein the electrical resistivity decreases with temperature. The minimum value of electrical resistivity for Yb0.1Pb0.9Te is 0.001 10 2 Vm. In the case of Nd0.1Pb0.9Te, the minimum value of electrical resistivity is 0.0007 10 2 Vm. As a result, the electrical conductivity of Ln-doped PbTe materials is higher than undoped PbTe at room temperature, and increases with temperature. 3. Conclusion Fig. 10. (a) Electrical and (b) thermoelectrical resistivity of LnxPb1 xTe compounds.
affected by the doping of lanthanide ions into a Pb–Te framework. Introducing lanthanide cations into a PbTe lattice changes the Pb–Te bond and increases the band gap. From Fig. 8, we note that as the impurity concentration increases, there is a shift in the absorption edge toward higher energy. The increase in the band gap with carrier concentration can be explained on the basis of the Burstein–Moss effect. As the doping level increases, electrons from the valence band must transit to electron states higher than those occupied by donor electrons near the conduction band edge. This results in optical band gap widening and is referred to as the Burstein–Moss band filling effect [24], which means that a higher energy photon is required to produce the same amount of absorption in order to move the absorption edge to higher energies. When a semiconductor nanoparticle is photo-excited, large numbers of charge carriers are photo-generated, and these photogenerated electrons occupy states at the bottom of the conduction band. After thermal relaxation of these charge carriers to the lower vibrational levels in the conduction band, there is a considerable population in the lower vibrational levels of the conduction band; i.e., the lower vibrational levels are well populated. The photo excitation of such a particle requires Eg + DEb energy, wherein DEb is the excess energy needed to transfer the electrons to the unoccupied electronic sublevels closest to the bottom of the conduction band. This shift in DEb is called the Burstein–Moss shift [23]. The blue shift in the absorption edge also supports the fact that Lanthanide cations act as donors in the PbTe host. The energy of the band gap of PbTe and doped PbTe nanoparticles can be estimated from the main absorption edges
In summary, a simple and efficient hydrothermal method was applied to the synthesis of cuboid PbTe, Yb, and Nd-doped PbTe as a thermoelectric material by employing Pb(NO3)2, Na2TeO3, NaBH4, NaOH, and H2O as the starting materials. Our results were evaluated using XRD, XPS, SEM, and TEM analyses, which confirm that the doping of Yb and Nd ions into the PbTe structure changed neither the crystal nature nor the morphology of PbTe nanoparticles. We have shown a series of Yb and Nd-doped PbTe samples which to estimate that a gap increase of up to 0.10 eV occurs at the highest doping levels appropriate to thermoelectrics. The electrical conductance of Ln-doped PbTe is higher than that of pure PbTe, and increases with temperature. Acknowledgments This work is funded by grant no. 2011-0014246 of the National Research Foundation of South Korea. The authors gratefully acknowledge this support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jiec.2014.09.038.
References [1] S.H. Ahmadian Fard-Fin, M. Salavati-Niasari, F. Mohandes, Mater. Res. Bull. 48 (2013) 4332. [2] Z. Lina, M. Wang, L. Wei, X. Songa, Y. Xuea, X. Yao, J. Alloys Compd. 509 (2011) 5047. [3] S. Ahmad, S.D. Mahanti, K. Hoang, M.G. Kanatzidis, Phys. Rev. B 74 (2005) 155205. [4] C. Wang, G. Zhang, S. Fan, Y. Li, J. Phys. Chem. Solids 62 (2001) 1957. [5] D. Lukovic´, P.M. Nikolic´, S. Vujatovic´, S. Savic´, D. Urosˇevic´, Sci. Sinter. 39 (2007) 161.
258
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
[6] Y. Pan, H. Bai, L. Pan, M.C. Tamargo, M. Sohel, J.R. Lombardi, J.Mater.Chem., 22 (2012) 23593–23601. [7] Z. Zong, H. Wang, L. Kong, Nanoscale Res. Lett. 8 (2013) 191. [8] M.V. Nikolic, K.M. Paraskevopoulos, T. Ivetic, T.T. Zorba, S.S. Vujatovic, E. Pavlidou, et al. J. Mater. Sci. 45 (2010) 5910. [9] L. Xu, Y. Zheng, J.C. Zheng, Phys. Rev. B 82 (2010) 195102. [10] A.I. Artamkin, A.E. Kozhanov, M. Arciszewska, W.D. Dobrowolski, T. Story, E.I. Slynko, et al. Acta Phys. Pol. A 106 (2004) 223. [11] G.J. Snyder, E.S. Toberer, J. Nat. Mater. 7 (2008) 105. [12] Y. Hanifehpour, S.W. Joo, B.K. Min, Nanoscale Res. Lett. 8 (2013) 141. [13] A. Alemi, A. Klein, G. Meyer, M. Dolatyari, A. Babalou, Z. Anorg. Allg. Chem. 637 (2011) 87. [14] A. Alemi, Y. Hanifehpour, S.W. Joo, B. Min, A. Colloids Surfaces, Physicochem. Eng. Asp. 390 (2011) 142. [15] A. Alemi, Y. Hanifehpour, S.W. Joo, A. Khandar, A. Morsali, B. Min, Physica B 407 (2012) 38.
[16] A. Khataee, A. Khataee, M. Fathinia, Y. Hanifehpour, S.W. Joo, Ind. Eng. Chem. Res. 52 (2013) 13357. [17] A. Khataee, S. Fathinia, M. Fathinia, Y. Hanifehpour, S.W. Joo, B. Soltani, Curr. Nanosci. 9 (2013) 780. [18] A. Khataee, Y. Hanifehpour, M. Safarpour, M. Hosseini, Sci. Adv. Mater. 5 (2013) 1. [19] N. Haldolaarachchige, A.B. Karki1, W. Adam Phelan, Y.M. Xiong, R. Jin, J.Y. Chan, et al. J. Appl. Phys. 109 (2011) 103712. [20] G.A. Akhmedova, D.Sh. Abdinov, J. Inorg. Mater. 45 (2009) 854. [21] Z. Dashevsky, S. Shusterman, M.P. Dariel, I.J. Drabkin, Appl. Phys. 92 (2002) 1424. [22] Z.K. Zlatanov, Mater. Chem. Phys. 103 (2007) 470. [23] K. Nouneh, K.J. Plucinski, M. Bakasse, I.V. Kity, J. Mater. Sci. 42 (2007) 6847. [24] G. Sharma, P. Chawla, S.P. Lochab, N. Singh, Bull. Mater. Sci. 34 (2011) 673. [25] D.T. Kim, K.S. Yu, W.T. Kim, C.D. Kim, H.L. Park, J. Mater. Sci. Lett. 11 (1992) 886.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
LnxPb1 xTe(Ln:Nd3+,Yb3+) nanomaterials: Synthesis, characterization, physical properties, and optical properties Younes Hanifehpour a,*, Nazanin Hamnabard a, Anita Veettikkunnu Chandran a, Sang Woo Joo a,*, Bong-ki Min b a b
School of Mechanical Engineering, WCU Nano Research Center, Yeungnam University, Gyeongsan 712-749, Republic of Korea Center for Research Facilities, Yeungnam University, Gyeongsan 712-749, Republic of Korea
A R T I C L E I N F O
Article history: Received 29 July 2014 Received in revised form 16 September 2014 Accepted 27 September 2014 Available online 5 October 2014 Keywords: PbTe Nanomaterial Blue shift Hydrothermal Electrical conductivity
A B S T R A C T
LnxPb1 xTe(Ln:Nd3+,Yb3+) nanomaterials were synthesized using the facile hydrothermal method. The prepared samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). XRD analysis demonstrated that PbTe phase is cubic. By increasing the concentration of Ln3+ ions, the DRS spectra of PbTe shows blue shifts instead of red shifts due to bonding changes. XPS analysis of doped PbTe samples confirms the incorporation of Yb and Nd into the lattice. The electrical conductivity of Lndoped PbTe is shown to be higher than that of pure PbTe, and increases with temperature. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Thermoelectric (TE) materials are attracting increasing attention in the fields of energy harvesting and heat-toelectricity energy conversion [1]. Much attention has been paid to the IV–VI lead chalcogenide for applications in solar cells, TE devices, telecommunications, field effect transistors (FETs), photodetectors, and photovoltaics. The lead chalcogenide, compared to many traditional II–VI and III–V materials, has smaller band gaps and a larger Bohr radius [2]. PbTe, as a member of the lead chalcogenide family, is a preferred thermoelectric material, and was one of the first materials studied by Ioffe and his colleagues in the middle of the last century [3]. This compound has attracted significant attention due to its small band gap (0.31 eV at 300 K), high dielectric constant, high mobility and face-centered cubic structure (FCC). Compared to other semiconductor materials, the quantum size effect can be observed in large structures as a result of the large exciton Bohr radius of PbTe (46 nm) [4–6].
* Corresponding authors. Tel.: +82 538101456. E-mail addresses: [email protected], [email protected] (Y. Hanifehpour), [email protected] (S.W. Joo).
The optical properties and thermoelectric efficiency of PbTe depend, in order, on the deep defect electronic states in the neighborhood of the band gap, and on larger values of the dimensionless figure of merit ZT [7]. ZT is given by ZT = S2T/rk, where S, r, T, and k are the Seebeck coefficient, electrical resistivity, absolute temperature, and thermal conductivity, respectively [8– 10]. Therefore, it is necessary to maximize the value of ZT in order to large Seebeck coefficient, low electrical resistivity, and low thermal conductivity of the PbTe material [11]. Rare earth-substituted nanomaterials with various compositions have become increasingly vital in diverse areas such as novel photocatalysts, luminescent devices, light-emitting displays, biological labeling, and imaging [12–18]. This is due to the introduction of dopant levels within the bandgap and modification of the band structure. Various investigations have been conducted of PbTe doped with a lanthanide with the goal of enhancing the thermoelectric and optical properties [19–21]. There is no documentation in the literature of doping PbTe with ytterbium (Yb) and neodymium (Nd) as lanthanide ions. The incorporation of large electropositive ions such as lanthanides atoms (i.e., Yb and Nd) into PbTe could be considered, in order to affect its electronic, electrical, and physical properties. In the present work, we considered the preparation, and the structural, electrical, and optical properties of Yb+3 and Nd+3 co-doped with lead tellouride using a co-reduction method at a hydrothermal condition.
http://dx.doi.org/10.1016/j.jiec.2014.09.038 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
252
200
200
220 222 311
331
422
220 222 311
111
X=0.1
Intensity
111
420
400
400
420 331
422
X=0.08 Intensity
X=0.08 X=0.06
X=0.06
X=0.04
X=0.04
X=0.02
X=0.02
X=0 20
30
40
50 2θ/Degree
60
X=0.1
70
X=0 80
20
30
40
50 2θ/degree
60
70
80
Fig. 1. XRD pattern of prepared PbTe and Nd-doped PbTe nanoparticles. Fig. 2. XRD pattern of as-prepared PbTe and Yb-doped PbTe nanoparticles.
Experimental details Materials All chemicals used in this study were of analytical grade and were used without further purification. Lead (II) nitrate (Pb(NO3)2
99%) was purchased from Kanto Chemical Company (Japan). Sodium hydroxide (NaOH 98%); sodium tellurite (Na2TeO3 100 mesh 99%); ytterbium (III) acetate hydrate (Yb(C2H3O2)3xH2O H2O 99.99%); Nd(CH3CO2)3xH2O Neodymium (III) acetate hydrate; and sodium borohydride (NaBH4 98%) were purchased from Sigma–Aldrich.
Fig. 3. (a) Structure of rock salt-type NaCl of the host lattice PbTe. For example, Nd or Yb is presented as a substitution impurity ion into the cation sublattice.
Fig. 4. SEM images of PbTe at two different magnifications.
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
Synthesis of PbTe, Yb, and Nd-doped PbTe samples PbTe, Yb, and Nd-doped PbTe nanoparticles were prepared with variable Yb and Nd mole fractions (0–10 mol%). In a typical experiment, stoichiometric Na2TeO3 powders (1 mmol), Pb(NO3)2 (1 mmol), Nd(CH3CO2)3 (1 mmol), Yb(CH3CO2)3 (1 mmol), and
253
NaOH (0.1 g) were added to 30 mL of distilled water in a beaker, and the mixture was dispersed to form a deposition by constant stirring. Subsequently, 0.5 g of NaBH4 was placed in a beaker. The color of the solution changed to purple/black after being stirred for 10 min. The mixture was transferred into a 50 mL stainless Teflonlined autoclave, maintained at 240 8C for 24 h, and then cooled to
Fig. 5. SEM images of (a) Nd0.04Pb0.98Te, (b) Nd0.1Pb0.9Te, (c) Yb.04Pb0.98Te, and (d) Yb0.1Pb0.9Te nanoparticles synthesized at 180 8C for 24 h.
Fig. 6. TEM images of (a), (b) PbTe, (c) Yb0.06Pb0.94Te, and (d) Nd0.06Pb0.94Te.
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
254
room temperature naturally. The final product was washed with distilled water and absolute alcohol several times to remove residual impurities, and then dried at 40 8C for 24 h. The final black powders were thus obtained. 1.1. Characterization instruments The X-ray diffraction (XRD) diffraction patterns of the obtained products were recorded on a diffractometer (D8 Advance, Bruker, Germany) with monochromatic high intensity Cu Ka radiation (l = 1.5406 A˚), an accelerating voltage of 40 kV, and an emission current of 30 mA. A scanning electron microscope (SEM) (S-4200,
Hitachi, Japan) was used to observe the surface state and morphology of the prepared nanoparticles. An high resolution transmission electron microscope (HRTEM) image and selected area electron diffraction (SAED) pattern were recorded using Cs-corrected high resolution transmission electron microscopy (TEM) (JEM-2200FS, JEOL, Japan) operated at 200 kV. The TEM sample was prepared by placing a drop of ethanol suspension of samples (PbTe and doped PbTe), dispersed with an ultrasonic bath, onto a carbon-coated copper grid and allowing it to dry. The absorption spectra were recorded with a UV–vis spectrophotometer (Varian Cary 3 Bio, Australia). Chemical compositions and the chemical states of the samples were
Fig. 7. Particle size histograms of (a) PbTe, (b) Nd0.04Pb0.98Te, and (c) Yb0.04Pb0.98Te nanoparticles.
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
255
Fig. 8. UV/vis absorption measurements for (a) Nd-doped PbTe. (b) Yb-doped PbTe with different molar concentrations of dopant.
analyzed using X-ray photoelectron spectroscopy (XPS) using a spectrometer (K-ALPHA, Thermo Scientific, U.K.). The four-point probe method was used to measure the electrical and thermoelectrical resistivity of the samples. A small oven was used to control the variation of temperature of the samples from room temperature to about 200 8C (maximum). A small chip (1 mm in thickness and 7 mm in length) was used for this analysis.
2. Results and discussion Figs. 1 and 2 show the XRD patterns of pure PbTe, Nd1 xPbxTe, and Yb1 xPbxTe nanoparticles. The strong and sharp peaks suggest that the samples are well crystallized. All the diffraction peaks can be assigned to the cubic phase of PbTe (JCPDF: 78-1905) as shown in Fig. 3 [22]. No peaks corresponding to other impurities of the Yb2O3 or Nd2O3 phase were observed in the pattern, indicating that the Yb and Nd were completely doped into the PbTe crystal lattice. From the X-ray diagram, we note that there was no change in the peak positions for the doped compounds. This shows that the pure and doped-PbTe systems have the same crystal symmetry. Therefore, all these samples show the similar basic XRD pattern. Fig. 4 shows SEM images of PbTe at two different magnifications using the hythrothermal method, and indicates that the sample is composed of homogenous nanocubes. Many nanocuboids were easily found. The mean size of the nanocuboids is 50 nm. Fig. 5 shows SEM images of the Yb-doped and Nd-doped PbTe samples, respectively. The figure shows uniform nanometer-scale particles with good size distribution in all cases. The assynthesized product exhibits cubic morphology with a narrow size distribution of 20–60 nm. The SEM images confirm that the doping of Yb and Nd into the structure of PbTe did not change the morphology of the PbTe nanoparticles (see supplementary data).
The SEM images of doped samples indicate that the samples are homogeneous with Yb or Nd dopants substituting for Pb sites in the PbTe compound. The samples do not contain any other dopant dominating phases. The shapes and sizes of the undoped and Nd and Yb-doped PbTe nanoparticles (1%) were analyzed using the TEM images shown in Fig. 6. The pure and Nd and Yb-doped PbTe nanoparticles both appeared similar sphere with good crystallinity confirming SEM results, respectively. The particles are irregular in shape, but are agglomerated in the case of the PbTe (Fig. 6(a)), and Yb, and Nd-doped PbTe samples (Figs. 6(c) and (d), respectively). For further investigation, size distribution histograms of these nano-particles were prepared using the manual microstructure distance measurement program. Fig. 7(a)–(c) shows the distribution size histograms of as-prepared samples. In all four cases, the particles are uniform and have a narrow range of distribution. The UV–vis diffuse reflectance spectra were used to evaluate photophysical properties of the as-synthesized nanoparticles. The absorption of light by the semiconductor nanoparticles depends on the gap between the conduction and valence band, and the impurity concentration in the host lattice [23]. The DRS spectra of
Table 1 Absorption edges for LnxPb1 xTe compounds. Material
Eg (eV)
PbTe Nd0.02Pb0.98Te Nd0.06Pb0.094Te Nd0.1Pb0.9Te Yb0.02Pb0.98Te Yb0.06Pb0.94Te Yb0.1Pb0.9Te
1.621 1.643 1.677 1.727 1.646 1.668 1.700
256
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
pure PbTe, Nd-doped PbTe, and Yb-doped PbTe are illustrated in Fig. 8(a) and (b). The reflectance characteristics of the doped PbTe sample were quite similar to that of the undoped sample. We note that all
samples showed a strong photoabsorption in the visible light range. There is a blue shift in the absorbance spectra of Ln-doped PbTe in comparison to PbTe, which was not expected for doped materials. The electronic properties of lead telluride could be
Fig. 9. XPS spectra of (a,b) Nd-doped and (c,d)Yb-doped PbTe compound.
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
257
of the UV–vis diffuse reflectance spectrum. Band gap energy is presented in Table 1. The band gap shows an increase with increased dopant concentration from 0.02 to 0.1 M. XPS analysis of 1% Nd-doped and Yb-doped PbTe nanoparticles was carried out to confirm the incorporation of Nd and Yb into the PbTe lattice, and to identify the oxidation state of Nd and Yb in the PbTe lattice. The XPS spectra of 1% Nd-doped and Yb-doped PbTe nanoparticles are shown in Fig. 9(a) and (b), respectively. The energy scale was calibrated with the C 1s peak of the carbon contamination at 286.22 eV. The peak near 540 eV is due to the specific species such as carbonate, adsorbed O2, and H2O [25]. Fig. 9(b) and (d) shows that weak peaks located at 982.59 and 189.52 eV, respectively, exist in the XPS spectrum of Ln-doped PbTe nanoparticles attributed to Nd3+ and Yb3+, respectively. The four probe method was used to measure the electrical and thermoelectrical resistivities of the samples. Fig. 10(a) shows the electrical resistivity of Yb and Nd-doped PbTe nanomaterials. The electrical resistivity measured at room temperature for pure PbTe was on the order of 0.09 10 2 Vm. The minimum value of electrical resistivity for Yb3+-doped compounds is 0.009 10 2 Vm and 0.007 10 2 Vm for Nd3+-doped PbTe. Fig. 10(b) shows the temperature dependence of the electrical resistivity for Ln-doped PbTe between 290 and 340 K, wherein the electrical resistivity decreases with temperature. The minimum value of electrical resistivity for Yb0.1Pb0.9Te is 0.001 10 2 Vm. In the case of Nd0.1Pb0.9Te, the minimum value of electrical resistivity is 0.0007 10 2 Vm. As a result, the electrical conductivity of Ln-doped PbTe materials is higher than undoped PbTe at room temperature, and increases with temperature. 3. Conclusion Fig. 10. (a) Electrical and (b) thermoelectrical resistivity of LnxPb1 xTe compounds.
affected by the doping of lanthanide ions into a Pb–Te framework. Introducing lanthanide cations into a PbTe lattice changes the Pb–Te bond and increases the band gap. From Fig. 8, we note that as the impurity concentration increases, there is a shift in the absorption edge toward higher energy. The increase in the band gap with carrier concentration can be explained on the basis of the Burstein–Moss effect. As the doping level increases, electrons from the valence band must transit to electron states higher than those occupied by donor electrons near the conduction band edge. This results in optical band gap widening and is referred to as the Burstein–Moss band filling effect [24], which means that a higher energy photon is required to produce the same amount of absorption in order to move the absorption edge to higher energies. When a semiconductor nanoparticle is photo-excited, large numbers of charge carriers are photo-generated, and these photogenerated electrons occupy states at the bottom of the conduction band. After thermal relaxation of these charge carriers to the lower vibrational levels in the conduction band, there is a considerable population in the lower vibrational levels of the conduction band; i.e., the lower vibrational levels are well populated. The photo excitation of such a particle requires Eg + DEb energy, wherein DEb is the excess energy needed to transfer the electrons to the unoccupied electronic sublevels closest to the bottom of the conduction band. This shift in DEb is called the Burstein–Moss shift [23]. The blue shift in the absorption edge also supports the fact that Lanthanide cations act as donors in the PbTe host. The energy of the band gap of PbTe and doped PbTe nanoparticles can be estimated from the main absorption edges
In summary, a simple and efficient hydrothermal method was applied to the synthesis of cuboid PbTe, Yb, and Nd-doped PbTe as a thermoelectric material by employing Pb(NO3)2, Na2TeO3, NaBH4, NaOH, and H2O as the starting materials. Our results were evaluated using XRD, XPS, SEM, and TEM analyses, which confirm that the doping of Yb and Nd ions into the PbTe structure changed neither the crystal nature nor the morphology of PbTe nanoparticles. We have shown a series of Yb and Nd-doped PbTe samples which to estimate that a gap increase of up to 0.10 eV occurs at the highest doping levels appropriate to thermoelectrics. The electrical conductance of Ln-doped PbTe is higher than that of pure PbTe, and increases with temperature. Acknowledgments This work is funded by grant no. 2011-0014246 of the National Research Foundation of South Korea. The authors gratefully acknowledge this support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jiec.2014.09.038.
References [1] S.H. Ahmadian Fard-Fin, M. Salavati-Niasari, F. Mohandes, Mater. Res. Bull. 48 (2013) 4332. [2] Z. Lina, M. Wang, L. Wei, X. Songa, Y. Xuea, X. Yao, J. Alloys Compd. 509 (2011) 5047. [3] S. Ahmad, S.D. Mahanti, K. Hoang, M.G. Kanatzidis, Phys. Rev. B 74 (2005) 155205. [4] C. Wang, G. Zhang, S. Fan, Y. Li, J. Phys. Chem. Solids 62 (2001) 1957. [5] D. Lukovic´, P.M. Nikolic´, S. Vujatovic´, S. Savic´, D. Urosˇevic´, Sci. Sinter. 39 (2007) 161.
258
Y. Hanifehpour et al. / Journal of Industrial and Engineering Chemistry 24 (2015) 251–258
[6] Y. Pan, H. Bai, L. Pan, M.C. Tamargo, M. Sohel, J.R. Lombardi, J.Mater.Chem., 22 (2012) 23593–23601. [7] Z. Zong, H. Wang, L. Kong, Nanoscale Res. Lett. 8 (2013) 191. [8] M.V. Nikolic, K.M. Paraskevopoulos, T. Ivetic, T.T. Zorba, S.S. Vujatovic, E. Pavlidou, et al. J. Mater. Sci. 45 (2010) 5910. [9] L. Xu, Y. Zheng, J.C. Zheng, Phys. Rev. B 82 (2010) 195102. [10] A.I. Artamkin, A.E. Kozhanov, M. Arciszewska, W.D. Dobrowolski, T. Story, E.I. Slynko, et al. Acta Phys. Pol. A 106 (2004) 223. [11] G.J. Snyder, E.S. Toberer, J. Nat. Mater. 7 (2008) 105. [12] Y. Hanifehpour, S.W. Joo, B.K. Min, Nanoscale Res. Lett. 8 (2013) 141. [13] A. Alemi, A. Klein, G. Meyer, M. Dolatyari, A. Babalou, Z. Anorg. Allg. Chem. 637 (2011) 87. [14] A. Alemi, Y. Hanifehpour, S.W. Joo, B. Min, A. Colloids Surfaces, Physicochem. Eng. Asp. 390 (2011) 142. [15] A. Alemi, Y. Hanifehpour, S.W. Joo, A. Khandar, A. Morsali, B. Min, Physica B 407 (2012) 38.
[16] A. Khataee, A. Khataee, M. Fathinia, Y. Hanifehpour, S.W. Joo, Ind. Eng. Chem. Res. 52 (2013) 13357. [17] A. Khataee, S. Fathinia, M. Fathinia, Y. Hanifehpour, S.W. Joo, B. Soltani, Curr. Nanosci. 9 (2013) 780. [18] A. Khataee, Y. Hanifehpour, M. Safarpour, M. Hosseini, Sci. Adv. Mater. 5 (2013) 1. [19] N. Haldolaarachchige, A.B. Karki1, W. Adam Phelan, Y.M. Xiong, R. Jin, J.Y. Chan, et al. J. Appl. Phys. 109 (2011) 103712. [20] G.A. Akhmedova, D.Sh. Abdinov, J. Inorg. Mater. 45 (2009) 854. [21] Z. Dashevsky, S. Shusterman, M.P. Dariel, I.J. Drabkin, Appl. Phys. 92 (2002) 1424. [22] Z.K. Zlatanov, Mater. Chem. Phys. 103 (2007) 470. [23] K. Nouneh, K.J. Plucinski, M. Bakasse, I.V. Kity, J. Mater. Sci. 42 (2007) 6847. [24] G. Sharma, P. Chawla, S.P. Lochab, N. Singh, Bull. Mater. Sci. 34 (2011) 673. [25] D.T. Kim, K.S. Yu, W.T. Kim, C.D. Kim, H.L. Park, J. Mater. Sci. Lett. 11 (1992) 886.