# Synthesis of Novel YbxSb2 − xTe3 Hexagonal Nanoplates: Investigation of Their Physical, Structural, and Photocatalytic Properties

Authors: Younes Hanifehpour; Sang Woo Joo
Journal: Journal of Nanomaterials
 (2014)

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## Abstract

Yb-doped Sb2Te3 nanomaterials were synthesized by a coreduction method in hydrothermal condition. Powder X-ray diffraction patterns indicate that the Yb
x
Sb
2
−
x
Te
3 crystals (x
=
0.00
–
0.05) are isostructural with Sb2Te3. The cell parameter a decreases for Yb
x
Sb
2
−
x
Te
3 compounds upon increasing the dopant content (x), while c increases. Scanning electron microscopy and transmission electron microscopy images show that doping of Yb3+ ions in the lattice of Sb2Te3 produces different morphology. The electrical conductivity of Yb-doped Sb2Te3 is higher than the pure Sb2Te3 and increases with temperature. By increasing concentration of the Yb3+ ions, the absorption spectrum of Sb2Te3 shows red shifts and some intensity changes. In addition to the characteristic red emission peaks of Sb2Te3, emission spectra of doped materials show other emission bands originating from f-f transitions of the Yb3+ ions. The photocatalytic performance of as-synthesized nanoparticles was investigated towards the decolorization of Malachite Green solution under visible light irradiation.


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## Body

## 1. Introduction

Antimony telluride (Sb2Te3) based compounds are very promising materials for thermoelectric (TE) applications in solid-state refrigeration and power generation, [1–3] but their extensive application is hindered by their low thermoelectric efficiency. Antimony telluride is a semiconductor with narrow band gap and layered structure. Possessing intrinsically a high figure-of-merit (ZT) because of its large Seebeck coefficient, this compound and its doped derivatives are considered to be the best candidates for near room-temperature TE applications [4–7]. Rare earth ions doped nanomaterials have become an increasingly important research topic and opened up the opportunity for creating new applications in diverse areas, such as light emitting displays, biological labeling, and imaging [8–10]. Investigations of impurity effects or doping agents on the physical properties of Sb2Te3 are attractive both for applied and basic research. Incorporating trivalent cations such as Sb3+ [11], In3+ [12], Fe3+ [13], Mn3+ [14], and some trivalent 3d elements [15] to the lattice of Bi2Se3 has been reported. Also, LnxBi2−xSe3 (Ln: Sm3+, Eu3+, Gd3+, Tb3+, and Nd3+) based nanomaterials were prepared by Alemi et al. [16, 17]. Recently, we have synthesized new luminescent nanomaterials based on doping of lanthanide (Ln: Ho3+, Nd3+, and Lu3+) into the lattice of Sb2S3 and (Ln: Ho3+, Nd3+, Lu3+, Sm3+, Er3+, and Yb3+) into the lattice of Sb2Se3 [18–21]. To the best of our knowledge, there is no study about doping of rare earth cations into the lattice of Sb2Te3. The electronic properties of antimony telluride could be affected by doping of lanthanide ions into a Sb–Te framework. Herein, we report synthesis of YbxSb2−xTe3 nanomaterials by a hydrothermal route. Structural and spectroscopic properties and electrical and thermal conductivity of the as-prepared materials are described. Also, the photocatalytic activity of YbxSb2−xTe3 nanomaterials was investigated towards Malachite Green (as a model organic dye) decolorization under visible light irradiation.


## 2. Experimental

All chemicals were of analytical grade and were used without further purification. Tellurium powder, Sodium Borohydride, SbCl3, Yb2O3, NaOH, and Malachite Green were obtained from Merck. The characteristic of this dye is presented in Table 1. Ethanol (99%). 4H2O were obtained from Aldrich.Table 1
Characteristics of Malachite Green.
Color index name
Chemical structure
Molecular formula
Color index number
λ
max
⁡ (nm)
M
w (g/mol)
C.I. Basic Green-4
C23H25N2HCL
42000
355
365


## 3. Synthesis of Sb2Te3 and Yb-Doped Sb2Te3 Samples

Tellurium powder (0.382 g) and NaOH (0.6 g) were added to distilled water (60 mL) and stirred well for 10 min at room temperature. Afterwards, Sodium Borohydride (4 g), SbCl3, and Yb2O3 with appropriate ratios were added, and the mixture was transferred to a 100 mL Teflon-lined autoclave. The autoclave was sealed, maintained at 180°C for 48 h, and then allowed to cool to room temperature naturally. The as-synthesized YbxSb2−xTe3 nanomaterials were collected and washed with distilled water and absolute ethanol several times in order to remove residual impurities and then dried at room temperature. The final black powders were obtained as a result.


## 4. Characterization Methods

The products yields were 85–95%. X-ray powder diffractometer (XRD D5000 Siemens AG, Munich, Germany) with Cu­K
α radiation was used for phase identification. The morphology of the materials was examined using a JEOL JSM-6700F Scanning Electron Microscope (SEM). A linked ISIS-300, Oxford EDS (energy dispersion spectroscopy) detector was used for elemental analyses. The SAED pattern and HRTEM image were performed by a Cs-corrected high-resolution TEM (JEM-2200FS, JEOL) operated at 200 kV. Photoluminescence measurements were carried out using a Spex FluoroMax-3 spectrometer. The absorption spectra were recorded with UV-Vis spectrophotometer (Varian Cary 3 Bio). The UV-Vis diffuse reflectance spectra were used for evaluation of photophysical properties of as-synthesized material. The electrical and thermoelectrical resistivity of samples was measured by Four Probe Method. An oven was required for the variation of temperature of the samples from the room temperature to about 200°C. Small chip with 1 mm thickness and 7 mm length was used for this analysis. This chip was obtained by pressing of 30 mg of sample under 30 kpa pressing device. Celref program (CCP14, London, UK) and WinXPOW program (STOE & CIE GmbH, Darmstadt, Germany) using a profile fitting procedure were used for calculation of cell parameters from powder XRD patterns and determination of reflections, respectively.


## 5. Photocatalytic Studies

The photocatalytic activity of undoped and YbxSb2−xTe3 nanomaterials was evaluated by the decolorization of Malachite Green (a triphenylmethane dye) in an aqueous solution under visible light. In a typical process, 0.1 g of the photocatalyst powder was added to 100 mL Malachite Green solution with an initial concentration of 5 mg/L. The suspension of photocatalyst and Malachite Green was magnetically stirred in a quartz photoreactor in the dark for 15 mins to establish an adsorption/desorption equilibrium of the dye. Then, the solution was irradiated by a 6 W fluorescent visible lamp (GK-140, China) as the light source. The color removal efficiency (CR (%)) was expressed as the percentage ratio of decolorized dye concentration to that of the initial one. During the photocatalytic process, 5 mL of the suspension was sampled at desired times and after centrifugation, the removal of color was evaluated by determining the absorbance of the solution at λ
max
⁡
=
619 nm by using UV-Vis spectrophotometer, Lightwave S2000 (England).


## 6. Results and Discussion

The lattice parameters were determined via reflections observed in2
θ
=
4
–
70°. An X-ray diffraction (XRD) pattern of the newly obtained Yb-doped Sb2Te3 is shown in Figure 1(a). All peaks can be perfectly indexed to rhombohedral Sb2Te3 (space group: R-3 m) with lattice constants a
=
4.264 Å and c
=
30.458 Å (Joint Committee on Powder Diffraction Standards (JCPDS) card number 15-0874). Additional unknown phases as shown by stars in Figure 1(b) were observed beyond doping levels of x
=
0.05 for Yb3+.Figure 1
Powder X-ray diffraction pattern of (a) Yb0.05Sb1.95Te3 and (b) impure Sb2Te3 synthesized at 180°C and 48 h.The calculation of cell parameters of the as-prepared materials was done from the XRD patterns. By increasing dopant content (x), the a parameter for Yb3+ decreases, while the c parameter increases, as shown in Figure 2. These changes of lattice constants can be attributed to the effective ionic radii of the Yb3+ ions and lattice shifts to various position of dopants or defects site. Figure 3 shows SEM image and EDX of Sb2Te3 nanoplates. The thickness of these plates is around 40–80 nm. The EDX analysis of the product confirms the ratio of Sb/Te to be 2 : 3, as expected. Doping of various Yb3+ concentrations into the structure of Sb2Te3 results in different morphology. At lower Yb3+ composition the morphology is hexagonal nanoplate as seen in Figure 4 in which the thickness of plates is around 40–90 nm and at higher Yb3+ concentration the product is nanoparticles. Figure 4 shows SEM, TEM image, and SAED pattern of Yb0.05Sb1.95Te3 nanoparticles whose diameter is around 20–50 nm. The TEM image and SAED pattern of Yb0.02Sb1.98Te3 confirm the result of SEM and shows crystallinity of product as seen at Figure 5. As expected, the EDX analysis of the product confirms purity and the ratio of Sb/Te/Yb (see Figure 6). The electronic properties of antimony telluride could be affected by doping of lanthanide ions into a Sb–Te framework. Doping of lanthanide cations into Sb2Te3 lattice results in decreasing the Sb–Te covalence bond. Due to different interaction in the doped Sb2Te3 lattice, there are different growth directions in lattice and production of various morphologies. The Four Probe Method was used for the measurement of electrical and thermoelectrical resistivity of samples (Figure 7). Figure 8(a) shows electrical resistivity of Yb-doped Sb2Te3 nanomaterials. The electrical resistivity measured at room temperature for pure Sb2Te3 was of the order of 0.09 Ω·m. The minimum value of electrical resistivity for Yb3+-doped compounds is 0.008 Ωm. Figure 8(b) shows the temperature dependence of the electrical resistivity for Yb-doped Sb2Te3 between 290 and 340 K in which the electrical resistivity decreases with temperature. The minimum value of electrical resistivity for Yb0.05Sb1.95Te3 is 0.0007 Ωm. As a result, the electrical conductivity of Yb-doped Sb2Te3 materials is higher than undoped Sb2Te3 at room temperature and increases with temperature. Selected absorption spectra of Sb2−xYbxTe3 (x = 0.02 and 0.05) are shown in Figure 9. The DRS spectra of Sb2Te3 lattice show an intensive peak around 480 nm. The absorption spectra in the spectral region 800–900 nm can be assigned to electronic transitions of Yb3+ from the 2
F
7/2 ground state to 2
F
5/2 excited level [22, 23]. As shown in Figure 9, there is a red shift in DRS spectra of Sb2−xYbxTe3 compounds, respectively. The calculated band gaps from absorbance spectra for Sb2−xYbxTe3 are E
g
=
2.587 eV (Yb-0.02) and 2.48 eV (Yb-0.05). Figure 10 exhibits the RT PL emission spectra of Sb2−xYbxTe3 compounds. Two peaks are shown in the PL spectra of Yb3+-doped compounds attributed to Sb2Te3 lattice centered at 560 nm and another is assigned tof-f transitions of Yb3+ ions from 2
F
5/2→2
F
7/2. There are red shifts in PL spectra by increasing concentration of Yb3+ [22, 23]. Figure 11 shows the typical evolution of the absorption spectra of C.I. Basic Green-4 under the irradiation of visible light using the Yb0.05Sb0.95Te3 nanoparticles as a photocatalyst. The absorption peak around 355 nm gradually weakened and decreased from the absorption spectra, indicating the degradation of the BG4. The loss of absorbance may be due to the destruction of the azo band and dye chromogen. Since no new peak was observed, the BG4 has been decomposed. Also, the photocatalytic activity of synthesized undoped and Yb-doped Sb2Te3 nanoparticles was compared and is presented in Figure 12. In a typical process, 100 mL of BG4 (5 mg/L) aqueous solution and 0.1 g of photocatalyst powder were mixed in a quartz photoreactor. It is clearly seen from Figure 12 that the color removal efficiency of the Yb-doped Sb2Te3 catalyst is much higher than that of pure Sb2Te3. The results demonstrated the good photocatalytic ability of these nanoparticles under visible light and can be compared with other new catalysts [24–26]. As can be seen, the decolorization efficiency is 16.30 and 75.62% after 120 min of treatment for Sb2Te3 and Yb0.05Sb1.95Te3, respectively.Figure 2
Thea and c lattice constants of YbxSb2 −xTe3 (0
≤
x
≤
0.05) dependent upon Yb3+doping on Sb3+ sites.The SEM image (a) and EDX (b) of as-prepared Sb2Te3 synthesized at 180°C and 48 h.
(a)
(b)The SEM image (a and b), TEM image (c), and SAED pattern (d) of as-prepared Yb0.02Sb1.98Te3 nanoplates at different magnifications synthesized at 180°C and 48 h.
(a)
(b)
(c)
(d)The SEM images (a), TEM image (b), and SAED pattern (c) of as-prepared Yb0.05Sb1.95Te3 nanoparticles at different magnifications.
(a)
(b)
(c)Figure 6
The EDX patterns of YbxSb2−xTe3 synthesized at 180°C and 48 h.Figure 7
Schematic of four-point probe.Electrical resistivity (a) and thermoelectrical resistivity (b) of YbxSb2−xTe3 nanomaterials at room temperature.
(a)
(b)Figure 9
Absorption spectra of YbxSb2−xTe3 nanoparticles at room temperature.Figure 10
Photoluminescence spectra of YbxSb2−xTe3 nanoparticles at room temperature.Figure 11
Absorption spectra of BG4 under the irradiation of visible light using the Yb0.05Sb0.95Te3 nanoparticles as a photocatalyst.Figure 12
Color removal efficiency (CR (%)) for a 5 mg/L BG4 solution by photocatalytic process under visible light (catalyst loading 1.0 g/L).


## 7. Conclusion

Novel thermoelectric YbxSb2−xTe3 based nanomaterials were synthesized by a simple and efficient coreduction method at 48 h and 180°C at basic media. According to SEM and TEM images, different morphologies were seen in Yb-doped Sb2Te3. Lanthanide doping promotes the electrical conductivity of Sb2Te3 as well as thermal conductivity. UV-Vis absorption and emission spectroscopy reveals mainly electronic transitions of the Yb3+ doped nanomaterials. Red shifts as well as increasing of intensity of absorption and emission peaks were seen in doped nanomaterial. Experiments showed that the as-obtained nanoparticles have high photocatalytic activity under visible light irradiation. The decolorization efficiency of BG4 solution using these photocatalysts was 16.30 and 75.62% after 120 min of treatment for Sb2Te3 and Yb0.05Sb1.95Te3, respectively.



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*Source: 452659-2014-05-29.xml*