Abstract
Gd3+-doped Sb2Se3 nanorods were synthesized by coreduction method at 180°C and for 48 h. Powder XRD patterns indicate that the Se3 crystals (–0.04) are isostructural with Sb2Se3. The cell parameters a and b increase for Gd3+ upon increasing the dopant content (x), while c decreases. SEM images show that doping of Gd3+ ions in the lattice of Sb2Se3 results in nanorods. High-resolution transmission electron microscopic (HRTEM) studies reveal that the Gd0.04Sb1.96Se3 is oriented in the [10-1] growth direction. UV-Vis absorption reveals mainly electronic transitions of the Gd3+ ions in doped nanomaterials. Emission spectra of doped materials show sharp emission band originating from f-f transition of the Gd3+ ions. The electrical conductance of Gd-doped Sb2Se3 is higher than undoped Sb2Se3 and increases with temperature.
1. Introduction
Investigations on semiconductor nanostructures have recently been in the focus of intensive research activities because of intrinsic fundamental interest and manifold possibilities for applications. Semiconductor selenides find applications as laser materials, optical filters, sensors, and solar cells. Antimony selenide, an important member of these V2VI3 compounds, is a layer-structured semiconductor of orthorhombic crystal structure, and exhibits good photovoltaic properties and high thermoelectric power (TEP) which allows possible applications for optical and thermoelectronic cooling devices [1–4]. Inorganic nanomaterials doped by lanthanide (Ln3+) ions with various compositions have become an increasingly important research subject, and opened up opportunities for creating new applications in diverse fields, including biological labelling and imaging and light-emitting displays, owing to their distinct optical properties [5–7]. Over the past two decades, many methods have been employed to prepare Sb2Se3 nanotubes, nanowires, nanosheets, nanorods, nanobelts, nanospheres, and nanoflakes including thermal decomposition [8], microwave irradiation [1, 9], complex decomposition approach [10], solvothermal reaction [11–14], hydrothermal method [15, 16], vacuum evaporation [17], colloidal synthetic method [18], and other chemical reaction approaches. Studies of impurity effects or doping agents on the physical properties of Sb2Se3 are interesting both for basic and applied research. Doping of trivalent cations such as Sb3+ [19], In3+ [20], Fe3+ [21], Mn3+ [22], and a number of further trivalent 3d elements [23] to the lattice of Bi2Se3 has been investigated, also EPR spectra of Gd-doped bulk Bi2Se3 [24]. Also, new (Ln: Sm3+, Eu3+, Gd3+, Tb3+, Nd3+) based nanomaterials were synthesized by Alemi et al. [25, 26]. Recently, we have reported novel luminescent nanomaterials based on doping of lanthanide (Ln: Ho3+, Nd3+, Lu3+) into the lattice of Sb2S3 and (Ln: Ho3+, Nd3+, Lu3+, Sm3+, Er3+, Yb3+) into the lattice of Sb2Se3 [27–30]. The incorporation of large cations such as lanthanides into antimony chalcogenide frameworks is expected to lead to materials with different optical and electrical properties. The incorporation of lanthanide ions into a Sb–Se framework could dramatically affect the electronic properties of that framework. In this research, nanorods of crystals (x = 0.00–0.04) were synthesized by introducing small amounts of Gd3+ to the Sb2Se3 lattice. Structural, spectroscopic properties and electrical conductivity of the synthesized materials are reported.
2. Experiment
All chemicals were of analytical grade and were used without further purification. Grey selenium (1 mmol) and NaOH (5 mmol) were added to distilled water (60 mL) and stirred well for 10 min at room temperature. Afterwards, hydrazinium hydroxide (2 mL, 40 mmol), SbCl3 (2, 1.99, 1.98, 1.96 mmol) and Gd2O3 (0.00, 0.01, 0.02, 0.04 mmol) 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 cooled to room temperature. The optimum conditions for this reaction are pH = 12, temperature 180°C, and reaction time 48 h. The black precipitate obtained was filtered and washed with ethanol and water. It was dried at room temperature. Yields for the products were 85–90%. Phase identification was performed with an X-ray powder diffractometer (XRD D5000 Siemens) with radiation. Cell parameters were calculated with Celref program from powder XRD patterns, and reflections have been determined and fitted using a profile fitting procedure with the Winxpow program. The reflections observed in 2θ = 4–70° were used for the lattice parameter determination. The morphology of materials were examined by a scanning electron microscope SEM (Hitachi S-4200). The HRTEM image and SAED pattern were recorded by a Cs-corrected high-resolution TEM (JEM-2200FS, JEOL) operated at 200 kV. Photoluminescence measurements were carried out using a Spex FluoroMax3 spectrometer after dispersing a trace amount of sample via ultrasound in distilled water. The Four-Probe Method was used for the measurement of electrical and thermoelectrical resistivity of samples. A small oven was needed for the variation of temperature of the samples from the room temperature to about 200°C (max.). Small chip with 1 mm thickness and 7 mm length was used for this analysis. This chip was obtained by pressing of 10 mg of sample under 30 kpa pressing device.
3. Results and Discussion
samples were prepared by a hydrothermal coreduction method. SbCl3 (99.99%), selenium powder, and Gd2O3 were used as starting materials, and hydrazinium hydroxide was used as the reducing agent. The powder X-ray diffraction (P-XRD) patterns (see. e.g. Figure 1) indicate that the Gd3+-doped powders have the same orthorhombic structure as Sb2Se3 and that single phase Sb2Se3 is retained at lower doping concentrations of Gd3+. All the peaks in the Figure 1 can be attributed to the orthorhombic phase of Sb2Se3 with pbnm space group and lattice parameters , , and (JCPDS card File: 72-1184).
The EDX analysis of the product confirms the ratio of Sb/ Se/Gd as expected (Figure 2). Also, ICP analysis confirms the exact amount of doping.
The cell parameters of the synthesized materials were calculated from the XRD patterns. With increasing dopant content (x), the a and b parameters for Gd3+ increase and the c decreases (Figure 3). The trend for lattice constants can be correlated to the effective ionic radii of the Gd3+ ions; assuming that the radius of Gd3+ is larger than that of Sb3+ results in greater amount of lattice parameters for Gd3+-doped materials.
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Scanning electron microscopic (SEM) images of Sb2Se3 nanorods show rods up to 5 μm lengths and diameter of 25–120 nm (Figure 4).
Also, Figures 5(a) and 5(b) show SEM images of Sb1.96Gd0.04Se3 nanorods in which the length of rods is about 6 μm lengths and diameters of 40–150 nm. Doping of Gd3+ into the structure of Sb2Se3 does not change the morphology of Sb2Se3 nanorods but the length and diameter of rods are changed.
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Figure 6(a) shows TEM image of as-prepared Sb1.96Gd0.04Se3 nanorods. The crystal lattice fringes are clearly observed and average distance between the neighboring fringes is 0.82 nm, corresponding to the [1–10] plane lattice distance of orthorhombic-structured Sb2Se3, which suggests that Sb1.96Gd0.04Se3 nanorods grow along the [10–1] direction. The SAED pattern of the nanorods indicates its single crystal nature and long axis is [10–1] (Figure 6(b)). Also, the typical HRTEM image and FFT pattern recorded from the same nanorods are shown in Figure 6(c). The HRTEM image and SAED pattern are the same for Sb2Se3, and show the similar growth direction (see the Supplementary Material available online at http://dx.doi.org/10.1155/2012/983150).
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UV-Vis spectra of Gd3+ doped Sb2Se3 are shown in Figure 7. The absorption bands of Gd3+ doped Sb2Se3 exhibit maxima at 850, 860, and 870 nm were assigned to gadolinium electronic transition of 8S7/2 → 6I5/2,13/2,11/2, 8S7/2→ 6I17/2, 9/2 and 8S7/2 → 6I7/2 [31, 32].
There is also a shift in the onset of absorption to lower energies (red shift) in Gd-doped samples compared to Sb2Se3 (see Figure 8). Band gap energies (Eg) for Sb2Se3 and Sb1.96Gd0.04Se3 are calculated from the UV-Vis absorption spectra as Eg = 1.55 eV for pure Sb2Se3 and Eg = 1.49 eV for Sb1.96Gd0.04Se3 [29, 30].
The sharp emission peak at 840 nm on excitation with the 835 nm in Figure 9 can be assigned to 6P7/2 → 8S7/2 transitions of Gd3+ [31, 32].
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In doped semiconductors, the two types of emissions are responsible for the dopant (impurity) luminescence. One can be observed only upon direct excitation of the dopant. The second type is obtained if energy transfer from host to dopant occurs. Scheme 1 shows the band-gap model for the luminescence of Gd-doped Sb2Se3. The excitation of electrons across the host lattice band gap from the valence to conduction band is followed by a radiative recombination with a deeply trapped hole at a defect state or a nonradiative relaxation into the lanthanide ion 5d level. Radiative transition from the excited state to the ground state of Gd3+ ions occurs, leading to luminescence of the Gd3+ ions [26].
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Binary compounds such as Sb2Se3 and its alloys are thermoelectric materials with layered crystalline structures. These materials have been investigated for direct conversion of thermal energy to electric energy and they specially are used for electronic refrigeration [33–36]. The Four-Probe Method was used for the measurement of electrical and thermoelectrical resistivity of samples (Scheme 2).
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The electrical resistivity of the compounds is shown in Figure 10. With the increase in the lanthanide cation concentration, the electrical resistivity of synthesized nanomaterials decreased obviously. At room temperature the electrical resistivity of pure Sb2Se3 was of the order of 0.2 and for Sb1.96Gd0.04Se3 was , respectively.
The temperature dependence of the electrical resistivity for Sb1.96Gd0.04Se3 between 290–350 K is shown in Figure 11 in which electrical resistivity decreases with temperature. As a result, the electrical conductivity of Gd-doped Sb2Se3 materials is higher than pure Sb2Se3 at room temperature, and increases with temperature.
Two factors including overlapping of wavefunctions of electrons in doped Sb2Se3 and acting as a charge carrier due to Gd atomic structure (having empty f orbitals) are important reasons for decreasing electrical resistivity.
4. Conclusions
Coreduction synthesis is a simple and efficient method for preparing luminescent nanomaterials of ( to 0.04). SEM images show that doping of Gd3+ into the sites of the Sb3+ does not change the morphology of Sb2Se3. HRTEM images show that the growth direction is the same for pure Sb2Se3 and Gd-doped samples. Emission and absorption spectra of doped materials, in addition to the characteristic red emission peaks of Sb2Se3, show other emission bands originating from f-f transitions of the Gd3+ ions. The electrical conductivity of Gd-doped materials is higher than pure Sb2Se3 at room temperature, and increases with temperature.
Acknowledgment
This work is funded by the 2011 Yeungnam University Research Grant.
Supplementary Materials
TEM, HRTEM images, SAED pattern of Sb2Se3 nanorods are provided.