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Journal of Nanomaterials
Volume 2013 (2013), Article ID 970489, 6 pages
Characterization of Cubic AgSbS2 Nanostructured Flowers Synthesized by Microwave-Assisted Refluxing Method
1Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
2Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
3Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
Received 29 April 2013; Revised 5 August 2013; Accepted 10 August 2013
Academic Editor: Sheng-Rui Jian
Copyright © 2013 Narongrit Tipcompor et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Cubic AgSbS2 nanostructured flowers of nanorods were successfully synthesized by microwave-assisted refluxing of AgNO3, Sb(CH3COO)3, and L-cysteine (C3H7NO2S) dissolved in ethylene glycol at the pH of 4, characterized by X-ray diffraction (XRD) and electron microscopy (EM). The 1.89 eV direct energy gap was determined by UV-visible absorption, including two 688 and 857 nm emission wavelengths by photoluminescence (PL) spectroscopy. A possible formation mechanism of AgSbS2 nanostructured flowers was also proposed according to the experimental results.
Presently, silver antimony sulfide (AgSbS2) is a very attractive material, due to its promising candidate for a number of applications: active recording films, micromechanical and optical memories, electrical switching and transmission in midinfrared [1–4]. AgSbS2 crystallizes as two main crystal systems: monoclinic and cubic .
Microwave-assisted refluxing is a distillation process of which the system is heated by a microwave radiation. It involves the condensation of vapors and the return of this condensate to its original. It has been developed and widely used in industrial and laboratory distillations . This process is very fast, simple, effective, and environmentally friendly. It can solve the problems of temperature and concentration gradients and provides uniform growth media.
To the best of our knowledge, there are not many reports on the synthesis of AgSbS2: amorphous AgSbS2 thin films by pulsed laser deposition , ternary AgSbS2 thin films by direct fusion process , and with by solid state heating process . Our motivation is to synthesize cubic AgSbS2 nanostructured flowers of nanorods by microwave-assisted refluxing method. No one has ever succeeded in synthesizing them by a solution method.
2. Experimental Procedures
To synthesize AgSbS2, 1 mmol AgNO3, 1 mmol Sb(CH3COO)3, and 2 mmol L-cysteine (C3H7NO2S, LC) (1 : 1 : 2 molar ratio Ag : Sb : S) were dissolved in 50 mL ethylene glycol (C2H6O2, EG) and followed by pH adjusting to 0, 1, 2, 3, 4, 5, and 6 using HCl and NaOH. These solutions were processed by refluxing method irradiated with a 300 W microwave radiation for 10, 30, and 60 min. Finally, black precipitates were synthesized, separated by filtration, washed with deionized water and ethanol, and dried at 70°C for 12 h, for further characterization.
Crystalline phases of the as-synthesized nanostructured products were analyzed by an X-ray diffractometer (XRD, Philips X’Pert MPD) operating at 20 kV 15 mA and using Cu-Kα line in deg. An XRD pattern was also simulated in order to show the real existence of cubic AgSbS2 phase in this research. The morphology investigation was carried out by a field emission scanning electron microscope (FE-SEM, JEOL JSM-6335F) operating at 35 kV and a transmission electron microscope (TEM, JEOL JEM-2010), high-resolution transmission electron microscope (HRTEM), and selected area electron diffractometer (SAED) operating at 200 kV. Their optical properties were studied by a UV-visible spectrometer (Lambda 25 PerkinElmer) using a UV lamp with the resolution of 1 nm and a fluorescence spectrophotometer (LS50B PerkinElmer) using 300 nm excitation wavelength at room temperature.
3. Results and Discussion
At the pH 6 and 5, different phases were detected in the products. Upon further decreasing in the pH to 4, cubic AgSbS2 (JCPDS number 17-0456)  was synthesized as the main product (Figure 1(a)). It was the only one cubic crystal system of the AgSbS2 phase  with no impurity detection. At the pH 3, 2, 1, and 0, the patterns were specified as cubic AgSbS2 mixed with Ag2S (JCPDS number 09-0422)  impurities.
In the solution containing Ag+, Sb3+, and L-cysteine (C3H7NO2S, LC) at the pH 6 and 5 (very close to the isoelectric point of LC), the solubility of LC was the lowest. The molecules often precipitated out of the solution  with the formation of different phases. At the pH 4, the H+ ions formed bonds with oxygen ions of carboxylate groups, leaving sulfur ions of L-cysteine (LC) to form complex ions with Ag+ and Sb3+ ions  at room temperature:
During microwave refluxing, these complex ions were decomposed with the formation of :
Due to their stabilities, the decomposition proceeded with rather slow rates. The concentration of was lower than that synthesized by the direct ion-exchange reaction. Finally, AgSbS2 was synthesized [9, 10]:
Alternately, Ag+ and Sb3+ formed complex ions with LC at room temperature:
Then the complex ions were decomposed by microwave refluxing to synthesize AgSbS2:
At the pH 3, 2, 1, and 0, additional impurities were synthesized. At high acidic solutions, H+ ions have much chance to form bonds with LC, leaving Ag+ as free ions. During refluxing, S2− ions were released from the LC and further reacted with Ag+ ions to synthesize Ag2S precipitates.
By reducing the refluxing time of the solution with the pH 4 from 60 min to 30 min and 10 min (Figure 1(b)), Sb2S3 (JCPDS number 02-0374)  precipitates were also detected caused by incomplete reaction.
In order to show the real existence of cubic AgSbS2 phase, its XRD pattern was simulated  and shown in Figure 2(a). The Bragg angles and peak intensities of the simulation, JCPDS database and experiment were summarized in Table 1. In this research, the experimental peaks were in good accordance with those of the simulation and the JCPDS database, including the intensities of the peaks that were controlled by different growth rates in different directions of unit cells. Simulated crystal structure  of AgSbS2 is also shown in Figure 2(b). Its unit cell was cubic structure with Ag or Sb coordinates at (0, 0, 0) and S at , and vice versa. For each of AgSbS2 unit cell, Ag or Sb atoms are located at each of the corners and the centers of all the cube faces, including S atoms located in between them. Ag and Sb atoms have the same probability to reside in any of the crystal lattice, and they both have the same occupancy as S atoms in the unit cell.
SEM and TEM images (Figures 3 and 4(a)) show nanostructured AgSbS2 synthesized in the solutions with different pH values. At the pH 0, the product was composed of a number of nanoparticles oriented in different directions. Upon increasing of the pH from 0 to different values, they gradually transformed into different morphologies: clusters of nanoparticles (pH 1), mixed nanoparticles and incomplete nanostructured flowers (pH 2), incomplete nanostructured flowers (pH 3), nanostructured flowers of nanorods grown out of cores (pH 4), and facet irregular microparticles with different orientations (pH 6). These products have different morphologies, controlled by the nucleation and growth processes. At the pH 4 and 60 min, nanorods of flower-like clusters were characterized by HRTEM (Figures 4(b) and 4(c)). The nanorods appeared as rough surfaces with the detection of the (111) and (200) crystallographic planes at an angle of 54.7 degree, specified as single crystal.
To synthesize nanostructured flowers, AgSbS2 molecules nucleated, followed by clustering of nuclei to form AgSbS2 cores. A number of nanorods grew out of these cores to form AgSbS2 flower-like clusters of nanorods (Figure 5). Due to the microwave-assisted refluxing process, the flower-like clusters were synthesized by the orientation growth, promoted by microwave refluxing. The atoms had more chance to arrange themselves in crystal lattice to form nanocrystalline flowers of nanorods. For the present research, microwave refluxing played a key role in the growth of flower-like crystals: decomposition of the complex ions, formation of nuclei, and orientation growth of AgSbS2 crystals in the shape of flower-like clusters [9, 10].
The SAED pattern (Figure 4(d)) appears as systematic spots corresponding to the (200), (202), and (002) crystallographic planes of a single crystalline nanorod of cubic AgSbS2 (JCPDS number 17-0456) , with the [0–10] as zone axis. A simulated electron diffraction pattern (Figure 4(e))  was in accordance with that obtained by the experiment, indicating that the interpreted pattern could exist in reality.
Figure 6(a) shows the photonic absorbance attenuating through AgSbS2 nanostructured flowers of nanorods. It should be noted that the absorption was controlled by two photon energy () ranges, the high and low energies. When photon energy is greater than the energy band gap (), the absorption is linearly increased with the increase in the photon energy. The steep inclination of the linear portion of the curve was caused by the UV absorption for charged transition from the topmost occupied state of valence band to the bottommost unoccupied state of the conduction band. For the photon energy with less than , the absorption curve was different from linearity, caused by the UV absorption for charged transition relating to defects. By extrapolating the linear portion curve of the versus plot to zero absorption , the direct energy gap () was determined to be 1.89 eV, very close to the 1.77–2 eV energy gap of AgSbS2 thin films reported by Ibrahim .
Photoluminescence (PL) of AgSbS2 nanostructured flowers (Figure 6(b)) was excited by 300 nm wavelength at room temperature. The emission was detected at 688 nm (1.8 eV) as main peak, including the minor peak at 857 nm, very close to the optical transmission of AgSbS2 thin films for optical recording media reported by Wagner et al. . The main emission peak was specified as the recombination of electrons and holes in trapped surface states residing in the forbidden region , but for the shoulders, they were caused by the shallow levels of donors and acceptors between the valence and conduction bands . Furthermore, PL emissions could be controlled by different morphologies of the products, including crystalline degree, temperature, different types and concentrations of defects, different shapes and sizes of the products, and excitation wavelengths.
AgSbS2 nanostructured flowers were successfully synthesized by microwave-assisted refluxing of AgNO3, Sb(CH3COO)3, and L-cysteine dissolved in ethylene glycol at the pH 4 for 60 min, according to the formation mechanism proposed in this report. Their direct energy gap was determined to be 1.89 eV, and their main and minor photoluminescence peaks were 688 and 857 nm, respectively.
The authors wish to thank the Thailand Research Fund (TRF) for providing financial support through the Royal Golden Jubilee Ph.D. Program and the TRF Research Grant BRG5380020 and the Thailand’s Office of the Higher Education Commission through the National Research University (NRU) Project, including the Graduate School of Chiang Mai University through a general support.
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