The metal selenides were produced first time by Kutscher in 1960s’ as producing PbSe with chemical bath deposition . After the Kutscher, the researchers synthesized the other metal selenides [2−6]. Chromium selenide is an inorganic compound. It exhibits semiconductor-metal transformations at elevated temperatures [7, 8]. The semiconductor-to-metal transition in CrS is accompanied by a transition to the NiA s structure. On the other hand, the resistance transition in CrSe is not associated with an identified structural transition. In order to study the possibility of pressure induced semiconductor-to-metal transitions, the high-pressure resistivities of these compounds were determined up to 70 kbar . Different structural forms of chromium selenide are well-documented (CrSe, Cr2Se3). The Cr2Se3 has a rhombohedral structure (space group: R3) based on NiAs-type structure [10, 11]. The CrSe crystallizes in the monoclinic structure immediate between NiAs- and PtS-type .
Na2SeO3, NaHSe, H2Se, Se(C2H5)2, selenium powder, bis (trimethylsilyl) selenium Se(TMS)2, selenourea (H2NC(Se)NH2) and SeO2 have been introduced as selenium reagents for the synthesis of metal selenides [13−18]. SeCl4 was selected in our experiments to provide a highly reactive selenium source in aqueous solution and has given good results. The aim of this paper was synthesis of the CrSe nanostructures by hydrothermal method. Over recent years, a great deal of research effort has been devoted to the research in metal selenide semiconductors. Hence, investigations on the synthesis and modification of nanosized Cr2Se3 have attracted tremendous attentions. However, properties of transition metal selenides are quite different from those of oxides since d electrons in selenides participate in covalent bonding. Such covalent bonding in selenides reduces the formal charge on transition metals and favors formation of metal-metal bonds. Metal-metal interactions indeed play a significant role in determining the properties of many of the transition metal selenides. The influence of such interactions on the electronic structure of these solids can be studied by means of their transport properties and these interactions become particularly important in transition elements selenides of the second and third series . To the best of our knowledge, it is the first time that SeCl4 is used as Se source for the synthesis of chromium selenides. The SeCl4 can be fast reduced by hydrazine and formed in to Se2− ions, which has influence on the sizes and morphologies of the resulting samples and is the basis of this synthetic route. This route may be extended to the fabrication of other metal selenides with novel morphologies and properties.
MATERIALS AND METHODS
All the chemicals used in our experiments were of analytical grade, were purchased from Merck and were used as received without further purification. GC-2550TG (Teif Gostar Faraz Company, Iran) were used for all chemical analyses. SEM images were obtained on Philips XL-30ESEM equipped with an energy dispersive X-ray spectroscopy. TEM images and SAED pattern were obtained on a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV. EDS analysis with 20 kV accelerated voltage was done.
Synthesis of chromium selenide nanoparticles
The chromium selenide nanoparticles were prepared as follows: first, an aqueous solution of Cr(NO3)3.9H2O was prepared, then a certain amount of surfactant was added the solution under strong magnetic stirring at room temperature. Second, an aqueous solution of SeCl4 was added to the above solution, then 4 ml of hydrazine was added drop-wise. The color of the colorless solution changed to black, indicating reduction of SeCl4. The solution was added to a Teflon-lined stainless steel autoclave and maintained at 180 ̊C for 12, 18 and 24h. After the autoclave was cooled to room temperature on its own, the black precipitate was separated by centrifugation. The precipitate was washed with deionized water and anhydrous ethanol several times and was dried at 60oC under vacuum for 4 h. Fig. 1 shows the formation process of the Cr2Se3 nanoparticles.
Fig. 1. Schematic diagram illustrating the formation of the chromium selenide nanoparticles.
Table 1. The reaction conditions of the products synthesized in this work.
RESULTS AND DISCUSSION
The morphology and particle size of the samples were investigated by SEM and TEM images. Fig. 2 shows the SEM images of samples 1 and 2 obtained in the presence of KBH4 and different surfactants including SDBS (Fig. 2a and b) and CTAB (Fig. 2c and d). We used four surfactants in our experiment to investigate their influence on samples morphology. In the presence of KBH4 when SDBS and CTAB are used, the products are in nano-size but their size and morphology are different: hexagons, prisms and spheres. As can be seen in Fig. 2, KBH4 is not a good surfactant in the current experiment, because it is clear that by using it, the products did not separate well and are agglomerated (Fig. 2).
Fig. 2. SEM images of prepared samples from SeCl4 and Cr(NO3)3.9H2O at 180 ˚C for 12h, in the presence of KBH4 and: (a,b) SDBS (sample 1) , (c,d) CTAB (sample 2).
With exchange of reductant from KBH4 to hydrazine (Fig. 3) at 180 ˚C for 12 h, agglomeration nanoparticles were formed, as shown in Fig. 3. Hydrazine limits the particle size, protects particles from further aggregation and plays an important role in the formation of nanoparticles. In the presence of PEG600, polymeric molecules adsorb preferentially on the nuclei surface to inhibit aggregation by steric hindrance mechanism (Fig. 3a and b). SDS monomers act as excess electrolyte, which in turn lowers the electrostatic repulsion barrier for coalescence to occur and can prevent particles from formation of big nanostructures (Fig. 3c and d).
Fig. 3. SEM images of prepared samples from SeCl4 and Cr(NO3)3.9H2O at 180 ˚C for 12h, in the presence of 4ml of hydrazine and: (a,b) PEG600 (sample 3) , (c,d) SDS (sample 4).
In other side, the effect of reaction time on the morphology and particle size of the products was investigated. Fig. 4 shows that reaction time has a small influence on the morphology and particle size of the Cr2Se3 nanostructures. For instance, when the reaction time increased from 12 to 18 (Fig. 4a and b) and then 24h (Fig. 4c and d) in samples 5 and 6, respectively, morphology of samples remained nearly constant; and particle size and agglomeration of nanoparticles were decreased and increased a little, respectively, as shown in Fig. 4. This figure shows that samples 5 and 6 with grain diameter about 50 nm have nearly even distribution.
Fig. 4. SEM images of prepared samples from SeCl4 and Cr(NO3)3.9H2O in the presence of 4ml of hydrazine and CTAB, at 180 ˚C for: (a,b) 18h (sample 5) , (c,d) 24 h (sample 6).
TEM image of sample 5 obtained in the presence of CTAB and hydrazine at 180 ˚C for 18h (Fig. 5a), shows the nanoparticles are agglomerated and their particle sizes are in the range 20–50 nm. The HRTEM image of Cr2Se3 nanoparticles in Fig. 5b shows the nanoparticles are highly crystalline and distance between the two adjacent planes is measured to be 0.125 nm. The high-order diffraction spots and diffused halo ring in the SAED spectrum in Fig. 5c indicate that the nanoparticles prepared in the presence of CTAB and hydrazine are well crystallized.
Fig. 5. (a) low-magnification, (b) high-magnification TEM images, (c) SAED pattern of sample 5.
EDS technique was used to determine the chemical composition of the products. Fig. 6 shows the EDS spectrum of the sample 6 obtained in the presence of CTAB and hydrazine. This figure shows the presence of Cr and Se elements in the products. In addition, neither N nor C signals were detected in the spectrum, which means there is no solvent or capping agent in the sample.
Fig. 6. EDS pattern of sample 6.
In our experiment, when Cr(NO3)3.9H2O and SeCl4 are added in the deionized water, a completely clear acidic solution is obtained that contain H2SeO3; however, H2SeO3 can be converted to Se, by N2H4.H2O, quickly up on heating, which has high reactivity and is easy to be disproportionate into Se2− ion under alkaline conditions (Eqs. (1)–(3)). Under the given condition, free Cr3+ ions can react with Se2− ions (Eq. (4)) to form Cr2Se3. The proposed mechanism for the synthesis of Cr2Se3 can be expressed in the following equations:
A novel hydrothermal synthetic route to Cr2Se3 semiconductors was developed. For the first time, SeCl4 was used as selenium source. The effect of some parameters such as, reductant, capping agent and reaction time on the particle size and morphology of the obtained products were investigated. The composition, structure and morphology of products were assigned with XRD, EDS, SEM and TEM.
Authors are grateful to the Kosar University of Bojnord for supporting this work by Grant no. (9509241612).
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.