Selenium Nanoparticles Prepared from Reverse Microemulsion Process

Chinese Chemical Letters Vol. 15, No.10, pp 1249- 1252, 20041249http://www.imm.ac.cn/journal/ccl.htmlSelenium Nanoparticles Prepared fromReverse M licroemulsion ProcessMing Zhu LIU, Sheng Yi ZHANG*. Yu Hua SHEN, Ming Liang ZHANGSchool of Chemisty and Chemical Enginering, Anhui University, Hefei 230039A bstract: Selenium nanoparticles were prepared by a reverse microemulsion system. Sodiumselenosulfate was used as selenium source. The results showed that hydrochloric acid concentrationand reaction temperature had great influence on the morphology of products. The crystallineselenium nanowires and amorphous selenium nanorods were obtained in given condition.Keywords: Reverse microemulsion, selenium nanoparticles, preparation.Selenium possesses excellent photoelectrical and semiconductor properties which make itextensively used in duplicate, photography, cells and rectifiers'. Selenium is also one ofessential trace elements in the human body and has great importance in nourishment andmedicine'. Like other nanoparticls, selenium nanoparticles would have some uniquemechanical, optical, electrical, biologic and chemical properties as compared with otherbulk materials. For example, it has been reported that the redness selenium nanoparticleshas high biological activities and low toxicity , and nanowires of trigonal selenium havenovel photoconductivity *. Thus selenium nanoparticles have caused the great interest ofrescarchers and a variety of synthesis methods have been exploited 5-8. This articledescribes the reverse microemulsion synthesis of selenium nanoparticles.Reverse microemulsion was first developed by Boutonnet' and has recently beenused to prepare various nanoparticlesl0-12.In reverse microemulsion system, waterdroplets, which dispersed in a continuous oil phase, not only work as nanoreactors for theformation of nanoparticles but also inhibit the aggregation of nanoparticles. As reported,the nanoparticles prepared with this method were mostly spherical or quadrate, since thesynthesizing was confined in the water droplets. In this paper, selenium nanorods andselenium nanowires were fabricated with this method by two processes: themicroemulsion reaction and the interface growth.ExperimentalMicroemulsion A, consisting of 10 mL cyclohexane, 0.25 mL 0.1 mol/L Na2SeSO3prepared as literature', 0.3 g sodium ddaemra (enc nd 02 L n-butanol, wasultrosized until it formed clear transp中国煤化工B, cnsising thesame materials except the Na2SeSO3, vHCNMHGicacid(HCI)with' E-mail: syzhangi@ mail.hf.ah.cn1250Ming Zhu LIU et al.certain concentation, was obtained by the same performance. The subsequent syntheticprocess involved two steps.First, microemulsion A and microemulsion B were mixed under ultrasonic treatmentand then this solution was placed in sillness for 20 minutes at 20°C or 409C. Second, anappropriate amount of the mixture of acetone and water (volume ratio 1:1) was added tothe reaction solution under stiring. After keeping for a while, the reaction solutiondivided into two transparent layers and was then kept stillness for 40 minutes at roomtemperature. The out products, suspended in the interface between two solution layers,were separated by centrifugation, washed with ethanol and water, dried in a desiccator andcharacterized by X-ray diffraction meter (XRD) and transmission electron microscopy(TEM).Results and DiscussionDuring the first step, the following reaction occurred through the collision of waterdroplets and the exchange of reactants between water droplets: Na2SeSO3+ 2HCl-→Se↓+SO2↑+ 2NaCl + H2O. When the reaction solution turned into orange colloid fromcolorless, it indicated that the amorphous spherical selenium nanoparticles were formed' .The TEM images showed that the products formed at first step were uniforn sphericity.In second step, the spheric nanoparticles could grow up into various shapes with thecollision, dissolution and reaggregation of the nanoparticles in the interface between twosolution layers.The HCI concentration affected the morphologies of products: the selenium nanorodswere obtained from 2.0 mol/L HCI solution (Figure 1a) and the chain-like seleniumnanoparticles were obtained from 3.0 molL HCl solution (Figure 1b). The results couldbe explained as follows. With 3.0 molL HCI solution, the reaction occurred so fast thatas-produced elemental selenium had no time to make nanorods, which lead elementalselenium to grow in various directions and fom the selenium spheres. In second step, theselenium spheres as-formed would aggregate to chain-like nanoparticles due to stackingthe nanocrystals in the same direction and provided energetically favorable link betweenthem!'. With 2.0 mol/L HCI solution, strand -rods assembled with litle spherical particles.These feeble nanorods could further grow by combining lttle particles and be transited tosolid nanorods.Reaction temperature also affected the morphologies of products. The productsobtained at 209C were nanorods (Figure 1a), which were further confirmed to be mostlyamorphous selenium by XRD. But at 40°C (with 2.0 mol/L HCI solution), nanowiresconsisting of the fibers bundling in parallel were obtained (Figure 2). Figure 3 showsthe XRD pattern obtained from the as-prepared nanowires. All the diffraction peaks inthe XRD pattern could be indexed as the trigonal phase of selenium nanowires. Thisresult suggested that the selenium nanowires were highly pure in chemical compositionwith all selenium atoms crystallized in the trigonal lattice. The abnormal intensity of the(100) peak indicated that these nanowi中国煤化I g the 001 direction.From this XRD pattern, the lttice conCNMH G7 nm, c=0.495 nm,which corresponded well to those of th3.495 nm) reportedin literature'.Selenium Nanoparticles Prepared from1251Reverse Microemulsion ProcessFigure 1 The TEM images of selinium nanoparticles prepared at 20°C with different HCIconcentration solution: (a) 2.0 mol/L; (b) 3.0 mol /L我一-200Em b; 100:amFigure 2 The TEM image of selenium nanowires.Figure 3 The XRD image of seleniumThe inset was the SARD pattem of se-nanowires.lenium nanowires.- lum。iinin20204020们20B02 θ°Here the influence of reaction temperature on morphologies of products wasdiscussed. The transformation of amorphous selenium into crystalline selenium requireshigher temperaturel. At 40°C, more amorphous selenium nanoparticles would bedissolved into the solution due to their higher free energy, which would favor theformation of crystal selenium nanoparticles.On the other hand, the hoist of thetemperature could weaken the adsorption of surfactants to nanoparticles in certaindirection and facilitate the escape of the SO2 gas produced in the reaction. These twofunctions of temperature could provide anisotropy microenvironment that played animportant roll in the formation of crystal selenium nanoparticles'. Thus more crystalselenium seeds were produced in first step. In second step, a various of nanoparticleswere released from the water droplets a中7 nttween two solutionlayers. The elemental selenium,中国煤化工tugo} Gnedissolution of amorphous selenium, deTYHCNMHGseds.Withtheinduction of surfactants and the thrust oI telmperaure, une crystal seeas assermbled alongthe same direction and translated into trigonal selenium and the nanowires were obtained.1252Ming Zhu LIU et al.It usually took a long time to form selenium nanowires+78, but here the seleniumnanowires were synthesized in one hour, if the local concentration of selenium in theinterface between two solution layers is higher and the temperature also can accelerate thegrowth speed of the nanowires.The mechanism of the formation of the wide nanowires is not clear yet. Abdelouasand his co-workers had obtained the wide selenium strand-wires contained severalnanoparticles in parallel due to attachment of nanoparticles to single strand wires'. Inour system, it may be speculated that fbers formed had the high free energy andaggregated in parallel to form wide nanowires in the interface of solutions.ConclusionsIn summery, we have provided a convenient and fast approach for the preparation ofselenium nanoparticles. The synthesis included two processes: the microemulsion reactionand the interface growth. The condition of the microemulsion reaction processinfluenced on the morphology of final products. Crystalline selenium nanowires andamorphous selenium nanorods were obtained by controlling HCI concentration andreaction temperature. Further study may extend this method for the preparation of othernanoparticles.AcknowledgmentsThis work was supported by the National Natural Science Foundation of China (No. 20031010 and29971001).References1. L.I. Berger, Semiconductor Materials, CRC Press, Boca Raton, FL 1997, pp.86-88.2. B. H. Xu, K.Y. Huang, Chemistry, Biochemistry of Selenium and its Application in LifeScience, Hua Fast University of Science & Technology Press (Ch). 1994.3. X.Y. Gao,J. s. Zhang, L. D. Zhang. M. x. Zhu, China Public Heailh, 2000, 16, 421.4. B. Gates, B. Mayers, B. Cattle, Y. N. Xia, Adv. Funct. Mater., 2002, 12, 219.5. V. V. Kopeikin, s.V. Valueva, A. L. Kipper, L. N. Borovikova, A. P. Filippov, PolymerScience, Series A, 2003, 45 (4), 374.5. X. Y. Gao, J. s. Zhang, L. D. Zhang, Adv. Mater, 2002, 14, 290.7. X. Zhang, Y. Xie, F. Xu, X. H. Liu, Chin. J. of lnorg. Chem, 2003, 19, 77.8. B. Gates, B. Mayers, A. Grossman, Y. N. Xia, Adv. Mater, 2002, 14, 1749.9. M. Boutonnet, J. Kizling, P. Stenius, G. Maire, Colloids Surf, 1982, 5, 209.10. s. Vaucher, M. Li, s. Mann, Angew. Chem. Int. Ed, 2000, 39(10),1793.1I. C. M. Bender, J. M. Burlitch, D. Bartber, C. Pollock, Chem. Mater,, 2000, 12, 1969.12. J. A. Johnson, M. L Saboungi, P. Thiyagarajan, et al, J.Phys. Chem. B, 1999, 103, 59.13. J.J. Zhu, H. Wang, S. Xu, H. Y. Chen, Langmuir, 2002, 18, 3306.14. A. Abdelouas, w. L. Gong, w. Lutze, et al, Chem. Mater., 2000, 12, 1510.15. J. Stuke, in Selenium, Van Nostrand Reinhold, New York, 1974, pp. 177.16. F. Li,Z. Hu, s. Jing, et al, Wuji Huaxue Xuebao (Chin. J. Inorg. Chem,), 2001, 17, 315.中国煤化工Received 18 August, 2003MYHCNMH G