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Synthesis of Cadmium Sulphide nanoparticles
Solar Energy Materials & Solar Cells 63 (2000) 309}314
Synthesis of Cadmium Sulphide nanoparticles Manjunatha Pattabi*, Jayasheela Uchil Materials Science Department, Mangalore University, Mangalagangotri 574 199, India
Abstract Nanoparticles form a link between molecular and bulk state of matter and exhibit size dependent physical and chemical properties. A novel technique of synthesizing nanoparticle "lms on biological membrane substrates is presented here. Cadmium Sulphide (CdS) nanoparticles were prepared by the chemical reaction of aqueous solutions of Cadmium Acetate and Thiourea. The reacting solutions were allowed to di!use across the membrane for di!erent periods to control the deposition time. The optical absorption spectra of the membrane after the reaction were recorded with bare membrane as reference. The optical absorption spectra show a clear shift in the absorption edge for "lms with di!erent deposition times at a "xed concentration. The band gaps calculated from the absorption spectra for "lms with smaller deposition time were higher than that for the bulk CdS. The particle size, estimated from the band gaps, lie in the nanometer range showing that the particle size and band gap can be tailored by controlling the deposition time and concentration of the precursors. 2000 Elsevier Science B.V. All rights reserved. Keywords: Nanoparticles; CdS; Bang gap; Egg membrane.
1. Introduction Nanoparticles, also known as nanocrystallites or Q-particles, with typical diameters up to 10}20 nm, form a link between molecular and bulk states of matter. Nanoparticles exhibit size-dependent physical and chemical properties, unlike their bulk counterparts. The nanoparticle has emerged as an exciting new "eld of materials research. In particular, semiconductor nanoparticle quantum dots have drawn considerable attention because of their interesting properties like the shift of excitonic
* Correspondence address: Dr. M. Pattabi, Solar-Hydrogen Fuel Cell Group, Centro de Invstigacion en Energia, Universidad Nacional Autonoma de Mexico, Temixco 62580, Morelos, Mexico.. E-mail address: [email protected] (M. Pattabi). 0927-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 0 5 0 - 7
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peak to higher energy [1], blue shift in absorption band edge [2], band gap tuning [3], below picosecond radiative rates of optical transitions [3] and enhanced non-linear optical properties [4] which may have potential applications in optical switching [5], single charge memories [6], single electron transistors [7], etc. Thus, preparation of size controlled, monodispersed semiconductor nanoparticles in a stable host matrix to prevent #occulation is of primary importance. Physical methods like sputtering and vapor-phase condensation are quite widely used for the preparation of nanoparticles. The chemical methods used for the preparation of semiconductor nanoparticles involve reactions in various media including aqueous media, microemulsions, Zeolites, gels, polymers and glass. A variety of chemical techniques are used for the synthesis of nanoparticles. But the properties of nanoparticles by any new synthetic route are di$cult to anticipate. The characteristics desirable in the "nal nanodispersive medium are high purity, monodispersity and controlled surface derivatization. Reports on the synthesis of nanoparticles using biological membranes are sparse. This paper presents a novel but simple technique of synthesizing CdS nanoparticles of predetermined size, in a permanent non-degradable (non-aging) host matrix of a biological membrane by di!usion of precursors across it.
2. Experimental details The chicken egg membrane has been used as the host matrix for the synthesis of CdS nanoparticles in the present study. The chicken egg is lined inside with a 40 lm thick double-layered membrane. Aqueous solutions of Cadmium Acetate and Thiourea are allowed to di!use across the membrane resulting in the formation of CdS particles. The reaction was allowed to take place for 1, 5, 10, 20, 40, 50, 60, 80 and 100 min, at a "xed concentration of the reactants. The reaction can be stopped at any desired time by removing the membrane from the reacting bath and washing in distilled water. An optical absorption spectrum is taken for all the samples, with an undeposited specimen of the membrane as reference, using a SHIMADZU UV3101PC UV-Vis-NIR Scanning Spectrophotometer. The two concentrations discussed here are: Concentration 1: 400 g/l Cadmium Acetate (1.5 M) and 100 g/l Thiourea (1.3 M) Concentration 2: 100 g/l Cadmium Acetate (0.375 M) and 25 g/l Thiourea (0.325 M).
3. Results and discussion Fig. 1 shows the variation of optical absorption as a function of wavelength for CdS particles formed on the egg membrane for di!erent reaction times with the reactants at concentration 1. Optical absorption as a function of wavelength for CdS particles formed on egg membrane for di!erent reaction times with the reactants at concentration 2 is shown in Fig. 2. One can see that the absorption spectra is blue shifted. It can be further seen that there is a clear and systematic shift in the absorption edge towards longer wavelengths for samples with increasing reaction time in both the
M. Pattabi, J. Uchil / Solar Energy Materials & Solar Cells 63 (2000) 309}314
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Fig. 1. Variation of optical absorption as a function of wavelength for CdS particles formed on the egg membrane for di!erent reaction times with the reactants at concentration 1.
Fig. 2. Optical absorption as a function of wavelength for CdS particles formed on the egg membrane for di!erent reaction times with the reactants at concentration 2.
concentrations. A comparison of the absorption spectra for these two concentrations shows that for the lower concentration, the absorption edge shifts to shorter wavelengths at a particular reaction time. It was found from the analysis of the absorption data that a best linear "t is obtained for (hla) as a function of hl , near the
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M. Pattabi, J. Uchil / Solar Energy Materials & Solar Cells 63 (2000) 309}314
absorption edge. A typical variation of (hla) as a function of hl for a sample with 80 minutes of reaction time at concentration 2 is shown in Fig. 3. The intercept in the energy axis by extrapolating the straight line, gives the band gap of the CdS particles. Fig. 4 gives the experimentally determined band gap values for samples with di!erent
Fig. 3. A typical variation of (hla) as a function of hl for a sample with 80 min of reaction time at concentration 2.
Fig. 4. Variation of band gap with reaction time for concentration 1 ("lled squares) and concentration 2 ( "lled circles).
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313
reaction times at both the concentrations. The band gap of CdS is a function of particle size in the nanoparticle regime, under both tight binding and e!ective mass approximation [8]. Therefore, it is possible to estimate the particle size from the band gap values determined from the optical absorption studies. The particle sizes were estimated from the band gap values using the band gap variation with size data under the e!ective mass approximation (EMA) [8]. The estimated particle sizes for samples with di!erent reaction times at both the concentrations are shown in Fig. 5. The optical absorption spectra showed no drift in the position of the absorption edge with time. A size regime of a cluster containing a few hundred to a few thousand atoms can be recognized in the evolution from isolated atoms to molecular and to a bulk phase wherein the interior of the cluster can be identi"ed with that of a bulk crystalline lattice with large fraction of surface atoms. Such a cluster is termed as a nanocrystal and the absorption spectra of semiconductor nanocrystals is blue shifted [9]. The blue shift in the absorption spectra of the CdS formed on the egg membrane (Figs. 1 and 2) indicates the formation of CdS nanoparticles. It is interesting to note that the absorption edge is strongly dependent on the reaction time at a particular concentration. Based on the fact that a smaller nanoparticle will exhibit an absorption edge at a smaller wavelength , it immediately follows that the particle size increases with the reaction time. This indicates the growth of initially formed nuclei rather than fresh nucleation as more cadmium and sulfur ions di!use across the membrane and are available for reaction. With the dilution of the reactants, the availability of the ions decrease and it takes a longer time to form a larger cluster. This fact is evident from
Fig. 5. Variation of particle size with reaction time for concentration 1 ("lled squares) and concentration 2 ("lled circles).
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the spectra at two di!erent concentrations for similar reaction times. Though the pore size in egg membrane is quite large compared with the size of the ions, the entangled nature of the pores in the membrane must be responsible for the slow di!usion of the ions across the membrane. It is interesting to note that the age of the membrane before deposition of CdS does not have any e!ect on the observed characteristics. Further, spectra recorded even three months after the formation of CdS particles do not show any drift indicating that coagulation has not taken place.
4. Conclusions The absorption spectra of the CdS formed on the chicken egg membrane by the di!usion of the precursors show a blue shift indicating the formation of CdS nanoparticles. The absorption edge shifts towards longer wavelengths for samples with increasing reaction time at a "xed concentration. At a lower concentration the absorption edge shifts to shorter wavelengths, for a particular reaction time. The band gaps calculated from the absorption spectra are higher than that for the bulk CdS and are dependent on the reaction time and concentration. The absorption edge does not show any drift with time even over a period of three months indicating the absence of coagulation after the nanoparticles are formed. The particle size, estimated from the band gaps, lies in the nanometer range. Thus, it is possible to prepare nanoparticles by the di!usion of precursors across a chicken egg membrane. The particle size and band gap can be tailored by controlling the deposition time and concentration of the precursors.
Acknowledgements The authors thank Prof. Jayagopal Uchil, Chairman, Department of Materials Science, Mangalore University, for providing the facilities and Mr. Ganesh Kumar for assistance.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Y. Wang, N. Herron, Phys. Rev. B. 42 (1990) 7253. B.G. Potter, J.H. Simmons, J. Appl. Phys. 68 (1990) 1218. T. Vossmeyer, L. Katsikas, M. Giersig, I.G. Popovic, H. Weller, J. Chem. Phys. 98 (1994) 7665. C.B. Murray, M. Nirma, M.G. Bwendi, Phys. Rev. Lett. 76 (1996) 1517. S. Schmitt-Rink, D.A.B. Miller, D.S. Chelma, Phys. Rev. B. 35 (1987) 8113. S. Tiwari, F. Rana, K. Chan, L. Shi, H. Hana", Appl. Phys. Lett. 69 (1996) 1232. D.L. Klien, J.E.B. Katani, R. Roth, A.P. Alvisatos, P.L. McEuen, Appl. Phys. Lett. 68 (1996) 2574. P.E. Lippons, M. Lannon, Phys. Rev. B. 39 (1989) 10935. A.C. Rastogi, in: Vikram Kumar, S. K. Agarwal (Ed.), Physics of Semiconductor Devices, Narosa Publishing House, London, 1998, p. 56.
Synthesis of Cadmium Sulphide nanoparticles Manjunatha Pattabi*, Jayasheela Uchil Materials Science Department, Mangalore University, Mangalagangotri 574 199, India
Abstract Nanoparticles form a link between molecular and bulk state of matter and exhibit size dependent physical and chemical properties. A novel technique of synthesizing nanoparticle "lms on biological membrane substrates is presented here. Cadmium Sulphide (CdS) nanoparticles were prepared by the chemical reaction of aqueous solutions of Cadmium Acetate and Thiourea. The reacting solutions were allowed to di!use across the membrane for di!erent periods to control the deposition time. The optical absorption spectra of the membrane after the reaction were recorded with bare membrane as reference. The optical absorption spectra show a clear shift in the absorption edge for "lms with di!erent deposition times at a "xed concentration. The band gaps calculated from the absorption spectra for "lms with smaller deposition time were higher than that for the bulk CdS. The particle size, estimated from the band gaps, lie in the nanometer range showing that the particle size and band gap can be tailored by controlling the deposition time and concentration of the precursors. 2000 Elsevier Science B.V. All rights reserved. Keywords: Nanoparticles; CdS; Bang gap; Egg membrane.
1. Introduction Nanoparticles, also known as nanocrystallites or Q-particles, with typical diameters up to 10}20 nm, form a link between molecular and bulk states of matter. Nanoparticles exhibit size-dependent physical and chemical properties, unlike their bulk counterparts. The nanoparticle has emerged as an exciting new "eld of materials research. In particular, semiconductor nanoparticle quantum dots have drawn considerable attention because of their interesting properties like the shift of excitonic
* Correspondence address: Dr. M. Pattabi, Solar-Hydrogen Fuel Cell Group, Centro de Invstigacion en Energia, Universidad Nacional Autonoma de Mexico, Temixco 62580, Morelos, Mexico.. E-mail address: [email protected] (M. Pattabi). 0927-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 0 5 0 - 7
310
M. Pattabi, J. Uchil / Solar Energy Materials & Solar Cells 63 (2000) 309}314
peak to higher energy [1], blue shift in absorption band edge [2], band gap tuning [3], below picosecond radiative rates of optical transitions [3] and enhanced non-linear optical properties [4] which may have potential applications in optical switching [5], single charge memories [6], single electron transistors [7], etc. Thus, preparation of size controlled, monodispersed semiconductor nanoparticles in a stable host matrix to prevent #occulation is of primary importance. Physical methods like sputtering and vapor-phase condensation are quite widely used for the preparation of nanoparticles. The chemical methods used for the preparation of semiconductor nanoparticles involve reactions in various media including aqueous media, microemulsions, Zeolites, gels, polymers and glass. A variety of chemical techniques are used for the synthesis of nanoparticles. But the properties of nanoparticles by any new synthetic route are di$cult to anticipate. The characteristics desirable in the "nal nanodispersive medium are high purity, monodispersity and controlled surface derivatization. Reports on the synthesis of nanoparticles using biological membranes are sparse. This paper presents a novel but simple technique of synthesizing CdS nanoparticles of predetermined size, in a permanent non-degradable (non-aging) host matrix of a biological membrane by di!usion of precursors across it.
2. Experimental details The chicken egg membrane has been used as the host matrix for the synthesis of CdS nanoparticles in the present study. The chicken egg is lined inside with a 40 lm thick double-layered membrane. Aqueous solutions of Cadmium Acetate and Thiourea are allowed to di!use across the membrane resulting in the formation of CdS particles. The reaction was allowed to take place for 1, 5, 10, 20, 40, 50, 60, 80 and 100 min, at a "xed concentration of the reactants. The reaction can be stopped at any desired time by removing the membrane from the reacting bath and washing in distilled water. An optical absorption spectrum is taken for all the samples, with an undeposited specimen of the membrane as reference, using a SHIMADZU UV3101PC UV-Vis-NIR Scanning Spectrophotometer. The two concentrations discussed here are: Concentration 1: 400 g/l Cadmium Acetate (1.5 M) and 100 g/l Thiourea (1.3 M) Concentration 2: 100 g/l Cadmium Acetate (0.375 M) and 25 g/l Thiourea (0.325 M).
3. Results and discussion Fig. 1 shows the variation of optical absorption as a function of wavelength for CdS particles formed on the egg membrane for di!erent reaction times with the reactants at concentration 1. Optical absorption as a function of wavelength for CdS particles formed on egg membrane for di!erent reaction times with the reactants at concentration 2 is shown in Fig. 2. One can see that the absorption spectra is blue shifted. It can be further seen that there is a clear and systematic shift in the absorption edge towards longer wavelengths for samples with increasing reaction time in both the
M. Pattabi, J. Uchil / Solar Energy Materials & Solar Cells 63 (2000) 309}314
311
Fig. 1. Variation of optical absorption as a function of wavelength for CdS particles formed on the egg membrane for di!erent reaction times with the reactants at concentration 1.
Fig. 2. Optical absorption as a function of wavelength for CdS particles formed on the egg membrane for di!erent reaction times with the reactants at concentration 2.
concentrations. A comparison of the absorption spectra for these two concentrations shows that for the lower concentration, the absorption edge shifts to shorter wavelengths at a particular reaction time. It was found from the analysis of the absorption data that a best linear "t is obtained for (hla) as a function of hl , near the
312
M. Pattabi, J. Uchil / Solar Energy Materials & Solar Cells 63 (2000) 309}314
absorption edge. A typical variation of (hla) as a function of hl for a sample with 80 minutes of reaction time at concentration 2 is shown in Fig. 3. The intercept in the energy axis by extrapolating the straight line, gives the band gap of the CdS particles. Fig. 4 gives the experimentally determined band gap values for samples with di!erent
Fig. 3. A typical variation of (hla) as a function of hl for a sample with 80 min of reaction time at concentration 2.
Fig. 4. Variation of band gap with reaction time for concentration 1 ("lled squares) and concentration 2 ( "lled circles).
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313
reaction times at both the concentrations. The band gap of CdS is a function of particle size in the nanoparticle regime, under both tight binding and e!ective mass approximation [8]. Therefore, it is possible to estimate the particle size from the band gap values determined from the optical absorption studies. The particle sizes were estimated from the band gap values using the band gap variation with size data under the e!ective mass approximation (EMA) [8]. The estimated particle sizes for samples with di!erent reaction times at both the concentrations are shown in Fig. 5. The optical absorption spectra showed no drift in the position of the absorption edge with time. A size regime of a cluster containing a few hundred to a few thousand atoms can be recognized in the evolution from isolated atoms to molecular and to a bulk phase wherein the interior of the cluster can be identi"ed with that of a bulk crystalline lattice with large fraction of surface atoms. Such a cluster is termed as a nanocrystal and the absorption spectra of semiconductor nanocrystals is blue shifted [9]. The blue shift in the absorption spectra of the CdS formed on the egg membrane (Figs. 1 and 2) indicates the formation of CdS nanoparticles. It is interesting to note that the absorption edge is strongly dependent on the reaction time at a particular concentration. Based on the fact that a smaller nanoparticle will exhibit an absorption edge at a smaller wavelength , it immediately follows that the particle size increases with the reaction time. This indicates the growth of initially formed nuclei rather than fresh nucleation as more cadmium and sulfur ions di!use across the membrane and are available for reaction. With the dilution of the reactants, the availability of the ions decrease and it takes a longer time to form a larger cluster. This fact is evident from
Fig. 5. Variation of particle size with reaction time for concentration 1 ("lled squares) and concentration 2 ("lled circles).
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M. Pattabi, J. Uchil / Solar Energy Materials & Solar Cells 63 (2000) 309}314
the spectra at two di!erent concentrations for similar reaction times. Though the pore size in egg membrane is quite large compared with the size of the ions, the entangled nature of the pores in the membrane must be responsible for the slow di!usion of the ions across the membrane. It is interesting to note that the age of the membrane before deposition of CdS does not have any e!ect on the observed characteristics. Further, spectra recorded even three months after the formation of CdS particles do not show any drift indicating that coagulation has not taken place.
4. Conclusions The absorption spectra of the CdS formed on the chicken egg membrane by the di!usion of the precursors show a blue shift indicating the formation of CdS nanoparticles. The absorption edge shifts towards longer wavelengths for samples with increasing reaction time at a "xed concentration. At a lower concentration the absorption edge shifts to shorter wavelengths, for a particular reaction time. The band gaps calculated from the absorption spectra are higher than that for the bulk CdS and are dependent on the reaction time and concentration. The absorption edge does not show any drift with time even over a period of three months indicating the absence of coagulation after the nanoparticles are formed. The particle size, estimated from the band gaps, lies in the nanometer range. Thus, it is possible to prepare nanoparticles by the di!usion of precursors across a chicken egg membrane. The particle size and band gap can be tailored by controlling the deposition time and concentration of the precursors.
Acknowledgements The authors thank Prof. Jayagopal Uchil, Chairman, Department of Materials Science, Mangalore University, for providing the facilities and Mr. Ganesh Kumar for assistance.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Y. Wang, N. Herron, Phys. Rev. B. 42 (1990) 7253. B.G. Potter, J.H. Simmons, J. Appl. Phys. 68 (1990) 1218. T. Vossmeyer, L. Katsikas, M. Giersig, I.G. Popovic, H. Weller, J. Chem. Phys. 98 (1994) 7665. C.B. Murray, M. Nirma, M.G. Bwendi, Phys. Rev. Lett. 76 (1996) 1517. S. Schmitt-Rink, D.A.B. Miller, D.S. Chelma, Phys. Rev. B. 35 (1987) 8113. S. Tiwari, F. Rana, K. Chan, L. Shi, H. Hana", Appl. Phys. Lett. 69 (1996) 1232. D.L. Klien, J.E.B. Katani, R. Roth, A.P. Alvisatos, P.L. McEuen, Appl. Phys. Lett. 68 (1996) 2574. P.E. Lippons, M. Lannon, Phys. Rev. B. 39 (1989) 10935. A.C. Rastogi, in: Vikram Kumar, S. K. Agarwal (Ed.), Physics of Semiconductor Devices, Narosa Publishing House, London, 1998, p. 56.