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Luminescent and photocatalytic properties of cadmium sulfide nanoparticles synthesized via microwave irradiation
Materials Chemistry and Physics 90 (2005) 155–158
Luminescent and photocatalytic properties of cadmium sulfide nanoparticles synthesized via microwave irradiation Huaming Yanga,∗ , Chenghuan Huanga , Xianwei Lib , Rongrong Shia , Ke Zhanga a
Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China b Institute of Resources and Environmental Engineering, Technology Centre, Baoshan Iron and Steel Co., Ltd., Shanghai 201900, China Received 2 August 2004; received in revised form 16 September 2004; accepted 5 October 2004
Abstract Uniform cadmium sulfide (CdS) nanoparticles of about 6 nm in crystal size have been successfully synthesized via microwave irradiation. The as-prepared sample has a uniform morphology and high purity. The red photoluminescence spectrum of the CdS nanoparticles displays a strong peak at 602 nm by using a 300 nm excitation wavelength. The photocatalytic oxidation of methyl orange (MeO) in CdS suspensions under ultraviolet illumination was investigated. The results indicate that a low pH value (pH 2.0) and low reaction temperatures (20–30 ◦ C) will facilitate the decolorization of the MeO solution. The photodegradation degree decreases with increasing the pH value and temperature of solution. The efficiency of the recycled CdS semiconductor becomes lower due to the deposit of elemental Cd on the CdS surface, which weakens the photocatalytic activity. The luminescent and photocatalytic mechanisms of the as-prepared CdS nanoparticles were primarily discussed. Microwave irradiation is proved to be a convenient, efficient and environmental-friendly one-step route to synthesize nanoparticles. © 2004 Elsevier B.V. All rights reserved. Keywords: CdS nanoparticles; Microwave irradiation; Luminescence; Photocatalysis; Decolorization
1. Introduction In recent years, metal chalcogenides have attracted considerable attention due to their proven and potential applications in electronic, optical and superconductor devices [1]. Among these materials, CdS, one of the most important IV–VI group semiconductors, is a potential candidate in solar cells, photoelectric devices and photocatalysts [2–4]. Recently, several methods have been successfully applied for the preparation of CdS nanosized materials, including hydrothermal synthesis [5], microemulsion method [6] and ultraviolet (UV) irradiation technology [7]. Microwave is an electromagnetic radiation with the frequency range 0.3–300 GHz. Since 1986, microwave irradiation has been widely applied as a heating method in chemistry and material synthesis. A thermal gradient during microwave heating can be avoided due to the properties of ∗
Corresponding author. Tel.: +86 731 8830549; fax: +86 731 8710804. E-mail address: [email protected] (H. Yang).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.10.028
internal and volumetric heating, providing a uniform and friendly environment for chemical reactions [8]. This method has been successfully applied for the organic synthesis [9] and the preparation of a variety of nanosized inorganic materials [10,11]. Compared with conventional heating, microwave heating possesses the advantage of high-efficiency and rapid formation of nanoparticles with a narrow size distribution. This paper reports a simple route for the preparation of cadmium sulfide (CdS) nanoparticles via the microwaveinduced heating in aqueous solution. Furthermore, the luminescent and photocatalytic properties of the as-prepared sample were also primarily investigated.
2. Experimental details All reagents were of analytical grade and used without further purification. All solutions were prepared with distilled water. Fifty millilitres aqueous solution of 0.05 M sodium sulfide was added dropwise into a 250-ml round-bottom flask
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H. Yang et al. / Materials Chemistry and Physics 90 (2005) 155–158
filled with 50 ml, 0.05 M cadmium chloride solution. Simultaneously, the mixture was stirred and ultrasonicated. Then this round-bottom flask connected with a refluxing system was placed in a domestic microwave oven. The reaction was carried out under microwave irradiation for 10 min. After cooling to ambient temperature, the yellow precipitate was centrifuged and washed, then dried in a vacuum at 60 ◦ C for about 10 h. A domestic microwave oven (2450 MHz,Galanz WP900) was modified. A water-cooled condenser outside the microwave oven cavity was connected by a glass joint to the round-bottomed flask stably set inside, which was a so-called refluxing system. In order to avoid the leaking of microwave, the glass joint was covered by an aluminium lamella. The crystal structure of the as-prepared CdS was examined by Xray diffraction (XRD) using a D/max-γA diffractometer (Cu K␣ radiation) at a scanning rate of 4◦ min−1 in the diffraction angle range 2θ = 10◦ –70◦ . The morphology of the sample was observed using transmission electron microscopy (TEM; JEM-200CX). The photoluminescence spectrum of the sample was recorded with a Hitachi M-3500 fluorescence spectrophotometer under ambient atmosphere. The slit was set at 5 nm. The sample was pressed into a thin slice and then placed in a special support for measurement. The photocatalytic oxidation of methyl orange (MeO) in suspension of CdS under UV illumination was studied as a test experiment, in order to test the photocatalytic activity of the as-prepared powder. Methyl orange, a well-known acid-base indicator, was chosen as a simple model of a series of common azodyes, largely used in the industry. Its structure is shown in Fig. 1. A 500 ml of a 2 × 10−3 mol l−1 MeO solution was employed as a target. The reaction temperature was controlled by a thermostatic apparatus. The mixture inside a 1000-ml beaker remained in suspension by magnetic stirring. A 125-W high-pressure mercury lamp (GYZ-125) fixed at a distance of 16 cm above the surface solution was used as UV light source. The absorbance of the MeO solution, which was drawn and then centrifugated to remove the semiconductor at every 15 min, was measured with a UV-visible spectrophotometer. The characteristic absorption wavelength of the MeO solution was 460 nm. The degree of MeO decolorization could be calculated according to the equation C = (A0 − A)/A0 × 100%, where C is the decolorization degree, A0 is the initial absorbance of methyl orange solution, and A is the absorbance of the methyl orange solution after photocatalysis. In fact, there exists a linear relationship between the absorbance and concentration of the MeO solution under the same condition in our experiment. Therefore, the degree of MeO decolorization indicated its photodegradation.
Fig. 1. Structure of methyl orange (MeO).
Fig. 2. XRD pattern of the as-prepared CdS nanoparticles.
3. Results and discussion Fig. 2 shows the XRD pattern of the as-prepared CdS powder. The three peaks with 2θ values of 26.496◦ , 43.699◦ and 52.000◦ correspond to the (1 1 1), (2 2 0) and (3 1 1) planes of the cubic phase -CdS (JCPDS01-0647), respectively. No other peaks were observed in the XRD patterns. The broadness of the peaks was attributed to the small dimensions of the CdS nanoparticles. The average crystal size of the powder was calculated to be about 6.1 nm according to Scherrer’s formula. A TEM image of the as-prepared CdS nanoparticles is shown in Fig. 3. The nanoparticles have a spherical morphology with an average diameter of ca.10 nm. But moderate agglomeration is also observed due to the high surface energy of the nanoparticles. The inserted selected-area electron diffraction pattern accounts for the random orientation of the nanoparticles. The three rings correspond to the (1 1 1), (2 2 0) and (3 1 1) planes of the cubic CdS phase. The photoluminescence spectrum for the sample measured using a 300 nm excitation wavelength, which was obtained under emission with the wavelength of 600 nm, is shown in Fig. 4. The spectrum displays a strong peak at 602 nm, which may be attributed to the recombination of an electron trapped in a sulfur vacancy with a hole in the valence band of CdS. In our experiment, the sulfur ion concentration
Fig. 3. TEM image of the as-prepared CdS nanoparticles.
H. Yang et al. / Materials Chemistry and Physics 90 (2005) 155–158
Fig. 4. Photoluminescence spectrum of the CdS nanoparticles (the excitation wavelength was 300 nm).
157
Fig. 6. Efficiency of recycled semiconductors.
may be insufficient to form stoichiometric CdS, and leads to the formation of sulfur vacancies [5]. The photo-generated electrons can be trapped into sulfur vacancies through a nonradiation decay and then recombine with the hole in the valence band, resulting in the red shift of photoluminescence spectrum. When CdS is under UV illumination (hν), electrons of the value band are excited to the conduction band, leaving positive holes in the conduction band of the semiconductor [12]: CdS(s) + hν → CdS(h+ + e− )
(1)
The direct oxidation of MeO by positive holes absorbed on the surface of CdS was expected to be the main oxidation pathway [13]. The effect of the pH value on the photocatalytic decolorization of the MeO solution is shown in Fig. 5, indicating that a low pH value facilitates the decolorization reaction. It can be considered that the oxidation ability of positive holes became stronger under a lower pH value [4]. Additionally, this also may be due to the amount of MeO absorbed on the surface of CdS. The adsorption density of MeO decreases with increasing pH value analogue to the report of Davis and Huang [14]. Fig. 6 shows that the efficiency of recycled CdS was significantly lower than that of new one. A possible explanation is the following [12]. During the photocatalytic oxidation, positive holes could result in CdS dissolution: CdS(s) + 2h+ → Cd2+ + S(s)
(2)
Fig. 5. Effect of the pH value on the photocatalytic decolorization of the MeO solution.
Fig. 7. Effect of the temperature on the degree of MeO decolorization.
When the soluble Cd2+ ion concentration is high, the reduction of the Cd2+ ion to elemental Cd could occur on the surface of CdS, which will reduce the photocatalytic activity of CdS: Cd2+ + 2e− → Cd(s)
(3) 30 ◦ C
Fig. 7 shows that a temperature between 20 and is sufficient for the photodegradation of the MeO solution. The efficiency of CdS remarkably decreases with increasing the solution temperature. A tentative explanation is as follows. The CdS dissolution is enhanced at high temperatures, which will result in a consumption of more positive holes. Hence, there will be an insufficient amount of positive holes to render the organic compound oxidization. 4. Conclusions CdS nanoparticles of about 6 nm in crystal size have been successfully synthesized via microwave irradiation. The red photoluminescence spectrum of the as-prepared CdS nanoparticles displays a strong peak at 602 nm, which may be attributed to the recombination of an electron trapped in a sulfur vacancy with a hole in the valence band of CdS. The photocatalytic oxidation of MeO in CdS suspensions under UV illumination was primarily investigated. The result shows that a low pH value (pH 2.0) and low reaction temperatures (20–30 ◦ C) are suitable for the photodegradation of the MeO solution. The photodegradation degree decreases with increasing pH value and solution temperature. The efficiency of recycled CdS semiconductors becomes lower due to the
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H. Yang et al. / Materials Chemistry and Physics 90 (2005) 155–158
deposit of elemental Cd on the CdS surface, which weakens the photocatalytic activity. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 50304014). References [1] R.S. Mane, C.D. Lokhande, Mater. Chem. Phys. 65 (2000) 1. [2] A. Olea, P.J. Sebastian, Sol. Energy Mater. Sol. Cells 55 (1998) 149.
[3] M.M. Soliman, M.M. Shaban, F. Abulfotuh, in: Proceedings of IV World Renewable Energy Congress (WREC), USA, 1996. p. 386. [4] W.Z. Tang, C.P. Huang, Chemosphere 30 (1995) 1385. [5] G.Q. Xu, B. Liu, J. Phys. Chem. Solids 61 (2000) 829. [6] A. Agostiano, M. Catalano, Micron 31 (2000) 253. [7] S.D. Wu, Z. Zhu, Mater. Sci. Eng. B 90 (2002) 206. [8] M.J. Blandamer, A.R. Butler, Chem. Soc. Rev. 20 (1991) 1. [9] P. Lidstr¨om, J. Tierney, Tetrahedron 57 (2001) 9225. [10] H. Wang, J.Z. Xu, J. Cryst. Growth 244 (2002) 88. [11] Q. Liu, Y. Wei, Mater. Res. Bull. 33 (1998) 779. [12] W.Z. Tang, C.P. Huang, Water Res. 29 (1995) 745. [13] L.B. Reutergardh, M. Iangphasuk, Chemosphere 35 (1997) 385. [14] A.P. Davis, C.P. Huang, Langmuir 6 (1990) 857.
Luminescent and photocatalytic properties of cadmium sulfide nanoparticles synthesized via microwave irradiation Huaming Yanga,∗ , Chenghuan Huanga , Xianwei Lib , Rongrong Shia , Ke Zhanga a
Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China b Institute of Resources and Environmental Engineering, Technology Centre, Baoshan Iron and Steel Co., Ltd., Shanghai 201900, China Received 2 August 2004; received in revised form 16 September 2004; accepted 5 October 2004
Abstract Uniform cadmium sulfide (CdS) nanoparticles of about 6 nm in crystal size have been successfully synthesized via microwave irradiation. The as-prepared sample has a uniform morphology and high purity. The red photoluminescence spectrum of the CdS nanoparticles displays a strong peak at 602 nm by using a 300 nm excitation wavelength. The photocatalytic oxidation of methyl orange (MeO) in CdS suspensions under ultraviolet illumination was investigated. The results indicate that a low pH value (pH 2.0) and low reaction temperatures (20–30 ◦ C) will facilitate the decolorization of the MeO solution. The photodegradation degree decreases with increasing the pH value and temperature of solution. The efficiency of the recycled CdS semiconductor becomes lower due to the deposit of elemental Cd on the CdS surface, which weakens the photocatalytic activity. The luminescent and photocatalytic mechanisms of the as-prepared CdS nanoparticles were primarily discussed. Microwave irradiation is proved to be a convenient, efficient and environmental-friendly one-step route to synthesize nanoparticles. © 2004 Elsevier B.V. All rights reserved. Keywords: CdS nanoparticles; Microwave irradiation; Luminescence; Photocatalysis; Decolorization
1. Introduction In recent years, metal chalcogenides have attracted considerable attention due to their proven and potential applications in electronic, optical and superconductor devices [1]. Among these materials, CdS, one of the most important IV–VI group semiconductors, is a potential candidate in solar cells, photoelectric devices and photocatalysts [2–4]. Recently, several methods have been successfully applied for the preparation of CdS nanosized materials, including hydrothermal synthesis [5], microemulsion method [6] and ultraviolet (UV) irradiation technology [7]. Microwave is an electromagnetic radiation with the frequency range 0.3–300 GHz. Since 1986, microwave irradiation has been widely applied as a heating method in chemistry and material synthesis. A thermal gradient during microwave heating can be avoided due to the properties of ∗
Corresponding author. Tel.: +86 731 8830549; fax: +86 731 8710804. E-mail address: [email protected] (H. Yang).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.10.028
internal and volumetric heating, providing a uniform and friendly environment for chemical reactions [8]. This method has been successfully applied for the organic synthesis [9] and the preparation of a variety of nanosized inorganic materials [10,11]. Compared with conventional heating, microwave heating possesses the advantage of high-efficiency and rapid formation of nanoparticles with a narrow size distribution. This paper reports a simple route for the preparation of cadmium sulfide (CdS) nanoparticles via the microwaveinduced heating in aqueous solution. Furthermore, the luminescent and photocatalytic properties of the as-prepared sample were also primarily investigated.
2. Experimental details All reagents were of analytical grade and used without further purification. All solutions were prepared with distilled water. Fifty millilitres aqueous solution of 0.05 M sodium sulfide was added dropwise into a 250-ml round-bottom flask
156
H. Yang et al. / Materials Chemistry and Physics 90 (2005) 155–158
filled with 50 ml, 0.05 M cadmium chloride solution. Simultaneously, the mixture was stirred and ultrasonicated. Then this round-bottom flask connected with a refluxing system was placed in a domestic microwave oven. The reaction was carried out under microwave irradiation for 10 min. After cooling to ambient temperature, the yellow precipitate was centrifuged and washed, then dried in a vacuum at 60 ◦ C for about 10 h. A domestic microwave oven (2450 MHz,Galanz WP900) was modified. A water-cooled condenser outside the microwave oven cavity was connected by a glass joint to the round-bottomed flask stably set inside, which was a so-called refluxing system. In order to avoid the leaking of microwave, the glass joint was covered by an aluminium lamella. The crystal structure of the as-prepared CdS was examined by Xray diffraction (XRD) using a D/max-γA diffractometer (Cu K␣ radiation) at a scanning rate of 4◦ min−1 in the diffraction angle range 2θ = 10◦ –70◦ . The morphology of the sample was observed using transmission electron microscopy (TEM; JEM-200CX). The photoluminescence spectrum of the sample was recorded with a Hitachi M-3500 fluorescence spectrophotometer under ambient atmosphere. The slit was set at 5 nm. The sample was pressed into a thin slice and then placed in a special support for measurement. The photocatalytic oxidation of methyl orange (MeO) in suspension of CdS under UV illumination was studied as a test experiment, in order to test the photocatalytic activity of the as-prepared powder. Methyl orange, a well-known acid-base indicator, was chosen as a simple model of a series of common azodyes, largely used in the industry. Its structure is shown in Fig. 1. A 500 ml of a 2 × 10−3 mol l−1 MeO solution was employed as a target. The reaction temperature was controlled by a thermostatic apparatus. The mixture inside a 1000-ml beaker remained in suspension by magnetic stirring. A 125-W high-pressure mercury lamp (GYZ-125) fixed at a distance of 16 cm above the surface solution was used as UV light source. The absorbance of the MeO solution, which was drawn and then centrifugated to remove the semiconductor at every 15 min, was measured with a UV-visible spectrophotometer. The characteristic absorption wavelength of the MeO solution was 460 nm. The degree of MeO decolorization could be calculated according to the equation C = (A0 − A)/A0 × 100%, where C is the decolorization degree, A0 is the initial absorbance of methyl orange solution, and A is the absorbance of the methyl orange solution after photocatalysis. In fact, there exists a linear relationship between the absorbance and concentration of the MeO solution under the same condition in our experiment. Therefore, the degree of MeO decolorization indicated its photodegradation.
Fig. 1. Structure of methyl orange (MeO).
Fig. 2. XRD pattern of the as-prepared CdS nanoparticles.
3. Results and discussion Fig. 2 shows the XRD pattern of the as-prepared CdS powder. The three peaks with 2θ values of 26.496◦ , 43.699◦ and 52.000◦ correspond to the (1 1 1), (2 2 0) and (3 1 1) planes of the cubic phase -CdS (JCPDS01-0647), respectively. No other peaks were observed in the XRD patterns. The broadness of the peaks was attributed to the small dimensions of the CdS nanoparticles. The average crystal size of the powder was calculated to be about 6.1 nm according to Scherrer’s formula. A TEM image of the as-prepared CdS nanoparticles is shown in Fig. 3. The nanoparticles have a spherical morphology with an average diameter of ca.10 nm. But moderate agglomeration is also observed due to the high surface energy of the nanoparticles. The inserted selected-area electron diffraction pattern accounts for the random orientation of the nanoparticles. The three rings correspond to the (1 1 1), (2 2 0) and (3 1 1) planes of the cubic CdS phase. The photoluminescence spectrum for the sample measured using a 300 nm excitation wavelength, which was obtained under emission with the wavelength of 600 nm, is shown in Fig. 4. The spectrum displays a strong peak at 602 nm, which may be attributed to the recombination of an electron trapped in a sulfur vacancy with a hole in the valence band of CdS. In our experiment, the sulfur ion concentration
Fig. 3. TEM image of the as-prepared CdS nanoparticles.
H. Yang et al. / Materials Chemistry and Physics 90 (2005) 155–158
Fig. 4. Photoluminescence spectrum of the CdS nanoparticles (the excitation wavelength was 300 nm).
157
Fig. 6. Efficiency of recycled semiconductors.
may be insufficient to form stoichiometric CdS, and leads to the formation of sulfur vacancies [5]. The photo-generated electrons can be trapped into sulfur vacancies through a nonradiation decay and then recombine with the hole in the valence band, resulting in the red shift of photoluminescence spectrum. When CdS is under UV illumination (hν), electrons of the value band are excited to the conduction band, leaving positive holes in the conduction band of the semiconductor [12]: CdS(s) + hν → CdS(h+ + e− )
(1)
The direct oxidation of MeO by positive holes absorbed on the surface of CdS was expected to be the main oxidation pathway [13]. The effect of the pH value on the photocatalytic decolorization of the MeO solution is shown in Fig. 5, indicating that a low pH value facilitates the decolorization reaction. It can be considered that the oxidation ability of positive holes became stronger under a lower pH value [4]. Additionally, this also may be due to the amount of MeO absorbed on the surface of CdS. The adsorption density of MeO decreases with increasing pH value analogue to the report of Davis and Huang [14]. Fig. 6 shows that the efficiency of recycled CdS was significantly lower than that of new one. A possible explanation is the following [12]. During the photocatalytic oxidation, positive holes could result in CdS dissolution: CdS(s) + 2h+ → Cd2+ + S(s)
(2)
Fig. 5. Effect of the pH value on the photocatalytic decolorization of the MeO solution.
Fig. 7. Effect of the temperature on the degree of MeO decolorization.
When the soluble Cd2+ ion concentration is high, the reduction of the Cd2+ ion to elemental Cd could occur on the surface of CdS, which will reduce the photocatalytic activity of CdS: Cd2+ + 2e− → Cd(s)
(3) 30 ◦ C
Fig. 7 shows that a temperature between 20 and is sufficient for the photodegradation of the MeO solution. The efficiency of CdS remarkably decreases with increasing the solution temperature. A tentative explanation is as follows. The CdS dissolution is enhanced at high temperatures, which will result in a consumption of more positive holes. Hence, there will be an insufficient amount of positive holes to render the organic compound oxidization. 4. Conclusions CdS nanoparticles of about 6 nm in crystal size have been successfully synthesized via microwave irradiation. The red photoluminescence spectrum of the as-prepared CdS nanoparticles displays a strong peak at 602 nm, which may be attributed to the recombination of an electron trapped in a sulfur vacancy with a hole in the valence band of CdS. The photocatalytic oxidation of MeO in CdS suspensions under UV illumination was primarily investigated. The result shows that a low pH value (pH 2.0) and low reaction temperatures (20–30 ◦ C) are suitable for the photodegradation of the MeO solution. The photodegradation degree decreases with increasing pH value and solution temperature. The efficiency of recycled CdS semiconductors becomes lower due to the
158
H. Yang et al. / Materials Chemistry and Physics 90 (2005) 155–158
deposit of elemental Cd on the CdS surface, which weakens the photocatalytic activity. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 50304014). References [1] R.S. Mane, C.D. Lokhande, Mater. Chem. Phys. 65 (2000) 1. [2] A. Olea, P.J. Sebastian, Sol. Energy Mater. Sol. Cells 55 (1998) 149.
[3] M.M. Soliman, M.M. Shaban, F. Abulfotuh, in: Proceedings of IV World Renewable Energy Congress (WREC), USA, 1996. p. 386. [4] W.Z. Tang, C.P. Huang, Chemosphere 30 (1995) 1385. [5] G.Q. Xu, B. Liu, J. Phys. Chem. Solids 61 (2000) 829. [6] A. Agostiano, M. Catalano, Micron 31 (2000) 253. [7] S.D. Wu, Z. Zhu, Mater. Sci. Eng. B 90 (2002) 206. [8] M.J. Blandamer, A.R. Butler, Chem. Soc. Rev. 20 (1991) 1. [9] P. Lidstr¨om, J. Tierney, Tetrahedron 57 (2001) 9225. [10] H. Wang, J.Z. Xu, J. Cryst. Growth 244 (2002) 88. [11] Q. Liu, Y. Wei, Mater. Res. Bull. 33 (1998) 779. [12] W.Z. Tang, C.P. Huang, Water Res. 29 (1995) 745. [13] L.B. Reutergardh, M. Iangphasuk, Chemosphere 35 (1997) 385. [14] A.P. Davis, C.P. Huang, Langmuir 6 (1990) 857.