- Email: [email protected]
Green hydrothermal synthesis and characterization of CdO2 nanoparticles
Materials Letters 64 (2010) 1779–1781
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Green hydrothermal synthesis and characterization of CdO2 nanoparticles Yan Liu, Yong Cai Zhang ⁎, Ming Zhang Key Laboratory of Environmental Material and Environmental Engineering of Jiangsu Province, College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China
a r t i c l e
i n f o
Article history: Received 5 April 2010 Accepted 27 May 2010 Available online 2 June 2010 Keywords: Semiconductors Nanomaterials Green synthesis Optical materials and properties CdO2 and Hydrothermal synthesis
a b s t r a c t A green hydrothermal method has been developed for the synthesis of CdO2 nanoparticles from Cd(OH)2 powder and 6 vol.% H2O2 aqueous solution at 80–150 °C. The characterization results from X-ray diffraction, transmission electron microscopy, and thermal gravimetric and differential scanning calorimetry analysis disclosed that the resultant products were pure cubic phase CdO2 nanoparticles with the sizes in the range of about 11–13 nm. The UV–vis absorption spectra revealed that the as-synthesized CdO2 nanoparticles had similar optical band gaps of about 3.85 eV. The Raman spectra of the as-synthesized CdO2 nanoparticles displayed two obvious peaks at about 348 and 830/833 cm–1, a characteristic of pyrite-type IIB-peroxides. © 2010 Elsevier B.V. All rights reserved.
1. Introduction CdO2 is a wide-band-gap semiconductor (Eg ≈ 3.6 eV [1]), and has been used as a photocatalyst [2], precursor to CdO [3,4], and an additive in plastic and rubber industry, etc. At present, there exist several kinds of methods to prepare CdO2 thin film or powder [1–5], including chemical bath deposition [1], hydrothermal method [2], reaction of cadmium salts with hydrogen peroxide in ammonia [3,5], electrochemical route [4], direct reaction of oxygen with cadmium in oxygen or in liquid ammonia, treating cadmium salts in liquid ammonia with alkali superoxides, and reaction of cadmium dimethyl in ethereal solution with hydrogen peroxide [5], etc. However, it is difficult for most of the existing methods to large-scale produce CdO2 nanoparticles at low cost in an environmental-friendly way: they used either ammonia or liquid ammonia as the reaction medium [1–3,5] or Cd metal or metallorganics as the source materials [5], or the fabrication procedures were complicated or dangerous [4,5], or even the obtained products were impure [5]. Besides, as far as is known, the reports on the properties of CdO2 nanoparticles are scarce up to now. Nowadays, there is obviously an increased emphasis on the topic of green chemistry and chemical processes [6,7], which aim at the total elimination or at least the minimization of generated waste and the implementation of sustainable processes through the adoption of twelve fundamental principles [7]. Herein, we report a green and simple synthesis of CdO2 nanoparticles via hydrothermal treatment of Cd(OH)2 powder in 6 vol.% H2O2 aqueous solution at 80–150 °C for
⁎ Corresponding author. Tel.: + 86 514 87962581; fax: + 86 514 87975244. E-mail address: [email protected] (Y.C. Zhang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.05.047
12 h, as well as the characterization of the resultant products by X-ray diffraction (XRD), transmission electron microscopy (TEM), thermal gravimetric and differential scanning calorimetry (TG-DSC), UV–vis absorption spectra and Raman. 2. Experimental The reagents used are analytically pure and bought from Sinopharm Chemical Reagent Co., Ltd. In a synthesis, 0.5 g of Cd(OH)2 powder was weighed into a Teflon-lined stainless steel autoclave of 50 ml capacity, then 40 ml of 6 vol.% H2O2 aqueous solution was added with stirring. The autoclaves were sealed and kept at 80–150 °C for 12 h, then air-cooled to room temperature. The resulting precipitates were filtered, washed with distilled water and ethanol, and dried in air at 80 °C. The obtained products were characterized by XRD (German Bruker AXS D8 ADVANCE X-ray diffractometer), TEM (Holland Philips Tecnai12 TEM), TG-DSC (German NETZSCH STA409PC differential thermal analyzer, in a N2 atmosphere at a heating rate of 10 °C/min), UV–vis absorption spectra (Japan Shimadzu UV-2550 UV–vis spectrophotometer, by ultrasonically dispersing the products in water), Raman spectra (Britain Renishaw Invia Raman spectrometer, excitation at 532 nm, 3 mW). 3. Results and discussion Fig. 1(a) and (b) show the XRD patterns of the products synthesized at 80 and 150 °C, respectively. Both of them displayed the characteristic XRD peaks corresponding to cubic phase CdO2 (JCPDS card no. 01-078-1125), and no XRD peaks arising from the reactant
1780
Y. Liu et al. / Materials Letters 64 (2010) 1779–1781
Fig. 1. XRD patterns of the products synthesized at (a) 80 and (b) 150 °C.
Cd(OH)2 were visible in Fig. 1(a) and (b), indicating the successful preparation of phase-pure CdO2 powders. Because the starting materials adopted in our synthesis of CdO2 were only Cd(OH)2 powder and 6 vol.% H2O2 aqueous solution (which was used here as the O2– 2 source), and CdO2 was the sole resulting solid after the hydrothermal reaction at 80–150 °C for 12 h, it was
Fig. 3. TG–DSC curves of the CdO2 nanoparticles synthesized at 80 °C.
believed that the formation mechanism of CdO2 may be described as the following equation: CdðOHÞ2 þ H2 O2 ¼ CdO2 þ 2H2 O Although there was no available information about the solubility constant of CdO2, it was likely that the solubility of CdO2 in water is smaller than that of Cd(OH)2 (this may be inferred from the facts that Cd2+ exists just in the form of Cd(NH3)2+ 4 , rather than Cd(OH)2) in concentrated ammonia, but the addition of H2O2 into the Cd(NH3)2+ 4 solution, CdO2 can be formed [5]. That is, the transformation reaction of Cd(OH)2 → Cd(NH3)2+ 4 → CdO2 in water is thermodynamically allowable). Therefore, Cd(OH)2 had a tendency to transform into CdO2 in the H2O2 aqueous solution, and this transformation process can be completed under the hydrothermal conditions of 80–150 °C for 12 h. Furthermore, because Cd(OH)2 and H2O2 were weakly alkaline and acidic in water, respectively, the acid-base reaction between them may also play an important role in the formation of CdO2. Fig. 2(a) and (b) show the TEM images of the CdO2 powders synthesized at 80 and 150 °C, respectively. As can be seen, the products in Fig. 2(a) and (b) comprised aggregated nanoparticles, whose sizes were calculated to be about 11 and 13 nm, respectively, using the Scherrer formula based on the half width of their (111) diffraction peak.
Fig. 2. TEM images of the CdO2 nanoparticles synthesized at (a) 80 and (b) 150 °C.
Fig. 4. UV–vis absorption spectra of the CdO2 nanoparticles synthesized at (a) 80 and (b) 150 °C.
Y. Liu et al. / Materials Letters 64 (2010) 1779–1781
1781
Furthermore, the peak at about 830 (Fig. 5(a)) or 833 (Fig. 5(b)) cm–1 had always been assigned to the O–O stretch vibration in metal peroxides [8]. 4. Conclusions
Fig. 5. Raman spectra of the CdO2 nanoparticles synthesized at (a) 80 and (b) 150 °C.
The thermal behavior of the CdO2 nanoparticles synthesized at 80 °C was investigated by TG-DSC. The TG curve in Fig. 3 shows that the weight loss below 190 °C was nearly 1.2%, which was attributed to the release of surface-adsorbed/bound water. The small amount of adsorbed/bound water in the sample was nearly impossible to be avoided owing to the method of synthesis, fine particle size and instability of the material [4]. The weight loss in the range of 190– 400 °C was about 11.4%, close to the calculated value (11.1%) based on the reaction of CdO2 → CdO + 1/2O2. In addition, the DSC curve in Fig. 3 exhibited a strong and broad exothermic peak at around 229 °C, corresponding to the decomposition of CdO2 and crystallization of CdO. Fig. 4(a) and (b) show the UV–vis absorption spectra of the CdO2 nanoparticles synthesized at 80 and 150 °C, respectively. Both of them exhibited an excitonic absorption feature at around 322 nm (3.85 eV), which was obviously blue-shifted in comparison with the band gap (3.6 eV) of the CdO2 thin film prepared by chemical bath deposition [1]. Fig. 5(a) and (b) show the Raman spectra of the CdO 2 nanoparticles synthesized at 80 and 150 °C, respectively. Both of them displayed two obvious peaks at about 348 and 830/833 cm–1. Although the data about the Raman spectra of CdO2 were almost blank so far, the Raman spectra in Fig. 5 agreed with the experimental and calculated results for ZnO2 with a similar pyrite-type structure [8].
CdO2 nanoparticles were synthesized by a green and simple hydrothermal method from Cd(OH)2 powder and 6 vol.% H2O2 aqueous solution at 80–150 °C, and characterized by the XRD, TEM, TG-DSC, UV–vis absorption spectra and Raman. The proposed method had at least two obvious advantages over the previous methods in synthesizing CdO2: (1) the synthesis was carried out simply in water, with no need of controlling the pH of the reaction solution or using ammonia or liquid ammonia, etc., as the reaction medium; (2) the starting Cd(OH)2 powder can be completely transformed into the target CdO2 nanoparticles under the current hydrothermal conditions, without the release of unwanted byproducts or pollution to the environment, except H2O. Acknowledgements Thanks to the University Natural Science Foundation of Jiangsu Province (No. 08KJB150019 and 06KJA15011), and the National Natural Science Foundation of China (No. 50873085). References [1] León-Gutiérrez LRD, Cayente-Romero JJ, Peza-Tapia JM, Barrera-Calva E, MartínezFlores JC, Ortega-López M. Some physical properties of Sn–doped CdO thin films prepared by chemical bath deposition. Mater Lett 2006;60:3866–70. [2] Liu Y, Zhang YC, Xu XF. Hydrothermal synthesis and photocatalytic activity of CdO2 nanocrystals. J Hazard Mater 2009;163:1310–4. [3] Zhang YC, Wang GL. Solvothermal synthesis of CdO hollow nanostructures from CdO2 nanocrystals. Mater Lett 2008;62:673–5. [4] Han X, Liu R, Xu Z, Chen W, Zheng Y. Room temperature deposition of nanocrystalline cadmium peroxide thin film by electrochemical route. Electrochem Commun 2005;7:1195–8. [5] Hoffman CWW, Ropp RC, Mooney RW. Preparation, properties and structure of cadmium peroxide. J Am Chem Soc 1959;81:3830–4. [6] Zhang YC, Wu X, Hu XY, Guo R. Low-temperature synthesis of nanocrystalline ZnO by thermal decomposition of a “green” single-source inorganic precursor in air. J Cryst Growth 2005;280:250–4. [7] Anastas PT, Warner JC. Green chemistry: theory and practice. New York: Oxford University Press Inc.; 1998. [8] Zhang S, Chen Y, Song S, Jin Y, Du W. A Raman study of ZnO powders with ZnO2 as precursor. Spectrosc Spectr Anal 2008;28:2569–73 in Chinese.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Green hydrothermal synthesis and characterization of CdO2 nanoparticles Yan Liu, Yong Cai Zhang ⁎, Ming Zhang Key Laboratory of Environmental Material and Environmental Engineering of Jiangsu Province, College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China
a r t i c l e
i n f o
Article history: Received 5 April 2010 Accepted 27 May 2010 Available online 2 June 2010 Keywords: Semiconductors Nanomaterials Green synthesis Optical materials and properties CdO2 and Hydrothermal synthesis
a b s t r a c t A green hydrothermal method has been developed for the synthesis of CdO2 nanoparticles from Cd(OH)2 powder and 6 vol.% H2O2 aqueous solution at 80–150 °C. The characterization results from X-ray diffraction, transmission electron microscopy, and thermal gravimetric and differential scanning calorimetry analysis disclosed that the resultant products were pure cubic phase CdO2 nanoparticles with the sizes in the range of about 11–13 nm. The UV–vis absorption spectra revealed that the as-synthesized CdO2 nanoparticles had similar optical band gaps of about 3.85 eV. The Raman spectra of the as-synthesized CdO2 nanoparticles displayed two obvious peaks at about 348 and 830/833 cm–1, a characteristic of pyrite-type IIB-peroxides. © 2010 Elsevier B.V. All rights reserved.
1. Introduction CdO2 is a wide-band-gap semiconductor (Eg ≈ 3.6 eV [1]), and has been used as a photocatalyst [2], precursor to CdO [3,4], and an additive in plastic and rubber industry, etc. At present, there exist several kinds of methods to prepare CdO2 thin film or powder [1–5], including chemical bath deposition [1], hydrothermal method [2], reaction of cadmium salts with hydrogen peroxide in ammonia [3,5], electrochemical route [4], direct reaction of oxygen with cadmium in oxygen or in liquid ammonia, treating cadmium salts in liquid ammonia with alkali superoxides, and reaction of cadmium dimethyl in ethereal solution with hydrogen peroxide [5], etc. However, it is difficult for most of the existing methods to large-scale produce CdO2 nanoparticles at low cost in an environmental-friendly way: they used either ammonia or liquid ammonia as the reaction medium [1–3,5] or Cd metal or metallorganics as the source materials [5], or the fabrication procedures were complicated or dangerous [4,5], or even the obtained products were impure [5]. Besides, as far as is known, the reports on the properties of CdO2 nanoparticles are scarce up to now. Nowadays, there is obviously an increased emphasis on the topic of green chemistry and chemical processes [6,7], which aim at the total elimination or at least the minimization of generated waste and the implementation of sustainable processes through the adoption of twelve fundamental principles [7]. Herein, we report a green and simple synthesis of CdO2 nanoparticles via hydrothermal treatment of Cd(OH)2 powder in 6 vol.% H2O2 aqueous solution at 80–150 °C for
⁎ Corresponding author. Tel.: + 86 514 87962581; fax: + 86 514 87975244. E-mail address: [email protected] (Y.C. Zhang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.05.047
12 h, as well as the characterization of the resultant products by X-ray diffraction (XRD), transmission electron microscopy (TEM), thermal gravimetric and differential scanning calorimetry (TG-DSC), UV–vis absorption spectra and Raman. 2. Experimental The reagents used are analytically pure and bought from Sinopharm Chemical Reagent Co., Ltd. In a synthesis, 0.5 g of Cd(OH)2 powder was weighed into a Teflon-lined stainless steel autoclave of 50 ml capacity, then 40 ml of 6 vol.% H2O2 aqueous solution was added with stirring. The autoclaves were sealed and kept at 80–150 °C for 12 h, then air-cooled to room temperature. The resulting precipitates were filtered, washed with distilled water and ethanol, and dried in air at 80 °C. The obtained products were characterized by XRD (German Bruker AXS D8 ADVANCE X-ray diffractometer), TEM (Holland Philips Tecnai12 TEM), TG-DSC (German NETZSCH STA409PC differential thermal analyzer, in a N2 atmosphere at a heating rate of 10 °C/min), UV–vis absorption spectra (Japan Shimadzu UV-2550 UV–vis spectrophotometer, by ultrasonically dispersing the products in water), Raman spectra (Britain Renishaw Invia Raman spectrometer, excitation at 532 nm, 3 mW). 3. Results and discussion Fig. 1(a) and (b) show the XRD patterns of the products synthesized at 80 and 150 °C, respectively. Both of them displayed the characteristic XRD peaks corresponding to cubic phase CdO2 (JCPDS card no. 01-078-1125), and no XRD peaks arising from the reactant
1780
Y. Liu et al. / Materials Letters 64 (2010) 1779–1781
Fig. 1. XRD patterns of the products synthesized at (a) 80 and (b) 150 °C.
Cd(OH)2 were visible in Fig. 1(a) and (b), indicating the successful preparation of phase-pure CdO2 powders. Because the starting materials adopted in our synthesis of CdO2 were only Cd(OH)2 powder and 6 vol.% H2O2 aqueous solution (which was used here as the O2– 2 source), and CdO2 was the sole resulting solid after the hydrothermal reaction at 80–150 °C for 12 h, it was
Fig. 3. TG–DSC curves of the CdO2 nanoparticles synthesized at 80 °C.
believed that the formation mechanism of CdO2 may be described as the following equation: CdðOHÞ2 þ H2 O2 ¼ CdO2 þ 2H2 O Although there was no available information about the solubility constant of CdO2, it was likely that the solubility of CdO2 in water is smaller than that of Cd(OH)2 (this may be inferred from the facts that Cd2+ exists just in the form of Cd(NH3)2+ 4 , rather than Cd(OH)2) in concentrated ammonia, but the addition of H2O2 into the Cd(NH3)2+ 4 solution, CdO2 can be formed [5]. That is, the transformation reaction of Cd(OH)2 → Cd(NH3)2+ 4 → CdO2 in water is thermodynamically allowable). Therefore, Cd(OH)2 had a tendency to transform into CdO2 in the H2O2 aqueous solution, and this transformation process can be completed under the hydrothermal conditions of 80–150 °C for 12 h. Furthermore, because Cd(OH)2 and H2O2 were weakly alkaline and acidic in water, respectively, the acid-base reaction between them may also play an important role in the formation of CdO2. Fig. 2(a) and (b) show the TEM images of the CdO2 powders synthesized at 80 and 150 °C, respectively. As can be seen, the products in Fig. 2(a) and (b) comprised aggregated nanoparticles, whose sizes were calculated to be about 11 and 13 nm, respectively, using the Scherrer formula based on the half width of their (111) diffraction peak.
Fig. 2. TEM images of the CdO2 nanoparticles synthesized at (a) 80 and (b) 150 °C.
Fig. 4. UV–vis absorption spectra of the CdO2 nanoparticles synthesized at (a) 80 and (b) 150 °C.
Y. Liu et al. / Materials Letters 64 (2010) 1779–1781
1781
Furthermore, the peak at about 830 (Fig. 5(a)) or 833 (Fig. 5(b)) cm–1 had always been assigned to the O–O stretch vibration in metal peroxides [8]. 4. Conclusions
Fig. 5. Raman spectra of the CdO2 nanoparticles synthesized at (a) 80 and (b) 150 °C.
The thermal behavior of the CdO2 nanoparticles synthesized at 80 °C was investigated by TG-DSC. The TG curve in Fig. 3 shows that the weight loss below 190 °C was nearly 1.2%, which was attributed to the release of surface-adsorbed/bound water. The small amount of adsorbed/bound water in the sample was nearly impossible to be avoided owing to the method of synthesis, fine particle size and instability of the material [4]. The weight loss in the range of 190– 400 °C was about 11.4%, close to the calculated value (11.1%) based on the reaction of CdO2 → CdO + 1/2O2. In addition, the DSC curve in Fig. 3 exhibited a strong and broad exothermic peak at around 229 °C, corresponding to the decomposition of CdO2 and crystallization of CdO. Fig. 4(a) and (b) show the UV–vis absorption spectra of the CdO2 nanoparticles synthesized at 80 and 150 °C, respectively. Both of them exhibited an excitonic absorption feature at around 322 nm (3.85 eV), which was obviously blue-shifted in comparison with the band gap (3.6 eV) of the CdO2 thin film prepared by chemical bath deposition [1]. Fig. 5(a) and (b) show the Raman spectra of the CdO 2 nanoparticles synthesized at 80 and 150 °C, respectively. Both of them displayed two obvious peaks at about 348 and 830/833 cm–1. Although the data about the Raman spectra of CdO2 were almost blank so far, the Raman spectra in Fig. 5 agreed with the experimental and calculated results for ZnO2 with a similar pyrite-type structure [8].
CdO2 nanoparticles were synthesized by a green and simple hydrothermal method from Cd(OH)2 powder and 6 vol.% H2O2 aqueous solution at 80–150 °C, and characterized by the XRD, TEM, TG-DSC, UV–vis absorption spectra and Raman. The proposed method had at least two obvious advantages over the previous methods in synthesizing CdO2: (1) the synthesis was carried out simply in water, with no need of controlling the pH of the reaction solution or using ammonia or liquid ammonia, etc., as the reaction medium; (2) the starting Cd(OH)2 powder can be completely transformed into the target CdO2 nanoparticles under the current hydrothermal conditions, without the release of unwanted byproducts or pollution to the environment, except H2O. Acknowledgements Thanks to the University Natural Science Foundation of Jiangsu Province (No. 08KJB150019 and 06KJA15011), and the National Natural Science Foundation of China (No. 50873085). References [1] León-Gutiérrez LRD, Cayente-Romero JJ, Peza-Tapia JM, Barrera-Calva E, MartínezFlores JC, Ortega-López M. Some physical properties of Sn–doped CdO thin films prepared by chemical bath deposition. Mater Lett 2006;60:3866–70. [2] Liu Y, Zhang YC, Xu XF. Hydrothermal synthesis and photocatalytic activity of CdO2 nanocrystals. J Hazard Mater 2009;163:1310–4. [3] Zhang YC, Wang GL. Solvothermal synthesis of CdO hollow nanostructures from CdO2 nanocrystals. Mater Lett 2008;62:673–5. [4] Han X, Liu R, Xu Z, Chen W, Zheng Y. Room temperature deposition of nanocrystalline cadmium peroxide thin film by electrochemical route. Electrochem Commun 2005;7:1195–8. [5] Hoffman CWW, Ropp RC, Mooney RW. Preparation, properties and structure of cadmium peroxide. J Am Chem Soc 1959;81:3830–4. [6] Zhang YC, Wu X, Hu XY, Guo R. Low-temperature synthesis of nanocrystalline ZnO by thermal decomposition of a “green” single-source inorganic precursor in air. J Cryst Growth 2005;280:250–4. [7] Anastas PT, Warner JC. Green chemistry: theory and practice. New York: Oxford University Press Inc.; 1998. [8] Zhang S, Chen Y, Song S, Jin Y, Du W. A Raman study of ZnO powders with ZnO2 as precursor. Spectrosc Spectr Anal 2008;28:2569–73 in Chinese.