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A simple way to prepare hydrophobic AgCl nanoparticles with multivalent metal ions on their surface
Materials Letters 58 (2004) 2049 – 2052 www.elsevier.com/locate/matlet
A simple way to prepare hydrophobic AgCl nanoparticles with multivalent metal ions on their surface Mei-Rong Song, Zhi-Jun Zhang * Laboratory of Special Functional Materials, Henan University, Kaifeng 475001, PR China Received 4 November 2003; accepted 6 January 2004
Abstract In this paper, we posed a new simple method to prepare AgCl nanoparticles with cupric 2-ethylhexoate (Cu 2-eh) capping layer, which could be well dispersed in various organic solvents. The Cu 2-eh capped AgCl (CEH-AgCl) particles have an average size of 80 nm, and the relationship between Cu 2-eh capped layer and the negative-charged AgCl core might be electrostatic force. UV – Visible (UV – Vis) spectra show that the prepared product presents a typical absorption of AgCl nanoparticles with an additional shoulder peak at longer wavelength. D 2004 Elsevier B.V. All rights reserved. Keywords: AgCl; Nanoparticles; Cupric 2-ethylhexoate; Hydrophobic; Optical property; Preparation
1. Introduction Synthesis and application of nanocomposite materials has recently been extensively focused on many areas in recent years because of their different physical and chemical properties from bulk materials or individual molecular [1– 3]. Among these, the synthesis of inorganic– organic composite materials has attracted much attention [4], since they combine the properties of both, inorganic in terms of its mechanical strength, thermal stability, charge generating properties (in the case of semiconductors) and the processability and flexibility of organics [5]. One popular synthesis method for such nanoparticles is the so-called reverse micelle technique [6]. For example, in 1993, Robert [7] employed a reverse micelle technique to form particles of silver halides (including AgCl, AgBr, AgI or mixtures thereof) that are capped with a layer of organic material by adding Cu2 +, Zn2 + as handles, which made silver halides soluble in various organic solvents and thus readily incorporable into a polymer matrix including PMMA, PC and CR-39 for some optical uses. However, this process involves surfactants and large amounts of organic solvent and is not very suitable for production on a large scale. Surface chemical modification of the nano-
* Corresponding author. Tel./fax: +86-378-2852533. E-mail address: [email protected] (Z.-J. Zhang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.01.002
particles developed in recent years is an easier means to obtain such oil-soluble nanoparticles [8]. In 2002, we prepared oil-soluble silver chloride nanoparticles by surface modification method [9], but failed in adding multivalent ions on the surface of silver halides particles that can often be used as sensitizer to help the separation of photoinduced electron-hole pairs [10 – 12]. In this paper, we present a simple way to prepare hydrophobic AgCl nanoparticles with multivalent metal ions on their surface. We also provide detailed characterization to study the structure, composition and optical absorption of the product.
2. Experimental 2.1. Chemicals and preparation The analytically pure silver nitrate (AgNO3), sodium chloride (NaCl) and ethanol were used as the raw materials for synthesis. Denatured cupric 2-ethylhexoate (Cu 2-eh) (Cu f 5 wt.%) was purchased without further treatment. In a typical preparation, the procedures were as follows: 20 ml of 0.1 M NaCl aqueous solution and 80 ml ethanol were added into a 250-ml reaction flask; then 5 ml of 0.1 M AgNO3 water solution was also added into the above mixtures quickly; after a period of 5– 10 s, the solution of 1 ml Cu 2-eh in 60 ml ethanol was introduced into the flask,
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and magnetic stirring was continuously applied throughout the entire process. Subsequently, a light blue precipitate was produced gradually. After 1 h, the light blue precipitate was filtered and rinsed by a great amount of distilled water and ethanol and turned into green finally. All procedures were performed at room temperature. The final target product was dried at 65 jC in oven for 12 h. Blank white AgCl particles were also prepared under the same procedures as above, except Cu 2-eh was not used. The Cu 2-eh capped AgCl (CEH-AgCl) nanoparticles can be partly dispersed into polar solvents, for example, ethanol and acetone, with ultrasonic dispersion (but soon precipitate), fully dispersed in weak-polar solvents, such as chloroform, benzene and methylbenzene, and easily dispersed in nonpolar solvents like liquid petroleum either without ultrasonic dispersion. However, in the case of the non-capped AgCl particles, they cannot be dispersed in all solvents above. Therefore, it was concluded that after adding Cu 2-eh, the dispersion capacity of nanosized AgCl in organics had been greatly improved. 2.2. Characterization of the synthesized nanoparticles Transmission electron microscopy (TEM) was carried out on a JEOL model JEM-2010 Ex/S electron microscope. The sample for TEM analysis was prepared by placing a drop of solution, CEH-AgCl dispersed in chloroform on a copper grid coated with a carbon film. Powder X-ray diffraction (XRD) measurement was performed on an X’pert Philips diffractometer using Cu Ka radiation, operating at 40 kV and 40 mA. Infrared (IR) analysis was carried out on AVATAR 360FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an AXIS ULTRO electron spectrometer (Kratos). The Al Ka radiation was used as the excitation source, and Flood gun had been used to reduce the charge effect. The prepared powder was pressed into a slice adhered on insulated tape. UV – Visible (UV – Vis) absorption spectra were collected on HEEIOSa spectrometer, and the result curves are obtained by reducing the absorption of solvent.
3. Results and discussion X-ray diffraction pattern of CEH-AgCl nanoparticles is shown in Fig. 1. It is seen that XRD pattern exhibits intense peaks that are in good agreement with the crystal planes of a face-centered cubic phase of silver chlorides (JCPDS 311238). The peaks represent different crystal planes that are assigned to according to the degree of 2h as follows: 24.75 (111), 32.26 (200), 46.14 (220), 54.85 (311), 57.71 (222) and 76.88 (420) crystal planes, respectively. According to Scherrer’s equation, average AgCl crystalline size was estimated to be about 70 nm.
Fig. 1. X-ray diffraction (XRD) pattern of CEH-AgCl nanoparticles.
Fig. 2 shows TEM image of CEH-AgCl nanoparticles dispersed in chloroform. The shape of the particles is irregular, and the sample has a wide size distribution in the range of 50 –200 nm. Average diameter of the particles is 80 nm, which is a little bigger than the result of XRD due to the presence of modified layer and aggregation of some particles. FT-IR spectra of Cu 2-eh and CEH-AgCl are shown in Fig. 3 as well as spectrum of 2-ethylhexoic acid as a reference. For 2-ethylhexoic acid, the wide and strong bands around 3200 –2500 cm 1 are assigned to stretching vibration absorption of O – H in dimer of 2-ethylhexoic acid. The absorption of tCjO is located at 1705 cm 1. The major differences of IR spectrum of Cu 2-eh from that of 2ethylhexoic acid are as follows: narrow bands around 2900 cm 1 instead of wide ones between 3200 and 2500 cm 1 appear, which are attributed to asymmetric and symmetric stretching vibration of – CH3 and – CH2; there are also three new peaks including one strong with two shoulder peaks at 1589, 1537 and 1506 cm 1, respectively. The former change indicates the disappearance of dimer, and the latter are attributed to the tCjO coordinated with Cu2 +, which is similar to the result reported by Gotoh [13]. In addition, there are absorption peaks of impurities; for example, 1704 cm 1 indicates that there is unreacted 2ethylhexoic acid in denatured Cu 2-eh. In the case of CEHAgCl, there are some differences compared with Cu 2-eh: first, peaks of impurities disappear; secondly, there is only one strong peak left between 1500 and 1600 cm 1 that is located at 1551 cm 1, which means the state of tCjO has been influenced by other substances besides Cu2 +. Since Cu2 + ions may be attracted by negatively charged AgCl nanoparticle by surplus Cl in synthesis procedure through electrostatic force, there might be a coordination effect involving Cu 2-eh and the surface of AgCl which results in the change of state of tCjO compared with that in Cu 2eh. In addition, peaks of long alkyl chain around 2900 cm 1 are also present. The broad peak at 3439 cm 1 and the weak peak at 1622 cm 1 indicate somewhat that ethanol and water exist in our samples. IR analysis suggests that Cu 2-eh
M.-R. Song, Z.-J. Zhang / Materials Letters 58 (2004) 2049–2052
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Fig. 2. TEM image of CEH-AgCl nanoparticles dispersed in chloroform.
has been chemically adsorbed on the surface of AgCl nanoparticles. The surface composition of CEH-AgCl nanoparticles was also studied by XPS measurements. Typical XPS spectrum is shown in Fig. 4a, which indicates that the surface of the nanoparticles are mainly composed of C, O and Cu, but Ag and Cl are hardly detected by XPS. Therefore, the product is not a simple mixture of AgCl nanoparticles and Cu 2-eh, and the latter is really capped on the surface of AgCl nanoparticles. We can conclude the valence of Cu is 2+ due to the presence of two shake-up peaks in Fig. 4b that are the feature of Cu2 + in addition to peaks of Cu2p1/2 and Cu2p3/2. Meanwhile, the atomic ratio of C, O and Cu calculated by XPS measurement is 9.65:2.28:1. Since the sample is easy to be contaminated by C and O that will lead to the increase of their concentration, we estimate that the actual ratio is close to 8:2:1, which suggests that only one C8H14O2 group is bonded to Cu2 +. Therefore, Cu 2-eh may exist on the surface of AgCl
Fig. 3. IR spectra of 2-ehylhexoic acid, Cu 2-eh and CEH-AgCl nanoparticles.
Fig. 4. XPS spectra of the CEH-AgCl nanoparticles.
as the form of [Cu(C8H14O2)]+ (abbreviated to (Cu-2-eh)+). Accompanied by analysis of FT-IR, the structure of the prepared CEH-AgCl nanoparticles is suggested as indicated in Fig. 5. As-prepared particle consists of an AgCl core and a modified layer of Cu 2-eh on the surface, and the relationship between the layer and the core might be electrostatic force. Fig. 6 shows UV – Vis absorption spectra of cupric 2ethylhexoate, CEH-AgCl nanoparticles and blank AgCl
Fig. 5. The suggested structure of the prepared CEH-AgCl nanoparticles.
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The results show that the modified particles have a mean size of 80 nm and good oil solubility. Cupric 2-ethylhexoate is capped on the surface of AgCl nanoparticles by electrostatic force in the form of (Cu-2-eh)+. Compared with previous methods, we present a simple and effective method to bond multivalent metal ions on the surface of hydrophobic AgX nanoparticles. The method can be applied to other metal ions, such as Co2 +, which we have demonstrated by using cobalt naphthenate as raw material (unpublished).
Acknowledgements
Fig. 6. UV – Vis absorption spectra of cupric 2-ethylhexote, CEH-AgCl nanoparticles and blank AgCl particles.
The financial support from the National Nature Science Foundation of China is gratefully acknowledged.
References particles. Cupric 2-ethylhexoate exhibits absorption at 256 nm attributed to the carboxyl group. For blank AgCl, the absorption peaks become so weak that they could hardly be discerned because they are not stable and soon precipitate. In the case of CEH-AgCl, it shows a typical absorption of AgCl nanoparticles at 244 nm which shifts to shorter wavelength due to quantum size effect compared with the value, 255 –265 nm, reported by Liu [14]. In addition, a shoulder peak at longer wavelength indicates that the absorption of AgCl is affected by (Cu-2-eh)+ on its surface.
4. Conclusions CEH-AgCl nanoparticles were prepared by adding a precipitate reagent to negative-charged AgCl sol solution.
[1] A.A. Eliseev, A.V. Lukashin, et al., Mater. Res. Innov. 3 (2001) 308. [2] O.B. Stephen, B. Louis, C.B. Murray, J. Am. Chem. Soc. 123 (2001) 12085. [3] P. Calandra, A. Longo, V.T. Liveri, Colloid Polym. Sci. 279 (2001) 1112. [4] S. Chen, W.M. Liu, L.G. Yu, Wear 218 (1998) 153. [5] M. Lal, M. Joshi, D.N. Kumar, C.S. Friend, et al., Mat. Res. Soc. Symp. Proc. 519 1998, pp. 217. [6] J.K. Lee, S.M. Choi, Bull. Korean Chem. Soc. 24 (1) (2003) 32. [7] E.S. Robert, B.S. Kevin, US Pat. 5252450.1993-10-12. [8] C.H. Wu, S.M. Wang, New Chem. Mater. 30 (7) (2002) 1. [9] M.R. Song, Z.R. Li, H. Yang, et al., Chem. Res. 13 (3) (2002) 1. [10] H. Tomonaga, T. Morimoto, Thin Solid Films 392 (2001) 355. [11] A. Kriltz, M. Muller, R. Fachet, H. Burger, J. Mater. Sci. 32 (1997) 169. [12] J. Weidlich, U. Dieckmann, O. Volker, US Pat. 5916487.1999-6-29. [13] Y. Gotoh, R. Igarashi, Y. Ohkoshi, et al., J. Mater. Chem. 10 (2000) 2548. [14] C.Y. Liu, X.Y. Chen, Y.J. Wang, Chin. J. Chem. Phys. 10 (1) (1997) 43.
A simple way to prepare hydrophobic AgCl nanoparticles with multivalent metal ions on their surface Mei-Rong Song, Zhi-Jun Zhang * Laboratory of Special Functional Materials, Henan University, Kaifeng 475001, PR China Received 4 November 2003; accepted 6 January 2004
Abstract In this paper, we posed a new simple method to prepare AgCl nanoparticles with cupric 2-ethylhexoate (Cu 2-eh) capping layer, which could be well dispersed in various organic solvents. The Cu 2-eh capped AgCl (CEH-AgCl) particles have an average size of 80 nm, and the relationship between Cu 2-eh capped layer and the negative-charged AgCl core might be electrostatic force. UV – Visible (UV – Vis) spectra show that the prepared product presents a typical absorption of AgCl nanoparticles with an additional shoulder peak at longer wavelength. D 2004 Elsevier B.V. All rights reserved. Keywords: AgCl; Nanoparticles; Cupric 2-ethylhexoate; Hydrophobic; Optical property; Preparation
1. Introduction Synthesis and application of nanocomposite materials has recently been extensively focused on many areas in recent years because of their different physical and chemical properties from bulk materials or individual molecular [1– 3]. Among these, the synthesis of inorganic– organic composite materials has attracted much attention [4], since they combine the properties of both, inorganic in terms of its mechanical strength, thermal stability, charge generating properties (in the case of semiconductors) and the processability and flexibility of organics [5]. One popular synthesis method for such nanoparticles is the so-called reverse micelle technique [6]. For example, in 1993, Robert [7] employed a reverse micelle technique to form particles of silver halides (including AgCl, AgBr, AgI or mixtures thereof) that are capped with a layer of organic material by adding Cu2 +, Zn2 + as handles, which made silver halides soluble in various organic solvents and thus readily incorporable into a polymer matrix including PMMA, PC and CR-39 for some optical uses. However, this process involves surfactants and large amounts of organic solvent and is not very suitable for production on a large scale. Surface chemical modification of the nano-
* Corresponding author. Tel./fax: +86-378-2852533. E-mail address: [email protected] (Z.-J. Zhang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.01.002
particles developed in recent years is an easier means to obtain such oil-soluble nanoparticles [8]. In 2002, we prepared oil-soluble silver chloride nanoparticles by surface modification method [9], but failed in adding multivalent ions on the surface of silver halides particles that can often be used as sensitizer to help the separation of photoinduced electron-hole pairs [10 – 12]. In this paper, we present a simple way to prepare hydrophobic AgCl nanoparticles with multivalent metal ions on their surface. We also provide detailed characterization to study the structure, composition and optical absorption of the product.
2. Experimental 2.1. Chemicals and preparation The analytically pure silver nitrate (AgNO3), sodium chloride (NaCl) and ethanol were used as the raw materials for synthesis. Denatured cupric 2-ethylhexoate (Cu 2-eh) (Cu f 5 wt.%) was purchased without further treatment. In a typical preparation, the procedures were as follows: 20 ml of 0.1 M NaCl aqueous solution and 80 ml ethanol were added into a 250-ml reaction flask; then 5 ml of 0.1 M AgNO3 water solution was also added into the above mixtures quickly; after a period of 5– 10 s, the solution of 1 ml Cu 2-eh in 60 ml ethanol was introduced into the flask,
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and magnetic stirring was continuously applied throughout the entire process. Subsequently, a light blue precipitate was produced gradually. After 1 h, the light blue precipitate was filtered and rinsed by a great amount of distilled water and ethanol and turned into green finally. All procedures were performed at room temperature. The final target product was dried at 65 jC in oven for 12 h. Blank white AgCl particles were also prepared under the same procedures as above, except Cu 2-eh was not used. The Cu 2-eh capped AgCl (CEH-AgCl) nanoparticles can be partly dispersed into polar solvents, for example, ethanol and acetone, with ultrasonic dispersion (but soon precipitate), fully dispersed in weak-polar solvents, such as chloroform, benzene and methylbenzene, and easily dispersed in nonpolar solvents like liquid petroleum either without ultrasonic dispersion. However, in the case of the non-capped AgCl particles, they cannot be dispersed in all solvents above. Therefore, it was concluded that after adding Cu 2-eh, the dispersion capacity of nanosized AgCl in organics had been greatly improved. 2.2. Characterization of the synthesized nanoparticles Transmission electron microscopy (TEM) was carried out on a JEOL model JEM-2010 Ex/S electron microscope. The sample for TEM analysis was prepared by placing a drop of solution, CEH-AgCl dispersed in chloroform on a copper grid coated with a carbon film. Powder X-ray diffraction (XRD) measurement was performed on an X’pert Philips diffractometer using Cu Ka radiation, operating at 40 kV and 40 mA. Infrared (IR) analysis was carried out on AVATAR 360FT-IR spectrometer. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an AXIS ULTRO electron spectrometer (Kratos). The Al Ka radiation was used as the excitation source, and Flood gun had been used to reduce the charge effect. The prepared powder was pressed into a slice adhered on insulated tape. UV – Visible (UV – Vis) absorption spectra were collected on HEEIOSa spectrometer, and the result curves are obtained by reducing the absorption of solvent.
3. Results and discussion X-ray diffraction pattern of CEH-AgCl nanoparticles is shown in Fig. 1. It is seen that XRD pattern exhibits intense peaks that are in good agreement with the crystal planes of a face-centered cubic phase of silver chlorides (JCPDS 311238). The peaks represent different crystal planes that are assigned to according to the degree of 2h as follows: 24.75 (111), 32.26 (200), 46.14 (220), 54.85 (311), 57.71 (222) and 76.88 (420) crystal planes, respectively. According to Scherrer’s equation, average AgCl crystalline size was estimated to be about 70 nm.
Fig. 1. X-ray diffraction (XRD) pattern of CEH-AgCl nanoparticles.
Fig. 2 shows TEM image of CEH-AgCl nanoparticles dispersed in chloroform. The shape of the particles is irregular, and the sample has a wide size distribution in the range of 50 –200 nm. Average diameter of the particles is 80 nm, which is a little bigger than the result of XRD due to the presence of modified layer and aggregation of some particles. FT-IR spectra of Cu 2-eh and CEH-AgCl are shown in Fig. 3 as well as spectrum of 2-ethylhexoic acid as a reference. For 2-ethylhexoic acid, the wide and strong bands around 3200 –2500 cm 1 are assigned to stretching vibration absorption of O – H in dimer of 2-ethylhexoic acid. The absorption of tCjO is located at 1705 cm 1. The major differences of IR spectrum of Cu 2-eh from that of 2ethylhexoic acid are as follows: narrow bands around 2900 cm 1 instead of wide ones between 3200 and 2500 cm 1 appear, which are attributed to asymmetric and symmetric stretching vibration of – CH3 and – CH2; there are also three new peaks including one strong with two shoulder peaks at 1589, 1537 and 1506 cm 1, respectively. The former change indicates the disappearance of dimer, and the latter are attributed to the tCjO coordinated with Cu2 +, which is similar to the result reported by Gotoh [13]. In addition, there are absorption peaks of impurities; for example, 1704 cm 1 indicates that there is unreacted 2ethylhexoic acid in denatured Cu 2-eh. In the case of CEHAgCl, there are some differences compared with Cu 2-eh: first, peaks of impurities disappear; secondly, there is only one strong peak left between 1500 and 1600 cm 1 that is located at 1551 cm 1, which means the state of tCjO has been influenced by other substances besides Cu2 +. Since Cu2 + ions may be attracted by negatively charged AgCl nanoparticle by surplus Cl in synthesis procedure through electrostatic force, there might be a coordination effect involving Cu 2-eh and the surface of AgCl which results in the change of state of tCjO compared with that in Cu 2eh. In addition, peaks of long alkyl chain around 2900 cm 1 are also present. The broad peak at 3439 cm 1 and the weak peak at 1622 cm 1 indicate somewhat that ethanol and water exist in our samples. IR analysis suggests that Cu 2-eh
M.-R. Song, Z.-J. Zhang / Materials Letters 58 (2004) 2049–2052
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Fig. 2. TEM image of CEH-AgCl nanoparticles dispersed in chloroform.
has been chemically adsorbed on the surface of AgCl nanoparticles. The surface composition of CEH-AgCl nanoparticles was also studied by XPS measurements. Typical XPS spectrum is shown in Fig. 4a, which indicates that the surface of the nanoparticles are mainly composed of C, O and Cu, but Ag and Cl are hardly detected by XPS. Therefore, the product is not a simple mixture of AgCl nanoparticles and Cu 2-eh, and the latter is really capped on the surface of AgCl nanoparticles. We can conclude the valence of Cu is 2+ due to the presence of two shake-up peaks in Fig. 4b that are the feature of Cu2 + in addition to peaks of Cu2p1/2 and Cu2p3/2. Meanwhile, the atomic ratio of C, O and Cu calculated by XPS measurement is 9.65:2.28:1. Since the sample is easy to be contaminated by C and O that will lead to the increase of their concentration, we estimate that the actual ratio is close to 8:2:1, which suggests that only one C8H14O2 group is bonded to Cu2 +. Therefore, Cu 2-eh may exist on the surface of AgCl
Fig. 3. IR spectra of 2-ehylhexoic acid, Cu 2-eh and CEH-AgCl nanoparticles.
Fig. 4. XPS spectra of the CEH-AgCl nanoparticles.
as the form of [Cu(C8H14O2)]+ (abbreviated to (Cu-2-eh)+). Accompanied by analysis of FT-IR, the structure of the prepared CEH-AgCl nanoparticles is suggested as indicated in Fig. 5. As-prepared particle consists of an AgCl core and a modified layer of Cu 2-eh on the surface, and the relationship between the layer and the core might be electrostatic force. Fig. 6 shows UV – Vis absorption spectra of cupric 2ethylhexoate, CEH-AgCl nanoparticles and blank AgCl
Fig. 5. The suggested structure of the prepared CEH-AgCl nanoparticles.
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The results show that the modified particles have a mean size of 80 nm and good oil solubility. Cupric 2-ethylhexoate is capped on the surface of AgCl nanoparticles by electrostatic force in the form of (Cu-2-eh)+. Compared with previous methods, we present a simple and effective method to bond multivalent metal ions on the surface of hydrophobic AgX nanoparticles. The method can be applied to other metal ions, such as Co2 +, which we have demonstrated by using cobalt naphthenate as raw material (unpublished).
Acknowledgements
Fig. 6. UV – Vis absorption spectra of cupric 2-ethylhexote, CEH-AgCl nanoparticles and blank AgCl particles.
The financial support from the National Nature Science Foundation of China is gratefully acknowledged.
References particles. Cupric 2-ethylhexoate exhibits absorption at 256 nm attributed to the carboxyl group. For blank AgCl, the absorption peaks become so weak that they could hardly be discerned because they are not stable and soon precipitate. In the case of CEH-AgCl, it shows a typical absorption of AgCl nanoparticles at 244 nm which shifts to shorter wavelength due to quantum size effect compared with the value, 255 –265 nm, reported by Liu [14]. In addition, a shoulder peak at longer wavelength indicates that the absorption of AgCl is affected by (Cu-2-eh)+ on its surface.
4. Conclusions CEH-AgCl nanoparticles were prepared by adding a precipitate reagent to negative-charged AgCl sol solution.
[1] A.A. Eliseev, A.V. Lukashin, et al., Mater. Res. Innov. 3 (2001) 308. [2] O.B. Stephen, B. Louis, C.B. Murray, J. Am. Chem. Soc. 123 (2001) 12085. [3] P. Calandra, A. Longo, V.T. Liveri, Colloid Polym. Sci. 279 (2001) 1112. [4] S. Chen, W.M. Liu, L.G. Yu, Wear 218 (1998) 153. [5] M. Lal, M. Joshi, D.N. Kumar, C.S. Friend, et al., Mat. Res. Soc. Symp. Proc. 519 1998, pp. 217. [6] J.K. Lee, S.M. Choi, Bull. Korean Chem. Soc. 24 (1) (2003) 32. [7] E.S. Robert, B.S. Kevin, US Pat. 5252450.1993-10-12. [8] C.H. Wu, S.M. Wang, New Chem. Mater. 30 (7) (2002) 1. [9] M.R. Song, Z.R. Li, H. Yang, et al., Chem. Res. 13 (3) (2002) 1. [10] H. Tomonaga, T. Morimoto, Thin Solid Films 392 (2001) 355. [11] A. Kriltz, M. Muller, R. Fachet, H. Burger, J. Mater. Sci. 32 (1997) 169. [12] J. Weidlich, U. Dieckmann, O. Volker, US Pat. 5916487.1999-6-29. [13] Y. Gotoh, R. Igarashi, Y. Ohkoshi, et al., J. Mater. Chem. 10 (2000) 2548. [14] C.Y. Liu, X.Y. Chen, Y.J. Wang, Chin. J. Chem. Phys. 10 (1) (1997) 43.