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A new approach for the determination of the iso-electric point of nanoparticles
Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 17–21
A new approach for the determination of the iso-electric point of nanoparticles Rongjun Pan a,∗ , Kongyong Liew b , Liusu Xu a , Yuran Gao b , Juying Zhou a , Hongwei Zhou a a
Institute of Application of Nanoscience & Nanotechnology, Department of Information and Computing Science, Guangxi University of Technology, Liuzhou 545006, China b Key Laboratory of Catalysis and Materials of Hubei Province, College of Chemistry and Material Science, South-central University for Nationalities, Wuhan 430074, China Received 23 October 2006; received in revised form 10 April 2007; accepted 17 April 2007 Available online 21 April 2007
Abstract A new and simple experimental approach is proposed for the determination of iso-electric point (IEP) of nanoparticles suspension using ultraviolet–visible absorption spectroscopy. The suspension was first ultrasonicated to obtain appropriate particle dispersion and the optimal ultrasonication time was investigated by measuring the UV–vis spectra. The absorbance of the suspensions was then determined as a function of pH values. The absorbance at λ = 245 nm changed from positive to negative after the weakest absorbance was reached. The point of intersection of y = 0 and the curve indicates the IEP of the nanoparticle dispersion. The value so obtained was close to those determined by the conventional electrokinetic method and from prediction. © 2007 Elsevier B.V. All rights reserved. Keywords: Iso-electric point; UV–vis absorption spectroscopy; Nanoparticles; Ultrasonication
1. Introduction In the past few decades, nanomaterials have received much attention due to their unique physical and chemical properties, which differ significantly from bulk materials, and have been widely used in the fields of catalysis, phototronics, optoelectronics, information storage and magnetic ferro-fluids. The challenge of nanoscience and nanotechnology seems to be to prevent the aggregation of nanomaterials. There are several solutions but essentially consist of steric or electrostatic stabilization. For electrostatic stabilization, iso-electric point (IEP) is a crucial parameter in preventing the nanomaterial from aggregation, especially for those structural and functional materials prepared from aqueous dispersion of the nanoparticles. The IEP of materials are usually determined by measuring the zeta potential by means of the electrophoresis and the electro-acoustic methods [1–7]. For some materials, the IEP may be at a very low pH value
∗
Corresponding author. Tel.: +86 772 2685876; fax: +86 772 2685876. E-mail address: [email protected] (R. Pan).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.035
[8], if it exists at all. Under these acidic conditions, particle dissolution is likely to interfere with the measurement. Hence, new methods were explored [9–11]. As far as we could ascertain, method utilizing UV–vis spectroscopy has not been reported. In this paper, an approach for measuring the IEP of nanoparticles dispersion using UV–vis is presented. 2. Method and experiment 2.1. Method According to the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory [12,13], when the pH value is higher or lower than the pristine point of zero charge (PZC), defined as the pH value at which the net proton charge of the particles equals zero, the particles will repulse each other because of their surface charges and be dispersed. The further away the pH is from the PZC, the better the particles will be dispersed resulting in larger particle number and hence higher concentration leading to enhancement of the absorbance for those materials absorbing in the ultraviolet–visible range. When the pH value is close to
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R. Pan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 17–21
the PZC, the particles tend to aggregate and the absorbance will decrease correspondingly. With very pure materials, the PZC matches the IEP obtained by means of the electrokinetic methods [6]; thus the larger the pH differs from the IEP of the particles, the better the nanoparticles’ dispersion and the larger the absorbance enhancement. Conversely, the closer the pH value is to the IEP, the weaker the absorbance will be.
1.85 to 11.20 to obtain another series of particle suspensions. The suspensions were exposed to the same irradiation for the optimal ultrasonication time, then stand for 10.0 min, followed by UV–vis measurement. Figures from Fig. 2b onwards were drawn using MatLab program.
2.2. Materials and instruments
Fig. 1 shows the TEM micrograph and the diameter distribution of the ITO nanoparticles obtained by co-precipitation. The particles were spherical in shape with a mean diameter of 11.4 nm. Fig. 2a shows the UV–vis spectra of the nanoparticles re-dispersed in dilute ammonia aqueous as a function of ultrasonication time. With the ultrasonication time increasing from 1.0 to 60.0 min, the absorbance increased initially and decreased after 15.0 min. The reasons might be that the ultrasonic irradiation broke up the aggregation of the nanoparticles resulting in an increase of the particle concentration which in turn led to an enhancement of the absorption intensity [17]. On further irradiation, the temperature of the system rose, resulting in an increase of the particle energy and re-aggregation, the concentration of particles decreased, hence the decrease of the UV–vis absorbance. The curve with the highest absorbance (curve e) was obtained after the suspension was exposed to ultrasonic irradiation for 15.0 min. Thus, 15.0 min was selected as the optimal ultrasonication time for the ITO nanoparticles dispersion. It is also observed from Fig. 2a that the peaks at λ ≈ 245 nm showed the most prominent change on exposure of the suspensions to ultrasonic irradiation for 1.0 to 60.0 min. Hence, the absorbance of the suspensions at λ ≈ 245 nm was selected as a measure of the degree of dispersion of the particles. Fig. 2b shows the absorption intensity of the suspensions as a function of the ultrasonication time also shows maximum absorbance at 15.0 min. Fig. 3a shows the absorbance of the suspensions at λ = 245 nm as a function of pH value after exposesure to ultrasonic irradiation for 15.0 min. When the pH value was increased from 1.85 to 11.20, the absorbance of the suspensions was observed
Hydrochloric acid and aqueous ammonia (both from Guangdong Guanghua Chemical Factory Co. Ltd.) were analytical grade reagents and used without further purification. The nanoparticles were indium tin oxide (ITO) prepared by a coprecipitation method [14,15] with 10 wt% of SnO2 . A Delta 320 pH meter (Mettler Toledo) was used to determine the pH value. Transmission electron microscopy (Technil G2 20) was used to investigate the morphology and size of the ITO nanoparticles. Ultraviolet–visible absorption spectra were measured with a Lambda 35 UV–visible spectrometer. Zeta potential was determined on Zetasizer 3000hs (Malvem Instrument Ltd.). 2.3. Experimental details 2.3.1. Determination of optimal ultrasonication time A series of ITO nanopartilce suspensions with a concentration of ∼0.7 mg/mL were prepared by re-dispersing a weighed amount of the nanoparticles in dilute aqueous ammonia. To reach appropriate particle dispersion [16], the suspensions were exposed to ultrasonic irradiation as a function of time. After each irradiation, the suspension was allowed to stand for 10.0 min before UV–vis absorption measurement was made. The time at which maximum absorbance was reached was taken to be the optimal time. 2.3.2. Investigation of IEP The same amount of ITO nanopartilces as the above experiment was re-dispersed in solutions with pH values varying from
3. Results and discussion
Fig. 1. TEM micrograph of ITO nanoparticles and its particle diameter distribution.
R. Pan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 17–21
19
Fig. 2. (a) UV–vis spectra and (b) absorbance intensities at a scattering wavelength of 245 nm of the suspensions of ITO nanoparticles as a function of ultrasonication time (a, 1.0 min; b, 2.0 min; c, 5.0 min; d, 10.0 min; e, 15.0 min; f, 30.0 min; g, 60.0 min).
to decrease from ∼0.7 initially and then increased up to ∼1.1, with a minimum absorbance observed at pH 7.50. As it is known that the greater the particle concentration is, the greater the absorbance will be [17]. Hence, when the pH of the solution is higher or lower than ∼7.50, the nanoparticles were well redispersed in the solution, resulting in the increase of the particle concentration. Since it is known that the closer the pH is to the IEP, the lower the particle concentration will be [6] hence the weaker absorbance. It is likely that the IEP of the ITO particles is close to pH 7.50. It is difficult experimentally to determine the pH at which the weakest absorbance occurred (meaning the IEP). Therefore, an assumption could be made that the absorbance will be extremely close to zero when the pH of the suspension is right at the IEP
of the nanoparticles. Since the surface charge of the particles is positive when the pH value of the suspension is lower than its IEP and negative when higher than the IEP, we could assume that the absorbance is positive when the pH value of the suspension is lower than its IEP and negative when higher than the IEP. Thus, Fig. 3b is obtained by changing the sign for the absorbance to negative from pH > 7.50. As showed in Fig. 3b, a point of intersection at zero absorbance is obtained at pH ∼7.35 which is the IEP of the nanoparticle. The following figure (Fig. 4) shows the variation of zeta potential with pH values for ITO particles obtained by means of electrokinetic method. For ITO nanoparticles dispersed in solution, the IEP is at a pH of ∼7.42. When the pH value is less than the IEP of the particles, the zeta potential is positive. Above the
Fig. 3. (a) Absorbance of the suspensions at 245 nm as a function of pH value and (b) the modified curve after the absorbance was assigned negative value at higher pH than that for the lowest absorbance.
20
R. Pan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 17–21
the CFSE as well as the surface defects and nonstoichiometry [18]. 4. Conclusions The IEP of ITO nanopartilces was measured by utilizing UV–vis absorption spectra. It well matched that by electrokinetic method. They are fairly close to the prediction value. This is a new approach to investigate the IEP of nanoparticles. Comparing to the traditional one, it is easy-operating and credible. Hence, it is worthy of being explored. This method allows qualitative results to be obtained rapidly. Acknowledgement This work was partly supported by Research Fund of Guangxi University of Technology.
Fig. 4. Zeta potential of ITO particles as a function of pH value.
IEP, the zeta potential is negative, reaching −26.8 mV at a pH of 11.20. Obviously, the IEP obtained by utilizing UV–vis spectra well matched the experimental result obtained by electrokinetic method although there was an error of 0.9% to the latter. Hence, it could be considered credible. Predictions for the IEP of simple metal oxides could be made using an electrostatic model, which takes into account the surface charges originating from amphoteric discussion of surface MOH groups (M means metal) and adsorption of the hydrolysis products of Mz+ (OH)z− (z is ionic charge) [8,18] z IEP = B − 11.5 + 0.0029 (CFSE) + a (1) R R = 2r0 + r+
(2)
where B and a are the parameters depending on the coordination number of metal ions, r0 is the radius of oxygen ion and r+ is that of metal ion and CFSE is the crystal field stabilization energy which was assumed to be zero in the calculations [19,20]. Because both In3+ and Sn4+ occupy octahedral interstices in SnO2 and In2 O3 , the coordination number for the metal ions is 6, and therefore B is equal to 18.6 and a is equal to zero [19]. Hence, the predicted IEPs for In2 O3 and SnO2 are 9.37 and 5.93, respectively. For complex oxides like ITO, the IEP can be described by the equation [18] IEP = si IEPi (3) where si is the mole ratio of the ith component and IEPi is the IEP of the ith component. The predicted IEP value here is IEP = 0.61 × 9.37 + 0.39 × 5.93 = 8.03 The experimental values for the IEP of ITO nanoparticles are consistent with the prediction, although a little lower than it. This difference may be caused by several factors. An enrichment of tin oxide on the surface might be one of the reasons. Another reason for the difference between the experimental values and the prediction is that the prediction neglects the contributions from
References [1] C. Goebbert, R. Nonninger, M.A. Aegerter, H. Schmidt, Wet chemical deposition of ATO and ITO coatings using crystalline nanoparticles redispersable in solutions, Thin Solid Films 351 (1999) 79–84. [2] Q. Tan, Z. Zhang, Z. Tang, S. Luo, K. Fang, Rheological properties of nanometer tetragonal polycrystal zirconia slurries for aqueous gel tape casting process, Mater. Lett. 57 (2003) 2375–2381. [3] E. Illes, E. Tombacz, The effect of humic acid adsorption on pH-dependent surface charging and aggregation of magnetite nanoparticles, J. Colloid Interface Sci. 295 (2006) 115–123. [4] J. Wang, L. Gao, Surface properties of polymer adsorbed zirconia nanoparticles, NanoStruct. Mater. 11 (4) (1999) 451–457. [5] J. Wang, L. Gao, Adsorption of polyethylenimine on nanosized zirconia particles in aqueous suspensions, J. Colloid Interface Sci. 216 (1999) 436–439. [6] M. Kosmulski, C. Saneluta, Point of zero charge/isoelectric point of exotic oxides: Tl2 O3 , J. Colloid Interface Sci. 280 (2004) 544–545. [7] M.C. Lamas, R.M. Torres Sanchez, Isoelectric point of soils determined by the diffusion potential method, Geoderma 85 (1998) 371–380. [8] J. Sun, B.V. Velamakanni, W.W. Gerberich, L.F. Francis, Aqueous latex/ceramic nanoparticle dispersions: colloidal stability and coating properties, J. Colloid Interface Sci. 280 (2004) 387–399. [9] E. McCafferty, J.P. Wightman, Determination of the surface isoelectric point of oxide films on metals by contact angle titration, J. Colloid Interface Sci. 194 (1997) 344–355. [10] A. Sillero, A. Maldonado, Isoelectric point determination of proteins and other macromolecules: Oscillating method, Comput. Biol. Med. 36 (2006) 157–166. [11] I.H. Huisman, G. Tragardh, C. Tragardh, A. Pihlajamaki, Determining the zeta-potential of ceramic microfiltration membranes using the electroviscous effect, J. Membr. Sci. 147 (1998) 187–194. [12] B.V. Derjaguin, L. Laudau, Theory of stability of highly charged lyophobic sols and adhesion of highly charged particles in solutions of electrolytes, Acta Physicochim. USSR 14 (1941) 633–662. [13] E.J.W. Verwey, J.Th.G. Overbeek, Theory of Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. [14] K.Y. Kim, S.B. Park, Preparation and property control of nano-sized indium tin oxide particle, Mater. Chem. Phys. 86 (2004) 210–221. [15] D. Yu, D. Wang, J. Lu, Y. Qian, Preparation of corundum structure Sn-doped In2 O3 nanoparticles via controlled co-precipitating and post-annealing route, Inorg. Chem. Commun. 5 (2002) 475–477. [16] T.S. Suzuki, Y. Sakka, K. Nakano, K. Hiraga, Effect of ultrasonication on the microstructure and tensile elongation of zirconia-dispersed alumina ceramics prepared by colloidal processing, J. Am. Ceram. Soc. 84 (2001) 2132–2134.
R. Pan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 17–21 [17] R. Pan, C. Sun, B. He, Y. Yu, Synthesis of solvent-stabilized copper nanoparticles, Chem. Bioeng. 23 (2006) 13–14. [18] A. Carre, F. Roger, C. Varinet, Study of acid/base properties of oxide, oxide glass, and glass-ceramic surfaces, J. Colloid Interface Sci. 154 (1992) 174–183.
21
[19] G.A. Parks, The isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems, Chem. Rev. 65 (1965) 177– 182. [20] M. Kosmulski, Chemical Properties of Materials Surface, Dekker, New York, 2001.
A new approach for the determination of the iso-electric point of nanoparticles Rongjun Pan a,∗ , Kongyong Liew b , Liusu Xu a , Yuran Gao b , Juying Zhou a , Hongwei Zhou a a
Institute of Application of Nanoscience & Nanotechnology, Department of Information and Computing Science, Guangxi University of Technology, Liuzhou 545006, China b Key Laboratory of Catalysis and Materials of Hubei Province, College of Chemistry and Material Science, South-central University for Nationalities, Wuhan 430074, China Received 23 October 2006; received in revised form 10 April 2007; accepted 17 April 2007 Available online 21 April 2007
Abstract A new and simple experimental approach is proposed for the determination of iso-electric point (IEP) of nanoparticles suspension using ultraviolet–visible absorption spectroscopy. The suspension was first ultrasonicated to obtain appropriate particle dispersion and the optimal ultrasonication time was investigated by measuring the UV–vis spectra. The absorbance of the suspensions was then determined as a function of pH values. The absorbance at λ = 245 nm changed from positive to negative after the weakest absorbance was reached. The point of intersection of y = 0 and the curve indicates the IEP of the nanoparticle dispersion. The value so obtained was close to those determined by the conventional electrokinetic method and from prediction. © 2007 Elsevier B.V. All rights reserved. Keywords: Iso-electric point; UV–vis absorption spectroscopy; Nanoparticles; Ultrasonication
1. Introduction In the past few decades, nanomaterials have received much attention due to their unique physical and chemical properties, which differ significantly from bulk materials, and have been widely used in the fields of catalysis, phototronics, optoelectronics, information storage and magnetic ferro-fluids. The challenge of nanoscience and nanotechnology seems to be to prevent the aggregation of nanomaterials. There are several solutions but essentially consist of steric or electrostatic stabilization. For electrostatic stabilization, iso-electric point (IEP) is a crucial parameter in preventing the nanomaterial from aggregation, especially for those structural and functional materials prepared from aqueous dispersion of the nanoparticles. The IEP of materials are usually determined by measuring the zeta potential by means of the electrophoresis and the electro-acoustic methods [1–7]. For some materials, the IEP may be at a very low pH value
∗
Corresponding author. Tel.: +86 772 2685876; fax: +86 772 2685876. E-mail address: [email protected] (R. Pan).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.035
[8], if it exists at all. Under these acidic conditions, particle dissolution is likely to interfere with the measurement. Hence, new methods were explored [9–11]. As far as we could ascertain, method utilizing UV–vis spectroscopy has not been reported. In this paper, an approach for measuring the IEP of nanoparticles dispersion using UV–vis is presented. 2. Method and experiment 2.1. Method According to the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory [12,13], when the pH value is higher or lower than the pristine point of zero charge (PZC), defined as the pH value at which the net proton charge of the particles equals zero, the particles will repulse each other because of their surface charges and be dispersed. The further away the pH is from the PZC, the better the particles will be dispersed resulting in larger particle number and hence higher concentration leading to enhancement of the absorbance for those materials absorbing in the ultraviolet–visible range. When the pH value is close to
18
R. Pan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 17–21
the PZC, the particles tend to aggregate and the absorbance will decrease correspondingly. With very pure materials, the PZC matches the IEP obtained by means of the electrokinetic methods [6]; thus the larger the pH differs from the IEP of the particles, the better the nanoparticles’ dispersion and the larger the absorbance enhancement. Conversely, the closer the pH value is to the IEP, the weaker the absorbance will be.
1.85 to 11.20 to obtain another series of particle suspensions. The suspensions were exposed to the same irradiation for the optimal ultrasonication time, then stand for 10.0 min, followed by UV–vis measurement. Figures from Fig. 2b onwards were drawn using MatLab program.
2.2. Materials and instruments
Fig. 1 shows the TEM micrograph and the diameter distribution of the ITO nanoparticles obtained by co-precipitation. The particles were spherical in shape with a mean diameter of 11.4 nm. Fig. 2a shows the UV–vis spectra of the nanoparticles re-dispersed in dilute ammonia aqueous as a function of ultrasonication time. With the ultrasonication time increasing from 1.0 to 60.0 min, the absorbance increased initially and decreased after 15.0 min. The reasons might be that the ultrasonic irradiation broke up the aggregation of the nanoparticles resulting in an increase of the particle concentration which in turn led to an enhancement of the absorption intensity [17]. On further irradiation, the temperature of the system rose, resulting in an increase of the particle energy and re-aggregation, the concentration of particles decreased, hence the decrease of the UV–vis absorbance. The curve with the highest absorbance (curve e) was obtained after the suspension was exposed to ultrasonic irradiation for 15.0 min. Thus, 15.0 min was selected as the optimal ultrasonication time for the ITO nanoparticles dispersion. It is also observed from Fig. 2a that the peaks at λ ≈ 245 nm showed the most prominent change on exposure of the suspensions to ultrasonic irradiation for 1.0 to 60.0 min. Hence, the absorbance of the suspensions at λ ≈ 245 nm was selected as a measure of the degree of dispersion of the particles. Fig. 2b shows the absorption intensity of the suspensions as a function of the ultrasonication time also shows maximum absorbance at 15.0 min. Fig. 3a shows the absorbance of the suspensions at λ = 245 nm as a function of pH value after exposesure to ultrasonic irradiation for 15.0 min. When the pH value was increased from 1.85 to 11.20, the absorbance of the suspensions was observed
Hydrochloric acid and aqueous ammonia (both from Guangdong Guanghua Chemical Factory Co. Ltd.) were analytical grade reagents and used without further purification. The nanoparticles were indium tin oxide (ITO) prepared by a coprecipitation method [14,15] with 10 wt% of SnO2 . A Delta 320 pH meter (Mettler Toledo) was used to determine the pH value. Transmission electron microscopy (Technil G2 20) was used to investigate the morphology and size of the ITO nanoparticles. Ultraviolet–visible absorption spectra were measured with a Lambda 35 UV–visible spectrometer. Zeta potential was determined on Zetasizer 3000hs (Malvem Instrument Ltd.). 2.3. Experimental details 2.3.1. Determination of optimal ultrasonication time A series of ITO nanopartilce suspensions with a concentration of ∼0.7 mg/mL were prepared by re-dispersing a weighed amount of the nanoparticles in dilute aqueous ammonia. To reach appropriate particle dispersion [16], the suspensions were exposed to ultrasonic irradiation as a function of time. After each irradiation, the suspension was allowed to stand for 10.0 min before UV–vis absorption measurement was made. The time at which maximum absorbance was reached was taken to be the optimal time. 2.3.2. Investigation of IEP The same amount of ITO nanopartilces as the above experiment was re-dispersed in solutions with pH values varying from
3. Results and discussion
Fig. 1. TEM micrograph of ITO nanoparticles and its particle diameter distribution.
R. Pan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 17–21
19
Fig. 2. (a) UV–vis spectra and (b) absorbance intensities at a scattering wavelength of 245 nm of the suspensions of ITO nanoparticles as a function of ultrasonication time (a, 1.0 min; b, 2.0 min; c, 5.0 min; d, 10.0 min; e, 15.0 min; f, 30.0 min; g, 60.0 min).
to decrease from ∼0.7 initially and then increased up to ∼1.1, with a minimum absorbance observed at pH 7.50. As it is known that the greater the particle concentration is, the greater the absorbance will be [17]. Hence, when the pH of the solution is higher or lower than ∼7.50, the nanoparticles were well redispersed in the solution, resulting in the increase of the particle concentration. Since it is known that the closer the pH is to the IEP, the lower the particle concentration will be [6] hence the weaker absorbance. It is likely that the IEP of the ITO particles is close to pH 7.50. It is difficult experimentally to determine the pH at which the weakest absorbance occurred (meaning the IEP). Therefore, an assumption could be made that the absorbance will be extremely close to zero when the pH of the suspension is right at the IEP
of the nanoparticles. Since the surface charge of the particles is positive when the pH value of the suspension is lower than its IEP and negative when higher than the IEP, we could assume that the absorbance is positive when the pH value of the suspension is lower than its IEP and negative when higher than the IEP. Thus, Fig. 3b is obtained by changing the sign for the absorbance to negative from pH > 7.50. As showed in Fig. 3b, a point of intersection at zero absorbance is obtained at pH ∼7.35 which is the IEP of the nanoparticle. The following figure (Fig. 4) shows the variation of zeta potential with pH values for ITO particles obtained by means of electrokinetic method. For ITO nanoparticles dispersed in solution, the IEP is at a pH of ∼7.42. When the pH value is less than the IEP of the particles, the zeta potential is positive. Above the
Fig. 3. (a) Absorbance of the suspensions at 245 nm as a function of pH value and (b) the modified curve after the absorbance was assigned negative value at higher pH than that for the lowest absorbance.
20
R. Pan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 17–21
the CFSE as well as the surface defects and nonstoichiometry [18]. 4. Conclusions The IEP of ITO nanopartilces was measured by utilizing UV–vis absorption spectra. It well matched that by electrokinetic method. They are fairly close to the prediction value. This is a new approach to investigate the IEP of nanoparticles. Comparing to the traditional one, it is easy-operating and credible. Hence, it is worthy of being explored. This method allows qualitative results to be obtained rapidly. Acknowledgement This work was partly supported by Research Fund of Guangxi University of Technology.
Fig. 4. Zeta potential of ITO particles as a function of pH value.
IEP, the zeta potential is negative, reaching −26.8 mV at a pH of 11.20. Obviously, the IEP obtained by utilizing UV–vis spectra well matched the experimental result obtained by electrokinetic method although there was an error of 0.9% to the latter. Hence, it could be considered credible. Predictions for the IEP of simple metal oxides could be made using an electrostatic model, which takes into account the surface charges originating from amphoteric discussion of surface MOH groups (M means metal) and adsorption of the hydrolysis products of Mz+ (OH)z− (z is ionic charge) [8,18] z IEP = B − 11.5 + 0.0029 (CFSE) + a (1) R R = 2r0 + r+
(2)
where B and a are the parameters depending on the coordination number of metal ions, r0 is the radius of oxygen ion and r+ is that of metal ion and CFSE is the crystal field stabilization energy which was assumed to be zero in the calculations [19,20]. Because both In3+ and Sn4+ occupy octahedral interstices in SnO2 and In2 O3 , the coordination number for the metal ions is 6, and therefore B is equal to 18.6 and a is equal to zero [19]. Hence, the predicted IEPs for In2 O3 and SnO2 are 9.37 and 5.93, respectively. For complex oxides like ITO, the IEP can be described by the equation [18] IEP = si IEPi (3) where si is the mole ratio of the ith component and IEPi is the IEP of the ith component. The predicted IEP value here is IEP = 0.61 × 9.37 + 0.39 × 5.93 = 8.03 The experimental values for the IEP of ITO nanoparticles are consistent with the prediction, although a little lower than it. This difference may be caused by several factors. An enrichment of tin oxide on the surface might be one of the reasons. Another reason for the difference between the experimental values and the prediction is that the prediction neglects the contributions from
References [1] C. Goebbert, R. Nonninger, M.A. Aegerter, H. Schmidt, Wet chemical deposition of ATO and ITO coatings using crystalline nanoparticles redispersable in solutions, Thin Solid Films 351 (1999) 79–84. [2] Q. Tan, Z. Zhang, Z. Tang, S. Luo, K. Fang, Rheological properties of nanometer tetragonal polycrystal zirconia slurries for aqueous gel tape casting process, Mater. Lett. 57 (2003) 2375–2381. [3] E. Illes, E. Tombacz, The effect of humic acid adsorption on pH-dependent surface charging and aggregation of magnetite nanoparticles, J. Colloid Interface Sci. 295 (2006) 115–123. [4] J. Wang, L. Gao, Surface properties of polymer adsorbed zirconia nanoparticles, NanoStruct. Mater. 11 (4) (1999) 451–457. [5] J. Wang, L. Gao, Adsorption of polyethylenimine on nanosized zirconia particles in aqueous suspensions, J. Colloid Interface Sci. 216 (1999) 436–439. [6] M. Kosmulski, C. Saneluta, Point of zero charge/isoelectric point of exotic oxides: Tl2 O3 , J. Colloid Interface Sci. 280 (2004) 544–545. [7] M.C. Lamas, R.M. Torres Sanchez, Isoelectric point of soils determined by the diffusion potential method, Geoderma 85 (1998) 371–380. [8] J. Sun, B.V. Velamakanni, W.W. Gerberich, L.F. Francis, Aqueous latex/ceramic nanoparticle dispersions: colloidal stability and coating properties, J. Colloid Interface Sci. 280 (2004) 387–399. [9] E. McCafferty, J.P. Wightman, Determination of the surface isoelectric point of oxide films on metals by contact angle titration, J. Colloid Interface Sci. 194 (1997) 344–355. [10] A. Sillero, A. Maldonado, Isoelectric point determination of proteins and other macromolecules: Oscillating method, Comput. Biol. Med. 36 (2006) 157–166. [11] I.H. Huisman, G. Tragardh, C. Tragardh, A. Pihlajamaki, Determining the zeta-potential of ceramic microfiltration membranes using the electroviscous effect, J. Membr. Sci. 147 (1998) 187–194. [12] B.V. Derjaguin, L. Laudau, Theory of stability of highly charged lyophobic sols and adhesion of highly charged particles in solutions of electrolytes, Acta Physicochim. USSR 14 (1941) 633–662. [13] E.J.W. Verwey, J.Th.G. Overbeek, Theory of Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. [14] K.Y. Kim, S.B. Park, Preparation and property control of nano-sized indium tin oxide particle, Mater. Chem. Phys. 86 (2004) 210–221. [15] D. Yu, D. Wang, J. Lu, Y. Qian, Preparation of corundum structure Sn-doped In2 O3 nanoparticles via controlled co-precipitating and post-annealing route, Inorg. Chem. Commun. 5 (2002) 475–477. [16] T.S. Suzuki, Y. Sakka, K. Nakano, K. Hiraga, Effect of ultrasonication on the microstructure and tensile elongation of zirconia-dispersed alumina ceramics prepared by colloidal processing, J. Am. Ceram. Soc. 84 (2001) 2132–2134.
R. Pan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 305 (2007) 17–21 [17] R. Pan, C. Sun, B. He, Y. Yu, Synthesis of solvent-stabilized copper nanoparticles, Chem. Bioeng. 23 (2006) 13–14. [18] A. Carre, F. Roger, C. Varinet, Study of acid/base properties of oxide, oxide glass, and glass-ceramic surfaces, J. Colloid Interface Sci. 154 (1992) 174–183.
21
[19] G.A. Parks, The isoelectric points of solid oxides, solid hydroxides, and aqueous hydroxo complex systems, Chem. Rev. 65 (1965) 177– 182. [20] M. Kosmulski, Chemical Properties of Materials Surface, Dekker, New York, 2001.