2212-6708 © 2014 Roman Marsalek. Published by Elsevier B.V.Selection and peer review under responsibility of Asia-Pacifi c Chemical, Biological & Environmental Engineering Societydoi: 10.1016/j.apcbee.2014.01.003

APCBEE Procedia 9 ( 2014 ) 13 – 17

Available online at www.sciencedirect.com

ScienceDirect

ICBEE 2013: September 14-15, New Delhi, India

Particle size and Zeta Potential of ZnO

Roman Marsalek Department of Chemistry, Faculty of Science, University of Ostrava, 30. Dubna 22, Ostrava, 701 03, Czech republic

Abstract

The influence of dispersant, pH and various additives on stability of ZnO suspension have been studied. Water and ethylene glycol were used as dispersants, sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB) and sodium carboxymethyl cellulose (NaCMC) have been adsorbed on ZnO particles. Equilibrium in the system is mostly influenced by pH. The lowest stability of suspensions of ZnO was found around isoelectric point which was confirmed by simultaneous measurement of particle size and zeta potential. The values of the zeta potential were also affected by the presence of surfactants, especially SDS and CTAB. © 2013 Published by Elsevier B.V. Selection and/or peer review under responsibility of Asia-Pacific Chemical, Biological & Environmental Engineering Society Keywords: ZINC OXIDE, PARTICLE SIZE DISTRIBUTION, ZETA POTENTIAL

1. Introduction

Zinc oxide (ZnO) has generated considerable attention because of its optical, magnetic, antibacterial and semiconducting properties. Its nanostructures exhibit interesting properties: high catalytic efficiency and strong adsorption capacity. This is extensively used in many applications such as cosmetics, paints, ceramics and electronics [1], [2]. Recently, much effort has been devoted to study ZnO as a very promising photocatalyst for photocatalytic degradation of water pollutants, owing to its high activity, low cost and environmental friendly feature. Some problems still remain to be solved in its application, such as degraded photocatalytic activity in aqueous solution introducing acid or base [3]. Zinc oxide is an important semiconductor suitable for a wide range of applications due to its unique electronic and optical properties.

Corresponding author. Tel.: +0-420-597-092-192; fax: +0-420-596-120-478. E-mail address: roman.marsalek@osu.cz

© 2014 Roman Marsalek. Published by Elsevier B.V.Selection and peer review under responsibility of Asia-Pacifi c Chemical, Biological & Environmental Engineering Society

14 Roman Marsalek / APCBEE Procedia 9 ( 2014 ) 13 – 17

ZnO is used in optical and photonic crystals, sensors, colloidal lithography, and catalyst support [4]. Great variety of colloid chemical methods and physical techniques are being used to synthesize zinc oxide particles with controlled size [5]. In pharmaceutical suspensions, when a solid active ingredient is dispersed in a liquid, the problems are related to agglomeration, flocculation (increase of particle size) and sedimentation: because of their high surface area, micro or nanoparticles form aggregates or agglomerates due to Van der Waals or other attractive forces [1]. Monitoring the presence of nanoparticles in dispersions having broad particle size distributions can be a problem for many measurement techniques because large particles or even aggregates of the smaller particles can mask the presence of the sought after nanoparticles [6].

There are many ways to affect the stability of suspensions of nanoparticles. It is reported that particle surface coating is one of the effective way to improve their stability and agglomeration condition [8]. Also pH is an important parameter for controlling the particle sizes of colloids because it affects the stability of surface charge and particle interactions [7]. The stability of suspensions can be further influenced by the addition of surfactants. The cationic surfactant (CTAB) exhibits adsorption on ZnO due to electrostatic attraction between the negative surface and positively charged surfactant. In the ZnO case, a small concentration of anionic surfactant causes a decrease of suspension stability. Fine oxide particles are aggregated by the presence of surfactants as a result of a strong hydrophobic interaction after adsorption [9]. Similar results were also observed in the interaction of cationic and anionic surfactants on clays and coals. [10]-[13]. The presence of surfactants in the synthesis of nano zinc oxide also affects the shape of the particles. ZnO nanostructures were synthesized by hydrothermal method using different molar ratios of cetyltrimethylammonium bromide (CTAB) and Sodium dodecyl sulfate (SDS) as structure directing agents. The results indicate that the mixture of cationic-anionic surfactants can significantly modify the shape and size of ZnO particles. Various structures such as flakes, sheets, rods, spheres, flowers and triangular-like particles sized from micro to nano were obtained. Recently, there have been lots of works on the synthesis of ZnO nanostructures using different surfactants like ethylenediaminetetraacetic acid (EDTA), sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), polyethylene glycol (PEG) and polyethyleneimine (PEI) [2]. To improve the stability of suspensions are also used polymers. The suspensions were formulated with ZnO at a fixed concentration (5 wt%), sodium poly-(acrylate), as a viscosifier, and sodium dodecylsulfate (SDS), as a wetting agent. Polymer adsorption serves as an effective way for modifying particle surface and hence improving the stability of pharmaceutical suspensions against flocculation [1].

The main purpose of sample preparation is to break up flocs and disperse the particles. This is achieved by increasing the surface charge of the particles and zeta potential measurements allow optimization of this procedure. The value of the zeta potential indicates possible behavior of the dispersion. Particles which have got the zeta potential between minus 30 to plus 30 mV show tendency to the coagulation. This fact is the most expressive at the isoelectric point, when the zeta potential equals zero mV. The isoelectric points (IEPs) of ZnO were found around at pH 8.0 [7], 9.7 [14], 9.8 [3], 10.3 [8]. Zeta potential is possible to affect in various ways also in the case of ZnO. After modification, the nanoparticles of ZnO/SiO2 were negatively charged over the whole pH value range examined [3]. In case of both series the zeta potential values of the samples increased with increasing indium ion content. It can be explained by the presence of indium ions on the surface of the crystals, due to their specific adsorption in the electric double layer [5]. It was obvious that the pH dependence of zeta potentials for ZnO nanoparticles shifted completely to that for TiO2 nanoparticles after TiO2 coating, also confirming the formation of core-shell structure [14]. The pH in isoelectric point was 10.3 for ZnO without coating, but it shifted to 6.0 for coated ZnO. The change of pH in isoelectric point would attribute to the highly monodisperse condition for ZnO nanoparticles coated with Al2O3 [8]. Zeta potential measurements indicated that addition of the cationic surfactant caused a positive increase of the zeta potential [9], [11]-[13]. The combined effect of the pH of a medium and the addition of commercial additives on the zeta potential and particle radius of aqueous suspensions zinc oxide was studied in this work.

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2. Experimental

2.1. Materials

We used ZnO powders from different manufacturers to prepare 1 wt.% dispersions (stock dispersions), each dispersed in water (or ethylene glycol). All samples were sonicated (5 minutes) to make them homogeneous. Sodium dodecyl sulfate (SDS, Sigma-Aldrich), cetyltrimethylammonium bromide (CTAB, Sigma-Aldrich) and sodium carboxymethyl cellulose (NaCMC) of analytical reagent grade were used for stabilization of the suspensions. Hydrochloric acid (HCl, Sigma-Aldrich) and sodium hydroxide (NaOH, Sigma-Aldrich) were used for adjusting the pH during zeta potential measurements. In addition, particle size distribution was also measured in ethylene glycol (EG, Sigma-Aldrich).

2.2. Methods

2.2.1. Particle size distribution For dynamic light scattering measurements 12 mm cell (DTS 0012) was used. One milliliter of distilled

water (or ethylene glycol) was added to the cell and then 50 μl from stock dispersions were added. Samples were again sonicated for 5 minutes. Size distributions of the nanoparticles were determined with a Malvern Zetasizer Nano ZS (Malvern Instruments Ltd., GB) by the DLS technique. The suitable parameters (viscosity, absorption and refractive index) were chosen for each ZnO and dispersants. For ZnO sample: absorption 0.1, refractive index 2.0, for water: viscosity 1.0031cP, refractive index 1.33, for ethylene glycol viscosity 1.98 cP, refractive index 1.44. The pH value of each suspension was adjusted by adding either NaOH or HCl. Histograms with the distributions of sizes were recorded.

2.2.2. Zeta potential measurements The zeta potential was measured by analyzing 0.1 g of ZnO in 10 ml of water (or additives solutions) using

the Zetasizer Nano ZS (Malvern Instruments Ltd., GB). Before zeta potential measurements all samples were sonicated for 5 minutes. Zetasizer Nano ZS uses Laser Doppler Velocimetry to determine electrophoretic mobility. The zeta potential was obtained from the electrophoretic mobility by the Smoluchowski equation.

The dynamic method consisted of using ZS Malvern Zetasizer device coupled with an automatic titrator (Malvern MPT-2). pH of the suspensions was automatically adjusted by this automatic titrator using hydrochlorid acid (0.25 and 0.025 mol/l) and sodium hydroxide (0.25 mol/l). Concentrations of additives (SDS, CTAB, NaCMC) were 5mmol/l.

Fig. 1. Size distribution of ZnO by number. Dispersants: water (red line), ethylene glycol (green line)

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3. Results and Discussion

3.1. Particle size distribution

Particle size distribution (and also coagulation) can be influenced in several ways: sonification , adding a stabilizer (e.g. sodium hexametaphosphate hexamethyl) [6]. Dispersant also plays very important role. Thefollowing picture shows the difference in the particle size distribution of ZnO in water and in ethylene glycol.

The picture shows great differences in the particle size distribution. The value of Z-average was 735nm for water and 340 nm in ethylene glycol, parameter Number mean was 151nm for water and 49 nm for ethyleneglycol, respectively. It was the same sample of ZnO. The probable cause of the difference is much higherviscosity of ethylene glycol compared to water (20 times higher). In the ethylene glycol the particles of ZnOget to each other more difficultly and rate of coagulation is lower. From this point of view, selecting theproper dispersant and setting methodology for measuring is very important.

3.2. The zeta potential

Fig. 2. (a) The influence of pH on the zeta potential (circles) and size (triangles) of ZnO particles. (b) The influence of additives on thezeta potential of ZnO particles from pH 7 to pH 12.

The picture 2(a) shows the changes of the zeta potential during NaOH titration. In addition particle size of our sample was followed. When we started titration, the pH was 7.3, the value of the zeta potential + 35 mVand diameter size approximately 50 nm. Addition of NaOH caused decrease of the zeta potential and increase of particle size. The maximum size was found at pH 10.2 near to the isoelectric point. In other words, for ZnOsuspensions it is necessary to know pH, adjust it and keep it at a value which the system is stable at. After the addition of SDS, CTAB and NaCMC, ions were adsorbed and caused a change to the zeta potential (picture 2b). In the case of adsorption of CTAB the zeta potential increased. In contrast, in the case of adsorption of SDS and NaCMC the zeta potential was shifted into negative values. The new values of IEP were: pH 11 in case of CTAB, pH 9.4 in case of NaCMC and pH 8.9 when we added SDS. With the help of additives, we areable to change the zeta potential of suspensions of ZnO and thus affect the stability of suspensions. Adding acationic surfactant extended stable region (zeta potential above |30mV| ) around neutral pH. Addition of anionic surfactant extended stable region in strongly alkaline pH. Adding NaCMC did not significantly affect the stability of ZnO suspension.

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4. Conclusions

Stability of suspensions of ZnO can be influenced by a variety of ways. Coagulated particles are influenced by the type of dispersant, pH and/or the presence of other substances. The stability of suspensions can be monitored by measuring the zeta potential. Suspensions of ZnO are stable in the neutral pH range. Suspensions are also more stable in the ethylene glycol with comparison with water. At a certain pH range it is also possible to create stable suspensions by adding surfactants, namely SDS and CTAB.

Acknowledgements

Article has been done in connection with project Institute of environmental technologies, reg. no. CZ.1.05/2.1.00/03.0100 supported by Research and Development for Innovations Operational Programme financed by Structural Founds of European Union and from the means of state budget of the Czech Republic.

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