Miniemulsion-Based Process for Controlling the Size and Shape of

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Industrial & Engineering Chemistry Research

Miniemulsion-based process for controlling the size and shape of zinc oxide nanoparticles Michael Fricke,†,‡ Andreas Voigt,∗,‡ Peter Veit,¶ and Kai Sundmacher†,‡ Max Planck Institute for Dynamics of Complex Technical Systems, Magdeburg, Germany, Process Systems Engineering, Otto-von-Guericke University Magdeburg, Germany, and Institute for Experimental Physics, Otto-von-Guericke University Magdeburg, Germany E-mail: [email protected] Phone: +49 (0)391 67-51435. Fax: +49 (0)391 67-11245

Abstract A miniemulsion-based approach to produce crystalline ZnO nanoparticles with narrowly distributed sizes and well-defined faces and shapes is presented. The synthesis reaction in the aqueous droplets of the w/o miniemulsion is induced by mass transfer of the base triethylamine (TEA) across the liquid/liquid interface. In a preliminary study, the dosage of TEA was determined to ensure the required pH range in the aqueous phase. Miniemulsions with a mean droplet size of about 200 nm were produced with a rotor-stator device. The aqueous zinc salt concentration was varied as the parameter in the experimental study. At small zinc salt concentrations, plate-like nanoparticles were obtained. An increase of the zinc ion concentration resulted in a decrease of the mean particle size. The shape of the ZnO crystals changed from plate-like to rod-like ∗

To whom correspondence should be addressed Max-Planck-Institute for Dynamics of Complex Technical Systems Magdeburg ‡ Otto-von-Guericke University Magdeburg ¶ Otto-von-Guericke University Magdeburg †

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particles. Based on the resulting particle size, it can be deduced that inter-droplet exchange is an essential part of the presented miniemulsion-based process.

Introduction The processing of inorganic nanoparticles with well-defined properties is of major importance for several high tech applications. 1–3 Miniemulsion-based processes have been found to be a promising precipitation technique for preparing nanoparticles with a controlled size and shape, in which the individual droplets can be considered as reactors with a confined reaction volume. 4–8 Miniemulsions are kinetically stabilized heterophase systems with mean droplet diameters in the range between 30 and 500 nm. 9 The particle formation in miniemulsions can be initiated by two different mechanisms. On the one hand the reactant mixing can be induced by forced droplet coalescence. On the other hand the precipitation reaction can be started by mass transfer of one of the reactants from the continuous phase of the emulsion into the aqueous droplets, where the particle synthesis takes place. This so called one-emulsion technique reveals a better process controllability compared to the coalescence-based method for miniemulsion systems. Hence, the ability to generate tailor-made nanoparticles with the one-emulsion technique is addressed here. A suitable substance that can be synthesized by using the one-emulsion technique is zinc oxide. Due to its specific solid state properties, zinc oxide nanoparticles have a number of high tech applications such as in catalysis, 10 solar cells 11 and sensors. 12 In particular, the particle size and shape affect the functionality of the nanocrystals. 13,14 The most important approaches to synthesize ZnO nanoparticles are vapor-phase 15,16 and solution-based processes. 17,18 Compared to the sophisticated vapor-phase techniques, the solution-based approach reveals lots of advantages in scalability, costs and handling. 18 The solution-based methods can basically be divided into the hydrothermal synthesis 19,20 and wet chemistry processes at low temperatures and under atmospheric conditions. 21,22 In the above-mentioned

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aqueous reaction routes, zinc salt solutions are used as the source of the zinc ions. Under alkaline reaction conditions, zinc ions react with hydroxide ions to different intermediate hydroxo-complexes. A comprehensive consideration of the reaction network with different main and side reactions like in a reaction landscape is presented in the literature. 23,24 A brief overview is given below. In the aqueous solution-based ZnO formation process, the OH- /Zn2+ mole ratio plays an important role. In case of a OH- /Zn2+ mole ratio of two, Zn (OH)2 is produced. If the mole ratio is less than two, zinc hydroxide salt complexes are formed. 25 For an increased hydroxide ion concentration, water soluble higher-order zinc hydroxo-complexes are synthesized which can decompose to ZnO. Under light alkaline conditions (pH < 12), solid Zn (OH)2 and ZnO are the energetically favored zinc species. Above is the most stable species. The solid state conversion of a pH of 12, soluble Zn (OH)2− 4 Zn (OH)2 to ZnO can be neglected for temperatures ranging from room temperature up to 90o C. In contrast, the addition of hydroxide ions in excess lead to the solubilization of solid Zn (OH)2 by chemical reactions to higher-order zinc hydroxo-complexes. In subsequent decomplexation reactions, crystalline ZnO and hydroxide ions are formed. 23 It is therefore of importance to provide a sufficient amount of hydroxide ions in the aqueous solution to crystallize solely ZnO nanoparticles at low temperatures. Only a few publications deal with miniemulsion-based processes to generate ZnO nanoparticles. Dolcet et al. 26 used an approach based on forced coalescence of miniemulsion droplets containing zinc chloride and sodium hydroxide at room temperature. The droplet coalescence was induced by sonication. The authors pointed out that the size and morphology of ZnO nanoparticles can be tuned with the application of different types of surfactants in the synthesis process. The synthesis of metal doped ZnO nanoparticles with a similar process is presented in another publication by Dolcet et al. 27 Polycrystalline platelets with an average crystallite size of of 15-20 nm were produced at room temperature with zinc nitrate hexahydrate and sodium hydroxide as educts. Winkelmann and co-workers 28,29 describe in two publications the synthesis of ZnO nanoparticles in miniemulsions. In the first publication, a

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process scheme based on the one-emulsion technique is presented, which allows for simultaneous emulsification and mixing in the disruption unit of a high pressure homogenizer. 28 The authors argue that the particle size can be controlled by variations of the mixing intensity of the miniemulsion droplets containing zinc sulphate and the organic base oleylamine. It was observed that the size of the resulting ZnO nanoparticles is minimized by increasing the mixing efficiency. Upon reaching a critical mixing quality, no further decrease of the particle size had been found. In the second publication by Winkelmann et al., 29 the precipitation of zinc oxide nanoparticles was induced by droplet coalescence. The coalescence of the two miniemulsions, one containing zinc sulfate and the other one loaded with sodium hydroxide in the droplet phase, was induced by a high pressure homogenizer device. Nanoparticles with a narrow size distribution were produced if complete coalescence was accomplished. Furthermore, a dependency of the educt concentration on the ZnO particle size was found. A concentration increase resulted in a reduction of the particle size. The authors explained this behavior by different nucleation and growth conditions. Our aim of the presented work here is to demonstrate the ability of the miniemulsionbased process to control the size and shape of ZnO nanoparticles. In contrast to the referred literature, narrowly distributed nanoparticles can be synthesized in a simple stirred batch reactor if the required chemical conditions in the droplet phase are provided. Based on a variation of the educt concentration, the observed change in particle size and shape can help to improve the understanding about possible particle forming mechanisms. This in turn will help to enhance the industrial applicability of this process route.

Materials and Methods Chemicals and Solutions N-decane with a purity of 94 % was purchased from Merck. Zinc acetate dihydrate (98 %) was obtained from Alfa Aesar. Deionized water was used to prepare the zinc salt solu4


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tions. The organic base triethylamine (TEA) with a purity of 99.7 % was purchased from Arcos Organics. The nonionic surfactant Span20 from Merck was used for preparing the miniemulsion. All chemicals were used without further purification.

Equipment For the determination of the chemical conditions of the ZnO synthesis, measurements of the pH value with the multipurpose electrochemical analytical meter S40 SevenMulti with an in Lab-Routine Pro electrode from Mettler-Toledo were accomplished. The droplet size distribution of the miniemulsions was measured with the dynamic light scattering device Horiba LB-500 based on a 650 nm laser diode irradiation and an 90 degree optical reflectance signal detection. The crystal structure of the ZnO nanoparticles was investigated with a powder diffractometer XRD X‘Pert PRO from PANalytical B.V. using a goniometer sampling stage with a Cu-Kα radiation source (λ = 0.154 nm). For the characterization of the size and shape of the ZnO nanocrystals, a transmission electron microscope (TEM) FEI Tecnai F20 (200 kV beam energy, signal detection with a CCD camera Gatan Orius 600) was used.

Experimental Procedure Measurements of the pH value were conducted in a surfactant-free aqueous solution/n-decane system to identify the required amount of TEA to add to the miniemulsion. For this purpose, samples consisting of Vw = 100 mL 0.05 mol/L zinc acetate solution and of Vo = 100 mL ndecane were mixed in a flask. Different amounts of TEA ranging from VTEA = 1.386 mL up to VTEA = 13.862 mL were added to the water/n-decane samples. These two-phase systems were mixed in a shaker for 24 h at room temperature. Thereafter, pH measurements were carried out directly in the aqueous phase of the two-phase samples. For the ZnO synthesis in the miniemulsion, the aqueous zinc acetate solution was acidified with acetic acid to prevent the formation of zinc hydroxide during heating of the solution. The preparation of the w/o miniemulsion was done as follows. The amount of 6 g of the non5



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ionic surfactant Span20 was mixed together with 204 g of n-decane. Subsequently, 90 g of the aqueous zinc acetate solution were added to the mixture. In order to obtain a homogeneous miniemulsion with submicron water droplets, the water-oil-surfactant system was emulsified with the rotor-stator mixer Ultra Turrax (Iker) at 17500 rpm for one minute. These obtained miniemulsions were the basis for the emulsion-assisted process. The application of this particular miniemulsion system is explained below (see Results and Discussion). The w/o miniemulsion was filled into a jacketed thermostated glass reactor and was then heated up to 75

o

C. For sufficient mixing conditions, the miniemulsion was stirred with a

Rushton turbine at 500 rpm. After reaching the temperature of 75 o C, 12.475 mL (0.09 mol) of the base TEA was added to the miniemulsion. The chemical reaction in the droplets is initiated by the mass transfer of TEA molecules across the o/w interface. The reaction mixture was maintained at 75

o

C under continuous stirring for 60 or 120 min. After that

period of time, samples for the characterization of the ZnO nanoparticles were taken. The complete procedure of ZnO preparation is depicted in figure 1. The experimental parameters of this study are summarized in table 1. Table 1: Experimental parameters of the study. experiment

E1 E2 E3a E3b E4

aqueous zinc acetate concentration 0.01 mol/L 0.025 mol/L 0.05 mol/L 0.05 mol/L 0.1 mol/L

TEA dosage

TEA/Zn2+ mole ratio

hold-up time

12.475 12.475 12.475 12.475 12.475

100 40 20 20 10

60 min 60 min 60 min 120 min 60 min

mL mL mL mL mL

The following procedure was carried out to prepare samples for the TEM investigations. 100 microL of the miniemulsion were taken with a glass syringe out of the reactor and were deposited onto a carbon-film coated copper grid. The sample was then immediately washed with 100 microL n-decane and 100 microL ethanol to remove surfactant residues. After drying the samples under vacuum conditions, the TEM images were obtained. About one 6


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Figure 1: The main steps of the experimental procedure from premixing, component addition, emulsification, reaction under heating and sample taking (from left to right) are depicted.

hundred particles per experiment were analyzed for the determination of the mean values of the particle length and diameter. In order to perform the XRD measurements, a sufficient amount of ZnO nanoparticles void of miniemulsion residues was required. To obtain this material, the particles were extracted from the reaction mixture by breaking the miniemulsion. The breaking of the miniemulsion was induced by repeating the following procedure. A volume of 120 mL final miniemulsion was centrifuged at 12000 rpm for 5 minutes on the lab centrifuge 3K30 from Sigma. The surfactant-rich supernatant was removed and pure n-decane was added. After a redispersion of the droplet phase on a vibro mixer, the procedure was started with the centrifugation again. After this a dispersion of ZnO nanoparticles sedimented at the bottom of the centrifuge tube. The remaining top liquid was removed and the particles were washed again three times with n-decane and ethanol. Finally, the remaining ZnO nanoparticles were dried and characterized by XRD. 7



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Results and Discussion Determination of the Reaction Conditions in the w/o System A special property of our presented miniemulsion-based process is that the particle formation takes place in the aqueous droplets. As explained in the introduction, the addition of hydroxide ions in excess of the stoichiometric ratio is indispensable for the formation of ZnO in low temperature conditions. Therefore, the mass transfer of the base TEA across the n-decane/water interface and the partition coefficient of TEA in the n-decane/water system are of major importance for the conversion of ZnO. Experimental investigations of the partition behavior of TEA in a pure water/n-decane system by Fricke and Sundmacher have shown that TEA is preferentially dissolved in the n-decane phase. 30 As a direct measurement of the pH value in the aqueous miniemulsion droplets is not possible we developed a model experiment consisting of a surfactant-free two-phase system to estimate the hydroxide ion concentration under reaction conditions. The measurement results of the pH value after termination of the ZnO formation in the surfactant-free two-phase system at room temperature are presented in figure 2. Instead of the OH- /Zn2+ mole ratio, the TEA/Zn2+ mole

Figure 2: Measurement results of the pH value in a surfactant-free system consisting of equal volumes of an aqueous 0.05 molar zinc acetate solution and n-decane after reaching the equilibrium state as function of the TEA/Zn2+ mole ratio in the two-phase system.

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ratio in the two phase system was chosen as the measure to illustrate the ability of TEA to cross the liquid/liquid interface under reaction conditions and to dissociate into the aqueous droplets. At a mole ratio of two, a pH value of about 7 was obtained. A distinct increase of the TEA amount, i.e. the TEA/Zn2+ mole ratio (notice the logarithmic scale on the x-axis), resulted in an drastic increase of the pH value from 7 up to 10. A sufficient amount of TEA above the stoichiometric level is necessary to achieve the required pH level in the aqueous droplet phase of the applied miniemulsion in our ZnO synthesis process. Consequently, the volume of 12.475 mL (0.09 mol) of the base TEA was added to the miniemulsion, which corresponds to TEA/Zn2+ mole ratios ranging from 10 to 100 depending on the zinc acetate concentration in the miniemulsion and leading to pH values of about 11.

Analysis of the Particle Formation For the miniemulsion system Span20 was chosen as the surfactant. Span20 with a HLB value of 8.6 is a readily available surfactant which is applied on a large scale in industrial applications. 31 Based on a previous optimization of the formulation, a sufficiently stable miniemulsion containing a large amount of water in dispersed droplets using only a relatively small amount of the surfactant Span20 could be obtained. The droplet size distributions of four different miniemulsions with different Zn acetate concentrations inside the water droplets were measured at room temperature after emulsification (for procedure see section: Experimental procedures). The results are shown in figure 3. Miniemulsions with a mean droplet size of about 200 nm could be produced with the rotor-stator mixer as the homogenization device. The results show that the zinc acetate concentration has no influence on the resulting droplet size distribution. The droplet size was not measured during the process, its sufficient kinetic stability for one or two hour experiments had been tested previously. As in the beginning of the investigation, the overall time scale of the particle formation process was unknown we varied the process hold-up time. For the zinc acetate concentration of 0.05 mol/L, we varied the hold-up time from one hour to two hours and obtained the final 9



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Figure 3: Droplet size distributions of the miniemulsion after emulsification for different zinc acetate concentrations: (E1) 0.01 mol/L; (E2) 0.025 mol/L; (E3) 0.05 mol/L and (E4) 0.1 mol/L.

ZnO particles after these two times. The results of this analysis are depicted in figure 4. No

Figure 4: ZnO formation for zinc acetate concentration of 0.05 mol/L at hold-up times of 60 min and 120 min: a) Particle length (circle) and diameter (square) as a function of the hold-up time; b) TEM image for hold-up time of 60 min and c) TEM image for hold-up time of 120 min.

significant changes in the size and shape of the particles after the two hold-up times of 60 and 120 min could be determined from the TEM image analysis. This observation indicates that the conversion to ZnO is completed within the reaction time of 60 min. Furthermore, coarsening of the particles by Ostwald ripening within a hold-up time of 60 min under 10


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reaction conditions seemed not to occur as no broader particle size distribution nor small particles could be detected in the TEM images. As a consequence, we set the hold-up time to 60 min for all further experiments on the particle synthesis process. Although the particle formation dynamics from the start to the one hour final time might be of interest, we currently limited our investigations on the variation of process parameters of importance to applications like the educt concentration. We carried out 4 different experiments (E1-E4) with varied zinc acetate concentrations inside the droplets (see table 1). Representative TEM images from all four experiments with ZnO nanoparticles obtained after the final process synthesis time of 60 min are shown in figure 5. Narrowly distributed nanoparticles with different sizes and well-defined crystal shape could be produced for all investigated concentrations. The corresponding XRD spectra for all four experiments E1-E4 (see table 1) are summarized in figure 6. The XRD pattern of the synthesized nanoparticles coincide with the hexagonal Wurtzite crystal structure of ZnO (JCPDS 36-1451). We write for all peaks the Miller indices like given in the JCPDS data sheet 36-1451, but point out that for hexagonal Wurtzite type lattices like ZnO the appropriate indexing is Miller-Bravais with four indices. For the main peaks in figure 6 these indices correspond to: (100) - (1011), (002) - (0002), (102) - (1012), (110) - (1120) and (103) - ((1013). Sharp and clearly visible reflections and the reflection intensity indicate that the ZnO nanoparticles are not amorphous and completely crystalline (even when prepared at relatively low temperatures and without any post-treatment of the samples such as calcination). The small peak broadening from sample E3 to E4 might be related to the observed decrease in particle size and related to the Scherrer effect for nanoscale material. 32 Based on the TEM images, the size and geometry of the ZnO nanoparticles were determined. The average length and diameter of the particles are plotted in figure 7 as function of the zinc ion concentration. In addition, the measured data is summarized in table 2. For a zinc acetate concentration of 0.01 mol/L, mainly hexagonal shaped plate-like

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Figure 5: TEM images of the synthesized particles for different zinc acetate concentrations: (E1) 0.01 mol/L; (E2) 0.025 mol/L; (E3a) 0.05 mol/L and (E4) 0.1 mol/L.

nanoparticles were produced. Some of the particles are characterized by the disappearance of one or two faces, which resulted in pentagonal or quadrilateral nanodisks. The average length and diameter of the ZnO nanodisks were L = 66 nm and D = 113 nm, respectively. Hence, the corresponding average aspect ratio was L/D = 0.58. An increase of the zinc acetate concentration in the aqueous droplet phase to 0.025 mol/L resulted in the decrease of the particle diameter (D = 66 nm), whereas the particle length (L = 60 nm) remained almost constant. The average aspect ratio of these nanoparticles was approximately 0.9, close to one. The increase of the zinc acetate concentration to a value of 0.05 mol/L led to a further decrease in the particle diameter D = 41 nm. The particle length (L = 77 nm) re-

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Figure 6: XRD spectra of the synthesized particles.

Figure 7: Analyzed particle morphology from TEM images: a) diameter and length of the ZnO nanoparticles and b) aspect ratio as function of the zinc ion concentration.

mained almost constant at this concentration. Here, the aspect ratio changed towards values of L/D > 1 and reached almost two. The particle shape changed now from mostly hexagonal plate-like nanoparticles to mostly hexagonal rod-like nanoparticles with this increase in zinc acetate concentration. At a further increase of the zinc acetate concentration to 0.1 mol/L, the particle shape and the aspect ratio (around 2.1) remained almost unchanged compared to 0.05 mol/L. The size of the nanoparticles was reduced in both dimensions to D = 20 nm and L = 42 nm. The resulting size and shape of ZnO nanoparticles strongly depend on the reaction condi13



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Table 2: Morphologic parameters of the ZnO nanoparticles obtained from TEM image analysis. experiment E1 E2 E3a E3b E4

Zn2+ -concentration [mol/L] 0.01 0.025 0.05 0.05 0.1

length L [nm] 66 ± 4 60 ± 6 76 ± 8 82 ± 8 42 ± 6

diameter D [nm] 113 ± 8 66 ± 8 43 ± 6 42 ± 10 20 ± 6

aspect ratio L/D [−] 0.58 0.91 1.76 1.95 2.10

tions in wet chemistry processes such as the nature of the solvent, 33 the temperature, 20,34,35 the pH value, 20,36 reactant concentrations 21,35,37 or the addition of shape controlling additives. 13,14,38 In our study, solely the concentration of zinc acetate in the aqueous droplets was varied (see column 2 in table 2). All other reaction parameters like the TEA concentration were kept constant. Consequently, only the mole ratios of zinc ions to other participating species like hydroxide ions, surfactant molecules or the molecules of the byproducts differ here. As these different ratios might influence the nucleation and growth kinetic, they may have led to the observed variation in crystal shape. The effect of different OH- /Zn2+ mole ratios on the morphology of ZnO nanoparticles in homogeneous media was the research objective of a number of publications. 20,23,36 Moezzi and co-workers 23 argue that at high OH- /Zn2+ mole ratios, the conversion to ZnO is completed faster and coarsening of the particles by the equilibrium dissolution/ recrystallization process occurs. Zhang et al. 20 explain the occurrence of different ZnO nanostructures by the varying amounts of growth units at different pH values. As outlined in the introduction, different zinc hydroxo-complexes may appear depending on the hydroxide ion concentration in aqueous solution media. For instance, at a pH value of 12, nucleation is the dominating crystallization step and therefore compact and small particles are formed. If growth is the prevailing crystallization mechanism, flower-like particles consisting of ZnO nanorods were obtained. Hence, the OH- /Zn2+ mole ratio significantly affects the particle shape. In our miniemulsionbased process, the hydroxide ion concentration can not be exactly determined. Therefore, the 14


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particle dimensions are depicted as function of the TEA/Zn2+ instead of the OH- /Zn2+ mole ratio in figure 8. With increasing TEA/Zn2+ mole ratio, an increase of the particle diameter

Figure 8: Diameter and length of the ZnO nanoparticles as function of the TEA/Zn2+ mole ratio.

is observed. In comparison to the arguments of Moezzi et al., 23 a coarsening effect of the particles sizes over time was not observed (see figure 4). Instead, an apparent attachment of growth units on the quadrilateral faces of the hexagonal shaped ZnO crystals may have occurred at higher TEA/Zn2+ mole ratios, which represent a higher OH- /Zn2+ ratio. Since the added quantity of TEA to the miniemulsion system remained unchanged, the same pH value and, therefore, the same solubility behavior of the hydroxo-complexes are maintained in our experiments. It follows that at high OH- /Zn2+ ratios, the smallest supersaturation level was established. With a decreasing OH- /Zn2+ ratio, a much higher supersaturation is present in the droplets. Consequently, we argue that the radial growth of the ZnO crystal is a function of the supersaturation level. A low supersaturation radial growth is preferred, whereas at higher supersaturation, axial growth is favored. The influence of applied surfactants in a miniemulsion-based process was investigated by Dolcet and co-workers. 26 The authors found that by changing the HLB value of the surfactant, ZnO nanostructures with different sizes and shapes could be synthesized. The here applied surfactant Span20 with its rather low HLB value of about 9 is less water soluble 15



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compared to the investigated surfactants in the work of Dolcet and co-workers. 26 As our observation of well-faceted and single-crystalline particles suggest that the particle formation takes place in the bulk of the water droplets, the influence of Span20 on the shape and size is assumed to be small. Beside the influence of surfactant molecules, the application of special shape control agents is another possibility that may affect the particle morphology. 13,14,38 These shapecontrolling agents preferentially adsorb at particular crystal faces and thus prevent the growth perpendicular to this adsorbed faces. The chemical compounds, which are not directly involved in the particle synthesis reaction in our considered miniemulsion-based process, are acetate ions and triethylammonium ions. Meagley and Garcia investigated the influence of mulidentate carboxyl ligands on the growth of ZnO crystal. 13 They observe that the addition of tricarboxylate ligands during the ZnO synthesis provoke a reduction in the length growth of the hexagonal crystal, which results in plate-like particles. The authors further reveal that the shape controlling effect is diminished by the reduction in the number of carboxylate groups of the ligand. Monocarboxylate ligands had no effect on the ZnO crystal shape according to their study. We argue therefore that the presence of acetate ions which are similar to monocarboxylate ligands will not affect the shape during particle growth. The influence of ethylenediamine and hexamine as ligands in the ZnO synthesis was described by Nicholas et al. 14 and Sugunan et al., 38 respectively. These compounds with multiple amino groups preferentially adsorb at the quadrilateral faces of the hexagonal shaped ZnO crystal. Consequently, needle-like particles with high aspect ratios are produced. As we find that an increase of the TEA/Zn2+ mole ratio results in a reduction of the aspect ratio a particular adsorption on the quadrilateral faces is not preferred. Currently, a clear correlation between zinc ions and another participating ionic species in the applied miniemulsion system on the control of the particle shape can not be derived from our experimental data. The most probable explanation based on our findings is that zinc ions themself may influence the growth perpendicular to the quadrilateral faces therefore inducing

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a length growth and an increase in the aspect ratio. More detailed investigations will improve the understanding of the face-specific growth of ZnO crystals in our miniemulsion system. The general decrease of the particle size with increasing zinc ion concentration in the droplet phase can be explained by different supersaturation levels during particle formation in the aqueous droplets. In figure 9 a) the equivalent spherical diameters of the ZnO nanoparticles are presented as a function of zinc ion concentration. Since TEA was added

Figure 9: Depiction of a) the equivalent spherical diameter of the ZnO nanoparticles and of b) the required number of droplets with the initial mean miniemulsion droplet size to synthesize one ZnO nanoparticle as function of the zinc ion concentration.

in excess in all experiments, a sufficient fast mass transfer across the liquid/liquid interface can be assumed. Consequently, a similar hydroxide ion concentration should be available in every experiment. Therefore, the supersaturation level, which determines the nucleation and growth conditions in the droplets, only depends on the zinc ion concentration. For the lowest zinc acetate concentration in the experimental series, the lowest supersaturation level is established. Thus, the ZnO synthesis was dominated by the growth of very few nucleated particles and, therefore, the largest particles in our experimental series were produced. In case of high supersaturation levels, the particle formation is controlled by nucleation of many ZnO crystals and, overall, smaller particles are formed. A reduction of nanoparticle size by increase of the initial ion concentrations has also been found in other emulsion-based investi-

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gations for example by Adityawarman et al. 39 in a barium sulfate nanoparticle precipitation study. In figure 9 b) we show another interesting outcome of our present study. For this we calculate the number of emulsion droplets required to build the corresponding final ZnO particle from given zinc ions. A given final ZnO particle with its known density and molar mass contains a required number of zinc ions. Assuming that a mean emulsion droplet of 200 nm size (compare to figure 3) contains a specific number of zinc ions related to the established zinc ion concentration we can calculate the number of emulsion droplets required for this particle build-up. The number of emulsion droplets required for the lowest zinc ion concentration is around 750. For the highest zinc ion concentration it is close to 1. For low zinc ion concentration that means that a single droplet does not contain enough zinc ions to build the final ZnO particles observed in our experiments. We argue that droplet coalescence induced by the TEA presence and the mild stirring conditions lead to material transport from droplets without particles to droplets with particles by coalescence. The zinc ions are redistributed in this way and then incorporated into ZnO particles by growth. The particle size distribution is quite narrow and no small particles are observed after one or two hours of experimentation. We believe therefore that Ostwald ripening, i.e. the zinc ion molecule transport by dissolution of smaller particles and the redistribution by diffusion through the droplet interface and across the continuous phase does not play an important role in our process. In contrast to the work of Winkelmann and co-workers, 28 here in our particle synthesis approach a conventional stirred reactor was sufficient to produce narrowly distributed ZnO nanoparticles. We obtained particles in a similar size range under equivalent concentration conditions as Winkelmann et al. produced in the combined emulsification and mixing device. Compared to the Winkelmann study, in our presented process the mixing intensity is not a critical process parameter in order to influence the particle size. Other influencing process parameters, such as the pH value and the more easily controllable zinc ion concentration, are

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the important variables in order to define the final particle properties in this miniemulsionbased process.

Conclusions Our presented miniemulsion-based method is a suitable process route to produce narrowly distributed, crystalline ZnO nanoparticle populations with well-defined faces at a rather mild process conditions of a temperature at 75

o

C and atmospheric pressure. One important

finding of our investigation is the necessity of a distinct overdosage of the base TEA to achieve a sufficient pH level in the aqueous phase. We could show that the zinc acetate concentration in the aqueous phase inside the emulsion droplets appeared as a parameter not only to control the particle size, but also to control the particle shape. For small zinc ion concentrations the largest particles of the experimental series with a mean particle size of about 95 nm and a plate-like shape were obtained. The increase of the zinc acetate concentration resulted in a size reduction of the particles down to 20 nm and in a shift of the particle shape to needle-like particles. We propose that the presence of zinc ions on specific faces of ZnO may lead to different growth rates of the ZnO faces and to the observed shape changes. A simplified relation of the final particle size to the initial mean emulsion droplet diameter revealed that, after the initialization of the particle synthesis by mass transfer of the base TEA across the o/w interface, droplet coalescence may have occurred for the required inter-droplet zinc ion mass exchange and corresponding particle growth. Different aspects of the particle formation dynamics should be studied in order to clarify this behavior in more detail. Our present data shows the applicability of a one-miniemulsion process for particle synthesis and even enables the control of size and shape of ZnO nanocrystals by relatively easy means, namely the zinc ion concentration. As pointed out in the introduction, the applicability of ZnO nanoparticles is manyfold and other physical properties like spectral absorption and emission of the material need to be considered in future studies to find out

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for which purposes these particularly shaped and sized ZnO nanoparticles material may fit best. In preliminary room-temperature TEM measurements it was observed that our ZnO nanoparticles can absorb energy from the electron beam and emit light for a while afterwards. New studies are planned to investigate these effects and their correlation with size and shape more deeply. The one-miniemulsion based approach can maybe also be applied to new and other material and material combinations or be used for more specific shape control purposes. We are sure that many interesting questions about this particle synthesis process are still ahead of us and the interested scientific community.

Acknowledgement The authors would like to thank Bianka Stein for supporting the experimental work and Luise Borchert for performing the XRD measurements.

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Miniemulsion-Based Process for Controlling the Size and Shape of

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