Efficient, One-Step Mechanochemical Process for the Synthesis of

Jan 18, 2008 - An efficient, mechanochemical reaction for synthesizing ZnO nanoparticles has been investigated. The starting materials were zinc sulfa...
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Ind. Eng. Chem. Res. 2008, 47, 1095-1101

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Efficient, One-Step Mechanochemical Process for the Synthesis of ZnO Nanoparticles Jun Lu and Ka M. Ng* Department of Chemical Engineering, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Hong Kong

Shihe Yang Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Hong Kong

An efficient, mechanochemical reaction for synthesizing ZnO nanoparticles has been investigated. The starting materials were zinc sulfate heptahydrate and potassium hydroxide, and potassium chloride served as the matrix salt. The reaction was carried out in a paste state at room temperature with a short grinding time and no postreaction calcination. The nanoparticles thus obtained had a mean diameter of 22.1 nm and exhibited excellent UV-blocking properties. A comparison of the costs of raw materials between this and other mechanochemical reactions is presented. Introduction Semiconductor nanocrystals have received much attention because of their special properties in comparison with those of bulk materials. In particular, ZnO is an important group II-VI semiconductor with a wide band gap (3.37 eV) and a large exciton binding energy of 60 meV at room temperature.1 ZnO nanocrystals have shown considerable potential as materials for light-emitting diodes, transparent conductive films, solar cells, and UV blockers.2-4 For these reasons, many methods for the synthesis of ZnO nanoparticles have been developed, including microwave method,5 sol-gel process,6 solvo/hydrothermal reactions,7 precipitation,8 solution-combustion method,9 spray pyrolysis,10 gas-phase condensation,11 thermal evaporation,12 direct milling,13-15 and mechanochemical reactions.16-21 However, most of these methods are not suitable for commercialization, and the resulting ZnO nanoparticles often show a high degree of agglomeration. It is thus highly desirable to devise a simple and cost-effective synthetic route for large-scale production. Mechanochemical or solid-state reactions are particularly suitable for the large-scale production of nanoparticles because of their simplicity and low cost. Because such reactions do not involve organic solvents, they are attractive from an environmental point of view. Not surprisingly, a wide variety of nanoparticles have recently been synthesized by mechanochemical processing, including ZnS, CdS, Ce2S3,22,23 LiMn2O4, SiO2, CeO2, and SnO2.24,25 A major drawback of such reactions is that the reaction time is long, ranging from several hours to several days.16-19,22 There are three common reaction routes for the synthesis of ZnO nanoparticles by mechanochemical methods:16-21 (1) milling of a mixture of ZnCO3‚2Zn(OH)2 and sodium chloride, followed by calcination of the milled powder mixture16 (this process has been commercialized); (2) solid-state reaction of ZnCl2 and Na2CO3 to form ZnCO3 and NaCl and subsequent thermal decomposition of ZnCO3;17-19 and (3) grinding of a * To whom correspondence should be addressed. Tel.: 852 2358 7238. Fax: 852 2358 0054. E-mail: [email protected].

mixture of zinc acetate and oxalic acid, followed by a thermal decomposition reaction.20,21 A milling time of 3-6 h is needed for the first and second reactions. This is followed by thermal decomposition at 400600 °C. The third reaction requires a milling time of 30 min, followed by thermal decomposition of ZnC2O4‚2H2O at 450 °C. Clearly, the calcination step consumes energy and prolongs the production cycle time. It has been reported that, if hydrated metal salts are used as starting materials, the grinding time and particle size can both be decreased.24,26,27 For example, in the synthesis of SnO2 nanoparticles, grinding of a mixture of KCl and SnCl4‚5H2O took 30 min. It took another 30 min to grind a mixture of KOH powder and the mixture obtained from the previous step.24 Along the same lines, we report here an energy-saving mechanochemical reaction with a total milling time of 30 min for the synthesis of ZnO nanoparticles. In this process, ZnSO4‚ 7H2O is the source of zinc. KOH (or NaOH) is used as the base to change zinc salt to zinc oxide, and KCl (or NaCl) serves as the matrix salt. The reaction mixture is a paste. Thus, this approach would allay fears that nanoparticles would become airborne during the milling process.28,29 Experimental Section Materials and Syntheses. Zinc sulfate heptahydrate (ZnSO4‚ 7H2O, 99.5%) was purchased from Riedel-de Haen. Potassium chloride (99.5%) was purchased from Aldrich. Sodium hydroxide (99.5%) and sodium chloride (99.5%) were purchased from BDH. Potassium hydroxide (g85%) was obtained from Fluka. A typical synthesis process was as follows: First, 0.02 mol of ZnSO4‚7H2O and 0.15 mol of KCl were mixed and ground for 10 min at room temperature. Then, 0.04 mol of KOH powder was added, and the mixture was ground for an additional 20 min. The resulting mixture was subjected to repeated sonication and washing with doubly deionized (DDI) water until no Clion could be detected. The sample was then filtered and vacuumdried at 70 °C for 4 h. This sample weighed 1.57 g and is designated as S4. To obtain a better understanding of the reaction mechanisms and to optimize the process conditions, a series of experiments

10.1021/ie071034j CCC: $40.75 © 2008 American Chemical Society Published on Web 01/18/2008

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Table 1. Process Conditions for the Mechanical Syntheses of Different Samples matrix salt/ matrix ZnSO4‚7H2O molar ratio sample salt S1 S2 S3 S4 S5a S6 a

KCl KCl KCl KCl KCl NaCl

7.5:1 7.5:1 7.5:1 7.5:1 7.5:1 10:1

base KOH KOH KOH KOH KOH NaOH

first-step grinding second-step time grinding (min) time (min) 10 10 10 10 NA 10

0 5 5 20 NA 10

washing step unwashed unwashed washed washed washed washed

S5 was synthesized in the solution phase.

was carried out. The samples obtained are denoted S1-S6, and the corresponding process conditions are listed in Table 1. The matrix salt used is identified in the second column. Listed in the third column is the molar ratio of matrix salt to zinc sulfate heptahydrate. The fourth column reports the base used. The molar ratio of base to zinc sulfate heptahydrate remained at 2:1 for all samples. Listed in the fifth column is the grinding time for mixing of the matrix salt and zinc sulfate heptahydrate, which remained constant at 10 min. The second-step grinding time listed in the sixth column refers to the grinding time for the reaction between zinc sulfate heptahydrate and the base. In the last column, “unwashed” means that the sample was directly characterized by powder X-ray diffraction without DDI water washing, whereas “washed” means that the sample was characterized after DDI water washing as stated above for sample S4. We also carried out an experiment in the solution phase. Solid powders of ZnSO4‚7H2O (0.02 mol) and KCl (0.15 mol) were dissolved in 100 mL of DDI water, and KOH powder (0.02 mol) was dissolved in 40 mL of DDI water. The KOH solution was added slowly to the solution of ZnSO4‚7H2O and KCl. After addition, the mixture was stirred for 2 h. Then, the resulting white precipitate was repeatedly washed with DDI water until no Cl- ion could be detected. After being washed, the sample was filtered and vacuum-dried at 50 °C for 10 h to obtain sample S5. Characterization. The structures of the synthesized nanomaterials were determined by powder X-ray diffraction (XRD, Philips PW-1830 X-ray diffractometer with Cu KR irradiation, λ ) 1.54056 Å, 40 kV and 40 mA). TEM observations were carried out with a JEOL JEM 2010 microscope. For TEM characterization, the powder sample was dispersed in ethanol by sonication for 20 min. For FTIR measurements (PerkinElmer, Spectrum One FT-IR spectrophotometer), the sample powders were ground together with KBr before measurement. Thermogravimetric analysis (TGA) was conducted in nitrogen on a Perkin-Elmer UNIX/TGA7 thermal analyzer at a heating rate of 10 °C/min from room temperature to 800 °C. We carried out the UV-vis characterization of our sample S4 and NanoZ_AQ40 for comparison. The latter is a water-based paint additive from Advanced Nanotechnology Limited. Sample S4 and NanoZ_AQ40 were dispersed in DDI (doubly deionized) water to form aqueous dispersions in a sonicator bath. The spectra were recorded in air at room temperature with a PerkinElmer Lambda 20 spectrophotometer. Results and Discussion Mechanochemical Reaction and Resulting Nanoparticles. To study the mechanochemical reaction of ZnO, a series of experiments was designed (Table 1). The sample for which ZnSO4‚7H2O and KCl were mixed and ground for 10 min is

Figure 1. (a) XRD patterns of samples S1-S3. (b) Magnified XRD pattern of sample S1.

designated as S1. After being ground, the sample mixture became a homogeneous white paste. In this step, ZnSO4‚7H2O was dissolved by its water of hydration after being milled with KCl. In the second step, KOH in powder form was added to the paste, and the mixture was ground for 5 min without washing away the KCl to form sample S2. During KOH addition and the subsequent grinding, significant amounts of heat and vapor were released. Throughout the entire grinding process, the mixture remained as a white paste. Sample S3 was obtained when sample S2 was washed with DDI water. Sample S4 was obtained using a second-step grinding time of 20 min while keeping other reaction conditions the same as for sample S3. The yield for sample S4, defined as the ratio of the amount of zinc(II) in ZnO to that in ZnSO4‚7H2O, was 97.5%. The overall reaction is as follows

ZnSO4(aq) + 2KOH(s) f ZnO(s) + K2SO4(s) + H2O(g) H2O is considered to be a gas because of vaporization. Using ∆fH0 values from Lange’s Handbook of Chemistry, 15th ed. (McGraw-Hill: New York, 1999), the ∆fH0 value of the reaction was calculated to be -127.4 kJ‚mol-1, supporting the observation that the reaction is highly exothermic. It can be seen from the XRD pattern of sample S1 in Figure 1a that no peaks can be attributed to ZnSO4‚7H2O (JCPDS file no. 22-1019). When the XRD pattern of sample S1 is magnified (Figure 1b), only peaks assigned to the characteristic (200) and (420) peaks of KCl (JCPDS file no. 01-0786) can be seen. This confirms that, after it had been ground for 10 min with KCl, all of the ZnSO4‚7H2O, along with most of the KCl, was dissolved by its water of hydration. The XRD pattern of sample S2 (Figure 1a) demonstrates the formation of KCl crystals from the reaction mixture after addition of KOH and subsequent grinding for 5 min. The (200), (220), (222), (400), and (420) peaks are obvious and can all be attributed to KCl (JCPDS file no. 01-0786). When the KCl in

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Figure 3. XRD pattern of sample S5.

Figure 4. FTIR spectra of the starting material and as-synthesized nanomaterials.

Figure 2. TEM images of the as-synthesized nanoparticles in sample S4: (a) large area TEM image, (b) high-resolution TEM image.

sample S2 was washed away and the product was vacuum-dried, only ZnO nanoparticles were obtained. This can be confirmed by the peaks that can be ascribed to the wurtzite ZnO structure (JCPDS file no. 36-1451) in the XRD pattern of sample S3 in Figure 1a. The broadening of the peaks in the XRD pattern of sample S3 reveals the size of the nanoparticles; using the Debye-Scherrer formula for spherical particles, we determined an average particle size of 20.4 nm. There are no obvious XRD peaks of ZnO in the diffraction pattern for sample S2. This is because the size and amount of the ZnO particles were smaller than those of the KCl particles, and the peaks of ZnO are dwarfed by those of KCl in the XRD pattern of sample S2. (1) Effect of Grinding Time on Product Characteristics. In the case of sample S4, the second-step grinding time was increased to 20 min. Its XRD pattern has no obvious difference from that of sample S3 and can also be attributed to wurtzite ZnO (see Supporting Information). By calculation with the Debye-Scherrer formula, an average particle size of 21.5 nm was obtained for sample S4, which is quite close to that of sample S3. Shown in Figure 2a and b are low- and high-resolution TEM images, respectively, of sample S4. There are both large and small nanoparticles, in the form of irregular spheres and short nanorods of size ranging from 10 to 80 nm, in Figure 2a. The

high-resolution image shows primarily irregular spheres with diameters ranging from 15 to 30 nm (Figure 2b). The larger particles in Figure 2a might just be aggregates of small nanoparticles. The TEM images of sample S3 are quite similar to those of sample S4 (see Supporting Information), further confirming that the difference in grinding time between 5 and 20 min makes no difference in the product. However, it is recommended that at least 10 min of grinding be used in the reaction step to minimize unreacted zinc precursors, especially when the process is carried out on a larger scale. (2) Reaction in the Solution Phase. As pointed out above, the solid-state reaction using hydrated metal salts was in a paste state. It is interesting to see what happens when the reaction is carried out in solution phase. Such a sample, denoted S5, was prepared using the same amounts of chemicals as samples S1S4. The only difference was that the reaction took place in 140 mL of DDI water. Figure 3 shows the XRD pattern of sample S5. Some peaks can be attributed to ZnO of wurtzite structure (JCPDS file no. 36-1451), and other peaks can be assigned to a type of zinc sulfate hydroxide hydrate (JCPDS file no. 09-0204). It is difficult to assign the remaining peaks, which probably arise from other types of zinc sulfate hydroxide hydrate. This is confirmed in Figure 4, which shows the FTIR spectra of the starting material ZnSO4‚7H2O, sample S5, sample S4, and sample S4 calcined at 400 °C (denoted as S4-400). The peaks at about 1632 and 3454 cm-1 found in all IR curves correspond to the bending and stretching vibrations of the -OH group, respectively. The 1120 and 609 cm-1 peaks in the FTIR spectra of ZnSO4‚7H2O and sample S5 are assigned to the stretching vibration of the S-O bond.30 The disappearance of the 1120 cm-1 peaks in the spectra of samples S4 and S4-400 demonstrates the total conversion of zinc sulfate hydroxide hydrate to ZnO nanoparticles. It is interesting to note that there

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Figure 5. (a) Large-area TEM image of sample S5. (b) High-resolution TEM image of the area indicated in Figure 5a.

are additional peaks at 1511 and 1399 cm-1 for sample S4. These bands can be assigned to the presence of carbonate species, which might be incorporated into the synthesized nanomaterials via absorption of CO2 from the air into the reaction product during the preparation process. The disappearance of these bands for sample S4-400 shows the loss of CO32from the surface of the ZnO nanoparticles during calcination.31 The intense broad band in the vicinity of 400-600 cm-1 and centered around 460 cm-1 for samples S4 and S4-400 is due to Zn-O vibrations. Such a band is also present for sample S5, but this band is weak compared with those for samples S4 and S4-400, implying that there is only a small amount of ZnO in sample S5. The morphologies of the zinc sulfate hydroxide hydrates and ZnO nanoparticles in sample S5 are shown in TEM images in Figure 5a and b. There are both large semitransparent large plates (from 100 nm to larger size) and small black particles in Figure 5a. The former is zinc sulfate hydroxide hydrates, and the latter is separated or agglomerated ZnO nanoparticles. Figure 5b is the high-resolution TEM image of the particle circled in the white square in Figure 5a. The size of this particle is 28 nm. Regular fringes are clearly visible in the nanoparticle with a spacing of 2.45 Å, which is quite near the (101) interplanar

distance of wurtzite ZnO [2.47 Å for (101) in JCPDS file no. 36-1451]. There are nanoparticles and nanoplates in the SEM image of sample S5 (Supporting Information). The particles are possibly agglomerated ZnO nanoparticles. The sheets are approximately several hundred nanometers to about 1 µm in length or width and are ascribed to zinc sulfate hydroxide hydrate.32 As stated previously, the metathesis reaction is highly exothermic and can self-propagate at ambient temperature in air. In the case of sample S5, the heat of reaction was absorbed by the solvent, water, and the zinc(II) in the zinc salt could not be totally converted to ZnO nanoparticles. Thus, only in the paste state could the zinc(II) in the zinc salt be completely converted into ZnO nanoparticles. This is what happened in samples S3 and S4, whereas intermediate products such as zinc sulfate hydroxide hydrates would be formed in the solutionphase reaction in the case of sample S5. Let us consider this observation from another angle. As mentioned, ZnO nanoparticles are often synthesized by thermal treatment of precursors such as Zn4(CO3)(OH)6, Zn4(CO3)(OH)6‚H2O, Zn5(OH)8(NO3)2(H2O)2-x(NH3)x, and Zn(OH)2.16-19,33 All of these precursors are in the form of nanosheets just like our product, sample S5. Such an intermediate product is no longer needed in the pastestate reaction. (3) Effect of Alternative Matrix Salt and Base on Product Characteristics. In large-scale synthesis, the cost and characteristics of the reactants are of great importance. For sample S6, sodium chloride was used as the matrix salt and sodium hydroxide as the base. The XRD pattern of sample S6 indicates that the product is ZnO nanoparticles in the wurtzite structure (see Supporting Information). The morphology and size of the nanoparticles of sample S6 are still similar to those of samples S3 and S4 (see the TEM image of sample S6 in the Supporting Information). They are in the form of irregular spheres as well as short nanorods. Most of the particles have sizes between 10 and 50 nm. These results show that the use of NaCl and NaOH as the matrix salt and base instead of KCl and KOH does not have much influence on the characteristics of the product. However, the KOH that we used was in the form of very fine powder, which made it easier to grind with the KCl. (4) Purity and Surface Chemistry of the Synthesized ZnO Nanoparticles. Purity is a very important characteristic of highquality ZnO nanoparticles, especially for use in cosmetics. Figure 6 shows XPS spectra of ZnO nanoparticles of sample S4. It can be seen in Figure 6a that, throughout the whole region of 0-1100 eV, only the characteristic peaks of Zn 2p1/2, Zn 2p3/2, and O 1s are present. A full survey scan did not reveal any other element peaks except a very weak C 1s peak, which comes from the tiny amount of absorbed CO2 on the sample surface. These results indicate that pure ZnO nanoparticles have formed. As shown in Figure 6b, the peak at the binding energy of 1022.5 eV corresponds to Zn 2p3/2.34 A high-resolution XPS spectrum of the O 1s binding energy is shown in Figure 6c. It can be fitted by two peaks centered at 532 and 531 eV. The high-binding-energy component can be attributed to the presence of loosely bound oxygen on the surface of the ZnO nanocystals. The low-binding-energy component is attributed to O2- ions on the wurtzite structure of the hexagonal Zn2+ ion array. The fact that the 532 eV peak has an area greater than that of the 531 peak implies that sample S4 has abundant surface hydroxyl groups.35 This can be further confirmed by the FTIR spectrum of sample S4 in Figure 4. The peaks at 1632 and 3438 cm-1 correspond to the bending and stretching vibrations of -OH groups, respectively. The TGA curve of ZnO

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Figure 7. TGA of sample S4.

Figure 8. UV-vis spectra of zinc oxide nanoparticles in sample S4 and NanoZ_AQ40 from Advanced Nano.

Figure 6. XPS spectra of sample S4: (a) survey scan in the energy range of 0-1100 eV, (b) high-resolution scan of the lines of Zn 2p3/2, (c) oxygen 1s peak.

is shown in Figure 7. The weight loss from room temperature to 120 °C was about 0.46%, which is attributed to the loss of surface absorbed water.24 The weight loss from 120 to 800 °C was about 5.79% and is attributed to the surface hydroxyl groups.35 In many applications, ZnO nanoparticles have to be uniformly dispersed in aqueous media in the presence of polyelectrolytes such as polymethacrylic acid and the ammonium salt of polymethacrylic acid.36-38 The strong interaction through hydrogen bonding between the hydroxylated nanoparticle surface and the COO- groups of the polyelectrolytes stabilizes the dispersion. With abundant hydroxyl groups, the ZnO nanoparticles synthesized by our method are expected to be more easily dispersed to form stable colloids. ZnO as a Sun Blocker. Fine particles of metal oxides (e.g., titanium, zinc, zirconium, iron, etc.) are extensively used as agents to attenuate (absorb and/or scatter) ultraviolet radiation

having a wavelength of 290-400 nm. Zinc oxide powder more effectively attenuates UV radiation in not only the UVB range (290-320 nm) but also the UVA range (320-400 nm) and has a lower refractive index, of about 1.9, than other metal oxides.4 The UV-blocking properties of our ZnO nanoparticles were studied by UV-vis spectrometry and compared with those of a commercial product (Figure 8). It can be seen that the curves of sample S4 and NanoZ_AQ40 exhibit excitonic absorption features at about 358 and 360 nm, respectively, which correspond to band gaps of 3.46 and 3.44 eV. The similar UVvis absorption characteristics of our sample and NanoZ_AQ40 stem from their similar particle sizes and morphologies (see the TEM image of NanoZ_AQ40 in the Supporting Information). The weak blue shift compared to the band gap of 3.37 eV for bulk ZnO signifies the small particle size. Process Flowsheet and Cost of Raw Materials. The flowsheet for producing ZnO nanoparticles is shown in Figure 9. ZnSO4‚7H2O and KCl are first added to the reactor, followed by KOH. As an example, assuming that all of the chemicals have purities of 100%, 5.75 g of ZnSO4‚7H2O, 11.2 g of KCl, and 2.24 g of KOH produce 1.63 g of ZnO as the product and 3.49 g of K2SO4 as a byproduct. The reactor effluent is then centrifuged to remove the liquid phase and washed to remove all of the KCl and K2SO4, which has a solubility about onethird that of KCl. The ZnO nanoparticles are then sent to a vacuum dryer to remove all residual water. A comparison of the new process and other processes is presented in Table 2. The raw material costs, including those of the zinc precursor and other chemicals, were estimated according to the amounts of materials required to produce 1 mol of ZnO. None of the excess matrix salt and base such as

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of NanoZ_AQ40 from Advanced Nano. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited

Figure 9. Flowsheet for producing ZnO nanoparticles with zinc sulfate heptahydrate as the zinc source and potassium chloride as the base. Table 2. Cost Comparison of Chemicals Required for Different Mechanochemical Reactions reaction

costa ($)

1b

5.61

3

2c

7.01

0.5

3d

6.11

1

4e

9.12

0.5

calcination at 300 °C for 1 h and washing with DDI water calcination at 450 °C for 0.5 h and washing with DDI water calcination at 400 °C for 0.5 h and washing with DDI water washing with DDI water

5f

7.26

0.5

washing with DDI water

milling time (h)

posttreatment

ref 16 20 18 this work

a

Cost of amount of chemical required to produce 1 mol of ZnO. All costs obtained from www.sciencelab.com are those for reagent grade (if applicable) and the largest batch size available. b Reaction 1: ZnCO3‚2Zn(OH)2 + 32.8NaCl f 3ZnO + CO2 + 2H2O + 32.8NaCl. c Reaction 2: Zn(CH3COO)2 + 1.2H2C2O4‚2H2O f ZnC2O4‚2H2O + 2CH3COOH + 0.2H2C2O4‚2H2O; ZnC2O4‚2H2O + 0.2H2C2O4‚2H2O + 0.5O2 f ZnO + 2.4CO2 + 2.4H2O. d Reaction 3: ZnCl2 + Na2CO3 + 8NaCl f ZnCO3 + 10NaCl; ZnCO3 + 10NaCl f ZnO + CO2 + 10NaCl. e Reaction 4: ZnSO4‚7H2O + 7.5KCl + 2KOH f ZnO + K2SO4 + 7.5KCl + 8H2O. f Reaction 5: ZnSO ‚7H O + 10NaCl + 2NaOH f ZnO + Na SO + 4 2 2 4 10NaCl + 8H2O.

KCl or K2SO4 was recycled. Whereas the new process requires a slightly higher cost of reactants, it has a considerably shorter milling time and does not require postreaction thermal treatment. Conclusions To summarize, ZnO nanoparticles with a mean diameter of about 20 nm were synthesized by a novel mechanochemical process. The room-temperature reaction is fast, with a reaction time of about 5 min; self-propagates; and does not require external energy input. The product zinc oxide nanoparticles have excellent UV-blocking properties and abundant surface hydroxyl groups; both of these properties make them an excellent candidate for cosmetics applications. The raw material costs of our process compare favorably with those of other mechanochemical processes. Indeed, the process cycle time, energy cost, and other factors must be considered in detail to determine the actual product cost. Other engineering issues, such as the effects of the raw-material particle size on ease of processing and of the molar ratio of matrix salt to zinc sulfate heptahydrate on product particle size, should also be studied. Efforts in these directions are underway. Supporting Information Available: XRD patterns of samples S4 and S6, TEM images of samples S3 and S6, SEM image of sample S5, and TEM image of zinc oxide nanoparticles

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ReceiVed for reView July 28, 2007 ReVised manuscript receiVed November 4, 2007 Accepted November 16, 2007 IE071034J