Preparation of Mesoporous ZnO Microspheres through a Membrane

Mesoporous ZnO microspheres with a diameter of about 10 μm were prepared through the combination of a membrane-dispersion microstructured reactor wit...
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Preparation of Mesoporous ZnO Microspheres through a Membrane-Dispersion Microstructured Reactor and a Hydrothermal Treatment Chunling Zhang,† Yujun Wang,‡,* Siwei Bi,† and Guangsheng Luo‡ † ‡

Department of Chemistry, QuFu Normal University, Shandong 273165, China The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Mesoporous ZnO microspheres with a diameter of about 10 μm were prepared through the combination of a membrane-dispersion microstructured reactor with a hydrothermal process. First, ZnSO4 reacted with NH4HCO3 in a membranedispersion reactor to produce the Zn5(CO3)2(OH)6 precursor, and then the precursor was treated at the temperature of 80 °C for 2 h to form the Zn5(CO3)2(OH)6 microspheres aided by the surfactant CTAB or F127; after the calcinations, mesoporous ZnO microspheres were obtained. The experimental results showed that the morphology of ZnO microspheres obtained using the membrane-dispersion microstructured reactor to mix the raw materials was better than that of those obtained using stirred tanks. Without surfactants and the hydrothermal treatment of the precursors, uniform ZnO microspheres could not be obtained. The SEM images showed that the monodispersed ZnO microspheres were assembled by the plates, and deeper observations revealed that these plates were aggregated by the ZnO nanorods formed by ZnO nanoparticles with a diameter of 9.0 nm. The adsorption desorption isotherm curve indicated that the ZnO microspheres had mesoporous structures with an average pore diameter of 13.8 nm when using CTAB as the surfactant, and the specific surface area of the ZnO microspheres could reach 71.3 m2/g, much higher than that of ZnO nanoparticles.

1. INTRODUCTION ZnO microspheres with mesopores have received considerable attention owing to their unique properties, including their high specific surface area, low density, good permeation, and potential applications such as high-efficiency catalysts,1,2 in drug delivery,3 6 and as sensors.7 ZnO microspheres also could promote light scattering and enhance photon absorption in dye-sensitized solar cells.8 As the sorbent for H2S, the higher the specific surface area and the pore volume of ZnO are, the higher the adsorption capacity is.9 There are two keys to preparing the ZnO microspheres with a high specific area. The first is how to obtain ZnO nanoparticles, while the second is how to assemble these nanoparticles into microspheres. For the first step, ZnO nanoparticles with small crystal size and narrow particle size distribution were successfully prepared in a membrane-dispersion microstructured reactor through the direct precipitation method.10 The second step is the assembly process, and many studies have shown that a surfactant can aid nanoparticles to form microspheres by self-assembly techniques in the hydrothermal process.11 14 For example, Barick et al.12 synthesized highly mesoporous self-aggregated nanoclusters of pure and transition metal (Mn, Co and Ni)-doped ZnO by refluxing their acetate precursors in diethylene glycol (DEG) medium. The porous spherical nanoclusters comprising numerous nanocrystals are fairly stable, and the specific BET surface areas of ZnO was 20.6 m2/g. Yan et al.15 fabricated ZnO hollow spheres via a homogeneous precipitation method under solvothermal conditions using zinc hydroxide carbonate as the precursor. The capping reagent of EG exerted a major influence on directing the formation of these unique microstructures. Li et al.16 prepared nanosheet-based flower-like microspheres on a layer of Zn5(CO3)2(OH)6 precursor via a hydrothermal process r 2011 American Chemical Society

using zinc nitrate and urea aqueous solutions as the feed stocks and polyethylene glycol (PEG) as the template in a mixed solution of ethanol and water, and the corresponding ZnO nanosheetassembled microspheres were obtained through the following calcination treatment. The mechanism was as follows: when Zn2+ and the PEG molecular chain of the coordination point connected, the inhibitor ability between the PEG molecules was small, so the PEG molecules were wound under intermolecular forces and formed a larger microspherical template. Finally, the crystal was nucleated and grew on the spherical template surface, and then the assembly of the microsphere structure was completed. Liu et al.14 developed an organization of ZnO nanobuilding blocks (either nanorods or nanoplatelets) into hollow, spherical dandelions using Zn(OH)2 as the precursor through the CTAB self-assembly approach. In the above studies, a stirred tank was often used to prepare the precursors. However, the stirred tank has some disadvantages, including the low rate of mass transfer and heterogeneous mixing; moreover, parts of the microspheres are likely broken by the violent agitation in the stirred tank. These disadvantages result in poor sphericity. The membrane-dispersion microstructured reactor developed by our lab exhibits numerous practical advantages in the reactor design and manufacturing process, including enhanced mixing efficiency and mass transfer, continuous operation, and low energy costs.17 19 In this paper, there are two differences from other Received: May 5, 2011 Accepted: October 5, 2011 Revised: August 25, 2011 Published: October 05, 2011 13355

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Industrial & Engineering Chemistry Research

Figure 1. Membrane reactor experimental setup and the principle of membrane dispersion (insert part): (1) the dispersed phase feed; (2) the continuous phase feed; (3) pump; (4) membrane dispersed reactor; (5) precipitation product; (6) the sealed flask; (7) the oven; (8) dispersed phase inlet; (9) continuous phase inlet; (10) microporous membrane; (11) microchannel; (12) mixture outlet.

studies. The first is the combination of the membrane-dispersion microstructured reactor with the hydrothermal process; the second is that the raw materials were ZnSO4 and NH4HCO3 and the precursor was zinc hydroxyl carbonate. Most of the studies cited above used zinc hydroxyl for the precursor. Because of these differences, special ZnO microspheres with mesopores consisting of plate-like nanoparticles were successfully prepared in this study.

2. EXPERIMENT AND ANALYSIS 2.1. Chemicals. Analytical-grade zinc sulfate heptahydrate (ZnSO4 3 7H2O), ammonium bicarbonate (NH4HCO3), and ethanol (CH3CH2OH) purchased from Beijing Chemical Works were used directly, without further purification. The triblock copolymer Pluronic F-127 and the cationic surfactant CTAB were obtained from Baier Di. 2.2. Experimental Methods. Figure 110 shows the membrane reactor experimental setup and the principle of membrane dispersion (insert part). The membrane-dispersion microreactor device consisted mainly of two polytetrafluoroethene (PTFE) sample plates with different thickness (30 mm  30 mm  0.5/2 mm), two stainless steel plates (30 mm  30 mm  2 mm), and a microfiltration membrane. First, one microchannel was fabricated using a machine on the PTFE plate with the thickness of 2 mm; the geometric size of the microchannel was 10 mm  2 mm  2 mm (length  width  height). This channel was used for the flowing of the continuous phase; then a square hole with a dimension of 15 mm 15 mm 0.5 mm (length  width  height) was made by a machine on the PTFE plates with a thickness of 0.5 mm, this square hole was used for the flowing of the dispersed phase. The stainless steel plate with an inlet stuck together with the thin PTFE plate, the inlet was connected with the square hole, this is the upper part of the microreactor, as shown in Figure 1; another stainless steel plate with an inlet and an outlet stuck together with the thick PTFE plate, the inlet and the outlet on the stainless steel plate were connected with the microchannel, this is the lower part of the microreactor, as shown in Figure 1. The microfiltration membrane made of nickel fiber with an average pore size of 5 μm was placed between two PTFE plates. The four plates were fixed with four screw bolts. The active area of the microporous membrane was 19 mm2, and the geometric size of the microchannel of the PTFE thick plate (2 mm in thickness) was 10 mm  2 mm  2 mm (length  width  height).

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In a typical experimental procedure, NH4HCO3 and ZnSO4 3 7H2O were dissolved in deionized water. A 2.0 mol/L portion of a NH4HCO3 solution (100 mL) was used as the dispersed phase feed, and 1.0 mol/L of ZnSO4 solution (100 mL) was used as the continuous phase feed. A 0.04 g portion of CTAB was added to the continuous phase feed and stirred until completely dissolved. Both of the feed flow rates were 10 mL/min. Under room temperature, the dispersed phase feed was pressed through the membrane into the microchannel to mix with the continuous phase feed coming from the continuous phase feed inlet. The two solutions were mixed in the microchannel, which caused a supersaturation of zinc hydroxy carbonate (0.1 mol/L) and the generation of the white product. Subsequently, this white product was transferred into a sealed flask followed by a uniform heating at 80 °C for 2 h, and then it was cooled to room temperature. The white precipitate was collected by filtration and washed with distilled water and ethanol several times under room temperature and then dried in a vacuum at 100 °C overnight. Finally, the white powder was calcined at 400 °C for 2 h. The stirred method used was as follows: first, under the room temperature, 0.1 mol of ZnSO4 3 7H2O and 0.04 g of CTAB were dissolved in deionized water and prepared as a blended solution whose volume was 100 mL. This solution was put into a beaker whose volume was 1000 mL, a JB-2 magnetic mixer (Ronghua instrument company, Jintan, Zhejing) was used to mix this solution and the NH4HCO3 solution, the length of cylinder stirrer was 2 cm and the diameter of the stirrer was 0.8 cm, and the turning speed of stirrer was 800 r/min. A 2.0 mol/L concentration of NH4HCO3 solution (100 mL) was added dropwise into the stirred solution, and the mixed solution was vigorously stirred for 30 min. Subsequently, this solution was transferred into a sealed flask followed by uniform heating at 80 °C for 2 h, and then it was left to cool at room temperature. The white precipitate was collected and washed with distilled water and ethanol several times at room temperature and then dried in a vacuum at 100 °C overnight. Finally, the white powder was calcined at 400 °C for 2 h. 2.3. Characterization. The morphology of microspheres was observed using a scanning electron microscope (SEM, JEM6301F, JEOL, Japan) with an accelerating voltage of 15 kV. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were taken using a transmission electron microscope (JEM-2010 type, JEOL, Japan). The XRD analyses were performed using an X-ray diffractometer (D/max-TTRIII, Rigaku, Japan) with Cu Kα (45 KV and 200 mA) at a scanning speed of 8°/min over the 2θ range of 10 80. Nitrogen adsorption desorption isotherms were measured on a surface area analyzer (Autosorb-1-C, Quantachrome, America). Before measurements were made, the samples were degassed at 150 °C in a vacuum for 2 h. The BET method was used to calculate the specific surface areas. The pore volume and pore size distributions were derived from the desorption branches of the isotherms using the Barrett Joyner Halenda (BJH) method. The total pore volume, Vp, was calculated from the nitrogen amount adsorbed at a relative pressure of 0.9.

3. RESULTS AND DISCUSSION 3.1. Comparison of ZnO Microspheres Prepared Using the Stirred Method and the Membrane-Dispersion Microstructured Reactor. SEM images of ZnO microspheres prepared using

the stirred method and the membrane-dispersion microstructured 13356

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Figure 2. SEM images of ZnO microspheres prepared by combining the stirred method with the hydrothermal treatment (a) and the membrane-dispersion microstructured reactor with the hydrothermal treatment (b).

Figure 4. XRD patterns of (a1) precursor and (a2) ZnO microspheres calcined by the precursor of a1. (b) The EDS spectrum of the sample in Figure 3g.

Figure 3. Low-magnification and high-magnification SEM images of ZnO microspheres prepared at different concentrations of CTAB. Lowmagnification images: (a) without CTAB; (b) 0.1 g/L CTAB; (c) 0.2 g/L CTAB; (d) 0.4 g/L CTAB; (e) 1.0 g/L CTAB; (f) 2.5 g/L CTAB. Highmagnification images: (g, h) synthesized using 0.4 g/L CTAB.

reactor are shown in Figure 2. It can be seen that ZnO microspheres (as shown in Figure 2a) prepared by combining the stirred method and the hydrothermal treatment had poor sphericity and that most of the microspheres were stuck together. However, uniform ZnO microspheres (Figure 2b) were synthesized by combining the membrane-dispersion microstructured reactor and the hydrothermal treatment. These results show that the mixing mode plays a key role in the preparation process for the uniform ZnO microspheres. The big difference between the stirred tanks and the membranedispersion reactor is the mixing mode. In a stirred tank, the ZnSO4 solution was added dropwise into another NH4HCO3 solution. This was done so that one small droplet containing ZnSO4 was instantly diluted into the whole NH4HCO3 solution, and therefore the concentration of Zn2+ in the NH4HCO3 solution was very low, which resulted in that the supersaturation ratio was relatively small. For the membrane dispersed microreactor, two solutions were

mixed very quickly, so the supersaturation ratio in a membranedispersion reactor was much larger than that in the stirred tanks. For the stirred tank, the operation mode is a batch mode, but for the membrane-dispersion microreactor, the operation mode is continuous. Another reason is that there was a violent shearing force when using a stirred tank to mix the solutions, the violent force was not suitable for forming the spheres, so the microspheres were not as good as those obtained using the membrane-dispersion microstructured reactor. 3.2. Effect of the Concentration of CTAB on the Morphology of Products. SEM images of ZnO microspheres prepared at different concentrations of CTAB are shown in Figure 3. The concentrations of CTAB are 0, 0.1, 0.2, 0.4, 1.0, and 2.5 g/L CTAB. As shown in Figure 3a, only the plate-like ZnO structures were obtained without CTAB after the hydrothermal treatment. However, when CTAB was used in the reaction process, uniform ZnO microspheres (Figure 3b f) could be obtained. It was found that the diameter of the microspheres decreased with an increase in the concentration of CTAB. The diameter of the microspheres was about 10 13 μm when 0.1 g/L CTAB was used, decreasing to 6 8 μm when the concentration of CTAB was increased to 1.0 g/L. The high-magnification images shown in Figure 3g and Figure 3h clearly indicate that ZnO microspheres with a diameter of 9 μm were formed by the aggregation of plates; after zooming in, it can be seen that there are pores between the ZnO plates. Figure 4a shows the XRD patterns of the precursor (the white powder) and ZnO microspheres when the concentration of CTAB was 0.2 g/L. It can be seen from Figure 4a1 that the three characteristic peaks of the precursor are located at 2θ, equaling 13.2°, 32.9°, and 59.2°; they were consistent with those 13357

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Industrial & Engineering Chemistry Research of JCPDS card No. 19-1458, proving that the precursor (the white powder) was zinc hydroxyl carbonate.20 Figure 4a2 shows that all diffraction peaks are in fairly good agreement with those of standard patterns for hexagonal wurtzite ZnO (JCPDS card No. 36-1451). No characteristic peaks from other impurities are detected. The sharpness of the peaks indicates that ZnO microspheres are highly crystallized. The (1 0 3) diffraction peak was used to calculate the crystal size through the Scherrer equation. In the Scherrer equation, τ = Kλ/β cos θ, where K is the shape factor, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (fwhm) in radians, and θ is the Bragg angle; τ is the mean size of the ordered (crystalline) domains. The results showed that the average crystallite size of ZnO is about 9.7 nm. Figure 4b shows the EDS spectrum taken for the ZnO powder of Figure 3g. The O- and Zn-related peaks are strong, revealing that the main composition of the powder was ZnO, there are some other small peaks in the EDS, which indicate that there was a little other compound in the powder; maybe because the calcination temperature was only 400 °C, the white precursor was not completely decomposed. 3.3. Possible Growth Mechanism of ZnO Microspheres. To investigate the growth of ZnO microspheres in the hydrothermal process, the SEM images of ZnO samples were taken at different times. As shown in Figure 5, when the hydrothermal time was 0 h, only the ZnO nanorod was obtained, and then parts of ZnO microspheres were formed when the hydrothermal time was 0.5 h; with the increase in time, uniform ZnO microspheres were

Figure 5. SEM images of the ZnO microspheres prepared at different hydrothermal time: (a) 0; (b) 0.5; (c) 1.0; (d) 1.5; (e) 2.0 h.

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prepared. These results prove that both the hydrothermal process and surfactant played key roles in the formation of ZnO microspheres. There are two main functions for CTAB in the whole process. First, CTAB can reduce the diameter of the crystals. According to our previous studies, without CTAB, the average crystallite size was 13.4 nm, calculated from the XRD spectrum. However, when the concentration of CTAB was 0.2 g/L in the original reaction solution, the average crystallite size reduced sharply to 9.7 nm. This is because there is an interaction between the surfactant molecules and the nanoparticles; the surfactant can disperse the nanoparticles and prevent them from becoming larger. Another important function for CTAB is in the assembly of the nanosized precursor particles in the hydrothermal process. According to these results, a possible growth process for ZnO microspheres is proposed. As shown in Figure 6, first, the oriented growth of the growth units led to ZnO nanorods. Sun et al.21 suggested that CTAB not only accelerates the reaction of the growth units but also leads to their oriented growth. CTAB is an ionic compound, which ionizes completely in water. While [Zn(OH)4]2 3 ZnCO3 exists in the form of negatively charged tetrahedrons, CTA+ particles are positively charged with a tetrahedral head and a long hydrophobic tail. At the beginning of the CTAB-assisted growth process, CTA+ Zn(OH)4]2 3 ZnCO3 ion pairs were formed by the electrostatic interaction. Since CTAB is a kind of strong acid weak base salt, it can accelerate the ionization of [Zn(OH)4]2 3 ZnCO3. The CTA+ [Zn(OH)4]2 3 ZnCO3 ion pairs form a combination of CTAB and ZnO. This leads to the oriented growth of the [Zn(OH)4]2 3 ZnCO3 along the [0001] direction.22 Thus, Zn5(CO3)2(OH)6 nanorods were prepared (Figure 6a), the nanorods formed the plate-like Zn5(CO3)2(OH)6 through the hydrophobic interaction (Figure 6b), and then the plate-like Zn5(CO3)2(OH)6 aggregated into Zn5(CO3)2(OH)6 microspheres (Figure 6c). So the microspheres synthesized by this method are assembled by the plate-like ZnO, and they are different from the microspheres assembled by the ZnO nanorods prepared through the direct precipitate method.23 When the triblock polymer F127 is used as the template, the polymer molecules surrounded the precursor-nanorods, the hydrophilic part contacted the outer face of nanorods because of the polar interaction, and the hydrophobic part makes nanorods aggregate to form a plate-like because of the hydrophobic interaction. Compared to the microspheres reported by others, these microspheres, whose diameters were 10 13 μm, are larger as well as more uniform. The diameters of other microspheres were almost 118 nm,24 200 600 nm,25,26 and 3 4 μm.8 3.4. Effect of the Hydrothermal Temperature on the Morphology of Products. The effect of the hydrothermal temperature on the morphology of products is shown in Figure 7. It can be seen that ZnO nanorods were synthesized at room

Figure 6. The possible growth process of ZnO microspheres. 13358

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Industrial & Engineering Chemistry Research temperature. Then plate-like ZnO (30 °C) and microspheres (50 °C) gradually increased with the increase of temperature. Finally, uniform microspheres were prepared at the temperatures

Figure 7. SEM images of the ZnO microspheres prepared at different hydrothermal temperatures: (a) room temperature (