NH4 + directed assembly of zinc oxide micro-tubes from nanoflakes
© Yang et al; licensee Springer. 2011
Received: 3 June 2011
Accepted: 11 August 2011
Published: 11 August 2011
A simple precipitation process followed with the heat treatment was developed to synthesize ZnO micro-tubes by self-assembly of nanoflakes composed of nanoparticles. The resulting ZnO micro-tubes demonstrated excellent photocatalytic performance in degrading methylene blue (MB) under UV illumination. It was found that NH4 + ion played a critical role in directing the assembly of the nanoflakes to form the micro-tube structure. A critical reaction ratio existed at or above which the ZnO micro-tubes could be obtained. For the mixtures of solutions of (NH4)2CO3 and zinc salt, the ratio ( ) was 2:1.
The zinc oxide (ZnO) has been widely investigated and utilized in various technical fields, including pigments, rubber additives, gas sensors, varistors, semiconductors, optoelectronic devices, light-emitting diodes, and solar cells, due to its catalytic, electrical, optoelectronic, and photochemical properties . With the development of nanotechnology, nano/micro-sized ZnO had attracted extensive research attentions over the past decade [2–30]. Abundant nanostructure morphologies exist for ZnO, such as flower-like nanostructures [5, 26, 30], nanorod [3, 12–15, 21], nanowires [4, 18], nanobridges and nanonails , tubular microstructural , nano/micro-sized particles [9, 11, 27, 28], and micro-tubes . A variety of methods had been developed to synthesize various ZnO nanostructures, including chemical vapor transport and condensation (CVTC) , electrodeposition , hydrothermal synthesis [25, 26], evaporation formation , chemical precipitation , and aqueous solution deposition . For example, nanohelixes, nanosprings, nanorings, and nanobelts had been synthesized by Kong and Wang via a solid-vapor process in 2003, which could have applications as one-dimensional nanoscale sensors, transducers, and resonators . In 2006, Wang and Song synthesized ZnO nanowires array by the vapor-liquid-solid process, which has the potential of converting mechanical, vibrational, and/or hydraulic energy into electricity for powering nanodevices .
In this work, a simple precipitation process followed with the heat treatment was developed to synthesize ZnO micro-tube structure by self-assembly of nanoflakes composed of nanoparticles. The formation mechanism of this interesting ZnO morphology was examined by systematically investigating the effects from zinc salt type, precipitation agent concentration, precipitation environment, and precipitation agent type. The study identified a key role played by NH4 + ion in the directional growth of the micro-tube structure. A critical reactant ratio ( ) was found at 2.0:1.0, below which no such micro-tube structure could be obtained. The photocatalytic performance of ZnO micro-tubes was demonstrated by their good photocatalytic degradation effect on MB under UV illumination. With the combination of the special catalytic, electrical, optoelectronic, and photochemical properties of ZnO and this interesting highly porous micro-tube structure, these ZnO micro-tubes may find potential applications in many technical areas.
Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, ≥99.0%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, People's Republic of China) and zinc sulfate heptahydrate (ZnSO4·7H2O, ≥99.5%, Kemiou Chemicals Co. Ltd., Shenyang, People's Republic of China) were used as the zinc source, and ammonium carbonate ((NH4)2CO3, NH3% ≥40.0%, Sinopharm Chemical Reagent Co., Ltd.) and sodium carbonate (Na2CO3, ≥99.8%, Sinopharm Chemical Reagent Co., Ltd.) were used as the precipitation reagents in the synthesis of self-assembled ZnO micro-tubes, respectively. Methylene blue trihydrate (C16H18ClN3S·3H2O, Kemiou Chemicals Co. Ltd.) was used as the model organic pollutant for the static photocatalytic degradation experiment with ZnO micro-tubes under UV irradiation. All the reagents were of analytical grade and used as received without further purification.
ZnO micro-tubes were synthesized by a simple precipitation method. In a typical synthesis process, a metal alkoxide, Zn(CH3COO)2·2H2O, was dissolved in deionized (DI) water to obtain solution #1 at the concentration of 1 M, and (NH4)2CO3 was dissolved in DI water to obtain solution #2 at the concentration of 1.8 M. While the mixture was stirred vigorously during the precipitation process, 100 mL of solution #1 was dropwise added into 200 mL of solution #2. After the addition of solution #1, the mixture was kept stirring for 30 min, and then the white precipitate was collected by centrifugation, washed with DI water repeatedly until neutral pH, and dried at 60°C to approximately 70°C for a day. The final ZnO product was obtained by calcination of the precipitate at 300°C for 2 h in air. To examine the effect of zinc salt on the morphology of obtained ZnO, an inorganic zinc salt, ZnSO4·7H2O, was also used in this synthesis processes with the same experimental setting as Zn(CH3COO)2·2H2O. To examine the precipitation reagent concentration effect on the formation of ZnO micro-tubes, (NH4)2CO3 solutions with different concentrations (from 1.8 to 0.5 M) were prepared and used in the precipitation process to obtain desired ratios. The chemical addition sequence in the precipitation process was examined with both zinc salts at the ratio of 3.2:1.0 to demonstrate the precipitation environment effect, in which both the addition of the zinc salt solution into the (NH4)2CO3 solution and the addition of the (NH4)2CO3 solution into the zinc salt solution were adopted. Na2CO3 was also used as the precipitation reagent to verify the effect of NH4 + in the formation of ZnO micro-tubes at the ratio of 3.2:1.0 for both zinc salts under the same experimental conditions.
The crystal structures of the precipitates and ZnO final products were analyzed by the D/MAX-2004-X-ray powder diffractometer (Rigaku Corporation, Tokyo, Japan) with Ni-filtered Cu (0.15418 nm) radiation at 56 kV and 182 mA. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were utilized to study their morphologies. SEM images were obtained with a SUPRA35 Field Emission Scanning Electron Microscope (Carl Zeiss NTS GmbH, Carl-Zeiss-Straße 56, 73447 Oberkochen, Germany). SEM samples were made by dispersing the precipitate or ZnO final product in ethanol, applying drops of the dispersion on a conductive carbon tape, and drying in air for 12 h. Before imaging, the sample was sputtered with gold for 120 s (Emitech K575 Sputter Coater, Emitech Ltd., Ashford Kent, UK). TEM observation was carried out on a JEOL 2010 transmission electron microscope (JEOL Ltd., Tokyo, Japan) operated at 200 kV, with point-to-point resolution of 0.28 nm. TEM samples were made by dispersing the precipitate or ZnO final product on a Cu grid. The UV-vis spectrum of ZnO micro-tubes was measured on a UV-2550 spectrophotometer (Shimadzu Corporation, Kyoto, Japan).
Photocatalytic degradation of methylene blue
The photocatalytic performance of ZnO micro-tubes was examined by their photodegradation of MB under UV irradiation. The initial concentration of MB aqueous solution is 1.46 × 105 mol/L (approximately 4.67 ppm) and a fixed concentration of 1 mg photocatalyst per milliliter. The average intensity of UV (254 nm) irradiance striking the MB solution was ca. 1.52 mW/cm2, measured by a Multi-Sense UV-B UV radiometer (Beijing Normal University Photoelectricity Instruments Plant, Beijing, China). The UV irradiation time varied from 20 to 180 min. At each time interval, ZnO micro-tubes were recovered by centrifugation at 12,600 rpm, and the light absorption of the clear solution was measured by the UV-2550 spectrophotometer. The remaining concentration of MB in the solution could be calculated by the ratio between the light absorptions of photocatalyst-treated and untreated MB solutions. For the comparison purpose, the concentration changes of MB solution were also investigated with the same experimental setup in the absence of ZnO micro-tubes and under UV light illumination, or with the presence of ZnO micro-tubes and no UV illumination.
Results and discussion
ZnO micro-tubes by self-assembled nanoparticles
The white Zn4CO3(OH)6·H2O precipitate demonstrates an interesting tube morphology at micrometer size, which is assembled by nanoflakes composed of nanoparticles (Figure 1B). These micro-tubes have a tri-pore structure, in which the largest pores are the tubes at micrometer size, the middle ones are the inter-nanoflake pores, and the smallest ones are the pores between nanoparticles in the nanoflakes.
Interestingly, the white ZnO final product has the similar micro-tube morphology as that of Zn4CO3(OH)6·H2O. Figure 1D, E shows the FESEM and TEM images of ZnO with different magnifications. From these observations, it is clear that the micro-tube morphology was kept during the heat treatment, while the diameter of these micro-tubes became smaller due to the contraction during the heat treatment. Thus, an interesting micro-tube structure for ZnO could be obtained by a simple precipitation process followed with the heat treatment, which has a highly porous structure and could find potential applications in many technical areas.
Effect of the type of zinc salt on ZnO structure morphology
The white Zn4CO3(OH)6·H2O precipitate obtained from ZnSO4·7H2O also demonstrates the similar tube morphology at micrometer size assembled by nanoflakes composed of nanoparticles (Figure 2B). After the heat treatment, similar highly crystallized ZnO micro-tubes were also obtained (Figure 2C, D), although no acetate ions were involved in this synthesis process. No obvious difference was observed on the crystal structure and morphology of the obtained ZnO final product. Thus, the type of zinc salts (organic or inorganic) is not the determining factor on the formation of ZnO micro-tubes.
Precipitation reagent concentration effect on ZnO structure morphology
The evolution of the morphology with the two zinc salts
Irregular agglomerated particles
Irregular agglomerated particles
Irregular agglomerated particles
Irregular agglomerated particles
Sphere-like microstructures consisted of nanoflakes
Sphere-like microstructures consisted of nanoflakes
Effect of the precipitation environment on ZnO structure morphology
Effect of the ammonium existence on ZnO structure morphology
In the precipitation process, CO3 2- ion is one of the key components to produce Zn4CO3(OH)6·H2O precipitate, which could then be converted to ZnO by the heat treatment. To form the micro-tube structure, however, CO3 2- ion shows little effect. The experimental result here suggests that NH4 + ion is the key factor in the formation of this micro-tube structure. Otherwise, the usage of Na2CO3 as the precipitation agent should also result in the formation of micro-tube structure as (NH4)2CO3 did. Thus, a possible mechanism could be proposed for the formation of these micro-tubes assembled by nanoflakes composed of nanoparticles based on the above experiment results. In the precipitation process, large amounts of NH4 + ions exist in the reaction mixture, which do not chemically participate in the formation of the Zn4CO3(OH)6·H2O precipitate. As suggested by Wang and Muhammed , NH4 + ions could adsorb onto Zn4CO3(OH)6·H2O nanoparticles just precipitated from the reaction mixture, form a monolayer on the surface of these nanoparticles, and hold the nanoparticles together by H-bonding. In their work, they observed that rod-shaped particles consisting of several spherical particles aligned in one direction. Here, the interaction between NH4 +-coated Zn4CO3(OH)6·H2O nanoparticles form nanoflakes first, and the interaction between NH4 +-coated Zn4CO3(OH)6·H2O nanoflakes bonds the nanoflakes together in one direction and produce micro-tube structures by self-assembly. This proposed mechanism could explain the huge difference observed on the precipitate morphology by the chemical addition sequence. When the Zn(CH3COO)2·2H2O solution was dropwise added into the (NH4)2CO3 solution, plenteous NH4 + ions existed that could adsorb onto Zn4CO3(OH)6·H2O precipitate to cover its surface and direct the formation of micro-tube morphology. When the (NH4)2CO3 solution was dropwise added into the Zn(CH3COO)2·2H2O solution, however, not enough NH4 + ions existed that could adsorb onto Zn4CO3(OH)6·H2O precipitate to cover its surface. Thus, the directional growth of Zn4CO3(OH)6·H2O was not achievable and no micro-tube structure was obtained.
Light absorbance property and photocatalytic performance of ZnO micro-tubes
The light absorption spectrum suggests that these ZnO micro-tubes may have a good photocatalytic performance under UV irradiation. The photocatalytic activity of these ZnO micro-tubes was investigated by its degradation effect on MB under UV irradiation. Figure 7B summarizes the residue MB concentration as a function of treatment time for three different treatments. When MB solution was under UV illumination without the addition of ZnO micro-tubes, no significant degradation could be observed. With the addition of ZnO micro-tubes, significant degradation still could not be observed when there was no UV illumination. This observation suggests that adsorption of MB will not contribute much to its concentration changes during the photocatalytic degradation treatment. Under UV light illumination, however, photodegradation of MB was clearly observed with the treatment of ZnO micro-tubes. After 3 h of treatment under UV illumination, the color of the MB solution changed from blue to almost colorless, and the concentration of residue MB was determined to near zero. From the comparison of these three treatments, it is clear that these ZnO micro-tubes have a good photocatalytic activity under UV illumination.
ZnO micro-tube structure was synthesized by a simple precipitation process followed with heat treatment. The micro-tube was formed by self-assembly of nanoflakes of ZnO nanoparticles, creating a highly porous structure. The formation mechanism of ZnO micro-tube structure was investigated, and the key role of NH4 + ion in the directional growth of this micro-tube structure was demonstrated. A critical reactant ratio ( ) was found at 2.0:1.0, below which no such micro-tube structure could be obtained. These ZnO micro-tubes demonstrated a good photocatalytic degradation effect on MB under UV illumination and could find potential applications in many technical areas.
This study was supported by the National Basic Research Program of China, Grant No. 2006CB601201, the Knowledge Innovation Program of Chinese Academy of Sciences, Grant No. Y0N5711171, and the Knowledge Innovation Program of Institute of Metal Research, Grant No. Y0N5A111A1.
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