Facile Growth of Porous Hierarchical Structure of ZnO Nanosheets on Alumina Particles via Heterogeneous Precipitation
Hamid Tajizadegan
1)*, Majid Jafari
1), Mehdi Rashidzadeh
2), Reza Ebrahimi-Kahrizsangi
1), Omid Torabi
1)1) Department of Materials Engineering, Najafabad Branch, Islamic Azad University, P.O. Box 517, Isfahan, Iran
2) Catalysis & Nanotechnology Research Division, Research Institute of Petroleum Industry (RIPI), P.O. Box 14665-137, Tehran, Iran [Manuscript received July 1, 2012, in revised form November 16, 2012, Available online 29 June 2013]
ZnO nanosheets and nanoflakes were grown on alumina particles in the absence of surfactants via heterogeneous precipitation using urea, zinc acetate and bayerite as precursors. Thermo-gravimetric analysis (TGA), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used and the results indicated the formation of only two phases: wurtzite-type ZnO andg-Al2O3. ZnO nanoflakes were grown on alumina particles in the samples with ZnO content of 40 and 60 wt%. By increasing the ZnO content to 80 wt%, a porous hierarchical structure of ZnO with nanosheet arrays appeared. Both of these nanoflakes and nanosheets were about 40e80 nm in thickness and about 1e2 mm in diameter. It was proposed that Zn5(CO3)2(OH)6nuclei undergo higher growth rates in thin sheets at edges of bayerite particles with a higher surface energy. The BrunauereEmmetteTeller (BET) measurements proved a reachable high surface area for hierarchical structures of ZnO nanosheets, which could mainly be attributed to their unique growth on alumina particles. Also, UV absorption results revealed that ZnOeAl2O3 compositions still show the UV characteristic absorption of ZnO, which can evidence the presence of photocatalytic properties in ZnOeAl2O3 compositions.
KEY WORDS: ZnO; Crystal growth; Heterogeneous nucleation; Hierarchical structures; High surface area; Morphology evolution
1. Introduction
Due to excellent properties such as wide band gap and large exaction binding energy, zinc oxide has become one of the most interesting materials for many applications such as optoelec- tronic devices[1]and photocatalysts[2]. Also, ZnO in single phase or in cooperation with other materials has been widely used as a catalyst in vast various applications such as gas, oil and petro- chemical industries[3,4]. With regard to the fact that catalytic and photocatalytic reactions generally take place on the surfaces of solid phase, synthesis of nanosized ZnO with high surface area has attracted intensive research interests[2,5]. Regardless of the morphology of ZnO, some researchers have attempted to employ other materials as a support or template in order to increase the surface area and aggregation resistance of ZnO active compo- nents[6,7]. However, the size and morphology of ZnO particles
play a key role in all applications[8,9]. In addition, it is well documented that ZnO can present a wide range of morphol- ogies[10]. This fact is mainly resulted from the preferential growth of ZnO which leads to one-dimensional structures such as nanorods, nanoneedles and nanotubes. In contrast, some re- searchers have attempted to control the preferential growth of ZnO in order to synthesize two-dimensional structures including sheets and discs with different properties[11]. Meanwhile, recently synthesis of hierarchical structures of these two- dimensional ZnO structures has attracted considerable interests due to their three-dimensional structures, higher surface area and resistance against aggregation[2,12]. Qu et al.[2] synthesized a hierarchical structure of ZnO nanosheets using trisodium citrate as surfactant via hydrothermal method. Lu et al.[12]synthesized a hierarchical structure of ZnO nanosheets by means of two sur- factants of sodium dodecyl sulfonic and PEG 600 via hydro- thermal method. The above two reports, together with most recent reports, concentrated on using expensive materials as surfactants, and complicated methods with specific conditions like hydrothermal, which are economically unfavorable, and especially difficult to transfer into an industrial scale. Therefore, synthesis of porous hierarchical structure of ZnO nanosheets with very high specific surface area remains a main subject.
*Corresponding author. Tel./Fax:þ98 331 229 1008; E-mail address:
[email protected](H. Tajizadegan).
1005-0302/$esee front matter CopyrightÓ2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
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Besides, it is well known that ZnO in combination with other materials gives different growth habits and morphologies. This fact is not only because of selection of precursors[13], calcination temperature[14], solvents[15]and surfactants[16], but can also be affected by support or template. Lee et al.[17] reported that morphology of ZnO can be controlled by adjusting Zn content in AleZn mixture. Moshfegh et al.[18]proved that the degree of substrate surface roughness affects the size and shape of zinc oxide nanostructure. Thus, the combination of ZnO with other materials is an effective way to control the structure and morphology of ZnO, which allows higher potentials and properties.
Herein, the motivation of this work is to design a simple and low cost approach to grow hierarchical structures of ZnO nanosheets with the cooperation of alumina particles (as sup- porting particles) and also, to avoid using high cost nanoparticles as supports and expensive surfactants as surface modifiers. In this regard, attempts werefirst made to grow ZnO on bayerite particles (Al(OH)3) via heterogeneous precipitation. Then, the effect of alumina particles and ZnO content on morphology, surface area and UV absorption were investigated. Also, the mechanism of growth and formation of ZnO was discussed.
2. Experimental
For synthesis of high purity ZnO (100Z sample), 0.3 mol/L zinc acetate dehydrate (Zn(CH3COO)2$2H2O, Merck, 99.5%) solution was mixed with urea (NH2CONH2, Merck, 98%) by mole ratio 1:6, respectively. Then, the solution was placed in an oil bath and refluxed under magnetic stirring at 90C for 1 h.
After refluxing, the obtained precipitation was filtered and washed with distilled water several times.
For preparation of ZnOeAl2O3samples with ZnO contents of 40, 60 and 80 wt% (40ZA, 60ZA and 80ZA samples, respec- tively), bayerite powder (Al(OH)3, Ardakan Industrial Ceramics Co, 98% purity, mean particle size 4mm) was added to the so- lution containing zinc acetate and urea. Then, the solution was stirred at room temperature for 2 h before refluxing. Subse- quently, the same procedure, as mentioned for pure ZnO, was followed.
All samples were dried at 40 C for 24 h and then were calcined at 400C for 3 h at a heating rate of 10C/min. Also, high purity Al2O3(100A sample) was synthesized by calcination of bayerite powder at 400 C for 3 h without any previous treatment.
The morphology of products was studied by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4160). The crystal structure was characterized by X-ray diffraction (XRD, Philips) using Cu-Ka radiation. Thermo-gravimetric analysis (TGA) was performed using METTLER TGA/SDTA 851E at a heating rate of 10C min1from room temperature to 900C.
Fourier transform infrared (FTIR) spectra of the samples were performed using an FT-IR-6300/JASCO spectroscope in the wave number of 400e4000 cm1. The specific surface area (SBET) was measured by BET method using N2 adsorption isotherms at 77 K (micromeritics ASAP-2010). The UVevis spectra were obtained on a Jasco V-670 spectrophotometer.
3. Results and Discussion
Fig. 1shows the TG curve of 60ZA sample at a heating rate of 10 C min1. Below 230C, the weight loss is related to the evaporation of physically adsorbed water molecules. The large
weight loss that is extended between 230 and 450C is related to the transformation of the precipitations of 60A sample to ZnO and Al2O3 phases together with evaporation of chemically adsorbed water and any residual organic (remained from urea precipitation agent). With regard to the TG curve showing no tangible weight loss at above 400C, all samples were calcined at 400 C. Also, all samples were kept at the maximum tem- perature for 3 h in order to ensure the complete formation of products.
The XRD patterns of samples with various ZnO contents are presented inFig. 2. The peaks corresponding tog-Al2O3(JCPDS 10-0425) and wurtzite-type ZnO (JCPDS 36-1451) can be clearly observed in XRD patterns of 100A and 100Z samples, respectively. The XRD pattern of 80ZA sample indicates sharp peaks of wurtzite-type ZnO and weak peaks ofg-Al2O3. As can be seen in the XRD patterns of 60ZA and 40ZA samples, by decreasing the ZnO content, the intensity of the peaks corre- sponding to ZnO becomes lower, and those corresponding to Al2O3become higher than the peak intensity of 80ZA sample.
This fact can be easily explained by the portion of each phase in X-ray diffraction. It should be mentioned that no other phases such as ZnAl2O4were observed in XRD patterns.
In order to confirm the XRD result, the 80ZA sample together with pure samples (100Z and 100A samples) was analyzed with an FTIR spectrometer at room temperature. Fig. 3 indicates several common absorption bands that exist in each sample such as the broad band centered at 3430 cm1due to OeH stretching vibration, and other bands at 1620, 1512 and 1377 cm1due to the CO, CO and CO2groups. The higher intensity of the OeH band in 100A and 80ZA samples refers to very high surface area of these samples which accelerate the rapid adsorption of moisture from air. As seen, the FTIR spectrum of 100Z sample shows an absorption peak at around 430 cm1 which is the characteristic absorption peak of ZneO bond and ZnO[19,20]. Also, the 100A sample showed a broad band in the range of 400e1100 cm1which was assigned to aluminum oxide[21]. In Fig. 1 TG curve of precipitation of 60ZA sample obtained after drying
step.
Fig. 2 XRD pattern of different samples: 100A, 40ZA, 60ZA, 80ZA and 100Z.
916 H. Tajizadeganet al.: J. Mater. Sci. Technol., 2013, 29(10), 915e918
details, the two broad absorption bands centered at about 597 and 781 cm1 were due to AleO stretching vibrations. The FTIR spectrum of 80ZA sample shows the superposition of characteristic absorption bands of ZnO and Al2O3. Due to very broad and intense bands of Al2O3, the characteristic absorption peak of ZnO could not appear except a weak shoulder at 430 cm1. The FTIR results suggested that ZnO and Al2O3
phases were formed after calcination for 3 h at 400C as esti- mated in TGA curve. Also, according to Cheng et al.[22], no characteristic vibration of ZneAl bond (three sharp peaks at 674, 548 and 495 cm1) was detected, which is in good agreement with XRD results.
Fig. 4(a) shows the FE-SEM micrograph of the 100Z sample after calcination, in which a porous structure of ZnO nano- particles (about 40e60 nm) with an interlaced configuration like wool was observed.Fig. 4(b) represents the FE-SEM micrograph of the 100A sample after calcination, which clearly shows alumina particles with irregular shapes.
Fig. 4(c) and (d) exhibits the FE-SEM micrographs of the precipitations of the 40ZA and 60ZA samples after drying step,
respectively. As shown inFig. 4, a lot of nanoflakes grow on bayerite particles in the precipitation process.Fig. 4(e) and (f) shows the FE-SEM micrographs of the 40ZA and 60ZA samples after calcination, respectively. It can be seen that both samples consist of ZnO nanoflakes grown on alumina particles. The ZnO nanoflakes have a thickness of about 40e80 nm and a diameter of about 1e2mm. It is obvious that the structure of samples and nanoflakes is retained after the calcination process. Therefore, the growth mechanism can be investigated in the precipitation process, in which no surfactant exists. Herein, the growth mechanism can be interpreted by decreasing particles surface energy[23,24]. The bayerite particles surfaces act as heterogeneous nucleation sites, and suppress additional nucleation in solution.
The precipitation method using urea hydrolysis leads to the formation of zinc carbonate hydroxide (Zn5(CO3)2(OH)6) as an intermediate product (reactions (1) and (2)), which is converted to ZnO during the calcination (reaction (3)).
COðNH2Þ2þH2O/2NH4þþHCO3þOH (1)
5Zn2þþ2CO32þ6OH/Zn5ðCO3Þ2ðOHÞ6 (2)
Zn5ðCO3Þ2ðOHÞ6/5ZnOþ2CO2[þ3H2O[ (3)
In this regard, the bayerite particles provide the areas with high surface energy like edges which are favorite areas for the growth of the Zn5(CO3)2(OH)6 nuclei. As a result, the nuclei extend along an edge in the form of a thin sheet called nanoflake.
Subsequently, the ZnO nanoparticles undergo higher growth rates at edges of alumina particles (generated from bayerite particles calcination (Eq.(4))) with high surface energy and also Fig. 3 FTIR spectra of 100Z, 100A and 80ZA samples obtained by
calcination at 400C for 3 h.
Fig. 4 FE-SEM micrographs: (a) 100Z after calcination, (b) 100A after calcination, (c) 40ZA after drying step, (d) 60ZA after drying step, (e) 40ZA after calcination, (f) 60ZA after calcination and (g and h) different magnifications of 80ZA sample after calcination.
H. Tajizadeganet al.: J. Mater. Sci. Technol., 2013, 29(10), 915e918 917
slower growth rates at the centers of faces with lower surface energy.
2AlðOHÞ3/Al2O3þ3H2O[ (4)
It was predicted that by further increasing of ZnO content, these nanoflakes grow and cover the alumina particles. This fact was confirmed by FE-SEM observations of 80ZA sample.
Fig. 4(g) and (h) shows the FE-SEM micrographs of 80ZA sample after calcination with different magnifications, in which a porous hierarchical structure of ZnO with nanosheet arrays can be clearly observed. Every nanosheet has a thickness of about 40e80 nm and a diameter of about 1e2mm. Another reason which can affect the growth of ZnO is that bayerite particles and Zn5(CO3)2(OH)6intermediate product produce too much vapor pressure in calcination process (Eqs.(3) and (4)) resulting in the formation of a porous zinc oxide with a high surface area. As a result, the formation of morphologies shown inFig. 4(g) and (h) are suitable to release these vapors.
In this regard, BET analysis was carried out on 100A (340 m2/g), 100Z (30 m2/g), 80ZA (237 m2/g), 60ZA (312 m2/g) and 40ZA (318 m2/g) samples. It is obvious that the increase in Al2O3content in ZnOeAl2O3composition enhances the amount of both BET surface area and stability against agglomeration. Therefore, the BET measurements proved a reachable high surface area in this approach.
The UVevis absorption spectra of the samples with different ZnO content are compared with pure ZnO inFig. 5. It is shown that, the 100Z sample exhibits an intense absorption below the wavelength of 400 nm, which is known as a characteristic ab- sorption of ZnO. In this regard, the absorption spectra of 40ZA, 60ZA and 80ZA samples still show the characteristic absorption of ZnO. It is evident that the absorption was decreased by decreasing the weight ratio of ZnO/Al2O3. This fact is related to lack of UV absorption of Al2O3 particles. However, UV ab- sorption results evidence the presence of photocatalytic proper- ties in these samples.
4. Conclusion
A simple and low cost approach was developed to synthesize ZnO supported on alumina particles. It was found that two
factors can control the growth of ZnO: (i) bayerite particles with higher surface energy at edges, and (ii) the vapor pressure pro- duced from calcination of bayerite particles and decomposition of zinc carbonate hydroxide. These factors result in the formation of a porous hierarchical structure of ZnO with nanosheet arrays at ZnO content of 80 wt% and also ZnO nanoflakes in lower content such as 40 wt% and 60 wt%. Due to high surface area and UVevis absorption of ZnOeAl2O3compositions, these as- prepared samples have important potential applications in future nanocatalysts or nano-photocalatyst. As a next result, this facile and low cost method can be potentially extended to prepare other metal oxides with novel geometrical structures and properties.
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Fig. 5 UVevis absorption spectra of the samples with different ZnO contents.
918 H. Tajizadeganet al.: J. Mater. Sci. Technol., 2013, 29(10), 915e918