Available online at SciVerse ScienceDirect

J. Mater. Sci. Technol., 2013, 29(10), 915e918

Facile Growth of Porous Hierarchical Structure of ZnO Nanosheets

on Alumina Particles via Heterogeneous Precipitation

Hamid Tajizadegan1)*, Majid Jafari1), Mehdi Rashidzadeh2), Reza Ebrahimi-Kahrizsangi1),Omid Torabi1)

1) Department of Materials Engineering, Najafabad Branch, Islamic Azad University, P.O. Box 517, Isfahan, Iran2) 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]

* Corresphamid_tj1005-03JournalLimited.http://dx

ZnO nanosheets and nanoflakes were grown on alumina particles in the absence of surfactants viaheterogeneous 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 resultsindicated the formation of only two phases: wurtzite-type ZnO and g-Al2O3. ZnO nanoflakes were grown onalumina particles in the samples with ZnO content of 40 and 60 wt%. By increasing the ZnO content to80 wt%, a porous hierarchical structure of ZnO with nanosheet arrays appeared. Both of these nanoflakesand nanosheets were about 40e80 nm in thickness and about 1e2 mm in diameter. It was proposed thatZn5(CO3)2(OH)6 nuclei undergo higher growth rates in thin sheets at edges of bayerite particles with a highersurface energy. The BrunauereEmmetteTeller (BET) measurements proved a reachable high surface area forhierarchical structures of ZnO nanosheets, which could mainly be attributed to their unique growth onalumina particles. Also, UV absorption results revealed that ZnOeAl2O3 compositions still show the UVcharacteristic 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 largeexaction binding energy, zinc oxide has become one of the mostinteresting materials for many applications such as optoelec-tronic devices[1] and photocatalysts[2]. Also, ZnO in single phaseor in cooperation with other materials has been widely used as acatalyst in vast various applications such as gas, oil and petro-chemical industries[3,4]. With regard to the fact that catalytic andphotocatalytic reactions generally take place on the surfaces ofsolid phase, synthesis of nanosized ZnO with high surface areahas attracted intensive research interests[2,5]. Regardless of themorphology of ZnO, some researchers have attempted to employother materials as a support or template in order to increase thesurface area and aggregation resistance of ZnO active compo-nents[6,7]. However, the size and morphology of ZnO particles

onding author. Tel./Fax: þ98 331 229 1008; E-mail address:z@yahoo.com (H. Tajizadegan).02/$e see front matter Copyright� 2013, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2013.06.008

play a key role in all applications[8,9]. In addition, it is welldocumented that ZnO can present a wide range of morphol-ogies[10]. This fact is mainly resulted from the preferentialgrowth of ZnO which leads to one-dimensional structures suchas nanorods, nanoneedles and nanotubes. In contrast, some re-searchers have attempted to control the preferential growth ofZnO in order to synthesize two-dimensional structures includingsheets and discs with different properties[11]. Meanwhile,recently synthesis of hierarchical structures of these two-dimensional ZnO structures has attracted considerable interestsdue to their three-dimensional structures, higher surface area andresistance against aggregation[2,12]. Qu et al.[2] synthesized ahierarchical structure of ZnO nanosheets using trisodium citrateas surfactant via hydrothermal method. Lu et al.[12] synthesized ahierarchical 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 mostrecent reports, concentrated on using expensive materials assurfactants, and complicated methods with specific conditionslike hydrothermal, which are economically unfavorable, andespecially difficult to transfer into an industrial scale. Therefore,synthesis of porous hierarchical structure of ZnO nanosheetswith very high specific surface area remains a main subject.

Fig. 1 TG curve of precipitation of 60ZA sample obtained after dryingstep.

Fig. 2 XRD pattern of different samples: 100A, 40ZA, 60ZA, 80ZAand 100Z.

916 H. Tajizadegan et al.: J. Mater. Sci. Technol., 2013, 29(10), 915e918

Besides, it is well known that ZnO in combination with othermaterials gives different growth habits and morphologies. Thisfact is not only because of selection of precursors[13], calcinationtemperature[14], solvents[15] and surfactants[16], but can also beaffected by support or template. Lee et al.[17] reported thatmorphology of ZnO can be controlled by adjusting Zn content inAleZn mixture. Moshfegh et al.[18] proved that the degree ofsubstrate surface roughness affects the size and shape of zincoxide nanostructure. Thus, the combination of ZnO with othermaterials is an effective way to control the structure andmorphology of ZnO, which allows higher potentials andproperties.Herein, the motivation of this work is to design a simple and

low cost approach to grow hierarchical structures of ZnOnanosheets with the cooperation of alumina particles (as sup-porting particles) and also, to avoid using high cost nanoparticlesas supports and expensive surfactants as surface modifiers. Inthis regard, attempts were first made to grow ZnO on bayeriteparticles (Al(OH)3) via heterogeneous precipitation. Then, theeffect of alumina particles and ZnO content on morphology,surface area and UV absorption were investigated. Also, themechanism of growth and formation of ZnO was discussed.

2. Experimental

For synthesis of high purity ZnO (100Z sample), 0.3 mol/Lzinc acetate dehydrate (Zn(CH3COO)2$2H2O, Merck, 99.5%)solution was mixed with urea (NH2CONH2, Merck, 98%) bymole ratio 1:6, respectively. Then, the solution was placed in anoil bath and refluxed under magnetic stirring at 90 �C for 1 h.After refluxing, the obtained precipitation was filtered andwashed with distilled water several times.For preparation of ZnOeAl2O3 samples with ZnO contents of

40, 60 and 80 wt% (40ZA, 60ZA and 80ZA samples, respec-tively), bayerite powder (Al(OH)3, Ardakan Industrial CeramicsCo, 98% purity, mean particle size 4 mm) was added to the so-lution containing zinc acetate and urea. Then, the solution wasstirred at room temperature for 2 h before refluxing. Subse-quently, the same procedure, as mentioned for pure ZnO, wasfollowed.All samples were dried at 40 �C for 24 h and then were

calcined at 400 �C for 3 h at a heating rate of 10 �C/min. Also,high purity Al2O3 (100A sample) was synthesized by calcinationof bayerite powder at 400 �C for 3 h without any previoustreatment.The morphology of products was studied by field-emission

scanning electron microscopy (FE-SEM, Hitachi S-4160). Thecrystal 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 aheating rate of 10 �C min�1 from room temperature to 900 �C.Fourier transform infrared (FTIR) spectra of the samples wereperformed using an FT-IR-6300/JASCO spectroscope in thewave number of 400e4000 cm�1. The specific surface area(SBET) was measured by BET method using N2 adsorptionisotherms at 77 K (micromeritics ASAP-2010). The UVevisspectra were obtained on a Jasco V-670 spectrophotometer.

3. Results and Discussion

Fig. 1 shows the TG curve of 60ZA sample at a heating rate of10 �C min�1. Below 230 �C, the weight loss is related to theevaporation of physically adsorbed water molecules. The large

weight loss that is extended between 230 and 450 �C is related tothe transformation of the precipitations of 60A sample to ZnOand Al2O3 phases together with evaporation of chemicallyadsorbed water and any residual organic (remained from ureaprecipitation agent). With regard to the TG curve showing notangible weight loss at above 400 �C, all samples were calcinedat 400 �C. Also, all samples were kept at the maximum tem-perature for 3 h in order to ensure the complete formation ofproducts.The XRD patterns of samples with various ZnO contents are

presented in Fig. 2. The peaks corresponding to g-Al2O3 (JCPDS10-0425) and wurtzite-type ZnO (JCPDS 36-1451) can beclearly observed in XRD patterns of 100A and 100Z samples,respectively. The XRD pattern of 80ZA sample indicates sharppeaks of wurtzite-type ZnO and weak peaks of g-Al2O3. As canbe seen in the XRD patterns of 60ZA and 40ZA samples, bydecreasing the ZnO content, the intensity of the peaks corre-sponding to ZnO becomes lower, and those corresponding toAl2O3 become higher than the peak intensity of 80ZA sample.This fact can be easily explained by the portion of each phase inX-ray diffraction. It should be mentioned that no other phasessuch as ZnAl2O4 were observed in XRD patterns.In order to confirm the XRD result, the 80ZA sample together

with pure samples (100Z and 100A samples) was analyzed withan FTIR spectrometer at room temperature. Fig. 3 indicatesseveral common absorption bands that exist in each sample suchas the broad band centered at 3430 cm�1 due to OeH stretchingvibration, and other bands at 1620, 1512 and 1377 cm�1 due tothe CO, CO and CO2 groups. The higher intensity of the OeHband in 100A and 80ZA samples refers to very high surface areaof these samples which accelerate the rapid adsorption ofmoisture from air. As seen, the FTIR spectrum of 100Z sampleshows an absorption peak at around 430 cm�1 which is thecharacteristic absorption peak of ZneO bond and ZnO[19,20].Also, the 100A sample showed a broad band in the range of400e1100 cm�1 which was assigned to aluminum oxide[21]. In

Fig. 3 FTIR spectra of 100Z, 100A and 80ZA samples obtained bycalcination at 400 �C for 3 h.

H. Tajizadegan et al.: J. Mater. Sci. Technol., 2013, 29(10), 915e918 917

details, the two broad absorption bands centered at about 597and 781 cm�1 were due to AleO stretching vibrations. TheFTIR spectrum of 80ZA sample shows the superposition ofcharacteristic absorption bands of ZnO and Al2O3. Due to verybroad and intense bands of Al2O3, the characteristic absorptionpeak of ZnO could not appear except a weak shoulder at430 cm�1. The FTIR results suggested that ZnO and Al2O3

phases were formed after calcination for 3 h at 400 �C as esti-mated in TGA curve. Also, according to Cheng et al.[22], nocharacteristic vibration of ZneAl bond (three sharp peaks at 674,548 and 495 cm�1) was detected, which is in good agreementwith 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 likewool was observed. Fig. 4(b) represents the FE-SEM micrographof the 100A sample after calcination, which clearly showsalumina 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,

Fig. 4 FE-SEM micrographs: (a) 100Z after calcination, (b) 100A after calciafter calcination, (f) 60ZA after calcination and (g and h) different m

respectively. As shown in Fig. 4, a lot of nanoflakes grow onbayerite particles in the precipitation process. Fig. 4(e) and (f)shows the FE-SEM micrographs of the 40ZA and 60ZA samplesafter calcination, respectively. It can be seen that both samplesconsist of ZnO nanoflakes grown on alumina particles. The ZnOnanoflakes have a thickness of about 40e80 nm and a diameterof about 1e2 mm. It is obvious that the structure of samples andnanoflakes is retained after the calcination process. Therefore,the growth mechanism can be investigated in the precipitationprocess, in which no surfactant exists. Herein, the growthmechanism can be interpreted by decreasing particles surfaceenergy[23,24]. The bayerite particles surfaces act as heterogeneousnucleation sites, and suppress additional nucleation in solution.The precipitation method using urea hydrolysis leads to theformation of zinc carbonate hydroxide (Zn5(CO3)2(OH)6) as anintermediate product (reactions (1) and (2)), which is convertedto ZnO during the calcination (reaction (3)).

COðNH2Þ2 þ H2O/2NH4þ þ HCO3� þ OH� (1)

5Zn2þ þ 2CO32� þ 6OH�/ Zn5ðCO3Þ2ðOHÞ6 (2)

Zn5ðCO3Þ2ðOHÞ6/ 5ZnOþ 2CO2[þ 3H2O[ (3)

In this regard, the bayerite particles provide the areas withhigh surface energy like edges which are favorite areas for thegrowth of the Zn5(CO3)2(OH)6 nuclei. As a result, the nucleiextend along an edge in the form of a thin sheet called nanoflake.Subsequently, the ZnO nanoparticles undergo higher growthrates at edges of alumina particles (generated from bayeriteparticles calcination (Eq. (4))) with high surface energy and also

nation, (c) 40ZA after drying step, (d) 60ZA after drying step, (e) 40ZAagnifications of 80ZA sample after calcination.

Fig. 5 UVevis absorption spectra of the samples with different ZnOcontents.

918 H. Tajizadegan et al.: J. Mater. Sci. Technol., 2013, 29(10), 915e918

slower growth rates at the centers of faces with lower surfaceenergy.

2AlðOHÞ3/ Al2O3 þ 3H2O[ (4)

It was predicted that by further increasing of ZnO content,these nanoflakes grow and cover the alumina particles. This factwas confirmed by FE-SEM observations of 80ZA sample.Fig. 4(g) and (h) shows the FE-SEM micrographs of 80ZAsample after calcination with different magnifications, in which aporous hierarchical structure of ZnO with nanosheet arrays canbe clearly observed. Every nanosheet has a thickness of about40e80 nm and a diameter of about 1e2 mm. Another reasonwhich can affect the growth of ZnO is that bayerite particles andZn5(CO3)2(OH)6 intermediate product produce too much vaporpressure in calcination process (Eqs. (3) and (4)) resulting in theformation of a porous zinc oxide with a high surface area. As aresult, the formation of morphologies shown in Fig. 4(g) and (h)are suitable to release these vapors.In this regard, BETanalysiswas carried out on 100A (340m2/g),

100Z (30 m2/g), 80ZA (237 m2/g), 60ZA (312 m2/g) and 40ZA(318m2/g) samples. It is obvious that the increase inAl2O3 contentin ZnOeAl2O3 composition enhances the amount of both BETsurface area and stability against agglomeration. Therefore, theBET measurements proved a reachable high surface area in thisapproach.The UVevis absorption spectra of the samples with different

ZnO content are compared with pure ZnO in Fig. 5. It is shownthat, the 100Z sample exhibits an intense absorption below thewavelength 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 absorptionof ZnO. It is evident that the absorption was decreased bydecreasing the weight ratio of ZnO/Al2O3. This fact is related tolack 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 synthesizeZnO supported on alumina particles. It was found that two

factors can control the growth of ZnO: (i) bayerite particles withhigher surface energy at edges, and (ii) the vapor pressure pro-duced from calcination of bayerite particles and decompositionof zinc carbonate hydroxide. These factors result in the formationof a porous hierarchical structure of ZnO with nanosheet arraysat ZnO content of 80 wt% and also ZnO nanoflakes in lowercontent such as 40 wt% and 60 wt%. Due to high surface areaand UVevis absorption of ZnOeAl2O3 compositions, these as-prepared samples have important potential applications in futurenanocatalysts or nano-photocalatyst. As a next result, this facileand low cost method can be potentially extended to prepare othermetal oxides with novel geometrical structures and properties.

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