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Applied Surface Science 258 (2012) 5455– 5461

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j our nal ho me p age: www.elsev ier .com/ loc ate /apsusc

lkali-dependent synthesis of flower-like ZnO structures with enhancedhotocatalytic activity via a facile hydrothermal method

in Suna, Rong Shaob, Zhidong Chena, Lanqin Tangb,∗, Yong Daib, Jianfei Dingb

Institute of Petrochemical Technology, Changzhou University, Changzhou 213164, PR ChinaCollege of Chemical and Biological Engineering, Yancheng Institute of Technology, 9 Yinbin Avenue, Yancheng 224051, PR China

r t i c l e i n f o

rticle history:eceived 12 January 2012eceived in revised form 10 February 2012ccepted 11 February 2012vailable online 18 February 2012

a b s t r a c t

Flower-like ZnO structures with high photocatalytic performance were successfully synthesized via afacile hydrothermal method. Alkaline environment played a critical role during the morphological trans-formation. When the molar ratio of Zn(CH3COO)2·2H2O to NaOH was set as 1:8 in the presence oftriethanolamine (TEA), and the molar ratio of Zn2+ to TEA was 1:9, the flower-like ZnO product was pro-duced. The hexagonal sphere-like, oblate-like, and hexagonal biprism-like samples were also obtained

2+

eywords:nOydrothermal methodrystal growthhotocatalysis

by adjusting the molar ratio of Zn to NaOH as 1:2, 1:5 and 1:12 with the presence of invariable amountof TEA, respectively. The prepared ZnO products were characterized by X-ray diffraction (XRD), scan-ning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer–Emmett–Teller(BET) surface area. Photodegradation experiments of the samples were carried out by choosing Methy-lene Blue (MB) as a model target under UV irradiation with homemade photocatalytic apparatus. Among

aped

these products, flower-sh

. Introduction

Photocatalysis oxidation, as one of the most promising tech-iques to confront with the challenges of environmental problems,as been extensively investigated in recent years [1]. ZnO hasecently been in the spotlight of research efforts, which is attributedo its outstanding advantages such as low cost and high quantumfficiency [2]. In addition, the features of its environmental friendlynd biodegradable are also desirable [3].

The well-controlled synthetic procedures of ZnO particles arelways been the focus of crystal synthesis, and some results haveevealed that ZnO particles with different morphologies wouldignificantly influence the photocatalytic activities [4–6]. To date,any commendable methods have been developed for preparingorphological ZnO particles such as sol–gel [7], chemical vapor

eposition [8], hydrothermal deposition [9], solvothermal deposi-ion [10] and thermal evaporation [11]. Among these pathways,ydrothermal method has emerged as a favorable technique to

abricate well-dispersed crystals with uniform sizes and control-able shapes [12]. Also, it presents a series of advantages suchs mild reaction conditions, less energy consuming, inexpensive

quipment and simple procedure [13]. Therefore, it is regarded as

prospective route for industrial application [14].

∗ Corresponding author. Tel.: +86 515 8829 8615; fax: +86 515 8829 8615.E-mail address: lanqin tang@ycit.edu.cn (L. Tang).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2012.02.034

samples exhibited the highest photocatalytic activity.© 2012 Elsevier B.V. All rights reserved.

More recently, ZnO architectures with different morphologiessynthesized via surfactant-assisted hydrothermal methods havebeen greatly highlighted. For instance, a cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal method was employed byWang et al. to synthesize ZnO particles with controllable shapes.With the concentration of CTAB increasing from 0.1 M to 0.5 M,the length of obtained rod-like samples increased from 3 �m to6 �m [15]. Yogamalar and Bose utilized a poly-ethylene glycol (PEG4000)-assisted hydrothermal method to prepare ZnO particles. PEGmolecules acted with different zinc precursors to obtain ZnO sam-ples with various shapes [16]. In brief, many efforts have beendevoted to examine the influences of surfactants during the pro-cess of morphological transformation. However, the morphologyand crystallinity of ZnO particles also critically depend on the typesof alkaline environments. Lu et al. have found that when the alka-line source used was monoethanol amine (MEA), diethanol amine(DEA) and TEA, the obtained ZnO powders exhibited a prismatichexagonal-like, flower-like and sphere-like morphology, respec-tively. Adding NH4OH into DEA greatly changed the morphologyand aspect ratios of ZnO powders. Therefore, it seems necessary toinvestigate the influences of alkaline during the process of shapetransition [17].

In the present work, we selected TEA and NaOH as alkalinesources, and employed a facile hydrothermal method to synthesize

flower-like ZnO microstructures with high photocatalytic perfor-mance. The different alkaline conditions played a great role duringthe morphology transition. A series of ZnO microcrystals with var-ious shapes such as hexagonal sphere-like, oblate-like, hexagonal

5456 L. Sun et al. / Applied Surface Science 258 (2012) 5455– 5461

Table 1Summarized reaction conditions and different morphologies.

Morphology Reaction conditions Designation

n (Zn2+):n (NaOH) n (Zn2+):n (TEA)

Nut-like Without NaOH 1:9 Sample-ANanorod-like 1:8 Without TEA Sample-BHexagonal sphere-like 1:2 1:9 Sample-C

bpat

2

2

at

2

saot3sartsd

oomatsdv

2

bs3tdwd6setlt(

be seen from Fig. 2a that the size of the structure is around 10 �m.Fig. 2b reveals that Sample-E consists of many hexagonal rod arrayswith an average diameter of about 2 �m.

(00

4)

(201

)(112

)

(20

0)

(10

3)

(11

0)

(102

)

(10

1)

(00

2)

(100

)

Inte

nsi

ty (

a. u

.)

Oblate-like 1:5Flower-like 1:8

Hexagonal biprism-like 1:12

iprism-like were successfully obtained. The as-synthesized sam-les were illuminated in terms of their morphology, crystallinitynd photocatalytic efficiency, the growth mechanism and pho-odegradation process were also discussed.

. Experimental

.1. Materials

All chemicals and reagents were of analytical purity and useds received without further purification, distilled water was usedhroughout.

.2. Preparation of flower-like ZnO particles

In a typical preparation, 4 mM of Zn(CH3COO)2·2H2O was dis-olved in 20 mL of distilled water, 36 mM of TEA was successivelydded to the above solution under magnetic stirring. Then, 25 mLf distilled water contained 32 mM of NaOH was added dropwiseo the emulsion. The resulting mixture was magnetically stirred for0 min at room temperature and transferred into a Teflon-linedtainless-steel autoclave with a capacity of about 60 mL, sealednd maintained at 140 ◦C for 12 h. After the autoclave cooled tooom temperature naturally, the as-formed precipitates were cen-rifuged at 5000 rpm for 5 min, washed with distilled water foreveral times, then dried at 60 ◦C in air for 8 h. The product wasenoted as Sample-E.

In order to investigate the influences of alkaline environmentsn the morphology transition of flower-like ZnO structures, a seriesf comparative experiments were carried out by adjusting theolar ratios of Zn2+ to NaOH. The obtained particles were denoted

s Sample-C, Sample-D, and Sample-F when the molar ratio of Zn2+

o NaOH was fixed as 1:2, 1:5, and 1:12, respectively. Two productsynthesized in the presence of single TEA and single NaOH wereesignated as Sample-A and Sample-B. The reaction conditions andarious morphologies are summarized in Table 1.

.3. Photochemical experiments

Photocatalytic activities of as-obtained samples were evaluatedy measuring the degradation of Methylene Blue (MB) in aqueousolution under UV light irradiation. A 6 W UV lamp of wavelength65 nm was employed as light source. All the experiments of pho-odegradation reaction were carried out in a 100 mL beaker with aiameter of 6.5 cm. For each experiment, 80 mg of photocatalystsas dispersed in 100 mL of 10 mg/L of the MB aqueous solution. Theistance between the light source and surface of the solution was

cm. Prior to the irradiation, the suspensions were magneticallytirred in the dark for 30 min to ensure the adsorption/desorption

quilibrium of MB aqueous solution with the photocatalyst. 6 mL ofhe aliquots were sampled at regular intervals of 30 min and ana-yzed by recording variations in the absorption band (663 nm) inhe UV–Vis spectra of MB using an UV-2100 spectrophotometerBeijing Beifen-Ruli Analytical Instrument Co., Ltd.).

1:9 Sample-D1:9 Sample-E1:9 Sample-F

2.4. Characterization

X-ray diffraction (XRD) patterns were recorded on a DX2700diffractometer using Cu K� radiation (� = 0.154056 nm) at 40 kVand 35 mA in the range of 2� value between 20◦ and 80◦. The mor-phologies of obtained samples were analyzed by scanning electronmicroscopy (SEM, QUANTA200). Transmission electron microscopy(TEM) using a JEOL-JEM-2100 operated at an accelerating voltageof 200 kV was employed to further examine the microstructures ofthe products. The Brunauer–Emmett–Teller (BET) specific surfaceareas of the samples were obtained by nitrogen adsorption on anitrogen adsorption apparatus (Beckman Coulter SA 3100). UV–Visabsorbance spectra of dye solutions were measured using a UV–Visspectrophotometer (UV 2300II).

3. Results and discussion

3.1. XRD analysis

Fig. 1 shows a typical XRD pattern of the flower-like ZnO parti-cles (Sample-E). It is confirmed that all diffraction peaks can beattributed to ZnO with the hexagonal wurtzite structure (spacegroup: P63mc (186): a = 0.3249 nm, c = 0.5206 nm). No other peakswere detected within the detection limit of the XRD instrument.The intense and sharp peaks demonstrate that the product is wellcrystallized.

3.2. SEM characterization

The morphology of Sample-E is characterized with SEM. It can

706050403020

2 θ (degree s)

Fig. 1. X-ray diffraction pattern of Sample-E.

L. Sun et al. / Applied Surface Science 258 (2012) 5455– 5461 5457

ages

3

3

sictm6

waiT

Fig. 2. SEM im

.3. Possible growth mechanism of ZnO architectures

.3.1. Effect of TEAFig. 3a and b is the typical SEM images of Sample-A. It can be

een that Sample-A contains lots of nut-like microstructures, whichs consistent with previous report [18]. When the growth process isarried out in the presence of single NaOH (the molar ratio of Zn2+

o NaOH is 1:8), ZnO nucleus will grow to form nano-rods with aean diameter of about 100 nm and an average length of around

00 nm (Fig. 3c and d).In order to investigate the roles of TEA in our synthetic system,

e carry out a further research on the precursor. As shown in Fig. 4a, heavy-white emulsion (denoted as P-1) is formed after adding TEAnto Zn2+ solutions at room temperature (the molar ratio of Zn2+ toEA is set as 1:9). The emulsion is centrifuged at 5000 rpm for 5 min,

Fig. 3. SEM images of Sample-A (a a

of Sample-E.

the formed precipitated substance is dried at room temperature for12 h, and then the white gel is obtained. Fig. 5 is a typical XRD pat-tern of the gel, the crystalline peaks correspond to Zn(OH)2, and noZnO peaks are detected. It is confirmed that the formed gel com-prises lots of Zn(OH)2 particles. Based on the above results, it isclearly illuminated that TEA molecules will hydrolyze to build upan alkaline environment. Also, the large disparity of morphologies(Sample-A and Sample-B) can be explained that only a portion ofTEA will hydrolyze to generate OH−, the precursor of Sample-B isformed on completely NaOH hydrolysis [19].

3.3.2. Effect of NaOHFig. 6a and b shows the typical SEM and TEM images of Sample-C.

It can be seen that Sample-C comprises large quantities of hexago-nal sphere-like particles with an average size of 100 nm. With the

nd b) and Sample-B (c and d).

5458 L. Sun et al. / Applied Surface Science 258 (2012) 5455– 5461

F (b) thr

mlaZma

tfNfNFtf

3

rtptfft

Fa

ig. 4. Images of precursor solutions prepared with (a) the absence of NaOH (P-1),espectively.

olar ratio of Zn2+ to NaOH increased from 1:2 to 1:5, lots of oblate-ike ZnO microstructures with an average diameter of around 5 �mre observed (Fig. 6c and d, Sample-D). When the molar ratio ofn2+ to NaOH is set as 1:12, Sample-F with hexagonal biprism-likeorphology is obtained. The length of the biprism is around 16 �m

nd the diameter is about 3 �m (as shown in Fig. 6e and f).It is considered that NaOH plays an important role in our reac-

ion system. As illustrated in Fig. 4b, a light-white emulsion isormed after adding NaOH into P-1 (the molar ratio of Zn2+ toaOH was 1:2). With the molar ratio of Zn2+ to NaOH increased

rom 1:2 to 1:5, clear solution is observed (Fig. 4c). It implies thataOH will further act with Zn(OH)2 to produce dissoluble ions.ig. 4a–c reveals that different alkaline environments have mucho do with the variety of precursor solutions, which is responsibleor the morphological transformation of ZnO structures.

.3.3. The growth process of Sample-EIn order to examine the growth procedure of Sample-E, the

eaction time is also investigated. From Fig. 7a, when the reac-ion maintains at 140 ◦C for 45 min, flower-like samples with moreicked tips are obtained. As the time prolongs from 45 min to 2 h,

he picked tips change into hexagons. It is well known that crystalaces with fast growth rate will occur easily. On the contrary, crystalaces whose growth rate is slow will disappear easily [20]. Hence,he observed results provide a direct evidence that the growth rate

80706050403020

♣

♣

♣

♣

Inte

nsi

ty (

a. u

.)

2θ (degrees)

♣Zn(O H)

2

ig. 5. X-ray diffraction pattern of the gel obtained by adding TEA into Zn2+ solutiont room temperature.

e molar ratio of Zn2+ to NaOH is 1:2, and (c) the molar ratio of Zn2+ to NaOH is 1:5,

along the c-axis direction is fast to form more picked tips in thebeginning. As the reaction proceeding, the growth rate along the c-axis direction is repressed, and the hexagonal flat plane is formed[21]. It is inferred that the formed [Zn(OH)4]2− complexes playa leading role initially, which is beneficial for the growth of ZnOnuclei along the c-axis. As time prolonging from 45 min to 2 h, lotsof [Zn(OH)4]2− complexes will decompose into ZnO nucleus, whichimplies that free [Zn(OH)4]2− complexes relatively decrease. As aresult, the growth rate along the c-axis direction is restrained.

The possible formation mechanism of ZnO architectures is pro-posed based on the investigation results. Firstly, a part of TEA willhydrolyze to produce OH− ions (reaction (1)). When the amount ofNaOH is zero, Zn(OH)2 precursors will be formed

TEA + H2O ↔ [TEA]+ + OH− (1)

Zn2+ + 2OH− → Zn(OH)2 (2)

Zn(OH)2decomposition−→ ZnO + H2O (3)

Zn(OH)2 + 2OH− ↔ [Zn(OH)4]2− (4)

ZnO + H2O + 2OH− ↔ [Zn(OH)4]2− (5)

by interactions between Zn2+ and OH− (reaction (2), OH− ionsare derived from the hydrolysis of TEA). Zn(OH)2 precursors willdecompose to ZnO nucleus under hydrothermal conditions (reac-tion (3)). With the amounts of NaOH increasing, the excess OH−

ions are in favor of the formation of [Zn(OH)4]2− ions (reaction(4)) [22]. A part of [Zn(OH)4]2− ions will directly transform intoZnO nuclei after the concentration above its critical solubility [23].Other formed [Zn(OH)4]2− complexes preferably adsorb on the sur-face of ZnO nuclei, which are beneficial for lifting the growth rate ofZnO nuclei along the c-axis direction [23] (hexagonal biprism-likesamples in Fig. 8). When [Zn(OH)4]2− ions are relatively insuffi-cient, TEA molecules will occupy the dominant position and serveas structure-directing and assembling agents to some extent, andresult in the formation of nut-like, hexagonal sphere-like, oblate-like and flower-like structures [24]. With NaOH further increasing,a portion of the obtained ZnO nuclei will be dissolved by OH− toform [Zn(OH)4]2− complexes directly (reaction (5)) [25]. As a result,the existed [Zn(OH)4]2− in terms of complexes in the reaction sys-tem will further increase. The possible growth mechanism has beenillustrated in Fig. 8.

3.4. Photocatalytic activity

To demonstrate the potential environmental application forremoval of printing and dyeing wastewater of prepared samples,

L. Sun et al. / Applied Surface Science 258 (2012) 5455– 5461 5459

Fig. 6. SEM and TEM images of Sample-C (a, b), Sample-D (c, d), and Sample-F (e, f), respectively.

Fig. 7. SEM images of Sample-E prepared at 140 ◦C for (a) 45 min, and (b) 2 h.

5460 L. Sun et al. / Applied Surface Science 258 (2012) 5455– 5461

matio

pfhpptUa

Ft(Mr

Fig. 8. Schematic description of the for

hotocatalytic activities of the catalysts synthesized with dif-erent molar ratios of Zn2+ to NaOH for degradation of MBave been performed under UV light irradiation at room tem-erature. As shown in Fig. 9, polygonal lines A–H representhotodegradation rates of MB in the presence of different pho-

ocatalysts. The blank experiment without catalysts but underV light irradiation shows that the decoloration ratio of MB islmost ignorable. Photodegradation rates of MB with different

200180160140120100806040200-20

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

(H)

(G)

(F)

(E)

(D)

(C)

(B)

(A)

Lig

ht

ON

Lig

ht

OF

F

C/C

0

Irradiation time (min)

ig. 9. Photocatalytic degradation of MB in the presence of (A) blank (without pho-ocatalysts), (B) Sample-B, (C) Sample-A, (D) Sample-F, (E) Sample-C, (F) Sample-D,G) Commercial ZnO nanoparticles, and (H) Sample-E. C0 and C represent initial

B concentration and evolution of MB concentration during photodegradation,espectively.

n mechanism of ZnO microstructures.

photocatalysts under UV light irradiation decrease in the followingorder: Sample-E > Commercial ZnO nanoparticles > Sample-D > Sample-C > Sample-F > Sample-A > Sample-B. Fig. 9 clearlyshows that photocatalysts with different morphologies willdisplay large differences of activities. We also measure the BETsurface area of several samples with nut-like, hexagonal sphere-like, oblate-like, flower-like, and hexagonal biprism-like shapes,the value is 3.200, 9.383, 1.656, 3.545, and 2.442 m2/g, respectively.It is interesting to find that catalysts with higher surface areas donot exhibit more impressive photocatalytic activities, which is notin according with commonly considered [26].

The possible account for flower-shaped samples with the high-est photocatalytic activity is that more polar faces of the crystalshave been exposed [27]. Therefore, more OH• radicals are gener-ated to participate in the typical redox reaction, which is formedby photoinduced electron–hole pairs. However, it is very inter-esting to find that not all dye stuffs will be decolored with thepresence of flower-like photocatalysts. The typical dyes such asMethyl Orange (MO), Methylene Blue (MB), and Malachite Green(MG) are employed as model targets with the same initial con-centration. From Fig. 10a, a large disparity of photodegradationefficiency is exhibited when using different dyes. MG displays themildest behavior to be degraded. However, the decoloration ratioof MO is almost negligible. Fig. 10b shows the variances in theabsorbance of MG with the flower-like photocatalyst under UVirradiation. The observed differences can be explained in termsof different electrostatic interactions between the various dyesand the photocatalyst surfaces [28]. It is revealed that some inor-ganic cationic ions have strong inhibition on the decolorization

of dye stuff solutions [29]. Consequently, we suppose that thesulfonic acid sodium salt on benzene ring will inhibit the photo-catalytic degradation efficiency, and this part of work is that we aredoing.

L. Sun et al. / Applied Surface Scie

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0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

C/C0

Irradiation time / min

a

b

Lig

ht

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F

Lig

ht

ON

(A)

(B)

(C)

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0.2

0.4

0.6

0.8

1.0

1.2

1.4

180min

150min

120min

90min

60min

30min

Abso

rban

ce

Wavelength (nm)

616nm

0mina

g

Fig. 10. (a) Photocatalytic performance of the flower-shaped photocatalyst(Sample-E) using different dye solutions as model targets: (A) Methyl Orange, (B)Methylene Blue, and (C) Malachite Green. (b) Absorption spectra of Malachite Greensolution taken at different photocatalytic degradation times using Sample-E as pho-t

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ocatalysts.

. Conclusions

The present study outlined a facile hydrothermal method toelectively fabricate flower-like ZnO microstructures with highhotocatalytic activity. The morphologies of obtained samplesould be controlled by adjusting the molar ratios of Zn2+ toaOH. The adsorption of free [Zn(OH)4]2− complexes to ZnO nucleilayed a significant role during the procedure of shape transi-ion. When the free [Zn(OH)4]2− complexes were sufficient, theod-like and hexagonal biprism-like samples were obtained. How-ver, the shapes of ZnO nucleus would transform into nut-like,exagonal sphere-like, oblate-like and flower-like when the freeZn(OH)4]2− complexes were relatively rare. Moreover, flower-likeamples exhibit excellent photocatalytic activity for decolorationf MB.

cknowledgment

This work was supported by The Talents Introduction Projectf Yancheng Institute of Technology under grant numberKR2011008.

[

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nce 258 (2012) 5455– 5461 5461

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