sol–gel assisted hydrothermal synthesis of zno microstructure

Original Research PaperSol–gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic activity Xiaohua Zhao, Meng Li, Xiangdong Lou⇑ School of Chem

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Original Research Paper

Sol–gel assisted hydrothermal synthesis of ZnO microstructures:

Morphology control and photocatalytic activity

Xiaohua Zhao, Meng Li, Xiangdong Lou⇑

School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang 453007, Henan, China

a r t i c l e i n f o

Article history:

Received 15 November 2012

Received in revised form 4 June 2013

Accepted 11 June 2013

Available online xxxx

Keywords:

Sol–gel assisted hydrothermal method

pH

Morphology

Photocatalysis

a b s t r a c t

ZnO microstructures of different morphologies were synthesized by the sol–gel assisted hydrothermal method using Zn(NO3)2, citric acid and NaOH as raw materials Twining-hexagonal prism, twining-hex-agonal disk, sphere and flower-like ZnO microstructures could be synthesized only through controlling the pH of the hydrothermal reaction mixture at 11, 12, 13 and 14, respectively The as-synthesized sam-ples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) Optical proper-ties were examined by UV–Vis absorption/diffuse reflectance spectroscopy and room-temperature photoluminescence measurements (PL) Photocatalytic activities of the samples were evaluated by deg-radation of Reactive Blue 14 (KGL) The results indicated that the flower-like ZnO composed of nano-sheets possessed superior photocatalytic activity to other ZnO microstructures and commercial ZnO, which could be attributed to the morphology, surface defects, band gap and surface area The formation mechanisms of different ZnO morphologies were also investigated based on the experimental results

Ó 2013 The Society of Powder Technology Japan Published by Elsevier B.V and The Society of Powder

1 Introduction

Zinc oxide (ZnO), an II–VI compound semiconductor with a

wide and direct band gap of 3.37 eV at room temperature, large

exciton binding energy of 60 meV[1], excellent chemical and

ther-mal stability, outstanding optical and electrical properties[2], has

been extensively studied and applied in transistors[3], solar cells

[4], piezoelectric transducers[5], gas sensors [6]and

photocata-lysts[7] As one of the most important semiconductor

photocata-lysts, ZnO has attracted considerable attention due to its high

photosensitivity, nontoxic nature and low cost[8] It is reported

that the photocatalytic activity of ZnO is strongly inﬂuenced by

the methods and conditions of preparation which have great

ef-fects on the microstructures of the materials, such as crystal size,

orientation and morphology, aspect ratio and even crystalline

den-sity[9] Therefore, it is essential to develop facile method to

pre-pare high quality ZnO with uniform morphologies

Among different methods [1–11], hydrothermal technique

which has several advantages over other growth processes such

as the simplicity of operation, low energy consumption and

poten-tial large-scale industrialization, has been successfully used in

preparation of ZnO with different morphologies[12] In the typical

hydrothermal process, the three most popular means to control the

morphology of ZnO are (i) Adding additives (such as polyelectro-lytes, polymers, anions, surfactants or amino acids) For example, Wang et al synthesized ZnO particles with controllable shape through a cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal method [13] Yogamalar et al prepared various shapes of ZnO by a poly-ethylene glycol (PEG 4000)-assisted hydrothermal method [14] (ii) Changing the alkaline environ-ments (such as monoethanolamine, diethanolamine) For example,

Lu et al used monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA) and NH4OH as the alkaline sources to obtain ZnO particles via the hydrothermal process[15] Sun et al selected TEA as alkaline sources to synthesize ﬂower-like ZnO through the hydrothermal method[16] (iii) Changing the pH of the reactants For example, Jang et al studied on the morphology change of ZnO nanostructures with different pH in the hydrothermal process

[17] Alver et al investigated the inﬂuence of pH on the optical and morphological properties of ZnO fabricated by hydrothermal method[18]

The surfactants or alkaline used in the reaction is inconsistent with the concept of ‘‘green’’ chemistry that is utilization of non-toxic, environmentally benign, easily available, and relatively inex-pensive chemicals[12] Among various additives, citric acid which could function as capping agent to manipulate the morphology of ZnO is a comparatively inexpensive and environmentally friendly reagent[2,19,20] Herein, the sol–gel assisted hydrothermal

meth-od was adopted to synthesis controllable morphologies of ZnO microstructures with citric acid as additive The major differences

0921-8831/$ - see front matter Ó 2013 The Society of Powder Technology Japan Published by Elsevier B.V and The Society of Powder Technology Japan All rights reserved http://dx.doi.org/10.1016/j.apt.2013.06.004

⇑ Corresponding author Tel.: +86 13782507167; fax: +86 3733326336.

E-mail addresses: xhzhao79@yahoo.com.cn (X Zhao), chemenglxd@126.com

(X Lou).

Contents lists available atSciVerse ScienceDirect

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Please cite this article in press as: X Zhao et al., Sol–gel assisted hydrothermal synthesis of ZnO microstructures: Morphology control and photocatalytic

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from the available literatures are: (1) Citric acid was used as a kind

of gelata in this work other than capping agent (2)

Twining-hexag-onal prism, twining-hexagTwining-hexag-onal disk, sphere and ﬂower-like ZnO

microstructures were obtained by only adjusting the pH of the

solution (3) To our best knowledge, ﬂower-like ZnO composed of

nanosheets has not been achieved by only using the raw materials

of Zn(NO3)2, citric acid and NaOH In addition, photocatalytic

deg-radation of dye pollutant (KGL, the chemical structure is presented

inFig 1) was performed by using these different morphologies of

ZnO as photocatalysts The experiment results revealed that the

ﬂower-like ZnO exhibited enhanced photocatalytic activity which

may be affected by the morphology, surface defects and surface

area of the ZnO materials

2 Experimental

2.1 Materials

All the chemicals were used as received without any further

puriﬁcation Distilled water was used in the reaction system as

the solvent medium

2.2 Preparation of different ZnO microstructures

ZnO samples were synthesized by two main procedures: (1)

sol–gel process, 3.756 g Zn(NO3)26H2O and 5.250 g citric acid

(C6H8O7) were dissolved in 100 mL distilled water and stirred at

the temperature of 70 °C to form the sol, then the sol was put into

the oven at 100 °C to form the gel (2) Hydrothermal process, 1 M

NaOH was directly dropped into the gel under constant stirring

The pH of the suspension was adjusted to 11, 12, 13 and 14,

respec-tively Then the resulting suspension (75 mL) was transferred into

100 mL Teﬂon-lined stainless steel autoclave and heated at 120 °C

for 17 h The product was then collected and washed by distilled

water thoroughly, then dried at 100 °C The ZnO samples prepared

at the pH of 11, 12, 13 and 14 were denoted as ZnO-11, ZnO-12,

ZnO-13 and ZnO-14, respectively

In order to investigate the inﬂuence of the sol–gel process on

the ﬁnal morphology of the ZnO microstructure, a comparative

experiment was carried out 1 M NaOH was directly dropped into

the mixed solution of 3.756 g Zn(NO3)26H2O and 5.250 g citric acid

(C6H8O7) to the pH of 14 With the same hydrothermal process, the

product thus obtained was denoted as ZnO-s

2.3 Characterization

The as-synthesized samples were characterized by X-ray

dif-fraction (XRD) (Bruker advance-D8 XRD with Cu Ka radiation,

k= 0.154178 nm, the accelerating voltage was set at 40 kV with a

100 mA ﬂux) Microstructures and morphologies were investigated

using JEOL JSM-6390LV scanning electronic microscopy (SEM) The

photoluminescence (PL) spectra were measured by SHIMADZU RF-5301PC ﬂuorescence spectrophotometer The UV–Vis absorption spectra were obtained on a UV-1700 PharmaSpec UV–Vis spectro-photometer The UV–Vis diffuse reﬂection spectra were obtained

on the Lambda 950 UV–Vis spectrophotometer Surface areas of the samples were determined from nitrogen adsoption–desorption isotherms using the ASAP 2000 instrument and the Brunauer–Em-mett–Teller (BET) method was used for surface area calculation 2.4 Photocatalytic experiments

The photocatalytic activities of the as-synthesized samples were evaluated by the degradation of aqueous KGL solution

100 mg photocatalyst was added into 250 mL, 20 mg/L KGL solu-tion and stirred for 20 min to reach the absorpsolu-tion equilibrium, then exposed to UV light (300 W high pressure Hg lamp, the stron-gest emission at 365 nm) In order to expel the temperature inﬂu-ence, water at room temperature was applied to absorb the heat generated from the UV light and the test tube containing KGL solu-tion was circled 10 cm away from lamp center Every 20 min, a sample was collected by centrifugation and characterized with UV–Vis spectrophotometer (UV-5100, Shanghai Metash Instru-ments Co., Ltd., China.) to monitor the degradation of KGL mole-cules The characteristic absorption peak of KGL at k = 608 nm was chosen to monitor the photocatalytic degradation process For comparison, ZnO powder purchased from Tianli Chemical re-agent Co., Ltd (99.0%, product number XK 13-201-00578) was used for the photocatalytic experiments

The photocatalytic degradation efﬁciency was calculated from the following expression (1):

Degradationð%Þ ¼C0 Ct

C0 100% ¼A0 At

where C0is the initial concentration of KGL, Ctis the KGL concentra-tion at certain reacconcentra-tion time t A0is the initial absorbance of KGL, At

is the KGL absorbance at certain reaction time t

3 Results and discussion 3.1 XRD analysis

XRD patterns of the as-syntheized samples are shown inFig 2 All the diffraction peaks of the samples can be well indexed as the

Fig 1 Chemical structure of Reactive Blue 14 (KGL).

Fig 2 XRD patterns of the as-synthesized ZnO samples.

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wurtzite-structrued hexagonal ZnO (JCPDS card No 36-1451) No

signiﬁcant characteristic peaks of impurities could be detected,

which indicates that all the samples were pure ZnO The high

intensities of the XRD peaks suggest that the ZnO phase

synthe-sized in this work is highly crystalline[21] Calculated via the

Sher-rer formula[10], the average crystalline size for ZnO-11, ZnO-12,

ZnO-13 and ZnO-14 samples is about 37 nm, 45 nm, 33 nm and

36 nm, respectively Compared with the standard card, the ratios

of relative XRD intensities of (1 0 0)/(0 0 2) are remarkably different,

which may attribute to different degrees of growth in preferred

orientation[22]

3.2 SEM analysis

The morphology and the size of the samples were examined by

SEM.Fig 3 presents the SEM images of the as-synthesized ZnO

samples It could be seen that adjustment of the solution pH led

to the formation of different morphologies Hexagonal prism-like

ZnO with twinning microstructures was obtained at the pH of 11

(Fig 3a) Each prism is about 2lm in length and 1lm in diameter

Fig 3b shows the morphology of the ZnO obtained at the pH of 12,

disk-like ZnO with twinning microstructures was formed under

this condition Each disk is about 2lm in length and 5lm in

diam-eter Meanwhile, there were some amorphous structures scattered

among these disks Comparing Fig 3a with b, the length of the

microstructures hardly changes but the diameter increases with

the addition of NaOH under the same condition With the further

increase of the solution pH to 13, sphere-like ZnO with 3–5lm

in diameter were obtained (Fig 3c) It should be noted that there exists sphere-like ZnO with twinning microstructures (arrow pointed inFig 3c).Fig 3d is the image of ZnO obtained at the pH

of 14 It reveals that the morphology of ZnO product is well-defined flower-like three-dimension (3D) microstructures with the diame-ter of 3–5lm, assembled by many nanosheets as ‘‘petals’’ These nanosheets are about 30 nm in thickness, and they alternately con-nect with each other to form the flower

3.3 Possible growth mechanisms of different ZnO microstructures The schematic illustration of the formation process is presented

inFig 4and the possible formation mechanisms of different mor-phologies of ZnO were proposed as follows Firstly, the sol–gel pro-cess surely has impact on the final morphology of the ZnO, which could be confirmed from the SEM result (Fig 3e) In the absence of the sol–gel process, the ZnO product (ZnO-s) had no specific mor-phology It is supposed that Zn(II) citric acid chelate complexes are formed during the gelation of sol[20], and then the gel was dis-solved by adding certain amount of NaOH In this process, part of

OHions in the solution might neutralize H+ions which come from citric acid, part of OHions might react with Zn(II) citric acid che-late complexes to form [Zn(OH)4]2complexes which will decom-pose into ZnO nuclei in the hydrothermal process (Eqs (2)–(4))

[23] The rest part of OHwill be left in the solution, which will af-fect the morphology of ZnO in the hydrothermal process

Fig 3 SEM images of the as-synthesized ZnO samples: (a) ZnO-11; (b) ZnO-12; (c) ZnO-13; (d) ZnO-14 (e) ZnO-s.

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Zn2þþ 4OH! ZnðOHÞ24 ð3Þ

Secondly, the OHleft in the solution must have played a key role in

the ZnO morphology formation process considering that the pH of

the solution was the only variable in this work It is known that

ZnO is a polar crystal with a positive polar (0 0 0 1) plane rich in

Zn2+cations and a negative polar ð0 0 0 1Þ plane rich in O2anions

[24] Under pH 11 hydrothermal circumstances (the molar ratio of

Zn2+/OHis about 1:6), [Zn(OH)4]2complexes preferably adsorb

on the positively charged Zn-(0 0 01) plane of ZnO nuclei, which

leads to form ZnO twining-hexagonal prism elongated along the

c-axis direction due to the intrinsic anisotropy in its growth ratev

withm[0 0 0 1] >>m½0 1 1 0 >>m½0 0 0 1[25] With the pH increasing

to 12 (the molar ratio of Zn2+/OHis about 1:6.5), the concentration

of OH presented in the aqueous solution increased, hence the

absorption of OH on the positively charged Zn-(0 0 0 1) plane

would compete with that of the [Zn(OH)4]2complexes [26] The

OHions could stabilize the surface charge and the structure of

Zn-(0 0 0 1) surfaces, leading to the growth rate along the c-axis

direction being suppressed to some extent, thus twining-hexagonal

disks were formed (Fig 2b) When pH = 13 (the molar ratio of Zn2+/

OH is about 1:7.5), based on surface energy minimization, the

nanoparticles would rearrange themselves on the surface of ZnO

nuclei for lowering the surface energy, so the sphere-like (Fig 2c)

ZnO nanostructure was formed[27] But for pH 14 (the molar ratio

of Zn2+/OHis about 1:12), the concentration of OHpresented in

the aqueous solution had a signiﬁcant increase, following the

de-crease in the concentration of [Zn(OH)4]2 due to the initial fast

nucleation of ZnO Hence, the absorption of OHon the positively

charged Zn-(0 0 0 1) plane would dominate in the competition with

[Zn(OH)4]2complexes Based on the above discussion, the growth

rate along the c-axis direction would be descended, leading to the

formation of nanosheets that are preferentially grown along the

[0 0 0 1] and ½0 1 1 0 directions within the f2 1 1 0g plane

Subse-quently, more and more nanosheets with a f2 1 1 0g-planar surface

interlaced and overlapped with each other into a multilayer and network structure, and the ﬂower-like ZnO nanostructures were shaped (Fig 3d)[26]

3.4 UV–Vis absorption analysis The UV–Vis absorption spectra of the as-synthesized samples were carried out for further analysis of the optical absorption prop-erties of the materials Fig 5 illustrates the UV–Vis absorption spectra of the samples at room temperature The absorption spec-tra of these samples have a narrow absorption peak located at about 370 nm, which is the characteristic of wide-band gap ZnO

[28] It is signiﬁcant that except characteristic wide-band gap ZnO absorption band, no other band is observed in the obtained UV–Vis spectra, which conﬁrms that the synthesized samples are pure ZnO[29]and also reveals the good optical properties of the as-synthesized ZnO[30]

Fig 4 Schematic illustration of the formation process of the as-synthesized ZnO samples.

Fig 5 UV–Vis absorption spectra of the as-synthesized ZnO samples.

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3.5 Photocatalytic activity

To demonstrate their potential environmental application in the

removal of contaminants from wastewater, the photocatalytic

activities of the as-synthesized ZnO samples were investigated by

degradation of KGL Because the yield of ZnO-11 was very small,

no other studies were taken on it The photocatalytic activity of

the commercial ZnO powders was also tested for comparison

Fig 6a shows the degradation rate of KGL as a function of

irradia-tion time in presence of different samples In the absence of light or

catalyst, the concentration of KGL had no obvious change for

120 min, indicating that both light and catalyst were necessary

for the effective photodegradation of KGL dye[31] FromFig 6a,

it is clear that the morphology of the ZnO sample has great

inﬂu-ence on its photocatalytic activity After irradiation for 120 min,

the degradation ratios of KGL were about 27.8% for ZnO-13, while

nearly 100% for ZnO-12, ZnO-14 and commercial ZnO It should be

noted that after irradiation for 40 min, only the degradation ratio

of ZnO-14 was nearly 100% Obviously, ZnO-14 showed the best

photocatalytic activity.Fig 6b shows the UV–Vis absorption

spec-tra of KGL for ZnO-14 sample It is obvious that the absorbance for

the maximum peak at 608 nm decreases, suggesting the

occur-rence of the destruction of KGL and the formation of some

interme-diates When the solution was irradiated for 40 min, the absorption

peaks in the curve almost disappeared (the KGL solution decolor-ized completely)

The reason may be explained as follows, when semiconductor materials are irradiated by light with energy higher or equal to the band gap (Eg), an electron e

cb

in the valence band (VB) can

be excited to the conduction band (CB) with the simultaneous gen-eration of a hole hþ

vb

in the VB Excited state e

cb and hþ

vb can recombine and get trapped in metastable surface states, or react with electron donors and electron acceptors adsorbed on the semi-conductor surface In other words, the photoelectron is easily trapped by electron acceptors like adsorbed O2, whereas the photo-induced holes can be easily trapped by electronic donors, such as

OHor organic pollutants, to further oxidize organic dye[7] There are many factors that could inﬂuence the photocatalytic activity of the ZnO, such as morphology, surface defects, surface areas, and so

on[32] Firstly, as reported in the literature, photocatalysts with higher surface energy show better photocatalytic performance [33] Among different crystal planes of ZnO, the (0 0 0 1) plane exhibits the highest surface energy[34] Therefore, ZnO samples with larger proportion of (0 0 0 1) plane would improve the photocatalytic activities From morphology analysis in 3.3 part, it is known that ZnO-14 in the shape of nanosheets composed ﬂower-like and ZnO-12 in the shape of twining-hexagonal disk both have larger area of (0 0 0 1) plane than ZnO-13 in the shape of sphere, so they exhibit enhanced photocatalytic activities than ZnO-13

Secondly, oxygen defects may be considered to be the active sites of the ZnO photocatalyst[35] Since proper amount of oxygen vacancies can entrap electrons from semiconductor, the holes can diffuse to the surface of the semiconductor and cause oxidation

of the organic dye Therefore, surface oxygen detects of high den-sity beneﬁt the efﬁcient separation of electron–hole pairs, mini-mize the radiative recombination of electron and hole and increase the lifetime of the charge carriers, hence improve the pho-tocatalytic activity[36] It has been reported that stronger exci-tonic PL intensity suggests there exists more surface defects[37]

Fig 7 is the photoluminescence spectrum of the as-synthesized ZnO samples measured at room temperature with the excitation wavelength of 350 nm It can be seen that there are two main emission peaks for all three samples, one is the strong peaks at

388 nm which correspond to the near band-edge emission (NBE) [38], the other is the wide band emission extending from

400 to 650 nm which covers the blue–yellow regions The emission

in the visible-light region is attributed to ZnO surface detects, in

Fig 6 (a) Photocatalytic degradation of KGL with different ZnO samples under

different conditions (b) Time-dependent absorption spectra of KGL solution in the

presence of ZnO-14 sample after UV irradiation Fig 7 PL spectra of the as-synthesized ZnO samples.

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which oxygen vacancies are the most suggested defects[39] As is

shown inFig 7, the PL intensity varies in the following order:

ZnO-14 > ZnO-12 > ZnO-13, suggesting that ZnO-ZnO-14 possesses surface

defects of the highest density, which is responsible for the best

photocatalytic activity, followed by ZnO-12, and then ZnO-13 This

result is in accordance with the photocatalytic activities

Thirdly, one of the other key factors controlling the

photocata-lytic activity of the ZnO samples is the optical absorption and the

migration of the light-induced electrons and holes[40] UV–Vis

dif-fuse reﬂectance spectroscopy was used to characterize the optical

absorption properties of the ZnO samples, and the spectrum is

dis-played in Fig 8 It demonstrates that all the deﬁned excitonic

absorption peaks could correspond to the wurtzite hexagonal

ZnO It has been recognized that the Egvalue occurred from an

electronic transition between the ﬁlled valence bands to the empty

conduction bands[11] According to the principle of the

photocat-alytic reaction, the smaller the band gap of the photocatalyst, the

better photogenerated of the electron–hole pairs, and the stronger

of the photocatalytic activity[41] The band gap energies (Eg) of the

as-synthesized ZnO samples could be obtained from the plots of (ahv)2versus photon energy (hv), and the values estimated from the intercept of the tangents to the plots were shown inTable 1 The samller Egvalue of ZnO-14 indicates that it could be better photogenerated than that of ZnO-12 or ZnO-13

In addition, surface area would also affect the photocatalytic activity of the catalyst[34] The surface areas of the as-synthesized ZnO samples are presented inTable 1 FromTable 1, one can see that ZnO-14 possesses much bigger surface area than ZnO-12 or ZnO-13 Furthermore, the nanosheet-composed micro-ﬂowers shape of ZnO-14 could effectively prevent aggregation, thus main-tain larger active surface area, offer more opportunity to the diffu-sion and mass transportation of KGL molecules and hydroxyl radicals (OH

) in the photocatalytic degradation process than those

of ZnO-12 and ZnO-13[25] Therefore, attributing to the largest area of (0 0 0 1) plane, nar-rowest band gap, highest density of surface defects and the largest surface area of ZnO-14, it exhibits the highest photocatalytic activ-ity among all the three samples As for ZnO-12 and ZnO-13, although the surface area of 12 is smaller than that of

ZnO-13, it exhibits more surface defects, larger areas of (0 0 0 1) plane and narrower band gap It can be concluded that in the photocata-lytic reactions, the inﬂuence of the integrated factors ((0 0 0 1) plane, Eg, surface defects) may exceed the impact factor of surface area, leading to the better photocatalytic activity of ZnO-12 than that of ZnO-13

4 Conclusion

In summary, we have synthesized twining-hexagonal prism, twining-hexagonal disk, sphere, ﬂower-like ZnO microstructures using sol–gel assisted hydrothermal method by just controlling the pH of the hydrothermal reaction mixture Possible growth mechanisms have been proposed The nanosheet composed ﬂow-er-like ZnO exhibits the best photocatalytic activity, which could

be attributed to the morphology, surface defects, band gap and sur-face areas The enhanced performance of the ﬂower-like ZnO indi-cates that it can be used as a promising photocatalyst for the practical application in photocatalytic degradation of organic dyes Acknowledgements

This project is supported by the National Natural Science Foun-dation of China (Grant No 21073055), Basic Scientiﬁc and Techno-logical Frontier Project of Henan Province, PR china (No 112300410207) and Henan Normal University Science Foundation for Young Scholars (No 2010qk06)

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