acetone sensing characteristics of zno hollow spheres prepared by

The removal of carbon via calcinations yields hollow zinc oxide spheres.. The results indicated that the products were pure hexagonal ZnO with the structure of hollow sphere, and the for

Acetone sensing characteristics of ZnO hollow spheres prepared by

one-pot hydrothermal reaction

, Qi Wang, Zhongxi Yang

School of Material Science and Engineering, Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China

a r t i c l e i n f o

Article history:

Received 4 June 2012

Accepted 18 July 2012

Available online 27 July 2012

Keywords:

ZnO

Hollow sphere

Microstructure

Sensors

a b s t r a c t

ZnO hollow spheres were one-pot fabricated by hydrothermal treatment Zinc nitrate were dissolved together with urea and glucose in water, and the mixtures were heated at 180 1C for 24 h During the hydrothermal treatment, carbon spheres are formed with zinc ions incorporated into the hydrophilic shell The removal of carbon via calcinations yields hollow zinc oxide spheres The as-prepared samples were characterized by X-ray diffraction and the field emission gun scanning electron microscope The results indicated that the products were pure hexagonal ZnO with the structure of hollow sphere, and the formation mechanism of ZnO hollow spheres was discussed Consequently, the ZnO hollow spheres exhibited good sensing performance towards acetone gas with rapid response when operated at 300 1C

&2012 Elsevier B.V All rights reserved

1 Introduction

Chemical sensors play an important role in the areas of

emissions control, environmental protection, public safety, and

human health[1] The general mechanism of the oxide

semicon-ductor sensors is based on the changes in electrical properties

before and after exposure to the target gases or vapors [2]

Acetone, a common reagent widely used in industries and labs,

is harmful to health Inhalation of acetone causes headache,

fatigue and even narcosis and harmfulness to nerve system

Hence it is necessary to monitor acetone concentration in the

environment for health and the workplace for safety[3– ]

Zinc Oxide, as an n-type semiconductor material, has been

widely investigated as a field-effect transistor[6], optical device

[7], dye-sensitized solar cell[8], and solid-state gas sensor[9,10]

Recently, ZnO-based sensors have been investigated for the

detection of acetone vapor at various concentration levels

[11–14] For chemical sensor applications, hollow structural

features provide enhanced surface activities, high

surface-to-volume ratio and fast diffusion, which allow easy gas penetration

into the sensing layers Furthermore, both the outer and inner

shells actively interact with target molecules So, several

promis-ing approaches have been developed to produce hollow

architec-tures such as spheres, hemispheres and inorganic tubes[15] Up

to now, the most important methods for hollow structures rely on

the use of sacrificial templates, and the desired hollow interiors

are generated upon the removal of templates by calcination or

dissolution[16] Recently, a novel method for the fabrication of metal oxide hollow spheres has been developed Titirici et al.[17]

have reported synthesis of various oxide hollow microspheres (such as Fe2O3, Co2O3, CeO2, MgO and CuO) using carbonaceous polysaccharide microspheres prepared from sacharide solution as template However, preparation of well-crystallized ZnO hollow spheres with controllable surface morphology and high gas response is still a great challenge In this contribution, ZnO hollow spheres are prepared by the one-pot hydrothermal reaction The study focuses on the formation mechanism of ZnO hollow spheres and the effect of hollow morphology on the acetone sensing characteristics

2 Experimental

Preparation of ZnO hollow sphere: ZnO hollow spheres were prepared by a hydrothermal approach using zinc nitrate (Zn(NO3)26H2O) as a zinc source In a typical synthesis, glucose monohydrate (C6H12O66H2O, 75 mmol), 5 mmol zinc nitrate and urea (CO(NH2)2, 50 mmol) were dissolved in 15 mL of distilled water under stirring, respectively The above two solutions were mixed immediately before the experiment and placed in a 50 mL capacity Teflon-lined stainless steel autoclave, which was heated

in an oven to 180 1C for 24 h After hydrothermal treatment, the black precipitates were centrifuged, and then with distilled water and absolute alcohol washed several times The washed precipi-tates were dried in a vacuum oven at 60 1C for 12 h After synthesis, the zinc-carbon composites were calcined in air at

500 1C (heating rate of 1 1C/min) for 4 h to remove the carbon core, leading to ZnO hollow spheres

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n

Corresponding author Tel.: þ86 531 82765473; fax: þ 86 531 87974453.

E-mail address: mse_songp@ujn.edu.cn (P Song).

Characterization: The crystalline phase in the samples were

characterized by an X-ray diffraction (XRD, Bruker D8 Advance)

using CuKaradiation (l¼0.15406 nm) at 30 kV and 40 mA at a

scanning rate of 21 at 2ymin1 ranging from 201 to 701 The

FESEM micrographs were obtained on a FEI Sirion 200 field

emission gun scanning electron microscope (FESEM, Hitachi

S4800) FESEM measurements were mounted on aluminum studs

using adhesive graphite tape and sputter coated with gold before

analysis

3 Results and discussion

Phase and morphology of the products: Typical XRD pattern of the

as-prepared ZnO is shown inFig 1, where one can see that all the

diffraction peaks are in good agreement with those of the hexagonal

structure of ZnO (JCPDS card 36-1451) No other diffraction peaks are

found, indicating that the products are pure ZnO and the carbon

microsphere templates were completely removed In addition, it can

be found that several diffraction peaks are strong and sharp, which

indicates that the prepared ZnO are highly crystallized yet

polycrys-talline The average crystallite size of ZnO samples was estimated

according to the line width analysis of the diffraction peaks based on

the Scherrer formula, D¼0.89l/bcosy, which was calculated to be

about 19.8 nm

Fig 2(a) shows FESEM image of the as-prepared zinc-carbon

composite microspheres obtained by the hydrothermal treatment

before calcinations The diameter of spheres is about 4–7mm Many

spheres are aggregated and linked to each other and their surfaces

are smooth After calcinations, we obtained ZnO hollow spheres

with diameters of about 1–2mm, as shown inFig 2(b) More details

can be found in Fig 2(c) and (d), some small openings in the

spheres can be seen clearly, implying the hollow structure of the

spheres And, this porous network is believed to be favorable for gas

sensor, which can facilitate the inward and outward gas diffusion

Furthermore, FESEM images of the spheres before and after

calcinations reveal a considerable shrinkage (from approximately

4–7mm to 1–2mm in diameter) of the structures during

calcina-tions treatment, showing a transition from loosely adsorbed

zinc ions to a dense zinc oxide network in the hollow spheres

The formation mechanism of ZnO hollow spheres is discussed, as

shown inFig 3 Firstly, the formation of the carbon spheres involves

the dehydration of the carbohydrate and subsequent carbonization

of the organic compounds The surface of carbon spheres is hydrophilic and has a distribution of OH and C¼O groups, which are formed from non- or just partially dehydrated carbohydrates The secondary step is the embedding of zinc precursors (Zn(OH)42) into the hydrophilic shell of as-prepared carbon spheres due to the fact that the functional groups in the surface layer can bind Zn cations through coordination or electrostatic interactions Finally, the removal of carbon core and densification of incorporating Zn cationic ions in the layer via calcinations results in the formation of hollow zinc oxide spheres As we all known, ZnO nuclei form from dehydration of Zn(OH)42ions in alkaline environment[18,19], and the reactions are as follows:

Zn2 þþ4OH

ZnðOHÞ42-ZnðOHÞ2þ2OH ð2Þ

Acetone sensing properties: Fig 4 displays the concentration dependent sensitivity of the sensor based on ZnO hollow spheres for acetone detection at an operating temperature of 300 1C It can

be seen that the sensitivity rapidly increases with increasing acetone concentration Furthermore, a quick response/recovery time was observed with this sensor The typical dynamic response curve of ZnO hollow spheres sensor toward 500 ppm acetone at 300 1C is shown in the inset ofFig 4 We can find that the response of the sensor increased abruptly on the injection of acetone, and then decreased rapidly and recovered to its initial value after the test gas was released The response time and recovery time of the sensor were less than 10 s

ZnO is well-known as an n-type semiconductor, characterized

by its high free carrier concentration When the ZnO hollow spheres were exposed to air, oxygen molecules are firstly adsorbed on the inner and outer surface of ZnO hollow spheres and capture free electrons from the conduction band to produce chemisorbed oxygen species (O

, O2 or O2) When ZnO hollow spheres are exposed to acetone gas at higher temperature (300 1C), acetone gas molecules can react with adsorbed oxygen species on the inner and outer surface This liberates free electrons in the conduction band, leading to an increase in the resistance of an n-type semiconductor The final reaction takes place as

C3H6O þ8OðadsÞ-3H2O þ 3CO2þ8e

ð4Þ

Therefore, the specific surface area of the sensors plays an important role in the contact and subsequent reaction of oxygen species with the tested gas In our case, the high response of the ZnO hollow spheres sensor can be ascribed to the larger specific surface area (not only the outer but also the inner surface) of the sensing materials Furthermore, the porous structure and open framework of ZnO hollow spheres may also contribute to the improved sensor response

4 Conclusions

In summary, ZnO hollow spheres were prepared by the glucose-mediated, one-pot hydrothermal synthesis of Zn-coated carbon spheres and then calcined at 500 1C, the hollow ZnO microspheres with diameters of 1–2mm were gradually transformed into solid microspheres The ZnO hollow spheres sensor shows high response,

2 Theta / degree

Fig 1 XRD pattern of as-prepared ZnO hollow spheres.

low detection and fast response and recovery towards acetone gas.

The excellent sensing performances are attributed to the hollow and

porous microstructure

Acknowledgment

This work was financially supported by National Natural Science Foundation of China (No 61102006), Natural Science Foundation of Shandong Province, China (No ZR2010EQ009), Shandong Distinguished Middle-aged and Young Scientist Encou-rage and Reward Foundation (No BS2009CL056)

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5 µm

1 µm

100 µm

500 nm

Fig 2 FESEM images of (a) zinc-carbon composite microspheres via hydrothermal synthesis, (b) low magnification image of ZnO hollow spheres after calcinations, (c) and (d) high magnification images of ZnO hollow spheres.

Glucose

Urea

Zinc nitrate

Hydrothermal

treatment

Zinc precursor

Carbon sphere

Calcination

ZnO hollow sphere

Fig 3 Synthetic scheme of ZnO hollow spheres fabricated by hydrothermal treatment.

2

4

6

8

10

12

14

16

Acetone concentration (ppm)

T = 300°C

0 10 20 30 40 50 60 0

2 4 6 8 10 12

Time (s)

500 ppm

Fig 4 Sensitivity of ZnO hollow spheres versus acetone concentration The inset

shows the response and recovery characteristics of the sensor to 500 ppm acetone at

300 1C.