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Guo, M. R. Liu and Y. X. Zheng, RSC Adv., 2014, DOI: 10.1039/C4RA12157K.

http://dx.doi.org/10.1039/c4ra12157k

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1

TOC

The facile synthesis of Sm2O3-doped ZnO nanocomposite provides a potentially new

approach for the development of ZnO-based sensor.

0

5

10

15

20

25

ZS-4

ZS-8ZS-6

ZS-2

Toluene

Ethanol

Methanol

Formaldehyde

ZS-0

Respons e (Ra/Rg)

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View Article OnlineDOI: 10.1039/C4RA12157K


1

Enhanced ethanol sensing properties based on Sm2O3-doped ZnO

nanocomposite

Cheng Penga,*

, Jiaojiao Guo, Mingrui Liu, Yixiong Zheng

a College of Materials Science and Engineering, Huaqiao University, Xiamen 361021, PR China

Abstract

ZnO doped with 2, 4, 6 and 8 wt % samaria (Sm2O3) were prepared by a novel sol spray

combustion method. The samples were characterized by XRD, FESEM, EDX, UV-vis DRS and

Raman techniques. The XRD results show that all samples containing various amount of Sm2O3

have hexagonal wurtzite structure. UV-vis spectra show that the addition of Sm2O3 did not affect

the band gap of ZnO. Raman spectra exhibits that the Sm2O3-doped ZnO nanocomposite keeps

the crystal structure of the bulk ZnO and possesses more surface defects. The influence of Sm2O3

dopant on the response and selectivity for ethanol detecting of the sensor based on ZnO

nanoparticles was investigated. The sensors’ responses were measured in presence of 100 ppm of

ethanol, formaldehyde, methanol and toluene. As 4 wt % Sm2O3 was added to ZnO, the response

to ethanol at various temperatures was significantly enhanced. The results reveal that doping

Sm2O3 may be a promising route for the production of ZnO-based gas sensor with good ethanol

sensing properties.

Keywords: Nanocomposite; ZnO; Sm2O3; Gas sensor; Ethanol Sensing.

1. Introduction

* Corresponding author. Tel.:+86 592 6162225; Fax: +86 592 6162225

E-mail address: [email protected]

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Recent years have witnessed great interest in semiconducting metal oxide based gas

monitoring devices for the detection of toxic pollutant gases, poisonous and hazardous gases [1].

As a wide band gap semiconductor, ZnO has been studied extensively owing to its outstanding

physical properties, chemical stability, low cost, and good flexibility in fabrication [2]. However,

the gas sensing properties of ZnO hardly meet the all requirements of gas sensors with high

sensitivity, short response and recovery time, good selectivity and long-term stability. For this

situation, more recent researches have been focused on improving the gas sensing performance

of ZnO.

Doping with suitable elements is an effective way to modify the electronic, optical and

magnetic properties of metal oxide semiconductor materials [3]. Owing to their superior optical,

electronic and magnetic properties, rare earth compounds have been widely used in various

applications, such as high-performance luminescent devices [4], varistor ceramics [5], catalysts

[6] and so on. Besides, previous studies have shown that rare earth elements introduced into the

metal oxide structure can overcome their disadvantages such as poor sensitivity and selectivity

[7]. Samarium ion with 4f5 electronic configuration usually exists in the form of triply ionized

ion (Sm3+

), which shows fast oxygen ion mobility and predominant catalytic properties [8].

It’s well known that ethanol plays an important role in many applications such as wine

industry, food industry, pigment and medical industries. However, people exposed to ethanol

may lead to some negative effects, nausea and vomiting, skin and eyes irritation, and even

central nervous system depression and acidosis [9]. Furthermore, ethanol is commonly used as

fuel in our daily life, which may pose threat to the security of our environment. Thus, controlling

and monitoring ethanol is of great importance. It’s urgent for us to develop a sensor with high

selectivity and stability, fast response and quick recovery in the detection of ethanol [10].


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It is widely accepted that the morphology, grain size and crystal structure of ZnO-based

materials, which are strongly depend on the method of preparation, play an important role in its

gas-sensing properties. So far, a variety of techniques have been used to synthesize ZnO-based

nanostructure, such as sol-gel [11], hydrothermal [12], microwave-assisted combustion [13],

ionothermal [14], electrospining [15], magnetron sputtering [16], laser ablation [17], MOCVD

[18] and etc. However, most of the reported experimental techniques are still limited to the

laboratory scale due to some unresolved problems, such as special conditions, tedious procedures

or complex apparatus, low yield and high cost.

In this paper, we report on the structural, optical and gas sensing properties of Sm2O3-doped

ZnO nanocomposite prepared by a novel sol spray combustion technology. This synthesis

method has some advantageous attributes, including low costs, simple experimental setups, high

yield, no residual impurities, high energy-efficiency and environmental friend. In addition, the

responses of Sm2O3-doped ZnO sensors, containing 2–8 wt% samaria, towards formaldehyde,

toluene, methanol, ethanol are also investigated in this work.

2. Experimental

2.1. Sample synthesis

All chemicals were of analytical grade and were used as purchased without further

purification. zinc nitrate hexahydrate (Zn(NO3)2·6H2O), samarium nitrate hexahydrate

(Sm(NO3)3·6H2O), ammonia, citric acid were all supplied by Shanghai Chemical Reagent

Company.

All the samples were prepared by a novel sol spray combustion method. In a typical

procedure, Zn(NO3)2·6H2O and citric acid were dissolved in deionized water at room

temperature under vigorously stirring. Proper amount of samarium nitrate to achieve 0, 2, 4 , 6


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and 8 wt % samaria was added to the solution under vigorous stirring. Then pH of the solution

was maintained around 7 by gradually adding ammonia followed by vigorous stirring. In the

process of sol spray combustion synthesis, sol was obtained by mixing the raw materials and

then sprayed onto the heating plate (600 oC) through a airbrush (Veda, WD-128P, PR China )

connected with mini pump (Veda, AC-18B, PR China). Small droplets were heated up and then

got ignited within several seconds. The combustion reaction resulted into light-yellow, final

fluffy samples. The samples are denoted as ZS-x, in which “x” specifies the wt % of Sm2O3.

2.2. Characterization

The crystallographic structures of the as-prepared samples were collected by powders X-ray

diffraction (XRD, Cu Ka radiation, SmartLab 3 kW, Rigaku, Japan). The powders morphology

observations were carried out on Field Emission Scanning Electron Microscopy (FESEM,

Su8010, Hitachi). Raman spectra were collected using a Renishaw InVia micro-Raman

spectrometer in backscattering geometry with an Ar+ laser (λex= 532 nm). UV–vis spectrum was

recorded on Ultraviolet Spectrophotometer (UV-2550, Shimadzu), using BaSO4 as a reference.

2.3. Fabrication of sensors

The WS-30A static gas-sensing system (Weisheng Electronics Co. Ltd, Henan, PR China)

was determined to test the sensing performance of the sensors. The structure of the gas sensor

belongs to the side-heated type. The preparation process is described as follows: ZS-x and

ZnO-NPs was mixed with a suitable amount of terpineol to form a paste and then coated onto the

surface of a ceramic tube where a pair of Au electrodes had been installed at each end, and a

Ni-Cr heating wire going through the tube was employed as a heating filament to control the

operating temperature by tuning the heating voltage. Then the sensors were dried and aged for

testing. The operating temperature was regulated automatically by the gas-sensing system and


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accurately controlled by adjusting the heating voltage. In the process of test, a given amount of

target gas were injected into a test chamber and uniformly dispersed by a fan. The gas response

was defined as the ratio of the electric resistance, namely S = Ra/Rg, where Ra and Rg is the

resistance of metal oxide semiconductors in air and the target gas, respectively. The response or

recovery time was estimated as the time taken for the sensor output to reach 90% of its saturation

after applying or switching off the target gas in a step function.

3. Results and discussion

3.1. Structural and compositional analyses

To identify chemically the as-prepared samples, XRD analysis was used to find phase and

crystal structure of the Sm2O3-doped ZnO samples prepared under the same conditions. The

X-ray patterns of the Sm2O3-doped ZnO with different Sm2O3 content are shown in Fig. 1. It is

indicated in the crystal planes (100), (002), (101), (102), (110) and (103) in the patterns, which

ban be perfectly indexed to the hexagonal wurtzite structure of ZnO according to the standard

JCPDS card (No. 36–1451). Also no diffraction peaks corresponding to samaria and other

impurities are observed in these patterns, even when the Sm2O3 content is 8 wt %. This suggests

that samaria may exists in amorphous phase or its crystrallites are too small to be detected.

Furthermore, no shifts were observed for (101) diffraction peak of ZnO doped with different

Sm2O3 content. Consequently, Sm2O3 has possibly deposited on the surface of ZnO nanoparticle

through the formation of Sm2O3-ZnO nanocomposites rather than Sm3+

substituted for Zn2+

or as

an interstitial atom. This could also be supported by UV-vis DRS measurement described below.

It is also worth noting that the full width at half maximum (FWHM) of the Sm2O3-loaded ZnO

samples is broader than that of pure ZnO and the peak intensity is weaker. Using Scherrer’s

equation for (101) diffraction peak, the average crystallite size of pure ZnO and ZS-x (x≠0) are


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estimated to be about 60 nm and 20 nm, respectively. This suggests that the crystallite growth of

ZnO could be suppressed when Sm2O3 is present in the samples, which may be attributed to the

preventing effect of samarium oxide in the formation of necks and the process of coalescence

between particles. This result is in accordance with other reports on Sm2O3-doped metal oxides

[19-20].

Fig. 2A shows the FESEM images of ZS-8 sample. As is shown, the product is foamy

porous in nature. The average particle size of ZS-8 samples is about 20 nm, which agrees well

with the results calculated by the Scherrer’s equation. It is noteworthy that some honey holes

appeared in the sample, which is attributed to the gases released from the combustion reaction. In

the process of sol-spray combustion a lot of gases can be formed, which will be very helpful for

the dispersion of final product. In addition, the EDX spectrum (Fig. 2B) confirms that the

as-prepared ZS-8 is composed of zinc, samarium and oxygen elements.

3.2. Optical properties.

UV-vis spectrum was carried out to investigate the effect of incorporation of Sm2O3 on

optical property of ZnO. The UV–vis reflectance spectra of as-prepared samples are shown in

Fig. 3A. All samples display fundamental absorption edges rising around 390 nm, which can be

attributed to the band-band transition of ZnO nanocrystals. It’s obvious that there is almost no

significant difference in the absorption edge of ZS-x samples. ZnO has a direct bandgap, we can

apply Tauc relation according to the following equation:

(αhυ)2 = C(hυ - Eg )

n (1)

Where α is the absorption coefficient of the ZnO at a certain value of wavelength λ, h is

Planck’s constant, C is a constant, υ is the frequency of light, Eg is the band gap energy and n =

1/2 for direct band gap semiconductor. We can draw for (αhυ)2

versus photon energy from Eq.


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(1). By extrapolating the linear part that meets the abscissa, the point that represents the value of

the band gap energy of the sample was obtained. Fig. 3B shows the Tauc plots of the ZS-x

samples, band gap energies of which are 2.873 (x=0), 2.871 (x=2), 2.878 (x=4), 2.886 (x=6),

2.894 eV (x=8), respectively. The band gap energy of as-prepared ZS-x is lower than that of bulk

ZnO (3.37eV), which may be related to surface defects density [21]. In addition, the results

indicate that the addition of samaria does not affect ZnO band gap. This may indicate that no

incorporation of Sm3+

in ZnO lattice has occurred because Sm3+

ions have a larger radius (0.0958

nm) than Zn2+

ions (0.074 nm) [22]. Based on above results, Sm2O3 may be loaded on ZnO as a

separate phase.

3.3. Raman spectra

Raman scattering is very sensitive to the microstructure of nanocrystalline materials and

used here for the structure elucidation of the Sm2O3-doped ZnO samples. Fig. 4 shows the

Raman spectra of ZS-x samples. Hexagonal wurzite ZnO belongs to the C6V4 space group

according to the group theory. There are two A1，two E1, two E2 and two B1 modes in the ZnO

Raman spectrum. The presence of a sharp peak at 437 cm-1

in the Raman spectra of ZS-x

samples can be attributed to the E2 (high) mode of non-polar optical phonons, which is the

characteristic peak of the hexagonal wurtzite phase [23]. As shown in Fig. 4, E2 (high) shows a

shift toward a high frequency (from 437 to ～439 cm-1

) in the following order: ZS-0< ZS-8<

ZS-6< ZS-2< ZS-4. Due to its high sensitivity to stress, the E2 mode (high) is usually applied to

investigate the stress state in films [24]. This frequency shift of the E2 mode (high) may be

ascribed to the stress variation. Since the E2 mode (high) of sample ZS-0 is similar to that (437

cm-1

of bulk ZnO [25], the stress within the sample ZS-0 can be neglected. It was reported that

under pressure the E2 (high) frequency of the wurzite ZnO crystal blue-shifted from that of free


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pressure and increased with the increase of biaxial compressive stress within the c-axis oriented

ZnO epilayers [26]. In our study, the E2 (high) frequencies of ZS-x (x ≠ 0) samples are all higher

than that of pure ZnO, which indicates that addition of Sm2O3 can imposed compressive stress to

ZnO. The more compressive stress could bring more defects (such as oxygen vacancy, V0) [27].

Furthermore, ZS-4 possesses the largest frequency of E2 mode (high) and undergoes the largest

compressive stress, which indicates that the density of V0 in ZS-4 is the highest in all samples.

This argument could also be supported by the observation that fwhm of E2 (high) increases after

doping Sm2O3, especially for ZS-4, because the increase of fwhm of E2 (high) means the increase

of defects [28].

3.4. Gas sensing behaviors

The response curves of the as-prepared samples to 100 ppm ethanol at various operating

temperatures are shown in Fig. 5. All samples exhibit different gas sensing performance at

different operating temperatures. As can be seen in Fig.5, the optimal operating temperature of

ZS-x for achieving the maximum ethanol response are obtained at about 340 °C, which is much

lower than that of pure ZnO, 370 °C. In addition, gas response is saturated to a maximum value

of 20.4 when the Sm2O3 concentration reaches 4 wt % at its optimal operating temperature, 340

°C. Apparently, loading Sm2O3 can not only effectively decrease the optimal operating

temperature of ZnO-based gas sensor, but also improve its gas sensing response. The gas sensing

response would decline above the optimal operating temperature, which might be ascribed to the

competing desorption of oxygen [29]. Performance of the ZnO-based sensors is known to be

dependent on the grain size of ZnO [15]. The results of XRD and FESEM confirmed that the

grain size of ZnO decreased when being loaded with Sm2O3, which could lead to an increase in

the adsorption sites and further an increase in the response. Fig. 6 gives the transient response of


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ZS-x-based sensors to 100 ppm ethanol at 340 °C. As shown in Fig. 6, the response increases

gradually when the contents of Sm2O3 increase from 0 to 4 wt %. However, the response

decreases gradually when the contents of Sm2O3 further increase from 4 to 8 wt %. The sensor

based on ZS-4 exhibits the best gas sensing performance.

Fig. 7 shows the electrical resistance of ZS-x samples in air (Ra) as a function of

temperature. It can be seen that the addition of Sm2O3 increases the resistance of ZnO sample in

the temperature range from 300 °C to 400 °C. The electrical resistances of the ZS-6 and ZS-8

samples are obviously much higher than those of ZS-2, ZS-4, and ZS-0 at 300 °C. At 400 °C the

electrical resistances of the ZS-0, ZS-2, ZS-4 and ZS-6 samples are very close, but still much

lower than that of ZS-8. Sm2O3 loaded on the surface of ZnO can prevent direct contact of ZnO

nanoparticles, which results into an increase in the height of Schottky barrier [30]. These results

are also confirmed by UV-vis results discussed above.

In order to elucidate the sensors selectivity, the responses of the sensors doping with various

amount of Sm2O3 towards different kinds of target gases at 340 °C are summarized in Fig. 8. As

shown in Fig.8, the sensors responses to ethanol are greatly enhanced with the addition of Sm2O3.

Furthermore, the responses of ZS-x sensor to ethanol are much larger than that of the sensor to

other target gases, which can be attributed to the number of electrons released upon exposure to

ethanol is more than those of other target gases [31]. The response values of ZS-4 samples to 100

ppm ethanol in the presence of all target gases reaches 20.4, which shows ZS-4 is highly

selective to ethanol.

3.5. Ethanol sensing mechanism

It’s well-known that the sensing mechanism of ZnO is the surface-controlled type. There are

some key factors influencing the gas sensing properties, such as surface state, the grain size,


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oxygen adsorption, lattice defects and so on. When n-type metal oxide semiconductors are

exposed in the air, various reactive oxygen species (such as O2−, O2−and O−) are formed on the

surface of metal oxide by capturing an electron from the conductance band. The electron

concentration on the surface of metal oxide decreases and forms a deplete layer, which caused

the potential barrier at the grain boundaries to increase and the carrier concentration to decrease

[32]. Consequently, the resistance of the ZS-x sensor increased. The target gas reacts with the

surface oxygen species, resulting into the concentration of surface oxygen ions decrease and

electron concentration increase. Thus the conductivity of the ZS-x increases.

ZnO is an n-type semiconductor in which oxygen vacancy acts as an electron donor to

provide electrons to the conduction band [33]. A larger quantity of V0 can induce more

adsorptions of oxygen without reducing the expansion level of depletion layer [20]. As a result,

ZX-4 has the best sensing performance among the ZS-x samples due to larger quantity of V0.

According to the literature [34], ethanol could be activated via dehydrogenation and

dehydration process, which can be described as follows:

C2H5OH(g)→ C2H4(g) + H2O(g) (acidicoxide) (2)

C2H5OH(g)→ CH3CHO(g) + H2(g) (basicoxide) (3)

Samaria loaded on ZnO plays as an additive to create more basic sites on the surface of

ZS-x samples. The products of dehydrogenation and dehydration process would oxidize to COx

and H2O. As the dehydrogenation product on samaria basic sites, the number of electrons

produced from oxidation of acetaldehyde is much higher that of ethylene according to the

following reactions:

C2H4 + 3O22−

(ad)→ 2CO2 + 2H2O + 6e− (4)

2CH3CHO + 5O22−

(ad)→ 4CO2 + 4H2O + 10e− (5)


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In this way, the ethanol sensing performance of ZS-x samples is much higher than that of

pure ZnO. In addition, smaller grain size, namely, a larger effective surface area of ZS-x samples

is also helpful for its better sensing performance than that of pure ZnO.

4. Conclusions

Sm2O3-doped ZnO nanocomposite has been successfully synthesized with a novel sol spray

combustion method. The XRD results show that all samples containing various amount of Sm2O3

have hexagonal wurtzite structure. UV-vis spectra show that the addition of Sm2O3 does not

affect the ZnO band gap and samaria may be dispersed on the surface of ZnO. Raman spectra

show that the ZS-4 possesses the largest frequency of E2 mode (high) and undergoes the largest

compressive stress, which indicates that the density of V0 in ZS-4 is the highest in all samples.

The gas sensing measurement indicates that the sensor fabricated by Sm2O3-ZnO nanocomposite

exhibits better gas sensing performance than that of pure ZnO nanoparticles. In addition, only a

proper amount of loading Sm2O3 can result into superior gas sensing performance. When

exceeding the optimal amount, further increase in the Sm2O3 concentration results in an adverse

effect. Results illustrates that Sm2O3 is a very promising additive for ZnO sensor. The facile

synthesis of Sm2O3-doped ZnO nanocomposite together with their superior sensing performance

provides a potentially new approach for the design and construction of ZnO-based sensor.

Acknowledgements

This work was supported by Scientific Research Fund of Huaqiao University

(No.10Y0195*), Fundamental Research Funds for the Central Universities (No. JB-ZR1138) and

The Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese

Scholars, State Education Ministry.


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Figure captions:

Figure 1 XRD patterns of the synthesized ZS-x samples.

Figure 2 SEM micrograph (A) and EDX spectrum (B) of ZS-4 sample.

Figure 3 UV-vis spectra (A) and Tauc plots (B) of the synthesized ZS-x samples.

Figure 4 Raman spectra of the synthesized ZS-x samples.

Figure 5 The responses of the ZS-x-based sensors to 100 ppm ethanol at various operating

temperatures.

Figure 6 The transient response of ZS-x-based sensors to 100 ppm ethanol at 340 °C.

Figure 7 The electrical resistance of ZS-x samples in air (Ra).

Figure 8 The responses of the ZS-x-based sensors towards different kinds of target gases at

340 °C.


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Figure 1 XRD patterns of the synthesized ZS-x samples.

Figure 2 SEM micrograph (A) and EDX spectrum (B) of ZS-4 sample.

(A)

(B)

20 30 40 50 60 70

(201)

(200)(112)

(103)(110)

(102)

(101)

(002)

ZS-2

ZS-4

ZS-6

ZS-8

ZS-0

Intensity (a. u.)

2θθθθ (degree)

(100)


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Figure 3 UV-vis spectra (A) and Tauc plots (B) of the synthesized ZS-x samples.

Figure 4 Raman spectra of the synthesized ZS-x samples.

300 400 500 600 700 800

Absorbance (a.u.)

Wavelength (nm)

ZS-0

ZS-2

ZS-4

ZS-6

ZS-8

(A)

2.0 2.5 3.0 3.5 4.0 4.5

0

(ahv)2(eV.cm-1)2

hv (eV)

ZS-0

ZS-2

ZS-4

ZS-6

ZS-8

(B)

400 425 450 475 500

Intensity (a. u.)

Raman shift (cm-1)

ZS-2

ZS-4

ZS-6

ZS-8

ZnO-NPs

E2


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Figure 5 The responses of the ZS-x-based sensors to 100 ppm ethanol at various operating

temperatures.

Figure 6 The transient response of ZS-x-based sensors to 100 ppm ethanol at 340 °C.

0 20 40 60 80 100 120

0

5

10

15

20

Response (Ra/Rg)

Time (s)

ZS-4

ZS-2

ZS-6

ZS-8

ZS-0

300 320 340 360 380 400 420

0

5

10

15

20

25

Response (Ra/Rg)

Operating temperature (OC)

ZS-2

ZS-4

ZS-6

ZS-8

ZS-0


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Figure 7 The electrical resistance of ZS-x samples in air (Ra).

Figure 8 The responses of the ZS-x-based sensors towards different kinds of target gases at

340 °C.

0

5

10

15

20

25

ZS-4

ZS-8ZS-6

ZS-2

Toluene

Ethanol

Methanol

Formaldehyde

ZS-0

Response (Ra/Rg)

300 320 340 360 380 400

0

4000

8000

12000

16000

T (0C)

ZS-0

ZS-2

ZS-4

ZS-6

ZS-8

Resistance (K

Ω

Ω

Ω

Ω )


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