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shell nanorods: Controlled-synthesis and low-temperature gas sensing properties
Sensors and Actuators B 155 (2011) 270–277
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
␣-MoO3 /TiO2 core/shell nanorods: Controlled-synthesis and low-temperature gas sensing properties Yu-Jin Chen a,∗ , Gang Xiao a , Tie-Shi Wang a , Fan Zhang a , Yang Ma a,b,c , Peng Gao b,∗ , Chun-Ling Zhu b , Endi Zhang c,∗ , Zhi Xu c , Qiu-hong Li c a b c
College of Science, Harbin Engineering University, Harbin 150001, China College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China Micro-Nano Technologies Research Center, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 24 September 2010 Received in revised form 16 December 2010 Accepted 19 December 2010 Available online 25 December 2010 Keywords: Heteronanostructures Sensor ␣-MoO3 TiO2 Ethanol sensing characteristics
a b s t r a c t Crystalline ␣-MoO3 /TiO2 core/shell nanorods are fabricated by a hydrothermal method and subsequent annealing processes under H2 /Ar flow and in the ambient atmosphere. The shell layer is composed of crystalline TiO2 particles with a diameter of 2–6 nm, and its thickness can be easily controlled in the range of 15–45 nm. The core/shell nanorods show enhanced sensing properties to ethanol vapor compared to bare ␣-MoO3 nanorods. The sensing mechanism is different from that of other one-dimensional metal oxide core/shell nanostructures due to very weak response of TiO2 nanoparticles to ethanol. The enhanced sensing properties can be explained by the change of type II heterojunction barrier formed at the interface between ␣-MoO3 and TiO2 in the different gas atmosphere. The present results demonstrate a novel sensing mechanism available for gas sensors with high performance. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Chemical sensors have played an important role in industrial, medical and domestic applications in detecting pollutant, toxic and combustible, and organic gases [1,2]. Now the chemical sensors are required not only to show high sensitivity and good selectivity, but also to detect a trace targeted gas at a relatively low temperature. One-dimensional (1D) metal oxide nanostructures with a high surface-to-volume ratio have attracted much attention because they have exhibited higher sensitivity and faster response and recovery [3–12]. However, for the sensors based on 1D nanostructures, the challenge to improve selectivity and reduce the operational temperature still exists. Nowadays, several methods such as catalyst functionalization, elemental doping, and heterostructure formation have been developed to enhance the selectivity and sensitivity and to decrease the working temperature of gas sensors. For catalyst functionalization, nanosized noble metal catalysts such as Pt, Pd, and Au are generally loaded on 1D nanostructures obtained previously [13–16]. After loading of the catalysts, the selectivity of sensors based on those
∗ Corresponding authors. Tel.: +86 451 82519754. E-mail addresses: [email protected] (Y.-J. Chen), [email protected] (P. Gao), endi [email protected] (E. Zhang).
sensing materials has been significantly enhanced [13]. However, it is supposed to an issue of uniform distribution of the catalysis in the whole system for high catalysis effect. For elemental doping, it is difficult to determine the location (on outer surface or in bulk) of the doped element, resulting in an unclear sensing mechanism. Recently, 1D hetero-nanostructure has attracted much attention for chemical sensor because the sensitivity and selectivity can be manipulated by the component phases [17–26]. The mechanism of the enhanced gas sensing properties is mainly divided into two basic categories. One is the sensing property tunable by the variety of p–n or n–n junction barrier in the different gas species [24]. For example, 1D SnO2 /CuO core/shell nanorods have exhibited extremely high sensitivity and selectivity to H2 S, which is attributed to the disappearance of p–n junction barrier as the hetero-nanostructures are exposed to H2 S species [24]. Another mechanism is based on a synergetic effect of different sensing materials in the hetero-nanostructures. Good sensing properties would be realized if the thickness of the outer shell material is less than or comparable to its Debye size [21–23]. Thus, 1D heteronanostructures are very promising materials for gas sensors with high performances. ␣-MoO3 and TiO2 are two kinds of important functional materials, and have practical applications in many areas. For example, ␣-MoO3 with double-layer planar structure can be used as gas sensors, cathodes of Li ion batteries, a photoelectric devices, etc.
0925-4005/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.12.034
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Fig. 1. Structural characterization of ␣-MoO3 nanorods. (a) XRD pattern, (b) SEM image, (c) magnified SEM image and (d) HRTEM image, inset: SAED pattern.
[27–31]. TiO2 nanostructures have been widely investigated for dye-sensitized solar cells, Li-ion batteries and photocatalysis, etc. [32–34]. Recently, Elder et al. fabricated TiO2 /MoO3 core/shell nanoparticles and found the decrease of photoabsorption energy of this system due to the electron transition from the valence band of TiO2 to the conduction band of MoO3 [35]. Thus, the nanocomposites have potential applications in photoluminescence, solar energy conversion and photocatalysis. However, the synthesis and the sensing properties about 1D ␣-MoO3 /TiO2 core/shell nanostructures have not been reported. In this work, crystalline ␣-MoO3 /TiO2 core/shell nanorods with various shell thickness were obtained for chemical gas sensors. The hetero-nanostructures exhibited enhanced ethanol sensing properties even at a low working temperature. The enhanced sensing mechanism is also discussed according to the theory of semiconductor heterojunctions and the catalytic nature of TiO2 . 2. Experimental ␣-MoO3 nanorods were first synthesized by a modified method reported by Fang et al. [36]. Simply, 7.2 g ␣-MoO3 powder was reacted with 55 ml of 30% aqueous H2 O2 and dissolved completely under stirring. 27 ml of concentrated nitric acid and 170 ml of distilled water were added to the solution above, respectively. The mixture was allowed to stand for 4 days at room temperature. 35 ml of the mixture was then transferred into a Teflon-lined stainless steel autoclave with a capacity of 50 ml for hydrothermal treatment at 170 ◦ C for 45 h. As the autoclave cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with distilled water and absolute ethanol, and dried in air. ␣-MoO3 /TiO2 core/shell nanorods were fabricated by a modified wet-chemical method [37–39]. Typically, 0.15 g of ␣-MoO3 nanorods was dispersed into 100 ml of distilled water under vigorously stirring. Then 25 ml of 0.2 mol/l Ti(SO4 )2 aqueous solution was dropped into the suspension over 1.5 h at 30 ± 5 ◦ C. The mixture was stirred for additional 3 h at that temperature, and then aged at room temperature for 2–3 h. The precipitates were sepa-
rated by centrifugation, washed with distilled water and absolute ethanol, and dried in air. After the process, the TiO2 in the products is amorphous. In order to obtain crystalline shells, the product above was annealed at 360 ◦ C for 6 h under a mixture of Ar/H2 flow, and then at 500 ◦ C for 2 h in the ambient atmosphere. The morphology and the size of the synthesized samples were characterized by scanning electron microscopy (SEM, JEOL-JSM6700F), and transmission electron microscope (TEM, JEOL 2010). The crystal structure of the sample was determined by X-ray diffraction (XRD, D/max 2550 V, Cu K␣ radiation). The fabrication process of the sensors based on the products has been described elsewhere [4]. Briefly, the ␣-MoO3 /TiO2 core/shell nanorods were dispersed in ethanol and a drop was spun on a ceramic tube between Pt electrodes to form a thin film. A resistance heater in the ceramic tube is used to control the working temperature of the sensors. The sensor response (S) to target gases is defined as S = Ra /Rg , where Ra is the sensor resistance in air, and Rg is the resistance in target–air mixed gas. 3. Results and discussion Fig. 1(a) shows XRD pattern of the as-synthesized ␣-MoO3 nanorods. Compared with the data in JCPDs No. 35-0609, all diffraction peaks in the pattern can be indexed to orthorhombic ␣-MoO3 . The SEM and magnified SEM images shown in Fig. 1(b) and (c), respectively, reveal that uniform ␣-MoO3 nanorods can be obtained. The average length and the diameter of the nanorods are about 12 m and 170 nm, respectively. Fig. 1(d) is a typical highresolution TEM (HRTEM) image of the nanorods. The (1 0 0) lattice fringes with the interplanar spacing of 0.393 nm are observed in the HRTEM image, implying that the nanorods have a single crystal nature. The selective-area electron diffraction (SAED) pattern in Fig. 1(d) demonstrates that the ␣-MoO3 nanorods grow preferentially along [0 0 1] direction. Fig. 2(a) shows XRD pattern of the as-synthesized ␣-MoO3 /TiO2 core/shell nanorods. Compared with the data in JCPDs No. 33-0664, all peaks in the pattern can be indexed to ␣-MoO3 . It should be
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* - TiO 2
Intensity (a.u.)
*
* (b)
**
*
(a)
10
20
30
40
50
60
70
2 Theta (degree) Fig. 2. (a) XRD pattern of ␣-MoO3 /TiO2 core/shell nanorods not treated in H2 and air and (b) XRD pattern of ␣-MoO3 /TiO2 core/shell nanorods treated in H2 and air.
noted that no peaks from other materials are presented in the pattern except the ones from ␣-MoO3 , implying that TiO2 is amorphous or not presented in the composites. However, the O, Mo and Ti peaks are observed in the energy dispersive spectroscopy (EDS) spectrum, as shown in Fig. 3(a). Furthermore, the average atomic ratio of Ti to Mo elements is about 1:1.1. Therefore, it can be determined that TiO2 in the composites is amorphous. Fig. 3(b) is the SEM image of the nanocomposites with an amorphous TiO2 shell. It can be seen that the nanocomposites are uniform and have very smooth surfaces. No impurities like particles are observed, suggesting that ␣-MoO3 nanorods are coated with uniformly amorphous TiO2 . This can also be proved by the TEM observations, as shown in Fig. 3(c). The nanocomposites have clear interfaces and thus exhibit a significant feature of a core/shell structure. The thickness of the outer shell is in range of 40–70 nm. The lattice fringes in the outer layer are not observed in the HRTEM image (not shown), which further confirms that the TiO2 shell is amorphous. It should be noted that the surface of most of the nanorods is uniformly coated with TiO2 layer. But, there are small parts of the nanorods whose ends are uncoated, as shown in the inset of Fig. 3(c). The crystalline TiO2 phase can be obtained after the core/shell nanorods are annealed under H2 /Ar flow and subsequently in the ambient atmosphere. Fig. 2(b) shows the XRD pattern of the core/shell nanorods after the annealing process. In the pattern, besides the diffraction peaks from ␣-MoO3 , the ones at 2 = 25.3, 38.5, 48.0, 53.9, 55.1◦ indicated by asterisks can be indexed to anatase TiO2 by comparison with the data in JCPDs No. 21-1272. The diffraction peaks at 2 = 25.3 and 38.5◦ are broadened and shift left compared to the as-synthesized nanocomposites owing to the transformation of TiO2 from amorphous phase into crystalline structure. XRD analysis reveals that the crystalline ␣-MoO3 /TiO2 core/shell nanorods can be obtained after the annealing processes described in Section 2. The morphology and the size of the crystalline core/shell nanorods were further characterized by SEM and TEM observations. Fig. 4(a) displays a typical SEM image of the crystalline ␣-MoO3 /TiO2 core/shell nanorods. No impurities like particles are observed in the SEM images, implying that the crystalline TiO2 structures are still uniformly coated on the surface of ␣-MoO3 nanorods. The average thickness of the TiO2 layer is about 45 nm, as shown in Fig. 4(b). The magnified TEM image (Fig. 4(c)) indicates the outer layer is composed of TiO2 nanoparticles with a diameter of 2–6 nm. Clear crystalline lattice fringes from those particles are observed in the HRTEM image, as shown in the inset of Fig. 4(b). It indicates the crystal nature of the TiO2 particles in the shell, which is agreement with the XRD analysis.
Fig. 3. Structural characterization of ␣-MoO3 /TiO2 core/shell nanorods not treated in H2 and air. (a) EDS pattern, (b) SEM image and (c) TEM image, and inset: TEM image of an individual nanorod with an uncoated end.
The XRD, SEM and TEM measurements above demonstrate that the crystalline ␣-MoO3 /TiO2 core/shell nanorods can be synthesized by the present method. Importantly, the thickness of TiO2 shell can be tuned by only varying the concentration of Ti(SO4 )2 aqueous solution. ␣-MoO3 /TiO2 core/shell nanorods with the shell thickness of about 40 nm can be obtained if the concentration of Ti(SO4 )2 aqueous solution is reduced to 0.1 mol/l, as shown in Fig. 5(a). After the annealing processes, although the morphologies of the core/shell nanorods have little change, the shell thickness decreases to 30 nm as shown in Fig. 5(b). Fig. 5(c) is a typical HRTEM image of the treated product. The lattice fringes from the outer layer can be clearly seen, indicating that the amorphous TiO2 can be also transformed into crystalline phase. In the image, the interfaces between ␣-MoO3 and TiO2 can be also observed, implying that the crystalline core/shell nanorods become dense compared to the
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Fig. 4. Structural characterization of ␣-MoO3 /TiO2 core/shell nanorods treated in H2 and air. (a) SEM image, (b) TEM image, inset: HRTEM image, and (c) magnified TEM image.
as-synthesized nanocomposites. ␣-MoO3 /TiO2 core/shell nanorods with smaller shell thickness will be obtained if the concentration of Ti(SO4 )2 aqueous solution is further reduced. Fig. 5(d) is a TEM image of the as-synthesized ␣-MoO3 /TiO2 core/shell nanorods as the concentration of Ti(SO4 )2 aqueous solution is decreased to 0.035 mol/l. It clearly shows that the thickness of the uniform TiO2
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shell is about 25 nm. After the annealing processes, the thickness of crystalline TiO2 shell decreases to 15 nm, as shown in Fig. 5(e) and (f). The results above demonstrate that crystalline ␣-MoO3 /TiO2 core/shell nanorods with shell thickness of about 45, 30 and 15 nm can be synthesized by the present method. For simplicity, those samples are named as MT-1, MT-2 and MT-3, respectively. For comparison, the sensors based on the bare ␣-MoO3 nanorods annealed under the same conditions as those of the core/shell nanorods, were also prepared. Fig. 6 shows the sensor response of those sensors to 200 ppm ethanol at the working temperature ranging from 100 to 350 ◦ C. For the bare ␣-MoO3 nanorods, the highest value of S occurs at 270 ◦ C. It slightly decreases at higher temperature, and dramatically decreases at lower temperature. When the working temperature is lower than 180 ◦ C, the bare nanorods have no response to 200 ppm ethanol. It shows that the optimum operating temperatures of the bare ␣-MoO3 nanorods is about 270 ◦ C. As for the hetero-nanostructures, they exhibit different sensing characteristics, which are significantly related to the thickness of TiO2 shell. The optimum operating temperature of MT-1 is higher than that of the bare ␣-MoO3 nanorods. Meanwhile, the values of S are lower than those of the pure ␣-MoO3 nanorods at all the tested temperatures. This reveals that larger shell thickness is disadvantageous to the improvement of ethanol sensing properties. However, the optimum operating temperatures of both MT-2 and MT-3 are decreased to 180 ◦ C. On one hand, the values of S of both samples are higher than those of the bare ␣-MoO3 nanorods at the working temperature lower than 220 ◦ C. On the other hand, they still have significant response to 200 ppm ethanol at the working temperature even lower 135 ◦ C. Thus, compared to the bare ␣-MoO3 , the core/shell nanorods with smaller thickness exhibit enhanced sensing properties including the decrease of the working temperature and the improvement of the sensor response. The enhanced sensing properties may be related to the following effects. (1) The enhanced sensing properties occurred after TiO2 coated on the surface of ␣-MoO3 nanorods. Thus, it is directly related to the nature of TiO2 nanostructures. The synergetic effect of different gas sensing materials is firstly considered because both ␣MoO3 and TiO2 are important sensing materials. The effect has been previously observed in other metal oxide core/shell nanostructures. For example, Fe2 O3 /SnO2 and Fe2 O3 /ZnO core/shell nanorods have shown higher sensitivities and lower working temperature than those of the bare Fe2 O3 nanorods [22,23]. However, the realization of the synergetic effect must meet the following points. Firstly, both core and shell materials should have strong response to the target gas. Secondly, the thickness of the shell material needs to be close to its Debye length (D ). In this case, oxygen molecules in air not only deplete completely electrons in bulk of the shell material, but also can further extract electrons in the core material, resulting in very high resistance of the nanocomposites in air. When the nanocomposites are exposed to ethanol vapor, the ethanol reacts with the adsorbed oxygen species, and then the extracted electrons of both core and shell materials are released back, leading to the significant decrease of the resistance of the nanocomposites. Consequently, the nanocomposites will exhibit enhanced sensing properties. The value of D can be calculated by the following equation, D = (εkT/q2 nc )1/2 , where ε is the static dielectric constant, k the Boltzmann’s constant, T an absolute temperature, q the electrical charge of the carrier, and nc the carrier concentration [40]. For metal oxides, the value of D is in the range of 3–30 nm depending on the categories and surface/bulk defects of sensing materials and the ambient temperature [40,41]. Thus the thickness of the shell is required to be less than several ten nanometers. In order to clarify if the ␣MoO3 /TiO2 core/shell nanorods follow the sensing mechanism, we prepared sensors based on TiO2 nanoparticles. Herein the TiO2
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Fig. 5. (a) TEM image of ␣-MoO3 /TiO2 core/shell nanorods obtained if the concentration of Ti(SO4 )2 aqueous solution is reduced to 0.1 mol/l, which is not annealed under H2 /Ar flow and subsequently in the ambient atmosphere, (b) and (c) HRTEM image of ␣-MoO3 /TiO2 core/shell nanorods obtained if the concentration of Ti(SO4 )2 aqueous solution is reduced to 0.1 mol/l, which is annealed under H2 /Ar flow and subsequently in the ambient atmosphere, (d) TEM image of ␣-MoO3 /TiO2 core/shell nanorods obtained as the concentration of Ti(SO4 )2 aqueous solution is 0.035 mol/l, which is not annealed under H2 /Ar flow and subsequently in the ambient atmosphere (e) TEM image and (f) HRTEM image of ␣-MoO3 /TiO2 core/shell nanorods obtained as the concentration of Ti(SO4 )2 aqueous solution is 0.035 mol/l, which is annealed under H2 /Ar flow and subsequently in the ambient atmosphere.
nanoparticles were prepared through the same processes of the preparation of the crystalline core/shell nanorods except that the ␣-MoO3 nanorods were not added. The measured results show that the TiO2 nanoparticles have no response even as they are exposed to 1000 ppm ethanol at all the tested temperatures. Therefore, the synergetic effect cannot well explain the enhanced sensitivity of ␣MoO3 /TiO2 core/shell nanorods. (2) The heterojunction presented at the interface between ␣-MoO3 and TiO2 , as shown in the TEM observations. Thus the change of heterojunction barrier at the different gas atmospheres may contribute to the enhanced sensing properties. The band gap and work function of anatase TiO2 are
3.2 and 4.2 eV [35,42], respectively; those ones of ␣-MoO3 are 2.75–2.95 and 4.33 eV [29], respectively. Accordingly, the electron transfer occurs from the conduction band of TiO2 to the conduction band of ␣-MoO3 , leading to the formation of a type II heterojunction at the interface. As the core/shell nanorods exposed to air, the barrier height (q˚) will further increase because the electron in the TiO2 bulk is trapped when TiO2 activate the dissociation of oxygen molecules into oxygen ions (Eq. (1)): TiO2
O2 + e− −→O−
(1)
Y.-J. Chen et al. / Sensors and Actuators B 155 (2011) 270–277
MoO3 nanorods MT-3 MT-2 MT-1
16
Sensor response (S)
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12
8
4
0 100
150
200
250
300
350
Temperature ( o C) Fig. 6. The sensor response dependent on the working temperature to 200 ppm ethanol for MT-1, MT-2, MT-3, and ␣-MoO3 nanorods.
According to semiconductor theory, the resistance (R) related to heterojunction barrier can be expressed by R ∝ B exp
q˚
(2)
kT
where B is a constant related to ambient temperature, ˚ heterojunction barrier, k the Boltzmann’s constant, T an absolute temperature. Therefore, the conductivity of the heterostructures in air is very low. Such increased resistance induced by the increased barrier height can be proved by the experiments that these heterostructures are exposed to the atmosphere with higher O2 concentration or other oxidizing gases such as NO. Thus we exposed the sensor based on MT-2 to 100 ppm NO vapor at 270 ◦ C. The experimental result showed that its resistance was about 10 times higher than that at the ambient atmosphere. −
C2 H5 OH + O → C2 H4 O + H2 O + e
−
(3)
When the heterostructures are exposed to ethanol, the reaction takes place between the adsorbed oxygen ions and the ethanol molecules (Eq. (3)). The released electrons will flow into the conduction band of TiO2 semiconductor, resulting in a decrease in the width and height of the barrier potential at the interfaces. In the case, the conductivity of the heterostructures will consequently be increased. Therefore, the ␣-MoO3 /TiO2 core/shell nanorods exhibit the enhanced sensing properties to ethanol. The sensing mechanism controlled by the change of the heterojunction barrier has also been found in other nanocomposites [43,44]. For example, Pt/MoO3 /SiC heterostructures showed high sensitivity to hydrogen based on the change of Schottky barrier [18]. (3) It is well known that TiO2 is a kind of effective catalyst, and it can photoelectrolyze water to produce H2 [34]. Therefore, the catalytic effect of TiO2 , like Au, Pt and Pd supported by metal oxides [13], may play an important role in the enhanced sensing properties. TiO2 nanoparticles here used as oxygen dissociation catalysts make more oxygen absorb on the ␣-MoO3 surface. The absorbed oxygen captures electrons from the bulk of ␣-MoO3 and become oxygen ions. This process increases the quantity of the absorbed oxygen, resulting in a greater and faster degree of electron depletion from ␣MoO3 nanorods, leading to the great changes in kinetics of surface reaction. It is also supported by the fact that TiO2 can catalytically activate the dissociation of oxygen molecules into oxygen ions at the temperature lower than 200 ◦ C [45]. Because of the dense TiO2 layer formed on the surface of ␣-MoO3 nanorods, the catalytic effect may play a relatively weak role in gas sensing properties of the core/shell nanorods though gas species can penetrate the dense layer through tiny spaces among the nanoparticles especially at a higher temperature. Such the possibility of gas diffusion can be
Fig. 7. The response of the crystalline ␣-MoO3 /TiO2 core/shell nanorods to ethanol at 180 ◦ C.
proved by the fact that the color of as-synthesized ␣-MoO3 /TiO2 core/shell nanorods has been changed from white into black under H2 /Ar flow at 270 ◦ C. However, the thermal electron transportation will play a main role in the resistance of the heterostructures at a high temperature, resulting in the negligible effect of the change of the barrier on the resistance (Eq. (2)). Consequently, the core/shell nanorods exhibited weaker response to ethanol than that of the bare ␣-MoO3 nanorods at a high temperature, as shown in Fig. 6. In addition, it should be noted that the core/shell nanorods have bad sensing characteristics if the thickness of TiO2 shell is larger than 45 nm. In this case, the thick TiO2 layer plays a dominant role in gas sensing properties of the core/shell nanorods. As described above, the TiO2 nanoparticles have very weak response to ethanol. Therefore, the thicker shell is disadvantageous to the improvement of gas sensing properties of the core/shell nanocomposites. The responses of MT-2 and MT-3 to ethanol with the different concentrations were further measured at the different working temperature. Fig. 7(a) and (b) shows the responses of MT-2 and MT3 under 100–500 ppm ethanol exposure at 180 ◦ C, respectively. The response time defines the time taken for the sensor to reach the saturation value after the core–shell nanorods are exposed to ethanol vapor, and the recovery time for the resistance recovery to 95% of the initial level after removal of ethanol vapor. It is found that both the response time and the recovery time of the core–shell structures are less than 40 s. Such fast response and recovery are in favor of the sensors continuously detecting the target gases. It can be also seen that both samples show very strong response to ethanol. The values of S of MT-2 to 10, 100, 200, 300 and 500 ppm ethanol are up to 4.8, 6.2, 11.0, 12.8 and 15.1, respectively. Those values are larger than or comparable to that of other 1D nanocomposites (Table 1). For example, the value of S is 3.7 for Pt/SnO2 core/shell nanorods as exposed to 10 ppm ethanol at 300 ◦ C, whereas it increases to 4.8 for
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Table 1 Comparison of the response of the different 1D nanocomposites to 10 ppm ethanol exposure. Materials
Temperature (◦ C)
Response
Reference
Pt/SnO2 Fe2 O3 /SnO2 Fe2 O3 /ZnO MT-2 MT-3
300 250 220 180 180
3.7 2.9 4.7 4.8 3.0
[16] [21] [22] This work This work
MT-2 at 180 ◦ C. In addition, the crystalline ␣-MoO3 /TiO2 core/shell nanorods still have strong response to ethanol at the working temperature lower than 135 ◦ C, as shown in Fig. 8. The values of S for MT-2 and MT-3 to 10 ppm ethanol are up to 3.8 and 5.8, respectively. The results above reveal that the crystalline ␣-MoO3 /TiO2 core/shell nanorods not only have strong response to ethanol, but also can work at a relatively low temperature. Gas sensors for practical applications are required not only to have strong sensor response, but also very good selectivity to the targeted molecules. Therefore, the response of the ␣-MoO3 /TiO2 core/shell nanorods to 1000 ppm of H2 , NH3 and CH4 were also measured at the working temperature of 180 ◦ C. No significant changes in the electric resistances of the nanocomposites were observed for three kinds of gases, indicating that the core/shell nanorods had very good selectivity to ethanol vapor. Strong sensor response, fast recovery, and good selectivity of ␣-MoO3 /TiO2 core/shell nanorods support their promising applications at the industrial level.
4 2 0
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300 ppm
200 ppm
100 ppm
6 10 ppm
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(a)
10
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15
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Time (s) Fig. 8. The response of the crystalline ␣-MoO3 /TiO2 core/shell nanorods to ethanol at 135 ◦ C. (a) MT-2 and (b) MT-3.
4. Conclusions In summary, the crystalline ␣-MoO3 /TiO2 core/shell nanorods with controlled shell thickness are prepared. The nanocomposites show enhanced sensing properties including strong response and low working temperature. The enhanced sensing properties can be explained by the change of heterojunction barrier as the core/shell nanorods are exposed to the different gas species. Our results implied that the core/shell nanostructures are good candidates for high-performance ethanol sensors. Acknowledgments The authors acknowledge the support from the National Natural Science Foundation of China (Grant Nos. 51072038, 50772025, 21001035, and 21003041), Outstanding Youth Foundation of Heilongjiang Province (Grant No. JC201008), Natural Science Foundation of Heilongjiang Province, China (Grant Nos. F200828 and E200839), Project supported by the Ministry of Science and Technology of China (Grant No. 2008DFR20420), the Fundamental Research Funds for the Central Universities (Grant Nos. HEUCFT1010 and HEUCF101016), ‘973’ National Key Basic Research Program of China (Grant No. 2007CB310500), and also Harbin Key Sci-tech Project (Grant No. 2010AA4BG004). References [1] S.B. Patil, P.P. Patil, M.A. More, Acetone vapour sensing characteristics of cobaltdoped SnO2 thin films, Sens. Actuators B 125 (2007) 126–130. [2] A.M. More, J.L. Gunjakar, C.D. Lokhande, Liquefied petroleum gas (LPG) sensor properties of interconnected web-like structured sprayed TiO2 films, Sens. Actuators B 129 (2008) 671–677. [3] C.H. Wang, X.F. Chu, M.M. Wu, Detection of H2 S down to ppb levels at room temperature using sensors based on ZnO nanorods, Sens. Actuators B 113 (2006) 320–323. [4] Y.J. Chen, L. Nie, X.Y. Xue, Y.G. Wang, T.H. Wang, Linear ethanol sensing of SnO2 nanorods with extremely high sensitivity, Appl. Phys. Lett. 88 (2006) 083105. [5] H.T. Wang, B.S. Kang, F. Ren, L.C. Tien, P.W. Sadik, D.P. Norton, S.J. Pearton, J. Lin, Hydrogen-selective sensing at room temperature with ZnO nanorods, Appl. Phys. Lett. 86 (2006) 243503. [6] Y.J. Chen, X.Y. Xue, Y.G. Wang, T.H. Wang, Synthesis and ethanol sensing characteristics of single crystalline SnO2 nanorods, Appl. Phys. Lett. 88 (2005) 233503. [7] Y.J. Chen, C.L. Zhu, G. Xiao, Ethanol sensing characteristics of ambient temperature sonochemically synthesized ZnO nanotubes, Sens. Actuators B 129 (2008) 639–642. [8] J. Chen, L. Xu, W.Y. Li, X.L. Gou, ␣-Fe2 O3 nanotubes in gas sensor and lithium-ion battery applications, Adv. Mater. 17 (2005) 582–586. [9] Y. Liu, M.L. Liu, Growth of aligned square-shaped SnO2 tube arrays, Adv. Funct. Mater. 15 (2005) 57–62. [10] Y.J. Chen, C.L. Zhu, G. Xiao, Reduced-temperature ethanol sensing characteristics of flower-like ZnO nanorods synthesized by a sonochemical method, Nanotechnology 17 (2006) 4537–4541. [11] A. Maiti, J.A. Rodriguez, M. Law, P. Kung, J.R. McKinney, P.D. Yang, SnO2 Nanoribbons as NO2 sensors: insights from first principles calculations, Nano Lett. 3 (2003) 1025–1028. [12] M. Law, H. Kind, F. Kim, B. Messer, P.D. Yang, Photochemical sensing of NO2 with SnO2 nanoribbon nanosensors at room temperature, Angew. Chem. Int. Ed. 41 (2002) 2405–2408. [13] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles, Nano Lett. 5 (2005) 667–673. [14] F.C. Huang, Y.Y. Chen, T.T. Wu, A room temperature surface acoustic wave hydrogen sensor with Pt coated ZnO nanorods, Nanotechnology 20 (2009) 065501. [15] M. Penza, C. Martucci, G. Cassano, NOx gas sensing characteristics of WO3 thin films activated by noble metals (Pd, Pt, Au) layers, Sens. Actuators B 50 (1998) 52–59. [16] X.Y. Xue, Z.H. Chen, C.H. Ma, L.L. Xing, Y.J. Chen, Y.G. Wang, T.H. Wang, Onestep synthesis and gas-sensing characteristics of uniformly loaded Pt@SnO2 nanorods, J. Phys. Chem. C 114 (2010) 3968–3972. [17] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B 140 (2009) 319–336. [18] N.V. Hieu, H.R. Kim, B.K. Ju, J.H. Lee, Enhanced performance of SnO2 nanowires ethanol sensor by functionalizing with La2 O3 , Sens. Actuators B 133 (2008) 228–234. [19] U.S. Choi, G. Sakai, N. Yamazoe, Sensing properties of Au-loaded SnO2 –Co3 O4 composites to CO and H2 , Sens. Actuators B 107 (2005) 397–401.
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Biographies Y.J. Chen has been a professor at Harbin Engineering University since 2007. He received his MS degree in 2003 from Harbin Engineering University and PhD degree in 2005 from Chinese Academy of Science. His research interests focus on the nanostructured materials for the nanodevice applications. G. Xiao has been a lecturer at Harbin Engineering University since 2003. He received his MS degree in 2004 from Harbin Engineering University. He is currently studying for PhD at Harbin Engineering University. His main research interest is in the development of nanostructured materials for their applications. T.S. Wang is currently studying for MS degree at Harbin Engineering University. His main research interest is in the development of nanostructured materials for their applications. F. Zhang is currently studying for MS degree at Harbin Engineering University. His main research interest is in the development of nanostructured materials for their applications. Y. Ma is currently studying for MS degree at Harbin Engineering University. His main research interest is in the development of nanostructured materials for their applications. P. Gao has been a professor at Harbin Engineering University since 2010. He received his PhD degree in 2006 from University of Science and Technology of China. His research interests focus on the nanostructured materials for the nanodevice applications. C.L. Zhu has been a lecturer at Harbin Engineering University since 2005. She received her MS degree in 2005 from Lanzhou University of Technology. She is currently studying for PhD at Harbin Engineering University. Her main research interest is in the development of nanostructured materials for their applications. E.D. Zhang has been a professor at Hunan University since 2010. He received his BA degree in 1988 from Tsinghua University. His research interests focus on the nanostructured materials for the nanodevice applications. Z. Xu has been a professor at Hunan University since 2010. He received his PhD degree in 1998 from Stanford University. His research interests focus on the nanostructured materials for the nanodevice applications. Q.H. Li has been a professor at Hunan University since 2008. He received his PhD degree in 2005 from Chinese Academy of Science. Her research interests focus on the nanostructured materials for the nanodevice applications.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
␣-MoO3 /TiO2 core/shell nanorods: Controlled-synthesis and low-temperature gas sensing properties Yu-Jin Chen a,∗ , Gang Xiao a , Tie-Shi Wang a , Fan Zhang a , Yang Ma a,b,c , Peng Gao b,∗ , Chun-Ling Zhu b , Endi Zhang c,∗ , Zhi Xu c , Qiu-hong Li c a b c
College of Science, Harbin Engineering University, Harbin 150001, China College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China Micro-Nano Technologies Research Center, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 24 September 2010 Received in revised form 16 December 2010 Accepted 19 December 2010 Available online 25 December 2010 Keywords: Heteronanostructures Sensor ␣-MoO3 TiO2 Ethanol sensing characteristics
a b s t r a c t Crystalline ␣-MoO3 /TiO2 core/shell nanorods are fabricated by a hydrothermal method and subsequent annealing processes under H2 /Ar flow and in the ambient atmosphere. The shell layer is composed of crystalline TiO2 particles with a diameter of 2–6 nm, and its thickness can be easily controlled in the range of 15–45 nm. The core/shell nanorods show enhanced sensing properties to ethanol vapor compared to bare ␣-MoO3 nanorods. The sensing mechanism is different from that of other one-dimensional metal oxide core/shell nanostructures due to very weak response of TiO2 nanoparticles to ethanol. The enhanced sensing properties can be explained by the change of type II heterojunction barrier formed at the interface between ␣-MoO3 and TiO2 in the different gas atmosphere. The present results demonstrate a novel sensing mechanism available for gas sensors with high performance. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Chemical sensors have played an important role in industrial, medical and domestic applications in detecting pollutant, toxic and combustible, and organic gases [1,2]. Now the chemical sensors are required not only to show high sensitivity and good selectivity, but also to detect a trace targeted gas at a relatively low temperature. One-dimensional (1D) metal oxide nanostructures with a high surface-to-volume ratio have attracted much attention because they have exhibited higher sensitivity and faster response and recovery [3–12]. However, for the sensors based on 1D nanostructures, the challenge to improve selectivity and reduce the operational temperature still exists. Nowadays, several methods such as catalyst functionalization, elemental doping, and heterostructure formation have been developed to enhance the selectivity and sensitivity and to decrease the working temperature of gas sensors. For catalyst functionalization, nanosized noble metal catalysts such as Pt, Pd, and Au are generally loaded on 1D nanostructures obtained previously [13–16]. After loading of the catalysts, the selectivity of sensors based on those
∗ Corresponding authors. Tel.: +86 451 82519754. E-mail addresses: [email protected] (Y.-J. Chen), [email protected] (P. Gao), endi [email protected] (E. Zhang).
sensing materials has been significantly enhanced [13]. However, it is supposed to an issue of uniform distribution of the catalysis in the whole system for high catalysis effect. For elemental doping, it is difficult to determine the location (on outer surface or in bulk) of the doped element, resulting in an unclear sensing mechanism. Recently, 1D hetero-nanostructure has attracted much attention for chemical sensor because the sensitivity and selectivity can be manipulated by the component phases [17–26]. The mechanism of the enhanced gas sensing properties is mainly divided into two basic categories. One is the sensing property tunable by the variety of p–n or n–n junction barrier in the different gas species [24]. For example, 1D SnO2 /CuO core/shell nanorods have exhibited extremely high sensitivity and selectivity to H2 S, which is attributed to the disappearance of p–n junction barrier as the hetero-nanostructures are exposed to H2 S species [24]. Another mechanism is based on a synergetic effect of different sensing materials in the hetero-nanostructures. Good sensing properties would be realized if the thickness of the outer shell material is less than or comparable to its Debye size [21–23]. Thus, 1D heteronanostructures are very promising materials for gas sensors with high performances. ␣-MoO3 and TiO2 are two kinds of important functional materials, and have practical applications in many areas. For example, ␣-MoO3 with double-layer planar structure can be used as gas sensors, cathodes of Li ion batteries, a photoelectric devices, etc.
0925-4005/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.12.034
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Fig. 1. Structural characterization of ␣-MoO3 nanorods. (a) XRD pattern, (b) SEM image, (c) magnified SEM image and (d) HRTEM image, inset: SAED pattern.
[27–31]. TiO2 nanostructures have been widely investigated for dye-sensitized solar cells, Li-ion batteries and photocatalysis, etc. [32–34]. Recently, Elder et al. fabricated TiO2 /MoO3 core/shell nanoparticles and found the decrease of photoabsorption energy of this system due to the electron transition from the valence band of TiO2 to the conduction band of MoO3 [35]. Thus, the nanocomposites have potential applications in photoluminescence, solar energy conversion and photocatalysis. However, the synthesis and the sensing properties about 1D ␣-MoO3 /TiO2 core/shell nanostructures have not been reported. In this work, crystalline ␣-MoO3 /TiO2 core/shell nanorods with various shell thickness were obtained for chemical gas sensors. The hetero-nanostructures exhibited enhanced ethanol sensing properties even at a low working temperature. The enhanced sensing mechanism is also discussed according to the theory of semiconductor heterojunctions and the catalytic nature of TiO2 . 2. Experimental ␣-MoO3 nanorods were first synthesized by a modified method reported by Fang et al. [36]. Simply, 7.2 g ␣-MoO3 powder was reacted with 55 ml of 30% aqueous H2 O2 and dissolved completely under stirring. 27 ml of concentrated nitric acid and 170 ml of distilled water were added to the solution above, respectively. The mixture was allowed to stand for 4 days at room temperature. 35 ml of the mixture was then transferred into a Teflon-lined stainless steel autoclave with a capacity of 50 ml for hydrothermal treatment at 170 ◦ C for 45 h. As the autoclave cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with distilled water and absolute ethanol, and dried in air. ␣-MoO3 /TiO2 core/shell nanorods were fabricated by a modified wet-chemical method [37–39]. Typically, 0.15 g of ␣-MoO3 nanorods was dispersed into 100 ml of distilled water under vigorously stirring. Then 25 ml of 0.2 mol/l Ti(SO4 )2 aqueous solution was dropped into the suspension over 1.5 h at 30 ± 5 ◦ C. The mixture was stirred for additional 3 h at that temperature, and then aged at room temperature for 2–3 h. The precipitates were sepa-
rated by centrifugation, washed with distilled water and absolute ethanol, and dried in air. After the process, the TiO2 in the products is amorphous. In order to obtain crystalline shells, the product above was annealed at 360 ◦ C for 6 h under a mixture of Ar/H2 flow, and then at 500 ◦ C for 2 h in the ambient atmosphere. The morphology and the size of the synthesized samples were characterized by scanning electron microscopy (SEM, JEOL-JSM6700F), and transmission electron microscope (TEM, JEOL 2010). The crystal structure of the sample was determined by X-ray diffraction (XRD, D/max 2550 V, Cu K␣ radiation). The fabrication process of the sensors based on the products has been described elsewhere [4]. Briefly, the ␣-MoO3 /TiO2 core/shell nanorods were dispersed in ethanol and a drop was spun on a ceramic tube between Pt electrodes to form a thin film. A resistance heater in the ceramic tube is used to control the working temperature of the sensors. The sensor response (S) to target gases is defined as S = Ra /Rg , where Ra is the sensor resistance in air, and Rg is the resistance in target–air mixed gas. 3. Results and discussion Fig. 1(a) shows XRD pattern of the as-synthesized ␣-MoO3 nanorods. Compared with the data in JCPDs No. 35-0609, all diffraction peaks in the pattern can be indexed to orthorhombic ␣-MoO3 . The SEM and magnified SEM images shown in Fig. 1(b) and (c), respectively, reveal that uniform ␣-MoO3 nanorods can be obtained. The average length and the diameter of the nanorods are about 12 m and 170 nm, respectively. Fig. 1(d) is a typical highresolution TEM (HRTEM) image of the nanorods. The (1 0 0) lattice fringes with the interplanar spacing of 0.393 nm are observed in the HRTEM image, implying that the nanorods have a single crystal nature. The selective-area electron diffraction (SAED) pattern in Fig. 1(d) demonstrates that the ␣-MoO3 nanorods grow preferentially along [0 0 1] direction. Fig. 2(a) shows XRD pattern of the as-synthesized ␣-MoO3 /TiO2 core/shell nanorods. Compared with the data in JCPDs No. 33-0664, all peaks in the pattern can be indexed to ␣-MoO3 . It should be
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* - TiO 2
Intensity (a.u.)
*
* (b)
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*
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2 Theta (degree) Fig. 2. (a) XRD pattern of ␣-MoO3 /TiO2 core/shell nanorods not treated in H2 and air and (b) XRD pattern of ␣-MoO3 /TiO2 core/shell nanorods treated in H2 and air.
noted that no peaks from other materials are presented in the pattern except the ones from ␣-MoO3 , implying that TiO2 is amorphous or not presented in the composites. However, the O, Mo and Ti peaks are observed in the energy dispersive spectroscopy (EDS) spectrum, as shown in Fig. 3(a). Furthermore, the average atomic ratio of Ti to Mo elements is about 1:1.1. Therefore, it can be determined that TiO2 in the composites is amorphous. Fig. 3(b) is the SEM image of the nanocomposites with an amorphous TiO2 shell. It can be seen that the nanocomposites are uniform and have very smooth surfaces. No impurities like particles are observed, suggesting that ␣-MoO3 nanorods are coated with uniformly amorphous TiO2 . This can also be proved by the TEM observations, as shown in Fig. 3(c). The nanocomposites have clear interfaces and thus exhibit a significant feature of a core/shell structure. The thickness of the outer shell is in range of 40–70 nm. The lattice fringes in the outer layer are not observed in the HRTEM image (not shown), which further confirms that the TiO2 shell is amorphous. It should be noted that the surface of most of the nanorods is uniformly coated with TiO2 layer. But, there are small parts of the nanorods whose ends are uncoated, as shown in the inset of Fig. 3(c). The crystalline TiO2 phase can be obtained after the core/shell nanorods are annealed under H2 /Ar flow and subsequently in the ambient atmosphere. Fig. 2(b) shows the XRD pattern of the core/shell nanorods after the annealing process. In the pattern, besides the diffraction peaks from ␣-MoO3 , the ones at 2 = 25.3, 38.5, 48.0, 53.9, 55.1◦ indicated by asterisks can be indexed to anatase TiO2 by comparison with the data in JCPDs No. 21-1272. The diffraction peaks at 2 = 25.3 and 38.5◦ are broadened and shift left compared to the as-synthesized nanocomposites owing to the transformation of TiO2 from amorphous phase into crystalline structure. XRD analysis reveals that the crystalline ␣-MoO3 /TiO2 core/shell nanorods can be obtained after the annealing processes described in Section 2. The morphology and the size of the crystalline core/shell nanorods were further characterized by SEM and TEM observations. Fig. 4(a) displays a typical SEM image of the crystalline ␣-MoO3 /TiO2 core/shell nanorods. No impurities like particles are observed in the SEM images, implying that the crystalline TiO2 structures are still uniformly coated on the surface of ␣-MoO3 nanorods. The average thickness of the TiO2 layer is about 45 nm, as shown in Fig. 4(b). The magnified TEM image (Fig. 4(c)) indicates the outer layer is composed of TiO2 nanoparticles with a diameter of 2–6 nm. Clear crystalline lattice fringes from those particles are observed in the HRTEM image, as shown in the inset of Fig. 4(b). It indicates the crystal nature of the TiO2 particles in the shell, which is agreement with the XRD analysis.
Fig. 3. Structural characterization of ␣-MoO3 /TiO2 core/shell nanorods not treated in H2 and air. (a) EDS pattern, (b) SEM image and (c) TEM image, and inset: TEM image of an individual nanorod with an uncoated end.
The XRD, SEM and TEM measurements above demonstrate that the crystalline ␣-MoO3 /TiO2 core/shell nanorods can be synthesized by the present method. Importantly, the thickness of TiO2 shell can be tuned by only varying the concentration of Ti(SO4 )2 aqueous solution. ␣-MoO3 /TiO2 core/shell nanorods with the shell thickness of about 40 nm can be obtained if the concentration of Ti(SO4 )2 aqueous solution is reduced to 0.1 mol/l, as shown in Fig. 5(a). After the annealing processes, although the morphologies of the core/shell nanorods have little change, the shell thickness decreases to 30 nm as shown in Fig. 5(b). Fig. 5(c) is a typical HRTEM image of the treated product. The lattice fringes from the outer layer can be clearly seen, indicating that the amorphous TiO2 can be also transformed into crystalline phase. In the image, the interfaces between ␣-MoO3 and TiO2 can be also observed, implying that the crystalline core/shell nanorods become dense compared to the
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Fig. 4. Structural characterization of ␣-MoO3 /TiO2 core/shell nanorods treated in H2 and air. (a) SEM image, (b) TEM image, inset: HRTEM image, and (c) magnified TEM image.
as-synthesized nanocomposites. ␣-MoO3 /TiO2 core/shell nanorods with smaller shell thickness will be obtained if the concentration of Ti(SO4 )2 aqueous solution is further reduced. Fig. 5(d) is a TEM image of the as-synthesized ␣-MoO3 /TiO2 core/shell nanorods as the concentration of Ti(SO4 )2 aqueous solution is decreased to 0.035 mol/l. It clearly shows that the thickness of the uniform TiO2
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shell is about 25 nm. After the annealing processes, the thickness of crystalline TiO2 shell decreases to 15 nm, as shown in Fig. 5(e) and (f). The results above demonstrate that crystalline ␣-MoO3 /TiO2 core/shell nanorods with shell thickness of about 45, 30 and 15 nm can be synthesized by the present method. For simplicity, those samples are named as MT-1, MT-2 and MT-3, respectively. For comparison, the sensors based on the bare ␣-MoO3 nanorods annealed under the same conditions as those of the core/shell nanorods, were also prepared. Fig. 6 shows the sensor response of those sensors to 200 ppm ethanol at the working temperature ranging from 100 to 350 ◦ C. For the bare ␣-MoO3 nanorods, the highest value of S occurs at 270 ◦ C. It slightly decreases at higher temperature, and dramatically decreases at lower temperature. When the working temperature is lower than 180 ◦ C, the bare nanorods have no response to 200 ppm ethanol. It shows that the optimum operating temperatures of the bare ␣-MoO3 nanorods is about 270 ◦ C. As for the hetero-nanostructures, they exhibit different sensing characteristics, which are significantly related to the thickness of TiO2 shell. The optimum operating temperature of MT-1 is higher than that of the bare ␣-MoO3 nanorods. Meanwhile, the values of S are lower than those of the pure ␣-MoO3 nanorods at all the tested temperatures. This reveals that larger shell thickness is disadvantageous to the improvement of ethanol sensing properties. However, the optimum operating temperatures of both MT-2 and MT-3 are decreased to 180 ◦ C. On one hand, the values of S of both samples are higher than those of the bare ␣-MoO3 nanorods at the working temperature lower than 220 ◦ C. On the other hand, they still have significant response to 200 ppm ethanol at the working temperature even lower 135 ◦ C. Thus, compared to the bare ␣-MoO3 , the core/shell nanorods with smaller thickness exhibit enhanced sensing properties including the decrease of the working temperature and the improvement of the sensor response. The enhanced sensing properties may be related to the following effects. (1) The enhanced sensing properties occurred after TiO2 coated on the surface of ␣-MoO3 nanorods. Thus, it is directly related to the nature of TiO2 nanostructures. The synergetic effect of different gas sensing materials is firstly considered because both ␣MoO3 and TiO2 are important sensing materials. The effect has been previously observed in other metal oxide core/shell nanostructures. For example, Fe2 O3 /SnO2 and Fe2 O3 /ZnO core/shell nanorods have shown higher sensitivities and lower working temperature than those of the bare Fe2 O3 nanorods [22,23]. However, the realization of the synergetic effect must meet the following points. Firstly, both core and shell materials should have strong response to the target gas. Secondly, the thickness of the shell material needs to be close to its Debye length (D ). In this case, oxygen molecules in air not only deplete completely electrons in bulk of the shell material, but also can further extract electrons in the core material, resulting in very high resistance of the nanocomposites in air. When the nanocomposites are exposed to ethanol vapor, the ethanol reacts with the adsorbed oxygen species, and then the extracted electrons of both core and shell materials are released back, leading to the significant decrease of the resistance of the nanocomposites. Consequently, the nanocomposites will exhibit enhanced sensing properties. The value of D can be calculated by the following equation, D = (εkT/q2 nc )1/2 , where ε is the static dielectric constant, k the Boltzmann’s constant, T an absolute temperature, q the electrical charge of the carrier, and nc the carrier concentration [40]. For metal oxides, the value of D is in the range of 3–30 nm depending on the categories and surface/bulk defects of sensing materials and the ambient temperature [40,41]. Thus the thickness of the shell is required to be less than several ten nanometers. In order to clarify if the ␣MoO3 /TiO2 core/shell nanorods follow the sensing mechanism, we prepared sensors based on TiO2 nanoparticles. Herein the TiO2
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Fig. 5. (a) TEM image of ␣-MoO3 /TiO2 core/shell nanorods obtained if the concentration of Ti(SO4 )2 aqueous solution is reduced to 0.1 mol/l, which is not annealed under H2 /Ar flow and subsequently in the ambient atmosphere, (b) and (c) HRTEM image of ␣-MoO3 /TiO2 core/shell nanorods obtained if the concentration of Ti(SO4 )2 aqueous solution is reduced to 0.1 mol/l, which is annealed under H2 /Ar flow and subsequently in the ambient atmosphere, (d) TEM image of ␣-MoO3 /TiO2 core/shell nanorods obtained as the concentration of Ti(SO4 )2 aqueous solution is 0.035 mol/l, which is not annealed under H2 /Ar flow and subsequently in the ambient atmosphere (e) TEM image and (f) HRTEM image of ␣-MoO3 /TiO2 core/shell nanorods obtained as the concentration of Ti(SO4 )2 aqueous solution is 0.035 mol/l, which is annealed under H2 /Ar flow and subsequently in the ambient atmosphere.
nanoparticles were prepared through the same processes of the preparation of the crystalline core/shell nanorods except that the ␣-MoO3 nanorods were not added. The measured results show that the TiO2 nanoparticles have no response even as they are exposed to 1000 ppm ethanol at all the tested temperatures. Therefore, the synergetic effect cannot well explain the enhanced sensitivity of ␣MoO3 /TiO2 core/shell nanorods. (2) The heterojunction presented at the interface between ␣-MoO3 and TiO2 , as shown in the TEM observations. Thus the change of heterojunction barrier at the different gas atmospheres may contribute to the enhanced sensing properties. The band gap and work function of anatase TiO2 are
3.2 and 4.2 eV [35,42], respectively; those ones of ␣-MoO3 are 2.75–2.95 and 4.33 eV [29], respectively. Accordingly, the electron transfer occurs from the conduction band of TiO2 to the conduction band of ␣-MoO3 , leading to the formation of a type II heterojunction at the interface. As the core/shell nanorods exposed to air, the barrier height (q˚) will further increase because the electron in the TiO2 bulk is trapped when TiO2 activate the dissociation of oxygen molecules into oxygen ions (Eq. (1)): TiO2
O2 + e− −→O−
(1)
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MoO3 nanorods MT-3 MT-2 MT-1
16
Sensor response (S)
275
12
8
4
0 100
150
200
250
300
350
Temperature ( o C) Fig. 6. The sensor response dependent on the working temperature to 200 ppm ethanol for MT-1, MT-2, MT-3, and ␣-MoO3 nanorods.
According to semiconductor theory, the resistance (R) related to heterojunction barrier can be expressed by R ∝ B exp
q˚
(2)
kT
where B is a constant related to ambient temperature, ˚ heterojunction barrier, k the Boltzmann’s constant, T an absolute temperature. Therefore, the conductivity of the heterostructures in air is very low. Such increased resistance induced by the increased barrier height can be proved by the experiments that these heterostructures are exposed to the atmosphere with higher O2 concentration or other oxidizing gases such as NO. Thus we exposed the sensor based on MT-2 to 100 ppm NO vapor at 270 ◦ C. The experimental result showed that its resistance was about 10 times higher than that at the ambient atmosphere. −
C2 H5 OH + O → C2 H4 O + H2 O + e
−
(3)
When the heterostructures are exposed to ethanol, the reaction takes place between the adsorbed oxygen ions and the ethanol molecules (Eq. (3)). The released electrons will flow into the conduction band of TiO2 semiconductor, resulting in a decrease in the width and height of the barrier potential at the interfaces. In the case, the conductivity of the heterostructures will consequently be increased. Therefore, the ␣-MoO3 /TiO2 core/shell nanorods exhibit the enhanced sensing properties to ethanol. The sensing mechanism controlled by the change of the heterojunction barrier has also been found in other nanocomposites [43,44]. For example, Pt/MoO3 /SiC heterostructures showed high sensitivity to hydrogen based on the change of Schottky barrier [18]. (3) It is well known that TiO2 is a kind of effective catalyst, and it can photoelectrolyze water to produce H2 [34]. Therefore, the catalytic effect of TiO2 , like Au, Pt and Pd supported by metal oxides [13], may play an important role in the enhanced sensing properties. TiO2 nanoparticles here used as oxygen dissociation catalysts make more oxygen absorb on the ␣-MoO3 surface. The absorbed oxygen captures electrons from the bulk of ␣-MoO3 and become oxygen ions. This process increases the quantity of the absorbed oxygen, resulting in a greater and faster degree of electron depletion from ␣MoO3 nanorods, leading to the great changes in kinetics of surface reaction. It is also supported by the fact that TiO2 can catalytically activate the dissociation of oxygen molecules into oxygen ions at the temperature lower than 200 ◦ C [45]. Because of the dense TiO2 layer formed on the surface of ␣-MoO3 nanorods, the catalytic effect may play a relatively weak role in gas sensing properties of the core/shell nanorods though gas species can penetrate the dense layer through tiny spaces among the nanoparticles especially at a higher temperature. Such the possibility of gas diffusion can be
Fig. 7. The response of the crystalline ␣-MoO3 /TiO2 core/shell nanorods to ethanol at 180 ◦ C.
proved by the fact that the color of as-synthesized ␣-MoO3 /TiO2 core/shell nanorods has been changed from white into black under H2 /Ar flow at 270 ◦ C. However, the thermal electron transportation will play a main role in the resistance of the heterostructures at a high temperature, resulting in the negligible effect of the change of the barrier on the resistance (Eq. (2)). Consequently, the core/shell nanorods exhibited weaker response to ethanol than that of the bare ␣-MoO3 nanorods at a high temperature, as shown in Fig. 6. In addition, it should be noted that the core/shell nanorods have bad sensing characteristics if the thickness of TiO2 shell is larger than 45 nm. In this case, the thick TiO2 layer plays a dominant role in gas sensing properties of the core/shell nanorods. As described above, the TiO2 nanoparticles have very weak response to ethanol. Therefore, the thicker shell is disadvantageous to the improvement of gas sensing properties of the core/shell nanocomposites. The responses of MT-2 and MT-3 to ethanol with the different concentrations were further measured at the different working temperature. Fig. 7(a) and (b) shows the responses of MT-2 and MT3 under 100–500 ppm ethanol exposure at 180 ◦ C, respectively. The response time defines the time taken for the sensor to reach the saturation value after the core–shell nanorods are exposed to ethanol vapor, and the recovery time for the resistance recovery to 95% of the initial level after removal of ethanol vapor. It is found that both the response time and the recovery time of the core–shell structures are less than 40 s. Such fast response and recovery are in favor of the sensors continuously detecting the target gases. It can be also seen that both samples show very strong response to ethanol. The values of S of MT-2 to 10, 100, 200, 300 and 500 ppm ethanol are up to 4.8, 6.2, 11.0, 12.8 and 15.1, respectively. Those values are larger than or comparable to that of other 1D nanocomposites (Table 1). For example, the value of S is 3.7 for Pt/SnO2 core/shell nanorods as exposed to 10 ppm ethanol at 300 ◦ C, whereas it increases to 4.8 for
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Table 1 Comparison of the response of the different 1D nanocomposites to 10 ppm ethanol exposure. Materials
Temperature (◦ C)
Response
Reference
Pt/SnO2 Fe2 O3 /SnO2 Fe2 O3 /ZnO MT-2 MT-3
300 250 220 180 180
3.7 2.9 4.7 4.8 3.0
[16] [21] [22] This work This work
MT-2 at 180 ◦ C. In addition, the crystalline ␣-MoO3 /TiO2 core/shell nanorods still have strong response to ethanol at the working temperature lower than 135 ◦ C, as shown in Fig. 8. The values of S for MT-2 and MT-3 to 10 ppm ethanol are up to 3.8 and 5.8, respectively. The results above reveal that the crystalline ␣-MoO3 /TiO2 core/shell nanorods not only have strong response to ethanol, but also can work at a relatively low temperature. Gas sensors for practical applications are required not only to have strong sensor response, but also very good selectivity to the targeted molecules. Therefore, the response of the ␣-MoO3 /TiO2 core/shell nanorods to 1000 ppm of H2 , NH3 and CH4 were also measured at the working temperature of 180 ◦ C. No significant changes in the electric resistances of the nanocomposites were observed for three kinds of gases, indicating that the core/shell nanorods had very good selectivity to ethanol vapor. Strong sensor response, fast recovery, and good selectivity of ␣-MoO3 /TiO2 core/shell nanorods support their promising applications at the industrial level.
4 2 0
100
200
300
300 ppm
200 ppm
100 ppm
6 10 ppm
Sensor response (S)
8
500 ppm
(a)
10
400
500
600
Time (s)
100 ppm
12 9 6
200 ppm
15
500 ppm
300 ppm
(b)
18
10 ppm
Sensor Response (S)
21
3 0 0
100
200
300
400
Time (s) Fig. 8. The response of the crystalline ␣-MoO3 /TiO2 core/shell nanorods to ethanol at 135 ◦ C. (a) MT-2 and (b) MT-3.
4. Conclusions In summary, the crystalline ␣-MoO3 /TiO2 core/shell nanorods with controlled shell thickness are prepared. The nanocomposites show enhanced sensing properties including strong response and low working temperature. The enhanced sensing properties can be explained by the change of heterojunction barrier as the core/shell nanorods are exposed to the different gas species. Our results implied that the core/shell nanostructures are good candidates for high-performance ethanol sensors. Acknowledgments The authors acknowledge the support from the National Natural Science Foundation of China (Grant Nos. 51072038, 50772025, 21001035, and 21003041), Outstanding Youth Foundation of Heilongjiang Province (Grant No. JC201008), Natural Science Foundation of Heilongjiang Province, China (Grant Nos. F200828 and E200839), Project supported by the Ministry of Science and Technology of China (Grant No. 2008DFR20420), the Fundamental Research Funds for the Central Universities (Grant Nos. HEUCFT1010 and HEUCF101016), ‘973’ National Key Basic Research Program of China (Grant No. 2007CB310500), and also Harbin Key Sci-tech Project (Grant No. 2010AA4BG004). References [1] S.B. Patil, P.P. Patil, M.A. More, Acetone vapour sensing characteristics of cobaltdoped SnO2 thin films, Sens. Actuators B 125 (2007) 126–130. [2] A.M. More, J.L. Gunjakar, C.D. Lokhande, Liquefied petroleum gas (LPG) sensor properties of interconnected web-like structured sprayed TiO2 films, Sens. Actuators B 129 (2008) 671–677. [3] C.H. Wang, X.F. Chu, M.M. Wu, Detection of H2 S down to ppb levels at room temperature using sensors based on ZnO nanorods, Sens. Actuators B 113 (2006) 320–323. [4] Y.J. Chen, L. Nie, X.Y. Xue, Y.G. Wang, T.H. Wang, Linear ethanol sensing of SnO2 nanorods with extremely high sensitivity, Appl. Phys. Lett. 88 (2006) 083105. [5] H.T. Wang, B.S. Kang, F. Ren, L.C. Tien, P.W. Sadik, D.P. Norton, S.J. Pearton, J. Lin, Hydrogen-selective sensing at room temperature with ZnO nanorods, Appl. Phys. Lett. 86 (2006) 243503. [6] Y.J. Chen, X.Y. Xue, Y.G. Wang, T.H. Wang, Synthesis and ethanol sensing characteristics of single crystalline SnO2 nanorods, Appl. Phys. Lett. 88 (2005) 233503. [7] Y.J. Chen, C.L. Zhu, G. Xiao, Ethanol sensing characteristics of ambient temperature sonochemically synthesized ZnO nanotubes, Sens. Actuators B 129 (2008) 639–642. [8] J. Chen, L. Xu, W.Y. Li, X.L. Gou, ␣-Fe2 O3 nanotubes in gas sensor and lithium-ion battery applications, Adv. Mater. 17 (2005) 582–586. [9] Y. Liu, M.L. Liu, Growth of aligned square-shaped SnO2 tube arrays, Adv. Funct. Mater. 15 (2005) 57–62. [10] Y.J. Chen, C.L. Zhu, G. Xiao, Reduced-temperature ethanol sensing characteristics of flower-like ZnO nanorods synthesized by a sonochemical method, Nanotechnology 17 (2006) 4537–4541. [11] A. Maiti, J.A. Rodriguez, M. Law, P. Kung, J.R. McKinney, P.D. Yang, SnO2 Nanoribbons as NO2 sensors: insights from first principles calculations, Nano Lett. 3 (2003) 1025–1028. [12] M. Law, H. Kind, F. Kim, B. Messer, P.D. Yang, Photochemical sensing of NO2 with SnO2 nanoribbon nanosensors at room temperature, Angew. Chem. Int. Ed. 41 (2002) 2405–2408. [13] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles, Nano Lett. 5 (2005) 667–673. [14] F.C. Huang, Y.Y. Chen, T.T. Wu, A room temperature surface acoustic wave hydrogen sensor with Pt coated ZnO nanorods, Nanotechnology 20 (2009) 065501. [15] M. Penza, C. Martucci, G. Cassano, NOx gas sensing characteristics of WO3 thin films activated by noble metals (Pd, Pt, Au) layers, Sens. Actuators B 50 (1998) 52–59. [16] X.Y. Xue, Z.H. Chen, C.H. Ma, L.L. Xing, Y.J. Chen, Y.G. Wang, T.H. Wang, Onestep synthesis and gas-sensing characteristics of uniformly loaded Pt@SnO2 nanorods, J. Phys. Chem. C 114 (2010) 3968–3972. [17] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B 140 (2009) 319–336. [18] N.V. Hieu, H.R. Kim, B.K. Ju, J.H. Lee, Enhanced performance of SnO2 nanowires ethanol sensor by functionalizing with La2 O3 , Sens. Actuators B 133 (2008) 228–234. [19] U.S. Choi, G. Sakai, N. Yamazoe, Sensing properties of Au-loaded SnO2 –Co3 O4 composites to CO and H2 , Sens. Actuators B 107 (2005) 397–401.
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Biographies Y.J. Chen has been a professor at Harbin Engineering University since 2007. He received his MS degree in 2003 from Harbin Engineering University and PhD degree in 2005 from Chinese Academy of Science. His research interests focus on the nanostructured materials for the nanodevice applications. G. Xiao has been a lecturer at Harbin Engineering University since 2003. He received his MS degree in 2004 from Harbin Engineering University. He is currently studying for PhD at Harbin Engineering University. His main research interest is in the development of nanostructured materials for their applications. T.S. Wang is currently studying for MS degree at Harbin Engineering University. His main research interest is in the development of nanostructured materials for their applications. F. Zhang is currently studying for MS degree at Harbin Engineering University. His main research interest is in the development of nanostructured materials for their applications. Y. Ma is currently studying for MS degree at Harbin Engineering University. His main research interest is in the development of nanostructured materials for their applications. P. Gao has been a professor at Harbin Engineering University since 2010. He received his PhD degree in 2006 from University of Science and Technology of China. His research interests focus on the nanostructured materials for the nanodevice applications. C.L. Zhu has been a lecturer at Harbin Engineering University since 2005. She received her MS degree in 2005 from Lanzhou University of Technology. She is currently studying for PhD at Harbin Engineering University. Her main research interest is in the development of nanostructured materials for their applications. E.D. Zhang has been a professor at Hunan University since 2010. He received his BA degree in 1988 from Tsinghua University. His research interests focus on the nanostructured materials for the nanodevice applications. Z. Xu has been a professor at Hunan University since 2010. He received his PhD degree in 1998 from Stanford University. His research interests focus on the nanostructured materials for the nanodevice applications. Q.H. Li has been a professor at Hunan University since 2008. He received his PhD degree in 2005 from Chinese Academy of Science. Her research interests focus on the nanostructured materials for the nanodevice applications.