Synthesis of metal and metal oxide nanostructures and their application for gas sensing

Materials Chemistry and Physics 127 (2011) 143–150

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Synthesis of metal and metal oxide nanostructures and their application for gas sensing N.M. Shaalan a,b , T. Yamazaki a,∗ , T. Kikuta a a b

Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan Physics Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt

a r t i c l e

i n f o

Article history: Received 11 August 2010 Received in revised form 13 December 2010 Accepted 19 January 2011 Keywords: Metal oxide nanostructure Nano-additives Thermal evaporation method Gas sensing

a b s t r a c t A method has been developed to synthesize metal and metal oxide nanostructures in high yields on the surface of SiO2 /Si substrate. In this method, starting materials in a covered alumina crucible are thermally evaporated under a high vacuum or a low pressure of ambient air. Spherical gold nanoparticles with a size of 15 nm and nanowires with a diameter of 70 nm were synthesized. SnO2 rough microwires, smooth nanowires, and nanoknives were synthesized by using Sn granules, SnO powder, and SnO2 powder as source materials, respectively. The microwires showed a quadrangular cross section and a length of several microns, while the nanowires showed a circular cross section and approximately the same length. The effects of source temperature and deposition time on nanostructure growth were studied. X-ray diffraction patterns suggested that the as-synthesized products consisted of crystalline nanostructure. Nanocomposite gas sensors on the base of noble metal and metal oxide were fabricated. These SnO2 nanowire gas sensors showed a reversible response to dilute NO2 gas at operating temperatures ranging between room temperature and 300 ◦ C even at high concentrations. The results demonstrated that gold doping improved the sensor response. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Miniaturization to the nanoscale has led to new functions in electronic devices such as sensors, solar cells, batteries and fuel cells. Recently, one-dimensional (1D) semiconductor nanostructures have become the focus of intensive research because of their unique application in the fabrication of electronic, optoelectronic, and sensor devices at the nanometer scale. They possess novel properties intrinsically associated with low dimensionality and size confinement, making the “bottom-up” construction of nanodevices possible [1–3]. Integration of nanostructures into devices is important and requires new creative methods for nanomaterial fabrication, stabilization and processing. So far, many kinds of 1D semiconductor nanomaterials, including single element [4–6] and compound semiconductors [7–9], have been successfully synthesized through wide variety of methods, and detailed research information on these 1D nanostructures can be readily found in pertinent literatures [10–13]. Among these nanostructures, SnO2 is an n-type semiconductor, which has been extensively studied for various applications, including gas sensors [14,15], catalyst support [16], transparent conducting electrodes [17] and lithium

∗ Corresponding author. E-mail addresses: [email protected] (N.M. Shaalan), [email protected] (T. Yamazaki). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.01.048

ion battery anode materials [18,19]. Studies have proven that the properties and performance of SnO2 -based devices can be dramatically influenced by structure features. Therefore, much attention has been paid to the synthesis of SnO2 nanostructure like nanowires, nanotubes, nanoribbons, nanobelts, nanorods, etc. Surface modification is one of the most effective methods used to improve the gas-sensing properties of metal oxides. It has been established that clusters of either noble metals or transition metal oxides can be used to create such gas sensors [20–22]. The use of noble metal modifiers such as Pd, Pt and Au have resulted in a sufficient growth in sensitivity and decrease in operating temperatures [20,23]. A recent study has shown that nanocomposites on the base of noble metals could also be essential for optimization of metal oxide properties [24]. At present, nanocomposites on the base of noble metals and metal oxides are being considered as prospective materials for applications in catalysis, electronics, fuel cells, and gas sensors [25,26]. In the present study, we introduce a synthesis of metal and metal oxide nanostructures under atmospheric gases (lab environment). Gold and tin dioxide nanostructures, including nanoparticles (NPs), nanowires (NWs), rough micowires (MWs) and nanoknives (NKns) were synthesized. The experiment was carried out under high vacuum (8 × 10−5 Pa) to fabricate metal nanostructures and under low pressures of ambient air (60–133 Pa) to fabricate metal oxide nanostructures. The effects of source temperature and deposition time on shape of nanostructure were studied. The influence of

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Fig. 1. Heating unit made of two crucibles and a heating element.

various source materials on SnO2 nanostructures was also investigated. Since the SnO2 nanowire sensors are superior for gas sensors, and the surface modification is one of the most effective methods used to improve the gas-sensing properties of metal oxides, we examined the influence of nanocomposites using a noble metal, like Au nanoparticles (AuNPs) and Au nanowires-nanoparticles (AuNWs–NPs), and metal oxide (SnO2 nanowires) on gas sensing properties. 2. Experimental details 2.1. Synthesis of Au nanostructures The experiments were carried out in a conventional vacuum evaporation system made of stainless steel work-chamber of about 210 mm in diameter and 300 mm in height. The deposition was performed in a vertically set heating furnace that consisted of crucibles and a heating element, as schematically outlined in Fig. 1. For synthesis of metal nanostructures, pieces of gold metal (with purity higher than 99.99%) were placed in the crucible, and then the crucible was covered by SiO2 /Si(1 0 0) substrate. Firstly, the chamber was evacuated to high vacuum (8 × 10−5 Pa) and then kept at this pressure for the entire experiment, i.e., from the beginning of crucible heating to the cooling of the sample. The crucible was heated from room temperature (RT) to 1140 ◦ C in about 2.5 min. The substrate was maintained at 300 ◦ C when the crucible was maintained at 1140 ◦ C for the reaction time and changed according the source temperature, see Table 1. After heating for 30 min, the crucible was cooled naturally to room temperature, resulting in a substrate surface coated with a thick

Fig. 2. SEM data of Au nanostructures: (a) 1140, (b) 1190, (c) 1240 and (d) 1340 ◦ C. (e) XRD patterns of the as-synthesized Au nanostructures.

layer-like powder of gold deposited with temperature-dependent thickness. The experiment was repeated for source temperatures of 1190, 1240, and 1340 ◦ C, see Table 1. 2.2. Synthesis of SnO2 nanostructures Starting material of Sn, SnO or SnO2 was placed in the crucible covered by SiO2 /Si substrate, and then the chamber was evacuated using a rotary vacuum pump. Then, the crucible was heated from RT to about 800 ◦ C in 2 or 3 min and maintained at this

Table 1 Experimental conditions, morphology and size of Au and SnO2 nanostructures. Source

Au

Nanomaterials

Au

P (Pa)

W (mg)

8 × 10−5

T1 (◦ C)

Ts (◦ C)

t (min)

Morphology

Size/diameter (nm)

1140 1190 1240 1340

300 330 370 417

30

NPs NPs NPs/NWs NPs/NWs

400

30 60 90

MWs

402 395

90

NWs NKns

15 90 70/70 100/70-100 1000 1200 2000 50 –

15 133

Sn SnO2 SnO SnO2

800 86 60

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Fig. 3. SEM data of SnO2 rough microwires: (a) 30, (b) 60 and (c) 90 min. (d) EDS data recorded from sample (a) indicates the existence of Sn and O.

temperature during the reaction for 90 min under a total pressure of 60–133 Pa of ambient air. The substrate was maintained at temperature depended on the source temperature for the reaction time. After the crucible was cooled to room temperature, a thick layer of white products was obtained on the substrate. For Sn granules, the evaporation was carried out systematically for 30, 60, and 90 min at 133 Pa. For SnO and SnO2 powders, the evaporation was carried out for 90 min at 86 and 60 Pa, respectively, see Table 1.

2.3. Gas sensor fabrication A pair of interdigitated Pt electrodes (15 fingers) with a thickness of 100 nm and a gap length of 120 ␮m was fabricated on an SiO2 /Si substrate. A few drops of ethanol suspended with SnO2 nanowires were deposited on the electrode. The weight of SnO2 nanowires dispersed on the substrate was 540 ␮g and the area of the sensing element was 7 × 10 mm2 . For doped sensors, AuNPs or AuNWs–NPs (40 ␮g) were mixed with SnO2 nanowires (500 ␮g) in 300 ␮L of ethanol. Undoped (S1), 8 wt.% AuNPs (S2) and 8 wt.% AuNWs–NPs–doped (S3) SnO2 nanowire gas sensors were fabricated. In order to investigate NO2 gas sensing properties, the sensors were placed in a quartz tube, which was inserted in an electric furnace. The operating temperature was varied from RT up to 300 ◦ C. The heating rate was 2 ◦ C min−1 during the change from an operating temperature to the next one. Dry synthetic air, mixed with different concentrations of NO2 gas, was passed at a rate of 200 ml min−1 through the quartz tube. The sensor sensitivity (S) was estimated as the ratio of electrical resistance, Rg /Ra , where Ra was the electrical resistance before the introduction of NO2 gas, and Rg was the maximum electrical resistance after the introduction of NO2 gas. The morphology, crystal structure, and components of undoped and gold-doped SnO2 nanowires were determined by X-ray diffraction (XRD) (Shimadzu XRD-6100) with the Cu K␣ radiation and field emission scanning electron microscopy (FESEM) (JEOL JSM-6700F) with energy dispersive X-ray spectroscopy (EDS).

3. Results and discussion Gold nanostructures obtained at various source temperatures were observed using FESEM. The high-magnification SEM images shown in Fig. 2 revealed that the products were significantly affected by the source temperature. At 1140 ◦ C, spherical nanoparticles with sizes averaging about 15 nm were formed (Fig. 1a). The size of Au particles increased up to 90 nm when the source temperature was increased to 1190 ◦ C (Fig. 2b). Gold formed hetero-nanostructure at higher temperature. Fig. 2c shows a few Au nanowires about 1 ␮m long and 70 nm in diameter formed at 1240 ◦ C. These nanowires, mixed with nanoparticles, were found to grow parallel to the substrate surface. These randomly oriented nanowires dominated at 1340 ◦ C and showed a uniform length of about 10 ␮m and diameters between 50 and 70 nm; note that nanoparticles with a diameter of about 100 nm cover the surface of nanowires (Fig. 2d). Table 1 summarizes the conditions of formation, morphology, and average size of the Au nanostructures, where T1 , Ts , P, w, and t are the source temperature, substrate temperature, deposition pressure, starting material weight and deposition time, respectively. The X-ray diffraction (XRD) patterns for the as-synthesized Au nanostructures are shown in Fig. 2e. Three peaks with different intensities were observed in every Au nanostructure. These peaks ˚ closed show that a face centered cubic of Au with a = 4.081 A, to the standard value of a = 4.078 A˚ indicated in JCPDS card no. 04-0784, is formed. It was observed from the calculation that

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Fig. 4. SEM data of SnO2 nanostructures synthesized by using different source materials: (a) SnO powder and (b) SnO2 powder. (c) EDS data recorded of (a) indicates the existence of Sn and O. (d) XRD patterns of the as-synthesized SnO2 nanostructures.

the FWHM of (2 0 0) peak decreased with increasing temperature up to 1190 ◦ C as shown in an inset figure, indicating that the size of Au nanoparticles increased with increasing temperature. We have investigated the effect of deposition time on the growth of SnO2 nanostructure formed at a fixed temperature and pressure using tin as a source material. Fig. 3 shows SnO2 microwires with diameters of 1.0, 1.2 and 2.0 ␮m for deposition times of 30, 60 and 90 min, respectively. Their surfaces were rough and their cross sections were rectangle-like with different sizes. The length of wires is not uniform, distributing between 15 and 25 ␮m. The usage of different source materials caused a growth of different shapes of nanomaterials. Various shapes of tin dioxide nanostructures were thus synthesized by using SnO or SnO2 powder as a source material. For SnO as a source material, the low-magnification SEM image shown in Fig. 4a reveals that the product consists of numerous wire-like nanostructures with a length of several microns. Some of the wires are straight and the others are curved. These nanowires were grown at 86 Pa and 800 ◦ C and were about 50 nm in diameter. Fig. 4b shows nanoknife-like structures of SnO2 rutile phase obtained at 60 Pa and 800 ◦ C by using SnO2 powder as a source material. The nanoknives have several microns in length, random

growth directions on the surface of substrate, and a small quantity of product. Figs. 3d and 4c show the chemical composition of the as-synthesized SnO2 nanostructures characterized by EDS. It is found that the nanostructures are mainly composed of Sn and O, and the small peak marked by Si is due to the Silicon substrate. XRD patterns observed for SnO2 products (Fig. 4d) suggested that the as-synthesized products consisted of SnO2 in the rutile ˚ which phase with lattice constants of a = 4.736 A˚ and c = 3.185 A, are approximately equal to the standard values of a = 4.738 A˚ and c = 3.187 A˚ (JCPDS card no. 41-1445). The appearance of most peaks of SnO2 in the rutile phase confirms the random growth directions of nanomaterials on the substrate, as shown in the SEM image. Because of the non-epitaxial relation between SnO2 nanostructures and SiO2 film of silicon substrate, SiO2 film with its amorphous structure is expected to lead to more non-homogenous nanostructures. The growth mechanism of our method is not well understood. However, we attempt to propose a general visualization for the growth mechanism of nanomaterials synthesized by this method, as shown in Fig. 5. It is recognized that the vapor pressure and the substrate temperature greatly influence the growth of the nanostructure formed by thermal evaporation [27]. Thus, we consider the growth of nanomaterial in the present method as follows. The atoms or molecules arriving at a substrate first adsorb on the surface of the substrate. Then, the adsorbed species react with each other and also with the substrate surface, forming bonds. The initial aggregation of the adsorbed species will be the nuclei of a nanostructure. At a low temperature, since the energy for growing 1D nanostructure is not sufficient, small nanoparticles were grown from limited nucleation sites, as shown from early SEM data observed after deposition time of 5 min (Fig. 5b). At a high temperature, some particles are condensed to form liquid droplets on the SiO2 surface and become nuclei for 1D nanostructures, as shown in Fig. 5c. Each nucleus forms one single nanowire, and the nanowires grow on the substrate randomly due to the non-epitaxial relation with SiO2 film. In the case of SnO2 used as a source material, since the vapor pressure may be low, the vapor pressure gradually decreases during evaporation. This results in a gradual decrease in the deposition rate, which seems to lead to the formation of nanoknives. It is shown that the sensitivity as well as the speed of response to NO2 largely depends on the nano-additives and the operating temperatures. Such characteristics are illustrated in Fig. 6a. It can be seen that the response of these sensors to 2 ppm NO2 varies not only with the operating temperature but also with the concentration of the nano-additives of Au. As the operating temperatures increased from room temperature, the response increased sharply up to 100 ◦ C and then decreased rapidly. Thus, all the curves present a maximum at 100 ◦ C. The monitored gas concentration, response and the operating temperature of doped and undoped SnO2 sensors for NO2 gas are currently available in pertinent literatures [28–39], as shown in Table 2. In the present data, the doped sensors exhibit much higher response than the undoped one. The AuNWs–NPs–doped SnO2 sensor shows its higher responses than the others at operating temperatures of RT, 50, 250 and 300 ◦ C, while the AuNPs–doped SnO2 sensor exhibits its higher responses than the others at operating temperatures of 100–200 ◦ C. It is clear that the doping with AuNWs–NPs has the same effect as the doping with AuNPs on the gas sensing properties of SnO2 nanowires, i.e., the addition of AuNWs to a sensor doped with AuNPs does not generate any additional effect, expect the increase in sensitivity at the lower temperatures (RT, 50 ◦ C). However, it is an attempt to seek a new type of doping (AuNWs–NPs). Generally, the result leads us to believe that the AuNPs or AuNWs–NPs–doped SnO2 nanowire sen-

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147

Fig. 5. (a) Illustration of the nanostructure growth mechanism. Early SEM image of gold: (b) for a low energy and (c) for a higher energy (shows condensed nanoparticles).

sors are superior to undoped one in monitoring a low concentration of NO2 gas, and the usage of Au-additives enhanced the sensor’s response to NO2 gas as was studied in Refs. [40,41] for WO3 sensors. In order to investigate the dispersion of gold nanoparticles through the aggregation of SnO2 nanowires, EDS analysis was performed. The results revealed that gold could disperse through the whole of the aggregation of SnO2 nanowires. To explain the improvement of the nanowire’s sensing performance upon Au-doped nanostructure, two possible sensitization mechanisms “chemical and electronic” were proposed [20]. The chemical sensitization is mediated by a “spillover effect”, and was

experimentally determined for hydrogen and oxygen [42–45], as schematically illustrated for molecular oxygen (as an oxidized gas) in Fig. 7. Au nanostructures are active catalysts with specific catalytic properties. An objective gas is first adsorbed on the surface of the metal additives to be activated or dissociated, followed by migration (spillover) to the semiconductor surface. The activated species thus followed to the semiconductor surface react with the adsorbed or surface oxygen, resulting in a decrease in the surface conductivity of the n-type semiconductor, in case of oxidized gases. Thus in the area close to contact, Au acts as a catalyst able to increase activity of SnO2 gas sens-

Table 2 Gas sensing properties of doped and undoped semiconductor SnO2 sensor for NO2 gas. Sensing material

Operating temperature range (◦ C)

Operating temp. at higher response (◦ C)

Gas concentration (ppm)

Higher response (Rg /Ra )

Ref.

SnO2 –5 wt.%WO3 SnO2 SnO2 SnO2 –2 wt.%Cd SnO2 –5 wt.%Al SnO2 –In2 O3 (1:9) SnO2 SnO2 nanobelts ZnO–SnO2 NWs SnO2 NPs–MWCNTs SnO2 NC SnO2 NRs SnO2 –8 wt.%AuNPs

100–250 120–220 220–480 200–400 300–400 200–500 300–400 RT-400 200 180–300 25–300 220–340

RT-300

500 5–100 300 2–1000 5–20 5–100 10–500 0.3 10 1.2 0.1 500 2 2–10 2 2–10

33,359 1150–2229 6.2 10–300 1.8–5.0 3–80 7–30 ∼5 66.3 5.35 >1000 ∼6 88 25–298 78 20–94

[28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

SnO2 –8 wt.%AuNWs–NPs

150 160 280 250 300 300 350 100 200 300 20 340 100 150 100 150

Present result

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100

a

S1 S2 S3

Response, Rg/Ra

80

2ppm NO2

60

40

20

0

0

50

100

150

200

250

300

Operating Temp. (°C) -5

10

Fig. 7. Scheme of the process taking place at a SnO2 nanowire surface: (1) ionosorption of oxygen at defect sites on the pristine SnO2 surface, (2) molecular oxygen dissociation on Au particles followed by spillover of the atomic species onto the oxide surface, (3) collection zone: making the surface of SnO2 nanowire an effective oxygen delivery system for Au-NP or Au-NW.

b

-6

10

-7

G (S)

10

-8

10

S1 before S1 after S2 before S2 after S3 before S3 after gas injection

-9

10 10

-10

10

-11

10

-12

-

-

O2 0

50

100

OO2

150

O 200

250

-

300

Opterating Temp. (°C) Fig. 6. (a) Response toward NO2 gas at different operating temperatures for S1: undoped, S2: 8 wt.% AuNPs and S3: 8 wt.% AuNWs–NPs doped SnO2 NWs samples. (b) Temperature dependence of SnO2 conductance before and after gas introduction.

ing. The use of AuNWs on SnO2 surface might rather increase the quantity of NO2 that can be activated on SnO2 surface, resulting in a greater degree of electron withdrawal from the SnO2 at the lower temperatures. This process is a significant enhancement of the probability of NO2 chemisorptions when SnO2 is covered with a noble metal (such as Au), which is reflected in the observed increase in nanowire response to objective gas at the lower temperatures. It is known that the conductance (G) of semiconductor increases with increasing temperature, whereas the transformation of physisorbed oxygen molecules into oxygen ions , due to charge exchange between adsorbed oxygen and the bulk of SnO2 [46,47], causes a decrease in conductance with increasing temperature. The conductance of SnO2 sensors before and after the injection of NO2 gas is shown in Fig. 6b. The conductance of SnO2 before the injection of gas increased with increasing temperature up to 200 ◦ C, and then started to decrease at 250 ◦ C. This decrease in a conductance above 250 ◦ C is probably due to the transformation of oxygen species on the surface of SnO2 , which causes an increase in the height of potential barriers at the SnO2 grain boundaries. Generally, the reaction of NO2 on SnO2 surface species depends on the surface temperature [47–50]. At low operating temperature

(<200 ◦ C), the predominant oxygen species on the sensor surface is O2 − . When an NO2 gas is adsorbed on the oxide as NO2 molecules at a low temperature, a part of the adsorbed molecules take electrons from the oxide to be NO2 − , leading to a decrease in conductance [47]: NO2 (g) + e− → NO2 − (s)

(1) − (s)

Because the formation of NO2 is thermally activated, the sensitivity increases with increasing temperature up to100 ◦ C (Fig. 6a). Another part of NO2 molecules react with O− (if it is formed), forming unidentate NO3 − (s), as observed by Raman spectroscopy and FTIR [48,49]: NO2 (g) + O− → NO3 − (s)

(2) − (s)

Upon exhaust of NO2 gas, NO2 on the oxide is desorbed through the reverse reaction of (1). Upon exhaust of NO2 gas, is dissociated to an NO2 gas molecule and O− through the reverse reaction of (2). As is well known, with the temperature farther increasing, O2 − is dissociated to two O− ions, and the density of O− increases. Thus, NO2 molecules easily react with O− ions, generating bidentate NO3 − (s) [50]: NO2 (g) + O− → NO3 − (s)

(3) − (s)

Accordingly, the formation of NO2 accompanied with the electron transfer from the oxide to NO2 becomes difficult, leading to a low sensitivity. Therefore, the difference between the samples is less pronounced at the higher temperatures. The response of the pure SnO2 sensor and that of the Au-doped one to different concentrations of NO2 (2–10 ppm) measured at 150 ◦ C are compared in Fig. 8a. The gas response of the AuNPs–doped SnO2 sensor is higher than that of the AuNWs–NPs–doped SnO2 sensor and undoped one. The response of the sensors increased with an increase of the NO2 gas concentration in the range of 2–10 ppm. Fig. 8b shows the response signals of undoped and Au-doped SnO2 nanowire sensors measured at various concentrations of NO2 at 150 ◦ C. Upon exposing the sensor to NO2 gas, the resistance of the sensor gives an increase dependent on NO2 concentration, and by switching off the respective gas, the resistance of the sensor decreases approximately to the ini-

N.M. Shaalan et al. / Materials Chemistry and Physics 127 (2011) 143–150

a

250

tial value. From Fig. 8b, we see that the response of the Au-doped sensors is quicker than that of the undoped one. Fig. 9 shows the response of SnO2 NW sensor (S1) for lower concentrations of NO2 gas (0.1–1.0 ppm) at 200 ◦ C. It appears that the sensor could respond to significantly lower concentrations of NO2 gas.

200

4. Conclusions

S1 S2 S3

Response, Rg/Ra

300

150 100 50 0

1

2

3

4

5

6

7

8

9

10

11

NO2 concentration (ppm)

b

9

10

5ppm

7ppm

10ppm

3ppm 8

10

R ( Ω)

2ppm

7

10

[1] [2] [3] [4]

S1

6

10

0

1000

2000

3000

4000

5000

Time (sec)

1.10

0.0 ppm

1.15

0.1 ppm

0.3 ppm

1.20

o

200 C 0.5 ppm

0.9 ppm 0.7 ppm

1.25

[5] [6] [7] [8]

Fig. 8. (a) Dependence of the response on NO2 concentration for S1: undoped, S2: 8 wt.% AuNPs and S3: 8 wt.% AuNWs–NPs doped SnO2 NWs samples. (b) Response curves toward 2–10 ppm of NO2 at 150 ◦ C.

1.30

In summary, a convenient method of thermal evaporation for the synthesis of metal and metal-oxide nanostructures is presented. Rough microwires, nanowires, nanoknives and nanoparticles have been synthesized on SiO2 /Si substrates with catalyst-free thermal evaporation method. Experimental results suggested that the temperature of substrate, source material and vapor concentration have a great influence on morphology of nanostructures. The as-synthesized Au and SnO2 nanostructures have the f.c.c and rutile structures, respectively. The capability for growth of nanomaterials by simple manner makes the present method attractive for a creation of new nanostructures for various nanoscale device applications. SnO2 nanocomposite sensors based on noble metal and metal oxide were fabricated. These SnO2 nancomposite gas sensors showed a reversible response to NO2 gas at an operating temperature ranging from room temperature to 300 ◦ C. The sensor response increased with Au doping, and the highest response upon exposure to 2 ppm NO2 gas obtained in this study was 88 at 100 ◦ C, which was measured for the 8 wt.% AuNPs–doped SnO2 nanowire gas sensor. The sensors would respond to significantly lower concentrations of NO2 gas. References

S2 S3

Response, Rg/Ra

149

[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

1.05

[27] [28] [29]

1ppm

1.00 0.95 0

500

1000

1500

2000

Time (sec) Fig. 9. The response of SnO2 NWs sensor (S1: undoped SnO2 ) to various concentrations of NO2 gas at 200 ◦ C.

[30] [31] [32] [33] [34] [35]

Y. Li, F. Qian, J. Xiang, C.M. Lieber, Mater. Today 9 (2006) 18. D.H. Zhang, Y.Y. Wang, Mater. Sci. Eng. B 134 (2006) 9. C. Thelander, P. Agarwal, Mater. Today 9 (2006) 28. G. Mingyuan, J.F. Liu, W. Haiping, Y. Changwa, Z. Yuewu, Z.D. Fu, S.L. Zhang, J.Z. Jiang, J. Phys. Chem. C 111 (2007) 11157. Y. Wu, J. Xiang, C. Yang, W. Lu, C.M. Lieber, Nature 430 (2004) 61. J.T. Miller, A.J. Kropf, Y. Zha, J.R. Regalbuto, L. Delannoy, C. Louis, E. Bus, J.A. Bokhoven, J. Catal. 240 (2006) 222. Q. Ahsanulhaq, S.H. Kim, Y.B. Hahn, J. Phys. Chem. Solids 70 (2009) 627. A. Qurashi, T. Yamazaki, E.M. El-Maghraby, T. Kikuta, Appl. Phys. Lett. 95 (2009) 153109. J. Sanghyun, L. Kangho, D.B. Janes, Nano Lett. 5 (2005) 2281. D. Meng, T. Yamazaki, Y. Shen, Z. Liu, T. Kikuta, Appl. Surf. Sci. 256 (2009) 1050. S. Vaddiraju, A. Mohite, A. Chin, Nano Lett. 5 (2005) 1625. M.Q. He, S.N. Mohammad, J. Chem. Phys. 124 (2006) 064714. J. Yang, T.W. Liu, C.W. Hsu, Nanotechnology 17 (2006) S321. D.E. Williams, Sens. Actuators, B 57 (1999) 1. M. Fleischer, H. Meixner, Sens. Actuators, B 43 (1997) 1. P.W. Park, H.H. Kung, D.W. Kim, M.C. Kung, J. Catal. 184 (1999) 440. T. Minami, Semicond. Sci. Technol. 20 (2005) S35. Y. Wang, F. Su, J.Y. Lee, X.S. Zhao, Chem. Mater. 18 (2006) 1347. S.J. Han, B.C. Jang, T. Kim, S.M. Oh, T. Hyeon, Adv. Funct. Mater. 15 (2005) 1845. N. Yamazoe, Y. Kurokawa, T. Seiyama, Sens. Actuators 4 (1983) 283. D. Kohl, Sens. Actuators, B 1 (1990) 158. A. Cabot, A. Diéguez, A. Romano-Rodrguez, J.R. Morante, N. Bârsan, Sens. Actuators, B 79 (2001) 98. G. Korotcenkov, V. Brinzari, Y. Boris, M. Ivanov, J. Schwank, J. Morante, Thin Solid Films 436 (2003) 119. M. Epifani, C. Giannini, L. Tapfer, L. Vasanelli, J. Am. Ceram. Soc. 83 (2000) 2385–2393. D.W. Hatchett, M. Josowicz, Chem. Rev. 108 (2008) 746. V. Viswanathan, T. Laha, K. Balani, A. Agarwal, S. Seal, Mater. Sci. Eng. R 54 (2006) 121. Z.R. Dai, Z.W. Pan, Z.L. Wang, Adv. Funct. Mater. 13 (2003) 9. J. Kaur, S.C. Roy, M.C. Bhatnagar, Sens. Actuators, B 123 (2007) 1090. J. Zhang, S. Wang, Y. Wang, B. Zhu, H. Xia, X. Guo, S. Zhang, W. Huang, S. Wu, Sens. Actuators, B 135 (2009) 610. G. Williams, G.S.V. Coles, Sens. Actuators, B 16 (1993) 349. G. Sberveglieri, S. Groppelli, P. Nelli, Sens. Actuators, B 4 (1991) 457. G. Faglia, P. Nelli, G. Sberveglieri, Sens. Actuators, B 19 (1994) 497. G. Sberveglieri, G. Faglia, S. Groppelli, P. Nelli, Sens. Actuators, B 8 (1992) 79. G. Leo, R. Rella, P. Siciliano, S. Capone, J.C. Alonso, V. Pankov, A. Ortiz, Sens. Actuators, B 58 (1999) 370. C. Baratto, E. Comini, G. Faglia, G. Sberveglieri, M. Zha, A. Zappettini, Sens. Actuators, B 109 (2005) 2.

150

N.M. Shaalan et al. / Materials Chemistry and Physics 127 (2011) 143–150

[36] I. Hwang, S. Kim, J. Choi, J. Choi, H. Ji, G. Kim, G. Cao, J. Lee, Sens. Actuators, B 148 (2010) 595. [37] K. Choi, J. Park, K. Park, H. Kim, H. Park, S. Kim, Sens. Actuators, B 150 (2010) 65. [38] M.J. Prades, E. Comini, E. Pellicer, M. Avella, P. Siciliano, G. Faglia, A. Cirera, R. Scotti, F. Morazzoni, J. Morante, J. Phys. Chem. C 112 (2008) 19540. [39] P. Sun, W. Fu, H. Yang, S. Liu, B. Luo, Y. Sui, Y. Zhang, M. Yuan, D. Ma, Mater. Chem. Phys. 119 (2010) 432. [40] H. Xia, Y. Wang, F. Kong, S. Wang, B. Zhu, X. Guo, J. Zhang, Y. Wang, S. Wu, Sens. Actuators, B 134 (2008) 133. [41] M. Penza, C. Martucci, G. Cassano, Sens. Actuators, B 50 (1998) 52. [42] S. Khoobiar, J. Phys. Chem. 68 (1964) 411. [43] M. Boudart, J. Mol. Catal. A: Chem. 138 (1999) 319.

[44] M. Bowker, L.J. Bowker, R.A. Bennett, P. Stone, A. Ramirez-Cuesta, J. Mol. Catal. A: Chem. 163 (2000) 221–232. [45] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Nano Lett. 5 (2005) 667. [46] H. Liu, S.P. Gong, Y.X. Hub, J.Q. Liu, D.X. Zhou, Sens. Actuators, B 140 (2009) 190. [47] B. Ruhland, T. Becker, G. Müller, Sens. Actuators, B 50 (1998) 85. [48] N. Sergent, M. Epifani, E. Comini, G. Faglia, T. Pagnier, Sens. Actuators, B 126 (2007) 1. [49] E. Leblanc, L. Perier-Camby, G. Thomas, R. Gibert, M. primet, P. Gelin, Sens. Actuators, B 62 (2000) 67. [50] L. Francioso, A. Forleo, S. Capone, M. Epifani, A.M. Taurino, P. Siciliano, Sens. Actuators, B 114 (2006) 646.