Ammonia sensors based on metal oxide nanostructures

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Ammonia sensors based on metal oxide nanostructures

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2007 Nanotechnology 18 205504 (http://iopscience.iop.org/0957-4484/18/20/205504) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 18 (2007) 205504 (9pp)

doi:10.1088/0957-4484/18/20/205504

Ammonia sensors based on metal oxide nanostructures Chandra Sekhar Rout, Manu Hegde, A Govindaraj and C N R Rao1 Chemistry and Physics of Materials Unit, DST Unit on Nanoscience and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur PO, Bangalore-5600 64, India E-mail: [email protected]

Received 14 February 2007, in final form 29 March 2007 Published 23 April 2007 Online at stacks.iop.org/Nano/18/205504 Abstract Ammonia sensing characteristics of nanoparticles as well as nanorods of ZnO, In2 O3 and SnO2 have been investigated over a wide range of concentrations (1–800 ppm) and temperatures (100–300 ◦ C). The best values of sensitivity are found with ZnO nanoparticles and SnO2 nanostructures. Considering all the characteristics, the SnO2 nanostructures appear to be good candidates for sensing ammonia, with sensitivities of 222 and 19 at 300 ◦ C and 100 ◦ C respectively for 800 ppm of NH3 . The recovery and response times are respectively in the ranges 12–68 s and 22–120 s. The effect of humidity on the performance of the sensors is not marked up to 60% at 300 ◦ C. With the oxide sensors reported here no interference for NH3 is found from H2 , CO, nitrogen oxides, H2 S and SO2 .

1. Introduction

for reliable and satisfactory ammonia sensors. Our success with the nanostructures of different semiconducting metal oxides for sensing gases such as H2 , oxides of nitrogen and hydrocarbons [13–15], suggested that it would be rewarding to explore nanostructures of certain metal oxides for sensing ammonia. In this paper, we report the gas-sensing properties of the nanostructures of ZnO, In2 O3 and SnO2 , of which those of SnO2 are particularly satisfactory, with good sensitivity and relatively short response and recovery times.

Detection of ammonia in the atmosphere is an extremely important problem with implications to the environment and medical practice as well as the automotive and chemical industries [1]. There have been reports on ammonia sensors, but they do not make use of nanostructures. Thus, thin films of ZnO and ZnO doped with different metals have been found to sense ammonia with sensitivities varying between 4 and 95 for 1–30 ppm of NH3 over the temperature range 30–300 ◦ C [2, 3]. Surface-ruthenated ZnO films appear to have a sensitivity of ∼440 for 1000 ppm of NH3 at 300 ◦ C [4, 5]. Thin films of sol– gel-derived SnO2 are reported to exhibit a linear relationship between the logarithm of sensitivity and NH3 concentration in the range of 0.05–10 volume% at 350 ◦ C [6]. SnO2 powder modified by Pt or SiO2 shows sensitivity between 12 and 25 for 200 ppm of NH3 at 160 ◦ C [7]. In2 O3 ceramics modified by Ti or loaded with Pt, Au show enhanced selectivity for 5– 1000 ppm at 145 ◦ C [8, 9]. Several other materials have also been tested for sensing ammonia, in particular single-walled nanotubes functionalized with poly-aminobenzene sulfonic acid and N-doped carbon nanotubes [10–12]. The nanotubes exhibit a sensitivity of 2 for 5 ppm of NH3 at 32 ◦ C. Based on the literature, there appears to be a clear need

2. Experimental details ZnO nanoparticles were prepared by the sol–gel technique starting with zinc acetate as the precursor as described earlier [16]. ZnO nanorods were synthesized by stirring a fine powder of Zn(CH3 COO)2 ·2H2 O (Qualigens, 98.5% pure) in 100 ml of methanol at 60 ◦ C for 1 h, followed by dropwise addition of 0.03 M KOH (Ranbaxy) to the solution [13]. The resulting solution was refluxed for 24 h to obtain the product, which was washed with ethanol and dried at 60 ◦ C in air. In2 O3 nanoparticles were prepared as follows [17]. 2 g of InCl3 ·4H2 O (Aldrich, 97% pure) and 5 ml of deionized water were placed in a Teflon-lined autoclave and the autoclave filled up to 80% volume with ethylene diamine (Merck, 99% pure). The autoclave was heated at 200 ◦ C for 24 h and cooled to room

1 Author to whom any correspondence should be addressed.

0957-4484/07/205504+09$30.00

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© 2007 IOP Publishing Ltd Printed in the UK

Nanotechnology 18 (2007) 205504

C S Rout et al

temperature. The product was centrifuged and washed with absolute ethanol several times. The product was finally dried in air at 60 ◦ C and then heated at 500 ◦ C in oxygen atmosphere for 2 h. In2 O3 nanorods were obtained by using anodic alumina membrane (AAM) templates with a pore size of 20 nm [14]. The method of preparation of these nanorods is as follows. In(OH)3 sol was prepared by adding ethylene diamine dropwise to InCl3 ·4H2 O. The sol was placed in 100 ml deionized water and stirred for 6 h at room temperature. After obtaining a light blue sol, alumina template membranes were immersed in the sol for 5 h under a pressure of 2 atm. The templates were then taken out and dried at 60 ◦ C and heated under argon atmosphere at 600 ◦ C for 4 h. The templates were dissolved in 0.6 M NaOH (Ranbaxy, 98% pure) solution to yield nanorods of In2 O3 . SnO2 nanoparticles were prepared by dissolving 1 g of SnCl4 ·5H2 O (Lobachemie, 98% pure) in 70 ml of H2 O, followed by addition of a few drops of ethylene diamine. The solution was placed in a 100 ml Teflon-lined autoclave and heated at 200 ◦ C for 24 h. After cooling the autoclave, the product was washed with alcohol and water and dried in air at 60 ◦ C overnight. SnO2 nanorods and flowerlike structures were obtained by following methods reported elsewhere [18, 19]. SnO2 nanorods were prepared starting with a mixture of 1 g of SnCl4 ·5H2 O in 70 ml of ethanol and water (5:1). After adding 10 M NaOH, the solution was transferred to a 100 ml autoclave and heated at 200 ◦ C for 24 h. After cooling the autoclave, the product was washed with absolute ethanol several times and then it was dried in vacuum to get the final product. Flower-like structures of SnO2 were obtained by taking 0.5 g of as-prepared SnO2 nanoparticles and 10 M NaOH in 80 ml ethanol and heating the mixture in an autoclave at 200 ◦ C for 36 h. The product was washed with dilute HCl and water to remove the by-products of sodium, and was dried in vacuum at 60 ◦ C. The nanostructures of ZnO, In2 O3 and SnO2 were characterized by x-ray diffraction (Cu Kα radiation), scanning electron microscopy (Leica S440i), transmission electron microscopy (JEOL JEM 3010), field emission scanning electron microscopy (Nova Nanosem 600) and microRaman spectroscopy (Labraman-HR) using an He–Ne laser (632.81 nm) in the back-scattering geometry. To fabricate thick film sensors, an appropriate quantity of diethyleneglycol (Merck, 99% pure) was added to the desired nanostructure of ZnO, In2 O3 or SnO2 to obtain a paste. The paste was coated on to an alumina substrate (5 mm×20 mm, 0.5 mm thick) attached with a comb-type Pt electrode on one side, the other side having a heater. The films were dried at 100 ◦ C and annealed at 300 ◦ C for 1 h. Gas sensing properties were measured using a home-built computer-controlled characterization system consisting of a test chamber, sensor holder, a Keithley multimeter-2700, a Keithley electrometer-6517A, mass flow controllers and a data acquisition system. The test gas was mixed with dry air to achieve the desired concentration and the flow rate was maintained at 200 sccm using mass flow controllers. The current flowing through the samples was measured using a Keithley multimeter 2700. The working temperature of the sensors was adjusted by changing the voltage across the heater side. By monitoring the output

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Figure 1. XRD patterns of nanoparticles and nanorods of ZnO, nanoparticles and nanorods of In2 O3 and nanoparticles, nanorods and flowers of SnO2 .

voltage across the sensor, the resistance of the sensor in dry air or in ammonia can be measured. The resistance of the oxides decreased on contact with NH3 . The sensitivity (response magnitude), S , was determined as the ratio, Rair /Rammonia , where Rair is the resistance of the thick film sensor in dry air and Rammonia is the resistance in the different concentration of ammonia. The resistance of the sensors based on ZnO nanoparticles and nanorods was in the 0.1–10 M range, whereas the resistance of the sensors based on In2 O3 and SnO2 nanostructures was in the 1–100 M range in dry air in the 100–300 ◦ C range. Response time is defined as the time required for the conductance to reach 90% of the equilibrium value after the test gas is injected and recovery time is taken as the time necessary for the sensor to attain a conductance 10% above the original value in air. The sensitivity of thick film sensors was also measured in atmospheres with different relative humidities.

3. Results and discussion The XRD patterns of ZnO nanoparticles and nanorods, In2 O3 nanoparticles and nanorods and SnO2 nanoparticles, nanorods and flowers are shown in figure 1. The x-ray diffraction (XRD) patterns correspond to the wurtzite structure (lattice parameters ˚ , c = 5.2 A, ˚ JCPDS no 36-1451). The XRD pattern a = 3.25 A of the ZnO nanorods shows sharp 002 reflections, indicating the formation of the rods along the c axis. In figure 2(a), we show a typical TEM image of ZnO nanoparticles with the inset showing the selected area electron diffraction (SAED) pattern. The SAED pattern indicates the particles to be single crystalline. Based on the widths of the reflections in the XRD pattern, the average diameter of the nanoparticle is found to be ∼15 nm. Figure 2(b) shows a TEM image of ZnO nanorods, with the SAED pattern as the inset. The SAED pattern indicates the single crystalline nature of the nanorods. The TEM image reveals that the diameter of the nanorods is in the range of 10–20 nm with the length in the 50–250 nm range. 2

Nanotechnology 18 (2007) 205504

C S Rout et al

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Figure 2. (a) TEM image of ZnO nanoparticles with the inset showing electron diffraction, (b) TEM image of ZnO nanorods with the inset showing electron diffraction pattern, (c) TEM image of In2 O3 nanoparticles with the inset showing electron diffraction pattern, (d) SEM image of In2 O3 nanorods with the inset showing TEM image and electron diffraction pattern.

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Figure 3. (a) TEM image of SnO2 nanoparticles with the inset showing the electron diffraction pattern, HREM image; (b) TEM image of SnO2 nanorods with the inset showing the electron diffraction pattern and HREM image; (c) FESEM image of SnO2 flowers.

pattern and an HREM image, confirming the single crystalline nature of the nanoparticles. In figure 3(b), we show a TEM image of SnO2 nanorods with the inset showing an HREM image and the SAED pattern. The SAED pattern shows the nanorods to be single crystalline in nature. The average diameter of the nanorods is ∼25 nm. An FESEM image of flower-like structures of SnO2 is shown in figure 3(c). The inset of figure 3(c) shows a high-resolution picture of a flower. The flowers consist of fine rod-like or fibre-like structures with diameters around 25 nm. In figure 4 we show the Raman spectra of the various metal oxide nanostructures studied by us. Raman bands are found at 332, 441 and 1076 cm−1 for the ZnO nanoparticles and nanorods [20]. Bulk ZnO shows Raman bands at 330 and 439 cm−1 [20, 21]. The nanoparticles and nanorods of In2 O3 show Raman bands at 305, 364, 495 and 630 cm−1 .

In2 O3 nanoparticles and nanorods have the cubic structure ˚ JCPDS no 06-0416) as revealed (cell parameters a = 10.11 A, by the XRD patterns (figure 1). The average diameter of the nanoparticles estimated to be ∼21 nm from the XRD pattern. Figure 2(c) shows a TEM image of In2 O3 nanoparticles, with the inset showing the SAED pattern. Figure 2(d) shows a SEM image of In2 O3 nanorods, with the inset showing the TEM image and the electron diffraction pattern. The nanorods are single crystalline as revealed by the SAED pattern and have an average diameter of around 20 nm. XRD patterns of SnO2 nanoparticles, nanorods and flowers could be indexed on the ˚,c = tetragonal rutile structure (cell parameters a = 4.738 A ˚ JCPDS no 41-1445). The average diameter of the 3.187 A, nanoparticles estimated from the XRD pattern is ∼23 nm. In figure 3(a), we show a typical TEM image of the SnO2 nanoparticles with the inset showing the electron diffraction 3

Nanotechnology 18 (2007) 205504

C S Rout et al

For bulk In2 O3 , the Raman bands are at 306, 366, 495 and 630 cm−1 [22]. Raman bands of SnO2 nanostructures are observed at 315, 472, 578, 632 and 773 cm−1 , in agreement with the literature [23]. Bulk SnO2 exhibits Raman bands at 472, 632 and 773 cm−1 [24], whereas for nanostructures two extra bands are found at 315 and 578 cm−1 . The Raman band positions of the nanostructures do not differ significantly from those of the bulk samples. This is not expected since phonon confinement occurs at much smaller sizes. In figure 5(a), we show the sensing characteristics of In2 O3 nanorods for 800 ppm of ammonia. The highest sensitivity is 70 at 300 ◦ C and ∼32 at 100 ◦ C. The variation of sensitivity of the In2 O3 nanorods with the concentration of NH3 (1– 800 ppm) at 300 ◦ C is shown in figure 5(b). The nanorods show a sensitivity of 4 for 1 ppm of NH3 at 300 ◦ C. The inset of figure 5(b) shows the variation of sensitivity of the In2 O3 nanoparticles with the concentration of NH3 at 300 ◦ C (1–800 ppm). The response times for In2 O3 nanoparticles and nanorods are 12 and 18 s respectively for 800 ppm NH3 at 300 ◦ C; the recovery times are 9 and 15 s respectively for In2 O3 nanoparticles and nanorods. Figure 6 shows the sensing characteristics of the ZnO nanoparticles and nanorods. ZnO nanoparticles show a highest sensitivity of ∼260 for 800 ppm of NH3 at 300 ◦ C and at 100 ◦ C the sensitivity becomes ∼19. For ZnO nanorods, the observed values of sensitivity are 80 and 18 for 800 ppm of ammonia at 300 ◦ C and 100 ◦ C respectively. The variation of sensitivity with the concentration of NH3 (1–800 ppm) at 300 ◦ C is shown in figures 7(a) and (b) respectively for ZnO nanoparticles and nanorods. The sensitivity values are 16

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Figure 5. (a) Gas sensing characteristics of In2 O3 nanorods for 800 ppm of ammonia, (b) variation of sensitivity with concentration of ammonia for In2 O3 nanorods, the inset showing variation of sensitivity with concentration of ammonia for the nanoparticles at 300 ◦ C.

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Nanotechnology 18 (2007) 205504

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nanoparticles does not show saturation behaviour for the first few cycles. This is due to the availability of a greater number of adsorbed oxygen species during initial measurements and after several cycles the sensitivity saturates at a particular value. We did not find any change in sensitivity after 20 cycles. In figure 9(a), we show ammonia-sensing characteristics of SnO2 nanorods while figure 9(b) shows the variation of sensitivity with concentration. The inset of figure 9(b) shows the variation of sensitivity with the concentration of ammonia for SnO2 flowers at 300 ◦ C. The sensitivity of SnO2 nanorods varies between 180 and 20 for 800 ppm of ammonia. For 1 ppm of ammonia a sensitivity of 18 is found at 300 ◦ C. The response and recovery times for the SnO2 nanoparticles are 22 and 10 s respectively for 800 ppm NH3 at 300 ◦ C. For the nanorods and flowers, the response times are 36 and 25 s respectively, whereas recovery times are 20 and 12 s. In figure 10, we compare the temperature variation of sensitivity in the 100–300 ◦ C range for ZnO, In2 O3 and SnO2 nanostructures. We see that ZnO nanoparticles show the highest values of sensitivity towards ammonia at 300 ◦ C. SnO2 nanoparticles, nanorods and flowers also show satisfactory values of sensitivity. The sensitivity is lowest with the In2 O3 nanoparticles and nanowires. The nanostructures of all the oxides show similar behaviour at low temperatures (