370 J. Zhang et al. / Sensors and Actuators B 139 (2009) 369–374nanoparticles with sizes of 5–15 nm were ﬁnally obtained afterannealing the amorphous precipitation precursor at 300 ◦ C. A gassensor was fabricated from the as-prepared SnO2 nanoparticlesand applied to ethanol sensing test. Sensing properties of thesensor, such as optimum operating temperature, sensitivity andresponse–recovery time, are systematically studied and comparedwith those reports in the literature.2. Experimental2.1. Synthesis of SnO2 nanoparticles SnO2 nanoparticles were prepared via a tin alkoxide (Sn(OEt)2 )hydrolysis process combined with subsequent calcination. In a typ-ical synthesis, 0.9 g of SnCl2 ·2H2 O was introduced into 40 mL ofethanol to form a transparent tin ethoxide solution, followed by theaddition of 1 mL of NH3 ·H2 O (25 wt%) under stirring to promote thehydrolysis process. After stirring for about 30 min, the white pre-cipitation precursor sol was centrifuged and washed by distilledwater and ethanol, then dried at 80 ◦ C in an electrical oven. Finally,SnO2 nanoparticles were obtained by annealing the precipitationprecursor in a mufﬂe furnace at 300 ◦ C for 2 h. Fig. 1. (a) Photograph of the sensor, (a: Pt wire; b: Ni–Cr alloy heater; c: Au electrode and d: alumina tube) and (b) test principle of the gas sensing measurement system2.2. Fabrication and analysis of gas sensor (Vh : heating voltage; Vc : circuit voltage; Vo : signal voltage and RL : load resistor). The gas sensor was fabricated as follows. A proper amount ofSnO2 nanoparticles was grinded with several drops of water in an possible formation mechanism of the precipitate precursor can beagate mortar to form a slurry. Then, the slurry was coated onto an depicted as follows :alumina tube with a diameter of 1 mm and length of 4 mm, posi- Sn2+ + 2EtOH → Sn(OEt)2 + 2H+ (1)tioned with two Au electrodes and four Pt wires on each end of the NH3 ·H2 Otube. A Ni–Cr alloy ﬁlament was put through the tube and used as Sn(OEt)2 + 2H2 O −→ Sn(OH)2 + 2EtOH (2)a heater by tuning the heating voltage. Gas sensing tests were per-formed on a static test system (HW-30A, HanWei Electronics Co., 300 ◦ C 2Sn(OH)2 + O2 −→ 2SnO2 + 2H2 O (3)Ltd., Henan Province, China) using air as the reference and dilutinggas at a relative humidity (RH) of 38%. The sensor was placed in Details of morphology and size of the nanoparticles can bea transparent testing chamber with a volume of 15 L and aged for observed in Fig. 3. From Fig. 3a and b, it can be seen that the SnO2several days before analysis. Target gas such as ethanol was injected nanoparticles have a nearly spherical morphology with sizes ininto the testing chamber by a microsyringe. The sensor signal volt- the range of 5–15 nm. The HRTEM images of the SnO2 nanopar-age (Vout ) was collected by a computer at a constant test voltage of ticles are displayed in Fig. 3c and d, showing that the particle size5 V. The sensor response is deﬁned as the ratio S = Rg /Ra , where Rg is ranging from 5 to 12 nm, in accordance with the TEM observa-and Ra are the electrical resistance of the sensor in test gas and in air, tions. The clear lattice fringes in the HRTEM images also conﬁrmrespectively. The response and recovery time is deﬁned as the time the high degree of crystallinity of the SnO2 nanoparticles. Fig. 3efor sensor to reach 90% of its maximum response and falls to 10% of displays the SAED pattern, conﬁrming the well crystallization andits maximum response, respectively. Fig. 1 shows the photograph of polycrystalline structure of the SnO2 nanoparticles.the sensor and working principle of the gas sensing measurementsystem.2.3. Characterization The morphology and size of the prepared SnO2 nanoparticleswas obtained by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selectedarea electron diffraction (SAED) on a Philips FEI Tecnai 20ST atan accelerating voltage of 200 kV. The crystal structure identiﬁca-tion was performed by X-ray diffraction (XRD, Rigaku D/max-2500diffractrometer, Cu K radiation, = 1.5418 Å).3. Results and discussion The XRD patterns of the precipitation precursor and heat-treated product are shown in Fig. 2. It can be seen that theprecipitation precursor has an amorphous structure, while the heattreated material is well crystallized and all diffraction peaks can bewell indexed to the tetragonal rutile structure of SnO2 . The averagecrystallite size is calculated to be 9.8 nm by Scherrer equation. The Fig. 2. XRD patterns of (a) precipitation precursor and (b) heat-treated products.
J. Zhang et al. / Sensors and Actuators B 139 (2009) 369–374 371 Fig. 3. TEM images (a and b), HRTEM images (c and d) and SAED pattern (e) of SnO2 nanoparticles. It should be point out that the prepared SnO2 nanoparti- exposed to air again and thus refreshed by air. The oxygen (O2 )cles after heat treatment have a larger average crystallite size in air will renewably capture electrons to deplete the particle sur-(9.8 nm) compared with those derived from hydrothermal con- face.ditions (2.5–3.5 nm) . This can be attributed to the calcination Fig. 6 shows the plots of sensor response versus ethanolprocess. It is well known that the average particle size increases concentration at different operating temperatures. Obviously,with increasing calcination temperature . When the amorphous at temperatures below 220 ◦ C, the response increases withprecursor was subjected to calcination at an elevated temperature, increasing temperature. At 220 ◦ C, the sensor shows the maxi-a signiﬁcant particle growth might take place in the calcination mum responses of 4.56, 24.35, 83.37 to 10, 100 and 1000 ppmprocess. Thus, it is possible that larger particles can be obtained by ethanol, respectively. At temperatures above 220 ◦ C, the responseincreasing the annealing temperature. Considering their easy fabrication with a high yield and goodsensing capabilities, we applied the prepared 5–15 nm SnO2nanoparticles for ethanol sensor applications. The gas sensor fab-ricated from the SnO2 nanoparticles was tested to 10, 100 and1000 ppm ethanol at different operating temperatures from 200to 250 ◦ C in a step of 10 ◦ C. The dynamic response–recoverycurves are shown in Fig. 4. It can be clearly seen that thecurves ascend or descend quickly when ethanol gas is in or out.This can be interpreted by the well accepted sensing mecha-nism for n-type semiconductor sensors. It is believed that thesensing process probably involves a three-step process, i.e., anadsorption–oxidation–desorption process. A schematic illustra-tion for this sensing mechanism is shown in Fig. 5. In air, SnO2nanoparticles are depleted of electrons from the conduction bandby oxygen species (O− , O− and O2− ) adsorbed on particle sur- 2face [9,31–33], forming an electron depletion layer on particlesurface which increases the sensor resistance. When the senoris exposed to reductive gases (e.g., ethanol gas in), the ethanolmolecules are oxidized by oxygen species into formaldehyde and simultaneously the depleted electrons are fed back into par- Fig. 4. Dynamic response–recovery curves of the sensor to different ethanol con-ticles, resulting in a narrowed depletion layer and therefore the centrations at different operating temperatures (for clarity, the curves are separatedsenor resistance is decreased. When gas is out, the sensor will be with a vertical offset of 1 V).
372 J. Zhang et al. / Sensors and Actuators B 139 (2009) 369–374Fig. 5. Schematic illustration for the sensing mechanism of the SnO2 nanoparticlesensor. Fig. 7. Dynamic response–recovery curves of the sensor to different gases at 220 ◦ C, all gas concentrations are 100 ppm (with a vertical offset of 0.25 V).Fig. 6. Sensor response to different ethanol concentrations at different operatingtemperatures.begins to decrease with increasing temperature. Consequently,the optimum operating temperature of the sensor is deﬁned as220 ◦ C. A comparison between the sensing performances of the sensor Fig. 8. Sensor response to different gases at 220 ◦ C.and literature reports is summarized in Table 1. It is noteworthythat the sensor prepared in this work exhibits comparable or bettersensing performances compared with those reported in the liter- (22s) is much longer than those of nanorod (1s) and nanowire (2s)ature, as shown in Table 1. From comparison, it can be seen that sensors, which may be induced by their high surface to volumethe SnO2 nanoparticle sensor has a relative low operating tem- ratio .perature of 220 ◦ C (<300 ◦ C), though still far higher than room Besides ethanol, the sensing performances of the sensor to sometemperature . Interestingly, it seems that all the nanoparti- other gases were also examined to depict its selectivity. Fig. 7 dis-cle sensors (Table 1) exhibit relatively higher responses compared plays the response–recovery curves of the sensor to methanol,with their one-dimensional counterparts [36–40]. In particular, the acetone, gasoline and n-butanol at 220 ◦ C, all gas concentrationshydrothermally generated SnO2 nanoparticle sensor possesses the were 100 ppm. It can be observed that the sensor signal voltagehighest response up to 65 , which probably results from its is increased when exposed to target gases. In Fig. 7, it should besmall crystallite size. However, the response time of the sensor also noted that the sensor is more sensitive to n-butanol than toTable 1Comparison of sensing performances towards ethanol of various SnO2 gas sensors. Average crystallite Operating Reference gas Ethanol Sensor Response Recovery size (nm) temperature (◦ C) (humidity) (ppm) response time (s) time (s)Nanoparticles (this work) 9.8 220 Air (38%) 100 24.3 22 70Nanoparticles  2.7–3.9 220 Air (48%) 100 65 28 100Nanoparticles  8.1 Room temperature Air 80 25.8 45 25Nanorods (single crystals)  – 300 Air (25%) 100 13.9 1 1Nanowhiskers (single crystals)  50–200 300 Air 50 23 >120 >600Nanoﬁbers polycrystalline  ∼11.5 330 Dry air 100 <20 <20 >100Nanowires (single crystals)  – 400 Dry air 100 10.5 2 136Nanotubes polycrystalline  ∼15 200 Air (40–50%) 100 <8 >40 <50
J. Zhang et al. / Sensors and Actuators B 139 (2009) 369–374 373 4. Conclusion In summary, highly sensitive SnO2 nanoparticles with a size of 5–15 nm that can be used for ethanol sensors were successfully synthesized by an alkoxide hydrolysis route combined with cal- cination treatment. Compared with other methods, there are at least two advantages. First, this hydrolysis procedure is a general route, which can be extended to the large-scale production of other metal oxides for sensor applications. In addition, this facile method is more economic, time-saving, human and environment-friendly as it requires no use of toxic organic solvents, surfactant or special laboratory apparatus such as microwave and sonication. However, further efforts are needed and currently in progress towards opti- mizing the sensor response to ethanol against n-butanol. 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