Ultrasensitive Detection of Volatile Organic Compounds by a Pore

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Ultrasensitive Detection of Volatile Organic Compounds by a Pore Tuning Approach Using Anisotropically Shaped SnO2 Nanocrystals Tetsuya Kida, Koichi Suematsu, Kazuyoshi Hara, Kiyoshi Kanie, and Atsushi Muramatsu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13006 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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ACS Applied Materials & Interfaces

Ultrasensitive Detection of Volatile Organic Compounds by a Pore Tuning Approach Using Anisotropically Shaped SnO2 Nanocrystals

Tetsuya Kidaa*, Koichi Suematsub, Kazuyoshi Harac, Kiyoshi Kaniec*, and Atsushi Muramatsuc

a

Division of Materials Science, Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan

b

Chemical and Textile Research Institute, Fukuoka Industrial Technology Center, Fukuoka 818-8540, Japan

c

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan

Corresponding Authors Tetsuya Kida E-mail: [email protected], FAX: +81-96-342-3679 Kiyoshi Kanie E-mail: [email protected], FAX: +81-22-217-5165

ACS Paragon Plus Environment


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Abstract Gas sensing with oxide nanostructures is increasingly important to detect gaseous compounds for safety monitoring, process controls, and medical diagnostics. For such applications, the sensor sensitivity is one major criterion. In this study, to sensitively detect volatile organic compounds (VOCs) at very low concentrations, we fabricated porous films using SnO2 nanocubes (13 nm) and nanorods with different rod lengths (50-500 nm) that were synthesized by a hydrothermal method. The sensor response to H2 increased with decreasing crystal size; the film made of the smallest nanocubes showed the best sensitivity, which suggested that the H2 sensing is controlled by crystal size. In contrast, the responses to ethanol and acetone increased with increasing crystal size and resultant pore size; the highest sensitivity was observed for a porous film using the longest nanorods. Using the Knudsen diffusion-surface reaction equation, the gas sensor responses to ethanol and acetone were simulated and compared with experimental data. The simulation results proved that the detection of ethanol and acetone was controlled by pore size. Finally, we achieved ultrahigh sensitivity to ethanol; the sensor response (S) exceeded S = 100,000, which corresponds to an electrical resistance change of five orders of magnitude in response to 100 ppm of ethanol at 250°C. The present approach based on pore size control provides a basis for designing highly sensitive films to meet the criterion for practical sensors that can detect a wide variety of VOCs at ppb concentrations.

Keywords: SnO2, gas sensor, nanorods, nanocubes, Knudsen diffusion, pore control


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INTRODUCTION Metal oxide nanostructures are drawing a great deal of attention because they often demonstrate unique properties that are different from their spherical forms. There have been many reports on oxide nanostructures, such as nanotubes, nanorods, nanowires, and nanosheets, among others, that can play a key role in energy and environmental applications, including batteries, fuel cells, photovoltaic cells, and gas sensors.1 Advances in synthesis technology allow the shape and size of oxide materials to be tailored with a wide variety of compositions and crystal structures.2 The design of oxide nanostructures, together with control of their composition and crystal phase, provides a promising avenue to achieve high performance devices. For resistive-type gas sensors, particle size control is principally important, as reported by Xu et al., who first showed that decreasing the oxide particle size from micro- to nanoscale caused a substantial increase in sensor responses.3 At high temperatures, oxygen adsorbs on oxide particle surfaces to form electrically depleted layers, leading to a high electrical resistance. The reaction of adsorbed oxygen with combustible gases induces a decrease in the electrical resistance of oxide particles, generating sensor signals (resistivity change) that are dependent on the concentration of combustible gases. Shrinking particle sizes to the nanoscale results in penetration of the electrically depleted layers through the entire particles, leading to a high electrical resistance. In this situation, the reaction of adsorbed oxygen with combustible gases causes a larger change in the electrical resistance, i.e., a higher sensor response. This phenomenon is called the “particle size effect”, and its validity has been theoretically demonstrated.4-6 The size and morphology of oxide particles also exert a strong influence on the sensor response. Gas sensor devices are composed of fine oxide particles that are assembled into films with mesopores ranging from 2 to 80 nm. Gases diffuse through the pores and react with adsorbed oxygen during diffusion, generating the sensor response. Shimizu et al.



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experimentally demonstrated that gas diffusion control in sensing films is critically important in obtaining high sensor responses.7-9 Sakai et al. experimentally and theoretically clarified the effect of film porosity on the sensor response (“pore size effect”).10-11 They assumed that the diffusion and surface reactions follow the Knudsen mechanism and first-order kinetics, respectively. Their calculations revealed that efficient gas diffusion facilitates the reaction of gases with adsorbed oxygen even near the bottom of a film, generating a higher sensor response. For example, hydrogen, which is a very small gas, tends to penetrate deep inside films and can be detected efficiently. On the other hand, the diffusion of larger gases such as ethanol and toluene is limited, especially in a dense film, leading to a smaller sensor response. Thus, controlling the pore size of sensing films is effective in achieving high sensitivity and selectivity in resistive-type gas sensors, as also confirmed by our previous studies.12,13 To fabricate a porous sensing film, we previously used TiO2 nanotubes for the sensing layer formation with the expectation that the assembly of nanotubes should produce a porous film due to the highly anisotropic shape of the nanotubes.14-16 Indeed, films made from the TiO2 nanotubes showed a high porosity that enabled the efficient diffusion of large gas molecules such as toluene. However, TiO2 requires high operating temperatures because of its high electrical resistance at lower temperatures. Thus, we aimed to prove the validity of the above approach by using SnO2, which is the most used material for gas sensing. To date, many gas sensors using oxide nanostructures have been reported, including nanotubes,17-21 nanorods,22-25 nanofibers/nanowires,26-31 nanosheets,32-33 and hollow spheres.34-37 However, porosity control when using those anisotropic particles has not yet been fully studied. There is still a broad opportunity to further enhance sensor performance by proper modification of the size and distribution of pores in sensing films. In this study, we synthesized SnO2 nanorods and nanocubes to form gas sensing films with different pore structures. In particular, the nanorod length was controlled to intensively tune the pore size in the films. Furthermore, Pd-loading on SnO2 nanocrystals was performed to increase the sensor response. Ethanol and


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acetone were selected as target gases because of their importance as biomarkers in diagnosing human disease. We carefully examined the gas sensitivity dependence on pore size, and we simulated gas diffusion profiles inside a film on the basis of gas diffusion and reaction kinetics to propose a model that leads to a high-performance gas sensor.

EXPERIMENTAL SECTION Synthesis of SnO2 nanorods SnO2 nanorods were synthesized by a hydrothermal method. SnCl4.5H2O (0.1 mol/L) was dissolved in an HCl solution (3.0 mol/L, 100 mL). Then, triethanolamine (33 mL, 250 mmol) was dropwise added to the solution. Ion-exchanged water and an HCl solution (6.0 mol/L, 100 mL) were further added into the mixture to produce the reaction solution (pH 0.4, 200 mL). The solution was placed in a Teflon-lined stainless steel container (volume: 20 mL) and hydrothermally treated at 200°C for 48 h. After the reaction, precipitates were recovered by centrifugation at 18,000 rpm for 10 min. The precipitates were washed with water three times and dried at 60°C in an oven. Nanorods with different rod lengths were also synthesized by a seeding method. See the Supporting Information for the detailed synthesis route.

Synthesis of SnO2 nanocubes SnO2 nanocubes were also synthesized by a hydrothermal method. SnCl4.5H2O (0.1 mol/L) was dissolved in an HCl solution (3.0 mol/L, 70 mL). Ethylenediamine (6.1 mL, 91 mmol) was dropwise added to the solution. Another HCl solution (6.0 mol/L, 1.0 mL) was added into the mixture, and then ion-exchanged water was further added to produce a reaction solution (140 mL). The solution was placed in a Teflon-lined stainless steel container (volume: 23 mL) and hydrothermally treated at 200°C for 48 h. After the reaction, precipitates were recovered by centrifugation at 18,000 rpm for 10 min. The precipitates were washed with water three times and dried at 60°C in an oven.



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Loading of Pd on SnO2 nanorods and nanocubes SnO2 nanorods or nanocubes (250 mg) were dispersed in distilled water (10 mL). PdCl2 (0.177 g, 1.0 mmol) was dissolved in an HCl solution (0.10 mol/L, 10 mL). A designated amount (60 µL) of the PdCl2 solution was added to the above suspension. Then, ethanol (0.5 mL) was added to the suspension, and its pH was controlled to 2.0 with an HCl solution (1.0 mol/L). The mixture was put in a quartz container with a septum cap and irradiated with UV light using a high-pressure Hg lamp for 2 h under vigorous stirring at room temperature. The products were washed with distilled water and dried at 60°C overnight to produce 1.0 mol% Pd-loaded SnO2 nanorods or nanocubes.

Sensor fabrication and sensing measurements Gas sensing films were fabricated by a screen printing method. The synthesized particles were mixed with α-terpineol to make a coating ink. The ink was coated through a screen mesh on alumina substrates (9× 13× 0.38 mm) equipped with a pair of comb-type Au microelectrodes (line width: 180 µm; distance between lines: 90 µm; sensing layer area: 64 mm2). The thickness of the films was ca. 20 µm. The surfaces of the alumina substrates were treated with an ammonia solution containing H2O2 at 80°C and then subjected to plasma cleaning (PS-601S, KASUGA ELECTRIC) prior to use. The Au electrodes were fabricated by screen-printing using a commercial Au paste, followed by calcination at 850°C immediately prior to the sensing film deposition. The sensing films deposited on the substrates were calcined at 350°C for 10 h under synthetic air flow (100 mL/min). Films with different pore sizes were fabricated by using nanocubes and the nanorods of different sizes. The sensing properties of the devices were examined using a conventional gas flow apparatus equipped with an electric furnace at a gas flow rate of 100 cm3/min. The flow rates of air and the sample gases were precisely controlled by mass flow controllers (SEC-series;


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HORIBA STEC). The sample gases of H2, ethanol, and acetone in air were prepared by diluting parent synthetic gas mixtures with synthetic air. The parent synthetic gas mixtures were purchased from Sumitomo Seika Chemicals. Each sensor device was connected with a standard resistor in series, and the voltage across the standard resistor was measured under an applied voltage of DC 4 V to evaluate the electrical resistance of the devices. The electrical signal of the sensor was acquired with an electrometer (TR2114; Advantest). The sensor response (S) was defined as the ratio of resistance in air (Ra) to that in air containing combustible gases (Rg) (S = Ra/Rg).

Materials characterization The obtained particles were analyzed by X-ray diffractometry using Cu Kα radiation (XRD; Ultima IV, Rigaku) and transmission electron microscopy (TEM; H-7650, Hitachi). The morphology of film surfaces was observed on a field emission scanning electron microscope (SEM; S-4800, Hitachi). Specific surface area of gas sensing films was measured by a nitrogen-gas-adsorption method using a specific surface area/pore size distribution analyzer (BELSORP-mini II, Bell Japan). The pore size distribution was obtained by the BJH (Barrett−Joyner− Halenda) method from adsorption/desorption isotherms. For this analysis, films were fabricated on an aluminum foil in the same manner as that used for gas sensing layer formation and the foil was removed before pore size distribution measurements.

RESULTS AND DISCUSSION Materials characterization Figure 1 shows transmission electron microscope (TEM) images and X-ray diffraction (XRD) patterns of the products. The TEM images (a) and (b) revealed the formation of cube and rod-like particles by the hydrothermal treatment at 200°C in the presence of triethanolamine and 1, 2-diaminoethane, respectively. The high magnification image in (a)



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also confirms the presence of ca. 13 nm cubic particles in the product. It should be noted that our method successfully produced cubic nanoparticles with a narrow size distribution. The nanorod length and diameter were in the range of ca. 200-500 nm and 10-40 nm, respectively. The nanorod length was also controlled to ca. 50, 150, and 500 nm by a seeding method, as shown in Figure S1. The detailed synthesis method is reported in Supporting Information. The corresponding XRD patterns (c) and (d) were in good agreement with that of tetragonal SnO2, indicating that the products are cubic and rod-shaped SnO2 nanoparticles. No impurity phases were seen in the patterns.

Figure 1. TEM images and XRD patterns of SnO2 nanocubes (a, c) and nanorods (b, d).


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Figure 2 (a) and (b) show high resolution (HR)-TEM images of the SnO2 nanocubes and nanorods, respectively. The images with visual lattice fringes clearly show their single-crystal nature. For the nanocubes, the distance between adjacent lattice planes is estimated to be 0.335 nm, which corresponds to the (110) plane of the tetragonal SnO2 phase. Its (001) plane is perpendicular to the (110) plane, as indicated in the crystal structure model in Figure 2 (a). Thus, the exposed surfaces of the SnO2 nanocubes are (110) and (001) faces. For the nanorods, the lattice spacing of the growth direction is 0.316 nm, corresponding to the (001) plane. On the other hand, the lattice spacing perpendicular to the growth direction is 0.335 nm, ascribable to the (001) plane. Thus, the main exposed surface of the SnO2 nanorods is a (110) face. The tips of the nanorods are terminated by (001) faces. To date, several syntheses of SnO2 nanorods and nanocubes by hydrothermal methods have been reported.38-41 The crystal structures of our materials are similar to those reported. Most studies used highly alkaline media in the presence of cations such as Na+, K+, NH3+, and N(CH3)+ to synthesize nanorods or nanocubes. A distinct difference in our method is that acidic media were used in the presence of organic molecules such as triethanolamine and ethylenediamine. Under acidic conditions, hydrolysis and condensation of SnCl4 occur via formation of tin chloride monomeric complexes such as [SnClx(H2O)6-x]4-x to form crystal nuclei.42 It is probable that the growth direction of the crystals was kinetically controlled by adsorption of organic molecules onto a specific lattice plane. The surface energies of each SnO2 lattice plane have been theoretically calculated; the energy increases in the order (001) > (101) > (100) > (110).43 Thus, the most stable and unstable planes are (110) and (001), respectively. Sato et al. proposed that N(CH3)+ adsorbed on (001) faces and lowered the surface energy to the level of (110), inducing the growth of SnO2 nanocubes whose surfaces are terminated by (001) and (110).43 Applying this idea to our work, it is likely that ethylenediamine preferentially adsorbed onto (001) faces of crystal nuclei to assist in the formation of the SnO2 nanocubes having terminated (001) and (110) faces. Ethylenediamine



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is a strong chelating ligand and thus can bind to the (001) plane, which has a higher number of dangling Sn bonds. Likewise, for the nanorod formation, it is possible that triethanolamine preferentially adsorbed onto (110) faces and further stabilized the surface energy. The uncapped reactive (001) surface would lead to an anisotropic growth of crystal nuclei along the [001] direction to form the SnO2 nanorods having terminated faces of (110).

Figure 2. High resolution TEM images of the SnO2 (a) nanocubes and (b) nanorods.

Pore size control using nanocubes and nanorods. Figure 3 shows SEM images of gas sensing films fabricated by a screen printing method using an ink containing the SnO2 nanocubes and nanorods. For the film made from the nanocubes, the crystals firmly packed to form a dense film. No obvious voids larger than 10 nm were seen in the high magnification SEM image (Figure 3 (b)). On the other hand, the nanorod film had a distinct porous morphology, and voids larger than 10 nm can be seen in


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the images. For both films calcined at 350°C, no significant crystal growth was observed. Due to this good thermal stability of the SnO2 nanocrystals, the film morphology was easily controlled by using the different shaped crystals. The nanorods with different rod lengths also showed good thermal stabilities; their rod-like morphology was preserved even after calcination, and the nanorods assembled to form porous films, as also shown in Figure S2.

Figure 3. SEM images of gas sensing films fabricated using (a, b) nanocubes and (c, d) nanorods. The films were calcined at 350°C in air. (a, c) Lower- and (b, d) higher-magnification images. To precisely analyze the pore structure, the pore size distribution of the films was measured, as shown in Figure 4 (a) and (b). The effect of Pd-loading on the pore structure was also examined. Tables 1 and 2 summarize the specific surface area, pore volume, and peak pore radius of the films made of the four different crystals with and without Pd-loading, respectively. The results show that the pore size of sensing films was well tuned using the synthesized SnO2 nanocrystals. The peak pore radius increased by using the nanorods, while the total pore volume did not significantly change. It should be noted that the surface area was



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largely decreased when using nanorods with longer rod lengths. The film with the long nanorods (500 nm) had mesopores with a wide distribution ranging from 10 to 100 nm. Another noteworthy feature is that for the 500 nm nanorods, the peak pore size significantly shifted from ca. 50 to 95 nm after Pd-loading. Concomitantly, the surface area and pore volume were decreased. One possible explanation is that aggregation of the nanorods occurred by the introduction of Pd nanoparticles, clogging pores ranging from 10 to 50 nm originally present in the film. In contrast, for films with smaller pore sizes, no such change in peak pore size was observed.

Figure 4. Pore size distribution of films fabricated with four different nanocrystals (a) without and (b) with Pd-loading. The films were calcined at 350°C in air.

Table 1. Pore properties of films fabricated from different SnO2 nanocrystals. Average Sample

rod Peak

pore Pore

volume Specific

length

radius (rP)

(VP)

surface area

/ nm

/ nm

/ cm3·g-1

/ m2·g-1

A. nanocubes

13

8.0

0.20

59.8

B. short nanorods

50

12.2

0.23

26.8


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C. middle nanorods

150

25.5

0.24

33.7

D. long nanorods

500

53.0

0.22

10.0

Table 2. Pore properties of films fabricated from different SnO2 nanocrystals loaded with Pd. Average Sample

rod Peak

pore Pore

volume Specific

length

radius (rP)

(VP)

surface area

/ nm

/ nm

/ cm3·g-1

/ m2·g-1

E. Pd/nanocubes

13

8.0

0.20

59.7

F. Pd/short nanorods

50

14.1

0.22

38.1

G. Pd/middle nanorods 150

25.5

0.31

34.5

H. Pd/long nanorods

95.3

0.07

6.4

500

Gas sensing properties of nanocubes and nanorods Electrical resistance values in air are one of the important parameters for resistive-type gas sensors. At high temperatures, oxygen adsorbs on the oxide surface, leading to a high device electrical resistance by extracting carrier electrons from the surface and forming electron depletion layers on the surface. Figure 5 (a) shows the electrical resistances in air for the devices using the SnO2 nanocubes and nanorods with different rod lengths. The electrical resistance of SnO2 is very high at lower temperatures. Thus, the measurements were performed at temperatures higher than 250°C. Without Pd-loading, the resistance increased in the following order: nanocubes (13 nm) > short nanorods (50 nm) > middle nanorods (150 nm) > long nanorods (500 nm). This tendency is in good agreement with the crystal sizes shown in Table 1. It is clear that the smallest nanocubes showed the highest resistance due to



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the extensive formation of an electron depletion layer throughout the crystals. For the larger nanorods, the depletion layer was confined to the surface (thin layer), making the device more conductive. On the other hand, Pd-loading increased the electrical resistances, as shown in Figure 5 (b). It is known that deposition of Pd on SnO2 forms a Schottky junction at the interface because of work function differences.44 The resulting energy barrier would limit the carrier transport, resulting in an increase in resistance. However, the devices using the nanorods (50 nm) showed a different behavior; the resistance was decreased by Pd-loading. One possible reason is that most of the Pd particles aggregated and separated from the nanotube hosts, unable to form the Schottky junction efficiently. This phenomenon may be associated with the pore clogging by Pd nanoparticles, as discussed above.

Figure 5. Electrical resistance in air as a function of operating temperature for the devices using four different nanocrystals (a) without and (b) with Pd-loading.

We first examined the sensor response to H2 at different temperatures because it is frequently used as a model gas. Figure 6 (a) shows the responses to 200 ppm H2 as a function of operating temperature for the different devices without Pd-loading. The sensor response (S) was defined as the ratio of resistance in air (Ra) to that in air containing target gases (Rg) (S = Ra/Rg). The sensor response at 300°C increased in the following order: nanocubes (13 nm) >


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short nanorods (50 nm) > middle nanorods (150 nm) > long nanorods (500 nm). The smallest nanocubes showed the best sensor response, i.e., sensitivity. This is apparently due to the “particle size effect”, as explained in the Introduction. The diffusion rate of H2 is very high, as shown in Table 3, and as such H2 can efficiently diffuse through pores in all films. Thus, the “pore size effect” was not observed for H2 detection using these porous films. The sensor responses went through a maximum at 300°C for all devices. This volcano-shaped dependence on temperature of the sensor response to combustible gases is often observed in SnO2 gas sensors without noble metal loading.12, 13, 45-47 Increasing the operating temperature improves the diffusion rate, as indicated by an increase in diffusion coefficients (Table 3), but an extensive thermal improvement in the surface reaction rate deteriorates the sensor response by decomposing most of the combustible gases at the film surface, limiting the gas diffusion deep inside the film. Thus, the optimum operating temperature is determined by the rates of both surface reaction and gas diffusion.11, 48 In the present case, both rates were well balanced at 300°C. In contrast, after Pd-loading, this volcano-shaped tendency disappeared; the operating temperature yielding the maximum sensor response shifted lower, as shown in Figure 6 (b). At temperatures higher than 300°C, the sensor response was drastically decreased. The activated catalytic activity of Pd at high temperatures should block the diffusion of H2 deep inside the films by combusting H2 near the film surface. At 250°C, all of the sensors with Pd-loading showed their maximum response. Particularly, the device using the long nanorods (500 nm) exhibited an improved sensor response. As revealed in the pore size distribution, this film has larger mesopores that allow for fast diffusion. Thus, both the improved diffusion and the Pd-activated surface reactivity are responsible for the enhanced sensor response.



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Figure 6. Sensor responses to 200 ppm H2 in air as a function of operating temperature for the devices using four different nanocrystals (a) without and (b) with Pd-loading.

Table 3. The Knudsen diffusion coefficients (Dk) of H2 and C2H5OH at 250 and 300°C in films with different pore sizes. Dk / ×1012 nm2s-1 Pore radius / nm

H2

C2H5OH

250°C

300°C

250°C

300°C

8.0

12.5

13.1

2.6

2.7

12.2

19.0

19.9

3.0

4.2

14.1

22.0

23.0

4.6

4.8

25.5

39.8

41.7

8.3

8.7

53.0 95.3

82.7 148.8

86.6 155.7

17.3 31.1

18.1 32.6

Figure 7 (a) shows the sensor responses to ethanol for the four devices without Pd-loading. The dependence of the sensor responses on temperature was different than that in H2 detection. All of the devices showed a maximum sensor response at 250°C. It should be noted that the device using the long nanorods (500 nm) delivered an ultrahigh sensor response to 100 ppm ethanol at 250°C. As shown in Table 3, the diffusion coefficient of ethanol is rather small compared with that of H2. Thus, its diffusion would be promoted in a highly


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porous film using the long nanorods (500 nm), increasing the utility factor of the film and thereby boosting the sensor response. The “pore size effect” likely prevails in this case. In contrast, at temperatures higher than 300°C, the sensor response was very low compared with the case at 250°C. The thermally activated catalytic activity of SnO2 itself accelerated the combustion of ethanol at the film surface, decreasing the utility factor of the films. Figure 7 (b) shows the sensor response for the devices with Pd-loading. The Pd-loading further improved the sensitivity of the films, particularly at 250°C. A moderate activation of the catalytic activity by Pd-loading may facilitate the surface reaction of adsorbed oxygen with ethanol.

Figure 7. Sensor responses to 100 ppm ethanol in air as a function of operating temperature for the devices using four different nanocrystals (a) without and (b) with Pd-loading.

Figure 8 (a) summarizes the sensor responses to ethanol at 250°C. It is clear that the sensor response increased with increasing peak pore size. The devices using the long nanorods (500 nm) without and with Pd-loading showed the largest sensor response. Those films have large mesopores peaking at 53 and 95 nm, respectively, easing the diffusion of ethanol through the pores. The observed maximum sensor response to 100 ppm ethanol



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exceeded S = 100,000, i.e., the electrical resistance changed by five orders of magnitude. Using SnO2 nanostructures, sensor responses (S) ranging from 10 to 65 have been reported for 100 ppm ethanol detection. 49-52) Thus, our sensor showed much higher sensor responses to ethanol than the reported sensors. Recently, Lee et al. have reported an ultrahigh sensor response to 5 ppm ethanol using Pt-doped SnO2 hollow nanospheres, reaching S = 1,400.53) Their results also indicate the importance of precise control of SnO2 nanostructures and noble metal-loading in obtaining high sensor responses to ethanol. Figure 8 (b) displays the cross sensitivities of the sensors to H2, CO, ethanol, and acetone at 250°C. The devices using the nanorods showed excellent selectivity to ethanol and acetone, while they had very low sensitivity to H2 and CO. The nanorod sensors also showed a good response speed, as shown in Figure S3. The 90% response times for the devices using Pd/nanorods (50 nm) and (150 nm) were 21 and 15 seconds, respectively. This high response speed should be due to the improved diffusivity of ethanol in the porous films. On the other hand, the recovery speed was rather sluggish; the 90% recovery times for the devices using Pd/nanorods (50 nm) and (150 nm) were 157 and 230 min, respectively. The accumulation of by-products formed after the ethanol surface reaction and their slow desorption from the film may hinder the re-adsorption of oxygen, ultimately leading to the slow recovery.


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Figure 8. (a) Sensor responses to 100 ppm ethanol in air at 250°C for the devices using four different nanocrystals without and with Pd-loading. (b) Cross sensitivity to 100 ppm H2, CO, ethanol, and acetone at 250°C for the devices using the short and middle nanorods with and without Pd-loading.

Simulated gas concentration profiles and sensor response To clarify the effect of pore size on the sensor responses to ethanol, we simulated the ethanol concentration inside films with different pore sizes. For the analysis, we used the following equation, which is based on the time dependence of the gas concentration in a mesoporous film being determined by the gas diffusion rate (Knudsen-type) and the surface reaction rate:11

∂C ∂ 2C = Dk 2 − kC ∂t ∂x

(1)

where C is the concentration of ethanol, t is the time, x is the distance (depth from the surface of a film), Dk is the diffusion coefficient, and k is the rate constant of the surface reaction. By



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solving the equation at the steady-state condition (

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∂C A = 0 ), the following gas concentration ∂t

profile can be obtained:

x )m L , cosh( m )

cosh(1 − C = CS

m=L

k Dk

(2)

where CS is the concentration of a combustible gas at the surface and L is film thickness. The profile shows that the gas concentration depends on the surface reaction rate, diffusion coefficient, and film thickness. The Knudsen diffusion coefficient is obtained as Dk =

4r 3

2 RT , where M is the molecular mass of the gas molecules, R is the gas constant, πM

T is temperature, and r is the pore radius. Thus, the gas concentration profile is finally dependent on pore size if k is constant. Figure 9 (a) shows the simulated dependence of normalized ethanol concentrations in films with different pore sizes as a function of depth from the surface. The gas concentration decreases as ethanol diffuses toward the bottom of the film, depending on pore size. For larger mesopores of more than 50 nm, the concentration difference between the surface and the bottom is approximately 40%. However, when the pore size decreases to 8 nm, the concentration difference increases, reaching 88%. This means that most of the ethanol cannot reach the bottom of the film when the film is dense. Increasing the rate of surface reaction, i.e., the k value, further increases the concentration difference between the surface and the bottom, as shown in Figure 9 (b). The observable sensor response is the integration of the sensor response of each particle. Thus, the film region where gases cannot reach does not contribute to the observable total sensor response (low utility factor of sensing film). It can be expected from the simulation results that increasing the pore size likely increases the ethanol concentration in the total volume of the film, thereby increasing the overall sensor response.


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Figure 9. Simulated ethanol concentration profile inside films with different pore sizes (rp = 8-100 nm). (a) k = 50,000 s-1, (b) k = 100,000 s-1. The film thickness (L) and surface concentration (CS) were set to 20 µm and 100 ppm, respectively.

To confirm this expectation, we then simulated the sensor response (S) using the following equation proposed by Sakai et al.:11

S=

Ra aC k = 1 + S tanh(m) , m = L Rg m Dk

(3)

where a is a constant, defined as a sensitivity coefficient. To use this equation, it is assumed that the sensing film is a uniform stack of infinitesimally thin sheets, and its sheet conductance (σ) is proportional to gas concentration with a sensitivity constant, a.

σ ( x) = σ 0 (1 + aC )

(4)

where σ0 is the conductance in air. Because resistance is the inverse of conductance, integration of 1/σ (x) from the surface (x = 0) to the bottom (x = L) gives the total resistance of the sensing film. The sensor response (S) can be obtained by the total resistances in gas and in air, as expressed in equation (3). Figure 10 (a) and (b) display the simulated sensor response to 100 ppm ethanol at 250°C as a function of pore radius in films with different k



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values. The simulated curves indicate that sensor response is a function of film pore radius because Dk is dependent on pore radius according to equation (3). The experimental data are plotted in Figure 10. For this simulation, two k values of 50,000 and 100,000 were used for SnO2 and Pd/SnO2, respectively, because Pd-loading should enhance the rate of surface reactions. Experimental data obtained using the long nanorods (500 nm) without and with Pd were well fitted on the simulated curves. The results clearly show that the detection of ethanol is diffusion-limited for the nanorod films. The same trend was also confirmed for acetone detection, as shown in Figures S4 and S5, suggesting the validity of our approach to detect larger gases such as ethanol and acetone by pore size control.

Figure 10. Simulated and experimental sensor responses to ethanol (100 ppm) at 250°C as a function of pore radius of the sensing films. The simulation was made with a = 1,500 and k = (a) 50,000, and (b) 100,000. Closed circles (●) correspond to experimental sensor responses for films with different pore radii using long nanorods (500 nm) (a) without and (b) with Pd.

As experimentally and theoretically revealed, increasing film pore sizes enhanced the sensor response to ethanol and acetone. However, it should be noted that the sensor response to ethanol tends to saturate after approximately 80 nm, as seen in the simulation curve (Figure 10). Highly porous thin films usually have high electrical resistances, which are disadvantageous for practical sensors that need a simple electrical circuit. Thus, the pore size


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should also be optimized from the practical point of view. Simultaneous control of the film thickness would provide another solution to produce a film with lower electrical resistances as well as larger pores. Increasing pore size was also beneficial in extending the detection limit. Figure S6 shows the dependence of the sensor responses on ethanol concentration in a log-log scale for the devices using the short and long nanorods (50 and 500 nm) with Pd-loading. Linear calibration curves were obtained by fitting experimental data. The detection limits of ethanol were estimated to be 330 and 3.1 ppb for the short and long nanorods, respectively, by extrapolating the curves to the value of 1.0 (baseline in air). Such very low detection limits may allow for the present sensors to detect alcohols as biomarkers at a ppb concentration in a patient's expired air.

CONCLUSION Hydrothermal methods starting from SnCl4.5H2O in the presence of morphology-directing agents such as ethylenediamine and triethanolamine successfully produced SnO2 nanocubes and nanorods, respectively, under acidic conditions at 200°C. The XRD and TEM measurements suggest that the nanocrystals were single crystalline and possessed good crystallinity. Gas sensing films having pore sizes ranging from approximately 8 to 100 nm were fabricated using the synthesized SnO2 nanocubes and nanorods with different rod lengths. The gas sensing film responses (S) to H2 showed a volcano-type dependence on temperature, with a maximum S at 300°C. The sensor responses increased in the order of nanocubes (13 nm) > short nanorods (50 nm) > middle nanorods (150 nm) > long nanorods (500 nm). This tendency is in good agreement with that of crystal size, suggesting that the sensor response to H2 is controlled by the crystal size. In contrast, the sensor responses to ethanol were strongly dependent on the pore size of the gas sensing films and showed a maximum at 250°C. Notably, the sensor device using the long nanorods showed an ultrahigh sensor response to 100 ppm ethanol, exceeding S = 100,000 at 250°C. The ethanol



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concentration profile inside the films was simulated on the basis of Knudsen diffusion and surface reaction kinetics. It was revealed that ethanol can easily diffuse through mesopores larger than 50 nm. The simulated sensor response to ethanol was found to increase as the pore size increases, which accords with the experimental data. The simulation results clearly revealed that the sensor response to ethanol is controlled by pore size for the nanorod films.

ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (B) (No. 26288107) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. This work was also financially supported by Industrial Technology Research Grant Program in 2011 (No. 11b15004d) from New Energy and Industrial Technology Development Organization (NEDO).

Supporting Information Synthesis method of nanorods with different rod lengths. TEM and SEM images of nanocubes and nanorods with different lengths. Simulated acetone concentration profile inside films, simulated and experimental sensor response to acetone, and dependence of sensor response on ethanol concentration. These materials are available free of charge via the Internet at http://pubs.acs.org.


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Table of Contents



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Fig.1 287x241mm (96 x 96 DPI)


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Fig.2 324x261mm (96 x 96 DPI)



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Fig.3 314x236mm (96 x 96 DPI)


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Fig.4 374x134mm (96 x 96 DPI)



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Fig.5 368x139mm (96 x 96 DPI)


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Fig.6 374x137mm (96 x 96 DPI)



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Fig.7 374x152mm (96 x 96 DPI)


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Fig.8 370x180mm (96 x 96 DPI)



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Fig.9 374x144mm (96 x 96 DPI)


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Fig.10 373x156mm (96 x 96 DPI)



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TOC 337x91mm (96 x 96 DPI)


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Ultrasensitive Detection of Volatile Organic Compounds by a Pore

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