Humidity-Independent Gas Sensors Using Pr-Doped In2O3

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 25322−25329

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Humidity-Independent Gas Sensors Using Pr-Doped In2O3 Macroporous Spheres: Role of Cyclic Pr3+/Pr4+ Redox Reactions in Suppression of Water-Poisoning Effect Jun-Sik Kim,† Chan Woong Na,‡ Chang-Hoon Kwak,† Hua-Yao Li,§ Ji Won Yoon,† Jae-Hyeok Kim,† Seong-Yong Jeong,† and Jong-Heun Lee*,† †

Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea Dongnam Regional Division, Korea Institute of Industrial Technology, Busan 46938, Republic of Korea § School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, P. R. China

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‡

S Supporting Information *

ABSTRACT: Pure and 3−12 at. % Pr-doped In2O3 macroporous spheres were fabricated by ultrasonic spray pyrolysis and their acetonesensing characteristics under dry and humid conditions were investigated to design humidity-independent gas sensors. The 12 at. % Pr-doped In2O3 sensor exhibited approximately the same acetone responses and sensor resistances at 450 °C regardless of the humidity variation, whereas the pure In2O3 exhibited significant deterioration in gas-sensing characteristics upon the change in the atmosphere, from dry to humid (relative humidity: 80%). Moreover, the 12 at. % Prdoped In2O3 sensor exhibited a high response to acetone with negligible cross responses to interfering gases (NH3, CO, benzene, toluene, NO2, and H2) under the highly humid atmosphere. The mechanism for the humidity-immune gas-sensing characteristics was investigated by X-ray photoelectron and diffuse reflectance infrared Fourier transform spectroscopies together with the phenomenological gas-sensing results and discussed in relation with Pr3+/ Pr4+ redox pairs, regenerative oxygen adsorption, and scavenging of hydroxyl groups. KEYWORDS: oxide semiconductor gas sensor, indium oxide, praseodymium, humidity dependence, cyclic redox reactions

1. INTRODUCTION

and suggested that the humidity-immune sensing mechanism originated from the scavenging of hydroxyl groups and formation of ionized oxygen on the In2O3 surface owing to cyclic redox reactions of trivalent/tetravalent redox pairs: Ce3+/Ce4+.12 Among lanthanide elements, only three elements, cerium (Ce), terbium (Tb), and praseodymium (Pr), have these tri/tetravalent states.13 Indeed, the doping of multivalent Tb significantly suppressed the degradation of gas-sensing characteristics by water vapors in a sensor using SnO2 yolk− shell spheres.8 This further supports the result that the coexistence of tri/tetravalent states has a key role in preventing the water-poisoning effect. In order to evaluate the general validity regarding the role of multivalency, it would be appropriate to check whether Pr addition is also effective to achieve gas-sensing characteristics of oxide semiconductor chemiresistors unhampered by water vapors.

Metal oxide semiconductor gas sensors have been intensively explored owing to their high sensitivity, rapid response, simple device structure, facile integration, and production flexibility.1,2 However, upon exposure to the ubiquitous water vapor in the atmosphere, the gas response significantly decreases and the sensor resistance significantly changes.3,4 Accordingly, the gassensing characteristics under humid conditions have been investigated using various materials such as SnO2,5 Pd−SnO2,6 Ni−SnO2,7 Tb−SnO2,8 WO3,9 Si−WO3,10 Pt−WO3,11 Pd− WO3,11 and Ce−In2O3.12 Note that most of the previous reports presented gas-sensing characteristics under different humidity conditions but only few studies explored the possible solution to suppress the humidity dependence of the gassensing characteristics. Therefore, it is important to develop a new approach to mitigate the water-vapor-poisoning effect in order to realize reliable gas sensors with humidity-independent sensing properties. In our previous contribution, we reported nanoceria-loaded In2O3 hollow spheres assisted by layer-by-layer coating process for stable acetone sensing regardless of the humidity variation © 2019 American Chemical Society

Received: April 12, 2019 Accepted: June 24, 2019 Published: June 24, 2019 25322

DOI: 10.1021/acsami.9b06386 ACS Appl. Mater. Interfaces 2019, 11, 25322−25329

Research Article

ACS Applied Materials & Interfaces The most stable form of praseodymium oxide is Pr6O11, exhibiting the highest oxygen mobility and high redox catalytic activity among the above three lanthanide oxides.14−16 This can be attributed to the high reducibility of Pr4+ to Pr3+. Accordingly, Pr has been used as an additive to enhance the catalytic activities of ceria-based materials.17 Likewise, owing to the excellent cyclic Pr3+/Pr4+ redox reactions, praseodymium has been widely used as a promoter in various fields such as oxidation catalysts,18,19 sonocatalysts,20 photocatalysts,21 solid electrolytes,22 and three-way catalysts.23 For gas sensor applications, only a few studies reported acetic acid detection using Pr-doped ZnO gas sensors and ethanol detection using Pr-doped ZnSn(OH)6 hollow microspheres.24,25 However, to the best of our knowledge, the design of humidity-independent gas sensors through praseodymium doping has not been investigated. In this study, pure and Pr-doped In2O3 gas sensors are prepared and their gas-sensing behaviors under dry and humid conditions are investigated at various temperatures. All the specimens are prepared by a single-step spray pyrolysis using polystyrene (PS) spheres as sacrificial templates to fabricate highly gas-accessible macroporous spheres. A 12 at. % Prdoped In2O3 gas sensor exhibits gas-sensing characteristics unaffected by water vapors over a wide range of relative humidity: (0−80%) at 450 °C, high response to acetone, and negligible cross responses to interfering gases. The main focus of this study is to understand the humidity-immune sensing mechanism in relation with redox reactions of Pr3+/Pr4+ and confirm the general applicability of multivalent lanthanide elements as hydroxyl scavengers for surface regeneration in metal oxide gas sensors.

Figure 1. XRD patterns of the (a,e) pure In2O3, (b,f) 3Pr−In2O3, (c,g) 6Pr−In2O3, and (d,h) 12Pr−In2O3 macroporous spheres.

concentration of praseodymium (Figure 1e−h). In3+ ions in the cubic In2O3 have a coordination number (CN) of 6; the ionic radius at the CN of 6 is 0.80 Å. A fine-scanned Pr 3d Xray photoelectron spectrum reveals that praseodymium ions exist in the form of Pr3+ or Pr4+ in all the samples, which is discussed below. The radii of Pr3+ and Pr4+ ions at the CN of 6 are 0.99 and 0.85 Å, respectively. Therefore, the down-shift of the (222) peak can be attributed to the expansion of the In2O3 lattice by the substitution of the larger Pr3+/Pr4+ ions at the sites of In3+. The crystallite sizes of the cubic In2O3 phase with various Pr contents (0, 3, 6, and 12 at. %) were calculated using the (222), (400), (440), and (622) peaks by Scherrer’s formula. The mean size of the crystallites monotonously decreased with the increase in the Pr concentration (In2O3: 22.7 ± 1.8 nm, 3Pr−In2O3: 20.1 ± 2.3 nm, 6Pr−In2O3: 18.1 ± 1.1 nm, and 12Pr−In2O3: 14.2 ± 3.1 nm). 3.2. Scanning Electron Microscopy and TEM Characterizations. All the specimens prepared by ultrasonic spray pyrolysis exhibited spherical morphologies (Figure 2). The average diameters of ∼200 spheres in the In2O3, 3Pr−In2O3, 6Pr−In2O3, and 12Pr−In2O3 specimens were 707 ± 327, 739 ± 355, 753 ± 282, and 717 ± 352 nm, respectively. In all the samples, macropores were observed on the surfaces of spheres, which were developed by the decomposition of the PS spheres (diameter ≈ 100 nm); no significant structural differences were observed between the specimens (insets shown in Figure 2). To investigate the inner structures of the spheres further, transmission electron microscopy (TEM) characterizations were performed (Figure 3). The macroporous morphologies of the In2O3 and Pr−In2O3 spheres were more clearly observed in TEM images. The macropores were expected to be interconnected by mesopores formed by outgassing from the decomposition of the PS templates during spray pyrolysis process and subsequent heat treatment. In addition, this can be confirmed by Brunauer−Emmett−Teller (BET) analysis on 12Pr−In2O3 specimen (Figure S1). Note that some parts of the spheres have hollow inner structures which can be explained by a brighter center part than an edge-side region (Figure 3b,e,h,k). These interconnected or hollow macroporous structures with high gas accessibility can be an excellent

2. EXPERIMENTAL SECTION The pure and praseodymium-doped In2O3 macroporous spheres were obtained by ultrasonic spray pyrolysis. A spray solution was produced by dissolving indium(III) nitrate hydrate (In(NO3)3·xH2O, 99.99%, Aldrich) and praseodymium(III) nitrate hexahydrate (Pr(NO3)3· 6H2O, 99.9%, Aldrich) in 250 mL of distilled water. PS (1.5 g) spheres (diameter: ∼100 nm) were then dispersed in the spray solution as sacrificial templates to obtain macropores. The atomic concentration of praseodymium was controlled by changing the mixing ratio of the two metal salts ([Pr]/([Pr] + [In]) = 0, 3, 6, and 12 at. %); the total concentration of metal salts in the spray solution was fixed at 0.03 M. The droplets of the spray solutions were nebulized by five ultrasonic transducers (resonance frequency: 1.7 MHz) and subsequently conveyed by a carrier gas (air, flow rate: 20 L m−1) to a quartz reactor (length: 1200 mm, diameter: 50 mm). The temperature of the quartz reactor was 700 °C. Precursor spheres were collected by a Teflon bag filter and annealed at 650 °C for 3 h to remove residual carbon components (heating rate: 10 °C min−1). For simplicity, In2O3 macroporous spheres with different praseodymium doping concentrations are referred to as In2O3, 3Pr−In2O3, 6Pr− In2O3, and 12Pr−In2O3. Experimental methods for the characterization and gas-sensing tests are described in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. XRD Patterns. The X-ray diffraction (XRD) pattern of In2O3 (Figure 1a) shows the highly crystalline cubic structure of In2O3 [Joint Committee on Powder Diffraction Standards (JCPDS) no. 06-0416]. Only the cubic In2O3 phase was also observed in 3Pr−In2O3, 6Pr−In2O3, and 12Pr−In2O3; no peak attributed to the addition of praseodymium was observed (Figure 1b−d). The position of the (222) peak was substantially shifted to a lower angle with the increase in the 25323


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Figure 2. Scanning electron microscopy images of the (a) pure In2O3, (b) 3Pr−In2O3, (c) 6Pr−In2O3, and (d) 12Pr−In2O3 macroporous spheres.

platform for gas sensors.26,27 In the lattice-resolved TEM image and fast Fourier transform electron diffraction pattern of the pure In2O3, the (112̅) and (21̅1̅) fringes separated by a distance of 4.12 Å correspond to the cubic In2O3 structure (Figure 3c). The TEM analysis further reveals that the (12̅1) fringes of In2O3 in 6Pr−In2O3 are separated by a distance of approximately 4.15 Å (Figure 3i), whereas the (11̅2) and (1̅12) fringes in 12Pr−In2O3 are separated by 4.20 and 4.19 Å, respectively (Figure 3l). The increase in the interplanar distance of the {211} lattice planes with the increase in the Pr concentration is consistent with the lattice expansion because of the Pr-substitution observed in the XRD analysis. An energy-dispersive X-ray spectroscopy elemental mapping of 3Pr−In2O3, 6Pr−In2O3, and 12Pr−In2O3 shows the uniform distributions of the Pr element (Figures S2 and 3m), confirming the doping of Pr into the In2O3 lattice. 3.3. X-ray Photoelectron Spectroscopy. Survey and fine-scanned X-ray photoelectron spectroscopy (XPS) profiles of In2O3, 3Pr−In2O3, 6Pr−In2O3, and 12Pr−In2O3 are shown in Figure S3 and Figures 4−5, respectively. We also carried out an XPS analysis on Pr6O11 (99.9%, Aldrich) to obtain reference spectra. The experimental Pr 3d5/2 XPS spectrum was curvefitted by Voigt profiles to investigate the chemical state of praseodymium in Pr6O11, 3Pr−In2O3, 6Pr−In2O3, and 12Pr− In2O3 (Figure 4a−d); the results are summarized in Table 1. According to the curve fitting of Pr 3d5/2 spectra in the Pr6O11 by Al Kutubi et al.,28 the XPS peaks of ∼931 and ∼935 eV should be corresponding to Pr4+ species, while the XPS peaks of ∼928 and ∼933 eV should be matched to Pr3+ species. As the Pr content increases in Pr−In2O3 (Figure 4b−d), the ratio of Pr3+/Pr4+ increased (Table 1). The area percentages of Pr3+ and Pr4+ components were 69.4 and 30.6% in the fitted Pr 3d5/2 spectra for 12Pr−In2O3 while those for Pr6O11 were 59.5 and 41.5%, respectively. The relatively high amount of Pr3+ in Pr6O11 in the present study is consistent with that reported by Al Kutubi et al. who suggested the formation of hydroxyl group

Figure 3. TEM images of the (a−c) pure In2O3, (d−f) 3Pr−In2O3, (g−i) 6Pr−In2O3, and (j−l) 12Pr−In2O3 macroporous spheres. (m) Elemental mapping of the 12Pr−In2O3 macroporous sphere.

near the Pr6O11 surface as the reason.28 Considering that XPS is a surface-weighted analysis, the highest Pr3+ area percentages in 12Pr−In2O3 specimen in the present study suggests the abundancy of Pr3+ at the surface. The peaks in the O 1s spectra for the In2O3, 3Pr−In2O3, 6Pr−In2O3, and 12Pr−In2O3 can be deconvoluted into four peaks (OI: Pr3+−O, OII: indium oxide lattice oxygen, OIII: oxygen vacancy and adsorption of oxygen complex, and OIV: Pr4+−O) (Figure 4e−f). The OI and OIV components at ∼528.5 and ∼531.5 eV belong to Pr3+−O and Pr4+−O, respectively.29 The OIII peak normally corresponds to oxygen vacancy and adsorption of OH−, CO32−, O−, O22−, and so forth.30 Praseodymium oxide exhibits a high OIII peak31,32 owing to the adsorption of hydroxyl contents owing to their highly basic33−35 and hygroscopic properties.36,37 Therefore, the OIII peak of the Pr-doped In2O3 spheres can be attributed to the hydroxyl group component. The area percentage of OIII monotonously increases with the increasing concentration of praseodymium (Table 2), implying that the adsorption of surface hydroxyl groups was promoted by the basic and hygroscopic nature of praseodymium. The In 3d5/2 peak of the pure In2O3 macroporous spheres is observed at 443.6 eV (Figure 5a). The position of the In 3d5/2 peak tends to shift from 443.6 to 443.9 eV with the increase in the Pr concentration (Figure 5, Table 3). This shift can be attributed 25324


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Table 1. Binding Energies and Compositions of the Pr 3d5/2 Photoelectron Lines of the Reference Pr6O11 and Pr-Doped In2O3 Macroporous Spheres Pr 3d composition (%)

Pr 3d5/2 binding energy (eV) 928 24.4 19.2 15.6 18.7

Pr6O11 3Pr−In2O3 6Pr−In2O3 12Pr−In2O3

931 27.9 32.9 26.9 23.3

933 35.1 39.6 48.7 50.7

935 12.6 8.3 8.6 7.3

Pr3+ 59.5 59.8 64.3 69.4

Pr4+ 41.5 41.2 35.5 30.6

Table 2. Area Percentages of O 1s Photoelectron Lines of the Pure and Pr-Doped In2O3 Macroporous Spheres area percentage (%) O 1s binding energy (eV)

OI (528.5)

OII (529.3)

OIII (530.5)

OIV (531.5)

In2O3 3Pr−In2O3 6Pr−In2O3 12Pr−In2O3

17.9 19.3 24.1

70.3 37.9 35.3 25.2

29.3 32.9 36 39.5

9.3 9.4 11.2

Table 3. Binding Energies of the In 3d3/2 and In 3d5/2 Photoelectron Lines of the Pure and Pr-Doped In2O3 Macroporous Spheres In 3d binding energy (eV) sample

Figure 4. Fine-scan XPS profile of (a−d) Pr 3d and (e−h) O 1s for the (a) reference Pr6O11, (e) pure In2O3, (b,f) 3Pr−In2O3, (c,g) 6Pr− In2O3, and (d,h) 12Pr−In2O3 macroporous spheres.

In 3d3/2 In 3d5/2

In2O3

3Pr−In2O3

6Pr−In2O3

12Pr−In2O3

451.2 443.6

451.2 443.6

451.4 443.8

451.5 443.9

in Figure 6a−d. The sensor resistance decreased upon exposure to the reducing gas and returned to the original value in the air. The gas response (S = Ra/Rg; Ra: resistance in the air, Rg: resistance in the gas) was calculated. For the pure In2O3 gas sensor, not only Ra but also S significantly decreased upon the change from the dry atmosphere to an RH of 80% (Figure 6e,i). The In2O3 sensor exhibited a high gas response to 20 ppm of acetone in the dry atmosphere (Sdry = 39.9−91.7) in the entire temperature range; however, the response significantly decreased at the RH of 80% (Swet = 7.8−16.7) (Figure 6i). In addition, the resistance of the sensor (Ra) substantially changed from Ra/dry = 327−472 kΩ to Ra/wet = 57−96 kΩ when the dry atmosphere was changed to the atmosphere with an RH of 80% (Figure 6e). These results suggest that the sensor cannot be operated in a precise and reliable manner without suppression of the moisture dependence. In contrast, the sensor resistances in air (Figure 6e−h) and gas responses (Figure 6i−l) in the dry and humid (RH = 80%) atmospheres tended to become similar with the increase in the Pr doping concentration. For a quantitative comparison of the gas-sensing characteristics in the dry and humid atmospheres, the resistance ratio (Ra‑wet/Ra‑dry) and response ratio (Swet/Sdry) were calculated (Figure 7). When the resistance and response ratios approach unity, we can regard that the sensor is humidity independent. The In2O3 gas sensor was easily poisoned by water vapor; that is, the overall Ra‑wet/Ra‑dry and Swet/Sdry values were the lowest. It is worth noting that the Ra‑wet/Ra‑dry and Swet/Sdry values increased with the Pr doping concentration, confirming that the Pr doping was effective to provide the gas-sensing

Figure 5. Fine-scan XPS profile of In 3d for the (a) pure In2O3, (b) 3Pr−In2O3, (c) 6Pr−In2O3, and (d) 12Pr−In2O3 macroporous spheres.

to the electron transfer from In2O3 to Pr ions, which will increase the Pr3+/Pr4+ ratio by the reduction of Pr4+ to Pr3+. This is consistent with the Pr3+/Pr4+ ratios attained by XPS results. 3.4. Gas-Sensing Characteristics. The sensing transients to 20 parts per million (ppm) of acetone at 450 °C are shown 25325


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500 °C. For all the sensors in this study, the resistances under dry and humid conditions became similar (Ra‑wet/Ra‑dry increased) with the increase in the sensing temperature, which is attributed to the promotion of hydroxyl group desorption. However, it should be noted that the waterpoisoning effect on the gas-sensing characteristics is still very high in the pure In2O3 sensor even at 450 °C, indicating that the complete removal of hydroxyl groups in the undoped In2O3 sensor is difficult or requires a higher sensing temperature. The humidity dependence of the gas response and sensor resistance obviously decreased with the increase of the Pr doping concentration, which implies that the Pr doping has an additional key role in the stable sensing characteristics of the Pr-doped In2O3 gas sensor, unaffected by water vapor. 3.5. Selectivity and Stability. The sensing characteristics of the In2O3 and 12Pr−In2O3 sensors to various gases [20 ppm of acetone, ammonia (NH3), carbon monoxide (CO), benzene, toluene, nitrogen dioxide (NO2), and hydrogen (H2)] were investigated under dry and humid conditions at 450 °C (Figure 8). Both sensors exhibited higher responses to

Figure 6. Dynamic gas-sensing transients, sensor resistances, and gas responses of the (a,e,i) pure In2O3, (b,f,j) 3Pr−In2O3, (c,g,k) 6Pr− In2O3, and (d,h,l) 12Pr−In2O3 macroporous spheres to 20 ppm of acetone at 450 and 350−450 °C, respectively, under dry and humid (RH = 80%) conditions.

Figure 8. Gas responses of the (a,b) pure In2O3 and (c,d) 12Pr− In2O3 macroporous spheres to 20 ppm of acetone (A), hydrogen (H), nitrogen dioxide (O), toluene (T), benzene (B), carbon monoxide (C), and ammonia (N) at 450 °C under (a,c) dry and (b,d) humid (RH = 80%) conditions.

acetone than those to the other gases. To quantify the selectivity of the sensor to acetone gas, Sacetone/Sother gas was calculated. The In2O3 gas sensor exhibited a high response to 20 ppm of acetone (S = 39.9) and high selectivity to acetone gas (Sacetone/Sother gas = 4.0−10.8) under the dry conditions (Figure 8a). However, under the humid conditions (RH = 80%), the response to the acetone gas significantly decreased (S = 16.7) (Figure 8b). For the 12Pr−In2O3 sensor, the gas responses under the dry and humid conditions were similar (Figure 8c,d); excellent selectivity values to acetone were observed under both dry (Sacetone/Sother gas = 2.95−6.41) and humid (Sacetone/Sother gas = 3.61−7.53) conditions. Acetone responses are generally high in metal oxide gas sensors because of highly polar nature of acetone and low dissociation energy of CH3−COCH3 bond.39,40 Thus, In2O3 is known as a sensing material to selectively detect acetone gas.41−43 These results indicate that the proposed 12Pr−In2O3 gas sensor can selectively detect acetone gas regardless of the humidity

Figure 7. (a) Resistance ratios (Ra/wet/Ra/dry) and (b) response ratios (Swet/Sdry) of the pure, 3Pr−, 6Pr−, and 12Pr−In2O3 macroporous spheres under 20 ppm of acetone measured in the range of 350−450 °C (dry: RH = 0%, wet: RH = 80%).

characteristics insusceptible to water vapors. In addition, the sensor became more robust against water poisoning with the increase in the sensing temperature to 450 °C. In particular, the 6Pr− and 12Pr−In2O3 sensors exhibited almost moistureindependent gas-sensing characteristics (Ra‑wet/Ra‑dry and Swet/ Sdry ≅ 1) at 450 °C. Yamazoe et al.38 studied the adsorption and desorption characteristics of surface hydroxyl groups on metal oxide surfaces and reported that surface hydroxyl groups begin to desorb at 250 °C but cannot be completely removed even at > 25326


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signal. Water vapor also reacts with the negatively charged surface oxygen competitively with other reducing gases, which generates hydroxyl groups on the surface and releases electrons to the materials, leading to substantial changes in gas response and sensor resistance. The typical water-poisoning mechanism of the n-type oxide semiconductor is expressed by the following reaction depending on the charged oxygen species on the surface

variation. The dynamic sensing transients of the 12Pr−In2O3 gas sensor upon exposure to 1−5 ppm of acetone at 450 °C under both dry and humid conditions showed the humidityindependent sensing characteristics (Figure 9a). The linear

H 2O + Oad−(or Oad 2 −) ↔ 2OH + e−(2e−)

(1)

The decreases in both Ra and S under the humid atmosphere observed in this study are in agreement with the above waterpoisoning reaction. The decrease in the moisture dependence of the gas-sensing characteristics with the increase in the sensing temperature to 450 °C can be explained by the promotion of dehydration reaction at elevated temperature.38 However, the suppression of the moisture dependence of the gas-sensing characteristics due to the Pr doping at a constant sensor temperature has a different origin. In a previous report, we proposed a regenerative In2O3 surface in a humid atmosphere by cyclic redox reactions of Ce3+/Ce4+ ions.12 Likewise, praseodymium has been used as a dopant in various catalyst applications owing to its excellent reversible redox cycle properties (Pr3+/Pr4+ redox pairs). For example, Jiang et al.19 suggested that reversible Pr3+/Pr4+ redox pairs on nanoceria supports significantly promote the oxidation of 3,3′,5,5′-tetramethylbenzidine. Hanifehpour et al.21 reported that doping of Pr into the ZnS lattice enhances the photocatalytic activity by facilitating the formation of oxygen radical owing to the facile Pr3+/Pr4+ redox reactions. On the basis of the above studies, we suggest the following reaction to explain the robust sensing characteristics of the Pr-doped In2O3 sensor against water vapors

Figure 9. (a) Dynamic sensing transients of the 12Pr−In2O3 macroporous spheres upon the exposure to 1−5 ppm of acetone at 450 °C under dry (red) and humid (RH = 80%) (blue) conditions. (b) Responses of the 12Pr−In2O3 macroporous spheres to various concentrations of acetone measured at 450 °C. (c) 15 repetitive sensing transients of the 12Pr−In2O3 macroporous spheres upon exposure to 20 ppm of acetone at 450 °C under humid conditions (RH = 80%).

regression line fitting of the acetone gas responses as a function of the acetone concentration under the humid conditions revealed that the detection limitation concentration was 85 ppb when Ra/Rg > 1.2 was used as the standard for gas detection (Figure 9b). Exhaled breath can contain acetone gas owing to many different factors such as diabetes, fasting, caloric restrictions, and intense exercise.44,45 For example, the exhaled breath acetone concentration of a type-I diabetic patient is higher than 1.8 ppm, whereas that of a healthy person is below 0.8 ppm.46 The acetone concentration in the exhaled breath can increase up to 40 ppm by ketogenic diet and to 170 ppm by fasting.47 Moreover, the exhaled breath contains large amounts of water vapor; therefore, humidity-immune acetone sensing could be very useful in the diagnosis of diabetes mellitus or monitoring of dietary fat loss in the body with no or less preconditioning steps such as dehumidification.48 Therefore, the 12Pr−In2O3 acetone sensor with a low detection limit and negligible water poisoning is very promising for a breath acetone analysis. Repetitive measurements were carried out at 450 °C at an RH of 80% using 20 ppm of acetone gas to confirm the stability of the sensor. The 12Pr−In2O3 gas sensor exhibited an excellent stability during 15 repetitions of gas injection and stable recovery characteristics (Figure 9c). In addition, we carried out measurements under different RH (20−80%); the 12Pr−In2O3 gas sensor exhibited similar responses independent of the humidity (Figure S4). These results indicate that the 12Pr−In2O3 gas sensor can detect acetone gas independent of the humidity level in a stable manner. 3.6. Sensing Mechanism. The n-type oxide semiconductor gas sensor forms an electron depletion layer near the surface by adsorption, dissociation, and ionization of oxygen at 200−500 °C, which leads to the increase in the sensor resistance. The reaction between the negatively charged surface oxygen and the reducing analyte gas releases electrons; the consequent decrease in sensor resistance is used as a sensor

Pr 3 + + 2OH → Pr 4 + + H 2O + Oad−

(2)

Pr 4 + + e− → Pr 3 +

(3) 3+

It is worth noting that Pr ions scavenge hydroxyl groups and Pr4+ ions can be easily reduced to Pr3+ by the electrons provided from the In2O3. Therefore, it can be concluded that Pr3+ has a key role for the suppression of the water-poisoning effect. In a previous study on nanoceria-loaded CeO2, the role of Ce3+ ions as hydroxyl scavengers was also emphasized.12 The most stable form of praseodymium oxide is Pr6O11, where substantial amounts of trivalent cations already exist in their stable phase,14 and the abundancy of Pr3+ in Pr-doped In2O3 specimens of the present study was confirmed by XPS analysis. In this perspective, praseodymium is a good hydroxyl scavenger in our system. The above cyclic redox reactions 2 and 3 can eliminate excess electrons and regenerate the adsorbed oxygen, which promotes the reverse process of the water-poisoning reaction 1. Accordingly, the sensor resistance and gas response are hardly affected by reaction 1 with the increase in the concentration of Pr. The reactants and products are equal when we sum reactions 1−3 if O− is assumed to be predominant; therefore, the gas response and resistance ratios would converge to unity when all the hydroxyl groups on the surface of In2O3 are removed by Pr3+ ions. It should be noted that the gas response and resistance ratio increase above 1 when an excess amount of praseodymium (24 at. % praseodymium) was added to In2O3, which suggests that the gas response and sensor resistance under humid conditions 25327



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are even higher than those under dry conditions (Figure S5). This implies that not only hydroxyl groups on the In2O3 surface but also those adsorbed on praseodymium ions are involved in reaction 2, which generate additional ionized oxygen and eliminate the higher concentration of electrons. To confirm the origin of hydroxyl groups on praseodymium ions in 12Pr−In2O3, a diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy measurement of 12Pr− In2O3 under the sensing conditions (450 °C and RH = 80%) was carried out. Two peaks around 3593 and 3624 cm−1 corresponding to Pr(OOH) and Pr(OH)3 were observed,49 respectively (Figure S6). These hydroxyl groups can be attributed to the high hygroscopicity and basicity of praseodymium, as discussed in the XPS analysis. Therefore, it is reasonable that the hydroxyl groups in reaction 2 include hydroxyl groups migrated from the matrix indium ions and directly bonded to dopant praseodymium ions; both kinds of hydroxyl groups can promote the reverse reaction of water poisoning by reacting with Pr3+ and provide the control of Ra and gas response in humid atmosphere, although further systematic study is needed to elucidate the detailed mechanism.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +82-2-928-3584. Phone: +82-2-3290-3282. ORCID

Jong-Heun Lee: 0000-0002-3075-3623 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This research was supported by the International Research & Development Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (MSIP) of Korea (NRF2017K1A3A1A49069947) and the Industrial Strategic Technology Development Program (10073068, Development of Miniaturized 10 mW TVOC/Alcohol Dual Gas Sensor and Module using Non-Silicon AAO Ceramic Substrate) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea).

4. CONCLUSIONS Oxide semiconductor gas sensors robust against water poisoning were designed using Pr-doped In2O3 macroporous spheres. The doping of Pr into the In2O3 sensor significantly suppressed the humidity dependence of the gas response and sensor resistance. The 12 at. % Pr-doped In2O3 sensor exhibited almost the same gas-sensing characteristics in the wide range of humidity (RH: 0−80%). According to the phenomenological gas-sensing characteristics and spectroscopic studies using XPS and DRIFT, the humidityindependent gas-sensing characteristics of the Pr-doped In2O3 gas sensors were attributed to the 3+/4+ redox pairs of the praseodymium ions, which facilitated the scavenging of surface hydroxyl groups, regenerative oxygen adsorption, and consumption of electrons, that is, the reverse reaction of water poisoning. The electron transfer from In ions to Pr ions and high hygroscopicity of Pr were also beneficial to achieve gassensing characteristics insusceptible to water vapors. The doping of Pr can be considered as a new promising strategy to design highly reliable oxide semiconductor gas sensors, which can be used in a wide range of ambient humidity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06386. BET analysis on 12Pr−In2O3 macroporous spheres; elemental mapping of 3Pr−, 6Pr−In2O3 and macroporous spheres; XPS survey scan spectra of the specimens; dynamic gas-sensing transients of 12Pr− In2O3 macroporous spheres exposed to 20 ppm acetone at 450 °C in different relative humidity conditions; dynamic gas-sensing transients of 24Pr−In2O3 macroporous spheres exposed to 20 ppm acetone at 450 °C in dry and humid conditions; and DRIFT spectra of the pure and 12Pr−In2O3 macroporous spheres exposed in humid condition at 450 °C (PDF) 25328


Research Article

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