Rh-catalyzed WO₃ with anomalous humidity dependence of gas sensing characteristics

Abstract

The sensing of volatile organic compounds is crucial in a variety of fields including disease diagnosis, food, and homeland security. However, the significant deterioration of gas response by water vapors often hinders the sensitive and reliable gas detection in a highly humid atmosphere. Herein, we report an Rh-loaded WO₃ hollow sphere chemiresistive sensor that can be potentially used for acetone gas analysis in a highly humid atmosphere. Pure WO₃ and Rh-loaded WO₃ hollow spheres are synthesized via a spray pyrolysis method. The Rh-loaded WO₃ sensor achieved a fast acetone response (2 s), high sensitivity, good linearity, high stability, low detection limit (40 ppb) and strong selectivity to acetone even under a highly humid (80% RH) atmosphere, compared with the unloaded WO₃ sensor. Interestingly, an abnormal phenomenon occurs only with the n-type Rh-loaded WO₃ sensor, where the resistance and gas response increases in humid atmospheres. The sensing mechanism by Rh loading is also addressed. The unusual improvement of gas response, selectivity, responding kinetics by Rh loading shows a good potential for the detection of acetone gas.

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1. Introduction

The sensing of volatile organic compounds (VOCs) is crucial in a numerous fields, including disease diagnosis, food, and homeland security.¹ Some VOCs are important for their role in disease biomarking (e.g. acetone for diabetes), toxicity, flammability or other hazardous characteristics, making the monitoring and detection of trace VOCs of prime importance.² Recently, metal–oxide semiconducting (MOS) materials have been indicated to be good candidates for VOC gas sensors because of their irreplaceable advantages, such as low cost, simple fabrication, miniaturized size, portability and good compatibility with Si processes.³

Nowadays, numerous researches have been focused on highly active nanomaterials, including nanoparticles,⁴ one-dimensional nanostructures,⁵ nanosheets⁶ and hollow spheres,⁷ as well as their building blocks,⁸ to develop VOC gas sensors with enhanced performance. It has been also identified that hollow and porous nanoarchitectures are more promising for large surface area to volume ratio, high gas accessibility and rapid gas responding kinetics.⁹ With such porous nanostructured MOS sensors, far-ranging gases and vapors can be efficiently detected and monitored with powerful sensitivity.¹⁰ However, the deterioration of sensing properties by water vapors in real-atmospheres may cause obstacles for the reliability of MOS sensors, which is an acute issue for MOS sensors. If the humidity-dependent problem could be adequately rectified, MOS hollow spheres sensors would be suitable to serve in various application areas because they possess advantages in terms of sensitivity, rapid response and miniaturization capabilities.⁷

Gas sensing films have been doped or surface-functionalized to achieve gas selectivity.¹¹ To date, numerous experimental and theoretical works are reported to detect acetone gas using modified MOS films, such as Pt–In₂O₃,¹² Pt–WO₃,¹³ Cr–WO₃,¹⁴ Si–WO₃,^15–17 graphene modified ZnFe₂O₄,¹⁸ and so forth.¹⁹ Some of them have demonstrated superior responses to acetone below 1 ppm in highly humid atmospheres (relatively humidity (RH) ≥ 80%) for the application of breath acetone monitoring.^16,17,20 Nevertheless, their gas sensing behaviors at varying humidity from RH 20% to RH 80% are rarely involved. In general, the introduction of water vapor into the atmosphere deteriorates gas response, response/recovery speed, and selectivity to the reducing gas in n-type oxide semiconductor sensors,²¹ which hinders the reliable and rapid detection of the trace concentration of analyte gases. It should be noted that the enhancement of gas response in highly humid atmosphere, which is highly advantageous to detect VOC gases in real atmosphere, has been scarcely reported, although there was a report on the enhancement of CO response by increasing humidity in a Pd-loaded SnO₂ sensor.²² The humidity-independent characteristics of gas sensors show a possibility of exhaled breath analysis because exhaled breath contains >80% humidity at 25 °C. Therefore, to materialize superior sensors, it requires not only a production of a preferable sensing material with VOCs selectivity and fast response, but also to make progressive investigations on the control of sensing properties in highly humid atmospheres.

In this paper, we report an Rh-loaded WO₃ hollow spheres (HWs) acetone sensor that can be potentially used for real-time VOCs analysis. This sensor achieves fast response (2 s), high sensitivity, low detection limit (40 ppb) and strong selectivity to trace acetone gas even under a highly humid (80% RH) atmosphere, comparing with the unloaded one. It is found that the n-type Rh-loaded WO₃ sensor exhibits an abnormal phenomenon, as the resistance and gas response increase in humid atmospheres. This unusual improvement can provide a good potential in the real-time acetone gas analysis for the application in diabetic diagnosis.

2. Experimental

2.1 Sample preparation

The WO₃ hollow spheres were prepared using a pre-suggested method.²³ Tungsten oxide (2.3185 g, WO₃, 99.995%, Sigma-Aldrich, USA) and citric acid monohydrate (2.1014 g, C₆H₈O₇·H₂O, 99.0%, Sigma-Aldrich, USA) were dissolved in 500 mL of a 2.8–3.0% diluted ammonium hydroxide aqueous solution and stirred until the solution was clear. The solution was transferred to a droplet generator for the spray pyrolysis. A large amount of droplets generated by six ultrasonic transducers (resonance frequency: 1.67 MHz) were carried to a high temperature (700 °C) quartz-tube reactor by a carrier gas (air, 40 L min⁻¹) and then condensed. The detailed experimental setup is shown elsewhere.²⁴ The condensed W-precursor was collected with a Teflon bag filter in the particle-collecting chamber and converted into WO₃ by heat treatment at 500 °C for 1 h. To load 0.5 at% Rh on the WO₃, an aqueous slurry containing rhodium trichloride hydrate (0.0113 g, RhCl₃·xH₂O, 99.9%, Sigma-Aldrich, USA) and WO₃ (0.5 g) were stirred at 80 °C for 2 h, dried at 70 °C for 12 h, and heat treated at 500 °C for 1 h.

2.2 Characterization

The morphologies and composition of the pure and Rh-loaded WO₃ hollow spheres were characterized using field emission scanning electron microscopy (FE-SEM, S-4700, Hitachi Co. Ltd., Japan) and high resolution transmission electron microscopy (HRTEM, JEM-ARM-200F, JEOL, USA). The phases of the samples were analyzed by X-ray diffraction (XRD, D/MAX-2500V/PC, Rigaku, Japan) and X-ray photoelectron spectroscopy (XPS, VG Multilab ESCA 2000 system, Thermo Fisher Scientific Inc., USA). To determine the precise loading concentration of Rh in WO₃ hollow spheres, an inductively coupled plasma spectrometer (ICP, iCAP 6300 SERIES, Thermo Fisher Scientific Inc., USA) was used.

2.3 Gas sensing characteristics

The pure or Rh-loaded WO₃ hollow spheres were dispersed in deionized water and the slurry was coated on alumina substrate (area: 1.5 × 1.5 mm², thickness: 0.25 mm) with two Au electrodes on its top surface (electrode widths: 1 mm, separation: 0.2 mm) and a micro-heater on its bottom surface. The sensors were dried at 70 °C for 12 h and heat-treated at 500 °C for 2 h. The sensor temperatures were controlled using the micro-heater underneath the substrate and were measured using an IR temperature sensor (Metis MP25, Sensortherm GmBH, Germany). Gas sensing characteristics were measured at 300–450 °C. The concentrations of gases and relative humidity were independently controlled by mixing the gases (100 ppm CH₃COCH₃, CO, NH₃, and 5 ppm H₂S, benzene, toluene, p-xylene, NO in air balance) and dry or humid synthetic air. The R_a/R_g (R_a: resistance in air, R_g: resistance in gas) values were used as the gas responses. A constant flow rate of 500 ml min⁻¹ was employed and a dc 2-probe resistance of the sensor was measured using a multimeter (Agilent 34970A) interfaced with a computer.

3. Results and discussion

3.1 Synthesis of the microstructures of Rh-loaded WO₃ HWs

The synthesis of the WO₃ hollow spheres starts from the hollow W-precursor spheres (Fig. 1a) that were prepared via ultrasonic spray pyrolysis based on our previous work.²³ After a heat treatment at 500 °C for 1 h, these precursors could be oxidized to crystalline WO₃ hollow spheres, and the morphology and sphere size are basically unchanged (Fig. 1b). The corresponding transmission electron microscope (TEM) image (Fig. 1c) clearly shows the hollow structures. These WO₃ hollow spheres are characterized by very thin shell thickness (∼25 nm in Fig. 1d), which must be beneficial to possess high gas response due to the effective electron depletion.²³ Fig. 1e displays that a hollow sphere consists of highly crystalline WO₃ nanoparticles. The interplanar spacing of 3.85 Å of the (001) plane and 3.69 Å of the (200) plane with an angle of 90° reveal an orthorhombic structure. In addition, the corresponding selected-area electron diffraction pattern (the inset of Fig. 1e) demonstrates that the WO₃ HWs are polycrystalline.

Fig. 1 The morphologies and microstructures of pure and Rh-loaded WO₃ HWs. (a) SEM image of pure WO₃ HWs. (b) SEM image of Rh-loaded WO₃ HWs. (c) TEM image of pure WO₃ HWs. (d) A high-magnification view of (c). (e) The high-resolution TEM image of (d), the inset is the corresponding selected-area electron diffraction pattern. (f) TEM image of Rh-loaded WO₃ HWs. (g) Elemental mapping images of Rh-loaded WO₃ HWs.

With the aim to improve the exhaled breath sensing characteristics, nanoscale rhodium oxide is functionalized on the surface of the WO₃ HWs by a simple slurry mixing and after-heating process. Fig. 1f represents the TEM image of the Rh-loaded WO₃ HWs with a designed 0.5 at% Rh concentration ([Rh]/[W]), showing a similar morphology, size distribution and shell thickness with prototype sample (Fig. 1c). The precise content of rhodium in Rh–loaded WO₃ hollow spheres is also confirmed as 0.45 at% by inductive coupled plasma emission spectrometer (ICP) analysis. Further, elemental mapping images (Fig. 1g) suggest that Rh was uniformly dispersed on the surface of the WO₃ hollow spheres.

3.2 Phase and chemical state analysis

Fig. S1† shows the X-ray diffraction (XRD) patterns for the as-synthesized WO₃ HWs and Rh-loaded WO₃ HWs. All the peaks for both the samples are well-matched with the standard WO₃ (JCPDS no. 20-1324), indicating an orthorhombic structure with the main growth directions along the (001) and (200) plane. It also shows a good consistency with the TEM results (Fig. 1e). With respect to the Rh-loaded one, no Rh-related phase can be found in its XRD spectrum because of the small amount of Rh.

Further, the X-ray photoelectron spectroscopy (XPS) measurements were carried out to detect Rh components and determine their chemical state (Fig. S2†). It demonstrates that the survey scanning energy spectra (Fig. S2a†) for both the samples are almost the same. The weak peaks located at 308.5 and 313.8 eV observed in Rh-loaded WO₃ are attributed to Rh 3d_5/2 and Rh 3d_3/2 in Rh³⁺, respectively, rather than the metal phase Rh⁰ (Fig. S2b†).²⁵ The binding energies of W 4f_7/2 and 4f_5/2 are 35.3 and 37.4 eV (Fig. S2c†) in the pure sample, respectively, indicating a 6+ state of tungsten. However, the binding energies of W 4f_7/2 and 4f_5/2 occur with a slight red chemical shift of 0.1 eV probably due to the increasing oxygen vacancy because of Rh loading, which is similar to a previously reported Au-loaded WO₃ sample.²⁶ Moreover, the O 1s peak at 530.2 eV of both samples is typically ascribed to W–O in WO₃ (Fig. S2d†). Consequently, rhodium is obtained as Rh₂O₃ rather than its incorporation in the lattice of WO₃, and the increased oxygen vacancy by Rh loading may bring an enhancement of gas sensing performance by promoting oxygen adsorption.²⁷

3.3 Gas sensing characteristics

Acetone gas sensing properties of pure and Rh-loaded WO₃ hollow spheres are measured in dry and various humid atmospheres. Fig. 2 shows the dynamic sensing transients of pure and Rh-loaded WO₃ sensors toward 10 ppm acetone gas at 400 °C in dry, 20% RH, 50% RH, and 80% RH atmospheres, respectively. In the dry atmosphere, the acetone response (R_a/R_g) of the Rh-loaded sensor is ca. 13.34 (Fig. 2b-1), which is about two times higher than that of the pure sensor, which shows response of 6.97 (Fig. 2a-1). Moreover, the response and recovery time (τ_res and τ_recov, time spans taken for the response to reach [in gas] and decrease by [return to air] 90% of its steady value, respectively) is decreased from 6 s and 158 s to 2 s and 129 s by Rh loading, respectively. Therefore, the sensor based on Rh-loaded WO₃ HWs exhibits enhanced acetone sensing performances than those of the prototype one.

Fig. 2 Transient responses of pure and Rh-loaded WO₃ sensors toward 10 ppm acetone gas at 400 °C in different humidity atmospheres, respectively. (a), (b), (c) and (d) Transient responses of pure WO₃ sensors in dry ambient, 20% RH, 50% RH, and 80% RH atmospheres, respectively. (c), (d), (e) and (f) Transient responses of Rh-loaded WO₃ sensors in dry ambient, 20% RH, 50% RH, and 80% RH atmospheres, respectively.

The acetone sensing properties of the pure sensor in which R_a/R_g and R_a are decreased in wet ambient (20–80% RH) (Fig. 2a-2, a-3, a-4). This is a general behavior of n-type MOS gas sensor. For confirmation, pure SnO₂, ZnO and In₂O₃ hollow spheres were prepared via spray pyrolysis and their gas sensing characteristics were measured (Fig. S3†). Indeed, all the pure sensors showed the significant decreases of R_a/R_g and R_a with increasing ambient humidity, which is a normal behavior for many MOS sensors.²¹ Righettoni et al.¹⁶ also reported the deterioration of the acetone sensing properties of Si-doped WO₃ in highly humid atmospheres. In a stark contrast, an abnormal phenomenon occurs on Rh-loaded WO₃ sensor, where the R_a/R_g and R_a increased in humid atmospheres (Fig. 2b-2, b-3, b-4). The sensing response at 80% RH increases from 5.70 to 20.68 after Rh loading. The τ_res of the pure WO₃ sensor gradually increased from 6 s to 11 s with increasing humidity, while the τ_res of Rh-loaded sensor remains at 2 s throughout the humidity range. The tendency was confirmed again by the continuous operation of the sensor with increasing relative humidity (Fig. S4†). The WO₃ HWs loaded with other noble metal catalysts (0.5 at% Pd or 0.5 at% Pt) showed a significant deterioration of acetone sensing characteristics (Fig. S5†), confirming that the role of the Rh catalyst to promote the gas sensing reaction under humid atmospheres is unique and unusual.

Fig. 3 presents an overview of 10 ppm acetone-sensing properties, such as R_a/R_g, R_a, τ_res and τ_recov, as functions of the working temperature and relative humidity. The R_a/R_g values of two sensors progressively increase with increasing temperature in dry and humid atmospheres (Fig. 3a). Moreover, the R_a/R_g values of the Rh-loaded sensor are ∼2 times higher than those of the pure sensor at all temperatures in the dry atmosphere. However, the two sensors displayed different R_a/R_g vs. humidity tendencies with varying sensor temperatures. Below 325 °C, the R_a/R_g values of both sensors decrease with increasing relative humidity. Beyond 350 °C, the Rh-loaded sensor shows an inverse behavior of the increase of R_a/R_g with the relative humidity, while the R_a/R_g of the unloaded WO₃ sensor still decreases with the increasing humidity. Fig. 3b shows the dependence of R_a on the working temperature at different ambient humidity. The resistances of two sensors in dry air are similar in the whole temperature range, indicating little influence on the resistances by Rh loading. In humid atmosphere, the R_a of pure WO₃ sensor is decreased by water vapors, while the R_a of the Rh-loaded sensor increases (marked by arrows in Fig. 2) in all the temperature range (Fig. 3b). In Fig. 3b, it can be observed that the resistance of the pure WO₃ sensor significantly increases by 27% when the operation temperature is declined from 400 °C to 300 °C in 80% RH condition. However, the increased rate of resistance of Rh-loaded WO₃ sensor is only 3% when the temperature decreases from 400 °C to 300 °C (Fig. 3b), exhibiting an anti-jamming and stable characteristic. The abnormal tendencies of R_a/R_g and R_a reflect that Rh loading can probably introduce a change of humidity cross sensing mechanism at different temperatures (see the next section).

Fig. 3 An overview of 10 ppm acetone-sensing properties: (a) gas response (R_a/R_g), (b) sensor resistance (R_a), (c) 90% response time (τ_res) and (d) 90% recovery time (τ_recov) as functions of the working temperature and relative humidity.

Further, we also observe the response and recovery rate of the two sensors, as shown in Fig. 3c and d, respectively. From dry to humid atmosphere, it is found that the response rate of the pure WO₃ sensor becomes 1.5–2 times more sluggish, while τ_res values of the Rh-loaded sensor are almost invariable, in particular above 350 °C (see Fig. 3c). It should be noted that the response speeds of the Rh-loaded sensor are 2–6 and 3–12 times faster than those of the pure sensor under dry and humid condition, respectively (Fig. 3c). In addition, both the sensors have faster recovery speed under humid conditions compared to dry atmosphere, moreover the τ_recov of the Rh-loaded sensor is shorter than that of the pure one in 300–400 °C range (Fig. 3d). Because all the gas sensing characteristics (R_a/R_g, τ_res and τ_recov) are significantly enhanced by Rh loading, this Rh-loaded WO₃ HWs sensor is applicable as disease diagnosis tool in an effective manner.

Fig. 4 presents the gas response of the two sensors to 0.2–20 ppm acetone in different humidities (0–80% RH) at 400 °C, indicating a good linear relation between R_a/R_g and the gas concentration. Both the sensors have stable response and recovery performances under various relative humidity conditions (Fig. S6 and S7†). The lowest acetone detection limit of the Rh-loaded sensor is calculated to be ca. 50 ppb and 40 ppb in dry air and 80% RH ambient when R_a/R_g > 1.2 was used as the criterion for gas detection, respectively, which are 4 times lower than those of the unloaded WO₃ sensor.

Fig. 4 Gas responses of pure and Rh-loaded WO₃ sensors as functions of acetone gas concentration at 400 °C in different humidity (0–80% RH) atmospheres.

The trace acetone detection limit, fast response and negligible temperature–resistance dependence in a highly humid atmosphere of the Rh-loaded WO₃ sensor can be applied for diabetes breath sensors. The key qualification for exhaled breath sensors is the selectivity of the target gas because of various interfering VOCs gases in human exhalation, such as CH₃COCH₃ (1.8 ppm for diabetes),²⁸ H₂S (∼0.5 ppm for halitosis),²⁹ CO (8.6 ppm for bronchiectasis),³⁰ NH₃ (14.7 ppm for renal failure),³¹ benzene, toluene, xylene (10 ppb for lung cancer),³² and NO (100 ppb for asthma),³³ although the precise criteria for medical diagnosis are still under investigation. Fig. 5 summarizes the sensing response for pure and Rh-loaded WO₃ sensors to these gases at 400 °C in 80% RH, respectively. The concentrations of these gases are as follows: 4 ppm CH₃COCH₃, 20 ppm CO, 20 ppm NH₃, 1 ppm H₂S, 20 ppb C₆H₆ (benzene), 20 ppb toluene (CH₃C₆H₅), 20 ppb xylene ((CH₃)₂C₆H₄), and 200 ppb NO. It should be noted that the responses toward ∼2 times higher concentration of biomarker gases than the detection limit for the diagnosis of diseases were measured (Table S1†) to more accurately examine the selectivity for CH₃COCH₃ and possible interferences from other gases. The response of the pure sensor to 4 ppm CH₃COCH₃ is 3.07 (see Fig. 5a), which is lower than those for 20 ppm NH₃ (Renal failure, R_a/R_g = 3.46) and 1 ppm H₂S (halitosis, R_a/R_g = 4.30). Therefore, if a patient is suffering from renal failure or halitosis, accurate diagnosis of diabetes is impossible. In contrast, the Rh-loaded sensor can overcome the above trouble as its R_a/R_g to 4 ppm CH₃COCH₃ (13.1) is 4 times higher than the prototype one (see Fig. 5b). This merit of the Rh-loaded sensor can possibly make an accurate identification of a diabetes patient even from patients suffering with either halitosis (R_a/R_g = 7.27) or renal failure (R_a/R_g = 3.46).

Fig. 5 Bar graph summarizing gas responses of (a) pure WO₃ sensors and (b) Rh-loaded WO₃ sensors at 400 °C to different analyte gases. Here, the concentrations of such gases are 4 ppm CH₃COCH₃, 20 ppm CO, 20 ppm NH₃, 1 ppm H₂S, 20 ppb benzene, 20 ppb toluene 20 ppb xylene, and 200 ppb NO.

3.4 Gas sensing mechanism

In Fig. 2a, 3a and b, we can observe that the R_a/R_g and R_a of the pure WO₃ sensor significantly decrease in highly humid atmosphere. According to the previous literature,³⁴ decrease in the resistances of n-type semiconductor materials results from adsorbed oxygen consumption and electron generation in the presence of water vapors as described by eqn (1). The R_a/R_g is also deteriorated due to competition between reducing gas and water.

H₂O_(g) + O⁻_(W) → 2OH_(W) + e⁻ (1)

Rh (111) has strong affinity for water compared with other metal surfaces,³⁵ and gets easily combined with hydroxyl group and hydrogen that are obtained by the decomposition of water.³⁶ This suppresses the direct reaction between the sensing surface (WO₃ in the present study) and water, which prevents the deterioration of gas sensing characteristics. Moreover, the decomposition of water by rhodium is accelerated because the co-adsorbed oxygen is cut down to nearly half of activation energy.³⁵ Therefore, more adsorbed oxygen can be formed through synergy between rhodium and surface oxygen adsorbed on metal oxide at high temperature, as depicted in eqn (2).

H₂O_(Rh) + O⁻_(W) → OH_(Rh) + OH_(W) + e⁻ → 2H_(Rh) + 2O⁻_(W) + h⁺ (2)

Because oxygen decomposed from water is adsorbed on the sensor (WO₃) surface and gets ionized, the depletion region on the surface of the sensor will be expanded, which leads to huge changes in resistance. This mechanism can well explain why the Rh-loaded sensor has higher R_a/R_g and R_a in humid atmospheres. This abnormal phenomenon suggests a new direction for the real-time developing of self-diagnosis sensor for diseases, using the direct analysis of highly humid exhaled breath.

If the hydroxyl group does not sufficiently decompose into hydrogen and oxygen due to the energy shortage (low temperature), R_a/R_g is reduced because of the competition between the target gas and water vapors reacting with surface adsorbed oxygen (O⁻_W). It results in the differences of R_a/R_g changes at different sensing temperature (Fig. 3a). At a low temperature like 300 °C, a decrease of R_a/R_g is induced by the adsorption of hydroxyl groups formed by water vapor (eqn (1)). On the contrary, R_a/R_g is gradually raised due to the increasing adsorbed oxygen formed by the decomposition of hydroxyl group and water above 350 °C (eqn (2)).

In addition, rhodium is known to play a role of securing the selective detection of acetone gas among other VOCs,³⁷ which is consistent with the present results. Therefore, the Rh-loaded WO₃ hollow spheres possesses a high response (R_a/R_g = 20.68 to 10 ppm CH₃COCH₃), rapid response time (τ_res = 2 s), excellent acetone selectivity and negligible temperature dependence of sensor resistance in a highly humid atmosphere (RH 80%), and low detection limit (40 ppb) at 400 °C which grants a rapid and reliable detection of trace acetone (Table 1).

Table 1 Gas response (R_a/R_g) to 5 ppm CH₃COCH₃, 90% response time (τ_res) for sensing 10 ppm CH₃COCH₃, resistance in air (R_a), and the detection limit of CH₃COCH₃ of pure and Rh-loaded WO₃ sensors in dry and RH 80% atmospheres

	Pure WO3		Rh-loaded WO3
	Dry	RH 80%	Dry	RH 80%
Ra/Rg	6.97	4.57	13.34	20.68
τres (s)	6	11	2	2
Ra (MΩ)	14.91	11.43	14.06	29.53
Detection limit (ppb)	167	201	52	40

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4. Conclusions

Pure WO₃ and Rh-loaded WO₃ hollow spheres were synthesized based on the spray pyrolysis method and their acetone gas sensing properties were measured in dry and various humid atmospheres (20–80% RH). The Rh-loaded WO₃ sensor exhibited enhanced acetone sensing performances, such as fast response (ca. 2 s), four times higher response, lower detection limit (40 ppb), good linearity, high stability and strong acetone selectivity even in highly humid atmospheres (80% RH), compared with the pure WO₃ sensor. Interestingly, an anomalous phenomenon occurred on the Rh-loaded sensor, where the R_a/R_g and R_a increased in humid atmospheres, compared with other sensors, which showed decreasing values of R_a/R_g and R_a. The abnormal sensing mechanism by Rh loading was also addressed. In addition, a low resistance (R_a)-temperature dependence of Rh-loaded sensor can grant a reliable detection of trace acetone in different external environments. This catalytic-Rh-induced unusual improvement of gas response, selectivity, and responding kinetics even in highly humid atmospheres can probably show a good potential for the real-acetone gas analysis containing high concentration of water vapor.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (no. 2013R1A2A1A01006545) and supported by Industrial Strategic Technology Development Program (no.10047868) funded by the Ministry of Trade, Industry & Energy (KEIT).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: X-ray diffraction patterns, X-ray photoelectron spectroscopy, gas sensing behaviors of pure and Rh-loaded WO₃ hollow spheres and other reference materials. See DOI: 10.1039/c4ra06654e

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