High performance chemiresistive H₂S sensors using Ag-loaded SnO₂ yolk–shell nanostructures

Abstract

SnO₂ yolk–shell spheres uniformly loaded with Ag nanoparticles were prepared by a facile one-pot ultrasonic spray pyrolysis of the source solution and the H₂S sensing characteristics were investigated. The Ag-loaded SnO₂ yolk–shell spheres showed ultrahigh and reversible response (R_a/R_g − 1 = 613.9, where R_a is the resistance in air and R_g is the resistance in gas) to 5 ppm H₂S with negligible cross-responses (0.6–17.3) to eight other interference gases at 350 °C. In contrast, pure SnO₂ spheres with dense inner structures and yolk–shell morphologies did not exhibit a high response/selectivity to H₂S nor reversible H₂S sensing. The highly sensitive, selective, and reversible H₂S sensing characteristics were explained in terms of the gas-accessible yolk–shell morphology and uniform loading of catalytic Ag nanoparticles. Namely, the gas-accessible yolk–shell morphology facilitated the rapid and effective diffusion of the analyte/oxygen gases and the uniform loading of Ag nanoparticles promoted the H₂S sensing reaction.

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Introduction

Hydrogen sulfide (H₂S) is a colorless, poisonous, corrosive, and flammable gas that smells like rotten eggs and is present in natural gases, volcanos, sewage, dumping grounds, decayed foods, and is caused by bacterial breakdown of organic matter.^1–3 Exposure to high concentrations of H₂S may induce immediate fainting and a high probability of death. Continuous exposure to low concentrations of H₂S can cause eye irritation, sore throat, nausea, headache, and dizziness. Moreover, the foul smell of H₂S creates an unpleasant environment.^2–4 Permissible exposure limits of H₂S suggested by the US Occupational Safety and Health Administration (OSHA) and US National Institute for Occupational Safety and Health (NIOSH) are 20 ppm (general industry ceiling limit) and 10 ppm (10 minute ceiling limit).^4,5 Accordingly, the detection of ubiquitous H₂S should be highly sensitive, selective, and reliable in order to monitor harmful pollutants, control the chemical reaction involving sulfur, and manage unpleasant odors.

Oxide semiconductor chemiresistors represent attractive platforms to detect trace concentrations of gases because of their high gas response, simple sensing mechanism, cost effectiveness, and facile integration.^6–13 The gas sensing characteristics of sensing materials can be improved significantly either by altering the morphological design of the nanostructures or by the addition of catalytic additives. In terms of morphology, oxide yolk–shell nanostructures, hollow spheres with movable cores and multiple shells,^14,15 are excellent nano-architectures for gas-sensor applications owing to their high surface area to volume ratio, high gas accessibility, and effective electron depletion.¹⁶ The high gas response of hollow or yolk–shell nanostructures as compared to agglomerated counterparts is attributed to the effective diffusion of the gasses over the entire sensing surface through the semi-permeable and thin shells.^17–21

To date, p-type CuO has been used as a representative additive to enhance the H₂S sensing characteristics of n-type oxide semiconductors such as SnO₂,^22,23 ZnO,²⁴ WO₃,²⁵ and MoO₃.²⁶ The selective and sensitive detection of H₂S by CuO-loaded n-type oxide semiconductors was attributed to the change of the resistive hetero-junction between p-type CuO and n-type oxide semiconductors into a conductive junction between metallic CuS and n-type oxide semiconductors due to the strong chemical interaction between CuO and H₂S.²⁷ The recovery after H₂S sensing, however, usually takes a relatively long time or is incomplete probably due to the sluggish kinetics of the oxidation of CuS into CuO ²⁸ or the irreversible adsorption/desorption of sulfur-containing species.²⁹ This suggests that the development of H₂S sensors with a high gas response, superior selectivity, and good reversibility remains a challenge and might be accomplished by combining highly gas accessible nano-architectures and/or new catalysts with strong chemical interactions with H₂S.

In previous contributions we have suggested that well-defined, pure, and catalyst-loaded yolk–shell nanostructures could be prepared by the simple, one-pot spray pyrolysis reaction of precursor solutions^30,31 and we demonstrated the potential of Pd-loaded SnO₂ yolk–shells as gas-sensing materials. Ag has been reported to be a potential additive that can promote H₂S sensing properties.³² However, little is known concerning the effect of Ag loading on the H₂S sensing characteristics of n-type oxide semiconductors.^33,34 Moreover, Ag-loaded SnO₂ hollow or yolk–shell nanostructures were never prepared or studied for the detection of H₂S until the present report.

Therefore, SnO₂ yolk–shell nanostructures uniformly loaded with Ag catalysts were prepared by a facile one-pot spray pyrolysis reaction and the H₂S sensing characteristics were investigated for the first time. Highly sensitive, selective, and reversible H₂S sensing was achieved using Ag-loaded SnO₂ yolk–shell nanostructures. The focus of the study was directed towards understanding the role of the Ag catalysts and the gas accessible yolk–shell nano-architectures in the design of high performance H₂S sensors.

Experimental

The Ag-loaded SnO₂ yolk–shell spheres were prepared via one-pot spray pyrolysis of an aqueous solution containing Sn(II) oxalate (SnC₂O₄, 98%, Sigma-Aldrich Co., Ltd., USA), nitric acid, Ag-nitrate (AgNO₃, 99%, Sigma-Aldrich Co., Ltd., USA), and sucrose (C₁₂H₂₂O₁₁, Junsei Co., Japan). Sucrose was used as the carbon source to form the yolk–shell spheres. The undoped SnO₂ yolk–shell spheres and SnO₂ spheres with dense inner structures were also prepared directly from the spray pyrolysis of a Sn precursor solution with and without sucrose. The nitric acid was added to SnC₂O₄ and stirred for 5 min. Subsequently, distilled water and sucrose was added and stirred for 5 min, which lead to the slightly turbid precursor solution. The concentrations of SnC₂O₄ and sucrose were 0.2 M and 0.5 M, respectively. The Ag content in the Ag-loaded yolk–shell SnO₂ spheres was fixed at 0.1 wt% (0.13 at%). The spray pyrolysis system (see Fig. S1 in the ESI†) was comprised of a droplet generator, quartz reactor, and powder collector. The length and diameter of the quartz reactor were 2000 and 100 mm, respectively. A 1.7 MHz ultrasonic spray generator with 20 vibrators was used to simultaneously generate a large quantity of droplets, which were carried into the high-temperature tubular reactor by air at a flow rate of 10 L min⁻¹. The reactor temperature was fixed at 1000 °C. For simplicity, hereafter, the pure SnO₂ spheres with dense inner structures, pure SnO₂ yolk–shell spheres, and Ag-loaded SnO₂ yolk–shell spheres after the spray pyrolysis reaction will be referred to as ‘D-SnO₂’, ‘YS-SnO₂’, and ‘Ag-YS-SnO₂’ specimens, respectively. Powders after spray pyrolysis reaction were dispersed in distilled water and the slurry was drop-coated on an alumina substrate (size: 1.5 × 1.5 mm²) with two Au electrodes on the top surface and a microheater on the bottom surface. The same thickness and uniformity of sensor film was maintained throughout the experiment. Prior to the measurements, the sensor was heated to 550 °C for 2 h to remove any hydroxyl contaminants and to stabilize the sensor.

The gas responses (S = R_a/R_g − 1; R_a: resistance in air, R_g: resistance in the analytic gas) to 5 ppm H₂S, ethanol (C₂H₅OH), formaldehyde (HCHO), trimethylamine (C₃H₉N), ammonia (NH₃), benzene (C₆H₆), toluene (C₇H₈), o-xylene (C₈H₁₀), hydrogen (H₂), and carbon monoxide (CO) were measured at 350–450 °C by switching the gas atmospheres. The crystal structures of the spheres were investigated using X-ray diffractometry (XRD, Rigaku DMAX-33). The morphologies of the spheres were characterized using scanning electron microscopy (SEM, JEOL JSM-6060) and high-resolution transmission electron microscopy (HR-TEM, FEI Tecnai™ 300 K). The specific surface areas and pore size distributions were determined from a Brunauer–Emmett–Teller (BET) analysis of nitrogen adsorption measurements (TriStar 3000, Micromeritics). The chemical state of Ag-loaded SnO₂ yolk–shell spheres was analyzed by X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, X-TOOL). The elemental compositions of the powders were investigated using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Thermo elemental, ICAP 6000).

Results and discussion

The Ag-YS-SnO₂ spheres were prepared by the spray pyrolysis of the solution containing the Sn-precursor, Ag-precursor, nitric acid, and sucrose at a high temperature. The formation of Ag-YS-SnO₂ spheres, which is similar to those of pure and Pd-loaded SnO₂ yolk–shell spheres described in previous contributions,³¹ can be described in the three following steps: (1) the formation of the Ag–C–Sn precursor composite spheres by polymerization and carbonization of sucrose, (2) the development of the Ag-loaded SnO₂ outer shells through the partial oxidation of carbon and the decomposition of the precursors near the surface, and (3) the subsequent oxidation/decomposition of the interior portion of the precursors into the Ag-loaded SnO₂ inner shells and yolks. The diameters of the Ag-YS-SnO₂ spheres ranged from 0.5 to 3 μm (Fig. 1a). The yolks were indirectly observed within the spheres through the semi-translucent shells (arrows in Fig. 1a) and the yolk–shell nanostructure was observed at the broken spheres (inset in Fig. 1a). TEM (Fig. 1b) clearly revealed that all spheres consisted of yolks and shells. From the magnified image (Fig. 1c), it was clear that the yolk–shell nanostructures with double shells and one core consisted of small primary particles and the thicknesses of the outer and inner shells were 40 and 100 nm, respectively. The lattice fringe with interplanar distances of 0.33 nm (Fig. 1d), corresponding to the (110) plane of the SnO₂ Cassiterite (rutile) crystal structures, suggested the crystalline nature of the Ag-YS-SnO₂ spheres. The uniform distribution of Ag components on the SnO₂ yolk–shell spheres, essential for the catalytic promotion of the gas sensing reaction, was confirmed by TEM elemental mapping (Fig. 1e–g), suggesting that the spray pyrolysis of the precursor solution containing sucrose provided a facile one-pot route to prepare yolk–shell oxide nanostructures uniformly loaded with noble metal catalysts.

Fig. 1 (a) SEM image, (b–d) TEM images, and (e–g) elemental mapping of Ag-loaded SnO₂ yolk–shell spheres (Ag-YS-SnO₂).

For comparison, the D-SnO₂ spheres were prepared from the precursor solution without sucrose. The solid inner structures were observed in the SEM image of a broken sphere (inset of Fig. S2a in the ESI†) and confirmed using TEM by the dark contour at the central portion of the spheres (Fig. S2b and S2c in the ESI†). The pure YS-SnO₂ spheres were also prepared by the spray pyrolysis reaction of the solution containing the Sn-precursor and sucrose. The yolk–shell morphology was observed from a broken YS-SnO₂ sphere (inset of Fig. S2d in the ESI†) and TEM images (Fig. S2e and S2f in the ESI†). The D-SnO₂ and YS-SnO₂ spheres had rutile-structured SnO₂ (JCPDS# 41-1445) as determined by X-ray diffraction (Fig. S3a and S3b in the ESI†). It was difficult to find the second phases such as Ag and Ag₂O in the Ag-YS-SnO₂ spheres probably due to the low detection limit of X-ray diffraction. The crystallite sizes of D-SnO₂, YS-SnO₂, and Ag-YS-SnO₂ spheres were determined by Scherrer's equation to be 13.3 ± 0.8 nm, 20.0 ± 3.4 nm, and 16.8 ± 1.6 nm, respectively. From ICP analysis, the presence of the Ag component was confirmed and the concentration of Ag was determined to be 0.13 at%. The valence state of Ag was investigated using XPS (Fig. S4 in the ESI†). No Ag related peak was observed in the SnO₂ yolk–shell spheres loaded with Ag (Fig. S4b in the ESI†). Accordingly, SnO₂ yolk–shell spheres loaded with 3.3 at% Ag were prepared by ultrasonic spray pyrolysis and heat treatment and their XPS spectra were studied. The binding energies of the Ag 3d_3/2 and Ag 3d_5/2 peaks were 374.4 and 368.5 eV, indicating that Ag existed in the form of Ag.³⁵

The size and volume of pores were analyzed by nitrogen adsorption/desorption isotherms (Fig. 2). The volumes of nitrogen adsorption/desorption in YS-SnO₂ and Ag-YS-SnO₂ spheres were similar and significantly higher than in the D-SnO₂ spheres (Fig. 2a). In the YS-SnO₂ and Ag-YS-SnO₂ spheres, the volume of nitrogen adsorption peaked at the pore sizes of 21.0 and 17.7 nm (Fig. 2c and e), respectively, and both showed high pore volumes (0.07 cm³ g⁻¹ and 0.10 cm³ g⁻¹). In contrast, a negligible pore volume (0.01 cm³ g⁻¹) was found in the D-SnO₂ spheres (Fig. 2g). The specific surface areas of the Ag-YS-SnO₂, YS-SnO₂, and D-SnO₂ spheres were 16, 16, and 1.5 m² g⁻¹, respectively (Fig. 2b, d, and f). Thus, the abundant pores in the spheres as well as the thin configuration of the shells were expected to significantly enhance the gas accessibility.

Fig. 2 (a) Nitrogen adsorption/desorption isotherms of D-SnO₂, YS-SnO₂, and Ag-YS-SnO₂ spheres and BET surface area and pore size distribution of (b and c) Ag-YS-SnO₂ spheres, (d and e) YS-SnO₂ spheres, and (f and g) D-SnO₂ spheres.

The gas responses of the D-SnO₂, YS-SnO₂, and Ag-YS-SnO₂ spheres to 5 ppm H₂S, C₂H₅OH, and HCHO were measured at 350–450 °C (Fig. 3). The D-SnO₂ spheres exhibited the lowest response and selectivity to H₂S, although the response to 5 ppm H₂S was higher than those to 5 ppm C₂H₅OH and 5 ppm HCHO at all sensing temperatures (Fig. 3a-1). The gas response to 5 ppm H₂S, C₂H₅OH, and HCHO was enhanced 1.7–2.2 times, 1.7–2.2 times, and 1.2–4.2 times, respectively, by employing the yolk–shell morphology (Fig. 3b-1). The thin and semi-permeable shells, abundant nano- and meso-pores, and high surface area of the SnO₂ yolk–shell nanostructures likely induced the effective diffusion of the analyte gasses on the sensing surface. Specifically, the increase of gas accessibility and surface area to volume ratio through the morphological design of nanostructures was important in achieving a high gas response and was consistent with reports on the enhancement of gas responses by employing hollow or yolk–shell nanostructures.^36,37

Fig. 3 Gas responses to 5 ppm H₂S, C₂H₅OH, and HCHO at 350–450 °C and selectivity to 5 ppm H₂S at 350 °C: (a) D-SnO₂ spheres, (b) YS-SnO₂ spheres, and (c) Ag-YS-SnO₂ spheres (the concentrations of all the gases: 5 ppm, E: C₂H₅OH, F: HCHO, TMA: trimethylamine, A: NH₃, B: benzene, T: toluene, X: o-xylene, H: H₂ and C: CO).

The response of the YS-SnO₂ spheres to 5 ppm H₂S at 350–450 °C was further enhanced by loading 0.13 at% Ag (Fig. 3c-1), while the variation in the response to 5 ppm C₂H₅OH and HCHO due to Ag loading was relatively small (Fig. 3b-2 and c-2). This led to the selective and sensitive detection of H₂S. The responses of three different sensors to 5 ppm H₂S, C₂H₅OH, HCHO, trimethylamine, NH₃, benzene, toluene, o-xylene, H₂ and CO were measured at 350 °C (Fig. 3a-2, b-2, and c-2) and the ratios between the responses to H₂S and other interference gases (SL = S_H2S/S_gas) were calculated in order to compare the selectivity to H₂S in a quantitative manner. The SL values of the Ag-YS-SnO₂ spheres (SL = 34.5–739.6) were markedly higher than those of the D-SnO₂ (SL = 2.5–31.8) and YS-SnO₂ spheres (SL = 2.3–36.0). In all three sensors, the response to 5 ppm C₂H₅OH was the second highest value (Fig. 3a-2, b-2, and c-2). Nevertheless, the S_H2S/S_ethanol value of the Ag-YS-SnO₂ spheres was still as high as 34.5, which was significantly higher than those of the YS-SnO₂ (2.46) and D-SnO₂ (2.33) spheres and guaranteed the ultra-selective detection of H₂S in the presence of interference gases. The enhancement of the SL values by Ag loading in YS-SnO₂ spheres strongly suggested that the chemical interaction between Ag and H₂S played a key role in the selective and sensitive detection of H₂S. This was supported by the similar SL values of the D-SnO₂ and YS-SnO₂ spheres regardless of the variation in the nano-architectures (Fig. 3a-2 and b-2), as well as the completely different H₂S sensing behaviors of YS-SnO₂ and Ag-YS-SnO₂ spheres in spite of their similar pore size distribution, pore volumes, and specific surface areas (Fig. 3b-2 and c-2).

The reversibility of the sensors after H₂S sensing was investigated (Fig. 4). The recovery characteristics depended on the morphology and Ag loading of the spheres. For quantitative comparison, recovery rate (%) = (R_air-recovery − R_gas-H2S)/(R_air-fresh − R_gas-H2S) × 100 (%), where R_air-fresh is the sensor resistance in air before exposure to H₂S, R_gas-H2S is the sensor resistance in 5 ppm H₂S, and R_air-recovery is the sensor resistance in air after 1 h exposure to air. As such, the recovery rate (%) was calculated from the sensing transients and the results are shown in Fig. 5. At 350 °C, the D-SnO₂ spheres did not completely recover the original sensor resistance in air after sensing 5 ppm H₂S (Fig. 4a-1). After increasing the sensor temperature from 350 to 450 °C, the recovery rate (%) of the D-SnO₂ spheres was increased from 28.7 to 95.8% (Fig. 4-a and 5a-1). The recovery rate (%) of the YS-SnO₂ spheres was 36.7% at 350 °C and increased to 61.6% as the sensor temperature increased to 450 °C (Fig. 4-b and 5b-1). This indicates that the H₂S response could be enhanced by the yolk–shell morphology, but at the expense of the reversibility. Finally, the recovery rate (%) of the Ag-YS-SnO₂ spheres was as high as 84.1% even at 350 °C and the recovery was complete at 450 °C (Fig. 4c and 5c-1).

Fig. 4 Dynamic sensing transients to 5 ppm H₂S at 350–450 °C: (a) D-SnO₂ spheres, (b) YS-SnO₂ spheres, and (c) Ag-YS-SnO₂ spheres.

Fig. 5 Sensor resistance in air (R_a), 90% response time (τ_res), and recovery rate [(R_air-recovery − R_gas-H2S)/(R_air-fresh − R_gas-H2S) × 100 (%)] where R_air-fresh: sensor resistance in air before exposure to H₂S, R_gas-H2S: sensor resistance in 5 ppm H₂S, and R_air-recovery: sensor resistance in air after 1 h exposure to air. (a) D-SnO₂ spheres, (b) YS-SnO₂ spheres, and (c) Ag-YS-SnO₂ spheres.

The sensing transients of Ag-YS-SnO₂ spheres to 0.25–5 ppm H₂S was measured at 350 °C (Fig. 6). The sensor showed reversible sensing and a high response to sub-ppm levels of H₂S. The detection limit of H₂S was determined to be <10.5 ppb when R_a/R_g − 1 > 0.2 was used as the sensing criterion (inset in Fig. 6). Permissible exposure limits of H₂S suggested by the US OSHA and US NIOSH are 20 ppm.^4,5 Recently, the American Conference of Governmental Industrial Hygienists changed the 8 h time weighted average recommended exposure limit from 10 ppm to 1 ppm based on a recent report.³⁸ Although humans can recognize the presence of H₂S at concentrations as low as 10–50 ppb,³⁹ the human nose cannot quantify the concentration of H₂S and is susceptible to olfactory fatigue. Accordingly, sub-ppm level detection of H₂S is indispensable for monitoring air quality. Towards this end, the Ag-YS-SnO₂ sensors provide a valuable sensing platform to detect ppb-levels of H₂S in a highly sensitive, selective, and reversible manner.

Fig. 6 Sensing transients of Ag-YS-SnO₂ spheres to 0.25–5 ppm H₂S at 350 °C.

The 90% response time (τ_res), the time to reach 90% variation of sensor resistance upon exposure to H₂S, was calculated (Fig. 5a-2, b-2, and c-2). The τ_res values of the D-SnO₂ spheres were higher than those of the YS-SnO₂ and Ag-YS-SnO₂ spheres. This can be attributed to the sluggish diffusion of gases through the dense agglomerates in the D-SnO₂ spheres. The sensor resistances in air (R_a) of the YS-SnO₂ spheres were ∼2 orders of magnitude higher than that of the D-SnO₂ spheres (Fig. 5a-3 and b-3), possibly due to the decrease in the cross-sectional area for conduction of the yolk–shell morphology. However, the ∼3 orders of magnitude increase in R_a by loading Ag (Fig. 5b-3 and c-3) was not attributed to the morphological variation considering the similar pore size and specific surface area of the YS-SnO₂ and Ag-YS-SnO₂ spheres (Fig. 2). The loading of Ag is known to extend the electron depletion layers of n-type oxide semiconductors adjacent to Ag nanoparticles.⁴⁰ Thus, the marked increase of R_a in Ag-loaded spheres might be attributed to the electronic interaction between Ag and the SnO₂ yolk–shell spheres.

Reversible H₂S sensing has been a challenging issue in the development of oxide semiconductors. For example, the D-SnO₂ and YS-SnO₂ spheres in the present study showed incomplete recovery from H₂S sensing at 350 °C, yet showed a complete recovery after sensing nine different gasses that did not contain sulfur (Fig. S5 in the ESI†). This strongly indicated that the incomplete recoveries from H₂S sensing in the D-SnO₂ and YS-SnO₂ sensors were related to sulfur-containing components during sensing and recovery. At 350–450 °C, O⁻ is regarded as the major oxygen adsorption species and the following H₂S sensing reaction can be considered:⁴¹

H₂S(g) + 3O_ad⁻ → SO₂(g) + H₂O(g) + 3e⁻ (1)

Note that SO₂(g) generated during the sensing reaction is known to adsorb on the surface sites of SnO₂, turn into inactive stable sulfate, and eventually decrease the active sites of oxygen adsorption for gas sensing.^42,43 Thus, the SO₂ poisoning after the H₂S sensing reaction makes it difficult to recover the sensor resistance at the sulfur-free fresh state. Thus, the high recovery rate (%) at high sensor temperatures can be explained by the increase of SO₂ desorption.⁴⁴ Notably, the recovery rates (%) of the YS-SnO₂ spheres from H₂S sensing at 375–450 °C (52.5–61.6%) (Fig. 5b-1) were relatively lower than those of the D-SnO₂ spheres (75.6–95.8%) (Fig. 5a-1). A higher amount of SO₂(g) will be generated from the large surface area of the YS-SnO₂ spheres, which in turn will be adsorbed on the sensing surfaces in an irreversible manner. Accordingly, the high surface area to volume ratio of yolk–shell nanostructures with high gas accessibilities has both a positive effect of enhancing the gas response and an adverse effect of deteriorating the reversibility of H₂S sensing. In contrast, the recovery rate (%) of the Ag-YS-SnO₂ spheres was as high as 84.1% at 350 °C, which increased to ∼100% as the sensor temperature increased to 450 °C (Fig. 5c-1). Two plausible explanations for this include the reduced SO₂ generation by the strong interaction between Ag and H₂S to form Ag₂S or (Ag–S)_ads ^45,46 and the preferred adsorption of SO₂ on Ag nanoclusters.⁴⁷

In both cases, the recovery involved the re-oxidation of sulfurized Ag, which could be facilitated by providing more oxygen to the fine Ag particles with high surface areas. Thus, the reversible H₂S sensing in the Ag-YS-SnO₂ spheres emanated from the uniform distribution of fine Ag nanoparticles with high surface area to volume ratios and the effective diffusion of oxygen through the semi-permeable thin shells and abundant pores. Therefore, the synergetic combination between the yolk–shell morphology of the SnO₂ spheres and the uniform loading of Ag nanoparticles provides an excellent strategy to design highly sensitive, selective, and reversible H₂S sensors.

The responses and selectivities to H₂S found in the literature are summarized in Fig. 7.^48–63 Note that only the results showing both response and selectivity to H₂S were considered for comparison. ‘R_a/R_g − 1’ and ‘R_g/R_a − 1’ were used as the responses of n-type and p-type semiconductor sensors, respectively. Considering both response and reversibility, R_a/R_g − 1 = 613.9 to 5 ppm H₂S (demonstrated in the current work) was among the highest values found in the literature (Fig. 7a). The selectivity to H₂S (S_H2S/S_{2nd gas}, S_H2S: response to H₂S, S_{2nd gas}: the highest response to interference gas) in the present study was as high as 34.5, which was again the highest value among those of the reversible H₂S sensors (Fig. 7b). This clearly shows that Ag-loaded SnO₂ yolk–shell spheres are a promising H₂S sensing material with a high response, excellent selectivity, and good reversibility.

Fig. 7 (a) Gas responses (R_a/R_g − 1 or R_g/R_a − 1) and (b) selectivity values (S_H2S/S_{2nd gas}) to H₂S found in the literature (R_a/R_g − 1: gas response of n-type semiconductors, R_g/R_a − 1: gas response of p-type semiconductors, S_H2S: response to H₂S, S_{2nd gas}: the highest response to interference gas).

Conclusions

Highly sensitive, selective, and reversible H₂S sensors were fabricated using Ag-loaded SnO₂ yolk–shell spheres. The H₂S gas response of pure SnO₂ yolk–shell spheres was higher than that of SnO₂ spheres with dense inner structures, which was attributed to the high surface area to volume ratio and gas accessibility of the spheres due to the yolk–shell morphology. However, highly selective and reversible H₂S sensing remains challenging both in pure SnO₂ spheres with dense inner structures and yolk–shell morphologies. In contrast, the SnO₂ yolk–shell spheres uniformly loaded with Ag nanoparticles, prepared by a facile one-pot spray pyrolysis reaction, exhibited ultrahigh responses and selectivities to ppm levels of H₂S as well as highly reversible H₂S sensing. The superior H₂S sensing was attributed to the enhancement of the H₂S response by the gas accessible yolk–shell morphology, selective and sensitive detection of H₂S via the strong chemical interaction between Ag and H₂S, and Ag-induced suppression of SO₂-related poisoning of SnO₂ surface.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2013R1A2A1A01006545). The work of Y.C. Kang was supported by NRF grant funded by MEST (no. 2012R1A2A2A02046367).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental setup of ultrasonic spray pyrolysis, SEM and TEM images of SnO₂ spheres with dense inner structures and yolk–shell morphologies, X-ray diffraction patterns, XPS results, and gas sensing transients to 8 interference gases. See DOI: 10.1039/c4ra01364f

‡ Both authors contributed equally to this work.

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