Highly Sensitive and Selective Sensing of H2S Gas Using Precipitation and Impregnation-Made CuO/SnO2 Thick Films

In this work, CuO-loaded tetragonal SnO2 nanoparticles (CuO/SnO2 NPs) were synthesized using precipitation/impregnation methods with varying Cu contents of 0–25 wt% and characterized for H2S detection. The material phase, morphology, chemical composition, and specific surface area of NPs were evaluated using X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller analysis. From gas-sensing data, the H2S responses of SnO2 NPs were greatly enhanced by CuO loading particularly at the optimal Cu content of 20 wt%. The 20 wt% CuO/SnO2 sensor showed an excellent response of 1.36 × 105 toward 10 ppm H2S and high H2S selectivity against H2, SO2, CH4, and C2H2 at a low optimum working temperature of 200 °C. In addition, the sensor provided fast response and a low detection limit of less than 0.15 ppm. The CuO–SnO2 sensor could therefore be a potential candidate for H2S detection in environmental applications.


Background
Hydrogen sulfide (H 2 S) is a highly toxic gas widely produced from several sources, such as manure fermentation plants, wastewater treatment systems, petroleum refineries, landfill sites, textile factories, stagnant water wells, extruded rubber industries, and other similar industrial facilities. H 2 S can be adsorbed through human lungs, gastrointestinal regions, and normal skin. Its funky odor will freeze the sense of smell leading to immediate paralysis and mortality when its concentration exceeds its threshold limit value (TLV) of 10 ppm [1]. Therefore, it is compelling to develop an effective and low-cost gas sensor that can detect H 2 S at sub-ppm concentrations with high response, short response time, high selectivity and good stability.
Copper Oxide (CuO) is a functional p-type metal oxide semiconductor with a moderate energy gap of 1.2-2.0 eV, remarkable sensitivity and selectivity toward H 2 S. CuO-loaded SnO 2 gas sensors have been extensively characterized toward H 2 S as presented in Table 1. CuO dopants provides relatively high enhancement of H 2 S response and selectivity for SnO 2 sensors [19][20][21][22][23][24][25][26][27]. The H 2 S-sensing performances also depend substantially on the synthesis method and the form of metal oxide materials. From the table, recent reports CuO/SnO 2 sensors are mostly in thick-film and thin-film forms, which offer similarly competitive H 2 S-sensing performances depending on synthesis method and preparation parameters. Between them, thick-film sensors are more preferred in practical applications due to their much lower production cost. Hence, it is compelling to investigate the H 2 S-sensing properties of thick-film CuO/SnO 2 materials prepared by other advanced techniques.
Precipitation and impregnation are attractive methods for production of thick-film nanocomposite materials because of ability to form diverse nanostructures, low processing temperature, and low cost. Some CuO-loaded SnO 2 nanomaterials synthesized by precipitation with NH 3 precipitant and impregnation were studied for H 2 S gas-sensing. However, the reported results still offered only modest response at high H 2 S concentrations due possibly to large particle sizes [28]. Herein, precipitated SnO 2 nanoparticles (NPs) were prepared using NaOH as a precipitant and impregnated with CuO at over a wide range of Cu contents to attain small nanoparticles and large responses at relatively low H 2 S concentrations. Thick-film sensors were fabricated by spin coating powder paste of synthesized CuO/SnO 2 nanoparticles and the effects of CuO loading level on H 2 S-sensing properties were explained based on CuO/SnO 2 heterojunctions.

Synthesis and Characterization of Nanoparticles
All chemicals with analytical grade were used directly without additional purification. Tin (IV) chloride pentahydrate (SnCl 4 ·5H 2 O) as a tin source was dissolved in deionized (DI) water under constant stirring to obtain the 0.1 M aqueous solution. An appropriate volume of 0.1 M sodium hydroxide (NaOH) aqueous solution was slowly dropped onto the SnCl 4 ·solution under vigorous stirring until white slurry appeared at the pH of 11. The slurry was washed thoroughly with DI water several times under centrifugation to remove chloride residues from the precipitate. The resulting precipitate was subsequently dried at 80 °C for 10 h in an oven and the obtained particles were calcined for 2 h at 600 °C at a heating rate of 10 °C/min. To impregnate CuO onto SnO 2 nanoparticles, 0.872 g of copper (II) acetate hydrate (98%; Aldrich) was dissolved in 30 mL of ethanol under This work vigorous agitation. The solution was then dropped onto 0.5 g of SnO 2 NPs with varying Cu concentrations from 5 to 25 wt%. Next, the suspension was continuously stirred until turning into homogeneous slurry and baked at 60 °C for 2 h in an oven. The resulting powders were annealed for 4 h at 300 °C at a heating rate of 10 °C/min. The structural characteristics of NPs were evaluated using X-ray diffraction (XRD) with a Cu Kα (λ = 1.54056 Å) X-ray source. The surface morphology and elemental distributions of NPs were examined using high-resolution scanning transmission electron microscopes (HR-TEM). The oxidation states of materials were studied using X-ray photoelectron spectroscopy (XPS) with an Al-K α (1486.8 eV) X-ray source. The specific surface area of the NPs was measured using a nitrogenadsorption analyser with Brunauer-Emmett-Teller analysis (SSA BET ).

Fabrication and Characterization of Gas Sensor
To fabricate SnO 2 and 5-25 wt% CuO/SnO 2 sensors, 60 mg of powder was thoroughly mixed with an α-terpineol (Aldrich, 90%)-based vehicle containing ethyl cellulose (30-70 mPa s, Fluka) to produce a homogeneous paste. Next, a sensing film was deposited on an alumina substrate (0.40 × 0.55 × 0.04 cm 3 ) with prepatterned interdigitated gold electrodes (0.24 cm × 0.5 cm) by spin coating of the paste at 700 rpm for 10 s and at 3000 rpm for 30 s. The 200-nm-thick interdigitated Au electrodes were deposited on alumina substrates by a sputtering process with argon gas at a pressure of 3 × 10 −3 mbar. The interdigit spacing, width, and length were 100 μm, 100 μm, and 0.24 cm, respectively. The resulting sensors as illustrated in Fig. 1 were annealed for 2 h at 450 °C at a ramping rate of 4 °C/min to eliminate organic components from the sensing layers. The microstructures of sensing films were characterized using field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDS).

Gas Sensing Measurement
The sensor characteristics toward H 2 S in the concentration range of 0.15-10 ppm was measured in a homemade sealed stainless-steel test chamber with an active volume of 0.5 L (Fig. 1). An external Ni/Cr heater operated with a computer-controlled power supply was used to control the working temperature ranging from 150 to 350 °C. The selectivity properties were evaluated against H 2 , SO 2 , CH 4 and C 2 H 2 . At a given working temperature, the sensors were initially settled in synthetic dry air for 10 min to obtain a steady resistance in air. Next, the dry air was mixed with a target gas sample to a desired gas concentration at a fixed total flow rate (2 L/min) using computerized multichannel mass flow controllers (Brook Instruments model 5850E). Each gas sample was applied to the sensors for 25 min and the dry air was resumed for 45 min. The sensor resistance was measured by the amperometric method at a bias of 10 V DC using a picoammeter (Kiethley model 6487). The performances of sensors with various Cu concentrations were characterized in terms of gas response, response time, selectivity, and stability. The gas response (S) was expressed as S = R a /R g for a reducing gas (H 2 S, H 2 , CH 4 , SO 2 and C 2 H 2 ), where R a and R g were the sensor resistance in air before and after exposure to a target gas, respectively. The response time (t res ) was the time taken to attain 90% of a steady-state response signal upon exposure to a target gas.

Structural Characteristics of Particles and Sensing Film
XRD patterns of CuO, SnO 2 , and 5-25 wt% CuO/ SnO 2 NPs are displayed in Fig. 2. The sharp diffraction peaks indicate the crystalline characteristic of all NPs. The diffraction patterns of SnO 2 and CuO correspond to tetragonal and monoclinic structures according to JCPDS files no 41-1445 and 45-0937, respectively. The SnO 2 powder exhibits three main peaks, while the CuO powder displays two distinct major peaks. The spectra for the 5-25 wt% CuO/SnO 2 NPs show the secondary CuO peaks of (002) and (111) planes together with the main SnO 2 peaks of (111), (101) and (211) planes, demonstrating the coexistence of CuO and SnO 2 phases. The mean crystallite sizes (d) of CuO/SnO 2 NPs were determined using Scherrer's equation (d = Kλ/(βcosθ) where K is the geometric factor of 0.89 for spherical particles, λ is the X-ray wavelength and β is the full width at half maximum of an XRD peak at the angle, θ. The mean crystallite diameter of unloaded SnO 2 is estimated to be 10 nm, while that of 20 wt% CuO/SnO 2 NPs is relatively small at 7 nm. The result indicates the inhibition of grain growth due to CuO loading on SnO 2 NPs. The chemical compositions and oxidation states of CuO and SnO nanoparticles will be evaluated further by EDX and XPS analyses.
BET specific surface areas (SSA BET ) and particle diameters (d BET ) of SnO 2 and 5-25 wt% CuO/SnO 2 NPs are shown in Fig. 3. SSA BET of CuO/SnO 2 NPs substantially increases from 39.9 to 44.21 m 2 /g, while the d BET reduces accordingly from 22.04 to 19.53 nm as the CuO content rises from 0 to 20 wt%. As the Cu content increases further to 25 wt%, SSA BET decreases slightly to 44.01 m 2 /g and d BET increases to 19.62 nm. The results agree with the XRD analysis of crystallite size. The influence of CuO loading level on SSA BET may be attribute to the inclusion of smaller CuO NPs produced by impregnation. The CuO NPs may act as separators to inhibit self-coagulation among SnO 2 NPs, resulting in the substantial increment of the effective surface area. Figure 4 shows typical surface morphologies of coprecipitation/ impregnation synthesized SnO 2 and 20 wt% CuO/SnO 2 NPs. The BF-TEM images show that most SnO 2 particles exhibit spheroidal shapes with different diameters ranging from 5 to 20 nm. After CuO loading, the diameters of SnO 2 NPs tend to be smaller but the secondary phase of CuO cannot be clearly identified ( Fig. 4d-f ). The related SAED patterns display dotted ring features of polycrystalline tetragonal SnO 2 structures with main diffraction rings corresponding to (110), (101), (200), (211) and (112) planes of SnO 2 as well as (002) plane of CuO in agreement the XRD analysis. The rings related to CuO were quite obscure due likely to weak diffraction signal from very small CuO secondary phase. Correspondingly, the HR-TEM images show lattice fringes on nanoparticles mainly associated with the planes of SnO 2 crystals. The secondary CuO phase structures cannot be observed in HR-TEM image due possibly to their very small sizes beyond the resolution of the TEM instrument.
Scanning transmission electron microscopy (STEM) and high-resolution EDS mapping analysis were employed to investigate the distributions of CuO in 20 wt% CuO/SnO 2 NPs as presented in Fig. 5. The STEM image illustrates a cluster of roughly round nanoparticles with diameters in the range of 5-15 nm in agreement with the TEM images but with relatively low image resolution due to scanning aberration. The corresponding EDS maps of Sn, O, and Cu elements demonstrate the detailed distribution of these species on various SnO 2 nanoparticles in the selected area. Apparently, Cu species are widely distributed on particles with similar density to Sn and O species. The results suggest that the CuO secondary nanoparticles are present and closely distributed on SnO 2 surfaces forming distributed CuO-SnO 2 junctions within the CuO/SnO 2 composite. However, the particles and related junctions are very small at molecular scales so that they cannot be exactly discerned by the TEM/STEM characterizations. of stoichiometric SnO 2 . With 20 wt% CuO loading, a Cu peak appears with a high Cu content of ~ 15.6 wt% or 7.04 at%, which is still smaller than that of Sn. Additionally, the Cu content from five different areas is found to vary from 14 to 18 wt%, indicating some variation of chemical composition within the film. Therefore, CuO loading by impregnation does not markedly influence particle morphologies but considerably changes the elemental composition.  [29]. In the case of 20 wt% CuO/SnO 2 NPs, the Cu2p core levels comprise Cu2p 3/2 and Cu2p 1/2 peaks centred at 933.5 eV and 953.4 eV along with the satellite peaks at ~ 942.9 and ~ 964.2 eV, corresponding to the Cu 2+ oxidation state of CuO [30]. The observed oxidation states affirm the coexistence of CuO and SnO 2 structures.

Gas-Sensing Characteristics
Figure 8a displays the changes in resistance of CuO, SnO 2 and 5-25 wt% CuO/SnO 2 films subjected to H 2 S pulses with varying concentrations from 0.15 to 10 ppm at a working temperature of 200 °C. The resistance in air of SnO 2 film increases by more than two orders of magnitude after loading CuO with 5-25 wt% contents. Additionally, it is observed that the baseline resistances of various CuO/SnO 2 sensors are not very different and only tend to increase slightly with increasing CuO loading level. To identify whether the resistance is changed due to the film geometry or material related properties, the film resistivity was additionally measured by the well-known four-probe method using 4-stripe Au/Cr electrodes with an interelectrode spacing of 100 μm and a bias current of 0.1 μA. The measured average resistivity values of CuO, SnO 2 , and 5-25 wt% CuO/SnO 2 films in air at 350 °C are ~ 8.1 × 10 3 , 2.1 × 10 4 and 7.4 × 10 7 − 1.8 × 10 8 Ω cm, respectively. The results confirm significant differences in resistivity among the three sets of materials and the similarities in resistivity among 5-25 wt% CuO/SnO 2 films. This behavior may be explained based on two effects including the percolation breaking of aggregated SnO 2 nanoparticles due to CuO secondary nanoparticles and the formation of CuO/SnO 2 (p-n) heterojunctions. The TEM/HR-TEM/STEM data suggest that CuO secondary nanoparticles may be formed surrounding the SnO 2 nanoparticles, thus breaking the percolation of agglomerated SnO 2 particles and forcing most conduction paths to be across the CuO nanoparticles. In addition, the formation of CuO/SnO 2 heterojunctions may induce carrier depletion regions throughout secondary CuO nanoparticles due to the work function difference, creating highly resistive conduction paths. Thus, an addition of CuO to SnO 2 particles at the level above the minimum value required to break the percolation of SnO 2 particles will cause a large increase of resistance as fully depleted CuO particles block electrical conduction. The lowest Cu content in this study of 5% is quite substantial and thus likely to exceed the percolation breaking threshold. Further addition of CuO will only slightly increase the resistance since the electrical conduction via fully depleted CuO is already nearly minimal. Other effects including particle/grain sizes, film thickness, electrode separation, and electrode contact may be neglected since they are not greatly changed according to the structural characterization results. Upon exposure to H 2 S, the sensor resistances decrease rapidly before recovering to the baseline levels after the resumption of dry air, confirming a typical n-type sensing characteristic. Interestingly, the baseline resistance of CuO sensor considerably drifts downward after several H 2 S pulses in contrast to the SnO 2 sensor that shows insignificant baseline drift. In the case of CuO/SnO 2 sensors, the baseline drift tends to increase with increasing Cu content. These behaviors may be related to the slow and incomplete CuO-CuS transformative reactions to be further discussed in Sect. 3.3.
The corresponding sensor response plotted versus H 2 S concentration at 200 °C is shown in Fig. 8b. All sensor responses increase monotonically with increasing H 2 S concentration. The response characteristics of all sensors conform well to the power law according to the equations as displayed along with the inset labels in Fig. 8b. The power-law exponent of CuO is close to 1, while that of SnO 2 sensors is around 1.5 and those of CuO-loaded SnO 2 sensors are larger than 2, suggesting differences in H 2 S reaction mechanisms on the surfaces of these materials [31]. Furthermore, the sensor response increases greatly as the CuO content increases from 0 to 20 wt% before slightly declining at a higher CuO content of 25 wt% and the 20 wt% CuO/ SnO 2 sensor offers the highest response of 1.36 × 10 5 to 10 ppm H 2 S at 200 °C. Moreover, it exhibits decent responses of ~ 2, 5, 20 and 230 at the lower H 2 S concentrations of 0.15, 0.3, 0.5 and 1 ppm, respectively. The excellent performances of 20 wt% CuO/SnO 2 sensor may be attributed to the increase of specific surface area due to CuO loading and the formation of CuO/ SnO 2 heterojunctions to be further discussed in the next section. Figure 9 presents the plot of the response versus working temperature of unloaded and CuO-loaded SnO 2 sensors at a H 2 S concentration of 10 ppm. The H 2 S responses of CuO/SnO 2 NPs sensors increase significantly with the increasing temperature from 150 to 200 °C and then reduce rapidly when the temperature further rises. Hence, 200 °C is the optimal working temperature of the CuO-loaded SnO 2 sensors. Specifically, the optimal 20 wt% CuO/SnO 2 sensor gives the highest response of 1.36 × 10 5 , which is much higher than those of other sensors at 200 °C. The optimal working temperature of 200 °C corresponds to the temperature that maximizes the H 2 S adsorption rate relative to desorption rate of CuO/SnO 2 surfaces. Furthermore, 5-25 wt% CuO/ SnO 2 sensors display a lower optimal working temperature than that of SnO 2 sensor (250 °C). The relatively low optimal working temperature will be subsequently explicated by CuO loading effects. Figure 10 summarizes the H 2 S selectivity of 0-25 wt% CuO/SnO 2 sensors against SO 2 , H 2 , CH 4 and C 2 H 2 . This type of sensor exhibits the highest H 2 S selectivity, i.e., more than three orders of magnitude higher H 2 S response than those of other gases. The data prove that CuO is the catalyst that selectively accelerates the reaction with H 2 S. The selectivity behavior may also be attributed to the increase of active sites for H 2 S adsorption due to the highest specific surface area of 20 wt% CuO/SnO 2 NPs. The enhancements for other tested gases are not significant due probably to relatively weak interactions between gas molecules and 20 wt% CuO/SnO 2 NPs. The attained H 2 S responses of 20 wt% CuO/SnO 2 sensors are substantially better than those of many other metal-loaded SnO 2 and CuO-loaded SnO 2 sensors made by distinct techniques as listed in Table 1. However, the achieved optimal working temperature of 200 °C is higher than the values of some reports at 100-150 °C. The lower working temperature is generally preferred in practical applications. Nevertheless, the 20 wt% CuO/SnO 2 sensor may operate at a lower working temperature of 150 °C where the sensor still exhibits a high response of 3.1 × 10 4 to 10 ppm H 2 S (Fig. 9), which is also higher than the response values of other sensors reported in Table 1. Therefore, the CuOloaded SnO 2 sensor is a highly promising candidate for H 2 S sensing due to its high H 2 S response, high H 2 S selectivity and low working temperature.
Finally, the stability, repeatability, and reproducibility of CuO/SnO 2 sensors were evaluated from four samples produced in the same batch. All sensors exhibited good stability with less than 15% drift in sensor response over 1 month under the same operating conditions. Moreover, each sensor showed good repeatability with less than 12% response variation from 8 repeated measurements. In addition, four sensors from the same batch were found to have fair response variation of less than 26% evaluated under the same test condition.

Gas-Sensing Mechanisms
The characterization results suggest the formation of CuO/SnO 2 composite comprising very small CuO species on SnO 2 nanoparticles. Thus, the mechanisms for electrical response of CuO/SnO 2 sensing films toward H 2 S may be described based on the composite junction theory of p-n junctions at the contacts between p-type CuO and n-type SnO 2 as depicted in Fig. 11. For undoped SnO 2 , chemisorbed oxygen species (O − ) are formed resulting in the creation of depletion regions on surface at a moderate temperature. Upon exposure to H 2 S, H 2 S molecules interact with adsorbed oxygen species on SnO 2 surface (H 2 S + 3O − → H 2 O + SO 2 + e − ), releasing electrons to SnO 2 conduction band and reducing the sensor resistance. At a low working temperature of 200 °C, the concentration of oxygen species is very low, leading to a low reaction rate and a low H 2 S response. With CuO loading, additional depletion regions will be formed at various p-n junctions around the surface of SnO 2 nanoparticles. In addition, carriers in secondary CuO nanoparticles, which can break percolation of aggregated SnO 2 nanoparticles, may be fully depleted, resulting in a high electrical resistance in air. In ambient with H 2 S, the gas molecules can  CuS is more conductive than CuO, leading to lower potential barriers at depletion regions around the interfaces. The induction of metallic CuS is equivalent to the injection of free electrons into the p-type material (CuO), making it less p-type. This encourages the electron transfer from CuS to SnO 2 , resulting in additional decrease of depletion width and increase of the electrical conductance of SnO 2 . The decrease of resistance due to the formation of CuS is much larger than the reduction due to the reducing reaction with oxygen species due to transfer of more electrons from CuS. At low CuO contents, there are relatively few and small CuO nanoparticles that are fully transformed into CuS surrounding SnO 2 particles. It will provide a limited amount of electrons to SnO 2 due to relatively few heterojunctions, resulting in small reduction of depletion region widths in SnO 2 and small resistance drop upon H 2 S exposure. As the CuO content increases, the numbers of transformed CuS nanoparticles and heterojunctions increase, leading to an increased number of conduction paths through CuS as well as much reduced SnO 2 depletion region widths and thus a higher resistance drop that can be achieved after H 2 S exposure. However, CuO particles may coalesce into large ones and the number of CuO/SnO 2 heterojunctions becomes lower at very high CuO content (> 20 wt%). The large CuO particles will not be fully transformed to CuS due to limited reaction depth with H 2 S and the depletion regions in CuO cores remain, limiting the conduction through CuO and reducing attainable resistance drop. In the case of CuO, the response is low despite the formation of CuS because the resistance of CuO is already low and is not much higher than that of CuS [20]. After H 2 S in atmosphere extinguishes, the electrical resistance returns to its original values as CuS can be reoxidized to CuO in air at an elevated temperature according to the reaction (Eq. 2) [26]: The oxidation of CuS is slow at a low working temperature. As the increase of working temperature, the oxidation rate increases and lead to the increase of recovery rate. Since the CuS-CuO transformative reaction (Eq. (2)) is slower than the CuO-CuS one (Eq. (1)) at this working temperature, residual CuS materials can remain after subjecting CuO to several H 2 S pulses. This results in a substantial downward baseline drift of CuO sensor and the increase of baseline drift with increasing Cu content of CuO/SnO 2 sensors as previously observed in Fig. 8a. However, there is an exception in the case of 5 wt% CuO/ SnO 2 sensor, which exhibits a small upward drift of baseline resistance after the first pulse. It may occur because the sensor did not fully reach the steady state before applying the first pulse leading to some upward recovery owing to oxidation in air while the drift due to CuS-CuO transformation at this low Cu content is relatively small due to a low residual CuS content. At higher Cu contents, the downward drifts due to residual CuS are large and overwhelm the small upward recovery. The baseline drift considerably reduces the validity, repeatability and stability of sensor response of CuO/SnO 2 as the response to a subsequent H 2 S pulse is affected by the residual CuS concentration after the previous H 2 S exposure leading to negative deviations from the ideal response behavior. Thus, the calculated responses of the CuO/SnO 2 sensors especially with high Cu contents in Fig. 8b are lower the theoretical values under no residual CuS condition. The problems can be reduced by increasing the working temperature. Thus, the sensors may operate above (2) CuS + O 2 → CuO + SO 2 Fig. 11 Energy band diagrams of CuO/SnO 2 heterojunctions a before and b after exposure to H 2 S (E f = Fermi-energy level, E C = conduction band and E V = valence band) the optimal working temperature at 250 °C when the drift is low, and response is still high. CuS structure can be formed at 103 °C and will be transformed into Cu 2 S, a less conductive ionic conductor, at the temperature above 220 °C [26]. Consequently, the sensor response of CuO/SnO 2 NPs decreases when the temperature rises above 200 °C. The observed high H 2 S selectivity against SO 2 , H 2 , CH 4 and C 2 H 2 can also be explained in relation to the working temperature. At the optimal working temperature of 200 °C, the rate of CuO-CuS transformation is high, while the reducing reaction rates of SO 2 , H 2 , CH 4 and C 2 H 2 are very low because these reactions require the chemisorbed oxygen species whose density is still very low at this working temperature.
Conclusions 0-25 wt% CuO/SnO 2 NPs were fabricated using the precipitation and impregnation method. XRD, BET, TEM, SEM, EDS and XPS data suggested the loading of very small CuO nanoparticles on larger SnO 2 NPs. The gassensing results demonstrated that CuO loading greatly enhanced the H 2 S response of SnO 2 NPs with an optimal Cu content of 20 wt%. The 20 wt%CuO/SnO 2 sensor can perceive low-ppm H 2 S concentrations with ultrahigh responses (1.4 × 10 5 at 10 ppm), short response times (35 s), fair recovery times (a few minutes), very high H 2 S selectivity against SO 2 , CH 4 , H 2 and C 2 H 2 and good stability. They could also offer a wide detection range (0.15-10 ppm) when compared with the unloaded one (3-10 ppm). Therefore, the CuO/SnO 2 sensors synthesized by precipitation and impregnation could be a promising candidate for H 2 S detection in environmental applications.