CuO/V2O5 hybrid nanowires for highly sensitive and selective H2S gas sensor

Vanadium pentoxide (V2O5) nanowires decorated with CuO nanoparticles on their surface have been prepared by a facile chemical route. The gas-sensing performance of the CuO/V2O5 nanohybrids has been examined against H2S, CO, and NO2 gases over a range of gas concentrations from 7 to 60 ppm and working temperatures from 100 to 400 C, and compared with that of pristine V2O5 nanowires without the decoration. The CuO/V2O5 nanohybrids exhibit a greatly enhanced sensitivity toward H2S gas selectively and are relatively indifferent to CO and NO2 gases. The gas-sensing response of the nanohybrids increases by nearly 18 times (from 1.84 to 31.86) when tested against 23 ppm of H2S gas at 220 C. The nanohybrids remain stable when detecting H2S gas for a period of two weeks. This selective enhancement is attributable to the local p–n junction formed at the interface together with the reversible chemical reaction that occurs when CuO is exposed to H2S gas at the temperature employed.


Introduction
Hydrogen sulphide (H 2 S) is a "broad-spectrum" poison, namely it is toxic to several systems in the human body, however the nerve system is most affected. Enzymes that exist in the human body are capable of detoxifying H 2 S by oxidation to harmless sulfates. However, the oxidative enzymes become overwhelmed when the H 2 S concentration exceeds a certain threshold level of about 300-350 ppm on average. 1 To avoid harmful incidents occurring, the design of commercial H 2 S gas sensors capable of detecting minute gas concentrations at ppm levels oen requires a detection level as low as 5-15 ppm under moderate working temperatures. In this regard, chemiresistor-type sensors based on one-dimensional metal oxide semiconductors (MOSs) are attractive since they are capable of providing gas-sensing ability for the stable detection of trace amounts of hazardous gases with a wide range of chemical attributes. To detect H 2 S gas specically, various MOS sensing materials such as WO 3 , SnO 2 , ZnO, In 2 O 3 and CuO have been explored. 2 The MOS sensors still suffer from major drawbacks such as cross-sensitivity, sensitivity to humidity, long-term signal driing and slow sensor response. 3 In particular, the poor selectivity remains a challenge to overcome and oen limits their practical applicability. [2][3][4] In the literature, the gas-sensing properties of onedimensional n-type vanadium pentoxide (V 2 O 5 ) has seldom been addressed, despite its promising potential in gas detection. [5][6][7][8][9][10] Liu et al. rst prepared V 2 O 5 nanobelts decorated with either Fe 2 O 3 , TiO 2 or SnO 2 nanoparticles by a hydrothermal method. 5,6 The hybrid nanobelts showed a moderately enhanced sensing response (S ¼ 2.0-3.1 at 100 ppm ethanol) when compared to the pristine V 2 O 5 nanobelts (S ¼ 1.7) at 200 C. Since then, V 2 O 5 nanostructures have been investigated as gas sensors for detecting mostly organic vapours. 7-10 Among them, Raible et al. reported a high sensitivity against organic amines such as 1-butylamine, with a detection limit as low as 30 ppb using V 2 O 5 nanobres. 7 Raj et al. chemically prepared V 2 O 5 nanostructures and found that V 2 O 5 showed a slightly improved sensitivity toward ethanol at ppm levels compared to ammonia with identical concentrations at room temperature. 8,9 Mondaferri et al. also demonstrated the sensing response against ammonia gas over a concentration range of 0.85-8.5 ppm at 200-250 C using electrospun V 2 O 5 nanobres. 10 Recently, the use of V 2 O 5 nanostructures for the detection of inorganic, hazardous gases has begun to emerge. [11][12][13] Yan et al. reported the room-temperature sensing capability of a porous silicon/V 2 O 5 nanorod composite with a selective, substantially enhanced sensitivity toward NO 2 gas at low concentrations (3 ppm). This compares favourably to that of organic gases such as ethanol, methanol and acetone with an even higher concentration (100 ppm). 11 Mane et al. prepared V 2 O 5 and Pd-decorated V 2 O 5 nanorods on glass substrates by chemical spraying and subsequent thermal treatment. 12,13 They also found an enhanced selectivity against NO 2 gas compared to other gases including NH 3 , H 2 S, CO, and SO 2 . 13 The use of V 2 O 5 nanostructures as sensors for detecting inorganic toxic gases requires more work to assess their applicability. We herein report the decoration of CuO nanoparticles onto V 2 O 5 nanowires to form hybrid nanostructures and investigate the gas sensitivity/selectivity of these nanostructures toward the toxic gases H 2 S, CO and NO 2 . The reason for selecting CuO for the hybrid structure lies in the fact that there are reports about enhanced selective sensing achieved with ptype CuO semiconductors exposed to H 2 S gas. 14-17 For the rst time, the CuO/V 2 O 5 nanohybrids have been found to exhibit highly sensitive and selective detection toward H 2 S gas at ppm levels continuously over a prolonged period of time (up to two weeks) and are relatively indifferent to CO and NO 2 gases over the moderate temperature range examined. The V 2 O 5 nanowires served as a backbone for the preparation of CuO/V 2 O 5 hybrids with decorative CuO nanoparticles. Fig. 3(a) shows that the CuO nanoparticles with a diameter of about 50 nm are well dispersed on the V 2 O 5 surface. Fig. 3(b) shows a typical TEM bright-eld image of the CuO/V 2 O 5 hybrids, in which discrete CuO nanoparticles seem to adhere onto the V 2 O 5 surface. The black dots are monoclinic CuO which was veried by TEM ( Fig. S1 in the ESI †). Fig. 3(c) displays an XRD pattern of the CuO/V 2 O 5 hybrids prepared via a deposition process, with a heat-treatment temperature of 400 C. Well-dened crystalline phases have been found for the orthorhombic V 2 O 5 and monoclinic CuO (JCPDS no. 48-1548), without the formation of any intermediate alloying compounds. It is worthwhile to note that the heat-treatment temperature turned out to be critically important for the successful preparation of CuO/V 2 O 5 hybrids. If the temperature was raised to 500 C, the hybrids became irregular fragments as shown in Fig. 3(d), suggesting that the V 2 O 5 nanowires had partially melted or soened (note that the melting temperature of pure bulk V 2 O 5 is about 690 C) so the structure of the hybrid was destroyed. Fig. 4 shows the sensing response of the pristine V 2 O 5 and the CuO/V 2 O 5 hybrid nanowires against 23 ppm H 2 S gas over a temperature range from 100 to 380 C. Two important ndings have been observed. Firstly, the bell-shaped distribution reveals an optimum working temperature at which the sensing response is highest. This bell-shaped temperature dependence is ascribed to two main reasons. 17 On the one hand, the adsorption, desorption and diffusion reactions involved, between the target gas and the semiconducting oxide surface, are all thermally activated processes. Hence the response increases with the working temperature. 3 On the other hand, electrons in oxide semiconductors are more likely to attain sufficient energy to jump to the conduction band from the valence band at elevated temperatures. This leads to an increased concentration of free electrons so that electrical resistance is reduced accordingly. A combination of the thermally activated process, together with the increased probability of free-electron formation as the temperature was increased, results in an optimal temperature at which the electrical resistance (and hence the sensing response) is highest. In this regard, we have demonstrated for the rst time that the optimal sensing temperature can be tailored to a lower temperature, against the model H 2 S gas at ppm levels, through the decoration  of CuO nanoparticles onto V 2 O 5 nanowires in Fig. 4. Secondly, we have found that the CuO/V 2 O 5 hybrids show a response as high as 31.86 against H 2 S gas at ppm levels at 220 C. For the rst time, this compares favourably with that of the pristine V 2 O 5 , for which the highest response is merely 1.84 at 300 C. The increased sensitivity is ascribed to the hybridization of V 2 O 5 with the CuO nanoparticles together with the decorative hybrid structure, and will be discussed in later sections.

Results and discussion
The CuO/V 2 O 5 hybrid nanowires also display a signicantly enhanced gas selectivity which is highly desirable in practical sensing applications. To illustrate this, the dynamic resistance behaviour of the pristine V 2 O 5 nanowires is displayed in Fig. 5 against H 2 S, CO, and NO 2 gases at 300 C. Responses of the V 2 O 5 nanowires are 1.84 for 23 ppm H 2 S gas, 1.48 for 60 ppm CO gas and 1.46 for 30 ppm NO 2 gas.
In addition, the response times toward detection of H 2 S, CO, and NO 2 gases are 196, 174, and 212 s, respectively, as shown in Fig. 6(a-c), for the above gas concentrations. The sensing properties are primarily ascribed to the exchange of charge carriers between the adsorbed gas molecules and the V 2 O 5 surface. In an air atmosphere, chemisorbed oxygen species, such as O 2À , O À and O 2 À , trap electrons from the conduction band of the n-type V 2 O 5 semiconductor, resulting in a spacecharge depletion near the surface. 18 Aer the introduction of the reducing H 2 S or CO gas into the testing chamber, the gas molecules would react with the previously chemisorbed oxygen  ions, so that the electrons trapped by the absorbed oxygen would subsequently be released into the bulk V 2 O 5 nanowires, according to the following reactions (1) and (2): This produces free electrons and hence decreases the electrical resistance. When the reducing gases were removed from the atmosphere, oxygen molecules spontaneously adsorb onto the V 2 O 5 surface reversibly to trap free electrons, which causes the electrical resistance to increase, returning back to its initial value. In contrast, the adsorption of oxidizing NO 2 gas would  deplete the number of charge carriers in the surface layer of the V 2 O 5 nanowires. This widens the depletion region, resulting in an increase in the electrical resistance accordingly. The desorption of gas molecules from the pristine V 2 O 5 appears to be more difficult than the adsorption, so that a "tail" was found in Fig. 6(a-c).
In addition, Fig. 5(d) summarizes the sensing response of the pristine V 2 O 5 nanowires. The lack of gas selectivity is apparent, however the sensor does provide a decent response to a change in concentration of the target gases over ppm concentrations, examined at the given working temperature. Fig. 7 shows the dynamic resistance change of the CuO/V 2 O 5 hybrids against H 2 S, CO, and NO 2 gases. The responses of the hybrids are 1.33 against 60 ppm CO gas, and 1.29 against 30 ppm NO 2 gas at 220 C. The result resembles that of the pristine V 2 O 5 nanowires at 300 C, despite the fact that the sensing temperature employed was reduced. A signicantly enhanced sensing performance was found for the CuO/V 2 O 5 hybrids toward H 2 S gas selectively. Fig. 7(a) shows the resistance change of the CuO/V 2 O 5 hybrids against H 2 S gas of various concentrations at 220 C; the responses are 31.86 at 23 ppm, 20.89 at 15 ppm and 7.86 at 7 ppm. This result indicates a dramatic enhancement of the sensing response from 1.84 for the pristine V 2 O 5 nanowires to 31.86 for the CuO/V 2 O 5 hybrids upon exposure to 23 ppm H 2 S. Compared to the rather indifferent sensitivity of the hybrid nanowires against CO and NO 2 gases, a signicantly increased sensing performance, nearly 18 times that of the pristine V 2 O 5 nanowires, has been discovered toward H 2 S gas selectively over the various gases and gas concentrations examined, as summarized in Fig. 8. Not to mention the fact that this occurs at a slightly lower working temperature (220 C). In addition, Fig. 6(d) shows that the response time of the hybrid is 130 s for H 2 S gas at a 23 ppm concentration. Therefore, the enhanced selective sensitivity is obtained with a simultaneously shortened response time. In comparison, the response times to detect CO and NO 2 gases are 268 and 211 s for the hybrids, respectively, which are similar to that of the pristine counterpart. Similar results have been found for the recovery times. The detection limit of the hybrid CuO/ V 2 O 5 nanowires has been determined experimentally, in which no response was detectable when the H 2 S concentration was below 3 ppm.
The selective enhancement and increased sensitivity toward H 2 S gas sensing of the CuO/V 2 O 5 hybrids may be explained by the p-n junction formed at the dissimilar interface and  sulfuration of the decorative CuO. The intrinsic V 2 O 5 and CuO are n-and p-type semiconductors, respectively. The decoration of CuO onto V 2 O 5 in the hybrids forms local p-n junctions at the interface. [14][15][16][17]19,20 The width of the space-charge region (W) at the interface can be estimated from eqn (3) and (4) for n-V 2 O 5 and p-CuO, respectively: 20,21 where 3 is the permittivity (2.25 Â 10 À11 F m À1 for V 2 O 5 and 2.123 Â 10 À10 F m À1 for CuO), 20,22 V o (¼ 0.6 eV) is the contact potential which is equal to the amount of the energy band bending between V 2 O 5 and CuO (f CuO À f V 2 O 5 ), q is the electrical charge of the carrier (1.6 Â 10 À19 C), and N is the carrier concentration (N V 2 O 5 ¼ 1.48 Â 10 18 cm À3 and N CuO ¼ 1 Â 10 19 cm À3 , at room temperature). 20,23 The calculated spacecharge widths are 9.97 nm for V 2 O 5 and 4.53 nm for CuO. These estimates are used for the plot of the energy-band diagram of CuO/V 2 O 5 p-n junction in air and H 2 S, in Fig. 9(a). Energy-band bending at the interface is apparent. The band gap (E g ), electron affinity (c) and work function (f) of V 2 O 5 are 2.3, 3.99, and 4.7 eV, respectively. 24,25 Those of CuO are 1.35, 4.07, and 5.3 eV, respectively. 20 Note that the calculated space-charge widths are an estimate since the carrier concentrations used in the calculation are the values at room temperature. A more precise determination of the W V 2 O 5 and W CuO needs information about the carrier concentrations at the working temperature (220 C) employed. Nonetheless, the electrical transport channel is suppressed by the presence of local p-n junctions, according to the estimates of the space-charge region. This would hence lead to a higher base resistance (as observed in Fig. 6 where the base resistance values of the CuO/V 2 O 5 hybrids were 10 to 20 times greater than that of the pristine V 2 O 5 ) and a larger change in electrical resistance (and hence sensing response) for the CuO/V 2 O 5 hybrid nanowires compared to that of the pristine V 2 O 5 counterpart.
An energy-band diagram shown in Fig. 9(b) further demonstrates how the sulfuration of the decorative CuO affects the response and recovery processes at the p-n junctions in the hybrids. In an air atmosphere, the potential barrier at the p-n junction impedes the electron ow through the interface. When H 2 S gas is introduced into the testing chamber with a concentration exceeding a certain critical level (about 7 ppm determined experimentally in the present study), the CuO is spontaneously converted to CuS by the following chemical reaction: [14][15][16] CuO + H 2 S / CuS + H 2 O CuS is a metallic conductor, thus, the p-n junction as well as the charge-depletion region transform into a metal-n-type heterostructure. The potential barrier at the p-n junction is then expected to be lowered; therefore, the conversion from CuO to CuS facilitates the electron ow, leading to the drastic decrease in the electrical resistance. Aer the inow of H 2 S gas is terminated and air is re-introduced into the chamber, CuS changes back to CuO by oxidation: The electrical resistance accordingly returns to its initial level. It may be interesting to note that the V 2 O 5 surface, without being decorated by the CuO nanoparticles as in the CuO/V 2 O 5 hybrid, may also be involved in the gas sensing when H 2 S is introduced. As yet, this appears not to contribute substantially to the H 2 S selectivity.
The gas-sensing stability of the CuO/V 2 O 5 nanohybrids has been examined against 15 ppm H 2 S for a long period of time. In Fig. 10, the sensing response remains relatively stable with an average sensitivity of 21.1 during the continuous two-week measurement. Note that every response value is in fact an average of seven measurements (as shown in the inset for the rst data point). This nding indicates that the CuO/V 2 O 5 nanohybrids may be used in practical sensing applications for the selective detection of hazardous H 2 S gas, despite more work being required to elucidate the applicability.

A. Materials
CuO-functionalized V 2 O 5 nanowires were prepared by a facile two-step route, involving hydrothermal and wet-deposition processes. In the hydrothermal process, crystalline V 2 O 5 nanowires were synthesized from a solution mixture consisting of 0.285 g V 2 O 5 powder in 9 ml de-ionized water, with dropwise addition of 1 ml hydrogen peroxide (35%) under vigorous agitation for 1 h. The solution was then placed in a Teon-lined stainless-steel autoclave and kept at 180 C for 24 h before being cooled down to room temperature naturally. The resultant V 2 O 5 nanowires were washed repeatedly with de-ionized water, centrifuged and then dried at 80 C. In the wet-deposition process, the as-prepared V 2 O 5 nanowires were dispersed in a mixed solution consisting of 10 ml of ethanol and 0.06 g of Cu(NO 3 ) 2 $3H 2 O. The suspension was then heated to 80 C for the removal of ethanol. The powder was then placed in an alumina boat and fed into the centre of a three-zone horizontal quartz-tube furnace. The tube was rst evacuated until the pressure inside reached 6 Â 10 À2 torr, before the introduction of a gaseous mixture containing 85 sccm argon (standard cubic centimetres per min at STP) and 15 sccm oxygen. The temperature was then increased to 400 C with a heating rate of 20 C per min and held isothermally for 1 h with a continuous ow of the mixed gas during the entire heating process to prepare CuO nanoparticles decorated on V 2 O 5 nanowires.

B. Characterization
The morphology and crystalline structure of the V 2 O 5 and CuO/ V 2 O 5 hybrid nanowires were examined by eld-emission scanning electron microscopy (FE-SEM), X-ray diffractometry (XRD) with Cu Ka radiation (l ¼ 0.15405 nm) and transmission electron microscopy (TEM).

C. Fabrication of the gas sensing devices
The gas sensors were fabricated by mixing the CuO/V 2 O 5 hybrids or pristine V 2 O 5 nanowires with methanol. The suspensions were then dispensed on alumina substrates with pre-congured, inter-digitated Ag electrodes. Analysis of the sensing properties was carried out in a self-assembled tubular furnace system along with a mass-ow controller, so that the concentration of various gases including air, H 2 S, CO and NO 2 gases, could be precisely tuned. The electrical resistance was recorded by a volt-amperometric Keithley 2410 every 1 s over a wide range of gas concentrations (7 to 60 ppm) and working temperatures (100 to 400 C) against target gases such as H 2 S, CO and NO 2 . For each test, the nanowire sensor was stabilized in air for 10 min before the target gas was introduced into the reaction chamber. The gas response (S) is dened as the ratio of the electrical resistance measured in fresh air (R a ) to the resistance measured in the presence of target gas (R g ). Either R a /R g or R g /R a was used for the determination of S, depending on the oxidising or reducing target gas used. The response time and recovery time are dened herein as the time it takes for the sensor to achieve 90% of its equilibrium level for the entire resistance change for adsorption and desorption of the target gas, respectively.

Conclusion
Hybrid p-CuO/n-V 2 O 5 nanowires were prepared via a facile chemical route and their gas-sensing properties were investigated. The formation of local p-n junctions at the nanometre scale CuO/V 2 O 5 interface, together with the reversible sulfuric/ oxidative reactivity when exposed to cyclic H 2 S gas/air at moderate temperatures, signicantly enhanced the response and selectivity toward H 2 S gas sensing. The hybrid nanowires showed a rather indifferent response when CO and NO 2 were used as the target gas. This unique selectivity toward H 2 S gas at precise ppm levels means that hybrid p-CuO/n-V 2 O 5 nanowires are anticipated to be used in a stable and reliable sensing device suitable for practical alarm applications.

Conflicts of interest
There are no conicts to declare.