- Nano Express
- Open Access
Sensitive, Selective, and Fast Detection of ppb-Level H2S Gas Boosted by ZnO-CuO Mesocrystal
© The Author(s). 2016
- Received: 16 September 2016
- Accepted: 13 October 2016
- Published: 26 October 2016
ZnO-CuO mesocrystal was prepared via topotactic transformation using one-step direct annealing of aqueous precursor solution and assembled into a H2S sensor. The ZnO-CuO mesocrystal-based sensor possesses good linearity and high sensitivity in the low-concentration range (10–200 ppb). Compared to pure CuO, the as-prepared ZnO-CuO mesocrystal sensor exhibited superior H2S sensing performance with a response ranging from 8.6 to 152 % towards H2S concentrations from 10 ppb to 10 ppm when applied at the optimized working temperature of 125 °C. The sensor showed excellent repeatability and good selectivity towards H2S gas even at a concentration four orders of magnitude lower than the interfering gases, such as H2, CO2, CO, NO2, acetone, and NH3. The improved sensitivity could be attributed partially to the effective diffusion of analyte gas through the mesocrystal surface and the abundant accessible active sites. Moreover, the nanoscale p-n junctions within the mesocrystal, which could effectively manipulate the local charge carrier concentration, are also beneficial to boost the sensing performance.
- p-n junction
- Gas sensor
- H2S detection
H2S is generally produced as a by-product from petroleum refining, farming, and biogas production [1, 2]. As one of the most toxic and flammable gases, H2S affects the nervous system of human beings and can cause people to lose consciousness at very low concentrations . The acceptable ambient limit for H2S (recommended by the Scientific Advisory Board on Toxic Air Pollutants, USA) is within the range of 20–100 ppb . Besides, small amount of H2S in exhaled breath is usually used as a signaling molecule for metabolic disorder called as halitosis [5, 6]. Furthermore, H2S with a concentration as low as 10 ppb is known to deteriorate the performance of hydrogen fuel cells . Therefore, there is a pressing need to explore efficient sensing devices with high sensitivity capable of detecting H2S at ppb level.
High response values have been achieved by using various metal oxide semiconductor sensing materials, including ZnO , In2O3 , CuO [10–12], SnO2 , and WO3 . Among these reported semiconductors, CuO is especially favored in selective detection of H2S due to its p-type semiconducting property  and strong affinity towards H2S molecules . The sensitivities are down to ppm and even sub-ppm levels; nevertheless, the response/recovery process is always quite long in the case of using individual metal oxide semiconductor. For example, CuO nanowire-based sensors are capable to detect H2S with a concentration as low as 10 ppb with a response of 4.8 % and 30.9 %; however, both the response/recovery times exceed 10 and 15 min, respectively. More seriously, a vertically aligned CuO nanowire array-based sensor is not recoverable when the concentration of H2S is higher than 1 ppm . There are some reports to reduce the response/recovery time by incorporating CuO with ZnO to form ZnO-CuO composites (ZnO nanofiber , nanowire , nanorod [19, 20], hollow sphere  decorated with CuO nanoparticles and CuO-ZnO micro/nanoporous film ), and ZnO is used as the transducing material in all these cases. Nevertheless, these composite structures could only achieve the detection of H2S with a concentration from 500 ppb to 7 ppm. In addition, the working temperature for most of the reported H2S gas sensor is in the range of 150–450 °C. Thus, a structure with appropriate ZnO and CuO arrangement is highly desirable to boost the response value and to reduce the response/recovery time towards ppb-level H2S gas and, possibly, working at a relatively lower temperature.
Mesocrystal is an ordered superstructure formed through the oriented alignment of nanoparticles [23, 24]. The excellent electrical conductivity and abundant adsorption sites induced by the mesocrystal structure are exactly expected for gas detection . So far, only a few research group realized the fabrication of mesocrystal gas sensors [26–28] and were based on one type of metal oxide nanoparticles. Recently, a unique ZnO-CuO mesocrystal was synthesized via topotactic transformation , and efficient charge transfer between n-type ZnO and p-type CuO nanoparticles was confirmed. It is expected that if this mesocrystal could be employed as a gas-sensing material, the sensing performance would be greatly benefited since the fundamental mechanism of chemiresistive gas sensing is the manipulation of charge transfer. Another prevailing advantage for this structure is the internal porosity , which allows both their outer and inner parts to participate in gas-sensing reactions, providing good diffusion and more accessible active sites, and thus a high response value. Nevertheless, this promising mesocrystal structure was largely ignored for ppb-level H2S gas sensing.
In this work, ZnO-CuO mesocrystal was prepared by merely using one-step direct annealing of aqueous precursor solution. The resulting gas sensor showed an enhanced sensing performance towards H2S even at a working temperature of 125 °C in terms of higher sensitivity, shorter response/recovery time in comparison with the pure CuO-based sensor and other nanostructure-based H2S sensors reported previously. The superior performance was attributed to a synergistic effect of the p-n junction built between ZnO and CuO, as well as the internal porosity for effective diffusion and adsorption induced by the unique structure of mesocrystal.
Preparation of ZnO-CuO Mesocrystal and Pure CuO
ZnO-CuO mesocrystal was prepared using a facile one-step direct annealing of aqueous precursor solution. In a typical procedure, polyethylene oxide/poly(p-phenylene oxide)/polyethylene oxide (P123, MW 5400, 0.104 g) was dissolved into deionized water (9.609 g) with stirring for 2.5 h, followed by the addition of Zn(NO3)2·6H2O (0.107 g) and Cu(NO3)2·6H2O (0.163 g). The mixture was stirred for 1.5 h, and NH4NO3 (0.636 g) was added to form a gel. The gel was stirred until it was homogeneous and then placed on a ceramic crucible to be treated under stage-temperature-programmed calcinations. The precursor solution was firstly heated from 300 to 525 K at a temperature ramp of 1.0 K/min and kept at 525 K for 40 min, followed by further heating to 775 k at a rate of 0.5 K/min and kept for 150 min at this temperature. Pure CuO was prepared using the same approach without adding Zn(NO3)2·6H2O.
X-ray diffraction (XRD) measurement was conducted using powder XRD (Bruker D8 Advance, with Cu-Kα radiation operating at 40 kV and 40 mA, scanning from 2θ = 30° to 80°). Field-emission scanning electron microscopy (FESEM, ZEISS SUPRA 55VP) and transmission electron microscope (TEM, JEM-2100) were used to characterize the morphology of the samples. N2 adsorption was performed at 77 K on NOVA 2200e (Quantachrome, USA), and the surface area data was calculated on the basis of the Brunauer-Emmett-Teller (BET) model. Inductively coupled plasma optical emission spectrometer (ICP-OES) analysis was carried out on PerkinElmer Optima 3300 DV.
Gas-Sensing Performance Evaluation
Initially, the prepared material was mixed with deionized water in a weight ratio of 4:1 and ground in a mortar for 15 min to form a paste. The paste was then coated on a ceramic substrate by a thin brush to form a sensing film on which silver interdigitated electrodes with both finger-width and interfinger spacing of about 200 μm were previously printed. The thickness of the film was controlled by the brushed cycles. The sample was dried naturally in air overnight. The sensor was aged under 4V voltage at room temperature for about 24 h. Finally, the sensor was fixed on a microheater to control the working temperature through modulating the current. The required amount of gas was injected into the conical flask (1.2 L) using a syringe and was mixed with air (relative humidity (RH) was about 20 %). For gas-sensing test, the sensor was inserted into the target gas chamber and measured by a CGS-1TP intelligent gas-sensing analysis system (Beijing Elite Tech Co., Ltd., China). After the sensor resistance reached a new constant value, the sensor was then inserted into a same size conical flask full of air to recover. The relative sensor response in resistance is defined as, Response = (R g − R a )/R a × 100 %, where R g and R a are the electrical resistances of the sensor in target gas and in air. The response time is defined as the period in which the sensor resistance reaches 90 % of the response value upon exposure to the target gas, while the recovery time is defined as the period in which the sensor resistance changes to 10 % of the response value after the target gas is removed.
To evaluate the sensitivity of the ZnO-CuO mesocrystal-based sensor, the typical dynamic response change measurements towards H2S with concentration ranging from 10 ppb to 10 ppm were conducted at 125 °C (Fig. 3b). The sensor showed apparent response of 8.6 % towards 10 ppb H2S. And with H2S concentration increasing to 10 ppm, the response of the sensor increased to 152 %. When the concentration is higher than 1 ppm, a saturation trend in response is observed, which might be resulted from the saturated adsorption of the active sites on the surface of the ZnO-CuO mesocrystal. Thus, the sensor is more suitable for the detection of ppb-level H2S. Figure 3c is the plot of the response of ZnO-CuO mesocrystal-based sensor as a function of H2S concentration with error bar. The trend roughly follows a Langmuir isotherm adsorption model as most commonly observed in chemiresistive sensors , a very fast linear increase in the low-concentration region, followed by a gentle slope in the high-concentration region. The capability of detecting low concentration of H2S (10–100 ppb) is critical in chemical diagnosis and quality control of industrial product. The linearity of the response values in the lower concentration range (10–200 ppb) possesses a slope of 0.0029 with a R-square value of 0.99 as shown in the inset of Fig. 3c. The limit of detection (defined as LOD = 3S D /m, where m is the slope of the linear part of the calibration curve (0.0029) and S D is the standard deviation of noise in the response curve (1.6 × 10−3)) of the ZnO-CuO mesocrystal-based sensor is determined to be 1.7 ppb. The good linearity and low detection limit are in favor of the future device integration applied in detection of ppb-level H2S. Figure 3d presents the response/recovery time with error bar towards different concentrations of H2S, the response time keeps in the range of 78–180 s, while the recovery time increases from 70 to 356 s with the increase of the H2S concentration. The quick response and short recovery time should be attributed to the internal porosity of the ZnO-CuO mesocrystal, which could provide direct diffusion channels and facilitate the diffusion of H2S molecules, as well as fast adsorption/desorption at the interface.
Various CuO and ZnO-CuO nanostructures employed for H2S sensing (C is the lowest tested concentration)
Response time (s)
Recovery time (s)
ZnO-CuO composite nanofiber
CuO-ZnO composite hollow sphere
CuO nanoparticle decorated ZnO nanorod
CuO/ZnO heterostructured nanorod
Ultrathin CuO layers modified ZnO nanowire
CuO-ZnO micro/nanoporous film
Network CuO-ZnO composite
However, the sensing mechanism of the ZnO-CuO mesocrystal is different. The electrons in ZnO and holes in CuO diffuse in opposite direction due to the great gradient of the same carrier concentration until the diffusion and drift of the carriers are finally balanced. Consequently, p-n junction is formed, and the energy band bends in the depletion layer to achieve a uniform Fermi level (EF) in the thermal equilibrium state (Fig. 6b). It can be calculated that the barrier height of the conduction band (ΔE c = E c2 − E c1) and the valence band (ΔE v = (E g2 − E g1) − ΔE c ) at the p-n junction are 0.77 and 0.73 eV, respectively . Therefore, compared with that of bare CuO, the sensitivity of ZnO-CuO mesocrystal for the detection of H2S can be enhanced in two ways: (1) the hole concentration in CuO can be lowered not only by the electron transfer from the H2S gas molecule (elector donor) but also by the electron injection from the conduction band of ZnO, leading to an increase in resistance; (2) the electron concentration in ZnO gets higher due to the electron transfer from H2S to ZnO, which renders a stronger p-n junction, thus helps to block the local hole transportation around the junction region in CuO and yields a further increase in resistance.
In summary, the ZnO-CuO mesocrystal was prepared via topotactic transformation using one-step direct annealing of aqueous precursor solution. The as-prepared sensor exhibited superior H2S sensing performance, such as low detection limit, high response, and fast response/recovery process. Moreover, the sensor showed excellent repeatability and good selectivity towards H2S gas even at a concentration four orders of magnitude lower than the interfering gases, including H2, CO2, CO, NO2, acetone, and NH3. The enhanced response of the ZnO-CuO mesocrystal to H2S is attributed partially to the effective diffusion of H2S molecules through the entire porous surface and the formation of uniform nanoscale p-n junction within the mesocrystal. Thus, the ZnO-CuO mesocrystal represents a promising sensing material for sensitive and selective detection of ppb-level H2S gas.
We thank the financial supports from the Xinjiang International Science & Technology Cooperation Program (20146002), the National Natural Science Foundation of China (21401213), the “Western Light” program of Chinese Academy of Sciences (XBBS201401), and Open Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (201532).
YG participated in the fabrication of the ZnO-CuO mesocrystal, the measurement of the sensing properties, the result analysis, and the writing of the manuscript. MG participated in the preparation of pure CuO. YL participated in the result analysis. XD participated in the design of the study and coordination of the work. All authors contributed to the interpretation of the results and drafting of the manuscript, and they read and approved the final version.
The authors declare that they have no competing interests.
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