Journal of ELECTRONIC MATERIALS https://doi.org/10.1007/s11664-018-6648-0 Ó 2018 The Minerals, Metals & Materials Society SO2 and H2S Sensing Properties of Hydrothermally Synthesized CuO Nanoplates PHAM VAN TONG,1,2 NGUYEN DUC HOA ,1,3,4 HA THI NHA,1 NGUYEN VAN DUY,1 CHU MANH HUNG,1 and NGUYEN VAN HIEU1 1.—International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No 1, Dai Co Viet Str., Hanoi, Vietnam 2.—Department of Physics, Faculty of Mechanical Engineering, National University of Civil Engineering (NUCE), No 55, Giai Phong Str., Hanoi, Vietnam 3.—e-mail: ndhoa@itims.edu.vn 4.—e-mail: hoa.nguyenduc@hust.edu.vn CuO nanoplates were synthesized by a facile hydrothermal method for a SO2 gas-sensing application The synthesized materials were characterized by field-emission scanning electron microscopy (FE-SEM), powder x-ray diffraction (XRD), Raman spectroscopy, and photoluminescence spectroscopy Gassensing characteristics were measured at various concentrations of SO2 and H2S at 200–350°C The results showed that rectangular CuO nanoplates with an average size of approximately 700 500 30 nm3 were synthesized FESEM and XRD analyses also depicted that the nanoplates were polycrystalline with an average crystal size of 12.85 nm Gas-sensing measurements demonstrated that the synthesized CuO nanoplates exhibited p-type semiconducting behavior, where the sensor resistance increased upon exposure to H2S and decreased when exposed to SO2 The sensor showed a considerably higher response to SO2 than to H2S in the measured concentrations ranging from ppm to 10 ppm, suggesting that the CuO nanoplates are suitable for high-sensitivity SO2 sensing We also clarified the sensing mechanism of the CuO nanoplate-based SO2 sensors Key words: CuO nanoplates, hydrothermal, SO2 sensing, sensing mechanism INTRODUCTION In recent years, the rapid growth in the economy and the industrialization of Vietnam have required a significant increase in energy supply; however, fossil fuels are limited, and the utilization of household biogas is a possible solution.1 Especially in Vietnam, the potential for biogas energy sources is very high because many pig farms and agricultural waste products are available.2 Biogas is extensively and readily produced and has been used in rural Vietnam in recent years3 thanks to the policy of the government to encourage the use of biogas for electric generation However, the biogas contains (Received June 27, 2018; accepted September 1, 2018) some toxic gases, such as sulfur dioxide (SO2), hydrogen disulfide (H2S), carbon monoxide (CO), and carbon dioxide (CO2).4 Furthermore, in big cities such as Hanoi, SO2 is also present in vehicle emissions as a result of fuel combustion in diesel buses and trucks.5 SO2 is a colorless gas with a strong odor and is considered an extremely toxic gas because, when combined with water, it becomes highly corrosive sulfuric acid which can damage the environment and constructions.6 The inhalation of low-concentration SO2 gas can cause chemical burns, as well as irritation of the nose, throat, and airways The occupational safety and health administration (OSHA, USA) designated permissible exposure limits (PEL) for SO2 to be only ppm Along with SO2, H2S is also an extremely toxic, explosive, and hazardous gas with an odor similar to that of bad eggs.7 It occurs naturally in Van Tong, Hoa, Nha, Van Duy, Hung, and Van Hieu crude petroleum and natural gas, and can be produced by the breakdown of organic matter and human/animal wastes in livestock biogas.8 Especially, in rural Vietnam, people use biogas for cooking purposes without desulfurization Biogas contains toxic H2S gas with a concentration of up to 0.5%,9 but most plants in Vietnam use it without any monitoring or desulfurization The PEL for H2S set by OSHA is 20 ppm; thus, its detection in the environment and monitoring are essential.10 Thus, monitoring of H2S11 and SO212 at low concentrations (ppm level) is very important and is the key issue in the safe use of biogas and industrial processes.13 Different materials and/or structures have been used for SO2 and H2S sensors For instance, an integrated microchip with Ru/Al2O3/ ZnO as the sensing material has been developed for SO2 sensors with a limit detection of ppm.14 An electrochemical sensor based on diamond-like carbon-modified polytetrafluoroethylene membranes has been fabricated for the detection of SO2.15 Fedoped metal oxides have also been prepared for conventional solid-state SO2 and CO2 gas sensors.16 Stabilized zirconia-based mixed potential-type sensors utilizing MnNb2O6-sensing electrodes have been prepared for the detection of low-concentration SO2, in which the sensor has a low detection limit of 50 ppb at an operating temperature of 700°C.17 Most of the developed SO2 sensors are solid electrolyte types, but such devices are limited by their short lifetime,18 bulky size,19 and high operation temperature.17 In contrast to the conventional electrochemical sensor, metal oxide-based resistive devices have some advantages, such as small size, simple operation, easily integration in circuits, good reproducibility, and good reversibility.20 Both ntype and p-type metal oxide semiconductors have been extensively studied for gas sensor applications.21 The n-type SnO2 dodecahedron was used for the monitoring of SO2, where the maximum response to 10 ppm SO2 at 183°C was only 1.92.22 Transition metal (Ni, Fe, and Co)-doped MoS2 nanoflowers have also been synthesized for roomtemperature SO2 gas sensors,23 where the responses are strongly dependent on the doped metal and reach the maximum value of approximately 20% to 4000 ppm SO2 We recently successfully synthesized different ptype semiconductors for gas sensors, such as Co3O4 nanorods,24 NiO nanosheets,25 and CuO nanowires.26 The advantages of p-type semiconductors27,28 over n-types include low-humidity dependence, high catalytic properties,29 and a high signal-to-noise ratio.30 Among others, CuO, a p-type semiconductor with a narrow band gap ($ 1.2 eV) has been the priority choice due to its robustness and abundance.31 CuO has been used as a sensing material in various sensors, such as VOCs,32 H2,33 H2S,34 CO,35 and NO2.36 Nanostructured CuO materials, such as core–shell nanoparticles,37 tadpoles, spindles, leaves/spheres and fusiform,38 nanotubes and nanocubes,39 nanowires,40 nanorods,41 nano-thin films,42 polyhedrons,43 urchin- and fiber-like,44 and nanoplates,45 have been extensively studied for gas-sensing applications utilizing the large specific surface area and effective adsorption sites for surface reactions.46 Different methods have been used to synthesize CuO nanostructures in which the sputtering or physical methods require a high vacuum and expensive equipment.37,42 The inexpensive wet chemical method is very advantageous in the synthesis and control of the morphology of CuO materials for gas sensor applications.38 However, the SO2-sensing characteristics of CuO nanoplates have not yet been studied In addition, sensing mechanisms of CuO-based sensors to sulfur-containing gases, such as H2S and SO2, remain unclear Here, CuO nanoplates were synthesized by a facile hydrothermal method and then spin-coated onto a thermally oxidized silicon substrate equipped with a pair of comb-type interdigitated Pt electrodes (Pt IDEs) for gas-sensing characterization Dynamic measurement of the change in resistance of sensors on exposure to different concentrations of SO2 and H2S gases was performed at various temperatures ranging from 200°C to 350°C The results demonstrated that the synthesized CuO nanoplates are excellent for monitoring low concentrations of SO2 gas The SO2 gas-sensing mechanism of the CuO nanoplates has also been discussed EXPERIMENTAL The materials used in this study were analytical copper(II) chloride (CuCl2), potassium hydroxide (KOH), and deionized (DI) water CuO nanoplates were synthesized by a facile hydrothermal method without any surfactant or post-thermal calcination Processes for the synthesis of CuO nanoplates are summarized in Scheme In a typical synthesis, 1.2 g of CuCl2 and 1.7 g of KOH were dissolved in DI water under magnetic stirring at a room temperature of $ 27°C The clear blue solution was transferred to a Teflon-lined autoclave (100 mL in volume) A hydrothermal process was carried out in an electric oven at 220°C for h After the reaction was completed, the autoclave was cooled naturally to room temperature The precipitated powders were collected and washed five times with DI water and subsequently two times with ethanol to remove unreacted ions by centrifuging at 4000 rpm for 15 Finally, the precipitate powders were dried at 45°C overnight before characterization The synthesized materials were characterized by powder x-ray diffraction (XRD; Advance D8, Bruker) and field-emission scanning electron microscopy (FE-SEM; JEOL 7600F), respectively Photoluminescence (PL) was measured at room temperature using an excited laser at 328 nm Raman spectroscopy was measured using the Renishaw Invia Confocal micro-Raman System SO2 and H2S Sensing Properties of Hydrothermally Synthesized CuO Nanoplates RESULTS AND DISCUSSION Scheme Processes for the hydrothermal synthesis of CuO nanoplates For gas-sensing characterization, the as-synthesized materials were dispersed in N-vinylpyrrolidone to form a colloidal solution, which was then spincoated onto a thermally oxidized Si substrate equipped with Pt IDEs to form the sensing devices.47 The Pt IDEs contain 37 digits with a length of approximately 780 lm, while the width and the space between two digits are 20 lm (Fig 1a) For gas-sensing measurement, the fabricated sensor was placed on a heating plate to control the working temperatures Prior to the gas-sensing measurement, the sensor was pre-heated at 400°C for h to stabilize the resistance and increase the contact between the sensing materials and the Pt IDEs The resistance of the sensor was continuously measured using a Keithley instrument (Model 2602) interfaced with a computer, while the dried air and analytic gases were switched on/off in each cycle Here, the analytic H2S and SO2, with a concentration of 100 ppm diluted in nitrogen were used as tested gases To obtain lower concentrations (1–10 ppm), the analytic gas was further diluted with dry air using a mixing system Details of the mixing system can be found elsewhere.48 The response time (sres) was estimated by fitting the sensor resistance versus time from the introduction of the SO2 gas until the resistance reached saturation, using Eq Rtị ẳ R0 expt=sres ị 1ị The recovery time (srec) was also estimated by fitting the sensor resistance versus time after stopping the introduction of SO2 gas until the resistance returns to the initial value, using Eq Rtị ẳ RS expðt=srec Þ ð2Þ where R0 and RS are the resistances of the sensor in air and the saturation value of the resistance in SO2, respectively After hydrothermal synthesis, the obtained products exhibited a black color, as shown in the inset of Fig 1a The morphologies and microstructure analysis of the obtained products were investigated by FE-SEM (Fig 1a–c) Obviously, the low-magnification FE-SEM image (Fig 1a) demonstrates that the as-prepared products are composed of homogenous nanoplates, which were well separated but not aggregated together because none of the surfactants was used during material synthesis The nanoplates exhibited an irregular rectangular shape with a size of approximately 700 500 nm2 and a thickness of approximately 30 nm (Fig 1b and c) The nanosheets were composed of nanocrystals of an average size of less than 20 nm (Fig 1d) In the study of Li et al.,31 the CuO nanoplates were synthesized by a hydrothermal method using an ionic liquid precursor, benzyltrimethylammonium hydroxide, and the nanoplates have the length of approximately 262 nm, depending on the synthesis conditions Here, we did not use an ionic liquid surfactant or structure-directing agent but still obtained a nanoplate structure During hydrothermal synthesis, the CuCl2 reacts with KOH to form Cu(OH)2 Subsequently, the Cu(OH)2 dehydrates to form CuO nanocrystals at high temperature, according to the following equations: CuCl2 ỵ KOH ẳ CuOHị2 DT CuOHị2 ! CuO þ H2 O ð3Þ ð4Þ The CuO crystals thus serve as seeds for the growth of nanoplates through the Ostwald ripening mechanism.49 Note that the dehydration of Cu(OH)2 can also occur at room temperature but at a very slow rate, and the produced CuO material has a flake morphology, but the nanoplates not During hydrothermal synthesis, the CuO nanocrystals (seeds) grew along two [100] and [010] preferential growth directions to form nanoplates according to the Ostwald-ripening mechanism.50 During growth, the CuO seeds aggregated together to form the nanoplates through van der Walls forces However, the mis-orientation of the nanocrystals during aggregation forms the polycrystalline nanoplates but not the single crystal structure The powder XRD pattern of the synthesized CuO nanoplates is shown in Fig 2a, where all the diffraction peaks were readily indexed to the monoclinic structure of CuO (JCPDS, No 80-1917) The diffraction peaks were broad as a result of the nanocrystalline nature of the synthesized materials.51 The average crystal size of the CuO nanoplates calculated using the Scherer equation was approximately 12.85 nm This value is considerably smaller than the size of the CuO nanoplates estimated by the SEM images, again confirming the poly-crystallinity of the nanoplates Van Tong, Hoa, Nha, Van Duy, Hung, and Van Hieu Fig (a, b) Low- and (c, d) high-magnification FE-SEM images of the synthesized CuO nanoplates-based sensor Inset in (a) is a photo of the hydrothermal product The Raman spectrum of the CuO nanosheets shown in Fig 2b displays two peaks, at 286.5 cmÀ1 and 337.7 cmÀ1, which were assigned to the active modes Ag and B1g, respectively.52 The Raman spectrum of the sheet-like CuO mesocrystals has three modes at around 281 cmÀ1, 345 cmÀ1, and 630 cmÀ1 belonging to the Ag and Bg modes of CuO, respectively.53 Note that, in the CuO Raman modes, the copper atoms remain stationary, and only the oxygen atoms take part in the motion because oxygen atoms are considerably lighter than the copper atoms The oxygen motion for the Ag mode is parallel to, and that for the Bg mode is perpendicular to, the monoclinic axis.54 Here, the wavelength numbers of those modes in the CuO nanoplates are shifted from the values reported in the bulk literature (298 cmÀ1, 345 cmÀ1, and 632 cmÀ1) due to the size effects.53,55 Here, the active mode B2g at 623 cmÀ1 did not appear, possibly due to the anisotropic structure of the CuO nanoplates.56 The PL spectrum of the synthesized CuO nanoplates shown in Fig 2c exhibits broad emission peaks centered at approximately 498 nm (2.48 eV) The band gap of bulk CuO ranged from 1.9 eV to 2.1 eV.57 The higher near-band-edge emission of nanocrystalline CuO was explained due to the Burstein–Moss effect.58 The broad emission at 498 nm (2.48 eV) is assigned to the energy levels of defect sites, such as VCu (copper vacancies), Cui (copper interstitial), and Vo (oxygen vacancy) in CuO nanoplates or the quantum confinement effect.59 This result is consistent with other CuO nanostructures reported to have an optical band gap of approximately 2.48 eV.60,61 The high defect level is expected to show a high gas-sensing performance The transient resistance versus time upon exposure to different concentrations of SO2 measured at temperatures ranging from 200°C to 350°C is shown in Fig 3a The base resistances of the sensor in air were 24.76 kX, 4.53 kX, 1.65 kX, and 0.80 kX for temperatures of 200°C, 250°C, 300°C, and 350°C, respectively The resistance of the CuO nanoplates decreases with increasing temperature and exhibits an obvious negative temperature coefficient of resistance in the measured range This result is consistent with other reports on CuO thin film.62 Prior to the introduction of the analytic gas, the resistance of the sensor is very stable and hardly changed with time However, upon exposure to SO2, the sensor resistance abruptly decreased and reacted to the saturation values within a minute, depending on the working temperatures and gas concentrations The sensor also shows good recovery characteristics where the resistances returned to the initial values when the flow of analytic gas was stopped Figure 3a also reveals that the response SO2 and H2S Sensing Properties of Hydrothermally Synthesized CuO Nanoplates Fig SO2-sensing characteristics of CuO nanoplates measured at different temperatures: (a) transient resistance versus time upon exposure to different SO2 concentrations, (b) sensor response as a function of SO2 concentrations Fig (a) XRD pattern, (b) Raman spectrum, and (c) PL spectrum of CuO nanoplates: average crystal size of 12.85 nm Inset in (b) shows an optical microscopic image of CuO nanoplate powder and recovery speed increases with increasing working temperature At all measured temperatures, the sensor shows reversible response characteristics Reversible adsorption of analytic gas molecules on the surface of the sensing material is very important in the practical application and reusability of gas sensors The sensor shows a remarkable response to low SO2 concentrations down to ppm, which is very effective because CuO nanosheets did not sense 40 ppm SO2 at room temperature.63 The sensing mechanism of SO2 based on metal oxides is complex because SO2 molecules can directly adsorb on the surface of CuO and/or sulfidate CuO into CuS In the study reported by Ma et al.,22 the resistance of n-type SnO2 nanocrystal-based sensors decreases with the introduction of SO2 gas because SO2 molecules can react with the chemisorbed oxygen species, and the trapped electrons are released back to the conduction band of SnO2, according to Eq 5ị SO2gasị ỵ O ẳ SO3adsị ỵ e According to this, given that CuO is a p-type semiconductor, upon exposure to SO2, the sensor resistance should increase and not decrease as observed in our study The result is opposite to that of n-type WO3, where the sensor resistance increased upon exposure to 10 ppm SO2 at 220°C.64 A decrease in the resistance of p-type semiconductor-based sensors upon exposure to SO2 gas was also found in MoS2 materials.23 We believe that SO2 gas can act as both an oxidizing agent and a reducing agent because of the multiple valences of sulfur Here, the SO2 exhibits characteristics of an oxidizing gas by direct adsorption The SO2 molecules act asan oxidizing agent, capturing electrons Van Tong, Hoa, Nha, Van Duy, Hung, and Van Hieu from the sensing material and adsorbing on the surface in the form of SOÀ , as follows: CuO ỵ SO2gasị ỵ e ẳ CuO SO 2adsị ð6Þ In addition, upon SO2 exposure, the SO2 molecules can react with CuO to form Cu2SO3, as shown below.65 2CuO ỵ SO2gasị ỵ 2e ẳ Cu2 SO3 ỵ 1=2O2 7ị The adsorption of SO2 molecules capture electrons and generate hole carriers, thereby decreasing the resistance of p-type CuO nanoplate-based gas sensors The sensor response (R0/R), as a function of SO2 concentrations measured at different temperatures, is shown in Fig 3b At all measured temperatures, the sensor response increased to the SO2 concentrations The response value increased from 1.5 to 2.8 when the SO2 concentration increased from ppm to 10 ppm at a measured temperature of 200°C The response value is considerably higher than that of the n-type SnO2 dodecahedron22 or SnO2 thin film loaded with metal oxide catalysts.66 At a given concentration, the sensor response decreases with increasing working temperatures The sensor response can be improved by decreasing the working temperature to below 200°C However, decreasing the working temperature increased the response and recovery time For practical application, the response and recovery time should be limited; thus, we did not check the sensor response at temperatures lower than 200°C Response and recovery times are important factors which determine the performance of gas sensors By using the exponential decay and growth functions, we could estimate the response and recovery times of the sensors, which were measured at different temperatures to various SO2 concentrations, as shown in Fig The response time of the sensor decreased significantly from approximately 40 s to about s with the increase of working temperature from 200°C to 350°C (Fig 4a) A higher SO2 concentration requires a shorter response time The fast response time at high working temperatures can be explained by the acceleration of thermal energy for the gas adsorption The sensor requires a longer recovery time of approximately 100–200 s at 200°C, depending on Sthe O2 concentrations, indicating the strong adsorption on the surface of the CuO However, those values decreased exponentially to around 15 s with the increase of working temperature from 200°C to 350°C (Fig 4b) The decrease of the response and recovery times is consistent with the Langmuir isotherm model, where the adsorption and desorption are exponentially dependent on the temperature.67 H2S is a reducing gas, which is mainly present in biogas, has high reactivity to CuO Thus, its sensing characteristics were also measured at different Fig (a) Response and (b) recovery times as functions of the working temperatures measured to different SO2 concentrations temperatures, and the data are shown in Fig Figure 5a shows the transient resistance versus time of the sensor measured at different temperatures upon exposure to various H2S concentrations As demonstrated, the sensor resistance increased with the exposure to H2S gas, again confirming the p-type characteristics of CuO This response trend was opposite to those of other reports, where the resistance decreases (conductance increases) upon exposure to H2S by the formation of CuS percolation paths.68 However, the sensor exhibited a relatively poor recovery characteristic, whereas the sensor resistance could not recover to the initial values after the H2S gas flow was stopped H2S gas is highly reactive to CuO69; thus, it can react with CuO surfaces to form CuSO4 or CuS depending on the analytic gas concentration and working temperatures.70 The working principle of the sensor was believed to be based on the phase transition of semiconducting p-type CuO to strongly degenerated p-type CuS with metallic conductivity,71 because, in the form of CuS,65 CuxS exhibits semi-metallic or semiconducting properties72 with a band gap of approximately 1.6 eV.73 Indeed, the sensing mechanism of H2S sensors based on CuO material relying on the sulfidation of CuO into CuS was reported in SO2 and H2S Sensing Properties of Hydrothermally Synthesized CuO Nanoplates Ref 68 However, in the study of Ramgir et al.,74 they found that, at intermediate concentrations (500 ppb to 50 ppm), the response curve of CuO thin film at room temperature is governed by both H2S oxidation and CuS formation mechanisms Here, the sensor resistance increased upon exposure to H2S gas Thus, we believe that the H2S response is mainly based on the following reaction: 2H2 Sgasị ỵ 3O 2adsị ẳ 2H2 Ogasị ỵ 2SO2gasị ỵ 3e 8ị The released electrons will neutralize free holes, and thus reduce the main carrier density, increasing sensor resistance Sensor response (R0/R), as a function of H2S concentrations measured at different temperatures (Fig 5b), reveals that the sensor has the highest response value at the low operating temperature of 250°C In addition, the sensor response increased from approximately 1.5 to 2.8 with an increase of H2S concentration from ppm to 10 ppm These values are relatively high when the response to ppm H2S of CuO thin film at room temperature is very low at approximately 1.29.74 Decreasing the temperature can increase the response value, but the recovery characteristic is very poor and not effective for practical application Fig H2S sensing characteristics of CuO nanoplates measured at different temperatures: (a) transient resistance versus time upon exposure to different H2S concentrations, (b) sensor response as a function of H2S concentrations The response and recovery times of the sensor when measured at different temperatures to various H2S concentrations are shown in Fig The response and recovery times of the sensors decrease with increasing working temperature, such as that of the SO2 response The response time of approximately 60 s at 250°C decreased to approximately 20 s when the temperature increased to 350°C The recovery time was longer (100–250 s) at 250°C, depending on the H2S concentration However, the recovery time decreased to approximately 24 s at 350°C for all H2S concentrations Such fast response and recovery times at a relative high temperature of approximately 350°C are effective for practical applications in the monitoring of H2S in environmental pollution A comparative result on the response of the sensor to various concentrations of SO2 and H2S measured at their optimal temperatures is plotted in Fig The response values to SO2 are approximately three times higher than those to the H2S gas despite its working temperature being lower In addition, hydrogen-sensing characteristics of the sensor were also tested at 200°C to different concentrations ranging from 50 ppm to 1000 ppm (Figure S1, Supplementary) The sensor showed very Fig (a) Response and (b) recovery times as functions of working temperatures measured for different H2S concentrations Van Tong, Hoa, Nha, Van Duy, Hung, and Van Hieu Fig Comparative result on the response of the sensor to H2S and SO2 at their optimal working temperatures low response values of 1.15–1.36 to the H2 concentration of 50–1000 ppm, respectively Such results suggest that the CuO nanoplates are effective for monitoring SO2 and/or H2S gases CONCLUSION We introduced a facile and saleable hydrothermal synthesis of CuO nanoplates for effective SO2 and H2S gas-sensing applications The CuO nanoplates are highly crystalline and allow reversible monitoring of low concentrations (1–10 ppm) of SO2 and H2S at moderate temperatures (250–350°C) The maximum response value to ppm SO2 at 200°C is 2.74 with the response time of less than 40 s, sufficient for practical applications The SO2-sensing mechanism mainly relied on the direct surface adsorption/desorption and not on the sulfidation process The developed sensor is suitable for monitoring toxic SO2 and H2S gases in biogas ACKNOWLEDGEMENT This study was supported by the Hanoi University of Science and Technology (Grant No T2017PC-171) ELECTRONIC SUPPLEMENTARY MATERIAL The online version of this article (https://doi.org/ 10.1007/s11664-018-6648-0) contains supplementary material, which is available to authorized users REFERENCES T.B Ho, T.K Roberts, and S Lucas, J Agric Sci Technol A 5, 387 (2015) T.T.T Cu, T.X Nguyen, J.M Triolo, L Pedersen, V.D Le, P.D Le, and S.G Sommer, Asian Aust J Anim Sci 28, 280 (2014) H Roubı´k, J Mazancova´, L.D Phung, and J Banout, Renew Energy 115, 362 (2018) J.-J Su and Y.-J Chen, Environ Monit Assess 187, 4109 (2015) P.D Hien, M Hangartner, S Fabian, and P.M Tan, Atmos Environ 88, 66 (2014) Z 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Figure 3a also reveals that the response SO2 and H2S Sensing Properties of Hydrothermally Synthesized CuO Nanoplates Fig SO2- sensing characteristics of CuO nanoplates measured at different temperatures:... monitoring SO2 and/ or H2S gases CONCLUSION We introduced a facile and saleable hydrothermal synthesis of CuO nanoplates for effective SO2 and H2S gas -sensing applications The CuO nanoplates are