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Enhancement of H2S sensing performance of p-CuO nanofibers by loading p-reduced graphene oxide nanosheets
Accepted Manuscript Title: Enhancement of H2 S sensing performance of p-CuO nanofibers by loading p-reduced graphene oxide nanosheets Authors: Jae-Hun Kim, Ali Mirzaei, Yifang Zheng, Jae-Hyoung Lee, Jin-Young Kim, Hyoun Woo Kim, Sang Sub Kim PII: DOI: Reference:
S0925-4005(18)31921-X https://doi.org/10.1016/j.snb.2018.10.144 SNB 25578
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
29-5-2018 20-10-2018 27-10-2018
Please cite this article as: Kim J-Hun, Mirzaei A, Zheng Y, Lee J-Hyoung, Kim J-Young, Kim HW, Kim SS, Enhancement of H2 S sensing performance of p-CuO nanofibers by loading p-reduced graphene oxide nanosheets, Sensors and amp; Actuators: B. Chemical (2018), https://doi.org/10.1016/j.snb.2018.10.144 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhancement of H2S sensing performance of p-CuO nanofibers by loading p-reduced graphene oxide nanosheets
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Jae-Hun Kima, Ali Mirzaeib,c, Yifang Zhenga, Jae-Hyoung Leea, Jin-Young Kima, Hyoun Woo Kimb,d,*, Sang Sub Kima,** a
Department of Materials Science and Engineering, Inha University, Incheon 22212, Republic of Korea
The Research Institute of Industrial Science, Hanyang University, Seoul 04763, Republic of Korea
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b
c
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Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea
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*
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Corresponding authors.
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d
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Department of Materials Science and Engineering, Shiraz University of Technology, Shiraz, Iran
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E-mail address: [email protected] (H.W.Kim), [email protected] (S.S. Kim).
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Highlights
For gas sensing, RGO loaded CuO NFs with different amounts of RGO were produced by electrospinning method and subsequent heating.
0.5wt% RGO loaded CuO NFs sensor had the maximum response to H2S gas.
Underlying sensing mechanism is discussed.
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Abstract Being compared to p-n heterojunctions, less attention has been paid to p-p heterojunctions for gas sensing studies. Motivated by this fact, herein, gas sensing characteristics of p-reduced
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graphene oxide (RGO) loaded p-CuO nanofibers (NFs) will be presented. In order to find the optimal value of RGO loading, different amounts of RGO (0.05-1.5 wt%) were added to the electrospun CuO NFs. The results of sensing tests showed that the CuO-0.5wt% RGO has the best sensitivity to H2S at 300°C. Also, low response to the interfering gases demonstrated the
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good selectivity of the optimal sensor towards H2S. Underlying sensing mechanism is discussed
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in details. High response of CuO-0.5wt% RGO sensor towards H2S was related to NF
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morphology, presence of RGO, high intrinsic sensitivity of CuO towards H2S gas, presence of a
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lot of CuO nanograins along with the existence of the plenty of p-p heterojunctions, which will be changed by transformation of CuO to CuS. The results of present study can be used for design
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and fabrication of novel RGO containing p-p heterojunction gas sensors.
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Keywords: CuO, Reduced graphene oxide, Nanofiber, Sensing mechanism, H2S
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1. Introduction
Nowadays gas sensors are vital in our life. They are extensively used for detection of
hazardous gases for environmental safety [1], home and public safety [2][3], vehicles [4] and air conditioning [5], and so on. Owing to very diverse applications of gas sensors, the need to develop of inexpensive, stable, sensitive, selective and high performance gas sensors, has
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triggered the researchers around the world to enhance the performance of such sensors mostly in term of sensitivity and selectivity. To this end, many different methodologies have been proposed so far. One common methodology is morphology engineering, where different
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nanostructures such as thin films [6] nanobelts [7], nanorods [7], nanowires [8] and nanofibers (NFs) [9,10] have been reported for enhancement in sensing performance.
In particular, NFs are very promising for sensing studies. They can be easily fabricated by electrospinning, which is an efficient and cost-effective technique to fabricate long metal oxide NFs. Furthermore, they possess large surface area, resulting from their one dimensional
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morphology. Accordingly, they have significant surface area for gas species and the presence of
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a lot of nanograins and large grain boundary areas, ensure high sensitivity in the sensors with
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NFs morphology [11-13]. Even though a lot of materials with NF morphology have been
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reported for gas sensing [14-21], the number of literature about CuO NFs for sensing purposes is
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rare. The main reported CuO NFs sensors are as follows. Choi et al. reported enhanced NO2 and H2S sensing performance of pristine CuO NFs and CuO-SnO2 NFs composites, respectively
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[22,23]. Zhao et al. reported porous CuO/SnO2 NFs composites with enhanced H2S sensing performance [24]. Katoch et al. [25], fabricated p-CuO/n-ZnO core-shell NFs to detect low
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concentrations of reducing gases such as H2S. As it can be seen, most of CuO-based gas sensors are reported to have good sensitivity towards H2S gas, which is a very poisonous and flammable gas causing dizziness, nausea, vomiting, and respiratory tract [26-28]. Sensing of H2S is vital
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especially in the fields of oil and natural gas exploration and breath analysis [29]. Generally p-type metal oxides such as CuO offer low response to target gases and they are
not very popular for sensing applications especially in pristine form [7]. A good strategy for sensing enhancement is generation of n-n [30] homojunction, p-p [31] or n-p [32]
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heterojunctions through loading semiconducting nanomaterials. In this regards, reduced graphene oxide (RGO) with p-type characteristic can be an interesting material. In addition to its high surface area, it has a lot of dangling oxygen atoms, acting as good binding sites for
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adsorption of the target gases [33]. Accordingly hybrid composites containing CuO and RGO can be promising materials for gas sensing studies [34]. Herein, for the first time, we report H2S gas sensing characteristics of RGO loaded CuO NFs. Besides large surface area of RGO and plenty of dangling bonds, work function difference between RGO and CuO will results in formation of plenty RGO-CuO heterojunctions, where electrons from CuO will transfer to RGO
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and accordingly a hole accumulation layer (HAL) will develop on CuO. When exposing to H2S,
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conversion of CuO to CuS changes the conductivity from semiconducting to metallic type and
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destroys the established heterojunction, leading to a high response to H2S.
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In our previous research [13], gas sensing features of RGO-loaded ZnO NFs were reported.
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In an attempt to extend our knowledge about RGO loaded gas sensors, in this study, the sensing characteristics of RGO-loaded CuO NFs to H2S is explored, and optimal RGO loading content
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and sensing mechanisms are presented. To this end, different amounts of RGO ranging from 0.05 wt% to 1.5 wt% were added to the electrospun CuO NFs and the gas sensors were fabricated.
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The sensing characteristics were studied at different RGO loading and sensing temperatures. The results obtained showed that the presence of CuO nanograins, existence of RGO and the formation of nano p-p heterojunctions can significantly enhance the sensing capacity of the CuO
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NFs.
2. Experimental Procedure
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2.1. Preparation of electrospinning solutions Polyvinyl alcohol (PVA- MW 80000 g/mol) copper acetate (Cu(CH3CO2)2) and p-RGO nanosheets were used as the starting chemicals. An aqueous solution containing 9 wt.% PVA at
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70°C. Next, a 4.3 wt.% (Cu(CH3CO2)2) aqueous solution and varying amounts of RGO (nominally 0.05, 0.1, 0.2, 0.3, 0.5, 1, and 1.5 wt%) were added to the PVA solution at 80°C and afterwards different mixed solutions of copper acetate, RGOs and PVA with high viscosity were prepared.
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2.2. Electrospinning process
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The details of experimental procedure of electrospinning can be found in our previous
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literature [35]. In summary, the prepared viscous solutions were transferred to a glass syringe
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having a stainless steel needle. During the electrospinning process, the needle was connected to a large positive voltage (+15 kV) and a large negative voltage (-10 kV) was applied to the
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aluminum collector. The solution feeding rate was constant (0.07ml/h), while the distance
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between the tip of needle to collector was 20 cm. Finally, in order to remove the solvent and polymer from the as synthesized NFs, samples were heat treated using a horizontal tube furnace
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at 600°C for 2 h.
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.2.3. Materials Characterization
Field-emission scanning electron microscopy (FE-SEM, Hitachi-S-4200) and transmission electron microscopy (TEM) were used to study the morphology of the prepared products. FESEM was equipped with Energy-dispersive X-ray spectroscopy (EDS) and EDS was utilized to evaluate the chemical composition. In order to obtain the work function of RFO and CuO,
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ultraviolet photoelectron spectroscopy (UPS) measurements (UPS, Thermo Fisher Scientific Co. Theta probe) at UHV (<10-10 Torr) were performed, using a HeI 21.2 eV UV light. 2.4. Gas sensing measurements
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For sensing studies, sputtered Ti (~50 nm thick)/Pt (~200 nm thick) bi-layers were applied onto the products that already were deposited on SiO2-grown Si substrates. The fabricated sensors’ sensing capabilities were investigated in the presence of H2S, CO, C6H6, and C7H8 gases. Sensing tests at different temperatures were carried out by a home-made sensing system,
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equipped with a gas-dilution and testing system. Gas concentrations were exactly controlled by
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varying the mixing ratio of the dry air and target gas (the total gas flow rate =500 sccm) by mass
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flow controllers. The resistance in air (Ra) and in the presence of target gas (Rg) was recorded,
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and the response of gas sensor was defined as R = Rg/Ra.
3. Results and discussion
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3.1. Morphological and chemical studies
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The morphologies of NFs were investigated by using FE-SEM. The micrographs are provided in Fig. 1(a)-(h). Fig. 1(a) displays the FE-SEM micrograph of pristine CuO NFs, where nanograins on the surface of NF can be clearly seen. Also, the inset shows the lower
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magnification image, demonstrating the successful synthesis of long, continuous CuO NFs having a diameter of ~50 nm. Figs. 1(b)-(h) provide the FE-SEM micrographs of different amounts RGO-loaded CuO NFs, where in all cases, long, continuous NFs comprising ultrafine grains are observed. Corresponding insets show lower magnification FE-SEM images, again
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demonstrating that almost the same diameter of electrospun NFs were prepared after the calcination treatment. The results of a typical EDS analysis for 1.5 wt% RGO-loaded CuO NFs are presented in
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Fig. 2(a). As can be seen, the weight percent of RGO is 1.53%, demonstrating successful loading of a desired amount of RGO in the CuO NFs. TEM analyses are presented in Fig. 2(b) and 2(c) for 0.5 wt% and 1.5 wt% RGO-loaded CuO NFs, respectively. RGO sheets along with CuO nanograins are clearly seen in Fig. 2(b). Lattice-resolved TEM micrograph in Fig. 2(c) shows the parallel fringes with spacing of 2.75Å and 2.32Å, which are consistent with the interplanar
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spacings of (110) and (111) planes, respectively, of CuO crystal structure.
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3.2. Gas sensing studies
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3.2.1. Optimal working temperature
Depending on the sensing temperature, the kinetics of gas adsorption, gas desorption and
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other related phenomena change and consequently different responses can be observed at
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different working temperatures [36]. However, there is only one temperature, so-called the optimal working temperature, in which the rate of gas adsoption is higher than the desorption
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rate and the highest sensitivity can be achieved. Accordingly, the responses of the RGO-loaded CuO NFs were tested at 150 to 400°C. Fig. 3(a) provides dynamic normalized resistance curves of 0.5 wt% RGO-loaded CuO NFs toward 10 ppm H2S gas at 150 to 400°C. When the sensor
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exposed to H2S gas, the resistance was increased, demonstrating p-type nature of sensor. Fig. 3(b) indicates the response versus the sensing temperature. It can be seen that up to 300°C, the response is increased with increasing of sensing temperature, reaching to a maximum value at 300°C and afterwards decreases. At T <300 °C, the H2S molecules have not enough energy to be
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adsorbed for subsequent reaction with the adsorbed oxygen. At T >300°C, the desorption rate is very high and H2S molecules escape away easily [37]. At 300°C, the rate of gas adsorption is high enough, resulting in the best response of gas sensor [37]. After finding the optimal
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temperature, the remaining experiments were carried out at 300°C. 3.2.2. Dependency of response of sensors on the RGO loading amount
To find the optimal value of RGO amount, various amounts RGO-loaded CuO NFs were exposed to H2S at 300°C and dynamic resistance curves are displayed in Fig. 4(a). All sensors
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show a reversible response to different concentrations of H2S. Fig. 4(b) indicates the calibration
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curves of different sensors. It is seen that the response values increase with the increase of RGO
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content up to 0.5 wt%, and with further increases of RGO loading the response is decreased. Fig.
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4(c) clearly shows this trend. RGO content strongly affect the response and the maximum response is observed at 0.5wt% RGO content. Details of sensing mechanism will be discussed in
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3.2.3. Selectivity studies
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sub-section of sensing mechanism.
To evaluate the selectivity, the optimal sensor based on 0.5wt% RGO-loaded CuO NFs was
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exposed to CO, C6H6 and C7H8 gases and the dynamic resistance curves can be seen in Fig. 5(a). Also, for comparison, the curve related to H2S is presented. All of interfering gases are reducing
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which have negligible effect on the sensor. (Fig. 5(b)). Fig. 5(c) shows the selectivity histogram of the 0.5wt% RGO-loaded CuO NFs to 10 ppm of H2S, CO, C6H6, and C7H8 gases, showing the responses of 11.2, 2.91, 3.17, and 2.66, respectively. Good selectivity of the 0.5 wt% RGO loaded CuO NFs sensor can be explained by the fact that in the presence of H2S, CuO will be converted to CuS with metallic conductivity, whereas other interfering gases have a reducing
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effect on the CuO, where they only slightly change the width of hole accumulation layer in CuO NFs. In the following section these effects will be explain in more details. The real selectivity is based on the sensor response among a mixture of these gases. Accordingly, the present sensor
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may not exhibit a high enough selectivity in the real situation, in which the sensitivity of H 2S is measured in a mixture of various gases. A better method with very high selectivity towards a specific gas is use of electronic nose which comprised of an array of semiconductor sensors. In fact, application of an electronic nose and artificial intelligence methods is a highly efficient way of analyzing the odorous gases. Nevertheless in comparison with simple metal-oxide gas sensors,
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it is more complex due to needs for signal processing unit [38].
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UPS is an efficient method for identifying the work function of materials [39,40]. Fig. 6(a)
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and 6(b) provide the UPS spectra of CuO and RGO, respectively. From Fig. 6(c) and 6(d), the cut-off values of 17.69 eV (CuO) and 16.74 eV (RGO), respectively can be obtained.
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Additionally, in order to revise instrumental broadening due to the analyzer, 0.1 eV is added to the work functions [41]. Accordingly, the work functions of CuO and RGO were calculated as
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3.61 eV (i.e., 21.2 eV-17.69 eV (+0.1) eV = 3.61 eV) and 4.56 eV (i.e., 21.2 eV – 16.74 eV
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(+0.1) eV = 4.56 eV), respectively.
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3.3. Gas sensing mechanism Adsorption of target gases on the surfaces of gas sensor changes its resistance and this is the
principle of sensing mechanism in metal-oxide gas sensors [42]. At first, we will describe the sensing mechanism in bare CuO gas sensor and then extend it to RGO-loaded SnO2 NFs sensors. In air, oxygen molecules wigh high large electron affinity (0.43 eV [43]), will be easily adsorbed
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on the surfaces of sensors and abstract electrons from the surfaces of CuO NFs, as a result, as shown in Eqs. (1)-(4): adsorbed oxygen ions will be appear on surface in molecular (O2−), or atomic (O− and O2−) forms: 𝑂2 (𝑔) → 𝑂2 (𝑎𝑑𝑠)
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(1)
𝑂2 (𝑎𝑑𝑠) + 𝑒 − → 𝑂2− (𝑎𝑑𝑠)
(2)
𝑂2− (𝑎𝑑𝑠) + 𝑒 − → 2𝑂− (𝑎𝑑𝑠)
(3)
𝑂− + 𝑒 − → 𝑂2− (𝑎𝑑𝑠)
(4)
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When T<100°C, the O2- is dominant, O- is dominant at 100
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higher temperatures [44]. Transfer of electron from the surface of CuO to the adsorbed oxygen
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will result in the decrease of the electrons and corresponding increase of holes in the surface of CuO. As a result a HAL having plenty of holes is formed on the outer surfaces of CuO.
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Accordingly current will be mainly passed throuth the HAL. When CuO NFs are exposed to H2S atmosphere, the H2S will provide holes to and/or take out electrons from the sensors surface,
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increasing the resistance of sensor.
Also, in the pristine CuO NFs, owing to the presence of CuO nanograins, there are a lot of
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CuO-CuO homojunctions. First, we expect that the CuO-CuO homojunctions will affect the sensing behaviors. As shown in Fig. 7a, since oxygen gas tends to penetrate into the CuO grain
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boundaries, O2- species are dominantly present at the grain boundaries, taking out electrons and lowering the Fermi levels. This will generate the potential barriers for both electrons and holes, decreasing the hole conductance across the grain boundaries and thus contributing to the decrease of conductivity.
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Upon the introduction of H2S gas, through the sulphurization reaction [45], CuO is converted to CuS. Sulphurization reaction can be shown as follows [23]: 𝐶𝑢𝑂 (𝑠) + 𝐻2 𝑆(𝑔) → 𝐶𝑢𝑆(𝑠) + 𝐻2 𝑂(𝑔)
(5)
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During the recovery period, by introduction of air, CuS is transformed to CuO: 𝐶𝑢𝑆 (𝑠) + 3/2𝑂2 (𝑔) → 𝐶𝑢𝑂(𝑠) + 𝑆𝑂2 (𝑔)
(6)
The generation of phase at the interfaces of the adjacent nanograins play a critical role in the gas detection mechanism of CuO NFs gas sensors. The work function of CuO was calculated to be
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3.61 eV, whereas that of CuS is reported to be in the range of 4.90-4.95 eV [46-49]. In the
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CuO/CuS henerointerfaces, due to the work-function difference, electrons in CuO will flow to
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CuS, establishing the potential barrier (Fig. 7(b) and 7(c)). When only the grain boundary
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regions are converted to CuS, the width of the potential barrier will be narrow (Fig. 7(b)). On the
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other hand, most part of CuO is converted to CuS, the potential barrier will be very broad (Fig. 7(c)). By the way, the potential barrier on the bottom side of the energy band will act as a barrier
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to the hole conduction. The contribution of CuO-CuO homojunctions to the CuS-generationinduced increase of the hole resistance will be dependent on the relative height of the hole
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potential barriers. If “b” in Fig. 7(b) and 7(c) is larger than “a” in Fig. 7(a), the hole resistance will be further increased by the transformation of CuO to CuS. However, if “b” is smaller than
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“a”, the hole resistance will be decreased. Accordingly, at this moment, we do not know whether the CuO-CuO homojunctions, by the introduction of H2S gas, leading to the enhancement of sensing behavior or not. In RGO-loaded gas sensors, in addition to the above mechanisms, the role of p-CuO/p-RGO heterojunctions should be considered. From the UPS measurement, we reveal that work
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functions of RGO and CuO in the present work are 4.56 and 3.61 eV, respectively (Fig. 8). When they are in intimate contact, electrons from CuO will flow to RGO (Fig. 8), resulting in equating of the Fermi levels, the bending of energy levels and expanding of HAL in CuO. When RGO
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loaded CuO NFs sensors are exposed to H2S gas, according to eq. (5), CuO will be transformed to CuS phase (Fig. 8).
Role of RGO can be explained based on the relative work functions of RGO, CuO, and CuS. It is noteworthy that CuO phase is continuous, whereas RGO is discrete. Accordingly, the main hole current will flow through the CuO structure. Upon the introduction of H2S gas, the
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adsorption of H2S molecules will transfer electrons to the CuO surfaces, decreasing the thickness
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of HAL. The work function of CuS is in the range of 4.90-4.95 eV [46-49]. Also, the CuS is
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regarded as a p-type material. Accordingly, if we assume that RGO and CuS are in contact,
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electrons in RGO will flow to CuS, decreasing its hole accumulation and thus increasing the hole
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resistance. Therefore, the introduction of H2S gas and the transformation of CuO to CuS will coincidentally increase the hole resistance, contributing to the improvement of sensor response.
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When exposing to fresh air, the reaction of eq. (6), take places and CuS is transformed to CuO and the resistance signal backs to the initial state.
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As it was shown in Fig. 4, the response of sensor greatly depended on the RGO content. RGOs are distributed among the CuO NFs and no direct contact is among the neighboring RGOs. Since the probability of direct contact among adjacent RGOs is very low, thus the role of
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p-RGO/p-RGO homojunctions is not considered here. Due to the discontinuous distribution of RGOs, the current in the sensor pass mainly through the CuO NFs. When amount of RGO is ≤0.5 wt%, the current flowing through CuO is blocked due to the presence of discrete RGOs, and the initial resistance is increased, consequently, in the presence of H2S a high modulation of the
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resistance will occur, depending on the RGO amount. However, at the sufficiently high content of RGO i.e. >0.5wt%, it would provide an additional path for current flow, resulting in a larger conduction path along the RGO, which would reduce the initial resistance and the response of
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sensor decreases relative to the optimal amount of RGO. Other factors which can enhance the response of the sensor should be considered. Discretely distributed RGOs, with a huge surface area, will induce the spillover effect and act as a catalyst for the dissociation and transportation of H2S gas molecules onto the CuO surfaces [13]. Furthermore, RGO has plenty of functional groups and defects on its surfaces. Accordingly, it
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can offer plenty of adsorption sites for the H2S gas molecules and accordingly will increase the
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sensitivity of the sensor [50]. Also, after the thermal annealing of RGO (during the calcination),
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the RGO can be partially reduced, therefore the chemical reactivity of RGO increases, leading to
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higher response of RGO loaded sensor [13]. Finally, the one-dimensional structure of NFs will
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provide a large surface area, contributing to the enhancement of sensing behavior. Table 1 [47-53], compares H2S sensing performance of the optimal gas sensor in this study
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with some H2S sensors having CuO. In a comparative point of view, the present sensor shows higher response to H2S than the reported sensors. However, the sensing temperature is relatively
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high.
4. Conclusion
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RGO loaded (0.05-1.5 wt%) CuO NFs with different amounts of RGO were produced by electrospinning method and subsequent heating. They were studied by FE-SEM, TEM and EDS. The gas sensing tests showed that the 0.5wt% RGO loaded CuO NFs sensor had the maximum
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response to H2S gas. Also the low responses to CO, C6H6 and C7H8 demonstrated a good selectivity of this sensor. High sensitivity towards H2S was mainly due to NF morphology, presence of RGO, presence of CuO/CuO homojunctions and p-RGO/p-CuO heterojunctions, where upon conversion of CuO to CuS, a high resistance modulation will be observed. The results obtained can be used for realization of inexpensive and high performance H2S gas
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sensors.
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Acknowledgments
This research was supported by Basic Science Research Program through the National Foundation
of
Korea
(NRF)
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Research
funded
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(2016R1D1A1B03935228).
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[53] P.S. Shewale, V.B. Patil , S.W. Shin, J.H. Kim, M.D. Uplane, H2S gas sensing
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properties of nanocrystalline Cu-doped ZnO thin films prepared by advanced spray
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pyrolysis. Sens. Actuators B. Chem. 186 (2013) 226- 234.
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Table and Figure Captions
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Table 1. Comparison among the H2S sensing properties of some sensors reported in the literatures with the present study.
Fig. 1. FE-SEM micrographs of (a) pristine CuO NFs and RGO-loaded CuO NFs with different amounts of RGO; (b) 0.05wt%, (c) 0.1 wt%, (d) 0.2wt%, (e) 0.3wt%, (f) 0.5wt%, (g) 1 wt%, and (h) 1.5 wt%. Insets show the corresponding lower magnification micrographs.
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Fig. 2. (a) EDS analysis of 1.5 wt.% RGO-loaded CuO NFs sensor, (b) a typical TEM image of
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0.5 wt.% RGO-loaded CuO NFs, and (c) a typical lattice-resolved TEM image of 1.5 wt.%
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RGO-loaded CuO NFs.
Fig. 3. (a) Normalized dynamic resistance curves of the 0.5wt% RGO-loaded CuO NFs sensor
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towards 10 ppm H2S at different sensing temperatures and (b) corresponding response curves versus the sensing temperature.
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Fig. 4. (a) Normalized dynamic resistance curves of the RGO-loaded CuO NFs sensors with
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different amounts of RGO towards 1, 5 and 10 ppm H2S at 300°C. (b) Corresponding calibration curves and (c) response of RGO-loaded CuO NFs sensor to 10 ppm H2S as a function of RGO loading at 300°C. For comparison the response of pristine CuO NFs is also included.
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Fig. 5. (a) Normalized dynamic resistance curves of the 0.5 wt% RGO-loaded CuO NFs sensor towards different concentrations of H2S, CO, C6H6, and C7H8 gases at 300°C. (b) Corresponding calibration curves and (c) selectivity histogram for 10 ppm H2S, CO, C6H6, and C7H8 gases at 300°C.
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Fig. 6. UPS spectra of (a) CuO and (b) RGO. Magnified views of the high binding energy part for (c) CuO and (d) RGO. Fig. 7. (a) Effect of oxygen adsorption on the band structure of CuO/CuS system. (b,c) Effect of
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CuO to CuS conversion on the band structure of CuO/CuS system; (b) when boundary regions are converted to CuS and (c) when most regions are converted to CuS.
Fig. 8. Changes of energy band structures in RGO-CuO system. Potential barriers have been
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established by the contact of RGO-CuO (the upper part) and RGO-CuS (the lower part).
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Sensor
Conc. (ppm)
T(°C)
0.5 wt% RGO-loaded CuO NFs
10
300
CuO-modified SnO2 nanoribbons
3
50
CuO-functionalized SnO2 NWs
20
300
CuO NRs
100
300
Porous CuO hollow spheres
1
190
ZnO-CuO NTs
20
50
ZnO-CuO NRs
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CuO decorated Fe2O3 NPs
5
11.7 (Rg/Ra)
This work
1.8 (ΔR/Ra)
[47]
8.09(ΔG/Ga)
[48]
4(ΔR/Ra)
[49]
3.5
[50]
0.45 (ΔR/Ra)
[51]
0.22 (ΔR/Ra)
[51]
150
4
[52]
5
150
2.5
[52]
100
250
~48
[53]
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Ref.
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50
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CuO-NiO core-shells
Response
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CuO NPs
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Table 1. Comparison of some H2S sensors based on CuO with the present study.
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Note: nanowires (NWs), nanotubes (NTs), nanorods (NRs).
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A
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S0925-4005(18)31921-X https://doi.org/10.1016/j.snb.2018.10.144 SNB 25578
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
29-5-2018 20-10-2018 27-10-2018
Please cite this article as: Kim J-Hun, Mirzaei A, Zheng Y, Lee J-Hyoung, Kim J-Young, Kim HW, Kim SS, Enhancement of H2 S sensing performance of p-CuO nanofibers by loading p-reduced graphene oxide nanosheets, Sensors and amp; Actuators: B. Chemical (2018), https://doi.org/10.1016/j.snb.2018.10.144 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhancement of H2S sensing performance of p-CuO nanofibers by loading p-reduced graphene oxide nanosheets
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Jae-Hun Kima, Ali Mirzaeib,c, Yifang Zhenga, Jae-Hyoung Leea, Jin-Young Kima, Hyoun Woo Kimb,d,*, Sang Sub Kima,** a
Department of Materials Science and Engineering, Inha University, Incheon 22212, Republic of Korea
The Research Institute of Industrial Science, Hanyang University, Seoul 04763, Republic of Korea
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b
c
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Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea
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*
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Corresponding authors.
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d
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Department of Materials Science and Engineering, Shiraz University of Technology, Shiraz, Iran
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E-mail address: [email protected] (H.W.Kim), [email protected] (S.S. Kim).
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Highlights
For gas sensing, RGO loaded CuO NFs with different amounts of RGO were produced by electrospinning method and subsequent heating.
0.5wt% RGO loaded CuO NFs sensor had the maximum response to H2S gas.
Underlying sensing mechanism is discussed.
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Abstract Being compared to p-n heterojunctions, less attention has been paid to p-p heterojunctions for gas sensing studies. Motivated by this fact, herein, gas sensing characteristics of p-reduced
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graphene oxide (RGO) loaded p-CuO nanofibers (NFs) will be presented. In order to find the optimal value of RGO loading, different amounts of RGO (0.05-1.5 wt%) were added to the electrospun CuO NFs. The results of sensing tests showed that the CuO-0.5wt% RGO has the best sensitivity to H2S at 300°C. Also, low response to the interfering gases demonstrated the
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good selectivity of the optimal sensor towards H2S. Underlying sensing mechanism is discussed
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in details. High response of CuO-0.5wt% RGO sensor towards H2S was related to NF
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morphology, presence of RGO, high intrinsic sensitivity of CuO towards H2S gas, presence of a
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lot of CuO nanograins along with the existence of the plenty of p-p heterojunctions, which will be changed by transformation of CuO to CuS. The results of present study can be used for design
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and fabrication of novel RGO containing p-p heterojunction gas sensors.
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Keywords: CuO, Reduced graphene oxide, Nanofiber, Sensing mechanism, H2S
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1. Introduction
Nowadays gas sensors are vital in our life. They are extensively used for detection of
hazardous gases for environmental safety [1], home and public safety [2][3], vehicles [4] and air conditioning [5], and so on. Owing to very diverse applications of gas sensors, the need to develop of inexpensive, stable, sensitive, selective and high performance gas sensors, has
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triggered the researchers around the world to enhance the performance of such sensors mostly in term of sensitivity and selectivity. To this end, many different methodologies have been proposed so far. One common methodology is morphology engineering, where different
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nanostructures such as thin films [6] nanobelts [7], nanorods [7], nanowires [8] and nanofibers (NFs) [9,10] have been reported for enhancement in sensing performance.
In particular, NFs are very promising for sensing studies. They can be easily fabricated by electrospinning, which is an efficient and cost-effective technique to fabricate long metal oxide NFs. Furthermore, they possess large surface area, resulting from their one dimensional
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morphology. Accordingly, they have significant surface area for gas species and the presence of
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a lot of nanograins and large grain boundary areas, ensure high sensitivity in the sensors with
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NFs morphology [11-13]. Even though a lot of materials with NF morphology have been
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reported for gas sensing [14-21], the number of literature about CuO NFs for sensing purposes is
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rare. The main reported CuO NFs sensors are as follows. Choi et al. reported enhanced NO2 and H2S sensing performance of pristine CuO NFs and CuO-SnO2 NFs composites, respectively
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[22,23]. Zhao et al. reported porous CuO/SnO2 NFs composites with enhanced H2S sensing performance [24]. Katoch et al. [25], fabricated p-CuO/n-ZnO core-shell NFs to detect low
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concentrations of reducing gases such as H2S. As it can be seen, most of CuO-based gas sensors are reported to have good sensitivity towards H2S gas, which is a very poisonous and flammable gas causing dizziness, nausea, vomiting, and respiratory tract [26-28]. Sensing of H2S is vital
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especially in the fields of oil and natural gas exploration and breath analysis [29]. Generally p-type metal oxides such as CuO offer low response to target gases and they are
not very popular for sensing applications especially in pristine form [7]. A good strategy for sensing enhancement is generation of n-n [30] homojunction, p-p [31] or n-p [32]
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heterojunctions through loading semiconducting nanomaterials. In this regards, reduced graphene oxide (RGO) with p-type characteristic can be an interesting material. In addition to its high surface area, it has a lot of dangling oxygen atoms, acting as good binding sites for
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adsorption of the target gases [33]. Accordingly hybrid composites containing CuO and RGO can be promising materials for gas sensing studies [34]. Herein, for the first time, we report H2S gas sensing characteristics of RGO loaded CuO NFs. Besides large surface area of RGO and plenty of dangling bonds, work function difference between RGO and CuO will results in formation of plenty RGO-CuO heterojunctions, where electrons from CuO will transfer to RGO
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and accordingly a hole accumulation layer (HAL) will develop on CuO. When exposing to H2S,
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conversion of CuO to CuS changes the conductivity from semiconducting to metallic type and
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destroys the established heterojunction, leading to a high response to H2S.
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In our previous research [13], gas sensing features of RGO-loaded ZnO NFs were reported.
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In an attempt to extend our knowledge about RGO loaded gas sensors, in this study, the sensing characteristics of RGO-loaded CuO NFs to H2S is explored, and optimal RGO loading content
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and sensing mechanisms are presented. To this end, different amounts of RGO ranging from 0.05 wt% to 1.5 wt% were added to the electrospun CuO NFs and the gas sensors were fabricated.
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The sensing characteristics were studied at different RGO loading and sensing temperatures. The results obtained showed that the presence of CuO nanograins, existence of RGO and the formation of nano p-p heterojunctions can significantly enhance the sensing capacity of the CuO
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NFs.
2. Experimental Procedure
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2.1. Preparation of electrospinning solutions Polyvinyl alcohol (PVA- MW 80000 g/mol) copper acetate (Cu(CH3CO2)2) and p-RGO nanosheets were used as the starting chemicals. An aqueous solution containing 9 wt.% PVA at
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70°C. Next, a 4.3 wt.% (Cu(CH3CO2)2) aqueous solution and varying amounts of RGO (nominally 0.05, 0.1, 0.2, 0.3, 0.5, 1, and 1.5 wt%) were added to the PVA solution at 80°C and afterwards different mixed solutions of copper acetate, RGOs and PVA with high viscosity were prepared.
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2.2. Electrospinning process
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The details of experimental procedure of electrospinning can be found in our previous
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literature [35]. In summary, the prepared viscous solutions were transferred to a glass syringe
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having a stainless steel needle. During the electrospinning process, the needle was connected to a large positive voltage (+15 kV) and a large negative voltage (-10 kV) was applied to the
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aluminum collector. The solution feeding rate was constant (0.07ml/h), while the distance
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between the tip of needle to collector was 20 cm. Finally, in order to remove the solvent and polymer from the as synthesized NFs, samples were heat treated using a horizontal tube furnace
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at 600°C for 2 h.
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.2.3. Materials Characterization
Field-emission scanning electron microscopy (FE-SEM, Hitachi-S-4200) and transmission electron microscopy (TEM) were used to study the morphology of the prepared products. FESEM was equipped with Energy-dispersive X-ray spectroscopy (EDS) and EDS was utilized to evaluate the chemical composition. In order to obtain the work function of RFO and CuO,
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ultraviolet photoelectron spectroscopy (UPS) measurements (UPS, Thermo Fisher Scientific Co. Theta probe) at UHV (<10-10 Torr) were performed, using a HeI 21.2 eV UV light. 2.4. Gas sensing measurements
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For sensing studies, sputtered Ti (~50 nm thick)/Pt (~200 nm thick) bi-layers were applied onto the products that already were deposited on SiO2-grown Si substrates. The fabricated sensors’ sensing capabilities were investigated in the presence of H2S, CO, C6H6, and C7H8 gases. Sensing tests at different temperatures were carried out by a home-made sensing system,
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equipped with a gas-dilution and testing system. Gas concentrations were exactly controlled by
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varying the mixing ratio of the dry air and target gas (the total gas flow rate =500 sccm) by mass
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flow controllers. The resistance in air (Ra) and in the presence of target gas (Rg) was recorded,
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and the response of gas sensor was defined as R = Rg/Ra.
3. Results and discussion
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3.1. Morphological and chemical studies
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The morphologies of NFs were investigated by using FE-SEM. The micrographs are provided in Fig. 1(a)-(h). Fig. 1(a) displays the FE-SEM micrograph of pristine CuO NFs, where nanograins on the surface of NF can be clearly seen. Also, the inset shows the lower
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magnification image, demonstrating the successful synthesis of long, continuous CuO NFs having a diameter of ~50 nm. Figs. 1(b)-(h) provide the FE-SEM micrographs of different amounts RGO-loaded CuO NFs, where in all cases, long, continuous NFs comprising ultrafine grains are observed. Corresponding insets show lower magnification FE-SEM images, again
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demonstrating that almost the same diameter of electrospun NFs were prepared after the calcination treatment. The results of a typical EDS analysis for 1.5 wt% RGO-loaded CuO NFs are presented in
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Fig. 2(a). As can be seen, the weight percent of RGO is 1.53%, demonstrating successful loading of a desired amount of RGO in the CuO NFs. TEM analyses are presented in Fig. 2(b) and 2(c) for 0.5 wt% and 1.5 wt% RGO-loaded CuO NFs, respectively. RGO sheets along with CuO nanograins are clearly seen in Fig. 2(b). Lattice-resolved TEM micrograph in Fig. 2(c) shows the parallel fringes with spacing of 2.75Å and 2.32Å, which are consistent with the interplanar
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spacings of (110) and (111) planes, respectively, of CuO crystal structure.
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3.2. Gas sensing studies
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3.2.1. Optimal working temperature
Depending on the sensing temperature, the kinetics of gas adsorption, gas desorption and
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other related phenomena change and consequently different responses can be observed at
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different working temperatures [36]. However, there is only one temperature, so-called the optimal working temperature, in which the rate of gas adsoption is higher than the desorption
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rate and the highest sensitivity can be achieved. Accordingly, the responses of the RGO-loaded CuO NFs were tested at 150 to 400°C. Fig. 3(a) provides dynamic normalized resistance curves of 0.5 wt% RGO-loaded CuO NFs toward 10 ppm H2S gas at 150 to 400°C. When the sensor
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exposed to H2S gas, the resistance was increased, demonstrating p-type nature of sensor. Fig. 3(b) indicates the response versus the sensing temperature. It can be seen that up to 300°C, the response is increased with increasing of sensing temperature, reaching to a maximum value at 300°C and afterwards decreases. At T <300 °C, the H2S molecules have not enough energy to be
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adsorbed for subsequent reaction with the adsorbed oxygen. At T >300°C, the desorption rate is very high and H2S molecules escape away easily [37]. At 300°C, the rate of gas adsorption is high enough, resulting in the best response of gas sensor [37]. After finding the optimal
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temperature, the remaining experiments were carried out at 300°C. 3.2.2. Dependency of response of sensors on the RGO loading amount
To find the optimal value of RGO amount, various amounts RGO-loaded CuO NFs were exposed to H2S at 300°C and dynamic resistance curves are displayed in Fig. 4(a). All sensors
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show a reversible response to different concentrations of H2S. Fig. 4(b) indicates the calibration
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curves of different sensors. It is seen that the response values increase with the increase of RGO
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content up to 0.5 wt%, and with further increases of RGO loading the response is decreased. Fig.
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4(c) clearly shows this trend. RGO content strongly affect the response and the maximum response is observed at 0.5wt% RGO content. Details of sensing mechanism will be discussed in
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3.2.3. Selectivity studies
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sub-section of sensing mechanism.
To evaluate the selectivity, the optimal sensor based on 0.5wt% RGO-loaded CuO NFs was
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exposed to CO, C6H6 and C7H8 gases and the dynamic resistance curves can be seen in Fig. 5(a). Also, for comparison, the curve related to H2S is presented. All of interfering gases are reducing
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which have negligible effect on the sensor. (Fig. 5(b)). Fig. 5(c) shows the selectivity histogram of the 0.5wt% RGO-loaded CuO NFs to 10 ppm of H2S, CO, C6H6, and C7H8 gases, showing the responses of 11.2, 2.91, 3.17, and 2.66, respectively. Good selectivity of the 0.5 wt% RGO loaded CuO NFs sensor can be explained by the fact that in the presence of H2S, CuO will be converted to CuS with metallic conductivity, whereas other interfering gases have a reducing
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effect on the CuO, where they only slightly change the width of hole accumulation layer in CuO NFs. In the following section these effects will be explain in more details. The real selectivity is based on the sensor response among a mixture of these gases. Accordingly, the present sensor
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may not exhibit a high enough selectivity in the real situation, in which the sensitivity of H 2S is measured in a mixture of various gases. A better method with very high selectivity towards a specific gas is use of electronic nose which comprised of an array of semiconductor sensors. In fact, application of an electronic nose and artificial intelligence methods is a highly efficient way of analyzing the odorous gases. Nevertheless in comparison with simple metal-oxide gas sensors,
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it is more complex due to needs for signal processing unit [38].
A
UPS is an efficient method for identifying the work function of materials [39,40]. Fig. 6(a)
M
and 6(b) provide the UPS spectra of CuO and RGO, respectively. From Fig. 6(c) and 6(d), the cut-off values of 17.69 eV (CuO) and 16.74 eV (RGO), respectively can be obtained.
ED
Additionally, in order to revise instrumental broadening due to the analyzer, 0.1 eV is added to the work functions [41]. Accordingly, the work functions of CuO and RGO were calculated as
PT
3.61 eV (i.e., 21.2 eV-17.69 eV (+0.1) eV = 3.61 eV) and 4.56 eV (i.e., 21.2 eV – 16.74 eV
CC E
(+0.1) eV = 4.56 eV), respectively.
A
3.3. Gas sensing mechanism Adsorption of target gases on the surfaces of gas sensor changes its resistance and this is the
principle of sensing mechanism in metal-oxide gas sensors [42]. At first, we will describe the sensing mechanism in bare CuO gas sensor and then extend it to RGO-loaded SnO2 NFs sensors. In air, oxygen molecules wigh high large electron affinity (0.43 eV [43]), will be easily adsorbed
9
on the surfaces of sensors and abstract electrons from the surfaces of CuO NFs, as a result, as shown in Eqs. (1)-(4): adsorbed oxygen ions will be appear on surface in molecular (O2−), or atomic (O− and O2−) forms: 𝑂2 (𝑔) → 𝑂2 (𝑎𝑑𝑠)
SC RI PT
(1)
𝑂2 (𝑎𝑑𝑠) + 𝑒 − → 𝑂2− (𝑎𝑑𝑠)
(2)
𝑂2− (𝑎𝑑𝑠) + 𝑒 − → 2𝑂− (𝑎𝑑𝑠)
(3)
𝑂− + 𝑒 − → 𝑂2− (𝑎𝑑𝑠)
(4)
N
U
When T<100°C, the O2- is dominant, O- is dominant at 100
A
higher temperatures [44]. Transfer of electron from the surface of CuO to the adsorbed oxygen
M
will result in the decrease of the electrons and corresponding increase of holes in the surface of CuO. As a result a HAL having plenty of holes is formed on the outer surfaces of CuO.
ED
Accordingly current will be mainly passed throuth the HAL. When CuO NFs are exposed to H2S atmosphere, the H2S will provide holes to and/or take out electrons from the sensors surface,
PT
increasing the resistance of sensor.
Also, in the pristine CuO NFs, owing to the presence of CuO nanograins, there are a lot of
CC E
CuO-CuO homojunctions. First, we expect that the CuO-CuO homojunctions will affect the sensing behaviors. As shown in Fig. 7a, since oxygen gas tends to penetrate into the CuO grain
A
boundaries, O2- species are dominantly present at the grain boundaries, taking out electrons and lowering the Fermi levels. This will generate the potential barriers for both electrons and holes, decreasing the hole conductance across the grain boundaries and thus contributing to the decrease of conductivity.
10
Upon the introduction of H2S gas, through the sulphurization reaction [45], CuO is converted to CuS. Sulphurization reaction can be shown as follows [23]: 𝐶𝑢𝑂 (𝑠) + 𝐻2 𝑆(𝑔) → 𝐶𝑢𝑆(𝑠) + 𝐻2 𝑂(𝑔)
(5)
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During the recovery period, by introduction of air, CuS is transformed to CuO: 𝐶𝑢𝑆 (𝑠) + 3/2𝑂2 (𝑔) → 𝐶𝑢𝑂(𝑠) + 𝑆𝑂2 (𝑔)
(6)
The generation of phase at the interfaces of the adjacent nanograins play a critical role in the gas detection mechanism of CuO NFs gas sensors. The work function of CuO was calculated to be
U
3.61 eV, whereas that of CuS is reported to be in the range of 4.90-4.95 eV [46-49]. In the
N
CuO/CuS henerointerfaces, due to the work-function difference, electrons in CuO will flow to
A
CuS, establishing the potential barrier (Fig. 7(b) and 7(c)). When only the grain boundary
M
regions are converted to CuS, the width of the potential barrier will be narrow (Fig. 7(b)). On the
ED
other hand, most part of CuO is converted to CuS, the potential barrier will be very broad (Fig. 7(c)). By the way, the potential barrier on the bottom side of the energy band will act as a barrier
PT
to the hole conduction. The contribution of CuO-CuO homojunctions to the CuS-generationinduced increase of the hole resistance will be dependent on the relative height of the hole
CC E
potential barriers. If “b” in Fig. 7(b) and 7(c) is larger than “a” in Fig. 7(a), the hole resistance will be further increased by the transformation of CuO to CuS. However, if “b” is smaller than
A
“a”, the hole resistance will be decreased. Accordingly, at this moment, we do not know whether the CuO-CuO homojunctions, by the introduction of H2S gas, leading to the enhancement of sensing behavior or not. In RGO-loaded gas sensors, in addition to the above mechanisms, the role of p-CuO/p-RGO heterojunctions should be considered. From the UPS measurement, we reveal that work
11
functions of RGO and CuO in the present work are 4.56 and 3.61 eV, respectively (Fig. 8). When they are in intimate contact, electrons from CuO will flow to RGO (Fig. 8), resulting in equating of the Fermi levels, the bending of energy levels and expanding of HAL in CuO. When RGO
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loaded CuO NFs sensors are exposed to H2S gas, according to eq. (5), CuO will be transformed to CuS phase (Fig. 8).
Role of RGO can be explained based on the relative work functions of RGO, CuO, and CuS. It is noteworthy that CuO phase is continuous, whereas RGO is discrete. Accordingly, the main hole current will flow through the CuO structure. Upon the introduction of H2S gas, the
U
adsorption of H2S molecules will transfer electrons to the CuO surfaces, decreasing the thickness
N
of HAL. The work function of CuS is in the range of 4.90-4.95 eV [46-49]. Also, the CuS is
A
regarded as a p-type material. Accordingly, if we assume that RGO and CuS are in contact,
M
electrons in RGO will flow to CuS, decreasing its hole accumulation and thus increasing the hole
ED
resistance. Therefore, the introduction of H2S gas and the transformation of CuO to CuS will coincidentally increase the hole resistance, contributing to the improvement of sensor response.
PT
When exposing to fresh air, the reaction of eq. (6), take places and CuS is transformed to CuO and the resistance signal backs to the initial state.
CC E
As it was shown in Fig. 4, the response of sensor greatly depended on the RGO content. RGOs are distributed among the CuO NFs and no direct contact is among the neighboring RGOs. Since the probability of direct contact among adjacent RGOs is very low, thus the role of
A
p-RGO/p-RGO homojunctions is not considered here. Due to the discontinuous distribution of RGOs, the current in the sensor pass mainly through the CuO NFs. When amount of RGO is ≤0.5 wt%, the current flowing through CuO is blocked due to the presence of discrete RGOs, and the initial resistance is increased, consequently, in the presence of H2S a high modulation of the
12
resistance will occur, depending on the RGO amount. However, at the sufficiently high content of RGO i.e. >0.5wt%, it would provide an additional path for current flow, resulting in a larger conduction path along the RGO, which would reduce the initial resistance and the response of
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sensor decreases relative to the optimal amount of RGO. Other factors which can enhance the response of the sensor should be considered. Discretely distributed RGOs, with a huge surface area, will induce the spillover effect and act as a catalyst for the dissociation and transportation of H2S gas molecules onto the CuO surfaces [13]. Furthermore, RGO has plenty of functional groups and defects on its surfaces. Accordingly, it
U
can offer plenty of adsorption sites for the H2S gas molecules and accordingly will increase the
N
sensitivity of the sensor [50]. Also, after the thermal annealing of RGO (during the calcination),
A
the RGO can be partially reduced, therefore the chemical reactivity of RGO increases, leading to
M
higher response of RGO loaded sensor [13]. Finally, the one-dimensional structure of NFs will
ED
provide a large surface area, contributing to the enhancement of sensing behavior. Table 1 [47-53], compares H2S sensing performance of the optimal gas sensor in this study
PT
with some H2S sensors having CuO. In a comparative point of view, the present sensor shows higher response to H2S than the reported sensors. However, the sensing temperature is relatively
A
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high.
4. Conclusion
13
RGO loaded (0.05-1.5 wt%) CuO NFs with different amounts of RGO were produced by electrospinning method and subsequent heating. They were studied by FE-SEM, TEM and EDS. The gas sensing tests showed that the 0.5wt% RGO loaded CuO NFs sensor had the maximum
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response to H2S gas. Also the low responses to CO, C6H6 and C7H8 demonstrated a good selectivity of this sensor. High sensitivity towards H2S was mainly due to NF morphology, presence of RGO, presence of CuO/CuO homojunctions and p-RGO/p-CuO heterojunctions, where upon conversion of CuO to CuS, a high resistance modulation will be observed. The results obtained can be used for realization of inexpensive and high performance H2S gas
A
N
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sensors.
M
Acknowledgments
This research was supported by Basic Science Research Program through the National Foundation
of
Korea
(NRF)
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Research
funded
A
CC E
PT
(2016R1D1A1B03935228).
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Ministry
of
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Table and Figure Captions
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Table 1. Comparison among the H2S sensing properties of some sensors reported in the literatures with the present study.
Fig. 1. FE-SEM micrographs of (a) pristine CuO NFs and RGO-loaded CuO NFs with different amounts of RGO; (b) 0.05wt%, (c) 0.1 wt%, (d) 0.2wt%, (e) 0.3wt%, (f) 0.5wt%, (g) 1 wt%, and (h) 1.5 wt%. Insets show the corresponding lower magnification micrographs.
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Fig. 2. (a) EDS analysis of 1.5 wt.% RGO-loaded CuO NFs sensor, (b) a typical TEM image of
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0.5 wt.% RGO-loaded CuO NFs, and (c) a typical lattice-resolved TEM image of 1.5 wt.%
M
A
RGO-loaded CuO NFs.
Fig. 3. (a) Normalized dynamic resistance curves of the 0.5wt% RGO-loaded CuO NFs sensor
ED
towards 10 ppm H2S at different sensing temperatures and (b) corresponding response curves versus the sensing temperature.
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Fig. 4. (a) Normalized dynamic resistance curves of the RGO-loaded CuO NFs sensors with
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different amounts of RGO towards 1, 5 and 10 ppm H2S at 300°C. (b) Corresponding calibration curves and (c) response of RGO-loaded CuO NFs sensor to 10 ppm H2S as a function of RGO loading at 300°C. For comparison the response of pristine CuO NFs is also included.
A
Fig. 5. (a) Normalized dynamic resistance curves of the 0.5 wt% RGO-loaded CuO NFs sensor towards different concentrations of H2S, CO, C6H6, and C7H8 gases at 300°C. (b) Corresponding calibration curves and (c) selectivity histogram for 10 ppm H2S, CO, C6H6, and C7H8 gases at 300°C.
22
Fig. 6. UPS spectra of (a) CuO and (b) RGO. Magnified views of the high binding energy part for (c) CuO and (d) RGO. Fig. 7. (a) Effect of oxygen adsorption on the band structure of CuO/CuS system. (b,c) Effect of
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CuO to CuS conversion on the band structure of CuO/CuS system; (b) when boundary regions are converted to CuS and (c) when most regions are converted to CuS.
Fig. 8. Changes of energy band structures in RGO-CuO system. Potential barriers have been
A
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PT
ED
M
A
N
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established by the contact of RGO-CuO (the upper part) and RGO-CuS (the lower part).
23
Sensor
Conc. (ppm)
T(°C)
0.5 wt% RGO-loaded CuO NFs
10
300
CuO-modified SnO2 nanoribbons
3
50
CuO-functionalized SnO2 NWs
20
300
CuO NRs
100
300
Porous CuO hollow spheres
1
190
ZnO-CuO NTs
20
50
ZnO-CuO NRs
20
CuO decorated Fe2O3 NPs
5
11.7 (Rg/Ra)
This work
1.8 (ΔR/Ra)
[47]
8.09(ΔG/Ga)
[48]
4(ΔR/Ra)
[49]
3.5
[50]
0.45 (ΔR/Ra)
[51]
0.22 (ΔR/Ra)
[51]
150
4
[52]
5
150
2.5
[52]
100
250
~48
[53]
N
U
Ref.
A
50
ED
CuO-NiO core-shells
Response
M
CuO NPs
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Table 1. Comparison of some H2S sensors based on CuO with the present study.
A
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PT
Note: nanowires (NWs), nanotubes (NTs), nanorods (NRs).
24
D
TE
CC EP
A
A
M
N U
EP T
CC
A
ED
D
TE
CC EP
A
A
M
D
TE
CC EP
A
A
M
N U SC
EP
CC
A TE
D
TE
EP
CC
A D
M
D
TE
CC EP
A
A
M
N U S
C
A