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Realization of H2S sensing by Pd-functionalized networked CuO nanowires in self-heating mode
Sensors & Actuators: B. Chemical 299 (2019) 126965
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Realization of H2S sensing by Pd-functionalized networked CuO nanowires in self-heating mode
T
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Jin-Young Kima, Jae-Hyoung Leea, Jae-Hun Kima, Ali Mirzaeib,c, Hyoun Woo Kimc,d, , ⁎ Sang Sub Kima, a
Department of Materials Science and Engineering, Inha University, Incheon 22212, Republic of Korea Department of Materials Science and Engineering, Shiraz University of Technology, Shiraz, Iran c Research Institute of Industrial Science, Hanyang University, Seoul 04763, Republic of Korea d Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: CuO Nanowire Pd-functionalization H2S sensing Self-heating test Gas sensor
In this study, we fabricated pristine and Pd-functionalized CuO nanowires (NWs) for H2S gas sensing. The CuO NWs were synthesized using a vapor–liquid–solid method and the Pd functionalization was achieved by ultraviolet irradiation of a Pd precursor solution in the presence of the CuO NWs. The characterization results confirmed the formation of pristine and Pd-functionalized CuO NWs with favorable compositions and morphologies. The gas-sensing response of the NWs towards H2S was enhanced by the Pd functionalization, at 25–100 °C. Moreover, self-heating tests showed that the Pd-functionalized gas sensors exhibited a selective response to H2S gas when a low voltage (5 V) was applied. The superior performance of the Pd-functionalized gas sensor was attributable to not only the catalytic effects of both CuS and Pd but also the CuO/Pd heterostructures. The results demonstrate that the fabricated H2S sensors are energy-efficient and could be used in various electronic devices.
1. Introduction Semiconducting metal oxides with one-dimensional morphologies, such as nanowires (NWs), have large surface areas and low production costs, and thus are promising for many applications, including gas sensing [1]. The very large surface areas, unique physical and chemical properties, diameters on the order of the Debye length, rapid diffusion kinetics, and single-crystalline characteristics of metal oxide NWs are the main advantages for realization of high-performance gas sensors [2–4]. Mostly n-type metal oxides, such as SnO2 and ZnO, are used for gas sensing applications [5]. Metal oxides with p-type conductivities have lower gas responses than those of n-type oxides [6]. Therefore, they have not been extensively investigated for sensing applications [5]. Among the p-type metal oxides, including Co3O4 [7], Cr2O3 [8], NiO [9], and CuO [10], the latter is the most promising for H2S sensing. Upon exposure to H2S, the semiconducting CuO (Eg =1.2 eV) transforms to CuS with metallic conductivity. The change in conductivity provides a high response to H2S gas [11]. The sensitivities of metal oxides can be increased using different strategies [12,13], such as the functionalization of the sensors by noble
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metals (Au [14], Pd [15], Pt [16], and Rh [17]). Noble metals have good catalytic activities, can facilitate the decomposition and adsorption of different gases, and can increase the concentration of gas molecules that reach the sensor surface in a process referred to as the spillover effect [18]. Moreover, noble metals can form heterojunctions with metal oxides because of their different work functions from those of the metal oxides, where the large resistance change of the sensor can improve the sensitivity [19]. In addition, compared to metal oxides such as NiO, noble metals have large numbers of charge carriers, and the electrons can redistribute very efficiently to counteract any charge build-up near the metal-metal oxide heterojunctions. Therefore, noble metals can effectively attract electronegative species like gaseous oxygen and accordingly increase the rate of the reactions [20]. Pd is a noble metal, which is less expensive than Pt [21] and Au [22] and has a good catalytic activity to H2S gas [23–26]. It is well known that Pd can further increase the probability of oxygen ionosorption on the metal oxide support by the back-spill-over effect, in addition to the regular spill-over effects [27]. In addition, PdO can be formed, acting as a stronger electron acceptor from the semiconductor, presumably promoting the gas response [28]. Pd functionalization can generate heterojunctions,
Corresponding authors. E-mail addresses: [email protected] (H. Woo Kim), [email protected] (S.S. Kim).
https://doi.org/10.1016/j.snb.2019.126965 Received 14 February 2019; Received in revised form 26 July 2019; Accepted 8 August 2019 Available online 16 August 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Sensors & Actuators: B. Chemical 299 (2019) 126965
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form PdO, and exert electronic and chemical sensitizations. Owing to the high operation temperatures (200–500 °C), metal oxide gas sensors consume high powers when are integrated into portable electronics, which often have low-power sources [29]. Thus, the development of self-heated gas sensors is required to reduce the power consumptions of the sensors and enable the integration in low-consumption and portable electronic devices. Recently, metal oxide NWs have gained considerable attention for the fabrication of self-heated gas sensors [30]. NWs generally have extremely low heat capacities. Therefore, they need a low electric power to be heated [31]. The selfheating effects in NWs can considerably decrease the power consumption compared to those of traditional gas sensors because the small NWs can be heated by a voltage in the microsecond time range [32,33]. Therefore, it is desirable to fabricate self-heated gas sensors with extremely low consumptions using metal oxide NWs with good performances. H2S is a flammable and highly poisonous gas [34]. The threshold limit for H2S is 10 ppm (ppm) for an exposure for 8 h [35]. At 100 ppm, it easily paralyzes the olfactory nerves, while at high concentrations (0.1%), it leads to suffocation and sudden physical collapse [36,37]. In addition, H2S is a biomarker for halitosis when its concentration in exhaled breath exceeds 2 ppm [38]. Accordingly, the detection of H2S is very important from different aspects. Previous studies have demonstrated the H2S gas sensing by CuO (Table S1 in Supporting Information). A few studies have demonstrated very high sensor responses at temperatures higher than 160 °C, while no extensive studies have been reported on room-temperature sensing. In this study, for the first time, we proposed a Pd catalyst for CuO sensors. Pristine and Pd-functionalized CuO NWs were realized and evaluated for H2S sensing. The gas sensing results at 25–100 °C show the higher sensitivity of the Pd-functionalized CuO NWs than that of the pristine sensor. Furthermore, for the first time, we carried out a self-heating analysis with respect to CuO sensing by applying voltage without external heat, which showed that the Pd-functionalized gas sensor could operate at a very low applied voltage (5 V). The underlying detection mechanism is described in detail. This study is valuable for the development of self-heated sensing devices with low power consumptions.
2. Experimental methods 2.1. Synthesis of networked CuO NWs Fig. 1. Schematic of the syntheses of the pristine and Pd-functionalized CuO NWs.
Fig. 1 illustrates the fabrications of the pristine and Pd-functionalized CuO NWs. The networked CuO NWs were synthesized by thermal oxidation on a patterned interdigital electrode (PIE) on a SiO2 grown (250 nm) Si substrate. The PIE was comprised of a tri-layered electrode pad, consisting of Cu (1 μm)/Pt (200 nm)/Ti (50 nm) layers. The Pt layer provided an electrode pathway, while the Ti layer provided adhesion between the SiO2 and Pt layers. The Cu layer acted as the source of CuO NWs. The substrate was then heated to 450 °C for 20 h in air in a horizontal-type tubular furnace. N2 and O2 gas flow rates of 300 and 10 sccm, respectively, were used during the growth of CuO from Cu. The Cu was gradually consumed, yielding CuO NWs.
2.3. Characterization The synthesized products were analyzed by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4200) and transmission electron microscopy (TEM, Philips CM-200). The crystallinity and phase formation were analyzed by X-ray diffraction (XRD, Philips X’Pert MRD). X-ray photoelectron spectroscopy (XPS, Specs XR50) was performed to analyze the chemical states of the materials. We obtained the work functions by ultraviolet photoelectron spectroscopy (UPS) using a UV source of He I (21.2 eV). The procedure used to obtain the work functions is presented at the end of Section 3.1.
2.2. Functionalization of Pd nanoparticles (NPs)
2.4. Gas sensing tests
For the deposition of Pd NPs, a palladium (II) solution was prepared by dissolving PdCl2 in a mixed solvent containing isopropanol and acetone (1:1 v:v). The CuO NWs were completely immersed in the Pd2+ solution and then ultraviolet-(UV)-light-irradiated with a halogen lamp (wavelength =360 nm, intensity =0.11 mW/cm2) at room temperature for 5 s. Subsequently, the CuO NWs were annealed at 500 °C for 0.5 h to remove the remaining solvents.
The sensing measurements were almost identical to those reported previously [39,40]. The gas concentrations were accurately controlled by changing the mixing ratio of the target gas and dry air using precise mass flow controllers at a total flow rate of 500 sccm. The resistances in the air (Ra) and in the presence of the target gas (Rg) were recorded and the sensor response was calculated as R = Rg/Ra. 2
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Fig. 2. FE-SEM images of the (a) pristine CuO NWs, (b) Pd-functionalized CuO NWs on the surface of the substrate, and (c) Pd-functionalized CuO NWs. (d) XRD pattern of the Pd-functionalized CuO NWs.
3. Results and discussion
XPS is a sensitive surface technique used for analyzing surface chemical compositions [42]. Fig. 4a indicates a full XPS survey spectrum of the Pd-functionalized CuO NWs. The XPS peaks related to Cu, O, Pd, C, and Si were observed. The C 1s peak is attributed to air dust, while the Si peaks are attributed to the Si substrate. In the Cu 2p corelevel XP spectrum (Fig. 4b), the peaks at 932.58 and 952.48 eV may be attributed to Cu 2p3/2 and Cu 2p1/2, respectively. The high satellite peaks at approximately 940–945 and 962.18 eV are the major features of CuO, demonstrating the existence of Cu2+ [43]. The O 1s XPS region is presented in Fig. 4c, where the peaks at 529.68 and 532.98 eV are attributed to the O2− present in the CuO lattice and the O species (as OH) present on the surfaces of the CuO NWs, respectively [44]. Fig. 4d shows the Pd 3d XPS region, comprising two peaks at 336.98 (Pd 3d5/2) and 342.18 (Pd 3d3/2) eV. The Pd 3d region was further deconvoluted to the corresponding subpeaks (Fig. 4e). In the deconvoluted spectrum, peaks corresponding to metallic Pd (40.98%) and Pd+2 (59.02%) are observed. Accordingly, both metallic and oxidized forms of Pd coexist on the surfaces of the CuO NWs. UPS was employed to determine the cut-off values of CuO, Pd, and PdO, which were 17.77, 16.25, and 16.10 eV, respectively (Fig. 5). The work functions were calculated by subtracting the cut-off values from 21.1 eV (reference). Also, 0.1 eV was added to the obtained values due to the broadening effect of the apparatus. Thus, the work functions of CuO, Pd, and PdO were determined to be 3.53, 5.05, and 5.20 eV, respectively.
3.1. Morphological, structural, and chemical analyses Fig. 2a exhibits a representative FE-SEM image of the pristine CuO NWs grown on the substrate. Fig. 2b indicates an FE-SEM image of the Pd-functionalized CuO NWs on the surface of the substrate, while Fig. 2c shows a typical FE-SEM image of the Pd-functionalized CuO NWs, with diameters of approximately 35–45 nm. Fig. 2d shows a typical XRD pattern of the Pd-functionalized CuO NWs, where the peaks related to the crystalline monoclinic CuO (Joint Committee on Powder Diffraction Standards (JCPDS) file no. 89-5899) are marked by the filled circles (•), demonstrating the formation of crystalline CuO NWs [3]. The peak related to the crystalline face-centered-cubic Pd (JCPDS file no. 87-0641) is marked by the filled triangle (▲) [41]. The XRD results show that crystalline CuO and Pd phases were formed without impurities or undesirable phases, demonstrating the successful synthesis. Fig. 3a presents a typical TEM image of the pristine CuO NWs. A typical CuO NW had a diameter of approximately 100 nm and smooth morphology. Fig. 3b indicates a selected-area electron diffraction (SAED) pattern of the pristine CuO NWs. The spotty pattern is attributed to the (111), (2¯02) , and (311¯) planes of CuO, demonstrating the single-crystalline structure of the CuO NWs. Fig. 3c presents an energydispersive X-ray (EDX) spectrum, demonstrating the presence of both Cu and O. Notably, the first peak in the EDX spectrum at 0.928 eV shows that the Cu (La line) and Ni (La line) peaks overlap. The Ni peaks are attributed to the Ni grid used for the analysis of the CuO NWs. The EDX spectroscopy elemental mapping confirmed the coexistence of Cu and O across the diameter of the NW (Fig. 3d). Fig. 3e shows a typical TEM image of the Pd-functionalized CuO NWs, with the Pd NPs on their surfaces. The lattice-resolved TEM image (Fig. 3f) shows parallel fringes with spacings of 0.23 and 0.275 nm, which can be attributed to the (111) plane of Pd and (110) plane of CuO, respectively. This demonstrates the coexistence of the crystalline Pd and CuO.
3.2. Gas sensing Preliminary tests were carried out to evaluate the optimal sensing temperatures (Topt) of the pristine and Pd-functionalized CuO NW gas sensors, considering the strong dependence of the sensing on the operation temperature [45]. To obtain Topt, the sensors were exposed to 1, 10, and 100 ppm of H2S gas at different temperatures, starting from room temperature, as shown in Fig. S1 (Supplementary Information). As the resistance increased upon the injection of the reducing gas, both sensors exhibited p-type behaviors. The responses strongly depended on the operation temperature. Fig. 6 summarizes the sensing responses of 3
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Fig. 3. (a) Low-magnification TEM image of a typical CuO NW and corresponding (b) SAED pattern, (c) EDX spectrum, and (d) EDX spectroscopy line profiles. (e) Low-magnification TEM and (f) lattice-resolved TEM images of a typical Pd-functionalized CuO NW.
responses to H2S gas for the two gas sensors. In case of pristine CuO NWs at the optimal temperature of 300 °C, the relative responses of H2S in comparison to C6H6, C7H8, C2H5OH, and CO gases are 1.547, 1.490, 1.602, and 1.216, respectively. In case of Pd-functionalized CuO NW at the optimal temperature of 100 °C, the relative responses of H2S in comparison to C6H6, C7H8, C2H5OH, and CO gases are 1.674, 1.720, 1.779, and 1.365, respectively. Both pristine and Pd-functionalized CuO NW gas sensors exhibited the highest responses to H2S gas. It is noteworthy that Pd-functionalized CuO NW gas sensors at 100 °C exhibited the higher relative response to H2S, compared to pristine CuO NWs at the optimal temperature of 300 °C. Further, the responses of the gas sensors under self-heating conditions were studied. In this condition, the Joule heating attributed to the applied voltage can increase the temperature of the gas sensor. In the self-heating experiments, voltages of 1 and 5 V were applied to the gas
the gas sensors to 100 ppm of H2S gas. The Topt values of the pristine and Pd-functionalized CuO NWs were 300 and 100 °C, respectively. At T < Topt, sufficient energy for an effective adsorption of H2S is not provided. On the other hand, at T > Topt, the desorption rate is higher than the adsorption rate, leading to a low performance of the sensor [46]. The maximum response of the pristine CuO NWs to 100 ppm of H2S was 1.861 (at 300 °C), which increased to 1.962 (at 100 °C) after the functionalization by the Pd NPs. Selectivity is crucial for practical applications, as the lack of selectivity can lead to false alarms and hinder the proper detection of the target gas [18]. To study the selectivities of the gas sensors, they were exposed to 1, 10, and 100 ppm of interfering gases (CO, C2H5OH, C7H8 (toluene), and C6H6) at their Topt values and the dynamic resistance curves were measured (Fig. 7a,b). Fig. 7c presents the selectivity patterns to 100 ppm of the different interfering gases along with the
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Fig. 4. (a) XPS survey spectrum of the Pd-functionalized CuO NWs, (b) Cu 2p, (c) O 1s, (d) Pd 3d core-level regions, and (e) deconvoluted Pd 3d region.
Fig. 9 shows the repeatability of the gas sensor. The responses to 100 ppm of H2S gas at 5 V in the first, second, and third cycles were 1.394, 1.391, and 1.368, respectively; the average response was 1.384. This shows the good repeatability of the gas sensor, which is important for practical applications. Fig. 10 reveals the long-term stability of the Pd-functionalized CuO NW gas sensor exposed to various concentrations of H2S gas at 5 V. Fig. 10a presents the transient resistance graphs of the sensor in the fresh state and after storage for six months in the
sensors without an external heating source. Fig. S2 (Supplementary Information) shows the dynamic resistances of the two gas sensors at 1 and 5 V. Fig. 8 displays the responses of the gas sensors to H2S as a function of the applied voltage. Both gas sensors exhibited the best responses at 5 V. Fig. 8b compares the responses of the two gas sensors to 100 ppm of H2S gas at different voltages. The pristine and Pd-functionalized CuO NW sensors exhibited responses of 1.08 and 1.40, respectively, at 5 V. 5
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Fig. 5. UPS spectra and cut-off values of (a, b) CuO, (c, d) Pd, and (e, f) PdO, respectively.
3.3. Gas sensing mechanism We explain the sensing mechanism of the pristine CuO NWs. As holes are the main charge carriers in the p-type CuO, the resistance in the hole accumulation layer (HAL) is decreased compared to that in the core regions of the CuO NWs [40,47,48]. When the CuO NW sensor is exposed to CO gas, by the reaction of the target gas with oxygen, CO + O− → CO2 + e−, electrons return to the surface of the sensor, combine with holes, and thus decrease the concentration of holes. Accordingly, the width of the HAL decreases and the resistance of the sensor increases, yielding the sensing response. Although the introduction of H2S may shrink the HAL on the CuO surface, the sulfurization reaction may also occur in the H2S ambient, CuO + H2S → CuS + H2O [49]. Consequently, CuO is converted to CuS, which has a metal-like conductivity [50]. The XPS results confirmed the conversion of CuO to CuS in the CuO-NW-based gas sensors upon the exposure to H2S gas (Fig. S3 and Text S1 in Supporting Information; [51–56]). Similar results were reported by other researchers [50,51,57–59]. For the Pd-functionalized CuO NW gas sensor, three results should be discussed: (i) Topt significantly decreased, to 100 °C, (ii) the response to H2S was enhanced, and (iii) the responses to the interfering gases was reduced. Pd is a noble metal that has a catalytic effect toward some
Fig. 6. Comparison of the responses of the pristine and Pd-functionalized CuO NW sensors (having optimal sensing temperatures of 300 and 100 °C, respectively) to 100 ppm of H2S gas as a function of the temperature.
laboratory. Fig. 10b compares the corresponding calibration curves. After the six months, the response changes were negligible, demonstrating the good stability of the gas sensor.
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Fig. 7. Dynamic responses to H2S, C6H6, C7H8, C2H5OH, and CO gases for the (a) pristine CuO NW gas sensor at 300 °C and (b) Pd-functionalized CuO NW gas sensor at 100 °C. (c) Selectivity histogram of the pristine and Pd-functionalized CuO NW sensors exposed to 100 ppm of interfering gases at Topt.
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Fig. 10. (a) Dynamic resistances of the Pd-functionalized CuO NW gas sensor exposed to different concentrations of H2S gas at an applied voltage of 5 V in the fresh state and after storage for six months in the laboratory. (b) Corresponding calibration curves.
Fig. 11. Spill-over effects of Pd, PdO, and CuS.
Fig. 8. (a) Responses of the pristine and Pd-functionalized CuO NW gas sensors to 100 ppm of H2S at 1 and 5 V. (b) Responses of the gas sensors to 100 ppm of H2S gas as a function of the applied voltage.
To elucidate the heterojunction-related phenomena, the electronic properties of each component should be understood. Regarding the electronic effects of Pd [63], as shown in the XPS scan (Fig. 4d), Pd is present in the oxidized (Pd2+ or (PdO)) and metallic (Pd0) forms. In the H2S atmosphere, PdO can be partially reduced to metallic Pd with a different work function from that of the oxidized form (PdO) (Tables S2 and S3 (Supporting Information)), changing the surface resistance of the CuO NWs and overall resistance of the gas sensor. The UPS results showed that the work functions of CuO, Pd, and PdO were 3.53, 5.05, and 5.20 eV, respectively. Pd and PdO phases could initially coexist. Upon the contact with CuO, Pd/CuO and/or PdO/CuO heterointerfaces are generated. As the work function of CuO is smaller than those of Pd and PdO, to equate the Fermi level, electrons flow from CuO to Pd/PdO, while holes flow from Pd/PdO to CuO (Fig. 12a,b). Accordingly, as CuO is a p-type material, a HAL is formed on the CuO side. In the present sensor system, CuO NWs are dominant sensing materials. The Pd(PdO) NPs are discretely deposited on the NW surfaces and thus cannot provide a continuous flow of current. Upon the exposure to the reducing gas, the HAL width in the CuO is decreased. In other words, the number of holes on the surface of the CuO is reduced. As the initial volume of the hole region or initial concentration of holes increased owing to the heterojunctions, the reduction of the same number of holes by the introduction of the reduction gas yielded a lower sensor response. This explains the reductions in sensor responses to reduction gases, including C2H5OH, C7H8, C6H6, and CO, upon the incorporation of Pd NPs. For the H2S gas, in addition to the heterojunction-induced change in
Fig. 9. Repeatability of the Pd-decorated CuO NW gas sensor exposed to 100 ppm of H2S gas at an applied voltage of 5 V.
gases [60]. Through chemical sensitization, Pd can dissociate and transfer the oxygen molecules and target gases by the spill-over effect (Fig. 11). Accordingly, more gas molecules can reach to the surface of the sensor, and thus, a stronger response is expected. Although Kim et al. reported that Pd appears to have a better catalytic effect toward H2S and that the response to H2S is stronger than that to other gases [61], the underlying mechanisms have not been revealed. Even though we did not use H2 gas in this study, it should be noted that Pd could also dissociate the H2 molecules into atomic H2 and increase the resistance by the spill-over effect [62]. 8
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Fig. 12. Energy levels of (a) CuO and Pd and (b) CuO and PdO and formations of the heterojunction barriers. Changes in (c) CuO/Pd and (d) CuO/PdO heterostructures upon the introduction of H2S gas.
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Fig. 13. (a) Generation of heat under self-heating conditions and thermograph of the CuO NW gas sensor at 5 V. (b) Induction of the self-heating effect along the length of the CuO NW and CuO–CuO homojunctions.
self-heating conditions, because a larger amount of heat was generated.
initial resistance, another mechanism is supposed to affect the sensing. Upon the exposure to H2S, CuO can be converted to CuS with the metallike conductivity, leading to a strong modulation of the resistance in the heterojunctions. The work function of CuS is 4.90–4.95 eV [64–67]. Fig. 12c,d show that the potential barriers of the CuO/Pd and CuO/PdO heterojunctions, respectively, are decreased in conjunction with the phase transformation of CuO to CuS. Accordingly, less electrons cross over from CuS to Pd, than from CuO. This leads to a larger electron concentration in the sensing layer (i.e., CuO/CuS) than in the pristine sensing layer (i.e., CuO), which leads to a smaller hole concentration and thus increased hole resistance of the sensing material. The initially smaller conduction volume of the hole region in the p-type sensor provides a higher sensor response. As the conversion of CuO to CuS contributes to the increase in hole resistance, the above mechanism can explain the improvement in H2S sensing. In addition, it is possible that the CuS particles are overall connected and form independent hole current paths. Considering the two different current paths (i.e., CuO and CuS) in parallel, the total resistance is the sum of the resistances (i.e., 1/RTOTAL = 1/RCuO + 1/RCuS). Accordingly, the total resistance is smaller than the individual resistances of CuO and CuS. However, this mechanism does not explain our observation of an increased resistance and should, therefore, be discarded. The CuS particles exhibit the spill-over effect and, thus, enhance the selective sensing of H2S gas (Fig. 11). The reaction of the H2S gas with the CuO leads to the nucleation and subsequent growth of the CuS phase [68], which indicates the possibility of discrete CuS particles being generated in the CuO mother phase. The formation of CuS from CuO is possible even at room temperature, which can be confirmed by thermodynamic calculations (Text S2, Supporting Information). Ramgir et al. reported the formation of CuS at room temperature in the H2S sensing system using thin CuO films [69]. In the case of other gases, the change in resistance of the heterojunction is considerably lower because the composition of CuO remains essentially unchanged, and only the hole concentration is changed. The results showed that developed the sensors can also operate under self-heating conditions without an external heating source. When a voltage is applied to the CuO NW gas sensor, heat is generated owing to the Joule heating effect (Fig. 13a). This effect could be due to two reasons (Fig. 13b): (i) the electron current flowing through the CuO NWs and (ii) a significant number of CuO NWs being networked and in direct contact with each other. Accordingly, Joule heating can be also generated in CuO–CuO homojunctions [32,33]. As the CuO NWs have a single-crystalline structure, as shown by the SAED pattern, no grain boundaries are present to act as additional sources of Joule heating. At a very low voltage (1 V), both sensors exhibited negligible responses to H2S gas because the generated heat was negligible. With the increase in voltage to 5 V, the sensors exhibited the maximum responses under the
4. Conclusion We fabricated gas sensors using pristine and Pd-functionalized CuO NWs. The NWs were synthesized by the vapor–liquid–solid method, with noble metals functionalized on their surfaces by UV irradiation. The characterizations verified the desired morphologies and chemical compositions of the fabricated sensors. The H2S sensing results demonstrated the promising effects of the Pd functionalization. In addition, the Pd-functionalized gas sensor exhibited a strong response to H2S gas under self-heating conditions at 5 V. The strong response of the Pd-functionalized gas sensor was attributed to the conversion of CuO to CuS and the strong catalytic effect of Pd. The CuS also had a catalytic role, and the Pd–CuO heterojunctions contributed to the enhancing the H2S-sensing capability. The obtained results are valuable for the development of sensitive and selective gas-sensing devices with NW morphologies, noble metal functionalization, and self-heating properties. Acknowledgments This study was supported by the Basic Science Research Program of the National Research Foundation (NRF) of Korea, funded by the Ministry of Education (2016R1A6A1A03013422), and by the NRF of Korea, funded by the Ministry of Science and ICT. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.126965. References [1] J.H. Bang, M.S. Choi, A. Mirzaei, Y.J. Kwon, S.S. Kim, T.W. Kim, H.W. Kim, Selective NO2 sensor based on Bi2O3 branched SnO2 nanowires, Sens. Actuators B Chem. 274 (2018) 356–369. [2] H.W. Kim, S.W. Choi, A. Katoch, S.S. Kim, Enhanced sensing performances of networked SnO2 nanowires by surface modification with atmospheric pressure Ar–O2 plasma, Sens. Actuators B Chem. 177 (2013) 654–658. [3] A. Katoch, J.-H. Kim, S.W. Choi, S.S. Kim, Growth and sensing properties of networked p-CuO nanowires, Sens. Actuators B Chem. 212 (2015) 190–195. [4] J.H. Kim, Z.U. Abideen, S.S. Kim, Optimization of metal nanoparticle amount on SnO2 nanowires to achieve superior gas sensing properties, Sens. Actuators B Chem. 238 (2017) 374–380. [5] H.-J. Kim, J.-H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: overview, Sens. Actuators B Chem. 192 (2014) 607–627. [6] C. Balamurugan, G. Bhuvanalogini, A. Subramania, Development of nanocrystalline CrNbO4 based p-type semiconducting gas sensor for LPG, ethanol and ammonia, Sens. Actuators B Chem. 168 (2012) 165–171. [7] Y.J. Kwon, H.G. Na, S.Y. Kang, M.S. Choi, J.H. Bang, T.W. Kim, A. Mirzaei,
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Ali Mirzaei received his Ph.D. degree in Materials Science and Engineering from Shiraz University in 2016. He was visiting student at Messina University, Italy in 2015. Since 2016 he is postdoctoral fellow at Hanyang University in Seoul. He is interested in the synthesis and characterization of nanocomposites for gas sensing applications. Hyoun Woo Kim joined the Division of Materials Science and Engineering at Hanyang University as a full professor in 2011. He received his B.S. and M. S. degreesfrom Seoul National University and his Ph.D. degree from Massachusetts Institute of Technology (MIT) in electronic materials in 1986, 1988, and 1994, respectively. He was a senior researcher in the Samsung Electronics Co., Ltd. from 1994 to 2000.He has been a professor of materials science and engineering at Inha Universityfrom 2000 to 2010. He was a visiting professor at the Department of Chemistryof the Michigan State University, in 2009. His research interests include the one-dimensional nanostructures, nanosheets, and gas sensors.
Jin-Young Kim received his B.S. degree from Inha University, Republic of Korea in 2017. He is now working as a M. S. degree at Inha University, Republic of Korea. He has been working on oxide nanowire gas sensors.
Sang Sub Kim joined the Department of Materials Science and Engineering, Inha University, in 2007 as a full professor. He received his B.S. degree from Seoul National University and his M.S and Ph.D. degrees from Pohang University of Science and Technology (POSTECH) in Material Science and Engineering in 1987, 1990, and 1994, respectively. He was a visiting researcher at the National Research in Inorganic Materials (currently NIMS), Japan for 2 years each in 1995 and in 2000. In 2006, he was a visiting professor at Department of Chemistry, University of Alberta, Canada. In 2010, he also served as a cooperative professor at Nagaoka University of Technology, Japan. His research interests include the synthesis and applications of nanomaterials such as nanowires and nanofibers, functional thin films, and surface and interfacial characterizations.
Jae-Hyoung Lee received his BS degree from Inha University, Republic of Korea in 2014. In February 2016, he received MS degree from Inha University, Republic of Korea. He is now working as a Ph. D. candidate at Inha University, Republic of Korea. He has been working on oxide nanofiber gas sensors. Jae-Hun Kim received his B.S. degree from Gyeongsang National University, Republic of Korea in 2013. In February 2015, he received M.S. degree from Inha University, Republic of Korea. He is now working as a Ph.D. candidate at Inha University, Republic of Korea. He has been working on oxide nanowire gas sensors.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Realization of H2S sensing by Pd-functionalized networked CuO nanowires in self-heating mode
T
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Jin-Young Kima, Jae-Hyoung Leea, Jae-Hun Kima, Ali Mirzaeib,c, Hyoun Woo Kimc,d, , ⁎ Sang Sub Kima, a
Department of Materials Science and Engineering, Inha University, Incheon 22212, Republic of Korea Department of Materials Science and Engineering, Shiraz University of Technology, Shiraz, Iran c Research Institute of Industrial Science, Hanyang University, Seoul 04763, Republic of Korea d Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea b
A R T I C LE I N FO
A B S T R A C T
Keywords: CuO Nanowire Pd-functionalization H2S sensing Self-heating test Gas sensor
In this study, we fabricated pristine and Pd-functionalized CuO nanowires (NWs) for H2S gas sensing. The CuO NWs were synthesized using a vapor–liquid–solid method and the Pd functionalization was achieved by ultraviolet irradiation of a Pd precursor solution in the presence of the CuO NWs. The characterization results confirmed the formation of pristine and Pd-functionalized CuO NWs with favorable compositions and morphologies. The gas-sensing response of the NWs towards H2S was enhanced by the Pd functionalization, at 25–100 °C. Moreover, self-heating tests showed that the Pd-functionalized gas sensors exhibited a selective response to H2S gas when a low voltage (5 V) was applied. The superior performance of the Pd-functionalized gas sensor was attributable to not only the catalytic effects of both CuS and Pd but also the CuO/Pd heterostructures. The results demonstrate that the fabricated H2S sensors are energy-efficient and could be used in various electronic devices.
1. Introduction Semiconducting metal oxides with one-dimensional morphologies, such as nanowires (NWs), have large surface areas and low production costs, and thus are promising for many applications, including gas sensing [1]. The very large surface areas, unique physical and chemical properties, diameters on the order of the Debye length, rapid diffusion kinetics, and single-crystalline characteristics of metal oxide NWs are the main advantages for realization of high-performance gas sensors [2–4]. Mostly n-type metal oxides, such as SnO2 and ZnO, are used for gas sensing applications [5]. Metal oxides with p-type conductivities have lower gas responses than those of n-type oxides [6]. Therefore, they have not been extensively investigated for sensing applications [5]. Among the p-type metal oxides, including Co3O4 [7], Cr2O3 [8], NiO [9], and CuO [10], the latter is the most promising for H2S sensing. Upon exposure to H2S, the semiconducting CuO (Eg =1.2 eV) transforms to CuS with metallic conductivity. The change in conductivity provides a high response to H2S gas [11]. The sensitivities of metal oxides can be increased using different strategies [12,13], such as the functionalization of the sensors by noble
⁎
metals (Au [14], Pd [15], Pt [16], and Rh [17]). Noble metals have good catalytic activities, can facilitate the decomposition and adsorption of different gases, and can increase the concentration of gas molecules that reach the sensor surface in a process referred to as the spillover effect [18]. Moreover, noble metals can form heterojunctions with metal oxides because of their different work functions from those of the metal oxides, where the large resistance change of the sensor can improve the sensitivity [19]. In addition, compared to metal oxides such as NiO, noble metals have large numbers of charge carriers, and the electrons can redistribute very efficiently to counteract any charge build-up near the metal-metal oxide heterojunctions. Therefore, noble metals can effectively attract electronegative species like gaseous oxygen and accordingly increase the rate of the reactions [20]. Pd is a noble metal, which is less expensive than Pt [21] and Au [22] and has a good catalytic activity to H2S gas [23–26]. It is well known that Pd can further increase the probability of oxygen ionosorption on the metal oxide support by the back-spill-over effect, in addition to the regular spill-over effects [27]. In addition, PdO can be formed, acting as a stronger electron acceptor from the semiconductor, presumably promoting the gas response [28]. Pd functionalization can generate heterojunctions,
Corresponding authors. E-mail addresses: [email protected] (H. Woo Kim), [email protected] (S.S. Kim).
https://doi.org/10.1016/j.snb.2019.126965 Received 14 February 2019; Received in revised form 26 July 2019; Accepted 8 August 2019 Available online 16 August 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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form PdO, and exert electronic and chemical sensitizations. Owing to the high operation temperatures (200–500 °C), metal oxide gas sensors consume high powers when are integrated into portable electronics, which often have low-power sources [29]. Thus, the development of self-heated gas sensors is required to reduce the power consumptions of the sensors and enable the integration in low-consumption and portable electronic devices. Recently, metal oxide NWs have gained considerable attention for the fabrication of self-heated gas sensors [30]. NWs generally have extremely low heat capacities. Therefore, they need a low electric power to be heated [31]. The selfheating effects in NWs can considerably decrease the power consumption compared to those of traditional gas sensors because the small NWs can be heated by a voltage in the microsecond time range [32,33]. Therefore, it is desirable to fabricate self-heated gas sensors with extremely low consumptions using metal oxide NWs with good performances. H2S is a flammable and highly poisonous gas [34]. The threshold limit for H2S is 10 ppm (ppm) for an exposure for 8 h [35]. At 100 ppm, it easily paralyzes the olfactory nerves, while at high concentrations (0.1%), it leads to suffocation and sudden physical collapse [36,37]. In addition, H2S is a biomarker for halitosis when its concentration in exhaled breath exceeds 2 ppm [38]. Accordingly, the detection of H2S is very important from different aspects. Previous studies have demonstrated the H2S gas sensing by CuO (Table S1 in Supporting Information). A few studies have demonstrated very high sensor responses at temperatures higher than 160 °C, while no extensive studies have been reported on room-temperature sensing. In this study, for the first time, we proposed a Pd catalyst for CuO sensors. Pristine and Pd-functionalized CuO NWs were realized and evaluated for H2S sensing. The gas sensing results at 25–100 °C show the higher sensitivity of the Pd-functionalized CuO NWs than that of the pristine sensor. Furthermore, for the first time, we carried out a self-heating analysis with respect to CuO sensing by applying voltage without external heat, which showed that the Pd-functionalized gas sensor could operate at a very low applied voltage (5 V). The underlying detection mechanism is described in detail. This study is valuable for the development of self-heated sensing devices with low power consumptions.
2. Experimental methods 2.1. Synthesis of networked CuO NWs Fig. 1. Schematic of the syntheses of the pristine and Pd-functionalized CuO NWs.
Fig. 1 illustrates the fabrications of the pristine and Pd-functionalized CuO NWs. The networked CuO NWs were synthesized by thermal oxidation on a patterned interdigital electrode (PIE) on a SiO2 grown (250 nm) Si substrate. The PIE was comprised of a tri-layered electrode pad, consisting of Cu (1 μm)/Pt (200 nm)/Ti (50 nm) layers. The Pt layer provided an electrode pathway, while the Ti layer provided adhesion between the SiO2 and Pt layers. The Cu layer acted as the source of CuO NWs. The substrate was then heated to 450 °C for 20 h in air in a horizontal-type tubular furnace. N2 and O2 gas flow rates of 300 and 10 sccm, respectively, were used during the growth of CuO from Cu. The Cu was gradually consumed, yielding CuO NWs.
2.3. Characterization The synthesized products were analyzed by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4200) and transmission electron microscopy (TEM, Philips CM-200). The crystallinity and phase formation were analyzed by X-ray diffraction (XRD, Philips X’Pert MRD). X-ray photoelectron spectroscopy (XPS, Specs XR50) was performed to analyze the chemical states of the materials. We obtained the work functions by ultraviolet photoelectron spectroscopy (UPS) using a UV source of He I (21.2 eV). The procedure used to obtain the work functions is presented at the end of Section 3.1.
2.2. Functionalization of Pd nanoparticles (NPs)
2.4. Gas sensing tests
For the deposition of Pd NPs, a palladium (II) solution was prepared by dissolving PdCl2 in a mixed solvent containing isopropanol and acetone (1:1 v:v). The CuO NWs were completely immersed in the Pd2+ solution and then ultraviolet-(UV)-light-irradiated with a halogen lamp (wavelength =360 nm, intensity =0.11 mW/cm2) at room temperature for 5 s. Subsequently, the CuO NWs were annealed at 500 °C for 0.5 h to remove the remaining solvents.
The sensing measurements were almost identical to those reported previously [39,40]. The gas concentrations were accurately controlled by changing the mixing ratio of the target gas and dry air using precise mass flow controllers at a total flow rate of 500 sccm. The resistances in the air (Ra) and in the presence of the target gas (Rg) were recorded and the sensor response was calculated as R = Rg/Ra. 2
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Fig. 2. FE-SEM images of the (a) pristine CuO NWs, (b) Pd-functionalized CuO NWs on the surface of the substrate, and (c) Pd-functionalized CuO NWs. (d) XRD pattern of the Pd-functionalized CuO NWs.
3. Results and discussion
XPS is a sensitive surface technique used for analyzing surface chemical compositions [42]. Fig. 4a indicates a full XPS survey spectrum of the Pd-functionalized CuO NWs. The XPS peaks related to Cu, O, Pd, C, and Si were observed. The C 1s peak is attributed to air dust, while the Si peaks are attributed to the Si substrate. In the Cu 2p corelevel XP spectrum (Fig. 4b), the peaks at 932.58 and 952.48 eV may be attributed to Cu 2p3/2 and Cu 2p1/2, respectively. The high satellite peaks at approximately 940–945 and 962.18 eV are the major features of CuO, demonstrating the existence of Cu2+ [43]. The O 1s XPS region is presented in Fig. 4c, where the peaks at 529.68 and 532.98 eV are attributed to the O2− present in the CuO lattice and the O species (as OH) present on the surfaces of the CuO NWs, respectively [44]. Fig. 4d shows the Pd 3d XPS region, comprising two peaks at 336.98 (Pd 3d5/2) and 342.18 (Pd 3d3/2) eV. The Pd 3d region was further deconvoluted to the corresponding subpeaks (Fig. 4e). In the deconvoluted spectrum, peaks corresponding to metallic Pd (40.98%) and Pd+2 (59.02%) are observed. Accordingly, both metallic and oxidized forms of Pd coexist on the surfaces of the CuO NWs. UPS was employed to determine the cut-off values of CuO, Pd, and PdO, which were 17.77, 16.25, and 16.10 eV, respectively (Fig. 5). The work functions were calculated by subtracting the cut-off values from 21.1 eV (reference). Also, 0.1 eV was added to the obtained values due to the broadening effect of the apparatus. Thus, the work functions of CuO, Pd, and PdO were determined to be 3.53, 5.05, and 5.20 eV, respectively.
3.1. Morphological, structural, and chemical analyses Fig. 2a exhibits a representative FE-SEM image of the pristine CuO NWs grown on the substrate. Fig. 2b indicates an FE-SEM image of the Pd-functionalized CuO NWs on the surface of the substrate, while Fig. 2c shows a typical FE-SEM image of the Pd-functionalized CuO NWs, with diameters of approximately 35–45 nm. Fig. 2d shows a typical XRD pattern of the Pd-functionalized CuO NWs, where the peaks related to the crystalline monoclinic CuO (Joint Committee on Powder Diffraction Standards (JCPDS) file no. 89-5899) are marked by the filled circles (•), demonstrating the formation of crystalline CuO NWs [3]. The peak related to the crystalline face-centered-cubic Pd (JCPDS file no. 87-0641) is marked by the filled triangle (▲) [41]. The XRD results show that crystalline CuO and Pd phases were formed without impurities or undesirable phases, demonstrating the successful synthesis. Fig. 3a presents a typical TEM image of the pristine CuO NWs. A typical CuO NW had a diameter of approximately 100 nm and smooth morphology. Fig. 3b indicates a selected-area electron diffraction (SAED) pattern of the pristine CuO NWs. The spotty pattern is attributed to the (111), (2¯02) , and (311¯) planes of CuO, demonstrating the single-crystalline structure of the CuO NWs. Fig. 3c presents an energydispersive X-ray (EDX) spectrum, demonstrating the presence of both Cu and O. Notably, the first peak in the EDX spectrum at 0.928 eV shows that the Cu (La line) and Ni (La line) peaks overlap. The Ni peaks are attributed to the Ni grid used for the analysis of the CuO NWs. The EDX spectroscopy elemental mapping confirmed the coexistence of Cu and O across the diameter of the NW (Fig. 3d). Fig. 3e shows a typical TEM image of the Pd-functionalized CuO NWs, with the Pd NPs on their surfaces. The lattice-resolved TEM image (Fig. 3f) shows parallel fringes with spacings of 0.23 and 0.275 nm, which can be attributed to the (111) plane of Pd and (110) plane of CuO, respectively. This demonstrates the coexistence of the crystalline Pd and CuO.
3.2. Gas sensing Preliminary tests were carried out to evaluate the optimal sensing temperatures (Topt) of the pristine and Pd-functionalized CuO NW gas sensors, considering the strong dependence of the sensing on the operation temperature [45]. To obtain Topt, the sensors were exposed to 1, 10, and 100 ppm of H2S gas at different temperatures, starting from room temperature, as shown in Fig. S1 (Supplementary Information). As the resistance increased upon the injection of the reducing gas, both sensors exhibited p-type behaviors. The responses strongly depended on the operation temperature. Fig. 6 summarizes the sensing responses of 3
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Fig. 3. (a) Low-magnification TEM image of a typical CuO NW and corresponding (b) SAED pattern, (c) EDX spectrum, and (d) EDX spectroscopy line profiles. (e) Low-magnification TEM and (f) lattice-resolved TEM images of a typical Pd-functionalized CuO NW.
responses to H2S gas for the two gas sensors. In case of pristine CuO NWs at the optimal temperature of 300 °C, the relative responses of H2S in comparison to C6H6, C7H8, C2H5OH, and CO gases are 1.547, 1.490, 1.602, and 1.216, respectively. In case of Pd-functionalized CuO NW at the optimal temperature of 100 °C, the relative responses of H2S in comparison to C6H6, C7H8, C2H5OH, and CO gases are 1.674, 1.720, 1.779, and 1.365, respectively. Both pristine and Pd-functionalized CuO NW gas sensors exhibited the highest responses to H2S gas. It is noteworthy that Pd-functionalized CuO NW gas sensors at 100 °C exhibited the higher relative response to H2S, compared to pristine CuO NWs at the optimal temperature of 300 °C. Further, the responses of the gas sensors under self-heating conditions were studied. In this condition, the Joule heating attributed to the applied voltage can increase the temperature of the gas sensor. In the self-heating experiments, voltages of 1 and 5 V were applied to the gas
the gas sensors to 100 ppm of H2S gas. The Topt values of the pristine and Pd-functionalized CuO NWs were 300 and 100 °C, respectively. At T < Topt, sufficient energy for an effective adsorption of H2S is not provided. On the other hand, at T > Topt, the desorption rate is higher than the adsorption rate, leading to a low performance of the sensor [46]. The maximum response of the pristine CuO NWs to 100 ppm of H2S was 1.861 (at 300 °C), which increased to 1.962 (at 100 °C) after the functionalization by the Pd NPs. Selectivity is crucial for practical applications, as the lack of selectivity can lead to false alarms and hinder the proper detection of the target gas [18]. To study the selectivities of the gas sensors, they were exposed to 1, 10, and 100 ppm of interfering gases (CO, C2H5OH, C7H8 (toluene), and C6H6) at their Topt values and the dynamic resistance curves were measured (Fig. 7a,b). Fig. 7c presents the selectivity patterns to 100 ppm of the different interfering gases along with the
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Fig. 4. (a) XPS survey spectrum of the Pd-functionalized CuO NWs, (b) Cu 2p, (c) O 1s, (d) Pd 3d core-level regions, and (e) deconvoluted Pd 3d region.
Fig. 9 shows the repeatability of the gas sensor. The responses to 100 ppm of H2S gas at 5 V in the first, second, and third cycles were 1.394, 1.391, and 1.368, respectively; the average response was 1.384. This shows the good repeatability of the gas sensor, which is important for practical applications. Fig. 10 reveals the long-term stability of the Pd-functionalized CuO NW gas sensor exposed to various concentrations of H2S gas at 5 V. Fig. 10a presents the transient resistance graphs of the sensor in the fresh state and after storage for six months in the
sensors without an external heating source. Fig. S2 (Supplementary Information) shows the dynamic resistances of the two gas sensors at 1 and 5 V. Fig. 8 displays the responses of the gas sensors to H2S as a function of the applied voltage. Both gas sensors exhibited the best responses at 5 V. Fig. 8b compares the responses of the two gas sensors to 100 ppm of H2S gas at different voltages. The pristine and Pd-functionalized CuO NW sensors exhibited responses of 1.08 and 1.40, respectively, at 5 V. 5
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Fig. 5. UPS spectra and cut-off values of (a, b) CuO, (c, d) Pd, and (e, f) PdO, respectively.
3.3. Gas sensing mechanism We explain the sensing mechanism of the pristine CuO NWs. As holes are the main charge carriers in the p-type CuO, the resistance in the hole accumulation layer (HAL) is decreased compared to that in the core regions of the CuO NWs [40,47,48]. When the CuO NW sensor is exposed to CO gas, by the reaction of the target gas with oxygen, CO + O− → CO2 + e−, electrons return to the surface of the sensor, combine with holes, and thus decrease the concentration of holes. Accordingly, the width of the HAL decreases and the resistance of the sensor increases, yielding the sensing response. Although the introduction of H2S may shrink the HAL on the CuO surface, the sulfurization reaction may also occur in the H2S ambient, CuO + H2S → CuS + H2O [49]. Consequently, CuO is converted to CuS, which has a metal-like conductivity [50]. The XPS results confirmed the conversion of CuO to CuS in the CuO-NW-based gas sensors upon the exposure to H2S gas (Fig. S3 and Text S1 in Supporting Information; [51–56]). Similar results were reported by other researchers [50,51,57–59]. For the Pd-functionalized CuO NW gas sensor, three results should be discussed: (i) Topt significantly decreased, to 100 °C, (ii) the response to H2S was enhanced, and (iii) the responses to the interfering gases was reduced. Pd is a noble metal that has a catalytic effect toward some
Fig. 6. Comparison of the responses of the pristine and Pd-functionalized CuO NW sensors (having optimal sensing temperatures of 300 and 100 °C, respectively) to 100 ppm of H2S gas as a function of the temperature.
laboratory. Fig. 10b compares the corresponding calibration curves. After the six months, the response changes were negligible, demonstrating the good stability of the gas sensor.
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Fig. 7. Dynamic responses to H2S, C6H6, C7H8, C2H5OH, and CO gases for the (a) pristine CuO NW gas sensor at 300 °C and (b) Pd-functionalized CuO NW gas sensor at 100 °C. (c) Selectivity histogram of the pristine and Pd-functionalized CuO NW sensors exposed to 100 ppm of interfering gases at Topt.
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Fig. 10. (a) Dynamic resistances of the Pd-functionalized CuO NW gas sensor exposed to different concentrations of H2S gas at an applied voltage of 5 V in the fresh state and after storage for six months in the laboratory. (b) Corresponding calibration curves.
Fig. 11. Spill-over effects of Pd, PdO, and CuS.
Fig. 8. (a) Responses of the pristine and Pd-functionalized CuO NW gas sensors to 100 ppm of H2S at 1 and 5 V. (b) Responses of the gas sensors to 100 ppm of H2S gas as a function of the applied voltage.
To elucidate the heterojunction-related phenomena, the electronic properties of each component should be understood. Regarding the electronic effects of Pd [63], as shown in the XPS scan (Fig. 4d), Pd is present in the oxidized (Pd2+ or (PdO)) and metallic (Pd0) forms. In the H2S atmosphere, PdO can be partially reduced to metallic Pd with a different work function from that of the oxidized form (PdO) (Tables S2 and S3 (Supporting Information)), changing the surface resistance of the CuO NWs and overall resistance of the gas sensor. The UPS results showed that the work functions of CuO, Pd, and PdO were 3.53, 5.05, and 5.20 eV, respectively. Pd and PdO phases could initially coexist. Upon the contact with CuO, Pd/CuO and/or PdO/CuO heterointerfaces are generated. As the work function of CuO is smaller than those of Pd and PdO, to equate the Fermi level, electrons flow from CuO to Pd/PdO, while holes flow from Pd/PdO to CuO (Fig. 12a,b). Accordingly, as CuO is a p-type material, a HAL is formed on the CuO side. In the present sensor system, CuO NWs are dominant sensing materials. The Pd(PdO) NPs are discretely deposited on the NW surfaces and thus cannot provide a continuous flow of current. Upon the exposure to the reducing gas, the HAL width in the CuO is decreased. In other words, the number of holes on the surface of the CuO is reduced. As the initial volume of the hole region or initial concentration of holes increased owing to the heterojunctions, the reduction of the same number of holes by the introduction of the reduction gas yielded a lower sensor response. This explains the reductions in sensor responses to reduction gases, including C2H5OH, C7H8, C6H6, and CO, upon the incorporation of Pd NPs. For the H2S gas, in addition to the heterojunction-induced change in
Fig. 9. Repeatability of the Pd-decorated CuO NW gas sensor exposed to 100 ppm of H2S gas at an applied voltage of 5 V.
gases [60]. Through chemical sensitization, Pd can dissociate and transfer the oxygen molecules and target gases by the spill-over effect (Fig. 11). Accordingly, more gas molecules can reach to the surface of the sensor, and thus, a stronger response is expected. Although Kim et al. reported that Pd appears to have a better catalytic effect toward H2S and that the response to H2S is stronger than that to other gases [61], the underlying mechanisms have not been revealed. Even though we did not use H2 gas in this study, it should be noted that Pd could also dissociate the H2 molecules into atomic H2 and increase the resistance by the spill-over effect [62]. 8
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Fig. 12. Energy levels of (a) CuO and Pd and (b) CuO and PdO and formations of the heterojunction barriers. Changes in (c) CuO/Pd and (d) CuO/PdO heterostructures upon the introduction of H2S gas.
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Fig. 13. (a) Generation of heat under self-heating conditions and thermograph of the CuO NW gas sensor at 5 V. (b) Induction of the self-heating effect along the length of the CuO NW and CuO–CuO homojunctions.
self-heating conditions, because a larger amount of heat was generated.
initial resistance, another mechanism is supposed to affect the sensing. Upon the exposure to H2S, CuO can be converted to CuS with the metallike conductivity, leading to a strong modulation of the resistance in the heterojunctions. The work function of CuS is 4.90–4.95 eV [64–67]. Fig. 12c,d show that the potential barriers of the CuO/Pd and CuO/PdO heterojunctions, respectively, are decreased in conjunction with the phase transformation of CuO to CuS. Accordingly, less electrons cross over from CuS to Pd, than from CuO. This leads to a larger electron concentration in the sensing layer (i.e., CuO/CuS) than in the pristine sensing layer (i.e., CuO), which leads to a smaller hole concentration and thus increased hole resistance of the sensing material. The initially smaller conduction volume of the hole region in the p-type sensor provides a higher sensor response. As the conversion of CuO to CuS contributes to the increase in hole resistance, the above mechanism can explain the improvement in H2S sensing. In addition, it is possible that the CuS particles are overall connected and form independent hole current paths. Considering the two different current paths (i.e., CuO and CuS) in parallel, the total resistance is the sum of the resistances (i.e., 1/RTOTAL = 1/RCuO + 1/RCuS). Accordingly, the total resistance is smaller than the individual resistances of CuO and CuS. However, this mechanism does not explain our observation of an increased resistance and should, therefore, be discarded. The CuS particles exhibit the spill-over effect and, thus, enhance the selective sensing of H2S gas (Fig. 11). The reaction of the H2S gas with the CuO leads to the nucleation and subsequent growth of the CuS phase [68], which indicates the possibility of discrete CuS particles being generated in the CuO mother phase. The formation of CuS from CuO is possible even at room temperature, which can be confirmed by thermodynamic calculations (Text S2, Supporting Information). Ramgir et al. reported the formation of CuS at room temperature in the H2S sensing system using thin CuO films [69]. In the case of other gases, the change in resistance of the heterojunction is considerably lower because the composition of CuO remains essentially unchanged, and only the hole concentration is changed. The results showed that developed the sensors can also operate under self-heating conditions without an external heating source. When a voltage is applied to the CuO NW gas sensor, heat is generated owing to the Joule heating effect (Fig. 13a). This effect could be due to two reasons (Fig. 13b): (i) the electron current flowing through the CuO NWs and (ii) a significant number of CuO NWs being networked and in direct contact with each other. Accordingly, Joule heating can be also generated in CuO–CuO homojunctions [32,33]. As the CuO NWs have a single-crystalline structure, as shown by the SAED pattern, no grain boundaries are present to act as additional sources of Joule heating. At a very low voltage (1 V), both sensors exhibited negligible responses to H2S gas because the generated heat was negligible. With the increase in voltage to 5 V, the sensors exhibited the maximum responses under the
4. Conclusion We fabricated gas sensors using pristine and Pd-functionalized CuO NWs. The NWs were synthesized by the vapor–liquid–solid method, with noble metals functionalized on their surfaces by UV irradiation. The characterizations verified the desired morphologies and chemical compositions of the fabricated sensors. The H2S sensing results demonstrated the promising effects of the Pd functionalization. In addition, the Pd-functionalized gas sensor exhibited a strong response to H2S gas under self-heating conditions at 5 V. The strong response of the Pd-functionalized gas sensor was attributed to the conversion of CuO to CuS and the strong catalytic effect of Pd. The CuS also had a catalytic role, and the Pd–CuO heterojunctions contributed to the enhancing the H2S-sensing capability. The obtained results are valuable for the development of sensitive and selective gas-sensing devices with NW morphologies, noble metal functionalization, and self-heating properties. Acknowledgments This study was supported by the Basic Science Research Program of the National Research Foundation (NRF) of Korea, funded by the Ministry of Education (2016R1A6A1A03013422), and by the NRF of Korea, funded by the Ministry of Science and ICT. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.126965. References [1] J.H. Bang, M.S. Choi, A. Mirzaei, Y.J. Kwon, S.S. Kim, T.W. Kim, H.W. Kim, Selective NO2 sensor based on Bi2O3 branched SnO2 nanowires, Sens. Actuators B Chem. 274 (2018) 356–369. [2] H.W. Kim, S.W. Choi, A. Katoch, S.S. Kim, Enhanced sensing performances of networked SnO2 nanowires by surface modification with atmospheric pressure Ar–O2 plasma, Sens. Actuators B Chem. 177 (2013) 654–658. [3] A. Katoch, J.-H. Kim, S.W. Choi, S.S. Kim, Growth and sensing properties of networked p-CuO nanowires, Sens. Actuators B Chem. 212 (2015) 190–195. [4] J.H. Kim, Z.U. Abideen, S.S. Kim, Optimization of metal nanoparticle amount on SnO2 nanowires to achieve superior gas sensing properties, Sens. Actuators B Chem. 238 (2017) 374–380. [5] H.-J. Kim, J.-H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: overview, Sens. Actuators B Chem. 192 (2014) 607–627. [6] C. Balamurugan, G. Bhuvanalogini, A. Subramania, Development of nanocrystalline CrNbO4 based p-type semiconducting gas sensor for LPG, ethanol and ammonia, Sens. Actuators B Chem. 168 (2012) 165–171. [7] Y.J. Kwon, H.G. Na, S.Y. Kang, M.S. Choi, J.H. Bang, T.W. Kim, A. Mirzaei,
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Ali Mirzaei received his Ph.D. degree in Materials Science and Engineering from Shiraz University in 2016. He was visiting student at Messina University, Italy in 2015. Since 2016 he is postdoctoral fellow at Hanyang University in Seoul. He is interested in the synthesis and characterization of nanocomposites for gas sensing applications. Hyoun Woo Kim joined the Division of Materials Science and Engineering at Hanyang University as a full professor in 2011. He received his B.S. and M. S. degreesfrom Seoul National University and his Ph.D. degree from Massachusetts Institute of Technology (MIT) in electronic materials in 1986, 1988, and 1994, respectively. He was a senior researcher in the Samsung Electronics Co., Ltd. from 1994 to 2000.He has been a professor of materials science and engineering at Inha Universityfrom 2000 to 2010. He was a visiting professor at the Department of Chemistryof the Michigan State University, in 2009. His research interests include the one-dimensional nanostructures, nanosheets, and gas sensors.
Jin-Young Kim received his B.S. degree from Inha University, Republic of Korea in 2017. He is now working as a M. S. degree at Inha University, Republic of Korea. He has been working on oxide nanowire gas sensors.
Sang Sub Kim joined the Department of Materials Science and Engineering, Inha University, in 2007 as a full professor. He received his B.S. degree from Seoul National University and his M.S and Ph.D. degrees from Pohang University of Science and Technology (POSTECH) in Material Science and Engineering in 1987, 1990, and 1994, respectively. He was a visiting researcher at the National Research in Inorganic Materials (currently NIMS), Japan for 2 years each in 1995 and in 2000. In 2006, he was a visiting professor at Department of Chemistry, University of Alberta, Canada. In 2010, he also served as a cooperative professor at Nagaoka University of Technology, Japan. His research interests include the synthesis and applications of nanomaterials such as nanowires and nanofibers, functional thin films, and surface and interfacial characterizations.
Jae-Hyoung Lee received his BS degree from Inha University, Republic of Korea in 2014. In February 2016, he received MS degree from Inha University, Republic of Korea. He is now working as a Ph. D. candidate at Inha University, Republic of Korea. He has been working on oxide nanofiber gas sensors. Jae-Hun Kim received his B.S. degree from Gyeongsang National University, Republic of Korea in 2013. In February 2015, he received M.S. degree from Inha University, Republic of Korea. He is now working as a Ph.D. candidate at Inha University, Republic of Korea. He has been working on oxide nanowire gas sensors.
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