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n-SnO2 core-shell nanowires for enhanced sensitive and selective formaldehyde detection
Sensors & Actuators: B. Chemical 290 (2019) 233–241
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Fabrication of heterostructured p-CuO/n-SnO2 core-shell nanowires for enhanced sensitive and selective formaldehyde detection
T
Li-Yuan Zhua,1, Kaiping Yuana,1, Jian-Guo Yanga, Hong-Ping Maa, Tao Wanga, Xin-Ming Jia, ⁎ Ji-Jun Fengb, Anjana Devic, Hong-Liang Lua, a
State Key Laboratory of ASIC and System, Shanghai Institute of Intelligent Electronics & Systems, School of Microelectronics, Fudan University, Shanghai 200433, China Shanghai Key Laboratory of Modern Optical System, School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China c Inorganic Materials Chemistry, Ruhr-University Bochum, Bochum 44780, Germany b
A R T I C LE I N FO
A B S T R A C T
Keywords: Core-Shell Nanowire Atomic layer deposition Gas sensor CuO-SnO2 Formaldehyde
Highly sensitive and selective gas sensors based on heterostructured p-CuO/n-SnO2 core-shell nanowires (NWs) with precisely controlled shell thickness were synthesized through a sequential process combining a solution processing and atomic layer deposition. The gas sensing devices were fabricated on micro-electromechanical systems, which has triggered great research interest for low power consumption and highly integrated design. The designed p-CuO/n-SnO2 core-shell NW structured gas sensors exhibited superior gas sensing performance, which is closely related to the thickness of the SnO2 shell. Specifically, p-CuO/n-SnO2 core-shell NWs with a 24 nm thick SnO2 shell displayed a high sensitivity (Ra/Rg) of 2.42, whose rate of resistance change (i.e. 1.42) is 3 times higher than the pristine CuO NW sensor when detecting 50 ppm formaldehyde (HCHO) at 250 °C. The enhanced gas sensing performance could be attributed to the formation of p-n heterojunction which was revealed by specific band alignment and the heterojunction-depletion model. Besides, the well-structured p-CuO/n-SnO2 core-shell NWs achieved excellent selectivity for HCHO from commonly occurred reducing gases. In a word, such heterostructured p-CuO/n-SnO2 core-shell NW gas sensors demonstrate a feasible approach for enhanced sensitive and selective HCHO detection.
1. Introduction Recently, semiconductor-based gas sensors have attracted tremendous research interest and already been widely applied to diverse application fields like gas leakage alarm, environment gas monitor, industrial gas analysis and so on [1,2]. During the past few years, significant research efforts have been made to fabricate various novel gas sensors based on metal oxide semiconductor materials with high specific surface area, excellent adsorption capacity and high carrier mobility [3–7]. Among various metal oxide semiconductor materials, copper oxide (CuO) is an excellent p-type multifunctional semiconductor, which has been extensively investigated for high performance gas sensors [8,9]. In order to improve the performance of CuObased gas sensors, substantial efforts have been focused on the design and fabrication of different nanostructures, such as nanowires, nanoflowers and nanocubes [10–13]. For example, Park et al. fabricated and characterized a ppb-level formaldehyde (HCHO) gas sensor based on
CuO nanocubes which were synthesized through a wet, facile and massproducible polyol process [13]. Kim et al. synthesized networked p-CuO nanowires (NWs) on patterned-electrode pads by thermal oxidation and demonstrated that the networked CuO NWs are comparable sensors detecting reducing gases when compared with networked n-SnO2 NWs [10]. However, the gas sensing performance of CuO is limited by its unsatisfactory stability and low response, which can not meet the demand of developing target gas monitor technologies [14]. Fabrication of composite structures is widely regarded as an effective strategy to improve the performance of gas sensors due to the collection of various properties of different materials. A considerable and widely accepted approach is to construct core-shell heterostructures consisted of two or more kinds of semiconductor materials, especially for the p-n heterostructures like CuO/SnO2, CuO/ZnO, CuO/TiO2, and so on [15–18]. For instance, the flower-like p-CuO/n-ZnO heterojunction nanorod sensor reported by Zhang et al. displayed a response 2.5 times higher than the
⁎
Corresponding author. E-mail address: [email protected] (H.-L. Lu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.snb.2019.03.092 Received 20 January 2019; Received in revised form 20 March 2019; Accepted 21 March 2019 Available online 30 March 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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2.2. Synthesis of heterostructured p-CuO/n-SnO2 core-shell NWs
pristine sample when detecting 100 ppm ethanol [17]. The formation of p-n core-shell heterostructures will introduce an additional wide depletion region, leading to a significant increase of the material resistance [19]. Besides, since the core CuO is susceptible to chemical change during exposure to surrounding environment, coating of an inert shell will significantly enhance the device stability and gas sensing response. Furthermore, significant research efforts have demonstrated that the shell thickness has an undeniable role in the gas sensing ability of the resulted core-shell materials. Therefore, to precisely control the shell thickness is of vital importance for obtaining highly sensitive core-shell gas sensing materials. It is remarkable that as a film forming technique of self-limiting growth, atomic layer deposition (ALD) technique possesses a great many significant advantages, especially for precise control of film thickness at the atomic level and uniform conformal coverage ability [20,21], which is beneficial to the accurate control of the shell thickness. For example, Kim et al. synthesized CuO/ZnO core-shell NWs with ALD technique and investigated the effect of the ZnO shell thickness on the gas sensing performance [22]. Among various gas sensing materials, n-type SnO2 is a promising candidate as a shell layer for high performance gas sensors due to its relatively large response and good stability for detecting both oxidizing and reducing gases [23]. On the other hand, compared with other metal oxide materials like ZnO and TiO2, SnO2 possesses a much larger work function (˜4.9 eV) [24], thus leading to an easier approach for reducing gases to be absorbed. For instance, Zhang et al. reported a novel gas sensor based on ZnO nanorods coated with a shell film of SnO2 synthesized via an ionic-layer adsorption and reaction method, whose response to 2000 ppm ethanol vapor was 7 times higher than that of pristine ZnO based sensor [25]. Herein, in this work, a facile route for the large-scale fabrication of heterostructured p-CuO/n-SnO2 core-shell NW gas sensors with high sensitivity and fast response is proposed combining a solution processing for the growth of CuO NWs and ALD of thickness-controlled SnO2 shells on the surface of CuO NWs. Particularly, the gas sensing devices were fabricated on micro-electromechanical systems (MEMS), which have triggered great research interest for low power consumption and highly integrated design. As a result, significantly improved sensing performance of heterostructured p-CuO/n-SnO2 core-shell NW gas sensors to HCHO were demonstrated comparing to the pristine CuO NW sensor. Specifically, the core-shell nanowire-based sensor with a 24 nm thick SnO2 shell displayed a high sensitivity (Ra/Rg) of 2.42, whose rate of resistance change (i.e. 1.42) is 3 times higher than the pristine CuO NW sensor when detecting 50 ppm HCHO at 250 °C. According to the surface-depletion and heterojunction-depletion models, the optimal shell thickness could be determined by the influence of both the radial modulation of the electron-depleted layer and the volume fraction of the electron-depleted layer in the core-shell structure [26]. Moreover, the obtained CuO/SnO2 core-shell NWs exhibited an excellent selectivity towards HCHO rather than other commonly occurred reducing gases.
The synthesis of the heterostructured p-CuO/n-SnO2 core-shell NWs was achieved by first fabricating Cu(OH)2 NWs via a modified facile wet chemical method reported before [27]. Briefly, a piece of Cu foam with moderate size was vertically immersed into an aqueous solution containing 1.25 M NaOH and 0.0625 M (NH4)2S2O8 for 10 min, followed by a rinsing and drying process. Secondly, the SnO2 shells of different thickness, namely 120, 180, 240, and 300 cycles, were deposited at 150 °C on as-prepared Cu(OH)2 NWs in a BENEQ TFS-200 ALD system. Briefly, TDMASn and DI water were used as the tin (Sn) and oxidant sources, respectively. The Cu(OH)2 NWs were alternately exposed to the vapor pulse of the TDMASn and DI water precursors in the ALD reactor chamber using the high purity nitrogen gas (N2) as the carrier gas. Meanwhile, the high purity N2 as the purge gas purged the gaseous by-products and residual gas out of chamber between two valid pulses, effectively avoiding unexpected gas reactions. For all samples, the deposition process in each growth cycle briefly includes a 0.5 s pulse of TDMASn, a 10 s purge, a 0.2 s pulse of DI water and a 10 s purge. The different desired thicknesses of SnO2 shell layers can be achieved by repeating different specific ALD growth cycles. Specifically during all ALD processes, the TDMASn source was held at 45 °C for evaporation. In the third step, Cu(OH)2/SnO2 NWs were ultrasonically separated from the Cu foam, uniformly dispersed in DI water, and baked in air at 80 °C on a piece of quartz substrate. Finally, the obtained samples on the quartz substrate were calcined in air at 550 °C for 2 h with 1 °C/min of the ramping rate, converting Cu(OH)2/SnO2 NWs into desired CuO/ SnO2 NWs. 2.3. Instruments and characterization The SnO2 shell layers were deposited in a BENEQ TFS-200 ALD system (Finland). The thicknesses of the SnO2 films deposited on flat silicon substrates were measured by a SOPRA GES-5E spectroscopic ellipsometry (SE) system (Hungary). Transmission electron microscope (TEM) characterization and energy-dispersive X-ray spectroscopy (EDS) analysis were carried out on a FEI Tecnai G2 F20 S-TWIN field-emission TEM (United States) with the acceleration voltage of 200 kV. Scanning electron microscope (SEM) images were recorded on a Zeiss SIGAMA HD field-emission SEM (Germany). Wide-angle X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance powder X-ray diffractometer (Germany) with Ni-filtered Cu-Kɑ radiation (40 kV, 40 mA, 1.5406 Å). X-ray photoelectron spectroscopy (XPS) measurement was conducted on a PHI 5000 VersaProbe system (United States) using an Mg-Kα X-ray source. 2.4. Gas sensing performance measurements For the gas sensing measurements, as-prepared p-CuO/n-SnO2 coreshell NWs were scratched from the quartz substrates, uniformly dispersed into DI water, and dropped onto the micro-electromechanical systems (MEMS) heating appliances. After thoroughly dried in air at room temperature, the MEMS heating appliances with as-prepared gas sensing materials were baked in an air-circulating oven at 45 °C for 24 h. Fig. 1 exhibits the schematic diagram and an optical microscope image of the MEMS heating appliance. Then, the fabricated MEMS device was connected to the external circuit by aluminium wire bonding. The sensing characteristics of the fabricated p-CuO/n-SnO2 core-shell NW sensors for reducing gases, namely HCHO, acetone (CH3COCH3), methylbenzene (C7H8) and ammonia (NH3), was measured using a JF02F gas sensing measurement system (China). The gas sensing response (R) in the reducing-gas measurements was estimated using the equation R = Ra/Rg (for n-type semiconductors) or R = Rg/Ra (for p-type semiconductors), where Ra and Rg are the resistance of gas sensing materials in air or in the atmosphere of the target gas. The response and recovery times were defined as the time taken for the
2. Experimental section 2.1. Chemicals and reagents Standardly cleaned copper (Cu) foams were used as the substrates in this work. Analytical grade sodium hydroxide (NaOH) and ammonium persulfate ((NH4)2S2O8, APS) were obtained from Shanghai Chemical Reagent Company. The precursor materials used for depositing SnO2 layer were tetrakis(dimenthylamido) tin (Ⅳ) ([N(CH3)2]4Sn, TDMASn, sigma Aldrich, 99.9999%) and de-ionized (DI) water. Other chemical reagents employed in our experiments were analytical grade and the gases were ultra high pure (99.999%). All aqueous solutions were prepared with DI water obtained from a Millipore Q purification system (resistivity > 18MΩ cm). 234
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Fig. 1. (a) The schematic diagram and (b) an optical microscope image of the MEMS heating appliance.
sensor output from 10% to 90% of the highest response variation upon exposure to the target gas and air, respectively. Moreover, in order to figure out the optimal operating temperature, the sensing performance under different temperature was investigated as well.
shell thickness shows a continuous increase when increasing the total number of ALD growth cycles with a slope of ˜0.1 nm/cycle. The corresponding sample are denoted as C/SX in this work, where X = 0, 12, 18, 24 and 31, representing the thicknesses of the SnO2 shell layers, respectively. Besides, it is notable that some of the single CuO NWs are relatively close to and even in contact with each other. Therefore, as it can be seen in Figs. 2(b)–(e) and S1(d), two or more CuO NWs were covered by SnO2 shell coverage fabricating one singe core-shell CuO/ SnO2 structure, which further confirming the excellent conformality of the ALD technique. The well-defined core-shell structure is further revealed by TEM characterization and EDS analysis results obtained on a typical sample C/S24 (Fig. 3(a)–(d)). The bright-field TEM and high-resolution TEM (HRTEM) images indicate the core-shell structured CuO/SnO2 NW with a uniform shell thickness of around 24 nm, which is consistent with the SEM results. As a contrast, the HRTEM image of the single pristine CuO NW is showed in Fig. S1(b), in which no shell layer could be found at all. A highly magnified TEM image (Fig. 3(b)) indicates somewhat roughness on the shell surface with a weak evidence of grains, further increasing the specific surface area for gas adsorbing. The HRTEM image (Fig. 3(c)) and selected-area electron diffraction (SAED) pattern (Fig. 3(c) inset) of the sample C/S24 reveal that the CuO core of the nanowire have a multi-crystalline structure. The lattice fringes exhibit 0.275 nm in the core and 0.334 nm in the shell region, which correspond to the d-spacing values of the (110) plane of tenorite CuO and (110) plane of cassiterite SnO2 phase, respectively. Fig. 3(d) further displays the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the sample C/S24 along with the collected elemental line scanning profiles. Obviously, the SnO2 shell looks brighter than the inside CuO core, which could be attributed to the much larger atomic number (Z) of Sn (50) than Cu (29). In this regards, the core-shell structure could be identified by this distinct Zcontrast as well. Meanwhile, the elemental line scanning profiles also confirm that the element Cu can be detected only in the central region, revealing the fact that CuO only exists in the core. The O and Sn elements were detected in both central and marginal regions while the concentration of Sn in the central is much lower in contrast to the marginal region, which further verifies the well-defined core-shell structure of the CuO/SnO2 NWs. For comparison, the HAADF-STEM image of the single pristine CuO NW is displayed in Fig. S1(c), in which only the Cu and O elements were detected in the overall NW during the elemental line scanning. The crystal structures of CuO/SnO2 core-shell NWs were measured by XRD, as shown in Fig. 4(a) (sample C/S0, C/S12, C/S18, C/S24 and C/S30). It can be seen that the diffraction peaks of pristine CuO NWs (i.e. sample C/S0) could be well indexed into a tenorite CuO phase (PDF#45-0937). All the samples show two characteristic diffraction peaks at 35.5° and 38.7°, which is corresponding to the (002) and (111) planes of the tenorite structure of CuO, respectively. Compared to the sample C/S0, other samples obtained after the ALD-SnO2 reaction display extra obvious peaks at 26.6° and 33.9°, which belong to the
3. Results and discussion 3.1. Microstructure characterization Scheme 1 illustrates the synthetic protocol of the heterostructured pCuO/n-SnO2 core-shell NWs based on the ALD process. Firstly, wellaligned Cu(OH)2 NWs were obtained on a piece of pre-cleaned Cu foam via a modified facile wet chemical method. After rinsing and drying, a uniform and light blue thin film could be seen from the surface of the Cu foam. The SEM image in Fig. S1(a) of the supporting information confirms that this film is composed of vertically aligned Cu(OH)2 NWs with a diameter of ˜132 nm. Secondly, these Cu(OH)2 NWs were conformably coated by amorphous SnO2 layers with different thicknesses through thermal ALD at 150 °C. After subsequent annealing in air, the reddish-brown core-shell structured CuO/SnO2 NWs were formed. The displayed SEM images (Fig. 2(b)–(e)) indicate the as-prepared CuO/ SnO2 NWs morphologies with uniform and conformal SnO2 shell layers. For comparison, the pristine CuO NWs without a SnO2 shell layer is representatively shown in Fig. 2(a). The average diameter of the pristine CuO NWs is measured to be ˜124 nm, which exhibits the shrinkage in nanowire structure due to reconstruction of Cu-OH to CuO during the annealing process at high temperature. The average diameter of the core-shell structured CuO/SnO2 NWs are calculated to be about 147, 160, 172 and 185 nm, respectively, which were summarized in Fig. 2(f). As a result, the thicknesses of SnO2 turned out to be 12, 18, 24 and 31 nm with different ALD growth cycles of 120, 180, 240 and 300, which are consistent with the results of the SnO2 layers grown on flat silicon wafers measured by SE system (Fig. S2). Evidently, the SnO2
Scheme 1. The synthetic protocol for the p-CuO/n-SnO2 core-shell NWs, combining a modified facile wet chemical method, an ALD process and a calcination process. 235
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Fig. 2. SEM characterization of p-CuO/n-SnO2 core-shell NWs with shell thicknesses ranging from 0 to 31 nm: (a) 0, (b) 12, (c) 18, (d) 24 and (e) 31 nm; (f) The relationship between the diameter of p-CuO/n-SnO2 core-shell NWs and the number of ALD growth cycles with a slope of ˜ 0.1 nm/cycle.
components, centered at 530.0 and 531.1 eV. The peak located at 530.0 eV could be assigned to oxygen in the CuO and SnO2 crystal lattice. The higher binding energy at 531.1 eV is usually attributed to the surficial adsorbed oxygen [28]. Besides, the XPS results of pristine CuO NWs shown in Fig. S3(b)–(d) demonstrate the well-defined CuO composition.
cassiterite SnO2 phase (PDF#41-1445). Besides, two additional weak peaks at 37.9° and 57.8° were detected as well, further confirming the cassiterite structure of SnO2. With the increasing thickness of the SnO2 layer, the intensity of the SnO2 characteristic diffraction peaks gradually increases. Meanwhile, there is almost no offset in the position of the characteristic diffraction peaks as the thickness of the SnO2 layer increases, implying the lattice mismatch between the CuO core NWs and the SnO2 shells is relatively small. In addition, any other impurity peak has not been found in the XRD patterns, which confirms the phase purity of the prepared samples. Figs. 4(b)–(d) and S3(a) typically display the XPS spectra of the sample C/S24. The whole survey spectrum in Fig. S3(a) simply shows the peak positions of copper, tin and oxygen in the CuO/SnO2 core-shell structure. The detailed XPS spectrum of Cu 2p in CuO (Fig. 4(b)) displays two distinct peaks at binding energies of 934.1 and 954.0 eV, corresponding to the Cu 2p3/2 and Cu 2p1/2 core levels, respectively. Additionally, there are two strong satellite peaks nearby, which is a characteristic of the CuO phase [28]. Specifically, the Sn 3d XPS spectrum (Fig. 4(c)) of the sample exhibits two separate peaks centered at 486.9 and 495.3 eV, which are corresponding to the characteristic Sn 3d5/2 and Sn 3d3/2 peaks, respectively. The results are identical with the typical binding energies of Sn in SnO2. The asymmetric O 1s peak of C/ S24 (Fig. 4(d)) was coherently fitted by two nearly Gaussian
3.2. Gas sensing performance To evaluate the gas sensing performance of the prepared heterostructured p-CuO/n-SnO2 core-shell NWs, the samples with different SnO2 shell thicknesses were dropped on the MEMS heating appliances and fabricated into MEMS-based gas sensors (Fig. 1). Moreover, the sensing properties of all the samples were measured using the JF02 F gas sensing measurement system. Typically, Fig. 5(a) shows the dynamic response curves of all the samples with varying shell thicknesses under the operating temperature of 250 °C, obtained for the reducing gas of HCHO with various concentrations ranging from 50 ppm to 1.5 ppm. Apparently, the response of all samples has turned out to be reduced as the HCHO concentration decreases, which is thoroughly presented in Table S1. More importantly, compared with the pristine CuO NW gas sensor (i.e. sample C/S0), the designed heterostructured pCuO/n-SnO2 core-shell NW structured gas sensors do exhibit different 236
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Fig. 3. TEM characterization of the p-CuO/nSnO2 core-shell NWs sample C/S24: (a) Lowmagnification and (b) high-magnification of a single core-shell NW; (c) HRTEM image obtained at the core-shell interface and the SAED pattern (inset) of a single core-shell NW; (d) HAADF-STEM image of a single core-shell NW and the corresponding EDS line scanning profiles of the Cu (red), O (blue) and Sn (green) elements along the orange line across the coreshell interface (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
continues to raise with the increasing temperature, resulting in a decline of the net adsorption rate. Secondly, as is generally known, the diffusion rate of the HCHO adsorbed on the C/S24 material surface will improve with the increasing temperature, leading to a deeper range for reactions between HCHO and C/S24. So considering above two factors comprehensively, when the operating temperature is 250 °C, the kinetics of the gas adsorption/desorption and diffusion achieves a superior equilibrium, leading to the best gas sensing performance of C/ S24. To investigate the time dependence of response, Fig. 5(c) shows the transient responses of the sample C/S24 exposed to 6 ppm HCHO at 250 °C. The response time and recovery time were measured as the time taken for the sensor output from 10% to 90% of the highest response variation. The response time of C/S24 is 52 s, which obviously exhibits a better response property in comparison to the pristine CuO NW sensor (i.e. 132 s, Fig. S4). Similarly, the recovery time of C/S24 (i.e. 80 s) is shorter than that of C/S0 (i.e. 164 s, Fig. S4). It indicates that comparing with pristine CuO sensors, the p-CuO/n-SnO2 core-shell structure exhibits rapider response when exposed to HCHO gas at the same concentration. At the same time, the p-CuO/n-SnO2 core-shell NW sensors possess a better recovery property when turning off the gas and flushing with air. From the viewpoint of practical application, a good selectivity is also a key parameter to a gas sensor, especially for distinguishing the given target gas from complex atmosphere [34]. Herein, the selectivity of the C/S24 sensor was studied towards HCHO and other various reducing gases (50 ppm) such as CH3COCH3, C7H8 and NH3, at an operating temperature of 250 °C, which is shown in Figs. 5(d) and S5. It can be observed apparently that the response of C/S24 to HCHO is significantly higher than other reducing gases. Generally, the selectivity of the sensor is affected by various factors, such as the bond dissociation energies of gas molecules and the adsorption capability on the surface
gas sensing performance, which is closely related to the thickness of the SnO2 shell. Specifically, when the HCHO concentration is fixed to 50 ppm, the response of the p-n core-shell samples keeps growing as the thickness of SnO2 increases from 12 nm to 24 nm (i.e. C/S12, C/S18 and C/S24). The sample C/S24 yielded a maximum response of 2.42, whose rate of resistance change (i.e. 1.42) is 3 times higher than that of the sample C/S0 (i.e. 0.48). However, the response begins to decrease as the thickness of SnO2 further increases above 24 nm (i.e. C/S31). What’s more, when the shell thickness is too thick (i.e. 31 nm of C/S31), the core-shell NW structured gas sensor possesses a worse performance than the pristine CuO NW sensor, which may be attributed to the lower sensitivity of pristine SnO2 than pristine CuO to HCHO. Table 1 [29–31] presents the sensing performance comparison of the prepared C/S24 sensor with other various CuO NWs-based gas sensors fabricated on MEMS. It can be found that when operating temperature and response values are considered comprehensively, the C/S24 sensor shows improved performance among the MEMS-type CuO NWs-based gas sensors. It has been widely demonstrated that the response of a sensing material is greatly affected by the operating temperature. As it can be seen from Fig. 5(b), the optimal operating temperature for C/S24 is 250 °C, and the response of C/S24 under other operating temperatures lower (i.e. 150 °C and 200 °C) or higher (i.e. 300 °C) than 250 °C has been turned out to be lower. Obviously, the operating temperature will affect the kinetics of the gas adsorption as well as the kinetics of diffusions, leading to the alteration of the material resistance [32,33]. Firstly, the kinetics of the gas adsorption is mainly affected by two factors, including adsorption rate and desorption rate. When the temperature gradually rises, the adsorption rate as well as discernible sensing variations will be increased. However, when the temperature exceeds 250 °C, the adsorption rate becomes saturated due to the limited oxygen vacancies on the material surface, but the desorption rate 237
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Fig. 4. (a) XRD patterns of p-CuO/n-SnO2 core-shell NWs with different shell thicknesses including C/S0, C/S12, C/S18, C/S24 and C/S31; (b)-(d) XPS survey spectra of the p-CuO/n-SnO2 core-shell NWs sample C/S24, namely (b) Cu 2p spectrum, (c) Sn 3d spectrum and (d) O 1s spectrum.
the material as typically shown in following Eqs. (1) and (2):
of sensing materials [23,35]. Compared with other gases, the smaller band dissociation energy of HCHO will make the reactions easier during chemical adsorption [23]. Besides, the molecular size of HCHO is relatively small among these gas molecules, which may contribute to a larger adsorption capacity as well as a better adsorption capability. In addition, maybe the first-principles calculations based on the density functional theory can provide accurate energetic and electronic properties of p-CuO/n-SnO2 NWs and further explain the selectivity towards HCHO according to the adsorption behaviours on the surface [36]. This result clearly exhibits the excellent selectivity of the heterostructured pCuO/n-SnO2 core-shell sensors for HCHO at 250 °C, confirming it is a promising material for sensitive and selective detection of HCHO gas.
HCHO + O(−ads) → HCOOH + e−
(1)
HCHO + 2O(−ads) → H2 O + CO2 + 2e−
(2)
Therefore, the resistance of the n-type sensor will achieve a rapid decrease when exposed to the reducing gas, which is in accordance with the experimental results shown in Fig. S6(c)–(f). Conversely, for a ptype sensor, it will show a relatively low resistance when exposed to air for the reason that holes are dominant carriers. When introducing the reducing gas of HCHO, the release of electrons will cause a decrease in the hole concentration in p-type semiconductor due to the electron-hole recombination, thus leading to an increase in the material resistance (Fig. S6(b)). The three-dimensional schematic of the p-n core-shell NWs exposed in HCHO gas is shown in Fig. 6(a). The experimental results (Fig. 5(a)) observably show that the sensing properties of CuO-based sensors were greatly enhanced when introducing a favorable thickness of SnO2 shell layer. The formation of the p-n heterojunction at the interface of the CuO/SnO2 core-shell structure satisfactorily account for the enhancement of gas sensing performance [38,39]. In general, CuO mainly shows p-type conductivity by holes [40] and SnO2 displays n-type conductivity by electrons [24], which are clearly exhibited in the band alignment diagram of Fig. 6(b). When the SnO2 shell film was deposited onto CuO NWs, the electrons in SnO2 and the holes in CuO would diffuse in opposite directions due to the great gradient of the carrier concentration, leading to the formation of the p-n heterojunction in thermal equilibrium (Fig. 6(c)). When the pn heterojunction was exposed to air, adsorbed oxygen would also capture the electrons from the conduction of p-CuO, thus actually leading to an increased concentration of holes according to the Law of mass action. Thus, the potential barrier height of the heterojunction depletion layer would have a rise due to the increase of concentration
3.3. Gas sensing mechanisms It was observed that the pristine p-type CuO NWs based sensor shows an increase in the resistance while the CuO/SnO2 core-shell NWs based sensors show a sudden decrease when introducing the reducing gas of HCHO (Fig. S6), indicating that the composite structure exhibits typical n-type conductivity behaviors [22]. According to the generally acknowledged surface reaction mechanism for semiconductor-based resistance-type gas sensors, the chemisorption of oxygen like O2−, O22−, O− or O2−, and the chemical reaction between adsorbed oxygen and gas molecules on the material surface are thought to be the cause of resistance change [37]. Initially, when an n-type sensor was exposed to air, the physisorbed oxygen captured electrons from the conduction band of sensing materials and turned into chemisorbed, resulting in forming an electron-depleted region underneath the material surface and exhibiting the high resistance state. Once the n-type sensor was exposed to the reducing gas of HCHO, the chemisorbed oxygen species would react with HCHO on the surface and release the electrons back to 238
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Fig. 5. (a) Dynamic response curves of all the samples (i.e. C/S0, C/S12, C/S18, C/S24 and C/S31) with varying shell thicknesses facing the reducing gas of HCHO with various concentrations ranging from 50 ppm to 1.5 ppm under the operating temperature of 250 °C; (b) Response of the sample C/S24 at different operating temperatures (i.e. 150 °C, 200 °C, 250 °C and 300 °C) facing the reducing gas of HCHO with various concentrations ranging from 50 ppm to 1.5 ppm; (c) Enlarged response of the sample C/S24 facing 6 ppm HCHO at 250 °C with response time of 52 s and recovery time of 80 s; (d) Response of the sample C/S24 facing other various reducing gases (50 ppm), namely acetone, methylbenzene and ammonia, compared with 50 ppm HCHO at the operating temperature of 250 °C.
Table 1 Sensing performance comparison of various CuO NWs-based gas sensors fabricated on MEMS. Material
Target gas
Response (Ra/Rg or Rg/Ra)
Operating temperature
Synthetic method
CuO [29] CuO [30] CuO [31] CuO/SnO2 (this work)
C2H5OH (500 ppm) CO (30 ppm) CO (100 ppm) HCHO (50 ppm)
1.5 1.36 1.2 2.42
220 °C 325 °C 300 °C 250 °C
Thermal Thermal Thermal Solution
oxidation oxidation oxidation processing & ALD
acknowledged theory combining the effects of the radial modulation of the electron-depleted layer and the volume fraction of the electrondepleted layer in the core-shell structure [26], the optimal shell thickness is nearly identical to the Debye length (λD) [22], which is constant for a given material at a specific temperature and carrier concentration. The Debye length can be expressed as Eq. (3) [43]:
gradient on both sides of the p-n heterojunction [41]. Therefore, the resistance of n-type p-CuO/n-SnO2 core-shell NW sensors achieved a further increase in air due to the additional existence of the depletion region formed at the CuO/SnO2 p-n heterojunction interface. Correspondingly, when the p-CuO/n-SnO2 core-shell NW sensors were exposed to the reducing gas of HCHO, not only the electron-depleted region below the surface but also the depletion layer at the p-n heterojunction interface became thinner (Fig. 6(d)), further decreasing the resistance of the core-shell NW sensors as well as improving the response which is defined as Rair/Rgas. In brief, the formation of the p-n heterojunction greatly increases the resistance of the composite in air and further decreases the resistance of the composite in HCHO gas, leading to a significant improvement of the sensing response. In addition, the existence of the p-n heterojunction will help improve the holeelectron separation rate at the interface, making electrons more easily captured by the chemisorbed oxygen [42]. Therefore, the recovery time will be reduced to some extent. Remarkably, as is shown in Fig. 5(a), the response of the sensors is highly dependent on the shell thickness. On the basis of the mostly
λD
εkT q 2n c
(3) −12
F/m where ε is the static dielectric constant (i.e. ˜14 × 8.85 × 10 for SnO2), k is the Boltzmann constant (i.e. 1.38 × 10−23 J/K), T is the absolute temperature (i.e. 523 K), q is the electron charge (i.e. 1.6 × 10−19 C), and nc is the carrier concentration (i.e. 5.71 × 1016 cm−3, whose value is obtained by Hall measurement for the SnO2 thin film prepared on the Si substrate by ALD). Hence, the λD value for 250 °C was calculated to be around 24.7 nm, which is in accordance with the experimental results (i.e. 24 nm). When the shell layer is close to the Debye length, the electrons in the SnO2 shell will be fully depleted by the combined effect of the chemisorbed oxygen on the 239
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Fig. 6. Schematics of the reducing gas sensing mechanism in the p-CuO/n-SnO2 core-shell NW gas sensors with a shell layer close to the Debye length: (a) The threedimensional schematic of the p-n core-shell NWs exposed in HCHO gas; The schematic energy band diagrams for the p-n core-shell NWs (b) in separate state and (c) in air; (d) Three-dimensional sensing mechanism diagrams of the carrier distribution in the p-CuO/n-SnO2 core-shell NWs with a shell layer close to the Debye length when exposed in air and in HCHO gas. The band structure data in figure (b) were determined from the literature [24,40].
demonstrated when comparing with pristine CuO NW gas sensors. Specifically, the p-CuO/n-SnO2 core-shell NW sensors with a SnO2 shell layer of 24 nm on CuO achieved a high sensitivity (Ra/Rg) of 2.42, whose rate of resistance change (i.e. 1.42) is around 3 times higher than the pristine CuO NW sensor when exposed to 50 ppm HCHO at 250 °C. In addition, the p-CuO/n-SnO2 core-shell NW gas sensors exhibited excellent selectivity for HCHO from commonly occurred reducing gases. Based on these results, it is believed that our precisely controlled heterostructured p-CuO/n-SnO2 core-shell NWs hold great potential for highly sensitive and selective HCHO gas sensors.
n-shell and the p-n heterojunction, as shown in Fig. 6(d). However, if the shell layer is much thinner than λD, although the shell layer will be completely electron-depleted as well, the resistance variation rate for the entire p-n core-shell NWs still remains relatively minor considering the small portion of the shell layer in the entire core-shell structure (Fig. S7(a)). On the other hand, although the p-n core-shell NWs with a shell layer thicker than λD will have an electron-depletion with a width close to λD, it is just partially depleted in shell layers as is described schematically in Fig. S7(b). Besides, the depletion layer at the p-n heterojunction interface is considered to be ineffective in view of the limitation of the diffusion depth. Therefore, it is essential to achieve superior gas sensing properties in the p-n core-shell NWs with a shell layer whose thickness is close to λD.
Competing financial interest The authors declare no competing financial interest.
4. Conclusions Acknowledgements In conclusion, highly sensitive and selective gas sensors based on structurally well-defined p-CuO/n-SnO2 core-shell NWs with ALD-confined shell thickness of SnO2 have been synthesized for the first time in this work. As a result, significantly improved sensing properties of the heterostructured p-CuO/n-SnO2 core-shell NW gas sensors have been
This work is supported by the National Key R&D Program of China (No. 2016YFE0110700), National Natural Science Foundation of China (No. U1632121, 51861135105, and 61874034), and Natural Science Foundation of Shanghai (No. 18ZR1405000). 240
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Appendix A. Supplementary data
[22] J.H. Kim, A. Katoch, S.S. Kim, Optimum shell thickness and underlying sensing mechanism in p-n CuO-ZnO core-shell nanowires, Sens. Actuators, B 222 (2016) 249–256, https://doi.org/10.1016/j.snb.2015.08.062. [23] Y.X. Li, N. Chen, D.Y. Deng, X.X. Xing, X.C. Xiao, Y.D. Wang, Formaldehyde detection: SnO2 microspheres for formaldehyde gas sensor with high sensitivity, fast response/recovery and good selectivity, Sens. Actuators, B 238 (2017) 264–273, https://doi.org/10.1016/j.snb.2016.07.051. [24] F. Li, X. Gao, R. Wang, T. Zhang, Design of WO3-SnO2 core-shell nanofibers and their enhanced gas sensing performance based on different work function, Appl. Surf. Sci. 442 (2018) 30–37, https://doi.org/10.1016/j.apsusc.2018.02.122. [25] B.W. Zhang, W.Y. Fu, H.Y. Li, X.L. Fu, Y. Wang, H. Bala, G. Sun, X.D. Wang, Y. Wang, J.L. Cao, Z.Y. Zhang, Actinomorphic ZnO/SnO2 core-shell nanorods: twostep synthesis and enhanced ethanol sensing propertied, Mater. Lett. 160 (2015) 227–230, https://doi.org/10.1016/j.matlet.2015.07.122. [26] J.H. Kim, S.S. Kim, Realization of ppb-scale toluene-sensing abilities with Pt-functionalized SnO2-ZnO core-shell nanowires, ACS Appl. Mater. Interfaces 7 (2015) 17199–17208, https://doi.org/10.1021/acsami.5b04066. [27] J. Bai, Y.P. Li, R. Wang, K. Huang, Q.Y. Zeng, J.H. Li, B.X. Zhou, A novel 3D ZnO/ Cu2O nanowire photocathode material with highly efficient photoelectrocatalytic performance, J. Mater. Chem. A 3 (2015) 22996–23002, https://doi.org/10.1039/ c5ta07583a. [28] M.H. Li, H.C. Zhu, J. Cheng, M.M. Zhao, W.P. Yan, Synthesis and improved ethanol sensing performance of CuO/SnO2 based hollow microspheres, J. Porous Mater. 24 (2017) 507–518, https://doi.org/10.1007/s10934-016-0286-9. [29] P. Raksa, A. Gardchareon, T. Chairuangsri, P. Mangkorntong, N. Mangkorntong, S. Choopun, Ethanol sensing properties of CuO nanowires prepared by an oxidation reaction, Ceram. Int. 35 (2009) 649–652, https://doi.org/10.1016/j.ceramint. 2008.01.028. [30] S. Steinhauer, A. Chapelle, P. Menini, M. Sowwan, Local CuO nanowire growth on microhotplates: in situ electrical measurements and gas sensing application, ACS Sens. 1 (2016) 503–507, https://doi.org/10.1021/acssensors.6b00042. [31] S. Steinhauer, E. Brunet, T. Maier, G.C. Mutinati, A. Köck, O. Freudenberg, C. Gspan, W. Grogger, A. Neuhold, R. Resel, Gas sensing properties of novel CuO nanowire devices, Sens. Actuators, B 187 (2013) 50–57, https://doi.org/10.1016/j. snb.2012.09.034. [32] G. Korotcenkov, Metal oxides for solid-state gas sensors: what determines our choice? Mater. Sci. Eng. B-Adv. 139 (2007) 1–23, https://doi.org/10.1016/j.mseb. 2007.01.044. [33] N. Joshi, L.F. da Silva, H.S. Jadhav, F.M. Shimizu, P.H. Suman, J.C. M’Peko, M.O. Orlandi, J.G. Seo, V.R. Mastelaro, O.N. Oliveira, Yolk-shelled ZnCo2O4 microspheres: surface properties and gas sensing application, Sens. Actuators, B 257 (2018) 906–915, https://doi.org/10.1016/j.snb.2017.11.041. [34] Z.D. Lin, N. Li, Z. Chen, P. Fu, The effect of Ni doping concentration on the gas sensing properties of Ni doped SnO2, Sens. Actuators, B 239 (2017) 501–510, https://doi.org/10.1016/j.snb.2016.08.053. [35] M.A. Ashraf, Z.L. Liu, W.X. Peng, Z. Parsaee, Design, preparation and evaluation of a high performance sensor for formaldehyde based on a novel hybride nonocomposite ZnWO3/rGO, Anal. Chim. Acta 1051 (2019) 120–128, https://doi.org/ 10.1016/j.aca.2018.11.014. [36] S.S. Li, Z.S. Lu, Z.X. Yang, X.L. Chu, The sensing mechanism of Pt-doped SnO2 surface toward CO: a first-principle study, Sens. Actuators, B 202 (2014) 83–92, https://doi.org/10.1016/j.snb.2014.05.071. [37] S.H. Yan, S.Y. Ma, W.Q. Li, X.L. Xu, L. Cheng, H.S. Song, X.Y. Liang, Synthesis of SnO2-ZnO heterostructured nanofibers for enhanced ethanol gas-sensing performance, Sens. Actuators, B 221 (2015) 88–95, https://doi.org/10.1016/j.snb.2015. 06.104. [38] C. Liu, L.P. Zhao, B.Q. Wang, P. Sun, Q.J. Wang, Y. Gao, X.S. Liang, T. Zhang, G.Y. Lu, Acetone gas sensor based on NiO/ZnO hollow spheres: fast response and recovery, and low (ppb) detection limit, J. Colloid Interface Sci. 495 (2017) 207–215, https://doi.org/10.1016/j.jcis.2017.01.106. [39] J.H. Kim, A. Katoch, S.H. Kim, S.S. Kim, Chemiresistive sensing behavior of SnO2 (n)-Cu2O (p) core-shell nanowires, ACS Appl. Mater. Interfaces 7 (2015) 15351–15358, https://doi.org/10.1021/acsami.5b03224. [40] S. Paul, J. Sultana, A. Bhattacharyya, A. Karmakar, S. Chattopadhyay, Investigation of the comparative photovoltaic performance of n-ZnO nanowire/p-Si and n-ZnO nanowire/p-CuO heterojunctions grown by chemical bath deposition method, Optik 164 (2018) 745–752, https://doi.org/10.1016/j.ijleo.2018.03.076. [41] D.X. Ju, H.Y. Xu, Q. Xu, H.B. Gong, Z.W. Qiu, J. Guo, J. Zhang, B.Q. Cao, High trimethylamine-sensing properties of NiO/SnO2 hollow sphere P-N heterojunction sensors, Sens. Actuators, B 215 (2015) 39–44, https://doi.org/10.1016/j.snb.2015. 03.015. [42] L.J. Wang, H.J. Zhai, G. Jin, X.Y. Li, C.W. Dong, H. Zhang, B. Yang, H.M. Xie, H.Z. Sun, 3D porous ZnO-SnS p-n heterojunction for visible light driven photocatalysis, Phys. Chem. Chem. Phys. 19 (2017) 16576–16585, https://doi.org/10. 1039/c7cp01687e. [43] S.W. Choi, A. Katoch, J.H. Kim, S.S. Kim, Striking sensing improvement of n-type oxide nanowires by electronic sensitization based on work function difference, J. Mater. Chem. C 3 (2015) 1521–1527, https://doi.org/10.1039/c4tc02057j.
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.03.092. References [1] L. Zhu, Y.Q. Li, W. Zeng, Hydrothermal synthesis of hierarchical flower-like ZnO nanostructure and its enhanced ethanol gas-sensing properties, Appl. Surf. Sci. 427 (2018) 281–287, https://doi.org/10.1016/j.apsusc.2017.08.229. [2] H.L. Tian, H.Q. Fan, M.M. Li, L.T. Ma, Zeolitic imidazolate framework coated ZnO nanorods as molecular sieving to improve selectivity of formaldehyde gas sensor, ACS Sens. 1 (2016) 243–250, https://doi.org/10.1021/acssensors.5b00236. [3] J. Zhang, X.H. Liu, G. Neri, N. Pinna, Nanostructured materials for room-temperature gas sensors, Adv. Mater. 28 (2016) 795–831, https://doi.org/10.1002/ adma.201503825. [4] A. Mirzaei, S.G. Leonardi, G. Neri, Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: a review, Ceram. Int. 42 (2016) 15119–15141, https://doi.org/10.1016/j.ceramint.2016.06.145. [5] Y.J. Jeong, W.T. Koo, J.S. Jang, D.H. Kim, H.J. Cho, I.D. Kim, Chitosan-templated Pt nanocatalyst loaded mesoporous SnO2 nanofibers: a superior chemiresistor toward acetone molecules, Nanoscale 10 (2018) 13713–13721, https://doi.org/10.1039/ c8nr03242d. [6] M.R. Alenezi, S.J. Henley, N.G. Emerson, S.R.P. Silva, From 1D and 2D ZnO nanostructures to 3D hierarchical structures with enhanced gas sensing properties, Nanoscale 6 (2014) 235–247, https://doi.org/10.1039/c3nr04519f. [7] M. Bao, Y.J. Chen, F. Li, J.M. Ma, T. Lv, Y.J. Tang, L.B. Chen, Z. Xu, T.H. Wang, Plate-like p-n heterogeneous NiO/WO3 nanocomposites for high performance room temperature NO2 sensors, Nanoscale 6 (2014) 4063–4066, https://doi.org/10. 1039/c3nr05268k. [8] B. Urasinska-Wojcik, J.W. Gardner, H2S sensing in dry and humid H2 environment with p-type CuO thick-film gas sensors, IEEE Sens. J. 18 (2018) 3502–3508, https:// doi.org/10.1109/JSEN.2018.2811462. [9] Y.J. Chen, F.N. Meng, H.L. Yu, C.L. Zhu, T.S. Wang, P. Gao, Q.Y. Ouyang, Sonochemical synthesis and ppb H2S sensing performances of CuO nanobelts, Sens. Actuators, B 176 (2013) 15–21, https://doi.org/10.1016/j.snb.2012.08.007. [10] J.H. Kim, A. Katoch, S.W. Choi, S.S. Kim, Growth and sensing properties of networked p-CuO nanowires, Sens. Actuators, B 212 (2015) 190–195, https://doi.org/ 10.1016/j.snb.2014.12.081. [11] J. Hu, C. Zou, Y.J. Su, M. Li, Y.T. Han, E.S.W. Kong, Z. Yang, Y.F. Zhang, An ultrasensitive NO2 gas sensor based on a hierarchical Cu2O/CuO mesocrystal nanoflower, J. Mater. Chem. A 6 (2018) 17120–17131, https://doi.org/10.1039/ c8ta04404j. [12] C. Yang, F. Xiao, J.D. Wang, X.T. Su, 3D flower- and 2D sheet-like CuO nanostructures: microwave-assisted synthesis and application in gas sensors, Sens. Actuators, B 207 (2015) 177–185, https://doi.org/10.1016/j.snb.2014.10.063. [13] H.J. Park, N.J. Choi, H. Kang, M.Y. Jung, J.W. Park, K.H. Park, D.S. Lee, A ppb-level formaldehyde gas sensor based on CuO nanocubes prepared using a polyol process, Sens. Actuators, B 203 (2014) 282–288, https://doi.org/10.1016/j.snb.2014.06. 118. [14] Y.F. Wang, F.D. Qu, J. Liu, Y. Wang, J.R. Zhou, S.P. Ruan, Enhanced H2S sensing characteristics of CuO-NiO core-shell microspheres sensors, Sens. Actuators, B 209 (2015) 515–523, https://doi.org/10.1016/j.snb.2014.12.010. [15] S.L. Bai, W.T. Guo, J.H. Sun, J. Li, Y. Tian, A.F. Chen, R.X. Luo, D.Q. Li, Synthesis of SnO2-CuO heterojunction using electrospinning and application in detecting of CO, Sens. Actuators, B 226 (2016) 96–103, https://doi.org/10.1016/j.snb.2015.11.028. [16] C. Wang, J.W. Zhu, S.M. Liang, H.P. Bi, Q.F. Han, X.H. Liu, X. Wang, Reduced graphene oxide decorated with CuO-ZnO hetero-junctions: towards high selective gas-sensing property to acetone, J. Mater. Chem. A 2 (2014) 18635–18643, https:// doi.org/10.1039/c4ta03931a. [17] Y.B. Zhang, J. Yin, L. Li, L.X. Zhang, L.J. Bie, Enhanced ethanol gas-sensing properties of flower-like p-CuO/n-ZnO heterojunction nanorods, Sens. Actuators, B 202 (2014) 500–507, https://doi.org/10.1016/j.snb.2014.05.111. [18] J.A. Deng, L.L. Wang, Z. Lou, T. Zhang, Design of CuO-TiO2 heterostructure nanofibers and their sensing performance, J. Mater. Chem. A 2 (2014) 9030–9034, https://doi.org/10.1039/c4ta00160e. [19] D.R. Miller, S.A. Akbar, P.A. Morris, Nanoscale metal oxide-based heterojunctions for gas sensing: a review, Sens. Actuators, B 204 (2014) 250–272, https://doi.org/ 10.1016/j.snb.2014.07.074. [20] Y. Zhang, H.L. Lu, T. Wang, Q.H. Ren, Y.Z. Gu, D.H. Li, D.W. Zhang, Facile synthesis and enhanced luminescent properties of ZnO/HfO2 core-shell nanowires, Nanoscale 7 (2015) 15462–15468, https://doi.org/10.1039/c5nr03656a. [21] K.P. Yuan, Q. Cao, H.L. Lu, M. Zhong, X.Z. Zheng, H.Y. Chen, T. Wang, J.J. Delaunay, W. Luo, L.W. Zhang, Y.Y. Wang, Y.H. Deng, S.J. Ding, D.W. Zhang, Oxygen-deficient WO3-x@TiO2-x core-shell nanosheets for efficient photoelectrochemical oxidation of neutral water solutions, J. Mater. Chem. A 5 (2017) 14697–14706, https://doi.org/10.1039/c7ta03878j.
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Fabrication of heterostructured p-CuO/n-SnO2 core-shell nanowires for enhanced sensitive and selective formaldehyde detection
T
Li-Yuan Zhua,1, Kaiping Yuana,1, Jian-Guo Yanga, Hong-Ping Maa, Tao Wanga, Xin-Ming Jia, ⁎ Ji-Jun Fengb, Anjana Devic, Hong-Liang Lua, a
State Key Laboratory of ASIC and System, Shanghai Institute of Intelligent Electronics & Systems, School of Microelectronics, Fudan University, Shanghai 200433, China Shanghai Key Laboratory of Modern Optical System, School of Optical-Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China c Inorganic Materials Chemistry, Ruhr-University Bochum, Bochum 44780, Germany b
A R T I C LE I N FO
A B S T R A C T
Keywords: Core-Shell Nanowire Atomic layer deposition Gas sensor CuO-SnO2 Formaldehyde
Highly sensitive and selective gas sensors based on heterostructured p-CuO/n-SnO2 core-shell nanowires (NWs) with precisely controlled shell thickness were synthesized through a sequential process combining a solution processing and atomic layer deposition. The gas sensing devices were fabricated on micro-electromechanical systems, which has triggered great research interest for low power consumption and highly integrated design. The designed p-CuO/n-SnO2 core-shell NW structured gas sensors exhibited superior gas sensing performance, which is closely related to the thickness of the SnO2 shell. Specifically, p-CuO/n-SnO2 core-shell NWs with a 24 nm thick SnO2 shell displayed a high sensitivity (Ra/Rg) of 2.42, whose rate of resistance change (i.e. 1.42) is 3 times higher than the pristine CuO NW sensor when detecting 50 ppm formaldehyde (HCHO) at 250 °C. The enhanced gas sensing performance could be attributed to the formation of p-n heterojunction which was revealed by specific band alignment and the heterojunction-depletion model. Besides, the well-structured p-CuO/n-SnO2 core-shell NWs achieved excellent selectivity for HCHO from commonly occurred reducing gases. In a word, such heterostructured p-CuO/n-SnO2 core-shell NW gas sensors demonstrate a feasible approach for enhanced sensitive and selective HCHO detection.
1. Introduction Recently, semiconductor-based gas sensors have attracted tremendous research interest and already been widely applied to diverse application fields like gas leakage alarm, environment gas monitor, industrial gas analysis and so on [1,2]. During the past few years, significant research efforts have been made to fabricate various novel gas sensors based on metal oxide semiconductor materials with high specific surface area, excellent adsorption capacity and high carrier mobility [3–7]. Among various metal oxide semiconductor materials, copper oxide (CuO) is an excellent p-type multifunctional semiconductor, which has been extensively investigated for high performance gas sensors [8,9]. In order to improve the performance of CuObased gas sensors, substantial efforts have been focused on the design and fabrication of different nanostructures, such as nanowires, nanoflowers and nanocubes [10–13]. For example, Park et al. fabricated and characterized a ppb-level formaldehyde (HCHO) gas sensor based on
CuO nanocubes which were synthesized through a wet, facile and massproducible polyol process [13]. Kim et al. synthesized networked p-CuO nanowires (NWs) on patterned-electrode pads by thermal oxidation and demonstrated that the networked CuO NWs are comparable sensors detecting reducing gases when compared with networked n-SnO2 NWs [10]. However, the gas sensing performance of CuO is limited by its unsatisfactory stability and low response, which can not meet the demand of developing target gas monitor technologies [14]. Fabrication of composite structures is widely regarded as an effective strategy to improve the performance of gas sensors due to the collection of various properties of different materials. A considerable and widely accepted approach is to construct core-shell heterostructures consisted of two or more kinds of semiconductor materials, especially for the p-n heterostructures like CuO/SnO2, CuO/ZnO, CuO/TiO2, and so on [15–18]. For instance, the flower-like p-CuO/n-ZnO heterojunction nanorod sensor reported by Zhang et al. displayed a response 2.5 times higher than the
⁎
Corresponding author. E-mail address: [email protected] (H.-L. Lu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.snb.2019.03.092 Received 20 January 2019; Received in revised form 20 March 2019; Accepted 21 March 2019 Available online 30 March 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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2.2. Synthesis of heterostructured p-CuO/n-SnO2 core-shell NWs
pristine sample when detecting 100 ppm ethanol [17]. The formation of p-n core-shell heterostructures will introduce an additional wide depletion region, leading to a significant increase of the material resistance [19]. Besides, since the core CuO is susceptible to chemical change during exposure to surrounding environment, coating of an inert shell will significantly enhance the device stability and gas sensing response. Furthermore, significant research efforts have demonstrated that the shell thickness has an undeniable role in the gas sensing ability of the resulted core-shell materials. Therefore, to precisely control the shell thickness is of vital importance for obtaining highly sensitive core-shell gas sensing materials. It is remarkable that as a film forming technique of self-limiting growth, atomic layer deposition (ALD) technique possesses a great many significant advantages, especially for precise control of film thickness at the atomic level and uniform conformal coverage ability [20,21], which is beneficial to the accurate control of the shell thickness. For example, Kim et al. synthesized CuO/ZnO core-shell NWs with ALD technique and investigated the effect of the ZnO shell thickness on the gas sensing performance [22]. Among various gas sensing materials, n-type SnO2 is a promising candidate as a shell layer for high performance gas sensors due to its relatively large response and good stability for detecting both oxidizing and reducing gases [23]. On the other hand, compared with other metal oxide materials like ZnO and TiO2, SnO2 possesses a much larger work function (˜4.9 eV) [24], thus leading to an easier approach for reducing gases to be absorbed. For instance, Zhang et al. reported a novel gas sensor based on ZnO nanorods coated with a shell film of SnO2 synthesized via an ionic-layer adsorption and reaction method, whose response to 2000 ppm ethanol vapor was 7 times higher than that of pristine ZnO based sensor [25]. Herein, in this work, a facile route for the large-scale fabrication of heterostructured p-CuO/n-SnO2 core-shell NW gas sensors with high sensitivity and fast response is proposed combining a solution processing for the growth of CuO NWs and ALD of thickness-controlled SnO2 shells on the surface of CuO NWs. Particularly, the gas sensing devices were fabricated on micro-electromechanical systems (MEMS), which have triggered great research interest for low power consumption and highly integrated design. As a result, significantly improved sensing performance of heterostructured p-CuO/n-SnO2 core-shell NW gas sensors to HCHO were demonstrated comparing to the pristine CuO NW sensor. Specifically, the core-shell nanowire-based sensor with a 24 nm thick SnO2 shell displayed a high sensitivity (Ra/Rg) of 2.42, whose rate of resistance change (i.e. 1.42) is 3 times higher than the pristine CuO NW sensor when detecting 50 ppm HCHO at 250 °C. According to the surface-depletion and heterojunction-depletion models, the optimal shell thickness could be determined by the influence of both the radial modulation of the electron-depleted layer and the volume fraction of the electron-depleted layer in the core-shell structure [26]. Moreover, the obtained CuO/SnO2 core-shell NWs exhibited an excellent selectivity towards HCHO rather than other commonly occurred reducing gases.
The synthesis of the heterostructured p-CuO/n-SnO2 core-shell NWs was achieved by first fabricating Cu(OH)2 NWs via a modified facile wet chemical method reported before [27]. Briefly, a piece of Cu foam with moderate size was vertically immersed into an aqueous solution containing 1.25 M NaOH and 0.0625 M (NH4)2S2O8 for 10 min, followed by a rinsing and drying process. Secondly, the SnO2 shells of different thickness, namely 120, 180, 240, and 300 cycles, were deposited at 150 °C on as-prepared Cu(OH)2 NWs in a BENEQ TFS-200 ALD system. Briefly, TDMASn and DI water were used as the tin (Sn) and oxidant sources, respectively. The Cu(OH)2 NWs were alternately exposed to the vapor pulse of the TDMASn and DI water precursors in the ALD reactor chamber using the high purity nitrogen gas (N2) as the carrier gas. Meanwhile, the high purity N2 as the purge gas purged the gaseous by-products and residual gas out of chamber between two valid pulses, effectively avoiding unexpected gas reactions. For all samples, the deposition process in each growth cycle briefly includes a 0.5 s pulse of TDMASn, a 10 s purge, a 0.2 s pulse of DI water and a 10 s purge. The different desired thicknesses of SnO2 shell layers can be achieved by repeating different specific ALD growth cycles. Specifically during all ALD processes, the TDMASn source was held at 45 °C for evaporation. In the third step, Cu(OH)2/SnO2 NWs were ultrasonically separated from the Cu foam, uniformly dispersed in DI water, and baked in air at 80 °C on a piece of quartz substrate. Finally, the obtained samples on the quartz substrate were calcined in air at 550 °C for 2 h with 1 °C/min of the ramping rate, converting Cu(OH)2/SnO2 NWs into desired CuO/ SnO2 NWs. 2.3. Instruments and characterization The SnO2 shell layers were deposited in a BENEQ TFS-200 ALD system (Finland). The thicknesses of the SnO2 films deposited on flat silicon substrates were measured by a SOPRA GES-5E spectroscopic ellipsometry (SE) system (Hungary). Transmission electron microscope (TEM) characterization and energy-dispersive X-ray spectroscopy (EDS) analysis were carried out on a FEI Tecnai G2 F20 S-TWIN field-emission TEM (United States) with the acceleration voltage of 200 kV. Scanning electron microscope (SEM) images were recorded on a Zeiss SIGAMA HD field-emission SEM (Germany). Wide-angle X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance powder X-ray diffractometer (Germany) with Ni-filtered Cu-Kɑ radiation (40 kV, 40 mA, 1.5406 Å). X-ray photoelectron spectroscopy (XPS) measurement was conducted on a PHI 5000 VersaProbe system (United States) using an Mg-Kα X-ray source. 2.4. Gas sensing performance measurements For the gas sensing measurements, as-prepared p-CuO/n-SnO2 coreshell NWs were scratched from the quartz substrates, uniformly dispersed into DI water, and dropped onto the micro-electromechanical systems (MEMS) heating appliances. After thoroughly dried in air at room temperature, the MEMS heating appliances with as-prepared gas sensing materials were baked in an air-circulating oven at 45 °C for 24 h. Fig. 1 exhibits the schematic diagram and an optical microscope image of the MEMS heating appliance. Then, the fabricated MEMS device was connected to the external circuit by aluminium wire bonding. The sensing characteristics of the fabricated p-CuO/n-SnO2 core-shell NW sensors for reducing gases, namely HCHO, acetone (CH3COCH3), methylbenzene (C7H8) and ammonia (NH3), was measured using a JF02F gas sensing measurement system (China). The gas sensing response (R) in the reducing-gas measurements was estimated using the equation R = Ra/Rg (for n-type semiconductors) or R = Rg/Ra (for p-type semiconductors), where Ra and Rg are the resistance of gas sensing materials in air or in the atmosphere of the target gas. The response and recovery times were defined as the time taken for the
2. Experimental section 2.1. Chemicals and reagents Standardly cleaned copper (Cu) foams were used as the substrates in this work. Analytical grade sodium hydroxide (NaOH) and ammonium persulfate ((NH4)2S2O8, APS) were obtained from Shanghai Chemical Reagent Company. The precursor materials used for depositing SnO2 layer were tetrakis(dimenthylamido) tin (Ⅳ) ([N(CH3)2]4Sn, TDMASn, sigma Aldrich, 99.9999%) and de-ionized (DI) water. Other chemical reagents employed in our experiments were analytical grade and the gases were ultra high pure (99.999%). All aqueous solutions were prepared with DI water obtained from a Millipore Q purification system (resistivity > 18MΩ cm). 234
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Fig. 1. (a) The schematic diagram and (b) an optical microscope image of the MEMS heating appliance.
sensor output from 10% to 90% of the highest response variation upon exposure to the target gas and air, respectively. Moreover, in order to figure out the optimal operating temperature, the sensing performance under different temperature was investigated as well.
shell thickness shows a continuous increase when increasing the total number of ALD growth cycles with a slope of ˜0.1 nm/cycle. The corresponding sample are denoted as C/SX in this work, where X = 0, 12, 18, 24 and 31, representing the thicknesses of the SnO2 shell layers, respectively. Besides, it is notable that some of the single CuO NWs are relatively close to and even in contact with each other. Therefore, as it can be seen in Figs. 2(b)–(e) and S1(d), two or more CuO NWs were covered by SnO2 shell coverage fabricating one singe core-shell CuO/ SnO2 structure, which further confirming the excellent conformality of the ALD technique. The well-defined core-shell structure is further revealed by TEM characterization and EDS analysis results obtained on a typical sample C/S24 (Fig. 3(a)–(d)). The bright-field TEM and high-resolution TEM (HRTEM) images indicate the core-shell structured CuO/SnO2 NW with a uniform shell thickness of around 24 nm, which is consistent with the SEM results. As a contrast, the HRTEM image of the single pristine CuO NW is showed in Fig. S1(b), in which no shell layer could be found at all. A highly magnified TEM image (Fig. 3(b)) indicates somewhat roughness on the shell surface with a weak evidence of grains, further increasing the specific surface area for gas adsorbing. The HRTEM image (Fig. 3(c)) and selected-area electron diffraction (SAED) pattern (Fig. 3(c) inset) of the sample C/S24 reveal that the CuO core of the nanowire have a multi-crystalline structure. The lattice fringes exhibit 0.275 nm in the core and 0.334 nm in the shell region, which correspond to the d-spacing values of the (110) plane of tenorite CuO and (110) plane of cassiterite SnO2 phase, respectively. Fig. 3(d) further displays the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the sample C/S24 along with the collected elemental line scanning profiles. Obviously, the SnO2 shell looks brighter than the inside CuO core, which could be attributed to the much larger atomic number (Z) of Sn (50) than Cu (29). In this regards, the core-shell structure could be identified by this distinct Zcontrast as well. Meanwhile, the elemental line scanning profiles also confirm that the element Cu can be detected only in the central region, revealing the fact that CuO only exists in the core. The O and Sn elements were detected in both central and marginal regions while the concentration of Sn in the central is much lower in contrast to the marginal region, which further verifies the well-defined core-shell structure of the CuO/SnO2 NWs. For comparison, the HAADF-STEM image of the single pristine CuO NW is displayed in Fig. S1(c), in which only the Cu and O elements were detected in the overall NW during the elemental line scanning. The crystal structures of CuO/SnO2 core-shell NWs were measured by XRD, as shown in Fig. 4(a) (sample C/S0, C/S12, C/S18, C/S24 and C/S30). It can be seen that the diffraction peaks of pristine CuO NWs (i.e. sample C/S0) could be well indexed into a tenorite CuO phase (PDF#45-0937). All the samples show two characteristic diffraction peaks at 35.5° and 38.7°, which is corresponding to the (002) and (111) planes of the tenorite structure of CuO, respectively. Compared to the sample C/S0, other samples obtained after the ALD-SnO2 reaction display extra obvious peaks at 26.6° and 33.9°, which belong to the
3. Results and discussion 3.1. Microstructure characterization Scheme 1 illustrates the synthetic protocol of the heterostructured pCuO/n-SnO2 core-shell NWs based on the ALD process. Firstly, wellaligned Cu(OH)2 NWs were obtained on a piece of pre-cleaned Cu foam via a modified facile wet chemical method. After rinsing and drying, a uniform and light blue thin film could be seen from the surface of the Cu foam. The SEM image in Fig. S1(a) of the supporting information confirms that this film is composed of vertically aligned Cu(OH)2 NWs with a diameter of ˜132 nm. Secondly, these Cu(OH)2 NWs were conformably coated by amorphous SnO2 layers with different thicknesses through thermal ALD at 150 °C. After subsequent annealing in air, the reddish-brown core-shell structured CuO/SnO2 NWs were formed. The displayed SEM images (Fig. 2(b)–(e)) indicate the as-prepared CuO/ SnO2 NWs morphologies with uniform and conformal SnO2 shell layers. For comparison, the pristine CuO NWs without a SnO2 shell layer is representatively shown in Fig. 2(a). The average diameter of the pristine CuO NWs is measured to be ˜124 nm, which exhibits the shrinkage in nanowire structure due to reconstruction of Cu-OH to CuO during the annealing process at high temperature. The average diameter of the core-shell structured CuO/SnO2 NWs are calculated to be about 147, 160, 172 and 185 nm, respectively, which were summarized in Fig. 2(f). As a result, the thicknesses of SnO2 turned out to be 12, 18, 24 and 31 nm with different ALD growth cycles of 120, 180, 240 and 300, which are consistent with the results of the SnO2 layers grown on flat silicon wafers measured by SE system (Fig. S2). Evidently, the SnO2
Scheme 1. The synthetic protocol for the p-CuO/n-SnO2 core-shell NWs, combining a modified facile wet chemical method, an ALD process and a calcination process. 235
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Fig. 2. SEM characterization of p-CuO/n-SnO2 core-shell NWs with shell thicknesses ranging from 0 to 31 nm: (a) 0, (b) 12, (c) 18, (d) 24 and (e) 31 nm; (f) The relationship between the diameter of p-CuO/n-SnO2 core-shell NWs and the number of ALD growth cycles with a slope of ˜ 0.1 nm/cycle.
components, centered at 530.0 and 531.1 eV. The peak located at 530.0 eV could be assigned to oxygen in the CuO and SnO2 crystal lattice. The higher binding energy at 531.1 eV is usually attributed to the surficial adsorbed oxygen [28]. Besides, the XPS results of pristine CuO NWs shown in Fig. S3(b)–(d) demonstrate the well-defined CuO composition.
cassiterite SnO2 phase (PDF#41-1445). Besides, two additional weak peaks at 37.9° and 57.8° were detected as well, further confirming the cassiterite structure of SnO2. With the increasing thickness of the SnO2 layer, the intensity of the SnO2 characteristic diffraction peaks gradually increases. Meanwhile, there is almost no offset in the position of the characteristic diffraction peaks as the thickness of the SnO2 layer increases, implying the lattice mismatch between the CuO core NWs and the SnO2 shells is relatively small. In addition, any other impurity peak has not been found in the XRD patterns, which confirms the phase purity of the prepared samples. Figs. 4(b)–(d) and S3(a) typically display the XPS spectra of the sample C/S24. The whole survey spectrum in Fig. S3(a) simply shows the peak positions of copper, tin and oxygen in the CuO/SnO2 core-shell structure. The detailed XPS spectrum of Cu 2p in CuO (Fig. 4(b)) displays two distinct peaks at binding energies of 934.1 and 954.0 eV, corresponding to the Cu 2p3/2 and Cu 2p1/2 core levels, respectively. Additionally, there are two strong satellite peaks nearby, which is a characteristic of the CuO phase [28]. Specifically, the Sn 3d XPS spectrum (Fig. 4(c)) of the sample exhibits two separate peaks centered at 486.9 and 495.3 eV, which are corresponding to the characteristic Sn 3d5/2 and Sn 3d3/2 peaks, respectively. The results are identical with the typical binding energies of Sn in SnO2. The asymmetric O 1s peak of C/ S24 (Fig. 4(d)) was coherently fitted by two nearly Gaussian
3.2. Gas sensing performance To evaluate the gas sensing performance of the prepared heterostructured p-CuO/n-SnO2 core-shell NWs, the samples with different SnO2 shell thicknesses were dropped on the MEMS heating appliances and fabricated into MEMS-based gas sensors (Fig. 1). Moreover, the sensing properties of all the samples were measured using the JF02 F gas sensing measurement system. Typically, Fig. 5(a) shows the dynamic response curves of all the samples with varying shell thicknesses under the operating temperature of 250 °C, obtained for the reducing gas of HCHO with various concentrations ranging from 50 ppm to 1.5 ppm. Apparently, the response of all samples has turned out to be reduced as the HCHO concentration decreases, which is thoroughly presented in Table S1. More importantly, compared with the pristine CuO NW gas sensor (i.e. sample C/S0), the designed heterostructured pCuO/n-SnO2 core-shell NW structured gas sensors do exhibit different 236
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Fig. 3. TEM characterization of the p-CuO/nSnO2 core-shell NWs sample C/S24: (a) Lowmagnification and (b) high-magnification of a single core-shell NW; (c) HRTEM image obtained at the core-shell interface and the SAED pattern (inset) of a single core-shell NW; (d) HAADF-STEM image of a single core-shell NW and the corresponding EDS line scanning profiles of the Cu (red), O (blue) and Sn (green) elements along the orange line across the coreshell interface (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
continues to raise with the increasing temperature, resulting in a decline of the net adsorption rate. Secondly, as is generally known, the diffusion rate of the HCHO adsorbed on the C/S24 material surface will improve with the increasing temperature, leading to a deeper range for reactions between HCHO and C/S24. So considering above two factors comprehensively, when the operating temperature is 250 °C, the kinetics of the gas adsorption/desorption and diffusion achieves a superior equilibrium, leading to the best gas sensing performance of C/ S24. To investigate the time dependence of response, Fig. 5(c) shows the transient responses of the sample C/S24 exposed to 6 ppm HCHO at 250 °C. The response time and recovery time were measured as the time taken for the sensor output from 10% to 90% of the highest response variation. The response time of C/S24 is 52 s, which obviously exhibits a better response property in comparison to the pristine CuO NW sensor (i.e. 132 s, Fig. S4). Similarly, the recovery time of C/S24 (i.e. 80 s) is shorter than that of C/S0 (i.e. 164 s, Fig. S4). It indicates that comparing with pristine CuO sensors, the p-CuO/n-SnO2 core-shell structure exhibits rapider response when exposed to HCHO gas at the same concentration. At the same time, the p-CuO/n-SnO2 core-shell NW sensors possess a better recovery property when turning off the gas and flushing with air. From the viewpoint of practical application, a good selectivity is also a key parameter to a gas sensor, especially for distinguishing the given target gas from complex atmosphere [34]. Herein, the selectivity of the C/S24 sensor was studied towards HCHO and other various reducing gases (50 ppm) such as CH3COCH3, C7H8 and NH3, at an operating temperature of 250 °C, which is shown in Figs. 5(d) and S5. It can be observed apparently that the response of C/S24 to HCHO is significantly higher than other reducing gases. Generally, the selectivity of the sensor is affected by various factors, such as the bond dissociation energies of gas molecules and the adsorption capability on the surface
gas sensing performance, which is closely related to the thickness of the SnO2 shell. Specifically, when the HCHO concentration is fixed to 50 ppm, the response of the p-n core-shell samples keeps growing as the thickness of SnO2 increases from 12 nm to 24 nm (i.e. C/S12, C/S18 and C/S24). The sample C/S24 yielded a maximum response of 2.42, whose rate of resistance change (i.e. 1.42) is 3 times higher than that of the sample C/S0 (i.e. 0.48). However, the response begins to decrease as the thickness of SnO2 further increases above 24 nm (i.e. C/S31). What’s more, when the shell thickness is too thick (i.e. 31 nm of C/S31), the core-shell NW structured gas sensor possesses a worse performance than the pristine CuO NW sensor, which may be attributed to the lower sensitivity of pristine SnO2 than pristine CuO to HCHO. Table 1 [29–31] presents the sensing performance comparison of the prepared C/S24 sensor with other various CuO NWs-based gas sensors fabricated on MEMS. It can be found that when operating temperature and response values are considered comprehensively, the C/S24 sensor shows improved performance among the MEMS-type CuO NWs-based gas sensors. It has been widely demonstrated that the response of a sensing material is greatly affected by the operating temperature. As it can be seen from Fig. 5(b), the optimal operating temperature for C/S24 is 250 °C, and the response of C/S24 under other operating temperatures lower (i.e. 150 °C and 200 °C) or higher (i.e. 300 °C) than 250 °C has been turned out to be lower. Obviously, the operating temperature will affect the kinetics of the gas adsorption as well as the kinetics of diffusions, leading to the alteration of the material resistance [32,33]. Firstly, the kinetics of the gas adsorption is mainly affected by two factors, including adsorption rate and desorption rate. When the temperature gradually rises, the adsorption rate as well as discernible sensing variations will be increased. However, when the temperature exceeds 250 °C, the adsorption rate becomes saturated due to the limited oxygen vacancies on the material surface, but the desorption rate 237
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Fig. 4. (a) XRD patterns of p-CuO/n-SnO2 core-shell NWs with different shell thicknesses including C/S0, C/S12, C/S18, C/S24 and C/S31; (b)-(d) XPS survey spectra of the p-CuO/n-SnO2 core-shell NWs sample C/S24, namely (b) Cu 2p spectrum, (c) Sn 3d spectrum and (d) O 1s spectrum.
the material as typically shown in following Eqs. (1) and (2):
of sensing materials [23,35]. Compared with other gases, the smaller band dissociation energy of HCHO will make the reactions easier during chemical adsorption [23]. Besides, the molecular size of HCHO is relatively small among these gas molecules, which may contribute to a larger adsorption capacity as well as a better adsorption capability. In addition, maybe the first-principles calculations based on the density functional theory can provide accurate energetic and electronic properties of p-CuO/n-SnO2 NWs and further explain the selectivity towards HCHO according to the adsorption behaviours on the surface [36]. This result clearly exhibits the excellent selectivity of the heterostructured pCuO/n-SnO2 core-shell sensors for HCHO at 250 °C, confirming it is a promising material for sensitive and selective detection of HCHO gas.
HCHO + O(−ads) → HCOOH + e−
(1)
HCHO + 2O(−ads) → H2 O + CO2 + 2e−
(2)
Therefore, the resistance of the n-type sensor will achieve a rapid decrease when exposed to the reducing gas, which is in accordance with the experimental results shown in Fig. S6(c)–(f). Conversely, for a ptype sensor, it will show a relatively low resistance when exposed to air for the reason that holes are dominant carriers. When introducing the reducing gas of HCHO, the release of electrons will cause a decrease in the hole concentration in p-type semiconductor due to the electron-hole recombination, thus leading to an increase in the material resistance (Fig. S6(b)). The three-dimensional schematic of the p-n core-shell NWs exposed in HCHO gas is shown in Fig. 6(a). The experimental results (Fig. 5(a)) observably show that the sensing properties of CuO-based sensors were greatly enhanced when introducing a favorable thickness of SnO2 shell layer. The formation of the p-n heterojunction at the interface of the CuO/SnO2 core-shell structure satisfactorily account for the enhancement of gas sensing performance [38,39]. In general, CuO mainly shows p-type conductivity by holes [40] and SnO2 displays n-type conductivity by electrons [24], which are clearly exhibited in the band alignment diagram of Fig. 6(b). When the SnO2 shell film was deposited onto CuO NWs, the electrons in SnO2 and the holes in CuO would diffuse in opposite directions due to the great gradient of the carrier concentration, leading to the formation of the p-n heterojunction in thermal equilibrium (Fig. 6(c)). When the pn heterojunction was exposed to air, adsorbed oxygen would also capture the electrons from the conduction of p-CuO, thus actually leading to an increased concentration of holes according to the Law of mass action. Thus, the potential barrier height of the heterojunction depletion layer would have a rise due to the increase of concentration
3.3. Gas sensing mechanisms It was observed that the pristine p-type CuO NWs based sensor shows an increase in the resistance while the CuO/SnO2 core-shell NWs based sensors show a sudden decrease when introducing the reducing gas of HCHO (Fig. S6), indicating that the composite structure exhibits typical n-type conductivity behaviors [22]. According to the generally acknowledged surface reaction mechanism for semiconductor-based resistance-type gas sensors, the chemisorption of oxygen like O2−, O22−, O− or O2−, and the chemical reaction between adsorbed oxygen and gas molecules on the material surface are thought to be the cause of resistance change [37]. Initially, when an n-type sensor was exposed to air, the physisorbed oxygen captured electrons from the conduction band of sensing materials and turned into chemisorbed, resulting in forming an electron-depleted region underneath the material surface and exhibiting the high resistance state. Once the n-type sensor was exposed to the reducing gas of HCHO, the chemisorbed oxygen species would react with HCHO on the surface and release the electrons back to 238
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Fig. 5. (a) Dynamic response curves of all the samples (i.e. C/S0, C/S12, C/S18, C/S24 and C/S31) with varying shell thicknesses facing the reducing gas of HCHO with various concentrations ranging from 50 ppm to 1.5 ppm under the operating temperature of 250 °C; (b) Response of the sample C/S24 at different operating temperatures (i.e. 150 °C, 200 °C, 250 °C and 300 °C) facing the reducing gas of HCHO with various concentrations ranging from 50 ppm to 1.5 ppm; (c) Enlarged response of the sample C/S24 facing 6 ppm HCHO at 250 °C with response time of 52 s and recovery time of 80 s; (d) Response of the sample C/S24 facing other various reducing gases (50 ppm), namely acetone, methylbenzene and ammonia, compared with 50 ppm HCHO at the operating temperature of 250 °C.
Table 1 Sensing performance comparison of various CuO NWs-based gas sensors fabricated on MEMS. Material
Target gas
Response (Ra/Rg or Rg/Ra)
Operating temperature
Synthetic method
CuO [29] CuO [30] CuO [31] CuO/SnO2 (this work)
C2H5OH (500 ppm) CO (30 ppm) CO (100 ppm) HCHO (50 ppm)
1.5 1.36 1.2 2.42
220 °C 325 °C 300 °C 250 °C
Thermal Thermal Thermal Solution
oxidation oxidation oxidation processing & ALD
acknowledged theory combining the effects of the radial modulation of the electron-depleted layer and the volume fraction of the electrondepleted layer in the core-shell structure [26], the optimal shell thickness is nearly identical to the Debye length (λD) [22], which is constant for a given material at a specific temperature and carrier concentration. The Debye length can be expressed as Eq. (3) [43]:
gradient on both sides of the p-n heterojunction [41]. Therefore, the resistance of n-type p-CuO/n-SnO2 core-shell NW sensors achieved a further increase in air due to the additional existence of the depletion region formed at the CuO/SnO2 p-n heterojunction interface. Correspondingly, when the p-CuO/n-SnO2 core-shell NW sensors were exposed to the reducing gas of HCHO, not only the electron-depleted region below the surface but also the depletion layer at the p-n heterojunction interface became thinner (Fig. 6(d)), further decreasing the resistance of the core-shell NW sensors as well as improving the response which is defined as Rair/Rgas. In brief, the formation of the p-n heterojunction greatly increases the resistance of the composite in air and further decreases the resistance of the composite in HCHO gas, leading to a significant improvement of the sensing response. In addition, the existence of the p-n heterojunction will help improve the holeelectron separation rate at the interface, making electrons more easily captured by the chemisorbed oxygen [42]. Therefore, the recovery time will be reduced to some extent. Remarkably, as is shown in Fig. 5(a), the response of the sensors is highly dependent on the shell thickness. On the basis of the mostly
λD
εkT q 2n c
(3) −12
F/m where ε is the static dielectric constant (i.e. ˜14 × 8.85 × 10 for SnO2), k is the Boltzmann constant (i.e. 1.38 × 10−23 J/K), T is the absolute temperature (i.e. 523 K), q is the electron charge (i.e. 1.6 × 10−19 C), and nc is the carrier concentration (i.e. 5.71 × 1016 cm−3, whose value is obtained by Hall measurement for the SnO2 thin film prepared on the Si substrate by ALD). Hence, the λD value for 250 °C was calculated to be around 24.7 nm, which is in accordance with the experimental results (i.e. 24 nm). When the shell layer is close to the Debye length, the electrons in the SnO2 shell will be fully depleted by the combined effect of the chemisorbed oxygen on the 239
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Fig. 6. Schematics of the reducing gas sensing mechanism in the p-CuO/n-SnO2 core-shell NW gas sensors with a shell layer close to the Debye length: (a) The threedimensional schematic of the p-n core-shell NWs exposed in HCHO gas; The schematic energy band diagrams for the p-n core-shell NWs (b) in separate state and (c) in air; (d) Three-dimensional sensing mechanism diagrams of the carrier distribution in the p-CuO/n-SnO2 core-shell NWs with a shell layer close to the Debye length when exposed in air and in HCHO gas. The band structure data in figure (b) were determined from the literature [24,40].
demonstrated when comparing with pristine CuO NW gas sensors. Specifically, the p-CuO/n-SnO2 core-shell NW sensors with a SnO2 shell layer of 24 nm on CuO achieved a high sensitivity (Ra/Rg) of 2.42, whose rate of resistance change (i.e. 1.42) is around 3 times higher than the pristine CuO NW sensor when exposed to 50 ppm HCHO at 250 °C. In addition, the p-CuO/n-SnO2 core-shell NW gas sensors exhibited excellent selectivity for HCHO from commonly occurred reducing gases. Based on these results, it is believed that our precisely controlled heterostructured p-CuO/n-SnO2 core-shell NWs hold great potential for highly sensitive and selective HCHO gas sensors.
n-shell and the p-n heterojunction, as shown in Fig. 6(d). However, if the shell layer is much thinner than λD, although the shell layer will be completely electron-depleted as well, the resistance variation rate for the entire p-n core-shell NWs still remains relatively minor considering the small portion of the shell layer in the entire core-shell structure (Fig. S7(a)). On the other hand, although the p-n core-shell NWs with a shell layer thicker than λD will have an electron-depletion with a width close to λD, it is just partially depleted in shell layers as is described schematically in Fig. S7(b). Besides, the depletion layer at the p-n heterojunction interface is considered to be ineffective in view of the limitation of the diffusion depth. Therefore, it is essential to achieve superior gas sensing properties in the p-n core-shell NWs with a shell layer whose thickness is close to λD.
Competing financial interest The authors declare no competing financial interest.
4. Conclusions Acknowledgements In conclusion, highly sensitive and selective gas sensors based on structurally well-defined p-CuO/n-SnO2 core-shell NWs with ALD-confined shell thickness of SnO2 have been synthesized for the first time in this work. As a result, significantly improved sensing properties of the heterostructured p-CuO/n-SnO2 core-shell NW gas sensors have been
This work is supported by the National Key R&D Program of China (No. 2016YFE0110700), National Natural Science Foundation of China (No. U1632121, 51861135105, and 61874034), and Natural Science Foundation of Shanghai (No. 18ZR1405000). 240
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Appendix A. Supplementary data
[22] J.H. Kim, A. Katoch, S.S. Kim, Optimum shell thickness and underlying sensing mechanism in p-n CuO-ZnO core-shell nanowires, Sens. Actuators, B 222 (2016) 249–256, https://doi.org/10.1016/j.snb.2015.08.062. [23] Y.X. Li, N. Chen, D.Y. Deng, X.X. Xing, X.C. Xiao, Y.D. Wang, Formaldehyde detection: SnO2 microspheres for formaldehyde gas sensor with high sensitivity, fast response/recovery and good selectivity, Sens. Actuators, B 238 (2017) 264–273, https://doi.org/10.1016/j.snb.2016.07.051. [24] F. Li, X. Gao, R. Wang, T. Zhang, Design of WO3-SnO2 core-shell nanofibers and their enhanced gas sensing performance based on different work function, Appl. Surf. Sci. 442 (2018) 30–37, https://doi.org/10.1016/j.apsusc.2018.02.122. [25] B.W. Zhang, W.Y. Fu, H.Y. Li, X.L. Fu, Y. Wang, H. Bala, G. Sun, X.D. Wang, Y. Wang, J.L. Cao, Z.Y. Zhang, Actinomorphic ZnO/SnO2 core-shell nanorods: twostep synthesis and enhanced ethanol sensing propertied, Mater. Lett. 160 (2015) 227–230, https://doi.org/10.1016/j.matlet.2015.07.122. [26] J.H. Kim, S.S. Kim, Realization of ppb-scale toluene-sensing abilities with Pt-functionalized SnO2-ZnO core-shell nanowires, ACS Appl. Mater. Interfaces 7 (2015) 17199–17208, https://doi.org/10.1021/acsami.5b04066. [27] J. Bai, Y.P. Li, R. Wang, K. Huang, Q.Y. Zeng, J.H. Li, B.X. Zhou, A novel 3D ZnO/ Cu2O nanowire photocathode material with highly efficient photoelectrocatalytic performance, J. Mater. Chem. A 3 (2015) 22996–23002, https://doi.org/10.1039/ c5ta07583a. [28] M.H. Li, H.C. Zhu, J. Cheng, M.M. Zhao, W.P. Yan, Synthesis and improved ethanol sensing performance of CuO/SnO2 based hollow microspheres, J. Porous Mater. 24 (2017) 507–518, https://doi.org/10.1007/s10934-016-0286-9. [29] P. Raksa, A. Gardchareon, T. Chairuangsri, P. Mangkorntong, N. Mangkorntong, S. Choopun, Ethanol sensing properties of CuO nanowires prepared by an oxidation reaction, Ceram. Int. 35 (2009) 649–652, https://doi.org/10.1016/j.ceramint. 2008.01.028. [30] S. Steinhauer, A. Chapelle, P. Menini, M. Sowwan, Local CuO nanowire growth on microhotplates: in situ electrical measurements and gas sensing application, ACS Sens. 1 (2016) 503–507, https://doi.org/10.1021/acssensors.6b00042. [31] S. Steinhauer, E. Brunet, T. Maier, G.C. Mutinati, A. Köck, O. Freudenberg, C. Gspan, W. Grogger, A. Neuhold, R. Resel, Gas sensing properties of novel CuO nanowire devices, Sens. Actuators, B 187 (2013) 50–57, https://doi.org/10.1016/j. snb.2012.09.034. [32] G. Korotcenkov, Metal oxides for solid-state gas sensors: what determines our choice? Mater. Sci. Eng. B-Adv. 139 (2007) 1–23, https://doi.org/10.1016/j.mseb. 2007.01.044. [33] N. Joshi, L.F. da Silva, H.S. Jadhav, F.M. Shimizu, P.H. Suman, J.C. M’Peko, M.O. Orlandi, J.G. Seo, V.R. Mastelaro, O.N. Oliveira, Yolk-shelled ZnCo2O4 microspheres: surface properties and gas sensing application, Sens. Actuators, B 257 (2018) 906–915, https://doi.org/10.1016/j.snb.2017.11.041. [34] Z.D. Lin, N. Li, Z. Chen, P. Fu, The effect of Ni doping concentration on the gas sensing properties of Ni doped SnO2, Sens. Actuators, B 239 (2017) 501–510, https://doi.org/10.1016/j.snb.2016.08.053. [35] M.A. Ashraf, Z.L. Liu, W.X. Peng, Z. Parsaee, Design, preparation and evaluation of a high performance sensor for formaldehyde based on a novel hybride nonocomposite ZnWO3/rGO, Anal. Chim. Acta 1051 (2019) 120–128, https://doi.org/ 10.1016/j.aca.2018.11.014. [36] S.S. Li, Z.S. Lu, Z.X. Yang, X.L. Chu, The sensing mechanism of Pt-doped SnO2 surface toward CO: a first-principle study, Sens. Actuators, B 202 (2014) 83–92, https://doi.org/10.1016/j.snb.2014.05.071. [37] S.H. Yan, S.Y. Ma, W.Q. Li, X.L. Xu, L. Cheng, H.S. Song, X.Y. Liang, Synthesis of SnO2-ZnO heterostructured nanofibers for enhanced ethanol gas-sensing performance, Sens. Actuators, B 221 (2015) 88–95, https://doi.org/10.1016/j.snb.2015. 06.104. [38] C. Liu, L.P. Zhao, B.Q. Wang, P. Sun, Q.J. Wang, Y. Gao, X.S. Liang, T. Zhang, G.Y. Lu, Acetone gas sensor based on NiO/ZnO hollow spheres: fast response and recovery, and low (ppb) detection limit, J. Colloid Interface Sci. 495 (2017) 207–215, https://doi.org/10.1016/j.jcis.2017.01.106. [39] J.H. Kim, A. Katoch, S.H. Kim, S.S. Kim, Chemiresistive sensing behavior of SnO2 (n)-Cu2O (p) core-shell nanowires, ACS Appl. Mater. Interfaces 7 (2015) 15351–15358, https://doi.org/10.1021/acsami.5b03224. [40] S. Paul, J. Sultana, A. Bhattacharyya, A. Karmakar, S. Chattopadhyay, Investigation of the comparative photovoltaic performance of n-ZnO nanowire/p-Si and n-ZnO nanowire/p-CuO heterojunctions grown by chemical bath deposition method, Optik 164 (2018) 745–752, https://doi.org/10.1016/j.ijleo.2018.03.076. [41] D.X. Ju, H.Y. Xu, Q. Xu, H.B. Gong, Z.W. Qiu, J. Guo, J. Zhang, B.Q. Cao, High trimethylamine-sensing properties of NiO/SnO2 hollow sphere P-N heterojunction sensors, Sens. Actuators, B 215 (2015) 39–44, https://doi.org/10.1016/j.snb.2015. 03.015. [42] L.J. Wang, H.J. Zhai, G. Jin, X.Y. Li, C.W. Dong, H. Zhang, B. Yang, H.M. Xie, H.Z. Sun, 3D porous ZnO-SnS p-n heterojunction for visible light driven photocatalysis, Phys. Chem. Chem. Phys. 19 (2017) 16576–16585, https://doi.org/10. 1039/c7cp01687e. [43] S.W. Choi, A. Katoch, J.H. Kim, S.S. Kim, Striking sensing improvement of n-type oxide nanowires by electronic sensitization based on work function difference, J. Mater. Chem. C 3 (2015) 1521–1527, https://doi.org/10.1039/c4tc02057j.
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.03.092. References [1] L. Zhu, Y.Q. Li, W. Zeng, Hydrothermal synthesis of hierarchical flower-like ZnO nanostructure and its enhanced ethanol gas-sensing properties, Appl. Surf. Sci. 427 (2018) 281–287, https://doi.org/10.1016/j.apsusc.2017.08.229. [2] H.L. Tian, H.Q. Fan, M.M. Li, L.T. Ma, Zeolitic imidazolate framework coated ZnO nanorods as molecular sieving to improve selectivity of formaldehyde gas sensor, ACS Sens. 1 (2016) 243–250, https://doi.org/10.1021/acssensors.5b00236. [3] J. Zhang, X.H. Liu, G. Neri, N. Pinna, Nanostructured materials for room-temperature gas sensors, Adv. Mater. 28 (2016) 795–831, https://doi.org/10.1002/ adma.201503825. [4] A. Mirzaei, S.G. Leonardi, G. Neri, Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: a review, Ceram. Int. 42 (2016) 15119–15141, https://doi.org/10.1016/j.ceramint.2016.06.145. [5] Y.J. Jeong, W.T. Koo, J.S. Jang, D.H. Kim, H.J. Cho, I.D. Kim, Chitosan-templated Pt nanocatalyst loaded mesoporous SnO2 nanofibers: a superior chemiresistor toward acetone molecules, Nanoscale 10 (2018) 13713–13721, https://doi.org/10.1039/ c8nr03242d. [6] M.R. Alenezi, S.J. Henley, N.G. Emerson, S.R.P. Silva, From 1D and 2D ZnO nanostructures to 3D hierarchical structures with enhanced gas sensing properties, Nanoscale 6 (2014) 235–247, https://doi.org/10.1039/c3nr04519f. [7] M. Bao, Y.J. Chen, F. Li, J.M. Ma, T. Lv, Y.J. Tang, L.B. Chen, Z. Xu, T.H. Wang, Plate-like p-n heterogeneous NiO/WO3 nanocomposites for high performance room temperature NO2 sensors, Nanoscale 6 (2014) 4063–4066, https://doi.org/10. 1039/c3nr05268k. [8] B. Urasinska-Wojcik, J.W. Gardner, H2S sensing in dry and humid H2 environment with p-type CuO thick-film gas sensors, IEEE Sens. J. 18 (2018) 3502–3508, https:// doi.org/10.1109/JSEN.2018.2811462. [9] Y.J. Chen, F.N. Meng, H.L. Yu, C.L. Zhu, T.S. Wang, P. Gao, Q.Y. Ouyang, Sonochemical synthesis and ppb H2S sensing performances of CuO nanobelts, Sens. Actuators, B 176 (2013) 15–21, https://doi.org/10.1016/j.snb.2012.08.007. [10] J.H. Kim, A. Katoch, S.W. Choi, S.S. Kim, Growth and sensing properties of networked p-CuO nanowires, Sens. Actuators, B 212 (2015) 190–195, https://doi.org/ 10.1016/j.snb.2014.12.081. [11] J. Hu, C. Zou, Y.J. Su, M. Li, Y.T. Han, E.S.W. Kong, Z. Yang, Y.F. Zhang, An ultrasensitive NO2 gas sensor based on a hierarchical Cu2O/CuO mesocrystal nanoflower, J. Mater. Chem. A 6 (2018) 17120–17131, https://doi.org/10.1039/ c8ta04404j. [12] C. Yang, F. Xiao, J.D. Wang, X.T. Su, 3D flower- and 2D sheet-like CuO nanostructures: microwave-assisted synthesis and application in gas sensors, Sens. Actuators, B 207 (2015) 177–185, https://doi.org/10.1016/j.snb.2014.10.063. [13] H.J. Park, N.J. Choi, H. Kang, M.Y. Jung, J.W. Park, K.H. Park, D.S. Lee, A ppb-level formaldehyde gas sensor based on CuO nanocubes prepared using a polyol process, Sens. Actuators, B 203 (2014) 282–288, https://doi.org/10.1016/j.snb.2014.06. 118. [14] Y.F. Wang, F.D. Qu, J. Liu, Y. Wang, J.R. Zhou, S.P. Ruan, Enhanced H2S sensing characteristics of CuO-NiO core-shell microspheres sensors, Sens. Actuators, B 209 (2015) 515–523, https://doi.org/10.1016/j.snb.2014.12.010. [15] S.L. Bai, W.T. Guo, J.H. Sun, J. Li, Y. Tian, A.F. Chen, R.X. Luo, D.Q. Li, Synthesis of SnO2-CuO heterojunction using electrospinning and application in detecting of CO, Sens. Actuators, B 226 (2016) 96–103, https://doi.org/10.1016/j.snb.2015.11.028. [16] C. Wang, J.W. Zhu, S.M. Liang, H.P. Bi, Q.F. Han, X.H. Liu, X. Wang, Reduced graphene oxide decorated with CuO-ZnO hetero-junctions: towards high selective gas-sensing property to acetone, J. Mater. Chem. A 2 (2014) 18635–18643, https:// doi.org/10.1039/c4ta03931a. [17] Y.B. Zhang, J. Yin, L. Li, L.X. Zhang, L.J. Bie, Enhanced ethanol gas-sensing properties of flower-like p-CuO/n-ZnO heterojunction nanorods, Sens. Actuators, B 202 (2014) 500–507, https://doi.org/10.1016/j.snb.2014.05.111. [18] J.A. Deng, L.L. Wang, Z. Lou, T. Zhang, Design of CuO-TiO2 heterostructure nanofibers and their sensing performance, J. Mater. Chem. A 2 (2014) 9030–9034, https://doi.org/10.1039/c4ta00160e. [19] D.R. Miller, S.A. Akbar, P.A. Morris, Nanoscale metal oxide-based heterojunctions for gas sensing: a review, Sens. Actuators, B 204 (2014) 250–272, https://doi.org/ 10.1016/j.snb.2014.07.074. [20] Y. Zhang, H.L. Lu, T. Wang, Q.H. Ren, Y.Z. Gu, D.H. Li, D.W. Zhang, Facile synthesis and enhanced luminescent properties of ZnO/HfO2 core-shell nanowires, Nanoscale 7 (2015) 15462–15468, https://doi.org/10.1039/c5nr03656a. [21] K.P. Yuan, Q. Cao, H.L. Lu, M. Zhong, X.Z. Zheng, H.Y. Chen, T. Wang, J.J. Delaunay, W. Luo, L.W. Zhang, Y.Y. Wang, Y.H. Deng, S.J. Ding, D.W. Zhang, Oxygen-deficient WO3-x@TiO2-x core-shell nanosheets for efficient photoelectrochemical oxidation of neutral water solutions, J. Mater. Chem. A 5 (2017) 14697–14706, https://doi.org/10.1039/c7ta03878j.
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