Tailorable Morphology of CoreâShell Nanofibers with Surface

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Cite This: ACS Appl. Nano Mater. 2018, 1, 6357−6367

Tailorable Morphology of Core−Shell Nanofibers with Surface Wrinkles for Enhanced Gas-Sensing Properties Changhui Zhao,†,‡ Jinglong Bai,§ Huimin Gong,† Sheng Liu,*,‡ and Fei Wang*,†,∥ †

Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen 518055, China School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China § School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China ∥ State Key Lab of Transducer Technology, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China Downloaded via UNIV OF WINNIPEG on December 4, 2018 at 04:51:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: One-dimensional core−shell nanostructures have attracted considerable attention because of their excellent carrier transport characteristics between core and shell layers. In this work, we report the preparation of surface-wrinkled In2O3@ZnO core−shell nanofibers (IZO CSNFs) in a facile way, where the as-spun polyacrylonitrile/indium nitrate (PAN/In(NO3)3) nanofibers are used as scaffolds directly in subsequent atomic layer deposition process. The thermal decomposition of the PAN can lead to the formation of the wrinkled ZnO shells. Especially, an evolvement has been observed from solid to hollow IZO CSNFs with the increase of shell thickness, where the ZnO thick shell is robust enough and may act as another type of scaffold to tailor their morphologies. Considering the different carrier transport features in various atmospheres, the IZO-x (x is the number of ALD cycles) sensors exhibit a clear shell thickness dependence on their gas-sensing properties compared to the cases of pure In2O3 and ZnO NFs. Furthermore, the IZO-50 CSNF-based sensor shows the highest response to NO2 (Rg/Ra = 78.6 at 50 ppm, Ra: resistance in air; Rg: resistance in target gas) at a low operating temperature of 200 °C. Meanwhile, the sensor based on IZO-300 CSNFs presents the superior ethanol sensing response (Ra/Rg = 29.3 at 100 ppm) at 320 °C. Possible gas-sensing mechanisms of these surface-wrinkled IZO CSNFs are discussed in detail. KEYWORDS: In2O3@ZnO core−shell nanofibers, surface wrinkles, electrospinning, atomic layer deposition, gas sensors, gas-sensing mechanisms

1. INTRODUCTION One-dimensional (1D) core−shell nanostructure-based gas sensors have attracted a great deal of attention over the past few decades because of their high aspect ratio and distinctive carrier transport properties between core and shell layers.1−3 To date, various 1D core−shell systems have been developed, such as ZnO−SnO2,4−6 In2O3−ZnO,7−10 MoO3−TiO2,11 α-Fe2O3−ZnO,12 CuO−SnO2,13 and Ga2O3−ZnO.14 Some core−shell systems display excellent gas-sensing performances compared with those of bare core or shell materials, especially when the thickness of the core or shell layer is close to its Debye length. Among them, In2O3 and ZnO are two multifunctional n-type metal oxide semiconductors, which have been investigated extensively for trace gas detection. Many researchers have provided important validation for the combination of these two materials including doping15−17 and building heterojunctions or core−shell nanostructures,18−21 which revealed dramatically enhanced gas-sensing properties. Singh et al. proved that In2O3−ZnO core−shell nanowires could enhance the sensitivity to reducing gases (CO, H2, and ethanol).7 Whereafter Park et al. © 2018 American Chemical Society

pointed out that the ethanol-sensing properties of In2O3/ZnO core−shell nanowires could be determined by the thickness of the ZnO shell.8 Recently, we have reported a coaxial electrospinning method to prepare ZnO@In2O3 core−shell nanofibers (CSNFs) for highly sensitive and selective detection of ethanol,10 which could be explained by the formation of a complete electron depletion layer in the In2O3 shell. Despite these notable efforts, a new type of In2O3@ZnO CSNFs with tailorable morphology may promote their gas-sensing properties and potentially enrich their sensing mechanisms from other perspectives, which may depend not only on the formed n−n junctions but also on their architectures. Especially, a systematic and comprehensive exploration of their gas-sensing behaviors toward both oxidizing and reducing gases is still of great importance. For 1D core−shell nanostructures with controlled shell thickness, it is a very promising approach to combine the Received: September 7, 2018 Accepted: October 29, 2018 Published: October 29, 2018 6357

DOI: 10.1021/acsanm.8b01573 ACS Appl. Nano Mater. 2018, 1, 6357−6367

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ACS Applied Nano Materials

Figure 1. (a) Schematic illustration of synthetic processes for pure In2O3 NFs as well as solid and hollow IZO CSNFs. SEM and TEM images of (b, c) pure In2O3 NFs, (d, e) IZO-50 CSNFs, and (f, g) IZO-300 CSNFs.

Moreover, the roles of porous/hollow core−shell structure and In2O3−ZnO junction are taken into account to analyze their sensing mechanisms, making it one of the applicable routes to achieve high-performance gas sensors.

electrospinning technique with a subsequent atomic layer deposition (ALD) step.22−24 Typically, the ALD films are deposited with controlled cycles onto the annealed electrospun products5 to adjust the shell thickness, while it is worth noting that the electrospun polymer nanofibers can also act as scaffolds for the preparation of metal oxide nanotubes.25−27 To take full advantage of the thermal decomposition of the electrospun polymers, in this regard, it is accepted that not only a hollow structure of electrospun core but also a wrinkled structure of the ALD shell can be formed during the core shrinkage procedure. Consequently, both the morphologies of core and shell may be tailored through a rational design of the preparation process. Herein, we report an approach to prepare surface-wrinkled In2O3@ZnO (IZO) CSNFs using electrospinning and ALD in sequence, where the electrospun polymer−metal salt nanofibers are used as scaffolds directly during the ALD process. By tuning the ALD cycles, both solid and hollow IZO CSNFs were successfully obtained with tailorable surface morphologies. The shell thickness dependences on morphological and gas-sensing properties of these IZO CSNFs are studied in different gases (NO2 and ethanol). Interestingly, the sensor performance of IZO CSNFs cannot always be attributed to the formed In2O3− ZnO n−n junctions because of their tailorable morphologies.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals were of analytical grade quality and were used as received without any further purification. Electrospinning of PAN/In(NO3)3 NFs. 1.0 g of polyacrylonitrile (PAN, Mw = 150000) was dissolved into 10 mL of N,N-dimethylformamide (DMF). Subsequently, 0.6 g of indium nitrate (In(NO3)3· 4.5H2O) was added to the above solution and stirred at room temperature overnight. A 15 kV dc voltage was applied between a syringe needle and metal roller collector (covered with aluminum foil) with a distance of 18 cm. The feeding rate was set as 0.4 mL h−1. Preparation of IZO CSNFs. The as-spun PAN/In(NO3)3 nonwovens were transferred from the collector to a Picosun R200 Series ALD reactor. The parameters of a single ALD cycle are as follows: 0.1 s pulse of diethylzinc (Zn(C2H5)2, DEZ), 4 s of purge with nitrogen (N2), 0.2 s pulse of deionized water, and 4 s of purge with N2 again. The pressure and temperature were 0.3 Torr and 200 °C, respectively. After a predetermined number of ALD cycles with above conditions, the PAN/In(NO3)3@ZnO CSNFs were obtained as illustrated in Figure 1a. As a typical example, the thickness of ZnO shell is about 20 nm in the case of 100 ALD cycles. The final products were annealed at 550 °C for 2 h in 6358


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Figure 2. (a) N2 adsorption−desorption isotherms and (b) pore size distribution plots of the In2O3, IZO-50, and IZO-300 products. Inset in (a) shows the corresponding specific surface area.

fibrous structure (Figure 1c). Then, the PAN/In(NO3)3 NFs were used directly as the scaffolds, and ZnO ALD coating was performed to prepare PAN/In(NO3)3@ZnO CSNFs. As shown in Figure 1d−g, SEM and TEM images directly demonstrate that the surface morphologies of IZO products change greatly after ZnO deposition. Because of the shrinkage of ZnO shells along with the decomposition of PAN scaffolds, the surface wrinkles will become more apparent in the case of thicker ZnO shells. For example, when a thin and continuous ZnO layer is deposited, the IZO-50 CSNFs (Figure 1d,e) will maintain the solid fibrous structure and the shell contacts with a slightly wrinkled surface. However, for IZO-300 CSNFs (Figure 1f,g), the ZnO thick shell is robust enough to balance the stress caused by core shrinkage, and the shell surface is corrugated after the thermal annealing.30 The In2O3 nanocrystallites tend to accumulate on the inner surface of the shell; in other words, the ZnO thick shell acts as another type of scaffold for the formation of hollow cores. Furthermore, the TGA results provide the proof of the weight loss behaviors for as-prepared PAN/In(NO3)3 NFs and PAN/ In(NO3)3@ZnO CSNFs (Figure S1). All precursors are relatively stable at least up to 165 °C. The main weight loss occurs with the decomposition of the nitrates and polymers from 450 to 600 °C. It can be seen that pure In2O3 shows ∼79% weight loss at 575 °C, IZO-50 shows ∼75% weight loss at 610 °C, and IZO-300 shows ∼40% weight loss at 600 °C. As a consequence, the IZO CSNFs with tailorable core or shell morphologies can be achieved in our work, which may provide some new routes for gas-sensing applications. To further analyze the morphologies of the materials, specific surface area and pore size distributions of the pure In2O3, IZO-50, and IZO-300 products were investigated by N2 adsorption−desorption measurements. Figure 2a shows that the isotherms of all products exhibit H3 type hysteresis loops, associated with the type IV isotherms, suggesting the presence of mesoporous structure. From the inset of Figure 2a, the specific surface area of IZO-50 CSNFs is 25.15 m2 g−1, which is close to that of pure In2O3 NFs (25.53 m2 g−1) and is clearly much higher than that of IZO-300 CSNFs (14.25 m2 g−1). Moreover, the pore size distribution plots (Figure 2b) indicate that the products contains a mean pore width of 17.58 nm for pure In2O3 NFs, 18.98 nm for IZO-50 CSNFs, and 32.77 nm for IZO-300 CSNFs. As mentioned above, there are more obvious wrinkles on the surface of IZO-300 CSNFs than those of IZO-50 CSNFs. However, the increased shell thickness in IZO-300 CSNFs may overcome the contribution of the hollow cores to their specific surface area. As a result, the IZO-300 CSNFs show an obvious decrease of specific surface area (but increase of pore width),

air to achieve IZO-x CSNFs, where x is the number of ALD cycles. For comparison, pure In2O3 and ZnO NFs were prepared using the same procedure except for skipping the ALD step. 2.2. Characterization and Measurement. The morphologies and chemical components of the products were characterized with a field-emission scanning electron microscope (FESEM, Zeiss, Gemini) and a high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30) equipped with an energy dispersive X-ray (EDX) analyzer. The N2 adsorption and the pore-size distribution were measured at 77 K with an ASAP 2020 system (Micromeritics). EDX elemental mappings and high angle annular dark field (HAADF) observations were carried out using the Tecnai G2 F30 TEM. The crystal structures of the products were recorded by X-ray diffraction (XRD, Smartlab) with Cu Kα radiation. X-ray photoelectron spectra (XPS) were measured using a Thermo Scientific ESCALAB 250Xi spectrometer with monochromatized Al Kα radiation. Thermogravimetic analyses (TGA) of the as-spun precursors were performed on a thermal analyzer (NETZSCH STA449F5) in flowing air at a heating rate of 10 °C min−1 in the range 30−700 °C. The products were mixed with a proper amount of binder (including 10 wt % ethylcellulose and 90 wt % terpilenol) to form a slurry. Then the slurry was coated on a patterned Cr/Au interdigitated electrode array (200 μm in both width and gap spacing).28 After drying at 80 °C, all sensors were annealed at 400 °C for 2 h in air to remove the residual organics. Gas-sensing properties were performed on an intelligent gassensing analysis system of CGS-4TPs (Beijing Elite Tech. Co., Ltd., China). In the case of NO2 testing, the operating temperature ranges from 160 to 320 °C. Dry synthetic air (O2/N2 = 21/79) was used as the diluting gas and controlled by a DGD-III system (Beijing Elite Tech. Co., Ltd., China). For ethanol measurement, a static test was performed under an air atmosphere, and the operating temperature ranges from 240 to 340 °C with a relative humidity of 40%. The sensor response is defined as the ratio of Rg/Ra for the oxidizing gas or Ra/Rg for the reducing gas, where Ra and Rg are the sensor resistance in air and in the target gas, respectively. The response (or recovery) time is defined as the time taken by the sensor to achieve 90% of the resistance change in the case of adsorption (or desorption) of the target gas.

3. RESULTS AND DISCUSSION 3.1. Structural and Morphological Characteristics. A schematic illustration (cross-section view) of three different schemes of synthetic processes is shown in Figure 1a. First of all, the In(NO3)3 is uniformly distributed in the as-spun PAN/ In(NO3)3 NFs. When annealing at 550 °C in air, the thermal decomposition of the PAN causes the shrinkage of PAN/ In(NO3)3 NFs and the formation of solid In2O3 NFs, which is similar to previous studies.29 The SEM image in Figure 1b confirms that the pure In2O3 NFs are made up of a number of nanocrystallites (average grain size ∼40 nm). Further observation with TEM shows that the In2O3 sample actually has a solid 6359


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Figure 3. HAADF-STEM images and the corresponding elemental mappings of (a−d) IZO-50 CSNFs and (e−h) IZO-300 CSNFs. (i, j) EDX spectra of two positions indicated in the STEM image (marked by red tick marks in panel e). XPS fine spectra of the IZO-50 CSNFs: (k) In 3d, (l) Zn 2p, and (m) O 1s.

Figure 4. SEM images of IZO CSNFs: (a, b) IZO-100, (d, e) IZO-200, and (g, h) IZO-400. TEM images of (c) IZO-100, (f) IZO-200, and (i) IZO-400.

which also confirms the influence of ZnO shell thickness on the morphology of IZO products. A detailed study of the structure and chemical elemental distribution of the IZO CSNFs was performed by HAADF-STEM

and EDX mapping techniques. Figure 3a shows the STEM image of IZO-50 CSNFs with a typical solid fibrous structure. Elemental mappings in Figure 3b,c reveal a similar distribution of In and Zn elements, revealing that the In2O3 core can be 6360


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Figure 5. (a) XRD patterns of pure In2O3, ZnO, and IZO-x products. HRTEM images of (b) IZO-50, (c) IZO-100, (d) IZO-200, (e) IZO-300, and (f) IZO-400; the inset shows the corresponding enlarged view of lattice fringes.

vacancies).19,35,36 Besides, Figure S2 displays the corresponding XPS survey spectrum, confirming the presence of the In, Zn, O, and C. The carbon peak comes from the residual carbon in the sample or adventitious hydrocarbon from the XPS instrument itself. To further study the effect of the ZnO shell thickness on the morphologies of products, we have also fabricated IZO-x CSNFs (x = 100, 200, and 400 for the ALD cycles). As shown in Figure 4a,b, the porous surface of IZO-100 CSNFs can be due to the formation of wrinkled ZnO shells. This morphological evolution can be confirmed by TEM observation (Figure 4c). When the number of ALD cycles reaches 200, the surface wrinkling characteristics become more evident (Figure 4d,e). Different from the solid fibers in IZO-50 and IZO-100 CSNFs, the IZO-200 CSNFs begin to show some features of hollow cores (Figure 4f). Further increasing the number of ALD cycles, the hollow fibrous structure can be clearly observed in IZO-400 CSNFs (Figure 4g−i). Meanwhile, the increased ALD cycles also contribute to the grain growth of ZnO (Figure 4h). XRD patterns of IZO CSNFs, pure In2O3, and ZnO NFs are shown in Figure 5a. Obviously, the relative peak intensities of In2O3 decrease gradually with increasing the number of ZnO ALD cycles. No obvious diffraction peaks attributed to ZnO are observed in both IZO-50 and IZO-100, which may be ascribed to the thin ZnO shells and high crystallinity of In2O3 cores. In these two curves, all peaks are in good agreement with cubic In2O3 (JCPDS No. 06-0416). Several diffraction peaks of ZnO (JCPDS No. 36-1415) become more apparent in the XRD

uniformly covered by the ZnO thin shell. Differently, the STEM image of IZO-300 CSNFs (Figure 3e) displays a wrinkled surface. Considering the superior contrast resolution of elemental mapping, the hollow structure of core region can be observed from the distribution of In element (Figure 3f). In particular, the distribution patterns of In and Zn elements (Figure 3f,g) also show clear core−shell structure, with In2O3 as the core and ZnO as the shell. Figure 3i,j shows the EDX spectra recorded from two selected positions (marked by red trick marks, in Figure 3e), which allows a direct comparison of In contents between the shell and core regions. Obviously, the intensity of In peaks at position i is relatively lower than that of position j. This result is also consistent with the elemental mapping results shown in Figure 3f,g. To illustrate the chemical state of the elements existing in the IZO-50 CSNFs, the XPS fine spectra of the In 3d, Zn 2p, and O 1s are characterized as shown in Figure 3k−m. The In 3d spectrum given in Figure 3k exhibits two peaks located at 443.55 and 451.1 eV (corresponding to the spin−orbit split of In 3d5/2 and In 3d3/2, respectively), indicating the normal state of In3+ in the IZO system.10,31 Similarly, as shown in Figure 3l, the peak centered at 1021.23 eV is indexed to Zn 2p3/2, and another peak centered at 1044.33 eV is assigned to Zn 2p1/2.32−34 The O 1s (Figure 3m) can be deconvoluted into four peaks, locating at 529.04, 529.75, 531.25, and 532.31 eV. The peaks at around 529.04 and 529.75 eV can be attributed to the lattice oxygen in In2O3 and ZnO, respectively, and the peaks at 531.25 and 532.31 eV can be assigned to the defective oxygen (or oxygen 6361


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Figure 6. (a) Responses of sensors based on IZO CSNFs, pure In2O3, and ZnO NFs exposed to 50 ppm of NO2 as a function of operating temperature (160−320 °C). Response transients of the sensors toward 50 ppm of NO2 at (b) 200 °C and (c) 300 °C. Inset in (b) shows the corresponding resistance curve of IZO-50 sensor. (d) Reproducibility of the IZO-50 sensor toward 50 ppm of NO2 at 200 °C (five cycles).

region (240−320 °C), which results in different optimum operating temperatures for IZO CSNF-based sensors. Figures 6b and 6c display the response-recovery characteristics of sensors exposed to 50 ppm of NO2 at 200 and 300 °C, respectively. It can be seen that the response of all sensors increases rapidly when the NO2 was injected, but instead declines gradually. This may be caused by the relatively low flow rate (300 sccm) of gases during the dynamic NO2 test. Apparently, the IZO-50 CSNF-based sensor shows the highest response value at 200 °C (Figure 6b). In the inset of Figure 6b, the resistance of IZO-50 sensor ranges from 0.356 MΩ (Ra) to 27.97 MΩ (Rg). In addition, the NO2 sensing performance of IZO-50, IZO-100, pure In2O3, and ZnO based sensors has been measured at 200 °C, and the results are shown in Figure S3. The sensor shows a significant decrease in response with decreasing NO2 concentration (from 100 to 2 ppm). For IZO-50 sensor, the response is about 144.9, 80.2, 37.7, 18.4, 9.3, and 4.4 for 100, 50, 20, 10, 5, and 2 ppm, respectively, which presents an approximately linear relation between the NO2 concentration and sensor response (Figure S3b). In comparison, the highest response value appears in the pure ZnO sensor at 300 °C (Figure 6c) due to the highly sensitive nature of ZnO to NO2 gas at a high operating temperature. In Figure 6d, it can be found that there is no obvious change of the sensor response (79.9 ± 1.3) during the five-cycle test for IZO-50 sensor, toward 50 ppm of NO2 at 200 °C, revealing a good reproducibility of sensor in our work. To meet the requirement of low-power consumption gas sensors, the IZO-50 CSNFs are a promising candidate for NO2 detection with lower optimum working temperature. To further analyze the influence of ZnO shells on the gassensing properties of In2O3 NFs, we have also measured these

patterns of IZO-200, IZO-300, and IZO-400. In particular, in the curve of IZO-400, the intensities of In2O3 peaks are weaker than those of ZnO, in accordance with the greatly increased ZnO shell thickness. To observe the surface characteristics of IZO CSNFs, HRTEM images have been measured. From the enlarged HRTEM images inset Figure 5b−f (marked by dashed boxes), the lattice fringes with d-spacings of 0.292−0.293 and 0.281−0.282 nm match well with the (222) plane of In2O3 and the (100) plane of ZnO, respectively. We can find that the lattice fringes of In2O3 core can be easily observed in IZO-50 (Figure 5b), even in IZO-100 (Figure 5c). That is to say, the surface of In2O3 cores is partially covered by ZnO thin shells in these two samples. On the other hand, only the fringes of ZnO can be seen in IZO-200 (Figure 5d), IZO-300 (Figure 5e), and IZO-400 (Figure 5f), indicating that In2O3 cores can be completely covered by ZnO thick shells. 3.2. Gas-Sensing Properties. Gas-sensing properties of the IZO CSNFs, pure In2O3, and ZnO NFs were studied for NO2 detection. As demonstrated in Figure 6a, the response of the IZO-50 sensor increases with the operating temperature, then reaches its maximum value of 78.6 to 50 ppm of NO2 at 200 °C, and decreases with further increase of temperature. Similar sensing behaviors can also be observed in the cases of pure In2O3 NFs (36.0) and IZO-100 (59.3); therefore, the optimum operating temperature of these sensors is around 200 °C. In contrast, the sensors based on IZO-300 (47.8), IZO-400 (55.8), and pure ZnO NFs (58.8) reach their maximum response values at 300 °C. Among them, the IZO-200 sensor exhibits a moderate response to NO2, even at its optimum temperature of 260 °C. It is obvious that the pure ZnO NF-based sensor presents a higher sensitivity than pure In2O3 NFs at high temperature 6362


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Figure 7. (a) Ethanol sensing properties of sensors as a function of operating temperature (240−340 °C). Gas-sensing properties of sensors at 320 °C: (b) sensor responses toward ethanol at various concentrations (5−5000 ppm); (c) the corresponding transient curves of the sensors to low concentration of ethanol (5−100 ppm); (d) selectivity of sensors to 100 ppm gases.

sensors in ethanol gas. Figure 7a displays the sensor responses toward 100 ppm ethanol at different operating temperatures (240−340 °C). The gas-sensing results indicate that both IZO50 and IZO-100 sensors have an optimum operating temperature of 320 °C, which is same with the pure In2O3 NFs. As a comparison, the responses of IZO-200, IZO-300, and IZO-400 sensors monotonically increase with the increase of operating temperature and reach their highest value at 340 °C, and there is a similar tendency for pure ZnO NFs. Therefore, these sensors can be classified into two groups. When the number of ALD cycles is IZO-400 > IZO-200 > IZO-100 > IZO50, particularly in the low ethanol concentration range (20−100 ppm). Besides, the response of IZO-400 sensor will gradually begin to lag behind that of IZO-200 and IZO-100 at high ethanol concentrations (Figure 7b). As a matter of practicality, it is also important to investigate the sensor selectivity at a certain operating temperature. Figure 7d shows the selectivity of sensors in various reducing gases at a fixed concentration of 100 ppm at 320 °C. Almost all sensors show a higher response to ethanol than to other gases, except that the pure ZnO sensor is more sensitive to acetone. Meanwhile, the pure In2O3 sensor is almost insensitive to all reducing gases. Interestingly, all of the IZO CSNF-based sensors present lower responses to methanol and acetone than that of ethanol and are insensitive to toluene and benzene. It should be noted that the sensors based on IZO-200, IZO-300, IZO-400, and pure ZnO obtain their highest ethanol response at 340 °C (Figure 7a). As a consequence, these four sensors were further 6363


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ACS Applied Nano Materials measured at 340 °C to investigate their selectivity. As can be seen from Figure S5a, the sensors may obtain higher responses to ethanol, methanol, and acetone at 340 °C than those at 320 °C. However, in Figure S5b, the ethanol selectivity (ResponseEthanol/ResponseGas) slightly decreases at 340 °C. Taking into account the sensing performance of the above sensors, the IZO-400 sensor shows the best selectivity at 320 °C; this may be suggested for the selective detection of ethanol. Table 1 lists the gas-sensing properties of In2O3−ZnO systems in related literatures and IZO CSNFs in this work. It can

In2O3 (λD(In2O3)).44,45 When the sensor is exposed to the oxidizing gas (NO2), the adsorbed NO2 will trap more electrons from the In2O3 NFs, which narrows the conduction region and increases the potential barrier with a corresponding increase in sensor resistance. On the contrary, when ethanol is injected, the reducing gas will react with the oxygen species and release free electrons back to the In2O3 NFs, resulting in the decrease of the λD1. This would also reduce the height of potential barrier and therefore cause a decrease in sensor resistance. Similar to previous reports, pure In2O3 sensor shows higher sensitivity to NO2 than that to ethanol at low operating temperatures.46,47 When a thin layer of ZnO is deposited on the In2O3 NFs, such as IZO-50 CSNFs, the corresponding sensing mechanisms can be illustrated in Figure 8b. Based on the analysis in Figure 2, the gas-sensing properties of IZO-50 and IZO-300 CSNFs are not enhanced by the specific surface area from the pore structure because their specific surface areas are actually decreased compared to that of pure In2O3 NFs. Because of the fact that tZnO is less than λD(ZnO) (Debye length of ZnO, about 22 nm),5 a complete EDL will generate in the ZnO thin shell, and the EDL will be further extended to the outer region of the In2O3 core. It has been reported that the formed n−n junction would facilitate the electron transfer from In2O3 to ZnO,48 where the EDL region generates in the In2O3 side and electron accumulation region generates in the ZnO side. This seems to suggest that the formed In2O3−ZnO n−n junctions will increase the sensor resistance in air (Ra), resulting in the inhibition of NO2 sensing properties. However, we can find that the EDL region built by the n−n junction (marked by a red dashed box, in Figure 8b) is completely covered by the whole EDL; therefore, the changes in sensor resistance induced by the n−n junction may be overcomed by the broad EDL region. At this time, the conduction region is still limited to the In2O3 cores, which is one of the main reasons why the IZO-50 sensor shows a similar sensing behavior to that of pure In2O3 NFs. For example, when the sensor is exposed to NO2 gas, the initial value of λD2 will increase, along with an increase in the height of potential barrier, which can be directly shown as the rapid increase of sensor resistance (Figure S6 and Figure 6). Obviously, the broad EDL should be more suitable for the modulation of electron transfer. Therefore, the IZO-50 sensor performance to both NO2 and ethanol gases can be improved because of the complete EDL in the ZnO thin shell, especially with the help of its porous and wrinkled surfaces, and the effect of the n−n junction is greatly weakened. When the ZnO shell is thick enough, hollow IZO CSNFs can be formed. As shown in Figure 8c, when the sensor is exposed to air, there may be five regions in the hollow IZO-300 CSNFs: the EDL at the inner surface of In2O3 core, the conduction region at the In2O3 core, the n−n junction at the interface of In2O3−ZnO, the conduction region at the ZnO shell, and the EDL at the outer surface of ZnO shell. With the increase of the thickness of ZnO shell, as compared with Figure 8b, the n−n junction region gradually separates from the complete EDL region and tends to form another conduction region in ZnO shell. Now the thickness of ZnO shell (tZnO) is larger than λD4 (λD(ZnO) = λD4). Based on this model, the sensing mechanisms of the IZO-300 sensor become more complex than those of the solid IZO-50 CSNFs. According to the above results, the gas-sensing enhancement of IZO-300 cannot be explained by the specific surface area, so a reasonable hypothesis should be deduced based on the n−n junction or hollow structure. When the sensor is exposed to NO2, the inner surface of In2O3 core may contribute to NO2 sensing, while the main body of the IZO-300 CSNFs has been

Table 1. Gas-Sensing Properties of In2O3−ZnO Systems in the Literature toward Ethanol and NO2 materials In2O3−ZnO core− shell nanowires In2O3−ZnO core− shell nanowires In2O3/ZnO core− shell nanowires In2O3/ZnO nanotubes ZnO@In2O3 core@ shell nanofibers In2O3−ZnO hollow microtubules In2O3/ZnO hollow nanostructures IZO-50 core−shell nanofibers IZO-300 core−shell nanofibers

response (Ra/ Rg or Rg/Ra)

gas concn (ppm)

temp (°C)

400 (ethanol)

350

1 (NO2)

300

3.2

7

1000 (ethanol)

300

1.96

8

100 (ethanol)

275

81.7

17

100 (ethanol)

225

31.87

10

100 (ethanol)

300

100 (ethanol)

260

68.19

9

50 (NO2)

200

78.6

100 (ethanol)

340

37.9

this work this work

265

∼32

ref 7

39

be found that most of the previous reports paid attention to the reducing gas such as ethanol, whereas the reported In2O3−ZnO materials showed a low response to NO2 (even lower than that of single oxide). Nevertheless, the sensor based on IZO-50 CSNFs exhibits a largely enhanced response to NO2 at 200 °C as compared to that of pure In2O3 NFs. In addition, the IZO-300 sensor shows an enhanced response to ethanol at high operating temperatures. 3.3. Gas-Sensing Mechanisms. Gas-sensing mechanisms of In2O3−ZnO systems have been intensively investigated in previous reports.7,24,31,34 It is a widely accepted notion that the electrons will transfer from In2O3 to ZnO, if they are in contact with each other, until an equal Femi energy level is reached for two materials. In general, the electron depletion layer (EDL) model and potential barrier model are suggested to explain the sensor performance to reducing and oxidizing gases in a certain situation.40 However, it may be inappropriate to apply these reported sensing mechanisms directly in the present work, where both the wrinkled surface and hollow core should be considered in the as-prepared IZO CSNFs. The EDL model is generally used to analyze the gas-sensing mechanisms of ultrafine In2O3 or ZnO NFs.41,42 However, the radius of In2O3 NFs (r(In2O3)) in this work is much larger than the thickness of the EDL (λD1); thus, the potential barrier (V) between the cross section of adjacent fibers plays a significant role in the conductivity of the sensing materials.43 As illustrated in Figure 8a, when the pure In2O3 NFs are exposed to air, oxygen molecules will adsorb on the surface of materials and trap free electrons from the conduction band of In2O3 to form oxygen species (O2−, O−, and O2−), causing the formation of EDL. At this point, the λD1 may approximate the Debye length of 6364


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Figure 8. Schematic illustrating the gas-sensing mechanisms of (a) pure In2O3 NFs as well as (b) solid and (c) hollow IZO CSNFs. r(In2O3) is the radius of In2O3 NFs, tZnO is the thickness of the ZnO shell, and λD (λD1, λD2, λD3, and λD4) represents the thickness of the electron depletion region (not drawn to scale).

IZO-200 is

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