Ag2V4O11 nanoheterostructures to ethanol gas

Journal of Alloys and Compounds 811 (2019) 151958

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Enhanced sensing performance of TiO2/Ag2V4O11 nanoheterostructures to ethanol gas Yun zhou a, b, 1, Qiujie Ding a, b, 1, Junyu Li a, b, Yuhang Wang c, Bing Wang a, b, Wenjun Zhu a, Xiaoping OuYang b, Lixin Liu a, **, Yuan Wang a, * a b c

Institute of Fluid Physics, China Academy of Engineering Physics, P.O. Box 919-111, Mianyang, 621900, Sichuan, China School of Materials Science and Engineering, Xiangtan University, Xiangtan, 411105, Hunan, China School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, 100083, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 March 2019 Received in revised form 19 August 2019 Accepted 21 August 2019 Available online 23 August 2019

High performance sensor for gas species detection is a significant task not only for fundamental sciences but also for industries. In this study, a new type of ethanol gas sensor based on one-dimensional TiO2/ Ag2V4O11 nanoheterostructures (NHSs) is reported. The sensitivity of the NHSs to ethanol is much higher than those of pure TiO2 and Ag2V4O11. By regulating the content of TiO2 and Ag2V4O11, it is found that TiO2/Ag2V4O11 NHSs with T/A (TiO2/Ag2V4O11) molar ratio of 2:1 exhibit excellent sensing performances to ethanol. The sensitivity at 300
C to 100 ppm ethanol is 25.6, about 6 times and 3 times higher than TiO2 and Ag2V4O11, respectively, and the sensor also possesses good selectivity to ethanol gas. Specifically, the NHSs exhibit efficient sensitivity (4.2e100 ppm ethanol gas) and excellent stability at room temperature. High-depth mechanism study reveals that besides the contribution of the fast electron transfer properties of the Ag2V4O11, the formation of nanoheterojunctions and the rich surface interfaces rooting from the hybridization of nanoparticles (NPs) and nanowires (NWs) can significantly enhance the sensing performance of the NHSs. © 2019 Elsevier B.V. All rights reserved.

Keywords: TiO2/Ag2V4O11 Hydrothermal synthesis Gas sensor Ethanol Heterojunction

1. Introduction Semiconductor gas sensors have captured tremendous interest because of elevated chemical and thermal stabilities, low fabrication cost, and high content in nature [1e7]. At present, TiO2 is considered to be one of the most promising gas sensitive materials [8,9], different types of gas sensors based on TiO2 nanostructures have been investigated, for instance, TiO2 nanorod [10], TiO2 nanotube [11], nest-like TiO2 nanostructure [12], TiO2/SnO2 coreshell heterostructure nanofiber [13], TiO2/V2O5 nanoheterojunction [14], lotus-like Au@TiO2 nanocomposite [15], and so on. These works illustrate that gas sensors based on TiO2 nanomaterials possess outstanding sensing property, such as excellent response to probe gas, ideal selectivity and short response/recovery (Res/Rec) time. Among these researches, building unique TiO2

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Liu), [email protected] (Y. Wang). 1 Yun Zhou and Qiujie Ding contributed equally to this work. https://doi.org/10.1016/j.jallcom.2019.151958 0925-8388/© 2019 Elsevier B.V. All rights reserved.

based nanoheterostructures (NHSs) is a preferable strategy for improving the sensitivity. Meanwhile, previous reports indicated that the surface modification of anatase TiO2 nanostructure with V species oxides can markedly improve the ethanol gas response compared with pure TiO2 [14,16,17]. As a transition V species oxide, Ag2V4O11 have acquired great attention because of its terrific electronic characteristic and applications. Thanks to its excellent discharge capacity, high-rate capability, and great stability, the Ag2V4O11 material has long been used in the field of lithium-ion battery electrodes [18,19]. In recent years, researchers discovered that Ag2V4O11 exhibits excellent photocatalytic performance and sensing property [20e22]. For instance, Shi et al. [20] demonstrated that the tubular Ag2V4O11 showed better photocatalytic property for isopropyl alcohol degradation in visible light, which could be attributed to its narrow bandgap energy (2.0 eV), the fast electron transfer properties and specific structure material. Fu et al. [22] described that Ag2V4O11 nanobelts showed higher gas-sensitive response to organic amines, the detectable minimum can be up to 5 ppm, and the selectivity is better than Ag0$35V2O5 and VOx@15%Ag nanomaterials. In addition, considering that the interactions between target gas

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molecules and surface-chemisorbed oxygen mainly take place on the surface sensing layers, design nanomaterials with tailored structure and high surface areas is an efficient approach to obtain enhanced sensing performance. For example, when the particle size is about twice the length of Debye depletion layer formed at the oxide surface, the gas sensitivity of the oxide will be greatly improved [23]. Owing to the fast electrons transmitting along the axial direction, one-dimensional nanowires (NWs) have been regarded to possess tremendous advantages in electrons conduction, resulting in the conductivity increase of the gas-sensitive material and reduction of the sensing response time [24,25]. Furthermore, nanoparticle (NP) is another kind of useful nanomaterial, its relative larger BET specific surface area, creating many active adsorption sites and evading the drawbacks [26e28], makes it an ideal candidate with greatly improved gas sensitivity. Therefore, it may be a promising strategy to hybridize the two kinds of nanomaterials to develop high-performance sensing materials. Motivated by this, in this paper, TiO2/Ag2V4O11 NPs/NWs-type NHSs have been designed and synthesized successfully by hydrothermal method, and the NHSs present outstanding sensing property to probe gas than single anatase TiO2 NPs and Ag2V4O11 NWs. 2. Experimental work 2.1. Materials and experiments TiO2 NPs were purchased from XFNANO company. Silver nitrate (AgNO3, analytical reagent) and ammonium metavanadate (NH4VO3, analytical reagent) were purchased from Alfa Aesar. All chemicals were used without further purification. TiO2/Ag2V4O11 NHSs were synthesized by facile hydrothermal method as shown in Fig. 1. Typically, 0.117 g NH4VO3 and 0.170 g AgNO3 powders were added into separate beakers containing 20 mL deionized water, keep magnetically stirring at 50
C for 20 min, after that TiO2 and NH4VO3 in a molar ratio of x:1 (x ¼ 1, 2, 4, 8) were added into the

as-obtained NH4VO3 solution and magnetically stirred for 20 min, setting as solution A. Next, a small amount of glacial acetic acid was slowly added into solution An until the pH value was 2 to form the clear aurantiaceous solution, and setting it as solution B. After stirring for 15 min, a mixture of the solution B and AgNO3 solution was transferred to a 50 mL Teflon lining stainless steel autoclave and heated at 180
C for 16 h. In the growth process, the size of the samples can be controlled by adjusting the hydrothermal temperature and growth time. Subsequently, cool to room temperature, the taupe products were collected and centrifuge several times with distilled water and ethanol, then dried at 70
C for 10 h to obtain the TiO2/Ag2V4O11 NHSs. TiO2/Ag2V4O11 NHSs with TiO2/ Ag2V4O11 M ratio of 1:1, 2:1, 4:1, and 8:1 were recorded as T/A ¼ 1:1, T/A ¼ 2:1, T/A ¼ 4:1, and T/A ¼ 8:1, respectively. 2.2. Characterization and gas sensing test The crystallinity and phase structure of the as-obtained samples were analyzed by x-ray diffraction (XRD, X'pert Pro, Holland Panalytical, CuKa, l ¼ 1.5406 Å), the morphology and microstructure of the samples were analyzed by transmission electron microscopy (TEM, Libra 200FE, Zeiss), high-resolution transmission electron microscopy (HRTEM), and field emission scanning electron microscopy (FESEM, Hitachi S-4800, Hitachi). Nitrogen adsorptiondesorption isotherm (TriStar II 3020, Micromeritics) was used to measure the Brunauer-Emmett-Teller (BET) specific surface areas of the samples. UVevisible absorption spectrum of the sample was recorded using UV-3150 spectrophotometer (Shimadzu instruments). Mott-Schottky experiment was employed to measure the flat-band potential and charge carrier density of as-prepared samples, the instrument is electronic chemistry workstation (RST5200, Zhengzhou Cerui). The relative dielectric constant of the material was obtained by Agilent 4294A Precision LCR meter (Agilent Technologies Inc.). The Fermi energy level was achieved by depositing Au on the sample surface and measuring their boundary

Fig. 1. Schematic illustration of the preparation process for TiO2/Ag2V4O11 NHSs.

Y. zhou et al. / Journal of Alloys and Compounds 811 (2019) 151958

at room temperature through Hitachi SII E-Sweep SPM system (Hitachi). The method of fabricating gas sensors was similar to the early article [29], and the schematic diagram of sensor is shown in Fig. S1(a). Detailed production process of gas sensor device is as follows: first, TiO2/Ag2V4O11 NHSs were dispersed into an ethanol solution to make a paste, and then applied it to the outside surface of the alumina tube which was printed with a pair of gold electrodes previously. The sensor was then placed into a drying oven and dried at 150
C for 3 h. Finally, the nickel-chromium alloy wire was inserted into the alumina tube and used as a heater by adjusting the heating voltage. The high precision detection system (WS-30A, as shown in Fig. S1(b)) was used to test the gas sensing characteristics. When the sensor resistance in air reached a steady state, the certain concentrations of target gas are injected into the 18 L test chamber by a micro-injector through a rubber plug, and two fans in the test chamber make the target gas mixed well. The ethanol gas was obtained by injecting the certain amount of ethanol liquid onto evaporation platform in the gas chamber by a micro-injector through a rubber plug. After the gas sensor resistance reaches a new steady state, the test chamber is opened to recover the gas sensor in air. The working temperature of gas sensor was measured from room temperature to 400
C and the relative humidity (RH) was measured from 0e60%. The relative humidity was controlled by adding certain amount of water onto evaporation platform in the gas chamber, and the value of RH was obtained by inner hygrometer. The sensor value of response could be written as S]Ra/Rg ¼ Ig/Ia. (testing voltage was 6 V), where Ra is the resistance in air atmosphere, Rg is the resistance in the target gas, Ia is the current passing the sensor in air atmosphere, and Ig is the current passing the sensor in the target gas. The response time was defined as the time required between the initial response value and after adding probe gas until reaches 90% of the stable response, the recovery time was defined as the time needed for the sensor to return to 90% above the original response in air after removing the target gas.

3. Results and discussion The crystallinity and phase structure of the samples are identified by XRD analysis and the results are shown in Fig. 2, where the sharp diffraction peaks reveal that the obtained materials have

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good crystallization degree. Furthermore, the pure TiO2 NPs exhibit standard diffraction patterns of the anatase phases TiO2 (JCPDS. NO. 73e1764), and the bare Ag2V4O11 NWs display standard Ag2V4O11 diffraction peaks (JCPDS. NO. 20e1386). In addition, it can be observed that all TiO2/Ag2V4O11 NHSs with different T/A molar ratios exist both TiO2 and Ag2V4O11 diffraction peaks, indicating TiO2/Ag2V4O11 NHSs have been successfully prepared by facile hydrothermal method. The XPS spectra are applied to further characterize the composition of materials, as shown in Fig. S2. Obviously, the high-resolution XPS spectra validate the coexistence of V5þ, Agþ, and Ti4þ in the sample, indicating the TiO2/ Ag2V4O11 is prepared successfully. Fig. 3 presents the typical FESEM patterns of TiO2/Ag2V4O11 NHSs with T/A molar ratios of 1:1, 2:1, 4:1, and 8:1, respectively. It is obviously that the samples possess NWs-like morphology with length of about several microns. As can be seen, in the same hydrothermal experimental parameter, the diameters of Ag2V4O11 in NHSs with different molar ratio are about 200 nm. The TiO2 NPs with diameter of about 30 nm are evenly distributed on the Ag2V4O11 NWs. Moreover, with the increase of T/A molar ratio, the number of NPs deposited on the NWs increases continuously and the agglomeration phenomenon for the NPs is aggravated too. TEM images are recorded to further confirm the microscopic morphology and formation of TiO2/Ag2V4O11 NHSs, as shown in Fig. 4. The TiO2/Ag2V4O11 NHSs can be clearly evidenced in Fig. 4(a), the diameter of TiO2 NPs is about 30 nm, and it is well distributed on the surface of Ag2V4O11 NWs with a diameter of about 200 nm. The selected area electron diffraction (SAED) pattern recorded from the single NPs-modified NWs is illustrated in inset picture of Fig. 4(b), it can be deduced that the sample possesses well crystalline character based on the clear diffraction stripes in the SAED pattern. In order to further observe the crystal structure of NHSs, HRTEM images are performed and presented in Fig. 4(c) and (d). Evidently, the measured lattice distances of 0.35 nm and 0.31 nm correspond to the (101) lattice distance of anatase TiO2 and the ð203Þ lattice distance of Ag2V4O11, respectively, and the result is consistent with the XRD result. Moreover, the good interfacial contact between TiO2 and Ag2V4O11 can also be observed, implying the successful construction of TiO2/Ag2V4O11 NHSs. To better understand the effect of the NHSs’ surface area on the gas sensing property, nitrogen adsorption-desorption isotherm was used to determine the specific surface area of the TiO2 NPs, Ag2V4O11 NWs, and TiO2/Ag2V4O11 NHSs with different molar ratios, as illustrated in Fig. 5. The specific surface area of TiO2 NPs was 95.9 m2/g based on the adsorption-desorption isotherm data, it is about four times larger than that of Ag2V4O11 NWs (23.4 m2/g). Thanks to the growth of TiO2 NPs on the Ag2V4O11 NWs surface, the TiO2/Ag2V4O11 NHSs display increased BET surface areas with increasing molar ratios, which are calculated to be 25.4 m2/g, 34.5 m2/g, 47.2 m2/g and 54.0 m2/g respectively. The large surface area and ample nanoheterojunctions of TiO2/Ag2V4O11 might make a synergistic effect on the enhancement of sensing performance. 3.1. Gas sensing properties

Fig. 2. XRD patterns of TiO2 NPs, Ag2V4O11 NWs, and TiO2/Ag2V4O11 NHSs.

The resistance change of the gas sensor is achieved by alternating the incoming air and the target gas, this reflects the gas sensing properties of the TiO2/Ag2V4O11 NHSs. The contact effect of TiO2/Ag2V4O11 NHSs with the gold electrode on the internal conductivity of the sensor seems to be negligible due to the good operating conditions of all parts. The gas sensitivity of the as-prepared materials was evaluated by optimizing the related parameters, such as concentration conditions, relative humidity, selectivity, repeatability, and dynamic Res/Rec time. Working temperature is often used as one of

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Fig. 3. FESEM images of TiO2/Ag2V4O11 NHSs with T/A molar ratios of (a) 1:1, (b) 2:1, (c) 4:1, and (d) 8:1, respectively.

Fig. 4. (a, b) Typical TEM images of as-prepared TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1), inset picture in Fig. (b) is the SAED pattern. (c, d) HRTEM images of the TiO2 part and Ag2V4O11 part.

indicators to reflect the performance of gas sensor, which greatly affects the sensitivity of the sensor. Therefore, in the beginning, we investigate the gas sensing property of various samples to ethanol gas at varying working temperatures to obtain the optimal working temperature. As clarified in Fig. 6(a), the sensitivities of Ag2V4O11 NWs and TiO2/Ag2V4O11 NHSs increase with the temperature and reach the highest value at 300
C, after that, the sensitivities decrease with the further increasing of temperature. This may be because large numbers of oxygen molecules adsorbed on the material's surface will escape before the oxidation and reduction reaction when the working temperature is too high, thus the change of resistance may decline. As for pure TiO2 NPs, the sensitivity increase with the temperature and optimize working temperature has not appeared until 400
C. The Ra and Rg of the samples at

different temperatures are illustrated in Table 1, it can be confirmed that all the sensors are semiconductor-type till 400
C according to the decrease of the Ra with the working temperature. Obviously, comparing with the pure TiO2 NPs, TiO2/Ag2V4O11 NHSs displays lower proper working temperature, this can be ascribed to the introduction of Ag2V4O11 and the formation of heterojunctions. Selectivity is another factor to measure the performance of gas sensors. To be specific, gas sensors need to be more sensitive to a certain gas under operating conditions, to avoid being influenced by other gases in the same surroundings. Fig. 6(b) shows the selectivity of different as-prepared sensors to five tested gases of 100 ppm, and the specific sensitivity values can be referred to Table 2. Evidently, the sensitivity of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) to ethanol is at least 2.5 times higher than that of other

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Table 2 Sensitivity of TiO2 NPs, Ag2V4O11 NWs, and TiO2/Ag2V4O11 NHSs based sensors towards 100 ppm various probe gases at 300
C and 30% RH. Material

TiO2 T/A ¼ 1:1 T/A ¼ 2:1 T/A ¼ 4:1 T/A ¼ 8:1 Ag2V4O11

Fig. 5. N2 adsorption-desorption isotherms of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1), the inset picture shows the BET surface areas of different as-prepared samples.

contradistinctive gases, such as acetone, ammonia, methanol, and toluene at the same testing conditions. Similar to TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1), other TiO2/Ag2V4O11 NHSs display low sensitive values towards contradistinctive gases too, implying the excellent selectivity of TiO2/Ag2V4O11 NHSs based sensors to ethanol.

Sensitivity (Ra/Rg) ethanol

acetone

ammonia

methanol

toluene

4.06 16.47 25.60 23.06 16.59 9.28

3.40 8.86 10.14 10.20 8.89 4.07

2.68 8.29 9.60 9.56 8.06 3.76

3.64 7.70 8.79 8.74 7.57 3.47

2.85 6.85 7.96 7.96 7.09 3.03

In order to study the influence of T/A molar ratio on response capability of the sensors, the sensing performance of TiO2/Ag2V4O11 with different T/A molar ratios to 100 ppm ethanol are investigated here (as shown in Fig. 7). It can be observed that the sensitivities of TiO2 NPs, Ag2V4O11 NWs, and TiO2/Ag2V4O11 NHSs with T/A molar of 1:1, 2:1, 4:1, and 8:1 at 300
C are 4.1, 9.3, 16.5, 25.6, 23.1, and 16.7, respectively. Furthermore, the Res/Rec time curves for different materials are explored, too. Among these different sensors, pure TiO2 NPs exhibits the longest Res/Rec time (18/15 s). By coupling of Ag2V4O11 material gradually, the Res/Rec time of TiO2/Ag2V4O11 NHSs becomes shorter, and the TiO2/Ag2V4O11 NHSs with T/A molar ratio of 2:1 exhibits the minimum value of 11/9 s. After that, further increasing the amount of Ag2V4O11 would lead to the increase of Res/Rec time again. The Ag2V4O11 NWs display longer Res/Rec time than TiO2/Ag2V4O11 NHSs but shorter than pure TiO2 NPs. The

Fig. 6. (a) Dynamic response of different as-prepared materials based sensors at different operating temperatures towards 100 ppm ethanol at 30% RH. (b) Selectivity tests of different as-prepared materials based sensors towards different detection gases of 100 ppm at 300
C and 30% RH.

Table 1 Resistance and sensitivity of TiO2 NPs, Ag2V4O11 NWs, and TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) based sensors at different temperature towards 100 ppm ethanol gas at 30% RH. Material

TiO2

Ag2V4O11

TiO2/Ag2V4O11

Temperature

Rg Ra Ra/Rg Rg Ra Ra/Rg Rg Ra Ra/Rg

200
C

250
C

300
C

350
C

400
C

18.17 MU 54.98 MU 3.03 1.16 MU 5.75 MU 4.96 1.40 MU 19.73 MU 14.09

14.46 MU 46.04 MU 3.18 0.72 MU 5.06 MU 7.03 0.81 MU 15.13 MU 18.68

9.94 MU 40.31 MU 4.06 0.5 MU 4.64 MU 9.28 0.49 MU 12.54 MU 25.60

7.31 MU 33.86 MU 4.63 0.71 MU 4.27 MU 6.01 0.85 MU 11.75 MU 20.26

6.59MU 31.09MU 4.72 0.96 MU 4.00 MU 4.17 0.92 MU 11.02 MU 11.97

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Fig. 7. (aef) Res/Rec curves of TiO2 NPs, Ag2V4O11 NWs, and TiO2/Ag2V4O11 NHSs with T/A molar of 1:1, 2:1, 4:1, and 8:1e100 ppm ethanol at 300
C and 30% RH, respectively.

results reveal that under an optimum T/A molar ratio of about 2:1, the sensor exhibits the highest sensitivity and best Res/Rec dynamics. The enhanced sensing property can be ascribed to the formation of a mass of heterjunctions and the much efficient charge transfer process of Ag2V4O11 NWs. To sum up, TiO2/Ag2V4O11 NHSs with T/A molar ratio of 2:1 exhibit most excellent sensing performance, thus future experiments will be focused on the TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) based sensor. To explore the effect of relative humidity on the sensitivity of sensor, the humidity dependence of the sensing performance is systematically studied and shown in Fig. 8(a). Obviously, the

calculated response value of the sensor decreases from 34.1 to 30.8, 28.4, 25.6, 22.9, 20.4, and 19.3 with the increase of relative humidity from 0% to 10%, 20%, 30%, 40%, 50%, and 60%, respectively. It can be observed the resistance value in humid conditions shows an increase compared to the initial resistance level under dry conditions (as shown in Table 3). This can be ascribed to the electrons extracting of the water molecules from the materials to form adsorbed oxygen species. Moreover, when the TiO2/Ag2V4O11 NHSs based sensor exposure to ethanol gas, the electrons will be released back to TiO2/Ag2V4O11 NHSs surface, whereas, the plentiful H2O molecules would strongly suppress the reaction and result in the

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Fig. 8. (a) Transient sensing response of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) at different relative humidity towards 100 ppm ethanol at 300
C. (b) Response curve of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) towards 5e1000 ppm concentrations of ethanol at 300
C and 30% RH. (c) Response of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) towards different concentrations of ethanol at 300
C and 30% RH. The inset picture shows the calibration curve to 5e100 ppm ethanol. (d) Plot of Log (S-1) and log C towards 5e1000 ppm of ethanol for TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) based gas sensor.

Table 3 Resistance and sensitivity of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) based sensor at different relative humidity towards 100 ppm ethanol gas at 300
C. Relative humidities (%)

Resistance in ethanol gas (Rg)

Resistance in air (Ra)

Sensitivity (Ra/Rg)

0 10 20 30 40 50 60

0.28 MU 0.34 MU 0.41 MU 0.49 MU 0.61 MU 0.75 MU 0.84 MU

9.55 MU 10.47 MU 11.63 MU 12.54 MU 14.09 MU 15.26 MU 16.19 MU

34.1 30.8 28.4 25.6 22.9 20.4 19.3

increase of Rg. Comparing the value of Ra and Rg, we can obtain that the increased rate of Rg is larger than that of Ra, implying the influence of the H2O molecules on TiO2/Ag2V4O11 NHSs resistance at ethanol condition is larger than that at air condition, thus resulting a response value decrease in the humid environment. Meanwhile, nonlinear relationship between sensitivity and relative humidity is presented and the reduction rate of response value decreases gradually with the increasing relative humidity, indicating the material possesses certain resistant function under high humid environment. Moreover, the dynamic response of TiO2/Ag2V4O11 NHSs (T/ A ¼ 2:1) with ethanol concentrations of 5e1000 ppm is investigated. As illustrated in Fig. 8(b), the sensitivities to 5, 10, 50, 100, 500, and 1000 ppm ethanol are 1.3, 2.8, 11.7, 25.6, 32.3, and 35.5,

respectively. Obviously, as shown in Fig. 8(c), the sensitivity increases linearly with ethanol concentration at concentration range of 5e100 ppm. Whereas when ethanol concentration is higher than 100 ppm, linear response behavior disappears due to the response saturation of the gas sensor. The inset figure of Fig. 8(c) shows the corresponding calibration curve of response vs. ethanol concentration (5e100 ppm), where the relationship of response (Y) vs. ethanol concentration (X) can be expressed as Y ¼ 0.25293X0.08346. Herein, the responstivity of 0.2593 ppm1 is obtained, and the correlation coefficient R2 is calculated as 0.99583, which presents the strong linear correlations between response and ethanol concentration at the concentration range of 5e100 ppm. indicating that the TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) sensor exhibits a nearly linear response at the ethanol concentration range of 5e100 ppm.

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When studying the relationship between different concentrations of gas and types of oxygen ions on the material surface, it is found that the sensing performance is closely related to adsorbed oxygen species. Usually, the relationship between gas sensitivity and concentration can be obtained from the following empirical equation [30,31]:

 S ¼ Ra Rg ¼ 1 þ aC b

(1)

which can be rewritten as follows:

Ra logð  1Þ ¼ blogC þ loga Rg

(2)

where, C is the partial pressure of the target gas which is positively correlated with concentration, the value of b is the main criterion for determining the type of negative oxygen ions adsorbed on the surface of materials. Researches find that as the value of b is about 0.5, O2 adsorbed on the surface of materials is the main component, whereas when b is about 1, O occupies the main adsorption mode. From Fig. 8(d), it can be calculated that the b value at the concentration range of 5e100 ppm is 1.39, which is close to 1, revealing the negative oxygen ions adsorbed on the NHSs surface are mainly O ions. Whereas at high concentration (100e1000 ppm), the b value is 0.34, which is close to 0.5, indicating that the negative oxygen ions adsorbed on the NHSs surface in high gas concentration is mainly O2 ions. Additionally, as shown in Fig. S4, we investigate the high resolution XPS spectra of the O 1s core-level binding energies of TiO2 NPs, Ag2V4O11 NWs, and TiO2/ Ag2V4O11 NHSs, respectively. It can be seen that the TiO2/Ag2V4O11 NHSs exhibit the excellent O2 absorption ability than TiO2 NPs and Ag2V4O11 NWs, which would provide more oxygen adsorption sites and improve the gas sensing performance. Moreover, the repeatability of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) based gas sensor is evaluated by repeating 10 times to 100 ppm ethanol (300
C, 30% RH), as shown in Fig. S3(a). It can be observed that the reversible cycles of response curve of NHSs show retained response of about 25.6, suggesting a steady and credible operation of gas sensor. The reproducibility TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) based sensors are investigated and displayed in Fig. S3(b). Obviously, all the sensors exist high sensitivity of 25.4e25.6, indicating TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) do exist outstanding reproducibility and promising application for ethanol detection. At this point, we summarize the work in this paper with the related work reported in recent literatures, as shown in Table 4, and the results show that TiO2/ Ag2V4O11 NHSs (T/A ¼ 2:1) possess relatively high comprehensive gas-sensitive performance compared with other semiconductor NHSs based sensors. This indicates that TiO2/Ag2V4O11 NHSs based ethanol sensor has potential application prospect. In addition, we further investigate the sensing performances of TiO2/Ag2V4O11 NHSs at room temperature. Fig. 9(a) illustrates the sensing response of the sensors to 100 ppm ethanol at room temperature. Obviously, the sensitivity of pure TiO2 NPs based sensor to

100 ppm ethanol is 1.9. As for pure Ag2V4O11 NWs based sensor, the sensitivity is 1.8. Thanks to the formed heterjunctions, the response values of TiO2/Ag2V4O11 NHSs to ethanol at room temperature are enhanced distinctly. For comparison, the sensing response of TiO2/ Ag2V4O11 NHSs with molar ratio of 2:1 exhibit the highest sensitivity of 4.2e100 ppm ethanol, and the Res/Rec time is about 14/ 12 s. Moreover, the dynamic response of TiO2/Ag2V4O11 NHSs (T/ A ¼ 2:1) to different ethanol concentration at room temperature was studied. As shown in Fig. 9(b), the sensitivities to 5, 10, 50, 100, 500, and 1000 ppm ethanol are 1.3, 1.6, 2.9, 4.2, 5.3, and 6.2, respectively. This indicates that the TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) still possess ability to detect ethanol at room temperature. Moreover, sensitivity curves of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) to 100 ppm ethanol at room temperature in different relative humidity were tested, as shown in Fig. 9(c). The NHSs based sensor exhibits sensitive response at different relative humidity, and the sensitivity of sensor decreases with the increase of relative humidity, this trend is similar to that at 300
C. Furthermore, the selectivity of the TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) based senor was tested to ethanol, acetone, ammonia, methanol, and toluene, as shown in Fig. 9(d). It is obvious that the sensitivity of the sensor to ethanol is about 2 times higher than other gases. Furthermore, we further study the room temperature cycling performance of the NHSs, as shown in Fig. S5(a). It can be observed that after 10 repeated tests, the sensitivity of the sensor remains about 4.2, indicating its good repeatability. As displayed in Fig. S5(b), the TiO2/ Ag2V4O11 NHSs (T/A ¼ 2:1) based sensors exist effective response of 3.5e4.2, indicating TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) possess a reproducibility at room temperature. The high sensing performance makes TiO2/Ag2V4O11 NHSs a potential candidate for ethanol detection under room temperature. In the above property characterization, the NHSs display excellent sensing properties compared with the bare materials. At this point, we further explore the energy band structure and surface adsorption process of the NHSs to analyze the sensing mechanism. First, in order to explore the electrical properties of materials, Mott-Schottky tests have been employed, as shown in Fig. 10(a) and Fig. 10(b). The conducting type and carrier concentration of the materials can be quantified by the Mott-Schottky equation [37]. 1/C2¼(2/eεε0Nc)[(E-Efb)-kT/e]

(3)

Where C is the space charge capacitance, e is the elementary charge, ε0 is the vacuum dielectric constant, Nc is the charge carrier density, E is electrode potential, and ε is the dielectric constant, which can be obtained via following equation [38]: ε ¼ Tl/(εopr2)

(4)

Where T is the capacitance, l is the thickness of the sample, εo is the permittivity of vacuum, r is the radius of the electrode. The test parameters are listed in Table S4, and the calculated ε at the

Table 4 Comparison of the sensitivity, response/recovery (Res/Rec) time of the TiO2/Ag2V4O11 NHSs based ethanol sensor and other metal oxide NHSs based ethanol sensors. NHSs based gas sensors

Ethanol concentration (ppm)

Work temperature (
C)

Sensitivity (Ra/Rg)

Res/Rec time (s)

Reference

TiO2/Ag2V4O11 NHSs TiO2/ZnO heterojunction TiO2/SnO2 hybrid oxide TiO2/MoS2 composites TiO2/V2O5 nanoheterostructure TiO2/a-Fe2O3 nanoheterostructure TiO2/CeO2 nanorods

100 100 100 100 100 100 300

300 320 320 150 350 370 300

25.6 13.2 10 14.2 24.6 14.2 5.44

11/9 5-10/5-10 e 8-10/8-10 6/7 5-7/4-6 8-10/8-10

This work [32] [33] [34] [14] [35] [36]

Y. zhou et al. / Journal of Alloys and Compounds 811 (2019) 151958

9

Fig. 9. (a) Sensing response of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) based sensor to 100 ppm ethanol at room temperature and 30% RH, inset picture shows a comparison of the response values of the different materials. (b) Response of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) based sensor towards different concentrations of ethanol at room temperature and 30% RH. (c) Transient sensing response of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) based sensor at different relative humidity towards 100 ppm ethanol at room temperature. (d) Selectivity tests of TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1) towards 100 ppm of different detection gases at room temperature and 30% RH.

frequency of 100 MHz for TiO2 and Ag2V4O11 are 42.5 and 213.8, respectively, the carrier concentration of TiO2 and Ag2V4O11 are calculated to be 2.1  1021 cm3 and 4.1  1020 cm3 via MottSchottky equation. Furthermore, both TiO2 and Ag2V4O11 possess positive slopes, manifesting the characteristic n-type semiconductor behavior of the samples. Fig. 10(c) and (d) display the electrochemical impedance spectroscopy of the TiO2, Ag2V4O11, and TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1), the results reveal that Ag2V4O11 possesses the smallest radius, indicating the highest carrier transfer capability. It can be deduced that although the carrier concentration of Ag2V4O11 is lower than that of TiO2, the carrier transfer ability of the Ag2V4O11 is much higher than TiO2. Thus the hybridization of the TiO2 and Ag2V4O11 may integrate the advantages of the two materials and thus improve the electron transfer efficiency, accordingly the gas sensitivity of the TiO2/Ag2V4O11 NHSs can be greatly enhanced. Secondly, as crucial structure information, band position is often the focus of sensing mechanism research. Thus, the energy bands matching of the heterojunction formed at the interface between TiO2 and Ag2V4O11 are analyzed here. According to formula (3), the flat band potential of the material can be obtained. Fig. 10(a) and (b) display that the flat band potential of TiO2 and Ag2V4O11 are about 0.53 eV and 0.76 eV vs. RHE (4.03 eV and 5.26 eV vs. vacuum level), respectively. It is well known that the flat band

potential is considered to be located just under the conduction band for n-type semiconductors. Hence, the conduction band of the TiO2 and Ag2V4O11 are estimated to be 4.03 eV and 5.26 eV vs. vacuum level. Fig. 11(a) displays the UVevis absorption spectrum of Ag2V4O11 NWs. Obviously, the sample shows an absorption band from UV light to visible-light, implying the possibility of its photoabsorption activity. The band gap energy of semiconductor can be acquired from the UVevis absorption spectrum according to the following equation [39]. (ahn)n ¼ A(hn-Eg)

(5)

Where a is the absorption coefficient, hn is the incident photo energy, A is a constant associated with the material, Eg is the band gap energy, n is determined by the type of the semiconductor and the value is 1/2 for indirect band gap semiconductor Ag2V4O11. The Eg extrapolated from the tangent line in Tauc's plot (Fig. 11(b)) of UVevis spectrum is about 1.86 eV, which is similar to the previous reported results [20,40]. Moreover, the Kelvin probe force microscopy measurement shows that the Fermi energy level of Ag2V4O11 is 0.2 eV lower than that of Au, considering the work function of 5.1 eV for Au, the work function of the Ag2V4O11 is determined to be 5.3 eV. Moreover, the standard literature energy levels of TiO2 (valence band of 7.2 eV, conduction band of 4.0 eV, the Fermi

10

Y. zhou et al. / Journal of Alloys and Compounds 811 (2019) 151958

Fig. 10. Mott-Schottky plots of (a) TiO2 NPs and (b) Ag2V4O11 NWs. Electrochemical impedance spectroscopy of (c) TiO2 NPs, TiO2/Ag2V4O11 NHSs (T/A ¼ 2:1), and (d) Ag2V4O11 NWs, respectively.

Fig. 11. (a) UVevis absorption spectrum of Ag2V4O11 NWs. (b) Tauc's plot of Ag2V4O11.

level of 4.2 eV vs. vacuum level, respectively) are employed here to analyze the matching of energy level [41]. Based on the above investigation, the energy band structure of the TiO2/Ag2V4O11 heterojunction is analyzed and presented in Fig. 12. As is demonstrated, the Fermi level of Ag2V4O11 is relatively lower than that of TiO2 (Fig. 12(a)), the electrons on the high level side will transfer to the low level side until the Fermi level reach the balance. Clearly, when two kinds of semiconductors constitute

heterojunctions, their energy levels will match each other and reach an equilibrium state (Fig. 12(b)). On this point, energy barrier will be formed at the interfaces of heterojunctions, which can influence the conduction channel of TiO2/Ag2V4O11 NHSs. The relationship between the energy barrier and the resistance can be obtained by the following equation [42]. R f R0exp(e4/kT)

(6)

Y. zhou et al. / Journal of Alloys and Compounds 811 (2019) 151958

11

Fig. 12. Plot of energy band structure and the formation of heterojunction (EC: conduction band energy level; EV: valence band energy level; EF: Fermi level).

Where R0 is the premier resistance, e stands for elementary charge, 4 reflects the height of energy barrier, k is the Boltzmann's constant, T is an absolute temperature. It can be deduced that the gas sensitivity of Ra/Rg is in direct proportion to the exp (De4). Because of the capture of free electrons by oxygen species in air atmosphere, the effective energy barrier (e4) would increase accordingly, and caused the widening of the depletion layer (Fig. 12(b)). While convert to ethanol condition, the ethanol gas reacts with the negative oxygen ions on the surface of the material and releases electrons, thus reduces the depletion layer and the height of the energy barrier (Fig. 12(c)). The additional reduction of the energy barrier will cause a significant additional alteration of the resistance, thus the gas sensitivity of the NHSs would be enhanced observably compared with the purity materials [43]. Finally, the surface reaction process is another important factor for affecting the gas sensitivity. We propose an analogous model of surface reaction processes for the Ag2V4O11 NWs and TiO2/ Ag2V4O11 NHSs based sensors, as shown in Fig. 13. When the sensors are placed in air, the oxygen molecules initially absorbed on the surface of material, and the absorbed oxygen molecules are  ionized into oxygen ions (O 2 and O ) via the reactions in equations (7)e(9). Due to the consumption of electrons in the surface of the semiconductor, a depletion layer can be formed (left figures in Fig. 13) and the resistance would increases accordingly. Ethanol acts as a reductive gas, upon the sensors exposure to ethanol gas, the electrons can be released back to the surface of the semiconductor by reactions in equations (10) and (11), thus the depletion layer would be thinned (right figures in Fig. 13) and the resistance would decrease [44]. O2 (gas)dO2 (ads)

(7)

O2 (ads) þ edO 2 (ads)

(8)

  O 2 (ads) þ e d2O (ads)

(9)

absorbed gas molecules can be induced. Furthermore, owing to the heterojunction effect, there are additional depletion layer in the heterojunction interface. This predicates much obvious depletion layer would be formed at the surface of the NHSs compared with the pure materials in air environment (left figures in Fig. 13). In addition, because the small particles size of the TiO2 NPs, the carriers in the TiO2 NPs may be outright depleted by surface adsorbed O2 molecules. While in ethanol ambience, the depletion layer would be narrowed more obviously than bare materials (right figures in Fig. 13). Thus, more evident change of the depletion layer in heterojunction interfaces and the entire depletion in NPs can lead to much more change for the resistance, therefore the NPs/NWs hybridized NHSs possess outstanding sensing performance. Additionally, TiO2 and Ag2V4O11 are effective photocatalyst, which could cause additional photocatalytic reaction in the gas sensing process. So the catalytic effect is also a considerable reason for the enhancing sensing performance [20]. From all the above, the superior sensing performance of TiO2/ Ag2V4O11 NHSs can be attributed to the more absorption site, electronic sensitization of Ag2V4O11, formation of TiO2/Ag2V4O11 heterojunction, and the catalytic effect. Additionally, T/A molar ratio can affect the sensing performance evidently, this can be ascribed to the competition relationship between carriers diffusion ability, quantity of heterojunctions and surface interfaces. In detail, with the increase of the TiO2 amount, the surface interfaces and heterojunctions would be improved, thus the sensing performance is enhanced. While with the superfluous TiO2 is introduced, although the BET surface of the NHSs is increased, the formation of heterojunctions would be saturated and superfluous TiO2 would agglomerate together. Furthermore, the reduction of the relative amount of Ag2V4O11 would lead to the decline of the carrier diffusion ability. Thus, the sensing performance is weakened obviously. Accordingly, the TiO2/Ag2V4O11 NHSs with molar ratio of T/A ¼ 2:1 based sensor exhibits excellent sensing performance. 4. Conclusions

 CH3CH2OH (gas) þ 3O 2 (ads)d2CO2 (gas) þ 3H2O þ3e

(10)

CH3CH2OH (gas) þ 6O (ads) d2CO2 (gas) þ 3H2O þ6e

(11)

As displayed in previous XPS analysis (Fig. S4), TiO2/Ag2V4O11 NHSs can provide more gas adsorption sites, consequently, more

In summary, one-dimensional TiO2/Ag2V4O11 NHSs have been successfully synthesized in this study. Characterization results indicate that the NHSs based sensor (TiO2/Ag2V4O11 NHSs) exhibits a lower optimized working temperature of 300
C, better selectivity (25.6) towards ethanol, and shorter Res/Rec time (11/9 s) compared

12

Y. zhou et al. / Journal of Alloys and Compounds 811 (2019) 151958

Fig. 13. Surface processes associated with the reaction with ambient oxygen and ethanol of Ag2V4O11 and TiO2/Ag2V4O11 NWs.

with pure anatase TiO2 NPs and Ag2V4O11 NWs based sensors. Specifically, the NHSs based sensor also exhibits good selectivity, efficient sensitivity (4.2e100 ppm ethanol), and excellent stability at room temperature. The enhanced sensing performance can be ascribed to the electronic sensitization of Ag2V4O11 NWs, formation of TiO2/Ag2V4O11 heterojunction, and the hybridization of the NWs and NPs. This study would administer to the basic sensing mechanism research of the NHSs and further application of ethanol sensors. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 11704354) and Hunan Provincial Innovation Foundation for Postgraduate (No. CX2018B351).

[10]

[11]

[12] [13]

[14]

[15]

[16]

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.151958.

[17]

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