Highly selective and sensitive xylene sensors based on Nb-doped NiO nanosheets

Journal Pre-proof Highly selective and sensitive xylene sensors based on Nb-doped NiO nanosheets Lei Qiu, Shendan Zhang, Jiangbo Huang, Chenhao Wang, Ruiyang Zhao, Fengdong Qu, Pei Wang, Minghui Yang

PII:

S0925-4005(19)31719-8

DOI:

https://doi.org/10.1016/j.snb.2019.127520

Reference:

SNB 127520

To appear in:

Sensors and Actuators: B. Chemical

Received Date:

26 July 2019

Revised Date:

6 November 2019

Accepted Date:

1 December 2019

Please cite this article as: Qiu L, Zhang S, Huang J, Wang C, Zhao R, Fengdong Q, Wang P, Yang M, Highly selective and sensitive xylene sensors based on Nb-doped NiO nanosheets, Sensors and Actuators: B. Chemical (2019), doi: https://doi.org/10.1016/j.snb.2019.127520

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Highly selective and sensitive xylene sensors based on Nb-doped NiO nanosheets

Lei Qiua,c, Shendan Zhangc,d, Jiangbo Huanga, Chenhao Wangc, Ruiyang Zhaob, Fengdong Quc,*, Pei Wanga,* and Minghui Yangc,d,*

Materials Science and Engineering Department, Dalian Maritime University,Dalian, 116026, P. R.

ro of

a

China b

State Key Laboratory Base of Eco-chemical Engineering, College of Chemical Engineering,

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo

re

c

-p

Qingdao University of Science and Technology, Qingdao, 266042, PR China

315201, PR China

Center of Material Science and Optoelectronics Engineering, University of Chinses Academy of

na

Sciences, Beijing 100049, China.

lP

d

ur

*Corresponding author email: [email protected] (F. Qu); [email protected] (P. Wang);

Jo

[email protected] (M.Yang)

Graphical abstract

ro of

Research Highlights 

Nb-doped NiO nanosheets with different dopant concentration were prepared through a facile

Dopant concentration variation influenced morphology, carrier concentration and sensing

re



-p

hydrothermal process.

property.

lP

Nb-NiO sensor exhibits excellent sensitivity and good sensitivity to xylene.

na



ur

Abstract: It’s demonstrated that doping of aliovalent atom can greatly influence the sensing performance of metal oxides-based gas sensors. In this work, Nb-doped nickel oxides with Nb

Jo

contents in the range of 6.2 to 29.1 at% have been synthesized by a one-step hydrothermal method. The gas sensing test results indicates that the 20.2 at% Nb doped NiO possesses an ultrahigh response (335.1 to 100 ppm), excellent selectivity and theoretical ppb-level detection limit (2 ppb) to xylene at 370 °C, which is much better than that of pure NiO sensor. The higher specific surface area and the enhanced catalytic activity caused by higher ratio Ni3+/Ni2+ are considered as the main

reasons for the enhanced gas sensor performance.

Keywords: Nb-doped NiO; nanosheets; xylene; electron sensitization; gas sensors

1. Introduction In recent decades, volatile organic compounds (VOCs) emissions such as benzene, xylene, toluene,

ro of

formaldehyde, and acetone have aroused widespread concern with the rapid development of economy and industrialization. Xylene, as a representative and vulnerable gas pollutant, is widely used and released in the field of industrial productions, including as chemical intermediates to

-p

synthesize polyester, a solvent in the paint, rubber and leather industries. It also can be found in the gasoline, cigarette smoke, building and decorating materials, etc [1-3]. As we all know, xylene

re

vapors can cause environmental pollution and damage human being health. The Center for Disease

lP

Control and Prevention indicates that long-term exposure to 14 ppm xylene and short-term inhalation of as low as 50 ppm xylene can impair the human respiratory system, the central nervous system,

na

liver, kidneys, eyes, and skin [4]. Therefore, it is remarkably necessary to develop high-performance

ur

gas sensors to detect xylene gas effectively. Nickel oxide (NiO), a typical p-type metal oxide semiconductor (MOS), has a widely adjustable

Jo

band gap (3.6 - 4.0 eV), distinguished electrical properties, and excellent chemical stability [5, 6]. It is widely used in the field of lithium-ion batteries [7], catalysis [8], and supercapacitor [9]. Moreover, NiO possesses a prominent catalytic activity for VOCs oxidation, which makes it serve as a good candidate to design and fabricate high-performance gas sensors for VOCs detection [10]. However, due to the special conduction through the parallel paths (i.e., the resistive particle cores and

semiconducting near-surface regions), the p-type semiconductor usually shows a relatively low response and selectivity [11]. Hence, it is very meaningful to find a method to improve NiO-based gas sensor performance. Currently, aliovalent doping are considered as the principal method for fabricating highly sensitive and selective chemical gas sensors [12-14]. When substitution occurs between ions of different valence states, electrons or holes will be generated to maintain electron balance, thereby changing

ro of

the concentration of charge carriers to achieve electronic sensitization. Both theoretical and

experimental studies have indicated that regulation of the electron donor/acceptor density by dope

-p

aliovalent metal is an efficient way to improve gas sensor performance. Niobium pentoxide (Nb2O5) is an n-type semiconductor with high dielectric constant and catalytic properties [15, 16]. Combined

re

Nb with NiO, it could be an efficient method to fabricate gas sensor material with excellent

lP

performance.

In this work, pure NiO and Nb-doped NiO nanosheets with different dopant concentration have

na

prepared successfully by facile hydrothermal processes. The gas sensing test results show that 20.2 at% Nb-doped NiO sensor exhibits the maximum response value of 335.1 to 100 ppm xylene at 370 oC,

ur

which is about 110 times larger than that of pristine NiO sensor. The improved gas sensing

Jo

performance can be attributed to the high specific surface area and electron sensitization caused by Nb-doping.

2.Experimental

2.1 Chemical reagent Ni(NO3)2·6H2O, NbCl5, urea and ethylene glycol (EG) are purchased from Beijing Chemicals Co,

Ltd. (Beijing, China). All of these chemicals are analytical grade and used as received without further purification. 2.2 Synthesis progress 2.2.1 Preparation of NiO nanosheets In a typical process, 2.12 g Ni(NO3)2.6H2O and 1.32 g urea are dissolved in a 45 ml mixture of

ro of

deionized (DI) water and ethylene glycol (EG) (v:v = 4:5). Then the mixture is stirred for one hour to form a homogeneous solution. The solution is transferred into a 50 mL Teflon-lined autoclave reactor and then placed in a 120 oC preheated oven for 4.5 h without active stirring. After that, the light

-p

green powder is collected by centrifugation and subsequently washed by DI water and isopropanol.

re

Then the powder is dried under vacuum at 60 oC overnight. Finally, the NiO nanosheets are obtained by sintering the precipitate at 450 oC for 2 h in air.

lP

2.2.2 Preparation of Nb-doped NiO nanosheets (denoted as Nb-NiO)

na

The synthetic process of Nb-NiO nanosheets is similar to the above procedure, except for the addition of NbCl5. The actual proportions of Nb/(Ni+Nb) are measured to be 6.2, 13.2, 16.0, 20.2,

ur

26.0, and 29.1 at% by inductively coupled plasma-atomic emission spectrometry (ICP-AES) (Table

Jo

S1), corresponding to input radio of Nb/(Ni+Nb) (5, 10, 15, 20, 25, and 30 at%) in solution mixture. The 6.2, 13.2, 16.0, 20.2, 26.0, and 29.1 at% Nb-NiO (defined as 6.2Nb-NiO, 13.2Nb-NiO, 16Nb-NiO, 20.2Nb-NiO, 26Nb-NiO and 29.1Nb-NiO) samples are used for gas sensing measurements. 2.3. Characterization

The crystallinities and phases of the samples are detected from X-ray diffraction analysis (XRD, Rigaku D/Max-2550) with Cu Kα radiation (λ = 1.541 Å; 40 kV, 350 mA). The morphology and size of the products are observed by a field emission scanning electron microscope (FESEM, JSM-7610F) and the detailed structural and morphology are performed using a transmission electron microscopy (TEM, JEOL-JEM-2100). In addition, the analysis of energy dispersive X-ray spectroscopic (EDS) are obtained from TEM attachment. Surface area and pore size distribution are evaluated using

ro of

Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively.

Meanwhile, the Thermo K-alpha X-ray photoelectron spectrometer (XPS) are conducted to

characterize the chemical compositions of the samples. The content of elemental is determined by

re

2.4. Fabrication and measurement of gas sensor

-p

inductively coupled plasma-atomic emission spectroscopy (ICP-AES, OPTIMA 3300DV).

lP

For gas sensing measurements, a resistivity-type sensor device is fabricated as follows: the sensing material is ground evenly and well dispersed in DI water by ultrasonic processing for about 30 min

na

to obtain a homogeneous suspension. Then, it is deposited in the middle of the interdigitated electrodes using dripping processing [17, 18]. The sensor is dried at 60 oC in a vacuum oven to

ur

remove DI water. After aging for 48 h, the as-prepared sensors are tested by CGS-8 intelligent gas

Jo

sensing analysis system (Beijing Elite Tech Co., Ltd., China) at a relative humidity (RH) range of 26–35% and room temperature (25 oC) [19]. The response of sensors is defined as S = Rg/Ra, where the Rg is the sensing resistance in target gas and the Ra is that in the air. The response time and recovery time are defined as the time spent for reaching 90% of total resistance variation when sensing materials exposed in the air and target gas, respectively.

3. Results and discussion 3.1. Structural and morphological characteristics The phase and crystal structure of the samples are characterized by X-ray diffraction (XRD) shown in Fig. 1. It can be observed that the diffraction peaks of all samples are consistent with the cubic phase of nickel oxide (NiO) (JCPDS card no. 01-071-1179). No other diffraction peaks

ro of

corresponding to Nb element can be observed. The content of Nb below the detect limitation of XRD or the incorporation of Nb into the NiO lattice may account for the absence of Nb-related peaks. According to ICP analysis, the reason for the pretty low content of Nb can be excluded. Therefore,

-p

Nb may be doped into the lattice of NiO. The substitution process requires that the ionic radii of the host and foreign cations be similar. The ionic radii of Ni2+ and Nb5+ at the coordination number of six

re

are 0.69 Å and 0.68 Å, respectively [20, 21]. Moreover, it can be clearly discovered that the (200)

lP

peak of Nb-NiO shifts to lower angel when Nb combines with NiO. This phenomenon indicates Nb metal atoms in the as-grown NiO are likely interstitial or substitute dopants, and they may have been

na

located inside the cages of cubic NiO or existed in the interstitial space, resulting in the local distortions as a result of slight lattice expansion [22]. Thus, it can be proved that Nb element has

ur

been incorporated into NiO lattice successfully. As shown in Fig. S1, the XRD pattern of

Jo

20.2Nb-NiO exhibits a broad peak at 20-30 degrees, which may belong to amorphous NbOx. Besides, with the increase of the Nb content, the peak intensity gets weaker and half peak width becomes larger, indicating that the decreasing of crystallinity and smaller grain size of Nb-NiO according to the Debye–Scherrer formula (𝐷 = 𝑘𝜆/𝑐𝑜𝑠𝜃) [23]. The particle size is calculated and exhibited in Table S2. It can be seen that the incorporation of Nb into NiO lattice can decrease particle size.

Morphologies and structure of the as-prepared samples are examined by scanning emission microscopy (SEM) and transmission electron microscope (TEM), shown in Fig. 2 and Fig. S2. It can be observed from Fig. 2a, c that pure NiO exhibits an excellent nanosheet structure of size about 1 2 μm. Fig. 2 b, d shows the SEM and TEM images of 20.2Nb-NiO, which exhibits a similar nanosheet structure with pure NiO but smaller in size (400 - 600 nm). Besides, as shown in Fig. S2, with the increase of Nb ratio, the size of nanosheets decreases continually. The detailed structure and

ro of

morphology analyzed by TEM. The TEM images are shown in Fig. 2c, d further reveal that these observed nanosheets have a 2D sheet structure. Moreover, the TEM also demonstrates the nanosheets contain some pores. The formation of the nanopores can be attributed to the dehydration of Ni(OH)2

-p

to form NiO during the process of annealing. Fig. 2e, f shows a high-resolution TEM (HRTEM)

re

image of pure NiO and 20.2Nb-NiO nanosheets. The lattice fringes are clearly observed. The fringe spacing is 0.209 nm and 0.241 nm, respectively, which are consistent with the interplanar spacing of

lP

(200) and (111) planes of cubic NiO. No Nb-related lattice fringes can be observed. Fig. 2g-j depict

na

the elemental distribution of Ni, O, and Nb, which reveals the Nb elemental distribute uniformly. Due to the relatively low content of Nb, the intensity is much lower than Ni and O. Furthermore, the

Jo

elements.

ur

EDS spectrum of 20.2Nb-NiO is exhibited in Fig. S2f and confirms the existence of Nb, Ni, and O

Fig. 3 and Fig. S3 show the N2 adsorption-desorption isotherms and pore size distributions of the samples. All these samples display a typical IV adsorption isotherm with a H3-type hysteresis loop [24]. The specific surface area of pure NiO, 6.2Nb-NiO, 13.2Nb-NiO, 16Nb-NiO, 20.2Nb-NiO, 26Nb-NiO and 29.1Nb-NiO calculated by the Brunauer-Emmett-Teller (BET) method are 61.53, 67.89, 120.53, 156.85, 175.47, 182.89 and 179.95 m2/g, respectively. Obviously, the introduction of

Nb increases the specific surface area. Moreover, when the content of Nb reaches to 20.2 at%, the specific surface area almost keeps stable with a further increase of Nb dopant. The large surface area can provide more active sites, which is conducive to improving gas sensing performance [25]. The total pore volume of different contents calculated by Barret–Joyner–Halenda (BJH) method are 0.22, 0.38, 0.73, 0.93, 1.26, 1.57 and 0.91 cm3g-1, respectively. Excessive Nb hinders the formation of nanosheets and reduces the pore content. More holes contribute to higher gas accessibility [26]. The

ro of

enormous specific surface areas and pore volume of Nb-NiO nanosheets make them candidates for high-performance gas sensing materials.

-p

3.2. Gas sensing properties

Operating temperature is an important parameter for semiconductor oxide gas sensors. To find the

re

optimum operating temperature, the response of the sensors based on pure NiO and different

lP

contents of Nb-NiO are tested toward 100 ppm xylene at different temperature (130 - 490 oC). From these curves from Fig. 4a, we can observe that 20.2Nb-NiO exhibits the maximum response of 335.1

na

to 100 ppm xylene at 370 oC. In addition, it can be seen that the response curve of all sensors shows an “increase-maximum-decrease” shape. The phenomenon can be explained as follows: under the

ur

low-temperature conditions, xylene molecules do not get enough energy to overcome the reaction of

Jo

the energy barrier with various oxygen species. With the increase of operating temperature, the molecular motion will accelerate. The kinds of oxygen absorbed on the surface of the material (such as O2 (ads), O2− (ads) and O− (ads)) will also increase meanwhile. On exceeding the best operating temperature, the stably adsorbed xylene molecules begin to desorb in large quantities, which leads to a low gas response [27]. Compared with pure NiO, the response enhances 110 times at the optimal Nb content and operating temperature. Therefore, the following sensing tests and discussions are

mainly concentrated on the 20.2Nb-NiO 2D nanosheets. Fig. 4b shows the response of to 100 ppm different VOCs, including methanol, ethanol, acetone, benzene, toluene and xylene. It can be seen that all Nb-NiO exhibits the enhanced response to all gas when compared with pure NiO. Moreover, all sensors show the highest response to xylene, which indicates that Nb plays a significant role in gas sensing progress. Apart from a high response to the analytical gas, gas selectivity is also not negligible. Effective selective detection of a single gas is of

ro of

great significance for the practical application of gas sensors [14]. Ethanol and benzene are one of the ubiquitous and representative indoor gases, which can be produced by culinary use, alcoholic

-p

beverages, cleaning products, and so on [28, 29]. Therefore, ethanol and benzene should be

considered as major interferent gases. Fig. 4c shows the gas selectivity (Sxylene/Sinterference gas) of pure

re

NiO and 20.2Nb-NiO at their optimal operating temperature. The sensor-based on pure NiO exhibits

lP

negligibly low responses to all analyte gases, nearly no selectivity at all. As for 20.2Nb-NiO, the selectivity enhanced 2.1-16.1 times than pure NiO. In particular, the selectivity to benzene is as high

na

as 43.6.

Dynamic sensing transients of 20.2Nb-NiO to 0.3 ppm - 100 ppm xylene are shown in Fig. 4d.

ur

The response of 20.2Nb-NiO increase with the concentration of xylene increase. Fig. S4 and S5

Jo

exhibits the reproducibility of the sensor on successive exposure to 100 ppm target gas and air. It can be observed that the sensors have good response/recovery characteristics and reproducibility. The gas responses as a function of gas concentration are plotted in Fig. 4e. It can be found that the sensor shows an initial rapid increase of response at low xylene concentration and a response plateau in high xylene concentration, which is similar to previous reports [30]. The inset in Fig. 4e indicates that the great liner relationship (S = 9.25C - 2.09) between xylene concentration and response at 0.3 ppm - 2

ppm. The correlation coefficient of the fitting result is R2 = 0.979, indicating an excellent fitting result. Besides, the standard deviation (σb) is calculated to be 0.062. In the case of a signal-to-noise ratio of three, the theoretical detection limit is calculated to be 2 ppb (LOD =

3σb

). The pretty

sensitivity

low detect limitation is good for real-time gas detection. Apart from this, the response and recovery curve can be observed in Fig. 4f. Accordingly, the response and recovery time of 20.2Nb-NiO to 100 ppm xylene at 370 °C can be calculated to be 63 s and 66 s, respectively.

ro of

Our 20.2Nb-NiO nanosheets exhibit the response of 335.1, selectivity to xylene over benzene or ethanol (Sxylene/Sbenzene = 43.6, Sxylene/Sethanol = 7.1), theoretical detection limit of 2 ppb, response time

-p

of 63 s and recover time of 66 s to 100 ppm xylene at 370 °C. Compared with detected-xylene other sensing material in Table 2, it can be clearly observed that the 20.2Nb-NiO possesses ultra-response

re

and excellent selectivity. This sensor-based on 20.2Nb-NiO nanosheets are superior to those reported

lP

in the literature. All of these excellent performance characteristics indicate its potential application for selective and sensitive xylene detection.

na

3.3. Gas sensing mechanism

ur

It’s widely accepted that the gas sensing mechanism is primarily related to the resistance change with the adsorption-desorption process of gas molecules [37]. The whole sensing process can be

Jo

clearly observed in Fig. 5. When pure NiO or Nb-NiO is exposed to air, it will absorb oxygen molecular and transform them into chemisorbed oxygen species (O−) by capturing electrons from the conduction band (Eq. (1)-(2)). This will lead to the formation of a hole accumulation layer (HDL) [38]. In a reducing atmosphere, it will react with the chemisorbed oxygen species, releasing electrons back to the sensing material, reducing the thickness of the HDL and increasing the electrical

resistance. (Eq. (3)-(4)). O2(gas) → O2(ads)

(1)

O2(ads) + 2e- → 2O-(ads)

(2)

C8H10(gas) → C8H10(ads)

(3)

C8H10(ads) + 21O- (ads) → 8CO2(g) + 5H2O + 21e-

(4)

ro of

As for the enhanced gas sensing performance always results from the co-effect of multi-factors such as the amount of reaction sites, electrical conductivity and carrier concentration of the sensing

-p

material, etc [6, 14, 39]. Here, the amount of reaction sites is considered here firstly. According to the test result of BET, the specific surface area is 61.53 and 175.48 m2g-1, respectively. The total pore

re

volume is 0.22 and 1.26 cm3g-1 respectively. The larger specific surface area can provide more

lP

reaction sites. The higher pore volume facilitates the gas transfer. Both of these are crucial for the interactions between sensing material and target gas, thus promoting the gas response.

na

Secondly, the change of carrier concentration is evaluated. According to some previous reports, doping with other mental will change the carrier concentration, which will affect the gas sensing

ur

performance [20, 40]. In order to further explore the possible mechanism, the X-ray photoelectron

Jo

spectroscopy (XPS) analysis is conducted. The high-resolution XPS scan of Nb 3d spectra of the 20.2Nb-NiO is shown in Fig. 6a. The binding energy of Nb 3d5/2 and Nb 3d3/2 are 207.05 eV and 209.75 eV, respectively, which assigned well to Nb5+ [41]. No other impurity peaks can be observed, which means Nb elemental has only one valence state. In Fig. S6, the O1s peaks are asymmetric and could be fitted into three different components. The binding energies nearly at 529.3 ± 0.1 eV (OL), 531.2 ± 0.6 eV (OV) and 533.2 ± 0.6 eV (OC) replace of lattice oxygen, deficient oxygen, and

chemisorbed oxygen species, respectively [20]. The introduction of Nb reduces the non-stoichiometry of the material and fills the ion vacancies in NiO, which leads to a decrease in OV content [42]. Fig. 6b exhibits the high-resolution Ni 2p3/2 XPS spectra of the pure NiO and 20.2Nb-NiO. It could be found that the binding energy of Ni2+ and Ni3+ are 853.48 eV and 855.33 eV, respectively, of pure NiO, as well as 853.97 eV and 855.62 eV of 20.2Nb-NiO [6]. A higher energy shift of binding energy can be explained by the presence of the metallic bond [43]. The ratio of

ro of

Ni3+/Ni2+ of pure NiO is determined to be 1.4 by calculating the peak area. However, the ratio of Ni3+/Ni2+ of 20.2Nb-NiO increased to 7.3. It is reported that Ni3+ can be induced by negatively

charged interstitial oxygen and/or by adsorption negatively charged oxygen on the surface of NiO

-p

[44]. The substitution of Nb5+ at the site of Ni2+ can be compensated by the electronic compensation

2NiO

3

X 2Nb••• Ni + 6e + 2OO + 2O2(g)

lP

Nb2O5 →

re

mechanism, which is described as follows:

1

• X O2(g) + 2NiO → O′′ i + 2NiNi + 2OO

2

(5) (6)

na

To Eq. (5), when Nb5+ substitute the Ni2+, the oxygen molecules are generated. The oxygen molecules generated from Eq. (5) will convert into negatively charge interstitial (or surface) oxygen

ur

through the oxidation of Ni2+ into Ni3+. The increased ratio of Ni3+/Ni2+ can be explained by Eq. (6).

Jo

Moreover, Ni3+ are favorable species to facilitate redox properties [45]. More content of Ni3+ contributes to gas sensing. Furthermore, electrons are generated to compensate for substituting Nb5+ into Ni2+ sites, which can neutralize holes in NiO and increase the resistance. This result agrees well with the Ra in Fig. S7. As the Nb content increases, Ra increases gradually. The decrease of the hole concentration may be the main reason for the enhancement of gas sensor performance. By the

previous report, when the hole concentration is very low, the injection of equal amounts of electrons by the sensing reaction between xylene molecules and chemisorbed oxygen ions will lead to a higher variation in sensor resistance. Therefore, the gas response is enhanced [46]. 4. Conclusions In conclusion, pure NiO and Nb-doped NiO with different dopant concentration nanosheets are

ro of

synthesized successfully. The 20.2Nb-NiO possesses an ultrahigh response (335.1-100 ppm) to xylene, excellent selectivity (Sxylene/Sbenzene = 43.6, Sxylene/Sethanol = 7.1) and pretty low theoretical detection limit (2 ppb) to 100 ppm xylene at 370 oC. Compared with pure NiO, the gas sensor

-p

response and selectivity of 20.2Nb-NiO increased by 110 and 2.1-16.1 times, respectively. The

enhanced gas sensing performance of 20.2Nb-NiO is attributed to the more amounts of active sites

re

and the effect of electron sensitization. It indicates that 20.2Nb-NiO can serve as a good candidate to

na

ur

Declaration of interests

lP

detect xylene efficiently.

Jo

he authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments This work is supported by National Natural Science Foundation of China (Grant No. 61971405),

National Key Research and Development Plan (Grant No. 2016YFB0101205) and Opened Fund of the State Key Laboratory on Integrated Optoelectronics (Grant No. IOSKL2017KF08M). M. Yang

Jo

ur

na

lP

re

-p

ro of

would like to thank for the Ningbo 3315 program.

Notes and reference

Jo

ur

na

lP

re

-p

ro of

[1] F.D. Qu, X.X. Zhou, B.X. Zhang, S.D. Zhang, C.J. Jiang, S.P. Ruan, et al., Fe2O3 nanoparticles-decorated MoO3 nanobelts for enhanced chemiresistive gas sensing, J. Alloy Compd. 782 (2019) 672-8. [2] C.S. Lee, H.Y. Li, B.Y. Kim, Y.M. Jo, H.G. Byun, I.S. Hwang, et al., Discriminative detection of indoor volatile organic compounds using a sensor array based on pure and Fe-doped In2O3 nanofibers, Sens. Actuators B 285 (2019) 193-200. [3] Y.Q. Zhang, J.H. Bai, L.S. Zhou, D.Y. Liu, F.M. Liu, X.S. Liang, et al., Preparation of silver-loaded titanium dioxide hedgehog-like architecture composed of hundreds of nanorods and its fast response to xylene, J. Colloid Interface Sci. 536 (2019) 215-23. [4] H. Gao, J. Guo, Y. Li, C. Xie, X. Li, L. Liu, et al., Highly selective and sensitive xylene gas sensor fabricated from NiO/NiCr2O4 p-p nanoparticles, Sens. Actuators B 284 (2019) 305-15. [5] S. Wang, D. Huang, S. Xu, W. Jiang, T. Wang, J. Hu, et al., Two-dimensional NiO nanosheets with enhanced room temperature NO2 sensing performance via Al doping, Phys. Chem. Chem. Phys. 19 (2017) 19043-9. [6] W.A. Shang, D.T. Wang, B.X. Zhang, C.J. Jiang, F.D. Qu, M.H. Yang, Aliovalent Fe(iii)-doped NiO microspheres for enhanced butanol gas sensing properties, Dalton T. 47 (2018) 15181-8. [7] G.M. Zhou, D.W. Wang, L.C. Yin, N. Li, F. Li, H.M. Cheng, Oxygen Bridges between NiO Nanosheets and Graphene for Improvement of Lithium Storage, ACS Nano 6 (2012) 3214-23. [8] R. Subbaraman, D. Tripkovic, K.C. Chang, D. Strmcnik, A.P. Paulikas, P. Hirunsit, et al., Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts, Nat. Mater. 11 (2012) 550-7. [9] X.H. Cao, Y.M. Shi, W.H. Shi, G. Lu, X. Huang, Q.Y. Yan, et al., Preparation of Novel 3D Graphene Networks for Supercapacitor Applications, Small 7 (2011) 3163-8. [10] G.M. Bai, H.X. Dai, J.G. Deng, Y.X. Liu, W.G. Qiu, Z.X. Zhao, et al., The microemulsion preparation and high catalytic performance of mesoporous NiO nanorods and nanocubes for toluene combustion, Chem. Eng. J. 219 (2013) 200-8. [11] H.J. Kim, J.H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview, Sens. Actuators B 192 (2014) 607-27. [12] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sens. Actuators B 5 (1991) 7-19. [13] M.D. Wang, Y.Y. Li, B.H. Yao, K.H. Zhai, Z.J. Li, H.C. Yao, Synthesis of three-dimensionally ordered macro/mesoporous C-doped WO3 materials: Effect of template sizes on gas sensing properties, Sens. Actuators B 288 (2019) 656-66. [14] X. Liu, S.T. Cheng, H. Liu, S. Hu, D.Q. Zhang, H.S. Ning, A Survey on Gas Sensing Technology, Sensors 12 (2012) 9635-65. [15] R. Ab Kabir, R.A. Rani, A.S. Zoolfakar, J.Z. Ou, M. Shafiei, W. Wlodarski, et al., Nb2O5 Schottky based ethanol vapour sensors: Effect of metallic catalysts, Sens. Actuators B 202 (2014) 74-82. [16] A. Jasik, R. Wojcieszak, S. Monteverdi, M. Ziolek, M.M. Bettahar, Study of nickel catalysts supported on Al2O3, SiO2 or Nb2O5 oxides, J. Mol. Catal. A-chem. 242 (2005) 81-90. [17] J. Wu, Y. Yang, C. Zhang, H. Yu, L. Huang, X. Dong, et al., Extremely sensitive and accurate H2S sensor at room temperature fabricated with In-doped Co3O4 porous nanosheets, Dalton T.

Jo

ur

na

lP

re

-p

ro of

(Cambridge, England : 2003), (2019). [18] S. Choi, M. Bonyani, G.J. Sun, J.K. Lee, S.K. Hyun, C. Lee, Cr2O3 nanoparticle-functionalized WO3 nanorods for ethanol gas sensors, Appl. Surf. Sci. 432 (2018) 241-9. [19] W.Y. Liu, J. Wu, Y. Yang, H. Yu, X.T. Dong, X.L. Wang, et al., Facile synthesis of three-dimensional hierarchical NiO microflowers for efficient room temperature H2S gas sensor, J. Mater. Sci.-Mater. El. 29 (2018) 4624-31. [20] C. Wang, X. Cui, J. Liu, X. Zhou, X. Cheng, P. Sun, et al., Design of Superior Ethanol Gas Sensor Based on Al-Doped NiO Nanorod-Flowers, ACS Sensors 1 (2016) 131-6. [21] S.-H. Kim, G.-I. Shim, S.-Y. Choi, Fabrication of Nb-doped ZnO nanowall structure by RF magnetron sputter for enhanced gas-sensing properties, J. Alloy Compd. 698 (2017) 77-86. [22] W.J. Mu, X. Xie, X.L. Li, R. Zhang, Q.H. Yu, K. Lv, et al., Characterizations of Nb-doped WO3 nanomaterials and their enhanced photocatalytic performance, RSC Adv. 4 (2014) 36064-70. [23] S.H. Kim, S.Y. Choi, Fabrication of Cu-coated TiO2 nanotubes and enhanced electrochemical performance of lithium ion batteries, J. Electroanal. Chem. 744 (2015) 45-52. [24] F. Qu, W. Shang, D. Wang, S. Du, T. Thomas, S. Ruan, et al., Coordination Polymer-Derived Multishelled Mixed Ni-Co Oxide Microspheres for Robust and Selective Detection of Xylene, ACS Appl. Mater. Inter. 10 (2018) 15314-21. [25] Z.H. Wang, Z.W. Tian, D.M. Han, F.B. Gu, Highly Sensitive and Selective Ethanol Sensor Fabricated with In-Doped 3DOM ZnO, ACS Appl. Mater. Inter. 8 (2016) 5466-74. [26] B.Y. Kim, J.W. Yoon, J.K. Kim, Y.C. Kang, J.H. Lee, Dual Role of Multiroom-Structured Sn-Doped NiO Microspheres for Ultrasensitive and Highly Selective Detection of Xylene, ACS Appl. Mater. Inter. 10 (2018) 16605-12. [27] J. Fu, Z. Chen, M. Wang, S. Liu, J. Zhang, J. Zhang, et al., Adsorption of methylene blue by a high-efficiency adsorbent (polydopamine microspheres): Kinetics, isotherm, thermodynamics and mechanism analysis, Chem. Eng. J. 259 (2015) 53-61. [28] E. Gallego, X. Roca, J.F. Perales, X. Guardino, Determining indoor air quality and identifying the origin of odour episodes in indoor environments, J. Environ. Sci. 21 (2009) 333-9. [29] M.B. Banerjee, S. Pradhan, R.B. Roy, B. Tudu, D.K. Das, R. Bandyopadhyay, et al., Detection of Benzene and Volatile Aromatic Compounds by Molecularly Imprinted Polymer-Coated Quartz Crystal Microbalance Sensor, IEEE Sens. J. 19 (2019) 885-92. [30] Y. Zhang, Y. Zhou, Z. Li, G. Chen, Y. Mao, H. Guan, et al., MOFs-derived NiFe2O4 fusiformis with highly selective response to xylene, J. Alloy Compd. 784 (2019) 102-10. [31] H. Chen, S. Ao, G.-D. Li, Q. Gao, X. Zou, C. Wei, Enhanced sensing performance to toluene and xylene by constructing NiGa2O4-NiO heterostructures, Sens. Actuators B 282(2019) 331-8. [32] C.H. Feng, C. Wang, H. Zhang, X. Li, C. Wang, P.F. Cheng, et al., Enhanced sensitive and selective xylene sensors using W-doped NiO nanotubes, Sens. Actuator B 221(2015) 1475-82. [33] B.L. Zhu, D.W. Zeng, J. Wu, W.L. Song, C.S. Xie, Synthesis and gas sensitivity of In-doped ZnO nanoparticles, J. Mater. Sci-Mater. El. 14 (2003) 521-6. [34] F. Qu, S. Zhang, B. Zhang, X. Zhou, S. Du, C.-T. Lin, et al., Hierarchical Co3O4@NiMoO4 core-shell nanowires for chemiresistive sensing of xylene vapor, Microchimi. Acta 186 (2019). [35] L. Liu, Z.C. Zhong, Z.J. Wang, L.Y. Wang, S.C. Li, Z. Liu, et al., Synthesis, Characterization, and m-Xylene Sensing Properties of Co-ZnO Composite Nanofibers, J. Am. Ceram Soc. 94 (2011) 3437-41. [36] B.L. Zhu, C.S. Xie, D.W. Zeng, W.L. Song, A.H. Wang, Investigation of gas sensitivity of

Jo

ur

na

lP

re

-p

ro of

Sb-doped ZnO nanoparticles, Mater. Chem. Phy. 89 (2005) 148-53. [37] C. Wang, R.Z. Sun, X. Li, Y.F. Sun, P. Sun, F.M. Liu, et al., Hierarchical flower-like WO3 nanostructures and their gas sensing properties, Sens. Actuators B 204 (2014) 224-30. [38] P.T. Moseley, Progress in the development of semiconducting metal oxide gas sensors: a review, Meas. Sci. Technol. 28 (2017) 15. [39] M.E. Franke, T.J. Koplin, U. Simon, Metal and metal oxide nanoparticles in chemiresistors: Does the nanoscale matter, Small 2 (2006) 36-50. [40] H.Y. Gao, D.D. Wei, P.F. Lin, C. Liu, P. Sun, K. Shimanoe, et al., The design of excellent xylene gas sensor using Sn-doped NiO hierarchical nanostructure, Sens. Actuators B 253 (2017) 1152-62. [41] C.Z. Guan, Y.D. Wang, K.F. Chen, G.P. Xiao, X. Lin, J. Zhou, et al., Molten salt synthesis of Nb-doped (La, Sr) FeO3 as the oxygen electrode for reversible solid oxide cells, Mater. Lett. 245 (2019) 114-7. [42] E. Heracleous, A.A. Lemonidou, Ni-Nb-O mixed oxides as highly active and selective catalysts for ethene production via ethane oxidative dehydrogenation. Part I: Characterization and catalytic performance, J. Catal. 237 (2006) 162-74. [43] A.S. Trifonov, A.V. Lubenchenko, V.I. Polkin, A.B. Pavolotsky, S.V. Ketov, D.V. Louzguine-Luzgin, Difference in charge transport properties of Ni-Nb thin films with native and artificial oxide, J. Appl. Phys. 117 (2015) 5. [44] M. Yang, H.F. Pu, Q.F. Zhou, Q. Zhang, Transparent p-type conducting K-doped NiO films deposited by pulsed plasma deposition, Thin Solid Films 520 (2012) 5884-8. [45] X. Wu, R.N. Wang, Y.L. Du, X.J. Li, H. Meng, X.M. Xie, NOx removal by selective catalytic reduction with ammonia over hydrotalcite-derived NiTi mixed oxide, New J. Chem. 43 (2019) 2640-8. [46] J.W. Yoon, H.J. Kim, I.D. Kim, J.H. Lee, Electronic sensitization of the response to C2H5OH of p-type NiO nanofibers by Fe doping, Nanotechnology 24 (2013) 8.

Biographies Lei Qiu received his B.S. degree in School of Material Science and Engineering in 2017. Now he is a master student under the supervision of Prof. Pei Wang at Materials Science and Engineering Department, Dalian Maritime University. His current research interest is gas sensors. Shendan Zhang received the B.S. degree in Faculty of Materials Science and Chemistry in China University of Geosciences in 2016. Now she is currently studying for her PhD degree in Ningbo Institute of Materials Technology and Engineering,

semiconducting nanomaterials functionated with noble metals.

ro of

Chinese Academy of Sciences. Her research interest is the gas sensors based on the

Jiangbo Huang is currently a master student under the supervision of Prof. Pei Wang at Materials Science and Engineering Department, Dalian Maritime University His current research interest is

-p

hydrogel.

Chenhao Wang received his B.S. degrees from South West University in 2013.Now he is a research

re

student in Ningbo institute of Materials Technology and Engineering, Chinese Academy of Sciences His present works mainly focus on synthesis of solid state functional materials, gas sensors, and

lP

machine design.

Ruiyang Zhao received the PhD degree from Jilin University in 2014. Now, he is a associate professor at Qingdao University of Science and Technology. His present works mainly focus on

na

synthesis functional materials.

Fengdong Qu received his B.S. and M.S. degrees from Jilin University in 2012 and

ur

2015, respectively. Now he is a research assistant in Ningbo Institute of Materials Author Biographies Technology and Engineering, Chinese Academy of Sciences. His present works mainly

Jo

focus on synthesis of solid state functional materials, gas sensors, and photoelectric devices. Pei Wang is associate professor of Materials Science and Engineering Department, Dalian Maritime University. He received his Ph.D. from Dalian University of Technology in 2005. From 2005-2018 he worked as associate professor in Dalian Maritime University. His research interests include the synthesis and modification of nano-hydrogel materials, preparation of high value-added coatings, construction of biological anti-fouling coating systems, and development of environmentally friendly materials.

Minghui Yang is a professor of Materials Chemistry at Ningbo Institute of Industrial Technology, Chinese Academy of Sciences. He is a graduate of the University of Liverpool and completed his Ph.D. in Materials Chemistry under the supervision of Professor J. Paul Attfield and Professor Amparo Fuertes at University of Edinburgh in 2010. His research interests include synthesis of metal (oxy)nitrides and application in

Jo

ur

na

lP

re

-p

ro of

photocatalysis, fuel cells, Li-S battery and gas sensors.

Figures caption: Fig. 1. (a) X-ray diffraction patterns of pure NiO and 6.2, 13.2, 16, 20.2, 26, 29.1Nb-NiO; (b)

re

-p

ro of

high-resolution of (200) peak of the pure NiO and 6.2, 13.2, 16, 20.2, 26, 29.1Nb-NiO.

lP

Fig. 2. (a) SEM (c) TEM and (e) HRTEM images of pure NiO; (b) SEM (d) TEM and (f) HRTEM of

Jo

ur

na

20.2Nb-NiO; (g-j) elements mapping of 20.2Nb-NiO from TEM.

ro of -p re lP na ur Jo Fig. 3. (a) N2 adsorption-desorption isotherms and (b) pore size distributions of the pure NiO and 20.2Nb-NiO.

ro of

Fig. 4. (a) Response of sensors based on pure NiO and different contents of Nb-NiO to 100 ppm xylene at different operating temperatures; (b) Responses of seven sensors to varieties of test gases

-p

(100 ppm) at 370 °C; (c) Selectivity of sensors based on NiO and 20.2Nb-NiO to 100 ppm of various

re

gases at 370 °C; (d) Sensing transients of the 20.2Nb-NiO sensors to different xylene concentrations

Jo

ur

na

time to 100 ppm xylene at 370 °C.

lP

at 370 °C; (e) gas responses as a function of gas concentration; (f) the typical response and recovery

Fig. 5. The schematic diagram of the xylene sensing process on the surface of pure NiO and Nb-NiO nanosheets.

ro of

Fig. 6. (a) The XPS spectra of Nb 3d in 20.2Nb-NiO; (b) Ni 2p3/2 XPS spectra of the pure NiO and

Jo

ur

na

lP

re

-p

20.2Nb-NiO.

Table 1 Comparison of the sensing performances of the sensors in this study and other studies Tsens (°C)

Sxylene

Sxylene/Sbenzene

Sxylene/Sethanol

Ref.

100

230

16.3

3.3

2.5

[31]

W-doped NiO nanotubes

200

375

8.74

7.3

2.9

[32]

Fe2O3/MoO3 nanobelts

100

233

22

8

2.7

[1]

100

375

6.49

2.45

1.3

[3]

100

420

10.1

1.68

0.39

[33]

100

255

24.6

16.4

3.8

[34]

100

320

14.8

4.1

3.5

[35]

100

420

12.2

1.69

0.38

[36]

100

370

335.1

43.6

NiGa2O4-NiO nanospheres

0.5wt% Ag/TiO2 nanorods In-doped ZnO nanoparticles Co3O4@NiMoO4 core-shell nanowires Co-ZnO composite nanofibers Sb-doped ZnO nanoparticles

Jo

ur

na

lP

re

20.2Nb-NiO nanosheets

ro of

Conc. (ppm)

-p

Sensing materials

7.1

This work

Table 2 Comparison of the sensing performances of the sensors in this study and other studies Conc. (ppm)

Tsens (°C)

Sxylene

Sxylene/Sbenzene

Sxylene/Sethanol

Ref.

NiGa2O4-NiO nanospheres

100

230

16.3

3.3

2.5

[31]

W-doped NiO nanotubes

200

375

8.74

7.3

2.9

[32]

Fe2O3/MoO3 nanobelts

100

233

22

8

2.7

[1]

0.5wt% Ag/TiO2 nanorods

100

375

6.49

2.45

1.3

[3]

100

420

10.1

1.68

100

255

24.6

16.4

100

320

14.8

4.1

100

420

12.2

100

370

Co3O4@NiMoO4 core-shell nanowires Co-ZnO composite nanofibers Sb-doped ZnO nanoparticles

335.1

Jo

ur

na

lP

20.2Nb-NiO nanosheets

-p

nanoparticles

1.69

re

In-doped ZnO

ro of

Sensing materials

43.6

0.39

[33]

3.8

[34]

3.5

[35]

0.38

[36]

7.1

This work