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Sensors and Actuators B 244 (2017) 983–991

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

esearch Paper

ighly sensitive and rapid chemiresistive sensor towards traceitro-explosive vapors based on oxygen vacancy-rich and defectiverystallized In-doped ZnO

uru Gea,b, Zhong Weia,∗, Yushu Lib,∗, Jiang Qub, Baiyi Zub, Xincun Doub

School of Chemistry and Chemical Engineering/ Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, Shihezi University,hihezi, 832003, ChinaLaboratory of Environmental Science and Technology, Xinjiang Technical Institute of Physics & Chemistry, Key Laboratory of Functional Materials andevices for Special Environments, Chinese Academy of Sciences, Urumqi, 830011, China

r t i c l e i n f o

rticle history:eceived 2 December 2016eceived in revised form 8 January 2017ccepted 12 January 2017vailable online 15 January 2017

eywords:ndium dopingnO nanoparticles

a b s t r a c t

In order to sensitively detect trace nitro-explosive vapors, the sensing properties of ZnO nanoparticles(NPs) are boosted by tailoring the doping level of indium (In). With the introduction of In, the shape of theZnO NPs changes from sphere with grain size of 55.2 ± 9.6 nm to irregular NPs with a reduced size. Thesensing performances of sensors towards room-temperature saturated nitro-explosive vapors generallyincrease firstly and then decrease, peaking at an atomic ratio of 1.29% (corresponding to 5% In in theprecursor). The 5% In-doped ZnO nanoparticle-based sensor exhibited remarkably enhanced responsestowards trace nitro-explosive vapors, including TNT of 9 ppb, DNT of 411 ppb, PNT of 647 ppb, PA of0.97 ppb and RDX of 4.9 ppt at room temperature. For instance, compared with ZnO, the responses to

as sensingitro-explosive vapors detectionxygen vacancies

nitro-explosive vapors were increased from 22.2, 8.5, 2.9, 4.9 and 9.8% to 54.7, 52.9, 57.2, 58.3 and 47.4%,respectively. Furthermore, much shorter response time (<6.3 s vs. 20–40 s) and recovery time (<14 s vs.20–40 s) were achieved, which is of vital importance for on-site explosive detection. Combining thesurface oxygen defects investigation, it is found that the remarkably increased oxygen vacancies areresponsible for the sensing performance improvement.

© 2017 Elsevier B.V. All rights reserved.

. Introduction

Nitro-explosives are important categories in military explo-ives and environmentally deleterious substances that have beenf pressing societal concern [1]. Thus, the exploration of theorresponding trace detection methods is of vital importance.urrently, most of the practically applied equipments for nitro-xplosives detection, such as ion mobility spectrometry (IMS) [2],as chromatograph-mass spectrometer [3], amplifying fluorescentonjugated polymers-based detector [4] and surface-enhancedaman spectroscopy (SERS) [5], are either huge or costly. There-

ore, the development of portable and low-cost sensing platforms,nd possibly, with sensitive and rapid characteristics, is urgently

eeded. Chemiresistive gas sensors are now playing signifi-ant roles in many fields, particularly detecting toxic pollutingases and nitro-explosive vapors due to their excellent sensing

∗ Corresponding authors.E-mail addresses: steven weiz@sina.com (Z. Wei), yushu0505@gmail.com

Y. Li).

ttp://dx.doi.org/10.1016/j.snb.2017.01.092925-4005/© 2017 Elsevier B.V. All rights reserved.

performances and portability. However, most of the gas sensorsreported previously only have limited applications in high concen-trations of toxic polluting gases at high temperatures (200–500 ◦C)or work at room temperature with the additional irradiation ofultraviolet light [6–10]. Frankly speaking, the detection of nitro-explosive vapors is quite difficult since the detection at hightemperature is unpractical and hazardous and the saturated vaporpressures of nitro-explosives are very ultralow at room tempera-ture. For instance, the room-temperature saturated vapor pressuresof TNT and RDX are only a few ppb (parts-per-billion) and ppt(parts-per-trillion), respectively. Thus, the development of a uniquechemiresistive sensing material to remarkably promote the sens-ing signals towards nitro-explosive vapors, and possibly, with rapidresponse/recovery process, would be of great significance.

As a representative semiconductor metal oxide, ZnO has beenwidely used in the detection of various high concentrations ofVOCs [11–13], owing to its full rational synthetic strategy design,

unique and novel electrical, optoelectronic and gas sensing proper-ties [14–16]. Furthermore, the surface states of ZnO could be easilyregulated in various ways, such as, surface modification [17,18],

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oping [19–22] and hybridization [23–25], among which dopingationic impurities, such as In, is an effective approach to improvehe sensitivity of ZnO for VOC detection [19–22]. Although ZnOanofibers with hollow structure or In2O3 nanospheres was usedo detect trace explosive nitro-compounds [26,27], to the best ofur knowledge, there is no systematic report on cationic impuri-ies doping in ZnO nanostructures to manipulate the surface statespecifically for nitro-explosive vapor detection.

In this work, the sensing properties of ZnO NPs are significantlyanipulated by regulating their surface states, which is realized by

djusting the doping level of Indium. The ZnO NPs with various Inoping levels were synthesized via a sol-gel route, followed by aost-annealing process for oxygen vacancy defect regulation. Theensing performances of the corresponding chemiresistive sensorsowards room-temperature saturated nitro-explosive vapors (TNT,NT, PNT, PA and RDX) are considerably enhanced compared with

he un-doped counterpart. The response of the In-doped ZnO NP-ased sensor towards 5 explosives generally increase firstly andhen decrease with the increase of In concentration, reaching thelimate when the starting atomic ratio of In/Zn is 5%. Comparedith that of the un-doped counterpart, the response towards nitro-

xplosive vapors increased for 2–20 times for 5% In doped ZnO NPsith shorter response time (<6.3 s vs. 20–40 s) and recovery time

<14 s vs. 20–40 s). The reason for this excellent performance cane attributed to the creation of abundant oxygen vacancies and theefective crystalline state which is beneficial for charge transfernd vapor adsorption.

. Experimental details

.1. Chemicals and reagents

Zincnitrate hydrate (Zn(NO3)2•6H2O), Indium nitrate hydrate

In(NO3)3•H2O), Lithium hydroxide monohydrate (LiOH•H2O),

ithium chloride (LiCl), Magnesium chloride (MgCl2), Magnesiumitrate (Mg(NO3)2), absolute ethanol, heptane, 2,4-DinitrotolueneDNT), p-Nitrotoluene (PNT) and Picric acid (PA) were purchasedrom Sigma-Aldrich. Raw 2,4,6-Trinitrotoluene (TNT) and HexogenRDX) were obtained from the National Security Department ofhina. Except for TNT was recrystallized with ethanol before use, allther explosives purchased are in analytical grade and were usedirectly without further purification.

Caution: TNT and other nitro-explosives used in the presenttudy are highly explosive and should be handled only in smalluantities [28].

.2. Preparation of sensing materials

The In-doped ZnO NPs were prepared mainly by two steps, preparation of ultrathin nanosheets and a post-annealing pro-ess. In a typical procedure, firstly, a mixture ethanol solution of0 mL of 0.1 M Zn(NO3)2

•6H2O and a different given amount (0,.005, 0.01, 0.02 and 0.04 M) of In(NO3)3

•H2O, and an ethanololution of 50 mL of 0.15 M LiOH•H2O were kept in a refrigeratornd maintained at 0 ◦C for 6 h, respectively. Then, the two solu-ions were mixed together, kept in an ice bath and stirred for

h. Afterward, the precipitation was washed with heptane andthanol in sequence for 5 times by redispersing the precipitatesn heptane or ethanol followed by centrifugalizing at 6000 r/minor 5 min. Finally, the precipitates were dried in the oven overnight

t 60 ◦C to get the In-doped ZnO ultrathin nanosheets. Secondly,he nanosheets were annealed for 1 h at 200 ◦C to obtain In-dopednO NPs. Pure ZnO nanosheets and NPs were prepared similarlyithout adding In(NO3)3

•H2O in the initial process.

rs B 244 (2017) 983–991

2.3. Characterization

X-ray diffraction (XRD) measurement was conducted usingpowder XRD (Bruker D8 Advance, with Cu K� radiation operatingat 40 kV and 40 mA, scanning from 2� = 10◦ to 90◦). Field-emissionscanning electron microscope (FESEM, ZEISS SUPRA 55VP) andtransmission electron microscope (JEM-2011 TEM, 200 kV) wereused to characterize the morphology of the samples. Energy dis-perse spectroscopy (EDS) and X-ray photoelectron spectroscopy(XPS, Thermo Scientific Escalab 250) were used to quantita-tively evaluate the composition of the impurity elements inthe In-doped ZnO NPs. The X-ray photoelectron spectroscopy(XPS) measurement was conducted on a Esca Lab 250Xi spec-trometer (Thermo Scientific) using monochromatic Al K� X-raysource (anode HT = 15 kV) operating at a vacuum higher than2 × 10−9 mbar. The nitrogen adsorption-desorption isotherms, thecorresponding pore size distributions and the Brunauer-Emmett-Teller (BET) specific surface area of the samples were determined bya mesoporous method on the AUTOSORB-IQ-MP physical adsorp-tion instrument.

2.4. Gas sensor fabrication and sensing tests

Initially, the In-doped ZnO or ZnO NPs were mixed with abso-lute ethanol in a weight ratio of 100:25 and grounded in a mortarfor 15 min to form a paste. The paste was then coated on a ceramicsubstrate to form a sensing film on which silver interdigitated elec-trodes with both finger-width and inter-finger spacing of about40 �m was previously printed. The fabricated device was driednaturally in air overnight. Here, the brush painting method usedfor coating is a traditional and classical method to obtain devices[29], in which the thickness of the film can be easily controlled byadjusting the ratio of solvent and material, as well as the brushedcycles. The sensors were aged at 4 V in air for approximate 24 h toensure the good stability by improving the grain boundary contact.The room temperature saturated explosive vapors were obtainedby putting solid explosive powder at the bottom of a conical flask(250 mL) and sealed for 48 h. For the humidity test, the saturatedsolutions of LiCl, MgCl2, and Mg(NO3)2 were kept in closed ves-sels to give approximate relative humidities of 11%, 33% and 54%,respectively [30]. Then the sensor was placed into the conical flask.All tests were performed in a small-sized vacuum glove-box forconsistent operating temperature (25 ± 2 ◦C) and relative humidity(RH, 30 ± 3%). For gas sensing test, the sensor was inserted into theflask with saturated explosive vapor first. After the sensor resis-tance reached a new constant value, the sensor was then insertedinto a same size conical flask full with air to recover. The test wasrepeated for 10–20 cycles in the same process. The electric signalof the sensor was measured by a Keithley 2636 B SourceMeter. Theschematic illustration of the measurement system is shown in Fig.S16. The relative sensor response in resistance is defined as,

Response = (Ra − Rg)/Ra × 100%

where Rg and Ra are the electrical resistances of the sensor inexplosive vapor and in air. The response time is defined as the

period in which the sensor resistance reaches 90% of the responsevalue upon exposure to the explosive vapor, while the recoverytime is defined as the period in which the sensor resistance changesto 10% of the response value after the explosive vapor was removed.

Y. Ge et al. / Sensors and Actuato

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cheme 1. Schematic illustration of the fabrication process of In-doped ZnOanosheets and NPs.

. Results and discussion

.1. Morphology and structure regulation via In-doping

The fabrication process of In-doped ZnO NPs is schematicallyhown in Scheme 1. The nucleation reaction occurred once thethanol solution of nitrate salts was mixed with alkaline solu-ion. Then the generated nucleuses aggregated and assembled intoanosheets with poor crystallinity. In-doped ZnO NPs were finallybtained upon post-annealing treatment of the nanosheets.

Through the XRD patterns of the ZnO NPs and four In-dopednO NPs with different doping levels (Fig. 1a), one can find thatll diffraction peaks can be well indexed to the hexagonal wurtzitenO (JCPDS, 36-1451) with no other peak observed. However, theiffraction peaks of In-doped ZnO NPs are obviously lower androader than the pure ZnO NPs, indicating the poor crystallinityf the doped ones, which is beneficial for the electronic transmis-ions process. X-ray photoelectron spectroscopy (XPS) and Energyisperse spectroscopy (EDS) were carried out to determine the ele-ental composition as well as the valence state of Zn, O and In

toms of ZnO and In-doped ZnO NPs with different doping levels.-ray photoelectron spectroscopy (XPS) was used to determine thealence state of Zn, O and In atoms in the ZnO or four In-doped ZnOanoparticles. The characteristic peaks corresponding to Zn 2p, Os and In 3d are clearly presented in the XPS spectra of the In-oped ZnO samples (Fig. S1). With increasing doping amount of

n, the noticed peaks around 444.7 and 452.4 eV, which representshe In 3d5/2 and In 3d3/2 states respectively, increase obviously,ndicating the successful doping of In element at the lattice posi-ion of Zn (Fig. 1b inset) [29,30]. The effective doping of In is alsovidenced by the characteristic L-series transition of In at 3.27 keVn EDS spectra (Fig. S2). According to the EDS results, the ratios ofn/Zn in In-doped ZnO NPs with starting ratios of 5, 10, 20 and 40%chieve 1.29, 3.30, 5.97 and 21.16%, respectively (Fig. 1b).

The effect of In element doping on the morphology and detailedtructure of ZnO NPs was studied using Field-emission scanninglectron microscope (FESEM) and Transmission electron micro-cope (TEM). SEM results indicate that In doping can significantlyeduce the size of both ZnO nanosheets (precursor) and NPs (Fig.3). The pure ZnO NPs have relatively regular spherical shape with

diameter of 40–80 nm as shown in TEM image and have perfect

rystallinity evidenced by the clear lattice fringes in the High reso-ution transmission electron microscopy (HRTEM) image (Fig. 1c).n contrast, In-doped ZnO NPs with different doping levels all haverregular shape and poor crystallinity to a certain extent (Fig. 1d–g).

rs B 244 (2017) 983–991 985

The size became smaller and the crystallinity became worse withthe increased In doping level. When the doping level of In reach 10%,the samples own a poor crystallinity, indicated by the indistinct lat-tice fringes in the HRTEM (Fig. 1e inset). Moreover, the grain sizes ofNPs are calculated according to Scherrer formula (Fig. 1h) [31]. Thegrain size decrease gradually from 55.2 ± 9.6 to 17 ± 7.3 nm whilethe In doping level increase from 0% to 40%.

In addition, the Brunauer-Emmett-Teller (BET) surface areaswere also studied by measuring N2 adsorption-desorptionisotherms (Fig. 2). It is observed that the adsorption-desorption vol-ume and the resulting BET surface area increase gradually with theincrease of In doping. For instance, The BET surface area of ZnO NPsincreased nearly by a factor of 5 from 10.256 m3/g to 47.65 m3/gwhen the In-doping level increase from 0 to 40%. This trend coin-cides well with the previous morphology study. The explanationfor this phenomenon could be that new nucleation centers createdby In ions during the growth process change the nucleation typefrom homogeneous to heterogeneous. Hence, with increasing Indoping level, smaller crystal grains formed yielding more defectiveand smaller nanoparticles with higher BET surface area, which areall beneficial for gas sensing properties [32,33].

3.2. Sensitive and rapid detection towards nitro-explosive vapors

The pure ZnO and four In-doped ZnO NPs with different dopinglevel were fabricated separately as chemiresistive sensors (Fig. S4).The device structure was schematically shown in Fig. 3a.

To evaluate the influence of In doping level on the gas sensingproperties of the In-doped ZnO NPs towards five representativenitro-explosive room temperature saturated vapors (TNT, DNT,PNT, PA and RDX), three successive cycles were measured basedon the fabricated chemiresistive sensors (Fig. S5–9). During thesystematical investigation, the operation temperature of 25 ± 3 ◦Cand relative humidity (RH) of 33 ± 3% are fixed to mimic the actualoperation environment for application, which is of great impor-tance to the popularization and application of the sensing materials.From the resistance change behavior, it is clearly shown that, onone hand, the resistance changes of different sensors to the sameconstituent change obviously with the varying of the doped Inconcentration, and on the other hand, the responses of the samesensor to different constituents change remarkably, which laid asolid foundation for the further sensory array-based discrimina-tive detection. According to the definition of relative response inresistance (Response = (Ra-Rg)/Ra × 100%), the response changes ofsensors based on the ZnO and four In-doped ZnO NPs with differ-ent In doping level towards the five nitro-explosive vapors werecalculated and presented in Fig. 3b–f. It is clearly shown that forthe pure ZnO nanoparticle-based sensor, an unstable, slow andpoor response was shown for all five nitro-explosive vapors, espe-cially for PNT (Fig. 3b). While with the immersing of the In-dopedZnO nanoparticle-based sensors into nitro-explosive vapors, theresponse increases rapidly. And with the immersing of the sensorinto air, the response decreases fast and can recover to the initialvalue, indicating the good reversibility of the sensor. The obvi-ous comparison among the response curves of pure and In-dopedZnO NP-based sensors indicated the decisive role of In dopingin improving the sensing performance towards nitro-explosivevapors. However, with higher In doping level, i.e. 20 and 40% (Fig. 3eand f), the In-doped ZnO NPs sensors exhibit worse response stabil-ity, which is probably caused by the incomplete desorption of theexplosive molecules when the sensor was removed from the explo-

sive vapor. With increase of In doping level, the BET surface areaand adsorption-desorption volume increased significantly (Fig. 2),making fully desorption harder. Therefore, the high In doping levelwas detrimental to the desorption process.

986 Y. Ge et al. / Sensors and Actuators B 244 (2017) 983–991

F in thef d In-d( nO NP

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ig. 1. (a) XRD patterns of ZnO NPs (0%) and In-doped ZnO NPs (ratio of the In/Zn

rom EDS results (inset is XPS spectra of In element). TEM image of (c) ZnO NPs aninset are HRTEM images). (h) The calculated grain sizes of ZnO NPs and In-doped Z

For more intuitive comparison of sensing performance ofensors, the statistical results of the response values andesponse/recovery times are shown in Fig. 4. Compared with theure ZnO NPs, the In-doped ZnO NPs generally show much higheresponses towards all nitro-explosive vapors, except for 40% In-oped ZnO NPs which show lower response towards DNT and

A (Fig. 4a). One can see that with the increase of In doping, theesponses towards every nitro-explosive vapor follow the generalrend to increase sharply first and then slowly decrease, with theighest responses existing at the In doping level of 5%. With In dop-

Fig. 2. (a) Nitrogen gas adsorption-desorption isotherms and (b) the volume sur

precursor: 5%, 10%, 20%, and 40%). (b) In/Zn ratio of In-doped ZnO NPs calculatedoped ZnO NPs with different In doping level: (d) 5%, (e) 10%, (f) 20%, and (g) 40%s.

ing level increasing from 0 to 5%, the response values towards TNT,DNT, PNT, PA and RDX increased for several times from 22.2, 8.5,2.9, 4.9, and 9.8% to 54.7, 52.9, 57.2, 58.3, and 47.4%, respectively.For instance, in the case of PNT, the response values of 5% In-dopedZnO NPs is 57.2%, which is nearly 20 times of the pure ZnO NPs.

With In doping, both the response time and recovery time

dropped a lot, indicating that In doping in ZnO NPs is efficient toaccelerate the response and recovery process, which is beneficialto continuous operation and real-time detection of trace explo-sive vapors (Fig. 4b and c). [34] The response times towards TNT,

face areas of ZnO NPs and four In-doped ZnO with different doping levels.

Y. Ge et al. / Sensors and Actuators B 244 (2017) 983–991 987

Fig. 3. (a) The schematic graph of the sensor structure. Response curves of ZnO and four different doping levels of In-doped ZnO nanoparticle-based sensors during threes , PA an

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uccessive cycles exposure to room-temperature saturated vapor of TNT, DNT, PNT

NT, PNT, PA and RDX dropped from 26, 36, 35, 36 and 38 s to.3, 4.3, 5.7, 4.3 and 6.3 s, respectively, when the In doping level

ncreased from 0 to 5%. More accurately, the response time periodsre within 8 s for all the explosive vapors, except for the 20% In-oped ZnO nanoparticle-based sensor towards DNT and PA. Similars the response time, compared with the pure ZnO nanoparticle-ased sensor, the doped ones with the recovery times for TNT, DNT,NT, PA and RDX are reduced from 26 ∼ 38 s to 4.3 ∼ 24 s. Thus, theresent In-doping tailoring strategy is clearly manifested to havebvious advantages to boosting the gas-sensing properties towardsitro-explosives by enhancing the response and accelerating theesponse and recovery process.

In addition, compared with other reported chemiresistive sen-ors for explosive vapor detection, the present sensor based on 5%n-ZnO NPs has many advantages (Table S1). [7,35–40] Firstly, theresent 5% In-ZnO nanoparticle-based sensor pertains to the bestNT and DNT sensor since the responses toward them are 54.7% and

2.9%, respectively. Secondly, the response time and recovery timere less than 14 s, which is remarkably superior than most of thether sensors. Especially, the response times for TNT and DNT are

d RDX: (b) 0, (c) 5, (d) 10, (e) 20 and (f) 40%.

5.3 and 4.3 s, respectively, which is remarkably superior to all othersensors in the list. Lastly, the present In-doped ZnO NP sensors areoperated at room temperature, which is more desirable for practicalenvironmental monitoring than those sensors operated at elevatedtemperature in the list [41]. This comparison further declared theconclusion that the present tailoring of ZnO NPs by In doping couldbe a very promising strategy to put the ZnO nanoparticle-basedexplosive sensors into practical application.

However, unfortunately, the pure ZnO NPs and In-doped ZnONPs with four different doping levels also exhibit cross-sensitivitytowards humidity as most metal oxides (Fig. S10). The humid-ity sensitivity results are obtained by measuring the resistancechanges of the pure ZnO and four different doping levels of In-doped ZnO nanoparticle-based sensors between dry air and air withhumidity of 11%, 33% and 54%, respectively (Fig. S11–15). Hence, thehumidity change of the environment is a huge distraction on theapplication of these kinds of materials. Therefore, the sensors must

meet with a special dehumidifying apparatus in front of the devicein practical applications of nitro-explosive vapor detection.

988 Y. Ge et al. / Sensors and Actuato

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ig. 4. (a) responses, (b) response time and (c) recovery time of sensors based on ZnOPs and In-doped ZnO NPs with different doping levels towards room-temperature

aturated vapors of TNT, DNT, PNT, PA, and RDX.

.3. Sensing mechanisms manipulated by In doping

It is known that the sensing performance of a gas sensor is highlyelated to the surface states of the sensing material. When sensoras exposed to air, the oxygen molecules in the air will be adsorbed

n the surface of sensing material to form adsorbed oxygen speciesincluding O2−, O2

−, and O−) by capturing free electrons from theonduction band of the semiconducting sensing material, leading tohe formation of a thick depletion layer (�) in the sensing material41]. The depth of the charge depletion layer, which is a dominatingactor for the sensing properties of chemiresistive sensors, could belassically expressed while the semiconductor grains are absorbedy gas molecules [14],

= LD(2�Vs)1/2,

where LD is the Debye length, Vs is the surface potential barrier,nd �=e/kT, where e, k, and T are the electron charge, Boltzmannonstant, and the absolute temperature, respectively. While, Vs cane expressed as the following equation:

s = eNt2/(2�0�rNd)

where Nt is the surface density of the adsorbed oxygen ions, �0s the permittivity of free space, �r is the dielectric constant, and Nds the charge carrier density. Apparently, the value of Vs depends on

rs B 244 (2017) 983–991

the density of the adsorbed oxygen ions and the dopant concentra-tion, which will influence � and finally the sensitivity [42]. Defects,such as the oxygen vacancy, is one of dominated factor to affect thethickness of Vs. Same material with higher concentration of defectswill adsorb more oxygen ions and achieve a higher value of Vs,which will lead to a higher value of �. [15]. Besides, a greater BETsurface area of sensing material can be favorable for gas-sensingperformance by providing more reaction site. However, in our case,the overall enhanced gas-sensing performance is inconsistent withthe trend of BET surface area, indicating that the smaller particlessize and the BET surface area with more reaction site are excludedas main factor for gas sensing performance of the In-doped ZnONPs. Therefore, the sensing properties tailored by In concentrationof In-doped ZnO NPs can be attributed to the dynamic competi-tion between the enhanced oxygen molecule adsorption and theincreasing carrier density.

Thus, to explore the states of the surface oxygen, XPS character-ization was conducted. The XPS spectrum of the O 1s in ZnO andfour In-doped ZnO NPs with different doping levels were shown inFig. 5a. The typical O 1s peak of all samples can be fitted approx-imately by three Gaussian curves (Fig. 5b–f). The component onthe low binding energy side of the O 1s spectrum around 530 or530.5 eV is attributed to O2− ions on wurtzite structure of hexago-nal Zn2+ ion array, that means, the intensity of this component is themeasure of the amount of oxygen atoms in a fully oxidized stoichio-metric surrounding [43,44]. The higher binding energy component,centered at 531.6 or 532.1 eV, is associated with O2− ions in theoxygen deficient regions within the matrix of ZnO [45,46]. On onehand, comparing the XPS spectra of O 1 s in pure ZnO NPs withthe doped ones, there is a shift of the high energy center after Indoping. For instance, the XPS spectra of O 1 s in the pure ZnO NPscentered at 530, 531.5 and 532.8 eV, while in the 5% In-doped ZnONPs centered at 531.2, 532.1 and 533.2 eV, respectively. Besides, thehigher binding energy component (531.6 or 532.1 eV) is more sig-nificant in In-doped ZnO NPs than in pure ZnO NPs, confirming thatmany oxygen atoms in a fully oxidized stoichiometric surroundingtransform into oxygen-deficient state in In-doped ZnO NPs. Amongthe doped samples, 5% In-doped ZnO NPs has more significant O 1speak shifts than others, which is correlated with the fact that the 5%In-doped ZnO NP-based sensor has the best gas sensing response.

With In doping, the structure of ZnO NPs was destroyed in vary-ing degree as evidenced by XRD results (Fig. 1a). However, a largernumber of In element in ZnO may form a structure of In2O3 whichlead to the oxygen-deficient state transform back into O2− ions onwurtzite structure of hexagonal Zn2+ or In3+ ion array. Schematicillustrations of the crystal structures of 5% In-doped ZnO and ZnONPs with oxygen vacancies on the surface were drew by Spartan10 V1.0.1 software (Fig. 6a), from which one can see that with thedoping of In, on one hand, more oxygen vacancies were generated,and on the other hand, the attraction of the same oxygen vacancytowards a TNT molecule becomes stronger. When sensor based onpure ZnO NPs or In-doped ZnO with four different doping levelsis exposed to explosive vapors, the explosive molecules will reactwith the adsorbed oxygen and release free electrons, leading to thedecrease of the depletion layer thickness and hence the electricalresistance (Fig. 6b).

The In doping manipulated sensing performance of ZnO NPsis realized upon complicated effects of In doping, such as reac-tion sites, charge depletion layer, crystallinity degree, as well asthe oxygen vacancy concentration, which is summarized in Fig. 6c.The amount of reaction sites increases with the increasing of Indoping due to the increased specific BET surface area and reduced

particle size evidenced by BET surface area and morphology obser-vation, which is beneficial to the gas sensing properties of theIn-doped ZnO NPs. The oxygen vacancy concentration increasedwith the increase of In doping no more than 5% due to the fact

Y. Ge et al. / Sensors and Actuators B 244 (2017) 983–991 989

F ped Zd

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ig. 5. (a) XPS spectra of O 1s in ZnO NPs and four different doping levels of In-dooping level: (c) 5%, (d) 10%, (e) 20% and (f) 40%.

hat the trivalent In3+ ions will replace the divalent Zn2+ ions andncrease free electrons, which is helpful for increasing the electronarrier concentration and the amount of adsorbed oxygen. How-ver, with further increase In doping level which is greater than 5%,he amount of adsorbed oxygen decreases with the oxygen vacancyoncentration gradually, which is consistent with the XPS resultsas shown in Fig. 6c). The decrease of the oxygen vacancy concen-ration at the higher doping concentration is probably due to theefective crystallinity (as shown in XRD pattern of Fig. 1a). Fur-hermore, higher doping level of In may lead poorer charge carrierensity of sensor, which can lead to a poor sensitivity [34]. Hence,s a combined result, the In doping manipulated oxygen vacancy,.e. the higher energy oxygen-deficient state, is the primary impactactor for gas sensing performance. The best 5% In-doped ZnO NP-ased sensor with the highest oxygen vacancy concentration anddsorbed oxygen amount has the highest sensitivity. Therefore, theethod of In doping in ZnO is efficient for modulate the sensing

erformance of the material towards nitro-explosive vapors.

nO, XPS spectra of O 1s in (b) pure ZnO NPs, and In-doped ZnO NPs with different

4. Conclusion

In summary, a cationic impurity doping method was adoptedto regulate the surface states of ZnO nanostructure and to greatlyimprove the sensing performance of the resulting chemiresistivegas sensing platform. Typically, a sol-gel route and a post-annealingprocess were employed to facilely prepare In-doped ZnO NPswith abundant oxygen vacancies. The resulting In-doped ZnOnanoparticle gas sensor shows enhanced responses towards room-temperature saturated vapors of nitro-explosives when comparedwith that of the pure ZnO nanoparticle-based sensor, such as, theresponses towards TNT, DNT, PNT, PA and RDX were increased from22.2, 8.5, 2.9, 4.9 and 9.8% to 54.7, 52.9, 57.2, 58.3 and 47.4%, respec-tively. Moreover, much shorter response time (<6.3 s vs. 20–40 s)and recovery time (<14 s vs. 20–40 s) were achieved through thisIn-doping process, which is of vital importance for on-site explosivedetection. The In doping produced small size, defective crystallinity

and abundant oxygen vacancies which are proved to be responsiblefor the sensing performance promotion. The present investigationpresents a totally new understanding in the underlying mecha-nisms influencing the sensor performance, and we expect that it

990 Y. Ge et al. / Sensors and Actuato

Fig. 6. (a) The schematic illustrations of the crystal structures of 5% In-doped ZnO(up part) and pure ZnO NPs (down part), (b) Schematic diagram of sensing on sur-fec

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ace of 5% In-doped ZnO (up part)and pure ZnO NPs (down part), and (c) Possibleffects of In3+ concentration on the reaction site, the oxygen vacancy concentration,rystallinity degree and charge depletion layer depth.

ould shine light on portable, real-time and cheap platforms forltra trace explosive vapor detection.

cknowledgments

We thank the financial supports from the Research Program ofhinese Academy of Sciences (CAS, CXJJ-16M122), National Naturalcience Foundation of China (51372273, 21401213), and Urumqicience and Technology Plan (Y151010019).

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.snb.2017.01.092.

eferences

[1] Z. Yang, X. Dou, Emerging and future possible strategies for enhancing 1Dinorganic nanomaterials-based electrical sensors towards explosives vaporsdetection, Adv. Funct. Mater. 26 (2016) 2406–2425.

[2] R.G. Ewing, D.A. Atkinson, G. Eiceman, G. Ewing, A critical review of ionmobility spectrometry for the detection of explosives and explosive relatedcompounds, Talanta 54 (2001) 515–529.

[3] J. Yinon, Field detection and monitoring of explosives, TrAC, Trends Anal.Chem. 21 (2002) 292–301.

[4] L. Guo, B. Zu, Z. Yang, H. Cao, X. Zheng, X. Dou, APTS and rGOco-functionalized pyrenated fluorescent nanonets for representative vaporphase nitroaromatic explosive detection, Nanoscale 6 (2014) 1467–1473.

[

rs B 244 (2017) 983–991

[5] Z. Li, G. Meng, Q. Huang, X. Hu, X. He, H. Tang, Z. Wang, F. Li, Agnanoparticle-grafted PAN-nanohump array films with 3D high-density hotspots as flexible and reliable SERS substrates, Small 11 (2015) 5452–5459.

[6] S. Some, Y. Xu, Y. Kim, Y. Yoon, H. Qin, A. Kulkarni, T. Kim, H. Lee, Highlysensitive and selective gas sensor using hydrophilic and hydrophobicgraphenes, Sci. Rep. 3 (2013) 1868.

[7] S.F. Liu, L.C. Moh, T.M. Swager, Single-walled carbonnanotube-metalloporphyrin chemiresistive gas sensor arrays for volatileorganic compounds, Chem. Mater. 27 (2015) 3560–3563.

[8] T. Li, W. Zeng, New insight into the gas sensing performance of SnO2

nanorod-assembled urchins based on their assembly density, Ceram. Int. 43(2017) 728–735.

[9] R. Xing, L. Xu, J. Song, C. Zhou, Q. Li, D. Liu, H.W. Song, Preparation and gassensing properties of In2O3/Au nanorods for detection of volatile organiccompounds in exhaled breath, Sci. Rep. 5 (2015) 10717.

10] T. Li, W. Zeng, H. Long, Z. Wang, Nanosheet-assembled hierarchical SnO2

nanostructures for efficient gas-sensing applications, Sens. Actuators B 231(2016) 120–128.

11] P. Rai, W.-K. Kwak, Y.-T. Yu, Solvothermal synthesis of ZnO nanostructuresand their morphology-dependent gas-sensing properties, ACS Appl. Mat.Interfaces 5 (2013) 3026–3032.

12] M.R. Alenezi, S.J. Henley, N.G. Emerson, S.R.P. Silva, From 1D and 2D ZnOnanostructures to 3D hierarchical structures with enhanced gas sensingproperties, Nanoscale 6 (2014) 235–247.

13] S.-L. Zhang, J.-O. Lim, J.-S. Huh, J.-S. Noh, W. Lee, Two-step fabrication of ZnOnanosheets for high-performance VOCs gas sensor, Curr. Appl. Phys. 13 (2013)S156–S161.

14] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Fabrication andethanol sensing characteristics of ZnO nanowire gas sensors, Appl. Phys. Lett.84 (2004) 3654–3656.

15] Z.L. Wang, ZnO nanowire and nanobelt platform for nanotechnology, Mater.Sci. Eng. R Rep. 64 (2009) 33–71.

16] J.X. Wang, X.W. Sun, Y. Yang, H. Huang, Y.C. Lee, O.K. Tan, L. Vayssieres,Hydrothermally grown oriented ZnO nanorod arrays for gas sensingapplications, Nanotechnology 17 (2006) 4995–4998.

17] J.W. Spalenka, P. Gopalan, H.E. Katz, P.G. Evans, Electron mobilityenhancement in ZnO thin films via surface modification by carboxylic acids,Appl. Phys. Lett. 102 (2013) 041602.

18] M. Luo, C. Shen, B.N. Feltis, L.L. Martin, A.E. Hughes, P.F. Wright, T.W. Turney,Reducing ZnO nanoparticle cytotoxicity by surface modification, Nanoscale 6(2014) 5791–5798.

19] L. Li, Z. Du, T. Wang, Enhanced sensing properties of defect-controlled ZnOnanotetrapods arising from aluminum doping, Sens. Actuators B 147 (2010)165–169.

20] E. S ennik, S. Kerli, Alver Ü, Z.Z. Öztürk, Effect of fluorine doping on theNO2-sensing properties of ZnO thin films, Sens. Actuators B 216 (2015) 49–56.

21] S. Pati, P. Banerji, S. Majumder, Properties of indium doped nanocrystallineZnO thin films and their enhanced gas sensing performance, RSC Adv. 5(2015) 61230–61238.

22] X. Xu, Y. Chen, S. Ma, W. Li, Y. Mao, Excellent acetone sensor of La-doped ZnOnanofibers with unique bead-like structures, Sens. Actuators B 213 (2015)222–233.

23] W. Tian, C. Zhang, T. Zhai, S.L. Li, X. Wang, J. Liu, X. Jie, D. Liu, M. Liao, Y. Koide,Flexible ultraviolet photodetectors with broad photoresponse based onbranched ZnS-ZnO heterostructure nanofilms, Adv. Mater. 26 (2014)3088–3093.

24] D. Fu, C. Zhu, X. Zhang, C. Li, Y. Chen, Two-dimensional net-like SnO2/ZnOheteronanostructures for high-performance H2S gas sensor, J. Mater. Chem. A4 (2016) 1390–1398.

25] M. Wang, C. Chen, H. Qin, L. Zhang, Y. Fang, J. Liu, L. Meng, Construction ofFeS2-sensitized ZnO@ZnS nanorod arrays with enhanced optical andphotoresponse performances, Adv. Mater. Interfaces 2 (2015) 1500163.

26] Y.-Y. He, X. Zhao, Y. Cao, X. -x. Zou, G.-D. Li, Facile synthesis of In2O3

nanospheres with excellent sensitivity to trace explosive nitro-compounds,Sens. Actuators B 228 (2016) 295–301.

27] Y. Cao, X. Zou, X. Wang, J. Qian, N. Bai, G.-D. Li, Effective detection of traceamount of explosive nitro-compounds by ZnO nanofibers with hollowstructure, Sens. Actuators B 232 (2016) 564–570.

28] G. He, N. Yan, J. Yang, H. Wang, L. Ding, S. Yin, Y. Fang, Pyrene-containingconjugated polymer-based fluorescent films for highly sensitive and selectivesensing of TNT in aqueous medium, Macromolecules 44 (2011) 4759–4766.

29] G. Machado, D. Guerra, D. Leinen, J. Ramos-Barrado, R. Marotti, E. Dalchiele,Indium doped zinc oxide thin films obtained by electrodeposition, Thin SolidFilms 490 (2005) 124–131.

30] S.S. Badadhe, I. Mulla, H2S gas sensitive indium-doped ZnO thin films:preparation and characterization, Sens. Actuators B 143 (2009) 164–170.

31] A. Patterson, The Scherrer formula for X-ray particle size determination, Phys.Rev. 56 (1939) 978.

32] L. Liao, H. Lu, J. Li, H. He, D. Wang, D. Fu, C. Liu, W. Zhang, Size dependence ofgas sensitivity of ZnO nanorods, J. Phys. Chem. C 111 (2007) 1900–1903.

33] K.-J. Chen, T.-H. Fang, F.-Y. Hung, L.-W. Ji, S.-J. Chang, S.-J. Young, Y. Hsiao, The

crystallization and physical properties of Al-doped ZnO nanoparticles, Appl.Surf. Sci. 254 (2008) 5791–5795.

34] Z. Wang, Z. Tian, D. Han, F. Gu, Highly sensitive and selective ethanol sensorfabricated with In-Doped 3DOM ZnO, ACS Appl. Mater. Interfaces 8 (2016)5466–5474.

ctuato

[

[

[

[

[

[

[

[

[

[

[

[

Y. Ge et al. / Sensors and A

35] P.C. Chen, S. Sukcharoenchoke, K. Ryu, A.L. d. Gomez, A. Badmaev, C. Wang, C.Zhou, 2,4,6-trinitrotoluene (TNT) chemical sensing based on alignedsingle-walled carbon nanotubes and ZnO nanowires, Adv. Mater. 22 (2010)1900–1904.

36] D. Wang, A. Chen, S.H. Jang, H.L. Yip, A.K.Y. Jen, Sensitivity of titania(B)nanowires to nitroaromatic and nitroamino explosives at room temperaturevia surface hydroxyl groups, J. Mater. Chem. 21 (2011) 7269–7273.

37] G.S. Aluri, A. Motayed, A.V. Davydov, V.P. Oleshko, K.A. Bertness, M.V. Rao,Nitro-aromatic explosive sensing using GaN nanowire-titania nanoclusterhybrids, IEEE Sens. J. 13 (2013) 1883–1888.

38] J.M. Schnorr, D.V.D. Zwaag, J.J. Walish, Y. Weizmann, T.M. Swager, Sensoryarrays of covalently functionalized single-walled carbon nanotubes forexplosive detection, Adv. Funct. Mater. 23 (2013) 5285–5291.

39] Y. Che, X. Yang, G. Liu, C. Yu, H. Ji, J. Zuo, J. Zhao, L. Zang, Ultrathin n-typeorganic nanoribbons with high photoconductivity and application inoptoelectronic vapor sensing of explosives, J. Am. Chem. Soc. 132 (2010)5743–5750.

40] Z. Yang, X. Dou, S. Zhang, L. Guo, B. Zu, Z. Wu, H. Zeng, Nanosensors: ahigh-performance nitro-explosives schottky sensor boosted by interfacemodulation, Adv. Funct. Mater. 25 (2015) 4039–4048.

41] N. Chen, X. Li, X. Wang, J. Yu, J. Wang, Z. Tang, S.A. Akbar, Enhanced roomtemperature sensing of Co3O4 – intercalated reduced graphene oxide basedgas sensors, Sens. Actuators B 188 (2013) 902–908.

42] P.P. Zhang, H. Zhang, X.H. Sun, A uniform porous multilayer-junction thin filmfor enhanced gas-sensing performance, Nanoscale 8 (2015) 1430–1436.

43] L.K. Rao, V. Vinni, Novel mechanism for high speed growth of transparent andconducting tin oxide thin films by spray pyrolysis, Appl. Phys. Lett. 63 (1993)608–610.

44] M. Chen, X. Wang, Y. Yu, Z. Pei, X. Bai, C. Sun, R. Huang, L. Wen, X-rayphotoelectron spectroscopy and auger electron spectroscopy studies ofAl-doped ZnO films, Appl. Surf. Sci. 158 (2000) 134–140.

45] R.N. Aljawfi, S. Mollah, Properties of Co/Ni codoped ZnO based nanocrystallineDMS, J. Magn. Magn. Mater. 323 (2011) 3126–3132.

46] C. Li, Z. Du, L. Li, H. Yu, Q. Wan, T. Wang, Surface-depletion controlled gassensing of ZnO nanorods grown at room temperature, Appl. Phys. Lett. 91(2007) 2101.

rs B 244 (2017) 983–991 991

Biographies

Yuru Ge received her bachelor’s degree in Hanan Normal University in 2014. Cur-rently, she is a postgraduate of Prof. Wei and she is also a joint-training student inProf. Dou’s group. Her research interests are constructing nanosensors for rapid andnon-contact sensing of military explosives.

Zhong Wei completed his Ph.D. in Polymer Chemistry and Physics in 2007 fromNankai University, China. He is now a professor at the School of Chemistry and Chem-ical Engineering, Shihezi University. His research interests are focused on polymermolecular structure design and synthesis.

Yushu Li completed her Ph.D. in 2014 at University of New Hampshire, U. S. A.Currently she is a postdoc researcher at the Laboratory of Environmental Scienceand Technology at The Xinjiang Technical Institute of Physics & Chemistry of theChinese Academy of Sciences. Her research interests are constructing nanosensorsfor rapid and non-contact sensing of improvised explosives.

Jiang Qu received his bachelor’s degree in 2013 at Nanjing Tech University, China.Now, he is a graduate student in Prof. Dou’s group. His research interest centersaround sensors by means of chemiresistive sensing materials and surface-enhancedRaman spectroscopy.

Baiyi Zu completed her Ph.D. at Nankai University, China. Currently she is an asso-ciate professor at the Laboratory of Environmental Science and Technology at TheXinjiang Technical Institute of Physics & Chemistry of the Chinese Academy of Sci-ences. Her research interests are constructing nanosensors for rapid and non-contactsensing of military and improvised explosives.

Xincun Dou obtained his Ph.D. in Materials Physics and Chemistry from Instituteof Solid State Physics in Chinese Academy of Sciences in 2009. Following postdoc-

toral work at Nanyang Technological University in Singapore in Materials Science,he joined the Laboratory of Environmental Science and Technology at Xinjiang Tech-nical Institute of Physics & Chemistry in 2011 as a “Hundred Talents Program”professor. His research interests are focused on the exploratory design of nano-materials and nanodevices, with an emphasis on explosives sensing applications.