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Sensors and Actuators B 263 (2018) 120–128

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo u r nal homep age: www.elsev ier .com/ locate /snb

nhanced formaldehyde detection based on Ni doping of SnO2

anoparticles by one-step synthesis

un Hu, Tao Wang, Yanjie Wang, Da Huang, Guili He, Yutong Han, Nantao Hu, Yanjie Su,hihua Zhou, Yafei Zhang, Zhi Yang ∗

ey Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information andlectrical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR China

r t i c l e i n f o

rticle history:eceived 30 September 2017eceived in revised form 30 January 2018ccepted 4 February 2018vailable online 5 February 2018

eywords:

a b s t r a c t

In this work, SnO2 nanoparticles with Ni doping were synthesized via a facile one-step hydrothermalroute. The addition of Ni to SnO2 lead to significant enhanced response to formaldehyde (HCHO). The Ni-doped SnO2 materials were used to fabricate HCHO gas sensors and the comprehensive HCHO-sensingproperties under diverse working temperatures were tested and studied. With suitable Ni concentration,the SnO2 nanoparticles exhibited a 10 times sensing enhancement with high gas response, excellentselectivity and good stability. Furthermore, the limit of detection (LOD) was experimentally figured out

ydrothermali-doped SnO2

anoparticlesas sensorormaldehyde

to be as low as 120 ppb, which is much lower than the threshold exposure limit of HCHO proposed byAmerican Conference of Governmental Industrial Hygienists. Importantly, the excellent performancesof the as-fabricated gas-sensing devices make Ni-doped SnO2 a promising candidate for HCHO sensingapplications. This synthesis strategy here can also give guidance for designing high-performance HCHOgas sensors.

© 2018 Elsevier B.V. All rights reserved.

. Introduction

Since the first report on the gas sensing behavior of metal oxideemiconductor (MOS) in the 1960s [1], the past several decadesave witnessed breakthroughs in both fundamental research andractical application of resistive gas sensors [2,3]. The mecha-ism of MOS resistive gas sensor can be described as follows:hen exposed to analyte gases, the oxidation-reduction reaction

etween the target gases and the surface of sensing materials willead to a resistant variation of MOS [4]. Up to now, gas sensors basedn MOS have received considerable attention in the detection ofazardous pollutants and flammable gases, such as formaldehydeHCHO) [5], NO2 [6], NH3 [7], H2S [8], SO2 [9], CO [10] and CH411]. Frequently-used MOS materials, such as SnO2, ZnO, TiO2, WO3,n2O3, Fe2O3, CeO2 and CuO, have been studied [12–14]. Among theseable MOS materials, SnO2 (Eg = 3.6 eV at 300 K), the most exten-

ively used material for commercially available sensors, is criticalo research from both theoretical and experimental perspectives14–16]. Gas sensors based on SnO2 materials exhibit some superi-

∗ Corresponding author.E-mail address: zhiyang@sjtu.edu.cn (Z. Yang).

ttps://doi.org/10.1016/j.snb.2018.02.035925-4005/© 2018 Elsevier B.V. All rights reserved.

orities, which have stimulated further potential research on its gassensing applications [3]. SnO2 is generally considered to be an n-type electrical conductivity due to its surface oxygen deficiency,which generates adequate oxygen vacancies acting as electrondonors [3,17].

However, SnO2 based gas sensors are still suffering from somerestrictions. Therefore, massive efforts have been made to enhancethe gas sensing performance of SnO2 based gas sensors, includingimproving sensor selectivity, reducing the operating temperatureand detection limit, enhancing sensor stability and shorteningresponse and recovery time [3]. A typical method used for achiev-ing these purposes is the modification of SnO2 with various noblemetals dopants like Au, Ag, Pt and Pd or other transition met-als elements such as Zn, Co, Cu, and Ni [9,12,18,19]. From thispoint of view, doping metallic elements into SnO2 nanostructuresshould be a promising approach, which alters the electronic prop-erties of SnO2 and stimulate more gas molecule adsorption sitesin turn resulting in a positive influence on the gas sensing prop-erties [19,20]. Among the metal dopants, Ni element plays a moreprominent role in improving the formation of oxygen vacancies in

the surface of SnO2 [19]. Sensors based on Ni-doped SnO2 havebeen found to exhibit higher gas response and better selectiv-ity towards the target gas with shorter response-recovery time

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nd lower working temperature [5,20]. Chen et al. studied theorous Ni-doped SnO2 hollow spheres with intensive ethanol sens-

ng properties [21]. Lin et al. reported that the gas sensors with mol% Ni doped SnO2 show obvious selectivity to n-butanol inomparison with the pure SnO2 gas sensor [22]. Li et al. preparedi-doped SnO2 hollow spheres with enhanced ethanol sensing per-

ormance [23]. It is believed that dopants in SnO2 materials improvehe surface reactivity with atmospheric oxygen and subsequentlyhe sensing response by residing on the surface in the form of metallusters or by modifying the crystalline structure [18].

As for HCHO, it is considered as a typical toxic chemicalubstance in environmental chemistry and atmosphere, whichndangers the health and safety of humans and other creatures5,13]. American Conference of Governmental Industrial Hygien-sts listed a threshold exposure limit of HCHO, and an ideal HCHOensor should have an experimental limit of detection (LOD) lowerhan 300 ppb [24]. Besides, to develop a HCHO gas sensor for prac-ical applications, its gas response, reversibility, selectivity, andtability should also be considered [24]. Herein, we reported aacile and low-cost hydrothermal route to synthesize a series ofi-doped SnO2 nanoparticles. After systematically studying of the

nfluence of the Ni-doped compositions, it is found that the pre-ared Ni-doped SnO2 gas sensors exhibited enhanced HCHO gasensing properties with outstanding gas response, selectivity, fastesponse/recovery and stability at 200 ◦C. Furthermore, when theddition amount of Ni reached 5.0 at%, the response value of Ni-oped SnO2 to 50 ppm of HCHO exceeds more than 10 timesompared with pure SnO2. The possible enhancement mechanismsor the effects of Ni doping are discussed.

. Experimental section

.1. Materials and synthesis

Tin dichloride hydrate (SnCl2·2H2O, 98.0 wt%), oxalic acid dehy-rate (H2C2O4·2H2O, 99.5 wt%), polyvinylpyrrolidone (PVP, K = 30)nd nickel (II) chloride hexahydrate (NiCl2·6H2O, 98.0 wt%) wereroduced by Sinopharm Chemical Reagent Co., Ltd, China. Alleagents are of analytical grade and can be used without furtherurification. Deionized water and absolute ethanol were used forll experiments.

Ni-doped SnO2 nanoparticles were obtained by one-stepydrothermal method. The typical synthesis steps were as fol-

ows. 0.226 g (1 mmol) of SnCl2·2H2O and 3.500 g (20 mmol) of

2C2O·2H2O were dissolved in 60 mL of deionized water and main-

ained magnetic stirring for about 0.5 h until the solution becameransparent. 1.000 g of PVP and an appropriate amount of nickel (II)hloride hexahydrate (NiCl2·6H2O) was then added to the mixture.

ig. 1. (a) Ceramic tube with heating coil. (b) Gas sensor unit that fixed on the electronic bystem.

rs B 263 (2018) 120–128 121

After continuous stirring for 1 h, the mixed solution was trans-ferred to a 100 mL Teflon-lined stainless steel autoclave and heatedat 200 ◦C for 12 h under stable conditions. Following the comple-tion hydrothermal reaction, the autoclave was then allowed to coolnaturally to room temperature. The precipitate was collected bycentrifugation and washed with deionized water and ethanol forseveral times, and finally dried overnight at 60 ◦C. All final sampleswere calcined in air at 450 ◦C for 0.5 h. The pure and Ni-doped SnO2nanoparticles with different doping concentrations of 0, 1, 2.5, 5.0,7.5 and 10 at% were synthesized and labeled as NS-0, NS-1, NS-2.5,NS-5, NS-7.5 and NS-10, respectively.

2.2. Characterization

The crystal structures of NS-0, NS-1, NS-2.5, NS-5, NS-7.5 andNS-10 nanoparticles was detected by X-ray diffraction (XRD, D8Advance, Bruker Corporation, Germany) with Cu K� source in a 2�range from 20◦ to 80◦. Analysis of elemental content was measuredby energy dispersive X-ray spectroscopy (EDS, Oxford InstrumentsINCA PentaFET × 3, Model: 7426). The surface state of the sam-ples was analyzed by an X-ray photoelectron spectrometer (XPS,Kratos Axis UltraDLD, Japan) with a monochromatic Al K� X-rayexcitation source (1486.6 eV). The morphology, nanostructures andcrystal lattices of as-prepared samples were characterized by fieldemission scanning electron microscopy (SEM, Ultra Plus, Carl Zeiss,Germany) and transmission electron microscopy (TEM, JEM-2100,JEOL, Japan).

2.3. Fabrication and measurement of gas sensors

The manufactured sensor is shown in Fig. 1a and b. The gas sen-sor was prepared by the following steps. Firstly, the sample wasmixed with ethanol and coated on the outer surface of the aluminatube that contains a pair of gold electrodes attached with Pt wires.Next, a small Ni-Cr alloy heating coil was inserted into the tube tosupply the operating temperature in the range of 100–500 ◦C. Thetube was then connected to the electronic stand as a sensor unit.The fabricated sensors were heated at an appropriate temperature(about 180 ◦C) for one week to ameliorate its long-term stabil-ity. Finally, gas detection was carried out in ambient air by usinga computer-controlled measurement system WS-30A (WeishengElectronics Co., Ltd., Zhengzhou, China), the schematic of measur-ing electric circuit is exhibited in Fig. 1c. The sensor unit was placedin a sealed chamber of the WS-30A before testing, and then the

calculated quantity of target gas was injected into the chamber,the chemical interaction on surface of the gas sensor results in anabrupt conductivity change. To control the experimental variables,the humidity in the testing was kept at 60% RH. The gas response

racket. (c) Schematic of measuring electric circuit of the gas sensing measurement

122 J. Hu et al. / Sensors and Actuators B 263 (2018) 120–128

Fig. 2. (a) XRD patterns of NS-0, NS-1, NS-2.5, NS-5, NS-7.5, NS-10 and standard tetragonal rutile SnO2 (JCPDS 41-1445). (b) EDX spectrum of NS-0, NS-5 and NS-10. (c) Thefull XPS survey graph of NS-0, NS-5 and NS-10. (d) Sn 3d XPS spectra of NS-0, NS-5 and NS-10.

(a) NS

(sbt

Fig. 3. Typical SEM images of as-obtained nanoparticles:

S) is defined as Ra/Rg (Ra and Rg are the electric resistance of theensors in air and in the target gas, respectively) [25–28], the S cane calculated by computer of WS-30A. Definition of the responseime (Tres) is that the time to reach 90% of the stable output of the

-0, (b) NS-1, (c) NS-2.5, (d) NS-5, (e) NS-7.5 and (f) NS-10.

sensor after the injection of detected gas, while the recovery time(Trec) is defined as the time to fall to 10% of its maximum outputafter the detected gas was discharged [7].

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. Results and discussion

.1. Structure and morphology

Fig. 2a shows the XRD patterns of the obtained pure SnO2 andi-doped SnO2 with well-resolved diffraction peaks in the range

rom 20◦ to 80◦. No additional peaks are observed for the Ni-dopednO2 and all the diffraction peaks can be assigned to SnO2 with

tetragonal rutile structure (JCPDS No. 41-1445, a = b = 4.738 Å, = 3.187 Å). This phenomenon can be explained by two reasons:irstly, the dopants can be in form of clusters. Secondly, the Sn4+

nd Ni2+ have the same ionic radius (0.69 Å), the Ni atoms can beocated in Sn positions in the crystallite bulk and bring about a

inor lattice deformation [8,18,23]. Moreover, according to previ-us studies, all diffraction peaks in XRD of Ni-doped SnO2 can be

till indexed to SnO2 tetragonal phase even if the Ni concentrationas up to 30 mol%, and only slight variation can be detected [29].

n this work, the absence of second phase in XRD can be attributedo the high solubility of Ni2+ in SnO2 bulk [8,22]. Although no extra

ig. 4. (a, b) TEM images and (c, d) HRTEM images of NS-5, (e-h) show the EDS elementaln, O and Ni.

rs B 263 (2018) 120–128 123

peak was observed for the XRD patterns, the Ni/Sn ratio of NS-5 andNS-10 were detected to be 1.76 at% and 3.38 at% respectively by EDSshown in Fig. 2b, implying that the Ni ions have been successfullydoped into SnO2 lattice.

XPS was applied to investigate the surface composition of NS-0, NS-5 and NS-10. From the full XPS spectra in Fig. 2c, the Sn 3d,O 1s and Ni 2p can be detected, confirming the presence of theNi component in the NS-5 and NS-10, which in agreement wellwith the results of EDS.As shown in Fig. 2d shows, the Sn 3d XPSspectra of NS-0, NS-5 and NS-10 can be assigned to Sn4+ of bulkSnO2 [30]. Note that two Sn 3d peaks of NS-0 (Sn 3d3/2 = 494.5 eV,Sn 3d5/2 = 486.5 eV) are shifted to a lower binding energy (NS-10: Sn3d3/2 = 495.0 eV, Sn 3d5/2 = 486.2 eV), which might be attributableto the increased oxygen deficiency caused by substitution of Sn4+

by Ni2+ that decreases the binding energy of Sn [8,31].

Typical SEM images of pure SnO2 and Ni-doped SnO2 are shown

in Fig. 3. The obtained elliptic microspheres are of well-separateddistribution with a uniform size ranging from 150 to 300 nm. How-ever, for samples with high Ni concentration (NS-7.5 and NS-10),

mapping images of 5.0 at% Ni-doped SnO2, indicating a uniform chemical phase of

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he size became larger and the morphology tend to bigger aggre-ation.

The detailed microstructures of the synthesized NS-5 are fur-her characterized by using TEM and high resolution TEM (HRTEM)Fig. 4a–d). The morphologies of the nanoparticles are well matchedo elliptic microspheres with a uniform width of about 200 nm, and

length of around 300 nm. From HRTEM image in Fig. 4c, the surfaceorphology NS-5 is observed coarse and irregular with no regu-

ar exposed facets, which can be concluded that Ni-doped SnO2anoparticles have an irregular oval shape of the exposed surface,hich provides surface defects [7,32–35]. HRTEM image in Fig. 4d

s an individual SnO2 nanoparticle of NS-5 with a lattice spacingf 0.334 nm, which matches the (110) planes of tetragonal rutilenO2 very well. These structural features of NS-5 can be indexedo a tetragonal rutile structure. Fig. 4(e–h) show the EDS elemental

apping images of 5.0 at%-Ni-SnO2, indicating a uniform chemicalhase of Sn, O and Ni. In addition, taking the coarse surface intoonsideration, surface defects can provide more adsorption site tohemisorbed oxygen and to some extent to enhance the gas sensingroperties of Ni-doped SnO2 nanoparticles.

.2. Gas sensing properties

The gas sensing properties of NS-0, NS-1, NS-2.5, NS-5, NS-7.5nd NS-10 were measured. Sensing properties of HCHO at differ-nt temperatures is evaluated to determine the optimum operatingemperature as shown in Fig. 5. As the temperature rises from 100o 300 ◦C, the gas response to HCHO is initially increased and thenegins to decrease. The best working temperature of NS-0, NS-1

s about 250 ◦C while others are about 200 ◦C. Therefore, 200 ◦C iselected as the testing temperature in the following experiments.

The resistance change of the sensors is displayed in Fig. 6. It cane seen that all these sensors exhibit typical n-type gas sensingehavior. As shown in Fig. 6a, the doping of Ni has a significant

nfluence on the baseline resistance of gas sensors. Compared withure SnO2, Ni-doped SnO2 exhibits a higher resistance in air but a

ower resistance in HCHO, which means that the detection signals more obvious. And the response to the baseline whether it is inhe air or the target gas can basically remain stable. The 5.0 at%i-doped SnO2 (NS-5) shows the best gas response with sensitiv-

ties calculated to be about 104 based on Ra/Rg to 50 ppm HCHOt 200 ◦C. The cyclic resistance changes of sensor based on NS-5as also tested as shown in Fig. 6b, implying a good reversible

esponse-recovery property. More detailed gas sensing properties

re displayed in Fig. 7.

Fig. 7a shows the dynamic response curve of different Ni-dopednO2 gas sensors exposed to HCHO gas in the concentration rangef 1–50 ppm at 200 ◦C. It is apparent that all the sensors show a

ig. 6. (a) Dynamic resistance curve of the NS-0, NS-1, NS-2.5, NS-5, NS-7.5 and NS-10 seensor based on NS-5.

Fig. 5. Response of NS-0, NS-1, NS-2.5, NS-5, NS-7.5 and NS-10 sensors to 50 ppmHCHO at various temperatures.

reversible response-recovery property. The addition of Ni to SnO2lead to significant enhanced response to HCHO. It is evident thatthe doping of Ni can effectively enhance the sensing performances.Especially, compared with other component sensors, the 5.0 at%Ni-doped SnO2 (NS-5) exhibits the best gas response with sen-sitivities calculated to be 9.887–104.103 when the concentrationof HCHO increase from 1 to 50 ppm, reaching a maximum of 10times more than that of pure SnO2 (NS-0). Nevertheless, sequen-tial increase of Ni doping results in decrease of the response forthe NS-7.5 and NS-10, which are even lower than the NS-5. Thelinear fitting between gas response (Ra/Rg) and concentration areshown in Fig. 7b. The corresponding response is linearly propor-tional to HCHO concentration in the region of lower than 50 ppm.There is a partial saturation in the higher the HCHO concentration,which caused the response distortion [24]. The theoretical limit ofdetection (LOD) can be figured out by a linear extrapolation of theresponse gas response as a function of HCHO concentration (theinset in Fig. 6b). An ultra-low formaldehyde detection concentra-tion of 120 ppb was figured out, this value is much lower than thethreshold exposure limit of HCHO (300 ppb) listed by AmericanConference of Governmental Industrial Hygienists [24]. The cyclicresponse curve of the sensor based on NS-5 towards 50 ppm HCHOat 200 ◦C is shown in Fig. 7c. The sensor show reversible responseand recovery for seven times, indicating that the sensor manufac-tured in this work possesses excellent stability and reversibility. In

addition, the response time and recovery time of the NS-5 basedgas sensor in 50 ppm HCHO at 200 ◦C is estimated to be 18 s and10s, respectively, which is shown in Fig. 7d.

nsor upon exposure to 50 ppm HCHO at 200 ◦C. (b) Cyclic resistance change of the

J. Hu et al. / Sensors and Actuators B 263 (2018) 120–128 125

Fig. 7. (a) Dynamic response curve of the NS-0, NS-1, NS-2.5, NS-5, NS-7.5 and NS-10 sensor device up to 1–50 ppm HCHO at 200 ◦C. (b) The corresponding response variationof NS-5 based gas sensor as a function of HCHO concentration (c) Cyclic response curve of the sensor based on NS-5. (d) The response and recovery time of the NS-5 basedgas sensor.

us gas

ao(C(aanwsmpgtr

Fig. 8. (a) Selectivity of NS-0 and NS-5 toward vario

Besides, selectivity and long-term stability are also considereds important parameters for gas sensors. Fig. 8a shows a bar graphf the response of un-doped (NS-0) and 5.0 at% Ni-doped SnO2NS-5) to various testing gases, including HCHO, ethanol (C2H6O,H3CH2OH), methanol (CH4O, CH3OH), triethylamine [C6H15N,CH3CH2)3N)], acetone (C3H6O, CH3COCH3), NH3, CO and NO. Allnalytes with the same concentration (100 ppm) were tested atn operating temperature of 200 ◦C. The sensor NS-5 exhibits sig-ificantly higher response to HCHO than those of other analytes,hereas the NS-0 shows no apparent selectivity. In addition, the

ensor based on NS-5 is stored in the ambient environment forore than 2 months to test its long-term stability. The sensing

erformance was measured every 5 days at 200 ◦C with the HCHOas concentration of 100 ppm. The results in Fig. 8b show thathe sensor has an excellent stability for HCHO detection with gasesponse of about 130, and there was no remarkable decrease in

es. (b) Long-term stability of NS-5 based gas sensor.

this detection. A comparison of the performances of different kindsof sensitive materials based HCHO sensors is listed in Table 1.Compared with other work, the Ni doping in this work not onlysignificantly improve the gas response but also enhance the selec-tivity of the SnO2 based sensor with excellent reversibility and fastresponse and recovery.

3.3. The gas sensing mechanisms

In general, n-type SnO2 works with a surface-controlled gassensing mechanism based on the oxidation–reduction reactionbetween the target gases and the surface of sensing materials [7].

According to the surface-depletion model [40], chemisorption ofoxygen (O2) is conducted on the surfaces of SnO2 nanocrystals. Bycapturing free electrons from the sensing materials at elevated tem-peratures, oxygen ions adsorbed from the air to the surface of the

126 J. Hu et al. / Sensors and Actuators B 263 (2018) 120–128

Table 1Comparison of the HCHO sensors based on SnO2.

Sensitive materials Synthesis method Workingtemperature (◦C)

S (Ra/Rg) andconcentration

Tres/Trec (s) References

Self-assembled In2O3 ammonolysis and re-oxidation process 420 1.35/5 ppm 48/58 [36]Ultrathin SnO2 nanosheets hydrothermal strategy 240 3.0/5 ppm 1/6 [37]Cd-doped TiO2-SnO2 sol-gel method 310 15.0/10 ppm 25/17 [38]

240 4.2/5 ppm 31.5/145 [39]s 260 65/100 ppm 14/13 [5]

200 9.9/1 ppm 18/10 This work

SwtarE

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Table 2Detailed XPS data of O 1s for NS-0, NS-5 and NS-10.

O 1s XPS spectra Peak 1(eV) Peak 2(eV) Peak 3(eV)

NS-0 533.264 531.705 530.298

La-doped SnO2 ball-milling solid chemical reaction

Ni-SnO2 chemical solution route and acid-washing procesNi-SnO2 Hydrothermal

nO2 nanoparticles can be ionized into adsorbed oxygen ions (Ox−),

hich involved in the redox reaction that occurs on the surface ofhe sensing materials, resulting in changes in the sensor resistancend the formation of a thick space charge layer that increase theesistance of sensors. The reaction can be described by the followingqs. (1)–(4) [7,12,36].

2 (gas) ↔ O2 (ads) (1)

2 (ads) + e− ↔ O−2 (ads) (2)

−2 (ads) + e− ↔ 2O− (ads) (3)

− (ads) + e− ↔ O2− (ads) (4)

Adsorbed oxygen ions (Ox−) are known to be the most common

efects in MOS. But the enhancement of HCHO sensing mechanismf Ni-doped SnO2 also can be attributed to structural sensitization41]. The electron density decreased while oxygen vacancy concen-ration increased with Ni doping. In the case of Ni doping samples,t is easy that Sn4+ ions in lattice are substituted by Ni2+, aboveeaction can be written in equation form (5) [23]:

iO + 2e′ ↔ Ni”Sn + O×O + V×

O (5)

From crystal structure analysis as shown in Eq. (5), the improve-ent of HCHO gas sensing performance by Ni doping is due to the

resence of Ni2+ in SnO2 crystals that capturing free electrons fromhe sensing materials to create one oxygen ion and one oxygenacancy, which changes the defect equilibrium of gas sensor andacilitate O2 adsorbed on the surface of SnO2 [23,41].

However, Liu et al. pointed out the amount of O2−, O− or O2−

s controlled by temperature [42]. For instance, O2− ions on the

urface of SnO2 nanoparticles maximize below 150 ◦C, while theonic species O− predominately exist on the surface from 150 ◦C to00 ◦C [42]. Therefore, O− ions on the surface of the prepared Ni-oped SnO2 sensor have been maximized at 200 ◦C [42]. When the

ensor is placed in a HCHO gas environment, the adsorbed oxygenpecies react with the reducing molecules, resulting in an abrupthange of sensor resistance. The process may be carried out by theq. (6) below [39,43]. The basic gas sensing mechanisms for HCHO

Fig. 10. O 1s spectra of (a) NS

NS-5 533.210 531.750 530.293NS-10 533.187 531.747 530.367

of the pure SnO2 and Ni-doped SnO2 based sensor are depicted inFig. 9.

HCHO + 2O− (ads) → CO2 + H2O + 2e− (6)

According to previous literatures, the gas sensing propertieswere closely related to the amount of the chemically adsorbed oxy-gen and the composition of oxygen vacancy [31,44,45]. In orderto further explore the sensing mechanisms of Ni-doped SnO2, the

high-resolution O 1s XPS spectrum is also investigated in detail.Fig. 10a–c shows the O 1s spectra that can be fitted into threekinds of oxygen species components with their binding energy ofO 1s presented in Table 2. The binding energies of about 530.3,

-0, (b) NS-5, (c) NS-10.

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31.7 and 533.2 eV are attributed to lattice oxygen (OL), oxygenacancy (OV), and chemisorbed oxygen (OC), respectively [44,45].he relative percentages of OL, OV and OC components in NS-0re approximately 46.57%, 41.12% and 12.31%, respectively, while8.85%, 50.51% and 20.64% in NS-5, and 34.84%, 48.52% and 16.64%

n NS-10. It is obvious that the concentration ratio of both oxy-en deficiency and chemisorbed oxygen increase with increasingmount of Ni doping and maximized in NS-5. Considering the gasensing performance mostly depends on the amount of O2 adsorbedn the surface of the sensor materials, while transition metals gen-rally serve as “accelerators” of various processes [18,44,45]. In thisase, the irregularly exposed surfaces of Ni-doped SnO2 nanopar-icles introduce more defects for chemisorbed oxygen. Increasedmount of OC means that more surface chemically adsorbed oxy-en species may participate in the redox reaction that occurs on theurface of the sensing materials and thus lead to greater changesn the sensor resistance. On the other hand, the increase of OV canupply more active sites to the gas reaction and adsorption on theurface of the sensing materials [44–47].

. Conclusions

In summary, a series of Ni-doped SnO2 nanoparticles were pre-ared by a one-step hydrothermal method, which have a greatotential in large-scale commercial production. The systematically

nvestigation of the effects of precursor compositions indicated thathe prepared Ni-doped SnO2 nanoparticles exhibit excellent HCHOensing properties, including enhanced gas response, selectivitynd stability towards HCHO with rapid response and recovery and

very low detection limit. Especially the 5.0 at% Ni doped SnO2ample Exhibits 10 times sensing enhancement and excellent selec-ivity with LOD of formaldehyde low to 120 ppb, which is muchower than the threshold limit of HCHO listed by American Con-erence of Governmental Industrial Hygienists, showing its greatotential for commercial applications in formaldehyde detection.

cknowledgments

The authors gratefully acknowledge financial supports fromhe National Natural Science Foundation of China (61671299),he National Key Research and Development Program of China2016YFC0102700), Shanghai Science and Technology Grant16JC1402000 and 17ZR1414100), the Program of Shanghaicademic/Technology Research Leader (15XD1525200), the Pro-ram for Professor of Special Appointment (Eastern Scholar) athanghai Institutions of Higher Learning (GZ2016005), and Shang-ai Jiao Tong University New Youth Teacher Initiative Project17X100040074). We also acknowledge analysis support from thenstrumental Analysis Center of Shanghai Jiao Tong University andhe Center for Advanced Electronic Materials and Devices of Shang-ai Jiao Tong University.

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Biographies

Jun Hu is a MS candidate in Shanghai Jiao Tong University, China. Her research focusnow includes nanomaterials and their application for gas sensors.

Tao Wang is a PhD candidate in Shanghai Jiao Tong University, China. His researchfocus now includes nanomaterials and their application for gas sensors.

Yanjie Wang is a UG candidate in Shanghai Jiao Tong University, China. Her researchfocus now includes nanomaterials and their application for gas sensors.

Da Huang is a PhD candidate in Shanghai Jiao Tong University, China. His researchfocus now includes nanomaterials and their application for gas sensors.

Guili He is a PhD candidate in Shanghai Jiao Tong University, China. Her researchfocus now includes nanomaterials and their application for gas sensors.

Yutong Han is a PhD candidate in Shanghai Jiao Tong University, China. Her researchfocus now includes nanomaterials and their application for gas sensors.

Nantao Hu is currently a associate professor in Shanghai Jiao Tong University, China.His research interests include synthesis of nanomaterials and their applications innanodevices.

Yanjie Su is currently a associate professor in Shanghai Jiao Tong University, China.His research interests include synthesis of nanomaterials and their applications innanodevices.

Zhihua Zhou is currently an assistant professor in Shanghai Jiao Tong University,China. His research interests include synthesis of nanomaterials and their applica-tions in nanodevices.

Yafei Zhang is currently a professor in Shanghai Jiao Tong University, China. His

research interests include synthesis of nanomaterials and their applications in nan-odevices.

Zhi Yang is currently a professor in Shanghai Jiao Tong University, China. Hisresearch interests include synthesis of nanomaterials and their applications in nan-odevices.