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Enhanced room temperature oxygen sensing properties of LaOCl-SnO2 hollow spheres by UV light illumination Ya Xiong, Wenbo Lu, Degong Ding, Lei Zhu, Xiaofang Li, Cuicui Ling, and Qingzhong Xue ACS Sens., Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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Enhanced room temperature oxygen sensing properties of LaOCl-SnO2 hollow spheres by UV light illumination Ya Xiong†‡, Wenbo Lu‡, Degong Ding‡, Lei Zhu‡, Xiaofang Li‡, Cuicui Ling*‡ and Qingzhong Xue*†‡ † State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, P. R. China ‡ College of Science, China University of Petroleum, Qingdao 266580, Shandong, P. R. China

* Corresponding authors:

E-mail

addresses:

[email protected]

[email protected] (C. C. Ling) Tel.: 86-0532-86981169; Fax: 86-0532-86981169

1

ACS Paragon Plus Environment

(Q.

Z.

Xue),

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ABSTRACT In this paper, a facile and elegant Green Chemistry method for the synthesis of SnO2 based hollow spheres has been investigated. The influences of doping, crystallite morphology and operating condition on the O2 sensing performances of SnO2 based hollow-sphere sensors were comprehensively studied. It was indicated that compared with undoped SnO2, 10 at.% LaOCl-doped SnO2 possessed better O2 sensing characteristics owing to an increase of specific surface area and oxygen vacancy defect caused by LaOCl dopant. More importantly, it was found that O2 sensing properties of the 10 at.% LaOCl-SnO2 sensor were significantly improved by ultra-violet light illumination, which was suited for room-temperature O2 sensing applications. Besides, this sensor also had a better selectivity to O2 with respect to H2, CH4, NH3 and CO2. The remarkable increase of O2 sensing properties by UV light radiation can be explained in two ways. On the one hand, UV light illumination promotes the generation of electron-hole pairs and oxygen adsorption, giving rise to high O2 response. On the other hand, UV light activates desorption of oxygen adsorbates when exposed to pure N2, contributing to rapid response/recovery speed. The results demonstrate a promising approach for room-temperature O2 detection.

KEYWORDS: LaOCl-doped SnO2; Hollow spheres; Oxygen sensing; Room temperature detection; UV light radiation

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Oxygen gas sensors are playing a more and more important role in our daily life. Their typical applications include the control of air-fuel ratio in automobile engines to enhance combustion efficiency, optimizing industrial boilers, medical ventilation assistance and so on. Therefore, different types of sensors including electrochemical,1, 2

optical3 and resistive sensors4 have been developed to monitor oxygen. However, the

structures of electrochemical sensors are too complicated while optical sensors are bulky and expensive. Resistive sensors based on metal oxide semiconductor (MOS) offer advantages in easy fabrication, miniaturization, simple operation, low production cost and so forth. Among various types of MOS materials, TiO2,5 ZnO6-10 and SnO24 are the most widely used to detect oxygen (Table 1) and other gases. However, these sensors usually have high working temperatures of 150-500 °C, which could lead to declined sensor stability, fast aging, increased power consumption and complicated circuitry required to maintain accurate temperature control. What’s worse, high-temperature operation limits the sensor’s utility in plant respiration, certain medical and food packaging industries where oxygen measurements have to be made at low temperature. Therefore how to develop high performance and reliable O2 gas sensors that can function at room temperature (RT) is of vital significance. Ahmed et al.11 reported oxygen gas detection at RT using Mn-ZnO nanorods fabricated by microwave-hydrothermal method. The results revealed that the Mn-ZnO nanorods based sensor had significantly better sensing performance than undoped ZnO nanorods based sensor due to high specific surface area. However, only three different oxygen contents (5 ppm, 10 ppm, 15 ppm) have been studied. Hu et al.12 synthesized 3


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SrTiO3 to monitor oxygen at near ambient temperature (40 °C) using physical high-energy ball milling technique. Nevertheless, this perovskite material was highly insulating, which made the reading of electrical signal much more complicated. Neri et al.13 developed Pt-In2O3 based oxygen sensor for RT operation via a nonaqueous sol-gel method. The sensor showed a response value of about 1.95 ( S =

R O2 R N2

) to 20 %

oxygen. Regretfully, the sensor exhibited slow kinetics with response/recovery time of 1080/2100 s, a problem that hitherto persists with the low temperature operation of MOS sensors. It is worth to mention that slow recovery speed may cause much inconvenience when the sensors are required to continuously detect target gas. Table 1. A comparison of the performance of O2 sensors based on MOS nanostructured materials Material

Synthesis route

T

Response/C

tres/trec

(°C)

(ppm)

(s)

Ref.

Nb-TiO2

Sol-gel

500

2.99/20

60/90

5

ZnO

Electrochemical

500

6.2*/80,000

NA

6

deposition 5mol.%P-TiO2

Sol-gel

116

29.6/100

35/20

14

Mn-ZnO

Micro-hydrother-

RT

3.8*/15

150/90

11

mal Pt-In2O3

Sol-gel

200

63.3/200,000

NA

15

1wt.%Pt

Sol-gel

RT

1.95/200,000

1080/

13

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-In2O3 10at.%LaOCl-

2100 Hydrothermal

RT

2.25/250

SnO2

182/

Current

1315

work

* denotes a value not explicitly stated in the study, but approximated from a graphical plot; NA denotes no specific data are available.

Improving gas-sensing performance with low power consumption is one of the main concerns for gas sensor applications. With this consideration in mind, the surface photo excitation technique is an effective method to study the changes of conductance (resistance) of the sensor caused by light illumination. In recent years, there have been a few reports on oxygen gas sensors based on ultra-violet (UV) light activated MOS.16, 17 For example, Wang et al.16 studied how UV light illumination affected the oxygen sensing behaviors of individual ZnO nanowire transistors. In his later work, Wang et al.17 reported a route to achieve quick oxygen response in individual β-Ga2O3 nanowire transistors by UV light activation. Illuminating these sensors with UV light is a feasible alternative to activate chemical reactions on metal oxide surface without the necessity of heating. With regard to SnO2 based sensor, so far some literatures are available on improving its sensing response toward LPG18 and NO219 by UV photo-stimulation. Nevertheless, surprisingly little work has been reported on its enhanced oxygen gas sensing behaviors under light illumination, which is desperately demanded to fundamentally understand the gas sensing mechanism and expand the potential applications of SnO2 based sensors. Previous studies proved that better response could be achieved by a hollow porous 5


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structure based sensor because of enhancement in active surface area and better gas diffusion.20, 21 Herein, undoped and 10 at.% LaOCl-doped SnO2 hollow spheres (HSs) were prepared by employing newly made carbon microspheres as templates and sensors based on these HSs were fabricated. The reason why we choose LaOCl dopant is that La compounds are excellent additives to MOS owing to their particular 4f-5d and 4f-4f electronic transition.22 Moreover, doping LaOCl can increase the amount of defect oxygen vacancies on the surface of SnO2.23 The influences of operating condition (RT, 100 °C, blue /UV light illumination), crystallite morphology (hollow spheres, solid nanoparticles) and relative humidity on the oxygen sensing performance of the SnO2 based sensors were systematically studied. In addition, the selectivity of the 10 at.% LaOCl-SnO2 HSs based sensor toward O2, H2, CH4, NH3 and CO2 was also investigated.

EXPERIMENTAL SECTION Chemicals All chemicals including tin (II) chloride dehydrate (SnCl2·2H2O, Sigma-Aldrich Co., UK), lanthanum chloride (LaCl3, Sigma-Aldrich Co., UK), glucose (Sinopharm, China), dimethylfomamide (DMF, Sigma-Aldrich Co., UK) and ethanol (Sinopharm, China) were analytical grade reagents and used as received without any further purifications. Synthesis and characterization of SnO2 based HSs Carbon microspheres were prepared as described elsewhere.24 In a typical procedure, glucose (8 g) was dissolved in deionized water (40 mL) and then the 6


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solution was sealed in a Teflon-lined stainless steel autoclave and maintained at 180 °C for 12 h. Black products were obtained after five cycles of centrifugation/washing/redispersion with deionized water and ethanol, seperately, and then oven-dried at 80 °C overnight. Synthesis of SnO2 based HSs.24 The typical synthesis process was as follows. 1.2 g of SnCl2 with LaCl3 with different atom ratios of La to Sn (0 and 10 at.%) was dissolved in 20 mL of DMF under stirring. 1.5 g of oven-dried carbon microspheres were uniformly dispersed in 40 mL of DMF by ultrasonication, then the SnCl2 based solution was added into the 40 mL of DMF at a speed of 5 s per drop. After ultrasonicating for about 30 min, a small amount of distilled H2O (e.g., 2 mL) was added to the solution dropwise to ensure hydrolyzation. After continuous ultrasonication for 1.5 h, the mixed solution was aged under ambient conditions for two days. The resulting precursor was collected by centrifugation and washed for five times with distilled water and ethanol, respectively, then oven-dried at 80 °C in vacuum overnight. SnO2 based HSs were finally obtained after the as-prepared precursor was calcined at 450 °C for 2 h in a muffle furnace. Characterization X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert diffractometer from 2θ = 10-80° using Cu-Kα radiation (λ = 1.5406 Å). The Brunauer-Emmett-Teller (BET) specific surface area was measured on a Quantachrome NOVA 2000e sorption analyzer. Energy-dispersive spectroscopy (EDS) was taken on a JEOL JEM-2010F. Transmission electron microscope (TEM) observations were performed with a 7


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JEM-2100 UHR microscope. X-ray photoelectron spectroscopy (XPS) studies were analyzed with a PHI5000 Versa Probe employing Al Kα radiation (ULVAC-PHI, Japan). Bonding energy was calibrated with reference to C1s peak (285.0 eV). Fabrication and gas sensing measurement of SnO2-based sensors The SnO2-based HSs were mixed with the solvent prepared by dissolving 0.2 g ethyl cellulose into 2.5 ml anhydrous terpineol, which was then subsequently printed onto commercial alumina substrate equipped with Pt-interdigitated electrodes and Pt heat element. Eventually, the SnO2-based films were sintered at 450 °C for 2 h in air to evaporate the organic solvent and improve the mechanical bond of the deposited samples. Four LED bulbs which can operate independently are installed in front of the four SnO2 based sensors (Figure 1). Different bulbs with wavelengths of 380 nm (UV light) and 460 nm (blue light) are used. The distance between the bulb and the sensor is fixed at 10 mm and the light is focused directly onto the sensor surface. Light intensity at the position of SnO2 based sensor is 30 mW/cm2.

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Figure 1. Scheme diagram of SnO2 based sensor array structure and test chamber. Light illumination and electrical resistance measurements are performed inside the glass chamber. Sensor elements are tested with a commercial gas detection apparatus (Huachuang Ruike Science and Technology Co. Ltd., Hubei Province, China). The measurement range of resistance is 100 Ω-1 GΩ with an error less than 5 %. Gas concentration is controlled by four mass flow controllers. An inlet and outlet port allows a gas supply to be introduced to continuously purge the apparatus at a total gas flow rate of 2 L/min. The moisture level of gas is controlled by mixing dry and wet air and the relative humidity (RH) in the test chamber is measured via a sensor integrated in the base plate. The O2 response of the sensor is defined as S = R 0 / R . While for reducing gases such as H2, CH4, NH3 and CO2, it is defined as S = R / R 0 , where R0 is the resistance of the material when exposed to a certain concentration of target gas and R is the resistance of the material in N2 or air background atmosphere. The response time (tres) is defined as the time needed for the sensor electrical resistance to change from R to R+ 90 % (R0-R) when shifted from N2 to oxygen while recovery 9


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time (trec) represents the time required for the sensor resistance to change from R0 to R0 -90 % (R0-R) when switched back to N2 from oxygen.

RESULT AND DISCUSSION Characterization of the samples XRD analyses of the pure SnO2 and 10 at.% LaOCl-SnO2 samples (Figure 2) show diffraction peaks that are characteristic of the tetragonal rutile structure of SnO2 (JCPDS 41-1445). The mean crystallite sizes of pure SnO2 and 10 at.% LaOCl-SnO2 deduced from the Scherrer equation are 34.64 and 30.94 nm, respectively. XRD pattern of the 10 at.% LaOCl-SnO2 sample mainly shows strong reflections of SnO2. However, the presence of small and well-dispersed tetragonal crystalline LaOCl nanoparticles is confirmed by the emergence of weak (110) and (103) reflections centred at 2θ = 30.46° and 45.44°. The inset shows the shift in the position of the major diffraction peak of LaOCl-SnO2 toward lower angle comparing that of pure SnO2, which suggests that LaOCl is strongly incorporated into the SnO2 crystal lattice. The Brunaeur-Emmet-Teller (BET) surface areas and pore sizes of pure SnO2 and 10 at.% LaOCl-SnO2 HSs are shown in Table 2. It is found that LaOCl doping can effectively increase surface area and decrease pore size of SnO2 based HSs.

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Figure 2. X-ray diffraction patterns of (a) pure SnO2, (b) 10 at.% LaOCl-SnO2 HSs (Inset shows the corresponding X-ray diffraction patterns of pure SnO2 and 10 at.% LaOCl-SnO2 HSs at 2θ = 25-29°). Table 2. Average BET surface areas and pore sizes of pure SnO2 and 10 at.% LaOCl-SnO2 HSs Material

BET surface area

Pore size

(m2/g)

(nm)

Pure SnO2

34.98

7.084

10 at.% LaOCl-SnO2

43.97

6.677

TEM images of pure SnO2 and 10 at.% LaOCl-SnO2 HSs are shown in Figure 3. It is clear that the shells are porous with a thickness of about 20 nm and consist of massive SnO2 based nanoparticles. Most of the SnO2 HSs have a rough morphology with a diameter of about 200 nm and the decrease of grain size is significant when doping LaOCl. EDS image in the inset of Figure 3 (b) clearly reveals the existence of La and Cl in 10 at.% LaOCl-SnO2. The left of C element may be due to the TEM carbon-coated Cu grid used to support the sample. 11


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Figure 3. TEM images of (a) pure SnO2 HSs, (b) 10 at.% LaOCl-SnO2 HSs (Insets show the corresponding EDS images). Since the chemical characteristics of the surface of MOS are decisive on the sensing properties, the chemical composition of different films was investigated using XPS. Figure 4 (a) shows typical XPS survey spectra recorded on pure SnO2 HSs and 10 at.% LaOCl-SnO2 HSs. One pronounced feature in the spectrum recorded on 10 at.% LaOCl-SnO2 HSs is the presence of La 3d core level. XPS spectrum of La element is shown in Figure 4 (b). Figure 4 (c-d) display the XPS spectra of oxygen for pure SnO2 HSs and 10 at.% LaOCl-SnO2 HSs, respectively. The spectrum of O1s can be fitted into nearly three peaks. The peak at 529.5 eV in the O1s spectrum corresponds to crystal lattice (OL) in the SnO2 phase while the medium binding energy of 531.1 eV is associated with oxygen vacancies (OV) within the matrix of SnO2.25 The peak located at 532.2 eV can be assigned to adsorbed and dissociated oxygen species (OC). From Table 3, we know the concentration of OV : 10 at.% LaOCl-SnO2 > pure SnO2.

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Figure 4. (a) XPS survey spectra recorded on pure SnO2 and 10 at.% LaOCl-SnO2 HSs, (b) XPS spectrum of La 3d for 10 at.% LaOCl-SnO2 HSs, corresponding peak fittings of O1s energy region of (c) pure SnO2 HSs and (d) 10 at.% LaOCl-SnO2 HSs. Table 3. Peak area ratios of the oxygen species (OL, OV, OC) of pure SnO2 and 10 at.% LaOCl-SnO2 Species

Position(eV)

Pure SnO2

10 at.% LaOCl-SnO2

OL

529.5

3.50 %

2.20 %

OV

531.1

53.94 %

56.18 %

OC

532.2

42.46 %

41.62 %

O2 sensing properties Figure 5 shows electrical resistance and response changes of 10 at.% LaOCl-SnO2 sensor to 1000 ppm O2 without light illumination (RT and 100 °C) and illuminated by 13


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blue and UV lights at RT with the same light density of 30 mW/cm2. It is worth mentioning that here the condition of no light illumination actually means no additional light but natural light. As can be seen in Figure 5 (a-b), the response of the sensor to O2 at RT without light illumination is insignificant and the sensor can hardly recover to its baseline resistance within the allotted time. However, when the sensor is illuminated under blue or UV light, obvious improvements in response time and especially recovery time are observed. In addition, the baseline resistance of the sensor is lower under UV light compared with that under blue light. Not only the sensor response but also the response/recovery speed is higher when exposed to shorter light. In order to compare the sensing characteristics of 10 at.% LaOCl-SnO2 sensor under light illumination and heat activation, the same sensor is heated at 100 °C and tested with the same O2 concentration under no light illumination. Compared with the sensor operating at 100 °C, the sensor illuminated by blue or UV light has a better response.

Figure 5. Electrical resistance change (a) and response change (b) of 10 at.% LaOCl-SnO2 HSs based sensor exposed to 1000 ppm O2 at different operating

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conditions (RT with no light illumination, 100 °C with no light illumination, RT illuminated by blue light, RT illuminated by UV light). To sum up, 380 nm UV light is the best choice to get the optimal sensor response while maintaining applausive response/recovery time. Therefore, RT UV light activation was chosen as the operation condition in all further experiments. Figure 6 shows the response/recovery characteristics of pure SnO2 and 10 at.% LaOCl-SnO2 sensors to 250 ppm O2 illuminated by 380 nm UV light. It is found that LaOCl doping not only increases the sensor response but also accelerates the response/recovery speed. For example, the response of 10 at.% LaOCl-SnO2 to 250 ppm O2 is 2.25 with response/recovery time of 161/1003 s, while that of pure SnO2 is only 1.14 with response/recovery time of 182/1315 s.

Figure 6. Electrical resistance changes of pure SnO2 and 10 at.% LaOCl-SnO2 HSs based sensors exposed to 250 ppm O2 with UV light illumination. The normalized resistance and response changes of pure SnO2 and 10 at.% LaOCl-SnO2 sensors with time under different O2 gas concentrations is shown in

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Figure 7. Apparently, the responses of two sensors increase with increasing gas concentration. Moreover, the response value and recovery speed of 10 at.% LaOCl-SnO2 sensor are much higher than those of pure SnO2 sensor. The inset in Figure 7 (a) shows an enlarged part of the measured data at 100 ppm O2, indicating that the 10 at.% LaOCl-SnO2 sensor responds well even to as low as 100 ppm O2 gas. As demonstrated in Figure 7 (b), the responses of two sensors increase rapidly with increasing O2 concentration from 100 to 1000 ppm, and then almost follow a linear increase with further increasing O2 concentration.

Figure 7. Electrical resistance changes (a) and response changes (b) of pure SnO2 and 10 at.% LaOCl-SnO2 HSs based sensors as a function of O2 concentration with UV light (Inset shows dynamic resistance curves of two sensors toward 100 ppm O2). The 10 at.% LaOCl-SnO2 HSs sensor is also tested to examine the selectivity against other gases, including H2, CH4, NH3 and CO2 (each gas concentration is 400 ppm, the response tests of H2, CH4, NH3 and CO2 are performed under air background atmosphere). It is found that the sensor shows high response to O2 but little response to H2, CH4, NH3 and CO2, as can be seen in Figure 8. Thus, it is believed that the 10 16


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at.% LaOCl-SnO2 HSs sensor has a good selectivity to O2. The large difference in response in our case can be attributed to different sensing mechanisms of O2 and other gases.

Figure 8. Cross-response of the sensor based on 10 at.% LaOCl-SnO2 HSs sensor to various gases under UV light irradiation. To substantiate the effect of structure on oxygen sensing performance of the sensors, a comparative study between 10 at.% LaOCl-SnO2 HSs and 10 at.% LaOCl-SnO2 solid nanoparticles was performed, keeping other conditions the same. The 10 at.% LaOCl-SnO2 particles prepared by ammonia precipitation method26 have a mean crystallite size (49.54 nm, calculated from Figure S1, Supporting Information) bigger than that of 10 at.% LaOCl-SnO2 HSs (30.94 nm, Figure 2). The BET surface area of the prepared 10 at.% LaOCl-SnO2 nanoparticles is 30.30 m2/g, which is smaller than that of 10 at.% LaOCl-SnO2 HSs (43.97 m2/g). As shown in Figure 9, the sensor based on 10 at.% LaOCl-SnO2 HSs not only has high response but also short response/recovery time compared with 10 at.% LaOCl-SnO2 particles. The enhancement in gas sensing properties lies in that HSs could facilitate UV light 17


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excitation and O2 gas diffusion, making O2 molecules accessible to both the exterior and interior surfaces of SnO2 based HSs.

Figure 9. Electrical resistance changes of 10 at.% LaOCl-SnO2 HSs and 10 at.% LaOCl-SnO2 nanoparticles based sensors to 250 ppm O2 with UV light illumination. The response variations of the 10 at.% LaOCl-SnO2 HSs sensor to 1000 ppm O2 at different relative humidities under UV light are also investigated (Figure S2, Supporting Information). It can be speculated that when the RH is below 30 %, moisture evidently has a positive influence on the O2 response, while the response remains almost the same when RH is higher than 30 %. The stability and reproducibility are also important parameters for the commercial use of any electronic devices. In order to check the stability of the 10 at.% LaOCl-SnO2 HSs sensor, its response toward 250 ppm O2 was measured for two months, at an interval of 3 days, as shown in Figure 10. It is found that O2 response of the 10 at.% LaOCl-SnO2 hollow-sphere sensor shows no obvious change during two months, confirming the good stability of the sensor.

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Figure 10. Responses of 10 at.% LaOCl-SnO2 HSs based sensor to 250 ppm O2 repeated with 20 times of test during two months under UV light illumination. Sensing mechanism Without light illumination It is known that the chemical potential of oxygen is below the conduction band of SnO2. As exposed to oxygen at RT, the majority of free electrons from SnO2 will be trapped by adsorbed oxygen on the surface with the formation of O2(ads)- (chemisorbed oxygen ion, Eqs. (1-2)),27 which induces the formation of depletion layers (Figure 11 (a)).

O2(g) ⇔ O2(ads)

(1)

O2(ads) +e- ⇔ O2(ads) -

(2)

Without light illumination, the carrier concentration on the surface of HSs is very low and almost all free electrons are trapped by adsorbed oxygen, further modulation of the sensors’ resistance by oxygen will be very small. As a result, the sensors are not 19


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sensitive to oxygen. Since the desorption activation energy is much higher than chemisorption activation energy, oxygen adsorbates are difficult to desorb from the surface of SnO2 based films at RT.28 Therefore an extremely sluggish recovery is observed at RT without light illumination. At 100 °C, molecular and/or adsorbed oxygen undergoes dissociation according to the reactions below (Eqs. (3-4)).11 The abstraction of electrons from SnO2 surface occurs with a relatively high rate, leading to a fast increase of oxygen adsorption content and great enhancement of sensor response. In addition, the temperature of 100 °C facilitates desorption of oxygen adsorbates when exposed to pure N2. Therefore, O2 sensing characteristics of the sensor are boosted when operating at 100 °C compared with those at RT. O 2(ads) - +e- ⇔ 2O (ads) -

(3)

O 2(ads) +2e - ⇔ 2O (ads) -

(4)

With light illumination When investigating the effects of light illumination on the sensing properties of SnO2 based sensors, it’s suggested that gas sensor performances are affected through the following ways: the generation of electron-hole pairs29 and the desorption of oxygen adsorbates.30 (1) Generation of electron-hole pairs As is known to all, the required energy for band to band excitation depends on the

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band gap energy of semiconductor. SnO2 exhibits a band gap of 3.6 eV. The photon energy is determined by light wavelength (E = hcλ−1). This means that the photon energies of 380 nm ( E = hν =

hc 4.136 × 10-15 × 3 × 1017 = eV = 3.3 eV < 3.6 eV ) UV λ 380

light and 460 nm (E=2.7 eV< 3.6 eV) blue light are not sufficient for band to band excitation of SnO2. However, previous Density Functional Theory (DFT) studies indicated that oxygen vacancies could form shallow donor levels, allowing optical excitation of electrons with light energies even below the band-gap energy.31, 32

hυ → e- + h +

(5)

O2 +e- ⇔ O2- (hυ)

(6)

SnO2 shows an n-type electrical conductivity, meaning that the material is non-stoichiometric with available oxygen vacancy defects. Doping of LaOCl further increases the amount of oxygen vacancy defects on the surface of 10 at.% LaOCl-SnO2, as proofed by the XPS results above. Therefore, we affirm that the excitation energy used in our experiments (380 nm, 3.3 eV; 460 nm, 2.7 eV) is sufficient for electron excitation from valence band to conduction band owing to the oxygen vacancies (Eq. (5)). In this case, substantial photogenerated electrons will be trapped by oxygen and photoinduced oxygen ions ( O 2 - (hυ) ) are thus created (Eq. (6)), as illustrated in Figure 11 (b). The above experimental results have shown that sensor response: S(RT illuminated by UV light) > S(RT illuminated by blue light) > S(100 °C without light illumination) > S(RT without light illumination). The improvement of R0/R under UV light radiation compared with that under other conditions can be explained by the largest resistance variation 21


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(∆R=R0-R) due to the largest number of adsorbed oxygen under UV light activation. (2) Desorption of oxygen adsorbates There are two mechanisms for the direct oxygen desorption under UV light, namely recombination with a light-generated hole in the valence band as reflected by Eq. (7) and direct excitation of a binding electron from oxygen adsorbates to the conduction band of SnO2 (Eq. (8)). It is believed that the adsorption energy of an oxygen atom on a metal oxide is about 1.5 eV33 and photoinduced oxygen ions O 2 (hν) is weakly adsorbed on the surface of material.34 Hence, Eqs. (7-8) are possible as the photons from the 380 nm (3.3 eV) UV light and 460 nm (2.7 eV) blue light are sufficiently energetic. As a result, the chemisorbed oxygen ions ( O2(ads)- ) and photoinduced oxygen ions O 2 - (hν) can be easily removed under light activation, giving rise to rapid recovery. In addition, O 2 - ( O2(ads)- and O 2 - (hν)) desorption will be faster when higher light energy (hν) is provided, which explains the reduced recovery time under UV light compared with that under blue light.

O-2 + h + → O2

(7)

O -2 + hυ ⇔ O 2 +e -

(8)

22


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Figure 11. Schematic illustration of the sensing process of the RT SnO2 HSs based sensor without (a) and with (b) UV light illumination. As for the effect of water vapour on the response of the 10 at.% LaOCl-SnO2 HSs based sensor, there are two mechanisms proposed by Heiland and Kohl35 to explain the increase of surface conductivity in the presence of water vapour. +

H 2O + Sn lat + O lat → ( HO - - Sn lat + ) + ( O lat H ) + e-

(9)

H 2 O + 2Sn lat + O lat → 2 ( HO - - Sn lat + ) + VO++ + 2e-

(10)

Where Snlat is denominated as lattice Sn, Olat represents lattice oxygen and VO++ represents bivalent oxygen vacancy. According to the two interaction mechanisms, water vapour offers the necessary condition for the creation of electrons and oxygen vacancy defects, contributing to higher oxygen adsorption amount. Therefore, to a 23


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certain extent (RH is below 30 %), water adsorption enhances the sensor response. However, RH higher than 30 % leads to a large coverage of hydroxyl groups on the sensor surface, limiting the adsorption of oxygen. Hence the sensor response remains unchanged when RH is beyond 30 %. To thoroughly clarify the effect of water vapor, a systematic investigation will be performed in our next study. The effect of LaOCl dopant on the oxygen sensing properties of the sensor can be mainly demonstrated in two ways. On the one hand, the grain size diminishes and specific surface area increases when doping LaOCl, which facilitate the adsorption of oxygen. On the other hand, doping LaOCl increases the amount of oxygen vacancy defects on the surface of SnO2, promoting the photogeneration of electrons and the increase of oxygen adsorption amount.

CONCLUSIONS In summary, sensors based on pure SnO2 and 10 at.% LaOCl-SnO2 HSs were fabricated using novel carbon microspheres as templates and tested toward O2 in different operating conditions. It was demonstrated that the 10 at.% LaOCl-SnO2 HSs based sensor showed applausive room temperature O2 sensing performances compared with pure SnO2. It was also found that 10 at.% LaOCl-SnO2 HSs sensor showed higher response and shorter response/recovery time when illuminated by UV light. The 10 at.% LaOCl-SnO2 HSs sensor also exhibited a good selectivity to O2 against H2, CH4, NH3 and CO2, which is believed to be a promising candidate for the effective detection of O2 gas at room temperature. Furthermore an explanation for the UV light activated O2 gas-sensing mechanism by adsorption-desorption model is 24


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proposed. UV light illumination not only promotes the generation of electron-hole pairs and oxygen adsorption, but also activates desorption of oxygen adsorbates when exposure to pure N2, thus contributing to high response and rapid response/recovery speed. We believe that our research can surely open up new opportunities for MOS sensors operating at room temperature.

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ASSOCIATED CONTENT Supporting Information X-ray diffraction patterns of 10 at.% LaOCl-SnO2 solid nanoparticles. Response variations of the 10 at.% LaOCl-SnO2 exposed to 1000 ppm O2 at RT under UV light with relative humidity from 0 to 50 %. This material is available free of charge on the ACS Publications website.

AUTHOR INFORMATION *Corresponding authors at: Department of College of Science, China University of Petroleum, Qingdao 266580, Shandong, P. R. China.

E-mail: [email protected] (Q. Z. Xue), [email protected] (C. C. Ling) Tel.: 86-0532-86981169. Fax: 86-0532-86981169

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This work is supported by Natural Science Foundation of China (11374372, 11604390),

Natural

Science

Foundation

of

Shandong

Province

(ZR2014EMQ006), Taishan Scholar Foundation (ts20130929), Fundamental Research Funds for the Central Universities (15CX08009A). 26


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For TOC Only

Table of contents text: LaOCl-SnO2 hollow spheres were synthesized by employing newly made carbon microspheres as templates, and sensor based on these HSs can effectively detect O2 gas at room temperature by ultra-violet light illumination.


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Enhanced Room Temperature Oxygen Sensing ... - ACS Publications

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