Temperature-Dependent Abnormal and Tunable p-n Response of

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Temperature-dependent abnormal and tunable p-n response of tungsten oxide-tin oxide based gas sensors Han Li, Wuyuan Xie, Tianjie Ye, Bin Liu, Songhua Xiao, Chenxia Wang, Yanrong Wang, Qiuhong Li, and Taihong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08211 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015

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ACS Applied Materials & Interfaces

Temperature-dependent abnormal and tunable p-n response of tungsten oxide-tin oxide based gas sensors Han Li, Wuyuan Xie, Tianjie Ye, Bin Liu, Songhua Xiao, Chenxia Wang, Yanrong Wang, Qiuhong Li and Taihong Wang* Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, China. E-mail: [email protected]; Fax: +86-0592-2187196; Tel: +86-0592-2183063

Abstract: Temperature-dependent abnormal p-n transition for its sensing response was observed with WO3-SnO2 hybrid hollow spheres based gas sensors for the first time. The sensors presented normal n-type response to ethanol at elevated temperature, whereas abnormal p-type like response in a wide operation temperature range from room temperature to about 95 ℃. By measuring various reducing gases and applying complex impedance plotting techniques, the abnormal p-type sensing behavior was demonstrated to be pseudo response resulted from the reaction between target gas and adsorbed water on the material surface. and the temperature controlled n-p switch is ascribed to the competition of intrinsic and extrinsic sensing behaviors, which are resulted from the reaction of target gas with adsorbed oxygen ions and protons from adsorbed water respectively. The former can modulate the intrinsic conductivity of the sensor by changing the electron concentration of the sensing materials, while the later can regulate the conduction of the water layer which contributes to the total conductivity as an external part. The hollow and hybrid nanostructures facilitated the observation of extrinsic sensing behaviors due to its large area active sites and abundant oxygen vacancies which could enhance the adsorption of water. This work might give a new insight into gas sensing mechanism, and opens up a promising way to develop practical temperature and humidity controllable gas sensors with little power consuming based on the extrinsic properties.

Keywords: WO3-SnO2, hollow nanospheres, gas sensors, p-n transition, humidity, proton transfer

1

Introduction

The selective detection of various gases is critically important for pollution monitoring, public safety assurance and personal healthcare.1-3 Metal oxide semiconductor (MOS) based gas sensors are considered to be the most promising technique, owing to the advantages of high sensitivity, low cost, easy implementation and good reversibility.4-8 Most semiconducting oxide gas sensors perform on the basis of the modification of the electrical properties of active sensing materials, brought about by the adsorption of an analyte on the surface of the sensor. Typically, for n-type oxides such as SnO2, ZnO, and WO3, the electrical resistance decreases upon surface reaction of combustible gases with adsorbed oxygen that capture electrons from the oxide surface.9-11 The surface trapped electrons are returned back to the oxide bulk by surface reaction, increasing the carrier (electron) concentration in the oxide and thus electrical conductance. In contrast, p-type oxides such as Cu2O and NiO show the opposite sensing behaviors due to the decrease of carrier (hole) concentration resulting from the recombination of holes and released electrons.12,13 This fundamental sensing mechanism leads to that MOS gas sensors usually need to operated at elevated temperatures (200 ℃ or higher).14 One of the current research focuses is on the reduced-temperature operation of the sensors to meet the demand of safety, low power consumption, and their further industrial applications. Up till now, several approaches are often used, for instance, noble metal (Pd, Pt) decoration,15-18 CNT-based combined structures,19,20 self-powered gas sensing system,21,22 metal element doping and UV light activation.14,23,24

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Herein, we firstly present an extrinsic sensing behavior with WO3-SnO2 composite hollow spheres (HNS) fabricated by a facile one-step hydrothermal reaction. Gas sensors based on these nanospheres show abnormal p-type sensing response to reducing gases at low temperature which can be switched to normal n-type with increasing operation temperature. In the past few years，the n-p transition has been recorded for In2O3- Fe2O3 mixture,25 Fe2O3 bulk samples,26 Fe2O3 nanomaterials，27 and the main explanation attributes the phenomenon to the surface adsorption of oxygen that leads to the formation of a surface inversion layer. This hypothesis may be suitable for narrow band semiconductor like Fe2O3, but improbable for wide band gap semiconductors like SnO2 or In2O3.28 Similar temperature controlled abnormal gas sensing characteristics also have been observed on uniformly loaded Pt@SnO2 nanorods, which is attributed to Pt-catalyzed morphological changes of ionsorbed oxygen.29 Moreover, nickel ferrite based quaternary transition metal oxides often show temperature dependent response switching as a function of gas reactivity due to the combined effect of hole transfer ( Ni 2+ + h + ↔ Ni 3+ ) and electron transfer ( Fe3+ + e− ↔ Fe2+ ).30,31 In our experiments, we found that the p-type sensing behavior is not in agreement with the well accepted mechanism of resistive-type gas sensors, but resulted from the proton transfer between adsorbed water molecules and target gas molecules on material surface. This so-called p-type sensing behavior actually should be interpreted in terms of pseudo p-type sensor response，since it only reflects the variation of external electrical property of sensing material. Humidity is usually considered as one of the greatest obstacles in gas–sensor applications,32 but plays a significant role in the abnormal sensing behavior here. This work might give a new insight into gas sensing mechanism, and opens up a promising way to design low temperature gas sensors.

2

Experimental details

2.1 synthesis of materials WO3-SnO2 HNS were synthesized using a facial hydrothermal method followed by calcination. Typically, 1 mmol Na2WO4, 0.5 mmol Na2SnO3 and 5 g glucose were added into 50 mL of deionized water in sequence. Afterwards, the homogeneous solution was transferred into a Teflon-lined stainless steel autoclave after stirring for several minutes. The autoclave was sealed and heated in an oven at 200 ℃ for 20 h. After naturally cooling to room temperature, the solid products were subjected centrifugation, washed with distilled water and ethanol, and finally dried in air at 65 ℃. Finally, the as-prepared dry products were annealed in Muffle furnace at 500 ℃ for 2 h in air.

2.2 Characterization of materials X-ray powder diffraction (XRD) data of the as-synthesized materials were collected on a Philips X’pert pro diffractometer by using Cu-Kα radiation at 35 kV and 25 mA. Scanning electron microscopy (SEM) images were recorded on a Hitachi S4800 scanning electron microscope. The transmission electron microscopy images (TEM) and energy dispersive spectra (EDS) were performed on JEOL JEM-2100.

2.3 Fabrication and measurement of gas sensors A sketch and detailed fabrication process of the temperature-tunable gas sensors have been given in our previous paper.33 The circumstances humidity was about 45% RH, and the temperature was 22 ℃. The response of the sensors was measured by monitoring the variation of the DC conductance with a ZhongKe NS-4003 smart sensor analyzer. The sensitivity was defined as S = Rg Ra or S = Ra Rg ( S > 1) , where Rg and Ra denote the resistance of the sensors in sensing gas-air mixture and air.


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3

Results and discussion

Scanning electron microscopy (SEM) observations show that the samples are composed of abundant nanospheres with good monodispersity, and all nanospheres are roughly uniform and spherical (Figure 1a and 1b). More details can be found in high magnification images as shown in Figure 1c. The hollow spheres are composed of many small crystal particles with the grain size of about 10 nm. The X-ray diffraction pattern of WO3-SnO2 HNS is shown as Figure 1d. All diffraction peaks can be found in the monoclinic WO3 and tetragonal SnO2 XRD patterns (references from ICDD database), indicating the existing of two mixture phases. The structure of the samples was further investigated by TEM. Typical TEM image confirms hollow structure of the nanosphere with diameter and thickness of about 550 nm and 30 nm as shown in Figure 2a. And the corresponding SAED pattern (inset in Figure 2a) shows that the nanosoheres are polycrystalline structures in nature. The composition of the nanospheres was examined by energy dispersive x-ray spectrometer (EDS), as shown in Figure 2b. Only three elements, O, W, and Sn, are identified in this spectrum. The high-resolution TEM (HRTEM) images shown in Figure 2c and Figure 2d indicate that the primary nanoparticles are highly crystalline. The lattice distances of 0.335 nm and 0.342 nm indicated in Figure 2c agree with the (110) lattice plane of tetragonal SnO2 (0.335 nm) while the lattice distance of 0.358 nm marked in Figure 2d correspond to the (200) crystalline plane of anorthic WO3 (0.365 nm).

Figure 1. (a-c) SEM images and (d) XRD pattern of WO3-SnO2 HNS.



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Figure 2. (a) TEM image and SAED pattern of WO3-SnO2 HNS. (b) EDS pattern of WO3-SnO2 HNS. (c and d) HRTEM images corresponding to SnO2 nanoparticles and WO3 nanoparticles of the nanospheres, respectively.

SnO2 and WO3 are well-known n-type semiconducting sensing materials whereas the WO3-SnO2 HNS show abnormal sensing behavior controlled by operation temperature. Figure 3a displays the conductance response curves of the sensor to 5000 ppm ethanol at different temperatures. The sensing response seems to be p-type at temperature below 95 ℃ with conductance value decreasing rapidly upon exposure to ethanol, while turns to n-type at the temperature above 185 ℃. Meanwhile, p-type response gradually decreases with increasing operation temperature. Typical responses toward various ethanol concentrations at temperatures of 22 ℃ (room temperature), 95 ℃ and 185 ℃ were measured respectively, as shown in Figure 3b, 3c, and 3d. The sensitivity increases with the ethanol concentration for both p-type and n-type response, showing the probability that the sensor can be used in broad temperature range. But, a large irreversible conductance reduction ( ∆R ) was observed in the first time detection at room temperature, as shown in Figure 3b.


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Figure 3. (a) Dynamic responses of WO3-SnO2 HNS based gas sensors toward 5000 ppm ethanol at various temperatures; (b-d) typical response curves of the sensor to various concentration of ethanol at room temperature, 95 ℃, and 185 ℃, respectively.

In the previous report, Wang et al. observed the p-type sensing behavior in ZnO nanotubes sensors for NO2 gas in low temperature of 30 ℃，and they attributed it to the existence of p-type ZnO nanotubes caused by the adsorption of oxygen and the incorporation of oxygen into the vacancies.34 But in our research, disparate sensing behaviors to different reducing gases were observed, as shown in Figure 4a. The sensors shows no response to CO, H2, and NO, p-type response to NH3 and ethanol, whereas n-type response to acetone at the temperature of 95 ℃. Further measurement performed at lower acetone concentration range from 500 ppm to 1700 ppm confirmed the n-type response, as shown in Figure 4b, indicating that the WO3-SnO2 HNS cannot be identified as p-type sensing materials and thereby different sensing mechanisms may be in action simultaneously in the sensing process. Remarkably, Hao et al. has reported the concentration controlled p-n transition sensing behaviors of porous α -Fe2O3 towards H2S,27 which does not consist with our results since the sensors shows n-type response to acetone with the concentration low to 500 ppm, whereas p-type response to ethanol with the concentration as high as 100000 ppm, as shown in Figure 3 and Figure 4. Moreover, in previous researches, some other structure tungsten oxide-tin oxide nanocomposites also have been prepared and their typical and enhanced n-type response to NO2 and ethylene have been reported.35-37 Sensing properties of SnO2 nanoparticles and WO3 nanoparticles (more detailed material synthesis and characterizations are shown in the Supporting Information) to acetone, ethanol, and ammonia at 95 ℃ were also investigated, as shown in Figure 4c and Figure 4d. The abnormal p-type sensing behavior was not observed with those two kind single component sensing materials, but they both showed higher response to acetone than ethanol and ammonia, indicating that acetone was more active to SnO2 and WO3 at low temperature which might lead to the n-type response of the WO3-SnO2 HNS based gas sensors to acetone. In addition, the abnormal sensing behavior could keep even the WO3 content in the composite was varied in a wide



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range, but become inconspicuous when WO3 was in a high proportion (see Figure S2 in the Supporting Information).

Figure 4. (a) Response curves of the sensor to different gases at the same concentration (5000 ppm) at 95 ℃. (b) Real-time response curves of the sensor to acetone at different concentrations at 95 ℃. Sensing properties of (c) SnO2 nanoparticles and (d) WO3 nanoparticles to 5000 ppm C3H6O, C2H5OH, and NH3 at 95 ℃.

Distinctly, the abnormal p-type sensing behavior cannot be explained by the conventional space charge model,38,39 so it should be treated as a pseudo p-type response. Some other researchers also found the p-type like sensing behavior of In2O3 to CO and H2, and attribute it to a catalytic conversion of chemisorbed oxygen form molecular from to atomic one,28 but the gas sensor show no response to both CO and H2 in our experiment as shown in Figure 4a. In order to understand the pseudo p-type response, two essential factors that affect the conductance of the gas sensor are taken into consideration. Normally, the intrinsic conductance of sensing material can be changed when exposed to active gases by modulating its carrier concentration, which has been generally considered as gas sensing mechanism. But due to the adsorption of water molecules, proton conduction on the material surface as an external part will play a significant role in enhancing the total conductivity of the material.40 Especially, Ling and Leach have reported that even at 300 ℃, relative humidity affects the sensitivity of a gas sensor based on the WO3-SnO2 heterojunction.41 The temperature dependence of the sensor conductance was shown in Figure 5a. The increase in conductance with temperature beyond 95 ℃ is ascribed to thermal excitation of the electron from the valence band under high temperature activation. And the decrease in conductance with the temperature raising from 20 ℃ to 95 ℃ was usually attributed to desorption of the water molecules from the material surface. So, the change of proton conduction resulted from some reaction between target gas and adsorbed water might give rise to the pseudo p-type sensing response.


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Figure 5. (a) Temperature-dependent conductance of the gas sensors. (b) Influence of air humidity on NH3 sensing characteristic at 95 ℃. (c) Enhanced gas sensitivity to 5000 ppm ethanol by increasing surrounding humidity from 20% RH to 95% RH.

To confirm the effect of adsorbed water on the pseudo p-type sensing behaviors, tests to 0.2 vol% NH3 and 5000 ppm ethanol at 95 with various values of RH were conducted. Gas sensor was successively exposed to humid air and humid NH3 from circumstance. As the same consequence, the resistance of the sensor in the humid NH3 ( Rgh ) was larger than that in humid air ( Rah ), conforming to the pseudo p-type gas sensing response. In addition, the sensitivity ( S = Rah Rgh ) was enhanced with increasing the humidity, as shown in Figure 5b. The promotion effect of humidity on the pseudo p-type sensing response was demonstrated identically in the measurement towards ethanol，as shown in Figure 5c. The sensitivity increases progressively with humidity as the values increase from 1.14 at 20% RH to 1.72 at 95% RH, revealing that high humidity facilitates the pseudo p-type response which might be inhibited at low humidity condition.

Figure 6. Complex impedance plots of the sensor exposed to different concentration of NH3 with RH of 11% (a) and 98% (b), respectively.

Since the sensor only shows p-type response to ethanol and ammonia as shown in Figure 4a, it can be recognized that those two species gases can dissolve into adsorbed water relative easily due to the formation of hydrogen bond during polar hydrogen atoms, oxygen atoms and nitrogen atoms which can enhance the interaction between target gas and water molecules. To verify the fact that target gas can react with adsorbed water, complex impedance plotting techniques were conducted which had been widely used to differentiate conduction processes and analyze the mechanism of the sensing materials.42-44 The complex impedances of the WO3-SnO2 HNS in various concentration NH3 atmospheres were measured at room temperature, with setting the humidity to 11% and 98% respectively. These measurements were performed on an Au interdigitated electrode device fixed on alumina ceramic substrate (inset in Figure 6a) with an impedance analyzer (Agilent 4294A). The frequency range is 100 Hz



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to 20 MHz, ReZ and ImZ are the real and imaginary parts of the complex impedance, respectively. At 11% RH, no apparent and regular change was observed with altering the gas concentration, as shown in Figure 6a. However, at the humidity of 98% RH, the impedance evidently decreased and the point of 10 KHz moved from semicircle (I) to a straight line (II) with increasing the concentration of NH3, as shown in Figure 6b. The straight line after the semicircle often appears in low frequency and represents Warburg impedance caused by the diffusion process of the ions at the electrode/sensing film interface.45-48 It is commonly accepted that adsorbed water cannot be polarized at higher frequencies, thus the impedance is mainly controlled by the geometric capacitance of the sensor.49 The movement of the point (10 KHz) position in the complex impedance curve may imply that new ions ( NH 4+ ) are created from the dissociation of NH 3 • H 2O in adsorbed water as NH3 is a well-known alkaline gas and much NH3 can dissolve into the adsorbed water in this case, which can be described as: +

NH 3 + H 2 O ⇔ NH 3 • H 2 O ⇔ NH 4 + OH − .

(1)

However, the dissociation process can only give the explanation to the n-type like response of the sensor to NH3 at room temperature and high humidity, and furthermore give the evidence that reducing gas can react with the adsorbed water and thus influences the total conductivity of the sensor. As organic molecules, ethanol cannot ionizes in water, but the protons can be transferred to C2 H 5OH from H 3O + due to the lager proton affinity of ethanol molecule (188.3 kcal·mol-1) compared to that of H2O (166.5 kcal·mol-1),50 which accounts for the pseudo p-type sensing response. A low energy crossed beam has been applied to the study of the proton transfer reactions of H 3O + with C2 H 5OH and CH 3OH which are proved to be direct.51

Figure 7. Schematic diagram of the sensing mechanism of the sensor at different work conditions: (A-B) at room temperature and high humidity; (C-D) low temperature (a slight thermal shock to the sensor); (E-F) high temperature.

Combining aforementioned facts and corresponded conclusions, we could attribute the abnormal p-type sensing behavior to an extrinsic effect in the form of reactions between the adsorbed water and targeted gas on the material


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surface that change the external conduction of sensing material, and the temperature controlled p-n switch can be interpreted as the transition between extrinsic and intrinsic sensing behavior, as illustrated as Figure 7. At room temperature and high humidity atmosphere, plenty water accumulates on the surface of WO3-SnO2 HNS, and ion-type conductivity is dominating,52,53 as shown in stage A. At this point, the gas sensor shows pseudo n-type response to NH3 due to the extra contribution of ions ( NH 4+ , OH − ) to conductance of the sensor, but pseudo p-type response to ethanol since protons will be hold on by ethanol molecules which leads to the formation of larger ions ( C2 H 5OH 2 + • ( H 2O) 2 ) with lower ionic diffusion coefficient D and thus decreases the conduction of the water layer on the surface, as shown in stage B. As for many liquids, including water, the self-diffusion coefficient satisfies the Stokes-Einstein relation:

D = kBT (6πηa ) where

a

is the effective radius of the molecule and

η

(2)

is the shear viscosity, and D is related to the equivalent

ionic conductivity by the Nernst equation.54 In addition, as this proton transfer reaction is exothermic, the sensing process is not completely reversible at room temperature due to no extra energy supply to recovery the sensor to the initial state, as shown in Figure 3b. With increasing the operation temperature, few layers of water molecules cover the material due to the desorption of the majority of water from the surface, and proton conduction according to the ion transfer mechanism of Grotthuss will play a role in the total conductivity of the material,40,55,56 which can be written as: H 3O + (l , x) + H 2O (l , x, ) ⇔ H 2O(l , x) + H 3O + (l , x, ) .

Where

(3)

l indicate the liquid phase, x and x , denote the different spatial positions. In this case, protons can

move freely in the physisorbed water since the initial and final states are the same and the energy is also equivalent, as shown in stage C. However, when probe gas molecules (NH3, C2H5OH) are introduced to the sensor, protons in the water layer could be trapped by them by the virtue of their larger proton affinity (PA-(NH3)=204 kcal·mol-1, PA-(C2H5OH)=188.3 kcal·mol-1) compared to that of H2O (PA-(H2O)=166.5 kcal·mol-1) [44], which can be described as: H 3O + + NH 3 ⇒ H 2O + NH 4+

(4)

and thus the proton freely movement will become irreversible and be inhibited，resulting the pseudo p-type sensing phenomenon, as shown in stage D. By further turning the operation temperature to a relative high degree, little water exists on the material surface, and the adsorbed oxygen molecules dissociated into more active oxygen ions ( O 2 − , O − ) under thermal activation,57,58 as shown in stage E. At this stage, intrinsic sensing behaviors take effect with reducing gases reacting with the adsorbed oxygen ions straightway. Electrons trapped by the oxygen ions are released back to the conduction band of SnO2, resulting in a significant increase in the conductance of sensing material, as shown in stage F. As to the other component of the composite, WO3 might also work in another sensing mechanism simultaneously as ethanol may break at H atoms which come in contact with WO3 to form hydrogen tungsten bronze:59,60

C2 H 5OH( gas) ⇔ H + C2 H 5O( surface)

(5)

C2 H5O → H + CH3CHO

(6)

xH ads + WO3 → H xWO3

(7)

The hydrogen atoms react as electron donors to WO3, which gives rise to the free carrier (electron) concentration in this n-type semiconductor.



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Owing to the contribution of water to the pseudo p-type sensing behavior, the excellent humidity sensing property of WO3-SnO2 HNS is noteworthy. In our previous report, we have demonstrated that the WO3-SnO2 HNS exhibit excellent humidity sensing performance at high temperature as compared to pure WO3 and SnO2.61 The hollow structure of the material is considered to facilitate the adsorption of water molecules, at the same time, much stronger interaction between water vapor and sensing materials is taken into account. Since, the work function of WO3 (4.41 eV) is slightly greater in comparison to that of SnO2 (4.18 eV), a depletion region in SnO2 layer is formed near the junction whereas an accumulation region is created in WO3 due to the transfer of electrons from SnO2 to WO3.62 Furthermore, the presence of WO3 nanoparticles is expected to capture oxygen from the surrounding SnO2 particles due to higher oxygen affinity of tungsten compared to tin, leading to abundant oxygen vacancies in SnO2,63 which will offer lots of active sites for the adsorption of oxygen and water molecules.64 On the other point, the shifting of lattice oxygen from SnO2 to WO3 gives rise to the enhancement of static electric field between SnO2 and WO3, resulting in stronger interactions between sensing materials and water dipoles, and thus the adsorption of water molecules even at temperature as high as 100 ℃ realizes.61

4

Conclusions

In conclusion, we have developed a simple approach to synthesize hybrid WO3-SnO2 HNS, and firstly observed extrinsic sensing behaviors with the WO3-SnO2 HNS based gas sensors. These extrinsic responses were treated as pseudo sensing responses and considered as the results of the interactions between adsorbed water and targeted gas. By increasing the work temperature, pseudo p-type response of the sensor to ethanol can transform into normal n-type, which results from the transition of sensing mechanisms. The excellent humidity sensing property of WO3-SnO2 HNS accounts for the observation of the extrinsic sensing behaviors. This work has proved an unconventional sensing mechanism based on the positive effect of humidity on gas sensing, and might indicate a new approach to reduce the operation temperature of gas sensors or develop room temperature gas sensors.

Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant No. 61376073) and the Fundamental Research Funds for Xiamen University (Grant No. 201412G010).

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Bondarenka,

V.;

Grebinskij,

S.;

Mickevičius,

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G.

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Films

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