Highly sensitive and selective NO2 sensor based on Au-impregnated WO3 nanorods

Accepted Manuscript Title: Highly sensitive and selective NO2 sensor based on Au-impregnated WO3 nanorods Authors: S. Kabcum, N. Kotchasak, D. Channei, A. Tuantranont, A. Wisitsoraat, S. Phanichphant, C. Liewhiran PII: DOI: Reference:

S0925-4005(17)31026-2 http://dx.doi.org/doi:10.1016/j.snb.2017.06.011 SNB 22478

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

27-2-2017 21-5-2017 2-6-2017

Please cite this article as: S.Kabcum, N.Kotchasak, D.Channei, A.Tuantranont, A.Wisitsoraat, S.Phanichphant, C.Liewhiran, Highly sensitive and selective NO2 sensor based on Au-impregnated WO3 nanorods, Sensors and Actuators B: Chemicalhttp://dx.doi.org/10.1016/j.snb.2017.06.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Highly sensitive and selective NO2 sensor based on Auimpregnated WO3 nanorods

S. Kabcum a,b, N. Kotchasak a,b, D. Channeic, A. Tuantranontd, A. Wisitsoraatd, S. Phanichphante, C. Liewhiran a,*

a

Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand b

Graduate School, Chiang Mai University, Chiang Mai 50200, Thailand

c

Department of Chemistry, Faculty of Science, Naresuan University, Phitsanulok, 65000, Thailand

d

Nanoelectronics and MEMS Laboratory, National Electronics and Computer Technology

Center, National Science and Technology Development Agency, Klong Luang, Pathumthani 12120, Thailand e

Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand

Tel.: +66-81-408-2324; Fax: +66-53-943-445 * Corresponding author: E-mail: [email protected]

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Graphical Abstract

Research Highlights

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Precipitated WO3 nanorods were impregnated with 0–2 wt% Au nanoparticles.



Structural characterizations confirmed that Au nanoparticles were loaded and well dispersed on WO3 nanorods.



Response to 5 ppm NO2 at 250°C was significantly enhanced from 141.8 to 836.6 with 0.5 wt% Au loading.



The optimal sensor exhibited high NO2 selectivity against NO, N2O, C2H5OH, CO, NH3, SO2 and H2.



The results were explained based on metal-semiconductor ohmic junctions and electronic sensitization effects.

ABSTRACT In this work, Au-impregnated WO3 nanorods with high-aspect-ratio were synthesized by a modified precipitation/impregnation method and systematically investigated for NO2 detection. Characterizations by electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy revealed the formation of 5–20 nm spherical and oval Au nanoparticles loaded on the surface of polycrystalline WO3 nanorods. WO3 sensing films with varying Au loading levels from 0 to 2 wt% fabricated by powder pasting and spin coating were tested towards NO2 over operating temperatures ranging from 25 to 350°C. It was found that an optimal Au loading of 0.5 wt% led to significant enhancement of NO2sensing performances. In particular, the optimal Au-loaded WO3 sensing film exhibited the highest response of 836.6 with response time of 64.2 s to 5 ppm NO2 at the optimal operating temperature of 250°C. Moreover, the sensor displayed high NO2 selectivity against NO, N2O, C2H5OH, CO, NH3, SO2 and H2. The observed enhancement could be attributed to the formation of metal-semiconductor ohmic junctions and electronic sensitization effects of Au

4 nanoparticles on the porous network of WO3 nanorods. Therefore, the Au-impregnated WO3 nanorods are highly potential for sensitive and selective NO2 detection. Keywords: Au nanoparticles; WO3 ; NO2; Gas sensors; Precipitation; Impregnation.

1. Introduction Nitrogen dioxide (NO2) is known as one of the most perilous polluting gases that can induce various health dangers including bronchoconstriction, airway inflammation, edema and death when its concentration exceeds the threshold limit value (TLV) of 0.1 ppm [1–11]. In addition, it cannot be detected by human sense due to its odorless and colorless at room and low temperatures. NO2 pollution has been progressively serious due to persistent use of fossil fuels, production of nitrogen-based chemicals and other natural causes [1]. There is thus a high demand of effective NO2 sensors for environmental monitoring and other industrial applications. Gas sensors based on semiconducting metal oxides including tin dioxide (SnO2), tungsten trioxide (WO3) and zinc oxide (ZnO) have been widely studied for NO2 detection due to good sensitivity, low cost and portability [1–11]. Among these, WO3 is particularly promising for NO2 sensing due to its relatively high catalytic activity and selectivity towards NO2 [1214]. However, its performances particularly sensitivity, selectivity, operating temperature and stability must still be further improved to meet the requirements of environmental monitoring [15, 16]. Hence, much research has been focusing on the improvement of its NO2-sensing performances by means of structural and chemical modifications using innovative preparation methods [1015]. Noble metals such as platinum (Pt), palladium (Pd) and gold (Au) are particularly effective chemical additives for selectivity

5 improvement towards specific gases. In particular, Au catalyst has been reported to provide a specific NO2 response enhancement superior to other catalysts including Ag, Pt and Pd [8, 1215]. Unloaded and Au-loaded WO3 nanostructures in different forms such as nanodots, nanowires, nanospheres and nanosheets have been synthesized by a variety of production techniques, namely magnetron sputtering, plasma enhanced chemical vapor deposition, electro-spinning, electrophoretic deposition, thermal oxidation, pulse laser deposition, thermal decomposition and precipitation [111, 1524]. NO2 responses of differently prepared unloaded and Au-loaded WO3 sensors are summarized in Table 1. Firstly, unloaded WO3 thin films deposited by various methods exhibit moderate responses in the range of 40100 to 1100 ppm NO2 at operating temperatures of 200400C [15]. Regarding nanostructures, unloaded WO3 nanoparticles prepared by electrophoretic deposition shows a good response of 49 to a relatively low NO2 concentration of 0.5 ppm at 200C [6]. Also, WO3-SnO2 nanolamellae made by an acidification method display a relatively high response of 190 to 1 ppm NO2 at 250C [7]. Moreover, WO3 nanosheets with interdigitated nanoelectrodes fabricated by precipitation and electron beam lithography methods show a high response of 28 to a much lower NO2 concentration of 0.01 ppm at 200C [8]. With Au loading, the response of WO3 thin film deposited by reactive RF sputtering increases from 1.6 to 4.8 towards 10 pm NO2 and the optimal working temperature decreases from 250 to 200C [9]. Similarly, Au loading enhances the response of another sputtered WO3 thin film from 21 to 76 to 3 ppm NO2 at 200C. Likewise, the response of WO3 nanowires produced by thermal evaporation is raised by Au loading from 2.9 to 3.36 towards 2 ppm NO2 at room temperature [11]. Furthermore, Au loading provides a relatively large response enhancement for WO3 nanoparticles synthesized by colloidal chemical methods from 25 to 420 towards 10 ppm of NO2 at 150C [12]. From these reports, Au loading improves NO2 response of WO3 sensors

6 and the response as well as the level of enhancement depends considerably on the production method. It is thus persuasive to explore alternative production methods that yield Au-loaded WO3 nanostructures with superior NO2-sensing performances. Precipitation is a widely-used chemical route that is potential for the synthesis of gassensing materials due to low cost, simplicity, low temperature, good reproducibility and wide variability. Various structural morphologies including nanoparticles, nanosheets, nanorods and nanoribbons can be controllably formed by the selections of precursor, solvent, dispersing agent, precipitant and annealing temperature [25]. In particular, WO3 nanorods with high aspect ratios are highly effective for gas sensing due to their large specific surface area and high stability because of low inclination to aggregation [26]. Recently, high-aspectratio WO3 nanorods have been successfully synthesized by precipitation using citric acid and ethylene glycol as precipitant and dispersing agents, respectively [27, 28]. Additionally, the synthesized nanorods have been loaded with Pd by an impregnation method and shown to display outstanding hydrogen-sensing performances [29]. However, they have never been loaded with Au by impregnation for gas sensing. In this work, high-aspect-ratio WO3 nanorods made by a citric acid-based precipitation method are impregnated with Au nanoparticles to develop highly-sensitive NO2 sensors. In addition, the influence of Auloading level on NO2 sensing performances is systematically studied over a wide range of temperatures from 25 (room temperature) to 350C.

2. Experimental 2.1. Preparation of unloaded and Au-loaded WO3 Nanopowder Unloaded and Au-loaded WO3 nanostructures were synthesized by the precipitation and impregnation procedure as depicted in Fig. 1. Firstly, the sodium tungstate solution was prepared by dissolving 5 g of sodium tungstate dihydrate (Na2WO4·2H2O) in the solvent

7 mixture containing 10 g of citric acid, 75 mL of ethylene glycol and 25 mL of deionized water under continuous stirring [2729]. 3 M HCl solution was then added under continuous stirring at 70C to the total pH of 1, leading to the precipitation of yellow-green WO3 nanostructures. Next, the precipitates were separated by centrifugation, thoroughly washed with deionized water/ethanol, dried at 60C for 24 hours and then calcined at 450C for 1 hour. To impregnate Au, a predetermined amount of gold (III) chloride (AuCl3) solution in ethanol was dropped onto the unloaded WO3 nanopowder, thoroughly mixed, dried at 80C for 24 hours in an oven and finally annealed at 300C for 2 h [29].

2.2. Particle characterizations The synthesized unloaded and Au-loaded WO3 nanoparticles with 0.25–2.0 wt% Au were designated as P-0 and P-0.25AuP-2Au, respectively. The phases of P-0 and P0.25AuP-2Au were investigated by X-ray diffraction (XRD) (Phillips X-‘pert) using CuKα radiation (20 kV, 20 mA) with a scanning speed of 5/min [30]. The morphologies and sizes of nanostructures were examined by field emission-scanning electron microscopy (FE-SEM: JSM-6335F, JEOL) and high-resolution transmission electron microscopy (HR-TEM: JSM2010, JEOL). The specific surface areas (SSABET) of Au/WO3 samples were measured by nitrogen absorption at 150C using Brunauer–Emmett–Teller (BET) analysis (Micromeritics Tristar 3000). Moreover, the oxidation states and relative chemical compositions of P-0 and P-0.25AuP-2Au were evaluated by X-ray photoelectron spectroscopy (XPS: AXIS UltraDLD, Kratos analytical, UK) using a monochromatic Al K X-ray excitation (1.4 keV) source (15 kV, 10 mA) [31].

2.3. Sensing film fabrication and characterizations

8 Sensing films were prepared by powder-pasting and spin-coating methods. For the paste preparation, ethyl cellulose (Fluka, 30–70 mPa·s) as a temporary binder and α-terpineol (Aldrich, 90%) as a solvent were homogeneously mixed by stirring and heating at 80°C for 12 h. 60 mg of nanopowder was then added to the binder solution (0.28 mL) and thoroughly ground in a mortar for 30 min to form a homogeneous paste for spin coating. The resulting paste was spin-coated at 700 rpm for 10 s and at 3,000 rpm for 30 s on an Al2O3 substrate (0.40 cm × 0.55 cm × 0.04 cm) equipped with interdigitated gold electrodes. The sample was then dried by baking on a hot plate at 90°C for 2 min and the spin-coating cycle was repeated one more time. Finally, the coated substrates were annealed in a furnace at 150°C for 1 h and at 450°C for 2 h with a heating rate of 1°C/min for binder removal. The sensors fabricated from P-0 and P-0.25AuP-2Au powder samples were designated as S-0 and S-0.25AuS2Au, respectively. After sensor testing, the surface and cross-sectional morphologies of sensing layers were further examined by SEM. In addition, the phase and crystallinity of sensing films were analyzed by grazing incident X–ray diffraction (GI-XRD) (Rigaku TTRAX III) using CuK radiation (30 kV, 15 mA) operated at 0.4 incident angle with a scanning speed of 3/min. Moreover, the oxidation state and relative chemical composition of the sensing films were also evaluated by XPS with the same instrument and condition as those of powders.

2.4. Gas-sensing measurement The gas-sensing characteristics of sensing films were characterized towards 0.1255 ppm NO2, 5 ppm NO, 50 ppm N2O, 2,000 ppm NH3, 50 ppm CO, 500 ppm SO2, 1,500 ppm C2H5OH and 1500 ppm H2 by the standard flow through technique. A flux of synthetic dry air as gas carrier was flowed to mix with the desired concentration of pollutants dispersed in synthetic dry air with a constant total flow rate of 2 L/min. All measurements were conducted

9 in a temperature-stabilized sealed chamber at 25C and a low humidity of 10%RH measured by a calibrated commercial humidity sensor. The gas flow rates were precisely manipulated using a computer-controlled multi-channel mass flow controllers. The external NiCr heater was heated by a regulated DC power supply to different operating temperatures ranging from 25C to 350C. The resistances of various sensors were continuously monitored with a computer-controlled system by voltage-amperometric technique with 1 V DC bias through a picoammeter with a built-in voltage source (Keithley 6487). The sensors were exposed to a gas sample for 10 min at each gas concentration and the air flux was then resumed for 25 min. The sensor response (S) is defined as the resistance ratio Rg/Ra, where Rg is the steadystate resistance in an oxidizing gas including NO2 and Ra is the stable resistance in dry air [3235]. The response definition is reversed for a reducing gas such as C2H5OH, NO, N2O, SO2, CO, and NH3. The response time (tres) is defined as the time required until 90% of the response signal is reached while the recovery time (trec) denotes the time needed to recover 90% of the original baseline signal.

3. Results and discussion 3.1.

Structural properties of Precipitation/Impregnation powders and sensing films The phase composition and crystallinity of unloaded WO3 (P-0) and 0.25–2 wt%

Au/WO3 (P-0.25Au–P-2Au) nanopowders were evaluated by XRD as demonstrated in Fig. 2(a). It is seen that all samples are highly crystalline and main XRD peaks are well matched with the monoclinic WO3 phase (JCPDS file no. 43-1305) having dominant planes at (0 2 0), (2 0 0) and (2 2 0) [1, 29]. In addition, the crystallite size of WO3 phase determined from Scherrer's equation depends on the Au loading concentration as displayed in the inset of Fig. 2(a). It is evident that the crystallite size of WO3 crystal increases monotonically from 21.4 nm to 26.2 nm as the Au content increases from 0 to 2 wt%, indicating grain growth due to

10 the presence of Au particles. When the Au-loading content becomes higher than 0.5 wt%, additional tiny peaks arise at 38.24 and 44.37, which can be very well matched with (1 1 1) and (2 0 0) planes of cubic gold phase (JCPDS files-no. 04-0784). Additionally, and the Au peak magnitudes tend to increase with increasing Au loading level, indicating that impregnated Au particles are loaded on WO3 surfaces. According to Scherrer’s principle, the appearance of Au-related peaks in XRD patterns also implies the presence of large crystallite of Au particles (>10 nm). The state of Au will be further confirmed by TEM and XPS analyses. The XRD patterns of corresponding unloaded and Au-loaded WO3 sensing films (S-0 and S-0.25Au–S-2Au) on Au/Al2O3 substrates after annealing and sensing test are shown in Fig. 2(b). It confirms that the sensing films have similar crystallinity to their respective powders with peaks matched to the same JCPDS file and diffraction peaks of Au phases are also present (JCPDS files NO. 04-0784) but with much higher magnitudes due to the dominant diffraction from Au electrodes. The XRD peaks of Al2O3 substrate (JCPDS file NO. 46-1212) are also present with slightly higher magnitudes compared with those of sensing films, demonstrating that the glazing-incident XRD technique can considerably reduce X-ray diffraction from the substrate, which would be much higher if the normal high incident angle was used. Bright-field (BF) and high-resolution (HR) TEM images of unloaded and 0.52.0 wt% Au-loaded WO3 nanopowders are illustrated in Fig. 3. It is seen that unloaded WO3 nanostructures are mainly aggregated nanorods whose length and diameter are in the ranges of 50200 nm and 520 nm, respectively (Fig. 3(a)). The inset selected area electron diffraction (SAED) pattern exhibits dotted rings that can be matched with (2 0 0), (2 0 2), (2 2 2), (1 4 0) and (4 2 0) planes of the monoclinic WO3 phase in agreement with the XRD data. From the respective HRTEM image (Fig. 3(b)), WO3 nanorods are seen to be highly crystalline exhibiting lattice fringes whose d-spacings can be assigned to some planes of the

11 monoclinic WO3 phase. With Au incorporation at the low content of 0.5 wt% (Fig. 3(c)), nanorods with various sizes are appeared to be more agglomerated while expected nanoparticles of secondary Au phase still cannot be observed. In addition, some round platelike structures are occasionally found within nanorod clusters. The corresponding SAED pattern displays dotted rings of only the monoclinic WO3 phase similar to those of unloaded one but with relatively high diffraction density. Also, the HR-TEM image (Fig. 3(d)) displays denser entangled highly crystalline nanorods having various lattices planes of WO3 crystals. Thus, the secondary-phase Au nanoparticles cannot be observed at this low loading level because the density of nanoparticles may be very low and/or the particle size may be very small. At the higher Au loading level of 1 wt% (Fig. 3(e)), few approximately round and oval nanoparticles with a light grey color are observed on some nanorods, suggesting that Au particles are loaded on WO3 nanorod surfaces. The phase of the secondary-phase nanoparticles may be identified from the inset SAED pattern. From SAED indexing, some weak diffraction rings can be designated as the (1 1 1) and (2 0 0) planes of Au phase while most others are corresponding to the planes of WO3 monoclinic phase. Moreover, it can be observed that WO3 nanorods are highly aggregated and more round plate-like structures are formed after Au loading, further suggesting the binding role of Au nanoparticles. The nanocrystalline Au phase can be further verified by the HRTEM image as illustrated in Fig. 3(f). From the image, secondary-phase nanoparticles on WO3 nanorods have the diameter in the range of 510 nm. In addition, the nanoparticle exhibits lattice fringes whose d-spacing can be matched well with the (1 1 1) plane in SAED pattern. With the highest Au content of 2 wt% (Fig. 3(g)), the size and number of nanoparticles increase accordingly with the Au loading level and diffraction rings of Au in the inset SAED pattern become more apparent. Moreover, plate-like WO3 nanostructures are increasingly evident, further suggesting the coalescence of WO3 nanorods due to the presence of Au nanoparticles. The respective HR-

12 TEM image (Fig. 3(h)) shows an approximately round nanoparticle with a relatively large diameter of 15 nm and similar lattice fringes of (1 1 1) Au, confirming the enlargement of Au nanoparticles with increasing Au content. The specific surface area (SSABET) and average BET equivalent particle diameter (dBET) of Au/WO3 samples were determined from BET analysis as shown in Fig 4. It is apparent that SSABET monotonically decreases from 40.43 to 32.64 m2/g while dBET increases from 20.73 to 24.83 nm as the Au loading level increases from 0 to 2.0 wt%. The dBET value is considerably smaller than the sizes of WO3 nanorods observed in TEM images. The discrepancy can be plausibly due to the presumption of spherical particle morphology of BET method, which is not valid for WO3 nanorods. Nevertheless, the dBET data can usefully indicate that the overall size of Au-loaded WO3 nanostructures tends to increase with increasing Au loading level in agreement with the crystallite size estimated from XRD data. A possible explanation for this observation is that Au nanoparticles deposited on the WO3 support by the impregnation process may act as binders, causing WO3 nanorods to partly aggregate and connect into larger cluster during thermal annealing treatment, resulting in larger agglomerated structures and lower specific surface area. In addition, it may be attributed to the formation of larger Au nanoparticles with increasing Au content as observed from TEM images. The chemical compositions and oxidation states of W, O and Au in nanopowders and sensing films after annealing and sensing test were evaluated by XPS as illustrated in Fig. 5. Fig. 5(a) shows the representative survey XPS spectra of 0.5 wt% Au loaded WO3 nanopowders and sensing films (P-0.5Au and S-0.5Au), confirming the presence of all expected elements (W, Au and O) and carbon contamination. By omitting the contamination peak, the contents of W, O and Au of P-0.5Au are determined from the XPS spectrum to be 77.62 wt% (23.69 at%), 21.7 wt% (76.12 at%) and 0.68 wt% (0.19 at%), respectively. The

13 corresponding values of S-0.5Au are similarly found to be 79.2 wt% (25.43 at%), 20.16 wt% (74.38 at%) and 0.64 wt% (0.19 at%), respectively. Hence, the measured values of Au content by XPS are in good agreement with the intended Au loading level of 0.5 wt%. For the W 4f elemental core level (Fig. 5(b)), the spin-orbit doublet pair, W 4f5/2W 4f7/2, can be individually decomposed into two pairs, indicating the presence of two different oxidation states. The main and minor W 4f5/2W 4f7/2 doublet pairs of P-0.5Au are positioned at 35.837.9 eV and 36.638.7 eV, respectively. The corresponding pairs of S-0.5Au are located at approximately the same binding energies. The main pair can be assigned to W6+, and the minor pair also corresponds to W6+ state but with surface defects [36]. In the case of Au element (Fig. 5(c)), the spin-orbit doublet components, Au 4f7/2Au 4f5/2, of P-0.5Au are situated at peak binding energies of 84.187.8 eV while the corresponding peaks of S-0.5Au are slightly shifted to 84.287.9 eV. The values confirm that the Au element is in a single metallic state [3638]. Regarding oxygen element (Fig. 5(d)), the O 1s core level of P-0.5Au can be decomposed into four contributions centered at 530.2 (main peak), 531.2, 532.1 and 533.1 eV while those of S-0.5Au are slightly moved to 530.4 (main peak), 531.4, 532.3 and 533.2 eV, respectively. The main peak of O 1s positioned at 530.2530.4 eV can be assigned to lattice oxygen ( O 2 ) on the outmost surface of Au-loaded WO3 nanorods while the minor components centered at 531.2531.4, 532.1532.3 and 533.1533.2 eV may be ascribed to the chemisorbed oxygen on WO3 surface, hydroxide surface functional groups associated with water adsorption and weakly bound oxygen species on the topmost surface, respectively [25]. Figs. 6(ad) illustrate top-view SEM micrographs of unloaded, 0.5, 1 and 2 wt% Auloaded WO3 powders (P-0, P-0.5Au, P-1Au and P-2Au) prepared by precipitation and impregnation methods (left) with insets of high-resolution images. Figs. 6(eh) show the

14 cross-sectional SEM micrographs of corresponding sensing films after annealing and sensing test (right). It can be seen that the unloaded WO3 nanopowder (Fig. 6(a)) comprises mainly nanorods with varying lengths in the range of 50200 nm [39]. The inset high-resolution image confirms the majority of nanorod structures with fine diameters of 10-30 nm in agreement with the TEM data. Upon impregnation with 0.5 wt% Au (Fig. 6(b)), the nanorod morphologies are roughly the same but the density of nanorods seems to slightly decrease and some plate-like nanostructures are occasionally seen. In addition, the enlarged SEM image (inset) shows relatively wide nanorods and plate-like structures with oval shapes having low aspect ratios. At relatively high Au contents of 12 wt% (Fig. 6(cd)), nanorods are apparently aggregated into round and plate-like clusters in accordance with TEM observation. The number and size of aggregates and plates seem to increase with increasing Au loading level as illustrated in the inset magnified images. For the cases of sensing films after annealing and sensing test (Fig. 6(eh)), it is seen that all sensing films similarly comprise very fine agglomerated nanostructures, which are uniformly arranged on a solid alumina substrates. However, the nanorod features observed in TEM are not clearly seen due to the dense aggregation of nanorods into secondary nanoparticles. In addition, it can be noticed that the size of clusters in the films tends to increase with increasing Au loading level. The thicknesses of sensing films are quite uniform and slightly varied in the range of 2.33 µm. The conformity in the film structure demonstrates that the precipitation/impregnation together with binder-powder pasting and spin coating methods are practical for gas sensor fabrication. This is in accordance with results from various nanostructured metal-oxide thick films reported in literature [39, 40].

3.2.

Gas-sensing properties

15 Au-loaded WO3 sensors have been tested towards NO2 over operating temperatures ranging from 25 to 350C and concentrations ranging from 0.125 to 5 ppm. The effect of operating temperature on NO2 response of WO3 sensing films with different Au loading concentrations are demonstrated in Fig. 7. It is seen that the NO2 responses of all WO3 sensors increase sharply as the operating temperature increases from 25 to 250C and then become rapidly decreasing as the temperature additionally increases to 350C. Regarding the influence of Au content, the NO2 response increases greatly as the Au loading level increases from 0 to 0.5 wt% but then decreases considerably as the loading content increases further. Specifically, the 0.5 wt% Au-loaded WO3 sensor (S-0.5Au) exhibits the highest response of 836.6 to 5 ppm NO2 at the optimal operating temperature of 250C. The effect of operating temperature and Au loading on NO2 sensing mechanisms will be further elaborated in the subsequent section. The influences of Au loading level on the NO2-sensing performances of WO3 nanorods are further evaluated in details at the optimal operating temperature. Fig 8(a) demonstrates the change in resistance of WO3 sensing films with different Au loading concentrations (S-0 and S-0.25AuS-2Au) subjected to various NO2 pulses. It is evident that the baseline sensor resistance decreases substantially by more than two orders of magnitude with increasing Au loading level from 0 to 2 wt% (S-0, S-0.25AuS-2Au). Similar trend of baseline resistance is observed at other operating temperatures and the resistance tends to decrease with increasing operating temperatures. The dependence of baseline sensor resistance on Au loading level will be further analyzed in the next section. After subjected to NO2 samples, unloaded and Au-loaded sensors exhibit a typical increase of resistance in response to an oxidizing gas confirming an n-type semiconducting behavior. This result is in accordance with some other reports of Au-WO3 composite sensors [8, 9, 11, 12]. In addition, the resistance change of WO3 sensor enhances significantly as the Au-loading concentration

16 increases up to 0.5 wt% before declining steadily when the Au content increases further to 2 wt%. The respective NO2-sensing characteristics in terms of sensor response (solid line, left axis) and response times (dash lines, right axis) of S-0 and S-0.25AuS-2Au as a function of NO2 concentration at the optimal operating temperature of 250C are displayed in Fig. 8(b). It can be seen that the sensor response and response time improve substantially as the Au loading content increases from 0 to 0.5 wt% but then steadily degrade when the Au loading level additionally increases to 2 wt%. Specifically, the optimal Au-loaded WO3 sensor (S0.5Au) offers relatively high NO2 response (S = 836.6 at 5 ppm) and short response time (tres = 64.2 s) compared with S-2Au (S = 544.4, tres = 73.2 s), S-1Au (S = 576.9, tres = 71.8 s), S0.75Au (S = 710.3, tres = 69.8 s), S-0.25Au (S = 463.5, tres = 76.6 s) and S-0 (S = 141.8, tres = 81.2 s). The achieved response value of 836.6 is markedly higher than other WO3-based sensors, which exhibited sensor response < 420 to 5 ppm NO2 [19, 11, 12]. Regarding the response dependency on NO2 concentration, the responses of all sensors increase monotonically with increasing H2S concentration according to the well-known power law equations as listed in the inset labels. It is seen that the exponent values of all WO3 sensors are close to 1.0. The exponent value of 1 infers a single gas molecule interaction of NO2 and the metal oxide surface [41]. At a low NO2 concentration of 0.125 ppm, the optimal sensor still offers a moderate response of ∼12.6, corresponding to the low NO2 detection limit of 0.013 ppm (projected at the response of 1.2 based on the power-law equation). The attained detection limit is considerably better than some of other highly sensitive NO2 sensors [19, 11, 12]. Thus, the optimal Au-loaded WO3 sensor has a great potential for sensitive NO2 detection at the moderate optimal working temperature of 250C. For the selectivity evaluation, the sensors were characterized towards NOx gases including 5 ppm NO2, 5 ppm NO, and 50 ppm N2O as well as other environmental gases

17 including 50 ppm CO, 2000 ppm, NH3, 1500 ppm H2, 1500 ppm C2H5OH and 500 ppm SO2, at 250C as displayed in Fig. 9. It is seen that unloaded WO3 nanorods exhibit relatively high NO2 response, modest responses to NO and NH3, and very low responses to other gases, displaying good NO2 selectivity. For Au-loaded WO3 nanorods, the response to NO2 increase greatly with increasing Au concentration up to 0.5 wt% and then slightly decreases as the Au content increases further while the responses to NO and NH3 also change in similar manners with few exceptions and much lower variations. Hence, the 0.5 wt% Au-loaded WO3 sensor exhibits the highest NO2 sensitivity and selectivity against other NOx and environmental gases. The catalyst selectivity of Au on precipitated WO3 nanorods are compared with those of Pt and Pd prepared by similar impregnation procedures at the optimal working condition as illustrated in Fig. 10. The inset graphs show the resistance responses to 5 ppm NO2 of WO3 nanorods loaded with 0.5 wt% Au, 0.5 wt% Pd and 0.5 wt% Pt. It is evident that the Auloaded WO3 sensor exhibits much higher and faster NO2 response than Pd-loaded and Ptloaded ones, confirming the unique catalytic property of Au for NO2 adsorption. From the selectivity histogram, it is seen that Au-loaded WO3 sensors offers high NO2 selectivity against H2 and NH3 in contrast to Pd-loaded and Pt-loaded ones, which exhibit high H2 selectivity against NO2 but limited selectivity relative to NH3. The plausible mechanisms for the observed selectivity will be discussed in the next section. Lastly, repeatability, reproducibility and stability of sensors particularly S-0.5Au were evaluated. Each sensor showed good repeatability with less than 18% response variation from five repeated measurements at the same gas concentration and operating temperature. In addition, six sensors from the same batch were found to have fair response variation of less than 29% measured under the same condition. Moreover, Au-loaded WO3 sensors exhibited moderate long-term stability with less than 30% response change after 6 months of periodic operations.

18

3.3.

Sensing mechanisms of Au-loaded WO3 nanorods The results demonstrate that the electrical resistivity of WO3 nanorods decreases

significantly with increasing Au loading while the NO2 response is enhanced and becomes optimal at the low Au loading level of 0.5 wt%. These observations may be described on the basis of Au-nanoparticle/WO3-nanorod metal-semiconductor (M-S) junctions and electronic sensitization effects of Au nanoparticles on the porous WO3 nanorod network as depicted in Fig. 11. For the unloaded WO3 sensor, the resistance of n-type WO3 structure is controlled by the number of electrons generated by the ionization of intrinsic defects (oxygen vacancies) as well as the type and amount of chemisorbed oxygen species ( O2 , O  , O 2  ) that extract electrons from the conduction band of WO3, inducing electron depletion regions on the surface (Fig. 11(a)). The defect ionization and oxygen chemisorption processes are thermally activated. Thus, the amounts of electrons and reactive oxygen species will increase with increasing temperature. As the temperature increases, O2 will be first formed and then begin to transform into O  at temperatures above 100C. Similarly, O  will be increasingly dominated as the temperature rises further before turning into O 2  at a temperature around 300C. Consequently, the intrinsic resistance tends to decrease with increasing the temperature [42, 43]. At the optimal operating temperature of 250C, the resistance of undoped WO3 nanorods is still quite high due to the low amount of thermally generated electrons and relatively large amount of preadsorbed O  species, resulting in wide depletion regions as illustrated in Fig. 11(a). With Au loading, metallic Au nanoparticlesWO3 nanorods ohmic M-S junctions are formed because the work function of WO3 at this operating temperature would be larger than that of Au [37, 38]. As a result, electrons are locally transferred from Au to WO3 around the

19 junctions, causing receded depletion regions as represented in Figs. 11(b)(d). This leads to the reduction of resistance upon Au loading in accordance with the observed experimental data (Fig. 8(a)). As the Au content increases (Fig. 11(b)(c)), the size and amount of Au nanoarticles simultaneously increase, leading to a higher number of ohmic M-S junctions. This results in more extensive recession of depletion regions in WO3 nanorods and the decrease of baseline resistance with increasing Au-loading concentration. At a very high Au loading level (Fig. 11(d)), many Au nanoparticles aggregate into larger particles, inducing wider receded depletion regions at various contacts and further lowering the baseline resistance. Upon NO2 exposure, NO2 molecules are chemisorbed by taking electrons from the conduction band to form NO2 ions at available adsorption sites on WO3 nanorods, leading to the extension of depletion regions and an increase of resistance according to the reaction [4447]. NO2  e    NO2

(1)

For undoped WO3 nanorods, there is a limited number of NO2 adsorption sites due to relatively large barrier height on surface and low number of remaining electrons that will have to be transferred to preadsorbed oxygen species as depicted in Fig. 11(a). As a result, there are rather few extended depletion regions due to NO2 adsorption, resulting in a small increase of resistance and a low NO2 response. With the presence of Au nanoparticles, NO2 will preferentially adsorb beside the ohmic M-S (Au-WO3) contacts due to the lowering of energy barrier height for electrons to overcome from the bottom of conduction band of WO3 to NO2 molecules and relatively high number of available electrons at the receded depletion regions (the electronic sensitization effect), leading to relatively large extended depletion regions due to NO2 adsorption [4853]. It should be noted that the NO2 adsorption is a

20 non-dissociative process so that its adsorption activation energy is mainly determined by the energy barrier height [54]. At a low Au loading level (Fig. 11(b)), some small Au nanoparticles are formed on WO3 nanorods and induce some additional extended depletion regions, resulting in some enhancement of resistance change and NO2 response. At the optimal Au loading level as represented Fig. 11(c), Au nanoparticles with moderate sizes are widely dispersed on the nanorods, leading to a large increase of extended depletion regions, a large increase of resistance and a high NO2 response. From the results, the Au-impregnated WO3 nanorods exhibit relatively high NO2 response compared with previously reported Au/WO3 nanostructures [812], demonstrating particularly efficient electronic sensitization mechanisms of this structure. The superior NO2-sensing performances may be attributed to the multi-directional connection of entangled nanorod network that allows multiple M-S ohmic interfaces as illustrated in Fig. 11(c). However, Au nanoparticles will agglomerate into larger particles or clusters at higher Au loading levels (12 wt%) as indicated by TEM images (Fig. 4). The number of NO2 adsorption sites around the agglomerated clusters are reduced as depicted in Fig. 11(d), leading to less extension of depletion regions upon NO2 exposure and degraded NO2 response. Regarding the effect of operating temperature on NO2 response, the experimental results reveal that the optimal operating temperature of unloaded and Au-loaded WO3 sensors are the same at 250°C, suggesting that their similar dependence of response on temperature. The similar temperature dependence behavior may be explained on the basis of the competitive chemisorption processes of oxygen species and NO2 . At low temperatures (< 250°C), NO2 adsorption can be easily activated with increasing temperature due to relatively low amount of preoccupied adsorption sites because the thermal energy is insufficient to induce active oxygen species. When the temperature is relatively high (< 250°C), NO2 adsorption will be largely inhibited by the dominating active oxygen species generated

21 and preadsorbed on the surface, leading to the decline of NO2 response with increasing temperature. The presence of Au additive does not change this temperature dependence behavior because Au nanoparticles only electronically enhance the number of adsorption sites on WO3 and does not affect the activation mechanisms of chemisorbed species. Concerning the selectivity, Au loading selectively enhances the NO2 response against NO, N2O, C2H5OH, CO, NH3, SO2 and H2 at the optimal working temperature of 250°C. Upon exposure to these environmental gases at this temperature, the gas molecules will react on the sensor surface according to the reactions: N 2O  e    N 2O    N 2  O 

(2)

NO  O    NO2  e 

(3)

C2 H 5OH  6O    2CO2  3H 2O  6e 

(4)

CO  O    CO2  e 

(5)

2 NH 3  7O    2 NO2  3H 2O  7e 

(6)

SO2  O    SO3  e 

(7)

H 2  O   H 2 O  e 

(8)

It can be seen that only N2O is an oxidizing gas and its reaction (eq. (2)) needs very large activation energy for dissociation of double bond of oxygen in N2O molecules so that Au, Pt and Pd cannot catalyze the reaction at this temperature, resulting in insignificant enhancement in N2O response [55]. All other gases are reducing ones and their reactions require preadsorbed O species (eqs. (3)-(8)). Unlike the case of NO2 , O adsorption requires the additional activation energy for dissociation of O2 into atomic oxygen (O) before accepting electrons to become O. The lowering of barrier height due to Au-WO3 ohmic contacts can supply electron but not energy for the dissociation process, which is normally acquired from the thermal energy. Thus, the density of O on WO3 surface remains limited

22 despite the presence of M-S ohmic junctions at a relatively low temperature of 250°C, leading to low responses to these gases for Au-loaded WO3 sensors. In addition, it has been reported by theoretical calculations that the O adsorption on gold has an endothermic chemisorption energy for oxygen (+0.54 eV) so that O adsorption on gold is still very weak at this temperature [56]. Thus, the M-S ohmic junctions by Au nanoparticles provide the selective enhancement for non-dissociative NO2 adsorption reaction but not for dissociative reactions for O and N2O, resulting in higher NO2 selectivity. In contrast, the O chemisorption rates on Pt and Pd are already significant at 250°C since Pt and Pd have exothermic oxygen chemisorption energies of -2.17 and -1.2, eV, respectively [56]. The inertness in oxygen chemisorption behavior of Au compared with Pt and Pd can be attributed to its unique electron configuration that has full 10 electrons in its 4d subshell. Moreover, Pt and Pd can strongly interact with hydrogen and hydrogen-containing gases via hydrogenation reactions, leading to their molecular dissociation and greatly enhanced reaction rate with O [13, 15, 29, 57]. Thus, Pt and Pd provide large improvements of H2 and NH3 responses. On the other hand, NO2 response is not significantly enhanced by Pt and Pd due to the fact that Pt and Pd form Schottky junctions with WO3, which results in lower electron concentrations at the interface, higher baseline resistances (see also Inset of Fig. 10) and lower NO2 adsorption rates (see also eq. (1)).

4. Conclusion In conclusion, unloaded WO3 and Au-loaded WO3 (0.252.0 wt%) nanoparticles were successfully prepared by precipitation/impregnation methods and systematically investigated for NO2 sensing. From TEM, XRD, SEM and XPS characterizations, 525 nm spherical or oval metallic Au nanoparticles were decorated on the surfaces of polycrystalline WO3 nanorods. The NO2-sensing performances of WO3 sensors were found to be strongly

23 dependent on the Au loading level and working temperature. The optimal Au loading level of 0.5 wt% resulted in a high response of 836.6 to 5 ppm NO2 with a short response time of 64.2 s at a moderate optimal operating temperature of 250C. In addition, the Au-loaded WO3 sensors exhibited high NO2 selectivity against NO, N2O, C2H5OH, CO, NH3, SO2 and H2. The attained results were explained by the formation of M-S ohmic junctions and electronic sensitization effects of Au nanoparticles on the porous network of WO3 nanorods. Therefore, Au-loaded WO3 nanorods prepared by precipitation/impregnation methods are highly promising for sensitive and selective NO2 detection.

Acknowledgements The authors gratefully acknowledge the financial support from the CMU Mid-Career Research Fellowship Program, Young Scientist and Technologist Program (YSTP), Thailand Graduate Institute of Science and Technology (TGIST) (YSTP: SCA-CO-2558-828-TH, SCA-CO-2559-2352-TH,

TG-44-10-59-068M),

National

Science

and

Technology

Development Agency (NSTDA), National Research Council of Thailand (NRCT), the Thailand Research Fund (TRF), the National Research University (NRU) Project under the Office of the Higher Education Commission (CHE), Ministry of Education, Thailand, Graduate School, the Materials Science Research Center, Department of Physics and Materials Science, Chiang Mai University. The special thanks should be given to the National Electronics and Computer Technology Center (NECTEC), Pathumthani, Thailand for sensor facility.

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31 Biographies Sathukarn Kabcum received his B.Sc. and M.S. degree in Materials Science from Department of Physics and Materials Science, Faculty of Science, Chiang Mai University in 2012 and 2014. At present, he is a PhD student majoring in Materials Science at Department of Physics and Materials Science, Faculty of Science, Chiang Mai University. His research interests involve the development and application of gas sensing materials.

Nataporn Kotchasak received her B.Sc. (2nd class honor) in Materials Science from Department of Physics and Materials Science, Faculty of Science, Chiang Mai University in 2016. She successfully received the Young Scientist and Technologist Program (YSTP-2015) Scholarship from National Science and Technology Development Agency (NSTDA), Thailand. She is currently a M.S. student in the Division of Materials Science, Department of Physics and Materials Science at Chiang Mai University under financial support from Thailand Graduate Institute of Science and Technology (TGIST) during 2016– 2018. Her research program focuses on the environmental gas sensor based on metal oxide.

Duangdao Channei received her B.Sc. and Ph.D. degrees from Department of Chemistry, Faculty of Science, Chiang Mai University in 2008 and 2014, respectively. During her Ph.D. study, she received the scholarship from the Royal Golden Jubilee Ph.D. program by

32 Thailand Research Fund (TRF). At present, she is a lecturer at Department of Chemistry, Faculty of Science, Naresuan University. Her current research interests include the designing of functional nanomaterials and nanocomposites and the development of highly efficient metal oxide photocatalysts for wastewater treatment.

Adisorn Tuantranont received the B.S. degree in Electrical engineering

from

King

Mongkut’s

Institute

of

Technology

Ladkrabang, Thailand, in 1995, and the M.S. and Ph.D. degrees in electrical engineering from the University of Colorado at Boulder in 2001. Since 2001, he has been the director of the Nanoelectronics and MEMS Laboratory, National Electronic and Computer Technology Center (NECTEC), Pathumthani, Thailand. His research interests are in

the

area

of

micro

electro-mechanical

systems

(MEMS),

nanoelectronics, lab-on-a-chip technology and printed electronics. He has authored more than 50 refereed journals, 150 proceedings, and holds five patents. He also received the Young Technologist Award in 2004 from the Foundation for the Promotion of Science and Technology under the patronage of H.M. the King, Thailand.

Anurat Wisitsoraat received his Ph.D., M.S. degrees from Vanderbilt University, TN, U.S.A., and B. Eng. Degree in Electrical Engineering from Chulalongkorn University, Bangkok, Thailand in 2002, 1997, and 1993, respectively. His research interests include microelectronic

33 fabrication, semiconductor devices, electronic and optical thin film coating, gas sensors, and micro-electromechanical systems (MEMS). Sukon Phanichphant is a Professor in Chemistry at Department of Chemistry, Faculty of Science, Chiang Mai University, since 1977. She is currently a senior researcher at the Materials Science Research Center, Faculty of Science, Chiang Mai University. Her research interests include synthesis and characterization of nanoparticles for use in medical and sensor applications as well as synthesis and characterization of conducting polymer for light-emitting devices.

Chaikarn Liewhiran received his B.Sc. from Srinakharinwirot University in 2002 on Physics, M.S. and Ph.D. degrees of Materials Science from the Chiang Mai University in 2004 and 2006, respectively. He received the Associate Professor in 2013 and was a lecturer in the Department of Physics and Materials Science at Chiang Mai University until the present, He currently received the TRF-CHE Young Scopus Researcher Award in Physical Science in 2014 from Thailand by the joining of Thailand Research Fund (TRF), Office of the Higher Education Com-mission (CHE) and Scopus. His research program focuses on the Nanoscience and Nanotechnology, the fundamentals of physical and chemical synthesis of metal oxide and metal–ceramic nanoparticles and their applications in nanocomposites, and the development of novel nanomaterials in selective bio- and chemical gas sensing for environmental monitoring.

34 Figures and Captions

Fig. 1. Schematic of precipitation/impregnation process for the synthesis of Au-loaded WO3 nanostructures.

35

Fig. 2. XRD data of (a) 0–2 wt% Au-loaded WO3 nanopowders (P-0 to P-2Au) (inset: crystallite size vs. Au content) and (b) corresponding sensing films (S-0 to S-2Au) after annealing and sensing test.

36

Fig. 3. BF-TEM images with corresponding SAED patterns and HR-TEM images of unloaded WO3 and 0.5, 1 and 2 wt% Au-loaded WO3 nanoparticles: (a)(b) P-0, (c)(d) P0.5Au, (e)(f) P-1Au and (g)(h) P-2Au.

37

Fig. 4. Specific surface area (SSABET) and calculated BET particle diameter (dBET) as a function of Au content ranging from 0 to 2 wt% of WO3 nanoparticles (P-0 to P-2Au).

38

Fig. 5. XPS spectra of 0.5 wt% Au-loaded WO3 nanoparticles and sensing films (P-0.5Au and S-0.5Au): (a) survey scan, (b) W 4f, (c) Au 4f and (d) O 1s.

39

Fig. 6. SEM top-view images of (a) P-0, (b) P-0.5Au, (c) P-1Au and (d) P-2Au (Insets: Corresponding high-resolution images) and SEM cross-sectional-view images of (e) S-0, (f) S-0.5Au, (g) S-1Au and (h) S-2Au on Au/Al2O3 substrate after annealing and sensing test.

40

Fig. 7. Sensor response vs. operating temperatures (25350°C) of 0–2 wt% Au-loaded WO3 sensors to 5 ppm NO2.

41

Fig. 8 (a) Change in resistance of 0–2 wt% Au-loaded WO3 sensors (S-0 to S-2Au) under exposure to 0.125–5 ppm NO2 at 250C and (b) the corresponding sensor response (left axis) and response time (tres) (right axis) vs. NO2 concentration

42

Fig. 9. The selectivity histograms of sensor responses to 50 ppm CO, 50 ppm N2O, 5 ppm NO, 2000 ppm NH3, 1500 ppm C2H5OH, 500 ppm SO2 and 1500 ppm H2 of unloaded and Au-loaded WO3 sensors with different Au loading levels (S-0, S-0.25Au to S-2Au) at the optimal operating temperature (250°C).

43

Fig. 10. The catalyst selectivity histograms of sensor responses to 5 ppm NO2, 50 ppm N2O, 5 ppm NO, 50 ppm CO, 2000 ppm NH3, 1500 ppm C2H5OH, 500 ppm SO2 and 1500 ppm H2 of Au-loaded, Pd-loaded and Pt-loaded WO3 sensors with the 0.5 wt% loading content (S0.5Au, S-0.5Pd and S-0.5Pt) at 250°C. Inset: Resistance of Au-loaded, Pd-loaded and Ptloaded WO3 sensors subjected to a 5 ppm NO2 pulse.

44

Fig. 11. Representative models for NO2 sensing mechanisms of (a) unloaded WO3 nanorods and WO3 nanorods loaded with Au at (b) very low, (c) optimal and (d) high Au concentrations at an optimal operating temperature.

45 Table 1. Summary of NO2-sensing performances of recently reported WO3-based gas sensors.

Sensing materials

Methods

Temp.

NO2 Concentration (ppm)

Unloaded WO3 thin film

DC magnetron sputtering (sensors)

200°C

1

40

Zeng et al.[1]

Unloaded WO3 thin film

Plasma enhanced vapor deposition

200°C

100

100

Tong et al.[2]

Unloaded WO3 thin film

Electrochemical methods

110°C 200°C

8 5

23 45

Gao et al.[3]

Unloaded WO3 thin film

DC reactive sputtering

300°C 400°C

5 5

75 100

Kim et al. [4]

Unloaded WO3 thin film

Pulse laser deposition

400°C

200

175

Kawasaki et al.[5]

Low frequency AC electrophoretic deposition

200°C

0.5

49

Heidiri et al.[6]

Acidification methods

250°C

1

190

Kida et al.[7]

WO3 nanosheets with interdigitated Au nanoelectrodes

Precipitation (powder)/Electron beam lithography (electrode)

200°C

0.01

28

Tamaki et al.[8]

Unloaded WO3 thin films Au-loaded WO3 thin films

Reactive RF sputtering

250oC 200°C

10 10

1.6 4.8

Penza et al.[9]

Reactive magnetron sputtering

200°C

3

21 76

Jin et al.[10]

Unloaded WO3 nanoparticles Au-loaded WO3 nanoparticles

Hydrothermal

Room Temp.

1 1

1 5.16

Yuan et al.[11]

Unloaded WO3 nanoparticles Au-loaded WO3 nanoparticles

Colloidal chemical method

150°C

10 10

25 420

Xia et al.[12]

Unloaded WO3 nanorods 0.5 wt% Au-loaded WO3 nanorods

Precipitation and Impregnation methods (powders), Spin-coating (sensors)

250°C

5

141.8 836.6

Present Work

Unloaded WO3 nanoparticles WO3-SnO2 nanolamellae

Pure WO3 nanothinfilms 0.2 wt% Au/WO3 nanothinfilms

NO2 Response**

Ref.

*Response = Ra/Rg for reducing gas including NO2. Ra = Baseline resistance in air before gas exposure, Rg = Steady-state resistance after gas exposure