NO₂ sensing properties of WO₃-decorated In₂O₃ nanorods and In₂O₃-decorated WO₃ nanorods

Abstract

In₂O₃ nanoparticle (NP)-decorated WO₃ nanorods (NRs) were prepared using sol–gel and hydrothermal methods. The In₂O₃ NRs and WO₃ NPs were crystalline. WO₃ NP-decorated In₂O₃ NRs were also prepared using thermal evaporation and hydrothermal methods. The NO₂ sensing performance of the In₂O₃ NP-decorated WO₃ NR sensor toward NO₂ was compared to that of the WO₃ NP-decorated In₂O₃ NR sensor. The former showed a high response to NO₂ due to a significant reduction of the conduction channel width upon exposure to NO₂. In contrast, the latter showed a far less pronounced response due to limited reduction of the conduction channel width upon exposure to NO₂. When the sensors were exposed to a reducing gas instead of an oxidizing gas (NO₂), the situation was reversed, i.e., the WO₃ NP-decorated In₂O₃ NR exhibited a stronger response to the reducing gas than the In₂O₃ NP-decorated WO₃ NR sensor. Thus, a semiconducting metal oxide (SMO) with a smaller work function must be used as the decorating material in decorated heterostructured SMO sensors for detection of oxidizing gases. The In₂O₃ NP-decorated WO₃ NR sensor showed higher selectivity for NO₂ compared to other gases, including reducing gases and other oxidizing gases, as well as showed high sensitivity to NO₂.

Introduction

Despite the numerous merits of semiconducting metal oxides (SMOs) as sensor materials there are still certain limitations, such as their relatively low response to gases at room temperature and dissatisfactory selectivity [1]. To address the dissatisfactory sensing properties, various strategies have been attempted, including noble metal catalyst doping, heterojunction formation, and radiation-assisted treatment with energetic particles including ion beams, electrons, and ultraviolet (UV) lights [2,3,4]. Of these techniques, heterostructure formation is plausibly most widely studied and is used for the fabrication of chemiresistive nanostructured gas sensors. There are several types of heterostructures including p-n, n–n and p–p heterostructures. Generally, p–p heterostructures are less commonly utilized because of their inferior sensing properties, whereas n–n heterostructures are as widely utilized as the p–n counterparts because of their superior sensing properties [5]. However, strangely, n–n heterostructures have not been studied as intensively as the p–n congeners. The enhanced sensing properties of n–n heterostructures are mainly due to the resistance modulation at the n–n heterojunctions in n–n heterostructures. Various heterostructure combinations are known, such as a simple mixture of two different types of n-SMOs [6], bi-layer type n–n nanostructures [7], n–n core–shell structures [8], a single type of n-SMO nanostructure decorated with another type of n-SMO nanoparticles (NPs) [9], etc.

This study focuses on, decorated n–n heterostructures. WO₃ and In₂O₃ are chosen as sensor materials for detecting a typical oxidizing gas, NO₂. The sensing properties of In₂O₃ NP-decorated WO₃ nanorods (NRs), WO₃ NP-decorated In₂O₃ NRs, pristine WO₃ NRs, and pristine In₂O₃ NRs are compared and the differences in the sensing properties of these four nanostructures are analyzed and the origin of the differences is discussed in detail.

Methods

Preparation of In₂O₃ nanoparticles-decorated WO₃ nanorods

High purity In₂O₃ NPs were synthesized using a sol–gel method [10]. Indium acetate ([In(C₂H₃O₂)2·H₂O]; 0.6695 g) was dissolved in diethylene glycol and stirred for 5 min. The solution was homogenized by heating to 130 °C and 3 mL of 3 N-nitric acid was added to the solution and stirred well. The solution was heated at 180 °C for 5 h, and pure yellowish In₂O₃ NPs were precipitated. The In₂O₃ NPs were dried at 400 °C for 2 h and then calcined at 500 °C for 1 h to obtain the pure In₂O₃ NPs. The WO₃ NRs were synthesized by using a low-temperature hydrothermal method [11]. Sodium tungstate (1.956 mL) and oxalic acid (1.512 mL) were dissolved in distilled water (50 mL). The solution was acidified to PH 0.7–0.9 by mixing with 3 mol/L HCl solution. A transparent precursor solution was formed and 3 g of K₂SO₄ was added to the solution. The mixed solution was maintained in an autoclave at 100 °C for 24 h, cooled to room temperature, and was centrifuged to collect the green product. The product was rinsed with ethanol and dried at 60 °C for 1 h to obtain pure WO₃ NRs. The substrate on which the WO₃ NRs were synthesized was placed on a spin coater and then rotated at 500 rpm. The In₂O₃ NPs synthesized via the sol–gel method were dispersed in ethanol with a micropipette and the ethanolic dispersion of In₂O₃ NPs was dropped on the rotating WO₃ NR substrate.

Preparation of WO₃ nanoparticles-decorated In₂O₃ nanorods

In₂O₃ NRs were synthesized using a thermal evaporation method [12]. A 3 mm thick gold film—coated p-type Si (100) substrate was placed on the top of an alumina boat containing a mixture of In₂O₃ powders and positioned at the center of a horizontal quartz tube furnace. The furnace was heated to 900 °C and maintained at that temperature for 30 min under argon gas at a constant flow rate of 200 cm³/min. The WO₃ NPs were synthesized using a hydrothermal method [13]. WO₃ powders (2 mL) were dissolved in 48 mL of hydrochloric acid in sonicater. The pH of the solution was controlled at 7 using sodium hydroxide. After sonication of the solution for 6 h the precipitated powders were collected by removing the liquid, leaving the powders behind. The powders were placed into a hydrothermal synthesizer containing ethanol and the synthesizer was placed in an oven and heated at 180 °C for 12 h. WO₃ NPs were synthesized in the hydrothermal synthesizer. The substrate on which the In₂O₃ NRs were synthesized by the thermal evaporation method was placed in a beaker containing ethanol and then ultrasonicated to separate the In₂O₃ NRs from the substrate. Meanwhile, the WO₃ NPs synthesized by the hydrothermal method were dispersed in ethanol. The two solutions (In₂O₃ NRs dispersed in ethanol and the WO₃ NPs dispersed in ethanol) were mixed and the mixed solution was exposed to UV (254 nm) irradiation for 12 h using a UV lamp. The mixed solution was then annealed under argon atmosphere at 400 °C for 1 h in an annealing furnace.

Fabrication of chemiresistive sensors

The In₂O₃ NP-decorated WO₃ NRs and WO₃ NP-decorated In₂O₃ NRs grown on the Si substrate were dispersed ultrasonically in isopropyl alcohol. A multiple-networked chemiresistive sensor was fabricated by pouring the solution containing the precursors of the two different nanostructures onto SiO₂/Si substrates with a patterned interdigital electrode with a double layer comprising separate layers of Ti (10 nm) and Au (100 nm): the assembly was dried at 150 °C for 1 min. For comparison of the sensing properties, pristine In₂O₃ and WO₃ NR sensors were also fabricated in a similar manner.

Characterization

The microstructures and phases of the synthesized NR samples were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The microstructures and phases of the samples were examined further by transmission electron microscopy (TEM).

Gas sensing tests

The NO₂ sensing performances of the fabricated sensors were examined using a custom-made gas sensing system. The concentration of NO₂ gas was controlled precisely in the concentration range of 5–200 ppm by mixing NO₂ with dry synthetic air using the mass flow controllers. Electrical measurements to examine the sensing properties of the sensors were conducted at room temperature under 50% relative humidity. The detailed sensing test procedure is described elsewhere [14]. The response of the sensors to NO₂ was evaluated by using the R_g/R_a ratio, where R_g and R_a are the resistances of the sensor measured in the presence of air and NO₂, respectively. The response and recovery times were determined by measuring the times required to reach 90% of the total change in the resistance of the sensor after exposure of the sensor to the analyte gas and ambient air, respectively.

Results and discussion

Figure 1a, b show low-magnification TEM images of the pristine and In₂O₃ NPs-decorated WO₃ NRs, respectively. The average diameter of the WO₃ NRs was ~ 50 nm and the length of the WO₃ NRs ranged from 200 to 1100 nm. The average diameter of the In₂O₃ NPs on the WO₃ NRs was 20 nm. The SEM images of the pristine and WO₃ NP-decorated In₂O₃ NRs are exhibited in Fig. 1c, d. The average diameter of the In₂O₃ NRs was 250 nm and the lengths of the In₂O₃ NRs ranged from 1 to 10 μm. The average diameter of the WO₃ NPs on the In₂O₃ NRs was 140 nm. Hence, the average diameter of the WO₃ NPs on the In₂O₃ NRs was ~ 7 times larger than that of the In₂O₃ NPs on the WO₃ NRs. The difference in size might be due to the different preparation methods (sol–gel versus hydrothermal methods).

Fig. 1

Low-magnification TEM images: a pristine and b In₂O₃ NPs-decorated WO₃ NRs and SEM images: c pristine and d WO₃ NPs-decorated In₂O₃ NRs

Figure 2a, b show the XRD patterns of the In₂O₃ NP-decorated WO₃ NRs and WO₃ NP-decorated In₂O₃ NRs, respectively. In the former pattern, the WO₃ NRs exhibited relatively sharp and intense reflection peaks, assigned to the primitive tetragonal structured WO₃ (JCPDS card No. 89-4481, a = 0.5275 nm, c = 0.7846 nm). In contrast, the In₂O₃ NPs exhibited relatively less sharp and less intense reflection peaks, assigned to body-centered cubic In₂O₃ with a lattice constant of a = 1.011 nm (JCPDS No. 89-4595). The lower intensity peaks for In₂O₃ compared to WO₃ might be due to the smaller volume of the In₂O₃ NPs relative to that of the WO₃ NRs. In contrast, in the latter pattern (Fig. 2b), In₂O₃ peaks were taller and sharper than WO₃ peaks, which might be due to the larger volume of In₂O₃ NRs than those of the WO₃ NPs.

Fig. 2

XRD patterns of a In₂O₃ NPs-decorated WO₃ NRs and b WO₃ NPs-decorated In₂O₃ NRs

Figure 3a, b present the high-resolution TEM image and corresponding selected area electron diffraction (SAED) pattern of the In₂O₃ NP-decorated WO₃ NRs. The regularly aligned fringes in both the WO₃ and In₂O₃ regions suggest that the WO₃ and In₂O₃ nanostructures are both crystalline. The corresponding spotty electron diffraction (ED) pattern in Fig. 3b reveals that the WO₃ and In₂O₃ nanostructures are single crystals.

Fig. 3

a High-resolution TEM image, b corresponding electron diffraction pattern of the In₂O₃ NPs-decorated WO₃ NRs

The temperature-dependent responses of all four different sensor materials to NO₂ are presented in Fig. 4. The responses of all the four sensor materials to NO₂ tended to increase with increasing temperature up to 300 °C, and then to decrease with further increases in the temperature. This result suggests that 300 °C is the optimal operating temperature of the sensors in detecting the NO₂. All the sensing tests hereafter were conducted at 300 °C. At too low operating temperature (250 °C or lower), the NO₂ molecules may not have enough energy to overcome the energy barrier of adsorption, and fail to be adsorbed on the surface of the sensor materials, WO₃ and In₂O₃. However, at too high operating temperature (350 °C or higher), adsorption failure might also occur because the rate of desorption may outweigh that of adsorption [15].

Fig. 4

Responses of In₂O₃ NPs-decorated WO₃ NRs and WO₃ NPs-decorated In₂O₃ NPs along with pristine WO₃ NRs and pristine WO₃ NRs at various operating temperatures to NO₂

Figure 5a–d present the dynamic response curves of the four different sensors toward NO₂. All the sensors showed stable and reversible response and recovery behavior. The resistances of the sensors increased when an oxidizing gas (NO₂) was supplied, and recovered to the initial value when the NO₂ supply was stopped and the sensors were exposed to ambient air. This response toward the oxidizing gas is in accord with the sensing behavior of n-type semiconductors. As is well known, both WO₃ and In₂O₃ are n-type semiconductors. The resistance changes increased as the NO₂ concentration was increased. The starting resistances of the pristine and WO₃ NP-decorated In₂O₃ NRs was markedly lower than the pristine and In₂O₃ NPs-decorated WO₃ NRs, respectively, which might be due to the much lower resistivity of In₂O₃ than that of WO₃.

Fig. 5

Dynamic response curves: a pristine WO₃ NRs to NO₂, b In₂O₃ NPs-decorated WO₃ NRs to NO₂, c pristine In₂O₃ NRs to NO₂, and d WO₃ NPs-decorated In₂O₃ NRs to NO₂

Figure 6 shows the responses of the four different sensors to NO₂ as a function of the NO₂ concentration. The response of the In₂O₃ NP-decorated WO₃ NRs to NO₂ far exceeded those of the other three sensors over the entire NO₂ concentration range. The more pronounced response of the In₂O₃ NP-decorated WO₃ NR sensor to NO₂ than that of the pristine WO₃ NRs and the greater response of the WO₃ NPs-decorated In₂O₃ NRs sensor to NO₂ than that of the pristine In₂O₃ NRs is plausibly due to the resistance modulation at the WO₃–In₂O₃ heterojunction formation [16]. Contrarily, the much stronger response of the In₂O₃ NP-decorated WO₃ NR sensor to NO₂ than that of the WO₃ NP-decorated In₂O₃ NR sensor is very interesting. The origin of this difference in the response of the heterostructured sensors with inverse configuration is discussed in detail in the next section.

Fig. 6

Calibration curves for responses of pristine and In₂O₃ NPs-decorated WO₃ to NO₂ as well as those of pristine and WO₃ NPs-decorated In₂O₃ NRs to NO₂ as a function of NO₂ concentrations

Figure 7a, b show the response and recovery times of the four different sensors toward NO₂ as a function of the NO₂ concentration. As expected, the response and recovery times of the In₂O₃ NP-decorated WO₃ NR sensor were shorter than those of the pristine WO₃ NRs. In contrast, the response and recovery times of the WO₃ NP-decorated In₂O₃ NR sensor were longer than those of the pristine In₂O₃ NR sensor. Comparison of the response and recovery times of the In₂O₃ NP-decorated WO₃ NR sensor with those of the WO₃ NP-decorated In₂O₃ NR sensor, interestingly, shows shorter response and recovery times for the former in the higher NO₂ concentration range, whereas longer response and recovery times for the lower NO₂ concentration range than the latter. Shorter response and recovery times are commonly associated with a higher response for gas sensors.

Fig. 7

Calibration curves for a response times and b recovery times of pristine and In₂O₃ NPs-decorated WO₃ NRs as well as pristine and WO₃ NPs-decorated In₂O₃ NRs to NO₂ as a function of NO₂ concentrations

The response of the In₂O₃ NP-decorated WO₃ NR sensor to various gases is shown in Fig. 8. The sensor showed a much stronger response to NO₂ than to the other oxidizing gases such as O₃ and SO₂ or reducing gases such as CO, CH₄ and H₂S, demonstrating the selectivity and sensitivity of the In₂O₃ NP-decorated WO₃ NR sensor toward NO₂. The selectivity of the sensor toward NO₂ against other gases might be related to the different optimal operating temperatures of the sensor for different target gases. The response of a sensor material to a certain gas might depend on many factors such as solid solubility of the gas in the material, the decomposition rate of the adsorbed molecule at the material surface, the charge carrier concentration in the material, the Debye length in the material, the catalytic activity of the material, the orbital energy of the gas molecule, etc. The dissociation (or reduction) rate of an oxidizing gas such as NO₂ is determined by these factors. Therefore, each gas has the characteristic optimal dissociation temperature at which its dissociation rate is maximized. The In₂O₃-decorated WO₃ nanorod sensor fabricated in this study showed higher response fortunately to NO₂ than other gases at 300 °C because of the higher dissociation rate of NO₂ at the surface of In₂O₃ and WO₃ at the temperature, but it might show higher responses to other gases than NO₂ at different temperatures [17,18,19,20,21,22].

Fig. 8

Selectivity patterns of pristine and In₂O₃ NPs-decorated WO₃ NRs toward NO₂ against other gases such as O₃, SO₂, CO, CH₄, and H₂S

Figure 9a–d illustrate the sensing mechanism of the In₂O₃ NP-decorated WO₃ NR sensor toward NO₂. Earlier studies reported that the response of a base sensor material could be enhanced by decoration with another type of SMO NPs, mainly because of the greater modulation of the width of the depletion layer or the conduction channel, resulting in the greater modulation of the sensor resistance [23,24,25]. n-Type WO₃ has a larger work function (qΦ) than n-type In₂O₃ (Fig. 9a). Accordingly, if WO₃ and In₂O₃ are in contact, even under vacuum, electron transfer from In₂O₃ (with a larger work function) to WO₃ (with a smaller work function) tends to occur until electronic equilibrium is attained between WO₃ and In₂O₃, as shown in Fig. 9a. Consequently, electron-accumulation and electron-depletion layers are formed in the WO₃ and In₂O₃ regions, respectively. The schematic shows the In₂O₃ NP-decorated WO₃ NRs with an accumulation layer with a width of W₁₁, formed by electron transfer from the In₂O₃ NPs to the WO₃ NRs (Fig. 9b). In ambient air, the surfaces of the WO₃ NR and In₂O₃ NP adsorb oxygen molecules and the adsorbed oxygen molecules are ionized by the capture of the free electrons in the WO₃ and In₂O₃ surface regions (Fig. 9c). Consequently, a depletion layer with a width of W₁₂ is formed in the surface region of WO₃. The schematic shows a decorated WO₃ NR with a depletion layer formed via ionization of adsorbed oxygen molecules and an accumulation layer formed by electron transfer from the In₂O₃ to the WO₃ (Fig. 9c). When NO₂ gas is supplied, NO₂ and O₂ molecules are both adsorbed by the In₂O₃ and WO₃ surfaces. The adsorbed NO₂ molecules are converted into NO₂⁻ or NO [26, 27] and the adsorbed oxygen molecules are converted into oxygen ions by capturing electrons from the WO₃ and In₂O₃ surface regions. Consequently, a thicker depletion layer (with a width of W₂₂) (Fig. 9d) is formed than that formed in ambient air. The schematic shows a WO₃ NR with a depletion layer with a width of W₂₂ as well as the accumulation layer with a width of W₂₁ formed by the electron transfer from the In₂O₃ to the WO₃ (Fig. 9d).

Fig. 9

Energy band diagrams of a WO₃-In₂O₃ couple a before and b after contact. Energy band diagrams and schematics of In₂O₃ NPs-decorated WO₃ NRs: c in air and d in NO₂, and of WO₃ NPs-decorated In₂O₃ NRs: e in air and f in NO₂

The sensing mechanism of the WO₃ NP-decorated In₂O₃ NR sensor toward NO₂ is illustrated in Fig. 9e, f. As discussed above, electron-accumulation and depletion layers are formed in the WO₃ and In₂O₃ regions, respectively. Thus, a depletion layer with a width of W₃₁ forms on the In₂O₃ side of the WO₃–In₂O₃ interface (Fig. 9e). In ambient air, oxygen molecules are adsorbed by the In₂O₃ NR surface and ionized by accepting the electrons from the In₂O₃ and WO₃ surface regions. Consequently, a depletion layer with a width of W₃₂ is formed in the In₂O₃ surface region. A WO₃ NP-decorated In₂O₃ NR with a depletion layer formed due to the ionization of adsorbed oxygen molecules and a depletion layer formed by electron transfer from the WO₃ NP is shown in Fig. 9e. Under NO₂ atmosphere, a thicker depletion layer (with a width of W₄₂) than that generated in ambient air is formed due to the adsorption and ionization of both NO₂ and O₂ molecules (Fig. 9f). Note that no electron-accumulation layer is formed in the In₂O₃ NR throughout the on–off cycling of the NO₂ gas supply.

Under ambient air and NO₂, there was no big difference in the basic response of the In₂O₃ NPs-decorated WO₃ NRs versus that of the WO₃ NPs-decorated In₂O₃ NRs. A relatively thin depletion layer is formed in both samples upon exposure to air and a thick depletion layer is generated upon exposure to NO₂. Consequently, the width of the conduction channel of the In₂O₃ NP-decorated WO₃ NRs formed upon exposure to NO₂ is much smaller than that of the WO₃ NP-decorated In₂O₃ NRs formed in ambient air. The conduction channel of the In₂O₃ NP-decorated WO₃ NRs has a room for substantial reduction upon exposure to NO₂ because the conduction channel width has already been expanded due to the formation of an accumulation layer by the transfer of electrons from the In₂O₃ NP to the WO₃ NR. In contrast, the conduction channel of the WO₃ NPs-decorated In₂O₃ NRs was already shrunken due to the formation of the electron-depletion layer via electron transfer from the In₂O₃ NR to the WO₃ NP. Accordingly, the conduction channel of the In₂O₃ NR has little room for further reduction upon exposure to NO₂ [28].

The response, S is defined as R_g/R_a. for the oxidizing gas NO₂ and S is proportional to A_a/A_g because the resistance R = ρl/A, where ρ, l and A are the density, length and cross-sectional area of the conductor (channel, here) [29]. S can be expressed as the ratio of the conduction channel width for an analyte gas to that for air, S = W ²_a /W ²_g because A = πW², where W is the conduction channel width. Therefore, the In₂O₃ NP-decorated WO₃ NR sensor has a higher response, S to NO₂ because of the far smaller conduction channel width, W_g in NO₂ atmosphere. In contrast, the WO₃ NP-decorated In₂O₃ NR sensor has a lower response, S because of the lower contraction of the conduction channel width, W_g in NO₂ atmosphere.

Conclusions

The sensing properties of the In₂O₃ NP-decorated WO₃ NR sensor toward NO₂ were compared to those of the WO₃ NP-decorated In₂O₃ NR sensor. The response of the former sensor to NO₂ was more pronounced than that of the latter due to the significant reduction of the conduction channel width of the former sensor upon exposure to NO₂. The conduction channel of the In₂O₃ NP-decorated WO₃ NR sensor had room for sufficient reduction as it was already expanded by electron transfer from the In₂O₃ NPs to the WO₃ NRs. In contrast, the WO₃ NP-decorated In₂O₃ NR sensor showed a lower response due to insufficient reduction of the conduction channel width upon exposure to NO₂. The conduction channel of the WO₃ NP-decorated In₂O₃ NR sensor had little room for further reduction due to prior shrinkage associated with electron transfer from the In₂O₃ NRs to the WO₃ NPs. For the detection of a reducing gas instead of an oxidizing gas, the magnitude of the sensor response would be reversed. Therefore, choosing a proper decorating material in fabricating n-SMO NR sensors decorated with n-SMO NPs is important in obtaining high sensitivity. An SMO with a smaller work function must be chosen as a decorating material in a decorated heterostructured sensor for oxidizing gas detection. In contrast, an SMO with a larger work function must be chosen as the decorating material for heterostructured sensors geared toward the detection of a reducing gas.