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).
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.
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.
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].
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.
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.
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.
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].
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).
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 Rg/Ra. for the oxidizing gas NO2 and S is proportional to Aa/Ag 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
2a
/W
2g
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, Wg 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, Wg in NO2 atmosphere.