Abstract
In this study, a two-layered thin-film structure consisting of a dispersed nanoscaled Ag2O phase and SnO2 layer (SA) and a mono-composite film layer (CSA) consisting of a nanoscale Ag2O phase in the SnO2 matrix are designed and fabricated for NO2 gas sensor applications. Two-layered and mono-layered SnO2–Ag2O composite thin films were synthesized using two-step SnO2 and Ag2O sputtering processes and Ag2O/SnO2 co-sputtering approach, respectively. In NO2 gas-sensing measurement results, both SA and CSA thin films that functionalized with an appropriate Ag2O content exhibit enhanced gas-sensing responses toward low-concentration NO2 gas in comparison with that of pristine SnO2 thin film. In particular, a gas sensor made from the mono-composite SnO2–Ag2O layer demonstrates apparently higher NO2 gas-sensing performance than that of double-layered SnO2–Ag2O thin-film sensor. This is attributed to substantially numerous p–n junctions of Ag2O/SnO2 formed in the top region of the SnO2 matrix. The gas-sensing response of the optimal sample (CSA270) toward 10 ppm NO2 gas is 5.91, and the response/recovery speeds in a single cycle dynamic response plot are 28 s/168 s toward 10 ppm NO2, respectively. Such a p–n thin-film configuration is beneficial to induce large electric resistance variation before and after the introduction of NO2 target gas during gas-sensing tests. The experimental results herein demonstrate that the gas-sensing performance of p–n oxide composite thin films can be tuned via the appropriate design of composite thin-film configuration.
1 Introduction
Metal oxide-based gas sensors are significant and broadly used in the industrial plant safety monitoring, hospital environmental monitoring, and chemical engineering process control. However, the performance of metal oxide-based gas sensors made from a single component is limited by their intrinsic characteristics. The development of approaches for the improved gas-sensing performance of metal oxide sensors toward target gases is therefore highly demanded. Integration of two or more materials to form a heterogeneous sensor structure has been shown to be a promising approach to improve the gas-sensing characteristics of metal oxide sensors [1,2,3,4]. At present, several metal oxide semiconductors have been widely used as gas sensor materials because of their stable physical and chemical properties in detecting environments [5,6,7]. Among various semiconductor oxides, SnO2 is an n-type semiconductor and has been widely used as gas-sensor material due to its high thermal stability, chemical stability, diverse morphologies, and diverse synthesis approaches [2,8,9]. Particularly, even though SnO2 is a widely known material for gas-sensing applications, there is still high demand for improving SnO2 with satisfactory performance to applying to gas sensors through various approaches. The type of heterojunction, surface structure, grain sizes, surface active, and relative element content are essential parameters to affect the semiconductor composite’s gas detection ability. Recent progress of SnO2-based p–n heterogeneous gas sensors has shown an improved gas-sensing ability toward target gas in comparison with the sensor made from pristine SnO2. For example, flower-like CeO2–SnO2 p–n composites show an enhanced triethylamine-sensing ability than SnO2 because of the formation of p–n heterojunction between CeO2 and SnO2 [10]. Furthermore, the SnO2/NiO nanocomposite-based sensor exhibits more prominent performances than the pristine constituent of NO2 gas [8]. Based on the aforementioned, the formation of SnO2-based p–n heterogeneous gas sensors is promising to improve the gas-sensing ability of the pristine SnO2. However, fewer literature on p–n-type composite thin films that give systematic discussions with microstructure-dependent gas sensibility are proposed. The thin-film structure of the gas sensor is very important and indispensable. Thus, how to investigate and integrate all the parameters relative to composite films for gas sensors is essential and is of potential interest to the sensor material community.
For applications of gas-sensing materials, the thin-film structure is the most commonly used and simple one and is nowadays compatible with the semiconductor industry process. For various thin-film processes, the thickness, composition, and microstructure of the film can be precisely controlled by a sputtering method, and the ability of large-area deposition can be achieved. The sputtering is therefore a promising approach to fabricate SnO2 thin-film structure for industrial gas sensor device applications. Moreover, the Ag2O has low bandgap energy and catalytic characteristic. It is promising to be integrated into wide-bandgap oxides to enhance the gas-sensing performance of wide-bandgap oxides toward the target gas. Surface modifying or incorporation of Ag2O with wide-bandgap oxide semiconductors are popular approaches for optimizing the performance of many metal oxide-based gas sensors [11,12]. The NO2 gas is exhausted during combustion in chemical factories and fuel vehicles. Exposure to NO2 gas with a low concentration still causes a threat to humans and the ecosystem. Thus, the design and development of oxide-based thin-film sensors to detect NO2 gas with high efficiency are in high demand in material technology. Therefore, Ag2O-functionalized SnO2 thin films are fabricated for NO2 gas detection in this study. The two-layered structure consisted of Ag2O and SnO2 layers, and a single composite film layer consisting of the Ag2O phase in the SnO2 matrix is designed and fabricated herein for NO2 gas sensor applications. There is no literature on integrating SnO2-based thin film with catalytic Ag2O by a sputtering process. There are only a few studies on SnO2 decorated or doped by Ag2O for gas sensor applications through chemical routes [13,14]. This work made systematic discussions on thin-film structure, P-type particle incorporation, surface layer decoration condition, gas-sensing ability, and gas-sensing mechanism, and is an important reference for SnO2 gas sensor development. In the meantime, there are some disadvantages to SnO2-based gas sensors synthesized by sol–gel, wet chemical synthesis, and spray pyrolysis methods which cannot precisely control the microstructures of the thin film during the in-situ thin-film growth process. Usually, a post-process with a high temperature is needed to obtain satisfying crystallinity to be used in sensor material for these preparation routes. Furthermore, integration of hetero-structure with SnO2 synthesized using these routes usually needs another synthesis step to achieve. These shortcomings reveal that the development of sputtering techniques to fabricate Ag2O particle incorporation in or surface decoration on SnO2 thin films is highly demanded for semiconductor industry applications. Based on the aforementioned, the sputtering thin-film process is a reliable process for realizing the practical application of gas-sensing devices, and the integration of Ag2O and SnO2 by the sputtering process has not been fully established. Based on the realization of the application of SnO2 thin-film template materials in gas-sensing elements, it is important to integrate the sputtering process development of Ag2O with SnO2 in this study.
2 Experimental procedures
2.1 Material synthesis process
SnO2–Ag2O composite films on the F-doped tin oxide (FTO) substrates were synthesized through different sputtering treatments, two-step sputtering and co-sputtering processes, named SA and CSA series, respectively. Both metallic Sn disc and metallic Ag disc with the size of 2 inches were used as sputtering targets to prepare SnO2–Ag2O composite films. During the growth of the SnO2−Ag2O composite films, the direct current sputtering power of the Sn target was fixed at 20 W; moreover, the radio-frequency sputtering power of the Ag target was fixed at 50 W. The thin-film growth temperature was maintained at 200°C with an Ar/O2 ratio of 5:2; the gas pressure during thin-film deposition was fixed at 20 mtorr. The sputtering guns are set to an incline angle of 60° over the sample surface. At first, the sputtering duration for the initially grown SnO2 film on the FTO substrate was fixed at 20 min. After that, two types of SnO2−Ag2O composite films were synthesized on the SnO2/FTO substrates via two-step sputtering and co-sputtering processes. The two-step sputtering and co-sputtering of SnO2 with the Ag2O sputtering time of 130, 200, and 270 s are named SA130, SA200, SA270, CSA130, CSA200, and CSA270, respectively. The thin-film configurations of SA and CSA series thin-film samples are shown in Figure 1. The resultant films herein have thicknesses of approximately 195–210 nm.
Schematics of thin-film configuration: (a) SA and (b) CSA series samples.
2.2 Material analysis
The surface morphology and the element content of various thin-film samples were investigated using scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM–XRD; JEOL 7900F). Furthermore, the surface roughness of the samples was investigated using atomic force microscopy (AFM; Veeco Dimension 5000 Scanning Probe Microscope (D5000)). The crystal structures of thin-film samples were investigated using grazing incidence small-angle (1°) X-ray scattering (XRD; Bruker D8 SSS). The optical absorption spectra of thin-film samples were conducted using ultraviolet-visible spectroscopy (UV-Vis; Jasco V-750). The chemical elemental binding states of various thin-film samples were investigated using X-ray photoelectron spectroscopy (XPS; PHI 5000 VersaProbe). The elemental depth profiling for the thin-film samples was investigated using secondary-ion mass spectrometry (SIMS; ION-TOF, TOF-SIMS V). A home-built computer-controlled gas-sensing measurement system was used to measure the NO2 gas-sensing properties of various thin-film samples. A vacuum chamber equipped with a pumping system, pressure gauges, heating holder, mass flow controllers, and sophisticated electric probe system together with a data acquisition system were integrated into the gas-sensing system. The gas sensors made from the SnO2–Ag2O composite thin films were coated with patterned metallic platinum onto the surface for electric contact with probes during gas-sensing measurements. Various concentrations of NO2 gas (1, 2.5, 5, 7.5, and 10 ppm) were introduced into the test chamber, using dry air as the carrier gas. The operating temperature of NO2 gas-sensing measurements varied from 175 to 300°C. A resistance meter (Agilent B2911A) was used to record the resistance changes of the sensor devices before and after the supply of the target gas. The gas-sensing response toward NO2 gas is defined as the resistance ratio of the sensor in the air (Ra) and target gas (Rg), i.e., Rg/Ra. For other reducing gases, the response is defined as Ra/Rg. The response/recovery speeds are described as the time in which the sensor resistance increases 90% from the initial value and decreases to 10% from the highest value with and without target gas, respectively.
3 Results and discussion
Figure 2 shows the SEM top-view images of the pristine SnO2 film and SnO2–Ag2O composite thin films with different sputtering treatment durations of Ag2O. The distinct grain boundaries among surface grains were observed for all samples, revealing well crystalline characteristics of the thin films. Figure 2a shows that the SnO2 thin-film surface is composed of irregular-shaped surface grains. The histogram of surface grain size reveals that the average SnO2 grain size was approximately 205 nm. Figure 2b–d presents the surface morphology of the SA samples in which SnO2 thin film surfaces were decorated with Ag2O particles. Grayscale contrast of the surface morphology clearly demonstrates the existence of Ag2O particles on the surface of SnO2 thin film. A longer sputtering time of Ag2O from 130 to 270 s will not only cause a higher density of Ag2O particles on the SnO2 surface but also cause the decorated Ag2O particles much bigger at the same time. From the inserted histograms of Ag2O particles, the average size of the Ag2O particles was approximately 38, 74, and 96 nm, for the SA130, SA200, and SA270, respectively. Figure 2e–g presents the surface morphology of CSA130, CSA200, and CSA270, respectively. The surface grain feature is similar to that of pristine SnO2 but the surface grain size of the samples was slightly decreased to 188, 175, and 160 nm for the CSA130, CSA200, and CSA270, respectively, according to the analysis of surface grain size histograms. The representative SEM–EDS spectra of SA200 and CSA270 are shown in Figure 2h and i, respectively. Moreover, the SEM–EDS analysis for estimating the composition of various samples is summarized in Table 1. An increased Ag content was demonstrated in SA and CSA series samples with a prolonged Ag2O sputtering duration. Notably, the Ag contents in the CSA thin-film samples are slightly lower than those of the SA thin-film samples under the same sputtering condition. This might be because the sputtering Ag atoms might encounter a larger scattering level from Sn atoms before reaching the thin solid film surface during the co-sputtering process in comparison with single Ag2O sputtering process for SA samples.
SEM top view images: (a) pristine SnO2, (b) SA130, (c) SA200, (d) SA270, (e) CSA130, (f) CSA200, and (g) CSA270. The corresponding SnO2 surface grain size, Ag2O particle size, and composite film surface grain size distribution histograms are also shown in the insets. The representative EDS spectra of Sn, Ag, and O elements taken from the composite films: (h) SA200 and (i) CSA270.
Composition (O, Sn, and Ag) of various thin-film samples from SEM–EDS spectra analysis
| Sample | Element (at%) | ||
|---|---|---|---|
| O | Sn | Ag | |
| SA130 | 71.14 | 28.37 | 0.49 |
| SA200 | 68.28 | 29.31 | 1.51 |
| SA270 | 69.01 | 28.52 | 2.47 |
| CSA130 | 68.88 | 30.66 | 0.46 |
| CSA200 | 69.46 | 29.45 | 1.09 |
| CSA270 | 69.18 | 28.80 | 2.02 |
XRD analysis was performed to investigate the crystallographic information associated with the heterostructure SnO2−Ag2O composite films. The results are depicted in Figure 3. The crystallographic phases of various samples are correlated with the diffraction data of the SnO2 JCPDS card (No. 002-1337) and Ag2O JCPDS card (No. 012-0793) herein. Several distinct diffraction peaks of tetragonal SnO2 are shown in Figure 2. The well-defined diffraction peaks possess crystallographic orientations in the SnO2 (100), (101), (200), (211), and (220). Figure 3a–c shows the XRD patterns of SA samples with different Ag2O sputtering times. The Ag2O (111) centered at approximately 32.7o is identified in the XRD patterns. Furthermore, the intensity of Ag2O (111) increased with the sputtering time of Ag2O. A similar crystallographic feature of SnO2 with Ag2O phases is shown in the XRD patterns of CSA samples (Figure 3d–f). The XRD patterns show that as-synthesized SnO2–Ag2O composite films (SA and CSA samples) are in a crystalline feature. The XRD results herein prove that both synthesis methods (SA and CSA) generate the perfect crystalline binary SnO2/Ag2O phases in the thin-film samples, and these thin-film samples show a feature of composite rather than alloy.
XRD pattern of various SnO2–Ag2O composite films: (a) SA130, (b) SA200, (c) SA270, (d) CSA130, (e) CSA200, and (f) CSA270. The S and A denote SnO2 and Ag2O, respectively.
Figure 4a shows the UV-vis absorbance spectra of pristine SnO2, SA series, and CSA series thin-film samples. It is evident that a steep absorption edge appears at the UV light region of approximately 325 nm for the SnO2. This is attributed to the wide-bandgap feature of pristine SnO2. Notably, the absorption edge appears to red-shift to some extent for the SnO2 decorated or incorporated with the Ag2O phase which has a visible light region band energy feature [15]. The UV-vis absorption spectra provide the evidence of the formation of SnO2–Ag2O heterostructures in SA and CSA series thin-film samples. In addition, the bandgap energies of the SnO2 and SnO2–Ag2O composite films are also evaluated by converting the Kubelka−Munk equation [16], as displayed in Figure 4b and c. The SnO2 has a bandgap energy of approximately 3.8 eV. The bandgap energies of SA130, SA200, and SA270 are approximately 3.77, 3.74, and 3.71 eV, respectively. Moreover, those of CSA130, CSA200, and CSA270 are 3.78, 3.76, and 3.73 eV, respectively. The higher Ag2O content in the composite thin films results in a narrower bandgap energy of the composite thin films.
(a) UV-Vis absorption spectra of various samples. Evaluation of bandgap energies of (b) SA series and (c) CSA series thin film samples. The pristine SnO2 thin film was added for comparison in each Tauc plot.
The dependence of surface structure and its roughness were also investigated using AFM for perspective images as shown in Figure 5. For pristine SnO2, the surface particles are in long strips and round shapes (Figure 5a). The root-mean-square (RMS) roughness is approximately 24.7 nm. The AFM micrographs shown in Figure 5b–d display abundant bright Ag2O particles of various sizes decorated on the SnO2 surface grains for the SA series samples. The increase in Ag2O particle size with sputtering duration is visibly distinguished for SA130–SA270 samples, which results in the surface roughness of the SA130 and SA270 increasing from 26.6 to 29.7 nm [17]. By contrast, the appearance of surface grains for the CSA samples is similar to that of pristine SnO2. Moreover, a decreased surface grain size was observed for the CSA130 to CSA270 as exhibited in Figure 5e–g. The RMS roughness is 22.3, 20.1, and 19.1 nm for the CSA130, CSA200, and CSA270, respectively. The RMS roughness in the SA series is found to be increased with an increase in Ag2O amount on the SnO2 films’ surfaces. However, there is a different trend in the CSA series, and the roughness is found to be decreased with an increased Ag2O amount in the composite films. The surface roughness evolution of various SnO2–Ag2O composite thin films is in agreement with the results of SEM observations.
AFM images: (a) pristine SnO2 film, (b) SA130, (c) SA200, (d) SA270, (e) CSA130, (f) CSA200, and (g) CSA270.
Figure 6(a) and (b) displays the SIMS depth profiles of representative SA200 and CSA270, respectively. The intensity variations of the Sn, Ag, O, and F signals are plotted as dependent on the depth of profiles. Both samples indicate three main layers from the film surface to the FTO substrate, i.e., SnO2/Ag2O composite layer (depth, 0−200 nm), pure SnO2 layer (depth, 200−500 nm), and FTO substrate (after 500 nm). The SIMS depth profiles indicate that SA200 has a substantially intense Ag signal on the surface, revealing the Ag2O particles decorated on the top of the SnO2 films through a two-step sputtering process. Comparatively, the distribution of Ag presents a relatively gentle gradient vertical distribution characteristic in the depth of the CSA270 film, demonstrating that Ag2O particles are incorporated into the SnO2 matrix through the co-sputtering process. The SIMS depth profiling analysis supports the successful synthesis of double-layered Ag2O/SnO2 composite film and mono-layer Ag2O/SnO2 composite film via different sputtering approaches in this study.
SIMS depth profiles of representative thin film samples: (a) SA200 and (b) CSA270.
The Ag 3d core-level spectra of various thin-film samples are displayed in Figure 7a. The two symmetric peaks of pure Ag2O that are centered at approximately 367.8 and 373.8 eV are ascribed to Ag 3d5/2 and 3d3/2, and these binding energies confirm the Ag+ binding state in the Ag2O lattice [11,15]. Moreover, Figure 7b demonstrates the narrow scan spectra of the Sn3d region of SA and CSA series samples. The symmetric Sn 3d5/2 and 3d3/2 peaks at the binding energies of 486 and 494.4 eV, respectively, confirm the Sn4+ binding state in the SnO2 lattice [2,9]. The binding energy shifts in the Ag 3d and Sn 3d spectra for the composite films in comparison with the binding energy of the pristine constituent ones could be explained by the different electro-negativities of metal ions causing the strong interaction (electron transfer) in the composite films [18,19]. The XPS analysis herein proved that the p–n junction is formed at the interface between the SnO2 and Ag2O. Under the possible surface oxygen-binding states of various samples, the O1s spectra of various samples are demonstrated. Herein, the O 1s spectra of SA samples in Figure 8a–c can be deconvoluted into three subpeaks with the binding energies centered at approximately 529.2(OI), 530.3(OII), and 531.7(OIII) eV. The species of OI and OII on the lower and medium binding energies of the O 1s spectra belong to O2− ions in Ag2O and SnO2, respectively [20,21]. The higher binding energy species of OIII are usually attributed to O x − ions from lattice defect or chemisorbed from the ambient air [4,22]. According to the O 1s spectra, in the SA series, the signal from Ag2O lattice oxygen is more obvious with the Ag2O decorating sputtering time. By contrast, in the CSA series, the lattice oxygen signal originating from Ag2O is not significant as exhibited in Figure 8d–f because of the cladding effect of the SnO2 matrix. The component of defective oxygen and the absorbed oxygen species of the samples increased with the co-sputtering time. The larger content of Ag2O particles in the SnO2 matrix might reduce the lattice accommodation ability between the SnO2/Ag2O. This further causes an increase of oxygen-binding component at the higher binding energy herein. A similar effect has been proposed in co-sputtering ZnO–Ag nanofilms [23].
High-resolution XPS spectra in Ag 3d and Sn 3d core-level regions of various thin film samples: (a) Ag 3d and (b) Sn 3d.
High-resolution O 1s XPS spectra and their deconvolution results of various thin film samples: (a) SA130, (b) SA200, (c) SA270, (d) CSA130, (e) CSA200, and (f) CSA270.
Gas-sensing experiments were performed at different temperatures in order to find out the optimum operating temperature of SnO2, SA, and CSA thin-film sensors for NO2 gas-sensing. For comparison, the operating temperature-dependent gas-sensing responses toward 10 ppm NO2 for the representative pristine SnO2, SA200, and CSA270 are demonstrated in Figure 9a. The operating temperature-dependent gas-sensing responses toward 10 ppm NO2 for all thin-film sensors have the same trend. The responses of thin-film sensors toward 10 ppm first increase with temperature and approach at the maxima value at 275°C, and then gradually decrease with further increase in the operation temperature. Therefore, the optimal operating temperature of thin-film sensors was chosen to be 275°C herein. Next, the gas-sensing responses of both SA and CSA series thin-film gas sensors toward different concentrations of NO2 gas at 275°C are summarized in Figure 9b and c, respectively. All the thin-film gas sensors show the responses increased with NO2 gas concentration. Notably, the SA200 and CSA270 show the best performance in SA series and CSA series thin-film gas sensors, respectively. This might be associated with the differences in Ag2O content and microstructure of thin-film gas sensors induced by process parameters. Furthermore, the gas-sensing response of metal oxide semiconductor gas sensors is usually empirically represented as S = a
(a) Operating temperature-dependent 10 ppm NO2 gas-sensing response for SnO2, SA200, and CSA270 sensors. Gas-sensing response versus NO2 gas concentration for (b) SA series and (c) CSA series sensors at 275°C. The NO2 gas sensing prediction and linear fitting by log(S−1) versus log(c) plot for (d) SA200 and (e) CSA270 sensors.
Figure 10a–c presents the dynamic response/recovery curves of pristine SnO2, SA200, and CSA270 toward different concentrations of NO2 gas at 275°C. All the thin-film sensors show well dynamic response and recovery characteristics of sensor resistance with introduction and removal of NO2 gas, respectively. Notably, the gas-sensing responses of the SnO2 sensor to 1, 2.5, 5, 7.5, and 10 ppm NO2 gas were 1.55, 1.79, 1.89, 2.31, and 2.69, respectively. For comparison, the gas-sensing responses of SA200 to 1, 2.5, 5, 7.5, and 10 ppm NO2 gas were 1.89, 2.56, 3.24, 3.68, and 4.67, respectively. In addition, the gas-sensing responses of CSA270 to 1, 2.5, 5, 7.5, and 10 ppm NO2 gas were 2.24, 2.98, 3.95, 4.75, and 5.91, respectively. The SnO2–Ag2O composite thin-film sensors markedly demonstrate superior gas-sensing ability toward NO2 gas at the same test concentration in comparison with that of the pristine SnO2 sensor. Furthermore, these thin-film sensors obviously display different response/recovery speeds in single cycle dynamic response plots, for example: 80 s/300 s (SnO2), 35 s/188 s (SA200), and 28 s/168 s (CSA270) toward 10 ppm NO2 gas. Notably, the composite thin-film sensors have more detective sensitivity toward NO2 gas than the pristine SnO2 thin-film sensor, and the CSA270 exhibits superior detective sensitivity among various sensors. Furthermore, Figure 10d and e shows that the cyclic response/recovery curves of the SA200 and CSA270 sensors could be repeated after five test cycles without any deterioration toward 10 ppm NO2 gas, respectively. The SA200 and CSA270 sensors demonstrate excellent stability and reproducibility of the gas-sensing performance toward NO2 gas. The responses of CSA270 upon exposure to 100 ppm ethanol vapor, 100 ppm ammonia, and 100 ppm hydrogen gas at 275°C, are 3.41, 1.03, and 1.07, respectively (Figure 10f). When the comparative target gases of ethanol vapor, ammonia gas, and hydrogen gas had a higher concentration of 100 ppm, the sensing responses of the CSA270 sensor to these gases were still substantially lower than the response to 10 ppm NO2, revealing the superior gas-sensing selectivity of CSA270 toward NO2 gas in this study. Table 2 displays the comparisons of NO2 gas-sensing responses of various SnO2-based composite films. Comparatively, CSA270 exhibits superior NO2 gas-sensing performance among various proposed works [27,28,29,30,31,32,33]. Also, the higher NO2 gas-sensing response for several referenced sensors is not only attributed to their p–n or n–n heterojunction effects, but also their material system dimensional structures offer advantages over thin films in the presented work. The comparison list shows that the CSA270 is of potential for application in gas sensors to detect low-concentration NO2 gas.
Dynamic NO2 gas-sensing curves: (a) SnO2, (b) SA200, and (c) CSA270 sensors toward different NO2 gas concentrations from 1 to 10 ppm at 275°C. Cycling gas-sensing curves for the (d) SA200 and (e) CSA270 sensors toward 10 ppm NO2 gas, and (f) gas-sensing selectivity of CSA270.
Comparisons of NO2 gas-sensing responses of various SnO2-based composite films [27,28,29,30,31,32,33]
| Material | Synthesis method | Temperature/NO2 concentration | Response | Ref. |
|---|---|---|---|---|
| SnO2–NiO | Sputtering | 200°C/10 ppm | 2.25 (Rg/Ra) | [27] |
| SnO2–MoO3 | Sol–gel | 200°C/500 ppm | 3.75 ((Rg−Ra)/Ra) | [28] |
| SnO2–In2O3 | FSP method | 250°C/50 ppm | ∼3 (Rg/Ra) | [29] |
| SnO2–SnO | Sputtering | 60°C/50 ppm | ∼6 ((Ig−Ia)/Ia) | [30] |
| SnO2–NiO | VLS + deposition | 250°C/10 ppm | 6.5 (Rg/Ra) | [31] |
| SnO2–WO3 | GPR method | 300°C/5 ppm | 6.5 (Rg/Ra) | [32] |
| SnO2–WO3 | Vapor growth method | 300°C/5 ppm | 12 (Rg/Ra) | [33] |
| SnO2–Ag2O | Sputtering | 275°C/10 ppm | 5.91 (Rg/Ra) | This work |
In the cases of SA200 and CSA270 composite thin-film sensors, these sensors exhibit superior sensing performance than that of the pristine SnO2 sensor under the same gas-sensing test condition. The gas-sensing mechanism of SnO2 is generally ascribed to its n-type semiconducting behavior and is dominated by negative charge carriers (electrons) [9]. However, the Ag2O on the contrast shows p-type sensing behavior [11]. In the results of gas-sensing analysis, the SnO2–Ag2O composite thin-film sensors show the n-type behavior toward NO2 gas. It was attributed to the fact that quantity of phase content is a great deal of difference between the SnO2 and Ag2O elements [7,11]. In Figure 11a and b, except for the surface depletion region of thin-film sensors caused by the absorbed O2− species formed at the optimal operating temperature of 275°C [34], the establishment of the heterojunction formed at the interface between n-type SnO2 grains and p-type Ag2O particles is the reason for enhancing the NO2 gas sensibility. Once NO2 target gas molecules pass through the interface of these heterocontacts, they lead to electronic sensitization by modulating depletion layers at the surface and heterojunctions as exhibited in Figure 11c and d. Similar to the study of NiO–SnO2 composite for C2H5OH gas sensor, it indicates the p–n junction depletion region size in the composite will be modulated by interacting with C2H5OH vapor; therefore, enhanced gas-sensing performance is achieved [35]. The interaction between NO2 gas molecules with the depletion layer can be depicted by the electron capture nature of the oxidizing gas, in which NO2 gas acts as an acceptor of negative charge. As the result, compared with sensors exposed to air ambient (Figure 11a and b), the total depletion region originated from the exposed SnO2 surface and SnO2/Ag2O interfaces will be substantially thickened when composite sensors acted with NO2 gas and caused bulk resistance of the sensors increased (Figure 11c and d). This explains the superior NO2 gas-sensing ability of the SnO2–Ag2O composite thin-film sensors with suitable Ag2O particle loading content. The improved gas-sensing ability because of the formation of p–n heterojunctions in the composite systems has been shown in TiO2–Ag2O and In2O3–Ag2O [11,36]. Furthermore, because of Ag2O functionalization, the gas-sensing response and recovery times are reduced in the SA200 and CSA270 in comparison with those of pristine SnO2. This might be associated with the fact that Ag2O will act as a catalyst and reduce the activation energies for both adsorption and desorption of the surface target gas species; moreover, the spillover effect of the Ag2O nanoparticles can enhance both the adsorption of gaseous substances and the ability of surface diffusion [37]. Notably, the surface electron condition deeply influences the gas-sensing ability. The surfaces and cross-section structures of SA200 and CSA270 are different as shown in previous structural analysis. From the semiconducting nature, SnO2 has the main electron concentration for reacting with NO2; by contrast, Ag2O nanoparticles are depleted when SnO2/Ag2O heterojunction was formed. The network of electrons should pass through potential barriers at heterojunctions; moreover, the potential barriers at the SnO2/Ag2O contact points effectively modulate the electron transport between two contact electrodes of the sensor by adsorbing or desorbing NO2 gas molecules. The abundant SnO2–Ag2O heterojunctions in the SnO2 matrix for the CSA270 provide many potential barriers, and therefore, higher gas-sensing sensitivity was observed in CSA270 than that of SA200 toward NO2 gas in this study.
Possible gas sensing mechanisms of SA200 and CSA270 sensors: (a) SA200 in air, (b) CSA270 in air, (c) SA200 in NO2 gas, and (d) CSA270 in NO2 gas.
4 Conclusions
In summary, the thin-film structure toward gas sensor is very important and indispensable. Thus, how to investigate and integrate all the parameters relative to composite films for gas sensors is essential and is of potential interest to sensor material community. The two-layer structured SnO2–Ag2O composite film and mono-layered SnO2–Ag2O composite film were synthesized via two-step sputtering and co-sputtering processes, respectively. The structural and compositional analyses prove the successful formation of the SA and CSA series thin-film composite structure. The operating temperature-dependent NO2 gas-sensing tests reveal the optimal thin-film sensor operating temperature is 275°C. Furthermore, the NO2 gas-sensing performance of the SnO2–Ag2O composite film was substantially improved compared with that of the pristine SnO2 at 275°C. The catalytic nature of Ag2O and the formation of p–n junctions at Ag2O/SnO2 interfaces might account for the improved NO2 gas-sensing performance of the composite thin films. Comparatively, the mono-composite layer of SnO2–Ag2O exhibits superior NO2 gas-sensing performance than that of double-layered SnO2–Ag2O composite film. The experimental results herein serve as a sound reference for the design of a heterogeneous SnO2–Ag2O thin-film sensor based on the control of structure and surface condition, with the effective detection of low-concentration NO2 gas.
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Funding information: This research was funded by the Ministry of Science and Technology of Taiwan (Grant No. MOST 108-2221-E-019-034-MY3).
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Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
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Conflict of interest: The authors state that there is no conflict of interest.
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© 2022 Yuan-Chang Liang and Yu-Wei Hsu, published by De Gruyter
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