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Abstract
The effectiveness of a highly sensitive sodium dodecyl benzenesulfonate (SDBS)–TiO2 thin film optical waveguide gas sensor assessed in detecting various organic gases. Gas sensing measurements indicated that the sensing element has good selectivity, high sensitivity, and a low detection limit of 1 ppb to xylene gas with fast response and short recovery times. Interference gas test results showed that the sensitive component can detect 1 ppm of xylene gas in a mixed system containing other interfering gases, thus demonstrating the effectiveness of the proposed sensor for organic gas detection.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Volatile organic compounds (VOCs) are environmental pollutants that are harmful to humans and other living organisms. Xylene is the most representative and common indoor pollutant, because of its extensive use as a solvent in rubber, paint, leather, printing, and adhesive materials [1–3]. Prolonged exposure to xylene can induce serious health problems, such as nerve damage, eye irritation, tinnitus, colon cancer, rectal cancer, and lung cancer, even at low concentrations [4–7]. Therefore, the maximum allowable exposure to xylene gas is strictly limited worldwide. For example, China’s limit is 0.2 mg/m3 (∼42 ppb) (GB/T 18883-2002) [8]. Hence, the fabrication of highly sensitive xylene gas sensors has become increasingly important. Various metal oxide materials, such as WO3 [9], NiO [10], ZnO [11], V2O5 [12], and TiO2 [13], have been used in the fabrication of xylene gas sensors owing to their good physico-chemical properties. Research on metal oxide (MOX) gas sensors has been extensively carried out.
TiO2 is the most commonly used material in automotive applications owing to its excellent gas sensing properties and chemical stability at high temperatures [14–17]. TiO2 is a typical n-type semiconductor metal oxide with a bandgap of 3.2 eV [13]. Therefore, it has a good photochemical activity in the visible region. Gas sensors based on TiO2 have been proposed for the detection of NO2 [18], acetone [19], NH3 [20], CO [21], and ethanol [22]. However, the gas sensing performance of such sensors is limited because of their low sensitivity and high working temperatures (which may be due to the low surface area and poor surface structure). Hence, it is necessary to develop highly sensitive gas sensors capable of detection at room temperatures.
Optical evanescent wave sensors have been widely used for humidity sensing [23], chemical sensing [24], biochemical sensing [25], and biosensing [26], because they are immune to electromagnetic interference and can be used in harsh environments [27–29]. The detector synthesized in this study is a planar optical waveguide (OWG) gas sensor based on the evanescent wave principle. Recently, there has been an interest in the application prospects of planar OWG sensors in detecting toxic gases, because planar waveguides present several advantages over conventional transmission devices for gas detection, particularly in terms of the sensitivity [30]. This type of sensor requires a sensitive material to be evenly laid on the substrate surface of the sensing element, to prepare the sensing material. In a previous work conducted by our group, we found that OWG sensitive materials with titanium dioxide need to be calcined at high temperatures. During the process of calcination, titanium dioxide forms a compact arrangement on the surface of tin-doped glass; this reduces the contact area between the gas and the sensitive material to some extent, thus affecting the gas sensing properties of the material [31]. The van der Waals attraction between the different particles can be effectively overcome by utilizing the hydrophobic and hydrophilic groups of the surfactant to form a steric hindrance; thus, the nanoparticles can be evenly dispersed in the base solution, and the size of the nanoparticle aggregates can be reduced [32–34]. Based on the above analysis, sodium dodecylbenzene sulfonate (SDBS), which is a commonly used inexpensive material, was selected as a dispersing agent to ultrasonically disperse titanium dioxide powder; SDBS not only improves the agglomeration of titanium dioxide, but also acts as a film-forming agent, because SDBS solution has a lower surface tension. The gas sensing property of the sensitive material was assessed toward different gases at ambient. To evaluate the gas sensing performance of the sensitive element in the presence of interfering gases, we simulated interfering gases and tested the sensitivity of the device. We explained the possible related mechanisms by illustrating the structure and morphology of the TiO2-based gas sensing device.
2 Experimental
2.1 Synthesis of TiO2 nanoparticles
The TiO2 nanoparticles were synthesized using a sol-gel method [35]. Initially, tetra-n-butyl titanate (C16H36O4Ti, 5 ml) was dispersed in 20 ml of ethanol (C2H5OH), and 1 ml of acetic acid (CH3COOH) was added. The mixture was magnetically stirred at room temperature and kept for 40 min. Deionized water (2 ml) was then added, and the mixture was stirred for another 5 min and left to mature for one day. The resulting sample was dried at 80 °C and grinded. The white TiO2 precursor obtained was calcined at 450 °C for 2 h in air to prepare TiO2 nanoparticles. All the reagents used were of analytical grade.
2.2 Preparation of SDBS-TiO2 nanoparticle suspension and sensing element
Different concentrations of the SDBS solution were configured. The SDBS–TiO2 nanoparticle suspension was prepared by dissolving 0.03 g of TiO2 powder in the solution by ultrasonic dispersion (under different conditions).
Before fabricating the sensing element, Sn-doped glass was washed with acetone and ethanol. To prepare the sensitive element, the suspension was coated on the Sn-doped glass surface by spin coating and dried at room temperature.
2.3 Gas detection device
The primary components of the OWG gas testing system present the Fig. 1, all tests were performed at room temperature. The planar OWG sensing element comprises a cladding (SDBS-TiO2), a waveguide layer((Sn-doped layer), and a substrate, The laser beam (λ = 650 nm) was coupled into the waveguide layer (Sn-doped layer) using a prism [36,37]. Light in the guided wave layer appears the total reflection of the evanescent wave after passing into the sensitive layer, when a laser beam of certain wavelength passes through the first prism and into the Sn-diffused guided layer [38]. For each measurement, a fresh syringe was used to inject 20 cm3 of the gas sample into the pipeline of the system (speed: 1 cm3/0.3 s), after which it was vented.
3 Result and discussion
3.1 Characterization of TiO2 powder and SDBS–TiO2 thin film
3.1.1 X-ray diffraction (XRD)
Figure 2 shows the XRD (Cu Kα source) patterns of the calcined, nanostructured TiO2 powder. TiO2 exhibits a characteristic reflection peak with planes at 25.1° (101), 37.7°(004), 47.7°(200), 53.8°(105), 55°(211), 62.6°(204), 68.4°(116), 70.2°(220), and 75°(215); their peak positions are consistent with the JCPDS Card #21-1272 for the pure tetragonal anatase phase of TiO2 [39], indicating that the titanium dioxide obtained was pure.
3.1.2 X-ray photoelectron spectroscopy (XPS)
The species and valency of the elements in the material were determined by XPS spectroscopy (shown in Fig. 3). The XPS survey spectrum of TiO2, shown in Fig. 3(a), suggests that the surface of TiO2 contains Ti, O, and carbon elements. Figure 3(b) shows the Ti2p core level XPS spectrum of the TiO2 powder, where the two broad peaks at 458.88 and 464.88 eV belong to Ti2p3/2 and Ti2p1/2, respectively. The splitting between the two core levels, i.e., Ti2p3/2 and Ti2p1/2, is 6 eV, indicating a +4 valence state for Ti, in the anatase TiO2, in an octahedral coordination with oxygen [40,41]. Figure 3(c) shows the O1s score level spectrum; the main curve with a binding energy of 530 eV is assigned to the oxygen in the TiO2 lattice (i.e., Ti-O-Ti) [39]. The second curve with a binding energy of 530.68. eV is attributed to the hydroxyl group (-OH) on the TiO2 surface. The peaks at 532.38 and 529.78 eV may be due to the C-O and C = O groups from the CO2 adsorption species of adventitious carbon (consistent with the C1s species located at 286.18 and 288.68 eV in Fig. 3(d)) [42–44].
3.1.3 Field emission scanning electron microscopy (FESEM)
The TiO2 powder and SDBS–TiO2 thin-film samples are further characterized using field emission scanning electron microscopy (FESEM, JEOL JSE-7500F). Figure 4(a) shows the FESEM images of the morphology of the TiO2 powder. As shown in Fig. 4(a), the agglomeration of the synthesized TiO2 powder is significant, and the size of the agglomerates is approximately between 10 and 2 µm. Figure 4(b) shows that the synthesized TiO2 sample has an average particle size of 30 nm.
3.2 Gas sensing performance of SDBS–TiO2 OWG element
3.2.1 Selection of OWG light source
The transmission of the SDBS–TiO2 thin film was monitored using an ultraviolet spectrophotometer (UV-2450 Japan) after exposure to different gases (xylene, toluene, methanol, isopropanol, trimethylamine, and dimethylamine) with the same concentration (1000 ppm). Figure 5 shows the results, where the change in the transmission is similar in the region of 300–800 nm. However, it is widely known that an OWG needs to exhibit high transmission in the optical spectral range. Hence, based on the transmittance and intensity of the laser, the semiconductor laser at λ = 650 nm was selected as the detection light source of the OWG system.
3.2.2 Optimization of the preparation conditions of sensitive elements
We optimized the preparation conditions because of the varied sensing performances due to the different preparation conditions employed for the sensitive element. The main factors affecting the gas sensing performance of the sensitive components include the mass fraction of the SDBS, dispersion time, dispersion temperature, and rotation speed. These parameters were optimized. Figure 6 shows the experimental results. The gas sensing performance of the sensitive element was optimum when the SDBS mass fraction was 1%, the dispersion time was 120 min, the dispersion temperature was 45 °C, and the rotation speed was 2000rpm. Therefore, in the following experiments, these were used as the optimal film forming conditions for the sensitive components (ΔI = Igas-Iair, where Igas and Iair are the signal intensities of the gas and ambient air, respectively). Table 1 lists the preparation conditions for the sensitive thin film.
Fig. 6. Preparation conditions for SDBS–TiO2 thin film/Sn-doped OWG sensor: (a) mass fraction of SDBS, (b) dispersion time, (c) dispersion temperature, and (d) rotation speed for the selective response to 1000 ppm of gas analytes.
Table 1. Preparation conditions for the sensitive thin film.
3.2.3 Gas sensitivity test of sensitive element
To determine whether SDBS affects the gas sensing performance of the sensitive materials, we determined the gas sensing performance of the SDBS thin film/Sn-doped OWG sensitive component by exposing it to different target gases with a concentration of 1000 ppm. Figure 7 shows the results. As OWG sensitive components with SDBS do not respond to the gas being tested, we can conclude that SDBS does not affect the selectivity of TiO2.
Fig. 7. Dynamic response of the SDBS thin-film/Sn-doped glass OWG sensor to 1000 ppm of gas analytes.
3.2.4 Response mechanism and selective response of sensor element
In the OWG sensor, the output light intensity (transmitted light intensity) is related to the refractive index of sensing film, thickness of the sensing film, as follows [45]:
Here, I is the output light intensity (transmitted light), I0 is the input light intensity, α is the absorption coefficient, N is the refractive number of the guided light on the surface of the OWG with length L, and de represents the real distance of light propagation. This relationship indicates that when the refractive index, and thickness of the sensitive layer increase, the output light intensity decreases; the intensity of the output light increases when the refractive index, a of the sensitive layer decreases. However, in order to make the response time of the optical waveguide gas sensor faster and the hysteresis time as small as possible, the thickness of the sensitive film should be designed as $\textrm{de}\sin {\theta _v}/2$, ${\mathrm{\theta }_v}$ is Modulus angle of v-order waveguide mode [46].The surface sensitivity (SOWG) of an OWG gas sensor can be expressed as follows [47]:
Figure 8 shows the surface of the film laid after ultrasonically dispersing the titanium dioxide powder in the SDBS solution. Compared with the morphology shown in Fig. 4(a), the agglomeration of the TiO2 powder is significantly improved after dispersion. Figure 8 shows that the agglomerates are not adhered to each other and that the agglomerate size distribution is uniform with an average size of approximately 1 µm. The agglomeration phenomenon of TiO2 is clearly improved, indicating that the re-aggregation of the TiO2 aggregates is effectively ameliorated, with many pore structures at the interface of each agglomerate. This increases the active on the surface of the sensitive film and the possibility of contact between the sensitive element and the gas, thus improving the gas sensing performance of the sensitive element.
Figure 9 shows the change in the output light intensity when the sensitive component contacts different gases with a concentration of 1000 ppm, at room temperature. The sensitive element has a good response to xylene with response and recovery times of 4 and 61 s, respectively.. The response of the sensor to the other gases is weaker, indicating that the sensitive element has a better selectivity toward xylene.
Fig. 9. Dynamic response of the SDBS–TiO2 thin-film/Sn-doped glass OWG sensor to 1000 ppm of gas analytes.
The planar waveguide sensor effect can be attributed to the interaction between the evanescent wave of the guided mode and the sensitive material. The evanescent wave changes the refractive index distribution near the waveguide surface. Thus, the effective refractive index of the guided mode is modified. Moreover, the transmitted light intensity is altered because the refractive index of the sensitive material is modified when the material is exposed to different vapor gases [48].
The sensitive component exhibited a good selective response to xylene. This can be attributed to the higher electronic cloud density of xylene gas over the other gases tested in this study, leading to a stronger electric field around it. Hence, according to Eq. (1), when xylene gas interacts with the sensitive materials, the electric field intensity around the waveguide layer changes significantly. Therefore, the sensitive component exhibits a high sensitivity to xylene gas.
As shown in Fig. 10, the sensitive element can detect 1 ppb of xylene gas, where the signal-to-noise ratio (S/N) is 4. Figure 11 shows the parallel test results in the form of a Y-axis error bar graph, where the standard deviation of the output light intensity measured five times represents the Y-axis error. Figure 11 shows that ${\textrm{I}_\textrm{g}} \cdot {\Delta }\,{\bar{\rm I}}$ and xylene gas concentration exhibit a good linear relationship, following the equation: y = (3.88 ± 0.0385) + (0.1732 ± 0.0059)x, with R2 = 0.9930, n = 5.
Fig. 10. Typical response of the SDBS–TiO2 thin film/Sn-doped glass OWG sensor when exposed to xylene vapor in air.
3.3 Gas sensing performance of sensitive components in the presence of interference gases
To evaluate the gas sensing performance of the sensitive components in the presence of interfering gases, we selected homologues of xylene gas (benzene, toluene, chlorobenzene, and styrene) with the same concentration as interference gases and carried out a simulation experiment. Initially, we maintained the same concentration of xylene gas and varied the concentrations of the interfering gases (Table 2). Figure 12(a) shows that the gas sensing property of the sensitive components is less affected when the interference gas concentration is reduced to 10 ppm. Figure 12(b) shows the five parallel test results (RSD ≤ 5.75). Subsequently, keeping the concentration of the interfering gas constant, we gradually reduced the concentration of xylene gas (Table 3). Figure 13 shows the results. When the concentration of xylene gas is 0.1 ppm, we cannot determine whether xylene gas is present in the mixture of gases, indicating that 1 ppm is the lower limit of the sensor element for xylene gas in the presence of interference gases. The output light intensity and the increasing concentration of xylene (in the concentration range of 1–1000 ppm) follow the equation: y = (3.827 ± 0.0347) + (0.1063 ± 0.0077)x, with R2 = 0.9845, n = 5. Figure 14 shows the results.
Fig. 12. (a) Spectrum of sensitive film in response to different concentrations of interference gases (with the xylene gas concentration kept constant), and (b) parallel test (n = 5).
Fig. 13. Spectrum of the sensitive film in response to mixed gases (with the interfering gas concentration kept constant and the concentration of xylene gas gradually reduced).
Table 2. Interference gas concentrations corresponding to various tests (Fig. 12(b)).
Table 3. Xylene gas concentrations corresponding to various tests (Fig. 13).
The proposed OWG gas sensor is more simpler than the other types of OWG gas sensors [49–51]; the preparation of the sensitive elements is straightforward; the sensor exhibits a faster response, better recovery times, and can detect gases at low levels. Table 4 gives a summary of the sensing performances of various TiO2 nanostructure-based gas sensors toward xylene. TiO2 and some TiO2-based composites have a poor gas response to xylene [13,52–56]. Moreover, most of the reported sensors have a high operating temperature and a poor response property. The detection limit of the proposed sensor is also better than the other sensors.
Table 4. Comparison of various titanium oxide nanostructure-based gas sensors in terms of xylene sensing performance.
4. Conclusions
The gas-sensing property of an SDBS–TiO2 thin film-based OWG gas sensor was studied. The structures and morphologies of the TiO2 powder and SDBS–TiO2 thin film was characterized by XRD, XPS, and FESEM. The experimental results showed that the sensor element exhibits a fast response and a high sensitivity. It can detect xylene gas at a concentration as low as 1 ppb, with response and recovery times of 4 and 61 s, respectively. Moreover, even in the presence of interfering gases, the sensor element can detect 1 ppm of xylene gas. Therefore, this type of OWG gas sensor is expected to be quite useful in detecting xylene in the future.
Funding
National Natural Science Foundation of China (21765021).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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