Citation

Qiuni Zhao, Zaihua Duan, Zhen Yuan, Xian Li, Si Wang, Bohao Liu, Yajie Zhang, Yadong Jiang, Huiling Tai. High performance ethylene sensor based on palladium-loaded tin oxide: Application in fruit quality detection[J]. Chin. Chem. Lett., 2020, 31(8): 2045-2049  

Qiuni Zhao^a, Zaihua Duan^a, Zhen Yuan^a, Xian Li^b, Si Wang^a, Bohao Liu^a, Yajie Zhang^a, Yadong Jiang^a, Huiling Tai^a,*     

Ethylene (C₂H₄) as a colorless and odorless phytohormone is closely related to the maturity and freshness of fruit [1]. For example, low concentration of C₂H₄ (from 10 ppb to 10 ppm) would be released from immature fruit, and C₂H₄ also triggers ripening genes resulting in changes in texture, color and taste of fruit [2]. With the fruit maturing, the C₂H₄ production increases gradually to over 100 ppm. Thus, the emission of C₂H₄ can be used as an indicator of fruit maturity [3]. Furthermore, excess C₂H₄ during fruit storage can accelerate respiratory rates of fruits resulting in senescence and even spoilage [4]. Therefore, in order to evaluate the quality of fruit efficiently and intelligently, it is important to develop a high performance gas sensor for detecting C₂H₄ concentration at trace level.

Up to now, various techniques, including optical, electrical and mass-sensitive methods, have been applied for C₂H₄ detection. Among them, gas chromatography [5], fluorescence [6], photoacoustic spectroscopy [7] could achieve high-sensitive detection of C₂H₄, but the expensive cost, bulky and non-portability of these methods largely impede their application in fruit detection. Instead, the resistive-type sensor are worth exploring because of its simple and affordable. Due to the limited physiochemical reactivity of C₂H₄, C₂H₄ sensors working at room temperature have poor performance [8-11]. In order to accelerate the adsorption and desorption of nonpolarC₂H₄molecules on sensitive materials, it is necessary to use assist antmethods such as heating [12] or ultraviolet irradiation[13]. Several C₂H₄ sensors based on heating have been reported in the literature [14-20]. For example, Kathirvelan et al. studied the C₂H₄ sensing properties of titanium dioxide and tungsten trioxide (TiO₂-WO₃) nanocomposites at 250 ℃, the minimum achievable concentration was 8 ppm for continuous detection and the response time is unknown [14]. Nimittrakoolchai et al. reported a WO₃ gas sensor prepared by precipitation method, the detection range is 3–10 ppm and the response time is about a few minutes for 3–5 ppm C₂H₄ [15]. Wang et al. employed a porous zinc oxide (ZnO) nanosheets based C₂H₄ gas sensor, it is of 5 ppm detection limit and the response less than 2 at 500 ℃ [16]. Progress has been made in resistive-type C₂H₄ sensors, but the detection limit, response value and response time still need to be improved.

In order to enhance the performance of gas sensor, using noble metals (e.g., Pt [17], Au [21], Ag [22] and Pd [23]) loaded metal oxide semiconductors (MOS) is a prevalent strategy. In particular, as a potential C₂H₄ acceptor and catalyst [24-26], Pd loaded MOS is expected to achieve high response, fast speed and low detection limit. Herein, Pd-loaded tin oxide (SnO₂) is used to prepare C₂H₄ gas sensor. The C₂H₄ sensing properties are investigated at 50–400 ℃, and the C₂H₄ gas sensing mechanism is discussed in this paper. Particularly, the potential application of Pd-loaded SnO₂ C₂H₄ sensor has been developed in fruit detection.

The Pd-loaded SnO₂ C₂H₄ sensor (Fig. S1a in Supporting information) was fabricated by simple coating method, and the details referred to the supplementary material. The micromorphologies, lattice images, crystallization and chemical valence states of pristine SnO₂ and Pd-loaded SnO₂ were characterized by various techniques including scanning electron microscopy (SEM, Zeiss, SUPRA 55), transmission electron microscope (TEM, FEI, Tecnai G2 F20 S-TWIN), X-ray diffraction (XRD, Bruker, D8 Advance) with a range of 2θ from 20° to 80° at 40 kV and 40 mA using Co-Kα as the irradiation source (λ = 0.1789 nm), and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, K-Alpha), respectively.

The C₂H₄ sensing performance of sensors were measured by a dynamic homemade system with the Keithley 2700 data acquisition instrument and a regulated power supply (Fig. S1b in Supporting information). The operating temperature of sensor was varied between 50 ℃ and 400 ℃. In order to simulate the conventional atmospheric environment, the testing relative humidity (RH) is controlled at 51.9% by air with bubbling distilled water [27, 28], and calibrated by a high-accuracy hygrometer (CEM, DT-625, Shenzhen Everbest Machinery Industry Co., Ltd., China). The response of gas sensor is calculated as following equation: Response = R_a/R_g (R_a: resistance under the air, R_g: resistance under the target gas). The sensitivity is defined as the slope of response– concentration linear fitting curve. And the response/recovery times are defined as the time taken by C₂H₄ gas sensor to achieve 90% of the total resistance change in the case of adsorption and desorption respectively.

The SEM images of pristine SnO₂ and Pd-loaded SnO₂ are shown in Fig. S2 (Supporting information). It can be seen that both of them are composed of nanoparticles, but there is no obvious difference in microscopic morphology of two SEM images. In order to confirm the existence of Pd, the XRD patterns of pristine SnO₂ and Pdloaded SnO₂ are characterized in Fig. 1a. All the diffraction peaks of SnO₂ can be well coincided with JCPDS card No. 41-1445 and match the tetragonal rutile structure [29], the diffraction peaks of Pd are tally with JCPDS data No. 46-1043, which reflect the face-centered cubic crystalline structure [30]. Compared with the prominent peaks of SnO₂, the weak diffraction peaks of Pd indicate that the trace existence of Pd in composite film. The TEM images of Pdloaded SnO₂ are performed as shown in Figs. 1b and c. The lattice fringes with an inter-planar distance of 0.33 nm coincide well with the (110) plane of SnO₂ in Fig. 1c. And the lattice distance of 0.22 nm is correspond to the (111) crystalline plane of Pd, which further suggests that the Pd-loaded SnO₂ nanoparticles have been successfully synthesized from the micro-morphology. It is beneficial to regulate the interface barrier between SnO₂ and Pd [31].

Figure 1

	Download: JPG larger image
Fig. 1. (a) XRD patterns of pristine SnO2 and Pd-loaded SnO2. (b) TEM image of Pd-loaded SnO2. (c) HRTEM image of Pd-loaded SnO2. (d) XPS fully scanned spectra. XPS spectra: O 1s spectra of (e) pristine SnO2 and (f) Pd-loaded SnO2.

The surface compositions and chemical valence states are analyzed by XPS. Fig. 1d is the full range XPS spectra of pristine SnO₂ and Pd-loaded SnO₂. Fig. 1e displays the O 1s spectra of SnO₂ and the binding energies at 530.2, 531.5 and 533.3 eV can be assigned to lattice oxygen species (O_L), oxygen vacancy (O_V) and the chemisorbed oxygen species (O_C, H₂O species and impurity oxygen) on the surface of sensing materials, respectively [32-34]. The oxygen vacancy has significant influence on the gas sensing properties of resistance-type MOS [33]. Compared Fig. 1f with Fig. 1e, the content of oxygen vacancy in Pd-loaded SnO₂ (40.3%) is higher than pristine SnO₂ (30.6%). The increase of oxygen vacancies of Pd-loaded SnO₂ is conducive to C₂H₄ reaction and further enhance the sensing performance [34]. The higher content of chemisorbed oxygen (15.3%) in pristine SnO₂ could be due to the adsorption of water molecules [35, 36]. The high-resolution XPS spectra of Pd 3d is shown in Fig. S3 (Supporting information), the Pd 3d _5/2 electron binding energies in palladium oxide (PdO) and metallic Pd are 335.9 and 334.5 eV, respectively [31]. The contents of PdO of Pd-loaded SnO₂ are calculated to be 58%, it is confirmed that Pd nanoparticles are easy to oxidize intoPdO in air due totheir strong electron transport ability. Moreover, the conversion of Pd and PdO in the oxygen environment plays an important role in MOS sensitization [24].

The working temperature has great influence on the response of MOS based gas sensors [37]. In order to investigate the relationship between operating temperature and the C₂H₄ sensing properties, the response of sensors based on pristine SnO₂ and Pd-loaded SnO₂ to 10 ppm C₂H₄ at operating temperatures ranging from 50 ℃ to 400 ℃ are shown in Fig. 2a. As can be seen, the addition of Pd reduces the optimum working temperature of gas sensor from 350 ℃ to 250 ℃. The phenomena of the increasing sensing response and the decreasing working temperature for SnO₂ are consistent with the previous researches by Zhang's group [38], which directly verifies the promotion effect of Pd nanoparticles. Fig. 2b displays the realtime resistance variation curves of two gas sensors to 20–100 ppm C₂H₄ at 250 ℃. When exposed to 100 ppm C₂H₄, the response of Pdloaded SnO₂sensor(11.1)is about 3 times higher than that of pristine SnO₂ (3.5). Moreover, the response time is also significantly shortened from 7 s to 1 s compared with pristine SnO₂. As shown in Fig. 2c, both of two sensors exhibit good linearity, but the Pdloaded SnO₂ gas sensor holds a larger sensitivity (0.05 ppm^-1) than that of pristine SnO₂ one (0.02 ppm^-1). Fig. 2d displays the linear fitting curves of two sensors to low concentration C₂H₄ (0.05–1 ppm), the Pd-loaded SnO₂ gas sensor exhibits better sensitivity (0.58 ppm^-1) with excellent linearity (R² = 0.9963). Especially, the Pd-loaded SnO₂ gas sensor achieves ultra-low detection concentration (50 ppb) (the inset of Fig. 2d). The real-time resistance variation curves of two sensors corresponding to Fig. 2d are shown in Figs. S4 and S5 (Supporting information). Fig. 2e exhibits the response and recovery curves for three cycles when the Pd-loaded SnO₂ gas sensor is exposed to 100 ppb, 1 ppm and 40 ppm C₂H₄ concentrations. As can be seen, the response and recovery curves can switch to the steady states after the C₂H₄ level changed, indicating that the sensor shows good repeatability in the continuous measurements. Fig. 2f presents the good stability and durability of the Pd-loaded SnO₂ gas sensor for low concentration C₂H₄ inaweek. Moreover, the other sensing performance(humidity, selectivity) of Pd-loaded SnO₂ gas sensor are studied in supplementary material. Compared with previous works [8-11, 14-20], the C₂H₄ sensor based on Pd-loaded SnO₂ shows high performance in terms of the response value (R_a/R_g = 11.1 for 100 ppm), detection limit (≤50 ppb) and response speed (1 s for 100 ppm), which are listed detailly in Table 1.

Figure 2

Download: JPG larger image

Fig. 2. (a) Response of the pristine SnO2 and Pd-loaded SnO2 sensors to 10 ppm C2H4 at different working temperature. (b) Real-time resistance variation curves and (c) linear fitting curves of response for pristine SnO2 and Pd-loaded SnO2 sensors to 20–100 ppm C2H4 at 250 ℃. (d) Linear fitting curves of response to 0.05–1 ppm C2H4 at 250 ℃, the inset of (d) is the resistance variation curves to 50 ppb C2H4. (e) Repeatability curves. (f) Stability curves in a week.

Table 1

Table 1 Gas sensing properties of reported resistive-type C2H4 gas sensors.

Table 1 Gas sensing properties of reported resistive-type C2H4 gas sensors.

A plausible explanation of C₂H₄ sensing mechanism for Pdloaded SnO₂ sensor is discussed as follows. As shown in Fig. 3a, since the work function of Pd (5.12 eV) is higher than that of SnO₂ (4.5 eV), the Schottky junction will be formed at the interface of Pd and SnO₂, which causes the part of electrons transfer from SnO₂ to Pd [39]. It is beneficial to the formation of depletion region around SnO₂. When the Pd-loaded SnO₂ gas sensor is exposed to air (Fig. 3b), oxygen molecules combine with electrons to form adsorbed oxygen (O^- at 250 ℃) on the surface area of Pd-loaded SnO₂, resulting in a thick electron-depleted layer [40]. Furthermore, Pd (PdO) as a catalyst activates the dissociation of oxygen molecules and enhances the adsorption activity of oxygen on the surface of SnO₂ [41]. The increase of oxygen vacancy of Pd-loaded SnO₂ is also proved by XPS analysis. The thicker electron-depleted layer results in the increase of air resistance of SnO₂ after Pd loading [24], which is beneficial to improve the C₂H₄ sensing response of SnO₂ sensor.

Figure 3

	Download: JPG larger image
Fig. 3. Schematic diagram of C2H4 sensing mechanism for Pd-loaded SnO2 gas sensor. Schottky barrier in (a) vacuum, (b) air and (c) C2H4.

When the Pd-loaded SnO₂ gas sensor is exposed to C₂H₄ gas (Fig. 3c), C₂H₄ molecules with a carbon-carbon double bond (C = C) are oxidized to ethylene oxide or acetaldehyde, then further reacted to produce carbon dioxide (CO₂) and water (H₂O), and the reaction process can be shown as follows [42]:

(1)

(2)

The process of C₂H₄ oxidation releases the trapped electrons back to the conduction band, leading to the reduction of depletion region and the decrease in resistance. As the potential receptors of C₂H₄, Pd nanoparticles can increase the number of efficient adsorption sites of C₂H₄ molecules near the surface of sensitive film [26]. Moreover, adsorbed C₂H₄ molecules on Pd will spillover onto the surface of SnO₂ because of the spillover effect [43], leading to the high sensing response. In summary, the C₂H₄ gas-sensing mechanism of Pd-loaded SnO₂ can be explained by depletion layer theory, the catalysis and receptor function of Pd.

The C₂H₄ emission can be used as an indicator to estimate the coloration, texture, and flavor taste of fruits [44]. Inspired by the demand for fruit freshness and ripeness monitoring, potential applications of Pd-loaded SnO₂ gas sensor are explored. The temperature and humidity of the laboratory are controlled at 25±2 ℃ and 50%±5% RH by an air conditioner, and recorded by the high-accuracy hygrometer. Fig. 4 displays the response curves in the application of fruit quality detection. In order to simulate the storage conditions and observe morphological changes, fruits are put into a transparent bottle packed with porous plastic wrap to avoid fermentation, and then place the gas sensor at the bottle mouth during detection. Fig. 4a shows the real-time response curves of the gas sensor to four fresh fruits (banana, lemon, apple and pear), the weight of each fruit is about 120–150 g. It can be seen that the Pd-loaded SnO₂ gas sensor presents different responses to different fruits. Fig. 4b displays the increasing response to gradually increasing banana weight (about 25 g, 50 g, 75 g, 100 g and 125 g). The nonlinear increase indicates that C₂H₄ emissions are mutually reinforcing [2]. To further support potential applications in fruit storage, Fig. 4c presents the variation in response to one banana at different ripening stages. As a comparison, the same test to lemon is shown in Fig. 4d. The results show that the response of banana changed significantly from non-climacteric to climacteric, while that of lemon changed smoothly. As a respiratory climacteric fruit, banana is susceptible to the influence of C₂H₄ concentration in senescence and metamorphosis, while the non-respiration climacteric lemon is more resistant to storage than banana due to the slow metabolism of lemon, which is consistent with the experimental results. In short, the above tests demonstrate that different species, quantities and storage states of fruits can be estimated by the Pd-loaded SnO₂ gas sensor like a smart nose, which suggest that the potential applications of Pd-loaded SnO₂ gas sensor in fruits quality evaluation.

Figure 4

	Download: JPG larger image
Fig. 4. Potential applications of the Pd-loaded SnO2 gas sensor: real-time response curves to (a) various fruits, (b) bananas with different quantities, (c) one banana in half a month, (d) one lemon in half a month.

In summary, a Pd-loaded SnO₂ C₂H₄ sensor for fruit quality evaluation is fabricated by coating method. The high sensing performance of Pd-loaded SnO₂ C₂H₄ sensor are obtained at 250 ℃, it is of high response (11.1 for 100 ppm), fast speed (1 s for 100 ppm), good linearity (R² = 0.9804 for 20-100 ppm), low detection limit (50 ppb) and good stability. The sensing mechanism of Pd-loaded SnO₂ sensor is discussed by the depletion layer theory, the catalysis and receptor function of Pd. Furthermore, the applications of the Pd-loaded SnO₂ sensor in detecting fruit species, quantities and storage states are explored. The results show that the prepared Pd-loaded SnO₂ sensor has great potential for developing high-performance C₂H₄ sensor in fruit quality monitoring.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work is supported by the National Science Funds for Excellent Young Scholars of China (No.61822106), National Science Funds for Creative Research Groups of China (No. 61421002), Natural Science Foundation of China (No. 61671115) and Central Public-interest Scientific Institution Basal Research Fund (No.Y2019XK18).

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.04.032.