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Pt-decorated rGO nanosheets composite film for detection of acetone
Sensors and Actuators B 255 (2018) 1482–1490
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Research Paper
Fully gravure-printed WO3 /Pt-decorated rGO nanosheets composite film for detection of acetone Lu Chen, Lei Huang ∗ , Youjie Lin, Liman Sai, Quanhong Chang, Wangzhou Shi, Qi Chen Joint Lab with Wuhu Token for Graphene Electrical Materials and Application, Department of Physics, Shanghai Normal University, Guilin Road 100, Shanghai 200234, China
a r t i c l e
i n f o
Article history: Received 2 March 2017 Received in revised form 13 August 2017 Accepted 19 August 2017 Available online 24 August 2017 Keywords: Acetone gas sensor Printed sensor rGO nanosheets WO3
a b s t r a c t With the emergence of non-invasive disease diagnosis, the development of a specific volatile organic compounds (VOCs) gas sensors in exhaled breath is on-going. Described herein is a flexible and lightweight chemiresistor made of a thin film based on WO3 /reduced graphene oxide (GO) nanosheets decorated with Pt nanoparticles, which were printed onto a flexible polyamide substrate by using gravure technique. In order to study the effect of graphene size on the performance of gas sensing, graphene with two sized such as large-sized reduced GO microsheets (GMs) and small-sized reduced GO nanosheets (GNs) were synthesized and used as modified materials incorporated into WO3 -based system. The printed WO3 /PtGNs sensor exhibits a good selectivity, a high response of 12.2 to 10 ppm acetone as well as a fast gas response/recovery time (14.1/16.8 s) at the operating temperature of 200 ◦ C. The sensing performance is attributed to the exposed (002) facets of WO3 which were not wrapped into GNs, the massive p-n junction active sites at the WO3 /GNs interface, as well as catalytic Pt nanoparticles. The printed WO3 /Pt-GNs sensor is a promising candidate for selective, fast and sensitive detection of acetone at relatively low operating temperature. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs) greatly threaten human health because these gases can adversely affect brain, nervous, endocrine and skin systems [1,2]. Among these VOCs, acetone gas is toxic to various organ systems and has been identified as a specific biomarker of diabetes [3]. The concentration of exhaled acetone from a healthy individual should be below 0.9 ppm and the concentration of more than 1.8 ppm could indicate diabetes [4,5]. However, an accurate detection of acetone in a complex environment of exhaled breath is quite difficult and very important for the fast and simple recognition of diabetes. Therefore, it is demanding to develop selective, low-power, sensitive, flexible and portable devices to detect acetone gas at ppb levels for non-invasive diagnosis of diabetes. The semiconducting metal oxide WO3 , especially hexagonal WO3 (h-WO3 ) exposed with (002) facets, has been proved to be one of the most efficient gas-sensing materials for acetone detection due to a strong interaction between acetone molecules and h-WO3 having high dipole moment on (002) facets [6]. However, the sensing performances including selectivity, sen-
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (L. Huang). http://dx.doi.org/10.1016/j.snb.2017.08.158 0925-4005/© 2017 Elsevier B.V. All rights reserved.
sitivity and operating temperature of h-WO3 still need to be further improved to meet the requirement of the low-power portable sensor device for health monitoring. Hence, a variety of strategies have been developed to design WO3 based composite by introducing other materials to enhance sensing performance of WO3 devices [7–9]. Recently, graphene has been widely studied as a new additive material of semiconducting metal oxide composites due to its large specific surface and high carrier mobility, thereby potentially providing superior sensitivity and rapid response time [10–14]. Metal oxides mixed with graphene have been demonstrated to be an effective strategy for enhancement of acetone sensing performances [15–17]. For example, Chao Wang et al. [15] demonstrated that modificated CuO-ZnO composite with reduced graphene oxide (GO) sheets could improve the gas response to acetone. The gas response value reached from 9.4 to 10 ppm of acetone, almost 1.5 times higher than pure CuO–ZnO. Seon-Jin Choi et al. [16] found that the WO3 hemitube decorated with graphene caused a 6.45fold enhanced response relative to pristine WO3 . However, the size of graphene sheets in their studies is in the micron range [15–17], these large-sized reduced GO microsheets (GMs) may wrapped around the metal oxides, resulting in a decreasing amount of active sites. Therefore, small-sized reduced GO nanosheets (GNs) may be a better choice as the loading materials. Besides, assembly
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of transition metal (e.g., Pd, Pt and Au) nanoparticles (NPs) into graphene also can be exploited to enhance sensing performance of chemiresistor-type devices, due to their effective catalytic properties which can dissociate adsorbed oxygen molecules into ionized oxygens (O− , O2− ) [18–22]. These facts, coupled with the typically efficient acetone gas sensing material h-WO3 , should inspire us to fabricate a ternary composite including WO3 , metal NPs and smallsized GNs for portable acetone sensor. To date, printed sensors onto flexible substrates have attracted considerable attention owing to low-cost technologies and potential application in flexible and portable devices [21,23–26]. Among printing techniques, the use of gravure printing of sensor devices is particularly attractive due to its high throughput, optimal control of sensing layer thickness and ability to use a wide range of inks [21]. Herein, we present a flexible chemiresistor-type acetone sensor based on WO3 /GNs decorated with Pt NPs (WO3 /Pt-GNs), which was printed onto a polyimide (PI) substrate by gravure printing technique. The structure characteristics and the gas-sensing performances of the printed WO3 /Pt-GNs sensor were studied. Furthermore, the underlying sensing mechanism towards acetone gas was also discussed.
2. Experimental 2.1. Synthesis of WO3 microspheres WO3 microspheres were prepared by a hydrothermal method as described previously [6]. The typical preparation procedures were as follows: 1.320 g (NH4 )10 W12 O4 ·xH2 O was dissolved in 40 mL of deionized H2 O. The value of pH was adjusted to 2.0 by the dropwise addition of 1 mol/L H2 SO4 under constant magnetic stirring at room temperature. Light yellow H2 WO4 was precipitated gradually. Then, 4.0 g (NH4 )2 SO4 and 1.008 g oxalic acid as the directing agents were added into the solution. The precursor solution was transferred into a Teflon-lined 50 mL autoclave for hydrothermal reaction at 180 ◦ C for 10 h. After cooling to room temperature, the prepared WO3 powders were centrifuged and washed with distilled water and ethanol several times. Finally, the obtained products were dried in air at 80 ◦ C for 4 h.
2.2. Preparation of WO3 /GMs composites GO sheets were synthesized by the chemical oxidation of natural graphite flakes (Alfa-Aesar) using the Hummers method [21,27]. GO was dispersed in deionized (DI) water via a mild sonication treatment to get a brown GO dispersion. The dispersion was centrifuged and rinsed with DI water several times and redispersed in 56 mL of glacial acetic acid via a mild sonication treatment. Then, 5 mL of hydriodic acid was mixed with the dispersion at 60 ◦ C for 40 h to obtain the large-sized reduced GO microsheets (GMs) product. The product was further centrifuged and rinsed with saturated sodium bicarbonate (NaHCO3 ), acetone and DI water several times and dried in air at 80 ◦ C for 4 h. The WO3 /GMs (0.25 wt% GMs) composite was formulated by dispersing obtained GMs and WO3 powders in DI water. The welldispersed composites were formed with ultrasonication using an ultrasonic cell disruptor (JYD-650L) for 1 h. The product was further centrifuged and dried in air at 80 ◦ C for 4 h. The WO3 /GMs composite inks were formulated by dispersing obtained WO3 /GMs powders in the mixture of ethanol, terpineol and EC (3 g EC powders were dispersed in 18 mL ethanol and 1 mL terpineol by bath sonication), following ultrasonication for 1 h.
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2.3. Preparation of WO3 /GNs composites Small-sized reduced GO nanosheets (GNs) were prepared by one-step ultrasonic synthesis route [28]. In a typical procedure, 1 mg/mL GO (50 mL) and 1 mol/L KMnO4 (50 mL) were mixed in a 250-mL round-bottom flask, forming a homogeneous mixture. This mixture was sonicated using the ultrasonic cell disruptor (JYD-650L) for 4 h, insuring the complete redox process. After the ultrasonic treatment, the mixture was centrifuged for 10 min at 3000 rpm. The supernatant was collected after being centrifuged for 30 min at 10000 rpm. Finally, the precipitate was dried in air at 80 ◦ C for 4 h to obtain the WO3 /GNs product. Similarly, the WO3 /GNs (0.25 wt% GNs) composite ink was obtained using the same method as aforementioned. 2.4. Preparation of WO3 /Pt-GNs composites GNs-Supported Platinum NPs (Pt-GNs) were synthesized by an ethylene glycol reduction method [29]. In a typical reaction, PtCl2 (66.5 mg) was dissolved in 5 mL of hydrochloric acid under heating. The solution was then condensed to about 2 mL and added into 100 mL of ethylene glycol, along with 80 mg of GNs under magnetic stirring. The pH was adjusted to 10 by concentrated ammonia, and the mixture was heated at 165 ◦ C for 30 min, then the obtained precipitates were collected, washed and dried at 80 ◦ C for 12 h to obtain Pt-GNs product. Likewise, the WO3 /Pt-GNs composite ink was obtained using the same method as aforementioned. The weight ratio of Pt-GNs powder was 0.5 wt%. 2.5. Device fabrication The sensor devices were fabricated on 125-m-thick flexible PI substrates by using a gravure printer (Schläfli Labratester, Switzerland). The principle of gravure printing machines is briefly described in Fig. 1a. In the first instance, the printing plate moves against the doctor blade, which removes the excess ink from the engraved cells by a doctor blade. Then, the ink is transferred to the surface of substrate, which is mounted in the nip between printing plate and impression cylinder [23]. The Ag-IDEs with the dimensions of 40 × 50 mm was made by gravure printing using commercial Ag ink (SS-5100, Shanghai Xinluyi Electronic Materials Co., Ltd.). Then the printed pattern was dried at 120 ◦ C for 1 h. Subsequently, the ink was overlaid on the Ag-IDEs and baked to form sensing layers and dried at 80 ◦ C for 0.5 h. The printed sensor structure and the practical image of the printed gas sensor were shown in Fig. 1b and c. 2.6. Gas sensing measurements The sensor testing was carried out using a homemade sensing system (EE-12b) [21,23]. Briefly, the printed sensor was placed in a cylindrical stainless steel chamber with a size of 12 L. Target gas (acetone, Wetry standard gas analysis technology Co., Ltd.) with a specific concentration (the different concentration of acetone is obtained by the method of static distribution) was introduced into the test chamber and the variation in sensor resistance was monitored using an Electrometer/High Resistance Meter (Keithley 6517B). Then, the upper lid of the chamber was opened for the sensor recovery. The sensor response (S) was calculated by the following equation: S = Ra/Rg
(1)
Where Ra and Rg were the resistances of the sensor element in the presence of atmospheric air and target gas, respectively. The response time and recovery time were defined as the time taken
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Fig. 1. (a) Schematic representation of the gravure printing process. (b) Schematic of the printed sensor. (c) Photo of a printed WO3 /Pt-GNs sensor on a PI substrate.
by the sensor to achieve 90% of the total resistance change after the sensor was exposed to the tested gas and air, respectively. In order to gain the selectivity of sensors to acetone gas, the other testing gases such as ethanol, benzene, formic acid, methanol and n-butanol were applied. 2.7. Characterization XRD was recorded on a Bruker D8-FOCUS diffractometer with a Cu-K␣1 radiation ( = 0.15406 nm). The morphology of the composites was examined by field emission scanning electron microscopy (FE-SEM, JSM-7000F) and transmission electron microscopy (TEM, JEM2010-HR). X-ray photoelectron spectroscopy (XPS) was performed using a PHI5000 Versa Probe. Raman analysis (Jobin Yvon LabRam-010, with a laser excitation wavelength of 632.8 nm and a spot size of 1 m2 ) was performed for the composites examination. 3. Results and discussion 3.1. Properties of samples Fig. 2a showed the SEM image of WO3 microspheres (Inset is a representative SEM image of a single WO3 sphere). It was clear to see that the microspheres of the WO3 sample had a diameter of 2–3 m, which were randomly assembled by numerous nanorods of a diameter of 10–20 nm. XRD pattern was used to analyze the crystal structures of WO3 in Fig. 2b. It was found that all the typical diffraction peaks of XRD were well indexed to standard pattern of PDF card nos. 85-2460, which indicated the profile of hexagonal WO3 . Furthermore, the (002) peak of WO3 samples was significantly stronger than this of the standard pattern. The relative texture coefficient of a specific crystal facet (TChkl ) is defined to assess the degree of the crystal facet exposure (Fig. S1, supporting information). The texture coefficients of (100) and (002) facets are given by [6]: TC002 =
0 I002 /I002 0 0 I002 /I002 + I100 /I100
(2)
TC100 = 1 − TC002
(3)
Here, TC100 and TC002 are the relative texture coefficients of (100) and (002) facets, Ihkl is the measured diffraction intensity of the (hkl) facet, and I0 hkl is the corresponding value of the standard XRD patterns (PDF card no. 85-2460). Fig. S1b displays the TChkl of h-WO3 , WO3 /GNs, and WO3 /Pt-GNs. As calculated by Eqs. (2) and (3), TC002 of h-WO3 , WO3 /GNs, and WO3 /Pt-GNs are 0.665, 0.644 and 0.613, indicating that h-WO3 , WO3 /GNs, and WO3 /Pt-GNs are all predominantly exposed with (002) facets. The larger proportion of (002) facets could enhance the interaction between large dipole moment acetone molecules and WO3 due to the asymmetric distribution of unsaturated coordinated O atoms in the O-terminated (002) facets, resulting in the enhancement of acetone sensing performance especially selectivity [6,30]. The TEM images of WO3 /GMs and WO3 /GNs were shown in Fig. S2a and 2b (supporting information), respectively. It was clear to see in Fig. S2a that WO3 nanorods with a diameter of ∼15 nm were randomly wrapped into around 10-m-sized GMs. In contrast, as shown in Fig. S2b, the WO3 nanorods were randomly dispersed around and partially overlapped with the GNs which exhibited a lateral size of dozens of nanometers. The TEM image of WO3 /PtGNs composites was further shown in Fig. 3a. It was found that the Pt NPs were decorated on the surface of GNs. Meanwhile, several WO3 nanorods were dispersed well with GNs. Lattice distances of (111) plane of Pt (0.382 nm) and (110) plane of h-WO3 (0.362 nm) were marked in the HRTEM image (Fig. 3b). The lattice distances of Pt were slightly larger than that of literature reports [4,29], presumably because of the formed PtO2 when the product was synthesized during the hydrothermal process. The survey-scanned XPS spectrum of WO3 /Pt-GNs was shown in Fig. 3c with all components including W, C, O and Pt, which was consistent with the input materials. Furthermore, the inset of Fig. 3c illustrated the high-resolution XPS spectrum of Pt core levels. It was found that the background noise was obvious, presumably because of the incorporation of Pt was extremely small. The peak binding energy of approximately 65.5 eV corresponds to the Pt 5p1/2 state [31]. Deconvoluting Pt 4f core level spectrum exhibited two characteristic peaks of 4f5/2 . The wide and weak peak at 75.6 eV as
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Fig. 2. (a) SEM image of WO3 microspheres. Inset is a representative SEM image of a single WO3 sphere. (b) XRD pattern of WO3 samples and the standard pattern of h-WO3 (PDF card no. 85-2460).
Fig. 3. (a) TEM and (b) HRTEM images of WO3 /Pt-GNs. (c) Survey-scanned XPS spectrum of WO3 /Pt-GNs samples, inset is the high-resolution XPS spectrum of Pt core levels. (d) Raman spectra of WO3 , WO3 /GNs, and WO3 /Pt-GNs samples.
well as a sharp peak at 76.6 eV were corresponding to Pt and PtO2 , respectively. The binding energy of 4f5/2 was consistent with the previous report by Takahiro Maruyama et al. [32], which showed 1.1 eV higher in comparison to some literatures [4,31]. This could be explained from the chemical interaction between the Pt NPs and
GNs, because of the strong -d hybridization between the Pt filled d orbitals and carbon p* empty orbitals [33]. Thus, although the initial purpose was to introduce Pt NPs, the results of TEM and XPS confirmed the partial oxidation of Pt catalysts during the hydrothermal process.
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Fig. 4. The UV–visible spectra of WO3 , WO3 /GNs and WO3 /Pt-GNs samples. Insert shows the optical band gap of WO3 , WO3 /GNs and WO3 /Pt-GNs samples.
Raman spectra of the WO3 , WO3 /GNs, and WO3 /Pt-GNs samples were shown in Fig. 3d. Three samples all exhibited Raman peaks at 266, 322 and 700–900 cm−1 , which could be attributed to O W O beading modes and O W O stretching modes. In addition, WO3 /GNs and WO3 /Pt-GNs exhibited obvious peak at ∼1335 and ∼1580 cm−1 , which could be attributed to D mode and G mode of graphene. The D peak, located at ∼1335 cm−1 was associated with structural defects and partially disordered structures of the sp2 domains. G peak located at ∼1580 cm−1 could be used to explain the degree of graphitization [34–36]. In this work, the ID /IG ratio of the WO3 /Pt-GNs was about 1.3, which exhibited a 30% enhancement in comparison to that of WO3 /GNs. The increased ID /IG ratio might be caused by the introduced Pt NPs which were synthesized by the hydrothermal method, resulting in a decrease of the sp2 domain and an increasing amount of defect sites. The defect sites could facilitate the adsorption of oxygen and acetone molecules, which was beneficial for gas sensing performance. The UV–visible spectra of pure h-WO3 , WO3 /GNs and WO3 /PtGNs composites was shown in Fig. 4. The optical band gap (Fig. 4, insert) was calculated from UV–vis absorption spectra using the Tauc plot (linear portion of the plot between (ahv)1/2 with hv where, a is absorption coefficient and v is the optical frequency). The value of the band gap was found to increase from 2.82 to 3.17 eV when GNs and Pt-GNs were incorporated into WO3 -based system. The increased band gap is due the change in the electronic interaction of WO3 microspheres with GNs and Pt-GNs [16]. 3.2. Gas sensing properties The gas sensing measurements of WO3 /Pt-GNs sensor were carried out in a homemade computer-controlled static gas sensing characterization system (EE-12b). The relationships between the operating temperature and gas response of the gas sensors based on WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs to 10 ppm acetone were tested, and the results were shown in Fig. 5. The responses of the three sensors to acetone varied obviously from 85 to 300 ◦ C. The WO3 /GMs and WO3 /GNs sensors reached their maximum response of 17.6 and 13.3 at the operating temperature of 255 ◦ C, and their responses then decreased after further increasing the operating temperature. However, the maximum response of the WO3 /Pt-GNs sensor jumped to 13.9 at the operating temperature of 230 ◦ C. Thus, the optimum operating temperature decreased after GNs decorated with Pt NPs. Furthermore, the response of WO3 /Pt-GNs to
Fig. 5. Response of WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs to 10 ppm acetone as a function of the operating temperature.
acetone was almost 30% higher than that of WO3 /GNs at the operating temperature of 200 ◦ C, which reflected the effect of catalytic Pt NPs decoration on the enhancement of the sensing properties toward acetone. Besides, the response (Ra /Rg ) of WO3 /GNs and WO3 /Pt-GNs to acetone was lower than that of WO3 /GMs when the operating temperature was above 180 ◦ C. The reason was presumably because the Rg of WO3 /GNs and WO3 /Pt-GNs were almost 4 times higher than that of WO3 /GMs (which could be seen in Fig. 6). The response transient of the gas sensor based on WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs samples to 10 ppm acetone were investigated and shown in Fig. 6. It could be seen that the response/recovery time of WO3 /GNs and WO3 /Pt-GNs were about 15.2/9.6 s and 14.1/16.8 s, which exhibited a shortened gas response/recovery time compared to that of WO3 /GMs. The fast response-recovery characteristics of the WO3 /GNs and WO3 /PtGNs sensors was mainly attributed to the massive p-n junction active sites at the WO3 /graphene interface which would quickly convert CH3 COCH3 with O− /O2− to CH3 CHO, CO2 and H2 O, resulting in a fast flow of charge carriers into the conduction band [15]. Besides, the background noise of WO3 /GNs and WO3 /Pt-GNs were remarkably higher than that of WO3 /GMs, presumably also because of the substantial numbers of p-n junctions which might reduce conducting channels to the charge carriers that caused the fluctuation of resistance. Fig. 7a showed dynamic response resistance of the printed WO3 /Pt-GNs sensor towards 1–100 ppm acetone concentration at 230 ◦ C. It could be seen that the sensor resistance significantly decreased after acetone exposure. When acetone gas was removed, the resistance increased and fully recovered its initial values. Besides, the saturation value of Rg decreased gradually with increasing gas concentration. The responses of WO3 /Pt-GNs sensor increased linearly with the increase of acetone concentration in a log–log plot as shown in the inset of Fig. 7a. However, the responses showed non-linearity as a function of acetone concentrations, presumably because of the complex structure of the composite which might possess a complicated influence on the change of resistance. We followed Vineet Dua’s reported method [37], the theoretical low detection limit (for signal-to-noise ratio of 3) for acetone was estimated to be 400 ppb. However, it was unable to measure acetone in the ppb range due to the large background noise. Thus, the actual low detection limit of WO3 /Pt-GNs sensor was 1 ppm. The influence of the relative humidity (RH) on sensing performance was
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GNs sensor could be operated without noticeable degradation in electrical performance when bent to the extreme. Selectivity is an important parameter of gas sensors, especially for breath analysis sensors which the sensitivity toward a specific gas needs to be distinctly higher than that of more than 200 different VOCs in human breath [2–4]. The gas selectivity of the WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs sensors through comparing the sensitivities toward various typical gases in the human breath were studied in Fig. 8. The WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs sensors showed a high response of 14.6, 9.8, and 12.2 to 10 ppm acetone, but a negligible response of below 3 towards 10 ppm of ethanol, benzene, formic acid, methanol and n-butanol. In order to compare the differences between the selectivity of the three sensors, the ratios of gas response values to acetone and other gases were calculated and shown in Table 1. It could be seen that the WO3 /Pt-GNs sensor exhibited the highest average ratio of 8.1, indicating that the selectivity of WO3 /Pt-GNs sensor was higher than that of WO3 /GMs and WO3 /GNs. The good selectivity of the WO3 /Pt-GNs sensor was mainly attributed to the exposed (002) facets of WO3 which was not wrapped into GNs, resulting in a strong interaction to acetone molecules compared to other gases [6]. Furthermore, the cross-selectivity between acetone and ethanol with different concentrations and testing temperatures were investigated (Fig. S5). A comparison between the cross-selectivity of WO3 /Pt-GNs and those reported in the literature for acetone detection was summarized in Table 2. To quantify the selectivity of several materials of literature reports, the ratios of gas response values to acetone and ethanol (Sac /Set ), methanol (Sac /Sme ) and benzene (Sac /Sbe ) were calculated. It is noticed that the printed WO3 /Pt-GNs shows 5.7 times of Sac /Set at 10 ppm, better than that of pure h-WO3 at 100 ppm [6], suggesting that Pt-GNs incorporated into h-WO3 enhanced the selectivity towards acetone than ethanol, as well as methanol and benzene. The enhanced selectivity maybe is attribute to the synergistic effect of exposed active sites, p-n junctions and even Pt NPs. Compared with other graphene composite sensors [15,17], the operating temperature of WO3 /Pt-GNs is the lowest among these materials, the best selectivity towards acetone than ethanol, methanol and benzene has been achieved. Especially, Seon-Jin Choi et al. demonstrated a similar WO3 /graphene material to our materials, which displayed good sensing performance towards acetone and H2 S. However, the operating temperature of WO3 /Pt-GNs is lower than WO3 /graphene [16]. The ZnFe2 O4 system seemed to exhibit a high Sac /Sbe value of 12.2 at the operating temperature near to 200 ◦ C [38]. However, the Sac /Set and Sac /Sme values were too low. In short, the WO3 /Pt-GNs sensor could better recognize as well as distinguish these VOCs and achieved higher selectivity compared with other materials reported at a relatively low operating temperature of 200 ◦ C.
3.3. Gas sensing mechanism
Fig. 6. Response transient of the gas sensor based on WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs samples to 10 ppm acetone at 200 ◦ C.
also investigated considering the exhaled breath gases containing highly water vapor. Fig. 7b showed WO3 /Pt-GNs sensor response to acetone as a function of RH at 230 ◦ C. The result showed that the sensitivities on high RH (95%) are lower than that in atmospheric condition. The limit of detection toward acetone was ∼600 ppb (linear extrapolation). Besides, the initial resistance of the WO3/PtGNs sensor at 230 ◦ C as a function of bending radius of curvature was shown in Fig. S4. The result showed that the printed WO3 /Pt-
It has been demonstrated that the printed WO3 /Pt-GNs sensor has a much faster response-recovery characteristics, higher sensitivity and selectivity at the operating temperature of 200 ◦ C. This may result from two factors: (i) catalytic Pt nanoparticle decoration; (ii) formation massive p-n junctions at the WO3 /GNs interfaces. WO3 is a typical n-type semiconducting sensor material. In ambient air, free oxygen molecules are absorbed on WO3 surface, and the absorbed oxygen molecules capture electrons from the conduction band of WO3 to form chemisorbed oxygen ions, such as O2 − , O− and O2− . The chemical reaction is listed below [39–41]: O2(ads) + e− ⇒ O2−
(4)
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Table 1 The ratios of gas response values to acetone and other gases including ethanol, benzene, formic acid, methanol and n-butanol. Sacetone Sethanol
Materials WO3 /GMs WO3 /GNs WO3 /Pt-GNs
5.2 8.1 5.7
Sacetone Sbenzene 4.8 8.6 10.1
Sacetone Sformicacid 11.6 7.9 9.1
Sacetone Smethanol 4.3 7.2 9.3
Sacetone Sn − butanol 10.5 7.6 7.7
Average 6.1 7.9 8.1
Table 2 The comparative analysis of gas selectivity of different materials in the present study and those reported in the literature, the ratios of gas response values of acetone to ethanol, methanol and benzene are labeled as Sac /Set , Sac /Sme and Sace /Sbe . Materials
Operating temperature ( ◦ C)
Sac :Set
Sac :Sme
Sac :Sbe
Reference
h-WO3 WO3 /Graphene CuO-ZnO/rGO SnO2 -Ni/Graphene ZnFe2 O4 ZnFe2 O4 WO3 /Pt-GNs
230 300 340 350 215 200 200
5.7 at 100 ppm 2.4 at 2 ppm 3.7 at 10 ppm 1.6 at 5 ppm 1.4 at 100 ppm 2.0 at 30 ppm 5.7 at 10 ppm
2.9 at 100 ppm N/A. 11.8 at 100 ppm N/A. 3.1 at 100 ppm 2.9 at 30 ppm 9.3 at 10 ppm
4.7 at 100 ppm N/A. N/A. N/A. 12.2 at 100 ppm 9.1 at 30 ppm 10.1 at 10 ppm
6 16 15 17 37 42 Current work
Fig. 7. (a) Response-recovery curve of WO3 /Pt-GNs to different concentrations of acetone measured at 230 ◦ C. Inset is a log–log plot of the response (S) curves to different acetone concentrations (C, 1 ppm–100 ppm) at the operating temperature of 230 ◦ C; (b) Sensor responses to different concentrations of acetone under different RHs.
O2(ads) + 2e− ⇒ O−
(5)
O2(ads) + 4e− ⇒ O2−
(6)
These three regions of the absorbed oxygen species are a function of the operating temperature. The first region (Eq. (4)) starts at room temperature and ends at ∼100 ◦ C. This temperature interval only allows the adsorption of O2 − on the surface through the chemical reaction; the second region (Eq. (5)) is situated in the 100–225 ◦ C, O− species are formed and are dominantly present; the third region (Eq. (6)) is located between 225 and 400 ◦ C, when the available thermal energy is enough to form O2− species [42]. When exposed to acetone, the acetone molecules can react with the absorbed oxygen ion (O− or O2− ) leading to the formation of CO2 and H2 O. The process can be expressed in the following reactions [17,43]: CH3 COCH3(gas) + 8O− ⇒ 3CO2(gas) + 3H2 O(gas) + 8e− (ads) CH3 COCH3(gas) + 8O2− (ads)
⇒ 3CO2(ads) + 3H2 O(gas) + 16e−
(7) (8)
As a result, it is precisely in the second and third regions (Eqs. (5) and (6)) that the greatest increment in sensitivity is displayed. This is the primary reason why the optimum operating temperature to acetone is usually above 250 ◦ C, which is consistent with the experimental results of WO3 /GMs and WO3 /GNs. However, the
introduced Pt NPs is an effect oxygen dissociation catalyst, which has high availability to activate the dissociation of molecular oxygen (O− or O2− ) at a relatively low temperature [44,45]. Moreover, the as-created O− or O2− species could spill onto the WO3 surface, which is known as the ‘spillover effect’ [4,17,46,47]. Therefore, the optimum operating temperature for WO3 /Pt-GNs sensors is lower as well as the response is higher than WO3 /GNs sensors. In this work, the relationships between the microstructure and gas sensing performance of WO3 functionalized by graphene with different sizes had been investigated. Large-sized GMs may wrap around the WO3 microspheres, resulting in the reduction of the amount of active sites on WO3 surface, which possesses a negative effect on the acetone selectivity. In contrast, more (002) facets of WO3 may be exposed due to the small size of GNs, resulting in a strong interaction to acetone molecules. Therefore, WO3 functionalized by GNs could achieve a high selectivity. Furthermore, it is well known that the p-n junctions formed at the interface between metal oxide and graphene would make a good contribution to the improvement of gas sensing performance [15–17]. A number of p-n junctions formed at the WO3 /GNs interface would quickly convert CH3 COCH3 with O− /O2− to CO2 and H2 O, resulting in a short gas response/recovery time. However, it should be mentioned that too many p-n junctions also have a negative effect on background noise (as described in Fig. 6). Thus, the reasonable design and control of
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Fig. 8. Response of the sensor based on WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs to various test gases with a concentration of 10 ppm at 200 ◦ C.
the p-n junction structure is a promising method to enhance the sensing performance. 4. Conclusions In conclusion, (002) facets exposed WO3 microspheres loaded with small-sized Pt-decorated graphene had been synthesized and exhibited a high selectivity and sensitivity to low concentration acetone gases at the operating temperature of 200 ◦ C. The superior performance of the printed sensors was attributed to a large number of exposed (002) facets of WO3 which was not wrapped into GNs as well as Pt catalysts. Thus, WO3 /Pt-GNs composite sensor is a promising selective, portable and low-cost chip for acetone detection. Acknowledgements This research was supported by the Natural Science Foundation of Shanghai (No. 16ZR1424400), Shanghai Municipal Education Committee Key Laboratory of Molecular Imaging Probes and Sensors for Shanghai Universities, and Industrial Research Fund from Wuhu Token Sciences Co., Ltd. Prof. Lei Huang appreciates the support of The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2017.08.158. References ˛ [1] B. Buszewski, M. Kesy, T. Ligor, A. Amann, Human exhaled air analytics: biomarkers of diseases, Biomed. Chromatogr. 21 (2007) 553–566. [2] W.Q. Cao, Y.X. Duan, Breath analysis: potential for cinical diagnosis and exposure assessment, Clin. Chem. 52 (2006) 800–811. [3] W. Miekisch, J.K. Schubert, F.E. Gabriele, N. Schomburg, Diagnostic potential of breath analysis-focus on volatile organic compounds, Clin. Chim. Acta 347 (2004) 25–39. [4] S.J. Choi, I. Lee, B.H. Jang, D.Y. Youn, W.H. Ryu, C.O. Park, I.D. Kim, Selective diagnosis of diabetes using Pt-functionalized WO3 hemitube networks as a sensing layer of acetone in exhaled breath, Anal. Chem. 85 (2013) 1792–1796. [5] S. Salehia, E. Nikana, A.A. Khodadadia, Y. Mortazavia, Highly sensitive carbon nanotubes-SnO2 nanocomposite sensor foracetone detection in diabetes mellitus breath, Sens. Actuators B 205 (2014) 261–267. [6] Q.Q. Jia, H.M. Ji, D.H. Wang, X. Bai, X.H. Sun, Z.G. Jin, Exposed facets induced enhanced acetone selective sensing property of nanostructured tungsten oxide, J. Mater. Chem. A 2 (2014) 13602–13611.
1489
[7] M. Righettoni, A. Tricoli, S.E. Pratsinis, Thermally stable, silica-doped -WO3 for sensing of acetone in the human breath, Chem. Mater. 22 (2010) 3152–3157. [8] T. Xiao, X.Y. Wang, Z.H. Zhao, L. Li, L. Zhang, H.C. Yao, J.S. Wang, Z.J. Li, Highly sensitive and selective acetone sensor based on C-doped WO3 for potential diagnosis of diabetes mellitus, Sens. Actuators B 199 (2014) 210–219. [9] X. Bai, H.M. Ji, P. Gao, Y. Zhang, X.H. Sun, Morphology, phase structure and acetone sensitive properties of copper-doped tungsten oxide sensors, Sens. Actuators B 193 (2014) 100–106. [10] F. Liu, X.F. Chu, Y.P. Dong, W.B. Zhang, W.Q. Sun, L.M. Shen, Acetone gas sensors based on graphene-ZnFe2 O4 composite prepared by solvothermal method, Sens. Actuators B 188 (2013) 469–474. [11] S. Mao, G.H. Lu, J.H. Chen, Nanocarbon-based gas sensors: progress and challenges, J. Mater. Chem. A 2 (2014) 5573–5579. [12] H. Zhang, J.C. Feng, T. Fei, S. Liu, T. Zhang, SnO2 nanoparticles-reduced graphene oxide nanocomposites for NO2 sensing at low operating temperature, Sens. Actuators B 190 (2014) 472–478. [13] E.C. Nallon, V.P. Schnee, C. Bright, M.P. Polcha, Q.L. Li, Chemical discrimination with an unmodified graphene chemical sensor, ACS Sens. 1 (2016) 26–31. [14] N.T. Hu, Z. Yang, Y.Y. Wang, L.L. Zhang, Y. Wang, X.L. Huang, H. Wei, L.M. Wei, Y.F. Zhang, Adsorption of gas molecules on transition metal embedded graphene: a search for high-performance graphene-based catalysts and gas sensors, Nanotechnology 25 (2014) 025502. [15] C. Wang, J.W. Zhu, S.M. Liang, H.P. Bi, Q.F. Han, X.H. Liu, X. Wang, Reduced graphene oxide decorated with CuO-ZnO hetero-junctions: towards high selective gas sensing property to acetone, J. Mater. Chem. A 2 (2014) 18635–18643. [16] S.J. Choi, F. Fuchs, R. Demadrille, B. Grévin, B.H. Jang, S.J. Lee, J.H. Lee, H.L. Tuller, I.D. Kim, Fast responding exhaled-breath sensors using WO3 hemitubes functionalized by graphene-based electronic sensitizers for diagnosis of diseases, ACS Appl. Mater. Interfaces 6 (2014) 9061–9070. [17] S. Singkammo, A. Wisitsoraat, C. Sriprachuabwong, A. Tuantranont, S. Phanichphant, C. Liewhiran, Electrolytically exfoliated graphene-loaded flame-made Ni-doped SnO2 composite film for acetone sensing, ACS Appl. Mater. Interfaces 7 (2015) 3077–3092. [18] H.T. Yu, P.C. Xu, D.W. Lee, X.X. Li, Porous-layered stack of functionalized AuNP-rGO (gold nanoparticles-reduced graphene oxide) nanosheets as a sensing material for the micro-gravimetric detection of chemical vapor, J. Mater. Chem. A 1 (2013) 4444–4450. [19] M.L. Zhang, H.C. Su, Y. Rheem, C.M. Hangarter, N.V. Myung, A rapid room-temperature NO2 sensor based on Tellurium-SWNT hybrid nanostructures, J. Phys. Chem. C 116 (2012) 20067–20074. [20] J.K. Zhang, L. Huang, Y.J. Lin, L. Chen, Z.Y. Zeng, L. Shen, Q. Chen, W.Z. Shi, Pencil-trace on printed silver interdigitated electrodes for paper-based NO2 gas sensors, Appl. Phys. Lett. 106 (2015) 143101. [21] L. Huang, Z.P. Wang, J.K. Zhang, J.L. Pu, Y.J. Lin, S.H. Xu, L. Shen, Q. Chen, W.Z. Shi, Fully printed, rapid-response sensors based on chemically modified graphene for detecting NO2 at room temperature, ACS Appl. Mater. Interfaces 6 (2014) 7426–7433. [22] M. Zhou, Y.H. Lu, Y.Q. Cai, C. Zhang, Y.P. Feng, Ultrafast and sensitive room temperature NH3 gas sensors based on chemically reduced graphene oxide, Nanotechnology 22 (2011) 385502. [23] Y.J. Lin, L. Huang, L. Chen, J.K. Zhang, L. Shen, Q. Chen, W.Z. Shi, Fully gravure-printed NO2 gas sensor on a polyimide foil using WO3 -PEDOT:PSS nanocomposites and Ag electrodes, Sens. Actuators B 216 (2015) 176–183. [24] Q. Zhang, L. Huang, Q.H. Chang, W.Z. Shi, L. Shen, Q. Chen, Gravure-printed interdigital microsupercapacitors on a flexible polyimide substrate using crumpled graphene ink, Nanotechnology 27 (2016) 105401. [25] Y.X. Xiao, L. Huang, Q. Zhang, S.H. Xu, Q. Chen, W.Z. Shi, Gravure printing of hybrid MoS2 @S-rGO interdigitated electrodes for flexible microsupercapacitors, Appl. Phys. Lett. 107 (2015) 013906. [26] A.P. Taylor, L.F. Velásquez-García, Electrospray-printed nanostructured graphene oxide gas sensors, Nanotechnology 26 (2015) 505301. [27] L. Huang, Y. Liu, L.C. Ji, Y.Q. Xie, T. Wang, W.Z. Shi, Pulsed laser assisted reduction of graphene oxide, Carbon 49 (2011) 2431–2436. [28] Y.H. Zhu, G.F. Wang, H. Jiang, L. Chen, X.J. Zhang, One-step ultrasonic synthesis of graphene quantum dots with high quantum yield and their application in sensing alkaline phosphatase, Chem. Commun. 51 (2015) 948–951. [29] G.Q. He, Y. Song, K. Liu, A. Walter, S. Chen, S.W. Chen, Oxygen reduction catalyzed by Platinum nanoparticles supported on graphene quantum dots, ACS Catal. 3 (2013) 831–838. [30] J. Zhu, S.L. Wang, S.H. Xie, H.X. Li, Hexagonal single crystal growth of WO3 nanorods along a [110] axis with enhanced adsorption capacity, Chem. Commun. 47 (2011) 4403–4405. [31] M. Zare, A. Mortezaali, A. Shafiekhani, Photoelectrochemical determination of shallow and deep trap states of Platinum-decorated TiO2 nanotube arrays for Photocatalytic applications, J. Phys. Chem. C 120 (2016) 9017–9027. [32] T. Maruyama, H. Kondo, R. Ghosh, A. Kozawa, S. Naritsuka, Y. Iizumi, T. Okazaki, S. Iijima, Single-walled carbon nanotube synthesis using Pt catalysts under low ethanol pressure via cold-wall chemical vapor deposition in high vacuum, Carbon 96 (2016) 6–13. [33] Y. Shen, Z.H. Zhang, R.R. Long, K.J. Xiao, J.Y. Xi, Synthesis of ultrafine Pt nanoparticles stabilized by pristine graphene nanosheets for electro-oxidation of methanol, ACS Appl. Mater. Interfaces 6 (2014) 15162–15170.
1490
L. Chen et al. / Sensors and Actuators B 255 (2018) 1482–1490
[34] Y.W. Zhu, S. Murali, W.W. Cai, X.S. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906–3924. [35] S. Some, Y. Kim, Y. Yoon, H.J. Yoo, S. Lee, Y. Park, H. Lee, High-quality reduced graphene oxide by a dual-function chemical reduction and healing process, Sci. Rep. 3 (2013) 1929. [36] K.S. Rao, J. Sentilnathan, H.W. Cho, J.J. Wu, M. Yoshimura, Soft processing of graphene nanosheets by glycine-bisulfate ionic-complex-assisted electrochemical exfoliation of graphite for reduction catalysis, Adv. Funct. Mater. 25 (2015) 298–305. [37] V. Dua, S.P. Surwade, S. Ammu, S.R. Agnihotra, S. Jain, K.E. Roberts, S. Park, R.S. Ruoff, S.K. Manohar, All-organic vapor sensor using inkjet-printed reducedgraphene oxide, Angew. Chem. Int. Ed. 49 (2010) 2154–2157. [38] X. Zhou, X.W. Li, H.B. Sun, P. Sun, X.S. Liang, F.M. Liu, X.L. Hu, G.Y. Lu, Nanosheet-assembled ZnFe2 O4 hollow microspheres for high-sensitive acetone sensor, ACS Appl. Mater. Interfaces 7 (2015) 15414–15421. [39] P.P. Sahay, Zinc oxide thin film gas sensor for detection of acetone, J. Mater. Sci. 40 (2005) 4383–4385. [40] M. Batzill, U. Diebold, The surface and materials science of tin oxide, Prog. Surf. Sci. 79 (2005) 47–154. [41] R.S. Khadayate, J.V. Sali, P.P. Patil, Acetone vapor sensing properties of screen printed WO3 thick films, Talanta 72 (2007) 1077–1081. ˜ [42] L. Castaneda, A. Maldonado, M.L. Olvera, Sensing properties of chemically sprayed TiO2 thin films using Ni, Ir, and Rh as catalysts, Sens. Actuators B 133 (2008) 687–693. [43] X. Zhou, J.Y. Liu, C. Wang, P. Sun, X.L. Hu, X.W. Li, K. Shimanoe, N. Yamazoe, G.Y. Lu, Highly sensitive acetone gas sensor based on porous ZnFe2 O4 nanospheres, Sens. Actuators B 206 (2015) 577–583. [44] R.Q. Xing, Q.L. Li, L. Xia, J. Song, L. Xu, J.H. Zhang, Y. Xie, H.W. Song, Au-modified three-dimensional In2 O3 inverse opals: synthesis and improved performance for acetone sensing toward diagnosis of diabetes, Nanoscale 7 (2015) 13051–13060. [45] R.Q. Xing, L. Xu, J. Song, C.Y. Zhou, Q.L. Li, D.L. Liu, H.W. Song, Preparation and gas sensing properties of In2 O3 /Au nanorods for detection of volatile organic compounds in exhaled breath, Sci. Rep. 5 (2015) 10717. [46] Y.H. Xiao, L.Z. Lu, A.Q. Zhang, Y.H. Zhang, L. Sun, L. Huo, F. Li, Highly enhanced acetone sensing performances of porous and single crystalline ZnO nanosheets: high percentage of exposed (100) facets working together with surface modification with Pd nanoparticles, ACS Appl. Mater. Interfaces 4 (2012) 3797–3804. [47] S. Mayavan, J.B. Sim, S.M. Choi, Simultaneous reduction, exfoliation and functionalization of graphite oxide into a graphene-Platinum nanoparticle hybrid for methanol oxidation, J. Mater. Chem. 22 (2012) 6953–6958.
Biographies Lu Chen received his B.S. degree from School of Resources and Environmental Engineering, East China University of Science and Technology, in 2013. He is currently a master candidate in Shanghai Normal University, China. His research interest is synthesis of graphene nanohybrids and their application in printed smart sensors. Lei Huang is a Professor and head of Joint Lab with Wuhu Token for Graphene Electrical Materials and Application in the Department of Physics at Shanghai Normal University. He received his B.S. in Physics from Hefei University of Technology in 1990 and M.S. in Thin Film Physics from Institute of Solid State Physics (Chinese Academy of Sciences), Ph.D. in Materials Science from University of Science and Technology of China in 1999. He has more than 50 publications in SCI journals and holds 10 patents. His research interests are graphene and nano-carbon; printed smart sensors and flexible nano energy devices. Youjie Lin received his M.S. degree from College of Mathematics and Physics, Shanghai Normal University, China in 2015. His current research interests include the preparation and functionalization of the nanostructured materials for printed sensors. Liman Sai is a lecturer of College of Mathematics and Science, Shanghai Normal University, China. She received his Ph.D. degree from Shanghai Jiao Tong University, China in 2015. Her current research interests are in the areas of quantum dot luminescence device. Quanhong Chang is a lecturer of College of Mathematics and Science, Shanghai Normal University, China. He received his Ph.D. degree from Harbin Institute of Technology, China in 2015. His current research interests include the preparation and functionalization of flexible nano energy devices. Wangzhou Shi is a professor and Vice Dean of College of Mathematics and Science, Shanghai Normal University, China. He received his Ph.D. degree from Institute of Solid State Physics, Chinese Academy of Sciences, in 1994. His research interests are in the areas of thin film devices. Qi Chen is an adjunct research professor of Joint Lab with Wuhu Token for Graphene Electrical Materials and Application from Wuhu Token Sciences Co. Ltd.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Research Paper
Fully gravure-printed WO3 /Pt-decorated rGO nanosheets composite film for detection of acetone Lu Chen, Lei Huang ∗ , Youjie Lin, Liman Sai, Quanhong Chang, Wangzhou Shi, Qi Chen Joint Lab with Wuhu Token for Graphene Electrical Materials and Application, Department of Physics, Shanghai Normal University, Guilin Road 100, Shanghai 200234, China
a r t i c l e
i n f o
Article history: Received 2 March 2017 Received in revised form 13 August 2017 Accepted 19 August 2017 Available online 24 August 2017 Keywords: Acetone gas sensor Printed sensor rGO nanosheets WO3
a b s t r a c t With the emergence of non-invasive disease diagnosis, the development of a specific volatile organic compounds (VOCs) gas sensors in exhaled breath is on-going. Described herein is a flexible and lightweight chemiresistor made of a thin film based on WO3 /reduced graphene oxide (GO) nanosheets decorated with Pt nanoparticles, which were printed onto a flexible polyamide substrate by using gravure technique. In order to study the effect of graphene size on the performance of gas sensing, graphene with two sized such as large-sized reduced GO microsheets (GMs) and small-sized reduced GO nanosheets (GNs) were synthesized and used as modified materials incorporated into WO3 -based system. The printed WO3 /PtGNs sensor exhibits a good selectivity, a high response of 12.2 to 10 ppm acetone as well as a fast gas response/recovery time (14.1/16.8 s) at the operating temperature of 200 ◦ C. The sensing performance is attributed to the exposed (002) facets of WO3 which were not wrapped into GNs, the massive p-n junction active sites at the WO3 /GNs interface, as well as catalytic Pt nanoparticles. The printed WO3 /Pt-GNs sensor is a promising candidate for selective, fast and sensitive detection of acetone at relatively low operating temperature. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Volatile organic compounds (VOCs) greatly threaten human health because these gases can adversely affect brain, nervous, endocrine and skin systems [1,2]. Among these VOCs, acetone gas is toxic to various organ systems and has been identified as a specific biomarker of diabetes [3]. The concentration of exhaled acetone from a healthy individual should be below 0.9 ppm and the concentration of more than 1.8 ppm could indicate diabetes [4,5]. However, an accurate detection of acetone in a complex environment of exhaled breath is quite difficult and very important for the fast and simple recognition of diabetes. Therefore, it is demanding to develop selective, low-power, sensitive, flexible and portable devices to detect acetone gas at ppb levels for non-invasive diagnosis of diabetes. The semiconducting metal oxide WO3 , especially hexagonal WO3 (h-WO3 ) exposed with (002) facets, has been proved to be one of the most efficient gas-sensing materials for acetone detection due to a strong interaction between acetone molecules and h-WO3 having high dipole moment on (002) facets [6]. However, the sensing performances including selectivity, sen-
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (L. Huang). http://dx.doi.org/10.1016/j.snb.2017.08.158 0925-4005/© 2017 Elsevier B.V. All rights reserved.
sitivity and operating temperature of h-WO3 still need to be further improved to meet the requirement of the low-power portable sensor device for health monitoring. Hence, a variety of strategies have been developed to design WO3 based composite by introducing other materials to enhance sensing performance of WO3 devices [7–9]. Recently, graphene has been widely studied as a new additive material of semiconducting metal oxide composites due to its large specific surface and high carrier mobility, thereby potentially providing superior sensitivity and rapid response time [10–14]. Metal oxides mixed with graphene have been demonstrated to be an effective strategy for enhancement of acetone sensing performances [15–17]. For example, Chao Wang et al. [15] demonstrated that modificated CuO-ZnO composite with reduced graphene oxide (GO) sheets could improve the gas response to acetone. The gas response value reached from 9.4 to 10 ppm of acetone, almost 1.5 times higher than pure CuO–ZnO. Seon-Jin Choi et al. [16] found that the WO3 hemitube decorated with graphene caused a 6.45fold enhanced response relative to pristine WO3 . However, the size of graphene sheets in their studies is in the micron range [15–17], these large-sized reduced GO microsheets (GMs) may wrapped around the metal oxides, resulting in a decreasing amount of active sites. Therefore, small-sized reduced GO nanosheets (GNs) may be a better choice as the loading materials. Besides, assembly
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of transition metal (e.g., Pd, Pt and Au) nanoparticles (NPs) into graphene also can be exploited to enhance sensing performance of chemiresistor-type devices, due to their effective catalytic properties which can dissociate adsorbed oxygen molecules into ionized oxygens (O− , O2− ) [18–22]. These facts, coupled with the typically efficient acetone gas sensing material h-WO3 , should inspire us to fabricate a ternary composite including WO3 , metal NPs and smallsized GNs for portable acetone sensor. To date, printed sensors onto flexible substrates have attracted considerable attention owing to low-cost technologies and potential application in flexible and portable devices [21,23–26]. Among printing techniques, the use of gravure printing of sensor devices is particularly attractive due to its high throughput, optimal control of sensing layer thickness and ability to use a wide range of inks [21]. Herein, we present a flexible chemiresistor-type acetone sensor based on WO3 /GNs decorated with Pt NPs (WO3 /Pt-GNs), which was printed onto a polyimide (PI) substrate by gravure printing technique. The structure characteristics and the gas-sensing performances of the printed WO3 /Pt-GNs sensor were studied. Furthermore, the underlying sensing mechanism towards acetone gas was also discussed.
2. Experimental 2.1. Synthesis of WO3 microspheres WO3 microspheres were prepared by a hydrothermal method as described previously [6]. The typical preparation procedures were as follows: 1.320 g (NH4 )10 W12 O4 ·xH2 O was dissolved in 40 mL of deionized H2 O. The value of pH was adjusted to 2.0 by the dropwise addition of 1 mol/L H2 SO4 under constant magnetic stirring at room temperature. Light yellow H2 WO4 was precipitated gradually. Then, 4.0 g (NH4 )2 SO4 and 1.008 g oxalic acid as the directing agents were added into the solution. The precursor solution was transferred into a Teflon-lined 50 mL autoclave for hydrothermal reaction at 180 ◦ C for 10 h. After cooling to room temperature, the prepared WO3 powders were centrifuged and washed with distilled water and ethanol several times. Finally, the obtained products were dried in air at 80 ◦ C for 4 h.
2.2. Preparation of WO3 /GMs composites GO sheets were synthesized by the chemical oxidation of natural graphite flakes (Alfa-Aesar) using the Hummers method [21,27]. GO was dispersed in deionized (DI) water via a mild sonication treatment to get a brown GO dispersion. The dispersion was centrifuged and rinsed with DI water several times and redispersed in 56 mL of glacial acetic acid via a mild sonication treatment. Then, 5 mL of hydriodic acid was mixed with the dispersion at 60 ◦ C for 40 h to obtain the large-sized reduced GO microsheets (GMs) product. The product was further centrifuged and rinsed with saturated sodium bicarbonate (NaHCO3 ), acetone and DI water several times and dried in air at 80 ◦ C for 4 h. The WO3 /GMs (0.25 wt% GMs) composite was formulated by dispersing obtained GMs and WO3 powders in DI water. The welldispersed composites were formed with ultrasonication using an ultrasonic cell disruptor (JYD-650L) for 1 h. The product was further centrifuged and dried in air at 80 ◦ C for 4 h. The WO3 /GMs composite inks were formulated by dispersing obtained WO3 /GMs powders in the mixture of ethanol, terpineol and EC (3 g EC powders were dispersed in 18 mL ethanol and 1 mL terpineol by bath sonication), following ultrasonication for 1 h.
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2.3. Preparation of WO3 /GNs composites Small-sized reduced GO nanosheets (GNs) were prepared by one-step ultrasonic synthesis route [28]. In a typical procedure, 1 mg/mL GO (50 mL) and 1 mol/L KMnO4 (50 mL) were mixed in a 250-mL round-bottom flask, forming a homogeneous mixture. This mixture was sonicated using the ultrasonic cell disruptor (JYD-650L) for 4 h, insuring the complete redox process. After the ultrasonic treatment, the mixture was centrifuged for 10 min at 3000 rpm. The supernatant was collected after being centrifuged for 30 min at 10000 rpm. Finally, the precipitate was dried in air at 80 ◦ C for 4 h to obtain the WO3 /GNs product. Similarly, the WO3 /GNs (0.25 wt% GNs) composite ink was obtained using the same method as aforementioned. 2.4. Preparation of WO3 /Pt-GNs composites GNs-Supported Platinum NPs (Pt-GNs) were synthesized by an ethylene glycol reduction method [29]. In a typical reaction, PtCl2 (66.5 mg) was dissolved in 5 mL of hydrochloric acid under heating. The solution was then condensed to about 2 mL and added into 100 mL of ethylene glycol, along with 80 mg of GNs under magnetic stirring. The pH was adjusted to 10 by concentrated ammonia, and the mixture was heated at 165 ◦ C for 30 min, then the obtained precipitates were collected, washed and dried at 80 ◦ C for 12 h to obtain Pt-GNs product. Likewise, the WO3 /Pt-GNs composite ink was obtained using the same method as aforementioned. The weight ratio of Pt-GNs powder was 0.5 wt%. 2.5. Device fabrication The sensor devices were fabricated on 125-m-thick flexible PI substrates by using a gravure printer (Schläfli Labratester, Switzerland). The principle of gravure printing machines is briefly described in Fig. 1a. In the first instance, the printing plate moves against the doctor blade, which removes the excess ink from the engraved cells by a doctor blade. Then, the ink is transferred to the surface of substrate, which is mounted in the nip between printing plate and impression cylinder [23]. The Ag-IDEs with the dimensions of 40 × 50 mm was made by gravure printing using commercial Ag ink (SS-5100, Shanghai Xinluyi Electronic Materials Co., Ltd.). Then the printed pattern was dried at 120 ◦ C for 1 h. Subsequently, the ink was overlaid on the Ag-IDEs and baked to form sensing layers and dried at 80 ◦ C for 0.5 h. The printed sensor structure and the practical image of the printed gas sensor were shown in Fig. 1b and c. 2.6. Gas sensing measurements The sensor testing was carried out using a homemade sensing system (EE-12b) [21,23]. Briefly, the printed sensor was placed in a cylindrical stainless steel chamber with a size of 12 L. Target gas (acetone, Wetry standard gas analysis technology Co., Ltd.) with a specific concentration (the different concentration of acetone is obtained by the method of static distribution) was introduced into the test chamber and the variation in sensor resistance was monitored using an Electrometer/High Resistance Meter (Keithley 6517B). Then, the upper lid of the chamber was opened for the sensor recovery. The sensor response (S) was calculated by the following equation: S = Ra/Rg
(1)
Where Ra and Rg were the resistances of the sensor element in the presence of atmospheric air and target gas, respectively. The response time and recovery time were defined as the time taken
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Fig. 1. (a) Schematic representation of the gravure printing process. (b) Schematic of the printed sensor. (c) Photo of a printed WO3 /Pt-GNs sensor on a PI substrate.
by the sensor to achieve 90% of the total resistance change after the sensor was exposed to the tested gas and air, respectively. In order to gain the selectivity of sensors to acetone gas, the other testing gases such as ethanol, benzene, formic acid, methanol and n-butanol were applied. 2.7. Characterization XRD was recorded on a Bruker D8-FOCUS diffractometer with a Cu-K␣1 radiation ( = 0.15406 nm). The morphology of the composites was examined by field emission scanning electron microscopy (FE-SEM, JSM-7000F) and transmission electron microscopy (TEM, JEM2010-HR). X-ray photoelectron spectroscopy (XPS) was performed using a PHI5000 Versa Probe. Raman analysis (Jobin Yvon LabRam-010, with a laser excitation wavelength of 632.8 nm and a spot size of 1 m2 ) was performed for the composites examination. 3. Results and discussion 3.1. Properties of samples Fig. 2a showed the SEM image of WO3 microspheres (Inset is a representative SEM image of a single WO3 sphere). It was clear to see that the microspheres of the WO3 sample had a diameter of 2–3 m, which were randomly assembled by numerous nanorods of a diameter of 10–20 nm. XRD pattern was used to analyze the crystal structures of WO3 in Fig. 2b. It was found that all the typical diffraction peaks of XRD were well indexed to standard pattern of PDF card nos. 85-2460, which indicated the profile of hexagonal WO3 . Furthermore, the (002) peak of WO3 samples was significantly stronger than this of the standard pattern. The relative texture coefficient of a specific crystal facet (TChkl ) is defined to assess the degree of the crystal facet exposure (Fig. S1, supporting information). The texture coefficients of (100) and (002) facets are given by [6]: TC002 =
0 I002 /I002 0 0 I002 /I002 + I100 /I100
(2)
TC100 = 1 − TC002
(3)
Here, TC100 and TC002 are the relative texture coefficients of (100) and (002) facets, Ihkl is the measured diffraction intensity of the (hkl) facet, and I0 hkl is the corresponding value of the standard XRD patterns (PDF card no. 85-2460). Fig. S1b displays the TChkl of h-WO3 , WO3 /GNs, and WO3 /Pt-GNs. As calculated by Eqs. (2) and (3), TC002 of h-WO3 , WO3 /GNs, and WO3 /Pt-GNs are 0.665, 0.644 and 0.613, indicating that h-WO3 , WO3 /GNs, and WO3 /Pt-GNs are all predominantly exposed with (002) facets. The larger proportion of (002) facets could enhance the interaction between large dipole moment acetone molecules and WO3 due to the asymmetric distribution of unsaturated coordinated O atoms in the O-terminated (002) facets, resulting in the enhancement of acetone sensing performance especially selectivity [6,30]. The TEM images of WO3 /GMs and WO3 /GNs were shown in Fig. S2a and 2b (supporting information), respectively. It was clear to see in Fig. S2a that WO3 nanorods with a diameter of ∼15 nm were randomly wrapped into around 10-m-sized GMs. In contrast, as shown in Fig. S2b, the WO3 nanorods were randomly dispersed around and partially overlapped with the GNs which exhibited a lateral size of dozens of nanometers. The TEM image of WO3 /PtGNs composites was further shown in Fig. 3a. It was found that the Pt NPs were decorated on the surface of GNs. Meanwhile, several WO3 nanorods were dispersed well with GNs. Lattice distances of (111) plane of Pt (0.382 nm) and (110) plane of h-WO3 (0.362 nm) were marked in the HRTEM image (Fig. 3b). The lattice distances of Pt were slightly larger than that of literature reports [4,29], presumably because of the formed PtO2 when the product was synthesized during the hydrothermal process. The survey-scanned XPS spectrum of WO3 /Pt-GNs was shown in Fig. 3c with all components including W, C, O and Pt, which was consistent with the input materials. Furthermore, the inset of Fig. 3c illustrated the high-resolution XPS spectrum of Pt core levels. It was found that the background noise was obvious, presumably because of the incorporation of Pt was extremely small. The peak binding energy of approximately 65.5 eV corresponds to the Pt 5p1/2 state [31]. Deconvoluting Pt 4f core level spectrum exhibited two characteristic peaks of 4f5/2 . The wide and weak peak at 75.6 eV as
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Fig. 2. (a) SEM image of WO3 microspheres. Inset is a representative SEM image of a single WO3 sphere. (b) XRD pattern of WO3 samples and the standard pattern of h-WO3 (PDF card no. 85-2460).
Fig. 3. (a) TEM and (b) HRTEM images of WO3 /Pt-GNs. (c) Survey-scanned XPS spectrum of WO3 /Pt-GNs samples, inset is the high-resolution XPS spectrum of Pt core levels. (d) Raman spectra of WO3 , WO3 /GNs, and WO3 /Pt-GNs samples.
well as a sharp peak at 76.6 eV were corresponding to Pt and PtO2 , respectively. The binding energy of 4f5/2 was consistent with the previous report by Takahiro Maruyama et al. [32], which showed 1.1 eV higher in comparison to some literatures [4,31]. This could be explained from the chemical interaction between the Pt NPs and
GNs, because of the strong -d hybridization between the Pt filled d orbitals and carbon p* empty orbitals [33]. Thus, although the initial purpose was to introduce Pt NPs, the results of TEM and XPS confirmed the partial oxidation of Pt catalysts during the hydrothermal process.
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Fig. 4. The UV–visible spectra of WO3 , WO3 /GNs and WO3 /Pt-GNs samples. Insert shows the optical band gap of WO3 , WO3 /GNs and WO3 /Pt-GNs samples.
Raman spectra of the WO3 , WO3 /GNs, and WO3 /Pt-GNs samples were shown in Fig. 3d. Three samples all exhibited Raman peaks at 266, 322 and 700–900 cm−1 , which could be attributed to O W O beading modes and O W O stretching modes. In addition, WO3 /GNs and WO3 /Pt-GNs exhibited obvious peak at ∼1335 and ∼1580 cm−1 , which could be attributed to D mode and G mode of graphene. The D peak, located at ∼1335 cm−1 was associated with structural defects and partially disordered structures of the sp2 domains. G peak located at ∼1580 cm−1 could be used to explain the degree of graphitization [34–36]. In this work, the ID /IG ratio of the WO3 /Pt-GNs was about 1.3, which exhibited a 30% enhancement in comparison to that of WO3 /GNs. The increased ID /IG ratio might be caused by the introduced Pt NPs which were synthesized by the hydrothermal method, resulting in a decrease of the sp2 domain and an increasing amount of defect sites. The defect sites could facilitate the adsorption of oxygen and acetone molecules, which was beneficial for gas sensing performance. The UV–visible spectra of pure h-WO3 , WO3 /GNs and WO3 /PtGNs composites was shown in Fig. 4. The optical band gap (Fig. 4, insert) was calculated from UV–vis absorption spectra using the Tauc plot (linear portion of the plot between (ahv)1/2 with hv where, a is absorption coefficient and v is the optical frequency). The value of the band gap was found to increase from 2.82 to 3.17 eV when GNs and Pt-GNs were incorporated into WO3 -based system. The increased band gap is due the change in the electronic interaction of WO3 microspheres with GNs and Pt-GNs [16]. 3.2. Gas sensing properties The gas sensing measurements of WO3 /Pt-GNs sensor were carried out in a homemade computer-controlled static gas sensing characterization system (EE-12b). The relationships between the operating temperature and gas response of the gas sensors based on WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs to 10 ppm acetone were tested, and the results were shown in Fig. 5. The responses of the three sensors to acetone varied obviously from 85 to 300 ◦ C. The WO3 /GMs and WO3 /GNs sensors reached their maximum response of 17.6 and 13.3 at the operating temperature of 255 ◦ C, and their responses then decreased after further increasing the operating temperature. However, the maximum response of the WO3 /Pt-GNs sensor jumped to 13.9 at the operating temperature of 230 ◦ C. Thus, the optimum operating temperature decreased after GNs decorated with Pt NPs. Furthermore, the response of WO3 /Pt-GNs to
Fig. 5. Response of WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs to 10 ppm acetone as a function of the operating temperature.
acetone was almost 30% higher than that of WO3 /GNs at the operating temperature of 200 ◦ C, which reflected the effect of catalytic Pt NPs decoration on the enhancement of the sensing properties toward acetone. Besides, the response (Ra /Rg ) of WO3 /GNs and WO3 /Pt-GNs to acetone was lower than that of WO3 /GMs when the operating temperature was above 180 ◦ C. The reason was presumably because the Rg of WO3 /GNs and WO3 /Pt-GNs were almost 4 times higher than that of WO3 /GMs (which could be seen in Fig. 6). The response transient of the gas sensor based on WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs samples to 10 ppm acetone were investigated and shown in Fig. 6. It could be seen that the response/recovery time of WO3 /GNs and WO3 /Pt-GNs were about 15.2/9.6 s and 14.1/16.8 s, which exhibited a shortened gas response/recovery time compared to that of WO3 /GMs. The fast response-recovery characteristics of the WO3 /GNs and WO3 /PtGNs sensors was mainly attributed to the massive p-n junction active sites at the WO3 /graphene interface which would quickly convert CH3 COCH3 with O− /O2− to CH3 CHO, CO2 and H2 O, resulting in a fast flow of charge carriers into the conduction band [15]. Besides, the background noise of WO3 /GNs and WO3 /Pt-GNs were remarkably higher than that of WO3 /GMs, presumably also because of the substantial numbers of p-n junctions which might reduce conducting channels to the charge carriers that caused the fluctuation of resistance. Fig. 7a showed dynamic response resistance of the printed WO3 /Pt-GNs sensor towards 1–100 ppm acetone concentration at 230 ◦ C. It could be seen that the sensor resistance significantly decreased after acetone exposure. When acetone gas was removed, the resistance increased and fully recovered its initial values. Besides, the saturation value of Rg decreased gradually with increasing gas concentration. The responses of WO3 /Pt-GNs sensor increased linearly with the increase of acetone concentration in a log–log plot as shown in the inset of Fig. 7a. However, the responses showed non-linearity as a function of acetone concentrations, presumably because of the complex structure of the composite which might possess a complicated influence on the change of resistance. We followed Vineet Dua’s reported method [37], the theoretical low detection limit (for signal-to-noise ratio of 3) for acetone was estimated to be 400 ppb. However, it was unable to measure acetone in the ppb range due to the large background noise. Thus, the actual low detection limit of WO3 /Pt-GNs sensor was 1 ppm. The influence of the relative humidity (RH) on sensing performance was
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GNs sensor could be operated without noticeable degradation in electrical performance when bent to the extreme. Selectivity is an important parameter of gas sensors, especially for breath analysis sensors which the sensitivity toward a specific gas needs to be distinctly higher than that of more than 200 different VOCs in human breath [2–4]. The gas selectivity of the WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs sensors through comparing the sensitivities toward various typical gases in the human breath were studied in Fig. 8. The WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs sensors showed a high response of 14.6, 9.8, and 12.2 to 10 ppm acetone, but a negligible response of below 3 towards 10 ppm of ethanol, benzene, formic acid, methanol and n-butanol. In order to compare the differences between the selectivity of the three sensors, the ratios of gas response values to acetone and other gases were calculated and shown in Table 1. It could be seen that the WO3 /Pt-GNs sensor exhibited the highest average ratio of 8.1, indicating that the selectivity of WO3 /Pt-GNs sensor was higher than that of WO3 /GMs and WO3 /GNs. The good selectivity of the WO3 /Pt-GNs sensor was mainly attributed to the exposed (002) facets of WO3 which was not wrapped into GNs, resulting in a strong interaction to acetone molecules compared to other gases [6]. Furthermore, the cross-selectivity between acetone and ethanol with different concentrations and testing temperatures were investigated (Fig. S5). A comparison between the cross-selectivity of WO3 /Pt-GNs and those reported in the literature for acetone detection was summarized in Table 2. To quantify the selectivity of several materials of literature reports, the ratios of gas response values to acetone and ethanol (Sac /Set ), methanol (Sac /Sme ) and benzene (Sac /Sbe ) were calculated. It is noticed that the printed WO3 /Pt-GNs shows 5.7 times of Sac /Set at 10 ppm, better than that of pure h-WO3 at 100 ppm [6], suggesting that Pt-GNs incorporated into h-WO3 enhanced the selectivity towards acetone than ethanol, as well as methanol and benzene. The enhanced selectivity maybe is attribute to the synergistic effect of exposed active sites, p-n junctions and even Pt NPs. Compared with other graphene composite sensors [15,17], the operating temperature of WO3 /Pt-GNs is the lowest among these materials, the best selectivity towards acetone than ethanol, methanol and benzene has been achieved. Especially, Seon-Jin Choi et al. demonstrated a similar WO3 /graphene material to our materials, which displayed good sensing performance towards acetone and H2 S. However, the operating temperature of WO3 /Pt-GNs is lower than WO3 /graphene [16]. The ZnFe2 O4 system seemed to exhibit a high Sac /Sbe value of 12.2 at the operating temperature near to 200 ◦ C [38]. However, the Sac /Set and Sac /Sme values were too low. In short, the WO3 /Pt-GNs sensor could better recognize as well as distinguish these VOCs and achieved higher selectivity compared with other materials reported at a relatively low operating temperature of 200 ◦ C.
3.3. Gas sensing mechanism
Fig. 6. Response transient of the gas sensor based on WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs samples to 10 ppm acetone at 200 ◦ C.
also investigated considering the exhaled breath gases containing highly water vapor. Fig. 7b showed WO3 /Pt-GNs sensor response to acetone as a function of RH at 230 ◦ C. The result showed that the sensitivities on high RH (95%) are lower than that in atmospheric condition. The limit of detection toward acetone was ∼600 ppb (linear extrapolation). Besides, the initial resistance of the WO3/PtGNs sensor at 230 ◦ C as a function of bending radius of curvature was shown in Fig. S4. The result showed that the printed WO3 /Pt-
It has been demonstrated that the printed WO3 /Pt-GNs sensor has a much faster response-recovery characteristics, higher sensitivity and selectivity at the operating temperature of 200 ◦ C. This may result from two factors: (i) catalytic Pt nanoparticle decoration; (ii) formation massive p-n junctions at the WO3 /GNs interfaces. WO3 is a typical n-type semiconducting sensor material. In ambient air, free oxygen molecules are absorbed on WO3 surface, and the absorbed oxygen molecules capture electrons from the conduction band of WO3 to form chemisorbed oxygen ions, such as O2 − , O− and O2− . The chemical reaction is listed below [39–41]: O2(ads) + e− ⇒ O2−
(4)
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Table 1 The ratios of gas response values to acetone and other gases including ethanol, benzene, formic acid, methanol and n-butanol. Sacetone Sethanol
Materials WO3 /GMs WO3 /GNs WO3 /Pt-GNs
5.2 8.1 5.7
Sacetone Sbenzene 4.8 8.6 10.1
Sacetone Sformicacid 11.6 7.9 9.1
Sacetone Smethanol 4.3 7.2 9.3
Sacetone Sn − butanol 10.5 7.6 7.7
Average 6.1 7.9 8.1
Table 2 The comparative analysis of gas selectivity of different materials in the present study and those reported in the literature, the ratios of gas response values of acetone to ethanol, methanol and benzene are labeled as Sac /Set , Sac /Sme and Sace /Sbe . Materials
Operating temperature ( ◦ C)
Sac :Set
Sac :Sme
Sac :Sbe
Reference
h-WO3 WO3 /Graphene CuO-ZnO/rGO SnO2 -Ni/Graphene ZnFe2 O4 ZnFe2 O4 WO3 /Pt-GNs
230 300 340 350 215 200 200
5.7 at 100 ppm 2.4 at 2 ppm 3.7 at 10 ppm 1.6 at 5 ppm 1.4 at 100 ppm 2.0 at 30 ppm 5.7 at 10 ppm
2.9 at 100 ppm N/A. 11.8 at 100 ppm N/A. 3.1 at 100 ppm 2.9 at 30 ppm 9.3 at 10 ppm
4.7 at 100 ppm N/A. N/A. N/A. 12.2 at 100 ppm 9.1 at 30 ppm 10.1 at 10 ppm
6 16 15 17 37 42 Current work
Fig. 7. (a) Response-recovery curve of WO3 /Pt-GNs to different concentrations of acetone measured at 230 ◦ C. Inset is a log–log plot of the response (S) curves to different acetone concentrations (C, 1 ppm–100 ppm) at the operating temperature of 230 ◦ C; (b) Sensor responses to different concentrations of acetone under different RHs.
O2(ads) + 2e− ⇒ O−
(5)
O2(ads) + 4e− ⇒ O2−
(6)
These three regions of the absorbed oxygen species are a function of the operating temperature. The first region (Eq. (4)) starts at room temperature and ends at ∼100 ◦ C. This temperature interval only allows the adsorption of O2 − on the surface through the chemical reaction; the second region (Eq. (5)) is situated in the 100–225 ◦ C, O− species are formed and are dominantly present; the third region (Eq. (6)) is located between 225 and 400 ◦ C, when the available thermal energy is enough to form O2− species [42]. When exposed to acetone, the acetone molecules can react with the absorbed oxygen ion (O− or O2− ) leading to the formation of CO2 and H2 O. The process can be expressed in the following reactions [17,43]: CH3 COCH3(gas) + 8O− ⇒ 3CO2(gas) + 3H2 O(gas) + 8e− (ads) CH3 COCH3(gas) + 8O2− (ads)
⇒ 3CO2(ads) + 3H2 O(gas) + 16e−
(7) (8)
As a result, it is precisely in the second and third regions (Eqs. (5) and (6)) that the greatest increment in sensitivity is displayed. This is the primary reason why the optimum operating temperature to acetone is usually above 250 ◦ C, which is consistent with the experimental results of WO3 /GMs and WO3 /GNs. However, the
introduced Pt NPs is an effect oxygen dissociation catalyst, which has high availability to activate the dissociation of molecular oxygen (O− or O2− ) at a relatively low temperature [44,45]. Moreover, the as-created O− or O2− species could spill onto the WO3 surface, which is known as the ‘spillover effect’ [4,17,46,47]. Therefore, the optimum operating temperature for WO3 /Pt-GNs sensors is lower as well as the response is higher than WO3 /GNs sensors. In this work, the relationships between the microstructure and gas sensing performance of WO3 functionalized by graphene with different sizes had been investigated. Large-sized GMs may wrap around the WO3 microspheres, resulting in the reduction of the amount of active sites on WO3 surface, which possesses a negative effect on the acetone selectivity. In contrast, more (002) facets of WO3 may be exposed due to the small size of GNs, resulting in a strong interaction to acetone molecules. Therefore, WO3 functionalized by GNs could achieve a high selectivity. Furthermore, it is well known that the p-n junctions formed at the interface between metal oxide and graphene would make a good contribution to the improvement of gas sensing performance [15–17]. A number of p-n junctions formed at the WO3 /GNs interface would quickly convert CH3 COCH3 with O− /O2− to CO2 and H2 O, resulting in a short gas response/recovery time. However, it should be mentioned that too many p-n junctions also have a negative effect on background noise (as described in Fig. 6). Thus, the reasonable design and control of
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Fig. 8. Response of the sensor based on WO3 /GMs, WO3 /GNs and WO3 /Pt-GNs to various test gases with a concentration of 10 ppm at 200 ◦ C.
the p-n junction structure is a promising method to enhance the sensing performance. 4. Conclusions In conclusion, (002) facets exposed WO3 microspheres loaded with small-sized Pt-decorated graphene had been synthesized and exhibited a high selectivity and sensitivity to low concentration acetone gases at the operating temperature of 200 ◦ C. The superior performance of the printed sensors was attributed to a large number of exposed (002) facets of WO3 which was not wrapped into GNs as well as Pt catalysts. Thus, WO3 /Pt-GNs composite sensor is a promising selective, portable and low-cost chip for acetone detection. Acknowledgements This research was supported by the Natural Science Foundation of Shanghai (No. 16ZR1424400), Shanghai Municipal Education Committee Key Laboratory of Molecular Imaging Probes and Sensors for Shanghai Universities, and Industrial Research Fund from Wuhu Token Sciences Co., Ltd. Prof. Lei Huang appreciates the support of The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2017.08.158. References ˛ [1] B. Buszewski, M. Kesy, T. Ligor, A. Amann, Human exhaled air analytics: biomarkers of diseases, Biomed. Chromatogr. 21 (2007) 553–566. [2] W.Q. Cao, Y.X. Duan, Breath analysis: potential for cinical diagnosis and exposure assessment, Clin. Chem. 52 (2006) 800–811. [3] W. Miekisch, J.K. Schubert, F.E. Gabriele, N. Schomburg, Diagnostic potential of breath analysis-focus on volatile organic compounds, Clin. Chim. Acta 347 (2004) 25–39. [4] S.J. Choi, I. Lee, B.H. Jang, D.Y. Youn, W.H. Ryu, C.O. Park, I.D. Kim, Selective diagnosis of diabetes using Pt-functionalized WO3 hemitube networks as a sensing layer of acetone in exhaled breath, Anal. Chem. 85 (2013) 1792–1796. [5] S. Salehia, E. Nikana, A.A. Khodadadia, Y. Mortazavia, Highly sensitive carbon nanotubes-SnO2 nanocomposite sensor foracetone detection in diabetes mellitus breath, Sens. Actuators B 205 (2014) 261–267. [6] Q.Q. Jia, H.M. Ji, D.H. Wang, X. Bai, X.H. Sun, Z.G. Jin, Exposed facets induced enhanced acetone selective sensing property of nanostructured tungsten oxide, J. Mater. Chem. A 2 (2014) 13602–13611.
1489
[7] M. Righettoni, A. Tricoli, S.E. Pratsinis, Thermally stable, silica-doped -WO3 for sensing of acetone in the human breath, Chem. Mater. 22 (2010) 3152–3157. [8] T. Xiao, X.Y. Wang, Z.H. Zhao, L. Li, L. Zhang, H.C. Yao, J.S. Wang, Z.J. Li, Highly sensitive and selective acetone sensor based on C-doped WO3 for potential diagnosis of diabetes mellitus, Sens. Actuators B 199 (2014) 210–219. [9] X. Bai, H.M. Ji, P. Gao, Y. Zhang, X.H. Sun, Morphology, phase structure and acetone sensitive properties of copper-doped tungsten oxide sensors, Sens. Actuators B 193 (2014) 100–106. [10] F. Liu, X.F. Chu, Y.P. Dong, W.B. Zhang, W.Q. Sun, L.M. Shen, Acetone gas sensors based on graphene-ZnFe2 O4 composite prepared by solvothermal method, Sens. Actuators B 188 (2013) 469–474. [11] S. Mao, G.H. Lu, J.H. Chen, Nanocarbon-based gas sensors: progress and challenges, J. Mater. Chem. A 2 (2014) 5573–5579. [12] H. Zhang, J.C. Feng, T. Fei, S. Liu, T. Zhang, SnO2 nanoparticles-reduced graphene oxide nanocomposites for NO2 sensing at low operating temperature, Sens. Actuators B 190 (2014) 472–478. [13] E.C. Nallon, V.P. Schnee, C. Bright, M.P. Polcha, Q.L. Li, Chemical discrimination with an unmodified graphene chemical sensor, ACS Sens. 1 (2016) 26–31. [14] N.T. Hu, Z. Yang, Y.Y. Wang, L.L. Zhang, Y. Wang, X.L. Huang, H. Wei, L.M. Wei, Y.F. Zhang, Adsorption of gas molecules on transition metal embedded graphene: a search for high-performance graphene-based catalysts and gas sensors, Nanotechnology 25 (2014) 025502. [15] C. Wang, J.W. Zhu, S.M. Liang, H.P. Bi, Q.F. Han, X.H. Liu, X. Wang, Reduced graphene oxide decorated with CuO-ZnO hetero-junctions: towards high selective gas sensing property to acetone, J. Mater. Chem. A 2 (2014) 18635–18643. [16] S.J. Choi, F. Fuchs, R. Demadrille, B. Grévin, B.H. Jang, S.J. Lee, J.H. Lee, H.L. Tuller, I.D. Kim, Fast responding exhaled-breath sensors using WO3 hemitubes functionalized by graphene-based electronic sensitizers for diagnosis of diseases, ACS Appl. Mater. Interfaces 6 (2014) 9061–9070. [17] S. Singkammo, A. Wisitsoraat, C. Sriprachuabwong, A. Tuantranont, S. Phanichphant, C. Liewhiran, Electrolytically exfoliated graphene-loaded flame-made Ni-doped SnO2 composite film for acetone sensing, ACS Appl. Mater. Interfaces 7 (2015) 3077–3092. [18] H.T. Yu, P.C. Xu, D.W. Lee, X.X. Li, Porous-layered stack of functionalized AuNP-rGO (gold nanoparticles-reduced graphene oxide) nanosheets as a sensing material for the micro-gravimetric detection of chemical vapor, J. Mater. Chem. A 1 (2013) 4444–4450. [19] M.L. Zhang, H.C. Su, Y. Rheem, C.M. Hangarter, N.V. Myung, A rapid room-temperature NO2 sensor based on Tellurium-SWNT hybrid nanostructures, J. Phys. Chem. C 116 (2012) 20067–20074. [20] J.K. Zhang, L. Huang, Y.J. Lin, L. Chen, Z.Y. Zeng, L. Shen, Q. Chen, W.Z. Shi, Pencil-trace on printed silver interdigitated electrodes for paper-based NO2 gas sensors, Appl. Phys. Lett. 106 (2015) 143101. [21] L. Huang, Z.P. Wang, J.K. Zhang, J.L. Pu, Y.J. Lin, S.H. Xu, L. Shen, Q. Chen, W.Z. Shi, Fully printed, rapid-response sensors based on chemically modified graphene for detecting NO2 at room temperature, ACS Appl. Mater. Interfaces 6 (2014) 7426–7433. [22] M. Zhou, Y.H. Lu, Y.Q. Cai, C. Zhang, Y.P. Feng, Ultrafast and sensitive room temperature NH3 gas sensors based on chemically reduced graphene oxide, Nanotechnology 22 (2011) 385502. [23] Y.J. Lin, L. Huang, L. Chen, J.K. Zhang, L. Shen, Q. Chen, W.Z. Shi, Fully gravure-printed NO2 gas sensor on a polyimide foil using WO3 -PEDOT:PSS nanocomposites and Ag electrodes, Sens. Actuators B 216 (2015) 176–183. [24] Q. Zhang, L. Huang, Q.H. Chang, W.Z. Shi, L. Shen, Q. Chen, Gravure-printed interdigital microsupercapacitors on a flexible polyimide substrate using crumpled graphene ink, Nanotechnology 27 (2016) 105401. [25] Y.X. Xiao, L. Huang, Q. Zhang, S.H. Xu, Q. Chen, W.Z. Shi, Gravure printing of hybrid MoS2 @S-rGO interdigitated electrodes for flexible microsupercapacitors, Appl. Phys. Lett. 107 (2015) 013906. [26] A.P. Taylor, L.F. Velásquez-García, Electrospray-printed nanostructured graphene oxide gas sensors, Nanotechnology 26 (2015) 505301. [27] L. Huang, Y. Liu, L.C. Ji, Y.Q. Xie, T. Wang, W.Z. Shi, Pulsed laser assisted reduction of graphene oxide, Carbon 49 (2011) 2431–2436. [28] Y.H. Zhu, G.F. Wang, H. Jiang, L. Chen, X.J. Zhang, One-step ultrasonic synthesis of graphene quantum dots with high quantum yield and their application in sensing alkaline phosphatase, Chem. Commun. 51 (2015) 948–951. [29] G.Q. He, Y. Song, K. Liu, A. Walter, S. Chen, S.W. Chen, Oxygen reduction catalyzed by Platinum nanoparticles supported on graphene quantum dots, ACS Catal. 3 (2013) 831–838. [30] J. Zhu, S.L. Wang, S.H. Xie, H.X. Li, Hexagonal single crystal growth of WO3 nanorods along a [110] axis with enhanced adsorption capacity, Chem. Commun. 47 (2011) 4403–4405. [31] M. Zare, A. Mortezaali, A. Shafiekhani, Photoelectrochemical determination of shallow and deep trap states of Platinum-decorated TiO2 nanotube arrays for Photocatalytic applications, J. Phys. Chem. C 120 (2016) 9017–9027. [32] T. Maruyama, H. Kondo, R. Ghosh, A. Kozawa, S. Naritsuka, Y. Iizumi, T. Okazaki, S. Iijima, Single-walled carbon nanotube synthesis using Pt catalysts under low ethanol pressure via cold-wall chemical vapor deposition in high vacuum, Carbon 96 (2016) 6–13. [33] Y. Shen, Z.H. Zhang, R.R. Long, K.J. Xiao, J.Y. Xi, Synthesis of ultrafine Pt nanoparticles stabilized by pristine graphene nanosheets for electro-oxidation of methanol, ACS Appl. Mater. Interfaces 6 (2014) 15162–15170.
1490
L. Chen et al. / Sensors and Actuators B 255 (2018) 1482–1490
[34] Y.W. Zhu, S. Murali, W.W. Cai, X.S. Li, J.W. Suk, J.R. Potts, R.S. Ruoff, Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906–3924. [35] S. Some, Y. Kim, Y. Yoon, H.J. Yoo, S. Lee, Y. Park, H. Lee, High-quality reduced graphene oxide by a dual-function chemical reduction and healing process, Sci. Rep. 3 (2013) 1929. [36] K.S. Rao, J. Sentilnathan, H.W. Cho, J.J. Wu, M. Yoshimura, Soft processing of graphene nanosheets by glycine-bisulfate ionic-complex-assisted electrochemical exfoliation of graphite for reduction catalysis, Adv. Funct. Mater. 25 (2015) 298–305. [37] V. Dua, S.P. Surwade, S. Ammu, S.R. Agnihotra, S. Jain, K.E. Roberts, S. Park, R.S. Ruoff, S.K. Manohar, All-organic vapor sensor using inkjet-printed reducedgraphene oxide, Angew. Chem. Int. Ed. 49 (2010) 2154–2157. [38] X. Zhou, X.W. Li, H.B. Sun, P. Sun, X.S. Liang, F.M. Liu, X.L. Hu, G.Y. Lu, Nanosheet-assembled ZnFe2 O4 hollow microspheres for high-sensitive acetone sensor, ACS Appl. Mater. Interfaces 7 (2015) 15414–15421. [39] P.P. Sahay, Zinc oxide thin film gas sensor for detection of acetone, J. Mater. Sci. 40 (2005) 4383–4385. [40] M. Batzill, U. Diebold, The surface and materials science of tin oxide, Prog. Surf. Sci. 79 (2005) 47–154. [41] R.S. Khadayate, J.V. Sali, P.P. Patil, Acetone vapor sensing properties of screen printed WO3 thick films, Talanta 72 (2007) 1077–1081. ˜ [42] L. Castaneda, A. Maldonado, M.L. Olvera, Sensing properties of chemically sprayed TiO2 thin films using Ni, Ir, and Rh as catalysts, Sens. Actuators B 133 (2008) 687–693. [43] X. Zhou, J.Y. Liu, C. Wang, P. Sun, X.L. Hu, X.W. Li, K. Shimanoe, N. Yamazoe, G.Y. Lu, Highly sensitive acetone gas sensor based on porous ZnFe2 O4 nanospheres, Sens. Actuators B 206 (2015) 577–583. [44] R.Q. Xing, Q.L. Li, L. Xia, J. Song, L. Xu, J.H. Zhang, Y. Xie, H.W. Song, Au-modified three-dimensional In2 O3 inverse opals: synthesis and improved performance for acetone sensing toward diagnosis of diabetes, Nanoscale 7 (2015) 13051–13060. [45] R.Q. Xing, L. Xu, J. Song, C.Y. Zhou, Q.L. Li, D.L. Liu, H.W. Song, Preparation and gas sensing properties of In2 O3 /Au nanorods for detection of volatile organic compounds in exhaled breath, Sci. Rep. 5 (2015) 10717. [46] Y.H. Xiao, L.Z. Lu, A.Q. Zhang, Y.H. Zhang, L. Sun, L. Huo, F. Li, Highly enhanced acetone sensing performances of porous and single crystalline ZnO nanosheets: high percentage of exposed (100) facets working together with surface modification with Pd nanoparticles, ACS Appl. Mater. Interfaces 4 (2012) 3797–3804. [47] S. Mayavan, J.B. Sim, S.M. Choi, Simultaneous reduction, exfoliation and functionalization of graphite oxide into a graphene-Platinum nanoparticle hybrid for methanol oxidation, J. Mater. Chem. 22 (2012) 6953–6958.
Biographies Lu Chen received his B.S. degree from School of Resources and Environmental Engineering, East China University of Science and Technology, in 2013. He is currently a master candidate in Shanghai Normal University, China. His research interest is synthesis of graphene nanohybrids and their application in printed smart sensors. Lei Huang is a Professor and head of Joint Lab with Wuhu Token for Graphene Electrical Materials and Application in the Department of Physics at Shanghai Normal University. He received his B.S. in Physics from Hefei University of Technology in 1990 and M.S. in Thin Film Physics from Institute of Solid State Physics (Chinese Academy of Sciences), Ph.D. in Materials Science from University of Science and Technology of China in 1999. He has more than 50 publications in SCI journals and holds 10 patents. His research interests are graphene and nano-carbon; printed smart sensors and flexible nano energy devices. Youjie Lin received his M.S. degree from College of Mathematics and Physics, Shanghai Normal University, China in 2015. His current research interests include the preparation and functionalization of the nanostructured materials for printed sensors. Liman Sai is a lecturer of College of Mathematics and Science, Shanghai Normal University, China. She received his Ph.D. degree from Shanghai Jiao Tong University, China in 2015. Her current research interests are in the areas of quantum dot luminescence device. Quanhong Chang is a lecturer of College of Mathematics and Science, Shanghai Normal University, China. He received his Ph.D. degree from Harbin Institute of Technology, China in 2015. His current research interests include the preparation and functionalization of flexible nano energy devices. Wangzhou Shi is a professor and Vice Dean of College of Mathematics and Science, Shanghai Normal University, China. He received his Ph.D. degree from Institute of Solid State Physics, Chinese Academy of Sciences, in 1994. His research interests are in the areas of thin film devices. Qi Chen is an adjunct research professor of Joint Lab with Wuhu Token for Graphene Electrical Materials and Application from Wuhu Token Sciences Co. Ltd.