2D Graphene Nanoplatelet-Tungsten Trioxide Hydrate

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2D/2D Graphene Nanoplatelet-Tungsten Trioxide Hydrate Nanocomposites for Sensing Acetone Shibin Sun, Xing Xiong, jingang han, Xueting Chang, nannan wang, mingwei wang, Yanhua Lei, Tao Liu, and Yanqiu Zhu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02185 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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ACS Applied Nano Materials

2D/2D Graphene Nanoplatelet-Tungsten Trioxide Hydrate Nanocomposites for Sensing Acetone Shibin Sun1, Xing Xiong1, Jingang Han1, Xueting Chang2*, Nannan Wang3, Mingwei Wang1, Yanhua Lei2, Tao Liu2, Yanqiu Zhu4* 1

College of Logistics Engineering, Shanghai Maritime University, Shanghai 201306, China 2 Institute

of Marine Materials Science and Engineering, College of Ocean Science and

Engineering, Shanghai Maritime University, Shanghai 201306, China 3

Fullerene Research Center, School of Resources, Environment and Materials, Guangxi University, Guangxi 530004, China

4

College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4SB, UK

1

ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT The development of miniaturized, low-cost, and high-performance acetone gas sensors is highly desirable for workshop safety management and human health diagnosis. Herein, we present a portable integrated system for real-time detection of acetone concentration based on a novel 2D/2D nanocomposite of tungsten oxide hydrate (WO3·H2O) nanoplates and graphene nanoplatelets (GNP). The GNP-WO3·H2O nanocomposites were firstly synthesized using a precipitation process at room temperature with sodium tungstate dihydrate as the precursor and GNP as the template. At low GNP content (0.5 and 1.0 wt.%), the GNP-WO3·H2O nanocomposites exhibited superior sensitivities to the pure WO3·H2O nanoplates. The GNP-WO3·H2O nanocomposite with 1.0 wt. % GNP addition could deliver a sensitivity of 6.7 towards 5 ppm acetone, with a fast response time of 7 s. After annealing at 200-400 ºC, the acetone-sensing performance of both the WO3·H2O nanoplates and GNP-WO3·H2O nanocomposites were obviously deteriorated, which can be attributed to the transformation of WO3·H2O to WO3. Based on the 400 ºC-annealed GNP-WO3·H2O nanocomposite, a portable integrated system that mainly consists a controller, an acetone gas sensor, a USB power supply module, and an OLED screen, was assembled. The integrated system is capable of monitoring acetone concentration in real time with low energy consumption. Keywords: Graphene nanoplatelets; tungsten oxide hydrate; annealing; acetone; gas sensor.

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1. INTRODUCTION Acetone, as one common reagent, has been widely used in the fields of plastics, rubber fiber, tissues, paint, explosives, and so on.1 However, acetone can anaesthetize human central nervous system, which may cause headache, narcosis, vertigo, temporary disturbance of consciousness, and even death.2 Acetone can also raise security problems since it can mix with air to form extreme flammable and explosive mixtures that can explode when they are exposed to open fire.3 In addition, the concentration of acetone in human breath is a crucial index for the diagnosis and health assessment of patients with diabetic ketosis.4 Therefore, it is of urgent importance to exploit high-performance acetone gas sensor to monitor acetone concentration for workplace safety and human health. In the past few decades, tungsten trioxide (WO3) nanocrystals-based gas sensors have been widely explored for acetone detection due to their low cost, diversity of crystal structures, excellent chemical stability, and high sensitivity.5-11 As is known, the gas-sensing effect of the WO3 nanocrystals originates from the reaction between the gas molecules and the oxygen ionic species adsorbed on their surfaces.12,

13

The morphology, dimension, size, and microstructure of the WO3

nanocrystals could significantly influence the adsorption of oxygen species on their surface, and therefore determine their gas-sensing performance.14 Among various WO3 nanocrystals, two-dimensional (2D) WO3 nanocrystals are very promising materials for gas sensing applications, since their unique lateral structure and high specific surface area could lead to a rapid and effective adsorption-desorption of gas molecules.15-20 For example, Chen et al. synthesized WO3 nanoplates using an intercalation-topochemical method, and the as-obtained WO3 nanoplates were highly sensitive to organic vapors including acetone, ethanol, methanol, and benzene.15, 16 Compared with the WO3 nanoparticles, the WO3 nanoplates had not only higher sensitivities but also faster response speed. Shendage et al. synthesized thin films of WO3 with 2D nanoplate-like morphology using a hydrothermal technique.17 The WO3 thin film sensor exhibited excellent NO2 detecting properties at low operating temperature of 100 ºC, with a sensitivity of 10 and 131.75 for 5 and 100 ppm NO2, respectively. Besides the WO3 nanocrystals, tungsten oxide hydrates (WO3·xH2O) nanocrystals are also 3



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promising sensitive materials for gas detection.21-23 Zeng et al. reported a hierarchical WO3·H2O architecture consisting of 2D nanosheets, which exhibited excellent gas-sensing properties to ethanol.22 Han et al. prepared a WO3·H2O microplate film on a glass substrate via a hydrothermal deposition process, and the film could detect 20 ppm NO2 at room temperature.23 Compared with WO3 nanocrystals, the WO3·xH2O nanocrystals have two advantages. Firstly, WO3·xH2O usually has a layered structure composed of corner-shared WO5(OH)2 or WO6 layers and interlayer water molecules.24, 25 Because the interlayer water increases the distance between adjacent WO5(OH)2 or WO6 layers, WO3·xH2O can be readily exfoliated into lamellar structure. Thus, the WO3·xH2O nanocrystals always have 2D plate-like morphology regardless of the synthesis method.25-28 Secondly, 2D WO3·xH2O nanocrystals can be prepared at a relatively low reaction temperature and short reaction time using inexpensive tungstate as precursor, with high homogeneity and controlled crystal structure and composition.29, 30 It should be noted that the interlayer water in the WO3·xH2O will be removed when the related sensors are used at relatively high temperatures, leading to the transformation of 2D WO3·xH2O nanocrystals to 2D WO3 nanocrystals. Accordingly, the gas-sensing properties of the WO3·xH2O nanocrystals could probably be influenced. However, little has been known about this. In order to develop WO3·xH2O nanocrystal-based gas sensors, it is essential to explore the influence of annealing or operating temperature on the gas-sensing properties of the WO3·xH2O nanocrystals. Recent studies have shown that hybridization of the WO3 nanocrystals with graphene (G), graphene oxide (GO), or reduced graphene oxide (RGO) is an effective strategy to improve their gas-sensing properties.31-36 Perfecto et al. prepared a WO3-RGO composite by coupling the bundle-like WO3 with RGO using a one-pot microwave-assisted hydrothermal.32 The sensitivity of the WO3-RGO sensor to 100 ppm acetone was 20% higher than that of the WO3 sensor. The WO3-GO composite nanofibers that were synthesized by an electrospinning technique displayed a sensitivity of 35.9 to 100 ppm acetone at 375 ºC, which was 4.3 times higher than that of the pure WO3 nanofibers.33 Kaur et al. reported an acetone sensor based on the Gd-doped WO3/RGO nanocomposite, which exhibited significant improvements in sensing response at an optimum operating temperature of 200 ºC.34 These achievements inspire us to harness the gas-sensing property 4


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of the WO3·xH2O nanocrystals. To date, very few works have been devoted to the gas-sensing behaviours of the hybrids of the WO3·xH2O nanocrystals and G or its derivatives. In this work, the 2D/2D graphene nanoplatelets (GNP)-WO3·H2O nanocomposites have been synthesized by using a facile, efficient precipitation method at room temperature with sodium tungstate dihydrate (Na2WO4·2H2O) and graphene nanoplatelets (GNP) as raw materials. The influences of GNP content and annealing temperature on the acetone-sensing properties of the GNP-WO3·H2O nanocomposites were then systematically investigated. Finally, a portable integrated system based on the 400 ºC-annealed GNP-WO3·H2O nanocomposite was then assembled, which could realize the real-time detection of acetone concentration. 2. EXPERIMENTAL 2.1 Chemicals Sodium tungstate dihydrate (Na2WO4·2H2O), concentrated hydrochloric acid (HCl, 36~38%), and acetone were provided by Sinopharm Chemical Reagent Co., Ltd. GNP was purchased from Nanjing XFNANO Materials Tech Co., Ltd. 2.2 Synthesis of 2D/2D GNP-WO3·H2O nanocomposites The GNP-WO3·H2O nanocomposites were fabricated via a simple precipitation method at room temperature, as illustrated in Fig. 1a. Firstly, the Na2WO4·2H2O solution was prepared by dissolving 0.8 g Na2WO4·2H2O in 20 ml distilled water. The GNP solution (0.5 g/L) was also prepared by dispersing the GNP in distilled water under ultrasonic vibration for 1 h. Then, different amount of the GNP solution was mixed with the Na2WO4·2H2O solution to achieve mixture solutions with different GNP weight ratio. After magnetic stirring for 10 min, 4 ml concentrated HCl was slowly added. After further stirring for 1 h, the grey precipitates containing the GNP-WO3·H2O nanocomposites were obtained. The GNP-WO3·H2O nanocomposites were washed three times with distilled water, collected by centrifugation, and dried in a drying box at 60 ºC for 24 h. The as-synthesized GNP-WO3·H2O nanocomposites were annealed in a muffle furnace at temperature ranging from 200 to 400 ºC for 1 h. The GNP-WO3·H2O nanocomposites containing 0.5, 1, and 1.5 wt. % GNP were labeled as 0.5GNP-WO3·H2O, 1.0GNP-WO3·H2O, and 1.5GNP-WO3·H2O, respectively. The annealed GNP-WO3·H2O nanocomposites at 200, 300, and 400 ºC are designated as 5



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GNP-WO3·H2O-200, GNP-WO3·H2O-300, and GNP-WO3·H2O-400, respectively. For comparison, the bare WO3·H2O nanoplates were also synthesized. The synthesis procedure of the WO3·H2O nanoplates was almost identical to that of the GNP-WO3·H2O nanocomposites, except that the GNP solution adding step was removed. The annealed WO3·H2O nanoplates at 200, 300, and 400 ºC are designated as WO3·H2O-200, WO3·H2O-300, and WO3·H2O-400, respectively.

Figure 1 Schematics: (a) synthesis of GNP-WO3·H2O nanocomposite, (b) deposition of gas-sensing film, and (c) sensor for gas-sensing tests. 2.3 Characterization The as-synthesized products were characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectrometery (XPS, PHI 5300), scanning electron microscopy (SEM, JSM 7500F, 20 kV), transmission electron microscopy (TEM, JEOL 2000FX, 200 kV), and atomic force microscopy (AFM, Park XE-70). Thermogravimetric (TGA) analysis was performed on a Netzsch 449 F3 Simultaneous Thermal Analyzer in N2 atmosphere from room temperature to 700 ºC at a heating rate of 10º/min. 2.4 Acetone-sensing tests The acetone-sensing measurements were performed on a WS-30A system. The deposition 6


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procedure of the sensing film is illustrated in Fig. 1b. Firstly, 5 mg of the GNP-WO3·H2O nanocomposites was dispersed in 2.5 ml ethanol to form a slurry dispersion. Then, the dispersion was dropped onto a ceramic tube, which was continuously rotated along with a coaxial rotating rod at a rotation rate of 15 r/min. During the dropping process, a 1000 W Xe lamp was used to dry the dispersion on the ceramic tube surface. In order to obtain a homogenous sensing film, the dropping process was repeated 3 times. Fig. 1c demonstrates the schematic of the gas sensor based on the GNP-WO3·H2O nanocomposites. Two Au electrodes and four Pt wires were connected to the ceramic tube. The operating temperature of the gas sensor was controlled by the Ni-Cr heating wire inserted in the middle of the ceramic tube. The circuit diagram of the sensor is shown in Fig. S1. The sensitivity (S) of the GNP-WO3·H2O-based acetone sensor was defined as Rair/Rgas, where Rair and Rgas are the resistances of the sensor exposed in air and in acetone, respectively. 3. RESULTS AND DISCUSSION 3.1 Characterization The XRD pattern of the WO3·H2O nanoplates is shown in Fig. 2a. The diffraction peaks are clearly assigned to WO3·H2O with orthorhombic structure (JCPDS: 01-084-0886).29 The main peaks located at 16.5°, 25.7°, 35.1°, 49.8°, 52.8°, 56.3°, and 57.3 ° correspond to the (020), (111), (131), (202), (222), (311) and (113) planes, respectively. The XRD patterns for the 0.5GNP-WO3·H2O, 1.0GNP-WO3·H2O, and 1.5GNP-WO3·H2O are almost identical to that of the WO3·H2O nanoplates, indicative of the successful formation of WO3·H2O with high phase purity in the GNP-WO3·H2O nanocomposites. The survey XPS spectra (Fig. 2b) verify that the GNP-WO3·H2O nanocomposites contains W, C, and O. The survey XPS spectrum (Fig. S2, see Supporting Information) of pure GNP shows the presence of C and O elements with a C/O ratio of 12.88, indicating that the GNP contains a small number of O-containing groups. Fig. 2c shows the W 4f core-level spectrum of 1.0GNP-WO3·H2O. The two main peaks located at 35.9 and 38 eV correspond to W 4f7/2 and W 4f5/2 of W3+ oxidation state for WO3, respectively.37 As shown in Fig. 2d, the C 1s spectrum for the 1.0GNP-WO3·H2O can be deconvoluted into three peaks. The dominant peak at 284.6 eV can be attributed to the C-C bond, while the other two weak peaks at 286.5 and 288.3 eV correspond to the C-O and COOH bonds, respectively.38 7



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Figure 2 (a) XRD patterns and (b) XPS survey spectra of WO3·H2O nanoplates and GNP-WO3·H2O nanocomposites. (c) W 4f and (d) C 1s spectra of 1.0GNP-WO3·H2O. SEM images (Fig. S3a, see Supporting Information) indicate that the pure WO3·H2O sample consists of a great number of nanoplates that are stacked layer by layer. One can see that most of the nanoplates are highly uniform with a quasi-circular shape, and with diameters of 1 μm, as shown in Fig. S3b (see Supporting Information). Fig. 3a is a typical TEM image of the WO3·H2O nanoplates, which further verifies their uniform, thick, and quasi-circular features. As shown in Fig. 4a and 4b, the AFM image and the corresponding height profile further demonstrated the plate-like morphology of the WO3·H2O nanoplates with thickness of ~200 nm. The HRTEM image in Fig. 3b shows the continuous and clear lattice fringe with a spacing of 5.3 Å which matches well with the (020) plane of orthorhombic WO3·H2O. For the 0.5GNP-WO3·H2O, it can be seen that the WO3·H2O nanoplates were densely decorated on the GNP surfaces, as shown in Fig. 3c, S4a and S4b (see Supporting Information). Notably, the shape of the WO3·H2O nanoplates became obviously irregular. Compared with the nanoplates that constitute pure WO3·H2O sample, most of the nanoplates grown on the GNP surfaces were much smaller, with sizes as small as tens of nanometers (Fig. S4c and S4d, see Supporting Information). The 1.0GNP-WO3·H2O possessed similar morphology to that of the 8


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0.5GNP-WO3·H2O, and most of the WO3·H2O nanoplates were grown on the GNP surfaces, as shown in Figs. 2d and S5a-S5d (see Supporting Information). In addition, few large aggregates can be found, as shown in Fig. S5e (see Supporting Information). High magnification TEM image (Fig. S5f, see Supporting Information) shows that the large aggregates were still composed of WO3·H2O nanoplates. HRTEM image in Fig. 3e shows that the well-crystallized 2D WO3·H2O nanoplates were strongly bonded with a 16 layered GNP. As confirmed by the AFM images (Fig. 4c, 4d, and S6), the WO3·H2O nanoplates grown on the GNP surface had thickness of 40-100 nm, which can further demonstrate the 2D/2D structure of the 1.0GNP-WO3·H2O. In the case of the 1.5GNP-WO3·H2O, the WO3·H2O nanoplates became larger and thicker, as shown in Fig. 3f and S7a. Obviously, a majority of the large WO3·H2O nanoplates were not grown on the GNP surfaces, and they were agglomerated and overlapped with the GNP (Fig. S7b and S7c, see Supporting Information). Similar to that of the 0.5GNP-WO3·H2O and 1.0GNP-WO3·H2O, a large number of thin WO3·H2O nanoplates were anchored onto the GNP surfaces (Fig. S7d, see Supporting Information).

Figure 3 TEM and HRTEM images: (a, b) WO3·H2O nanoplates, (c) 0.5GNP-WO3·H2O, (d, e) 1.0GNP-WO3·H2O, and (f) 1.5GNP-WO3·H2O. 9



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Figure 4 (a) AFM image and (b) height profile of WO3·H2O nanoplates. (c) AFM image and (d) height profile of 1.0GNP-WO3·H2O. 3.2 Acetone-sensing performance Fig. 5a-5d displays the response-recovery curves of the sensors based on different sensing materials towards 100 ppm acetone vapor at an operating temperature in the range of 200-400 ºC. Both the WO3·H2O and GNP-WO3·H2O sensors behave as n-type sensors because their Uout sharply increased when exposing to the acetone vapor (i.e. electrical resistance of the sensors decreased). As plotted in Fig. 5e, the sensitivities of all the sensors increased with increasing the operating temperature to the highest value of 340 ºC, beyond which they decreased again. Among all the tested sensors, the 1.0GNP-WO3·H2O sensor exhibited the best acetone-sensing activity. At 340 ºC, the sensitivities of the WO3·H2O, 0.5GNP-WO3·H2O, 1.0GNP-WO3·H2O, and 1.5GNP-WO3·H2O sensors towards 100 ppm acetone are 8.3, 11.5, 14.5, and 5.9, respectively. Fig. 5f shows the response and recovery times of the 1.0GNP-WO3·H2O sensor to 100 ppm acetone. Evidently, both the response and recovery rates were accelerated when the operating temperature was increased. At 200 ºC, the response and recovery times were 44 and 48 s, respectively, which decreased to 5 and 13 10


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s when operated at 300 ºC. The sensing performance of the WO3·H2O and 1.0GNP-WO3·H2O sensors towards other organic vapors such as ethanol and ammonia were also studied, as shown in Fig. S8. Obviously, the sensitivities of the two sensors towards ethanol and ammonia were much lower than those towards acetone, suggesting that they have good selectivity for acetone.

Figure 5 (a)-(d) Response-recovery curves of different sensors towards 100 ppm acetone at operating temperature ranging from 200 to 400 ºC. (e) Plots of sensitivities of different sensors towards 100 ppm acetone as a function of operating temperature. (f) Response and recovery times of 1.0GNP-WO3·H2O sensor towards 100 ppm acetone at varied operating temperature.

Figure 6 (a) Response-recovery curves and (b) plots of sensitivities of WO3·H2O and GNP-WO3·H2O sensors vs concentrations of acetone vapor at 300 ºC. 11



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Fig. 6a presents the response-recovery curves of different sensors operated at 300 ºC. As shown in the response profiles, the Uout for the all the sensors increased instantly to a stable value upon exposed to the acetone. As the acetone vapor was expelled, the Uout decreased shortly to the original value. This result confirms the rapid response and recovery characteristics of all the tested acetone sensors. Obviously, the sensitivities of all the tested sensors increased with increasing vapor concentration, and the increasing sensitivity order was 1.5GNP-WO3·H2O < WO3·H2O < 0.5GNP-WO3·H2O < 1.0GNP-WO3·H2O sensor, as displayed in Fig. 6b. The 1.0GNP-WO3·H2O sensor could deliver a sensitivity of 6.7 even the acetone concentration was as low as 5 ppm. The response and recovery times here was 7 and 18 s, respectively. Increasing the acetone vapor concentration to 600 ppm, the sensitivity of the 1.0GNP-WO3·H2O sensor can be as high as 24.5. As is well-known, the gas-sensing performance of oxide semiconductors depends strongly on the adsorption of oxygen species on their surfaces.

39-41

We here take the 1.0GNP-WO3·H2O sensor as

the example to investigate the possible acetone-sensing mechanism. When the 1.0GNP-WO3·H2O sensor is exposed in air, O2 molecules can be adsorbed on the 1.0GNP-WO3·H2O surface. The O2 molecules will then capture the electrons from the conduction band of WO3, forming the chemisorbed oxygen species such as O-, O2-, and O2-. Since the gas sensor is normally operated at relatively high temperature, O2- can be easily be transformed to O- or O2-. That is to say, O- and O2are the main chemisorbed species. The possible reactions can be expressed using the equations (1)-(3). Due to the decreased concentration of electrons, the resistance of the 1.0GNP-WO3·H2O sensor decreases. O2(ads) + e- → O2- (ads)

(1)

O2- (ads) + e- → 2O- (ads)

(2)

O- (ads) + e- → O2- (ads)

(3)

When the 1.0GNP-WO3·H2O sensor is exposed in acetone vapor, a reducing gas, the acetone will react with the oxygen species to release the electrons, which could decrease the resistance of the 1.0GNP-WO3·H2O. The reaction between acetone and chemisorbed oxygen species can be expressed using the equations (4) and (5). CH3COCH3 + 6O2-(ads) → 3CO2 + 3H2O(ads) + 12e12


(4)

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CH3COCH3 + 6O-(ads) → 3CO2 + 3H2O(ads) + 6e-

(5)

The amount of GNP could significantly influence the acetone-sensing properties of the GNP-WO3·H2O nanocomposites. With low GNP content (0.5 and 1.0 wt. %), the GNP-WO3·H2O nanocomposites (0.5GNP-WO3·H2O and 1.0GNP-WO3·H2O) exhibited superior sensitivities to the WO3·H2O nanoplates. However, the 1.5GNP-WO3·H2O with high GNP content possessed decreased sensitivities compared with the WO3·H2O nanoplates. The improvement in the acetone-sensing properties of the 0.5GNP-WO3·H2O and 1.0GNP-WO3·H2O should be attributed to two factors. First, the 2D GNP provided ideal templates for the growth of the WO3·H2O nanostructures, resulting in the formation of the GNP-WO3·H2O nanocomposites with large specific surface area. 35, 42-47 SEM and TEM results have confirmed that most of the WO3·H2O nanoplates were densely decorated on the GNP surfaces, as shown in Figs. 3 and S3-S5 (see Supporting Information). Thus, the GNP-WO3·H2O nanocomposites could provide adequate active sites for the acetone-sensing response, leading to enhanced sensitivities. Second, n-type WO3·H2O and p-type GNP could form a great number of p-n heterojunctions, which afford supplementary sites for the electron transfer between the acetone vapor and the oxygen ionic species adsorbed on surfaces of both the WO3·H2O and GNP. 48-51

Consequently, the acetone-sensing performance of the GNP-WO3·H2O nanocomposites was

improved. When high GNP content (1.5 wt. %) was used, very thick WO3·H2O nanoplates were formed and most of them were not grown on the GNP surfaces (Fig. S7, see Supporting Information). Particularly, serious agglomeration of the WO3·H2O nanoplates happened, which could significantly deteriorate the gas-sensing activities of the 1.5GNP-WO3·H2O nanocomposite. Therefore, the 1.5GNP-WO3·H2O sensor exhibited the lowest sensitivity among all the tested sensors. 3.3 Effect of annealing on the acetone-sensing performance It is well-known that phase transformation (from WO3·H2O to WO3) or possible morphology change of the WO3·H2O nanoplates could happen at high temperature, which may lead to the variation of sensing activities for both the WO3·H2O and GNP-WO3·H2O sensors. In order to understand the effect of temperature on the phase transformation of the sensing materials, TGA analysis was firstly performed in N2 atmosphere at temperature ranging from room temperature to 700 ºC, as shown in Fig. 7a. The TGA curve of pure WO3·H2O sample shows a continuous weight 13



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loss before 330 ºC, corresponding to the loss of crystal water from WO3·H2O. XRD results (Fig. 7b) confirms that WO3 with monoclinic crystal structure (JCPDS: 01-083-0951) was obtained after annealing WO3·H2O at 300 ºC for 1 h. After 330 ºC, the TGA curve of WO3·H2O is close to a straight line, showing that WO3 is stable from 330 to 700 ºC. The total weight loss is ~7.89%, in accordance with the weight loss (7.2%) of one water molecule from WO3·H2O. It should be noted that, after annealing at higher temperature (for example 400 ºC), the XRD peaks of the as-obtained WO3 became sharper, indicative of its higher degree of crystallinity.

Figure 7 (a) TGA curves of WO3·H2O and 1.0GNP-WO3·H2O. XRD patterns of unannealed and annealed samples: (b) WO3·H2O and (c) 1.0GNP-WO3·H2O. (d) W 4f core-level spectra of unannealed and annealed 1.0GNP-WO3·H2O. Different from that of of the bare WO3·H2O sample, the TGA curve of the 1.0GNP-WO3·H2O shows a very slight weight loss before 150 ºC. When the temperature reached 394 ºC, the maximum weight loss of ~8.62% was obtained, indicative of the complete loss of crystal water from the WO3·H2O grown on GNP. The higher total weight loss of the 1.0GNP-WO3·H2O than that of the WO3·H2O may be attributed to the higher amount of absorbed water of the former from air. In addition, minor amount of CO2 or H2O vapor generated from the oxygen-containing groups of the 14


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GNP under thermal processing should be considered. Survey XPS spectra (Fig. S9a, see Supporting Information) of the annealed 1.0GNP-WO3·H2O demonstrate that the relative intensity ratio of C 1s peak to O 1s peak increased with increasing annealing temperature. Also, C 1s core-level XPS spectra (Fig. S9b, see Supporting Information) of show that the C-O (or COOH) group almost disappeared after annealing. These results further identify the loss of the oxygen-containing groups of GNP. As shown in Fig. 7c, XRD patterns verify the formation of WO3 when annealing temperature exceeds 300 ºC. Interestingly, after 366 ºC, the weight of the 1.0GNP-WO3·H2O increased with increasing temperature, which should be attributed to the nitrogen-doping of the GNP. After annealing at 400 ºC for 1 h, WO3 with high degree of crystallinity can be obtained for the 1.0GNP-WO3·H2O. Fig. 7d shows a comparison of the W 4f XPS spectra for unannealed and annealed 1.0GNP-WO3·H2O. After annealing, the binding energies of W 4f7/2 and W 4f5/2 were obviously shifted, which indicates that the annealing process may influence the interaction between GNP and WO3·H2O or WO3.

Figure 8 Plots of sensitivities of sensors based on (a) annealed WO3·H2O and (b) annealed 1.0GNP-WO3·H2O as function of operating temperature towards 100 ppm acetone. Fig. 8a presents the plots of sensitivities of the WO3·H2O, WO3·H2O-200, WO3·H2O-300, and WO3·H2O-400 sensors towards 100 ppm acetone as a function of operating temperature. Compared with those of the WO3·H2O sensor, the sensitivities of the WO3·H2O-200 sensors decreased slightly. For the WO3·H2O-300 and WO3·H2O-400 sensors, dramatic decreases in sensitivities occurred. At 300 ºC, the sensitivities of the WO3·H2O-200, WO3·H2O-300, and WO3·H2O-400 sensors towards 100 ppm acetone were 6.9, 2.3, and 2.4, respectively, which were 1.1, 3.4, 3.25 times lower than that 15



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(7.8) of the WO3·H2O sensor. Similar to the WO3·H2O sensor, the sensitivities of the sensors increased firstly with increasing operating temperature and then reached the highest value at 340 ºC. The influence of annealing temperature on the acetone-sensing activities of the 1.0GNP-WO3·H2O was also studied. As plotted in Fig. 8b, the annealed 1.0GNP-WO3·H2O also exhibited reduced sensitivities in comparison with those of the unannealed 1.0GNP-WO3·H2O. For example, the sensitivity of the 1.0GNP-WO3·H2O sensor to 100 ppm acetone at 300 ºC was 12.1, which reduced to

9.7,

5.2,

and

6.8

for

the

1.0GNP-WO3·H2O-200,

1.0GNP-WO3·H2O-300,

and

1.0GNP-WO3·H2O-400 sensors, respectively. The sensitivities of the 1.0GNP-WO3·H2O and 1.0GNP-WO3·H2O-400 sensors are comparable to those of the previously reported acetone sensors, as listed in Table 1. Notably, the method used in our work for the synthesis of the GNP-WO3·H2O nanocomposites is very simple with cheap raw materials, which may find wide potential applications in gas sensors. Table 1 Comparison of sensing performance of 1.0GNP-WO3·H2O and 1.0GNP-WO3·H2O-400 with other sensing materials. Sensing materials

Sensitivity

Concentration (ppm)

Temperature (ºC)

Refs.

GO-WO3 composite nanofibers

~14

100

300

33

Gd doped WO3/RGO

~70

100

200

34

SnO2-rGO nanocomposites

~4.7

200

Room temperature

39

ZnO/Co3O4 hollow polyhedron

30

100

300

40

Co-doped ZnO nanofibers

16

100

360

41

Sn-doped ZnO nanorods

6.3

200

300

52

α-Fe2O3 nanoparticles

7

100

300

53

rGO-ZnO composites

9.5

100

260

54

G-W18O49 nanocomposites

15.012

100

300

55

SnO2@rGO nanocomposites

2.5

100

160

56

1.0GNP-WO3·H2O

12.1

100

300

This work

1.0GNP-WO3·H2O-400

6.8

100

300

This work

16


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Figure 9 (a) Response-recovery curves and (b) plot of sensitivities of annealed WO3·H2O-based sensors as a function of acetone concentration. (c) Response-recovery curves and (d) plot of sensitivities of annealed 1.0GNP-WO3·H2O-based sensors as a function of acetone concentration. The operating temperature was 300 ºC. Fig. 9a presents the response-recovery curves of the annealed WO3·H2O-based sensors towards 5-600 ppm acetone vapor at 300 ºC. All the annealed WO3·H2O-based sensors exhibited fast response rates because their Uout responded much quickly to the acetone vapor. The WO3·H2O, WO3·H2O-200, WO3·H2O-300, WO3·H2O-400 sensors had a response time of 5, 7, 9, and 10 s towards 5 ppm acetone, respectively. As shown in Fig. 9b, the sensitivities of all the annealed WO3·H2O-based sensors increased with increasing acetone concentrations. Taking the WO3·H2O-300 sensor for example, its sensitivities towards 5, 10, 25, 50, 100, 200, 300, 400, 500, and 600 ppm were 2.62, 3.27, 3.69, 4.12, 5.53, 5.88, 7.64, 8.92, 9.14, and 10.45, respectively. The response-recovery and sensitivities vs acetone concentration curves of the annealed 1.0GNP-WO3·H2O-based sensors are shown in Fig. 9c and Fig. 9d, respectively. Compared with the annealed WO3·H2O-based sensors, the annealed 1.0GNP-WO3·H2O-based sensors could produce much higher Uout values when exposing to acetone vapor, indicative of their higher sensitivities. For example, the sensitivity of the 17



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1.0GNP-WO3·H2O-400 sensor to 200 ppm acetone was 9.65, which was three times higher than that (2.96) of the WO3·H2O-400 sensor. Overall, the deceasing order in sensitivity was 1.0GNP-WO3·H2O

>

1.0GNP-WO3·H2O-200

>

1.0GNP-WO3·H2O-400

>

WO3·H2O

>

WO3·H2O-200 > 1.0GNP-WO3·H2O-300 > WO3·H2O-400 > WO3·H2O-300 sensors. According to the above analyses, the annealing temperature could greatly influence the acetone-sensing properties of both the WO3·H2O and GNP-WO3·H2O sensors. As shown in the SEM and TEM images (Fig. S10 and S11, see Supporting Information), no obvious morphological change of the WO3·H2O nanoplates and the 1.0GNP-WO3·H2O nanocomposite was observed after annealing. As confirmed from the XRD and TGA results, the WO3·H2O nanoplates could lose the inter-structural water molecules when annealing at temperatures above room temperature and then be transformed to the WO3 nanoplates when the annealing temperatures exceed 300 ºC. That is to say, phase transformation of WO3·H2O mainly affected the acetone-sensing characteristics of the WO3·H2O sensor. After annealing, the sensitivities of all the samples were reduced, proving that the WO3·H2O nanocrystals have superior acetone-sensing performance to the WO3 nanocrystals. After annealing at 200 ºC, a mixture of the WO3·H2O nanoplates and the WO3 nanoplates may occur, but the amount of the WO3 nanoplates should be very low since no WO3 phase can be detected from the XRD pattern. As a result, the sensitivity of the WO3·H2O-200 (or 1.0GNP-WO3·H2O-200) sensor is slightly lower than that of the WO3·H2O (or 1.0GNP-WO3·H2O) sensor. After annealing at temperature above 300 ºC, the WO3·H2O nanoplates can be completely transformed to WO3 nanoplates, leading to the dramatic decrease of sensitivities. Due to the high stability of WO3, the annealed GNP-WO3·H2O possessed higher sensing stability than that of the unannealed one. Fig. S12 presents the reproducibility of the WO3·H2O, 1.0GNP-WO3·H2O, and 1.0GNP-WO3·H2O-400 sensors to 100 ppm acetone at 300 ºC. For both the WO3·H2O and 1.0GNP-WO3·H2O sensors (Fig. S12a and S12c), the response-recovery curves gradually attenuated with increasing cycles, indicative of their decreased sensing performance. The sensitivities of the two sensors obviously decreased after certain cycles (20 cycles for the WO3·H2O sensor and 27 cycles for the 1.0GNP-WO3·H2O sensor), and then decreased slightly and continuously, as shown in Fig. S12b and S12d. By contrast, the response-recovery curves of the 1.0GNP-WO3·H2O-400 sensor were repeatable and no obvious 18


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change occurred (Fig. S12e), indicative of its high stability. Accordingly, the sensitivity of the 1.0GNP-WO3·H2O-400 sensor remained relatively stable during the continuous tests (Fig. S12f). It is noteworthy that the WO3·H2O-400 (or 1.0GNP-WO3·H2O-400) sensor showed higher sensitivities than those of the WO3·H2O-300 (or 1.0GNP-WO3·H2O-300) sensor, which may be attributed to the stronger interaction between GNP and WO3 of the former. Normally, the binding between GNP and WO3 should increase with increased annealing temperature (>300 ºC). Thus, the formation of the p-n heterojunctions between the n-type WO3·H2O and p-type GNP would be promoted, resulting in the enhancement of the acetone-sensing performance. 3.4 Portable integrated system for detecting acetone Among all the tested samples, the 1.0GNP-WO3·H2O-400 showed distinct advantage in terms of phase stability, crystallinity, and sensing performance. Therefore, the 1.0GNP-WO3·H2O-400 was selected as the sensitive material to assemble the portable integrated system. The logical diagram and photographs of the integrated system are presented in Fig. 10a and 10b, respectively. The system is mainly composed of a controller, a gas sensor, an A/D conversion module, a USB/TTL module (power supply, 5 V), an OLED screen, and a buzz alarm. The operating temperature of the acetone sensor is 300 ºC. The acetone sensor is connected with a load resistor (4.7 kΩ). When the gas sensor is exposed to acetone vapor, the resistance of the gas sensor decreases, leading to the voltage increase of the load resistor. Through the A/D conversion module, the electrical signals of the load resistor are converted to digital signals. The controller will process the real-time signals by comparing them with the fitted sensitivity curve and then input the concentration value. Finally, the real-time concentration value of the acetone vapor was visually presented on the OLED screen. Fig. 10c shows the photographs of the system for real-time acetone concentration analysis under different conditions (no gas, 260 and 568 ppm acetone). When exposed in ambient air or the acetone concentration was below 5 ppm, “no gas” was displayed on the OLED screen. The alarm value of the system was set to be 500 ppm, beyond which the buzz alarm will be triggered and “warning” is displayed on the OLED screen. Notably, the alarm value can be randomly adjusted as needed by simply modifying the software program running in the background. The integrated system also exhibited fast response, high accuracy and stability. As shown in Video S1 (see supporting 19



Page 20 of 28

information), the testing system responded quickly to the acetone vapor, with a response time of 6 s. When repeatedly sensing 100 ppm acetone for 10 cycles, the same value (99 ppm) was displayed on the OLED screen (Fig. S13), indicative of the high reliability and stability of the testing system.

Figure 10 (a) Logical diagram and (b) photographs of the portable integrated system. (c) Real-time acetone concentration analysis and display under different conditions (no gas, 260 and 568 ppm acetone). 4. CONCLUSIONS In summary, we have successfully prepared the GNP-WO3·H2O hybrids by using a facile, efficient precipitation process at room temperature with Na2WO4·2H2O as the precursor, GNP as templates, and distilled water as solvent. Compared with the WO3·H2O nanoplates, the GNP-WO3·H2O nanocomposites with 0.5 and 1.0 wt. % GNP exhibited obvious enhancement in the acetone-sensing performance. After annealing, the acetone-sensing sensitivities of both the WO3·H2O nanoplates and the GNP-WO3·H2O nanocomposites were obviously reduced. Using the 400 ºC-annealed GNP-WO3·H2O nanocomposite as the sensitive material, we have demonstrated the design and assembly of a portable integrated system that are capable of real-time detection of acetone 20


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concentration in the range of 5 to 600 ppm. This work provides novel 2D/2D hybrids for the development of miniaturized, low-cost, high-performance of acetone gas sensors. AUTHOR INFORMATION Corresponding author. E-mail: [email protected]; [email protected]. Notes. The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is financially supported by the National Key Research and Development Program of China (Nos. 2016YFB0300700 and 2016YFB0300704), the National Natural Science Foundation of China (Nos. 51202142, 51202144 and 51602195), the National Natural Science Foundation of Shanghai (No. 17ZR1440900). REFERENCES (1) Osica, I.; Imamura, G.; Shiba, K.; Ji, Q. M.; Shrestha, L. K.; Hill, J. P.; Kurzydłowski K. J.; Yoshikawa G.; Ariga K. Highly Networked Capsular Silica-Porphyrin Hybrid Nanostructures as Efficient Materials for Acetone Vapor Sensing, ACS Appl. Mater. Interfaces 2017, 9, 9945–9954. (2) Deng, C.; Zhang, J.; Yu, X.; Zhang, W.; Zhang, X. Determination of Acetone Inhuman Breath by Gas Chromatography-Mass Spectrometry and Solid-Phase Microextraction with On-Fiber Derivatization, J. Chromatogr. B 2004, 810, 269–275. (3) Jeong, Y. J.; Koo, W. T.; Jang, J. S.; Kim, D. H.; Kim, M. H.; Kim, I. D. Nanoscale PtO2 Catalysts-Loaded SnO2 Multichannel Nanofibers Toward Highly Sensitive Acetone Sensor, ACS Appl. Mater. Interfaces 2018, 10, 2016–2025. (4) Güntner, A. T.; Sievi, N. A.; Theodore, S. J.; Gulich, T.; Kohler, M.; Pratsinis, S. E. Noninvasive Body Fat Burn Monitoring from Exhaled Acetone with Si-doped WO3-sensing Nanoparticles, Anal. Chem. 2017, 89, 10578–10584. (5) Jia, Q. Q.; Ji, H. M.; Wang, D. H.; Bai, X.; Sun, X. H.; Jin, Z. G. Exposed Facets Induced Enhanced Acetone Selective Sensing Property of Nanostructured Tungsten Oxide, J. Mater. Chem A 2014, 2, 13602–13611. 21



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Microstructures of SnO2@RGO Nanocomposites and Their Formaldehyde-Sensing Performance, Sens. Actuators B 2018, 269, 223–237.

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TOC Graphic

Photographs of the portable integrated system and its modules.

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2D Graphene Nanoplatelet-Tungsten Trioxide Hydrate

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