J Mater Sci COMPOSITES Composites
WO3 nanolamellae/reduced graphene oxide nanocomposites for highly sensitive and selective acetone sensing Jasmeet Kaur1, Kanica Anand1, Kanika Anand1, and Ravi Chand Singh1,* 1
Department of Physics, Guru Nanak Dev University, Amritsar, Punjab 143005, India
Received: 19 January 2018
ABSTRACT
Accepted: 8 June 2018
WO3 nanolamellae/reduced graphene oxide (RGO) nanocomposites have been synthesized by employing hydrothermal method where partial reduction in the graphene oxide and anchoring of nanolamellae on RGO sheets occur simultaneously. Nanocomposites with different amounts of RGO have been characterized by TEM, XRD, Raman spectroscopy, XPS, TGA-DTA, BET and PL spectroscopy. Chemiresistive sensors comprising of a thick layer of synthesized material have been fabricated on alumina substrates and investigated for acetone sensing. Sensing characteristics reveal that the sensor based on 2 wt% RGO nanocomposite not only exhibits high sensitivity, excellent selectivity and low optimum operating temperature but low detection limit (down to 1 ppm) as well. The mechanism for enhanced sensing performance of nanocomposite to acetone may be attributed to the presence of RGO sheets which facilitates large specific surface area for gas adsorption, superior conductivity, faster carrier transport and formation of heterojunctions at the interface between the RGO sheets and WO3 nanolamellae.
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Introduction Recently, reduced graphene oxide (RGO) has been extensively implemented in improving sensing properties of metal oxide gas sensors due to its gigantic specific surface area, superior electron transportation and extraordinary thermal, electrical and sensing characteristics. The hybridization of metal oxide semiconductors with reduced graphene oxide (RGO) shows significant potential in the enactment of practical low-temperature chemical gas
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sensors. Moreover, the nanocomposites effectively inhibit the irreversible restacking of RGO sheets and the clustering of metal oxide nanostructures. Several reports on implementation of RGO–metal oxide as gas-sensing materials are available in the literature for detection of different gases [1–4]. For example, Qin et al. [5] reported graphene-wrapped nanoparticles with improved sensing properties towards alcohol. Anand et al. [6] reported hydrogen sensor based on graphene/ZnO nanocomposite which showed better sensing response at relatively lower
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temperature in comparison with bare ZnO. Lately, Gu et al. [7] reported In2O3–graphene nanocomposite-based gas sensor for selective detection of NO2 at room temperature. Wang et al. [8] also reported ZnO nanosheets/graphene oxide nanocomposites for highly effective acetone vapour detection. Among various metal oxides, tungsten trioxide (WO3) is an important n-type semiconductor for gassensing application due to its good response to reducing or oxidizing gases, low cost and being friendly to the environment [9–13]. In order to accomplish high sensing response and excellent selectivity, doping and morphological modulation of WO3 nanostructures have already been employed [14–19]. Hierarchical flower-like WO3 nanostructures [20], WO3 nanoplates [15], Cu-doped WO3 [21], Indoped WO3 [22], Au-doped WO3 [23] have been efficaciously reported for improvement in gas-sensing performance of WO3. Though lots of advancements have been made in improving sensing performance of WO3 nanostructures, still there exist some unresolved issues such as poor selectivity and high operating temperature, which may eventually hinder its practical applicability. To overcome these hurdles, hybrids of WO3 nanostructures and RGO have been studied for detection of various gases [24]. The synergistic effects of RGO and WO3 yield improved sensing response. Recently, Chu et al. investigated the influence of graphene concentration on gas-sensing properties of graphene/WO3 nanocomposites towards acetaldehyde vapour and found that the presence of graphene reduced the optimum operating temperature with improved selectivity [25]. Su et al. [26] fabricated a room-temperature NO2 gas sensor based on WO3 and reduced graphene oxide (RGO/WO3) nanocomposite films. Shi et al. [27] investigated H2S sensing by employing reduced graphene oxide/ hexagonal WO3 nanosheet composites. Although there are several reports on graphene/ WO3 nanocomposites for the detection of different gases, but only a few are available for acetone detection. Acetone (C3H6O), a volatile organic compound, is widely used in industries and laboratories, and its exposure is extremely harmful to various organ systems. Acetone is also a selective breath indicator for type 1 diabetes. Recently, Choi et al. [28] reported that 0.1 wt% graphene-functionalized WO3 hemitubes showed higher response towards 5 ppm acetone at 300 °C when compared to WO3 hemitubes.
Perfecto et al. [29] also investigated the acetone sensing properties of WO3-based structures and their RGO composites synthesized by a microwave-assisted hydrothermal method and found that the sensor response based on RGO and WO3 nanocomposite was 20% larger than WO3 towards 100 ppm acetone. There are also reports in the literature on acetone sensors based on doped WO3 and RGO nanocomposite. Recently, our group reported rare earth (gadolinium)-doped WO3 and RGO nanocomposite prepared by self-assembly approach for acetone detection which is quite time-consuming and lengthy synthesis process [30]. For example, Chen et al. [31] reported acetone sensor based on WO3/Ptdecorated rGO nanosheets. Moreover, the use of rare earths and noble metals is a major cost barrier in practical application. The application of WO3 nanolamellae and RGO nanocomposite, synthesized through simple method, for the detection of acetone, has not been investigated yet and advancement in such novel RGO/WO3 nanocomposites which display superior gas-sensing performance to acetone is highly significant. Also, the nanocomposite proposed in this communication is cost-effective, making it commercially viable alternative. In this work, we employed a hydrothermal method to successfully synthesize WO3 nanolamellae and RGO nanocomposites by varying GO contents (0.5, 1.0, 2.0 and 4.0 wt%). During synthesis process, GO reduced to RGO with attachment of WO3 nanolamellae to RGO sheets. Afterwards, we investigated their gas-sensing properties by optimizing the operating temperature and GO content. The experiment results show that gas sensor based on optimized RGO/WO3 content can detect as low as 1 ppm of acetone in air and exhibits maximum sensing response at a lower optimum working temperature (200 °C) as compared to WO3 (350 °C). The selectivity, stability and repeatability of RGO/WO3 (2.0 wt%) sensor have also been checked. In addition, gas-sensing mechanism behind enhanced sensing response of RGO/WO3 nanocomposite has also been proposed.
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Experimental section Materials used Graphite fine powder (extra pure), sulphuric acid (98%), sodium nitrate (99.5%), potassium permanganate (99%), hydrazine hydrate (80% AR), ammonia solution (30%) were obtained from Loba Chemie Mumbai, India. Sodium tungstate dihydrate (Na2 WO42H2O) was purchased from Sigma-Aldrich. Hydrochloric acid (35–38%) was procured from Finar Limited Gujarat, India. All the chemicals were used as received without further purification.
Synthesis of RGO/WO3 nanocomposites Graphite oxide (GO) has been prepared from graphite powder using a modified Hummer’s method [32]. RGO/WO3 nanocomposites were synthesized via hydrothermal method by using different contents of GO (0.5, 1.0, 2.0 and 4.0 wt%). Firstly, an appropriate amount of GO was ultrasonicated in 50 mL of distilled water for 1 h to achieve a homogeneous suspension. Then, 2 g of Na2WO42H2O was dissolved in the above suspension and kept on stirring for 0.5 h. Subsequently, an acidic solution (10 mL HCl in 20 mL distilled water) was added dropwise to above suspension solution under vigorous stirring until the pH was adjusted to 2.0. Afterwards, the acquired mixture was shifted to 100 mL Teflon-lined stainless steel autoclave with the addition of 1 mL hydrazine hydrate and kept at 180 °C for 24 h. The obtained products were centrifuged and washed with deionized water and ethanol for several times, followed by overnight drying at 60 °C. For comparison, WO3 nanolamellae were also prepared by the same procedure in the absence of GO. Corresponding photographs of obtained five powder samples are presented in Fig. 1 which shows the change in colour of WO3 with the addition of RGO. It should be specially mentioned that the RGO/ WO3 nanocomposite with 2.0 wt% GO was found to be most optimal for gas-sensing application. Thus,
Figure 1 Photographs of as-synthesized powder samples.
the material characterizations of the nanocomposites were based on the 2.0 wt% nanocomposite thereinafter.
Characterization techniques and gassensing measurements Morphology of the samples was analysed with transmission electron microscopy (TEM) and highresolution microscopy (HRTEM) on JEOL JEM 2100. The X-ray diffraction (XRD) patterns of as-synthesized samples were collected on Shimadzu 7000 powder diffractometer operated at 40 kV/30 mA with Cu-Ka radiation. The Raman spectra were recorded with Renishaw Invia spectrometer (514 nm argon-ion laser source). X-ray photoelectron spectroscopy (XPS) was employed to study surface chemical analysis by using XPS PHI 5000 system with an Al-Ka radiation source. TGA/DTA analysis was performed on Hitachi STA 7200 thermal analyser in N2 atmosphere at a heating rate of 10 °C min-1 at the temperature ranging from room temperature to 800 °C. The specific surface area was measured on Micromeritics ASAP 2020 instrument. Photoluminescence emission spectra (PL) were obtained using 310 nm as an excitation wavelength on Lambda 45, Perkin-Elmer Fluorescence spectrometer. The gas-sensing measurements were taken using home built equipment. To fabricate thick film sensors, a paste was prepared by mixing a proper amount (2–3 mg) of synthesized powder samples with distilled water. The paste was painted onto an alumina substrate (12 mm 9 5 mm size) having predeposited gold electrical contacts to obtain a thick film. In order to deposit gold contacts, liquid bright gold was procured from Hobby Colorobbia Bright Gold manufacturer and was painted on alumina substrates leaving a 2-mm gap in middle. After that, the sensor was dried in air at ambient temperature which was then followed by an annealing at 300 °C for 2 h to improve stability. The identical geometry of sensors has been maintained by masking alumina substrates suitably with the help of polymer film. After painting them with sensing material, extra wet material has been removed. The comprehensive explanation of gas-sensing setup has been reported in our previous publication [33]. In the present case, the sensor response is defined as the ratio of Rair/Rgas, where Rair and Rgas are the resistances of sensor in air and air–gas environment, respectively. The time
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acquired by the sensor to achieve 90% of the total resistance change during adsorption and desorption of gas is defined as response and recovery times, respectively.
Results and discussion Characterization of WO3 nanolamellae and RGO/WO3 nanocomposite The morphological and structural characteristics of WO3 and RGO/WO3 nanocomposites were probed with TEM and HRTEM and micrographs are displayed in Fig. 2. TEM micrographs of WO3 reveal the nanolamellae-like morphology (Fig. 2a), while WO3 nanolamellae are entrenched in crumpled RGO sheets in case of RGO/WO3 nanocomposite (Fig. 2c). Further, the HRTEM image of RGO/WO3 nanocomposite (Fig. 2c) clearly displays the edge of RGO sheets and WO3 nanolamellae, revealing strong interaction between WO3 nanolamellae and RGO sheets. The HRTEM images of WO3 nanolamellae and RGO/WO3 nanocomposite shown in Fig. 2b, d Figure 2 TEM and HRTEM images of WO3 nanolamellae (a, b) and RGO/WO3 nanocomposite (c, d).
indicate that spacing of the clear lattice fringe is 0.364 nm which corresponds to (020) plane of monoclinic WO3. The crystal structure and phase purity were characterized by XRD. Figure 3 illustrates the XRD patterns of GO, RGO, WO3 and RGO/WO3 nanocomposite. A strong peak at 2h = 10.90o in XRD pattern of GO is related to (001) reflection of GO (Fig. 3a). This peak vanishes upon reduction in GO into RGO [34]. The RGO/WO3 nanocomposite exhibits similar XRD pattern which are in good agreement with monoclinic phase of WO3 (JCPDS-431035). No peak corresponding to GO or RGO was observed in nanocomposite which may be ascribed to low amount and proper exfoliation of RGO sheets [35, 36]. Raman spectroscopy offers a non-destructive and fast technique to characterize graphene and graphene derivatives. Figure 4 displays Raman spectra of GO, RGO, WO3 and RGO/WO3 nanocomposite. The Raman spectra of GO show two distinguished peaks positioned at 1358 and 1604 cm-1 corresponding to renowned D and G bands of graphitic materials,
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Figure 4 Raman spectra of GO, RGO, WO3 and RGO/WO3 nanocomposite.
Figure 3 a XRD patterns of GO and RGO, b XRD patterns of WO3 and RGO/WO3 nanocomposite.
respectively [37]. Shifting of these bands to lower wavenumbers has been observed in RGO and RGO/ WO3 nanocomposite confirming the effective reduction in GO into RGO. The intensity ratio of D and G bands (ID/IG) has increased in RGO and RGO/WO3 nanocomposite in comparison with GO which also affirms the effective reduction in GO. Feng et al. [38] relate this increase in value of ID/IG ratio to the presence of larger amount of defects and disorder for rGO–SnO2 nanocomposite. Higher ratio of ID/IG accounts for higher degree of defects and disorder in carbon materials. These defects aid the adsorption of oxygen and gas molecules which could be encouraging for enhanced sensing response [31]. The
characteristic bands of WO3 are also present in nanocomposite. Raman bands at about 806 and 717 cm-1 are allocated to O–W–O stretching vibrational modes, while the bands near 326 and 272 cm-1 relate to O–W–O bending vibrational modes of monoclinic WO3 [18]. In comparison with RGO, shifting of D and G bands in nanocomposite (see zoomed Raman spectra of RGO and RGO/WO3 nanocomposite in Fig. 4) has been observed which indicate the intimate interaction between RGO and WO3. Similar observations have been reported by Zhu et al. for hierarchical ZnO and graphene composites [39]. Further, to investigate the surface compositions and chemical states of RGO/WO3 nanocomposite, XPS analysis has been employed. The full survey spectrum of RGO/WO3 nanocomposite is presented in Fig. 5a, suggesting the presence of W, O and C in the nanocomposite. The C 1s peak is deconvoluted into three peaks located at 284.2, 285.1 and 288.2 eV which are related to C–C, C–O and C=O, respectively (Fig. 5b) [3]. XPS spectrum of the O 1s is displayed in Fig. 5c. The deconvolution of O 1s yields four peaks positioned at 529.33, 529.83, 530.5 and 531.6 eV. The first two peaks refer to lattice oxygen [40, 41], while the peak at 530.5 eV corresponds to combined contribution from the O=C surface groups in GO and the oxygen in WO3 [5]. The 531.6 eV peak belongs to chemisorbed oxygen species which are present at the surface of nanocomposites [8]. The high-resolution spectrum of W 4f region has also been acquired and is shown in Fig. 5d which exhibits two strong peaks of W6? at 35.14 and 37.27 eV, corresponding to W
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Figure 5 Survey scan XPS spectrum of RGO/WO3 nanocomposite (a) and highresolution XPS spectra in the vicinity of the C 1s peaks (b), O 1s (c) and W 4f (d).
4f7/2 and W 4f5/2, respectively. The low-intensity peaks located at 34.05 and 36.03 eV are characteristics of W5? [5]. All these observations confirm the successful formation of RGO/WO3 nanocomposite. TG/DTG/DTA analysis has been employed to investigate the thermal stability of RGO/WO3 nanocomposite, and respective curves are shown in Fig. 6. Initially, the nanocomposite shows weight loss at around 110 °C which is associated with elimination of adsorbed water molecules. Then, the nanocomposite exhibits a progressive weight reduction in temperature range between 200 and 500 °C which is ascribed to elimination of oxygen-containing
Figure 6 TGA/DTG/DTA curves of RGO/WO3 nanocomposite.
functional groups [42]. Pyrolysis of these labile oxygen-containing functional groups yield CO, CO2 and steam [43] and accounts for 3% of weight loss which is also accompanied by broad DTA peak, showing minima around at 400 °C. Above 500 °C, high pyrolysis of carbon skeleton has occurred which is confirmed by the corresponding exothermic peak at 571 °C in DTA curve. Different research groups have reported similar results in the literature [44–47]. It can be concluded from TG/DTG/DTA analysis that RGO/WO3 nanocomposite exhibits significant thermal stability at higher temperatures and can sustain 400 °C thermal annealing without much degradation of the original weight. This thermal stability results from strong interaction between WO3 nanolamellae and RGO sheets. Liu et al. [48] reported similar reasons to show excellent thermal stability of graphene/ polymer composite. For gas-sensing application, surface area is also a significant factor. For example, Zhang et al. [49, 50] reported that large surface area significantly improves the interactions between gas species and sensing materials and results in enhanced sensor response [50]. Therefore, considering the advantage of high specific area in gas-sensing application, we also examined the BET specific surface areas of WO3
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and RGO/WO3 nanocomposite. From the measurements, the obtained BET specific areas are 6.6 and 25.52 m2/g for WO3 and RGO/WO3 nanocomposite, respectively. From BET analysis, it is concluded that introduction of RGO provides more active sites for gas adsorption and thus contributes to enhanced sensing response [51, 52]. The room-temperature PL emission spectra of the WO3 and RGO/WO3 nanocomposite were recorded with 310 nm as an excitation wavelength and are shown in Fig. 7. Three prominent bands at 460, 484 and 528 nm are present in PL spectrum of WO3. The origins of first two bands are near-band-edge and band-to-band transition, respectively, while the presence of third band indicates the existence of oxygen vacancies in WO3. Similar PL emission spectra for WO3 have been reported by Ahmed et al. [53]. Compared to WO3, the PL emission intensity of RGO/WO3 nanocomposite is considerably quenched, proposing the presence of an additional pathway for the electron transfer from the conduction band of excited WO3 to highly conducting RGO sheets. Such reduction in PL emission intensity has been earlier observed in graphene–metal oxide nanocomposites [54, 55]. Due to its two-dimensional planar p-conjugation framework, the presence of RGO in RGO/ WO3 nanocomposite can efficiently hinder the recombination of electron–hole pairs. This extends the lifetime of charge carriers, and as a consequence, these electron–hole pairs can reduce oxygen to create
Figure 7 PL emission spectra for WO3 and RGO/WO3 nanocomposite at an excitation wavelength of 310 nm.
superoxide radical species with proficient oxidation ability for gas reaction.
Gas-sensing characteristics In order to investigate the sensing characteristics of WO3 and RGO/WO3 sensors, preliminary experiments have been performed. To obtain optimum operating temperature, the sensing response of the different gas sensors towards 20 ppm acetone has been measured as a function of operating temperature. The results obtained are presented in Fig. 8a, and the optimized working temperatures for which WO3 and RGO/WO3 show maximum response are found to be 350 and 200 °C, respectively. The hybridization of WO3 with RGO has reduced the optimum operable temperature of WO3 sensor with improvement in sensing response. The variation in sensing response with different GO contents (0.5, 1.0, 2.0 and 4.0 wt%) in RGO/WO3 nanocomposite for 20 ppm of acetone at 200 °C is displayed in Fig. 8b. It has been observed that 2 wt% RGO/WO3 gas sensor displayed the best response (8.1) and therefore has been selected for further sensing characterizations. The dynamic responses of WO3 and RGO/WO3 sensors samples for 20 ppm acetone at respective optimum temperatures are shown in Fig. 8c. The response and recovery time of WO3 and RGO/WO3 sensors are listed in Table 1. Figure 8d shows the variation of resistance of WO3 and RGO/WO3 nanocomposite at their respective optimum operable temperatures towards 20 ppm of acetone. The sensor resistance reduces upon introduction of acetone and attains initial value upon its removal for both the samples, indicating the n-type behaviour of the synthesized sample. The incorporation of RGO provides maximum resistance change as compared to WO3 (Fig. 8d). The striking enhanced response with RGO incorporation will be discussed in next section. Figure 9a shows the variation of sensor response when exposed to different concentrations of acetone at 200 °C. It can be seen that the response of RGO/ WO3 nanocomposite increases progressively at low concentration of acetone and tends to saturate at higher acetone content. The detection of low acetone concentrations is highly significant. We know that the average acetone concentration in a healthy human breath is 0.35–0.85 ppm and in diabetic people it is more than 2–2.5 ppm. Considering the potential of
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Figure 8 a Response versus operating temperature plots for WO3 and RGO/WO3-based nanocomposites towards 20 ppm acetone, b sensor response variation, for 20 ppm of acetone, with RGO content (0.5, 1.0, 2.0 and 4.0 wt%) in RGO/WO3 nanocomposite at 200 °C, c sensor response versus time graphs for WO3 and Table 1 Response and recovery time of pure WO3 and RGO/WO3 nanocomposites
RGO/WO3 at their respective optimum operating temperatures towards 20 ppm of acetone, d sensor resistance variations with time for WO3 and RGO/WO3 at their respective optimum operating temperatures towards 20 ppm of acetone.
S. no
Sample
Response time (s)
Recovery time (s)
1 2 3 4
WO3 0.5 wt%RGO/WO3 1.0 wt%RGO/WO3 2.0 wt%RGO/WO3
4.55 2.83 2.39 1.76
21.45 13 9.66 8
the RGO/WO3 nanocomposite sensor to detect low concentration of acetone efficaciously at low temperatures, this sample was further tested to recognize its ability as a practical acetone sensor. Inset of Fig. 9a shows the response of RGO/WO3 nanocomposite sensor to low concentration of acetone. It is worth
noting that the RGO/WO3 nanocomposite sensor is able to detect even 1 ppm of acetone, thus showing its practical prospective in gas-sensing arena. Further, to assess the selectivity of RGO/WO3 sensor, it was exposed to 20 ppm of ammonia, ethanol, hydrogen and LPG and results are represented in
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Fig. 9b. The sensor response to above gases is very less in comparison with acetone, thereby showing that RGO/WO3 sensor is highly selective to acetone gas. Additionally, for practical application, the longterm stability is one of the important aspects for sensing devices. In order to investigate the stability of WO3 and RGO/WO3 nanocomposite gas sensors, the sensing response towards 20 ppm acetone have been recorded at different intervals for 50 days and outcomes are displayed in Fig. 9c. From Fig. 9c, it is clear that the sensors show excellent stability.
The repeatability of RGO/WO3 nanocomposite has also been tested towards 20 ppm of acetone for five successive cycles, and results are presented in Fig. 9d. The sensor response remains almost same even after five repeated cycles, showing good repeatability of RGO/WO3 nanocomposite. So far, acetone sensors based on different metal oxides and RGO nanocomposites have been reported in the literature and some results are represented in Table 2 [8, 28, 29, 51, 56–61]. Compared with other sensing materials, the RGO/WO3 nanocomposite reported in the present study shows good sensing properties.
Figure 9 a Variation of sensing response for RGO/WO3 nanocomposite when exposed to different ppm of acetone (inset figure represents results for lower conc. of acetone) at 200 °C, b histogram showing sensor response of WO3 and RGO/WO3
towards 20 ppm ammonia, hydrogen, ethanol, acetone and LPG, c long-term stability curves for WO3 and RGO/WO3 nanocomposite, d repeatability test for RGO/WO3 nanocomposite on successive exposure (5 cycles) to 20 ppm of acetone at 200 °C.
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Table 2 Comparison of acetone sensing performance of the current work with the literature S. no
Sensing material
Concentration (ppm)
Sensor response
Working Response/ temperature (°C) recovery time (s)
Specific surface area (m2/g)
Reference
1 2 3 4 5
rGO–ZnO Graphene–ZnFe2O4 ZnO/GO GR–WO3 RGO–h-WO3
100 1000 100 5 200
*9.5 9.1 35.8 6.96 1.55
N.A 0.73/24.72 13/7 8.5/\ 30 14/N.A
114 N.A N.A N.A 23
[56] [57] [8] [28] [29]
6
SnO2–RGO
N.A
153.4
[51]
7 8 9
CuO–ZnO/rGO rGO/a-Fe2O3 Chitosan
10/14 2/20 10/\ 200
N.A 5.8 N.A
[58] [59] [60]
10
Calix [4] pyrrole-decorated carbon nanotubes RGO/WO3
37/85
N.A
[61]
1.76/8
25.52
This work
11
60
14.1%
260 275 240 300 Room temperature Room temperature 340 225 Room temperature 25
20
8.1
200
10
*2
10 100 20
9.4 13.9 47%
N.A means not available
Gas-sensing mechanism In order to understand the gas-sensing mechanism, I– V characteristics of pure WO3 and RGO/WO3 (2 and 4 wt%) sensors before and after acetone exposure have been investigated at their optimum working temperatures. The respective curves are displayed in Fig. 10. The linear and symmetrical I–V curves indicate the good ohmic contact between the samples and the test electrodes [58]. Evidently, in comparison with pure WO3, RGO-based nanocomposites present large value of current (low resistance) which is due to conductive nature of RGO. Similar decrease in value of resistance with graphene incorporation has already reported in the literature [5, 28, 62]. The RGO/WO3 nanocomposite sensor also shows a typical decrease in resistance on exposure to reducing gas acetone, thus confirming an n-type semiconducting behaviour. As WO3 is considered to be involved in the receptor function, so sensing characteristics shows n-type dominant behaviour. The existence of RGO mainly offers the electronic conduction path. When pure WO3 is exposed to air, the adsorbed oxygen species capture electrons from conduction band of WO3 and this results in the formation of a depletion layer with high resistance [63]. The specific reactions [18] are represented by Eqs. (1)–(4).
O2 gas $ O2 ðadsÞ
ð1Þ
O2 ðadsÞ + e $ O 2 ðadsÞ
ð2Þ
O 2 ðadsÞ + e $ 2O ðadsÞ
ð3Þ
O ðadsÞ + e $ 2O2 ðadsÞ CH3 COCH3 gas þ 8O ðchemisorbed) ! 3CO2 + 3H2 O þ 8e
ð4Þ ð5Þ
However, on exposure of WO3 to a reducing gas like acetone, the adsorbed oxygen species react with acetone molecules and alter the space-charge layer in the grain boundaries, releasing electrons back to conduction band (Eq. 5) leading to decrease in resistance. Therefore, by monitoring this change in resistance of sensor before or after acetone, the magnitude of sensor response can be measured. In case of RGO-based nanocomposite, the stronger interaction between WO3 nanolamellae and RGO facilitates the migration of electrons from WO3 to RGO. The presence of RGO sheets significantly increases the electronic conduction in the nanocomposite which facilitates the faster movement, capture and release of electrons. Qin et al. [5] have reported similar enhancement in conductivity of WO3 and graphene nanocomposites for superior sensing response. When the concentration of RGO is less than 2 wt%, WO3 nanolamellae are discretely dispersed in conductive RGO sheets. The presence of conducting
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Figure 10 I–V curves of WO3 and RGO/WO3 (2 and 4 wt%) sensors before and after exposure to 20 ppm acetone at their optimum working temperatures.
RGO decreases the barrier between two neighbouring WO3 grains and aids in the faster movement of charge carriers and results in a relatively low resistance in comparison with pure WO3 nanolamellae. On exposure to acetone, besides on surface of WO3, extra gas molecules will be adsorbed at interface between RGO and WO3 which provide extra active sites due to RGO’s oxygen functional groups and defects. Moreover, it has been already established from BET analysis that the surface area of RGO/WO3 nanocomposite is larger than WO3. This will amplify the chemisorbed oxygen amount, and when sensor is exposed to acetone, a large number of electrons will be released, resulting in significant improvement in sensor response. It is to be noted that up to 2 wt% RGO, the conduction path is still mainly through WO3 nanolamellae. RGO sheets serve as a two-
dimensional conductive platform to electrically interconnect the WO3 nanolamellae. But as the content of RGO is more than 2 wt%, the conductivity of sample increases to a large magnitude as a result of shorter resistance paths through RGO sheets and the small variation in resistance upon gas exposure is not noticeable. Similar effect of graphene concentration on gas-sensing performance of SnO2 and graphene nanocomposite has been reported by Tammanoon et al. [64]. Another plausible reason for enhanced sensing response might be the creation of p–n heterojunctions at the interface between p-type RGO and n-type WO3 for which I–V curves are presented in Fig. 10. Wang et al. [58] reported similar I–V characteristics in case of RGO decorated CuO–ZnO heterojunctions. Various research groups reported above-mentioned reasons for enhanced sensing response in RGO-based nanocomposites [65–67].
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Conclusion In conclusion, a highly sensitive and selective acetone sensor based on WO3 nanolamellae/reduced graphene oxide (RGO) nanocomposites has been successfully synthesized via hydrothermal method. As compared to WO3, the nanocomposites have exhibited enhanced performance at a relatively lower temperature. The optimum concentration of RGO, i.e. 2.0 wt%, has been proposed for the fabrication of acetone sensor based on RGO/WO3 nanocomposite. The availability of large specific surface area for gas adsorption, higher conductivity, faster carrier transport and the creation of heterojunctions at the interface between the RGO sheets and WO3 nanolamellae are the possible reasons for such excellent sensing performance.
[4]
[5]
[6]
[7]
[8]
Acknowledgements
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The authors sincerely acknowledge Central Instrumental Facility, Guru Nanak Dev University, Amritsar, India, for providing different characterization techniques under UGC-UPE and DST-FIST schemes. Authors thank Dr. P. K Diwan, UIET, Kurukshetra University, for providing TGA/DTA facility. One of the authors, Jasmeet Kaur, wishes to thank University Grants Commission (UGC), India, for awarding fellowship in carrying out Ph.D. research work.
[10]
[11]
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Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest.
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