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Atomic layer deposition of Rh nanoparticles on WO3 thin film for CH4 gas sensing with enhanced detection characteristics
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Atomic layer deposition of Rh nanoparticles on WO3 thin film for CH4 gas sensing with enhanced detection characteristics Yao Tan∗, Yan Lei School of Environmental and Chemical Engineering, Chongqing Three Gorges University, Chongqing, 404020, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: WO3 thin film Rh nanoparticles Atomic layer deposition Gas sensing Methane
CH4 gas sensor was fabricated by depositing WO3 film (200 nm) on a ceramic substrate through E-beam evaporation, followed by depositing Rh nanoparticles on WO3 through Atomic Layer Deposition (ALD). The amount of Rh nanoparticles was regulated by ALD cycles. The TEM characterization showed that Rh nanoparticles were homogeneously distributed on WO3 thin film, and the XPS investigation indicated the interactions between Rh nanoparticles and WO3 thin film. The gas-sensing performance testing showed that, when the Rh ALD deposition cycle is 20, the sample showed the highest selectivity, which is increased by~110% compared with pure WO3, which could be attributed to the surface catalytic effect contributed by Rh nanoparticles, while over high concentration of Rh could be harmful to the gas sensing performance, which blocks the active sites on the surface of WO3 thin film. The sensing stability of WO3/Rh composite (with ALD cycle of 20) was further investigated by exposing the sensor in CH4 gas environment for one week.
1. Introduction Metal oxide semiconductor (MOS) materials have been widely applied for gas sensing and environmental protection, because of their wide availability and high sensitivity to low ppm concentration of gases [1–6]. Regarding gas sensing, previous research has shown that ZnO nanowires can be used for NH3 gas sensing [7]; In2O3 nanofibers can be used for CO gas sensing [8]; CoFe2O4 nanocrystals can be used for acetone vapor detection [9]. Recent research has shown that the mechanism of gas sensing through MOS materials is based on surface reactions [10]. Different MOS materials have different reactions and selectivity to different gases, which could be used to detect various gases. For example, Cu2O/CuO cages have been used for ethanol vapor sensing applications [11–13]; TiO2 based nanoparticles have been used to detect low concentrations of NO2 in the air [14]; Galatsis et al. have applied MO3, TiO2 and WO3 sol-gel gas sensor for O3, CO, and NO2 gas sensing applications [15]; Zhang et al. synthesized SnO2 nanoparticles, and studied its gas sensitivity characteristics in ethanol vapor [16]. Even though the fundamental mechanism of the surface reaction is still controversial, the essential step is that the gas adsorption and surface reaction on the MOS surface results in the electron flow, which changes MOS electric resistance from a base value [7,17]. However, a reversed electron flow would decrease the electric resistance change and decrease gas response sensitivity [18]. Therefore, if the electron trapping
∗
could be improved during gas sensing, the gas sensor response towards a specific gas would be increased. It is known that methane (CH4) is playing an important role in modern society, as it is widely used for fuels and industrial raw materials. Meanwhile, CH4 is also a typical greenhouse gas, which contributes to global warming. Therefore, it is a great necessity and desire to develop a highly reliable and sensitive CH4 gas sensor. Based on different mechanisms, such as catalytic combustion and flame ionization, various types of CH4 gas sensors have been proposed [19–21]. However, MOS CH4 sensor has attracted the most attention because of its high sensitivity, high reliability, and wide availability [22]. Many types of MOS materials have been used for CH4 sensing, including SnO2, ZnO, WO3, In2O3, and etc [22–25]. Among these materials, WO3, an ntype semiconductor, is a popular candidate for gas sensing in recent years [26–28]. The adsorption and reaction of CH4 on WO3 surface result in the change of its electric resistance [29]. Therefore, the gas response could be further increased by facilitating catalytic reaction on WO3 surface. It is known that Rh nanoparticles have been widely used for CH4 reforming and oxidation reaction, which results in the generation of a large number of free electrons [30,31]. Meanwhile, a Schottky barrier between Rh (metal) and WO3 (semiconductor) could be formed, and electrons would flow to Rh and be trapped over there [32]. Therefore, introducing Rh nanoparticles on WO3 thin film could potentially
Corresponding author. E-mail address: [email protected] (Y. Tan).
https://doi.org/10.1016/j.ceramint.2019.12.094 Received 27 August 2019; Received in revised form 4 December 2019; Accepted 9 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yao Tan and Yan Lei, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.094
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10 min, followed by exposing in the air to desorb the gas on the sensor. The stabilized electric resistance of the sensor was recorded. The gassensing performance could be evaluated by the resistance ratio of Rair/ Rgas (Rair: the sensor electrical resistance in the air; Rgas: the sensor electrical resistance in the testing gas). The resistance was recorded by an electrical data acquisition system (HP, 3520).
increase the gas sensing performance for CH4 detection. However, the performance of WO3/Rh composites for CH4 gas sensing has never been reported. In the study, Rh nanoparticles were deposited on WO3 thin film surface by Atomic Layer Deposition (ALD), which could provide better surface distribution and adhesion of the nanoparticles to substrate than other methods, such as spin coating and dip coating [33]. The impacts of different Rh loadings on the CH4 sensing performance of WO3/Rh composites were investigated. Finally, the basic sensitization mechanism for CH4 gas sensing by WO3/Rh composite is explained.
3. Results and discussion The WO3 thin film is deposited by E-beam evaporation, and the characterization results are presented in Fig. 1a–c. The SEM image in Fig. 1a indicates that the film is flat and continuous, and individual grains are observed. In the AFM image in Fig. 1b, and the corresponding surface roughness profile is presented in Fig. 1c. The surface roughness is around ± 20 nm. During the E-beam evaporation, the surface roughness could be controlled by the deposition rate. In this research, the WO3 deposition rate is controlled as 0.5 nm/s, a relatively high deposition rate for a higher surface roughness to increase the surface area for gas adsorption. The TEM and STEM images of WO3/Rh-3 are displayed in Fig. 1d and e. The Rh nanoparticles are homogeneously distributed among WO3. The particle size distribution based on the counting of 100 Rh nanoparticles is presented in Fig. 1f, and the size follows Gauss distribution. The average particle size is calculated as 5.85 nm. HRTEM images for WO3 thin film and Rh nanoparticle are presented in Fig. 1g and h. The lattice distance of 0.38 nm and 0.21 nm in Fig. 1g and h correspond to (020) and (111) planes in WO3 and Rh crystal structures. The corresponding EDX line scan regarding Rh nanoparticles in Fig. 1h is presented in Fig. 1i, it is observed that Rh is evenly distributed. The XRD patterns regarding WO3, WO3/Rh-1, WO3/Rh-2, and WO3/ Rh-3 are presented in Fig. 2. For pure WO3, its XRD patterns are consistent with standard diffraction patterns presented in JCPDS card (NO. 43–1035) [35], and secondary or impurity phases are not observed, indicating the high purity crystalline of the thin film. Moreover, the XRD patterns of WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 are similar to WO3, and XRD peaks of Rh nanoparticles are not observed, which could be explained by that the amount of Rh nanoparticles is below the detection limit of the X-ray diffractometer. The chemical interactions between WO3 and Rh could be studied by XPS, and the spectra for WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 regarding W and Rh are presented in Fig. 3. In Fig. 3a, two peaks at 37.8 and 35.7 eV are observed in WO3 thin film, which could be the characteristic peaks for W 4f5/2 and W4f7/2 [36]. In WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3, the two peaks are shifted to lower binding energy. This could be attributed to the bonding between WO3 and Rh, which changes the surface chemical states and electronic structure of WO3 [37]. More details were explained in the energy band diagram in Fig. 6. In Fig. 3b, for pure WO3, no Rh XPS peaks are observed because of the absence of Rh nanoparticles. Two XPS peaks are observed in WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 at 312.6 and 307.7 eV, which come from Rh 3d3/2, and Rh 3d5/2, respectively. Compared with the peak positions of pure Rh at 312.1 and 307.2 eV, for Rh 3d3/2, and Rh 3d5/2, the peak shift in Fig. 3b could also be explained by the interactions between WO3 and Rh [38]. In Fig. 4a, the CH4 sensing performance of WO3, WO3/Rh-1, WO3/ Rh-2, and WO3/Rh-3 is investigated between 250 and 450 °C, and the CH4 concentration is controlled as 1 ppm. It is found that, generally, the introduction of Rh nanoparticles onto WO3 thin film could increase the CH4 gas sensing response, and WO3/Rh-2 shows the highest signal response among the four samples, as high as ~110% increase in sensitivity, compared with pure WO3 sensor. When the amount of Rh nanoparticles is further increased, the gas sensing performance is decreased, as shown in WO3/Rh-3. The highest response in WO3/Rh-2 could be attributed to the methane surface catalytic effect contributed by Rh [39,40]. However, Rh nanoparticles could also partially cover the WO3 thin film surface. When the Rh amount is further increased from
2. Experimental 2.1. WO3 thin film deposition WO3 thin film was deposited on the electrodes by E-beam evaporation, with the target of WO3 (99.99%) [34]. During the deposition, the vacuum was less than 5E-5 Torr, and the deposition rate was around 0.4 nm/s. An intentional high growth rate was applied to increase the surface roughness and surface area. The thin film thickness was controlled as 200 nm. 2.2. Rh nanoparticle deposition The Rh nanoparticles were deposited on WO3 thin film by ALD. Rh (acac)3 was used as the Rh source, and O2 was used as the reactant. During the ALD, the substrate temperature was 250 °C, and the vacuum was 300 mTorr. To vaporize Rh(acac)3, the container temperature was holding at 150 °C, and Ar was used as the carrying gas. The typical four pulses for the ALD were summarized and presented in Table 1, and a simplified ALD process was presented in Scheme 1. In three batches, the Rh deposition cycle was 10, 20 and 30, and the products were noted as WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3. The sample without Rh deposition was named as WO3. 2.3. Thin-film characterization The thin film surface morphology and Rh nanoparticle size were characterized by SEM (JEOL 7000), AFM, and TEM (Hitachi HT7700). The crystal structure was determined by XRD, and surface conditions of the thin film were studied by XPS. 2.4. Gas sensing testing The gas sensing performance of WO3 and WO3/Rh was evaluated in CH4, H2S, C7H8, CO, NH3, and n-C5H7 gas environment, the gas concentration was controlled between 1 and 5 ppm, and the temperature was controlled between 250 and 400 °C. The standard concentration of gases used for the experiment was 5 ppm (supplied by EGAS DEPOT, balanced with air). Based on the typical condition of the ambient environment, the humidity level was controlled as 80% RH during gas sensing testing. Different concentrations of CH4 gas (1–4 ppm) were obtained by diluting 5 ppm CH4. The two Au electrodes with a width of 25 μm, a gap of 150 μm, and a thickness of 200 nm, were deposited on an alumina substrate by sputtering with a shallow mask. A heater was attached to the backside of the substrate. For each gas sensing performance testing, the sensor was stabilized with baseline condition for 12 h. During the gas sensing, the sensor was exposed in the gas for Table 1 The details of the four pulses in the ALD process.
Chemical/gas Time
1st pulse
2nd pulse
3rd pulse
4th pulse
Rh(acac)3/Ar 4s
Ar 60 s
O2 4s
Ar 60 s
2
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Scheme 1. The four pulses of the ALD process.
Fig. 1. WO3 characterization: (a). SEM; (b). AFM; (c). Surface roughness profile of as-deposited; WO3/Rh-3 characterization: (d). TEM; (e). STEM; (f). Rh nanoparticle size distribution; (g). HRTEM for WO3; (h) HRTEM for Rh nanoparticles; (i). EDX line scan for Rh nanoparticle in (h).
3
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that, when the CH4 concentration decreases to 0.1 ppm, the WO3/Rh-2 sensor will lose sensing capability, with the Response of 1.07. This indicates that WO3/Rh-2 has higher capability to detect lower concentrations of CH4. The gas sensing sensitivity of WO3–Rh-2 at 350 °C is investigated with the gas concentration of 5 ppm, and the result is presented in Fig. 4d. The responses to CH4, H2S, C7H8, CO, NH3, n-C5H7, NO and NOx are observed as 63.1, 34.2, 28.5, 20.6, 15.6, 10.9, 5.1 and 4.0, respectively. It is concluded that WO3/Rh-2 has high selectivity towards both reducing gases and oxidizing gases, but shows the highest selectivity towards CH4 gas. This could be attributed to the Rh's catalytic effects towards CH4, and more details will be discussed below. The response time is the time that the gas sensor takes to reach the 90% of the stabilized signal intensity, while the recovery time is the time that the gas sensor takes to decrease the 90% of the stabilized signal. The general definitions for the response and recovery time are presented in Fig. 5a, and the corresponding response and recovery time regarding WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 between 100 and 1000 ppm are presented in Fig. 5b and c, respectively. In Fig. 5b, it is observed that a higher gas concentration results in a shorter response time. WO3 has the longest response time among all the samples. After the introduction of Rh nanoparticles, the response time of CH4 sensing for WO3/Rh is decreased, and WO3/Rh-2 has the shortest response time. Similar results are also observed with the recovery time, as shown in Fig. 5c. This demonstrates that the presence of Rh nanoparticles could decrease both responding and recovery time. The CH4 sensing stability testing for WO3/Rh-2 is studied by exposing the sensor to 5 ppm CH4 and 350 °C for 7 days, and the Rresponse is recorded and plotted in Fig. 5d. After running for 7 days, Rresponse is reduced by 0.63% (from 63.1 to 62.7), indicating its higher stability than other reported gas sensors. For example, Biswas et al. reported a decreased sensitivity of 5.5% for the graphene-semiconductor sensor for NH3 sensing in one week [41]. Based on the experiment above, the gas sensing mechanism for the increased CH4 sensing characteristics in WO3/Rh composite could be summarized. For general metal oxide semiconductor gas sensor, the response can be calculated by the ratio of the electric resistance in air and in target gas. The lattice oxygen, Olat, in WO3 could be reactive for CH4 oxidation, while the adsorbed oxygen species, including O2− and O−, on the surface of WO3, are more reactive [42]. Generally, the reactions could be summarized:
Fig. 2. X-ray diffraction patterns of WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh3.
WO3/Rh-2, the covered WO3 surface could be over high, resulting in decreased signal response. Moreover, it is observed in Fig. 4a tat, the temperature-dependence gas sensing response curves exhibit reversed “V” shape, and the optimum operating temperature is shown as 350 °C for all samples. Cyclic response transition properties of the four samples are presented in Fig. 4b. In the five pulses from the left to the right, the CH4 concentration is controlled as 5, 4, 3, 2, and 1 ppm, respectively. It is observed that Rh nanoparticles could increase the CH4 sensing signal intensity of WO3/Rh composites, and WO3/Rh-2 shows the highest performance. Specifically, the sample of WO3/Rh-2 shows the highest response of 63.1 at 5 ppm, which is around twice the response from pure WO3. Similar results are also observed with the CH4 concentration at1 ppm. The detection limit of the gas sensors could be determined by plotting a linear curve regarding the gas concentration v.s. the response, and the results for the four samples are presented in Fig. 4c. It is noted that when the Response is close or around 1, the gas sensor loses the sensing ability because it could not distinguish the signals from air and sensing gas. For WO3 sensor, it is calculated that when CH4 concentration is 0.16 ppm, the Response is 1.06, losing the gas sensing capability. With the same gas concentration (CH4: 0.16 ppm), the Response is 1.7 for WO3/Rh-2. This indicates that WO3/Rh-2 has a higher sensibility for low concentration CH4 detection. It can also be calculated
CH4 + 2Olat → CH3-Olat + H-Olat,
(1)
CH3-Olat + 2Olat + 2O− → CO2(gas) + 2H-Olat + 2e.
(2)
Due to the catalytic effect of Rh for methane oxidation [30,40], the
Fig. 3. XPS spectra of WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 for (a). W; (b). Rh. 4
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Fig. 4. (a). Temperature-dependent response with 1 ppm CH4; (b). Cyclic response transition properties under 5, 4, 3, 2, and 1 ppm of CH4 at 350 °C; (c). Limit of detection at 350 °C. (d). Selective CH4 detection characteristics for CH4, H2S, C7H8, CO, NH3, n-C5H7, NO and NOx under 5 ppm and 350 °C.
The energy band diagram of the barrier height of WO3/Rh junction could be expressed as:
presence of Rh could speed up the Reaction (2), which results in higher flow of electrons, therefore a lower electric resistance in the target gas, and an increased Rair/Rgas. Meanwhile, the increased reaction rate in Reaction (2) also results in a shorter time reaching the reaction equilibrium, therefore, a decreased response time. The decreased recovery time in WO3/Rh-2 could be also be attributed to the presence of Rh nanoparticles, which facilitate the transfer and regeneration of the electrons [43]. However, when the amount of Rh nanoparticles is further increased from WO3/Rh-2, the recovery time is increased, as shown in WO3/Rh-3 in Fig. 5b. A possible explanation is that, lattice oxygen (Olat) within WO3 plays a key role in CH4 adsorption and surface reaction, as shown in Reaction (1) and Reaction (2). When the Rh nanoparticles are excessive on the surface of WO3 (such as in WO3/Rh3), a portion of lattice oxygen is covered and blocked by Rh nanoparticles, and the Reaction (1) and Reaction (2) are prevented, generating decreased electron flow. Therefore, the response time of WO3/ Rh-3 is increased. It is observed that Rh nanoparticles have both positive (transferring electron) and negative (blocking lattice oxygen) impacts on WO3/Rh gas sensing. The optimum amount of Rh nanoparticles occurs in WO3/Rh-2. Furthermore, ideally, to verify this mechanism, it is necessary to detect and monitor the CO2 generation during CH4 sensing. However, due to the limitation of hardware, we are unable to monitor the CO2 at the current stage. The mechanism for the increased selectivity of WO3/Rh over pure WO3 could be further explained by the energy band diagrams. Specifically, the energy band diagrams of WO3/Rh is before and after the thermal equilibrium are presented in Fig. 6a and b. A Schottky contact is formed between n typed WO3 semiconductor and the metal Rh. In the above figures, ФRh and ФWO3 refer to the work function of Rh and WO3, respectively. Ec, Ev, and EF refer to the conduction band, valence band, and Fermin energy, and χ refers to WO3 electron affinity.
ФB = ФRh – χ,
(3)
Meanwhile, during the methane adsorption and dissociation, protons (H+) are formed and diffused into the metal nanoparticles, and capture their electron to form H2 [44]. When the electrons are consumed within Rh, the work function of Rh (ФRh) is reduced, which results in a decreased barrier height of WO3/Rh junction (ФB). The electrons generated in the semiconductor (WO3) would flow into metal (Rh). Therefore, the presence of Rh decreases the barrier height of WO3/Rh junction, speeds up CH4 surface reaction, and results in an increased gas sensing selectivity, as shown in Fig. 4a, b, and c. 4. Conclusions WO3/Rh gas sensors with different amount of Rh nanoparticles were fabricated, and their gas sensing performance was investigated in CH4 gas environment. The resulted indicated that all gas sensors had the optimum operating temperature of 350 °C. Among the four samples, WO3/Rh-2 showed the highest sensitivity for CH4 sensing, which could be attributed to the surface catalytic effect contributed by Rh nanoparticles, while over high concentration of Rh could be harmful to the gas sensing performance, because of blocking the active sites on WO3 thin film. Moreover, WO3/Rh-2 presented the shortest response time and recovery time among the four sensors. Finally, the gas sensing mechanism was explained. Declaration of competing interest We confirm that there are no known conflicts of interest associated 5
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Fig. 5. (a). The response/recovery time definition in the gas sensor; (b). The response time for WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 between 100 and 1000 ppm; (c). The recovery time for WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 between 100 and 1000 ppm; (d). The CH4 sensing stability testing for WO3/Rh-2 at under 5 ppm and 350 °C for 7 days.
Fig. 6. Energy band diagram of WO3/Rh: (a). Before the thermal equilibrium condition; (b). Under the thermal equilibrium condition.
with this publication and there has been no significant financial support for this work that could have influenced its outcome.
[2]
Acknowledgment
[3]
This research was financially supported by the National Natural Science Foundation of China (NO. 625413580), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (NO. KJQN201801222), the Chunhui Project from Education Ministry of China (NO. Z2015140), and the Key Cultivation Project of Chongqing Three Gorges University (NO. 17ZD12).
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Short communication
Atomic layer deposition of Rh nanoparticles on WO3 thin film for CH4 gas sensing with enhanced detection characteristics Yao Tan∗, Yan Lei School of Environmental and Chemical Engineering, Chongqing Three Gorges University, Chongqing, 404020, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: WO3 thin film Rh nanoparticles Atomic layer deposition Gas sensing Methane
CH4 gas sensor was fabricated by depositing WO3 film (200 nm) on a ceramic substrate through E-beam evaporation, followed by depositing Rh nanoparticles on WO3 through Atomic Layer Deposition (ALD). The amount of Rh nanoparticles was regulated by ALD cycles. The TEM characterization showed that Rh nanoparticles were homogeneously distributed on WO3 thin film, and the XPS investigation indicated the interactions between Rh nanoparticles and WO3 thin film. The gas-sensing performance testing showed that, when the Rh ALD deposition cycle is 20, the sample showed the highest selectivity, which is increased by~110% compared with pure WO3, which could be attributed to the surface catalytic effect contributed by Rh nanoparticles, while over high concentration of Rh could be harmful to the gas sensing performance, which blocks the active sites on the surface of WO3 thin film. The sensing stability of WO3/Rh composite (with ALD cycle of 20) was further investigated by exposing the sensor in CH4 gas environment for one week.
1. Introduction Metal oxide semiconductor (MOS) materials have been widely applied for gas sensing and environmental protection, because of their wide availability and high sensitivity to low ppm concentration of gases [1–6]. Regarding gas sensing, previous research has shown that ZnO nanowires can be used for NH3 gas sensing [7]; In2O3 nanofibers can be used for CO gas sensing [8]; CoFe2O4 nanocrystals can be used for acetone vapor detection [9]. Recent research has shown that the mechanism of gas sensing through MOS materials is based on surface reactions [10]. Different MOS materials have different reactions and selectivity to different gases, which could be used to detect various gases. For example, Cu2O/CuO cages have been used for ethanol vapor sensing applications [11–13]; TiO2 based nanoparticles have been used to detect low concentrations of NO2 in the air [14]; Galatsis et al. have applied MO3, TiO2 and WO3 sol-gel gas sensor for O3, CO, and NO2 gas sensing applications [15]; Zhang et al. synthesized SnO2 nanoparticles, and studied its gas sensitivity characteristics in ethanol vapor [16]. Even though the fundamental mechanism of the surface reaction is still controversial, the essential step is that the gas adsorption and surface reaction on the MOS surface results in the electron flow, which changes MOS electric resistance from a base value [7,17]. However, a reversed electron flow would decrease the electric resistance change and decrease gas response sensitivity [18]. Therefore, if the electron trapping
∗
could be improved during gas sensing, the gas sensor response towards a specific gas would be increased. It is known that methane (CH4) is playing an important role in modern society, as it is widely used for fuels and industrial raw materials. Meanwhile, CH4 is also a typical greenhouse gas, which contributes to global warming. Therefore, it is a great necessity and desire to develop a highly reliable and sensitive CH4 gas sensor. Based on different mechanisms, such as catalytic combustion and flame ionization, various types of CH4 gas sensors have been proposed [19–21]. However, MOS CH4 sensor has attracted the most attention because of its high sensitivity, high reliability, and wide availability [22]. Many types of MOS materials have been used for CH4 sensing, including SnO2, ZnO, WO3, In2O3, and etc [22–25]. Among these materials, WO3, an ntype semiconductor, is a popular candidate for gas sensing in recent years [26–28]. The adsorption and reaction of CH4 on WO3 surface result in the change of its electric resistance [29]. Therefore, the gas response could be further increased by facilitating catalytic reaction on WO3 surface. It is known that Rh nanoparticles have been widely used for CH4 reforming and oxidation reaction, which results in the generation of a large number of free electrons [30,31]. Meanwhile, a Schottky barrier between Rh (metal) and WO3 (semiconductor) could be formed, and electrons would flow to Rh and be trapped over there [32]. Therefore, introducing Rh nanoparticles on WO3 thin film could potentially
Corresponding author. E-mail address: [email protected] (Y. Tan).
https://doi.org/10.1016/j.ceramint.2019.12.094 Received 27 August 2019; Received in revised form 4 December 2019; Accepted 9 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yao Tan and Yan Lei, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.094
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10 min, followed by exposing in the air to desorb the gas on the sensor. The stabilized electric resistance of the sensor was recorded. The gassensing performance could be evaluated by the resistance ratio of Rair/ Rgas (Rair: the sensor electrical resistance in the air; Rgas: the sensor electrical resistance in the testing gas). The resistance was recorded by an electrical data acquisition system (HP, 3520).
increase the gas sensing performance for CH4 detection. However, the performance of WO3/Rh composites for CH4 gas sensing has never been reported. In the study, Rh nanoparticles were deposited on WO3 thin film surface by Atomic Layer Deposition (ALD), which could provide better surface distribution and adhesion of the nanoparticles to substrate than other methods, such as spin coating and dip coating [33]. The impacts of different Rh loadings on the CH4 sensing performance of WO3/Rh composites were investigated. Finally, the basic sensitization mechanism for CH4 gas sensing by WO3/Rh composite is explained.
3. Results and discussion The WO3 thin film is deposited by E-beam evaporation, and the characterization results are presented in Fig. 1a–c. The SEM image in Fig. 1a indicates that the film is flat and continuous, and individual grains are observed. In the AFM image in Fig. 1b, and the corresponding surface roughness profile is presented in Fig. 1c. The surface roughness is around ± 20 nm. During the E-beam evaporation, the surface roughness could be controlled by the deposition rate. In this research, the WO3 deposition rate is controlled as 0.5 nm/s, a relatively high deposition rate for a higher surface roughness to increase the surface area for gas adsorption. The TEM and STEM images of WO3/Rh-3 are displayed in Fig. 1d and e. The Rh nanoparticles are homogeneously distributed among WO3. The particle size distribution based on the counting of 100 Rh nanoparticles is presented in Fig. 1f, and the size follows Gauss distribution. The average particle size is calculated as 5.85 nm. HRTEM images for WO3 thin film and Rh nanoparticle are presented in Fig. 1g and h. The lattice distance of 0.38 nm and 0.21 nm in Fig. 1g and h correspond to (020) and (111) planes in WO3 and Rh crystal structures. The corresponding EDX line scan regarding Rh nanoparticles in Fig. 1h is presented in Fig. 1i, it is observed that Rh is evenly distributed. The XRD patterns regarding WO3, WO3/Rh-1, WO3/Rh-2, and WO3/ Rh-3 are presented in Fig. 2. For pure WO3, its XRD patterns are consistent with standard diffraction patterns presented in JCPDS card (NO. 43–1035) [35], and secondary or impurity phases are not observed, indicating the high purity crystalline of the thin film. Moreover, the XRD patterns of WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 are similar to WO3, and XRD peaks of Rh nanoparticles are not observed, which could be explained by that the amount of Rh nanoparticles is below the detection limit of the X-ray diffractometer. The chemical interactions between WO3 and Rh could be studied by XPS, and the spectra for WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 regarding W and Rh are presented in Fig. 3. In Fig. 3a, two peaks at 37.8 and 35.7 eV are observed in WO3 thin film, which could be the characteristic peaks for W 4f5/2 and W4f7/2 [36]. In WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3, the two peaks are shifted to lower binding energy. This could be attributed to the bonding between WO3 and Rh, which changes the surface chemical states and electronic structure of WO3 [37]. More details were explained in the energy band diagram in Fig. 6. In Fig. 3b, for pure WO3, no Rh XPS peaks are observed because of the absence of Rh nanoparticles. Two XPS peaks are observed in WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 at 312.6 and 307.7 eV, which come from Rh 3d3/2, and Rh 3d5/2, respectively. Compared with the peak positions of pure Rh at 312.1 and 307.2 eV, for Rh 3d3/2, and Rh 3d5/2, the peak shift in Fig. 3b could also be explained by the interactions between WO3 and Rh [38]. In Fig. 4a, the CH4 sensing performance of WO3, WO3/Rh-1, WO3/ Rh-2, and WO3/Rh-3 is investigated between 250 and 450 °C, and the CH4 concentration is controlled as 1 ppm. It is found that, generally, the introduction of Rh nanoparticles onto WO3 thin film could increase the CH4 gas sensing response, and WO3/Rh-2 shows the highest signal response among the four samples, as high as ~110% increase in sensitivity, compared with pure WO3 sensor. When the amount of Rh nanoparticles is further increased, the gas sensing performance is decreased, as shown in WO3/Rh-3. The highest response in WO3/Rh-2 could be attributed to the methane surface catalytic effect contributed by Rh [39,40]. However, Rh nanoparticles could also partially cover the WO3 thin film surface. When the Rh amount is further increased from
2. Experimental 2.1. WO3 thin film deposition WO3 thin film was deposited on the electrodes by E-beam evaporation, with the target of WO3 (99.99%) [34]. During the deposition, the vacuum was less than 5E-5 Torr, and the deposition rate was around 0.4 nm/s. An intentional high growth rate was applied to increase the surface roughness and surface area. The thin film thickness was controlled as 200 nm. 2.2. Rh nanoparticle deposition The Rh nanoparticles were deposited on WO3 thin film by ALD. Rh (acac)3 was used as the Rh source, and O2 was used as the reactant. During the ALD, the substrate temperature was 250 °C, and the vacuum was 300 mTorr. To vaporize Rh(acac)3, the container temperature was holding at 150 °C, and Ar was used as the carrying gas. The typical four pulses for the ALD were summarized and presented in Table 1, and a simplified ALD process was presented in Scheme 1. In three batches, the Rh deposition cycle was 10, 20 and 30, and the products were noted as WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3. The sample without Rh deposition was named as WO3. 2.3. Thin-film characterization The thin film surface morphology and Rh nanoparticle size were characterized by SEM (JEOL 7000), AFM, and TEM (Hitachi HT7700). The crystal structure was determined by XRD, and surface conditions of the thin film were studied by XPS. 2.4. Gas sensing testing The gas sensing performance of WO3 and WO3/Rh was evaluated in CH4, H2S, C7H8, CO, NH3, and n-C5H7 gas environment, the gas concentration was controlled between 1 and 5 ppm, and the temperature was controlled between 250 and 400 °C. The standard concentration of gases used for the experiment was 5 ppm (supplied by EGAS DEPOT, balanced with air). Based on the typical condition of the ambient environment, the humidity level was controlled as 80% RH during gas sensing testing. Different concentrations of CH4 gas (1–4 ppm) were obtained by diluting 5 ppm CH4. The two Au electrodes with a width of 25 μm, a gap of 150 μm, and a thickness of 200 nm, were deposited on an alumina substrate by sputtering with a shallow mask. A heater was attached to the backside of the substrate. For each gas sensing performance testing, the sensor was stabilized with baseline condition for 12 h. During the gas sensing, the sensor was exposed in the gas for Table 1 The details of the four pulses in the ALD process.
Chemical/gas Time
1st pulse
2nd pulse
3rd pulse
4th pulse
Rh(acac)3/Ar 4s
Ar 60 s
O2 4s
Ar 60 s
2
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Scheme 1. The four pulses of the ALD process.
Fig. 1. WO3 characterization: (a). SEM; (b). AFM; (c). Surface roughness profile of as-deposited; WO3/Rh-3 characterization: (d). TEM; (e). STEM; (f). Rh nanoparticle size distribution; (g). HRTEM for WO3; (h) HRTEM for Rh nanoparticles; (i). EDX line scan for Rh nanoparticle in (h).
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that, when the CH4 concentration decreases to 0.1 ppm, the WO3/Rh-2 sensor will lose sensing capability, with the Response of 1.07. This indicates that WO3/Rh-2 has higher capability to detect lower concentrations of CH4. The gas sensing sensitivity of WO3–Rh-2 at 350 °C is investigated with the gas concentration of 5 ppm, and the result is presented in Fig. 4d. The responses to CH4, H2S, C7H8, CO, NH3, n-C5H7, NO and NOx are observed as 63.1, 34.2, 28.5, 20.6, 15.6, 10.9, 5.1 and 4.0, respectively. It is concluded that WO3/Rh-2 has high selectivity towards both reducing gases and oxidizing gases, but shows the highest selectivity towards CH4 gas. This could be attributed to the Rh's catalytic effects towards CH4, and more details will be discussed below. The response time is the time that the gas sensor takes to reach the 90% of the stabilized signal intensity, while the recovery time is the time that the gas sensor takes to decrease the 90% of the stabilized signal. The general definitions for the response and recovery time are presented in Fig. 5a, and the corresponding response and recovery time regarding WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 between 100 and 1000 ppm are presented in Fig. 5b and c, respectively. In Fig. 5b, it is observed that a higher gas concentration results in a shorter response time. WO3 has the longest response time among all the samples. After the introduction of Rh nanoparticles, the response time of CH4 sensing for WO3/Rh is decreased, and WO3/Rh-2 has the shortest response time. Similar results are also observed with the recovery time, as shown in Fig. 5c. This demonstrates that the presence of Rh nanoparticles could decrease both responding and recovery time. The CH4 sensing stability testing for WO3/Rh-2 is studied by exposing the sensor to 5 ppm CH4 and 350 °C for 7 days, and the Rresponse is recorded and plotted in Fig. 5d. After running for 7 days, Rresponse is reduced by 0.63% (from 63.1 to 62.7), indicating its higher stability than other reported gas sensors. For example, Biswas et al. reported a decreased sensitivity of 5.5% for the graphene-semiconductor sensor for NH3 sensing in one week [41]. Based on the experiment above, the gas sensing mechanism for the increased CH4 sensing characteristics in WO3/Rh composite could be summarized. For general metal oxide semiconductor gas sensor, the response can be calculated by the ratio of the electric resistance in air and in target gas. The lattice oxygen, Olat, in WO3 could be reactive for CH4 oxidation, while the adsorbed oxygen species, including O2− and O−, on the surface of WO3, are more reactive [42]. Generally, the reactions could be summarized:
Fig. 2. X-ray diffraction patterns of WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh3.
WO3/Rh-2, the covered WO3 surface could be over high, resulting in decreased signal response. Moreover, it is observed in Fig. 4a tat, the temperature-dependence gas sensing response curves exhibit reversed “V” shape, and the optimum operating temperature is shown as 350 °C for all samples. Cyclic response transition properties of the four samples are presented in Fig. 4b. In the five pulses from the left to the right, the CH4 concentration is controlled as 5, 4, 3, 2, and 1 ppm, respectively. It is observed that Rh nanoparticles could increase the CH4 sensing signal intensity of WO3/Rh composites, and WO3/Rh-2 shows the highest performance. Specifically, the sample of WO3/Rh-2 shows the highest response of 63.1 at 5 ppm, which is around twice the response from pure WO3. Similar results are also observed with the CH4 concentration at1 ppm. The detection limit of the gas sensors could be determined by plotting a linear curve regarding the gas concentration v.s. the response, and the results for the four samples are presented in Fig. 4c. It is noted that when the Response is close or around 1, the gas sensor loses the sensing ability because it could not distinguish the signals from air and sensing gas. For WO3 sensor, it is calculated that when CH4 concentration is 0.16 ppm, the Response is 1.06, losing the gas sensing capability. With the same gas concentration (CH4: 0.16 ppm), the Response is 1.7 for WO3/Rh-2. This indicates that WO3/Rh-2 has a higher sensibility for low concentration CH4 detection. It can also be calculated
CH4 + 2Olat → CH3-Olat + H-Olat,
(1)
CH3-Olat + 2Olat + 2O− → CO2(gas) + 2H-Olat + 2e.
(2)
Due to the catalytic effect of Rh for methane oxidation [30,40], the
Fig. 3. XPS spectra of WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 for (a). W; (b). Rh. 4
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Fig. 4. (a). Temperature-dependent response with 1 ppm CH4; (b). Cyclic response transition properties under 5, 4, 3, 2, and 1 ppm of CH4 at 350 °C; (c). Limit of detection at 350 °C. (d). Selective CH4 detection characteristics for CH4, H2S, C7H8, CO, NH3, n-C5H7, NO and NOx under 5 ppm and 350 °C.
The energy band diagram of the barrier height of WO3/Rh junction could be expressed as:
presence of Rh could speed up the Reaction (2), which results in higher flow of electrons, therefore a lower electric resistance in the target gas, and an increased Rair/Rgas. Meanwhile, the increased reaction rate in Reaction (2) also results in a shorter time reaching the reaction equilibrium, therefore, a decreased response time. The decreased recovery time in WO3/Rh-2 could be also be attributed to the presence of Rh nanoparticles, which facilitate the transfer and regeneration of the electrons [43]. However, when the amount of Rh nanoparticles is further increased from WO3/Rh-2, the recovery time is increased, as shown in WO3/Rh-3 in Fig. 5b. A possible explanation is that, lattice oxygen (Olat) within WO3 plays a key role in CH4 adsorption and surface reaction, as shown in Reaction (1) and Reaction (2). When the Rh nanoparticles are excessive on the surface of WO3 (such as in WO3/Rh3), a portion of lattice oxygen is covered and blocked by Rh nanoparticles, and the Reaction (1) and Reaction (2) are prevented, generating decreased electron flow. Therefore, the response time of WO3/ Rh-3 is increased. It is observed that Rh nanoparticles have both positive (transferring electron) and negative (blocking lattice oxygen) impacts on WO3/Rh gas sensing. The optimum amount of Rh nanoparticles occurs in WO3/Rh-2. Furthermore, ideally, to verify this mechanism, it is necessary to detect and monitor the CO2 generation during CH4 sensing. However, due to the limitation of hardware, we are unable to monitor the CO2 at the current stage. The mechanism for the increased selectivity of WO3/Rh over pure WO3 could be further explained by the energy band diagrams. Specifically, the energy band diagrams of WO3/Rh is before and after the thermal equilibrium are presented in Fig. 6a and b. A Schottky contact is formed between n typed WO3 semiconductor and the metal Rh. In the above figures, ФRh and ФWO3 refer to the work function of Rh and WO3, respectively. Ec, Ev, and EF refer to the conduction band, valence band, and Fermin energy, and χ refers to WO3 electron affinity.
ФB = ФRh – χ,
(3)
Meanwhile, during the methane adsorption and dissociation, protons (H+) are formed and diffused into the metal nanoparticles, and capture their electron to form H2 [44]. When the electrons are consumed within Rh, the work function of Rh (ФRh) is reduced, which results in a decreased barrier height of WO3/Rh junction (ФB). The electrons generated in the semiconductor (WO3) would flow into metal (Rh). Therefore, the presence of Rh decreases the barrier height of WO3/Rh junction, speeds up CH4 surface reaction, and results in an increased gas sensing selectivity, as shown in Fig. 4a, b, and c. 4. Conclusions WO3/Rh gas sensors with different amount of Rh nanoparticles were fabricated, and their gas sensing performance was investigated in CH4 gas environment. The resulted indicated that all gas sensors had the optimum operating temperature of 350 °C. Among the four samples, WO3/Rh-2 showed the highest sensitivity for CH4 sensing, which could be attributed to the surface catalytic effect contributed by Rh nanoparticles, while over high concentration of Rh could be harmful to the gas sensing performance, because of blocking the active sites on WO3 thin film. Moreover, WO3/Rh-2 presented the shortest response time and recovery time among the four sensors. Finally, the gas sensing mechanism was explained. Declaration of competing interest We confirm that there are no known conflicts of interest associated 5
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Fig. 5. (a). The response/recovery time definition in the gas sensor; (b). The response time for WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 between 100 and 1000 ppm; (c). The recovery time for WO3, WO3/Rh-1, WO3/Rh-2, and WO3/Rh-3 between 100 and 1000 ppm; (d). The CH4 sensing stability testing for WO3/Rh-2 at under 5 ppm and 350 °C for 7 days.
Fig. 6. Energy band diagram of WO3/Rh: (a). Before the thermal equilibrium condition; (b). Under the thermal equilibrium condition.
with this publication and there has been no significant financial support for this work that could have influenced its outcome.
[2]
Acknowledgment
[3]
This research was financially supported by the National Natural Science Foundation of China (NO. 625413580), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (NO. KJQN201801222), the Chunhui Project from Education Ministry of China (NO. Z2015140), and the Key Cultivation Project of Chongqing Three Gorges University (NO. 17ZD12).
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