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Fabrication of highly sensitive and selective H2 gas sensor based on SnO2 thin film sensitized with microsized Pd islands
Accepted Manuscript Title: Fabrication of highly sensitive and selective H2 gas sensor based on SnO2 thin film sensitized with microsized Pd islands Author: Nguyen Van Toan Nguyen Viet Chien Nguyen Van Duy Hoang Si Hong Hugo Nguyen Nguyen Duc Hoa Nguyen Van Hieu PII: DOI: Reference:
S0304-3894(15)30068-6 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.09.013 HAZMAT 17086
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
22-6-2015 30-8-2015 6-9-2015
Please cite this article as: Nguyen Van Toan, Nguyen Viet Chien, Nguyen Van Duy, Hoang Si Hong, Hugo Nguyen, Nguyen Duc Hoa, Nguyen Van Hieu, Fabrication of highly sensitive and selective H2 gas sensor based on SnO2 thin film sensitized with microsized Pd islands, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.09.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of highly sensitive and selective H2 gas sensor based on SnO2 thin film sensitized with microsized Pd islands Nguyen Van Toan1, Nguyen Viet Chien1, Nguyen Van Duy1, Hoang Si Hong3, Hugo Nguyen2, Nguyen Duc Hoa1*, Nguyen Van Hieu1* 1
International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No 1, Dai Co Viet Road, Hanoi, Viet Nam 2
Division of Microsystems Technology, Department of Engineering Sciences, Uppsala University, 75237, Uppsala, Sweden 3
School of Electrical Engineering (SEE), Hanoi University of Science and Technology (HUST), Hanoi, Viet Nam *
Corresponding authors.
Highlights H2 gas sensors based on SnO2 thin film sensitized with Pd islands were fabricated. The sensors could monitor hazardous H2n gas at low concentrations of 25–250 ppm. H2 response of Pd/SnO2 is higher than that of Pt/SnO2 and Au/SnO2 sensors Enhancement of sensor performance was discussed based on spillover and diffusion mechanisms.
Abstract Ultrasensitive and selective hydrogen gas sensor is vital component in safe use of hydrogen that requires a detection and alarm of leakage. Herein, we fabricated a H2 sensing devices by adopting a simple design of planar–type structure sensor in which the heater, electrode, and sensing layer were patterned on the front side of a silicon wafer. The SnO2 thin film–based sensors that were sensitized with microsized Pd islands were fabricated at a wafer–scale by using a sputtering system combined with micro–electronic techniques. The thicknesses of SnO2 thin film and microsized Pd islands were optimized to maximize the sensing performance of the devices. The optimized sensor could be used for monitoring hydrogen gas at low concentrations of 25–250 ppm, with a linear dependence to H2 concentration and a fast response and recovery time. The sensor also showed excellent selectivity for monitoring H2 among other gases, such as CO, NH3, and LPG, and satisfactory characteristics for ensuring safety in handling hydrogen. The hydrogen sensing characteristics of the sensors sensitized with Pt and Au islands were also studied to clarify the sensing mechanisms. Keywords: H2 gas sensor; Sputtering; SnO2; Pd islands; wafer–scale fabrication
1.
Introduction
Hydrogen gas is expected to become a green and renewable energy source in upcoming years for several applications, including fuel cell vehicles, space crafts, automobiles, power generators, and aircrafts [1]. This light and odorless gas is highly flammable; hydrogen gas leaks can result in disastrous consequences, such as explosion [2]. Therefore, there has been a huge demand on effective gas sensor that can be used for detection and alarming of H2 leakage during production, storage, transportation and usage [3-6]. Resistive sensors operate while relying on the change of conductivity in the nanostructured metal oxide semiconductors upon adsorption/desorption of analytic molecules; hence, such sensors appear to be one of the simplest, low–cost and effective devices for real–time monitoring or detection of gas leaks [7]. Thus, different nanostructures of metal oxide semiconductors have been prepared for hydrogen gas sensing [8,9]. Gas sensors based on nanorods, nanowires, and/or nanotubes have an extensively high sensitivity, but they are currently under development and unsuitable for commercialization because of limited fabrication techniques toward mass production [10]. The most investigated nanostructures for gas sensing is the thin film form because of their simple design and configuration, compatibility with silicone techniques, and scalable fabrication for commercialization. For instance, sol–gel thin film of SnO2 was prepared for hydrogen gas sensor [11]. ZnO thin film was also prepared for the hydrogen–sensing application [12]. Bare metal oxides show a relatively low sensitivity to hydrogen gas, with a high detection limit [13–15]. However, the gas–sensing characteristics of materials are improved significantly by adding sufficient noble metals, such as Pd, to bare metal oxide semiconductors [16]. For instance, Lin et al. [17] used a hydrothermal method to decorate Pd nanoparticles on the surface of ZnO nanowire arrays for enhanced ethanol sensing performance. Xing et al. [18]
reported that the ZnO nanoflowers decorated with Pd nanoparticles exhibited the highest response to ethanol, followed by H2, CO and CH4, respectively. The enhancements of hydrogen depend on the catalytic activity of Pd on dissociation of hydrogen and oxygen molecules and/or formation of the carrier depletion region that modifies the conduction of materials [19]. The most common and general method for adding (doping, functionalization, or decoration) noble metal to bare metal oxide semiconductors is wet chemical pathway, in which the metal ion precursors are in the form of a solution [21]. Bulk doping or surface modification of noble metal is usually attained using wet chemical method. An advantage of wet chemical method for modification is the low–cost and simple synthesis method; however, controlling the decoration region and thickness of catalytic layer is difficult. Recently, physical methods have been used for decoration/functionalization of metal oxide surface. Sub–millimeter sizes of catalytic islands have been used to enhance the gas–sensing characteristics of SnO2 thin film [22]. The advantages of using thin film catalytic islands in sensor fabrication include the ease in controlling the thickness of the catalytic layer or amount of additive metal, density, and position of islands [23]. This thin film fabrication technique also enables large–scale fabrication for mass production [24]. Nevertheless, optimization of a simple and wafer–scale synthesis process for maximizing the hydrogen–sensing performance is also important and necessary for practical application. Herein, we demonstrate a process for fabrication of high–sensitive and high–selective H2 gas sensors based on thin film of SnO2 sensitized with micro–sized Pd islands. The advantage of using Pd islands as a catalyst for enhancing H2–sensing performance of SnO2 thin film is the simple synthesis, in which the thicknesses of SnO2 film and Pd islands may be controlled to achieve the highest sensitivity of the sensors. The enhancement of the sensor sensitivity is also
discussed based on spillover and diffusion mechanisms. We response of SnO2–Pd film is also compared with that of sensors sensitized with Pt and Au islands as catalyst to clarify the sensing mechanism. 2.
Experimental
2.1 Sensor fabrication The H2 gas sensor composes of a micro–heater and a pair of electrodes of Pt/Cr layers deposited on a thermally oxidized silicon wafer [24]. Process for the fabrication of Pd/SnO2 thin film sensors is shown in Figure S1 (Supplementary material). A sensing layer of SnO2 thin film was patterned and deposited by reactive sputtering, followed by an ordinary sputter deposition of functionalizing Pd islands. The SnO2 thin films of different thicknesses ranging from 20 to 80 nm were deposited from a Sn target under the following conditions: based pressure of 10−6 torr; working pressure of 5×10−3 torr; and Ar/O2 flow ratio of 50:50. Pd islands of different thicknesses were deposited using a Pd target, and pure Ar was used as the sputter gas. Sputtering conditions were similar to that of the SnO2 deposition, but the thicknesses of Pd islands were controlled to be approximately 5, 10, 25, and 40 nm. Thicknesses of the SnO2 and Pd thin films were measured using a Veeco Dektak 150 Surface Profilometer (Veeco Instruments Inc., USA) with an accuracy of ±0.6 nm. The size of the sensing area was pre–defined as 150 µm×150 µm, whereas the diameter and distance between the Pd islands were both 5 µm. Fabrication of sensor wafers involved the following process reported in ref. [24]: Finally, heat treatment at 400°C was conducted for 2 h in air to ensure stability of the sensors. The materials and devices were characterized using some advanced techniques, including field–emission scanning electron microscopy (FE–SEM, JEOL model 7600F). Elemental analyses were conducted using energy–
dispersive X–ray spectroscopy (EDS) that was integrated in the FE–SEM instrument. The crystal structure of materials was studied through a wide–angle powder X–ray diffraction (XRD) using CuKα X–radiation with a wavelength of 1.54178 Å. 2.2. Gas sensing measurements Sensor measurements were obtained using a flow–through technique [24]. Briefly, the measurement system was a chamber with a volume of about one liter. Inside the sensing chamber, two tungsten needles were used as the electrical connection to the device for gas– sensing measurement. A series of mass flow control was used to control the injection of analytic gas into the sensing chamber. Prior to these measurements, dry air (commercial product, Cryotech Vietnam, JSC) was blown through the sensing chamber until the desired stability of the sensor resistance was reached. Sensor resistance was continuously measured using a Keithley instrument (model 2602) that was connected to a computer while switching dried air and analytic gases on and off during each cycle. The total gas flow rate was 400 sccm. The sensor response is defined as S=Ra/Rg, where Ra and Rg are the resistances of the sensor in dry air and analytic gas, respectively. In this experiment, we used the parent H2 gas with calibrated concentration of 10000 ppm in dry air balance (commercial product, Cryotech Vietnam, JSC). The target gas concentration was controlled by changing the mixing ratio of the parent gas and dry air using series of mass–flow controllers. The target gas concentration was calculated as follows: C(ppm)=Cstd(ppm)×f/(f+F), where f and F are the flow rates of the parent gas and dry air, respectively, and Cstd(ppm) is the calibrated concentration of the parent gas. The selectivity of the fabricated sensor against other gases, such as CO, NH3, and liquefied petroleum gas (LPG) was also studied through separate measurement of the variation in sensor resistance upon exposure to each gas.
3.
Results and discussion
Morphology of the fabricated SnO2–Pd thin film sensor was characterized using SEM; the images are shown in Figure 1. A SEM image of a sensor chip shows the clear patterned micro– heater and a defined sensing area [Figure 1(A)]. A high–magnification SEM image of the thin film region reveals the homogeneity of the film [Figure 1(B)], which was deposited on thermally oxidized silicon substrate. The thin film has high porosity because of the polycrystalline nature of the oxide, which was obtained using the sputtering deposition method. The thickness of the thin film was approximately 80 nm, which was estimated from the debris of the film [Figure 1(C)]. The boundary between the SnO2 film and the Pd island can be seen clearly in the inset of Figure 1(D). The size of Pd island is about 5 µm in diameter. The SnO2 thin film composes of nanograins with an average size of less than 10 nm as revealed in a high–magnification SEM image [Figure 1(D)]. The grain size of fabricated SnO2 thin film is much smaller than that of the film prepared by sputter deposition of Sn and subsequent calcination at high temperature [25]. The smaller grain size of SnO2 thin film is expected to show a higher sensitivity. SEM images of the SnO2–Pd thin films with different Pd thicknesses are shown in Figure 2. The SEM images zoomed in the Pd/SnO2 boundary and the Pd and SnO2 regions were marked by Pd and SnO2. It is impossible to measure exactly the thickness of Pd payer by plan view SEM images but we can qualitatively estimate from the difference in contrast between Pd and SnO2 regions of different samples [Figure 2(A)-(D)]. The Pd/SnO2 boundaries became clearer with increasing of Pd thickness. The Pd layer with a thickness of 5 nm is not a dense film but looks like composing of dis-continuous nanogranules [Figure 2(A)]. The Pd layer became continuously and denser with increasing the Pd film thickness to 10 nm [Figure 2(B)]. However, there was no much difference in the surface morphology of the 25 and 40 nm Pd layers [Figure 2(C,D)].
Results of the crystal structure and elemental analyses of the thin film using XRD and EDS are shown in Figures 3(A) and 3(B), respectively. The XRD pattern confirms the tetragonal structure of the fabricated SnO2 thin film (JCPDS, No. 41–1445). No detectable peak of the Pd or PdO phase can be observed in the XRD pattern. This result is possibly because of the low crystallinity or small content of the Pd. To confirm the existence of Pd in the thin film, EDS analysis was performed for the area shown in the inset of Figure 3(B). The EDS analysis shows the existence of C, O, Pd, Si, Pt, and Sn. The peaks of C and Si originated from the contaminated carbon on the surface and silicon substrate, respectively. The presence of Pt was ascribed to the Pt electrode and Pt–coating for SEM analysis. O, Pd, and Sn were components of the prepared material. The intensities of Pd and Sn are weak, thus the quantitative estimation of composition from the EDS analysis is not presented.
The hydrogen gas–sensing characteristics of the base SnO2 thin film sensors were tested in different concentrations (100–1000 ppm) of H2 at temperatures of 300, 350, and 400°C. As shown in Figure 3, the plots of transient resistance versus time upon exposure to H2 for different sensors exhibit similar trends, in which the resistance of sensors decreases significantly in H2 and recovers to the initial values when the flow of analytic gas stops. As exposed to 250 ppm H2 gas, the response time was 15, 14, and 14 s whereas the recovery time was 4, 3, and 3 s at temperature of 300, 350, and 400°C, respectively. This significantly fast response and recovery times are a result of high working temperature, which accelerates adsorption and desorption of hydrogen molecules. The sensors show almost 100% recovery at all measured temperatures, which confirms the reversible adsorption of H2 molecules on the sensor surface.
The response to 250 ppm H2 of different sensors measured at 300, 350, and 400°C is plotted in Figure 5(A). The response to H2 increases with the increase of working temperature up to 400°C. Further increase of working temperature can increase the sensor response but would require a high power consumption to maintain the temperature of the sensor chip [26]. Figure 5(A) also demonstrates that the magnitude of increment in sensor response of the film SnO2 (40 nm) is largest followed by the SnO2 (80 nm), SnO2 (60 nm) and SnO2 (20 nm). The magnitude of the incensement of the sensor response is dependent on the optimal working temperature, and the sensitivity of the sensors. In the report by Ansari et al. [27], they investigated the effect of grain size on the H2 gas sensitivity of the SnO2 film, and found that the larger particle size lead to a lower optimal working temperature. Such those characteristics were attributed to the effect of grain boundaries between the nanoparticles. It is difficult to find the relationship between the grain size and the magnitude of the incensement of the sensor response. Herein, the variation in magnitude of increment in sensor response of different films can be a result of the difference in thin film thickness. However, a systematic study about the effect of such those characteristics on the gas-sensing performance of the SnO2 thin films are necessary to clarify our claim. The responses of all SnO2 sensors as a function of H2 concentration at 300°C are shown in Figure 5(B). The sensor response increases nearly linearly with increasing H2 concentration in the measured range [Figure 5(B)]. The sensor with a thickness of 40 nm has the highest response, followed by the 20, 60, and 80 nm sensors. The maximum response of the 40 nm sensor to 1000 ppm H2 at 300°C is approximately 6.5. This value is comparable with that of the bare SnO2 thin film prepared using sol–gel method, in which the induced response was approximately 5 to 1000 ppm H2 at 150°C [28]. Du and George investigated the effect of thickness on the CO sensing of SnO2 thin films that were prepared using an atomic layer deposition, and they found that the
sensor with approximately 2.5 nm thickness achieved the highest response [29]. In the current study, the films may be thicker to maximize response because of their high porosity, which enables the analytic gas molecules to defuse deeply into the film, resulting in a resistant change sensor. The diffusion constant (DK) can be represented as DK=4r/3(2RT/πM)1/2, where r is the pore size, R is the universal gas constant, T is the temperature, and M is the molecular weight of diffusing gas [30]. A high porosity of granule films leads to a high diffusion constant; thus, response may be maximized by increasing the film thickness. Based on above investigation, we varied the thickness of Pd islands from 5 nm to 40 nm and maintained the thickness of SnO2 thin film at 40 nm to study the effect of Pd islands on sensor performance. Given that the SnO2 thin film sensors that were sensitized with Pd islands have a superior sensitivity, thus low concentrations of H2 (25–250 ppm) were tested. The transient resistance vs. time upon exposure to various H2 concentrations of the SnO2–Pd sensors with different thicknesses of Pd islands is shown in Figure 6. At all measured temperatures from 200 to 400°C, the fabricated sensors showed similar response characteristics to those of the bare SnO2 thin film. Nevertheless, the SnO2–Pd sensors exhibited much lower detection limit, with a significant response to 25 ppm H2. This low detection limit with high response not only enables use of the sensors in ensuring safe usage of hydrogen (Lower explosive limit of H2 is 4%), but also in diagnosis of diseases through monitoring of hydrogen gas in exhaled breath [31]. The temperature dependence of sensor response to 250 ppm H2 of different sensors is plotted in Figure 7(A). For all of the sensors excepted the SnO2–Pd(5 nm) and the SnO2–Pd(10 nm) devices, the responsivity increases with the increase of working temperature. However, the maximum response was attained at approximately 350 and 300°C for the SnO2–Pd(5 nm) and SnO2–Pd(10 nm) sensors, respectively. The SnO2–Pd(10 nm) sensor showed the highest
responsivity of approximately Ra/Rg=28 at 300°C. This optimal sensor not only exhibited enhanced responsivity but also functioned effectively at lower working temperature. The sensor responses as a function of H2 concentration (25 ppm to 250 ppm) for different sensors are shown in Figure 7(B). For all of the sensors, the sensor response increases linearly with the H2 concentration in the measured range. However, the SnO2–Pd(10 nm) sensor showed the highest response with the best linear dependence on H2 concentration. The response and recovery times to 250 ppm H2 of the SnO2–Pd sensors with different thicknesses of Pd islands measured at temperature ranging from 200 to 400oC are shown in Figure 7(C). We can see that the response time is shorter than the recovery time, and they decreased with increasing of working temperatures. The response time is in the range of 3 to 25 s whereas the recovery time is in the range of 12 to about 250 s. The dependence of the response and recovery times on the H2 concentrations measured at temperature of 300oC is plotted in Figure 7 (D). We can see that the response time decreases with H2 concentrations, whereas the recovery time increases. The result is consistent with the report by Zhao et al. [32], where the sensing properties were explained based on the diffusion model. The response and recovery times of the sensor are approximately 3 and 50 s at 300°C, those are comparable with the values of the bare SnO2 sensors.
The use of Pd islands as catalyst in the SnO2 thin film also improved the selectivity of the sensors. In Figure 8(A), the selectivity of the sensors was studied by comparing the response of the bare SnO2 and SnO2 sensitized with Pd islands sensors to different gases, such as 250 ppm CO, 250 ppm NH3, and 2500 ppm LPG. The SnO2–Pd(10 nm) sensor showed significantly enhanced selectivity at working temperatures of approximately 300 and 400°C. Further comparison of the response to different concentrations of H2 gas for the bare SnO2 and SnO2–
Pd(10 nm) sensors is shown in Figure 8(B). The SnO2–Pd(10 nm) sensor showed a high response even at a low H2 concentration. The stability of the SnO2–Pd(10 nm) sensor was also studied after 6 months stored in ambient environment by measuring the transience resistance vs. time at temperature of 300oC. The data (Figure S2, Supplementary material) show that no significant distortion in sensor response can be observed after ten cycles switching on/off from dry air to H2 and back to air. Results indicate that the fabricated SnO2–Pd thin film sensor is excellent for applications in selectively monitoring H2 gas at low concentration (ppm levels) with high sensitivity and good stability. The sensing mechanism of the device can be explained using a depletion conduction model. In the bare SnO2 thin film sensor, the H2 molecules interacted with pre–adsorbed oxygen (O2−, O−, and O2−) upon exposure to H2 and released electrons to increase the total carriers, thus reducing the electron depletion region, resulting in a decrease in the sensor resistance [10]. However, two mechanisms contribute to the sensing characteristics of the SnO2–Pd sensors: (1) the chemical and (2) the electronic mechanisms [20]. The chemical mechanism relies on the catalytic activity of Pd through the spillover principle, which accelerates dissociation of (i) oxygen and (ii) hydrogen molecules to enhance the number of pre–adsorbed oxygen on the surface of SnO2, as well as the interaction between hydrogen and pre–adsorbed oxygen to increase the response of sensors to H2. Providing experimental measurement to clarify whether the catalytic dissociation of either oxygen or hydrogen determines the enhancement of sensor response is difficult. If the dissociation of oxygen using a Pd catalyst is dominant, then the sensor response to all other gases, such as CO, NH3, and LPG, should be improved. However, the results showed the highest improvement to H2 gas because the Pd catalyzes the dissociation of H2 molecule into two active hydrogen atoms better than other gases. Thus, the SnO2 thin film that was sensitized with Pd
islands exhibited a high response to hydrogen. Meanwhile, the electronic mechanism is based on formation of poor conduction region formed at the interface between Pd islands and SnO2 thin film. The work function of Pd (~5.2 eV) is higher than that of the n–type SnO2 (~4.4 eV), thus Schottky barrier is formed at the interface of Pd and SnO2 [33]. As a result, the conduction channel of the device is modulated, which facilitates the response to the analytic gas. Therefore, the formation of Pd–SnO2 heterojunction junction and the high catalytic dissociation of H2 molecules into active hydrogen atoms dominate the significant enhancement of hydrogen sensing of SnO2 thin films through sensitization with microsized Pd islands. However, the reason why the sensor with 10 nm thick Pd islands showed the highest response is still unclear. In our opinion, the 10 nm thickness of Pd islands is optimal for adsorption of analytic molecules on the surface of Pd, which can effectively modulate the thickness of depletion region at the Pd/SnO2 interface. As discussed above, both chemical and electronic mechanisms contribute to the characteristics of the sensitized sensors [33]. To identify the mechanism that dominates the enhancement of H2– sensing performance of SnO2 thin films, we fabricated three sensors that were sensitized with different noble metals: Pd, Pt, and Au, with work functions of approximately 5.2, 5.6, and 4.8 eV, respectively. Such those metals have been recently used as catalyst for enhanced gas-sensing performances due to their high catalytic activity [34–36]. The SnO2–Pd, SnO2–Pt, and SnO2–Au sensors were fabricated with a fixed the thickness of Pd, Pt, and Au islands at 10 nm. The Schottky barrier formed at the interface of Pt–SnO2 is the highest (~ 1.2 eV), followed by that of SnO2–Pd (0.8 eV) and SnO2–Au (0.4 eV). Responses of different sensors (SnO2–Pd, SnO2–Pt, SnO2–Au) to various concentrations of H2 measured at 300oC are shown in Figure 9. Details about the transient response to H2 of SnO2–Pt and SnO2–Au sensors are shown in Figure S3, and
Figure S4, respectively (Supplementary material). We expected that the higher Schottky barrier formed at the interface between catalyst and SnO2 result a higher sensitivity when the electronic mechanism dominates the enhancement of the sensing performance. However, the data shows opposite results, in which the SnO2–Pd sensor showed the highest response, followed by SnO2– Pt and SnO2–Au (Figure S5, Supplementary material). Such results reveal that the electronic mechanism is not dominant in the enhancement of the sensing performance of SnO2 thin films; instead, the chemical mechanism dominates. Thus to enhance the sensitivity of the sensor, the noble metal islands should have a high catalytic activity to the desired gas. The H2 gas–sensing mechanism of the Pd-SnO2 thin films with different thicknesses of Pd islands is shown in Figure 10. Due to the spillover catalytic activity of Pd in the dissociation of oxygen, the number of pre-adsorbed oxygen on the surface of Pd/SnO2 (Figure 10B,C,D) is higher than that in the bare SnO2 (Figure 10A), thus the SnO2–Pd sensors exhibited higher response. In addition to the catalytic activity, the formation of heterojunction between Pd and SnO2 also contribute on the gas–sensing performance because the reaction between analytic H2 molecules and Pd layer can modulate the Schottky barrier at the Pd/SnO2 heterojunction. However, at a relatively small thickness of Pd (5 nm), the contribution of Pd/SnO2 heterojunction in the modulation the Schottky barrier is low because the discontinuous Pd layer [Figure 10(B)]. With increase the thickness of Pd islands to about 10 nm, the adsorption of H2 molecule on the surface of Pd sufficient enough to modulate the Schottky barrier height, thus significantly increase the sensor response (Figure 10(C)). However, with increase of Pd thickness to about 25 nm or more, the adsorption of H2 molecules on the surface of Pd islands became less effective in modulation the Schottky barrier of the Pd/SnO2 heterojunction because the H2 adsorption just modulate the top layer of Pd island, resulting in a decrease of sensor response [Figure 10(D)].
4.
Conclusion
We have introduced a scalable synthesis process for fabrication of highly sensitive and selective H2 sensors based on SnO2 thin film sensitized with microsized Pd islands. The thicknesses of SnO2 and Pd films were optimized to maximize the sensor response. The SnO2(40 nm)–Pd(10 nm) sensor exhibited the highest sensitivity and selectivity to H2 thus should be a strong candidate for monitoring of H2 at low concentration of ppm level. The enhancement of sensor performance was also discussed based on spillover and diffusion mechanisms. We pointed out that the chemical mechanism or the catalytic activity of the metal islands dominates the enhancement of H2 sensing performance of the SnO2–Pd thin film sensors. Acknowledgments: The present research was funded by the Vietnam National Foundation for Science and Technology Development (Nafosted Code: 103.02–2014.06), and the research project of Vietnam Ministry of Education and Training under code B2015–01–92.
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Figure Captions Figure 1. SEM images of (A) a full chip, (B) SnO2 thin film on SiO2, (C) cross sectional image, and (D) SnO2–Pd. Inset of (D) is a Pd island of about 5 µm in diameter. Figure 2. SEM images of the SnO2–Pd thin films with different Pd thicknesses: (A) 5 nm; (B) 10 nm; (C) 25 nm; (D) 40 nm Figure 3. The XRD pattern (A) and EDS analysis (B) of SnO2 thin film sensitized with Pd islands. Inset of (B) is SEM image of the correspondent EDS analysis. Figure 4. Transient resistance vs. time upon exposure to various H2 concentrations of the bare SnO2 sensors with different thicknesses: (A) 20 nm, (B) 40 nm, (C) 60 nm, and (D) 80 nm. Figure 5. A comparative sensing performance of the base SnO2 sensors with different thicknesses: (A) sensor response as a function of operating temperatures; (B) Sensor response as a function of H2 concentrations measured at 300oC. Figure 6. Transient resistance vs. time upon exposure to various H2 concentrations of the SnO2 – Pd sensors with different thicknesses of Pd islands: (A) 5 nm, (B) 10 nm, (C) 25 nm, and (D) 40 nm. Figure 7. A comparative sensing performance of the SnO2–Pd sensors with different thicknesses of Pd islands: (A) sensor response as a function of operating temperatures; (B) Sensor response as a function of H2 concentrations measured at 300oC; (C) temperature dependence of the response and recovery time of different sensors; (D) response and recovery times as a function of H2 concentrations. Figure 8. (A) A comparison of the response to different gases [NH3 (250 ppm), CO (250 ppm), LPG (2500 ppm) and H2 (250 ppm)] of the bare SnO2 and SnO2–Pd (10 nm) thin film sensors; (B) a comparision of gas response as a function of H2 concentration of SnO2 and SnO2–Pd at 300oC. Figure 9. Response of different sensors (SnO2–Pd, SnO2–Pt, SnO2–Au) to various concentrations of H2 measured at 300oC. Figure 10. The gas–sensing mechanism of the SnO2 sensor sensitized with Pd islands of different thicknesses: (A) Pd = 0 nm; (B) Pd=5 nm; (C) Pd = 10 nm; (D) Pd≥25 nm.
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S0304-3894(15)30068-6 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.09.013 HAZMAT 17086
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
22-6-2015 30-8-2015 6-9-2015
Please cite this article as: Nguyen Van Toan, Nguyen Viet Chien, Nguyen Van Duy, Hoang Si Hong, Hugo Nguyen, Nguyen Duc Hoa, Nguyen Van Hieu, Fabrication of highly sensitive and selective H2 gas sensor based on SnO2 thin film sensitized with microsized Pd islands, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.09.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of highly sensitive and selective H2 gas sensor based on SnO2 thin film sensitized with microsized Pd islands Nguyen Van Toan1, Nguyen Viet Chien1, Nguyen Van Duy1, Hoang Si Hong3, Hugo Nguyen2, Nguyen Duc Hoa1*, Nguyen Van Hieu1* 1
International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No 1, Dai Co Viet Road, Hanoi, Viet Nam 2
Division of Microsystems Technology, Department of Engineering Sciences, Uppsala University, 75237, Uppsala, Sweden 3
School of Electrical Engineering (SEE), Hanoi University of Science and Technology (HUST), Hanoi, Viet Nam *
Corresponding authors.
Highlights H2 gas sensors based on SnO2 thin film sensitized with Pd islands were fabricated. The sensors could monitor hazardous H2n gas at low concentrations of 25–250 ppm. H2 response of Pd/SnO2 is higher than that of Pt/SnO2 and Au/SnO2 sensors Enhancement of sensor performance was discussed based on spillover and diffusion mechanisms.
Abstract Ultrasensitive and selective hydrogen gas sensor is vital component in safe use of hydrogen that requires a detection and alarm of leakage. Herein, we fabricated a H2 sensing devices by adopting a simple design of planar–type structure sensor in which the heater, electrode, and sensing layer were patterned on the front side of a silicon wafer. The SnO2 thin film–based sensors that were sensitized with microsized Pd islands were fabricated at a wafer–scale by using a sputtering system combined with micro–electronic techniques. The thicknesses of SnO2 thin film and microsized Pd islands were optimized to maximize the sensing performance of the devices. The optimized sensor could be used for monitoring hydrogen gas at low concentrations of 25–250 ppm, with a linear dependence to H2 concentration and a fast response and recovery time. The sensor also showed excellent selectivity for monitoring H2 among other gases, such as CO, NH3, and LPG, and satisfactory characteristics for ensuring safety in handling hydrogen. The hydrogen sensing characteristics of the sensors sensitized with Pt and Au islands were also studied to clarify the sensing mechanisms. Keywords: H2 gas sensor; Sputtering; SnO2; Pd islands; wafer–scale fabrication
1.
Introduction
Hydrogen gas is expected to become a green and renewable energy source in upcoming years for several applications, including fuel cell vehicles, space crafts, automobiles, power generators, and aircrafts [1]. This light and odorless gas is highly flammable; hydrogen gas leaks can result in disastrous consequences, such as explosion [2]. Therefore, there has been a huge demand on effective gas sensor that can be used for detection and alarming of H2 leakage during production, storage, transportation and usage [3-6]. Resistive sensors operate while relying on the change of conductivity in the nanostructured metal oxide semiconductors upon adsorption/desorption of analytic molecules; hence, such sensors appear to be one of the simplest, low–cost and effective devices for real–time monitoring or detection of gas leaks [7]. Thus, different nanostructures of metal oxide semiconductors have been prepared for hydrogen gas sensing [8,9]. Gas sensors based on nanorods, nanowires, and/or nanotubes have an extensively high sensitivity, but they are currently under development and unsuitable for commercialization because of limited fabrication techniques toward mass production [10]. The most investigated nanostructures for gas sensing is the thin film form because of their simple design and configuration, compatibility with silicone techniques, and scalable fabrication for commercialization. For instance, sol–gel thin film of SnO2 was prepared for hydrogen gas sensor [11]. ZnO thin film was also prepared for the hydrogen–sensing application [12]. Bare metal oxides show a relatively low sensitivity to hydrogen gas, with a high detection limit [13–15]. However, the gas–sensing characteristics of materials are improved significantly by adding sufficient noble metals, such as Pd, to bare metal oxide semiconductors [16]. For instance, Lin et al. [17] used a hydrothermal method to decorate Pd nanoparticles on the surface of ZnO nanowire arrays for enhanced ethanol sensing performance. Xing et al. [18]
reported that the ZnO nanoflowers decorated with Pd nanoparticles exhibited the highest response to ethanol, followed by H2, CO and CH4, respectively. The enhancements of hydrogen depend on the catalytic activity of Pd on dissociation of hydrogen and oxygen molecules and/or formation of the carrier depletion region that modifies the conduction of materials [19]. The most common and general method for adding (doping, functionalization, or decoration) noble metal to bare metal oxide semiconductors is wet chemical pathway, in which the metal ion precursors are in the form of a solution [21]. Bulk doping or surface modification of noble metal is usually attained using wet chemical method. An advantage of wet chemical method for modification is the low–cost and simple synthesis method; however, controlling the decoration region and thickness of catalytic layer is difficult. Recently, physical methods have been used for decoration/functionalization of metal oxide surface. Sub–millimeter sizes of catalytic islands have been used to enhance the gas–sensing characteristics of SnO2 thin film [22]. The advantages of using thin film catalytic islands in sensor fabrication include the ease in controlling the thickness of the catalytic layer or amount of additive metal, density, and position of islands [23]. This thin film fabrication technique also enables large–scale fabrication for mass production [24]. Nevertheless, optimization of a simple and wafer–scale synthesis process for maximizing the hydrogen–sensing performance is also important and necessary for practical application. Herein, we demonstrate a process for fabrication of high–sensitive and high–selective H2 gas sensors based on thin film of SnO2 sensitized with micro–sized Pd islands. The advantage of using Pd islands as a catalyst for enhancing H2–sensing performance of SnO2 thin film is the simple synthesis, in which the thicknesses of SnO2 film and Pd islands may be controlled to achieve the highest sensitivity of the sensors. The enhancement of the sensor sensitivity is also
discussed based on spillover and diffusion mechanisms. We response of SnO2–Pd film is also compared with that of sensors sensitized with Pt and Au islands as catalyst to clarify the sensing mechanism. 2.
Experimental
2.1 Sensor fabrication The H2 gas sensor composes of a micro–heater and a pair of electrodes of Pt/Cr layers deposited on a thermally oxidized silicon wafer [24]. Process for the fabrication of Pd/SnO2 thin film sensors is shown in Figure S1 (Supplementary material). A sensing layer of SnO2 thin film was patterned and deposited by reactive sputtering, followed by an ordinary sputter deposition of functionalizing Pd islands. The SnO2 thin films of different thicknesses ranging from 20 to 80 nm were deposited from a Sn target under the following conditions: based pressure of 10−6 torr; working pressure of 5×10−3 torr; and Ar/O2 flow ratio of 50:50. Pd islands of different thicknesses were deposited using a Pd target, and pure Ar was used as the sputter gas. Sputtering conditions were similar to that of the SnO2 deposition, but the thicknesses of Pd islands were controlled to be approximately 5, 10, 25, and 40 nm. Thicknesses of the SnO2 and Pd thin films were measured using a Veeco Dektak 150 Surface Profilometer (Veeco Instruments Inc., USA) with an accuracy of ±0.6 nm. The size of the sensing area was pre–defined as 150 µm×150 µm, whereas the diameter and distance between the Pd islands were both 5 µm. Fabrication of sensor wafers involved the following process reported in ref. [24]: Finally, heat treatment at 400°C was conducted for 2 h in air to ensure stability of the sensors. The materials and devices were characterized using some advanced techniques, including field–emission scanning electron microscopy (FE–SEM, JEOL model 7600F). Elemental analyses were conducted using energy–
dispersive X–ray spectroscopy (EDS) that was integrated in the FE–SEM instrument. The crystal structure of materials was studied through a wide–angle powder X–ray diffraction (XRD) using CuKα X–radiation with a wavelength of 1.54178 Å. 2.2. Gas sensing measurements Sensor measurements were obtained using a flow–through technique [24]. Briefly, the measurement system was a chamber with a volume of about one liter. Inside the sensing chamber, two tungsten needles were used as the electrical connection to the device for gas– sensing measurement. A series of mass flow control was used to control the injection of analytic gas into the sensing chamber. Prior to these measurements, dry air (commercial product, Cryotech Vietnam, JSC) was blown through the sensing chamber until the desired stability of the sensor resistance was reached. Sensor resistance was continuously measured using a Keithley instrument (model 2602) that was connected to a computer while switching dried air and analytic gases on and off during each cycle. The total gas flow rate was 400 sccm. The sensor response is defined as S=Ra/Rg, where Ra and Rg are the resistances of the sensor in dry air and analytic gas, respectively. In this experiment, we used the parent H2 gas with calibrated concentration of 10000 ppm in dry air balance (commercial product, Cryotech Vietnam, JSC). The target gas concentration was controlled by changing the mixing ratio of the parent gas and dry air using series of mass–flow controllers. The target gas concentration was calculated as follows: C(ppm)=Cstd(ppm)×f/(f+F), where f and F are the flow rates of the parent gas and dry air, respectively, and Cstd(ppm) is the calibrated concentration of the parent gas. The selectivity of the fabricated sensor against other gases, such as CO, NH3, and liquefied petroleum gas (LPG) was also studied through separate measurement of the variation in sensor resistance upon exposure to each gas.
3.
Results and discussion
Morphology of the fabricated SnO2–Pd thin film sensor was characterized using SEM; the images are shown in Figure 1. A SEM image of a sensor chip shows the clear patterned micro– heater and a defined sensing area [Figure 1(A)]. A high–magnification SEM image of the thin film region reveals the homogeneity of the film [Figure 1(B)], which was deposited on thermally oxidized silicon substrate. The thin film has high porosity because of the polycrystalline nature of the oxide, which was obtained using the sputtering deposition method. The thickness of the thin film was approximately 80 nm, which was estimated from the debris of the film [Figure 1(C)]. The boundary between the SnO2 film and the Pd island can be seen clearly in the inset of Figure 1(D). The size of Pd island is about 5 µm in diameter. The SnO2 thin film composes of nanograins with an average size of less than 10 nm as revealed in a high–magnification SEM image [Figure 1(D)]. The grain size of fabricated SnO2 thin film is much smaller than that of the film prepared by sputter deposition of Sn and subsequent calcination at high temperature [25]. The smaller grain size of SnO2 thin film is expected to show a higher sensitivity. SEM images of the SnO2–Pd thin films with different Pd thicknesses are shown in Figure 2. The SEM images zoomed in the Pd/SnO2 boundary and the Pd and SnO2 regions were marked by Pd and SnO2. It is impossible to measure exactly the thickness of Pd payer by plan view SEM images but we can qualitatively estimate from the difference in contrast between Pd and SnO2 regions of different samples [Figure 2(A)-(D)]. The Pd/SnO2 boundaries became clearer with increasing of Pd thickness. The Pd layer with a thickness of 5 nm is not a dense film but looks like composing of dis-continuous nanogranules [Figure 2(A)]. The Pd layer became continuously and denser with increasing the Pd film thickness to 10 nm [Figure 2(B)]. However, there was no much difference in the surface morphology of the 25 and 40 nm Pd layers [Figure 2(C,D)].
Results of the crystal structure and elemental analyses of the thin film using XRD and EDS are shown in Figures 3(A) and 3(B), respectively. The XRD pattern confirms the tetragonal structure of the fabricated SnO2 thin film (JCPDS, No. 41–1445). No detectable peak of the Pd or PdO phase can be observed in the XRD pattern. This result is possibly because of the low crystallinity or small content of the Pd. To confirm the existence of Pd in the thin film, EDS analysis was performed for the area shown in the inset of Figure 3(B). The EDS analysis shows the existence of C, O, Pd, Si, Pt, and Sn. The peaks of C and Si originated from the contaminated carbon on the surface and silicon substrate, respectively. The presence of Pt was ascribed to the Pt electrode and Pt–coating for SEM analysis. O, Pd, and Sn were components of the prepared material. The intensities of Pd and Sn are weak, thus the quantitative estimation of composition from the EDS analysis is not presented.
The hydrogen gas–sensing characteristics of the base SnO2 thin film sensors were tested in different concentrations (100–1000 ppm) of H2 at temperatures of 300, 350, and 400°C. As shown in Figure 3, the plots of transient resistance versus time upon exposure to H2 for different sensors exhibit similar trends, in which the resistance of sensors decreases significantly in H2 and recovers to the initial values when the flow of analytic gas stops. As exposed to 250 ppm H2 gas, the response time was 15, 14, and 14 s whereas the recovery time was 4, 3, and 3 s at temperature of 300, 350, and 400°C, respectively. This significantly fast response and recovery times are a result of high working temperature, which accelerates adsorption and desorption of hydrogen molecules. The sensors show almost 100% recovery at all measured temperatures, which confirms the reversible adsorption of H2 molecules on the sensor surface.
The response to 250 ppm H2 of different sensors measured at 300, 350, and 400°C is plotted in Figure 5(A). The response to H2 increases with the increase of working temperature up to 400°C. Further increase of working temperature can increase the sensor response but would require a high power consumption to maintain the temperature of the sensor chip [26]. Figure 5(A) also demonstrates that the magnitude of increment in sensor response of the film SnO2 (40 nm) is largest followed by the SnO2 (80 nm), SnO2 (60 nm) and SnO2 (20 nm). The magnitude of the incensement of the sensor response is dependent on the optimal working temperature, and the sensitivity of the sensors. In the report by Ansari et al. [27], they investigated the effect of grain size on the H2 gas sensitivity of the SnO2 film, and found that the larger particle size lead to a lower optimal working temperature. Such those characteristics were attributed to the effect of grain boundaries between the nanoparticles. It is difficult to find the relationship between the grain size and the magnitude of the incensement of the sensor response. Herein, the variation in magnitude of increment in sensor response of different films can be a result of the difference in thin film thickness. However, a systematic study about the effect of such those characteristics on the gas-sensing performance of the SnO2 thin films are necessary to clarify our claim. The responses of all SnO2 sensors as a function of H2 concentration at 300°C are shown in Figure 5(B). The sensor response increases nearly linearly with increasing H2 concentration in the measured range [Figure 5(B)]. The sensor with a thickness of 40 nm has the highest response, followed by the 20, 60, and 80 nm sensors. The maximum response of the 40 nm sensor to 1000 ppm H2 at 300°C is approximately 6.5. This value is comparable with that of the bare SnO2 thin film prepared using sol–gel method, in which the induced response was approximately 5 to 1000 ppm H2 at 150°C [28]. Du and George investigated the effect of thickness on the CO sensing of SnO2 thin films that were prepared using an atomic layer deposition, and they found that the
sensor with approximately 2.5 nm thickness achieved the highest response [29]. In the current study, the films may be thicker to maximize response because of their high porosity, which enables the analytic gas molecules to defuse deeply into the film, resulting in a resistant change sensor. The diffusion constant (DK) can be represented as DK=4r/3(2RT/πM)1/2, where r is the pore size, R is the universal gas constant, T is the temperature, and M is the molecular weight of diffusing gas [30]. A high porosity of granule films leads to a high diffusion constant; thus, response may be maximized by increasing the film thickness. Based on above investigation, we varied the thickness of Pd islands from 5 nm to 40 nm and maintained the thickness of SnO2 thin film at 40 nm to study the effect of Pd islands on sensor performance. Given that the SnO2 thin film sensors that were sensitized with Pd islands have a superior sensitivity, thus low concentrations of H2 (25–250 ppm) were tested. The transient resistance vs. time upon exposure to various H2 concentrations of the SnO2–Pd sensors with different thicknesses of Pd islands is shown in Figure 6. At all measured temperatures from 200 to 400°C, the fabricated sensors showed similar response characteristics to those of the bare SnO2 thin film. Nevertheless, the SnO2–Pd sensors exhibited much lower detection limit, with a significant response to 25 ppm H2. This low detection limit with high response not only enables use of the sensors in ensuring safe usage of hydrogen (Lower explosive limit of H2 is 4%), but also in diagnosis of diseases through monitoring of hydrogen gas in exhaled breath [31]. The temperature dependence of sensor response to 250 ppm H2 of different sensors is plotted in Figure 7(A). For all of the sensors excepted the SnO2–Pd(5 nm) and the SnO2–Pd(10 nm) devices, the responsivity increases with the increase of working temperature. However, the maximum response was attained at approximately 350 and 300°C for the SnO2–Pd(5 nm) and SnO2–Pd(10 nm) sensors, respectively. The SnO2–Pd(10 nm) sensor showed the highest
responsivity of approximately Ra/Rg=28 at 300°C. This optimal sensor not only exhibited enhanced responsivity but also functioned effectively at lower working temperature. The sensor responses as a function of H2 concentration (25 ppm to 250 ppm) for different sensors are shown in Figure 7(B). For all of the sensors, the sensor response increases linearly with the H2 concentration in the measured range. However, the SnO2–Pd(10 nm) sensor showed the highest response with the best linear dependence on H2 concentration. The response and recovery times to 250 ppm H2 of the SnO2–Pd sensors with different thicknesses of Pd islands measured at temperature ranging from 200 to 400oC are shown in Figure 7(C). We can see that the response time is shorter than the recovery time, and they decreased with increasing of working temperatures. The response time is in the range of 3 to 25 s whereas the recovery time is in the range of 12 to about 250 s. The dependence of the response and recovery times on the H2 concentrations measured at temperature of 300oC is plotted in Figure 7 (D). We can see that the response time decreases with H2 concentrations, whereas the recovery time increases. The result is consistent with the report by Zhao et al. [32], where the sensing properties were explained based on the diffusion model. The response and recovery times of the sensor are approximately 3 and 50 s at 300°C, those are comparable with the values of the bare SnO2 sensors.
The use of Pd islands as catalyst in the SnO2 thin film also improved the selectivity of the sensors. In Figure 8(A), the selectivity of the sensors was studied by comparing the response of the bare SnO2 and SnO2 sensitized with Pd islands sensors to different gases, such as 250 ppm CO, 250 ppm NH3, and 2500 ppm LPG. The SnO2–Pd(10 nm) sensor showed significantly enhanced selectivity at working temperatures of approximately 300 and 400°C. Further comparison of the response to different concentrations of H2 gas for the bare SnO2 and SnO2–
Pd(10 nm) sensors is shown in Figure 8(B). The SnO2–Pd(10 nm) sensor showed a high response even at a low H2 concentration. The stability of the SnO2–Pd(10 nm) sensor was also studied after 6 months stored in ambient environment by measuring the transience resistance vs. time at temperature of 300oC. The data (Figure S2, Supplementary material) show that no significant distortion in sensor response can be observed after ten cycles switching on/off from dry air to H2 and back to air. Results indicate that the fabricated SnO2–Pd thin film sensor is excellent for applications in selectively monitoring H2 gas at low concentration (ppm levels) with high sensitivity and good stability. The sensing mechanism of the device can be explained using a depletion conduction model. In the bare SnO2 thin film sensor, the H2 molecules interacted with pre–adsorbed oxygen (O2−, O−, and O2−) upon exposure to H2 and released electrons to increase the total carriers, thus reducing the electron depletion region, resulting in a decrease in the sensor resistance [10]. However, two mechanisms contribute to the sensing characteristics of the SnO2–Pd sensors: (1) the chemical and (2) the electronic mechanisms [20]. The chemical mechanism relies on the catalytic activity of Pd through the spillover principle, which accelerates dissociation of (i) oxygen and (ii) hydrogen molecules to enhance the number of pre–adsorbed oxygen on the surface of SnO2, as well as the interaction between hydrogen and pre–adsorbed oxygen to increase the response of sensors to H2. Providing experimental measurement to clarify whether the catalytic dissociation of either oxygen or hydrogen determines the enhancement of sensor response is difficult. If the dissociation of oxygen using a Pd catalyst is dominant, then the sensor response to all other gases, such as CO, NH3, and LPG, should be improved. However, the results showed the highest improvement to H2 gas because the Pd catalyzes the dissociation of H2 molecule into two active hydrogen atoms better than other gases. Thus, the SnO2 thin film that was sensitized with Pd
islands exhibited a high response to hydrogen. Meanwhile, the electronic mechanism is based on formation of poor conduction region formed at the interface between Pd islands and SnO2 thin film. The work function of Pd (~5.2 eV) is higher than that of the n–type SnO2 (~4.4 eV), thus Schottky barrier is formed at the interface of Pd and SnO2 [33]. As a result, the conduction channel of the device is modulated, which facilitates the response to the analytic gas. Therefore, the formation of Pd–SnO2 heterojunction junction and the high catalytic dissociation of H2 molecules into active hydrogen atoms dominate the significant enhancement of hydrogen sensing of SnO2 thin films through sensitization with microsized Pd islands. However, the reason why the sensor with 10 nm thick Pd islands showed the highest response is still unclear. In our opinion, the 10 nm thickness of Pd islands is optimal for adsorption of analytic molecules on the surface of Pd, which can effectively modulate the thickness of depletion region at the Pd/SnO2 interface. As discussed above, both chemical and electronic mechanisms contribute to the characteristics of the sensitized sensors [33]. To identify the mechanism that dominates the enhancement of H2– sensing performance of SnO2 thin films, we fabricated three sensors that were sensitized with different noble metals: Pd, Pt, and Au, with work functions of approximately 5.2, 5.6, and 4.8 eV, respectively. Such those metals have been recently used as catalyst for enhanced gas-sensing performances due to their high catalytic activity [34–36]. The SnO2–Pd, SnO2–Pt, and SnO2–Au sensors were fabricated with a fixed the thickness of Pd, Pt, and Au islands at 10 nm. The Schottky barrier formed at the interface of Pt–SnO2 is the highest (~ 1.2 eV), followed by that of SnO2–Pd (0.8 eV) and SnO2–Au (0.4 eV). Responses of different sensors (SnO2–Pd, SnO2–Pt, SnO2–Au) to various concentrations of H2 measured at 300oC are shown in Figure 9. Details about the transient response to H2 of SnO2–Pt and SnO2–Au sensors are shown in Figure S3, and
Figure S4, respectively (Supplementary material). We expected that the higher Schottky barrier formed at the interface between catalyst and SnO2 result a higher sensitivity when the electronic mechanism dominates the enhancement of the sensing performance. However, the data shows opposite results, in which the SnO2–Pd sensor showed the highest response, followed by SnO2– Pt and SnO2–Au (Figure S5, Supplementary material). Such results reveal that the electronic mechanism is not dominant in the enhancement of the sensing performance of SnO2 thin films; instead, the chemical mechanism dominates. Thus to enhance the sensitivity of the sensor, the noble metal islands should have a high catalytic activity to the desired gas. The H2 gas–sensing mechanism of the Pd-SnO2 thin films with different thicknesses of Pd islands is shown in Figure 10. Due to the spillover catalytic activity of Pd in the dissociation of oxygen, the number of pre-adsorbed oxygen on the surface of Pd/SnO2 (Figure 10B,C,D) is higher than that in the bare SnO2 (Figure 10A), thus the SnO2–Pd sensors exhibited higher response. In addition to the catalytic activity, the formation of heterojunction between Pd and SnO2 also contribute on the gas–sensing performance because the reaction between analytic H2 molecules and Pd layer can modulate the Schottky barrier at the Pd/SnO2 heterojunction. However, at a relatively small thickness of Pd (5 nm), the contribution of Pd/SnO2 heterojunction in the modulation the Schottky barrier is low because the discontinuous Pd layer [Figure 10(B)]. With increase the thickness of Pd islands to about 10 nm, the adsorption of H2 molecule on the surface of Pd sufficient enough to modulate the Schottky barrier height, thus significantly increase the sensor response (Figure 10(C)). However, with increase of Pd thickness to about 25 nm or more, the adsorption of H2 molecules on the surface of Pd islands became less effective in modulation the Schottky barrier of the Pd/SnO2 heterojunction because the H2 adsorption just modulate the top layer of Pd island, resulting in a decrease of sensor response [Figure 10(D)].
4.
Conclusion
We have introduced a scalable synthesis process for fabrication of highly sensitive and selective H2 sensors based on SnO2 thin film sensitized with microsized Pd islands. The thicknesses of SnO2 and Pd films were optimized to maximize the sensor response. The SnO2(40 nm)–Pd(10 nm) sensor exhibited the highest sensitivity and selectivity to H2 thus should be a strong candidate for monitoring of H2 at low concentration of ppm level. The enhancement of sensor performance was also discussed based on spillover and diffusion mechanisms. We pointed out that the chemical mechanism or the catalytic activity of the metal islands dominates the enhancement of H2 sensing performance of the SnO2–Pd thin film sensors. Acknowledgments: The present research was funded by the Vietnam National Foundation for Science and Technology Development (Nafosted Code: 103.02–2014.06), and the research project of Vietnam Ministry of Education and Training under code B2015–01–92.
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Figure Captions Figure 1. SEM images of (A) a full chip, (B) SnO2 thin film on SiO2, (C) cross sectional image, and (D) SnO2–Pd. Inset of (D) is a Pd island of about 5 µm in diameter. Figure 2. SEM images of the SnO2–Pd thin films with different Pd thicknesses: (A) 5 nm; (B) 10 nm; (C) 25 nm; (D) 40 nm Figure 3. The XRD pattern (A) and EDS analysis (B) of SnO2 thin film sensitized with Pd islands. Inset of (B) is SEM image of the correspondent EDS analysis. Figure 4. Transient resistance vs. time upon exposure to various H2 concentrations of the bare SnO2 sensors with different thicknesses: (A) 20 nm, (B) 40 nm, (C) 60 nm, and (D) 80 nm. Figure 5. A comparative sensing performance of the base SnO2 sensors with different thicknesses: (A) sensor response as a function of operating temperatures; (B) Sensor response as a function of H2 concentrations measured at 300oC. Figure 6. Transient resistance vs. time upon exposure to various H2 concentrations of the SnO2 – Pd sensors with different thicknesses of Pd islands: (A) 5 nm, (B) 10 nm, (C) 25 nm, and (D) 40 nm. Figure 7. A comparative sensing performance of the SnO2–Pd sensors with different thicknesses of Pd islands: (A) sensor response as a function of operating temperatures; (B) Sensor response as a function of H2 concentrations measured at 300oC; (C) temperature dependence of the response and recovery time of different sensors; (D) response and recovery times as a function of H2 concentrations. Figure 8. (A) A comparison of the response to different gases [NH3 (250 ppm), CO (250 ppm), LPG (2500 ppm) and H2 (250 ppm)] of the bare SnO2 and SnO2–Pd (10 nm) thin film sensors; (B) a comparision of gas response as a function of H2 concentration of SnO2 and SnO2–Pd at 300oC. Figure 9. Response of different sensors (SnO2–Pd, SnO2–Pt, SnO2–Au) to various concentrations of H2 measured at 300oC. Figure 10. The gas–sensing mechanism of the SnO2 sensor sensitized with Pd islands of different thicknesses: (A) Pd = 0 nm; (B) Pd=5 nm; (C) Pd = 10 nm; (D) Pd≥25 nm.
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