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Fabrication and characterization of thermochemical hydrogen sensor with laminated structure
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Fabrication and characterization of thermochemical hydrogen sensor with laminated structure Seil Kim a,1, Yoseb Song a,1, Hyo-Ryoung Lim a, Young-Tae Kwon a, Tae-Yeon Hwang a, Eunpil Song a, Songjun Lee b, Young-In Lee c, Hong-Baek Cho a, Yong-Ho Choa a,* a
Department of Fusion Chemical Engineering, Hanyang University, Ansan 426-791, Republic of Korea Department of Electronic Systems Engineering, Hanyang University, Ansan 426-791, Republic of Korea c Department of Materials Science and Engineering, Seoul National University of Science & Technology, Seoul 139743, Republic of Korea b
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abstract
Article history:
In this study, we reported the simple and cost-effective fabrication of thermochemical
Received 26 May 2016
hydrogen (TCH) sensors composed of chalcogenide thin films and Pt/Al2O3 powders.
Received in revised form
Chalcogenide thin films of two types, composed of Bi2Te3 (monomorphic-type) and Bi2Te3e
29 August 2016
Sb2Te3 (four-leg PN junction-type), were prepared by electrochemical deposition. The Pt/
Accepted 31 August 2016
Al2O3 powder, which acts as a heating catalyst, was synthesized by impregnation of an
Available online xxx
Al2O3 powder with an aqueous solution of platinum (IV) chloride pentahydrate. Its heating process was optimized via a hydrogen-sensing evaluation to control the size of the Pt
Keywords:
particles. The monomorphic-type TCH sensor showed an output signal of 14.2 mV in
Thermochemical hydrogen sensor
response to 10 vol% hydrogen gas, whereas an output signal of 39.6 mV was obtained from a
Chalcogenide thin film
four-leg PN junctionetype TCH sensor. Even though the nep junction-type had the same
Pt/Al2O3 catalyst
deposition area as that of the monomorphic-type, the output signal of the nep junction
Recovery time
TCH sensor was greater by a factor of 2.8. In addition, the monomorphic-type TCH sensor
Response time
had an inferior response time (T90) of 31 s and a longer recovery time (D10) of 38 s; the four-
Reliability
leg PN junction-type TCH sensor had a lowest response time of 27 s and a fastest recovery time of 9 s (in 3% H2/air at room temperature). © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has been recently spotlighted as an alternative energy source, instead of fossil fuels such as coal and petroleum, due to the fact that it can be easily produced by
electrolysis of water, biomass gasification, and steam reforming of methane. It is also regarded as a clean energy source because it generates electricity with no pollution and does not produce global warming emissions [1e3]. Hydrogen uses include providing energy source for vehicles, airplanes,
* Corresponding author. Fax: þ82 31 418 6490. E-mail address: [email protected] (Y.-H. Choa). 1 Co-first authors. http://dx.doi.org/10.1016/j.ijhydene.2016.08.216 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216
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electric utilities and factories [4]. Especially, hydrogen vehicle can be operated with fuel cells, which are three times higher than gasoline engine [5]. However, during its transport, storage, and use, it is important to handle hydrogen with care because it is a highly explosive gas when it reaches concentrations of 4e75 vol% in an air atmosphere [6]. Accordingly, the development of hydrogen gas sensors that possess rapid and accurate performance has become necessary to use hydrogen safely. There are many different types of hydrogen sensors, including semiconductor [7], optical [8] and electrochemical [9] ones, etc. Recently, thermoelectric (TE) materials have been widely investigated for a variety of applications, such as power generation and cooling devices, based on the Seebeck and Peltier effects, which is are highly reliable physical phenomena [10]. Because hydrogen gas sensors are required to be highly dependable, a number of studies investigating hydrogen gas sensors that utilize the Seebeck effect have been undertaken [11e16]. It was found that these sensors have many advantages including wide-range detection, long-term stability, and reliability. TE hydrogen gas sensors consist of a TE film and a heating catalyst that is integrated onto the film. When the hydrogen gas reacts with the catalyst (e.g., Pt or Pd), heat energy is released via an exothermic reaction of the catalyst on one side of the film. This results in the formation of two zones in the TE film, where a hot zone and cold zone are formed on both sides. This temperature gradient is converted into a voltage signal by the Seebeck effect of the TE film (without requiring any external power), which is the reason why TE hydrogen gas sensors are considered to be superior low-power sensors compared to other semiconductor- or catalytic combustion-type gas sensors [16]. Numerous studies have focused on high vacuum-based processes, such as thermal and electron-beam evaporation, to synthesize TE films [17e20]. However, it is generally more expensive and relatively difficult to synthesize thick films with vacuum processes as compared to wet processes (e.g., electrodeposition processes), which have many advantages. Benefits of wet processing include easy scale up, low costs, simple maintenance, and room temperature operation [21]. Additionally, it is easy to synthesize vertically-deposited, relatively thick TE thin films on a substrate. The composition and morphology of these films, which play a major role in the sensing properties of TE material-based hydrogen gas sensors, can be controlled by varying the applied potential and electrolyte concentration. In a previous work, we reported a vertically-aligned chalcogenide nanowire arraybased TCH gas sensor, which possessed a wide detection range (from 400 ppm to 45 vol%) and a fast response time (1.3 s) at high concentrations of hydrogen gas [15]. Here, we focus on reducing the response and recovery times of TCH sensors during hydrogen sensing at room temperature. Two types of TCH sensors, composed of TE thin films (Bi2Te3 and Sb2Te3) and Pt/Al2O3 powder, were synthesized by electrochemical deposition and an impregnation method, respectively. Pt/Al2O3 is applied on the top of TE thin films as a heating catalyst. In this study, the hydrogen sensing properties of two-type TCH hydrogen sensors with optimizing Pt/ Al2O3 catalyst were systemically studied with various reliability tests at room temperature.
Experimental Preparation of Pt/Al2O3 catalyst The Pt/Al2O3 catalyst, which is applied to the top of TE thin films, was synthesized by impregnation of an alumina powder (<10 mm, 99.7%, Sigma Aldrich) with an aqueous solution of platinum (IV) chloride pentahydrate (1% w/v). First, the alumina powder (0.5 g) and Pt precursor solutions (0, 1, 5, 10, 20 ml) were mixed together and vigorously stirred at 100 C for 30 min. The mixture was then dried at 120 C for 2 h in an air atmosphere to make the Pt precursor-decorated alumina powder. To reduce the Pt precursor, calcination was carried out at 300 C for 2 h in a 5% H2/Ar atmosphere with a slow heating rate of 30 C/h (to prevent growth of the Pt particles). Finally, Pt/Al2O3 powders with different Pt contents (0.933, 4.501, 8.614, and 15.862 wt%) were synthesized.
Fabrication of the TCH sensor The electrolyte used to synthesize the BixTe1x film was prepared by dissolving 10 mM TeO2 (99.995%, Alfa Aesar, Inc.) and Bi(NO3)3$5H2O (98%, Acros Organic) of 0, 10, 40 mM in a concentrated nitric acid (HNO3, 60%, Junsei Chemical) solution. The electrolyte used to synthesize the SbxTe1x film was prepared by separately dissolving TeO2 in concentrated nitric acid and dissolving Sb2O3 (99.9%, Sigma Aldrich) in an L-tartaric acid (C4H6O6, 99.5%, Sigma Aldrich) solution. These two solutions were then mixed together to make an electrolyte consisting of 9 mM HTeO2 þ with different Sb3þ concentrations (3 mM and 9 mM) in 1 M HNO3 and 0.5 M C4H6O6. All electrochemical depositions were performed in a 200 ml electrochemical cell using a potentiostat (AMETEK, VeraSTAT3) with a standard three-electrode cell that consisted of a platinumcoated titanium strip as the counter electrode, Ag/AgCl (4 M KCl sat. AgCl) as the reference electrode, and Au(80 nm)/ Ti(20 nm) deposited on an SiO2(300 nm)/Si wafer as the working electrode. Here, we fabricated two types of TCH sensors by applying the Pt/Al2O3 catalyst onto the top of the TE thin films: a monomorphic-type TCH sensor (composed of an n-type Bi2Te3 thin film) and a junction-type sensor (composed of an n-type Bi2Te3 and p-type Sb2Te3 thin film that are connected in series with a four-leg configuration). Schematic illustrations depicting the synthesis method used to prepare the monomorphic-type TCH sensor are shown in Fig. 1(aef). For the monomorphic-type sensor, a seed layer (Au/Ti) was deposited onto the SiO2/Si wafer (a-b). This was followed by electrochemical deposition of a BixTe1x film, which was carried out by applying a suitable voltage (c). A gold thin film was then deposited on the BixTe1x film using a gold electrolyte (Techni Gold 25 RTU, TECHNIC) (d), and a thermal grease (TC 10, EVERCOOL THERMAL CO., LTD) was put on the gold thin film in order to avoid electrical contact and to increase the thermal current (e). Next, copper conducting wires were connected to both the top and bottom gold electrodes to obtain the output voltage signal of the TE thin film. Finally, the Pt/Al2O3 powder was applied onto the thermal grease to detect hydrogen gas (f). The process used to fabricate the four-leg PN junction TCH sensor is shown in Fig. 1(gen). Here, square-
Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216
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Fig. 1 e Schematic illustration of the preparation processes for the two types of thermochemical hydrogen sensors: (aef) monomorphic-type and (gen) four-leg PN junction-type.
shaped gold thin films were selectively deposited onto the SiO2/Si wafer (h). The unwished sides of deposition on the gold film were sealed with stop-off lacquer (Microstop, Pyramid Plastics, Inc.) and dried for 1 h in an air atmosphere. Then, electrochemical deposition of the BixTe1x film was carried out on the gold-deposited side (i). After deposition, the stop-off lacquer was removed from the gold-deposited side, and the SbxTe1x thin film was deposited onto the golddeposited side (while blocking the BixTe1x side of the thin film) (j). Next, the gold thin film was deposited on the BixTe1x and SbxTe1x films simultaneously (h), and a copper conducting wire was attached on the gold thin film to connect the BixTe1x and SbxTe1x legs under the gold electrode (l). Finally, like the process used to fabricate the monomorphic-type TCH sensor, thermal grease and Pt/Al2O3 powder were sequentially applied onto the gold thin film (m, n).
no. 00-005-0712). It was found that the Pt peak increases as the amount of the Pt precursor in the Pt/Al2O3 increases. Scherrer's equation was employed to calculate the particle size of Pt. As a result, we observed that the Pt particles constantly
Results and discussion Characterization of Pt/Al2O3 catalyst X-ray diffraction patterns of the as-synthesized Pt/Al2O3 powder as a function of the Pt content are shown in Fig. 2. All catalyst patterns display diffraction lines of FCC Pt [(111), (200), and (220)] (JCPDS no. 00-004-0802) and an a-Al2O3 phase (JCPDS
Fig. 2 e X-ray diffraction patterns of Pt/Al2O3 powder as a function of the Pt content; (a) 0 wt%, (b) 0.933 wt%, (c) 4.501 wt%, (d) 8.614 wt% and (e) 15.682 wt%.
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Fig. 3 e TEM images of Pt/Al2O3 powder as a function of the Pt content: (a) 0 wt%, (b) 0.93 wt%, and (c) 15.8 wt%, (d) highresolution image of Pt/Al2O3 powder and (e) FFT-converted SAED pattern of Pt/Al2O3 powder.
increased in size from 13.87 to 24.83 nm as the amount of Pt precursor increased. This was due to agglomeration of the Pt nanoparticles during the annealing process. FE-SEM images of Al2O3 and Pt/Al2O3 powders with different amounts of the Pt precursor are shown in Fig. S1(aee). The Al2O3 powder possessed a rhombohedral structure with a particle size smaller than 5 mm. As the Pt content increased from 0.93 to 15.8 wt%, the presence of Pt particles on the surface of the Al2O3 powder increased. The point EDS spectra were measured at the area marked (1) in Fig. S1(f). It was found that Pt particles are well-attached to the alumina surface with a content of 26.44 wt%. Fig. 3 shows the HR-TEM and Fourier transform (FFT)-converted SAED patterns of the Pt/Al2O3 powder. It was found that the size and amount of Pt nanoparticles in the Pt/Al2O3 powder increased as the concentration of the Pt precursor increased (Fig. 3(aec)). The FFT patterns taken from the HR-TEM images of these particles correlated with the d-spacing values of the (200) and (111) lattice planes of face-centered cubic (FCC) Pt and with the dspacing values of the (012) and (113) lattice planes of the Al2O3 phase (Fig. 3(dee)). These results were consistent with the XRD data. The hydrogen-sensing properties of the assynthesized Pt/Al2O3 powders with different Pt contents were studied as a function of the hydrogen concentration (1e5 vol%). The amount of Pt/Al2O3 powder used to detect hydrogen was fixed at 0.1 g. The release of the H2/air mixture gas (used for sensing) was repeated at regular time intervals of 120 s on followed by 120 s off; this was done at fixed flow rate of 500 sccm (Fig. 4). It was found that the temperature constantly increased with respect to changes in the concentration of hydrogen gas. In particular, the temperature variation of the Pt/Al2O3 powder with 0.93 wt% Pt was more significantly increased compared to the increases observed at other conditions. This is attributed to the increased specific surface area
of the small Pt particles; hydrogen gas can more effectively react with the high specific surface area of Pt particles at the same gas concentration. By increasing the Pt content in the powder, the agglomeration of Pt particles was accelerated, which prevented the effective reaction between the Pt catalyst and hydrogen gas. Here, we chose the Pt/Al2O3 powder with 0.93 wt% Pt to fabricate the TCH sensor due to its good sensing performance at increased hydrogen concentrations.
Synthesis of stoichiometric chalcogenide thin film Prior to electrochemical deposition of the TE film, cyclic voltammetry (CV) was carried out with the working electrode in electrolytes with different Bi3þ concentrations (0, 10, and 40 mM) and a fixed HTeO2 þ concentration of 10 mM. As shown in Fig. 5 (a), the reduction peak was observed at approximately 0.14 V in the electrolyte consisting of 10 mM HTeO2 þ and 1 M HNO3; this reduction peak corresponds to the reduction of
Fig. 4 e Hydrogen-sensing properties of Pt/Al2O3 powder with different Pt contents as a function of the hydrogen concentration (catalyst loading amount is 0.1 g).
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Fig. 5 e Cyclic voltammograms of the (left) electrolytes containing 10 mM HTeO2 þ with different Bi3þ concentrations (0 mM, 10 mM, and 40 mM) in 1 M HNO3 (dotted lines are the oxidation slopes) and (right) the electrolyte containing 9 mM HTeO2 þ with different Sb3þ concentrations (3 mM and 9 mM) in 1 M HNO3 and 0.5 M C4H6O6. [CE: Ag/AgCl (sat. KCl), scan rate: 10 mV/s.].
Fig. 6 e X-ray diffraction patterns of (a) BixTe1¡x films deposited in electrolytes containing 10 mM HTeO2 þ , 10 mM Bi3þ, and 1 M HNO3 at different applied voltages: (1) 50 mV, (2) 0 mV, and (3) ¡50 mV. (b) SbxTe1¡x films deposited in electrolytes containing 9 mM HTeO2 þ , 3 mM Sb3þ, 1 M HNO3, and 0.5 M C4H6O6 at different voltages: (4) ¡150 mV, (5) ¡175 mV, and (6) ¡200 mV. The deposition time was fixed at 1 h.
Fig. 7 e FE-SEM images of BixTe1¡x films deposited in electrolytes containing 10 mM HTeO2 þ , 10 mM Bi3þ, and 1 M HNO3 at different applied voltages: (a) 0.05 V, (b) 0 V, and (c) ¡0.05 V. SbxTe1¡x films deposited in electrolytes containing 9 mM HTeO2 þ , 3 mM Sb3þ, 1 M HNO3, and 0.5 M C4H6O6 at different applied voltages: (d) ¡150 mV, (e) ¡175 mV, and (f) ¡200 mV. The deposition time was fixed at 1 h. Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216
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Bi2Te3 (DGf 0 ¼ 899.088 kJ mol1) [22,23]. In order to confirm the CV results, potentiostatic electrodeposition was carried out between 50 mV and 50 mV vs. Ag/AgCl (sat. KCl) with a step size of 50 mV. To deposit the Sb2Te3 thin film, linear sweep voltammetry (LSV) was performed with the seed layerdeposited working electrode in an electrolyte with different Sb3þ concentrations (3 and 9 mM) and a fixed HTeO2 þ concentration of 10 mM (Fig. 5 (b)). Reduction peaks were observed between 150 mV and 200 mV in the electrolyte consisting of 9 mM HTeO2 þ , 3 mM Sb3þ, 1 M HNO3, and 0.5 M C4H6O6. Fig. 6 (a) shows the X-ray diffraction patterns taken between 37 and 45 for BixTe1x films deposited in an electrolyte containing 10 mM HTeO2 þ and 1 M HNO3 at different applied potentials. When potentials of 0 and 50 mV vs. Ag/AgCl (sat. KCl) were applied, the presence of Bi4Te5 (JCPDS no. 00-022-0115) and BiTe (JCPDS no. 00-050-0602) phases, respectively, were confirmed. Through this, the optimal potential needed to deposit near-stoichiometric Bi2Te3 thin films was determined to be 50 mV. X-ray diffraction patterns of SbxTe1x thin films with different potentials are shown in Fig. 6 (b). When a 150 mV potential was applied, a deposit with a mixed structure of Sb2Te3 (JCPDS no. 00-015-0874) and Te (JCPDS no. 00-004-0554) was observed. For an applied potential of 200 mV, a Sb0.405Te0.595 phase (JCPDS no. 00-045-1228) was observed due to the increased Sb content, which was deposited with a more negative potential. A near-stoichiometric Sb2Te3 (JCPDS no. 00-015-0874) phase, without other phases such as tellurium, was synthesized by deposition at an applied potential of 175 mV. Fig. 7(aec) show the cross-section images of BixTe1x thin films deposited by electrochemical deposition as a function of the applied potential; here, the deposition time was fixed at 1 h and the electrolyte was 10 mM HTeO2 þ , 10 mM Bi3þ, and 1 M HNO3. It was found that the thickness of the BixTe1x films was almost same at potentials of 50 mV and 0 V. Additionally, a BixTe1x film could be grown with a dendritic-like structure at a potential of 50 mV. The cross-section images of SbxTe1x thin films as a function of the applied potential (with an electrolyte containing 9 mM HTeO2 þ , 3 mM Sb3þ, 1 M HNO3, and 0.5 M C4H6O6) are shown in Fig. 7(def). In the case of the SbxTe1x film, the morphological change observed (from a smooth surface to a rough surface) can be mainly attributed to the severe ion depletion near the surface, which was caused by the more negative potential [24]. Fig. 8 e Hydrogen-sensing properties of monomorphictype (black line) and four-leg PN junction-type (red line) thermochemical hydrogen sensors: (a) voltage signal as a function of the H2 concentration, (b) voltage signal with a fixed gas flow of 3% H2/air, and (c) voltage signal with a regular gas flow interval of 3% on and 0% H2/air off. The flow rate of hydrogen was fixed at 500 sccm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
HTeO2 þ ions by elemental Te (s). Bi2Te3 deposition occurs at an applied potential that is more positive than the potential required for the overpotential deposition (OPD) of elemental Bi; this is due to the negative Gibbs free energy of formation of
Hydrogen gas sensing properties The hydrogen-sensing properties, including gas detection, response time (T90), recovery time (D10), and reliability, of the TCH sensor were measured using a custom-made apparatus at room temperature. The output voltage signal of the TCH sensor was measured using a nanovoltmeter (2182 A, Keithley) that was placed in contact with the TCH sensor. The sensing properties of the two types of TCH sensors were studied as a function of the hydrogen concentration (1e10 vol % H2/air), as shown in Fig. 8 (a). For both sensor types, the voltage signal increased with increasing hydrogen concentration (at a flow rate of 500 sccm). For the monomorphic-type TCH sensor, the voltage signal reached 14.2 mV at a hydrogen concentration of 10 vol%. Alternatively, for the four-leg PN junction-type TCH sensor, the voltage signal reached
Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216
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Table 1 e Comparison of response and recovery times of various thermoelectric hydrogen sensors. Materials
Alkali-doped NiO thick film Li-doped NiO thin film Li-doped NiO Bi2Te3/Sb2Te3 thin film Phosphorus-doped Si0.8Ge0.2 Phosphorus-doped Si0.8Ge0.2 Bi2Te3 thin film SiGe pattern Bi2Te3 thin film (monomorphic-type) Bi2Te3/Sb2Te3 thin film (fourleg PN junction-type)
Synthesis method
Hydrogen concentration (vol.%)
Response time T90 (s)
Recovery time D10 (s)
Operating temperature ( C)
Ref.
Paste printing RF sputtering Molten salt method Magnetron sputtering RF sputtering RF sputtering Magnetron sputtering RF magnetron sputtering Electrochemical deposition
3 3 3 3 3 3 3 3 3
55 47e35 <25 28 46 45 40 <2.5 31
50 40 18 55 e e 60 e 38
RT 100 RT RT 100 100 RT 100 RT
[25] [26] [12] [14] [27] [16] [13] [28] This work
Electrochemical deposition
3
27.4
9.6
RT
This work
39.6 mV at 10 vol%; this represents a 2.8-fold increase compared to the monomorphic-type TCH sensor. This improvement can be explained by the difference in the electrical connections, such as the parallel and serial conversions, of the two types of TCH sensors. A repeatability test of the TCH sensors for hydrogen gas detection was also performed under 3% H2/air conditions, as shown in Fig. 8 (b). The performance changes of the two types of TCH sensors were collected for 16000 s by continuously measuring the hydrogen gas. For the junction-type TCH sensor, the relative error between the initial signal and the final signal was 3.5%; for the monomorphic-type, the relative error was 30% and the final voltage signal was higher than the initial voltage. The outstanding error observed for the monomorphic-type sensor is due to its difficulty in releasing the thermal energy that is generated as a result of the exothermic oxidation of hydrogen gas. The release of thermal energy from the TE material is important to maintain the stability of TCH sensors. The monomorphic-type of sensor has a large amount of thermal energy and a large area during hydrogen sensing. For the junction-type sensor, heat was released easily by dividing the region of the thin film into four areas; this can be confirmed in Fig. 8 (a). In the case of the monomorphic-type sensor, the voltage signal was not decreased to zero by the flowing air gas, despite the low hydrogen concentration. Alternatively, the voltage signal of the junction-type sensor was decreased to almost zero. As a result, in our system, the junction-type TCH sensor displayed good reliability and stability compared to the monomorphic-type TCH sensor. The response time and recovery time of the two types of TCH sensors were calculated using the time-dependent voltage signal at a concentration of 3 vol% H2/air. It was found that the response time and recovery time of the four-leg PN junction-type sensor were 27 s and 9 s, respectively, which were faster than those of the monomorphic-type (T90: 31 s and D10: 38 s). This improvement was attributed to the excellent heat release of the junctiontype TCH sensor compared with that of the monomorphictype. Also, the fast response time and recovery time of the TCH sensor were caused by the thickness of the film. Many studies concerning TE-based hydrogen sensors have a TE layer that enables heat to flow in a horizontal direction. However, our sensor has a laminated structure with a
vertically-deposited thin film, which enables heat to flow vertically. To verify the test, the release of the 3% H2/air mixture gas (used for sensing) was repeated at regular time intervals of 120 s on and 120 s off at a fixed flow rate of 500 sccm. In the case of the junction-type TCH sensor, the relative error was 3.4% after repeating the test 10 times. For the monomorphic-type sensor, the relative error was 12.1% after repeating the sensing test 10 times. Table 1 shows a comparison of the response and recovery times of various TE hydrogen sensors. These response and recovery times are among the fastest that have been reported for TE hydrogen sensors [12e14,16,25e28]. Table 1 compares the results obtained in this study to those obtained in previous studies. This shows that our TCH sensor has relatively fast response and recovery times compared to those of other thermoelectric sensors.
Conclusions In this study, we fabricated lamination-structured TCH sensors composed of a chalcogenide thin film and a Pt/Al2O3 catalyst that acts as a heating element. Chalcogenide films of two types, composed of Bi2Te3 (monomorphic-type) and Bi2Te3eSb2Te3 (four-leg PN junction-type), respectively, were prepared by cost-effective electrochemical deposition. The monomorphic-type TCH sensor showed a voltage signal of 14.2 mV in response to 10 vol% hydrogen gas, whereas an output signal of 39.6 mV was obtained from the four-leg PN junctionetype TCH sensor. Even though the nep junctiontype has the same deposition area as that of the monomorphic-type, the voltage signal of the four-leg PN junction-type TCH sensor was more than 2.8 times as high. Also, the best response and recovery times of the monomorphic-type TCH sensor were 31 s and 38 s, respectively. In the case of the four-leg PN junction-type TCH sensor, the lowest response time was 27 s and the fastest recovery time was 9 s (in 3% H2/Air at room temperature). This improvement is attributed to the excellent heat release of the junction-type TCH sensor compared with the monomorphictype sensor.
Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216
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Acknowledgements This work was supported by the Technological innovation R&D program (S2307196, Development of High Durability and High Sensitive Sensor Platform Having an Embedded Sensing Electrode for NO2, NH3 Gas) of the Small and Medium Business Administration (SMBA, Korea), and supported by NanoMaterial Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2016M3A7B4900044).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.08.216.
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[12] Han CH, Han SD, Singh I. Thermoelectric hydrogen sensor using LixNi1xO synthesized by molten salt method. Korean J Chem Eng 2006;23(3):362e6. [13] Zhang J, Luan W, Huang H, Qi Y, Tu ST. Preparation and characteristics of Pt/ACC catalyst for thermoelectric thin film hydrogen sensor. Sens Actuator B Chem 2007;128:266e72. [14] Huang H, Luan W, Zhang JS, Qi YS, Tu ST. Thermoelectric hydrogen sensor working at room temperature prepared by bismuthetelluride PeN couples and Pt/g-Al2O3. Sens Actuator B Chem 2008;128:581e5. [15] Kim S, Lee YI, Choi YM, Lim HR, Lim JH, Myung NV, et al. Thermochemical hydrogen sensor based on chalcogenide nanowire arrays. Nanotechnology 2015;26:145503. [16] Tajima K, Qiu F, Shin W, Izu N, Matsubara I, Murayama N. Micromechanical fabrication of low-power thermoelectric hydrogen sensor. Sens Actuator B Chem 2005;108:973e8. [17] Lin JM, Chen YC, Lin CP. Annealing effect on the thermoelectric properties of Bi2Te3 thin films prepared by thermal evaporation method. J Nanomater 2013;2013:1e6. [18] Goncalves LM, Couto C, Alpuim P, Rolo AG, Volklein F, Correia JH. Optimization of thermoelectric properties on Bi2Te3 thin films deposited by thermal co-evaporation. Thin Solid Films 2010;518:2816e21. [19] Chang H, Kao MJ, Peng CH, Kuo CG, Huang KD. Thermoelectric properties of Sb2Te3 thin films by electron beam evaporation. J Nanosci Nanotechnol 2011;11:7491e4. [20] Santos R, Loureiro J, Nogueira A, Elangovan E, Pinto JV, Veiga JP, et al. Thermoelectric properties of V2O5 thin films deposited by thermal evaporation. Appl Surf Sci 2013;282:590e4. [21] Park DY, Myung N, Schwartz M, Nobe K. Nanostructured magnetic conip electrodeposits: structureeproperty relationships. Electrochim Acta 2002;47:2893e900. lez MS, Prieto AL, Gronsky R, Sands T, [22] Martı´n-Gonza Stacy AM. Insights into the electrodeposition of Bi2Te3. J Electrochem Soc 2002;149(11):C546e54. € sele U, et al. [23] Lee J, Farhangfar S, Lee J, Cagnon L, Scholz R, Go Tuning the crystallinity of thermoelectric Bi2Te3 nanowire arrays grown by pulsed electrodeposition. Nanotechnology 2008;19:365701. [24] Ma Y, Ahlberg E, Sun Y, Iversen BB, Palmqvist AEC. Thermoelectric characteristics of electrochemically deposited Bi2Te3 and Sb2Te3 thin films of relevance to multilayer Preparation. J Electrochem Soc 2012;159:D50e8. [25] Shin W, Imai K, Izu N, Murayama N. Thermoelectric thickfilm hydrogen gas sensor operating at room temperature. Jpn J Appl Phys Part 2 Lett 2001;40:L1232e4. [26] Matsumiya M, Qiu F, Shin W, Izu N, Murayama N, Kanzaki S. Thin-film Li-doped NiO for thermoelectric hydrogen gas sensor. Thin Solid Films 2002;419:213e7. [27] Tajima K, Qiu F, Shin W, Sawaguchi N, Izu N, Matsubara I, et al. Thermoelectric properties of RF-sputtered SiGe thin film for hydrogen gas sensor. J Appl Phys Part 1 Regul Pap Short Notes Rev Pap 2004;43(9A):5978e83. [28] Nishibori M, Shin W, Izu N, Itoh T, Matsubara I, Yasuda S, et al. Robust hydrogen detection system with a thermoelectric hydrogen sensor for hydrogen station application. Int J Hydrogen Energy 2009;34:2834e41.
Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216
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Fabrication and characterization of thermochemical hydrogen sensor with laminated structure Seil Kim a,1, Yoseb Song a,1, Hyo-Ryoung Lim a, Young-Tae Kwon a, Tae-Yeon Hwang a, Eunpil Song a, Songjun Lee b, Young-In Lee c, Hong-Baek Cho a, Yong-Ho Choa a,* a
Department of Fusion Chemical Engineering, Hanyang University, Ansan 426-791, Republic of Korea Department of Electronic Systems Engineering, Hanyang University, Ansan 426-791, Republic of Korea c Department of Materials Science and Engineering, Seoul National University of Science & Technology, Seoul 139743, Republic of Korea b
article info
abstract
Article history:
In this study, we reported the simple and cost-effective fabrication of thermochemical
Received 26 May 2016
hydrogen (TCH) sensors composed of chalcogenide thin films and Pt/Al2O3 powders.
Received in revised form
Chalcogenide thin films of two types, composed of Bi2Te3 (monomorphic-type) and Bi2Te3e
29 August 2016
Sb2Te3 (four-leg PN junction-type), were prepared by electrochemical deposition. The Pt/
Accepted 31 August 2016
Al2O3 powder, which acts as a heating catalyst, was synthesized by impregnation of an
Available online xxx
Al2O3 powder with an aqueous solution of platinum (IV) chloride pentahydrate. Its heating process was optimized via a hydrogen-sensing evaluation to control the size of the Pt
Keywords:
particles. The monomorphic-type TCH sensor showed an output signal of 14.2 mV in
Thermochemical hydrogen sensor
response to 10 vol% hydrogen gas, whereas an output signal of 39.6 mV was obtained from a
Chalcogenide thin film
four-leg PN junctionetype TCH sensor. Even though the nep junction-type had the same
Pt/Al2O3 catalyst
deposition area as that of the monomorphic-type, the output signal of the nep junction
Recovery time
TCH sensor was greater by a factor of 2.8. In addition, the monomorphic-type TCH sensor
Response time
had an inferior response time (T90) of 31 s and a longer recovery time (D10) of 38 s; the four-
Reliability
leg PN junction-type TCH sensor had a lowest response time of 27 s and a fastest recovery time of 9 s (in 3% H2/air at room temperature). © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has been recently spotlighted as an alternative energy source, instead of fossil fuels such as coal and petroleum, due to the fact that it can be easily produced by
electrolysis of water, biomass gasification, and steam reforming of methane. It is also regarded as a clean energy source because it generates electricity with no pollution and does not produce global warming emissions [1e3]. Hydrogen uses include providing energy source for vehicles, airplanes,
* Corresponding author. Fax: þ82 31 418 6490. E-mail address: [email protected] (Y.-H. Choa). 1 Co-first authors. http://dx.doi.org/10.1016/j.ijhydene.2016.08.216 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216
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electric utilities and factories [4]. Especially, hydrogen vehicle can be operated with fuel cells, which are three times higher than gasoline engine [5]. However, during its transport, storage, and use, it is important to handle hydrogen with care because it is a highly explosive gas when it reaches concentrations of 4e75 vol% in an air atmosphere [6]. Accordingly, the development of hydrogen gas sensors that possess rapid and accurate performance has become necessary to use hydrogen safely. There are many different types of hydrogen sensors, including semiconductor [7], optical [8] and electrochemical [9] ones, etc. Recently, thermoelectric (TE) materials have been widely investigated for a variety of applications, such as power generation and cooling devices, based on the Seebeck and Peltier effects, which is are highly reliable physical phenomena [10]. Because hydrogen gas sensors are required to be highly dependable, a number of studies investigating hydrogen gas sensors that utilize the Seebeck effect have been undertaken [11e16]. It was found that these sensors have many advantages including wide-range detection, long-term stability, and reliability. TE hydrogen gas sensors consist of a TE film and a heating catalyst that is integrated onto the film. When the hydrogen gas reacts with the catalyst (e.g., Pt or Pd), heat energy is released via an exothermic reaction of the catalyst on one side of the film. This results in the formation of two zones in the TE film, where a hot zone and cold zone are formed on both sides. This temperature gradient is converted into a voltage signal by the Seebeck effect of the TE film (without requiring any external power), which is the reason why TE hydrogen gas sensors are considered to be superior low-power sensors compared to other semiconductor- or catalytic combustion-type gas sensors [16]. Numerous studies have focused on high vacuum-based processes, such as thermal and electron-beam evaporation, to synthesize TE films [17e20]. However, it is generally more expensive and relatively difficult to synthesize thick films with vacuum processes as compared to wet processes (e.g., electrodeposition processes), which have many advantages. Benefits of wet processing include easy scale up, low costs, simple maintenance, and room temperature operation [21]. Additionally, it is easy to synthesize vertically-deposited, relatively thick TE thin films on a substrate. The composition and morphology of these films, which play a major role in the sensing properties of TE material-based hydrogen gas sensors, can be controlled by varying the applied potential and electrolyte concentration. In a previous work, we reported a vertically-aligned chalcogenide nanowire arraybased TCH gas sensor, which possessed a wide detection range (from 400 ppm to 45 vol%) and a fast response time (1.3 s) at high concentrations of hydrogen gas [15]. Here, we focus on reducing the response and recovery times of TCH sensors during hydrogen sensing at room temperature. Two types of TCH sensors, composed of TE thin films (Bi2Te3 and Sb2Te3) and Pt/Al2O3 powder, were synthesized by electrochemical deposition and an impregnation method, respectively. Pt/Al2O3 is applied on the top of TE thin films as a heating catalyst. In this study, the hydrogen sensing properties of two-type TCH hydrogen sensors with optimizing Pt/ Al2O3 catalyst were systemically studied with various reliability tests at room temperature.
Experimental Preparation of Pt/Al2O3 catalyst The Pt/Al2O3 catalyst, which is applied to the top of TE thin films, was synthesized by impregnation of an alumina powder (<10 mm, 99.7%, Sigma Aldrich) with an aqueous solution of platinum (IV) chloride pentahydrate (1% w/v). First, the alumina powder (0.5 g) and Pt precursor solutions (0, 1, 5, 10, 20 ml) were mixed together and vigorously stirred at 100 C for 30 min. The mixture was then dried at 120 C for 2 h in an air atmosphere to make the Pt precursor-decorated alumina powder. To reduce the Pt precursor, calcination was carried out at 300 C for 2 h in a 5% H2/Ar atmosphere with a slow heating rate of 30 C/h (to prevent growth of the Pt particles). Finally, Pt/Al2O3 powders with different Pt contents (0.933, 4.501, 8.614, and 15.862 wt%) were synthesized.
Fabrication of the TCH sensor The electrolyte used to synthesize the BixTe1x film was prepared by dissolving 10 mM TeO2 (99.995%, Alfa Aesar, Inc.) and Bi(NO3)3$5H2O (98%, Acros Organic) of 0, 10, 40 mM in a concentrated nitric acid (HNO3, 60%, Junsei Chemical) solution. The electrolyte used to synthesize the SbxTe1x film was prepared by separately dissolving TeO2 in concentrated nitric acid and dissolving Sb2O3 (99.9%, Sigma Aldrich) in an L-tartaric acid (C4H6O6, 99.5%, Sigma Aldrich) solution. These two solutions were then mixed together to make an electrolyte consisting of 9 mM HTeO2 þ with different Sb3þ concentrations (3 mM and 9 mM) in 1 M HNO3 and 0.5 M C4H6O6. All electrochemical depositions were performed in a 200 ml electrochemical cell using a potentiostat (AMETEK, VeraSTAT3) with a standard three-electrode cell that consisted of a platinumcoated titanium strip as the counter electrode, Ag/AgCl (4 M KCl sat. AgCl) as the reference electrode, and Au(80 nm)/ Ti(20 nm) deposited on an SiO2(300 nm)/Si wafer as the working electrode. Here, we fabricated two types of TCH sensors by applying the Pt/Al2O3 catalyst onto the top of the TE thin films: a monomorphic-type TCH sensor (composed of an n-type Bi2Te3 thin film) and a junction-type sensor (composed of an n-type Bi2Te3 and p-type Sb2Te3 thin film that are connected in series with a four-leg configuration). Schematic illustrations depicting the synthesis method used to prepare the monomorphic-type TCH sensor are shown in Fig. 1(aef). For the monomorphic-type sensor, a seed layer (Au/Ti) was deposited onto the SiO2/Si wafer (a-b). This was followed by electrochemical deposition of a BixTe1x film, which was carried out by applying a suitable voltage (c). A gold thin film was then deposited on the BixTe1x film using a gold electrolyte (Techni Gold 25 RTU, TECHNIC) (d), and a thermal grease (TC 10, EVERCOOL THERMAL CO., LTD) was put on the gold thin film in order to avoid electrical contact and to increase the thermal current (e). Next, copper conducting wires were connected to both the top and bottom gold electrodes to obtain the output voltage signal of the TE thin film. Finally, the Pt/Al2O3 powder was applied onto the thermal grease to detect hydrogen gas (f). The process used to fabricate the four-leg PN junction TCH sensor is shown in Fig. 1(gen). Here, square-
Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216
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Fig. 1 e Schematic illustration of the preparation processes for the two types of thermochemical hydrogen sensors: (aef) monomorphic-type and (gen) four-leg PN junction-type.
shaped gold thin films were selectively deposited onto the SiO2/Si wafer (h). The unwished sides of deposition on the gold film were sealed with stop-off lacquer (Microstop, Pyramid Plastics, Inc.) and dried for 1 h in an air atmosphere. Then, electrochemical deposition of the BixTe1x film was carried out on the gold-deposited side (i). After deposition, the stop-off lacquer was removed from the gold-deposited side, and the SbxTe1x thin film was deposited onto the golddeposited side (while blocking the BixTe1x side of the thin film) (j). Next, the gold thin film was deposited on the BixTe1x and SbxTe1x films simultaneously (h), and a copper conducting wire was attached on the gold thin film to connect the BixTe1x and SbxTe1x legs under the gold electrode (l). Finally, like the process used to fabricate the monomorphic-type TCH sensor, thermal grease and Pt/Al2O3 powder were sequentially applied onto the gold thin film (m, n).
no. 00-005-0712). It was found that the Pt peak increases as the amount of the Pt precursor in the Pt/Al2O3 increases. Scherrer's equation was employed to calculate the particle size of Pt. As a result, we observed that the Pt particles constantly
Results and discussion Characterization of Pt/Al2O3 catalyst X-ray diffraction patterns of the as-synthesized Pt/Al2O3 powder as a function of the Pt content are shown in Fig. 2. All catalyst patterns display diffraction lines of FCC Pt [(111), (200), and (220)] (JCPDS no. 00-004-0802) and an a-Al2O3 phase (JCPDS
Fig. 2 e X-ray diffraction patterns of Pt/Al2O3 powder as a function of the Pt content; (a) 0 wt%, (b) 0.933 wt%, (c) 4.501 wt%, (d) 8.614 wt% and (e) 15.682 wt%.
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Fig. 3 e TEM images of Pt/Al2O3 powder as a function of the Pt content: (a) 0 wt%, (b) 0.93 wt%, and (c) 15.8 wt%, (d) highresolution image of Pt/Al2O3 powder and (e) FFT-converted SAED pattern of Pt/Al2O3 powder.
increased in size from 13.87 to 24.83 nm as the amount of Pt precursor increased. This was due to agglomeration of the Pt nanoparticles during the annealing process. FE-SEM images of Al2O3 and Pt/Al2O3 powders with different amounts of the Pt precursor are shown in Fig. S1(aee). The Al2O3 powder possessed a rhombohedral structure with a particle size smaller than 5 mm. As the Pt content increased from 0.93 to 15.8 wt%, the presence of Pt particles on the surface of the Al2O3 powder increased. The point EDS spectra were measured at the area marked (1) in Fig. S1(f). It was found that Pt particles are well-attached to the alumina surface with a content of 26.44 wt%. Fig. 3 shows the HR-TEM and Fourier transform (FFT)-converted SAED patterns of the Pt/Al2O3 powder. It was found that the size and amount of Pt nanoparticles in the Pt/Al2O3 powder increased as the concentration of the Pt precursor increased (Fig. 3(aec)). The FFT patterns taken from the HR-TEM images of these particles correlated with the d-spacing values of the (200) and (111) lattice planes of face-centered cubic (FCC) Pt and with the dspacing values of the (012) and (113) lattice planes of the Al2O3 phase (Fig. 3(dee)). These results were consistent with the XRD data. The hydrogen-sensing properties of the assynthesized Pt/Al2O3 powders with different Pt contents were studied as a function of the hydrogen concentration (1e5 vol%). The amount of Pt/Al2O3 powder used to detect hydrogen was fixed at 0.1 g. The release of the H2/air mixture gas (used for sensing) was repeated at regular time intervals of 120 s on followed by 120 s off; this was done at fixed flow rate of 500 sccm (Fig. 4). It was found that the temperature constantly increased with respect to changes in the concentration of hydrogen gas. In particular, the temperature variation of the Pt/Al2O3 powder with 0.93 wt% Pt was more significantly increased compared to the increases observed at other conditions. This is attributed to the increased specific surface area
of the small Pt particles; hydrogen gas can more effectively react with the high specific surface area of Pt particles at the same gas concentration. By increasing the Pt content in the powder, the agglomeration of Pt particles was accelerated, which prevented the effective reaction between the Pt catalyst and hydrogen gas. Here, we chose the Pt/Al2O3 powder with 0.93 wt% Pt to fabricate the TCH sensor due to its good sensing performance at increased hydrogen concentrations.
Synthesis of stoichiometric chalcogenide thin film Prior to electrochemical deposition of the TE film, cyclic voltammetry (CV) was carried out with the working electrode in electrolytes with different Bi3þ concentrations (0, 10, and 40 mM) and a fixed HTeO2 þ concentration of 10 mM. As shown in Fig. 5 (a), the reduction peak was observed at approximately 0.14 V in the electrolyte consisting of 10 mM HTeO2 þ and 1 M HNO3; this reduction peak corresponds to the reduction of
Fig. 4 e Hydrogen-sensing properties of Pt/Al2O3 powder with different Pt contents as a function of the hydrogen concentration (catalyst loading amount is 0.1 g).
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Fig. 5 e Cyclic voltammograms of the (left) electrolytes containing 10 mM HTeO2 þ with different Bi3þ concentrations (0 mM, 10 mM, and 40 mM) in 1 M HNO3 (dotted lines are the oxidation slopes) and (right) the electrolyte containing 9 mM HTeO2 þ with different Sb3þ concentrations (3 mM and 9 mM) in 1 M HNO3 and 0.5 M C4H6O6. [CE: Ag/AgCl (sat. KCl), scan rate: 10 mV/s.].
Fig. 6 e X-ray diffraction patterns of (a) BixTe1¡x films deposited in electrolytes containing 10 mM HTeO2 þ , 10 mM Bi3þ, and 1 M HNO3 at different applied voltages: (1) 50 mV, (2) 0 mV, and (3) ¡50 mV. (b) SbxTe1¡x films deposited in electrolytes containing 9 mM HTeO2 þ , 3 mM Sb3þ, 1 M HNO3, and 0.5 M C4H6O6 at different voltages: (4) ¡150 mV, (5) ¡175 mV, and (6) ¡200 mV. The deposition time was fixed at 1 h.
Fig. 7 e FE-SEM images of BixTe1¡x films deposited in electrolytes containing 10 mM HTeO2 þ , 10 mM Bi3þ, and 1 M HNO3 at different applied voltages: (a) 0.05 V, (b) 0 V, and (c) ¡0.05 V. SbxTe1¡x films deposited in electrolytes containing 9 mM HTeO2 þ , 3 mM Sb3þ, 1 M HNO3, and 0.5 M C4H6O6 at different applied voltages: (d) ¡150 mV, (e) ¡175 mV, and (f) ¡200 mV. The deposition time was fixed at 1 h. Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216
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Bi2Te3 (DGf 0 ¼ 899.088 kJ mol1) [22,23]. In order to confirm the CV results, potentiostatic electrodeposition was carried out between 50 mV and 50 mV vs. Ag/AgCl (sat. KCl) with a step size of 50 mV. To deposit the Sb2Te3 thin film, linear sweep voltammetry (LSV) was performed with the seed layerdeposited working electrode in an electrolyte with different Sb3þ concentrations (3 and 9 mM) and a fixed HTeO2 þ concentration of 10 mM (Fig. 5 (b)). Reduction peaks were observed between 150 mV and 200 mV in the electrolyte consisting of 9 mM HTeO2 þ , 3 mM Sb3þ, 1 M HNO3, and 0.5 M C4H6O6. Fig. 6 (a) shows the X-ray diffraction patterns taken between 37 and 45 for BixTe1x films deposited in an electrolyte containing 10 mM HTeO2 þ and 1 M HNO3 at different applied potentials. When potentials of 0 and 50 mV vs. Ag/AgCl (sat. KCl) were applied, the presence of Bi4Te5 (JCPDS no. 00-022-0115) and BiTe (JCPDS no. 00-050-0602) phases, respectively, were confirmed. Through this, the optimal potential needed to deposit near-stoichiometric Bi2Te3 thin films was determined to be 50 mV. X-ray diffraction patterns of SbxTe1x thin films with different potentials are shown in Fig. 6 (b). When a 150 mV potential was applied, a deposit with a mixed structure of Sb2Te3 (JCPDS no. 00-015-0874) and Te (JCPDS no. 00-004-0554) was observed. For an applied potential of 200 mV, a Sb0.405Te0.595 phase (JCPDS no. 00-045-1228) was observed due to the increased Sb content, which was deposited with a more negative potential. A near-stoichiometric Sb2Te3 (JCPDS no. 00-015-0874) phase, without other phases such as tellurium, was synthesized by deposition at an applied potential of 175 mV. Fig. 7(aec) show the cross-section images of BixTe1x thin films deposited by electrochemical deposition as a function of the applied potential; here, the deposition time was fixed at 1 h and the electrolyte was 10 mM HTeO2 þ , 10 mM Bi3þ, and 1 M HNO3. It was found that the thickness of the BixTe1x films was almost same at potentials of 50 mV and 0 V. Additionally, a BixTe1x film could be grown with a dendritic-like structure at a potential of 50 mV. The cross-section images of SbxTe1x thin films as a function of the applied potential (with an electrolyte containing 9 mM HTeO2 þ , 3 mM Sb3þ, 1 M HNO3, and 0.5 M C4H6O6) are shown in Fig. 7(def). In the case of the SbxTe1x film, the morphological change observed (from a smooth surface to a rough surface) can be mainly attributed to the severe ion depletion near the surface, which was caused by the more negative potential [24]. Fig. 8 e Hydrogen-sensing properties of monomorphictype (black line) and four-leg PN junction-type (red line) thermochemical hydrogen sensors: (a) voltage signal as a function of the H2 concentration, (b) voltage signal with a fixed gas flow of 3% H2/air, and (c) voltage signal with a regular gas flow interval of 3% on and 0% H2/air off. The flow rate of hydrogen was fixed at 500 sccm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
HTeO2 þ ions by elemental Te (s). Bi2Te3 deposition occurs at an applied potential that is more positive than the potential required for the overpotential deposition (OPD) of elemental Bi; this is due to the negative Gibbs free energy of formation of
Hydrogen gas sensing properties The hydrogen-sensing properties, including gas detection, response time (T90), recovery time (D10), and reliability, of the TCH sensor were measured using a custom-made apparatus at room temperature. The output voltage signal of the TCH sensor was measured using a nanovoltmeter (2182 A, Keithley) that was placed in contact with the TCH sensor. The sensing properties of the two types of TCH sensors were studied as a function of the hydrogen concentration (1e10 vol % H2/air), as shown in Fig. 8 (a). For both sensor types, the voltage signal increased with increasing hydrogen concentration (at a flow rate of 500 sccm). For the monomorphic-type TCH sensor, the voltage signal reached 14.2 mV at a hydrogen concentration of 10 vol%. Alternatively, for the four-leg PN junction-type TCH sensor, the voltage signal reached
Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216
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Table 1 e Comparison of response and recovery times of various thermoelectric hydrogen sensors. Materials
Alkali-doped NiO thick film Li-doped NiO thin film Li-doped NiO Bi2Te3/Sb2Te3 thin film Phosphorus-doped Si0.8Ge0.2 Phosphorus-doped Si0.8Ge0.2 Bi2Te3 thin film SiGe pattern Bi2Te3 thin film (monomorphic-type) Bi2Te3/Sb2Te3 thin film (fourleg PN junction-type)
Synthesis method
Hydrogen concentration (vol.%)
Response time T90 (s)
Recovery time D10 (s)
Operating temperature ( C)
Ref.
Paste printing RF sputtering Molten salt method Magnetron sputtering RF sputtering RF sputtering Magnetron sputtering RF magnetron sputtering Electrochemical deposition
3 3 3 3 3 3 3 3 3
55 47e35 <25 28 46 45 40 <2.5 31
50 40 18 55 e e 60 e 38
RT 100 RT RT 100 100 RT 100 RT
[25] [26] [12] [14] [27] [16] [13] [28] This work
Electrochemical deposition
3
27.4
9.6
RT
This work
39.6 mV at 10 vol%; this represents a 2.8-fold increase compared to the monomorphic-type TCH sensor. This improvement can be explained by the difference in the electrical connections, such as the parallel and serial conversions, of the two types of TCH sensors. A repeatability test of the TCH sensors for hydrogen gas detection was also performed under 3% H2/air conditions, as shown in Fig. 8 (b). The performance changes of the two types of TCH sensors were collected for 16000 s by continuously measuring the hydrogen gas. For the junction-type TCH sensor, the relative error between the initial signal and the final signal was 3.5%; for the monomorphic-type, the relative error was 30% and the final voltage signal was higher than the initial voltage. The outstanding error observed for the monomorphic-type sensor is due to its difficulty in releasing the thermal energy that is generated as a result of the exothermic oxidation of hydrogen gas. The release of thermal energy from the TE material is important to maintain the stability of TCH sensors. The monomorphic-type of sensor has a large amount of thermal energy and a large area during hydrogen sensing. For the junction-type sensor, heat was released easily by dividing the region of the thin film into four areas; this can be confirmed in Fig. 8 (a). In the case of the monomorphic-type sensor, the voltage signal was not decreased to zero by the flowing air gas, despite the low hydrogen concentration. Alternatively, the voltage signal of the junction-type sensor was decreased to almost zero. As a result, in our system, the junction-type TCH sensor displayed good reliability and stability compared to the monomorphic-type TCH sensor. The response time and recovery time of the two types of TCH sensors were calculated using the time-dependent voltage signal at a concentration of 3 vol% H2/air. It was found that the response time and recovery time of the four-leg PN junction-type sensor were 27 s and 9 s, respectively, which were faster than those of the monomorphic-type (T90: 31 s and D10: 38 s). This improvement was attributed to the excellent heat release of the junctiontype TCH sensor compared with that of the monomorphictype. Also, the fast response time and recovery time of the TCH sensor were caused by the thickness of the film. Many studies concerning TE-based hydrogen sensors have a TE layer that enables heat to flow in a horizontal direction. However, our sensor has a laminated structure with a
vertically-deposited thin film, which enables heat to flow vertically. To verify the test, the release of the 3% H2/air mixture gas (used for sensing) was repeated at regular time intervals of 120 s on and 120 s off at a fixed flow rate of 500 sccm. In the case of the junction-type TCH sensor, the relative error was 3.4% after repeating the test 10 times. For the monomorphic-type sensor, the relative error was 12.1% after repeating the sensing test 10 times. Table 1 shows a comparison of the response and recovery times of various TE hydrogen sensors. These response and recovery times are among the fastest that have been reported for TE hydrogen sensors [12e14,16,25e28]. Table 1 compares the results obtained in this study to those obtained in previous studies. This shows that our TCH sensor has relatively fast response and recovery times compared to those of other thermoelectric sensors.
Conclusions In this study, we fabricated lamination-structured TCH sensors composed of a chalcogenide thin film and a Pt/Al2O3 catalyst that acts as a heating element. Chalcogenide films of two types, composed of Bi2Te3 (monomorphic-type) and Bi2Te3eSb2Te3 (four-leg PN junction-type), respectively, were prepared by cost-effective electrochemical deposition. The monomorphic-type TCH sensor showed a voltage signal of 14.2 mV in response to 10 vol% hydrogen gas, whereas an output signal of 39.6 mV was obtained from the four-leg PN junctionetype TCH sensor. Even though the nep junctiontype has the same deposition area as that of the monomorphic-type, the voltage signal of the four-leg PN junction-type TCH sensor was more than 2.8 times as high. Also, the best response and recovery times of the monomorphic-type TCH sensor were 31 s and 38 s, respectively. In the case of the four-leg PN junction-type TCH sensor, the lowest response time was 27 s and the fastest recovery time was 9 s (in 3% H2/Air at room temperature). This improvement is attributed to the excellent heat release of the junction-type TCH sensor compared with the monomorphictype sensor.
Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216
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Acknowledgements This work was supported by the Technological innovation R&D program (S2307196, Development of High Durability and High Sensitive Sensor Platform Having an Embedded Sensing Electrode for NO2, NH3 Gas) of the Small and Medium Business Administration (SMBA, Korea), and supported by NanoMaterial Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2016M3A7B4900044).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.08.216.
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Please cite this article in press as: Kim S, et al., Fabrication and characterization of thermochemical hydrogen sensor with laminated structure, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.216