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Pd bimetallic ultra-thin films
Accepted Manuscript Title: Fast-response hydrogen sensors based on discrete Pt/Pd bimetallic ultra-thin films Author: Kamrul Hassan A.S.M Iftekhar Uddin Gwiy-Sang Chung PII: DOI: Reference:
S0925-4005(16)30678-5 http://dx.doi.org/doi:10.1016/j.snb.2016.05.013 SNB 20161
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
2-1-2016 21-4-2016 5-5-2016
Please cite this article as: Kamrul Hassan, A.S.M Iftekhar Uddin, Gwiy-Sang Chung, Fast-response hydrogen sensors based on discrete Pt/Pd bimetallic ultra-thin films, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.05.013 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.
Fast-response hydrogen sensors based on discrete Pt/Pd bimetallic ultra-thin films
Kamrul Hassan, A. S. M. Iftekhar Uddin, Gwiy-Sang Chung*
School of Electrical Engineering, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 44610, Republic of Korea
* Corresponding author. Tel.: +82-52-259-1248; Fax: +82-52-259-1686 E-mail address: [email protected] (G.-S. Chung) URL:http://home2.ulsan.ac.kr/user/gschung
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Abstract This study presents the fast response-recovery characteristics of an ultra-thin discrete platinum/palladium (Pt/Pd) bimetallic film-based hydrogen (H2) sensor. Pt/Pd bimetallic nanoparticles (NPs) with a size of 2-11 nm were deposited on an alumina (Al2O3) substrate using a pulsed laser deposition (PLD) method. The metal loading and the composition of the Pt/Pd NPs were carefully controlled by varying the deposition conditions of the PLD system. The resultant discrete Pt/Pd bimetallic ultra-thin film was applied as a new H2 detection material, in which the fabricated Pt (3 nm)/Pd (3 nm) film showed noteworthy advantages, such as a large detection range of 10 to 40,000 ppm, high response magnitude, and fast response-recovery time. For a 10,000 ppm (1 vol.%) H2 concentration at 150 oC, a maximum sensor response of 13.56% and a response-recovery time of 4/5 sec were observed. The observed fast response-recovery time characteristics can be attributed to the enhanced hydrogen-induced changes in the work function of the Pt/Pd ultra-thin film. The fabricated sensor also exhibits good reproducibility and a negligible humidity effect within the entire detection range.
Keywords: Pt/Pd bimetal, ultra-thin film, hydrogen sensor, fast response-recovery time, PLD
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1. Introduction Hydrogen (H2) is a colorless, odorless, and tasteless gas, which can cause a safety hazard when mixed with air because of its large flammability range (4−75%) where high burning rates and explosive tendencies increases the risks [1]. At high concentration it can also act as an asphyxiant. In the chemical industry, H2 is used as a reactant, for the processing of ammonia, petrochemicals, and methanol. It is also used as an energy carrier, with remarkable applications in fuel cell technologies and rocket propulsion systems. With renewed emphasis being placed on the production of H2 through photo catalytic water splitting [2−4], the development of a hydrogen economy powered by sunlight is more reasonable. Moreover, H2 derived from renewable sources of clean energy can also act as a feedstock for reactions to convert CO2 into a useable fuel through catalytic hydrogenation [5, 6], which not only produces fuels with a greater energy density than H2, but also consumes a damaging greenhouse gas. If such an energy infrastructure is to develop then it will necessarily be monitored by a network of sensing devices, which can detect H2 levels where it is used, transported, and stored. If, in this situation, distributed H2 power generation systems become conventional, then such point-of-use consumption could finally make H2 sensors as essential as carbon monoxide and smoke detectors. Various types of H2 sensors have already been developed by observing the H2-induced changes in electrical [7-9], electrochemical [10, 11], thermal conductivity [12], surface plasmon resonances [13], surface acoustic waves [14], and mechanical [15] properties. Commercially available H2 sensors are mainly based on Pd thin films and are limited by their response time (tens of seconds) and physical sizes (inches) [16]. Because of their unique physical and chemical properties and their potential for improving sensor performance, nanostructured materials are in high demand for gas sensing applications due to in part to their miniaturized device dimensions, which bring about low power consumption. Numerous Pd nanostructures have been used in H2 sensors, including ultrathin films made of continuous [17] or discontinuous [18] nanocrystals, nanowires [19], and nanotubes [16], etc. Due to the kinetics of hydrogen absorption, the absorption energy, and the hydrogen diffusion coefficient of palladium (Pd), among the H2 gas sensors it generally exhibits one of the faster response-recovery times [20]. Along with this, the incorporation of platinum (Pt) promises to enhance the absorption of H2 gas molecules [21, 22].
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In addition, highly porous or truly nanoscale sensing materials can contribute to the enhancement of the response and recovery times of the sensor [23]. Alternatively, precisely controlled bimetallic composition and structure offers the possibility of combining the unique advantages of each counterpart, allowing enhanced catalytic activity, selectivity, and stability [24-27]. Supported Pt-Pd nanoparticles (NPs) are considered to be the most promising bimetallic heterogeneous catalysts in H2 sensing technology due to their extraordinary core-shell like nanostructure, which preferentially support individual sensing properties rather that alloying phenomenon. Importantly, the Pd/Pt core-shell boundary has been observed to bring about interesting physical and chemical properties that have importance for hydrogen storage [28]. Due to the modification of the structure and the electronic states, such as atomic arrangements and chemical potentials of the interfacial region, it can be expected that the hydrogen absorption properties can be quite different compared to the bulk counterpart nanoparticles. Unfortunately, the synthesis of uniform bimetallic nanoparticles with diameters below 5 nm is a challenging task for traditional catalyst synthesis methods, such as wet impregnation [29, 30] and colloidal chemistry [31, 32]. In the current work, we have fabricated uniform Pt/Pd bimetallic NPs in a discrete ultra-thin film manner on alumina (Al2O3) substrate using a pulsed laser deposition (PLD) technique. It was expected that the discrete Pt/Pd ultra-thin film would show possible enhancements due to the synergetic interplay between the bi-functional capping configuration, the large surface-to-volume ratio, and the quantum size of the nanoparticles. The as-fabricated structure was used as a resistivity-type sensor with the aim of developing a high performance H2 sensor with a fast response-recovery time.
2. Experiment 2.1. Ultra-thin film and sensor fabrication Pt/Pd ultra-thin film was deposited on the 2 × 2 cm2 Al2O3 substrate from the Pd (iTASCO, 99.95% purity) and Pt (iTASCO, 99.98% purity) targets using a PLD method. As in a typical PLD system, a krypton fluoride (KrF) excimer laser (Lambda Physik Compex Pro 201) was used to produce laser pulses of 5 ns duration at a wavelength of 248 nm with the energy of 0.35 J per pulse. The laser beam was focused on the target at an angle of 45o, whereas the target holder 4
rotates such that subsequent laser pulses hit the target at different positions. The detailed deposition parameters are listed in Table 1. Five different samples with different Pt/Pd bimetal thicknesses were fabricated by varying the number of laser shots in the PLD system and labeled as S1 = Pt (1 nm)/Pd (1 nm), S2 = Pt (2 nm)/Pd (2 nm), S3 = Pt (3 nm)/Pd (3 nm), S4 = Pt (3 nm)/Pd (4 nm), and S4 = Pt (3 nm)/Pd (8 nm), respectively. Finally, to fabricate a simple resistivity-type sensor, several pieces of Al2O3 substrate with the dimensions of 6 × 12 mm2 were cut from the base substrate and two contact electrodes of silver (Ag) were painted at a distance of 2 mm.
2.2. Ultra-thin film characterization The water droplet contact angles were measured by contact angle goniometry (Kruss DSA 100 Drop Shape Analyzer) using the sessile drop method at room temperature. Distilled deionized water droplets (about 3 µL) were dropped on the thin film surfaces using a microsyringe. The average value of the water contact angle was determined by experimental drop profiles at five different positions for the same sample. Surface morphology of the as deposited film and the film thickness were measured using atomic force microscopy (AFM, NT-MDT, Ntegra) in the semi-contact (tapping) mode. The compositional analyses of the as-prepared samples were examined with a JEOL JEM-2010F energy-dispersive spectrometer (EDS). Structural properties of the sensing materials were investigated using X-ray diffractometer (XRD, Rigaku Ultima IV) with Cu Kα (λ = 0.154 nm) radiation with a 2θ scanning range of 10-90o. X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha) was performed using Al Kα radiation as the x-ray source.
2.3. Transport measurement Gas sensing measurements of the as-fabricated sensors were conducted in an in-house gas sensor assembly (chamber) at atmospheric pressure within a temperature range of 20-200oC, using the flow-through technique with and without various H2 concentrations. A Keithley probe station (SCS-4200) with a bias voltage fixed at 1V was used for all measurements and data acquisition. A programmable heater integrated with the sensor holder in the chamber was used to adjust the temperature. A computerized mass flow controller (ATO-VAC, GMC 1200) system was used to vary the concentration of H2 in synthetic air. The gas mixture was delivered to the 5
chamber at a constant flow rate of 50 sccm (standard cubic centimeters per minute) with different H2 concentrations. The gas concentration was controlled and measured using the following equation: 𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝐺𝑎𝑠𝑐𝑜𝑛. (𝑝𝑝𝑚) =
𝐹𝑙𝑜𝑤 𝑟𝑎𝑡𝑒𝑔𝑎𝑠 × 𝑆𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝐺𝑎𝑠𝑐𝑜𝑛. (𝑝𝑝𝑚) (𝐹𝑙𝑜𝑤 𝑟𝑎𝑡𝑒𝑔𝑎𝑠 + 𝐹𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑠𝑦𝑛𝑡ℎ𝑒𝑡𝑖𝑐 𝑎𝑖𝑟 )
The gas chamber was purged with synthetic air between each H2 pulse to allow the surface of the sensor to return to atmospheric conditions. The sensor response (S%) was calculated using (Rg-Ra )/Ra × 100, where Ra and Rg are respectively the resistances of the sensor in the presence of synthetic air and H2 at certain concentrations. The response-recovery time of the sensor was defined as the time to reach 90% of the total resistance change. The humidity level inside the chamber was monitored using a Testo 625 hygrometer. The relative humidity (RH) in the gas chamber was varied from 40-80% using a water bubbler controller.
3. Results and discussion 3.1. Ultra-thin film structure and morphology In order to determine the wettability on the surface of the Al2O3 substrate, characterization with water droplets was carried out. The contact angle was determined by using the following formula, cos 𝜃𝑤 = 𝛾𝑠𝑣 −
𝛾𝑠1 𝛾1𝑣
(1)
where, θw is water contact angle. The contact angle images of glass, Si, and Al2O3 substrate are shown in Fig. 1. The contact angle of the Pt/Pd/Al2O3 substrate was measured to be 142.4o, which was much higher than the contact angles of glass (21.3o), silicon (50.6o), and glass (103.4o) substrates. It is well known that if θw > 90o, the surface is hydrophobic, otherwise hydrophilic, which revealed that the examined Pt/Pd/Al2O3 substrate exhibits a highly hydrophobic property. Additionally, it has been reported in the literature that surface roughness is directly proportional to the contact angle and thereby inversely proportional to the surface energy. The surface roughness and the chemical composition are the main factors in determining the surface energy of thin films [33]. By using the Owens-Own method we calculate the surface free energy and summarized in supplementary information (Table S1). Thus, the contributions of 6
both factors reveal that the surface structure of the Al2O3 substrate leads to lower surface energy, hence higher hydrophobicity [34-37]. Besides, this lower surface energy made the grain size small and reduced the intergrain distance. Consequently, with deposition of the Pt/Pd thin film on the Al2O3 substrate, the Pt capped the Pd nanoparticles (NPs) to form discrete bimetal nanoparticles of high uniformity with almost equal distances within the grains. This situation leads to create shortest current path by contacting neighboring grains at different H2 concentration ranges. For this reason, after hydrogen absorption electron scattering was reduced and resistance change was fast. Moreover, the discrete ultra-thin film manner creates more rooms for the desorption of hydrogen gas within shorter period of time. The AFM images of the as-deposited samples confirm the deposition of very smooth, compact, and discrete Pt, Pd, and Pt-coated Pd thin films, in Fig. 2 (a, b, c) respectively. The appearance of all the films was partially mirror-like, in which the observed average grain size was found to be about 3 nm for both Pt and Pd films and about 6 nm for the Pt/Pd film. The root mean square (rms) surface roughness of these samples was observed to be nearly 0.095, 0.105, and 0.938 nm for Pt, Pd, and Pt/Pd films, respectively. From the figures, we can see that the NPs were deposited uniformly over the entire substrate with uniform distances between adjacent nanoparticles. The elemental composition of the as-deposited Pt/Pd bimetal thin film on the Al2O3 substrate was investigated by EDS and elemental mapping, as is shown in Fig. 3. A selected area of the EDS spectrum from Fig. 3(a) was taken, and the result is shown in Fig. 3(b). The presence of various well-defined peaks of aluminum (Al), oxygen (O), and Pt and the absence of peaks related to contaminations confirmed the formation of a high-purity sensing material. The low content or the absence of the Pd peak in Fig. 3(b) might be attributed to the capping and overlapping of Pt over Pd. To confirm the distribution of Pt and Pd atoms on the lattice surface, elemental mapping of the area shown in Fig. 3(a) was carried out, and the results are depicted in Fig. 3(c-f). The analysis shows that Al, O, Pt and Pd are distributed homogeneously over the whole substrate, which further confirmed the formation of a highly uniform Pt/Pd film. The XRD pattern of the Pt/Pd thin film is shown in Fig. 4. A broad peak centered at 2θ = 34.94o can be indexed to the (104) plane of α-Al2O3. In addition, the diffraction peaks observed at 2θ values of 39.79o, 46.22o, and 67.7o correspond to the face-centered cubic (fcc) structure of Pt or Pd (111), (200), and (220) (Pt: JCPDS card no. 04-0802; Pd: JCPDS card no. 7
46-1043) [38]. The estimated crystallite size of the bimetal nanoparticles was calculated from the dominant (111) peak using Scherrer’s formula and was found to be about 5.6 nm. The composition of Pt:Pd was calculated from Vegard’s law and found to be 58:42. XPS analysis was carried out to examine the surface composition of the Pt/Pd bimetal with the Pt/Pd thickness ratios of 2/2, 3/3, 3/4, and 3/8 (samples S2-S5), respectively. Four deconvoluted peaks were considered for the fitting of Pt (Pt 4f5/2 and Pt 4f7/2) and Pd (Pd 3d3/2 and 3d5/2) peaks as shown in Fig. 5. The observed peaks with relatively higher intensity correspond to the predominant species of the metallic Pd (Fig. 5(a, c, e, and g) for the respective thickness ratios) and the metallic Pt (Fig. 5(b, d, f, and h) for the respective thickness ratios). Pd2+ and Pt2+ correspond to the ionization state of Pd and Pt atoms. With the increasing Pt/Pd ratio a slight shift in the Pd 3d and Pt 4f to a higher energy level was observed, which might be attributed to the thicker layer of Pt, the induced strain on the Pd and the electronic interactions between the Pt shell and Pd core. This phenomenon demonstrates the Pt/Pd core-shell arrangement. However, the absence of Pd-O and Pt-O indicates the high-purity physical deposition of bimetallic Pt/Pd nanostructures and the effective assembly of the Pt/Pd bimetal onto the surface of the Al2O3 substrate.
3.2 Gas sensor studies In the presence of synthetic air, H2 physisorbs on the Pd surface, dissociates, chemisorbs, and subsequently forms the Pd-hydride (Pd-H) as follows, 𝐻2 (𝑔) → 𝐻2 (𝑎𝑑𝑠)
(2)
1
𝑃𝑑 + 2𝐻2 (𝑎𝑑𝑠) → 𝑃𝑑-𝐻
(3)
The electrical resistivity of PdH0.7 is almost a factor of 2 higher than that of Pd, which allows the detection of H2 due to the increase in the resistance of the Pd resistor [39]. In an air environment, the limit of detection of H2 (𝐿𝑂𝐷𝐻2 ) can be increased due to the presence of oxygen that enables the formation of catalytic water molecules at the Pd surface, thereby reducing the steady state surface coverage of the chemisorbed H2 that is available to be absorbed into the bulk of the PdHx (reaction 3) [40-42]. 3 𝑃𝑑-𝑂 + 𝐻2 (𝑎𝑑𝑠) → 𝑃𝑑-𝐻 + 𝐻2 𝑂(𝑎𝑑𝑠) 2
(4)
𝐻2 𝑂(𝑎𝑑𝑠) → 𝐻2 𝑂(𝑔)
(5) 8
Chemisorbed oxygen (reaction 4) also blocks the Pd adsorption sites, impeding the H2 adsorption and extending the time required for equilibration of H2 in the gas phase with the PdHx. The strong influence of oxygen on the sensing behavior of Pd reveals that the surface chemistry is a critical factor in determining the performance of the resistance-based H2 gas sensors. Johansson et al. [43] have shown that in dry air at T = 100oC, Pt is a better catalyst than Pd for reaction 4. Pt is also a superior catalyst for reaction 3, which is the rate-limiting reaction for sensor response [44]. Yang et al. synthesized Pt nanowires and evaluated their H2 sensing performance, which revealed better sensing performance compared to sensors with Pd nanowires and sensing performance that improved at higher temperatures [45]. Therefore, operating temperature is one possible way for accelerating the sensing performance of Pd thin film [44]. In addition, the fabrication of Pt/Pd bimetallic ultra-thin films has the potential of favorably altering the surface chemistry of the Pd thin film. The operating temperature has a significant influence on the fundamental sensing mechanism of bimetal based gas sensors. In order to determine the optimum working temperature, the fabricated sensors were investigated over a temperature range of 20-200oC. Fig. 6 depicts the relationship between the response magnitudes of the sensors and the operating temperature, measured with a 10,000 ppm H2 gas concentration. The tested samples showed no response at lower temperature ( 100oC). It is well established that hydride formation of Pt is relatively faster than that of Pd under identical conditions of 𝑃𝐻2 and T ≥ 100 °C due to the rapid water formation rate on Pt than Pd. Besides, at T < 100 oC, stable bulk hydride phase does not exist on Pt, hence the transduction mechanism for the detection of H2 that operates at Pd will not exist at Pt. Heating of the samples is a possible mean that not only enhance the response of the sensor, but also it accelerates the sensor’s response time [44]. However, with the increase of Pt shell thickness, baseline resistance of the as-fabricated samples shifted to lower values (see Fig. S1), which impeded the sensitivity of the sensor. With increasing temperature, the response values gradually increased, reached to maximum values at 150 oC, and then decreased with further increase in operating temperature. Therefore, 150 oC was selected as the optimum working temperature of the Pt/Pd bimetallic ultra-thin film sensor, and was used for further sensing characteristics examinations. When the Pd surface was exposed to an H2 gas environment, the Pd phase changed to Pd hydride and expanded in volume, which significantly reduced the nano-gap between the particles 9
and the resistance between the electrodes. A Pt cap with good H2 dissociation allows H2 to pass through and interact with the Pd in the core, leading to a volume expansion of the Pt/Pd bimetal NPs. Moreover, the synergistic effect between the Pd and Pt enhanced the catalytic activity of the Pt/Pd bimetal, which preferentially facilitated the absorption of H2 molecules at the interface of the bimetal boundary of Pt/Pd nanoparticles and played an important role in enhancing the H2 sensing properties [28, 46-48]. In this work, we assessed the influence of an ultra-thin Pt film on a Pd ultra-thin film with regard to the hydrogen sensing performance. It was found that the Pt layer accelerated both the response and recovery time characteristics of the as-fabricated sensor. Importantly, Pt increased the 𝐿𝑂𝐷𝐻2 at 150 oC and enabled the simultaneous acceleration of response-recovery with virtually no degradation of the sensitivity to H2. In order to realize the thickness effect of ultra-thin Pt/Pd film on H2 sensing characteristics, theoretical analysis of the as-fabricated sensors was carried out using Comsol Multiphysics electrostatics simulation software. Fig. 7(a) illustrates the distribution of electrical permittivity in the film (electric impedance tomography), which reveals that at a certain applied potential difference on the boundaries of the chamber creates a surface charge density depending on the permittivity distribution inside the chamber. As seen from the color legend in Fig. 7(a), the surface charge density of the Pt/Pd bimetallic thin film is comparatively higher than the bare Pt and Pd films due to higher relative permittivity of the Pt/Pd bimetal (Ɛr = 67.38) in comparison to bare Pt (Ɛr = 17.19) and Pd (Ɛr = 13.698) films. Fig. 7(b) shows the relationships of surface charge density with the Pt/Pd film thickness at an operating temperature of 150 oC. It was observed that surface charge density of the Pt/Pd bimetal thin film with a thickness of 3/3 nm exhibited the maximum value of 0.65 µC/m2 in air medium compared to the other films, while in H2 medium this value was the minimum (0.18 µC/m2). This phenomenon reveals that the uniform organization of the Pt/Pd bimetal with 3/3 nm thickness provides the maximum amount of dissociations-chemisorptions of the H2 molecules and hydride formation, hence effectively increases the electrical resistance of the film surface. To relate this simulated result to the realtime phenomenon, the as-fabricated sensors were exposed to a certain concentration of H2. Fig. 7(c) shows the variation of response magnitude of the sensors to 10,000 ppm H2 at 150 oC with respect to film thickness. A maximum response of 13.56% was obtained for S3 sample (3/3 nm), which was much higher than that of S2 (2%), S4 (9.1%), and S5 (5.5%) samples. However,
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sample S1 did not show any response, which might be attributed to the large gaps between the nanoparticles where charge transfer paths were not formed. Fig. 8 shows the real-time resistance variation of the as-fabricated sensors. It can be seen that the S3 sample had a faster response-recovery time (4/5 sec) than the S2, S4, and S5 samples. Fig. S1 (I-V characteristics curve) shows that the baseline resistance of the sensors (S2-S5) drifts to lower values with the increase of Pt shell thickness. The Pt/Pd (3/3 nm) film thickness indicates a comparatively higher performance due to the synergies between the hydrogen gas species dissociated by if not one then the other of the Pt and Pd catalysts. More importantly, physisorption and chemisorption process is vital within H2 and metal before hydrogen molecules interaction into the metal lattice. During chemisorption process, H2 molecules get incorporated into the crystal structure of metal and form metal hydride (MHx), while the physisorbed molecules interact with the interstitial sites of the Pt/Pd NPs [49]. These sites can act as electron scattering centers and result decrease in carrier mobility. Both processes together cause a hydrogen-induced increase in electrical resistance of the Pt/Pd ultra-thin films, which is normally termed as an electronic effect during sensing. Moreover, formation of a β-MH phase in the tetragonal sites can results in an increased lattice constant. The formation of MHx also causes a decrease in the work function, which ultimately changes the electronic properties of the bimetallic nanoparticles [50]. Therefore, the Pt capping of the Pd plays the role of a trade-off factor between the sensor characteristics of response and recovery rate. Detailed comparisons among the tested samples are summarized in Table 2. Fig. 9(a) shows the dynamic resistance variation of S3 with various H2 concentrations at 150 o
C. The sensor exhibited increased resistance after exposure to H2 gas and a clear response from
10 to 40,000 ppm H2. Fig. 9(b) illustrates the repeatability of the S3 sensor with a constant response value of 13.56% at a concentration of 10,000 ppm H2 at 150 oC. The as fabricated sensor was kept during a period of twenty two weeks and the functionality of the H2 sensor towards 1000 ppm and 10,000 ppm of H2 gas at 150 oC was checked in every two weeks. Fig. 10 (a) reveals that the H2 sensor exhibits good working stability during over five months of operation, which can be attributed to the high purity sensing layer formation (less possibility of occurring contaminations) while using PLD system. Moreover, in ambient condition (for example, T ≤ 100 oC), Pt-O is unavailable. In addition, the as-fabricated sensors are highly hydrophobic in nature. Due to these reasons, environmental effect is almost negligible 11
on the sensing surface, which ultimately offers good stability to the sensor. In order to assess the reproducibility behavior of our sensor, we investigated the sensing characteristics of five individual sensors and the results are summarized in Fig. 10 (b). Similar outcomes are observed for all the tested sensors, which reveal excellent reproducibility characteristics of the asfabricated sensors. This phenomenon might be attributed to the controlled and high purity deposition environment during sensor fabrication using PLD system. Fig. 10 (c) represents the selectivity property of the S3 sensor under exposure to 1,000 ppm test gases, including hydrogen, oxygen, carbon mono-oxide, carbon dioxide, nitrogen, and 100 ppm nitrogen dioxide. The asfabricated sensor showed excellent selectivity towards H2 due to high selective absorption capability of Pd and Pt catalysts with H2 molecules. At a preeminent temperature of 150 oC, catalytic activity of the S3 sensor toward H2 decomposition might be strongly facilitated and might be an exothermic process, in which the sufficient surface energy of the sensor surface satisfactorily supported to be reacted with the bond energy of H2 compared to other test gases [51]. In real-world applications, the humidity effect is a great concern in the performance evaluation of the sensor, as the adsorption of water molecules decelerates the sensor response significantly. In order to get rid of this degradation phenomenon we have used hydrophobic Al2O3 substrate and a thermally-activated Pt/Pd bimetallic thin film. Fig. 11 shows the humidity effect on the as-prepared bimetallic film. However, at 150 oC temperature no significant degradation in response magnitude was observed with the relative humidity (RH) up to 80%, which suggests that we have indeed fabricated a high-performance sensor.
4. Conclusions This study demonstrated the potential of using discrete Pt/Pd bimetallic ultra-thin films on Al2O3 substrates for H2 sensing. The fast sensing and recovery of our Pt/Pd ultra-thin films could be attributed to the bi-functional capping configuration, the large surface-to-volume ratio, and the nanoscale aspect of surface features. In addition, the thickness of the Pt/Pd layers plays an important role in sensor performance. The discrete Pt/Pd (3/3 nm/nm) ultra-thin film showed enhanced performance that included a response value of 13.56% to 10,000 ppm of H2 at 150 oC, a broad detection range of 10-40,000 ppm, and fast response-recovery times of 4/5 sec (to 1% 12
H2). The fabricated sensor also exhibits good reliability and a negligible humidity effect within the entire detection range with operation at 150 oC. Our results may also provide valuable guidelines for designing high performance catalysts for fuel cell applications.
Acknowledgements This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded in 2014 by the Ministry of Science, ICT, and Future Planning (NRF-2014R1A2A2A01002668 and the MOTIE (Ministry of Trade, Industry and Energy), Korea, under the Eco-friendly Fuel Cell Test-bed program supervised by the KEA (Korea Energy Agency).
Conflict of interest The authors declare no competing financial interest.
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[28] M. Yamauchi, H. Kobayashi, H. Kitagawa, Hydrogen storage mediated by Pd and Pt nanoparticles, Chem. Phys. Chem. 10 (2009) 2566-2576. [29] N. Castillo, R. Perez, M.J. Martinez-Ortiz, L. Diaz-Barriga, L. Garcia, A. Conde-Gallardo, Structural analysis of platinum–palladium nanoparticles dispersed on titanium dioxide to evaluate cyclo-olefines reactivity, J. Alloys Compd. 495 (2010) 453-457. [30] B.D. Adams, C.K. Ostrom, A. Chen, Highly active PdPt catalysts for the electrochemical reduction of H2O2, J. Electrochem. Soc. 158 (4) (2011) 34-39. [31] Y. Liu, M. Chi, V. Mazumder, K.L. More, S. Soled, J.D. Henao, S. Sun, Compositioncontrolled synthesis of bimetallic PdPt nanoparticles and their electro-oxidation of methanol, Chem. Mater. 23 (2011) 4199-4203. [32] A.X. Yin, X.Z. Min, Y.W. Zhang, C.H. Yan, Shape-selective synthesis and facet-dependent enhanced electrocatalytic activity and durability of monodisperse sub-10 nm Pt-Pd tetrahedrons and cubes, J. Am. Chem. Soc. 133 (2011) 3816-3819. [33] A. Sanger, A. Kumar, S. Chauhan, Y.K. Gautam, R. Chandra, Fast and reversible hydrogen sensing properties of Pd/Mg thin film modified by hydrophobic porous silicon substrate, Sens. Actuators B 213 (2015) 252–260. [34] D.K. Owens, R.C. Wendt, Estimation of the surface free energy of polymers, J. Appl. Polym. Sci. 13 (1969) 1741-1747. [35] S.M. Chiu, S.J. Hwang, C.W. Chu, D. Gan, The influence of Cr-based coating on the adhesion force between epoxy molding compounds and IC encapsulation mold, Thin Solid Films 515 (2006) 285-292. [36] J.S. Chen, S.P. Lau, Z. Sun, G.Y. Chen, Y.J. Li, B.K. Tray, J.W. Chai, Metal-containing amorphous carbon films for hydrophobic application, Thin Solid Films 398-399 (2001) 110-115. [37] Z. Lodziana, N.Y. Topsoe, J.K. Norskov, A negative surface energy for alumina, Nat. Mater. 3 (2004) 289-293. [38] R. Kumar, D. Varandani, B.R. Mehta, V.N. Singh, Z. Wen, X. Feng, K. Muller, Fast response and recovery of hydrogen sensing in Pd-Pt nanoparticles-graphene composite layers, Nanotechnology 22 (2011) 275719. [39] F.A. Lewis, The palladium hydrogen system, Academic Press, London and New York 71 (1967) 1160-1161.
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[40] J.L. Gland, B.A. Sexton, G.B. Fisher, Oxygen interactions with the Pt (111) surface, Surf. Sci. 95 (1980) 587-602. [41] G.B. Fisher, J.L. Gland, The interaction of water with the Pt (111) surface, Surf. Sci. 94 (1980) 446-455. [42] K.M. Ogle, J.M. White, The low-temperature water formation reaction of Pt (111): A static SIMS and TDS study, Surf. Sci. 139 (1984) 43-62. [43] M. Johansson, L. Ekedahl, Hydrogen adsorbed on palladium during water formation studied with palladium membranes, Appl. Surf. Sci. 173 (2001) 122-133. [44] F. Yang, D. Taggart, R. Penner, Joule heating a palladium nanowire sensor for accelerated response and recovery to hydrogen gas, Small 6 (2010) 1422-1429. [45] F. Yang, K.C. Donavan, S.-C. Kung, R.M. Penner, The surface scattering-based detection of hydrogen in air using a platinum nanowire, Nano Lett. 12 (2012) 2924-2930. [46] M. Yamauchi, R. Ikeda, H. Kitagawa, M. Takata, Nanosize effects on hydrogen storage in palladium, J. Phys. Chem. C 112 (2008) 3294-3299. [47] N.V. Long, T. Duy Hien, T. Asaka, M. Ohtaki, M. Nogami, Synthesis and characterization of Pt-Pd alloy and core-shell bimetallic nanoparticles for direct methanol fuel cells (DMFCs): Enhanced electrocatalytic properties of well-shaped core-shell morphologies and nanostructures, Int. J. Hydrogen Energy 36 (2011) 8478-8491. [48] N.V. Long, M. Ohtaki, T.D. Hien, J. Randy, M. Nogami, A comparative study of Pt and PtPd core-shell nanocatalysts, Electrochim. Acta 56 (2011) 9133-9143. [49] F.A. Lewis, The Hydrogen Palladium System, Academic, London (1967). [50] P.F. Ruths, S. Ashok, S.J. Fonash, J.M. Ruths, A study of Pd/Si MIS Schottky barrier diode hydrogen detector IEEE Trans. Electron Devices 28 (1981) 1003–1009. [51] A.S.M.I. Uddin, U. Yaqoob, D.-T. Phan, G.-S. Chung, A novel flexible acetylene gas sensor based on PI/PTFE-supported Ag-loaded vertical ZnO nanorods array, Sens. Actuators B 222 (2016) 536-543.
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Biographies Kamrul Hassan received his B.Sc. Eng. from the school of Electrical and Electronics Engineering, Chittagong University of Engineering & Technology (CUET), Bangladesh, in 2012. He joined as a researcher in the department of refrigerator R&D in Walton, Bangladesh, in 2013. He is now working as an M.S candidature in the School of Electrical Engineering, University of Ulsan, Ulsan, South Korea. His research interests include simulation modeling of nanosensors, bimetal, metal/metal oxide; localized surface plasmon resonance (LSPR) based nanosensors. A.S.M. Iftekhar Uddin received his B.Sc. Eng. from the Faculty of Engineering, International Islamic University Chittagong, Chittagong, Bangladesh, in 2005 and M.E. from the School of Electrical Engineering, University of Ulsan, Ulsan, Republic of Korea, in 2015. He joined as lecturer in Sylhet International University, Sylhet, Bangladesh, in 2006 and promoted as Assistant professor in 2010. He is now working as a Ph.D candidature in the School of Electrical Engineering, University of Ulsan, Ulsan, Republic of Korea. His research interests include metal/metal oxide and graphene, localized surface plasmon resonance (LSPR), and piezoelectrictriboelectric based active flexible nanosensors. Gwiy-Sang Chung received his B.E. and M.E. Degrees in Electronic Engineering from Yeungman University, Kyongsan, South Korea, in 1983 and 1985, respectively, and his Ph.D. Degree from Toyohashi University of Technology, Toyohashi, Japan, in 1992. He joined the Electronics and Telecommunications Research Institute (ETRI), Daejon, South Korea, in 1992, where he worked on Si-on-insulator materials and devices. Moreover, he also worked as a visiting scholar at UC Berkeley and Stanford University, CA, USA, in 2004 and 2009, respectively. He is now a professor in the School of Electrical Engineering, University of Ulsan, Ulsan, South Korea. His research interests include Si, SiC, ZnO, AlN-M/NEMS, flexible self-powered wireless sensors nodes, energy harvesting, localized surface plasmon resonance (LSPR), and graphene/MoS2based composites. He is the author or co-author of more than 248 peer-reviewed journal articles.
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Figure captions Fig. 1. The water droplet contact angle images of glass, silicon, Al2O3, and Pt/Pd/Al2O3. Fig. 2. AFM images of the (a) Pt, (b) Pd, and (c) Pt/Pd bimetallic thin films. Fig. 3. Typical EDS spectrum and elemental mapping of the Pt/Pd thin film on the Al2O3 substrate. Fig. 4. XRD pattern of the Pt/Pd film on the Al2O3 substrate. Fig. 5. XPS spectra of Pt/Pd bimetal thin films with different Pt/Pd thickness ratios. XPS profiles for S2, S3, S4, and S5 samples: (a, c, e, and g) represent the Pd 3d and (b, d, f, and h) represent the Pt 4f doublets, respectively. Fig.6. Response variation of S3 to 10,000 ppm H2 versus operating temperature. Fig. 7. (a) Simulation model of the film in a closed chamber and the surface charge distribution for Pd, Pt, and Pt/Pd thin film. Color legend represents the charge distribution on the surface of the film. (b) Simulation result of the surface charge density as a function of Pt/Pd film thickness. (c) Experimental result on response magnitude versus Pt/Pd film thickness. Fig. 8. Response-recovery time characteristics of the Pt/Pd films to 10,000 ppm H2 at 150 oC. Fig. 9. (a) Real-time resistance variation of S3 for various H2 concentrations and (b) repeatability behavior with 10,000 ppm H2 at 150 oC. Fig 10. (a) Working stability and (b) reproducibility of the S3 sensor to 10,000 ppm H2 at 150 o
C. (c) Selectivity histogram of the S3 sensor with various test gases.
Fig. 11. Transient response of S3 at different humidity concentrations at 150 oC. Inset shows the enlarged portion of the curve within the 20,000-40,000 ppm range of H2 concentrations.
19
Figures
Fig. 1
20
Fig. 2
21
Fig. 3
22
Fig. 4
23
Fig. 5 24
Fig. 6
25
(a)
26
(b)
27
(c)
Fig. 7
28
Fig. 8
29
(a)
30
(b)
Fig. 9
31
(a)
32
(b)
33
(c) Fig.10
34
Fig. 11
35
Table 1.
Deposition parameters of the PLD system for fabricating the Pt/Pd ultra-thin
films. Sample
Pt/Pd ultra-thin film
Target
Pd (99.95%), Pt (99.98%)
Substrate
Al2O3
Substrate temperature (oC)
100
Target-substrate distance (cm)
3.5
Base pressure (Torr)
2×10-6
Laser energy per pulse (mj)
350
No. of shots
2500 (Pd), 3500 (Pt)
Table 2.
Summary of the as-fabricated H2 sensors under different testing conditions.
Thickness of Pt/Pd thin film (nm) (1/1)
(2/2)
(3/3)
(3/4)
(3/8)
150
150
150
150
2
13.5
9.1
5.5
Response/recovery time (sec)
9/6
4/5
12/13
16/19
Detection range (ppm)
100-40,000
10-40,000
10-40,000
100-10,000
RH 80
RH 90
RH 90
RH 85
Operating temperature (oC) Sensor response @ 10,000 ppm (%)
No response
Humidity effect Negligible change of performance up to (%) *All
the characterizations were carried out within 10,000 ppm H2 gas concentration.
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S0925-4005(16)30678-5 http://dx.doi.org/doi:10.1016/j.snb.2016.05.013 SNB 20161
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
2-1-2016 21-4-2016 5-5-2016
Please cite this article as: Kamrul Hassan, A.S.M Iftekhar Uddin, Gwiy-Sang Chung, Fast-response hydrogen sensors based on discrete Pt/Pd bimetallic ultra-thin films, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.05.013 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.
Fast-response hydrogen sensors based on discrete Pt/Pd bimetallic ultra-thin films
Kamrul Hassan, A. S. M. Iftekhar Uddin, Gwiy-Sang Chung*
School of Electrical Engineering, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 44610, Republic of Korea
* Corresponding author. Tel.: +82-52-259-1248; Fax: +82-52-259-1686 E-mail address: [email protected] (G.-S. Chung) URL:http://home2.ulsan.ac.kr/user/gschung
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Abstract This study presents the fast response-recovery characteristics of an ultra-thin discrete platinum/palladium (Pt/Pd) bimetallic film-based hydrogen (H2) sensor. Pt/Pd bimetallic nanoparticles (NPs) with a size of 2-11 nm were deposited on an alumina (Al2O3) substrate using a pulsed laser deposition (PLD) method. The metal loading and the composition of the Pt/Pd NPs were carefully controlled by varying the deposition conditions of the PLD system. The resultant discrete Pt/Pd bimetallic ultra-thin film was applied as a new H2 detection material, in which the fabricated Pt (3 nm)/Pd (3 nm) film showed noteworthy advantages, such as a large detection range of 10 to 40,000 ppm, high response magnitude, and fast response-recovery time. For a 10,000 ppm (1 vol.%) H2 concentration at 150 oC, a maximum sensor response of 13.56% and a response-recovery time of 4/5 sec were observed. The observed fast response-recovery time characteristics can be attributed to the enhanced hydrogen-induced changes in the work function of the Pt/Pd ultra-thin film. The fabricated sensor also exhibits good reproducibility and a negligible humidity effect within the entire detection range.
Keywords: Pt/Pd bimetal, ultra-thin film, hydrogen sensor, fast response-recovery time, PLD
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1. Introduction Hydrogen (H2) is a colorless, odorless, and tasteless gas, which can cause a safety hazard when mixed with air because of its large flammability range (4−75%) where high burning rates and explosive tendencies increases the risks [1]. At high concentration it can also act as an asphyxiant. In the chemical industry, H2 is used as a reactant, for the processing of ammonia, petrochemicals, and methanol. It is also used as an energy carrier, with remarkable applications in fuel cell technologies and rocket propulsion systems. With renewed emphasis being placed on the production of H2 through photo catalytic water splitting [2−4], the development of a hydrogen economy powered by sunlight is more reasonable. Moreover, H2 derived from renewable sources of clean energy can also act as a feedstock for reactions to convert CO2 into a useable fuel through catalytic hydrogenation [5, 6], which not only produces fuels with a greater energy density than H2, but also consumes a damaging greenhouse gas. If such an energy infrastructure is to develop then it will necessarily be monitored by a network of sensing devices, which can detect H2 levels where it is used, transported, and stored. If, in this situation, distributed H2 power generation systems become conventional, then such point-of-use consumption could finally make H2 sensors as essential as carbon monoxide and smoke detectors. Various types of H2 sensors have already been developed by observing the H2-induced changes in electrical [7-9], electrochemical [10, 11], thermal conductivity [12], surface plasmon resonances [13], surface acoustic waves [14], and mechanical [15] properties. Commercially available H2 sensors are mainly based on Pd thin films and are limited by their response time (tens of seconds) and physical sizes (inches) [16]. Because of their unique physical and chemical properties and their potential for improving sensor performance, nanostructured materials are in high demand for gas sensing applications due to in part to their miniaturized device dimensions, which bring about low power consumption. Numerous Pd nanostructures have been used in H2 sensors, including ultrathin films made of continuous [17] or discontinuous [18] nanocrystals, nanowires [19], and nanotubes [16], etc. Due to the kinetics of hydrogen absorption, the absorption energy, and the hydrogen diffusion coefficient of palladium (Pd), among the H2 gas sensors it generally exhibits one of the faster response-recovery times [20]. Along with this, the incorporation of platinum (Pt) promises to enhance the absorption of H2 gas molecules [21, 22].
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In addition, highly porous or truly nanoscale sensing materials can contribute to the enhancement of the response and recovery times of the sensor [23]. Alternatively, precisely controlled bimetallic composition and structure offers the possibility of combining the unique advantages of each counterpart, allowing enhanced catalytic activity, selectivity, and stability [24-27]. Supported Pt-Pd nanoparticles (NPs) are considered to be the most promising bimetallic heterogeneous catalysts in H2 sensing technology due to their extraordinary core-shell like nanostructure, which preferentially support individual sensing properties rather that alloying phenomenon. Importantly, the Pd/Pt core-shell boundary has been observed to bring about interesting physical and chemical properties that have importance for hydrogen storage [28]. Due to the modification of the structure and the electronic states, such as atomic arrangements and chemical potentials of the interfacial region, it can be expected that the hydrogen absorption properties can be quite different compared to the bulk counterpart nanoparticles. Unfortunately, the synthesis of uniform bimetallic nanoparticles with diameters below 5 nm is a challenging task for traditional catalyst synthesis methods, such as wet impregnation [29, 30] and colloidal chemistry [31, 32]. In the current work, we have fabricated uniform Pt/Pd bimetallic NPs in a discrete ultra-thin film manner on alumina (Al2O3) substrate using a pulsed laser deposition (PLD) technique. It was expected that the discrete Pt/Pd ultra-thin film would show possible enhancements due to the synergetic interplay between the bi-functional capping configuration, the large surface-to-volume ratio, and the quantum size of the nanoparticles. The as-fabricated structure was used as a resistivity-type sensor with the aim of developing a high performance H2 sensor with a fast response-recovery time.
2. Experiment 2.1. Ultra-thin film and sensor fabrication Pt/Pd ultra-thin film was deposited on the 2 × 2 cm2 Al2O3 substrate from the Pd (iTASCO, 99.95% purity) and Pt (iTASCO, 99.98% purity) targets using a PLD method. As in a typical PLD system, a krypton fluoride (KrF) excimer laser (Lambda Physik Compex Pro 201) was used to produce laser pulses of 5 ns duration at a wavelength of 248 nm with the energy of 0.35 J per pulse. The laser beam was focused on the target at an angle of 45o, whereas the target holder 4
rotates such that subsequent laser pulses hit the target at different positions. The detailed deposition parameters are listed in Table 1. Five different samples with different Pt/Pd bimetal thicknesses were fabricated by varying the number of laser shots in the PLD system and labeled as S1 = Pt (1 nm)/Pd (1 nm), S2 = Pt (2 nm)/Pd (2 nm), S3 = Pt (3 nm)/Pd (3 nm), S4 = Pt (3 nm)/Pd (4 nm), and S4 = Pt (3 nm)/Pd (8 nm), respectively. Finally, to fabricate a simple resistivity-type sensor, several pieces of Al2O3 substrate with the dimensions of 6 × 12 mm2 were cut from the base substrate and two contact electrodes of silver (Ag) were painted at a distance of 2 mm.
2.2. Ultra-thin film characterization The water droplet contact angles were measured by contact angle goniometry (Kruss DSA 100 Drop Shape Analyzer) using the sessile drop method at room temperature. Distilled deionized water droplets (about 3 µL) were dropped on the thin film surfaces using a microsyringe. The average value of the water contact angle was determined by experimental drop profiles at five different positions for the same sample. Surface morphology of the as deposited film and the film thickness were measured using atomic force microscopy (AFM, NT-MDT, Ntegra) in the semi-contact (tapping) mode. The compositional analyses of the as-prepared samples were examined with a JEOL JEM-2010F energy-dispersive spectrometer (EDS). Structural properties of the sensing materials were investigated using X-ray diffractometer (XRD, Rigaku Ultima IV) with Cu Kα (λ = 0.154 nm) radiation with a 2θ scanning range of 10-90o. X-ray photoelectron spectroscopy (XPS, Thermo Fisher K-Alpha) was performed using Al Kα radiation as the x-ray source.
2.3. Transport measurement Gas sensing measurements of the as-fabricated sensors were conducted in an in-house gas sensor assembly (chamber) at atmospheric pressure within a temperature range of 20-200oC, using the flow-through technique with and without various H2 concentrations. A Keithley probe station (SCS-4200) with a bias voltage fixed at 1V was used for all measurements and data acquisition. A programmable heater integrated with the sensor holder in the chamber was used to adjust the temperature. A computerized mass flow controller (ATO-VAC, GMC 1200) system was used to vary the concentration of H2 in synthetic air. The gas mixture was delivered to the 5
chamber at a constant flow rate of 50 sccm (standard cubic centimeters per minute) with different H2 concentrations. The gas concentration was controlled and measured using the following equation: 𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝐺𝑎𝑠𝑐𝑜𝑛. (𝑝𝑝𝑚) =
𝐹𝑙𝑜𝑤 𝑟𝑎𝑡𝑒𝑔𝑎𝑠 × 𝑆𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝐺𝑎𝑠𝑐𝑜𝑛. (𝑝𝑝𝑚) (𝐹𝑙𝑜𝑤 𝑟𝑎𝑡𝑒𝑔𝑎𝑠 + 𝐹𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑠𝑦𝑛𝑡ℎ𝑒𝑡𝑖𝑐 𝑎𝑖𝑟 )
The gas chamber was purged with synthetic air between each H2 pulse to allow the surface of the sensor to return to atmospheric conditions. The sensor response (S%) was calculated using (Rg-Ra )/Ra × 100, where Ra and Rg are respectively the resistances of the sensor in the presence of synthetic air and H2 at certain concentrations. The response-recovery time of the sensor was defined as the time to reach 90% of the total resistance change. The humidity level inside the chamber was monitored using a Testo 625 hygrometer. The relative humidity (RH) in the gas chamber was varied from 40-80% using a water bubbler controller.
3. Results and discussion 3.1. Ultra-thin film structure and morphology In order to determine the wettability on the surface of the Al2O3 substrate, characterization with water droplets was carried out. The contact angle was determined by using the following formula, cos 𝜃𝑤 = 𝛾𝑠𝑣 −
𝛾𝑠1 𝛾1𝑣
(1)
where, θw is water contact angle. The contact angle images of glass, Si, and Al2O3 substrate are shown in Fig. 1. The contact angle of the Pt/Pd/Al2O3 substrate was measured to be 142.4o, which was much higher than the contact angles of glass (21.3o), silicon (50.6o), and glass (103.4o) substrates. It is well known that if θw > 90o, the surface is hydrophobic, otherwise hydrophilic, which revealed that the examined Pt/Pd/Al2O3 substrate exhibits a highly hydrophobic property. Additionally, it has been reported in the literature that surface roughness is directly proportional to the contact angle and thereby inversely proportional to the surface energy. The surface roughness and the chemical composition are the main factors in determining the surface energy of thin films [33]. By using the Owens-Own method we calculate the surface free energy and summarized in supplementary information (Table S1). Thus, the contributions of 6
both factors reveal that the surface structure of the Al2O3 substrate leads to lower surface energy, hence higher hydrophobicity [34-37]. Besides, this lower surface energy made the grain size small and reduced the intergrain distance. Consequently, with deposition of the Pt/Pd thin film on the Al2O3 substrate, the Pt capped the Pd nanoparticles (NPs) to form discrete bimetal nanoparticles of high uniformity with almost equal distances within the grains. This situation leads to create shortest current path by contacting neighboring grains at different H2 concentration ranges. For this reason, after hydrogen absorption electron scattering was reduced and resistance change was fast. Moreover, the discrete ultra-thin film manner creates more rooms for the desorption of hydrogen gas within shorter period of time. The AFM images of the as-deposited samples confirm the deposition of very smooth, compact, and discrete Pt, Pd, and Pt-coated Pd thin films, in Fig. 2 (a, b, c) respectively. The appearance of all the films was partially mirror-like, in which the observed average grain size was found to be about 3 nm for both Pt and Pd films and about 6 nm for the Pt/Pd film. The root mean square (rms) surface roughness of these samples was observed to be nearly 0.095, 0.105, and 0.938 nm for Pt, Pd, and Pt/Pd films, respectively. From the figures, we can see that the NPs were deposited uniformly over the entire substrate with uniform distances between adjacent nanoparticles. The elemental composition of the as-deposited Pt/Pd bimetal thin film on the Al2O3 substrate was investigated by EDS and elemental mapping, as is shown in Fig. 3. A selected area of the EDS spectrum from Fig. 3(a) was taken, and the result is shown in Fig. 3(b). The presence of various well-defined peaks of aluminum (Al), oxygen (O), and Pt and the absence of peaks related to contaminations confirmed the formation of a high-purity sensing material. The low content or the absence of the Pd peak in Fig. 3(b) might be attributed to the capping and overlapping of Pt over Pd. To confirm the distribution of Pt and Pd atoms on the lattice surface, elemental mapping of the area shown in Fig. 3(a) was carried out, and the results are depicted in Fig. 3(c-f). The analysis shows that Al, O, Pt and Pd are distributed homogeneously over the whole substrate, which further confirmed the formation of a highly uniform Pt/Pd film. The XRD pattern of the Pt/Pd thin film is shown in Fig. 4. A broad peak centered at 2θ = 34.94o can be indexed to the (104) plane of α-Al2O3. In addition, the diffraction peaks observed at 2θ values of 39.79o, 46.22o, and 67.7o correspond to the face-centered cubic (fcc) structure of Pt or Pd (111), (200), and (220) (Pt: JCPDS card no. 04-0802; Pd: JCPDS card no. 7
46-1043) [38]. The estimated crystallite size of the bimetal nanoparticles was calculated from the dominant (111) peak using Scherrer’s formula and was found to be about 5.6 nm. The composition of Pt:Pd was calculated from Vegard’s law and found to be 58:42. XPS analysis was carried out to examine the surface composition of the Pt/Pd bimetal with the Pt/Pd thickness ratios of 2/2, 3/3, 3/4, and 3/8 (samples S2-S5), respectively. Four deconvoluted peaks were considered for the fitting of Pt (Pt 4f5/2 and Pt 4f7/2) and Pd (Pd 3d3/2 and 3d5/2) peaks as shown in Fig. 5. The observed peaks with relatively higher intensity correspond to the predominant species of the metallic Pd (Fig. 5(a, c, e, and g) for the respective thickness ratios) and the metallic Pt (Fig. 5(b, d, f, and h) for the respective thickness ratios). Pd2+ and Pt2+ correspond to the ionization state of Pd and Pt atoms. With the increasing Pt/Pd ratio a slight shift in the Pd 3d and Pt 4f to a higher energy level was observed, which might be attributed to the thicker layer of Pt, the induced strain on the Pd and the electronic interactions between the Pt shell and Pd core. This phenomenon demonstrates the Pt/Pd core-shell arrangement. However, the absence of Pd-O and Pt-O indicates the high-purity physical deposition of bimetallic Pt/Pd nanostructures and the effective assembly of the Pt/Pd bimetal onto the surface of the Al2O3 substrate.
3.2 Gas sensor studies In the presence of synthetic air, H2 physisorbs on the Pd surface, dissociates, chemisorbs, and subsequently forms the Pd-hydride (Pd-H) as follows, 𝐻2 (𝑔) → 𝐻2 (𝑎𝑑𝑠)
(2)
1
𝑃𝑑 + 2𝐻2 (𝑎𝑑𝑠) → 𝑃𝑑-𝐻
(3)
The electrical resistivity of PdH0.7 is almost a factor of 2 higher than that of Pd, which allows the detection of H2 due to the increase in the resistance of the Pd resistor [39]. In an air environment, the limit of detection of H2 (𝐿𝑂𝐷𝐻2 ) can be increased due to the presence of oxygen that enables the formation of catalytic water molecules at the Pd surface, thereby reducing the steady state surface coverage of the chemisorbed H2 that is available to be absorbed into the bulk of the PdHx (reaction 3) [40-42]. 3 𝑃𝑑-𝑂 + 𝐻2 (𝑎𝑑𝑠) → 𝑃𝑑-𝐻 + 𝐻2 𝑂(𝑎𝑑𝑠) 2
(4)
𝐻2 𝑂(𝑎𝑑𝑠) → 𝐻2 𝑂(𝑔)
(5) 8
Chemisorbed oxygen (reaction 4) also blocks the Pd adsorption sites, impeding the H2 adsorption and extending the time required for equilibration of H2 in the gas phase with the PdHx. The strong influence of oxygen on the sensing behavior of Pd reveals that the surface chemistry is a critical factor in determining the performance of the resistance-based H2 gas sensors. Johansson et al. [43] have shown that in dry air at T = 100oC, Pt is a better catalyst than Pd for reaction 4. Pt is also a superior catalyst for reaction 3, which is the rate-limiting reaction for sensor response [44]. Yang et al. synthesized Pt nanowires and evaluated their H2 sensing performance, which revealed better sensing performance compared to sensors with Pd nanowires and sensing performance that improved at higher temperatures [45]. Therefore, operating temperature is one possible way for accelerating the sensing performance of Pd thin film [44]. In addition, the fabrication of Pt/Pd bimetallic ultra-thin films has the potential of favorably altering the surface chemistry of the Pd thin film. The operating temperature has a significant influence on the fundamental sensing mechanism of bimetal based gas sensors. In order to determine the optimum working temperature, the fabricated sensors were investigated over a temperature range of 20-200oC. Fig. 6 depicts the relationship between the response magnitudes of the sensors and the operating temperature, measured with a 10,000 ppm H2 gas concentration. The tested samples showed no response at lower temperature ( 100oC). It is well established that hydride formation of Pt is relatively faster than that of Pd under identical conditions of 𝑃𝐻2 and T ≥ 100 °C due to the rapid water formation rate on Pt than Pd. Besides, at T < 100 oC, stable bulk hydride phase does not exist on Pt, hence the transduction mechanism for the detection of H2 that operates at Pd will not exist at Pt. Heating of the samples is a possible mean that not only enhance the response of the sensor, but also it accelerates the sensor’s response time [44]. However, with the increase of Pt shell thickness, baseline resistance of the as-fabricated samples shifted to lower values (see Fig. S1), which impeded the sensitivity of the sensor. With increasing temperature, the response values gradually increased, reached to maximum values at 150 oC, and then decreased with further increase in operating temperature. Therefore, 150 oC was selected as the optimum working temperature of the Pt/Pd bimetallic ultra-thin film sensor, and was used for further sensing characteristics examinations. When the Pd surface was exposed to an H2 gas environment, the Pd phase changed to Pd hydride and expanded in volume, which significantly reduced the nano-gap between the particles 9
and the resistance between the electrodes. A Pt cap with good H2 dissociation allows H2 to pass through and interact with the Pd in the core, leading to a volume expansion of the Pt/Pd bimetal NPs. Moreover, the synergistic effect between the Pd and Pt enhanced the catalytic activity of the Pt/Pd bimetal, which preferentially facilitated the absorption of H2 molecules at the interface of the bimetal boundary of Pt/Pd nanoparticles and played an important role in enhancing the H2 sensing properties [28, 46-48]. In this work, we assessed the influence of an ultra-thin Pt film on a Pd ultra-thin film with regard to the hydrogen sensing performance. It was found that the Pt layer accelerated both the response and recovery time characteristics of the as-fabricated sensor. Importantly, Pt increased the 𝐿𝑂𝐷𝐻2 at 150 oC and enabled the simultaneous acceleration of response-recovery with virtually no degradation of the sensitivity to H2. In order to realize the thickness effect of ultra-thin Pt/Pd film on H2 sensing characteristics, theoretical analysis of the as-fabricated sensors was carried out using Comsol Multiphysics electrostatics simulation software. Fig. 7(a) illustrates the distribution of electrical permittivity in the film (electric impedance tomography), which reveals that at a certain applied potential difference on the boundaries of the chamber creates a surface charge density depending on the permittivity distribution inside the chamber. As seen from the color legend in Fig. 7(a), the surface charge density of the Pt/Pd bimetallic thin film is comparatively higher than the bare Pt and Pd films due to higher relative permittivity of the Pt/Pd bimetal (Ɛr = 67.38) in comparison to bare Pt (Ɛr = 17.19) and Pd (Ɛr = 13.698) films. Fig. 7(b) shows the relationships of surface charge density with the Pt/Pd film thickness at an operating temperature of 150 oC. It was observed that surface charge density of the Pt/Pd bimetal thin film with a thickness of 3/3 nm exhibited the maximum value of 0.65 µC/m2 in air medium compared to the other films, while in H2 medium this value was the minimum (0.18 µC/m2). This phenomenon reveals that the uniform organization of the Pt/Pd bimetal with 3/3 nm thickness provides the maximum amount of dissociations-chemisorptions of the H2 molecules and hydride formation, hence effectively increases the electrical resistance of the film surface. To relate this simulated result to the realtime phenomenon, the as-fabricated sensors were exposed to a certain concentration of H2. Fig. 7(c) shows the variation of response magnitude of the sensors to 10,000 ppm H2 at 150 oC with respect to film thickness. A maximum response of 13.56% was obtained for S3 sample (3/3 nm), which was much higher than that of S2 (2%), S4 (9.1%), and S5 (5.5%) samples. However,
10
sample S1 did not show any response, which might be attributed to the large gaps between the nanoparticles where charge transfer paths were not formed. Fig. 8 shows the real-time resistance variation of the as-fabricated sensors. It can be seen that the S3 sample had a faster response-recovery time (4/5 sec) than the S2, S4, and S5 samples. Fig. S1 (I-V characteristics curve) shows that the baseline resistance of the sensors (S2-S5) drifts to lower values with the increase of Pt shell thickness. The Pt/Pd (3/3 nm) film thickness indicates a comparatively higher performance due to the synergies between the hydrogen gas species dissociated by if not one then the other of the Pt and Pd catalysts. More importantly, physisorption and chemisorption process is vital within H2 and metal before hydrogen molecules interaction into the metal lattice. During chemisorption process, H2 molecules get incorporated into the crystal structure of metal and form metal hydride (MHx), while the physisorbed molecules interact with the interstitial sites of the Pt/Pd NPs [49]. These sites can act as electron scattering centers and result decrease in carrier mobility. Both processes together cause a hydrogen-induced increase in electrical resistance of the Pt/Pd ultra-thin films, which is normally termed as an electronic effect during sensing. Moreover, formation of a β-MH phase in the tetragonal sites can results in an increased lattice constant. The formation of MHx also causes a decrease in the work function, which ultimately changes the electronic properties of the bimetallic nanoparticles [50]. Therefore, the Pt capping of the Pd plays the role of a trade-off factor between the sensor characteristics of response and recovery rate. Detailed comparisons among the tested samples are summarized in Table 2. Fig. 9(a) shows the dynamic resistance variation of S3 with various H2 concentrations at 150 o
C. The sensor exhibited increased resistance after exposure to H2 gas and a clear response from
10 to 40,000 ppm H2. Fig. 9(b) illustrates the repeatability of the S3 sensor with a constant response value of 13.56% at a concentration of 10,000 ppm H2 at 150 oC. The as fabricated sensor was kept during a period of twenty two weeks and the functionality of the H2 sensor towards 1000 ppm and 10,000 ppm of H2 gas at 150 oC was checked in every two weeks. Fig. 10 (a) reveals that the H2 sensor exhibits good working stability during over five months of operation, which can be attributed to the high purity sensing layer formation (less possibility of occurring contaminations) while using PLD system. Moreover, in ambient condition (for example, T ≤ 100 oC), Pt-O is unavailable. In addition, the as-fabricated sensors are highly hydrophobic in nature. Due to these reasons, environmental effect is almost negligible 11
on the sensing surface, which ultimately offers good stability to the sensor. In order to assess the reproducibility behavior of our sensor, we investigated the sensing characteristics of five individual sensors and the results are summarized in Fig. 10 (b). Similar outcomes are observed for all the tested sensors, which reveal excellent reproducibility characteristics of the asfabricated sensors. This phenomenon might be attributed to the controlled and high purity deposition environment during sensor fabrication using PLD system. Fig. 10 (c) represents the selectivity property of the S3 sensor under exposure to 1,000 ppm test gases, including hydrogen, oxygen, carbon mono-oxide, carbon dioxide, nitrogen, and 100 ppm nitrogen dioxide. The asfabricated sensor showed excellent selectivity towards H2 due to high selective absorption capability of Pd and Pt catalysts with H2 molecules. At a preeminent temperature of 150 oC, catalytic activity of the S3 sensor toward H2 decomposition might be strongly facilitated and might be an exothermic process, in which the sufficient surface energy of the sensor surface satisfactorily supported to be reacted with the bond energy of H2 compared to other test gases [51]. In real-world applications, the humidity effect is a great concern in the performance evaluation of the sensor, as the adsorption of water molecules decelerates the sensor response significantly. In order to get rid of this degradation phenomenon we have used hydrophobic Al2O3 substrate and a thermally-activated Pt/Pd bimetallic thin film. Fig. 11 shows the humidity effect on the as-prepared bimetallic film. However, at 150 oC temperature no significant degradation in response magnitude was observed with the relative humidity (RH) up to 80%, which suggests that we have indeed fabricated a high-performance sensor.
4. Conclusions This study demonstrated the potential of using discrete Pt/Pd bimetallic ultra-thin films on Al2O3 substrates for H2 sensing. The fast sensing and recovery of our Pt/Pd ultra-thin films could be attributed to the bi-functional capping configuration, the large surface-to-volume ratio, and the nanoscale aspect of surface features. In addition, the thickness of the Pt/Pd layers plays an important role in sensor performance. The discrete Pt/Pd (3/3 nm/nm) ultra-thin film showed enhanced performance that included a response value of 13.56% to 10,000 ppm of H2 at 150 oC, a broad detection range of 10-40,000 ppm, and fast response-recovery times of 4/5 sec (to 1% 12
H2). The fabricated sensor also exhibits good reliability and a negligible humidity effect within the entire detection range with operation at 150 oC. Our results may also provide valuable guidelines for designing high performance catalysts for fuel cell applications.
Acknowledgements This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded in 2014 by the Ministry of Science, ICT, and Future Planning (NRF-2014R1A2A2A01002668 and the MOTIE (Ministry of Trade, Industry and Energy), Korea, under the Eco-friendly Fuel Cell Test-bed program supervised by the KEA (Korea Energy Agency).
Conflict of interest The authors declare no competing financial interest.
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Biographies Kamrul Hassan received his B.Sc. Eng. from the school of Electrical and Electronics Engineering, Chittagong University of Engineering & Technology (CUET), Bangladesh, in 2012. He joined as a researcher in the department of refrigerator R&D in Walton, Bangladesh, in 2013. He is now working as an M.S candidature in the School of Electrical Engineering, University of Ulsan, Ulsan, South Korea. His research interests include simulation modeling of nanosensors, bimetal, metal/metal oxide; localized surface plasmon resonance (LSPR) based nanosensors. A.S.M. Iftekhar Uddin received his B.Sc. Eng. from the Faculty of Engineering, International Islamic University Chittagong, Chittagong, Bangladesh, in 2005 and M.E. from the School of Electrical Engineering, University of Ulsan, Ulsan, Republic of Korea, in 2015. He joined as lecturer in Sylhet International University, Sylhet, Bangladesh, in 2006 and promoted as Assistant professor in 2010. He is now working as a Ph.D candidature in the School of Electrical Engineering, University of Ulsan, Ulsan, Republic of Korea. His research interests include metal/metal oxide and graphene, localized surface plasmon resonance (LSPR), and piezoelectrictriboelectric based active flexible nanosensors. Gwiy-Sang Chung received his B.E. and M.E. Degrees in Electronic Engineering from Yeungman University, Kyongsan, South Korea, in 1983 and 1985, respectively, and his Ph.D. Degree from Toyohashi University of Technology, Toyohashi, Japan, in 1992. He joined the Electronics and Telecommunications Research Institute (ETRI), Daejon, South Korea, in 1992, where he worked on Si-on-insulator materials and devices. Moreover, he also worked as a visiting scholar at UC Berkeley and Stanford University, CA, USA, in 2004 and 2009, respectively. He is now a professor in the School of Electrical Engineering, University of Ulsan, Ulsan, South Korea. His research interests include Si, SiC, ZnO, AlN-M/NEMS, flexible self-powered wireless sensors nodes, energy harvesting, localized surface plasmon resonance (LSPR), and graphene/MoS2based composites. He is the author or co-author of more than 248 peer-reviewed journal articles.
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Figure captions Fig. 1. The water droplet contact angle images of glass, silicon, Al2O3, and Pt/Pd/Al2O3. Fig. 2. AFM images of the (a) Pt, (b) Pd, and (c) Pt/Pd bimetallic thin films. Fig. 3. Typical EDS spectrum and elemental mapping of the Pt/Pd thin film on the Al2O3 substrate. Fig. 4. XRD pattern of the Pt/Pd film on the Al2O3 substrate. Fig. 5. XPS spectra of Pt/Pd bimetal thin films with different Pt/Pd thickness ratios. XPS profiles for S2, S3, S4, and S5 samples: (a, c, e, and g) represent the Pd 3d and (b, d, f, and h) represent the Pt 4f doublets, respectively. Fig.6. Response variation of S3 to 10,000 ppm H2 versus operating temperature. Fig. 7. (a) Simulation model of the film in a closed chamber and the surface charge distribution for Pd, Pt, and Pt/Pd thin film. Color legend represents the charge distribution on the surface of the film. (b) Simulation result of the surface charge density as a function of Pt/Pd film thickness. (c) Experimental result on response magnitude versus Pt/Pd film thickness. Fig. 8. Response-recovery time characteristics of the Pt/Pd films to 10,000 ppm H2 at 150 oC. Fig. 9. (a) Real-time resistance variation of S3 for various H2 concentrations and (b) repeatability behavior with 10,000 ppm H2 at 150 oC. Fig 10. (a) Working stability and (b) reproducibility of the S3 sensor to 10,000 ppm H2 at 150 o
C. (c) Selectivity histogram of the S3 sensor with various test gases.
Fig. 11. Transient response of S3 at different humidity concentrations at 150 oC. Inset shows the enlarged portion of the curve within the 20,000-40,000 ppm range of H2 concentrations.
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28
Fig. 8
29
(a)
30
(b)
Fig. 9
31
(a)
32
(b)
33
(c) Fig.10
34
Fig. 11
35
Table 1.
Deposition parameters of the PLD system for fabricating the Pt/Pd ultra-thin
films. Sample
Pt/Pd ultra-thin film
Target
Pd (99.95%), Pt (99.98%)
Substrate
Al2O3
Substrate temperature (oC)
100
Target-substrate distance (cm)
3.5
Base pressure (Torr)
2×10-6
Laser energy per pulse (mj)
350
No. of shots
2500 (Pd), 3500 (Pt)
Table 2.
Summary of the as-fabricated H2 sensors under different testing conditions.
Thickness of Pt/Pd thin film (nm) (1/1)
(2/2)
(3/3)
(3/4)
(3/8)
150
150
150
150
2
13.5
9.1
5.5
Response/recovery time (sec)
9/6
4/5
12/13
16/19
Detection range (ppm)
100-40,000
10-40,000
10-40,000
100-10,000
RH 80
RH 90
RH 90
RH 85
Operating temperature (oC) Sensor response @ 10,000 ppm (%)
No response
Humidity effect Negligible change of performance up to (%) *All
the characterizations were carried out within 10,000 ppm H2 gas concentration.
36