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Response Characteristics of Hydrogen Sensors Based on PMMA-Membrane-Coated Palladium Nanoparticle Films Minrui Chen, Peng Mao, Yuyuan Qin, Jue Wang, Bo Xie, Xiuzhang Wang, Deyan Han, Guo-hong Wang, Fengqi Song, Min Han, Jun-Ming Liu, and Guanghou Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07641 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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ACS Applied Materials & Interfaces

Response Characteristics of Hydrogen Sensors Based on PMMA-Membrane-Coated Palladium Nanoparticle Films Minrui Chen,†,‡ Peng Mao, ∥ , ⊥ Yuyuan Qin,‡ Jue Wang,‡ Bo Xie,*,† Xiuzhang Wang,† Deyan Han,§ Guo-hong Wang,§ Fengqi Song,‡ Min Han,‡ Jun-Ming Liu‡ and Guanghou Wang‡ †

Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, PR China

‡

National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR

China §

Hubei Collaborative Innovation Center for Rare Metal Chemistry, Hubei Normal University,

Huangshi 435002, PR China ∥

Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of

Birmingham, Birmingham B15 2TT, UK ⊥

College of Electronic Science and Engineering, Nanjing University of Posts and

Telecommunications, Nanjing 210023, PR China *

Corresponding author; E-mail: [email protected]

KEYWORDS: Hydrogen sensors, PMMA membrane layer, Pd nanoparticle films, H2-sensing kinetics, Electrical transport mechanism

ACS Paragon Plus Environment

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ABSTRACT: Coating polymeric membrane for gas separation is a feasible approach to fabricate gas sensors with selectivity. In this study, poly(methyl methacrylate) (PMMA) membrane-coated palladium (Pd) nanoparticle (NP) films were fabricated for high-performance hydrogen (H2) gas sensing by carrying out gas phase cluster deposition and PMMA spin coating. No changes were induced by the PMMA spin coating in the electrical transport and H2 sensing mechanisms of the Pd NP films. Measurements of H2 sensing demonstrated that the devices were capable of detecting H2 gas within the concentration range of 0‒10% at room temperature and showed high selectivity to H2 due to the filtration effect of the PMMA membrane layer. Despite the presence of the PMMA matrix, the lower detection limit of the sensor is less than 50 ppm. A series of PMMA membrane layers with different thicknesses were spin coated onto the surface of Pd NP films for the selective filtration of H2. It was found that the device sensing kinetics were strongly affected by the thickness of the PMMA layer, with the devices with thicker PMMA membrane layers showing a slower response to H2 gas. Three mechanisms slowing down the sensing kinetics of the devices were demonstrated to be present: diffusion of H2 gas in the PMMA matrix, nucleation and growth of the β phase in the α phase matrix of Pd hydride, and stress relaxation at the interface between Pd NPs and the PMMA matrix. The retardation effect caused by these three mechanisms on the sensing kinetics relied on the phase region of Pd hydride during the sensing reaction. Two simple strategies, minimizing the thickness of the PMMA membrane layer and reducing the size of the Pd NPs, were proposed to compensate for retardation of the sensing response.


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Introduction Hydrogen gas (H2), as the most important new alternative energy sources, and one of the cleanest, has attracted considerable attention because of its high energy density, low cost, and renewability. However, H2 is highly flammable and explosive when its concentration exceeds the lower explosion limit in air of 4% at room temperature. Developing a reliable, accurate, fast, and wide-range hydrogen gas leakage detector is very important for hydrogen generation, storage, and utilization. Many types of H2 sensor have been reported, including sensors based on metal oxides,1‒2 acoustic waves,3‒5 and metal and semiconductor thin films.6‒8 However, these sensors have a number of significant drawbacks, notably their response time, selectivity, sensitivity, and repeatability, that make them unsuitable for many applications. Recently, palladium (Pd) nanostructures have been used to fabricate high-performance H2 sensors,9‒18 such as the sensor devices we developed through the gas phase deposition of Pd nanoparticle (NP) films with controlled coverage on gold interdigital electrodes,9, 19‒21 and the H2 sensor based on Pd nanowire arrays.13‒14, 22‒28 In these devices, the sensing mechanism is based on the tunneling or/and hopping of electrons between the nanogaps which are closed by hydrogen-induced lattice expansion (HILE) during H2 exposure.22 Changes in the gap size owing to lattice expansion or dwindling of Pd during H2 absorption or desorption result in changes in the electron barrier, leading to a significant and measurable change in the conductance of the nanostructures.22 Although Pd-nanostructure-based H2 sensors exhibit excellent sensing performance at room temperature, cross-sensitivity with carbon monoxide (CO), water vapor, and methane (CH4) remains a critical issue. More importantly, among the many cross-sensitive gases, CO is highly


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toxic to the Pd catalyst. As a consequence, the H2-sensing ability of Pd is destroyed by CO, which is attributed mainly to a reduction in the available dissociation sites on the Pd metal surface caused by CO adsorption.29‒32 It is fortunate that developments in H2 separation technologies based on polymeric membranes hold promise for smaller, cheaper, and more feasible H2 sensors with high selectivity.33‒36 Recent reports have shown that polymeric membranes could act as a gas-selective polymer barrier in H2 gas sensors, enabling them to absorb and release H2 without cross-sensitivity and toxicity.33,

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In this regard, Lee et al.

reported the first demonstration of a H2 sensor based on polymer membrane-coated Pd NP/single-layer graphene (SLG) hybrid structure achieved high selectivity toward H2,37 but the understanding of the sensing kinetics of the H2 sensors is still quite limited. In the case of the nanostructured HILE based H2 sensors, further investigations are required in order to confirm whether the electrical transport and sensing mechanisms have been changed or not due to the encapsulation of the PMMA matrix before the PMMA membrane layer would be used as a selective gas separator. Also, the fast response speed of the nanostructured HILE based H2 sensors may be lost due to the barrier effect of selective filtration membrane. Based on the fact that rapid response is a major challenge in the development of H2 sensors, the effect of the thickness of polymeric membranes on the H2-sensing kinetics, still require close examination in order to establish the feasibility of the use of this method for the fabrication of nanostructured HILE based H2 sensors with high selectivity and fast response. In this report, Pd NP films were fabricated by the gas phase deposition of Pd nanoclusters with controlled coverage on gold interdigital electrodes.20 A thin layer of poly(methyl methacrylate) (PMMA) was spin coated onto a Pd NP film, to fabricate a high-performance H2 sensor with gas selectivity. The electrical transport and H2-sensing mechanisms of the Pd NP films before and


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after PMMA coating were investigated. Further, the sensing behavior of this device was examined, focusing on the kinetic characteristics of the Pd NP film coated by the PMMA membrane when responding to H2. The sensors with the PMMA membrane layers clearly exhibited prolonged response time which is strongly dependent on not only the thickness of the PMMA membrane layer but also the phase region of Pd hydride during the sensing reaction. Through the analysis of experimental data, three mechanisms slowing down the sensing kinetics of the devices were proposed. Meanwhile, two simple strategies, minimizing the thickness of the PMMA membrane layer and reducing the size of the Pd NPs, were suggested to compensate for the prolonged response time in consideration of the retardation effect caused by the PMMA matrix.

Experiment Device fabrication Figure 1 is a schematic illustration of the preparation of the sensor devices. The Pd NPs were deposited on prefabricated gold interdigital electrodes by a clusters beam deposition system. The gold interdigital electrodes, 80 nm thick and with 4 µm electrode separation, were patterned onto a silicon substrate with a 300 nm silicon dioxide (SiO2) insulating surface layer using a standard lithographic lift-off procedure. Pd NPs were formed in a high-vacuum chamber by a magnetron plasma gas aggregation cluster source. Argon (100 sccm) was introduced into the aggregation tube to maintain a stable pressure of 120 Pa. A stable magnetron discharge was carried out with an input power of 47 W. Atoms were sputtered from the Pd target, and Pd nanoclusters formed through the aggregation process in the argon gas. During deposition, the current across the electrode gap was measured in real time with a source meter (Keithley 2601B) by applying a bias


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Figure 1. Schematic illustration of the procedure used to fabricate the hydrogen sensor based on a PMMA-membrane-coated Pd NP film. (a) The deposition of Pd NPs. (b) The Pd NP film formed on the interdigital electrode substrate after the NP deposition. (c) The PMMA-membrane-coated Pd NP film fabricated by spin coating. (d) The structure of the hydrogen sensor based on the PMMA-membranecoated Pd NP film.

of 1 V, as shown in Figure 1a. The Pd NP film was formed after the deposition, which was stopped when the predetermined conductance values defined as the initial conductance G0 were attained (Figure 1b). For the PMMA coating, 4 g of PMMA powder from Alfa Aesar with molar mass 400,000 to 550,000 g mol−1, manufacturer data, was dispersed in 100 ml of Anisole. The mixture, agitated by a magnetic stirrer, was heated using a 60 °C water bath for 6 h to accelerate the dissolution of the PMMA. The mixture was then coated on the surface of the sensor devices (pre-deposited Pd NP films) by spin coating (Spin Coater, KW-4A, Institute of Microelectrons of the Chinese Academy of Sciences) for 30 s. A PMMA membrane layer was formed on the surface of the Pd


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NP film deposited on the interdigital electrode substrate after drying in air for 24 h, as shown in Figure 1c and d. The thickness of the PMMA membrane layer could be varied by adjustment of the spin velocity. Structure characterization The scanning transmission electron microscopy (STEM) investigation was performed using a JEOL instrument (JEM2100F) with a spherical-aberration corrector (CEOS GmbH). The images were acquired using high angle annular dark field (HAADF) and bright field (BF) detectors. To facilitate STEM observation, Pd NPs were deposited on SiO2 films supported by copper grids at the same time as sensor device fabrication. The thickness of the PMMA membrane layer was measured by scanning edge sections using atomic force microscopy (AFM; 175 NTEGRA Probe NanoLaboratory, NT-MDT Co.). Electron transport measurement The investigation of electron transport property of Pd NP films was carried out by measuring temperature-dependence on resistance. Temperature-dependent DC resistance was measured using a closed cycle cryostat (Janis CCS450) with a temperature controller (Cryogenic Model 32B) over the range 20–300 K. The resistance was monitored using a source meter (Keithley 2601B). Gas sensing tests The H2-sensing response of the fabricated sensors was studied at room temperature in a gas flow chamber fabricated in our laboratory. The tested target gases included pure H2 and mixtures of CO/nitrogen (N2), CH4/N2, H2/N2 and H2/air. For pure H2, since the sensor responds to the H2


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partial pressure only, it is easy to convert the data measured for pure H2 gas to an H2 concentration response for a gas mixture with a minor deviation by assuming a 1 atm ambient pressure. The flow of the target gases to the test chamber was regulated by a mass flow controller (Sevenstar D07-7A/ZM). The pressure of pure H2 gas (PH2) in the test chamber was monitored by a piezo-resistive gauge. During H2-sensing measurement, the current across the electrode gap was measured in real time with a source meter by applying a bias of 1 V. The relative change in conductance is defined as the sensing response S:

S = ∆G G0 = (G − G0 ) G0

(1)

where G is the conductance of the sensor exposed to the target gas and G0 is the initial conductance of the Pd NP film without target gas loading. A rapid loading of H2 is necessary for measuring the response time of the sensors. As shown in Figure S1 in the Supporting Information, the samples were therefore sealed inside a small measurement cell that was connected to a much larger chamber. The capacity of the large chamber and small measurement cell is about 22.4 L and 9.6 mL, respectively. For each H2 pulse measurement, firstly, keeping the electromagnetic valve open, the large chamber and small measurement cell were pumped until the vacuum is below 1 Pa. Next, the electromagnetic valve was closed and then the large chamber was filled with H2 to a predetermined pressure. An H2 pulse could be formed and entered the small measurement cell as the electromagnetic valve was rapidly opened. Because of the great difference in volume between the large chamber and measurement cell, the pressure changes in the large chamber are negligible during the formation of H2 pulse. Simultaneously, both PH2 and the conductance of the Pd NP films were recorded.


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The response time is defined as the time required to reach a conductance change (∆G) of 90% at a given H2 pulse pressure.

Results and Discussion The structural characterization of a typical Pd NP film sample is shown by the HAADF-STEM images in Figure 2a. As seen, Pd NPs are randomly distributed in the substrate and form numerous closely spaced particle-assembling areas. These randomly distributed Pd NPs in the substrate resulted in the random formation of a large number of nanogaps, which constitute numerous quantum transport39 pathways based on percolation.40 The charge transport in our

Figure 2. (a) Typical HAADF-STEM image of Pd NP films. (b) Typical HAADF-STEM image (upper left) and BF-STEM image (lower right) of a single Pd NP. The inset shows the fringes of the Pd NP. (c) SAED pattern. (d) HAADF-STEM image of Pd NP film after PMMA coating. (e) HAADF-STEM image of few closely spaced Pd NPs after PMMA coating. (f) Size distribution of Pd NPs. The red line is the log-normal fitting of the distribution.


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sensor occurs through tunneling or/and hopping between the Pd NPs. The exponential dependence of the tunneling or/and hopping probability41 on the size of the nanogaps allows the potential development of the high-performance sensor.22 Figure 2b shows the HAADF-STEM and BF-STEM images of a single Pd NP. The lattice fringes have an interplanar spacing of 0.226 nm, corresponding to the (111) planes of the face-centered cubic (FCC) structure of metallic Pd, which clearly reveals the crystalline nature of the Pd NPs synthesized in the present study. According to the selected-area electron diffraction (SAED) pattern (Figure 2c), it can also be deduced that the Pd NPs were in a crystalline state. The main diffraction rings can be assigned to the Pd FCC phase corresponding to the (111), (200), (220), and (311) crystal planes. The morphology of the Pd NP film after PMMA coating is showed in Figure 2d. Form the comparison between Figure 2a and d, the Pd NPs exhibited the same organization before and after PMMA coating, indicating that PMMA has no obvious effect on the surface morphology of the Pd NP film. Figure 2e shows the details of few closely spaced Pd NPs embedded in the PMMA matrix. This image ensures that the nanogaps between the closely spaced Pd NPs were not visibly disturbed. As mentioned above, the fabrication of Pd NPs was carried out by clusters beam deposition system. This formation process of nanoclusters involves a gas expanding from a high pressure source into vacuum, which makes the NPs deposited onto substrate at high flying speed. As a result, the NPs are adsorbed on the substrate with high adhesive force. The maintaining of the film morphology after PMMA coating, which can be attributed to the adhesive force between NPs and substrate, make it possible for this nanostructure to act as sensing devices. We statistically analyzed the size of the Pd NPs, and the distribution fitting a log-normal dependence is presented in Figure 2f with a probable size of 10 nm.


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Figure 3. The resistance of the Pd NP film before and after PMMA coating plotted against T–1/4. The solid curves fit the experimental data with R ∝ exp[(TM/T)1/4].

We measured temperature-dependent resistance and H2-sensing response of a typical Pd NP film before and after PMMA coating to check whether the electrical transport mechanism and sensing behavior remain unchanged after PMMA coating. The thickness of PMMA membrane layer is about 0.45 µm. Figure 3 presents the temperature-dependence of resistance for the typical Pd NP film before and after PMMA coating. We can clearly see that the resistance R of the Pd NP film both before and after PMMA coating increase monotonically with decreasing temperature T over the whole measurement temperature range of 20–300 K, indicating that the Pd NP film exhibited the electrical transport phenomenon similar to semiconductors with a negative temperature coefficient. By plotting R (on log scale) VS T−1/4, we can see that the R-T curve is well described by the Mott variable-range hopping (VRH) mechanism42 because of the linear relationship between logR and T−1/4 as shown in Figure 3. Mott VRH model gave a temperature dependence conduction relationship42: R = RM exp[(TM / T )1/4 ]

(2)


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where RM is the resistance parameter, and TM is the Mott characteristic temperature whose value depends on the electronic density of states at the Fermi level and the localization length. The values of TM of the Pd NP film before and after PMMA coating are 2.57 × 104 and 1.33 × 104, respectively. In addition, the resistance of the Pd NP film increased after PMMA coating. A detailed study of the physical mechanism that result in the value change of R and TM by PMMA coating needs further investigation, but is beyond the aims of this study. However, no matter coated or not coated by PMMA, the Pd NP film exhibited the electrical transport behavior dominated by Mott VRH mechanism, indicating that the electronic transport mechanism of Pd NP film is essentially unchanged after PMMA coating. As a result, the unique quantum transport behavior of such Pd NP film with the PMMA membrane layer presented an opportunity for fabricating high performance HILE-based H2 sensors. Figure 4 shows comparisons of the H2-sensing response for a typical Pd NP film before and after PMMA coating. As shown in Figure 4a and b, the sensor was exposed to several pure H2 loading and deloading cycles; the measured gas pressures are noted at the peak of each response cycle. The positive sign of ∆G/G0 indicates that the conductance of the Pd NP film before and after PMMA coating increased and decreased quickly during H2 loading and deloading for each cycle. The H2-sensing response curves of the Pd NP film before and after PMMA coating are both similar to the results of our previous studies.9, 20 The relative changes in conductance (∆G/G0) versus PH2 are plotted in Figure 4c, in which shows the calibration curves for the sensors. The calibration curves can be divided into three discrete response regimes corresponding to the three phase regions of Pd hydride, i.e., an α phase region, an α–β phase coexistence region, and a β phase region, in which the sensor exhibited different sensitivities to H2. As PH2 increased from 102 to 105 Pa, the sensor both before and after PMMA coating exhibited excellent response


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Figure 4. (a, b) Response curves for a typical Pd NP film exposed to pure H2 loading and deloading cycles before and after PMMA coating. The pressure (in pascals) for each hydrogen loading is marked at the top of each response peak. (c) Lin-log calibration plots of ∆G/G0 versus PH2 for the same typical Pd NP film before and after PMMA coating shown in (a) and (b).

sensitivity, indicating that the devices were capable of detecting H2 gas over a wide dynamic range (0–10%) at room temperature. The response of the Pd NP film to low concentration (50‒600 ppm) H2/air mixture after PMMA coating was tested in order to obtain the lower detection limit of the sensor with PMMA


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membrane layer. The sensor exhibited significant response to 50, 150, 250, 400 and 600 ppm concentration H2 with about 0.37, 0.62, 0.86, 1.25 and 1.64% relative changes in conductance, respectively (Figure S2 in the Supporting Information). In this measurement, lower concentration H2/air mixture was not tested, due to experimental limitations. However, based on the fact that the respond signal to 50 ppm H2 is much higher than the conductance noise (Figure S2 in the Supporting Information), it can be concluded that the lower detection limit of the sensor with PMMA membrane layer is less than 50 ppm. As seen in Figure 4c, the calibration curve of the Pd NP film after PMMA coating is almost identical to that before PMMA coating until PH2 reached 3000 Pa, when Pd hydride entered the β phase region. Thereafter, the response of the Pd NP film after PMMA coating dropped slightly. The results indicate that the Pd NP film maintained its response sensitivity to some extent even after PMMA coating. Previous studies have shown that Pd NPs undergo a large lattice expansion and form a β phase hydride43‒44 when the H2 concentration is increased to above a certain value. We thus hypothesize that the response drop (in Figure 4c) of the Pd NP film after PMMA coating is due to the hindering effect of the PMMA layer on the large-volume expansion of Pd hydride.45‒47 Collectively, the data in Figure 4 support the conclusion that the sensing mechanism of Pd NP films remains based on the hopping of electrons after PMMA coating, and that the PMMA coating had no effect on the mechanism. In fact, the maintenance of the sensing mechanism is essential for the remarkable H2-sensing performance of Pd NP films. To inspect the selective H2 filtration effect of the PMMA membrane layer, the response to gas mixtures of CO/N2, CH4/N2, H2/N2 and the mixture of H2, CO and CH4 in N2 ((H2 + CO + CH4)/N2) for Pd NP films with and without the PMMA membrane layer spin-coated was compared, and the results are shown in Figure 5. The thickness of PMMA membrane layer is


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Figure 5. The response of sensors with and without a PMMA membrane layer to target gas mixtures including CO/N2, CH4/N2, H2/N2 and (H2 + CO + CH4)/N2. The concentration of CO/N2, CH4/N2 and H2/N2 was 1000 ppm. The tested mixed gas of H2, CO and CH4 in N2 was formed by filling the three kinds of gas into the test chamber simultaneously.

about 0.45 µm. For testing the response to (H2 + CO + CH4)/N2, the mixtures of CO/N2, CH4/N2 and H2/N2 were simultaneously fed into the test chamber through three mass flow controllers at 1.5 L min−1 flow rate. As shown in Figure 5, the response to H2 of sensors with and without the PMMA membrane layer is approximately 9.1%, indicating little negative influence of the PMMA membrane layer on the response sensitivity of Pd NP films to H2. The response of the sensor without the PMMA membrane layer to CO, and CH4 is −3.4 and 4.5%, respectively. However, the response of the sensor with the PMMA membrane layer to CO and CH4 has decreased by roughly an order of magnitude. Such a huge decrease of response to CO and CH4 observed for the sensor with the PMMA membrane layer is due to the filtration effect of PMMA matrix. In addition, the response of sensors with and without the PMMA membrane layer to (H2 + CO + CH4)/N2 is 2.9 and −0.7%, respectively. The negative response of the sensor without the PMMA membrane layer to (H2 + CO + CH4)/N2 indicates that the CO adsorb on the Pd metal surface more effectively than the other two kinds of gas, which can be attributed to the higher binding energy of CO on Pd. In the presence of the PMMA membrane layer, the sensor exhibited positive response to (H2 + CO + CH4)/N2 which is less than the response to H2/N2 because of the


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dilution of H2 in (H2 + CO + CH4)/N2. It at least proves that CO and CH4 adsorption on the surface of Pd NPs was prevented during the sensors exposed to (H2 + CO + CH4)/N2 due to the filtration effect of the PMMA membrane layer. Hence, these measurements revealed the remarkable fact that the PMMA membrane layer, as a selective gas filter for H2, can protect Pd NP films from responding to CO and CH4 without significant degradation of their response sensitivity to H2. To study the effect of the thickness of the PMMA membrane layer on the H2-sensing kinetics of Pd NP films, four samples with the PMMA membrane layer spin coated at rotational speeds of 2000, 3000, 4000, and 5000 rpm (denoted samples a, b, c, and d, respectively), and one sample without a PMMA membrane layer (denoted sample e), were prepared. The initial conductance G0 of all five samples was 0.2 µA during Pd nanocluster deposition, to eliminate the influence of NP coverage on sensing performance.20‒21 Figure 6a shows the AFM three-dimensional (3D) image of the edge section of the PMMA membrane layer on the surface of sample b. We can obtain the thickness of the PMMA layer by quantitatively measuring the step height of the PMMA edge section in the image. In the same way, the thickness of three other samples was measured. Figure 6b shows the thickness of the PMMA layer versus the rotational speed of the spin coater for the four samples. In Figure 6b, the thickness of the PMMA membrane layer of samples a, b, c, and d is 0.55, 0.49, 0.41, and 0.36 µm, respectively. It can be seen that there is a decrease in the thickness of the PMMA layer with increasing rotational speed for the four samples. In Principle, gas permeability decreases with increased polymer membrane thickness. For thicker PMMA membrane layer, the filtration effect is better but the permeability of H2 is lower, leading to the reducing of sensitivity and response speed during H2-sensing. To qualify the PMMA membrane layer to act as gas filter, the Pd NPs


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Figure 6. (a) AFM 3D image of the edge section of the PMMA membrane layer coated on the surface of sample b. (b) The thickness of PMMA membrane layers versus rotational speed.

have to be fully sealed into the PMMA matrix. Therefore, the thickness of PMMA membrane layer should be at least larger than the size of the Pd NPs in our devices. From the STEM images of Pd NPs in Figure 2 and the data from Figure 6, since the thickness of the PMMA membrane layer is much larger than the size of Pd NPs, it was deduced that the Pd NPs were fully embedded in the PMMA matrix. This full encapsulation enabled the PMMA membrane layer coated on the top of the Pd NP films to act as a gas separator, allowing H2 to penetrate.34‒36, 48 Generally speaking, the response time is one of most crucial technical parameters for an H2 safety sensor, for obvious reasons. Although the PMMA membrane layer provided enhanced performance in selectivity for H2 sensing, based on the importance of the response time, it is


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Figure 7. (a) Response time as a function of PH2 for samples a–e on a lin-log scale. The dashed lines indicate the boundaries of the three hydride phase regions. (b) Plots of the response time versus the thickness of the PMMA membrane layer for sample a–e at PH2 = 1, 3, and 2.3 kPa. The solid lines are the linear fitting of the dependence of the response time on the thickness of the PMMA membrane layer from 0.36 to 0.55 µm. The slopes of the linear fitting (ux, where x is the value of PH2) are indicated. The dashed lines serve to guide the eye.

necessary to evaluate the effect of the PMMA membrane layer on the sensor response time. Therefore, the response time of the aforementioned five samples a–e was measured. Figure 7a displays the response time of the five samples versus PH2. The five samples all exhibited strong dependence of the response time on PH2, similar to the behavior of H2 sensors based on Pd nanostructures.9, 14, 19, 49 However, two remarkable features of the data are shown in


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Figure 7a. First, the response time of all samples was found to fall rapidly with an increase in PH2, except for a slow-response peak for the PH2 range 1–3 kPa, which corresponds to the α to β phase transition of Pd hydride with coexistence of the α and β phases of Pd hydride. This slow sensor response can be attributed mainly to the large lattice expansion caused by nucleation and growth of the β phase in the α matrix because of H2 absorption.44, 50‒51 Second, samples a‒d exhibited significant prolonging of the response time compared with sample e. This slowing of the sensor response depended strongly on not only the thickness of the PMMA membrane layer but also PH2. As the thickness of the PMMA membrane layer increased, the response time prolonged significantly. Moreover, we noticed that this slowing of the response time induced by the PMMA membrane layer became increasingly prevalent as PH2 was increased, especially in the β phase region (PH2 ≥ 3000 Pa). To obtain further insights into the effect of the PMMA membrane layer on the response time of the sensors in the three hydride phase regions, we plotted the response time as a function of the thickness of the PMMA membrane layer at three PH2 values (1, 3, and 2.3 kPa), corresponding to the start and end points of the α to β phase transition and the point of lowest response in the α–β phase coexistence region, respectively (Figure 7b). It is clear that the linear relationship between the response time and the thickness of the PMMA membrane layer is in the range 0.36–0.55 µm for the three PH2 values. As quantitatively analyzed in Figure 7b, the slopes of the three fitting curves, which represent the slowing effect of the PMMA membrane thickness on the response time, are 22.84, 69.34, and 111.18 s·µm−1 at PH2 = 1, 2.3, and 3 kPa, respectively, indicating that for higher PH2 values, especially in the β phase region, the retardation effect becomes stronger.


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In the case of PH2 = 1 kPa, the response time of sample e, which was not coated with a PMMA membrane layer, coincides with the linear relationship fitted within samples a–d (the red dashed line in Figure 7b). In the process of H2 sensing, H2 molecules diffused through the PMMA filter before adsorption on the surface of Pd NPs. The reduction in the response speed is mainly induced by the diffusion process of H2 molecules within the PMMA matrix. Therefore, the response time t is composed of two terms: the diffusion time through the PMMA layer tD = Dh and the hydrogenation reaction time of the Pd NP film t0, where the coefficient D refers to the diffusibility of H2 in the PMMA matrix and h is the thickness of the PMMA membrane layer, which is essentially the diffusion distance of H2 molecules in the PMMA matrix. As a consequence, in the α phase region (PH2 ≤ 1 kPa), the response time t consists of the hydrogenation reaction time t0 and the diffusion time through the PMMA layer tD. The retardation of the response time in the α phase region mainly results from the diffusion of H2 molecules through the PMMA layer, but this does not appear to be the case in the β phase region, as indicated by the deviation of the response time of sample e from the linear fitting curve of samples a–d as shown in Figure 7b (the green dashed line). In the case of PH2 = 3000 Pa, the starting point of the β phase region, the Pd hydride has undergone a large lattice expansion of more than 3.47% of the lattice parameter (3.89 Å), which is much larger than the expansion in the α phase region (less than 0.13%) during absorption of H2.52 In addition, because of the presence of the PMMA matrix, the large-volume expansion of Pd NPs in the β phase region suffers from suppression induced by mechanical stress on the interface between the PMMA matrix and Pd NPs.53‒55 Consequently, once the large-volume expansion occurs in Pd NPs embedded in the PMMA matrix, the response time will be retarded due to stress relaxation. Therefore, we attribute the slowing of the response time in the β phase region to stress relaxation


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induced by the lattice expansion, in addition to the diffusion process of H2 molecules within the PMMA matrix. Based on this analysis, in the β phase region (PH2 ≥ 3 kPa), the response time t has three terms: the hydrogenation reaction time t0, the diffusion time through the PMMA layer tD and the prolonged response time induced by the mechanical stress relaxation tR. This phenomenon agrees with the result that the response of the Pd NP film after PMMA coating dropped in the β phase region as shown in Figure 4c, indicating that the suppression of the volume expansion of Pd NPs in the β phase region not only prolonged the response time but also lowered the sensitivity. It is clear that the PMMA membrane inhibits the expansion of the Pd NPs by H2 absorption. As a result, in these devices coated by PMMA, the conductance changes induced by H2 absorption decrease with the increase in PH2. This means that a quantitative determination of the PH2 from the conductance measurement would be unreliable at a higher range of hydrogen pressure. Accordingly, it is reasonable to conclude that the upper detection limit of the devices with PMMA membrane layers is lower than that of the devices without PMMA membrane layers due to the inhibitory action of PMMA matrix on the expansion of Pd NPs by H2 absorption. For the same reason, the lower detection limit of the devices is also affected by the PMMA matrix. However, the PMMA matrix affects the upper detection limit more strongly than the lower detection limit due to the much larger volume expansion of Pd NPs in the β phase region than in the α phase region. With regard to Figure 7a, the slow-response peak indicates a huge delay in the response time for the α–β phase coexistence region of Pd hydride for all five samples. Associated with the diffusion of H2 molecules in the PMMA matrix and stress relaxation at the interface between Pd NPs and the PMMA membrane layer, the slow-response peak can be mainly attributed to the


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nucleation and growth of the β phase in the α phase matrix of Pd hydride, which plays a key role in the sensing kinetics. In the α–β phase coexistence region, the response time t is composed of four terms: the hydrogenation reaction time t0, the diffusion time through the PMMA layer tD, the prolonged response time induced by the mechanical stress relaxation tR and the prolonged time induced by the nucleation and growth tN. The response time of a sensor is mainly determined by tN, as it is larger than the other terms. The microscopic mechanism leading to the slow-response peak in Figure 7a remains unexplained. Although all the Pd NP films with a PMMA membrane layer in our study clearly exhibited up to tens of seconds of retardation of the sensing response, in view of the improvement of selectivity towards H2, this retardation is acceptable. Langhammer et al. studied the hydriding and dehydriding kinetics of Pd NPs in different size.56 It was proved that smaller Pd NPs are faster in the hydriding and dehydriding kinetic processes. In our previous study, the Pd NP films with different mean diameters were obtained by controlling the NPs deposition. The response time of the devices was found to depend strongly on the Pd NP size: smaller Pd NP shows faster response to H2.9 In fact, the response time depends partly on the hydrogen diffusion in NPs. The diffusion length can be shortened by reducing the NP size. Therefore, we suggest that it is feasible to compensate for the retardation of the sensing response by reducing the size of the Pd NPs. Minimizing the thickness of the PMMA membrane layer, which results in shortening of the diffusion length in PMMA matrix, is also crucial for maintaining the high-speed response of the sensor. However, based on the fact that the permeability increases with decreased polymer membrane thickness, the sensors lose gas selectivity once the PMMA membrane thickness is reduced to a critical dimension. Based on this point, the PMMA filtration membrane layers should be as thin as possible on the premise of the selective filtration of H2.


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Conclusions In summary, we fabricated H2 sensors by depositing a Pd NP film on interdigital electrodes, and then a PMMA membrane, which acted as a selective filtration layer, was spin coated on the surface of the Pd NP film. The sensing behavior of this sensor was investigated. We demonstrated that no changes to the electric transport and sensing mechanisms of Pd NP films resulted from the presence of the PMMA membrane layer. In addition, the sensors with a PMMA membrane layer showed good selectivity for H2 with little decrease in the H2 response sensitivity in the β phase region of Pd hydride. It was observed that the sensors exhibited retardation of the sensing response induced by the PMMA membrane layer. As the thickness of the PMMA membrane layer increased, the response time exhibited a tendency toward significant prolongation. Based on our experimental data, the retardation of the response time caused by the PMMA membrane layer differs significantly in the three phase regions of Pd hydride. We ascribe the increased response time to three mechanisms: diffusion of H2 molecules in the PMMA matrix, nucleation and growth of the β phase in the α phase matrix of Pd hydride, and stress relaxation at the interface between Pd NPs and the PMMA membrane layer.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Figure S1: the schematic diagram of the measurement configuration used for response time measurements.


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Figure S2: response curve for the Pd NP film to 50‒600 ppm H2/air mixture loading and deloading cycles after PMMA coating.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Tel: +86-0714-6576185

ORCID Bo Xie: 0000-0002-3862-0411 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Minrui Chen and Peng Mao contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge financial support from the National Natural Science Foundation of China (grant 61301015, 11604161 and 11627806), the Training Program of the Major Research Plan of the National Natural Science Foundation of China (grant 91622115), the Natural Science Foundation of Jiangsu Province (grant BK20160914) and the Promotion Program of Hubei Normal University.


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FIGURE CAPTIONS Figure 1. Schematic illustration of the procedure used to fabricate the hydrogen sensor based on a PMMA-membrane-coated Pd NP film. (a) The deposition of Pd NPs. (b) The Pd NP film formed on the interdigital electrode substrate after the NP deposition. (c) The PMMAmembrane-coated Pd NP film fabricated by spin coating. (d) The structure of the hydrogen sensor based on the PMMA-membrane-coated Pd NP film. Figure 2. (a) Typical HAADF-STEM image of Pd NP films. (b) Typical HAADF-STEM image (upper left) and BF-STEM image (lower right) of a single Pd NP. The inset shows the fringes of the Pd NP. (c) SAED pattern. (d) HAADF-STEM image of Pd NP film after PMMA coating. (e) HAADF-STEM image of few closely spaced Pd NPs after PMMA coating. (f) Size distribution of Pd NPs. The red line is the log-normal fitting of the distribution. Figure 3. The resistance of the Pd NP film before and after PMMA coating plotted against T–1/4. The solid curves fit the experimental data with R ∝ exp[(TM/T)1/4]. Figure 4. (a, b) Response curves for a typical Pd NP film exposed to pure H2 loading and deloading cycles before and after PMMA coating. The pressure (in pascals) for each hydrogen loading is marked at the top of each response peak. (c) Lin-log calibration plots of ∆G/G0 versus PH2 for the same typical Pd NP film before and after PMMA coating shown in (a) and (b). Figure 5. The response of sensors with and without a PMMA membrane layer to target gas mixtures including CO/N2, CH4/N2, H2/N2 and (H2 + CO + CH4)/N2. The concentration of CO/N2, CH4/N2 and H2/N2 was 1000 ppm. The tested mixed gas of H2, CO and CH4 in N2 was formed by filling the three kinds of gas into the test chamber simultaneously. Figure 6. (a) AFM 3D image of the edge section of the PMMA membrane layer coated on the surface of sample b. (b) The thickness of PMMA membrane layers versus rotational speed. Figure 7. (a) Response time as a function of PH2 for samples a–e on a lin-log scale. The dashed lines indicate the boundaries of the three hydride phase regions. (b) Plots of the response time versus the thickness of the PMMA membrane layer for sample a–e at PH2 = 1, 3, and 2.3 kPa.


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The solid lines are the linear fitting of the dependence of the response time on the thickness of the PMMA membrane layer from 0.36 to 0.55 µm. The slopes of the linear fitting (ux, where x is the value of PH2) are indicated. The dashed lines serve to guide the eye.


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Table of Contents


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Figure 1. Schematic illustration of the procedure used to fabricate the hydrogen sensor based on a PMMAmembrane-coated Pd NP film. (a) The deposition of Pd NPs. (b) The Pd NP film formed on the interdigital electrode substrate after the NP deposition. (c) The PMMA-membrane-coated Pd NP film fabricated by spin coating. (d) The structure of the hydrogen sensor based on the PMMA-membrane-coated Pd NP film. 95x55mm (300 x 300 DPI)



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Figure 2. (a) Typical HAADF-STEM image of Pd NP films. (b) Typical HAADF-STEM image (upper left) and BFSTEM image (lower right) of a single Pd NP. The inset shows the fringes of the Pd NP. (c) SAED pattern. (d) HAADF-STEM image of Pd NP film after PMMA coating. (e) HAADF-STEM image of few closely spaced Pd NPs after PMMA coating. (f) Size distribution of Pd NPs. The red line is the log-normal fitting of the distribution. 1267x831mm (72 x 72 DPI)


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Figure 3. The resistance of the Pd NP film before and after PMMA coating plotted against T−1/4. The solid curves fit the experimental data with R ∝ exp[(TM/T)1/4]. 62x47mm (300 x 300 DPI)



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Figure 4. (a, b) Response curves for a typical Pd NP film exposed to pure H2 loading and deloading cycles before and after PMMA coating. The pressure (in pascals) for each hydrogen loading is marked at the top of each response peak. (c) Lin-log calibration plots of ∆G/G0 versus PH2 for the same typical Pd NP film before and after PMMA coating shown in (a) and (b). 154x286mm (300 x 300 DPI)


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Figure 5. The response of sensors with and without a PMMA membrane layer to target gas mixtures including CO/N2, CH4/N2, and H2/N2 and (H2+CO+CH4)/N2. The concentration of CO/N2, CH4/N2 and H2/N2 was 1000 ppm. The tested mixed gas of H2, CO and CH4 in N2 was formed by filling the three kinds of gas into the test chamber simultaneously. 51x32mm (300 x 300 DPI)



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Figure 6. (a) AFM 3D image of the edge section of the PMMA membrane layer coated on the surface of sample b. (b) The thickness of PMMA membrane layers versus rotational speed. 103x129mm (300 x 300 DPI)


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Figure 7. (a) Response time as a function of PH2 for samples a–e on a lin-log scale. The dashed lines indicate the boundaries of the three hydride phase regions. (b) Plots of the response time versus the thickness of the PMMA membrane layer for sample a–e at PH2 = 1, 3, and 2.3 kPa. The solid lines are the linear fitting of the dependence of the response time on the thickness of the PMMA membrane layer from 0.36 to 0.55 µm. The slopes of the linear fitting (ux, where x is the value of PH2) are indicated. The dashed lines serve to guide the eye. 127x196mm (300 x 300 DPI)


Response Characteristics of Hydrogen Sensors ... - ACS Publications

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