Pd Nanoparticle Film on a Polymer Substrate for Transparent and

Subscriber access provided by Imperial College London | Library

Functional Nanostructured Materials (including low-D carbon)

Pd Nanoparticle Film on a Polymer Substrate for Transparent and Flexible Hydrogen Sensor Bo Xie, Peng Mao, Minrui Chen, Zhaoguo Li, Juanjuan Han, Lun Yang, Xiuzhang Wang, Min Han, Jun-Ming Liu, and Guanghou Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15445 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Pd Nanoparticle Film on a Polymer Substrate for Transparent and Flexible Hydrogen Sensor Bo Xie*,†, Peng Mao§,∥, Minrui Chen‡, Zhaoguo Li⊥, Juanjuan Han†, Lun Yang†, Xiuzhang Wang†, Min Han*,‡, Jun-Ming Liu‡, and Guanghou Wang‡ †

Institute for Advanced Materials and Hubei Key Laboratory of Pollutant Analysis & Reuse Technology, Hubei Normal University, Huangshi 435002, P. R. China

‡

National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China

§

School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom

∥

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

Nanjing 210023, P. R. China ⊥

Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, P. R.

China

*Corresponding

authors:

E-mail: [email protected] (B. Xie), [email protected] (M. Han).

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Alongside the rise in fully automated equipment and the wearable devices, there is currently a high demand for optically transparent and flexible gas sensors operating at room temperature. Nanoparticle films are ideal H2-sensing materials that can be coupled with flexible substrates because of their discrete nanogranular structure and unique interparticle electrical responsiveness. In this work, we present an optically transparent and flexible H2 sensor based on a Pd nanoparticle film, prepared on a polyethylene terephthalate sheet using a straightforward nanocluster deposition technique. Hundreds of bending cycles demonstrated that the sensor has good electrical stability and mechanical robustness without significant degradation in H2-sensing performance. The H2-sensing behaviors under bent state were systematically evaluated. The loading of tensile and compressive strains under bent state produced a positive and negative influence, respectively, on the sensing performances. The possible influence mechanism of the tensile and compressive strains on H2-sensing performance was attributed to the changes in the percolation network topology and the interparticle space induced by the strains. The ability to detect an H2 concentration as low as 15 ppm, dynamic response range as wide as 0−10% and sub-10 s response time were achieved. In addition, the sensor can be operated in the relative humidity range of 0−90% at room temperature. These results demonstrate that the sensor exhibits significant potential for next-generation transparent and flexible H2 detector.

KEYWORDS: palladium nanoparticle, flexible, transparent, hydrogen gas sensor, nanogranular


Page 2 of 29

Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Hydrogen energy, as the most promising and one of the cleanest new alternative energy sources, has attracted unprecedented interest over the last few decades. However, hydrogen gas (H2) is colorless, odorless, and flammable at concentrations above 4% in air.1 Due to the high risk of explosion, H2 sensors have an essential role in the development of the hydrogen economy. Studies of H2-sensing materials have drawn considerable attention owing to the requirements and safety issues of H2-based applications. Recently, wearable electronic devices on flexible substrates have attracted increasing attention due to their potential in various applications, such as in medical devices,2 civilian or military sensors,3 consumer electronics,4‒5 and energy devices.6 Chemical sensor devices on flexible substrates have also been increasingly reported.7‒12 By coupling nanostructured materials with flexible substrates, these devices show a range of advantages, including a relatively simple fabrication process, cost effective manufacturing, extreme portability, and good wearability. The H2 sensors were also involved in the development of flexible wearable electronics due to their potential to be integrated into various smart wearable devices for healthcare applications. For instance, the H2, produced by intestinal bacteria, are coexist in the breath gas.13 A wearable H2 sensor provides a potentially cheap way for medical examination such as evaluation of colonic flora by detecting and analyzing the H2 concentration in human breath. Furthermore, another current trend is to make gas sensors transparent due to aesthetic reasons and the requirements of the integration and compatibility with the various optoelectronics devices such as displays, light emitters, and solar cells. Instead of rigid and opaque sensors, the technology of gas sensing is gradually drifting toward flexible transparent sensors. Now, advances in low-dimensional nanostructured materials have opened new opportunities to flexible transparent conductive materials with gas sensing properties, mainly including semiconducting oxides, metal nanostructures, carbon-based materials, and conducting polymers, all of which have demonstrated superior performance.14‒20 Among these literatures, the H2 sensors with good flexibility and optical transparency is still few. Besides flexibility and transparency, a promising gas sensor for futuristic wearable sensing technology should



possess the following significant features: (i) operation at room temperature; (ii) low detection limit, high sensitivity, fast response, and reproducibility; (iii) low power consumption; (iv) cost effective and environment-friendly, etc. Based on the growing importance of H2 as a fuel and the rise of hydrogen economy,21‒22 persistent efforts should be devoted to searching for flexible and transparent H2 sensing materials with high sensitivity, stability, and selectivity at room-temperature. Among various H2-sensing materials, palladium (Pd) as a catalytic metal appeared as the most frequently used sensing active material because it can selectively and reversibly adsorb H2 molecules, forming interstitial hydrogen atoms in the Pd hydride (PdHx), at room temperature. With regard to the Pd-based H2-sensing materials, there are two mechanisms that govern H2 sensing. In the case of continuous Pd films, the electron flow is scattered by the interstitial hydrogen atoms, inducing an increase of resistance.23 In the case of Pd-based nanostructures, another sensing mechanism based on the cotunneling of electrons has been discovered in discontinuous Pd nanowires with cracks and gaps that are closed by hydrogeninduced lattice expansion (HILE) during H2 exposure, resulting in an decrease of resistance.24 In general, Pd nanostructured materials render the H2 sensors superior sensing performance due to the enhanced chemical activity and shortened diffusion path of the interstitial hydrogen atoms within the Pd hydride induced by the decrease of the dimension and size of materials. Moreover, it is fortunate that both transparency and flexibility of the devices can be achieved by nanostructuring the sensing materials. Therefore, nanostructured materials are well suited for manufacturing the flexible and transparent sensors. In this work, we report the fabrication of a flexible, optically transparent, and resistive type H2 sensor using Pd nanoparticles (NPs) as sensing active material and polyethylene terephthalate (PET) as substrate. To achieve light transmittance and flexibility of our H2 sensors we controllably deposited Pd NPs on PET substrates, forming random percolative NP films with the transmittance of above 80% in the visible region of the spectrum. Because of the discontinuous granular architecture, the existing nanogaps between the closely spaced Pd NPs resulted in HILE-based sensing behavior. As a flexible gas sensor, subjected to hundreds of bending cycles, our sensor showed great tolerance toward repeated bending, good


Page 4 of 29

Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrical stability and mechanical robustness without significant degradation in H2-sensing performance. The influence of tensile and compressive strains under bent state on H2-sensing performance was systematically evaluated. Interestingly, a tensile strain applied induced enhanced H2-sensing performance with respect to the response amplitude, sensitivity, and response speed. The possible mechanism was attributed to changes in the percolation network topology and the interparticle space induced by the strains. The ability to sensing H2 concentrations as low as ∼15 ppm, dynamic response range as wide as 0−10% and a sub-10 s response time were achieved. In addition, the sensor can be operated in the relative humidity range of 0−90% at room temperature.

RESULTS AND DISCUSSION Sensor Fabrication and Characterization. Briefly, the preparation process for the transparent and flexible H2 gas sensor in our research was similar to that in our earlier contributions.25‒26 A PET sheet with prepatterned interdigital electrodes (IDEs) prepared by mask evaporation deposition (Figure 1a,b) served as the flexible substrate. As shown in Figure 1c, a magnetron plasma gas aggregation cluster source27 was employed to generate the Pd NPs. A Pd NP film with well-controlled NP coverage of the flexible substrate was achieved by in situ monitoring the current across the IDE gap by a computer-interfaced sourcemeter (Keithley 2636B). A typical current evolution of the NP arrays at bias of 1 V, which can be described by the percolation model, as a function of deposition time is shown in Figure S1. During the deposition process, the NPs produce a circular deposit on the substrate, its thickness being larger in the center.28 For fabricating the sensors efficiently, the PET substrate should be placed near the section center of the cluster-beam spot as possible as it can be.



Figure 1. The fabrication of the H2 sensor based on Pd NP film on a PET substrate. (a) Fabrication of Ag IDEs on a flexible PET sheet. (b) The bent IDE substrate. (c) Controllable deposition of Pd NPs via monitoring the current in situ.

Representative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the Pd NP film, which was prepared by depositing Pd nanoclusters at deposition rate of 0.2 ± 0.05 Å/s for ~8 min, are shown in Figure 2a. From the high-magnification STEM image, it is clearly seen that the Pd NPs are predominantly cuboid, which corresponds to face-centered cubic (fcc) structures. The Pd NPs were closely packed and formed a discontinuous film in a disordered manner. Although a large number of NPs were in contact with each other in the NP aggregation areas, the high-resolution STEM (HRSTEM) image (Figure S2) clearly shows that these neighboring NPs did not coalesce to form larger metallic islands or a continuous film but maintained their own individual shapes and/or formed interfaces between adjacent NPs. As a result, these individual NPs and aggregates formed numerous electron-conducting pathways based on percolation in the flexible substrate. We stochastic captured eight transmission electron microscopy (TEM) images at different areas of this sample at low-magnification, as shown in Figure S3, for statistical analysis of the size distribution of the NPs. The diameters of the NPs were measured from these TEM images one by one, and the statistical result are shown in Figure 2b. The size distribution of NPs fitted a log-normal dependence with probable size of ∼7.2 nm. To confirm that the same morphology is always present in the flexible PET substrate, scanning electron microscopy (SEM) characterization of the Pd NP film on the PET substrate


Page 6 of 29

Page 7 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


was carried out. Figure 2c is the SEM image of the Pd NP film on the PET substrate that was prepared simultaneously with the sample on the copper TEM grid. This image shows that the Pd NP film on the PET substrate exhibits nonuniform-like morphology that is consistent with the microstructure observed in the sample on the copper TEM grid, although it is difficult to achieve high-quality SEM images at high magnification due to the high electric insulation of the PET sheet.

Figure 2. The fabrication and characterization of a Pd NP film on formvar-coated copper grid and a PET substrate. (a) HAADF-STEM image of the Pd NP film. (b) The size distribution of the NPs, where the blue line is the log-normal fitting of the distribution. (c) Field emission SEM image of the Pd NP film on PET substrate. (d) Optical transmittance of the Pd NP films on PET sheets for different deposition times. The inset shows the photograph of the sample with the university badge as the background.

Pd is widely used to detect and quantify H2 concentration due to its specific interactions (catalytic surface reactivity and solubility) with H2. However, bulk metal Pd is susceptible to mechanical damage originating from volume changes associated with repeated H2 absorption and desorption.29 On the basis of the above microstructure characterization (Figure 2a,c), Pd NPs obtained by reducing the size and dimensions of the bulk metal can be assembled to form a granular film—an artificial material with unique electronic properties. The individual grain-like nanostructure endows the Pd NP films with the ability to withstand mechanical deformation, which makes the Pd NP films can be used as flexible H2-sensing element. A further advantage of Pd NP films is superior H2-sensing performance owing to their increased surface-area-to-volume ratio. To ensure the transparency, and three samples were prepared on PET substrates with different



deposition times of 2, 4 and 8 min at deposition rate of 0.15 ± 0.05 Å/s, and their TEM images and transmittances from visible to near infrared wavelength region (400–1500 nm) are showed in Figure S4. The experimental details are provided in Figure S5 in the Supporting Information. We observed that longer deposition time induced drop in transmittance. Within the visible wavelength region (400–800 nm), the transmittance slightly increased with an increasing wavelength, as seen in Figure 2d. At 550 nm, the transmittance of the sample with deposition time of 8 min was above 86%. The high transmittance of the Pd NP film based H2 sensor emerge from the sub-monolayer structure composed of metal NPs because of its extremely thin thickness. Here, light absorption of the Pd NP films induced by plasmon resonance was not observed in the visible wavelength region. The inset of Figure 2d shows the photograph of a Pd NP film on PET sheet with a university badge as the background. The high transmittance of the Pd NP film allows its practical applications as gas sensors integrated with other optical devices.


Page 8 of 29

Page 9 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Electrical stability and repeatability of H2 response. (a) Schematic diagram of the overall H2sensing experimental setup. (b) The relative change in baseline current ΔI0/I0 of the Pd NP film-based flexible H2 sensor at 8.5 mm bending radius over 500 bending cycles. The inset photographs show the sample under bending and flat conditions. The bending-unbending cycles were driven by test equipment manufactured in our laboratory, comprised of two motor-controlled sliders. The dashed line indicates the average value (around 5.9%) at which ΔI0/I0 stabilized after the bending cycles. A PET sheet with 0.25 mm thickness was used as the substrate owing to its greater flexibility compared with 0.5 mm PET sheet. (c) Relative current variation ΔI/I0 versus hydrogen pressure PH2, corresponding to the calibration curve, for the sensor before and after the bending cycles (the data were extracted from parts d and e). The sensor exhibited sigmoidal calibration curves before (blue line) and after (red line) the bending cycle test. (d, e) Current amplitude versus time for the sensor before and after the bending cycles. The hydrogen pressure PH2 for each hydrogen loading is indicated against each column.



Electrical Stability and Repeatability of the H2 Sensor. Electrical stability and mechanical robustness are of extreme importance in a number of practical applications, such as wearable and portable electronic devices to detect flammable and combustible gases in the ambient atmosphere, especially for a flexible gas sensor. To evaluate the reliability of the H2 sensor and the influence of bending cycles on sensor performance, we performed dynamic H2 response measurements up to 5 kPa using a test system of our own design (Figure 3a) before and after 500 bending cycles. First, we surveyed the effects of bending cycles on the baseline current (at 1 V bias). A Pd NP film with an initial current of 21.41 nA was built upon a 0.25 mm-thick PET sheet by performing deposition of Pd nanoclusters at deposition rate of 0.2 ± 0.05 Å/s for 5 min. Analyzing from the TEM image of the TEM sample prepared under the same fabricating conditions, as shown in Figure S6 in the Supporting Information, the thickness and roughness of this Pd NP film are 2.1 and 5.3 nm, respectively. Electrical current of the Pd NP film was measured during 500 bending cycles and compared to the baseline current before starting the test. The strain ε induced by the bending can be expressed as   Ts 2 Rb , where Ts is the substrate thickness and Rb is the radius of curvature of the sample.30 The maximum strain is about 1.47% when the sample was bent at maximum degree of bending, corresponding to minimum radius of curvature 8.5 mm (the inset in Figure 3b). Here, we performed this test at device bending orientations with respect to the microelectrode orientation: perpendicular and parallel to the fingers of the IDE (Supporting Information, Figure S7). Figure 3b shows the relative baseline current change (ΔI0/I0) as a function of bending cycles, where I0 is baseline current of the sample (the error bars of the plots at parallel bending orientation are too small to be distinguished in Figure 3b). First, the device was tested at perpendicular bending orientation. Notably, a gradually increasing trend of ΔI0/I0 was observed during the first 120 bending cycles, and then the ΔI0/I0 remained stable at about 5.9%. As a result, the baseline current increased from approximately 21.41 to 22.68 nA. The microscopic mechanism causing the baseline current variation in the bending cycles may be due to a few NPs’ rearrangement, resulting in the change in the electrical conductivity. The bending cycle test at parallel bending orientation was also performed and the result indicated


Page 10 of 29

Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a negligible decrease in baseline current which is much less than that at perpendicular orientation. Such a large difference in baseline current change between perpendicular and parallel bending orientations may be attributed to the anisotropic rearrangement of NPs. The sample was characterized by SEM before and after the bending cycle test. We compared the morphology of the NPs film before and after the bending cycle test, as shown in Figure S8. These two SEM images were taken from two different regions of the sample as it is hard to focus the field of view on the same area of the sample before and after the bending cycle test. However, we can see in Figure S8 that there is not much difference between the overall morphology of the Pd NP film on the PET sheet before and after the bending cycle test. The maintaining of the film morphology after bending cycles can be attributed to the discretegrain structure of the film as well as the strong adhesive force between the NPs and the substrate. The strong adhesive force stems partly from high flying speed of the nanoclusters during deposition process.31 The stability and robustness of film morphology makes it possible for this nanostructure to act as a flexible sensing device. Siffalovic et al.

32

showed that an NP monolayer was preserved on a stretched Mylar

membrane strained up to 11% and that no disintegration into separate islands took place. Referring to this result, we surmise that the undisturbed baseline current following the upward trend enables the sensor to run stably and reliably without performance degradation, although a slight baseline drift was observed in the initial stage of the bending cycle test process. To confirm this, we compared the H2-sensing response before and after the bending cycles. Figure 3c−e compares the H2-sensing response of the Pd NP film before and after the bending–unbending

recycle

test.

The

relative

change

in

the

current

signal

I /I 0 =[( I  I 0 ) / I 0 ] 100% is defined as the sensing response amplitude, where I is the

current of the sensor exposed to the target gas. Overall, the device exhibited a very similar H2-sensing response before and after the bending–unbending cycle test, as shown in Figure 3d,e. The current of the sensor increased in the presence of H2, indicating that the H2-sensing mechanism of the Pd NP film was not changed by the bending cycles and remained based on HILE. Further, there was negligible deviation between the calibration curves of the H2 sensor before and after the bending–unbending cycle test, as demonstrated in Figure 3c, which



indicates that the sensor is able to withstand repeated mechanical deformation without significant degradation in H2-sensing performance. The calibration curves, similar to the results of our previous studies,25‒26 exhibit three discrete response regimes with different slopes corresponding to the three phase regions of Pd hydride, i.e., the α phase region, the α−β phase coexistence region, and the β phase region, which further confirms that the bending cycles had no effect on the H2-sensing mechanism with respect to the Pd NP film and flexible polymer combination. The stable H2-sensing behavior can be attributed to the discontinuous structure of the Pd NP monolayer, in which the discrete NPs shift with increasing substrate stress and are preserved on the strained polymeric membrane rather than induce cracks as in continuous-metal layers or bulk metals.32 Repeated deformations have no irreversible effects on the surface morphology of the Pd NP film due to the discrete nanogranular structure. These results reveal that our H2 sensor not only possesses prominent mechanical robustness and flexibility but also exhibits stable and reproducible H2-sensing performance even after hundreds of bending cycles. Effect of Strain on the H2-Sensing Behaviors. Essentially, the electro-mechanical responsiveness of the flexible device derives from the interparticle electronic coupling that plays a key role in the charge transport properties of the metallic NP assemblies. On the basis of the fact that the interparticle electron coupling strength can be significantly altered by adjusting the interparticle separation,33‒35 it seems reasonable to expect that the strains applied to substrates have a marked influence on H2-sensing behavior due to the potential strong correlation between H2-sensing mechanism and tunneling process. To evaluate the effect of bending-induced strain on H2-sensing performance, the response to H2 of the strained device, which was attached to cylinder wall as depicted in Figure 4a, was measured. Ignoring the thickness of the substrate and cylinder wall, the radius of curvature is 50 mm, half of the diameter of the cylinder, which resulted in about 5‰ tensile or compressive strains. The calibration plots for the device in the bent and flat states are compared in Figure 4b. The response characteristics of the device in the bent states are overall quite similar to that in flat state. Sigmoidal calibration curves were observed for all bent and flat states of the sensor, but, ignoring this basic similarity, differences in the


Page 12 of 29

Page 13 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


response amplitude and slopes of the calibration curves were also observed. Here, we are more concerned with the sensing behaviors at low range of hydrogen pressures because there is a nonnegligible need for accurate detection of H2 at low concentration range due to the safety issues associated with the H2-based applications. Taking greater insight into the calibration curves in the α phase region (PH2 < 1000 Pa), the response amplitude ΔI/I0 was linearly correlated with PH21/2, as shown in Figure 4c. The results can be explained by applying Sievert’s law and the conductance model of electronic tunneling transfer (see the Supporting Information, section of Theoretical Analysis of the Calibration Curve and Figure S9). Interestingly, in the case of the α phase region (PH2 < 1000 Pa), the slopes of these calibration curves, which are associated with the sensitivity of the sensor according to the definition by the International Union of Pure and Applied Chemistry,36 are 0.685, 0.418, and 0.257 % Pa−1/2 in tensile, flat, and compressive states, respectively, indicating that the sensitivity of the H2 sensor was enhanced by up to ~64%, compared with that in the unstrained situation, by applying a tensile strain. The slope of the calibration curve and the response amplitude (ΔI/I0) for tensile strain was larger than that in the flat state, whereas the slope of the sensor calibration curve and the response amplitude for compressive strain was smaller than that in the flat state, which suggests that the tensile and compressive strain had a positive and negative influence, respectively, on the sensitivity of the H2 sensor.



Figure 4. H2-sensing of a sensor with a baseline current of 41.67 nA under strain. (a) A flexible sensor device showing convex and concave bending. The corresponding lower diagrams show the device exposed to H2 molecules (red) under strain; the Pd NPs are represented by the white spheres while the IDEs are simplified as a pair of yellow plane electrodes. (b) Calibration plots of ΔI/I0 versus PH2 for the sensor under tensile and compressive strain on a log−log scale which yields a huge difference in the α regime. As the error bars are decreased in the β regime the lines are not overlapping at all. Three discrete response regimes, corresponding to the three phase regions of Pd hydride, were observed. (c) ΔI/I0 versus PH21/2 in the α phase region (PH2 < 1000 Pa). The dashed lines are the linear fitting of the dependence of ΔI/I0 on PH21/2. The intercepts of all the fitting lines were fixed at 0, since the response to 0 Pa of H2 should be 0. The slopes of the linear fitting are indicated. (d) ΔI/I0 versus PH21/2 in the α phase region at different temperatures for the bent sensor applied 1% tensile strain.

In view of the HILE-based H2-sensing mechanism, the sensitivity depends on lattice expansion induced changes in the cotunneling current and the initial current flowing through the percolation network in the Pd NP film. For concave bending of the device, the I0 increases with increasing compressive strain because of the shorter interparticle distance. Additionally, the changes in the I caused by the lattice expansion will decrease because the interparticle space is squeezed by the compressive strain, limiting swelling of the Pd NPs. Consequently, under the effects of decreased variation in the response current and the incremental increase in the initial current induced by compressive strain, the sensitivity will clearly be reduced. In


Page 14 of 29

Page 15 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


contrast, the tensile strain provides more freedom for the NPs to swell in space, by loosening the NP percolation network and also reducing the I0, resulting in a higher overall sensitivity than that in flat state. In the case of the β phase region, a monotonic correlation of the response amplitude with H2 pressure at the same slope value of 0.0031 ± 0.0002 % Pa−1 and no saturation response was observed up to 10 kPa (Figure S10), indicating that the device is capable of quantitatively detecting H2 in this high hydrogen pressure range which corresponding to 10% hydrogen concentration. This result reveals that our sensor is able to response to H2 with the dynamic response range as wide as 0−10%. Furthermore, the device under 5‰ tensile strain was tested to various pressures of H2 in the α phase region at room temperature (RT, ∼ 22 °C), 35 °C and 50 °C to assess the effect of temperature on sensing properties due to the time and temperature dependent viscoelastic nature of PET substrate.37 The experimental setup was the same as for the tests in Figure 3a, except that a ceramic heating cylinder with 25 mm diameter, controlled by a proportionintegral-differential (PID) controller, was used to provide different ambient temperatures and a known deformation, ε = 1%, as shown in Figure S11a. The raw data of the response signal at various temperatures can be found in Figure S11b−d. In general, the H2 sensor exhibited positive response (ΔI > 0) under all the test temperature. The response amplitude versus H2 pressures graph of the bent device exposed to H2 at indicated test temperatures was plotted in Figure 4d. The response amplitude and sensitivity of the bent device decreased concomitantly with the increasing temperature. This can be mainly attributed to the reduced solubility of H in PdHx at elevated temperature.38 On the basis of the natural absorption behavior of Pd materials with H2 molecules, it is reasonable to propose that, at higher temperatures, a lower solubility of H atoms in PdHx can produce smaller changes in crystal grain volume during gas absorption, thus reducing the response amplitude of the NP film to H2. However, the reduction in response amplitude was much smaller at 35 °C compared to that at 55 °C. Thanks to the negligible degradation in response amplitude at 35 °C, which is close to human body temperature, our sensors showed potential application prospects in wearable devices. We now investigate the performance of the H2 sensor with respect to response time. The



response of the flexible sensor to a series of rapid H2 loadings was measured to evaluate its kinetic characteristics in different strain states. To test this, the sample was sealed inside a small cell that was connected to the test chamber through a high-speed electromagnetic valve, as shown in Figure S12 in the Supporting Information. The response time is defined as the time required for the increase in the current ΔI to reach 0.9ΔImax (where ΔImax is the maximum change in the response current for H2 loading) after the sensor is exposed to a given H2 pressure. The normalized response signal ΔI/ΔImax versus time plots for the device in the strain and flat states are shown in Figure 5a. It is evident that the change in current of the sample initially increased with time and then saturated at a value that depended on the H2 pressure. Fast response times of 4.62 and 2.74 s were measured at H2 pressures of 500 ± 25 and 10000 ± 500 Pa, respectively, in the flat state. It is worthy of note that the detector response time not only depended on the H2 pressure but was also greatly affected by the applied strain. As the sensor was bent at tensile strain, the response time diminished, with values of 3.44 and 1.41 s at H2 pressures of 500 ± 25 and 10000 ± 500 Pa, respectively, whereas in the case of compressive strain the response time was prolonged to 9.36 and 7.6 s, respectively. To further investigate the sensing kinetic characteristics of our flexible H2 sensor, the comprehensive trends of the response time as functions of H2 pressure in the flat and bent states are shown in Figure 5b. As might be expected, the response time in all strained states is noticeably shortened with increasing H2 pressure, except for a slow-response peak due to the large lattice expansion caused by nucleation and growth of the β phase in the α matrix, similar to the behavior of H2 sensors based on Pd nanostructures on rigid substrates.25, 39‒41


Page 16 of 29

Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. The device sensing kinetics under strain. (a) Normalized response curves at H2 pressures of 500 ± 25 Pa and 10000 ± 500 Pa in the flat, tensile strain, and compressive strain states. The horizontal dashed lines indicate ΔI/ΔImax = 0.90. (b) Response time versus hydrogen pressure plots for the sensor in the flat, tensile strain, and compressive strain states on a linear−log scale. The inset shows the schematic illustration of the effect of strain on the diffusion length. The dashed lines denote the diffusion paths of hydrogen atoms in the Pd NPs.

More importantly, the device exhibited significant prolonging of the response time with compressive strain, while the reverse was observed for tensile strain. We attribute this phenomenon to the effects of strain on the diffusion path (the inset in Figure 5b) and release of stress on the interface between the flexible substrate and Pd NPs. When the device was subjected to tensile strain, the interparticle distance increased and created more space as well as adsorption sites, especially among neighboring NPs in the NP aggregation areas. An in situ small angle X-ray scattering (SAXS) study of the nanoparticle displacement was carried out and confirmed that the interparticle distance increased with loading tensile strain.32 Due to this, the diffusion paths of hydrogen atoms in Pd NPs were shorter (the inset in Figure 5b), resulting in considerable improvement in the response speed. On the other hand, a greater impact of compressive strain on the response time was observed (Figure 5b). Compressive strain hindered the sensing process by not only diminishing H2 reaction sites and extending



the diffusion distance but also suppressing stress relaxation at the interface between Pd NPs and the PET substrate. In fact, H2 uptake in metallic Pd can result in compressive stresses in the gigapascal range 42. Additional compressive stress originating from the concave bending of the PET substrate will significantly prolong stress relaxation at the interface between Pd NPs and the PET substrate, leading to slowing of the response time. Collectively, affected by the extension of the diffusion distance and the additional stress relaxation, the response time was more sensitive to compressive strain than to tensile strain. These experimental results unambiguously show that a flexible H2 sensor with a sub-10 s response time has been achieved. It is possible, but unconfirmed, that the flexible polymeric substrate with a low elastic modulus is mainly responsible for the fast response, due to the relatively mild stress relaxation.

Figure 6. Real time response to H2/Ar mixture. The concentration (in ppm) of for each hydrogen loading is marked at the top of each response peak. The inset shows the magnified image of the response peak for 130 ppm H2, from which the signal-to-noise ratio (SNR = H/h = 8.85) is extracted.

Limit of Detection for Hydrogen Gas. Now, we turn to the limit of detection for H2 (LODH2). The response to a low-concentration (130−760 ppm) H2/Ar mixture was investigated (Figure 6). The sensor exhibited a significant response to a 130 ppm concentration of H2 with about 0.487% change in the response amplitude, which is much higher than the electronic noise. This indicates that the LODH2 of the sensor is less than 130 ppm. To obtain a reliable LODH2, we determined its value from the experimental data and by calculation using the signal-to-noise method.43‒44 In the case of the response to 130 ppm H2, the signal-to-noise ratio (SNR) was about 8.85 (the inset in Figure 6). The detection limit can


Page 18 of 29

Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


thus be conservatively estimated as LOD H = 130 ppm / SNR  15 ppm . This value is low 2

enough for H2 leak detection, since the lower explosion limit for H2 is 4%. Response to H2/air Mixture in Different Humidity Environment. Finally, the influence of relative humidity (RH) on the sensing behaviors was investigated for its importance in developing sensors operable at atmospheric environment. Figure 7 shows the response to H2 in air with concentrations of 1%, 0.6% and 0.3% measured at various humidity levels from 0 to 90% at room temperature. The response signals, in which current growth curves upon H2 exposure were observed in Figure 7a, are similar to the responses to pure H2. This illustrates that the water vapor present in the humid air should not have effect on the sensing mechanism during the hydriding and dehydriding of Pd NPs. However, the response amplitude (ΔI/I0) of the flexible H2 sensor decreased with the RH increased at the testing concentrations of H2. Up to 90% RH, the response and recovery speed became significantly slower. This result was reasonable because the physisorbed water vapor occupied the active sites of the Pd NPs, weakening the ability of the surface to adsorb H2 molecules, and thus slowing the device sensing kinetics.45 But overall, our sensors can be operated in the relative humidity range of 0−90%.



Page 20 of 29

Figure 7. The effect of humidity on the sensor performance. (a) Sensor response to H2/air mixture with concentrations of 1%, 0.6% and 0.3% in dry atmosphere (RH = 0%) and in humid atmosphere (RH = 30%, 60%, 90%). (b) Relationship of sensor response amplitude with H2 concentrations under dry and RH conditions.

Comparison of Different Nanomaterial-based H2 Sensors. We compared the key metrics of our sensor with recently developed nanomaterial-based H2 sensors, as summarized in Table 1.18‒19,

41, 46‒51

Although the H2-sensing performance of our sensor is predominantly

influenced by the applied strain, its overall performance is better than other sensors, to the best of our knowledge (see Table 1). Furthermore, the sensitivity, response time, and response amplitude can easily be regulated by applying strains to the flexible substrate without the use of additional complicated and expensive technologies. The coupling of Pd NP film assemblies and flexible substrates not only reduces cost and complexity but also confers tunability of the sensing performance. Table 1. Sensing performance for nanomaterial-based resistive H2 sensors Sensing elementa Single Pd NW48

Response amplitude (%) 3 (0.1%)

Detecting rangec

LODH2 (ppm)

Response time (s)

Flexibility

2

~400 (0.1%)

No

2 ppm−10%


Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CNT network49

1000 (311 ppm)

0.89−311 ppm

0.89

7 (311ppm)

No

10 (0.1%)

50 ppm−0.8%

50

12 (0.1%)

No

Pd NPs/graphene46

54.1 (2%)

0.2−2%

2000

6 (2%)

No

Pd/Mg NWs mesh19

~11.7 (10000 ppm)

10−40000 ppm

10

∼6 (10000 ppm)

Yes

Pd NWs@ZIF-851

0.3 (0.1%)

1000 ppm−1%

1000

13 (0.1%)

No

Pd@Ag HNWs18

0.89 (900 ppm)

100−900 ppm

100

120

Yes

~100 (2%)

600 ppm−10%

600

~0.38 (2%)

Yes

~4 (0.1%)

0.1−100%

1000

~27 (0.1%)

No

~5.5 (0.1%)b

15 ppm−10% b

15

~6 (1000 ppm) b

Yes

Pd NPs@Si nanomesh50

Pd-PMMA hybrid film47 Network of Pd/Cr NWs41 Pd NP film (this work) aAbbreviations: bPure

CNT = carbon nanotube; HNW = hollow nanowire; NP = nanoparticle; NW = nanowire.

H2 pressures were converted to hydrogen concentrations by assuming a 1 atm ambient pressure. The key metrics of our

sensor were extracted from the experimental data in the flat state. cThe

detecting range is the gas concentration measured in the study. It may not exactly reflect the actual limited working

range for the sensors.

CONCLUSION We have realized an optically transparent and flexible HILE-based H2 sensor by depositing a Pd NP film onto a PET substrate. The H2 responsiveness showed no performance degradation after 500 bending cycles, showing good flexibility, robust electromechanical properties, and stable H2-sensing behavior due to its discrete nanogranular structure and unique interparticle electrical responsiveness. The effect of strain on the H2-sensing behaviors was investigated. In the α phase region, a linear dependence between the response amplitude ΔI/I0 and PH21/2 was observed. The influence of tensile and compressive strains under bent state on H2sensing performance was systematically evaluated. A tensile strain was found to produce greater sensitivity, which was enhanced by up to ~64% compared with the unstrained situation, owing to more freedom to swell in space and the lower initial current baseline induced by tensile strain. Moreover, the loading of tensile and compressive strain produced a positive and negative influence, respectively, on the sensing kinetics of the flexible H2 sensor. The ability to detect an H2 concentration as low as 15 ppm, dynamic response range as wide



Page 22 of 29

as 0−10% and sub-10 s response time were achieved. In addition, the flexible transparent sensor can be operated in the relative humidity range of 0−90%. Compared with other conventional rigid-substrate-based H2 sensors, our sensor can be fabricated using a controllable low-cost process, and also displays superior sensing performance. The nanogranular film described in this work holds promise for the development of flexible, transparent, and wearable sensors for chemical detection in the coming era of the Internet of Things.

EXPERIMENTAL SECTION Fabrication of the Flexible H2 Sensors. Commercial PET sheet (Shanghai Haomiao Industrial Co., Ltd.) of 0.5 mm thickness (except where noted) was purchased and used as the flexible substrate. This was ultrasonically cleaned in ethanol and deionized water for 30 min and dried with nitrogen. A pair of silver IDEs with a 35 μm gap was deposited by mask evaporation deposition, as schematically depicted in Figure 1a. The Pd NPs were generated from a magnetron plasma gas aggregation cluster source

27

(Figure 1c) and extracted to a

high-vacuum deposition chamber with a differential pumping system

27, 52.

Pure argon (80

sccm) was introduced into the aggregation tube by a mass flow controller (Sevenstar, CS200), to maintain a stable pressure of 100 Pa. A direct-current magnetron sputtering power supply (Advanced Energy, MDX-500) was used to sustain a stable magnetron discharge with a preset input power. The deposition rate was precisely controlled by adjusting the sputtering power and monitored by a quartz crystal microbalance. The current across the IDE gap was monitored in situ with a computer-interfaced sourcemeter (Keithley 2636B) by applying a bias of 1 V during deposition, as shown in Figure 1c. The deposition was stopped by operating a shutter when the predetermined current values were attained. Device Characterization. An amorphous carbon film on a 300-mesh formvar-coated copper grid was placed alongside the IDEs on the PET substrate, and the Pd NPs were deposited onto it simultaneously. The microstructure of the Pd NPs was characterized using a scanning transmission electron microscopy (STEM, JEOL, JEM2100F). The image were acquired in the high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) mode. For further analysis of the surface topography of the Pd NP films, we


Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


performed transmission electron microscope (TEM, FEI Tecnai F20s Twin) at low magnifications. The surface of the Pd NP films on the flexible PET substrate was characterized using a field emission scanning electron microscope (FE-SEM, Hitachi, SU8010). Optical characterization was carried out using an ultraviolet-visible-near-IR spectrophotometer (Agilent, Carry 5000) in the wavelength range of 400 to 1500 nm. Experimental details of the measurement of spectral transmittance are provided in Figure S5 in the Supporting Information. Electro-mechanical Properties Measurement. In the bending cycle test, the sensor, clamped by two programmable stepper-motor-controlled sliders (see the inset in Figure 3b), was subjected to bending–unbending cycles in which the two sliders moved back and forth. The current (at 1 V bias) of the flexible sensor in the bent and flat states was measured by the sourcemeter (Keithley 2636B). Gas-Sensing Measurement. As shown in Figure 3a, characterization of the pure H2-sensing behavior of our devices was carried out in a test chamber of our laboratory, which was connected to a mechanical pump by a valve and could be evacuated in the H2 loading and deloading cycles. Pure H2 or an H2/Ar mixture (the test gas) was introduced to the test chamber at a constant gas flow rate of 200 sccm using a mass flow controller (Sevenstar, D07-7A/ZM). The pressure of pure H2 (PH2) in the test chamber was monitored by a piezoresistive gauge (Zhenghua, ZJ-1P), since the sensor responds, in principle, only to H2 partial pressure. H2/Ar mixtures with different concentrations were obtained by adjusting the H2 and Ar gas flow with the help of high-precision mass flow controllers. The sensor was connected to the sourcemeter (Keithley 2636B) by an electrical feedthrough, to measure the current (at bias voltage of 1.0 V) during H2 sensing. In order to measure the H2-sensing properties under strain, the flexible device was bent by wrapping it around the external (convex bending) and internal (concave bending) surfaces of a cylinder, as shown in Figure 4a. The schematic diagram of the measurement configuration used for response time measurements is showed in Figure S12. The tested samples were sealed inside a small cell (~100 mL) which was connected to the intrinsic testing chamber (~30 L) through a highspeed electromagnetic valve. The large chamber was filled with H2 to a predetermined pressure, and



then a rapid H2 loading were switched into the small cell as the electromagnetic valve was rapidly opened. Because the small cell’s volume is far smaller than that of the large chamber, the H2 pressure changes are negligible during the formation of the H2 loading. For measuring the LODH2 of the sensor, a gas mixer was equipped between the test chamber and mass flow controllers in order to obtain low concentration H2, as shown in Figure S13. During the tests, 300 sccm Ar gas, controlled using a mass flow controller, was continually flowed through the gas mixer into the chamber, and H2/Ar mixture of 1% concentration was also flowed through the gas mixer into the chamber in the form of pulse at different flow rate. The concentration of H2 can be obtained from the flow ratio of the two gases. The setup for H2 sensing in different humidity environment is schematically shown in Figure S14. By bubbling dry air in water and changing the flow rates through the mass flow controllers, the humidity of the target gas is controlled. The mixture of dry, wet, and hydrogen-containing air flowed through the test chamber with the flow rate of 1000 sccm. A hygrometer was placed nearby the sample to obtain the ambient humidity.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Evaluation of the current between the interdigital electrodes during Pd NPs deposition; HRSTEM image of neighboring NPs; TEM images of the same Pd NP film characterized by STEM in Figure 2 at randomly selected 8 regions; Spectral transmittance of the Pd NP films on PET sheets for different deposition times from visible to near infrared wavelength region; Schematic diagram of the transmission measurement; The analysis of the thickness and roughness of this Pd NP film; Illustrations of the device bending orientations in the bending cycle test; SEM images of the Pd NP film on PET sheet before and after the bending cycle test; Theoretical Analysis of the Calibration Curve; Plots of the data from Figure 4b in the β phase region on a linear scale; Schematic diagram of the measurement configuration used for gas-sensing measurement (Figure S11−14).


Page 24 of 29

Page 25 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (B.X.) *E-mail: [email protected] (M.H.) 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. B.X., P.M., and M.C. contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant numbers 51871091, 11627806, 21802038, 11604161, and 11604310), the Scientific Research Project of Education Department of Hubei Province (grant number Q20182505), the Hubei Key Laboratory of Pollutant Analysis and Reuse Technique (Hubei Normal University) (grant number PA20170204), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (grant number 16KJB140009), and the Natural Science Foundation of Jiangsu Province (grant number BK20160914).



REFERENCES 1.

Buttner, W. J.; Post, M. B.; Burgess, R.; Rivkin, C., An Overview of Hydrogen Safety Sensors and

Requirements. Int. J. Hydrogen Energy 2011, 36 (3), 2462‒2470. 2.

Khan, Y.; Ostfeld, A. E.; Lochner, C. M.; Pierre, A.; Arias, A. C., Monitoring of Vital Signs with

Flexible and Wearable Medical Devices. Adv. Mater. 2016, 28 (22), 4373‒4395. 3.

Trung, T. Q.; Lee, N.-E., Materials and Devices for Transparent Stretchable Electronics. J. Mater.

Chem. C 2017, 5 (9), 2202‒2222. 4.

Lee, J. H.; Hwang, J.-Y.; Zhu, J.; Hwang, H. R.; Lee, S. M.; Cheng, H.; Lee, S.-H.; Hwang, S.-W.,

Flexible Conductive Composite Integrated with Personal Earphone for Wireless, Real-Time Monitoring of Electrophysiological Signs. ACS Appl. Mater. Interfaces 2018, 10 (25), 21184‒21190. 5.

Zhang, S.; Li, Y.; Tian, Q.; Liu, L.; Yao, W.; Chi, C.; Zeng, P.; Zhang, N.; Wu, W., Highly Conductive,

Flexible and Stretchable Conductors Based on Fractal Silver Nanostructures. J. Mater. Chem. C 2018, 6 (15), 3999‒4006. 6.

Wu, W., Inorganic Nanomaterials for Printed Electronics: a Review. Nanoscale 2017, 9 (22),

7342‒7372. 7.

Kano, S.; Kim, K.; Fujii, M., Fast-Response and Flexible Nanocrystal-Based Humidity Sensor for

Monitoring Human Respiration and Water Evaporation on Skin. ACS Sens. 2017, 2 (6), 828‒833. 8.

Guo, H.; Lan, C.; Zhou, Z.; Sun, P.; Wei, D.; Li, C., Transparent, Flexible, and Stretchable WS2 Based

Humidity Sensors for Electronic Skin. Nanoscale 2017, 9 (19), 6246‒6253. 9.

Segev-Bar, M.; Haick, H., Flexible Sensors Based on Nanoparticles. ACS Nano 2013, 7 (10), 8366‒8378.

10. Nakata, S.; Arie, T.; Akita, S.; Takei, K., Wearable, Flexible, and Multifunctional Healthcare Device with an ISFET Chemical Sensor for Simultaneous Sweat pH and Skin Temperature Monitoring. ACS Sens. 2017, 2 (3), 443‒448. 11. Duy, L. T.; Trung, T. Q.; Dang, V. Q.; Hwang, B. U.; Siddiqui, S.; Son, I. Y.; Yoon, S. K.; Chung, D. J.; Lee, N. E., Flexible Transparent Reduced Graphene Oxide Sensor Coupled with Organic Dye Molecules for Rapid Dual-Mode Ammonia Gas Detection. Adv. Funct. Mater. 2016, 26 (24), 4329‒4338. 12. Perillo, P.; Rodríguez, D., TiO2 Nanotubes Membrane Flexible Sensor for Low-Temperature H2S Detection. Chemosensors 2016, 4 (3), 10‒19. 13. Riordan, S. M.; McIver, C. J.; Duncombe, V. M.; Thomas, M. C.; Bolin, T. D., Evaluation of the Rice Breath Hydrogen Test for Small Intestinal Bacterial Overgrowth. Am. J. Gastroenterol 2000, 95 (10), 2858‒2864. 14. Zhang, J.; Song, L.; Zhang, Z.; Chen, N.; Qu, L., Environmentally Responsive Graphene Systems. Small 2014, 10 (11), 2151‒2164. 15. Liu, H.; Li, M.; Voznyy, O.; Hu, L.; Fu, Q.; Zhou, D.; Xia, Z.; Sargent, E. H.; Tang, J., Physically Flexible, Rapid-Response Gas Sensor Based on Colloidal Quantum Dot Solids. Adv. Mater. 2014, 26 (17), 2718‒2724. 16. Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S., Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6 (9), 652‒655. 17. Zhou, X.; Lee, S.; Xu, Z.; Yoon, J., Recent Progress on the Development of Chemosensors for Gases.

Chem. Rev. 2015, 115 (15), 7944‒8000. 18. Jang, J. S.; Qiao, S.; Choi, S. J.; Jha, G.; Ogata, A. F.; Koo, W. T.; Kim, D. H.; Kim, I. D.; Penner, R. M., Hollow Pd-Ag Composite Nanowires for Fast Responding and Transparent Hydrogen Sensors. ACS Appl.

Mater. Interfaces 2017, 9 (45), 39464‒39474.


Page 26 of 29

Page 27 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19. Hassan, K.; Uddin, A. S. M. I.; Chung, G.-S., Mesh of Ultrasmall Pd/Mg Bimetallic Nanowires as Fast Response Wearable Hydrogen Sensors Formed on Filtration Membrane. Sens. Actuators, B 2017, 252, 1035‒1044. 20. He, Q.; Liu, J.; Liu, X.; Li, G.; Deng, P.; Liang, J., Preparation of Cu2O-Reduced Graphene Nanocomposite Modified Electrodes towards Ultrasensitive Dopamine Detection. Sensors 2018, 18 (1), 199‒211. 21. Salvi, B. L.; Subramanian, K. A., Sustainable Development of Road Transportation Sector Using Hydrogen Energy System. Renewable & Sustainable Energy Reviews 2015, 51, 1132‒1155. 22. Kammen, D. M.; Lipman, T. E., Assessing the Future Hydrogen Economy. Science 2003, 302 (5643), 226‒229. 23. Öztürk, S.; Kılınç, N., Pd Thin Films on Flexible Substrate for Hydrogen Sensor. J. Alloys Compd. 2016, 674, 179‒184. 24. Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M., Hydrogen Sensors and Switches from Electrodeposited Palladium Mesowire Arrays. Science 2001, 293 (5538), 2227‒2231. 25. Chen, M.; Mao, P.; Qin, Y.; Wang, J.; Xie, B.; Wang, X.; Han, D.; Wang, G. H.; Song, F.; Han, M.; Liu, J. M.; Wang, G., Response Characteristics of Hydrogen Sensors Based on PMMA-Membrane-Coated Palladium Nanoparticle Films. ACS Appl. Mater. Interfaces 2017, 9 (32), 27193‒27201. 26. Xie, B.; Liu, L. L.; Peng, X.; Zhang, Y.; Xu, Q.; Zheng, M. Y.; Takiya, T.; Han, M., Optimizing Hydrogen Sensing Behavior by Controlling the Coverage in Pd Nanoparticle Films. J. Phys. Chem. C 2011,

115 (32), 16161‒16166. 27. Han, M.; Xu, C. H.; Zhu, D.; Yang, L.; Zhang, J. L.; Chen, Y. P.; Ding, K.; Song, F. Q.; Wang, G. H., Controllable Synthesis of Two-Dimensional Metal Nanoparticle Arrays with Oriented Size and Number Density Gradients. Adv. Mater. 2007, 19 (19), 2979‒2983. 28. Kousal, J.; Shelemin, A.; Schwartzkopf, M.; Polonskyi, O.; Hanuš, J.; Solař, P.; Vaidulych, M.; Nikitin, D.; Pleskunov, P.; Krtouš, Z.; Strunskus, T.; Faupel, F.; Roth, S. V.; Biederman, H.; Choukourov, A., Magnetron-Sputtered Copper Nanoparticles: Lost in Gas Aggregation and Found by In Situ X-ray Scattering. Nanoscale 2018, 10 (38), 18275‒18281. 29. Cizek, J.; Prochazka, I.; Vlach, M.; Zaludova, N.; Dobron, P.; Chmelik, F.; Brauer, G.; Anwand, W.; Mücklich, A.; Nikitin, E.; Gemma, R.; Pundt, A.; Kirchheim, R., Multi-Scale Analysis of Hydrogen-Induced Buckling in Pd Films. Procedia Eng. 2009, 1 (1), 99‒103. 30. Yin, J.; Hu, P.; Luo, J.; Wang, L.; Cohen, M. F.; Zhong, C. J., Molecularly Mediated Thin Film Assembly of Nanoparticles on Flexible Devices: Electrical Conductivity Versus Device Strains in Different Gas/Vapor Environment. ACS Nano 2011, 5 (8), 6516‒6526. 31. Binns, C., Nanoclusters Deposited on Surfaces. Surf. Sci. Rep. 2001, 44 (1‒2), 1‒49. 32. Siffalovic, P.; Chitu, L.; Vegso, K.; Majkova, E.; Jergel, M.; Weis, M.; Luby, S.; Capek, I.; Keckes, J.; Maier, G. A.; Satka, A.; Perlich, J.; Roth, S. V., Towards Strain Gauges Based on a Self-Assembled Nanoparticle Monolayer--SAXS Study. Nanotechnology 2010, 21 (38), 385702‒385706. 33. Huth, M., Granular Metals: from Electronic Correlations to Strain-Sensing Applications. J. Appl. Phys. 2010, 107 (11), 113709‒113715. 34. Beloborodov, I. S.; Lopatin, A. V.; Vinokur, V. M.; Efetov, K. B., Granular Electronic Systems. Rev.

Mod. Phys. 2007, 79 (2), 469‒518. 35. Xie, B.; Mao, P.; Chen, M.; Li, Z.; Liu, C.; Qin, Y.; Yang, L.; Wei, M.; Liu, M.; Wang, X.; Han, D.; Li, S.; Song, F.; Han, M.; Liu, J.-M.; Wang, G., A Tunable Palladium Nanoparticle Film-Based Strain Sensor in a Mott Variable-Range Hopping Regime. Sens. Actuators, A 2018, 272, 161‒169.



36. Currie, L. A., Nomenclature in Evaluation of Analytical Methods Including Detection and Quantification Capabilities (IUPAC Recommendations 1995). Pure Appl. Chem. 1995, 67 (10), 1699‒1723. 37. Duan, L.; D’hooge, D. R.; Spoerk, M.; Cornillie, P.; Cardon, L., Facile and Low-Cost Route for Sensitive Stretchable Sensors by Controlling Kinetic and Thermodynamic Conductive Network Regulating Strategies.

ACS Appl. Mater. Interfaces 2018, 10 (26), 22678‒22691. 38. Fisser, M.; Badcock, R. A.; Teal, P. D.; Hunze, A., Optimizing the Sensitivity of Palladium Based Hydrogen Sensors. Sens. Actuators, B 2018, 259, 10‒19. 39. Penner, R. M., A Nose for Hydrogen Gas: Fast, Sensitive H2 Sensors Using Electrodeposited Nanomaterials. Acc. Chem. Res. 2017, 50 (8), 1902‒1910. 40. Xie, B.; Zheng, M. Y.; Liu, F.; Peng, X.; Wang, G. H.; Han, M., Fast Response Characteristics of Hydrogen Sensors Based on Pd Nanoparticle Films with Controlled Coverage. J. Nanopart. Res. 2013, 15 (6), 1746‒1754. 41. Zeng, X. Q.; Wang, Y. L.; Deng, H.; Latimer, M. L.; Xiao, Z. L.; Pearson, J.; Xu, T.; Wang, H. H.; Welp, U.; Crabtree, G. W.; Kwok, W. K., Networks of Ultrasmall Pd/Cr Nanowires as High Performance Hydrogen Sensors. ACS Nano 2011, 5 (9), 7443‒7452. 42. Wagner, S.; Kramer, T.; Uchida, H.; Dobron, P.; Cizek, J.; Pundt, A., Mechanical Stress and Stress Release Channels in 10‒350 nm Palladium Hydrogen Thin Films with Different Micro-Structures. Acta

Mater. 2016, 114, 116‒125. 43. Long, G. L.; Winefordner, J. D., Limit of Detection a Closer Look at the IUPAC Definition. Anal.

Chem. 1983, 55 (07), 712A‒724A. 44. Alankar, S.; Vipin, B. G., Methods for the Determination of Limit of Detection and Limit of Quantitation of the Analytical Methods. Chron. Young Sci. 2011, 2 (1), 21‒25. 45. Lange, U.; Hirsch, T.; Mirsky, V. M.; Wolfbeis, O. S., Hydrogen Sensor Based on a Graphene– Palladium Nanocomposite. Electrochim. Acta 2011, 56 (10), 3707‒3712. 46. Kumar, R.; Malik, S.; Mehta, B. R., Interface Induced Hydrogen Sensing in Pd Nanoparticle/Graphene Composite Layers. Sens. Actuators, B 2015, 209, 919‒926. 47. Jang, B.; Lee, K. Y.; Noh, J.-S.; Lee, W., Nanogap-Based Electrical Hydrogen Sensors Fabricated from Pd-PMMA Hybrid Thin Films. Sens. Actuators, B 2014, 193, 530‒535. 48. Yang, F.; Taggart, D. K.; Penner, R. M., Fast, Sensitive Hydrogen Gas Detection Using Single Palladium Nanowires that Resist Fracture. Nano Lett. 2009, 9 (5), 2177‒2182. 49. Xiao, M.; Liang, S.; Han, J.; Zhong, D.; Liu, J.; Zhang, Z.; Peng, L., Batch Fabrication of Ultrasensitive Carbon Nanotube Hydrogen Sensors with Sub-ppm Detection Limit. ACS Sens. 2018, 3 (4), 749‒756. 50. Gao, M.; Cho, M.; Han, H. J.; Jung, Y. S.; Park, I., Palladium-Decorated Silicon Nanomesh Fabricated by Nanosphere Lithography for High Performance, Room Temperature Hydrogen Sensing. Small 2018, 14 (10), 1703691‒1703700. 51. Koo, W.-T.; Qiao, S.; Ogata, A. F.; Jha, G.; Jang, J.-S.; Chen, V. T.; Kim, I.-D.; Penner, R. M., Accelerating Palladium Nanowire H2 Sensors Using Engineered Nanofiltration. ACS Nano 2017, 11 (9), 9276‒9285. 52. de Heer, W. A., The Physics of Simple Metal Clusters: Experimental Aspects and Simple Models. Rev.

Mod. Phys. 1993, 65 (3), 611‒676.


Page 28 of 29

Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents


Pd Nanoparticle Film on a Polymer Substrate for Transparent and

Recommend Documents