Hollow PdâAg Composite Nanowires for Fast Responding and

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Hollow Pd-Ag Composite Nanowires for Fast Responding and Transparent Hydrogen Sensors Ji-Soo Jang, Shaopeng Qiao, Seon-Jin Choi, Gaurav Jha, Alana F Ogata, Won-Tae Koo, Dong-Ha Kim, Il-Doo Kim, and Reginald M. Penner ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10908 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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ACS Applied Materials & Interfaces

Hollow Pd-Ag Composite Nanowires for Fast Responding and Transparent Hydrogen Sensors Ji-Soo Jang, †Shaopeng Qiao,¶ Seon-Jin Choi,§ Gaurav Jha,‡ Alana F. Ogata,‡ Won-Tae Koo, † Dong-Ha Kim, † Il-Doo Kim†,* and Reginald M. Penner‡,*

†

Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Department of Chemistry, University of California, Irvine, CA 92697, United States ¶ Department of Physics, University of California, Irvine, CA 92697, United States § Biomaterials Biomaterials Innovation Research Center, Division of Engineering in Medicine, Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA 02139, United States *E-mail: [email protected] and [email protected]

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ABSTRACT

Pd based alloy materials with hollow nanostructures are ideal hydrogen (H2) sensor building blocks because of their double-H2 senisng active sites (interior and exterior side of hollow Pd alloy) and fast response. In this work, for the first time, we report a simple fabrication process for preparing hollow Pd-Ag alloy nanowires (Pd@Ag HNWs) by using the electrodeposition of lithographically patterned silver nanowires and followed galvanic replacement reaction (GRR) to form palladium. By controlling the GRR time of aligned Ag NWs within an aqueous Pd2+-containing solution, the compositional transition and morphological evolution from Ag NWs to Pd@Ag HNWs simultaneously occurred, and the relative atomic ratio between Pt and Ag was controlled. Interestingly, a GRR duration of 17 h transformed Ag NWs into Pd@Ag HNWs, that showed enhanced H2 response and faster sensing response time - reduced 2.5-fold – as compared to silver nanowires subjected to a shorter GRR period of 10 h. Furthermore, Pd@Ag HNWs patterned on the colorless and flexible polyimide (cPI) substrate showed highly reversible H2 sensing characteristics. To further demonstrate the potential use of Pd@Ag HNWs as sensing layers for all-transparent, wearable H2 sensing devices, we patterned the Au NWs perpendicular to Pd@Ag HNWs to form heterogeneous grid-type metallic NWs electrode which showed reversible H2 sensing properties in both bent and flat state. Keywords: LPNE, galvanic replacement reaction, alloy metal, hollow nanowire, sensor


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INTRODUCTION Bimetallic materials with various structures such as core-shell (independent distribution), alloy (random distribution), and cluster-in-cluster (dumbbell shape) have attracted much attention in the past several decades due to their intriguing applications in catalysts, gas/bio sensors, optoelectronic devices, and energy storages.1-6 Among them, Pd based alloys as promising hydrogen (H2) gas sensing materials have been received considerable attention due to highly reliable H2 sensing characteristics and particulary enhaned sensing speed.2, 7 For example, Wang et al. 8 reported that Pd-Ag alloy film with high H2 permeability showed 60-fold faster H2 sensing speed than pure Pd film. Li et al.9 developed Pt-covered Pd NWs using lithographically patterned nanowire electrodeposition (LPNE) technique, which showed 2-fold faster H2 sensing speed compared to pristine Pd NWs. Pt catalysts reduced the activation energies of Pd sensors, leading to faster response/recovery kinetics. Even though Pd based alloys effectively accelerated hydrogen responding speed, but the hydrogen solubility in alloy matrix compared to pristine Pd is rather reduced. This feature suppresses phase transition from ɑ-Pd to the β-Pd phase, thereby resulting in decreased hydrogen response.10 To overcome these limitations, nanostructured H2 sensing layers with large surface areas and high porosity have been suggested. For example, Lim et al.11 synthesized Pd nanotubes array for H2 sensisng via sacrificial ZnO templating route followed by selective ethching of ZnO. However, the simultaneous and accurate manipulation in size, porosity, and morphology of Pd sensing layers is challenging. Galvanic replacement reaction (GRR), which is generally induced by electrochemical potential difference between two metallic species, is one of the most attractive techniques for formation of hollow nanostructures with high gas accessibility.12, 13 For instance, Sun et al. reported that sacrificial Ag nanoparticles (NPs) could be morphologically evolved to hollow Pd-Ag alloy (hearafter, Pd@Ag) NPs via GRR.14 Since the formation of nanostructured Pd 3 ACS Paragon Plus Environment


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alloys with open porosity can be simply achieved through GRR, one can expect faster and more sensitive H2 sensing performance. In parallel with morphological evolution, compositional tuning between Pd and Ag can be also easily achieved by adjusting GRR time and precursors concentration. So far, one-dimensional (1D) metallic NWs synthesized by various methods such as hydrothermal, electrospinning, LPNE, polyol methods and electrodeposition methods,

15-20

have been used as H2 sensing layers. Among them, LPNE technique is highly attrative because the numbers, height, thickness, and compositions of NWs can be precisely controlled.21 To the best of our knowledge, LPNE technique combined with GRR for rational designing of the porous Pd alloy NWs as H2 sensing layers has never been studied. In this work, we report on the enhanced H2 sensing properties by employing the hollow and porous Pd-Ag alloy NWs (Pd@Ag HNWs). Aligned Ag NWs were prepared by conventional LPNE and transferred to Pd@Ag HNWs through the subsequent GRR. During the GRR, macro-sized and meso-sized pores were formed on the Pd@Ag HNWs. Interestingly, we found that continuous replacement reaction from Ag to Pd induced more sensitive and faster H2 response, attributing these enhanced sensing characteritics to (i) minimization of dead interfacial sensing sites through hollow structures and (ii) alloying effect between Pd and Ag. To further investigate the potential use of thin-walled Pd@Ag HNWs as flexible and transparent H2 sensing layers, Pd@Ag HNWs directly pattenred on colorless polyimide (cPI) substrate were combined with LPNE-assisted Au NWs as transparent sensing electrodes, showing the bendable H2 sensing performance.

RESULTS AND DISCUSSION Figure 1 illustrates fabrication process of 1-D Pd@Ag HNWs integrated on the flexible cPI substrate. The aligned Ag NWs were prepared by LPNE process.22 First, the cPI substrate was 4 ACS Paragon Plus Environment

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physically attached on the glass substrate (amorphous SiO2) using adhesive 1 Mil Kapton tape. A Ni film and positive photoresist (PR) films were then sequentially deposited on the cPI (Figure 1a and 1b) using thermal evaporatation and spin coating, respectively. After patterning the photoresist using a contact mask and ultraviolet (UV) light, the exposed Ni was etched by nitric acid to form a 1-D trench at the edges of the PR layer (Figure 1c). Using these trenches as a template, Ag NWs were selectively electrodeposited, and residual PR and Ni were removed by acetone and nitric acid, respectively (Figure 1d). The patterned Ag NWs on the cPI were then immersed in the aqueous Pd precursor solution in which the galvanic replacement reaction (GRR) occurred. After the GRR, compositional and structural transitions from Ag NWs to Pd@Ag HNWs were observed. (Figure 1e). The porosity, relative composition ratio (Pd/Ag), and morphology were optimized by controlling the GRR time (10 min, 1 h, 10 h, and 17 h). Finally, Au NWs contact electrode lines perpendicular to Pd@Ag HNWs were directly electrodeposited on Pd@Ag HWNs-coated cPI substrate by using 2nd LPNE process. Finally Au NWs-integrated Pd@Au HNWs on cPI substate was detached and characterized as a wearable H2 sensing device (Figure 1f and 1g).

Morphological Features Figure 2 presents the morphological changes of the Pd@Ag HNWs as a function of various GRR time. As shown in Figure 2a, polycrystalline Ag NWs consisted of densely packed nanoparticles are transferred to hollow and porous NWs via the GRR between Ag and Pd. Due to the higher standard reduction potential of the Pd2+/Pd (0.915 V vs. standard hydrogen electrode (SHE)) than AgCl/Ag (0.22 V vs. SHE), Pd ions can be reduced to metallic Pd on the outer surface of Ag NWs, while metallic Ag is transformed to AgCl. The overall reactions are expressed in chemical reaction 1, 2, and 3.23-25 (PdCl4)2- → Pd2+ + 4Cl-

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2Ag (s) + 2Cl- → 2AgCl (s) + 2e-

(2)

Pd2+ + 2e- → Pd (s)

(3)

Based on the previous reports, (PdCl4)2- can be thermally decomposed in the form of Pd2+ and Cl-, resulting in the spontaneous formation of AgCl from a sedimentation reaction.26 During the formation of AgCl, electrons are formed simultaneously, leading to reduction of Pd2+ on the surface of Ag NWs. Not only the dissolution of Ag NWs but also the growth of metallic Pd contributes to the formation of hemi-tubular structure. Furthermore, the high miscibility of Ag with Pd can lead to alloying, thereby forming the hollow Pd-Ag alloy NWs (Pd@Ag HNWs).27 To investigate the formation of hemi-tubular Pd@Ag NWs, we carried out ex-situ scanning electron microscope (SEM) analysis. Bare Ag NWs with bumpy surface and ~180 nm width was successfully fabricated on the flexible cPI substrate (Figure 2b). Through the low magnified SEM image of Ag NWs, aligned Ag NWs with 10 μm gap were well formed on the cPI substrate (Figure S1). After 1 h GRR of Ag NWs in aqueous Pd precursor solution, metallic Pd was partially deposited on the Ag NWs, thereby showing thicker width than bare Ag NW and two separate regimes with different color contrasts (Figure 2c). However, an obvious morphological change in these NWs was not observed. On the other hand, when GRR was continued for longer times, i.e., 10 h and 17 h reaction time, morphological changes of the Pd@Ag HNWs were noticable. 17 h GRR driven Pd@Ag HNWs (hearafter, Pd@Ag HNWs_17 h) exhibited larger grain size of Pd@Ag on the surface, thicker width and more rough surface compared with those of 10 h GRR driven Pd@Ag HNWs (hearafter, Pd@Ag HNWs_10 h). Moreover, meso- and macro-sized pores were formed on the surface of Pd@Ag HNWs (green and red arrows in Figure 2d and 2e). The SEM image of Figure 2f indicates that the NW morphology was conserved in Pd@Ag HNWs_17 h, i.e., continuous and well-aligned features, even after solution based GRR for 17 6 ACS Paragon Plus Environment

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h at 90 oC. Moreover, during the GRR at 90 oC, we observed that a few Pd@Ag HNWs_17 h were physically broken and overturned. By using these points, we can directly demonstrate that final product showed hemi-tubular morphological characteristic. As shown in Figure 2g (tiltedSEM image), edge site of Pd@Ag HNWs_17 h obviously showed the inlet of a cavern like morphology, implying a hollow hemi-tube structure. In addition, overturned part of Pd@Ag HNWs_17 h clearly showed tunnel-like continuous open pores in the Pd@Ag HNWs, directly demonstrating hollow morphology of Pd@Ag HNWs_17 h (Figure 2h). However, longer time GRR such as 18 h or 20 h, most of the Pd@Ag HNWs were peeled off from the cPI substrate, showing discontinuous 1D structures (Figure S6); the broken Pd@Ag HNWs cannot be used as H2 sensing layers. The use of LPNE process combined with GRR allows easy fabrication of the aligned hollow bi-metallic NWs on the flexible substrate.

Structural Features and Phase Information The energy-dispersive X-ray spectroscopy (EDS) analysis of reaction time-dependent samples (Pd@Ag HNWs_10 min, Pd@Ag HNWs_1 h, Pd@Ag HNWs_10 h, and Pd@Ag HNWs_17 h) was carried out to investigate their relative atomic ratio (Pd/Ag). As shown in Figure 3a, the visible intensity of Pd is much weaker than Ag component, meaning that Ag component is major component in Pd@Ag HNWs_10 min. However, as GRR time (1 h to 17 h) increased, the intensity of Pd increased relative to Ag while the NW thickness also gradually increased (Figure 3b–3d). As shown in Figure 3e, the relative atomic ratio of Pd to Ag was increased from 2 at% to 61 at%. In the case of Pd@Ag HNWs_17 h, Pd content was more dominant element in Pd-Ag composite NWs. Interestingly, not only the mean width but also the mean height of Pd@Ag HNWs were simultaneously increased with increasing GRR time as shown in SEM and AFM images, respectively (Figure S2). Since the electrons from the oxidation of Ag components are continuously supplied for Pd2+ reduction during GRR, metallic 7 ACS Paragon Plus Environment


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Pd was first nucleated on the surface of Ag NWs and gradually grown by Ostwald ripening behavior, showing increased mean height and width. After 17 h GRR, final Pd@Ag HNW showed mean width of 350±5 nm as well as mean height of 187±5 nm, while bare Ag NW exhibited 200±5 nm (mean width) and 41±3 nm (mean height) as shown in Figure 3f. These results are well matched to our suggested GRR growth mechanism of Pd on Ag NWs and consistant with a previous report.23 To further investigate the the crystalline structure and chemical bonding state of Pd and Ag, ex-situ X-ray photoelectron spectroscopy (XPS) was performed for the bare Ag NWs and Pd@Ag HNWs samples prepared at different GRR time. The double peak of bare Ag NWs was observed at ~ 368 eV, corresponding to metallic Ag 3d5/2 state (Figure 4a (i)). On the other hand, the peak of Ag is slightly shifted to lower binding energy (~367.8 eV) due to the formation of Pd@Ag alloy phase. A similar shift phenomenon was also observed in the previous report.21 Interestingly, the peak intensity of Ag was gradually decreased depending on the GRR time, indicating that the dissolution of Ag was continuously occurring during the replacement reaction (Figure 4a (ii) to 4a (iv)). For the Pd 3d region in each sample except bare Ag, two distinctive double peaks with a spin-orbit coupling energy between Pd 3d3/2, and 3d5/2 of 5.2 eV are shown at 335.8, and 338.3 eV, corresponding to Pd 3d 5/2, and Pd2+ 3d5/2, respectively (Figure 4b). The Pd2+ peaks may mainly come from core (Pd)-shell (PdO) structure. In fact, the oxygen adsorption energy of Pd based alloy materials is lower than metallic Pd, thereby resulting in formation of very thin PdO layers on alloy Pd-Ag layers.28 Therefore, the very thin oxide layers can be formed on the surface of metallic Pd, thereby showing Pd2+ peaks in XPS data. Contrary to Ag phase, the peak intensity of Pd was gradually increased as increasing the GRR time. In case of Pd@Ag HNWs_17 h sample, XPS peak intensity of Ag is extremely low, while atomic % of Ag is approximately 40 at % as shown in Figure 3e and 4a (iv). It can be explained by difference of depth profiling ability between EDS and XPS. Because 8 ACS Paragon Plus Environment

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the XPS analysis is a surface-sensitive spectroscopic technique, we interpreted that Pd elements are more dominantly distributed in exterior shell of Pd@Ag HNWs_17 h, thereby showing extremely low Ag peak intensity. However, because the core part in Pd@Ag HNWs showed a Pd@Ag alloy skeleton, higher at % of Ag can be indicated in EDS mapping analysis. These suggestions are well supported by the ex-situ EDS mapping data (Figure 3a–d) of Pd and Ag, and relative ratio of Pd/Ag data (refer to Figure 3e). Optical Properties To evaluate the transmittance properties of these NWs, UV-Vis measurements were conducted on bare Ag NWs, Pd@Ag_10 min HNWs, Pd@Ag_1 h HNWs, Pd@Ag_10 h HNWs, and Pd@Ag_17 h HNWs on cPI substrates. In this work, we use the bare cPI film, which showed a mean thickness of 30 μm and exhibited above 89.0 % transmittance at 550 nm (Figure S3). However, after the fabrication on LPNE processed Ag NWs on cPI substrate, a slightly reduced transmittance of 87±1 % was observed (black line in Figure 5a). Here, the additional light scattering at surface of Ag NWs results in slight degradation of the transmittance value (see schematic image in Figure 5b (i)). Pd@Ag HNWs on cPI film showed a similar transmittance – wavelength trace. These results imply that the transmittance of aligned NWs with 10 μm gap on cPI film is not severely affected even after GRR. The Figure 5b indicates the transmittance value depending on the GRR time in detail. Although the transmittance trace of each sample looks similar, the tendency of transmittance was found through the comparison of GRR time-dependent Pd@Ag HNWs on cPI film. After 10 min GRR with bare Ag NWs, the transmittance of Pd@Ag_10 min HNWs showed further reduced transmittance value of 82.6±0.7 %. The deposition of Pd on the surface of Ag NWs may possibly be a main cause of more reduced transmittance (Figure 5b (ii)). However, interestingly, we observed that longer time GRR induced the slight recovery of transmittance. 1 h and 10 h GRR driven Pd@Ag HNWs showed average transmittance of 84.9±0.6 % and 85.3±0.5 %, 9 ACS Paragon Plus Environment


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respectively. Due to the fact that simultaneous reduction of Pd and dissolution of Ag during GRR can lead to formation of thin-walled hollow structures and macro-porous structures, which is beneficial for light penetration into hollow structure compared to densely packed Pd@Ag_10 min HNWs, thereby showing higher transmittance values (Figure 5b (iii)).29 However, Pd@Ag_17 h HNWs exhibited relatively thick average width (350±5 nm) and wall thickness (187±5 nm), showing reduced transmittance value (83.2±0.6 %) again. In this sense, we realize that the formation of thin-walled hollow porous structure and increase of mean width as well as wall thickness is a trade-off in terms of transmittance of the Pd@Ag on cPI film system. Chemical Sensing Properties To clearly demonstrate the feasibility of Pd@Ag HNWs on cPI film as a wearable H2 sensor, we measured sensing characteristics toward H2 gas molecules using the 5 different sensors (bare Ag NWs, Pd@Ag HNWs_10 min, Pd@Ag HNWs_1 h, Pd@Ag HNWs_10 h, and Pd@Ag HNWs_17 h). Note that the sensing properties of all samples on cPI film were measured with interdigitated Au sensing electrodes. As shown in Figure 6a (i), no resistance variation of bare Ag NWs on cPI was observed when H2 gas was exposed to the sensing devices. This demonstrates that bare Ag NWs are not active in detecting H2. Likewise, Pd@Ag HNWs_10 min and Pd@Ag HNWs_1 h didn’t show H2 sensing properties (Figure S4). On the other hands, Pd@Ag HNWs_10 h on cPI showed dynamic H2 response as shown in Figure 6a (ii). When H2 gas with concentration ranging from 100 ppm to 900 ppm exposed to Pd@Ag HNWs_10 h, reversible resistance variation was occurred even at room temperature (RT). This can be explained by the effect of relative Pd ratio in Ag NWs. In case of other three samples (Ag NWs, Pd@Ag HNWs_10 min, and Pd@Ag HNWs_1 h), relative Pd atomic ratio in Pd@Ag NWs was lower than 5 at% (refer to Figure 3e). Due to the fact that the Pd component in Pd@Ag HNWs is a substantive active material for H2 detection while Ag didn’t possess H2 10 ACS Paragon Plus Environment

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detection capability, we conclude that Pd@Ag HNWs prepared with low Pd concentrations (< 5 at% Pd in this study) are incapable of detecting H2. Accordingly, the best H2 sensing characteristic in terms of response speed as well as sensitivity was observed for Pd@Ag_17 h HNWs, which have 64 at% of Pd in Pd@Ag HNWs (Figure 6a(iii)). To investigate the feasibility as a flexible H2 sensor, the H2 sensing characteristics were also evaluated in a bent state of Pd@Ag HNWs on cPI film at an angle (θb) of 30o (Figure S5). In the case of sensing device with bare Ag NWs on cPI film, there was no characteristic response to H 2 gas in the bent state (Figure 6b (i)). Similar to flat state of Pd@Ag HNWs, Pd@Ag HNWs_10 h and Pd@Ag HNWs_17 h sensing devices in the bent state showed reversible resistance dynamics toward each concentration of H2 gas (Figure 6b (ii) and 6b (iii)). However, due to the mechanical stress toward Pd@Ag HNWs, their base resistance and sensing characteristics such as sensing speed looks relatively unstable during the H2 sensing reaction. The quantitative hydrogen response of each sample was compared at Figure 7 Note that we carried out the H2 sensing measurement test at below [H2] < 0.1 %. Therefore the increase of resistance after exposure to H2 can be explained by formation of α-PdHx (x < 0.015) through the hydrogen diffusion in the interstitial sites of metallic Pd.30 In terms of response defined as ∆R/R0 × 100 (%, where R0 is the initial resistance of the NWs) and sensing speed (response time and recovery time), the Pd@Ag HNWs_17 h sensing device showed 0.89±0.01 % of response at 900 ppm of H2 and 120±3.53 s of response time, while Pd@Ag HNWs_10 h sensor showed relative low response (0.268±0.01% @ 900 ppm of H2) and slow speed (300±2.83 s) (Figure 7a and 7c). Here, response time is defined as time necessary for the increase of resistance from R0 to 0.9∆Rmax, while recovery time is defined as time for the decrease of resistance from ∆Rmax to 0.1R0. In addition, Pd@Ag_17 h HNWs also showed faster recovery value (102±12.73 s) compared with that (225±9.20 s) of Pd@Ag HNWs_10 h (Figure 7e). These tendencies were also observed at all concentration range of H2 11 ACS Paragon Plus Environment


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(100–900 ppm) in our system. In case of the bent state of Pd@Ag HNWs_17 h, the response of 0.65±0.03% at 900 ppm of H2 and sensing speed of 120±14.14 s were observed (Figure 7b and 7d). Generally, the response speed at [H2] < 900 ppm and recovery properties of Pd@Ag HNWs were degraded in the bent state (Figure 7d and 7f). This can be explained by strain effect on the adsorption as well as diffusion of H2 on the Pd surface. According to the previous research, when the tensile strain was applied to Pd, applied tensile stress can induce the deep potential wells of H adsorption sites in Pd, thereby showing difficulty of H2 desorption kinetics, which are main cause of slow recovery behavior.31,

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Furthermore, since the location of

adsorbed H2 sites tend to change from on the surface of metal to in the surface of metal through the tensile strain, the H2 molecules should more diffuse to inner side of Pd, resulting in slower response time.31 These previous results are well correspond to our experimental sensing results, i.e., slow response and recovery in the bent state. Although sensing characteristics were slightly degraded by applied tensile strain to Pd@Ag HNWs, the detection of H2 in the bent state can be achieved through their reversible H2 sensing behaviors. On the basis of flat state sensing properties, 2.5 fold faster sensing speed (120±3.53 s response/102±12.73 s recovery @ 900 ppm) was observed in Pd@Ag HNWs_17h compared with that of Pd@Ag HNWs_10 h. In addition, this value is much faster response speed compared to pure Pd NWs (over 400 s response and recovery @ 900 ppm), which were described in our previous work.9 The detailed comparison of H2 sensing characteristics among metal based H2 sensors were indicated in Table 1. In terms of sensing mechanism, the reason for enhanced (i) sensitivity toward H2 (over 300 % increase @ 900 ppm) and (ii) sensing speed (250 % increase @ 900 ppm) of Pd@Ag HNWs_17 h compared to Pd@Ag HNWs_10 h can be explained by increased Pd component effect with hollow structures and Pd/Ag alloy effect, respectively. Firstly, since substantive H 2 sensing active layer is Pd in Pd@Ag HNWs, increased Pd component in Pd@Ag HNWs system can 12 ACS Paragon Plus Environment

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provide the more H2 reaction sites. Note that the H2 molecules can be spill-overed by catalytic Pd effect, forming PdHx phase even at RT. Therefore, increased Pd can induce the larger resistance variation of Pd@Ag HNWs.33 Furthermore, morphological change from densely packed fibrous structures to hollow fibrous structures with high porosity can also contribute to provision of the more H2 reaction sites (Figure 8a (ii)). Especially, H2 is one of the smallest gas molecules (kinetic diameter = 0.289 nm) in the nature, showing great diffusivity through the path way of numerous pores as well as hollow structures. Due to the effective sensing reaction sites, i.e., interior and exterior side of Pd@Ag HNWs, and numerous pore zones on the surface of hollow Pd@Ag HNWs, H2 gas with lower [H2] < 0.1 % can be detected in our system. Secondly, formation of Pd@Ag alloy phase during GRR can improve the permeation of H 2 gases in the Pd@Ag HNWs system according to the previous research (Figure 8a (i)). 34 In particular, larger Pd component than Ag in Pd@Ag alloy can lead to optimized permeation toward H2 gas molecules because diffusion coefficient of H in PdAg alloy decreases with increasing Ag components.35 In fact, Pd0.6Ag0.4 alloy showed the best H2 permeation characteristics in the previous research.30,

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In this sense, Pd@Ag HNWs_17 h with

approximately 60 at% Pd and 40 at% Ag can exhibit the highest permeation toward H 2 gas compared with that of bare Ag NWs or Pd@Ag HNWs_10 h, thereby showing the fastest H 2 sensing properties (250 % reduced response time in the flat state). In terms of mechanical property such as flexibility, Pd@Ag alloy exhibited the higher mechanical strength compared to pristine Pd because the lattice of Pd@Ag alloy has already been expanded by the silver atom (Pd (111): lattice distance of 0.39 nm, PdAg (111): lattice distance of 0.40–0.41 nm), thereby showing less brittle than the pure Pd.36, 37 Accordingly, the Pd@Ag HNWs showed stable and reversible H2 sensing dynamics even at the bent state (refer to the Figure 6b). As shown in Figure 8b and 8c, the enhanced sensing speed (response/recovery) through the alloy effect was experimentally demonstrated. Due to the high permeation activity in Pd@Ag HNWs_17h 13 ACS Paragon Plus Environment


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having Pd0.6Ag0.4, we obviously confirm that Pd@Ag HNWs_17h showed much faster response/recovery time once again (Figure 8b and 8c). To further investigate the relationship between tensile strain and sensing characteristics, we additionally analyzed our sensitivity versus H2 concentration data by using Sievert’s Law.11 In case of the low concentration of H2 such as below [H2] < 1 %, we can assume that resistance variation of Pd based materials is linearly proportional to [H2]1/2.35 By using this theory, we plotted the ∆R/R0 (%) versus [H2]1/2 normalized graph in the both bent and flat state of Pd@Ag HNWs as shown in Figure 8d and 8e. Both sensitivity of Pd@Ag HNWs_17 h and Pd@Ag HNWs_10 h showed approximately linear behavior in the graphs. However, the slope of Pd@Ag HNWs_17 h and Pd@Ag HNWs_10 h in the bent state were generally decreased. This may be possibly explained by tensile stress effect. Since H2 adsorption energy can be increased by applied tensile strain to Pd surface (-0.4 ~ -0.5 to -0.6 eV), the number of reacted H atoms with bent Pd@Ag HNWs can be decreased compared to flat Pd@Ag HNWs, resulting in lower H2 sensitivity (lower slope in ∆R/R0 (%) versus [H2]1/2 graph).31 Application for transparent and flexible H2 sensor To demonstrate the feasibility as transparent bendable H2 sensor, we fabricated the crossed Au NWs corresponding to Pd@Ag HNWs_17 h, which have the best H2 sensing performances, and measured their transmittance properties and H2 sensing characteristics. Note that Au NWs perpendicular to Pd@Ag HNWs_17 h used as a transparent sensing electrode. Due to their nanoscale size, crossed Au NWs are able to show the transparent properties. As shown in Figure 9a, the crossed Au NWs (a vertical line in SEM image) were formed on the Pd@Ag NWs well. The mean thickness of Au NWs is 200 nm. Although Au NWs were additionally deposited on Pd@Ag HNWs, their transmittance value is still over 80 % (81 %±0.56 at wavelength of 500 nm) at the visible light wavelength range (Figure 9b). The texts in book were obviously visible through the crossed NWs on cPI film as shown in inset image in Figure 14 ACS Paragon Plus Environment

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9b. Finally, we carried out H2 sensing measurement test with Au NWs loaded Pd@Ag HNWs_17 h sensing device in both bent and flat state with 900 ppm H2 gas. Due to the nanoscale sensing electrode and polycrystalline structures of Au NWs with nano-sized grains, the larger base resistance of Pd@Ag HNWs (89.33 kΩ) than Pd@Ag HNWs (16.56 kΩ) with bulky gold sensing electrode was measured. However, characteristic H2 response and recovery properties were clearly observed using Au NWs loaded Pd@Ag HNWs on cPI film during the cyclic exposure to 900 ppm H2 in both bent and flat state (Figure 9c). Although the sensitivity and sensing speed of Pd@Ag HNWs with Ag NWs electrode is lower than Pd@Ag HNWs with bulky gold electrode, we demonstrate that crossed Au NWs with Pd@Ag HNWs on PI film can potentially be used as a transparent and flexible H2 detector. CONCLUSION A galvannic replacement reaction (GRR) provides a highly controllable method for introducing an active metal component, Pd in this case, into otherwise inactive Ag NWs in this study. Pd@Ag HNWs prepared using this approach starting with Ag precursor NWs showed rapid and sensitive detection of H2 in air. An advantage of this approach is that the GRR process produces Pd@Ag HNWs possessing a porous morphology with a high surface area which may contribute to the excellent H2 sensing performance of these NWs. We also employed a double-LPNE technique to obtain grid-type metallic NWs composed of Pd@Ag HNWs with perpendicular Au NWs for application in transparent and flexible sensing layers. In parallel with H2 sensing properties in the flat state, the characteristic H2 response and recovery were also observed in the bent state of sensing devices. Interestingly, Au NWs successfully read the real-time resistance of Pd@Ag HNWs in the both bent and flat state, enabling the reversible and repetitive H2 detection at the 900 ppm of H2 gas. In this sense, we believe that transparent property of our sensing device can be potentially applied to smart



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window and the wall of winding objects, i.e., gas pipes, safety helmet, and safety glasses, as next-generation H2 sensor.

EXPERIMENTAL SECTION Chemicals and Materials A commercial silver and gold plating solution was prepared from Clean Earth SolutionsTM. The potassium tetrachloropalladate (II, K2PdCl4), 4,4’-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA), 3,3’-diaminodi-phenyl sulfone (APS), and N,N-dimethylformamide (DMF) were purchased by Sigma-Aldrich. Acetone, and nitric acid were used as received from Fisher (ACS Certified). Positive photoresists (Shipley S-1808) and MF-319 (developer) were prepared from Microchem Corporation. Nickel pellets was purchased by Kurt J. Lesker Coporation. All chemicals were used without further purification. Preparation of Polyimide Substrate As a precursor for coloress polyimide (cPI) substrate, polyamic acid (PAA) solution was prepared by dissolving 2.0 g of 6FDA and 1.0 g of APS in 4.0 g of DMF solution. The asprepared solution was stirred at 500 rpm with a magnetic stirrer for 5 h at room temperature. After stirring process, the transparent PAA solution was coated on the glass substrate (1’ x 1’) by using the doctor’s blade with a thickness range of 25–30 μm. Finally, a polyimide film was achieved after imidization process at followed 100 oC, 200 oC, and 300 oC for 1 h at each temperature in the furnace. Preparation of hollow Pd@Ag Nanowires on the Polyimide Substrate Ag NWs were fabricated on the flexible polyimide (PI) substrate by using the conventional lithographically patterned NWs electrodeposition (LPNE).20 Firstly, 40 nm thick of Ni film was thermally evaporated on the transparent PI substrate. After then, a positive photoresist (PR, Shipley, S1808) solution was spin coated on the as-prepared Ni layer, followed by soft-baking 16 ACS Paragon Plus Environment

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at 90 oC for 30 min. As-prepared sample was optically patterned by contact mask, having patterned lines with 10 μm gaps, and UV light source combined with a shutter and photolithographic alignment fixture (Newport, 8320i-line, 2.3 s). The exposed PR part was chemically developed by MF-319 solution for 20 s and rinsed with Millipore water. And then, prepared sample was immersed in 0.8 M nitric acid solution to etch the exposed Ni parts for 6 min. After etching the Ni layer and making the undercut, Ag was potentiostatically electrodeposited under the nanoscale Ni layer by using Gamry Series G 300 potentiostat at 0.75 V versus saturated calomel electrode (SCE, reference electrode) with a Pt foil counter electrode for 600 s. After then, the surface of product was rinsed with acetone to remove residual photoresist layer and Ni was completely removed by immersion in nitric acid for 10 min. Final product was rinsed with Millopore water and followed air-dried. After fabricating the Ag NWs on the PI substrate, this device was immersed in the aqueous Pd (II) solution at 90 oC for 17 h to achieve the hollow Pd@Ag HNWs through the galvanic replacement reaction (GRR) between Pd and Ag. Here, 6 mg of K2PdCl4 was dissolved in 200 mL Millipore water. After GRR, Pd@Ag NWs integrated on PI substrate was rinsed by Millipore water and airdried. Reference Pd@Ag NWs were prepared by controlling the GRR time (10 min, 1 h and 10 h). Preparation of Perpendicular Au Nanowires electrode on the Pd@Ag Hollow Nanowires In order to fabricate the perpendicular Au NWs on the Pd@Ag HNWs as contact pad electrodes, 50 nm Ni layer was additionally evaporated on the Pd@Ag HNWs. And then, PR layer coating, patterning by UV-vis lamp, developing of exposed PR layer, etching of Ni, and electrodeposition of Au were sequentially carried out similar to the previous LPNE process. Note that Au NWs were perpendicularly electrodeposited with Pd@Ag HNWs at -0.9 V versus SCE with a Pt foil counter electrode for 900 s. After then, all PR layer and Ni layer were removed by acetone and nitric acid solution, respectively. 17 ACS Paragon Plus Environment


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Characterization A field-emission scanning electron microscopy (FE-SEM) images were acquired using a FEI Magellan 400 XHR system. Energy dispersive spectroscopic (EDS) images were acquired by EDS detector (Oxford Instruments, 80 mm2, with Aztec software) in SEM system. All SEM specimens were coated by 2 nm Cr. For UV-Vis measurements, a Hewlett Packard 8453 UVVisible spectrophotometer was used. The chemical elements and their bonding states were investigated by X-ray photoelectron spectroscopy (XPS, Sigma Probe, Thermo VG Scientific) with Al Kα radiation (1486.6 eV). Evaluation of sensing performance The gas sensing properties were tested by a sensor testing equipment which is introduced elsewhere.39 Gas responses of 5 different sensing layers (Ag NWs, Pd@Ag NWs_10 min, Pd@Ag NWs_1 h, Pd@Ag NWs_10 h, and Pd@Ag NWs_17 h) were examined by resistance modulation (∆R/R0 x 100, where R0 is the sensing resistance in the baseline air) in both flat and bent state. All the sensors were stabilized in baseline ambient air before the sensing measurement. H2 gas exposed to all the sensors with concentration ranging from 100 ppm to 900 ppm with a 10 min on/off interval. The concentration of H2 gas was controlled by injection of air with H2 gas (Figure S9). All sensing measurements were carried out at room temperature. ASSOCIATED CONTENT Supporting Information Supporting Information is available online from the http://pubs.acs.org or from the author. SEM of Ag NWs, AFM data, additional raw H2 sensing data, dital image of bent state sensing device can be available. AUTHOR INFORMATION Corresponding Author 18 ACS Paragon Plus Environment

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*E-mail: [email protected] *E-mail: [email protected] ORICD Reginald M. Penner: 0000-0003-2831-3028 Il-Doo Kim: 0000-0002-9970-2218

Notes The authors declare no competing financial interest. Acknowledgements This work supported by Global Ph.D Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. NRF2016H1A2A1907718), Wearable Platform Materials Technology Center (WMC) funded by National Research Foundation of Korea (NRF) Grant of the Korean Government (MSIP) (No. 2016R1A5A1009926), and the NRF of Korea grant funded by the Ministry of Science, ICT and Fugure Planning (NRF-2015R1A2A1A16074901).

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Figure 1. Schematic illustration of synthetic process for the hollow Pd@Ag HNWs with crossed Au NWs; (a) physically attached cPI film on glass, (b) Ni and PR plated cPI substrate, (c) electrodeposition of Ag on the undercut, (d) etching of Ni and PR process, (e) formation of Pd@Ag HNWs via GRR, (f) 2nd LPNE process for perpendicular Au NWs, (g) lift off of Pd@Ag HNWs and Au NWs loaded cPI film, the operation principal of Au NWs as sensing electrode on the Pd@Ag HNWs was indicated in yellow box schematic image.



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Figure 2. (a) Schematic diagram illustrating morphological evolution from bare Ag NW to hollow Pd@Ag NW, SEM image of (b) bare Ag NW (c) Pd@Ag_1 h HNW, (d) Pd@Ag_10 h HNW, (e) Pd@Ag_17 h HNW, and (f) low magnified Pd@Ag_17 h HNWs, (g) tilted SEM image of Pd@Ag_17 h HNW, (h) SEM image of overturned Pd@Ag_17 h HNW with magnified image


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Figure 3. EDS elemental mapping images of (a) Pd@Ag_10 min HNWs, (b) Pd@Ag_1 h HNWs, (c) Pd@Ag_10 h HNWs, and (d) Pd@Ag_17 h HNWs, (e) plot of Pd and Ag atomic ratio versus GRR, (f) plot of mean width and mean height of NWs versus GRR time



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Figure 4. XPS spectra of (a) Ag in (i) Ag NWs, (ii) Pd@Ag_1h HNWs, (iii) Pd@Ag_10h HNWs, (iv) Pd@Ag_17 h HNWs, (b) Pd in (i) Ag NWs, (ii) Pd@Ag_1h HNWs, (iii) Pd@Ag_10h HNWs, (iv) Pd@Ag_17 h HNWs


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Figure 5. (a) Transmittance of Ag NWs, Pd@Ag_10 min HNWs, Pd@Ag_1h HNWs, Pd@Ag_10 h HNWs, and Pd@Ag_17 h HNWs, (b) normalized transmittance versus GRR time plots



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Figure 6. (a) Raw H2 sensing responses for three NWs at room temperature in the flat state: (i) Ag NWs, (ii) Pd@Ag_10 h HNWs, and (iii) Pd@Ag_17 h HNWs, (b) Raw H2 sensing responses for three NWs at room temperature in the bent state: (i) Ag NWs, (ii) Pd@Ag_10 h HNWs, and (iii) Pd@Ag_17 h HNWs


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Figure 7. Plotted are the normalized resistance change versus [H2] for (a) pure Ag NWs, Pd@Ag HNWs_10 h, and Pd@Ag HNWs_17h in flat state, (b) pure Ag NWs, Pd@Ag HNWs_10 h, and Pd@Ag HNWs_17h in bent state, response time versus [H2] with Pd@Ag HNWs_10 h and and Pd@Ag HNWs_17h in (c) flat state, and (d) bent state, recovery time versus [H2] with Pd@Ag HNWs_10 h and and Pd@Ag HNWs_17h in (e) flat state, and (f) bent state



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Figure 8. (a) Suggested H2 sensing mechanism of Pd@Ag HNWs (i) porosity & Pd@Ag alloy effect, (ii) hollow structure effect. Normalized curve of response (b) and recovery (c) for Pd@Ag HNWs_10 h and Pd@Ag HNWs_17 h in both flat and bent state. Note that the [H2] = 900 ppm. Sensitivity versus [H2]1/2 in flat state (d) and bent state (e)


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Figure 9. (a) SEM image of heterogeneous grid-type NWs composed of Pd@Ag HNWs and Au NWs with magnified SEM image and corresponding schematic image, (b) Transmittance of heterogeneous grid-type NWs composed of Pd@Ag HNWs and Au NWs, (c) raw H2 sensing data of heterogeneous grid-type NWs composed of Pd@Ag HNWs and Au NWs in both flat and bent state 33 ACS Paragon Plus Environment


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Table 1. Sensing properties for metal based hydrogen sensors operated in air. Sensing layer Pd NWs Pt NWs Pd/Ni film Pd HNWs Pd@Pt NWs (θPt=0.1 ML) Pd@Ag HNWs_10 h Pd@Ag HNWs_17h

Dimensions 25 nm (h) × 85 nm (w) 20 nm (h) × 130 nm (w) t = 50 nm 1–2 μm of height and 50–100 nm of diamter 40 nm (h) × 100 nm (w) 142.6 nm (h) × 250 nm (w) 187.5 nm (h) × 350 nm (w)

Temp (K)

τresp/τrec @ [H2] ≈0.1%

LODH2

Ref.

RT

400 s/1000 s

100 ppm

40

550 K

150 s/1100 s

10 ppm

41

RT

120 s/20 s

500 ppm

9

RT

180 s/n/r

100 ppm

10

294 K

150 s/1100 s

n/r

8

RT

300 s/225 s

100 ppm

this work

RT

120 s/102 s

100 ppm

this work

Abbreviations: HNWs = hollow nanowires, NWs =nanowires, t = thickness, h = height of nanowire, w = width of nanowire, n/r = not reported, RT=room temperature, θPt=0.1 ML means that Pt coverage of 0.1 monolayer. τresp is the time necessary for the resistance to increase from R0 to the 0.9ΔRmax, and τrec is the time for the resistance to decrease from ΔRmax to 0.1R0.


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