Letter pubs.acs.org/ac
Well-Defined and High Resolution Pt Nanowire Arrays for a High Performance Hydrogen Sensor by a Surface Scattering Phenomenon Hae-Wook Yoo,‡,† Soo-Yeon Cho,‡,† Hwan-Jin Jeon,‡,⊥ and Hee-Tae Jung*,‡ ‡
Department of Chemical and Biomolecular Engineering (BK-21 Plus), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ⊥ Semiconductor R&D Center, Samsung Electronics, Samsungjeonja-ro, Hwaseong-si, Gyeonggi-do 445-330, Republic of Korea S Supporting Information *
ABSTRACT: Developing hydrogen (H2) sensors with a high sensitivity, rapid response, long-term stability, and high throughput is one of the critical issues in energy and environmental technology [Hübert et al. Sens. Actuators, B 2011, 157, 329]. To date, H2 sensors have been mainly developed using palladium (Pd) as the channel material because of its high selectivity and strong affinity to the H2 molecule [(Xu et al. Appl. Phys. Lett. 2005, 86, 203104), (Offermans et al. Appl. Phys. Lett. 2009, 94, 223110), (Yang et al. Nano Lett. 2009, 9, 2177), (Yang et al. ACS Nano 2010, 4, 5233), and (Zou et al. Chem. Commun. 2012, 48, 1033)]. Despite significant progress in this area, Pd based H2 sensors suffer from fractures on their structure due to hydrogen adsorption induced volumetric swelling during the α → β phase transition, leading to poor long-term stability and reliability [(Favier et al. Science 2001, 293, 2227), (Walter et al. Microelectron. Eng. 2002, 61−62, 555), and (Walter et al. Anal. Chem. 2002, 74, 1546)]. In this study, we developed a platinum (Pt) nanostructure based H2 sensor that avoids the stability limitations of Pd based sensors. This sensor exhibited an excellent sensing performance, low limit of detection (LOD, 1 ppm), reproducibility, and good recovery behavior at room temperature. This Pt based H2 sensor relies on a highly periodic, small cross sectional dimension (10−40 nm) and a welldefined configuration of Pt nanowire arrays over a large area. The resistance of the Pt nanowire arrays significantly decreased upon exposure to H2 due to reduced electron scattering in the cross section of the hydrogen adsorbed Pt nanowires, as compared to the oxygen terminated original state. Therefore, these well-defined Pt nanowire arrays prepared using advanced lithographic techniques can facilitate the production of high performance H2 sensors.
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stable and reproducible sensor response.14−16 Therefore, due to such technical barriers, Pt nanostructures have only been used as assistant catalytic materials attached to other conducting channels such as metal oxides, graphene, and metal nanowires, to enhance the affinity of H2 to the main conducting channels.17 In the study described below, we developed a novel and facile procedure for the construction of highly sensitive H2 sensors based on Pt nanowire arrays, with high resolution in the cross sectional dimension. Robust Pt nanowire arrays in the range of 10 to 40 nm were fabricated by using advanced lithographic techniques with the secondary sputtering phenomenon.18 Through the post dry-etching process, we were able to control dimensions of the periodic nanowire arrays and their sensitivity in H2 sensing. The resulting Pt nanowire arrays exhibit an excellent limit of detection (LOD; 1 ppm) at room temperature (298 K) as compared to Pd nanostructure based H2 sensors reported so far. This outstanding sensing performance is the
he rate of formation of water with platinum (Pt) is faster than that with palladium (Pd) for the same feature dimensions,1−3 making Pt a potential H2 sensing material. Unlike Pd nanostructures, however, Pt does not form a stable bulk hydride phase by H2 adsorption. Therefore, the transduction mechanism responsible for the detection of H2 in Pd nanostructures should not exist in Pt nanostructures.4−9 Recently, it was reported that a different transduction mechanism might be present in Pt nanowires, creating a potential sensing channel for H2 sensors.4,10−12 This may be the result of the H2 adsorption on the Pt surface, which has a significantly lower inelastic electron scattering in the nanowire cross section compared to the oxygen-terminated Pt surface, leading to a significant reduction in the electrical resistance of the Pt sensing channel. Despite the availability of Pt nanostructures for a gas sensing channel, a delicate design and configuration is essential for the use of Pt nanostructures as H2 sensors. A Pt based H2 sensor can only be used when the cross sectional dimension of the channel is near the mean free path of metal electrons (λPt ∼ 5 nm).13 In addition, not only high resolution nanostructures but also uniformity and fine alignment should be ensured for a © 2015 American Chemical Society
Received: November 19, 2014 Accepted: January 12, 2015 Published: January 12, 2015 1480
DOI: 10.1021/ac504367w Anal. Chem. 2015, 87, 1480−1484
Letter
Analytical Chemistry
shadow metal mask (Figure 1f). Finally, sensing devices with a wide assortment of Pt nanowire arrays were loaded on the gas sensing chamber that was connected to a gas delivery system fabricated in-house (Figure 1g). The resistance of the sensing channels, or the Pt nanowire arrays, was recorded in real time by using a multiplexing data-acquisition module (Agilent 34970A) with a constant flow of reference gas (air or N2) to observe the sensor response of H2 gas.
result of the well-defined configuration of the Pt nanowire arrays (namely, high periodicity, uniformity, and high resolution in the cross sectional dimension compared to the mean free path of metal (λPt ∼ 5 nm)), which is difficult to achieve using conventional techniques. Through this study, a significant step was taken toward the commercialization of ultrasensitive H2 sensors using Pt nanostructures.
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EXPERIMENTAL SECTION Figure 1 illustrates the process used to fabricate well-aligned Pt nanowire arrays with dimensions ranging from 10 to 40 nm in
RESULTS AND DISCUSSION Figure 2a shows a representative image of an H2 sensor device chip composed of a Pt nanowire sensing channel and Au/Ti
Figure 1. Schematic illustration showing the fabrication of a wellaligned high resolution Pt nanowire array. (a) The PS prepattern is created by PDMS molding. (b) A uniform 10 nm thick Pt layer is deposited onto a polymer prepatterned substrate using e-beam evaporation. (c) Pt particles are emitted on the side-surface of the prepatterns during the Ar-ion bombardment process. (d) With the RIE process, the PS prepattern residue is removed, resulting in thin Pt nanowire arrays. (e) The post dry-etching process is performed on the Pt nanowire arrays to reduce the line dimension. (f) Finally, a high resolution Pt nanowire array is generated and Au electrodes are deposited onto the array. (g) The completed sensor chips are loaded in a gas sensing chamber connected to a gas delivery system fabricated in-house.
Figure 2. Investigation of the Pt nanowire arrays. (a) Photograph of a Pt nanowire array (5 × 5 mm2) for the H2 sensor on the SiO2/Si substrate with Au/Ti electrodes. (b) SEM images of the Pt nanowire array (inset: tilted view). (c) AFM and SEM images of the as prepared Pt nanowire array. The height of the Pt nanostructures was measured as ∼40 nm. (d) AFM and SEM images of the Pt nanowire array after the post dry-etching process. The dimension of the Pt nanostructures was reduced to approximately 10 nm. (e) EDAX measurement for an as prepared Pt nanowire array; the Pt component was clearly observed on the Pt nanowire array only (left) and not on the substrate (right).
the cross section. Prepatterned polystyrene (PS) of width 500 nm was created by pattern-transfer using a poly(dimethylsiloxane) (PDMS) mold (Figure 1a). A uniform 10 nm-thick Pt layer was deposited onto the PS prepatterned substrate using electron beam evaporation (Figure 1b). This Pt layer was etched and emitted to the side surface of the PS prepatterns with wide angle distribution, through the secondary sputtering phenomenon during an Ar ion-bombardment process (Figure 1c).18−22 After the PS residue was removed by the oxygen reactive ion etching (RIE) process under low vacuum conditions (Figure 1d), periodic Pt nanowire arrays were generated with a 40 ± 5 nm cross sectional dimension and 500 nm spacing. To further reduce the feature dimensions of the Pt nanowires, a post dry-etching process (Ar ion-milling) was implemented with varying etching times (Figure 1e). By increasing the dry-etching time from 0 to 50 s, we were able to efficiently manipulate the cross sectional dimensions of the Pt nanowires from 40 to 10 nm. A thick gold electrode (100 nm) with a Ti layer was deposited on the resulting Pt nanowire arrays using electron beam evaporation with a predetermined
electrodes on a SiO2 substrate. The yellow dashed-square part of the image is the Pt nanowire patterned area on a large scale (25 mm2 in this study) prepared using the advanced lithography techniques as described above. The scanning electron microscope (SEM) images demonstrate that the Pt nanowires are highly ordered with uniform spacing (500 nm), creating a stable electrical channel of sensor devices (Figure 2b, inset image: tilted view of Pt nanostructures). The as-prepared Pt nanowire arrays before the postetching process have a cross sectional dimension of 40 ± 5 nm with 500 nm spacing. Atomic force microscope (AFM) and SEM images of the as-prepared Pt nanowire array and postetched Pt nanowire array show a reduced cross sectional dimension of 10 ± 3 nm (Figure 2c,d). During the post ion-milling process, a certain amount of Pt 1481
DOI: 10.1021/ac504367w Anal. Chem. 2015, 87, 1480−1484
Letter
Analytical Chemistry
restored to the former state within a given purging time (10 min) after the second cycle in a repeated sensing test, which distinctly demonstrates the high reproducibility of the Pt nanowire array as a sensing channel. The distinct negative response of the Pt nanowire array for H2 gas is due to the change in electron scattering in the Pt nanowires, as discussed in a previous study.4 The surface of the Pt nanowires is slightly oxidized in air, and the oxygen atoms can be easily substituted into hydrogen in the presence of H2 gas by strong chemisorption of the hydrogen originating from the high affinity between Pt and H2.23−25 Electron scattering in the cross section of the Pt nanowires significantly decreases due to the substitution of hydrogen atoms on the Pt surface instead of oxygen. This phenomenon occurs due to the large difference in atomic weight between the two components resulting from the diffusive scattering of conduction electrons that depend on the molecules adsorbed on the metal surface (Figure 3b(i,ii)).26,27 Therefore, the negative response in the resistance change for the Pt nanowire arrays is inevitable due to H2 exposure. To further confirm the H2 sensing mechanism, we used pure N2 (without O2) as the purging gas instead of air (Figure 3c,d). The Pt nanowire array showed a negative response on initial exposure to H2 even with N2 purging. The mild N2 purging was insufficient to detach the oxygen preadsorbed on the Pt surface (slightly oxidized Pt surface), resulting in the negative resistance change by hydrogen substitution as described above. However, the decreased resistance of the Pt nanowire array was barely recovered by N2 purging after the Pt sensing channel was exposed to H2 (Figure 3c,d(i)). These results prove that the oxygen in the reference gas is essential to replace the adsorbed hydrogen on the Pt surface, which was clearly verified from the last cycle of responses from the H2 sensor (Figure 3c,d(ii)) and the distinct recovery behavior on purging with air containing oxygen. In other words, the H2 sensing in the Pt nanowire arrays operates by changing the degree of electron scattering from the reversible replacement of surface atoms (oxygen or hydrogen). We found that the amplitude of the sensor response of the Pt sensing channel for H2 was strongly affected by the feature dimensions of the Pt nanowires (Figure 4a). We measured the sensor responses to 1000 ppm of H2 gas in three different cross sectional dimensions (10 ± 3, 25 ± 4, 40 ± 5 nm) of the Pt nanowire arrays. As expected, as the dimension of the Pt nanowires became smaller, higher sensor responses were registered for H2 gas. In the case of the 40 ± 5 nm Pt nanowire arrays (green line), the change in resistance to H2 gas was only 0.7%, but it increased to 1.7% for the 25 ± 4 nm Pt sensing channel (red line). Interestingly, the sensor response was significantly enhanced for the 10 ± 3 nm Pt nanowire array (blue line) with a 5.2% change in resistance, which was an enhancement in sensitivity of 3 times and 7.4 times, respectively, compared to the 25 ± 4 and 40 ± 5 nm Pt nanowire arrays. This tendency of the sensor response with a varying feature size is due to the change in the rate of electron scattering in Pt nanowires (Figure 4b). As the feature dimension of the nanowires is reduced, these high resolution nanostructures provide favorable conditions for increasing the rate of electron scattering in the cross section of the Pt nanowires,27,28 resulting in increased baseline resistance and an enhanced response to the adsorbed gas molecules. In this study, about 10 nm scale Pt nanowire arrays with a highly enhanced sensor response are likely to be ideal for maximizing electron
particles is etched away from the Pt nanowires, resulting in a reduced cross sectional dimension as clearly seen in the SEM images (right images of Figure 2c,d). The resulting high resolution Pt nanowire array has feature dimensions comparable to a mean free path of Pt (λPt ∼ 5 nm), which is a highly effective feature for the H2 sensor. Energy dispersive Xray (EDAX) spectroscopic results show that the Pt nanowires are well isolated and that no other components exist on the substrate (Figure 2e). The Pt peak was only observed in the regions where the Pt nanowires were formed, and no Pt peaks were observed on bare substrate regions (+; center image of Figure 2e). This confirms the formation of periodic and wellisolated Pt nanowire arrays on the defined position along the PS prepattern. The cyclic test for H2 gas was performed under two different reference gases (air or N2) to investigate the sensor response of the Pt nanowire arrays, as described in Figure 3. 1000 ppm of
Figure 3. H2 sensing mechanism of the Pt nanowire array from the sensor responses to 1000 ppm of H2. (a) Pt nanowire array shows a negative sensor response for H2 in a cyclic test under dry air conditions. (b) Schematic description of the surface scattering of electrons in Pt nanostructures. A Pt−H surface (ii) greatly reduces surface scattering as compared to a Pt−O surface (i), resulting in a decreased resistance to H2 adsorption. (c) Sensor responses to H2 with N2 as a reference gas. The decreased resistance from the H2 reaction is rarely recovered under N2 conditions. (d) The hydrogen on the Pt surface is not detached by N2 purging (i), while the adsorbed hydrogen is easily substituted by oxygen from the O2 containing air (ii).
H2 and reference gas were alternately injected into the sensing chamber for 5 and 10 min, respectively, while the flow rate of each gas was maintained at 400 sccm, as regulated by a selfdesigned gas delivery system. When the Pt nanowire array was exposed to H2 gas, it showed a negative response in resistance change and it was recovered again by purging with manufactured air (20% O2 containing N2). As shown in the first cycle of the sensing test, it may be difficult to attain the initial value of the pristine Pt nanowire array as there are quasipermanent reaction sites for the strong chemisorption of hydrogen, which is rarely desorbed under general purging conditions. Except for the portion of quasi-permanent resistance change from the pristine state, the decreased resistance of the Pt sensing channel from H2 exposure was 1482
DOI: 10.1021/ac504367w Anal. Chem. 2015, 87, 1480−1484
Letter
Analytical Chemistry
defined Pt nanowire arrays used in this study can be used in high performance H2 sensors. In summary, we fabricated well-defined, highly periodic, high resolution (width
