Bismuth Nanowires with Rough Surface for Stable

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Palladium/Bismuth Nanowires with Rough Surface for Stable Hydrogen Sensing at Low Temperatures Lingling Du, Lijun Zheng, Hongrei Wei, Shizheng Zheng, Zhengyou Zhu, Jian Chen, and Dachi Yang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02029 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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ACS Applied Nano Materials

Palladium/Bismuth Nanowires with Rough Surface for Stable Hydrogen Sensing at Low Temperatures Lingling Du, Lijun Zheng, Hongrei Wei, Shizheng Zheng, Zhengyou Zhu, Jian Chen, and Dachi Yang* Tianjin Key Laboratory of Optoelectronic Sensor and Sensing Network Technology and Department of Electronics, College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, P. R. China KEYWORDS: Palladium/Bismuth; Rough surface; Low-temperature; Stability; Hydrogen sensor.

ABSTRACT: Smart and reliable palladium (Pd) based sensors that are able to operate in a large temperature range especially at low temperatures are highly desired, but remain challenging due to the “reverse sensing behaviors” caused by the α-β phase transition in PdHx. Heteroatom doping and morphology tuning are effective ways to improve the stability of Pd-based hydrogen sensors at low temperatures. Here, we have developed rough-surfaced palladium/bismuth nanowires (RS-PdBi NWs) with uniform diameters and even Bi atoms distribution via first AAO-confined electrodeposition and subsequent chemical etching. Remarkably, the hydrogen sensors built with RS-PdBi NWs are able to work in a larger temperature range of 194.3 - 400 K and much lower critical temperature (⁓194.3 K) of “reverse sensing behavior” is achieved compared with the pristine Pd NWs sensors (~287 K), exhibiting improved low-temperature stability. This superior sensing stability is attributed to the synergistic effect: Bi atoms dopant modified the electronic structure of Pd atoms and the rough surface provided ample space for the PdHx expansion. The RS-PdBi NWs sensors with high stability are potential for the reliable and rapid response towards hydrogen leakage in a wide low-temperature range (194.3 - 400 K).

sensing behavior” induced by hydrogen percolations further depresses the response stability of Pd based sensing materials. 14,

Introduction Hydrogen (H2) is one of the clean and renewable energy sources with high energy density and zero air pollution, and the liquid and gaseous hydrogen has been use in various fields such as hydrogenation processes, petroleum transformation, subzero refrigeration as well as aerospace technologies. 1, 2 Particularly, the detection towards hydrogen leakage is of great importance under various temperatures. Smart and reliable sensors not only enable to detect below explosion limit of hydrogen of the 4% mass concentration but also well adapt to various temperatures.

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Doping and alloying of Pd with other metals (e.g. Ag, Au, Cu) 16, 17 have been demonstrated as an effective way to suppress α-β phase transition. Under such cases, the dopants may modify the electronic performance of Pd atoms and weaken the hydrogen percolation, hence depress or avoid the “reverse sensing behavior” on exposure to H2 at low temperatures, and finally achieve improved sensing stability of hydrogen sensors. 18-20 However, the H2 adsorption capacity of the Pd-alloys decrease due to the other metal atoms partially substituting for Pd atoms on the surface of sensing materials, leading to a decreased sensing sensitivity. 20 To expose more Pd atoms for enhancing the H2 adsorption, the Pd-based nanostructures with modified morphology, such as Pd nanosheets, 21 Pd nanotubes 22 and nanocauliflower, 23 are employed in the hydrogen sensors. Furthermore, the modified morphology (e.g., screw-threaded shapes and porous NWs) may offer necessary region for the PdHx expansion and thus could lessen or even avoid the α-β phase transition. 19, 24 Additionally, it is reported that the thermodynamic and phase transition of low-dimensional (1D) Pd-based nanostructure may differ from those of its bulk structure owing to the size effect, for improving the hydrogen sensing stability. 25, 26 Thus, 1D pristine and alloyed Pd nanostructure is widely investigated for the advanced H2 sensors. 27-29 To date, although further inhibiting α-β phase transition and decreasing the critical temperature

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To date, Palladium (Pd) has been widely studied and employed as hydrogen sensing material due to its excellent selectivity and sensitivity to H2. 6, 7 The hydrogen sensing mechanism is descripted as follows: H2 molecules are adsorbed onto Pd atomic surface, followed by dissociated into H atoms, and then diffuse into Pd lattice to form PdHx intermediate, which leads to the resistance variation. 8 ࡼࢊ

ࡴ૛ ሱሮ ࡴ െ ࡴሺ૚ሻ ࡼࢊ ൅ ࢞ࡴ ՜ ࡼࢊࡴ࢞ሺ૛ሻ The insertion of H atoms into the Pd lattice causes a lattice expansion in PdHx, and the H atoms act as electron-scattering sources disturbing the free electrons flow in Pd atoms to alter the resistance. 9 Unfortunately, the α-β phase transition of PdHx causes the irreversible deformation of Pd lattice and the hysteresis response to H2, 10-13 which decreases the stability and reproducibility of a hydrogen sensor. Additionally, the “reverse

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of “reverse sensing behavior” is highly desired, little is reported on employing the synergistic effects of doping heteroatoms and tuning morphology towards Pd-based nanostructure, to further improve low- temperature stability of hydrogen sensors. In this study, we reported rough-surfaced palladium/bismuth NWs (RS-PdBi NWs) for improving low-temperature stability of H2 sensors, which have been developed via first AAOconfined electrodeposition followed by chemical etching, as is schematically shown in Figure 1 (a). The RS-PdBi NWs possess rough surface in the shape of bristle and alloyed structure with lattice expansion. The hydrogen sensors built with RS-PdBi NWs are able to stably work in a large temperature range of 193.4 - 400 K. Significantly, the critical temperature (⁓193.4 K) of “reverse sensing behavior” is much lower than that of pristine Pd NWs (287 K). The improved stability is attributed to the synergistic effects of rough-surfaced morphology providing ample space for the PdHx expansion and Bi-atoms dopant modifying electronic structure of Pd atoms.

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alloy NWs are optimized contents. Considering that the “galvanic cell” reaction of Bi species occurs in NaOH aqueous solution as follows, the morphological modification on etching Bi species off successfully is achieved, which is committed to the rough surface for the sensitive response toward H2. ࡮࢏ ൅ ૜ࡻࡴ ή՜ ࡮࢏ሺࡻࡴሻ૜ ൅ ࢋ ൉ ሺ૚ሻ ࡮࢏ሺࡻࡴሻ૜ ൅ ࡻࡴ ή՜ ࡮࢏ࡻ૛ ή ൅૛ࡴ૛ ࡻሺ૛ሻ Crystalline structure and chemical state To gain insight into the effect of Bi species in the crystal structure, the X-ray diffraction (XRD) analysis was carried out. The XRD pattern of RS-PdBi NWs in Figure 3 (a) displays distinct diffraction peaks corresponding to (111), (200), (220) and (311) facets of the face-centered cubic (fcc) of Pd, 31 which are consistent with the result of SAED analysis. It is noteworthy that a lower shift of ‫ ׽‬0.45 2θ degrees in the diffraction peaks occurs compared with those of the pristine Pd NWs (Figure S3, Supporting Information), which reveals the lattice expansion in PdBi alloy due to the Bi atoms doping into Pd lattice, though no characteristic diffraction peaks of Bi species are observed for its low loading content. The similar lattice expansion in alloy structure has also been identified in Pd-Bi, and Pd-Sn systems. 31, 32 The alloyed lattice expansion can restrain the expansion or contractions of PdHx system, thus suppresses the α-β phase transformation, 16, 17 and then improves the sensing stability of the hydrogen sensor at low temperatures. We further investigated the influence of Bi on the chemical state and electronic structure of Pd species present in the NWs by the X-ray photoelectron spectroscopy (XPS), and all the results mentioned below are calibrated with C 1s at 284.8 eV (Figure S4 (a), Supporting Information). As seen from the survey spectrum of the RS-PdBi NWs in Figure 3 (b), apart from the appearance of palladium, bismuth, and carbon peaks, the presence of oxygen element can be ascribed to O2 from the air during electrodeposition. Figure 3 (c) further shows the fitting curves of the high-resolution XPS spectrum of Pd 3d (335.36 and 340.67 eV, respectively) in RS-PdBi NWs. Compared to the pristine Pd NWs, the binding energies of the Pd 3d in RSPdBi NWs shift negatively ~ 0.16 eV (Figure S4 (b), Supporting Information). According to the integration area, the palladium oxidation in RS-PdBi NWs (~ 33.3%) is significantly less than that in pristine Pd NWs (~ 71.4%) (Figure S4 (c), Supporting Information). The results indicate that the electronic structure of Pd atoms is modified after the Bi atoms doping into Pd lattice and the Pd atoms are in electron-rich state. 33 In addition, the chemical states of Bi element in RS-PdBi NWs were further studied as well. As shown in Figure 3 (d), the oxidized state is detected up to ~ 54.23% in Bi species. By contrast, the metallic state (66.7%) is dominant in Pd species. It indicates that Bi element is easier to adsorb oxygen and even be oxidized in air. 34 There could be an oxidation competition between the Pd and Bi elements: when the oxygen atom is at the interface between the Pd atoms and the Bi atoms, it migrates to Bi element more easily, which will be in favor of an electrontransfer from Bi species to the Pd atoms. It can explain that the bonding energy between palladium and the strongly adsorbed oxygen is weakened even the oxidation of Pd is prevented in PdBi alloy. 35 The large amount of metallic Pd (0) and the rich

Results and discussion Morphology and chemical composition PdBi NWs were firstly prepared via AAO-confined electrodeposition. After removing AAO templates, we can observe that the PdBi NWs with smooth surface exhibit uniform cylindrical geometry with the diameter of ~ 45 nm (Figure S1 (a) (b), Supporting Information). The elemental mappings (Figure S1 (c) - (f), Supporting Information) reveal that the Pd and Bi elements in PdBi NWs are uniformly distributed. After chemical etching, the bundle of RS-PdBi NW arrays with similar shapes are observed in Figure 1 (b) - (c). From the transmission electron microscope (TEM) image (Figure 1 (d)), we can clearly see that the etched PdBi NW with rough surface and continuous small bumps structure in the shape of bristle. The corresponding elemental mappings (Figure 1 (e) - (g)) and the energy dispersive spectroscopy (EDS) analysis results demonstrate the Bi atomic content on the RS-PdBi NWs decreases to ~ 3 at% (Figure 2 (b)), wherever, it is ~ 15 at% in the PdBi NWs with smooth surface before etching (Figure 2 (a)). As a result, the Bi species covering on the NWs surface are partially etched off successfully, which indicates more Pd atoms for hydrogen adsorption are exposed on the PdBi NWs’ surface. The measured lattice spaces of 0.249 nm, 0.198 nm and 0.325 nm, 0.236 nm in the high-resolution TEM (HR-TEM) image (Figure 1 (h)) correspond to Pd (111), (200) and Bi (012), (014) planes, respectively. It suggests that the Bi atoms have been successfully doped into Pd lattice and the binary alloy has been formed. Moreover, the selected area electron diffraction (SAED) pattern further confirms the well-defined poly-crystallization structure exists in PdBi alloy. Actually, the Bi/Pd ratio plays a crucial role in the hydrogen sensing performance of the desired solid PdBi NWs. Based on the previous research works, 16,19,20 the excessive amount for heteroatom dopant in Pd based materials is unfavorable for the adsorption and diffusion of hydrogen. 6, 30 In this study, PdBi NWs with various Bi/Pd ratios were tried to prepared by modulating the electrolyte contents. However, with either higher or the lower Bi content, no desired RS-PdBi NWs were obtain after Bi atoms etched off (Figure S2, Supporting Information). Accordingly, we found that the Bi atomic ratio of ~ 15% in PdBi


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electrons around Pd atoms are favorable of the hydrogen adsorption, devoting to the superior hydrogen response of the RSPdBi NWs sensor at low temperatures. Hydrogen sensing stability at low temperatures As shown in Figure 4 (a), the hydrogen sensors are built via firstly integrating multiple RS-PdBi NWs between IDEs and then assembling onto chip-holder using wire-bonding process. From the partially magnified SEM images of the IDE region in Figure 4 (b) - (c), well-separated and overlapped RS-PdBi NWs across the electrodes can be clearly seen, which constructs a conductive path between two electrodes and allows large amounts of RS-PdBi NWs to be fully exposed to gas environment. 14 Figure 4 (d) shows the hydrogen sensing schematic illustration, the NWs lattice expand along with hydrogen adsorption and diffusion in the RS-PdBi NWs when H2 is open and the reversible hydrogen desorption occurs when H2 is switched off. To investigate the sensing stability of the PdBi NWs sensors towards H2 at low temperatures, the transient sensing test of the PdBi NWs sensors to H2 was performed using a sealed dynamic system as presented in Figure S5 of the Supporting Information. The relevant sensing parameters were descripted in Figure S6 of the Supporting information. As shown in Figure S7 of the Supporting Information, in the temperature range of 160 K - 400 K, there is significant difference for RS-PdBi NWs sensor in the sensing response behavior dependent temperatures. Specifically, when the testing temperature is above 220 K, the testing current decreases as the H2 is introduced into the sample chamber, and after the current saturated, the H2 flow is switched off for the recovery process, the testing current increases and essentially resumes the baseline (Figure S7 (a) - (h), Supporting Information). By contrast, when the testing temperature was lower to 190 K, the testing-current variation behaves the opposite trend (Figure S7 (i) - (j), Supporting Information). Similarly, such hydrogen sensing behaviors described above are also discovered in pristine Pd NWs and PdCu NWs, and the “reverse sensing behavior” decreases the stability and reproducibility hydrogen sensor. 19, 20 Before the reverse sensing behavior occurring, the testing current quickly changed with the “on” and “off” towards H2 with different concentration at various temperatures and the sensing response was going in ΔI (-) mode. Figure S8 of the Supporting Information displays the response time (Tres) and the recovery time (Trec) of RS-PdBi NWs sensor. As the temperature decreased, Tres was significantly prolonged. Until the operating temperature lowed to 190 K, a hydrogen sensing hysteresis and a low response in ΔI (+) mode (Figure 4 (i)) caused by the α-β phase transition appeared along with the reverse sensing behavior. 37 Figure 4 (e) – (h) additionally clearly show the sensing response ΔI (ΔI = Ig - I0, where I0 and Ig are the real-time peak currents in background Ar gas and H2 with various concentration, respectively.) dependent on H2 concentrations of the RSPdBi NWs sensor at several representative temperatures (others shown in Figure S9 of the Supporting Information). As we can see, the sensing response (ΔI) was not affected by the baseline current drifting. To a given H2 concentration for two cycles, the sensor showed similar sensing response (ΔI) even the baseline current drifted (Figure S9, Supporting Information). Figure 4 (i) - (j) clearly show that the critical temperature of the reverse sensing behavior of the RS-PdBi NWs sensors lowers to ~194.3 K, compared with those of other Pd-based materials reported

previously (Table 1). Size reduction from bulk to the nanoscale and alloying with other metal (Ag, Cu) lead to the lower critical temperatures of Pd-based materials. 37-40 Though the critical temperatures of the various shaped-controlled PdCu NWs are much lower than that of pristine Pd bulk, they are still far higher than 194.3 K. 19, 20 Such facts suggest that our RS-PdBi NWs sensors are able to work in a wide temperature range. As comparison, the hydrogen sensing of the PdBi NWs with smooth surface was investigated at various temperature (Figure S10 - S11, Supporting Information). The results reveal that the reverse sensing behavior of the PdBi NWs with smooth surface appears at 200 K, and the critical temperature shown in Figure S12 of the Supporting Information is ~201.3 K. After partially etching off Bi atoms to build rough surface, the RS-PdBi NWs exhibit an excellent hydrogen sensing response in wide lowtemperature range. The selectivity of H2 detection with respect to other potentially interfering gases with 3% concentration such as carbon monoxide, ethanol, methylbenzene, ammonia and acetone was studied at 298 K. As seen in Figure S13 (a) of the Supporting Information, the RS-PdBi NWs sensor shows much better selectivity to H2 over other gases. Furthermore, we re-tested and gained sensing response (ΔI) of the RS-PdBi NWs sensor at 310 K after working continuously for 100 h at various temperatures. We can see, in Figure S7 (i) of the Supporting Information, that the ΔI still keeps repeatable and stable when compared with the former result (Figure 4 (e)). In addition, the long-term stability testing for hydrogen sensing was performed at 298 K. The results in Figure S13 (b)of the Supporting Information reveal that the RS-PdBi NWs sensor continuously working for 15 days, the sensing response (ΔI) towards 3% and 0.1% H2 concentration displays minor changes. The above results indicate that our sensor possesses excellent sensing stability. The Figure S13 (c) of the Supporting Information shows the shape of RS-PdBi NWs after hydrogen sensing tests by SEM characterization. The RSPdBi NWs were not found fracture upon exposure to H2 at various temperatures, which reveals that there is no apparent hydrogen embrittlement happened, enabling far superior H2 sensing performance. Combined with the characterization of the RS-PdBi NWs, the improved stability of the hydrogen sensors can be ascribed to the following two aspects. On the one hand, the rough surface via chemical etching exposes more Pd atoms on the NWs’ surface and thus achieve improved sensing sensitivity. Additionally, such rough surface provides sufficient space for the PdHx expansion, which could inhabit the “reverse sensing behavior” even in the case of saturated hydrogen at low temperatures. On the other hand, the Bi dopant into Pd lattice modulates the electronic structure of Pd atoms, so that the Pd atoms are in an electron-rich state, which is beneficial to the hydrogen sensing. Meanwhile, the Bi content inside the NWs reduce the excessive hydrogen penetration in PdHx even if a high hydrogen concentration reaches 3%. As a result, the synergistic effects above lead to the lower critical temperature of reverse sensing behavior and the superior stability of the RS-PdBi NWs sensors. Conclusions In summary, the RS-PdBi NWs with a unique architecture by doping heteroatoms and tuning morphology were successfully



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developed for improving the low-temperature stability of hydrogen sensors. When working at 160 - 400 K, the RS-PdBi NWs hydrogen sensors demonstrate an excellent repeatable and durable sensing stability at low temperatures and the critical temperature of the reverse sensing behavior lowers to 194.3 K. The doping of Bi atoms into Pd lattice induces the alloy lattice expansion as well as the different electronic properties of alloyed Pd from those of monometallic Pd. The rough surfaced 1D nanostructure obtains a higher surface-exposure of Pd atoms and provides the space for PdHx expansion. The synergistic effects contribute to the lower critical temperature of the reverse sensing behavior and the superior sensing stability of the RSPdBi NWs hydrogen sensors. Our approaches to improve temperature-dependent stability could be employed for other Pdbased sensing materials, which can be synthesized and modified via wet-chemical routines. Moreover, our study provides experimental base for highly stable hydrogen sensors adapted in critical environment such as those work at low temperatures.

ASSOCIATED CONTENT Supporting Information: Experimental Section including the material synthetic process, the sensor building, characterizations and hydrogen sensing measurement. The morphology and composition of the cylindrical PdBi NWs before chemical etching. XRD patterns of RS-PdBi NWs and Pd NWs. Carbon peak calibration and Pd 3d XPS spectra for Pd NWs. The schematic diagram of the hydrogen sensing test system. The parameter definition of hydrogen sensors. The raw data of the sensing response and the fitted sensing response curves of RS-PdBi NWs and PdBi NWs with smooth surface. The response time and recovery time of the RS-PdBi NWs sensors towards H2 and the.

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

ORCID Dachi Yang: 0000-0001-6842-449X

Author Contributions L.D. and D.Y. conceived the idea; L. D., D.Y. and H.W. carried out experiments; L.D. and D.Y. analyzed results, L.D., L.Z. and D.Y. wrote the article; S.Z., Z.Z. and J.C. carried out XRD and TEM characterization. All authors reviewed the manuscript.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 21473093), Fundamental Research Funds for the Central Universities, Tianjin Research Program of Application Foundation and Advanced Technology (Grant No. 14JCYBJC41300).

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30. Lupan, O.; Postica, V.; Adelung, R.; Labat, F.; Ciofini, I.; Schürmann, U.; Kienle, L.; Chow, L.; Viana, B.; Pauporté, T., Functionalized Pd/ZnO Nanowires for Nanosensors. Phys. Status Solidi RRL. 2018, 12, 1700321 (1) - 1700321 (9). 31. Xu, H.; Zhang, K.; Yan, B.; Wang, J.; Wang, C.; Li, S.; Gu, Z.; Du, Y.; Yang, P., Ultra-uniform PdBi Nanodots with High Activity towards Formic Acid Oxidation. J. Power Sources. 2017, 356, 27-35. 32. Du, W.; Mackenzie, K. E.; Milano, D. F.; Deskins, N. A.; Su, D.; Teng, X., Palladium–Tin Alloyed Catalysts for the Ethanol Oxidation Reaction in an Alkaline Medium. ACS Catal. 2012, 2, 287-297. 33. Al-Odail, F. A.; Anastasopoulos, A.; Hayden, B. E., Hydrogen Evolution and Hydrogen Oxidation on Palladium Bismuth Alloys. Top. Catal. 2011, 54, 77-82. 34. Tripković, A. V.; Popović, K. D.; Stevanović, R. M.; Socha, R.; Kowal, A., Activity of a PtBi Alloy in the Electrochemical Oxidation of Formic Acid. Electrochem. Commun. 2006, 8, 1492-1498. 35. Karski, S.; Witońska, I., Bismuth as an Additive Modifying the Selectivity of Palladium Catalysts. J. Mol. Catal. A: Chem. 2003, 191, 87-92. 36. 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 PMMAMembrane-Coated Palladium Nanoparticle Films. ACS Appl. Mater. Interfaces 2017, 9, 27193-27201. 37. Wicke, E.; Brodowski, H., Hydrogen in Palladium and Palladium Alloys. Top. Appl. Phys. 1978, 29, 73-155. 38. Feenstra, R.; Bruin-Hordijk, G. J.; Bakker, H. L.; Griessen, R.; Groot, D. G., Critical Point Lowering in Thin PdHx Films. J. Phys. F: Met. Phys. 1983, 13, L13-L18. 39. Buck, H.; Alefeld, G., Hydrogen in Palladium–Silver in the Neighborhood of the Critical Point. Phys. Status Solidi B. 1972, 49, 317-327. 40. Mazzolai, F.M.; Lewis, F.A., Elastic Energy Dissipation in the Palladium–Silver–Hydrogen (Deuterium) System. I. Hydrogen– Dislocation Interaction Effects. J. Phys. F: Met. Phys. 1985, 15, 12491260.



Page 6 of 11

(a) I

AAO

Ⅲ

II

PdBi NWs in AAO

(b) (a) (a)

PdBi NWs

RS-PdBi NWs

(c)

(d d) (d)

100 nm

500 nm

(d)

(e)

(f)

(311) (200)

(g)

(h)

(111) (220)

Pd

Bi

5 nm

O

Figure 1. (a) Schematic of the synthetic process. (Ⅰ) Electrodeposition. (Ⅱ) Dissolving AAO template. (Ⅲ) Chemical etching.(b) SEM and (c) amplified SEM images of RS-PdBi NWs. (d) TEM image of individual PdBi RS-NW with elemental mappings: (e) Pd, (f) Bi and (g) O. (h) The HR-TEM image of RS-PdBi NW and the inset SAED pattern taken from the dashed rectangle and circle in (c), respectively. The scale bars in (d) - (g) are all 20 nm, and the inset one in (h) is 1 / (5 nm).


Page 7 of 11

(a)

Before etching

Pd

Element Mass (%) Atom (%)

C

Pd Bi

Intensity (a.u.)

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


Bi

O

Bi

(b)

Pd

74

85

Bi

26

15

Total amount

100

100

After etching

Pd

Element Mass (%) Atom (%)

C

Pd O

0

Bi

1

2

Bi

Bi 3

4

Pd

95

97

Bi

5

3

Total amount

100

100

5

6

7 keV

Figure 2. EDS analysis of the PdBi NWs. (a) before being etched with the Bi atomic content of ~ 15 at% and (b) after being etched in 4 M NaOH with the Bi atomic content of ~ 3 at%.


C 1s Bi 4f

Pd 4p Bi 5d

Bi 4p Pd 3p O 1s Bi 4d

O KLL

Intensity (a.u.)

(311)

(220)

(200)

Intensity (a.u.)

RS-PdBi NWs

C KLL Pd MNN

(b)

(111)

(a)

200

0

Pd-PDF#89-4897

20

(c)

30

40

50

60

70

2-Theta (degree)

3d 5/2 3d 3/2

80 Pd

1200 1000 800

90

(d)

Pd (0) O2-Pd Pd (Ⅱ)

344

342

340

338

336

Binding Energy (eV)

600

400

Binding Energy (eV)

Intensity (a.u.)

10

Intensity (a.u.)

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

Page 8 of 11

Pd 3d


Bi

O2-Bi Bi (Ⅲ)

166

334

4f 7/2 Bi (0)

4f 5/2

164

162

160

158

Binding Energy (eV)

156

Figure 3. (a) XRD pattern of RS-PdBi NWs. (b) XPS survey spectrum of RS-PdBi NWs. (c) - (d) The high-resolution XPS spectra of Pd 3d (c) and Bi 4f (d) of RS-PdBi NWs.


Page 9 of 11

(b)

(a)

(c)

(d)

H2 absorption

H2 exposure

2 μm (g)

0.000 On

-0.010 -0.015

T=298.15 K

-0.020

On : H2 Open

-0.025 -0.030

On

On

On

On

On

3% Off

0

2% Off

-0.0015 -0.0020

0.5

On : H2 Open

1.0

1.5

2.0

Time (ksec)

2.5

3

4

5

3% Off

0.0006

2% Off Off : H2 Close 1% Off

0.0002

3.0

-0.0002

On

On

On On 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

Time (ksec)

3% 2% 1% 0.5% 0.2% 0.1%

150

(j)

On : H2 Open

0.0004

PdHx

0.005 0.000 -0.005 -0.010 -0.015 -0.020 -0.025 -0.030 -0.035 -0.040

6

0.0000

Off : H2 Close

3% Off

0.0

ΔI (mA)

1% Off 0.5% Off T=400 K

-0.0010

2

(i)

Off : H2 Close

Time (ksec)

H2 coverage

H2 desorption

T=190 K

0.0008

-0.0005

1

2% Off

(h) 0.0010

0.0000

On On

On On

0.1% 0.5% 0.2% Off Off 1% Off Off

Time (ksec)

(f) 0.0005

On

On

-0.005

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

RS-PdBi NWs

ΔI (mA)

0.002 OnOn On On On On On On On On On 0.000 0.1% -0.002 0.2%Off 0.2% -0.004 0.5%Off Off 0.5% Off 1% -0.006 1%Off Off Off -0.008 T=310 K 2% 2% -0.010 Off Off On : H2 Open -0.012 Off : H2 Close -0.014 3% Off 3% Off

ΔI (mA)

ΔI (mA)

(e)

10 μm

ΔI (mA)

5 mm

ΔI (mA)

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


200

250

300

350

Temperature (K)

400

0.001 0.000 -0.001

194.3 K

-0.002

3% 2% 1%

-0.003 160

170

180

190

200

210

220

Temperature (K)

Figure 4. (a) A hydrogen sensor prototype. (b) - (c) The sequentially magnified SEM images of the IDE integrated with the multiple RS-PdBi NWs. (d) Schematic illustration of the RS-PdBi NWs for H2 adsorption and desorption cycles. (e) - (h) The representative hydrogen sensing response (ΔI) curves dependent on H2 concentrations for RS-PdBi NWs sensor at (e) 310 K, (f) 400 K, (g) 298.15 K and (h) 190 K, respectively. (i) The sensing response (ΔI) plots of RS-PdBi NWs dependent on H2 concentration under various temperatures. (j) The enlarged plots from the dashed box in (i), exhibiting the region of critical temperature of reverse sensing behaviors.



Page 10 of 11

Table 1. The critical temperature of various Pd-based NWs sensors

Pd-based sensors

Composition

Diameter

The critical temperature

Refs.

Pd bulk

Pd

565 K

37

Pd film

Pd

ͳʹͲ

460 K

38

Individual Pd NW

Pd

‫ ׽‬65 nm

263 K

14

Multiple Pd NWs

Pd

‫ ׽‬65 nm

287 K

14

Pd90Ag10 foil

PdAg

ͳͲ

446 K

39

Pd77Ag23 foil

PdAg

ʹ

298 K

40

screw-threaded PdCu NWs

PdCu

‫ ׽‬65 nm

259.4 K

19

random-gapped PdCu NWs

PdCu

‫ ׽‬70 nm

261 K

19

P-PdCu NWs

PdCu

‫ ׽‬60 nm

264.2 K

20

PS-PdCu NWs

PdCu

‫ ׽‬60 nm

257.2 K

20

PM-PdCu NWs

PdCu

‫ ׽‬60 nm

239.9 K

20

Smooth-surfaced PdBi NWs

PdBi

‫ ׽‬45 nm

201.3 k

This work

RS-PdBi NWs

PdBi

‫ ׽‬45 nm

194.3 k

This work


Page 11 of 11

Table of Content

ΔI (mA)

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


0.005 0.000 -0.005 -0.010 -0.015 -0.020 -0.025 -0.030 -0.035 -0.040

2 μm

194.3 K 3% 2% 1% 0.5% 0.2% 0.1%

150

200

250

300

350

Temperature (K)

RS-PdBi NWs

400

11Environment ACS Paragon Plus

Bismuth Nanowires with Rough Surface for Stable

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