Amorphous Metal-Hydrides for Optical Hydrogen Sensing: The Effect

Letter pubs.acs.org/acssensors

Amorphous Metal-Hydrides for Optical Hydrogen Sensing: The Effect of Adding Glassy Ni−Zr to Mg−Ni−H Mercedes Victoria,† Ruud J. Westerwaal,† Bernard Dam,† and Jacobus L. M. van Mechelen*,‡ †

Department of Chemical Engineering, Materials for Energy Conversion and Storage, Delft University of Technology, Julianaweg 136, 2628 BL Delft, The Netherlands ‡ ABB Corporate Research, Segelhofstrasse 1K, 5405 Baden-Dättwil, Switzerland S Supporting Information *

ABSTRACT: Optical hydrogen sensors have a promising future in a society where hydrogen detection becomes increasingly essential. Sophisticated designs have been reported, which traditionally use Pd as sensing material. Upon hydrogenation, Pd remains metallic and is characterized by a small optical contrast and low sensitivity. Here we report on a new generation hydrogen sensing materials, with a large optical contrast and high sensitivity in a broad hydrogen pressure sensing range. We show that the sensitivity of transparent hydrides is strongly increased by changing the intrinsic mechanism of hydrogenation. Using the robust behavior of the metallic glass Ni−Zr upon hydrogenation, we made amorphous Mg−Ni−Zr where the interplay of transparent Mg2NiH4 and glassy Ni−Zr provides unprecedented advantages. We demonstrate practical usage of the novel hydride Mg−Ni−Zr in gas and liquid environments corresponding to realistic applications. KEYWORDS: optical hydrogen sensing, Mg−Ni−Zr, hydrogen sensor, amorphous metal-hydride, optical sensitivity, optical contrast

O

optical variation to the electronic properties of the hydride state. This allows defining two quality parameters, the sensitivity and the optical contrast. The aim is to find materials where both are substantially large. Here we report on optimizing the sensing material by modifying the mechanism of hydrogenation while assuring a final insulating hydride state. Hereto, we add metallic glassiness of the Ni−Zr system to the well-known transparent Mg−Ni hydrides. This procedure reveals a new class of glassy materials that, as compared to traditionally used materials, benefits from a wide sensing range and large optical variations at adequate hydrogen partial pressures. In addition, amorphization brings along mechanical stability6 through reduction of volume changes upon hydrogenation, leading to an optical response that is repeatable and reversible within 0.1% for hundreds of cycles. We test practical usage of the sensing material in gas and liquid environments as a function of temperature. The optical response of a metal-hydride depends largely on its thermodynamic properties.7 With increasing hydrogen partial pressure pH2, the hydrogen concentration cH2 increases in a nonlinear way for most materials. For single element hydrides at intermediate partial pressures, coexistence of several

ptical hydrogen sensing has a promising potential for applications in industry and (future) consumer products. Inherently safe, it enables detection of hydrogen in versatile configurations from surfaces to difficult accessible locations. In this regard, sophisticated optical designs have been reported that demonstrate meticulous levels of engineering. Examples are nanostructures (dots, rods, etc.) where (surface) plasmons are induced,1 fibers with tapered regions that modify the waveguiding conditions,2 and fiber-based micromirrors that directly sense the transfer function.3 These structures can (partly) be made from the sensing material and thereby sense hydrogen directly, or be coated with the sensing material, which indirectly changes the response conditions. Despite the potential of these designs, the properties of the sensing material are hardly optimized. Moreover, no qualification parameters exist to indicate their performance. The material currently mostly used is Pd, although sometimes also Mg and Y.4 Hydrogenation of these materials causes an abrupt change of optical functions, which makes the material sensitive only in a very small hydrogen pressure range. Recent work has shown that alloying Mg with Ti5 and Pd with Au4 broadens the sensing range. However, in both cases the absolute variation of the optical properties is strongly reduced by adding metallicity to the original semitransparent hydrides MgH2 and PdH0.7. The sensing range is principally related to the intrinsic mechanism of hydrogenation of the metal-hydride and the © XXXX American Chemical Society

Received: December 3, 2015 Accepted: January 11, 2016

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DOI: 10.1021/acssensors.5b00265 ACS Sens. XXXX, XXX, XXX−XXX

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Figure 1. Pressure−transmission isotherms of selected Mg−Ni−Zr compositions (a) Zr poor compositions Mg64NiAZrB with ξ = A/B and A+B = 36, (b) Zr rich compositions Mg52NiAZrB at 21 °C in a gaseous environment. (c) Normalized reflection of Mg52NiAZrB with ξ = 1.0 during repeated hydrogenation at 40 mbar H2. Green lines indicate a response that varies less than 0.1%. The range between 909 and 910 min has been enlarged and shows four consecutive hydrogenation curves in black and green to evidence the reproducibility. (d) Temperature dependence of PTI’s of Mg52NiAZrB with ξ = 1.0. The inset shows the temperature dependence of the optical response ; at selected levels of pH2.

pressure−composition isotherms instead of a plateau.6,9 In the presence of H2, however, the Ni−Zr alloy remains metallic10 and thus manifests a small optical change that is not ideal for optical sensing purposes. More suited would be to start with the Mg−Ni−H system and to make it amorphous by alloying with Zr. The advantage of the Mg−Ni−H system for an optical sensor material is its large optical change upon hydrogenation,11 due to formation of both transparent Mg2NiH4 and MgH2.12 At room temperature, the amorphous phase of Ni−Zr is energetically more favorable than its crystalline form only for ξ = Ni/Zr ratios between 0.6 and 3.5.13 Therefore, in order to influence the degree of crystallinity of Mg−Ni by adding Zr, this ratio needs to be kept, contrary to ref 14 where Zr only adds some metallicity to the system. We have sputtered Pd covered Mg−Ni−Zr gradient thin films with ξ = 0.7−2.9, as described in the Supporting Information. X-ray diffraction in the as-deposited, hydrogenated, and unloaded state was performed to investigate the crystalline structure. Besides the (111) and (220) reflections of Pd, none of the examined states showed any other reflections (see Figure S2). This contrasts with earlier work where for Mg−Ni thin film alloys without Zr, for similar Mg/Ni ratios, crystallinity was indicated by the Mg2Ni (003) and (006)

phases generally causes a sharp increase of cH2. Represented as pH2 vs cH2, as commonly done for pressure−composition isotherms, this corresponds to a plateau region. For hydride forming alloys, the relative transmissivity ; = ln(It/It,0), where It and It,0 are the transmitted intensities of the hydrogenated and as-deposited film, respectively, resembles the behavior of cH2.8 Optical hydrogen sensors correlate pH2 and ; in a so-called pressure-transmission isotherm (PTI), being the optical equivalent of the pressure−composition isotherm. An optical sensing device is most sensitive in the plateau region and quickly decreases for pressures beyond this range. A sensing material of broader utility, with a wide sensing range and a sensitivity independent of the value of the hydrogen pressure, would need to show a monotonously increasing PTI. That is, the existing plateau needs to be strongly inclined. Recently, it has been shown that Mg−Ti alloys at Mg/Ti ≈ 1 manifest a slightly inclined plateau as compared to pure MgHx.5 However, the sensing range is around 100−101 mbar, which is at the high end for most applications. The optical contrast of Mg−Ti−H is small compared to MgH2 due to the formation of metallic TiH2. Amorphous alloys with an attractive hydrogen−hydrogen interaction, such as Ni−Zr, are shown to have steep hydrogen B

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Figure 2. (a) Comparison of the optical sensitivity S of Mg−Ni−Zr−H to often used hydrogen sensing materials calculated by eq 1. (b) Extinction coefficient kH at 635 nm for most materials of (a). The ballpark of kH of Mg52NiAZrB, indicated by an ellipse, has been estimated using the measured Δ; (Figure 1b) and k0 of Mg−Ni.15

reflections.12 This implies amorphousness of all studied Mg− Ni−Zr films and points to the remarkable observation that adding Mg to the Ni−Zr glass morphologically does not change its nature. Figure 1a shows the PTI’s for Mg−Ni−Zr compositions with ξ in the range where Ni−Zr forms a glass. Similar to most metal-hydrides, such as MgH2, however, the PTI’s show a pronounced plateau. It turns out that the influence of Ni−Zr becomes significant when the Mg/Ni ratio is close to stoichiometry and the Ni/Zr ratio within the range of metallic glass formation. Figure 1b shows that now the plateaus of the PTI’s have changed to monotonous and steep slopes at the expense of slightly decreased maximum values of ; . Hereby, the hydrogen pressure range for which there is a measurable optical change has enormously increased as compared to traditional sensing materials such as Pd and its alloys. With increasing Zr content, the PTI’s in Figure 1a,b shift down and broaden their range of ; . The down shift can be understood within a simple substitution picture for an amorphous compound as the enthalpy of formation of ZrH2 (−165 kJ/mol H2) is substantially lower than that of NiH0.5 (−6 kJ/mol H2).16 The transparency increase is probably due to the increase of Mg relative to Ni, and thus the formation of MgH2. The gradual variation of the optical properties during hydrogenation observed for the Mg−Ni−Zr system largely resembles the inclined pressure−composition isotherms of glassy Ni−Zr, opposite to crystalline Ni−Zr.6 Vitrification, and

its known influence on hydrogenation, seems a robust property and is independent of Mg. This makes Mg−Ni−Zr substantially different from Mg−Ti that at specific ratios also possesses somehow inclined isotherms.5 Amorphization of Ni−Zr relates to disorder that causes a spread in the energies of the configurational sites that hydrogen can occupy17 and leads to gradual hydrogenation. Mg−Ti−H is an immiscible polycrystalline alloy8b with some level of chemical segregation,18 opposite to the randomness of Ni−Zr. For sensing purposes, an ideal PTI would be a linear increase on a log−log scale: the latter since optical detectors possess logarithmic sensitivity, and hydrogen partial pressure levels required for applications typically span several orders of magnitude and are also logarithmically spaced. We therefore define an optical sensitivity S of the sensing material that can be calculated from the PTI’s in the following way: S=

∫;

;max

min

d log pH d;

2

d;

(1)

where ; is normalized to its largest value max (; ). S is thus in fact a measure of the absence of plateau regions. Figure 2a shows S for Mg−Ni−Zr thin film compositions of Figure 1b. For compositions with Mg/Ni ∼ 2, S has a minimum for ξ = 0.8−1.0 and strongly increases with ξ. For Mg−Ni−Zr compositions of Figure 1a, which possess a plateau region, S is much smaller. In order to put these values in the context of typically used sensing materials, we have determined S based on reported PTI’s for hydrogenated Pd,7 Pd−Cu,19 and Mg−Ti7,20 C

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ACS Sensors (see Figure 2a). Although Mg−Ti and Pd−Cu have slightly inclined plateaus, their sensitivity is far below the newly found class of amorphous Mg−Ni−Zr with Mg/Ni ∼ 2. Besides the sensitivity S, equivalently important is the optical contrast between the as-deposited and completely hydrogenated state, Δ; = |max(; ) − min(; )|. In analogy with S, for Δ; we only consider the sensing material. Through the Lambert−Beer law, Δ; ∝ (k0 − kH), where k0 and kH are the extinction coefficients in the as-deposited and hydrogenated state, respectively. min (; ) is determined by k0 ≈ 4.0 ± 0.5 for all Pd- and Mg-based alloys.7,15,21 max(; ) depends strongly on the sensing material properties and thus on kH. Insulating MgH2,22 for instance, has kH < 0.01 (see Figure 2b) and thus Δ; is large. This contrasts with the sensitivity S for MgH2 that is rather small due to the abrupt change of ; while hydrogenating Mg, manifested as a plateau in the PTI. Figure 2b shows that kH for Mg−Ni−Zr compositions of Figure 1 is in the vicinity of transparent Mg2NiH4, that is, midway between MgH2 and very metallic Pd−Cu−H. Mutual comparison of kH for the shown alloys indicates that also from an optical contrast point of view Mg−Ni−Zr compositions with Mg/Ni ∼ 2 are by far the best for sensing applications. Palladium, the most widely used sensing material, has low sensitivity and very low transmission. Alloying increases the sensitivity, as reported,4 but decreases the optical contrast even more. For a specific application, the sensor performance can thus be tuned primarily by the Ni/Zr ratio which is of leading influence for the sensitivity at Mg/Ni ∼ 2. The absolute amount of Zr further regulates the optical contrast. Similar results using this novel approach of optimizing metal-hydrides by introducing glassiness are expected for other alloys that are composed of an early and late transition metal element.23 A reproducible behavior is a key asset for a sensor material. Figure 1c shows the behavior of repeated hydrogenation of a Pd-capped Mg52Ni24Zr24 thin film manifested by the change of the reflectivity during 50 periods. During the first ∼25 cycles, the optical contrast slightly increases, most probably due to relaxation of the thin film microstructure. Beyond this initial behavior, the response becomes highly reproducible, and Δ; is stable within 0.1%. At this stage, the gradual variation of ; , and thus the sensitivity S, is also very reproducible during hydrogen absorption (see Figure 1c). However, during desorption, ; of Mg−Ni−Zr manifests hysteresis comparable to other metalhydrides4 (see Figure S4). This extreme level of reproducibility is due to the mechanical stability of the glassy Ni−Zr alloy,9 as confirmed by AFM measurements (see Figure S3). Figure 1d shows the temperature dependence of the PTI’s of Mg52Ni24Zr24 between 21 and 100 °C. With temperature Δ; decreases, but S increases. Industrial applications often deal with dissolved hydrogen in liquids that range from water, (edible) oil, to bitumen and liquid aluminum.24 We show application of amorphous Mg− Ni−Zr integrated in a micromirror configuration (see Supporting Information) in mineral oil. Figure 3 shows the optical response of a Mg52Ni24Zr24 thin film with Pd catalyst and a thin liquiphobic PTFE layer at 21 and 60 °C as a function of time under hydrogen partial pressures 25−1300 ppm in oil. The effect of the inclined PTI’s caused by the glassiness of Ni− Zr is that each hydrogen level corresponds to a distinct optical reflection. Amorphous Mg−Ni−Zr shows good sensitivity better than Mg−Ti20in these hydrogen and temperature ranges corresponding to application in power transformers.

Figure 3. Optical reflection of a Ti/Mg52Ni20Zr28/Pd/PTFE fiber optic sensor in transformer oil at selected hydrogen partial pressures (a) at 21 °C and (b) at 60 °C, at 635 nm. The legend values in square brackets are the hydrogen concentrations in oil calculated from the concentration of the gas mixture. Dashed lines are fits to the data using a sigmoid function.

In conclusion, we reported on a route for improving the sensing material of optical hydrogen sensors. In order to enable mutual comparison with existing materials we introduced two quality parameters, sensitivity and optical contrast. Palladium is the most widely used sensing material, although both its sensitivity and optical contrast are marginal. Transparent hydrides, such as MgH2, have a large optical contrast but low sensitivity. We showed that amorphization of transparent Mg2NiH4, introduced by the metallic glass Ni−Zr, increases the sensitivity through making the hydrogenation more gradual, and preserves the optical contrast. We validated performance of a fiber optic hydrogen sensor based on Mg−Ni−Zr in mineral oil for electrical transformer applications. The demonstrated approach can be used generally to optimize the sensing performance of metal-hydrides that rely on the characteristics of the pressure−composition isotherm.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.5b00265. Figure S1: schematic representation of the hydrogen sensing material layout. Figure S2: X-ray diffraction spectra for selected compositions in the as-deposited state, after hydrogenation, and hydrogenated. Figure S3: AFM images in the as-deposited state and after 50 hydrogenation cycles. Figure S4: pressure−transmission D

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(13) Materials interfaces: Atomic-level structure and properties. Chapman and Hall: London, 1992. (14) Goo, N. H.; Lee, K. S. The electrochemical hydriding properties of Mg−Ni−Zr amorphous alloy. Int. J. Hydrogen Energy 2002, 27 (4), 433−438. (15) Lohstroh, W.; Westerwaal, R. J.; van Mechelen, J. L. M.; Chacon, C.; Johansson, E.; Dam, B.; Griessen, R. Structural and optical properties of Mg-Ni-H switchable mirrors upon hydrogen loading. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70 (16), 165411. (16) Buschow, K. H. J.; Bouten, P. C. P.; Miedema, A. R. Hydrides formed from intermetallic compounds of two transition metals: a special class of ternary alloys. Rep. Prog. Phys. 1982, 45 (9), 937. (17) (a) Kirchheim, R.; Sommer, F.; Schluckebier, G. Hydrogen in amorphous metalsI. Acta Metall. 1982, 30 (6), 1059−1068. (b) Griessen, R. Phase separation in amorphous metal hydrides: A Stoner-type criterion. Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 27 (12), 7575−7582. (18) Baldi, A.; Gremaud, R.; Borsa, D. M.; Baldé, C. P.; van der Eerden, A. M. J.; Kruijtzer, G. L.; de Jongh, P. E.; Dam, B.; Griessen, R. Nanoscale composition modulations in MgyTi1−yHx thin film alloys for hydrogen storage. Int. J. Hydrogen Energy 2009, 34 (3), 1450− 1457. (19) Westerwaal, R. J.; den Besten, C.; Slaman, M.; Dam, B.; Nanu, D. E.; Böttger, A. J.; Haije, W. G. High throughput screening of Pdalloys for H2 separation membranes studied by hydrogenography and CVM. Int. J. Hydrogen Energy 2011, 36 (1), 1074−1082. (20) Mak, T.; Westerwaal, R. J.; Slaman, M.; Schreuders, H.; van Vugt, A. W.; Victoria, M.; Boelsma, C.; Dam, B. Optical fiber sensor for the continuous monitoring of hydrogen in oil. Sens. Actuators, B 2014, 190 (0), 982−989. (21) (a) Lohstroh, W.; Westerwaal, R. J.; van Mechelen, J. L. M.; Schreuders, H.; Dam, B.; Griessen, R. The dielectric function of MgyNiHx thin films. J. Alloys Compd. 2007, 430 (1−2), 13−18. (b) von Rottkay, K.; Rubin, M.; Duine, P. A. Refractive index changes of Pd-coated magnesium lanthanide switchable mirrors upon hydrogen insertion. J. Appl. Phys. 1999, 85 (1), 408−413. (22) Isidorsson, J.; Giebels, I. A. M. E.; Arwin, H.; Griessen, R. Optical properties of MgH2 measured in situ by ellipsometry and spectrophotometry. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68 (11), 115112. (23) Petö, G.; Bakonyi, I.; Tompa, K.; Guczi, L. Photoemission investigation of the electronic-structure changes in Zr-Ni-Cu metallic glasses upon hydrogenation. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 52 (10), 7151−7158. (24) Gupta, R. E. Hydrogen Fuel - Production, Transport, and Storage; CRC Press, 2008.

isotherms shown during absorption and desorption. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

R.J.W. and B.D. designed experiments; M.V. and R.J.W. performed experiments; J.L.M.vM. and M.V. analyzed data and wrote the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge P. Ngene, C. Boelsma, H. Schreuders, R. Gremaud, G.M. Müller, A. Kramer and Th. Paul for their technical and scientific support.

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DOI: 10.1021/acssensors.5b00265 ACS Sens. XXXX, XXX, XXX−XXX