Pulsed Laser Deposition of PdCuAu Alloy ... - ACS Publications

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Pulsed Laser Deposition of PdCuAu Alloy Membranes for Hydrogen Absorption Study J. Galipaud, M. H. Martin, L. Roué, and D. Guay* INRS-Énergie, Matériaux et Télécommunications, Institut National de la Recherche Scientifique, 1650 Lionel-Boulet Blvd., Varennes, QC J3X 1S2, Canada S Supporting Information *

ABSTRACT: Finding suitable binary and ternary alloys for hydrogen purification membranes remains a challenge due to the vast compositional range to be studied. In this context, we proposed a new combination of alloy fabrication (as thin film) by pulsed laser deposition (PLD) and subsequent hydrogen solubility characterization by electrochemical in situ X-ray diffraction (E in situ XRD) able to rapidly screen alloys in a broad range of compositions. In this study, we demonstrated the capabilities of this new technique on the PdCuAu system. A FCC solid solution was formed in the full explored composition range (Pd = 15−100 at. %, Cu = 0−80 at. %, Au = 0−30 at. %). Structural reorganization, relieving some strain in the lattice, was observed during the first electrochemical hydrogen−dehydrogenation cycles. Using E in situ XRD to study the hydrogen solubility of PdCuAu alloys, we confirmed that the Pd content in the alloy is the primary parameter driving hydrogen solubility. Replacing Cu with Au slightly enhanced the hydrogen solubility of PdCuAu.

1. INTRODUCTION The potential use of hydrogen as an energy vector develops strong incentives to improve hydrogen gas production, separation, purification, and storage. Purification membranes are a key element in this development, which seeks to reduce significantly hydrogen production costs and thus provide H2 at a competitive price compared to gasoline.1,2 Commercially available membranes such as pure palladium or PdAg alloy membranes still fail to ensure long-term efficiency, particularly during temperature cycling, and generate mechanical failure in the device due to hydrogen embrittlement. Surface poisoning remains an issue, particularly due to sulfur compounds which tend to react readily with pure palladium to irreversibly form Pd sulfides and prevent further hydrogen permeation.3,4 In recent years, extensive research has been devoted to the study of palladium alloys to achieve suitable hydrogen solubility and diffusion and alleviate some of the issues noted above (low hydride formation critical temperature; minimal stress development; resistance to poisoning).5,6 However, despite a wide variety of binary alloys investigated both experimentally7−10 and theoretically,11−14 researchers have failed to identify an alloy which fulfills all current requirements. Numerous studies15,16 have focused on PdCu alloys, largely due to the improved sulfur resistance they provide.17,18 However, increasing the Cu content in a FCC PdCu alloy dramatically decreases hydrogen solubility and consequently the membrane’s permeability to hydrogen.19 In researching an adequate compromise between hydrogen permeability, sulfur © 2015 American Chemical Society

resistance, and mechanical stability, the exploration of palladium ternary alloys proves to be an interesting avenue. However, considerably less experimental and theoretical data are available. Moreover, investigation of a decent composition range for each ternary alloy can only be reached through the preparation of numerous samples. Some theoretical work has been initiated in this direction, by Sholl et al. among others, building on well-known PdCu alloys (resistant to poisoning) to investigate PdCuM alloys with the potential to increase the permeation capabilities of PdCu.20,21 Addition of 4 at. % Au and Ag appeared to increase permeability in Pd70Cu30 alloys. Various experimental studies investigated such alloys; however, morphology variations (roughness, density, preferential surface segregation) and difficulties with conditioning for permeation tests prevented the broad screening of numerous alloys.22,23 Often experimental work is restrained to a narrow composition range of the ternary element. Another approach, based on novel fabrication and characterization techniques, focuses on the broad screening and rapid testing of large numbers of alloys to easily select promising compositions in terms of solubility and diffusion of hydrogen.24,25 In this article, we present a novel approach to rapidly investigating the hydrogen solubility of palladium ternary alloys in a broad composition range. We used pulsed laser deposition Received: August 3, 2015 Revised: November 2, 2015 Published: November 2, 2015 26451

DOI: 10.1021/acs.jpcc.5b07511 J. Phys. Chem. C 2015, 119, 26451−26458

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to easily and rapidly prepare palladium ternary alloys in a broad range of compositions with controlled morphologies. We then used electrochemical in situ X-ray diffraction (E in situ XRD) to measure the hydrogen content in the PdCuAu alloys. E in situ XRD was already successfully used to investigate hydrogen absorption in PdCu (with Cu content from 0 to 85 at. %).26 We relied on the electrochemical absorption of hydrogen to measure hydrogen solubility, which occurs in a matter of minutes, thanks to out-of-equilibrium conditions and the use of thin films. The screening of ternary alloys is thus facilitated.

(1)

where e is the charge of the electron, NA Avogadro’s number, Y the sputter yield of the ions under study, w the atomic weight of the ablated species, d the density of the ablated material, I the ionic current of the beam, and S the sputtered surface. Using this technique, information about the element chemical states was also extracted. To ascertain the XPS results, X-ray reflectometry was also performed, which enabled a more complete view of the film structures. These experiments were carried out with a Panalytical X’pert PRO diffractometer with a monochromatic Cu source (λ = 1.5418 Å). In this technique, using the interference pattern caused by changes in refractive index in the film at low 2θ angle, it is possible to draw a schematic view of the different layers in the film.30 Finally, electrochemical in situ XRD (E in situ XRD) was used to explore the behavior of PdCuAu alloys upon hydrogen absorption and desorption. A custom-designed electrochemical cell was fitted in a Bruker D8 advance X-ray diffractometer. In this configuration, the electrolytic absorption and desorption of hydrogen in and out of the thin film was possible, while X-ray diffraction experiments could be carried out. All the details of the experiment may be found elsewhere.26 The electrolyte used was 0.1 M H2SO4, renewed with the help of two peristaltic pumps. All experiments were carried out at room temperature. Electrochemical and X-ray diffraction experiments were performed sequentially. First, a diffraction pattern was taken in the 38°−45° region with the system at open circuit potential (OCP). Next, absorption of hydrogen was enabled by applying a constant cathodic potential between 0.2 and −0.3 V vs RHE. Then, X-ray diffraction experiments were carried out while maintaining the potential to ensure no hydrogen desorption was occurring. The hydrogen was then desorbed by applying a more positive potential (typically 0.5 V vs RHE), and another diffraction pattern was recorded.

2. EXPERIMENTAL SECTION All samples were prepared on heavily doped (ρ = 0.001 Ω.cm) silicon (100) wafers (University wafers) and titanium substrates (thickness = 0.5 mm, 99% Alfa Aesar). They were cut to 1.3 cm × 1.3 cm squares and cleaned as follows: (i) Si substrates were consecutively rinsed under sonication in acetone, methanol, and deionized water for 10 min; (ii) Ti substrates were first polished with a series of SiC papers down to Grit 600, then soaked in hot oxalic acid solution (10 wt %) for 1 h, and finally rinsed in deionized water. Both substrates were then stored in dry conditions until further use. The films were deposited by pulsed laser deposition (PLD). A custom-made vacuum chamber was pumped down to a pressure of 2 × 10−5 mbar. A KrF laser (248 nm, 16 ns, 500 mJ) was focused on a copper target (99.99%, Kurt J. Lesker) partially covered by a palladium foil (99.9%, Alfa Aesar) and a gold foil (99.95%, Alfa Aesar) so the exposed relative surface area of Cu, Pd, and Au could be varied at will to achieve the desired PdCuAu composition. Alloy compositions were varied in the following ranges: 15 < at. % Pd < 100; 0 < at.% Cu < 80; 0 < at.% Au < 30. Silicon and titanium substrates were placed facing the target at a distance of 5 cm. The laser energy was set to provide a fluence of approximately 8 J cm−2 on the target. The depositions were performed at a repetition rate of 40 Hz for 38 min at room temperature for all samples, resulting in film thicknesses of ca. 100−150 nm. X-ray diffraction (XRD) was performed with a Brücker D8 Advance diffractometer equipped with a Gö bel mirror (weighted Cu Kα1 and Cu Kα2 at 1.5418 Å). Bragg−Brentano as well as grazing incidence (5°) configurations were used. The latter was required to enhance the film to substrate signal ratio in certain cases. Williamson−Hall analyses (uniform stress deformation model, USDM27,28) helped characterize the stress and crystallite size of the films. The same apparatus was employed for in situ XRD experiments. A JEOL JSM 6300F scanning electron microscope (SEM) enabled the imaging of both top and side views of the samples. From these images, the mean thickness of the films could be extracted. Coupled to the microscope, energy dispersive X-ray spectroscopy was performed to assess the elemental compositions of the alloys. For each sample, an average of three measurements was taken to reduce uncertainties. Precision on the composition was however only ca. 3% due to the low amount of signal emanating from the thin films. A VG Escalab 220I-XL X-ray photoelectron spectroscope (XPS) was used to more thoroughly analyze the composition variations within the films. Depth concentration profiles were performed using argon ion etching. Etching was performed for 10 min between each XPS measurement. This procedure was carried out until the substrate was reached. The probing depth was estimated from the etching time and the etch rate (γ) given by the formula29

3. RESULTS AND DISCUSSION 3.1. Physicochemical Characterization. Figure 1 presents a sampling of X-ray diffractograms taken at 5° grazing incidence on PdCuAu thin films of various compositions produced by pulsed laser deposition. All the samples presented in this figure has the same crystalline structure, as depicted by the five characteristic peaks of a FCC phase (space group Fm3m ̅ ) shown in the patterns. However, their Pd, Cu, and Au contents differ considerably, and such homogeneity in the structure is unexpected. In fact, the PdCuAu ternary phase diagram predicts a range of different structures with space group Pm3m ̅ , Imma, Fm3m ̅ , and P4/mmm in the studied range.31−33 Most of these phases are not observed in the XRD patterns of the PLD deposited samples, indicating that a FCC metastable phase has been formed. The formation of metastable phases is commonly achieved by PLD since, in this technique, the atoms’ mobility at the substrate surface may be dramatically decreased, quenching the reorganization of the species to form a kinetically stable compound (the alloy is stable by ways of very slow kinetics to return to equilibrium state). This would explain the observation of structures unreported in the phase diagrams.34,35 Heating the samples for 4 h at 400 °C enables them to recover their thermodynamically stable phases (see Figure S1). 26452

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solution at a particular composition, calculated as a linear combination of the lattice parameters of Pd, Cu, and Au according to the relative amount of each element as measured by EDX. A clear linear correlation exists between the lattice parameter and the composition of the ternary alloys. The lattice parameter appears to follow Vegard’s law for solid solutions over the entire range of study (15 < at. % Pd < 100; 0 < at. % Cu < 80; 0 < at. % Au < 30). It is common to see an extension in the solid solubility range for alloys deposited by PLD.35 In the present deposition conditions, the solid solubility of PdCuAu appears to extend over the entire tested composition range. This feature was already shown for PdCu alloys, and it appears the addition of gold does not modify the film crystallization behavior. A metastable FCC solid solution of PdCuAu is formed in the full explored range. Although most of the lattice parameters in the different crystallographic directions follow this 1-to-1 relationship, the lattice parameter in the (200) direction appears to be shifted toward higher values by roughly 1%, which translates to a state of macroscopic strain in the crystal. Both the deposition technique and the deposited materials have an impact on such behavior. PLD deposited films show frequent nonuniform stress and texturation.36 Moreover, Pd, Cu, and Au have Zener anisotropy ratios (characterizing the ratio between the biggest and smallest elastic moduli in the crystal) of respectively 2.8, 3.2, and 2.8, inferring that if stress buildup occurs in the film, it is likely to preferentially translate into strain in the softer [100] direction. Macrostrain is not the only result of stress buildup in the sample. The broad peaks present in the XRD patterns predict a non-negligible amount of microstrain in the lattice as well. Williamson−Hall analyses coupled with USDM (uniform stress deformation model) were employed to estimate the crystallite size and microstrain in the lattice.27,28,37 In USDM, a uniform stress value in all directions is assumed, with Hooke’s law used to translate it to microstrain. This accounts for the possibility of different elastic moduli in each crystallographic direction. From these analysis models, as shown in Figure 3, the crystallite size was estimated at about 20 nm at all compositions except pure metals, which tend toward larger crystallites in the same deposition conditions (55 nm for pure Pd, 80 nm for pure Cu). Stress has a tendency to increase with the amount of alloying elements. In all of the fabricated films, the microstrain appears to be higher at compositions of roughly 40−50% Pd. This is expected in a solid solution, since the median composition is the one at which each atom’s first-neighbor shell will vary the most; i.e., one Pd atom will be surrounded by the broadest variety of Pd and Cu configurations. Consequently, this is the composition at which lattice distortions due to different bond lengths will be highest.27 In principle, addition of Au to the alloy should increase disorder in the lattice since new bond lengths are introduced, which should as a consequence induce even more microstrain. However, as experiments show, replacement of Cu by Au does not appear to have a major effect on the occurrence of microstrain in the films. PLD has the ability to deposit extremely dense films, regardless of their composition and their thicknesses (between 1 nm and 1 μm). Analysis by SEM shows that no pinhole or pores are seen in the film’s depth or on its surface. The thickness is estimated at around 100 ± 40 nm depending on the composition (data not shown).

Figure 1. XRD patterns taken in grazing incidence (5°) of several PdCuAu alloys.

Some smaller, sharper peaks may be observed at smaller angles (at 2θ = 38.2°, 44.5°, 64.6°, and 77.9°) on some patterns presented in Figure 1. They most likely relate to the presence of Au droplets on the sample. PLD is known to give rise to the formation of droplets, particularly in low melting temperature metals, which deposit onto the film. As we use pure metal targets to fabricate PdCuAu films, it is not surprising to observe small droplets. In the present case, only gold droplets may be identified through X-ray diffraction. Figure 2 shows the lattice parameter in each crystallographic direction, extracted from XRD patterns such as those in Figure 1, plotted versus the lattice parameter expected in a solid

Figure 2. Lattice parameter as measured by X-ray diffraction in different crystallographic directions, compared to the expected lattice parameter based on the alloy’s composition, taking into consideration Vegard’s law of solid solutions. The black curve represents ideal solid solution behavior. 26453

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Figure 4. XPS depth profiling showing the Si, Pd, Cu, and Au contents as a function of depth (A) and the relative Pd, Cu, and Au contents (B). Figure 3. Results of USDM treatment on Williamson−Hall analyses performed on PdCuAu alloys.

PLD has the crucial drawback of producing droplets.38,39 In our case, since pure metal targets were used, the droplets could be composed of pure Pd, Cu, or Au. As described in Figure 1, some of these droplets generate a XRD signal, and the majority of them are composed of pure Au. However, the presence of these droplets will not interfere with the hydrogen solubility measurements since they are going to be performed by E in situ XRD. Indeed, only the hydrogen solubility in the alloy phase will be assessed since we are going to evaluate it from the displacement of the alloy XRD peak (the XRD peaks of pure metals originating from the presence of droplets are not taken into account). It should be noted that as a deposition technique, PLD’s unique characteristics enable us to change the films’ composition at will without affecting the morphologya tremendous advantage over other widely used deposition techniques.40 To investigate the sample’s composition homogeneity along its depth, XPS depth profiling was performed on a typical PdCuAu sample. From XPS data presented in Figure 4, three different regions may be observed: a surface region, systematically enriched in copper; a bulk region with steady compositions representing the thin film under study; and a substrate region characteristic of the substrate on which the film is deposited. In the surface region, representing less than 5 nm between the atmosphere and the thin film, systematic copper segregation is observed. Similar results were observed in PdCuAu prepared by other deposition techniques and other PdCu binary and ternary alloys.41−43 Moreover, on high-resolution XPS spectra (Figure 5), characteristic copper oxide peaks are observed. Since no heat treatment was performed, Cu presence at the surface might be related to its higher affinity for oxygen.44 This does not present an issue for further hydrogen solubility measurements, however, as the bulk region is much thicker (typically 140 nm) than the surface region (smaller than 5 nm). Moreover, as mentioned earlier, hydrogen solubility measure-

Figure 5. High-resolution XPS spectra taken in the Au 4f (75−110 eV), Pd 3d (330−360 eV), and Cu 2p (925−970 eV) core level regions at three characteristic depths.

ments will be performed by focusing on a distinct XRD peak of the alloy (and not on the surface copper oxide phase). Furthermore, in acidic medium and at the electrochemical potentials used in the characterization experiments (0.1 M H2SO4, −0.2 V vs RHE), copper oxide is not stable and dissolves readily. The bulk region presents a very steady composition, with the Pd to Cu and Au ratio not varying by more than 3 at. %. This region extends over 140 nm. As measured by X-ray reflectometry, the density is 13.7 g cm−3, close to ρcalculated ≈ 13 g cm−3, the density deduced for a nonporous Pd73Cu9Au18 alloy (mean composition in the bulk region). In the final region, XPS readings indicate the Pd, Cu, and Au signals decrease down to 0% to show only Si. The interface 26454

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behavior of the samples may be divided into two groups: (i) In the first one, situated at low H/M and made of samples with composition able to absorb only low amounts of hydrogen, there is none or little irreversible strain as (aas‑dep − acycled) is close to zero. (ii) In the second group, which include several samples in a broad range of H/M (approximately from H/M = 5 to 68 at. %), the (aas‑dep − acycled) is ca. 0.02 Å, representing roughly a 0.5% change of the lattice parameter. Comparing these H/M values with the known α to β transition values in PdCu (2−5 at. % depending on the Cu content50,51), a good match may be noted with the H/M value at which plastic deformation in the film start to occur. This turns out to be an elegant way of sensing the H/M value at which the transition from α to β phase is appearing. Interestingly, the lattice parameter after cycling is closer to the one expected from the composition of the sample as determined by EDX. This means that a relaxation of the initially strained lattice occurs with hydrogen absorption/desorption cycling. To investigate this behavior further, more careful analyses were performed, in the form of sin2 ψ curves to determine the corresponding residual stress in the film. Figure 7 displays the

between the bulk and substrate regions appears rather large by XPS, with the Si signal transition from 0 to 100% taking at least 100 nm. It should be recalled that XPS is not the ideal tool to investigate interfaces, and many phenomena may contribute to a widespread intermixing between silicon and the metallic atoms. Preferential sputtering, roughness induced by the etching, or a signal from unetched areas may all potentially artificially broaden the interface between the film and the substrate.29 In summary, this thorough physical characterization of the films indicates that the deposition of PdCuAu alloys with the same crystallographic structure, in a solid solution of Pd, Cu, and Au with crystallite size of ca. 20 nm, may be achieved by PLD in a wide range of compositions. PLD allows us to rapidly and easily produce dense films regardless of their composition. This facilitates the screening of these alloys for hydrogen solubility measurements much more than other deposition techniques such as autocatalytic plating or chemical vapor deposition, where reoptimization of the preparation technique is required when adding a third element or for different compositions.45,46 The next section is devoted to analyzing the hydrogen absorption capabilities of the formed alloys. 3.2. Hydrogen Solubility Measurements. E in situ XRD presents all the advantages of conventional X-ray diffraction. Accordingly, this is an excellent tool for studying crystal orientation, stress, and strain in samples that absorb hydrogen. There is interest in FCC alloys in the study of the transition from α-H solid solution phase to β-hydride phase. The β-phase is detrimental to membranes, as the lattice parameter of the host lattice increases sharply, in turn inducing mechanical failure of the membrane in most cases. It was shown that as long as the hydrogenated film remains in the α-phase region, only elastic deformations are observed.47,48 However, if the film absorbs enough hydrogen to reach β-phase, irreversible plastic deformations are observed.49 Figure 6 shows the deviation of the lattice parameter of the samples in dehydrogenated state after one and several hydrogenation/dehydrogenation cycles compared to the asdeposited lattice parameter. This deviation is plotted as a function of the hydrogen content of the film at saturation. The

Figure 7. Evolution of the relative variation of the interplanar distance (dhkl − d0)/d0 as a function of sin2 ψ. The data are shown for a Pd65Cu6Au29 sample (with H/M = 22.5 at. %) and in two different states: right after deposition (as-deposited) and after three hydrogen absorption/desorption cycles.

relative variation of the interplanar distance as a function of sin2 ψ, with ψ representing the angle of inclination of the normal to the sample surface with respect to the diffraction vector. This relationship is given for two different crystallographic directions, namely [111] and [200] on a Pd65Cu6Au29 sample (with H/M = 22.5 at. %) and in two different states: right after deposition (as-deposited) and after three hydrogen absorption/ desorption cycles. Such analysis provides insight on the macrostress present in the film. Provided the stress is homogeneous in a polycrystalline randomly oriented film, the following relationship between stress and strain can be defined: dhkl − d0 1+ν σhkl sin 2 ψ = d0 Ehkl

Figure 6. Evolution of the difference between the interplanar distance of as-deposited films and after cycling as a function of their maximum H/M ratio. The red vertical lines represent the hydrogen amount where phase transitions occur at room temperature.

(2)

where v is the alloy’s Poisson ratio, Ehkl is its elastic modulus in the hkl crystallographic direction, dhkl is the interplanar distance 26455

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The Journal of Physical Chemistry C of the hkl crystallographic planes as measured by XRD, d0 is the relaxed interplanar distance value, and σhkl is the stress in the hkl crystallographic direction. As it can be inferred by an inspection of eq 2, the slope of the curves shown in Figure 7 is a measure of the macroscopic stress in the different crystallographic planes. Macrostress in the film is not negligible. In particular, the steep negative slope in the as-deposited [200] curve indicates that compressive stress is induced by the preparation method. A compressive stress value of 1 GPa in the [100] direction is calculated. In comparison, no stress is observed in the [111] direction. Similar behavior is observed in metals and alloys produced by PLD and explained by the segregation of interstitial defects at grain boundaries.52 After several hydrogen absorption/desorption cycles, a small positive slope is observed in both directions, indicating that tensile stress has appeared and showing the effect of hydrogen on the film’s structure. Naturally, the effect of H insertion is much stronger in the [200] direction. As the initial compressive stress was so significant, it appears hydrogen-induced plastic deformations have released some stress in the film. Hydrogen induces a cold annealing of some sort. These analyses reveal that hydrogen absorption and desorption affect the as-deposited structure of the films during the first cycles. It is thus preferable to perform several hydrogen absorption/desorption cycles before measuring the actual hydrogen solubility in the film. In the following, hydrogen solubility values were all recorded after one or two such absorption/desorption cycles. Using the in situ XRD technique, it was possible to extract hydrogen solubility data from the difference in lattice parameter between a sample’s hydrogen-saturated state and its hydrogenfree state. Process details and calculations may be found elsewhere.26 This procedure is useful for the measurement of hydrogen solubility data, since the technique’s precision relies solely on XRD signal quality and not on the amount of hydrogen absorbed, as is the case in conventional electrosorption measurements, for example.16 Theory states that two main factors guide the absorption of hydrogen in FCC alloys: the nature of the elements surrounding hydrogen in an octahedral site (both nearest and next-nearest neighbors surrounding the hydrogen affect its absorption energy in the site)53 and the size of this octahedral site (the larger the site, the better the absorption). Hydrogen solubility is therefore dependent on these two factors as well. Experimentally, however, it is difficult to dissociate these two parameters, as a change in the alloy’s composition will accordingly affect the occurrence of nearest and next-nearestneighbor configurations and the volume of an octahedral site. Yet, the Pd−Cu−Au system has this particular feature of alloying Pd (with a lattice parameter a = 3.896 Å) with an element with a smaller lattice parameter, Cu (a = 3.621 Å), and another with a larger lattice parameter, Au (a = 4.079 Å). It is therefore possible to obtain a glimpse on the main effect driving hydrogen solubility in these alloys because lines of constant lattice parameter (and constant volume of the octahedral sites) but of different nearest- and next-nearest-neighbor configurations can be drawn and compare to the hydrogen maximum solubility data. In Figure 8, the maximum hydrogen solubility, expressed as H/M ratio, in PdCu and PdCuAu samples are displayed in a ternary phase diagram. The background color represents a contour plot of the H/M ratio. As an initial determination drawn from overall observation of these curves, we may notice

Figure 8. Variation of the maximum hydrogen solubility (given as H/ M ratio) as a function of the composition in the Pd−Cu−Au system. Blue dotted line represent curves of equal lattice parameter. The blue stars represent the experimental data points.

that pure palladium performs better that any Cu- or Aucontaining PdCuAu alloy. The pure Pd sample is the only one where H/M is higher than 60%. This result agrees with the literature, as all experimental work on PdCu and PdAu alloys has found that the addition of Cu or Au decreases the alloy’s overall hydrogen solubility.54,55 In the case of PdAu alloys, it is interesting to note this contradicts first-principles studies, which predict a larger maximum solubility in Pd80Au20.56 Second, in the contour plot, the lines between two colors describing compositions with the same H/M values are almost parallel to the lines describing samples with the same palladium composition. This highlights the fact that the greatest share of the H/M variation is described by a variation in Pd content. The relative Au/Cu content has a minor effect in hydrogen absorption capabilities. From these observations, we conclude that while palladium content is the principal driving force in hydrogen absorption capability, the presence of Pd neighboring the octahedral sites is the determining factor for hydrogen absorption. To explore the effect of Au on hydrogen solubility in greater depth, Figure 9 compares PdCu and PdCuAu samples in terms of H/M ratio. In this figure, samples with equal palladium content share the same abscissa, meaning that a Pd60Cu40 sample will have the same x value as a Pd60Cu20Au20 sample. Figure 9 indicates that for 40 < at. % Pd < 80 replacing Cu with Au is beneficial to hydrogen solubility. This is an unexpected result since, according to the literature, almost no change in the hydrogen solubility was observed in PdCu and PdAu alloys upon introduction of a third element (Au or Cu).19,54,57−59 Since both Au and Cu have a very low affinity for hydrogen, it is hypothesized that this beneficial effect is mainly due to the larger-sized octahedral sites in PdCuAu. According to the data presented above, it is interesting to investigate Au-rich PdCuAu alloys as they exhibit larger maximal hydrogen solubility. Yet, other important properties such as hydrogen diffusivity and resistance of the material to poisoning must be studied in order to fully characterize them. This will be the subject of future investigations. Previous works 26456

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Strategic Grant Program, the NSERC Hydrogen Canada (H2CAN) Strategic Research Network and Air Liquide Canada.

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(1) Driscoll, D. J.; Ackiewicz, M. Hydrogen from Coal Program Multi-Year RD&D Plan, 2010. (2) Ruth, M.; Joseck, F. Hydrogen Threshold Cost Calculation, 2011. (3) Basile, A.; Iulianelli, A.; Longo, T.; Liguori, S.; De Falco, M. PdBased Selective Membrane State-of-the-Art. In Membrane Reactors of Hydrogen Production Processes; De Falco, M., Marelli, L., Iaquaniello, G., Eds.; Springer: Berlin, 2011; pp 21−56. (4) Gao, H.; Lin, Y.; Li, Y.; Zhang, B. Chemical Stability and Its Improvement of Palladium-Based Metallic Membranes. Ind. Eng. Chem. Res. 2004, 43 (22), 6920−6930. (5) Lewis, F. A. The Palladium Hydrogen System; Academic Press: New York, 1967. (6) Gabitto, J. F.; Tsouris, C. Sulfur Poisoning of Metal Membranes for Hydrogen Separation. Int. Rev. Chem. Eng. 2009, 1 (5), 394. (7) Sakamoto, Y.; Chen, F. L.; Ura, M.; Flanagan, T. B. Thermodynamic Properties for Solution of Hydrogen in Palladium Based Binary Alloys. Ber. Bunsenges. Phys. Chem. 1995, 99, 807−820. (8) Sakamoto, Y.; Ohishi, T.; Kumashiro, E.; Takao, K. Diffusivity and Solubility of Hydrogen in Pd-Fe and Pd-V Alloys. J. Less-Common Met. 1982, 88, 379−385. (9) Roa, F.; Way, J. D. Influence of Alloy Composition and Membrane Fabrication on the Pressure Dependence of the Hydrogen Flux of Palladium−Copper Membranes. Ind. Eng. Chem. Res. 2003, 42 (23), 5827−5835. (10) Knapton, A. G. Palladium Alloys for Hydrogen Diffusion Membranes. Platinum Met. Rev. 1977, 21 (2), 44−50. (11) Alfonso, D. R.; Cugini, A. V.; Sholl, D. S. Density Functional Theory Studies of Sulfur Binding on Pd, Cu and Ag and Their Alloys. Surf. Sci. 2003, 546 (1), 12−26. (12) Kamakoti, P.; Sholl, D. S. A Comparison of Hydrogen Diffusivities in Pd and CuPd Alloys Using Density Functional Theory. J. Membr. Sci. 2003, 225 (1−2), 145−154. (13) Xu, S.; Sood, P.; Liu, M. L.; Bongiorno, A. First-Principles Study of Hydrogen Permeation in Palladium-Gold Alloys. Appl. Phys. Lett. 2011, 99 (18), 181901. (14) Sonwane, C. G.; Wilcox, J.; Ma, Y. H. Solubility of Hydrogen in PdAg and PdAu Binary Alloys Using Density Functional Theory. J. Phys. Chem. B 2006, 110 (48), 24549−24558. (15) Howard, B. Hydrogen Permeance of Palladium-Copper Alloy Membranes over a Wide Range of Temperatures and Pressures. J. Membr. Sci. 2004, 241 (2), 207−218. (16) Martin, M. H.; Galipaud, J.; Tranchot, A.; Roué, L.; Guay, D. Measurements of Hydrogen Solubility in CuxPd100-X Thin Films. Electrochim. Acta 2013, 90 (2010), 615−622. (17) Brien, O.; Casey, P. Sulfur Poisoning of Palladium and Palladium Copper Alloy Hydrogen Separation Membranes, 2011. (18) She, Y.; Emerson, S. C.; Magdefrau, N. J.; Opalka, S. M.; Thibaud-Erkey, C.; Vanderspurt, T. H. Hydrogen Permeability of Sulfur Tolerant Pd−Cu Alloy Membranes. J. Membr. Sci. 2014, 452, 203−211. (19) Flanagan, T. T. B.; Chisdes, D. M. Solubility of Hydrogen (1 Atm, 298 K) in Some Copper/palladium Alloys. Solid State Commun. 1975, 16 (5), 529−532. (20) Kamakoti, P.; Sholl, D. S. Towards First Principles-Based Identification of Ternary Alloys for Hydrogen Purification Membranes. J. Membr. Sci. 2006, 279 (1−2), 94−99.

Figure 9. Maximum H/M ratio in PdCu and PdCuAu as a function of their palladium content.

seem to indicate a better poisoning resistance in PdCu than in PdAu.60 As a consequence, an optimum will need to be found between hydrogen solubility and resistance to poisoning in PdCuAu alloys.

4. CONCLUSIONS This study has demonstrated that the rapid fabrication of PdCuAu alloys over a large composition range is possible using PLD. Dense, smooth, and homogeneous thin films with no sign of cracks or pinholes were produced regardless of their composition. All of the films crystallize in a metastable FCC solid solution, which facilitates comparison of the different alloys. The PLD technique causes the appearance of macrostrain, seemingly concentrated in the [100] direction of the crystal. Microstrain and nanocrystallinity are also a consequence of these highly energetic growth conditions. This study has also demonstrated that E in situ XRD is a rapid and efficient technique for determining the hydrogen content in the thin films even in low amounts, as the technique precision relies only on the accuracy of the XRD patterns. Comparisons between PdCuAu and PdCu indicate that the substitution of Au for Cu is beneficial to hydrogen solubility. However, this effect is quite minimal compared to the major impact of the alloy Pd content on the hydrogen solubility. However, no information could be obtained on the diffusion of hydrogen through this technique. Additionally, with E in situ XRD, it is also possible to study the effect of hydrogen absorption/desorption cycling on the alloy structure, such as the relaxation of strain.

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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/acs.jpcc.5b07511. Figure S1 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.G.). 26457

DOI: 10.1021/acs.jpcc.5b07511 J. Phys. Chem. C 2015, 119, 26451−26458

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DOI: 10.1021/acs.jpcc.5b07511 J. Phys. Chem. C 2015, 119, 26451−26458