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Abstract
For hydrogen sensor and storage applications, films of Au and Pd were (i) co-sputtered at different rates or (ii) deposited in a sequentially alternating fashion to create a layered structure on a cover glass. Peculiarities of hydrogen uptake and release were optically monitored using 1.3 μm wavelength light. Increase of optical transmission was observed for hydrogenated Pd-rich films of 10–30 nm thickness. Up to a three times slower hydrogen release took place as compared with the hydrogen uptake. Compositional ratio of Au:Pd and thermal treatment of films provided control over the optical extinction changes and hydrogen uptake/release time constants. Higher uptake and release rates were observed in the annealed Au:Pd films as compared to those deposited at room temperature and were faster for the Auricher films. Three main parameters relevant for sensors: sensitivity, selectivity, stability (reproducibility) are discussed together with the hydrogenation mechanism in Au:Pd alloys.
© 2017 Optical Society of America
1. Introduction
Plasmonic applications are expanding into a wider spectral range from UV to THz by harnessing peculiarities of dielectric permittivity, , in different materials at the wavelengths of interest. Strong changes of the real and imaginary parts of the permittivity occur as a result of phase transitions, phase intermixing or chemical reactions [1]. For example, the affinity of Pd to hydrogen, which can be absorbed with an anomalously high volume ratio of ~ 600 for solid Pd [2], alters the optical response of Pd via changes in effective permittivity [3–5]. For future solar hydrogen applications and in fuel cells [6–9], storage and monitoring of hydrogen is of paramount importance. Hence, sensing capabilities have to keep pace with the rapid development in solar-to-hydrogen conversion, which, in current state of the art industrial installation, has reached 24.4% [10].
Detection of hydrogen using non-contact optical readout was demonstrated by orientational optical plasmonic scattering [11] and spectral shifts in polarization sensitive extinction [3, 4]. However, a more sensitive optical detection of Pd hydrogenation can be obtained by transmission measurements [12]. A “switchable mirror” comprised of a rare metal (Gd or Y) layer with an overlaying Pd thin film exhibits a large transmission change in visible wavelength region upon exposure to hydrogen [13, 14]. Such sensors can provide a highly desirable non-contact method for hydrogen sensing and monitoring, especially in micro-chip applications where standard electrical resistance measurement is not desirable due to possibility of a dielectric breakdown/discharge [15, 16].
Despite the exceptional affinity of Pd towards hydrogen other important parameters, such as response times and stability should ideally be fine-tuned for specific applications. The primary way of controlling the properties of metals is through creation of alloys. Permittivity control by co-sputtering of Ag and Au has been demonstrated to provide a possibility to engineer the optical response of an binary alloy [17–19] and was recently extended to ternary Au, Ag and Cu alloys [20,21]. Solid solutions of Rh and Ag mixed at the atomic level can store hydrogen much like Pd, a functionality non-existent either in pure Rh or Ag [22]. Another development of great interest is that Pd and Ru alloy nano-crystals enable CO oxidization catalysis superior to that of Rh [23]. Also, Au and Pd alloy nanoparticles enhance electro catalytic activities [24]. Alloys tailored at the nanoscale are expected to bring forward new functionalities due to particular geometric configuration of atoms and strain resulting from the mismatch of lattice constants [25], as well as due to different electron binding and charge distribution which alters their chemical behavior [26].
Here, we prepare films of Au and Pd at different intermixing ratios, as well as layered Au/Pd films deposited in an alternating fashion for prospective applications in hydrogen detection and storage. Optical readout of transmittance changes during hydrogen uptake and release was monitored using a simple non contact method. Thereby the effect of composition as well as alloying conditions on sensitivity to ambient hydrogen was elucidated. Detailed understanding of basic reactions with hydrogen are important for designing optical hydrogen sensor devices.
2. Samples and methods
A series of Au and Pd intermixed samples were prepared by magnetron sputtering (AXXIS, JK-Lesker) using two modes of deposition: (i) co-sputtering (Au:Pd) and (ii) alternating Au and Pd layers (Au-Pd) of different thicknesses deposited onto a 0.4-mm-thick cover glass (Micro cover glass, Matsunami). During co-sputtering, the substrate was either kept at room temperature (RT) or heated at 250°C. In the latter case of alternating deposition, substrate was kept as room temperature. The numbers of alternation cycles were: 2, 4, 10, 20 (a pair of Au and Pd layers per one cycle) with corresponding effective thicknesses of 7.50, 3.75, 1.50, 0.75 nm/cycle, respectively. Pd was terminating top surface layer [27]. Sputtering rate was calibrated using the optical interference microscope 3D profiler (Bruker). Also the thicknesses of each layer of alternating layer samples were analyzed using X-ray diffraction (XRD).
The density of states (DOS) for Au, Pd, and Au:Pd alloys are calculated at the structures optimized using the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE) [28] and the projector augmented wave method [29] as implemented in the VASP code [30–33]. The Au-Pd alloys are constructed by alternating Au and Pd layers in 2-by-2 manner in the 〈001〉 direction using the unit cell consisting of Au4Pd4 and 1-by-2 in the 〈111〉 direction, using the unit cell consisting of AuPd2. For both types of alloy systems, a 21 × 21 × 11 k-point mesh is used with the cutoff energy of 500 eV.
Optical permittivity of the samples was determined by transmission and reflection measurements using setup and analysis method reported earlier [21] at normal incidence. For hydrogenation of samples, the entire optical setup was put into a sealed container with dimensions 50×50×15 cm3. The spectral range from 450 to 1000 nm was used as a result of limits imposed by the available detectors.
Due to the flow rate limitation, the time dependence of hydrogen uptake was measured using a setup, shown in Fig. 1(a), with a 21.4 cm3 sample chamber equipped with a gas mixing system to provide 500 mL/hour flow rate of a 4% H2:N2 mixture at a pressure of 0.1 MPa (~1 bar), a 1.31 μm wavelength laser emitting diode (ILX lightwave Co. MPS- 8012) and a Ge photo diode detector (Ando Electric Co., Ltd., AQ2150A) was used for in situ monitoring of transmission changes. A fiber optic collimator/focuser with a numerical aperture NA = 0.25 (Edmund Optics, Ltd.) was used to define the 2-mm-diameter spot on the metal film inside the gas chamber [34]. The probe wavelength is out of the range used for the permittivity measurements. However for most metals permittivity can be approximated by the Drude-Lorenz theory. Therefore behavior at 1.31 μm can be extrapolated from the results obtained in the visible stectral range following a generic dependence:
where the first two terms represent the Drude free electron model and the last term is the Lorentz contribution accounting for the bound electrons participating in interband transitions. Here ε(∞) is the permittivity at the high frequency limit (infinity), ωp is the plasma frequency, τ is the relaxation time of the free electrons; j denotes the index of a Lorentz oscillator, Aj, ω0,j and τj are the amplitude, the resonant frequency and the relaxation time of the given oscillator j, respectively, ħ is the reduced Plank constant, i is the imaginary unit, and ω is the optical cyclic frequency.
Fig. 1 (a) Setup for measurement of hydrogenation. (b) Extinction losses Ext ∼ −10 lg(It/I0) [dB] where It,0 are the transmitted and reference intensities, respectively. Hydrogenated Pd becomes more transparent. ΔExt is the change of extinction losses between hydrogen saturated and depleted states.
3. Results and discussion
3.1. Au:Pd alloy formation during sputtering
Hydrogen uptake and storage properties of both co-sputtered and alternating layer films were investigated. Their morphological differences were also apparent in XRD profiles, summarized in Fig. 2. Pd and Au are known to be completely miscible to form solid solutions [25, 35]. Structure of the films was inspected using XRD and showed a continuous intermixing of Au and Pd in the case of films co-sputtered onto a 250°C heated substrate at different sputtering rates. A continuous shift of the 〈111〉 peak without a significant change of its width is indicative of well controlled alloying of Au and Pd and was better than was obtained by chemical exchange reactions on colloidal nanoparticles [36]. On the other hand, for Au-Pd films made by alternating sputtering of Au and Pd, a complex XRD pattern was observed. This reflects a nano-cluster structure, since the constituent 2–3 nm thickness films (as judged by sputtering time) do not result in a continuous fully intermixed material.
Fig. 2 (a) XRD of Au:Pd films co-sputtered onto a 250°C pre-heated glass substrate at different sputtering rates. Vertical lines mark 〈111〉 position of the fcc-Au and Pd peaks. (b) XRD of layered Au:Pd films in the range 40 to 70 nm thickness deposited at layer cycles: 2, 4, 10, 20 (corresponding thicknesses are 7.50, 3.75, 1.50, 0.75 nm, respectively) and co-sputtering deposition at RT. (c) The experimentally measured XRD spectra (Meas.) and simulation (Sim.) results of alternating sputtering
Alloying of Au and Pd was previously studied as a diffusion constant, D, dependent process [37]. The intermixing rate was governed by D values of ∼ 10−10cm2/s at 1000°C and D ∼ 10−15cm2/s at 350°C. At the lower temperature of 250°C, the expected values of D range from 10−16 to 10−17cm2/s, and are even more significantly diminished at RT. Therefore, appreciable migration of atoms is not expected at RT.
By alternating sputtering multi layers of Au and Pd were formed as observed by XRD. The samples made by 2 and 4 cycles of sputtering showed characteristic separate Au and Pd XRD profiles which were not forming a periodic layered structure. However, in the case of 10 and 20 cycles, the pure Au and Pd profiles have disappeared and satellite peaks in XRD identified formation of layered structure.
The multiple satellite peaks are well known in the field of semiconductor super-lattice materials with similar XRD profiles [38–41]. From the satellite peaks, it is possible to calculate the thickness of each layer by using the Bragg equation:
where λ = 0.15148 nm is the characteristic X-ray wavelength of the Cu Kα line. The suffixes m or n respectively define the m-th or n-th peak orders of the XRD spectra. The diffraction multi-peaks were analyzed using a simulation program LEPTOS, Bruker AXS, which is based on the dynamical theory of diffraction. The results are shown in Fig. 2 (c). Such analysis, provides possibility to cross check results of numerical calculations with XRD experimental data. The thickness of a single layer Λ was deduced to be 20 nm (for 2 cycles), 12 nm (4 cycles), 6.4 nm (10 cycles), and 3.4 nm (20 cycles), respectively. Total thickness of the film has been estimated to range from 40 to 68 nm.Similar satellite XRD peaks to those resulting from alternating deposition were likewise observed for Pd:Au films co-sputtered onto a RT substrate. The origin of those satellite peaks in the case of simultaneous co-sputtering needs more detailed analysis. However, qualitatively they can be explained by the relative position of the rotating substrate stage in relation to the sputtering sources. As the substrate plane is tilted with respect to both of the sources, a gradient of deposition rates develops along the diameter of the sample [18]. So, as the substrate rotates during sputtering, periodic oscillations in composition emerge and, at RT, diffusion proceeds too slowly to efface them. The films prepared by sputtering, each with different structural properties, was tested next for their hydrogen uptake and release capability.
3.2. Hydrogen storage and optical properties of metals
3.2.1. Steady state optical transmission/reflection spectra of films
Hydrogen storage has to be both reversible and rapid for practical applications [42]. The interaction between Pd and H affects their optical properties. Figure 3 shows optical spectroscopic change occurred due to a hydrogen uptake. Typically, transmittance increased while reflectance decreased. An optical transmission measurement of Pd film hydrogenation is sensitive due to the exponential dependence of transmittance, T, on the imaginary part of the refractive index of a strongly absorbing film (Pd with hydrogen) on a non-absorbing substrate of refractive index, ns (glass) [12]:
where d is the thickness of the Pd film and λ is the wavelength of light. With these results, applying the Drude-Lorenz model, the real and imaginary parts of the permittivity can be determined in the measured wavelength range. The Drude part, which consists of the ωp and τ, accounts for the contribution arising from free electron behavior. Here ωp is the ratio of free carrier density and effective mass, τ is the relaxation time, which incorporates the information on electron scattering by electrons, holes, grain boundaries and other scatterers. When hydrogen was absorbed into the Pd, ωp slightly increased from 1.5×1016 s−1 to 1.9×1016 s−1, whereas τ decreased from 4.3× 10−16 s to 3.0× 10−16 s. Such changes in optical parameters are related to the resonance wavelength shift of plasmonic materials [17, 43, 44].Fig. 3 (a) Optical transmission and reflection of Pd thin films (thickness 20, 30 40 nm) with 4% H2 or N2 (without H2) condition, and (b) optical permittivity of the Pd with/without H2
This increase of ωp means a commensurate increase of the free electron density and/or a decrease of the effective mass of electrons. Hydrogen might donate a free electron to Pd or soften the bonding as discussed below. The decreasing τ is caused by the an increase of scattering inside the solid phase. The internally trapped hydrogen causes the scattering of electrons, therefore, a more rapid relaxation of free electron oscillation is obtained.
The absorbed hydrogen causes the binding between Pd and hydrogen, thereby creating a complex tentatively described as PdHx. Based on thermodynamic arguments there is a possibility of H+ or species being present inside the metal [45, 46]. Earlier studies showed that upon hydrogenation of Pd heat generation is observed, while for H2 release an elevated temperature of > 150°C is required [47]. It is noteworthy that recent density functional calculations [48] of H2 interaction with Au:Pd binary clusters revealed that H2 molecules donate electrons to metal clusters during molecular adsorption. Hydrogen enters interior of Pd with larger inner surface area. Similar to longevity of fuel cells, batteries, and super-capacitors, reversibility of processes at the microscopic level during charging-discharging are critically important. In this case, the phenomenon of interest is the reversibility of hydrogen uptake and release.
According to thermoelectric power experiments, hydrogen uptake causes an increase of electron density in the Pd host. However, the electrical resistivity is known to increase upon hydrogen uptake and, at room temperature, can be almost doubled at full hydrogenation, which depends on pressure [49, 50]:
where is the atomic ratio of H and Pd, pH [bar] is the hydrogen pressure, and p0 is the total pressure of the gas mixture. The electrons and hydrogen cations in Pd create an interesting guest-host system with an amazingly high ratio of at high 16.5 × 103 bar pressure [50]; at a hydrogen pressure equal to 1 bar (at normal conditions). The results of this study on the optical response of the hydrogen uptake/release agrees well with electrical response.For hydrogen release from the solid Pd phase, a dramatic increase in size occurs from protons to H2 directly or via an intermediate . This causes a steric hindrance for hydrogen release at the surface of Pd and, consequently, it takes longer. It is noteworthy, that the (bulk ⊖|⊕ gas) electron-ion pair mechanism is fully reversible, hence, is promising for practical storage applications where reversibility is the key requirement.
3.2.2. Dynamics in optical readout / release of hydrogen
As explained in the experimental section, a simple flow chamber with 4% H2 in a N2 carrier gas was set up with the possibility to monitor in real time optical transmission changes upon hydrogenation of Au and Pd films at λ = 1.31 μm wavelength. Extinction losses were determined by measuring optical transmission: Ext ≡ −10 lg(It/I0) [dB] where It,0 respectively are the transmitted and reference intensities; reference intensity I0 = 12.2 μW was measured without the sample in a N2 filled chamber. Following the introduction of H2 no measurable change in optical transmission was observed for pure Au films with thickness in the range of 10–30 nm. The hydrogen response of alternating layer films is summarized in Fig. 4, whereas Fig. 5, Fig. 6 and Fig. 7 illustrate the optically detected hydrogen response of Au:Pd layers co-sputtered at RT and 250°C. In the case of Pd containing films the presence of H2 induced an increase in their transmittance. Figure 4(a) and 5 shows the total extinction of pure Pd as well as alloyed Au:Pd films and Ext changes as H2 flow was cycled through the chamber. Of note is that the trend of decreased extinction with progressive hydrogenation enables increased precision of the measurement due to a better signal-to-noise ratio as more hydrogen is absorbed by Pd and a larger change in the extinction, ΔExt, is measured. Temporal evolution of hydrogen uptake and release were well fitted by single exponential transients with time constants τin,out, respectively. This is due to the hydrogen uptake process being rate limited by the surface reaction. For the Au:Pd film, from the results of Fig. 4(a) and Fig. 5, there was an obvious asymmetry τin < τout.
Fig. 4 Hydrogen response of alternating layer Au-Pd film deposited over 2, 4, 20 cycles with total layer thickness ranging from 40 to 70 nm. (a) Extinction [dB] changes during ON/OFF cycling of H2 flow (marked by arrows). (b) Transient of the Au-Pd 4-cycle alternating layer sample fitted using single exponential uptake and release time constants τin,out, respectively. (c) Hydrogen uptake time constants τin for Au-Pd alternating layer films deposited over different number of cycles. (d) Hydrogen release time constants τout for alternating layer Au-Pd films. (e) Extinction change (ΔExt) during hydrogen reaction.
Fig. 5 Extinction losses, Ext, during hydrogenation of pure Pd and Au:Pd (1:1) co-sputtered films of different thickness on pre-heated 250°C and RT glass substrates. For Au, Ext is constant and is unaffected by the presence of H2 (horizontal line for Au of 20 nm).
Fig. 6 Change of extinction, ΔExt, for simultaneously co-sputtered Au:Pd films. Error bars are ±20% and the lines are guides for eye with starting point at origin of coordinates (0;0).
Fig. 7 Hydrogen uptake (a) and release (b) time constants τin,out, respectively, for Au:Pd films prepared by co-sputtering with and without annealing.
In the case of alternating Au-Pd films, cycle number clearly affected the hydrogen response. A higher number of cycles resulted in an increase of ΔExt as well as of the hydrogen uptake and release time constants τin and τout. Hence, hydrogen response is affected by diffusion dynamics inside the multi-layer stack.
Films of Pd and Au:Pd alloys co-sputtered onto a pre-heated substrate showed smaller optical losses in transmission, as can be seen in Fig. 5, presumably due to a better homogeneity and smaller nano-porosity, as tested for several different thicknesses. In turn, Fig. 6 shows the change of extinction losses due to hydrogenation (an increase in transmission) for co-sputtered alloy samples of different thickness. Extinction [dB] was linearly dependent on thickness of Pd as one would expect from optical absorption. Interestingly, mixture Au:Pd (1:1) co-sputtered on substrate at RT showed the largest changes of extinction as can be seen in Fig. 6(a), even larger than in the case of pure Pd. When deposition of Pd and Au layers was carried out on a pre-heated substrate, smaller ΔExt values were observed as depicted in Fig. 6(b). This could also be attributed to a greater extent of alloy lattice ordering in the case of alloy formation on a pre-heated substrate. The value of ΔExt was found to scale in proportion to the amount of Pd in the intermixed films.
Fig 7 shows experimentally obtained Hydrogen uptake and release time constants of co-sputtered Au:Pd samples. The largest ΔExt observed in Au:Pd (1:1) deposited on a RT substrate, however, it had the longest H2 uptake time, τin, markedly slower than in the pure Pd film [36]. In general, the presence of Au in the film decreased τin, most probably, due to a larger electro negativity of Au (2.54) as compared with Pd (2.22; same as H) according to the Pauling scale [51]. Hydrogen release time, τout, was approximately 2–3 times longer compared to the hydrogen uptake time. A similar result was reported in [52], where stacked Pd and Au nano-layers were deposited by thermal evaporation onto optical fibers and attenuation changes in evanescent wave upon hydrogenation of a alternating layer film were measured.
3.2.3. Surface adsorption / desorption on Au-Pd alloy
The hydrogen read out was found to be strongly influenced by Au:Pd alloying. The hydrogen diffusion constant D in Pd is on the order of 10−5 cm2/s [53]. Therefore, the diffusion time in a thin 10 nm film could be estimated at the order of ∼ 10μs. This is much more rapid than observed in experiments. The rate limiting reaction is adsorption/desorption of hydrogen on the surfaces of metal thin films. Mullins et al. revealed that temperature defined the hydrogen desorption rate. A lower temperatures desorption has been observed on the surface of Au:Pd alloying site. This indicated that alloying Au and Pd resulted in a decrease of affinity to hydrogen [54]. This effect can be also explained by the density of state (DOS) changes in Au and Pd alloy. Quaino et al. showed by density functional theory (DFT) calculations for Au(111) surface on Pd, that the Fermi level of an alloy was lower in energy due to the Au:Pd bond formation [55]. We have also confirmed this prediction by DFT calculations presented in Fig. 8. Hence, an alloy of Au:Pd would be expected to have a reduced activation energy for hydrogen adsorption and desorption on the surface. As a result we have obtained faster uptake and release times in Au:Pd alloy films especially with annealed and homogeneously alloyed samples.
Fig. 8 The density of state (DOS) for Au:Pd alloy and pure Au, and Pd systems for the 〈001〉 (left column) and 〈111〉 (right column) orientations. Gold and silver spheres show Au and Pd, respectively. Zero in the energy x axis indicates the top of the occupied valence band.
Conclusions
The mixture of Au and Pd made by co-sputtering can improve (shorten) the response time during H2 uptake and release stages, most probably, due to electro negativity of Au which facilitates the adsorption of positively charged hydrogen species and H+ at the inner surface of the alloy phase. Alternating sputter-deposited layered films have higher sensitivity as compared with co-sputtered samples. Stability in H2 uptake-release performance showed no degradation on a day-to-day basis. This is consistent with earlier observations that embitterment during hydrogenation can be avoided [47]. The fully reversible mechanism of hydrogen uptake into Pd phase is promising for applications where long term stability is of paramount importance. Using Au:Pd alloy, it is possible to enable more rapid response in optical hydrogen monitoring devices. Even if such enhancement comes at the cost of sensitivity, an overall performance improvement can be attained. This is due to there being no shortage of potential signal enhancing transducers, such as optical resonator devices (i.e. microring resonators, photonic crystals and plasmonic materials).
Funding
Japan Society for the Promotion of Science (JSPS) KAKENHI JP16712939 and JP13312041; Grants-in-Aid for Scientific Research, Open Partnership Joint Projects of JSPS Bilateral Joint Research Projects JP14544510; Tateishi Foundation Research Grant C; Amada Foundation Grant for General Research and Development; Australian Research Council Discovery Project DP130101205; Startup Funding of Nanotechnology Facility by Swinburne University.
Acknowledgments
The computation in this work has partly been done using the facilities of the Supercomputer Center, the Institute for Solid State Physics, the University of Tokyo. Authors greatly thank to Dr. Hitoshi Morioka of Bruker AXS K.K., and Professor Masahiro Yoshimoto and Assistant Professor Hiroyuki Nishinaka of Kyoto Institute of Technology for their helps with the simulation analysis of XRD spectra.
References and links
1. S. K. Earl, T. D. James, T. J. Davis, J. C. McCallum, R. E. Marvel, R. F. Haglund, and A. Roberts, “Tunable optical antennas enabled by the phase transition in vanadium dioxide,” Opt. Express 21(22), 27503–27508 (2013). [CrossRef] [PubMed]
2. A. Baumel, P. Drodten, E. Heitz, and R. Bender, Corrision Handbook, Platinum Metals (Ir, Os, Pd, Rh, Ru) (John Wiley and Sons, 2008), Chap. A 35.
3. A. Tittl, C. Kremers, J. Dorfmuller, D. N. Chigrin, and H. Giessen, “Spectral shifts in optical nanoantenna-enhanced hydrogen sensors,” Opt. Mat. Express 2(2), 111–118 (2012). [CrossRef]
4. C. Wadell and C. Langhammer, “Drift-corrected nanoplasmonic hydrogen sensing by polarization,” Nanoscale 7(25), 10963–10969 (2015). [CrossRef] [PubMed]
5. N. Strohfeldt, J. Zhao, A. Tittl, and H. Giessen, “Sensitivity engineering in direct contact palladium-gold nano-sandwich hydrogen sensors,” Opt. Mater. Express 5(11), 2525–2535 (2015). [CrossRef]
6. N. S. Lewis and D. G. Nocera, “Powering the planet: chemical challenges in solar energy utilization,” Proc. Nat. Acad. Sci. U. S. A. 103(47), 15729–15735 (2006). [CrossRef]
7. K. Juodkazis, J. Juodkazyte, P. Kalinauskas, E. Jelmakas, and S. Juodkazis, “Photoelectrolysis of water: solar hydrogen-achievements and perspectives,” Opt. Express 18(S2), A147–A160 (2010). [CrossRef]
8. G. Gahleitner, “Hydrogen from renewable electricity: an international review of power-to-gas pilot plants for stationary applications,” Int. J. Hydrog. Ener. 38(5), 2039–2061 (2013). [CrossRef]
9. M. M. May, H. J. Lewerenz, D. Lackner, F. Dimroth, and T. Hannappel, “Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure,” Nat. Commun. 6, 8286 (2015). [CrossRef] [PubMed]
10. A. Nakamura, Y. Ota, K. Koike, Y. Hidaka, K. Nishioka, M. Sugiyama, and K. Fujii, “A 24.4 solar to hydrogen energy conversion efficiency by combining concentrator photovoltaic modules and electrochemical cells,” Appl. Phys. Express 8(10), 107101 (2015). [CrossRef]
11. A. D. Assl, D. E. Gomez, A. Roberts, and T. J. Davis, “Frequency-dependent optical steering from subwavelength plasmonic structures,” Opt. Lett. 37(20), 4206–4208 (2012). [CrossRef]
12. J. I. Avila, R. J. Matelon, R. Trabol, M. Favre, D. Lederman, U. G. Volkmann, and A. L. Cabrera, “Optical properties of Pd thin films exposed to hydrogen studied by transmittance and reflectance spectroscopy,” J. Appl. Phys. 107(2), 023504 (2010). [CrossRef]
13. I. Aruna, B. R. Metha, and L. K. Malhotra, “Faster H recovery in Pd nanoparticle layer based Gd switchable mirrors: Size-induced geometric and electronic effects,” Appl. Phys. Lett. 87(2), 103101 (2005). [CrossRef]
14. I. Aruna, B. R. Mehta, L. K. Malhotra, and S. M. Shivaprasad, “A color-neutral. Gd nanoparticle switchable mirror with improved optical contrast and response time,” Adv. Mater. 16(2), 169–173 (2004). [CrossRef]
15. S. Tomonari, H. Yoshida, M. Kamakura, K. Yoshida, K. Kawahito, M. Saitoh, H. Kawada, S. Juodkazis, and H. Misawa, “Efficient microvalve driven by a Si-Ni bimorph,” Jpn. J. Appl. Phys. 42(7A), 4464–4468 (2003). [CrossRef]
16. S. Tomonari, H. Yoshida, M. Kamakura, K. Yoshida, K. Kawahito, M. Saitoh, H. Kawada, S. Juodkazis, and H. Misawa, “Miniaturization of a thermally driven Ni-Si bimorph,” Jpn. J. Appl. Phys. 42(7A), 4593–4597 (2003). [CrossRef]
17. Y. Nishijima and S. Akiyama, “Unusual optical properties of the Au/Ag alloy at the matching mole fraction,” Opt. Mat. Express 2(9), 1226–1235 (2012). [CrossRef]
18. C. Gong and M. S. Leite, “Noble metal alloys for plasmonics,” ACS Photon. 3(4), 507–513 (2016). [CrossRef]
19. C. Gong, M. Rebello, S. Dias, G. C. Wessler, J. A. Taillon, L. G. Salamanca-Riba, and M. S. Leite, “Near-field optical properties of fully alloyed noble metal nanoparticles,” Adv. Opt. Mater. 5(1), 1600568 (2017). [CrossRef]
20. Y. Nishijima, Y. Hashimoto, G. Seniutinas, L. Rosa, and S. Juodkazis, “Engineering gold alloys for plasmonics,” Appl. Phys. A 117, (2)641–645 (2014). [CrossRef]
21. Y. Hashimoto, G. Seniutinas, A. Balcytis, S. Juodkazis, and Y. Nishijima, “Au-Ag-Cu nano-alloys: tailoring of permittivity,” Sci. Rep. 6, 25010 (2016). [CrossRef] [PubMed]
22. K. Kusada, M. Yamauchi, H. Kobayashi, H. Kitagawa, and Y. Kubota, “Solid solution alloy nanoparticles of immiscible Pd and Ru elements neighboring on Rh: changeover of the thermodynamic behavior for hydrogen storage and enhanced CO-oxidizing ability,” J. Am. Chem. Soc. 136(5), 1864–1871 (2014). [CrossRef] [PubMed]
23. K. Kusada, M. Yamauchi, H. Kobayashi, H. Kitagawa, and Y. Kubota, “Hydrogen-storage properties of solid-solution alloys of immiscible neighboring elements with Pd,” J. Am. Chem. Soc. 132(45), 15896–15898 (2010). [CrossRef] [PubMed]
24. M. Hirano, K. Enokida, K. Okazaki, S. Kuwabata, H. Yoshida, and T. Torimoto, “Compositio-dependent electrocatalytic activity of AuPd alloy nanoparticles prepared via simultaneous sputter deposition into an ionic liquid,” Phys. Chem. Chem. Phys. 15(19), 7286–7294 (2013). [CrossRef] [PubMed]
25. M. F. Juarez, G. Soldano, H. Guesimi, F. Tielens, and E. Santos, “Catalytic properties of au electrodes modified by an uderlayer of Pd,” Surf. Sci. 631, 235–247 (2015). [CrossRef]
26. H. Kobayashi, K. Kusada, and H. Kitagawa, “Creation of novel solid-solution alloy nanoparticles on the basis of density-ofstates engineering by interelement fusion,” Accounts Chem. Res. 48(6), 1551–1559 (2015). [CrossRef]
27. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions (National Association of Corrosion Engineers, 1974).
28. J. P. Perdew, K. Burke, and M. Ernzerhof, “Generalized gradient approximation made simple,” Phys. Rev. Lett. 77(18), 3865–3868 (1996). [CrossRef] [PubMed]
29. P. E. Blochl, “Projector augmented-wave method,” Phys. Rev. B 50, (24)17953–17979 (1994). [CrossRef]
30. G. Kresse and J. Hafner, “Ab initio molecular dynamics for liquid metals,” Phys. Rev. B 47, (1)558–561 (1993). [CrossRef]
31. G. Kresse, “Ab initio molekular dynamik fur flussige Metalle,” Ph.D. thesis, Diss., Techn. Universitat Wien (1993).
32. G. Kresse and J. Furthmuller, “Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set,” Comput. Mat. Sci. 6(1), 15–50 (1996). [CrossRef]
33. G. Kresse and J. Furthmuller, “Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set,” Phys. Rev. B 54, (16)11169–11186 (1996). [CrossRef]
34. S. Okazaki and S. Johjima, “Temperature dependence and degradation of gasochromic responsebehavior in hydrogen sensing with Pt/WO3 thin film,” Thin Solid Films 558, 411–415 (2014). [CrossRef]
35. A. E. Bon and I. K. Kagan, Stroyeniye I Svoistva Dvoinykh Metalicheskikh Sistem (Nauka, 1976).
36. X. Teng, Q. Wang, P. Liu, W. Han, A. I. Frenkel, W. Wen, N. Marinkovic, J. C. Hanson, and J. A. Rodriguez, “Formation of Pd/Au nanostructures from pd nanowires via galvanic replacement reaction,” J. Am. Chem. Soc. 130(3), 1093–1101 (2008). [CrossRef]
37. M. Murakami, D. de Fontaine, and J. Fodor, “X-ray diffraction study of interdiffusion in bimetallic Au/Pd thin films,” J. Appl. Phys. 47(7), 2850–2856 (1976). [CrossRef]
38. H. Nakajima, H. Fujimori, and M. Koiwa, “Interdiffusion and structural relaxation in Mo/Si multilayer films,” J. Appl. Phys. 63(4), 1046–1051 (1988). [CrossRef]
39. I. K. Schuller, “New class of layered materials,” Phys. Rev. Lett. 44(24), 1597–1600 (1980). [CrossRef]
40. B. M. Clemens and J. G. Gay, “Effect of layer-thickness fluctions on superlattice diffraction,” Phys. Rev. B 35, (17)9337–9340 (1987). [CrossRef]
41. Y. Tominaga, Y. Kinoshita, K. Oe, and M. Yoshimotoa, “Structural investigation of GaAs1−x Bix/GaAsGaAs1−x Bix/GaAs multiquantum wells,” Appl. Phys. Lett. 93(13), 131915 (2008). [CrossRef]
42. Z. Zhao, M. Carpenter, H. Xi, and D. Welch, “All-optical hydrogen sensor based on a high alloy content palladium thin film,” Sens. Actuat. B-Chem. 113(1), 532–538 (2006). [CrossRef]
43. Y. Nishijima, A. Balčytis, G. Seniutinas, S. Juodkazis, T. Arakawa, S. Okazaki, and R. Petruškevičius, “Plasmonic Hydrogen Sensor at Infrared Wavelength,” Sens. Mater. 29(9), 1269–1274 (2017).
44. A. Tittl, P. Mai, R. Taubert, D. Dregely, N. Liu, and H. Giessen, “Palladium-based plasmonic perfect absorber in the visible wavelength range and its application to hydrogen sensing,” Nano Lett. 11(10), 4366–4369 (2011). [CrossRef] [PubMed]
45. K. Juodkazis, J. Juodkazyte, B. Sebeka, and S. Juodkazis, “Reversible hydrogen evolution and oxidation on Pt electrode mediated by molecular ion,” Appl. Surf. Sci. 290(30), 13–17 (2014). [CrossRef]
46. K. Juodkazis, J. Juodkazyte, A. Grigucevicien, and S. Juodkazis, “Hydrogen species within the metals: role of molecular hydrogen ion ,” Appl. Surf. Sci. 258(2), 743–747 (2011). [CrossRef]
47. I. Wei and J. Brewer, “Desorption of hydrogen from palladium plating,” AMP J. Tech. 5, 49–53 (1996).
48. S. Zhao, Z. Z. Tian, J. N. Liu, Y. L. Ren, and J. J. Wang, “Interaction of H2 with gold-palladium binary clusters: Molecular and dissociative adsorption,” Comput. Theor. Chem. 1055(1), 1–7 (2015). [CrossRef]
49. R. M. Holmes, “The effect of absorbed hydrogen on the thermoelectric properties of palladium,” Science 56(1442), 201–202 (1922). [CrossRef] [PubMed]
50. D. A. Otterson and R. J. Smith, NASA Report, Absorption of Hydrogen by Palladium and Electrical Resistivity up to H-Pd Atom Ratios of 0.97 (National Aeronautics and Space Administration, 1969).
51. L. Pauling, “The nature of the chemical bond. iv. the energy of single bonds and the relative electronegativity of atoms,” J. Am. Chem. Soc. 54(9), 3570–3582 (1932). [CrossRef]
52. D. Monzon-Hernandez, D. Luna-Moreno, and D. Martinez-Escobar, “Fast response fiber optic hydrogen sensor based on palladium and gold,”" Sens. Actuat. B-Chem. 136(2), 562–566 (2009). [CrossRef]
53. L. Holleck, “Diffusion and solubility of hydrogen in palladium and palladium-silver alloys,” J. Phys. Chem. 74(3), 503–511 (1970). [CrossRef]
54. W. Y. Yu, G. M. Mullen, and C. B. Mullins, “Hydrogen adsorption and absorption with Pd-Au bimetallic surfaces,” J. Phys. Chem. C 117, (38)19535–19543 (2013). [CrossRef]
55. P. M. Quaino, R. Nazmutdinov, L. F. Peiretti, and E. Santos, “Unravelling the hydrogen absorption process in Pd overlayers on a Au(111) surface,” Phys. Chem. Chem. Phys. 18(5), 3659–3668 (2016). [CrossRef] [PubMed]
