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Membrane concept for environmental surface science
Journal of Alloys and Compounds 742 (2018) 518e523
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Review
Membrane concept for environmental surface science Olga Sambalova a, b, Andreas Borgschulte a, b, * a Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Advanced Analytical Technologies, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland b University of Zürich, Department of Chemistry, Winterthurer Strasse, 190, CH-8057 Zürich, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2017 Received in revised form 4 January 2018 Accepted 10 January 2018
The electronic structure of hydrogen chemisorbed to surfaces can be measured using standard surface science techniques, because the required hydrogen pressure is compatible with the UHV-technology. However, processes relevant for energy conversion and storage take place at several atmospheres hydrogen pressure; and thus valuable information on these systems is not accessible by commonly used surface science methods due to their incompatibility with high pressures. In this paper, we review the membrane approach for high pressure XPS for Lab-based systems. The method is based on a specimen holder, which is a metallic, hydrogen permeable membrane fed on one side with a high hydrogen pressure and exposed on the other side to the X-ray beam at UHV-pressures. We discuss a quantitative model for the permeation of hydrogen through a Pd-membrane and the future prospects and limitations of the method for hydrogen related surface reactions. © 2018 Elsevier B.V. All rights reserved.
This paper has been presented in Symposium O: Functional metal hydrides, at the EMRS Fall Meeting and Exhibit, Warsaw, September 2016. Keywords: Metal hydrides High-pressure XPS
Contents 1. 2. 3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518 Membrane approach for hydrogen related surface reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519 Modelling surface coverage and permeation rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 520 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523
1. Introduction The understanding of gasesolid interactions is a central scientific challenge in various fields of research, such as catalysis and electro-catalysis, corrosion, hydrogen embrittlement and the development of various types of hydrides (i.e. metallic, complex, and organic) for hy- drogen detection, permeation and storage. The static thermodynamic properties, such as chemisorption and absorption enthalpies, are relatively easily accessible by state-of-the-
* Corresponding author. Empa, Laboratory for Advanced Analytical Technologies, CH-8600 Dübendorf, Switzerland. E-mail address: [email protected] (A. Borgschulte). https://doi.org/10.1016/j.jallcom.2018.01.160 0925-8388/© 2018 Elsevier B.V. All rights reserved.
art methods such as the measurements of pressure-composition isotherms (pcT), calorimetry, etc. [1e3]. Methods to follow dynamical phenomena such as hydrogen permeation [4] and the meas- urements and modelling of sorption kinetics [5e7] exist as well. The limits of the state-of-the-art methods are reached when it comes to the rational design and optimisation of the materials suitable for the above-mentioned applications. One reason is that the investigation of the surface as gateway for gases between (bulk) solid and gas requires so- phisticated technology. In particular, in situ information about chemical bonds and/or the electronic configuration of valence states is of utmost importance. This includes the active solid material itself, adsorbates and reaction intermediates on the surface. X-ray photoelectron spectroscopy (XPS)
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is a powerful surface characterisation technique, providing insightful in- formation about the elements present on a given surface, including their respective chemical states and concentrations across as many as a few tens of atomic layers. However, the mean- free path of electrons with energies below 1500 eV in a gaseous environment is typically be- low a micrometer at ambient pressure, rendering XPS technique unsuitable for investigation under realistic experimental conditions [8,9]; furthermore, UltraHigh-Vacuum (UHV)- conditions are required for the safe operation of X-ray anodes and channeltrons [8,9]. These problems can be partly overcome by performing so-called “environmental”, “near am- bient pressure” or “high pressure” XPS [9]. These techniques are based on the use of differ- ential pumping stages, which lower the high pressure at the sample stepwise down to the re- quired vacuum level. The most important step (“key component”, see Fig. 1) consists of a small orifice at a distance z to the sample similar to its diameter d. The distance must be be- low the mean free path length of electrons at the desired gas pressure [9]. In state-of-the-art systems, the pressure at the sample may reach a few tens of mbar, with acceptable photoelec- tron intensity losses [9]. Despite their success, these techniques suffer from drawbacks, such as reduced energy resolution and specimen freedom of movement, high cost and impeded ac- cessibility to the facilities, as most of them are limited to operation at synchrotron light sources. The latter is a consequence of the small orifice diameter requirement (d and z are typically less than 1 mm): electrons, which are able to enter the aperture of the analyzer, can only come from an area similar to the opening of the orifice. The maximum distance corresponds to the mean free path of electrons le , which scales with the pressure: for electrons of 400 eV kinetic energy is about 4 mm in 1 mbar oxygen, and reduces to 4 mm in 1000 mbar. The X-rays exciting those electrons have to be focused onto the accordingly small area on the sample [9]. This is finitely possible with special laboratory-sources, but is perfect for synchrotron radiation. However, the so-called “pressure gap” in XPS can still only be partially overcome with a limited maximum pressure, and high cost. An alternative principle of accessing high pressure phenomena by XPS is as follows. The window separates near ambient pressure (sample) mini-cell from the UHV (analysis) side, while X-rays and the thus generated photoelectrons penetrate through the window and allow for analysis of the window-sample interface. A number of
519
developments have been achieved in applying robust ultrathin XPS window for AP XPS [10e13]. The general approach has been recognized by the community to be a valuable tool and various windows have been investigated for this purpose, among others silicon [10], graphene [11,12] and graphene oxide [13]. However, one of the major drawbacks of this technique is the limited variety of materials that can be used as the window. The materials have to be able to withstand the pressure difference while still being permeable not only to X-rays, but more importantly to the generated photoelectrons. Thus, even in case of ultrathin membranes (10e100 nm), the applicability of the technique is limited to synchrotron-based or other high photon energy sources while only the interfaces can be studied [10e13], which may be different to a free surface. Recently, we developed a different membrane-based approach to study materials exposed to high hydrogen pressure of 1 bar while keeping analysis chamber under high vacuum - thus effectively achieving high pressure XPS analysis. The core idea of the approach is similar to this used in the window approach, as described above: the membrane device, which is exposed to UHV in the analysis chamber on one side and ambient pressure hydrogen on the other. However, the substrate permeates through the membrane and is analysed close to the UHV, rather than ambient pressure side. The hydrogen flux from the surface into the vacuum is desorption, rather than diffusion, limited, leading to ambient pressure conditions at the surface of the membrane on the side exposed to UHV [14]. The validity of the approach for hydrogen related surface reactions has been demonstrated [14]. In this paper, we review the concept in view of possible applications, extending the idea to reactants other than hydrogen, i.e. water. This includes the quantitative description of the process parameters, such as temperature, pressure, and membrane properties. From this, the setup is then adapted to the sought applications. 2. Membrane approach for hydrogen related surface reaction The underlying physics of the membrane approach for surface reaction is the continuity of the chemical potential from the feed side to the vacuum side of the membrane (Fig. 2). The slope of the chemical potential inside the membrane depends on the hydrogen diffusion in it; if diffusion is not rate limiting, the slope, and thus the
Fig. 1. Top: Principle, key component and commercial realisation of near ambient X-ray photo-electron spectroscopy (NAPXPS). Photographs from Specs [29] Bottom: Corresponding counterparts of “high-pressure” XPS based on the membrane approach [14].
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Fig. 2. The left panel is a cross section through a bare Pd membrane depicting the energy potential surface, the chemical potential, and the concentration of hydrogen. The chemical potential is in quasi-equilibrium. The chemical potential of hydrogen in Ca on Pd-membranes (right panel) can be considered to be nearly constant, although the hydrogen concentration varies greatly, because of the low desorption probability of H2 from Ca(H2) due to the high activation barrier. The latter is depicted by a simplified energy-potential surface.
differences between the chemical potentials is small [14,19e22]. This allows assuming similar hydrogen coverages on both sides of the membrane. However, the detailed materials properties may vary on a microscopic scale. Using technical membranes, grain boundaries and other defects may undermine the assumption. An upper limit is found by geometrical arguments: the continuity argument is valid on the surface as well. Therefore, the chemical potential parallel to the surface is averaged within a distance similar to the thickness of the membrane (typically of the order of 100 mm [14]). Vice versa, if the surface roughness is much below this value, the surface can be assumed to have equal hydrogen chemical potential. The hydrogenation properties of the surface (e.g., coverage) as the result of the constant chemical potential, though, might change upon the microscopic properties (crystal orientation, termination, contaminants). To consider these changes, it is helpful to recapitulate the conditions for surface reactions and UHV-technology. Although these two matters are related to different topics, that are science on one hand and technology on the other, the key parameters originate from the same thermodynamic relations. To simplify discussion, we first consider only hydrogen and metals, but similar estimates are valid for other gases, such as water, and other materials, such as polymers, as well. Hydrogen is physisorbed at a surface with a binding energy of around 5e10 kJ/mol [6,15]. Substantial amount of hydrogen is thus physisorbed only at high pressures or low temperatures. The enthalpy of chemisorption is higher by one order of magnitude than that of physisorption [6,15]. Hydrogen chemisorption is therefore thermodynamically possible at room temperature in pressures as low as 106 mbar for most electropositive metals. As a consequence, even under ultra-high vacuum conditions, most materials will eventually be covered by (atomic) hydrogen or other impurity gases. It is possible, though, to investigate clean metals by surface science methods [15]. Kinetics is the key. As a rule of thumb, pressure of 106 mbar corresponds to an impingement rate of one molecule per second and per surface site [16]. With typical measurement times of 1000 s, pressures below 109 mbar give acceptably clean results for freshly prepared surfaces. Further conditions for UHV-technology include the use of materials with a low vapor pressure, which mostly results from a high binding energy of the surface atoms to the bulk atoms (facilitated by either the atoms of the materials, e.g. copper, or by a strong binding of oxygen, which is forming an inert oxide skin as is the case of stainless steel or aluminium) [16]. The upper limit is
usually given by the mean free path of electrons in the chamber, which should be longer than the path length of the electrons through the spectrometer, and lies around 105 mbar [16]. Obviously, UHV-conditions do not coincide with experimental conditions to investigate gas e solid interactions with a high binding energy such as chemisorption. The enthalpy of formation and thus the equilibrium pressure of hydrogen e metal bulk system vary greatly. Bulk systems such as highly stable metal hydrides, e.g. CaH2, have plat- eau pressures compatible with UHV. “It is, however, virtually impossible to prepare the alka- line earth hydrides and related compounds by direct reaction with gaseous hydrogen” [16]. The reason lies in the relatively slow kinetics. Here the low pressure is a disadvantage: the impingement rate is directly proportional to the pressure. Furthermore, hydrogen molecules have to be split first, which is usually performed by a catalytically active cap layer, or by pre- viously hydrided Pd substrates [16]. In this approach, the Pd substrate is exposed to high hy- drogen pressure in a reaction cell, and is then quickly transported to a UHVchamber, where the alkaline earth metal is deposited onto the Pd hydride substrate and is simultaneously hy- drogenated [16]. This method cannot, however, control the chemical potential of hydrogen. Some metals, such as yttrium and other rare earth metals, can be hydrogenated directly from the gas phase because of their high reactivity [17]. However, though advantageous for hydro- genation, this is a major drawback for surface analysis, since increased surface reactivity also means increased adsorption of residual gases, such as oxygen and water, which are practically omnipresent in any hydrogen gas. Therefore, an additional hydrogen purification step is required. The desired high pressure of 1 bar implies that in hydrogen gas with purity level as high as 109, each surface atom is hit by one residual gas molecule per second. If the material is hydrogenated via the membrane, the vacuum level in front of the sensitive capping layer remains in the UHV range, i.e. << 106 mbar. In this respect Pd membranes have an advantage, as they are highly selective for hydrogen permeation, and thus provide very pure and atomic hydrogen.
3. Modelling surface coverage and permeation rates To model the surface kinetics of hydrogen permeation through a Pd-membrane, it is crucial to consider the site dependence of
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521
hydrogen in the materials (see Fig. 2): Hydrogen has a high binding energy at the surfaces, which corresponds to a high surface coverage even at high UHV-pressures (compare discussion above). The energy potential surface defines the forward and reverse reaction rate of hydrogen on the high pressure side of the membrane and of hydrogen desorbing into vacuum [6,18,19]:
E E þ Echem 2 Apfeed exp diss $ð1 qlow Þ2 2B$exp diss qlow kT kT
E Rf ¼ ApH2 $exp diss $ð1 qH Þ2 ; Rr kT Ediss þ Echem 2 ¼ B$exp qH kT
This equation is equal to the situation in which we have only one surface with negligible bulk interaction (except the factor 2). In particular, it follows that the coverage on the low vacuum side is defined by the applied pressure a:
2 E E þ Echem 2 Apfeed exp diss $ 1 qhigh B$exp diss qhigh kT kT E þ Echem 2 $qlow ¼ B$exp diss kT (2) The hydrogen coverage on high and low pressure side, respectively, qhigh, qlow define the chemical potentials at the surfaces. Their difference is the driving force for diffusion through the membrane, which is given by Fick's law [6]:
vc Dc zD vx d
(3)
where j is the diffusion flux, D is the diffusion coefficient, c is the concentration and x is the position. Here, we neglected the nonlinearity of the diffusion gradient through the membrane. We can give quantitative data. The flux through such a 0.15 mm thick Pd membrane was determined by the membrane approach using the mol [8]. The diffusion parameter of Pd is Sieverts method: jz108 cm 2s
106cms at the measurement temperature (320 K) [17]. Therefore, 2
8
2
mol ¼ 1020 H . To trans$cm concentration difference, Dc ¼ 10 $1:5$10 3 cm3 106 late this to the surface coverage, we compare the value to the maximum concentration of hydrogen in palladium, which is around one H per Pd [20]. The corresponding reference concen22
H. tration (concentration at around 200 mbars) is c0 ¼ 6:8$10 cm3 The continuity of the chemical potentials leads to the following relation between surface coverage and bulk concentration [9]:
q
msurface
e 1q
kT
¼
c mbulk e kT 1c
(4)
where msurface and mbulk are surface and bulk chemical potentials respectively. For small concentration changes, the relative concentration difference is equal to the relative coverage difference, i.e.
Dc c0
¼
Dq < 1% q0
(6)
(1)
where Rf and Rr are the rates of the forward and reverse reactions respectively, A and B are constants, Ediss and Echem are dissociation and chemisorption energies respectively, k is Boltzmann constant, T is temperature and qH is site coverage. In kinetic equilibrium, neglecting the pressure on the vacuum side (compare also Fig. 2) and defining the hydrogen pressure on the feed side as pfeed, one obtains
j ¼ D
¼0
(5)
This result is in agreement with the experimental result that hydrogen permeation through Pd membranes is limited by desorption at low temperatures [21,22]. Furthermore, one concludes that the concentration difference between the coverages at high and low vacuum side is negligible, i.e., qhigh zqlow , and thus
q¼
1 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ Ap2B exp Echem kT
(7)
feed
It is worth noting that the dissociation energy cancels out, because it is equal on both sides. Some expected dependencies result from the equation, such as the coverage on the low vacuum side approaches unity at high pressures, and/or large chemisorption enthalpy. The assumption that only two hydrogen sites per surface are required has recently shown to be too simple, at least three sites are needed [23]. However, this changes the math slightly (q2 /q3 , etc.), but not the overall picture. The situation is qualitatively different when the properties of the membrane sides are different, e.g. due to contaminants, which block desorption of hydrogen on the vacuum side. This was used to empirically demonstrate the shift in the rate limiting step by comparing the pressure dependence of the flux through clean and contaminated membranes [14]. To include this in the above described model calculations, we have to modify the number of available sites on the vacuum side. As not all sites are available anymore, the maximum coverage reduces by q00 ¼ uq0, in which u is one for a clean surface, and zero for a surface completely covered by, e.g., CO (see also Fig. 3). The desorption rate, R, at the low vacuum side is then developed from eqs. (1) and (2) using qlow ¼ 1uqhigh to
chem B$exp Ediss þE kT sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R¼ 1þ 12 B u exp Echem 1þ ApH kT
(8)
2
The model equation is plotted in Fig. 3. At very high pressures, the pressure dependence flattens out because all of the sites are occupied. Pressure dependence is only observed at lower pressures. The calculations confirm the experimental observations that (i) the overall rate from a contaminated surface is lowered, but (ii) the slope of the pressure dependence is increased for the contaminated surface compared to the clean surface as a result of a de- creased desorption rate on the low vacuum side. The approach was recently extended for ambient pressure XAS analysis [24]. Hydrogen permeation through AgPd alloy membrane clearly showed hydrogen pressure-dependent evolution of a spectral feature that corresponded to Pd 4p states, suggesting interaction of hydrogen with the latter. It is noteworthy, that, via this relatively simple system, a spectral connection between XPS and XAS can be established, which is in agreement with the theoretical connection of the two techniques through the electron binding energy [24]. This idea of membrane-based XPS was further expanded to include investigation of high pressure hydrogenation of materials other than Pd membranes by coating the surface with, e.g. Ca, as sketched in Fig. 2. In this case, the coverage difference induced by diffusion is negligible, as is desorption term on the vacuum side. The coverage on the high pressure side is then equal to a normal
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Fig. 3. Left graph: Flux (dp/dt) through a Pd membrane, measured as freshly built in (“dirty”) and after long-term sputtering (“clean”) adapted from Ref. 14. nsputt, nvac are the slopes of the double log plot of dp ¼ f ðpÞ. The right graph shows the rates as calculated using equation (8) with u ¼ 1 (clean) and u ¼ 0.1 (contaminated). dt
surface science setup, and the coverage on the low pressure side is defined by the corresponding chemical potential. It is worth noting that due to the low dissociation barrier of Pd, the assumption of equal chemical potentials on both side of the Pd-membrane coating system applies to virtually all materials. To summarize, the membrane approach is based on the following materials parameters: 1. The diffusion through the membrane must be fast enough to not be rate-determining. Thus, for hydrogen, Pd or similar materials have suitable properties. Simultaneously, the membrane thickness should be as thin as possible; we demonstrated the functioning of 150 mm thick Pd-membranes. 2. The surface coverage of stand-alone membranes is a consequence of the balance be- tween the adsorption and desorption kinetics of the surface. The coverage on the low vacuum side is defined by the applied pressure and chemisorption properties. 3. Any material, which can be deposited on such a membrane and has a hydrogen disso- ciation barrier higher than Pd (which is the case for most materials), can be investigat- ed. Important parameter here is the membrane thickness, as it has to be thin enough to allow fast diffusion. This may be a challenge, as the surface layer of just 1e10 nm thickness may be enough to represent bulk properties [25]. The following systems have been investigated by this method: Pd, and Pd-Ag membranes [14]. Hydrogen reduction of cobalt nitrate to metallic Co followed by XPS [14]. Hydrogenation of a metal-organic water reduction catalyst (Cobalt dioxime complex- es) [14]. Nanometer-thin polytetrafluoroethylene (PTFE) coating on Pd membrane. Hydrogen reduction of molybdenum oxide at room temperature. The PTFE coating has been demonstrated to increase catalytic activity via enhancement of Pd membrane permeability. The effect was attributed to lower hydrogen desorption barrier. Membranebased XPS could reveal the electronic structure of PTFE and Pd-
PTFE complex, allowing to identify the Pd-PTFE interaction [26]. In case of molybdenum oxide reduction, membrane-based XPS approach was applied for studying hydrogen gas facilitated reduction of MoO3 thin films. It could be established, that hydrogen intercalation into MoO3 results in HxMoO3 products, which slowly decompose into MoO2 þ1/2 H2O. The obtained results shed light on the debated mechanism of the hydrogen reduction in bulk molybdenum oxide [25]. The concept is not exhausted by the investigation of hydrogenation reactions. Key factor for the applicability of these methods is the availability of a membrane with fast diffusion of the target molecule. The membrane approach opens the possibility to perform XPS analysis of reactions in aqueous environment, which were previously inaccessible due to the UHV requirements of XPS. One of the problems that can be addressed by the given approach is mechanistic, time- resolved insight into, for example, heterogeneous solar water splitting reactions in aqueous environment. The field of artificial photosynthesis and a crucial part of it, solar water split- ting, is increasingly researched by the scientific community, driven by the ever increasing demand for sustainable sources of energy [27,28]. The membrane-based approach has the po- tential to provide detailed mechanistic insight via operando analysis of chemical states and electrochemically active regions in heterogeneous solar water splitting catalysis. Therefore, the adaptability of the membrane approach for investigating reactions in H2O, rather than H2 environment is currently being examined in our group. The reaction can be driven photo- as well as electrochemically. In case of the latter, an electrochemical micro-cell can be studied [10]. Briefly, inert thin gold membrane acts as one of the electrodes, while the stainless steel of the sample chamber acts as a counter electrode. This system has previously been demonstrated to provide quantitative information on in situ Si oxide growth at the Si-water interface under controlled electrochemical conditions. An important asset is the comparison with other techniques, in particular with near ambient pressure XPS [9]. Near ambient pressure XPS probes a sample environment, which reflects the situation of a surface in a high pressure gas environment. The sample surface studied by the membrane approach has the chemical potential of a sample exposed to the corresponding gas pressure. However, certain dynamic effects are thus missing or different, which has dis- advantages as well as advantages. The
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membrane surface is not identical to the gas exposed surface. Conclusions on dynamic phenomena are thus allowed only after careful modelling (see above), and are usually restricted to desorption phenomena. The membrane delivers highly pure atomic hydrogen to the sample, while nearby walls and compartments are exposed to UHV-compatible pressures only. This allows the hydrogenation of dissociatively less active surfaces. Furthermore, the sample is less prone to contaminations, which are produced at high gas pressures due to gas-solid interaction of the gas with the chamber walls. The applied hydrogen pressure of even 1 bar is compatible with the UHV-side analysis of the membrane approach. This allows for unperturbed XPS measurements as demonstrated. Furthermore, the membrane-based technique has a potential to be applied for other surface sensitive methods such as electron diffraction, reflecting high resolution energy loss electron spectroscopy, and X-ray absorption spectroscopy using low energy photons. Furthermore, the approach is based on a specific membrane holder, which can be adapted to the concrete dimensions of the analysis system. The capital investment is relatively low, when compared to ambient pressure photoemission systems. 4. Conclusions In this overview paper we review the fundamentals and demonstrated applications of the membrane approach to highpressure XPS as well future prospects and limitations. The quantitative model to describe the permeation rate based on surface kinetics is introduced to underline the general applicability of the approach. Acknowledgements This work was partly supported by the UZH-UFSP program LightChEC. Financial support from the Swiss National Science Foundation (grant numbers 144120 and 172662) is acknowledged. References [1] M. Bielmann, S. Kato, P. Mauron, A. Borgschulte, A. Züttel, Characterization of hydrogen storage materials by means of pressure concentration isotherms based on the mass flow method, Rev. Sci. Instrum. 80 (2009), 083901. [2] C. Rongeat, I. Llamas-Jansa, S. Doppiu, S. Deledda, A. Borgschulte, L. Schulz, O. Gutfleisch, Determination of the heat of hydride formation/decomposition by high-pressure differential scanning calorimetry (HP-DSC), J. Phys. Chem. B 111 (2007) 13301e13306. [3] R. Gremaud, C.P. Broedersz, D.M. Borsa, A. Borgschulte, P. Mauron, H. Schreuders, J.H. Rector, B. Dam, R. Griessen, Hydrogenography: an optical combinatorial method to find new light-weight hydrogen-storage materials, Adv. Mater. 19 (2007) 2813e2817. [4] V.M. Gryaznov, Surface catalytic properties and hydrogen diffusion in Palladium alloy membranes, Z. Phys. Chem. 147 (1986) 123e132. [5] M.H. Mintz, Y. Zeiri, Hydriding kinetics of powders, J. Alloy. Comp. 216 (1994) 159e175. [6] M. Martin, C. Gommel, C. Borkhardt, E. Fromm, Absorption and desorption
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kinetics of hydrogen storage alloys, J. Alloy. Comp. 238 (1996) 193e201. [7] A. Borgschulte, F. Pendolino, R. Gremaud, A. Züttel, Hydrogen diffusion in NaH as derived from isotope exchange experiments, Appl. Phys. Lett. 94 (2009), 111907. [8] F. Mangolini, J.B. Åhlund, G.E. Wabiszewski, V.P. Adiga, P. Egberts, F. Streller, F. Backlund, P.G. Karlsson, B. Wannberg, R.W. Carpick, Angle-resolved environmental X-ray photoelectron spectroscopy: a new laboratory setup for photoemission studies at pressures up to 0.4 Torr, Rev. Sci. Instrum. 83 (2012), 093112. €gl, Ambient pressure photoelectron spectroscopy: a new [9] M. Salmeron, R. Schlo tool for surface science and nanotechnology, Surf. Sci. Rep. 63 (2008) 169e199. [10] T. Masuda, T. Masuda, H. Yoshikawa, H. Noguchi, T. Kawasaki, M. Kobata, K. Kobayashi, K. Uosaki, In situ x-ray photoelectron spectroscopy for electrochemical reactions in ordinary solvents, Appl. Phys. Lett. 103 (2013), 111605. [11] J. Velasco-Velez, V. Pfeifer, M. Haevecker, R.S. Weatherup, R. Arrigo, C.H. Chuang, E. Stotz, G. Weinberg, M. Salmeron, R. Schlçgl, A. Knop-Gericke, Photoelectron spectroscopy at the grapheneeliquid interface reveals the electronic structure of an electrodeposited cobalt/graphene electrocatalyst, Angew. Chem. Int. Ed. 54 (2015) 14554e14558. [12] R.S. Weatherup, B. Eren, Y. Hao, H. Bluhm, M.B. Salmeron, Graphene membranes for atmospheric pressure photoelectron spectroscopy, J. Phys. Chem. Lett. 7 (2016) 1622e1627. [13] A. Kolmakov, D.A. Dikin, L.J. Cote, J. Huang, M. Kazemian Abyaneh, M. Amati, L. Gregoratti, S. Guenther, M. Kiskinova, Graphene oxide windows for in situ environmental cell photoelectron spectroscopy, Nat. Nanotechnol. 6 (2011) 651e657. [14] R. Delmelle, B. Probst, R. Alberto, A. Züttel, D. Bleiner, A. Borgschulte, Closing the pressure gap in X-ray photoelectron spectroscopy by membrane hydrogenation, Rev. Sci. Instrum. 86 (2015), 053104. [15] K. Christmann, Interaction of hydrogen with solid surfaces, Surf. Sci. Rep. 9 (1988) 1. [16] H. Lüth, Surfaces and Interfaces of Solid Materials, Springer, Berlin, 1995. [17] J. Hayoz, C. Koitzsch, D. Naumovi c, L. Schlapbach, P. Aebi, Electronic structure of the YH3 phase from angle-resolved photoemission spectroscopy, Phys. Rev. Lett. 90 (2003), 196804. [18] H. Conrad, G. Ertl, E.E. Latta, Adsorption of hydrogen on palladium single crystal surfaces, Surf. Sci. 41 (1974) 435e446. [19] A. Borgschulte, R. Gremaud, R. Griessen, Interplay of diffusion and dissociation mechanisms during hydrogen absorption in metals, Phys. Rev. B 78 (2008), 094106. [20] T.L. Ward, T. Dao, Model of hydrogen permeation behavior in palladium membranes, J. Membr. Sci. 153 (1999) 211e231. [21] D. Tomanek, S. Mukherjee, V. Kumar, K.H. Bennemann, Calculation of chemisorption and absorption induced surface segregation, Surf. Sci. 114 (1982) 11e22. [22] D. McLean, Grain Boundaries in Metals, Oxford University Press, London, 1957. [23] A. Krozer, A. Fischer, L. Schlapbach, Experimental study of the valence-band region of Mg-Pd and Ba-Pd interfaces with and without hydrogen and of Mg and Ba hydrides, Phys. Rev. B 53 (1996) 13808. [24] O. Sambalova, Y. Arbelo Pena, R. Delmelle, C. Cirelli, B. Patterson, F. Barbato, D. Bleiner, A. Borgschulte, X-ray absorption spectroscopy probing hydrogen in metals, Proc. SPIE 102430P (2017). [25] A. Borgschulte, O. Sambalova, R. Delmelle, S. Jenatsch, R. Hany, F. Nüesch, Hydrogen reduction of molybdenum oxide at room temperature, Sci. Rep. 7 (2016) 40761. [26] R. Delmelle, P. Ngene, B. Dam, D. Bleiner, A. Borgschulte, Promotion of hydrogen desorption from Pd surfaces by fluoropolymer coating, Chem. Cat. Chem. 8 (2016) 1646e1650. [27] A. Borgschulte, The hydrogen grand challenge, Front. Energy Res. 4 (2016) 11. [28] D.G. Nocera, The artificial leaf, Acc. Chem. Res. 45 (2012) 767e776. [29] www.specs.de; SPECS Surface Nano Analysis GmbH, Voltastrasse 5, 13355 Berlin, Germany, accessed 10.01.2017.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Review
Membrane concept for environmental surface science Olga Sambalova a, b, Andreas Borgschulte a, b, * a Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Advanced Analytical Technologies, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland b University of Zürich, Department of Chemistry, Winterthurer Strasse, 190, CH-8057 Zürich, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2017 Received in revised form 4 January 2018 Accepted 10 January 2018
The electronic structure of hydrogen chemisorbed to surfaces can be measured using standard surface science techniques, because the required hydrogen pressure is compatible with the UHV-technology. However, processes relevant for energy conversion and storage take place at several atmospheres hydrogen pressure; and thus valuable information on these systems is not accessible by commonly used surface science methods due to their incompatibility with high pressures. In this paper, we review the membrane approach for high pressure XPS for Lab-based systems. The method is based on a specimen holder, which is a metallic, hydrogen permeable membrane fed on one side with a high hydrogen pressure and exposed on the other side to the X-ray beam at UHV-pressures. We discuss a quantitative model for the permeation of hydrogen through a Pd-membrane and the future prospects and limitations of the method for hydrogen related surface reactions. © 2018 Elsevier B.V. All rights reserved.
This paper has been presented in Symposium O: Functional metal hydrides, at the EMRS Fall Meeting and Exhibit, Warsaw, September 2016. Keywords: Metal hydrides High-pressure XPS
Contents 1. 2. 3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518 Membrane approach for hydrogen related surface reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519 Modelling surface coverage and permeation rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 520 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523
1. Introduction The understanding of gasesolid interactions is a central scientific challenge in various fields of research, such as catalysis and electro-catalysis, corrosion, hydrogen embrittlement and the development of various types of hydrides (i.e. metallic, complex, and organic) for hy- drogen detection, permeation and storage. The static thermodynamic properties, such as chemisorption and absorption enthalpies, are relatively easily accessible by state-of-the-
* Corresponding author. Empa, Laboratory for Advanced Analytical Technologies, CH-8600 Dübendorf, Switzerland. E-mail address: [email protected] (A. Borgschulte). https://doi.org/10.1016/j.jallcom.2018.01.160 0925-8388/© 2018 Elsevier B.V. All rights reserved.
art methods such as the measurements of pressure-composition isotherms (pcT), calorimetry, etc. [1e3]. Methods to follow dynamical phenomena such as hydrogen permeation [4] and the meas- urements and modelling of sorption kinetics [5e7] exist as well. The limits of the state-of-the-art methods are reached when it comes to the rational design and optimisation of the materials suitable for the above-mentioned applications. One reason is that the investigation of the surface as gateway for gases between (bulk) solid and gas requires so- phisticated technology. In particular, in situ information about chemical bonds and/or the electronic configuration of valence states is of utmost importance. This includes the active solid material itself, adsorbates and reaction intermediates on the surface. X-ray photoelectron spectroscopy (XPS)
O. Sambalova, A. Borgschulte / Journal of Alloys and Compounds 742 (2018) 518e523
is a powerful surface characterisation technique, providing insightful in- formation about the elements present on a given surface, including their respective chemical states and concentrations across as many as a few tens of atomic layers. However, the mean- free path of electrons with energies below 1500 eV in a gaseous environment is typically be- low a micrometer at ambient pressure, rendering XPS technique unsuitable for investigation under realistic experimental conditions [8,9]; furthermore, UltraHigh-Vacuum (UHV)- conditions are required for the safe operation of X-ray anodes and channeltrons [8,9]. These problems can be partly overcome by performing so-called “environmental”, “near am- bient pressure” or “high pressure” XPS [9]. These techniques are based on the use of differ- ential pumping stages, which lower the high pressure at the sample stepwise down to the re- quired vacuum level. The most important step (“key component”, see Fig. 1) consists of a small orifice at a distance z to the sample similar to its diameter d. The distance must be be- low the mean free path length of electrons at the desired gas pressure [9]. In state-of-the-art systems, the pressure at the sample may reach a few tens of mbar, with acceptable photoelec- tron intensity losses [9]. Despite their success, these techniques suffer from drawbacks, such as reduced energy resolution and specimen freedom of movement, high cost and impeded ac- cessibility to the facilities, as most of them are limited to operation at synchrotron light sources. The latter is a consequence of the small orifice diameter requirement (d and z are typically less than 1 mm): electrons, which are able to enter the aperture of the analyzer, can only come from an area similar to the opening of the orifice. The maximum distance corresponds to the mean free path of electrons le , which scales with the pressure: for electrons of 400 eV kinetic energy is about 4 mm in 1 mbar oxygen, and reduces to 4 mm in 1000 mbar. The X-rays exciting those electrons have to be focused onto the accordingly small area on the sample [9]. This is finitely possible with special laboratory-sources, but is perfect for synchrotron radiation. However, the so-called “pressure gap” in XPS can still only be partially overcome with a limited maximum pressure, and high cost. An alternative principle of accessing high pressure phenomena by XPS is as follows. The window separates near ambient pressure (sample) mini-cell from the UHV (analysis) side, while X-rays and the thus generated photoelectrons penetrate through the window and allow for analysis of the window-sample interface. A number of
519
developments have been achieved in applying robust ultrathin XPS window for AP XPS [10e13]. The general approach has been recognized by the community to be a valuable tool and various windows have been investigated for this purpose, among others silicon [10], graphene [11,12] and graphene oxide [13]. However, one of the major drawbacks of this technique is the limited variety of materials that can be used as the window. The materials have to be able to withstand the pressure difference while still being permeable not only to X-rays, but more importantly to the generated photoelectrons. Thus, even in case of ultrathin membranes (10e100 nm), the applicability of the technique is limited to synchrotron-based or other high photon energy sources while only the interfaces can be studied [10e13], which may be different to a free surface. Recently, we developed a different membrane-based approach to study materials exposed to high hydrogen pressure of 1 bar while keeping analysis chamber under high vacuum - thus effectively achieving high pressure XPS analysis. The core idea of the approach is similar to this used in the window approach, as described above: the membrane device, which is exposed to UHV in the analysis chamber on one side and ambient pressure hydrogen on the other. However, the substrate permeates through the membrane and is analysed close to the UHV, rather than ambient pressure side. The hydrogen flux from the surface into the vacuum is desorption, rather than diffusion, limited, leading to ambient pressure conditions at the surface of the membrane on the side exposed to UHV [14]. The validity of the approach for hydrogen related surface reactions has been demonstrated [14]. In this paper, we review the concept in view of possible applications, extending the idea to reactants other than hydrogen, i.e. water. This includes the quantitative description of the process parameters, such as temperature, pressure, and membrane properties. From this, the setup is then adapted to the sought applications. 2. Membrane approach for hydrogen related surface reaction The underlying physics of the membrane approach for surface reaction is the continuity of the chemical potential from the feed side to the vacuum side of the membrane (Fig. 2). The slope of the chemical potential inside the membrane depends on the hydrogen diffusion in it; if diffusion is not rate limiting, the slope, and thus the
Fig. 1. Top: Principle, key component and commercial realisation of near ambient X-ray photo-electron spectroscopy (NAPXPS). Photographs from Specs [29] Bottom: Corresponding counterparts of “high-pressure” XPS based on the membrane approach [14].
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Fig. 2. The left panel is a cross section through a bare Pd membrane depicting the energy potential surface, the chemical potential, and the concentration of hydrogen. The chemical potential is in quasi-equilibrium. The chemical potential of hydrogen in Ca on Pd-membranes (right panel) can be considered to be nearly constant, although the hydrogen concentration varies greatly, because of the low desorption probability of H2 from Ca(H2) due to the high activation barrier. The latter is depicted by a simplified energy-potential surface.
differences between the chemical potentials is small [14,19e22]. This allows assuming similar hydrogen coverages on both sides of the membrane. However, the detailed materials properties may vary on a microscopic scale. Using technical membranes, grain boundaries and other defects may undermine the assumption. An upper limit is found by geometrical arguments: the continuity argument is valid on the surface as well. Therefore, the chemical potential parallel to the surface is averaged within a distance similar to the thickness of the membrane (typically of the order of 100 mm [14]). Vice versa, if the surface roughness is much below this value, the surface can be assumed to have equal hydrogen chemical potential. The hydrogenation properties of the surface (e.g., coverage) as the result of the constant chemical potential, though, might change upon the microscopic properties (crystal orientation, termination, contaminants). To consider these changes, it is helpful to recapitulate the conditions for surface reactions and UHV-technology. Although these two matters are related to different topics, that are science on one hand and technology on the other, the key parameters originate from the same thermodynamic relations. To simplify discussion, we first consider only hydrogen and metals, but similar estimates are valid for other gases, such as water, and other materials, such as polymers, as well. Hydrogen is physisorbed at a surface with a binding energy of around 5e10 kJ/mol [6,15]. Substantial amount of hydrogen is thus physisorbed only at high pressures or low temperatures. The enthalpy of chemisorption is higher by one order of magnitude than that of physisorption [6,15]. Hydrogen chemisorption is therefore thermodynamically possible at room temperature in pressures as low as 106 mbar for most electropositive metals. As a consequence, even under ultra-high vacuum conditions, most materials will eventually be covered by (atomic) hydrogen or other impurity gases. It is possible, though, to investigate clean metals by surface science methods [15]. Kinetics is the key. As a rule of thumb, pressure of 106 mbar corresponds to an impingement rate of one molecule per second and per surface site [16]. With typical measurement times of 1000 s, pressures below 109 mbar give acceptably clean results for freshly prepared surfaces. Further conditions for UHV-technology include the use of materials with a low vapor pressure, which mostly results from a high binding energy of the surface atoms to the bulk atoms (facilitated by either the atoms of the materials, e.g. copper, or by a strong binding of oxygen, which is forming an inert oxide skin as is the case of stainless steel or aluminium) [16]. The upper limit is
usually given by the mean free path of electrons in the chamber, which should be longer than the path length of the electrons through the spectrometer, and lies around 105 mbar [16]. Obviously, UHV-conditions do not coincide with experimental conditions to investigate gas e solid interactions with a high binding energy such as chemisorption. The enthalpy of formation and thus the equilibrium pressure of hydrogen e metal bulk system vary greatly. Bulk systems such as highly stable metal hydrides, e.g. CaH2, have plat- eau pressures compatible with UHV. “It is, however, virtually impossible to prepare the alka- line earth hydrides and related compounds by direct reaction with gaseous hydrogen” [16]. The reason lies in the relatively slow kinetics. Here the low pressure is a disadvantage: the impingement rate is directly proportional to the pressure. Furthermore, hydrogen molecules have to be split first, which is usually performed by a catalytically active cap layer, or by pre- viously hydrided Pd substrates [16]. In this approach, the Pd substrate is exposed to high hy- drogen pressure in a reaction cell, and is then quickly transported to a UHVchamber, where the alkaline earth metal is deposited onto the Pd hydride substrate and is simultaneously hy- drogenated [16]. This method cannot, however, control the chemical potential of hydrogen. Some metals, such as yttrium and other rare earth metals, can be hydrogenated directly from the gas phase because of their high reactivity [17]. However, though advantageous for hydro- genation, this is a major drawback for surface analysis, since increased surface reactivity also means increased adsorption of residual gases, such as oxygen and water, which are practically omnipresent in any hydrogen gas. Therefore, an additional hydrogen purification step is required. The desired high pressure of 1 bar implies that in hydrogen gas with purity level as high as 109, each surface atom is hit by one residual gas molecule per second. If the material is hydrogenated via the membrane, the vacuum level in front of the sensitive capping layer remains in the UHV range, i.e. << 106 mbar. In this respect Pd membranes have an advantage, as they are highly selective for hydrogen permeation, and thus provide very pure and atomic hydrogen.
3. Modelling surface coverage and permeation rates To model the surface kinetics of hydrogen permeation through a Pd-membrane, it is crucial to consider the site dependence of
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521
hydrogen in the materials (see Fig. 2): Hydrogen has a high binding energy at the surfaces, which corresponds to a high surface coverage even at high UHV-pressures (compare discussion above). The energy potential surface defines the forward and reverse reaction rate of hydrogen on the high pressure side of the membrane and of hydrogen desorbing into vacuum [6,18,19]:
E E þ Echem 2 Apfeed exp diss $ð1 qlow Þ2 2B$exp diss qlow kT kT
E Rf ¼ ApH2 $exp diss $ð1 qH Þ2 ; Rr kT Ediss þ Echem 2 ¼ B$exp qH kT
This equation is equal to the situation in which we have only one surface with negligible bulk interaction (except the factor 2). In particular, it follows that the coverage on the low vacuum side is defined by the applied pressure a:
2 E E þ Echem 2 Apfeed exp diss $ 1 qhigh B$exp diss qhigh kT kT E þ Echem 2 $qlow ¼ B$exp diss kT (2) The hydrogen coverage on high and low pressure side, respectively, qhigh, qlow define the chemical potentials at the surfaces. Their difference is the driving force for diffusion through the membrane, which is given by Fick's law [6]:
vc Dc zD vx d
(3)
where j is the diffusion flux, D is the diffusion coefficient, c is the concentration and x is the position. Here, we neglected the nonlinearity of the diffusion gradient through the membrane. We can give quantitative data. The flux through such a 0.15 mm thick Pd membrane was determined by the membrane approach using the mol [8]. The diffusion parameter of Pd is Sieverts method: jz108 cm 2s
106cms at the measurement temperature (320 K) [17]. Therefore, 2
8
2
mol ¼ 1020 H . To trans$cm concentration difference, Dc ¼ 10 $1:5$10 3 cm3 106 late this to the surface coverage, we compare the value to the maximum concentration of hydrogen in palladium, which is around one H per Pd [20]. The corresponding reference concen22
H. tration (concentration at around 200 mbars) is c0 ¼ 6:8$10 cm3 The continuity of the chemical potentials leads to the following relation between surface coverage and bulk concentration [9]:
q
msurface
e 1q
kT
¼
c mbulk e kT 1c
(4)
where msurface and mbulk are surface and bulk chemical potentials respectively. For small concentration changes, the relative concentration difference is equal to the relative coverage difference, i.e.
Dc c0
¼
Dq < 1% q0
(6)
(1)
where Rf and Rr are the rates of the forward and reverse reactions respectively, A and B are constants, Ediss and Echem are dissociation and chemisorption energies respectively, k is Boltzmann constant, T is temperature and qH is site coverage. In kinetic equilibrium, neglecting the pressure on the vacuum side (compare also Fig. 2) and defining the hydrogen pressure on the feed side as pfeed, one obtains
j ¼ D
¼0
(5)
This result is in agreement with the experimental result that hydrogen permeation through Pd membranes is limited by desorption at low temperatures [21,22]. Furthermore, one concludes that the concentration difference between the coverages at high and low vacuum side is negligible, i.e., qhigh zqlow , and thus
q¼
1 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ Ap2B exp Echem kT
(7)
feed
It is worth noting that the dissociation energy cancels out, because it is equal on both sides. Some expected dependencies result from the equation, such as the coverage on the low vacuum side approaches unity at high pressures, and/or large chemisorption enthalpy. The assumption that only two hydrogen sites per surface are required has recently shown to be too simple, at least three sites are needed [23]. However, this changes the math slightly (q2 /q3 , etc.), but not the overall picture. The situation is qualitatively different when the properties of the membrane sides are different, e.g. due to contaminants, which block desorption of hydrogen on the vacuum side. This was used to empirically demonstrate the shift in the rate limiting step by comparing the pressure dependence of the flux through clean and contaminated membranes [14]. To include this in the above described model calculations, we have to modify the number of available sites on the vacuum side. As not all sites are available anymore, the maximum coverage reduces by q00 ¼ uq0, in which u is one for a clean surface, and zero for a surface completely covered by, e.g., CO (see also Fig. 3). The desorption rate, R, at the low vacuum side is then developed from eqs. (1) and (2) using qlow ¼ 1uqhigh to
chem B$exp Ediss þE kT sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R¼ 1þ 12 B u exp Echem 1þ ApH kT
(8)
2
The model equation is plotted in Fig. 3. At very high pressures, the pressure dependence flattens out because all of the sites are occupied. Pressure dependence is only observed at lower pressures. The calculations confirm the experimental observations that (i) the overall rate from a contaminated surface is lowered, but (ii) the slope of the pressure dependence is increased for the contaminated surface compared to the clean surface as a result of a de- creased desorption rate on the low vacuum side. The approach was recently extended for ambient pressure XAS analysis [24]. Hydrogen permeation through AgPd alloy membrane clearly showed hydrogen pressure-dependent evolution of a spectral feature that corresponded to Pd 4p states, suggesting interaction of hydrogen with the latter. It is noteworthy, that, via this relatively simple system, a spectral connection between XPS and XAS can be established, which is in agreement with the theoretical connection of the two techniques through the electron binding energy [24]. This idea of membrane-based XPS was further expanded to include investigation of high pressure hydrogenation of materials other than Pd membranes by coating the surface with, e.g. Ca, as sketched in Fig. 2. In this case, the coverage difference induced by diffusion is negligible, as is desorption term on the vacuum side. The coverage on the high pressure side is then equal to a normal
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Fig. 3. Left graph: Flux (dp/dt) through a Pd membrane, measured as freshly built in (“dirty”) and after long-term sputtering (“clean”) adapted from Ref. 14. nsputt, nvac are the slopes of the double log plot of dp ¼ f ðpÞ. The right graph shows the rates as calculated using equation (8) with u ¼ 1 (clean) and u ¼ 0.1 (contaminated). dt
surface science setup, and the coverage on the low pressure side is defined by the corresponding chemical potential. It is worth noting that due to the low dissociation barrier of Pd, the assumption of equal chemical potentials on both side of the Pd-membrane coating system applies to virtually all materials. To summarize, the membrane approach is based on the following materials parameters: 1. The diffusion through the membrane must be fast enough to not be rate-determining. Thus, for hydrogen, Pd or similar materials have suitable properties. Simultaneously, the membrane thickness should be as thin as possible; we demonstrated the functioning of 150 mm thick Pd-membranes. 2. The surface coverage of stand-alone membranes is a consequence of the balance be- tween the adsorption and desorption kinetics of the surface. The coverage on the low vacuum side is defined by the applied pressure and chemisorption properties. 3. Any material, which can be deposited on such a membrane and has a hydrogen disso- ciation barrier higher than Pd (which is the case for most materials), can be investigat- ed. Important parameter here is the membrane thickness, as it has to be thin enough to allow fast diffusion. This may be a challenge, as the surface layer of just 1e10 nm thickness may be enough to represent bulk properties [25]. The following systems have been investigated by this method: Pd, and Pd-Ag membranes [14]. Hydrogen reduction of cobalt nitrate to metallic Co followed by XPS [14]. Hydrogenation of a metal-organic water reduction catalyst (Cobalt dioxime complex- es) [14]. Nanometer-thin polytetrafluoroethylene (PTFE) coating on Pd membrane. Hydrogen reduction of molybdenum oxide at room temperature. The PTFE coating has been demonstrated to increase catalytic activity via enhancement of Pd membrane permeability. The effect was attributed to lower hydrogen desorption barrier. Membranebased XPS could reveal the electronic structure of PTFE and Pd-
PTFE complex, allowing to identify the Pd-PTFE interaction [26]. In case of molybdenum oxide reduction, membrane-based XPS approach was applied for studying hydrogen gas facilitated reduction of MoO3 thin films. It could be established, that hydrogen intercalation into MoO3 results in HxMoO3 products, which slowly decompose into MoO2 þ1/2 H2O. The obtained results shed light on the debated mechanism of the hydrogen reduction in bulk molybdenum oxide [25]. The concept is not exhausted by the investigation of hydrogenation reactions. Key factor for the applicability of these methods is the availability of a membrane with fast diffusion of the target molecule. The membrane approach opens the possibility to perform XPS analysis of reactions in aqueous environment, which were previously inaccessible due to the UHV requirements of XPS. One of the problems that can be addressed by the given approach is mechanistic, time- resolved insight into, for example, heterogeneous solar water splitting reactions in aqueous environment. The field of artificial photosynthesis and a crucial part of it, solar water split- ting, is increasingly researched by the scientific community, driven by the ever increasing demand for sustainable sources of energy [27,28]. The membrane-based approach has the po- tential to provide detailed mechanistic insight via operando analysis of chemical states and electrochemically active regions in heterogeneous solar water splitting catalysis. Therefore, the adaptability of the membrane approach for investigating reactions in H2O, rather than H2 environment is currently being examined in our group. The reaction can be driven photo- as well as electrochemically. In case of the latter, an electrochemical micro-cell can be studied [10]. Briefly, inert thin gold membrane acts as one of the electrodes, while the stainless steel of the sample chamber acts as a counter electrode. This system has previously been demonstrated to provide quantitative information on in situ Si oxide growth at the Si-water interface under controlled electrochemical conditions. An important asset is the comparison with other techniques, in particular with near ambient pressure XPS [9]. Near ambient pressure XPS probes a sample environment, which reflects the situation of a surface in a high pressure gas environment. The sample surface studied by the membrane approach has the chemical potential of a sample exposed to the corresponding gas pressure. However, certain dynamic effects are thus missing or different, which has dis- advantages as well as advantages. The
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membrane surface is not identical to the gas exposed surface. Conclusions on dynamic phenomena are thus allowed only after careful modelling (see above), and are usually restricted to desorption phenomena. The membrane delivers highly pure atomic hydrogen to the sample, while nearby walls and compartments are exposed to UHV-compatible pressures only. This allows the hydrogenation of dissociatively less active surfaces. Furthermore, the sample is less prone to contaminations, which are produced at high gas pressures due to gas-solid interaction of the gas with the chamber walls. The applied hydrogen pressure of even 1 bar is compatible with the UHV-side analysis of the membrane approach. This allows for unperturbed XPS measurements as demonstrated. Furthermore, the membrane-based technique has a potential to be applied for other surface sensitive methods such as electron diffraction, reflecting high resolution energy loss electron spectroscopy, and X-ray absorption spectroscopy using low energy photons. Furthermore, the approach is based on a specific membrane holder, which can be adapted to the concrete dimensions of the analysis system. The capital investment is relatively low, when compared to ambient pressure photoemission systems. 4. Conclusions In this overview paper we review the fundamentals and demonstrated applications of the membrane approach to highpressure XPS as well future prospects and limitations. The quantitative model to describe the permeation rate based on surface kinetics is introduced to underline the general applicability of the approach. Acknowledgements This work was partly supported by the UZH-UFSP program LightChEC. Financial support from the Swiss National Science Foundation (grant numbers 144120 and 172662) is acknowledged. References [1] M. Bielmann, S. Kato, P. Mauron, A. Borgschulte, A. Züttel, Characterization of hydrogen storage materials by means of pressure concentration isotherms based on the mass flow method, Rev. Sci. Instrum. 80 (2009), 083901. [2] C. Rongeat, I. Llamas-Jansa, S. Doppiu, S. Deledda, A. Borgschulte, L. Schulz, O. Gutfleisch, Determination of the heat of hydride formation/decomposition by high-pressure differential scanning calorimetry (HP-DSC), J. Phys. Chem. B 111 (2007) 13301e13306. [3] R. Gremaud, C.P. Broedersz, D.M. Borsa, A. Borgschulte, P. Mauron, H. Schreuders, J.H. Rector, B. Dam, R. Griessen, Hydrogenography: an optical combinatorial method to find new light-weight hydrogen-storage materials, Adv. Mater. 19 (2007) 2813e2817. [4] V.M. Gryaznov, Surface catalytic properties and hydrogen diffusion in Palladium alloy membranes, Z. Phys. Chem. 147 (1986) 123e132. [5] M.H. Mintz, Y. Zeiri, Hydriding kinetics of powders, J. Alloy. Comp. 216 (1994) 159e175. [6] M. Martin, C. Gommel, C. Borkhardt, E. Fromm, Absorption and desorption
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