Conference-ALC ’ 15-Hydrogen Storage of Pd / Mg composite Nanoparticles Fabricated by Gas Evaporation Method

Mg nanoparticles covered with the Pd nanoparticles (Pd/Mg NPs) have been fabricated by the gas evaporation method using He gas. The coaxial formation of the Mg and Pd NPs enables to fabricate the Pd/Mg NPs. The surfaces of the Pd and Mg NPs are quite clean because only pure He gas has been used during the fabrication of the NPs. The surface and interface chemical states of the Pd/Mg NPs have been investigated by X-ray photoelectron spectroscopy (XPS) without the exposure to the air. The XPS investigation has revealed that the formation of the interfacial alloy state between the Mg and Pd NPs. The Pd/Mg NPs can absorb the hydrogen at the room temperature by the hydrogenation of the most of the Mg atoms inside the Pd/Mg NPs. [DOI: 10.1380/ejssnt.2016.150]


I. INTRODUCTION
Nanoparticles (NPs) attract scientific interests in many fields such as the surface science and catalytic chemistry because of the high surface activity and size effect.The NPs also show the remarkable aspect as the hydrogen storage material in the last two decades [1].For instance, Pd NPs exhibit interesting hydrogen absorption properties [2,3].Pd is well known as the hydrogen storage material which can absorb and desorb the hydrogen reversibly at room temperature.The Pd bulk forms PdH 0.7 as the result of the hydrogen absorption.The size down of the Pd crystal grain causes drastic decrease of hydrogen storage capacity.The hydrogen storage capacity of the Pd NPs (average diameter: 4.1 nm) is up to 0.4 under 760 Torr of hydrogen and room temperature [2,3].This decrease of the hydrogen storage capacity is attributed to the change of the electronic structure of Pd near fermi level due to the size down of the Pd crystal grain [3].On the contrary, the advantage of the reduction of the size of crystal grain has been reported in the case of the hydrogen storage of the Mg NPs [4,5].Mg is most promising material for the practical hydrogen storage application because Mg is light and inexpensive metal.Moreover, Mg stores the hydrogen up to 7.6 wt% by forming the hydride (MgH 2 ).The problem for the practical use of Mg is the high temperature more than 300  C because of the enhancement of the kinetics of hydrogenation by the decreasing of the diffusion path length of hydrogen atoms [5].
Further improvement of the hydrogen storage property of the Mg NPs can be established by the combination with the catalytic metals for the hydrogen dissociation reaction such as Pd.The Mg NPs suffer from the low surface activity which causes the difficulty of the hydro-ISSN 1348-0391 c ⃝ 2016 The Surface Science Society of Japan (http://www.sssj.org/ejssnt)gen dissociation reaction on the surface of the Mg NPs.The Pd NPs have not shown the high storage capacity compared with the Pd bulk, however, the surface activity is greatly enhanced [6].The ideal NP for the hydrogen storage is the Mg NP covered with the Pd NPs (Pd/Mg NPs).The hydrogen molecules dissociate on the surface of the Pd NPs and the hydrogen atoms diffuse into the Mg NPs through the interface between the Pd NPs and Mg NPs.
In this study, we have fabricated the Pd/Mg NPs by the gas evaporation method [6].The gas evaporation method provides the NPs without any chemical compounds such as the ligands or surfactants which cause the loss of the surface activity of NPs.Furthermore, the gas evaporation method is suitable for the fabrication of the composite materials compared with the wet chemical methods.It is difficult to control the nucleation, alloying and deposition processes by the wet chemical methods due to the variation of the reduction rate of the metal elements which limits the fabrication of the nano-composites.We can carry out the each process individually in the separated chambers by the gas evaporation method.The NP of the most of metal elements and these composites can be created if the evaporation of each metal is possible.
The characterization of the surface and interface chemical state of the Pd/Mg NPs has been carried out by X-ray photoelectron spectroscopy (XPS).The hydrogen storage property of the Pd/Mg NPs has been investigated using the quartz crystal microbalance (QCM) which enables us to obtain the mass change of the Pd/Mg NPs during the hydrogenation reaction without handling of the NPs in the air.

II. EXPERIMENTAL
Pd/Mg NPs were fabricated by the gas evaporation method using He gas (99.99995%) [6]. Figure 1 shows the schematic view of the apparatus for the fabrication of the Pd/Mg NPs.The apparatus was composed of the fabrication chambers and the deposition chamber.The fabrication chambers were filled with He gas and evacuated by differential pumping by the vacuum pumps via the deposition chamber.The rod shaped Mg was evaporated under the He gas atmosphere (80 Torr) in the fabrication chamber 1 in Fig. 1.The evaporated Mg atoms collided with He atoms and aggregated with each other to form the Mg NPs.The Mg NPs were transferred toward the fabrication chamber 2. The fabrication chamber 2 equipped two Pd wires which act as the evaporation source of the Pd atoms.The Mg NPs passed between the Pd wires and were covered with Pd NPs to form the Pd/Mg NPs.The Pd/Mg NPs were deposited on the Si wafer in the deposition chamber.The deposition rate of the NPs was monitored by the QCM.
The shape of the Pd/Mg NPs was observed by the transmission electron microscopy (TEM, JEM-2500SE JEOL, 200 kV).The Pd/Mg NPs deposited on the Cu grid were transferred from the fabrication chamber to the TEM instrument via the ambient air.
The analysis for the surface and interface chemical states of the Pd/Mg NPs was carried out by the X-ray photoelectron spectroscopy (XPS).The XPS chamber was connected with the deposition chamber of the NPs.The Pd/Mg NPs were transferred to the XPS chamber via high vacuum path which enabled minimization of the surface oxidation of the Pd/Mg NPs.The Mg Kα line (1253.6eV) was used as the X-ray source.The photoelectron spectra were obtained using the concentric hemispherical analyzer (CHA, Phoibos 100 SPECS GmbH) with the channeltron detectors.
In order to investigate the hydrogen absorption property of the Pd/Mg NPs, we measured the pressurecomposition isotherm (PCI) of the Pd/Mg NPs using the QCM [2,7].The QCM is sensitive for the mass change of the loaded materials on the quartz crystal (less than 1 ng).Moreover, the other favorable point of the QCM is that the QCM sensor head can be installed into the vacuum chamber which enables us to obtain the PCI of the NPs without the air oxidations.The Pd/Mg NPs were deposited on the Au coated quartz crystal and exposed to the pure H 2 gas (99.99999%) at 303 K.The amount of the Pd/Mg NPs was 26.8 µg.The nominal weight ratio (Mg/Pd) obtained by QCM was 0.70.

III. RESULTS AND DISCUSSIONS
Figure 2 shows the typical TEM images of the Mg NPs and the Pd/Mg NPs.The shape of the Mg NPs is typically hexagonal or tetragonal [8].The Mg NPs show the clear facets corresponding to the crystal faces of hcp structure.On the other hand, the surface of the Pd/Mg NP is quite rough and the fine particles are deposited on the surface.The difference of the contrast in TEM images represents that the smaller particles are composed by the heavier elements compared with Mg.As the results, the TEM images in Fig. 2 reveal that the Pd NPs cover the surface of the Mg NPs partially.The sizes of the Mg NPs and the Pd NPs distribute between 20-50 nm and 2-5 nm, respectively.In addition to the Pd/Mg NPs, many Pd NPs can be seen in the TEM images in Fig. 2 (B) which means that the amount of the Pd NPs is enough to cover the surface of the Mg NPs.
Figure 3 shows Pd 3d and Mg 2p core electron spectra of the Pd/Mg NPs obtained by XPS.The peak position of the Pd 3d spectrum for the Pd/Mg NPs corresponds with that of the Pd sheet as the standard of the metallic state (Pd 0 ).The surface chemical state of the sample is quite similar with the Pd 0 state because of the many Pd NPs in the sample which can be seen in Fig. 2. The surface chemical state of the Pd/Mg NPs can be seen more clearly in the Mg 2p spectrum.Note that the Pd 4p spectrum overlaps with the energy region of the Mg 2p spectrum (∼50 eV).In order to extract the Mg 2p from the composite spectrum of the Mg 2p and Pd 4p, the Pd 4p spectrum of the Pd sheet has been subtracted from the composite spectrum of the Mg 2p and Pd 4p of the Pd/Mg NPs.The peak area of the Pd 4p spectrum of the Pd sheet has been normalized with respect to the peak area ratio of the Pd 3d spectra between the Pd/Mg NPs and the Pd sheet.The subtracted spectrum represents the Mg 2p core electron spectrum and it can be deconvoluted into two components, one is the peak at 50.9 eV which can be attributed to the MgO or Mg(OH) 2 (Mg 2+ ) and the other peak at 49.6 eV which is located slightly lower binding energy side compared with the metallic state of Mg (Mg 0 at 49.8 eV).This component can be assigned to the Mg-Pd intermetallic alloy state at the interface between Mg NPs and Pd NPs.The stable intermetallic phases are reported such as MgPd(P m 3m), Mg 5 Pd 2 (P 63/mmc) and Mg 6 Pd(F m 3m) [9].The slight peak shift toward lower binding energy side implies the transfer of the valence electron from Pd to Mg which is consistent with the peak shift of Pd3d spectrum of the Mg-Pd alloy state [11].
Previous studies about thin films have reported that the Mg-Pd interfacial alloy state brings the benefits to the hydrogen storage property [11,12].The Mg thin film capped with Pd layer has the interfacial alloy between Mg film and Pd layer.The interfacial alloy enables the thin film to absorb the hydrogen gas at room temperature because the Mg-Pd alloy improves the diffusion of the hydrogen from Pd to Mg [11].Furthermore, the enthalpy of the hydride forming reaction is tunable by the thickness of the interfacial alloy layer [12].The equilibrium pressure for the hydrogenation depends on the degree of the intermixing between Mg and Pd.This means that the thermodynamics of the hydrogen storage of the Mg-Pd system can be controlled by the alloying between Mg and Pd.
The PCI for the hydrogen absorption of the Pd/Mg NPs is shown in Fig. 4. The PCI for the Pd NPs and the equilibrium pressure of the Mg and Pd bulk are also shown in Fig. 4. The PCI for the Pd/Mg NPs has two plateau regions at 5 × 10 −2 Torr and 14 Torr which represent the equilibrium pressures of the hydrogenation.These plateau regions can be attributed to the hydrogenation of the Mg and Pd in the Pd/Mg NPs, respectively [2,13,14].The equilibrium pressure of Mg in the Pd/Mg NPs is enhanced compared with that of the Mg thick film capped by Pd (less than 1×10 −2 Torr [13]) due to the formation of the interfacial alloy between Mg NPs and Pd NPs [12].In addition to the enhancement of the equilibrium pressure, the plateau region of the Mg is seen over the wide pressure range from 0.01 to1 Torr.This wide plateau implies that the Mg NPs have the wide distribution of the site energies for the hydrogen occupation in the Mg NPs [3,15] is also shown as the reference [2].The horizontal broken lines represent the equilibrium pressure of the hydrogenation of Mg and Pd bulk [13,14].
the Mg atoms (surface or bulk) and the particle size.The wide size distribution of the Mg NPs leads the wide distribution of the site energy and thermodynamical properties for the hydrogenation of the Mg.The dehydrogenation of the Mg can not be observed in this system at the room temperature due to the irreversible change of the interface chemical state between Mg and Pd [16].
If the Mg atoms in the Pd/Mg NPs could be fully hydrogenated, the hydrogen storage capacity of Mg is expected to be 3.1 wt% which can be calculated simply with weight ratio (Mg/Pd= 0.70) and the ideal hydrogen storage capacity of Mg (7.6 wt%).The PCI shows that the hydrogen storage capacity of the Mg atoms is 2.8 wt% in Fig. 4.This result indicates that most of the Mg atoms in the Pd/Mg NPs (89%) have been hydrogenated at the room temperature (303 K).The Mg NPs can not absorb the hydrogen at the room temperature due to the low surface activity for the dissociation reaction of H 2 molecules.In the case of the Pd/Mg NPs, as shown in TEM image in Fig. 2, the Mg NPs have been covered partially with Pd NPs.The Pd NPs enhance the surface activity and enable hydrogen absorption of the Mg NPs at the room temperature.The composite Mg NPs with the catalytic NPs can exhibit the remarkable hydrogen storage properties.In analogy with Pd, the improvement of the surface activity of the Mg NPs will be established by Ti NPs.Ti is also well known as the good hydrogen storage metal.In addition to the scientific attraction, Ti is inexpensive compared with Pd which is favorable for the reduction of the cost of the materials.

IV. CONCLUSION
The Pd/Mg NPs have been fabricated by the gas evaporation method using He gas.The surface of the Mg NPs has been covered with the Pd NPs partially and enhanced the surface activity for the dissociation reaction of H 2 molecules.The surface chemical state of the Mg NPs is composed of the interfacial alloy with Pd and the oxide (Mg 2+ ).The investigation of the hydrogen storage property of the Pd/Mg NPs has revealed that the most of the Mg atoms inside the Mg NPs can be hydrogenated at the room temperature by the covering with Pd NPs.

FIG. 1 .
FIG.1.Schematic view of the fabrication chamber of the Pd/Mg NPs by the gas evaporation method using pure He gas (99.99995%).

FIG. 2 .
FIG. 2. Bright field image of (A) Mg NP and (B) Pd/Mg NP obtained by TEM observation.

FIG. 3 .
FIG. 3. XPS spectra for (a) Pd 3d and (b) Mg 2p core electron of the Pd/Mg NPs.The Pd 3d spectrum of the Pd sheet is also shown as the reference.The Mg 2p spectrum has been obtained by the subtraction of the Pd 4p (Pd sheet) spectrum from the Mg 2p+Pd 4p spectrum (Pd/Mg NPs).The Mg 2p spectrum is deconvoluted into two components and the background is approximated by the Shirley method using CasaXPS software [10].
FIG.4.PCI for the hydrogen absorption of the Pd/Mg NPs at 303 K.The PCI has been obtained by QCM without the exposure of the Pd/Mg NPs to the air.The PCI for the hydrogen absorption of the Pd NPs (average diameter is 4.1 nm) is also shown as the reference[2].The horizontal broken lines represent the equilibrium pressure of the hydrogenation of Mg and Pd bulk[13,14].
• C required for hydrogen absorption and desorption reactions.This problem can be solved by the formation of NP.Wagemans et al. have revealed by the theoretical calculation that the size down of the Mg crystal grain leads the thermodynamical destabilization of Mg and MgH 2 [4].Jeon et al. have reported that Mg NPs (average diameter: 5 nm) can absorb and desorb the hydrogen at 200