Hydrogenation Effect on Mg/Co Multilayer
Thin Films
M. K. JANGID, S.P. NEHRA AND M.SINGH
Thin Film & Membrane Science Laboratory, Department Of Physics, University of Rajasthan, Jaipur-302004 (India)
Abstract:
Multilayer Mg/Co thin films have been prepared using thermal evaporation method at pressure 10-5torr. Annealing
of structure has been performed in atmospheric condition at 600 K constant temperature for one hour. Hydrogenation of annealed thin films has been performed by keeping these in hydrogenation cell at different hydrogen pressures for 30 min. The UV–VIS absorption spectra of thin films have been carried out at room temperature in the wavelength range of 300–800 nm. The optical band gap was found to be increase and conductivity has been found to be decreased with hydrogen pressure. The relative resistivity varies nonlinearly with time and increases with hydrogen pressure. Raman spectra of these sample shows decreasing intensity of peaks with hydrogenation. These results suggested that multilayer Mg/Co thin films structures can be used for hydrogen storage as well as solar collector materials.
Keywords: Multilayer; Hydrogenation; Optical properties; I–V characteristics; resistivity.
1. Introduction
The use of thin films for hydrogen storage has become very important as the main process of absorption and desorption of hydrogen takes place on the surface of the material. Alloying of magnesium with transition metals can result in a substantially improved dissociation activity due to the presence of d-electrons in transition metals [Huot et. al.(2001)]. The hydriding / dehydriding kinetics of metal hydrides are relevant to areas of design and applications of various metal hydride devices, especially in energy conversion devices such as heat pumps, refrigerators, automobiles, power generators, batteries and thermal energy storage units [Goodell et.al.(1980)]. Several methods have been employed to overcome the kinetic limitation and thermodynamic stability of Mg-based hydrogen storage materials, including surface modifications addition of catalysts, and formation of metastable structures or multi-phase materials. A very interesting development in the metal hydrides was found [Huiberts et.al.(1996)] with the discovery of a spectacular change in the optical and electrical properties of thin films of Y and La hydrides near their metal–insulator transition as hydrogen is absorbed. The study of these metal–insulator transitions has been extended now to intermetallic hydrides such as magnesium based alloys [Giebels (2004)]. The electrical resistivity increases by almost three orders of magnitude between metallic Mg2Ni and transparent Mg2NiH4[Enache
et.al.(2004)]. Mg based films absorb hydrogen very easily , While in bulk samples hydrogenation requires high temperatures and high pressures, for thin films it occurs readily at room temperature and low pressures, and it opened the way to the third-generation switchable mirrors[Blomqvist (2003)]. So far all alloys of Mg with the transition metals (TM) Ni, Co, Fe, Mn, V have been found to switch with hydrogen [Richardson et. al.(2002)]. when exposed to H2, Pd-capped Mg-Ti thin films switch from a reflecting metallic state to a black, light absorbing,
semiconductor well agrees as reported by Pankove et al [Pankove et. al.(1983)]. The interaction of hydrogen in a metal hydride can be understood by using the anionic model [Sinha et. al.(1982)]. The increase in resistivity ratio with increasing hydrogen pressure means that hydrogen takes more electrons from the conduction band of the intermetallic compound and accelerates the hydrogen absorption capacity of samples [Singh et. al.(1991a); Singh et. al.(1991b); Singh et. al.(1992)]. In the present work have been studied the effect of hydrogenation on optical band gap, current-voltage characteristics and electrical resistivity of Mg/Co multilayer thin film structure for hydrogen storage.
2. Experimental
Multilayer Mg/Co thin films have been prepared by thermal evaporation technique. Mg granules (99.99%), Co powder (99.98%) pure were purchased from Alfa Aesar, Jonson Matthay Company, U.S.A. used for the present study, are placed into two different boats of tungsten in the vacuum chamber and Mg/Co (500nm) multilayer thin films are deposited on glass and ITO substrates by stacked layer method in situ evaporation and for this purpose vacuum chamber contains pressure of the order of 10-5torr. Multilayer Mg/Co was annealed in atmospheric
condition at 600 K constant temperature for one hour to get homogeneous mixture and inter-diffusion of thin film structures. For annealing used temperatures have been obtained and maintained with the help of Metrex Muffle furnace automatic controlled by digital microprocessor having temperature precision of ±1 K. Hydrogenation of Mg/Co multilayer has been performed by keeping these in hydrogenation cell, where hydrogen gas was introduced at different pressure (15, 30, 45 & 60psi) for half an hour. UV–VIS absorption spectra of thin films are taken at room temperature in the wavelength range of 300–800 nm with the help of Hitachi-330 spectrophotometer. Transverse I-V characteristics of samples have been recorded using Keithley-238 high current source measuring unit. The applied voltage was within the range of -2.0 to +2.0 volts with increasing step of 0.1 volt. For I-V characteristics, electrode contacts have been made using silver (Ag) paste on the thin films. I-V characteristics of thin film structure have been monitored with the help of SMU Sweep computer software. The resistivity of the thin films was measured by the Van der Pauw method at 300 K & various constant hydrogen pressures. Raman spectra of annealed and hydrogenated Mg/Co bilayer structure are taken by a continuous wave-Green laser with a constant wavelength 532 nm at room temperature by help of R-3000 Raman system. All the measurements have been performed at room temperature.
3. Results and discussion
3.1 Optical absorption and band gap of Mg/Co multilayer
The optical absorption spectra of Mg/Co multilayer thin films deposited onto a glass substrate were studied at room temperature in the range of wavelengths 300– 800nm. The variations of absorption % with wavelength (λ) are shown in Figure1. This shows that the absorption is decreased with hydrogen pressure. By studying absorption spectra of Mg/Co multilayer thin films, very useful information can be obtained about the energy gap, position of impurity levels in forbidden band, characteristics of optical transitions, etc. The optical band gap of these films have been calculated by using Tauce relation [Tauc (1974)] αhν = A (Eg-hν)1/2 where α is the absorption coefficient, hν is
the photon energy, Eg is the optical band gap andA is a constant. In this work, direct band gap was determined by plotting (αhν)2 vs hν curves, with the extrapolation of the linear region to low energies. Figure2 shows the graph
between (αhν)2 versus energy (eV) of photon on Mg/Co thin films assuming it to be the direct band gap material.
The intercept of straight line to energy axis has been used to find out the optical band gap. The value of optical energy band gap increases due to hydrogenation (Fiure3 & Table1). The band gap increase can be explained by two reasons: (i)The hydrogen is increased, the film structure transforms gradually to the crystalline phase, which has a larger band gap than the amorphous one and (ii) It may be due to hydrogen accumulates at interface and takes electrons from the conduction band of thin film which support the anionic model for hydrogen interaction with intermetallics[Bastide et. al.(1980);Bortz et. al.(1999);Sinha et. al.(1982);Trung et.al.(2009)].
3.2 Current- Voltage characteristics
300 400 500 600 700 800
0.2 0.3 0.4 0.5 0.6 0.7 Abso rp ti on % Wavelength (nm) a b c d
Mg/Co multilayer thin films (a) at 15 psi H2
(b) at 30 psi (c) at 45 psi (d) at 60 psi
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
0 2 4 6 8 ( h ) 2
h(eV)
a b c d Mg/Co multilayer thin films
(a) at 15 psi H 2 (b) at 30 psi (c) at 45 psi (d) at 60 psi
Fig.1. Optical absorption spectra of Mg/Co multilayer thin films. Fig. 2. Optical Band gap spectra of Mg/Co multilayer thin films.
10 15 20 25 30 35 40 45 50 55 60 65 1.90 1.95 2.00 2.05 2.10 2.15 2.20 B a nd G a p (e V)
Hydrogen pressure (psi) ) Mg/Co Multilayer thin films
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
-0.0012 -0.0008 -0.0004 0.0000 0.0004 0.0008 0.0012 Cu rren t (mA) Voltage (volt) a b c d
Mg/Co multilayer thin films (a) at 15 psi H2
(b) at 30 psi (c) at 45 psi (d) at 60 psi
Fig. 3. Variation in Optical Band gap with hydrogen pressure. Fig. 4. I-V characteristics of Mg/Co multilayer thin films.
3.3 Resistivity Measurements
Figure5 shows that hydrogen absorption increases with hydrogen pressure, which is indicated by the increasing resistivity ratio (ρH/ρo) due to the net effect of hydrogen absorption with pressure, where ρo is the resistivity before
hydrogen absorption and ρH is the resistivity of the sample after hydrogen absorption. The resistivity ratio increases
slowly with hydrogen absorption time and finally reaches the equilibrium position. It was also observed that resistivity increases with hydrogen pressure. The increase in resistivity ratio with increasing hydrogen pressure means that hydrogen takes more electrons from the conduction band of the intermetallic compound and accelerates the hydrogen absorption capacity of samples. It is suggested that the hydriding process needs a higher pressure for more hydrogen absorption in the same samples at the same temperature [Singh et. al.(1991a); Singh et. al.(1991b); Singh et. al.(1992)].
3.4 Raman spectroscopy
hydogen may change the phase or make the bonding with metal interstitial as well as surface locations [fukata et. al.(1997)].
0 10 20 30 40 50
0.5 1.0 1.5 2.0 2.5 3.0
Re
la
ti
ve Res
isti
v
it
y
Time (min.)
(a) at 15 psi H2 (b) at 30 psi (c) at 45 psi (d) at 60 psi
a b c d
0 100 200 300 400 500
1000 1500 2000 2500 3000
Int
e
ns
it
y
(
C
ou
nt
s
)
Wavenumber (cm-1)
a
b
c d Mg/Co multilayer thin film (a) at 15 psi H2 (b) at 30psi (c) at 45 psi (d) at 60 psi
Fig. 5. Hydriding kinetics of Mg/Co multilayer thin films. Fig. 6. Raman spectra of Mg/Co multilayer thin films.
H2 Pressure
(psi)
Band Gap (eV)
Conductivity (10-5Ώ-1-m-1)
Intensity of Raman peaks (Counts)
15 1.9755 8.9641 2825.2321
30 2.0146 6.7322 2273.0748
45 2.0697 5.0209 2125.0682
60 2.1248 3.4151 1806.1168
Table 1. Variations in optical band gap, Conductivity and Intensity of Raman peaks with H2 pressure
4. Conclusion
It is concluded from the above study that hydrogen gas tailored the optical and electrical properties of Mg/Co multilayer. The optical absorption% decreased with hydrogen pressure and also the band gap of thin films found to be increase with hydrogen pressure. I–V characteristics of annealed samples show semiconductor behavior and the conductivity have been found to be decreased with hydrogen pressure. The resistivity measurements show that hydrogen interacts with Mg/Co multilayer according to the anionic model and relative resistivity increases with hydrogen pressure, due to the net effect of hydrogen absorption with pressure. Decreasing in intensity of Raman peak may be due to phase transformation. The enhanced hydrogen affinity at the film substrate interface and suggested the study of complex hydride formation in thin films might give valuable information for the use of these Mg/Co multilayer thin film structures for hydrogen storage as well as solar collector materials.
5. Acknowledgments
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