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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 9 9 8e6 0 0 4
Avai lab le at www.sc iencedi rect .com
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Enhanced oxidation resistance of magnesium nanorodsgrown by glancing angle deposition
Salih U. Bayca a,*, Mehmet F. Cansizoglu b, Alexandru S. Biris b,c, Fumiya Watanabe c,Tansel Karabacak b
aUniversity of Celal Bayar, Soma Vocational School, Soma, Manisa 45500, TurkeybUniversity of Arkansas at Little Rock, Department of Applied Science, Little Rock, AR 72204, USAcNanotechnology Center, University of Arkansas at Little Rock, Little Rock, AR 72204, USA
a r t i c l e i n f o
Article history:
Received 4 July 2010
Received in revised form
26 January 2011
Accepted 27 January 2011
Available online 31 March 2011
Keywords:
Magnesium
GLAD
Hydrogen storage
Oxidation
Thin film
TGA
* Corresponding author. Tel.: þ90 236 612 00E-mail address: [email protected] (S
0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.01.152
a b s t r a c t
Oxidation behavior of magnesium thin films and nanorods were investigated in the
temperature range of 25e550 �C by using thermal gravimetric analysis. Arrays of vertical
magnesium nanorods were deposited by the DC magnetron sputtering glancing angle
deposition technique, while the magnesium thin films were deposited using the same
system but at normal incidence. The morphologies and corresponding crystal structure of
the samples were analyzed by scanning electron microscopy, transmission electron
microscopy and X-ray diffraction methods, respectively. We report that the Mg thin films
showed oxidation induced weight gain starting from room temperature. On the other
hand, Mg nanorods did not show any indication of significant oxidation at temperatures
below 350 �C. Enhanced oxidation resistance of Mg nanorods was also confirmed by quartz
crystal microbalance measurements. At temperatures higher than 350 �C, Mg nanorods
started to get oxidized and their weight increased at a similar rate to that of Mg thin films.
We argue that reduced oxidation of Mg nanorods is mainly attributed to their single crystal
nature. Magnesium nanorods’ reduced oxidation can potentially play a key role in
hydrogen storage and gas sensing applications.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction low and can lead to poor material properties especially at
Magnesium is among the lightest of all metals and as a result
it is used in an increased number of industrial applications
and processes as well as for products for which reduced
overall weight is essential, such as automobile parts, sporting
goods, and aerospace equipment [1]. Magnesium and its
alloys have also generated a high interest as advanced
materials for hydrogen storage and detection [2e11].
However, the oxidation resistance of magnesium is rather
63; fax: þ90 236 612 2002..U. Bayca).2011, Hydrogen Energy P
elevated temperatures. Several researchers have studied the
oxidation properties of magnesium and magnesium alloys
[12e15]. The oxide layer that can form at low temperatures
on the surface of magnesium and magnesium alloys is
generally in the form of MgO. The morphology of MgO layer is
porous and therefore it cannot act as an efficient barrier to
prevent the further oxidation of the metallic structure
[16e19]. Notable weight gain due to the oxidation process
was reported for Mg alloys at temperatures higher than
ublications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 9 9 8e6 0 0 4 5999
200 �C [20e22] for Mg ultrafine particles and Mg thin films [23]
at temperatures higher than 400 �C, respectively [24]. Studies
on the oxidation rate of magnesium and its alloys also
reported that they get further enhanced at temperatures
higher than about 400 �C [19,20,22e25]. However, there have
been reports indicating that thin films of Mg have been
observed to significantly get oxidized at temperatures as low
as 160 �C [26].
One of the most promising applications of the magnesium
based material systems is their use for solid-state hydrogen
storage environments because of their high storage capacity
(7.6 wt% for Mg), low material density, low cost, and vast
availability [27]. However, typical hydrogen absorption/
desorption temperatures of magnesium are elevated (�300 �C)due to the poor kinetics [28]. Recently, nanostructured
magnesium fabricated by the glancing angle deposition
(GLAD) technique [29e31] has been an attractive structure.
GLAD Mg nanostructures and nanostructures also incorpo-
rating catalyst additives [36e39] have been on the focus of
research for hydrogen storage and have significantly lower
absorption temperatures. Recently, Cansizoglu and Karabacak
reported significantly lower absorption temperatures of
hydrogen in Mg nanotrees produced by GLAD, where
a hydrogen storage value of w4.8 wt% was measured at
temperatures as low as 100 �C [40]. On the other hand, these
hydrogen storage experiments on nanostructured Mg either
with or without catalyst additives have been performed under
oxygen-free environments. There is a concern that high
surface area Mg nanostructures with small feature sizes can
seriously suffer from surface oxidation when exposed to
ambient or oxygen rich environments leading to low interac-
tion with gaseous hydrogen and poor absorption/desorption
kinetics. Therefore, there is a need to investigate the oxidation
properties of magnesium nanostructures in the presence of
oxygen, which will be quite critical especially for hydrogen
storage applications. It has been previously reported that
atomic hydrogen cannot diffuse trough the crystalline struc-
ture of the magnesium oxide films [26,41e43] which makes
MgO a diffusion barrier for H. In addition, Pranevicius et al. [44]
found that even small impurity levels of oxygen during
hydrogenation could result in the formation of a surface oxide
barrier.
The aim of the present study is to provide a detailed
investigation of oxidation properties of magnesium nanorods
fabricated by GLAD technique and compare this to that of
conventional magnesium thin films. Oxidation behavior of
magnesium samples has been studied using thermal gravi-
metric analysis (TGA) in dehumidified dry air supplied by
Airgas at temperatures ranging from room temperature
(w25 �C) to 550 �C. Mg nanorods and thin films have also been
characterized by X-ray diffraction (XRD) and scanning elec-
tron microscopy (SEM). The results of this study are believed
to be significant in elucidating the thermal behavior and
stability of GLAD-generated Mg nanostructures under various
temperature conditions. Such an understanding of the
behavior of various Mg structures in gaseous environments
could provide solutions to the current limitations faced by this
material for large scale usage in energy related applications
with a focus on their usage as the active component in high
efficiency hydrogen storage systems.
2. Experimental details
Glancing angle deposition (GLAD, also known as oblique angle
deposition) has been an effective technique to produce arrays
of various 3D nanostructures, in the form of nanosprings,
nanobeads, and nanorods [32e35]. During GLAD, the incident
beam of atoms is sent at a high angle (typically larger than 70�)as measured from the surface normal of a rotating or
a stationary substrate. Due to geometrical “shadowing effect”,
the incident vapor is preferentially deposited onto higher
surface features, which are better exposed to incident beam of
atoms. This preferential growth dynamic gives rise to the
formation of well-separated nanostructures, with dimensions
typically ranging from tens to hundreds of nanometers,
depending on the detailed growth parameters and material
being deposited.
In our experiment, samples of Mg thin films and GLAD
nanorods were deposited using a DC magnetron sputter
deposition unit. The substrates were polished alumina pieces
that were annealed at about 200 �C for 5 h under ambient
conditions before deposition in order to fully oxidize any
impurities present in the substrate. During the sputter depo-
sition of Mg, the base pressure was about 7.0 � 10�7 mbar, and
argon pressure was 2.7 � 10�3 mbar. The DC power used was
80 W and the distance between the center point of the
substrate holder and the Mg source (diameter of about 5 cm)
was about 20 cm. Conventional thin films samples were
deposited at an angle of q ¼ 0� and nanorods were deposited
using GLAD method at a deposition angle of q ¼ 84� (as
measured from the substrate surface normal). In addition,
both types of depositions were accompanied with an
azimuthal substrate rotation around the central substrate
surface normal axis with a speed of 1 rpm. Deposition rates
were measured to be 10.8 Ao/sec and 2.8 Ao/sec for normal
incidence and GLAD experiments, respectively. Silicon wafer
pieces have been used as substrates for the samples to be
analyzed for SEM and XRD. For TEM analysis, nanorods
deposited onto alumina substrates were scrapped off into
isopropyl alcohol and sonicated. Later this suspension was
placed on to holey carbon grids.
Thin films and nanostructured samples of Mg were
analyzed using SEM (FESEM-7000F, JEOL) for the morpholog-
ical investigation and XRD system (Bruker D8 discover) for the
crystal orientation (texture) investigation. TEM analysis was
performed by a JEOL JEM 2100F. Film thicknesses were
measured using the cross sectional SEM images.
Oxidation experiments of the Mg nanorods and Mg thin
films were performed by using TGA (METLER TOLEDO 815e) at
temperatures ranging 25e550 �C in dry air. A heating rate of
3 �C/min was used for the experiments. The weight change
was recorded as a function of time (for isothermal TGA
measurements) or temperature (for variable temperature
measurements). Isothermal TGA experiments were carried
out at constant temperatures of 150, 250, 300 and 350 �C. Thefinal mass values presented in this work were obtained by
subtracting the bare alumina weight data from that of coated
alumina samples of identical dimensions.
As it will be discussed in the following section, during some
of the TGA experiments, we observed a weigh loss for our
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 9 9 8e6 0 0 46000
nanorod samples. Mg has a high vapor pressure and may
show evaporation around 400 �C [23,52]. The origin of weight
loss although not entirely clear is believed to be due to either
the sublimation of Mg (due to magnesium’s high vapor pres-
sure at low temperatures) or the evaporation of water vapor
adsorbed onto the high surface area of the nanorod samples.
In order to investigate this observation, we utilized a new
quartz crystal microbalance (QCM) system that enabled us to
measure possible weight changes of the Mg nanorod coatings
under vacuum (i.e. free of water vapor on the surface of
nanorods) conditions. QCM crystals are typically used in
conventional thin film thickness monitoring units and are
sensitive to very small changes in the mass of a coating as
small as in the scale of nanograms. To perform the mass
change detection vs. temperature experiments using QCM
crystals as a substrate for our nanorod and thin film coatings,
we have developed a custom made chamber that can operate
at temperatures ranging from room temperature up to 500 �Cas well as at pressures from vacuum (10�3 bar) up to 50 bar.
The chamber has a dual QCM sample holder configuration.
One holder is reserved for a bare QCM crystal used for the
control sample while a 2nd holder is for the actual sample
measurements.We depositedMg nanorods onto QCM crystals
using GLAD as described above. Before Mg deposition, the
oscillation frequency of the bare QCM crystal wasmeasured at
atmospheric pressure and at a pressure of 10�3 bar while at
room temperature in order to calculate the pressure effect on
the crystal. The total Mg deposition amount on the crystal was
calculated comparing the frequency values before and after
Fig. 1 e Top view (top row) and cross section view (bottom row) s
of thickness about a) 250 nm and b) 1700 nm that were deposit
deposition. The sample to be tested was placed inside the
chamber and evacuated for 2 days at 10�3 bar of pressure. This
enabled us to remove the water present on the sample surface
and inside the chamber. Once the frequency stabilization was
achieved in both QCM crystal oscillations, the temperature
was increased from room temperature to 250 �C in 3 h at
a steady rate. Then the system was cooled down to room
temperature. The initial and finalmass values before and after
QCM tests were compared under the same conditions in order
to calculate the weight percentage changes in the mass of Mg
nanorod coatings.
3. Results and discussion
Both thin films and nanorod-structured samples of Mg were
grown with various thicknesses in order to understand their
morphological and crystal orientation evolution. Fig. 1 shows
the top view and cross sectional SEM images of some of theMg
thin film samples deposited at normal incidence. Mg thin
films quickly form a rough morphology during the early
deposition stages (Fig. 1a). As the film gets thicker (Fig. 1b),
a highly textured microstructure incorporating a cheese like
fine porosity was found to develop. This porosity might be due
to the limited mobility of the Mg atoms during the sputter
deposition process, while the substrate temperatures are
typically low (i.e. lower than about 50e100 �C), which is
correlated to lower adatommobility values [45] Sputtered thin
films also show a columnar microstructure as can be seen
canning electron microscopy (SEM) images of Mg thin films
ed at normal incidence.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 9 9 8e6 0 0 4 6001
from the cross sectional SEM images in Fig. 1 (bottom row). In
addition, the XRD analysis presented in Fig. 2a for the sputter-
deposited Mg thin films show a pattern that promotes
a texture formation in the (002) direction which is indepen-
dent of the film thickness value.
On the other hand, GLAD Mg nanorods show a different
morphology than that of the Mg thin films. Unlike continuous
Mg films, as can be seen in Figs. 4 and 5, the nanorods form in
the shape of isolated columnar arrays with an average diam-
eter of about 120 nm, and separated by gaps in the range of
a few tens of nanometers. In addition, the XRD patterns of the
GLAD nanorods differed significantly from that of the thin
films deposited at normal incidence (Fig. 2b). Contrary to
strongly (002) oriented Mg thin films, the crystallographic
analysis of the nanorods presents peaks corresponding to the
crystalline directions of (101), (102) and (103) in addition to the
(002) peaks. These peaks other than (002) become more
intense when the nanorods get longer, while the change in
(002) orientation is minimal for thicknesses larger than
900 nm. The formation of crystal orientations other than the
preferential (002) texture in sputtered GLAD Mg nanorods is
Fig. 2 e X-ray diffraction (XRD) profiles of (a) Mg thin films
deposited by magnetron sputtering at normal incidence
and (b) Mg nanorods deposited by glancing angle
deposition (GLAD) show the evolution crystal orientation
as a film gets thicker. Line profiles have been off-set in the
y-axis for clarity.
believed to be due the shadowing effect and limited adatom
mobility of these crystal orientations [46,47]. During the initial
stages of GLAD, Mg atoms can form randomly oriented
islands. Some of these islands have higher vertical growth
rates due to the lower adatommobility on their crystal planes
(e.g. due to a more atomically rough crystal plane), where
others have faster lateral growth rate due to higher adatom
mobilities (e.g. on a atomically smooth closed packed crystal
plane like for example on Mg (002)). Due to shadowing effect,
obliquely incident atoms preferentially deposit on higher
islands of lower adatom mobility and higher vertical growth
rates, leading to nanorods with energetically un-preferential
crystal orientations. However, in our case it seems that
roughness of the GLAD Mg samples, and therefore the shad-
owing effect couldn’t develop strong enough at thicknesses
smaller than 900 nm, leading to still a dominant (002) texture.
As the nanorods get longer and surface gets rougher, shad-
owing effect can become more effective to promote the
growth of nanorods with other crystalline orientations.
TEM analysis on the GLAD Mg nanorods revealed a single
crystal structure (Fig. 3). In Fig. 3, single set of spots in the
selective area diffraction (SAD) pattern and high resolution
TEM image of a single nanorod both show that individual
nanorods of Mg have a single crystal formation. SAD pattern
corresponds to the hexagonal crystal structure on the sidewall
where the diffraction pattern was taken. Single crystal nature
of the individual Mg nanorods is similar to the previously
reported results on metallic nanorods deposited by GLAD
[46,48e50]. Single crystal structure is an important property in
explaining the oxidation behavior of the Mg nanorods.
Variable temperature TGA result of a conventional Mg thin
film is shown in Fig. 6. TGA datawas recorded forMg thin films
with a thickness of about 1100 nm. Actual mass values were
normalized to the initial value of the Mg thin film. These films
were found to continue gain weight due to the oxidation for
temperatures of up to 550 �C, with the exception of a small
plateau between 250 and 350 �C. Total change in the weight
was as high as about 85 wt% (weight percent) indicating an
oxygen rich stoichiometry compared to the crystalline MgO.
The rate of weight gain slightly slowed down as the temper-
ature increased from room temperature. This parabolic
behavior takes place due to the slower oxidation rate of Mg,
which originates from the diffusion barrier properties of the
oxide layer [23,52]. As the oxide layer gets thicker, it impedes
the oxygen from passing through and reaching the metallic
magnesium underneath the oxide layer [22,51]. The plateau
observed between temperatures of about 250e350 �Cmay also
be ascribed to the competing reactions of already slowed
down oxidation rate (i.e. smaller weight gain) and evaporation
of water vapor trapped in the small pores of Mg film (i.e.
weight loss).
The weight change profile of Mg nanorods (1050 nm in
height and morphology similar to the one shown in Fig. 5b),
for variable temperature TGA has a quite different behavior
than that of theMg films. In Fig. 6 it is observed that theweight
of the Mg nanorods decreased by about 2 wt% as the
temperature increased from 25 to 150 �C. This slight weight
lost is believed to be the result of the evaporation of water
vapor adsorbed on the surface of the magnesium nanorods.
The possibility of sublimation of magnesium has been
Fig. 3 e Transmission electron microscopy (TEM) results of an individual GLAD Mg nanorod are shown: (a) TEM selective
area diffraction (SAD) pattern showing an hcp crystal structure, (b) High resolution lattice imaging, and (c) TEM image.
Fig. 4 e Top view (a) and cross section view (b) SEM images
of Mg nanorods by GLAD of thickness about 200 nm.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 9 9 8e6 0 0 46002
discarded after our QCM tests performed at temperatures
from 25 to 250 �C (described above) where no measurable
mass loss has been observed under water vapor free condi-
tions. Fig. 7, showed that the mass of the nanorods remained
relatively constant between temperatures from 150 to 350 �C.Above 350 �C, magnesium nanorods begin to gain weight with
increasing temperature up to 550 �C. This weight gain of about
6 wt%, corresponds to an oxidation process which is much
smaller than that of conventional Mg film (w85 wt%). We also
have recorded a weight gain due to the structural oxidation of
the Mg nanorod samples of about 12 wt% during the QCM
measurements. Although the QCM experiments were per-
formed under vacuum (10�3 bar) we believe that this level of
vacuum was not high enough to avoid oxidation.
Reduced oxidation values of Mg nanorods compared to
conventional thin films are believed to mainly originate from
their single crystal property. In a single crystal structure, there
are no grain boundaries present and therefore the diffusion
rate of oxygen is significantly reduced. In addition, possible
crystal orientation on the sidewalls of the nanorods might
present a less reactive plane for oxygen penetration and oxide
formation. For example, Mg basal (001) plane is about 25%
more reactive than (100) prismatic plane [23] in oxidation. On
the other hand, some of theMg nanorods have un-preferential
crystal orientations (Fig. 2) with non-closed pack crystal
planes, which could normally enhance diffusion of oxygen,
yet it seems that single crystal property dominated the
process leading to enhanced resistance to oxidation.
In addition, isothermal TGA analyses were carried out to
examine weight changes of the Mg nanorods at constant
temperatures of 150, 250, 300 and 350 �C. The TGA results for
these temperatures are shown in Fig. 7 for up to 180 min. All
the isothermal lines show a similar behavior. At the initial
times there is a mass loss mainly due to the evaporation of
water vapor followed by a slightly fluctuating behavior espe-
cially at temperatures higher than 150 �C. The fluctuation at
later times can originate from the competition between the
Fig. 5 e Top view (top row) and cross section view (bottom row) SEM images of Mg nanorods by GLAD of thickness about a)
900 nm and b) 1050 nm.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 9 9 8e6 0 0 4 6003
weight gain due to the low oxidation of nanorods and evap-
oration of trapped water vapor in the smaller gaps of the
structures. However, the final weight gain after the initial
release of the water vapor is still insignificant (max value
0 100 200 300 400 500 600
1.0
1.2
1.4
1.6
1.8
Mg nanorods
).u.a( ssam dezila
mroN
Temperature (oC)
Mg thin film
Fig. 6 e Thermogravimetric analysis (TGA) results of
magnesium thin film andmagnesium nanorods are shown
as a function of temperature. Actual mass values of the
coatings were normalized to their initial values for
comparison.
being about 3 wt% at 300 �C). These results also show the
reduced oxidation property of Mg nanorods produced by
GLAD, which is consistent with the variable temperature TGA
and QCM results discussed above.
Fig. 7 e Isothermal TGA plots of magnesium nanorods at
150, 250, 300, and 350 �C show the change in dynamic
weight of nanorods as a function of time. Actual mass
values of the coatings were normalized to their initial
values for comparison.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 9 9 8e6 0 0 46004
4. Conclusions
In this study, we report that Mg thin films showed significant
oxidation with weight gains of as high as about 85 wt%, in
temperature ranges from 25 to 550 �C. On the other hand, the
oxidation of Mg nanorods grown by GLAD was minimal with
a weight change of as little as 6 wt% in similar conditions.
Our QCM studies also further confirmed the reduced oxida-
tion of Mg nanorods. Because of their nanostructured large
surface area morphology, one might normally expect an
enhanced oxidation in nanorods of Mg. The unusual resis-
tance of Mg nanorods to oxidation is believed to originate
mainly from their single crystal nature where no grain
boundaries for fast-diffusion of oxygen are present. In addi-
tion, possible crystal orientation on the sidewalls of the
nanorods might present less reactive planes for oxygen
penetration and oxide formation. These results suggest that
GLAD Mg nanorod arrays with or without catalyst additives,
or their alloys can be attractive candidates especially for
hydrogen storage applications where reduced oxidation
property is highly desirable.
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