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Structural and hydrogenation studies of ZnO and Mg dopedZnO nanowires

Jai Singh, M.S.L. Hudson, S.K. Pandey, R.S. Tiwari, O.N. Srivastava*

Hydrogen Energy Centre, Unit on Nanoscience & Technology, Department of Physics, Banaras Hindu University, Varanasi 221005, India

a r t i c l e i n f o

Article history:

Received 10 March 2011

Received in revised form

30 March 2011

Accepted 2 April 2011

Available online 7 July 2011

Keywords:

ZnO nanowires

Hydrogen storage

XRD

Rietveld

X-ray diffraction

* Corresponding author.E-mail address: hepons@yahoo.com (O.N

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.04.010

a b s t r a c t

In this work, Mg doped zinc oxide (MgxZn1�xO, x ¼ 5, 10 and 20 at. %) nanowires were

successfully prepared by two step process. Initially, ZnO nanowires were grown by thermal

evaporation of Zn powder under oxygen atmosphere. Mg powder was doped in as grown

ZnO through solid state diffusion at low temperature. Energy dispersive x-ray spectroscopy

(EDAX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UVeVisible

absorption spectra analysis reveals that the Mg doping on ZnO nanowires induces lattice

strain in ZnO. Rietveld analysis of XRD data confirms the wurtzite structure and a contin-

uous compaction of the lattice (in particular, the c-axis parameter) as x increases. The

hydrogenation properties of ZnO nanowires and Mg doped ZnO (MgxZn1�xO, x ¼ 0, 5, 10 and

20 at. %) nanowires were studied. The hydrogenated samples were further investigated

through XRD and Fourier transform infrared spectroscopy (FTIR). The hydrogen storage

capacity of as grown ZnO nanowires has been estimated to be 0.57 wt. % H2 at room

temperature. However, the hydrogen storage capacity gets increased to w1 wt. % upon

doping ZnO with 10 at. % Mg. Further increase in Mg concentration decreases the hydrogen

storage capacity of ZnO nanowires. Thus for 20 at. % Mg doped ZnO; the hydrogen

absorption capacity gets decreased from w1 wt. % to 0.74 wt. %. The mechanism of

hydrogen storage in ZnO nanowires and Mg doped samples of ZnO has been discussed.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction respect to energy storage. Nanostructured systems including

As nanotechnology progresses and complex nanosystems

fabricated, a rising impetus is being given to the development of

multi-functional and size-dependent materials. Recent devel-

opments in nanoscience and nanotechnology have not only

brought potential building blocks for nanoscale electronic,

optoelectronic and medicines, but also offer a breakthrough in

hydrogenstorage [1,2].Thesurface/volumeratio increasesas the

material dimension decreases to nano-order. The high surface/

volume ratio of nanomaterials has significant implications with

. Srivastava).2011, Hydrogen Energy P

carbon nanostructures and metal organic frameworks are

considered to be potential candidates for hydrogen storage [3].

Additionally, nanomaterials are found to be superior catalyst for

hydrogen storagematerials [4]. One class of nanomaterialwhich

is receiving considerable attention recently is, 1D ZnO nano-

structures, having wide band gap (w3.37 eV). The potential

application of thiswide band gap semiconductor oxide includes

ultraviolet optoelectronic devices, light-emitting diodes and

photodetectors [5e10]. The ZnO nanostructures are also being

used in several industrial applications such as medicine, gas

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e (a) Schematic illustration of the experimental setup

used for synthesis of ZnO Nanowires. (b) Photographic

image of the ZnO product synthesized by thermal

evaporation method, as synthesized cotton-like product is

easily observed.

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 7 ( 2 0 1 2 ) 3 7 4 8e3 7 5 4 3749

sensors, varistors, etc [5e10]. The availability of a rich genre of

nanostructures makes ZnO as an important material [5e8].

Furthermore, ZnO and Mg substituted ZnO are reported to

exhibit hydrogen storage behaviour [11,12]. The doping of ZnO

withametalcouldchange itsproperties,dopingwiththeGroupII

elements (Cd, Mg) maymodulate the value of the band gap and

increase the UV luminescence intensity [13]. One of the inter-

esting features of ZnO is the possibility to tune its band gap by

substituting (alloying)bivalentmetalssuchasCdandMginplace

of Zn. While Cd is known to reduce the band gap [14], Mg

substitution leads to the enhancement in band gap [15]. The

bandgapofzincoxidecanbewidenedbyalloyingzincoxidewith

magnesium oxide which has a wider band gap of 7.7 eV or nar-

rowedbyalloyingwithcadmiumoxidewhichhasasmallerband

gap of 2.3 eV [13e21]. Therefore, the Zn1�xMgxO films have been

suggested as one of the promising barrier materials of ZnO for

band gap engineering as well as heterostructure device design.

Out of several interesting physical properties which may arise

due to the substitution of Mg for Zn in ZnO, two namely (a)

hydrogen storage (gravimetric hydrogen storage capacity) char-

acteristics and (b) optical properties (variation in band gap) have

been studied.

There are number of reports on the growth and properties

of the ZnO [22e25] and Zn1�xMgxO nanostructurs [26]

synthesized by various techniques [27]. Synthesis of ZnO: Mg

alloys for optoelectronics applications is generally done

through vapour phase deposition with or without catalyst.

Routes that have been used to prepare ZnO and Zn1�xMgxO

alloys are molecular beam epitaxy [28], vapour liquid solid

phase growth [29], pulsed laser deposition [30], and metal

organic vapour phase epitaxial (MOVPE) growth [31].

Theoretical studies predict that nanostructures are

attractive candidates for high-density hydrogen storage [32].

Earlier experiments reported considerable H2 storage capacity

of carbon-based nanomaterials, but the highest storage

capacity demonstrated experimentally after excluding

experimental errors was only 0.43 wt. % H2 at room temper-

ature [33].

Recent studies reveal that ZnO nanostructures are able to

reversibly uptake hydrogen up to 0.8 wt. % [11] at ambient

condition and up to 2.4 wt. % upon Mg doping [12]. Keeping

these aspects in view, the present investigation deals with the

possible hydrogen absorption behaviour of the as synthesized

ZnO nanowires andMg doped ZnO (Zn1�xMgxO, x¼ 0, 5, 10 and

20 at. %) nanowires at room temperature under constant

hydrogen pressure (60 atm). The present study reveals that

the as synthesized ZnO nanowires absorbs only 0.57 wt. % H2.

However, doping of 10 at. %Mg in ZnO increases the hydrogen

absorption capacity to w1 wt. % under identical condition.

Furthermore, there is no detailed study on the effect of Mg

doping on the structure and hydrogen storage properties of the

Zn1�xMgxOnanostructures. The presentwork therefore focuses

on a simple and rapid method for the mass production of ZnO

nanowires based on the thermal evaporation of metal zinc

powder at w900 �C and Mg doping of ZnO nanostructures by

solid state diffusion at 600 �C. It is also experimentally demon-

strated that up tow1 wt. % hydrogen could be stored in the Mg

doped ZnO nanostructures at room temperature (28 �C) underthe pressure of about 60 atm and about w75% of the stored

hydrogen can be released upon heating at 40�C under 1 atm.

2. Experimental details

The experimental setup for the synthesis of ZnO nanowires

consistsofahorizontal tube furnace (30cmin lengthand4cmin

diameter), a large quartz tube (50 cm in length and 3 cm in

diameter) having two gas inlets and a gas-flow control system.

Fig. 1(a) shows a schematic diagram of the system. The

temperature distribution of the furnacewas fully characterized

and stable within ca. w10 �C for time-to-time operation. For

removing air in the system, the quartz tube was purged for

w5 min by a flow of high-purity argon (w99.99%, w500 SCCM)

from higher-temperature end of the quartz tube. Afterwards,

the Ar flow rate was kept constant (w1000 SCCM) and at the

same time the tube furnace was heated to w900 �C at a rate of

10 �Cmin�1. Oxygenwas inserted into quartz tubewith theflow

of 100 SCCM at the w900 �C temperature. 2 g of Zn (99.9%)

powder contained inanaluminaboatwas inserted into the tube

from the downstream end and placed at the centre of the tube.

The reaction began within about 2e3 min and continued for

8e10min usually, depending on the amount of sourcematerial

and the temperature.White cotton-like productwas formed (as

shown in Fig. 1(b)) and carried by the flowing gas continuously

deposited out-side furnace, was collected carefully for charac-

terization andmeasurement purposes.

For Mg doping ZnO nanowires with nominal composition

(Zn1�xMgxO, x ¼ 5, 10 and 20 at. %) have been synthesized by

solid-state reaction method at vacuum pressure using high-

purity powder of Mg (99.99%). The Mg powder was fully mixed

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in an agatemortar for 30min and cold pressed (1 tons/in2) into

small rectangular pellets (10 � 5 � 1 mm3). The pellet config-

ured was wrapped in a tantalum (Ta) foil and sealed in silica

tube evacuated up to 10�5 torr. Thereafter, silica tube was

heated (put) in a programmable tube-type furnace at w600 �Cfor w12 h. The silica tube was cooled to room temperature at

the rate of 10 �C/min and Mg doped ZnO samples were

retrieved for further studies.

The as-prepared products were characterized by using

scanning electron microscopy (SEM), transmission electron

microscopy (TEM) and powder X-ray diffractometer. The

morphologies were directly examined by using SEM (ESEM

Techni 200QuantaattachedwithEDAX). ForTEMobservations,

the nanowire products were ultrasonically dispersed in

ethanol and then dropped onto carbon-coated copper grids.

TEM observations were carried out on high-resolution trans-

mission electron microscope (HRTEM, Tecnai 20 G2 FEI). The

XRD analyses were performed on a Philips X’PERT PRO PAN

Analytical X-ray diffractometer with CuKa irradiation (wave-

length, l ¼ 1.5406 A) at a scanning speed of 0.02� s�1.

Hydrogen absorption behaviour of ZnO and Mg doped ZnO

nanowires have been studied by using computerized P-C-T

instrument supplied by Advanced Materials Corporation,

Pittsburgh, USA. A quantity of 0.5 gm of the material was

loaded in the sample chamber of AMC P-C-I instrument,

which was heated in a programmable furnace at 200 �C for 2 h

in order to remove the moisture content in as-synthesized

ZnO. The P-C-I machine was then programmed to monitor

the amount of hydrogen absorbed/adsorbed at 60 atm H2 in

autosoak mode.

Fig. 2 e (a) XRD patterns of Zn1LxMgxO (x [ 0, 5, 10 and

20 at. %) powder. (b) Rietveld refinement plot undoped ZnO

and doped Zn1LxMgxO (x [ 5, 10 and 20 at. %) samples.

Observed (solid lines) and calculated (dot lines) data are

overlapped, with the difference pattern and the expected

peak positions (j) shown at the bottom.

3. Results and discussion

3.1. Structural characterization (XRD analysis) of ZnOand Mg doped ZnO samples

X-ray powder diffraction measurements confirmed the phase

formationofZn1�xMgxO.PolycrystallinesamplesofZn1�xMgxO

(x ¼ 0, 5, 10 and 20 at. %) were synthesized through two step

process. As shown in Fig. 2(a), the observed powder data for

Zn1�xMgxO has been indexed to ZnO wurtzite structure indi-

cating the single phase and polycrystalline samples of

Zn1�xMgxO formwith Mg incorporation.

Detailed structural analyseswere performed for Zn1�xMgxO

(x ¼ 0, 5, 10 and 20 at. %) by Rietveld refinement using P63mc

spacegroupofhexagonal structurewithZn/Mgat2b (1/3, 2/3, 0)

and O at 2b (1/3, 2/3, u) [32e34] over the 2q range of 30�e80�.Fig. 2 (b) shows the Rietveld fits for pure ZnO and Zn1�xMgxO

solid solution. As can be seen from this figure, the fit between

observed and calculated profiles is very good for all the

compositions. Very good fit between observed and calculated

profiles by considering Mg occupying Zn site confirms that Mg

is indeed substituting Zn in the formation of Zn1�xMgxO solid

solution.

The substitution ofMgdid not causemarked changes in the

diffraction patterns as expected from the similar four-

coordination ionic radii [34] of Zn2þ (0.60 A) and Mg2þ (0.57 A).

Due tonon-negligible correlationeffects, theoccupanciesofZn

andMg in Zn1�xMgxO, x> 0 could only be refinedwith the scale

factor fixed to obtain pure ZnO, Zn0.950Mg0.050O, Zn0.90Mg0.10O,

and Zn0.8 0Mg0.20O. In all four samples, the final Rietveld Rwp

valueswereobtainedas<4%. the refinedstructuralparameters

(Table1andFig. 2 (b)), it is observed thatMgsubstitution results

in an elongation of the a- axis and a contraction of the c-axis.

The overall consequence is a more pronounced wurtzite

distortion, with the (Zn, Mg)O4 tetrahedral uniformly

compressed along the c axis. While the parent ZnO is already

substantiallydistorted, as indicatedby thedeviationof c/a ratio

(1.6021) from that of a standard geometry 1.633, the hexagonal

Table 1 e Structural parameters for polycrystalline Zn1LxMgxO (x [ 0, 5, 10 and 20 at. %) determined by the Rietveldrefinement of XRD data in space group P63mc with Zn/Mg at (1/3, 2/3, 0) and O at (1/3, 2/3, u).

Parameter Standard ZnO X ¼ 0 at. % X ¼ 5 at. % X ¼ 10 at. % X ¼ 20 at. %

a (A) 3.249858 3.2498 � 0.0002 3.2530 � 0.0004 3.2549 � 0.0006 3.2549 � 0.0003

c (A) 5.206519 5.2060 � 0.0003 5.2060 � 0.0002 5.2054 � 0.0002 5.2010 � 0.0006

Volume (A3) 47.622830 47.6173 � 0.0003 47.708 � 0.0005 47.7582 � 0.0004 47.7194 � 0.0002

c/a 1.6021 1.6018 1.6008 1.5996 1.5978

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 7 ( 2 0 1 2 ) 3 7 4 8e3 7 5 4 3751

lattice is furtherdeformeduponMgsubstitution. Fromexisting

wurtzite structures, it is well known that when the bonding

character becomes more ionic, the c/a ratio moves away from

the ideal value [35]. Another important distortion in the wurt-

zite structure arises from the c-axis cation displacement,

which is measured by the deviation of the anion positional

parameter u from an ideal value of 0.375. The four nearest

cationeanion pairs are equidistant when u ¼ a2/(3c2) þ 0.25,

whereas the centres of cation and anion charge within each

isolated ZnO4 unit coincide if u ¼ 0.375, regardless of the

c/a ratio.

3.2. SEM and TEM analysis

Fig. 3 (aed) show the SEM and TEM images of as synthesized

ZnO nanowires without Mg doping. Fig. 3(a) shows ZnO nano-

wires with a diameter ranging fromw50e100 nm and a length

of up to w10 mm. Fig. 3 (b) shows a low-magnification TEM

image of ZnO nanowires with diameters ranging from

w50e100nm.Thediameterof thenanowire is almost the same

throughout the length. Fig. 3(c) exhibits a high-resolution TEM

(HRTEM) image of nanowire corresponding to encircled region

Fig. 3(b). The image shows well-defined lattice fringes with

lattice spacing of 0.52 nm corresponding to the d spacing of

(00.1) crystal plane, this confirms that the grown nanowires

have low density of defect and have grown along [00.1] direc-

tion. The HRTEM image and the selected area electron

diffraction (SAED) pattern (Fig. 3 (d)) reveal that nanowire of

ZnO is a single crystal hexagonal wurtzite structure.

For the flowing gas phase reaction technique, the gas flow

rate and temperature are the critical parameters for realizing

continuous and substrate-free synthesis. Under a lowflow rate,

the reaction product cannot be carried out in the silica tube by

flowing gas, and they will drop down onto the substrate or tube

wall [36e40]. The nanowires will continue their growth on the

substrate or on the tube wall, and the non-uniformity of local

precursor concentration and local temperature will cause the

morphologyof theproducts todependonthegrowthlocationon

the tube. Very high flow rate of gas prevents the complete

reaction of zinc with oxygen. To realize continuous growth and

simplify the collection procedure, the products are required to

be carried out without touching the tube. The carrying force

produced by certain gas flow rate is a function of the tube

diameter. In the present used furnace with a diameter of 4 cm,

a suitable gas flow rate ranges from 500 to 1500 SCCM. No

catalyst particles are found on the tip of the wires, suggesting

that the nanowires are grown via a vapour-solid (VS) process.

Nanowires formed by Mg doping in ZnO following the above

foresaid process show similar morphology, with a diameter of

about 70e100 nm and a length of up to 10 mm (Fig. 3 (e and f)).

3.3. EDAX analysis of Zn1�xMgxO (x ¼ 0, 5, 10 and20 at. %) samples

The energy-dispersive X-ray spectrum (EDAX) of without Mg

sample (Fig. 4(a)) exhibits the Zn and O signals peak of the

elements presented with an approximate atomic ratio of

51.05:48.95, which is consistentwith the stoichiometry of ZnO.

EDAX spectrum of 5% dopedMg:ZnO sample (Fig. 4(b)) exhibits

the Zn, O and Mg signal peaks of the elements presented with

an approximate atomic ratio of 45.86:49.77:4.37 which is again

consistent with the stoichiometry of w5 at. % doped Mg in

ZnO. EDAX spectrum as shown in Fig. 4(c) indicates that the

as-prepared 10%Mg doped ZnO product is pure phase with an

approximate stoichiometry of 45.44:45.54:09.02. In order to

obtain further evidence to the composition, EDAX measure-

ment was performed to detect the ratio of the elements. The

corresponding EDAX spectrum shown in Fig. 4(d) reveals the

atomic ratio of the involving elements: Zn, O and Mg, which is

45.44:35.54:19.02, presenting the doping of corresponding Mg

nominal stoichiometry. Compositional analysis using EDAX

was performed for all the samples and good agreement has

been observed between nominal doping level and atomic

percentage of Zn, Mg.

3.4. Hydrogen uptake of ZnO and Zn1�xMgxONanowires

In the present investigation the hydrogenation behaviour of

ZnO and Mg substituted ZnO (Zn1-xMgxO, x ¼ 5, 10, 20 at. %) at

room temperature under 60 atm H2 pressure. A representative

hydrogen absorption curve is shown in Fig. 5. As it is evident

from Fig. 5, ZnO absorbs only 0.57 wt. % H2. However, 5, 10 and

20 at. % Mg doped ZnO absorbs 0.73, 1.0 and 0.75 wt. % H2,

respectively. Thus it becomes clear that 10 at. % Mg doped ZnO

exhibits the higher hydrogen uptake. The mechanism for

hydrogen absorption in ZnO is not well known or understood

so far. However, based on hydrogenation behaviour of other

hydrogen storage materials e.g. intermetallic hydrides such as

LaNi5H6, complex hydrides (e.g. NaAlH4) and carbon nano-

structures particularly carbon nanotubes/nanofibres have been

studied by us and others [41e44]. It can be said that hydrogen

uptake by storage materials can take place by (a) physisorption

of hydrogen molecules (H2) and (b) chemisorptions absorption

of hydrogen atoms (ions) involving dissociation of H2 at the

surface of the material and then diffusion of H atoms in the

lattice to get eventually lodged at interstitial sites.

The hydrogen absorption kinetics obtained for ZnO and Zn

(Mg) O are shown in Fig. 5. The chemisorption of hydrogen is

a fast process [41]. Therefore, hydrogen storage through

chemisorption in ZnO and Zn (Mg) O can be ruled out. It may

Fig. 3 e (a) Show the SEM (b) TEM images of as synthesized ZnO nanowires without Mg doping (c) high-resolution TEM

(HRTEM) image of nanowire and (d) corresponding SADP of ZnO nanowire. (e and f) SEM and TEM images of as synthesized

ZnO nanowires 10% at. Mg doping and inset SADP.

Fig. 4 e (aed) EDX analyses of Zn1LxMgxO (x [ 0, 5, 10 and 20 at. %) samples, respectively.

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 7 ( 2 0 1 2 ) 3 7 4 8e3 7 5 43752

Fig. 5 e Hydrogen absorption kinetics of Zn1LxMgxO (x [ 0,

5, 10 and 20 at. %) at room temperature and 60 atm H2

pressure.

Fig. 6 e (a and b) FTIR spectra of ZnO and Zn1LxMgxO (x[ 0,

5, 10 and 20 at. %) samples before and after the

hydrogenation process, respectively at room temperature.

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 7 ( 2 0 1 2 ) 3 7 4 8e3 7 5 4 3753

be pointed out that this is contrary to proportions made by

other workers for hydrogen storage where chemisorptions of

hydrogen is taken to be a feasible process for ZnO and Zn (Mg)

O [11,12].

This absorption of hydrogen atoms and ions, Ho, Hþ and H�

(located in interstitials) may lead to hydrogen storage in the

present case. The H� ionsmay be located nearly Zn2þ ions, and

also at oxygen vacancies which are invariably present in ZnO

[38]. Hþ may be located near O2� ions and Ho in the nearly

neutral interstitial positions/configurations. The FTIR spectro-

scopic studies have been carried out in the present investiga-

tions in additions to native ZneO and MgeO bonds, OH bonds

were invariably found to be present (Fig. 6). This suggests the

hydrogen storage through bonding of Hþ at oxygen sites. As

indicated a favourable site of location of H� may be nearly

oxygen vacancies. Whereas H located in OHmay not get easily

dislodged, the H� at oxygen vacancies because of absence of

formation of any bond may get dislodged through application

ofmoderate temperatures. It, therefore, appears that hydrogen

uptake in ZnO and Zn (Mg) O takes place due to association of

H0, Hþ and H�, preferably H0 and H�. The higher storage

capacity of Zn (Mg) O may be due to higher unit cell volume

(w0.2842%) which will result in enlargement of hydrogen

containing bonds. This will lead to lowering of energy which is

required for the said bond formation. However, the above

suggested mechanism for hydrogen uptake is only tentative

and more extensive work needs to be done for unrevealing the

exact mechanism of hydrogen uptake.

The Fourier Transform Infrared Spectra (FTIR) of the

prepared samples is recorded in the 4000e500 cm�1 region to

confirm the presence of doped material. Fig. 6 (a and b) shows

the FTIR spectrum of ZnO and Mg doped ZnO (x ¼ 5, 10 and

20% at.) samples in the region of 4000e500 cm�1 before and

after the hydrogenation process. The KBr pellet technique has

been used to record the spectra. Awide absorption band in the

region of 3550e3250 cm�1 has been assigned to the stretching

vibration mode of hydroxyl group. The band at 1685 cm�1

which appears at wavelength just half of the 3550e3250 cm�1,

has been assigned to the first overtone of fundamental

stretchingmode of OH. These vibrations indicate the presence

of bound H2O on the surface of the sample [45]. The band

located at 560e600 cm�1 can be attributed to the ZneO

stretching mode in the ZnO lattice [45]. There is an indication

of slight peak shift of ZneO band towards high energy side on

increasing the Mg content. In plots for x ¼ 5, 10 and x ¼ 20%

Mg, wide bands at 1426e1435 cm�1 are attributed to the MgeO

stretching vibration and is completely absent in the pure ZnO

sample. The FTIR spectrum of Mg doped and undoped ZnO

sample revealed that there is a significant increase in the eOH

signal (3400 cm�1) after the hydrogenation process.

4. Conclusions

In summary, ZnO nanowireswith andwithoutMg dopingwere

synthesized by two step process first using a simple thermal

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 7 ( 2 0 1 2 ) 3 7 4 8e3 7 5 43754

oxidation of zinc powder and then doping of Mg by solid state

diffusion. Both ZnO nanowires with and without Mg doping

possess wurtzite crystalline structure. The effect of Mg substi-

tution on the lattice parameters of wurtzite ZnO has been

observed based on Rietveld analysis of XRD of polycrystalline

Zn1�xMgxO. Increase inMgconcentration results inpronounced

c-axis compression of the hexagonal lattice. Hydrogen absorp-

tion measurements reveal that Mg doped ZnO nanowires have

the highest uptake and release capability at room temperature.

As for the precise hydrogen storage mechanism of ZnO nano-

wires, further investigations need to be done.

Acknowledgements

The authors will like to acknowledge Professor T.N. Veziroglu

(President, IAHE Florida USA) and ProfessorM. Groll (Stuttgart,

Germany) for helpful discussions. Financial assistance from

Ministry of New and Renewable Energy (MNRE) and DST

(UNANST), New Delhi (India) is gratefully acknowledged.

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