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Room temperature sensing properties of networked GaNnanowire sensors to hydrogen enhanced by the Ga2Pd5

nanodot functionalization

Sang Sub Kim, Jae Young Park, Sun-Woo Choi, Hyo Sung Kim, Han Gil Na,Ju Chan Yang, Chongmu Lee, Hyoun Woo Kim*

School of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea

a r t i c l e i n f o

Article history:

Received 25 August 2010

Received in revised form

9 November 2010

Accepted 11 November 2010

Available online 10 December 2010

Keywords:

Nanowires

GaN

Ga2Pd5

Annealing

Hydrogen sensors

* Corresponding author. Tel.: þ82 32 860 754E-mail address: hwkim@inha.ac.kr (H.W.

0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.11.050

a b s t r a c t

Multiple-networked GaN nanowires with excellent sensing properties to hydrogen were

realized by functionalizing their surfaces with Ga2Pd5-related nanodots. Compared to the

bare-GaN nanowire sensors, functionalization improved the relative resistance responses

by a factor of >50 at H2 concentrations ranging from 100 to 2000 ppm. At room tempera-

ture, the nanodot-functionalized GaN nanowire sensors exhibited a relative resistance

response of 34.1% at 100 ppm H2. Interestingly, a shell layer was transformed mostly into

Ga2Pd5-phased nanodots, which was confirmed by X-ray diffraction and transmission

electron microscopy. The mechanisms responsible for the improvement induced by

nanodot functionalization are proposed in terms of the hydrogen spillover effect.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction an infinitesimal sensing signal, and less reliability in current

Better designs of new materials, as well as structural and

architectural innovations, are needed to overcome the serious

limitations imposed by thin film gas sensors, including rela-

tively low sensitivity, long response and recovery time, and

poor selectivity [1]. One-dimensional (1D) nanostructures

exhibit excellent gas sensitivity due to their exceptionally high

surface-to-volume ratio, single-crystalline nature, and semi-

conducting electrical behavior [2e7]. In particular, a relative

resistance response (|DR|/R) of ∼71% was achieved at 100 ppm

H2 using single nanowires of metal oxides [6]. However, gas

sensors with single 1D nanostructures still have a number of

drawbacks, such as an expensive photolithography process,

4; fax: þ82 32 862 5546.Kim).ssor T. Nejat Veziroglu. P

values with significant variation. Accordingly, networked

nanowires, where multiple nanowires are involved in the

sensing process, were used to circumvent the shortcomings

caused by the use of a single nanowire in gas sensors [8].

Despite playing an important role as a clean, abundant, and

promising energy source, hydrogen (H2) is a dangerous gas for

transport and storage because it is flammable and explosive

and it easily leaks fromgas-handling equipment upon careless

treatment. Accordingly, H2 gas sensors play an important role,

particularly for fuel leak detection in spacecrafts, automobiles,

and aircrafts, as well as in fire detectors and in the diagnosis of

exhaust and emissions from industrial processes [9]. More-

over, it is essential to develop high-performance H2 gas

ublished by Elsevier Ltd. All rights reserved.

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 ) 2 3 1 3e2 3 1 92314

sensors that can be operated at room temperature with a high

sensitivity [10] for domestic and industrial applications.

As a wide bandgap material, gallium nitride (GaN) exhibits

a high breakdown voltage, high thermal conductivity, and

a large saturation electron drift velocity [11]. In addition, GaN

provides the high thermal and environmental stability needed

for high-performance devices. Furthermore, GaN exhibits

a stronger Fermi-level pinning effect than the others [12],

where the pinning effect would limit the Schottky barrier

height variation within a small range. Accordingly, GaN has

excellent potential for H2 gas sensing because it can be oper-

ated within large temperature ranges, but is also sensitive to

surface charge. Despite the above advantages [11], there are

few reports on H2 gas sensors based on GaN nanowires.

Johnson et al. reported that GaN nanowires have higher

sensitivity to H2 gas compared to ZnO nanowires [9]. Lim et al.

used Pd-coated GaN multiple nanowires and achieved a DR/R

of ∼7.4% at 200 ppmH2 in N2 after 10min of exposure, whereas

uncoated GaN nanowires exhibited a DR/R of ∼0.48% under the

same conditions [2].

In general, the ability of a H2 sensor is enhanced using

a catalyst [13]. Noble metals are the most commonly used

catalysts [13,14]. In this paper, a shell layer was sputtered on

GaNnanowires using a Pd target, and, subsequently, nanodots

were formed by thermal annealing. This study compared the

sensing characteristics of functionalized GaN nanowires and

bare ones in terms of H2 gas. An attempt was made to use

multiple GaN nanowires to overcome the drawbacks caused

by the use of a single nanowire in gas sensors.

Fig. 1 e a) SEM image and b) TEM image of functionalized

GaN nanowires.

2. Materials and methods

First, the core GaN nanowires were fabricated in a tube

furnace. The Au (approximate thickness ¼ 3 nm)-coated Si

substrate was kept at the preset reaction temperature of

1000 �C for 1�h under a constant flow of NH3 (flow rate:

20 sccm) and Ar (flow rate: 100 sccm). The GaN powders used

as the source material were placed in an alumina boat in the

quartz tube. Subsequently, the substrates were transferred to

a turbo sputter coater (Emitech K575X, Emitech Ltd., Ashford,

Kent, UK) that was used in a previous study [15,16]. With a Pd

target at room temperature, sputteringwas carried out for 40 s

in high-purity (99.999%) argon (Ar) ambient at 25 �C. Subse-quently, the core-shell nanowires were annealed at 800

�C for

0.5 h in N2 ambient.

The collected products were characterized by X-ray

diffraction (XRD, Philips X’pert MRD diffractometer), scanning

electron microscopy (SEM, Hitachi S-4200), and transmission

electron microscopy (TEM, Philips CM-200) equipped with an

energy-dispersive X-ray spectrometer (EDX). For the sensing

measurements, Ni (∼200 nm in thickness) and Au (∼50 nm)

double layer electrodes were deposited sequentially by sput-

tering on the specimens using an interdigital electrode mask.

The response of the networked GaN nanowire sensors to H2

was measured using a homemade gas dilution and sensing

system. A known amount of highly purified H2 (>99.999%) was

introduced from a container with N2 gas acting as a diluting

agent to obtain the required H2 concentration in the

measuring system. The sensing characteristics were then

recorded at 298 K (room temperature) with various H2

concentrations ranging from 100 to 2000 ppm. This configu-

ration is the same as the experimental setup reported previ-

ously [17e20]. The relative resistance response (|DR|/R) was

used to evaluate the sensing capability. Herein, |DR|/

R¼ (Rg� R0)/R0, where R0 is the initial resistance in the absence

of H2 and Rg is the resistance measured in the presence of H2.

In addition, the conventional definition of sensitivity for

reducing gases (i.e. R0/Rg) was used [21].

3. Results and discussion

Fig. 1a shows an SEM image of the 800 �C-annealed core-shell

nanowires (i.e. functionalized nanowires), showing that the

GaN-core/Pd-shell nanowires maintain their continuous 1D

morphology despite subsequent thermal annealing. Fig. 1b

shows a low-magnification TEM image indicating that the

nanowire surface is relatively rough. Nanodot- or nanocluster-

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 ) 2 3 1 3e2 3 1 9 2315

like structures were observed on the surface of the nanowires.

Although not shown here, EDX showed that the nanodot- or

nanocluster-like structures on their surfaces comprised Pd

elements (Supplementary material S-1). An associated XRD

spectrum showed that all recognizable reflection peaks can be

indexed to a hexagonal GaN structure (JCPDS: 06-0416) or

orthorhombic Ga2Pd5 phase with a lattice parameter of

a ¼ 5.485 A, b ¼ 18.396 A, and c ¼ 4.083 A (JCPDS: 27-0232)

(Fig. 2a). The results suggest that the shell of the annealed

nanowires is composed mainly of a Ga2Pd5 phase because Pd

was reported to transform completely into Pd gallides, such as

Ga2Pd5 and Ga5Pd, at an annealing temperature of 700��C [22]

and the XRD data revealed the presence of a Ga2Pd5 phase.

Fig. 2b shows the high-resolution TEM lattice image of a region

near the outer surface of the nanowire shown in Fig. 1b. The

spacing between the lattice planes is approximately 0.29 nm

and 0.23 nm, corresponding to the d131 and d201 spacing of an

orthorhombic Ga2Pd5 phase (JCPDS card: No. 27-0232).

Fig. 3a and b show the time dependence of the resistance of

the bare and functionalized GaN nanowires sensors, respec-

tively, at different H2 concentrations ranging from 100 to

2000 ppm. The resistance decreased when the sensors were

exposed to H2 gas. On the other hand, the resistance recovered

completely to the initial value when the H2 supply was

Fig. 2 e a) XRD spectrum and b) lattice-resolved TEM image

taken from a region near the outer surface of the 800�C-annealed core-shell nanowire.

Fig. 3 e Dynamic response at various hydrogen

concentrations for the sensors fabricated from a) bare-GaN

nanowires and b) functionalized GaN nanowires.

stopped. Table 1 shows |DR|/R at different H2 concentrations,

which was calculated from Fig. 3. The bare-GaN nanowires

showed |DR|/R of 0.58e1.22% at H2 concentrations of

100e2000 ppm. In contrast, functionalized GaN nanowires

showed |DR|/R of 34.11e63.04% in the H2 range, 100e2000 ppm.

A simple calculation revealed that nanodot functionalization

improved the sensitivity by factors of 58.8, 59.4, 57.5, 55.8, 54.5,

53.7, and 51.7 at H2 concentrations of 100, 200, 300, 500, 1000,

1500, and 2000 ppm, respectively.Wang et al. reported that the

bare and Pd-coated ZnO nanorods showed |DR|/R of ∼0.25%and ∼4.2%, respectively, for 500 ppm H2 in N2 after 10 min of

exposure at room temperature [23]. Lim et al. reported that

bare and Pd-coated GaN nanowires had relative |DR|/R of

∼0.57% and ∼8.2%, respectively, for 500 ppm H2 in N2 at room

temperature [2]. Accordingly, the nanodot-functionalized GaN

nanowires in this study becamemuchmore sensitive to H2 by

functionalization compared to Pd-coated ZnO nanorods and

Pd-coated GaN nanowires, which were previously reported

[2,24]. Furthermore, the nanodot functionalization signifi-

cantly improved the sensing capability (in terms of |DR|/R) by

factors of 58.8 at very low H2 concentrations (100 ppm). So far,

the detection of 100 ppm H2 has been only rarely reported

despite having been achieved using the Pd-coated multiple

ZnO nanowires [24].

Table 1 e The relative resistance response (|DR|/R) measured at different hydrogen concentrations, for bare-GaN nanowiresand functionalized GaN nanowires.

H2 concentration (ppm) 100 200 300 500 1000 1500 2000

|DR|/R For bare samples (%) 0.58 0.67 0.76 0.87 0.96 1.08 1.22

|DR|/R For functionalized samples (%) 34.11 39.82 43.57 48.57 52.32 58.04 63.04

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 ) 2 3 1 3e2 3 1 92316

Fig. 4 shows the response/recovery times for bare and

functionalized GaN nanowires. For bare-GaN nanowires, the

response and recovery times were approximately 600 and

300 s, respectively. For functionalized GaN nanowires, the

response and recovery times were approximately 200 and

800 s, respectively. The response time of the functionalized

GaN nanowire sensor was as short as ∼200 s, which is even

considerably shorter than the bare-GaN nanowire sensor of

600 s. On the other hand, the recovery time of the function-

alized GaN nanowire sensor was longer than that of the bare-

GaN3 nanowire sensor. The response times were investigated

for a variety of sensors. For example, Sennik et al. reported

that the highly-ordered TiO2 nanotubes’ H2 sensors exhibited

a response time of 65 min at room temperature [25]. However,

to our knowledge, there are no reports of the response times of

GaN sensors for H2 gas. In addition, in the case of function-

alized GaN nanowires, Fig. 3b showed that the resistance

decreases very rapidly in the initial stages after H2 exposure,

and the rate of the reduction decreases with time thereafter.

On the other hand, for uncoated GaN nanowires (Fig. 3a), the

rate of decreasing resistance was not significantly dependent

on the exposure time. Supplementary material (S-2) shows

enlarged figures for the fall and rise times measured at

100 ppm H2.

Fig. 5 shows the sensitivity as a function of the H2

concentration, in which the sensitivity was estimated using

the relationship, S ¼ R0/Rg. Most importantly, the sensitivity

was enhanced significantly by functionalizing the surface of

the GaN nanowires with nanodots. The sensitivity of a semi-

conductor gas sensor can be described as S ¼ A[C]N þ B, where

A and B are constants and [C] is the concentration of the target

gas [26]. The data fitting indicated S ¼ 3.097[C] þ 1.006 and

Fig. 4 e Response/recovery times for the unfunctionalized

and functionalized GaN nanowires.

S ¼ 5.700[C] þ 1.562 for the bare-GaN and functionalized

nanowire sensors, respectively, showing that the enhance-

ment of sensitivity by the functionalization becomes more

evident at higher H2 concentrations.

Fig. 6a shows a schematic and a TEM image of the sputter-

coated GaN nanowires. After the growth of the bundles of GaN

nanowires on the Si substrate, the shell layers were coated

continuously along the GaN core nanowires. For functionali-

zation, the core-shell nanowires were heated thermally and

the shell layer was agglomerated to generate nanodot-like

Ga2Pd5 structures on the surface of the core nanowires

(Fig. 6b), in which the activation energy for the transformation

was provided by thermal annealing. Since the morphology

and characteristics of the surface clusters/nanodots can be

tailored by changing the shell thickness, shell materials, and

surface properties of the core nanowires, this technique can

be applied to other core/shell combinations to examine the

extraordinary properties.

The sensing mechanism of GaN can be explained by the

oxidizing/reducing gas effect, which has been mentioned in

a previous report on ZnO nanowire sensors [27]. In air

ambient, the GaN surface will adsorb oxygen species (i.e. O2�,

O2�, O�, etc) [28]. Using the surface reactions, including

O2�(ads) þ e� / 2O�(ads), the conducting electrons are trap-

ped,making the nanowire less conductive [29]. In otherwords,

adsorbed or dissociated oxygen molecules will extract elec-

trons from GaN nanowires. Furthermore, the following reac-

tion is expected when oxygen is adsorbed at the N vacancies

of GaN: 1/2mO2 þ {vacant site} þ e� / {Om�}, where the species

in the braces are bound to oxygen species and m is an integer.

On the other hand, with the introduction of H2 gas, the

reductive gas decreases the concentration of oxygen species

Fig. 5 e Sensitivities of bare-GaN nanowires and nanodot-

functionalized GaN nanowires.

Fig. 6 e a) Schematic diagram of the as-synthesized core-

shell nanowires. b) Schematic diagram showing the

formation of nanodots by heating the core-shell

nanowires. c) Schematic diagram explaining the

functionalization effects by the nanodots. The left-hand-

and right-hand-side figures explain the changes in GaN

nanowires without and with nanodots, respectively.

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 ) 2 3 1 3e2 3 1 9 2317

on the nanowire surface, resulting in an increase in electron

concentration and a decrease in resistance. Active hydrogen

species will react with the surface oxygen species, generating

gaseous H2O. For example, Al-Hardan et al. suggested that

a reaction of H2 with adsorbed oxygen species (O�(ads)) will

release captured electrons back to the conduction band,

thereby increasing the electron concentration and decreasing

the resistance according to the following reaction:

H2O�(ads) / H2O(g) þ e� [30]. When the gas ambient was

switched from H2 to air, the resistance of the nanowire

changed back to the original value.

In regard to the functionalization-induced changes, the

sensitivity will be enhanced greatly by the two effects of the

chemical sensitization mechanism and the electronic sensi-

tization mechanism [31]. The enhanced sensitivity by the

functionalization can be explained by the electronic sensiti-

zationmechanism, in which a change in the oxidation state of

metal additives eventually alters the conductivity of the GaN

nanowires. For example, in the case of Pd, air ambient

facilitates the generation of a PdO phase, whereas H2 ambient

reduces it to the Pd phase [32]. The electronic sensitization

process involves the generation of depletion zones around the

particles, and the improvement in sensing can be attribute to

the modulation of the Schottky barriers (and, here, the width

of the conduction channel) due to changes in the oxidation

state of the Pd (and, therefore, its work function) [33]. Electron

transfer between the Pd/PdO phase and core materials will

affect the sensitivity. In the present study, a considerable

portion of the Pd phase was transformed into the Ga2Pd5

phase, as shown in Fig. 2. However, upon the alternative

introduction of air/pure H2 gases, it is possible that the

oxidation state of Pd or Ga2Pd5 phase can be changed, which

includes not only the oxidation of Pd or Ga2Pd5 phases, but

also the transformation between Pd and Ga2Pd5 phases.

On the other hand, by a chemical sensitizationmechanism

[34], metal additives catalytically activate the dissociation of

H2 molecules through a “spillover effect” and ultimately

generateatomichydrogen.A rangeofmetals andmetal oxides,

including Pt [31], Pd [35], PdO [36], and PteRh [14], have been

suggested to have a hydrogen spillover effect. However, to our

knowledge, the spillover effect of intermetallic compounds

has rarely been reported. Although a systematic investigation

is needed, Ga2Pd5 will easily adsorb and dissociate H2. Subse-

quently, the dissociated hydrogen species (such as atomic

hydrogen (H)) diffuse to the surface of the GaN nanowires,

activating the reaction between hydrogen and adsorbed

oxygen. This causes effective shrinkage of the depletion layer

at the surface of the nanowire with an enlargement of the

underlying conduction channel. This decreases the resistance

of the functionalized sensors, which are converted into

a reduction of sensing signals, leading to higher sensitivity.

Accordingly, the nanodot-enhanced sensitivitywith respect to

H2 gas in this study can be dominated not only by an electronic

sensitizationmechanism, but also by a chemical sensitization

mechanism. Fig. 6c describes the change in the conducting

channel by functionalization.

From Figs. 3b and S-2, for functionalized nanowires, a rapid

decrease in resistance was observed in the initial stage after

H2 exposure and the subsequent time variation. In the early

stages of H2 introduction, atomic hydrogenmoves onto a GaN

surface to generate the H2O vapor. However, in the later

stages, the GaN surface could have been saturated with

atomic hydrogen, retarding further reactions. Fig. 4 shows

that the recovery time of the bare-GaN nanowire sensor is as

short as 300 s, which is much shorter than the functionalized

GaN nanowire sensor (800 s). Hydrogen spillover is effective,

and surplus hydrogen species remain on the GaN surface.

Accordingly, the oxygen species generated during the

recovery period will be consumed by a reaction with surface

hydrogen species, and, thus, their contribution to the increase

in resistivitywill be suppressed. It is surmised that the heating

will reduce the recovery time because of faster kinetics and

easier desorption of species. However, the temperature effects

are complicated and further optimization will be necessary.

The gas purging will also be an effective method to reduce the

recovery time, by which the adsorbed species can be

detached. In addition, the surface modification was reported

to shorten the recovery time, by blocking or reducing the

surface active sites for the formation of hydroxyl groups [37].

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 ) 2 3 1 3e2 3 1 92318

4. Conclusions

Nanodot-functionalized networked GaN nanowires were

fabricated to enhance the sensitivity to H2 gas. First, core-shell

nanowires were synthesized on the surface of networked GaN

nanowires by sputtering from a Pd target at room tempera-

ture. Secondly, continuous shell layers were transformed into

nanodots by heat treatment. Characterization showed that

the nanodots were composed mainly of an orthorhombic

Ga2Pd5 structure. The H2 sensing characteristics of the sensors

fabricated from functionalized and bare-GaN nanowires,

respectively, were compared at room temperature. At a H2

concentration of 100 ppm, bare and functionalized GaN

nanowires exhibited |DR|/R of ∼0.58% and ∼34.11%, respec-

tively, at room temperature, showing that the functionaliza-

tion greatly improves the sensitivity in GaN nanowire-based

gas sensors. The data fitting to the conventional sensitivity

equation indicates that the enhanced sensitivity by func-

tionalization becomes more evident at higher H2 concentra-

tions. The response of the functionalized nanowires to H2 was

faster than that of the uncoated devices. On the other hand,

the recovery time of the functionalized nanowires was longer

than that of the bare-GaN nanowires. These results suggest

that Ga2Pd5 facilitates the dissociation of H2 into H-species on

its surface, ultimately enhancing the H2 sensitivity.

Acknowledgement

This study was supported by Basic Science Research Program

through the National Research Foundation of Korea (NRF)

funded by the Ministry of Education, Science and Technology

(2009-0073723).

Appendix. Supplementary material

Supplementary data related to this article can be found online

at doi:10.1016/j.ijhydene.2010.11.050.

r e f e r e n c e s

[1] Zheng W, Lu X, Wang W, Li Z, Zhang H, Wang Y, et al. Ahighly sensitive and fast-responding sensor based onelectrospun In2O3 nanofibers. Sens Actuators B Chem 2009;142:61e5.

[2] Lim W, Wright JS, Gila BP, Johnson JL, Ural A, Anderson T,et al. Room temperature hydrogen detection using Pd-coatedGaN nanowires. Appl Phys Lett 2008;93:072109.

[3] Vomiero A, Bianchi S, Comini E, Faglia G, Ferroni M, Poli N,et al. In2O3 nanowires for gas sensors: morphology andsensing characterization. Thin Solid Films 2007;515:8356e9.

[4] Law M, Kind H, Messer B, Kim F, Yang PD. Photochemicalsensing of NO2 with SnO2 nanoribbon nanosensors at roomtemperature. Angew Chem Int Ed Engl 2002;41:2405e8.

[5] Comini E, Faglia G, Sberveglieri G, Pan ZW, Wang ZL. Stableand highly sensitive gas sensors based on semiconductoroxide nanobelts. Appl Phys Lett 2002;81:1869e71.

[6] Rout CS, Kulkarni GU, Rao CNR. Room temperature hydrogenand hydrocarbon sensors based on single nanowires of metaloxides. J Phys D Appl Phys 2007;40:2777e82.

[7] Lee JM, Park J-E, Kim S, Kim S, Lee E, Kim S-J, et al. Ultra-sensitive hydrogen gas sensors based on Pd-decorated tindioxide nanostructures: room temperature operatingsensors. Int J Hydrogen Energy 2010;35:12568e73.

[8] Sysoev VV, Schneider T, Goschnick J, Kiselev I, Habicht W,Hahn H, et al. Percolating SnO2 nanowire network as a stablegas sensor: direct comparison of long-term performanceversus SnO2 nanoparticle films. Sens Actuators B Chem 2009;139:699e703.

[9] Johnson JL, Choi Y, Ural A, Lim W, Wright JS, Gila BP, et al.Growth and characterization of GaN nanowires for hydrogensensors. J Electron Mater 2009;38:490e4.

[10] Favier F, Walter EC, Zach MP, Benter T, Penner RM. Hydrogensensors and switches from electrodeposited palladiummesowire arrays. Science 2001;293:2277e81.

[11] Das SN, Pal AK. Hydrogen sensor based on thin filmnanocrystalline n-GaN/Pd Schottky diode. J Phys D Appl Phys2007;40:7291e7.

[12] Song JH, Lu W, Flynn JS, Brandes GR. AlGaN/GaN Schottkydiode hydrogen sensor performance at high temperaturewith different catalytic metals. Solid State Electron 2005;49:1330e4.

[13] Tien LC, Wang HT, Kang BS, Ren F, Sadik PW, Norton DP.Room-temperature hydrogen-selective sensing using singlePt-coated ZnO nanowires at microwatt power levels.Electrochem Sol Stat Lett 2005;8:G230e2.

[14] Al-Saleh MA, Hossain MM, Shalabi MA, Kimura T, Inui T.Hydrogen spillover effects on PteRh modified Co-claycatalysts for heavy oil upgrading. Appl Catal A Gen 2003;253:453e9.

[15] Kim HW, Shim SH, Lee JW. SiOx-sheathed carbonnanotubes prepared via a sputtering technique. Carbon2007;45:2695e8.

[16] Kim HS, Na HG, Yang JC, Lee C, Kim HW. GaN-based core-shell nanowires sputtered with Pd target and their annealingcharacteristics. Inha: Surface Coatings Technology Press;2010.

[17] Kim HW, Shim SH, Lee JW, Park JY, Kim SS. Bi2Sn2O7

nanoparticles attached to SnO2 nanowires and used ascatalysts. Chem Phys Lett 2008;456:193e7.

[18] Choi SW, Park JY, Kim SS. Synthesis of SnO2eZnO core-shellnanofibers via a novel two-step process and their gas sensingproperties. Nanotechnology 2009;20:465603e8.

[19] Park JY, Choi SW, Lee JW, Lee C, Kim SS. Synthesis and gassensing properties of TiO2eZnO core-shell nanofibers. J AmCeram Soc 2009;92:2551e4.

[20] Park JY, Choi SW, Kim SS. Fabrication of highly selectivechemical sensor based on ZnO nanorod arrays. NanoscaleRes Lett 2010;5:353e9.

[21] Wang C, Yin L, Zhang L, Xiang D, Gao R. Metal oxide gassensors: sensitivity and influencing factors. Sensors 2010;10:2088e106.

[22] Kim CC, Je JH, Kim DW, Baik HK, Lee SM, Ruterana P.Annealing behavior of Pd/GaN (0001) microstructure. MaterSci Eng B 2001;82:105e7.

[23] Wang HT, Kang BS, Ren F, Tien LC, Sadik PW, Norton DP,et al. Appl Phys Lett 2005;86:243503.

[24] Wang HT, Kang BS, Ren F, Tien LC, Sadik PW, Norton DP,et al. Detection of hydrogen at room temperature withcatalyst-coated multiple ZnO nanorods. Appl Phys A 2005;81:1117e9.

[25] Sennik E, Colak Z, Kilinc N, Ozturk ZY. Synthesis of highly-ordered TiO2 nanotubes for a hydrogen sensor. Int JHydrogen Energy 2010;35:4420e7.

[26] Williams DE. Solid state gas sensors. Bristol: Hilger; 1987.

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 ) 2 3 1 3e2 3 1 9 2319

[27] Das SN, Kar JP, Choi JH, Lee TI, Moon KJ, Myung JM.Fabrication and characterization of ZnO single nanowire-based hydrogen sensor. J Phys Chem C 2010;114:1689e93.

[28] Lee DS, Lee JH, Lee YH, Lee DD. GaN thin films as gas sensors.Sens Actuators B 2003;89:305e10.

[29] Neri G, Bonavita A, Micali G, Rizzo G, Pinna N,Niederberger M. In2O3 and PteIn2O3 nanopowders for lowtemperature oxygen sensors. Sens Actuators B 2007;127:455e62.

[30] Al-Hardan NH, Abdullah MJ, Abdul Aziz A. Sensingmechanism of hydrogen gas sensor based on RF-sputteredZnO thin films. Int J Hydrogen Energ 2010;35:4428e34.

[31] Shen Y, Yamazaki T, Liu Z, Meng D, Kikuta T. Hydrogensensor made of undoped and Pt-doped SnO2 nanowires.J Alloys Compd 2009;488:L21e5.

[32] Matsushima S, Teraoka Y, Miura N, Yamazoe N. Electronicinteraction between metal additives and tin oxide in tindioxide-based gas sensors. Jpn J Appl Phys 1988;27:1798e802.

[33] Kolmakov A, Klenov DO, Lilach Y, Stemmer S, Moskovits M.Enhanced gas sensing by individual SnO2 nanowires andnanobelts functionalized with Pd catalyst particles. NanoLett 2005;5:667e73.

[34] Yamazoe N, Kurogawa Y, Seiyama T. Effects of additives onsemiconductor gas sensors. Sens Actuators B 1983;4:283e9.

[35] Du AJ, Smith SC, Yao XD, Lu GQ. Hydrogen spillovermechanism on a Pd-doped Mg surface as revealed by abinitio density functional calculation. J Am Chem Soc 2007;129:10201e4.

[36] Chen LF, Wang JA, Valenzuela MA, Bokhimi X, Acosta DR,Novaro O. Hydrogen spillover and structural defects ina PdO/zirconia nanophase synthesized through a surfactant-templated route. J Alloys Compd 2006;417:220e3.

[37] Chakraborty S, Mandal I, Ray I, Majumdar S, Sen A, Maiti HS.Improvement of recovery time of nanostructured tindioxide-based thick film gas sensors through surfacemodification. Sens Actuators B 2007;127:554e8.