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Hydrogen sensing properties of ZnO nanorods:Effects of annealing, temperature and electrodestructure

Sadullah Ozturk a, Necmettin Kılınc a,b,*, _Imren Torun a, Arif Kosemen a,c,Yasin S‚ahin

a, Zafer Ziya Ozturk a,d

aGebze Institute of Technology, Science Faculty, Department of Physics, 41400 Gebze, Kocaeli, TurkeybKoc University, Electrical and Electronics Engineering, 34450 Sariyer, Istanbul, TurkeycMus‚ Alparslan University, Department of Physics, 49100 Mus‚, TurkeydTUB_ITAK-Marmara Research Center, Materials Institute, 41470 Gebze, Kocaeli, Turkey

a r t i c l e i n f o

Article history:

Received 6 August 2013

Received in revised form

10 January 2014

Accepted 13 January 2014

Available online 11 February 2014

Keywords:

ZnO

Nanorods

Hydrothermal process

H2 sensor

Gas sensor

* Corresponding author. Gebze Institute of TTurkey. Tel.: þ90 2626051312; fax: þ90 262 6

E-mail address: nkilinc@gyte.edu.tr (N. K0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2014.01.0

a b s t r a c t

In this study, the hydrogen (H2) sensing properties of vertically aligned zinc oxide (ZnO)

nanorods were investigated depending on annealing, Pd coating, temperature and elec-

trode structure. ZnO nanorods were fabricated by using hydrothermal method on a glass

substrate and an indium tin oxide (ITO) coated glass substrate. In order to determine the

effects of annealing on the H2 sensor performance, the nanorods were heated at 500 �C in

dry air. H2 sensing measurements were done in the temperature range of 25e200 �C. It was

found that, the sensor response of Pd coated ZnO nanorods were much higher than the un-

coated nanorods due to the catalytic effect of Pd thin film. Moreover, the un-annealed

samples showed better sensor response than the annealed samples due to the number

of oxygen deficiency. In addition, the lateral electrode structure showed higher sensor

response than the sandwich electrode structure.

Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

The use of hydrogen (H2) as a clean, efficient, and sustainable

energy source has been expanding into various fields, such as

the chemical industries, the food industry, the semiconductor

industry and the transportation [1]. The detection of H2 is so

crucial in safety issue due to the flammable and explosive

properties of H2 gas and also in a H2 source for leak detection

[2]. In addition, there are many different H2 production

echnology, Science Facu538490.ılınc).2014, Hydrogen Energy P66

processes such as steam reforming of natural gas, catalytic

decomposition of natural gas, partial oxidation of heavy oils,

coal gasification, steam-iron coal gasification, thermochem-

ical, photochemical, photoelectro-chemical and photobiolog-

ical processes [1,3,4,5,6] and the detection of H2 is also very

important for real-time quantitative analysis of H2 produc-

tion. The H2 sensors can be classified into several categories,

according to physicochemical detection mechanism: Cata-

lytic, electrochemical, resistor based, work function based,

mechanical, optical, acoustic [7,8,9,10,11,12,13,14,15,16,17,18].

lty, Department of Physics, P.O. Box 141, 41400 Gebze, Kocaeli,

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

Fig. 1 e Schematic illustration of ZnO nanorod sensor

structures (lateral (a) and sandwich (b) electrodes types).

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 9 ( 2 0 1 4 ) 5 1 9 4e5 2 0 1 5195

Semiconducting metal oxides have been extensively using as

sensitive materials for H2 detection.

Although semiconducting metal oxide materials can be

effect very easily by physical and chemical effects in

ambient, they can conserve their structure stability. With

this property, metal oxide materials attracted the

attention of researchers, and interest on these materials has

been increased day by day. Metal oxide materials have

been used in many application areas such as solar cells,

transistors, light emitting diodes and gas sensors

[19,20,21,22,23,24,25,26]. Semi-conductive metal oxide ma-

terials are often selected as sensing materials for detecting

of pollutant, oxidized, flammable and explosive agents

[19,20,21,22,23,24,25,26]. These materials are attracted

considerable attention because they show high sensitivity

to the gases and resistant to physical and chemical defor-

mation [27,28,29,30]. In general, sensing mechanism of

metal oxide gas sensors depends on adsorbed oxygen ions

(O2�, O�, O2�) on the surface. Metal oxides have been used at

high temperatures due to the different forms of adsorbed

oxygen ions at the different temperature ranges

[31,32,33,34]. Operating metal oxide sensors at high tem-

peratures causes an increase the costs. In order to improve

the sensor performance (sensitivity and low operating

temperature) of metal oxide materials, the nanostructured

metal oxides have been fabricated by using different

methods [35,36,37,38]. Metal oxide nanostructures have

extraordinarily large surface-to-volume ratio. This causes

dominant surface effect due to increase of their specific

surface. Hence, high surface area leads to the enhancement

of the surface related properties, such as catalytic activity or

surface adsorption. The selectivity of metal oxide based gas

sensors is another drawback due to cross response to many

gas species [32,34]. To overcome this problem, researchers

have been focused on doping metal oxide with catalytic

materials or fabricating heterostructures [39,40,41,42,43]. In

semi-conductive metal oxide materials, SnO2, WO3, TiO2

and ZnO etc are most preferred materials for gas sensing.

Among metal oxides, ZnO has large band gap (3.37 eV) and

high exciton binding energy (60 meV) and has been used in

many application such as used in optoelectronics, transparent

electronics, spintronic devices, and sensor applications [13].

ZnO nanostructures have been intensively studied for sensing

different gases such as H2, NOx, CO, H2S, O2, ethanol, etc in the

temperature range between room temperatures to 400 �C [13].

In order to improve the sensor parameters of ZnO based

sensor such as sensitivity, response time, recovery time and

reducing optimal working temperatures at low temperatures,

researchers have been focused on generally two ways. The

first, metal oxides have been doped with noble metals such as

palladium (Pd), platinum (Pt), and the second, metal oxides

have been synthesized in nanostructure forms such as

nanowire, nanotube, nanorods. In this study, ZnO nanorod

were fabricated with hydrothermal method and then func-

tionalize the nanorod with Pd thin film to enhance sensor

performance of ZnO based H2 sensor.

Many research groups have focused on gas sensor based on

nanostructured ZnO and doped ZnO [25]. For instance, Xing

et al. fabricated PdeZnOnanoflowers and the gas sensor of the

nanoflowers presented high sensitivity, short response time,

high selectivity and low operating temperatures [44]. In gen-

eral, chemical vapor deposition (CVD), solegel and laser

deposition techniques have been used to dope metal oxides

materials with metals [44]. In addition, Wei et al. prepared

pure and PdeZnO nanofibers by using electro-spinning pro-

cess [45]. They found that the sensor parameters of PdeZnO

nanofibers were better than pure ZnO nanofibers at 220 �C for

low concentration range (1e20 ppm) of CO [45]. Liao et al.

prepared ZnO nanorods by reactive vapor deposition of pure

Zn powder in an oxygen-containing ambience with different

diameters and investigated the structure, optical, and gas-

sensing properties of the nanorods [46]. They found that the

thin nanorods have a significantly better sensing performance

than the thick nanorods for the detection of C2H5OH and H2S

in air. The H2 gas sensing properties of ZnO nanorods were

investigated depending on temperature, size of the nanorods,

electric field and Pd nanoparticle density on the nanorods

surface [13,43,47e51]. Besides that, Chang et al. fabricated ZnO

nanorods with controlling Pd nanoparticle density in nano-

rods with photochemical deposition method, and tested

PdeZnO nanorod sensors to H2 gas at different temperature

ranges for clarifying the sensing mechanism [43]. Wang et al.

synthesized ZnO nanorod on a ZnO thin film through a simple

hydrothermalmethod and found that the ZnO nanorod arrays

show a high sensitivity and reversible response to H2, NH3 and

CO at a relatively low operating temperature [51].

In this study, ZnO nanorods were fabricated by using hy-

drothermal process on to a glass and ITO substrates. H2 gas

sensing properties of ZnO nanorodwere studied depending on

annealing, Pd film coating, operating temperature, electrode

structure and H2 concentration. Work function based (Pd/ZnO

nanorod/ITO) and resistance-based (IDE on ZnO nanorod)

electrodes structures were used for understanding sensing

mechanism of ZnO nanostructures.

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 9 ( 2 0 1 4 ) 5 1 9 4e5 2 0 15196

2. Experimental details

ZnO nanorods were synthesized on ZnO thin film (seed layer)

coated glass substrate by hydrothermal method at 90 �C for

3 h, and the details were given in previous works [52e54]. The

glass and indium tin oxide (ITO) substrates were cleaned with

acetone, methanol and deionized water in an ultrasonic bath

for 10 min each, respectively. In order to fabricate ZnO

nanorod, zinc nitrate hexahydrate (Zn(NO3)2・6H2O) and

hexamethylenetetramine (HMTA; C6H12N4) were used as re-

agents in equi-molar. ZnO nanorods were synthesized on the

seed layer by using hydrothermal process at 90 �C for 3 h. The

fabricated ZnO nanorods are approximately 60 nm in diam-

eter and 800 nm in length. Some of the ZnO nanorod samples

were annealed at 500 �C in dry air ambient for 2 h in order to

investigate the effects of annealing to gas sensing properties.

Two types of electrode were used. Lateral and sandwich

type electrode structures are called as resistive and work

function based sensors respectively. Fig. 1 shows a schematic

diagram for the lateral (a) and sandwich (b) type electrodes. In

order to fabricate resistive based sensor; annealed and as-

prepared ZnO nanorods were coated with approximately

15 nm Pd thin film by using thermal evaporator (Leybold

Univex 450). Then gold (Au) inter digital electrodes were

deposited on to as-prepared and Pd film coated ZnO nanorods

using the same evaporator system. The thickness of Au elec-

trodes was 250 nm. For work function based sensor mea-

surements, ZnO nanorods that fabricated on ITO, were then

coated with approximately 100 nm Pd film by using the

evaporator system. The ITO and Pd were used as top and

bottom electrode of device respectively (Fig. 1(b)). ZnO nano-

rods were characterized by scanning electron microscope

(SEM-Jeol JSM 6335) and UVevis spectrophotometer (Scinco

UVevis spectrophotometer).

Five type samples were tested for H2 gas sensor in a tem-

perature interval of 25e200 �C for concentration range from

200 to 4000 ppm H2 in dry air. The sample notation and the

preparation conditions of samples are depicted in Table 1. For

instance, RAs sample was as-prepared, without Pd film depo-

sition, and then lateral Au electrode (IDE) coated ZnO nano-

rods. Pd/RAs sample was as-prepared and 15 Pd thin film

coated on to ZnO nanorods, and then lateral Au IDE deposited.

Table 1 e The sensor device notation and the preparationconditions of the devices.

No Device Operatingprinciple

Electrodestructure

Pd film Heattreatment

1 RAs Resistance-

Based

IDE/ZnO

nanorod

No No

2 Pd/RAs Resistance-

Based

IDE/ZnO

nanorod

Yes Yes

3 Pd/RHeated Resistance-

Based

IDE/ZnO

nanorod

Yes Yes

4 Pd/WAs Work

Function

Based

Pd/ZnO

Nanorod/ITO

Yes No

5 Pd/WHeated Work

Function

Based

Pd/ZnO

Nanorod/ITO

Yes Yes

For H2 gas sensing measurements, a constant voltage was

applied to sensor devices and the current was monitored

continuously and recorded using an IEEE 488 data acquisition

system incorporated into a PC during the measurements.

3. Results and discussion

3.1. Structural characterization

The morphologies of ZnO nanorods on ZnO seed layer were

studied by SEM. Fig. 2(a) shows the SEM image of ZnO nano-

rods. ZnO nanorods with average diameters of 60 nm have

been synthesized conveniently, and the ZnO nanorods are

vertically arranged with hexagonal shape on the substrate.

The detailed structural characterization, fabrication process

and the nanorod growth mechanism were discussed previ-

ously [52e54]. Fig. 2(b) shows UVevis spectrum of as-prepared

and annealed ZnO nanorods. UVevis analysis of the nanorods

was carried out at room temperature. A sharp UV absorption

at a wavelength of 374 nm (ca. 3.31 eV) and 382 nm (ca. 3.24)

was found for annealed and as-prepared ZnO nanorods,

respectively as seen in Fig. 2(b). This near UV absorption for

Fig. 2 e (a) SEM image of as-prepared ZnO nanorods and (b)

UVevis spectrum of as-prepared and annealed ZnO

nanorods.

Fig. 3 e The currentevoltage characteristic of as-prepared

(RAs) ZnO nanorods under dry air and exposed to 1000 ppm

H2 at 200 �C.

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 9 ( 2 0 1 4 ) 5 1 9 4e5 2 0 1 5197

annealed and as-prepared ZnO nanorods agreeswith the band

gap of bulk ZnO [55] which mainly originated from the free

carrier or excitonic transition between the valance band and

the conduction band. The band gap of ZnO nanorods

increasedwith annealing from 3.24 eV to 3.31 eV. The increase

in the band gap may be related to decline in the oxygen va-

cancy of the nanorods with annealing in air. The annealing of

ZnOunder oxygen ambient causes the decrease in the number

of oxygen vacancy or oxygen deficiency [56].

3.2. Hydrogen sensing measurements

3.2.1. Lateral electrode typeGenerally, Au, Pd and Ag metals make Schottky contacts to

ZnO material [57]. Fig. 3 shows the currentevoltage (IeV)

Fig. 4 e The current versus time graphs of as-prepared (RAs) (ae

nanorod samples exposed to 500 ppm H2 at room temperature

characteristic of as-prepared (RAs) ZnO nanorods that coated

with Au IDE electrodes under dry air and exposed to 1000 ppm

H2 at 200 �C. The symmetric IeV relation from the

AueZnOeAu confirms Schottky junction formation between

Au and ZnO nanorods both under dry air and exposed to

1000 ppm H2. The IeV characteristics of a Schottky diode are

represented by Ref. [58]

I ¼ I0

�exp

�qVnkT

�� 1

�(1)

where I0 is the saturation current based on thermionic emis-

sion theory, n is the ideality factor, k is the Boltzmann’s con-

stant, T is the absolute temperature. The magnitude of the

current of RAs ZnO nanorods sample towards 1000 ppm H2 is

higher than that of the sample under dry air in the whole

voltage range as seen in Fig. 3. The increase in the currentwith

exposing H2 could be explained with the decrease in the bar-

rier height between contact electrode and ZnO [13,49e51].

Fig. 4 shows current versus time graphs of as-prepared (RAs)

and 15 nm Pd coated as-prepared (Pd/RAs) ZnO nanorod

samples exposed to 500 ppm H2 at room temperature and

200 �C. After the current of the sensor samples was measured

under dry air flow for 25 min to obtain base line, 500 ppm H2

was flowed into the chamber for enough time to observe

response time. After the sensors had been come to saturation,

the sensorswere purgedwith dry air. This process that defined

as exposure to H2 and purging with dry air repeated two times

for observing repeatability of the sensors as seen in Fig. 4. It is

clearly seen from Fig. 4(a) that the current of RAs sample

increased slowly from 32 pA to 36 pAwhen expose to 500 ppm

H2, but the current of RAs sample did not recoverwhile purging

with dry air. The second exposure to 500 ppm H2 and purging

with dry air showed similar behavior. On the other hand, RAs

sensor sample exhibited recover response to H2 at a temper-

ature of 200 �C as seen in Fig. 4(b). The current of RAs sample

b) and 15 nm Pd coated as-prepared (Pd/RAs) (ced) ZnO

and 200 �C.

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 9 ( 2 0 1 4 ) 5 1 9 4e5 2 0 15198

increased rapidly from 20 nA to 34 nAwhen expose to 500 ppm

H2, and then came to saturation. The current of RAs sample

fully recovered to the base line while purging with dry air. The

sensing of H2 by ZnO nanorods could be explained as follow;

when oxygen adsorbed on ZnO nanorod surface as O2�, O� and

O2� ions, an electron extracted from the conduction band

[59,60]. The O2�, O� and O2� ions are stable 100 �C, between 100

and 300 �C and above 300 �C respectively [61]. The sensing

mechanism of a reduced gas such as hydrogen is based on

interaction between adsorbed oxygen on ZnOnanorod surface

and hydrogen gas. The reaction between hydrogen and oxy-

gen ions produces H2O molecules and consumes adsorbed

oxygen on the surface by releasing electrons. These electrons

will be back to the conduction band and causes an increase in

the current of ZnO nanorods. These electrons also provide in a

reduction of surface depletion region and increase conduc-

tivity of the nanorods.

Pd/RAs sensor sample shows fully recover and repeatable

response to 500 ppm H2 at both room temperature and 200 �Cas given in Fig. 4(c) and (d). The current of Pd/RAs sample

increased rapidly from approximately 5 mA and 7 mA to 65 mA

and 465 mA at room temperature and 200 �C respectively when

expose to 500 ppm H2, and then came to saturation. The cur-

rent of RAs sample fully recovered to the base line while

purging with dry air for both indicated temperature as seen in

Fig. 4(c) and (d). Pd film coating on ZnOnanorod provides rapid

and easy recoverability compared to uncoated nanorods at

room temperature as seen in Fig. 4(a) and (c). The H2 sensing

mechanism of Pd/RAs at room temperature can be explained

with catalytic properties of Pd to H2 and dissociation oxygen

molecules at low temperatures by Pd thin film. Similarly, Pd-

coated ZnO nanostructures showed high and fast hydrogen

response at room temperature [10,43,44]. The increase in the

current of Pd/RAs samplewas higher than that of RAs sample at

both room temperature and 200 �C as seen in Fig. 4.

The sensing properties of ZnO nanorods can be compared

by determining their sensor response which is defined as

Sensor responseðRÞ ¼ DI=I0 (2)

Fig. 5 e The temperature dependence the sensor response

graph for Pd/RAs (a) and Pd/RHeated (b) samples exposure to

500 ppm H2.

where DI is the change in current and I0 is the reference

value (baseline) of the nanorod devices exposed to H2. The

sensor response of RAs sample was in the range of 0.8e1.1 at

the temperature interval of 100e200 �C and lower than the

sensor response of both Pd/RAs and Pd/RHeated samples.

Fig. 5 shows temperature dependence the sensor response

graph for Pd/RAs and Pd/RHeated samples exposure to

500 ppm H2. It is clearly seen that Pd/RAs sample is more

sensitive to H2 than Pd/RHeated. The sensor response of as-

prepared sample was higher than that of annealed ones

for all measured temperature. As a result, Pd/RAs (as-pre-

pared and Pd film coated ZnO nanorods) sample shows

highest sensor response at all measured temperatures. The

low sensor response for annealed sample could be

explained with decreasing oxygen vacancies of the ZnO

nanorod by heating in air. The annealing process in dry air

significantly increases both the quantity and the rate of

oxygen, and this causes a decrease in oxygen vacancies on

the surface. On the other hand, the sensor response of both

Pd/RAs and Pd/RHeated samples increases with increasing

temperature as given in Fig. 5.

Fig. 6 e The sensor response versus time graphs of as-

prepared (Pd/WAs) (a) and annealed (Pd/WHeated) (b) ZnO

nanorod samples exposed to 200e3000 ppm H2 at room

temperature.

Fig. 7 e The sensor responses of Pd/WAs (a) and Pd/WHeated

(b) samples as a function of concentration at room

temperature and 200 �C.

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 9 ( 2 0 1 4 ) 5 1 9 4e5 2 0 1 5199

3.2.2. Sandwich electrode typeFig. 6 shows sensor response versus time graphs of as-

prepared (Pd/WAs) and annealed (Pd/WHeated) ZnO nanorod

samples exposed to 200e3000 ppm H2 at room temperature.

The H2 sensing mechanism of sandwich electrode was

differed from lateral electrode type. The mechanism for

sandwich electrode is based on Schottky barrier height be-

tween metal electrode and ZnO. The Pd thin film acts as a

Table 2 e A summary of sensor response for all sensordevices exposure to 500 ppm H2 at indicatedtemperatures.

Device Temperature (�C)

25 100 200

RAs No recovery 0.9 0.8

Pd/RAs 11.4 22.2 79.7

Pd/RHeated 1.4 4.4 27

Pd/WAs 13.5 2.21 14.94

Pd/WHeated 0.25 1.1 5.89

catalyst where the dissociation of hydrogenmolecules occurs.

The dissociation of H2 in to Pd electrode causes a decrease in

work function of the electrode and then Schottky barrier

height at the interface between the electrode and ZnO de-

creases [10]. Das et al. fabricated individual ZnO NW for

Schottky diode type hydrogen sensor by using Pt top electrode,

and reported a high sensitive and fast response at RT [62].

Sensor responses of both Pd/WAs and Pd/WHeated samples

increased with enhancing concentration as given in Fig. 6.

Sensor response of Pd/WAs sample was higher than that of Pd/

WHeated sample for measured concentrations as seen in Fig. 6.

Sensor responses of Pd/WAs and Pd/WHeated samples for

3000 ppm H2 were approximately 32 and 0.6 respectively. The

low sensor response for annealed sample could be explained

with decreasing oxygen vacancies of the ZnO nanorod by

heating in air. Similar behavior was observed for ZnO nanorod

with lateral electrode type.

Fig. 7 shows the sensor responses of Pd/WAs and Pd/

WHeated samples as a function of concentration at room tem-

perature and 200 �C. The sensor responses of the samples

increased with increasing concentration at the indicated

temperatures. The sensor response of Pd/WAs sample was

higher than that of Pd/WHeated sample at the temperature in-

terval of 25e200 �C as seen in Fig. 7. The enhance in the

temperature causes an increase in the sensor response for

measured concentration. Table 2 shows a summary of sensor

response for all samples exposure to 500 ppm H2 at various

temperatures. The highest sensor response was observed for

Pd/RAs sensor device. The sensors with lateral electrode type

showed higher sensor response for both as-prepared and

annealed samples compare to sandwich type electrode. The

Pd film coating enhanced H2 sensor response and decreased

working temperature to even room temperature.

4. Conclusions

Hydrogen gas sensor based on ZnO nanorods were investi-

gated depending on annealing, Pd coating, temperature and

electrode structure. ZnO nanorods were fabricated by using

hydrothermal process at 90 �C for 3 h. The Pd coated ZnO

nanorods are suitable for practical applications in high

hydrogen sensitive at low hydrogen concentrations at room

temperature. Annealing of ZnO nanorod showed low sensor

response compare to as-prepared ZnO nanorod due to the

decreasing oxygen vacancies of the ZnO nanorod by heating

in air. The sensor response of ITO/ZnO nanorods/Pd sand-

wich type sensor was lower than the response of Au IDE

coated ZnO nanorods lateral electrode structure. The sensing

mechanism for both electrode structures could be explained

with the change in the barrier height between contact elec-

trode and ZnO.

Acknowledgments

This study was supported by Scientific and Technological

Research Council of Turkey (TUB_ITAK) entitled “Development

of Automotive Gas Sensors Based on Nano-Metal-Oxide

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 9 ( 2 0 1 4 ) 5 1 9 4e5 2 0 15200

Semiconductor with increased Selectivity, Sensitivity and

Stability” (Project Number: 111M261). The authors thank to Dr.

Anıl Gunay-Demirkol for SEM measurements.

r e f e r e n c e s

[1] Momirlan M, Veziroglu TN. Current status of hydrogenenergy. Renew Sustain Energy Rev 2002;6:141e79.

[2] Buttner WJ, Post MB, Burgess R, Rivkin C. An overview ofhydrogen safety sensors and requirements. Int J HydrogenEnergy 2011;36(3):2462e70.

[3] Mazloomi K, Gomes C. Hydrogen as an energy carrier:prospects and challenges. Renew Sustain Energy Rev2012;16:3024e33.

[4] Rosales-Colunga LM, Razo-Flores E, Ordonez LG, Alatriste-Mondragon F, De Leon-Rodriguez A. Hydrogen production byEscherichia coli DhycA DlacI using cheese whey as substrate.Int J Hydrogen Energy 2010;35:491e9.

[5] Holladay JD, Hu J, King DL, Wang Y. An overview of hydrogenproduction technologies. Catal Today 2009;139:244e60.

[6] Davila-Vazquez G, Arriaga S, Alatriste-Mondragon F, Leon-Rodriguez A, Rosales-Colunga L, Razo-Flores E. Fermentativebiohydrogen production: trends and perspectives. RevEnviron Sci Biotechnol 2008;7:27e45.

[7] Linke S, Dallmer M, Werner R, Moritz W. Low energyhydrogen sensor. Int J Hydrogen Energy2012;37(22):17523e8.

[8] Lee EB, Hwang IS, Cha JH, Lee HJ, Lee WB, Pak JJ, et al.Micromachined catalytic combustible hydrogen gas sensor.Sens Actuators B Chem 2011;153:392e7.

[9] Korotcenkov G, Do Han S, Stetter JR. Review ofelectrochemical hydrogen sensors. Chem Rev2009;109:1402e33.

[10] Gu HS, Wang Z, Hu YM. Hydrogen gas sensors based onsemiconductor oxide nanostructures. Sensors2012;12:5517e50.

[11] Noh JS, Lee JM, Lee W. Low-dimensional palladiumnanostructures for fast and reliable hydrogen gas detection.Sensors 2011;11:825e51.

[12] Kilinc N. Resistive hydrogen sensors based onnanostructured metals and metal alloys. NanosciNanotechnol Lett 2013;5:825e41.

[13] Shafiei M, Yu J, Arsat R, Kalantar-zadeh K, Comini E,Ferroni M, et al. Reversed bias Pt/nanostructured ZnOSchottky diode with enhanced electric field for hydrogensensing. Sens Actuators B Chem 2010;146:507e12.

[14] Chiu S-Y, Huang H-W, Huang T-H, Liang K-C, Liu K-P, Tsai J-H, et al. Comprehensive investigation on planar type ofPdeGaN hydrogen sensors. Int J Hydrogen Energy2009;34(13):5604e15.

[15] Henriksson J, Villanueva LG, Brugger J. Ultra-low powerhydrogen sensing based on a palladium-coatednanomechanical beam resonator. Nanoscale 2012;4:5059e64.

[16] Silva SF, Coelho L, Frazao O, Santos JL, Malcata FX. A reviewof palladium-based fiber-optic sensors for molecularhydrogen detection. IEEE Sens J 2012;12:93e102.

[17] Jakubik WP, Urbanczyk MW, Kochowski S, Bodzenta J.Bilayer structure for hydrogen detection in a surfaceacoustic wave sensor system. Sens Actuators B Chem2002;82:265e71.

[18] Sadek AZ, Wlodarski W, Li YX, Yu W, Li X, Yu X, et al. A ZnOnanorod based layered ZnO/64� YX LiNbO3 SAW hydrogengas sensor. Thin Solid Films 2007;515:8705e8.

[19] Aroutiounian V. Metal oxide hydrogen, oxygen, and carbonmonoxide sensors for hydrogen setups and cells. Int JHydrogen Energy 2007;32(9):1145e58.

[20] Comini E, Faglia G, Sberveglieri G, Pan Z, Wang ZL. Stable andhighly sensitive gas sensors based on semiconducting oxidenanobelts. Appl Phys Lett 2002;81:1869e71.

[21] Tiginyanu IM, Lupan O, Ursaki VV, Chow L, Enachi M. 3.11-Nanostructures of metal oxides. In: Bhattacharya Pallab,Fornari Roberto, Kamimura Hiroshi, editors. Comprehensivesemiconductor science and technology. Amsterdam:Elsevier; 2011. pp. 396e479.

[22] Kolmakov A, Moskovits M. Chemical sensing and catalysis byone-dimensional metal-oxide nanostructures. Annu RevMater Res 2004;34:151e80.

[23] Lu JG, Chang P, Fan Z. Quasi-one-dimensional metal oxidematerialsdsynthesis, properties and applications. Mater SciEng 2006;52:49e91.

[24] Comini E, Baratto C, Faglia G, Ferroni M, Vomiero A,Sberveglieri G. Quasi-one dimensional metal oxidesemiconductors: preparation, characterization andapplication as chemical sensors. Prog Mater Sci 2009;54:1e67.

[25] Wang ZL. Zinc oxide nanostructures: growth, properties andapplications. J Phys-Condens Matter 2004;16:R829e58.

[26] Pearton SJ, Ren F, Wang Y-L, Chu BH, Chen KH, Chang CY,et al. Recent advances in wide bandgap semiconductorbiological and gas sensors. Prog Mater Sci 2010;55:1e59.

[27] Tricoli A, Righettoni M, Teleki A. Semiconductor gas sensors:dry synthesis and applications. Angew Chem Int Ed2010;49:7632e59.

[28] Franke ME, Koplin TJ, Simon U. Metal and metal oxidenanoparticles in chemiresistors: does the nanoscale matter.Small 2006;2:36e50.

[29] Fine GF, Cavanagh LM, Afonja A, Binions R. Metal oxidesemiconductor gas sensors in environmental monitoring.Sensors 2010;10:5469e502.

[30] Kanan SM, El-Kadri OM, Abu-Yousef IA, Kanan MC.Semiconducting metal oxide based sensors for selective gaspollutant detection. Sensors 2009;9:8158e96.

[31] Afzal A, Cioffi N, Sabbatini L, Torsi L. NOx sensors basedon semiconducting metal oxide nanostructures: progressand perspectives. Sens Actuators B Chem2012;171e172:25e42.

[32] Forleo A, Francioso L, Capone S, Siciliano T, Lommens P,Hens Z. Synthesis and gas sensing property of ZnO quantumdots. Sens Actuators B Chem 2010;146:111e5.

[33] Oh E, Choi HY, Jung SH, Cho S, Kim JC, Lee KH, et al. Highperformance NO2 gas sensor based on ZnO nanorod grownby ultrasonic irradiation. Sens Actuators B Chem2009;141:239e43.

[34] Chang SC. Oxygen chemisorption on tin oxide: correlationbetween electrical conductivity and E.P.R. measurements. JVac Sci Technol Arch 1980;17:366e9.

[35] Yang Z, Li L-M, Wan Q, Liu Q-H, Wang T-H. High-performance ethanol sensing based on an aligned assemblyof ZnO nanorods. Sens Actuators B Chem 2008;135:57e60.

[36] Huang MH, Mao S, Feick H, Yan H, Wu Y, Kind H, et al. Room-temperature ultraviolet nanowire nanolasers. Science2001;292:1897e9.

[37] Wan Q, Li QH, Chen YJ, Wang TH, He XL, Li JP, et al.Fabrication and ethanol sensing characteristics of ZnOnanowire gas sensors. Appl Phys Lett 2004;84:3654e6.

[38] Cao Y, Hu P, Pan W, Huang Y, Jia D. Methanal and xylenesensors based on ZnO nanoparticles and nanorods preparedby room-temperature solid-state chemical reaction. SensActuators B Chem 2008;134:462e6.

[39] Liewhiran C, Camenzind AR, Teleki A, Pratsinis SE,Phanichphant S. Doctorbladed thick films of flame-made Pd/ZnO nanoparticles for ethanol sensing. Curr Appl Phys2008;8:336e9.

[40] Tamaekong N, Liewhiran C, Wisitsoraat A, Phanichphant S.Acetylene sensor based on Pt/ZnO thick films as prepared by

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 9 ( 2 0 1 4 ) 5 1 9 4e5 2 0 1 5201

flame spray pyrolysis. Sens Actuators B Chem2011;152:155e61.

[41] Bittencourt C, Llobet E, Ivanov P, Correig X, Vilanova X,Brezmes J, et al. Influence of the doping method on thesensitivity of Pt-doped screen-printed SnO2 sensors. SensActuators B Chem 2004;97:67e73.

[42] Wongrat E, Pimpang P, Choopun S. Comparative study ofethanol sensor based on gold nanoparticles: ZnOnanostructure and gold:ZnO nanostructure. Appl Surf Sci2009;256:968e71.

[43] Chang C-M, Hon M-H, Leu I-C. Outstanding H2 sensingperformance of Pd nanoparticle-decorated ZnO nanorodarrays and the temperature-dependent sensingmechanisms. ACS Appl Mater Interfaces 2013;5:135e43.

[44] Xing L-L, Ma C-H, Chen Z-H, Chen Y-J, Xue X-Y. High gassensing performance of one-step-synthesized PdeZnOnanoflowers due to surface reactions and modifications.Nanotechnology 2011;22:215501.

[45] Wei SH, Yu Y, Zhou MH. CO gas sensing of Pd-doped ZnOnanofibers synthesized by electrospinning method. MaterLett 2010;64:2284e6.

[46] Liao L, Lu HB, Li JC, He H, Wang DF, Fu DJ, et al. Sizedependence of gas sensitivity of ZnO nanorods. J Phys ChemC 2007;111:1900e3.

[47] Motaung DE, Mhlongo GH, Kortidis I, Nkosi SS, Malgas GF,Mwakikunga W, et al. Structural and optical properties ofZnO nanostructures grown by aerosol spray pyrolysis:candidates for room temperature methane and hydrogen gassensing. Appl Surf Sci 2013;279:142e9.

[48] Huh J, Park J, Kim GT, Park JY. Highly sensitive hydrogendetection of catalyst-free ZnO nanorod networks suspendedby lithography-assisted growth. Nanotechnology2011;22:085502.

[49] Yu J, Ippolito SJ, Wlodarski W, Strano M, Kalantar-zadeh K.Nanorod based Schottky contact gas sensors in reversed biascondition. Nanotechnology 2010;21:265502.

[50] Lupan O, Chai G, Chow L. Novel hydrogen gas sensor basedon single ZnO nanorod. Microelectron Eng 2008;85:2220e5.

[51] Wang JX, Sun XW, Yang Y, Huang H, Lee YC, Tan OK, et al.Hydrothermally grown oriented ZnO nanorod arrays for gassensing applications. Nanotechnology 2006;17:4995.

[52] Ozturk S, Kilinc N, Tasaltin N, Ozturk ZZ. A comparativestudy on the NO2 gas sensing properties of ZnO thin films,nanowires and nanorods. Thin Solid Films 2011;520:932e8.

[53] Ozturk S, Kilinc N, Tasaltin N, Ozturk ZZ. Fabrication of ZnOnanowires and nanorods. Phys Low-Dimens Syst Nanostruct2012;44:1062e5.

[54] Ozturk S, Kilinc N, Ozturk ZZ. The effects of annealing on gassensing properties of ZnO nanorod sensors coated with Pdand Pt. Procedia Eng 2012;47:434e7.

[55] Wong EM, Searson PC. Electrophoretic deposition of ZnOquantum particle thin films. Appl Phys Lett 1999;74:2939.

[56] [a]Lupan O, Pauporte T, Chow L, Viana B, Pelle F, Ono LK,et al. Effects of annealing on properties of ZnO thin filmsprepared by electrochemical deposition in chloride medium.Appl Surf Sci 2010;256:1895e907;[b]Wei S, Lian J, Wu H. Annealing effect on thephotoluminescence properties of ZnO nanorod arrayprepared by a PLD-assistant wet chemical method. MaterCharact 2010;61:1239e44;[c]Li J, Huang J-H, Zhang Y-L, Yang Y, Song W-J, Li X-M.Effects of rapid thermal annealing in different ambients onstructural, electrical, and optical properties of ZnO thin filmsby solegel method. J Electroceram 2011;26:84e9.

[57] Pearton SJ, Norton DP, Ip K, Heo YW, Steiner T. Recentprogress in processing and properties of ZnO. Prog Mater Sci2005;50:293e340.

[58] Sze SM, Ng KK. Physics of semiconductor devices. NewJersey: John Wiley & Sons; 2007.

[59] Raju AR, Rao CNR. Gas-sensing characteristics of ZnO andcopper-impregnated ZnO. Sens Actuators B Chem1991;3:305e10.

[60] Saito S, Miyayama M, Kuomoto K, Yanagida H. Gas sensingcharacteristic of porous ZnO and Pt/ZnO ceramics. J AmCeram Soc 1985;68:40e3.

[61] Wetchakun K, Samerjai T, Tamaekong N, Liewhiran C,Siriwong C, Kruefu V, et al. Semiconducting metal oxides assensors for environmentally hazardous gases. SensActuators B Chem 2011;160:580e91.

[62] Das SN, Kar JP, Choi J-H, Lee TI, Moon K-J, Myoung J-M.Fabrication and characterization of ZnO single nanowire-based hydrogen sensor. J Phys Chem C 2010;114:1689e93.