Fe dopants enhancing ethanol sensitivity of ZnO thin filmdeposited by RF magnetron sputtering

M. Mehedi Hassan • Wasi Khan • A. H. Naqvi •

Prabhash Mishra • S. S. Islam

Received: 13 February 2014 / Accepted: 22 May 2014 / Published online: 13 June 2014

� Springer Science+Business Media New York 2014

Abstract Pure and 5 % Fe-doped ZnO thin films (TFs)

have been successfully deposited on Al2O3 substrate from

pre-doped target material by RF magnetron sputtering

technique. X-ray diffraction (XRD) patterns confirm the

formation of both films in single phase wurtzite structure

without any extra impurity peak. The calculated average

crystallite sizes are found to be 22 and 17 nm for pure and

Fe-doped ZnO TFs, respectively. The broadening in XRD

peaks of Fe-doped ZnO TF occurs due to decrease in crys-

tallite size and increase in lattice strain. Field emission

scanning electron microscopy images exhibit that the par-

ticles growth in Fe-doped ZnO TF is more uniform and

smaller than pure ZnO. Energy dispersive X-ray spectros-

copy and Fourier transform infrared spectroscopy results

confirm the existence of Fe dopants into ZnO matrix. The

doping effect enhances the sensitivity of ZnO sensor almost

three times for ethanol gas sensing, the improvement in the

response time and recovery time is noticeable as the size

reduction effect increases the surface to volume ratio, and

resulting more numbers of ethanol gas molecules are

adsorbed to produce a higher concentration of oxygen ions

which leads a larger deviation in capacitance.

Introduction

Modern advancements in nanomaterials offer an outlook to

dramatically boost up the response of these materials,

because of their performance is directly communicated

with surface to volume ratio. The availability of various

metal oxide nanostructures materials proposes new

opportunities for enhancing the properties and perfor-

mances of gas sensors because of higher surface to volume

ratio in nanomaterials compared to bulk materials. A very

long time zinc oxide (ZnO) is being used as promising

materials for gas sensing applications because of its high

chemical stability, suitability to doping, variety of different

structures, non-toxicity, abundance in nature, low cost,

large binding energy of 60 meV, and a wide band gap of

3.37 eV [1–3]. Beside all these, ZnO sensors offer a large

multiplicity of advantages, such as short response time,

easy manufacturing, small in size, and good detection

capabilities of combustible and toxic gases compared with

the as usual analytical instruments [4–6]. Gas sensors based

on low dimensional ZnO nanostructures in forms of single

crystals, sintered pellets, thick films, and thin films, [7–11]

have been already developed to monitor the gases such as

ethanol, H2, CO, NH3, etc. [12, 13]. Among all these

structures, thin films are most suitable as they have good

sensitivity, can be fabricated in small dimensions on a large

scale, low cost, and are usually integrated to microelec-

tronic technology and circuits.

M. M. Hassan � W. Khan (&) � A. H. Naqvi

Centre of Excellence in Materials Science (Nanomaterials),

Department of Applied Physics, Z.H. College of Engineering

and Technology, Aligarh Muslim University, Aligarh 202002,

India

e-mail: wasiamu@gmail.com

M. M. Hassan

e-mail: mehedinano@gmail.com

A. H. Naqvi

e-mail: alimhnaqvi@yahoo.com

P. Mishra � S. S. Islam

Nano-Sensor Research Laboratory, F/O Engineering and

Technology, Jamia Millia Islamia (Central University),

New Delhi 110025, India

e-mail: prabhash786@gmail.com

S. S. Islam

e-mail: safiul_el@rediffmail.com

123

J Mater Sci (2014) 49:6248–6256

DOI 10.1007/s10853-014-8349-2

Ethanol gas (vapor) sensors have many applications in

biomedical, chemical, to control of fermentation processes

[14], detect leaks in industrial distribution lines, safety

testing of food packaging, mainly quality monitoring of

wine, and also fix to monitor drunken driving [15, 16]. To

meet all these requirements, ethanol gas sensors have been

extensively studied. However, several problems such as

high operating temperature, poor selectivity, and relatively

low response have been detected in traditional ZnO gas

sensors, which limit their application against more expen-

sive approaches [17]. Most of these thin film-based gas

sensors operate at the temperature of over 200 �C, which

implement thermometer structure in sensor device for

heating. The heating effect also reduces the lifetime as well

as long-term stability of the sensors [18]. Hence, two

approaches are commonly utilized to overcome all these

problems. One approach is to prepare porous ZnO struc-

tures as huge amount of gas diffusion and mass transport

can take place because of the large surface to volume ratio

and high porosity [19, 20]. Another useful approach is

doping with metal or other metal oxides related to sensing

materials due to their catalytic activities or formation of

p–n hetero-junctions, they can improve gas sensitivity by

varying energy band gap, renovating the structural and

surface morphology, and creating more active center at the

grain boundaries [21, 22]. Compared with other metals, Fe

has often been doped in ZnO to improve optical, electrical,

and magnetic properties [23–26]. There are only a few

papers reported the improvement in ethanol gas sensing of

the Fe–ZnO sensor. That is why we aspired to explore the

ethanol sensing by Fe–ZnO TF in details. Fe-doped ZnO

TF can be deposited by different kinds of method such as

metal organic chemical vapor deposition (MOCVD) [27],

pulsed laser deposition (PLD) [28], sol–gel [29], atomic

layer deposition [30], radio-frequency (RF) magnetron

sputtering [24, 25], and laser molecular beam epitaxy (L-

MBE) [31]. Among them, the RF magnetron sputtering is

widely employed to grow ZnO TFs due to its easy control

over the preferred crystalline orientation, growing at rela-

tively low temperature, good interfacial adhesion to the

substrate, and the high packing density of the grown film.

In this work, we reported the ethanol gas sensing prop-

erties of ZnO and Fe-doped ZnO TFs deposited by RF

sputtering, which is demonstrating an excellent improve-

ment in ethanol gas sensing characteristics with Fe doping.3

Experiment and characterization

Thin film preparation

Fe-doped ZnO (0 and 5 %) TFs were deposited on

10 mm 9 10 mm Al2O3 (001) substrates by radio-

frequency (RF) magnetron sputtering at room temperature

using pure ZnO and Fe pre-doped ZnO targets instead of

the Zn and Zn–Fe strip targets that have previously been

used by many other researchers [24, 25]. For this present

work, the targets were prepared using a simple sol–gel

method. The diameter of the target was 50 mm and the

thickness was *3 mm. The substrate was thoroughly

cleaned using acetone, rinsed in deionized water, and dried.

The distance between target to substrate was fixed at

7.0 cm and the chamber pressure was kept at *5 9 10-3

mbar. The system was further pumped to keep the ultimate

pressure at 1 9 10-6 mbar, which was later raised to

3 9 10-2 mbar by adding high purity argon (Ar) gas in the

chamber. Prior to start the deposition on the substrate, the

target was pre-sputtered for 15 min to remove the con-

tamination of target surface keeping the substrate covered.

Both TFs were deposited at a fixed RF power of 100 W and

deposition time of 60 min keeping the Ar gas flow rate

constant. After completing sputtering, the deposited TFs

were taken out of the chamber and further transferred to a

furnace for annealing in the air for 2 h at 500 �C.

Characterization

The structural and morphological characterizations of pure

and 5 % Fe-doped ZnO TFs were carried out using X-ray

diffraction (XRD), Field emission scanning electron

microscopy (FESEM) embedded with energy dispersive

X-ray spectroscopy (EDS), and Fourier transform infrared

spectroscopy (FTIR). The crystalline phase, structure, and

crystallite size of Fe-doped ZnO TFs were investigated by

XRD (Rigaku Miniflex-II) with Cu-Ka radiations

(k = 1.5406 A) in 2h range from 30� to 70� operated at a

voltage of 30 kV and current 15 mA. The particle size and

surface morphology were analyzed by FESEM (NANO

NOVA). The elemental composition was explored by EDS.

The different chemical bonding present in the thin films

were investigated by FTIR spectra (Bruker Vertex 70 V) in

diffuse reflectance mode.

Gas sensing performance

In order to study the gas sensing properties, the capacitive

response of both TFs, 2 mm 9 2 mm electric points were

made with aluminum wire thermally evaporated through a

metal mask on the surface, and external wire contacts were

taken from these points. The ethanol vapor was formed by

heating the liquid solvents contained in a vessel of known

volume and was exposed to the sensor placed in a closed

chamber with nitrogen as a carrier gas flowing at a constant

rate. The schematic diagram of the gas sensing setup is

shown in Fig. 1. The response of the pure and Fe-doped

ZnO TFs for different concentration of ethanol gas was

J Mater Sci (2014) 49:6248–6256 6249

123

studied by the change in its capacitance through Keithley

590 CV analyzer connected to a computer, and the data

storage was controlled by Lab-View software. Both sensors

were tested with various concentrations of ethanol gas

(50–300 ppm) at 100 �C. It should be noted that techno-

logical application requires the ethanol to detect at least

200 ppm (*0. 6 g/L in the human blood).

Results and discussion

XRD studies

Figure 2 shows the XRD patterns of pure and 5 % Fe-

doped ZnO TFs. The XRD patterns have been analyzed

using PowderX software which shows that the both sam-

ples exhibit single phase wurtzite structure and are indexed

on the basis of JCPDS card No. 36-1451 [32]. No extra

phases corresponding to iron oxides were detected in the

XRD patterns which clarify that Fe incorporation does not

affect the crystal structure and has been successfully

substituted into the ZnO host structure at the Zn2? site.

The crystallite size and lattice parameters were calcu-

lated by the Debye–Scherrer’s formula [33]

D ¼ k � kb� cos h

ð1Þ

where k is the wavelength of X-ray radiation, b is the full

width at half maximum (FWHM) of the peaks at the dif-

fracting angle h, k is the shape constant. The average

crystallite size of pure ZnO TF is 22 nm and decreased to

17 nm for 5 % Fe doping as given in Table 1. The lattice

parameters of Fe–ZnO TF measured from the XRD data

are a = 3.265 A, c = 5.237 A, which are also slightly

smaller than pure ZnO (a = 3.272 A; c = 5.245 A). The

size reducing effects can be explained on the basis of ionic

radii. The ionic radius of Fe2? is 78 pm and Fe3? is 68 pm

in a tetrahedral coordination, whereas that of Zn2? is

74 pm. So it may be considered that Fe3? ions cause the

reduction of the lattice parameters due to the smaller ionic

radius which is already reported by some authors [34, 35].

XRD patterns also exhibit that, as the Fe ions induce, the

intensity of the most intense peak (002) decreases and the

width increases, which shows the degradation of crystal-

linity and decrease in crystallite size. The broadening of the

XRD peaks may also be explained on the basis of the effect

of microstrain (g). The average microstrain (g) of pure and

Fe-doped ZnO TFs was calculated using the Williamson-

Hall equation [36]:

b cos h ¼ kkDþ 4g sin h ð2Þ

With the help of this equation, b cos h is plotted against

in h. Drawing a linear extrapolation, the slope of the plot

gives the value of microstrain (4g) as shown in Fig. 3. The

strain of Fe-doped ZnO (0.2265) TF is greater than

undoped ZnO (0.1966) thin film, as Zn2? ions in the ZnO

Fig. 1 Schematic diagram of

sensing setup

Table 1 Variation of crystallite size, lattice parameters, and lattice

strain with doping concentration

Fe

doping

(%)

Crystallite

size (nm)

FWHM

(002)

Lattice

strain

Lattice

parameter

c/

a ratio

a (A) c (A)

0 21.58 0.3894 0.1966 3.272 5.245 1.60

5 17.02 0.4882 0.2265 3.265 5.237 1.60

6250 J Mater Sci (2014) 49:6248–6256

123

lattice has been substituted by Fe3? ions. This is already

proved by some authors that when crystallize size is

reducing and strain is increasing then Zn?2 is substituted

by Fe?3, not by Fe?2 [25, 26, 34]. It means that the doping

effect creates crystal defects and charge imbalance around

the dopants. The c/a ratio of both samples is 1.6 which is

quite equal to the packing fraction of the hexagonal crystal

structure. It can also be seen from Fig. 2, the intensity of

the peaks (101), (102), and (103) reduced on Fe doping,

which indicated that Fe-ZnO TF exhibited preferential

orientation along c-axis perpendicular to the substrate

surface [37]. So, we can conclude that Fe doping may

change the lattice parameters of ZnO TF because of the

different ionic radius of the dopant and host atoms but the

crystallinity remains significantly unaffected.

SEM and EDS analyses

The morphology and elemental analyses of both TFs were

studied by FESEM embedded with EDS. Figure 4a, b show

the FESEM micrographs for the pure and Fe-doped ZnO TFs,

respectively. Figure 4a shows that the particles in pure ZnO

is largely agglomerated, and some pores are visible. The

particles are almost spherical in shape but not uniform, on the

other hand FESEM image of Fe-doped ZnO TF as shown in

Fig. 4b is less agglomerated, equally distributed, and exhibit

uniform shape. The particle size is also smaller as compare to

pure ZnO. Few numbers of pores are also visible in the Fe-

ZnO TF which is very significant for gas sensing. Figure 5

shows the energy dispersive X-ray spectrum (EDS) for 5 %

Fe-doped ZnO TF. The EDS pattern reveals only the pre-

sence of Zn, Fe, and O elements, which confirms the high

purity of the product and the successful doping of Fe ions into

ZnO host structure. The Al peaks appeared in the EDS

spectrum from the substrate.

Fig. 2 XRD patterns of pure

and Fe-doped ZnO thin films

Fig. 3 Determination of lattice strain for pure and Fe-doped ZnO thin

films

J Mater Sci (2014) 49:6248–6256 6251

123

FTIR analysis

FTIR is a powerful technique to find out the information

about the chemical bonding in a material, identify the

elemental constituents of the sample, and it also supple-

ments the information obtained from XRD and EDS [24].

The FTIR spectra of both pure and Fe-doped ZnO TFs

were recorded in diffuse reflectance mode in the wave-

number region 400–4000 cm-1 and shown in Fig. 6. This

spectral region is very important because of several stretch

modes involving hydroxyl bond, carbon to oxygen, and

metal oxide bonds are obtained clearly in this range. A

strong absorption band informing about the vibrational

properties of ZnO is observed for each sample at

516 cm-1, which is assigned to the Zn–O stretching band

which is consistent with that reported elsewhere. This band

is shifted at 526 cm-1 for Fe-doped ZnO TF confirms Fe

doping into ZnO matrix. A significant spectroscopic band

at 632 cm-1 appears which is identified as absorption

bands of Al–O presence in Al2O3 substrate [38]. A peak

also visible near 771 cm-1, which is assigned to Fe–O in

Fe-doped ZnO TF confirms the existence of Fe ions [39].

Sensing analysis

The response and sensing characteristics of Fe (0 and 5 %)-

doped ZnO sensors to different concentration of ethanol

gas are shown in Figs. 7 and 8, respectively. Here, the

sensitivity (S), response time (TRes), and recovery time

(TRec), shown in Table 2 were calculated from the capac-

itance (C) time (T) data using the following equations:

S ¼ C90 % Gas � C10 % Gas

C10 % Gas

ð3Þ

TRes ¼ Tð10 % Gas�to�90 % GasÞ ð4Þ

TRec ¼ Tð100 % Gas�to�10 % GasÞ ð5Þ

To avoid the signal noise and errors, we have assumed

10 and 90 % change of capacitance value for each con-

centration of ethanol gas flow. It is clear from the Figs. 7

and 8 that the Fe-doped ZnO sensor shows significant

improvement in response to ethanol gas as compared to the

pure ZnO sensor. The stepwise regular decrease of the

electrical capacitance is very steady with the decrease of

ethanol concentration. The capacitance value becomes high

when a large number of electrons introduce in the surface

by ethanol oxidation. The XRD and FESEM results show

that when Fe ions is doped into ZnO structure the particle

size reduces and increases the surface to volume ratio

compare to the pure ZnO. As the TF has a higher specific

surface area, more numbers of ethanol gas molecules are

adsorbed and produce a higher concentration of oxygen

ions which lead to a larger deviation in capacitance and

higher response, thus improving its response as well as

recovery characteristics. The sensitivity (S) of Fe-ZnO

sensor (0.164) is three times greater than ZnO sensor

(0.0503) at 50 ppm of ethanol gas concentration, whereas it

is four times at 300 ppm (ZnO = 0.7279; Fe–ZnO =

2.913). It is also clear from the Table 2 as well as in the

Figs. 9 and 10 that Fe-ZnO sensor shows faster response

and recovery. The response and recovery time of ZnO

sensor reduces to 20 s from 29 and 60 to 38 s respectively

after Fe doping. From Fig. 7, it is also noticeable that the

total time for gas inject and release of undoped ZnO TF is

reasonably higher than Fe-doped ZnO TF, which again

confirms the improvement of ZnO based ethanol gas sensor

on Fe doping. This result is quite comparable with ZnO

based sensors reported previously [17]. In fact, oxygen

adsorption plays an important role in the electrical prop-

erties of pure and Fe-doped ZnO TFs by varying the

Fig. 4 FESEM images of a pure and b 5 % Fe-doped ZnO thin film

6252 J Mater Sci (2014) 49:6248–6256

123

conduction electrons. Reactive type oxygens such as O2-,

O2- and O- are adsorbed on the surface of ZnO TF at

higher temperature and the quantities strongly depend on

temperature. At low temperature only O2- is chemisorbed,

whereas at high temperature O2-and O- are chemisorbed,

and the O2- vanishes and the whole oxygen adsorption

mechanism can be illustrated as [40]:

O2ðgasÞ $ O2ðadsorbedÞ ð6Þ

O2ðadsorbedÞ þ e� $ O2ðadsÞ� ð7Þ

O2ðadsÞ2� þ e� $ 2OðlatÞ2� ð8Þ

where the subscripts gas, ads, and lat mean gas, adsorbed

and lattice, respectively. The oxygen species confine the

conduction electrons and creating an electron depleted

layer at the surface, results a change in the electron con-

centration. The ethanol sensing method of ZnO-based gas

sensor can be described as follows:

CH3CH2OHads þ 6Oads� ! 2CO2 þ 3H2Oþ 6e� ð9Þ

As ethanol gas flows into the test chamber and is con-

sequently adsorbed on the surface of the ZnO sensor the

capacitance increases. It is also noticed that at a relatively

lower operating temperature, our Fe-doped ZnO sensor still

shows a comparatively higher response to ethanol. When

Fe ions are induced to ZnO nuclei, the synergetic effect

from both Fe and ZnO sensing materials can efficiently

activate the dissociation of oxygen molecules at lower

temperature, and thus lowering the sensor operating tem-

perature and enhancing the sensor response to the target

gas. In our case, Fe-doped ZnO gas sensor is taking the

relevant ionic radius (Zn2? = 74 pm, Fe3? = 68 pm) into

account as Fe3? generally substitutes the Zn2? partially in

the ZnO crystallites and forms Fe donors, where more

electrons might be originated at lower temperature through

the O2- and O- are not generated [41]. In order to explain

the doping of Fe3? into ZnO structure, the equations will

be written using Kroger and Vink’s notation [42], and

considering the different possibilities of doping of Fe3? in

the ZnO structure, i.e., may occupy a regular site of the

Zn2? or an interstitial site, or still both positions simulta-

neously, as follows [43]:

Fe2O3 þ V��o �!ZnO

2Fe0Zn þ 3O�o ð10Þ

Fe2O3�!ZnO

2Fe��Zn þ V 00Zn þ 3Oo: ð11Þ

Thus, the extra oxygen will add to the capacitance of the

film and increase the response signal. After gaining the

Fig. 5 EDS image of 5 % Fe-

doped ZnO thin film

Fig. 6 FTIR spectra for pure and Fe-doped ZnO thin films

J Mater Sci (2014) 49:6248–6256 6253

123

maximum value, the capacitance returns almost the origi-

nal value when ethanol gas flowing is stopped. This fact

proves the reversibility of the process. In this case, Fe-ZnO

sensor also shows better reversibility compare to the pure

ZnO sensor. All these data suggest that the Fe-ZnO is a

promising sensor for detecting ethanol gas at a compara-

tively lower temperature, and Fe additive plays a crucial

role to improve the sensing parameters.

Conclusion

To sum up, pure and 5 % Fe-doped ZnO TFs have been

successfully deposited by RF sputtering. We have used

single pre-doped target material for Fe-doped ZnO TF

which suggests a new approach to deposit doped metal

oxide TFs by RF sputtering. It has been observed from

XRD results that both TFs are in single phase without any

Fig. 7 Real-time measurement

of capacitive response for

ethanol vapor

Table 2 Variation of sensitivity, response time, and recovery time

with ethanol concentration for both films

Ethanol

concentration

(ppm)

Sensitivity (S) Response time

(s)

Recovery time

(s)

ZnO Fe–ZnO ZnO Fe–ZnO ZnO Fe–ZnO

50 0.0503 0.164 21 12 177 60

75 0.1944 0.393 18 9 154 30

100 0.1135 0.858 16 10 146 32

125 0.1293 1.279 13 11 69 34

150 0.1713 1.694 15 12 66 31

175 0.1874 1.966 17 14 64 40

200 0.2529 2.229 19 15 62 35

225 0.382 2.418 21 16 73 29

250 0.6337 2.539 24 18 75 40

275 0.6525 2.734 26 19 73 42

300 0.7279 2.913 29 20 60 39Fig. 8 Variation of sensitivity to ethanol concentration for both

sensors

6254 J Mater Sci (2014) 49:6248–6256

123

impurity and the crystallite size has decreased on Fe dop-

ing. The gas sensing measurements showed that compared

with pure ZnO, Fe-ZnO sensor exhibits quicker and higher

response and better reversibility to ethanol at a relatively

low temperature. Fe ions greatly influence the sensing

mechanism by increasing the active adsorption center on

the surface of ZnO TF. The present work provides some

new light on the development of low cost and highly

sensitive practical gas sensor.

Acknowledgements The authors are grateful to the Council of

Science & Technology (CST), Govt. of UP, India for financial support

to Centre of Excellence in Materials Science (Nanomaterials). One of

the authors (M. Mehedi Hassan) is thankful to the University Grants

Commission (UGC) for providing financial support in the form of

Maulana Azad National Fellowship. The authors gratefully

acknowledge the financial support provided by Ministry of Commu-

nication and Information Technology (MCIT), Govt. of India to

Nano-Sensor Research Laboratory.

References

1. Gedamu D, Paulowicz I et al (2014) Rapid fabrication technique

for interpenetrated ZnO nanotetrapod networks for fast UV sen-

sors. Adv Mater 26:1541–1550

2. Jin X et al (2013) A novel concept for self-reporting materials:

stress sensitive photoluminescence in ZnO tetrapod filled elas-

tomers. Adv Mater 25:1342–1347

3. Mishra YK et al (2012) Crystal growth behaviour in Au-ZnO

nanocomposite under different annealing environments and

photoswitchability. J Appl Phys 12:064308-5

4. Rout CS, Raju AR, Govindaraj A et al (2007) Ethanol and

hydrogen sensors based on ZnO nanoparticles and nanowires.

J Nanosci Nanotechnol 7:1923–1929

5. Fan Z et al (2004) Photoluminescence and polarized photode-

tection of single ZnO nanowires. Appl Phys Lett 85:6128–6130

6. Xing XY, Zheng KB, Chen GR et al (2006) Synthesis and

electrical properties of ZnO nanowires. Micron 37:370–373

7. Alenezi MR, Henley SJ et al (2014) From 1D and 2D ZnO

nanostructures to 3D hierarchical structures with enhanced gas

sensing properties. Nanoscale 6:235–247

8. Baruwati B, Kumar DK et al (2006) Hydrothermal synthesis of

highly crystalline ZnO nanoparticles: a competitive sensor for

LPG and EtOH. Sens Actuators B 119:676–682

9. Liu X, Zhang J, Wang L et al (2011) 3D hierarchically porous

ZnO structures and their functionalization by Au nanoparticles

for gas sensors. J Mater Chem 21:349–356

10. Suchea M, Christoulakis S, Moschovis K et al (2006) ZnO

transparent thin films for gas sensor applications. Thin Solid

Films 515:551–554

11. Mridha S, Basak D (2006) Investigation of a p-CuO/n-ZnO thin

film heterojunction for H2 gas-sensor applications. Semicond Sci

Technol 21:928–932

12. Wan Q, Li QH, Chen YJ, Wang TH (2004) Fabrication and

ethanol sensing characteristics of ZnO nanowire gas sensors.

Appl Phys Lett 84:3654–3656

13. Wang JX, Sun XW, Yang Y et al (2006) Hydrothermally grown

oriented ZnO nanorod arrays for gas sensing applications.

Nanotechnology 17:4995–4998

14. de Lacy Costello BPJ, Ewen RJ et al (2000) The development of

a sensor system for the early detection of soft rot in stored potato

tubers. Meas Sci Technol 11:1685–1691

15. Zeng W, Liu T, Wang ZC (2010) UV light activation of TiO2-

doped SnO2 thick film for sensing ethanol at room temperature.

Mater Trans 51:243–245

16. Ho JJ, Fang YK, Wu KH, Hsieh WT et al (1998) High sensitivity

ethanol gas sensor integrated with a solid-state heater and thermal

isolation improvement structure for legal drink-drive limit

detecting. Sens Actuators B 50:227–233

17. Zhang W-H, Zhang W-D, Zhou J-F (2010) Solvent thermal

synthesis and gas-sensing properties of Fe-doped ZnO. J Mater

Sci 45:209–215. doi:10.1007/s10853-009-3920-y

18. Siciliano T, Di Giulio M, Tepore M (2009) Room temperature

NO2 sensing properties of reactively sputtered TeO2 thin films.

Sens Actuators B 137:644–648

19. Mun Y, Park S et al (2013) NO2 gas sensing properties of Au-

functionalized porous ZnO nanosheets enhanced by UV irradia-

tion. Ceram Int 39:8615–8622

Fig. 9 Response time of both gas sensors to different concentration

of ethanol

Fig. 10 Recovery curve of both sensors to various concentrations of

ethanol

J Mater Sci (2014) 49:6248–6256 6255

123

20. Mishra YK, Kaps S et al (2013) Fabrication of macroscopically

flexible and highly porous 3D semiconductor networks from

interpenetrating nanostructures by a simple flame transport

approach. Part Part Syst Charact 30:775–783

21. Han N, Chai L, Wang Q et al (2010) Evaluating the doping effect

of Fe, Ti and Sn on gas sensing property of ZnO. Sens Actuators

B 147:525–530

22. Jebril S, Kuhlmann H et al (2010) Epitactically Interpenetrated

High Quality ZnO Nanostructured Junctions on Microchips

Grown by the Vapor- Liquid- Solid Method. Cryst Growth Des

10:2842–2846

23. Hassan MM, Khan W et al (2014) Effect of size reduction on

structural and optical properties of ZnO matrix due to successive

doping of Fe ions. J Lumin 145:160–166

24. Gong H, Hu JQ, Wang JH, Ong CH, Zhu FR (2006) Nano-

crystalline Cu-doped ZnO thin film gas sensor for CO. Sens

Actuators B 115:247–251

25. Kim KJ, Park YR (2004) Optical investigation of Zn1-xFexO

films grown on Al2O3(0001) by radio-frequency sputtering.

J Appl Phys 96:4150–4153

26. Xu L, Li X (2010) Influence of Fe-doping on the structural and

optical properties of ZnO thin films prepared by sol–gel method.

J Cryst Growth 312:851–855

27. Chen H, Gu SL, Liu W, Zhu SM, Zheng YD (2008) Influence of

unintentional doped carbon on growth and properties of N-doped

ZnO films. J Appl Phys 104:113511–113516

28. Chen SH, Yu CF et al (2008) Nanoscale surface electrical

properties of aluminum zinc oxide thin films investigated by

scanning probe microscopy. J Appl Phys 104:114314–

114316

29. Sonawane BK et al (2008) Influence of post annealing on the

structural andoptical properties of MgZnO films. Optoelectron

Adv Mater-Rapid Commun 2:714–718

30. Lim SJ, Kwon SJ, Kim H, Park JS (2007) High performance thin

film transistor with low temperature atomic layer deposition

nitrogen-doped ZnO. Appl Phys Lett 91:183517-3

31. Zhang XA et al (2008) Enhancement-mode thin film transistor

with nitrogen-doped ZnO channel layer deposited by laser

molecular beam epitaxy. Thin Solid Films 516:3305–3308

32. Powder Diffraction File (2001) Joint Committee Power Diffrac-

tion Standards, ICDD, Card 36-1451. Newtown Square, PA

33. Cullity BD, Stock RS (2001) Elements of X-ray diffraction, 3rd

edn. Prentice Hall, Englewood Cliffs

34. Cheng W, Ma X (2009) Structural, optical and magnetic prop-

erties of Fe-doped ZnO. J Phys Conf Ser 152:012039-7

35. Hassan MM, Ahmed AS, Chaman M et al (2012) Structural and

frequency dependent dielectric properties of Fe3? doped ZnO

nanoparticles. Mater Res Bull 47:3952–3958

36. Williamson GK, Hall WH (1953) X-ray line broadening from

filed aluminium and wolfram. Acta Metall 1:22–31

37. Rambu AP et al (2013) Structure and gas sensing properties of

nanocrystalline Fe-doped ZnO films prepared by spin coating

method. J Mater Sci 48:4305–4312. doi:10.1007/s10853-013-

7245-5

38. Djelloul A, Aida M-S, Bougdira J (2010) Photoluminescence,

FTIR and X-ray diffraction studies on undoped and Al-doped

ZnO thin films grown on polycrystalline a-alumina substrates by

ultrasonic spray pyrolysis. J Lumin 130:2113–2117

39. Ivanova T, Harizanova A, Koutzarova T, Vertruyen B (2010)

Study of ZnO sol–gel films: effect of annealing. Mater Lett

64:1147–1149

40. Hsueh TJ, Hsu CL, Chang SJ, Chen IC (2007) Laterally grown

ZnO nanowire ethanol gas sensors. Sens Actuators B Chem

126:473–477

41. Zhu BL, Xie CS, Wang WY et al (2004) Improvement in gas

sensitivity of ZnO thick film to volatile organic compounds

(VOCs) by adding TiO2. Mater Lett 58:624–629

42. Kroger FA (1974) The chemistry of imperfect crystalls. North-

Holland Publishing Co., Amsterdam, Netherlands

43. Sabioni ACS et al (2003) Comparative study of high temperature

oxidation behaviour in AISI 304 and AISI 439 stainless steels.

Mater Res 6:173–185

6256 J Mater Sci (2014) 49:6248–6256

123