Review Article Metal/Metal-Oxide Nanoclusters for …downloads.hindawi.com/journals/jnm/2016/2359019.pdfReview Article Metal/Metal-Oxide Nanoclusters for Gas Sensor Applications AhmadI.Ayesh

Review ArticleMetalMetal-Oxide Nanoclusters for Gas Sensor Applications

Ahmad I Ayesh

Department of Mathematics Statistics and Physics Qatar University Doha Qatar

Correspondence should be addressed to Ahmad I Ayesh ayeshqueduqa

Received 21 July 2016 Revised 7 October 2016 Accepted 31 October 2016

Academic Editor Yu-Lun Chueh

Copyright copy 2016 Ahmad I Ayesh This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The development of gas sensors that are based on metalmetal-oxide nanoclusters has attracted intensive research interest in thelast years Nanoclusters are suitable candidates for gas sensor applications because of their large surface-to-volume ratio that can beutilized for selective and rapid detection of various gaseous species with low-power consuming electronics Herein nanoclusters areused as building blocks for the construction of gas sensor where the electrical conductivity of the nanoclusters changes dramaticallyupon exposure to the target gas In this review recent progress in the fabrication of size-selected metallic nanoclusters and theirutilization for gas sensor applications is presented Special focus will be given to the enhancement of the sensing performancethrough the rational functionalization and utilization of different nanocluster materials

1 Introduction

A sensor is a device that produces a response upon exposureto some stimulus through introducing functionally relatedoutput The response is an alert in one or more of thesensor properties such as mass electrical conductivity andcapacitanceTherefore sensors enable us tomonitor the envi-ronment around us and to use that information for differentpurposes [1] Nanotechnology is enabling the productionof efficient sensors with broad range of applications Theunique properties of the nanomaterials make them suitablecandidates for sensitive detection of chemical and biologicalspecies [2] because they exhibit great adsorptive capacity dueto the large surface-to-volume ratio produce great modula-tion of the electrical signal upon exposer to analytes due tothe great interaction zone over the cross sectional area (Debyelength) enable tuning electrical properties by controllingthe composition and the size of the nanomaterial and easeconfiguration and integration in low-power microelectronicsystems

In this review we will focus on the conductometric (orresistive) transducers Therefore the review will introducethe operation principle of the nanostructure conductometricgas sensors and the fabrication of nanocluster devices Nextrecent progress in conductometric nanocluster gas sensorswill be presented Lastly the review will be summarized withpossible future developments in gas sensors

2 Nanostructure Conductometric Gas Sensors

Nanomaterials are classified depending on their dimensionsinto three categories zero- one- and two-dimensional nano-materials In order to use those materials for gas sensingapplications they should have suitable composition andmorphologies [3] This review focuses on nanomaterials ofzero-dimensions (or nanoclusters) Nanoclusters are definedas aggregates of atoms (or small nanoparticles) that arein nanometer size and their properties are different fromtheir bulk equivalents The synthetic conditions to obtainnanoclusters are so broad ranging from chemical methods atdifferent temperatures andor pressures to physical methodswhere nanoclusters could be formed from the atomic vapor

A conductometric transduce consists of (i) layer of gassensitive material (ii) substrate (iii) electrodes to measurethe electrical signals and (iv) heater The basic structure ofa typical conductometric transducer is shown schematicallyin Figure 1 The contacts could be of Ohmic or Schottky typeand their geometry controls the sensor operationalmodeTheconductance (or the resistance) of the sensor is dependenton the properties of the sensitive material concentrationof the target gas and the measurement parameters such asthe temperature and applied voltage The reaction of thetarget gas with the sensitive material takes place at differentsites of the structure depending on the morphology and istransduced into electrical signal

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2016 Article ID 2359019 17 pageshttpdxdoiorg10115520162359019

2 Journal of Nanomaterials

Powersupply

Ammeter

Insulating substrateMetallic

electrodes

Sensitive layer

Figure 1 Schematic representation of the conductometric trans-ducer

The gas sensitivematerial could bemade of bulk or grainsthat have sizes in the micro- or nanorange The sensitivematerial could be completely or partially depleted dependingon its thickness porosity and the Debye length 120582D Varioussensitive materials are prepared and deposited as thick filmthin film or incorporated into transducers for gas sensingapplications [4ndash9] The sensor performance depends on theporosity and the sensitivity of the material In additioncharge transport depends on the percolation path throughintergranular regions Therefore by changing small detailsin the preparation process each sensor differs in its sensingcharacteristics

The sensitive layer could be compact of porous materialsee Figure 2 [10] When the sensitive layer consists mainlyof compact material with a thickness larger than the Debyelength it can only partially be depleted when exposed to agas thus the interaction does not affect the entire sensitivelayer Accordingly two levels of the resistance into parallelare introduced with only the underneath layer of the sensitivelayer being in contact with the electrodes Therefore thinporous sensitive layer should function better for the gassensing application

A main advantage of the gas sensors made of porousnanostructured thin films is that the volume of the nanos-tructure is accessible to the gases where the active surface ismuch higher than the geometric one unlike the sensorsmadeof compact layers where the interaction takes place only at thesurface layer of the sensor (this is the case formost of the thickfilm based sensors) For gas sensors with nanostructuredporous structure where necks might be present between thegrains it is possible to have interaction between the target gaswith surfacebulk for large necks grain boundaries for largegrains and flat bands for small grains and small necks Forsmall grains and narrow necks a surface influence on chargecarrier mobility should be taken into consideration when themean free path of free charge carriers becomes comparablewith the dimension of the grains The number of collisionsexperienced by the free charge carriers in the bulk of the grainbecomes comparable with the number of surface collisions

Large grains

Gas

Product

Small grainsEn

ergy

Currentflow

x

x

z

qVS

xg gt 120582D

xg

2x0

Eb

Figure 2 Schematic representation of a porous sensing layer withgeometry and energy band 120582D is the Debye length 119909g is the grainsize and 1199090 is the depth of the depletion layer copy IOP PublishingReproduced with permission All rights reserved [11]

Consequently the number of collisions may be influenced byadsorbed species acting as additional scattering centres

Contacts in gas sensors made of thin film of nanoclustershave dominating effect on the resistance Direct currentmeasurements and AC impedance spectroscopy could beused to identify the contact related elements They also canbe used to identify the presence of surface regions sincethe depletion region behaves like a capacitor [8] Each typeof contribution in a sensing layer can be simulated to anequivalent circuit Those equivalent circuits can be extrap-olated from detailed critical analysis of the experimentalelectrical measurements morphology of the sensing layerand microscopic characteristics of the sensing layer andsensor For Schottky contact type a simple approximation canbe performed based on the fact that the total charge trappedon the surface level 119876119878 can be written as [18]

119876119878 = 1199021198991198871199041199110 (1)

where 119902 is the electron charge 119899119887 is the electron concentra-tion 119904 is the total surfacewhere the adsorption take place and1199110 is the depth of the depletion region The relation betweenthe surface charge and band bending (119881119878) is [10]

119876119878 = 119904radic21199021205761205760119899119887119881119878 (2)

where 1205760 and 120576 are the air permittivity and the media relativepermittivity It should be noted that in this approximation itis assumed that all the electrons in the conduction band from

Journal of Nanomaterials 3

the depletion layer are captured on the surface trap levelsThecapacitance of the depletion region can be given as

119862119904 = 1119904 (120597119876119878120597119881119878 ) = radic11990212057612057601198991198872119881119878 (3)

AC impedance spectroscopy is a useful characterizationtechnique that can be used to identify the sensor equivalentcircuit and the determination of the values of circuit elementsthat is resistances and capacitances Once equivalent circuitsare accompanied with the above equations one can optimizethe sensorsrsquo parameters via modifying layer fabrication tech-nology [19ndash22] and isolate the influence of the target gas (forexample O2 H2O CO CH4 O3 and NO) on the differentcomponents of the sensor [11 23ndash26]

Following the above discussion the sensing performanceof a transducer is greatly affected by the choice of the sensitivelayer thickness on different transducer platforms [27ndash32] aswell as the electrode positionspacing [28 33 34] The effectof the former was investigated at different ambient conditions[18] In the latter [11] sensing layers with interdigitatedelectrodes having different spacing in the range 10ndash50 120583m forSnO2 filmswere used [11]The results show that the resistanceincreases with the electrode spacing and decreases with thethickness of the sensitive layer for air and NO2 althoughthe results show that the resistance is independent of layerthickness or electrode spacing for the CO case

Typically gas sensors are operated in ambient air wherethey are exposed to humidity and interfering gases such asoxygen and carbon dioxide Those gases may form bondswith the surface of the nanoclusters by exchanging electronsthus they may form dipoles Since dipoles do not affect theconcentration of the free charge carriers they do not havean effect on the resistance of sensor sensitive layer As anexample Figure 3 shows the case of oxygen and hydroxylgroups (as dipoles) where they are bounded to the surfaceof an n-type semiconductor [11] Their effects are mainlythe band bending and change of the electronic affinity ofthe semiconductor when compared to the situation existingbefore the adsorption

In the temperature range between 100 and 500∘C oxygenmay ionosorb over the surface of the nanoclusters eitherin the molecular (O2

minus) or in atomic (Ominus) forms At hightemperatures Ominus is dominant while at temperatures of 200∘Cor below O2

minus is more dominant since it has lower activationenergy The interaction with oxygen creates a depletion layerat the nanocluster surface thus a barrier potential has to beovercome by electrons to reach the nanocluster surface [10]The chemisorption of oxygen can be described as [10]

1205732Ogas2 + 120572119890minus + 119878 larrrarr Ominus120572120573119878 (4)

where Ogas2 is oxygen in themolecular form in ambient atmo-

sphere 119890minus is the electron that has sufficient energy to reachto the nanocluster surface 119878 is an unoccupied chemisorptionsite for oxygen Ominus120572120573119878 is the chemisorbed oxygen species and120572 = 1 or 2 for singly or doubly ionized forms respectively120573 = 1 or 2 for atomic or molecular forms respectively

EVac

EVS

Φ120594

qVSECb

EVb

120583

ECs

Ed

EFESS

zz0

Figure 3 Band bending after chemisorption of charged species (egionosorption of oxygen on 119864SS levels)Φ denotes the work function120594 is the electron affinity and 120583 is the electrochemical potential copyIOP Publishing Reproduced with permission All rights reserved[11]

For some reducing gases gas detection is related to thereactions between the species to be detected and ionosorbedsurface oxygen When a reducing gas like CO comes intocontact with the surface the following reactions may takeplace

COgas 997888rarr COads (5)

COads +Oadsminus 997888rarr CO2gas + 119890minus (6)

These consume ionosorbed oxygen which change thedensity of ionosorbed oxygen that is detected and in turnchange the electrical conductance of metal nanocluster

Direct adsorption is also possible for gases such as NO2which is strongly electronegative

NO2gas larrrarr NO2ads (7)

119890minus + NO2ads larrrarr NO2adsminus (8)

Therefore the occupation of surface states which aremuch deeper in the band gap than oxygen increases thesurface potential and reduces the overall sensor conductanceUsing the Schottky approximations for nanoclusters withdiameter (119863) less than or equal to Debye length (119863 le 120582D)the energy barrier Δ119864 can be written as [35]

Δ119864 sim 119896B119879( 1198772120582D) (9)

with 120582D = radic1205761205760119896B1198791199022119899119887 Here 119896B is the Boltzmann con-stant119879 is the temperature in Kelvin and119877 is the radius of thenanocluster Therefore if Δ119864 is comparable with the thermalenergy a homogeneous electron concentration in the grainwill result which in turn produces flat band energy

4 Journal of Nanomaterials

Nanocluster synthesis

Examples(1) Inert-gas condensation(2) Ball milling(3) Laser ablation(4) Microwave(5) Ultrasonication(6) Irradiation(7) Lithography

Physical methods Chemical methods Biological methods

Examples(1) Sol-gel method(2) Chemical reduction(3) Hydrolysis

Examples(1) Using algae(2) Synthesis by plant

extracts(3) Synthesis by bacteria(4) Synthesis by fungi

Figure 4 Illustration of the different nanocluster synthesis methods

3 Nanocluster Device Fabrication

Because of the wide availability of synthesis and processing ofnanomaterials a careful selection of methodology to preparenanoclusters of sufficiently fine dispersion porous structurehigh crystallinity and bulk quantity is required Neverthelessnewmaterial science and physics await discovery and remainto be explored based on the newly acquired nanoscience andnanotechnology knowledge

Nanoclusters can be synthesized using different chemicaland physical methods with many examples that can be foundin the literatures [36ndash41] Those synthesis methods include(a) physical synthesis such as inert gas condensation ballmilling laser ablation and others (b) chemical synthesissuch as sol-gel chemical reduction hydrolysis and othersand (c) biological synthesis that can be established usingalgae plant extracts bacteria fungi yeast and others Asummary of thosemethods is presented in Figure 4 Althoughexamples of the usage of nanoclusters synthesized using dif-ferent methods for gas sensing applications will be presentedin this review special focus will be given to nanoclustersynthesis using the inert gas condensation technique becauseof its many advantages as discussed below [42ndash45]

Sputtering and inert gas condensation inside an ultra-high vacuum chamber is a unique technique for producinghigh quality nanoclusters of many advantages including thefollowing [46 47] (i) the nanoclusters are of high purityas they are prepared by inert gas inside ultra-high vacuum[48] (ii) the size of the nanoclusters that can be tuned easilywithin a range of sizes corresponding to the source designby controlling the source conditions (as discussed below)[49] (iii) the produced nanoclusters being charged whichallow the size selection of the nanoclusters using a suitablemass filter [48] (iv) the narrow size distribution of the

produced nanoclusters [13] (v) the produced nanoclusterswhich may self-assemble directly on device substrate withoutthe need of any additional experimental steps [50] (vi)the coverage of the deposited nanoclusters on the substrate(and thus the sensitive layer thickness is quit controlled bycontrolling the deposition time) [51] (vii) the composition ofthe produced nanoclusters which is controlled by controllingthe composition of the target material [52] and (viii) thetechnique that could be used on a commercial scale

A typical example of an ultra-high vacuum system thatcan be used for nanocluster fabrication is shown in Figure 5[12] The system consists of the following main parts (i)source chamber where nanoclusters are produced (ii) depo-sition chamber where the nanoclusters are deposited on thesubstrate and (iii) quadrupole mass filter (QMF) that is usedfor investigating the nanocluster size distribution or selectingnanoclusters of a particular size

31 Nanocluster Production To produce nanoclusters of aparticular metal a target of the metal is fixed on the sputterhead [53] The system is then pumped down to a desirablepressure prior the nanocluster production A high negativeDC voltage is applied to the target and an inert gas (typicallyargon) is injected inside the source chamber Consequentlyplasma is ignited [54] Herein the inert gas plays three majorroles (i) producing the plasma required to sputter the metalfrom the target (ii) establishing inert gas condensation ofthe sputtered material and (iii) creating pressure gradientbetween the source chamber and the deposition chamberwhich introduce the nanoclusters to the deposition chamberthrough the QMF

The main factors that determine the nanocluster sizeare the distance from the target surface to the exit nozzleof the source (defined as the aggregation length (119871)) inert

Journal of Nanomaterials 5

Nanoclusterbeam

Pumping port

Lineartranslation

Vaporgeneration Aggregation

region

Expansionregion

Nozzles

L

Cooling

Depositionchamber

TP

TP

SampleQCM

QMF

Faraday cup

Electricalfeedthrough

Waterin amp out

Source

Ar amp He inlets

Water in amp out

Cryo

stat

mm5 amp 6

Gas

Gas

Figure 5 Schematic diagram of the ultra-high vacuum compatiblenanocluster system and the nanocluster source Reprinted withSpringer permission from [12]

gas flow rate (119891) and sputtering discharge power (119875) Thesputter head is mounted on a linear translator with a motorwhere its position and thus 119871 can be varied without ventingthe source chamber An example of the investigation ofthese factors for Pd nanoclusters is shown in Figure 6 [49]Here the three factors (119871 119891 and 119875) could be tuned togenerate nanoclusters of the required average size In generalincreasing the aggregation length increases the time spent bythe nanoclusters inside the source chamber (growth region)thus their sizes increase However the relation between thenanocluster size and both 119891 and 119875 is not a simple relationand it depends on the nanocluster formation mechanism(s)Hence it is subject of investigation by different researchgroups [55ndash57] Herein the above three factors have tobe fine-tuned for each type of nanoclusters to achieve therequired nanocluster size

The QMF consists of four parallel metal rods where eachpair of opposite rods is connected together electrically topotentials of (119880 + 119881 cos(120596119905)) and minus(119880 + 119881 cos(120596119905)) here119880 is a DC voltage and 119881 cos(120596119905) is an AC voltage [58] Ineach size distribution scan the ratio 119880119881 is fixed and themass distribution is scanned by varying the frequency 120596Theresolution of the filter is adjusted for a mass scan by settingthe 119880119881 ratio The 119880119881 ratio is activated as a function of themass number such that the actual resolutionΔ119872119872 does notremain constant but Δ119872 does [59] A grid located at the exitof the mass filter (Faraday cup) can be used to measure theion flux of the selected masssize and the resultant currentis measured by a picoammeter Therefore the current signalreflects the populationnumber of the produced nanoclusters

32 Substrate Preparation The most common gas sensorstructures presented in the literature are the two-point andfield-effect transistor (FET) structures (see Figure 7) Sensorsbased on both structures rely on changes of electrical resistiv-ityconductivity of the gas sensitive layer (nanocluster film)due to the interaction with the surrounding atmosphere

Gas sensors based on the two-point structure are made oftwometallic electrodeswith proper spacing and a filmof nan-oclusters Herein planar metallic electrodes of interdigitatedstructure are appropriate to be used for the electrical contactof materials [60] The dimensions of the spacing between theelectrodes need to be optimized for each type of sensor asdiscussed below

A typical structure of the FET sensor consists of aconducting substrate coated with an insulating layer that rep-resents the gate (eg SiO2doped-Si) Twometallic electrodesare deposited on an insulating substrate and they serve assource and drain The active nanocluster layer is depositedbetween the source and drain electrodes and it acts as thechannel of the FET The resistance of the active layer can bechanged by the field-effect created by applying a potential tothe gate here the gate is the doped-Si substrate The chargetransfer process induced by surface reactions determines thesensor resistance The current flows parallel to the surfaceand is modulated by the gate voltage for the channel of aFET or by increasing the sensor temperature for the two-point structure When the channel is fully depleted carriersthermally activated from surface states are responsible forconduction

The metal electrodes can be made by photolithographyand microfabrication technology on an insulator substratesuch as glass (for the two-point structure) and SiO2doped-Si(for FET and two-point structures) Figure 7 shows schematicdiagram of an interdigitated electrode structure and the stepsincluded in the photolithography process Photolithographyis the process of transferring geometric shapes on a maskto the surface of a substrate The steps involved in thephotolithographic process are substrate cleaning insulatorlayer formation (for the case of doped-Si wafers) photoresistapplication exposure to UV light through a mask anddevelopment metal contact evaporation and lift-off all of thephotoresist In the first step the substrateswafers are chem-ically cleaned to remove particulate matter on the surface aswell as any traces of organic ionic and metallic impurities IfSi wafers are used silicon dioxide (or silicon nitride) whichserves as an insulating layer is grown on the surface of thewafer After the formation of the insulating layer positivephotoresist is applied to the surface of the wafer using spincoatingwhich produces a thin uniform layer of photoresist onthe substratewafer surfaceThe photoresist is exposed to UVlight through amask (a glass plate with a patterned emulsionof metal film on one side) here the exposed photoresist willbe removed Exposure to the UV light changes the chemicalstructure of the photoresist so that it becomes more solublein the developer The exposed resist is then washed away bya developer solution leaving windows of the bare underlyingsubstrate The mask therefore contains an exact copy of themetallic contact pattern which is to remain on the waferNext step includes metallic contact fabrication by a thin

6 Journal of Nanomaterials

30 40 50 60 7020

Diameter (nm)

68 W116 W189 W

240 W320W376W

024

026

028

030

032

Inte

nsity

(nA

)

(a)

10 20 30 400

P (W)

30

35

40

45

Peak

dia

met

er (n

m)

(b)

20

40

60

80

100

Peak

dia

met

er (n

m)

30 50 70 90 11010

f (sccm)

L = 30mmL = 40mmL = 50mm

L = 60mmL = 70mmL = 80 mm

L = 90mmFit 60 mmFit 70 mm

(c)

40 60 80 10020

L (mm)

f = 20 sccmf = 30 sccmf = 50 sccm

Fit 20 sccmFit 30 sccm

20

40

60

80Pe

ak d

iam

eter

(nm

)

(d)

Figure 6 (a) The effect of the sputtering discharge power on the nanocluster size distribution (b) The dependence of the peak diameter onthe sputtering discharge power (c) The dependence of the peak diameter on Ar flow rate for aggregations lengths in the range of 30ndash90mmThe dashed lines are the theoretical nanocluster size calculation for 119871 = 60 and 70mmThe solid lines serve as guide to the eye to show zone Iincrease in the peak diameter with 119891 for 119871 = 60mm and 119891 = 40 sccm and zone II decrease in the peak diameter with 119891 for 119871 = 80mm or for119871 = 60mm and 119891 = 40 sccm (d)The dependence of the peak diameter on 119871 for 119891 = 20 30 and 50 sccmThe dashed lines are the theoreticalnanocluster size calculations for 119891 = 20 and 30 sccm Reprinted with AIP permission from [13]

film evaporator that utilizes either thermal evaporation orsputtering Finally the substrate is placed inside hot acetoneto left-off all of the photoresist keeping the substrate with themetallic electrodes according to the pattern on the usedmask

33 Device Fabrication A directed beam of nanoclustersexits from the QMF and is deposited on the substrate (seeFigure 6) The produced nanoclusters are deposited on thesensor substrate that is fixed on a cryostat finger or on asample holder which is mounted on a vertical motorizedlinear translatorThe nanocluster deposition rate is measuredusing a quartz crystal monitor (QCM) The QCM is fixed ona motorized linear translator that enables driving the QCM

in front of the exit nozzle check the deposition rate and thendrive it back away from the beam path

The produced nanoclusters are deposited on an insulatingsubstrate with preformed AuNiCr contacts The electricalconductivity of the sample is observed during nanoclusterdeposition see Figure 8 A sharp rise in the conductivityindicates completion of at least one continuous networkof nanoclusters between the contacts Consequently thenanocluster deposition can be suddenly stopped using anautomatic shutter at the onset of conduction (percolationthreshold) or keep the deposition for longer time to create athicker film of nanoclusters Electrical measurements can beperformed subsequently on the sample as a function of thetarget gas type and concentration at controlled temperature

Journal of Nanomaterials 7

Met

al el

ectro

de

Met

al el

ectro

de

(a)

Insulating substrate

Metal electrode

(b)

Insulating substrateMetal electrode

Doped-Si back gate

(c)

Doped-Si

Doped-Si

Doped-Si

Doped-Si

Doped-Si

Photoresist

Mask

UV radiation

Exposed photoresist

Developed photoresist

AuMetal deposition

AuLift-off

SiO2

SiO2

SiO2

SiO2

SiO2

(d)

Figure 7 Schematic diagram of the metallic electrode structures (a) Top view of the interdigitated structure (b) Side view of substrate usedfor the two-point structure (c) Side view of substrate used for the FET structure (d) Steps of the photolithography process

8 Journal of Nanomaterials

Met

al el

ectro

deA

Met

al el

ectro

de

Time (s)

Onset of conductionSource measuring unit I

(120583A

)

Figure 8 Schematic diagram of the onset of conduction measure-ment The source measuring unit applies a voltage to the metallicelectrodes and measures the electric current A sharp rise in theelectric current indicates the completion of at least one continuousnetwork of nanoclusters between the contacts

4 Progress in ConductometricNanocluster Gas Sensors

Recently highly stable and sensitive sensors have beenmade by incorporating various nanocluster materials intosensors [61ndash65] Their novel fundamental phenomena andsize dependent properties make them ideal candidates for thethird-generation gas sensors However not all nanoclustermaterials are effective sensors The selection of optimal sens-ing material is highly dependent on the design manufactur-ing chemical activity stability and so forth In this sectionresearch works recently published on the conductometrictwo-point and FET gas sensors that utilize nanoclusters willbe reported

Many types of nanoclusters were produced and testedwith particular regard to their electrical properties in con-trolled atmosphere for gas sensing applications Table 1reports a list of the nanocluster gas sensors made ofmetalmetal-oxides that have been found in the literaturetogether with the target gas(es) chosen It is interesting tonotice that tin oxide conductometric gas sensors are by farone of the most studied and also of the few that have beencommercialized due to their better performances in termsof sensitivity and stability compared to other nanoclustersZinc oxide nanoclusters and nanowires are of the moststudied [66ndash77] This is due to the easiness of preparingnanoclustersnanowires and multiple intriguing nanostruc-tures and furthermore to the biocompatibility of zinc oxidethat makes it promising for medical and in vivo applicationsCuO is a striking p-type metal-oxide semiconductor thathas unique optical electrical and catalytic properties [16]Here CuO nanoclusters would further endorse the chemicalreactivity of the nanoclusters because as the surface-to-volume ratio of the particle increases the number of reactivesites increases This metal-oxide is known for its low costand the antifouling effect which is effective for reducing thenegative microorganisms [16] Therefore gas sensors of CuOnanoclusters can be used for implementable devices that are

Table 1 Nanocluster gas sensors made of metalmetal-oxides

Year Nanocluster Target gas Reference2003 CeO2 O2 [80]

2004 Cu- and La-dopedTiO2

CO [81]

2004 SrTiO3 O2 [82]2004 Pd-Pt loaded SnO2 CH4 [83]2004 SnO2 O2 [84]2005 In-doped SnO2 H2 CH4 C3H8 [85]

2005ZnO doped withMnO2 TiO2 and

Co2O3Alcohol [86]

2005 F-doped SnO2H2 CO CH4

C3H8[87]

2005 SrTiO3 O2 [88]2005 Ce1minus119909Zr119909O2 O2 [89]2005 Pd and porous Si H2 [90]2005 SnO2 H2 [91 92]2005 F-doped SnO2 H2 [93]2005 SrTiO3 O2 [94]

2005 Scandia dopedSnO2

CO [95]

2006 SnO2 CO O2 [96]2006 Cu-doped ZnO CO [97]

2006 ZnFe2O4CH3COHCH3

C2H5OH[98]

2006 CoFe2O4 Ethanol [99]2006 Pd-doped SnO2 CO O2 [100]2006 TiO2 WO3 H2 [101]2006 Ba doped SmCoO3 CO2 O2 [102]2007 Al-doped TiO2 CO [103]2007 Pd H2 [15]2008 SnO2SBA-15 H2 [104]2008 SnO2 O2 CO NO2 H2O [105]2009 SnO2 H2 [78]

2009 Pd and SnO2nanowires H2 [79]

2009 ZnO H2O [106]2009 Cu2O and CuO Acetone [107]2009 PdO-doped SnO2 CO [108]2009 Y-doped TiO2 CO [109]2009 Au-Pt Methanol [110]

2010 Pt and ZnOnanwires Ethanol NH3 [111]

2011 Pd and Ag H2 [112]

2011 SnO2Ethanol CH3OH

C2H5OH[113]

2011 Fe- and Co-dopedHAp CO [114]

2012 SnO2 CO [115]2013 Au and TiO2 CO [116]

Journal of Nanomaterials 9

Table 1 Continued

Year Nanocluster Target gas Reference2013 ZnO Ethanol [117]2014 Pd and VO Ethanol [118]2014 Pd and SnO2 H2 [119]2014 In2O3 and WO3 H2S [120]2014 Sm2O3 and ZnO Ethanol [121]2014 SnO2 Ethanol [122]2015 PdSnO2 CO [123]

2015 Au-Pt on GO Uric acid anddopamine [124]

2015 PtndashSnO2 H2 CO and LPG [125]2015 Pt-graphene H2 [17]2015 Au-Pd H2 [126]2016 CuFe2O4 H2 [127]2016 CuO H2S [16]2016 120572-iron oxide Ethanol [128]

2016 In4Sn3O12 andTeO2

CO [129]

2016 AuTiO2 CO [130]

biocompatible In addition palladium nanoclusters are ofgreat importance for hydrogen sensing at room temperatureswith optimal sensitivity and selectivity (this is important fromsafety point of view when dealing with explosive gases suchas hydrogen)Therefore this review will give special focus onsome examples of those three nanoclusters SnO2 ZnO CuOPd Au and Pt

41 Tin Oxide Nanoclusters Conductometric gas sensorsbased on SnO2 nanoclusters that utilize the two-point struc-ture were fabricated by Yeow et al [78] The nanoclusterswere synthesized based on the hydrothermal method usingpotassium stannate trihydrate as a precursor in an ethanol-deionizedwatermixed solventThe nanoclusters were dilutedand dispersed in water before drop casting a few monolayersonto electrical electrodes The response time is dependenton the operating temperature 90 of resistance change(119877air-119877gas) was achieved within the first 13ndash30min A higheroperating temperature leads to a greater change in con-ductance and hence greater response On the other handdesorption of all oxygen ionic species previously adsorbedoccurs at high temperatures which explains the reduction inresponse as the operating temperature is increased beyondthe optimum valueThe results show none linear relationshipof the response A possible explanation of the response is thatthe inner surfaces of the porous nanocluster films are not fullyutilized for gas detection due to the limitation of diffusion ofthe analyte gas through the nanopores

The dependence of the gas sensor sensitivity on thesize of nanoclusters used as a sensitive layer is a majorfactor controlling the performance of the gas sensor As anexample Tan et al have reported size dependence of the SnO2nanoclusters of sizes 20 30 and 40 nm as shown in Figure 9[14] They showed that the sensor exhibits highly consistent

responses over many cycles and the sensors made of smallernanoclusters possess higher gas sensitivity due to the increasein effective surface area

Shen et al reported on two-point sensors that are basedon SnO2 nanowires which were synthesized by thermalevaporation at 900∘C [79] The nanowires were doped withpalladiumnanoclusters at 350∘C for 30min in air Gas sensorsbased on SnO2 nanowires with 0wt 08 wt and 2wt Pddoping were fabricatedThese gas sensors showed a reversibleresponse to H2 gas at an operating temperature of 150∘CThesensor response increased with Pd concentration The 2wtPd-doped SnO2 nanowire sensor showed a response of twoorders of magnitude for 1000 ppmH2 gas at 100

∘C Pd dopinghas demonstrated to improve the sensor response and lowerthe operating temperature

42 Palladium Nanoclusters Hydrogen gas sensors that uti-lize Pd nanoclusters with average sizes between 35 and6 nm were reported by Van Lith et al [15] The nanoclusterswere prepared by sputtering and inert gas condensationThe sensors are based on tunneling between discontinu-ous networks of nanoclusters They demonstrated that theconduction through the nanocluster film is dominated bytunneling gaps see Figure 10(a)The sensor operated at roomtemperature which make them usable for H2 sensing safelyIn addition the sensor detected H2 with concentration aslow as 05 as depicted in Figure 10(b) The work showedthat the sensor response is dependent on the nanocluster size(Figure 10(c)) The sensor resistance was found to decreasewith increasing hydrogen concentration a factor of sim10 at5 of hydrogen thus the response is shown as the relativedecrease in resistance (Δ119877119877) This was explained in termsof the increase in the nanocluster size upon exposure to H2thus decreasing the tunneling barriers which decrease theresistance of the sensor

43 Copper Oxide Nanoclusters H2S gas sensors based onCuO nanoclusters are impeded in polymer membranesof poly-vinyl-alcohol (PVA) and glycerol ionic liquid (IL)[16] The nanoclusters were fabricated with a precise con-trol of nanoclusters size by the colloid microwave assistedhydrothermal method Different concentrations of nanoclus-ters in polymer solutions of PVA and 5 IL were preparedThe solutions that contain nanoclusters were used to fabricatethin polymer membranes by the solution casting methodwhere the membranes hold semiconducting properties andwere flexible Each membrane was integrated between twoelectrical electrodes of capacitor structure as shown in theinset of Figure 11 Herein the top and bottom electrodes weremade of stainless steel grid and copper sheet respectivelyThe conductance measurements revealed that those sensorswere sensitive to H2S gas with concentrations as low as10 ppm they are operational at low temperatures and theirsensing behavior was reversible which allowed multiple useof the produced sensors (see Figure 11)The produced sensorswere also selective to H2S and they revealed reasonably fastresponse of 204plusmn128 sThe above sensors were reliable withlow cost manufacturing thus they can be used for industrialfield applications

10 Journal of Nanomaterials

500 1000 1500 20000

Time (sec)

2E minus 02

3E minus 02

4E minus 02

5E minus 02

6E minus 02

7E minus 02

8E minus 02

9E minus 02

1E minus 01

1E minus 01

1E minus 01

Curr

ent (

mA

)

(a)

20 nm30nm40nm

250 300 350 400 450 500200

Temperature (∘C)

0

2

4

6

8

10

12

14

16

18

20

Sens

itivi

ty (R

airRga

s)

(b)

Figure 9 (a) Typical current response of a SnO2 hydrogen sensor (b) Sensitivity-temperature relationship before Pd functionalization copyIOP Publishing Reproduced with permission All rights reserved [14]

165

170

175

180

185

190

ln(R

)

8020 40 60 120100 160140

1kT (eVminus1)

Ec = 25 meV

(a)

05

113

16

2

5

00

02

04

06

08

10

minusΔRR

0

50 100 150 200 2500

Time (minutes)(b)

D = 35 nm D = 6nm D = 5nmT = 50∘C

2 4 6 8 10 12 140

H2 pressure ()

0

02

04

06

08

1

minusΔRR

0

1 2 3 4 50

H2 pressure ()

0

02

04

06

08

1

minusΔRR

0

1 2 3 4 50

H2 pressure ()

0

02

04

06

08

1

minusΔRR

0

(c)

Figure 10 (a) Temperature dependence of the sensor resistance The circles are experimental data and the line is a linear fit for activationenergy of 25meV Inset schematic illustration of a cluster film between two contacts The main conduction paths are illustrated by the blacklines with tunnel gaps depicted by zigzag lines (b) Transient response of a typical Pd cluster tunneling sensor to the hydrogen levels indicated(c) Response at room temperature as a function of hydrogen pressure given as the percentage of atmospheric pressure for 35 nm clusters and6 nm nanoclusters Response of a sensor fabricated with 5 nm clusters measured at 50∘CThe solid lines are experimental data and the filledcircles with dashed lines are theoretical fits Reprinted with AIP permission from [15]

Journal of Nanomaterials 11

A

Cu sheet

PVA-IL-CuO membrane

Stainlesssteel grid

Silver paste

PVA + 5 + 3CuO

Air

300

ppm

200

ppm

100

ppm

50pp

m

25pp

m

10pp

m

00E + 00

10E minus 06

20E minus 06

30E minus 06

I(A

)

80 160 240 320 4000Time (min)

Figure 11 Electrical current response of PVA-IL-3CuO sensorwhen exposed to H2S gas with different concentrations measuredat 80∘C The inset is a schematic diagram of the produced sensorand the electrical measurement circuit Reprinted with Elsevierpermission from [16]

44 ZincOxideNanoclusters ZnOnanoclusters were utilizedfor recognition of Chinese liquors by Zhang et al [86]The nanoclusters were prepared by the renovated hybridinduction and laser heating The sensitivity of the sensorwas enhanced by doping with MnO2 TiO2 and Co2O3 Thedoped nanoclusters were coated onto Al2O3 tubes (4mmlength 12mm external diameter and 08mm internal diam-eter) on which Pt electrodes had been fixed at each end Thethick filmswere sintered at 650∘C for 2 hours after being driedunder air to remove water The film thickness observed bylight microscopy was about 35 120583m A small Ni-Cr alloy coilwith a resistance of about 33Ωwas placed through the tube asa heaterThe sensors were tested against five different Chineseliquors namely Baiyunbian Beijing Erguotou Red StarErguotou Zhijiangdaqu Jianliliangjiu alcohol and dilutedalcohol (forged liquor) The significant sensitivity of each gassensor to alcohol is obtained at about 320∘C It is shownthat doping ZnO nanoclusters with MnO2 TiO2 and Co2O3greatly improves the sensitivities to alcohol Furthermorethey showed that the optimum operating temperature ofthe doped nano-ZnO gas sensors can be further reducedby suitable doping The normalized principal componentanalysis (PCA) results of training data set projected ontotheir first two PCs have been proved to be effective fordiscriminating the response of gas sensor array to simple andcomplex odors The PCA results depict that alcohol dilutedalcohol and different flavor type liquors can be distinguished

In a different work [106] ZnO nanostructure was pre-pared through the hydroxide precipitation using dropwisemethod and used to fabricate humidity sensors The vari-ations in resistance with the variations in humidity andtemperature were tested The curves for sensing elementannealed at 119879 = 150 and 300∘C reveal that variation ofresistance is slow in the region from70 to 95However thesample annealed at 450∘C shows linearity in resistance versusRH which is suitable for device fabrication Sensitivitycurve for nanoclusters annealed at a temperature of 550∘C

shows that as RH increases resistance decreases sharply upto 40 RH and shows highest sensitivity in this range thenit decreases less rapidly up to 95 RH as relative humidityincreasesThe sensing behavior was explained in terms of theadsorption ofmoisture which affects the protonic conductionon the surface and conductivity with varying amounts ofwater adsorbed by it A hysteresis of sensing element versushumidity for sensing element prepared at 550∘C was alsoobserved The sensing element shows less hysteresis (plusmn5)as compared to others Curve ldquoardquo represents the values ofresistance when RH increases and curve ldquobrdquo represents thevalues of resistances whenRHdecreasesThe response timeof sensor is 80 sec for sensing element prepared at 550∘Cand this sensing element also shows repeatability within plusmn5accuracy

45 Gold and PlatinumNanoclusters Au and Pt nanoclustersare commonly used as catalysts and sensors in many appli-cations Normally either or both nanoclusters are integratedor alloyed with different nanomaterials to enhance theirsensitivity and selectivity and to increase their kinetic oxygenreduction limitation [17 124 131] Wang et al fabricatedhydrogen gas sensors using graphene oxide (GO) assembledwith platinum (Pt) nanoclusters between a pair of prepat-terned TiAu electrodes with microgap as shown in Figure 12[17]They assembled the nanomaterials by alternating currentdielectrophoresis (DEP) method The signal measurementsfor devices produced at different parameters that includeprocessing time of a device peak-to-peak voltage and fre-quency were tested The optimum sensing response of thedevice to hydrogenwas at themeasurement parameters of 5V500 kHz and 30 s respectivelyThe fabricated device exhibitsa sensitivity of (sim10) to 200 ppm hydrogen gas (measuredusing optimized parameters) at room temperature

Recently Liu et al [124] synthesized nanocomposites ofgraphene oxide (GO) decorated with Au-Pt hybrid bimetallicnanoclusters with enhanced catalytic activity by an elec-trochemical reduction process on glassy carbon (ERGO)electrode They investigated the synergistic electrocatalyticeffect for electrodeposited bimetallic Au-Pt nanoclusters andGO utilized for detection of uric acid (UA) and dopamine(DA) The role of Au-Pt nanoclusters was to speed up theelectron transfer for increasing the sensitivity while GO-ERGO allowed broader separation of the oxidation peakpotentials for sensing DA and UA

Conductometric hydrogen gas sensor based on Pt-SnO2nanoclusters was fabricated [125] The results are that thosesensors are sensitive to lowhydrogen concentrations at sensoroperating temperature of 85∘C with response time of 05 sHerein Pt promotes dissociation of hydrogen molecules andactivates reaction between adsorbed hydrogen and oxygenspecies on the surface of nanoclusters The sensor was selec-tive to hydrogen compared with CO and LPG (at 150 ppm)with the lowest response for LPG thus it can be used inhydrogen leak detection devices

Lotus-like AuTiO2 nanoclusters were produced byhydrothermal reaction by controlling the ratio of Au to TiF4(without surfactant) [130] The produced sensors were testedagainst O2 H2 NO and CO The best sensing performance

12 Journal of Nanomaterials

1mm 100 120583m

TiAuTiAu285 nm SiO2

n++ Si

TiAu

TiAu

285 nm SiO2n++ Si

Functiongenerator

OscilloscopeH2 moleculePt nanoparticle

200 ppm 300 ppm 500 ppm

Response timeRecovery time

Response timeRecovery time

Response timeRecovery time

Processing time 30 s 60 sFrequency 100 kHz 500 kHz 1000 kHz

AC voltage 5V 10 V2V5V10 V

0

5

10

15

Sens

itivi

ty (

)

0 500 20001000 25001500

Time (s)

40

45

50

55

60

65

Resp

onse

tim

e (m

in)

3

4

5

6

Reco

very

tim

e (m

in)

200 400 500300

Gas concentration (ppm)

30

35

40

45

50

55

60

Reco

very

tim

e (m

in)

40

45

50

55

60

65

Resp

onse

tim

e (m

in)

200 400 500300

Gas concentration (ppm)

3

4

5

6

Reco

very

tim

e (m

in)

4

5

6

7

Resp

onse

tim

e (m

in)

200 400 500300

Gas concentration (ppm)

(a) (b) (c) (d)

(e) (f)

(g) (h)

Figure 12 ((a) and (b)) Optical microscopy images of the prepatterned microgap electrodes (c) Schematic diagram of DEP experiment(d) Schematic diagram of the device (e) Hydrogen gas response of the devices produced using 500 kHz frequency 30 s processing time atvarious measurement voltages (f)ndash(h) Response and recovery times of fabricated device at various voltage frequency and processing timerespectively Reprinted with American Chemical Society permission from [17]

was for CO where the response rose to 17 for 500 ppm COat 325∘C that is 85 times better than that of pure TiO2Moreover the selectivity the response and recovery timeare improved greatly which confirmed that the lotus-like

AuTiO2 nanostructures had promising potential in gassensing applicationsThe enhancement in the sensing behav-ior was contributed to the catalytic activity of Au nanoclustersand unique lotus-like nanostructure

Journal of Nanomaterials 13

Methanol sensor was developed using two porous Auelectrodes with Pt nanoclusters that produces a micronano-porous Au-Pt system [110] Each Au-Pt electrode acts as acurrent collector and gas diffusion layer for methanol Thesensor was fabricated by hot-pressing of the electrodes withNafion film The sensor showed response current in thetemperature between 20 and 100∘C for concentration ofmethanol between 0 and 2M The sensor has sensitivity of96mAmMsdotcm2 and a response time around 10 s with asensor area of 025 cm2

5 Summary and Outlook

The article reviewed recent progress in gas sensors based onnanoclusters During the past years numerous new data onnanocluster gas sensor properties towards common targetspecies such as H2 O2 and CO have been published Theuse of nanoclusters allows the fabrication of an array ofsensors in a chip with high sensitivity The greater surface-to-volume ratio the better stoichiometry and greater levelof crystallinity compared to bulk materials make the newlydeveloped nanocluster gas sensors very promising for betterunderstanding of sensing principles and development of anew generation of sensors The selectivity of course remainsa concern for metal and metal-oxide based gas sensor Thismay be improved by fabricating sensor arrays using severaldifferent nanoclusters or by composite materials The reviewfocused on nanocluster production by inert gas condensationtechnique as a novel synthesis method However gas sensorsbased on nanoclusters synthesized by different methods werepresented

Nanocluster based sensors outperform their bulk compo-nent There are still many parameters to be addressed Forcommercial sensors better control of the growth is requiredwith a thorough understanding of the growth mechanismthat can lead to a control in size distributions shape crystalstructure and atomic termination In addition attention hasto be paid to issues like the electrical contacts and nanoma-nipulation that allow reliable production and integration ofsensors Other parameters such as limits of detection limitsof quantification dynamic range response and recoverytimes and lifetime have to be improved In addition thedrift of the sensors are frequently related to high workingtemperatures and exposure to chemically active ambient gasIn conclusion future conductometric gas sensors shouldoperatewithmultipurpose sensing and high selectivity recog-nition of specific chemical species Sensors that are operatedas standalone portable sensors are paramount for industrialapplications

Competing Interests

The author declares that there is no conflict of interestsregarding the publication of this paper

References

[1] Z X Cheng X H Ren J Q Xu and Q Y Pan ldquoMesoporousIn2O3 effect of material structure on the gas sensingrdquo Journalof Nanomaterials vol 2011 Article ID 654715 6 pages 2011

[2] N D Hoa N V Duy S A El-Safty and N V Hieu ldquoMeso-nanoporous semiconductingmetal oxides for gas sensor appli-cationsrdquo Journal of Nanomaterials vol 2015 Article ID 97202514 pages 2015

[3] A B Kashyout HM A Soliman H Shokry Hassan and AMAbousehly ldquoFabrication of ZnO and ZnOSb Nanoparticles forgas sensor applicationsrdquo Journal of Nanomaterials vol 2010Article ID 341841 8 pages 2010

[4] S Semancik and R E Cavicchi ldquoThe growth of thin epitaxialSnO2 films for gas sensing applicationsrdquo Thin Solid Films vol206 no 1-2 pp 81ndash87 1991

[5] N S Baik G Sakai N Miura and N Yamazoe ldquoHydrother-mally treated sol solution of tin oxide for thin-film gas sensorrdquoSensors andActuators B Chemical vol 63 no 1 pp 74ndash79 2000

[6] M Ivanovskaya P Bogdanov G Faglia and G SberveglierildquoFeatures of thin film and ceramic sensors at the detection ofCO and NO2rdquo Sensors and Actuators B Chemical vol 68 no 1pp 344ndash350 2000

[7] K Ihokura and I Watson The Stannic Oxide Gas SensormdashPrinciples and Applications CRC Press Boca Raton Fla USA1994

[8] W Gopel and K D Schierbaum ldquoSnO2 sensors current statusand future prospectsrdquo Sensors and Actuators B Chemical vol26 no 1-3 pp 1ndash12 1995

[9] G Korotcenkov ldquoPractical aspects in design of one-electrodesemiconductor gas sensors status reportrdquo Sensors andActuatorsB Chemical vol 121 no 2 pp 664ndash678 2007

[10] N Barsan and U Weimar ldquoConduction model of metal oxidegas sensorsrdquo Journal of Electroceramics vol 7 no 3 pp 143ndash1672001

[11] N Barsan and U Weimar ldquoUnderstanding the fundamentalprinciples of metal oxide based gas sensors the example of COsensing with SnO2 sensors in the presence of humidityrdquo Journalof Physics Condensed Matter vol 15 no 20 pp R813ndashR8392003

[12] A I Ayesh S Thaker N Qamhieh and H Ghamlouche ldquoSize-controlled Pd nanocluster grown by plasma gas-condensationmethodrdquo Journal of Nanoparticle Research vol 13 no 3 pp1125ndash1131 2011

[13] A I Ayesh S Thaker N Qamhieh and H GhamloucheldquoInvestigation of the formation mechanisms of Pd nanoclustersproduced using a magnetron sputtering sourcerdquo AdvancedMaterials Research vol 324 pp 145ndash148 2011

[14] E T H Tan GW Ho A SWWong S Kawi and A T SWeeldquoGas sensing properties of tin oxide nanostructures synthesizedvia a solid-state reaction methodrdquo Nanotechnology vol 19 no25 Article ID 255706 2008

[15] J Van Lith A Lassesson S A Brown M Schulze J GPartridge andA Ayesh ldquoA hydrogen sensor based on tunnelingbetween palladium clustersrdquo Applied Physics Letters vol 91 no18 Article ID 181910 2007

[16] A I Ayesh A F S Abu-Hani S T Mahmou and Y HaikldquoSelective H2S sensor based on CuO nanoparticles embeddedin organic membranesrdquo Sensors amp Actuators B Chemical vol231 pp 593ndash600 2016

[17] J Wang S Rathi B Singh I Lee H-I Joh and G-HKim ldquoAlternating current dielectrophoresis optimization of Pt-decorated graphene oxide nanostructures for proficient hydro-gen gas sensorrdquo ACS Applied Materials and Interfaces vol 7 no25 pp 13768ndash13775 2015

14 Journal of Nanomaterials

[18] N Barsan D Koziej and U Weimar ldquoMetal oxide-based gassensor research how tordquo Sensors and Actuators B Chemicalvol 121 no 1 pp 18ndash35 2007

[19] A Chiorino GGhiotti F PrinettoM C Carotta GMartinelliand M Merli ldquoCharacterization of SnO2-based gas sensors Aspectroscopic and electrical study of thick films from commer-cial and laboratory-prepared samplesrdquo Sensors and Actuators BChemical vol 44 no 1-3 pp 474ndash482 1997

[20] T P Hulser H Wiggers F E Kruis and A Lorke ldquoNanostruc-tured gas sensors and electrical characterization of depositedSnO2 nanoparticles in ambient gas atmosphererdquo Sensors andActuators B Chemical vol 109 no 1 pp 13ndash18 2005

[21] M Ramzan and R Brydson ldquoCharacterization of sub-stoichiometric tungsten trioxide (WO3-X) using impedancespectroscopyrdquo Sensors and Actuators A Physical vol 118 no 2pp 322ndash331 2005

[22] A Chandra Bose P Balaya P Thangadurai and S RamasamyldquoGrain size effect on the universality of AC conductivity inSnO2rdquo Journal of Physics and Chemistry of Solids vol 64 no4 pp 659ndash663 2003

[23] G Martinelli M C Carotta L Passari and L Tracchi ldquoAstudy of the moisture effects on SnO2 thick films by sensitivityand permittivity measurementsrdquo Sensors and Actuators BChemical vol 26 no 1-3 pp 53ndash55 1995

[24] GGhiotti AChiorinoGMartinelli andMCCarotta ldquoMois-ture effects on pure and Pd-doped SnO2 thick films analysedby FTIR spectroscopy and conductancemeasurementsrdquo Sensorsand Actuators B Chemical vol 25 no 1-3 pp 520ndash524 1995

[25] A Labidi C Jacolin M Bendahan et al ldquoImpedance spec-troscopy onWO3 gas sensorrdquo Sensors and Actuators B Chemi-cal vol 106 no 2 pp 713ndash718 2005

[26] L Chen and S C Tsang ldquoAg dopedWO3-based powder sensorfor the detection of NO gas in airrdquo Sensors and Actuators BChemical vol 89 no 1-2 pp 68ndash75 2003

[27] P Montmeat R Lalauze J-P Viricelle G Tournier and CPijolat ldquoModel of the thickness effect of SnO2 thick film on thedetection propertiesrdquo Sensors and Actuators B Chemical vol103 no 1-2 pp 84ndash90 2004

[28] MHausner J Zacheja and J Binder ldquoMulti-electrode substratefor selectivity enhancement in air monitoringrdquo Sensors andActuators B Chemical vol 43 no 1-3 pp 11ndash17 1997

[29] F Hossein-Babaei and M Orvatinia ldquoAnalysis of thicknessdependence of the sensitivity in thin film resistive gas sensorsrdquoSensors and Actuators B Chemical vol 89 no 3 pp 256ndash2612003

[30] S-S Park and J D Mackenzie ldquoThickness and microstructureeffects on alcohol sensing of tin oxide thin filmsrdquo Thin SolidFilms vol 274 no 1-2 pp 154ndash159 1996

[31] S M A Durrani E E Khawaja and M F Al-Kuhaili ldquoCO-sensing properties of undoped and doped tin oxide thin filmsprepared by electron beam evaporationrdquo Talanta vol 65 no 5pp 1162ndash1167 2005

[32] G G Mandayo E Castano F J Gracia A Cirera A Cornetand J R Morante ldquoStrategies to enhance the carbon monoxidesensitivity of tin oxide thin filmsrdquo Sensors and Actuators BChemical vol 95 no 1-3 pp 90ndash96 2003

[33] X Vilanova E Llobet J Brezmes J Calderer and X CorreigldquoNumerical simulation of the electrode geometry and positioneffects on semiconductor gas sensor responserdquo Sensors andActuators B Chemical vol 48 no 1-3 pp 425ndash431 1998

[34] D E Williams and K F E Pratt ldquoTheory of self-diagnosticsensor array devices using gas-sensitive resistorsrdquo Journal of theChemical Society Faraday Transactions vol 91 no 13 pp 1961ndash1966 1995

[35] K Kalantar-zadeh and B Fry Nanotechnology Enabled SensorsSpringer Berlin Germany 2008

[36] A N Shipway E Katz and I Willner ldquoNanoparticle arrayson surfaces for electronic optical and sensor applicationsrdquoChemPhysChem vol 1 no 1 pp 18ndash52 2000

[37] M Lai and D J Riley ldquoTemplated electrosynthesis of nanoma-terials and porous structuresrdquo Journal of Colloid and InterfaceScience vol 323 no 2 pp 203ndash212 2008

[38] A Jaworek ldquoMicro- and nanoparticle production by electro-sprayingrdquo Powder Technology vol 176 no 1 pp 18ndash35 2007

[39] D G Shchukin and G B Sukhorukov ldquoNanoparticle synthesisin engineered organic nanoscale reactorsrdquo Advanced Materialsvol 16 no 8 pp 671ndash682 2004

[40] J A Gerbec D Magana A Washington and G F StrouseldquoMicrowave-enhanced reaction rates for nanoparticle synthe-sisrdquo Journal of the American Chemical Society vol 127 no 45pp 15791ndash15800 2005

[41] B Xia I W Lenggoro and K Okuyama ldquoNovel route tonanoparticle synthesis by salt-assisted aerosol decompositionrdquoAdvanced Materials vol 13 no 20 pp 1579ndash1582 2001

[42] C Binns ldquoNanoclusters deposited on surfacesrdquo Surface ScienceReports vol 44 no 1-2 pp 1ndash49 2001

[43] F Baletto and R Ferrando ldquoThe surface of helium crystalsrdquoReviews of Modern Physics vol 77 no 1 p 317 2005

[44] A Banerjee and B Das ldquoAn ultrahigh vacuum complemen-tary metal oxide silicon compatible nonlithographic systemto fabricate nanoparticle-based devicesrdquo Review of ScientificInstruments vol 79 no 3 Article ID 033910 2008

[45] E Perez-Tijerina S Mejıa-Rosales H Inada and M Jose-Yacaman ldquoEffect of temperature on AuPd nanoparticles pro-duced by inert gas condensationrdquo Journal of Physical ChemistryC vol 114 no 15 pp 6999ndash7003 2010

[46] WA deHeer ldquoThephysics of simplemetal clusters experimen-tal aspects and simple modelsrdquo Reviews of Modern Physics vol65 no 3 article 611 1993

[47] H Haberland US Patent No 5110435 1992[48] S Pratontep S J Carroll C Xirouchaki M Streun and R

E Palmer ldquoSize-selected cluster beam source based on radiofrequencymagnetron plasma sputtering and gas condensationrdquoReview of Scientific Instruments vol 76 no 4 Article ID 0451032005

[49] A I Ayesh N Qamhieh H Ghamlouche S Thaker and MEl-Shaer ldquoFabrication of size-selected Pd nanoclusters usinga magnetron plasma sputtering sourcerdquo Journal of AppliedPhysics vol 107 no 3 Article ID 034317 2010

[50] A I Ayesh ldquoElectronic transport in Pd nanocluster devicesrdquoApplied Physics Letters vol 98 no 13 Article ID 133108 2011

[51] J Schmelzer Jr S A Brown A Wurl M Hyslop and R JBlaikie ldquoFinite-size effects in the conductivity of cluster assem-bled nanostructuresrdquo Physical Review Letters vol 88 no 22Article ID 226802 2002

[52] A I Ayesh N Qamhieh S T Mahmoud and H AlawadhildquoProduction of size-selected Cu119909Sn1minus119909 nanoclustersrdquo AdvancedMaterials Research vol 295ndash297 pp 70ndash73 2011

[53] A I Ayesh H A Ahmed F Awwad S I Abu-Eishah and S TMahmood ldquoMechanisms of Ti nanocluster formation by inert

Journal of Nanomaterials 15

gas condensationrdquo Journal of Materials Research vol 28 no 18pp 2622ndash2628 2013

[54] M Ohring Materials Science of Thin Films Academic PressCambridge Mass USA 2nd edition 2002

[55] J B Chen J F Zhou A Hafele et al ldquoMorphological studiesof nanostructures from directed cluster beam depositionrdquo TheEuropean Physical Journal D vol 34 no 1 pp 251ndash254 2005

[56] O Kamalou J Rangama J-M Ramillon P Guinement and BA Huber ldquoProduction of pulsed mass-selected beams of metaland semiconductor clustersrdquo Review of Scientific Instrumentsvol 79 no 6 Article ID 063301 2008

[57] A N Banerjee R Krishna and B Das ldquoSize controlleddeposition of Cu and Si nano-clusters by an ultra-high vacuumsputtering gas aggregation techniquerdquoApplied Physics A vol 90pp 299ndash303 2008

[58] A I Ayesh N Qamhieh S T Mahmoud and H AlawadhildquoFabrication of size-selected bimetallic nanoclusters usingmag-netron sputteringrdquo Journal of Materials Research vol 27 no 18pp 2441ndash2446 2012

[59] P H Dawson Quadrupole Mass Spectrometry and Its Applica-tions Elsevier Press Amsterdam The Netherlands 1976

[60] W J Liu J Zhang L J Wan et al ldquoDielectrophoretic manip-ulation of nano-materials and its application to micronano-sensorsrdquo Sensors and Actuators B Chemical vol 133 no 2 pp664ndash670 2008

[61] N Pinna G Neri M Antonietti and M Niederberger ldquoNon-aqueous synthesis of nanocrystalline semiconducting metaloxides for gas sensingrdquoAngewandte ChemiemdashInternational Edi-tion vol 43 no 33 pp 4345ndash4349 2004

[62] L P Sun L H Huo H Zhao S Gao and J G ZhaoldquoPreparation and gas-sensing property of a nanosized titaniathin film towards alcohol gasesrdquo Sensors and Actuators BChemical vol 114 no 1 pp 387ndash391 2006

[63] Z Liu T Yamazaki Y Shen T Kikuta N Nakatani and Y LildquoO2 and CO sensing of Ga2O3 multiple nanowire gas sensorsrdquoSensors and Actuators B Chemical vol 129 no 2 pp 666ndash6702008

[64] C S Rout A R Raju A Govindaraj and C N R RaoldquoEthanol and hydrogen sensors based on ZnO nanoparticlesand nanowiresrdquo Journal of Nanoscience and Nanotechnologyvol 7 no 6 pp 1923ndash1929 2007

[65] M S Arnold P Avouris ZW Pan andZ LWang ldquoField-effecttransistors based on single semiconducting oxide nanobeltsrdquoJournal of Physical Chemistry B vol 107 no 3 pp 659ndash6632003

[66] J Zhong SMuthukumar Y Chen et al ldquoGa-dopedZnO single-crystal nanotips grown on fused silica bymetalorganic chemicalvapor depositionrdquoApplied Physics Letters vol 83 no 16 p 34012003

[67] S Y Bae C W Na J H Kang and J Park ldquoComparativestructure and optical properties of Ga- In- and Sn-doped ZnOnanowires synthesized via thermal evaporationrdquoThe Journal ofPhysical Chemistry B vol 109 no 7 pp 2526ndash2531 2005

[68] S Y Li P Lin C Y Lee T Y Tseng and C J Huang ldquoEffectof Sn dopant on the properties of ZnO nanowiresrdquo Journal ofPhysics D Applied Physics vol 37 no 16 p 2274 2004

[69] D K Hwang H S Kim J H Lim et al ldquoStudy of thephotoluminescence of phosphorus-doped p-type ZnO thinfilms grown by radio-frequencymagnetron sputteringrdquoAppliedPhysics Letters vol 86 no 15 Article ID 151917 2005

[70] W Lee M-C Jeong S-W Joo and J-M Myoung ldquoArsenicdoping of ZnO nanowires by post-annealing treatmentrdquo Nan-otechnology vol 16 no 6 pp 764ndash768 2005

[71] W Lee M C Jeong and J M Myoung ldquoOptical characteristicsof arsenic-doped ZnO nanowiresrdquo Applied Physics Letters vol85 no 25 article 6167 2004

[72] D W Zeng C S Xie B L Zhu et al ldquoControlled growth ofZnO nanomaterials via doping Sbrdquo Journal of Crystal Growthvol 266 no 4 pp 511ndash518 2004

[73] G Shen J H Cho J K Yoo G-C Yi and C J Lee ldquoSynthesisand optical properties of S-doped ZnO nanostructures nanon-ails and nanowiresrdquo Journal of Physical Chemistry B vol 109 no12 pp 5491ndash5496 2005

[74] J B Cui and U J Gibson ldquoElectrodeposition and room tem-perature ferromagnetic anisotropy of Co and Ni-doped ZnOnanowire arraysrdquo Applied Physics Letters vol 87 no 13 ArticleID 133108 2005

[75] C X Xu X W Sun Z L Dong M B Yu Y Z Xiong and J SChen ldquoMagnetic nanobelts of iron-doped zinc oxiderdquo AppliedPhysics Letters vol 86 no 17 Article ID 173110 2005

[76] D A Schwartz K R Kittilstved and D R Gamelin ldquoAbove-room-temperature ferromagnetic Ni2+-doped ZnO thin filmsprepared from colloidal dilutedmagnetic semiconductor quan-tum dotsrdquo Applied Physics Letters vol 85 no 8 pp 1395ndash13972004

[77] C Ronning P X Gao Y Ding Z L Wang and D SchwenldquoManganese-doped ZnO nanobelts for spintronicsrdquo AppliedPhysics Letters vol 84 no 5 pp 783ndash785 2004

[78] S C Yeow W L Ong A S W Wong and G W HoldquoTemplate-free synthesis and gas sensing properties of well-controlled porous tin oxide nanospheresrdquo Sensors andActuatorsB Chemical vol 143 no 1 pp 295ndash301 2009

[79] Y Shen T Yamazaki Z Liu et al ldquoMicrostructure and H2 gassensing properties of undoped and Pd-doped SnO2 nanowiresrdquoSensors and Actuators B Chemical vol 135 no 2 pp 524ndash5292009

[80] N Izu W Shin and N Murayama ldquoFast response of resistive-type oxygen gas sensors based on nano-sized ceria powderrdquoSensors and Actuators B Chemical vol 93 no 1-3 pp 449ndash4532003

[81] A M Ruiz A Cornet and J R Morante ldquoStudy of La and Cuinfluence on the growth inhibition and phase transformationof nano-TiO2 used for gas sensorsrdquo Sensors and Actuators BChemical vol 100 no 1-2 pp 256ndash260 2004

[82] Y Hu O K Tan W Cao and W Zhu ldquoA low temperaturenano-structured SrTiO3 thick filmoxygen gas sensorrdquoCeramicsInternational vol 30 no 7 pp 1819ndash1822 2004

[83] S-J Hong and J-I Han ldquoEffect of low temperature compositecatalyst loading (LTC2L) on sensing properties of nano gassensorrdquo Sensors and Actuators A Physical vol 112 no 1 pp80ndash86 2004

[84] T G G Maffeıs M Penny K S Teng S P Wilks H S Ferkeland G T Owen ldquoMacroscopic and microscopic investigationsof the effect of gas exposure on nanocrystalline SnO2 at elevatedtemperaturerdquo Applied Surface Science vol 234 no 1ndash4 pp 82ndash85 2004

[85] K S Yoo S H Park and J H Kang ldquoNano-grained thin-filmindium tin oxide gas sensors for H2 detectionrdquo Sensors andActuators B Chemical vol 108 no 1-2 pp 159ndash164 2005

[86] Q Zhang C Xie S Zhang et al ldquoIdentification and patternrecognition analysis of Chinese liquors by doped nano ZnO gas

16 Journal of Nanomaterials

sensor arrayrdquo Sensors and Actuators B Chemical vol 110 no 2pp 370ndash376 2005

[87] C-H Han S-D Han I Singh and T Toupance ldquoMicro-beadof nano-crystalline F-doped SnO2 as a sensitive hydrogen gassensorrdquo Sensors and Actuators B Chemical vol 109 no 2 pp264ndash269 2005

[88] C Xiangfeng J Dongli G Yu and Z Chenmou ldquoEthanolgas sensor based on CoFe2O4 nano-crystallines prepared byhydrothermal methodrdquo Sensors and Actuators B Chemical vol120 no 1 pp 177ndash181 2006

[89] N Izu N Oh-Hori M Itou W Shin I Matsubara and NMurayama ldquoResistive oxygen gas sensors based on Ce1minusxZrxO2nano powder prepared using new precipitation methodrdquo Sen-sors and Actuators B Chemical vol 108 no 1-2 pp 238ndash2432005

[90] K Luongo A Sine and S Bhansali ldquoDevelopment of a highlysensitive porous Si-based hydrogen sensor using Pd nano-structuresrdquo Sensors and Actuators B Chemical vol 111-112 pp125ndash129 2005

[91] S Shukla R AgrawalH J Cho and S Seal ldquoEffect of ultravioletradiation exposure on room-temperature hydrogen sensitivityof nanocrystalline doped tin oxide sensor incorporated intomicroelectromechanical systems devicerdquo Journal of AppliedPhysics vol 97 no 5 Article ID 054307 2005

[92] S P Zhang H J Cho Z Rahman C Drake and S Seal ldquoHy-drogen-discriminating nanocrystalline doped-tin-oxide room-temperature microsensorrdquo Journal of Applied Physics vol 98no 10 Article ID 104306 2005

[93] C-H Han S-D Han I Singh and T Toupance ldquoMicro-beadof nano-crystalline F-doped SnO2 as a sensitive hydrogen gassensorrdquo Sensors and Actuators B Chemical vol 109 no 2 pp264ndash269 2005

[94] Y Hu O K Tan J S Pan H Huang and W Cao ldquoTheeffects of annealing temperature on the sensing properties oflow temperature nano-sized SrTiO3 oxygen gas sensorrdquo Sensorsand Actuators B Chemical vol 108 no 1-2 pp 244ndash249 2005

[95] G Xu Y-W Zhang X Sun C-L Xu and C-H Yan ldquoSynthesisstructure texture and CO sensing behavior of nanocrystallinetin oxide doped with scandiardquoThe Journal of Physical ChemistryB vol 109 no 8 pp 3269ndash3278 2005

[96] J C Belmonte J Manzano J Arbiol et al ldquoMicromachinedtwin gas sensor for CO and O2 quantification based on catalyti-cally modified nano-SnO2rdquo Sensors and Actuators B Chemicalvol 114 no 2 pp 881ndash892 2006

[97] H Gong J Q Hu J HWang C H Ong and F R Zhu ldquoNano-crystalline Cu-doped ZnO thin film gas sensor for COrdquo Sensorsand Actuators B Chemical vol 115 no 1 pp 247ndash251 2006

[98] X Chu D Jiang and C Zheng ldquoThe gas-sensing properties ofthick film sensors based on nano-ZnFe2O4 prepared by hydro-thermal methodrdquoMaterials Science and Engineering B vol 129no 1-3 pp 150ndash153 2006

[99] Y Hu O K Tan J S Pan H Huang and W Cao ldquoThe effectsof annealing temperature on the sensing properties of lowtemperature nano-sized SrTiO3 oxygen gas sensorrdquo Sensors andActuators B Chemical vol 108 no 1-2 pp 244ndash249 2005

[100] J Cerda Belmonte J Manzano J Arbiol et al ldquoMicromachinedtwin gas sensor for CO and O2 quantification based on catalyti-cally modified nano-SnO2rdquo Sensors and Actuators B Chemicalvol 114 no 2 pp 881ndash892 2006

[101] G N Chaudhari A M Bende A B Bodade S S Patiland V S Sapkal ldquoStructural and gas sensing properties of

nanocrystalline TiO2WO3-based hydrogen sensorsrdquo Sensorsand Actuators B Chemical vol 115 no 1 pp 297ndash302 2006

[102] E Delgado and C R Michel ldquoCO2 and O2 sensing behavior ofnanostructured barium-doped SmCoO3rdquoMaterials Letters vol60 no 13-14 pp 1613ndash1616 2006

[103] Y J Choi Z Seeley A Bandyopadhyay S Bose and S AAkbar ldquoAluminum-doped TiO2 nano-powders for gas sensorsrdquoSensors and Actuators B Chemical vol 124 no 1 pp 111ndash1172007

[104] J Yang K Hidajat and S Kawi ldquoSynthesis of nano-SnO2SBA-15 composite as a highly sensitive semiconductor oxide gassensorrdquoMaterials Letters vol 62 no 8-9 pp 1441ndash1443 2008

[105] S Palzer E Moretton F H Ramirez A Romano-Rodriguezand J Wollenstein ldquoNano- and microsized metal oxide thinfilm gas sensorsrdquoMicrosystem Technologies vol 14 no 4-5 pp645ndash651 2008

[106] B C Yadav R Srivastava C D Dwivedi and P PramanikldquoSynthesis of nano-sized ZnO using drop wise method andits performance as moisture sensorrdquo Sensors and Actuators APhysical vol 153 no 2 pp 137ndash141 2009

[107] E Comini G Sberveglieri C Sada et al ldquoChemical vapordeposition of Cu2O and CuO nanosystems for innovative gassensorsrdquo in Proceedings of the IEEE Sensors 2009 Conference(SENSORSrsquo09) pp 111ndash113 October 2009

[108] M Yuasa T Masaki T Kida K Shimanoe and N YamazoeldquoNano-sized PdO loaded SnO2 nanoparticles by reverse micellemethod for highly sensitive CO gas sensorrdquo Sensors and Actua-tors B Chemical vol 136 no 1 pp 99ndash104 2009

[109] Z Seeley Y J Choi and S Bose ldquoCitrate-nitrate synthesis ofnano-structured titanium dioxide ceramics for gas sensorsrdquoSensors and Actuators B Chemical vol 140 no 1 pp 98ndash1032009

[110] J D Kim Y J Lee and J Y Park ldquoExtremely small methanolsensor with micronano porous Au-Pt electrodes for compactDMFC applicationsrdquo in Proceedings of the IEEE SENSORSConference pp 1943ndash1946 2009

[111] S J Chang W Y Weng C L Hsu and T J Hsueh ldquoHighsensitivity of a ZnO nanowire-based ammonia gas sensor withPt nano-particlesrdquoNano Communication Networks vol 1 no 4pp 283ndash288 2010

[112] R Dasari F J Ibanez and F P Zamborini ldquoElectrochemicalfabrication of metalorganicmetal junctions for molecularelectronics and sensing applicationsrdquo Langmuir vol 27 no 11pp 7285ndash7293 2011

[113] L Mahdavian ldquoThermodynamic study of alcohol on SnO2(100)-based gas nano-sensorrdquo Physics and Chemistry of Liquidsvol 49 no 5 pp 626ndash638 2011

[114] R Khairnar R Mene S Munde and M Mahabole ldquoNano-hydroxyapatite thick film gas sensorsrdquo in Proceedings of the 4thNanoscience and Nanotechnolgy sSymposium (NNS rsquo11) vol 1415of AIP Conference Proceedings pp 189ndash192 Bali IndonesiaDecember 2011

[115] M A Andio P N Browning P A Morris and S A AkbarldquoComparison of gas sensor performance of SnO2 nano-structures on microhotplate platformsrdquo Sensors and ActuatorsB Chemical vol 165 no 1 pp 13ndash18 2012

[116] Y-S Kim P Rai and Y-T Yu ldquoMicrowave assisted hydrother-mal synthesis of AuTiO2 core-shell nanoparticles for hightemperature CO sensing applicationsrdquo Sensors and ActuatorsB Chemical vol 186 pp 633ndash639 2013

Journal of Nanomaterials 17

[117] N J Ridha M H H Jumali A A Umar and F K MohamadldquoEthanol sensor based on ZnO nanostructures prepared viamicrowave ovenrdquo in Proceedings of the 7th International Confer-ence on Sensing Technology (ICST rsquo13) pp 121ndash126 WellingtonNew Zealand December 2013

[118] W Jin S YanW Chen S Yang C Zhao and Y Dai ldquoEnhancedethanol sensing characteristics by decorating dispersed Pdnanoparticles on vanadium oxide nanotubesrdquoMaterials Lettersvol 128 pp 362ndash365 2014

[119] A I Ayesh S T Mahmoud S J Ahmad and Y Haik ldquoNovelhydrogen gas sensor based on Pd and SnO2 nanoclustersrdquoMaterials Letters vol 128 pp 354ndash357 2014

[120] L Yin D Chen M Hu et al ldquoMicrowave-assisted growthof In2O3 nanoparticles on WO3 nanoplates to improve H2S-sensing performancerdquo Journal of Materials Chemistry A vol 2no 44 pp 18867ndash18874 2014

[121] M Bagheri N F Hamedani A R Mahjoub A A Khodadadiand YMortazavi ldquoHighly sensitive and selective ethanol sensorbased on Sm2O3-loaded flower-like ZnO nanostructurerdquo Sen-sors and Actuators B Chemical vol 191 pp 283ndash290 2014

[122] N Rajesha J C Kannanb T Krishnakumar S G Leonar-did and G Neri ldquoSensing behavior to ethanol of tin oxidenanoparticles prepared by microwave synthesis with differentirradiation timerdquo Sensors and Actuators B Chemical vol 194pp 96ndash104 2014

[123] Q Wang C Wang H Sun et al ldquoMicrowave assisted synthesisof hierarchical PdSnO2 nanostructures for CO gas sensorrdquoSensors and Actuators B Chemical vol 222 Article ID 18846pp 257ndash263 2016

[124] Y Liu P She J Gong et al ldquoA novel sensor based onelectrodeposited AundashPt bimetallic nano-clusters decorated ongraphene oxide (GO)-electrochemically reduced GO for sensi-tive detection of dopamine and uric acidrdquo Sensors and ActuatorsB vol 221 pp 1542ndash1553 2015

[125] S Rane S Arbuj S Rane and S Gosavi ldquoHydrogen sensingcharacteristics of Pt-SnO2 nano-structured composite thinfilmsrdquo Journal of Materials Science Materials in Electronics vol26 no 6 pp 3707ndash3716 2015

[126] C Wadell F A A Nugroho E Lidstrom B Iandolo J BWagner and C Langhammer ldquoHysteresis-free nanoplasmonicpd-au alloy hydrogen sensorsrdquo Nano Letters vol 15 no 5 pp3563ndash3570 2015

[127] M A Haija A I Ayesh S Ahmed and M S KatsiotisldquoSelective hydrogen gas sensor using CuFe2O4 nanoparticlebased thin filmrdquo Applied Surface Science vol 369 pp 443ndash4472016

[128] S G Leonardi A Mirzaei A Bonavita et al ldquoA comparisonof the ethanol sensing properties of -iron oxide nanostructuresprepared via the sol-gel and electrospinning techniquesrdquo Nan-otechnology vol 27 no 7 Article ID 075502 2016

[129] A Mirzaei S Park G-J Sun H Kheel and C Lee ldquoCO gassensing properties of In4Sn3O12 and TeO2 composite nanopar-ticle sensorsrdquo Journal of Hazardous Materials vol 305 pp 130ndash138 2016

[130] H Liu W Yang M Wang L Xiao and S Liu ldquoFabricationof lotus-like AuTiO2 nanocomposites with enhanced gas-sensing propertiesrdquo Sensors and Actuators B vol 236 pp 490ndash498 2016

[131] A I Ayesh Z Karam F Awwad and M A Meetani ldquoCon-ductometric graphene sensors decorated with nanoclustersfor selective detection of Hg2+ traces in waterrdquo Sensors andActuators B Chemical vol 221 Article ID 18652 pp 201ndash2062015

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