N

Ma

b

c

d

a

ARRAA

KNNECSG

1

mccohsgppiagsssstZhm

0h

Sensors and Actuators B 177 (2013) 286– 294

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h o me pa ge: www.elsev ier .com/ locate /snb

on-aqueous synthesis of hexagonal ZnO nanopyramids: Gas sensing properties

uhammad Z. Ahmada,d,∗, Jin Changb, Muhammad S. Ahmadc, Eric R. Waclawikb, Wojtek Wlodarskia

School of Electrical & Computer Engineering, RMIT University City Campus, GPO Box 2476V, Melbourne, 3001, AustraliaSchool of Chemistry, Physics & Mechanical Engineering, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland, 4001, AustraliaSchool of Bioprocess Engineering, Universiti Malaysia Perlis, Kompleks Pusat Pengajian Jejawi 3, 02600 Arau, Perlis, MalaysiaMechanisation & Automation Research Center, MARDI HQ, 43400 Serdang, Selangor, Malaysia

r t i c l e i n f o

rticle history:eceived 16 August 2012eceived in revised form 11 October 2012ccepted 7 November 2012vailable online 19 November 2012

eywords:

a b s t r a c t

Zinc oxide (ZnO) nanopyramids were synthesized by a one-pot route in a non-aqueous and surfactant-free environment. The synthesized metal oxide was characterized using SEM, XRD, and TEM to investigatethe surface morphology and crystallographic phase of the nanostructures. It was observed that the ZnOnanopyramids were of uniform size and symmetrical, with a hexagonal base and height of ∼100 nm. Gassensing characterization of the ZnO nanopyramids when deposited as thin-film onto conductometrictransducers were performed towards NOx and C2H5OH vapor of different concentrations over a temper-

◦

anopyramidsitrogen dioxidethanolonductometricolvothermal

ature range of 22–350 C. It was observed that the sensors responded towards NO2 (10 ppm) and C2H5OH(250 ppm) analytes best at temperatures of 200 and 260 ◦C with a sensor response of 14.5 and 5.72,respectively. The sensors showed satisfactory sensitivity, repeatability as well as fast response and recov-ery towards both the oxidizing and the reducing analyte. The good performance was attributed to the lowamount of organic impurities, large surface-to-volume ratio and high crystallinity of the solvothermally

amid

as sensing synthesized ZnO nanopyr

. Introduction

Zinc oxide (ZnO) is a widely investigated and interestingetal oxide due to its stable chemical characteristics, rich defect

hemistry, and it is relatively low-cost. It is non-toxic, biologically-ompatible, and easily fabricated from solution into a wide rangef interesting shapes and morphologies [1–3]. Nanostructured ZnOas been explored for various applications such as field emis-ion displays, light-emitting diodes, energy storage devices andas sensors [2,4–7]. In chemical sensing applications, the use oforous ZnO nanostructure layers are crucial to enhance sensingerformance [8]. High porosity allows rapid diffusion of molecules

nside the nanostructured films as well as creating higher inter-ction volumes with analytes. Recently, efforts to enhance ZnOas sensor performance include employing types of ZnO nano-tructures of controlled crystal sizes and shapes with increasedurface-to-volume ratios. Another common method to enhance theensor response is by inserting or adding catalytic noble metalsuch as platinum (Pt), palladium (Pd), and gold (Au) [6]. By doinghis, better dissociation rates can be achieved. Reports of porous

nO nanostructures fabricated by various deposition techniquesave garnered tremendous interest. These techniques include ther-al decomposition [9], thermal evaporation [10], rf sputtering

∗ Corresponding author. Tel.: +61 03 9925 3690; fax: +61 03 9925 2007.E-mail address: zamharir@gmail.com (M.Z. Ahmad).

925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2012.11.013

s.© 2012 Elsevier B.V. All rights reserved.

[11], pulsed laser deposition (PLD) [12], spray pyrolysis [13], andchemical vapor deposition (CVD) [14]. However, aqueous as wellas non-aqueous synthesis have been widely employed to controlnanostructures growth at low temperatures [3]. These techniquesare known to be capable of creating ZnO morphologies of differ-ent shapes and size [15–19]. Some of ZnO nanostructures that havebeen successfully developed employing these techniques includesnanowires (NW) [20], nanorods (NR) [19,21], nanofibers [22], nano-tubes (NT) [23], nanoplates (NP) [19], nanobullets (NB) [19], andnanoparticles [24]. In this study, ZnO nanopyramids (NPys) weresynthesized employing a non-aqueous benzylamine route with theaim of producing nanostructures with a narrow range morpholog-ical sizes and shapes. Furthermore, with respect to gas sensingor catalysis applications, the low amount of organic impuritiesimbued by this method was a particularly important considerationand necessary in order to gain good accessibility of the nanostruc-ture surface [8].

Gas sensing characterization employing zinc oxide nanopyra-mids (ZnO-NPys) as active layers in chemical sensing applicationshas not previously been reported in literature. In the presentwork, the authors synthesized ZnO-NPys using a non-aqueousroute by employing standard Schlenk techniques [19,25]. Thestructural, morphological characteristics of synthesized ZnO-NPys

were investigated using electron microscopy and X-ray powderdiffraction (XRD) analysis techniques. Finally, for the gas sensingcharacterization, ZnO-NPys were spin coated onto SiO2 substrateswith pre-patterned gold (Au) interdigitated transducers (IDTs) and

d Actu

t0

2

2

blSowStsCascrs1

2

aXCtmemaC

2

dssfr(swpcv

M.Z. Ahmad et al. / Sensors an

ested towards NO2 and ethanol vapor of concentration in the range.5–10 and 10–200 ppm, respectively.

. Experimental

.1. Chemical synthesis

Anhydrous zinc acetate (Zn(CH3COO)2, Sigma–Aldrich, 99.99%),enzyl ether ((C6H5CH2)2O, Sigma–Aldrich, 98%), and benzy-

amine (C6H5CH2NH2, Sigma–Aldrich, 99%), were purchased fromigma–Aldrich Chemical Corporation and used as received with-ut further purification or distillation. All synthetic proceduresere conducted under nitrogen atmosphere using standard

chlenk techniques. ZnO-NPys were prepared by aminolysis reac-ion between zinc acetate and benzylamine in benzyl etherolution. Typically, anhydrous (Zn(CH3COO)2 (0.915 g, 5 mmol),6H5CH2NH2 (5.35 g, 50 mmol) and (C6H5CH2)2O (20 mL) weredded into a 100 mL round bottom flask. Then the mixture wastirred and heated at 180 ◦C for 24 h. After reaction, the solution wasooled to room temperature and the white precipitate was sepa-ated by centrifugation. The resulting white powder was washedeveral times with ethanol and dried in vacuum oven at 90 ◦C for2 h.

.2. Characterization techniques

The structure and morphology and of the samples were char-cterized by X-ray diffraction spectrometry (XRD, PANanalyticalPert Pro Multi Purpose Diffractometer) with monochromatizedu K� radiation (� = 0.154178 nm), field-emission scanning elec-ron microscopy (FE-SEM, JEOL 7001F), transmission electron

icroscopy (TEM, JEOL 1010) and high resolution transmissionlectron microscopy (HR-TEM, JEOL 2100). A resistance measure-ent test employing conductometric transducers (8 finger pairs)

nd tested towards oxidizing and reducing analytes (NOx and2H5OH) were performed.

.3. Sensor fabrications and gas testing

To fabricate the gas sensor device, 20 mg of the ZnO-NPys wereispersed in 10 mL ethanol and then spin coated onto a SiO2 sub-trate (8 mm × 12 mm) with pre-patterned Au IDTs. The spin coatedamples were annealed for 12 h at 400 ◦C to ensure remaining sur-ace organic contaminants were removed. After annealing, goldibbons (99.9% purity) were attached to the Au pads using silverAg) epoxy and resin mixture paste (1:1 ratio) and left to dry andolidify in a heated environment of 100 ◦C for 15 min. The sensors

ere then connected to the real-time data acquisition system afterlacement on a micro-heater in a custom made Teflon based gashamber set-up and sealed in a quartz lid (Fig. 1). The approximateolume for the gas testing chamber connected to a computerized

Fig. 1. Schematic drawing of the ZnO-NPy ba

ators B 177 (2013) 286– 294 287

mass flow controller was 30 mL. A gas calibration system was usedto deliver known concentration of NO2 or ethanol balanced in syn-thetic air at a constant gas flow of 200 standard cubic centimetersper minute (sccm). Devices were operated at every 20 ◦C interval fora range of temperatures from 22 to 350 ◦C. It was exposed towardsNO2 of 10 ppm in sequence before increasing the temperature. Oncethe optimized sensor operating temperature for NO2 was obtainedand dynamic response was performed. Consequently, these stepswere repeated to obtain the sensing performance towards C2H5OH(12.5 ppm). Optimization and dynamic response of the sensor wasperformed in a chamber that was maintained using an externalheater and a thermocouple was used to monitor the operating tem-perature in situ. Film resistance was measured with a Keithley 2001multimeter connected to a data acquisition system for a real timedata logging.

3. Results and discussion

3.1. ZnO formation

The growth mechanism of the ZnO nanocrystal can be under-stood on the basis of the following reactions and crystal habits ofwurtzite ZnO. Wurtzite-structured ZnO crystal is comprised of anumber of alternating planes of Zn2+ ions surrounded by four O2−

ions stacked alternatively along c-axis (Fig. 2(a)) [26].The positive polar plane (0 0 0 1) and the negative polar plane

(0 0 0 1) is abundant in Zn2+ and O2−, respectively. As oppositepolarity attracts, it is possible that positive zinc complexes areadsorbed on the negative plane by electrostatic force [27]. It hasbeen reported that the change of the ZnO particles morphology bythe stabilization of certain planes can be readily achieved throughvarying the capping agent or the precursor ratio in a typical colloidsynthesis [28].

Our proposed aminolysis synthesis mechanism (Scheme 1) usedto prepare the ZnO nanocones could be considered as arisingthrough formation of quaternary structure on the carbonyl groupof the zinc acetate precursor. It is reasonable to assume that thebenzylamine could play multiple roles in complexing, structure-directing, and as an assembling agent in the present syntheticsystem. Prior to solvothermal treatment, benzylamine acted in asimilar way as an alcohol solvent, as a coordinating and relativelyweak ligand which acted to form a complex in the precursor solu-tion. During the first step, benzylamine attacks the carbonyl groupof the acetate (Eq. (1)), resulting in a bond between the carbonyland the nitrogen of the amine. Protonation of oxygen of the zincacetate by hydrogen of the positively charged amine follows. Fol-lowing this reaction, zinc hydroxide (Eq. (2)) in the form of white

precipitate is formed. Dehydration of the zinc (II) hydroxide occursin the final step of this scheme (Eq. (3)) and the powdery precipi-tate product was further heated in a calcination step to remove 1mole of water per each mole of zinc oxide.

sed sensor and the gas chamber set-up.

288 M.Z. Ahmad et al. / Sensors and Actuators B 177 (2013) 286– 294

Fig. 2. (a) Schematic diagram of the ZnO wurtzite structure; (b) hexagonal pyramid stacking formation of Zn2+ and O2−; (c) side view of pyramid.

Scheme 1. Proposed aminolysis mechanism of ZnO nanopyramids by the reaction between zinc acetate and benzylamine.

Fig. 3. Growth schematic diagram of the ZnO-NPys by the solvothermal process.

M.Z. Ahmad et al. / Sensors and Actuators B 177 (2013) 286– 294 289

F hite –h

tnmma(ge

3

pwNws(Z

F

ig. 4. ZnO nanopyramids: (a) deposited onto conductometric transducer, (right/womogeneous size nanopyramids (c) author’s illustration of sketched ZnO-NPys.

In this experiment, it is evident that the temperature andhe solvents used are towards the self assembly of ZnO intoanocone forms. Chang and Waclawik have reported multiple ZnOorphologies that were successfully synthesized through onlyinor changes in these parameters. Shapes which resulted in the

minolysis synthesis can be nanobullet, nanopyramids or nanorodsrespectively ZnO NB, NPy, and NR) [19]. Fig. 3 shows the schematicrowth diagram of the hexagonal based ZnO-NPys fabricated heremploying this solvothermal process route.

.2. Structural and morphological characterization

SEM images in Fig. 4(a) and (b) reveal that the white ZnOrecipitate aminolysis product were of the nanopyramid form,ith a distinct, hexagonal base. Further illustrations of the ZnO-Pys features (Fig. 4(c)) of approximate 100 nm length and 100 nm

idth were characterized by XRD and HR-TEM. The XRD analysis

hown in Fig. 5 exhibited the pattern of the planes (0 1 0), (0 0 2),0 1 1), (0 1 2), (1 1 0), (0 1 3), (0 2 0), (1 1 2) and (0 2 1) for wurtzitenO. A preferential (0 0 2) orientation with the high intensities

ig. 5. XRD patterns of the ZnO-NPys samples (a) as prepared and (b) annealed.

on IDTs) and (dark/right – on SiO2 substrates); (b) high magnification image of the

of (0 1 0) and (0 1 1) diffraction peaks were apparent in the pat-terns. The analyzed pattern was consistent with that of the wurtzite(P63mc) ZnO crystal structure (a = 3.249 A, c = 5.206 A) which is ingood agreement with ICDD 36-1451. Further investigation of thenanostructure fine details employing TEM also confirmed the ZnO-NPys morphology in detail, the hexagonal base and the pyramidforms, can be seen in the images supplied in Fig. 6(a) and (b).The HR-TEM image of the hexagonal based ZnO-NPys (shown inFig. 6(c)) reveals that the ZnO is single crystalline of wurtzite crys-tal structure with a lattice spacing of 2.606 A, corresponding tothe (0 0 0 2) plane of the hexagonal ZnO. This implies that thegrowth of the hexagonal based ZnO-NPys took place along thec-axis direction. The image of the corresponding selected areaelectron diffraction (SAED) in Fig. 6(d) shows that the {0 0 0 1}zone axis of the hexagonal cone was surrounded by {1 0 1 0}planes.

3.3. Gas sensing characterization of ZnO-NPys

3.3.1. NO2 sensing characteristicsSensor response (R) towards NO2 were calculated according to

the equation R = Rg/Ra, where Rg and Ra are the resistance of the sen-sor in NO2-air mixed gas and air respectively. The response (�res)or recovery time (�rec) was defined as the time taken for the sensorto achieve 90% of its maximum response, or to decrease to 10% ofits maximum response, respectively. Fig. 7(a) shows the dynamicresponse of the sensor towards NO2 concentrations and the shortterm stability of this sensor. It was found that these devices weremost stable at 200 ◦C and the dynamic response of the sensorstowards NO2 was subsequently obtained at this operating tem-perature. The dynamic response of the sensors was obtained at anexposure time of 240 s and sufficient recovery time with expo-

sure to 0.5, 1.25, 2.5, 5, and 10 ppm NO2 gas. Response times (�res)of 60, 64, 64, 68, and 60 s were measured from the synthetic airbaseline to a stable exposure of the aforementioned concentrationsof NO2 gas and similarly the recovery times of 60, 52, 36, 28, and

290 M.Z. Ahmad et al. / Sensors and Actuators B 177 (2013) 286– 294

F e conp

3rmNaotfticitIaa[

F(

ig. 6. TEM images of: (a) ZnO-NPys, (b) HR-TEM image of the basal section of thattern of the nanopyramids.

2 s were measured from the same set of NO2 gas to synthetic air,espectively. It is accepted that, upon exposure to NO2, gasolecules are directly absorbed on the active sites on the ZnO-Py surfaces. Charge transfer is likely to occur from ZnO tobsorbed NO2 because of the strong electron-withdrawing powerf the NO2 molecules, which leads to the increase of thickness ofhe depletion layer. The nanopyramids structure can be almostully depleted by exposing to NO2 gas (Fig. 9(b)). As a result,he barrier heights at the boundaries between the nanopyramidsncrease significantly, resulting in the large increase in electri-al resistance, i.e., the high sensor response. One should keepn mind that the mechanism of NO2 depends on the operatingemperature of the sensors and on the gas concentration range.

n this experiment, it is assumed that the sensors work withdsorption/desorption processes previously proposed for the rel-tively low operating temperature when exposed towards NO2 gas29].

ig. 7. Dynamic response of the ZnO-NPys based sensor and the short term repeatability

b) C2H5OH at 260 ◦C.

e, (d) the lattice fringes of the ZnO on (0 0 0 2) plane, and (c) corresponding SAED

3.3.2. Ethanol (C2H5OH) sensing characteristicsAs a reducing vapor, ethanol sensing characteristics were cal-

culated according to the equation R = Ra/Rg, where Ra and Rg arethe resistance of the sensor in air and ethanol–air mixed gas,respectively. Fig. 7(b), shows the dynamic response of the ZnO-NPy based sensor towards different ethanol concentrations andthe short term stability as well as the repeatability of the sen-sor measurement. It was found that the sensors were most stablein the range of 240–260 ◦C when exposed towards ethanol. Thedynamic response of the sensors was performed at the operatingtemperature of 260 ◦C due to the slightly improved sensor responseand recovery times at this temperature. The sensors were exposedtowards ethanol vapor for 120 s at concentrations of 12.5, 31.3,

62.5, 187.5, 250, 375 and 500 ppm followed by a sufficient recov-ery time. Response times, �res, of 8 s (3 first concentrations) and12 s (the remaining) were measured for the from the synthetic airbaseline to a stable exposure of the aforementioned concentrations

behavior at optimized sensor operating temperature towards (a) NO2 at 200 ◦C and

M.Z. Ahmad et al. / Sensors and Actu

F(

o8viseNrsl

aggrvbpsrtaeSGtctTadlpst

Fci

ig. 8. Response of ZnO-NPys based sensor towards NO2 (10 ppm) and C2H5OH12.5 ppm) with increasing operating temperatures.

f ethanol vapor and similarly the recovery times of 48, 48, 64, 76,8, 112, and 116 s were measured from the same set of ethanolapor to synthetic air, respectively. Upon exposure to a reduc-ng gas ethanol vapor, the oxygen ionic species adsorbed on theurface of ZnO-NPys can be expected to react with the reducingthanol molecules. The electrons released are injected into ZnO-Pys conduction band. When ethanol was removed, the resistance

eturned to the original value due to re-adsorption of oxygen ionicpecies. Large ethanol molecules adsorbed onto the surfaces fol-ows typical acid–base behavior [29].

Fig. 8 represents the sensor response behavior at these temper-ture ranges towards NO2 and ethanol. Upon exposure of oxidizingas (NO2) the resistance of the increases and exposure of reducingas (ethanol) decreases the resistance of the sensors. The sensoresponse versus operating temperature curve shows a maximumalue which depends on the target analyte. This could be explainedy the temperature dependence of the adsorption and desorptionrocess on the metal-oxide surface [30]. It was observed that theensitivity towards NO2 and C2H5OH is highest at 200 and 260 ◦C,espectively, and are supported by other performance parame-ers such as fast response and recovery time, good repeatability,nd stable baseline resistance. Thus, the tradeoff between differ-nt parameters is needed in choosing the operating temperature.uch behavior is common in zinc oxide resistance-based sensors.enerally, the resistance change of the sensitive layer is based on

he principle of conductance variation of the sensing element. Theonductance change depends on gas atmosphere and operatingemperature of the semiconductor material exposed to the test gas.hese conditions resulted in space-charge layer changes as wells band modulation. The schematic of the mentioned behavior isepicted in Fig. 9. Common phenomena taken place at the surface

evel such as physisorption, chemisorption, and electron transferrocesses are often involved in such situations. As ZnO is an n-typeemiconducting metal oxide, oxygen ions species were adsorbedo the surface during exposure to air, and then were ionized into

ig. 9. Schematic design of gas-sensing features on ZnO-NPys: (a) in synthetic air, (b) inases of (b) oxidizing and (c) reducing gas on the surface of sensing materials (the red regn this figure legend, the reader is referred to the web version of the article.)

ators B 177 (2013) 286– 294 291

(O−, O2−, O2−) by capturing free electrons from the particles, thus

leading to the formation of a thick space charge layer with anincrease of potential barrier. At a molecular level, the reactionactivities at vacancy sites can result in withdrawal from (oxidizingenvironment (Fig. 9(b)) and donation (reduced environment(Fig. 9(c)) of electrons to the ZnO-NPy surface leading to anincrease/decrease in resistance, respectively [3]. According to thespace charge model, �D (the Debye length) can be expressed by thefollowing equation:

�D = (εε0kBT)1/2

q2nb(7)

where ε and ε0 is the electrical permittivity of the material in atmo-sphere and vacuum, respectively; kB is the Boltzmann’s constant;T is the temperature (in Kelvin), q is the electrical charge of thecarrier, and nb is the free carrier concentration.

In considering the response of these ZnO-NPy sensors to NO2and ethanol, gas diffusion is also a contributing factor which canstrongly contribute to the sensor’s response and recovery time. Inthe present work, the sensing layer fabricated from ZnO-NPys washighly porous. This porosity provided ready access for gaseous ana-lyte molecules to reach deep into the thin film layer through theporous network. Diffusion of NO2/ethanol into the sensing layerwas strongly believed to be an important factor which resulted inthe high response of this nanostructured thin-film sensor. There-fore, limited gas diffusion effects that can sometimes occur inhighly compact, sintered semiconductor thin film sensors waslikely to be negligible and so surface phenomena such as adsorp-tion/desorption of NO2/ethanol molecules at active sites on theZnO-NPy surfaces are expected to be the dominating factor of thesensor performance. This assumption is in fact supported by thefast response of the sensors towards both analytes. The deviceresponded reversibly towards NO2 with changes in the resistanceat 200 ◦C. It is also significant that the response time for the sensorincreased with NO2 concentration. Meanwhile, the recovery timedecreased with higher NO2 concentrations. This behavior is likelyto be due to the uniformity in size, shape and surface structures ofthe ZnO-NPys. The faceted surfaces of the nanopyramids made itpossible to achieve an effective interaction with gas/vapor analytedue to their large surface-to-volume ratio [31,32].

Performance parameters such as long �res and �rec, repeatabil-ity, and baseline stability was the basis for considering 200 and260 ◦C the optimized sensor operating temperatures for NO2 andethanol, respectively. Thus, there was a tradeoff that needed tooccur when choosing the best set of parameters to measure sen-sor response and so choose the best operating temperature. The

comparisons of sensing responses of various ZnO nanostructureswithout an additional catalytic layer are summarized in Table 1(NO2 sensing) and Table 2 (ethanol sensing). It is observed that theoverall sensor operating temperatures for both the NO2 and ethanol

NO2, and (c) in ethanol vapor. Note: the interaction of the gas molecules for eachion is the Debye length (�D) thickness. (For interpretation of the references to color

292 M.Z. Ahmad et al. / Sensors and Actuators B 177 (2013) 286– 294

Table 1Sensor response of ZnO nanostructures toward NO2.

ZnO nanostructure type Lowest gas concentration(ppm)

Optimized operatingtemperature (◦C)

Sensor response (R) �res/�rec (s) Ref.

Nanowires 0.5 225 12 24/12 [33]Nanobelts 0.5 350 0.8 180/268 [30]Nanorods 1.0 350 1.8 180/– [34]Nanobarbed fibers 0.03 210 1.5 96/36 [22]Nanoparticles 1 300 2.7 33/7 [35]ZnO-NPys (this work) 0.5 200 2 60/60 This work

Table 2Sensor response of ZnO nanostructures toward ethanol vapor.

ZnO nanostructure type Lowest vaporconcentration (ppm)

Optimized operatingtemperature (◦C)

Sensor response (R) �res/�rec (s) Ref.

Nanoflowers 1 320 4.1 2/15 (100 ppm) [36]Nanosheets 0.01 400 3.05 ± 0.21 7/19 (200 ppm) [37]Nanorods 10 300 1.57 –/– [38]Nanoparticles 50 300 7 94/– [35]Porous nanosheets 200 320 36.5 3/15 [39]Nanowires 1 300 1.9 –/– [40]ZnO-NPs (this work) 12.5 260 2 8/48 This work

Fig. 10. The response of ZnO-NPys based sensor towards (a) NO2 and (b) C2H5OH as a function of different concentrations at their optimized operating temperature.

F ferent

is

torsSc(

Ttt

ig. 11. The response and recovery time of the ZnO-NPys based sensor towards dif

n the tables are higher with these ZnO-NPys based sensors than forensors constructed from ZnO with different morphologies.

Fig. 10 shows the response (R) of the sensor when exposedowards some concentration values during the dynamic test. It isbserved that the sensor response increased almost linearly withespect to the concentrations of each analyte. The measured sensorensitivity S, given in this almost linear case can be expressed as:

= �R/�C where �R is the sensor response and �C is the analyte con-entration [41]. The sensitivity for the lowest concentration of NO20.5 ppm) and C2H5OH (12.5 ppm) is 3.65 and 0.16, respectively.

The �res and �rec of each concentration are provided in Fig. 11.he measured sensor �res are approximately 60 s when exposedowards different concentration of NO2 in (Fig. 11(a)). Meanwhilehe �rec measured for the sensors recovered to the baseline much

concentrations at their optimized operating temperature (a) NO2 and (b) C2H5OH.

quicker as the NO2 concentration increased. In Fig. 11(b), the�res and �rec of the sensors when exposed towards ethanol vaporbehaved the opposite way. The �res observed, with average mea-surement are approximately ∼10 s. The measured �rec was longeras the ethanol concentration increased. Nevertheless, all �res and�rec for both NO2 and ethanol testing performed on the sensor haveproven to be lower than 70 s and 120 s, respectively.

4. Conclusions

We have successfully employed a simple, inexpensive non-aqueous synthesis that yielded ZnO nanopyramids with hexagonalbase (ZnO-NPys) which were deposited onto conductometric trans-ducers for gas characterization. NO2 and ethanol as oxidizing and

d Actu

rTaFeoaois

A

QtaGac

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

M.Z. Ahmad et al. / Sensors an

educing agents, respectively, were employed as target analytes.he fabricated sensors showed satisfactory sensing properties suchs equal to 14.5 (10 ppm of NO2) and 5.72 (250 ppm of C2H5OH).ast response and recovery, good repeatability, and linear depend-nce between sensitivity and ethanol concentration at optimizedperating temperature were also observed. So far, the sensor waslso found to perform at lower temperature compared to reportsf other ZnO nanostructures. This non-aqueous route of ZnO-NPyss proposed to be a good candidate for applications in chemicalensing.

cknowledgements

The authors would like to acknowledge RMIT University,ueensland University of Technology, and Malaysian Agricul-

ural Research and Development Institute (MARDI) for financialssistance. ERW acknowledges the financial support of the QLDovernment Smart Futures (NIRAP) program. Jin Chang gratefullycknowledges the financial support of the China Scholarship Coun-il (CSC).

eferences

[1] L.E. Greene, B.D. Yuhas, M. Law, D. Zitoun, P. Yang, Solution-grown zinc oxidenanowires, Inorganic Chemistry 45 (2006) 7535–7543.

[2] Z.L. Wang, Splendid one-dimensional nanostructures of zinc oxide: a new nano-material family for nanotechnology, ACS Nano 2 (2008) 1987–1992.

[3] L. Schmidt-Mende, J.L. MacManus-Driscoll, ZnO – nanostructures, defects, anddevices, Materials Today 10 (2007) 40–48.

[4] C. Klingshirn, J. Fallert, H. Zhou, J. Sartor, C. Thiele, F. Maier-Flaig, D. Schneider,H. Kalt, 65 years of ZnO research – old and very recent results, Physical StatusSolidi B 247 (2010) 1424–1447.

[5] M. Niederberger, G. Garnweitner, N. Pinna, G. Neri, Non-aqueous routes tocrystalline metal oxide nanoparticles: formation mechanisms and applications,Progress in Solid State Chemistry 33 (2005) 59–70.

[6] E. Comini, G. Sberveglieri, Metal oxide nanowires as chemical sensors, MaterialsToday 13 (2010) 36–44.

[7] M. Ahmad, D. Rafi ud, C. Pan, J. Zhu, Investigation of hydrogen storage capabil-ities of ZnO-based nanostructures, Journal of Physical Chemistry C 114 (2010)2560–2565.

[8] I. Bilecka, M. Niederberger, New developments in the nonaqueous and/or non-hydrolytic sol–gel synthesis of inorganic nanoparticles, Electrochimica Acta 55(2010) 7717–7725.

[9] L. Vayssieres, K. Keis, S.-E. Lindquist, A. Hagfeldt, Purpose-built anisotropicmetal oxide material: 3D highly oriented microrod array of ZnO, Journal ofPhysical Chemistry B 105 (2001) 3350–3352.

10] Z. Peng, G. Dai, P. Chen, Q. Zhang, Q. Wan, B. Zou, Synthesis, characterization andoptical properties of star-like ZnO nanostructures, Materials Letters 64 (2010)898–900.

11] N.H. Al-Hardan, M.J. Abdullah, A.A. Aziz, Sensing mechanism of hydrogen gassensor based on RF-sputtered ZnO thin films, International Journal of HydrogenEnergy 35 (2010) 4428–4434.

12] D. Valerini, A. Cretì, A.P. Caricato, M. Lomascolo, R. Rella, M. Martino, Opticalgas sensing through nanostructured ZnO films with different morphologies,Sensors and Actuators B 145 (2010) 167–173.

13] P. Singh, A. Kaushal, D. Kaur, Mn-doped ZnO nanocrystalline thin films preparedby ultrasonic spray pyrolysis, Journal of Alloys and Compounds 471 (2009)11–15.

14] D. Barreca, D. Bekermann, E. Comini, A. Devi, R.A. Fischer, A. Gasparotto, C.Maccato, G. Sberveglieri, E. Tondello, 1D ZnO nano-assemblies by Plasma-CVDas chemical sensors for flammable and toxic gases, Sensors and Actuators B 149(2010) 1–7.

15] L. Zhang, J. Zhao, J. Zheng, L. Li, Z. Zhu, Hydrothermal synthesis of hierarchicalnanoparticle-decorated ZnO microdisks and the structure-enhanced acetylenesensing properties at high temperatures, Sensors and Actuators B 158 (2011)144–150.

16] Y.H. Ni, J.S. Zhu, L. Zhang, J.M. Hong, Hierarchical ZnO micro/nanoarchitectures:hydrothermal preparation, characterization and application in the detection ofhydrazine, CrystEngComm 12 (2010) 2213–2218.

17] S.-H. Hu, Y.-C. Chen, C.-C. Hwang, C.-H. Peng, D.-C. Gong, Development of a wetchemical method for the synthesis of arrayed ZnO nanorods, Journal of Alloysand Compounds 500 (2010) L17–L21.

18] H. Zhao, X. Su, F. Xiao, J. Wang, J. Jian, Synthesis and gas sensor properties of

flower-like 3D ZnO microstructures, Materials Science and Engineering B 176(2011) 611–615.

19] J. Chang, E. Waclawik, Facet-controlled self-assembly of ZnO nanocrystalsby non-hydrolytic aminolysis and their photodegradation activities, CrystEn-gComm 14 (2012) 4041–4048.

ators B 177 (2013) 286– 294 293

20] S. Kar, A. Dev, S. Chaudhuri, Simple solvothermal route to synthesize ZnOnanosheets, nanonails, and well-aligned nanorod arrays, Journal of PhysicalChemistry B 110 (2006) 17848–17853.

21] R. Wahab, Y.-S. Kim, H.-S. Shin, Fabrication, characterization and growth mech-anism of heterostructured zinc oxide nanostructures via solution method,Current Applied Physics 11 (2011) 334–340.

22] H.-U. Lee, K. Ahn, S.-J. Lee, J.-P. Kim, H.-G. Kim, S.-Y. Jeong, C.-R. Cho, ZnOnanobarbed fibers: fabrication, sensing NO2 gas, and their sensing mechanism,Applied Physics Letters 98 (2011) 193114.

23] C.-Y. Su, A.M. Goforth, M.D. Smith, P.J. Pellechia, H.-C. zur Loye, Exceptionallystable, hollow tubular metal-organic architectures: synthesis, characterization,and solid-state transformation study, Journal of the American Chemical Society126 (2004) 3576–3586.

24] J. Lian, Y. Liang, F.-L. Kwong, Z. Ding, D.H.L. Ng, Template-free solvothermalsynthesis of ZnO nanoparticles with controllable size and their size-dependentoptical properties, Materials Letters 66 (2012) 318–320.

25] T.T. Tidwell, Wilhelm Schlenk: the man behind the flask, Angewandte ChemieInternational Edition 40 (2001) 331–337.

26] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Recent progress in processingand properties of ZnO, Progress in Materials Science 50 (2005) 293–340.

27] L. Xu, Y. Guo, Q. Liao, J. Zhang, D. Xu, Morphological control of ZnO nano-structures by electrodeposition, Journal of Physical Chemistry B 109 (2005)13519–13522.

28] T. Andelman, Y. Gong, M. Polking, M. Yin, I. Kuskovsky, G. Neumark, S. O’Brien,Morphological control and photoluminescence of zinc oxide nanocrystals, Jour-nal of Physical Chemistry B 109 (2005) 14314–14318.

29] A. Gurlo, N. Barsan, U. Weimar, M. Ivanovskaya, A. Taurino, P. Siciliano, Poly-crystalline well-shaped blocks of indium oxide obtained by the sol–gel methodand their gas-sensing properties, Chemistry of Materials 15 (2003) 4377–4383.

30] A.Z. Sadek, S. Choopun, W. Wlodarski, S.J. Ippolito, K. Kalantar-zadeh, Char-acterization of ZnO nanobelt-based gas sensor for H2, NO2, and hydrocarbonsensing, IEEE Sensors Journal 7 (2007) 919–924.

31] A. Gurlo, Nanosensors: towards morphological control of gas sensing activity.SnO2, In2O3, ZnO and WO3 case studies, Nanoscale 3 (2011) 154–165.

32] M. Tiemann, Porous metal oxides as gas sensors, European Journal of Chemistry13 (2007) 8376–8388.

33] M.W. Ahn, K.S. Park, J.H. Heo, J.G. Park, D.W. Kim, K.J. Choi, J.H. Lee, S.H. Hong,Gas sensing properties of defect-controlled ZnO-nanowire gas sensor, AppliedPhysics Letters 93 (2008) 263103.

34] P.-S. Cho, K.-W. Kim, J.-H. Lee, NO2 sensing characteristics of ZnO nanorods pre-pared by hydrothermal method, Journal of Electroceramics 17 (2006) 975–978.

35] N. Tamaekong, C. Liewhiran, A. Wisitsoraat, S. Phanichphant, Flame-spray-made undoped zinc oxide films for gas sensing applications, Sensors 10 (2010)7863–7873.

36] J. Huang, Y. Wu, C. Gu, M. Zhai, K. Yu, M. Yang, J. Liu, Large-scale synthesisof flowerlike ZnO nanostructure by a simple chemical solution route and itsgas-sensing property, Sensors and Actuators B 146 (2010) 206–212.

37] L. Zhang, J. Zhao, H. Lu, L. Li, J. Zheng, H. Li, Z. Zhu, Facile synthesis and ultra-high ethanol response of hierarchically porous ZnO nanosheets, Sensors andActuators B 161 (2012) 209–215.

38] H. Ahn, H. Clyde Wikle III, S.-B. Kim, D. Liu, S. Lee, M. Park, D.-J. Kim, Geometriceffect of ZnO nanorods on ethanol sensing properties: the relative role of a seedlayer, Journal of the Electrochemical Society 159 (2012) E23–E29.

39] J. Huang, Y. Wu, C. Gu, M. Zhai, Y. Sun, J. Liu, Fabrication and gas-sensing prop-erties of hierarchically porous ZnO architectures, Sensors and Actuators B 155(2011) 126–133.

40] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Fabrication andethanol sensing characteristics of ZnO nanowire gas sensors, Applied PhysicsLetters 84 (2004) 3654–3656.

41] A. D’Amico, C. Di Natale, A contribution on some basic definitions of sensorsproperties, Sensors Journal, IEEE Sensors Journal 1 (2001) 183–190.

Biographies

Muhammad Z. Ahmad received his BSc degree in electrical engineering from Uni-versity Wisconsin of Milwaukee, USA (2000). Currently he is employed as a R&DEngineer at Malaysian Agricultural Research and Development Institute (MARDI) inKuala Lumpur, Malaysia. He is also pursuing his PhD research at RMIT University,Melbourne, Australia on characterization of nanostructured materials for vapor/gassensing. His interests include nanotechnology, chemical sensors, conductive poly-mers, and composite materials.

Jin Chang obtained his MSc degree in organic chemistry from Nanjing Universityof Technology, China (2010). Currently he is pursuing his PhD research at Queens-land University of Technology, Brisbane, Australia. His main research includes themorphology and facets control of nanomaterials by solvothermal methods and theirapplications in chemical sensors, solar cells and light emitting diode. His researchalso includes the quantum chemistry simulation for organic molecules and inorganiccrystals based on first-principles calculation.

Muhammad S. Ahmad received the BSc degree in chemical engineering from Pur-due University, West Lafayette, Indiana, USA in 2000 and PhD degree from Universityof Wisconsin, Milwaukee, Wisconsin, USA in 2008. His PhD thesis is in Chiral Cataly-sis and Syntheses of Organic Compound. He is a senior lecturer at University MalaysiaPerlis (UniMAP) in the School of Bioprocess Engineering. His research interests

2 d Actu

ip

ESzTahap

chapters, over 450 papers and holds 29 patents. He is a full professor atRMIT University, Melbourne, Australia, and heads the Sensor Technology labo-ratory at School of Electrical and Computer Engineering. His research interests

94 M.Z. Ahmad et al. / Sensors an

nclude chemical and biochemical catalysis, chemical and biological sensors, naturalroduct, extraction and organic syntheses.

ric R. Waclawik received his doctoral degree in 1997 from Flinders University ofouth Australia, where he worked on ultra-cold gas-phase collisions between ben-ene and small atoms and molecules. After postdoctoral fellows at the University of

oronto and the University of Exeter, he was a lecturer at Flinders University in 2000nd has been working in the Queensland University of Technology since 2003. Nowe is an associate professor in the Science and Engineering Faculty where he leads

research program studying a range of nanomaterials systems for photovoltaics,hotonics and sensing applications.

ators B 177 (2013) 286– 294

Wojtek Wlodarski has worked in the areas of sensor technology and instru-mentation for over 35 years in the USA, Canada, Holland, France, Poland, andcurrently in Australia. He has published 4 books and monographs, several book

include chemical, physical and biosensors, nanotechnology, materials science andinstrumentation.