Morphology-dependent humidity adsorption kinetics of ZnO nanostructures

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Sensors and Actuators A 187 (2012) 37–42

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators A: Physical

journa l homepage: www.e lsev ier .com/ locate /sna

orphology-dependent humidity adsorption kinetics of ZnO nanostructures

. Asara, A. Erola,∗, S. Okurb, M.C. Arikana

Physics Department, Istanbul University, Faculty of Science, Vezneciler, 34134 Istanbul, TurkeyDepartment of Metallurgy, Faculty of Engineering, Izmir Katip Celebi University, Cigli, 35620 Izmir, Turkey

r t i c l e i n f o

rticle history:eceived 20 May 2012eceived in revised form 6 August 2012ccepted 9 August 2012vailable online 19 August 2012

eywords:nO

a b s t r a c t

The humidity-sensing characteristics of ZnO nanostructures are investigated using a quartz crystalmicrobalance (QCM) measurement. ZnO nanostructures are synthesized via sol–gel route in nanoparti-cle (ZnO-NP) and nanowire (ZnO-NW) morphologies with diameter about 20–30 nm. Scanning electronmicroscopy (SEM) and X-ray diffraction (XRD) methods are used to determine the morphology and crys-tal structure of ZnO nanostructures. Humidity sensing capabilities are discussed in terms of the differentmorphologies. The results show that ZnO-NP is more sensitive to humidity changes than ZnO-NW. QCMresults are analyzed using Langmuir adsorption model to determine adsorption rates, Gibbs free energy

anowireanoparticleensorumidity sensorangmuir adsorption modelCM

of adsorption (�G), and adsorbed mass amount by the synthesized ZnO nanostructures. Negative valueof �G for humidity adsorption on ZnO nanostructures indicates that the process is spontaneous andadsorption capacity increases with size reduction. Gibbs free energy of the ZnO-NP is found to be morenegative, indicating that the ZnO-NP has more favorable adsorption sites compared to the ZnO-NW.Experimental and theoretical results exhibit that humidity-sensing properties of ZnO nanostructures aremorphology-dependent.

. Introduction

The use of nanostructures as sensor materials has attracted sig-ificant attention due to their enhanced sensitivity and selectivity,iniaturized size, fast response and low cost compared to their

ulk and thin film counterparts [1–3]. One of the reasons why nano-tructures show superior chemical and physical properties is thathey have high surface to volume ratio which is directly related toarticle size and morphology. When the size of nanostructure iseduced below 5 nm, surface to volume ratio becomes larger dras-ically. For a 5 nm particle, 50% of atoms are on its surface andurface energy increases with the amount of low coordination sur-ace atoms. Therefore, nanostructures are more chemically reactivehan bulk materials [4].

Humidity monitoring is very important in wide range of fieldsuch as meteorology, agriculture, automotive industry, food pro-essing, textile, medicine and device manufacturing. It is possible toeasure relative humidity based on changing resistance or capac-

tance of sensitive material. On the other hand, using QCM forensing analysis has advantages of high sensitivity, simple and

oom temperature operation compared to the conventional analy-is methods. QCM is a mass sensitive tool that utilizes piezoelectricroperty of quartz crystal to measure frequency shift due to mass

∗ Corresponding author. Tel.: +90 2124555700; fax: +90 2124400069.E-mail addresses: [email protected], [email protected] (A. Erol).

924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.sna.2012.08.019

© 2012 Elsevier B.V. All rights reserved.

loading. Quartz resonators are capable of measuring mass changesas small as a fraction of a monolayer of atoms related to frequencychange by Sauerbrey relation [5]:

�m = −A√

��

2f 20

�f (1)

where �m and �f represent mass and frequency changes, f0, �and � denote fundamental frequency, shear modulus and densityof quartz crystal, respectively. A net change of 1 Hz corresponds to1.34 ng of materials adsorbed onto the crystal surface area (A) of0.196 cm2.

ZnO is one of the most promising metal oxide semiconductorsfor gas/vapor/humidity-sensing applications and has pronouncedsensitivity to gases such as NH3, NO2, CO, H2, and ethanol [6–9].It has been observed that ZnO nanostructures are more sensitivedue to their high surface to volume ratio and have more chemi-cally active centers [10]. In the literature, there are several papersfocused on QCM-based ZnO nanostructure humidity sensors. Zhanget al. [11] studied frequency responses of ZnO nanowire andnanorod coated QCM humidity sensors with diameters of 30–40 nmand 300 nm, respectively. They observed larger frequency responsefrom ZnO nanowires at 97% RH, due to rough and larger surfacearea. Wang et al. [12] investigated humidity sensitivity of ZnO

nanotetrapods depending on film thickness in RH changing from30 to 80%. They found that sensitivity increased up to a certainthickness of 91 nm and then saturated. Zhou et al. [13] devel-oped a wireless humidity sensor prototype using combination of

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3 Actuators A 187 (2012) 37–42

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Table 1Saturated salt solutions and corresponding relative humidity values at 24 ◦C.

Relative humidity (%RH) Saturated salts Calculated vapor molarconcentration (M)

11 LiCl 1.06 × 10−4

22 KAc 2.11 × 10−4

43 K2CO3 4.13 × 10−4

53 Mg(NO3)2 5.28 × 10−4

75 NaCl 7.20 × 10−4

84 KCl 8.06 × 10−4

8 N. Asar et al. / Sensors and

ower-like ZnO nanostructure and QCM with good reproducibilitynd stability. Erol et al. [14,15] investigated the humidity adsorp-ion/desorption kinetics of ZnO nanoparticles (diameter ∼ 5 nm)nd ZnO nanowires (diameter varying between 20 and 50 nm andengths up to micrometers) via QCM measurements at room tem-erature, observing nearly 40 Hz/%RH and 3 Hz/%RH, respectively.

Although humidity sensing properties of ZnO material haveeen intensively studied, the determination of humidity adsorp-ion/desorption kinetic parameters from experimental data using

odified Langmuir adsorption model has been first introducedo literature by us [14]. We have applied this theoretical modelo several different material systems in order to obtain adsorp-ion/desorption kinetic parameters of humidity sensing process14–18]. Here, our aim is to investigate the effects of morphology onumidity sensing properties of ZnO nanoparticles and nanowiresnd support experimental findings with Langmuir adsorptionodel.In this work, we report the humidity-sensing properties of

nO nanostructures synthesized at different morphologies usingol–gel method. The crystal structures and morphologies of ZnOanostructures have been investigated by XRD and SEM analy-is. The humidity-sensing capability of the synthesized structuresas investigated using QCM measurements under various rela-

ive humidity conditions. Langmuir adsorption model was used toalculate adsorption rates, Gibbs free energies and adsorbed massmount for both nanostructures. To the best of our knowledge, its the first time humidity-sensing properties of ZnO having differ-nt morphologies were investigated by both experimentally andheoretically.

. Experimental

.1. Synthesis and structural characterization of ZnOanostructures

ZnO-NP and ZnO-NW were synthesized by sol–gel method,sing zinc acetate dihydrate (ZnAc) and sodium hydroxide (NaOH)s starting materials [19]. Precursor solutions were prepared byissolving ZnAc and NaOH in ethanol at 70 ◦C. Subsequently, OH−

ol was dropped into Zn2+ sol with magnetic stirring in an iceath. Resultant precipitate of ZnO nanostructures was washed with-heptane to eliminate the excess acetate and sodium ions then col-

ected by centrifugation and re-dispersed in ethanol for drying inmbient air. Chemical reaction during the process is

n(CH3COO)2 · 2H2ONaOH−→ ZnO + 2CH3COOH + H2O (2)

Lin et al. [20] showed that the shapes and sizes of ZnO nano-tructures can be controlled by using different concentrations ofrecursor. Therefore, Zn2+ molarities of two solutions were pre-ared at different molarities as 0.08 M and 0.04 M and OH− molarityas kept at a fixed value of 0.5 M in order to synthesize ZnO withifferent morphologies. pH values of the solutions were measureds 11 (for 0.08 M) and 14 (for 0.04 M).

SEM and XRD (with Cu K�1 line, � = 1.54056 A) were used toetermine the morphology and crystal structure of the synthesizednO nanostructures, respectively.

.2. Monitoring of humidity-sensing capabilities of ZnOanostructures

In order to monitor humidity-sensing capabilities of syn-

hesized ZnO nanostructures, QCM measurements are carriedut under different %RH environments at room temperature.T-cut quartz crystals with resonant frequency of 7.995 MHzandwiched between two gold electrodes were used for QCM

94 KNO3 8.37 × 10−4

97 K2SO4 9.31 × 10−4

measurements. Density (�) and shear modulus (�) of quartz crys-tals are 2.684 g/cm3 and 2.947 × 1011 g/cm s2.

ZnO nanostructures were dispersed in ethanol, then drop-castedon ultrasonically cleaned quartz crystals and dried under argon gasflow to prepare humidity sensors. Measurements were carried outby CHI-400A series QCM system interfaced with a computer viaserial port for data acquisition and storage. The QCM system con-sists of a quartz crystal oscillator, digital function generator, dataacquisition circuitry, a potentiostat and a galvanostat. Several glassvessels have been filled with different saturated salt solutions tohave different humidity ambient. ZnO nanostructure-coated quartzcrystal was placed into sensing test cell. The frequency of ZnO-coated quartz crystal was monitored until it become stable. BeforeZnO-coated QCM exposure to humidity, amount of the loadedmass was determined using Eq. (1). In order to make a compar-ison of the effect of morphology on humidity sensing properties,loaded amount of mass values was recorded before exposing tohumidity, then obtained QCM values were normalized according tothe loaded mass amount. ZnO sensors were periodically exposedto various saturated salt solutions as given in Table 1 [21]. Fre-quency responses of sensors to RH changing between 11 and 97%were measured at room temperature (24 ◦C) and RH values wererecorded by EI-1050 commercial sensor simultaneously. The molarvapor concentration C was calculated using ideal gas equation atthe corresponding partial values for the relative humidity valuesmeasured at the equilibrium point.

3. Results and discussion

3.1. Structural characterization of ZnO nanostructures

SEM analysis was used to determine morphology and the sizeof synthesized ZnO nanostructures. As seen from Fig. 1(a) and (b),synthesized ZnO samples are composed of nanoparticles (for 0.08 Mof Zn2+) and nanowires (for 0.04 M of Zn2+) with lengths between200 and 400 nm. Both structures have similar diameters in the20–30 nm range. At high and low pH conditions solubility of ZnOchanges as a consequence of amphoteric behavior. It is shown thatan increase in pH value results in an increase of ZnO solubilityand at pH > 13.5, growth of ZnO is governed by dissolution–re-precipitation mechanism [22,23]. Therefore, the morphology ofZnO synthesized using 0.04 M of Zn2+ (pH 14) resulted as nanowire.

XRD pattern of the synthesized ZnO nanostructures is given inFig. 2. Diffraction peaks in the XRD pattern are indexed to hexagonalwurtzite structured ZnO (space group P63mc (186); a = b = 0.33 nm,c = 0.52 nm) which agrees well with the JCPDS card (36-1451) andno other diffraction peaks from any kind of side products wereobserved.

3.2. Humidity-sensing properties of ZnO nanostructures

Fig. 3(a) and (b) show the frequency shift of ZnO-NP and ZnO-NW film-coated QCM at increasing and decreasing steps of %RH

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N. Asar et al. / Sensors and Actuators A 187 (2012) 37–42 39

vtsi

Fig. 1. SEM images of (a) ZnO-NP and (b) ZnO-NW.

alues between 11% and 97% in equal time intervals (200 s) at room

emperature, respectively. As seen from the figures, although bothensors turn back to initial frequency value at 11% RH after increas-ng and decreasing steps of humidity change, at some %RH values,

Fig. 2. XRD patterns of (a) ZnO-NP and (b) ZnO-NW.

Fig. 3. The frequency response to different %RH values of (a) ZnO-NP and (b) ZnO-NW at room temperature.

a noticeable hysteresis is observed, which can be an indication ofdifference between time constants of adsorption and desorptionprocesses.

The frequency shift amount per %RH is determined using alinear fit to the average value of frequency shift during the adsorp-tion process for every %RH value and plotted in Fig. 4(a) and(b). The frequency response of ZnO-NP and ZnO-NW is found asapproximately 1.0 Hz/%RH and 0.4 Hz/%RH, respectively. ZnO-NPhas higher response with a factor of 2.5 because of the morphologyadvantage of sphere-like nanoparticles compared to ZnO-NWs.

Fig. 5(a) and (b) show the frequency response of ZnO-NP andZnO-NW films-coated QCM at six different %RH values withinlonger exposure time (1000 s) in order to observe saturationvalue of the sensors. As seen from the figures, frequency shiftincreases with %RH and recovers back to the initial value afteradsorption–desorption cycle for both sample and for the same%RH value, ZnO-NP has larger response than ZnO-NW. Adsorp-tion occurs very fast initially then tends to saturate, because of thereduced number of vacant sites.

As shown in Fig. 5(a), increasing RH values causes a slower pro-cess to reach the equilibrium value for both ZnO-nanostructures.This characteristic becomes more noticeable for %RH values big-ger than 75%, especially for ZnO-NP. Two different scenarios maybe considered to explain this sensing characteristic observed athigh humidity levels: (I) When there are more available adsorption

sites through the coated film, diffusion process might contributeto the adsorption process. As a result of diffusion of humid-ity molecules into the film, humidity molecules continue to beadsorbed not only by the surface of the film, but also through the

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40 N. Asar et al. / Sensors and Actuators A 187 (2012) 37–42

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a dWhen the system reaches equilibrium, adsorption and desorption

Fig. 4. Frequency shift versus %RH of (a) ZnO-NP and (b) ZnO-NW sensors.

lm. Therefore reaching an equilibrium condition may take moreime by increasing %RH. (II) Adsorption surface can be saturatedith humidity molecules and then adsorption continues on this

urface via dipole–dipole interactions of the adsorbed moleculesnd environment humidity molecules.

Because the diffusion through ZnO film is much slower processhan the layer by layer adsorption, it is expected to observe a similarharacteristic at desorption process and an appreciable hysteresisetween adjacent adsorption/desorption cycles at high humidity

evels. However, neither ZnO-NP nor ZnO-NW exhibits significantlower desorption process at high %RH values. A negligible hystere-is is observed for both ZnO films, but this can be also related to theayer by layer adsorption. Moreover, if one examine the humid-ty cycles given at RH% values ≥84% in Fig. 5(a), it is clear thatdsorption process seems to approach the saturation after a cer-ain time, but then start to increase again, indicating that a secondtep of adsorption process starts. Therefore, the second proposedrocess is more suitable considering the humidity responses of ZnOanostructures at high humidity levels, especially for ≥84% RH inig. 5.

Langmuir adsorption model was used to determine kineticarameters of ZnO nanostructures by utilizing response and recov-ry curves of sensors. Concerning the adsorption isotherm curvesf the synthesized ZnO nanostructures, of the several adsorptionsotherms, Langmuir adsorption isotherm is the best matched oneo our adsorption–desorption curves and has been applied success-ully to determine the adsorption/desorption kinetics of several

ynthesized structures by us [14,17,24,25]. Langmuir model haseveral assumptions [26]:

Fig. 5. Response and recovery cycles of (a) ZnO-NP and (b) ZnO-NW sensors to11–43, 53, 75, 84, 94 and 97% RH.

i. Monolayer adsorption is formed on a uniform surface.ii. There are active adsorption centers on adsorbent surface and

they are energetically equivalent.iii. There is no interaction between adsorbed molecules.

When a gas phase adsorbate is in contact with a solid adsor-bent after a while an equilibrium condition will occur between gasmolecules that are adsorbed on solid surface and free gas molecules.Increasing surface coverage results in reduced amount of favorableadsorption sites and adsorption rate decreases. Adsorption equi-librium between gas molecules (A), empty surface states (S) andoccupied surface sites (SA) is given by

S + Aka�kd

SA (3)

Assuming that there is a certain number of adsorption centerson the surface, an equilibrium constant (K) can be derived as

K = [SA][S][A]

(4)

K is proportional to amount of surface coverage (�), number ofvacant sites (1 − �) and gas pressure (P). According to Langmuirmodel, adsorption and desorption rates are given by,

ra = ka(1 − �)P (5)

rd = kd� (6)

k and k represent Langmuir adsorption and desorption constants.

rates must be equal to each other.

ka(1 − �)P = kd(�) (7)

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N. Asar et al. / Sensors and Actuators A 187 (2012) 37–42 41

Table 2Calculated Langmuir kinetic parameters and Gibbs free energies of ZnO-NP and ZnO-NW sensors.

Sample �fmax (Hz) �mmax (ng) C (×10−4 M) ka (M−1 s−1) kd (×10−3 s−1) Kequ (M−1) Gibbs free energy, �G (kJ/M)

2.68 1.53 × 105 −11.941.59 5.18 × 104 −10.86

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ZnO-NP 67.18 90.71 9.05 409.71ZnO-NW 26.51 35.30 9.05 82.39

Reaction rate for adsorption related to fractional surface cover-ge can be expressed as:

d�

dt= ka(1 − �)C − kd� (8)

Eq. (8) is integrated to determine time dependency of surfaceoverage as

(t) = K ′(e − e−kobst) (9)

here K′ and kobs are the association constant and the inverse ofelaxation time as given in Eqs. (10) and (11):

′ = C

C + (kd/ka)(10)

obs = kaC + kd (11)

Gas pressure depends on molar concentration (C) so that it cane used instead of pressure. In this study, measure of the surfaceoverage is frequency shift (�f) during the adsorption and desorp-ion processes. �fmax is also the maximum adsorbed amount ofumidity on the surface. Therefore, Eq. (9) takes the following form:

f (t) = �fmaxK ′(1 − e−kobst) (12)

In order to obtain the parameters of the adsorption process, theumidity cycles below 94% RH are chosen, because the isothermselow 94% RH obey Langmuir adsorption isotherms, on the con-rary, the isotherms above 94% RH seem like BET adsorptionsotherms.

Experimental data of humidity adsorption on ZnO nanostruc-ures obtained at 75% RH is fitted to Langmuir model as seen fromig. 6(a) and (b). As seen from the fitting curves in Fig. 6, there ismismatch just before the saturation for both structures. Lang-uir adsorption isotherm has a steep increase compared to the

xperimental one. That means that adsorption process becomeslower before reaching saturation and experimental conditions doot exactly fulfill with the assumptions of Langmuir model justefore the saturation condition is established.

We determined K′ and kobs determined from the curve fits andhen using Eqs. (10) and (11), ka, kd and equilibrium constant Kequ

Kequ = ka/kd) values were calculated. The Gibbs free energy �G ofdsorption/desorption process in terms of Kequ at a constant tem-erature is defined as

G = −RT ln Kequ (13)

R is the universal gas constant (8.314 J/mol K), T is the tempera-ure (K).

Using Sauerbrey equation (Eq. (1)), the maximum adsorbedmount of the humidity molecules on the surface of ZnO-NP andnO-NW at 24 ◦C is calculated as 90 ng and 35 ng, respectively. Allalculated values are tabulated in Table 2.

Negative Gibbs energy values calculated as −11.94 kJ/mol and10.86 kJ/mol are indicative of spontaneous reaction. Larger nega-

ive Gibbs free energy and adsorbed mass amount values obtainedor ZnO-NP sensor represent that ZnO-NP has more favorabledsorption sites [21,27,28].

It was theoretically found by Xiong et al. [29] that Gibbs free

nergy for nanomaterials changes with particle size and shape.he size and shape effects on the Gibbs free energy become morebvious with decreasing size. For the nanoparticles similar in size,ibbs free energy is affected by the effect of shape factor, which is

Fig. 6. The least square fit (solid line) of the adsorption part at 75% RH given in Fig. 5to the Langmuir adsorption isotherm model given in Eq. (12) for (a) ZnO-NP and (b)ZnO-NW sensors.

defined as the surface area ratio between non-spherical and spher-ical nanoparticles. The shape factor increases with the amount ofthe deviation from sphere-like shape. It was proved that the Gibbsfree energy decreases with the shape factor and increases withdecreasing particle size. Therefore, spherical nanoparticles havelarger Gibbs free energy and more prone to adsorb molecules perunit area onto their surface to decrease the total free energy andto become more stable. Hence, adsorption on smaller sphericalparticle has a higher adsorption coefficient and Gibbs free energy.The basis of the calculations, stemmed from Young–Laplace equa-tion, states that chemical potential of an atom in a convex surface(positive curvature) is higher than that on a flat surface [27–29].These theoretical finding supports our experimental and theoret-ical results. Although both ZnO nanoparticles and ZnO-NWs arein the similar size, spherical-like ZnO-NPs have more availableadsorption sites compared to the ZnO NWs, therefore response ofthe ZnO NP is observed to be higher and its Gibbs free energy isfound to be larger.

4. Conclusions

We synthesized ZnO nanostructures via sol–gel method innanoparticle and nanowire morphologies under different growth

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onditions. QCM measurements were carried out over the rangef 11–97% RH to monitor humidity-sensing properties of the sen-ors. Larger frequency response was obtained from ZnO-NP sensorhich is nearly four times higher than response of ZnO-NW sen-

or due to having more adsorption sites. Adsorption parametersnd Gibbs free energies of sensors were calculated by applyingangmuir model to the QCM results at 75% RH. Negative valuesf Gibbs free energy demonstrated that both adsorption processesere spontaneous and larger negative value of Gibbs free energy

or ZnO-NP obtained as −11.937 confirms that ZnO-NP has morenergetically favorable adsorption. Experimental and theoreticalvidences indicate that adsorption capacity of ZnO nanostructuresepends on the morphology.

cknowledgments

This work was supported by Scientific Research Projects Coor-ination Unit of Istanbul University under the project numbers907 and 24861 and TUBITAK (Turkish Scientific Association) underroject number TBAG 109T240.

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Biographies

N. Asar received her B.Sc. degree and her M.S. degree in physics from Istanbul Uni-versity, Istanbul, Turkey, in 2008 and 2012, respectively. She has been studying onnanostructure synthesizing and investigation of sensor properties of nanostructure.

A. Erol received her doctor of philosophy degree in physics from Istanbul Univer-sity, Turkey, in 2002. Her Ph.D. thesis work focused mainly on electrical and opticalcharacterization of hot electron light emitting devices. During her Ph.D. she alsohas worked at Essex University, UK, as research fellow. Currently, she is workingat Physics Department of Istanbul University as an associate professor. Her schol-arly output includes papers mainly focused on optical and electrical properties oflow dimensional III–V group semiconductors, and synthesis of II–VI semiconduc-tor nanostructures, sensor properties of nanostructures, two refereed internationalconference proceedings, an edited book published by Springer in 2008, a chapterin a book published by Springer in 2012, and a popular physics book published in2006.

S. Okur received his bachelor’s degree in physics education from Hacettepe Univer-sity in Ankara, Turkey, in 1989, his master’s degree in physics from Ankara Universityin Ankara, Turkey, in 1992, and master’s degree in physics from Illinois Institute ofTechnology (IIT) in Chicago, IL, USA, in 1996, and his Ph.D. in physics from IIT in1998. He is currently an associate professor in the Department of Physics at IzmirInstitute of Technology in Izmir, Turkey. His current research interests include SAM,LB, organic semiconducting thin film interfaces and their application to electronicdevices such as organic photovoltaic, OLEDs, OTFTs, nano-dots, and nano-wires.

M.C. Arıkan received his Ph.D. degree from University of Essex, UK, in 1980. His Ph.D.

thesis focused mainly on investigation of photoconductive properties of GaAs. Cur-rently he is with Istanbul University as a professor of solid state physics. He is activelycontinuing his research on characterizations and applications of the low dimen-sional semiconductors. His scholarly output includes papers published in refereedinternational journals, and refereed international conference proceedings.