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Sensors and Actuators B 193 (2014) 774– 777

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l h om epage: www.elsev ier .com/ locate /snb

eneral

n electrodeless quartz crystal resonator integrated with UV/Vispectroscopy for the investigation of the photodecomposition ofethylene blue

ooree Ko, Sangmin Jeon ∗

epartment of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Nam-gu, Pohang, Kyungbuk, Korea

r t i c l e i n f o

rticle history:eceived 7 October 2013eceived in revised form1 December 2013ccepted 11 December 2013vailable online 19 December 2013

a b s t r a c t

The photodecomposition of organic molecules (methylene blue) was investigated using an integratedsystem consisting of a UV–vis spectrometer and an electrodeless quartz crystal microbalance (EL-QCM).ZnO nanorods were directly grown on a bare quartz crystal and methylene blue was coated onto thenanorods by drop-casting. Ring-type remote electrodes were used to enable the transmission of light.As the coated methylene blue was photo decomposed by UV light, the changes in mass and light trans-mittance were measured with the EL-QCM and UV–vis spectrometer respectively. These simultaneousmeasurements revealed that the rate of change in optical transmission is greater than that of the mass in

eywords:lectrodeless quartz crystal microbalanceV–vis absorbancenO nanorodshotodegradationhotodecompositionethylene blue

the early stage of photodecomposition, which indicates that the structural degradation of methylene blueis dominant in the early stage. In contrast, the changes in light transmittance were linearly proportionalto those in mass during the later stages, which is attributed to the mineralization of methylene blue.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Since the discovery of the photocatalytic water splitting reac-ion, there has been extensive research to develop highly activehotocatalysts for applications in water/air remediation [1–3],ydrogen production [3,4], solar cells [5], self-cleaning materi-ls [6,7], and sensors [8,9]. A photocatalyst generates electrone−) and hole (h+) pairs through bandgap excitation upon absorp-ion of light energy greater than or equal to its bandgap energy.hese charge carriers can migrate to the surface of the photo-atalyst and react with adsorbed molecules; the holes oxidizedsorbed water molecules to hydroxyl radicals (•OH) and thelectrons reduce molecular oxygen to superoxide radical anions•O2

−). These radicals are strong oxidants that completely miner-lize organic contaminants to CO2 and H2O [10–12].

Photocatalytic activity is typically evaluated by measuring thehotodecomposition rate of an organic dye such as methylenelue (MB) or methyl orange (MO) upon light irradiation. The opti-

al technique such as ultraviolet-visible (UV–vis) spectroscopy isidely used in the measurement of photodecomposition because

t is inexpensive, rapid, and straightforward [13,14]. UV–vis

∗ Corresponding author. Tel.:+82 54 279 2392; fax: +82 54 279 5528.E-mail address: jeons@postech.ac.kr (S. Jeon).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.12.041

spectroscopy is used to measure changes in the color intensityof the organic dyes upon decomposition. However, the opticalmethod is unable to distinguish the photocatalytic mineraliza-tion of dye molecules from their partial structural degradation. Toaddress this drawback of optical measurements, some high sensi-tivity gravimetric instruments such as microcantilevers or quartzcrystal microbalances (QCMs) have been used in the direct mea-surement of photocatalytic mineralization by monitoring absolutechanges in mass [15,16].

In this study, we have investigated the photodecompositionof organic molecules using an integrated system consisting ofa UV–vis spectrometer and a QCM. Instead of a conventionalQCM with gold electrodes on both the top and bottom sides ofa quartz crystal plate, electrodeless QCM (EL-QCM) configurationwas adopted; the electrodes are physically separated and placed inclose proximity to the surfaces of the quartz plate [17–19]. Becausethe electrodeless configuration does not require the depositionof metal electrodes on the quartz plate, it is optically transpar-ent and more chemically stable than the conventional QCM. Inthis study, ZnO nanorods were directly synthesized on the quartzplate and ring-type electrodes were used to allow the transmission

of light. The simultaneous measurements of the transmittance oflight and the change in resonance frequency of the QCM enablethe determination of the correlation between the actual photocat-alytic mineralization and the structural degradation of the organic

W. Ko, S. Jeon / Sensors and Actuators B 193 (2014) 774– 777 775

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which is characteristic of the methyl and methylamine groups inMB [14]. With increases in the UV irradiation time, the transmit-tance increases to values close to 100% (its original level before MB

ig. 1. Schematic diagram of the integrated UV–vis spectroscopy and EL-QCM mea-urement set-up. Ring-shaped remote electrodes are used for the actuation of QCMnd for the transmission of the UV–vis light.

ye molecules adsorbed on the ZnO nanorods during the photode-omposition.

. Experimental

.1. Materials

Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%), ammoniumydroxide (28 wt.% NH3 in water), and methylene blue (MB) wereurchased from Sigma–Aldrich. Deionized water (18.3 M� cm) wasbtained from a reverse osmosis water system (Human Science,orea) and used to prepare the MB solution. For UV light irradi-tion, a UV lamp (� = 254 nm, 4.5 mW/cm2, Spectroline, NY) wassed.

.2. Instrumental set-up

Fig. 1 shows a schematic diagram of the UV–vis and EL-QCMntegrated measurement set-up. The integrated chamber was fab-icated with polymethyl methacrylate to accommodate a 5 MHz.5 cm diameter quartz plate. One disk-shaped 20 mm diameterlectrode was built into each of the top and bottom sides of thehamber with a hole (9 mm in diameter) in the center for UV–vispectroscopy. The top side of the chamber is removable to enablehe full access of UV light to the ZnO nanorods pattern. A quartzlate was mounted parallel to the electrodes at a distance ofa. 0.8 mm. The UV–vis absorption spectra were obtained with aV–vis spectrometer (Ocean Optics) and the resonance frequencyas monitored by connecting the electrodes to a custom-built PC-

ased conductance measurement system consisting of a functionenerator (NI PXI-5406), a digitizer (NI PXI-5114), and a multi-lexer (NI PXI-2593) assembled in a PXI-1033 chassis (National

nstruments (NI), Texas). The measurement software was pro-rammed in LabVIEW, and the conductance spectra were obtainedy measuring the admittance and phase of the quartz crystal overhe selected frequency span. The resonance frequencies were cal-ulated from Lorentzian fits of the conductance spectra.

.3. Synthesis of ZnO nanorods on the quartz crystal plate

ZnO nanorods were grown in a circular central pattern (20 mmn diameter) on the quartz plate. First, seed layers consisting ofircular patterns of Ti (5 nm thick) and ZnO (30 nm thick) wereequentially sputtered onto one side of a 5 MHz quartz crystal. Theputtering conditions were set at a base pressure of 3 × 10−6 Torrnd a working pressure of 7 × 10−3 Torr (Ar gas) with an RF power

f 100 W at room temperature. The ZnO nanorods were hydrother-ally grown on the coated quartz crystal at 90 ◦C for various lengths

f time in a 10 mM Zn(NO3)·6H2O solution at a pH of 10.6 by addingmmonia to the solution. The quartz crystals with ZnO nanorods

Fig. 2. An image of the ZnO nanorods directly synthesized on a quartz crystal plate(1 in. in diameter) and a scanning electron microscope (SEM) image of ZnO nanorodsgrown for 2 h.

were then rinsed with deionized water and ethanol prior to use.Fig. 2 shows an image of the ZnO nanorods synthesized on thequartz crystal plate and a scanning electron microscope (SEM)image of the ZnO nanorods grown for 2 h, which resulted in 1.9 �mlong ZnO nanorods. Note that the synthesized ZnO nanorods aretransparent.

3. Results and discussion

The photodecomposition of MB was investigated by coating0.3 mM MB solution onto the ZnO nanorods with drop-casting.Fig. 3(a) and (b) show the variation in the transmittance spectra ofthe MB-coated quartz plate and its optical photodecomposition rate(�TD/�TL) with UV irradiation time. In the transmittance spectra,the minimum transmittance appears at a wavelength of 663 nm,

Fig. 3. (a) Transmittance spectra of the MB-coated quartz plate and (b) its opticalphotodecomposition rate (�TD/�TL) at 663 nm when exposed to UV light.

776 W. Ko, S. Jeon / Sensors and Actuators B 193 (2014) 774– 777

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ig. 4. (a) Variation in the frequency (�fD) of the MB-coated ZnO nanorods, (b) auartz plate with MB-coated ZnO nanorods before and after 45 min of UV irradiation.

oating). Some values are higher than 100% due to the difference inhe position of the quartz plate before and after coating methylenelue. These results show that the structural degradation and pho-ocatalytic mineralization of MB are occurring simultaneously. Thetructural degradation of MB is evident not only from the variationn transmittance but also in the blue shift in the transmittance bandfrom 663 to 635 nm) with increases in the UV irradiation time. Thislue shift occurs as the MB molecules are modified to intermediateshrough N-demethylation by the radicals generated in the photo-atalyst upon exposure to UV light [14]. Fig. 3(b) shows that mostf the optical photodecomposition of MB occurs in the early stagef UV irradiation, with subsequent saturation.

The degree of photodecomposition can be determined byonitoring the change in mass (�m) of MB adsorbed on the photo-

atalyst. Fig. 4(a) shows the variation in the resonance frequency ofhe MB-coated quartz resonator during photodecomposition (�fD).ote that �fD was measured at the same time as the transmittance

pectra. Because UV irradiation induces a temperature increase inhe system, the frequency shift was measured once the tempera-ure had been stabilized after each exposure to UV light. The changen resonance frequency can be related to the change in mass dueo photocatalytic mineralization by using the Sauerbrey equation20]:

f = 2f 2o

A√

�q�q�m (1)

here fo is the resonance frequency of the unloaded quartz crystal,q is the density of quartz (2.648 g/cm3), �q is the shear modu-

us of quartz (2.947 × 1011 g/cm s2), and A is the active area of theuartz crystal. The initial MB coating on the quartz plate induces a

requency shift (�fL) of approximately −95 Hz, and the resonancerequency gradually increases with UV irradiation time. In contrasto the transmittance, the resonance frequency does not recoverompletely to its original value before coating with MB. This result

Fig. 5. A comparison of the optical photodecomposition rate with the gravimetricphotodecomposition rate. Structural degradation dominates photocatalytic miner-alization in the initial stage of photodecomposition.

is attributed to the coffee stain effect, which produces a high con-centration of MB molecules in a ring shape on the quartz plate, asshown in Fig. 4(b). The QCM detects the change in mass over theentire quartz plate whereas UV–vis spectroscopy is sensitive to onlya small part of the quartz plate, that is, the central area. The concen-trated MB molecules in the ring shape far from the central area ofthe quartz plate are not directly adsorbed onto the surfaces of ZnOnanorods, and therefore are not subject to photodecomposition,which is a surface-limited reaction. Note that the gravimetricallymeasured photodecomposition rate undergoes small changes inthe initial stages of photodecomposition and dramatically increaseswith further UV irradiation.

Fig. 5 compares the optical photodecomposition rate (�TD/�TL)with the gravimetric photodecomposition rate (�fD/�fL). In theinitial stage of photodecomposition, the transmittance changesrapidly whereas the change in the resonance frequency is verysmall. However, after further UV irradiation, the optical and gravi-metric photodecomposition rates increase linearly. As mentionedabove, the transmittance spectra measure the photodecomposi-tion of MB, which arises from the concomitant occurrence of theN-demethylation of MB and its oxidative photocatalytic mineral-ization under UV irradiation. However, the changes in resonancefrequency result only from the photocatalytic mineralization oforganic compounds to volatile compounds such as H2O and CO2.The negligible change in the gravimetric photodecomposition rateand the dramatic change in the optical photodecomposition ratein the initial stage of photodecomposition imply the dominanceof structural degradation with negligible mineralization of theorganic molecules during this stage. In contrast, the relatively largechange in the gravimetric photodecomposition rates after furtherUV irradiation indicates that photocatalytic mineralization is moredominant than structural degradation.

4. Conclusions

We have investigated the photodecomposition of organicmolecules using an integrated system consisting of a UV–vis spec-trometer and an electrodeless quartz crystal microbalance forthe first time to the best of our knowledge. After coating MBonto ZnO nanorods directly grown on a quartz crystal plate, thephotocatalytic decomposition of MB under UV irradiation wasinvestigated by measuring the changes in mass and transmittance

of the quartz plate. The photodecomposition through photocat-alytic mineralization is reflected in the change in mass while thephotodecomposition through both photocatalytic mineralizationand structural degradation of the MB molecules is reflected in

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he change in transmittance. These synchronous measurementsf optical and gravimetric properties during photodecompositionere used to distinguish the structural degradation of the MBolecules from their photocatalytic mineralization. The structural

egradation of the organic molecules occurs dominantly in thearly stage of photodecomposition followed by concomitant struc-ural degradation and mineralization. This integrated system hasreat potential for the study of materials that undergo both opticalnd gravimetric changes.

cknowledgment

This research was supported by Basic Science Research Programhrough the National Research Foundation of Korea (NRF) fundedy the Ministry of Education (NRF-2011-0011246)

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Biographies

Wooree Ko received her Ph.D. degree in chemical engineering in 2013 fromPOSTECH, Korea. She is currently working for Samsung Display.

Sangmin Jeon received his Ph.D. degree in Materials Science and Engineering in2002 from Univeristy of Illinois at Urbana, USA. After 2 years of postdoctoral researchat Oak Ridge National Laboratory, he joined POSTECH, where currently he is anassociate professor chemical engineering.