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Effect of Static Magnetic Field on Photocatalytic

Degradation of Methylene Blue over ZnO and TiO2Powders

Supawan Joonwichien Eiji Yamasue

Hideyuki Okumura Keiichi N. Ishihara

Received: 13 May 2011 Springer-Verlag 2011

Abstract Magnetic field effects (MFEs) on photocatalytic degradation of meth-

ylene blue (MB) solution over ZnO and TiO2 powders are investigated under static

magnetic field up to 0.7 T with light irradiation by ultraviolet (UV) light emitting

diode. The UVvisible-near-infrared spectrometer is used to monitor the MB con-

centrations. The positive MFE is observed for ZnO, while the negative MFE for

TiO2, and both MFEs are increased with the increase in the magnetic field applied.

By increasing the settling time (the time interval between the preparation of MBsolution and the powder dispersion into the MB solution), the photodegradation

abilities under MFEs are decreased for both the catalysts. The cause of MFE is

discussed in terms of dissolved oxygen in the MB solution and magnetic adsorption

of the constituent molecules.

1 Introduction

There have been many experimental and theoretical studies for the effects ofmagnetic field on chemical and biological systems [13]. The magnetic field effects

(MFEs) upon photochemical reactions are a newly developed research field that has

shown rapid progress since 1976 when the singlet sensitized photodecomposition

reaction of dibenzoyl peroxide in toluene was found to be dependent on magnetic

field [4].

As for the mechanism of MFEs, the kinetics of photocatalytic reactions have long

been considered [5, 6] not only in homogeneous systems [7], but also in

heterogeneous systems [8, 9]. It was reported that one of the key factors of MFE

proposed is the interaction between the radicals and the magnetic field [10, 11] and

S. Joonwichien (&) E. Yamasue H. Okumura K. N. IshiharaDepartment of Socio-Environmental Energy Science, Graduate School of Energy Science,

Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606-8501, Japan

e-mail: [email protected]

123

Appl Magn Reson

DOI 10.1007/s00723-011-0270-0

Applied

Magnetic Resonance


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also documented that magnetic field possibly affects the recombination process of

electrons and holes [12]. However, as far as investigated, most of the previous

publications on MFEs for heterogeneous photocatalytic reactions are limited to the

TiO2-based catalyst system. Although authors have already reported existence of

MFEs on photocatalytic degradation of methylene blue (MB) solution over ZnOpowder [13, 14], the quantitative analyses as well as the examination on the possible

cause were not performed, with less regard on the experimental temperature variation.

Thus, the aim of the present study is to provide further details of MFE on the

photodegradation of MB over ZnO powders compared with TiO2, both of which

exhibit wide band gaps and similar photodegradation mechanisms through utilizing

radical species [15, 16].

2 Experimental

2.1 Materials

Powder zinc oxide (ZnO 99.9%, wurtzite structure, Wako Pure Chemical Ind. Ltd.)

and titanium dioxide [TiO2 99.9%, ST-41 (anatase structure), Ishihara Sangyo Ltd.]

were used as photocatalysts. The BET (BrunauerEmmettTeller) surface areas for

ZnO and TiO2 powders were 3.8 m2/g (corresponding to a mean particle size of less

than 5 lm) and 9.5 m2/g (0.2 lm), respectively. The granular MB solid and distilled

water were received from Nacalai Tesque Inc., Kyoto, Japan, and the MB structureis given in Fig. 1. MB solution was prepared by authors as described in the

following section.

2.2 Photocatalytic Measurement

The granular MB was dissolved in distilled water to be 0.02 mmol/L followed by

manual shaking for 5 min. After settling in the dark for another 5 min, 0.005 g of

the photocatalyst powder was immediately mixed with the MB solution in a small

reaction cell made of transparent quartz (inner volume of about 4 mL) with a plastic

cap in order to prevent evaporation. The pH of 6.7 0.04 was observed for the MB

solution, and no changes in pH values were registered during the experiment.

Hereafter, the time interval between the preparation of the MB solution and the

powder dispersion into the MB solution is called settling time. The concentration

Fig. 1 Molecular structure of MB

S. Joonwichien et al.

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of dissolved oxygen was monitored after the settling process by a multifunction

meter equipped with dissolved oxygen (DO) electrode (OE-270AA, DKK-TOA

Corp.). When the settling time is 5 min, the DO is 7.57 0.04 mg/L.

The experimental setup is schematically shown in Fig. 2. The cell with the MB

solution dispersed with a photocatalyst was irradiated from the bottom side by

ultraviolet (UV) light emitting diode (LED) lamp (OMRON, ZUV-L8V, Kyoto,Japan) with the center light wavelength of 365 nm and with the light intensity of

600 mW/cm2. The distance from the UV light source to the cell was set to be

10 mm. As for the applied magnetic field, external static magnetic field intensities

of 0.3, 0.5, and 0.7 T were applied by sandwiching the cell.

For measurement of the MB concentration, the reaction cell with the sample was

directly put into UVVisible (Vis)-near-infrared (NIR) spectrometer (Perkin-Elmer

lambda 900) and then MB concentration was determined by comparison with the

predetermined calibration curves between the MB concentration and the absorbance

at 664 nm. Since the slope of the calibration curve decreases with temperature from

15 to 35C (the maximum increase of temperature due to light irradiation is 6C),

another calibration curve for the slope and the temperature was also determined.

3 Results

The photodegradation of MB over ZnO and TiO2 powders as a function of UV

irradiation time is illustrated in Fig. 3a and b, respectively. The MFEs on the

photodegradation are reproducibly observed (at least three times) for both photocat-

alysts. The ZnO powder exhibits positive MFE, while it is negative for TiO2.Interestingly, both the MFEs are gradually increased with the applied magnetic field.

In order to clarify the mechanism of MFEs, the settling time was varied from

5 min to 1, 3, and 24 h. Figures 4 and 5 show the photodegradation results after

various MB settling times for ZnO and TiO2 particles, respectively. It is clear that

Fig. 2 Schematic diagram of the experimental apparatus

MFE on Photocatalytic Degradation of Methylene Blue

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the MFEs are decreased with increasing the settling time for both catalysts. Also the

photodegradation abilities under 0 T magnetic field are decreased with increasing

the settling time for both catalysts.

Although the opposite MFE tendency for the cases between ZnO and TiO2 seems

to be curious, similar results have already been observed by Wakasa et al. [12, 17].

They investigated the MFE on photocatalytic decomposition of tert-butanol over

TiO2 and Pt/TiO2 powders, and both systems similarly generated acetone andmethane as the main products. However, the yields of acetone for the former were

increased with increasing the magnetic field strength from 0 to 1.5 T, while for the

latter the decrease of acetone was observed. As for the possible mechanism, Wakasa

et al. [12, 16] mentioned that the observed MFEs over both the photocatalysts are

not caused by the reencounter of free radicals but are rather explained by

magnetically induced acceleration of recombination of electrons and holes since the

same particle types are used.

4 Discussion

4.1 Effect of DO under 0 T Magnetic Field

Firstly, the relationship between the photodegradation ability under 0 T magnetic

field and the initial DO concentration will be considered. Figure 6 shows the

relationship between the amount of DO in the initial MB solution and the settling

time. It is found that the DO level decreases with increasing the settling time.

Namely, through the shaking process, supersaturated small air bubbles (containing

21 vol% of oxygen gas) are introduced into the solution, which may react (possibly

indirectly) with photogenerated electrons, resulting in the production of superoxide

anions on and/or near the photocatalyst surface. Thus, the higher the DO

concentration is, the more effective the photodegradation becomes. However,

elongated settling time may cause the dissolution of air bubbles toward a steady-

Fig. 3 Magnetic field dependences of the MB photodegradation using a ZnO and b TiO2 powders


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state equilibrium, resulting in the reduction of photodegradation ability. Figure 7

shows the relationship between the change in concentration of MB after the reaction

time of 80 min under 0 T magnetic field and the initial DO concentration before

light irradiation. They exhibit linear tendencies for both photocatalysts. This result

supports the above speculation. Furthermore, similar slopes for different photocat-

alysts in Fig. 7 indicate that the photodegradation of MB is a diffusion-controlled

process of DO species.

Gerischer and Heller [18] investigated in detail the role of oxygen in the

photodestruction of organics on catalyst surfaces. They developed kinetics models

to predict the maximum electron uptake by oxygen and found that the latter depends

on catalyst particle sizes and oxygen concentration in the solution. Early studies

have shown that the adsorption of oxygen on illuminated TiO2 surface depends on

the number of hydroxyl groups of the surface [1921]. The dependence of

degradation rate constants of organics on the dissolved oxygen concentration can be

Fig. 4 Photodegradation of MB using ZnO under magnetic field of 00.7 T for settling times: a 5 min

(selected data from Fig. 2a), b 1 h, c 3 h, and d 24 h


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Fig. 5 Photodegradation of MB using TiO2 under magnetic field of 00.7 T for settling times: a 5 min

(selected data from Fig. 2b), b 1 h, c 3 h, and d 24 h

Fig. 6 Relationship betweenthe DO levels and the settling

duration


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well described by the LangmuirHinshelwood (LH) equation [2224]. In order to

further study the effect of the DO variation on the kinetics of the MB

photodegradation, the apparent reaction rate constant (k00 ) based on the LH

approximation is plotted against the DO concentration (Fig. 8) where the second-

order kinetics is assumed as below:

1=Ct 1=C0 k00t; 1

where Ct is the concentration of MB at any time t (min), C0 is the initial dye

concentration (mmol/L), and k00 is the rate constant for second-order reaction

(L/mmol min). The plot of 1/Ct versus time represents a straight line, where the

slope obtained from the linear regression analysis equals the apparent second-order

rate constant [13, 2528]. The best fit of MB degradation agreed well with the

Fig. 7 Relationship between

the change in concentration of

MB after the reaction time of 80

min under 0 T magnetic field

and the DO concentration

Fig. 8 Relationship between

the apparent second-order rate

constant of MB degradation and

the DO concentration


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second-order kinetics, indicating that the reaction takes place between the two

adsorbed substances (MB molecule and DO) or the reaction, occurs between a

radical in solution and an adsorbed substrate molecule (MB molecule and oxygen

radical species). Figure 8 shows that the reaction rate constant decreases in

proportion to the DO concentration regardless of the type of photocatalyst used. Onthe other hand, the reaction rate is changed at the lower DO concentration, which

suggests that the diffusion of DO is the rate determining step through the liquid

boundary layer.

4.2 Effect of DO on MFE and Mechanism of MFE

As described in Sect. 1, MFEs were explained mostly in terms of the radical pair

mechanism in the previous studies. Kiwi [8] explained that when radical species

diffuse to the photocatalyst surface, an applied magnetic field interferes with thereactivity of the radical species and the recombination rate of the photogenerated

radicals become dependent on the strength of the magnetic field. It is also reported

that the MFEs may occur through variations on the reencounter of reactive free

radicals [29, 30] or the recombination of electrons and holes [12]. Wakasa et al. [12,

16] claimed that the observed MFEs over the photocatalysts are not caused by the

reencounter of free radicals, but by magnetically induced acceleration of recom-

bination of photoexcited electrons and holes.

Our experiments of varying the settling time, however, suggest that one of the

critical factoring for MFE such as DO must be considered. In this respect, thefollowing interesting reports may support our speculation; the diffusion of

paramagnetic DO molecules in water [3133] (and also the dissolution of oxygen

molecules into water [34]) is accelerated by a strong gradient static magnetic field.

Thus, it is possible that diffusion of DO molecules is increased and the dissolution

of oxygen quickens under the magnetic field [35, 36], resulting in the acceleration of

the reaction due to the faster supply of consumed oxygen toward the powder surface

[37]. Since the DO level is almost stable at 3 h settling time, but the MFE can be

Fig. 9 Adsorption of MB on a ZnO and b TiO2 in the dark under magnetic field of 0 and 0.7 T


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Table1

Compar

isonofphotocatalystsusedinthisworkandRefs.

[12,

16]

Effect

Pre

sentwork

Referencedworks

Positiveeffect

ZnO

TiO

2

(ana

taseform)

Par

ticlesize(\5,0

00nm)

Particlesize(*25nm)

BETsurfacearea(3.8m

2/g)

BETsurfacearea(notmentioned)

Negativeeffect

TiO

2

(ST-41)(anataseform)

Pt/TiO

2(anataseform)

Par

ticlesize(*200nm)

Particlesize(\100,0

00nm)

BETsurfacearea(9.5m

2/g)

BETsurfacearea(notmentioned)

Possiblereaction:

(presentwork)

Possiblereaction:(referencedwork)

P

photocatalyst(ZnO,

TiO

2),BET

Brunauer-Emme

tt-Teller


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observed, indicating involvement of other possible factors, i.e. the complex

formation of dye with oxygen molecules prior to the UV irradiation must influence

the photocatalysis [38] as well as the corresponding MFE. From this point of view,

the speculation is, the amount of oxygen molecules chemically or physically

adsorbed on the photocatalyst powder and the complex formation of the MB-dyewith oxygen molecules is formed [14], so-called MB-O2 complex before UV

illuminating. In order to confirm the possibility of this speculation, the concentration

of DO in the solution for 5 min, 1 h, 3 h, and 24 h are calculated to be 0.47, 0.42,

0.39, and 0.38 mmol/L, respectively. This suggested that the amount of DO in the

solution is sufficient to form MB-O2 complex since the concentration of oxygen is

much larger than MB dye concentration (0.02 mmol/L) for all cases. The fast supply

of oxygen molecules is prompted by magnetic field around the powder, the

produced electron by light irradiation could be efficiently consumed through DO.

Besides, the possible involvement of MB-O2 complex can be represented by thesecond-order kinetics reaction, considering each elementary step involved with the

MB-O2 degradation. Moreover, the variation of settling time could be ascribed to

the MB-O2 complex formation as well [39]. However, this explains only the

positive MFE as exhibited in the case of ZnO.

In order to explain the negative MFE exhibited in this study for TiO2, the

disincentive factor of the reaction is further considered, where the magnetic

adsorptionthe MFE on acceleration or deceleration of the MB adsorption toward

the powder surfaceis focused. Figure 9a and b shows the decrease in MB

concentration for ZnO and TiO2, respectively, when each powder is dispersed intothe solution in the dark. In the case of ZnO, the concentration of MB under magnetic

field (0.7 T) reproducibly (at least three times) decreases more than that without

magnetic field (0 T), indicating the acceleration of MB adsorption on the surface of

ZnO under magnetic field, while slight deceleration for TiO2. Although the MFE on

adsorption of DO is not measured directly, it is possible that some sort of change is

generated because magnetic adsorptions of gas were sometimes reported [4042]. It

is also reported that the increment of adsorption oxygen on the catalyst surface

increased under magnetic field of 80 mT, depending on types of metal oxide and its

structure [43]. If this speculation holds, the magnetic field affects the adsorption of

DO molecules and the reaction is influenced by the settling time. Since an opposite

MFEs is found between ZnO and TiO2 according to our analysis, this effect may

also caused by the unique physical properties of the photocatalyst and the quantity

of oxygen adsorbed [43].

Consequently, it can be said that the direction of MFE on the photodegradation is

explained by the combination of positive and negative effects. As shown in Table 1,

although opposite MFEs are observed between the present work and Wakasas

report [12, 16] even for similar TiO2 photocatalysts, it can be phenomenologically

explained by a combination of positive and negative factors under magnetic field.

The cause of the MFE is the action of DO molecule, which involves DO short-range

diffusion (micron-order distance) or molecular transport, adsorption including MB-

O2 complex formation, chemical reaction (photodegradation), scavenging (or

supplying) electrons or holes, and the spin-state (singlet-triplet) variation [4446].

One of them could be also the deciding factor for the recombination of electron and


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hole [12, 17]. However, the detailed discussion of MFE on the photoreaction is

beyond the scope of this paper. Further investigation is required to elucidate the

mechanism of the novel phenomena.

5 Conclusion

MFEs on photocatalytic degradation of MB solution over ZnO and TiO2 powders

are investigated under static magnetic field up to 0.7 T with light irradiation by

UV-LED. With the conditions investigated in this study, the ZnO powder exhibits a

positive MFE, while it is negative for TiO2, and the results are reproducible. Both

MFEs are increased with the increase in the applied magnetic field. By increasing

the settling time of the MB solution, the photodegradation abilities under MFEs are

significantly reduced for both catalysts. The cause of MFE is phenomenologicallyexplained by alteration of the dissolved oxygen molecules in the MB solution and

the magnetic adsorption of the constituent molecules that include water, MB dye,

and oxygen.

Acknowledgments Financial support from Global Center of Excellence (GCOE) Program and Grant-

in-Aid for Scientific Research (B) by the Ministry of Education, Culture, Sports, Science and Technology,

Japan are gratefully acknowledged.

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