Materials Research Bulletin 46 (2011) 2523–2526

Photocatalytic properties of TiO2–P2O5 glass ceramics

Jie Fu *

R & D Department, Ohara Inc., Sagamihara-shi, Kanagawa 252-5286, Japan

A R T I C L E I N F O

Article history:

Received 6 July 2011

Received in revised form 30 July 2011

Accepted 15 August 2011

Available online 22 August 2011

Keywords:

A. Glasses

C. X-ray diffraction

D. Optical properties

A B S T R A C T

Binary TiO2–P2O5 glasses with 69 mol% and 76 mol% TiO2 were prepared and converted into glass

ceramics by heat-treatments. XRD measurements show that the main crystalline phases precipitated in

the glass ceramics are anatase-type TiO2 crystals or (TiO)2P2O7 crystals, depending on the concentration

of titanium constituent. Photocatalytic activities of the glass ceramics were evaluated by the

decomposition of methylene blue (MB) and measuring the water contact angle. It is found that the

glass ceramics containing anatase crystals exhibit both photocatalytic oxidation activity and highly

photo-induced hydrophilicity under UV irradiation with intensity of 1.0 mW/cm2.

� 2011 Elsevier Ltd. All rights reserved.

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Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Among the photocatalysts reported so far, titania with nano-sized particles shows the best performance. It exhibits both strongoxidation ability and super-hydrophilicity, and hence has foundwide applications in environment fields such as air/waterpurification, deodorization, sterilization, as well as self-cleaning[1]. Titania is also an important component for glass ceramics. It isoften introduced as nucleating agent to the base glass to boost thenucleation process. Recently, glass ceramics containing titaniacrystalline phase has been investigated as photocatalytic materials[2]. It was shown that rutile-type titania was precipitated in SiO2-based glasses and the resultant glass ceramics show photocatalyticoxidation ability. Titania has three crystalline forms: rutile, anataseand brookite. Rutile is stable phase, but anatase and brookite aremeastable phases. However, the photocatalytic activity of rutile islower than anatase or brookite, it is therefore expected that higherphotocatalytic activity could be obtained if anatase or brookitecrystalline phase is precipitated in glass ceramics. The purpose ofthe present work is to prepare such glass ceramics.

Binary TiO2–P2O5 glasses can be prepared containing highcontent of titania in the ranges from approximately 50 to 70 mol%[3,4]. Investigations on the crystallization of the glass containing65 mol% indicated that the crystalline phases are (TiO)2P2O7 andTiP2O7 with small amount of anatase [3]. In this work, glasses withhigher TiO2 contents were prepared and converted into glassceramics by heat-treatments. Crystalline phases and photocata-lytic activities of the obtained glass ceramics have been examined.

* Tel.: +81 42 772 2101; fax: +81 42 772 2280.

E-mail address: fu@ohara-inc.co.jp.

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.08.016

2. Experimental procedure

2.1. Preparation of glass ceramics

Glasses with mol% compositions 69TiO2–31P2O5 and 76TiO2–24P2O5 were prepared by melt-quenching method using reagent-grade NH4H2PO4 and TiO2 as starting materials. The well mixedbatches were placed in platinum crucibles and melted in anelectrical furnace. The batches were first kept at 700 8C for 2 h inorder to release volatile products coming from the startingmaterials. Then, they were heated to 1450 8C and melted at thistemperature for 3 h. Finally, the melts were poured directly fromthe furnace into water to quench them to room temperature,thereby avoiding crystallization and preserving the amorphousnature of the glasses. After being oven dried at 80 8C in air for oneday the resultant glass frits were further crushed, and particleswith a size of 1.0–3.0 mm were used for heat-treatments andproperty measurements. The heat-treatments were performed attemperatures between 750 8C and 900 8C for 4 h. Bulk glasses werealso prepared from the composition 76TiO2–24P2O5 doped with3 mol% and 5 mol% SiO2, respectively, by pouring the melts onto apreheated stainless steel plate and pressing them quickly. Afterbeing polished the glasses were converted to glass ceramics by theheat-treatments at 750 8C for 2 h.

2.2. Characterization

The glass transition temperature (Tg), crystallization tempera-ture (Tc) and melting temperature (Tm) were determined bydifferential thermal analysis (DTA) at a heating rate of 10 K/min.

Crystalline phases were identified by X-ray diffraction analysis(XRD) (Philips Diffractometer). CuKa radiation was obtained from acopper X-ray tube operated at 40 kV and 40 mA. The measurements

10 15 20 25 30 35 40 45 50 55 60

Degree (2 )

Inte

nsity

(a. u

.): (TiO)2P2O7

900 -4h

800 -4h

750 -4h

As cast

(a)

J. Fu / Materials Research Bulletin 46 (2011) 2523–25262524

were made in step-scanning mode and the diffraction angle (2u)ranged from 108 and 608 with a step size of 0.018 and a step timeof 2 s.

2.3. Evaluation of photocatalytic activities

Photocatalytic activities of the glass ceramics were evaluated bythe decomposition of methylene blue (MB) and measuring thewater contact angle. Before these measurements, the glassceramics were soaked in 4.6 wt% HF aqueous solution for 1 minin order to eliminate the residual glassy phase in the surface. 0.5 gof the glass ceramic particles were dispersed in 5 ml of MB solution(0.01 mmol/L), and the concentration changes of MB under darkconditions and UV irradiation were measured at a interval of30 min using UV spectrometer (V-650, JASCO Corporation, Japan)to evaluate the adsorption and photodecomposition properties.Photo-induced hydrophilicity on bulk glass ceramics was evaluat-ed from the water contact angle value using a commercialautomatic contact angle system (DM-501, Kyowa InterfaceScience, Japan). The UV light source used for above measurementsis Toshiba FL-10BLB and the irradiation intensity is 1.0 mW/cm2.

3. Results and discussion

Both glasses in the binary system investigated in the presentstudy had strong tendency to devitrify, and only glass particlescould be prepared by quickly quenching the melts in cool water.The stability of the glasses against crystallization was increased bythe addition of a small amount of SiO2, and bulk glasses withthickness of about 1.5 mm doped with 3 mol% and 5 mol% SiO2

respectively were obtained.Fig. 1 shows DTA curves of 69TiO2–31P2O5 and 76TiO2–24P2O5

glasses. Glass transition temperature (Tg) is located at 650 8C forthe former glass and at 645 8C for the latter glass. Almost the samemelting temperature is observed at about 1200 8C. However, thereis a large difference in crystallization behavior; only onecrystallization peak at 790 8C for the former glass, but two peaksat 775 8C and 845 8C for the latter glass. XRD measurementsrevealed that the crystallization peak in the former glass was dueto the formation of (TiO)2P2O7 phase and the first crystallizationpeak in the latter glass to the anatase-type TiO2, indicating that theanatase phase can be easily precipitated from latter glass byheating the glass around the first crystallization temperature.

Fig. 1. DTA curves of (a) 69TiO2–31P2O5 and (b) 76TiO2–24P2O5 glasses.

Fig. 2 shows the XRD patterns of (a) 69TiO2–31P2O5 and (b)76TiO2–24P2O5 glasses heat-treated at 750 8C to 900 8C for 4 h. Nocrystal phase is detectable in either glass before the heat-treatments. Crystallization took place after the heat-treatments,but much different crystallization behavior is observed betweenboth glasses. For the glass containing 69 mol% TiO2, the precipi-tated phase is mainly (TiO)2P2O7 and it increases with the heat-treatment temperature. However, for the glass containing 76 mol%TiO2, anatase-type TiO2 is obtained as single or main phase, andhigher heat-treatment temperature results in more formation ofthe anatase phase. Structural studies on the binary TiO2–P2O5

glasses have showed that the P–O–P bonds are gradually replacedby the P–O–Ti bonds and Ti–O–Ti bonds with the increase of TiO2

content [4]. Therefore, easy precipitation of anatase phase in theglass containing higher TiO2 content is probably due to theexistence of many Ti–O–Ti bonds.

The photocatalytic oxidation activity of the obtained glass-ceramics was evaluated by the decomposition of MB. Changes inMB concentrations were measured under both dark conditions andUV light irradiation. Typical results are shown in Fig. 3. Thedecrease of MB concentration in the dark is due to adsorption whilethat under UV irradiation results from both adsorption and

10 15 20 25 30 35 40 45 50 55 60

Degree (2 )

Inte

nsity

(a. u

.)

(b)

As cast

750 -4h

900 -4h

800 -4h

: Anatase Ti O2 : (TiO)2P2O7 : TiP2O7

Fig. 2. XRD patterns of (a) 69TiO2–31P2O5 and (b) 76TiO2–24P2O5 glasses before and

after heat-treatments at 750 8C to 900 8C for 4 h.

0

20

40

60

80

100

0 30 60 90 120 150UV irradiation time (min.)

Cha

nges

in th

e C

once

ntra

tions

of M

B (%

)

Under the darkUnder UV irradiation

Fig. 3. Changes in MB concentrations under both dark condition and UV irradiation

for 76TiO2–24P2O5 glass heat-treated at 900 8C for 4 h.

0

10

20

30

40

50

60

70

0 60 120 180 24 0 300UV irradiation time (min.)

Wat

er c

onta

ct a

ngle

()

3mol% SiO2 5mol% SiO2

Fig. 5. Variation of water contact angle under UV irradiation for 76TiO2–24P2O5

glass-ceramics doped with 3 mol% and 5 mol% SiO2. The glass ceramics were

obtained by heat-treating the mother glass at 750 8C for 4 h.

10 20 30 40 50 60

Inte

nsir

ty (a

. u.)

(a)

(b)

: Anatase TiO2 : TiP2O7 : (TiO)2P2O7

750 -4h

J. Fu / Materials Research Bulletin 46 (2011) 2523–2526 2525

decomposition of MB. The former did not change much and nearlyreached saturation in about 30 min, but the latter changedmarkedly, indicating that the glass ceramic efficiently decomposesMB. To compare the decomposition ability for different heat-treatment temperature, the decomposed fraction of MB after UVirradiation for 120 min was calculated and the results are shown inFig. 4. For the glasses containing 69 mol% TiO2, the decomposedfraction of MB is below 10% and remains almost unchanged inwhole range of the heat-treatment temperature, indicating littlephotocatalytic oxidation activity in these samples. For the glassescontaining 76 mol% TiO2, however, the decomposed fraction of MBincreases from 20% to 50% with the increase of the heat-treatmenttemperature, indicating an increase of the photocatalytic oxidationactivity. As illustrated in Fig. 2(b), the anatase crystalline phaseincreased with the increase of the heat-treatment temperature.Therefore, the increase of the anatase crystalline phase could beone reason for the enhanced photocatalytic oxidation activity.

Fig. 5 shows changes of the water contact angle of SiO2-dopedglass ceramics as a function of UV irradiation time. Large decrease

Fig. 4. Decomposed fraction of MB after UV irradiation for 120 minutes as a function

of the heat-treatment temperature in both (a) 69TiO2–31P2O5 and (b) 76TiO2–

24P2O5 glasses.

Degree (2 )

Fig. 6. XRD patterns of 76TiO2–24P2O5 glass ceramics doped with (a) 3 mol% and (b)

5 mol% SiO2. The glass ceramics were obtained by heat-treating the mother glass at

750 8C for 4 h.

in the contact water angle is clearly observed in both samples afterUV irradiation, indicating these two samples exhibit photo-induced hydrophilicity. To the present author’s knowledge, thisis the first observation of photo-induced hydrophilicity in a glassceramic material. From the XRD pattern as shown in Fig. 6, it isconfirmed that the main crystalline phase precipitated in thesetwo SiO2-doped glass ceramics is also anatase TiO2. Therefore, it isreasonable to consider that the anatase phase is responsible for thephoto-induced hydrophilicity.

It is considered that the photocatalytic properties of the presentglass ceramics would be affected largely by the size and volumefraction of the anatase crystallites exposed on the surfaces.Investigations on these factors will be carried out in the future.

4. Conclusions

Crystallization and photocatalytic properties were examined inthe binary TiO2–P2O5 glasses with high concentrations of TiO2.Glass ceramics containing anatase crystalline phase has been

J. Fu / Materials Research Bulletin 46 (2011) 2523–25262526

obtained by heat-treating the glass containing 76 mol% TiO2 atvarious temperatures. The glass ceramics exhibit both photocata-lytic oxidation activity and highly photo-induced hydrophilicity.

Acknowledgments

The author would like to thank N. Watanabe and T. Morita forthe measurements of the photocatalytic properties.

References

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Am. Ceram. Soc. 93 (2010) 461.[3] A. Kishioka, S. Ueki, M. Aizawa, K. Itatani, F.S. Howell, Phosphorus Res. Bull. 5

(1995) 87.[4] R.K. Brown, D.R. Tallant, W.L. Warren, A. McIntyre, D.E. Day, Phys. Chem. Glasses 38

(1997) 300.