photocatalytic energy storage ability of tio2-wo3 composite prepared

As a result, the prepared TiO2-WO3had good energy storage ability while pure TiO2 showed no capacity and pure WO3 showed quite low performance.. The photocatalytic energy storage perform

TiO2-WO3 hybrid photocatalysts were prepared using wet-chemical technique, and their energy storage performance was characterized by electrochemical galvanostatic method TiO2 powder was coupled with WO3 powder, which was used as electron pool and the reductive energy could be stored in As a result, the prepared TiO2-WO3had good energy storage ability while pure TiO2

showed no capacity and pure WO3 showed quite low performance The energy storage ability was affected by the crystal structure

of WO3and calcination temperature The photocatalyst had better capacity when WO3had low degree of crystallinity, since its loose structure made it easier for electrons and cations to pass through The photocatalytic energy storage performance was also affected by the molar ratio of TiO2to WO3 Energy storage capacity was significantly dependent on the composition, reaching the maximum value

at TiO2/WO31:1 (mol/mol)

Key words: photocatalyst; TiO2-WO3; energy storage

DOI: 10.1016/S1001-0742(09)60129-7

Introduction

TiO2is a promising photocatalyst for the conversion of

the light energy into chemical energy, and has attracted

extensive attention for its application to water purification

(Herrmann, 1999), metal protection (Yuan and Tsujikawa,

1995), anti-bacterial (Rincon and Pulgarin, 2004),

self-cleaning (Rincon and Pulgarin, 2004) and so on But it has

photocatalytic effect only under light illumination

There-fore, researchers focus more on new photocatalysts, which

have energy storage ability and can have the photcatalytic

effect in dark condition They combined photo-responsive

semiconductors (like TiO2 or SrTiO3) (Tatsuma et al.,

2001; Ohko et al., 2002) with an energy storage material,

which has redox activity, and a more positive conduction

band potential than that of the photo-responsive

semicon-ductor in order to accept electrons from the irradiated

semiconductor (Tatsuma et al., 2001) WO3 shows these

characteristics and can be used as an energy storage

ma-terial Reductive energy (excited electrons) generated by

UV-irradiated photocatalyst can be stored in WO3, which

retains the reductive energy for a certain period even after

the light is turned off (Fig 1) (Tatsuma et al., 2001, 2002)

The energy storage ability of TiO2/WO3 thin film

photocatalyst was firstly reported by Fujishima and

Tat-suma (TatTat-suma et al., 2001, 2003; Ohko et al., 2002;

Ngaotrakanwiwat and Tatsuma, 2004; Takahashi et al.,

* Corresponding author E-mail: shangguan@sjtu.edu.cn

2004) TiO2/WO3film was coated on the indium tin oxide -coated (ITO-coated) glass plate and was applied to anti-bacterial and anti-corrosion in darkness The study works showed that the interfacial contact state largely affected the energy storage ability, and the crystal structure of WO3

in TiO2/WO3film had great influence on its photo-charge ability and the reversibility of the film (Ngaotrakanwiwat

et al., 2003; Higashimoto et al., 2005, 2007)

TiO2 photocatalyst powder used for interior and exte-rior wall paint has attracted much attention because of its potential application in the removal of indoor and outdoor pollution In order to develop the photocatalyst paint having the energy storage property, in the present work, TiO2-WO3powder was prepared using simple wet-chemical technique The crystal structure of WO3 of the photocatalyst could be changed by controlling heat-treatment temperature during sample preparation, and the molar ratio of WO3 to TiO2 could also be changed to the desired by simply adjusting the dosage of soluble tungstic acid and TiO2powder The energy storage ability

of the TiO2-WO3 photocatalyst samples with different

WO3crystal structure and various molar ratio of WO3 to TiO2were evaluated

1 Experimental 1.1 Preparation of TiO2-WO3powder Soluble tungstic acid solution was first made using

Fig 1 Energy storage mechanism of TiO 2 -WO 3 photocatalyst.

cation-exchange technique (Choi et al., 2002; Zou, 2005)

Na2WO4·2H2O (AR, Sinopharm Chemical Reagent Co.,

Ltd., China) was dissolved into de-ionized water and

formed 0.25 mol/L aqueous solution of sodium tungstate

The aqueous solution was then let to flow down at a certain

rate through the glass column packed with protonated

cation-exchange resin

Soluble tungstic acid solution was dried at 80°C into

solid state, and ground into powder which was marked as

W-0 Then the W-0 sample was heat-treated for 2 hr under

different temperatures (150, 250, 350, 450, 550°C), and the

heat-treated samples were in turn marked as 150,

W-250, W-350, W-450, W-550, respectively

TiO2powder (P25, Degussa), at a certain molar ratio of

Ti to W, was put into the soluble tungstic acid solution

prepared as above, along with stirring Then the

well-distributed suspension was dried at 80°C, and ground into

powder which was then marked as TixWy-0 (x:y equals

the molar ratio of TiO2 to WO3) TiW-0 sample was

also heat-treated with same condition as W-0 And the

samples heat-treated at different temperatures were in turn

marked as 150, 250, 350,

TixWy-450, TixWy-550.

1.2 Characterization

The XRD patterns were obtained using an X-ray

diffrac-tometer (D/max-2200/PC, Riguku, Japan) with Cu Kα

radiation in a scan range of 15–45◦ and a scan speed of

2◦/min operated at 40 kV and 20 mA; UV-Vis

spectroscop-ic measurement in diffuse reflectance mode were carried

out using a UV-Vis double-beam spectrophotometer

(TU-1901, Beijing Purkinje General Instrument, China) with

a Ulbricht sphere (Radius; 60 mm) from 230 to 850 nm

For measurement of UV-Vis absorption spectra, a pressed

piece of BaSO4 was used as reference; Te

Brunauer-Emmett-Teller (BET) surface area was measured from

N2 adsorption method on Quantachrome NOVA

1000-TS (USA); The TEM was recorded on JEM-2010/INCA

OXFORD Analytical Transmission Electron Microscope

(JOEL, Japan; Oxford, U.K.) Thermogravimetric analyses

(TGA) were carried out with a TGA 2050 (TA, USA) at a

heating rate of 15°C/min under air

1.3 Measurement of photocatalytic activity

The ability of photocatalytic energy storage was

measured by electrochemical galvanostatic method, using

three pole system of Pt auxiliary electrode, SCE reference electrode and working electrode of TiO2-WO3 photocata-lyst powder

The working electrode of TiO2-WO3 powder was pre-pared as follows The TiO2-WO3powder was mixed with ethanol and ground to form slurry, and then the slurry was spread on a conductive indium tin oxide glass (ITO, 2 cm× 2.5 cm) by squeegee method (Smestad and Gratzel, 1998), followed by thermal treatment at 80°C for 2 hr in air The mass of ITO glass was measured both before and after coating The net weight of photocatalyst powder, which was coated onto ITO glass, was measured For one kind

of TiO2-WO3 powder sample, several working electrodes were prepared

All working electrodes of photocatalysts above were photo-charged in 3 wt.% NaCl electrolyte solution (pH 5) for 60 min with 350 W-Xe lamp (Spherical Xenon Lamp, Shanghai DianGuang Device Co., Ltd., China) Discharge ability of those photo-charged photocatalysts

on working electrodes were measured by electrochemical galvanostatic method with a speed of 2μA/min in the three pole system The discharge ability of unit mass sample was then calculated

2 Results and discussion 2.1 Structure and physicochemical property of TiO2

-WO3 Figure 2 shows the thermal gravimetric analysis (TGA)

of as dried W-0 and Ti1W1-0 solid at a heating rate of 15°C/min in air atmosphere in the temperature range of 40–800°C The TGA curve of W-0 sample showed the mass losses below 270°C, which was due to the release of absorbed water and the decomposition of tungsten oxide hydrate It indicated that decomposition of tungsten oxide hydrate all occurred below 270°C and only tungsten oxide existed after being treated over 270°C The TGA curves of

as dried Ti1W1-0 sample generally showed the mass losses below 240°C, indicating that decomposition of tungsten oxide hydrate was completed under 240°C

XRD patterns of pure WO3 samples are shown in Fig 3a The sample calcinated at 150°C generally consists

of tungsten oxide hydrate and some monoclinic WO3 The sample calcinated at 250°C mainly contains WO3

Fig 2 TG curves of solid-state samples W-0 and Ti1W 1 -0 which was

dried at 80°C.

which has quite low degree of crystallinity With thermal

temperature rising, the crystallinity degree increased

XRD patterns of TiO2-WO3(1:1, mol/mol) samples are

shown in Fig 3b It shows that the peak intensity of WO3

in TiO2-WO3 increases with calcination temperatures

in-creasing from 150 to 550°C, being in agreement with pure

WO3 shown in Fig 3a No phase transformation of TiO2

occurred in all TiO2-WO3samples prepared in the process,

and the TiO2in all samples had the same phase structure

of anatase

In order to investigate the photo absorption

perfor-mance of TiO2-WO3, the UV-Vis absorption spectra of

1:1 (mol/mol) TiO2-WO3 samples calcinated at various

temperatures was measured As a comparison, the spectra

of P-25 (TiO2) was also measured and shown together

in Fig 4 Compared with P-25, the absorption edges of

all TiO2-WO3 samples shifted to longer wavelength The

absorption edges of TiO2-WO3 calcinated from 250 to

550°C are around 490 nm, which is in agreement with

pure WO3 The adsorption edge (520 nm) of TiO2-WO3

calcinated at 150°C is attributed to the tungsten oxide

hydrate

The specific surface area (SSA) of TiO2-WO3 samples

with different thermal temperature was measured As

Fig 4 Di ffuse reflection spectra of P25, and 1:1 (mol/mol) TiO 2 -WO 3

samples which were heat-treated for 2 hr at di fferent temperatures (150,

250, 350, 450, 550°C).

Table 1 Specific surface area (SSA) of TiO 2 -WO 3 samples heat-treated for 2 hr at di fferent temperatures

TiO 2 -WO 3

TiO 2 -WO 3

shown in Table 1, the SSA showed a slight variation with calcination temperature

Micrographs of pure WO3and TiO2-WO3samples were investigated by TEM measurement TEM image of WO3 samples in Fig 5a indicated that WO3 generally formed large particles in square shape and was accompanied with

a bit small particles From TEM images and EDS analysis results shown in Fig 5 b–d, it seemed that TiO2 particles were stuck on the surface of WO3 and formed a contact interface between them

2.2 Energy storage performance The energy storage capacity of TiO2-WO3samples with different heat-treatment temperature is shown in Fig 6 Single TiO2 did not have energy storage performance while single WO3 had quite low energy storage capacity

Fig 3 XRD patterns of pure WO 3 (a) and 1:1 (mol /mol) TiO 2 -WO 3 samples (b), respectively heat-treated for 2 hr at di fferent temperatures (150, 250,

350, 450, 550°C).

Fig 5 TEM image of pure WO 3 and TiO 2 -WO 3 samples, and EDS spectrum images of TiO 2 -WO 3 samples, The unlabeled peaks are adventitious carbon and copper (a) WO 3 ; (b) 5:1 (mol /mol) TiO 2 -WO 3 ; (c) 1:1 (mol /mol) TiO 2 -WO 3 ; (d) 1:3 (mol /mol) TiO 2 -WO 3

Fig 6 Energy storage capacity of pure WO 3 , 1:1 (mol /mol) TiO 2

-WO 3 and 2:1 (mol /mol) TiO 2 -WO 3 samples heat-treated for 2 hr at

di fferent temperatures (150, 250, 350, 450, 550°C), compared with the

performance of pure TiO 2

By comparison, TiO2-WO3 samples showed much more

energy storage capacity than sum capacity of TiO2 and

WO3, which evidently indicated the energy storage ability

of hybrid TiO2-WO3samples It also could be found that

TiO2-WO3 samples treated at 250°C have best energy

storage ability while other samples have less capacity

Generally, main reaction of energy storage goes in three

steps: generation of electrons, storage of

photo-excited electrons and the release of storage electrons

(Tatsuma et al., 2001)

WO3+ xe- + xNa+—— Na

First, electrons in TiO2 valence band are excited to the

conduction band under UV irradiation Then, the

photo-excited electrons are transferred and injected to WO3

(Nenadovic et al., 1984; Tada et al., 2004), because it has a

more positive conduction band than TiO2, and the electrons

are then stored by WO3along with a redox reaction After

the UV light is turned off, the stored electrons are released,

and they can react just as the photo-electrons do

For all TiO2-WO3 samples treated below 550°C in this

work, when the molar ratio of TiO2 to WO3 (x:y) was

fixed, samples had same amount of TiO2 Because the TiO2

had same crystal structure which could be known from

XRD analysis, the generation of photo-electrons by TiO2

in the first step went in the same way Thus, the difference

of energy storage ability was attributed to the different

electron-storage ability of samples, which was affected by

the crystal structure of WO3

As known from Eq (2), the photo-electrons and Na+

transferred within channels of WO3 or entered into its

frame The crystal structure of WO3was monoclinic and

it belonged to the pseudo-cubic ReO3 type (Solonin et

al., 2001) It could be represented as continuous frame

constructed from [WO]6 octahedron which was linked by

corner sharing oxygen atom, and the arrangement results in

a simple cubic symmetry Within the frame, there existed

outspread tunnels which could be used as circulation

higher crystallization degree of WO3 The structure tunnel became small and compact, and this made it difficult for ions and electrons to transfer or enter into This resulted

in worse energy storage ability When WO3 was applied

to electrochromics, it had the similar behavior of the per-formance changing with its crystallization degree (Chen et al., 1991) It concluded that the excellent energy storage ability of TiO2-WO3 heat-treated at 250°C resulted from the WO3crystal phase and its low degree of crystallinity

To observe the influence of molar ratio of TiO2to WO3

on photocatalytic energy storage performance, TiO2-WO3

samples with various molar ratios were heat-treated at 250°C As shown in Fig 7, the molar ratio of TiO2 to

WO3 has influence on the performance Pure TiO2 had

no capacity, and pure WO3 had quite low capacity The TiO2-WO3photocatalyst samples had a best capacity when molar ratio of TiO2 to WO3 was 1:1, and had lower capacity with either larger or smaller ratio This could be explained by the different function of TiO2and WO3 (Tat-suma et al., 2001, 2002; Takahashi et al., 2004) TiO2acts

an electron generator to supply photo-generated electrons under irradiation, while WO3 plays the role of electron receiver which determines the quantity of electrons re-ceived and stored So the molar ratio of WO3 to TiO2

would affect the utilization efficiency of TiO2 and WO3, and have influence on the quantity of stored electrons which determined materials’ energy storage performance Besides, the storage of photo energy is achieved through

Fig 7 Energy storage capacity of pure WO 3 and TiO 2 -WO 3 sam-ples with di fferent TiO 2 /WO 3 molar ratio, which were heat-treated at 250°C for 2 hr, compared with the energy storage capacity of P25.

the transfer of photo-generated electrons from TiO2 to

WO3(Nenadovic et al., 1984; Tada et al., 2004) Therefore,

it is necessary for excellent photocatalysts with energy

storage ability to have suitable ratio of TiO2/WO3and good

contact between TiO2and WO3

3 Conclusions

TiO2-WO3 photocatalyst powder samples have been

successfully made from soluble tungstic acid and TiO2

powder by wet-chemical technique The crystal structure

of WO3could be changed through changing heat-treatment

temperature Molar ratio of TiO2 to WO3 was changed

via adjusting the dosage of soluble tungstic acid and TiO2

powder

The prepared TiO2-WO3 photocatalyst showed energy

storage ability in electrochemical measurement while pure

TiO2 showed no capacity and pure WO3 was low The

energy storage ability of TiO2-WO3was dependent on the

crystal structure of WO3and the molar ratio of WO3/TiO2

When sample was composed of TiO2-WO3 in 1:1 molar

ratio and was heat-treated at 250°C, it gave the best the

energy storage ability This might be attributed to the loose

structure of WO3crystal phase, which might be helpful to

the electrons transfer during the process of photocatalytic

energy storage

Acknowledgments

The work was supported by the National Basic Research

Program of China (973 Program) (No 2007CB613305),

the National High Technology Research and Development

Program of China (863 Program) (No 2007AA061405)

and the Special Foundation of Nanometer Technology

(No 0752nm005) from Shanghai Municipal Science and

Technology Commission (STCSM) of China

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