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Journal of Crystal Growth 310 (2008) 4319–4324

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

� Corr

Univers

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

A simple route to synthesize highly crystalline N-doped TiO2 particles underlow temperature

Jingjing Xu a,b,c,d, Yanhui Ao a,b,c,d, Degang Fu a,b,c,d,�, Chunwei Yuan a,b

a State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, Chinab School of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096, Chinac Key Laboratory of Environmental and Bio-Safety in Suzhou, Research Institute of Southeast University, Dushu Lake Higher Education Town, Suzhou 215123, Chinad Jiangsu Laboratory for Biomaterials and Devices, Nanjing 210096, China

a r t i c l e i n f o

Article history:

Received 3 January 2008

Received in revised form

3 July 2008

Accepted 16 July 2008

Communicated by S. Udaelectron spectroscopy (XPS), diffuse reflectance spectra (DRS), and TG–DSC analysis. The results

Available online 18 July 2008

PACS:

81.07.�b

Keywords:

A1. Crystal morphology

A1. Crystal structure

A2. Growth from solution

B1. Nanomaterials

B1. Oxides

B2. Semiconductor materials

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.jcrysgro.2008.07.045

esponding author at: State Key Laboratory

ity, Nanjing 210096, China. Tel.: +86 25 83794

ail addresses: xujj@seu.edu.cn (J. Xu), fudegan

a b s t r a c t

A simple route was developed for the synthesis of highly crystalline N-doped TiO2 nanoparticles under

low temperature (i.e. p348 K) and ambient pressure, through hydrolysis of titanium-n-butoxide in

acidic ethylamine solution. The as-prepared particles were characterized by X-ray diffraction (XRD),

transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) measurement, X-ray photo-

indicated that N atoms had got incorporated into TiO2 lattices as evidenced by the formation of O�Ti�N

bonds. This process occurred under mild condition, i.e. without further calcinations. The N-doped TiO2

particles exhibited large BET surface area. Photocatalytic activity was evaluated by degradation of dye

X-3B under artificial sunlight irradiation, and N-doped TiO2 samples showed much higher activity than

that of pure TiO2 and P25.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Currently, there is much interest in the development of TiO2

heterogeneous photocatalysts to deal with environmental reme-diation due to its high photo-stability, low-cost, and environ-mental friendly feature [1]. The photocatalytic activity of TiO2

strongly depends on its crystalline structure and size [2]. Amongthe three phases of TiO2, anatase is thought to be the most activeform. Smaller crystallite size usually exhibits better photocatalyticactivity due to larger surface area.

However, owing to its wide band gap (3.2 eV for anatase), TiO2

can only be excited under ultraviolet irradiation, which occupieso5% of the total solar irradiance at the Earth’s surface. For thesake of efficient use of sunlight, many groups had focused ondeveloping synthesis routes to modified TiO2 with visible lightresponsive activity by metal doping, and coupling with othersemiconductors of band gap narrower than that of TiO2. Recently,

ll rights reserved.

of Bioelectronics, Southeast

310; fax: +86 25 83793091.

g@seu.edu.cn (D. Fu).

studies report that substitutional doping of non-metallic ele-ments, like N, C, F, or S [3–7], seem to be a very promisingapproach for the production of TiO2 operating at visiblewavelength, despite the origin of high activity of these materialsbeing still the subject of debate [8]. According to Asahi et al. [3],doping TiO2 with nitrogen leads to a narrowing in the bang gapdue to the mixing of p states of nitrogen with O 2p states.However, Irie et al. [9] argued that a combination of titaniareduction and/or formation of an isolated N 2p narrow band isprincipally responsible for the observed visible-induced photo-catalytic activity.

At present, N-doped TiO2 has been prepared by a variety ofmethods such as spray pyrolysis [6] and PLD method [10], whichneed special equipments. By sputtering of TiO2 precursor under anitrogen/argon gas mixture at 550–600 1C [3,9], Diwald et al. [11]doped TiO2 (110) single crystals with NH3 at 870 K. Furthermore,most of the wet synthesis methods are followed by calcinations ofthe resulting materials [12–14]. There are little reports onpreparing N-doped TiO2 under low-temperature (p373 K) condi-tion. In this article, a highly crystalline N-doped TiO2 wasdeveloped in moderate condition (ambient pressure, withoutfurther calcinations process) with the ability of visible light

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J. Xu et al. / Journal of Crystal Growth 310 (2008) 4319–43244320

response. X-ray diffraction (XRD), transmission electron micro-scopy (TEM), Brunauer–Emmett–Teller (BET), X-ray photoelectronspectroscopy (XPS), diffuse reflectance spectra (DRS) and TG–DSCanalysis were used to discuss the performance of this N-dopedTiO2.

20

A

AA

Inte

nsity

(a.

u.)

2 Theta (degree)

A: Anatase

B: Brookite

A(101)

B(121)a

b

A

806040

Fig. 1. XRD patterns of (a) N-doped TiO2 (NTO) and (b) pure TiO2 (TIO).

2. Experimental procedure

2.1. Materials

Titanium (IV) n-butoxide (Ti (OBu)4) was of chemically puregrade, and obtained from Shanghai Meixing Company. Ethylaminewas of analytical reagent grade, and was obtained from J and KChina Chemical Ltd. Isopropanol (i-PrOH) was purchased fromShanghai Chemical Ltd. companies with analytical reagent grade.Degussa P25 TiO2, a mixture of anatase and rutile (8:2) whosesurface area was 50 m2/g, and mean diameter was 30 nm, wassupplied by Degussa Co., Germany. All the other reagents were ofanalytical grade, and used without further treatments. Water usedin the experiments was distilled.

2.2. Photocatalysts preparation

Ti(OBu)4 was chosen as a Ti precursor, and ethylamine wasemployed as N source. The pH of the ethylamine solution wasadjusted to 1.5. The mixture of Ti (OBu)4 and i-PrOH was addedgradually into the as-prepared acid ethylamine solution undervigorous stirring until Ti(OBu)4 was hydrolyzed completely. Themolar ratio of N/Ti was controlled at 0.1. The solution was keptrefluxing at 348 K for 24 h. The sol was then dried in the oven at333 K and the fine N-doped TiO2 powders (defined as NTO) wereobtained eventually. For comparison, pure TiO2 (defined as TIO)was prepared by the same process without addition of ethyla-mine. The color of NTO turns to yellow compared to the white ofTIO due to the N doping.

2.3. Characterization

The crystalline structure of NTO was measured by XRD (XD-3A,Shimadazu Corporation, Japan) using graphite monochromaticcopper radiation (Cu-Ka) at 40 kV, 30 mA over the 2y range of20–801. The morphology and size were observed by TEMmicrographs with a JEOL-JEM 2000EX microscope, working at a100 kV accelerating voltage. The specific surface area and pore sizedistribution of the samples were obtained using the BET surfacearea measuring apparatus at the boiling point of liquid nitrogen.The BET surface area was determined by multipoint BET methodusing the adsorption data in the relative pressure (P/P0) range of0.05–0.3. Desorption isotherm was used to determine the poresize distribution using the Barret–Joyner–Halender (BJH) methodwith cylindrical pore size. The nitrogen adsorption volume at therelative pressure (P/P0) of 0.994 was used to determine the porevolume and average pore diameter. The chemical nature of N inTiO2 was studied employing XPS in a VG Microtech MultilabESCALB-250 with Mg Ka radiation. A UV–vis spectrophotometer(Shimadzu UV-4100) was used to record the DRS of samples. Thecrystallization behavior of NTO was also monitored using aTG–DSC instrument (TG 2000/2960, TA Instruments).

2.4. Photocatalytic activity

The photocatalytic activity of all samples was evaluatedby photocatalytic degradation of reactive brilliant red dye X-3Bsolution. A 70 mg sample was added into 200 ml of 50 mg/l

X-3B solution. The suspension was stirred in dark for 30 min toobtain adsorption equilibrium of X-3B before illumination.A 250 W halogen lamp (Instrumental Corporation of BeijingNormal University) was used as artificial solar light source. AUV-lamp (HG-250-UV Mejiro Precision Inc., l ¼ 365 nm,P ¼ 250 W) was used as UV light source with the intensity of1.7 mW/cm2. At a defined time interval, 5 ml suspension wasremoved and the concentration of X-3B was analyzed using theUV–vis spectrophotometer at 535 nm.

3. Results and discussion

The XRD patterns of different TiO2 samples are provided inFig. 1. It can be found that the as-prepared NTO sample mostlybelongs to anatase TiO2 with a little brookite phase (at 30.71).There is a significant intensity increase in XRD spectra of NTOsample due to its higher anatase phase crystallinity. The enhancedanatase crystallinity in NTO reflects that the transition fromanatase to rutile phase would be suppressed due to N doping,which forms the defects and/or segregates at the grain boundaryand then increases the potential energy for atomic diffusion.Meanwhile, NTO shows diffraction peak broadening compared toTIO, which indicates the formation of smaller NTO nanoparticles[12]. As shown in Table 1, using Debye–Scherrer equation, thecrystallite sizes of NTO and TIO are determined to be 3.5 and5.5 nm, respectively, which denotes that N doping may inhibit thegrowth of particles to some extent.

It is further confirmed from the TEM (Fig. 2) that the particlesize is about 4–5 nm for NTO and about 6 nm for TIO. SAEDanalysis of the two regions gives a pattern consistent with thepresence of both anatase and brookite crystal phase. The radialprofile identified the 101, 004, 112, 200, 211, and 204 for anatase,as well as 121 for brookite. These results are in good agreementwith those determined by XRD.

Fig. 3 shows the nitrogen adsorption–desorption isotherms ofTIO and NTO. As seen from Fig. 3(a), TIO shows the isotherm oftype IV (BDDT classification) [15]. At high relative pressures from0.4 to 0.8, the isotherms exhibit hysteresis loops of type H3,indicating that the powders contained mesopores (2–50 nm)[16,17]. Fig. 3(b) shows that the isotherm of NTO is a combinationof type I and IV (BDDT classification) with two very distinctregions. At low relative pressure the isotherm exhibits highadsorption, indicating that the NTO contains micropores (type I),

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Table 1Results of XRD, BET, XPS analysis and kapp and R data for each sample

TIO NTO P25

Crystallite size (nm) 5.4 3.5 —

Anatase crystallinity (%) 88.6 94.4 —

BET surface area (m2/g) 229.2 267.9 —

Pore volume (cm3/g) 0.17 0.15 —

Average pore size (nm) 3.2 2.5 —

Eb (eV) (ri/%) Ti–O 530.0 (65.8) 530.0 (47.6) —

O–H 531.6 (34.2) 531.6 (52.4) —

Apparent rate constant kapp (min�1) UV 0.009 0.010 0.009

Solar 0.004 0.028 8.743�10�4

Regression relative coefficient R UV 0.996 0.985 0.998

Solar 0.894 0.999 0.990

Fig. 2. TEM images and SAED patterns of NTO and TIO.

J. Xu et al. / Journal of Crystal Growth 310 (2008) 4319–4324 4321

while at high relative pressure from 0.4 to 1.0 the curve exhibits avery small hysteresis loop, indicating the presence of mesopores(type IV). The formation of mesoporous structure in the sampleTIO and NTO is attributed to the aggregation of TiO2 particles[18,19]. Fig. 3(c) shows the BJH pore diameter distribution derivedfrom the adsorption and desorption branch of the isotherm ofNTO. It can be seen that the pore diameter ranges from 1.8 to4.0 nm and the average pore diameter of 2.5 nm is smaller thanthat of TIO (as shown in Table 1), while the pore volume decreasesfrom 0.17 cm3/g in TIO to 0.15 cm3/g in NTO. The result can beexplained by the observation that the aggregation of biggercrystallites can form bigger pores [18]. In addition, the BET surfacearea becomes larger with nitrogen doping, i.e. the value changesfrom 229.2 m2/g in TIO to 267.9 m2/g in NTO as listed in Table 1.

In order to analyze the chemical composition and purity of theprepared samples, most of the studies about nitrogen-dopedtitanium oxide systems have been performed using XPS, recently[20]. A linear increase of the visible-light response to the XPS

intensity of the nitrogen at 396 eV was reported recently in NTO[9,21]. The XPS survey spectrum of NTO (Fig. 4(a)) shows that NTOcontains only Ti, O, N, and C elements. The C element can beascribed to the residual carbon from precursor solution, as oursamples are all prepared at low temperature. The high-resolutionXPS spectra of the N1s and O1s region on the surface of NTOsamples are shown in Fig. 4(b) and (c). The wide and asymmetricN1s region indicates that there are at least two kinds of chemicalstates. Using Origin software with Gaussian rule, N1s region isfitting to two peaks. One peak, at 396 eV, corresponds to the so-called b-N state, which represents atomic N in the form of mixedtitanium oxide–nitride (TiO2�xNx), O–Ti–N [22]. This indicatesthat the process used in this study can lead the substitution ofsome oxygen sites by nitrogen under low temperature. Anotherpeak at 399.2 eV may account for the presence of oxidized state ofN or C–N bonds [23]. This can be attributed to the decompositionof unbound ethylamine molecule chemically absorbed on thesurface of NTO. As shown in Fig. 4(c), the O1s spectra of both TIO

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0.020

40

60

80

100

120 TIO

Qua

ntity

Ads

orbe

d (c

m3

g-1 S

TP)

Relative Pressure (P / Po)

0.0

40

50

60

70

80

90

100NTO

Qua

ntity

Ads

orbe

d (c

m3

g-1 S

TP)

Relative Pressure (P / Po)

5

0.0

0.1

0.2

0.3

0.4

0.5

Pore

Vol

ume

(cm

3 g-1

)

Pore Diameter (nm)

BJH Adsorption BJH Desorption

0.2 0.4 0.6 0.8 1.0

0.2 0.4 0.6 0.8 1.0

10 15 20

Fig. 3. Adsorption–desorption isotherm of TIO (a) and NTO (b); BJH pore size

distribution derived from the adsorption and desorption branch of the isotherm of

NTO (c).

600

Ti 3pTi

3s

C1sN

1s

Ti 2s

O1s

Ti 2s

Inte

nsity

(a.

u.)

Binding Energy (eV)

402

N 1s

absorbed-N

Ti-N

Inte

nsity

(a.

u.)

Binding Energy (eV)

536

Inte

nsity

(a.

u.)

Binding energy (eV)

TIO

NTO

O 1s

O-H

Ti-O

Ti-O

O-H

400 200 0

400 398 396 394

534 532 530 528 526

Fig. 4. XPS survey spectrum of NTO sample (a); XPS details collected from NTO

sample: (b) N 1s, (c) O 1s core levels.

J. Xu et al. / Journal of Crystal Growth 310 (2008) 4319–43244322

and NTO samples are fitted to two peaks, and the correspondingdata are listed in Table 1. The lower binding energy of 530.0 eV isattributed to Ti–O in TiO2 crystal lattice, and the 531.6 eV is relatedto the O–H resulting from chemisorbed water [24]. As seen from

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20020

40

60

80

100

Abs

orba

nce

(a.u

.)

Wavelength (nm)

TIO

NTO

300 400 500 600 700 800

Fig. 5. DRS spectra of TIO and NTO nanoparticles.

20090

92

94

96

98

100

-4

-2

0

2exo

TG

/ %

DSC

/ (m

W/m

g)

400 600 800 1000Temperature / °C

Fig. 6. TG/DSC curve of NTO particles.

00.0

0.2

0.4

0.6

0.8

1.0

C /

C0

Irradiation time (min)

P25-UV

P25-S

TIO-UV

TIO-S

NTO-UV

NTO-S

0

0.0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

2.4

2.7

Irradiation time (min)

P25-UV

P25-S

TIO-UV

TIO-S

NTO-UV

NTO-S

ln (

C0

/ C)

20 40 60 80

20 40 60 80

Fig. 7. Kinetic of X-3B degradation for different samples (a); variations in ln(C0/C)

as a function of irradiation time and linear fits of different samples (b).

J. Xu et al. / Journal of Crystal Growth 310 (2008) 4319–4324 4323

Table 1, the ratio of O–H/Ti–O is enhanced in NTO compared tothat in TIO. It illuminates that doping N in TiO2 can increase thenumber of surface hydroxyl groups, and subsequently increaseconcentration of free hydroxyl radicals, which is proved to bebeneficial for photocatalytic reactions [25].

The UV–vis–DRS spectra of TIO and NTO are shown in Fig. 5and there is a significant shift in the onset absorption towards thehigher wavelength for NTO compared to TIO. Therefore, it can beexpected that the NTO catalyst should have higher photocatalyticactivity under visible light illumination. There are two factors thatcontribute to this enhanced visible light absorption in NTOsample. One may be the increase in crystallinity [24], which isconfirmed by XRD analysis, and the other is localized impuritylevels near or above the valence band edge formed by doped Natoms [26]. As known, the valence and conduction bands of TiO2

are mainly formed by completely filled oxygen 2p orbital and theempty Ti 3d orbital. The energy level of N 2p orbital lies below theconduction band and valence band edge of TiO2. During N doping,the 2p orbital of the doped N atom significantly interacts with thatof O 2p, leading to a charge transfer between a dopant and aconduction or valence band. As a result, there is a red-shift in theband gap transition. Meanwhile, the absorbed intensity of NTO inthe UV region does not decrease; it can be hypothesized that thephotocatalytic activity of NTO may not be lower than that of TIOunder UV light.

The thermal behavior of the samples was investigated with aTG–DSC technique from room temperature to 1100 1C and theresults are shown in Fig. 6. It is well known that the thermalbehavior of TiO2 usually depends on the chemical composition,preparation condition and existing phases [27–29]. The endother-mic peak at about 100 1C is due to the desorption of the physicallyadsorbed water and alcohol [28]. A sharp exothermal peakappearing at about 250 1C is assigned to the decomposition oforganic substances in the sample. The relative broad exothermalpeak at 800 1C owing to the phase transformation from anatase torutile, while the usual transformation temperature is around600 1C. The TG curve can be divided into three stages. The firststage ranges from room temperature to 200 1C with a mass loss of6.8%, which represents the dehydration and loss of residualsolvent. In the temperature range of 200–400 1C, the mass loss isabout 12.1%, which is attributed to the loss of organic substances,corresponding to the sharp exothermal process from DSC curve.The third mass loss of 1.6% in the range of 400–1100 1C is owing tothe gradual removal of the organic residues. It can be concludedfrom TG–DSC results that the phase of sample NTO is mainly

anatase, and the doping of nitrogen can inhibit the anatase torutile transformation with transition temperature 800 1C.

The photocatalytic activities of NTO, TIO, and P25 sampleswere studied by decomposition of X-3B in aqueous solution under

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UV light and artificial solar light illumination. It can be seen inFig. 7(a) that under UV light radiation the NTO shows comparablephotocatalytic activity to that of P25, while better activity thanTIO. In the solar light illumination, NTO sample exhibits muchhigher photocatalytic activity than that of P25 and TIO. Thephotocatalytic degradation percents of X-3B are 89.9%, 6.3%, and24.9% for NTO, P25 and TIO, respectively. The apparent rateconstant (kapp) has been chosen as the basic kinetic parameter forthe evaluation of different photocatalysts, since it enables one todetermine a photocatalytic activity independent of the previousadsorption period in the dark and the concentration of X-3Bremaining in the solution. The apparent first-order kinetic Eq. (1)is used to fit experimental data (Fig. 7(a)):

lnðC0=CÞ ¼ kapp � t (1)

where C is the concentration of X-3B remaining in the solution atirradiation time t, and C0 is the initial concentration at t ¼ 0 [30].The variations in ln(C0/C) as a function of irradiation time aregiven in Fig. 7(b) and kapp data for NTO, TIO, and P25 are listed inTable 1. It shows that under solar light illumination the kapp ofNTO obviously increased. The enhanced photocatalytic activityof NTO can be attributed to the results of the synergetic effects ofhigher crystallinity, N atoms doping, more hydroxyl groupsexisting, stronger absorption, and red-shift in adsorption edge.

4. Conclusion

In summary, a sol–gel method under mild condition, i.e. lowtemperature and ambient pressure, has been used for preparationof nitrogen-doped anatase TiO2 nanoparticles. The doping Natoms could inhibit the grain growth, the transformation fromanatase to rutile phase, and form a new phase of O–Ti–N in theTiO2 lattices. After nitrogen doping, the sample exhibits higherBET surface area, while the absorbance spectra of N-doped TiO2

exhibits significant red-shift to visible region. The photocatalyticactivity of N-doped TiO2 under solar light was highly improvedcompared to P25 and pure TiO2.

Acknowledgment

This work is financial supported by the National NaturalScience Foundation of China (No. 60121101) and the Jointproject of Guangdong Province and Education Department(No. 2007A090302018).

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