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Visible-light photocatalytic activity of N/SiO2–TiO2 thin films on glass

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2010 Adv. Nat. Sci: Nanosci. Nanotechnol. 1 015004

(http://iopscience.iop.org/2043-6262/1/1/015004)

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 015004 (5pp) doi:10.1088/2043-6254/1/1/015004

Visible-light photocatalytic activity ofN/SiO2–TiO2 thin films on glassThi My Dung Dang, Duy Dam Le, Vinh Thang Chauand Mau Chien Dang

Laboratory for Nanotechnology, Vietnam National University, Ho Chi Minh City, Community 6,Linh Trung Ward, Thu Duc District, Ho Chi Minh City, Vietnam

E-mail: dtmdung@vnuhcm.edu.vn

Received 15 January 2010Accepted for publication 17 April 2010Published 13 May 2010Online at stacks.iop.org/ANSN/1/015004

AbstractNanocrystalline N-doped SiO2/TiO2 visible-light photocatalyst thin films were synthesizedusing the sol-gel method on glass substrates. The synthesized catalysts were thencharacterized using several analytical techniques like x-ray Diffraction (XRD), ScanningElectron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic ForceMicroscopy (AFM), and UV–vis absorption spectroscopy (UV–vis). The experimental resultsrevealed that the maximum optical response of the synthesized SiO2/TiO2 thin films shiftedfrom the ultraviolet (UV) to the visible-light region (λ> 420 nm). The photocatalytic activityof N-doped SiO2–TiO2 photocatalyst was considerably higher than that of SiO2–TiO2, andthis result was obtained with an optimal concentration of 40 mol% of N. The enhancedphotocatalytic activity was attributed to the increasing surface area and forming morehydroxyl groups in the doped catalyst.

Keywords: visible light, N-doped SiO2–TiO2, thin films, photocatalysts

Classification numbers: 4.10, 5.07

1. Introduction

Titanium dioxide (TiO2) has been widely used as aphotocatalyst for solar energy conversion and environmentalapplications because of its low cost, non-toxicity andgood photoactivity. When TiO2 is irradiated by sunlightwith a wavelength of less than 387 nm (ultraviolet range),electrons are passed across the band gap into the conductionband, leaving holes in the valence band [1]. These holeshave high oxidation power, thus can easily react withadsorbed hydroxide ions to produce hydroxyl radicals,the main oxidizing species which are responsible for thephotooxidation of organic compounds [2]. The addition ofa second metal oxide like SiO2, ZrO2 or Al2O3, etc, wasalso found to be an effective route to improve the thermalstability and UV photocatalytic activity of TiO2 [3]. Amongthem, SiO2–TiO2 materials were most widely investigatedin the photocatalysis field because they exhibited higherphotocatalytic activity than pure TiO2. This could beexplained by the addition of SiO2 into TiO2 retarding orinhibiting the crystallization of anatase phase. A contact angle

of SiO2–TiO2 thin films with 15 mol% SiO2 concentration isless than 2◦ and these films can maintain a super-hydrophilicproperty for a long time in dark conditions, thus exhibitingexcellent antifogging capabilities [4]. The effects of texture,surface state and activity of the N-doped SiO2–TiO2 wereinvestigated in detail.

The major problem that only about 4% of the solarspectrum falls in the useful ultraviolet (UV) range leads tolow efficiency use of the catalyst TiO2. Therefore, improvingefficient use has become an appealing challenge [5]. One ofthe approaches to achieving this objective is to dope TiO2

with nonmetal atoms such as S, C, and N. Among theseanion-doped TiO2 photocatalysts, N-doped TiO2 has been themost extensively studied, and considerable success has beenachieved in enhancing the visible-light-driven photocatalyticactivity by decreasing the band gap of N-doped TiO2 [5]. Ithas been discussed that doped N atoms help to improve thevisible-light absorption capability of the TiO2 catalyst. Thesynergetic effects of doped N on optical shift, crystallinity,surface areas, and the activity of TiO2 have also beenstudied.

2043-6254/10/015004+05$30.00 1 © 2010 Vietnam Academy of Science & Technology

Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 015004 T M D Dang et al

TEOS EtOH + H2O +

HCl 1%

Sol TEOS

N-SiO2-TiO2 sol

EtOH + TTIP

Urea

Stirring 30 min

Stirring 60 min

Stirring 30 min Stirring 30 min

Figure 1. Synthetic process of N/SiO2–TiO2 solution.

In the present study, nanocrystalline N-doped SiO2–TiO2

were prepared by the sol-gel method. The synthesizedfilms were tested for their photocatalytic activities usinga degrading Methylene Blue (MB), which is known tobe difficult to degrade under irradiation and is often usedas a model dye contaminant to evaluate the activity of aphotocatalyst [6].

2. Experimental procedures

2.1. Preparation of N-SiO2–TiO2 thin films

All chemicals used in the study were procured from Aldrich,Germany and used as received. Titanium tetraisopropoxide(TTIP) and tetraethylorthosilicate (TEOS) were used asprecursors for titania and silica, respectively. First, TEOSwas hydrolyzed in an aqueous HCl solution, and a TTIPethanol mixture (1 mol TTIP per 20 mol ethanol) wasslowly introduced dropwise. The molar concentration ofSi/Ti in the solutions was chosen at an optimal value of15 mol% [4]. Urea was also added to the solution with variousN/SiO2 + TiO2 molar concentrations from 10 to 50 mol%.The detailed synthesis procedure is shown in figure 1.N/SiO2–TiO2 thin films were deposited on glass substrateswith dimensions of 26 × 76 mm2 by a dip coating processat room temperature. The substrates were immersed in theas-prepared N-SiO2–TiO2 solution for 1 min, and withdrawnfrom the solution at a velocity of 4 mm s−1. After each layerwas deposited, the film was annealed at 300 ◦C on a hotplatefor 5 min. The procedure from coating to drying was repeatedthree times to achieve a film thickness of about 150 nm.Afterward, the substrates were calcined at 500 ◦C for 2 h. Inour work, N/SiO2–TiO2 powders were also prepared usingthe same calcination procedure.

2.2. Characterization of film structure and morphology

X-ray diffraction (XRD) patterns of the calcined gelswere obtained with Cu-Kα radiation (Siemens Kristalloflexdiffractometer) to determine the crystal phase compositionof the prepared photocatalysts. The average crystal size wasestimated by applying the Scherrer equation to the apparentfull-width-at-half-maximum intensity (FWHM) of the (101)peak of anatase TiO2 [7], as follows:

d = (kλ) / (β cos θ), (1)

where d denotes the average crystallite size, k = 0.9,λ = 0.15405 nm is the x-ray wavelength of Cu-Kα, β isthe full-width of the peak measured at half-maximumintensity (FWHM) and θ is the Bragg angle of the peak.Synthesized samples were also studied by using UV–visabsorption spectra with Jasco UV–vis V530 double beamspectrophotometer in a wavelength range from 190 to1100 nm. Transmission Electron Microscopy (TEM) was usedto study the particle size. Samples for TEM measurementwere suspended in ethanol and ultrasonically dispersed. Dropsof the suspensions were placed on a copper grid coatedwith carbon. These were analyzed using a JEOL 1400 FieldEmission Electron Microscope. In addition, the morphologyof the as-prepared photocatalyst films was observed usinga Scanning Electron Microscope (Jeol JMS-6480LV) andAtomic Force Microscope (Nanotech Electonica SL).

2.3. Photocatalytic activity measurements

The self-cleaning activities of synthesized films wereevaluated by analyzing the decrease of MB concentrationduring exposure to visible-light irradiation. Glass samples(26 × 26 mm2) were placed in a container filled with 10 mlof 10 ppm MB. This container was then exposed to thevisible light provided by a compact Philips lamp (18 W).The MB solution was then taken out after being exposedfor 2 h, and the concentration of MB was determined usingUV–Visible spectrometry. The changes of MB concentrationwere estimated by the absorbance peak for MB at 664 nm.The obtained results showed that the concentration of MBdecreased when the exposure time was increased. Theefficiency of decomposition of MB can be calculated byapplying the following formula [8]:

R0 =A0 − At

A0100%, (2)

where R0 is the efficiency of decomposition, A0 is theabsorbance of the peak for MB at 664 nm before exposureand At is the absorbance of the peak for MB at 664 nm after texposing hours.

3. Results and discussions

3.1. X-ray diffraction

The powder XRD diagrams of samples with different Ncontents calcined at 500 ◦C are shown in figure 2. Allsamples showed only the anatase phase with high intensityand other crystal phases (rutile or brookite) weren’t detected.Bragg reflections at angles of 25.4◦, 38◦, 48◦, 54◦ and55◦ corresponded to (101), (004), (200), (105) and (211)tetragonal crystal planes of anatase phase TiO2, respectively.To obtain transparent self-cleaning TiO2 films on glasssubstrates, a calcination procedure is carried out at 500 ◦C.The existence of anatase phase in our synthesized filmsis a very important result, because it is well known thatanatase exhibits the highest photocatalytic activity. From thefull-width-at-half-maximum (FWHM) of the strongest peak(101) anatase phase, crystallite sizes were calculated usingScherrer’s equation (table 1).

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 015004 T M D Dang et al

Table 1. Crystallite sizes of N-SiO2–TiO2 powders with different Ncontent.

Sample Mol% N Crystallite size (nm)

N0 15 mol% SiO2–TiO2 9 [9]N10 TiO2–15 mol% SiO2–10 mol% N 9.25N20 TiO2–15 mol% SiO2–20 mol% N 10.13N30 TiO2–15 mol% SiO2–30 mol% N 10.36N40 TiO2–15 mol% SiO2–40 mol% N 15.59N50 TiO2–15 mol% SiO2–50 mol% N 13.07

Figure 2. XRD diagrams of N/SiO2–TiO2 powders with differentN content.

The effects of N-doped concentration on the crystallitesize of TiO2 powders are demonstrated in table 1. It isrevealed that the size of TiO2 powders increases from 9.25 to15.59 nm when the N-doped concentration increases from 10to 40 mol%, and the size decreases to 13.07 nm at sample N50.It can be said that the larger the amount of N-doping, the betterthe crystallization that takes place, and the larger the grain sizeof the TiO2 powders. But when the N-doped concentrationincreased more than 40%, the crystalline size decreased. Thisproves the retarding effect of N on the crystallinity of TiO2

when the concentration of the dopant is too high.

3.2. Optical properties of as-prepared N/SiO2–TiO2

thin films

The optical absorbance spectra of the N/SiO2–TiO2 thin filmswith different N-doped concentrations measured in the regionof 300–800 nm are shown in figure 3. The transmittancewithin the visible and near infrared region is higher than 70%,which reveals the superior optical properties of N/SiO2–TiO2

produced in this work. The spectra show shoulders near350 nm and bases which approach zero at about 300 nm.The transmittance quickly decreases below 350 nm due tothe absorption of light caused by the excitation of electronsfrom the valence band to the conduction band of TiO2.The absorption edge shifted towards longer wavelengths (i.e.red shift) with the increase of N-doped concentration from0 to 40 mol%. A shift towards shorter wavelengths (i.e. blueshift) is observed when the amount of N-doping is more than40 mol%.

Figure 3. Optical transmission spectra of N/SiO2–TiO2 films withdifferent N content.

b

a

Figure 4. SEM image (a) and TEM image (b) of N40 thin film.

3.3. Surface morphology observation

As far as the geometry of the surface is concerned, thehydrophilic properties are well known to be enhanced forfilms with fine roughness. Therefore, controlling the surfacemicrostructure of the films is a solution to improve thehydrophilic property of the synthesized TiO2 films [6].Figure 4 shows SEM and TEM images of N40. We observedthat the film surface is smooth and homogeneous withoutcracks. Moreover, the surface consists of little granularcrystallization with a size of 15 nm. This result of SEM andTEM observation is similar to that obtained by XRD study. Itcan be said that synthesizing films without the agglomeration

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 015004 T M D Dang et al

a b c

Figure 5. AFM image of (a) N20 film, (b) N30 film and (c) N40 film.

a b c

Figure 6. The contact angle of water on normal glass substrate (a), on N40 film (b) and on N40 film stored overnight (c).

Figure 7. The antifogging ability of N40 film and normal glasssubstrate with cold water vapor (I), and hot water vapor (II). (a)–(c)are the results for N40 films, while (b)–(d) are the results for normalglass substrate, respectively.

of large clusters results in an increase of specific surfacearea, and subsequently enhances the desired photocatalyticproperties of the films.

The AFM images from figure 5 indicate that the filmroughness changed according to the same rule in XRD orUV–Vis: the larger the amount of N-doping, the better thecrystallization, the larger the grain size of TiO2 powders,the smaller the grain boundary, and the smaller the surfaceroughness. But there is a threshold limit value of N-dopedconcentration at which the surface roughness (RMS) doesn’tdecrease, that is threshold N40. However, these RMS valuesare around 1 nm, so that the rate of the oxidation-reductionreaction substrate by e− and h+ is similar.

3.4. Hydrophilic properties of N40 thin films

Figure 6 illustrates the dependence of photo-induced changeon the water contact angle of N40 films, which were treated

Figure 8. Photocatalysis decomposition of MB with N/SiO2–TiO2

films.

after 2 h with visible-light irradiation and then kept overnightin a dark environment. The hydrophilic ability of the samplemay be explained by the contact angle of water on the surface.The super-hydrophilic property of the surface allows water tospread completely across the surface rather than remaining asdroplets. The observed result means that N40-coated glass isa good material for antifogging and self-cleaning purposes.Moreover, figure 6(b) shows a very low contact angle (<2◦)of water on the N40-coated sample, while high values ofwater contact angles resulted when water was deposited onnormal glass substrates. In addition, after storing overnightin ambient conditions, the contact angle of water on N40film-coated glass slowly increased to about 6◦ (figure 6(c)).This result means that the coated sample could maintainthe super-hydrophilic capability for a long time in a darkenvironment.

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 015004 T M D Dang et al

Figure 9. Photocatalysis decoloration of MB with N40 film on glass substrate. (a) N40 film/glass; (b) glass substrate.

An 18 W compact lamp is used to irradiate one N40thin film coating and one normal glass substrate for 2 h. Toinvestigate the antifogging ability of coating films, hot or coldwater vapor is used. Figure 7 shows the result of this test.With the N40 film-coated glass sample (a, c), we can readthe letter behind clearly. In contrast, the letters behind theglass substrate sample without coating (b, d) cannot be read.This result exhibits the excellent antifogging ability of N40film produced in this work. These transparent self-cleaningTiO2 films on glass substrates have great potential for practicalapplications such as mirrors, window glass and windshields ofautomobiles.

3.5. Photocatalysis property of N/SiO2–TiO2 film on glasssubstrate

Photocatalytic performance capabilities of the N/SiO2–TiO2

thin films and without pores were evaluated by degradingMB under visible-light irradiation. It should be noted thatMB may decompose itself under visible-light irradiation,and physical adsorption can also occur during the process.Figure 8 shows the variation of photo degradation of MBunder visible-light irradiation. This result was obtained aftertaking into account both the physical adsorption and theself-degradation of MB under visible light. The obtainedresults show that the percentage of degradation increased withincreasing visible-light exposure time. Moreover, the pureTiO2 film showed about 60.44% degradation of MB after6 h of visible-light exposure. The photocatalytic propertiesof the films increased to about 71.76, 82.29 and 75.8%with an addition of 30, 40 and 50 mol% of N, respectively.Therefore, it can be said that a significant improvementof photocatalytic performance can be achieved by choosingappropriate compositions for the synthesized films. It is alsointeresting to note that the addition of N further improved thephotocatalytic properties of the TiO2 films.

Finally, we qualitatively analyse the photocatalysisproperty of N/SiO2–TiO2 film on glass substrate bydecoloration of MB, and figure 9 shows the photocatalysisdecoloration of MB with N40-coated glass substrate. It canbe seen that the concentration of MB on the experimental

substrate decreased about 50% after a period of about 15 minand almost no MB was detected after a period of 34 min.

4. Conclusion

Nanocrystalline N/SiO2–TiO2 thin films were preparedby a combination of the sol-gel and dipping coatingtechnique. The N/SiO2–TiO2 film with 40 mol% N-dopedconcentration gives optimal results on crystalline structure,optical property, surface area, and photocatalysis property.The photocatalytic properties of the prepared thin films wereevaluated by degrading MB under visible light. It is notedthat photocatalytic performance can be improved by addingactive N elements. In the present study, the N40-coatedfilms exhibited the highest photocatalytic performance,demonstrated by the fact that the MB was nearly completelydecomposed after only 6 h of visible-light exposure.

Acknowledgments

The authors appreciate the financial support of the Departmentof Science and Technology, Ho Chi Minh City, Vietnam.

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