MATERIALS Revista Mexicana de Fısica S59 (1) 158–162 FEBRUARY 2013

Effect of Ti content in the photocatalytic behavior of Fe/TiO2-SiO2 systems

A. Leon Chorenoa, N. Portillo Veleza, I. Hernandez Pereza,b, R.Suarez Parrac, M. May Lozanoa,L. Gonzalez Reyesa, and R. Luna Paza,b

aDepartment of Basic Sciences, UAM-A, 02200, Mexico, D.F.e-mail: ihp@correo.azc.uam.mx; ihernandez@correo.ler.uam.mx

bDivision of Basic Sciences and Engineering, UAM-L, 5200, Lerma Mexico.cCentro de Investigacion en Energıa, UNAM, 62580, Temixco-Morelos, Mexico.

Received 30 de junio de 2011; accepted 9 de septiembre de 2011

In this work we report the synthesis of Fe/TiO2-SiO2 systems with different concentrations of TiO2 in order to determine the influence oftitanium content on the structural, textural, optical properties and their photocatalytic behavior. The materials were synthesized by the sol-gelmethod and their modification was carried out by incipient impregnation. All samples were characterized by means of X ray diffraction, N2

physisorption (BET method), DR-UV-Vis and Raman spectroscopy. The modifications of the structural and optical properties are discussedon the basis of long-range order reduction, suggesting the formation of highly dispersed TiO2 species. On the other hand, it was observed thatthe energy of the optical band gap decreases by introducing Fe. On the basis of these phenomena, the photocatalytic activity was measured,employing the degradation of orange II azo dye as a model reaction.

Keywords: Fe-TiO2-SiO2; photocatalysis; semiconductors.

En este trabajo se presenta la sıntesis de sistemas Fe/TiO2-SiO2 con diferentes concentraciones de TiO2 Con el fin de determinar la influenciadel contenido de titanio en las propiedades estructurales, texturales,opticos y de su comportamiento fotocatalitico. Los materiales fueronsintetizados por el metodo sol-gel y su modificacion se llevo a cabo por impregnacion incipiente. Todas las muestras fueron caracterizadas pormedio de difraccion de rayos X, metodo de Bet, el RD-UV- Vis y espectroscopia Ramen. Las modificaciones de las propiedades estructuralesy opticas se discuten en la base de la reduccion de orden de largo alcance, lo que sugiere la formacion de especias altamente dispersa TiO2

Por otro lado, se observo que la energıa de la brecha de bandaoptica disminuye mediante la introduccion de Re. Sobre la base de estosfenomenos, la actividad fotocatalıtica se midio, empleando la degradacion de colorante naranja II azo, como una reduccion modelo.

Descriptores:Fe/TiO2-SiO2; photocatalysis; semiconductors.

PACS: 82.50.Hp

1. Introduction

The control of pore size, distribution and structure of mate-rials are important aspects to consider during research anddevelopment of new materials. These factors also play animportant role in the production of adsorbents and catalysts,since they are intimately related to its catalytic activity. SiO2-TiO2 system has been studied extensively because of its mul-tiple applications, such as the manufacture of special ce-ments, paints and UV absorbers in creams and cosmetics foreveryday use [1,2]. It has also been used in developing of re-fractory materials, glass with controlled refractive index andlow expansion coefficient. Recent studies have found thatcatalytic systems including TiO2 and TiO2-SiO2 show excel-lent results in oxidation and acid-base reactions [3-5].

In recent years, the wastewater discharged from the dyestuff manufacturing, printing and textile industries representone of the most serious environmental problems, due to con-tain chemicals that exhibit mutagenic effects and toxic to hu-man and aquatic life [6]. Unfortunately, azo-dyes compoundsare not biodegradable and resist to conventional treatment.Heterogeneous photocatalysis is emerging as promising tech-nologies to wastewater treatment due to it is not selective andtherefore it is capable of destroy a wide variety of organiccontaminants. In this process, a semiconductor is activatedby UV or Visible radiation (with the adequate frequency) to

photogenerate electron-hole pairs and HO• radicals, whichare very strong oxidants. There are several studies where ithas investigated the removal of organic pollutants on semi-conductor TiO2, TiO2-SiO2 or Fe2O3 catalysts [7-12]. How-ever, there is no information on the use of TiO2-SiO2 systemmodified with Fe. This study shows the effect of Ti contentand the unique structure and electronic properties that con-fer Fe doping TiO2-SiO2 system on the photodegradation ofOrange II textile azo-dye.

2. Experimental

2.1. Material Synthesis

In order to have a reference, TiO2 was synthesized by the fol-lowing procedure: a homogeneous mixture of titanium iso-propoxide in i-propanol was hydrolyzed at room temperaturein the presence of HNO3. After gelation, the sample wasaged for 24 h, then dried at 120◦C and calcined at 450◦Cunder flowing air during 4 h.

To synthesize TiO2-SiO2 system, a mixture of titaniumisopropoxide (in the necessary amounts to obtain materialswith: 5, 10, 15, 25, 40, 60, 70 and 80 wt % of TiO2) andtetraethyl ortho-silicate in i-propanol was hydrolyzed andaged to obtain the corresponding gels. The samples thus ob-

EFFECT OF TI CONTENT IN THE PHOTOCATALYTIC BEHAVIOR OF Fe/TiO2-SiO2 SYSTEMS 159

tained were dried for 24 hours at 120◦C, to be finally calcinedat 450◦C for 4 hours in air stream.

The TiO2-SiO2 (with different Ti/Si mol ratio) powderswere modified with a 1 % wt of Fe by means of a wet im-pregnation method at 25◦C, employing a FeSO4.7H2O aque-ous solution. After impregnation the materials were driedovernight at 120◦C and subsequently calcined for 4 hours at450◦C in an air flow.

2.2. Characterization

The structural characterization of all synthesized materialswas carried out by powder XRD using a Philips X’Pertdiffractometer, operating in 2θ mode, samples were analyzedin the range of 5 to 80, 2θ degrees using a Cu tube Kaα ra-diation (35 kV, 25 mA) at 2◦ s−1 scan rate and wavelengthλ= 0.15405 nm. The textural properties were determined bynitrogen physisorption at 77 K, employing the BET methodin a Micromeritics ASAP 2020 apparatus. Before analysisthe samples were out-gassed at 100◦C for 12 h.

The optical properties of materials were determined usingan UV-Vis (Varian Cary I double-beam) spectrophotometeroperating in the diffuse reflectance mode. The absorption co-efficient F (R∞) was obtained from diffuse reflectance datathrough the Kubelka-Munk equation. In order to determinethe vibrational modes of Fe/TiO2-SiO2 systems and con-firm their structural changes, dispersive Raman spectroscopywas used (Thermo-Nicolet Almega model, equipped with alaser source in the visible region 532 nm, using potential ofmedium intensity, spectral shift of 1 cm−1 and with a resolu-tion of 0.5 cm−1)

2.3. Photocatalytic Test

All experiments were carried out in a batch glass reactor of100 mL, equipped with magnetic stirring, cooling jacket andLummi UV lamp 7 W. The reactions were performed at roomtemperature (25◦C) using an aqueous solution of Orange IIdye with an initial concentration of 17.51 ppm and 2 mL ofH2O2 (commercial solution of 30% vol). The discolorationreaction was monitored by the evolution in absorption band

at 484 nm of UV-Vis spectra, while the detoxification wasmonitored in the 365 nm.

3. Results and Discussion

3.1. Textural Properties

The composition and results of the textural, structural andband gap energy (Eg) of the Fe/TiO2-SiO2 systems are shownin Table I.

From this Table, it was noticed that the incorporation ofSiO2 increases the specific area of synthesized materials andthe higher value of specific area is developed by the sam-ple with 25 % of TiO2, this behavior may be associated toformation of SiO2 tetrahedral species, which has the abilityto form polymeric arrangements, leaving voids during ther-mal treatment (calcinations), and permit the expansion of theTiO2 structure. For this reason, there are observed impor-tant variations in textural, structural and electronic propertiesdepending in TiO2 content.

According to the IUPAC classifications, the isotherms ob-tained for all Fe/TiO2-SiO2 systems are identified as type IV,which are characteristic of mesoporous solids. In Figure 1are illustrated the adsorption-desorption isotherms, addition-ally it was found that the sample Fe/TiO2-SiO2 with 5% ofTiO2 have a hysteresis loop type H3, where it presents a lim-ited adsorption at high values ofp/po so it is suggested thatthe system consists of aggregates of plate-like particles, re-sulting in slit-shaped pores.

The hysteresis loop of the sample Fe/TiO2-SiO2-25 is oftype H1, this type has been associated with porous materialsconsisting with uniform and compact clusters arranged fairlyregular and very narrow pore distribution.

On the other hand, the materials with 40 and 55 TiO2 con-tent have a loop of type H4, associated with narrow pores asslit and a significant microporosity. Adsorption isotherms forsamples with 62, 70 and 80% of TiO2 presents a hysteresisloop of type H2, which corresponds to a homogeneous distri-bution with a high porous degree of interconnection. Theseresults show that the TiO2 content has an important influenceon the size, shape and porosity distribution, which dependslargely on the arrangement or symmetry long-range (crys-

TABLE I. Textural and electronic properties of Fe/Si-TiO2 systems

Photocatalyst TiO2(W % ) SiO2 (W %) Surface Area m2/g Pore Volume cm3/g Pore Size (A) Eg(eV)

TiO2 100 - 65 0.098753 56.2617 3.21

Fe/Si-Ti 80 80 20 205.749 0.155089 30.1511 2.87

Fe/Si-Ti 70 70 30 150.023 0.114958 30.6509 2.92

Fe/Si-Ti 62 62 38 318.777 0.288629 36.217 3.03

Fe/Si-Ti 55 55 45 201.881 0.114288 22.6447 2.94

Fe/Si-Ti 40 40 60 237.460 0.126721 21.3461 2.90

Fe/Si-Ti 25 25 75 440.534 0.645548 18.6161 3.27

Fe/Si-Ti 5 5 95 27.560 0.072325 104.9692 3.31

Rev. Mex. Fis. S59 (1) (2013) 158–162

160 A. LEON CHORENO et al.,

FIGURE 1. Adsorption-desorption isotherms for the Fe/TiO2-SiO2

systems with different TiO2 content.

talline structure) formed during the synthesis stage. Addi-tionally, it was revealed that the surface area also dependson the size of crystals and how these are added, because asmaller particle size or crystal, generate a more number ofdefects and therefore the specific surface area increases.

3.2. Structural Characterization

Figure 2 shows the X-ray diffraction patterns of samplesFe/TiO2-SiO2. From these results we can see that sampleswith lower silicon content (rich in TiO2), have very intensereflection at 25 2θ degrees, which are characteristics of ti-tanium oxide in anatase phase, it is important to notice thatthere are no reflections corresponding to rutile or brookitephases.

These intense and narrow peaks reveal the presence ofTiO2 crystals larger than 20 nm. Besides, the increase in

FIGURE 2. XRD Pattern of Fe/TiO2-SiO2 samples with differentTiO2 content.

FIGURE 3. Optical band gap energy for Fe/TiO2-SiO2 samplesa) Fe/TiO2, b) Fe/TiO2-SiO2 80, c) Fe/TiO2-SiO2 70, d) Fe/TiO2-SiO2 62, e) Fe/TiO2-SiO2 55, f) Fe/TiO2-SiO2 40, g) Fe/TiO2-SiO2 25 and h) Fe/TiO2-SiO2 5.

silica content produce wide and low intensity reflections inthe same positions, indicating that there is a loss of long-range ordering. This behavior can be attributed to the for-mation of nanocrystalline materials (less than 5 nm) of TiO2,which are highly dispersed on silicon oxide surface due tothe latter has an amorphous structure. Additionally, it wasfound, that the sample whit 5% of TiO2 content was com-pletely amorphous (spectrum is not showed).

3.3. Optical Characteristics

The calculation of the optical band gap energy (Eg) of synthe-sized materials was obtained from extrapolation of the linearregion of the Kubelka-Munk transformation of the UV-Visabsorption spectra (F(R) = 0), as shown in Fig. 3. As it isknown, the absorption edge is the necessary energy to pro-mote an electron from the HOMO (highest occupied molec-ular orbital) or valence band to a LUMO (lowest unoccupiedmolecular orbital) or conduction band. In Table I are summa-rized the values of Eg obtained for all systems, finding thatthe TiO2-SiO2 systems with lower TiO2 content show thehighest Eg values, this situation was expected, since the SiO2

is an insulating material. Furthermore, we find that sampleswith low TiO2 content (Fe/TiO2-SiO2 25 and Fe/TiO2-SiO2

5 Eg = 3.27 and 3.31 eV respectively) present a band gap en-ergy higher than the bulk Deggusa P-25 TiO2 (3.2 eV), thisincrease can be attributed to quantum confinement effect (de-crease in crystallite size observed by XRD analysis).

For the samples modified with Fe it was observed a sig-nificant decrease in the Eg (red shift), attributed to the forma-tion of new electronic states located in the interband, whichfavors the electronic transitions and the generation of charge

Rev. Mex. Fis. S59 (1) (2013) 158–162

EFFECT OF TI CONTENT IN THE PHOTOCATALYTIC BEHAVIOR OF Fe/TiO2-SiO2 SYSTEMS 161

FIGURE 4. Raman spectra of: a) Fe/TiO2, b) Fe/Si-Ti 80, c) Fe/Si-Ti 70, d) Fe/Si-Ti 62, e) Fe/Si-Ti 55, f) Fe/Si-Ti 40, g) Fe/Si-Ti 25and Fe/Si-Ti 5.

carriers electron-hole pairs (e− h+ respectively). This found-ing opens a range of applications such as photocatalysis andphotovoltaic processes that can operate in the visible re-gion [13-16]. Additionally, it was observed that the positionof absorption edge is sensitive to the type of bond betweenmetal oxides polyhedral,i.e. the coordination number de-termines not only the Eg but also their size and electronicbehavior.

Figure 4, shows the Raman vibrational modes forFe/TiO2-SiO2 samples. The vibrational bands at 156, 408,527 and 650 cm−1 are assigned to Ti-O-Ti bonds, character-istic to crystalline TiO2 in anatase phase [17,18]. In Fig. 4it is evident the disappearance of these bands as SiO2 con-tent increases, which reflects the loss of long-range orderingintroduced by the incorporation of Si within TiO2 structureand the corresponding increasing in Ti-O-Si bonds, which isin good agreement with the results observed by XRD analy-ses. Besides, no vibrational modes for Ti-O-Fe bonds weredetected; thus could be due to the low Fe content.

These observations lead us to think that as the Si contentincreases in the Ti-O-Ti structure, it also increases the degreeof amorphous species associated with Ti-O-Si bonds, whichincorporate a high degree of local disorder into the structureof the anatase phase, suggesting the existence of highly dis-persed Ti species in the SiO2 matrix, so it can be considerednegligible, the latter may be associated to the limited amountof photogenerated HO• radicals via dissolved oxygen.

Figure 5 shows a comparative photocatalytic performanceof Fe/TiO2-SiO2 systems, where it is noticed that the higherRB-5 degradation was obtained employing the Fe/ TiO2-SiO2 40 sample. This behavior can be associated with thelow optical band gap energy. Although the sample with 80%

FIGURE 5. Influence of TiO2 content on RB-5 photodegradation at30 min.

of TiO2 has the lowest Eg, this one achieves only a RB-5 azo-dye remotion of 68%, this behavior can be associated to thepresence of Fe/TiO2-SiO2 aggregates of different sizes (thecorresponding UV-Vis spectrum shows different absorptionedges), other reason may be related to their porous structure,whose interconnections prevents the light pass to the activesites. Finally, we notice that despite its low specific area, thesample with 5% TiO2 content has a similar photoactivity thanbulk TiO2, this may be due to the presence of a large numberof superficial active sites, which are available for reaction.

4. Conclusions

Titanium content in the synthesized materials plays a signif-icant role both in the crystal structure in the shape and dis-tribution of pores, as well as electronic and photocatalyticbehavior. The photoactivity should be associated with the Egand the amount of surface species from Ti-O-Fe and Ti-O-Ti bonds and Fe2O3 nanoparticles. The addition of Fe leadsto a decrease in edge energy and the generation of internalelectronic inter-band states (red shift), allowing the possibil-ity to apply this systems for photovoltaic processes or photo-catalysis using visible irradiation to remove different organicpollutants as Orange II azo-dye.

Acknowledgements

A. Leon Choreno thanks to Conacyt the support for master’sstudies at UAM-A. We appreciate the collaboration of Dr. J.Flores Moreno and Dr. G. Negron Silva for some XRD andBET measurements respectively. This contribution was alsorealized with the financial support of the Conacyt 82909-Yand PAPIIT IN114609-3 projects. Technical assistance ofDrausin Jimenez and R. Moran Elvira for the reactor con-struction and related apparatus is also acknowledged.

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