Surface, optical and photocatalytic properties of silica-supported TiO2 treated with electron beam

Radiation Physics and Chemistry 109 (2015) 40–47

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Surface, optical and photocatalytic properties of silica-supported TiO2 treated with electron beam Pawel Wronski a, Jakub Surmacki a, Halina Abramczyk a, Agnieszka Adamus a, Magdalena Nowosielska b, Waldemar Maniukiewicz b, Marcin Kozanecki c, Magdalena Szadkowska-Nicze a,n a

Institute of Applied Radiation Chemistry, The Faculty of Chemistry, Lodz University of Technology, Wroblewskiego 15, 93-590 Lodz, Poland Institute of General and Ecological Chemistry, The Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland c Department of Molecular Physics, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland b

H I G H L I G H T S







Titanium (IV) n-butoxide and silica were used for synthesis of TiO2/SiO2 samples. Preparation conditions affected surface and optical properties of TiO2/SiO2 samples. Electron beam red shifted light absorption of TiO2/SiO2 samples. Electron beam enhanced the photoactivity of TiO2/SiO2 in azo dye discoloration.

art ic l e i nf o

a b s t r a c t

Article history: Received 1 September 2014 Received in revised form 15 December 2014 Accepted 17 December 2014 Available online 18 December 2014

The influence of high-energy electron beam, (EB), treatment, in the dose range of 100–1000 kGy, on the physicochemical properties of silica-supported TiO2 was examined. TiO2/SiO2 supported oxides were obtained by impregnation of commercial silica gel (2–4 mm) with titanium (IV) n-butoxide. Surface and optical properties of prepared TiO2/SiO2 systems were analyzed using SEM, BET, XRD, Raman and UV–vis spectroscopy. The photoactivity under visible light was tested in discoloration of azo dye solution. No significant structural changes of the TiO2/SiO2 surface were detected as a result of EB treatment. Effect of EB irradiation was observed as an increase of photocatalytic activity in dye decomposition under visible light for TiO2/SiO2 samples containing ca. 23 wt% TiO2. The enhancement of activity was assigned to EBinduced defects and C-modification of TiO2 particles. & 2014 Elsevier Ltd. All rights reserved.

Keywords: TiO2/SiO2 composites Electron beam irradiation Visible-light-photocatalysis

1. Introduction Titanium dioxide is one of the most intensively studied photocatalysts for air and water purification. Practical application of titania creates a need to design a reactor which would maximize photocatalytic efficiency and minimize the energy consumption. The large surface area of the photocatalyst is one of the most important factors in achieving a high efficiency in the photocatalytic reaction. The photocatalytic activity of TiO2 tend to be increased when surface area of TiO2 is larger (Carp et al., 2004). Incorporating the TiO2 into an adsorbent material, such as silica, has many advantages over using a TiO2 suspension for water n

Corresponding author. Fax: þ48 42 684 00 43. E-mail address: [email protected] (M. Szadkowska-Nicze).

http://dx.doi.org/10.1016/j.radphyschem.2014.12.009 0969-806X/& 2014 Elsevier Ltd. All rights reserved.

purification. Silica is an unique support because it occurs in many physical forms and chemical compounds, posses large surface area and high porosity and is transparent to the UV radiation. SiO2-supported TiO2 materials have been extensively used as catalysts for a wide variety of reactions because their physicochemical properties are superior than those of the single oxides (Kim et al., 2005). Moreover TiO2 supported on SiO2 surface is effective on the recovery of photocatalyst. Different types of commercial silica gels (Lepore et al., 1996; Kobayakawa et al., 1998; Yamashita et al., 1998; Chun et al., 2001; Chen et al., 2004; Wang et al., 2006; Qourzal et al., 2009; Bellardita et al., 2010; Bai et al., 2011) were used as matrices for TiO2 immobilization, and various TiO2 precursors: titanium alkoxides (Lepore et al., 1996; Bellardita et al., 2010; Jaroenworaluck et al., 2012), titanium chlorides (Kobayakawa et al., 1998; Yamashita et al., 1998; Bellardita et al., 2010) or commercial TiO2 powders (Lepore et al., 1996) were employed to

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prepare titania/silica systems. Both, type of TiO2 and SiO2 precursors as well as preparation method used for synthesis of TiO2/SiO2 materials affect their structure and properties (Gao and Wachs, 1999). The surface structures of TiO2/SiO2 supported oxides under various environments were extensively investigated by spectroscopic techniques (Raman, UV–vis–NIR, DRS, XANES, XPS), which revealed the consumption of surface Si–OH groups and the formation of Ti–O–Si bridging bonds (Gao et al., 1998). These Ti– O–Si bonds could modify the band-gap energy and local structure of the catalyst. The photocatalytic activity enhancement of silicasupported TiO2 could be explained by sum of many factors such as surface area, adsorption, band gap energy and local structure (Kim et al., 2005). Use of TiO2 as a photocatalyst for environmental applications under solar irradiation requires tuning its band gap response to the visible region. In order to shift the optical response of titania from the UV to the visible spectral range, modifications of TiO2 with metal or non-metal compounds were proposed (Anpo and Takeuchi, 2003; Tachikawa et al., 2007; Kuznetsov and Serpone, 2009). The ionizing irradiation such as gamma rays or electron beam, (EB), can also enhance the catalytic activity of pristine (Jun et al., 2006; Nho et al., 2009; Kim et al., 2010b; Khan et al., 2014) and doped (Kim et al., 2010a; Bzdon et al., 2012; Surmacki et al., 2013) TiO2 under UV and visible light in comparison to the nontreated titania. Especially EB was found as an effective tool for narrowing the band gap of TiO2 making it suitable for the decomposition of toxic chemicals under visible light illumination (Kim et al., 2010b; Khan et al., 2014). It is well known that during EB treatment, formation of defects in the bulk and on the surface of irradiated crystals occurs. Production of defects within the bulk of TiO2 particles is associated with the formation of color centers and Ti3 þ interstitial ions and an overall reduced stoichiometry as a result of the loss of oxygen atoms. The oxygen vacancies affect surface adsorption of adsorbates such as O2 or H2O on TiO2 (Thompson and Yates, 2005), which have an impact on the catalytic activity of titania (Pan et al., 2013). In the present work, we have investigated the effect of EB irradiation on surface, optical and catalytic properties of TiO2 immobilized on silica gel. TiO2 has been produced by hydrolysis of titanium (IV) n-butoxide introduced into silica grains. The changes in physicochemical properties of TiO2/SiO2 samples induced by EB treatment were examined by: SEM microscopy, BET, XRD, Raman and UV–vis spectroscopy. Photocatalytic activity under visible light was examined on the basis of an azo dye solution discoloration, as long as this kind of compounds is considered to have negative environmental impact (Konstantinou and Albanis, 2004).

2. Experimental 2.1. Preparation and EB irradiation of silica-supported TiO2 TiO2/SiO2 samples were prepared by impregnation of commercial silica gel (Chempur) with titanium (IV) n-butoxide, (Ti(OC4H9)4, TTB) (ACROSS Organics) in cyclohexane (C6H12, cHx) (POCH Gliwice) solutions. Silica gel beads were sieved to limit the size to 2–4 mm, immersed in cHx solutions of TTB, stirred for 24 h and aged at ambient temperature for 48 h. Then, the samples were dried at 70 °C for cHx removal and immersed in distilled water (Milipore system) for hydrolysis of TiO2 precursor. After 72 h of soaking, the TiO2/SiO2 supported oxides were rinsed with distilled water and dried at 100 °C for next 72 h. Ti4 þ content of TiO2/SiO2 samples was determined by colorimetric analysis using H2O2 as a complex agent. All chemicals were used as received without additional purification. The concentrations of TTB/cHx solutions per 1 g of silica gel and appropriate Ti4 þ contents in TiO2/SiO2 samples

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Table 1 Nomenclature for the samples and their composition. Sample name

[TTB] per 1 g SiO2 mM/1 g SiO2

[Ti] in SiO2 mmol/g

TiO2 percentagea wt%

A B C D

2.48 6.03 9.54 14.80

0.67 1.87 2.95 2.88

5.3 14.0 23.6 23.0

a TiO2 contents of TiO2/SiO2 samples were calculated by assuming that all Ti4 þ ions were transformed into TiO2 molecules.

are presented in Table 1. Dried TiO2/SiO2 granules enclosed in glass Petri plates were placed 1.6 m before linear accelerator ELU-6E LINAC, (Elektronika Company, Russia) and irradiated by electron beam, (EB), characterized by mean beam power, electron peak current, mean peak current and electron energy of 0.6 kW, 800 mA, 70 mA and 6 MeV, respectively. The EB irradiation was performed in a continuous regime (20 pps, 4 μs duration). EB dose rate was determined by radiochromic dosimeter using WINdose GEX B3 radiochromic films. The films were placed 1.6 m from the front of the accelerator in covering area of ca. 20 cm2 and subsequently irradiated with EB for 5 min, heated for 25 min in 60 °C to fix the color and analyzed using Genesys 20 Spectronic/Unicam spectrophotometer. The dose rate, calculated from average absorbance at 552 nm, was equal 5.8 70.2 kGy min  1. EB irradiation time was 17 min. 14 s, 43 min. 6 s, 86 min. 12 s and 172 min. 25 s for doses of 100, 250, 500 and 1000 kGy, respectively. For comparison of EB irradiation effect to the thermal treatment, part of TiO2/SiO2 granules was calcined at 400 °C for 4 h. 2.2. Characterization The specific surface area (SSA) of the samples was determined using Micromeritics ASAP 2020 equipment. SSA analysis was based on BET model of N2 low temperature adsorption and assumption that nitrogen molecules cover 0.162 nm2 of adsorbent surface. Size and volume of pores between 0.85 nm and 150.00 nm radius were determined using BJH desorption cumulative volume of pores and BJH desorption average pore radius. During the analysis, 0.4–0.5 g of the granules TiO2/SiO2 were placed in measurement ampoule and degassed for 4 h at 100 °C. Then, the ampoule was attached to the instrument and the adsorption process was carried out at  195 °C. Powder X-ray diffraction, (XRD), patterns were collected using a PANalytical X'Pert Pro MPD diffractometer in the Bragg–Brentano reflection geometry. Copper CuKα radiation from a sealed tube was used. Data were collected in the 2θ range 15–80° with a step of 0.0167° and exposure per step of 27 s. Crystalline phases were identified by comparison with the JCPDS data files. Size of the crystallites were estimated according to the Scherrer equation. The Raman spectra were recorded with Ramanor U1000 (JobinYvon) and Spectra Physics 2017-04S argon ion laser operating at 514 nm at power of 11 mW, step of scanning of 2 cm  1, and integration time of 0.5 s. Absorption spectra of TiO2/SiO2 granules were determined using Cary 5000 UV–vis–NIR Spectrometer (Varian Inc.) equipped with external diffusive reflectance accessory (Ulbricht sphere). For all measurements the reflectance ports were used. Samples were placed inside polystyrene Petri plates (50 mm in diameter, 6 mm thick). The baseline was recorded with reflectance port covered by empty Petri plate. Due to granulate form of the samples, the no standard procedure was applied which allowed only for qualitative analysis. Therefore, spectra were normalized to the their maxima.

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For all spectra the constant background was subtracted to 0 absorbance at 670 nm. Microstructure of TiO2/SiO2 granules was evaluated using a scanning electron microscopy (Hitachi TM-1000). 2.3. Photocatalytic activity measurements Discoloration of a dye aqueous solution was chosen to characterize the photocatalytic activity of the EB-irradiated TiO2/SiO2 samples. The anionic azo dye Reactive Blue 81, (RB81), H14C25N7Cl2S3O10Na3, M¼808.0 g/mol, containing three sulfonate groups, dichlorotriazine ring and the azo-group connecting to phenyl and naphtyl, was synthesized in the Institute of Polymer and Dye Technology (Lodz University of Technology). RB81 was degraded under visible light illumination with 11 W fluorescence lamp (3500 K). Photocatalytic tests of TiO2/SiO2 samples were performed as follow: 1 g of TiO2/SiO2 was saturated with water on glass Petri plate, poured with 40 mL of RB81 solution (4.5  10  5 M) and placed above the lamp. The plate was covered with glass lid to minimize the liquid evaporation. When distance between the lamp and reaction vessel was 1 cm, the temperature of RB81 solution increased from room temperature to ca. 39 °C during the first hour of illumination and maintained at this level till the end of the process. Adsorption tests were performed simultaneously without the access of light. Both, adsorption and photodegradation of RB81 was carried out for 5 hours. The RB81 concentration was determined spectrophotometrically at 582 nm, (ε582 ¼2.9  104 L mol  1 cm  1) using an Perkin-Elmer Lambda 750 Spectrophotometer. Samples for concentration measurement (3 mL in volume) were taken by sucking off the supernatant at 1 h intervals and returned to the system after the analysis.

3. Results and discussion 3.1. Surface and optical properties of TiO2/SiO2 samples Table 1 presents that TiO2 loading in prepared TiO2/SiO2 samples is limited to ca. 23 wt%. This probably originates from the silica pore structure which could limit the diffusion of TTB/cHx solution into the silica gel and restrict the formation of TiO2 crystals. Fig. 1 shows the SEM images and photographs of TiO2/SiO2 samples with the lowest (A) and maximum (C and D) TiO2 content. As it is shown in Fig. 1 the optical properties of TiO2/SiO2 samples depend on TiO2 loading. Change in the color of the samples from white to pale yellow is observed. One can see, that although samples C and D contain the same amount of TiO2, their appearance and surface texture are different. Sample D, prepared from the most concentrated solution of TTB/cHx, is pale yellow, glassy and its microstructure is more uniform in comparison to sample C, which granules are whiter and rough. This visual assessment suggests that concentration of titania precursor influence the size and distribution of TiO2 species on the SiO2 surface. The first step of impregnation involves reaction of TTB with the surface hydroxyls on silica. Thus, the Ti atoms bind to the silica via oxygen as a bridge. The titration of the surface hydroxyls with Ti-precursors is either monofunctional (one TTB molecule titrating one OH group) or bifunctional (one TTB molecule titrating two OH group) depending on reaction conditions. Two types of Ti species, highly dispersed surface TiOx species and TiO2 crystallites, are possibly present on the silica surface (Gao et al., 1998). Our results suggests that size and distribution of TiO2 particles in samples C and D may be different due to solute/solvent competition in occupation of silica surface. The influence of EB irradiation on

Fig. 1. SEM micrographs of TiO2/SiO2 samples and their photographs before (I) and after EB irradiation with doses of 250, 500 and 1000 kGy (II).

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Fig. 2. Absorption spectra of TiO2/SiO2 beads: (I) effect of Ti4 þ content on absorption spectra of non-irradiated samples A–D, (II) dose [kGy] effect on absorption spectrum of sample D.

TiO2/SiO2 samples was observed as color intensification (Fig. 1). Seemingly, homogenous white or pale yellow beads after being irradiated with higher doses of EB, have become a mixture of orange and red beads. Fig. 2. shows the influence of TiO2 loading and the dose of EB irradiation on the TiO2/SiO2 absorption spectra. The increase of TiO2 loading slightly shifts the absorption of prepared samples towards longer wavelengths (Fig. 2(I)). Effect of TiO2 loading on the absorption spectra of TiO2/SiO2 composites produced by deposition of TiO2 from titanium (IV) isopropoxide or TiCl4 solutions in the SiO2 sol was observed earlier (Bellardita et al., 2010; Zou and Gao, 2011). The absorption spectra of EB-irradiated sample D presented in Fig. 2(II) show, that increase of EB dose up to 1000 kGy broadens the absorption band towards longer wavelengths and shifted the maximum from ca. 440 to 490 nm. It might suggest, that defects generated in TiO2/SiO2 beads by EB irradiation absorb in the visible region above 450 nm wavelength. It was found earlier, that after EB or gamma irradiation of TiO2 deposited on silicon substrates, the density of Ti3 þ ion increased and that of Ti4 þ ion decreased at the TiO2/Si interface (Zhang et al., 2002). A number of studies (Wang et al., 2006; Onda et al., 2004; Jun et al., 2006; Khan et al., 2014) have shown, that EB-irradiating of TiO2, generated Ti3 þ states and oxygen vacancies. Gamma-irradiation of TiO2 sols generated the broad absorption band in the range of 500–700 nm, which was attributed to electrons stabilized on TiO2 colloidal particles or Ti3 þ states (Safrany et al., 2000; Huang et al., 2001). It has been accepted that absorption of various reduced TiO2 specimens in the visible spectral range was associated with oxygen vacancies (F-type color center) and Ti-related centers (Kuznetsov and Serpone, 2009). The latter species are easily oxidized by oxygen so their red-shifted absorption declines and absorption band at ca. 450 nm remains the dominant feature of the TiO2 spectrum in air atmosphere. XRD patterns presented in Fig. 3 show presence of anatase crystallites in TiO2/SiO2 samples containing ca. 23 wt% of titania. The broad halo, in the 2 theta range from 17° to 30°, with maximum at ca. 22°, recorded for all samples has been assigned to amorphous SiO2 and the peaks at 2θ ¼25.3°, 38°, 48.1°, 54.3°, 63,2° corresponding to the anatase are well pronounced in samples C and D. Similar peaks attributed to anatase crystals produced in silica gels were observed earlier (Lepore et al., 1996; Chen et al., 2004; Wang et al., 2006; Shifu and Gengyu, 2006; Qourzal et al., 2009; Xu et al., 2009; Zou and Gao, 2011). The average size of anatase crystallites calculated according to the Scherrer equation

Fig. 3. XRD patterns of TiO2/SiO2 samples. Upper part: effect of EB irradiation (curve 2) and thermal treatment (curve 3) on XRD profile of sample C (curve 1). Lower part: influence of TiO2 loading on XRD patterns of TiO2/SiO2 samples (A–D) EB-irradiated with dose of 250 kGy.

was in the range of 4–5 nm. Our results are consistent with data reported by Chen et al. (2004), where sizes of anatase crystallites produced in silica gel were in the range of 4–6 nm. It is worth to take into account, that crystallites with sizes lower than 4 nm are rather undetectable in XRD experiments because they are below the detection sensitivity of this technique (Gao et al., 1998). Influence of EB and thermal treatment on XRD patterns of sample C is shown in upper part of Fig. 3. No changes in crystalline structure of TiO2/SiO2 samples after EB irradiation has been observed using the XRD method (Fig. 3 curves 1–2). Small increase of the peak at 2 theta equal 25.3° observed for sample C heated at 400 °C for 4 h (Fig. 3 curve 3) may suggest thermal sintering of the crystallites. Raman spectra, presented in Fig. 4(I), show that TiO2 in the samples C and D has well-crystallized anatase structure, which is consistent with XRD results. The anatase titania is characterized by the apparent peaks at 142 (Eg), 398 (B1g), 515 (A1g, B1g) and 638 cm  1 (Eg) (Kelly et al., 1997, Kumar et al., 2006). We evidently observed the shift of Raman active modes of Eg from 142 (pure anatase, TiO2) to 152 cm  1(sample C and D, TiO2/SiO2). We did not record peaks at ca. 920 and 1080 cm  1, which are indicative of the

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Fig. 4. Raman spectra of the bare SiO2 and TiO2/SiO2 samples (A–D) irradiated with EB dose of 250 kGy (I); influence of EB irradiation on Raman spectrum of sample D (II).

formation of Ti–O–Si bonds (Gao et al., 1998). The effect of EB irradiation on Raman spectra of TiO2/SiO2 samples was insignificant as it is shown on the right side of Fig. 4 where sample D was chosen as an example. The surface properties of TiO2/SiO2 composites, investigated by low temperature nitrogen adsorption, are summarized in Table 2. N2 adsorption–desorption isotherms and pore size distributions are shown in Fig. 5. Increase of SSA with simultaneously diminishing of total pore volume and average pore radius of sample A in comparison to silica host support (Table 2) and nearly equal adsorption–desorption isotherms for bare SiO2 and sample A (Fig. 5 (III)) suggests that small individual TiO2 particles might be grafted onto the pore surface of sample with TiO2 content of ca. 5 wt%. Increase of TiO2 loading results in decreasing of total pore volume, average pore radius and changing in hysteresis loop shapes. Narrowing of the pores (Table 2, Fig. 5 (IV)) diminishes the pore volume, so it is logical to conclude that the titania is deposited inside the pores. Differences in pore size distributions and isotherm shapes reflect the differences in distributions and sizes of anatase crystals formed in silica grains. Comparison of obtained isotherms (Fig. 5 (III)) with shapes of hysteresis loops provided by IUPAC shows, that H1 type (characteristic of cylindrical pores) isotherms recorded for silica gel support and sample A transform to H4 type hysteresis for samples with higher TiO2 loading. H4 hysteresis loops are assigned to materials containing non-rigid aggregates of plates-like particles or assemblages of slit-shaped pores (Thommes, 2010). Notable discrepancies in surface properties of sample C and D, especially in SSA and average size of pores confirm the conclusion based on Fig. 1 that although TiO2 loading in those samples is similar distribution and size of TiO2 crystallites may be different.

Influence of EB and thermal treatment on surface properties of TiO2/SiO2 systems is presented on the example of sample C (Table 2; Fig. 5 parts (I) and (II)). Negligible reduction after EB irradiation and small increase of pore radius due to calcination at 400 °C has been observed. These changes were in the range of ca. 1.5 nm with comparison to non-treated sample. Slight decrease of SSA and rise of total pore volume after the calcination at 400 °C might be noticed. These results might suggest, that hydrocarbon residues included in silica pores in the course of TiO2 synthesis from TTB/cHx solution could be removed from the system during thermal treatment. The influence of EB irradiation on properties of TiO2/SiO2 in water solution was monitored by measuring of pH values and water sorption. pH of aqueous supernatant above the commercial silica gel was measured as 7.370.1 and a similar pH values were obtained when silica was doped with TiO2. When TiO2/SiO2 samples were irradiated with EB the pH of supernatant decreased to values ca. 4.4 70.2. Sorption of water in commercial silica gel was ca. 1.66 g per 1 g of SiO2. Doping of silica gel with TiO2 decreased the water sorption to ca. 1.10 g per 1 g of TiO2/SiO2 for samples with the highest TiO2 content. It is reasonable because titania is more hydrophobic in comparison to silica gel. EB irradiation was found to increase slightly the sorption of water. It might be suggested, that EB irradiated TiO2/SiO2 samples became more hydrophilic due to oxygen vacancies which create additional sites for chemical adsorption of water (Pan et al., 2013). It was reported (Dohshi et al., 2005) that γ-ray irradiation has increased the wettability of TiO2 single crystals.

Table 2 Surface characteristic of samples determined by the N2 adsorption–desorption method.

Visible light induced catalytic activity of EB-irradiated TiO2/SiO2 samples was investigated in discoloration of RB81 dye in aqueous solutions. It is well known, that color of some dyes fades under visible light. We observed no RB81 discoloration in the absence of TiO2/SiO2 granules and in the presence of non-doped silica, although the lamp emission spectrum overlapped the absorption spectrum of the dye. Discoloration of RB81 solution due to adsorption and visible light induced catalysis in the presence of non-irradiated and EBirradiated TiO2/SiO2 samples is shown in Fig. 6. One can see that non-irradiated samples were practically inactive in RB81 discoloration under visible light because photocatalytic discoloration

Sample Treatment

SiO2 A B C

D

– EB, 250 kGy EB, 250 kGy – EB, 250 kGy 400 °C/4 h EB, 250 kGy

SSA [m2 g  1] Total pore volume [cm3 g  1]

Average pore radius [nm]

371 72 402 7 1 3677 1 3377 3 340 7 3 3317 1 382 7 3

3.62 2.99 2.87 2.25 2.26 2.72 1.92

0.925 0.810 0.695 0.513 0.515 0.615 0.504

3.2. Photocatalytic activity of TiO2/SiO2 samples

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Fig. 5. Effect of preparation conditions on the N2 adsorption–desorption isotherms ((I), (III)) and pore size distributions ((II), (IV)). Part (I) and (II) treatment influence on sample C. Part (III) and (IV) influence of EB irradiation (250 kGy) on isotherms and pore size distributions of samples A–D.

was, in the range of experimental error, the same as discoloration subsequent to dye adsorption (Fig. 6 (I) and (II)). The effect of EB irradiation (250 kGy) on the adsorptive and photocatalytic

properties of TiO2/SiO2 samples in the dye solution discoloration presented in parts (III) and (IV) of Fig. 6 show that EB irradiation has improved the adsorption capacity and photocatalytic activity

Fig. 6. Discoloration of RB81 solution (4.5  10  5 M) during 5 h of adsorption (left column) and photocatalysis (right column) in the presence of TiO2/SiO2 samples (A (∎), B( ), C ( ), D ( )) non-irradiated (parts (I) and (II)) and EB-irradiated with dose of 250 kGy (parts (III) and (IV)).

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visual assessment (Fig. 1), that some of the beads in samples C and D irradiated with doses in the range of 500–1000 kGy were less transparent than others due to an intense color. Taking into account above mentioned data we have recognized 250 kGy as an optimal dose for improvement of prepared TiO2/SiO2 samples photoactivity. It was found earlier that the dose equal to 500 kGy was optimal for photocatalytic dye decomposition on TiO2 P25 under UV irradiation (Jun et al., 2006). Degradation of RB81 adsorbed on the TiO2/SiO2 surface illuminated with visible light can occur due to photosensitization process involving initial excitation of the dye molecule which is subsequently converted into a radical cation after electron injection into the TiO2 conduction band (Zhao et al., 2005) according to scheme:
þ (TiO2(ecb)/SiO2) RB81ads(TiO2/SiO2)þhν-RB81nads(TiO2/SiO2)-RB81ads Fig. 7. Discoloration degree of RB81 solution (4.5  10  5 M) after 5 h of adsorption (empty bars) and photocatalysis (filled bars) in the presence of sample D EB-irradiated in the dose range of 100–1000 kGy or calcined at 400 °C for 4 h.

of TiO2/SiO2 samples, especially for samples with the highest TiO2 content. Strong correlation between curves of RB81 adsorption and photocatalytic discoloration confirms that photodegradation of dyes is closely related to the adsorption of these compounds on the surface of TiO2 particles, and the degradation takes place at, or near to the TiO2 particles surface, rather than in the bulk solution (Chun et al., 2001). Influence of EB dose and calcination at 400 °C on adsorptive and photocatalytic properties of sample D is presented in Fig. 7. We have compared discoloration degree of RB81 solution calculated as [(C0  C)/C0] where C0 and C denote dye concentrations before and after 5 h of the process, respectively. Sample D was chosen as the most susceptible for EB treatment. As it can be seen, the increase of EB irradiation dose up to 250 kGy has resulted in the highest reduction of RB81 concentration due to photocatalysis as well as adsorption. Sample calcined at 400 °C exhibited lower activity than non-treated ones. An increase of dye adsorption onto EB-irradiated samples could be partly related to decreasing pH values of supernatant over these samples. It was reported that adsorption of anionic azo dye onto silica increased with decrease in pH (Parida et al., 2006). The enhanced photocatalytic activity of EB-irradiated TiO2/SiO2 samples with high TiO2 content could be related to radiation-induced defects in the TiO2 particles. It has been revealed that oxygen vacancies can behave as important adsorption and active sites for heterogeneous catalysis (Pan et al., 2013). It was found that EB-induced changes in the structure of the C impurities on the surface of TiO2 are closely related to the variation in its photocatalytic activity (Kim et al., 2010). One can suppose that TiO2/SiO2 samples produced by hydrolysis of titanium (IV) n-butoxide contain some amounts of residual carbon from alkoxy chains. It was found that TiO2 (anatase, P25 Degussa) doped with carbon from ethanol exhibited enhanced adsorption and photocatalytic activity for azo dyes decomposition (Janus and Morawski, 2007; Janus et al., 2009). During calcination at 400 °C hydrocarbon residues are removed from TiO2/SiO2 samples and photocatalytic activity of calcined samples was lower than those of EB-irradiated and non-treated (Fig. 7). These results confirm the positive role of carbon in improvement of TiO2 photoactivity. Subsequent rise of EB dose up to 1000 kGy has diminished adsorptive and photocatalytic properties of sample D. It might be due to the fact that an excessive internal structural defects introduce charge carrier recombination centers thus affect the charge separation process and the photocatalytic activity. The diminishing of photocatalytic activity of TiO2/SiO2 samples irradiated with doses above 250 kGy can be partly related to decreasing transparency of catalysts beads. It has been observed by

The electron from TiO2 conduction band can reduce surfaceadsorbed oxygen to yield the oxidizing species such as H2O2, O2
 /HO2
, and
OH radicals, which are presumed to be responsible for degradation of the dye cationic radical. EB irradiation of TiO2/SiO2 composites extended their absorption towards longer wavelength so TiO2 can be directly excited by visible light and initiate the degradation reactions like in the conventional photocatalytic process. Our results might suggest that EB-induced surface defects affected the interaction between TiO2/SiO2 surface and RB81 dye. The increase of RB81 adsorption onto EB-irradiated TiO2/SiO2 samples restricted light access to photocatalyst surface and photosensitization process seems to be favorable in the case of this dye degradation. Moreover, detailed inspection of RB81 absorption spectra recorded during photocatalytic discoloration in the presence of non-irradiated as well as EB-irradiated TiO2/SiO2 samples showed no changes in the shape and position of absorption bands. The absorption diminished gradually, without appearance of any new bands. These observations may suggest that discoloration processes in the case of both types of TiO2/SiO2 samples (non-irradiated and EB-irradiated) were due to disruption of RB81 conjugated structure. In order to check the catalytic stability of EB-irradiated TiO2/SiO2 beads, the sample D irradiated with the dose of 250 kGy was used in several runs of photocatalytic degradation of RB81. After the first run (5 h of photocatalysis), TiO2/SiO2 granules were extracted from solution, rinsed with water and reused in the next catalytic cycle to discoloration of a new RB81 solution with initial concentration of 4.5  10  5 M. After each photocatalytic reaction the pH of supernatant was measured. We have observed that after four photocatalytic runs the activity of sample D and pH of supernatant over this sample approached values obtained for the non-irradiated sample D. Depletion of activity might be resulted from the fact that TiO2 surface defects created by EB irradiation may be removed by oxygen and water molecules (Onda et al., 2004; Thompson and Yates, 2005; Pan et al., 2013). It was also found that organic molecules containing a Cl moiety, adsorbed on the surface of TiO2–SiO2 mixed oxides, are able to remove trapped electrons (Panayotov and Yates, 2003). In order to elucidate the effect of water and dye adsorption on the photocatalytic performance of EB-irradiated samples, the sample D irradiated with dose of 250 kGy was soaked in water or RB81 solution for 24 h before photocatalytic reaction. We have observed that contact of EB-irradiated TiO2/SiO2 sample with water or with the dye solution without access of light did not change its photoactivity. However, when sample was immersed in water and illuminated with visible light for 5 h, its activity was reduced remarkably. Results of above experiments suggest that EB-induced defects in TiO2/SiO2 system are repaired during RB81

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photodegradation in water and catalytic activity of EB-irradiated TiO2/SiO2 samples decays.

4. Conclusions TiO2/SiO2 composites have been prepared by hydrolysis of titanium (IV) n-butoxide previously included into commercial silica gel. XRD and Raman spectra showed nano-sized anatase crystals in TiO2/SiO2 samples containing ca. 23 wt% of TiO2. N2 low temperature adsorption–desorption analysis indicates that TiO2 crystallites were incorporated into silica pores. Distributions and sizes of anatase crystals formed in silica grains depended on composition of impregnating solution. No significant structural changes of the TiO2/SiO2 surface were detected as a result of EB treatment. EB irradiation shifted absorption of TiO2/SiO2 samples towards longer wavelengths. The photoactivity of TiO2/SiO2 granules under visible light increased with increase of TiO2 loading and with dose of EB irradiation up to 250 kGy. Enhancement of adsorption and photocatalytic activity for azo dye decomposition was assigned to EBinduced defects (oxygen vacancies, Ti3 þ species) and C-modification of TiO2 particles.

Acknowledgments The work was partially financed from the Young Scientists' Fund, The Faculty of Chemistry, Lodz University of Technology. The authors are grateful to dr K. Blus (Institute of Dye and Polymer Technology, Lodz University of Technology) for synthesis of Reactive Blue 81 dye.

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