SiO2-TiO2-B2O3-BaO glass-ceramic system with Fe2O3 or CuO additives as photocatalyst

Journal of Environmental Chemical Engineering 4 (2016) 3106–3113

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SiO2-TiO2-B2O3-BaO glass-ceramic system with Fe2O3 or CuO additives as photocatalyst Fatma H. Marghaa,* , Ahmed A. Abdelsamadb , Tarek A. Gad-Allahb a b

Department of Glass Research, National Research Centre, 33El Bohouth st. (former El Tahrir st.), P.O. 12622, Dokki, Giza, Egypt Water Pollution Research Department, National Research Centre, 33El Bohouth st. (former El Tahrir st.), P.O. 12622, Dokki, Giza, Egypt

A R T I C L E I N F O

Article history: Received 10 April 2016 Received in revised form 2 June 2016 Accepted 14 June 2016 Available online 17 June 2016 Keywords: Glass-ceramics Heat-treatment Optical properties Photocatalysis Humic acid

A B S T R A C T

Different amounts of Fe2O3 or CuO have been added over the glass batch composed mainly of SiO2, B2O3, BaO and the photoactive TiO2. The obtained glass samples were subsequently heat treated at the onset of nucleation temperature (490  C) for 1–3 h followed by heating at 550  C for the same period in order to obtain nanocrystalline glass-ceramic materials. X-ray diffraction analysis of the obtained materials revealed amorphous structure; however, tiny crystals could be detected under high-resolution electron microscope (HR-TEM). All samples showed absorption in UV with variable absorption in the visible light regions depending on the composition. The prepared glass-ceramic materials displayed photocatalytic efficiency for the degradation of Humic Acid, though Fe doping is much favorable than Cu doping. About 46% removal of humic acid could be achieved when using sample containing 0.5 wt.% of Fe2O3 and heat treated at 490  C/1 h followed 550  C/1 h. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction The most popular and promising photocatalyst is crystalline TiO2 because of its high efficiency, low cost and wide gap which can be activated by UV light from solar or artificial radiation sources [1,2]. Two main criteria were investigated for improving the efficiency of TiO2: minimizing photogenerated electron–hole recombination rates and expanding the range of operation into visible light wavelengths [3]. Both processes can be achieved by coupling TiO2 with other materials such as semiconductor oxides that show a clear energy difference between their valence bands as well as between their conduction bands (the two differences-shifts being to the same direction) [2,3]. The strong absorptivity of Fe2O3 in the visible range, along with its abundance and low cost, narrow band gap (Eg  2.2 eV [4]) has stimulated considerable interest in its use as a photocatalyst and a photoelectrode [5]. But, the position of the valence band of Fe2O3 does not allow the generation of OH radical from water making it an ineffective photocatalyst for organics oxidation, when used by itself [6]. Therefore, its combination with other semiconductors (e.g. TiO2) is obligatory [7]. TiO2–Fe2O3 binary mixed oxides were reported to be a good catalyst with improved photocatalytic

* Corresponding author. E-mail addresses: [email protected] (F.H. Margha), [email protected] (T.A. Gad-Allah). http://dx.doi.org/10.1016/j.jece.2016.06.020 2213-3437/ã 2016 Elsevier Ltd. All rights reserved.

properties and enhanced visible light response. Studies with TiO2– Fe2O3 mixed oxide catalyst have shown an increased photocatalytic activity for dichloroacetic acid destruction at 450 nm [8]. Recently, Wodka et al. [9] synthesized Fe2O3/P25 composite containing 1 wt.% of the iron(III) oxide nanoparticles on the Evonic-Degussa P25 titania surface using precipitation method. According to their findings, doping with Fe(III) enhanced the activity of TiO2 toward the removal of oxalic acid (OxA) and formic acid (FA) from water under UV or artificial sunlight (halogen lamp). Also, Liu et al. [10] prepared Fe2O3–TiO2 composite photocatalyst using hydrothermal method, which showed excellent photocatalytic activity for the degradation of auramine under visible and solar light irradiation. Cu2O (and CuO) is also an abundant p-type semiconductor with band-gap energy of 1.8–2.5 eV (and 1.21–2.00 eV, respectively) [11] that absorb visible light. CuO/TiO2 nanorods has been prepared via electrospinning process and showed excellent antibacterial abilities under visible light [12]. In a similar study, TiO2/CuO electrospun nanofibers was successfully fabricated and displayed efficient concurrent photocatalytic organic degradation and clean energy production from dye wastewater [13]. Because photocatalysis usually occurs on the surface of photocatalyst, the higher the surface area is, the better is the efficiency of the photocatalyst due to presence of more active sites for the adsorption of water and contaminants as well as for the formation of hydroxyl radicals [14]. One method for increasing the

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surface area is to diminish the particle size as possible using surfactants and polymers as structure-directing agents [14]. However, this small particle size leads to difficulties in separation of the photocatalyst from the treated solution. Precipitation of photoactive crystals in a glassy matrix in the form of glass-ceramic ensures the confinement of crystal growth and hence small crystals could be obtained. In mean time, the glass-ceramic composite as a whole possesses large particle size which allows its separation by simple methods [15]. In this study, we propose a glass-ceramic composition in which TiO2, as photocatalyst, was coupled with another low band gap semiconductor (Fe2O3 or CuO) to enhance TiO2 activity. The glass composition is SiO2, TiO2, B2O3, Na2O, K2O, P2O5, Li2O and BaO with either Fe2O3 or CuO. The prepared photocatalytic glass-ceramic materials were investigated for the degradation of Humic acid (HA) which is the major constituent of natural organic matter found in all surface waters [16,17] and represent the major precursor of the carcinogenic halogenated disinfection by-products (DBPs) [18]. 2. Experimental 2.1. Preparation of glass samples The glass samples were prepared using reagent-grade chemicals (Loba Chemie, India). All chemicals were used as received without further purifications. The composition (24.69 SiO2, 24.29 TiO2, 12.95 B2O3, 7.69 Na2O, 4K2O, 1.2 P2O5, 0.8 Li2O, 24.28 BaO in wt.%) was selected because of its good transparency and photocatalytic activity according to our previous study [15]. Fe2O3 or CuO was added to the mentioned composition with 0.25, 0.5 and 1% over the batch to study their effects on the final glass-ceramic products. The prepared materials were labeled as TFx and TCx for Fe2O3 and CuO additions, respectively, where x refers to the addition percentage. The glass samples were prepared by melting the composition batch in a platinum crucible. The furnace temperature was raised to 1200  C gradually to avoid sputtering or splashing of the batch materials during melting. After complete melting, the molten batch was held for 2–3 h at constant temperature with occasional swirling to ensure homogenization of the melt. The melted samples were then casted onto hot steel moulds and then rapidly transferred to a muffle furnace adjusted at desired temperature for annealing to obtain strain-free glass samples. Photographs of the prepared materials, taken by Sony Cybershot DSC-H9 digital still camera (Japan), are presented in Fig. 1. 2.2. Characterization of prepared materials Temperatures of crystallization and growth of the prepared samples were determined using differential scanning calorimetry (DSC) (SETRAM Instrumentation Regulation, LabsysTM TG-DSC16 under inert gas). The crystal structure was identified from the collected XRD patterns using Bruker diffractometer (Germany)

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with graphite monochromatized Cu-Ka radiation. Optical properties of polished glass and glass-ceramics slabs (1  0.3  3 cm3) were measured using JASCO spectrophotometer (Model V630, Japan). 2.3. Evaluation of photocatalytic performance Photocatalytic experiments were carried out using one–liter photo-reactor. The reactor consists of two parts; the first part is an inner quartz double-wall jacket with inlet and outlet for the water circulation to maintain the temperature constant during the experiment. This jacket has an empty chamber at the center for immersion of a medium pressure mercury lamp (150 W, lmax  365 nm). The second part is the outer borosilicate glass container (volume 750 mL after insertion of the inner part) in which the reaction takes place. HA solution was mixed with the glass-ceramic materials (0.5– 2 mm grain size), stirred for 60 min prior to experiment to allow adsorption equilibrium of HA on the glass-ceramic surface. Then, the UV lamp was switched on to start photodegradation process. After definite irradiation times, suitable volumes of the solution were sampled using a syringe and the glass-ceramic materials were allowed to settle down. After that, the solution was analyzed according to the UV–vis absorbency at 254 nm. 3. Results and discussion 3.1. Characterization of the prepared materials Differential scanning calorimetry (DSC) is a rapid tool for studying the crystallization nature through the determination of temperature range of crystallization and the suitable heattreatment schedule. This is achieved by finding out the onset of glass transition temperature (Tg), onset of first crystallization peak (Tp) and endotherm endpoint liquidus temperature (Tliq) [19]). Fig. 2 shows the DSC curves of TF glass samples containing various amounts of Fe2O3. It is obvious that adding Fe2O3 causes a marked shift on the DSC data to higher values, which is evidenced by the shift of the endothermic and crystallization peaks to higher temperatures. Specifically, the glass transition temperature (Tg) increased from 507 to 516 and up to 520  C when the weight ratio of Fe2O3 were 0.25%, 0.5% and 1 wt.%, respectively. The increase in glass transition temperature can be ascribed to the increase in average coordination number with the addition of Fe2O3. According to Sun’s bond-strength criterion for glass formation [20], the fundamental condition for glass formation is the existence of strongly bonded large networks or long chains for atoms in the liquid. The increase in iron oxide content creates strongly bonded networks which are responsible for the increase in thermal stability of the glass. Similar behavior was observed for the glass systems 35TeO2–(65-x)V2O5–xFe2O3 [21] and 20ZnO–(80-x)TeO2– xFe2O3 [22]. On the other hand, the peak of crystallization (Tp) appeared at 704  C and 832  C in case of 0.25 and 0.5% Fe2O3,

Fig. 1. Photographs of the prepared TF and TC glass samples.

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respectively; no remarkable change in TP was observed by increasing the weight ratio of Fe2O3 to 1%. This effect may be related to the increase in the viscosity of the glass which consequently leads to less mobility of the structural units in the glass [23]. According to the DSC results in Fig. 2, the heat-treatment schedule for TF was proposed to be carried out at the onset of the nucleation temperature (490  C) for 1–3 h followed by heating at 550  C for the same period. These temperatures were selected at the start of nucleation peak 490  C to produce small nuclei then continue heating up to 550  C to allow the growth of these nuclei. The heating rate was 10  C/min for both steps. TC group was heat treated at the same temperature as TF group to keep both groups at the same condition and to clarify the role of additives on the photocatalytic activity. Photocatalyst should be in a crystalline form to be active for the degradation of organic contaminants. The type of phases and their extent of crystallization also affect greatly on the efficiency of the photocatalyst. On the other hand, the surrounding phases may alter the photocatalytic behavior of the photocatalyst. For these reasons, crystallization behaviors of the prepared glass materials were examined by XRD technique in this work. The recorded XRD patterns of TF and TC groups are depicted in Fig. 3. The diffractograms of the samples do not show any peak even after the heat-treatment for 3 h, but only a broad diffused scattering characteristic of amorphous structure. In relation to DSC data (Fig. 2), the proposed heat treatment schedules should lead to the formation of small crystals. Therefore, the broad diffraction hump implies that the as-casted rod is basically amorphous. Nevertheless, the existence of a small amount of crystals in the heat treated samples cannot be absolutely excluded due to the limitation of the resolution of normal XRD technique, which may not be able to detect the crystalline diffractions, especially, if crystal size is small down to nanosacle and its amount is less than a critical level [24]. This type of crystal is preferable as it will uphold the transparency of the formed glass-ceramic. The transparent glass-ceramic materials are most wanted because the photocatalytic oxidation process requires clear solutions with minimum turbidity to be efficient. Furthermore, the transparent materials will allow the light to penetrate the photocatalyst and consequently both sides of the materials will be available for the generation of hydroxyl radicals. These hydroxyl radicals will then react with the organic contaminants in the medium to undergo the oxidation and decontamination processes.

In order to investigate the morphologies of the prepared glassceramic materials, scanning electron microscopic (SEM) examinations were carried out on the powder of samples at the upmost heating time to allow the recognition of formed crystals. The SEM image of TF1 and TC1 samples heat-treated at 450  C/3 h followed by another heat-treatment step at 550  C/3 h is provided in Fig. 4. The image shows that both samples contain the same spherical shapes that might be crystals embedded in glassy matrix. This puts forward that both samples may have the same crystal structure and addition of Fe2O3 or CuO did not effect on the morphology of the crystals. More information about the crystallization process could be obtained from HR-TEM images of the same samples. According to the images presented in Fig. 5, two different regions were found in the heat treated samples. The first region, which comprises the majority of the sample, is of typically amorphous structure. In addition to the amorphous structure, other region containing a number of nanocrystals with average diameter of 15 nm in the amorphous matrix has been observed. The small crystal structure elucidates the absence of distinctive diffraction peaks in the XRD patterns. It is also notable that both samples have the same morphology and crystal structure indicating that the Fe and Cu addition has no effect on the crystal growth. The optical properties of the glass and the corresponding glass ceramic materials rely mainly on the composition of glass and the structure of the crystals and of the residual glass phases in glassceramics. The presence of photocatalyst in transparent glassceramic matrix ensures that entire surface will be available for hydroxyl radical generation. The UV–vis spectra of TF groups before and after applying the supposed heat-treatment schedules are presented in Fig. 6a–c. All TF samples were transparent before and after heat treatment which confirms that the crystals formed by the applied heat-treatment schedule are of tiny sizes that cannot change the absorption cutoff. The cutoff of the prepared samples were at 470 nm, 510 nm and 610 when the iron ratio increased from 0.25, 0.5 and 1%, respectively. This red shift might be attributed to the iron ions in the material. All samples have strong absorption in UV and reasonable absorption in the visible light region. This gives an indication that these materials may show good photocatalytic activity under both UV and visible irradiation. Hence, the materials can be considered as a promising photocatalyst under solar irradiation. The effects of the copper ratio and the applied heat-treatment on the optical properties of the TC group are elucidated in Fig. 6d–f. All samples showed high absorption in the UV light region along with a broad absorption peak with a maximum absorption at 780 nm. Increasing the Cu ions in the glass materials caused an extending of the UV absorption to the near visible light (200–500 nm) region. Same behavior was observed by ElBatal et al. [25]) in the CuO-doped lead borate glass system. The applied heat-treatment did not affect the position of neither the UV absorption nor the visible absorption peaks. This might related to the size of the formed crystals. The strong absorption in both UV and visible regions of the glassceramic materials suggests that these materials are promising for the decontamination of water/wastewater under solar irradiation. Hence, these materials may help in reducing the cost of water/ wastewater treatment. 3.2. Photocatalytic degradation of HA Photocatalytic activity of the prepared glass-ceramic materials for the removal of HA using UV photoreactor was investigated. The conditions of the experiments were: initial concentration of HA = 20 ppm, photocatalyst dose = 1500 ppm at neutral pH. Prior to perform photocatalytic degradation of HA using the prepared photocatalysts, control experiment was carried out in absence of photocatalyst but under UV irradiation in order to investigate the

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Fig. 3. XRD patterns of TF and TC groups heat treated for different time intervals.

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Fig. 4. SEM microgrph of (a) TF0.5 and (b) TC1 samples heat-treated at 450  C/3h + 550  C/3 h.

Fig. 5. HR-TEM of (a) TF0.5 and (b) TC0.5 heat treated at 490  C/3 h + 550  C/3 h.

photolytic HA removal from water. The adsorption capacity of the prepared glass ceramics materials was assessed as well. The obtained results showed that there is no remarkable removal of HA from water via neither photolysis nor adsorption over the prepared glass-ceramic materials. Therefore, the presence of photocatalyst together with suitable irradiation was proved to be essential for the removal of HA from water. The photocatalytic degradation of HA over TF or TC glassceramic materials containing different ratios of iron or copper and heat treated at different temperatures for different times was investigated through 120 min of UV irradiation. The collected results are illustrated in Fig. 7. The time of heat treatment affected clearly on the photocatalytic efficiency of TF materials (Fig. 7a–c). For example, the efficiency of TF0.25 sample increased when the heat treatment time increased from 1 h to 2 h and decreased by further increase to 3 h. This might be due to that 2 h heat treatment allowed the crystallization of photoactive phase of TiO of TiO but further heat treatment time caused the increase in crystal size and lower effective surface area. The iron addition enhanced the photocatalytic efficiency which was clear by comparing the HA removal using TF0.25 and TF0.5. It is also interesting to note that the increase in heat treatment time did not affect on the efficiency of TF0.5 sample. The addition of iron to be 1% of the composition affected adversely on the photocatalytic efficiency. This is because there is critical doping ratio of each dopant. Beyond this ratio, the dopant act as electron-hole recombination center rather than trap

center [26]. In this case (TF1), the time of heat treatment could enhance the efficiency by improving the crystallization of the prepared samples. Evaluating the HA removal using TC glass-ceramic material that subjected to heat treatment at 490  C followed by heating at 550  C for different times (see Fig. 7d–f) reveals that the increase in the time of heat treatment causes a development in the photocatalytic efficiencies. This improvement could be attributed to better crystallinity. Similar to doping with Fe, the higher the content of Cu, the lower is the photocatalytic activity. This is because of the increased recombination rate between charge carriers (electrons and holes) at the Cu atoms that are dispersed in the glass-ceramic material. It is worth noting that the maximum HA removal in case of Fe was 46% while the best achieved removal in case of Cu was only 27%. This indicates that Fe doping is much favorable than Cu doping for the removal of HA from water. 4. Conclusion In this study, photocatalytic transparent nanocrystalline materials based on SiO2-TiO2-B2O3-Na2O-K2O-P2O5-Li2O-BaO glass ceramic system with the addition of Fe2O3 or CuO were prepared successfully. XRD analysis of the materials before and after heattreatment showed no peaks which can be credited to the small crystallite sizes and dispersion of the formed crystals observed in SEM and TEM images. All materials showed strong absorption in

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UV and reasonable absorption in the visible light region. The effect of doping ratio in TC and TF group was found to play crucial role in the final photocatalytic activity of the material. The maximum HA removal (46%) could be achieved by using TF1 (490  C/1 h + 550  C/

1 h) compared to only 27% removal when TC0.5 (490  C/ 2 h + 550  C/2 h) was used. This reveals that Fe doped SiO2-TiO2B2O3-BaO glass-ceramic system is much favorable than that doped with Cu for the removal of HA from water.

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