Bismuth fluoride on SrO-Bi2O3-B2O3transparent glass ceramic

Accepted Manuscript Title: Controlled Crystallization of Photocatalytic Active Bismuth Oxyfluoride/Bismuth Fluoride on SrO-Bi2 O3 -B2 O3 transparent Glass ceramic Authors: V.P. Singh, Rahul Vaish PII: DOI: Reference:

S0955-2219(18)30164-X https://doi.org/10.1016/j.jeurceramsoc.2018.03.031 JECS 11788

To appear in:

Journal of the European Ceramic Society

Received date: Revised date: Accepted date:

10-12-2017 12-3-2018 18-3-2018

Please cite this article as: Singh VP, Vaish R, Controlled Crystallization of Photocatalytic Active Bismuth Oxyfluoride/Bismuth Fluoride on SrO-Bi2 O3 B2 O3 transparent Glass ceramic, Journal of The European Ceramic Society (2010), https://doi.org/10.1016/j.jeurceramsoc.2018.03.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Controlled Crystallization of Photocatalytic Active Bismuth Oxyfluoride/Bismuth Fluoride on SrO-Bi2O3-B2O3transparent Glass ceramic

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V.P. Singha,band Rahul Vaisha* aSchool bGovt.

of Engineering, Indian Institute of Technology Mandi, Himachal Pradesh, 175005, India Engineering College, Bharatpur, Rajasthan, 321001, India

Corresponding Author

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E-mail: [email protected] (Dr. Rahul Vaish)

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ABSTRACT

SrO-Bi2O3-B2O3 (SBBO) transparent glass ceramics are fabricated via conventional melt-

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quenching technique. Simply via surface fluorination of these glass ceramics, by using HF

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solution, we obtained the coating of cubic BiO0.51F1.98over SBBO glass ceramic. Further,

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with successive fluorination by using concentrated HF solutions the crystal structure of coating tends towards the cubic α-BiF3 structurefrom the cubic BiO0.51F1.98. Both the

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structures of cubic BiO0.51F1.98 and cubic α-BiF3possess totally different morphologies; first having compact spheres and the other shows densely packed cubes, respectively. Along with

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fluorination, besides the structural and microstructure transformations, we have attained the improved transparency. Synergy effects of structural transformation on the contact angle and the photocatalytic properties are studied. The fluorinated SBBO transparent glass ceramics can be used as self-cleaning structural application.

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KEYWORDS: Glass-ceramics, Crystal growth, Photocatalysis, Contact angle, Self-cleaning.

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I. INTRODUCTION Glasses and glass ceramics are known for their unique characteristics such as easy fabrication, transparency, tuning of compositions and wide range of properties with different specific applications [1, 2]. Among these, self-cleaning applications of different glasses having photocatalytic property and photo induced surface wettability are under the prime focus for the

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researchers [3, 4]. As compare to photocatalytic powders, glass showing photocatalytic property

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is easy to mould in different shapes like tubes or 2-D sheets or elongated rods for easy

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fabrication of bigger photocatalytic panels and reactors at large scale having low cost of

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manufacturing and ability to use it multiple times.TiO2 in its powders form or thin films coated

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over glass or tiles is the most utilised material till date [5-7]. Including TiO2, most of the

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photocatalytic materials respond only under UV light irradiation with their restricted functions at low scale. While, for cleaning waste water or air pollution at large scale, for example drugs and

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chemical waste water, printing, dying and textile waste water, pesticides, NOx and SOx etc, we need to develop visible-light active low-cost reactors or panels with high solar energy conversion

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efficiency.

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Incidentally, few resent studies show Bi-based compounds like oxides, halides or oxyhalides having quick and better visible light active photocatalytic performances and could be the suitable alternative of TiO2[8]. For example, nanospheres of Bi2O3are reported for an excellent photocatalytic performance under visible light irradiation as compare to nitrogen doped TiO2 and Degussa P25 [9]. Better photocatalytic performance of BiOCl than commercial TiO2 is severally

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reported [10-13]. More specifically, Mu et al. compared BiOCl nanosheet arrays and P25 TiO2 film by Methyl orange (MO) degradation and confirmed better durability and degradation rate for BiOCl [14]. Zou et al. have found 3.88 times superior photocatalytic performance in BiOF

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than that of commercial TiO2[15]. Bi is known for strong anisotropy with large Fermi wavelength, unusual long mean free path, low effective mass and less charge carrier density. In Bi-based compounds, Bi3+ ion have 6s2 outer cell configuration with stereo chemically active electron lone pairs resulting extraordinary electronic properties in the materials. Due to large electronegative difference in-between Bi3+ ion and the other oxide and/or halide ions, such

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compounds form polar covalent bonding with highly intrinsic polarized lattice structure which

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lead the separation of photo induced electron-hole pairs with improved mobility and showing

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better photocatalytic property [15-17].

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In the same time, fluorination of Bi based compound has shown further improvement in the

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photocatalytic performance [18-21]. Bi2O3, BiVO4, BiPO4, Bi2WO6 and many other compounds

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have already been reported for exceptional enhancement in the photocatalytic performance after fluorination [18-22].Jiang et. al. reported fluorinated Bi2O3 with efficient photocatalytic

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performance [18]. Fluorination leads the better surface adsorption with the enhancement of interfacial charge transfer dynamics [19]. In addition, fluorination is also implemented to

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synthesize some complex Bi based compounds which are known for the efficient photocatalytic

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performance. Bervas et al. synthesized BiOF via fluorination of B2O3 or BiF3 under the treatment of saturated aqueous solution of NH4F [23]. Another report shows the fluorination via ion exchange method to convert BiOCl to BiOF, Bi7F11O5 andBiF3, respectively [24]. Many other techniques are adopted for the fluorination of compounds for example, doping [25], hydrothermal process with NH4F [26], heat treatment in presence of NaF [27], reductive

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fluorination using XeF2[28], HF treatment [29]e.t.c. Using simple fluorination and tuning the stoichiometric compound of BixOyFz Ren et. al have prepared six bismuth-containing compounds: Bi2O3, BiOF, Bi7O5F11, BiO0.67F1.66, BiO0.51F1.98 and BiF3[30]. They controlled

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fluorination by tweaking the concentration of NH4F in the medium through hydrothermal method. Fluorination results in decreasingO/F ratio and wider band gaps in BixOyFz, which is proposed for enhance photocatalytic activity [30]. Feng et al. reported the degradation of rhodamine B (RhB) for BiF3 crystals with 2.1 times higher photocatalytic activity as compared to commercial TiO2 [26]. The better performance in BiF3 is prevailed by fluorine.

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This article reports simple and effective way to introduce photocatalytic properties on the

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surface of transparent SrO-Bi2O3-B2O3 (SBBO) glass ceramics. To introduce the photocatalytic

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performance in SBBO glass ceramic, fluorination was obtained via etching through different

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concentrations of hydrofluoric acid. Selection of glass for the photocatalytic applications was

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due to the good glass formability with high mechanical strength and its stability. We studied the

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photocatalytic performance of fluorinated SBBO glass ceramics using methylene blue (MB) under visible light (420 nm) irradiation. Also, we performed contact angle measurement for self-

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cleaning application point of view.

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II.EXPERIMENTAL SECTION Materials preparation. SBBO transparent glass ceramics having equimolar ratio of oxides

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with composition SrO-Bi2O3-B2O3were prepared by conventional melt quench technique. According to the molar ratio of glass composition of SrO-Bi2O3-B2O3 the calculated amounts of constituents ie. Sr, Bi and B were obtained from the compounds SrCO3, Bi2O3 and H3BO3, respectively. All materials were of AR grade. Appropriate amounts of each

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precursor were mixed homogeneously in a ball mill which possesses a jar and 20 balls of agate material. Acetone was used as a mixing medium. Mixing was obtained for 5 hr at 200 rpm. The milled powder was poured in a platinum crucible for melting at 1000 °C in a Nabertherm

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furnace. Finally, the melt was quenched on a preheated stainless steel plate maintained at 200 oC resulting with the plates of smooth and transparent yellowish thin glass ceramics, which further annealed at 200 oC for 5 hr to remove the thermal stress presented in the glass plates.

Next, the fluorinations of SBBO glass ceramics were obtained. For this purpose, we used water solution of HF with 1.5, 3, 6 and 12 M concentrations. Transparent plates of SBBO glass

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ceramics were etched for 10 min by using HF solutions of different concentration, respectively.

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Thus, along with the etching we obtained the successive fluorination. All samples were sonicated

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in acetone for 10 min to remove unwanted HF solution and contaminations from the surface.

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consecutive fluorination, respectively.

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Samples were named as SBBO-HF-1, SBBO-HF-2, SBBO-HF-3 and SBBO-HF-4 after

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To investigate the photocatalytic activities of as quenched and fluorinated SBBO glass

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ceramics the solution of a typical Methylene blue(MB) in deionized water with concentration 5 ppm was used. All samples having 1 cm2 plate size were immersed into 10 ml of MB solution,

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separately. Before photocatalytic experiment, to attain equilibrium of adsorption-desorption process all the samples dipped in MB solution were subjected under dark for 6 hours and

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monitored at specific time intervals. Photocatalytic experiment was performed in a chamber with white tubes of Luzchem LZC-420having maximum emission of 420 nm.The lamps were separated about at 12 cm of height from the containers having the exposure of 3200 lx intensity over the samples. During photocatalytic degradation samples were equally exposed to 420 nm light irradiation and examined at a different time intervals.

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Characterization. As quenched and fluorinated SBBO glass ceramics were characterized using X-ray diffraction (XRD) at 9 kW rotating anode (Cu Kα) based Rigaku powder diffractometer. Raman scattering was performed on HORIBA (Model -LabRAM HR Evolution) with a grating

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of 1800 lines/mm and a Peltier cooled charge coupled device (CCD) detector working at -60o C. Roughness of different glass ceramics were tested by using HOMMEL-ETAMIC W5 instrument. Turbo Datawave 1.51 software is used to evaluate all the parameters and extensive graphical profile analysis obtained through the HOMMEL-ETAMIC W5 instrument. The micrographs were recorded with FE-SEM Inspect™S50. Optical absorption as

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well as band gap calculation was estimated by using double beam UV–visible

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spectrophotometers SHIMADZU-2450. The contact angle was measured with a Ramé-Hart

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III. RESULTS AND DISCUSSION

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Goniometer model 250.

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Fig. 1 represents the X-ray diffraction (XRD) patterns of as quenched SBBO and fluorinated

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SBBO glass ceramics named as SBBO-HF-1, SBBO-HF-2, SBBO-HF-3 and SBBO-HF-4, respectively. The XRD of as quenched SBBO shows few broad as well as sharp peaks. Broad

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peaks bellow 2θ at 25o is due to amorphous nature of glass. On the other hand, sharp peaks represent the crystallization in the sample which shows the ceramic nature. Peak positions are

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well matched with the JCPDS file no. 60-0269 and corresponded to (004), (113), (006), (306),

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(413), and (103) planes of SrBi2B2O7. It is important to note that it was desirable to have only glass phase after quenching but not any crystals. In order to avoid crystallization in our glass sample, we could perform quenching from higher temperature (above 1000oC). However, as bismuth starts evaporation above 1000 oC, we could not perform melting at elevated temperature. Interestingly as quenched glasses were yellowish and transparent even after having surface

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crystallization. As compared to JCPDS file no. 60-0269, peak at 41.28o for (006) plane shows enormously high intensity, relatively. Similar observations have also reported by Majhi et.al.[31].This anomaly in peak intensity is reported due to isolated growth of crystals in glass

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matrix with preferred orientation along (006) plane [31]. XRD pattern of SBBO-HF-1 glass ceramic is exactly matched with the diffraction peaks corresponding to (111), (200), (220), (311), (222), (400), (331) and (420) planes, which belongs to the cubic structure of BiO0.51F1.98 (JCPDS file no. 024-0147). Increasing the fluorinetion leads the XRD peaks shifting towards the lower angle side for SBBO-HF-2, SBBO-HF-3 and SBBO-HF-4 samples, respectively. Shifting

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of a typical peak about the plane (220) is shown in the magnified 2θ range of XRD from 41o

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to 45o, as shown in Fig. 1. Figure demonstrates the broad view of the peak related with the

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(006) plane of SrBi2B2O7 phase in SBBO sample as well as the peak of (220) plane related to

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BiO0.51F1.98 phase in HF etched SBBO samples. During the precise comparison of XRD patterns between SBBO-HF-1 to SBBO-HF-4 samples and the available JCPDS files and

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literatures[23, 24, 26, 32-34], the shifted peak positions and intensity pattern of SBBO-HF-4

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sample match well with cubic α-BiF3 structure (JCPDS file no. 073-1988). The peak patterns of

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α-BiF3 than BiO0.51F1.98 are almost same, belong to similar planes, and only differ by peak positions of ~0.1o sifted towards lower angle side. The peaks of SBBO-HF-1 glass

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ceramicsample is well matched with the cubic structure of BiO0.51F1.98 and gradually transform to cubic structure of α-BiF3 with constant peak shifting towards lower angle side in conjunction

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with the fluorination of SBBO-HF-2, SBBO-HF-3 and SBBO-HF-4, respectively. No other characteristic peaks of any intermediate or secondary phase could be detected from the diffraction patterns of any fluorinated SBBO glass ceramic samples. Initially with dilute etching of HF the XRD of SBBO-HF-1 and SBBO-HF-2 samples show broad peak with

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noisy base line, which is due to thin layer of crystals growth of BiO0.51F1.98/BiF3 over the surface of SBBO glass ceramics. However, increasing the etching concentration of the HF, maximizes the crystal growth of BiF3, resulting sharp XRD peaks with less noisy baseline

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for SBBO-HF-3 and SBBO-HF-4 glass ceramics, respectively. In addition, we have not seen any XRD signals of SrBi2B2O7 crystals related with the as quenched SBBO glass ceramic in the XRD patterns of any HF etched SBBO glass ceramics. This must be due to the uniform coating of BiO0.51F1.98/BiF3 over the SBBO glass ceramics which prevent the X-ray to penetrate as well as overcome to the diffraction intensity of SrBi2B2O7 related phase.

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Due to the almost similar structures of BiOF and BiF3 in the solid solution of BiOxF(3−2x),

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the exact compositions of these compounds are very hard to distinguish by XRD, where ‘x’

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can be as low as 0.02. As well, to prepare the single compound of either BiOF or BiF 3 is

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very difficult. However, recently Bervas et al. have been able to prepare pure tysonite BiF3

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through high-energy milling by introducing defects in the lattice [23]. Feng et al. have

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reported formation of BiF3 (JCPDS: 51-0944) by a simple water-bath method by showing no impurity of Bi2O3, BiOF and Bi [26]. For this, they maintain the molar ratio of Bi:F is

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1:3, bellow to this the impurity of Bi2O3 and BiOF are appeared. Similarly, just by using NH4F in different molar ratios (RF = F/Bi), Kan et al. have obtained pure BiF3 (JCPDS: 73-

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1988) at RF= 8 [24]. Bellow to RF=8, samples consist small amount of Bi7F11O5 in addition

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to the BiF3 compound. Pure phase formation of BiF3 (JCPDS: 73–1988) is also reported by Yang et al. by mixing the BiOCl and NH4F in the ratio of 1: 6 (Bi: F) [32]. Sarkar et al. have synthesized BiF3 nanoparticles of cubic structure (space group Fm3m, JCPDS: 731988) by using hydrothermal method [33]. Poly vinyl pyrrolidone (PVP) is used for the encapsulation of these nanoparticles for the stabilization. In another work, Zhao et al. have

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reported the evolution of the BiF3 nanocrystals via a simple novel solvent extraction method [34]. In this work, the formation of solid solution BiOxF(3−2x) or BiO0.51F1.98/BiF3 crystals over

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the surface of SBBO glass ceramics is due to the presence of SrBi 2B2O7 nano crystals in the amorphous glass matrix. The Bi present in SrBi2B2O7 nano crystals easily reacts with concentrated HF solution and dissolves the Bi from SrBi2B2O7 nano crystals in accordance to the Thomson–Freundlich equation. In HF solution, positively charge Bi3+ion react with F– ion and form BiF3. However, in diluted HF solution immediately the BiF3 hydrolyze with

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water and form BiOF structure (eqs (1 & 2)).Presence of BiO0.51F1.98 phase in SBBO-HF-1

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sample is due to the etching treatment of SBBO glass ceramics using diluted/hydrated HF.

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Etching through HF at low concentration results in partial fluorination and BiO0.51F1.98 phase

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formation. While etching of SBBO surface by concentrated HF (ie. 3M, 6 M and 12 M) leading

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deep diffusion in the glass with advanced fluorination and phase transformation to α-BiF3,

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relatively. However, at highest etching concentration used in present work for SBBO-HF-4 glass ceramic, due to the limitation of XRD resolution and lacking of any other specified

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characterization techniques, we cannot claim for pure structure of cubic-αBiF3. It would be the mixture of BiO0.51F1.98 and cubic-αBiF3 phases. Increasing the HF concentration only

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leads more and more crystal growth of cubic-αBiF3 than BiO0.51F1.98. 2Bi3+ + 6F- → 2BiF3

(Concentrated HF)

2Bi3+ + 2H2O+ 6F-→ 2BiOF↓+ 4HF (Diluted HF)

(1)

(2)

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Fig. 2 (a) shows the UV–Vis absorption spectra of as quenched and fluorinated SBBO glasses. SBBO having gradual absorption of light from wavelength 800 nm to 550 nm range and depicted sharp absorption increase from 550 nm to 420. Wavelength between 420 nm to 225 nm the

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absorption remains constant and shows few small characteristic shoulders of SBBO and below 225 nm the absorption decreases significantly. Fluorinated SBBO glasses show sharp absorption than SBBO in 800 nm to 420nm wavelength range, relatively. Apart from SBBO, fluorinated glasses show another sharp absorption edge between 325 nm to 275 nm wavelength. This absorption is must be due to the crystals grown after fluorination. Below 275 nm, all fluorinated

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glasses show similar absorption to SBBO. Further the optical band gap energy is determined

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from the absorption coefficient(α) measured as a function of the incident photon energy E (hν) by

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using the Kubelka-Munk model (Fig. 1(b)) [35]. Direct bandgap of as quenched SBBO glass

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ceramic is calculated by plotting (αhν)2 vs. E (hν) and found to be 2.7 eV. In correspond to two band edges in fluorinated SBBO glasses, two band gaps E1 and E2 are calculated and shown in

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the Fig. 2 (b). Band gap E2 increases with the fluorination. Fig. 3 (a to f) demonstrates the typical digital optical photographs of as quenched and fluorinated

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SBBO glasses along with the histogram of their surface roughness. As quenched SBBO glass ceramic looks transparent with yellowish appearance (Fig. 3 (a). After fluorination, SBBO glass

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ceramic loses its transparency and the sample SBBO-HF-1 appears as translucent (Fig. 3 (b)).

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However, further fluorinations recover the transparency in SBBO-HF-2, SBBO-HF-3 and SBBO-HF-4 glass ceramics, respectively (Fig. 3 (c-e)). SBBO-HF-4 sample shows almost similar transparency and yellowish look to as quenched SBBO glass ceramic. In correlation of transparency, first the surface roughness of SBBO glass ceramic increases with fluorination for

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SBBO-HF-1 sample and decreases for SBBO-HF-2, SBBO-HF-3 and SBBO-HF-4 glass ceramics, respectively, shown as Fig. 3 (f). Surfaces morphology of as quenched and fluorinated SBBO glass ceramics is observed through

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FE-SEM along with the color mapping for the elements detection, shown in Fig. 4 (a-e).The focused views corresponding to the local region of images shown in Fig. 4 (a-e) are shown in Fig. 5 (a-e). SEM images of as quenched SBBO are having nanosized particles of order 10-20 nm, which are evenly distributed throughout the surface, as shown in Fig. 4 (a) and Fig. 5 (a). SEM images of SBBO-HF-1 for fluorinated SBBO glass ceramics how spherical structures with

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0.8 to 1.2 µm diameters, shown as Fig. 4 (b) and Fig. 5 (b). Fig. 5 (b) is the enlarged image of

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Fig. 4 (b). Image shown in Fig. 5 (b) display the spheres are densely packed and uniformly

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distributed throughout the surface and the outer shell of the sphere is composed of semi

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circular plates of nano size. Plates are emerging from the sphere and spread homogeneously

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over each sphere. These plates are crossing each other and forming net like structure over the

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sphere. More fluorination reduces the size of the spheres possessing the diameter in-between 50 to 100 nm range for SBBO-HF-2 sample, shown in Fig. 4 (c) and Fig. 5 (c). Fig. 5 (c) is the

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enlarged image of Fig. 4 (c). Spheres are agglomerated and densely packed to each other. Along with spherical structures, two dimensional elongated semi circular nanoplates are also grown and

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dispersed uniformly. Further increasing fluorination ensuing two types of morphologies in

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SBBO-HF-3 sample, images shown in Fig. 4 (d) and Fig. 5 (d). Both of these morphologies possess very small domains throughout the surface. We have highlighted these domains as 'A' and 'B', clearly shown in Fig. 4 (d). However, it was difficult to distinguish between both type of morphologies image at present scale therefore we also obtained images at higher magnification corresponding to highlighted domains 'A' and 'B', as shown in Fig. 5 (d). One is similar to the

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morphology of SBBO-HF-2 sample containing spheres of smaller diameter in the range of 40 to 80 nm and other includes the cluster of cubes in addition to small spherical particles. The edges of the cubes are varied from 100 nm to 200 nm. Surface morphology of SBBO-HF-4 sample, for

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maximum fluorination, displays bigger cubes with 150 nm to 250 nm size, images are shown in Fig. 4 (e) and Fig. 5 (e). A close view indicates few agglomerated particles of 10 to 20 nm size are also spread in between the cubes. EDS color mapping discloses the quantification and dispersion of Sr, Bi, O and F elements in as quenched and fluorinated SBBO glass ceramics, shown as inset of Fig. 4 (a-e). Dark red color in all samples is due to dominating Bi while pink

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and yellow colors represent the O and Sr, respectively. Under the detection limit of EDS,

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quantification of B (lighter element) is not possible. Color mapping of fluorinated SBBO glass

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ceramics reveals an additional blue color with homogeneous distribution. Blue color belongs to

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F element which emerges further along with the fluorination of SBBO glass ceramic, respectively. Fig. 6(a-b) is the cross-sectional black/white image across the thickness of

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fluorinated plate of SBBO-HF-4glass ceramic. The upper portion of the plate appears as

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white colored layer having a uniform thickness of ~3 μm, while the lower part appears as

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gray (black) (Fig. 6(a)). Line mapping for elements detection along the thickness up to 6 μm confirms Bi and F in major fraction with very less O and Sr constituents in white colored layer of

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thickness 3 μm, as shown in Fig. 6 (b). Along the line mapping above 3 μm thickness the quantity of F reduces significantly with considerable increase in Sr content. Concentration of

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Bi and O remains almost constant above 4 μm throughout the line mapping. XRD reveals the phase alteration from BiO0.51F1.98 to α-BiF3 structure. FE-SEM results demonstrate a clear transformation of morphology from spherical to cubical structure along with the fluorination of SBBO glass surface. First, the size of the spheres reduces with fluorination for

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SBBO-HF-1, SBBO-HF-2 and SBBO-HF-3 samples, respectively. Alongside the spheres, cubic morphology appears in the same SBBO-HF-3 glass ceramic. Finally, fluorination leads the removal of spherical morphology completely in SBBO-HF-4 sample, only having cubic

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morphology of bigger size. In addition to above structural and microstructural transformation along with the fluorination of the surface of SBBO glass ceramic, we have also observed the improved transparency. As compare to transparent as quenched SBBO glass ceramic, first we observed translucent glass ceramic of SBBO-HF-1 sample along with fluorination. While, further fluorination lead the improvement in transparency, respectively. This unique progression

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from transparent to translucent and then again transparent is because of the decrease in the

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mismatching of refractive indices in-between growing crystals and the residual glass [36]. In

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present case the high transparency with fluorination could be due to the control of homogeneous

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nucleation and growth of α-BiF3 crystals with cubic shape, however, cause of recovery of transparency with fluorination need to be further investigated. Similar results of high

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transparency with cubical crystals than spherical one is reported by the research group of Edgar

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D. Zanotto [36]. They showed cubical as well as spherical crystallization up to 97% volume

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fraction in glass–ceramics with larger size of the crystals of micrometric order, respectively. According to them the high transparency in cubical crystallized glass is due to the synchronized

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change in the compositions of glass matrix and crystals during crystallization so that the

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respective refractive indices do not change considerably and remain almost similar to glass. Fig. 7 (a-f) illustrates the contact angle of water droplets of size 15 µl over the several regions of each plate of glass ceramics for as quenched and fluorinated SBBO glass ceramics. Water contact angle is the measurement of hydrophilicity or hydrophobicity. Surface contact angle bellow 90o represents the hydrophilic and above is for the hydrophobic materials. The measured

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water contact angle for as quenched and fluorinated plates of SBBO glass ceramics are 63.3o ,70.1o, 96.6o, 101.6o, and 120.4o, respectively. This result suggests that the fluorination lead hydrophilic to hydrophobic surface of as quenched SBBO glass ceramic. Further, the water

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droplet contact angle over the surface of SBBO-HF-4 glass ceramic is measured under the light illumination of 420 nm wavelength for 60 minute duration, shown as Fig. 7 (g). Contact angle is estimated at every 15 min, shows decreasing trend 120.4o, 101.2o, 88.1o, 50.2o and 27.9o. Thus, under light illumination the water droplet spread over the glass surface with time and the wettability transforms from hydrophobicity to hydrophilicity. This photo-induced surface

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important required aspect for self cleaning applications.

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wettability of fluorinated SBBO glass ceramic is due to its photocatalytic nature, which is an

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Next, to analyze photocatalytic performance, we have obtained absorption study from UV-vis for

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as quenched and fluorinated plates of SBBO glass ceramics immersed in MB solution. First, we

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have studied the adsorption desorption equilibrium for the samples immersed in MB solution

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subjected under dark for 6 hr and then photocatalytic degradation under 420 nm irradiation, respectively, shown as Fig. 8 (a-e). Using the above associated UV-vis absorptions further we

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have studied the rate of adsorption under dark as well as the photocatalysis under 420 nm light irradiation, shown as Fig. 9. Here ‘Co’ denotes the initial concentration of MB while ‘Ct’ is for

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the concentration at time ‘t’. Under dark, the initial adsorption of dye is due to the strong

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electrostatic interaction between dye molecules and the surface of glass ceramic. Fluorinated SBBO glass ceramics show more adsorption of MB than as quenched SBBO glass ceramic is due to the increased surface roughness. For fluorinated SBBO glass ceramics, under 420 nm light irradiation the photocatalytic degradation initiated and increases significantly. It is important to note that the photolysis of MB under 420 nm light irradiation is negligible. The

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reaction rates confirm the superior photocatalysis of MB for all fluorinated SBBOglass ceramic than as quenched SBBO glass ceramic. We have calculated the reaction rate constant corresponding to the photocatalytic degradation of MB (not shown here) using the reaction

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kinetics represented as eq. ln(Ct/Co) = kf t. Where, kf is the reaction rate constant. The reaction rate constant for the degradation of MB corresponding to as quenched and fluorinatedSBBOHF-1, SBBO-HF-2, SBBO-HF-3 and SBBO-HF-4 glass ceramics are 0.00292, 0.01509, 0.02226, 0.01607 and 0.01451 min-1, respectively. Fluorination leads improvement in photocatalytic reaction rate from SBBO, SBBO-HF-1 and SBBO-HF-2, respectively. While with

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further fluorination, photocatalytic reaction rate decreases for SBBO-HF-3 and SBBO-HF-4,

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respectively. Relatively the fast decrease in Ct/Co curve and increasing photocatalytic reaction

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rate from SBBO to SBBO-HF-2 is due to the reducing particle size and bandgap E1 of

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BiO0.51F1.98 along with structural transformation to cubic α-BiF3. Further fluorination causes slow relative decrease in Ct/Co and photocatalytic reaction rate is due to increasing relative

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hydrophobic nature of SBBO-HF-3 and SBBO-HF-4 glass ceramics. Here hydrophobic

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properties is performing contrary to photocatalytic property of the fluorinated glass ceramic and

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therefore we obtained the optimized photocatalytic and hydrophobic results in fluorinated SBBO-HF-2 glass ceramic for self-cleaning applications. Thus our results demonstrate the

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collective effect of fluorination on the surface of SBBO glass ceramic and stabilization of cubic α-BiF3 structure with cubic morphology which lead the improvement of transparency and

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photoinduced wettability from hydrophobicity to hydrophilicity. IV. CONCLUSIONS A systematic, uniform and tuneable growth of α-BiF3 from BiO0.51F1.98 structure over SrO-Bi2O3B2O3 glass ceramic was obtained via fluorination through controlled etching. The transparent

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coating of BiF3 over SrO-Bi2O3-B2O3 glass ceramic improved the photo induced wettability and visible light driven high photocatalytic performance. Here we proposed a easy method of fluorination for glass materials and their potential applications for industrial photocatalytic and

ACKNOWLEDGMENT RV thanks to DST-SERB, New Delhi for funding this project. REFERENCES

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self-cleaning applications instead of using traditional powder or coating of thin films.

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FIGURES

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FIG. 1.(a)X-ray diffraction patterns of as quenched SBBOglass ceramic and after etching SBBO glass ceramics from HF solutions of different concentration named as SBBO-HF-1, SBBO-HF-2, SBBO-HF-3,SBBO-HF-4, respectively.

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FIG. 2.(a)Absorption vs wavelength spectra and (b) α2(hν)2 vs. hν plots for band gap (Eg) calculation of as quenched SBBO glass ceramic andetched SBBO glass ceramics from HF solutions of different concentration named as SBBO-HF-1, SBBO-HF-2, SBBO-HF-3,SBBOHF-4, respectively.

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FIG. 3. (a)-(e) typical digital photographs and (f) surface roughness of as quenched SBBO glass ceramic and etched SBBO glass ceramics from HF solutions of different concentrations named as SBBO-HF-1, SBBO-HF-2, SBBO-HF-3,SBBO-HF-4, respectively.

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FIG. 4.SEM images along with the colour mapping of (a) as quenched SBBO glass ceramic and etched SBBO glass ceramics from HF solutions of different concentrations named as (b) SBBOHF-1, (c) SBBO-HF-2, (d) SBBO-HF-3,(e) SBBO-HF-4, respectively.

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FIG. 5. Focused SEM images corresponding to the images shown in fig. 4 for (a) as quenched SBBO glass ceramic, and etched SBBO glass ceramics from HF solutions of different concentrations named as (b) SBBO-HF-1, (c) SBBO-HF-2, (d) SBBO-HF-3 (showing two different highlighted regions A and B related to fig 4(d)), (e) SBBO-HF-4, respectively.

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FIG. 6. (a) SEM image of the cross-section along the thickness ofSBBO-HF-4 glass ceramic, and (b) elemental mapping along the green line shown in the SEM image.

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FIG. 7. Water droplet contact angles for (a) as quenched SBBOglass ceramic, and etched SBBO glass ceramics from HF solutions of different concentrations named as (b) SBBO-HF1,(c) SBBO-HF-2, (d) SBBO-HF-3,(e) SBBO-HF-4, respectively. (f) The bar chart of contact angle for all SBBO and SBBO-HF glass ceramics. (g) Water droplet contact angle on SBBO-HF4 glass ceramic under 420 nm light irradiation at different time intervals.

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Figure 8. Absorption vs wavelength spectra for (a)photolysis of MB solution, and the adsorption-desorption of MB solution under dark as well as photocatalysis degradation under 420 nm irradiation for (b) as quenched SBBO glass ceramic, and etched SBBO glass ceramics from HF solutions of different concentration named as (c) SBBO-HF-1, (d) SBBO-HF-2, (d) SBBO-HF-3 and (e) SBBO-HF-4 glass ceramics, respectively.

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Figure 9. The Ct/Co curves for the adsorption-desorption of MB solution under dark as well as photocatalysis degradation under 420 nm irradiation for as quenched SBBO glass ceramic and etched SBBO glass ceramics from HF solutions of different concentration named as SBBO-HF1, SBBO-HF-1, SBBO-HF-1, SBBO-HF-1.

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