Synthesis, characterization, and photocatalytic ...

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Cite this: New J. Chem., 2017, 41, 2498

Synthesis, characterization, and photocatalytic degradation of Rhodamine B dye under sunlight irradiation of porous titanosilicate (TS)/bismuth vanadate (BiVO4) nanocomposite hybrid catalyst† Ajay Kumar Adepu, Vamsi Katta and Venkatathri Narayanan* Novel mesoporous titanosilicate/bismuth vanadate (BVTS) inorganic hetero-structures were successfully synthesized under mild conditions. The synthesized mesoporous BVTS hetero-structures were characterized by various analytical techniques for structural and chemical properties. We confirmed from TEM results that a good heterojunction formed between titanosilicate and bismuth vanadate. More significantly, the BVTS hetero-structures exhibited enhanced photocatalytic activity in the degradation of Rhodamine B dye (RhB) under sunlight irradiation. The optimum photocatalytic activity of BVTS 1 under visible light is almost 3.5 and

Received 6th January 2017, Accepted 15th February 2017

6.5-fold higher than pure titanosilicate (TS) and pure bismuth vanadate (BVO), respectively. These composites and their photocatalytic Rhodamine-B dye degradation under sun light irradiation is reported for the first

DOI: 10.1039/c7nj00071e

time. The popularity of fabrication of heterojunction structures is an inevitable trend. A RhB solution (100 mL, 105 M) was degraded in 60 min using the BVTS nanocomposites photocatalyst with visible light irradiation.

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The BVTS nanocomposites responded well and thus remarkably enhanced photocatalytic efficiency.

1. Introduction Organic dyes in textile and other industrial effluents are major environmental contaminants. As many dyes are highly watersoluble, traditional treatment methods including flocculation, activated carbon adsorption, and biological treatment do not work efficiently. Recently, a photocatalysis method played an important role in the degradation of organic dyes in wastewater.1,2 Compared with other treatments, photocatalytic degradation has several advantages such as using an environmentally friendly oxidant O2, complete mineralization, no waste disposal problems, and using only mild temperature and pressure conditions.3–6 Moreover, the photocatalytic degradation can work even at much lower concentrations of organic dyes. Therefore, photocatalytic degradation is a promising solution to degrade organic dyes. Early studies on photocatalysts mainly focused on the Ultraviolet-driven TiO2 photocatalyst.7,8 However, UV light takes up only ca. 4% of the solar energy while visible-light is ca. 43%; thus, visible-lightdriven photocatalysts are a new focus.9 Our research group is working on photocatalytic studies of titanosilicate (TS), which was exclusively used in the past as the catalyst in cyclohexanone ammoximation because of its extremely high catalytic activity and selectivity.10–12 It has been proven that Department of Chemistry, National Institute of Technology, Warangal 506 004, Telangana, India. E-mail: [email protected]; Tel: +91 9491319976 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj00071e

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the active sites inside the mesoporous channels of TS are easily accessible to reactants when particle size is controlled in a range of 100–200 nm. Titanosilicate is a semiconducting material that is chemically activated by light. It has an energy band gap of 3.0–3.2 eV, similar to titanium dioxide. We successfully synthesized highly porous titanosilicate by a template-assisted method using hydrothermal synthesis. Characterized by wide angle XRD, HRTEM, FT-IR, UV-vis DRS, and calculated band gap, it was similar to titanium dioxide. Titanosilicate (TS) mesoporous material was found to be able to catalyze the oxidation of aromatics, olefins, and alcohols with hydrogen peroxide.13–15 The new discovery is that TS is also useful for photocatalytic dye degradation studies and the material is a composite. In each reaction, TS showed different physicochemical properties. From the photocatalysis point of view, it was previously reported that it caused photocatalytic decomposition of NO. Also, Mesoporous TS/reduced graphene oxide composites were reported by ThuyDuong Nguyen-Phan et al.16 to have a doping effect on methylene blue dye removal from water. Since BiVO4 was found to be an active photocatalyst for O2 evolution from aqueous AgNO3 solution under visible-light irradiation, more and more attention has been attracted to the synthesis of a visible-light-driven BiVO4 photocatalyst.17–25 Among the three crystalline phases of BiVO4, tetragonal zircon (z-t), tetragonal scheelite (s-t), and monoclinic scheelite (s-m) structures,26 it is found that the monoclinic scheelite BiVO4 (m-BiVO4) exhibits much higher photocatalytic activity than the

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other two tetragonal phases.27 Therefore, many methods have been employed for the synthesis of m-BiVO4, such as a solidstate reaction, hydrothermal or solvo-thermal method, aqueous method, ultrasound- or microwave-assisted route, metalorganic decomposition, flame spray pyrolysis, and a solution combustion method.28–41 Compared with other methods, the aqueous method provided a milder environment for the synthesis of monoclinic BiVO4 and the reaction parameters, as well as properties of the products, could be easily tuned. Herein, we report a facile, effective synthesis of novel and highly porous stable titanosilicate/bismuth vanadate inorganic hybrid photocatalysts with a Z-scheme mechanism to achieve an improved visible light responsive photocatalysis with prolonged life time of charge carrier transfer. The novel BVTS inorganic hybrid nanocomposites were characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS or EDX), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and UV-visible diffuse reflectance spectroscopy (UV-vis DRS). Based on our experimental results, we proposed a plausible mechanism and this is discussed for enhanced visible light photocatalysis. However, to the best of our knowledge there is no previous report on the synthesis of a BVTS inorganic hybrid nanocomposite photocatalyst.

2. Experimental section Tetraethyl orthosilicate (TEOS, 98.0%) and titanium isopropoxide (TTIP, 97.0%), bismuth nitrate (Bi(NO3)35H2O, 99.0%), and Rhodamine B (95.0%) were purchased from Sigma-Aldrich (USA). Ammonium vanadate (NH4VO3, 98.0%), cetyltrimethylammonium bromide (CTAB, 98.0%), and ammonium hydroxide solution (26.0%) were purchased from SD fine chemicals Ltd. All aqueous solutions were prepared with Millipore purity water. 2.1

Synthesis of photocatalysts

Porous TS was synthesized using a hydrothermal method. In a typical procedure, 0.3 g of CTAB was dissolved in 60 mL of Millipore water to form a homogenous solution and then 1.38 mL of ammonium hydroxide was added. About 0.60 g of tetraethyl orthosilicate (TEOS) was added dropwise into this mixture under vigorous stirring, followed by the addition of 0.40 g of titanium isopropoxide (TTIP); then stirring was continued for another 6 h. The mixture was placed into a 100 mL stainless steel autoclave and heated at 120 1C for 12 h. The solid products were collected by centrifugation and washed with water and warm ethanol, followed by drying at 80 1C overnight. The resulting powder was calcined at 550 1C for 5 h using a programmable tubular furnace. In a typical procedure, 2 mmol of NH4VO3 was first dissolved into 20 mL of deionized water at 96 1C and then the solution was cooled to room temperature. The NH4VO3 solution was added dropwise into a 250 mL round bottom flask (RB) containing 40 mL of CTAB solution (0.05 M) in an oil bath at 60 1C. Afterwards, 2 mmol of Bi(NO3)35H2O was added into 20 mL of deionized water and stirred for about 10 min to form a

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hydrolyzed white floccule. The floccule suspension was added dropwise into the RB flask at 60 1C with stirring. After that, the flask was connected to a condenser and heated in oil bath at 80 1C for 12 h. The final product was centrifuged, washed with deionized water and absolute ethanol for several times, and dried in air. The as-prepared m-BiVO4 crystals were reported and denoted as BVO.42 An appropriate amount of TS was suspended in 50 mL of water and ultrasonicated for 1 h. The TS solution was added dropwise into a 250 mL RB flask containing 40 mL of CTAB solution (0.05 M) in an oil bath at 60 1C. Separately, 2 mmol of NH4VO3 was first dissolved into 20 mL of deionized water at 96 1C and then the solution was cooled to room temperature. The NH4VO3 solution was added dropwise into a 250 mL flask containing CTAB and TS solutions. Afterwards, 2 mmol Bi(NO3)35H2O was added into 20 mL of deionized water and stirred for about 10 min to form a hydrolyzed white floccule. The floccule suspension was added dropwise into the flask at 60 1C with stirring. After that, the flask was connected to a condenser and heated in an oil bath at 80 1C for 12 h. The final product was centrifuged, washed with de-ionized water and absolute ethanol for several times, and dried in air. The as-prepared TS/BiVO4 composites BVTS 1 (TS/BiVO4 in a 2 : 1 ratio), BVTS 2 (TS/BiVO4 in a 1 : 1 ratio), and BVTS 3 (TS/BiVO4 in 1 : 2 ratio) were prepared and reported. The titanosilicate was named as TS and BiVO4 as BVO in a composite. 2.2 Characterization Powder X-ray diffraction (PXRD) patterns were recorded on a PAN Analytical Advance X-ray diffractometer, Netherlands using Ni filtered Cu Ka (l = 1.5406 Å) radiation in a 2y scan range between 101 and 701. High resolution FESEM with EDX measurements were conducted on a Carl Zeiss model Merlin Compact 6027 FESEM, Germany with beam voltage of 3.0 kV. The transmission electron microscopic (TEM EDX) analyses were carried out on an FEI Tecnai G2 Spirit transmission electron microscope, Netherlands with an acceleration voltage of 200 kV. Fourier transform infrared (FT-IR) spectra were recorded in transmission mode from 4000 to 400 cm1 on a PerkinElmer Spectrum 100 FT-IR spectrophotometer, USA using the KBr pellet technique. UV-vis diffuse reflectance spectra (UV-vis DRS) were obtained on a Thermo Scientific Evolution 600 diffuse reflectance spectrophotometer, USA and BaSO4 was used as a reference standard. UV-visible absorption spectra (UV-vis) were recorded on a Thermo Scientific Evolution 600 UV-vis NIR spectrophotometer, USA. 2.3

Dye photodegradation experiment

Rhodamine B (RhB) shows a major absorption band centered at 553 nm which was used to monitor photocatalytic degradation of RhB. Photocatalytic efficiencies of BVTS were evaluated by the degradation of RhB under sunlight irradiation. Then 0.1 g of BVTS photocatalyst was added into 100 mL of RhB solution with a concentration of 5 ppm. Prior to irradiation, solutions suspended with photocatalysts were stirred under dark conditions for 30 min to ensure that the surface of the catalyst was saturated with RhB. During photocatalytic processes, a sample

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was periodically withdrawn, centrifuged to separate the photocatalyst from the solution, and used for absorbance measurements. Concentrations were recorded with a PerkinElmer (Lamda25) UV-vis spectrophotometer.

3. Results and discussion 3.1

Morphology and composition characterization

We developed hybrid nanocomposites of porous titanosilicate/ bismuth vanadate for inorganic photocatalysts applications of RhB dye degradation using sunlight irradiation. Fig. 1 shows the XRD patterns of pure TS, pure BiVO4, and inorganic hybrid nanocomposites BVTS 1, BVTS 2, and BVTS 3 which were used to elucidate phase and structural parameters. With pure BiVO4 all the diffraction peaks were indexed to m-BiVO4 which was identical to the standard (JCPDS no. 14-0688). Characteristic splitting of peaks at 18.51, 351, and 461 of 2y was observed for m-BiVO4;42 the sharp and narrow diffraction peaks indicated a high crystallinity of the m-BiVO4. A broad peak centered at 231 was ascribed to mesoporous TS. Similar bands were found for the BVTS composites. Moreover, no other impurity phase was seen, indicating the BVTS to be a two-phase composite. Morphological features of pure TS, pure BiVO4 and BVTS are shown in Fig. 2. Spherical shapes with the highly porous nature of TS were observed in both pure TS and BVTS composites. The morphology and particle size of the synthesized BVTS sample was examined by taking FE-SEM images at different magnifications. Particle size of synthesized hetero structures was estimated by taking FE-SEM images as shown in Fig. 2. Images obtained at higher magnifications suggested that the nanoparticles formed aggregates or clusters. An EDS spectrum indicated that the synthesized coupled catalyst is only made up of Ti, Si, Bi, V, and O; no other notable impurities were observed. From the FE-SEM images we concluded that the synthesized sample was composed of agglomerated particles. Porous pure TS is well distributed over the BiVO4 surface and forms a heterojunction between this inorganic TS and BiVO4 which enables facile electron transfer for various catalytic applications.

TEM image (Fig. 3a) is porous structure of pure titanosilicate. Fig. 3(b and c) indicated BVTS 1 is a hybrid nanocomposite; from these results we confirmed that a good heterojunction formed between TS and BVO. HRTEM studies confirm the formation of the interface between Titanosilicate and BiVO4 in the composite system (Fig. 3d). Furthermore, the selected area electron diffraction (SAED) pattern (inset of Fig. 3d) shows a faint but full diffraction ring which is indexed to the characteristic planes of Titanosilicate/ BiVO4. However, Titanosilicate/BiVO4 hybrid nanocomposites also exhibit a certain degree of particle agglomeration. FT-IR spectra of the prepared samples are shown in Fig. 4. Pure BiVO4 has a broad band between 650 and 850 cm1, which is attributed to Bi–O and V–O vibrations42 and FT-IR spectra of the synthesized pure TS as-synthesized sample shows peaks around 1700 and 3430 cm1 corresponding to carboxyl and hydroxyl groups, respectively. The strong peaks near 1100, 802, and 467 cm1 agree with a Si–O–Si bond which implies the condensation of silicon alkoxide. From the spectra, it was observed that the Ti peak (TS) formed at wavelength 960 cm1. Thangaraj et al. demonstrated that the IR band around 960 cm1 exhibited by TS zeolites can also be attributed to a stretching mode of a [SiO4] unit bonded to a Ti4+ ion [O3SiOTi].43 The characteristic peaks of Titanosilicate and BiVO4 also were retained in TS/BiVO4 in organic hybrid nanocomposite samples. The energy band structure feature of a semiconductor is considered as a key factor in determining its photocatalytic activity. Fig. 5 shows the UV-vis diffuse reflectance absorption spectra of the as-prepared samples. All the samples show strong absorption in the UV and visible light regions. The steep shape of the spectrum indicates that visible light absorption is not due to transition from the impurity level, but instead is due to the band gap transition.44,45 For a crystalline semiconductor, the optical absorption near the band edge follows the equation ahn = 14A (hn  Eg)n/2, where a, n, Eg and A are an absorption coefficient, a light frequency, the band gap, and a constant, respectively. In this work, the band gaps are estimated to be 3.1 eV, 2.2 eV, 2.40 eV, 2.36 eV, and 2.34 eV from the absorption edge corresponding to pure TS, pure BiVO4 (BVO), BVTS 1, BVTS 2, and BVTS 3 samples, respectively. Compared with the morphology, the samples with slab morphology possess a much higher band gap energy (2.40 eV), which may be ascribed to a size effect and crystal defects. 3.2

Fig. 1 XRD patterns of pure Titanosilicate, pure BiVO4, and BVTS nanocomposites.

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Photocatalytic activity and the mechanism

These results indicate that the visible-light-response of our BVTS 1 composite photocatalyst is greatly improved by coupling an appropriate amount of TS. Therefore, the BVTS 1 composite photocatalyst exhibits a higher photocatalytic efficiency than the others. The photocatalytic performance of TS/BiVO4 powder synthesized by the combustion route was evaluated in terms of the photocatalytic degradation of RhB in aqueous solution under sunlight irradiation. Fig. 6 shows the absorption spectra of RhB dye solution with TS/BiVO4 nanocomposite under direct sunlight irradiation for different time intervals. The dye degraded at a faster rate under sunlight (60 min); concentrations were recorded on a PerkinElmer (Lambda 25) UV-vis spectrophotometer.

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Fig. 2

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FESEM images of (a) pure TS, (b) pure BiVO4, (c) BVTS 1 (d) BVTS 2, and (e) BVTS 3; (f) EDX analysis of TS/BiVO4 nanocomposites.

To demonstrate the influence of TS on photodegradation, the degradation of RhB with pure BiVO4 was studied under visible light. Fig. 6 shows the variation of C/C0 with time for pure TS, Pure BiVO4, and TS/BiVO4 nanocomposites, where C0 is the initial concentration of the dye solution and C is the concentration of the dye solution with respect to the degradation time ‘t’. It is obvious from Fig. 6 that the rate of degradation was significantly faster with the composite as compared to pure BiVO4, which shows the beneficial impact of TS on photocatalytic performance of the composite. Photocatalytic activity of the TS/BiVO4 composite prepared by the present method is higher than the BiVO4-based composites prepared by the hydrothermal method.

Photocatalytic performances of TS, BiVO4, and TS/BiVO4 composites were evaluated by photocatalytic removal of RhB in liquid phase as shown in Fig. 6. The blank experiment (absence of photocatalysts) demonstrated that no RhB was photodegraded by sunlight. All the samples exhibited photocatalytic activity and RhB degradation under visible light irradiation. However, the photocatalytic activity of TS or BiVO4 is low. All the TS/BiVO4 composites exhibit higher photocatalytic activities than that of TS or BiVO4. Based on the above experimental results, a possible photocatalytic mechanism is proposed to explain the enhanced photocatalytic activity of the as-synthesized TS/BiVO4 for the photocatalytic degradation of RhB. The visible-light driven electron–hole separation

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Fig. 3

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TEM images of (a) pure TS, (b) BVTS 1, (c) magnified BVTS 1, and (d) HR TEM of BVTS 1 (SAED pattern of BVTS inset figure).

Fig. 4 FT-IR spectra of pure titanosilicate, pure BiVO4, and BVTS nanocomposites.

Fig. 5 UV-vis DRS spectra of pure titanosilicate, pure BiVO4 (BVO), and BVTS nanocomposites.

ECB = EVB  Eg

where EVB and ECB are valance and conduction band potentials, X is the electronegativity of the semiconductor, Ee is the energy of free electrons on the hydrogen scale (4.5 eV), and Eg is the bandgap energy of the semiconductor. The absolute electronegativity of TS and BiVO4 is 5.81 and 6.04 eV. The calculated CB and VB of TS is 0.24 and 2.96 eV, whereas CB and VB of BiVO4 is 0.44 and 2.64 eV, respectively. The conduction band (CB) of TS (0.24) is more negative than that of BiVO4 (0.44).

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and transport of photo generated charge carriers on the TS/BiVO4 nanocomposite are illustrated in Fig. 7. The conduction band (CB) and valance band (VB) potentials of TS and BiVO4 were calculated using the following equation:46,47 EVB = X  Ee + 0.5(Eg)

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Fig. 6 Photodegradation of RhB dye under irradiation of direct sunlight with pure TS, pure BiVO4 (BVO), and TS/BiVO4 (BVTS) hybrid nanocomposites.

Thus, the photo generated electrons transfer from the CB of TS to that of BiVO4, while the holes remain in the VB of TS. RhB

Sun Light

! hn

RhB

RhB* + TS - RhB* + TS(eCB) BiVO4 + hn - BiVO4(hVB+ + eCB) eCB + O2 - O2  O2  + H+ - HO2

superoxide radicals (O2 ). These radicals can oxidize the pollutant due to their high oxidative capacities. The possible photocatalytic reactions are similar to reports available in the literature and Fig. 7 shows the schematic illustration of the photocatalytic degradation of RhB over TS/BiVO4 photocatalyst under sunlight irradiation. The energy band diagram of the TS/BiVO4 heterostructure photocatalyst after thermodynamic equilibrium is presented in Fig. 7. BiVO4 is considered as an intrinsic semiconductor, so the Fermi level in BiVO4 lies in the middle of the conduction band and valence band, which is approximately equal to 1.6 eV. According to a literature survey, band energy levels of BiVO4, EC and EV increase from 0.4 eV to 1.3 eV and 2.8 eV to 1.1 eV, respectively.47 Obviously, the difference of ECB between BiVO4 and TS allowed the transfer of an electron from the conduction band of BiVO4 to that of TS. When the system is irradiated with visible light, an electron (e) is promoted from the valence band into the conduction band of BiVO4 leaving a hole (h+) behind. Then the excited-state electrons produced by BiVO4 can be injected into the conduction band of the coupled TS due to the joint of the electric fields between these two materials. Conduction electrons are good reductants which could capture adsorbed O2 on the surface of the catalyst and reduce it to O2 . It can be further dissociated or reduced by photoinduced electrons, so there would be a constant stream of the surface  OH groups being produced. In the presence of hydroxyl radicals, the efficient photocatalytic degradation of RhB can be carried out smoothly. In addition, the photogenerated hole in BiVO4 also may activate some unsaturated organic pollutants, resulting in their subsequent decomposition.

eCB + HO2 -  OH + OH RhB*+ + (hVB+, O2 , HO2, eCB and OH) - Degraded Products

4. Conclusion

The photo generated electrons and holes are easily separated in the transfer process, thereby enhancing the activity of the photocatalyst for degradation of RhB. The electron would subsequently transfer to the photocatalyst surface to react with water and oxygen to generate some active species such as hydroxyl radicals ( OH) and

In summary, novel wide spectrum responsive TS/BiVO4 inorganic hetero-structures were successfully synthesized under mild conditions. This composition, and the photocatalytic degradation of RhB using this material under sunlight irradiation, is reported for the first time. Results revealed that this catalyst possesses higher

Fig. 7 Schematic illustration of the photocatalytic degradation mechanism of RhB over TS/BiVO4 under sunlight irradiation.

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photocatalytic activity than other catalysts towards the removal of RhB. Hence, the combination of strong visible light absorption by the introduction of BiVO4 and existing activated species enabled BiVO4/TS to have a high activity for photodegradation of RhB. The popularity of the fabrication of heterojunction structures is an inevitable trend. Monoclinic BiVO4 crystals were synthesized by a CTAB-assisted aqueous method. A RhB solution (100 mL, 105 M) was degraded in 60 min using the TS/BiVO4 nanocomposite photocatalyst under visible light irradiation and BVTS 1 activity was much higher than that of the BVTS 2 or BVTS 3 composites. The TS/BiVO4 nanocomposites could respond and thus remarkably enhanced the photocatalytic efficiency.

Acknowledgements The authors are thankful to the DST-SERB (EMR/2014/000629), New Delhi and MHRD, New Delhi for the partial financial support.

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