Microwave Synthesis, Photoluminescence, and Photocatalytic Activity

In addition to possessing interesting photoluminescence properties, a comparison of the photocatalytic ..... Monatshefte für Chemie - Chemical Monthl...
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Microwave Synthesis, Photoluminescence, and Photocatalytic Activity of PVA-Functionalized Eu3+-Doped BiOX (X = Cl, Br, I) Nanoflakes Armita Dash, Shyam Sarkar, Venkata N. K. B. Adusumalli, and Venkataramanan Mahalingam* Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER)-Kolkata, Mohanpur Campus, Nadia district, Mohanpur, West Bengal 741252, India S Supporting Information *

ABSTRACT: We report a facile microwave-assisted green synthetic route for colloidal poly(vinyl alcohol) (PVA)-coated europium (Eu3+)-doped luminescent heavy metal bismuth oxyhalide (BiOX; X = Cl, Br, I) nanoflakes at low temperature and examine their structural, optical, and photocatalytic characteristics. PVA coating onto the surface of the nanoflakes endows them with hydrophilic nature. Both Eu3+-doped BiOCl and BiOBr nanoflakes exhibit strong optical properties related to Eu3+ and Bi3+ which are quenched in case of Eu3+-doped BiOI matrix. These results are supported by Eu3+ photoluminescence lifetime values of 0.61 ms, 0.59 ms, and 8.9 μs, respectively. The former two matrices have quite similar crystal field environments as deduced from the asymmetric ratios of 5D0 → 7F2 (614 nm) and 5D0 → 7F1 (591 nm) transitions. In addition to possessing interesting photoluminescence properties, a comparison of the photocatalytic activity of Eu3+-doped BiOX (X = Cl, Br, I) nanoflakes, with corresponding estimated band gaps of 3.36, 2.74, and 1.67 eV has been evaluated using Rhodamine B (RhB) dye under visible light irradiation. The nanoflakes exhibited 100% dye degradation under visible light irradiation. Eu3+-doped BiOCl nanoflakes manifested higher photocatalytic efficiency compared to the other matrices following apparent first-order kinetics. Such a boost in efficiency is attributed to their high surface area to volume ratios, layered crystalline structures, indirect band gap nature, and ability to utilize broad bands in the solar spectrum.

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INTRODUCTION Nanocrystals doped with lanthanide ions (Ln3+) have gained considerable interest in the recent years owing to their unique optical properties which facilitate promising applications in laser materials, optoelectronic devices, solar cells, optical amplifiers in telecommunication, etc.1−3 Luminescence in these materials arises due to the parity forbidden intraconfigurational 4f−4f transitions in Ln3+ ions.4,5 Reports have revealed that Eu3+-doped matrices show Stokes luminescence, i.e., emission of lower energy radiation upon excitation by higher energy radiation.6−10 The crystal field and coordination environment around Eu3+ ion in its host lattice can be analyzed from its luminescence properties arising out of transitions between 5D0 and 7FJ (J = 0, 1, 2, 3, 4) energy levels.11−16 Among various inorganic host lattices of Ln3+ ion-doped fluorides and oxides are widely studied.17−23 However, oxyhalides are less explored as hosts for lanthanide ions. Oxyfluorides are interesting as they combine the advantageous features of low phonon energy of halides with chemically and thermally robust oxides. For example, Gao et al. have compared the luminescent properties of Ln3+ in LaF3 and LaOF.24 Similarly, a detailed spectroscopic study has been carried for sol−gel-derived Eu3+-doped LaOF nanocrystals.25 Extension of © 2014 American Chemical Society

the study toward heavy metal oxyhalides, particularly bismuth oxyhalides, will be interesting due to the size and charge compatibility of Bi3+ with that of Ln3+ ions as well as bismuth is nontoxic.26 Because of comparable ionic radius (1.17 Å) and high polarizable nature of Bi3+ ions, they can easily replace Ln3+ ions. In addition, bismuth oxyhalides, BiOX (X = Cl, Br, I), are very good photocatalysts. Bismuth oxyhalides, BiOX (X = Cl, Br, I), are ternary semiconductors of the V−VI−VII family having tetragonal structures with [X−Bi−O−Bi−X] layers stacked one above the other by nonbonding van der Waals interaction through the halogen atoms along c-axis.27−31 Electric fields are induced through polarization of atoms in the space between these layers which assist in effective separation of photogenerated electron− hole pairs, thus, augmenting photocatalytic efficiency of these materials. Moreover, the electron−hole recombination is prohibited due to the indirect band gap nature of BiOX where the excited electron in conduction band has to gain the crystal momentum in k-space with the emission or absorption Received: October 15, 2013 Revised: January 14, 2014 Published: January 21, 2014 1401

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Figure 1. Rietveld refined, experimental, and difference XRD patterns of PVA coated Eu3+-doped (A) BiOCl and (B) BiOBr nanoflakes. The corresponding standard patterns are shown in the bottom of the respective figures. Briefly, 0.05 mmol of Eu2O3 (17.6 mg) was stirred with concentrated HNO3 and heated at 95 °C to evaporate HNO3. The resulting Eu(NO3)3 was added with 0.95 mmol of Bi(NO3)3.5H2O in 25 mL of deionized water under continuous stirring for 10 min. Then 2 mmol of KX (X = Cl, Br, I; to prepare BiOCl, BiOBr, or BiOI) was added to this mixture, and the solution was stirred vigorously for 30 min. PVA (0.1 g) was added to this colloidal mixture and stirred for another 30 min and subsequently transferred into a 30 mL glass vial capped tightly with silicone septum. The sealed vial was kept in the microwave reactor (Anton Paar Monowave 300), and the reaction was carried out for 15 min at a low temperature of 70 °C. Subsequently, the reaction system was allowed to come down to room temperature. The product was collected by centrifugation and washed twice with deionized water and ethanol. The product was dried under vacuum. Characterization. The crystallinity and phase analysis of the PVAcoated BiOX (X = Cl, Br, I) nanoflakes was carried out using powder X-ray diffraction (XRD) measurements using a Rigaku-Smartlab diffractometer with Cu Kα operating at 50 kV and 40 mA at a scanning rate of 1° min−1 in the range of 10°−70°. The samples were completely well powdered and spread evenly on a quartz slide. The morphology of the nanoflakes was characterized by scanning electron microscopy (SEM). SEM images were taken using FEI Quanta FEG 200 high-resolution scanning electron microscope. The FTIR spectra were recorded using PerkinElmer Spectrometer RX1 spectrophotometer with KBr disk technique in the range of 4000−400 cm−1. For recording the FTIR spectra 10 mg of the samples was mixed with 200 mg of KBr to make the pellets. The photoluminescence (PL) measurements were performed using Horiba Jobin Yvon Fluorolog. All the emission spectra were recorded using steady state 450 W xenon lamp as excitation source and exciting the samples at 394 nm wavelength. The lifetime measurements were performed with the Horiba Jobin Yvon Fluorolog CP machine equipped with a pulsed Xe source operating at 25 W. The sample was excited at 394 nm with time per flash is 60 per second with an initial delay of 0.05 μs. The emission collected at 612 nm with a R928P PMT. Diffuse reflectance UV−vis spectra (DRS) of the samples were taken with a Varian Cary 100 spectrophotometer equipped with a diffuse reflectance accessory in the region 200−800 nm, with boric acid as reference. The reflectance spectra were converted into Kubelka−Munk function [F(R)] which is proportional to the absorption coefficient for low values of F(R). The photodegradation of dye (Rhodamine B) was analyzed in a Hitachi UV-4100 UV−vis− NIR spectrophotometer. All the measurements were performed at room temperature. Photocatalytic Activity. The photocatalytic activities of the Eu3+doped BiOX (X = Cl, Br, I) nanoflakes were evaluated by the degradation of Rhodamine B (RhB) under visible irradiation using a Xe lamp (Newport, Standford) with 260 W power (70 mA). To allow only the visible light to fall on the samples a 400 nm cutoff filter (AM 1.5G) was used. Prior to irradiation, the suspensions were stirred in dark for 30 min to reach adsorption−desorption equilibrium. After that, the solution was exposed to radiation using a solar simulator with

of phonon before annihilating a hole in valence bond radiatively.32 However, there are few reports which show that BiOX matrices have good photocatalytic efficiency under sunlight irradiation. Solar radiation is clean, renewable, and the most inexpensive source of energy. Photocatalytic reaction mediated by sunlight leaves no residues and hence an appealing green chemical route to degrade organic pollutants.33 For example, Tian et al. demonstrated the photodegradation behaviors of bisphenol A mediated by BiOBr microspheres under simulated sunlight.34 Yu and co-workers examined the photocatalytic activity of BiOI nanosheets in sunlight-driven removal of sodium pentachlorophenate.35 Further, among the manifold synthetic techniques employed to prepare BiOX nanostructures, reports on green synthetic route using microwave irradiation at low temperature are scarce. Microwave reactions are fast, with products having narrow size distribution, and uniform morphology and composition which can be tuned facilely.36−38 In this article, we report the synthesis of hydrophilic poly(vinyl alcohol) (PVA)-coated Eu3+-doped BiOX (X = Cl, Br, I) nanoflakes for the first time via green synthetic route employing microwave irradiation. Their structure, morphology, and surface functionalization were analyzed by PXRD, SEM, and FTIR, respectively. Eu3+-doped BiOCl and BiOBr matrices show bright Eu3+ luminescence properties in addition to that of Bi3+ which makes them potential candidates to host lanthanide ions for biological applications due to the hydrophilicity and biocompatibility imparted by PVA coating. Furthermore, the band gap energies of Eu3+-doped BiOCl, BiOBr, and BiOI nanoflakes are determined as 3.36, 2.74, and 1.67 eV, respectively, by optical absorption. Efficient photocatalytic activity of these three materials evaluated via degradation of Rhodamine B dye under visible light using a solar simulator proves that they have prospective role in “green” technology.

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EXPERIMENTAL DETAILS

Chemicals. All the chemicals used in this work, such as bismuth nitrate [Bi(NO3)3·5H2O], europium oxide (Eu2O3, 99.99%), poly(vinyl alcohol) [PVA, Mw = 27 000], and Rhodamine B (RhB) were purchased from Sigma-Aldrich. Ethylene glycol (EG), potassium chloride (KCl), potassium bromide (KBr), potassium iodide (KI), and nitric acid (HNO3) were purchased from Merck. All the chemicals were used without further purification. Europium nitrate was obtained by mixing appropriate proportions of europium oxide with HNO3, and residual HNO3 was removed by evaporation. Synthesis. Eu3+(5%)-doped BiOX (X = Cl, Br, I) nanoflakes were prepared using europium nitrate and potassium halide as precursors in deionized water with PVA (poly(vinyl alcohol)) as the capping agent. 1402

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continuous magnetic stirring. In each experiment, 0.05 g of photocatalysts was added into 50 mL of RhB solution with a concentration of 5 × 10−6 mol L−1. At each irradiation time interval, a 3 mL aliquot of the suspension was collected, and the slurry samples, including the photocatalyst and RhB solution, were centrifuged (4500 rpm, 3 min) to remove the photocatalyst particles in order to assess rate of decolorization and degradation photometrically. The solutions were analyzed by a Hitachi UV-4100 UV−vis−NIR spectrophotometer, and the characteristic absorption of RhB at 554 nm was used to monitor the photocatalytic degradation. All measurements were carried out at room temperature.

two bismuth ions in adjacent BiOX layers inside a single unit cell is found to be 4.85, 5.67, and 6.73 Å for BiOCl, BiOBr, and BiOI crystals, respectively. Scanning electron microscopy revealed the morphology of Eu3+-doped BiOX (X = Cl, Br, I) nanostructures. Figures 3A,B show the flower-like assembly of BiOCl nanoflakes. BiOCl nanoflakes have a size range of 300−500 nm. A higher magnification of the images reveals the thickness of each nanoplate varying in the range of 15−18 nm approximately. As seen in Figures 3C,D, Eu3+-doped BiOBr and BiOI nanoflakes possess similar assembled structures of size range 800 nm−1.1 μm. To confirm the presence of elements, we have done the EDX analysis for all the three nanoflake samples. The results are shown in Figure S2. However, the presence of Eu3+ ions is not detected. This may be due to interference from other ions. The presence of Eu3+ ions is confirmed with the photoluminesce and lifetime analyses (vide inf ra). Surface Functionalization. Vibrational analysis of PVA coated Eu3+-doped BiOX (X = Cl, Br, I) nanoflakes was carried out with FTIR analysis (Figure 4). A typical FTIR spectrum of pure PVA has vibrations at 3400, 2930, 1718, and 1442 cm−1. A broad and strong peak observed at 3595 cm−1 is due to stretching vibrations of hydroxyl groups ν(OH) with strong hydrogen bonding of intra- and intermolecular type. The peak at 2930 cm−1 is assigned to alkyl C−H stretching of PVA. Characteristic peaks observed at 1718 and 1442 cm−1 with weak intensity are associated with C−O stretching and CH2 bending deformations of PVA. The peak present at 1120 cm−1 is due to C−H in-plane bending. The peaks at 844 and 586 cm−1 correspond to C−H out-of-plane bending vibrations. The red-shift from 1718 to 1621 cm−1 of C−O stretching vibration bands is seen in case of PVA-coated BiOX:Eu3+ (X = Cl, Br, I). This implies that the O atom of C−O−H group in PVA strongly binds to the surface of the nanoflakes. The Bi−O stretching mode in BiOX is assigned to the band at 528 cm−1. A gradual shift in the peak of 528 cm−1 toward lower frequency is seen from Eu3+-doped BiOCl to BiOBr and then BiOI nanoflakes. This is probably due to heavier atoms tend to cause the attached bonds absorb at lower frequencies. Photoluminescence Studies. Excitation and emission spectra of 5% Eu3+-doped BiOCl and BiOBr nanoflakes are shown in Figures 5 and 6, respectively. Highly intense spectral patterns justify successful doping of Eu3+ ion into BiOCl and BiOBr matrices. A broad peak centered at 350 nm is observed in the excitation spectra of Eu3+-doped BiOCl nanoflakes, monitoring the emission at 614 nm. This band is attributed to the Eu3+−O2− charge transfer which arises from the transition of 2p electrons of O2− to the empty 4f orbitals of Eu3+ ions. The broad charge transfer band lies at longer wavelength due to the presence of Bi3+ in the matrix.43 Shallow peaks at 362, 394, 464, and 506 nm were observed which correspond to 5D4 ← 7 F0, 5D2 ← 7F0, 5D3 ← 7F0, and 5D1 ← 7F0 transitions of Eu3+, respectively (see Figure 5A). Upon excitation at 394 nm, the Eu3+-doped BiOCl nanoflakes show strong emission peaks near 591, 614, 650, and 695 nm which are characteristics of Eu3+ ions and are due to 5D0 → 7FJ transitions where J = 1, 2, 3, and 4, respectively (Figure 5B). Similarly, the excitation and the emission spectra of Eu3+-doped BiOBr display the characteristic Eu3+ peaks (see Figure 6). The 5D0 → 7F1 (591 nm) transition is a magnetic-dipole transition whereas 5D0 → 7F2 (614 nm) transition is a hypersensitive electric-dipole transition whose intensity is influenced by the crystal field. For Eu3+-doped BiOI nanoflakes, the observed Eu3+ emission intensity is quite weak

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RESULTS AND DISCUSSION Structure and Morphology. The crystallinity and purity of the as-synthesized bismuth oxyhalide samples were analyzed by powder XRD. Figure 1 shows the Rietveld refined PXRD patterns of the Eu3+-doped BiOCl and BiOBr nanoflakes with their corresponding standard patterns while the corresponding patterns Eu3+-doped BiOI nanoflakes are shown in Figure S1. All the diffraction peaks could be perfectly indexed with the standard pattern of the tetragonal phase of BiOX (X = Cl, Br, I) with a space group P4/nmm. There is no presence of Bi2O3 or any other impurity peak observed in the refined patterns. The lattice parameters (a and c) for BiOCl are 3.898 Å and 7.424 Å, while the corresponding values for BiOBr and BiOI are 3.929 Å, 8.132 Å and 3.992 Å, 9.15 Å, respectively. These values are closely matching with that of the reported values.39,40 The average crystallite sizes of the Eu3+-doped BiOX nanoflakes were calculated using Debye−Scherrer equation, D = 0.9λ/(β cos θ), where D denotes the average crystallite size, λ (= 1.5418 Å) is the wavelength of incident X-ray, β is the corrected full width at half-maximum, and θ is the diffraction angle for the (hkl) plane. Crystallite sizes obtained for Eu3+-doped BiOCl, BiOBr, and BiOI are 25.4, 16.0, and 31.9 nm, respectively. The tetragonal structure of BiOX (X = Cl, Br, I) nanoflakes is also evident from the basic unit cell crystal structure shown in Figure 2 which was obtained using VESTA program41,42 taking

Figure 2. Unit cell crystal structure of tetragonal BiOX (X = Cl, Br, I) drawn using the VESTA program.

the unit cell lattice constants (a, b, c) and the atomic coordinates (x, y, z) of bismuth, oxygen, and halogen (X = Cl, Br, I), as given in Table S1. The unit cell has four BiOX molecules. The BiOX layers are stacked over one another by the nonbonding van der Waals interaction through halide atoms (X = Cl, Br, I) along the c-axis. The crystal structures of the three BiOX (X = Cl, Br, I) matrices are the same, but the interplanar lattice spacings are different. The distance between 1403

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Figure 3. SEM images of Eu3+-doped (A, B) BiOCl, (C) BiOBr, and (D) BiOI showing nanoflakes assembled in flower-like arrangement.

Figure 4. FTIR spectra of PVA coated Eu3+-doped BiOX (X = Cl, Br, I) nanoflakes along with pure PVA molecule.

presumably due to its high hygroscopic nature (Figure S3). Hygroscopicity of the oxyhalides follows the trend I > Br > Cl. A significant multiphonon relaxation occurs which is the nonradiative energy transfer from the excited states of Eu3+ to the different vibration modes of O−H bond in the surrounding H2O species as water molecules are the primary hubs of nonradiative transitions.44 The lifetime of the 5D0 level of Eu3+ in both BiOCl and BiOBr nanoflakes is around 0.16 ms The decay curves can be fitted well using the equation I = A1 exp(−t/τ1) where τ is the decay lifetime and are shown in Figures 7A,B. Eu3+ ion has a simple energy level structure. The ground state multiplet (7F0) and the principal emitting state (5D0) are nondegenerate. Hence, the transitions 5DJ → 7F0 in the excitation spectra and 5D0 → 7FJ in the emission spectra are unaffected even in the low-symmetry host environment. The magnetic dipole transition 5D0 → 7F1 (591 nm) is independent of the host environment while the electric dipole transitions 5 D0 → 7FJ (J = 0, 2, 4) are largely sensitive to host structure and

Figure 5. (A) Excitation and (B) emission spectra of Eu3+-doped BiOCl monitored at 614 and 394 nm, respectively.

symmetry. To understand the variation of symmetry and coordination environment around Eu3+ ion doped in BiOX (X = Cl, Br, I) matrices, the asymmetric ratio was calculated which is a relative ratio of integrated areas under the peaks of 5D0 → 7 F2 (614 nm) and 5D0 → 7F1 (591 nm) transitions.14−16 The asymmetric ratios for Eu3+-doped BiOCl, BiOBr, and BiOI 1404

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enhancement in the relative intensity of the hypersensitive electric dipole transition 5D0 → 7F2 and thus a high asymmetric ratio. Lowering of symmetry in crystal field around Eu3+ ion can be attributed to formation of anion vacancies causing distortion and affecting asymmetric ratios. Comparable asymmetric ratios and lifetime values of Eu3+-doped BiOCl and BiOBr imply that both the host lattices have similar crystal field. Eu3+-doped BiOCl and BiOBr are interesting matrices in which Bi3+ luminescence was observed at room temperature. Such phenomenon occurs due to 6s and 6p transitions in its energy level structure.26,45−48 Figure 8 shows Bi3+ excitation and emission spectra in Eu3+-doped BiOCl nanoflakes. A broad and highly intense excitation band appears in the region from 270 to 360 nm with a shoulder at 310 nm, monitoring emission wavelength at 420 nm. Two weak excitation peaks appear at 251 and 368 nm. The peak at 251 nm corresponds to the 1S0 → 1 P1 transition, while the peaks centered at 320 and 368 nm correspond to two components of the 1S0 → 3P1 excitation band, which are caused by the crystal-field splitting of the 3P1 excited state. On the other hand, the emission band in the UV region is obtained with a maximum at 420 nm which corresponds to 3P1 → 1S0 transition, monitoring at 310 nm excitation wavelength. Similar spectra are obtained for Eu3+doped BiOBr nanoflakes as shown in Figure S4. The inset in Figure 8B represents a schematic diagram showing electronic transitions inside Bi3+ ions. However, no energy transfer process has taken place from host matrix (containing Bi3+) to the dopant ion Eu3+ since the Bi3+ excitation peak was not observed while monitoring Eu3+ emission (excitation spectrum). Contrary to such observations in Eu3+-doped BiOCl and BiOBr matrices, Bi3+ emission is completely quenched in Eu3+doped BiOI matrix. The reason for the difference in the optical characteristics of the three matrices is unclear at this stage. However, we believe that the observed results are due to the differences in hygroscopicity, covalency (increases from Cl to I) and bond strength of Bi−X in [X−Bi−O−Bi−X] stacks.49 Diffuse reflectance spectroscopy (DRS) is a useful tool for semiconductor materials to characterize their optical absorption property, the key parameter to evaluate their photocatalytic activity. UV−vis diffuse reflectance spectra of Eu3+-doped BiOX (X = Cl, Br, I) nanoflakes are shown in Figure 9A. There is a clear indication of red-shift of the UV−vis absorption edges of the Eu3+-doped BiOX nanoflakes with increasing atomic number of X. Eu3+-doped BiOCl and BiOBr nanoflakes have intense absorption edges at 369 and 452 nm which lie in the near-UV and visible regions, respectively. On the other hand, Eu3+-doped BiOI nanoflakes have absorption edge clearly in the

Figure 6. Excitation and emission spectra of Eu3+-doped BiOBr nanoflakes monitored and excited at 614 and 394 nm, respectively.

Figure 7. Photoluminescence decay curve of Eu3+-doped (A) BiOCl and (B) BiOBr nanoflakes monitoring at λem = 614 nm (5D0 → 7F2).

nanoflakes are 3.37, 2.74, and 1.88, respectively. Distortion in symmetry around Eu3+ ion in BiOCl and BiOBr caused an

Figure 8. (A) Excitation and (B) emission spectra of Bi3+ ion in Eu3+-doped BiOCl nanoflakes monitored at 411 and 320 nm, respectively. The inset in (B) is the energy level diagram of Bi3+ ion showing allowed transitions (solid arrows) and spin-forbidden transitions (dotted arrows). 1405

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Figure 9. (A) UV−vis diffuse reflectance spectra and (B) plots of (αhν)1/2 versus the photon energy (hν) of as-synthesized Eu3+-doped (a) BiOCl, (b) BiOBr, and (c) BiOI nanoflakes.

Figure 10. Temporal evolution of UV−vis spectra of RhB degradation under visible light in the presence of 0.05 g of Eu3+-doped (A), BiOCl, (B) BiOBr, and (C) BiOI nanoflakes. (D) shows the concentration change of RhB over time in different photocatalyst solutions.

literature.51,52 It is clearly seen that the band gap energy decreases with increasing atomic number of X in BiOX. Further, 5% Eu3+ doping does not affect the optical absorption properties of bismuth oxyhalides. Figure S5 shows the UV−vis diffuse reflectance spectrum and plot of (αhν)1/2 versus the photon energy (hν) of BiOCl nanoparticles without europium doping. It shows an absorption edge at 364 nm, and the band gap energy is estimated to be 3.31 eV. These values closely match with the results obtained for Eu3+-doped BiOCl nanoflakes. Photocatalytic Activity. The photocatalytic activities of the Eu3+-doped BiOX (X = Cl, Br, I) nanoflakes were evaluated by following the degradation of Rhodamine B (RhB) under visible light irradiation with a 150 W Xe lamp (400 nm cutoff). The UV−vis spectra of RhB photodegradation and digital images of its color change at different irradiation time on Eu3+doped BiOCl, BiOBr, and BiOI nanoflakes (0.05 g) are

visible region. These results are also consistent with their respective colors as white/colorless, pale yellow, and dark red in solid powdered phase. For a crystalline semiconductor, the optical absorption near the band edge follows the formula αhv = A(hv − Eg)n/2, where α, hν, A, and Eg are absorption coefficient, photon energy, a constant, and band gap energy, respectively.50 The variable n determines the characteristics of transition in the semiconductor, i.e., n = 1 for direct interband transition and n = 4 for indirect interband transition.51 Eu3+-doped BiOX nanoflakes follow n = 4, i.e., indirect band gap transition, owing to the steep sharpness of their absorption onset. Figure 9B depicts the plots of (αhν)1/2 versus the photon energy (hν). The intercept of the tangent to the x-axis gives good approximation of the band gap energies of Eu3+-doped BiOCl, BiOBr, and BiOI nanoflakes as 3.36, 2.74, and 1.67 eV, respectively, which are close to the reported values in the 1406

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constant (mg/L min). If C is very small, then the above equation is simplified to

illustrated in Figures 10A, 10B, and 10C, respectively. All the spectra show characteristic absorption peak of RhB at 554 nm whose intensity decreased dramatically as the exposure time to irradiation increased in the presence of the photocatalysts. The Eu3+-doped BiOCl took 20 min to degrade completely while the corresponding values for BiOBr and BiOI are 25 and 100 min. The visual trace of the dye degradation is shown in the inset of corresponding UV−vis spectra (Figures 10A−C). Figure 10D shows the variation in RhB concentrations (C/C0) with irradiation time over different photocatalysts, where C0 is the initial concentration of RhB and C is the concentration of RhB at t time. A control experiment was performed to verify any degradation of RhB under direct irradiation of light source without any photocatalyst. The plot of blank experiment in Figure 10D clearly indicates that the RhB concentration hardly changed with the increase of irradiation time in the absence of photocatalysts. No degradation of the dye had taken place either in dark or when exposed to solar light. This shows that these nanoflakes possess high photocatalytic efficiency under visible ligh (λ > 400 nm). During photocatalytic reaction, the colloidal nanoflakes showed good stability in solution. The photocatalytic degradation efficiency D was calculated using the formula D=

ln

Figure 11 is a plot of ln(C0/C) versus time resulting in a straight line, and the slope is equal to the apparent first-order

Figure 11. Kinetics of RhB dye degradation for first-order linear fitted plot ln(C0/C) versus t. The corresponding slope values for Eu3+-doped BiOCl, BiOBr, and BiOI photocatalysts are 0.27, 0.23, and 0.10 min−1.

A0 − At C − Ct = 0 A0 C0

rate constant kapp which are 0.27, 0.23, and 0.10 min−1 for Eu3+doped BiOCl, BiOBr, and BiOI nanoflakes, respectively. These values substantiate the photocatalytic reaction rate being highest for BiOCl matrix, closely followed by BiOBr and least for BiOI. Though the exact reason for the difference in the photocatalytic behavior of BiOI is not clear we believe the difference in the band gap might be the reason. We extended the photocatalytic effect to understand whether doping other lanthanide ions have any effect. In this regard we chose one from lower, middle, and higher in the series of lanthanides. We prepared La3+-, Gd3+-, and Lu3+-doped BiOCl nanoflakes under identical condition and performed the photocatalytic studies. The UV−vis absorbance results and the graph of ln(C0/C) vs time are shown in Figure S8. The results suggest that there is only a slight difference in the observed photocatalytic effect. We believe this slight difference may be attributed to the difference in the microscopic properties like crystallite size of the nanoflakes. However, we emphasize a more detailed study is needed to completely understand this.

where A0 and C0 are the corresponding initial absorbance and concentration before irradiation while A and C are the absorbance and concentration at time t of irradiation. Stirring in the dark resulted only in the adsorption process of photocatalyst on to the dye. The color of the Eu3+-doped BiOCl and BiOBr powder obtained after centrifugation changed from white to pink. While such change in case of Eu3+-doped BiOI could not be distinguished since the initial color was dark brown. This shows that Eu3+-doped BiOX nanoflakes have good adsorption capacity. To check whether the adsorption of the dyes to the nanoflakes depends on the isoelectic point (IEP) (as IEP depends on the nature of the halogens), we calculated the isoelectronic point by measuring the zeta potential measurements at various pH of the dispersion. The graph shown in Figure S6 indicates that IEPs are 5.5, 3.6, and 2.4 for BiOCl, BiOBr, and BiOI, respectively. However, to check whether this difference has any effect on the adsorption efficiency, we calculated the adsorption efficiency of the dye over all the three nanoflakes (after 30 min stirring) using UV−vis analysis. The calculated efficiencies are 9.6%, 10.4%, and 11.5% for Eu3+-doped BiOCl, BiOBr, and BiOI nanoflakes, respectively. This suggests that there is only slight difference in the adsorption efficiency between the nanoflakes and may not be a main reason for the difference in the photocatalytic effect. The kinetics of RhB degradation under visible light was explored to present a numerical difference among the degradation rates by the photocatalysts. The general Langmuir−Hinshelwood model depicting apparent first-order kinetics is as follows53,54 r=−

C0 = kKt = kappt C

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CONCLUSION Hydrophilic lanthanide (Eu3+)-doped bismuth oxyhalide (X = Cl, Br, I) nanoflakes were synthesized via the low-temperature microwave method for the first time. Eu3+ doping into these BiOX matrices showed strong photoluminescence. Comparable lifetime values and asymmetric ratios of Eu3+-doped BiOCl and BiOBr matrices evince that Eu3+ ionic sites have similar crystal field in both the host lattices. The three matrices, bearing band gap energies of 3.36, 2.74, and 1.67 eV (for X = Cl, Br, and I, respectively), exhibit 100% photocatalytic efficiency under visible light irradiation via RhB degradation. The results suggest that both BiOCl and BiOBr degraded the dye close to 20 min, whereas it took almost 100 min for the BiOI. These results suggests that both BiOCl and BiOBr are good host matrices for lanthanide ions to exhibit luminescence properties as well as good photocatalytic materials compared to BiOI. The present work provides a facile route to fabricate Ln3+-doped

dC kKC = dt 1 + kC

where r is the degradation rate of dye (mg/L min), C the concentration of dye (mg/L), t the irradiation time, K the adsorption coefficient of dye (L/mg), and k the reaction rate 1407

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bismuth oxyhalide nanostructures, possessing both promising luminescent and photocatalytic properties.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns; unit cell constants and atomic coordinates of BiOX (X = Cl, Br, I); photoluminescence spectra and decay curve; diffuse reflectance spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax (+91) 33-25873020; e-mail mvenkataramanan@yahoo. com (V.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.V. thanks the Department of Science and Technology (DST) and IISER-Kolkata for the funding. S.S. thanks UGC for the fellowship. We thank Dr. Dipti P. Das of Institute of Minerals and Materials Technology (IMMT) Bhubaneswar for helping with diffuse reflectance measurements. The authors thank Dr. V. Sudarsan at BARC for helping with Rietveld refinements. The authors thank Sagar Ganguli for helping with photocatalytic measurements.

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REFERENCES

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