Hydrothermal Synthesis , Characterization , and Optical Properties of Ce Doped Bi 2 MoO 6 Nanoplates

1 Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand 2Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 3 Electron Microscopy Research and Service Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 4Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 5Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand


Introduction
Aurivillius family of structurally related oxides with chemical formula of Bi 2 A −1 B  O 3+3 (A = Ca, Sr, Ba, Pb, Bi, Na, K, and B = Ti, Nb, Ta, Mo, W, and Fe) was originally attractive material due to its layered structure and unique properties [1,2].The perovskite-type blocks lead to variable layers along the -axis due to the integer  with  = 0, 1, 2, 3, 4, 5 and a typical "mica-like" two-dimensional structure [1].Bi 2 MoO 6 with narrow band gap of 2.9 eV is a typical Aurivillius phase with its structure consisting of perovskite layers [3,4].Bi 2 MoO 6 is an interesting material due to its unique physical properties for solar energy conversion, ion conduction, and photocatalysis for water splitting under visible-light irradiation and gas sensors [1,2].Various synthetic methods for this material have been reported such as hydrothermal/solvothermal [1,3,5], aerosol-spraying [4], coprecipitation [6], thermal evaporation [7], and hard-template method [8].Recently, rare earth dopants have been excessively applied to modify optical properties of nanomaterials due to their possible transition of 4f electron configuration.Among them, cerium is one of the most interesting dopants due to its different electronic structure between Ce 3+ (4f 1 5d 0 ) and Ce 4+ (4f 0 5d 0 ), leading to different optical properties.It generates oxygen vacancies and bulk oxygen species, which have relatively high mobility.Thus they are more active for oxidation processes [9,10].
In this paper, 0-3% Ce doped Bi 2 MoO 6 crystallites were successfully synthesized by the hydrothermal process.Phase,

Experimental Procedures
All the reagents were of analytical grade and used as received without further purification.In a typical experiment, 0.5 mmol Na 2 MoO 4 ⋅2H 2 O and 1 mmol Bi(NO 3 ) 3 ⋅5H 2 O were dissolved in 60 mL deionized water to form solution A under 20 min magnetic stirring at room temperature.Concurrently, 1 and 3% by weight Ce(NO 3 ) 3 ⋅6H 2 O were dissolved in 40 mL deionized water each to form solution B under 20 min magnetic stirring at room temperature.Then, solution B was slowly added to solution A to form homogeneous solutions with further stirring for 30 min.Each solution of both with and without Ce 3+ dopants was adjusted the level of acid or alkali until reaching at the pH of 10 using 3 M NaOH, poured into each of stainless steel autoclave with a Teflon liner, and heated at 180 ∘ C for 20 h.At the conclusion of the process, the autoclaves were cooled to room temperature.The products were separated centrifugally, washed with deionized water and absolute ethanol several times, and dried at 80 ∘ C for 12 h.
The phase of the samples was characterized by Xray diffraction (XRD) using a Philips X'Pert MPD X-ray diffractometer with CuK  irradiation at  = 1.5406Å.The surface morphology was investigated by field emission scanning electron microscope (FE-SEM, JEOL JSM 6335F) and transmission electron microscope (TEM, JEOL, JEM2100) operated at the accelerating voltage of 35 and 200 kV, respectively.Raman and FTIR spectra were recorded on HORIBA JOBIN YVON T64000 Raman spectrometer with 50 mW and 514.5 nm wavelength Ar green laser and BRUKER TENSOR27 Fourier transform inferred (FTIR) spectrometer with KBr as a diluting agent and operated in the ranges of 100-1,000 cm −1 and 400-4,000 cm −1 , respectively.X-ray photoelectron spectroscopy (XPS) of the products was carried out via an Axis Ultra DLD, Kratos Analytical Ltd., with a monochromated Al K  (1486.6 eV) radiation as the excitation source at 15 kV.All obtained spectra were calibrated to a C1s electron peak at 285.1 eV.UV-visible absorption spectra of an ethanol suspension of 0-3% Ce doped Bi 2 MoO 6 samples were recorded under a Lambda 25, Perkin Elmer UV-visible spectrophotometer.

Results and Discussion
The typical XRD patterns as shown in Figure 1 reveal the phase and purity of the as-obtained 0-3% Ce doped Bi 2 MoO 6 samples.All peaks of the undoped and Ce doped Bi 2 MoO 6 samples were specified as the single phase orthorhombic Bi 2 MoO 6 structure (JCPDS card number 73-2020 [11]).The presence of sharp and intense peaks confirmed the formation of highly crystalline nanomaterials.Furthermore, the absence of any impurity related peaks indicates that Ce 3+ ions were successfully doped into Bi 2 MoO 6 nanostructure.However, the intensity ratio of the (060) peak to the (131) peak of 3% Ce doped Bi 2 MoO 6 sample is 1.66, obviously larger than the undoped Bi 2 MoO 6 which is equivalent to 0.60 [12].This important result indicates that the crystal has special anisotropic growth along the [0b0] direction.
The morphology and particle sizes of the Ce doped Bi 2 MoO 6 with different contents of Ce ions were investigated by SEM as shown in Figure 2. It can be seen that the samples were comprised of a large number of nanoplates with diameters ranging between 0.1 and 0.3 m and <100 nm thick.The surfaces of these nanoplates are smooth.Interestingly, when the samples were doped by different Ce concentrations, Ce doped Bi 2 MoO 6 are still to be nanoplates.These show that Ce doping concentration had little effect on the shape of the products.Clearly, morphology and particle sizes of the Ce doped Bi 2 MoO 6 nanoplates were consistent with pure Bi 2 WO 6 .
More information of the structure was obtained by TEM observation as shown in Figure 3.It confirms that the undoped Bi 2 MoO 6 nanoplates have an average diameter of about 0.2 m, in accordance with the SEM analysis.Obviously, some of lighter color parts can be seen, due to the difference in the contrast in TEM, mainly related to the difference in thickness of the samples.Furthermore, the 3% Ce doped Bi 2 MoO 6 sample was composed of square nanoplates with ∼100 nm thick edge.The selected area electron diffraction (SAED) patterns clearly demonstrate the single crystalline nature of the nanoplates.Interestingly, the SAED patterns taken on the whole single nanoplate show single crystalline patterns with sharp diffraction bright spots, giving the [100] zone axis character of orthorhombic Bi 2 MoO 6 .Based on the above XRD results, it is reasonable to conclude that the nanoplates preferentially grew along the [010] direction.
The chemical composition of 3% Ce doped Bi 2 MoO 6 nanoplates was investigated by XPS spectroscopy as shown in Figure 4 and was calibrated using C1s peak at 285.1 eV.The Bi4f peaks of the 3% Ce doped Bi 2 MoO 6 nanoplates appear at 159.52 eV of 4f 7/2 and 164.80 eV of 4f 5/2 , corresponding to Bi 3+ [4, [13][14][15].Additional weak spin-orbit doublet peaks with binding energy of 157.92 eV for Bi 4f 7/2 and 163.40 eV for Bi 4f 5/2 are also detected, suggesting that some of bismuth exist as the (+3 − ) valence state [16].Probably, the Bi (+3−) formal oxidation state could be attributed to the substoichiometric phase within the microsized plates [16].The production of lower oxidation state results in the presence of oxygen vacancies inside.The Mo3d spectrum showed spin-orbit splitting of the Mo3d levels at 232.84 eV and 236.00 eV, corresponding to the 3d 5/2 and 3d 3/2 orbitals [4,13,17,18].The spin-orbit splitting between Mo3d 5/2 and Mo3d 3/2 signals of Ce doped Bi 2 MoO 6 nanoplates was set to 3.16 eV which are consistent with the previous reports [17].However, single spin-orbit doublets showed peaks with binding energies of 231.3 eV (Mo3d 5/2 ) and 234.6 eV (Mo3d 3/2 ).These peaks are associated with Mo in formal (+6) oxidation state [19,20].The O 1s binding energy of 530.60 eV was in agreement with the literature value [4] Therefore, from the above results it is quite clear that there is coexistence of Ce 3+ and Ce 4+ in this sample [10,21].Bi 2 MoO 6 crystal is built up of perovskite-like (MoO 4 ) 2− and fluorite-like (Bi 2 O 2 ) 2+ layers.Its room temperature and ambient pressure structure is orthorhombic (space group symmetry P2 1 ab).A standard group theoretical analysis for the P2 1 ab room temperature phase of Bi 2 MoO 6 unit cell leads to 108 degrees of freedom at the Brillouin zone center (Γ point).The optical modes are distributed among the irreducible representation of the factor group C 2V as 26A 1 + 27A 2 + 26B 1 + 26B 2 .Selection rules state that the A 1 , B 1 , and B 2 are both Raman and IR active whereas the A 2 modes are only Raman active [23][24][25].
Raman spectra of 0-3% Ce doped Bi 2 MoO 6 samples are shown in Figure 5.It is well known that the bands in the 180-500 cm −1 range originated from the bending, wagging, and external modes by directly correlating the Mo-O bonds, and the 700-900 cm −1 region originated from the stretching vibration modes of the MoO 6 octahedrons.Raman peaks at 323, 345, and 400 cm −1 corresponded to the   symmetry bending modes.Raman modes near 293 cm −1 seemed to be from the   bending vibration.The band at 144 cm −1 was assigned as the lattice modes of Bi 3+ atoms mainly in the direction normal to the layers.The strong band at 792 cm −1 was assigned to A 1 mode of Mo-O stretching vibration of the distorted MoO 6 octahedrons.The shoulder peak at 715 cm −1 was identified to the   asymmetric stretching of MoO 6 octahedrons involving the vibration of the equatorial oxygen atoms within the layers.The band at 841 cm −1 was assigned as the A 2 symmetric and asymmetric stretching vibrations of the MoO 6 octahedrons, relating to the motion of the apical oxygen atoms normal to the layers.When the Ce was doped into the samples, the strong bands at 792 cm −1 and two shoulder peaks at 715 and 840 cm −1 also slightly shifted to 713, 791, and 838 cm −1 , confirming an effective substitution of Bi 3+ ions by Ce 3+ ions in the as-prepared nanocrystals, as also revealed by the XRD analysis [23][24][25][26].FTIR spectra of the samples (Figure 6) show the band in the 400-900 cm −1 range, corresponding to Bi-O stretching and bending, Mo-O stretching, and Mo-O-Mo bridging stretching modes of Bi 2 MoO 6 .The bands at 843 and 797 cm −1 were assigned as the asymmetric and symmetric stretching modes of MoO 6 relating to vibrations of apical oxygen atoms, respectively.The 731 cm −1 mode was attributed to the asymmetric stretching vibration of the equatorial oxygen atoms of MoO 6 octahedrons.Those at 603 and 570 cm −1 were specified as the bending vibrations of MoO 6 .Weak bands at 409 and 448 cm −1 were attributed to the stretching and bending vibrations of BiO 6 octahedrons [2,26].
The UV-visible absorption spectra of the undoped and Ce doped Bi 2 MoO 6 are shown in Figure 7.They show the strong absorption in the UV and visible-light regions.It should be noted that the maximum absorption was detected at 321 nm for 3% Ce doped Bi 2 MoO 6 , obviously blue shifted compared to that of Bi 2 MoO 6 at 383 nm.For a crystalline semiconductor, the optical absorption near the band edge follows the equation ℎ] = (ℎ] −   ) /2 , where , ],   , and  are the absorption coefficient, photonic frequency, energy gap, and a constant, respectively [2,3].For Bi 2 MoO 6 , the value of  is 1 for the direct transition.The plot of (ℎ]) 2 versus photon energy (ℎ]) of undoped and Ce doped Bi 2 WO 6 was estimated from the intercepts of the tangents to the plots which are 1.86 eV for pure Bi 2 MoO 6 and 2.04 eV for 3%
Ce doped orthorhombic Bi 2 MoO 6 nanoplates were successfully synthesized by the hydrothermal method.The experimental results presented that the as-synthesized products were orthorhombic Bi 2 MoO 6 with the growth along the [010] direction.UV-visible absorption spectra show strong absorption due to the intrinsic energy gap transition of Bi 2 MoO 6 .250300 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) Absorbance (a.u.)