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Enhanced organic dye removal of the W and N co-doped NaTaO3 under visible light irradiation
Journal of Alloys and Compounds 681 (2016) 225e232
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Enhanced organic dye removal of the W and N co-doped NaTaO3 under visible light irradiation Sai Wang a, b, Xuewen Xu a, b, *, Han Luo a, b, Yinghao Bai a, b, Saleem Abbas a, b, Jun Zhang a, b, Jianling Zhao a, b, Chengchun Tang a, b a b
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China Hebei Key Laboratory of Boron Nitride Micro and Nano Materials, Tianjin 300130, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2016 Received in revised form 7 April 2016 Accepted 22 April 2016 Available online 24 April 2016
We carried out both the experimental and theoretical investigations on the synthesis and photocatalytic performance of the visible-light responded, W and N co-doped NaTaO3 powders. The nanocubic NaTa1xWxO3xNx (x ¼ 0.00, 0.01, 0.03, 0.05, 0.1) particles with a range of 100e200 nm in size have been successfully synthesized at 700 C via the NaOH flux reaction method. The microstructure, chemical composition and optical absorption of the as-prepared photocatalysts were discussed in detail. The photocatalytic performance of as-prepared powders was evaluated by decolarization of rhodamine B (RhB) under visible light irradiation with the wavelengths longer than 400 nm. The optical band gap of NaTaO3 could be significantly reduced through the co-doping of W and N. The degradation efficiency of RhB over the NaTaO3 with co-doping of W and N in the charge-compensated mode was remarkably improved. Furthermore, the first-principle calculations were also performed to discover the effects of codoping of W and N on the electronic structure of NaTaO3. These present results provide a simple and feasible route to design new photocatalysts based on NaTaO3 or other oxide photocatalysts. © 2016 Elsevier B.V. All rights reserved.
Keywords: Photocatalysis NaTaO3 Dye removal First-principle calculations Band engineering
1. Introduction The perovskite-type oxides containing d0 transition-metal ions, such as Nb, Ti and Ta, have been reported as the effective photocatalysts for degradation of organic dyes and water splitting [1e4]. Tantalate sodium (NaTaO3) seems to be more advantageous due to its excellent effectiveness in carrier separation [5e10]. However, as a wide-gap (~4.1 eV) semiconductor, the pure NaTaO3 photocatalyst only works efficiently under ultraviolet light irradiation [5e8]. To improve the photocatalytic efficiency of NaTaO3, numerous approaches have been introduced, including the doping of anions or cations [9,11e20], forming composites [15,21e23], designing various nanostructures [24,25], and forming solid solutions [26e29]. The doping of NaTaO3 has been considered as a promising way
* Corresponding author. School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China. E-mail addresses: [email protected] (S. Wang), [email protected] (X. Xu), [email protected] (H. Luo), [email protected] (Y. Bai), saleem_ [email protected] (S. Abbas), [email protected] (J. Zhang), zhaojl@hebut. edu.cn (J. Zhao), [email protected] (C. Tang). http://dx.doi.org/10.1016/j.jallcom.2016.04.231 0925-8388/© 2016 Elsevier B.V. All rights reserved.
to improve the photoactivity under visible light irradiation. The doping of anions, including N3 [17e19] and S2 [11], usually resulted in obvious red shift and additional add-on shoulder at the absorption edge of NaTaO3. The La-doped NaTaO3 exhibited enhanced photocatalytic activity on water splitting, even though its optical band gap (4.13e4.17 eV) was slightly larger than that of pure NaTaO3 [13,16]. By comparison, the absorption edge of NaTaO3 with the codoping of La and Cr was shifted to 420 nm, which was contributed to the charge transfer from Cr 3d orbital to Ta 5d orbital [30]. The self-doped Ta4þ cation in NaTaO3 narrowed the band gap to 1.70 eV [31]. However, the mono-doped mode for oxide photocatalysts usually resulted in new recombination centers associated with dopants due to charge imbalance [32e34], which had also been confirmed for doped NaTaO3 system with the theoretical results [35,36]. According to the calculations based on density functional theory (DFT), the doped N anion leaded to reduced band gap of NaTaO3 and the isolated midgap states which had adverse effect on the photocatalytic activity [35, 36]. Meanwhile, Modak et al. [35] and Kanhere et al. [37] suggested the codoping of both anion and cation in the charge-compensated mode was a feasible route to design visible-light driven, NaTaO3-based photocatalysts with high activity. Very recently, Zhou et al. verified that the photocatalytic
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activity of NaTaO3 was greatly enhanced with this co-doped mode [38]. However, it is still lack of experimental studies on the effects of the codoping of both cations and anions on the photocatalytic performance for NaTaO3. In the present work, (W, N)-co-doped and mono-doped NaTaO3 powders were successfully synthesized with a flux method at 700 C. The prepared temperature is much lower than that adopted in other researches, in which the solid-state reaction method has applied [10,29]. The codoped NaTaO3 had reduced band gap of 2.3 eV, and high photocatalytic activity on decolorization of organic dye under visible light irradiation. The first principle calculations were also carried out to discuss the electronic structures of (W, N)co-doped NaTaO3. 2. Experimental and calculation details The NaTa1xWxO3xNx (x ¼ 0.00, 0.01, 0.03, 0.05, 0.1) samples were prepared with a NaOH- flux method. In a typical process, NaOH (Alfa Aesar), Ta2O5 (Alfa Aesar), Na2WO4 $2H2O (Alfa Aesar) and Ta3N5 powders prepared by ammoniation of Ta2O5 at 1100 C were applied as raw materials. As shown in the supplementary materials (Figs. S1 and S2), we also studied the effect of molar ratios of raw materials and prepared temperature on the crystalline and morphology of pure NaTaO3 to obtain optimum flux process. In the present work, the molar ratio of NaOH to Ta2O5 was fixed to 2.5: 1. The amounts of W and N were in 20% excess to compensate the loss during preparation. These powders were carefully grinded in an agate mortar. Then the mixed powers were put in the alumina crucible and reacted at 700 C for 4 h under N2 flow. The products were washed with diluted HNO3 and deionized water for several times to remove residual raw materials. Finally, the co-doped NaTaO3 powders were obtained after drying process at 60 C for 12 h. For comparison, the W- and N- monodoped NaTaO3 samples were also synthesized with the same process, as shown in the supplementary materials (Fig. S3). The phase compositions of the as-prepared powders were confirmed by X-ray Power Diffractometry (XRD, Cu Ka, Bruker D8) with a scan rate of 5 /min and a step of 0.02 at room temperature. X-ray photoelectron spectroscopy (XPS) measurements were performed using a VG ESCALAB 210 electron spectrometer. The morphologies of the NaTa1xWxO3xNx powders were observed by transmission electron microscope (TEM, JEM-2100F). The ultravioletevisible diffuse reflectance spectra (DRS) of the co-doped samples were measured by using an ultravioletevisible spectrometer (UVeVis, Hitachi U-3900) at room temperature. The photocatalytic performances of the doped NaTaO3 powders were evaluated by degradation of rhodamine B (RhB) under visible light irradiation. A 300 W xenon lamp (CHF-XM-300W, Trusttech, China) with the cut-off filter (l 400 nm) was applied as the visible-light source with the energy intensity around 100 mW/cm2. In each experiment, 100 mg of the catalyst was added into 200 mL of RhB (10 ppm) solution. Prior to degradation, the solutions were stirred continuously in the dark for ca. 60 min to ensure the adsorption/desorption equilibrium. Then 5 ml aliquots were sampled every 30 min and centrifugated to remove the co-doped NaTaO3 particles. The absorbance changes of the RhB solutions were also measured using the UVeVis spectrophotometer (U3900H) with the absorption band maximum of 554 nm. The degradation efficiency was defined as follows:
decolorize RhB aqueous solution for five successive runs. After each cycle, the centrifugated photocatalyst powders were washed, dried and weighted. The mass loss (about 10%) of the photocatalyst was inevitable during this process. Thus, the volumes of RhB solution used for the repeated cycle were proportionally reduced to ensure the constant concentration of photocatalyst powders. The electronic structures of the doped NaTaO3 systems were calculated with the full-potential linearized augmented planewave method plus local orbital (FLAPW þ lo) implemented in the WIEN2k code [39]. As well known, NaTaO3 exhibits a rich polymorphism over a wide range of temperatures. In the present work, the NaTaO3 powders fabricated with the flux method crystallized in monoclinic structure with the space group of P2/m [10,38]. Therefore, we built a 2 2 2 supercell of monoclinic NaTaO3. And then, one Ta5þ cation and one O2 anion were replaced by one W6þ and one N3, respectively, to form the Na8Ta7WO23N supercell. The full geometry optimizations of cell volumes, c/a ratios and the internal coordinates were carried out using the Broyden-FletcherGoldfarb-Shenno (BFGS) algorithm. The exchange-correlation (XC) energy of electron-electron was treated under the generalized gradient approximation (GGA) corrected with the modified-BeckeJohnson (mBJ) potential to accurately calculate band gap of doped NaTaO3. Special points sampling integration over Brillouin zone (BZ) was performed with 8000 k-points meshes using the tetrahedron method. The convergence tolerances for self-consistency cycles were set as the changing of total energy within 0.1 mRy per unit cell.
3. Results and discussions Fig. 1(a) shows the XRD patterns of co-doped NaTaO3 with various doping amounts of W and N. NaTaO3 powders had been prepared with different methods, including solid state reaction [10,13,29,40], sol-gel method [10,40], hydrothermal synthesis [19,20,31], and flux synthesis [16,17,41,42]. As pointed out by Hu et al. [10,40], the crystal structure of NaTaO3 is sensitive to the prepared temperature. In other words, NaTaO3 powders synthesized at high temperatures (~1200 C) usually crystallize in orthorhombic structure with the TaeOeTa angle of ~163 , while the products prepared at low temperatures have monoclinic structure. To clarify the phase of NaTaO3 synthesized at 700 C with the NaOH flux method, the magnified patterns at 2q ¼ 40 , 57.8 and 67.9 belonged to NaTaO3 and NaTa0.95W0.05O2.95N0.05 samples are shown in Fig. 1(b), respectively. The shape of these peaks is closely similar to that of the NaTaO3 powders prepared with sol-gel
h ¼ (C0Ct)/ C0 100% where C0 and Ct are the initial and residual concentrations of dye aqueous solution, respectively. To investigate the reusability of the co-doped NaTaO3, the used powders were repeatedly applied to
Fig. 1. (a) XRD patterns of NaTa1xWxO3xNx (x ¼ 0.00, 0.01, 0.03, 0.05, 0.1) samples. (b) The magnification of the AeC peaks for NaTaO3 and NaTa0.95W0.05O2.95N0.05 sample.
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method, and is different with the high-temperature phase [10]. Thus, All XRD patterns of the NaTa1xWxO3xNx (x 0.05) samples can be well indexed to the monoclinic NaTaO3 (JCPDS 74-2479) with the lattice constants of a ¼ 3.8895 Å, b ¼ 3.8885 Å, c ¼ 3.8895 Å, and b ¼ 90.367. As the content of the dopants increases to 0.1, the diffraction peaks belonged to Na2W4O13 phase at 2q ¼ 25 can be observed. The Rietveld refinements of the crystal structure parameters were carried out using the GSAS program [43] (shown in supplementary materials). The refined lattice constants of mono-doped and co-doped NaTaO3 are tabulated in Table 1. Obviously, the fitting lattice constants of undoped NaTaO3 are well agree with the theoretical data. Although the radius of N3 (1.61 Å) is larger than that of O2 (1.40 Å), the substitution of N3 for O2 in the unit cell of NaTaO3 only has slight effects on the lattice constants, which is due to the high lattice tolerance of perovskite structure [12,17,19]. Furthermore, the radius of W6þ is much close to that of Ta5þ. Thus, both the mono-doping and co-doping for NaTaO3 with W6þ and N3 result in slight lattice expansion. Fig. 2 shows the morphology of the pure NaTaO3 and NaTa0.95W0.05O2.95N0.05 samples. The as-prepared samples are wellcrystallized and have nanocubic shape, which is similar to the products synthesized with the solid state reaction method [10,13] and hydrothermal method [19,20]. The edge length of the nanocube prepared in the present work is around 100e200 nm, which is much smaller than the size of the samples prepared via hightemperature solid state reaction method. Moreover, the co-doped samples exhibit more regular microstructure, as shown in Fig. 2(c). High resolution TEM images in Fig. 2(b) and (d) demonstrate the structural perfection of NaTaO3 and NaTa0.95W0.05O2.95N0.05 samples, respectively. The observed lattice fringe widths are about 0.27 nm for both NaTaO3 and NaTa0.95W0.05O2.95N0.05 samples, which is assigned to the (101) plane of monoclinic NaTaO3. Both the ordered nanocubic morphology and the clear fringes indicate the high crystallinity of the as-prepared photocatalysts, which can be contributed to the liquid reaction environment supplied by NaOH flux at low temperature. The XPS measurement was carried out to investigate the chemical bonding of as-prepared samples. Fig. 3 shows the XPS spectrum of NaTa0.95W0.05O2.95N0.05 powders. The similar spectra of the other samples are not shown here. In the global range XPS spectra (Fig. 3(a)), the C 1s peak located at 284.5 eV was applied as a standard peak to calibrate all binding energies. The main peaks for Ta4f, W4f, Ta4p3/2, N1s, O1s and Na1s are located at 24e30, 34e40, 405, 398.2, 530 and 1075 eV, respectively. Fig. 3(b) shows typical Ta5þ features containing the Ta 4f5/2 and Ta 4f7/2 peaks at 28.1 and 25.58 eV, respectively. Furthermore, compared with previous reports, the Ta 4f7/2 peak shifts to a lower bonding energy, which indicates the partial formation of TaeN bond [44]. As shown in Fig. 3(c), the strong W 4f7/2 and W 4f5/2 XPS peaks also indicate the partial replacement of W6þ at Ta5þ site. Fig. 3(d) illustrates both Ta4p3/2 and N 1s peaks observed at 396e405 eV, implying the formation of TaeN bonding [38,45]. As listed in Table 1, the surface
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atomic compositions from the XPS spectra for different co-doped samples are very close to the designed stoichiometry except for the NaTa0.9W0.1O2.9N0.1 sample. The above results indicated that NaTaO3 nanocubes with co-doping of both W6þ and N3 in chargecompensated mode were successfully synthesized via the NaOH flux method. The comparative spectra of UVeVis DRS of the as-prepared photocatalysts are shown in Fig. 4. The band gap energy (Eg) for each sample is calculated from the inserted plots in Fig. 4(a) using the following equation, ahv ¼ A(hvEg)n/2
(1)
where, a, n, A are absorption coefficient, light frequency and a constant, respectively. For monoclinic NaTaO3 with the indirect-gap [46], the value of n is equal to 4. The undoped NaTaO3 sample shows strong absorption in UV light region with the absorption edge of ~310 nm. The band gap of undoped sample is estimated to be 3.8 eV, which is well consistent with the previous report [47,48]. For the co-doped NaTaO3 samples, the add-on shoulder is imposed onto the cutoff edge of the absorption spectrum, which extends the absorption range up to about 500 nm. With increasing the amount of W and N, the red shift of the shoulder peak can be obviously observed. The optical band gap of NaTa0.95W0.05O2.95N0.05 sample is correspondingly reduced from 3.8 to 2.3 eV, implying the visiblelight driven activity of this sample. Fig. 4(b) shows DRS spectra of NaTa0.95W0.05O3 and the NaTaO2.95N0.05. The absorption range for these two mono-doped NaTaO3 samples expands up to 420 nm and 460 nm, respectively. Therefore, the mono-doping of W or N can also improve the activity of NaTaO3 under visible-light irradiation as reported in Refs. [17,19,38]. However, by comparison, the band gap of these two mono-doped NaTaO3 samples is much larger than that of co-doped sample, as tabulated in Table 1. The DFT calculations were also performed to further analyze the effect of co-doping of both W and N on the electronic structure of monoclinic NaTaO3. Fig. 5(a) shows the schematic diagram of the optimized Na8Ta7WO23N supercell. As well-known, the DFT calculations usually overestimate the lattice constants and underestimate the band gap of semiconductors. It is also true for the W, N codoped NaTaO3 case in the present work. The optimized lattice constants of the co-doped 2 2 2 supercell are as fellow, a ¼ 8.094 Å, b ¼ 8.033 Å and c ¼ 8.030 Å, which are slightly higher than the refined data. The interaxial angles for the co-doped supercell are also deviated from the theoretical values. Of particular importance is the twisted TaeOeTa bond in Na8Ta8WO23N supercell. For monoclinic NaTaO3, the TaeOeTa angle is close to the ideal value of 180 . As pointed out in the previous studies [7,10], the larger the bond angles of TaeOeTa is, the higher the mobility of photo-generated carriers is in NaTaO3. However, the TaeOeTa angles in the co-doped NaTaO3 vary from the ideal value to about 165 , which is much close to that for orthorhombic NaTaO3. The bond distortion is further confirmed with FTIR spectra in
Table 1 Featured lattice constants of as-prepared NaTaO3 samples refined with GSAS code, chemical composition of the doped photocatalysts measured using XPS, and the optical band gap derived from UVeVis diffuse reflectance spectroscopy for as-prepared doped NaTaO3 samples. Samples
NaTaO3 NaTaO2.95N0.05 NaTa0.95W0.05O3 NaTa0.99W0.01O2.99N0.01 NaTa0.97W0.03O2.97N0.03 NaTa0.95W0.05O2.95N0.05
Lattice constants a (Å)
b (Å)
c (Å)
b (o)
3.8929 3.8959 3.8960 3.9017 3.8964 3.8954
3.8917 3.8926 3.8978 3.8987 3.8937 3.8973
3.8897 3.8899 3.8960 3.8963 3.8930 3.8927
90.347 90.299 90.301 90.313 90.319 90.344
Chemical composition
Band gap (eV)
NaTa1.02O2.96 NaTa1.01O2.93N0.04 NaTa0.98W0.04O2.96 NaTa1.01W0.01O2.95N0.02 NaTa0.97W0.02O2.93N0.03 NaTa0.98W0.04O2.92N0.04
3.8 3.0 3.2 2.8 2.6 2.3
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Fig. 2. TEM and HTEM images of (a, b) undoped-NaTaO3, and (c, d) NaTa0.95W0.05O2.95N0.05 sample.
supplementary materials (Fig. S5). Therefore, the charge-carrier mobility in the co-doped case should be decreased, and consequently to the photocatalytic activity. As shown in Fig. 5(b) and (c), the calculated total and partial densities of states (DOSs) are plotted for pure and co-doped NaTaO3, respectively. The aforementioned mBJ potential was applied to accurately calculate the electronic structure of tantalates. For monoclinic NaTaO3, the calculated band gap of ca. 3.9 eV is well consistent with the above experimental results and other previous studies [10,24,35,37]. The theoretical value of band gap for the codoped supercell is ca. 2.9 eV. The good agreement between the DFT calculated values and derived results from DRS spectra for the band gap of co-doped NaTaO3 further confirms the reliability of the DFT calculation adopted in the present work. For a semiconductor photocatalyst, the electronic structure in vicinity of Fermi level plays a key role in its photocatalytic performance. The top of valence band for monoclinic NaTaO3 is mainly contributed by O 2p state, while the conduction band minimum (CBM) is originated from Ta 5d stats. Doping of N at the O site produces the band-like states just above the valence band maximum (VBM), and does not bring any shift of CBM (shown in supplementary materials) [35]. The substitution of Ta by W introduces continuum band below CBM and thus reduces the band gap by about 0.3 eV, as shown in Fig. 5(c). With co-doping of W and N, the band gap of NaTaO3 is efficiently reduced to respond visible light. In addition, this chargecompensated co-doping mode can inhibit the formation of charge-
related defects which are also important to suppressing the photocatalytic activity [37]. In a word, the co-doping of both W and N has opposite effects on the photocatalytic performance of NaTaO3. The twisted TaeOeTa bonds resulted from the co-doping have adverse effect on the performance. While the reduced band gap gives rise to enhancement of visible light activity. The photocatalytic activity of the as-prepared NaTa1xWxO3xNx samples was evaluated by degradation of RhB aqueous as a model reaction under visible-light irradiation. The concentration changes of RhB are monitored by examine the variations in maximal absorption in UVevis spectra at 554 nm. The temporal evolution of RhB degradation by NaTa1xWxO3xNx (x ¼ 0.05) sample is shown in Fig. 6(a). It can be seen that the concentration of RhB decreases with increasing the irradiation time and the peak at 553 nm completely disappears after irradiation for 3.5 h. As shown in Fig. 6(b), for the blank experiment, only a small amount of RhB was degraded. By comparison, after exposure to visible light for 3.5 h, the degradation efficiencies of RhB over undoped NaTaO3 and NaTa0.95W0.05O2.95N0.05 samples were 54% and 93%, respectively. To further discuss the effect of doped mode on the photocatalytic performance, Fig 6(c) compared the photocatalytic degradation of RhB over the NaTa0.95W0.05O3 and NaTa0.95W0.05O2.95N0.05 under visible light irradiation, respectively. Obviously, the degradation efficiency of RhB over these two mono-doped samples is also improved, which is ascribed to both the visible-light responded ability and doped level acted as photocarriers trap. However, the
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Fig. 3. XPS spectrum of the as-prepared NaTa0.95W0.05O2.95N0.05 sample. (a) global; (b, c, d) Gaussian fitting of the enlarged areas in the red frames of (a) for the Ta 4f, W 4f and Ta 4p3/2/N 1s peaks, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. UVeVis diffuse reflectance spectra of (a) as-prepared NaTa1xWxO3xNx photocatalysts (Insert: plots of (ahn)1/2 vs. photon energy (hn)) and (b) NaTaO2.95N0.05 and NaTa0.95W0.05O3 photocatalysts.
photocatalytic activity of these two mono-doped samples under visible light irradiation is still inferior to that of NaTa0.95W0.05O2.95N0.05 sample. According to above discussion, the high activity of co-doped NaTaO3 can be contributed to the following factors. The first is the narrowed band gap of about 2.3 eV for NaTa0.95W0.05O2.95N0.05 sample, which results in large improvement of visible light
absorption. The second is charge-compensated co-doping mode. Owing to the charge imbalance for the mono-doping system, the charge-related defects are usually formed and reduce the photocatalytic activity by accelerate the recombination of photogenerated electron and hole [32e35]. By comparison, the codoping of W and N in same molar ratio ensures the charge balance, and thus decreases the defect concentration of doped NaTaO3
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Fig. 5. (a) The schematic diagram of 2 2 2 supercell of NaTaO3 with the codoping of W and N. (b) The calculated total and partial density of states of monoclinic NaTaO3 corrected with mBJ potential. (c) The calculated partial density of states for W, N, Ta and O, respectively. The Fermi level is assigned to 0 eV.
Fig. 6. (a) UVevis spectral changes of RhB in aqueous NaTa0.95W0.05O2.95N0.05 dispersion as a function of visible light irradiation time; (b) Normalized concentration of RhB versus visible light irradiation time over NaTa1xWxO3xNx photocatalysts. (c) Photocatalytic degradation of RhB for the NaTaO2.95N0.05 and NaTa0.95W0.05O3 under visible light irradiation. (d) The cyclic photocatalytic test of NaTa0.95W0.05O2.95N0.05 for the degradation of RhB.
products, which is favorable to the separation of photogenerated electrons and holes. Furthermore, the high crystallinity of the codoped NaTaO3 samples synthesized with the NaOH flux method is also contributed to the inhibition of volume recombination of carriers. The reusability of a photocatalyst has been considered as one of the key factors for applications. As shown in Fig. 6(d), the reusability of NaTa0.95W0.05O2.95N0.05 photocatalyst was proved with the successive photocatalytic runs. The degradation of RhB over NaTa0.95W0.05O2.95N0.05 powders displayed steady performance
during five cycles with the fixed irradiation time of 5 h. The good stability of the co-doped NaTaO3 is comparable to that of C3N4/ NaTaO3 composite [22]. The deactivation of NaTaO3 can be usually attributed to the elution of Naþ on the surface of NaTaO3 during photocatalytic process, which leads to the formation of the surface defects, and thus accelerating the recombination of photogenerated carriers [49e51]. In the present work, the co-doped NaTaO3 powders containing stoichiometric or slightly excess Na were synthesized at a relatively low temperature with excess NaOH. Meanwhile, as mentioned above, the charge-compensated doping
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mode was also beneficial to suppressing the formation of Na vacancy. Thus, the sufficient Naþ on the surface of as-prepared photocatalyst is assumed to give rise to the stable recycling performance of co-doped NaTaO3. In addition, as described above, the volume of RhB solution was sharply reduced for the last two cycles, which resulted in relatively rapid self-degradation (also shown in Fig. 6(b) under long-time irradiation) under same conditions for these runs. Therefore, the photocatalytic efficiency should be overestimated for these two cycles. Nevertheless, the (W, N)-co-doped NaTaO3 powders can be used as high-performance and visible-light driven photocatalysts for environmental applications. The doping of anion and/or cation has been widely employed to tune the band gap of photocatalysts. The above discussion on NaTaO3 indicates the superiority of co-doping of both cation and anion in charge-compensated mode over the mono-doping. Furthermore, the NaOH flux method is also confirmed to be a feasible route to fabricate doped tantalates or other oxide photocatalysts with designed stiochiometry ratio. It is expected that the route presented here can be applied to design and fabricate doped oxide photocatalysts driven by visible light. 4. Conclusions In summary, we discussed the synthesis, phase composition, microstructure, electronic structure and photocatalytic performance of the W,N-co-doped NaTaO3 in the present work. The NaTaO3 nanopowders with the co-doping of both W and N in charge-compensated mode were synthesized with a NaOH flux method at a temperature as low as 700 C. The as-prepared samples crystallized in monoclinic structure and exhibited nanocubic shape in a size of 100e200 nm. The chemical compositions of doped NaTaO3 were close to the designed stiochiometry. The accurate DFT calculation results indicated that the doping of W and N introduced the states below CBM and bands above VBM, respectively. Thus, the as-prepared NaTa1xWxO3xNx (x 0.03) samples exhibited strong absorption extended up to 500 nm with the corresponding band gap of ca. 2.3 eV. The co-doped NaTaO3 showed superior activity for decolorization of rhodamine B in aqueous solution under irradiation at wavelengths longer than 400 nm. The remarkable improvement of photocatalytic activity were contributed to high absorption to visible light, high crystallinity, as well as low concentration of introduced defects resulted from both the co-doping in charge-compensated mode and the NaOH flux synthesis. The present work is experimentally valuable to band engineering of NaTaO3 and other oxide photocatalysts. Acknowledgements This work was supported by the Science and Technology Innovation Fund for Outstanding Youth in Hebei University of Technology (No. 2013006), the Natural Science Foundation of Hebei Province (No. E2014202155), and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, No. IRT13060). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.04.231. References [1] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253e278.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced organic dye removal of the W and N co-doped NaTaO3 under visible light irradiation Sai Wang a, b, Xuewen Xu a, b, *, Han Luo a, b, Yinghao Bai a, b, Saleem Abbas a, b, Jun Zhang a, b, Jianling Zhao a, b, Chengchun Tang a, b a b
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China Hebei Key Laboratory of Boron Nitride Micro and Nano Materials, Tianjin 300130, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2016 Received in revised form 7 April 2016 Accepted 22 April 2016 Available online 24 April 2016
We carried out both the experimental and theoretical investigations on the synthesis and photocatalytic performance of the visible-light responded, W and N co-doped NaTaO3 powders. The nanocubic NaTa1xWxO3xNx (x ¼ 0.00, 0.01, 0.03, 0.05, 0.1) particles with a range of 100e200 nm in size have been successfully synthesized at 700 C via the NaOH flux reaction method. The microstructure, chemical composition and optical absorption of the as-prepared photocatalysts were discussed in detail. The photocatalytic performance of as-prepared powders was evaluated by decolarization of rhodamine B (RhB) under visible light irradiation with the wavelengths longer than 400 nm. The optical band gap of NaTaO3 could be significantly reduced through the co-doping of W and N. The degradation efficiency of RhB over the NaTaO3 with co-doping of W and N in the charge-compensated mode was remarkably improved. Furthermore, the first-principle calculations were also performed to discover the effects of codoping of W and N on the electronic structure of NaTaO3. These present results provide a simple and feasible route to design new photocatalysts based on NaTaO3 or other oxide photocatalysts. © 2016 Elsevier B.V. All rights reserved.
Keywords: Photocatalysis NaTaO3 Dye removal First-principle calculations Band engineering
1. Introduction The perovskite-type oxides containing d0 transition-metal ions, such as Nb, Ti and Ta, have been reported as the effective photocatalysts for degradation of organic dyes and water splitting [1e4]. Tantalate sodium (NaTaO3) seems to be more advantageous due to its excellent effectiveness in carrier separation [5e10]. However, as a wide-gap (~4.1 eV) semiconductor, the pure NaTaO3 photocatalyst only works efficiently under ultraviolet light irradiation [5e8]. To improve the photocatalytic efficiency of NaTaO3, numerous approaches have been introduced, including the doping of anions or cations [9,11e20], forming composites [15,21e23], designing various nanostructures [24,25], and forming solid solutions [26e29]. The doping of NaTaO3 has been considered as a promising way
* Corresponding author. School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, PR China. E-mail addresses: [email protected] (S. Wang), [email protected] (X. Xu), [email protected] (H. Luo), [email protected] (Y. Bai), saleem_ [email protected] (S. Abbas), [email protected] (J. Zhang), zhaojl@hebut. edu.cn (J. Zhao), [email protected] (C. Tang). http://dx.doi.org/10.1016/j.jallcom.2016.04.231 0925-8388/© 2016 Elsevier B.V. All rights reserved.
to improve the photoactivity under visible light irradiation. The doping of anions, including N3 [17e19] and S2 [11], usually resulted in obvious red shift and additional add-on shoulder at the absorption edge of NaTaO3. The La-doped NaTaO3 exhibited enhanced photocatalytic activity on water splitting, even though its optical band gap (4.13e4.17 eV) was slightly larger than that of pure NaTaO3 [13,16]. By comparison, the absorption edge of NaTaO3 with the codoping of La and Cr was shifted to 420 nm, which was contributed to the charge transfer from Cr 3d orbital to Ta 5d orbital [30]. The self-doped Ta4þ cation in NaTaO3 narrowed the band gap to 1.70 eV [31]. However, the mono-doped mode for oxide photocatalysts usually resulted in new recombination centers associated with dopants due to charge imbalance [32e34], which had also been confirmed for doped NaTaO3 system with the theoretical results [35,36]. According to the calculations based on density functional theory (DFT), the doped N anion leaded to reduced band gap of NaTaO3 and the isolated midgap states which had adverse effect on the photocatalytic activity [35, 36]. Meanwhile, Modak et al. [35] and Kanhere et al. [37] suggested the codoping of both anion and cation in the charge-compensated mode was a feasible route to design visible-light driven, NaTaO3-based photocatalysts with high activity. Very recently, Zhou et al. verified that the photocatalytic
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activity of NaTaO3 was greatly enhanced with this co-doped mode [38]. However, it is still lack of experimental studies on the effects of the codoping of both cations and anions on the photocatalytic performance for NaTaO3. In the present work, (W, N)-co-doped and mono-doped NaTaO3 powders were successfully synthesized with a flux method at 700 C. The prepared temperature is much lower than that adopted in other researches, in which the solid-state reaction method has applied [10,29]. The codoped NaTaO3 had reduced band gap of 2.3 eV, and high photocatalytic activity on decolorization of organic dye under visible light irradiation. The first principle calculations were also carried out to discuss the electronic structures of (W, N)co-doped NaTaO3. 2. Experimental and calculation details The NaTa1xWxO3xNx (x ¼ 0.00, 0.01, 0.03, 0.05, 0.1) samples were prepared with a NaOH- flux method. In a typical process, NaOH (Alfa Aesar), Ta2O5 (Alfa Aesar), Na2WO4 $2H2O (Alfa Aesar) and Ta3N5 powders prepared by ammoniation of Ta2O5 at 1100 C were applied as raw materials. As shown in the supplementary materials (Figs. S1 and S2), we also studied the effect of molar ratios of raw materials and prepared temperature on the crystalline and morphology of pure NaTaO3 to obtain optimum flux process. In the present work, the molar ratio of NaOH to Ta2O5 was fixed to 2.5: 1. The amounts of W and N were in 20% excess to compensate the loss during preparation. These powders were carefully grinded in an agate mortar. Then the mixed powers were put in the alumina crucible and reacted at 700 C for 4 h under N2 flow. The products were washed with diluted HNO3 and deionized water for several times to remove residual raw materials. Finally, the co-doped NaTaO3 powders were obtained after drying process at 60 C for 12 h. For comparison, the W- and N- monodoped NaTaO3 samples were also synthesized with the same process, as shown in the supplementary materials (Fig. S3). The phase compositions of the as-prepared powders were confirmed by X-ray Power Diffractometry (XRD, Cu Ka, Bruker D8) with a scan rate of 5 /min and a step of 0.02 at room temperature. X-ray photoelectron spectroscopy (XPS) measurements were performed using a VG ESCALAB 210 electron spectrometer. The morphologies of the NaTa1xWxO3xNx powders were observed by transmission electron microscope (TEM, JEM-2100F). The ultravioletevisible diffuse reflectance spectra (DRS) of the co-doped samples were measured by using an ultravioletevisible spectrometer (UVeVis, Hitachi U-3900) at room temperature. The photocatalytic performances of the doped NaTaO3 powders were evaluated by degradation of rhodamine B (RhB) under visible light irradiation. A 300 W xenon lamp (CHF-XM-300W, Trusttech, China) with the cut-off filter (l 400 nm) was applied as the visible-light source with the energy intensity around 100 mW/cm2. In each experiment, 100 mg of the catalyst was added into 200 mL of RhB (10 ppm) solution. Prior to degradation, the solutions were stirred continuously in the dark for ca. 60 min to ensure the adsorption/desorption equilibrium. Then 5 ml aliquots were sampled every 30 min and centrifugated to remove the co-doped NaTaO3 particles. The absorbance changes of the RhB solutions were also measured using the UVeVis spectrophotometer (U3900H) with the absorption band maximum of 554 nm. The degradation efficiency was defined as follows:
decolorize RhB aqueous solution for five successive runs. After each cycle, the centrifugated photocatalyst powders were washed, dried and weighted. The mass loss (about 10%) of the photocatalyst was inevitable during this process. Thus, the volumes of RhB solution used for the repeated cycle were proportionally reduced to ensure the constant concentration of photocatalyst powders. The electronic structures of the doped NaTaO3 systems were calculated with the full-potential linearized augmented planewave method plus local orbital (FLAPW þ lo) implemented in the WIEN2k code [39]. As well known, NaTaO3 exhibits a rich polymorphism over a wide range of temperatures. In the present work, the NaTaO3 powders fabricated with the flux method crystallized in monoclinic structure with the space group of P2/m [10,38]. Therefore, we built a 2 2 2 supercell of monoclinic NaTaO3. And then, one Ta5þ cation and one O2 anion were replaced by one W6þ and one N3, respectively, to form the Na8Ta7WO23N supercell. The full geometry optimizations of cell volumes, c/a ratios and the internal coordinates were carried out using the Broyden-FletcherGoldfarb-Shenno (BFGS) algorithm. The exchange-correlation (XC) energy of electron-electron was treated under the generalized gradient approximation (GGA) corrected with the modified-BeckeJohnson (mBJ) potential to accurately calculate band gap of doped NaTaO3. Special points sampling integration over Brillouin zone (BZ) was performed with 8000 k-points meshes using the tetrahedron method. The convergence tolerances for self-consistency cycles were set as the changing of total energy within 0.1 mRy per unit cell.
3. Results and discussions Fig. 1(a) shows the XRD patterns of co-doped NaTaO3 with various doping amounts of W and N. NaTaO3 powders had been prepared with different methods, including solid state reaction [10,13,29,40], sol-gel method [10,40], hydrothermal synthesis [19,20,31], and flux synthesis [16,17,41,42]. As pointed out by Hu et al. [10,40], the crystal structure of NaTaO3 is sensitive to the prepared temperature. In other words, NaTaO3 powders synthesized at high temperatures (~1200 C) usually crystallize in orthorhombic structure with the TaeOeTa angle of ~163 , while the products prepared at low temperatures have monoclinic structure. To clarify the phase of NaTaO3 synthesized at 700 C with the NaOH flux method, the magnified patterns at 2q ¼ 40 , 57.8 and 67.9 belonged to NaTaO3 and NaTa0.95W0.05O2.95N0.05 samples are shown in Fig. 1(b), respectively. The shape of these peaks is closely similar to that of the NaTaO3 powders prepared with sol-gel
h ¼ (C0Ct)/ C0 100% where C0 and Ct are the initial and residual concentrations of dye aqueous solution, respectively. To investigate the reusability of the co-doped NaTaO3, the used powders were repeatedly applied to
Fig. 1. (a) XRD patterns of NaTa1xWxO3xNx (x ¼ 0.00, 0.01, 0.03, 0.05, 0.1) samples. (b) The magnification of the AeC peaks for NaTaO3 and NaTa0.95W0.05O2.95N0.05 sample.
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method, and is different with the high-temperature phase [10]. Thus, All XRD patterns of the NaTa1xWxO3xNx (x 0.05) samples can be well indexed to the monoclinic NaTaO3 (JCPDS 74-2479) with the lattice constants of a ¼ 3.8895 Å, b ¼ 3.8885 Å, c ¼ 3.8895 Å, and b ¼ 90.367. As the content of the dopants increases to 0.1, the diffraction peaks belonged to Na2W4O13 phase at 2q ¼ 25 can be observed. The Rietveld refinements of the crystal structure parameters were carried out using the GSAS program [43] (shown in supplementary materials). The refined lattice constants of mono-doped and co-doped NaTaO3 are tabulated in Table 1. Obviously, the fitting lattice constants of undoped NaTaO3 are well agree with the theoretical data. Although the radius of N3 (1.61 Å) is larger than that of O2 (1.40 Å), the substitution of N3 for O2 in the unit cell of NaTaO3 only has slight effects on the lattice constants, which is due to the high lattice tolerance of perovskite structure [12,17,19]. Furthermore, the radius of W6þ is much close to that of Ta5þ. Thus, both the mono-doping and co-doping for NaTaO3 with W6þ and N3 result in slight lattice expansion. Fig. 2 shows the morphology of the pure NaTaO3 and NaTa0.95W0.05O2.95N0.05 samples. The as-prepared samples are wellcrystallized and have nanocubic shape, which is similar to the products synthesized with the solid state reaction method [10,13] and hydrothermal method [19,20]. The edge length of the nanocube prepared in the present work is around 100e200 nm, which is much smaller than the size of the samples prepared via hightemperature solid state reaction method. Moreover, the co-doped samples exhibit more regular microstructure, as shown in Fig. 2(c). High resolution TEM images in Fig. 2(b) and (d) demonstrate the structural perfection of NaTaO3 and NaTa0.95W0.05O2.95N0.05 samples, respectively. The observed lattice fringe widths are about 0.27 nm for both NaTaO3 and NaTa0.95W0.05O2.95N0.05 samples, which is assigned to the (101) plane of monoclinic NaTaO3. Both the ordered nanocubic morphology and the clear fringes indicate the high crystallinity of the as-prepared photocatalysts, which can be contributed to the liquid reaction environment supplied by NaOH flux at low temperature. The XPS measurement was carried out to investigate the chemical bonding of as-prepared samples. Fig. 3 shows the XPS spectrum of NaTa0.95W0.05O2.95N0.05 powders. The similar spectra of the other samples are not shown here. In the global range XPS spectra (Fig. 3(a)), the C 1s peak located at 284.5 eV was applied as a standard peak to calibrate all binding energies. The main peaks for Ta4f, W4f, Ta4p3/2, N1s, O1s and Na1s are located at 24e30, 34e40, 405, 398.2, 530 and 1075 eV, respectively. Fig. 3(b) shows typical Ta5þ features containing the Ta 4f5/2 and Ta 4f7/2 peaks at 28.1 and 25.58 eV, respectively. Furthermore, compared with previous reports, the Ta 4f7/2 peak shifts to a lower bonding energy, which indicates the partial formation of TaeN bond [44]. As shown in Fig. 3(c), the strong W 4f7/2 and W 4f5/2 XPS peaks also indicate the partial replacement of W6þ at Ta5þ site. Fig. 3(d) illustrates both Ta4p3/2 and N 1s peaks observed at 396e405 eV, implying the formation of TaeN bonding [38,45]. As listed in Table 1, the surface
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atomic compositions from the XPS spectra for different co-doped samples are very close to the designed stoichiometry except for the NaTa0.9W0.1O2.9N0.1 sample. The above results indicated that NaTaO3 nanocubes with co-doping of both W6þ and N3 in chargecompensated mode were successfully synthesized via the NaOH flux method. The comparative spectra of UVeVis DRS of the as-prepared photocatalysts are shown in Fig. 4. The band gap energy (Eg) for each sample is calculated from the inserted plots in Fig. 4(a) using the following equation, ahv ¼ A(hvEg)n/2
(1)
where, a, n, A are absorption coefficient, light frequency and a constant, respectively. For monoclinic NaTaO3 with the indirect-gap [46], the value of n is equal to 4. The undoped NaTaO3 sample shows strong absorption in UV light region with the absorption edge of ~310 nm. The band gap of undoped sample is estimated to be 3.8 eV, which is well consistent with the previous report [47,48]. For the co-doped NaTaO3 samples, the add-on shoulder is imposed onto the cutoff edge of the absorption spectrum, which extends the absorption range up to about 500 nm. With increasing the amount of W and N, the red shift of the shoulder peak can be obviously observed. The optical band gap of NaTa0.95W0.05O2.95N0.05 sample is correspondingly reduced from 3.8 to 2.3 eV, implying the visiblelight driven activity of this sample. Fig. 4(b) shows DRS spectra of NaTa0.95W0.05O3 and the NaTaO2.95N0.05. The absorption range for these two mono-doped NaTaO3 samples expands up to 420 nm and 460 nm, respectively. Therefore, the mono-doping of W or N can also improve the activity of NaTaO3 under visible-light irradiation as reported in Refs. [17,19,38]. However, by comparison, the band gap of these two mono-doped NaTaO3 samples is much larger than that of co-doped sample, as tabulated in Table 1. The DFT calculations were also performed to further analyze the effect of co-doping of both W and N on the electronic structure of monoclinic NaTaO3. Fig. 5(a) shows the schematic diagram of the optimized Na8Ta7WO23N supercell. As well-known, the DFT calculations usually overestimate the lattice constants and underestimate the band gap of semiconductors. It is also true for the W, N codoped NaTaO3 case in the present work. The optimized lattice constants of the co-doped 2 2 2 supercell are as fellow, a ¼ 8.094 Å, b ¼ 8.033 Å and c ¼ 8.030 Å, which are slightly higher than the refined data. The interaxial angles for the co-doped supercell are also deviated from the theoretical values. Of particular importance is the twisted TaeOeTa bond in Na8Ta8WO23N supercell. For monoclinic NaTaO3, the TaeOeTa angle is close to the ideal value of 180 . As pointed out in the previous studies [7,10], the larger the bond angles of TaeOeTa is, the higher the mobility of photo-generated carriers is in NaTaO3. However, the TaeOeTa angles in the co-doped NaTaO3 vary from the ideal value to about 165 , which is much close to that for orthorhombic NaTaO3. The bond distortion is further confirmed with FTIR spectra in
Table 1 Featured lattice constants of as-prepared NaTaO3 samples refined with GSAS code, chemical composition of the doped photocatalysts measured using XPS, and the optical band gap derived from UVeVis diffuse reflectance spectroscopy for as-prepared doped NaTaO3 samples. Samples
NaTaO3 NaTaO2.95N0.05 NaTa0.95W0.05O3 NaTa0.99W0.01O2.99N0.01 NaTa0.97W0.03O2.97N0.03 NaTa0.95W0.05O2.95N0.05
Lattice constants a (Å)
b (Å)
c (Å)
b (o)
3.8929 3.8959 3.8960 3.9017 3.8964 3.8954
3.8917 3.8926 3.8978 3.8987 3.8937 3.8973
3.8897 3.8899 3.8960 3.8963 3.8930 3.8927
90.347 90.299 90.301 90.313 90.319 90.344
Chemical composition
Band gap (eV)
NaTa1.02O2.96 NaTa1.01O2.93N0.04 NaTa0.98W0.04O2.96 NaTa1.01W0.01O2.95N0.02 NaTa0.97W0.02O2.93N0.03 NaTa0.98W0.04O2.92N0.04
3.8 3.0 3.2 2.8 2.6 2.3
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Fig. 2. TEM and HTEM images of (a, b) undoped-NaTaO3, and (c, d) NaTa0.95W0.05O2.95N0.05 sample.
supplementary materials (Fig. S5). Therefore, the charge-carrier mobility in the co-doped case should be decreased, and consequently to the photocatalytic activity. As shown in Fig. 5(b) and (c), the calculated total and partial densities of states (DOSs) are plotted for pure and co-doped NaTaO3, respectively. The aforementioned mBJ potential was applied to accurately calculate the electronic structure of tantalates. For monoclinic NaTaO3, the calculated band gap of ca. 3.9 eV is well consistent with the above experimental results and other previous studies [10,24,35,37]. The theoretical value of band gap for the codoped supercell is ca. 2.9 eV. The good agreement between the DFT calculated values and derived results from DRS spectra for the band gap of co-doped NaTaO3 further confirms the reliability of the DFT calculation adopted in the present work. For a semiconductor photocatalyst, the electronic structure in vicinity of Fermi level plays a key role in its photocatalytic performance. The top of valence band for monoclinic NaTaO3 is mainly contributed by O 2p state, while the conduction band minimum (CBM) is originated from Ta 5d stats. Doping of N at the O site produces the band-like states just above the valence band maximum (VBM), and does not bring any shift of CBM (shown in supplementary materials) [35]. The substitution of Ta by W introduces continuum band below CBM and thus reduces the band gap by about 0.3 eV, as shown in Fig. 5(c). With co-doping of W and N, the band gap of NaTaO3 is efficiently reduced to respond visible light. In addition, this chargecompensated co-doping mode can inhibit the formation of charge-
related defects which are also important to suppressing the photocatalytic activity [37]. In a word, the co-doping of both W and N has opposite effects on the photocatalytic performance of NaTaO3. The twisted TaeOeTa bonds resulted from the co-doping have adverse effect on the performance. While the reduced band gap gives rise to enhancement of visible light activity. The photocatalytic activity of the as-prepared NaTa1xWxO3xNx samples was evaluated by degradation of RhB aqueous as a model reaction under visible-light irradiation. The concentration changes of RhB are monitored by examine the variations in maximal absorption in UVevis spectra at 554 nm. The temporal evolution of RhB degradation by NaTa1xWxO3xNx (x ¼ 0.05) sample is shown in Fig. 6(a). It can be seen that the concentration of RhB decreases with increasing the irradiation time and the peak at 553 nm completely disappears after irradiation for 3.5 h. As shown in Fig. 6(b), for the blank experiment, only a small amount of RhB was degraded. By comparison, after exposure to visible light for 3.5 h, the degradation efficiencies of RhB over undoped NaTaO3 and NaTa0.95W0.05O2.95N0.05 samples were 54% and 93%, respectively. To further discuss the effect of doped mode on the photocatalytic performance, Fig 6(c) compared the photocatalytic degradation of RhB over the NaTa0.95W0.05O3 and NaTa0.95W0.05O2.95N0.05 under visible light irradiation, respectively. Obviously, the degradation efficiency of RhB over these two mono-doped samples is also improved, which is ascribed to both the visible-light responded ability and doped level acted as photocarriers trap. However, the
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Fig. 3. XPS spectrum of the as-prepared NaTa0.95W0.05O2.95N0.05 sample. (a) global; (b, c, d) Gaussian fitting of the enlarged areas in the red frames of (a) for the Ta 4f, W 4f and Ta 4p3/2/N 1s peaks, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. UVeVis diffuse reflectance spectra of (a) as-prepared NaTa1xWxO3xNx photocatalysts (Insert: plots of (ahn)1/2 vs. photon energy (hn)) and (b) NaTaO2.95N0.05 and NaTa0.95W0.05O3 photocatalysts.
photocatalytic activity of these two mono-doped samples under visible light irradiation is still inferior to that of NaTa0.95W0.05O2.95N0.05 sample. According to above discussion, the high activity of co-doped NaTaO3 can be contributed to the following factors. The first is the narrowed band gap of about 2.3 eV for NaTa0.95W0.05O2.95N0.05 sample, which results in large improvement of visible light
absorption. The second is charge-compensated co-doping mode. Owing to the charge imbalance for the mono-doping system, the charge-related defects are usually formed and reduce the photocatalytic activity by accelerate the recombination of photogenerated electron and hole [32e35]. By comparison, the codoping of W and N in same molar ratio ensures the charge balance, and thus decreases the defect concentration of doped NaTaO3
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Fig. 5. (a) The schematic diagram of 2 2 2 supercell of NaTaO3 with the codoping of W and N. (b) The calculated total and partial density of states of monoclinic NaTaO3 corrected with mBJ potential. (c) The calculated partial density of states for W, N, Ta and O, respectively. The Fermi level is assigned to 0 eV.
Fig. 6. (a) UVevis spectral changes of RhB in aqueous NaTa0.95W0.05O2.95N0.05 dispersion as a function of visible light irradiation time; (b) Normalized concentration of RhB versus visible light irradiation time over NaTa1xWxO3xNx photocatalysts. (c) Photocatalytic degradation of RhB for the NaTaO2.95N0.05 and NaTa0.95W0.05O3 under visible light irradiation. (d) The cyclic photocatalytic test of NaTa0.95W0.05O2.95N0.05 for the degradation of RhB.
products, which is favorable to the separation of photogenerated electrons and holes. Furthermore, the high crystallinity of the codoped NaTaO3 samples synthesized with the NaOH flux method is also contributed to the inhibition of volume recombination of carriers. The reusability of a photocatalyst has been considered as one of the key factors for applications. As shown in Fig. 6(d), the reusability of NaTa0.95W0.05O2.95N0.05 photocatalyst was proved with the successive photocatalytic runs. The degradation of RhB over NaTa0.95W0.05O2.95N0.05 powders displayed steady performance
during five cycles with the fixed irradiation time of 5 h. The good stability of the co-doped NaTaO3 is comparable to that of C3N4/ NaTaO3 composite [22]. The deactivation of NaTaO3 can be usually attributed to the elution of Naþ on the surface of NaTaO3 during photocatalytic process, which leads to the formation of the surface defects, and thus accelerating the recombination of photogenerated carriers [49e51]. In the present work, the co-doped NaTaO3 powders containing stoichiometric or slightly excess Na were synthesized at a relatively low temperature with excess NaOH. Meanwhile, as mentioned above, the charge-compensated doping
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mode was also beneficial to suppressing the formation of Na vacancy. Thus, the sufficient Naþ on the surface of as-prepared photocatalyst is assumed to give rise to the stable recycling performance of co-doped NaTaO3. In addition, as described above, the volume of RhB solution was sharply reduced for the last two cycles, which resulted in relatively rapid self-degradation (also shown in Fig. 6(b) under long-time irradiation) under same conditions for these runs. Therefore, the photocatalytic efficiency should be overestimated for these two cycles. Nevertheless, the (W, N)-co-doped NaTaO3 powders can be used as high-performance and visible-light driven photocatalysts for environmental applications. The doping of anion and/or cation has been widely employed to tune the band gap of photocatalysts. The above discussion on NaTaO3 indicates the superiority of co-doping of both cation and anion in charge-compensated mode over the mono-doping. Furthermore, the NaOH flux method is also confirmed to be a feasible route to fabricate doped tantalates or other oxide photocatalysts with designed stiochiometry ratio. It is expected that the route presented here can be applied to design and fabricate doped oxide photocatalysts driven by visible light. 4. Conclusions In summary, we discussed the synthesis, phase composition, microstructure, electronic structure and photocatalytic performance of the W,N-co-doped NaTaO3 in the present work. The NaTaO3 nanopowders with the co-doping of both W and N in charge-compensated mode were synthesized with a NaOH flux method at a temperature as low as 700 C. The as-prepared samples crystallized in monoclinic structure and exhibited nanocubic shape in a size of 100e200 nm. The chemical compositions of doped NaTaO3 were close to the designed stiochiometry. The accurate DFT calculation results indicated that the doping of W and N introduced the states below CBM and bands above VBM, respectively. Thus, the as-prepared NaTa1xWxO3xNx (x 0.03) samples exhibited strong absorption extended up to 500 nm with the corresponding band gap of ca. 2.3 eV. The co-doped NaTaO3 showed superior activity for decolorization of rhodamine B in aqueous solution under irradiation at wavelengths longer than 400 nm. The remarkable improvement of photocatalytic activity were contributed to high absorption to visible light, high crystallinity, as well as low concentration of introduced defects resulted from both the co-doping in charge-compensated mode and the NaOH flux synthesis. The present work is experimentally valuable to band engineering of NaTaO3 and other oxide photocatalysts. Acknowledgements This work was supported by the Science and Technology Innovation Fund for Outstanding Youth in Hebei University of Technology (No. 2013006), the Natural Science Foundation of Hebei Province (No. E2014202155), and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, No. IRT13060). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.04.231. References [1] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253e278.
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