Bi4TaO8Cl Nano-Photocatalyst: Influence of Local, Average, and

Apr 21, 2017 - Research & Development Department, University of Petroleum and Energy Studies (UPES), Bidholi, Dehradun 248007, India ... The average s...
0 downloads 15 Views 4MB Size
Download PDF
Article pubs.acs.org/IC

Bi4TaO8Cl Nano-Photocatalyst: Influence of Local, Average, and Band Structure Swetha S. M. Bhat,† Diptikanta Swain,‡ Mikhail Feygenson,§ Joerg C. Neuefeind,§ Abhishek K. Mishra,∥ Janardhan L. Hodala,† Chandrabhas Narayana,‡ Ganapati V. Shanbhag,† and Nalini G. Sundaram*,† †

Materials Science Division, Poornaprajna Institute of Scientific Research, Bidalur, Near Devanahalli, Bengaluru, Karnataka, India CPMU, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru, Karnataka, India § Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Research & Development Department, University of Petroleum and Energy Studies (UPES), Bidholi, Dehradun 248007, India ‡

S Supporting Information *

ABSTRACT: The average structure, local structure, and band structure of nanoparticles of photocatalyst Bi4TaO8Cl, an Aurivillius−Sillen layered material, has been studied by powder neutron Rietveld refinement, neutron pair distribution function technique, Raman scattering, and density functional theory calculations. A significant local structural deviation of nano-Bi4TaO8Cl was established in contrast to the local structure of bulk-Bi4TaO8Cl. Local structure was further supported by Raman scattering measurements. Through DFT calculations, we identify specific features in the electronic band structure that correlate lower secondary structural distortions in nano-Bi4TaO8Cl. Increased distortion of TaO6, decreased Ta−O−Ta bond angle, and increased octahedral tilt in the local structure of nano-Bi4TaO8Cl influence the band structure and the electron hole pair migration. Therefore, in addition to morphology and size, the local structure of a nanomaterial contributes to the photocatalytic performance. Trapping experiments confirm the role of superoxide radical in the photocatalysis mechanism of this material. Such studies help in developing new functional materials with better photocatalytic efficiency to address energy and environmental issues.

1. INTRODUCTION Sunlight-driven heterogeneous photocatalysis is drawing more attention of researchers to address the twin concerns of global energy demands and environmental pollution.1 Semiconductor materials, owing to their chemical stability2 and inertness, could be potential candidates for photocatalytic applications.3 The wide band gap benchmark photocatalyst TiO2 suffers from utilizing broad spectrum solar energy and hence motivated research in designing new materials for sunlight-driven photocatalysis. In this context numerous alternative photocatalysts4 to TiO2 have been synthesized and showed efficient photocatalytic performances.5−7 The majority of photocatalytic semiconductors comprise transition metals such as Ta, Nb, W,8 Mo, and V along with main group elements having s electrons such as Bi, Ag, and Sn. Considerable research has been carried out on layered materials since they provide optimum band structure and promote electron hole separation conducive for photocatalysis. Bismuth-based materials such as BiOX9−12 have been investigated and found to show excellent photocatalytic activity. Their layered structure, consisting of interleaved Bi2O2 slabs, responsible for effective production of electron hole pairs, plays a major role in improving photocatalytic performance.11 In this © 2017 American Chemical Society

context, the Aurivillius−Sillen phases ([Bi2O2][TaO3n+1][Bi2O2][Cl])13−15 are found to be promising catalysts for sunlight-driven photocatalytic dye degradation. Less attention has been paid to these phases though they exhibit a variety of crystal structures and also provide a facile environment for enhanced photocatalysis. The static electric field produced between Bi2O2 and halogen ions resulting in polarization between the layers is presumed to help in separation of excitons and improve the photocatalytic performance.16 However, in order to understand the fundamental processes that drive photocatalysis, it is essential to study the structural influence on photocatalytic performance. In the literature there are several studies on the correlation of crystal structure and photocatalytic activity for bulk materials.17−19 Superior photocatalytic activity is shown by ATaO3 over ANbO3 for photocatalytic splitting of water.19 This was explained in terms of M−O−M bond angle, where the degree of linearity toward 180° was deduced to be responsible for the enhanced photocatalytic activity. Similarly, perovskite systems such as Sr2M2O7 [M = Ta, Nb]20 are known to exhibit structure dependent photocatalytic performance. The Received: August 15, 2016 Published: April 21, 2017 5525

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536

Article

Inorganic Chemistry

Bi4TaO8Cl. The photocatalytic mechanism has been carried out to find the active species triggering the dye degradation. The current study demonstrates that in addition to particle size the local structure and electronic band structure have remarkable influence in controlling the photocatalytic activity of the nanomaterials.

electron hole pair charge separation and delocalization of electrons in the conduction band improve upon increase in the M−O−M bond angle of MO6 corner shared octahedra. Additionally, tilt in the octahedra also influences the band gap,21 which in turn impacts photocatalysis. The consequence of tilt and distortion in MO6 octahedra modulates band structure, eventually promoting tuning of photocatalytic activity. Most of the work on the structural influence of photocatalysts has been studied using traditional powder diffraction techniques.20,21 It is widely accepted that the photocatalytic activity can be tuned by synthesizing nanomaterials with a particular morphology.22−27 Layered nanomaterials would induce the photogeneration of excitons to a large extent and promote photocatalytic performance. The influence of size and morphology on photocatalytic activity of Bi4TaO8Cl for degradation of Congo red dye has been explained elsewhere.28 Beyond particle size and morphology, to the best of our knowledge, nano-photocatalysts have hardly been investigated for their structural features on the origin of their photocatalytic activity. Due to the size effect of the nanoparticles, broadened peaks appear in the X-ray powder diffraction, making it difficult to extract accurate structural information, especially for lighter atoms. On the contrary, neutron powder diffraction helps to determine accurate oxygen positions in the presence of heavy elements even for broadened diffraction data of nanoparticles. It is well known that the neutron pair distribution function (PDF) elucidates the local structure of material with a high degree of confidence and provides atomic scale structural information that could help in the fundamental understanding of the structural features of nanomaterials. It appears from the literature that there are no studies on the local structure of Aurivillius−Sillen nano-photocatalysts using the nontraditional PDF technique. In this article, we present average structure, local structure, and band structure of nano-Bi4TaO8Cl photocatalysts. The average crystal structure of nano-Bi4TaO8Cl has been analyzed utilizing Rietveld refinement of neutron powder diffraction (NPD) data, and the PDF technique has been employed to study the local structure of nano- and bulk-Bi4TaO8Cl. For the sake of convenience we use the following abbreviations in order to distinguish average and local structure: Average structure of nano-Bi4TaO8Cl will be A-nano-Bi4TaO8Cl and for local structure L-nano-Bi4TaO8Cl; similarly, L-bulk-Bi4TaO8Cl will be used wherever necessary. The off-centering and the interaction between d states of transition metal and p states of oxygen in perovskite materials are the key determining factors responsible for different technologically important properties.29,30 Recently, it has been shown that electronic structure can be used to assess the cation−anion interactions that control primary and secondary structural distortions relevant to the inversion symmetry in such materials.31 In the present density functional theory (DFT) calculations, we have calculated the band structure of both bulk- and nano-Bi4TaO8Cl and identify specific electronic interactions responsible for high photocatalytic activity in nano-Bi4TaO8Cl. The presence of distortion in TaO6 octahedra and octahedra tilt were evaluated and their implications on observed photocatalysis have been explored for local and average structures. Further to support the local structure analysis, the Raman scattering tool was employed. Electronic band structure has been estimated through DFT for both nano- and bulk-

2. EXPERIMENTAL SECTION 2.1. Synthesis. Nano-Bi4TaO8Cl has been previously synthesized by solution combustion technique.28 The particle size was found to be in the range from 30 to 70 nm. Stoichiometric amounts of Bi(NO3)3· 6H2O, TaCl5, and BiOCl precursors were dissolved in water and stirred for 1 h. Urea was used as a fuel. The obtained suspension was transferred to a crucible and fired at 650 °C, and calcination was continued for 4 h to obtain a yellowish fine powder of nanoBi4TaO8Cl. Bulk-Bi4TaO8Cl was synthesized by solid-state technique as described in a previous report.28 Stoichiometric amounts of Bi2O3, Ta2O5, and BiOCl were ground and heated at 700 °C for 14 h, which yielded an intense yellow powder of bulk-Bi4TaO8Cl. Bulk-Bi4TaO8Cl exhibits aggregated microstructures with a particle size of 400 nm due to higher temperature and longer calcination time.28 2.2. Characterization. Phase purity has been ascertained previously28 using a D2 Phaser Bruker with Cu Kα radiation of wavelength 1.5418 Å. For NPD data collection, time of flight (TOF) powder neutron diffraction data measurements for 200 mg of Bi4TaO8Cl bulk and nanoparticles were collected on the nanoscale ordered materials diffractometer (NOMAD) at the Spallation Neutron Source (SNS) in Oak Ridge National Laboratory , USA.32 Room-temperature data were collected for 2 h for each sample in a quartz capillary. AXIS ultra DLD X-ray photoelectron spectroscopy (XPS) was applied to confirm the presence of Cl in nano-Bi4TaO8Cl. A PerkinElmer UV−visible spectrophotometer was used for the investigation of the photocatalytic mechanism. A PerkinElmer Lamda 45 spectrophotometer was utilized for photoluminescence analysis. Pellets of both nano- and bulk-Bi4TaO8Cl samples were obtained to record the room-temperature photoluminescence data. Raman spectra of the bulk and nano forms were recorded in the 180° backscattering geometry, using a 532 nm excitation from a diodepumped frequency-doubled Nd:YAG solid-state laser (model GDLM5015 L, Photop Suwtech Inc., China) and a custom-built Raman spectrometer equipped with a SPEX TRIAX 550 monochromator and a liquid-nitrogen-cooled CCD (Spectrum One with CCD 3000 controller, ISA Jobin Yovn - SPEX). Surface area was measured by the N2 sorption technique. 2.3. Band Structure. Computational Methodology. The DFT calculations of nano- and bulk-Bi4TaO8Cl were performed using the Quantum Espresso package33 which utilizes a plane-wave basis set for the description of the valence electrons with periodic boundary conditions. For the description of valence electrons charge density, norm-conserving (for Bi)34 and ultrasoft pseudopotentials (for Ta, Cl, and O)34 were used with core corrections. The Perdew−Burke− Ernzerhof (PBE)35 method was used for the exchange−correlation functional. Relaxation toward equilibrium was carried out with the Broyden−Fletcher−Goldfarb−Shenno (BFGS)35−37 quasi-Newton algorithm, based on the trust radius procedure. The Davidson method38 with overlap, until the forces were less than 0.01 eV/Å, was used for diagonalization. Cell volume and cell dimensions were fixed during relaxation to minimize the forces on atoms. The Brillouin zone was sampled using 12 × 12 × 2 Monkhorst−Pack39 mesh kpoints for both bulk- and nano-Bi4TaO8Cl relaxation calculations. A total energy convergence better than 10−7 eV has been achieved during relaxations of the ions. For density of states (DOS) and band structure calculations 24 × 24 × 4 Monkhorst−Pack mesh k-points were used. Such dense grids and a kinetic energy cut-off of 450 eV for the plane waves ensured an accurate description of properties that are influenced by sharp features in the density of states. 5526

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536

Article

Inorganic Chemistry 2.4. Photocatalytic Reactor. The photocatalytic reactor comprises a quartz tube with a 20 cm length consisting of an outer water jacket for continuous cooling. The description of the reactor is explained elsewhere.28 2.5. Photocatalytic Dye Degradation Mechanism Experiment. Photocatalytic dye degradation of Congo red by Bi4TaO8Cl has been reported,28 and in continuation, understanding the mechanistic pathway has been investigated for nano-Bi4TaO8Cl. In order to detect the active species involved in the photocatalytic degradation, various scavengers, isopropyl alcohol (IPA), KI, and benzoquinone (BQ) have been used.

parameters for nano-Bi4TaO8Cl obtained from Rietveld refinement of NPD data are shown in Table 1. Refined atomic Table 1. Crystallographic Data Obtained from Rietveld Refinement and PDF Analysis

a (Å) b (Å) c (Å) volume of unit cell (Å3) Rwp (%) Rp (%) χ2

3. RESULTS AND DISCUSSION 3.1. Crystal Structure of Nano-Bi4TaO8Cl. Neutron diffraction is utilized for accurately determining the position of oxygen and chlorine atoms in the presence of heavier atoms such as Bi and Ta. Rietveld refinement of neutron data collected for nano-Bi4TaO8Cl has been performed using the GSAS/EXPGUI40 suite. Figure 1 represents the observed,

A-bulkBi4TaO8Cl Rietveld

A-nanoBi4TaO8Cl Rietveld

L-bulkBi4TaO8Cl PDF

L-nanoBi4TaO8Cl PDF

5.454(3) 5.498(2) 28.734(5) 861.50(7)

5.459(6) 5.439(7) 28.786(7) 854.69(3)

5.445(1) 5.504(1) 28.855 (3) 864.61(2)

5.392(3) 5.474(2) 28.791(5) 849.5(3)

5.14 4.05 4.01

3.66 3.16 3.20

13.19

14.10

positions and thermal parameters are listed in Table S1. Rietveld refinement of A-bulk-Bi4TaO8Cl has been performed (Figure S1), and crystallographic information (Table 1) and atomic coordinates are listed in Table S3. There is a significant difference between the average structure of nano- and bulkBi 4TaO8Cl with respect to cell parameters and bond distribution. Since we focus on average structure of nanophotocatalysts structural features of nano-Bi4TaO8Cl have been discussed in detail. As reported by Kusianova et al.,41 Bi4TaO8Cl comprises layers of Bi2O2 sandwiched between the TaO4 perovskite layers and chlorine layer. Table 2 shows bond lengths for Ta−O, and Table 2. Selected Bond Distances of Bi4TaO8Cl bond type in (Å)

A-Bbulk Bi4TaO8Cl

A-nano Bi4TaO8Cl

L-bulk Bi4TaO8Cl

L-nano Bi4TaO8Cl

Figure 1. Observed, calculated, and difference plot obtained from Rietveld refinement of TOF neutron diffraction data of nanoBi4TaO8Cl.

Ta−O(5) Ta−O(6) Ta−O(7)

calculated, and difference plot obtained from Rietveld refinement. The model proposed by Lightfoot et al.41 was adopted as an initial model. Nano-Bi4TaO8Cl crystallizes in the orthorhombic phase with the P21cn space group.41 Background was refined by using a shifted Chebyshev function using 30 coefficients, and the profile was fitted by using a pseudo-Voigt function. All atoms in the general positions were refined. Thermal parameters were refined keeping the occupancy fixed. Bond length restraints were used between the Ta and O(5), O(6), and O(8) bonds. Since the reaction conditions of nano-Bi4TaO8Cl are violent, there is a possibility of oxygen vacancy in the system, which can lead to structural distortion. Although the neutron scattering technique is very sensitive to light atoms, it is noteworthy that Rietveld refinement does not show any oxygen vacancy in this material Therefore, defectsinduced structural distortion can be considerably negligible. The presence of Cl in nano- and bulk-Bi4TaO8Cl has been confirmed by XPS (Figure S2), and surface composition ratios are listed in Table S2. Slight deviations in the ratios of Ta/O, Bi/O, and Cl/O obtained by XPS from the calculated value could be associated with a limited sensitivity of XPS to a few atomic layers at the surface of our sample. It is also noticed that there is a slight Bi enrichment on the surface of the nanoBi4TaO8Cl compared to bulk-Bi4TaO8Cl. Crystallographic

Ta−O(8)

1.97(1) 1.99(1) 1.89(5) 2.07(1) 2.10(1) 1.85(4)

1.81(1) 1.69(1) 1.90(1) 2.08(1) 2.16(1) 1.94(1)

2.08(1) 1.86(1) 2.03(1) 1.93(1) 2.21(1) 1.93(1)

1.79(1) 1.80(1) 1.99(1) 2.00(1) 2.21(1) 1.79(1)

Table S4 shows bond lengths of Bi−O and Bi−Cl. The Ta−O bond lengths in TaO6 octahedra range from 1.69 to 2.16 Å. These bond length distributions indicate that the TaO6 octahedra in A-nano-Bi4TaO8Cl are more distorted than that of A-bulk-Bi4TaO8Cl. Further, Ackerman et al.42 reported that as the M−ligand−M bond angles in corner-shared octahedra approach 180°, the excited electron would have higher mobility. This effect has been observed in Sr2Ta2O720 and Sr2Nb2O7,20 where the larger Ta−O−Ta angle increases electron−hole mobility, thus resulting in higher photocatalytic activity in Sr2Ta2O7. Similarly here, Ta−O−Ta bond angles were investigated (Table 3) for Bi4TaO8Cl to evaluate their contribution to the mobility of Table 3. Selected Bond Angles and Tilts

Ta−O−Ta (deg) average tilt (deg) 5527

A-bulkBi4TaO8Cl

A-nanoBi4TaO8Cl

L-bulkBi4TaO8Cl

L-nanoBi4TaO8Cl

159.60(1) 154.69(1) 22.86

142.84(1) 154.42(1) 31.37

158.10(1) 155.18(1) 23.36

153.40(1) 151.94(1) 27.33

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536

Article

Inorganic Chemistry electron hole pairs. The Ta−O−Ta bond angles are 142.84° and 154.42° for A-nano-Bi4TaO8Cl (Table 3). In addition to the bond angle, the octahedral tilt between the octahedra also contributes to the photocatalytic activity.21 It is obvious that if the Ta−O−Ta bond angle decreases, the octahedral tilt increases. Therefore, for A-nano-Bi4TaO8Cl, tilt (31.37°) in the TaO4 layers increased as the bond angle Ta−O−Ta (142.84° and 154.42°) decreased (Table 3) compared to Abulk-Bi4TaO8Cl. Hence, it is noteworthy that with decreasing particle size an increase in TaO6 tilt was observed. The decreased octahedral tilt is known to influence the band gap of the material.21 The surface area of nano- and bulk-Bi4TaO8Cl was measured and found to be 4 and 2 m2/g, respectively. Interestingly, Aurivillius−Sillen phase materials16 show a low surface area, which is unlikely to play any major role in improving the photocatalytic activity. Therefore, it is necessary to explore the key factor, for these layered materials, contributing to photocatalytic activity other than surface area. The PDF technique has been adopted to reveal the short range ordering within the material, which can influence the photocatalytic activity. 3.2. Local Structure of Nano- and Bulk-Bi4TaO8Cl. Pair distribution function analysis was carried out using the PDFgui43 suite in order to obtain structural information in the short range order. Accurate local structure elucidation is crucial to understand the distortion of TaO6 octahedra on the short range atomic scale. Neutron total scattering provides precise and adequate information on structural features such as disorder and distortion present in the local structure. Experimental PDF values of L-nano-Bi4TaO8Cl and L-bulkBi4TaO8Cl were obtained by a Fourier transform of total scattering structure functions for Qmax = 36 Å−1. PDF refinements, though similar to Rietveld refinements, have one important distinction: the structural model is strictly valid only for the length scales in which the refinement is carried out.44−46 This makes the technique extremely valuable, as structural deviations from the average structure, if any, at various length scales can be easily modeled. Therefore, in order to investigate the distortions in the TaO6 octahedra, known to influence the photocatalytic behavior,20,21 PDF refinements at smaller length scales (1.7 to 5 Å) have been carried out. Careful inspection of Figure 3 indicates that L-nanoBi4TaO8Cl exhibits broadened scattering intensity. It is known from the literature44 that as the particle size is reduced, coherently scattered peaks become broadened. The broadened peaks observed for L-nano-Bi4TaO8Cl in the lower “r” range indicate the reduced particle size. These differences are also supported by the broadened peaks in the X-ray pattern of the nano- compared to bulk-Bi4TaO8Cl. The results obtained from Rietveld refinement of Bi4TaO8Cl with the P21cn space group were used to fit experimental PDF for L-nano-Bi4TaO8Cl and L-bulk-Bi4TaO8Cl. Symmetry constraints have been removed, and the entire unit cell has been considered for the refinement. Figure 3 shows the PDF refinement fit of the G(r) function for the low r range from 1.7 to 5 Å for bulk and nanostructures. There is a significant decrease in unit cell volume of L-nano-Bi4TaO8Cl, indicating a deviation from the long-range order. However, it is well known that significant changes in the local structure sometimes could be due to the different instrument resolution function and asymmetric shape of the diffraction peak.47 Hence, in order to

Figure 2. Structural representation of Bi4TaO8Cl along [010] in (a) Lnano-Bi4TaO8Cl and (b) L-bulk-Bi4TaO8Cl.

confirm that there is a change in the short range behavior of the material, higher r range refinement was also carried out. It is found that the lattice parameters at high r range agree with the average structure, clearly indicating that the decreased unit cell is indeed a local behavior of the material. Details of the crystallographic parameters obtained from the PDF refinement are listed in Table 1, and atomic coordinates are in Tables S5 and S6. Figure 2a and b are the structural representations of L-nano-Bi4TaO8Cl and L-bulk-Bi4TaO8Cl. From Figure S3, it can be observed that the Ta−O bond distribution is from 1.79 to 2.21 Å, and the PDF pattern indicates expanded Ta−O bond lengths in L-nano-Bi4TaO8Cl, resulting in a wider Ta−O bond length distribution. Interestingly, the Bi−Cl bond length distribution is also higher for L-nano-Bi4TaO8Cl (Table S4). Other atomic coordination spheres such as Bi−Bi, O−O, Ta−Ta, and Bi−Cl occur above 2.5 Å (Figure S3); however, there is no report on the contribution of Bi−Bi, O−O, Ta−Ta, Bi−O, and Bi−Cl correlations toward photocatalytic activity, whereas the influence of Ta−O on photocatalytic activity can be found widely.20,21 Hence, the explanation is restricted only to the Ta− O coordination sphere. Increased intensity for L-bulk-Bi4TaO8Cl demonstrates a narrower distribution of bond lengths. Closer inspection of the first peak in the PDF (Figure 4) for L-nano-Bi4TaO8Cl reveals the appearance of a broadened peak (as compared to L-bulkBi4TaO8Cl), which could be due to the smaller particle size. The shape and size of the particle usually contribute to the change in the shape and relative intensity of the peak.47−51 Billinge52 reported that the broadness of the PDF peak appears when there is strain in the molecule. Here also, the signature of strain in TaO6 is observed as the peak is broadened from an r range of 1.79 Å−2.21 Å for L-nano-Bi4TaO8Cl. Hence the combination of particle size and strain in the molecule might have resulted in broadening of the first peak, although both contributions are not easy to separate. However, the shift of the peak to lower r provides stronger evidence of the presence of strain with a decrease in particle size, as seen in Figure 4. Peak shift toward the lower r range represents compressive homogeneous strain,48 and broadening of the peak could be 5528

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536

Article

Inorganic Chemistry

Figure 3. Pair distribution function fit for (a) nano-Bi4TaO8Cl and (b) bulk-Bi4TaO8Cl.

of Ta−O−Ta, we observe enhanced tilt of TaO6 octahedra in local and average structures of nano-Bi4TaO8Cl (Figure 5). It is well known that the octahedral tilt can govern the electronic band structure of the material, and the tilt observed in these Aurivillius−Sillen nano-photocatalysts help in optimizing the photocatalytic activity. We presume that the increased tilt in TaO6 is a consequence of the strain in the molecule in short range order. It is noteworthy that this study shows that when the particle size becomes smaller for this layered material, the local structural behavior is different from that of bulk-Bi4TaO8Cl. Usually such behavior is observed for simple systems such as CdSe,49 where heavy disorder is found in the local environment. However, to our knowledge, the local structure of mixed metal oxychlorides such as Aurivillius−Sillen phases has not been investigated. Interestingly, we found that in addition to a decrease in the size and the unique interconnected spherical morphology, structurally there is a decrease in the unit cell volume of L-nano-Bi4TaO8Cl compared to L-bulk-Bi4TaO8Cl. Moreover, the PDF technique not only provides adequate information about atomic ordering in different length scales but is also sensitive to the shape of the particle. Strain in the molecule is evidenced in the short range order that is induced by the spherically shaped interconnected particles. From the literature, the influence of the local structure toward the photocatalytic activity is not clear. However, since photocatalysis is a local phenomenon based on electron−hole production followed by redox reactions, we believe that the increased octahedral tilt in the local structure influences the electron−hole pair migration and band structure.21,53 In nanoBi4TaO8Cl, there is a competitive interplay of two effects, namely, the electron−hole recombination due to increased octahedral tilt and the short migration length of the excitons due to decreased particle size. Hence, these structural phenomena could be essential in our understanding of the photocatalytic activity of these nanomaterials. In order to corroborate the observed changes occurring in the local

Figure 4. Pair distribution function indicating strain due to wider distribution of bond lengths in the TaO6 octahedra.

due to inhomogeneous strain. Presumably, nano-Bi4TaO8Cl comprises homogeneous and inhomogeneous strain. From the above observation it can clearly be deduced that as the size decreases, strain in the molecule increases. It should also be noted that these differences could also be due to different kinds of defects, charge accumulation grain boundaries, etc., for which further proof can be obtained by using a defect model in the DFT, which is beyond the scope of this work. Table 3 indicates the bond angle Ta−O−Ta change in the local and average structure. The average Ta−O−Ta angle for Lbulk-Bi4TaO8Cl tends more toward 180° than for L-nanoBi4TaO8Cl. Since these bond angles offer higher delocalization of the electrons, one can predict the possibility of enhanced photocatalytic activity for bulk-Bi4TaO8Cl. The octahedral tilt in the local and average structure of the bulk-Bi4TaO8Cl do not exhibit any significant difference. However, the particle size and morphology play crucial roles in improving the photocatalytic activity for nano-Bi4TaO8Cl. With the change in the bond angle

Figure 5. TaO6 tilted octahedra shown with ball-and-stick models: (a) A-nano-Bi4TaO8Cl and (b) L-nano-Bi4TaO8Cl (TaO6 tilt is more for A-nanoBi4TaO8Cl than L-nano-Bi4TaO8Cl). 5529

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536

Article

Inorganic Chemistry

octahedra present in between the [Bi2O2]2+ layers. Similarly, the Raman mode (Figure 6) in the frequency range 800−900 cm−1 can be related to the stretching vibrations of axial Ta−O bonds, which are projected into the [Bi2O2]2+ layers and are shown in the inset of Figure 6. Further, from the previous reports,59,60 it is observed that the higher polyhedral distortions lead to broadened peaks in the higher frequency region. It is evident from the inset of Figure 6 that nano-Bi4TaO8Cl exhibits a broadened and intense peak around 840 cm−1 compared to bulk-Bi4TaO8Cl, indicating tantalum octahedra of nanoBi4TaO8Cl are more distorted than those of the bulkBi4TaO8Cl. The difference in local structure between nano- and bulkBi4TaO8Cl is apparent from the Raman spectra (Figure 6). Despite the slight differences, the signatures of the peak for both nano- and bulk-Bi4TaO8Cl are similar, indicating that they share the same layered tantalate structure. However, Raman modes corresponding to nano-Bi4TaO8Cl are broadened and also split, indicating a wide distribution of Ta−O bond length; consequently a wider distribution of oxygen atoms in the crystal can be deduced. Hence, the Raman study supports the wider distribution in bond lengths of TaO6 observed in the pair distribution function analysis of L-nano-Bi4TaO8Cl. In addition to these changes in local structure due to a decrease in the particle size, the electronic structure of the material is also modified; especially the band gap of the nanomaterials significantly differs from that of the bulk. The experimental band gap of nano- and bulk-Bi4TaO8Cl has been measured by diffused reflectance spectra (DRS), and the results show a wider band gap for nano-Bi4TaO8Cl (2.80 eV) than bulk-Bi4TaO8Cl (2.59 eV).28 In order to further investigate the band structure of these materials, we have performed density functional theory calculations.

structure of nano-Bi4TaO8Cl, Raman scattering measurement was conducted. 3.3. Raman Study. We have carried out a Raman scattering study to further confirm the deviation found in the local structure of nano-Bi4TaO8Cl from its bulk-Bi4TaO8Cl. It is well known that Raman scattering is very much sensitive to the distortion of octahedra and strain in the crystal.54 Although less attention has been paid to Aurivillius−Sillen phases, some of the Aurivillius55 phases and perovskites54 have been studied by Raman scattering to investigate tantalate sheets. Figure 6

Figure 6. Raman shift for (a) nano-Bi4TaO8Cl and (b) bulkBi4TaO8Cl.

depicts the Raman spectra of nano-Bi4TaO8Cl and bulkBi4TaO8Cl collected at room temperature in the frequency range 100−1000 cm−1. According to previous reports TaO6 octahedra56−58 show Raman modes in the frequency range from 150 to 1000 cm−1. The bending mode of axial Ta−O−Ta bond linkage appears in the frequency range 150−300 cm−1. However, the frequency around 600 cm−1 is associated with the stretching vibration of the Ta−O bond of the corner-shared

Figure 7. (a) Band structure of nano-Bi4TaO8Cl, (b) band structure of bulk-Bi4TaO8Cl, (c) DOS of nano-Bi4TaO8Cl, and (d) DOS of bulkBi4TaO8Cl. 5530

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536

Article

Inorganic Chemistry

Figure 8. Charge density plot of LUMO states in (a) bulk-Bi4TaO8Cl and (b) nano-Bi4TaO8Cl at a 0.0015 electron/Å3 isovalue (rotated by 45° with respect to the b direction).

3.4. Density Functional Theory Calculations of Electronic Band Structure. DFT calculated electronic band structure plots of bulk- and nano-Bi4TaO8Cl are shown in Figure 7a and b, respectively. The structural model has been adopted from the average structure of nano- and bulkBi4TaO8Cl obtained by Rietveld refinement. Relaxed coordinates and selected bond lengths are given in Tables S7 and S8 and Figures S7 and S8, respectively. Even after the relaxation, there are significant differences in the bond lengths and angles, which is reflected as changes in band gap values for the bulkand nano-Bi4TaO8Cl. The obtained band gap values from the DFT calculations follow the trend of the experimental band gap. We found that the calculated band gap of 1.48 eV for nanoBi4TaO8Cl is 0.10 eV higher than that of bulk-Bi4TaO8Cl (1.38 eV) and follows the same trend as observed from DRS measurements (2.8 and 2.59 eV for nano- and bulk-Bi4TaO8Cl, respectively).28 This difference in the band gap is solely due to the changes in the cell parameters and atomic positions of nano-Bi4TaO8Cl. It is well known that DFT-calculated band gaps for insulators and semiconductors are underestimated by about 50% (or sometimes more).61,62 However, this does not affect the accuracy of the description of the total energy and the related properties of crystals and molecules. Valence band states in semiconductors that contribute to the electronic ground-state density do tend to agree with experiment, although the total valence bandwidth may be underestimated. On the other hand, conduction band states of a semiconductor are often found to agree, though generally not in location, with respect to the valence bands.63 As expected, our DFT-GGA values are underestimated for both bulk- and nano-Bi4TaO8Cl. Earlier reports show that the Bi-based oxyhalides have unusually high valence band maxima compared to their oxide counterparts due to the high dispersive O 2p orbitals in addition to the lower electronegativity of halides making their p

orbitals of higher energy than the O 2p orbitals, resulting in a reduced band gap.64 Apart from that, the lowering of the conduction band minimum is observed in the Pb2+ organic− inorganic hybrid perovskites due to spin−orbit coupling. A similar effect may be seen in the case of Bi4TaO8Cl, as Bi is isoelectronic with Pb2+.65 This reduced band gap makes it a better photocatalyst compared to its oxides or oxyhalides. In the present calculations we have not included the spin−orbit interaction while calculating the electronic band structure of both bulk- and nano-Bi4Ta8OCl and believe that comparison of electronic bands will remain pertinent even without the inclusion of this interaction. Electronic DOS for nano- and bulk-Bi4TaO8Cl are shown in Figure 7c and d, respectively. In order to understand how the orbitals contribute to the total density of states of the systems, we project out the contribution of individual atoms to the total electronic density of states (Figures S4 and S5). The bond lengths of Ta−O and Bi−O obtained after the relaxation are shown in Figures S6 and S7. For both bulk- and nanoBi4TaO8Cl structures, we note that the valence band comprises O 2p and Cl 3p orbitals, while conduction band consists of significant contribution of Ta 5d orbitals along with Bi 6p states, as observed in the Bi4NbO8Cl16 system. The striking difference between the DOS of bulk- and nano-Bi4TaO8Cl is the change of position of the lowest unoccupied states in nanoBi4TaO8Cl, and in order to further characterize these bands and to understand this difference, we plotted the charge densities associated with these bands in both bulk- and nano-Bi4TaO8Cl (Figure 8). While looking at charge densities of LUMO states in nanoBi4TaO8Cl, we observed higher charge densities on Bi p orbitals, connected with oxygens in the Bi4O2 units. LUMO orbitals are unoccupied; however the interaction between the frontier molecular orbitals (HOMO and LUMO) is an important and controlling part of the total orbital interaction in the molecules, and the strength of the interaction is directly related to the HOMO−LUMO energy separation. While 5531

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536

Article

Inorganic Chemistry

structure of bulk- and nano-Bi4TaO8Cl. From the literature,18,20 it is evident that the photocatalytic property of the material depends on the crystal structure, as the three-dimensional corner-shared octahedra control the band structure of the nanophotocatalyst. As the M−O−M bond angle becomes linear, the mobility of the electron along the corner-shared octahedra increases. Hence, it can be predicted that the material possessing a bond angle M−O−M approaching 180° would increase the probability of the electron−hole pair taking part in the reaction. In the case of Bi4TaO8Cl, as the particle size decreases, the octahedral tilt on average as well as the local structure increased compared to its bulk counterpart. As the tilt increases, the conduction band narrows, making the band gap wide. Generally, increased tilt restricts the fair interaction of the orbital between Ta−O−Ta and increases the antibonding character, leading to an increased band gap. Increased tilt of the tantalum octahedra in nano-Bi4TaO8Cl increases the band gap of the material, as evidenced by DFT calculations. This could indicate the role of the structural distortion of the material and particle size on the band structure of Bi4TaO8Cl. It is also noticed from the literature that the increase in the Bi content on the semiconductor could activate the molecular oxygen to form the superoxide to trigger the dye degradation.66,67 XPS analysis shows that there is a slight Bi enrichment on the surface of nano-Bi4TaO8Cl (Table S2). Therefore, it can enhance the super oxide production from the molecular oxygen for the dye degradation. 3.5.1. Trapping Experiment. Photocatalytic dye degradation of Congo red was carried out, and it was inferred that Bi4TaO8Cl nanoparticles show enhanced performance over the bulk counterpart (Figure 9). The error bar indicates the standard deviation after the three repetitions of the degradation experiments in the presence of the scavengers. Active species are responsible for the photocatalytic degradation of any dyes. Generally there are three active species, •OH, h+ (holes), and superoxide, produced during photodegradation, which play a dominant role in the degradation.68−70 A trapping experiment has been carried out to analyze the mechanistic pathway of Congo red dye degradation of both nano- and bulk-Bi4TaO8Cl. Figure 10 represents trapping experiments using various scavengers under UV irradiation. It can be observed from Figure 10 that addition of IPA71 and KI72 did not affect the degradation of Congo red dye. However, dye degradation was almost inhibited upon superoxide scavenger BQ73addition. This trapping experiment reveals

HOMO states are the same for both the nano and bulk compounds, we note a shift in the LUMO states, and in order to further investigate the differences in the bonding, we investigated the electronic bands and observed differences. The stronger Bi−O pΠ−pΠ interactions within the Bi4O2 units are indicative of strong Bi−O bonds within the Bi4O2 units. As a result of these stronger Bi−O bonds, the binding of Bi ions, connected with oxygens in TaO6 octahedra, weakens, which in turn is indicative of weak secondary structural distortion (second-order Jahn−Teller distortions)63 in nano-Bi4TaO8Cl. Hence, the nano-Bi4TaO8Cl structure maintains the higher primary structural distortions in TaO6 octahedra, due to weak interactions with the external environment. However, in bulkBi4TaO8Cl, we observed weak electrostatic interactions within the Bi−O bonds of the Bi4O2 units and stronger Bi−Bi pΠ− pΠ interactions between the two Bi4O2 units. The nature of the electronic bands reveals that nano-Bi4TaO8Cl exhibits weak secondary structural distortions, similar to CsNaNbOF5 and KNaNbOF5 materials, where electronic bands are indicative of primary and secondary structural distortions causing breaking of centrosymmetry.36 3.5. Photocatalysis. It has been found that nanoBi4TaO8Cl degrades Congo red dye faster than the bulk under sunlight irradiation (Figure 9).28 Key factors contributing to the increased photocatalytic activity is usually particle size and morphology. Nano-Bi4TaO8Cl has unique morphology from bulk-Bi4TaO8Cl.28

Figure 9. Photocatalytic dye degradation of nano-Bi4TaO8Cl.

In addition to the morphological difference, dissimilarities in structural features have been established between the local

Figure 10. Bar diagram with the error bar showing changes in the concentration of dye in the presence of (a) nano-Bi4TaO8Cl and (b) bulkBi4TaO8Cl with different scavengers. 5532

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536

Article

Inorganic Chemistry that superoxide is responsible for triggering the oxidation of Congo red dye. 3.6. Photoluminescence (PL) Study. Photoluminescence emission spectra result from the recombination rate of electron−hole pairs of the photocatalyst. Therefore, PL is an effective tool for analyzing the rate of recombination of photogenerated excitons. It is well known that emission intensity increases if the number of recombinations of the excitons increases.74 It has been reported75,76 that the decrease in the emission intensity in the PL spectrum of a mixed oxide photocatalyst indicates an increase in the lifetime of the charge carriers. Adopting the same principle, PL analysis of nano- and bulk-Bi4TaO8Cl has been investigated to analyze the improved photocatalytic activity of nano-Bi4TaO8Cl. It is clear from the normalized emission spectra (Figure 11) excited at 400 nm that

Figure 12. Schematic representation of the photocatalytic mechanism for nano-Bi4TaO8Cl.

Bi4TaO8Cl, electron−hole pairs are generated. The local structure of nano-Bi4TaO8Cl could contribute to the carrier migration through Ta−O−Ta as the tilt is lowered from the average structure. The smaller particle size of nano-Bi4TaO8Cl enhances the diffusion of the electron−hole pair toward the surface compared to bulk-Bi4TaO8Cl. When the photogenerated electron combines with the oxygen, it produces superoxide, which is confirmed by a trapping experiment. The reactive species superoxide aids in the degradation of the dye. The photogenerated holes on the surface of the semiconductor would react with the hydroxyl molecules and form hydroxyl radicals, helping further oxidation of the dye.28

Figure 11. Normalized photoluminescence spectra of (a) nanoBi4TaO8Cl and (b) bulk-Bi4TaO8Cl.

the intensity of bulk- is greater than nano-Bi4TaO8Cl. This layered material exhibits two broad peaks at 530 and 545 nm. Although the PL properties for these layered materials not reported, the peaks match with other tantalate compounds.77,78 The emission transitions of tantalates are usually an intrinsic property of the tantalum octahedra present in the material. A decrease in the PL intensity implies that migration of excitons is faster toward the surface of the photocatalyst than the recombination process. Once the movement of free excitons increases, photocatalytic activity also increases. It can be deduced from the above observation that the electron−hole pair produced in bulk-Bi4TaO8Cl needs a longer time to reach the surface than the nano-Bi4TaO8Cl. In addition to the radiative transitions, an increase of the nonradiative decay could also contribute to the decreased PL intensity due to the presence of defects.79 These nonradiative recombination centers reduce the photocatalytic activity of the nanomaterials. However, nano-Bi4TaO8Cl shows enhanced photocatalytic activity, demonstrating that such defect-induced nonradiative transitions are insignificant. In addition to smaller particles, which enhance the photocatalytic activity, the reduced rate of recombination in nano-Bi4TaO8Cl helps promote the degradation rate of Congo red dye. Moreover, it can be concluded that as the dimensions of the particles decrease, i.e., from 400 nm to 40 nm, the PL intensity also diminishes, making the nano-photocatalyst more active. Since there are no reports available for photoluminescence of Bi4TaO8Cl, further studies are necessary to assign the corresponding transitions. A possible photocatalytic mechanism is proposed (Figure 12) based on the above results; when light shines on the nano-

4. CONCLUSION The average and local structure of nano-Bi4TaO8Cl was analyzed in detail by utilizing neutron diffraction measurement, the neutron pair distribution function, and Raman studies. Shrinking of the unit cell volume was observed for nanoBi4TaO8Cl, obtained from Rietveld refinement of NPD and PDF analysis of the local structure. The corner-shared TaO4 layers were found to influence the photocatalytic performance. The Raman study supports the observed deviation in local structure of nano-Bi4TaO8Cl from bulk-Bi4TaO8Cl. Increased octahedral tilt was found in the local structure of nanoBi4TaO8Cl. DFT band structure calculations reveal that in nano-Bi4TaO8Cl the strong Bi−O interactions within the Bi4O2 units result in elongation of the Bi−O bonds with the TaO6 octahedra and, hence, weaken the secondary structural distortion. These strong structural distortions in nanoBi4TaO8Cl result in decreased Ta−O−Ta bond angle. Increased octahedral tilt in local and average structures influences the increased band gap of nano-Bi4TaO8Cl, which was further confirmed by DFT. This study helps in identifying the key features in the crystal structure that control the electronic structure and thereby the band gap. Trapping experiments demonstrate the role of superoxide in triggering the photocatalytic activity. This study establishes that this unique morphology and particle dimension influences the average and local structure of the nano-photocatalyst. This fundamental understanding of nanomaterials would help in the design and fabrication of new efficient photocatalysts for fulfilling energy and environmental demands. 5533

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536

Article

Inorganic Chemistry

■

(11) Mi, Y.; Zhou, M.; Wen, L.; Zhao, H.; Lei, Y. A Highly Efficient Visible-light Driven Photocatalyst: Two Dimensional Square-like Bismuth Oxyiodine Nanosheets. Dalton Trans. 2014, 43, 9549−56. (12) Li, J.; Yu, Y.; Zhang, L. Bismuth Oxyhalide Nanomaterials: Layered Structures Meet Photocatalysis. Nanoscale 2014, 6, 8473−88. (13) Ackerman, J. F. The Structures of Bi3PbWO8Cl and Bi4NbO8Cl and The Evolution of The Bipox Structure Series. J. Solid State Chem. 1986, 62, 92. (14) Fan, J.; Hu, X.; Xie, Z.; Zhang, K.; Wang. Photocatalytic Degradation of Azo Dye By Novel Bi-Based Photocatalyst Bi4TaO8I under Visible-Light Irradiation. Chem. Eng. J. 2012, 179, 44−51. (15) Hu, X. Y.; Fan, J.; Zhang, K. L.; Wang, J. J. Photocatalytic Removal of Organic Pollutants in Aqueous Solution by Bi4NbxTa1‑XO8I. Chemosphere 2012, 87, 1155−60. (16) Lin, X.; Huang, T.; Huang, F.; Wang, W.; Shi, J. Photocatalytic Activity of A Bi-Based Oxychloride Bi4NbO8Cl. J. Mater. Chem. 2007, 17, 2145. (17) Huang, F.; Wu, J.; Lin, X.; Zhou, Z. A Novel Bi-Based Oxychloride CdBiO2Cl: Crystal Structure, Electronic Structure And Photocatalytic Activity. J. Alloys Compd. 2011, 509, 764−768. (18) Sato, J.; Inoue, H. K. Y. Photocatalytic Activity for Water Decomposition of Indates with Octahedrally Coordinated d10 Configuration. II. Roles of Geometric and Electronic Structures. J. Phys. Chem. B 2003, 107, 7970. (19) Kato, H.; Kudo, A. Water Splitting into H2 and O2 on Alkali Tantalate Photocatalysts ATaO3 (A = Li, Na, and K). J. Phys. Chem. B 2001, 105, 4285. (20) Akihiko Kudo, H. K.; Seira, N. Water Splitting into H2 and O2 on New Sr2M2O7 (M = Nb and Ta) Photocatalysts with Layered Perovskite Structures: Factors Affecting the Photocatalytic Activity. J. Phys. Chem. B 2000, 104, 571. (21) Balaz, S.; Porter, S. H.; Woodward, P. M.; Brillson, L. J. Electronic Structure of Tantalum Oxynitride Perovskite Photocatalysts. Chem. Mater. 2013, 25, 3337−3343. (22) Kubacka, A.; Fernandez-Garcia, M.; Colon, G. Advanced Nanoarchitectures for Solar Photocatalytic Applications. Chem. Rev. 2012, 112, 1555−614. (23) Baxter, J.; Chen, G.; Danielson, D.; Mildred, S. D.; Andrei, G. F.; Fisher, T. S.; Jones, C.; Maginn, E.; Kortshagen, U.; Manthiram, A.; Nozik, A.; Sands, T.; Shi, L.; Shollh, D.; Yiying, Wu. Nanoscale Design To Enable The Revolution In Renewable Energy. Energy Environ. Sci. 2009, 2, 559−588. (24) Park, T. J.; Rajendra, C. P.; Suhee, K.; Sunyong Lee, S. UltraThin Coating Of G-C3N4 On An Aligned Zno Nanorod Film For Rapid Charge Separation And Improved Photodegradation Performance. RSC Adv. 2016, 6, 89944−89952. (25) Mishra, Y. K.; Modi, Y. K.; Cretu, V.; Postica, V.; Lupan, O.; Reimer, T.; Paulowicz, I.; Hrkac, V.; Benecke, W.; Kienle, L.; Adelung, R. Direct Growth of Freestanding ZnO Tetrapod Networks for Multifunctional Applications in Photocatalysis, UV Photodetection, and Gas Sensing. ACS Appl. Mater. Interfaces 2015, 7, 14303−14316. (26) Reimer, T.; Paulowicz, I.; Rö der, R.; Kaps, S.; Lupan, O.; Chemnitz, S.; Benecke, W.; Ronning, C.; Adelung, R.; Mishra, Y. K. Single Step Integration of ZnO Nano- and Microneedles in Si Trenches by Novel Flame Transport Approach: Whispering Gallery Modes and Photocatalytic Properties. ACS Appl. Mater. Interfaces 2014, 6, 7806−7815. (27) Kang, S.; Pawar, R. C.; Lee, C. S. Decoration Of Au Nanoparticles Onto Biocl Sheets For Enhanced Photocatalytic Performance Under Visible Irradiation For The Degradation Of Rhb Dye. J. Exp. Nanosci. 2016, 11, 853−871. (28) Bhat, S. S. M.; Sundaram, N. G. Efficient Visible Light Photocatalysis Of Bi4TaO8Cl Nanoparticles Synthesized By Solution Combustion Technique. RSC Adv. 2013, 3, 14371. (29) Nguyen, S. D.; Kim, S. H.; Halasyamani, P. S. Synthesis, Characterization, and Structure− Property Relationships in Two New Polar Oxides: Zn2(MoO4) (SeO3) and Zn2(MoO4) (TeO3). Inorg. Chem. 2011, 50, 5215−22.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01970.

■

Rietveld and PDF refinement fits, atomic coordinates, XPS, density of states, and bond distances (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail (N. G. Sundaram): [email protected]. ORCID

Nalini G. Sundaram: 0000-0001-9380-2202 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS S.S.M.B. thanks UGC, Govt. of India, for a fellowship and Manipal University for allowing her to carry out this research work as a part of her Ph.D. program. N.G.S. acknowledges funding from DST, India, to carry out this work. S.S.M.B. and N.G.S. thank CENSE, IISc, Bengaluru, India, for XPS measurements. Neutron-scattering experiments were conducted at the SNS, which is operated with the support from the Division of Scientific User Facilities, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DEAC05-00OR22725. A.K.M. acknowledges a startup SEED grant from University of Petroleum and Energy Studies (UPES) to finalize this project.

■

REFERENCES

(1) Nebel, C. E. Photocatalysis: A Source of Energetic Electrons. Nat. Mater. 2013, 12, 780−1. (2) Jing, L.; Zhou, W.; Tian, G.; Fu, H. Surface Tuning for Oxidebased Nanomaterials as Efficient Photocatalysts. Chem. Soc. Rev. 2013, 42, 9509−49. (3) Qu, Y.; Duan, X. Progress, Challenge and Perspective of Heterogeneous Photocatalysts. Chem. Soc. Rev. 2013, 42, 2568−80. (4) Hernández-Alonso, M. D.; Fresno, F.; Suárez, S.; Coronado, J. M. Development of Alternative Photocatalysts to TiO2: Challenges and Opportunities. Energy Environ. Sci. 2009, 2, 1231−57. (5) Song, J.-M.; Mao, C. J.; Niu, H.-L.; Shen, Y.-H.; Zhang, S.-Y. Hierarchical Structured Bismuth Oxychlorides: Self-Assembly From Nanoplates to Nanoflowers via A Solvothermal Route and their Photocatalytic Properties. CrystEngComm 2010, 12, 3875−3881. (6) Zhang, Z.; Wang, W.; Shang, M.; Yin, W. Low-temperature Combustion Synthesis of Bi2WO6 Nanoparticles as a Visible-lightdriven Photocatalyst. J. Hazard. Mater. 2010, 177, 1013−8. (7) Wang, S.; Huang, B.; Wang, Z.; Liu, Y.; Wei, W.; Qin, X.; Zhang, X.; Dai, Y. A New Photocatalyst: Bi2TiO4F2 Nanoflakes Synthesized By A Hydrothermal Method. Dalton Trans. 2011, 40, 12670−5. (8) Zheng, H.; Ou, J. Z.; Strano, M. S.; Kaner, R. B.; Mitchell, A.; Kalantar-zadeh, K. Nanostructured Tungsten Oxide − Properties, Synthesis, and Applications. Adv. Funct. Mater. 2011, 21, 2175−2196. (9) Zhang, H.; Liu, L.; Zhou, Z. Towards Better Photocatalysts: FirstPrinciples Studies Of The Alloying Effects On The Photocatalytic Activities of Bismuth Oxyhalides under Visible Light. Phys. Chem. Chem. Phys. 2012, 14, 1286−92. (10) Ye, L.; Chen, J.; Tian, L.; Liu, J.; Peng, T.; Deng, K.; Zan, L. BiOI Thin Film via Chemical Vapor Transport: Photocatalytic Activity, Durability, Selectivity And Mechanism. Appl. Catal. B: Environ. 2013, 130, 1−7. 5534

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536

Article

Inorganic Chemistry (30) Welk, M. E.; Norquist, A. J.; Arnold, F. P.; Stern, C. L.; Poeppelmeier, K. R. Out-of-Center Distortions in d0 Transition Metal Oxide Fluoride Anions. Inorg. Chem. 2002, 41, 5119−5125. (31) Billeter, S. R.; Turner, A. J.; Thiel, W. Linear Scaling Geometry Optimisation and Transition State Search in Hybrid Delocalised Internal Coordinates. Phys. Chem. Chem. Phys. 2000, 2, 2177−2186. (32) Neuefeind, J.; Feygenson, M.; Carruth, J.; Hoffmann, R.; Chipley, K. K. The Nanoscale Ordered Materials Diffractometer Nomad At The Spallation Neutron Source SNS. Nucl. Instrum. Methods Phys. Res., Sect. B 2012, 287, 68−75. (33) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; R, M. QUANTUM ESPRESSO: A Modular And Open-Source Software Project For Quantum Simulations Of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. (34) We used the pseudopotentials Ta.pbe-nsp-van.UPF, Bi.pbe-dmt.UPF, Cl.pbe-n-van.UPF, O.pbe-van-ak.UPF from http://www. quantum-espresso.org. (35) Perdew, J. P.; Kieron, B.; Matthias, E. Phys. Rev. Lett. 1996, 77, 3865−69. (36) Fletcher, R. Practical Methods of Optimization; Wiley: New York, 1987. (37) Billeter, S. R.; Curioni, A.; Andreoni, W. Efficient Linear Scaling Geometry Optimization And Transition-State Search For Direct Wavefunction Optimization Schemes In Density Functional Theory Using A Plane-Wave Basis. Comput. Mater. Sci. 2003, 27, 437. (38) Davidson, E. R. Super-Matrix Methods. Comput. Phys. Commun. 1989, 53, 49. (39) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (40) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS), 2004. (41) Kusainova, A. M.; Yu, S.; Stefanovich; Dolgikh, V. A.; Mosunov, A. V.; Hervoches, C. H.; Lightfoot, P. Dielectric Properties And Structure of Bi4NbO8Cl and Bi4TaO8Cl. J. Mater. Chem. 2001, 11, 1141. (42) Ackerman, J. F. The Structures of Bi3PbWO8Cl and Bi4NbO8Cl and The Evolution of The Bipox Structure Series. J. Solid State Chem. 1986, 62, 92. ́ P.; Liu, J. W.; Bryndin, D.; Bǒzin, E. S.; (43) Farrow, C. L.; Juhas, Bloch, J.; Proffen, T.; Billinge, S. J. L. PDFfit2 and PDFgui: computer programs for studying nanostructure in Crystals. J. Phys.: Condens. Matter 2007, 19, 335219. (44) Kim, H.; Nakamura, J.; Shao, H.; Nakamura, Y.; Akiba, E.; Chapman, K. W.; Chupas, P. J.; Proffen, T. Local Structural Evolution of Mechanically Alloyed Mg50Co50 Using Atomic Pair Distribution Function Analysis. J. Phys. Chem. C 2011, 115, 7723−7728. (45) Page, K.; Kim, Y.; Thomas, P.; Patrick, M. W.; Anthony, K. C.; Ram, S. Local Atomic Ordering in BaTaO2N Studied by Neutron Pair Distribution Function Analysis and Density Functional Theory. Chem. Mater. 2007, 19, 4037. (46) Billinge, S. The Atomic Pair Distribution Function: Past And Present. Z. Kristallogr. - Cryst. Mater. 2004, 219, 117−121. (47) Jeong, I. K.; Graf, M. J.; Heffner, R. H. Effects of Bragg Peak Profiles And Nanoparticle Sizes on The Real-Space Pair Distribution Function. J. Appl. Crystallogr. 2005, 38, 55. (48) Tiano, A.; Georgia, C. P.; Crystal, S. L.; Jinkyu, H.; Cheng, Z.; Qiang, L.; Chenyang, S.; Milinda, A. M.; Billinge, S.; Stach, E.; Thomas, J.; Guerrero, K.; Munayco, P.; Munayco, J.; Scorzelli, R. B.; Burnham, P.; Arthur, J. V.; Wong, S. S. Correlating Size and Composition- Dependent Effects with Magnetic, Mössbauer, and Pair Distribution Function Measurements in a Family of Catalytically Active Ferrite Nanoparticles. Chem. Mater. 2015, 27, 3572−3592. (49) Yang, X.; Masadeh, A. S.; McBride, J. R.; Bozin, E. S.; Rosenthal, S. J.; Billinge, S. J. Confirmation of Disordered Structure of Ultrasmall Cdse Nanoparticles from X-Ray Atomic Pair Distribution Function Analysis. Phys. Chem. Chem. Phys. 2013, 15, 8480−6. (50) Gilbert, B. Finite Size Effects on the Real-Space Pair Distribution Function of Nanoparticles. J. Appl. Crystallogr. 2008, 41, 554.

(51) Masadeh, A. S.; Farrow, E. S.; B, C. L.; Paglia, G.; Juhas, P.; Billinge, S. J. L. Quantitative Size-dependent Structure and Strain Determination of Cdse Nanoparticles Using Atomic Pair Distribution Function Analysis. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 115413. (52) Proffen, T.; Billinge, S. J. L.; Egami, T.; Louca, D. Structural analysis of complex materials using the atomic pair distribution function − a practical guide. Z. Kristallogr. - Cryst. Mater. 2003, 218, 132. (53) He, J.; Borisevich, A.; Kalinin, S. V.; Pennycook, S. J.; Pantelides, S. T. Control of Octahedral Tilts and Magnetic Properties of Perovskite Oxide Heterostructures by Substrate Symmetry. Phys. Rev. Lett. 2010, 105, 1−4. (54) Kumar, S.; Kundu, S.; Ochoa, D. A.; Garcia, J. E.; Varma, K. B. R. Raman Scattering, Microstructural and Dielectric Studies on Ba1−xCaxBi4Ti4O15 Ceramics. Mater. Chem. Phys. 2012, 136, 680. (55) Long, C.; Fan, H.; Li, M. High Temperature Aurivillius Piezoelectrics: The Effect of (Li, Ln) Modification on the Structure and Properties of (Li, Ln)0.06(Na, Bi)0.44Bi2Nb2O9(Ln = Ce, Nd, La and Y). Dalton Trans. 2013, 42, 3561−70. (56) Repelin, Y.; Husson, E.; Bennani, F.; Proust, C. Raman Spectroscopy of Lithium Niobate and Lithium Tantalate. Force Field Calculations. J. Phys. Chem. Solids 1999, 60, 819−25. (57) Ma̧czka, M.; Ptak, M.; Majchrowski, A.; Hanuza, J. Raman and IR Spectra of K4Nb6O17 and K4Nb6O17·3H2O Single Crystals. J. Raman Spectrosc. 2011, 42, 209. (58) Bizeto, M. A.; Leroux, F.; Shiguihara, A. L.; Temperini, M. L. A.; Sala, O.; Constantino, V. R. L. J. Phys. Chem. Solids 2010, 71, 560−564. (59) Xu, T.; Zhang, C.; Shao, X.; Wu, K.; Zhu, Y. MonomolecularLayer Ba5Ta4O15 Nanosheets: Synthesis and Investigation of Photocatalytic Properties. Adv. Funct. Mater. 2006, 16, 1599. (60) Dobal, P. S.; Katiyar, R. S.; Jiang, Y.; Guo, R.; Bhalla, A. S. Raman Scattering Study of A Phase Transition in Tantalum Pentoxide. J. Raman Spectrosc. 2000, 31, 1061. (61) Perdew, J. P.; Levy, M. Physical Content of the Exact KohnSham Orbital Energies: Band Gaps and Derivative Discontinuities. Phys. Rev. Lett. 1983, 51, 1884−1887. (62) Sham, L. J.; Schlüter, M. Density-Functional Theory of the Energy Gap. Phys. Rev. Lett. 1983, 51, 188810.1103/PhysRevLett.51.1888. (63) Zhang, J.; Zhang, Z.; Zhang, W.; Zheng, Q.; Sun, Y.; Zhang, C.; Tao, X. Polymorphism of BaTeMo2O9: A New Polar Polymorph and the Phase Transformation. Chem. Mater. 2011, 23, 3752−61. (64) Fujito, H.; Kunioku, H.; Kato, D.; Suzuki, H.; Higashi, M.; Kageyama, H.; Abe, R. Layered Perovskite Oxychloride Bi4NbO8Cl: A Stable Visible Light Responsive Photocatalyst for Water Splitting. J. Am. Chem. Soc. 2016, 138, 2082−2085. (65) Even, J.; Pedesseau, L.; Jancu, J.-M.; Katan, C. Charge Carriers in Hybrid Organic−Inorganic Lead Halide Perovskites Might Be Protected as Large Polarons. J. Phys. Chem. Lett. 2013, 4, 2999−3005. (66) Zhou, J.; Tian, G.; Chen, Y.; Shi, Y.; Tian, C.; Pan, K.; Fu, H. Growth Rate Controlled Synthesis of Hierarchical Bi2S3/In2S3 Core/ shell Microspheres with Enhanced Photocatalytic Activity. Sci. Rep. 2014, 4 (4027), 1−9. (67) Xiaoli, J.; Liqun, Y.; Hui, W.; Yurong, S.; Haiquan, X.; Zhiguo, Z.; He, Z. Appl. Catal., B 2015, 165, 668−675. (68) WenWu, Z.; YanFang, L.; ShiFeng, J.; WenJun, W.; LiWei, G. Sci. Rep. 2016, 6, 23235. (69) Hao, Q.; Niu, X.; Nie, C.; Hao, S.; Zou, W.; Ge, J.; Chen, D.; Yao, W. A Highly Efficient G-C 3 N 4 /SiO2 Heterojunction: The Role of SiO2 in the Enhancement of Visible Light Photocatalytic Activity. Phys. Chem. Chem. Phys. 2016, 18, 31410. (70) Wang, Y.; Zhang, P. Photocatalytic Decomposition of Perfluorooctanoic Acid (PFOA) by TiO2 in the Presence of Oxalic Acid. J. Hazard. Mater. 2011, 192, 1869−75. (71) Joshi, U. A.; Darwent, J. R.; Yiu, H. H. P.; Rosseinsky, M. J. The Effect of Platinum on the Performance of WO3 Nanocrystal Photocatalysts for the Oxidation of Methyl Orange and Iso-Propanol. J. Chem. Technol. Biotechnol. 2011, 86, 1018−1023. 5535

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536

Article

Inorganic Chemistry (72) Wang, D.; Guo, L.; Zhen, Y.; Yue, L.; Xue, G.; Fu, G. AgBr Quantum Dots Decorated Mesoporous Bi2WO6 Architectures with Enhanced Photocatalytic Activities for Methylene Blue. J. Mater. Chem. A 2014, 2, 11716−11727. (73) Yang, M.; Zhang, Y.; Zhang, N.; Tang, Z.; Xu, Y. Visible-LightDriven Oxidation of Primary C−H Bonds over CdS with Dual Cocatalysts Graphene and TiO2. Sci. Rep. 2013, 3, 3314. (74) Wang, Y.; Deng, K.; Zhang, L. Visible Light Photocatalysis of BiOI and its Photocatalytic Activity Enhancement by in Situ Ionic Liquid Modification. J. Phys. Chem. C 2011, 115, 14300−8. (75) Martha, S.; Hemalata, R. K.; Biswal, N.; Parida, K. Facile Synthesis of InGaZn Mixed Oxide Nanorods for Enhanced Hydrogen Production Under Visible Light. Dalton Trans. 2012, 41, 14107− 14116. (76) Li, L.; Wang, Y.; Li, H.; Huang, H.; Zhao, H. Synthesis Of ZnO Nanosheets Decorated With Au Nanoparticles And Its Application In Recyclable 3d Surface-Enhanced Raman Scattering Substrates. RSC Adv. 2015, 5, 57193−200. (77) Shimizu, K.; Tsuji, Y.; Hatamachi, T.; Toda, T.; Kodama, T.; Satob, M.; Kitayama, Y. Photocatalytic Water Splitting on Hydrated Layered Perovskite Tantalate A2SrTa2O7·nH2O (A = H, K, and Rb). Phys. Chem. Chem. Phys. 2004, 6, 1064−1069. (78) Wiegel, M.; Emond, H. J.; Stobbe, R.; Blasse, G. Luminescence of Alkali Tantalates And Niobates. J. Phys. Chem. Solids 1994, 55, 773− 778. (79) Chen, D.; Wang, Z.; Tiezhen, R.; Ding, H.; Yao, W.; Zong, R.; Zhu, Y. Effect of Native Defects on Photocatalytic Properties of ZnO. J. Phys. Chem. C 2014, 118, 15300−15307.

5536

DOI: 10.1021/acs.inorgchem.6b01970 Inorg. Chem. 2017, 56, 5525−5536