Synthesis, structure and optical properties of layered M4Nb6O17⋅nH2O (M = K, Rb, Cs) hexaniobates

Journal Pre-proof Synthesis, structure and optical properties of layered M4Nb6O17⋅nH2O (M = K, Rb, Cs) hexaniobates Eduardo Caetano C. Souza PII:

S0925-8388(19)34398-1

DOI:

https://doi.org/10.1016/j.jallcom.2019.153152

Reference:

JALCOM 153152

To appear in:

Journal of Alloys and Compounds

Received Date: 10 July 2019 Revised Date:

21 November 2019

Accepted Date: 22 November 2019

Please cite this article as: E.C.C. Souza, Synthesis, structure and optical properties of layered M4Nb6O17⋅nH2O (M = K, Rb, Cs) hexaniobates, Journal of Alloys and Compounds (2019), doi: https:// doi.org/10.1016/j.jallcom.2019.153152. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Synthesis, structure and optical properties of layered M4Nb6O17⋅nH2O (M = K, Rb, Cs) hexaniobates Eduardo Caetano C. Souza Institute of Chemistry, University of São Paulo 05508-000, São Paulo, SP, Brazil Potassium hexaniobate (K4Nb6O17) with 3D-layered and 2D-nanostructured morphological features has been primarily of academic interest due to its catalytic and photophysical properties. Although its microscopic properties are well described in the literature, there remains an absence of support information on the spectroscopic nature arising from the insertion of larger counterions in the crystalline lattice. In the present work, a new protocol for the preparation of layered M4Nb6O17⋅3H2O (M = K, Rb, Cs) hexaniobates is reported, starting from a citric acid aqueous solution comprising the alkali cations and a niobium citrate complex. X-ray powder diffraction, Raman and photoluminescence spectroscopies were employed to examine the implications of alkali metal incorporation. Crystal structures determination was accomplished by Rietveld profile refinement method. The observed occurrence of a pseudo Jahn-Teller effect (PJTE) implies a reduction of NbO6 octahedral symmetry due to the shortening of terminal Nb−OM bond lengths along with O−Nb−O bond angles tilted from 180°. The Raman spectra point to a local structural disorder by increasing the metal radius with characteristic frequencies assigned to variations of Nb−O bond order. Crystal lattice compositions Rb4Nb6O17⋅3H2O and Cs4Nb6O17⋅3H2O materially induce changes in the luminescent properties and strongly distorted octahedral groups yield a brighter PL blue emission at low temperature (77K).

1

1. Introduction Layered transition metal oxides are of particular interest for a wide range of applications, including optoelectronic devices, battery electrodes and electrochemical sensors [1-5]. Like graphite that consists of stacked graphene monolayers, layered hexaniobates consist of twodimensional pillared plates forming three-dimensional fractal structures. In order to obtain freely dispersed nanometer-thin monolayers the layered structure must be exfoliated. The delamination gives rise to nanomaterials with high surface area and enhanced electronic and optical properties from the confinement of electrons in 2D- dimensional sheets [6,7]. Additionally, the physical properties of layered functional materials may be altered through cations exchange on the surface or at the interlayer spacing by revealing a new electronic band structure with change in the energy states of atomic-thin layers. The pertinent structures related to the origin of photophysical characteristics in transition metal oxides, such as electrochromism and photoluminescence, fall into layered configurations with MO6 octahedra (M denotes metal). Octahedral symmetry is essential to describe the electronic properties of oxides and induces the splitting of metal d levels into eg and t2g bands. The insertion of ions and charge-balancing electrons leads to filling of the unoccupied t2g states and, therefore, may change the HOMO-LUMO energy gap [8-10]. In a seminal paper published by Nassau, Shiever and Bernstein [11] at Bell Labs in 1969, the dipolar structure of K4Nb6O17⋅nH2O single crystals was first investigated as an alternative material to mica due to its dielectric properties. Afterwards, Gasperin and Le Bihan [12] studied the hydration mechanism in alkali niobates of composition M4Nb6O17⋅nH2O (M = K, Rb, Cs). They confirmed the hydrated forms for the rubidium and cesium niobates and unveiled the atomic structure for the polycrystalline K4Nb6O17, assigned to the point group symmetry mm2 with space group P21nb. Despite the dielectric properties have earned more attention in the beginning, more than 150 papers have emerged describing the structure,

2

catalytic and optical properties of materials based on the potassium hexaniobate. Until quite recently, the number of works on these topics further increases. This type of layered structure has been of academic interest due to the possibility of ions exchange and intercalation of guest molecules in the basal spacing. Moreover, the incorporation of H+ ions by a softchemical process allows obtaining well defined nanoscale morphologies with improved macroscopic properties [13,14]. Even the first study on the alkylammonium ions intercalation into the lamellar K4Nb6O17 has been reported in 1976 [15], only 20 years later the exfoliation of this layered structure from n-tetrabutylammonium hydroxide (TBAOH) solution was published [16]. In 1997, Domen et al. [17] revealed the TEM micrograph of the exfoliated layers showing the pictures of the resulting nanoscrolls with rod like morphology. Posteriorly, the Mallouk group [18] confirmed the self-rolling of 2D- dimensional sheets in the presence of TBAOH, showing the irreversible transformation of K0.8H3.2Nb6O17 single sheets to niobate nanoscrolls. The discovery of this novel class of nanostructures has opened the door for new possibilities of application as building blocks for solar cell electrodes, smart windows and electro-optic modulator [19-21]. The K4Nb6O17 has been the main precursor used for the preparation of acid hexaniobates and other derivatives [22,23]. Information on the influence of alkali metals of larger radii and polarizabilities on the structure and physical properties still remains missing even now [24,25]. In the present study, we pursue an alternative synthesis procedure with a chemical approach to isolate layered niobates of M4Nb6O17 (M = K, Rb or Cs) composition. The chemical solution route used in this work has been described as an efficient method to obtain homogeneous solid solutions at lower crystallization temperatures in comparison to the conventional solid-state reaction [26]. Crystal structures determination was accomplished by X-ray powder diffraction refinement using Rietveld analysis. Raman spectroscopy, a

3

technique sensitive to molecular distortions and bond order variations, was employed to investigate short-range structural disorder. Photoluminescence spectroscopy is also an important method for measuring the amount of disorder in a system. It was applied to further evaluate the implications of cations exchange on the macroscopic photophysical properties of the layered phases. 2. Experimental 2.1 Synthesis of the layered hexaniobates The layered hexaniobates of composition K4Nb6O17⋅nH2O, Rb4Nb6O17⋅nH2O and Cs4Nb6O17⋅nH2O were synthesized via chemical solution route with the desired cations dissolved in the presence of citric acid. The water soluble ammonium niobium oxalate complex

NH4[NbO(C2O4)2(H2O)2]⋅(H2O)n

(Companhia

Brasileira

de

Metalurgia

e

Mineração, CBMM) was employed as a precursor of niobium ions. 25 g of niobium complex was dissolved into 500 mL of deionized water at 80 oC under stirring. After cooling, the Nb(V) species were precipitated by adding 80 mL of 6.0 mol L-1 ammonia water solution, as a gelatinous hydrated niobium oxide. The precipitate was filtered and washed twice with deionized water. The cations were then complexed with citrate anions by dissolving the precipitate within 500 mL citric acid (C6H8O7, Synth 99.5%) aqueous solution of concentration 0.5 mol L-1. The pH value of the mixture was raised to 9.0 by addition of 6.0 mol L-1 ammonia solution. After filtering the resulting niobium citrate solution (hereafter named stock solution), the niobium concentration was determined by gravimetric analysis. The reagents K2CO3 (99,9%, Merck), Rb2CO3 (99,9%, Aldrich), Cs2CO3 (99,9%, Aldrich) were used as source of alkali metals. Specific quantity of an alkali salt was dissolved into deionized water containing citric acid. The molar ratio between the alkali metal and citric acid was fixed to 1:2. Specific quantity of the stock niobium citrate solution was added with

4

further dropwise addition of ammonia solution to adjust the pH value to ∼9. The solvent was mostly removed after heat treatment at 100 oC up to reduce the solution volume to near 100 mL. Next, the vessel was placed into a drying oven at 90 oC for 12 h. The resulting semirigid gel was firstly heat treated at 450 oC in a furnace at the fume hood and the produced dark brown powder was grinded before placing it into a platinum crucible. The materials were calcined at selected temperatures from 800 to 1100 oC for 5 h in air and exposed under laboratory conditions prior analyzes. 2.2 Characterization The thermal behavior of the as prepared semirigid gel and of the hexaniobate powders were studied by thermogravimetric analysis (TGA) through a Netzsch STA (simultaneous thermal analyzer) TG/DSC 490 PC Luxx coupled to a mass spectrometer (MS) Aëolos 403C, under synthetic air with a flow rate of 50 mL.min.-1 and heating rate of 10 oC.min.-1. The structural evolution was accompanied by the X-ray diffraction patterns acquired from a Bruker D8 Discover diffractometer with Cu Kα radiation, Twin optic and LynxEye detector. The measurements were carried out with the X-ray source operating at 40 kV and 30 mA, Goebel mirror at the end of the X-ray tube and Soller slits at the entrance of the detector; Xray beam with 2° as an incidence angle over the sample and step of 0.05 degree in the goniometer, from 2° to 60°. Crystal structure refinement was performed using the Rietveld method by the Material Analysis Using Diffraction (MAUD) computer program. Data were collected over the range 3° < 2θ > 60° with step of 0.02 degree and counting time of 10 s. The layered niobates morphology was observed by a JEOL JSM-7410 field emission scanning electron microscope (FE-SEM) with a secondary electron lower detector (LEI) under accelerating voltage of 5 kV, from Central Analítica of Instituto de Química (IQ-USP). For the analysis, the powder was spread over a carbon tape affixed onto an aluminum stub. The

5

Raman spectra with an excitation wavelength of 1064 nm (laser Nd:YAG, Coherent Compass 1064-500 N) were obtained using a FT-Raman Bruker RFS 100 spectrometer with germanium detector. The laser output was fixed to 30 mW and for each spectral acquisition was taken 1024 accumulated scans. The photoluminescent (PL) excitation and emission spectra were attained on a Horiba Spectra Lux Fluorolog 3 spectrofluorometer using a Xe lamp of 450 W, simple monochromator and CCD detector. Powder samples were inserted into a quartz tube and the spectra were recorded at room and low temperatures (77 K).

3. Results and discussion 3.1 Thermal Analysis The “citrate gel process” employed for the preparation of the hexaniobates is a simple and inexpensive chemical route quite useful to produce a large amount of powder from a multicomponent metal-chelate precursor solution with cations having different equilibrium constant of hydrolysis. This because complexing the cations by citric acid improves their stability against hydrolysis or precipitation [26]. The chelating citrate anions provide the uniform distribution of the metals in aqueous media that remain uniformly distributed in the final gel after drying, resulting in homogeneous mixed oxides after thermal treatment. In Fig. 1a, the thermal behavior (TGA-DSC) of the semirigid gel obtained in the preparation of the potassium hexaniobate can be observed. The Fig. 1b shows the MS curve to H2O (m/z:18) and CO2 (m/z:44) fragments attained simultaneously with the TGA analysis. The first endothermic event of weight loss up to 200 oC is related to the elimination of the residual water followed by the initial of the thermal decomposition of the citrate complexes. The final step, between 550 and 600 oC, is associated to the strong exothermic peak at 565 oC in the DSC curve. It is attributed to the oxidative cleavage reaction of the residual −C−C−H bonds under synthetic air atmosphere, resulting in CO2 and H2O as evidenced by the MS data. From

6

the methodology employed, the initial of the crystallization process is expected to take place at the temperature of total decomposition. Potassium hexaniobate crystalline powders without any apparent carbon impurity were attained after 5 h of heat treatment at temperature as low as 600

o

C (XRD pattern is in the supplementary information). Under a favorable

thermodynamic process, the incorporation of three up to five water molecules of hydration into the lamellar spacing is anticipated, according to previous study [12]. Thermogravimetric analysis may be applied for a first quantitative statement on the number of H2O molecules per formula. As the hydration takes place under the normal laboratory conditions of temperature, pressure and humidity, all events of mass change are mainly related to the structural water loss. Therefore, the concentration of adsorbed H2O was considered negligible. The results in Fig. 1c show the dehydration analyzes of the layered materials K4Nb6O17⋅nH2O, Rb4Nb6O17⋅nH2O and Cs4Nb6O17⋅nH2O, obtained after calcination at 1100, 1100 and 900 oC, respectively, and posterior exposition to ambient conditions. The aspect of the curves indicates that the dehydration process is not the same for the three hexaniobates. The phases with K and Cs cations have thermal behavior more similar: two main stages of water molecules loss. In the former, the total dehydration follows up to 150 oC, according to early studies [11]. In the later, the introduction of Cs ion seems to increase the binding energy of structural water and the dehydration extends until 200 oC. Somehow, the introduction of Rb does not show the same trend and the total water loss could not exceed 100 oC. The experimental weight change relative to the hexaniobates dehydration was determined as about 4.8% to K4Nb6O17⋅nH2O, 3.9% to Rb4Nb6O17⋅nH2O and 3.5% to Cs4Nb6O17⋅nH2O. Considering the estimated values for the most stable materials expected under the laboratory conditions as 5.2% to K4Nb6O17⋅3H2O, 4.4% to Rb4Nb6O17⋅3H2O and 3.8% to Cs4Nb6O17⋅3H2O, we can presume that all synthesized hexaniobates are present in the trihydrated form.

7

a

100

Mass/%

Exo

60

4

40 0

QMID*10-10/A

20 0 2.0

DSC/(mW/mg)

8 80

m/z:18

b

m/z:44

1.5 1.0 0.5 0.0 200

400

600

800

1000

o

Temperature/ C

Fig. 1 a) TGA and DSC curves of the semirigid gel obtained in the synthesis of K4Nb6O17, b) MS-curves showing the H2O (m/z:18) and CO2 (m/z:44) released during heat treatment under synthetic air and c) thermogravimetric analyzes of the synthesized hexaniobates containing different alkali metals. The layered-type morphology of the hexaniobate can be seen by the SEM image in Fig. 2, that shows a particle of Rb4Nb6O17⋅3H2O with a lateral topography consistent with the fracture planes of a lamellar structure.

Fig. 2 SEM image showing the layered morphology of Rb4Nb6O17⋅3H2O particle.

8

3.2 X-ray powder diffraction The potassium hexaniobate presents an orthorhombic crystalline structure and it has been described in the literature mainly from the space group P21nb (No 33) [27]. Asymmetrically, the unit cell dimensions of rubidium and cesium hexaniobates have been determined from Pmnb symmetry (Table 1). In fact, it is not trivial to suppose the space group in which the hexaniobate crystallizes. Here, X-ray powder diffraction patterns were refined in the Pmnb (No 62) space group, with a < b > c and Z = 4, for solving the crystal structures of all synthesized M4Nb6O17⋅nH2O (M = K, Rb, Cs) hexaniobate phases. This symmetry was appointed for describing more adequately the structures experimentally obtained by XRD. The phases K4Nb6O17⋅3H2O and Rb4Nb6O17⋅3H2O were attained after heating at 1100 oC for 5 h in air. The lower temperature of 900 oC (for 5 h) was required as the most suitable for Cs4Nb6O17⋅3H2O phase formation to avoid thermal decomposition (please, see supp. info. for further details). Table 1 Comparison of unit cell dimensions of orthorhombic hydrated hexaniobates. K4Nb6O17⋅3H2O

Space group P21nb

a, Å 7.78(2)

b, Å 37.60(8)

c, Å 6.46(2)

V, Å3 1889.7

Ref. 12

Rb4Nb6O17⋅3H2O

Pmnb

7.83(1)

39.06(5)

6.57(1)

2009.4

27

Pmnb 7.90(3) Cs4Nb6O17⋅3H2O * Standard uncertainties are in parentheses.

41.05(1)

6.55(3)

2124.1

27

Compound

3.3 Rietveld Refinement The XRD patterns of the layered hexaniobates show an unconventional example of solving a crystal structure that make structural and profile parameters refinement more laborious. Broad and overlapped higher angle Bragg peaks along with preferential crystal growth along [020] and [040] directions are observed due to a platelet-like shape (Fig. 2) of the crystallites. The non-linear least squares method required initial approximation of several free variables, including fourth degree polynomials for background, scale factor which was kept as a free variable during refinement, zero shift, lattice parameters and peak profile

9

functions (Caglioti U, V, W and third degree asymmetry). Microstructural parameters as crystallite size and microstrain also contributed to minimize peak shape residuals and starting values were 950 Å and 10-4, respectively. Initially, the atomic positions were refined independently and updated in subsequent cycles, site occupancies were fixed as 1.0. Refinement of individual isotropic atomic displacement parameters did not show significant reduction of discrepancy terms as by applying anisotropic approximation. The quality of the refinement using the Rietveld method was quantified by the respective figures of merit: weighted profile residual, Rwp, expected residual, Rexp and goodness of fit, χ2. The unit cell data and structure refinement parameters are given in Table 2. The implications of peak asymmetry and background have reflected in the differences observed in the results of χ2. However, the relatively low values determined for Rexp may ensure that the calculated models are chemically feasible [28,29]. Pattern decomposition resulted in at least 126 distinguishable Bragg reflections. Therefore, there is a relevant number of unresolved reflections due to significant peak broadening. Table 2 Crystal structure data and refinement parameters of synthesized K4Nb6O17⋅3H2O, Rb4Nb6O17⋅3H2O and Cs4Nb6O17⋅3H2O powders.

K4Nb6O17⋅3H2O

S.G. Pmnba

a, Å 7.48(3)

b, Å 37.68(1)

c, Å 6.45(3)

V, Å3 1818

Rwp 14.5%

Rexp 3.2%

χ2 20.53

Rb4Nb6O17⋅3H2O

Pmnb

7.85(6)

38.92(5)

6.49(8)

1984

14.1%

3.8%

13.77

Cs4Nb6O17⋅3H2O

Pmnb

7.88(1)

41.08(7)

6.51(3)

2108

13.2%

4.8%

7.56

Compound

a

transformed from the reported space group symmetry P21nb.

The corresponding plots of the observed and calculated powder diffraction data for K4Nb6O17⋅3H2O, Rb4Nb6O17⋅3H2O and Cs4Nb6O17⋅3H2O phases are shown in Fig. 3. Vertical bars indicate the positions of Kα1 components of all calculated Bragg reflections. The difference between the observed and calculated intensities, Yobs – Ycalc, is displaced at negative counts for clarity. The chemical models of crystal structures obtained through the 10

refinement data are illustrated in ball-stick representation. The unit cell is projected along the b axis perpendicular to the lamellae plans. The NbO6 groups are depicted as octahedra with oxygen atoms at the corners and bonds are shown between niobium and oxygen. The alkali metals M1 and M2 are represented as larger spheres within a narrow interlamellar (nonhydrated) region II while terminal M3 and M4 atoms are within the open (hydrated) region I.

K4Nb6O17·3H2O

Rb4Nb6O17·3H2O

11

Cs4Nb6O17·3H2O

Fig. 3 The observed (red) and calculated (black) diffraction patterns of K4Nb6O17⋅3H2O, Rb4Nb6O17⋅3H2O and Cs4Nb6O17⋅3H2O. The intensity difference, Yobs – Ycalc, is displaced at negative counts for clarity. Vertical bars (green) indicate calculated positions of Kα1 components of Bragg reflections. Crystal structure models simulated from the XRD refinement data are presented. The structure of a crystalline hexaniobate holds chains of highly distorted NbO6 octahedra with an apical Nb−OM bond and an opposing axial Nb−O bond and four bridging Nb−O bonds lying roughly in a plane. The Nb−O bond lengths in the NbO6 octahedra were determined from the crystal structures data to measure the extension of the octahedral distortions due to the insertion of alkali metals at interlayers. The Fig. 4 illustrates two octahedral groups containing terminal O1 and O8 oxygen ions bonded to M1 and M3 alkali metal cations from distinct crystallographic sites positioned at the interlayered spacing. The anions O2, O4, O5, O6, O9, O12 frame the base of the polyhedron with edges perpendicular to the orthorhombic b axis. Table 1 summarizes all calculated cation-oxygen distances along with O−Nb−O bond angles.

12

Fig. 4 Illustration of two corner sharing NbO6 octahedral groups with terminal oxygen atoms bonded to alkali metal cations, M1 and M3, within the interlamellar spacing.

Table 3 Bond lengths (in Å) and bond angles (in degrees) calculated for M4Nb6O17⋅3H2O, where M = K, Rb and Csa Bond lengths (Å) Bonds K1–O1 Rb1–O1 Cs1–O1 Nb1–O1 Nb1–O2 Nb1–O4 Nb1–O5 Nb1–O9 Nb1–O10

K4Nb6O17 3.037(17)

Rb4Nb6O17

Bond angles (degrees) Cs4Nb6O17

3.164(46) 1.654(05) 2.071(46) 1.946(09) 1.887(58) 1.850(51) 2.070(10)

1.712(01) 2.093(51) 2.042(83) 1.957(89) 1.913(89) 2.140(11)

3.253(35) 1.789(34) 2.120(61) 2.055(31) 1.970(49) 1.940(46) 2.258(74)

Bonds K4Nb6O17 Rb4Nb6O17 O1–Nb1–O10 164.2 163.8 O2–Nb1–O5 165.2 165.0 O4–Nb1–O9 163.8 163.0 O8–Nb3–O10 162.6 162.0 O6–Nb3–O5 161.8 162.1

Cs4Nb6O17 163.0 164.7 162.1 161.6 161.1

K3–O8 2.850(03) Rb3–O8 2.952(23) Cs3–O8 3.010(05) Nb3–O5 1.872(15) 1.962(90) 1.978(47) Nb3–O6 2.032(64) 2.156(20) 2.185(20) Nb3–O8 1.690(20) 1.781(26) 1.832(06) Nb3–O10 2.104(79) 2.176(16) 2.306(29) Nb3–O12 1.952(78) 1.976(34) 2.007(54) a The estimated standard uncertainties in the final digits are given in parentheses.

13

It is well known that the origin of occurrence of a pseudo Jahn-Teller effect (PJTE) in transition metal oxides depends on local atomic structure and vibronic coupling [30]. Consequently, electronic and nuclear motions result in atomic displacement and, therefore, the reduction in symmetry is accompanied by variations of metal−oxygen bond lengths. 3.4 Raman Spectroscopy Raman spectroscopy was employed for the analysis of the influence of the alkali metals on the structure and on the vibrational modes relatives to the bonds between niobium and oxygen. Variations in the Nb−O bond strength result in structural changes due to distortions in the NbO6 octahedra that lead to different chemical and physical properties. The Raman spectra of the hexaniobates M4Nb6O17⋅3H2O (M = K, Rb , Cs) are presented in Fig. 5. Based on previous studies [31,32], the bands in lower frequencies (<150 cm-1) may be attributed to the lattice vibration and to the translation of the K, Rb and Cs atoms. The bands in the range from 150 and 480 cm-1 most likely correspond to internal bending modes of O−Nb−O groups in the NbO6 octahedra. The vibrational frequencies observed in the region from 500 and 750 cm-1 are presumed to result primarily from larger Nb−O distances while the higher-frequency stretching modes (800 − 900 cm-1) are believed to reflect modes involving lower Nb−O distances. Thus, it is possible to make a qualitative assignment to the vibrational frequencies resulting from the insertion of cations bonded to the terminal oxygen atom in the NbO6 octahedra as Nb−OM (M = K, Rb, Cs). Band positions in the vibrational spectra are indicated in the accompanying Raman plots and the assignments are presented in Table 4. The broadening of the Raman bands with the insertion of larger alkali metals is apparent even before any assignment of bands to specific motions is made, suggesting an overall greater degree of disorder. The extension of this observed symmetry breaking follows the electronic structure of the octahedral [NbO6]7group that can be described by 2pπ (t1u, t2u, t1g) atomic orbitals of six oxygen atoms in the

14

highest occupied molecular orbitals. The LUMO is comprised by 4d (t2g) atomic orbitals of Nb5+. The HOMO-LUMO orthogonality induces the off-center displacement of Nb5+ ion, implying a bond angle tilt from 180 oC (Table 3) and lowering of orthorhombic symmetry. The additional covalency due to orbital overlap between displaced Nb and the terminal oxygen atom reduces substantially the terminal Nb1−O1 and Nb3−O8 bond lengths, as observed in Table 3. The intensity of the bands is increased with the polarizability of the counterion, especially when Cs is introduced. Cations exchange affects the electron distribution around oxygen atoms of Nb−OM bonds, that actively inflects the degree of polarizability of the associated O−M vibrations. Changing degrees of NbO6 distortions may also influence the Raman intensity due to variations of molecular symmetry [33]. These observed changes seem to be mostly related to the chemical nature of the metals that appear do not affect the orthorhombic symmetry by expanding the unit cell volume. As the shortening of Nb−OM bond distances in the octahedra rise as a quantum implication due to a second-order Jahn-Teller effect, the bond order variation is a consequence of the additional covalency summed to this terminal bond, as mentioned previously. Raman spectra alter significantly the band position in the wavenumber range from 800 to 900 cm-1 with frequencies ascribed to shortest Nb−O lengths of highly distorted octahedrally coordinated niobates. Higher intensity sharp bands between 860 and 900 cm-1 move to lower wavenumber, as expected considering the calculated Nb−OM distances presented in Table 3. On the other hand, band positions from 820 − 860 cm-1 move to higher frequency. This unexpected observation may not be properly interpreted only by describing the direct correlation of single Nb−O bond lengths and the Raman frequency. The interpretation of the Raman spectra by a diatomic model approximation may not be qualitatively accurate to analyze complex transition metal-oxides. The influence of the chemical environment and of the vibrational modes of neighboring atoms cannot be neglected. Similar remark has been

15

reported after proton exchange reaction by replacing K+ for H+ of small size [34,35]. The dispersion observed in the calculated Nb−O distances, considering all distinct crystallographic sites, is a consequence of the structural disorder in the octahedra and it can be noted as a tendency of bands displacement to higher and lower frequencies, as observed in Fig. 5b.

Fig. 5 Raman spectra recorded from K4Nb6O17⋅3H2O, Rb4Nb6O17⋅3H2O and Cs4Nb6O17⋅3H2O layered hexaniobate powders. In response to the reduction of the Nb−OM bond length as a result of a PJTE, the remaining Nb−O bonds are expected to be much larger, according to the calculated distances. Thereby, the vibrational frequencies associated to νNb−O stretching and O−Nb−O bending modes would appear as Raman bands shifted to lower frequencies with the insertion of Rb and Cs ions. This expectation can be mostly observed in Fig. 5c, in the range 500 − 550 cm-1 and 600 − 675 cm-1, and Fig. 5d between 200 and 450 cm-1. The results support the structural

16

changes observed from X-ray diffraction analysis due to distortions in the NbO6 octahedra induced by variations in the Nb−O bond lengths when larger group 1 metal cations are introduced into the lattice. Table 4 Wavenumber (cm-1) and relative intensity (between parenthesis) attributed for the observed Raman modes of K4Nb6O17⋅3H2O, Rb4Nb6O17⋅3H2O and Cs4Nb6O17⋅3H2O with proposed qualitative assignment. K4Nb6O17 100(12) 117(14) 140(15) 153(17) 175(22) 192(36) 213(38) 229(39) 240(33) 276(24) 319(14) 400(17) 435(7) 537(18) 568(17) 643(16) 693(18) 844(29) 878(100) 900(11)

Rb4Nb6O17 cm-1 100(17) 124(12)

Cs4Nb6O17 97(22) 125(46) 142(59)

Qualitative Assignment Lattice vibration

151(28) 184(44) 194(49) 209(62)

171(100) 184(78) 204(86)

234(63) 280(30) 320(18) 396(27) 435(10) 529(24) 570(26) 635(21)

229(64) 285(36) 319(28) 391(30) 425(18) 526(31) 572(38) 624(66)

846(43) 876(100) 894(16)

851(64) 871(91)

O–Nb–O bending modes

Nb–O stretching modes Nb–OM stretching modes

3.5 Photoluminescence Spectroscopy

The PL emission spectra attained at 77 Kelvin under the excitation wavelength of 315 nm are shown in Figure 6. All hexaniobates showed low blue emission at room temperature with similar spectral profile characterized by a broad band with low resolution compared to those recorded at 77 K. Broad emission bands in the PL spectra imply a different chemical bonding character among the ground and excited states. For the observed bandwidth

17

shortening in Fig. 6 that points to an increasing in the photoluminescent activity, any attempt for a qualitative argument may be reasonably directed to the localized NbO6 luminescent center where the emission is generated from [36]. In the case of optical processes involving electronic states that participate in the chemical bonding, variations in the metal-ligand bond order and the symmetry of the site play an important role. The initial and final points of an optical transition should reflect the vibrational frequency of an oscillator and the redistribution of the electronic states after excitation. The increase in the force constant of the Nb−OM bond would change the excited photon by an energy ħωe, where ħ is the reduced Planck´s constant and ωe accounts for the oscillation frequency in the excited state. Increasing the density of these states results in the change of the relaxation energy and the emission spectrum would shift to lower wavelength. In previous work, A. Kudo and T. Sakata [34] reported the implications of local NbO6 disorder on the photoluminescent activity of H+-exchanged K4Nb6O17. The authors have applied the concepts of configurational coordinate to explain the energy gap between the ground and excited electronic states observed to the precursor K4Nb6O17 and to its acid form. It helps one to understand how an excited luminescence center interacts with the lattice vibrational modes in its vicinity. The density of the electronic states may be altered through distortions in the octahedra and the resulting distribution alters the occupation of those states, which may therefore change the donor-acceptor transition energy [37]. The shortening of PL bands might be related to the increase of quantum efficiency in the O2- 2pπ → 4d Nb5+ charge transfer process due to lowering of NbO6 symmetry as higher the octahedra distortion with insertion of Rb and Cs. The extension of this reduction in symmetry is evaluated by the measured bond lengths along with O−Nb−O bond angles tilted from 180° shown in Table 3, which are supported by the Raman data. As a result, a blue-shift in the emission spectra of the hexaniobates can be observed in the PL plot by the bands apex shifting from 500 nm (2.48 eV) to 476 nm (2.61

18

eV). Further description on the Configurational Coordinate Diagram can be found in the Supplementary Information.

Fig. 6 PL emission spectra of the synthesized hexaniobate powders recorded at low temperature (77 K) 4. Conclusions In the present work, layered K4Nb6O17⋅nH2O, Rb4Nb6O17⋅nH2O and Cs4Nb6O17⋅nH2O hexaniobate powders were successfully synthesized by the citrate gel process, from a starting aqueous solution with the desired cations dissolved in the presence of citric acid. The most stable trihydrated forms Rb4Nb6O17⋅3H2O and Cs4Nb6O17⋅3H2O could be achieved by limiting calcination temperatures to 1000 oC or lower. Rietveld analysis was performed to solve the crystal structures of the corresponding layered phases. Given the diversity of chemistries in these family of layered materials, the spectroscopic nature arising from the insertion of counterions with larger radii and polarizabilities could be evaluated in terms of variations of Nb−O distances along with O−Nb−O bond angles at local NbO6 octahedra. Observations of occurrence of a pseudo Jahn-Teller effect (PJTE) were confirmed by the structural changes determined from the refinement of XRD powder diffraction data. As a

19

response to the reduction in symmetry with addition of larger group 1 metal cations, rubidium and cesium hexaniobates showed a brighter luminescent blue emission at 77 Kelvin.

Acknowledgements

The author acknowledges Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for financial support (2012/15785-0). Acknowledgments to Laboratório de Espectroscopia Molecular of the University of São Paulo for the Raman spectra, Laboratório dos Elementos do Bloco-f for the Photoluminescence measurements and Laboratório de Sólidos Lamelares for additional experimental facilities.

References [1] J.N. Coleman, M. Lotya, A. O’Neill, S.D. Bergin, P.J. King, U. Khan, K. Young, A. Gaucher, S. De, R.J. Smith, I.V. Shvets, S.K. Arora, G. Stanton, H.-Y. Kim, K. Lee, G.T. Kim, G.S. Duesberg, T. Hallam, J.J. Boland, J.J. Wang, J.F. Donegan, J.C. Grunlan, G. Moriarty, A. Shmeliov, R.J. Nicholls, J.M. Perkins, E.M. Grieveson, K. Theuwissen, D.W. McComb, P.D. Nellist, V. Nicolosi, Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials, Science 331 (2011) 568–571. doi:10.1126/SCIENCE.1194975. [2] T.K. Townsend, E.M. Sabio, N.D. Browning, F.E. Osterloh, Improved Niobate Nanoscroll Photocatalysts for Partial Water Splitting, ChemSusChem. 4 (2011) 185– 190. doi:10.1002/cssc.201000377. [3] R. Abe, K. Shinmei, N. Koumura, K. Hara, B. Ohtani, Visible-Light-Induced Water Splitting Based on Two-Step Photoexcitation between Dye-Sensitized Layered Niobate and Tungsten Oxide Photocatalysts in the Presence of a Triiodide/Iodide

20

Shuttle Redox Mediator, J. Am. Chem. Soc. 135 (2013) 16872–16884. doi:10.1021/ja4048637. [4] C. Hu, L. Zhang, L. Cheng, J. Chen, W. Hou, W. Ding, A comparison of H+-restacked nanosheets and nanoscrolls derived from K4Nb6O17 for visible-light degradation of dyes, J. Energy Chem. 23 (2014) 136–144. doi:10.1016/S2095-4956(14)60128-5. [5] C. Zhou, Y. Zhao, L. Shang, Y. Cao, L.-Z. Wu, C.-H. Tung, T. Zhang, Facile preparation of black Nb4+ self-doped K4Nb6O17 microspheres with high solar absorption and enhanced photocatalytic activity, Chem. Commun. 50 (2014) 9554–9556. doi:10.1039/C4CC04432K. [6] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R.A. Dilanian, T. Sasaki, Superconductivity in two-dimensional CoO2 layers, Nature. 422 (2003) 53–55. doi:10.1038/nature01450. [7] N. Sakai, Y. Ebina, K. Takada, T. Sasaki, Electronic Band Structure of Titania Semiconductor Nanosheets Revealed by Electrochemical and Photoelectrochemical Studies, J. Am. Chem. Soc. 126 (2004) 5851–5858. doi:10.1021/JA0394582.

[8] C.-G. Granqvist, Electrochromic Metal Oxides: An Introduction to Materials and Devices, in: Electrochromic Mater. Devices, 1st ed., Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2015: pp. 1–40. doi:10.1002/9783527679850.ch1.

[9] A.H. Khan, S. Pal, A. Dalui, B. Pradhan, D.D. Sarma, S. Acharya, Tuning copper sulfide nanosheets by cation exchange reactions to realize two-dimensional CZTS dielectric layers, J. Mater. Chem. A. 7 (2019) 9782–9790. doi:10.1039/C9TA00370C.

21

[10] A.R. Sotiles, L.M. Baika, M.T. Grassi, F. Wypych, Cation Exchange Reactions in Layered Double Hydroxides Intercalated with Sulfate and Alkaline Cations (A(H2O)6)[M62+Al3(OH)18(SO4)2]·6H2O (M2+ = Mn, Mg, Zn; A+ = Li, Na, K), J. Am. Chem. Soc. 141 (2019) 531–540. doi:10.1021/jacs.8b11389.

[11] K. Nassau, J.W. Shiever, J.L. Bernstein, Crystal Growth and Properties of Mica-Like Potassium Niobates, J. Electrochem. Soc. 116 (1969) 348–353. doi:10.1149/1.2411844. [12] M. Gasperin, M.T. Le Bihan, Mecanisme d’hydratation des niobates alcalins lamellaires de formule A4Nb4O17 (A = K, Rb, Cs), J. Solid State Chem. 43 (1982) 346–353. doi:10.1016/0022-4596(82)90251-1. [13] S.K. Sahu, L.A. Boatner, A. Navrotsky, Formation and Dehydration Enthalpy of Potassium Hexaniobate, J. Am. Ceram. Soc. 100 (2017) 304–311. doi:10.1111/jace.14465. [14] R. Uppuluri, A. Sen Gupta, A.S. Rosas, T.E. Mallouk, Soft chemistry of ionexchangeable layered metal oxides, Chem. Soc. Rev. 47 (2018) 2401–2430. doi:10.1039/C7CS00290D. [15] G. Lagaly, K. Beneke, Cation exchange reactions of the mica-like potassium niobate K4Nb6O17, J. Inorg. Nucl. Chem. 38 (1976) 1513–1518. doi:10.1016/00221902(76)90019-1. [16] K. Domen, Y. Ebina, S. Ikeda, A. Tanaka, J.N. Kondo, K. Maruya, Layered niobium oxides pillaring and exfoliation, Catal. Today. 28 (1996) 167–174. doi:10.1016/0920-5861(95)00223-5.

22

[17] R. Abe, K. Shinohara, A. Tanaka, M. Hara, J.N.K. And, Kazunari Domen, Preparation of Porous Niobium Oxides by Soft-Chemical Process and Their Photocatalytic Activity, 9 (1997) 2179–2184. doi:10.1021/CM970284V. [18] G.B. Saupe, C.C. Waraksa, H.-N. Kim, Y.J. Han, D.M. Kaschak, A. D.M. Skinner, T.E. Mallouk, Nanoscale Tubules Formed by Exfoliation of Potassium Hexaniobate, 12 (2000) 1556–1562. doi:10.1021/CM981136N. [19] M.A. Bizeto, A.L. Shiguihara, V.R.L. Constantino, Layered niobate nanosheets: building blocks for advanced materials assembly, J. Mater. Chem. 19 (2009) 2512– 2525. doi:10.1039/b821435b. [20] N. Kimura, Y. Kato, R. Suzuki, A. Shimada, S. Tahara, T. Nakato, K. Matsukawa, P.H. Mutin, Y. Sugahara, Single- and Double-Layered Organically Modified Nanosheets by Selective Interlayer Grafting and Exfoliation of Layered Potassium Hexaniobate, Langmuir. 30 (2014) 1169–1175. doi:10.1021/la404223x. [21] B.P. Bastakoti, Y. Li, M. Imura, N. Miyamoto, T. Nakato, T. Sasaki, Y. Yamauchi, Polymeric Micelle Assembly with Inorganic Nanosheets for Construction of Mesoporous Architectures with Crystallized Walls, Angew. Chemie Int. Ed. 54 (2015) 4222–4225. doi:10.1002/anie.201410942. [22] B.N. Nunes, C. Haisch, A. V. Emeline, D.W. Bahnemann, A.O.T. Patrocinio, Photocatalytic properties of layer-by-layer thin films of hexaniobate nanoscrolls, Catal. Today. 326 (2019) 60–67. doi:10.1016/J.CATTOD.2018.06.029. [23] L.A. Faustino, B.L. Souza, B.N. Nunes, A.-T. Duong, F. Sieland, D.W. Bahnemann, A.O.T. Patrocinio, Photocatalytic CO 2 Reduction by Re(I) Polypyridyl Complexes

23

Immobilized on Niobates Nanoscrolls, ACS Sustain. Chem. Eng. 6 (2018) 6073– 6083. doi:10.1021/acssuschemeng.7b04713. [24] G.J.-P. Deblonde, A. Chagnes, M.-A. Roux, V. Weigel, G. Cote, Extraction of Nb(v) by quaternary ammonium-based solvents: toward organic hexaniobate systems, Dalt. Trans. 45 (2016) 19351–19360. doi:10.1039/C6DT03873E. [25] D.J. Sures, S.A. Serapian, K. Kozma, P.I. Molina, C. Bo, M. Nyman, Electronic and relativistic contributions to ion-pairing in polyoxometalate model systems, Phys. Chem. Chem. Phys. 19 (2017) 8715–8725. doi:10.1039/C6CP08454K. [26] M. Kakihana, Preparation of high temperature superconducting oxides, J. Sol-Gel Sci. Technol. 6 (1996) 7–55. doi:10.1007/BF00402588. [27] M. Gasperin, M.-T. le Bihan, Un niobate de rubidium d’un type structural nouveau: Rb4Nb6O17·3H2O, J. Solid State Chem. 33 (1980) 83–89. doi:10.1016/00224596(80)90550-2.

[28] B.H. Toby, R factors in Rietveld analysis: How good is good enough, Powder Diffr. 21 (2006) 67–70. doi:10.1154/1.2179804.

[29] V.K. Pecharsky, P.Y. Zavalij, Fundamentals of powder diffraction and structural characterization of materials, Springer US, 2005. doi:10.1007/b106242.

[30] I.B. Bersuker, Pseudo-Jahn–Teller Effect—A Two-State Paradigm in Formation, Deformation, and Transformation of Molecular Systems and Solids, Chem. Rev. 113 (2013) 1351–1390. doi:10.1021/cr300279n.

24

[31] M. Maczka, M. Ptak, A. Majchrowski, J. Hanuza, Raman and IR spectra of K4Nb6O17and K4Nb6O17·3H2O single crystals, J. Raman Spectrosc. 42 (2011) 209–213. doi:10.1002/jrs.2668. [32] J.M. Jehng, I.E. Wachs, Structural chemistry and Raman spectra of niobium oxides, Chem. Mater. 3 (1991) 100–107. doi:10.1021/cm00013a025.

[33] E. Flores, P. Novák, E.J. Berg, In situ and Operando Raman Spectroscopy of Layered Transition Metal Oxides for Li-ion Battery Cathodes, Front. Energy Res. 6 (2018) 1– 16. doi:10.3389/fenrg.2018.00082.

[34] A. Kudo, T. Sakata, Effect of Ion Exchange on Photoluminescence of Layered Niobates K4Nb6O17 and KNb3O8, J. Phys. Chem. 100 (1996) 17323–17326. doi:10.1021/jp9619806.

[35] M.A. Bizeto, F. Leroux, A.L. Shiguihara, M.L.A. Temperini, O. Sala, V.R.L. Constantino, Intralamellar structural modifications related to the proton exchanging in K4Nb6O17 layered phase, J. Phys. Chem. Solids. 71 (2010) 560–564. doi:10.1016/j.jpcs.2009.12.036.

[36] N.G. Basov, Luminescence Centers in Crystals, 1st ed., Springer US, 1976.

[37] I. Pelant and J. Valenta, “Luminescence Spectroscopy of Semiconductors”, Oxford University Press, 1st edition, 2012.

25

Author contributions Use this form to specify the contribution of each author of your manuscript. A distinction is made between five types of contributions: Conceived and designed the analysis; Collected the data; Contributed data or analysis tools; Performed the analysis; Wrote the paper. For each author of your manuscript, please indicate the types of contributions the author has made. An author may have made more than one type of contribution. Optionally, for each contribution type, you may specify the contribution of an author in more detail by providing a one-sentence statement in which the contribution is summarized. In the case of an author who contributed to performing the analysis, the author’s contribution for instance could be specified in more detail as ‘Performed the computer simulations’, ‘Performed the statistical analysis’, or ‘Performed the text mining analysis’. If an author has made a contribution that is not covered by the five pre-defined contribution types, then please choose ‘Other contribution’ and provide a one-sentence statement summarizing the author’s contribution.

Manuscript title: Synthesis, structure and optical properties of layered M4Nb6O17.nH2O

(M=K,Rb,Cs) hexaniobates

Author 1: Eduardo Caetano C. Souza (corresponding author) ☒

Conceived and designed the analysis Conceived the presented idea, performed and verified analytical methods and synthesis

☒

Collected the data Collected XRD, SEM, TGA-DTG-MS and PL data

☒

Contributed data or analysis tools Performed computations, structure refinements and simulations

☒

Performed the analysis Performed all analytical methods

☒

Wrote the paper Paper was fully written by corresponding author

☒

Other contribution Criticisms have been answered and paper was rewritten by the author

Author 2: Enter author name ☐

Conceived and designed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Collected the data Specify contribution in more detail (optional; no more than one sentence)

☐

Contributed data or analysis tools Specify contribution in more detail (optional; no more than one sentence)

☐

Performed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Wrote the paper Specify contribution in more detail (optional; no more than one sentence)

☐

Other contribution Specify contribution in more detail (required; no more than one sentence)

Author 3: Enter author name ☐

Conceived and designed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Collected the data Specify contribution in more detail (optional; no more than one sentence)

☐

Contributed data or analysis tools Specify contribution in more detail (optional; no more than one sentence)

☐

Performed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Wrote the paper Specify contribution in more detail (optional; no more than one sentence)

☐

Other contribution Specify contribution in more detail (required; no more than one sentence)

Author 4: Enter author name ☐

Conceived and designed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Collected the data Specify contribution in more detail (optional; no more than one sentence)

☐

Contributed data or analysis tools Specify contribution in more detail (optional; no more than one sentence)

☐

Performed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Wrote the paper Specify contribution in more detail (optional; no more than one sentence)

☐

Other contribution Specify contribution in more detail (required; no more than one sentence)

Author 5: Enter author name ☐

Conceived and designed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Collected the data Specify contribution in more detail (optional; no more than one sentence)

☐

Contributed data or analysis tools Specify contribution in more detail (optional; no more than one sentence)

☐

Performed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Wrote the paper Specify contribution in more detail (optional; no more than one sentence)

☐

Other contribution Specify contribution in more detail (required; no more than one sentence)

Author 6: Enter author name ☐

Conceived and designed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Collected the data Specify contribution in more detail (optional; no more than one sentence)

☐

Contributed data or analysis tools Specify contribution in more detail (optional; no more than one sentence)

☐

Performed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Wrote the paper Specify contribution in more detail (optional; no more than one sentence)

☐

Other contribution Specify contribution in more detail (required; no more than one sentence)

Author 7: Enter author name ☐

Conceived and designed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Collected the data Specify contribution in more detail (optional; no more than one sentence)

☐

Contributed data or analysis tools Specify contribution in more detail (optional; no more than one sentence)

☐

Performed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Wrote the paper Specify contribution in more detail (optional; no more than one sentence)

☐

Other contribution Specify contribution in more detail (required; no more than one sentence)

Author 8: Enter author name ☐

Conceived and designed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Collected the data Specify contribution in more detail (optional; no more than one sentence)

☐

Contributed data or analysis tools Specify contribution in more detail (optional; no more than one sentence)

☐

Performed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Wrote the paper Specify contribution in more detail (optional; no more than one sentence)

☐

Other contribution Specify contribution in more detail (required; no more than one sentence)

Author 9: Enter author name ☐

Conceived and designed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Collected the data Specify contribution in more detail (optional; no more than one sentence)

☐

Contributed data or analysis tools Specify contribution in more detail (optional; no more than one sentence)

☐

Performed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Wrote the paper Specify contribution in more detail (optional; no more than one sentence)

☐

Other contribution Specify contribution in more detail (required; no more than one sentence)

Author 10: Enter author name ☐

Conceived and designed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Collected the data Specify contribution in more detail (optional; no more than one sentence)

☐

Contributed data or analysis tools Specify contribution in more detail (optional; no more than one sentence)

☐

Performed the analysis Specify contribution in more detail (optional; no more than one sentence)

☐

Wrote the paper Specify contribution in more detail (optional; no more than one sentence)

☐

Other contribution Specify contribution in more detail (required; no more than one sentence)

Research Highlights -

Synthesis of M4Nb6O17·nH2O (M=K,Rb,Cs) phases by sol-gel technique;

-

Crystal structures determination by Rietveld refinement;

-

Occurrence of a pseudo Jahn-Teller effect (PJTE) in the NbO6 octahedra.

-

Bond order variations and reduction of NbO6 octahedral symmetry induce changes in the photoluminescent properties;

Dear Editor, I declare that the article is original and it has not been published previously, or it is under consideration elsewhere. The author is aware about its content and declares that not conflicts of interest exist and, if accepted, it will not be published elsewhere, in any language, without the written consent of the publisher.

Sincerely yours, Dr. Eduardo Caetano C. Souza (Corresponding author) E-mail: [email protected] Institute of Chemistry – University of São Paulo Av. Prof. Lineu Prestes, 748, Butantã, São Paulo, 05508-000, SP, Brazil