Preparation and Physical and Photocatalytic Activity of a New Niobate Oxide Material Containing NbO4 Tetrahedra

The shape and connection type of MOx are critical to the physical and chemical properties. A series of new material Sr2-xNaxNbO4 containing NbO4 tetrahedra was prepared by controlling the ratio of SrCO3 to sodium niobate under ambient air. With increasing the content of Sr in the sample, the MOx shape will change from NbO6 octahedra to NbO4 tetrahedra, which is confirmed by the Raman scattering spectra. With increasing the content of NbO4 in the sample, the lattice parameter increases, optical band gap becomes larger, and the surface changes to be more active for oxygen adsorption, resulting in a higher photocatalytic activity.


Introduction
Complex metal oxide (AMO y ) is built upon the framework of MO x with sharing edges or corners and the inserted metal ion.The shape of MO x is diverse, such as octahedron and tetrahedron.The shape and connection types of MO x are critical to the physical and chemical properties [1][2][3][4][5].Recently, theoretical predictions and experimental results confirmed that the TaS 2 built from the edge-sharing of TaS 6 trigonal prisms exhibits superior catalytic performance in comparison with that of TaS 6 octahedra [6,7].Those early studies suggested a potential way to improve the catalytic activity via tuning the shape of MO x .The above hypothesis is rarely investigated in niobates, which is one kind of photocatalyst for water splitting and organic degradation, such as NaNbO 3 , SrNb 2 O 6 , and Cs 2 Nb 4 O 11 [8][9][10][11][12][13][14][15].Most of the niobate photocatalysts contain NbO 6 octahedra.For instance, NaNbO 3 contains the formwork of NbO 6 octahedra sharing corners with a slight distortion [16,17].The NbO 6 octahedra in SrNb 2 O 6 share edges and corners simultaneously [14].Cs 2 Nb 4 O 11 is the one that coexisted with NbO 6 octahedra and NbO 4 tetrahedra in the structure [12,18].The tetrahedral NbO 4 structure is rarely found in niobium oxide compounds because the Nb 5+ atom is usually too large to fit into an oxygen-anion tetrahedron and exists in the rare earth ANbO 4 (A = Y, Sm, and La) compounds [1,2,19].To investigate the effect of MO x shape on photocatalytic activity, the challenge is preparing the sample containing NbO 4 tetrahedra.
Here, we reported a series of new material Sr 2-x Na x NbO 4 containing NbO 4 tetrahedra, which was prepared from sodium niobate and SrCO 3 with different ratios under ambient air.The MO x shape will change from NbO 6 octahedra to NbO 4 tetrahedra.The Raman results identified that NbO 4 tetrahedra existed in the samples.The change from the NbO 6 octahedra to NbO 4 tetrahedra results in the different physical and chemical properties, such as optical band gap, adsorption property, and photocatalytic activity.

Experimental Section
2.1.Sample Preparation.Sodium niobate precursor was synthesized via a simple hydrothermal process.Typically, the prepared mixture of 50 mL of NaOH solution (8 M) and 2 g of Nb 2 O 5 powder was poured into a 100 mL teflon-lined stainless steel autoclave.The hydrothermal reaction was performed in a drying oven at 120 °C for 3 h [20][21][22].After naturally cooling down to room temperature, we obtained the samples and rinsed the samples with deionized water and absolute ethanol to remove the residual unreacted precursor.The sodium niobate precursor was confirmed as the Na 2 Nb 2 O 6 •xH 2 O from the XRD.SrCO 3 and sodium niobate precursor were mixed in the mortar at different molar ratios and grounded for 15 min.Then, they were heated at 500 °C for 3 h and calcined at 800 °C for 10 h in a muffle furnace.After cooling, we got the samples.For simplicity, we denoted the samples prepared from the ratios of SrCO 3 to sodium niobate precursor of 1 : 2, 2 : 2, and 4 : 2 as A, B, and C, respectively.For comparison, the sample was also prepared by solid state reaction method using NaNbO 3 and SrCO 3 as starting materials at 900 °C for 10 h.

Characterizations.
The crystal structures were identified by X-ray diffraction (XRD).Raman scattering spectrum was measured using a laser Raman spectrophotometer at room temperature.The X-ray photoelectron spectra (XPS) were measured by a Kratos AXIS Ultra photoelectron spectroscope.The diffusion reflection spectrum was recorded with a UV-vis spectrophotometer (Shimadzu 2550) using BaSO 4 as the reference and transformed to the absorption spectra automatically.The temperature programmed desorption (TPD) was carried out on Chembet Pulsar TPD using He pretreatment at 300 °C for 1 h.Inductively coupled plasma (ICP) was performed on Agilent 720/730 after pretreatment in mixture of HCl and HNO 3 .
Photocatalytic activity for the decomposition of RhB in an aqueous solution was evaluated in the presence of samples A, B, and C under full arc light irradiation of Xe lamp as reported previously [23].The initial concentration and pH value of RhB solution were about 2.5 mg L −1 and 4.5.Before illumination, the reagent was left in the dark for 30 min to achieve adsorption-desorption equilibrium.After adsorption of dye, the pH value is 6.7 for sample A, 6.6 for sample B, and 8.5 for sample C. The light emitted from the 300-W Xe lamp (the spectra is the same as that in our previous report [24]) directly irradiated on the solution in the reactor.The stirring was on during the reaction.The variation in the concentration of RhB was recorded by measuring the absorbance of the main peak in the UV-vis spectrum (UV-2550, Shimadzu) every 30 min.

Results and Discussions
All the samples show the similar XRD patterns with that of Sr 2 NbO 4 (PDF#28-1245), as shown in Figure 1(a) [25].A slight amount of SrCO 3 impurity was found in sample C due to too much SrCO 3 in the starting mixture.The compositions of samples were determined as Sr 0.6 Na 1.4 NbO 4 , Sr 0.97 Na 1.03 NbO 4 , and Sr 1.73 Na 0.27 NbO 4 for A, B, and C, from the ICP results.The total charge at A site will increase from 2.8 for sample A, 2.97 for sample B, and up to 3.73 for sample C.Moreover, the positions of diffraction peaks were successively shifted towards smaller 2θ with increasing the content of SrCO 3 , as shown in Figure 1(b).Two factors, namely strains and lattice parameter changes, will contribute to the shift of diffraction peaks [26].The strains could be estimated by the following equations.
where β is the full width at half-maximum (FWHM) of the θ-2θ peak; θ is the diffraction angle; λ is the X-ray wavelength; η is the effective strain, and D is the crystallite size.The strain (η) is calculated from the slope, and the crystallite size (D) is calculated from the intercept of a plot of β cos θ/λ against sin θ/λ.The calculated effective strain of samples A, B, and C are 1.15%, 0.87%, and 0.41%, respectively.However, the decrease in the effective strain is not reasonable.Kumar et al. found that the diffraction peak of NaNbO 3 /CdS core/ shell particles shifted to larger 2θ in comparison with pristine NaNbO 3 , where the effective strain of NaNbO 3 /CdS core/ shell particles will decrease down to −0.76% from 0.89% for NaNbO 3 [26].Our result is opposite to the above tendency, so we thought the shift of diffraction peak is not caused by the strains.According to the Bragg's law, we calculated the lattice parameter using Sr 2 NbO 4 as the parent structure, as shown in Figure 1(c).It is clearly seen that the lattice parameter increases with the rise in the content of Sr and trend towards 4.11 Ǻ that of Sr 2 NbO 4 .
Raman scattering is an effective method of investigating the changes at local structure.Figure 2(a) shows Raman scattering spectra of samples prepared from different ratios.The Raman band near 780-840 cm −1 appears in all the samples, whereas the band near 500-600 cm −1 only existed in sample A. The Raman band around 780-830 cm −1 is corresponding to the vibrational modes of a regular NbO 4 tetrahedron, and the band near 500-700 cm −1 is assigned as the symmetric stretching mode of the NbO 6 octahedra [1,18,19].The correlation between the Raman wavenumbers for the stretching bands in niobate compounds and the Nb-O bond length has previously been established.We calculated the Nb-O bond length using the empirical equation ν/cm −1 = 29522 × exp −1 9168R , where ν is the Nb-O stretching wavenumber, and R is the Nb-O bond length [13,27].The results are shown in Figure 2(b), and all of the Nb-O length satisfied the requirement of Nb-O bond length in NbO 4 tetrahedron of 1.83-1.93Ǻ, indicating that all the samples contain NbO 4 tetrahedron [1].The precursor of sodium niobate only has the band near 500-600 cm −1 , and the YNbO 4 contains the Raman band near 780-840 cm −1 .Above results confirmed that sample A contains NbO 6 octahedran and NbO 4 tetrahedra, and samples B and C mainly contain NbO 4 tetrahedron.The formation mechanism will be discussed later.The Nb-O bond length increases with the rise in the content of Sr.
The optical properties of samples are displayed in Figure 3.The absorption edge of sample A locates at ~324 nm corresponding to ~3.8 eV, whereas that of samples B and C is ~310 nm corresponding to ~4.0 eV, implying that sample A has a narrower band gap in comparison with samples B and C. The increase of lattice distance usually results in the larger band gap due to the smaller expansion of the energy levels according to the principle of solid state physics.The O1s XPS lines obtained from samples are shown in Figure 4.The binding energies were calibrated to the C1s peak at 284.8 eV.The O1s spectra can be divided into two peaks at 530.0 and 531.4 eV, respectively.The peak at 530.0 eV represents the binding energy of lattice oxygen ions [28][29][30][31].The peak at 531.4 eV was considered as the binding energy of oxygen defects [31] or surface oxygen, such as O 2 − [29,30].The ratio of O ads to O latt increases with the rise in the content of Sr.We used the TPD to investigate the property of  Photocatalytic activity was evaluated by the photocatalytic RhB photodegradation under 300-W Xe lamp. Figure 5(a) shows the RhB concentration varied with reaction time.Obviously, after irradiation, the RhB concentration is decreased.
When the reagent solution is dilute, the reaction rate (r) can be expressed as r = kC, where k is the apparent rate constant, and C is the instantaneous concentration of the reactant.The plots of −ln(C/C 0 ) vs. time are shown in Figure 5  International Journal of Photoenergy 2.8 × 10 −2 h −1 for samples A, B, and C, respectively.Sample B exhibits 2.6 times higher activity than sample A. Dichlorid phenol (DCP), a colorless toxic pollutant, was used as a model pollutant, as shown in Figure 5(d).The DCP concentration was decreased over the sample B. We will address the formation mechanism of NbO 4 tetrahedra and how to affect the photocatalytic activity.Usually the Sr 2 NbO 4 could be synthesized under vacuum or reduced atmosphere [25].In this work, we used sodium niobate as one of the starting materials and got Sr 2- x Na x NbO 4 under ambient air.When sodium niobate reacts with SrCO 3 , the Sr 2+ will substitute Na + .The Sr atom will give one excess charge and create an equivalent reduced atmosphere, resulting in the change of NbO 6 octahedra to NbO 4 tetrahedra.With increasing the content of Sr in the structure, the lattice parameter will be increased due to larger ionic radius of Sr 2+ (1.18 Ǻ) than Na + (1.02 Ǻ) [16].More amount of Sr inserted into the structure will cause the unbalance charge in A site (AMOx), leading to more defects generated in the sample, as that observed in sample C.Moreover, we could get the Na-doped Sr 2 NbO 4 via the solid state reaction method using pseudoperovskite NaNbO 3 as the starting material, indicating that Na is very important to get Sr 2-x Na x NbO 4 , not the structure of sodium niobate.With increasing the content of NbO 4 in the sample, supported by the results of Raman spectra, the lattice parameter increases, optical band gap becomes larger, and the surface changes to be more active for oxygen adsorption, resulting in a higher photocatalytic activity.Above trends could be obtained from the results of samples A and B, supporting the catalytic property of MO 4 tetrahedron is superior to that of MO 6 octahedron.Due to more defects in sample C, its photocatalytic activity decreases in comparison with sample B.

Conclusions
We prepared a series of Sr 2-x Na x NbO 4 photocatalytic materials containing NbO 4 tetrahedra by controlling the ratio of SrCO 3 to sodium niobate under ambient air.With increasing the content of NbO 4 in the sample, the lattice parameter increases, optical band gap becomes larger, and the surface changes to be more active for oxygen adsorption, resulting in a higher photocatalytic activity.The efficiency of this catalyst can also be due to the photosensibilisation of RhB and should be tested by using another organic molecule as model pollutant.

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International Journal of Photoenergy Consequently, it is reasonable to get the bigger optical band gap in samples B and C because they have larger lattice parameter.Samples B and C have the similar absorption edge and band gap, which is possibly attributed to their common properties of containing NbO 4 tetrahedra.Sample C has a tail around 310-500 nm, indicating that sample C possibly has more defects in comparison with sample B.

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International Journal of Photoenergy oxygen adsorption.The amount of adsorbed oxygen on samples is consistent with the XPS results.

Figure 4 :Figure 5 :
Figure 4: (a) O1s XPS lines obtained from samples and (b) the ratio of O ads to O latt change with Na : Sr ratio in the sample.