International Journal of Photoenergy
Volume 2018 (2018), Article ID 8516356, 6 pages
https://doi.org/10.1155/2018/8516356

Research Article

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

Hengkai Pan,1,2 Bei Wang,1,2 Feng Zhang,1,2 Weifeng Zhang,1,2 and Guoqiang Li1,2

¹Henan Key Laboratory of Photovoltaic Materials, Henan University, Kaifeng 475004, China
²Laboratory of Low-Dimensional Materials Science, Henan University, Kaifeng 475004, China

Correspondence should be addressed to Feng Zhang; zhangfeng.home@163.com and Guoqiang Li; gqli1980@henu.edu.cn

Received 2 April 2018; Revised 19 July 2018; Accepted 9 August 2018; Published 4 September 2018

Academic Editor: Leonardo Palmisano

Copyright © 2018 Hengkai Pan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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.

1. Introduction

Complex metal oxide (AMOy) is built upon the framework of MOx with sharing edges or corners and the inserted metal ion. The shape of MOx is diverse, such as octahedron and tetrahedron. The shape and connection types of MOx are critical to the physical and chemical properties [1–5]. Recently, theoretical predictions and experimental results confirmed that the TaS₂ built from the edge-sharing of TaS₆ trigonal prisms exhibits superior catalytic performance in comparison with that of TaS₆ octahedra [6, 7]. Those early studies suggested a potential way to improve the catalytic activity via tuning the shape of MOx. The above hypothesis is rarely investigated in niobates, which is one kind of photocatalyst for water splitting and organic degradation, such as NaNbO₃, SrNb₂O₆, and Cs₂Nb₄O₁₁ [8–15]. Most of the niobate photocatalysts contain NbO₆ octahedra. For instance, NaNbO₃ contains the formwork of NbO₆ octahedra sharing corners with a slight distortion [16, 17]. The NbO₆ octahedra in SrNb₂O₆ share edges and corners simultaneously [14]. Cs₂Nb₄O₁₁ is the one that coexisted with NbO₆ octahedra and NbO₄ tetrahedra in the structure [12, 18]. The tetrahedral NbO₄ structure is rarely found in niobium oxide compounds because the Nb⁵⁺ atom is usually too large to fit into an oxygen-anion tetrahedron and exists in the rare earth ANbO₄ (A = Y, Sm, and La) compounds [1, 2, 19]. To investigate the effect of MOx shape on photocatalytic activity, the challenge is preparing the sample containing NbO₄ tetrahedra.

Here, we reported a series of new material Sr2-xNaxNbO₄ containing NbO₄ tetrahedra, which was prepared from sodium niobate and SrCO₃ with different ratios under ambient air. The MOx shape will change from NbO₆ octahedra to NbO₄ tetrahedra. The Raman results identified that NbO₄ tetrahedra existed in the samples. The change from the NbO₆ octahedra to NbO₄ tetrahedra results in the different physical and chemical properties, such as optical band gap, adsorption property, and photocatalytic activity.

2. 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 Nb₂O₅ 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 [20–22]. 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 Na₂Nb₂O₆•xH₂O from the XRD.

SrCO₃ 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 SrCO₃ 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 NaNbO₃ and SrCO₃ as starting materials at 900°C for 10 h.

2.2. 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 BaSO₄ 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 HNO₃.

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.

3. Results and Discussions

All the samples show the similar XRD patterns with that of Sr₂NbO₄ (PDF#28-1245), as shown in Figure 1(a) [25]. A slight amount of SrCO₃ impurity was found in sample C due to too much SrCO₃ in the starting mixture. The compositions of samples were determined as Sr0.6Na1.4NbO₄, Sr0.97Na1.03NbO₄, and Sr1.73Na0.27NbO₄ 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 SrCO₃, 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 is the full width at half-maximum (FWHM) of the θ-2θ peak; is the diffraction angle; is the X-ray wavelength; is the effective strain, and is the crystallite size. The strain () is calculated from the slope, and the crystallite size () is calculated from the intercept of a plot of against . 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 NaNbO₃/CdS core/shell particles shifted to larger 2θ in comparison with pristine NaNbO₃, where the effective strain of NaNbO₃/CdS core/shell particles will decrease down to −0.76% from 0.89% for NaNbO₃ [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 Sr₂NbO₄ 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 Sr₂NbO₄.

(a)

(b)

(c)

Figure 1: (a) XRD patterns, (b) enlarged view of main peak, (c) lattice parameter vs. Na : Sr ratio of samples prepared from different ratio of SrCO₃ to sodium niobate of (A) 1 : 2, (B) 2 : 2, and (C) 4 : 2. The standard XRD pattern of Sr₂NbO₄ (PDF#28-1245) was plotted at the bottom in Figure 1(a).

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 NbO₄ tetrahedron, and the band near 500–700 cm⁻¹ is assigned as the symmetric stretching mode of the NbO₆ 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 , where is the Nb-O stretching wavenumber, and 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 NbO₄ tetrahedron of 1.83–1.93 Ǻ, indicating that all the samples contain NbO₄ tetrahedron [1]. The precursor of sodium niobate only has the band near 500–600 cm⁻¹, and the YNbO₄ contains the Raman band near 780-840 cm⁻¹. Above results confirmed that sample A contains NbO₆ octahedran and NbO₄ tetrahedra, and samples B and C mainly contain NbO₄ tetrahedron. The formation mechanism will be discussed later. The Nb-O bond length increases with the rise in the content of Sr.

(a)

(b)

Figure 2: (a) Raman scattering spectra of samples prepared from different ratios and (b) plot of Raman peak position vs. Na : Sr ratio.

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. 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 NbO₄ tetrahedra. Sample C has a tail around 310–500 nm, indicating that sample C possibly has more defects in comparison with sample B.

Figure 3: UV-vis spectra of samples prepared from different ratios.

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 [28–31]. The peak at 531.4 eV was considered as the binding energy of oxygen defects [31] or surface oxygen, such as O₂⁻ [29, 30]. The ratio of Oads to Olatt increases with the rise in the content of Sr. We used the TPD to investigate the property of oxygen adsorption. The amount of adsorbed oxygen on samples is consistent with the XPS results.

(a)

(b)

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

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 () can be expressed as , where is the apparent rate constant, and is the instantaneous concentration of the reactant. The plots of −ln() vs. time are shown in Figure 5(b), and the rate constants of RhB degradation are also displayed in Figure 5(c). is 2.0 × 10⁻², 5.2 × 10⁻², and 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.

(a)

(b)

(c)

(d)

Figure 5: (a) RhB concentration varied with reaction time, (b) plot of −ln() vs. time, (c) the rate constant of all the samples, and (d) dichlorid phenol concentration varied with reaction time over sample B.

We will address the formation mechanism of NbO₄ tetrahedra and how to affect the photocatalytic activity. Usually the Sr₂NbO₄ could be synthesized under vacuum or reduced atmosphere [25]. In this work, we used sodium niobate as one of the starting materials and got Sr2-xNaxNbO₄ under ambient air. When sodium niobate reacts with SrCO₃, the Sr²⁺ will substitute Na⁺. The Sr atom will give one excess charge and create an equivalent reduced atmosphere, resulting in the change of NbO₆ octahedra to NbO₄ tetrahedra. With increasing the content of Sr in the structure, the lattice parameter will be increased due to larger ionic radius of Sr²⁺ (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 Sr₂NbO₄ via the solid state reaction method using pseudoperovskite NaNbO₃ as the starting material, indicating that Na is very important to get Sr2-xNaxNbO₄, not the structure of sodium niobate. With increasing the content of NbO₄ 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 MO₄ tetrahedron is superior to that of MO₆ octahedron. Due to more defects in sample C, its photocatalytic activity decreases in comparison with sample B.

4. Conclusions

We prepared a series of Sr2-xNaxNbO₄ photocatalytic materials containing NbO₄ tetrahedra by controlling the ratio of SrCO₃ to sodium niobate under ambient air. 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. 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.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the Young Core Instructor Foundation from the Education Commission of Henan Province (2015GGJS-021), the Program for Science & Technology Innovation Talents in Universities of Henan Province, China (17HASTIT014), and Henan University funding (0000A40374).

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