The Polymerization Effect on Synthesis and Visible-Light Photocatalytic Properties of Low-Temperature β-BiNbO4 Using Nb-Citrate Precursor
© Zhai et al. 2015
Received: 4 September 2015
Accepted: 15 November 2015
Published: 1 December 2015
Low-temperature β-BiNbO4 powders (denoted as Low-β) were prepared by citrate and Pechini methods using homemade water-soluble niobium precursors. The addition of ethylene glycol and the resultant polymerization effect on the synthesis and visible-light photocatalytic performance of β-BiNbO4 powders were fully investigated. The polymerization effect is beneficial to lower the phase formation temperature and obtain smaller particle catalysts. Both methods can synthesize catalysts with excellent performance of visible-light degradation of methyl violet (MV). The Low-β BiNbO4 powder prepared by citrate method shows better degradation rate of about 1 h to decompose 80 % of MV and also displays good photocatalytic stability. The photodegradation of MV under the visible-light irradiation followed the pseudo-first-order kinetics according to the Langmuir-Hinshelwood model, and the obtained first-order rate constant and half-time are 2.85 × 10−2 min−1 and 24.3 min, respectively. The better photocatalytic performance of BiNbO4 powders prepared by citrate method can be attributed to its smaller band gap and better crystallinity.
In recent years, environmental pollution, especially organic pollutants, has attracted much attention due to its deleterious effect on human health . TiO2, one of the most popular photocatalysts, can solely absorb ultraviolet (UV) light. In order to make good use of solar light source, many visible-light active photocatalysts have been deeply investigated, such as quantum dot-based photocatalysts [2–5]. Among these photocatalysts, much attention has been paid on bismuth-based photocatalytic materials, such as BiOBr [6, 7], Bi2O2CO3 , BiVO4 , BiNbO4 [10–12], and BiTaO4 [13, 14], due to their excellent photooxidation ability for organic contaminant degradation via a visible-light photocatalytic process.
BiNbO4 has orthorhombic α and triclinic β phases. In general, the low-temperature α phase synthesized at 900 °C irreversibly transforms to the high-temperature β phase (denoted as High-β) at 1020 °C . While in our former work, we first synthesized the low-temperature β phase (denoted as Low-β) at 700 °C and observed the phase transition from β to α phase . The visible-light photocatalytic performance test also shows that the Low-β exhibits the best photocatalytic efficiency compared with α phase and High-β . The formation of a pure triclinic phase of BiNbO4 at a low temperature of 700 °C can be attributed to the advantage of the citrate method.
The citrate method is a simple way to obtain stable precursors and reactive and stoichiometric fine powders which has been widely used in the fabrication of various simple and complicated oxides . Compared to the citrate method, the Pechini method is superior to obtain a homogeneous multicomponent gel without any phase segregation throughout the processing, which is beneficial to prepared stoichiometric oxides with a much lower temperature and smaller particle size. The difference between these two methods is the introduction of polyhydroxy alcohol, such as ethylene glycol, and the polymerization process between metal citrate and polyhydroxy alcohol. The polymerization results in immobilization of metal complexes in rigid organic polymer nets, thus ensuring the compositional homogeneity .
In this work, Low-β BiNbO4 powders were prepared by citrate and Pechini methods using homemade water-soluble niobium precursors. The addition of ethylene glycol and the resultant polymerization effect on the synthesis and visible-light photocatalytic performance of Low-β BiNbO4 powders were investigated, including the phase formation process, particle size, optical properties, and the photocatalytic degradation of methyl violet (MV) under visible light.
The precursor materials were bismuth nitrate (Bi(NO3)3·5H2O), ammonia, ethylene glycol (EG), and a Nb-citrate (Nb-CA) aqueous solution. The synthesis of water-soluble Nb-CA has been described in detail in a previous work . Low-β BiNbO4 powders were prepared by citrate and Pechini methods. Bi(NO3)3·5H2O was first dissolved in Nb-CA. Then the solution was kept stirring at 60 °C, and ammonia was added to adjust the pH value to 7–8. Finally, a stable solution was obtained. The above is the citrate method process; for the Pechini method, another step is added: after the stable and transparent solution was obtained, EG was dropped as chelating agent and stirring at 80 °C to promote the polymerization between metal CA and EG. The two kinds of solutions were finally dried at 180 °C to evaporate the solvent and calcined at various temperatures from 500 to 900 °C for 3 h with a ramp rate of 3 °C min−1.
The structure of the thermally treated powders was characterized by X-ray diffraction (XRD) with Cu Kα radiation. The grain sizes of the powders were examined using transmission electron microscopy (TEM). The photocatalytic activity of the BiNbO4 powders for the decomposition of MV was evaluated under irradiation of a 150-W Xe lamp at the natural pH value; the details have been described in the previous work, and the degradation process was monitored by an ultraviolet-visible near infrared (UV-vis-NIR) spectrophotometer .
Results and Discussion
For BiNbO4 particles, the mechanism of MV degradation under visible-light irradiation involves photocatalytic and photosensitization pathways, and the latter has a dominant role in the degradation . Both catalysts exhibit excellent performance of visible-light degradation of MV, compared with the degradation of MV without a catalyst. The Low-β BiNbO4 powder prepared by the citrate method shows a better degradation rate of about 1 h to decompose 80 % of MV. The better photocatalytic performance can be attributed to its smaller band gap and better crystallinity than that of catalysts prepared by the Pechini method. Better crystallinity plays an important role in the degradation of MV, corresponding to the adsorption ability test of BiNbO4 powders, as seen in the inset in Fig. 4. Though the catalysts prepared by the Pechini method are with a smaller particle size, the adsorption ability is equal to that of catalysts by citrate method.
The effect of operating parameters such as the amount of catalyst loading, pH value, and the additive H2O2 concentration on the photocatalytic performance of Low-β BiNbO4 powders prepared by the citrate method has been investigated in our former work . It shows that the optimal operation conditions are catalyst loading of 1 g L−1, pH value of 8, and the additive H2O2 concentration of 2 mmol L−1.
Low-β BiNbO4 powders were prepared by citrate and Pechini methods using homemade water-soluble niobium precursors. The addition of ethylene glycol and the resultant polymerization effect on the synthesis and visible-light photocatalytic performance of Low-β BiNbO4 powders were investigated. The polymerization effect is beneficial to lower the phase formation temperature and obtain smaller particle catalysts. Both methods can synthesize catalysts with excellent performance of visible-light degradation of MV. The Low-β BiNbO4 powder prepared by the citrate method shows a better degradation rate of about 1 h to decompose 80 % of MV and also displays good photocatalytic stability. The photodegradation of MV under the visible-light irradiation followed the pseudo-first-order kinetics according to the Langmuir-Hinshelwood model, and the obtained first-order rate constant and half-time are 2.85 × 10−2 min−1 and 24.3 min, respectively. The better photocatalytic performance of BiNbO4 powders prepared by the citrate method can be attributed to its smaller band gap and better crystallinity.
This project was supported by the Natural Science Foundation of China (No. 51202107), a grant from the Opening Funding of National Laboratory of Solid State Microstructure (No. M26017), and the Doctoral Scientific Research Foundation of Henan Normal University (No. 5101029170260).
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