Efficient Visible-Light Photocatalytic Properties in Low-Temperature Bi-Nb-O System Photocatalysts
Low-temperature Bi-Nb-O system photocatalysts were prepared by a citrate method using homemade water-soluble niobium precursors. The structures, morphologies, and optical properties of Bi-Nb-O system photocatalysts with different compositions were investigated deeply. All the Bi-Nb-O powders exhibit appreciably much higher photocatalytic efficiency of photo-degradation of methyl violet (MV), especially for Bi-Nb-O photocatalysts sintered at 750 °C (BNO750), only 1.5 h to completely decompose MV, and the obtained first-order rate constant (k) is 1.94/h. A larger degradation rate of Bi-Nb-O photocatalysts sintered at 550 °C (BNO550) can be attributed to the synergistic effect between β-BiNbO4 and Bi5Nb3O15. Bi5Nb3O15 with small particle size on β-BiNbO4 surface can effectively short the diffuse length of electron. BNO750 exhibits the best photocatalytic properties under visible-light irradiation, which can be attributed to its better crystallinity and the synergistic effect between β-BiNbO4 and α-BiNbO4. The small amount of α-BiNbO4 loading on surface of β-BiNbO4 can effectively improve the electron and hole segregation and migration. Holes are the main active species of Bi-Nb-O system photocatalysts in aqueous solution under visible-light irradiation.
KeywordsBi-Nb-O photocatalyst Citrate method Visible-light photocatalytic property Synergistic effect
Recent years, much attention has been focused on the environmental remediation due to the increasing pollution problems caused by the industries. Organic pollutants, particularly dyes, have a deleterious effect on human health . In 1972, photosensitized decomposition of water into H2 and O2 using TiO2 semiconductor electrode was first reported by Fujishima and Honda . Since then, a large number of semiconductor materials have been investigated as active catalysts for the reduction and/or elimination of environmental pollution in water and air due to their potential in the conversion of light energy. TiO2, as one of the most popular photocatalysts, can solely absorb the UV light, which accounts for only 4 % of the total sunlight. It greatly inhibits its practical applications for the decomposition of toxic and hazardous organic pollutants. Hence, it is very necessary to develop photocatalysts with high catalytic activities under the visible light. Currently, numerous strategies, such as doping [3, 4, 5], dye sensitization [6, 7], and growths of TiO2-based heterostructures [8, 9], have been developed, aiming to promote their photo-response performance to the visible range. Besides, other novel photocatalysts with outstanding visible-light photocatalytic properties have been developed, such as quantum dot-based photocatalysts [10, 11, 12, 13, 14].
Bismuth-based photocatalysts, due to their excellent photo-degradation performance for organic contaminant using visible light, have attracted much attention, such as BiWO6 , BiOX (X = Cl, Br, I) [16, 17], Bi2O2CO3 , BiNb3O15 , and BiNbO4 [20, 21, 22, 23, 24, 25]. Among these materials, BiNbO4 is investigated for H2 generation and contaminant degradation under visible light, exhibiting greater photocatalytic performance than TiO2. In general, BiNbO4 has orthorhombic α and triclinic β phases; α phase synthesized at 900 °C irreversibly transforms to the high-temperature β phase (denoted as High-β) at 1020 °C . Compared with β phase, the α phase always shows better photocatalytic performance due to the formation of a narrow conducting band and the electron and holes can effectively reach reaction sites on the surface in orthorhombic structure . While in our former work, we first synthesized the pure low-temperature β phase (denoted as Low-β) at 700 °C and the visible-light photocatalytic performance test shows that the Low-β exhibits better photocatalytic efficiency compared with α phase [20, 28]. The formation of pure triclinic phase of BiNbO4 at low temperature can be attributed to the formation of intermediate Bi5Nb3O15 phase.
Compared with BiNbO4, the research of Bi5Nb3O15 as photocatalyst is rare, though it is expected to have high photocatalytic efficiency due to the composition of the conduction band (CB) and valance band (VB) same as BiNbO4 . Because of the volatilization of Bi element at high temperature, the conventional solid state method requires critical control of Bi content to obtain stoichiometric Bi-Nb-O compounds; also, the resulting bulk products of several micrometers are harmful to efficient electronic diffusion, which inhibits the research and applications of Bi-Nb-O system photocatalysts. Citrate method, a simple way to obtain stable precursors and reactive, stoichiometric fine powders, has been widely used in the fabrication of various complicated oxides .
In this paper, low-temperature Bi-Nb-O system photocatalysts were prepared by the citrate method using homemade water-soluble niobium precursors. The structures, morphologies, optical properties of Bi-Nb-O system photocatalysts with different composition were investigated deeply. The visible-light photocatalytic properties were evaluated with the degradation of methyl violet (MV) under visible light irradiation. The synergistic effect between different compositions in Bi-Nb-O system was also proposed to explain the efficient visible-light photocatalytic properties.
Bismuth nitrate (Bi(NO3)3·5H2O), citric acid (CA), ammonia (NH3·H2O), and Nb-citrate (Nb-CA) aqueous solution were used as starting materials. The synthesis of water-soluble Nb-CA has been described in details in our previous work . The Bi-Nb-O powders were prepared using the citrate method. Bi(NO3)3·5H2O was first dissolved in Nb-CA aqueous solution, followed by addition of CA. Then the solution was kept stirring at 60 °C, using ammonia to adjust the pH value to 7~8. Finally, the stable and transparent precursor solution was dried at 180 °C and then sintered at various temperatures from 500 to 800 °C for 3 h to obtain the Bi-Nb-O powders.
The structures of the Bi-Nb-O powders were characterized by X-ray diffraction (XRD; Rigaku-D/Max 2000) using Cu Kα radiation. The scanning electron microscope (SEM; JSM-6700F) and transmission electron microscope (TEM; Tecnai F20 S-Twin, FEI) were used to examine the morphologies and grain sizes of the powders. The specific surface area was measured on a surface area apparatus (Micromeritics TriStar 3000, Shamidzu) at 77 K by N2 adsorption/desorption method (BET method). The photoluminescence (PL) spectra were detected using an F-280 fluorescence spectrophotometer with excitation wavelength of 320 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed on Thermo Fisher K-Alpha equipment.
To evaluate the visible-light photocatalytic activities of Bi-Nb-O powders, the decomposition reaction of MV aqueous solution was carried out under irradiation of a 150-W Xe lamp (LA-410UV-3, Hayashi, Japan) at the natural pH value. The details have been described in the previous work and the photo-degradation process was monitored by an ultraviolet-visible near infrared (UV-vis-NIR) spectrophotometer (UV-3600, Shimadzu, Japan) . The concentration of the residual MV in solution was determined as a function of irradiation time by measuring the maximum absorption at 582 nm.
To detect the active species during photocatalytic reactivity, hydroxyl radicals (·OH) and holes (h+) were investigated by adding 5 mM tert-butyl alcohol (t-BuOH; a quencher of ·OH) and EDTA-2Na (a quencher of h+) . The method was similar to the former photocatalytic activity test with 30-min visible-light irradiation.
Results and Discussion
The grain sizes, specific surface areas, and UV-vis absorption data of Bi-Nb-O powders sintered at different temperatures
Grain size (nm)
Specific surface areas (m2/g)
Band gap (eV)
λ a (nm)
where A is the band edge constant and n is an index which assumes the values 1/2 and 2 for direct allowed and indirect allowed transitions, respectively. Because Low-β as the major phase in Bi-Nb-O system is the indirect band gap semiconductor, the value of n is taken as 2. The energy band gaps of Bi-Nb-O powders are estimated, as listed in Table 1. The energy band gap is consistent with that obtained using density functional theory computation . The band gaps of Bi-Nb-O powders suggest all of them have the visible-light photocatalytic performance through direct photo-absorption, and the critical absorbance wavelength is above 400 nm. The color of the powders is pale yellow, which is consistent with the band gaps.
As for Bi-Nb-O compounds, no matter Bi5Nb3O15 or α, β phase BiNbO4, their electronic structures are composed of VB by O 2p state and CB by hybridization states of Bi 4p, Nb 4d, and O 2p . In other group’s research, Bi5Nb3O15 powder with a small size has higher photocatalytic activity than P25 and bulk α phase BiNbO4, due to short diffuse length of electron and efficient visible-light harvesting . In our Bi-Nb-O system, Bi5Nb3O15 phase was first formed with a smaller particle size at 500 °C. With the sintering temperature increase, the major phase Bi5Nb3O15 decompose, which results in Low-β phase BiNbO4 coexisting with a smaller amount of Bi5Nb3O15 particles in BNO550. The better photocatalytic property of BNO550 can be attributed to the synergistic effect between β-BiNbO4 and Bi5Nb3O15. Bi5Nb3O15 with a smaller particle size on β-BiNbO4 surface can effectively short the diffuse length of electron, subsequently decreasing the recombination rate of electrons and holes. When the sintering temperature is above 600 °C, the content of Bi5Nb3O15 is negligible and the photocatalytic properties are mainly from Low-β phase BiNbO4.
Compared with the High-β, the crystal evolution of Low-β has not completed yet. The photocatalytic properties are promoted with the sintering temperature, which means better grain crystallinity. As in Fig. 4, the adsorption test in “dark” shows that the adsorption ability is nearly the same for BNO powders sintered between 600 and 750 °C. So the improved grain crystallinity has the dominant role in the promotion of photocatalytic properties. As the sintering temperature increase to 750 °C, the major phase is Low-β phase BiNbO4 coexists with a smaller amount of α phase BiNbO4. For BNO800, the content of α phase increases and the adsorption ability decreases, as shown in Fig. 4. As described in our former work, Low-β phase BiNbO4 prepared by the citrate method has better photocatalytic performance than α phase BiNbO4 . BNO750 having the best photocatalytic properties under visible light may be attributed to two aspects: one is the better crystallinity and the other is the synergistic effect between Low-β phase BiNbO4 and α phase BiNbO4. Compared with β phase, the α phase with [NbO4] chains favors the formation of a narrow conducting band and the electrons and holes can effectively reach reaction sites of photocatalysts . For BNO750, the small amount of α phase BiNbO4 loading on the surface of Low-β phase BiNbO4 can effectively improve the electron and hole segregation and migration. So BNO750 exhibits the best visible-light photocatalytic properties in low-temperature Bi-Nb-O system photocatalysts.
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-temperature Bi-Nb-O photocatalysts has been investigated also, similar to that of pure Low-β phase BiNbO4 described in our former work . It shows that the optimal operation conditions are catalyst loading of 1 g/L, pH value of 8, and the additive H2O2 concentration of 2 mmol/L.
Bi-Nb-O system photocatalysts were prepared by the citrate method using homemade water-soluble niobium precursors. The structures, morphologies, and optical properties of Bi-Nb-O system photocatalysts with different compositions were investigated deeply. All the Bi-Nb-O powders exhibit appreciably much higher photocatalytic efficiency of photo-degradation of MV, especially for BNO750, only 1.5 h to completely decompose MV, and the obtained k is 1.94/h. Larger degradation rate of BNO550 can be attributed to the synergistic effect between β-BiNbO4 and Bi5Nb3O15. Bi5Nb3O15 with a smaller particle size on β-BiNbO4 surface can effectively short the diffuse length of electrons. BNO750 exhibits the best photocatalytic properties under visible-light irradiation, which can be attributed to its better crystallinity and the synergistic effect between β-BiNbO4 and α-BiNbO4. The small amount of α phase BiNbO4 loading on surface of Low-β phase BiNbO4 can effectively improve the electrons and holes segregation and migration. Holes are the main active species of Bi-Nb-O system photocatalysts in aqueous solution under visible-light irradiation.
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 Foundation of Henan Educational Committee (No. 16A140028).
HZ carried out the main part of the experimental work and XRD measurements and the photocatalytic activity test. SS carried out the XPS and SEM tests. LZ, PL, HLi, and HLu participated in the preparation of water-soluble Nb-CA precursors. JK carried out the TEM image measurements. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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