The upconversion and enhanced visible light photocatalytic activity of Er³⁺-doped tetragonal BiVO₄

Abstract

Er³⁺-doped BiVO₄ with tetragonal structure is prepared by the microwave hydrothermal method. X-ray diffraction and Rietveld refinement demonstrate that the structure is transformed from the monoclinic (C2/c:c3) phase to the tetragonal (I41/amd:2) phase by doping with Er³⁺ ions. Er³⁺ doping also influences the morphology change of BiVO₄ from irregular flake-like crystal to rod-like crystal, which leads to the increase of the surface areas from 3.25 to 11.96 m² g⁻¹. Compared with the monoclinic BiVO₄, the upconversion of the Er³⁺-doped tetragonal BiVO₄ occurs through the transitions from the ⁴I_15/2 ground state to ⁴F_7/2, ²H_11/2, and ⁴F_9/2 states, respectively. The photocatalytic experiment indicates that the tetragonal BiVO₄ (8 at.%) with a larger specific surface area (9.88 m² g⁻¹) shows the best photocatalytic activity under visible light irradiation, which can efficiently improve the degradation rate of RhB up to 97.2% at 150 min.

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1. Introduction

Semiconductor photocatalysts are widely used in the degradation of organic contaminants in wastewater.^1–3 According to early reports, monoclinic BiVO₄ (m-BiVO₄) is the best visible-light-driven photocatalyst, while the photocatalytic activity under visible light irradiation of tetragonal BiVO₄ (t-BiVO₄) is negligible.^4,5 Therefore monoclinic BiVO₄ has been extensively studied as a promising photocatalyst for organic photocatalytic degradation due to its narrow band gap (2.4 eV).^6,7 However, the rapid recombination rate of electron–hole pairs is always the main drawback for monoclinic BiVO₄ to be applied. To solve this problem, different methods have been used to improve the photocatalytic activity of monoclinic BiVO₄, such as co-catalyst loading, impurity doping, and the formation of a heterojunction.^8,9 Recently, S. Usai and Sergio Obregón reported that Y³⁺ or Er³⁺ doping could induce the monoclinic–tetragonal heterostructure and improve the photocatalytic performance under sun-like excitation.^10,11 On this basis, we obtained the tetragonal BiVO₄ after doping Er³⁺ ions. This phenomenon is quite different from the previous reports. In this work, the Er³⁺-doped tetragonal BiVO₄ system exhibits higher photocatalytic activity than that of monoclinic BiVO₄ for the degradation of RhB under visible light irradiation.

2. Experimental

All the reagents were of analytical grade and were used without any further purification. In a typical preparation process, 0.01 mol Bi(NO₃)₃·5H₂O was dissolved in 25 mL deionized water and stirred for 20 min at room temperature. A quantity of 0.01 mol NH₄VO₃ was dissolved in 25 mL 100 °C deionized water and stirred for 20 min until the dissolution was complete. Then the NH₄VO₃ solution was added into the Bi(NO₃)₃·5H₂O solution with magnetically stirring for 20 min, thus forming the jacinth slurry A. The pH of the slurry A was adjusted to 8.5 by adding concentrated NaOH (5 mol L⁻¹). After stirred for several minutes, the appropriate contents of Er(NO₃)₃·6H₂O were added to slurry A until the molar ratio of n_Er to n_Bi was 0–12 at.%, generating precursors B. Then each precursor was transferred into the polytetrafluoroethylene (PTFE) autoclave (the filling density was 60%) and heated under microwave irradiation using a microwave hydrothermal reactor (Model MDS-8, Sineo, ShangHai). The specific program was as follow. Firstly, the microwave hydrothermal reaction power was set to 300 W and the temperature was from room temperature up to 100 °C for 6 min. Secondly, the microwave hydrothermal reaction temperature was from 100 °C to 150 °C for 6 min and from 150 °C to 200 °C for 40 min, and then it was allowed to cool to room temperature naturally. Finally, the obtained precipitates were washed three times with deionized water and absolute ethyl alcohol, and then dried at 80 °C for 12 h in air.

The structural properties of the as-prepared samples were determined by an X-ray diffractometer (XRD, D/Max-2200, Rigaku, Japan) with Cu Kα radiation (λ = 0.15406 nm). The morphology was analyzed by using a scanning electron microscope (SEM, S-4800, Hitachi, Japan). The microstructures were investigated by a transmission electron microscopy (TEM) and a high resolution transmission electron microscopy (HRTEM, J EM-2100, Japan). The specific surface area was measured by a specific surface area analyzer (3H-2000BET-A, BJ, China). The adsorbate was N₂ while the carrier gas was He. The UV-vis diffuse reflection spectra were recorded on a UV-vis spectrophotometer (Hitachi U-3900H, Japan), using BaSO₄ as a reference. The upconversion fluorescence spectra were measured using YAG:Nd³⁺ (Quanta Ray Lab-170) pulse laser and Ti sapphire sapphire femtosecond laser (Mira-900) as excitation sources, and the output wavelength was 980 nm. The collection and detection of luminescence were carried out on a spectrometer (SP2750i) with a spectral resolution of 0.008 nm. All of the measurements were performed at room temperature. The Zeta potential was identified by Nano Particle and Zeta potential analyzer (Malvern Nano-ZS, UK).

In a process, a 500 W xenon lamp was used as a visible-light source. 0.05 g photocatalyst was dispersed in 50 mL RhB solution (5 mg L⁻¹) under ultrasonic treatment. Prior to the irradiation, the solution was magnetically stirred in the dark for 30 min to achieve the adsorption–desorption equilibrium. The concentration of RhB was determined by recording the absorbance at 553 nm using a UV-vis spectrophotometer (Model SP-756p).

3. Results and discussion

Fig. 1(a) shows the XRD patterns of pure BiVO₄ and Er³⁺-doped BiVO₄ photocatalysts. It is worth noting that the monoclinic BiVO₄ (JCPDS no. 14-0688) was changed into the tetragonal BiVO₄ (JCPDS no. 14-0133) when Er³⁺ is incorporated. The observed evolution of the crystalline structure of Er³⁺-doped BiVO₄ is significantly different from that previously reported,¹¹ that is merely the tetragonal phase is present when Er³⁺ content is above 4 at.%. In other words, Er³⁺-doped BiVO₄ is considered to stabilize the tetragonal structure. In addition, the intensities of the (101) and (200) peaks of the tetragonal BiVO₄ are reduced gradually so that the crystallinity of the composite is decreased.

Fig. 1 (a) XRD patterns of different Er³⁺-doped contents. (b) Rietveld refined XRD patterns and the insets show schematics of the monoclinic BiVO₄ and Er³⁺-doped BiVO₄ (8 at.%). (c) Zeta potential of different Er³⁺-doped contents. (d) XRD patterns of monoclinic BiVO₄ and Er³⁺-doped BiVO₄ (8 at.%) in the drying oven at 80 °C.

The Zeta potential of the precursor slurry is 41.20 with the Er³⁺-doped BiVO₄ (0 at.%) in Fig. 1(c), suggesting the larger electrostatic repulsion, which is responsible for the shortage of BiVO₄ crystal nuclei. The Bi³⁺ and VO₃⁻ will grow up into more stable thermodynamic monoclinic BiVO₄ crystal grains according to certain spatial configurations and then irregular monoclinic BiVO₄ in the microwave hydrothermal process.¹² The Zeta potentials of Er³⁺-doped BiVO₄ (4–12 at.%) are 35.71, 32.76, 32, 10.09, 1.94, respectively, suggesting the decreasing electrostatic repulsion, which can form a large number of BiVO₄ crystal nuclei. The crystal nucleus will generate amorphous BiVO₄ precipitate, which can be crystallized into tetragonal BiVO₄ and Er₈V₂O₁₇ in the drying oven at 80 °C (Fig. 1(d)). The tetragonal BiVO₄ and Er₈V₂O₁₇ are dissolved and crystallized into the tetragonal Er³⁺-doped BiVO₄. The increase of Er³⁺ doping content give rise to the decrease of Er³⁺-doped BiVO₄ crystal grains with recrystallization process (Table 1), resulting in the widened diffraction peak with the Er³⁺-doped BiVO₄ (12 at.%).

Table 1 Rietveld refined structural parameters and crystallite size of the catalysts

Samples	Crystal structure	Space group	Lattice parameters	Tetragonal crystallite size (nm)
Undoped	Monoclinic	C2/c:c3	a = 5.1881 Å b = 5.0788 Å c = 11.6757 Å
4 at.% Er^3+	Tetragonal	I41/amd:2	a = b = 7.2769 Å c = 6.4413 Å	67.9
6 at.% Er^3+	Tetragonal		a = b = 7.2854 Å c = 6.4424 Å	69.4
8 at.% Er^3+	Tetragonal		a = b = 7.2894 Å c = 6.4464 Å	69.1
10 at.% Er^3+	Tetragonal		a = b = 7.2876 Å c = 6.4468 Å	59.3
12 at.% Er^3+	Tetragonal		a = b = 7.2852 Å c = 6.4465 Å	49.4

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To further analyze the crystal structure of the Er³⁺-doped BiVO₄, the Rietveld refinement on the XRD patterns of pure BiVO₄ and the Er³⁺-doped BiVO₄ (8 at.%) is performed using the Maud program.^13,14 In the light of the refinement results, the schematics of the BiVO₄ crystal structure are shown in Fig. 1(b). The structural transition of Er³⁺-doped BiVO₄ can be found, where the monoclinic BiVO₄ and the Er³⁺-doped BiVO₄ (8 at.%) having C2/c:c3 and I41/amd:2 space groups, respectively, can be observed.

The morphologies of the samples synthesized by microwave-hydrothermal process were characterized by a scanning electron microscopy (SEM), as shown in Fig. 2. When the content of Er³⁺ is 0 at.%, the crystal possesses irregular flake-like morphology with 100 nm thickness (Fig. 2(a)). The distribution of crystal grains is nonuniform and the gain surface is smooth, resulting in small specific surface area (3.25 m² g⁻¹). With the increase of doping Er³⁺ content (4–12 at.%), the flake-like crystal grains are dissolved gradually and recrystallized to the well-defined rod-like crystals of the tetragonal BiVO₄ (Fig. 2(b)–(f)). This implies that the formation of crystals in nucleation–dissolution–recrystallization may be predominated by Er³⁺ ions. The increase of the specific surface area (from 7.42 m² g⁻¹ to 11.96 m² g⁻¹) can therefore be correlated with these changes of the morphology and smaller gain size. The changes of morphologies and the specific surface area attribute to the different Zeta potentials, resulting in the different growth of BiVO₄ crystal. The tetragonal BiVO₄ and Er₈V₂O₁₇ crystals are dissolved gradually and recrystallized to form the well-defined rod-like crystals. These results are in accordance with the XRD patterns.

Fig. 2 SEM images of Er³⁺-doped BiVO₄ (a) 0 at.% (b) 4 at.% (c) 6 at.% (d) 8 at.% (e) 10 at.% (f) 12 at.%. (g) Specific surface areas of the monoclinic BiVO₄ and Er³⁺-doped BiVO₄.

It can also be seen in the TEM images that the irregular flake-like crystal is the monoclinic BiVO₄ (0 at.%) with 100 nm thickness (Fig. 3(a)) and the rod-like crystal is Er³⁺-doped BiVO₄ (8 at.%) with the length of 300 nm (Fig. 3(c)). From the insets in Fig. 3(b) and (d), the SEAD diffraction patterns are the bright spots of electron diffraction, which indicates that both monoclinic and tetragonal BiVO₄ are crystalline. The HRTEM images obtained from the (b and d) areas marked with rectangles are shown in Fig. 3(b) and (d). An interval of 0.2910 nm between the uniform fringes is in good agreement with the (040) lattice plane of monoclinic BiVO₄. Meanwhile, the spaces of 0.4810 nm and 0.3626 nm of the clear lattice fringes with corresponding to (101) and (200) planes of tetragonal BiVO₄ imply the high crystallinity. These results are consistent with the initial XRD analyses.

Fig. 3 (a and b) TEM images and HRTEM of monoclinic BiVO₄ (0 at.%); (c and d) TEM images and HRTEM of Er³⁺-doped BiVO₄ (8 at.%) (the insets in (b) and (d) show SEAD patterns).

The optical properties of monoclinic BiVO₄ and Er³⁺-doped tetragonal BiVO₄ are measured by using UV-vis diffuse reflection spectrum in the wavelength range of 200–800 nm. From the diffuse reflectance spectra (Fig. 4(a)), two clear absorption edges can be noticed both in the UV and visible light regions, which corresponds to the band gap energy of 2.4 eV and 2.9 eV of the monoclinic and tetragonal phases, respectively.¹⁵ Compared with the monoclinic BiVO₄, three new absorption bands at ca. 490, 524 and 655 nm of Er³⁺-doped BiVO₄ can be observed, which can be assigned to the Er³⁺ transitions from the ⁴I_15/2 ground state to ⁴F_7/2, ²H_11/2, and ⁴F_9/2 states, respectively.¹⁶

Fig. 4 (a) UV-vis diffuse reflection absorption spectra of Er³⁺-doped BiVO₄. (b) The upconversion fluorescence spectra of Er³⁺-doped tetragonal BiVO₄ (8 at.%) upon 980 nm laser excitation.

In order to understand the luminescent property of Er³⁺ doping in the catalysts, we have performed the upconversion fluorescence spectra of Er³⁺-doped tetragonal BiVO₄ (8 at.%) upon 980 nm laser excitation (Fig. 4(b)). In the spectrum, there are two emission parts: the green emission from 490 nm to 570 nm is corresponding to the transitions from ²H_11/2/⁴F_7/2 to ⁴I_15/2 when Er³⁺ ions are doped and the red one in a range 640–690 nm is assigned to the transition of ⁴F_9/2–⁴I_15/2 when Er³⁺ ions are doped.¹⁷ The result indicates the existence of the upconversion process in the Er³⁺-doped tetragonal BiVO₄ photocatalyst. Generally, the Er³⁺-doped upconversion process can increase the number of incoming photons with adequate energy. Then the photons can be absorbed by the photocatalyst, which will generate more photogenerated electron–hole pairs.¹⁸ Therefore, the photocatalytic activities of Er³⁺-doped tetragonal BiVO₄ with up-conversion effect are expected to be efficiently enhanced.

The photocatalytic activities of the Er³⁺-doped BiVO₄ are evaluated by measuring the degradation of RhB aqueous solution under visible light irradiation shown in Fig. 5(a). For the monoclinic BiVO₄, the photodegradation of RhB is 63.13% after 150 min irradiation. By introducing the up-conversion Er³⁺ and optimizing the contents (4–12 at.%), the photodegradation rates of RhB have promptly reached to 96.80%, 96.98%, 97.2%, 96.38% and 88.35% respectively, which exhibits much higher photocatalytic activity than that of the monoclinic BiVO₄. When Er³⁺ is increased to 10 at.%, the photocatalytic activity begins to be decreased because the excessive Er³⁺ ions may cover the active sites on BiVO₄ surface and also may act as the recombination centers of photogenerated electrons and holes, resulting in the decrease of photocatalytic activity.¹⁹ Therefore, the optimum content of doping Er³⁺ is 8 at.%.

Fig. 5 (a) Photocatalytic degradation to RhB with different Er³⁺ doping contents; (b) absorption spectra changes during the photocatalytic degradation of the Er³⁺-doped tetragonal BiVO₄ (8 at.%) in RhB solution under visible light irradiation (inset show the TOC analysis of the degradation of RhB solution by the Er³⁺-doped tetragonal BiVO₄ (8 at.%) photocatalyst) (c) cycling runs in the photocatalytic degradation of RhB in the presence of Er³⁺-doped tetragonal BiVO₄ (8 at.%) photocatalyst. (d) A schematic illustration of the energy conversion in the Er³⁺-doped BiVO₄ photocatalyst.

The temporal evolution of spectral changes during the photodegradation process of RhB for Er³⁺-doped BiVO₄ (8 at.%) is displayed in Fig. 5(b). From the spectra, it can be seen that the absorbance of RhB at the maximum absorption wavelength is gradually decreased, which realizes the de-ethylation. Under visible illumination, the intense bright cherry-red color of the starting solution gradually disappears with increasing exposure time. The de-ethylation of the N,N,N,N-tetraethylated rhodamine molecules appears in the wavelength position of its major absorption band moving toward the blue region, and it is as follows: λ_max, RhB, 554 nm; N,N,N-triethylated rhodamine, 539 nm; N,N-diethylated rhodamine, 522 nm; and N-ethylated rhodamine, 510 nm; and rhodamine, 498 nm. RhB has completely been changed into rhodamine.²⁰ Then through the destruction of their conjugate structure, the further degradation is achieved. After a series of complicated oxidation reaction, the RhB is decomposed to smaller organics and minerals, which can be proved by the results of the TOC measurement.²¹ The inset in Fig. 5(b) displays that the ability of Er³⁺-doped tetragonal BiVO₄ (8 at.%) to mineralize RhB is evaluated by monitoring changes of organic carbon in TOC. As can be seen, TOC diminishes gradually and ca. 36% TOC removal is obtained after 150 min irradiation, implying that RhB degradation is accompanied by partial mineralization.

The recycle experiments are performed to evaluate the photocatalytic repeatability and stability of Er³⁺-doped tetragonal BiVO₄ (8 at.%) photocatalyst under visible light irradiation.²² Fig. 5(c) shows the results from the four successive runs of the photodegradation of RhB under the same experimental conditions. It is clear to see that no significant photoactivity loss is observed after four times of successive recycles. The results indicate that Er³⁺-doped tetragonal BiVO₄ (8 at.%) photocatalyst is stable during the photodegradation of RhB.

Fig. 5(d) shows the mechanism of the upconversion of Er³⁺-doped tetragonal BiVO₄. For monoclinic BiVO₄, the absorbable light is limited to 519 nm (Fig. 4(a)), while the proper doping content of Er³⁺ in the Er³⁺doped BiVO₄ with can effectively be converted from the long wavelength light to the short wavelength light so as to generate the excited electrons with higher energy and sequentially to capture low energy photons as shown in Fig. 5(d). When these excited electrons are on the ground state by means of irradiation or energy transformation, the emitted light or the transformed energy will make the tetragonal BiVO₄ yield more photoinduced electron–hole pairs so as to enhance the photocatalytic activities. Then the highly oxidative holes accumulated in the BiVO₄ can directly decompose the organic pollutants or react with H₂O molecules to produce ˙OH,^23,24 which indicates that the upconversion process can enhance the utilization of efficient photons of Er³⁺-doped tetragonal BiVO₄ photocatalysts. On the basis, the upconversion plays a significant role in the improvement of the photocatalytic activities in the Er³⁺-doped tetragonal BiVO₄ system.

4. Conclusions

The tetragonal BiVO₄ with different Er³⁺-doped contents has been successfully synthesized by microwave hydrothermal method. The morphology of Er³⁺-doped BiVO₄ has been changed from irregular flake-like to rod-like crystals with the structural transition from monoclinic to tetragonal BiVO₄. Compared with the monoclinic BiVO₄, the upconversion of the Er³⁺-doped tetragonal BiVO₄ occurs through the transitions from the ⁴I_15/2 ground state to ⁴F_7/2, ²H_11/2, and ⁴F_9/2 states, which makes the tetragonal BiVO₄ generate more photoinduced electron–hole pairs so as to enhance the photocatalytic activities. Furthermore, the Er³⁺-doped tetragonal BiVO₄ exhibits significantly higher photocatalytic activity than that of the monoclinic BiVO₄ for the degradation of RhB under visible light irradiation.

Acknowledgements

This work is supported by the Project of the National Natural Science Foundation of China (Grant no. 51172135); the Academic Leaders Funding Scheme of Shaanxi University of Science & Technology (2013XSD06); Doctorate Scientific Research Initial Fund Program of Shaanxi University of Science & Technology(BJ4-13); the Graduate Innovation Fund of Shaanxi University of Science & Technology (SUST-A04).

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