Enhanced Visible Light-Driven Photocatalysis by Eu3+-Doping in

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The Enhanced Visible Light-driven Photocatalysis by Eu doping in BaNbVO with Layered Mixed-anion Structure 2

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Lin Qin, Peiqing Cai, Cuili Chen, Cheng Han, Jing Wang, Sun Il Kim, and Hyo Jin Seo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12628 • Publication Date (Web): 31 May 2016 Downloaded from http://pubs.acs.org on June 1, 2016

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The Journal of Physical Chemistry

The Enhanced Visible Light-Driven Photocatalysis by Eu3+-doping in BaNb2V2O11 with Layered Mixed-anion Structure

Lin Qin, Peiqing Cai, Cuili Chen, Han Cheng, Jing Wang, Sun Il Kim, Hyo Jin Seo*

Department of Physics and Interdisciplinary Program of Biomedical, Mechanical &

Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea

*

Corresponding author: [email protected] (Hyo Jin Seo) Tel.: +82-51-629 5568;

fax:+82-51-6295549;

1

ACS Paragon Plus Environment


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ABSTRACT Europium (Eu)-doped BaNb2V2O11 powders with layered mixed-anion structure were prepared through the solid state method successfully. The obtained powders were characterized using X-ray Powder Diffraction (XRD), Structural Refinement, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photo Electron Spectroscopy (XPS), ultraviolet visible (UV-vis) absorption spectra and photoluminescence (PL) measurements. The experimental results demonstrate that BaNb2V2O11 powders could absorb the UV-vis light effectively with the band gap energy of 2.219 eV. The introduction of Eu3+ in the host contributes to the decrease of the band gap energy (2.137 eV). Moreover, the photocatalytic activities of the obtained powders were evaluated through the photocatalytic degradation of the methylene blue (MB) solutions under visible light irradiation as a function of time. All the results indicate that the BaNb2V2O11 powders can be used as a potential visible-light-driven photocatalyst. Notably, due to the introduction of Eu3+, the photocatalytic degradation rate of MB solutions could be improved dramatically. The influence of the existence of Eu3+ on the photocatalytic properties is further discussed in detail based on the crystal structure characteristic.

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1 INTRODUCTION Due to the pollutants from anthropogenic sources, whether air borne or water soluble, serious problems have been exposed to both human health and living conditions. Degradation and decomposition of anthropogenic sources with photocatalytic oxidation technique have been studied widely.1-3 As an active and efficient photocatalyst, TiO2 has only been used largely for organic pollutants degradation and wastewater purification under UV irradiation.4,5 The development of novel visible-light-driven photocatalysts has been studied from the viewpoint of the utilization of solar light energy extensively.6,7 Numerous works of rare earth (RE) compounds have been published, mainly focused on their luminescence properties, due to the RE can offer some superiorities transitions in UV-vis region.8,9 Recently, some researchers have begun to pay more attention to RE compounds or RE doped compounds as photocatalysts. As reported, the photocatalytic activities of photocatalysts can be significantly enhanced by doping RE ions, mainly due to their rich energy levels in the f-orbital.10,11 For example, Kudo reported that the water splitting activities of NiO-NaTiO3 photocatalysts can be improved by doping with RE ions obviously.12 Karunakaran concluded that the RE based metal salts have significant and insatiable interest in photocatalysis field.13 Mahalingan and his coworkers indicated that the RE doped CaMoO4 nanocrystals with high water dispersibility and quantum efficiency can be used as potential materials for photocatalysis.14 Otherwise, RE systems also can decrease the particle size giving rise to increase the surface area of the photocatalysis, to modify the local energy levels and to promote the separation of e- and h+ to delay the lifetime of exciton, or to decrease the band gap to absorb more visible light, which are important factors for photocatalysts. 3



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In 1985, the compounds BaNb2V2O11 with trigonal structure were reported by Trunov firstly.15 And then in 2007, Villars described the crystal structure of BaNb2V2O11 as slabs, where vertex-linked NbO6 octahedra share vertices with single VO4 tetrahedra form double layers on both sides, and the element Ba are existed among the slabs.16 In current work, we investigate BaNb2V2O11 as a novel kind of potential photocatalyst and an enhanced photocatalytic activity could be expected by doping with Eu3+ on the following motivations. Firstly, the framework of trigonal BaNb2V2O11 is constructed by two optical active groups, NbO6 and VO4, both of these two optical active groups can absorbed UV-vis light efficiently. The photoinduced electrons can be created easily, from the full occupied 2p orbitals of oxygen(O) to the empty 3d orbitals of vanadium(V) and the empty 4d orbitals of niobium(Nb) under the light excitation.17,18 As reported, the transition metals M (M=V, Nb or Ta) containing complex oxides can absorb UV-vis light and then exhibit photocatalytic activities efficiently.19 Such as Ag3VO4,20 Sr2M2O7 (M= Nb and Ta),21 etc. Secondly, such an excellent photocatalytic activities can be attributed to the structure characteristic of BaNb2V2O11. As Domen reported,22,23 layered materials such as K4Nb6O17 and A4TaxNb6-2xO17 (A = K, Rb) show high quantum yields (5%), which are more efficient than any other photocatalytic materials. The main reason is due to their interlayer spaces as reaction sites, which will benefit for photocatalysis.24 Thirdly, although the introduction of Eu3+ in BaNb2V2O11 cannot modify the position of valence band (VB) and conduction band (VB) edges, it will introduce new energy levels into the band gap to decrease the band gap energy, thus enhancing the absorption in the visible region. Moreover, when Eu3+ (with smaller ionic radius) were introduced in the lattice to replace Ba2+, defects were unavoidable in the lattices such as cation vacancies or interstitial oxygen. When the defects reach a high level with the increasing of Eu3+ doping 4


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concentration, the impurities could be induced in order to keep the integrity of the final products. It is now recognized that defects always play an essential role in capturing the electrons and holes to decrease the recombination rate and to further increase the activity of photocatalyst. In addition, Eu3+ ions gathered on the surface of particles will increase the photocatalytic activity but decrease the photoluminescence quantum yield.25 To our best knowledge, the photocatalytic activities of BaNb2V2O11 and Eu3+-doped BaNb2V2O11, have not been reported by others. In this work, the efficient photocatalytic degradation rate of MB solutions in the presence of BaNb2V2O11 and Eu3+-doped BaNb2V2O11 were investigated on the base of crystal structure, morphologies, UV-vis absorption spectra and photoluminescence properties.

2 EXPERIMENTAL 2.1 Synthesis of BaNb2V2O11:xEu3+(x=0-7%) The BaNb2V2O11:xEu3+(x=0-7%) powders were prepared using a solid state method. The raw materials were highly pure Ba2CO3, Eu2O3, Nb2O5 and NH4VO3 (99.99%). Firstly, the stoichiometric mixture was slowly sintered at 450 °C for 6 h. After the obtained powders had been mixed again, they were fired at 900 oC for 8 h. Finally, the BaNb2V2O11:xEu3+ (x=0-7%) fine powders were obtained after cooling to room temperature naturally.

2.2 Characterization The XRD of BaNb2V2O11:xEu3+ (x=0-7%) powders were carried out by XRD measurements, which were collected on a Rigaku D/Max diffractometer operating at 36 kV and 25 mA using Cu Kα radiation (λ=1.5405 Å). The surface morphologies of the obtained powders were characterized by the SEM, TEM, and HRTEM. The UV-vis absorption spectra were characterized using a Cary 5



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5000 UV-vis-NIR absorbance spectrometer (Varian Cary 6000i). The XPS analyses were measured using XPS program, Axis Ultra HAS (Kratos) monochromatic Al Kα radiation at a reduced power of 100 W. The PL spectra and luminescence decay curves were measured through a pulsed Nd:YAG laser at 266 nm with different temperatures. The photocatalytic activities were evaluated through the photocatalytic degradation of MB solution (10 mg/L, 100 mL) with small amount of photocatalyst (0.1 g). Ahead of visible light irradiation with a 300 W Xenon lamp, the obtained powders should be stirred in the dark environment for 30 min to exclude desorption equilibration and adsorption. The concentration of MB solution can be obtained by recording the absorption band maximum (around 665 nm) in the absorption spectra. And the photodegradation efficiency (η) was calculated according to the following equation:

η=

C0 − C ×100% C0

(1)

where C0 was the initial MB concentration, C was the residual MB concentration after photodegradation for certain period of time.

3 RESULTS AND DISCUSSIONS 3.1 Structural and Particulate Properties The room temperature XRD patterns of BaNb2V2O11:xEu3+(x=0-7%) are shown in Figure 1, along with the corresponding standard BaNb2V2O11 (standard PDF card No. 39-0081). All the reflections could be identified and assigned to the trigonal BaNb2V2O11. When the doping concentration is higher than 5%, a small amount of impurity appeared.

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♥ impurity

Intensity (a. u.)

24.0

24.2

24.4

24.6

24.8

25.0

Two Theta (degree)

♥

♥

3+

7% Eu

3+

5% Eu

3+

3% Eu

3+

1% Eu

3+

0.01% Eu

pure sample PDF#39-0081 10

20

30

40

50

60

70

Two Theta (degree) Figure 1. XRD patterns of the obtained BaNb2V2O11:xEu3+(x=0-7%) and the standard data of

10

20

(110)

Obs Calc Bckgr diff

30

40

(217) (303)

(113) (116) (202) (119) (027) (208)

(107)

(015) (009)

(018)

(012) (101) (104)

BaNb2V2O11 (JCPDS 39-0081).

Intensity(a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

60

70

Two Theta (degree) Figure 2. Rietveld refinement of pure BaNb2V2O11 based on the XRD pattern, showing the experimental and the calculated profile.

According to the experimental XRD patterns of BaNb2V2O11, the atom positions and structure parameters can be obtained through the GSAS program shown in Figure 2. The BaNb2V2O11 sample is in a trigonal crystal system with a space group of R3m (166) (a=5.5383(2) 7



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Å, b=5.5383(2) Å, c=28.1484(11) Å, α=β=90 °, γ=120 ° and V=748.41(5) Å3 ), as shown in Table 1 and Table 2, which were conformed to paper reported by Villars.16 The structure sketch map of BaNb2V2O11 is shown Figure 3 modeled based on the atomic coordinates. As figure shows, the framework of the structure is mainly formed by ideal tetrahedral [VO4] and hexahedral [NbO6] groups. Actually, there are two kinds of chains in the structure, one is vertex-linked NbO6 octahedra sharing vertices with single VO4 tetrahedra generating tunnel [VO4] and [NbO6] chains running parallel to [100], and the other chains were formed by Ba atoms themselves, which were also parallel to [100]. As reported, the chains formed by Ba atoms can prohibit the energy diffusion and enhance the separation of e- and h+.26 Moreover, the chains formed by Ba is expected to be beneficial for stabilizing the overall framework and increasing Madelung energy which are sensitive to photo-corrosion.26 The two chain constructions develop a double layered structure, such a structure was reported to be useful for photocatalysis.27

Table 1. Refined Crystallographic Parameters of BaNb2V2O11. Formula

Refined-BaNb2V2O11

radiation

Cu Ka

2θ range(degree)

10-70

symmetry

trigonal

space group#

R3m (166) 5.5383(2)

a/Å b/Å

5.5383(2)

c/Å

28.1484(11)

α/°

90

β/°

90

γ/°

120

Z

3

Rp

0.0756

Rwp

0.0856

2

X

V /Å

1.725 3

747.71(4) 8


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Table 2. The Crystallographic Parameters Obtained from Rietveld Refinement for BaNb2V2O11. Atom

Wyck.

Site

x/a

y/b

z/c

U [Å2]

O1

18h

.m

0.50500

0.49500

0.24900

0.0250

O2

9d

.2/m

1/2

0

1/2

0.0250

Nb1

6c

3m

0

0

0.13120

0.0018(20)

O3

6c

3m

0

0

0.34020

0.0250

V1

6c

3m

0

0

0.39900

0.016(4)

Ba1

3a

-3m

0

0

0

0.0176(23)

Figure 3. Schematic diagram of the typical layered BaNb2V2O11.

The surface morphologies of BaNb2V2O11 powders prepared through the solid state method at 900 oC were examined by SEM, TEM and HRTEM measurements shown in Figure 4. The particle shape of the obtained powders was irregular. Such a wide variety of particle sizes, between 1 and 10 µm approximately, is mainly due to the high temperature solid state method with grinding process. The morphologies of the obtained powders are mostly ruleless with high crystallinity. The high resolution (HR) TEM image shown in Figure 4b confirms the single crystalline nature of the BaNb2V2O11 powders, and the spacing corresponds to the (020) 9



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reflections of the BaNb2V2O11 powders is calculated to be 0.5 nm (Figure 4c). The corresponding selected area electron diffraction (SAED) pattern shown in Figure 4d exhibits the trigonal symmetric diffraction pattern ascribed to BaNb2V2O11 powders.

Figure 4. (a, b). The SEM images, (c). The HRTEM image, (d). The SAED pattern of BaNb2V2O11 powders.

The

chemical

compositions

and

the

ionic

states

of

all

ions

presented

in

BaNb2V2O11:0.05Eu3+ powders were determined by XPS measurement, as shown in Figure 5. There are only six elements, Ba, Eu, Nb, V, O and C, can be identified in the overall XPS spectrum (Figure 5a), indicating the purity of the compound. Figure 5b, c, d, e, f show the core level spectra of each ions. It is worth noting that the core level spectrum of V-2p3/2 shows an obvious asymmetry curve, indicating that there are multiple valence states in V-2p3/2 core level.27 As figure shows, such a curve could be decomposed into two broad band located around 517.7 and 515.4 eV, which could be assigned to V5+ and V4+, respectively.28 The existence of V4+ can be attributed to the high vacancy and such high vacancy is mainly due to the substitution of Ba2+ by 10


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Eu3+, and the existence of oxygen vacancy can be confirmed by the XPS spectrum of O 1s (Figure 5e). As figure shows, the asymmetrical peak can be decomposed into three symmetric curves, labeled as 1, 2, 3. The peak around 529.4 eV can be regarded as the characteristic peak of O 1s in BaNb2V2O11 lattices. However, the other two peaks at binding energy (BE)=528.7 and 531.0 eV should be assigned to adsorbed oxygen species and surface lattice oxygen (such as O2−, O−, or O22−), respectively;29,30 As well known, in the coexistence of V5+ and V4+ system, along with high vacancy concentration, photocatalytic activity can improve obviously.31

Figure 5. The core level XPS spectra for chemical compositions in BaNb2V2O11: Survey XPS spectrum (a), Ba 3d (b), V 2p (c), Nb 3d (d), O 1s (e) and Eu 3d (f).

3.2 Effect of Eu3+ Concentration on Optical Properties The optical properties of undoped and Eu3+-doped BaNb2V2O11 were measured through the UV-vis absorption spectra, as shown in Figure 6. Clearly, in the 7 mol% Eu3+-doped BaNb2V2O11, an relative broad peak in the range of 500-600 nm indicates the existence of the impurity, 11



corresponding to the results from XRD data. Changes in the optical properties also can be easily justified from the colors of the obtained powders in this work. Pure BaNb2V2O11 powder is yellowish, while grey color of the powder gradually indicates the presence of Eu3+ in the host.

Intensity (a. u.)

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x=0 x=0.001 x=0.01 x=0.03 x=0.05 x=0.07

200

300

400

500

600

700

800

Wavelength (nm) Figure 6. The UV-vis absorption spectra of BaNb2V2O11:Eu3+ with different concentrations.

The band gap can be calculated through the following formula:32 Eg (eV) = 1240/λg (nm)

(2)

where λg represents the absorption edge wavelength obtained from absorbance spectrum. The calculated band gaps values are 2.219, 2.193, 2.157, 2.151, 2.137 eV for undoped, 0.1%, 1%, 3%, 5% Eu3+ doped samples, respectively. Compared with pure BaNb2V2O11, the band gap of all doped samples decreased with increasing the concentration of Eu3+, indicating that the introduction of Eu3+ can affect the band gap structure evidently. Actually, the band gap structure of photocatalysts is an important influencing factor to the photocatalytic efficiency. According to the references,17,22 the valence band (VB) and conduction band (CB) potentials of all Eu3+-doped BaNb2V2O11 powders should be quite similar due to the same crystal structure, the same O 2p 12


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orbits comprising the CB and the same orbits (the hybridization of Nb 4d and V 3d orbitals accompanied by a few O 2p states) comprising the VB. Thus, the difference of band gap energies is mainly due to the introduction of Eu3+ ions. Actually, the introduction of Eu3+ in BaNb2V2O11 cannot modify the position of VB and CB edges in BaNb2V2O11. Instead, the decrease of the band gap could be attributed to the charge-transfer transition between the 4f orbits of Eu3+ and the CB of BaNb2V2O11. That is to say, the interband transition in Eu3+ doped BaNb2V2O11 might be approximately viewed as one from VB or impurity levels to CB.33 According to the references, the impurity levels could be the defects due to the charge imbalance and mismatching ion size substitution of Eu3+ in Ba2+ sites.34,35 Such a substitution will result in a negative charge (Eu3+Ba)•, which requires charge compensation. As reported, the required charge compensation can be obtained through the following possible mechanisms: one possible method for maintaining charge neutrality in the lattice is maintained by creating the Ba vacancy (3Ba2+→2Eu3++VBa′′), forming the dipole complexes [2(EuBa3+)•–VBa′′]. Another required charge compensation mechanism is related to the interstitial oxygen Oi′′: 2Ba2+→2Eu3++Oi′′, forming the dipole complexes of [2(EuBa3+)•–Oi′′]. An important defect inevitably induced is V4+ defects to maintain the charge balance, which can be clarified from the XPS curves of oxygen and vanadate components. As reported, V4+ in the lattices always acts as holes and electrons traps through altering the photogenerated electron-hole pairs recombination rate.36 As reported, the positions of valence band (VB) and conduction band (CB) can be calculated by eq 3 and 4:37

EVB = X − E e + 0.5Eg

(3)

ECB = X − E e − 0.5Eg

(4)

13



Here X represents the absolute electronegativity, Ee represents the energy of free electrons on the hydrogen scale (~4.5 eV vs NHE) and Eg represents the energy band gap. As calculated from the UV-vis absorption spectrum, the experimental Eg is 2.219 eV for BaNb2V2O11. The corresponding VB and CB positions of BaNb2V2O11 are calculated to be 2.522 and 0.303 eV vs NHE, respectively.

Potential (eV) vs NHE (pH=7) -1.0

0.0

BaNb2V2O11

-.

-0.5

O2/O2

V3d+Nb4d

-0.046eV 0.5 1.0 1.5

.

-

OH /OH 2.0

0.303 eV

2.219 eV

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+1.99eV

2.5

O 2p

2.522 eV

3.0

Figure 7. The suggested band components and positions of BaNb2V2O11.

As the values shown in Figure 7, the results clearly show that BaNb2V2O11 cannot convert photo-generated electrons into reactive superoxide anions. However, VB potential (2.522 eV vs NHE) is much higher than the redox potential OH-/•OH (1.99 eV vs NHE). Actually, oxidation ability of photo-induced holes always plays leading roles in the environmental photocatalysis.38

3.3 Effects of Eu3+ Concentration on Photoluminescence(PL) Spectra As reported, there is a certain relationship between the photoluminescence and the photocatalysis, which can be revealed on the basis of photoluminescence characteristics.38 Usually, the weaker the PL intensity showed, the higher separation rate of photo-induced electrons and 14


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holes obtained, and possibly, the higher photocatalytic activity could be expected.38 However, although BaNb2V2O11 has an efficient UV-vis light absorption, almost no emission can be observed under any excitation wavelengths, even under the excitation of a YAG:Nd laser. A very weak emission can be detected under the excitation of a 266 nm YAG:Nd laser at the lower temperature(< 120 K).

(a)

0.0012

Intensity (a. u.)

0.0010

λex =266 nm 0.0008

Em 1

0.0006

Em2

0.0004

0.0002

0.0000

400

450

500

550

600

650

700

750

Wavelength (nm)

(b) Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


λex=266 nm

0.0000

0.0005

0.0010

0.0015

480 nm 500 nm 580 nm 600 nm

0.0020

Time (s) Figure 8. (a) The Luminescence spectrum of BaNb2V2O11 powders under the excitation of 266 nm pulsed YAG:Nd Laser at 10 K, (b) The decay curves of BaNb2V2O11 powders by monitoring the wavelength 480, 500, 580 and 600 nm under the excitation of 266 nm at 10 K, respectively. 15



As shown in Figure 8a, the obtained powder exhibits a broad yellow emission band under the excitation of a 266 nm YAG:Nd laser at 10 K. As reported, the luminescence in niobate and vanadate based compound originates from the ligand-metal charge transfer (CT) bands (O 2p →V 3d and Nb 4d).9,39 According to the crystal structure of BaNb2V2O11, there are two kinds of luminescent centers, [VO4]3- and [NbO6]7- group. However, the decay curves, shown in Figure 8b, monitored by different emission wavelengths exhibit nearly the same profile, indicating that the emission comes not from different luminescent centers, but from only one kind of luminescent center. As well-known, vanadate group, [VO4]3−, always show yellow emission, while, niobate group, [NbO6]7-, always show blue emission.17,21 So, the yellow emission can be suggested to [VO4]3−. In addition, niobates, with high phonon energies, are very inefficient emitters compared with the vanadates that have much lower phonon energies. In conclusion, the yellow emission can be attributed to [VO4]3−. And the emission band can be decomposed into two peaks: Em1=520 nm and Em2=570 nm, which is similar to the other vanadates. And the full-width at half-maximum (FWHM) of BaNb2V2O11 was calculated to be 110.48 nm in 10 K. 0.0012

Max. Intensity (a. u.)

(a)

10K 50K 30K 60K 75K 90K 100K 120K 150K

0.0010

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0008

0.0006

0

20

40

60

80

100

120

140

160

Temperature (K)

0.0004

0.0002

0.0000 400

450

500

550

600

650

700

Wavelength (nm)

16


750

800

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(b) Ln[Intensity (a. u.)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10K 30K 50K 60K 75K 90K 100K 120K

λem=480 nm

30 µs

300 µs

500 µs

600 µs

18 µs

0.0000

0.0004

0.0008

0.0012

0.0016

Time (s) Figure 9. (a) The luminescence spectra of BaNb2V2O11 powders under the excitation of 266 nm pulsed YAG:Nd Laser with different temperatures, (b) Luminescence decay curves of BaNb2V2O11 powders by monitoring the emission wavelength of 480 nm under the excitation of 266 nm with different temperatures.

Temperature-dependent emission spectra were measured, as shown in Figure 9a. The emission intensities show obvious decrease with the increasing of temperature. Inset in Figure 9a shows the luminescence intensity at different temperatures. The thermal quenching temperature, Tq, defined as the temperature when the emission intensity is 50% compared with its original value, is about 90 K for BaNb2V2O11. Such a lower quenching temperature was attributed to the crystal structure, corner-sharing NbO6 and VO4 groups always lead to shift of the optical absorption to lower energies, smaller ∆ST; lower quenching temperatures, energy migration, exciton delocalization and consequently luminescence quenching.39 On the contrary, such a material can be a promising candidate for photocatalysis. Temperature-dependent decay curves were also measured, as shown in Figure 9b, which have a similar phenomenon with the 17



luminescence spectra.

(a)0.0010

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Tq (K)

10K 30K 50K 60K 75K 90K 100K 120K

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λex=266 nm

0.0000

0.0005

0.0010

0.0015

480 nm 617.5 nm

0.0020

0.0025

Time (s) Figure 10. (a) The luminescence spectra of BaNb2V2O11:0.05Eu3+ under the excitation of 266 nm pulsed YAG:Nd Laser, (b) Decay curves of BaNb2V2O11:0.05Eu3+ by monitoring the different emission wavelengths under the excitation of 266 nm at the temperature of 10 K.

The typical temperature-dependent emission spectra of Eu3+- doped BaNb2V2O11 were also measured, shown in Figure 10a, which is similar to the spectra of pure sample. However, some weak peaks in the region of 580-620 nm can be observed, which can be assigned to the f-f transition of Eu3+.8 An inefficient energy transfer from [VO4] to Eu3+ can be observed, which 18


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allows both the [VO4] and Eu3+ emissions at any temperature, since the intensity of the f-f emission almost keeps nearly the same. The FWHM of 0.05 mol% Eu3+- doped BaNb2V2O11 measured at 10 K was calculated to be 112.45 nm, compared the value with pure BaNb2V2O11 (110.48 nm), these two values are nearly the same indicating the presence of Eu3+ ions in BaNb2V2O11 did not modified the position of valence band and conduction band edges. The thermal quenching temperature, Tq, of BaNb2V2O11 doping with different Eu3+ concentrations were also measured as listed in the insert of Figure 10a. With increasing the doping concentration of Eu3+, the Tq value decreased, and both of these values are smaller than the Tq value of pure sample. This is mainly due to the impurities defects introduced by Eu3+. Figure 10b shows the decay curves of 0.05 mol% Eu3+- doped BaNb2V2O11 measured by monitoring at different wavelengths at 10 K. As Figure 10b shows, the similar decay curves were observed, and the characteristic decay curve of Eu3+ cannot be detected, this may be due to the high intensity of [VO4] compared with Eu3+ and the inefficient energy transfer in the host. Usually, the energy transfer efficiency of [VO4]→Eu3+ depends on the bond angle of Eu-O-V.40 The energy transfer efficiency will be enhanced if the angle of M-O-Eu is closer to 180o.41 As shown in Figure 3, the bond angle of Eu-O-V is about 93.447°, so it is easy to understand the low energy transfer efficiency, which allows both the [VO4] and Eu3+ emissions. Theoretically, the longer luminescence lifetime showed, the smaller recombination probability obtained. The rate for electron-hole recombination τ-1e-h can be calculated through the decay time. As mentioned above, the decay time of the pure BaNb2V2O11 under the excitation of 266 nm at 10 K was calculated to be 600 us, and the decay time of 5 mol% Eu3+ doped BaNb2V2O11 was calculated to be 700 us. So the rates for electron-hole recombination without and 19



with the introduction of Eu3+ were estimated to be 1667 s-1 and 1423 s-1, respectively. Due to the introduction of Eu3+, a long lifetime of the exciton could be expected, giving more chances for electron-hole separations, then further react with dye molecules to oxidize the dye pollutant into non-toxic products.

3.4 Effect of Eu3+ Concentration on Photocatalytic Activity

Intensity (a. u.)

(a)

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Degradation rate

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0.8

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3+

Eu -doped concentration (%) Figure 11. (a) The absorption spectra of MB solution after irradiation for 4 h. (b) Degradation of the MB solution vs. time in typical photocatalytic experiments performed using Eu3+ doped BaNb2V2O11 under visible light illumination.

20


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The lower PL emission spectra for BaNb2V2O11 are responsible for higher photocatalytic activity. The photocatalytic efficiencies of undoped and Eu-doped BaNb2V2O11 were characterized by the photodegradation of MB solution under the visible light irradiation for 4 h followed by spectrophotometric monitoring, as shown in Figure 11a.42 As Figure 11b shows, initially, the greater the percentage of Eu in the BaNb2V2O11, the more MB will be degraded, indicating that the introduction of Eu3+ really play an important role in improving the photocatalytic activity of pure BaNb2V2O11. Such phenomenon is in agreement with the luminescence properties. According to previous studies, the probable mechanism for the enhanced photocatalysis of BaNb2V2O11 is proposed as follows. Firstly, conceivable reason is mainly due to the ideal NbO6 octahedron and VO4 tetrahedron. Both of NbO6 octahedron and VO4 tetrahedra are activated optical centers, however, there is almost no emission can be observed under any excitation wavelengths, even under the excitation of a YAG:Nd laser. Actually, a very weak emission can be detected under the excitation of a 266 nm YAG:Nd laser at the lower temperature. As reported, vanadate and niobate always exhibit excellent photocatalytic properties with an ideal VO4 tetrahedron and NbO6 octahedra, respectively. In the structure of BaNb2V2O11, there is only one kind of VO4 tetrahedron, each of them is isolated and far from each other ( > 4.8781 Å), all VO4 tetrahedrons keep the ideal tetrahedron Td symmetry in BaNb2V2O11. However, for vertex-linked NbO6 octahedra, the bond angle of O-Nb-O in BaNb2V2O11 is measured to be 171.604o, which is close to an ideal octahedron. Moreover, Xu reported that niobates consisting of NbO6 chains or layers favor of forming a narrow CB and a possible delocalization of the charge carriers.43 And the deviation of O-Nb-O angles from the ideal 90o was adverse to delocalization. Ye reported a similar result in the NiM2O6 (M = Nb; Ta) consisting of MO6 (M = Nb, Ta) octahedron. The closer 21



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the bond angle of M-O-M is to the ideal 180o, the more the excitation energy will be delocalized.44 Such a high electron-hole pairs mobility in BaNb2V2O11 will be benefit for photocatalytic activity. Secondly, since UV light is only a small fraction of the solar spectrum, it’s essential to develop new photocatalyst to make full use of solar energy. As the UV-vis absorption spectrum shows, BaNb2V2O11 can absorb visible light effectively, leading to generate more electron-hole pairs to take participate in the photocatalysis. When doped with Eu3+, the band gap will be reduced dramatically. Thirdly, theoretically, upon excitation of illumination, electrons can be created from the VB to the CB of BaNb2V2O11, and holes (h+) will be produced, then the separated electrons and holes will arrive to the powder surface to participate in photocatalytic reaction. However, the separated electrons and holes are easily trapped by the lattice defects on the way to the surface. Even if they succeeded in migrating to the surface, recombination is unavoidable. To suppress the recombination phenomenon and prove the activity of the photocatalyst, Eu3+ ions are introduced to the host of the BaNb2V2O11 to form defect levels, acted as an electron trapped sites to reduce the recombination. The mechanism is shown in Figure 12 and eq 5 - 8:

Figure 12. The photocatalytic and luminescence mechanism induced by Eu/BaNb2V2O11 powders. VB and CB denote valence and conduction bands, respectively. 22


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Eu3+/BaNb2V2O11 + visible light → Eu3+/BaNb2V2O11 (e- + h+)

(5)

H2O → H+ + OH-

(6)

h+ + OH- → OH•

(7)

OH• + MB → photodegraded products

(8)

Here, four different kinds of samples were chosen to measure the photolysis of MB solutions: (I) without photocatalyst, (II) with a well-known commercial photocatalysts (P25 sample), (III) with pure BaNb2V2O11, (IV) with Eu-doped BaNb2V2O11 (Eu=5%). As shown in Figure 13, only a small quantity of MB solution can be degraded without any photocatalyst (9.6% degradation), however, MB concentration decreased to 46.6% and 87 for pure BaNb2V2O11 and Eu3+-doped BaNb2V2O11 in 240 min, respectively. Compared with the well-known commercial photocatalysts (P25 sample), both of the prepared photocatalysts show higher photocatalytic efficiency, shown in Figure 13a.

(a) 1.0 without any catalyst PM25 pure BaNb2V2O11

0.9 0.8

Ddgradation rate

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(b) without any catalyst PM25 pure BaNb2V2O11

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Eu doped BaNb2V2O11

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Irradiation time (min) Figure 13. (a) Degradation of MB without any photocatalyst, and in the presence of pure BaNb2V2O11, 0.05 Eu-doped BaNb2V2O11 at room temperature (initial MB concentration, 10 mg/L; photocatalysts, 0.01 g). (b) ln(C0/C) vs irradiation time for degradation of MB under different conditions.

The decomposition processes can be modeled as the pseudo-first order reaction rate equation with the kinetic expression ln(C0/Ct)=kt, where C0 represents the initial concentration of MB and Ct denotes the concentration at the time t. Plots of ln(C0/Ct) vs time for no photocatalyst, P25 sample, pure BaNb2V2O11 and Eu3+-doped BaNb2V2O11 are shown in Figure 13b. From the linear extrapolations, all these four photocatalysts show good linear relation meeting a pseudo-first-order reaction, and the kinetic constants were calculated to be 0.00042 min-1(no photocatalyst), 0.00124 min-1(P25 sample), 0.00386 min-1(BaNb2V2O11) and 0.00816 min-1(Eu3+-doped BaNb2V2O11), respectively. And Eu3+-doped BaNb2V2O11 shows the highest photocatalytic performance which was nearly 20 times than that with no photocatalyst, indicating Eu3+-doped BaNb2V2O11 could dramatically enhance the photocatalytic performance under visible light irradiation. 24


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4 CONCLUSIONS In conclusion, different ratios of Eu-doped BaNb2V2O11 powders have been prepared through a simple and efficient solid state method. The structural characteristics of the obtained powders have been characterized by XRD, SEM, TEM and XPS, which confirm the successful preparation. Optical properties of the obtained powders indicated a red shift of the band gap energies, from 2.219 eV to 2.137 eV, mainly due to the defect levels. The layered BaNb2V2O11 compound, with an ideal NbO6 and VO4 groups, exhibits high photocatalytic efficiency for degrading MB solutions. The photocatalytic activities can be improved significantly through the introduction of Eu3+ due to the decrease of band gap and the defect levels. However, when the doping concentration was higher than 5%, small amount of impurities appeared. The optimal atomic ratio of Eu3+ was considered to be 5%, with a narrowed band gap energy (2.137 eV), show the highest photocatalytic activity. Under irradiation with visible light for 4 h, the MB solution can be removed nearly 90%, and the photodegradation reactions followed the first-order kinetics. All these results show that such two novel photocatalysts (BaNb2V2O11 and Eu/BaNb2V2O11) could be used as efficient candidates for the degradation of environmental pollutants.

ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013-R1A1A2009154).

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λex=266 nm x=0.07 x=0.05 x=0.03 x=0.01 x=0.001 x=0

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Enhanced Visible Light-Driven Photocatalysis by Eu3+-Doping in

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