Thin Solid Films 519 (2011) 5792–5795
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Visible photocatalytic properties of vanadium doped zinc oxide aerogel nanopowder R. Slama a,b, F. Ghribi a, A. Houas b, C. Barthou c, L. El Mir a,d,⁎ a Laboratoire de Physique des Matériaux et des Nanomatériaux appliquée à l'Environnement, Faculté des Sciences de Gabès, Université de Gabès, Cité Erriadh Manara Zrig, 6072 Gabès, Tunisie b Unité de Recherche Environnement, Catalyse et Analyse des procédés URECAP (UR/99/11-20), Ecole Nationale d'Ingénieurs de Gabès, Université de Gabès, Route de Médenine 6029 Gabès, Tunisie c Institut des NanoSciences de Paris (INSP), UPMC Université Paris 6, CNRS UMR 7588, 140 rue de Lourmel, F-75015 Paris France d College of Sciences, Department of Physics, Al-Imam Muhammad Ibn Saud University, Riyadh 11623, Saudi Arabia
a r t i c l e
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Available online 31 December 2010 Keywords: ZnO:V Nanoparticles Sol-gel Aerogel Visible photo-degradation Water treatment Catalyst Photoluminescence
a b s t r a c t Vanadium-doped zinc oxide nanoparticles have been synthesized by sol-gel method. In our approach the water for hydrolysis used in the synthesis of nanopowder was slowly released followed by a thermal drying in ethyl alcohol at 250 °C. The obtained nanopowder was characterized by various techniques such as particle size analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and photoluminescence (PL). In the as-prepared state, the powder with an average particle size of 25 nm presents a strong luminescence band in the visible range. From photoluminescence excitation (PLE) the energy position of the obtained PL band depends on the excitation wavelength and this PL band can be also observed under visible excitations. This result is very promising for visible photo catalysis applications, which was confirmed by methylene blue photo-degradation using visible lamp as a light source. © 2010 Published by Elsevier B.V.
1. Introduction Nanoparticulate oxides such as TiO2 and ZnO are increasingly being used as photocatalysts for mineralization of toxic organic and inorganic compounds. However, it is generally known that these semiconductors are poor absorbers of photons in the solar light due to their wide band gap (3.2 eV) [1,2]. In the last decade, a large effort has been focused on the research of how to make these semiconductors active under visible light. For this aim various techniques have been employed to make them absorb photons of lower energy as well [3]. One strategy that has been investigated for improving photocatalytic activity is to dope semiconductors with different elements such as transition metals. In the literature, it is mentioned that ZnO was used as a photo catalyst and it can be doped by several elements such as La [4], Mn [5], Co [6], Ag [7], and Ta [8] to improve its photocatalytic activity. Since vanadium is one of the typical transition metals, it has been frequently investigated because it can lead to conspicuous absorption in the visible region [9,10], and it can increase carrier lifetime [11]. A number of studies have revealed that vanadium doped ZnO improve optical and structural properties of ZnO [12–14]. However, many papers have investigated the photocatalytic activities of vanadium doped TiO2 [9,11].
⁎ Corresponding author. E-mail address:
[email protected] (L. El Mir). 0040-6090/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.tsf.2010.12.197
Up to the present, conventional methods, such as ultrasonic irradiation, thermal chemical vapor deposition, ion implantation, electrochemical deposition, and hydrothermal method, have been used to fabricate transition-metal doped ZnO structured materials [15]. Since ZnO is a suitable alternative to TiO2 as it has a similar band gap and it is a dilute semiconductor [1,2], vanadium doped ZnO will be investigated in this paper. The photocatalytic activities of undoped and vanadium doped zinc oxide with varied vanadium doping levels are systematically studied. We have synthesized photocatalysts by a modified sol-gel method under supercritical conditions. The crystalline structure and the optical properties of photocatalysts have been investigated to explain the photodegradation of methylene blue (MB) under visible irradiation.
2. Experimental 2.1. Materials Zinc acetate dehydrate and ammonium metavanadate precursors were purchased respectively from Sharlau and Analyticals Carlo Erba. Methanol and ethanol solvents were purchased from Aldrich. For activity tests, methylene blue (MB) dye was purchased from Fluka. Reference photo catalyst TiO2 (Degussa P25) was obtained from Degussa Chemical Germany. All chemical compounds are used as received without any further purification.
R. Slama et al. / Thin Solid Films 519 (2011) 5792–5795
2.2. Preparation of undoped and V-doped ZnO (ZnO:V) Vanadium doped ZnO catalyst was prepared by modified sol-gel method (according to L. El Mir et al. [12]) with the following procedure: 16 g of zinc acetate dehydrate was dissolved in 112 ml of methanol. After magnetic stirring at room temperature, the appropriate amount of ammonium metavanadate was added, and the solution was placed in an autoclave and dried under supercritical conditions of ethyl alcohol. Then the obtained powder was heated in a furnace under air conditions at 500 °C for 2 h. Pure ZnO was prepared using the same procedure. 2.3. Catalysts characterization The XRD patterns were recorded using the Co Kα radiation (λ = 1.78901 Å) of a bruker D5005 diffractometer. Typical θ–2θ spectra were collected between 2θ = 20° and 70° in 0.02° steps. UV– VIS DRS were recorded on Shimadzu UV3101PC Visible spectrophotometer in the wavelength range of 200–800 nm. BaSO4 was used as a standard for these measurements. The aerogel powders were also characterized using a JEOL JSM-6300 scanning electron microscope (SEM) and a JEM-200CX transmission electron microscope (TEM). For PL measurements, a 450-W Xenon lamp was used as an excitation source. 2.4. Photoreactor and experimental procedure The photocatalytic activities of pure ZnO and vanadium doped ZnO catalysts were tested by using methylene blue (MB) degradation. Experiments were performed in a photocatalytic reactor equipped with a Pyrex cell with a circulating water jacket. The irradiation source was a 250 W halogen lamp with a maximum emission at 640 nm, placed inside the Pyrex cell. Experimental conditions are as follows: initial dye concentration ethyl alcohol is C = 0.03 g/L; catalyst concentration is 0.5 g/L and a magnetic stirring. The concentration of the dye was determined by UV–VIS spectrophotometer (Shimadzu) at its maximum wavelength (664.5 nm). 3. Results and discussion
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Table 1 The grain size of samples with different atomic percentages of vanadium doped zinc oxide. ZnO:Vx%
0
5
10
15
G(nm)
28.58
19.87
19.75
19.91
similar to those of pure ZnO which could be indexed to the hexagonal wurtzite structure ZnO for undoped ZnO and ZnO:V1%. For vanadium percentages lower than 5% no characteristic peaks of V or vanadium oxide are detected, which may demonstrate that V ions have entered into the ZnO lattices. It is found that for more than 5% doping percentages, a secondary additional phase was detected corresponding to Zn3 (VO4) (JCPDS Card 37–1485). The nanoparticle sizes are determined by the use of the Debye–Scherrer formula. G=
0:9λ ; B cosθB
where λ is the X-ray wavelength (λ = 1.78901 Å for Co Kα), θβ is the maximum of Bragg diffraction peak (in rad) and B is the line width at half maximum. The grain sizes of nanoparticles are shown in Table 1. 3.1.2. TEM and SEM SEM and TEM images in Figs. 2 and 3 have shown that ZnO crystallites present very similar prismatic shapes with a narrow particle size distribution assembled in aggregates; the sizes vary between 15 and 30 nm in good agreement with the results of crystallite size obtained by Debye–Scherrer formula. In these aggregates, the presence of vanadium was confirmed by energy dispersive spectroscopy analysis during SEM observations shown in inset in Fig. 3. 3.2. Optical study The optical properties of nanopowders are of most importance because of its wide relationship with photocatalytic activity. 3.2.1. UV–VIS absorption spectra The UV–VIS diffuse absorption spectra of undoped ZnO and vanadium doped ZnO are shown in Fig. 4. It is found that the
3.1. Structural analysis of the catalysts
(101)
3.1.1. XRD The XRD patterns of vanadium doped ZnO nanopowders heated at 500 °C were shown in Fig. 1. It can be seen that all peaks were quite
Zn3(VO4)2
(102)
(110)
(002)
Intensity (arbit.unit)
(100)
ZnO
15% 10% 5% 0%
20
30
40
ð1Þ
50
60
70
2θ θ (degree) Fig. 1. XRD patterns of pure zinc oxide and the V doped ZnO.
Fig. 2. TEM picture of ZnO:V nanopowder with 10 at.% vanadium.
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R. Slama et al. / Thin Solid Films 519 (2011) 5792–5795 Table 2 Band gap energies of samples with different mol percentages of vanadium doped zinc oxide. ZnO–Vx% x =
0
5
10
15
Eg (eV)
3.210
3.183
3.176
3.126
absorption of all ZnO:V are higher than undoped ZnO. Compared with the undoped ZnO, the ZnO:Vx% samples exhibit a red-shift of absorption edge and a significant enhancement of light absorption in the visible light region (Fig. 5). The ZnO:V10% demonstrated the higher absorption in the visible region. The band gap energies are determined using the Kubelka Munck function, and shown in Table 2. Since Eg of vanadium oxide (2.5 eV) is lower than that of ZnO (3.2 eV), it is reasonable to expect that the Eg decrease with vanadium doping.
Fig. 3. SEM image of ZnO:V nanopowder with 10 at.% vanadium and EDX analysis.
Intensity (arbit.unit)
ZnO ZnO:V 5% ZnO:V 10% ZnO:V 15%
200
400
800
600
3.2.2. Photoluminescence study The photoluminescence (PL) property of the ZnO is one of the most interesting and important properties that has been intensively investigated recently [16]. PL emission spectra have been widely used to understand the fate of electron-hole pairs in semiconductor particles [2]. The PL spectrum of ZnO:V10% nanopowder after thermal treatment in a furnace for 2 h at 500 °C in air, with the excitation wavelength of λexc = 371 nm is shown in Fig. 6. The PL spectra consist on a sharp peak centered at 580 nm, showing the strong emission ability. According to Zheng et al. this broad emission band revealed in the visible region is due to the superposition of green and yellow emissions [17]. Wang et al. found that the emission band in the visible spectral range is due to the different intrinsic or extrinsic defects [18]. In the literature, the origin of visible emissions (green, yellow, orange and red) is still highly controversial. Zheng et al. and Wang et al. reported that many hypotheses have been proposed to explain the origin of different visible emissions such as oxygen vacancy, interstitial oxygen, zinc vacancy and interstitial zinc. The green emission is typically associated with oxygen deficiency. It is well known that introducing impurity into wide band gap semiconductor can change his structural and optical properties, so the maximum emission at 385 nm for pure ZnO in the inset of Fig. 6 became dominated by second emission at 580 nm for ZnO:V10%.
Wavelength (nm) Fig. 4. UV–VIS DRS spectra of undoped ZnO and V-doped ZnO.
20
140
Intensity (a.u.)
αhν)2 cm-2 (eV)2
10 ZnO:V
15%
5
ZnO:V
10%
ZnO:V
Intensity (a.u.)
30
15
20
120 100 80 60 40 20 0 -20
400
600
800
λ (nm)
1000
1200
10
5%
ZnO 0 3.0
3.1
3.2
3.3
3.4
0 400
500
600
700
800
900
wavelength (nm)
hν (ev) Fig. 5. Plots of (αhν)2 vs (hν) for 10% vanadium doped ZnO.
Fig. 6. PL spectrum of ZnO:V10% nanoparticles at room temperature. The inset shows the PL spectrum of pure ZnO.
R. Slama et al. / Thin Solid Films 519 (2011) 5792–5795 Table 3 Apparent first order kinetic rate for the degradation of MB.
1,0 ZnO ZnO:V5% ZnO:V10% ZnO:V15%
0,8
MB C/C0
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ZnO:Vx% x =
0
5
10
15
K 10− 3 (min− 1)
18.91
12.01
25.99
12.82
Table 3, the photocatalytic efficiency of these catalysts is ranked in order from the highest to the lowest: ZnO:V10%, ZnO, ZnO:V15% and ZnO:V5%.
0,6
0,4
5. Conclusion 0,2
0,0
0
20
40
60
80
100
120
140
160
180
200
Irradiation time(min) Fig. 7. Photocatalytic degradation of MB using different percentages of vanadium doped zinc oxide under visible irradiation.
4. Photocatalytic degradation of MB The photocatalytic activities of pure ZnO and ZnO:V10% catalysts were examined by the photodegradation of methylene blue under visible light irradiation (Fig. 7). MB was used as an indicator for the photocatalytic activities [7,19] owing to its absorption peaks in the visible range. A global first order kinetic model (Eq. (2)) was used to study the degradation reaction kinetics, where Ct and C0 are respectively the residual dye concentrations at instant t and t = 0, k (listed in Table 3) is the apparent first order constant relative to degradation kinetics. Ct = expð−kt Þ: C0
ð2Þ
It was found that the optimal V percentages for attaining the highest photocatalytic activity is 10%. When the V percentages are below or up to this level, the photocatalytic activity decreases with the decrease or increase of V percentages. It is well known that photocatalytic efficiency largely depends on the optical properties [5]. However, it can be clearly seen that as a whole the ZnO:V10% catalyst has the optimal structural and particular optical characteristics; so the strong visible emission peak and the highly blue shift absorbance of ZnO:V10% can demonstrate their optimal photocatalytic efficiency. This highest photocatalytic efficiency may be attributed to the better performance of the absorption in visible range and to the larger content of oxygen vacancies or defects produced by vanadium. From
In summary, vanadium doped zinc oxide prepared by modified sol-gel process under supercritical conditions is more efficient under visible light than undoped ZnO. A systematic study of the structural, morphological, optical and photocatalytic properties of ZnO:V10% sample was investigated. The enhancement of photoactivity observed mainly for the optimum amount of vanadium equal to 10% is due to the incorporation of vanadium in the structure of ZnO, which is demonstrated by optical and structural characterizations. This material may be considered a good candidate to improve ZnO technological applications. In fact, ZnO:V was used in optical and magnetic devices; with this visible photo-degradation properties, it can be considered as a multi functional material. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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