ZnO ratio-induced photocatalytic behavior of TiO2–ZnO nanocomposite

Superlattices and Microstructures 62 (2013) 192–199

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ZnO ratio-induced photocatalytic behavior of TiO2–ZnO nanocomposite M. Jlassi ⇑, H. Chorfi, M. Saadoun, B. Bessaïs Photovoltaic Laboratory, Research and Technology Center of Energy, Borj-Cedria Science and Technology Park, BP. 95, 2050 Hammam-Lif, Tunisia

a r t i c l e

i n f o

Article history: Received 22 March 2013 Received in revised form 17 July 2013 Accepted 29 July 2013 Available online 6 August 2013 Keywords: Nanocomposites Microstructures Photocatalysis Titanium dioxide Zinc oxide

a b s t r a c t The aim of this study is to examine the photocatalytic activity of TiO2 (P25)–ZnO nanocomposite. The precursors of the TiO2–ZnO nanocomposite were deposited on a low cost ceramic substrate using the simple roll-coating method. We seek to improve the photocatalytic performance and the mechanical adherence of the TiO2 nanoparticles by adding ZnO. The photocatalytic properties of the nanocomposite were tested through the bleaching of polluted water. These properties were optimized by varying the composition of the nanocomposite precursors, deposition conditions and temperature annealing. A systematic study of the nanocomposites was made using ultraviolet–visible spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). These characterizations allowed us to establish a relationship between the photocatalytic performances and the ZnO ratio using an azo-dye (methyl orange). It was found that the kinetic degradation increases with the increasing of the ZnO ratio. The Photodegradation of the dye using the sole ZnO was found to be more efficient than the P25 TiO2 and the TiO2– ZnO nanocomposite itself. The discussions were based on the mobility and lifetime of the charge carriers generated in the ZnO or in TiO2–ZnO nanocomposite. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Semiconductor photocatalysis has attracted considerable attention in recent years due to its great potential to solve environmental problems [1]. Combining some semiconductors to form ⇑ Corresponding author. E-mail address: [email protected] (M. Jlassi). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.07.020

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heterojunctions in photocatalytic systems has become a primary focus of researchers in recent years because of their somewhat unique properties inexistent in the individual nanomaterial arising from the interfacial interaction at the nanoscale [2]. TiO2 [3,4] and ZnO [5,6], as two well-known semiconductors, have been intensively investigated in the fields of photocatalysis, solar cells and gas sensors due to their special electronic and optical properties. Among these, TiO2-based photocatalysts received more attention because of the particularities of high photo-oxidization, photostablility, low cost and non-toxicity. The TiO2 P25 powder, which is widely used in photocatalysis, is considered as a standard in this area [7]. When a suspension of TiO2 is applied in photocatalysis system, the suspended TiO2 has to be yet separated upon completion of each reaction cycle. However, this problem could be avoided by using the TiO2 films applied on different types of substrates. The preparation of TiO2 thin film is one of the major researches in photocatalysis field in order to identify an optimum condition for coating of TiO2 film on various substrates. A number of methods have been employed to fabricate TiO2 films, including dip-coating method [8], sputtering [9], chemical vapor deposition [10], and roll coating process [11]. Among these methods, the roll coating process is one of the most appropriate technologies to prepare thin oxide coating. The interest in application of roll coating method is due to several advantages including; good homogeneity, ease of composition control, low processing temperature, large area coatings and low equipment cost [12]. In previous works, a TiO2 powder-based ink was mixed with a dissolved inert polymer and deposited on the same polymer substrate [13]. Lately, the inactive polymer binder was replaced by photoactive ITO nanoparticles having acceptable photocatalytic activity [14]. This paper, aims to investigate the effects of the preparation conditions on the surface morphology and photocatalytic properties of nanocomposites TiO2/ZnO catalysts. Roll coating procedure was used for the preparation of TiO2/ZnO -coated ceramic. The effect of the ZnO ratio in the TiO2 film on the photocatalytic activity was studied. The prepared TiO2–ZnO nanocomposite was deposited on a low cost ceramic substrate and tested as a photocalytist for the discoloration of an azo-dye. All results were discussed in light of the effects of ZnO content on the structure and morphology of the nanocomposite.

2. Experimental procedure 2.1. Preparation of the samples The preparation of the TiO2–ZnO nanocomposite film consists of mixing inks containing 2 g of TiO2 (Degussa, P25 powder) and different proportions of a ZnO colloidal solution. The preparation of a ZnObased colloidal solution consists of mixing 15 g of zinc acetate dihydrate [Zn(CH3COO)2, 2H2O] (99.99%, Aldrich) with 6 g of high purity zinc chloride [ZnCl2] (99.999%; Sigma–Aldrich) in 100 ml of methanol (CH3OH). After stirring and heating, the mixture turns into a transparent solution. The ZnO-based powder was prepared by dissolving 10 g of zinc chloride in 200 ml of deionized water; the obtained solution was stirred while adding drop by drop an aqueous ammonia solution until we obtain a white precipitate. After filtering the solution and rinsing with ethanol and deionized water several times, and annealing at a temperature of 120 °C for 12 h we finally obtain a ZnO dried powder. After drying, the powder is crushed and filtered with a strainer of 100 lm then of 60 lm. The ZnO ink is prepared by mixing 1 g of the previously synthesized ZnO-based colloidal solution with 1 g of ZnO powder and 2 g of glycerol. The obtained TiO2–ZnO and ZnO inks were deposited on ceramic substrates using the roll-coating method. After heat treatment in air at 550 °C for 15 min we obtain homogeneous and adherent layers. The TiO2 films were prepared a paste composed of Degussa P25 powder, a binder and an organic excipient. The TiO2-based ink was produced by mixing the TiO2 powder with a polymer-based solution melt in acetone. The choice of the binder and the excipient should not modify the photocatalytic properties of P25 powder. The Structural composition of the as-formed TiO2 films depends on the melting point of the binder and the evaporation temperature of the excipient. Hence, The obtained TiO2 ink were deposited on ceramic substrates using the roll-coating method and treated at a temperature of 120 °C to remove organic excipient. The photocatalytic activities of the TiO2–ZnO nanocomposite samples were compared with those of the TiO2 and ZnO in order to try understanding

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Table 1 Experimental details of the samples. Sample

Proportion of TiO2 and ZnO

Compounds forming the inks

S0 S1 S2 S3 S4 S5

TiO2 ink TiO2 (2 g)–ZnO (2 g) TiO2 (2 g)–ZnO (4 g) ink TiO2 (2 g)–ZnO (6 g) ink TiO2 (2 g)–ZnO (8 g) ink ZnO (2 g)–ZnO (1 g) ink

2 g of TiO2 powder + solvent + glycerol S0 + 2 g of ZnO colloidal solution S0 + 4 g of ZnO colloidal solution S0 + 6 g of ZnO colloidal solution S0 + 8 g of ZnO colloidal solution 2 g of ZnO powder + 1 g of ZnO colloidal solution

the origin of these activities. We obtained homogeneous and adherent layers. All details concerning the samples are summarized in Table 1. 2.2. Photocatalysis experiments The photocatalytic activity of the TiO2/ZnO was evaluated by photocatalytic decolorization of MO in aqueous solution under simulated a light source irradiation. To evaluate the photocatalytic activity of the products at identical experimental conditions we designed and implemented a fixed bed reactor irradiated with three 15 W UV–C lamps. The reactor is inclined by an angle of about 40°. The degradation of the well known organic azo-dye methyl orange (MO), a typical pollutant used in the textile industry, was investigated as a probe reaction under visible light irradiation. Another reason for the selection of methyl orange as model compound is due to its relatively high toxicity and complex structure, which makes it difficult to be treated by physical or biological methods. The volume and concentration of the solution containing the methyl orange is 2 l and 10 mg/l, respectively. The irradiation distance between the UV lamps and the sample is 10 cm. The radial flux was measured by a power meter. The average light intensity is about 22 mW cm2. These experimental parameters were kept stable during the photocatalytic tests. The solution to be treated circulates in the reactor container in darkness for 60 min to establish adsorption–desorption equilibrium. During illumination, the sample was taken at different time intervals until total discoloration of the treated solution. The absorbance of the methyl orange was measured at 465 nm using a UV–vis–NIR spectrophotometer. 2.3. Characterization of materials The crystallographic structure was examined by means of X-ray diffraction XRD technique using a Bruker D 8 advance X-ray diffractometer with Cu Ka (k = 1.5418 Å) a radiations for 2h values in the range of 20°–70°. The surface morphology of the prepared films examined by Scanning Electron Microscopy (SEM) (FEG–SEM JEOL 7400F) was operated at 20 kV, with an approximate resolution of 10 mm. High-resolution transmission electron microscopy (HRTEM) photographs were obtained on a JEOL 2010 machine at an accelerating voltage of 200 kV. The measurements of the absorbance A (k) of the films were measured by Lambda 950 UV–vis–NIR spectrophotometer which is equipped with an integrating sphere in the wavelength range 300–600 nm. All the measurements were carried out at room temperature. 3. Results and discussion 3.1. UV–vis spectral analysis Fig. 1 illustrates the absorption spectra of TiO2–ZnO; TiO2 and ZnO layers were deposited by the roll-coating on glass substrates. The absorption edge of all samples (S0 to S4) begins approximately at 385 nm, although the absorption edge of ZnO is narrower than that of TiO2 and TiO2–ZnO. On the other hand, the shape of the absorption edge of the TiO2–ZnO nanocomposite does not appear to be modified by the increase of the ZnO ratio.

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S0

Absorbance (arb.unit)

S1 S2 S3 S4 S5

300

350

400

450

500

550

600

Wavelength (nm) Fig. 1. Absorption spectra of the various TiO2–ZnO nanocomposites.

Table 2 Crystallite size and optical band-gap of the TiO2–ZnO nanocomposite for various ZnO ratio. Sample

S0 S1 S2 S3 S4 S4

Crystallite size (nm)

Optical band gap (eV)

TiO2 structure anatase

TiO2 structure rutile

ZnO

21 22 24 17 20 –

20 29 30 22 25 –

– 23 30 24 25 29

3.3 3.3 3.3 3.3 3.3 3.3

The value of the optical band-gap of the ZnO–TiO2 nanocomposite, evaluated from the absorbance, is about 3.3 eV; it seems to be independent of the ZnO ratio (Table 2). 3.2. XRD analysis Fig. 2 shows XRD patterns of the P25 TiO2 powder following annealing at 550 °C during 15 min. The annealing temperature may change the percentage of anatase and rutile phases in the structure of the P25 TiO2 powder; it is then necessary to check that the percentage of anatase and rutile are not modified. For this purpose we referred to the intensity of the XRD peak line of (1 0 1) anatase and (1 1 0) rutile crystallographic planes. The weight percentage of anatase in the sample can be estimated from the respective integrated XRD peak intensities using the following equation [13]:

X A ð%Þ ¼ 100=½1 þ 1:265ðIR =IA Þ

ð1Þ

where IA is the intensity of the anatase peak at 2H = 25.64°, IR is the intensity of the rutile peak at 2H = 27.74°. We notice that the ratio anatase/rutile in the TiO2 P25 powder is preserved (78% anatase and 22% rutile) after heat treatment in air at 550 °C, whatever the ZnO ratio may be. It is worth noting the presence of ZnO does not affect the crystalline structure of the TiO2 P25 in the TiO2–ZnO nanocomposite, consequently two distinct ZnO and TiO2 phases were pointed out (Fig. 2). Moreover, one may notice that the increase of the ZnO ratio enhances the relative intensity of the ZnO XRD patterns. The crystallite size of the TiO2–ZnO nanocomposite was estimated from the Debye–Scherrer equation:

D ¼ kk=b cos h

ð2Þ

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rutile

anatase

ZnO

Intensity (arb.unit)

S0 S1 S2 S3 S4 S5

20

25

30

35

40

45

50

55

60

65

70

2θ θ (degrees) Fig. 2. XRD patterns of the TiO2–ZnO nanocomposite versus ZnO ratio.

Fig. 3. SEM view of sample S4 corresponding to a maximum ratio of ZnO in the TiO2–ZnO nanocomposite film deposited on glass substrate (10,000).

where k is the X-ray wavelength (kCu = 1.5405 Å), k is the factor shape, D is the average diameter size of the crystallites (in Å), h is the Bragg angle (in radians) and b is the width at half height of the XRD line. The value of the coefficient k is 0.89. Table 2 summarizes the estimated average crystallite size and the optical band-gap of all compounds forming the TiO2–ZnO nanocomposite. 3.3. Microstructure and morphology of the nanocomposite Fig. 3 shows a SEM view of the TiO2–ZnO nanocomposite film deposited onto glass substrate. One may notice (Fig. 3) that the TiO2–ZnO nanocomposite film is not homogenous, but seems to be granular and porous. The TiO2–ZnO nanocomposite film seems to present a large specific surface area offering a great number of catalytic sites. Fig. 4 shows TEM views of TiO2, ZnO and TiO2–ZnO nanocomposites where the TiO2 P25 powder was heat treated at 550 °C during 15 min. One may notice that the TiO2 particle size varies from 10

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Fig. 4. TEM images of the TiO2–ZnO nanocomposite (S1–S4) versus ZnO content (Table 1). Samples S0 and S5 corresponds to the TiO2 and ZnO powders.

Table 3 Composition of element Zn and Ti in TiO2–ZnO nanocomposite. Element

TiK ZnK Total

S1

S2

S3

S4

Weight (%)

Atomic (%)

Weight (%)

Atomic (%)

Weight (%)

Atomic (%)

Weight (%)

Atomic (%)

95.7 4.3 100.00

96.8 3.2 100.00

80.8 19.2 100.00

85.2 14.8 100.00

69.1 30.9 100.00

75.3 24.7 100.00

56.1 43.9 100.00

63.6 36.4 100.00

to 60 nm (Fig. 4 S1). According to estimation from the Scherrer law (Table 2) the average particle size is around 20 nm, knowing that according to Degussa TiO2 has an average particle size of 30 nm. Fig. 4 S1 shows TEM image of the TiO2–ZnO nanocomposite. We notice that the size of the nanoparticles varies from 10 to 50 nm. These results are in agreement with the average crystallite size estimated from XRD, which is the range of 22–29 nm. Fig. 4 S2–S4 show that the nanoparticles tend to agglomerate further and further as the ZnO ratio increases. The nanoparticle agglomeration is rather complex, so that one cannot easily estimate the size of the nanoparticles. Table 3 shows the percentage of Zn and Ti in the TiO2–ZnO nanocomposite, as determined by XTEM quantitative microanalysis. The percentage of Zn in the TiO2–ZnO nanocomposite increases with increasing the ZnO ratio. 3.4. Photocatalytic activity of the TiO2–ZnO nanocomposite The photocatalytic activity of the TiO2–ZnO nanocomposite was compared with that of TiO2 and ZnO (Fig. 5) in the same experimental conditions. The optical absorption spectra of the MO dye solution (without any catalyst) under UV irradiation shows a negligible photochemical discoloration, mainly due to the fact that the UV lamp weakly emits in the 400–550 nm range, which coincides with the main spectral absorption band of the dye. The UV photocatalytic degradation activities (discoloration) of the MO dye (C0 = 10 mg L1) of ZnO and TiO2 are compared to that of the TiO2–ZnO nanocomposite, in the same experimental conditions. The decolorization percentage of MO (g%) were calculated according to the following equation:

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decolorization percentage (%)

100

80

60

S0 S1

40

S2 S3 S4

20

S5 0 -60 -30

0

30

60

90 120 150 180 210 240 270 300 330 360 390

Time of UV irradiation (min) Fig. 5. Decolorization of MO as a function of reaction time for different samples S0 (P25 TiO2), S1 (TiO2 (2 g)–ZnO (2 g)), S2 (TiO2 (2 g)–ZnO (4 g)), S3 (TiO2 (2 g)–ZnO (6 g)), S4 (TiO2 (2 g)–ZnO (8 g)) and S5 (ZnO).

g% ¼ ½ðC 0  CÞ  100=C 0

ð3Þ

Studying the dependence of decolorization efficiency on the initial dye concentration is important from an application point of view [15]. Fig. 5 shows the effect of the initial MO concentration on the photocatalytic decolorization in the presence of 10 mg L1 TiO2/ZnO. The decolorization percentage decreased markedly with the increase in the initial dye concentration, especially when the initial dye concentration varied from 5 to 10 mg L1. An explanation for this change was that as the initial concentration of methyl orange increased, the path length of the photons entering the solution decreased. The reversal occurred at lower concentration, and thereby increased the number of photons absorbed by the catalyst [15,16]. The same effect was observed by Wang et al. during the photocatalytic decolorization of commercial dyes using zinc oxide power as photocatalyst [17]. One may notice that the photocatalytic activity of the TiO2–ZnO nanocomposite improves with the increasing of the ZnO ratio. This behavior could be due to the formation of a junction between TiO2 and ZnO that leads to further separation of photogenerated carriers. On the other hand, the minimum energy of the conduction bands of ZnO and TiO2 are approximately at the same level, while their optical band-gap are almost identical (3.2 eV) [13,18]. These data let us presume that, compared to the sole TiO2, the photocatalytic activity enhancement of the TiO2–ZnO nanocomposite (Fig. 5) is due to the increase of the ZnO ratio. Furthermore, the sole ZnO presents better photocatalytic activity than the nanocomposite and TiO2 (Fig. 5). This would suppose that the carrier lifetime of the electrons in the conduction band of ZnO is much higher than in TiO2 [19], enabling more electrons and holes to be in contact with the organic species adsorbed on the surface before they recombine.

4. Conclusions In this work we succeeded preparing adequate inks for the deposition of TiO2–ZnO nanocomposite on low cost ceramic substrates. TEM observations of ZnO and TiO2–ZnO based powders show that the addition of ZnO has a significant impact on the shapes of the nanocomposite, which in turn has a considerable effect on the improvement of its photocatalytic activity during discoloration of an azo-dye pollutant. The photocatalytic activity improves as the ZnO ratio increases, probably due to an electronic interaction between TiO2 and ZnO or eventually to the high (but not stable) photocatalytic activity of ZnO as compared to TiO2.

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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

Y.L. Liao, W.X. Que, J. Alloys Compd. 505 (2010) 243–248. H. Kim, J. Kim, W. Kim, W. Choi, J. Phys. Chem. C 115 (2011) 9797–9805. Guidong. Yang, Zifeng. Yan, Tiancun Xiao, Appl. Surf. Sci. 258 (2012) 8704–8712. G. Yang, Z. Jiang, H. Shi, M.O. Jones, T. Xiao, P.P. Edwards, Z. Yan, Appl. Catal. B: Environ. 96 (2010) 458–465. M. Batzill, U. Diebold, Prog. Surf. Sci. 79 (2005) 47–154. Q. Zhang, C.S. Dandeneau, X. Zhou, G. Cao, Adv. Mater. 21 (2009) 4087–4108. Fujishima, K. Honda, Nature 238 (1972) 37. Puangrat Kajitvichyanukul, Jirapat Ananpattarachai, Siriwan Pongpom, Sci. Technol. Adv. Mater. 6 (2005) 352–358. Z. Ding, X. Hu, P.L. Yue, G.Q. Lu, P.F. Greenfield, Catal. Today 68 (2001) 173–182. G. Pecchi, P. Reyes, P. Sanhueza, J. Villasenor, Chemosphere 43 (2001) 141–146. R.S. Sonawane, S.G. Hegde, M.K. Dongare, Mater. Chem. Phys. 77 (2002) 744–750. Cheng-Yang Tsai, Jen-Shou Liu, Pei-Li Chen, Chao-Sung Lin, Surf. Coat. Technol. 205 (2011) 5124–5129. M. Saadoun, H. Chorfi, L. Bousselmi, B. Bessaïs, Phys. Status Solidi C 4 (2007) 2029. M.H. Habibi, N. Talebian, Dyes Pigm. 73 (2007) 186. Huayue Zhu, R u Jiang, Yongqian Fu, Yujiang Guan, Jun Yao, Ling Xiao, Guangming Zeng, Desalination 286 (2012) 41–48. S.K. Kansal, M. Singh, D. Sud, J. Hazard. Mater. 141 (2007) 581–590. H.H. Wang, C.S. Xie, W. Zhang, S.Z. Cai, Z.H. Yang, Y.H. Gui, J. Hazard. Mater. 141 (2007) 645–652. F. Lu, W.P. Cai, Y.G. Zhang, Adv. Funct. Mater. 18 (2008) 1047–1050. P.V.V. Jayaweera, A.G.U. Perera, K. Tennakone, Inorg. Chim. Acta (2008) 361.