Calorimetric, optical and catalytic activity studies of europium chloride–polyvinyl alcohol composite system

Journal of Physics and Chemistry of Solids 72 (2011) 1057–1065

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Calorimetric, optical and catalytic activity studies of europium chloride–polyvinyl alcohol composite system Khaled H. Mahmoud a,n, Zeinhom M. El-Bahy b,1, Ahmed I. Hanafy b,1 a b

Physics Department, Faculty of Science, Taif University, Taif, Saudi Arabia Chemistry Department, Faculty of Science, Taif University, Taif, Saudi Arabia

a r t i c l e i n f o

abstract

Article history: Received 27 February 2011 Received in revised form 5 May 2011 Accepted 3 June 2011 Available online 12 June 2011

Polyvinyl alcohol (PVA) films doped with europium chloride (EuCl3) have been prepared by casting from their aqueous solutions. The phase transitions and thermal decomposition behavior of the prepared samples were investigated by thermal analysis and the interactions between the host PVA and Eu3 þ were examined by FTIR spectroscopy. The optical absorption was recorded at room temperature in the range of 190–1000 nm. From the absorption edge studies, the values of the Urbach energy (Ee) were found to be 0.56 eV in case of the pure polymer; however, its value increased to be in the range of 1.21–1.75 eV. These energy values indicate that the model based on electronic transitions between localized states is not preferable and transitions are made between band tails. Optical parameters such as refractive index and complex dielectric constant have been determined. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple-DiDomenico model. Color properties of the prepared samples are discussed in the framework of CIE L*u*v* color space. The prepared samples have been used as catalysts in the photocatalytic degradation of p-nitrophenol (PNP) in aqueous solution under UV light irradiation using H2O2 as oxidizing agent. The catalytic activity of the Eu-polymer towards the photodegradation of PNP greatly increased after doping with Eu3 þ ions. The highest catalytic activity was noticed at the optimum pH value of 5.5. & 2011 Elsevier Ltd. All rights reserved.

Keywords: D. Optical properties

1. Introduction Combination of polymers with metal salts gives complexes, which are useful for development of the advanced high energy electrochemical devices such as batteries, fuel cells, electrochemical display devices and photo electrochemical cells [1]. Especially there has been much interest in rare-earth doped polymer because of its applications in optical communication systems, amplifiers, lasers and optical sensors especially in local networks and data communications [2,3]. Polymers are promising host candidates for those applications because of their excellent properties such as high transparency, low cost and easy fabrication. Polyvinyl alcohol (PVA) is recognized as one of the very few water soluble vinyl polymers. Besides, it is semicrystalline, fully biodegradable, non toxic and biocompatible properties, it has broad spectrum of applications [4]. Moreover, it contains a carbon backbone with hydroxyl groups, which can be used as a source of

n Corresponding author. Permanent address: Physics Department, Cairo University, Faculty of Science, Cairo, Egypt. E-mail address: [email protected] (K.H. Mahmoud). 1 Permanent address: Chemistry Department, Al-Azhar University, Nasr City 11884, Cairo, Egypt.

0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.06.007

hydrogen bonding and assist in the formation of polymer composites [5]. The optical uses of PVA are concerned with the retardation, polarization and filtration of light as well as with photography and related imaging fields. Eu3 þ ions in particular doped materials have large attention because of their potential use in optical devices, hole-burning high density memory and projection color television [6]. Besides, Eu3 þ ions are one of the well known active elements for red emitting phosphors. Doping with Eu3 þ ions is important for studying the structure and bonding characteristics of different materials due to their unsplit ground state and relatively simple system of electronic levels [7]. Photocatalysis is a promising technique to deal with contaminated waters. Water purification by photocatalysis has been attracting an immense research interest in recent years. Nitroaromatic compounds are considered as harmful water pollutants, which have environmental concerns. They are widely used as herbicides, explosives, solvents and industrials chemicals or precursors [8]. As such, it is necessary to eliminate these pollutants from industrial wastewater. Various technologies are used for that purpose, i.e. ultrasonic degradation [9–11], photocatalytic degradation [12–14], photo-Fenton degradation [15], photoelectrocatalytic degradation [16], advanced oxidation process with UV/H2O2 (AOP) [17,18], catalytic oxidation [19] and microwave enhanced advanced oxidation processes [20–23]. Clearly, photocatalysis is

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one of the most promising technologies that can be used for the degradation of p-nitrophenol (PNP). Therefore, the photocatalytic degradation of (PNP) was studied as a probe reaction to evaluate the catalytic activity of the prepared samples towards the photodegradation of the harmful water pollutants. To the best of our knowledge, few works are known in literature about rare earth metal salts-containing polymer. The objective of the present work is to prepare a composite system between EuCl3 and polymer (PVA) and study the effect of Eu3 þ ion on thermal and spectroscopic properties of composites to probe their practical uses in the industrial applications. Another imperative aim of the present work is to test the photocatalytic activity of the prepared solids toward the degradation of p-nitrophenol under UV light irradiation using H2O2 as oxidizing agent.

2. Experimental PVA with approximate molecular weight of 17,000 was supplied by BDH chemical Ltd. Poole England. Anhydrous EuCl3 (F.W. 258.31) was supplied by BDH Chemicals Company; Japan. 160 Po P1 p2 P3 P4 P5

140 120 DTA (a.u)

100 80 60 40 20 0 -20 -40

0

100

200

300

400

500

600

T (°C) Fig. 1. DTA curves of (100  X) PVA-XEuCl3 composite system.

700

The solution method was used to obtain film samples. Weighed amounts of natural granules PVA were dissolved in a mixture of distilled water and ethanol with the ratio of 4:1 using a magnetic stirrer at 50 1C on water bath for 4 h. The appropriate weighed amounts of EuCl3 were dissolved in distilled water at room temperature. Solutions of EuCl3 and PVA were mixed together with to give 1, 2, 3, 4 and 5 wt% of Eu-doped polyvinyle alcohol using magnetic stirrer at 60–80 1C. Pure polyvinyl alcohol and Eu-doped polyvinyle alcohol will be referred as P0 and Px, respectively, where x is the Eu wt%. Films of suitable thickness (  30 mm) were casted onto stainless steel Petri dishes, and then dried in an open air at room temperature (about 25 1C) for about 8 days until solvent was completely evaporated. TG/DTG/DTA measurements were carried out using Schimadzu DTG-60H thermal analyzer. About 3 mg of sample was placed in platinum crucible in presence of a-Al2O3 as reference. Nitrogen was used as a carrier gas with flow rate of 20 ml/min and the ramp rate was 10 1C/min. The temperature was elevated from room temperature to 600 1C. The IR spectra were measured using PYE spectrophotometer over the range of 400–4000 cm  1. The absorption measurements of the samples were performed using PerkinElmer lambda 4b spectrophotometer over the range of 190–1000 nm. The tristimulus transmittance values (X,Y,Z) were calculated using the transmittance data obtained in the visible range according to CIE L*u*v* system. Also, the CIE three dimensional (L*, U*, V*) color constants, whiteness (W), yellowness (Y), chroma (C*), hue and color difference (DE) were performed. The evaluation of the photocatalytic activity was carried out in a cylindrical Pyrex glass reactor. Irradiation experiments were performed using an 8 W medium pressure Hg lamp. Unless otherwise stated, the reaction was carried out at room temperature under the conditions of 0.06 g of the Eu-containing polymer solid catalyst in 100 ml solution of 5  10  3 M of PNP, 61.6 mmol/l of H2O2 and the pH of the solution was initially adjusted at 5.5. Generally, (0.1 M) HCl and (0.1 M) NaOH were used to adjust the pH value in the beginning of all experiments. The concentration of PNP was measured by UV spectrophotometer JASCO V-570. Prior to irradiation, after adjusting pH, the suspensions were magnetically stirred in the dark for 30 min to establish the adsorption/desorption equilibrium of PNP. The aqueous suspensions containing

Table 1 Transition temperatures and enthalpy associating each phase transition for (100–X) PVA-XEuCl3 composite system. Polymer

Glass phase transition

Melting phase transition (Tm)

New phase transitions

Tg (1C)

Tm (1C)

Tp1 (1C)n

DH (J/g)

Tp2 (1C)nn

DH (J/g)

DH (J/g)

DH (J/g)

P0

94

3.93

223

63.75

361 417 437 496

463 128 462 1590

145

P1

116

403.09

212

74.65

409 428 453 510

271 1040 389 2350

–

P2

107

260.28

212

60.62

424 503

720 2290

261

81.31

P3

95

99.67

204

102.57

493

3190

253 386

78.71 314.61

P4

117

2.41

206

68.23

492

4610

246

146.82

P5

105

3

208

79.72

280 322

93 49

250 317

200.19 40.51

n nn

¼ Exothermic peak. ¼ Endothermic peak.

5.61

–

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PNP were irradiated with constant aerating. At given irradiation time intervals, samples were taken from the suspension, and then passed through a 0.45 mm Millipore filter to remove the particles. The concentration of PNP in the filtrate was measured by applying the following equation: % degradation efficiency ¼ ½ðC0 2CÞ=C0   100 where C0 is the initial PNP content and C is the retained PNP in solution. The PNP concentration was estimated by measuring the absorbance at 400 nm.

3. Results and discussion 3.1. Thermal analysis 3.1.1. Differential thermal analysis (DTA) DTA analysis is sensitive enough to record thermal events such as glass and melting phase transitions. Fig. 1 shows DTA thermograms of (100 X) PVA–XEuCl3 composite system from room temperature up to 600 1C .The different phase transitions and associated enthalpies are given in Table 1. For pure PVA (sample P0) there are two distinct endothermic phase transitions at 94 and 223 1C with an enthalpy 3.93 and 63.75 J/g, respectively. These transitions are corresponding to glass and melting phase transitions, respectively, and are in good agreement with previous reports [24,25]. The endothermic peak observed at 145 1C with enthalpy 5.61 J/g may be assigned to the a-relaxation associated with the crystalline regions. The exothermic peaks occurring at temperatures of 361, 417, 437 and 496 1C with associated enthalpies of 463.61, 128.18, 462.02 and 1590 J/g, respectively, are attributed to different degradation processes in PVA [26]. Under addition of EuCl3 to the pure polymer there is an irregular change of glass transition temperature and its values are higher than those for pure polymer P0. This suggests that the segmental mobility of amorphous PVA becomes more rigid segments [27]. Also there is a decrease in melting temperatures after addition of EuCl3. The reduction in melting temperature may arise due to reduction in molecular weight of PVA and/or inner microstructure transition, breaking of inter and intra molecular hydrogen bonding [28]. The changes in the phase transitions temperatures and

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associated enthalpies give an indication of the structural modification of PVA with EuCl3. 3.1.2. Thermogravimetry (TG) and differential thermogravimetry (DTG) TG analysis is commonly used to measure absorbed moisture content, residual solvent levels, degradation temperatures and the amount of non-combustible (inorganic) filler in polymer or composite materials compositions. Fig. 2(a) and (b) shows the TG and DTG thermograms of composite system heated in nitrogen atmosphere. According to these findings, the thermal degradation of PVA film and its composites show four weight loss stages, as shown in Fig. 2. The first weight loss takes place between 49–145 1C with weight loss in the range of 4.96–15.80% due to the loss of adsorbed moisture and/or evaporation of the trapped water; the second stage at 153–389 1C with weight loss in the range of 21.95–56.26% involves the elimination reactions of H2O and residual acetate groups, which are most likely due to partially hydrolyzed form of PVA. It is noticed that the thermal degradation of sample P5 finished rapidly at 358 1C and this is expected because of the small weight of polymer. For other samples namely P0–P4 the third region occurs between 301–465 1C with weight loss of 18.8–38.1% and the final degradation step occurs between 451–573 1C and weight loss of 15.3–25.4%, which is more complex and includes the further degradation of polyene residues to yield the hydrocarbons and metallic residues [29]. It is noted that the addition of EuCl3 has no strong effect on the thermal stability of PVA. Generally speaking; the thermal stability of composites is almost the same for all samples. 3.2. Spectroscopic studies 3.2.1. Infrared spectroscopy analysis A substantial part of the knowledge concerning the mode of binding in metallopolymer can be deduced from the infrared spectra. Fig. 3 shows a comparison between the IR spectral data of the pure polymer PVA (P0) and that of Eu3 þ -containing polymer (P1–P5). The spectrum of the pure polymer is well consistent with that previously reported in previous literatures [30–32]. With the careful inspection of the spectra of P0 and Eu3 þ containing polymer, one can notice several changes with increase in the Eu3 þ content. An increase in the intensity and a slight red-shift of

Fig. 2. (a) TG. (b) DTG curves of (100 X) PVA-XEuCl3 composite system.

K.H. Mahmoud et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1057–1065

the band characteristic to carbonyl group at 1711 cm  1 [31] were noticed. A blue-shift of the peak at 1093 cm  1 characteristic to nC-O [33] was observed. The n(OH) band centered at 3316 cm  1 becomes larger and splits into two terminal peaks. The peak at 1330 cm  1 characteristic to d(OH) (in plane) and peak at 568 cm  1 due to d(OH) (out of plane) [33] underwent a slight shift of the center of the peak to lower wavenumber and it was accompanied by an increase of the peak intensity. All the forgoing IR spectroscopy data revealed that the Eu3 þ ion binds to the polymer chain through a hydroxyl group together with the residual carbonyl oxygen atom and elucidate the complexation between Eu3 þ and polymer. This is supported by the thermal properties data in the previous section. The general increase in the peaks intensity may be due to the increase in water content and OH group associated with the Eu3 þ ion in the Eu3 þ -polymer complex. For such materials, the mass of the ion concerned in trivalent metal salts plays an important role in determining the features of absorption spectra [34,35].

3.2.2. Optical absorption Fig. 3 shows the optical absorption spectrum of films under study from 190–1000 nm. The measurements of the optical absorption and the absorption edge are important, especially in connection with the theory of electronic structure of amorphous materials. The non-sharp edges in the optical absorption spectra (Fig. 4) of the present polymer composite system are clear indication of the amorphous nature of samples [36]. Most commercial PVA have strong absorption in the range of 190–400 nm due to the presence of unhydrolyzed acetate group in PVA backbone [37]. The spectrum of P0 sample contains two absorption bands at 202 and 276 nm together with a shoulder at 360 nm. These absorption bands identify the carbonyl groups of the –[CH¼CH]nC¼O type (n¼1, 2 or 3), which arises from the presence of vinyl acetate monomer during polymerization [38]. The absorption band at

1.2 P0 P1 P2 P3 P4 P5

1 Absorbance (a.u.)

1060

0.8 0.6 0.4 0.2 0 190

290

390

490

590

690

790

890

990

Wavelength (nm) Fig. 4. Spectral absorbance of (100–X) PVA-XEuCl3 composite system.

276 nm is assigned to the carbonyl groups associated with ethylene unsaturation of the –[CH¼CH]2C¼O type, which is indicative of the presence of conjugated double bonds on PVA backbone [39]. On the other hand the peak at 360 nm is assigned to the –[CH¼ CH]3C¼ O– carbonyl group. Upon addition of EuCl3 the bands at 202 and 276 nm disappear and another absorption band arises at 330 nm for P3, P4 and P5 samples. The absorption band at 202 nm and those at 276 and 360 nm are assigned to n–p* and p–p* transition, respectively. The change in absorption spectrum of pure polymer (P0) under addition of EuCl3 is an indication of chelate formation of Eu3 þ coordinated with the hydroxyl group of PVA [40]. This is also supported by the thermal and the infrared spectroscopy data. 3.2.3. Optical parameters The absorption coefficient, a (n) below and near the edge of each curve was determined, using the relation [41] sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 ð1RÞ2 ð1RÞ4 1 ln ð1Þ aðnÞ ¼ þ þ 2 2 d R 2TR 4T 2 R4 where d is the thickness of the film sample; R is the reflectance; T is the transmittance of sample for incident photon. The absorption edge is observed in the UV region (Fig. 4) and can be divided into two regions, depending upon the value of the absorption coefficient (a) for many amorphous materials. The first region, usually known as Urbach tail [42], which is considered with a o104 cm  1 and depends exponentially on the photon energy (hn) as

aðVÞ ¼ cehv=Ee

ð2Þ

where ao is constant; Ee is the width of band tail. In the second region, where 104 r a r106 cm  1, the following relation is obeyed [43]:

aðnÞ ¼ BðhnEopt Þn

Fig. 3. IR spectra of (100–X) PVA-XEuCl3 composite system.

ð3Þ

where B is constant called band tailing parameter; Eopt is the optical band gap energy; n is the index, which takes different values depending on the mechanism of inter-band transitions n¼2, 3, 1/2 and 1/3 corresponding to indirect allowed, indirect forbidden, direct allowed and direct forbidden transitions, respectively. In amorphous materials, at higher levels of (a Z104 cm  1) the absorption edge is usually interpreted in terms of indirect transitions across the optical gap [44]. The width of the band tails (Urbach energy Ee) associated with valence band and conduction bands was believed to be originated from the same physical origin. This origin is attributed to phonon-assisted indirect electronic

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3.2.4. Wemple and DiDomenco model parameters The refractive index is one of the fundamental properties in material science. It is closely related to the electronic polarizability of ions and the local field inside the material. Evaluation of the refractive indices of optical materials is important for applications in integrated optics devices such as switches, filters, modulators, etc. Thus, the refractive index of a material is a key parameter for device design [21]. The values of refractive index n and extinction coefficient k have been calculated using the theory of reflectivity of light. According to this theory, the reflectance of light of a material can be expressed as [47] R¼

½ðn1Þ2 þ k2 

ð4Þ

½ðn þ1Þ2 þ k2 

where k¼ al/4p is the extinction coefficient of materialand l is the wavelength of incident photon. The values of n at different wavelengths are plotted in Fig. 6. It is clear that the refractive index decreases with increasing wavelength of incident photon until a constant value is reached at longer wavelengths (n0) (see Table 2).

ln (α)

7

P0

P1

P2

P3

P4

P5

6

5

2 1.9 1.8 1.7 1.6 n

transitions between localized states, where the density of these states is exponentially dependent on energy [45]. The Urbach energy arises from the random potential fluctuations in the material into the band gap [46]. The values of Urbach energy (Ee) were calculated by determining the reciprocal of the slopes of the linear regions of the curves as shown in Fig. 5 and represented in Table 2. It is noted that the sample P0 has the smallest Urbach energy (Ee ¼ 0.56 eV) and sample P1 has the largest one (Ee ¼1.75 eV). This can be explained on the basis that Urbach energy is generally used to characterize the degree of disorder in amorphous and crystalline solids. Materials with larger values of Ee would have a great tendency to convert weak bonds into defects. Therefore, the value of Urbach energy is considered as a measure of defects concentration.

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1.5 1.4 P1 P2 P3 P4 P5 P0

1.3 1.2 1.1 1 400

450

500

550

600

650

700

750

800

Wavelength (nm) Fig. 6. Refractive index (n) against wavelength (l) of (100–X) PVA-XEuCl3 composite system.

It is clear that P0 has the largest refractive index and P1 has the lowest value, this is attributed to disorder occurred in P1 sample as a result of addition of EuCl3 salt to pure polymer. Neglecting the lattice contribution, for a wavelength longer than the phonon resonance, Wemple and DiDomenco [48,49] have examined the refractive index data below the inter-band absorption edge. They found that the normal dispersion of the optical dielectric constant in materials and the energy dependence of refractive index satisfy a Sellmeier relation of the form n2 ðEÞ ¼ 1þ

Eosc Ed E2osc E2

ð5Þ

where Eosc is the single oscillator energy (average oscillator energy for electrons); E is the photon energy in eV; and Ed is the dispersion energy parameter of the material. The parameter Eosc is directly related to the optical band gap. The parameter Ed is a measure of the inter-band optical transitions. Experimental verification of Eq. (5) can be obtained by plotting (n2–1)  1 versus E2 as illustrated in Fig. 7(a) (for sample P4 only for the sake of brevity), which yields a straight line for normal behavior and allows the determination of oscillator parameters. These parameters are listed in Table 3. The obtained curves in Fig. 6(a) and (b) show positive deviation from linearity. A positive deviation from linearity at longer wavelengths is usually observed due to the negative contribution of lattice vibrations on the refractive index. The refractive index at infinite wavelength (nN), average oscillator wavelength (lo) and oscillator strength So can be calculated through the relation [50] 2

4 4.8

5

5.2

5.4

5.6

5.8

6

n21 1 l ¼ 1 o2 n2 1 l

ð6Þ

Rearranging Eq. (6) gives

hν (eV) Fig. 5. Urbach plots of (100–X) PVA-XEuCl3 composite system.

Table 2 Values of energy tail (Ee) and refractive index (n0) for (100–X) PVA-XEuCl3 composite system. Polymer

Ee (eV)

n0

P0 P1 P2 P3 P4 P5

0.56 1.75 1.43 1.21 1.24 1.58

1.7 1.38 1.39 1.44 1.42 1.64

2

n2 1 ¼

S2o lo

ð7Þ

2 2 ð1lo Þ=l 2

where So ¼ ðn21 1Þ=l0 . The values of nN and So are derived from linear plot of (n2–1)  1 versus 1/l2 as shown in Fig. 7(b) and the date are presented in Table 3. It is observed that these parameters change irregularly with composition. 3.2.5. Determination of complex dielectric constant The real and imaginary parts of complex dielectric constant can be calculated from the following relations [51]:

e0 ðlÞ ¼ n2 ðlÞk2 ðlÞ

ð8Þ

K.H. Mahmoud et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1057–1065

1.2

1.2

1

1

0.8

0.8 1/(n2-1)

11(n2-1)

1062

0.6 0.4

0.6 0.4

0.2

0.2

0 0

10

20 (hν)

0 0.0E+00

30

2

1.0E-05

2.0E-05

3.0E-05

λ-2 (nm)

30

1.2E-02

25

1.0E-02

20

8.0E-03

dielectric loss

15

6.0E-03

10

4.0E-03

5

2.0E-03

0

dielectric loss

dielectric constant

dielectric constant

0.0E+00 0

1

2

3

4

5

6

7

hν (eV) 2

Fig. 7. Plots of the refractive index factor (n –1)

1

2

versus (a) (hn) , (b) (l)

2

and (c) dielectric constant, dielectric loss versus photon energy hn for P4 sample.

20

Table 3 Single oscillator model parameters for (100–X) PVA-XEuCl3 composite system.

P0 P1 P2 P3 P4 P5

Ed (eV) 13.31 7.07 6.49 7.7 6.17 8.82

Eosc (eV) 6.42 7.85 6.87 7.92 6.2 5.15

nN 1.75 1.38 1.39 1.43 1.41 1.64

eN 3.06 1.89 1.93 2.04 1.98 2.68

l0 (nm) 193.42 158.1 182.3 170.0 200 241

So (nm  2)

16 14

5

5.55  10 3.61  10  5 2.84  10  5 3.65  10  5 2.48  10  5 2.95  10  5

12 Yt

Polymer number

P0 P1 P2 P3 P4 P5

18

10 8 6

e00 ðlÞ ¼ 2nðlÞkðlÞ

ð9Þ

where e0 and e00 are calculated for polymer composite system at different incident photon energies and for sake of brevity we represented one figure only for P4 as an example. From Fig. 7(c), it is noticed that as incident photon energy increases e0 and e00 also increase. The dielectric constant at infinite wavelengths ðe01 Þ is derived from Eq. (7) where e01 equals the value of n21 . The obtained values are listed in Table 3. It is clear that e01 changes irregularly with composition and the samples P0 and P1 have the highest and the lowest static dielectric constant values, respectively.

3.2.6. Color measurements Fig. 8 illustrates the variation of the tristimulus transmittance (Yt) with wavelength in the range of 380–760 nm for composite samples (P0–P5). It is noticed that the behavior of Yt for all

4 2 0 100

200

300

500 400 Wavelength (nm)

600

700

800

Fig. 8. Tristimulus transmittance of (100–X) PVA-XEuCl3 composite system.

samples is similar since they have the same peak position at about 450 nm. Also it is observed that Yt(max) varies with composition. The smallest and the highest Yt(max) values were observed for the samples P0 and P1, respectively, and this was attributed to the transmission values. Fig. 8 shows the position of all samples on the chromaticity diagram and their distance to the white point. We see that all samples lie near to white point (C). The locations of the samples

K.H. Mahmoud et al. / Journal of Physics and Chemistry of Solids 72 (2011) 1057–1065

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can be considered as a reflection of the electron disorder in the solid samples. The high Ee value of Eu-doped polymer leads to the increase of the electron disorder and defects of the samples and hence increases the conductivity; therefore, it increases the catalytic activity. The general increase of the catalytic activity of Eu-polymer samples can also be attributed to the enhanced absorption in the UV region, which may lead to the suppressed recombination between photogenerated electrons and holes in composite samples (e  /h þ recombination) via trapping of the photogenerated electrons in the catalyst by Eu dopant [53]. In addition, Eu3 þ ions may create acidic centers, which are considered as active centers for the degradation process. The reaction mixture was filtered and the solution was tested for Eu3 þ ions using Varian atomic absorption spectrometer (SHIMADZU AA-6400F). The Eu3 þ ions could not be detected in the filtrate, which may be due to the complete absence of Eu3 þ ions or its concentration is very low and less than the detection limit of the Atomic absorption instrument and hence the leaching effect can be neglected.

are very near to each other, so they overlap indicating a very small color gradient Fig. 9. Table 4 represents the color parameters L*, U*, V*, hue, W and Ye [52] and color difference data, DL*, DU*, DV*, DC* and DE between all samples and sample P0. It is observed that the color parameters change noticeably with composition. Moreover, the color difference data indicate that the sample P1 is lighter and greener than the other samples. In addition, it is hue, bluer and more saturated than the rest of the samples. The sample P1 also has the highest DE value among all the employed samples. 3.3. Catalytic activity of Eu-polymer samples 3.3.1. Effect of Eu3 þ loading The prepared samples have been used as catalysts in the photocatalytic degradation of p-nitrophenol. The same abovementioned experimental conditions were used. The photocatalytic degradation % was plotted against the Eu-polymer samples, which contain different concentrations of Eu3 þ (P0–P5) and the plots are shown in Fig. 10. The data shows an increase in the photodegradation % of PNP with increase in the loading amount of Eu3 þ within 90 min of irradiation time. The highest photocatalytic activity was noticed for the sample containing 5% Eu3 þ (P5), which showed 95% of PNP degradation after 90 min. These results indicated that most of PNP molecules in the solution had been decomposed. The degradation of PNP on the surface of Eu containing polymer is much higher than that of the Eu-free polymer (P0). The catalytic activity of the former is approximately 2 times more than that over the later. It is clear that the presence of Eu increased the Urbach energy (Ee) value, Table 2. The Ee value

3.3.2. The influence of the amount of catalyst As previously mentioned, the Eu containing polymer exhibited higher catalytic activity than that of the pure polymer. Therefore, it is interesting to make further studies on these catalysts using P5, which has the highest catalytic activity. The effect of the amount of catalyst on the degradation of PNP was carried out and the degradation % is plotted as a function of the catalyst mass as presented in Fig. 11. It can be seen that the degradation % increases gradually with increasing the mass of catalyst used in each run (100 ml of 5  10  3 M PNP solution) until the catalyst mass of 0.06 gm. Moreover, the PNP degradation decreased after increasing the catalyst mass more than 0.06 gm. Therefore, this catalyst concentration is considered as the optimum catalyst mass. The decrease in degradation % was attributed to the aggregation of the 5% Eu-polymer particles at high concentration (more than 0.06 gm) causing a decrease in the number of surface active sites and

Fig. 10. Photocatalytic degradation of PNP over Eu-PVA containing different amounts Eu ions using: 0.06 g catalyst, 5  10  3 M of PNP, 61.6 mmol/l of H2O2, pH¼ 5.5 at room temperature.

Fig. 9. Chromaticity diagram of (100–X) PVA-XEuCl3 composite system.

Table 4 Color parameters of (100-X) PVA-XEuCl3 composite system. Polymer number

Ln

Un

Vn

Cn

DLn

DUn

DVn

DCn

DE

hue

W

Ye

Po P1 P2 P3 P4 P5

88.08 96.06 95.56 94.98 95.11 89.19

0.76 0.16 0.38 0.19 0.44 1.04

1.54 0.46 0.33 0.53 1.01 1.5

1.72 0.49 0.5 0.57 1.11 1.83

– 7.98 7.48 6.9 7.03 1.11

–  0.6  0.38  0.57  0.32 0.28

–  1.08  1.21  1.01  0.53  0.04

– 1.23 1.26 1.16 0.61 0.29

– 8.07 7.58 6.99 7.05 1.14

63.8 70.86 41.07 70.45 66.27 55.27

 864.157  1078.45  1064.09  1047.48  1051.32  891.78

0.021 0.005 0.005 0.007 0.012 0.022

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catalyst and the substrate again and consequently the degradation efficiency decreases. In other words, the rate of e  /h þ recombination increases in acidic and basic media and hence decreases the rate of reaction. In neutral medium, the charges over the catalyst and PNP is minimum and hence the rate of e  /h þ recombination is minimum which causes the increase of PNP degradation%.

4. Conclusions

Fig. 11. Photocatalytic degradation of PNP over different amounts of 5%Eu-PVA using: 0.06 g catalyst, 5  10  3 M of PNP, 61.6 mmol/l of H2O2, pH¼ 5.5 at room temperature.

Fig. 12. Photocatalytic degradation of PNP at different pH values over 5%Eu-PVA using: 0.06 g catalyst, 5  10  3 M of PNP, 61.6 mmol/l of H2O2 at room temperature.

increase the opacity and light scattering of the catalyst particles at high concentration. This tends to decrease the passage of irradiation through the sample [54,55]. From forgoing results it is important to mention that, from the economic point of view, the optimized photocatalyst loading is 0.6 g/L.

3.3.3. Influence of pH on the PNP degradation Fig. 12 represents the degradation % of PNP over P5 using different initial pH values in the range of 2–8 using 0.06 gm of the solid catalyst under the same above-mentioned experimental conditions. It is clear that the degradation of the PNP increases with raising pH value, i.e. the degradation was pH dependent till the pH value reaches 5.5 after which, the degradation % decreases with further increase of the pH. These results are in agreement with a previous report by Ali et al. [56]. In low pH value (very acidic) and high pH value (very basic) media, the degradation % was lower than neutral medium. In acidic medium, the low degradation % is due to the presence of similar charges on both the solid catalyst and the PNP molecules by adsorbing H þ ions [57]. With raising pH value in acidic medium, the charge density decreases. This is accompanied by an increase of the degradation % until the neutral medium is reached. In the neutral medium, the degradation % is maximum because the similar charge density is minimum. Again after pH¼6, the increase of the pH leads to an increase of the similar charge density on the catalyst due to the adsorption of OH  . This may increase the repulsion between the

Polymer composite system based on PVA-EuCl3 was prepared using a solvent casting technique. Thermal analysis techniques (DTA, TG and DTG) were performed in order to find out the different phase transitions and the thermal decomposition behavior of samples under study. DTA thermograms revealed that position of Tg was shifted to higher temperatures and Tm to lower ones as EuCl3 content is increased. The IR spectral analysis indicated the formation of complex between Eu3 þ ions and PVA. Optical measurements indicate the presence of n–p* and p–p* electronic transitions, which were interpreted in terms of Urbach transition mechanism. The refractive index values decreased with increasing wavelength of the incident photon. In the same time the PVA has higher value of the refractive index than that of the other samples. The dispersion of refractive index is discussed in terms of Wemple and DiDomenco single oscillator model and its parameters Ed, Eosc, nN, lo, So and e01 were found to be dependent on EuCl3 content. The dielectric constant and dielectric loss increased with photon energy. The color measurements show that no color gradient between PVA and other composite samples. Owing to the compositional dependence on optical measurements, these composites may be suitable for optical data storage. Also, it was interesting to test its catalytic activity towards the degradation of harmful p-nitrophenol. The catalytic activity tests showed that the presence of Eu with polyvinyl alcohol increased the catalytic activity approximately twice more than that of Eu-free polyvinyle alcohol. The increase of the catalytic activity is mainly due to the increase of the Urbach energy which reflected the increase of the electron disorder and the increase of the conductivity on the surface of solid catalysts. The e  /h þ recombination rate was minimal in case of the catalysts containing Eu3 þ and at neutral medium. References [1] E.M. Abdelrazek, I.S. Elashmawi, A. El-khodary, A. Yassin, Curr. Appl. Phys. 10 (2010) 607. [2] A.M. Shehap, K. Atef, K.H. Mahmoud, F.M. Abdel-Rahim, Phil. Mag. 89 (2009) 989. [3] Z. Zheng, H. Liang, H. Mikg, Q. Zhang, Y. Yu, S. Lin, Y. Zhang, J. Xie, Chin. Opt. Lett. 2 (2004) 67. [4] C.A. Finch, Polyvinyl alcohol: Properties and Applications, Wiley Interscience, New York, 1973. [5] S. Rajendran, M. Sivakuma, R. Subadevi, Mater. Lett. 58 (2004) 641. [6] M. Nogami, T. Nagakura, T. Hayakkawa, J. Lumin. 86 (2000) 117. [7] W.A. Pisarski, J. Pisarska, G.D. Dzik, M. Maczka, W.R. Romanowski, J. Phys. Chem. Solids 67 (2006) 2452. [8] M.B. Cassidy, H. Lee, T. Zablotowicz, J. Ind. Biotechnol. 23 (1999) 232. [9] N.N. Mahamuni, A.B. Pandit, Ultrason. Sonochem. 13 (2006) 165. [10] C. Berberidou, I. Poulios, N.P. Xekoukoulotakis, D. Mantzavinos, Appl. Catal. B 74 (2007) 63. [11] J.K. Kim, F. Martinez, I.S. Metcalfe, Catal. Today 124 (2007) 224. [12] M.S. Vohra, K. Tanaka, Water Res. 37 (2003) 3992. [13] C. Adan, A. Bahamonde, M.F. Garcia, A.M. Arias, Appl. Catal. B72 (2007) 11. [14] J. Lukac, M. Klementova, P. Bezdicka, S. Bakardjieva, J. Subrt, L. Szatmary, Z. Bastl, J. Jirkovsky, Appl. Catal. B 74 (2007) 83. [15] M.P. Moya, M. Graells, L.J. Valle, E. Centelles, H.D. Mansilla, Catal. Today 124 (2007) 163. [16] X. Zhao, T. Xu, W. Yao, C. Zhang, Y. Zhu, Appl. Catal. B72 (2007) 92. [17] J. Matos, J. Laine, J.M. Herrmann, Appl. Catal. B18 (1998) 281. [18] A. Kunz, P.P. Zamora, N. Duran, Adv. Environ. Res. 7 (2002) 197. [19] J.F. Akyurtlu, A. Akyurtlu, S. Kovenklioglu, Catal. Today 40 (1998) 343. [20] D.H. Han, S.Y. Cha, H.Y. Yang, Water Res. 38 (2004) 2782. [21] J.G. Mei, S.M. Yu, J. Cheng, Catal. Commun. 5 (2004) 437. [22] Z. Ai, P. Yang, X.H. Lu, Chemosphere 60 (2005) 824.

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