Microwave assisted combustion synthesis of coupled ZnO–ZrO2 nanoparticles and their role in the photocatalytic degradation of 2,4-dichlorophenol

Available online at www.sciencedirect.com

CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 5681–5691 www.elsevier.com/locate/ceramint

Microwave assisted combustion synthesis of coupled ZnO–ZrO2 nanoparticles and their role in the photocatalytic degradation of 2,4-dichlorophenol E.D. Sherlya, J. Judith Vijayaa,n, N. Clament Sagaya Selvama, L. John Kennedyb a Catalysis and Nanomaterials Research Laboratory, Department of Chemistry, Loyola College, Chennai 600034, India Materials Division, School of Advanced Sciences, Vellore Institute of Technology (VIT) University, Chennai Campus, Chennai 600127, India

b

Received 4 September 2013; received in revised form 1 November 2013; accepted 1 November 2013 Available online 12 November 2013

Abstract Zinc oxide (ZnO), zirconium oxide (ZrO2) and their coupled oxides in the molar ratio 1:1, 2:1 and 1:2 (labeled as ZnZr, Zn2Zr, and ZnZr2 respectively) were successfully prepared by a microwave assisted urea–nitrate combustion synthesis. The structure and morphology of the pure ZnO, ZrO2 and coupled ZnZr, Zn2Zr, and ZnZr2 were characterized by powder X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), high resolution scanning electron microscopy (HRSEM), energy dispersive X-ray spectrometry (EDX) and Brunauer–Emmett–Teller (BET) methods. The results of the photocatalytic degradation of 2,4-dichlorophenol (2,4-DCP) in aqueous solution indicated that the coupled metal oxide, Zn2Zr is more effective towards the degradation of 2,4-DCP when compared to ZnO, ZrO2, ZnZr and ZnZr2. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Microwave combustion synthesis; ZnO–ZrO2 coupled metal oxides; Photocalytic degradation; 2,4-Dichlorophenol

1. Introduction Environmental remediation is the need of the hour, especially for waste water decontamination. There are number of ways to address this issue and heterogeneous photocatalysis by semiconductor nanoparticles plays a vital role in meeting this challenge and attaining the goal of a cleaner and better environment [1–3]. Large band gap oxide and sulphide semiconductor materials such as TiO2, WO3, ZnO, CuO, ZrO2, CdS, ZnS etc. are commonly used as photocatalysts [4]. These materials are excited by light of energy equal to or more than the band gap energy. Once electron/hole pairs are generated by light excitation, most of them recombine generating heat. Only a small fraction is successfully transferred to the interface to initiate the redox reactions. The efficiency of

n

Corresponding author. Tel.: þ91 44 28178200; fax: þ 91 44 28175566. E-mail addresses: [email protected], [email protected] (J.J. Vijaya).

the photocatalytic processes depends upon the life time of the photo-generated electron/hole pairs [5]. TiO2 is one of the most commonly used photocatalysts as it is very robust and stable under irradiation. ZnO is another efficient, low cost n-type semiconductor with a band gap of around 3.2 eV and a large exciton binding energy (60 MeV). Compared to TiO2, ZnO is of lower cost and absorbs a large fraction of the UV spectrum [6,7]. But both these materials have the disadvantage of having a wide band gap
3.2 eV, so that the photo-excitation process requires light in the UV region of the electromagnetic spectrum [8,9]. Moreover, a high recombination rate of the photogenerated charge carriers (electrons and holes) makes the photocatalytic process inefficient. So there arises the need to make photocatalysts with suitable band gap, which can undergo electronic excitation with absorption in the visible region facilitating the effective use of solar energy and a mechanism to improve the electron hole pair separation for enhancing photocatalytic efficiency. This issue has been addressed through different approaches for e.g., dye sensitization [10–12], doping the metal oxide with

0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.11.006

5682

E.D. Sherly et al. / Ceramics International 40 (2014) 5681–5691

different metals [13,14] and by coupling metal oxides of suitable band gaps [15–17]. Coupled semiconductors (two different semiconductors present together in a system as hetero-junctions), show enhanced photocatalytic activity. On illumination, both the semiconductors are simultaneously excited and the electrons slip to the low-lying conduction band of one semiconductor, while holes move to the less anodic valence band. And thus, the improved electron–hole pair separation enhances the photocatalytic efficiency [18–23]. Karunakaran et al. [24] have reported that phenol degradation can take place on ZrO2 upon illumination with light of wavelength 365 nm even though the band gap of ZrO2 is
5.0 eV, and would normally require UV-C light (o 280 nm) for the generation of electron–hole pairs. The diffuse reflectance spectral study suggested phenol-sensitized activation of ZrO2 with 365 nm light. They have further reported that TiO2, Fe2O3, CuO, ZnO, ZnS, Nb2O5 and CdO particles could enhance the photodegradation on ZrO2, indicating the inter-particle charge-transfer and improved electron– hole pair separation [18,25–28]. 2,4-Dichlorophenol (2,4-DCP) is a precursor in the manufacture of the widely used herbicide, 2,4-Dichlorophenoxy acetic acid (2,4-D) and is also the major transformation product of 2,4-D caused by the solar photolysis and also microbial activities in soil or natural water. It is one of the 129 priority pollutants listed by the United States EPA, because of its carcinogenicity, toxicity and persistence [29–31]. Numerous in vitro and in vivo tests have demonstrated that 2,4DCP can modulate the transcription of steroid genetic genes in both human and fish and disrupted the steroid genesis which in turn causes the impaired reproduction [32]. Some of the reported methods to synthesize coupled metal oxides are impregnation method [15,33], solid state dispersion method [34], sol–gel method [20,35], hydrothermal method [36], co-precipitation method [37–39] mechanical mixing method [40], photo-deposition method [41], and flame spray pyrolysis [42]. While these methods have their own advantages, most of them involve lengthy, complicated procedures, some of them require high temperature, specialized instruments, toxic reagents and external additives during the reaction. Solution combustion synthesis (SCS) is an effective, lowcost method for the production of various industrially important nanomaterials. This process involves a self-sustained reaction in homogeneous solution of different oxidizers (e.g., metal nitrates) and fuels (e.g., urea, glycine, hydrazides). Microwave assisted combustion synthesis is becoming an increasingly popular and versatile method for the preparation of metal oxide nanoparticles. In a microwave assisted combustion synthesis, the microwave induces oscillation of molecular dipoles leading to higher rate of molecular collisions generating enormous amount of heat within the sample. Since, the microwave interaction is at a molecular level, the temperature distribution is homogeneous inside the solution causing an explosion reaction followed by vigorous evolution of gases to form nanostructures. The reaction can be performed in a domestic microwave oven and the operation is clean, fast, and cheap and as prepared metal oxide nanoparticles are highly pure and the yield is also good. The reaction is completely

done in aqueous medium and therefore no harmful organic solvents are used [43–45]. The goal of the manuscript is to present the microwave combustion synthesis as a fast, one-pot, single step, environmental friendly and a low cost method for the synthesis of coupled metal oxides of ZnO and ZrO2. As far as we could find, this is the first successful attempt to prepare ZnO–ZrO2 coupled metal oxide by microwave combustion synthesis using urea as the fuel. 2. Experimental section 2.1. Synthesis of nano- ZnO, ZrO2 and coupled ZnO and ZrO2 2.1.1. Synthesis of nano-ZnO All the reagents used were of analytical grade obtained from Merck, India and were used as received without further purification. For the preparation of ZnO nanoparticles, zinc nitrate (Zn(NO3)2  6H2O) and urea (CH4N2O) were taken as starting materials. The stoichiometric compositions of the solution components (fuels and oxidizer) were calculated according to the principle of propellant chemistry, keeping the oxidizer (metal nitrate) to fuel (urea) ratio as unity [46]. Oxidizing valency of Zn(NO3)2: 1Zn ¼ þ 2, 2N ¼ 0, 6O ¼  12, Total ¼ þ 2  12¼  10. Reducing valency of CH4N2O; 1C ¼ þ 4, 4H ¼ þ 4, 2N ¼ 0, 1O ¼  2, Total ¼ þ 4 þ 4  2¼ þ 6. (φe) (O/F)¼ 10/6¼ 1.67 i.e., for every one mole of Zn(NO3)2, 1.67 mol of urea is required. Zinc nitrate (2.97 g) and urea (1.096 g) were dissolved separately in 10 mL of deionized water, mixed and stirred for 1 h to obtain a clear solution. This was placed in a domestic microwave-oven (2.45 GHz and 850 W) for 7 min. Initially, the solution boiled and underwent dehydration followed by decomposition with the evolution of gases. When the solution reached the point of spontaneous combustion, it vaporized and instantly became a solid. The obtained solid was washed well with alcohol and dried in a hot air oven at 80 1C for 2 h. During combustion, the gaseous products released were N2, NO2, CO2 and H2O as water vapor. 2.1.2. Synthesis of nano-ZrO2 Same procedure as above, instead of zinc nitrate, zirconyl nitrate (2.31 g in 10 mL deionized water) was used. 2.1.3. Synthesis of coupled ZnO and ZrO2 Zinc nitrate and zirconyl nitrate in 1:1, 2:1 and 1:2 M ratios were dissolved in deionized water and mixed with required amount of urea (for every one mole of Zn(NO3)2 or ZrO (NO3)2, 1.67 mol of urea) and treated in a microwave oven to synthesize coupled ZnO–ZrO2 in 1:1, 2:1 and 1:2 M ratios and were labeled as ZnZr, Zn2Zr and ZnZr2 respectively. 2.2. Characterization of ZnO, ZrO2 and coupled ZnO–ZrO2 The crystallinity of pure ZnO, ZrO2 and coupled ZnZr, Zn2Zr and ZnZr2 were determined by using a Philips X'pert

E.D. Sherly et al. / Ceramics International 40 (2014) 5681–5691

X-ray diffractometer with Cu Kα radiation at λ¼ 1.540 Å. Morphological studies and energy dispersive X-ray analysis of ZnO, ZrO2 and coupled ZnO–ZrO2 have been performed with a Jeol JSM 6360 high resolution scanning electron microscope. The diffuse reflectance UV–visible spectra of the samples were recorded using Cary 100 UV–visible spectrophotometer to estimate their energy band gap. The photoluminescence properties were recorded using Varian Cary Eclipse Fluorescence Spectrophotometer. The surface area, pore volume, pore size distribution, and the pore diameter were determined using an automated gas sorption system (Quantachrome Corp. Nova1000 gas sorption analyzer). 2.3. Photocatalytic reactor setup and degradation procedure

Lamda 25). The effect of the pH of the solution was studied by adjusting the pH of the phenol solution containing the catalyst, using dilute HCl and NaOH (both from Merck, India). The pH of the solution was measured using a HANNA Phep (Model H 198107, 0.2–0.5 pH unit accuracy) digital pH meter. The PCD efficiency (η) was calculated from the following expression: η ¼ C 0  Ct =C 0  100 where C0 is the initial concentration of 2,4-dichlorophenol, and Ct, the concentration of 2,4-dichlorophenol after time ‘t’. To check the stability and reusability of catalyst, the resulting suspension was centrifugalized at the end of experiment. The separated catalyst was reused for repeated tests. All measurements were repeated twice and the results were reproducible within the experiments errors (7 3%). 3. Results and discussion 3.1. X-ray diffraction analysis Fig. 2 represents the XRD diffraction patterns of ZnO, ZrO2, ZnZr (ZnO and ZrO2 1:1 ratio), Zn2Zr (ZnO and ZrO2 2:1 ratio) and ZnZr2 (ZnO and ZrO2 1:2 ratio). Diffraction peaks, corresponding to both ZnO (JCPDS: 36-1451) and ZrO2 (JCPDS: 27-0997), are clearly observed in the individual as °ZnO • ZrO2

•

•

•

•

ZrO2

•

• •

• ° ° ° Intensity (a.u)

PCD experiments were carried out in a Multilamp photocatalytic reactor (Heber Scientific), as shown in Fig. 1. The cylindrical photocatalytic reactor tubes were of quartz/borosilicate with a dimension of 36–1.6 cm (height–diameter). The top portion of the reactor has ports for sampling, gas-purging, and gas outlet. The aqueous 2,4-dichlorophenol (2,4-DCP) solution containing an appropriate quantity of pure ZnO, ZrO2 or coupled ZnO/ZrO2 was taken in the quartz/borosilicate tube and subjected to aeration for thorough mixing. This was then placed inside the reactor setup. The lamp housing has low pressure mercury lamps (8  8 W) emitting 254, 312 or 365 nm with polished anodized aluminum reflectors and black cover to prevent UV leakage. The PCD was carried out by mixing 100 mL of aqueous 2,4-dichlorophenol solution and a fixed weight of pure ZnO, ZrO2 or coupled ZnO/ZrO2 photocatalysts. Prior to irradiation, the slurry was stirred for 30 min to reach the adsorption equilibrium followed by UV irradiation. Aliquots were withdrawn from the suspension at specific time intervals and centrifuged immediately at 1500 rpm. The extent of phenol degradation was monitored by using a UV–visible spectrophotometer (Perkin-Elmer,

5683

•

°

°

ZnZr2

•°

°

°

• °

° •

°

•

°

° • •

°

• •°

°

°

Zn2Zr

°°

• ° ° °•

• °

ZnZr

°°

° °

30

°

° 40

50

60

ZnO

°

° 70

80

2 Theta (degree) Fig. 1. Schematic diagram of the photocatalytic reactor used in the present study.

Fig. 2. XRD pattern of ZnO, ZrO2 and the coupled metal oxides ZnZr, Zn2Zr and ZnZr2.

E.D. Sherly et al. / Ceramics International 40 (2014) 5681–5691

3.2. Optical studies (diffuse reflectance spectroscopy) The band gap energy (Eg) of ZnO, ZrO2, ZnZr, Zn2Zr and ZnZr2 was obtained from the optical diffuse reflectance spectra (DRS) recorded at room temperature. The Tauc plot of the same is displayed in Fig. 3. The value of Eg could be thus estimated to be 3.2 eV and 4.5 eV for ZnO, ZrO2 respectively, and 3 eV for the samples ZnZr, Zn2Zr and ZnZr2. The addition of ZnO changes the absorption edge of ZrO2 bringing it closer to ZnO as observed also in the ZnO–ZrOx prepared by hydrothermal method [36]. When comparing the DRS of pure ZrO2 with that of phenol adsorbed ZrO2 (Fig. 4), the latter shows smaller band gap. This could be due to the phenol sensitized excitation of ZrO2 [24]. This study is carried out in order to propose the mechanism for the photocatalytic

10

8

6

(f(r)*hv)1/2

well as in the coupled diffraction patterns. Sharp diffraction peaks indicate that the samples have high crystallinity. It can be deduced from the locations of the peaks that the incorporation of ZrO2 does not lead to a solid solution with ZnO, and the samples ZnZr, Zn2Zr and ZnZr2 can be regarded as composite powders of crystalline ZrO2 and ZnO. Similar trend is shown by ZrOx/ZnO composite metal oxides prepared by Liao et al. by hydrothermal method [36]. For pure ZnO, the diffraction peaks are located at 2θ ¼ 31.841, 34.521, 36.331, 47.631, 56.711, 62.961, 68.131, and 69.181 are associated with [100], [002], [101], [102], [110], [103], [112] and [201] planes respectively [47]. This pattern has been indexed as hexagonal wurtzite phase of ZnO with lattice constants a ¼ b¼ 0.324 nm and c ¼ 0.521 nm (JPCDS card number: 36-1451), and further it is also confirmed that the synthesized nano-powder is free of impurities as it does not contain any characteristic XRD peaks other than ZnO peaks. ZrO2 exists in three phases, namely monoclinic, tetragonal, and cubic. Crystallization of ZrO2 in a particular phase depends upon the temperature and pH conditions employed during the synthesis and the time of reaction [48]. The observed diffraction peaks of ZrO2 at 2θ ¼ 30.301, 35.401, 50.601, 59.81, and 62.851 are associated with [111], [200], [220], [311], and [222] plane respectively. These planes are then associated with d-spacing values of 2.93, 2.55, 1.80, 1.53, and 1.47 Å, respectively, which can be readily assigned to a cubic phase of ZrO2 (JCPDS no. 27-0997) with the lattice parameter of a¼ 0.497 and c ¼ 0.320 [49]. Furthermore, no characteristic peaks from other crystalline impurities were detected by XRD, thus suggesting high purity. The crystallite size of ZnO, ZrO2 and coupled metal oxides was calculated using Debye–Scherrer formula, d¼ 0.89λ/ β cos θ [50], where ‘d’ is the crystallite size, 0.89, Scherrer's constant, λ, the wavelength of X-rays, θ, the Bragg diffraction angle, and β, the full width at half-maximum (FWHM) of the diffraction peak. The average crystallite size of pure ZnO was found to be 49 nm, which is derived from the FWHM of more intense peaks. The crystallite size of ZrO2 is calculated to be 43 nm. For the coupled metal oxides, it is 40 nm. Lowering of the crystallite size on coupling between two metal oxides is seen also in other preparation methods [36,38].

ZnO

4

ZrO2 ZnZr Zn2Zr

2

ZnZr2

0 1

2

3

4

5

6

7

band gap (eV) Fig. 3. Tauc plot of ZnO, ZrO2, ZnZr, Zn2Zr and ZnZr2.

ZrO2 - Phenol

ZrO2

(f(r)*hv)1/2

5684

2

3

4

5

6

7

8

band gap (eV) Fig. 4. Tauc plot of ZrO2 and phenol adsorbed ZrO2.

degradation of 2,4-DCP on ZrO2 and coupled oxides which will be discussed in the later part of the present work. In the coupled oxides, there is an inter-particle electron transfer from the conduction band of ZrO2 to the low lying conduction band of ZnO and this inter-particle electron transfer explains the efficient photocatalytic degradation of 2,4-DCP by the coupled metal oxide Zn2Zr compared to that of the pure ZnO or ZrO2 [25–28]. 3.3. Photoluminescence (PL) spectroscopy PL signals of semiconductor materials result from the recombination of photo-induced charge carriers. The band–band PL spectrum can directly reflect the separation situation of photoinduced charge carriers, viz. the stronger the band–band PL

E.D. Sherly et al. / Ceramics International 40 (2014) 5681–5691

5685

ZnZr2

ZnZr

Photoluminescence intensity (a.u)

Photoluminescence intensity (a.u)

Zn2Zr

ZnZr2

Zn2Zr

ZnZr

ZrO2

ZrO2

ZnO ZnO

350

400

450

500

550

600

Wavelength (nm)

250

300

350

400

450

500

550

600

650

Wavelength (nm)

Fig. 5. PL spectra of ZnO, ZrO2, ZnZr, Zn2Zr and ZnZr2 (excitation wavelength 320 nm).

Fig. 6. PL spectra of ZnO, ZrO2, ZnZr, Zn2Zr and ZnZr2 (excitation wavelength 224 nm).

signal, the higher the recombination rate of photo-induced carriers. The excitonic PL spectrum cannot directly reflect the separation situation of photo-induced carriers. However, it can reveal some important information about the surface defects, oxygen vacancies and surface states, which can strongly affect the photocatalytic reactions [51]. The PL spectra were recorded at two excitation wave lengths at 224 and 320 nm for ZnO, ZrO2 and the coupled ZnZr, Zn2Zr and ZnZr2. On examining the PL spectra at the excitation wavelength of 320 nm (Fig. 5), the UV band gap emission at 392 nm of ZnO and the coupled metal oxides results from the radiative recombination of an excited electron in the conduction band with the valence band hole or the band–band PL spectrum. The visible emissions at 435, 482, 495, 520, and 540 nm are due to the excitonic PL signal in which the non-radiative transitions of excited electrons from the CB bottom to the different subbands or the surface states occur first, and subsequent radiative transitions from the sub-band to the VB occurs. The energy of the radiative photon, which is the energy difference between the sub-band and VB top, is lower than the band gap energy. These excitonic PL signals result from the surface oxygen vacancies and the defects of the semiconductors [52]. Oxygen vacancies and defects can easily bind the photoinduced electrons to form excitons so that the PL signal can easily occur and larger the content of oxygen vacancy or defect, the stronger will be the PL signal. These oxygen vacancies can promote O2 adsorbing, and the adsorbed O2 to capture photo-induced electrons and thereby simultaneously

producing O2 radical groups. The radical groups are active to promote the oxidation of organic substances [51]. Thus, it can be suggested that the oxygen vacancies and the defects are in favor of the photocatalytic reactions. Fig. 5 shows that the pure ZnO, ZrO2 as well as the coupled metal oxides have excitonic PL signals indicating the oxygen vacancies and surface defects. The band–band and excitonic emission intensities are lower in the coupled metal oxides. This could be due to the suppression of recombination of photogenerated electron hole pairs in the coupled metal oxides. Fig. 6 shows the PL spectra at the excitation wavelength of 224 nm. The emission below 300 nm is observed only for ZrO2 and this corresponds to band–band emission of ZrO2 as its band gap is found to be around 4.5 eV from the DRS measurements. There is not much difference in the position of excitonic PL signals at 320 nm and 224 nm. Thus, the employed microwave combustion method can produce plenty of defects in ZnO, ZrO2 and the coupled metal oxides. 3.4. Morphology of ZnO, ZrO2 and coupled metal oxides The surface morphology of the as prepared ZnO, ZrO2 and coupled metal oxide, Zn2Zr (with higher photocatalytic activity) was examined by HR-SEM method. Growth of the nanoparticles cannot be controlled in the combustion synthesis so that particles with varying sizes are obtained. Fig. 7(a and b) shows the SEM micrographs of ZnO nanoparticles, which are of irregular spherical shape with varying sizes between 50 and

5686

E.D. Sherly et al. / Ceramics International 40 (2014) 5681–5691

1 μm

1μm

1 μm

500 nm

500 nm

500 nm

Fig. 7. HR-SEM images of (a and b) ZnO, (c and d) ZrO2 and (e and f) Zn2Zr.

150 nm. Fig. 7(c and d) shows the SEM micrograph of ZrO2 nanoparticles which are spherical with sizes ranging between 10 and 40 nm and Fig. 7(e and f) shows the SEM micrograph of coupled metal oxide (Zn2Zr) nanoparticles with sizes ranging from 30 to 150 nm. In the coupled metal oxide (Zn2Zr), ZrO2 nano-spheres are embedded on the matrix of ZnO nano-particles. The HR-SEM images show that the particles are aggregated, which is due to the enormous heat generated during the combustion reaction. The difference in the particle sizes shown by XRD analysis and SEM analysis for ZnO nanoparticles could be due to the particle aggregation. It is also because, the SEM measurements are based on the difference between the visible grain boundaries, while XRD calculations measure the extended crystalline region that diffracts X-rays coherently. Hence the XRD method has a more stringent criterion and leads to smaller sizes [53]. The

composition of the prepared ZnO, ZrO2 and coupled metal oxide Zn2Zr was analyzed by means of energy dispersive X-ray analysis (EDX) as shown in Fig. 8(a–c). The EDX result showed the presence of Zn and O, Zr and O and Zn, Zr and O in Fig. 8a, b and c, respectively by showing the corresponding characteristic peaks and compositions. No other extra peaks appeared which shows that the prepared samples are free from impurities. 3.5. BET isotherm and surface area measurements Fig. 9(a–d) shows the nitrogen adsorption–desorption isotherms of ZnO, ZrO2, Zn2Zr and ZnZr2. All four samples showed type IV isotherms and the hysteresis loops are clearly shown in Fig. 9. ZrO2 and Zn2Zr showed H3 hysteresis loops. A distinguishing feature of H3 hysteresis loops is that they

E.D. Sherly et al. / Ceramics International 40 (2014) 5681–5691

Element

Wt%

At%

OK

19.99

50.51

Zn L

80.01

49.49

5687

KeV

Element

Wt%

At%

OK

32.81

73.57

Zr L

67.19

26.43

KeV

Element Wt%

At%

OK

25.59

60.69

Zn L

50.78

29.48

Zr L

23.63

9.83

KeV

Fig. 8. EDX of (a) ZnO, (b) ZrO2 and (c) Zn2Zr.

often continue into the low pressure region. Adsorption at low pressures mainly occurs at slit like micro-pores [54–56]. This indicates that ZrO2 and Zn2Zr contain micro-pores along with meso-pores. This could be the reason for the higher BET surface area for ZrO2 and Zn2Zr. It can be seen from Table 1 that the surface area and the pore volume is the highest for ZrO2 and Zn2Zr. The higher BET surface area of Zn2Zr when compared to other samples (ZnO, ZnZr2) is the main reason for its higher photocatalytic activity observed in the present study. Though, ZrO2 has a comparative higher surface area of 17.99 m2/g, it shows poor photocatalytic activity when irradiated with 365 nm light as its band gap is around 5 and it cannot undergo direct band gap excitation at that particular wavelength. The coupled metal oxides showed larger surface area when compared to the pure metal oxides [37,38].

3.6. Photocatalytic activity In order to determine the photocatalytic activity of ZnO, ZrO2 and the coupled metal oxides ZnZr, Zn2Zr and ZnZr2, a series of experiments were carried out with 2,4-DCP in aqueous suspension with the light of wavelength 365 nm. The photocatalytic degradation follows a pseudo-first order reaction and its kinetics can be expressed using ln(C0/Ct)=kt, where k is the apparent reaction rate constant, C0, the initial concentration of 2,4-DCP, t, the reaction time and Ct, the concentration of 2,4-DCP at time t. Fig. 10 shows the kinetic fit for the degradation of 2,4-DCP over pure ZnO, ZrO2 and coupled ZnZr, Zn2Zr and ZnZr2 with the catalyst dosage of 30 mg, 2,4-DCP concentration of 75 mg/L and at pH 5. Rate constant (k) values are given in Table 1.

E.D. Sherly et al. / Ceramics International 40 (2014) 5681–5691

3.6.1. Photocatalytic degradation (PCD) efficiency and composition of the catalyst Photocatalytic degradation of 2,4-dichlorophenol has been carried out on the surfaces of pure ZnO, ZrO2 and the coupled ZnZr, Zn2Zr and ZnZr2 at different conditions such as varying pH, concentration of the catalyst and concentration of phenol. Maximum degradation (up to 90%) was achieved with the coupled catalyst Zn2Zr (ZnO and ZrO2 in 2:1 ratio) at pH 5 for 75 mg/L phenol and 30 mg catalyst as shown in Fig. 11. The reason could be that Zn2Zr has more surface area and pore size volume compared to ZnO and ZnZr2 as given in Table 1. ZrO2, though has better surface area and smaller particle size, cannot be a good photocatalyst at 365 nm light due its large band gap, as discussed earlier. 3.6.2. Effect of pH on PCD efficiency Fig. 12 illustrates the degradation of 2,4-DCP under various pH conditions using the coupled metal oxide Zn2Zr. Maximum degradation of all phenol peaks is observed at pH around 5. At higher pH values, there is a degradation of only the peak at 199 nm (absorption peak due to Π-Πn transition), but not the peak at 284 nm (due to n-Πn transition). The degradation of the peak at 284 nm is observed at acidic pH. The pH value has a profound importance on the photodegradation efficiency of 2,4-DCP on pure or coupled ZnO and ZrO2 catalysts. There are number of researches to show that the removal efficiency of chlorophenols decreased with increasing pH values [57–59].

At low pH values (below pHzpc), the semiconductor surface carries a net positive charge, which facilitates better adsorption of the neutral or negatively charged chlorophenols or their 0.1 0.0 -0.1 -0.2

log(Ct/Co)

5688

-0.3 ZnO

-0.4

ZrO2 ZnZr

-0.5

Zn2Zr

-0.6

ZnZr2

-0.7 -0.8 0

2

1

3

4

Time (h) Fig. 10. Kinetic curves of ln(C0/Ct) vs. irradiation time (experimental conditions: 2,4-DCP¼ 75 mg/L, catalyst dose¼ 30 mg/100 mL, pH 5, λ ¼365 nm).

100

24

Adsorption

22

Desorption

20

Adsorption

(c)

Desorption

18

Adsorption

16

Desorption

14

Adsorption

(a) (d)

Desorption

12

80

(b) PCD efficiency (%)

Volume (cc/g)

26

60

ZnO ZrO2 ZnZr Zn2Zr

40

ZnZr2

10

20

8 6 4

0

2 0 0.0

0.2

0.4

0.6

0.8

0

1

Fig. 9. Adsorption–desorption isotherms of ZnO, ZrO2, Zn2Zr and ZnZr2.

3

4

5

Time (h)

1.0

Relative Pressure, p/p0

2

Fig. 11. PCD efficiency for different composition of catalyst (experimental conditions: 2,4-DCP¼ 75 mg/L, catalyst dose¼ 30 mg/100 mL, pH 5, λ ¼ 365 nm), inset shows the degradation pattern for the coupled metal oxide Zn2Zr.

Table 1 Surface area, pore volume, pore diameter values and photocatalytic activity results of pure ZnO, ZrO2 and coupled Zn2Zr, ZnZr2. Sample

BET surface area (m2/g)

Pore volume (cm3/g)

Average pore diameter (nm)

kobserved (h  1)

ZnO ZrO2 Zn2Zr ZnZr2

9.46 17.99 13.37 7.30

0.0277 0.0369 0.0376 0.0262

2.91 2.45 2.19 2.46

0.3855 0.0787 0.4467 0.2903

E.D. Sherly et al. / Ceramics International 40 (2014) 5681–5691

intermediates on the catalyst. This in turn can lead to their efficient photcatalytic degradation. But there are also reports about the better removal efficiency of chlorophenols under alkaline conditions, possibly due to the abundance of hydroxyl radicals. Therefore, two mechanisms operate in the photocatalytic degradation of chlorophenols. At low pH range, the direct oxidation of organic compounds by photo-generated holes may be the predominant mechanism. Upon the adsorption of phenol on the catalyst surface, charge transfer from the aromatic ring to the positive hole could take place. After the direct charge transfer, highly electrophilic groups such as chlorine and hydroxyl could dissociate from the aromatic ring and lead to the formation of chloride and hydroxide ions. With increasing pH, oxidation by hydroxyl radicals becomes more and more important. The decrease of overall oxidation rates of 2,4-DCP in this region may be attributed to the decrease of 100 90

PCD efficiency (%)

80 70 60

2,4-DCP adsorption on the catalyst surface as at this pH, the catalyst surface attains a negative charge and phenol is dissociated into phenoxide ion [60]. 3.6.3. Effect of phenol concentration on PCD efficiency The rate of degradation is found to increase with the increase in the initial concentration of 2,4-DCP, from 30 to 75 ppm and decreased as the concentration is increased further (Fig. 13). At low concentrations of phenol, the number of catalytic sites will not be the limiting factor and the rate of degradation will be proportional to the substrate concentration. As the concentration of 2,4-DCP increases, more and more phenol molecules are adsorbed on the surface of the photocatalyst. The reactive species (dOH and dO2 ) required for the degradation of the pollutant on the catalyst surface remains constant for a given light intensity, catalyst amount and duration of irradiation. Hence, the available OH radicals are inadequate for the pollutant degradation at higher concentrations. As a result, the phenol degradation rate decreases as the concentration increases [3,61]. In addition, an increase in the substrate concentration can lead to the generation of the intermediates, which may adsorb on the surface of the catalyst. Slow diffusion of the generated intermediates from the catalyst surface can result in the deactivation of active sites on the photocatalyst and result in a reduction of the degradation rate.

50 40 30 1

2

3

4

5

6

7

8

9

10

pH Fig. 12. PCD efficiency at varying pH for Zn2Zr (experimental conditions: 2,4-DCP¼ 75 mg/L, catalyst dose¼30 mg/100 mL, λ ¼365 nm).

100

3.6.4. Effect of catalyst loading on PCD efficiency Fig. 14 shows the effect of increasing the catalyst concentration on the photocatalytic degradation rate. As the catalyst loading increases, the photocatalytic degradation rate initially increases then decreases. The maximum degradation is observed with 30 mg of catalyst for 75 ppm phenol. With increased catalyst loading, the number of active sites in the solution will increase, but the penetration of light decreases, because of the excessive particle concentration. Moreover, at 100

100ppm 75ppm 50ppm 30ppm

90 80

20mg 30mg 40mg 50mg

80

70

PCD efficiency (%)

PCD efficiency (%)

5689

60 50 40

60

40

30 20

20

10 0 0

1

2

3

4

5

Time (h) Fig. 13. PCD efficiency for varying concentration of 2,4-DCP (experimental conditions: catalyst dose¼30 mg/100 mL, pH 5, λ ¼ 365 nm).

0

0

1

2

3

4

5

Time (h) Fig. 14. PCD efficiency with varying concentration of catalyst (experimental conditions: 2,4-DCP¼75 mg/L, pH 5, λ ¼365 nm).

5690

E.D. Sherly et al. / Ceramics International 40 (2014) 5681–5691

References

ZrO2 ZnO

e¯ CB

e¯ CB

e¯ e¯

.

Defect

h+ VB

energy level

PhO + H

Phenol energy level

h+ VB

hν (365 nm)

Fig. 15. Schematic energy level diagram of ZnO–ZrO2 coupled metal oxide for the photocatalytic degradation of phenol.

high catalyst concentration, there is a reduction in catalyst surface area, due to agglomeration of the particles [61]. Based on the above experimental observations and analysis, we have proposed a mechanism with a schematic energy level diagram to represent the charge transfer process in ZnO–ZrO2 coupled metal oxide for the degradation of of 2,4-DCP as given in Fig. 15. When ZnO–ZrO2 coupled metal oxide was irradiated by UV light (365 nm), the electrons in the valence band (VB) of ZnO can be excited to its conduction band (CB). As ZrO2 band gap is higher, this energy is not sufficient to cause the electron transfer from its VB to CB, but electronic excitation can take place to the defect levels present in its band gap. DRS spectrum of phenol adsorbed-ZrO2 (Fig. 4) points out that there is an electronic excitation from nonbonding orbitals of phenol to the conduction band of ZrO2 and these electrons can be transferred to the low lying CB of ZnO. Electron transfer can take place from the CB of ZnO to the low lying defect levels of ZrO2 and the hole transfer from ZrO2-VB band to the VB band of ZnO. This in turn leads to the efficient separation of photogenerated electron–hole pairs. As a result, the photocatalytic activity of the ZnO–ZrO2 coupled metal oxide is significantly enhanced.

4. Conclusions Pure ZnO, ZrO2 and their coupled oxides ZnZr, Zn2Zr and ZnZr2 were successfully prepared by a microwave assisted combustion synthesis using urea as the fuel. The prepared nano-particles are well characterized by XRD, DRS, PL, BET, HR-SEM and EDX analysis. This method has formed extremely pure, porous nano-particles in good yield. Microwave combustion synthesis has been used for the first time for the preparation of ZnO–ZrO2 coupled metal oxides. The coupled metal oxide Zn2Zr showed the maximum efficiency towards the degradation of 2,4-dichlorophenol (90%). The catalysts exhibited good stability and reusability. The recommended process parameters such as the catalyst concentration, phenol concentration and pH are 30 mg/100 mL of the catalyst, 75 mg/L of 2,4-DCP and pH 5.

[1] M.A. Fox, M.T. Dulay, Heterogeneous photocatalysis, Chem. Rev. 93 (1993) 341–357. [2] K. Orloff, H. Falk, An international perspective on hazardous waste practices, Int. J. Hyg. Environ. Health 206 (2003) 291–302. [3] S. Ahmed, M.G. Rasul, R. Brown, M.A. Hashib, Influence of parameters on the heterogeneous photocatalytic degradation of pesticides and phenolic contaminants in wastewater: a short review, J. Environ. Manage. 92 (2011) 311–330. [4] R.M. Mohameda, D.L. McKinney, W.M. Sigmund, Enhanced nanocatalysts, Mater. Sci. Eng. R 73 (2012) 1–13. [5] H.H. Mohamed, D.W. Bahnemann, The role of electron transfer in photocatalysis: fact and fictions, Appl. Catal. B: Environ. 128 (2012) 91–104. [6] M.L. Curri, R. Comparelli, P.D. Cozzoli, G. Mascolo, A. Agostiano, Colloidal oxide nanoparticles for the photocatalytic degradation of organic dye, Mater. Sci. Eng. C 23 (2003) 285–289. [7] C.C. Chen, H.J. Fan, J.L. Jan, Degradation pathways and efficiencies of acid blue 1 by photocatalytic reaction with ZnO nanopowder, J. Phys. Chem. C 112 (2008) 11962–11972. [8] J.M. Herrmann, Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants, Catal. Today 53 (1999) 115–129. [9] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96. [10] R. Vinu, Sneha Polisetti, Giridhar Madras, Dye sensitized visible light degradation of phenolic compounds, J. Chem. Eng. 165 (2010) 784–797. [11] R. Velmurugan, M. Swaminathan, An efficient nanostructured ZnO for dye sensitized degradation of Reactive Red 120 dye under solar light, Sol. Energy Mater. Sol. Cells 95 (2011) 942–950. [12] S. Kaur, V. Singh, Visible light induced sonophotocatalytic degradation of Reactive Red dye 198 using dye sensitized TiO2, Ultrason. Sonochem. 14 (2007) 531–537. [13] K.T. Ranjit, B. Viswanathan, Synthesis, characterization and photocatalytic properties of iron-doped TiO2 catalysts, J. Photochem. Photobiol. A 108 (1997) 79–84. [14] C. Siriwong, N. Wetchakun, B. Inceesungvorn, D. Channei, T. Samerjai, S. Phanichphant, Doped-metal oxide nanoparticles for use as photocatalysts, Prog. Cryst. Growth Charact. Mater. 58 (2012) 145–163. [15] P. Sathishkumar, R. Sweena, J.Wu. Jerry, S. Anandan, Synthesis of CuO–ZnO nanophotocatalyst for visible light assisted degradation of a textile dye in aqueous solution, J. Chem. Eng. 171 (2011) 136–140. [16] Chao Xu, Lixin Cao, Ge Su, Wei Liu, Hui Liu, Yaqin Yu, Xiaofei Qu, Preparation of ZnO/Cu2O compound photocatalyst and application in treating organic dyes, J. Hazard. Mater. 176 (2010) 807–813. [17] Xu Jing-jing, Chen Min-dong, Fu De-gang, Preparation of bismuth oxide/ titania composite particles and their photocatalytic activity to degradation of 4-chlorophenol, Trans. Nonferrous Met. Soc. China 21 (2011) 340–345. [18] C. Karunakaran, R. Dhanalakshmi, P. Gomathisankar, G. Manikandan, Enhanced phenol-photodegradation by particulate semiconductor mixtures: interparticle electron-jump, J. Hazard. Mater. 176 (2010) 799–806. [19] X. Lin, F. Huang, J. Xing, W. Wang, F. Xu, Heterojunction semiconductor SnO2/SrNb2O6 with an enhanced photocatalytic activity: the significance of chemically bonded interface, Acta Mater. 56 (2008) 2699–2705. [20] B. Neppolian, Q. Wang, H. Yamashita, H. Choi, Synthesis and characterization of ZrO2–TiO2 binary oxide semiconductor nanoparticles: application and interparticle electron transfer process, Appl. Catal. A 333 (2007) 264–271. [21] D. Robert, Photosensitization of TiO2 by MxOy and MxSy nanoparticles for heterogeneous photocatalysis applications, Catal. Today 122 (2007) 20–26. [22] H.H. Ou, S.L. Lo, C.H. Wu, Exploring the interparticle electron transfer process in the photocatalytic oxidation of 4-chlorophenol, J. Hazard. Mater. B 137 (2006) 1362–1370.

E.D. Sherly et al. / Ceramics International 40 (2014) 5681–5691 [23] X.H. Ou, C.H. Wu, S.L. Lo, Photodegradation of 4-chlorophenol by UVphotocatalysts: the effect of the interparticle electron transfer process, React. Kinet. Catal. Lett. 88 (2006) 89–95. [24] C. Karunakaran, R. Dhanalakshm, P. Gomathisankar, Phenolphotodegradation on ZrO2 – enhancement by semiconductors, Spectrochim. Acta A 92 (2012) 201–206. [25] C. Karunakaran, R. Dhanalakshmi, P. Gomathisankar, Semiconductor photocatalyzed degradation of carboxylic acids: enhancement by particulate semiconductor mixtures, Int. J. Chem. Kinet. 41 (2009) 716–726. [26] C. Karunakaran, S. Senthilvelan, Photocatalysis with ZrO2: oxidation of aniline, J. Mol. Catal. A 233 (2005) 1–8. [27] H.R. Pouretedal, Z. Tofangsazi, M.H. Keshavarz, Photocatalytic activity of mixture of ZrO2/SnO2, ZrO2/CeO2 and SnO2/CeO2 nanoparticles, J. Alloys Compd. 513 (2012) 359–364. [28] Z. Zhan, H.C. Zeng, A catalyst-free approach for sol–gel synthesis of highly mixed ZrO2–SiO2 oxides, J. Non-Cryst. Solids 243 (1999) 26–38. [29] J. Jia, S. Zhang, P. Wang, H. Wang, Degradation of high concentration 2,4-dichlorophenol by simultaneous photocatalytic–enzymatic process using TiO2/UV and laccase, J. Hazard. Mater. 205–206 (2012) 150–155. [30] M.P. Titus, V.G. Molina, M.A. Banos, J. Gimenez, S. Esplugas, Degradation of chlorophenols by means of advanced oxidation processes: a general review, Appl. Catal. B: Environ. 47 (2004) 219–256. [31] S. Chaliha, K.G. Bhattacharyya, Catalytic wet oxidation of 2-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol in water with Mn(II)MCM 41, J. Chem. Eng. 139 (2008) 575–588. [32] J.C. Sin, S.M. Lam, K.T. Lee, A.R. Mohamed, Photocatalytic performance of novel samarium-doped spherical-like ZnO hierarchical nanostructures under visible light irradiation for 2,4-dichlorophenol degradation, J. Colloid Interface Sci. 401 (2013) 40–49. [33] J.C. Tristao, F. Magalhaes, P. Corio, M.T.C. Sansiviero, Electronic characterization and photocatalytic properties of CdS/TiO2 semiconductor composite, J. Photochem. Photobiol. A: Chem. 181 (2006) 152–157. [34] A. Kambur, G.S. Pozan, I. Boz, Preparation, characterization and photocatalytic activity of TiO2–ZrO2 binary oxide nanoparticles, Appl. Catal. B: Environ. 115–116 (2012) 149–158. [35] H. Zou, Y.S. Lin, Structural and surface chemical properties of sol–gel derived TiO2–ZrO2 oxides, Appl. Catal. A: Gen. 265 (2004) 35–42. [36] W. Liao, T. Zheng, P. Wang, S. Tu, W. Pan, Efficient microwave-assisted photocatalytic degradation of endocrine disruptor dimethyl phthalate over composite catalyst ZrOx/ZnO, J. Environ. Sci. 22 (2010) 1800–1806. [37] H. Wang, S. Baek, J. Lee, S. Lim, High photocatalytic activity of silverloaded ZnO–SnO2 coupled catalysts, Chem. Eng. J. 146 (2009) 355–361. [38] K. Vignesh, R. Priyanka, M. Rajarajanc, A. Suganthi, Photoreduction of Cr(VI) in water using Bi2O3–ZrO2 nanocomposite under visible light irradiation, Mater. Sci. Eng. B 178 (2013) 149–157. [39] W. Zheng, Z. Bingru, L.I. Fengting, A simple and cheap method for preparation of coupled ZrO2/ZnO with high photocatalytic activities, Front. Environ. Sci. Eng. China 1 (2007) 454–458. [40] L. Song, S. Zhang, A simple mechanical mixing method for preparation of visible-light sensitive NiO–CaO composite photocatalysts with high photocatalytic activity, J. Hazard. Mater. 174 (2010) 563–566. [41] J. Wang, X.M. Fana, D.Z. Wu, J. Daia, H. Liu, H.R. Liu, Z.W. Zhou, Fabrication of CuO/T-ZnOw nanocomposites using photodeposition and their photocatalytic property, Appl. Surf. Sci. 258 (2011) 1797–1805. [42] C. Chaisuk, A. Wehatoranawee, S. Preampiyawat, S. Netiphat, A. Shotipruk, J. Panpranot, B. Jongsomjit, O. Mekasuwandumrong, Preparation and characterization of CeO2/TiO2 nanoparticles by flame spray pyrolysis, Ceram. Int. 37 (2011) 1459–1463.

5691

[43] S.T. Aruna, A.S. Mukasyan, Combustion synthesis and nanomaterials, Curr. Opin. Solid State Mater. Sci. 12 (2008) 44–50. [44] Y. Koseoglu, A simple microwave-assisted combustion synthesis and structural, optical and magnetic characterization of ZnO nanoplatelets, Ceram. Int. (2013). [45] K.D. Bhatte, D.N. Sawant, R.A. Watile, B.M. Bhanage, A rapid, one step microwave assisted synthesis of nanosize zinc oxide, Mater. Lett. 69 (2012) 66–68. [46] S.R. Jain, K.C. Adiga, V.R.P. Verneker, A new approach to thermochemical calculations of condensed fuel–oxidizer mixtures, Combust. Flame 40 (1981) 71–79. [47] J. Zhou, F. Zhao, Y. Wang, Y. Zhang, L. Yang, Size-controlled synthesis of ZnO nanoparticles and their photoluminescence properties, J. Lumin. 122–123 (2007) 195–197. [48] M.N. Tahir, L. Gorgishvili, Jixue Li, T. Gorelik, U. Kolb, L. Nasdala, W. Tremel, Facile synthesis and characterization of monocrystalline cubic ZrO2 nanoparticles, Solid State Sci. 9 (2007) 1105–1109. [49] N.C.S. Selvam, A. Manikandan, L. John Kennedy, J.J. Vijaya, Comparative investigation of zirconium oxide (ZrO2) nano and microstructures for structural, optical and photocatalytic properties, J. Colloid Interface Sci. 389 (2013) 91–98. [50] B.D. Cullity, Elements of X-ray Diffraction, 3rd edition, AddisonWesley, Reading, Mass, USA, 1967. [51] J. Liqiang, Q. Yichun, W. Baiqi, L. Shudan, J. Baojiang, Y. Libin, F. Wei, F. Honggang, S. Jiazhong, Review of photoluminescence performance of nano-sized semiconductor materials and its relationships with photocatalytic activity, Sol. Energy Mater. Sol. Cells 90 (2006) 1773–1787. [52] N.C.S. Selvam, J.J. Vijaya, L.J. Kennedy, Effects of morphology and Zr doping on structural, optical, and photocatalytic properties of ZnO nanostructures, Ind. Eng. Chem. Res. 51 (2012) 16333–16345. [53] G. Sangeetha, S. Rajeshwari, R. Venckatesh, Green synthesis of zinc oxide nanoparticles by Aloe barbadensis miller leaf extract: structure and optical properties, Mater. Res. Bull. 46 (2011) 2560–2566. [54] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Reporting physisorption data for gas/ solid systems, Pure Appl. Chem. 57 (1985) 603–619. [55] Z. Miroslaw, Pd and ZnAl2O4 nanoparticles prepared by microwavesolvothermal method as catalyst precursor, J. Alloys Compd. 439 (2007) 312–320. [56] V.V. Kutarov, E. Robens, Y.I. Tarasevich, E.V. Aksenenko, Adsorption hysteresis in open slit like super micropores, Theor. Exp. Chem. 47 (2011) 163–168. [57] R. Doong, C. Chen, R. Maithreepala, S. Chang, The influence of pH and cadmium sulfide on the photocatalytic degradation of 2-chlorophenol in titanium dioxide suspensions, Water Res. 35 (2001) 2873–2880. [58] Y. Ku, R.M. Leu, K.C. Lee, Decomposition of 2-chlorophenol in aqueous solution by UV irradiation with the presence of titanium dioxide, Water Res. 30 (1996) 2569–2578. [59] E. Leyva, E. Moctezuma, M.G. Ruiz, L. Martinez-Torres, Photodegradation of phenol and 4-chlorophenol by BaO–Li2O–TiO2 catalysts, Catal. Today 40 (1998) 367–376. [60] W.Z. Tang, C.P. Huang, Photocatalyzed oxidation pathways of 2,4dichlorophenol by CdS in basic and acidic aqueous solutions, Water Res. 29 (1995) 745–756. [61] M.P. Titus, V. Garcıa-Molina, M.A. Banos, J. Gimenez, S. Esplugas, Degradation of chlorophenols by means of advanced oxidation processes: a general review, Appl. Catal. B: Environ. 47 (2004) 219–256.