Comparative studies on influence of morphology and La doping on structural, optical, and photocatalytic properties of zinc oxide nanostructures

Journal of Colloid and Interface Science 407 (2013) 215–224

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Comparative studies on influence of morphology and La doping on structural, optical, and photocatalytic properties of zinc oxide nanostructures N. Clament Sagaya Selvam a, J. Judith Vijaya a,⇑, L. John Kennedy b a b

Catalysis and Nanomaterials Research Laboratory, Department of Chemistry, Loyola College, Chennai 600 034, India Materials Division, School of Advanced Sciences, Vellore Institute of Technology (VIT) University, Chennai Campus, Chennai 600 127, India

a r t i c l e

i n f o

Article history: Received 9 April 2013 Accepted 4 June 2013 Available online 18 June 2013 Keywords: La-doped ZnO Photocatalyst Electron microscopy Bisphenol A Photocatalytic degradation

a b s t r a c t A simple, low temperature co-precipitation method was developed to synthesize ZnO nanomaterials with different morphologies such as nanoflakes, spherical nanoparticles (SNPs), and nanorods. The concentration of the capping agent, Triton X-100, is a key factor in the morphological control of ZnO nanostructures. The formation of different morphologies of ZnO was confirmed by HR-SEM and HR-TEM. XRD data showed the formation of single-phase ZnO with the wurtzite crystal structure. The influence of La contents on the structure, morphology, absorption, emission, and photocatalytic activity of ZnO SNPs was investigated systematically. The influence of the ZnO morphologies on the photocatalytic degradation (PCD) of Bisphenol A (BPA) as a model reaction is evaluated and discussed in terms of surface area, crystal growth habits, particle size, and oxygen defects. The results indicated that the particle size is an important factor for the enhancement of PCD. Furthermore, the effect of different photocatalytic reaction parameters on the resulting PCD efficiency of ZnO SNPs was investigated. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction The fabrication of hierarchical inorganic micro- and nanostructures has attracted considerable interest, because of its morphology, size-dependent properties, and applications [1–3]. The structure of nanomaterials, including morphology, particle size, and two-dimensional and three-dimensional architectures, can play the important roles in determining the electrical, optical, and catalytic properties. In this context, semiconductor nanoparticles, group II–VI in particular, have attracted a great deal of attention because of its size-tunable luminescence and photocatalytic properties [4–8]. Generally, oxide nanoparticles can exhibit unique chemical properties due to their limited size and a high density of corner or edge surface sites. When the size of the semiconductor particles becomes smaller, the properties of the semiconductor, such as its optical absorption and luminescence emission, undergo drastic changes [9]. Among the nanostructured semiconductors, zinc oxide (ZnO) is a versatile and interesting semiconductor material to study because it possesses very attractive physical properties such as a wide direct band gap (3.37 eV), a large exciton binding energy of 60 meV at room temperature, and unique electronic, catalytic, optoelectronic, and photocatalytic properties [10–13]. In recent years, a wide variety of synthetic routes have ⇑ Corresponding author. Fax: +91 44 28175566. E-mail address: [email protected] (J. Judith Vijaya). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.06.004

thus been proposed to prepare specific nanostructures of ZnO, including nanorods [14], nanowires [15], nanobelts [16], tetrapods [17], and many other anisotropic prototypes in order to increase the development of ZnO nanostructures. Most of the preparation methods, however, involve complicated synthetic procedures, which may hinder the applicability of the products. Therefore, the development of a simple and environmentally friendly method to prepare ZnO nanostructures with controllable morphology is crucial with respect to their practical applications and has thus become an important topic of investigation. Precipitation approach compared with other traditional methods provides a facile way for low-cost and large-scale production, and it does not need expensive raw materials and complicated equipments [18]. In this work, a series of ZnO nanoparticles with different morphologies were prepared via a simple precipitation route by adjusting the concentration of the capping agent. Among the various applications of ZnO nanostructures, photocatalysis is the most important application for the environmental protection. Although TiO2 is universally recognized as the most photo-active catalyst, ZnO is a suitable alternative to TiO2, as it has similar band gap energy (3.2 eV) and absorption over a larger fraction of the solar spectrum than TiO2 [19]. Moreover, it is well known that ZnO exhibits the richest range of morphologies among the wide band gap semiconductors. Over the past few years, tremendous effort has been made to control the shape of ZnO nanocrystals in order to investigate the effect of morphology-

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dependent photocatalytic activities [20–24]. Metal dopants have also been used to improve the morphology and photocatalytic activity of nano-doped ZnO [25–30]. When La was doped into ZnO, more surface defects are produced, which hindered the recombination of photo-induced electron–hole pairs. This contributed to the improvement of the photocatalytic activity [31,32]. However, the studies concerning the effect of ZnO morphology and La doping on its photocatalytic activity are still of great importance and challenge to explore, as it plays an important role in determining the absorption, emission, and photocatalytic activity. Therefore, we have investigated the relationship between photocatalytic activity, crystallinity, particle size, defects, surface area, and morphology of ZnO particles in detail. In addition, the influence of La contents on the structure, morphology, absorption, emission, and photocatalytic activity of ZnO SNPs was investigated systematically in the present study.

2. Experimental section 2.1. Preparation of photocatalysts All the chemicals were obtained from Merck, India (Analytical grade). The commercially available TiO2 (Degussa P-25) was obtained from Degussa Chemical, Germany. The typical synthesis procedure for pure and La-doped ZnO is as follows: Zinc acetate dihydrate (Zn(Ac)22H2O) and lanthanum(III)nitrate hexahydrate (La(NO3)36H2O) were taken as the precursors of zinc and lanthanum, respectively. Zn(Ac)22H2O and NaHCO3 were dissolved separately in double distilled water to obtain 0.1 mol/L solutions. Zinc acetate solution (250 mL of 0.1 mol/L) was slowly added into vigorously stirred NaHCO3 (250 mL of 0.1 mol/L) and Triton X 100 mixed solution. The Triton X 100 concentrations in the mixed solutions differed within the range of 0–2 mmol/L, which correspond to the concentration for the structural changes in the surfactant from pre-micelle concentration (PMC) to critical micelle concentration (CMC) such as spherical micelle (CMC1) and spherical to rod like micelle concentration (CMC2). Based on this, PMC, CMC1, and CMC2 in Triton X-100 aqueous solution were obtained. The values are, respectively, close to 2.1  104 mol/L, 3.2  104 mol/L, and 1.3  103 mol/L. Lanthanum(III) nitrate in the required stoichiometry was slowly added into the above solution, and a white precipitate was obtained. The precipitate was filtered, repeatedly rinsed with distilled water, and then washed twice with ethanol. The resultant solid product was dried at 70 °C for 2 h and calcined at 200 °C for 3 h. Pure ZnO was also prepared by the same procedure without the addition of lanthanum(III) nitrate solution. The doping concentrations of lanthanum are expressed in wt%.

2.3. Photocatalytic reactor setup and degradation procedure PCD experiments were carried out in a self-designed photocatalytic reactor as shown in Fig. 1. The cylindrical photocatalytic reactor tube was made up of quartz/borosilicate with a dimension of 36–1.6 cm (height-diameter). The top portion of the reactor tube has ports for sampling, gas purging, and gas outlet. The aqueous BPA solution containing appropriate quantity of either pure ZnO or La-doped ZnO 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 either 254 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 BPA solution and fixed weight of pure ZnO or La-doped ZnO photocatalysts. Prior to irradiation, the slurry was aerated for 30 min to reach 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 BPA degradation was monitored by using UV–Visible spectrophotometer (Perkin–Elmer, Lamda 25) and high performance liquid chromatography (HPLC) (Shimadzu LC10 ATVP series equipped with UV– Visible detector). The effect of pH of the solution was studied by adjusting the pH of BPA solution containing the catalyst, using dilute HCl and NaOH (both from Merck, India). The pH of the solution was measured using HANNA Phep (Model H 198107, 0.2–0.5 pH unit accuracy) digital pH meter. The intermediates were identified using gas chromatograph coupled with mass spectrometer (GC– MS) (Perkin–Elmer Clarus 500). The temperature of the column was programmed as follows: initial column temperature was held for 2 min at 70 °C, ramped at 10 °C/min to 280 °C, with final hold for two minutes at 280 °C. The extent of mineralization was determined using a total organic carbon analyzer (TOC) (Shimadzu VCPN). The PCD efficiency (g) was calculated from the following expression

g ¼ Ci  Ct=Ci  100 ðorÞ g ¼ TOCi  TOCt=TOCi  100 where Ci or TOCi is the initial concentration of BPA and Ct or TOCt, concentration of BPA after ‘‘t’’ minutes.

2.2. Characterization of photocatalysts The structural characterization of pure and La-doped ZnO was performed using a Philips X’pert X-ray diffractometer with Cu Ka radiation at k = 1.540 Å. The particle size and morphology of pure and La-doped ZnO samples were observed using high resolution scanning electron microscope (HR-SEM) (Stereoscan LEO 440) and high resolution transmission electron microscope (HR-TEM) (JEOL JEM 3010). The diffuse reflectance UV–Visible spectra of pure and La-doped ZnO samples were recorded using Cary100 UV–Visible spectrophotometer to estimate their energy band gap. The emission spectra of the pure and La-doped ZnO photocatalysts were recorded using Varian Cary Eclipse Fluorescence Spectrophotometer at an excitation wavelength of 372 nm.

Fig. 1. Schematic diagram of the photocatalytic reactor.

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3. Results and discussion 3.1. Size and morphology of ZnO nanostructures HR-SEM observations confirm the morphology of ZnO prepared under different concentrations of capping agent, as presented in Fig. 2a–d. It is obvious that the morphology of ZnO changes with the increase in the concentration of capping agent. When the concentration of capping agent is 2.1  104 mol/L (PMC), a high yield of ZnO nanoflakes is obtained with diameters of 35–40 nm, as shown in Fig. 2a. When the concentration of capping agent is increased to 3.2  104 mol/L (CMC1), the morphology of ZnO changes to spherical shaped particles with the diameters of 10– 12 nm as shown in Fig. 2b. When the concentration of capping agent is further increased to 1.3  103 mol/L (CMC2), the morphology of ZnO becomes nanorods with diameters of 12–15 nm as shown in Fig. 2c. Generally, the concentration where aggregation of surfactant and monomers into micelles occurs is called as the critical micelle concentration (CMC). CMC is a key parameter for the optimization of capping agent in morphology formulations. It has also been reported [33,34] that the concentration of capping agent determines the size and morphology of nanoparticles. Quin et al. reported that [35] when the concentration of a capping agent is increased, its structure may change from single molecules to spherical and rod like as observed in the present study. The chemical growth of nanometer-sized ZnO inevitably involves the process of precipitation. In the present study, precipitation process consists of a nucleation step followed by particle growth step. During the nucleation step, nuclei are formed from the solution and they grow via molecular addition, which also depends on micelles. The confinement of growing ZnO in the spherical micelles leads to more uniform growth with smaller size distribution (around 10– 12 nm). It is well known that the spherical micelles structure can

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change to rod structure by aggregation of spherical micelles when the concentration of a capping agent is increased. Therefore, the growth of ZnO in the rod like micelles leads to the formation of rod like structure with broad size distribution (diameter of 12– 15 nm and length around 100 nm). The decrease in the particle size to the nanometer length scale increases the surface-to-volume ratio. This makes them potentially useful in the field of photocatalysis. In the present study, the concentration of capping agent has been varied, and the reaction conditions (concentration of reactants, temperature, and time) were kept as the same for all the reactions. Therefore, it is concluded that three concentrations of capping agent (Triton X-100), which represent the pre-micelle, spherical micelle, and rod like micelle, respectively, are responsible for the formation of flakes, spherical, and rod like morphology. In addition, the La doping on ZnO SNPs results in the formation of elongated ZnO SNPs as shown in Fig. 2d, because the presence of small amounts of La ions on ZnO SNPs alters the growth rate and resulted in the elongated SNPs. To provide further evidence in the formation of ZnO nanostructures with different morphologies, HR-TEM analysis was carried out. A HR-TEM image of typical ZnO nanoflakes is presented in Fig. 3a, indicating that the nanoflakes are self-assembled. The inset of Fig. 3a shows the corresponding selected area electron diffraction (SAED) pattern. The pattern implies that the ZnO nanoflakes are good single crystalline material. A HR-TEM image of ZnO nanorods is presented in Fig. 3c, indicating that the nanorods are selfaggregated. The corresponding SAED pattern is shown as the inset in Fig. 3c, indicating the single crystalline nature of ZnO nanorod. The collective behavior of van der Waals forces and electrostatic interactions would have favored the self-aggregation/self-assembly of the ZnO nanoflakes and nanorods as observed from the electron microscopy studies [36]. A HR-TEM image of ZnO SNPs is

Fig. 2. HR-SEM images of the as-synthesized ZnO nanostructures in different capping agent (Triton X-100) concentrations: (a) ZnO nanoflakes (PMC – 2.1  104 mol/L), (b) ZnO SNPs (CMC1-3.2  104 mol/L), (c) ZnO nanorods (CMC2 – 1.3  103 mol/L), and (d) 1.5 wt% La-doped ZnO SNPs (CMC1 – 3.2  104 mol/L).

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Fig. 3. HR-TEM images and corresponding SAED pattern of the as-synthesized ZnO nanostructures (a) ZnO nanoflakes (b) ZnO SNPs, (c) ZnO nanorods, and (d) 1.5 wt% Ladoped ZnO SNPs.

presented in Fig. 3b, indicating that the SNPs are self-assembled without agglomeration. Due to the smaller dimension of the spherical shaped granular particles, the polar fields generated in each particle were weaker [37]. Consequently, a less pronounced tendency of agglomeration between single particles was expected, leading to an un-agglomerated assembly of ZnO nanoparticles formation as shown in HR-TEM images. The inset of Fig. 3b shows corresponding SAED pattern. The pattern implies that the ZnO SNPs is good single crystalline material. In addition, the La doping on ZnO SNPs results in the formation of elongated nanoparticles as shown in Fig. 3d. The inset of Fig. 3d shows the corresponding SAED pattern. The pattern shows that the La-doped ZnO spherical nanoparticles are single crystalline in nature.

3.2. Crystal structure of ZnO nanostructures The XRD patterns of the ZnO nanostructures with different morphologies are shown in Fig. 4a. The entire diffraction peaks match with the standard data for a hexagonal ZnO wurtzite structure (JCPDS 36-1451), and no characteristic peaks of any other impurities are detected in the patterns, which indicates that all the samples have high phase purity. The XRD pattern of ZnO SNPs clearly shows that the slight shifting of diffraction peaks indicates an expansion of unit cell as a result of size effect. In addition, the peak width broadens, due to the smaller particle size distribution. The XRD pattern of the La-doped ZnO SNPs (Fig. 4b) clearly shows the shifting of diffraction peaks, slightly toward lower angle, on La doping in comparison with that of pure ZnO SNPs. Furthermore, the intensity of the diffraction peaks decreases, and the width broadens due to the formation of smaller average diameters of ZnO SNPs as a result of La doping. The shifting and broadening of XRD lines with doping strongly suggest that La3+ ions were successfully incorporated into the ZnO host structure at the Zn2+ site. The crystal size and unit cell parameters are given in Table 1. It is

clearly seen from Table 1 that the lattice constants of ZnO SNPs were found to be slightly larger than those of other ZnO nanostructures, due to the crystal size effect. The lattice constants of Ladoped ZnO samples were also found to be slightly larger than those of pure ZnO SNPs. This is consistent with the fact that an ionic radius of La3+ is larger (1.06 Å) than that of Zn2+ (0.74 Å) as reported in the literature for La-doped ZnO [38,39].

3.3. UV–Visible absorption and photoluminescence spectroscopy We have already investigated [40] the UV–Visible absorption and PL measurements to verify how the morphology of ZnO affects their absorption and emission properties. This relationship has already been investigated for a number of ZnO nanostructures in order to understand whether the substantially increased surfaceto-volume ratio in a nanostructure leads to significant rearrangement of absorption and PL emission characteristics [41,42]. For the ZnO samples taken for this study, influence of La doping on absorption and emission properties of ZnO was illustrated in Figs. 5 and 6. Fig. 5 shows diffuse reflectance spectra (DRS) of the pure and La-doped ZnO SNPs. Pure ZnO SNPs exhibit a sharp absorption edge at about 372 nm. Inset of Fig. 5 shows diffuse reflectance spectra of La-doped ZnO SNPs. The figure indicates that the maximum absorbance band shifts toward lower wavelength by increasing the La loading. Consequently, the band gap of La-doped ZnO increases gradually with increase in the La loading and is much higher as compared to that of pure ZnO SNPs as shown in Table 1. This is mainly attributed to the reduction in size of ZnO SNPs (Quantum confinement effect) as La contents increase. Thus, the use of sizequantized doped semiconductor particles may result in increased photocatalytic activity. It is interesting to investigate the PL spectra of the as-prepared pure and La-doped ZnO nanostructures as shown in Fig. 6. The

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1.5 wt% La-ZnO 360

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30

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Fig. 5. UV–Visible absorption spectra of pure and La-doped ZnO SNPs.

Intensity (a.u.)

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(103) (112) (200)

Therefore, the band gap and PL of ZnO SNPs can be manipulated by controlling their morphologies and doping with metal ions and thus indicate the capabilities of these nanostructures for diverse applications in various fields.

1.5 wt% La-ZnO

3.4. Influence of the morphologies on the PCD performance of the ZnO nanostructures

1 wt% La-ZnO 0.5 wt% La-ZnO

Pure ZnO

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2 Theta (degree) Fig. 4. X-ray diffraction patterns of ZnO: (a) Different morphologies of pure ZnO and (b) pure and La-doped ZnO SNPs.

strong UV band gap emission (375–395 nm) results from the radiative recombination of an excited electron in the conduction band with the valence band hole. The broad visible or deep-trap state emissions (410–440 nm and 540–580 nm) are commonly defined as the recombination of the electron-hole pair from localized states with energy levels deep in the band gap, resulting in lower energy emission. These deep-trap emissions indicate the presence of defects or oxygen vacancies of ZnO nanostructures [43]. There are reports [44,45] stating that smaller sized ZnO nanostructures might favor a high-level surface defects, which account for the increase in the defect emission relative to the UV emission as seen in the present case of ZnO SNPs. All the emission bands were slightly blueshifted and broadened with higher PL intensity due to the increase in La content. This can be attributed to the increased density of surface defect states, because of the presence of dopants.

The influence of ZnO morphologies on the PCD efficiency is shown in Fig. 7. It has been reported that the morphologies of ZnO catalysts play an important role in the photocatalytic activity [20–24]. However, the studies concerning the various factors of ZnO morphology on PCD efficiency are still of great importance. Therefore, the influence of the ZnO morphologies on the photocatalytic degradation (PCD) of BPA as a model reaction is evaluated and discussed below in terms of surface area, crystal growth habits, particle size, and oxygen defects in the present study. 3.4.1. Effect of surface area on the photocatalytic activity Generally, a high specific surface area has a beneficial effect on the activity for catalysts. In this work, the surface areas of ZnO with spherical, rods, and flakes shaped morphologies are 45.64, 39.26, and 32.16 m2/g, respectively. However, the photocatalytic performance followed the same order of spherical shaped morphology > rods like morphology > flakes like morphology as shown in Fig. 7. High specific surface area of ZnO was beneficial to photocatalytic activity via enhancing the adsorption of BPA, which is the determining step in the heterogeneous photocatalytic reaction. 3.4.2. Effects of the crystal habits on the photocatalytic performance Generally, catalysts with higher surface energy show better catalytic performance [46]. Non-faceted particles have higher surface

Table 1 Physical characteristics of pure ZnO and La-doped ZnO SNPs. Catalysts

Crystal size (nm)

Lattice parameter (a) (nm)

Lattice parameter (l) (nm)

kmax (nm)

Band gap (eV)

ZnO Nanoflakes ZnO nanorods ZnO SNPs 0.5 wt% La-ZnO 1.0 wt% La-ZnO 1.5 wt% La-ZnO 2.0 wt% La-ZnO

36.5 36.5 26.5 18.0 14.5 11.0 10.5

0.3241 0.3246 0.3251 0.3300 0.3311 0.3316 0.3319

0.5201 0.5206 0.5219 0.5222 0.5228 0.5236 0.5241

372 370 367 365 364 362 362

3.33 3.35 3.37 3.39 3.40 3.42 3.42

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Intensity (a.u.)

100

Intensity (a.u.)

1.5 wt% La-ZnO 1.0 wt% La-ZnO

80

370

380

390

PCD efficiency (%)

2 wt% La-ZnO

400

Wavelength (nm)

0.5 wt% La-ZnO Pure ZnO

60

50 mg/L 100 mg/L 150 mg/L 200 mg/L 250 mg/L

40

20

0 360

390

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510

540

570

600

630

660

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energies than the faceted ones [47]. In the present work, spherical shaped ZnO morphology showed the best photocatalytic activity among all of the ZnO nanostructures synthesized, presumably because of the formation of non-faceted morphologies with higher surface energies. The rods like morphology showed better photocatalytic activity than flakes like morphology. The reason is that the Zn-terminated (0 0 1) and O-terminated (0 0 1) polar faces are facile to adsorb oxygen molecules and OH ions, resulting in the greater production rate of H2O2 and OH radicals, and hence enhancing the PCD efficiency. In addition, the surface energies (E) of the facets in ZnO crystals follow the sequence  > E(1 0 1  0) > E(1 0 1  1)  > E(0 0 0 1).  E(0 0 0 1) > E(1 0 1 1) Therefore, the rod like morphology having 0 0 0 1 plane with high surface energy showed better photocatalytic activity than that of flakes morphology. ZnO with flakes like morphology showed the least catalytic activity. This is due to the formation of nanoparticles with smooth (1 1 0 1) and (1 0 1 0) facets, which did not have higher surface energy, resulting in poor activity.

PCD efficiency (%)

3.4.3. Effect of particle sizes on the photocatalytic activity The particle sizes of ZnO catalysts play an important role in the photocatalytic activity. Moreover, as the particle size decreases, the number of active surface sites increases. Thus, it is expected that ZnO SNPs with very smaller particle size distribution would be a

ZnO SNPs ZnO Nanorods ZnO Nanoflakes

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Fig. 6. Room temperature PL spectra of pure and Zr doped ZnO SNPs.

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Time (min) Fig. 7. Influence of morphologies on the PCD (experimental conditions: photocatalyst = 30 mg/100 mL, BPA = 200 mg/L, pH of suspension = 6–6.5 (natural pH), k = 365 nm).

Fig. 8. Effect of concentration of BPA on PCD efficiency (experimental conditions: pure ZnO SNPs = 30 mg/100 mL, initial pH of suspension = 6–6.5 (natural pH), k = 365 nm).

potentially efficient photocatalyst. Moreover, among the ZnO nanostructures with different morphologies synthesized in this study, the ZnO with spherical morphology had smaller particle size distribution, as indicated by the XRD, HR-SEM, and HR-TEM data, which showed considerably higher photocatalytic activity for the degradation of BPA than other ZnO morphologies as shown in Fig. 7. Because of its homogeneous and small particle size, the charge carrier recombination rate decreases, and this counteracts the increased photocatalytic activity. In addition, when the size of ZnO particles decreases, the amount of the dispersion of particles per volume in the solution will increase, resulting the enhancement of the photon absorbance. Therefore, the photocatalytic activity of ZnO SNPs is high, and the photocatalytic efficiency of different ZnO morphologies is in the order of spherical shaped morphology > Nanorods like morphology > Nanoflakes like morphology. 3.4.4. Effects of oxygen vacancies on the photocatalytic activity Photocatalytic activity is highly related to the concentration of defects on the surface of the nanomaterials. Influences of defects have been proposed in the literature for the enhanced photocatalytic activity of ZnO [48]. Usually, higher photocatalytic activity of ZnO nanostructures has been attributed to the high concentration of surface donor defects (oxygen vacancies and zinc interstitials) [49,50]. In this case, the higher activity in the presence of more surface defects (oxygen vacancies) was attributed to the lower recombination between photo-generated electrons and holes with oxygen vacancies serving as electron traps [51]. All the ZnO nanostructures synthesized in this study had higher oxygen vacancies. However, ZnO with spherical shaped morphology showed best activity than the other ZnO morphologies as shown in Fig. 7. The reason is that the ZnO spherical morphology with smaller particle size distribution has a high-level surface defects, attributed to the increased concentration of both electron traps (oxygen vacancies) and hole traps (oxygen interstitials) as indicated by the photoluminescence data. Thus, ZnO SNPs were selected as best catalyst for the study of other parameters. 3.5. Effect of reaction parameters on the photocatalytic degradation of BPA Photocatalytic oxidation is a promising alternative technique to the conventional methods for the complete mineralization of BPA.

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radicals available for attacking BPA molecules becomes less, and consequently, PCD efficiency decreases. Also, as the concentration of BPA increases, the photons get interrupted before they can reach the photocatalyst surface; hence, absorption of photons by the photocatalyst decreases, and consequently, the PCD efficiency reduces [21]. Hence, under the given set of conditions, the maximum concentration of BPA that could be degraded by 30 mg/L of ZnO SNPs is found to be 200 mg/L. Thus, 200 mg/L BPA was selected as optimum concentration for the study of other parameters.

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10 mg/100 mL 30 mg/100 mL 50 mg/100 mL 70 mg/100 mL

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3.5.2. Effect of the catalyst dosage Blank experiments were carried out without photocatalyst to examine the extent of degradation (Photolysis). There was no evidence of PCD of BPA in aqueous solution in the absence of ZnO SNPs. When aqueous solution of BPA containing ZnO SNPs was irradiated with UV light, PCD of BPA was observed. The PCD of BPA was found to increase with increase in the amount of ZnO SNPs up to 30 mg/100 mL, and further increase in photocatalyst amount showed negative effect as illustrated in Fig. 9. The reason for this is that the increase in the amount of catalyst increases the number of active sites on the photocatalyst surface, which in turn increase the number of hydroxyl and superoxide radicals to degrade BPA. When the concentration of the catalyst increases above the optimum value, the degradation decreases due to the interception of the light by the suspension. Sun et al. [52] reported that as the excess catalyst (turbidity) prevent the illumination of light, OH radical, a primary oxidant in the photocatalytic system decreased and the efficiency of the degradation reduced accordingly. Furthermore, the increase in catalyst concentration beyond the optimum may result in the agglomeration of catalyst particles; hence, the part of the catalyst surface becomes unavailable for photon absorption, and thereby, PCD efficiency decreases [53].

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Time (min) Fig. 9. Effect of the amount of photocatalyst on PCD efficiency (experimental conditions: photocatalyst = pure ZnO SNPs/100 mL, BPA = 200 mg/L, pH of suspension = 6–6.5 (natural pH), k = 365 nm).

The effect of various operating parameters such as initial concentration of BPA, catalyst loading, pH, and light wavelength on the degradation of BPA was investigated. 3.5.1. Effect of the initial concentration of BPA The PCD of BPA at different initial concentrations in the range of 50–250 mg/L was investigated as a function of UV light irradiation time at the natural pH of suspension (6.2, without adjustment). The results are illustrated in Fig. 8. It was found that the PCD efficiency of BPA was strongly depended on the initial concentration of BPA. The PCD efficiency of BPA was decreased from 99.5% to 82.5% with the increase in the initial BPA concentration from 50 to 250 mg/L after 210 min. This may be due to the fact that as the initial concentration of BPA increases, more and more BPA molecules are adsorbed on the surface of ZnO SNPs, but the number of  OH and  O 2 radicals formed on the surface of ZnO and the irradiation time is constant. Therefore, relative number of OH and  O 2

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90 80

3.5.3. Effect of initial pH The pH of the suspension has strong effect on the PCD process as shown in Fig. 10. The role of pH on the PCD is studied in the pH range limited to 2–11 with optimized experimental conditions (200 mg/L BPA solution and 30 mg/100 mL ZnO SNPs loading) by

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pH Fig. 10. Effect of pH on PCD efficiency (experimental conditions: BPA = 200 mg/L, pure ZnO SNPs = 30 mg/100 mL, k = 365 nm).

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9.0 ± 0.3 [54]. Below pH 9, active sites on the positively charged catalyst surface are preferentially covered by BPA molecules. Thus, surface concentration of the BPA is relatively high, while those of OH and hydroxyl radical (OH) are low. Hence, PCD decreases at acidic pH. On the other hand, above pH 9, catalyst surface is negatively charged by means of metal-bound OH, consequently the surface concentration of the BPA is low, and that of hydroxyl radical is high. In addition, BPA is not protonated above pH 9, the electrostatic repulsion between the surface charges on the adsorbent and the adsorbate hinders the amount of BPA adsorption, consequently surface concentration of the BPA decreases, which results in the decrease of PCD at pH 11. In conclusion, pH 8 can provide moderate surface concentration of BPA and hydroxyl ions (OH), which react with the holes to form hydroxyl radicals (OH), thereby enhancing the PCD of BPA [55].

PCD efficiency (%)

100

80

60 Pure ZnO

40

0.5 wt% La-ZnO 1

wt% La-ZnO

1.5 wt% La-ZnO

20

2

wt% La-ZnO

TiO 2 (Degussa P-25)

0 0

30

60

90

120

150

180

210

240

Time (min) Fig. 11. Effect of La doping on the PCD efficiency (experimental conditions: BPA = 200 mg/L, photocatalyst = 30 mg/100 mL, k = 365 nm. pH of suspension = 8).

considering the solubility of ZnO SNPs in acidic as well as in highly basic solutions. The pH of the suspension is adjusted initially, and it is not controlled during the course of the reaction. In acidic medium, less PCD of BPA was observed. The extent of PCD of BPA was found to increase with increase in initial pH of suspension exhibiting maximum PCD at pH 8 and decrease at pH 11 (Fig. 10). The possible explanation is that the pH at zero point charge (zpc) of ZnO is

3.0

3.0

Commercial ZnO 0 min 30 min 60 min 90 min 120 min 150 min 180 min 210 min 240 min 270 min

2.0 1.5 1.0

Commercial TiO 2

2.5

0 min 30 min 60 min 90 min 120 min 150 min 180 min 210 min 240 min 270 min

2.0

Absorbance

2.5

Absorbance

3.5.4. Effects of La doping on the photocatalytic activity The influence of La doping on ZnO SNPs on the PCD efficiency is evaluated as shown in Fig. 11. The PCD efficiency of commercial TiO2 (Degussa P-25) is also evaluated for the purpose of comparison as shown in Fig. 11. The PCD efficiency of ZnO SNPs increases with an increase in the La loading and shows a maximum activity at 1.5 wt% and then decreases on further La doping to 2 wt%. The reason can be explained as follows: As demonstrated by HRTEM and XRD, the synthesized La-doped ZnO catalyst possesses smaller particle size distribution than pure ZnO SNPs. Apart from their small size, as La3+ was doped in ZnO, more surface defects are produced as demonstrated in PL spectra,

0.5

1.5 1.0 0.5

0.0

0.0 210

240

270

300

330

360

210

Wavelength (nm)

240

270

300

330

360

Wavelength (nm) 3.5

3.0

Pure ZnO 0 min 30 min 60 min 90 min 120 min 150 min 180 min 210 min 240 min 270 min

2.0 1.5 1.0 0.5

0 min 30 min 60 min 90 min 120 min 150 min 180 min 210 min 240 min 270 min

2.5

Absorbance

Absorbance

2.5

1.5 wt% La-ZnO

3.0

2.0 1.5 1.0 0.5 0.0

0.0 210

240

270

300

Wavelength (nm)

330

360

210

240

270

300

330

360

Wavelength (nm)

Fig. 12. UV-Visible absorption spectra of PCD of BPA (experimental conditions: k = 365 nm, BPA = 200 mg/L, photocatalyst = 30 mg/100 mL, pH of suspension = 8).

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1.0

HO

C

OH

CH3

(TOCt)/(TOC0 )

0.8

H2O, hυ

ZnO / La - ZnO

OH

0.6 OH

CH3

CH3

0.4 HO

C

OH

CH3

0.2

365 nm 254 nm

0.0

HO

C

HO

CH3

OH OH

OH

Aromatic intermediates 0

30

60

90

120

150

180

210

240

270

300

OH

Time (min) Fig. 13. Comparison of photocatalytic mineralization of BPA (experimental conditions: BPA = 200 mg/L, 1.5 wt% La-ZnO SNPs = 30 mg/100 mL, pH of suspension = 8).

Ring opening reaction

Aliphatic acids

OH

and a space charge layer could be formed on the surface. Consequently, the migration of the photo-induced electrons and holes toward surface defects is reasonable [48]. Thus, the separation efficiency of the electron–hole pairs of La-doped ZnO with more oxygen defects should be more than that of the pure ZnO SNPs. Therefore, the enhancement in the PCD efficiency of ZnO as La doping increases was attributed to the small particle size and higher defect concentration. However, excessive amounts of dopants can retard the photocatalysis process, because excess amount of dopants deposited on the surface of ZnO increases the recombination rate of free electrons and energized holes, thus inhibiting the photodegradation process. Hence, further increase in La doping to 2 wt% results in the decrease of PCD efficiency. All the experiments were carried out following the batch-wise procedure, and the products were analyzed using UV–Visible spectrophotometer to evaluate the effect of doping on PCD. The UV–Visible spectra of PCD of BPA using commercial TiO2 and ZnO, pure ZnO and 1.5 wt% La-doped ZnO are shown in Fig. 12. The results show that 1.5 wt% La-doped ZnO has maximum activity as compare to other photocatalysts. 3.5.5. Photocatalytic mineralization of BPA The extent of photocatalytic mineralization of BPA at optimized conditions (BPA = 200 mg/L, photocatalyst = 30 mg/100 mL, pH = 8, k = 365 nm) over pure and La-doped ZnO under the light of wavelengths at 254 and 365 nm is shown in Fig. 13. The mineralization rate at 365 nm is considerably higher than at 254 nm. Moreover, one can clearly see in Fig. 11 that BPA degrades into small fragments as irradiation time increases, and the complete mineralization of BPA is achieved in 240 min at 365 nm. On the other hand, the complete mineralization is not achieved even after 280 min at 254 nm (Fig. 13). Since the band gap excitation of electrons in ZnO or La-doped ZnO with 254 nm can promote electrons to the conduction band with high kinetic energy, they can reach the solid-liquid interface easily, suppressing electron–hole recombination in comparison with 365 nm. Hence, the observation of low rate at 254 nm is therefore unexpected. This can be accounted by considering partial absorption and wasting of light of 254 nm by BPA itself. Generally, the pollutant must have negligible absorption close to the wavelength of irradiation source. Hence, the entire light of irradiation at 254 nm in the reactor is not used for the excitation of ZnO particles. Hence, low absorption and wasting of light at 254 nm by BPA might be the actual cause for less rate of degradation as reported earlier [56]. On the other hand, the complete mineralization of BPA is achieved in 150 min at 365 nm as shown in Fig. 13.

CO2 , H2O Scheme 1. Photocatalytic degradation pathway of Bisphenol A.

It is observed experimentally that the brown color formed during the reaction became faint and finally turned into colorless after 180 min at 365 nm. This is because BPA does not absorb light at wavelength 365 nm to a significant extent. The complete light absorption by the La-doped ZnO semiconductor results in the generation of more number of hydroxyl radicals and superoxide free radicals. The photodegradation pathway of BPA has been investigated by many researchers [57,58]. Generally, BPA photocatalytic degradation was occurred by hydroxyl radicals attack at the phenyl group of BPA. Therefore, in the present study, the PCD of BBA is believed to be initiated through attacks by hydroxyl radicals at the phenyl group of BPA, which may result in the formation of mono hydroxylated or dihydroxylated BPA. This is followed by the cleavage of the two phenyl groups into intermediates. Finally, the mineralization to CO2 would have occurred via oxidative processes involving the cleavage of intermediates to aliphatic acids as shown in Scheme 1. 4. Conclusion In summary, high-quality self-assembled ZnO and La-doped ZnO nano-spherical particles have been synthesized by a facile, low-cost, co-precipitation approach. It is found that the structural, optical, and PCD properties are sensitively dependent on the incorporation of La3+ ions in the Zn2+ lattice site. ZnO morphology with spherical shaped nanocrystals showed enhanced photocatalytic activity than the other ZnO morphologies due to the small crystal size distribution, high surface area, and more oxygen vacancies. However, 1.5 wt% La-doped ZnO shows superior performance toward degradation and mineralization of BPA than the other doped ZnO, pure ZnO, and commercial TiO2 (Degussa P-25) at 365 nm than 254 nm. It has been concluded that novel self-assembled morphology, smaller particle size, high crystallinity, and surface defects ZnO upon La loading have a significant influence on the enhanced photocatalytic activity of La-doped ZnO catalysts. Acknowledgment The authors duly acknowledge the financial support rendered by University Grants Commission (UGC) (Ref. F. No. 38-118/2009 (SR)), New Delhi.

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