Superlattices and Microstructures 51 (2012) 512–522

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Effects of morphology on photocatalytic performance of Zinc oxide nanostructures synthesized by rapid microwave irradiation methods Amir Kajbafvala a,b,⇑, Hamed Ghorbani b, Asieh Paravar b, Joshua P. Samberg a, Ehsan Kajbafvala b, S.K. Sadrnezhaad b a b

Department of Materials Science and Engineering, North Carolina State University, 911 Partners Way, Raleigh, NC 27695-7907, USA Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave., P.O. Box 11365-9466, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 16 November 2011 Received in revised form 20 January 2012 Accepted 26 January 2012 Available online 3 February 2012 Keywords: Zinc oxide nanostructures Microwave synthesis Photocatalytic performance Methylene blue (MB)

a b s t r a c t In this study, two different chemical solution methods were used to synthesize Zinc oxide nanostructures via a simple and fast microwave assisted method. Afterwards, the photocatalytic performances of the produced ZnO powders were investigated using methylene blue (MB) photodegradation with UV lamp irradiation. The obtained ZnO nanostructures showed spherical and flower-like morphologies. The average crystallite size of the flower-like and spherical nanostructures were determined to be about 55 nm and 28 nm, respectively. X-ray diffraction (XRD), scanning electronic microscopy (SEM), Brunauer–Emmett–Teller (BET), room temperature photoluminescence (RT-PL) and UV–vis analysis were used for characterization of the synthesized ZnO powders. Using BET N2adsorption technique, the specific surface area of the flower-like and spherical ZnO nanostructures were found to be 22.9 m2/gr and 98 m2/gr, respectively. Both morphologies show similar band gap values. Finally, our results depict that the efficiency of photocatalytic performance in the Zinc oxide nanostructures with spherical morphology is greater than that found in the flower-like Zinc oxide nanostructures as well as bulk ZnO. Ó 2012 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Department of Materials Science and Engineering, North Carolina State University, 911 Partners Way, Raleigh, NC 27695-7907, USA. Tel.: +1 919 917 6061. E-mail addresses: [email protected], [email protected] (A. Kajbafvala). 0749-6036/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2012.01.015

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1. Introduction At present, environmental pollution has become a major threat to the lives of humans. Controlling environmental pollution will be imperative due to urban development, population growth and expanding industries. Pollutants, especially from polluted air and industrial effluents, pose a severe ecological problem as the bio-degradation of these pollutants is generally very slow and conventional treatments are mostly ineffective and not environmentally benign. Generally, there are various processes for the purification of pollutants in both water and air. One such process for treatment of these pollutants is advanced oxidation. Producing highly active species such as hydroxyl radicals are the basis of this process. Among the advanced oxidation processes, heterogeneous photocatalysis is used as a successful method for analysis of organic pollutants. Heterogeneous photocatalysis refers to cases where the photosensitizer is a semiconductor [1–4]. Photocatalytic purification by semiconducting oxides of organic pollutants from industrial wastewater shows great potential for environmental remediation due to their peculiar and fascinating physicochemical properties [5–7], allowing the ‘‘green’’ mineralization of organic pollutants. By considering that under light irradiation, photocatalytic reactions mainly occur on the surface of the catalyst, for instance, a nanosize ZnO material is believed to perform much better than its bulk counterpart in photolysis processes due to a higher surface-to-volume (S/V) ratio. Moreover, when the size of the catalyst reaches the nanoscale, the probability of recombination of photo-generated electron–hole pairs diminishes owing to their fast arrival at reaction sites on the surface [8]. Therefore, much effort has been devoted in order to find suitable ways for controlled synthesis of various ZnO nanostructures with high S/V ratio [9–16]. Among various oxide semiconductor photocatalytic materials, ZnO occupies a special place due to various properties which include its photosensitive nature, non-toxicity and wide band gap. In addition to having a wide band gap (3.37 eV), ZnO also has a high activation energy [17–19]. This material has several applications in various fields such as electronics [20], catalysts [21] and optical devices [22,23]. Photocatalytic properties, oxidation and the removal of pollutants are some of the properties, which make ZnO desirable. When a photon with energy higher or equal to the band gap of ZnO is irradiated on the particle, valance band electrons are excited into the conduction band, creating electron– hole pairs (EHP). When these EHPs combine, energy is released as heat or can react with an absorbed electron donor on the surface of the Zinc oxide. ZnO has a hexagonal closed pack structure where zinc atoms occupy half of the tetrahedral sites and all of the octahedral sites are empty. This yields a lot of interstitial spaces where other atoms or defects (such as VO, VZn, ZnO and OZn) may exist. Therefore, ZnO typically has a significant amount of chemical defects [24–29]. These defects cause the formation of sub-bands and assist the photocatalytic property of ZnO. There are several oxidation and reduction reactions that cause the degradation of pollutants and convert them to non-toxic compounds [1,2,30,31]. Different methods have been used to synthesize variety of ZnO nanostructures in the literature [4– 37]. Here, we report the synthesis of two ZnO nanostructures (with flower-like and spherical morphologies) by a simple, fast and low cost microwave irradiation method. Microwaves are electromagnetic waves containing electric and magnetic fields propagating in the same direction perpendicular to one another. Microwave synthesis has many advantages such as fast crystallization, cost efficiency and low waste production. After interacting with matter microwaves can be reflected, passed or absorbed by the material. Polar molecules have molecular dipole moments which interact with the high frequency electromagnetic radiation. This interaction causes the molecules to vibrate and rotate which, in turn, causes the polar solution to heat [32–37]. In this study, polar molecules like water and NH4OH were used in the synthesis of a flower-like zinc oxide, while methanol was used in synthesis of a spherical. During microwave irradiation, interaction of molecular dipole moments with the high frequency electromagnetic radiation will create heating. As-synthesized ZnO nanostructures were characterized in details in terms of their morphological, structural and photocatalytic properties by X-ray diffraction, SEM, BET, UV–visible and photoluminescence measurements. Finally, the efficiency of the morphological effect on the photocatalytic performance of synthesized ZnO nanostructures was compared.

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2. Experimental procedure In this present study, two ZnO nanostructures, sample I and II, have been synthesized using different chemical methods. All of the raw materials were purchased from MERCK Chemical Co. Ltd. (Darmstadt, Germany) and were of analytical grade. Furthermore, a domestic microwave oven was used (Samsung, CE285, 900 W) as the heating source. During the fabrication process for sample I, synthesis was performed using zinc acetate dehydrate ((CH3COO)2 Zn 2H2O), ammonium hydroxide (NH4OH), and deionized water. First, 25 ml of ammonium hydroxide was dissolved in 75 ml of deionized water while vigorous stirring at room temperature. The pH of this solution was measured to be 10.6. Then, zinc acetate dehydrate was added gradually to the above solution under vigorous stirring at room temperature until the final pH was adjusted to 8.0. The prepared solution was colorless in this step. This solution was then heated by a domestic microwave oven for 180 s. A white precipitate was formed and separated by filtration. The obtained white jell, was washed with deionized water and acetone several times to remove any impurities. Finally, it was dried in an oven at 60 °C for 24 h to obtain a powder. Sample II was synthesized using zinc acetate dehydrate ((CH3COO)2 Zn 2H2O), methanol (CH3OH), triethanolamine (N(CH2CH2OH)3) and sodium hydroxide (NaOH) as initial ingredients. Ten milliliters triethanolamine was dissolved in 90 ml methanol while vigorous stirring at room temperature. The pH of this solution was measured to be 10.5. Next, Zinc acetate dihydrate was slowly added while vigorously stirring until the pH of the solution reached 8.0. Then NaOH pellets were slowly added into the above solution to adjust the final pH to 11.0. After this step, a white and dense solution was formed. This solution was heated by a microwave oven for 120 s. After heating, a white dense jell was formed and separated by centrifugation. Again, this white jell was washed with deionized water and acetone several times to remove any impurities. The obtained white jell was dried in an oven at 60 °C for 24 h to obtain powder. 2.1. Characterization After preparation; structural, chemical and phtocatalytic properties of the ZnO powders, were investigated. Same studies were performed for bulk ZnO for comparison. Morphologies of these synthesized powders were studied by scanning electron microscopy (SEM, Philips XL30). X-ray diffraction (XRD) analysis (JEOL, JDX-8030) with Cu-Ka radiation (k = 1.54178 Å) was used to examine the crystalline structure of the products. UV–vis spectrophotometer was used to measure the band gap of the prepared samples. The UV–vis absorption spectra were recorded on a Nexus 670 spectrophotometer (Nicolet Co.). To determine the specific surface area of the powders the Brunauer–Emmett–Teller (BET) method was carried out using nitrogen gas adsorption at 196 °C while measuring the degassing of the samples at room temperature. The surface area was measured by an automatic analyzer (Tristar 3000, Micrometrics). A desorption isotherm was used to determine the pore size distribution by the Barret–Joyner–Halender (BJH) method, assuming a cylindrical pore model [38]. The nitrogen adsorption volume at the relative pressure (P/P0) of 0.8 was used to determine the pore volume and average pore size. Optical properties were studied by use of room temperature photoluminescence (RT-PL, Cecil, CE7200) measurements. The room temperature photoluminescence spectrum was measured using a He–Cd laser with the wavelength of 325 nm as the excitation source. Lastly, photocatalytic activities of the powders were studied by measuring the amount of methylene blue (MB) photodegradation with irradiation by UVA lamp into a Pyrex (World Kitchen, inCommand Technologies, Inc.) photoreactor. 2.2. Evaluation of the photocatalytic properties Photocatalysis properties of the obtained powders were investigated by photodegradation of methylene blue (MB), which is an aromatic compound (C16H18N3ClS) [39,40]. For comparison, the same experiments were performed for bulk ZnO. A solution of water and MB is blue, and as photodegradation proceeds, this will slowly become colorless. Fig. 1 illustrates a schematic of our experimental setup for measuring photocatalytic behaviors of synthesized ZnO nanostructures. Tests were performed

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Power

UV Lamp Orifice plate Quartz sleeve ZnO+water+ MB Air

Agitator Fig. 1. Schematic diagram of the photoreactor setup we were used in this study.

in a 2 L cylindrical Pyrex container. A solution containing 10 ppm MB and 100 ppm of ZnO was mixed and poured into an ultrasonic bath and kept for 15 min, before transfer to the photoreactor. Irradiation was carried out with a 8 W (UVA, k = 253.7 nm) mercury lamp. During irradiation, vigorous agitation was maintained by a magnetic stirrer and air blower to keep the suspension homogenous. The concentration of the MB in each samples was determined with a spectrophotometer (UVA/VisBeckman DU series-500) at kmax = 665 nm and a calibration curve. The percent of photo-degradation (R) was calculated by using Bearlamber law;

%R ¼ ½1  C 2 =C 1  100

ð1Þ

where C2 is concentration of MB at time t and C1 is the initial concentration of MB, In addition;

Lnð½MB=½MB0  ¼ K obs

ð2Þ

was used where Kobs is the kinetic rate constant [2,3]. 3. Results and discussions Fig. 2 shows the XRD patterns of both prepared powders. All of the diffraction peaks are well matched with the standard hexagonal structure of ZnO (JCPDS card No. 36-1451). Both samples show wurtzite crystal structure of ZnO and belong to the space group of P63mc, with lattice parameters of a = 3.25 Å, c = 5.21 Å. Sharp diffraction peaks in both XRD curves indicate the two samples have high crystallinity. For two curves, the average crystalline sizes were calculated using the Debby–Scherer equation (d ¼ kk=B cos h, where d is the mean crystalline size of the powder, k is the wavelength of Cu-Ka (k = 1.54178 Å), B is the full width at half maximum (FWHM) intensity of the peak in radian, h the Bragg’s diffraction angle and k is a constant usually equal to 0.9). The average crystallite size of samples I and II were determined to be about 55 and 28 nm, respectively. The scanning electron microscopy (SEM) images of the synthesized ZnO powders are presented in Fig. 3(a–d). Sample I shows a uniform flower-like ZnO nanostructure (Fig. 3a and b) with each single flower having several petals attached in the center with lengths in the range of 700–950 nm (Fig. 3a); and a width in the range of 130–230 nm. Looking closer to these flowers is shown that each single petal is composed of many smaller nanoparticles with lengths in the range of (45–95) nm. Fig. 3c and d demonstrate SEM images for sample II. These are uniform and well dispersed ZnO spheres with nice symmetrical configurations. The average diameter of the ZnO spheres was measured in the range of 250–400 nm. Repeating the synthesizing procedure for three times confirmed that both methods are reproducible.

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Fig. 2. XRD patterns of the synthesized ZnO nanostructures. All peaks were indexed according to the JCPDS card No. 36-1451.

Fig. 3. SEM images of synthesized ZnO nanostructures, (a) Low magnification, and (b) high magnification images of ZnO with flower-like morphology, (c) low magnification, and (d) high magnification images of ZnO with spherical morphology.

Fig. 4a and b represent the UV–vis absorption spectrum of the flower-like (sample I) and spherical (sample II) ZnO nanostructures, respectively. Band gaps of both nanostructures were calculated using the following equation;

Eg ¼

hc kc

ð3Þ

where h is plank’s constant, c is the speed of light, and kc is cut-off wavelength [41,42]. Fig. 4a illustrates a broad absorption band centered on 226 nm and a small but sharp peak at 396 nm for the flower-like ZnO nanostructure. The first and second band gaps were calculated to be

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Fig. 4. UV–vis spectra of synthesized ZnO nanostructures, (a) with flower-like morphology, (b) with spherical morphology.

3.04 and 2.92 eV, respectively. Fig. 4b shows a broad absorption band centered on 230 nm and a small but sharp peak at 384 nm for spherical ZnO nanostructure. In this case, the first and second band gaps were calculated 3.27 and 2.95 eV, respectively. It has been concluded that both morphologies have nearly similar band gaps, which may be due to having similar particle sizes. Next, we were measured the photodegradation percent of MB over synthesized ZnO nanopowders as well as bulk ZnO (Fig. 5). Degradation percent of MB over spherical ZnO nanostructures quickly increases to 78% after UV irradiation for 4 h. Whereas, degradation percent of ZnO for flower-like nanostructure was measured to be only 17% with the same irradiation time. This value was about 2.5% for bulk ZnO. Using a BET N2-adsorption technique, the specific surface area of the spherical, flower-like, and bulk ZnO nanostructures was measured to be 98, 22.9, and 4.8 m2/gr, respectively. The specific surface area of the spherical ZnO nanostructure is larger than the flower-like ZnO nanostructures as well as bulk ZnO. This difference could be due to the porous structure of ZnO spheres, as can be seen in the higher magnification SEM micrograph (Fig. 3d). Conversely, bonding of small particle constituents of the nanostructures are flat form in the flower-like nanostructure, thus they are including less porosity. We were also measured pore volumes of flower-like and spherical ZnO morphologies and bulk ZnO by measuring the nitrogen adsorption volume. The average pore volumes of the flower-like and spherical ZnO nanostructures were measured to be 0.06 and 0.28 cm3/gr, respectively. This pore volume was measured to be 0.0002 cm3/gr for bulk ZnO. Fig. 6 shows the variation of Ln([MB]/[MB]0) versus irradiation time for the MB solutions treated by both synthesized ZnO nanostructures and bulk ZnO. In addition, the values of the measured specific surface areas, pore volumes, MB degradation efficiencies after 60 min and Kobs constants of photocatalytic reaction are listed in Table 1. As can be seen from these data, spherical ZnO nanostructures,

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Fig. 5. Photodegradation percent of MB over ZnO for two synthesized ZnO morphologies as well as bulk ZnO.

Fig. 6. Variation of Ln ([MB]/[MB]0) versus irradiation time for the MB over ZnO for two synthesized ZnO morphologies as well as bulk ZnO.

Table 1 Physical properties of synthesized ZnO nanostructures along with bulk ZnO. Sample

BET (m2/gr)

[Photocatalyst]0 (ppm)

[MB]0 (ppm)

Pore volume (Cm3/gr)

R (60 min)

Kobs (hrs1)

Bulk ZnO Flower-like ZnO Spherical ZnO

4.8 22.9

100 100

10 10

0.0002 0.06

0.004 0.022

0.0015 0.0478

98

100

10

0.28

0.25

0.3911

which have had larger specific surface area, show a higher amount of Kobs for MB degradation than the flower-like ZnO nanostructures as well as bulk ZnO. Likewise, both nanostructure ZnO powders depict Kobs higher than bulk ZnO. As we mentioned earlier in introduction, a nanostructure ZnO material is believed to perform much better than its bulk counterpart in photocatalysis processes due to a higher surface-to-volume (S/V) ratio. This is in a well match with the presented data in Table 1.

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According to the previous reported studies on crystallization process of ZnO in alkali medium, external condition such as solution pH, temperature, solvent and precursor constituents could affect the size, dimension and morphology of the final products [43–47]. As we mentioned in the above paragraphs, in this present study, various morphologies of ZnO nanostructures have been prepared by controlling the pH value and using different precursors. During the microwave heating of alkali solution, Zn(OH)2 will form by the reaction between Zn2+ and OH ions that coexist in the solution by the following reaction;

Zn2þ þ 2OH ! ZnðOHÞ2

ð4Þ

then OH ions will reacted with Zn(OH)2 to form [Zn(OH)4]2 complex,

ZnðOHÞ2 þ 2OH ! ½ZnðOHÞ4 2

ð5Þ

Finally, ZnO nuclei will form through the decomposition of [Zn(OH)4]2,

½ZnðOHÞ4 2 ZnO þ H2 O þ 2OH

ð6Þ

Surfactants with high molecular weights (e.g. triethanolamine (TEA)), when exist in solution, can change the way a crystal grows. During the synthesis of spherical ZnO nanostructures, TEA acts as a capping agent and prevents the expansion on nuclei along specific crystal surfaces and finally during the crystal growth direction –C axes– [0 0 0 1] is hindered [43–51]. Due to various defects, the band gap of prepared ZnO nanostructures is lower than bulk ZnO (3.37 eV). This phenomenon is also affected by the size effect. In general, in lower band gap structures, electrons and holes transitions are easier [52–55]. Conversely, electron–hole recombination rates would determine the photocatalytic efficiency of the material. Presence of defects will help to create sub-bands inside the structure. These defects act as traps and thus the electron–hole recombination could not take place. Therefore, photocatalytic characteristics of the synthesized ZnO nanostructures are better than bulk ZnO. Photocatalytic reactions occur at the surface of a photocatalyst. Thus, specific surface area of photocatalyst would determine the amount of organic molecules absorbed on the surface of the material. It can be seen in Table 1 that the photocatalytic activity of synthesized ZnO nanostructures increases with increasing specific surface area, pore sizes and R value. In this study, the photocatalytic activity of spherical ZnO (with larger surface area) is more than the flower-like morphology as well as bulk ZnO (See Fig. 5). As a conclusion, the obtained results from photocatalytic experiments are compatible with the result from BET (See Table 1). Fluorescence studies were applied for both synthesized ZnO nanostructures. The room temperature photoluminescence (RT-PL) spectrums are shown in Fig. 7. In most cases, the PL spectra of ZnO consists of an ultraviolet (UV) emission peak in the range of 360–390 nm, and a broad visible light emission band centered at around 500–560 nm which is associated with oxygen vacancies [56–58]. In our cases, the RT-PL curves exhibits shoulder peaks centered at about 544 and 537 nm for flower-like and spherical ZnO nanostructures, respectively. These peaks are associated with visible green light emission which is possibly associated with oxygen vacancies. Furthermore, the two other sharper emission peaks, at about 372 and 365 nm for flower-like and spherical ZnO nanostructures, respectively, are due to the recombination of photo-generated electrons and holes [59,60]. In general, if the photo-generated electron–hole pairs were recombined, the photocatalytic activity would be decreased or eliminated. Presence of various defects like oxygen vacancies in ZnO structure, can act as the recombination centers to capture photo-induced electrons, here, the lower RT-PL intensity at UV region for spherical ZnO morphology (365 nm peak) indicates the rate of recombination of electrons and holes is comparatively lower in this case, so that the photocatalytic activity can be stabilized or improved in some way in the case of spherical ZnO as compared to the flower-like morphology. In general, light has energies coincide with its wavelength. By absorption of this light energy, Zinc oxide will excite to a higher energy state. If the absorbed energy was larger than the band gap, valance band electrons will transit to the conduction band and produce EHPs on the surface (Eq. (7)). Holes can move to the surface where there are electron acceptors like Hydroxyl groups (OH) and H2O that will absorb on the ZnO surface (Eqs. (8) and (9)). These reactions result in the formation of OH radicals, a

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Fig. 7. Room temperature photoluminescence spectra of the as-synthesized ZnO powders.

strong oxidant which could partially or completely mineralize the organic compounds like MB (Eq. (10)). þ

ZnO þ hm $ ZnOðh þ e Þ

ð7Þ

þ

ð8Þ

þ

ð9Þ

h þ H2 O ! OH þ Hþ h þ OH ! OH OH þ MB ! degradation of MB

ð10Þ

In addition, electrons on the surface of ZnO can also act to reduce any electron acceptors, usually dissolved oxygen on the ZnO surface (Eq. (11)). This also results in the formation of OH radicals (Eq. (12)).

e þ O2 ! O_ 2

ð11Þ

 þ O 2 þ H ! HO2

ð12Þ

2H2 O ! O2 þ H2 O2

ð13Þ

H2 O2 þ O_ 2 ! OH þ OH þ O2

ð14Þ



According to Eq. (14), OH radicals can degrade the MB [2–4,18,27]. In conclusion, despite the particle size and band gap measurements that showed almost the same values for both synthesized ZnO nanostructures, it can be concluded that the efficiency of the spherical ZnO morphology is much better than flower-like ZnO, due to the its higher specific surface area and pore volume rather than flower-like nanostructure. Therefore, adsorption of pollutants is likely to occur faster on the ZnO with spherical morphology and in this case, the rate of oxidation and reduction is greater than the ZnO with flower-like morphology. So, performance of the spherical ZnO nanostructure is better than that of the flower-like nanostructure. 4. Conclusions We were synthesized two morphologies of ZnO nanostructures by a simple microwave assisted method. This chemical method is simple, fast and cost effective, and thus is suitable for large-scale productions with high energy efficiencies. Continuing fabrication of nanostructures, photocatalytic performance of the prepared ZnO powders were investigated by measuring the MB photodegradation.

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In general, BET and UV–vis analysis could be useful measurements for investigating the photocathalyst behavior of nanostructure materials. The BET tests were showed that specific surface area of the spherical sample (98 m2/gr) was more than the flower-like ZnO (22.9 m2/gr), and bulk ZnO (4.8 m2/gr). Using UV–vis analysis, the band gap for both of spherical and flower-like ZnO samples were determined to be 3.27 and 3.04 eV, respectively. Finally, we come to this conclusion that the functional efficiency of the spherical ZnO nanostructure is greater than the flower-like ZnO nanostructure. Acknowledgment The authors would like to thank Pars Oil and Gas Company (POGC) for financial supports. References [1] C. Wang, B.Q. Xu, X. Wang, J. Zhao, Preparation and photocatalytic activity of ZnO/TiO2/SnO2 mixture, J. Sol. St. Chem. 178 (2005) 3500–3506. [2] M.M. Udin, M.A. Hasnat, A.J.F. Samed, R.K. 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