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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 16 (2013) 1542–1550

1369-80http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/mssp

Self-assembly fabrication of ZnO hierarchical micro/nanospheres for enhanced photocatalytic degradation ofendocrine-disrupting chemicals

Jin-Chung Sin, Sze-Mun Lam, Keat-Teong Lee, Abdul Rahman Mohamed n

School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Pulau Pinang, Malaysia

a r t i c l e i n f o

Keywords:ZnOHierarchicalPhotocatalysisEndocrine-disrupting chemical

01/$ - see front matter & 2013 Elsevier Ltd.x.doi.org/10.1016/j.mssp.2013.05.008

esponding author. Tel.: +60 45996410; fax:ail address: chrahman@eng.usm.my (A.R. Mo

a b s t r a c t

ZnO hierarchical micro/nanospheres were successfully synthesized via a facile andsurfactant-free chemical solution route. The field emission scanning electron microscopyand transmission electron microscopy observations showed that the ZnO micro/nano-spheres were assembled by large amounts of interleaving nanosheets with the thicknessof about 17 nm. The X-ray diffraction, energy dispersion X-ray and Raman results revealedthat the as-synthesized products were well-crystalline and possessing wurtzite hexagonalphase pure ZnO. Under UV irradiation, the ZnO micro/nanospheres showed an enhancedphotocatalytic performance compared with the ZnO nanorods and commercial TiO2 in thedegradation of phenol. The photocatalytic enhancement of ZnO micro/nanospheres wasattributed to their unique hierarchical porous surface structure and large surface areawhich can enhance the electron–hole separation and increased the yield of hydroxylradical quantities as evidenced by the photoluminescence spectra. By using a certain ofradical scavengers, hydroxyl radical was determined to play a pivotal role for the phenoldegradation. Moreover, the as-synthesized ZnO micro/nanospheres could be easilyrecycled without any significant loss of the photocatalytic activity. Other endocrine-disrupting chemicals such as resorcinol, bisphenol A and methylparaben were alsosuccessfully photodegraded under identical conditions. These characteristics showed thepractical applications of the ZnO micro/nanospheres in environmental remediation.

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1. Introduction

Nowadays, “green life” has been inspiring people to paymore and more attention to the removal of hazardoussubstances in the wastewater, especially organic pollutants.The photocatalytic reaction has become a desirable methodto convert the organic pollutants into simpler and harmlesscompounds to eliminate the environmental pollution. As animportant functional oxide, ZnO with a direct wide bandgap (∼3.3 eV) has received broad interest as a photocatalystdue to its unique chemical and physical properties, envir-onmental stability and low cost, as compared to other metal

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+60 45941013.hamed).

oxides [1–3]. It was reported that the nanoscale ZnOmaterials which have special fine nanostructure and largespecific surface area were more effective than that of thebulk materials due to the photocatalysis always being ableto occur at interfaces between the catalyst and organicpollutants [4,5]. Much effort has been devoted to thesynthesis of ZnO nanostructures with tunable size andshape parameter to effectively degrade numerous organicpollutants in the wastewaters [5–8]. However, the lowdimensional nanoscaled building blocks (such as nanopar-ticles, nanorods and nanosheets) tend easily to aggregateduring the preparation and photocatalysis processes, result-ing in reduction of their surface area and photocatalyticefficiency. Therefore, three-dimensional (3D) ZnO materialswith hierarchical structure have aroused great concern dueto their fine structure and larger size that can avoid the

Fig. 1. Schematic diagram of the photoreactor.

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aggregation of low dimensional nanoblocks in the commer-cial application as photocatalysts [9–11]. For example, Luet al. [9] reported that flower-like ZnO hierarchical archi-tecture synthesized via a citrate-mediated hydrothermalroute showed a higher photocatalytic activity comparedwith the mono-morphological nanostructures. Wang et al.[10] prepared cuboid-shaped ZnO hierarchical structurepossessing enhanced photocatalytic performance for thedegradation of methyl orange by a mixed surfactantsmediated hydrothermal method. Hitherto, most of 3Dhierarchical architectures were developed via the surfac-tants or structure directing reagents assisted assemblymechanism, and self-assembly of nanoscaled buildingblocks into the 3D structured morphologies without anysurfactants still remains an intricate challenge.

In this study, we synthesized the ZnO hierarchicalmicro/nanospheres (ZHNs) using a facile chemical solutionroute without any organic solvent or surfactant. A possibleexplanation of the formation of the hierarchical structurewas presented. The photocatalytic activities of ZHNs wereevaluated by degrading various endocrine-disrupting che-micals (EDCs) such as phenol, resorcinol (ReOH), bisphenolA (BPA) and methylparaben (MP) under a UV irradiation.To date, there is insufficient information available on theapplication of ZnO hierarchical structures as photocata-lysts especially on EDCs degradation. It is believed that thiswill be the pioneering work on the catalytic properties,photocatalytic activity and reusability of the ZnO hierarch-ical structure on EDCs degradation. A significant improve-ment of photocatalytic activity was observed when ZHNswere used as photocatalyst compared to 1D ZnO nanorods(ZNRs) synthesized in our previous study [7].

2. Experimental

2.1. Preparation of ZHNs

All the reagents were of analytical grade withoutfurther purification. The detailed synthesis procedurewas as follows: 5 mmol Zn(NO3)2 �6H2O was first dissolvedin 80 mL of deionized water. Then 30 mmol NaOH wasadded into the above solution and stirred continuously for3 h at room temperature. After stirring, the as-formedprecipitates were filtrated, washed with deionized waterfor several times, dried at 60 1C for 12 h and finallycalcined at 450 1C for 2 h.

2.2. Characterization

The as-synthesized products were characterized by X-ray diffraction (XRD) analysis on a Philips PW1820 dif-fractometer equipped with Cu Kα radiation over a rangefrom 201 to 801. The field emission scanning electronmicroscopy (FESEM) analysis was carried out using aQuanta FEG 450 together with an energy dispersion X-ray spectrum (EDX) analysis. Transmission electron micro-scopy (TEM) image was taken on a Philips CM 12 instru-ment operating at 120 keV. The diffuse reflectancespectroscopy (DRS) of catalysts was tested in a PerkinElmer Lambda 35 UV–vis spectrometer. The spectra wererecorded timely in the range of 300–600 nm using BaSO4

as the reference standard. The specific surface area andpore size distribution were obtained based on N2 adsorp-tion–desorption data at 77 K using a Micromeritics ASAP2020 instrument. Raman scattering experiment was com-pleted at room temperature using a Renishaw inVia RamanMicroscope with laser excitation at 532 nm.

2.3. Measurement of photocatalytic activity

All experiments were carried out in an immersion wellphotoreactor (Fig. 1). The photoreactor is made of Pyrex glasswith dimensions of 200�100�60 mm3 (height� outerdiameter� inner diameter). In the center of cylindricalphotoreactor, a 15W UV Pen-Ray (UVP) lamp with a max-imum emission at about 365 nmwas used as UV source. Thetotal UV output intensity at distance 10 mm away from UVlight, measured by radiometer was 0.840 mW cm−2. In atypical experiment, the reaction mixture was prepared byadding 350 mg of catalyst into the photoreactor containing350 mL of 20 mg L−1 substrate solution. Prior to the photo-reaction, the solution was magnetically stirred in dark for 1 huntil the adsorption–desorption equilibrium was reached;then it was irradiated and bubbled with air at a fixed flowrate of 5 mL min−1. The photocatalytic reaction temperaturewas kept at room temperature using a water circulation toprevent any thermal catalytic effect. After the elapse of aperiod of time, 5 mL of the solution were drawn andcentrifuged immediately to separate the suspended solids.Initial substrate degradation was followed by HPLC (PerkinElmer Series 200) using C18 column (length 150 mm� innerdiameter 4.6 mm�particle size 5 mm) at a flow rate of1 mL min−1. Experimental conditions for the liquid chroma-tography methods are shown in Table 1. The dissolution ofZnO in the course of photoreaction was estimated on aShimadzu AA-6650 atomic absorption spectrophotometer

Table 1Experimental conditions for the HPLC analysis.

Analyte Mobile phase composition Wavelength (nm)

Phenol 30% CH3CN; 70% H2O 254Resorcinol 30% CH3CN; 70% H2O 238Bisphenol A 80% CH3CN; 20% H2O 225Methylparaben 50% CH3OH; 50% H2O 254

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(AAS). In order to determine the reproducibility of all theresults, at least duplicated runs were carried out for eachcondition for averaging the results, and the experimentalerror was found to be within 74%. The degradation effi-ciency of each product was calculated using the equation:

% Degradation¼ 100� ½ðCo–CÞ=Co� ð1Þwhere Co is the equilibrium concentration of substrates after1 h dark adsorption and C is the concentration of substratesafter irradiation in selected time interval.

2.4. Detection of active species

The roles of photogenerated positively charged holes(hvb

þ), hydroxyl radicals (dOH) and superoxide anionradicals (O2

d−) in the photocatalytic reactions were eval-uated by adding 2 mM of different scavengers in a mannersimilar to the above photodegradation experiment. Ter-ephthalic acid-photoluminescence (TA-PL) probing tech-nique was used in the detection of dOH. TA readily reactedwith dOH to produce highly fluorescent product, 2-hydroxyterephthalic acid. The method relied on the PLsignal at 425 nm of 2-hydroxyterephthalic acid. The PLintensity of 2-hydroxyterephthalic acid was proportionalto the amount of dOH formed. In the detection experi-ment, a basic TA solution was added to the reactor insteadof aqueous solution of EDCs and the concentration of TAwas set at 5�10−4 M in 2�10−3 M NaOH solution. The PLspectra of generated 2-hydroxyterephthalic acid weremeasured by a Perkin Elmer Lambda S55 spectrofluorom-eter. At the intervals of given irradiation time, the reactionsolution was used to measure the increase in the PLintensity at 425 nm by excitation with a wavelength of315 nm.

3. Results and discussion

3.1. Characterization of the as-synthesized products

Fig. 2a shows the XRD pattern of as-synthesized pro-ducts. All the diffraction peaks were labeled and can bereadily indexed to hexagonal wurtzite ZnO structure (JCPDSCard no. 36-1451) with lattice parameters of a¼3.25 Å,c¼5.21 Å. The sharp and narrow peaks showed that theproducts obtained to be well in a crystallized form. No othercrystalline impurities were detected in the pattern, indicat-ing the phase purity of the ZnO products. The averagecrystallite size (D) of the products was calculated using theScherrer equation D¼0.9λ/(β cos θ), where λ is the wave-length of the incident X-ray radiation, β is the full width athalf maximum (FWHM) and θ is the Bragg angle [12]. By

taking the FWHM at (101), the average crystallite size of theproducts was determined to be 26 nm. Further evidence ofthe formation of ZnO came from the EDX analysis. The zinc(Zn) and oxygen (O) peaks can be easily observed (Fig. 2b).According to the estimation of the peak areas, the atomicratio of Zn/O was approximated to 1:1. The weak carbon (C)peak was also detected, which originated from the support-ing carbon tape.

Fig. 3 shows the FESEM and TEM images of the as-synthesized products. A panoramic morphology of theproducts is presented in Fig. 3a, indicating the synthesizedproducts were spherical-shaped and grown in large quan-tity. The diameters of micro/nanospheres varied from870 nm to 2.80 μm, which has a relatively narrow sizedistribution. The magnified image in Fig. 3b showed thatthe microspheres presented a hierarchical structure. Thehigh magnification FESEM image (Fig. 3c) clearly demon-strated that the surface structure of microspheres wasaccumulated by lots of interleaving 2D nanosheets withaverage thickness of ∼17 nm, forming an open porousstructure through self-assembly. Fig. 3d revealed the TEMimage of the product consisted of a large number ofnanosheets which were arranged in such an order wayto form spherical-shaped morphologies. The observedTEM image was also dark in its center which could mainlydue to the large density of ZnO nanosheets in the middlepart of the product. The TEM examination of as-synthesized ZnO product was in good agreement withthe FESEM observations in terms of its morphologies anddimensionalities.

On the basis of these results, the growth of ZHNs couldbe proposed based on the chemical reactions involved andcrystal growth habits of ZnO. The reaction process can beexpressed as follows:

Zn2þ þ 2OH−-ZnðOHÞ2↓ ð2Þ

ZnðOHÞ2 þ 2OH−-ZnðOHÞ42− ð3Þ

ZnðOHÞ2−4 -ZnOþ H2Oþ 2OH− ð4Þ

At the early stage of the reaction, ZnO nucleatedspontaneously from the solution of ZnðOHÞ42− to multi-nuclei aggregates. With the reaction proceeding, thesemultinuclei aggregates could serve as the sites for thegrowth of 2D ZnO nanosheets along two directions includ-ing [0001] plane [13]. From the thermodynamics point ofview, the surface energy of an individual nanosheet wasquite high with two main exposed planes and therefore,they tended to aggregate into self-assemble forms todecrease the surface energy by reducing exposed areas.Hence, the synthesized ZHNs were constructed fromnanosheets made by the self-assemble processes. Fig. 3eshows the schematic growth diagram of the ZHNs. In ourwork, the 3D ZnO structures with nanosheets wereassembled without any surfactant and structure-directingreagents, and it is also a facile way for large-scale synthesisof 3D ZnO structures. In order to further understand theexact growth mechanism of the ZnO hierarchical micro/nanospheres, a series of experiments for morphologyevolution of the products over time were in going.

Fig. 2. (a) XRD pattern and (b) EDX spectrum of ZHNs.

Fig. 3. (a)–(c) FESEM images and (d) TEM image of ZHNs. (e) The schematic growth diagram of the ZHNs.

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Fig. 4a shows the UV–vis DRS spectrum of the as-synthesized products. The synthesized ZHNs demonstratedstrong absorption in the ultraviolet region which lay between370 and 380 nm. The band gap energy of as-synthesized ZnOproduct can be calculated according to the equation Eg (eV)¼1240/λ (nm), where λ is the wavelength (nm) of absorption

onset and Eg is the band gap energy of synthesized product[7]. The Eg was measured to be 3.32 eV (Fig. 4a inset) whichwas consistent with that of the reported bulk ZnO [14].

Fig. 4b is the N2 adsorption–desorption isotherms andthe Barrett–Joyner–Halanda (BJH) pore size distribution ofas-synthesized ZHNs. As can be seen, the isotherms belonged

Fig. 4. (a) UV–vis DRS spectrum of ZHNs. Inset of (a) is the plot of R% versus photon energy. (b) N2 adsorption–desorption isotherms and the correspondingpore size distribution (inset of b) for the ZHNs. (c) Raman spectrum of ZHNs.

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to type IV, indicating the existence of abundant mesopores inthe ZnO product [9]. In the inset of Fig. 4b, the typical poresize distribution curve of ZHNs was shown, which indicatesan average pore size of 36.7 nm. The specific surface area ofthe synthesized ZHNs was measured to be 17.4 m2 g−1 by theBrunauer–Emmett–Teller (BET) method, which was largerthan that of the ZNRs synthesized in our previous study(7.3 m2 g−1) and other reported nanostructured ZnO[7,15,16].

Fig. 4c shows the Raman spectrum which covered therange of 200–800 cm−1. A dominant sharp peak at438 cm−1 was ascribed to the nonpolar optical phononsof E2H mode, which was one of the characteristic peaks ofwurtzite ZnO and further confirmed that the synthesizedproducts were pure ZnO [17]. The peaks located at 330 and380 cm−1 were corresponded to the multi-phonon scatter-ing process E2H−E2L and A1 (TO) phonons of ZnO crystal,respectively [18]. A peak at 576 cm−1 was assigned to theE1L mode, which was caused by defects such as oxygenvacancies, zinc interstitials, or their complexes [8]. Finally,the high intensity at E2 mode suggested the excellentoptical and crystalline properties of the synthesized ZHNs.

3.2. Photocatalytic experiments

The photocatalytic activities of the as-synthesized pro-ducts were evaluated by the degradation of phenol underUV light irradiation. Phenol is one such of the EDC whichproduced worldwide in millions of tons each year andwidely used in manufacturing of resins, insulation panels,pesticides, paints and lubricants. The extensive use andpoor biodegradability of phenol have resulted in its ubi-quitous presence in the environment and have led to

contamination of surface and ground waters [19,20].Evidence of phenol effect came from observation ofincreased chromosome aberrations in spermatogonia andprimary spermatocytes of mice treated with a solution ofphenol in water [21]. Thus, phenol was chosen as themodel substrate to evaluate the photocatalytic activities ofthe as-synthesized ZnO products in this work. Fig. 5ashows the HPLC profiles during the photodegradation ofphenol over the ZHNs. The phenol showed a characteristicpeak at retention time (RT) 5.6 min; it gradually decreasedwith increasing reaction time, indicating the excellentphotocatalytic activity of the product. In addition to theabove-mentioned main compound, the peaks at RT 1.1, 1.6,2.2, 2.6 and 3.3 min could be assigned to muconic acid,pyrogallol, hydroquinone, resorcinol and benzoquinoneintermediates, respectively when compared with the stan-dard chemicals. The measured solution pH changedslightly from initial pH 5.2 to pH 4.8, indicating the lowmolecular weight acid compound was formed during thedegradation. These analytical results were consistent withthose reported in literatures [22,23]. Fig. 5b shows that thedegradation efficiency reached 84.1% in 150 min. Underidentical experimental conditions, the ZHNs showed muchhigher activities than ZNRs (56.9% degradation efficiency)and commercial TiO2 (42.1% degradation efficiency).Further comparative experiments were also performed toevaluate the catalytic activity. It was shown that theconcentration of phenol hardly changed when the solutionwithout the catalysts was irradiated or when the solutionwith the ZHNs was kept in the dark. The high photocata-lytic activity of the present ZHNs can be attributed to theirlarge surface area which can increase the number ofsurface activation sites and allow the efficient transport

Fig. 5. (a) Time-dependent HPLC chromatogram of phenol solution with ZHNs ([phenol]¼20 mg L−1; volume of phenol¼350 mL; photocatalystdosage¼350 mg). (b) Phenol concentration dependence on irradiation time using different photocatalysts ([phenol]¼20 mg L−1; volume of phe-nol¼350 mL; photocatalyst dosage¼350 mg).

Fig. 6. (a) Photocatalytic activity of the ZHNs for phenol degradation with five times of cycling uses ([phenol]¼20 mg L−1; volume of phenol¼350 mL;photocatalyst dosage¼350 mg). (b) FESEM image of ZHNs after reused for five cycles.

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of photogenerated electron–hole pairs to the substratemolecules. In addition, the hierarchical porous structuresZHNs assembled with intercrossed nanosheets can bemore easily penetrated by the UV light source andabsorbed more photons, thus increasing the quantity ofphotogenerated electron–hole pairs available to partici-pate in the photocatalytic degradation of phenol [24].These factors are beneficial to form more dOH to enhancethe photocatalytic degradation of phenol as inferred fromthe following PL test in this study and other reports [9,25].

Cycling uses as well as maintaining high photocatalyticactivity are critical issues for long-term use in practicalapplications of the catalyst. Therefore, two criteria arerequired to be considered: (i) The stability of the catalystto maintain its high activity over time. As shown in Fig. 6a,84.1% of phenol was degraded by ZHNs at the first cycle. Asincreasing the recycling times, the efficiency of phenoldegradation decreased slightly, which was still higher than70.0% after being used for five cycles. It can be concludedthat the photocatalytic ability of ZHNs decreased little, ifany, because it was very difficult to avoid no any loss ofcatalyst materials in the experiments. Thus, the reducedcatalysis ability could be explained by the tiny loss of the

photocatalyst. Fig. 6b shows the FESEM image of ZHNsafter the fifth catalytic reaction. It can be seen that thehierarchical micro/nanospheres morphology was still wellmaintained after recycling test, which illustrated that thephotocatalyst has good stability. In addition, the ZnOdissolution data collected from AAS revealed a little lossof ZnO under UV irradiation (o4.0% loss of zinc wasobserved after the fifth catalytic reaction). (ii) The easewith which the catalyst could be recycled from solution. Inthis study, the hierarchical micro/nanospheres morphol-ogy of the product could prevent the agglomeration ofnanosheets and achieve the easy solid/liquid separation. Itwas indicated that the as-synthesized ZHNs showed effi-cient photocatalytic activity for the degradation of organicpollutants and could easily be recycled for reuse.

In addition to phenol degradation, the ZHNs could beapplied in photocatalytic degradation of other EDCs suchas ReOH, BPA and MP as shown in Fig. 7a. Like phenol,these EDCs are resistant to biodegradation and mayundergo natural reductive anaerobic degradation to yieldpotentially carcinogenic aromatic intermediates [26].ReOH and BPA were efficiently degraded under the iden-tical experimental conditions as those in the degradation

Fig. 7. (a) Photocatalytic degradation of several other EDCs by ZHNs. (b) Pseudo first-order kinetics in photocatalytic degradation of various EDCs. Inset of(b) shows the corresponding k value and linear fit (R2). The concentration and volume of substrate and dose of the photocatalyst were kept at 20 mg L−1,350 mL and 350 mg, respectively for each EDC solution.

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of phenol. For MP, the ZHNs increased the degradationefficiency from 1.4% (for direct photolysis) to 58.4% in150 min. Apparently, ZHNs demonstrated different activ-ities in photocatalytic degradation of different EDCs, how-ever it could improve the degradations of all EDCs statedabove due to the catalytic ability. The kinetics of all EDCsdegradation reactions are presented in Fig. 7b, whichfollow pseudo-first-order kinetics in agreement with theliterature [27]. For a first-order reaction, −dc/dt¼kt, or ln(Co/C)¼kt, where Co is the equilibrium concentration ofEDC after 1 h dark adsorption, C is the concentration ofEDC remaining in the solution at irradiation time t and k isthe observed rate constant. A plot of ln(Co/C) versus tgenerates a straight line and the slope is the k. As shown inFig. 7b, the linear relationship was achieved with morethan 98% linear fit (R2) (Fig. 7b inset) revealing an excellentconcurrence with the given model.

3.3. Possible photocatalytic mechanism

3.3.1. Role of active speciesTo investigate the photocatalytic mechanism over

ZHNs, the influence of active species such as dOH, O2d−

and hvbþ in the photodegradation of phenol was explored.

Different scavengers were used individually to remove thecorresponding active species so that the role of differentactive species in the photocatalytic process based on thechange of phenol degradation could be understood. In thisstudy, ethanol was added to the reaction system as dOHscavenger [28], sodium iodide (NaI) was introduced asscavenger of hvb

þ [29] and p-benzoquinone (BQ) wasadopted to quench O2

d− [30]. Fig. 8a shows that thedegradation efficiency of phenol on ZHNs decreased sig-nificantly from 84.1% to 42.2% after 150 min irradiationwhen ethanol was added into the photocatalytic reactionsolution, reflecting the participation of dOH in the reactionmechanism. Differently, the degradation efficiencies ofphenol were slightly retarded to 74.6% and 80.9% withthe addition of NaI and BQ, respectively in the sameduration, revealing hvb

þ and O2d− contributed to lesser

extent in phenol degradation. Furthermore, the role ofactive species involved in the photodegradation of phenol

over ZNRs was also studied and the result is shown inSupporting Information (Fig. S1). The degradation efficien-cies of phenol decreased from 56.9% to 23.2%, 48.7% and54.4% in the presence of ethanol, NaI and BQ, respectively.Hence, the photogenerated active dOH were the maindriving for the photocatalytic degradation of phenol overZHNs and ZNRs in these present experiments.

3.3.2. Hydroxyl radical analysisFig. 8b shows the PL spectral changes observed after

each product was irradiated for 150 min of UV light in theaqueous basic solution of TA. An obvious difference in PLintensity at about 425 nm was observed using differentcatalysts. It was clear that the formation rate of dOH on theZHNs was higher than that of ZNRs. This implied thehierarchical porous surface of ZHNs was helpful to producethe dOH and favorable for improving its photocatalyticactivity compared to ZNRs. The same results are verified inSection 3.2. In addition, the inset of Fig. 8b illustrates thechange of PL spectra with irradiation time for the case ofZHNs. A gradual increase in PL intensity was observed withincreasing irradiation time, which suggested that thefluorescence was caused by chemical reactions of TA withdOH formed during photoilluminated reactions. Therefore,these results further confirmed the evidence of dOHformation and indeed participated in degradation process.

3.3.3. Possible degradation mechanismCombining our experiment results with the related

literatures [3,26,31], a plausible schematic illustration forEDCs degradation over the ZHNs could be proposed inFig. 9. Under the UV irradiation, the efficient chargeseparation of electron–hole (ecb−–hvb

þ) pairs and genera-tion of large number of ecb−–hvb

þ pairs played pivotal rolein EDCs degradation. The photogenerated ecb− in theconduction band ZnO can move towards the surface andwas scavenged by the ubiquitous O2 promoting the O2

d−

formation and then converted to active dOH. At the sametime, the photogenerated hvb

þ can trap on the catalystsurface undergoing charge transfer with adsorbed watermolecules or with surface bound hydroxide species togenerate active dOH. The dOH have been formed by many

Fig. 8. (a) Effects of different scavengers on degradation of phenol in the presence of ZHNs ([phenol]¼20 mg L−1; volume of phenol¼350 mL;photocatalyst dosage¼350 mg). (b) PL spectra of aqueous basic solution of TA with an excitation at 315 nm for ZHNs and ZNRs. Inset of (b) shows thePL spectra changing with irradiation time for the case of the ZHNs.

Fig. 9. A schematic diagram of photocatalytic degradation reactionmechanism over the surface of ZHNs.

J.-C. Sin et al. / Materials Science in Semiconductor Processing 16 (2013) 1542–1550 1549

semiconductors like TiO2, ZnO, etc. via generation ofecb−–hvb

þ pairs under irradiation [26,31], which acted asstrong oxidizing agents to degrade the EDC substrates. Thephotocatalytic degradation of EDCs was believed to takeplace according to the following mechanism:

ZnOþ hv-ZnO ðecb−–hvbþÞ ð5Þ

hvbþ þ H2O-dOHþ Hþ ð6Þ

hvbþ þ OH−-dOH ð7Þ

ecb− þ O2-O2

d− ð8Þ

O2d−þHþ-dOOH ð9Þ

dOOHþ Hþ þ ecb−-H2O2 ð10Þ

H2O2 þ ecb−-dOHþ OH− ð11Þ

EDCs+dOH-peroxy or hydroxylate intermediate-miner-alized product (12)

In our study, the ZHNs first absorbed the UV light andefficiently generated the separation of ecb−–hvb

þ pairs.Additionally, the unique hierarchical porous surface struc-ture and large surface area of ZHNs can increase thequantity of photogenerated ecb− and hvb

þ available toparticipate in the photocatalytic degradation of EDCs.

Subsequently, enhanced the yield of dOH quantities in thedegradation of EDCs, which further improved the photo-catalytic activity of ZHNs.

4. Conclusions

In the absence of any surfactants or structure directingreagents, ZnO hierarchical micro/nanospheres (ZHNs)were successfully obtained by a facile chemical solutionroute, and confirmed by XRD, EDX, FESEM, TEM, UV–visDRS, N2 adsorption–desorption, Raman and PL measure-ments. The proposed method was rather simple, mild,cost-effective and particularly suited for industrial produc-tion of ZnO. By utilizing the as-synthesized ZHNs asphotocatalysts, considerable photocatalytic degradationwas observed towards EDCs. Under UV irradiation, theZHNs exhibited significantly improved photocatalyticactivity compared to ZNRs and commercial TiO2. Theimproved photocatalytic activity was attributed to theunique hierarchical porous surface structure and largesurface area of synthesized ZHNs, which can enhance theecb−–hvb

þ separation and increased the yield of dOHquantities as evidenced by the PL spectra. The dOH weredetermined as main active species during the photocata-lyic degradation process. Furthermore, the as-synthesizedZHNs could be easily recycled without any significant lossof the photocatalytic activity, which was favorable for thepotential practical applications.

Acknowledgments

This research was supported by a Research UniversitiGrant (No. 814176) and a Post Graduate Research Scheme(no. 8045032) from Universiti Sains Malaysia as well as aMy Ph.D. scholarship through Malaysia Government.

Appendix A. Supporting information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.mssp.2013.05.008.

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