Materials Letters 81 (2012) 239–241

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Materials Letters

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ZnO nanocapsules for photocatalytic degradation of thionine

Suzan A. Khayyat a, M.S. Akhtar b, Ahmad Umar c,d,⁎a Department of Chemistry, Faculty of Sciences, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabiab New and Renewable Energy Materials Development Center (New REC), Chonbuk National University, South Koreac Department of Chemistry, Faculty of Sciences and Arts, Najran University, P.O. Box 1988, Najran, 11001, Kingdom of Saudi Arabiad Promising Centre for Sensors and Electronic Devices (PCSED), Najran University, P.O. Box 1988, Najran, 11001, Kingdom of Saudi Arabia

⁎ Corresponding author at: Department of ChemistryNajran University, P.O. Box 1988, Najran, 11001, Kingdo534 574 597; fax: +966 7 5442 135.

E-mail address: ahmadumar786@gmail.com (A. Um

0167-577X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.matlet.2012.04.039

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 March 2012Accepted 9 April 2012Available online 14 April 2012

Keywords:ZnO nanocapsulesThionine dyePhotocatalystsPhotodegradationIllumination

This paper reports the successful synthesis of ZnO nanocapsules and their utilization as a photocatalyst forphotocatalytic degradation of thionine. The nanocapsules were synthesized by facile hydrothermal processand characterized in detail in terms of their morphological, structural and optical properties. The detailedstudies revealed that the synthesized nanocapsules possess well-crystalline and good optical properties. Byutilizing the as-synthesized ZnO nanocapsules as a photocatalyst, significant photocatalytic degradationwas observed towards thionine dye under light illumination. The considerable photo-degradation of thioninedye was due to the unique morphology and high surface area of synthesized ZnO nanocapsules which mightimport the effective electron/hole separation and might generate the large number of oxy-radicals.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Recently, considerable attentions have been made by the environ-mental scientists regarding the pollutions caused by water solubleorganic dyes and organic chemicals as when these pollutants releaseinto the environment, they effectively disturb the ecosystem andpose a direct threat to human beings and animals [1]. Amongvarious or-ganic pollutants, the thionine (TH) dye is commonly used as a sensitizerin photographic processes, laser components, photographic filter layers,chemotherapy, etc. [2,3]. Hence, it is needed to search for a new photo-catalyst for the efficient degradation of this harmful and widely useddye. Generally, the photocatalytic reaction occurs on the surfaces ofphotocatalysts especially semiconductor metal oxides and due to lightabsorption, the electrons were excited from valence band to theconduction band and generate the electron–hole pairs which helpin the degradation of organic pollutants [4,5].

Among various semiconductor metal oxides, zinc oxide (ZnO)presents itself as one of the promising photocatalyst for the photocatalyticdegradation of organic dyes and chemicals due to its versatile propertiessuch asdirect andwide bandgap (~3.37 eV), large excitonbinding energy(60 meV), semiconducting, piezoelectric and pyroelectric properties andso on. Moreover, ZnO exhibits almost similar band gap and degradationmechanism as of TiO2 [6–13]. Therefore, several ZnO nanostructures areutilized as efficient photocatalyst for the photocatalytic degradation ofvarious organic pollutants and reported in the literature [6–13]. This

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paper reports, for the first time, the utilization of ZnO nanocapsules forthe photocatalytic degradation of very harmful and widely used thioninedye. A significant (~79%) photocatalytic degradation of thionine dye wasobserved when ZnO nanocapsules were used as a photocatalyst.

2. Experimental details

Well-crystalline ZnO nanocapsules were synthesized by facilehydrothermal process. In a typical reaction process, 0.01 Mzinc chloride(ZnCl2.2H2O) was mixed with 0.001 M urea, both made in 50 ml DIwater, under continuous stirring. After 15 min of stirring, to maintainthe pH=10, few drops of ammonium hydroxide was added in theresultant solution and stirred again. After 20 min of vigorous stirring,the obtained solution was transferred to a teflon-lined autoclave andheatedup to 150±10 °C for 3 h. After desired reaction time, the autoclavewas allowed to cool at room-temperature. White colored precipitatewas obtained which was washed with DI water, ethanol and acetone,sequentially and dried at 80 °C for 2 h. The as-synthesized productswere characterized in terms of theirmorphological, structural and opticalproperties and were used as photocatalyst for the photocatalytic degra-dation of thionine dye.

The photocatalytic degradation of TH dye was carried out in a threeneck pyrex flask reactor using as-synthesized ZnO nanocapsules asphotocatalyst under the UV illumination of xenon arc lamp (300 W,Hamamatsu: L 2479). TH dye (10 ppm) solution was prepared in100 ml DI water and added the 0.2 g of synthesized ZnO nanocapsulepowder in it. The whole reaction mixture was purged with Ar gas for30 min under continuous stirring. The resulting suspension wasequilibrated by stirring for 30 min to stabilize the absorption of THdye over the surface of photocatalyst, i.e. ZnO nanocapsules before

Fig. 2. (a) UV–vis. spectrum(b) FTIR spectrumand (c) Raman-scattering of as-synthesizedZnO nanocapsules.

240 S.A. Khayyat et al. / Materials Letters 81 (2012) 239–241

exposing to the light. To evaluate the photocatalytic performances,photocatalytic degradation of TH was performed under irradiationof UV light for various time intervals (20 min) andmeasured the absor-bance by using UV–vis. spectrophotometer (2550 Shimadzu, Japan). Thedegradation rate of TH dye was estimated using the following equa-tion [14]:

Degradation rate %ð Þ ¼ Co−C=Coð Þ � 100% ¼ Ao−A=Aoð Þ � 100%

where Co represents the initial concentration, C the variable concen-tration, Ao the initial absorbance, and A the variable absorbance.

3. Results and discussion

The general morphologies of as-synthesized ZnO product werecharacterized by field emission scanning electron microscopy (FESEM)which reveal the synthesis of ZnO nanocapsules. The nanocapsuleswere synthesized in large quantity and possessed pointed tips at bothof their ends (Fig. 1(a)).Most of the nanocapsules exhibit almost uniformmorphologies, lengths and diameters. The average diameters and lengthsof as-synthesized nanocapsules are ~150±20 nm and ~700±100 nm,respectively (Fig. 1(b)). The nanocapsules weremade of zinc and oxygenonly as was confirmed by energy dispersive spectroscopy (EDS)(Fig. 1(c)). No other peak related with any impurity was detected inthe spectrumwhich further confirms the formation of pure ZnO. Sev-eral well-defined diffraction reflections which correspond to ZnOwere observed in the obtained X-ray diffraction pattern (Fig. 1(d)). Thereflections appeared at 31.6°, 34.4°, 36.1°, 47.6°, 56.7°, 62.9°, 66.4°,68.0°, 69.2°, 74.2° and 78.3° correspond to the lattice planes of (100),(002), (101), (102), (110), (103), (200), (112), (201), (004) and (202),respectively. The observed diffraction reflections are well-matchedwith the standard JCPDS data card no. 36–1451. No other reflection relat-ed with any impurity was detected in the pattern which confirms thewell-crystallinity and purity of as-synthesized ZnO nanocapsules.

The UV–Vis spectrum (Fig. 2(a)) depicts a strong narrow absorptionpeak at ~375 nmwithout any other absorption peak. The observed peakis a significant and characteristic peak for the absorption band of thewurtzite hexagonal pure ZnO [6–9]. The presence of single peak sub-stantiates that the synthesized ZnO nanocapsules are pure withoutimpurities. Various significant IR bands appeared in the observedFTIR spectrum of as-synthesized ZnO nanocapsules (Fig. 2(b)). Thesharp peak that appeared at 533 cm−1 is related with the Zn–Ostretching mode [6]. Appearance of a short peak at 897 cm−1 is dueto the carbonate moieties which generally are observed when FTIR

Fig. 1. Low (a) and (b) high-magnification FESEM images, (c) EDS spectrum and (d) XRDpattern of as-synthesized ZnO nanocapsules.

samples are measured in air [15]. In addition to this, appearance oftwobands at 1628 and 3395 cm−1 could be ascribed as bending vibrationof absorbed water and surface hydroxyl and O–H stretching mode,respectively. The synthesized ZnO nanocapsules were further charac-terized by Raman-scattering spectroscopy (Fig. 2(c)). The appearance ofthe strongest peak at 438 cm−1 attributed to E2 mode, corresponds towurtzite hexagonal phase ZnO, and confirms that the synthesized nano-capsules are pure ZnO. A band at 331 cm−1 has been observed which isassigned as E2H–E2L (multi-phonon process) and can be found onlywhen the ZnO is single crystal [1]. A very broad and suppressedpeak at 567 cm−1 attributed as E1L mode has also been seen in thespectrum. Finally, the high intensity at E2 mode suggests the excellentoptical and crystalline properties of the synthesized ZnO nanocapsules.

To evaluate the photocatalytic activity of ZnO nanocapsules, thephoto-degradation of TH has been employed. To the best of ourknowledge, this is first report which demonstrates the photocatalyticdegradation of TH using ZnO nanocapsules. The photocatalytic decom-position was monitored by measuring the absorbance at regular timeintervals by using UV–vis. at a wavelength of 598 nm. Interestingly, itwas observed that the relative absorption intensity continuously de-creases as the UV illumination exposure time increases, which indicatesthat the TH dye decomposes gradually over the surface of the photoca-talyst, i.e. ZnO nanocapsules (Fig. 3(a)). A significant degradation(~79%) of TH dye was observed in 140 min. in the presence of ZnOnanocapsules as photocatalyst (Fig. 3(b)). Fig. 3(c) shows the plot ofthe variation in the relative concentration (A/Ao) versus time intervalfor the photo-degradation of TH dye over the surface of ZnO nanocap-sules under UV irradiation. Interestingly, it was observed that in thepresence of ZnO nanocapsule catalyst, the photo-degradation rate ofthionine dye gradually increases with increasing of the UV exposuretime. However, there was no self-degradation of TH dye in the absenceof ZnO nanocapsules (Fig. 3(c)). Therefore, one can conclude that ZnOnanocapsules act as an efficient photocatalyst for significant degrada-tion of THdye to the environment friendly residueswhichmight ascribethe efficient charge separation of electron–hole pair and generation oflarge number of electron–hole pairs under the UV-light illumination[16]. These kinds of dye molecules are usually degraded by the genera-tion of hydroxyl (OH•) and oxy radicals such as superoxide (O2•, HO2•)radicals under illumination. The hydroxyl and oxy radicals are formedby the semiconducting metal oxides like TiO2, ZnO etc. via generationof electron–hole pairs under illumination [17], which act as the strongoxidizing agent to break the large organic materials into less harmfulsmall organic materials. In our study, the ZnO nanocapsule catalyst

Fig. 3. (a) UV–Vis absorbance spectra of decomposed TH dye solution by light over as-synthesized ZnO nanocapsules, (b) degradation rate (%) and (c) extent of decomposition(A/Ao) of THdyewith respect to time intervals, (d)A schematic illustration of photocatalyticactivity of ZnO nanocapsules.

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firstly absorbs the UV light and efficiently generates the separation ofelectron (ē) and hole (h+) pairs.

The schematic representation has been illustrated to understand thephoto-degradation of TH dye over the ZnO nanocapsules (Fig. 3(d)).Firstly, the TH dye molecules substantially absorbs on the surface of theZnO nanocapsules and under UV light, the TH dye and water moleculesover the surface of nanocapsules form dye cation and OH• radicals.Thus, the formation of dye cation radicals is responsible for theeasy transformation or oxidation of harmful organic or dye to lessharmful or complete degradation of the chemicals. Herein, the uniquemorphology and high surface area of ZnO nanocapsules might importthe effective electron and hole separation which might form a largenumber of hydroxyl, superoxide and dye cation radicals. Thus, theseradicals result in the sufficiently high degradation of TH dye over thesurface of ZnO nanocapsules. Therefore, it is expected that ZnO nano-capsules are one of the excellent active catalysts for the photocatalyticdegradation of TH dye.

4. Conclusion

Well-crystalline ZnO nanocapsules were synthesized by simple hy-drothermal process and utilized as efficient catalyst for the photocatalyticdegradation of thionine dye. The synthesized nanocapsules exhibit well-

crystalline morphologies and optical properties. Considerable photo-degradation of thionine dye was observed which resulted from theunique morphology, high surface area and excellent crystallinity of as-synthesized ZnOnanocapsuleswhichmight import the effective electron/hole separation and might generate the large number of oxyradicals.

Acknowledgments

AhmadUmarwould like to acknowledge the support of theMinistryof Higher Education, Kingdom of Saudi Arabia for the Promising Centrefor Sensors and Electronic Devices (PCSED) at Najran University, SaudiArabia, dated 24/3/1432H, 27/02/2011. Suzan A. Khayyat would liketo acknowledge the support of the Deanship of Scientific Research,King Abdulaziz University for a research grant no. 313/363/1432.

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