z Catalysis

Improved Photocatalytic Degradation of Organic Dyes byZnO-NanoflowersAkbar Mohammad,[a] Kshipra Kapoor,[a] and Shaikh M. Mobin *[a, b, c]

Recently, we reported one pot synthesis of ZnO nanoflowers(ZnO-NFs) by using asymmetric Zn(II) dimeric complex, [Zn(hmp-H)2(H2O)(m-Cl)Zn(m-Cl)(Cl)3],(A), [hmp-H = 2-(2-hydrox-ymethyl)pyridine)] as a single molecular precursor at room tem-perature (RT). The prepared ZnO-NFs catalyst was systemati-cally probed for its structural, morphological and functionalproperties. Our primary aim was to utilize ZnO-NFs to find asuperior photocatalyst for the degradation of different dyessuch as Methyl orange (MO), Congo red (CR), Chicago sky blue

(CSB) and Eosin B (EO) which are commonly found in industrialwaste and are major contributor to the river pollutant. In orderto overcome this problem, we explored the photocatalytic per-formance of ZnO-NFs, which results in improved photocatalyticactivity towards MO dye (15 mg/L, 99.46 %) compared to otherdyes CR, CSB and EO. Based on the photocatalytic evaluations,ZnO-NFs could possess versatile photocatalytic properties forremediation of contaminated water.

Introduction

The draining of organic pollutant from different industries suchas leather, paper, pharmaceutical, steel plant to surroundingsmay cause hazardous effect on the human health, aquatic lifeas well as environment.[1–5] The organic dyes containing azogroup have gained much attention due to their in ability to de-grade because of highly stable nature towards temperature,light and other chemicals.[5–7] To remove the dyes from con-taminated water, several methods such as physical, chemical,and biological approaches have been used.[8] Among them,physical adsorption and degradation is considered as a com-petitive method because of high efficiency, economic suitabilityand easy to design and operation. Due to its high efficiencyand low cost method, photocatalytic degradation has attractedmuch attention to degrade organic dyes.[9, 10] The semi-conductor based photocatalysis is always preferable due to itsnecessity to elucidate the energy and environment relatedproblems with the help of infinite solar energy.[11, 12] Recently,semiconductor based heterogeneous photocatalysts ZnO, CuO,Fe2O3, TiO2, In2O3, Bi2O3 were utilized in photocatalytic and pho-toelectrochemical processes.[13–19] ZnO with its wide band gap

and large exciton binding energy is suitable for a wide range ofapplications including UV lasers, dye sensitized solar cells, gassensors and UV sensors.[20] Currently, the development of pho-tocatalysts with unusual morphology and characteristics are ofan extent important because of their unique chemical andphysical properties.[21] Based on the above considerations, wedeveloped a cost effective photocatalyst for degradation of dif-ferent organic dye contaminants.

Herein, we report one pot synthesis of ZnO-nanoflowers(ZnO-NFs) using our recently reported single molecular pre-cursor (A). The photocatalytic performances of ZnO-NFs havebeen evaluated on Methyl orange (MO), Congo red (CR), Chica-go sky blue (CSB) and Eosin B (EO). Additionally, possible degra-dation product of MO was analyzed by Mass spectroscopy anal-ysis. Moreover, ZnO-NFs photocatalyst can be recycled up to 3cycles. The proposed catalyst is cost effective and can be usedas an effective catalyst for the remediation of contaminatedwater from dyes.

Results and Discussion

Synthesis

Asymmetric dimer [Zn(hmp-H)2(H2O)(m-Cl)Zn(m-Cl)(Cl)3], (A) wasobtained by the reaction of ZnCl2 with hmp-H {hmp-H (hmp-H = 2-(2-hydroxymethyl)pyridine} in MeOH at RT (Scheme 1,Path 1).[22] The two Zn(II) centers are present in hexa- and tetra-coordinated environment. The complete structural aspect wasdiscussed in our recent paper.[22]

One pot reaction was carried out by dissolving A in distilledwater under stirring at room temperature to form a uniformclear solution. On addition of an aqueous NaOH solution drop-wise, the clear solution turns into precipitation within few mi-nutes. The obtained product was repeatedly washed and driedat 50 8C (ZnO-NFs) (Scheme 1, Path 2).[22]

[a] A. Mohammad, K. Kapoor, Dr. S. M. MobinDiscipline of ChemistryIndian Institute of Technology IndoreSimrol, Indore 452020, IndiaTel: + 91 731 2438 762E-mail: xray@iiti.ac.in

[b] Dr. S. M. MobinCentre for Material Science and EngineeringIndian Institute of Technology Indore, Simrol, Indore 452020, India

[c] Dr. S. M. MobinCentre for Biosciences and Bio-Medical EngineeringIndian Institute of Technology IndoreSimrol, Indore 452020, India

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/slct.201600476

Full PapersDOI: 10.1002/slct.201600476

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Further, the photocatalytic activities of as synthesized ZnO-NFs were performed for the degradation of different dyesMethyl orange (MO), Congo red (CR), Chicago sky blue (CSB)and Eosin B (EO).

Characterization

ZnO-NFs have been characterized for confirmation of phaseand purity, morphology, optical properties and surface areaanalysis. The crystalline and pure phase nature of ZnO-NFs wasconfirmed by the XRD pattern with well indexed diffractionpeaks similar to previous report (JCPDS 36–1451). ZnO-NFsshow hexagonal form with average crystallite size of 35 nm,calculated by using Scherrer’s equation and having the latticeparameters of a = b = 3.247 � and c = 5.204 � (Figure 1).

The FESEM image shows presence of uniform ZnO nano-structures which self-assembled to form the flower like mor-phology with approximate size of ~1.2 mm (Figure 2).

The ZnO-NFs have well-organized morphology consists ofseveral petals which were self-assembled into nanoflowers.TEMresults show the flower like morphology which is consistentwith SEM results. The higher magnification image of ZnO-NFsshow obvious lattice fringes which further confirmed the crys-talline nature of ZnO. The SAED pattern also confirmed thecrystallinity of ZnO-NFs (Figure 3).

The surface area and band gap of ZnO-NFs are supposedto be the essential aspect in determining their photocatalyticactivity. Specific surface area and pore size of the ZnO-NFs wascalculated by using N2 adsorption-desorption isotherm at 77 Kas a BET model (Figure 4, Table 1). ZnO-NFs were degassed at

300 8C and then the adsorption isotherm was acquired. Fig-ure 4 reveals presence of H3 hysteresis loop at high relativepressure (P/P0 = 0.9) which suggest formation of nanoflowerswith porous surface.

Moreover, the surface area and pore size in diameter of theZnO-NFs were found to be 41.09 m2/g and 48.38 nm, re-spectively (Table 1). This implies that ZnO-NFs are mesoporous

Scheme 1. Schematic diagram for the synthesis of A via Path 1 and for thesynthesis of ZnO-Nanoflowers (ZnO-NFs) via Path 2.[22]

Figure 1. XRD pattern of ZnO-NFs.

Figure 2. FE-SEM image of ZnO-NFs at different magnifications.

Table 1. Surface based properties of ZnO-NFs.

Photocatalyst ZnO-NFs

BET Surface Area (m2/g) 41.09Pore Volume (cm3/g) 0.1336Pore Diameter (nm) 48.38

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in nature with large surface area, which trigger the photo-catalytic activity.

The absorption spectra of the ZnO-NFs showed a band at376 nm, which corresponds to the formation of ZnO nano-structures (Figure 5). The bandgap was also calculated usingthe equation E(eV) = 1240/l (nm), and found to be 3.29 eV.

Photocatalytic activity

The photocatalytic activity of ZnO-NFs was studied by thedegradation of MO under UV light irradiation with respect totime. The aqueous solution of MO initially shows absorptionmaximum at 464 nm. After addition of photocatalyst ZnO-NFs,the absorption maxima of MO with 15 mg/L of concentration

(Figure 6a) shows tremendous decrease in absorption maximaresulting 99.46 % of degradation in just 50 mins. For compar-ison sake, we have also performed the similar experiment usingcommercial ZnO and degradation reached to only 76.86 % in130 min (Figure S1).

Further, photocatalytic activity of ZnO-NFs towards varyingconcentration range 30–60 mg/L of MO dye was exploredwhich result in 94.28 % for 30 mg/L in 80 min, 45.88 % for45 mg/L in 120 min, 40.11 % for 60 mg/L in 120 min (Figure 6b-d). The orange color of the initial solution changes to almostcolorless in presence of ZnO-NFs under UV light irradiationwith respect to time indicates destruction of dyes.

The excellent photodegradation response of ZnO NFs overMO prompted us to extend activity of ZnO-NFs on Congo Red(CR), Chicago Sky Blue (CB), Eosin B (EO) dyes. The fixed con-centration, 15 mg/L of Congo Red (CR), Chicago Sky Blue (CB)and Eosin B (EO) dyes were exposed to 20 mg of photocatalystZnO-NFs under UV light irradiation at different time interval(Figure 7-9). The resulting UV spectra reveals decrease in max-imum absorbance peak corresponds to each dye with respectto time. The maximum absorbance peak of CR at 500 nm de-creases rapidly and decomposition reached to 99.37 % after 80mins (Figure 7). Similarly, the maximum absorption peak of CSBand EO at 620 nm and 515 nm, respectively decreases and de-graded upto 96.68 % and 95.97 %, respectively in 80 mins (Fig-ure 8 and 9).

The difference in photocatalytic activities of ZnO-NFs to-wards all the dyes used (MO, CR, CSB and EO), may be due tothe difference in kinetic behavior of these dyes for photo-catalytic degradation reaction.

The rate of photocatalytic activity of ZnO-NFs was inves-tigated using pseudo-first-order kinetics viz. ln (C0/C) = kt–(2)where, k = Pseudo first-rate kinetic constant (min�1), t = irradi-ation time (min), C = concentration and C0 = initial concen-tration (mg L�1). The variation of ln(C/C0) as a function of irradi-ation time for the degradation of MO is plotted in Figure 10.The reaction rate constant of ZnO-NFs for the MO concen-

Figure 3. TEM images of ZnO-NFs at different magnification (A�C) and SAEDpattern (D).

Figure 4. N2 adsorption-desorption isotherm.

Figure 5. UV-vis spectra for ZnO-NFs.

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tration of 15 mg/L, 30 mg/L, 45 mg/L and 60 mg/L were calcu-lated which corresponds to 0.05485 min�1, 0.02348, 0.00454and 0.004 min�1respectively (Figure 10).

It was observed that as the dye concentration was in-creased the reaction rate constant value decreases with decom-position rate of 15 mg/L > 30 mg/L > 45 mg/L > 60 mg/L (Fig-

ure 11). This implies that the photocatalyst ZnO-NFs possessesa faster reaction rate at 15 mg/L of MO dye concentration.

The kinetic rate curves of degradation of MO, CR, CSB andEO dyes are summarized in Figure 11, Table 2.

Figure 6. a–d). Absorption spectra of Methyl orange (MO) solution at varying concentration (15-60 mg/L) under UV light in the presence of ZnO-NFs (20 mg).

Figure 7. Absorption spectra of Congo Red (CR) solution (15 mg/L) under UVlight in the presence of ZnO-NFs (20 mg).

Figure 8. Absorption spectra of Chicago Sky Blue (CSB) solution (15 mg/L)under UV light in the presence of ZnO-NFs (20 mg).

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The mixture was magnetically stirred in dark for 30 mins toobtain adsorption-desorption equilibrium. After the dark ad-sorption for 30 min, ZnO-NFs reveals 42.41 % adsorption to-wards CR, while the adsorption decreases to 30.60 %,17.10 %and 5.50 % for MO,CSB and EO, respectively (Figure 12).

But, Congo red dye required more time to degrade viz.80 min as compared to MO, it takes 50 min to degradion, weused MO as our model dye and tested it using varying concen-trations. The reaction rate constants of MO, CR, CSB and EO are0.05485, 0.04611, 0.03182 and 0.02884 min�1, respectively (Ta-ble 2), which suggest that ZnO-NFs show the excellent photo-catalytic degradation capability for MO compared to other dyesused (CR, CSB and EO) (Figure 11). This high photocatalytic deg-radation rate of ZnO-NFs may be depends on its large surfacearea (41.09 m2 g�1), small crystallite size and band gap energyof 3.29 eV.

Sunlight driven degradation of MO

The MO dye (15 mg/L) in presence of photocatalyst ZnO-NFs(20 mg) was also exposed to sun light (36 8C, 25 % humidity).The suspension was magnetically stirred in dark for 30 mins inorder to establish the adsorption/desorption equilibrium of MOmolecules on the surface of the catalyst. The sample was col-

Figure 9. Absorption spectra of Eosin B (EO) solution (15 mg/L) under UVlight in the presence of ZnO-NFs (20 mg).

Figure 10. Kinetic linear simulation curves of ZnO nanoflowers (ZnO-NFs)with different Methyl orange (MO) concentrations.

Figure 11. Kinetic linear simulation curves of ZnO-NFs with four different or-ganic dyes.

Table 2. Rate constant for different contaminates using ZnO-NFs as Photo-catalyst.

Organic Dye Rate constant (min�1)

Methyl Orange (MO) 5.485*10�2

Congo Red (CR) 4.611*10�2

Chicago Sky Blue (CSB) 3.182*10�2

Eosin B 2.884*10�2

Figure 12. Photocatalytic degradation rate of ZnO-NFs on MO, CR, CSB andEO.

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lected to measure the methyl orange degradation using UV-visspectrophotometer. It was observed that 99.05 % of MO dyewas degraded in 180 mins (Figure S2-S4).

Photocatalytic degradation mechanism

We proposed probable photocatalytic degradation mechanismby using ZnO-NFs with 3.29 eV band gap for degradation oforganic dye used in the following stepwise manners(Scheme 2):[23] (i) on irradiation of ZnO-NFs with UV light,

the excited electrons move to the conduction band (eCB�) leav-

ing behind the holes in the valence band (hVB+) (ii) the reaction

of holes in the valence band (hVB+) either with adsorbed H2O

from aqueous solution or with surface OH� on the surface ofZnO-NFs (iii) high oxidation potential of the holes (hVB

+) in thephotocatalyst permits the direct oxidation of the dye to re-active intermediates (iv) the hydroxyl radicals play vital role indegradation of organic dyes such as MO via formation of inter-mediates and finally generation of non-toxic products CO2 andH2O (Scheme 2).Therefore, ZnO-NFs exhibit higher photo-catalytic activities under UV light irradiation.

Further, the degradation of MO dye was monitored by us-ing Mass spectrometry at an interval of 10 mins which revealsthe formation of different degraded fragmental products of MOdye, which have been obtained after degradation under UVlight irradiation in presence of ZnO-NFs (Scheme 3, Figure S5).

Additionally, we have also recorded the percentage compo-sition of degraded dye solution by using elemental analysis as

shown in Table S1. The elemental results further confirmed thatstudy of MO degradation using ZnO-NFs under UV light irradi-ation is degradation, instead of decolorization. The photo-catalyst (ZnO-NFs) have showed nearly consistent (~99 %) pho-tocatalytic activity after 3 cycles and recycled catalyst wasfurther analyzed by SEM to see the morphological features (Fig-ure S6a and S6b). The reused ZnO-NFs have retained theircharacteristic features nearly the same. Thus, ZnO-NFs are notphoto-corroded and could be repeatedly used for photo-catalytic degradation of organic dyes molecules used withoutany noticeable decay in the efficiency. It is to be noted that thephotocatalyst ZnO-NFs synthesized in a facile manner exhibitsimproved photocatalytic performance towards degradation oforganic dye used (MO) when compared to reported values forthe same dye (Table 3).[24–31] Previous reports suggest that mor-phology (size, shape etc.), uniformity and surface based proper-ties of the photocatalyst plays important role on the photo-catalytic degradation efficiency and the time required todegrade the pollutants. The degradation of the dye also de-pends on the type of source used like- visible or UV, the type oflamp and its intensity. The ZnO nanorods take 180 min to de-grade only 32 %[24] of MO dye under UV irradiation while in oth-er case nanorods take 80 min to degrade 95 % of MO.[25] In an-other report the morphology resembling to ZnO nano-mushrooms shows the 92 % of MO degradation under UV lightin 210 mins.[26] Moreover, photocatalytic degradation study onZnO nanoplate and ZnO nanoplate-nanowire architecture hadshown that MO degraded more than 99 % under UV light irradi-ation in 90 and 50 mins, respectively.[27] In some cases, porousZnO nanobelt film and ZnO nanobelt film had shown morethan 99 % degradation ability against MO but the time requiredto degrade the MO was very high 160 and 400 mins for porousZnO nanobelt and ZnO nanobelt, respectively.[28] The ZnO thinfilms with different annealing temperatures have also beenstudied for the photocatalytic degradation of MO under UVlight and showed the degradation up to 88 %.[29] In addition, we

Scheme 2. Schematic showing the photodegradation of MO under UV lightirradiation using ZnO-NFs.

Scheme 3. Possible products obtained by photocatalytic degradation re-action.

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have also noticed some composite materials were also exam-ined such as ZnO/TiO2 composite had photocatalytic degrada-tion efficiency (98.88 %) in 100 mins.[30] The other compositeslike TiO2/chitosan, ZnO/chitosan and TiO2/ZnO/chitosan, undersolar light irradiation for 240 min also shows MO degradationto 71.8 %, 58.2 % and 97.8 %, respectively (Table 3).[31] Addition-ally, we also made the comparison using commercial ZnOwhich showed relatively low degradation efficiency (76.86 %)with longer time of exposure (130 min) to UV light (Table 3). Inpresent work, we have fabricated uniform morphology of ZnO-NFs which proved to be efficient photocatalyst when com-pared to the other morphologies reported. The photocatalyticdegradation of MO dye using ZnO-NFs under UV light irradi-ation is facile and showed the enhanced response (99.46 %,50 min).Thus, we could able to conclude that both the mor-phology and surface properties of the material can play a vitalrole for the degradation of organic pollutants.

Conclusions

ZnO nanoflowers (ZnO-NFs) have been constructed using ourrecently reported precursor, (A). The MO dye was adopted asthe model dye and photocatalysis behavior was carried out indetail. ZnO-NFs showed enhanced photocatalytic ability inpresence of UV light irradiation. In addition, different degrada-tion products have been analyzed by the mass spectroscopy.Moreover, the photocatalyst also shows the excellent activityagainst CR, CSB and EO. Thus, ZnO-NFs reported here could ac-tively participate to remediate in range of contaminants andmake the water pure.

Suppoting Information

Experimental section, other figures and table (Figure S1–S9 andTable S1) are present in supporting information.

Acknowledgements

The authors are grateful to the CSIR, New Delhi and IIT Indorefor funding. We would like to thank to Sophisticated In-strumentation Centre (SIC), IIT Indore for providing character-ization facilities. We gratefully acknowledge Advance ImagingCenter, IIT Kanpur for TEM results. A.M. is also grateful toMHRD, Government of India, for research fellowship.

Keywords: precursor · ZnO-Nanoflowers · photocatalyst · dyes-degradation · mass spectroscopy

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Materials Dye Degradation effi-ciency

Time Type of Irradiation References

Pure ZnO nanorods on polyester fibers MO 32 % 180 min UV [24]ZnO nanorods MO 95 % 80 min UV [25]ZnO nano-mushrooms MO 92 % 210 min UV [26]ZnO Nanoplateand ZnO Nanoplate-Nanowire architecture

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MO

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MO 71.8 %58.2 %97.8 %

240 min Simulated Solar irradi-ation

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ZnO commercial MO 76.86 % 130 min UVPresentwork

ZnO Nanoflowers(ZnO-NFs)

MO 99.46 % 50 min UV

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Submitted: May 5, 2016

Accepted: July 21, 2016

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