hexagonal zno nanorods assembled flowers for

Hexagonal ZnO nanorods assembled flowers for photocatalytic dye degradation: Growth, structural and optical properties Qazi Inamur Rahman a, ⇑ , Musheer Ahmad b , Sunil Kumar Misra c , Minaxi B. Lohani a, ⇑ a Department of Chemistry, Integral University, Lucknow 226026, India b Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India c Department of Chemistry, D.A.V College, Kanpur 208001, India article info Article history: Received 4 July 2013 Received in revised form 21 September 2013 Accepted 7 October 2013 Available online 14 October 2013 Key words: Nanostructures Chemical synthesis Catalytic properties Surfaces abstract A facile hydrothermal method was used to synthesize highly crys- talline hexagonal ZnO nanorods assembled flowers by the reaction of zinc acetate and hexamethylenetetraamine (HMTA) at 105 °C. The morphological characterizations revealed that well defined ZnO nanorods were assembled into flowers morphology. X-rays diffraction patterns showed the highly crystalline nature of ZnO with hexagonal wurtzite structure. The structural and optical prop- erties of hexagonal ZnO nanorods assembled flowers were mea- sured by Fourier transform infra-red (FT-IR) and ultraviolet– visible (UV–Vis) measurements. The as-synthesized hexagonal ZnO nanorods assembled flowers were applied as an efficient pho- tocatalyst for the photodegradation of organic dyes under UV-light irradiation. The methylene blue (MB) and rhodamine B (RhB) over the surface of hexagonal ZnO nanorods assembled flowers consid- erably degraded by 91% and 80% within 140 min respectively. The degradation rate constants were found to be k app (0.01313 mint 1 ) and k app (0.0104 mint 1 ) for MB and RhB dye respectively. The enhanced dye degradation might be attributed to the efficient charge separation and the large number of oxyrad- icals generation on the surface of the hexagonal ZnO nanorods assembled flowers. Ó 2013 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.10.011 ⇑ Corresponding author. Tel.: +91 9889307703. E-mail addresses: qaziinamur06@gmail.com (Q.I. Rahman), minaxilohani@gmail.com (M.B. Lohani). Superlattices and Microstructures 64 (2013) 495–506 Contents lists available at ScienceDirect Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices 1. Introduction Recently, the effluents of textile and dye industries are the main pollutants in water which cause serious damage to both flora and fauna life [1,2]. The colored organic dyes are heavily polluted the water system and imbalanced the eutrophication in the aquatic life [3,4]. The complete remediation of these dyes into less harmful chemicals is required to overcome these problems [5–7]. Among various dye remediation process, the heterogeneous photocatalytic process is well known method for the decomposition of hazardous waste materials especially organic compounds into less harmful chemicals [8]. In general, the semiconducting materials are required to facilitate the heterogeneous photocatalytic reaction. So far, many semiconductor materials such as TiO 2 , ZnO, Fe 2 O 3 , CdS, and ZnS are effectively used as photocatalysts because of their electronic structure of the metal atoms in chemical combination such as a filled valence band and an empty conduction band at ground state. Recently, the metal oxide semiconductors (TiO 2 , ZnO, Fe 2 O 3 ) have shown good photocatalytic activity toward the decomposition of harmful organic dyes into less harmful molecules under light illumination [9]. Zinc oxide (ZnO) semiconducting materials with the wurtzite hexagonal phase are recently gained the special attention among the various metal oxides due to presence of some exotic and fascinating properties. ZnO is well known n-type semiconductor and showing unique properties such as a wide band gap of 3.37 eV, high-excitation binding energy (60 meV), high thermo-mechanical stability, piezoelectric and optoelectric properties [10]. These fascinating properties of ZnO semiconductor make one of most important multifunctional materials which are used in various applications like fabrication of light emitting diodes (LEDs), laser diodes, surface acoustic wave filter, photonic crystal, photodetector, optical modular solar cells and chemical sensor [11]. The ZnO materials in nanoscale have shown a variety of nanostructures such as nanorods [12], nanobelts [13], nanotubes [14], nano- springs and nanospirals [15], polyhedral cages [16], porous webby [17], sea urchin and comb like [18] and other complicated morphologies [19,20]. However, it was reported that ZnO nanomaterials could absorb more fraction of solar spectrum as compared to other metal oxide nanomaterials [21]. ZnO nanomaterials have presented the impressive catalytic activity and quantum efficiency [22,23]. Usui reported the surfactant assisted chemical synthesis of ZnO nanorods and used as photocatalyst for efficient degradation of methylene blue (MB) dye with 90% degradation rate in 7 h [24]. While, Sun et al. demonstrated the photocatalytic degradation of MB dye over the surface of ZnO nanobelts with 94% degradation rate in 5 h under UV light illumination [25]. Recently, Zhang and Oh reported a comparative study of degradation of MB, Rhodamine-B and Methylene Orange dye by the activated carbon composite of TiO 2 [26]. Among various ZnO nanostructures, the nanorods morphology exhibit the high volume to surface ratio which might helpful for the generation of active sites during the photocatalytic degradation. In this work, we report on the synthesis of hexagonal shaped ZnO nanorods through the cationic surfactant cetyltrimethylammonium bromide (CTAB) assisted hydrothermal method. The possible growth mechanism has illustrated to explain the growth of hexagonal ZnO nanorods assembled flowers morphology. The synthesized hexagonal ZnO nanorods assembled flowers have been applied as photocatalysts for the degradation of MB and RhB dyes under UV light illumination. The kinetics of decoloration of organic dyes such as MB and RhB dyes are discussed. 2. Experimental details For the synthesis of hexagonal ZnO nanorods assembled flower morphology, 0.05 M zinc acetate (Zn(CH 3 COO) 2 2H 2 O, 99.8%), 0.05 M hexamethylenetetramine (HMTA; C 6 H 12 N 4 , 99.8%) and 1 M so- dium hydroxide (NaOH, Pellets, Sigma–Aldrich) solutions were prepared separately in deionized (DI) water under continuous stirring at ambient temperature. Firstly, HMTA solution was gradually added into Zn(CH 3 COO) 2 solution with constant stirring followed by drop wise addition of NaOH solu- tion to maintain the pH 11–12. The above solution was further stirred for 30 min and then gradually added into the cetyltrimethylammonium bromide (CTAB, 0.75 mM) solution. Afterward, the whole reaction mixture was transferred into a 250 ml Teflon-lined stainless steel autoclave and heated up 496 Q.I. Rahman et al. /Superlattices and Microstructures 64 (2013) 495–506 to 105 ± 5 °C for 10 h. After completion of period, the autoclave was allowed to cool at the room tem- perature and white precipitate was washed repeatedly with DI water followed by ethanol and acetone and dried in oven at 50 °C for 30 min. The morphological properties of as-synthesized hexagonal ZnO nanorods assembled flowers were examined by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high resolution TEM (HRTEM). For TEM analysis, the synthesized ZnO nanorods dispersed into acetone and a few drops of acetone containing ZnO nanorods poured on the TEM grid and exam- ined. The crystal phase and crystallinity were analyzed by powder X-ray diffraction (PXRD) with Cu K a radiations k = 1.54178 Å with 4°/min scanning rate in the range of 20–80°. The chemical composition was analyzed by using energy dispersive spectroscopy (EDX) coupled with FESEM. The quality and composition were examined by Fourier transform infrared (FTIR) spectroscopy in the range of 400– 4000 cm 1 . The optical property as-prepared ZnO nanorods were analyzed by UV–Vis absorption spec- troscopy at ambient temperature. The photocatalytic experiments of synthesized hexagonal ZnO nanorods assembled flower were measured by monitoring the decomposition of organic dyes (MB and RhB). The photocatalytic reaction was performed in Pyrex flask reactor under the light illumination with xenon arc lamp (Thoshiba, SHLS-1002) at ambient temperature. For the photodegradation of organic dyes such as MB and RhB dye, 0.15 g of synthesized ZnO photocatalyst was added in 10 ppm aqueous dye solution (100 mL) un- der constant stirring. Prior to illumination, dye suspension was continuously stirred for about 1 h to develop adsorption–desorption equilibrium between dye and photocatalyst under the dark. After- ward, the suspension was purged by oxygen to provide the stability of aqueous suspension in order to scavenge electron from the catalyst surface. Then, the stable aqueous dye suspension was exposed to UV light illumination under constant stirring. The sample was successively taken out from Pyrex reactor after every 10 min of time interval and subjected to centrifuge at 10,000 rpm to filter out ZnO powder, and then measured absorption spectroscopy of degrade dye solution using UV–Vis spec- trophotometer. Generally, the photo-catalytic degradation of dye followed the pseudo-first order kinetics and rate constant was determined by following relation; lnðC  =CÞ¼kt The k was calculated from graph between ln(C°/C) vs. time interval, where C° and C denote the dye concentration at time, t = 0 and t = t respectively. 3. Results and discussion 3.1. Structural and optical properties of synthesized hexagonal ZnO nanorods assembled flowers Fig. 1 shows the FESEM images of synthesized hexagonal ZnO Nanorods assembled flowers. The low magnification FESEM image (Fig. 1(a)) depicts the uniform and dense growth of hexagonal ZnO nanorods wherein these hexagonal ZnO nanorods are assembled into flower morphology. From Fig. 1(b), well defined hexagonal ZnO nanorods make an assembly of flower shape. Each ZnO nanorods possess the average diameter in the range of 250 ± 20 nm. Moreover, the hexagonal ZnO nanorods are tapering at their end and exhibit clean and smooth surface throughout their lengths. The crystallinity and crystal phase of as-synthesized hexagonal ZnO nanorods assembled flowers are analyzed by XRD, as shown in Fig. 1(c). The major reflections are appeared at 32.41°, 35.19°, 36.96°, 48.25°, 57.28°, and 63.41° corresponding to lattice planes of (1010), (0002), (1011), (1012), (1120), and (1 013) respectively. All the diffraction peaks in XRD are fully matched with pure wurtzite phase of ZnO crystals (JCPDS: 36-1451). No other peaks have been detected in XRD patterns, confirming that the synthesized nanorods exhibit pure ZnO crystal with hexagonal wurtzite phase. Furthermore, the energy dispersive X-ray (EDX) spectroscopy has been executed to investigate the chemical composition of synthesized hexagonal ZnO nanorods assembled flowers, as shown in Fig. 1(d). Only Zn and O peaks are observed in EDX spectrum, indicating the full agreement of stoichi- ometric value of zinc and oxygen in ZnO nanorods. It is noticed that any stray peaks in EDX spectrum are not detected; confirming the high purity of the synthesized hexagonal ZnO nanorods assembled Q.I. Rahman et al. /Superlattices and Microstructures 64 (2013) 495–506 497 flowers. Thus, XRD pattern and EDX spectrum clearly deduce that the synthesized hexagonal ZnO nanorods assembled flowers are constituted of Zn and O atoms only. The detailed morphological characterizations of synthesized hexagonal ZnO nanorods assembled flowers have further analyzed by TEM, as shown in Fig. 2. The low magnification TEM image (Fig. 2(a)) depicts the hexagonal ZnO nanorods which assembled in flowers manner which is consis- tent with FESEM results in terms of their morphology and dimensions. Fig. 2(b) shows the HR-TEM Fig. 1. FE-SEM (a) low magnification image, (b) high magnification image, (c) X-ray diffraction pattern of ZnO, and (d) EDX spectrum of synthesized hexagonal ZnO nanorods assembled flowers via hydrothermal method. Fig. 2. Distinctive (a) low magnification TEM image, (b) HR-TEM images of hexagonal ZnO nanorods assembled flowers. 498 Q.I. Rahman et al. /Superlattices and Microstructures 64 (2013) 495–506 image of ZnO nanorods which exhibits very clear lattice fringes with the distance between two par- allel lattice fringes of 0.52 nm. This value belongs to [0001] crystal plane of wurtzite phase of ZnO [27], indicating the defect free and good crystallinity of hexagonal ZnO nanorods. Hence, these observations affirm that synthesized hexagonal ZnO nanorods exhibit typical wurtzite single crystal- line structure with the preferential growth oriented along c axis [0001]. The quality and structure of as-synthesized hexagonal ZnO nanorods assembled flowers are char- acterized by Fourier transform infrared (FTIR) spectroscopy in the range of 400–4000 cm 1 as shown in Fig. 3(a). The appearance of strong IR band at 561 cm 1 represents the Zn–O stretching, indicating the formation of wurtzite phase ZnO [28]. The weak IR band at 862 cm 1 is attributed to presence of carbonate ion CO 2 3  [29]. Additionally, the IR band at 1424 cm 1 is due to the C–O bond in stretching mode which is usually come from the atmosphere. The synthesized hexagonal ZnO nanorods assem- bled flowers is further characterized by UV–Vis absorption spectrum to explain the structural and optical properties which is shown in Fig. 3(b). A strong absorption band at 372 nm is obtained by syn- thesized ZnO, which is matched up with the characteristic UV absorption band for bulk ZnO [30].No other absorption peak is detected in the spectrum, confirming the high purity of synthesized hexag- onal ZnO nanorods assembled flowers. 3.2. Plausible growth mechanism of hexagonal ZnO nanorods assembled flowers To investigate the growth of synthesized hexagonal ZnO nanorods assembled flower morphology, a possible mechanism is illustrated as Fig. 4. The mechanism can be explored by considering the in- volved reactions in the synthesis process. In this hydrothermal process, the hexamethylenetetramine (HMTA) solution gradually pours into zinc acetate solution, and subsequently release of hydroxyl ions by the thermal degradation of HMTA which further react with Zn 2+ ions to form zinc hydroxide (Zn(OH) 2 ) [31]. The sequential reactions are as follows; ZnðCH 3 COOÞ 2  2H 2 O þ H 2 O ! Zn 2þ þ 2CH 3 COO  ðCH 2 Þ 6 N 4 þ 6H 2 O $ 6HCHO þ 4NH 3 NH 3 þ H 2 O $ NH þ 4 þ OH  Zn 2þ þ 2OH  $ ZnðOHÞ 2 Later, Zn(OH) 2 again reacts with hydroxyl ions to form ZnðOHÞ 2 4 , Fig. 3. (a) FTIR spectrum and (b) UV–Vis spectrum of synthesized hexagonal ZnO nanorods assembled flowers via hydrothermal method. Q.I. Rahman et al. /Superlattices and Microstructures 64 (2013) 495–506 499 ZnðOHÞ 2 þ 2OH  ðfrom NaOHÞ!ZnðOHÞ 2 4 The addition of cetyltrimethylammonium bromide (CTAB) solution as a surfactant in reaction mixture reduces the surface tension and inhibits the formation of new phase due to its cationic behavior. Usu- ally, CTAB surfactant plays two pivotal roles in the synthesis, (i) CTAB can effectively control the mor- phology of building blocks for synthesis of hexagonal ZnO nanorods by selective adsorption of surfactants [32] and (ii) CTAB can facilitate the molecular aggregation above Critical Micelle Concen- tration (CMC) producing spherical micelles at relatively low concentration. The CTAB also facilitates to transport of ZnðOHÞ 2 4 growth units which come together to form individual hexagonal rod-like struc- tures which are further self-assembled into flower morphology. As the reaction aged at 105 °C for 10 h, ZnðOHÞ 2 4 ions dissociate to form ZnO nuclei as follow; ZnðOHÞ 2 4 ! ZnO þ H 2 O þ 2OH  The formation of ZnO nanorods assembled flower is usually preceded by two steps mechanism in aqueous solution. First nucleation involves the formation of aggregated ZnðOHÞ 2 4 nanoparticles and then covered by CTAB micelles which might take part of forming the hexagonal rod morphology. The second nucleation involves the formation of ZnO nanorods from the aggregated nanoparticle fol- lowed by thermal dissociation of ZnðOHÞ 2 4 to ZnO nuclei. In general, ZnO is polar crystal in which O 2 ions are in hexagonal close packing and Zn 2+ ions lie in the tetrahedral hole of four O 2 ions. The growth velocities of ZnO crystal in the different planes are as follows; [0001] > [00  1  1] > [01  10] > [01  11] > [000  1] in the aqueous phase condition [33]. The [000 1] faces are the most rapid-growth-rate planes of hexagonal ZnO crystals as compared to other growth facets [34]. However, the morphology of ZnO nanostructures is greatly affected by the growth velocity into the different directions. It is reported that Zn and O atoms are arranged alternatively along the c-axis Fig. 4. Plausible growth mechanism of hexagonal ZnO nanorods assembled flowers. 500 Q.I. Rahman et al. / Superlattices and Microstructures 64 (2013) 495–506 and top surface-plane where Zn is terminated to [0001] and is catalytically active along with chem- ically inert terminated O at the bottom surface [35]. At high pH and temperature, ZnO nuclei grow very rapid and form hexagonal rods like morphology. At prolonged heating, the hexagonal ZnO nano- rods might be assembled into flower shape due to electrostatic interaction between ions and polar surface. Importantly, the synthesized ZnO nanorods follow the same growth pattern as reported in the literature for ideal growth of hexagonal wurtzite ZnO crystals [36]. 3.3. Photocatalytic decomposition of organic dyes (methylene blue and rhodamine-B) using hexagonal ZnO nanorods assembled flowers The synthesized hexagonal ZnO nanorods assembled flowers are used as catalyst to study photo- catalytic activity towards the efficient degradation of organic dyes (MB and RhB). Fig. 5 shows the UV–Vis absorption spectra of degraded MB and RhB dye over the surface of synthesized ZnO under UV light illumination. In case of MB dye (Fig. 5(a)), the maximum absorption at k max = 661 nm of MB dye is continuously decreased with the increase of expose time from 0 to 140 min. Similarly, the maximum absorption at k max = 554 nm of RhB dye decrease as increasing the expose time from 0 to 140 min, as depicted in Fig. 5(b). The decreased in the relative intensities of absorption indicates the degradation of MB and RhB dye by synthesized hexagonal ZnO nanorods assembled flower as pho- tocatalyst under UV light illumination. Figs. 6 and 7 exhibit the typical time dependent photodegradation reaction efficiency plots with and without ZnO photocatalysts for MB and RhB dye. In general, the extent of dye degradation reaction mediated over the surface of ZnO nanorods is calculated by the following relation: Extent of degradation ð%Þ¼ðC o  C=C o Þ100 ¼ðA o  A=A o Þ100 where C o represents the initial concentration at time t = 0, while denotes C is the concentration at time = t, and A o shows initial absorbance and A corresponds to absorbance at time = t respectively. In both cases, no appreciable degradation occurs under UV light illumination in the absence of ZnO photocatalyst, suggesting the degradation facilitates by the ZnO nanomaterials. The degradation rates of MB and RhB dye are gradually increased with the increase of exposed time. The synthesized hex- agonal ZnO nanorods assembled flowers show the high degradation rate of 91% for MB dye as com- pared to RhB dye degradation (80%) within 140 min of expose time under UV-light illumination. The pie charts of MB and RhB dye degradation are shown in Figs. 6(b) and 7(b). The pie charts reveal that most of MB dye is degraded within 70 min (Fig. 6(b)) and then slower down, whereas most of RhB dye Fig. 5. (a) UV–Vis absorbance spectra of methylene blue, and (b) rhodamine B dye solution as function of time under UV-light illumination over hexagonal ZnO nanorods assembled flowers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Q.I. Rahman et al. /Superlattices and Microstructures 64 (2013) 495–506 501 degrades within 100 min (Fig. 7(b)). This suggests that the synthesized hexagonal ZnO nanorods assembled flowers presents highly active catalyst for MB dye as compared to RhB dye. Furthermore, the kinetics of organic dyes (MB and RhB) degradation reactions are presented in Fig. 8. Usually, the organic dyes follow apparent first order kinetics which is in good agreement with a general Langmuir–Hinshelwood mechanism; r ¼dC=dt ¼ kKC=1 þ KC where, r is the degradation rate of reactant (mg/1 min), C concentration of reactant (mg/l), t illumina- tion time, K adsorption coefficient of reactant (l/mg) and k reaction rate constant (mg/l min). If C is very small then the above equation could be simplified into; lnðC o =CÞ¼kKt  k app t In general, the plots between ln(C o /C) vs. time represents a straight line and the slope is equal to the apparent first order rate constant. The rate constants for MB and RhB dye are estimated to k app Fig. 6. (a) Extent of degradation rate of methylene blue dye in every successive time interval, and (b) pie chart of methylene blue dye degradation as function of time. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 7. (a) Extent of degradation rate of rhodamine B dye in every successive time interval, (b) pie chart of rhodamine B dye degradation as function of time. 502 Q.I. Rahman et al. / Superlattices and Microstructures 64 (2013) 495–506 (0.01313 mint 1 ) and k app (0.0104 mint 1 ) respectively, showing the first order kinetics. These results are consistent with previously reported works for MB and RhB dye degradation [37,4]. The mechanism of dye degradation over ZnO under UV light illumination is explained on the basis of reported literatures [4]. Upon illumination, an electron is excited and moves to conduction band (CB) and leaves hole in valence band (VB). This phenomenon creates the electron–hole pairs which might help in the degradation of dye. It is known that the adsorbed oxygen on the surface of ZnO play a pivotal role in the dye degradation reaction by combining with electron (  e) in conduction band and generate superoxide radical anion O Å 2 , instantaneously superoxide radical anion O Å 2 ÀÁ get protonated to yield HOO Å radicals [37]. On the other hand, the photo generated h + in VB reacts with H 2 O/OH  and dye molecules to generate an active species such as OH Å and dye + . The following steps are possible in the dye degradation over ZnO semiconductor under light illumination; ZnO þ hv !  eðconduction bandÞþh þ ðvalence bandÞ O 2 þ  e ! O  2 þ H þ ! HOO  h þ þ H 2 O !  OH þ H þ e  þ HOO  þ H þ ! H 2 O 2 ðFormation of H 2 O 2 Þ H 2 O 2 þ  e !  OH þ OH  ðFormation of  OH radicalsÞ Dye þ þfO 2 ; O  2 ; HOO  ; or  OHg!peroxy or hydroxylated Intermediate ! ! Mineralized product Therefore, the formation of active oxygen species fO 2 ; O  2 ; HOO  ; or  OHg over the surface of ZnO pho- tocatalyst significantly leads to the degradation of organic dye into less harmful material. Inclusively, the good optical and structural properties of synthesized hexagonal ZnO nanorods assembled flowers greatly affects their photocatalytic activity towards the degradation of MB and RhB dye under UV-light illumination. Additionally, the enhanced photocatalytic degradation might result from the high con- centration of defects over the surface of synthesized hexagonal ZnO nanorods assembled flowers [38]. Herein, as compared to degradation rate of RhB dye, the high degradation rate towards MB dye might be associated to its simple chemical structure. Moreover, the degradation rate with synthe- sized hexagonal ZnO nanorods assembled flowers is better than of those literatures on dye degrada- tion with ZnO flowers [39–41]. In order to check the stability and reproducibility of synthesized ZnO Fig. 8. Kinetics study of dye degradation of (a) methylene blue and (b) rhodamine B dye, exhibiting typical first order liner plot ln C o /C = f(t). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Q.I. Rahman et al. /Superlattices and Microstructures 64 (2013) 495–506 503 photocatalysts, the used photocatalysts are further characterized by XRD patterns (as presented in Fig. 9) which present the almost similar patterns of as-synthesized ZnO photocatalysts. No other dif- fraction peak is detected, confirming the stability of ZnO nanorods assembled flowers and reproduc- ibility of photocatalysts. Thus, the synthesized hexagonal ZnO nanorods assembled nanorods could be a suitable and excellent photocatalyst for dye remediation due to its good crystal quality, optical and structural properties. 4. Conclusion Highly crystalline hexagonal ZnO nanorods assembled flowers were synthesized by a facile hydro- thermal method using zinc acetate and hexamethylenetetraamine (HMTA) at 105 °C. Well defined ZnO nanorods were self-assembled into flowers morphology during the synthetic process. FT-IR and UV–Vis studies revealed good structural and optical properties of hexagonal ZnO nanorods assembled flowers. The photocatalytic activities of synthesized hexagonal ZnO nanorods assembled flowers car- ried out for the photodegradation of organic dyes (MB and RhB dye) under UV-light irradiation. The MB and RhB dye over the surface of hexagonal ZnO nanorods assembled flowers exhibited the degra- dation rates of 91% and 80% within 140 min respectively. The degradation rate constants were found to be k app (0.01313 mint 1 ) and k app (0.0104 mint 1 ) for MB and RhB dye respectively. The en- hanced dye degradation might be attributed to the efficient charge separation, generation of the large number of oxyradicals on the surface of the hexagonal ZnO nanorods assembled flowers and the chemical structure of dye molecule. Acknowledgement We are expressing our sincere thanks for Dr. A.R. Khan chairman of department of chemistry Inte- gral University Lucknow India, for his kind support and encouragement for our research work. References [1] J. McCann, B.N. Ames, Detection of carcinogens as mutagens in the salmonella/microsome test: assay of 300 chemicals: discussion, Proc. Natl. Acad. Sci. USA 73 (1975) 950–954 . [2] M.R. Hoffman, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69–96 . [3] S. Ameen, M.S. Akhtar, Y.S. Kim, H.S. Shin, Nanocomposites of poly(1-naphthylamine)/SiO 2 and poly(1-naphthylamine)/ TiO 2 : comparative photocatalytic activity evaluation towards methylene blue dye, Appl. Catal. 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From Fig. 1(b), well defined hexagonal ZnO nanorods make an assembly of flower shape. Each ZnO nanorods possess. hexag- onal ZnO nanorods assembled flowers. 3.2. Plausible growth mechanism of hexagonal ZnO nanorods assembled flowers To investigate the growth of synthesized hexagonal ZnO nanorods assembled flower. to explain the growth of hexagonal ZnO nanorods assembled flowers morphology. The synthesized hexagonal ZnO nanorods assembled flowers have been applied as photocatalysts for the degradation of MB