Photocatalysis of two-dimensional honeycomb-like ZnO nanowalls on zeolite Zhichao Liu a , Zhifeng Liu a, ⇑ , Ting Cui a , Junwei Li a , Jing Zhang a , Tao Chen a , Xingchen Wang a , Xiaoping Liang b a School of Materials Science and Engineering, Tianjin Chengjian University, 300384 Tianjin, China b School of Materials Science and Engineering, Tianjin Polytechnic University, 300387 Tianjin, China highlights  ZnO nanowalls were supported on synthetic zeolite from fly ash.  ZnO nanowalls/zeolite prepared by sol–gel and hydrothermal synthesis method.  Degradation of methylene blue in water can reach 90% after 30 min. graphical abstract article info Article history: Received 31 May 2013 Received in revised form 22 August 2013 Accepted 4 September 2013 Available online 20 September 2013 Keywords: Photocatalysis Two-dimension ZnO nanowalls Zeolite Fly ash Wastewater treatment abstract Recent years have seen a series of new materials and technologies in wastewater treatment. Among var- ious materials and technologies, the preparation and application of composite photocatalytic materials has received significant attention. We focus on ZnO/zeolite composite photocatalysts because of their superiority in wastewater treatment. Two-dimensional honeycomb-like ZnO nanowalls were fabricated on porous material of zeolite synthesized from fly ash by simple sol–gel and hydrothermal synthesis method in order to maximize the specific surface area and photocatalytic performance as well as easy to separation or recovery. The degradation of methylene blue dye in water can rapidly reach 90% with two-dimensional honeycomb-like ZnO nanowalls on zeolite composite materials after 30 min under UV light irradiation, which implies its huge potential application of photocatalysts in wastewater treatment. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction In recent years, the world is facing water crisis due to lacking of clean drinking water. With the fast development of various indus- tries, a huge quantity of wastewater has been produced from industrial processes and was discharged into soils and water sys- tems. Wastewater usually contains many pollutants such as cat- ionic and anionic ions, oil and organics, which have poisonous and toxic effects on ecosystems [1,2]. Because of this, purification and stabilization of environmental waste by titanium dioxide (TiO 2 ) photocatalysis has gained increasing attention due to its bio- logical and chemical inertness and strong oxidizing power. As is similar to TiO 2 , zinc oxide (ZnO) is also an important semiconduc- tor material, it has a promising outlook and has attracted much attention in solar cells [3], gas sensors [4,5], photocatalysts [6]. This is due to their many unique properties, for example, high elec- trochemical stability and excellently electronic properties. As is well known, when the particle sizes of many semiconductors decrease to nanometer or sub-nanometer scales, these materials 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.09.022 ⇑ Corresponding author. Tel.: +86 22 23085236; fax: +86 22 23085110. E-mail address: tjulzf@163.com (Z. Liu). Chemical Engineering Journal 235 (2014) 257–263 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej usually exhibit quantum size effects, presenting different electric and optical properties from bulk materials [7–9]. ZnO clusters have a lot of defects including small particle size, instability and very susceptible to aggregate, so many materials such as glass, polymers and zeolites are usually used as supports or stabilizers in various preparation methods [10–12]. Among the various supports, zeolite is attractive candidate due to its unique uniform pores and channel sizes, high adsorption capacity, and hydrophobic and hydrophilic properties to confine the ZnO clusters and limit the growth of par- ticles [11–13]. As is known from a large number of literatures, ZnO has been attracting intensive interest not only because of its excellent elec- trical and optical performances but also because of its various mor- phologies [14]. It exhibits one of the largest morphology families of nanostructure system, such as nanopowders [15], nanofilms [16], nanotubes [17], nanobelts [18,19], nanowires [19,20], nanorods [21–23] and nanowalls [24,25]. These nanostructures are more conducive to response due to the increase of contact area with the external environment. So the one- or two-dimensional self- assembled ZnO structures with a controllable size and morphology has also become a hot topic. To the best of our knowledge, little attention is paid to the two- dimensional ZnO nanostructure/zeolite composite material. In the previous works, enough of the zeolite has been prepared using fly ash as original material by hydrothermal synthesis technique. In this paper, the highly oriented two-dimensional honeycomb-like ZnO nanowalls are grown on preexisting textured ZnO nanoparti- cles seed layer on the surface of zeolite synthesized from fly ash via sol–gel and hydrothermal synthesis method. As a two-dimen- sional nanostructure with a high porosity, the vertically aligned ZnO nanowalls exhibit great promise for photocatalytic applica- tion, and the performance of ZnO/zeolite composite material is much higher than individual ZnO or zeolite. To illustrate the effect of the two-dimensional honeycomb-like ZnO nanowalls/zeolite, a series of composite photocatalysts were designed and synthesized. The purpose of this study was to detailedly examine the synthesis process and the effect of technical parameters, such as calcining temperature, the concentration of the growth solution and reaction time on photocatalytic reactivity of the supported ZnO nanowalls for the degradation of methylene blue dye in water under UV light irradiation. 2. Experimental section 2.1. Materials All chemicals were of analytical reagent grade and used without further purification. And all the aqueous solutions were prepared using distilled water. 2.2. Preparation of synthetic zeolite The pure-form zeolites were synthesized using fly ash by the al- kali fusion method. Firstly, the mixture of hydrochloric acid solu- tion (50%) and fly ash (ratio, 10:1 (mL/g)) was sealed in a beaker, which was kept in a water bath at 80 °C for 2 h. Then the fly ash would be filtered, washed repeatedly with distilled water and dried at 100 °C for 12 h. Secondly, 10 g of the treated fly ash was mixed with 13 g of NaOH to obtain a homogeneous mixture, after which the mixture would be heated in a crucible in air at 600 °C for 120 min. Thirdly, the fusion product was dissolved in distilled water (ratio, 1:10 (g/mL)), and then there would be an aging pro- cess with vigorous agitation at 25 °C for 24 h. The mixture was then crystallized at 100 °C for 12 h. Finally, the as-prepared sample is separated by filtering, washing with distilled water (until the fil- trate pH reached to 7), dried at 100 °C and being kept in powder form for further use. 2.3. Synthesis of two-dimensional honeycomb-like ZnO nanowalls/ zeolite The preparation of two-dimensional honeycomb-like ZnO nano- walls/zeolite composite material includes the coating of ZnO seed layer on surface of zeolite and the growth of two-dimensional ZnO. First of all, ZnO seed layer was synthesized by a sol–gel meth- od using zinc acetate dehydrate (Zn(CH 3 COO) 2 , 15 mmol) as the starting material, 2-methoxyethanol as the solvent (CH 3 OCH 2 CH 2- OH, 50 mL) and ethanolamine (MEA, NH 2 CH 2 CH 2 OH, 1.43 mL) as the stabilizer. The seed solution was stirred at 50 °C for 2 h until yielding a clear and homogeneous solution. Secondly, ZnO nano- particles/zeolite composites were obtained by mixing the appro- priate amount of ZnO seed layer solution with the powder form of synthetic zeolite at 50 °C for 15 min. After drying at 90 °C for 12 h, the ZnO nanoparticles/zeolite composites were obtained after annealing in air at different calcining temperatures (280, 320 and 400 °C) for 1 h. Finally, two-dimensional honeycomb-like ZnO nanowalls were obtained in the aqueous solution of zinc nitrate hexahydrate (Zn(NO 3 ) 2 Á6H 2 O) and hexamethylenetetramine (C 6 H 12 N 4 ) with different concentrations (0.01, 0.02 and 0.05 mol l À1 ) after heating at 90 °C for different reaction time (2, 6 and 12 h). The as-obtained products were finally dried in air. 2.4. Characterization The morphology of the samples was observed using a HITACHI S-4800I field emission scanning electron microscope (FE-SEM) and HITACHI H-7650 transmission electron microscopy (TEM) operated at an accelerating voltage of 100 kV. The EDS spectra of the sam- ples analysis were also performed during the FE-SEM observation. The X-ray diffraction (XRD) analysis of the nanostructures was per- formed using a Rigaku D/max-2500 using Cu K a radiation (k = 0.154059 nm). The total surface area, pore size distribution and nitrogen ad/desorption isotherms were calculated using Nova 3000e Surface Area Analyzers. IR adsorption spectra with transmis- sion mode were recorded on BIO-RAD FTS3000 IR spectrometer. Photodegradation of methylene blue was performed by UV irradi- ation using a 30 W ultraviolet lamp (k max = 365 nm). The photode- composition reactions were carried out in a quartz reactor, equipped with a cold finger to avoid thermal reactions. In a typical reaction, 0.1 g of the catalyst and 100 mL of dye solution with a concentration of 10 mg/L were stirred and irradiated for several hours. Aliquots were collected at different times during the irradi- ation, and the concentration of the residual methylene blue was monitored by UV–visible spectrophotometry. 3. Results and discussion 3.1. Characteristics of two-dimensional honeycomb-like ZnO nanowalls/zeolite X-ray diffraction of ZnO/zeolite with the composition of differ- ent forms is depicted in Fig. 1a–c. It can be seen that all these sam- ples maintain good zeolite crystal structure. There are some obvious peaks of zeolite at 2h = 5.6°, 11.2°, 16.2°, 28.2° and 31.4° (JCPDS, No. 52-0142) (Fig. 1a), moreover, these peaks of zeolite are also observed in Fig. 1b and c. Meanwhile, in Fig. 1b and c the obvious peaks are also displayed at 2h = 31.8, 34.4, 36.3, which is regarded as an attributive indicator of ZnO (100), (002) and (10 1) (JCPDS, No. 36-1451) (Fig. 1b and c). The intensities of (10 0) peak and (002) peak in ZnO are very strong compared with 258 Z. Liu et al. /Chemical Engineering Journal 235 (2014) 257–263 those of other peaks (Fig. 1c). Further structure characterization of the ZnO crystals was performed by TEM (Fig. 1d). The high-resolu- tion TEM image further confirms that the ZnO nanorod is a single- crystal characteristic and the lattice spacing of 0.26 nm and 0.28 nm corresponds to the (002) and (10 0) of ZnO. So it is not much difference between Fig. 1b and c in the XRD patterns when ZnO microstructure is composed by a hexagonal prism-type single crystal. And TEM also provides the evidence that ZnO has a pre- ferred orientation along (100) and (0 02) direction. Fig. 2 gives the EDS of two-dimensional honeycomb-like ZnO nanowalls/zeolite, which shows correct stoichiometry of ZnO to Al 2 O 3 and SiO 2 in the nanocomposite structure. The mass percent- age of the element of Zn, O, Al and Si in the nanocomposite struc- ture was 18.49 wt.%, 42.91 wt.%, 13.31 wt.% and 17.99 wt.%, respectively. So these four elements of total quality percentage is 92.7 wt.%. Meanwhile, these four elements of total atomic percent- age is 94.73 at.%. This result means that the phases of the samples are almost pure. Fig. 3 presents the SEM images of zeolite (Fig. 3a), ZnO/zeolite (Fig. 3b) and ZnO nanowalls/zeolite (Fig. 3c and d). As shown in Fig. 3a–c, ZnO nanostructure is just supported on the surface of the zeolite, the specific surface area (in Table 1) of the composite materials will decrease when some zeolite holes are plugged by ZnO nanostructure. However, the specific surface area of the com- posite materials will greatly increase when there is ZnO two- dimensional honeycomb-like nanostructure (Fig. 3d). Additional Fig. 1. X-ray diffraction patterns of synthetic zeolite (a), ZnO/zeolite (b), two-dimensional honeycomb-like ZnO nanowalls/zeolite (c) and high-resolution TEM image of two- dimensional honeycomb-like ZnO nanowalls (d). Fig. 2. EDS elemental analysis of two-dimensional honeycomb-like ZnO nanowalls/zeolite. Z. Liu et al. /Chemical Engineering Journal 235 (2014) 257–263 259 data can also come to it, Table 1 shows the surface area of ZnO/zeo- lite is 102 m 2 g À1 , however, the data of the zeolite and ZnO nano- walls/zeolite are 197 m 2 g À1 and 395 m 2 g À1 , respectively. The pore size distribution and nitrogen ad/desorption isotherms of two-dimensional honeycomb-like ZnO nanowalls/zeolite are depicted in Fig. 4. As can be seen from Fig. 4a, the pore size of as-synthesized samples distribution at about 5 nm, this proves that the two-dimensional honeycomb-like ZnO nanowalls/zeolite composite materials are mesoporous materials. Fig. 4b shows the nitrogen ad/desorption isotherms are categorized as Type H3 (IU- PAC) hysteresis loops. Type H3 is formed by fissure hole material or slice flaky particulate material, and critical increase appeared at high relative pressure. Consequently, this also supports the structure characteristics of the two-dimensional honeycomb-like ZnO nanowalls/zeolite composite materials. Fig. 5 gives the infrared spectra of zeolite, ZnO/zeolite and ZnO nanowalls/zeolite, respectively. It can be seen that the characteris- tic peaks of the zeolite are not changed when ZnO nanostructure loads on the zeolite. A shoulder between 3550 and 3400 cm À1 is as- signed to the asymmetrical stretching of H–O–H or O–H bonds, and the bending vibration of the water molecules appear in the 1700– 1600 cm À1 , in the peak of 1050–950 cm À1 is Si–O and Al–O bonds respectively. And Zn–O bond appears in the peaks of 500– 400 cm À1 . So it can be concluded that ZnO has little effect on the structure of the zeolite during the growth process of ZnO. Fig. 6 demonstrates the photocatalytic activities of zeolite, ZnO/zeolite, ZnO nanowalls/zeolite under UV light irradiation. Also, in order to explain the adsorption performance of zeolite, the homologous photocatalytic experiment of zeolite is texted without UV light irradiation. The degradation rate of ZnO nano- walls/zeolite has reached to nearly 90% when adsorption biodegra- dation test was carried out for 30 min, meanwhile the degradation rate of zeolite and ZnO/zeolite has just reached to nearly 70% and 30% respectively. The degradation rate of ZnO/zeolite slowly close Fig. 3. SEM images of synthetic zeolite (a), ZnO/zeolite (b) and two-dimensional honeycomb-like ZnO nanowalls/zeolite (c and d). Table 1 Brunauer–Emmett–Teller of zeolite, ZnO/zeolite and two-dimensional honeyco mb- like ZnO nanowalls/zeolite. Specimens Synthetic zeolite ZnO/ zeolite Two-dimensional honeycomb-like ZnO nanowalls/zeolite Datum of BET (Surface Area/m 2 g À1 ) 197 102 395 Fig. 4. The pore size distribution and nitrogen ad/desorption isotherms of two-dimensional honeycomb-like ZnO nanowalls/zeolite. 260 Z. Liu et al. /Chemical Engineering Journal 235 (2014) 257–263 to the ZnO nanowalls/zeolite with the increase of photocatalytic time. However, it should be noted that the degradation rate of zeo- lite gradually leveling off in nearly 80%. Because ZnO is also an important semiconductor material, many pollutants can be de- graded due to its biological and chemical inertness and strong oxi- dizing power. The two-dimensional honeycomb-like ZnO nanostructure has greatly improved the contact area with the out- side environment than other ZnO nanostructures. In order to verify whether such degradation was caused by adsorption or photoca- talysis, the adsorption profile of zeolite is provided through meth- ylene blue decoloration experiment without UV light irradiation. As can be seen from Fig. 5, the absorption efficiency of zeolite al- most no difference between without UV light irradiation and with UV light irradiation conditions. 3.2. Effect of calcining temperature on performances of two- dimensional honeycomb-like ZnO nanowalls/zeolite Fig. 7 shows the decoloration of methylene blue for different calcining temperatures (280, 320 and 400 °C) treatment on performances of two-dimensional honeycomb-like ZnO nano- walls/zeolite. It can be seen that the degradation efficiency of two-dimensional honeycomb-like ZnO nanowalls/zeolite at 320 °C is slightly stronger than these samples at 280 °C and 400 °C. However, the degradation rate of ZnO nanowalls/zeolite with different calcining temperatures (280, 320 and 400 °C) have reached to more than 80% when adsorption biodegradation test was carried out for 30 min. Other studies have shown that the crys- tal structure transition temperature of ZnO is between 300 °C and 400 °C [23,26]. Above results also demonstrated that the calcining temperature can change the crystalline structure of ZnO loading on the zeolite, resulting in the difference on methylene blue discolor- ation under UV irradiation. 3.3. Effect of reaction time on performances of two-dimensional honeycomb-like ZnO nanowalls/zeolite Fig. 8 gives the effect of reaction time (2, 6, 12 h) on methylene blue decoloration for two-dimensional honeycomb-like ZnO nano- walls/zeolite composite photocatalysts. It is obvious that the deg- radation efficiency of two-dimensional honeycomb-like ZnO nanowalls/zeolite increases with the increasing of growth time from Fig. 8. The morphology of ZnO nanostructures will be affected by the reaction time during the ZnO growth solution. The ZnO nucleates typically show two groups of crystal surface: (1 00) and (002). The ZnO can grow along the two groups of planes but with different rates. The (100) direction takes the lead in growth when ZnO seeds start to grow, then ZnO is also growing along (00 2) direction as time goes on. So the specific surface area of ZnO nanostructures is expanded as the increasing of growth time, thereby the degradation efficiency will also be increasing in such conditions. 3.4. Effect of the concentration of the growth solution on performances of two-dimensional honeycomb-like ZnO nanowalls/zeolite Fig. 9 demonstrates the photocatalytic activities of two-dimen- sional honeycomb-like ZnO nanowalls/zeolite composite photocat- alysts for different concentrations (0.01, 0.02 and 0.05 mol l À1 )of Fig. 5. Transmission FT-IR spectra of ZnO/zeolite with the composition of different forms. Fig. 6. Decoloration of methylene blue for ZnO/zeolite with the composition of different forms. Fig. 7. Decoloration of methylene blue for different calcining temperature of two- dimensional honeycomb-like ZnO nanowalls/zeolite. Fig. 8. Decoloration of methylene blue for two-dimensional honeycomb-like ZnO nanowalls/zeolite with different reaction time. Z. Liu et al. / Chemical Engineering Journal 235 (2014) 257–263 261 the growth solution. However, the degradation efficiency of two- dimensional honeycomb-like ZnO nanowalls/zeolite for different concentrations of the growth solution have not too much of a dif- ference. Concluded from this figure, the concentration of the growth solution should be moderate, which is controlled by the concentration of ZnO growth solution and kinetics. So integrating all factors, 0.02 mol l À1 was chosen as the concentration of the growth solution in our experiment. 3.5. Investigations on the growth mechanism of two-dimensional honeycomb-like ZnO nanowalls/zeolite In our study, two-dimensional honeycomb-like ZnO nanowalls were supported on synthetic zeolite from fly ash by a simple sol– gel and hydrothermal synthesis method, which are illustrated in Fig. 10. Fig. 10a and b display the ball model of zeolite molecular sieve, zeolite is an attractive sorptive material owing to its unique uniform pores and channel sizes, high adsorption capacity, and hydrophobic and hydrophilic properties, which may provide selec- tive exclusion of undesired molecules or ions. The preparation of two-dimensional honeycomb-like ZnO nanowalls/zeolite compos- ite material includes the coating of ZnO seed layer on surface of zeolite and the growth of two-dimensional ZnO (Fig. 10a–f). In this growth process, The ZnO nucleates typically show two groups of crystal surface: (10 0) and (002), where the (00 2) surface is per- pendicular to another. The ZnO can grow along the two groups of planes but with different rates, which are controlled by the ZnO kinetics. The (10 0) direction takes the lead in growth when ZnO seeds start to grow (Fig. 10c), then ZnO is also growing along (00 2) direction as time goes on (Fig. 10d). The growth of ZnO nanowalls form aqueous solution is based on the formation of solid phase from a solution, in this experiment, Zn(NO 3 ) 2 is used as source of zinc and C 6 H 12 N 4 as source of OH À . With the increase of grow temperature, The C 6 H 12 N 4 begins to decompose into ammonia and then Zn(OH) 2 occurred. The ZnO film grows from the nuclei precipitation on the substrate because this solution is heated (These can be represented by the following reactions) [23,26,27]. After an appropriate time, ZnO nanostructure will form the two-dimensional honeycomb-like ZnO nanowalls structure (Fig. 10e and f). Fig. 10e is a plan view of such a nanostructure, Fig. 10g is a honeycomb in order to more visual expresses the im- age of such two-dimensional honeycomb-like ZnO nanowalls/zeo- lite structures. CH 2 N 4 þ 6H 2 O ! 6HCHO þ NH 3 ð1Þ NH 3 þ H 2 O $ NH þ 4 þ OH À ð2Þ 2OH À þ Zn 2þ ! ZnðOHÞ 2 ð3Þ ZnðOHÞ 2 ! ZnO ðsÞ þ H 2 O ð4Þ There are two kinds of reaction routes including adsorbent pro- cess and photocatalytic degradation process in the experimental process for catalytic degradation of methylene blue by using two-dimensional honeycomb-like ZnO nanowalls/zeolite composite materials. The methylene blue organic molecules were firstly adsorbed on the surface of ZnO nanowalls and the outside or inside of zeolites. Meanwhile, the photocatalytic degradation process would take place in the surface of ZnO nanowalls, Then methylene blue organic molecules in the inside or outside of zeo- lites would be transferred to the surface of ZnO nanowalls when Fig. 9. Decoloration of methylene blue for two-dimensional honeycomb-like ZnO nanowalls/zeolite with different concentrations of the growth solution. Fig. 10. Schematic diagram of two-dimensional honeycomb-like ZnO nanowalls/zeolite. 262 Z. Liu et al. / Chemical Engineering Journal 235 (2014) 257–263 the concentration of methylene blue organic molecules on the sur- face of ZnO nanowalls would be gradually reduced, this is based on the principle of diffusion. So these processes played a role in the methylene blue adsorption degradation experiment repeatedly un- til the end of the experiment. 4. Conclusions Two-dimensional honeycomb-like ZnO nanowalls were sup- ported on synthetic zeolite from fly ash by a simple sol–gel and hydrothermal synthesis method in order to maximize the specific surface area and photocatalytic performance as well as the retriev- ability. The technologic parameters, such as calcining temperature, reaction time and the concentration of the growth solution, have an important effect on the structure and photocatalytic activity of the two-dimensional honeycomb-like ZnO nanowalls/zeolite composite photocatalysts. 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Ya, ZnO/CuInS 2 Core/Shell heterojunction nanoarray for photoelectrochemical water splitting, International Journal of Hydrogen Energy 37 (2012) 15029–15037 . Z. Liu et al. / Chemical Engineering Journal 235 (2014) 257–263 263 . Synthesis of two-dimensional honeycomb-like ZnO nanowalls/ zeolite The preparation of two-dimensional honeycomb-like ZnO nano- walls/zeolite composite material includes the coating of ZnO seed layer on. the concentration of the growth solution in our experiment. 3.5. Investigations on the growth mechanism of two-dimensional honeycomb-like ZnO nanowalls/ zeolite In our study, two-dimensional honeycomb-like. Decoloration of methylene blue for two-dimensional honeycomb-like ZnO nanowalls/ zeolite with different concentrations of the growth solution. Fig. 10. Schematic diagram of two-dimensional honeycomb-like