photocatalysis of two-dimensional honeycomb-like zno nanowalls on

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 maximiz

Photocatalysis of two-dimensional honeycomb-like ZnO nanowalls on

zeolite

Zhichao Liua, Zhifeng Liua,⇑, Ting Cuia, Junwei Lia, Jing Zhanga, Tao Chena, Xingchen Wanga,

Xiaoping Liangb

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

h i g h l i g h t s

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

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

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

a b s t r a c t

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 ancat-ionic 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 (TiO2) photocatalysis has gained increasing attention due to its bio-logical and chemical inertness and strong oxidizing power As is similar to TiO2, 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.

⇑ Corresponding author Tel.: +86 22 23085236; fax: +86 22 23085110.

E-mail address: tjulzf@163.com (Z Liu).

Contents lists available atScienceDirect

Chemical Engineering Journal

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c e j

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-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(NO3)26H2O) and hexamethylenetetramine (C6H12N4) with different concentrations (0.01, 0.02 and 0.05 mol l1) 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 Ka 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 (kmax= 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 inFig 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 inFig 1b and c Meanwhile, inFig 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 (1 0 0), (0 0 2) and (1 0 1) (JCPDS, No 36-1451) (Fig 1b and c) The intensities of (1 0 0) peak and (0 0 2) peak in ZnO are very strong compared with

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 (0 0 2) and (1 0 0) of ZnO So it is not

much difference betweenFig 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 (1 0 0) and (0 0 2) direction

Fig 2gives the EDS of two-dimensional honeycomb-like ZnO

nanowalls/zeolite, which shows correct stoichiometry of ZnO to

Al2O3and SiO2in 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 3presents 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 (inTable 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), dimensional honeycomb-like ZnO nanowalls/zeolite (c) and high-resolution TEM image of two-dimensional honeycomb-like ZnO nanowalls (d).

data can also come to it,Table 1shows the surface area of

ZnO/zeo-lite is 102 m2g1, however, the data of the zeolite and ZnO

nano-walls/zeolite are 197 m2g1and 395 m2g1, 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 fromFig 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 5gives 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 cm1is 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 cm1, in the peak of 1050–950 cm1is Si–O and Al–O bonds respectively And Zn–O bond appears in the peaks of 500–

400 cm1 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 degrabiodegra-dation 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

honeycomb-like ZnO nanowalls/zeolite.

Specimens Synthetic

zeolite ZnO/

zeolite Two-dimensional honeycomb-like ZnO nanowalls/zeolite

Datum of BET (Surface

Area/m 2

g 1 )

197 102 395

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 fromFig 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 7shows 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 irradidiscolor-ation

3.3 Effect of reaction time on performances of two-dimensional honeycomb-like ZnO nanowalls/zeolite

Fig 8gives 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 fromFig 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 0 0) and (0 0 2) The ZnO can grow along the two groups of planes but with different rates The (1 0 0) direction takes the lead in growth when ZnO seeds start to grow, then ZnO is also growing along (0 0 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 9demonstrates 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 l1) 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

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 l1 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

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

NH3þ H2O $ NHþ

There are two kinds of reaction routes including adsorbent pro-cess and photocatalytic degradation propro-cess 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.

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 The degradation rate of ZnO

nano-walls/zeolite has reached to nearly 90% when adsorption

biodegra-dation test was carried out for 30 min under UV light irradiation

when the calcining temperature is 320 °C, the hydrothermal

reac-tion time is 12 h and the concentrareac-tion of the growth solureac-tion is

0.02 mol l1 The results show that the as-prepared

two-dimen-sional honeycomb-like ZnO nanowalls/zeolite composite

photocat-alysts present a huge potential application in wastewater

treatment

Acknowledgements

The authors gratefully acknowledge financial support from

Na-tion Nature Science FoundaNa-tion of China (No 51102174) and

Nat-ural Science Foundation of Tianjin (11JCYBJC27000)

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