solgel based hydrothermal method for the synthesis of 3d flowerlike zno microstructures

Author's Accepted ManuscriptSol gel-based hydrothermal method for the synth-esis of 3D flower-like ZnO microstructures com-posed of nanosheets for photocatalytic applications Xiaohua Z

Author's Accepted Manuscript

Sol gel-based hydrothermal method for the

synth-esis of 3D flower-like ZnO microstructures

com-posed of nanosheets for photocatalytic applications

Xiaohua Zhao, Feijian Lou, Meng Li, Xiangdong Lou

, Zhenzhen Li, Jianguo Zhou

DOI: http://dx.doi.org/10.1016/j.ceramint.2013.10.140

Reference: CERI7567

To appear in: Ceramics International

Received date: 13 August 2013

Revised date: 29 October 2013

Accepted date: 29 October 2013

Cite this article as: Xiaohua Zhao, Feijian Lou, Meng Li, Xiangdong Lou, Zhenzhen Li, Jianguo Zhou, Sol gel-based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures composed of nanosheets for photocatalytic applications, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2013.10.140

This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply

to the journal pertain.

www.elsevier.com/locate/ceramint

Sol gel-based hydrothermal method for the synthesis of 3D flower-like ZnO microstructures

composed of nanosheets for photocatalytic applications

Xiaohua Zhaoa, b, Feijian Louc, Meng Lib, Xiangdong Loub, *, Zhenzhen Lib, Jianguo Zhoua, *

School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210097, PR China

Corresponding authors Tel.: +86 13 623731736; fax: +86 37 33326336

chemenglxd@126.com

xhzhao79@163.com

Abstract: Self-assembled 3D flower-like ZnO microstructures composed of nanosheets have been

prepared on a large scale through a solgel-assisted hydrothermal method using Zn(NO3)2·6H2O, citric acid, and NaOH as raw materials The product has been characterized by X-ray powder diffraction

(XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy

(TEM), and scanning electron microscopy (SEM) The optical properties of the product have been

examined by room temperature photoluminescence (PL) measurements A possible growth mechanism

of the 3D flower-like ZnO is proposed based on the results of experiments carried out for different

hydrothermal treatment times Experiments at different hydrothermal treatment temperatures have also

been carried out to investigate their effect on the final morphology of the ZnO The photocatalytic

activities of the as-prepared ZnO have been evaluated by photodegradation of Reactive Blue 14 (KGL)

under ultraviolet (UV) irradiation The experimental results demonstrated that self-assembled 3D

*

Corresponding author Tel.: +86 13623731736; Fax: +86 3733326336

E-mail address: chemenglxd@126.com (Xiangdong Lou)

E-mail address: xhzhao79@163.com (Jianguo Zhou)

flower-like ZnO composed of nanosheets could be obtained over a relatively broad temperature range

(90150 ºC) after 17 h of hydrothermal treatment All of the products showed good photocatalytic

performance, with the degree of degradation of KGL exceeding 82% after 120 min In particular, the

sample prepared at 120 ºC for 17 h exhibited superior photocatalytic activity to other ZnO samples and

commercial ZnO, and it almost completely degraded a KGL solution within 40 min The relationship

between photocatalytic activity and the structure, surface defects, and surface areas of the samples is also

discussed

Key words: nanosheets; self-assembled 3D flower-like structures; ZnO; solgel-based hydrothermal

method; photocatalysis

1 Introduction

As one of the most important metal oxides and semiconductors, zinc oxide (ZnO) has found

applications in a wide range of fields, including light-emitting diodes [1], nanolasers [2], field-effect

transistors [3], solar cells [4], gas sensors [5,6], among others [7] Besides the above applications, ZnO is

also regarded as a promising photocatalytic material in the UV spectral range due to its wide direct band

gap (3.37 eV), high exciton binding energy (60 meV), excellent chemical/thermal stability, high transparency, and non-toxicity [810] It has been proven that the activity of photocatalysts is strongly influenced by the microstructures of the photocatalytic materials, such as crystal size, orientation and

morphology, aspect ratio, and even crystalline density [11] Therefore, study of the microstructure of

ZnO is highly relevant to research and applications in photocatalysis

The self-assembly of nanoscaled building blocks into complex structures has been a recent hot topic in

research Much attention has been paid to the organization of complex micro-/nanoarchitectures,

especially three-dimensional (3D) hierarchical architectures [12] Compared with low-dimensional

structures, 3D ZnO hierarchical architectures provide an effective means of maintaining high specific

surface area and preventing aggregation during photocatalytic reaction processes, leading to enhanced

photocatalytic performance [13] One of the 3D hierarchical architectures adopted by ZnO has a

flower-like appearance, and there have been many reports on the synthesis of flower-like ZnO during the

past few years [5,1236] However, most of these reports have been concerned with flower-like ZnO composed of nanorods [1424], including hexagonal nanorods [1518], sword-like nanorods [1722], needle-like nanorods [23,24], or other forms of flower-like ZnO [5,2531] Reports about flower-like ZnO composed of nanosheets have been rare [12,13,3235] Moreover, some methods have been based

on tedious operations and rigorous experimental conditions, and have required expensive substrates, complex template agents, or high temperatures [3335] Compared with other flower-like ZnO, self-assembled flower-like ZnO composed of nanosheets usually shows more of the (0001) plane and a

greater surface area, which should improve its photocatalytic activity [13] There is still a need to

develop a method for preparing 3D flower-like ZnO microstructures composed of nanosheets that avoids

the use of toxic reagents or expensive substrates In our previous work [37], we synthesized ZnO with

different microstuctures, including 3D flower-like ZnO composed of nanosheets However, the detailed

formation mechanism and photocatalytic activity of this flower-like ZnO were not investigated

Herein, we report the further use of this facile, low-cost, green solgel-based hydrothermal method and

an investigation of the dependence of the morphology evolution of the self-assembled flower-like ZnO composed of nanosheets on the hydrothermal treatment time (017 h) and temperature (90150 ºC)

Moreover, the corresponding photocatalytic activities have also been studied The experimental results

have indicated that the self-assembled flower-like ZnO composed of nanosheets could be obtained from

90 to 150 ºC after 17 h of hydrothermal reaction, or at 120 ºC after 4 h The ZnO synthesized at 120 ºC

for 17 h exhibited superior photocatalytic activity to other ZnO samples, such that the degree of

degradation of KGL reached almost 100% after 40 min of UV irradiation This could be attributed to the

particular morphology, surface defects, and surface area of the photocatalyst

2 Experimental section

2.1 Materials

All chemicals were purchased from Shanghai Chemical Industrial Co Ltd (Shanghai, China), and

were used without further purification Distilled water was used in the reaction system as the solvent

medium

2.2 Synthesis of 3D flower-like ZnO microstructures composed of nanosheets

<1> In this work, 3D flower-like ZnO samples were synthesized by the following procedure A sol

was first prepared by adding Zn(NO3)2·6H2O (3.756 g) and citric acid (C6H8O7; 5.250 g) to distilled water

(100 mL) and stirring at 70 ºC The sol was then placed in an oven at 100 ºC to form the gel Secondly, 1 m

NaOH was directly dropped into the dry gel under constant stirring, until a suspension of pH 14 was

obtained The resulting suspension was then transferred to a 100 mL Teflon-lined stainless steel autoclave

and heated at 120 ºC for 17 h Finally, the white precipitate was collected, washed thoroughly with

distilled water, and then dried at 100 °C to obtain the final sample

<2> In order to reveal the growth mechanism of the 3D flower-like ZnO, experiments were

conducted for different hydrothermal treatment times (0, 4, 8, and 12 h) while the other conditions were

kept unchanged

<3> In order to investigate the influence of hydrothermal treatment temperature on the final

morphology of the ZnO, experiments at different hydrothermal treatment temperatures (90, 150 ºC) were

also carried out while the other conditions were kept unchanged

2.3 Characterization

The as-synthesized samples were characterized by X-ray diffraction (XRD) (Bruker Advance-D8

XRD with Cu-K radiation, =0.154178 nm, the accelerating voltage was set at 40 kV with a 100 mA

flux) Microstructures and morphologies were investigated by field-emission scanning electron

microscopy (FESEM; JSM-6701F, JEOL), transmission electron microscopy (TEM; JEM-2100, JEOL),

and scanning electron microscopy (SEM; JSM-6390LV, JEOL) Photoluminescence (PL) spectra were

measured on a Shimadzu RF-5301PC fluorescence spectrophotometer The surface areas of the samples were determined from nitrogen adsorptiondesorption isotherms using an ASAP 2000 instrument and the BrunauerEmmettTeller (BET) method was used for surface area calculation

2.4 Photocatalytic experiments

The photocatalytic activities of the as-synthesized samples were evaluated by the degradation of

aqueous KGL solution Photocatalyst (100 mg) was added to 250 mL of 20 mg/L KGL solution and the

mixture was stirred for 20 min to reach absorption equilibrium and then exposed to UV light (300 W

high-pressure Hg lamp; maximum emission at 365 nm) In order to minimize temperature fluctuations,

water at room temperature was employed to absorb the heat generated from the UV light and the test tube

containing the KGL solution was rotated at a distance of 10 cm from the center of the lamp Samples

were collected at intervals of 20 min, centrifuged, and the supernatants were characterized by UV/Vis

spectrophotometry (UV-5100, Shanghai Metash Instruments Co Ltd., China) to monitor the degradation

of the KGL The characteristic absorption peak of KGL at =608 nm was chosen to monitor the

photocatalytic degradation process For comparison, commercial ZnO powder purchased from Tianli

Chemical Reagent Co Ltd (99.0%; product number XK 13-201-00578; BET: 2.93) was also used for

 u 100%

0

A A A

 u 100%

(1)

where C0 and A0 are the initial concentration and absorbance of KGL, and Ct and At are the concentration

and absorbance of KGL at a certain reaction time t

3 Results and discussion

3.1 Structure and morphology

Figs 1 ac show FESEM images of the ZnO microstructures synthesized at 120 ºC for 17 h at low, medium, and high magnifications, respectively From the FESEM images, it can be seen that the ZnO

product consisted of numerous 3D flower-like aggregates, with single flowers having diameters in the range 23 m In addition, each flower was made up of many thin nanosheets as “petals”, and these nanosheets were about 30 nm in thickness Further information about the ZnO product was obtained

from TEM and HRTEM images and the associated SAED patterns Fig 1d shows a typical TEM image

of a flower-like ZnO microstructure, confirming the 3D structure with a diameter of about 2 m and its

construction from numerous nanosheets The SAED pattern shown in the inset of Fig 1d indicates the

single-crystalline nature of the nanosheet The HRTEM image shown in Fig 1e exhibits well-resolved

lattice fringes with a spacing of 0.26 nm, which is in good agreement with the interplanar spacing of the

(0001) plane The XRD pattern of the flower-like ZnO microstructure is displayed in Fig 1f All of the

diffraction peaks could be well indexed to hexagonal wurtzite ZnO (JCPDS Card No 36-1451) No

characteristic peaks from any impurities were detected In addition, the strong and sharp peaks indicated

that the prepared ZnO was highly crystalline

3.2 Effect of hydrothermal treatment time

In order to reveal the formation mechanism of the 3D flower-like ZnO, SEM images and XRD

patterns were acquired at appropriate intervals during the time-dependent evolution process Fig 2a

shows relatively uniform microspheres with an average diameter of 2 m, which were collected before

being transferred to the Teflon-sealed autoclave From the magnified image shown as an inset in Fig 2a,

one can see that the microspheres were composed of tiny nanosheets When the reaction time was

extended to 4 h (Fig 2b), these tiny nanosheets were gradually extended When the hydrothermal

treatment time was increased to 8 h and 12 h, more and larger nanosheets grew and the shapes of the

flower-like ZnO microstructures were further developed Finally, well-defined 3D flower-like

microstructures were obtained after extending the reaction time to 17 h (Fig 2e) Therefore, it can be

concluded that flower-like ZnO is produced after a hydrothermal treatment time of 4 h, and further

increasing the hydrothermal treatment time makes the diameters of the nanosheets more uniform and the

flower-like ZnO more defined in accordance with the Ostwald ripening mechanism [25] The surface

areas of the samples increased accordingly with extending the hydrothermal time; Table 1 lists the

surface areas of different samples It can be seen that the surface areas of the samples increased from 2.25

to 11.05 m2/g when the hydrothermal time was increased from 0 h to 17 h XRD patterns of the samples

obtained after different reaction times are shown in Fig 2f It is worth noting that all of the diffraction

peaks could be well indexed to hexagonal wurtzite ZnO (JCPDS card No 36-1451)

3.3 Possible growth mechanism of the 3D flower-like ZnO

A schematic illustration of the formation process is presented in Fig 3, and a possible formation

mechanism for the 3D flower-like ZnO is proposed as follows Firstly, in the solgel process, it is

supposed that Zn(II)citric acid chelate complexes are formed during the gelation of the sol [38], and

then the gel is dissolved by adding a certain amount of NaOH Some of the OH ions in the solution

might neutralize H+ ions derived from the citric acid, while other OH ions might react with the Zn(II)citric acid chelate complexes to form [Zn(OH)4]2 complexes, which will decompose into ZnO nuclei (Eqs (2)(4)) [39,40]

Zn(OH)42- oZnO + H2O + 2OH- (4)

At the same time, further OH left in the solution will affect the morphology of the ZnO It is known that

ZnO forms polar crystals, with a positive polar (0001) plane rich in Zn2+ cations and a negative polar

(0001) plane rich in O2 anions [41] Usually, hexagonal rod-like ZnO elongated along the c-axis

direction would be obtained due to the intrinsic anisotropy in its growth rate v with [0001] >>

[0110] >> [0001] [42] However, in the present case, because the molar ratio of Zn2+ to OH is about 1:12 (in solution at pH 14), a very high concentration of OH is present in the aqueous medium

Following the decrease in the concentration of Zn(OH)4  due to the initial fast nucleation of ZnO, the

absorption of OH ions on the positively charged Zn-(0001) plane would dominate in the competition

with Zn(OH)4  Therefore, the excess OH ions stabilize the surface charge and the structure of the

Zn-(0001) surfaces to some extent, allowing fast growth along the [0110] direction, which leads to the

formation of ZnO nanosheets with a {21 10}-plane surface [12] In order to minimize the total surface

energy, numerous spherical ZnO aggregates composed of tiny nanosheets are then formed in the reaction

system (Fig 2a) [40, 43]

Secondly, in the hydrothermal process, the ZnO aggregates would tend to further decrease their

energy through surface reconstruction, which would provide more active sites for further heterogeneous

nucleation and growth Thus, the nanosheets would grow out continuously from the surface of the

primary structures (Fig 2b) [40] Subsequently, with increasing hydrothermal treatment time, more and

more nanosheets with a {21 10}-planar surface become interlaced and overlapped with each other to

form a multilayer network structure, and thereby the flower-like ZnO nanostructures are shaped (Figs 2c2e)

3.4 Photocatalytic activities of ZnO samples synthesized for different hydrothermal treatment times

To demonstrate their potential environmental application in the removal of contaminants from

wastewater, the photocatalytic activities of the as-synthesized ZnO samples were investigated by the

degradation of KGL From Fig 4(a), it is clear that the ZnO samples synthesized for different

hydrothermal treatment times exhibited different photocatalytic activities Their photocatalytic activities

increased with increasing hydrothermal treatment time The ZnO synthesized for a hydrothermal time of

17 h showed superior photocatalytic activity, degrading KGL by 96.7 % after irradiation for 60 min

It is known that when semiconductor materials are irradiated with light of energy higher than or equal

to the band gap, an electron (ecb) in the valence band (VB) can be excited to the conduction band (CB)

with the simultaneous generation of a hole (hvb+) in the VB Excited-state ecb and hvb+ can recombine and

become trapped in metastable surface states, or react with electron donors and electron acceptors

adsorbed on the semiconductor surface In other words, the photoelectron is easily trapped by electron

acceptors such as adsorbed O2, whereas the photoinduced holes can be easily trapped by electron donors

such as OH or organic pollutants, to further oxidize organic dyes [44] There are many factors that may

influence the photocatalytic activity of ZnO, such as morphology, surface area, surface defects, and so on

[45] On the one hand, the larger surface area (Table 1) in the ZnO (120 ºC, 17 h) sample can provide

more active sites for the adsorption of KGL, and then facilitate the diffusion and mass transportation of

KGL molecules and hydroxyl radicals during the photochemical reaction [14] On the other hand,

oxygen defects may be considered to be the active sites of the ZnO photocatalyst [46] Since an

appropriate amount of oxygen vacancies can entrap electrons from the semiconductor, the holes can

diffuse to the surface of the semiconductor and cause oxidation of the organic dye Therefore, a high

density of surface oxygen defects is beneficial for efficient separation of electron-hole pairs, minimizes

the radiative recombination of electrons and holes, and increases the lifetime of the charge carriers,

thereby improving the photocatalytic activity [47] To study the surface defects of the as-synthesized

ZnO samples, room temperature photoluminescence (PL) spectra were measured using Xe light (350 nm)

as the excitation source, and representative spectra are shown in Fig 4(b) Two main emission peaks are

seen for all three samples, a strong peak at around 388 nm, corresponding to the near band-edge emission

(NBE) [48], and a broad band emission extending from 400 to 650 nm, covering the blue to yellow

region The emission in the visible-light region is attributed to ZnO surface defects, among which oxygen

vacancies are likely to be the most prominent [49] The PL intensity varied in the following order: 17 h >

12 h > 8 h > 4 h > 0 h, suggesting that ZnO (17 h) possessed the highest density of surface defects, and

this may have been responsible for its superior photocatalytic activity

3.5 Effect of hydrothermal treatment temperature

In order to further study the effects of the hydrothermal treatment temperature on the ZnO product,

samples were also synthesized at 90 ºC and 150 ºC for 17 h The morphologies of the ZnO samples

prepared at different hydrothermal treatment temperatures are shown in Figs 5a and 5b It can be

observed that the sample prepared at 90 ºC (Fig 5a) showed a flower-like morphology with a diameter of

12 m, but the edges of the nanosheets were not obvious When the temperature was increased to

150 ºC (Fig 5b), the diameter of the sample reached 34 m, and the nanosheets accumulated more

densely and became thicker Comparing Figs 5 a,b with Fig 2e, it can be concluded that flower-like ZnO can be obtained over a relatively wide hydrothermal temperature range (90150 ºC), but that the interspace of nanosheets in the ZnO sample synthesized at 120 ºC is larger than that in the samples

synthesized at either lower (90 ºC) or higher hydrothermal temperature (150 ºC) This may be because

the growth rate of nanosheets is slower at low reaction temperature (90 ºC) [50], and faster at high

reaction temperature (150 ºC); consequently, in the same hydrothermal time, the nanosheets in the

flower-like microstructures are either not fully formed or accumulate more densely A larger interspace

between nanosheets means greater surface area, which will influence the photoactivity of the sample to

some extent Comparison of the surface areas listed in Table 2 shows that the ZnO sample synthesized at

120 ºC indeed had the largest surface area The XRD pattern of this sample (Fig 5c) could also be well

indexed to hexagonal wurtzite ZnO (JCPDS Card No 36-1451)

3.6 Photocatalytic activity of ZnO samples synthesized at different hydrothermal temperatures

Fig 6a shows the degradation rates of KGL as a function of irradiation time in the presence of ZnO

samples synthesized at different hydrothermal treatment temperatures In the absence of light or catalyst,

the concentration of KGL showed no obvious change over 120 min, indicating that both light and catalyst

were necessary for effective photodegradation of the KGL dye [51] After irradiation for 120 min, the

degrees of degradation of KGL were about 82.17% for the ZnO (150 ºC) sample, and almost 100% for

ZnO (120 ºC) and ZnO (90 ºC) samples and a commercial ZnO sample It should be noted that after

irradiation for 40 min, only for the ZnO (120 ºC) sample did the degree of degradation approach 100%,

and accordingly the color of the KGL had disappeared (Fig 6b) The ZnO (120 ºC) sample clearly

showed the best photocatalytic activity The reasons can be explained as follows Firstly, as mentioned

above, the ZnO (120 ºC) sample had a larger surface area and a greater interspace between the

nanosheets than the other samples A greater interspace between the nanosheets can effectively prevent

aggregation during the photodegradation, and a larger surface area can provide more active sites for the

adsorption of KGL Hence, the photocatalytic activity of the ZnO (120 ºC) sample was enhanced

Secondly, as shown in Fig 6c, the PL intensity varied in the following order: 120 ºC > 90 ºC > 150 ºC,

meaning that the ZnO (120 ºC) sample separated electrons and holes more efficiently than the other

samples Hence, the ZnO (120 ºC) sample showed the best photocatalytic activity, followed by the ZnO

(90 ºC) sample and the ZnO (150 ºC) sample, respectively

4 Conclusions

In summary, 3D flower-like ZnO composed of nanosheets has been successfully synthesized by a solgel-based hydrothermal method over a relatively broad temperature range (90150 ºC) after 17 h of hydrothermal reaction or at 120 ºC after 4 h A possible formation mechanism has been proposed based

on the experimental results Compared with the other samples, the ZnO prepared at 120 ºC for 17 h

exhibited enhanced photocatalytic activity, indicating that it may potentially be used as a promising

photocatalyst for practical application in the photocatalytic degradation of organic dyes